FABRICATION AND ASSEMBLY OF ONE-DIMENSIONAL SEMICONDUCTOR
NANOSTRUCTURES AND THEIR APPLICATION TO MULTI-FUNCTIONAL DEVICES
By
Yaozhong Zhang

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
Mechanical EngineeringDoctor of Philosophy
2017

ABSTRACT
FABRICATION AND ASSEMBLY OF ONE-DIMENSIONAL SEMICONDUCTOR
NANOSTRUCTURES AND THEIR APPLICATION TO MULTI-FUNCTIONAL DEVICES
By
Yaozhong Zhang
One-dimensional (1D) semiconducting nanostructures have been extensively studied in the
past few decades due to their excellent electrical, chemical and optical properties, which enable
remarkable performance in various applications such as electronic/optoelectronic devices,
chemical/biological sensors and energy harvesting. Since water pollution becomes as one of the
most challenging global issues, photocatalytic degradation has received considerable attentions as
an efficient water treatment technology with regard to removing organic contaminants. Upon
receiving UV or sunlight irradiation with energy higher than their bandgap, semiconducting
materials generate charge carriers such as electrons and holes that react with water and produce
reactive species for decomposition. TiO2 and ZnO are the most widely used photocatalysts because
of their higher degradability and lower production cost. Especially, TiO2 and ZnO nanostructures
exhibit better degradability thanks to their low recombination of charge carriers and large active
surface areas. However, the challenges like low utilization of sunlight (mainly responsive to UV
light) and non-recyclability limit their large-scale use as photocatalysts. In this work, we tackle
these limitations by developing a new immobilized photocatalytic system to improve their
recyclability and investigating a novel semiconductor-semiconductor heterogeneous material to
enhance their optical response in the visible region. First, ZnO nanowires (NWs) have been
synthesized using a hydrothermal process and hybridized with silicon nanocrystals (SiNCs). This
heterojunction lowers the ZnO bandgap (more active under visible light) and exhibits superior
photodegradation performance under the visible and white light conditions compared to original

ZnO NW photocatalysts. Second, we have developed a novel fabrication technique to create a
vertically-aligned ZnO NW array on a polymer substrate with strong adhesion. The proposed twostep fabrication process allows the part of NWs to be embedded into the polymer matrix, securing
the nanomaterials for harsh operating environments. A ZnO-NW/Polydimethylsiloxane (PDMS)
film presents the unique immobilized photocatalytic system that can float on the water surface,
targeting buoyant pollutants. In particular, crude oil has been used a model pollutant for
degradation experiment. A strong adhesion of ZnO NWs to the polymer substrate also enables two
new implementations of the photocatalytic system including the application with high shear
stresses and piezoelectrocalysis. The latter application is particularly interesting as organic
pollutants were degraded via mechanical vibration without resorting to light energy. Finally, this
two-step synthesis technique combined with strain engineering allows us to create multifunctional
soft micromotors, i.e., the ZnO-NWs/PDMS submillimeter 3D structures integrated with various
nanomaterials (metal nanoparticle catalysts, magnetic nanoparticles, etc.). The micromotors
possess multiple functionalities such as photocatalysis, piezoelectrocatalysis, locomotion/selfpropulsion, and magnetic response. This level of multifunctionality integration on a single
platform can be rarely found in the literature. In the end, an immobilized photocatalytic system
with high surface areas, improved mass transfer, and easy recyclability has been developed, which
can find its uses in biomedical and environmental applications. Apart from the photocatalytic
applications, the research has also been oriented to address some technical challenges in
deterministic assembly of the solution-processed 1D nanostructures for device integration.

Copyright by
YAOZHONG ZHANG
2017

ACKNOWLEDGEMENTS

All my accomplishments and honors in these years are coming from the help and the
support that I have received from all professors, colleagues, friends and family. First, I would like
to thank my advisor, Dr Junghoon Yeom, for offering me a great opportunity of working in his
group. His academic guidance and continuous support has greatly helped me in pursuing my dream
as a researcher. After working with him, my research scope is much broadened and my research
skills are significantly improved. I am deeply impressed and inspired by his rigorous academic
attitude and I believe my work experience in his laboratory will be of benefit to my future academic
career. Next, I would like to thank all my committee members, Dr Ranjan Mukherjee, Dr Rebecca
Anthony and Dr Chuan Wang, for their help in my PhD research. In last few years I received many
helps and supports from my collaborators. Here I would like to specially thank Dr Peter Lillehoj,
Dr Per Askeland, Dr Daniel Ratchford, Dr Alexandra Zevalkink, Dr Lik Chuan Lee, Dr Karl
Dersch, Dr Baokang Bi, Dr Le Cai and Dr Rajib Mandal. And many thanks to my colleagues,
Tung-Yi Lin, Alex Ho, Alborz Izadi, Amberley Xiaolu Huang, Aryan Mehboudi and Haocheng
Sun. In addition, I would express my gratitude to my parents. Their love is always my most
important support in my PhD study. I also want to thank all my friends for their help and
encouragement throughout my PhD career.

v

TABLE OF CONTENTS

LIST OF TABLES ......................................................................................................................... ix
LIST OF FIGURES ........................................................................................................................ x
CHAPTER 1 ................................................................................................................................... 1
INTRODUCTION .......................................................................................................................... 1
1.1 Synthesis and frication of one-dimensional semiconductor nanostructures ...................... 1
1.2 Fabrication challenges of NW assemblies for environmental remediation ....................... 3
1.3 Fabrication challenges of NW assemblies for electronic and optoelectronic devices ....... 5
1.4 Research objectives and scopes of the dissertation ........................................................... 6
REFERENCES ............................................................................................................................. 10
CHAPTER 2 ................................................................................................................................. 14
LITERATURE REVIEW ............................................................................................................. 14
2.1 Approaches of ZnO-based hybrid photocatalysts ............................................................ 14
2.1.1 The mechanism of photocatalytic degradation using ZnO ....................................... 15
2.1.2 The mechanism of charge separation using hybrid ZnO .......................................... 16
2.1.3 The factors influence the performance of ZnO photocatalyst .................................. 19
2.1.4 ZnO/metal oxide hybrid ............................................................................................ 21
2.1.4.1 ZnO/TiO2 hybrid structure ................................................................................. 21
2.1.4.2 ZnO/WO3 hybrid structure ................................................................................. 26
2.1.4.3 ZnO/Fe2O3 hybrid structure ............................................................................... 28
2.1.5 ZnO/carbon-based hybrid ......................................................................................... 31
2.1.6 ZnO/metal hybrid ...................................................................................................... 34
2.1.7 Summary ................................................................................................................... 37
2.2 The improvement of photocatalysts in various systems .................................................. 38
2.2.1 Slurry system ............................................................................................................ 39
2.2.2 Immobilized system .................................................................................................. 42
2.2.2.1 Immobilized photoreactors ................................................................................. 42
2.2.2.2 Buoyant photocatalysts ...................................................................................... 44
2.2.3 Micromotor system ................................................................................................... 46
2.2.4 Summary and outlook ............................................................................................... 48
REFERENCES ............................................................................................................................. 51
CHAPTER 3 ................................................................................................................................. 73
SYNTHESIS AND APPLICATION OF SINGLE 1D SEMICONDUCTOR
NANOSTRUCTURES ................................................................................................................. 73
3.1 Synthesis and application of 1D TiO2 nanostructures ..................................................... 73
3.1.1 Hydrothermal synthesis of TiO2 nanowires .............................................................. 73
3.1.2 Electrochemical synthesis of TiO2 nanotubes........................................................... 78
3.2 Synthesis and application of 1D ZnO nanostructures ..................................................... 84
3.2.1 Hydrothermal synthesis of ZnO NWs for photocatalysts ......................................... 84
3.2.2 Hydrothermal synthesis of ZnO NWs for electronic devices ................................... 91
vi

REFERENCES ............................................................................................................................. 96
CHAPTER 4 ............................................................................................................................... 102
ZNO NANOWIRES/SI NANOCRYSTALS HETEROSTRUCTURES AS IMMOBILIZED
PHOTOCATALYSTS FOR PHOTODEGRADATION ............................................................ 102
4.1 Introduction ................................................................................................................... 102
4.2 Experimental .................................................................................................................. 104
4.2.1 Synthesis of ZnO NWs ........................................................................................... 104
4.2.2 Synthesis of Si nanocrystals.................................................................................... 105
4.2.3 Surface Functionalization Process for Si NCs ........................................................ 106
4.2.4 Preparation of Hybrid Structure .............................................................................. 106
4.2.5 Materials Characterization ...................................................................................... 107
4.2.6 Photodegradation Assessment ................................................................................ 108
4.3 Result and discussion..................................................................................................... 109
4.4 Conclusion ..................................................................................................................... 123
REFERENCES ........................................................................................................................... 125
CHAPTER 5 ............................................................................................................................... 133
IMMOBILIZED FLOATING PHOTOCATALYSTS: A NOVEL CONFIGURATION OF
ZNO/PDMS HETEROSTRUCTURES ...................................................................................... 133
5.1 Introduction ................................................................................................................... 133
5.2 Experimental .................................................................................................................. 135
5.2.1 Synthesis of ZnO nanowire array ........................................................................... 135
5.2.2 Secondary growth of ZnO nanowire array.............................................................. 136
5.2.3 Materials characterization ....................................................................................... 136
5.2.4 Photocatalytic assessment and activities ................................................................. 137
5.2.5 Residue inspection .................................................................................................. 138
5.3 Result and discussion..................................................................................................... 138
5.4 Conclusion ..................................................................................................................... 152
REFERENCES ........................................................................................................................... 153
CHAPTER 6 ............................................................................................................................... 159
MULTIFUNCTIONAL SOFT MICROMOTORS FOR PHOTOCATALYTIC DEGRADATION
..................................................................................................................................................... 159
6.1 Introduction ................................................................................................................... 159
6.2 Experimental .................................................................................................................. 162
6.2.1 Preparation of ZnO NWs ........................................................................................ 162
6.2.2 Fabrication of ISMs ................................................................................................ 163
6.2.3 Characterization ...................................................................................................... 164
6.2.4 Photocatalytic assessment and activities ................................................................. 164
6.3 Result and discussion..................................................................................................... 165
6.5 Conclusion ..................................................................................................................... 181
REFERENCES ........................................................................................................................... 182
CHAPTER 7 ............................................................................................................................... 187
RAPID AND LARGE-SCALE ASSEMBLY PROCESS FOR 1D NANOSTRUCTURES ..... 187
7.1 Introduction ................................................................................................................... 187
vii

7.2 Experimental .................................................................................................................. 190
7.2.1 Blanket transfer process .......................................................................................... 190
7.2.2 Patterning of the MCE membrane .......................................................................... 192
7.2.3 SWCNT solution preparation and vacuum filtration .............................................. 193
7.2.4 SWCNT transfer and transistor fabrication ............................................................ 195
7.2.5 Materials and device characterization ..................................................................... 195
7.3 Result and discussion..................................................................................................... 196
7.4 Conclusion ..................................................................................................................... 210
REFERENCES ........................................................................................................................... 212
CHAPTER 8 ............................................................................................................................... 218
CONCLUSION AND OUTLOOKS........................................................................................... 218

viii

LIST OF TABLES

Table 2.1. General summary of ZnO/TiO2 hybrid photocatalysts ............................................... 22
Table 2.2. General summary of ZnO/WO3 hybrid photocatalysts ............................................... 27
Table 2.3. General summary of ZnO/Fe2O3 hybrid photocatalysts .............................................. 30
Table 2.4. General summary of ZnO/CNT hybrid photocatalysts ................................................ 33
Table 2.5. General summary of ZnO/G and ZnO/rGO hybrid photocatalysts.............................. 33
Table 2.6. General summary of ZnO/metal hybrid photocatalysts ............................................... 36
Table 2.7. Summary of ZnO-based photocatalysts in slurries ...................................................... 41
Table 2.8. Summary of immobilized ZnO-based photocatalysts .................................................. 44
Table 2.9. Recent approaches with respect to ZnO buoyant photocatalysts ................................. 46
Table 2.10. Comparison of three immobilized systems ................................................................ 48
Table 2.11. Comparison of slurry and immobilized systems ....................................................... 48
Table 3.1. A brief comparison of the three techniques with respect to the through-out free-standing
TiO2 NT array films. ..................................................................................................................... 81
Table 7.1. Comparison of the MCE, AAO and polycarbonate membrane in SWCNT filtration tests.
..................................................................................................................................................... 205

ix

LIST OF FIGURES

Figure 2.1. Schematic of photocatalysis process on single-component ZnO photocatalysts, where
chemical equations represent detail photocatalysis process ......................................................... 16
Figure 2.2. Band diagram of ZnO/TiO2 hybrid nanostructures, where the heterojunction formed
between ZnO and TiO2 promotes charge separation. (Adapted from Reference 9) .................... 17
Figure 2.3. Three types of band alignments. (a) type I, (b) type II and (c) type III. (Adapted from
Reference 10) ................................................................................................................................ 18
Figure 2.4. Band diagram of n-ZnO/p-CuO hybrid nanostructures, where the heterojunction
formed between ZnO and CuO promotes charge separation. (Adapted from Reference 11) ....... 19
Figure 3.1. The correlation parameter towards TiO2 NW growth, (a) wire dimension as a function
of molarity of the solute, (b) wire length as a function of growth time. Scale bar: 1 Âľm. ............ 73
Figure 3.2. (a) A SEM image of the TiO2 NWs via the hydrothermal method. (b) XRD pattern of
the as-prepared TiO2 NWs. Scale bar: 1 Îźm. ................................................................................ 74
Figure 3.3. Photocatalytic degradation of MB dye at room temperature. (a) the UV-Visible
absorbance spectra of a MB dye with dispersed TiO2 NWs. (b) normalized MB dye concentration
measured at 665nm from (a) with and without TiO2 NWs as a function of time. ........................ 75
Figure 3.4. The schematic of the fabrication of the TiO2 NWs on the glass substrate. ................ 76
Figure 3.5. SEM images of the TiO2 NWs via the hydrothermal method grown. (a) on a glass
substrate, (b) on a polyimide film. Scale bar: (a) 100 Îźm on a large view and 10 Îźm in the inset,
(b) 20 Îźm on alarge view and 5 Îźm in the inset. .......................................................................... 77
Figure 3.6. Photocatalytic degradation of TiO2 NWs grown on the glass substrate at room
temperature. (a) the UV-Visible absorbance spectra change of MB dye with dispersed TiO2 NWs.
(b) degradation of the MB dye with and without TiO2 NWs catalysts measured by the normalized
peak height of (a) at 665 nm as a function of time. ...................................................................... 78
Figure 3.7. SEM images of the TiO2 NTs via the electrochemical anodization method. (a) one-step
anodization, (b) two-step anodization. Scale bar: (a) 1 Îźm on the large view and 100 nm in the
inset, (b) 5 Îźm on the large view and 500 nm in the inset. ........................................................... 79
Figure 3.8. SEM images of the TiO2 NT array. (a) cross-sectional view, (b) bottom view. Scale
bar: 200 nm in (a) and 100 nm in (b). ........................................................................................... 80
Figure 3.9. Optical images of the as-prepared free-standing film of TiO2 NT array. ................... 82

x

Figure 3.10. SEM images of the as-prepared free-standing TiO2 NT film. (a) a Ti substrate after
detachment, (b) the detached TiO2 NT film. Scale bar: (a) 10 Îźm on the large view and 1 Îźm in
the inset, (b) 10 Îźm on the large view and 2 Îźm in the inset. ....................................................... 82
Figure 3.11. SEM images of the as-prepared free-standing TiO2 NT film and its corresponding.
Scale bar: 1Âľm. ............................................................................................................................. 83
Figure 3.12. (a) SEM image of the ZnO NWs on the Zn foil. (b) XRD pattern of the as-prepared
ZnO NWs. (Scale bar: 200 Îźm on the large view and 300 nm in the inset) ................................. 85
Figure 3.13. The schematic of the fabrication sequence for the patternable ZnO NW arrays on the
Zn foil. A vertically-aligned ZnO NW array can be obtained as the PDMS cover is removed
afterwards. ..................................................................................................................................... 86
Figure 3.14. (a) SEM images of the patterned ZnO NWs on the Zn foil, (b) boundary of the
patterned ZnO NWs. Scale bar: (a) 50 Îźm on the large view and 10 Îźm in the inset, (b) 20 Îźm on
the large view and 5 Îźm in the inset ............................................................................................. 87
Figure 3.15. Schematic diagrams of the fabrication for the differential growth of the ZnO NW
arrays on the Zn foil. ..................................................................................................................... 87
Figure 3.16. SEM images of the ZnO NWs grown on the ZnO seeding layer. (a-b) the coated
seeding layer was facing up during the reaction, (c-d) ) the coated seeding layer was facing down
during the reaction. Scale bar: (a) 50 Îźm on the large view and 5 Îźm in the inset, (b) 5 Îźm on the
large view and 300 nm in the inset, (c) 10 Îźm on the large view and 500 nm in the inset, (d) 10
Îźm on the large view and 3 Îźm in the inset. ................................................................................. 88
Figure 3.17. The schematics of the fabrication sequence for the patternable ZnO NW arrays on the
glass substrate. .............................................................................................................................. 89
Figure 3.18. (a) A SEM image of the patterned ZnO NWs via the ZnO seeding layer, (b) XRD
pattern of the as-prepared ZnO NWs. Scale bar: 1 mm on the large view and 10 Îźm in the inset.
....................................................................................................................................................... 90
Figure 3.19. The scheme of the fabrication for the ZnO NW arrays sensor. Highly ordered ZnO
NWs were grown on the zinc substrate vertically. ....................................................................... 93
Figure 3.20. SEM of the ZnO NW array on the zinc substrate (a) after 10 hours hydrothermal
treatment. ZnO NWs in high aspect ratio were distributed uniformly on the substrate, (b) after Al
electrode fabricated. Porous is observed along each ZnO nanowires, which size is corresponding
to the diameter of the nanowires. Angled view of the ZnO NW array after FIB patterning. ZnO
NWs at the boundary was partially cleaved by FIB in (a). Multiple ZnO NW network layers are
observed in (b). Scale bar: (a-b) 10 Âľm in the large view and 3Âľm in the inset, (c-d) 1 Âľm........ 94
Figure 3.21. I-V curve of as-prepared ZnO NW array, which behavior is corresponding to the
typical diode. ................................................................................................................................. 95

xi

Figure 4.1. Top-down and cross-sectional SEM images of the as-prepared SiNCs/ZnO-NWs
hybrid structures: (a) and (e) for ZS0, (b) and (f) for ZS1, (c) and (g) for ZS2, (d) and (h) for ZS3.
Scale bar: 10 μm in (a) – (d); 500 nm in the inset of (a) – (d); 2 μm in (e) – (h). ...................... 109
Figure 4.2. Top-down and cross-sectional SEM images of the as-prepared SiNCs/ZnO-NW hybrid
structures with heavy Si loading, Scale bar: 10 Îźm in (a); 500 nm in the inset of (a); 2 Îźm in (b).
..................................................................................................................................................... 110
Figure 4.3. SEM/EDS elemental mapping of bare ZnO-NW and Si NCs/ZnO NWs hybrid samples:
(a) ZS0; (b) ZS1; (c) ZS2; (d) ZS3. Zn in Red, O in blue and Si in yellow. The respective weight
fractions of Zn and Si are: Zn 85.2% in ZS0; Zn 80.1% and Si 1.7% in ZS1; Zn 70.7% and Si 5.6%
in ZS2; Zn 61.2% and Si 8.5% in ZS3. Scale bar: 5 Îźm. ........................................................... 110
Figure 4.4. TEM images of (a) bare ZnO NW sample (ZS0), (b) synthesized and functionalized Si
NCs, (c) Si-NCs/ZnO-NW hybrid structure prepare from ZS2. Scale bar: 5 Îźm in (a) and (c). (d)
XRD patterns of ZS0 and ZS3, (e) FTIR of ZS0. ....................................................................... 112
Figure 4.5. Raman-scattering spectra of (a) the bare ZnO-NWs (ZS0) and three SiNCs/ZnO-NWs
hybrid structures (ZS1, ZS2, and ZS3), (b) ZS0 showing four Raman peaks of bare ZnO........ 113
Figure 4.6. (a) Room-temperature PL spectra of the bare ZnO NW sample and three Si-NCs/ZnONWs hybrid samples, (b) the same PL spectra with the magnified scale to better show the peaks
related to oxygen defects of ZnO; (c-e) schematic diagrams of ZS0 (c), ZS1 (d), and ZS2/ZS3 (e);
(f) time-resolved PL decay of the SiNCs on the ZnO NWs for the three hybrid samples (signal
filtered at 675 nm +/- 50 nm), (g) a bi-exponential fit of the PL decay...................................... 114
Figure 4.7. UV-Vis diffuse reflectance spectra of the bare ZnO NW sample (ZS0) and three SiNCs/ZnO-NWs hybrid samples (ZS1, ZS2, and ZS3). (a) diffuse reflectance spectra, (b)
corresponding Kubelka-Munk plot as a function of the exciting light energy. .......................... 118
Figure 4.8. Degradation profiles of methylene blue dye solution for the bare ZnO NWs (ZS0) and
SiNCs/ZnO-NWs hybrid samples (ZS1, ZS2, ZS3) under (a) UV light, (b) visible light and (c)
white light. (d) Schematic energy band diagram of charge carrier movements in Si-NCs/ZnO-NWs
hybrid structure. .......................................................................................................................... 119
Figure 5.1. Schematic of the proposed two-step fabrication process of a vertically-aligned ZnO
NW array on a polymer substrate. (a) A ZnO NW array was vertically grown on the glass slide via
the hydrothermal synthesis, (b) a certain amount of a PDMS solution was spin-casted onto the ZnO
NW array and thermally cured afterwards, (c) a PET film was brought into intimate contact with
the PDMS film, (d) the PDMS/PET stack was detached from the glass slide simultaneously, (e)
the PDMS/PET layer became flat due to the recovery of PET film, (f) the ZnO NW array embedded
in the PDMS film was re-grown to a desired length via the same hydrothermal process, (g) ZP film
was obtained after the PET film was removed from the PDMS film, (h) the photograph of asprepared ZP film (1” x 1”). ......................................................................................................... 139
Figure 5.2. SEM images of the ZnO NWs (a) on the glass substrate after first growth, and (b) on
the PDMS before secondary growth, (c) on the PDMS substrate (~300 Âľm in thickness) after
secondary growth and (d) on the PDMS substrate after secondary growth (failed detachment).
xii

Scale bar: (a) and (c) 50 Âľm in the large view and 3 Âľm in the inset, (b) 20 Âľm in the large view
and 5 Âľm in the inset, (d) 50 Âľm in the large view and 2 Âľm in the inset. ................................. 140
Figure 5.3. (a) XRD patterns of ZnO NWs, (b) photoluminescence spectrum of ZnO NWs. .... 141
Figure 5.4. (a) degradation profiles of MB dye (at 667nm) in four consecutive cycles, (b)
photodegradation kinetics of MB dye in four consecutive cycles, (c) constant k. ..................... 144
Figure 5.5. SEM images of the ZP film (a) before thermal and mechanical test and (b) after thermal
and mechanical test. (c) and (d) are the photographs of ZP films (1”x1”) imaged in (a) and (b),
respectively. Scale bar: 10 Âľm in the large view and 1 Âľm in the inset. .................................... 145
Figure 5.6. Schematics of ZP film (a) before secondary growth of ZnO NWs, (b) after secondary
growth of ZnO NWs, (c) degradation profiles of MB dye (at 667 nm) before/after secondary
growth, (d) degradation profiles of MB dye with/without magnetic stirring. ............................ 146
Figure 5.7. Schematics of ZP film (a) without vibration, (b) with vibration, (c) degradation profiles
of MB dye (at 667nm) in dark condition with ultrasonication, (d) degradation profiles of MB dye
(at 667nm) under UV light irradiation with ultrasonication. ...................................................... 148
Figure 5.8. Schematic of pizeo-photocatalytic mechanism of ZP film....................................... 149
Figure 5.9. Schematics of (a) mixer setup and (b) float setup, (a1) and (a2) the photograph of mixer
ZP film used in (a), (b1) and (b2) are the photograph floated ZP film used in (b). All the ZP films
are in 1” x 1”. .............................................................................................................................. 150
Figure 5.10. (a) spectral changes of crude oil UV light irradiation, (b) degradation profiles of crude
oil at 288nm and 293nm under UV light irradiation................................................................... 151
Figure 5.11. crude oil samples (a) before and (b) after degradation ........................................... 151
Figure 6.1. Fabrication schematic of ISM under uniaxial SPB and producible shapes. (a) verticalaligned ZnO NWs grow on the substrate, (b) spin-cast thin PDMS layer, (c) pre-stretched one thin
PDMS film and plasma-bond to ZnO-PDMS substrate, (d) pattern shapes and remove residue
edges, (e) peel off the bilayer from the substrate and obtain the micromotors, (f) process for the
semi-circular ISMs, (g) process for the circular ISMs, (h) process for the over-rolled ISMs. Scale
bar: 500 Îźm. ................................................................................................................................ 166
Figure 6.2. (a-d) SEM images of the ISM produced by uniaxial process, (b) outer surface of the
ISM, (c) boundary of Layer P and Layer Z, (d) side surface of Layer P, (e) Photoluminescence
spectrum of the ZnO NWs on ISM and glass, (f) XRD pattern of the ZnO NW on ISM and glass.
Scale bar: 500 Îźm in a; 30 Îźm in b, 10 Îźm in c and 20 Îźm in d; 5 Îźm in the inset of b............. 168
Figure 6.3. (a) photocatalytic activity of ISMs in different quantities, (b) numbers of ISMs with
various shapes on the degradation rate of MB. Experiments proceeded at room temperature. .. 169
Figure 6.4. Time-lapsed images of a micromotor in the 70% ethanol solution with 5% H2O2. (a) t
= 1s, (b) t = 10s, (c) t = 20s and (d) t = 30s. Scale bar: 2 mm. ................................................... 170
xiii

Figure 6.5. Time-lapsed images of a micromotor atop water. (a) t = 1s, (b) t = 20s, (c) t = 40s and
(d) t = 60s. Scale bar: 2 mm. ....................................................................................................... 173
Figure 6.6. Photocatalytic activity of ISMs (a) with/without self-propulsion, (b) with/without
magnetic propulsion. Experiments proceeded at room temperature. .......................................... 174
Figure 6.7. Schematics of the SPB process. (a) Layer P was stretched at a certain ratio, (b) the
bilayer structure was formed between Layer P and Layer Z, (c) The deformation of bilayer
structure took place right after the releasing of stretching force on Layer P, (d) The deformation
process stopped until bilayer structure was fully relaxed, where a curved bilayer structure was
obtained. ...................................................................................................................................... 174
Figure 6.8. (a) the comparison of experimental curvatures and analytical curvatures as a function
of the stretching ratio of Layer P, (b) detail comparison of model 2 and model 3, (c) circular ISM
with L=1500 Âľm, (d) over-rolled ISM with L=2000 Âľm. Scalebar:100 Âľm. ............................. 178
Figure 6.9. (a-g) schematics of biaxial SPB process. (h-l) hollow and semi-closed ZCM with
various shapes and sizes, Scalebar: (h-j) 500 Âľm, (k,l) 1 mm. ................................................... 179
Figure 7.1. A schematic diagram of the fabrication sequences showing a versatility of PR blanket
transfer in different settings; (a) a PR film spin-coated and partially cured on a flat PDMS stamp;
and the fabrication of exemplary multilayer or freestanding microstructures using the PR BT
process such as (b1-b4) multilayer polymeric structures, (c1-c4) free-standing polymeric
structures, (d1-d4) PR patterns on a porous membrane. ............................................................. 190
Figure 7.2. (a) An optical micrograph of an array of PR 1518 lines patterned on the AAO
membrane; light jade – PR lines, dark jade – AAO membrane, (b) an SEM image of the PR patterns
in (a) showing the open porous structures; scale bar in inset image is 500 nm, (c) an optical
micrograph of an array of SWNT mats after the vacuum filtration of SWNT suspension through
the PR-patterned AAO membrane; light green – PR pattern, dark jade – SWNT mat on the AAO
membrane, (d) an SEM image of the single square window on which the SWNTs are selectively
deposited to form a CNT mat; scale bar in inset image is 500 nm. ............................................ 192
Figure 7.3. (a) The schematic diagram of the filtration system for depositing SWCNTs. (b) Photo
of the filtration system in the experiment. .................................................................................. 194
Figure 7.4. A schematic of the filtration guided assembly (FGA) process; (a) spin-coating of PR
on the flat PDMS stamp, (b) conformal contact between PR/PDMS and the MCE filter membrane,
(c) slow peeling of PDMS transferring the PR layer onto the membrane, (d) patterning PRs using
photolithography, (e-f) vacuum filtration to guide nanomaterials onto the pattern openings. ... 196
Figure 7.5. Optical microscope images of the PR-patterned MCE membrane before SWCNT
filtration (a),(c),(e) and after filtration (b),(d),(f). Various SWCNT patterns were formed onto the
opened regions of the MCE membrane. Scale bars are 100 Îźm in the large views and 10 Îźm in the
inset. ............................................................................................................................................ 198
Figure 7.6. Photographic images of the 47-mm MCE membrane patterned with the PR layer (a)
before filtration and (b) after filtration of the SWCNT solution; SEM images of the individual linexiv

array pattern (c) before SWCNT filtration and (d) after the filtration; (e-g) SEM images of the
SWCNT films deposited on the opened pattern for three different SWCNT loadings – 0.1, 0.25,
0.8 mg/L, respectively. Scale bar: 8 mm in (a) and (b), 100 Îźm in large view of (c) and (d), 20 Îźm
in the inset (top right) of (c) and (d), 200 nm in (e-g). ............................................................... 199
Figure 7.7. Photographs of the 47-mm-diameter MCE membranes (a) after filtration of 40 mL of
highly concentrated (~8 mg/L) SWCNT solution and (b) after filtration of the waste solvent
collected from (a). (c) Raman spectroscopy measurements performed on the bare MCE membrane,
after 1st filtration of the SWCNT solution (a) and after re-filtration of the wasted solvent (b). The
characteristic Raman peak (G band) is missing in the re-filtrated MCE membrane. ................. 200
Figure 7.8. (a) A photograph of the 47-mm MCE membrane patterned with the PR layer before
filtration, (b) an optical microscope image of the individual line array pattern from (a), (c) the same
MCE membrane as (a) but after filtration of the SWCNT solution, (d) the SWCNT line-array
pattern from (c). Scale bar: 8 mm in (a) and (c), 100 Îźm in (b) and (d). .................................... 201
Figure 7.9. (a) The average filtration speed measured for the various SWNT loadings with the
fixed filtration volume (40 mL), (b) the surface profilometry scan of SWCNT patterns, (c) the
sheet resistances of the unsorted SWCNT patterns from the five locations indicated in the inset
(after transferred onto the glass substrate), (d) the percolation behavior of unsorted SWCNT
pattern. ........................................................................................................................................ 206
Figure 7.10. The surface profilometry scans for 6 SWCNT line patterns at 4 different random
locations. The SWCNT patterns were transferred to the silicon surface before the measurements.
The 6 line patterns at each location have the following averages and standard deviations of the
measured heights: (average = 330 nm, standard deviation = 34 nm), (325 nm, 44 nm), (358 nm, 21
nm), (344 nm, 27 nm) ................................................................................................................. 207
Figure 7.11. Electrical characteristics of the bottom-gated SWCNT transistors. (a) Transfer
characteristics of a representative device (channel length = 5 Âľm, width = 20 Âľm) measured with
VDS varying from –5 to 0 V (Inset image – the false-colored SEM image of the transferred
SWCNTs patterns bridging on the electrodes; The golden color region represents the Au contacts
and three horizontal strips are the SWCNTs patterns. The dark vertical strip in the middle indicates
the gap space between two electrodes, which corresponds to the transistor channel length; Scale
bar = 20 μm). (b) Output characteristics of the same device with VGS varying from 20 to –20 V.
Histogram of the distributions of transistor performance of the on/off current ratio (c) and fieldeffect mobility (d). ...................................................................................................................... 209

xv

CHAPTER 1
INTRODUCTION
1.1 Synthesis and frication of one-dimensional semiconductor nanostructures
In the past decades, one-dimensional (1D) semiconductor nanostructures have attracted
considerable attention due to their promising applications in electrochemistry[ 1], gas sensing[2],
photocatalysis [3], and optoelectronics [4]. Various devices enabled by 1D semiconductor
nanostructures, such as light emitting diodes, solar cells, and chemical sensors, have demonstrated
remarkable enhancements in their performance thanks to their unique physical properties that are
not available in the bulk counterparts. Devices made out of individual 1D nanostructures typically
exhibit these nascent electrical, mechanical, and optical properties and are suitable for the proofof-the-concept devices. But the fabrication of such devices relies on the “pick and place” method
to position individual 1D nanomaterials and is painstakingly time consuming and ineffective.
Moreover, these individual nanodevices are prone to device-to-device variation and low signal-tonoise ratio due to charge fluctuation, material inhomogeneity and defects [5]. It is therefore not
suitable for many device applications or production on a large scale, at low cost and high
throughput. Instead, the use of networks or ensembles of 1D nanostructures presents a highly
promising route to addressing these issues. The objective of the dissertation is to present novel
nanomanufacturing approaches to 1D nanostructure assembly. The notion of nanomanufacturing
encompasses both top-down and bottom-up processes that deal with the production of
nanomaterials (e.g. nanoparticles, nanowires, nanotubes, etc.), the integration or assembly of these
nanoscale building blocks, and the interconnection techniques to link nanoscale features to the
macroscopic world.

1

The 1D nanostructure networks or ensembles take various assembled forms including
vertically-aligned 1D nanostructure array and aligned or random 2D networks of 1D
nanostructures. The former assembly approach is advantageous because it offers high surface-areato-volume ratio with minimal interface with the substrate and the variation of individual
nanostructures becomes averaged and less susceptible to noise. The latter assembly approach is
most common especially with random orientations of 1D nanostructures laying on the substrate.
In this dissertation, each assembled structure (vertically-aligned and 2D random network) will be
investigated with an emphasis on development of novel fabrication methods tailored for target
applications. For vertically-aligned, semiconductor 1D nanostructures, we pay our attention to the
growth of zinc oxide (ZnO) and titanium dioxide (TiO2) nanowires (NWs) on various substrates.
Likewise, for developments of a random network of 1D nanostructures, single-walled carbon
nanotubes will be considered as a model 1D structure for 2D assembly.
A number of techniques have been developed to create vertically-aligned NWs on different
substrates, which can be roughly classified in two primary strategies: directly growth vs. postprocessing. Direct growth indicated the generation of desired nanostructures through chemical
synthesis. For TiO2 and ZnO NWs, gas-phase and solution-phase methods are widely employed
to create vertically-aligned NW array structures [6,7]. In general, gas phase methods such as
chemical vapor deposition (CVD) [8,9], metal organic chemical vapor deposition (MOCVD) [10,11]
and vapor-liquid-solid (VLS) [12,13] processes provide well-aligned, polycrystalline semiconductor
directing grown onto the substrates. However, the synthesis temperature is rather high (typically
over 500°C), limit the type of substrates that nanomaterials can grow on. In contrast, solutionbased approaches such as the hydrothermal method [14,15] and solvothermal synthesis [16,17] can be
used to grow NWs at relatively low temperature (< 200°C), expanding the options for growth

2

substrates [6,18]. For example, ZnO NWs can be directly synthesized on polymeric substrates using
the hydrothermal method [19]. Polymer substrates cannot be used in a high-temperature, gas-phase
reaction approach. Specifically, this dissertation will address the issues associated with the
adhesion of hydrothermally-grown NWs to polymeric substrates.
Another aspect of nanomanufacturing is to assemble the solution-processed nanomaterials,
in particular 1D nanostructures. Due to ease of handling, transporting, and patterning, 1D
nanostructures are typically suspended in solutions before further manipulation. Post-processing
of the solution-processed 1D nanostructures refers to assembling NWs (or nanotubes, NTs) onto
different substrates after synthesis, by which external stimuli and the assistance of templates are
usually employed. The attempts on using external stimuli, such as electrical and magnetic fields,
enabled precise placement of NWs but worked inefficient in a large area [20,21]. Although templatebased assembly method worked better in a wafer scale, it has drawbacks in difficulty control of
NW density and uniformity [22,23]. Here we will address these issues by proposing a new assembly
approach based on soft-lithographic patterning of porous nanomembranes and vacuum filtration
of 1D nanostructures through the patterned membrane. The fabrication and assembly methods for
NWs/NTs in this dissertation will be investigated for two main applications, photocatalytic
degradation and electronic devices.

1.2 Fabrication challenges of NW assemblies for environmental remediation
Since water pollution has been considered as one of the major global challenges,
photocatalytic degradation, as one of the prospective approaches, has been extensively studied
with the semiconductor nanostructures that harness photons to produce reactive species for
degradation. As one of the most studied semiconductor materials, ZnO has been suggested as an

3

efficient catalyst for the photocatalytic degradation [24–27] due to its high UV light absorption as
well as compatible band structure, offering excellent photocatalytic activities in decomposing
organic compounds. Although promising photocatalysis performance has been observed, ZnO has
limitations due to the high rate recombination of photogenerated electron-hole pairs and low
utilization of solar spectrum [28]. Numerous methods have been developed to extend the
absorbance spectrum to the visible region and circumvent surface charge recombination of the
photogenerated electronhole pairs within ZnO. Amongst, hybriding ZnO with other materials is
regarded as one potential way to counter those defects in ZnO photocatalysis.
In Chapter 2, recent progresses regarding ZnO-based hybrid photocatalysts are summarized
including various foreign materials and different kinds of photocatlayst systems. On the basis of
this extensive literature review, we extract some important factors that can elevate the
photocatalytic performance of ZnO (and TiO2) photocatalysts. A novel hybrid photocatalayst
systems consisting of ZnO NWs and Si nanocrystals is than introduced, whose improvements in
charge transportation and spectral response were explored and the degradabilities with respect to
different light-irradiation condition were analyzed. Aside from the abovementioned challenges,
there are particular demands in fabrication of ZnO photocatalysts for different working conditions.
Usually, solution-based hydrothermal synthesis has a merit of low synthesis temperature, which
allows the ZnO NWs to be created on a flexible polymeric substrate with highly ordered array.
However, one main challenge of this process is weak adhesion between the ZnO NW array and
the substrate. It was often observed that the NWs after growth were detached from the polymer
surface and re-suspended in the solution. With this respects, an immobilized-platform of ZnO
photocatalysts is discussed in the following chapter, describing a novel method to partially embed

4

ZnO NWs in flexible polymeric films for better adhesion. The capability and versatility of the
novel immobilized photocatalysts were investigated afterwards.

1.3 Fabrication challenges of NW assemblies for electronic and optoelectronic devices
Many nanoelectronic devices utilize the uniformly patterned film (or network) of
nanomaterials because the device performance depends on uniformity of conductivity as well as
transparency across the patterned substrates. The previous methods to assemble networks of
nanomaterials, including magnetic-field guided assembly, electric-chemical guided assembly, and
flow-field-based approach, provide some successes in obtaining patterned random network
structures[29]. However, it is still difficult to produce predictable patterns distributed over a large
area, and even more challenging is to create a uniformly patterned film of hybrid nanostructures.
In a later chapter, we describe a solution-based filtration process for the scalable production of
patterned single-walled carbon nanotubes (SWCNTs) networks. A vacuum filtration process has
been originally developed for the separation of solutes from solvents and has been extensively
used in chemistry laboratories. This filtration technique has been used to produce a large-area sheet
of various nanomaterials including carbon nanotubes [30] and graphene/graphene oxide [31]. In
order to create desired devices, a sheet of nanomaterial obtained from filtration needs to be further
patterned typically via photolithography, liftoff or etching, unavoidably generating nanomaterial
wastes. Here, the filter membrane is pre-patterned using the soft-lithography-based technique,
called blanket transfer, previously reported by our group [32], permitting directed assembly of the
nanomaterials and patterning. By means of filtrating suspended SWCNTs solutions over the prepatterned filter membrane and the subsequent transfer technique, we demonstrate an array of
SWCNT patterns with uniform distribution into predetermined windows on the filter membrane.

5

This integration creates a larger number of standard SWCNT transistors across a 2” wafer with
reasonable performance.

1.4 Research objectives and scopes of the dissertation
Heterogeneous photocatalytic degradation working as an effective treatment towards toxic
organic pollutants in industrial wastewaters, has attracted increasing attention in the past 40 years.
Photocatalysts are typically semiconductor materials whose bandgap can absorb the light energy
(i.e. photons) and separate charge carriers such as electrons and holes. Among a number of
semiconductor materials, TiO2 and ZnO have been well investigated and widely used due to their
environmental benign nature, nontoxicity, low cost and remarkable photocatalytic activity. Their
typical low dimensional nanostructures, including nanoparticles and nanowires, have been
experimentally verified to serve as efficient photocatalysts, whereas the issues still remain. First,
for the majority of photocatalysis study, nanosized TiO2 or ZnO materials are used in suspension
during degradation experiments, and therefore, they need to be separated from the cleaned water.
Various immobilized photocatalytic platforms have been studied extensively as well, but their
performance is far below that of the suspension systems due to the decreased surface areas and
mass transfer rate. Second, for both TiO2 and ZnO, their spectral response to sunlight is limited to
the UV region. Therefore, a separate UV source has been implemented for photodegradation,
significantly hampering a widespread adoption in wastewater treatment facilities or other units. In
addition, low degradability against pollutants with high hydrophobicity is also a main limitation
[33]. Unlike TiO2, ZnO suffers from photocorrosion (is particularly weak under acids and bases
[34]) and thus hindering recyclability and long-term use.

6

In Chapter 3 we will present the synthesis strategies of TiO2 and ZnO nanostructures
including nanowires andnanotubes. Series of experiments including SEM, TEM, EDS, XRD,
Raman and photoluminescence were carried out to characterize the as-synthesized TiO2 and ZnO
nanostructures. The photocatalytic degradation and other applications are studied basing on their
various shapes.
Among the variety of ZnO nanostructures, one-dimensional (1D) ZnO NWs has been
proposed as a promising material system owing to its compatibility to various substrates, large
surface area to volume ratio, and rich materials chemistry developed for forming ZnO-based
heterogenous nanostructures [35,36]. In Chapter 4, we report for the first time that the hybrid
structure of Si nanocrystal/ZnO Nanowires (Si NCs/ZnO NWs) is synthesized through the
hydrothermal method and low pressure plasma process, consecutively. The Si NCs boost the
ZnO’s spectral response to more visible light, thus absorbing more solar radiation. In addition,
heterojunction formed by ZnO NWs and Si NCs facilitate the charge carrier separation by proper
band alignment. The Si NCs-decorated ZnO NWs exhibit the better degradability in comparison
to pure ZnO NWs under all lighting conditionos (UV, visible and white-light irradiations). The
characterizations of the morphology and crystal structures are presented. The loading effect of Si
NCs on the optical response of ZnO NWs is systematically studied using UV/Vis absorption
spectra and photoluminescence. Possible mechanism of the charge carriers transport and enhanced
photo-degradability will be proposed and discussed.
Chapter 5 illustrates a novel method to fabricate vertically-aligned ZnO NW array on the
ultra-thin flexible polymeric film as an immobilized photocatalytic system. The NW array has
reliable adhesion to the substrate as well as excellent photo catalytic degradation capability. The
key strategy is that instead of directly growing ZnO NW array from the seed layer on the polymeric

7

substrate, a two-step growth process is utilized to encapsulate hydrothermally-grown ZnO NWs in
a thin polymer layer and use the exposed region of the NWs as a seed layer to regrow them in the
same solution to a desired length. As-grown ZnO NW arrays exhibit the wurtzite crystallites which
are found possessing excellent photocatalytic activities and structural stability. Investigation on a
decomposition rate in four cycles demonstrates not only the stability of the catalytic activity but
also the minimal loss of the as-grown ZnO NW array from the flexible substrate. A strong adhesion
of ZnO NWs to the flexible substrate allows us to mount the film onto a circular rod. The NWs
can survive from harsh operation conditions with high shear stresses such as overhead mixing and
sonication. This also allows us to test the piezocatalytic effect of ZnO, demonstrating that
pollutants are degraded under mechanical vibration without any light energy. This system provides
a pathway to further improve photocatalytic performance of ZnO-based hybrid photocatalysts.
Finally, this immobilized system is lighter than water and thus floating on the surface of water,
targeting hydrophobic or lightweight pollutants such as oil droplets. It has been shown that the
ZnO-NW-based floating catalysts can degrade crude oil on the water surface.
In Chapter 6 we reports a novel photocatalytic platform of micromotors with multiple
functionalities. The proposed micromotor consists of ZnO NWs as photocatalysts and
Polydimethylsiloxane (PDMS) as a base substratefor catalyst immobilization. A ZnO NWsembedded PDMS layer and another pre-stretched PDMS layer are bonded together with an oxygen
plasma surface treatment, which induces an out-of-plane deformation to form 3D shaped
microstructures. The immobilization technique for ZnO NWs has been introduced in Chapter 5.
The micromotors with the predetermined shape, especially tubular shape, can be created by the
interfacial stresses associated with the initial strain mismatch. Self-propelling micromotors created
by the rolled-up technique are composed of the multiple layers where the bubble-generating

8

catalysts (e.g. Pt, Ag) are located at inner wall. Bubbles are generated when H2O2 is catalytically
decomposed into O2 and propel micromotors along the axis. Another functionality, which is remote
navigation, was conferred to micromotors by incorporating Fe-based materials for themagnetic
control [37]. We also show that this modified ZnO catalytic micromotors (ISMs) created by means
of the stretched PDMS bonding method (SPB) have remarkable photocatalytic degradability owing
to its significant increment of active surface area and reduction in mass transfer resistance.
Moreover, the SPB supports to fabricate diverse shapes of ISMs, which may open a door for
creating multifunctional micromotors towards other applications.
The Filtration-Guided Assembly (FGA) method is described in Chapter 7, describing the
fabrication of SWCNT films with specified patterns. By using both the modified soft lithographyic
technique, called blanket transfer (BT) method, and standard photolithography, an array of
microscale patterns is fabricated onto a mixed cellulose ether (MCE) filter membrane. A solution
containing the SWCNTs is filtered through the patterned filter membrane, and the SWCNTs are
deposited onto the open region in the filter subsequently, resulting in an array pattern of SWCNT
networks. Soft lithographic (using PDMS) or other processes have been developed to pick up the
majority of patterned SWCNTs from the membrane and transfer print to a target substrate with an
alignment to an electrode-array. An array pattern of homogeneous SWCNT network films is
created over an area of 20 cm2 within 10 min. Herein, a 2”×2” size array of SWCNT transistors
is fabricated with the critical pattern size of 5 Âľm and the pattern transfer yield is more than 80%.
The electrical performance of the back-gated transistors made from the FGA and transfer method
of 95% pure SWCNTs is then demonstrated. It is believed that the combination of BT and FGA is
compatible to a variety of low-dimensional nanostructures and is an effective way to build an array
of numerous electronic devices on a wafer-scale area.

9

REFERENCES

10

REFERENCES

1. Wang, X. et al. Highly sensitive, temperature-dependent gas sensor based on hierarchical
ZnO nanorod arrays. J Mater Chem C 3, 11397–11405 (2015).
2. Mani, G. K. & Rayappan, J. B. B. A highly selective room temperature ammonia sensor
using spray deposited zinc oxide thin film. Sens. Actuators B Chem. 183, 459–466 (2013).
3. Shen, W. et al. Photocatalytic degradation for methylene blue using zinc oxide prepared by
codeposition and sol–gel methods. J. Hazard. Mater. 152, 172–175 (2008).
4. Ameen, S., Akhtar, M. S., Seo, H.-K., Nazeeruddin, M. K. & Shin, H.-S. An Insight into
Atmospheric Plasma Jet Modified ZnO Quantum Dots Thin Film for Flexible Perovskite
Solar Cell: Optoelectronic Transient and Charge Trapping Studies. J. Phys. Chem. C 119,
10379–10390 (2015).
5. Conley, J. F., Stecker, L. & Ono, Y. Directed integration of ZnO nanobridge devices on a Si
substrate. Appl. Phys. Lett. 87, 223114 (2005).
6. Ge, M. et al. A review of one-dimensional TiO2 nanostructured materials for environmental
and energy applications. J. Mater. Chem. A 4, 6772–6801 (2016).
7. Udom, I., Ram, M. K., Stefanakos, E. K., Hepp, A. F. & Goswami, D. Y. One dimensionalZnO nanostructures: Synthesis, properties and environmental applications. Mater. Sci.
Semicond. Process. 16, 2070–2083 (2013).
8. Shi, J. & Wang, X. Growth of Rutile Titanium Dioxide Nanowires by Pulsed Chemical
Vapor Deposition. Cryst. Growth Des. 11, 949–954 (2011).
9. Fan, Z., Wang, D., Chang, P.-C., Tseng, W.-Y. & Lu, J. G. ZnO nanowire field-effect
transistor and oxygen sensing property. Appl. Phys. Lett. 85, 5923–5925 (2004).
10. Wu, J.-J. & Yu, C.-C. Aligned TiO2 Nanorods and Nanowalls. J. Phys. Chem. B 108, 3377–
3379 (2004).
11. Lee, W., Jeong, M.-C. & Myoung, J.-M. Catalyst-free growth of ZnO nanowires by metalorganic chemical vapour deposition (MOCVD) and thermal evaporation. Acta Mater. 52,
3949–3957 (2004).
12. Xu, C., Zhan, Y., Hong, K. & Wang, G. Growth and mechanism of titania nanowires. Solid
State Commun. 126, 545–549 (2003).
13. Li, S. Y., Lee, C. Y. & Tseng, T. Y. Copper-catalyzed ZnO nanowires on silicon (100)
grown by vapor–liquid–solid process. J. Cryst. Growth 247, 357–362 (2003).

11

14. Wu, W.-Q. et al. Hydrothermal Fabrication of Hierarchically Anatase TiO2 Nanowire arrays
on FTO Glass for Dye-sensitized Solar Cells. Sci. Rep. 3, (2013).
15. Sun, Y., George Ndifor-Angwafor, N., Jason Riley, D. & Ashfold, M. N. R. Synthesis and
photoluminescence of ultra-thin ZnO nanowire/nanotube arrays formed by hydrothermal
growth. Chem. Phys. Lett. 431, 352–357 (2006).
16. Wen, B.-M., Liu, C.-Y. & Liu, Y. Solvothermal synthesis of ultralong single-crystalline TiO
2 nanowires. New J. Chem. 29, 969–971 (2005).
17. Cheng, H.-M., Chiu, W.-H., Lee, C.-H., Tsai, S.-Y. & Hsieh, W.-F. Formation of Branched
ZnO Nanowires from Solvothermal Method and Dye-Sensitized Solar Cells Applications. J.
Phys. Chem. C 112, 16359–16364 (2008).
18. Innocent Udom, M. K. R. One dimensional-ZnO nanostructures: synthesis, properties and
environmental applications. Mater Sci Semicond Process. Mater. Sci. Semicond. Process. 16,
2070–2083. (2013).
19. Greene, L. E. et al. Low-Temperature Wafer-Scale Production of ZnO Nanowire Arrays.
Angew. Chem. Int. Ed. 42, 3031–3034 (2003).
20. Englander, O., Christensen, D., Kim, J., Lin, L. & Morris, S. J. S. Electric-Field Assisted
Growth and Self-Assembly of Intrinsic Silicon Nanowires. Nano Lett. 5, 705–708 (2005).
21. GarcĂ­a-SĂĄnchez, P., Arcenegui, J. J., Morgan, H. & Ramos, A. Self-assembly of metal
nanowires induced by alternating current electric fields. Appl. Phys. Lett. 106, 023110
(2015).
22. Cha, N.-G., Echegoyen, Y., Kim, T.-H., Park, J.-G. & Busnaina, A. A. Convective Assembly
and Dry Transfer of Nanoparticles Using Hydrophobic/Hydrophilic Monolayer Templates.
Langmuir 25, 11375–11382 (2009).
23. Mann, S. Self-assembly and transformation of hybrid nano-objects and nanostructures under
equilibrium and non-equilibrium conditions. Nat. Mater. 8, 781–792 (2009).
24. Lee, K. M., Lai, C. W., Ngai, K. S. & Juan, J. C. Recent developments of zinc oxide based
photocatalyst in water treatment technology: A review. Water Res. 88, 428–448 (2016).
25. Sirelkhatim, A. et al. Review on Zinc Oxide Nanoparticles: Antibacterial Activity and
Toxicity Mechanism. Nano-Micro Lett. 7, 219–242 (2015).
26. Mohd, A. M. A., Julkapli, N. M. & Abd, H. S. B. Review on ZnO hybrid photocatalyst:
impact on photocatalytic activities of water pollutant degradation. Rev. Inorg. Chem. 0,
(2016).
27. Johar, M. A. et al. Photocatalysis and Bandgap Engineering Using ZnO Nanocomposites,
Photocatalysis and Bandgap Engineering Using ZnO Nanocomposites. Adv. Mater. Sci. Eng.
Adv. Mater. Sci. Eng. 2015, 2015, e934587 (2015).
12

28. Mohd, A. M. A., Julkapli, N. M. & Abd, H. S. B. Review on ZnO hybrid photocatalyst:
impact on photocatalytic activities of water pollutant degradation. Rev. Inorg. Chem. 36, 77–
104 (2016).
29. Trotsenko, O., Tokarev, A., Gruzd, A., Enright, T. & Minko, S. Magnetic field assisted
assembly of highly ordered percolated nanostructures and their application for transparent
conductive thin films. Nanoscale 7, 7155–7161 (2015).
30. Wang, Z., Liang, Z., Wang, B., Zhang, C. & Kramer, L. Processing and property
investigation of single-walled carbon nanotube (SWNT) buckypaper/epoxy resin matrix
nanocomposites. Compos. Part Appl. Sci. Manuf. 35, 1225–1232 (2004).
31. Eda, G., Fanchini, G. & Chhowalla, M. Large-area ultrathin films of reduced graphene oxide
as a transparent and flexible electronic material. Nat. Nanotechnol. 3, nnano.2008.83 (2008).
32. Zhang, Y., Han, J.-H., Zhu, L., Shannon, M. A. & Yeom, J. Soft lithographic printing and
transfer of photosensitive polymers: facile fabrication of free-standing structures and
patterning fragile and unconventional substrates. J. Micromechanics Microengineering 24,
115019 (2014).
33. Dong, H. et al. An overview on limitations of TiO2-based particles for photocatalytic
degradation of organic pollutants and the corresponding countermeasures. Water Res. 79,
128–146 (2015).
34. Lee, K. M., Lai, C. W., Ngai, K. S. & Juan, J. C. Recent developments of zinc oxide based
photocatalyst in water treatment technology: A review. Water Res. 88, 428–448 (2016).
35. Zhang, Y. et al. Synthesis, Characterization, and Applications of ZnO Nanowires, Synthesis,
Characterization, and Applications of ZnO Nanowires. J. Nanomater. J. Nanomater. 2012,
2012, e624520 (2012).
36. Kołodziejczak-Radzimska, A. & Jesionowski, T. Zinc Oxide—From Synthesis to
Application: A Review. Materials 7, 2833–2881 (2014).
37. Tottori, S. et al. Magnetic Helical Micromachines: Fabrication, Controlled Swimming, and
Cargo Transport. Adv. Mater. 24, 811–816 (2012).

13

CHAPTER 2
LITERATURE REVIEW
2.1 Approaches of ZnO-based hybrid photocatalysts
In the past few decades, water pollution has been becoming a serious global issue due to
the rapid development of the industry and the widespread use of chemical and biological synthetic
materials. Therefore, the research on the aspect of pollutants removal for the natural water system
is highly urgent and essential. Among numerous strategies photocatalysis is one promising
technology of disinfection and detoxification and has been demonstrated to be effective to various
types of organic pollutants originated from the industrial and agricultural wastes. This process
mainly involves two reactions, oxidation and reduction. With the assistance of solar energy
photocatalysis is economic and applicable to versatile environmental conditions so that it has
attracted considerable interest in recent years. Up to now a number of photocatalysts, including
TiO2, ZnO, WO3 and Fe2O3, have been synthesized and modified by various micro/nano
fabrication techniques.
ZnO serving as a prospective photocatalyst has been applied to a variety of organic
pollutants [1–4]. Comparing to other candidates, ZnO as a metal oxide semiconductor has a lot of
advantages, such as well-band-alignment to redox, tunable band gap by doping, high quantum
yield in the near-UV spectral region and diverse nanostructures whereby its photocatalytic
performance is easier elevated by incorporation with other nanomaterials. Besides, the particular
properties like biocompatible, environmental benign, biodegradable and low cost fabrication make
ZnO industrial available to address large-scale wastewater problem.
ZnO is a wide band gap (3.2-3.5 eV) semiconductor and has three major crystal structures,
including rocksalt, wurtzite and zinc blende [5]. Amongst, wurtzite ZnO in hexagonal crystal

14

geometry is the most common structure at ambient temperature which possesses better
thermodynamic stability. As of now a number of works with respect to the characterization and
test of wurtzite ZnO have been reported. Although excellent photocatalytic activity of wurtzite
ZnO has been observed in comparison with other photocatalysts, the drawbacks such as narrow
spectral response at visible region and high recombination rate of charge carriers impede the range
of application. In these years considerable efforts have been dedicated to diverse ZnO
nanostructures in order to counter these defects as working for photocatalyst. It is already known
that one effective way to modify ZnO is the formation of hybrid structures using appropriate
nanostructures and foreign materials so that the photocatalysis process can be facilitated.

2.1.1 The mechanism of photocatalytic degradation using ZnO
ZnO is capable of decomposing a variety of organic pollutants by use of the heterogeneous
photocatalysis process ascribe to the sufficient build-in potential in comparison to the reduction
potential of the H+/H2 redox pair as well as the oxidation potential of the O2/H2O redox pair under
normal hydrogen electrode (NHE) [6–8]. This process generally takes place when photogenerated
electron-hole pairs are excited under sunlight irradiation and subsequently transported to the
surface of ZnO nanostructures, where the surrounded organic pollutants are gradually mineralized
into water, CO2 and other inorganic molecules in the presence of reactive oxidants, such as
hydroxyl radicals (¡OH). The process detail can be represented by a series of chemical equations
(see Figure 2.1), which consists of a number of oxidation and reduction reactions occurring at the
interface of ZnO nanostructures and water. As seen in Figure 2.1, upon the irradiation of sunlight,
the photons energy greater than the bandgap of ZnO can stimulate electrons (e-) escaping from
valence band (VB) to conduction band (CB), resulting holes (h+) generated at the corresponding

15

original positions. When transporting to the active sites on the surface of ZnO nanostructures, these
electron-hole pairs are highly probable to convert hydroxyl anions in water and oxygen into active
superoxide radicals and hydroxyl radicals, respectively, and other oxidants in subsequent redox
reactions. Eventually, the organic pollutants surrounding these oxidants are mineralized into water,
carbon dioxide and some inorganic molecules. It is worthwhile noticing that a considerable
quantity of photogenerated electron-hole pairs recombines and dissipates in the form of heat prior
to the participation of reaction. Therefore one potential way to improve the photocatalytic activity
of ZnO is to suppress the recombination rate of electron-hole pairs.

Figure 2.1. Schematic of photocatalysis process on single-component ZnO photocatalysts, where
chemical equations represent detail photocatalysis process

2.1.2 The mechanism of charge separation using hybrid ZnO
The rapid recombination of photogenerated electron-hole pairs is one primary issue
impeding the quantum yield of ZnO that has attracted much attention in recent years. Apart from
the solutions such as thermal treatment and element doping, hybridization with foreign
nanomaterials (e.g. metal, semiconductor, carbon, and polymer) has been demonstrated as an
effective methodology in this regard. As shown in Figure 2.2, the constituent such as TiO2 in ZnObased hybrid structure is employed to prolong the lifetime of charge carriers [9]. A heterojunction
16

bridging ZnO and TiO2 enables photogenerated electrons and holes to transport in accordance with
the direction in band diagram (Figure 2.2). In detail, the electrons generated at the CB of TiO2 is
likely to flow towards the CB of ZnO due to the potential difference. Likewise, the holes at the
VB of ZnO moves to that of TiO2 under the sunlight irradiation. It can be seen that the
transportation of photogenerated electron-hole pairs is able to work continuously as long as the
heterojunction is valid. As a result, the electrons and holes accumulated at the CB and VB can
respectively react with oxygen and water for reactive oxidants. Furthermore, the formation of
heterojunction decreases the bandgap of ZnO as well as TiO2, resulting an increment yield of
charge carriers on both components. In this manner, the photogenerated electron-hole pairs are
effectively separated prior to the possible recombination whereby improves the photocatalytic
activity of ZnO.

Figure 2.2. Band diagram of ZnO/TiO2 hybrid nanostructures, where the heterojunction formed
between ZnO and TiO2 promotes charge separation. (Adapted from Reference 9)

Figure 2.3 illustrates three types of band alignments in the field of semiconductor hybrid
structures, including straddling gap (type-I), staggered gap (type-II) and broken gap (type-III) [10].
As seen, all three scenarios possess the capability to separate photogenerated electron-hole pairs
derived from one (or both) photocatalyst through the heterojunction. However, the photogenerated
electrons and holes in type-I heterostructures are separated from wider bandgap component and

17

directed to the same side of the heterojunction, resulting charge accumulation at the narrower
bandgap component thus has the higher probability of recombination. In other words, type-I
heterostructures promote the charge recombination instead of separation which is unlikely to be
pursued by researchers in photocatalysis. In contrast, type-II and type-III heterostructures are the
desired band alignments for charge separation. In type-III the CB potential of one component is
even lower than the VB potential of the other component, which is capable of separating
photogenerated electron-hole pairs. Type-II heterostructures render a pathway to confine the
photogenerated electrons and holes on different sides of the heterojuntion after separation that
reduces the recombination rate efficiently. Such the band alignment as ZnO/TiO2 hybrid structures
is belonged to type-II heterostructures, which is favored to elevate the photocatalysis performance.
In addition, comparing to n-n heterojuntion (e.g. TiO2/ZnO), p-n heterojunction base on type-II
and type-III heterostructures enable further enhancement in charge separation due to the opposite
build-in potential in components.

Figure 2.3. Three types of band alignments. (a) type I, (b) type II and (c) type III. (Adapted from
Reference 10)

As displayed in Figure 2.4, p-CuO/n-ZnO formed in p-n heterojuntion has particular
advantages in charge separation and free carriers promotion [11]. For instance, an internal build-in
electrical field established at the interface of the p-n heterojunction promotes the separation of
photoinduced electrons and holes. In addition, the resultant bandgap by the formation of
18

heterojunction can yield more free electrons and holes respective to p-type and n-type
semiconductors under sunlight irradiation, directly increasing the reactive oxidants.

Figure 2.4. Band diagram of n-ZnO/p-CuO hybrid nanostructures, where the heterojunction
formed between ZnO and CuO promotes charge separation. (Adapted from Reference 11)

2.1.3 The factors influence the performance of ZnO photocatalyst
On the basis of relevant studies in the field of photocatalytic degradation, some primary
factors are worthwhile noticed that can significantly affect the performance of photocatalysts,
which includes the aspects of material, solution and light source [12]. In detail, material involves
mass loading, surface area, and morphology, defect and so on. Solution refers to PH value and
temperature. Light source contains light intensity and wavelength. For slurry system, optimal
loading and uniform distribution of photocatalysts are always desired. The problems such as light
scattering and absorption, agglomeration and sedimentation derived from too much loaded
photocatalysts may cause the serious reduction of photocatalytic activity. Moreover, it can be
imagined that excessive loading in practical application will increase the cost of applied
photocatalysts as well as post-treatment process. In contrast, proper photocatalyst loading allows
sufficient photons and spaces accommodating them, thus resulting best performance in water
cleanliness. With a given loading, photocatalysts in larger surface area is able to generate larger
quantity of reactive oxidants under sunlight irradiation and increase adsorption of pollutants as
19

well. The relevant works have indicated that larger specific surface area of photocatalyst can
elevate the degradation rate of pollutant [13–15]. Therefore, surface area is another essential factor
regarding slurry system as well as immobilized system. The detailed comparison of slurry system
and immobilized system will be discussed in next chapter, where catalytic surface area is one
critical factor. In the aspect of solution, PH value and temperature are two key factors influencing
the type of dominate oxidants and degradation rate. It has been found that photogenerated holes
(h+) are the major oxidants in acidic solution (low PH value) while hydroxyl radicals (¡OH) are
responsible for the oxidation for the rest conditions [16]. The appropriate PH value can reduce the
effect of charge carrier scavenger in pollutants thus elevate the degradability. On the other hand,
the increment of temperature can lead to an improvement of degradation rate which obeys Van’t
Hoff-Arrhenius law [17]. In general, ZnO as photocatalyst works best under UV light irradiation
because the photons possess sufficient energy to activate the formation of photogenerated electronhole pairs, meanwhile its photocatalytic activity in visible spectrum has also been confirmed
thanks to the surface defects and oxygen vacancy [18]. And the irradiation light in higher intensity
promotes the yield of oxidants whereby accelerates the degradation process. However, in some
cases, such as the excitation light with wavelength matching with the absorption spectrum of
pollutant, may cause the evaluation difficulty due to the unexpected byproducts via the natural
decomposition of pollutant. Additionally, high intensity light requires extra power which is not
economic in large-scale application. Taking the research analysis and cost-effectiveness into
consideration, suiTable light source is necessary in the process of photocatalytic degradation.
Apart from the abovementioned aspects, the proper utilization of photocatalysts is another option
to improve photocatalysis performance. The suspension and immobilization are two major
methods widespread utilized for evaluation of photocatalysts, which however have intrinsic

20

defects in particular working conditions. The recent progress regarding their modification and the
development of potential method will be discussed in the following chapter.

2.1.4 ZnO/metal oxide hybrid
ZnO has become one of the most attractive materials over the past few decades and its
unique properties in photoelectronics and electrochemicals has been applied to various devices. In
the field of environmental remediation, ZnO is also prospective thanks to non-toxicity,
biodegradability and biocompatiblity. ZnO has remarkable performance as a photocatalyst in
decomposing a wide range of organic pollutants, which however exhibits a long-standing issue,
the limited photocatalytic activity. Hybrid structure is effective to elevate the photocatalytic
activity of ZnO through enhancement of charge separation and extension of response spectrum.
As mentioned above, type-II and type-III heterostructures are desired band alignments with respect
to the modification of ZnO. In this section some major metal oxide semiconductors serving as
foreign materials formed these special band alignments when coupling with ZnO, which are
reviewed in terms of the performance and capability.

2.1.4.1 ZnO/TiO2 hybrid structure
TiO2 is a well-documented material [19] which has been widely used in solar cells, batteries,
supercapacitors and biomedical devices. Other than these applications, so far, considerable efforts
have been devoted to water purification [8]. TiO2 has three crystal structures, including anatase
(3.2 eV), rutile (3.0 eV) and brookite (3.4 eV). The first two structures are prior photocatalyst
candidates since they are sTable in ambient condition and are capable of working under both UV
and visible light [20]. Comparing to ZnO, TiO2 is more sTable in the use of potocatalysis in harsh

21

conditions. However some fundamental challenges such as fast recombination and slow
transportation of charge carriers still remain. Like ZnO, the major strategies including thermal
treatment and element doping have been proposed to counter these problems, in which the
hybridization is identified as a very promising method to facilitate the photocatalytic activity of
TiO2. In ZnO-based hybrid photocatalyst system, TiO2 is employed as a foreign material mostly
in view of two specialties, the similarity of its properties to ZnO and the appropriate band
configuration in a good agreement with the redox potential under NHE. Moreover, the
heterojunction of ZnO/TiO2 performs type-II band alignment that is favored in photocatalysis
optimization.
Table 2.1. General summary of ZnO/TiO2 hybrid photocatalysts
Structure
ZnO/TiO2
nanoparticles

Component
ZnO/TiO2
ZnO/TiO2
ZnO/TiO2
Mn/ZnO/TiO2
ZnO/TiO2
ZnO/TiO2
ZnO/TiO2
ZnO/TiO2/Clay
ZnO/TiO2/SiO2

ZnO/TiO2

ZnO/TiO2
ZnO/TiO2/Ag2O
ZnO/TiO2/CNT

Pollutant
Acridine Orange
(AO)
Acid Red 14 (AR
14)
Sodium Methyl
Red (SMR)
Methylene Blue
4-chlorophenol
Diazinon
Methyl Orange
Methyl Blue
Methyl Green
Methyl Blue
Rhodamine B
(RhB)
Isomerization of
Norbornadiene
(NBD)
Methyl Blue
Methyl Blue
Methyl Orange

22

Light source

Reference

Solar

21

UV

22

UV

23

UV
UV
UV

24

UV

27

UV
UV, Visible

28

UV, Visible

30

UV, Visible
UV
UV

31

25
26

29

32
33

Table 2.1. (cont’d)

ZnO/TiO2

ZnO/TiO2

ZnO/TiO2
ZnO/TiO2/Er2O3/
Y2O3/Fe
S/ZnO/TiO2
ZnO/TiO2/Fe3O/P
ANI
ZnO/TiO2

Ag/ZnO/TiO2

ZnO/TiO2
ZnO/TiO2/Er3+/Y
AIO3
ZnO/TiO2

ZnO/TiO2

ZnO/TiO2
ZnO/TiO2/graphe
ne

Azo dye sunset
yellow
Heterocyclic dye
methyl blue
Xanthene dye
rhodamine B
Azo dye Remazol
Red Brilliant F3B
(RR180)
Pesticide 2,4dichlorophenoxya
cetic acid (2,4-D)
Methyl Orange

UV

34

UV

35

UV

36

Acid Red B

Sunlight

37

Rhodamine B

Visible

38

Methyl Orange

Visible

39

Methyl Orange
Rhodamine B
Crystal Violet
(CV)
Methyl Blue
Methyl Orange
4-chlorophenol
(4-CP)
Acid Red B
Methyl Orange
Rhodamine B
Azo fuchsine
(AF)
Methyl Orange
Brilliant golden
yellow (BGY)
(azo dye
widespread for
textile/dyeing
industries)
Rhodamine B
Malachite green
Rhodamine B
Texbrite BAC-L
Texbrite NFW-L

UV

40

UV, Visible,
Solar

41

UV

42

Solar

43

UV

44

Sunlight

45

Visible

46

Visible

47

23

Table 2.1. (cont’d)
ZnO/TiO2
nanowires/nanoro
ds/nanotubes

I/ZnO/TiO2
ZnO/TiO2
ZnO/TiO2
ZnO/TiO2/CdS

ZnO/TiO2/CNT
ZnO/TiO2
ZnO/TiO2
ZnO/TiO2
ZnO/TiO2 film

ZnO/TiO2

Rhodamine B
Bromo-Pyrogallol
Red (Br-PGR)
Rhodamine B
Rhodamine B
Atrazine
Textile
wastewater
Acid Orange 7
(AO7)
Methyl Red
Rhodamine B
Methyl blue
Methyl orange
4-nitrophenol (4NP)

Visible

48

UV, Visible

49

UV

50

UV, Visible

51

UV

52

UV

53

UV
UV

54

UV, Visible

56

55

As shown in Table 2.1, up to now, ZnO/TiO2 hybrid photocatalysts have exhibited
remarkable performance towards the decomposition of numerous organic pollutants with the
assistance of the enhanced phototcatalysis. Owing to the various conditions researchers applied to
their works, the quantitative comparison is difficult. Herein we will discuss the potential
development of ZnO/TiO2 by summarizing some recent progress reported in terms of various
pollutants and degradation under varied working conditions.
Azo dyes are commonly used as probe pollutants to evaluate the photocatalytic activity of
ZnO-based hybrid photocatalysts. And optical measurement of pollutant is the primary method to
analyze capability of photocatalysts. During degradation process, the characteristic absorbance
peak of pollutant was observed diminishing by using UV/Vis spectrometer, which reflected the
decrement concentration of polluntants in the presence of photocatalysts. Note that Bouguer–
Lambert–Beer law reveals the relationship of attenuation of spectrum to pollutant concentration,

24

with which the degradation rate can be eventually obtained from the diagram of the spectrum
intensity change at characteristic absorbance peak as a function of the length of light irradiation.
In this manner, single-component ZnO photocatalyst and ZnO/TiO2 hybrid photocatalysts were
studied, from which ZnO/TiO2 hybrid photocatlysts performed higher degradation rate than singlecomponent ones in the case of valid heterojunction [36,54]. In recent years, many attempts have
been made to practically utilize ZnO/TiO2 hybrid photocatalysts. As a way, a lot of pollutants that
contained complex chemical components have been employed as probe pollutants. Consequently,
the results regarding the removal of majority pollutants are promising under UV light irradiation
[26,42,50]. On the other hand, the studies associated with the ZnO/TiO2 hybrid photocatalysts under
the irradiation of visible light demonstrated the functional synergy effect in promoting the
degradation of pollutants [30,31,56]. Especially, the research towards the textile/dying pollutant of
using the sunlight paves a way for ZnO/TiO2 hybrid photocatalyst in practical application [45].
Notably, as of now, a number of strategies with respect to the improvement of ZnO/TiO2 hybrid
photocatalyst have been implemented successfully. For instance, core-shell structure was exploited
to counter the drawbacks of ZnO, such as photocorrosion and slow charge mobility, which showed
prospective results in chemical stability and photocatalytic activity [21–23]. Moreover, the
facilitation of the band-band charge separation using piezotronic effect was proved in ZnO/TiO2
nanoplates as a potential way to further improve the photocatalysis [27].
Except for the structural/morphologic modification, the combination of numerous
methods, including doping, phase mixing and foreign materials coupling were also introduced to
further modify ZnO/TiO2 hybrid photocatalysts in regard of the spectral response and charge
separation. For example, ZnO/TiO2 hybrid photocatalysts doped by particular species would
change the electronic band configuration which caused promotion of electron-hole pair generation

25

and transportation [24,38,57]. Meanwhile it has been proved that the mixed multiple crystalline
phases of TiO2 could broaden the range of light absorption owing to the change of band structures
[46]. When additional foreign materials were introduced to ZnO/TiO2 to form ternary composites,
it would not only reinforce the hybrid photocatalysis but also bring advantages such as better
chemical stability, larger surface adsorption and Fenton reaction compatibility [ 39,41,51]. It
worthwhile notices that the recent trend about modifying ZnO/TiO2 hybrid photocatalyts is to add
extra components, which can effectively address some drawbacks emerged in two-component
hybrid photocatalysts.

2.1.4.2 ZnO/WO3 hybrid structure
Over the past few years WO3 nanostructures have displayed great capabilities in the
application of gas sensors, super capacitors, solar cells, electrochromic devices and photocatalysis
[58,59]. When using as photocatalysts, TiO2 and ZnO with wide bandgap electrical property have
drawbacks in regard of spectral response, which can only utilize a small fraction of solar spectrum
(3-5%). At this point WO3 is a valuable photocatalyst since it has remarkable photocatalytic
activity under the presence of sunlight irradiation thanks to its proper bandgap [60] in spite of its
flaws in rapid charge recombination and inappropriate CB level. As yet, various WO3 crystal
structures have been discovered (monoclinic, orthorhombic, tetragonal, triclinic, hexagonal and
cubic) with band gaps varied from 2.5 eV to 2.8 eV, which are correspond to UV region and blueregion of solar spectrum [60,61]. Notably, among these phases, monoclinic-WO3 has displayed the
best photocatalytic activity and chemical stability whereby attracts a lot of attention in the research
of water treatment. The recent research associated with the ZnO/WO3 hybridization implies that
monoclinic-WO3 is a suiTable foreign material due to the appropriate bandgap alignment (type-

26

II), which made significant contributions in broadening the spectral response and facilitating
charge separation in hybrid photocatalysts.
Table 2.2. General summary of ZnO/WO3 hybrid photocatalysts
Structure
ZnO/WO3
nanoparticles

Component

Pollutant

Light source

ZnO/WO3

Methyl Orange

UV, Visible

62

Visible

63

UV
UV, Visible

64

UV

66

Sunlight

67

UV

68

Sunlight

69

UV

70

Sunlight

71

UV
UV

72

ZnO/WO3
ZnO/WO3/ZnWO4
ZnO/WO3
Ag/ZnO/WO3

Ag/ZnO/WO3
ZnO/WO3
Ag/ ZnO/WO3
Ag/ZnO/WO3
ZnO/WO3
nanowires/nanoro
ds/nanotubes

ZnO/WO3
ZnO/WO3
ZnO/WO3

Methyl blue
Anionic Orange G
(OG)
Methyl Orange
Methyl Orange
Reactive Red 120
(RR120)
Reactive Orange 4
(RO4)
Naphthol Blue
Black (NBB)
Acid orange II
Reactive red 120
(RR120)
Acid black 1 (AB
1)
2,4dichlorophenoxya
cetic acid (2,4-D)
resorcinol (ReOH)
phenol

Reference

65

73

So far, a number of attempts have been made towards finding the optimal combinations of
ZnO and WO3 in terms of their morphology and proportion. And numerous works regarding the
evaluation of their degradation efficiency against a verity of pollutants have been reported. Table
2.2 summarized some updates about the ZnO/WO3 hybrid photocatalysts in view of pollutants and
light conditions.
Upon sunlight irradiation, when photons in a wide range of energy were converted into the
photogenerated carriers by hybrid photocatalysts, electrons and holes were able to be respectively

27

transported to ZnO and WO3 due to difference potential of their band structures. In this manner,
the enhanced charge separation derived from ZnO/WO3 heterojunctions promoted the
photocatalytic activity of photocatalysts. As reported, the enhancement of degradation efficiency
contributed by WO3 was usually above 25% [74]. More specifically, ZnO/WO3 heterojunctions
could boost the degradation rate to some pollutants which is resistive to single-component ZnO
photocatalysts [70,72]. In these years, one kind of ternary composites, Ag/ZnO/ WO3, was
intensively investigated to remove particular pollutants under sunlight irradiation. The perspective
result indicated that the noble metal in hybridization can further facilitate the charge separation
[67]. Besides, the study on hybridization of extra foreign materials, ZnWO4, confirmed double
heterojunctions between ZnO and WO3 phases, which revealed a potential mechanism to suppress
the charge recombination [64]. In view of the above, strategies in multiple composites and multiple
heterojunctions will bring more promising results in modifying ZnO/WO3 hybrid photocatalysts
in the near future.

2.1.4.3 ZnO/Fe2O3 hybrid structure
Motivated by the electronic recording devices for data storage, iron oxide were extensively
investigated in the past decades because of the distinguishable chemical stability and magnetic
property. Nowadays, with the development of iron oxide nanostructures, the application has been
widely extended to biomedicine and environmental remediation. The typical crystal structures of
iron oxide include wustite (FeO), hematite (Îą-Fe2O3), maghemite (Îł-Fe2O3) and magnetite (Fe3O4)
[75,76]. In photocatalysis study, hematite and maghemite belonging to iron(III) oxide are two main
classes of iron oxide which are of interest to researchers due to their natural magnetic behavior as
well as diverse geometries. One primary obstacle that hinders photocatalysts in practical

28

application is the recyclability (or recovery). Most of techniques regarding the separation of micro/nano- scale particles currently used in research and industrial would significantly increase the cost
in large-scale water treatment. In the light of magnetic property, the fabrication of iron(III) oxide
for photocatalysts could be one potential solution to counter this challenge [77–79]. In addition, the
bandgap energy of iron(III) oxide varies from 1.9 eV to 2.2 eV, which is capable of absorbing a
wide range of sunlight, approximately from deep UV region to visible yellow region. Therefore,
iron(III) oxide serving as foreign materials are getting an increased attention. However, there are
some drawbacks of iron(III) oxide, such as in-direct bandgap and high charge recombination rate,
that have been noticed when using as photocatalysts independently. Due to this reason, the
hybridization of iron(III) oxide to other photocatalysts is being explored. The recent progress of
ZnO/Fe2O3 is tabulated (Table 3).
Recently both ZnO/Îą-Fe2O3 and ZnO/Îł-Fe2O3 hybrid photocatalysts have been
demonstrated to be effective to degrade various pollutants. In particular, the enhancement of
photocatalysis has been elaborated that the photogenerated charge carriers were separated through
the ZnO/Fe2O3 heterojunction, resulting a larger number of active ¡OH and O2-¡ [80]. On the other
hand, the investigations towards the dye degradation in the presence of visible light irradiation
confirmed the extension of spectral response of ZnO photocatalysts as incorporating with Fe2O3.
As a result, with the assistance of facilitated charge separation and light absorption, the degradation
efficiency of ZnO/Fe2O3 photocatalysts was evidently improved in case of sunlight irradiation.
Aside the abovementioned studies, the assessment of Ag/ZnO/ Fe2O3 ternary photocatalysts has
be confirmed to address some issues existed in ZnO/Fe2O3 hybrid photocatlysts [81]. To our best
knowledge, not many ZnO/Fe2O3 ternary photocatalysts have been reported. ZnO/Fe2O3-based

29

hybrid photocatalysts has a high potential in large-scale application as considering the
commercialization.
Table 2.3. General summary of ZnO/Fe2O3 hybrid photocatalysts
Structure

Component

Light source

Reference

UV

82

Sunlight

83

UV
UV
UV, Visible

80

Visible

79

Solar

86

UV, Visible

87

UV

88

UV, Visible

89

UV

90

ZnO/Îł-Fe2O3

Pollutant
2,4dichloropenoxyace
tic acid (2,4-D)
2,4dichlorophenoxya
cetic acid (2,4-D)
Salicylic acid (SA)
Methyl orange
Rhodamine B
Methylene blue
Rhodamine B
Methyl orange
3,6dichlorophenoxya
cetic acid
(Dicamba)
2,4dichlorophenoxya
cetic acid (2,4-D)
Pentachlorophenol
(PCP)
Chlorophenoxyace
tic acid
Pentachlorophenol
(PCP)
2,4dichlorophenoxya
cetic acid (2,4-D)
chlorophenol

ZnO/Fe2O3
nanoparticles

ZnO/Îł-Fe2O3

Sunlight

91

ZnO/Fe2O3

Methylene blue

visible

92

Ag/ZnO/Fe2O3

Methyl orange
Iodoform (CHI3)

UV, Visible

81

ZnO/Fe2O3
ZnO/Fe2O3
ZnO/Îą-Fe2O3
ZnO/Îą-Fe2O3
ZnO/Îł-Fe2O3

ZnO/Fe2O3

ZnO/Îą-Fe2O3
ZnO/Îł-Fe2O3
ZnO/Îą-Fe2O3
ZnO/Îł-Fe2O3

ZnO/Fe2O3
nanowires/nanoro
ds/nanotubes

30

84
85

2.1.5 ZnO/carbon-based hybrid
Although carbon-based nanomaterials have been investigated for many decades, the
excellent capabilities originated from their unique properties are keep emerging in every relevant
field. As representative nanomaterials, each breakthrough regarding diamond, fullerene, carbon
nanotubes (CNTs), graphene and so on, has brought numerous opportunities for fabricating highperformance devices and further raised research interest in widespread applications. Comparing to
metal oxide nanomaterials, carbon-based nanomaterials are usually known as stronger (covalent
bond) and lighter (atomic weight) thanks to their structures and constituents. Furthermore, some
special properties, such as superconductivity [93], charge carrier mobility [94], nonlinear/linear
optical property [95] and biocompatibility [96] have been widely used to various applications. In
the field of photcatalysis, the combination of ZnO and carbon-based nanomaterials has achieved
many objectives involving overcome some intrinsic defects and improved photocatalytic activity
of single-component ZnO photocatalysts. In this section we will briefly summarize the ZnO-based
photocatalysts hybridizing with some typical carbon-based nanomaterials and discuss their up-todate challenges and potentials.
When working as photocatalysts, CNTs were found to be advantageous in chemical
stability, electron scavenger and diffusion of reacting species boosted by mesoporous character
[97–99]. In last few decades a number of photocatalysis approaches have implied the remarkable
degradation performance of various photocatalysts in the presence of hybridization with CNTs
(see Table 2.4) [100]. As known, the heterostructures formed by CNTs have enhancements not only
in efficient charge separation but also the extension of spectral response especially to wide bandgap
materials like TiO2 and ZnO, which makes them functional upon broaden solar spectrum [101].
Moreover, studies associated with synergetic effect of ternary composite photocatalysts indicated

31

that CNTs could further suppress the recombination of charge carriers with the assistance of
metal/nonmetal ions, through which the degradation rate of numerous pollutants were significantly
increased [102,103].
As a result of the excellent electronic and optical properties, graphene has also successfully
been employed to wastewater treatment [104]. The recent research towards hybrid photocatalysts
revealed that photogenerated electrons favor to transfer through π bonds in the presence of
graphene, which is effective to suppress the charge recombination [105]. Furthermore, the superior
optical transparency and large surface area of graphene in comparison with CNTs allowed more
reactive sites forming hence accelerated the degradation process [106–108]. Table 2.5 summarizes
the recent works about the ZnO/graphene hybrid photocatalysts. Like ZnO/CNTs hybrid
photocatalysts, ZnO/graphene hybrid photocatalysts are also capable of utilizing the majority of
solar spectrum and degrading a verity of textile pollutants. To date, ZnO/graphene ternary
composites have attracted considerable attention because of their degradation efficiency in various
pollutants. It has been demonstrated that significant improvements in charge separation and
spectral response have been obtained by using the formation of ternary composites. Basing on
relevant works, both metals and semiconductors are suiTable to form ZnO/graphene-based ternary
photocatalysts [109,110].
Except for carbon nanotubes and graphene, some other carbon-based nanomaterials, such
as carbon fibers and carbon quantum dots, also displayed attractive degradability [111–114]. Overall,
carbon-based nanomaterials are promising as foreign materials in coupling with ZnO for hybrid
photocatalysts. More specifically, ZnO/carbon-based hybrid photocatalysts with the combination
of extra metal/metal oxide materials are of interest to researcher in order for higher photocatalytic
activity.

32

Table 2.4. General summary of ZnO/CNT hybrid photocatalysts
Structure
ZnO/CNT
nanoparticles

Component

Pollutant

Light source

Reference

ZnO/CNT

Rhodamine B

Sunlight

115

ZnO/MWCNT
ZnO/MWCNT
Ag/ZnO/CNT/PI
Cu/ZnO/MWCNT

Cyanide (CN)
Rhodamine (RhB)
Methyl orange
Methyl orange
Methylene blue
Rhodamine 6G
(Rh6G)

UV
UV
UV
Visible

116

UV

119

Methyl orange

UV

102

Methyl blue
acetaldehyde
Methyl blue

120

125

ZnO/MWCNT
Mg/ZnO/MWCN
T
Co/ZnO/MWCNT
ZnO/MWCNT
ZnO/MWCNT
ZnO/CNT/rGO

ZnO/CNT
nanowires/nanoro
ds/nanotubes

ZnO/CNT

MB

ZnO/CNT

phenol

UV
UV
UV
UV
Visible
Solar

ZnO- carbon
nanofiber

Rhodamine B

UV

117
103
118

121
122
123

Table 2.5. General summary of ZnO/G and ZnO/rGO hybrid photocatalysts
Structure
ZnO/G, ZnO/GO,
ZnO/rGO
nanoparticles

Component

Pollutant

Light source

Reference

ZnO/rGO

Methyl blue

UV

121

ZnO/rGO
ZnO/G
ZnO/G

Orange II
Methyl blue
Methyl blue
Methyl blue
Methyl orange
Methyl Orange
Methyl blue
Methyl blue
Methyl orange
MB,MO
MB
MO
4-nitrophenol (4NP)

Solar
UV
UV

124

Visible

128

UV
UV
UV
UV
UV
Visible
UV, Visible

109

UV

135

ZnO/G
Ag/ZnO/G
ZnO/GO
ZnO/rGO
ZnO/G
ZnO/rGO
ZnO/G
ZnO/GO
ZnO/GO

33

126
127

129
130
131
132
133
134

Table 2.5. (cont’d)
ZnO/G
ZnO/GO
ZnO/rGO
ZnO/GO/g-C3N4
ZnO/GO
ZnO/G
ZnO/GO
ZnO/GO/TiO2
ZnO/rGO
ZnO/rGO/RuO2
ZnO/rGO
ZnO/G
ZnO/rGO/TiO2
Ag/ZnO/GO

ZnO/GO/CuAu

ZnO/GO
ZnO/rGO
ZnO/rGO
ZnO/GO/nanocell
ulose
ZnO/rGO
ZnO/rGO
ZnO/GO
Ag/ZnO/rGO
ZnO/GO

MB
Rhodamine(RhB)
MB
MB
MO
RhB
MB
MB
Texbrite BAC-L
Texbrite BBU-L
Texbrite NFW-L
RhB
MB
MB
MO
Pyrogl Red(PR)
MB
MB
MO
MB
Indigotin (IN)
Sunset (SY)
Tartrazine (TT)
Brilliant Red X3B (X3B)
CH3CHO
MB
MO
RhB
Ciprofloxacin
(CF)
MB
MB
MB
RhB
Deoxynivalenol
(DON)

UV, Visible
UV
UV, Visible
Visible
Solar
UV
UV

136

Ultrasonic

141

Solar
Solar
UV, Sunlight

142

Visible

145

UV
UV, Visible

146

Solar

148

UV

149

UV, Visible

150

Visible

105

Solar

151

UV
UV
UV
Visible

152

UV

156

137
108
110
138
139
140

143
144

147

153
154
155

2.1.6 ZnO/metal hybrid
Up to date works associated with the ZnO/metal hybrid nanostructures have attracted
considerable attention owing to their outstanding properties in electronics and optics that can well
34

fit for various optoelectronic applications. In the research of environment remediation, taking the
advantages of this hybrid nanostructures, such as effective charge separation and extended light
absorption, ZnO/metal hybridization have also become one pathway for fabricating highperformance photocatalysts. As can been in Table 6, the recent studies imply that ZnO/metal
photocatalysts are able to treat a variety of pollutants in water under the presence of suiTable light
irradiation, which have comparable degradation performance like other ZnO-based
heterostructures. Differ from ZnO/metal oxide hybrid photocatalysts, the major promotion of
ZnO/metal hybrid photocatalysts are derived from the special functionalities, mostly electron
scavenger and surface plasmonic resonance (SPR), of decorated metal particles. In detail, upon
light irradiation, the electrons in metal particles excited by SPR effect allow an increased
possibility to generate reactive oxidants such as ¡OH. Meanwhile the misaligned band structure
enables a positive transportation of photogenerated electrons from ZnO to metal, suppressing the
recombination of charge carriers. Except for the improvement of the photocatalytic activity, these
effects caused by metal particles are also effective to overcome the photocorrosion of ZnO,
offering a pathway for further enhancement of degradation [157]. Thus, hybridization with metal is
a promising route to modify ZnO-based photocatalysts.
So far many kinds of metal particles, including Au, Ag and Pt have been employed to
assistant ZnO photocatalysts in order for desired performance. On the basis of these works, two
primary modification strategies are suggested to further optimize ZnO/metal hybrid
photocatalysts. One way is to build binary composites by taking account the effects of the
dimension, morphology and synthesis method of metal particles. Such work as ZnO/Au hybrid
photocatalysts revealed a possible restriction about the particle size with respect to photocatalysis
promotion from charge separation and SPR, from which optimal degradation performance could

35

be realized under the proper combination [158]. Moreover, it is reported that the solution-based
process is favored since it provides a slow growth rate for metal particles to uniformly decorate
ZnO nanostructures [159]. Another way, same as other abovementioned hybrid photocatalyts, is to
fabricate ternary (or quaternary) composites to approach multiple synergetic effects. According to
relevant reports, the extra metal particles serving as electron sinks could further suppress charge
recombination as well as produce more oxidants [160], while the extra metal oxide/carbon-based
materials with narrow bandgap allowed the hybrid photocatalysts working under a broader range
of light [161]. In this manner, since the charge separation, the spectral response and the chemical
stability were facilitated simultaneously and effectively, ternary (or quaternary) composites
exhibited excellent photocatalytic activity as desired (see Table 2.6).
Table 2.6. General summary of ZnO/metal hybrid photocatalysts
Structure
ZnO/Au
nanoparticles

ZnO/Ag
nanoparticles

Component
ZnO/Au

Pollutant
RhB

Light source
UV, Visible

Reference

ZnO/Au
ZnO/Au
ZnO/Au
ZnO/Au
ZnO/Au

RhB
RhB, MO, MB
MB
MB
Effluent treatment
plant (CETP)
RhB
RhB
phenol
MO
RhB
RhB
RhB
PF

UV
Visible
Sunlight
Solar, UV
Sunlight

163

Solar
Visible
UV, Visible
UV
UV
UV
Solar
UV, Visible

168

UV
Solar

175

UV

177

ZnO/Au/rGO
ZnO/Au
ZnO/Au
ZnO/Au
ZnO/Au/NiO
ZnO/Au
ZnO/Au
ZnO/Ag
ZnO/Ag
ZnO/Ag
ZnO/Ag

RhB
Metamitron,
metribuzin
MB
36

162

164
165
166
167

169
170
171
172
173
158
174

176

Table 2.6. (cont’d)
4-nitrophenol
RhB
MB
RhB
RhB, MB,MO,
fuchsine
Ketorolac
tromethamine
(KTC)
MB
MB, Cr(VI)
MO,4-NP
MO,phenol
MO
RhB,MB,MO
RhB

UV
UV, visible
Visible
Solar, UV, Visible
visible

160

Solar

181

Visible
Visible
Visible
UV
UV
Solar
Visible

182

UV
Visible

157

ZnO/Pt/C

MB
Acid orange 7
(AO7)
MB

UV

190

ZnO/Pt
ZnO/Pt
ZnO/Pt
ZnO/Pt
ZnO/Pt

RhB
MB
R6G
MB
Malachite green

UV, Visible
UV
UV
UV
Visible

191

ZnO/Ag/Ba
ZnO/Ag
ZnO/Ag
ZnO/Ag
ZnO/Ag/Ag2WO4
ZnO/Ag

ZnO/Ag/GO
ZnO/Ag
ZnO/Ag/In2O3
ZnO/Ag
ZnO/Ag
ZnO/Ag
ZnO/Ag
/rGO/TiO2
ZnO/Ag
ZnO/Ag/CNT
ZnO/Pt
nanoparticles

178
179
180
161

183
184
185
186
187
188

189

159
192
193
194

2.1.7 Summary
In summary, multi-component hybridization is effective to improve the photocatalytic
performance of ZnO. The valid heterojunctions between ZnO and foreign materials are able to
reduce the charge recombination whereby promote generation of reactive oxidants. In addition,
the modifications in nanostructure morphology, such as core-shell structures and hollow structures,
have solved many problems of single-component ZnO in using as photocatalysts. Apart from the
abovementioned hybrid photocatalysts, many other efficient ZnO-based hybrid photocatalysts,
such as ZnO/CuO, ZnO/CdS, ZnO/ZnS and etc, have also been developed and examined under
37

various working conditions. Owing to the coupling of foreign materials, a lot of ZnO-based hybrid
photocatalysts possessed superb optical, electronical, chemical and physical properties and extra
functionality that have been working in other fields. As examples, foreign materials such as NiO,
Ag2O and SiO2 have shown attractive capabilities in magnetic response, bacterial control and
electron tunneling, which are extremely desired not only in wastewater treatment but also in
biosensors and electronic devices. In current situation, considering the complex environment
conditions, compatibility and versatility are primary demands to photocatalysts. An increased
number of attempts towards selecting and modifying appropriate photocatalysts for special cases,
involving particular pollutants, wide spectrum of light irradiation and varied light intensity and
working temperatures have been devoting in these years.

2.2 The improvement of photocatalysts in various systems
Over the past decades photocatalysis has been widely introduced in the fields of watersplitting hydrogen evolution, self-cleaning surface and wastewater treatment thanks to its
advantages like infinite energy source, environmental-friendship, and cost-effectiveness. To
achieve high efficiency in wastewater cleanliness, numerous types of photocatalysts are being
developed while their photocatalytic activities with respect to different organic pollutants are being
explored. On the basis of the previous research regarding photocatalysts, some factors have been
recognized playing important roles in affecting photocatalytic degradation performance, including
mass loading and defect content, thermal and chemical stability, surface contaminants and area,
particle adsorption and photon conversion efficiency. Besides, there are two more important points
to evaluate the photocatalysts in practical application, which are degradation efficiency with
respect to particular pollutants and cost originated from the fabrication and recycling process of

38

photocatalysts. Taking account all these conditions some efforts are devoted to study efficient
photocatalysts, such as TiO2, ZnO and Fe2O3, and their corresponding large-scale fabrication
methods. Meanwhile, investigations are also carried out on proper manners for utilizing
photocatalysts. Presently, there are two primary photocatalyst systems, slurry system and
immobilized system, which are widely adopted in photocatalyst research [195]. Briefly, slurry
system has the photocatalysts uniformly dispersed into the solution, which usually gives a fast
degradation process but suffers from the potential agglomeration and difficult recovery of
photocatalysts. In contrast, immobilized system stands for photocatalysts fixed on some sTable
substrates, partially losing catalytic surface area but making the photocatalysts recyclable. In this
chapter we will summarize the recent progress about the ZnO-based photocatalysts basing on these
two systems and discuss some modifications and other potential utilization methods.

2.2.1 Slurry system
Upon reviewing progress on photocatalysis, various pollutants are found to be rapidly
decomposed by photocatalyst suspension under the presence of light irradiation in lab-scale
experiment. With the assistant of post-treatments like sedimentation and filtration, these suspended
photocatalyst particles are able to be completely removed, leaving purified and cleaned water
eventually. As known from the relevant studies, slurry system is an efficient methodology for
organic dye decomposition. In order for desired performance and reproducibility, photocatalytic
degradation are usually carried out in special reactors which has homogenous particle dispersion
and controllable working environment. Those special reactors, namely photoreactor, is a device
that can effectively utilize photocatalysts during the entire photodegradation process. In general,
for slurry photoreactor the polluted solution should always have photocatalysts uniform suspended

39

after injected to the reaction chamber. In addition, there are many other factors that helps to
improve the photocatalysis. For instance, the filled chamber should be transparent and thin enough
for the maximal penetration depth of light irradiation. Benefiting from effective light utilization,
high photocatalytic area to reactor volume ratio and low mass transfer limitation, photocatalysts
in slurries usually exhibit remarkable photocatalytic activity than that of immobilized ones under
the same working conditions [196–199]. Furthermore, owing to the advantages of photoreactors,
photocatalytic degradation performance deriving from slurry system are established as
benchmarks for the evaluation of photocatalysts [200]. One typical example is Degussa P25 powder,
which in certain weight percentage with respect to particular concentration of pollutants is usually
used to assess other developed photocatalysts [201–203]. To date the most of photocatalysts has been
tested in slurry photoreactor in research, among which ZnO and its hybrid structures have been
intensively studied. Table 2.7 shows the recent progress of ZnO-based photocatalysts in view of
slurry systems, which includes a wide range of pollutants and various working conditions.
Although the superb compatibility and capability have been demonstrated, some
limitations hinder the slurry system from large-scale application. As an example, the separation of
photocatalysts is always challenging when the system is scaled up, causing unexpected residues
and increased cost. For small-size photocatalyst particles the problems as agglomeration
significantly influence their photocatalytic activity [204]. The shadow effect penetration depth of
the irradiation light raised by the absorption and light scattering of suspended photocatalysts and
pollutants can result non-homogeneous and poor photocatalytic reaction. As one potential solution,
there are a lot of modified slurry photoreactors that contain functionalities such as continuous
stirring and ultrafiltration, by which some drawbacks faced in slurry system can be overcome
[205,206]. And many attempts are focused on improving particle uniform distribution and recovery

40

functionalities in slurry system. At the same time, the investigation on the immobilized
photocatalysts is being carried out as an alternative solution.
Table 2.7. Summary of ZnO-based photocatalysts in slurries
Type
ZnO

Morphology
Particle plus
membrane

Pollutant
Reactive black 5
(RB5) and etc.
Real refinery
wastewater with
pretreatment
MB
MO and MB
formaldehyde
MB
phenol
Malachite green
(MG)
Arsenite (As III,
As V)

ZnO

particle

Ag/ZnO
Ag/ZnO/CdO
N-ZnO
Y (yttrium)-ZnO
ZnO/Fe3O4

particle
particle
particle
particle
particle

ZnO

particle

CuO/ZnO

particle

Dy (dysprosium)ZnO

particle

CuO/ZnO

NWs

ZnO/Ag3VO4/AgI
ZnO
ZnO/Fe3O4/AgBr
ZnO/TiO2

particle
particle
particle
particle

La/ZnO, Se/ZnO

particle

ZnO/NiFe2O4
ZnO
ZnO/AgBr/Fe3O4/
Ag3VO4

particle
particle

Hexavalent
chromium (Cr VI)
RhB and MB
Textile effluent
RhB
MB
Acid yellow 29,
coomasie brilliant
blue G250, acid
green 25
Congo red (CR)
MB

particle

RhB

ZnO/kaolin clay

NWs

ZnO/g-C3N4

particle

ZnO/GO
ZnO
ZnO/nanoclinoptil
olite zeolite (NCP)

Acid red (AR17)

Light

Reference

UV

207

UV

208

Visible
Visible
Visible
UV
UV+visible

209

UV

213

UV

214

Visible

215

Visible

216

Visible
UV
Visible
UV

217

Visible

220

Solar
UV

221

Visible

222

210
202
211
212

203
218
219

12

UV

223

Visible

224

NWs
particle

Hexavalent
chromium
MO and humic
acid(HA)
MB
MO

UV
UV

225

particle

4-nitrophenol

UV

227

41

226

2.2.2 Immobilized system
In these years the research associated with photocatalysts has oriented towards the
immobilized system, which refers to the photocatalysts in form of film or nanowire array structures
are fixed onto substrates before applying to photo degradation. The immobilized ZnO
nanostructures on various substrates possess remarkable degradation capabilities thanks to their
diverse morphologies such as particle-gathered films, vertical-grown nanowire arrays, multibrunches tree array and so on [228]. Comparing to slurry system, the immobilized photocatalysts
are more advantageous in fewer post-treatment for separation and recycling meanwhile retaining
the comparable photocatalytic capability in photocatalytic degradation, satisfying the demands of
practical application. Nevertheless, the ordinary immobilized system which simply grow
photocatalysts on substrates without any further setup assistance usually displayed the decrement
of photocatalytic activity attributed to limited catalytic surface area and mass transfer. In this
regards, two modified immobilized systems including immobilized photoreactors and buoyant
photocatalysts are developed to optimally utilize and recover the immobilized photocatalysts in
various aspects.

2.2.2.1 Immobilized photoreactors
Immobilized photoreactor has been proven to be effective to solve the emerged problems
of slurry system and immobilized photocatalysts. Differ from slurry photoreactor, immobilized
photoreactor doesn’t require complicated recovery process for photocatalysts after photocatalytic
degradation. As for the design of the majority of these works, the photocatalysts were used to be
immobilized onto the reactor wall where the incident light had concentrated irradiation to elevate
the photocatalytic activity. The probe pollutants were injected along with a continuous flow system

42

which established a circulated flow to enhance mass transfer to surroundings. Consequently, the
polluted solution was thoroughly cleaned after flown across the immobilized photocatalysts
multiple times. It worthwhile notices that the immobilized photoreactors has no post-treatment
demand in comparison to the slurry one whereby suiTable to large-scale application. It has been
found that there are many factors such as surface-to-volume ratio of photocatalysts, connection
between photocatlayst and reactor, depth of light penetration and flow rate of polluted solution
that potentially affect the device performance. For instance, approaches on using high aspect ratio
capillary with the combination of porous- or nanowire-structural photocatalysts have
accomplished faster degradation rate because of large active catalyst surface area [ 229–231]. On the
other hand, the fabricated thin layer of immobilized photocatalysts in combining transparent
substrate has greatly reduced the depletion of photon energy from the incident light hence boost
the reaction rate [229–231].
With an increased interest in this field, the modification of photoreactors in terms of
photocatalysts with multiple composites are being conducted in order for reduced charge
recombination and enhanced chemical stability [232–234]. Table 2.8 is the summary of immobilized
ZnO-based photocatalysts in these years, which indicated that the research are focusing on ZnObased hybrid structures as targeting low cost, high photocatalytic activity and diverse morphology.
More specifically, considerable attention is given to the exceptional degradability towards various
pollutants under the presence of solar light to confirm potential performance in practical
application [235,236]. In chapter 4 we will discuss a novel prototype of an immobilized photoreactor,
which in superb photocatalytic performance is believed as a pathway to overcome some limitations
currently faced by immobilized photocatalysts.

43

Table 2.8. Summary of immobilized ZnO-based photocatalysts
Type
ZnO/PMMA
ZnO/TiO2/Glass
ZnO/TiO2/Glass
N-ZnO/Polyester
Fabric (PF)
ZnO, TiO2
ZnO
ZnO
ZnO/Polyvinyl
Pyrrolidone (PVP)
ZnO
ZnO
ZnO/glass spiral

Morphology
Nanorods
composites
Coat on nanorods

Pollutant
MO
phenol
MO

Light
UV
UV
UV

Reference

Particle film

Amaranth (AM)

Solar light

235

membrane
Particles coat on
disk

Textile wastewater
Reactive Yellow
81 (RY81)
Humic substances
(HS)

UV

230

UV

237

UV

238

particles

Congo red (CR)

UV

239

nanorods
Particle formed
film
Particle formed
film

MO

UV

231

MO

UV

240

Solar light

236

visible

234

UV

241

particles

Ag/ZnO/Metal
Mesh

NWs

ZnO

Particle formed
film

Reactive red 120
(RR-120)
Food black 2
(FB2) and MO
carbamazepine

229
232
233

2.2.2.2 Buoyant photocatalysts
Although the approaches associated with photoreactors have proven the continuality and
reusability of immobilized photocatalysts in degrading various pollutants, photoreactors still face
challenges in some practical cases. In order for best performance the artificial light with high
output power energy is usually employed to supply intense and concentrate light irradiation.
However this scenario is hardly realized by solar light in natural environment ascribe to the limited
light receiving area of the reactor and low light density that fails to achieve expected light
absorption. Besides, the scalable utilization of reactors brings more technical problems such as
external power supplies for the circulated water flow system and post-treatment process. Aside
from the aforementioned disadvantages, there are some severe restrictions in slurry system and

44

immobilized photoreactor. For instance, the rapid attenuation of incident light in water, especially
that of UV light, would significantly affect the activity of photocatalysts distanced from the water
surface [242], which confines the scale of slurry system. In addition, both slurry system and
immobilized photoreactor are ineffective to deal with insoluble pollutants (e.g. oil splits) floating
at water surface. In recent years buoyant photocatalysts has been demonstrated as an ideal model
to serve to large-scale application owing to the excellent recyclability, optimal sunlight utilization
and effectiveness to oil pollutants.
Buoyant photocatalysts refer to the photocatalysts immobilized on floatable substrates
which can maximally utilize incident light during the entire degradation process. This system is
designed to prevent incident light from scattering and absorption before arriving to the surface of
photocatalysts. As known, for immobilized photocatalysts a good support possesses the features
include reliable substrate-to-photocatalyst contact, stable property against generated oxidants, long
lifetime in water and environmental benign. As a modified immobilized system one more feature
that need to take account in fabrication buoyant photocatalysts is the lightweight. So far, except
for many kinds of polymers [243–245], other types of materials have also been used as floatable
substrates [246–250]. However some defects such as low surface-area morphology of photocatalysts
and complex fabrication for substrate-to-photocatalysts attachment are still remain. Moreover, to
our knowledge, ZnO working as one type of buoyant photocatalyst is rarely explored. Comparing
to TiO2 and other photocatalysts, ZnO has obvious advantages in cost-effectiveness, substrate
compatibility and superior degradability. As seen the recent approaches with respect to ZnO
buoyant photocatalysts (Table 2.9), although a variety of organic dyes have been successfully
decomposed using ZnO-based photocatalysts, few of them have been tested by ZnO in form of
buoyant photocatalysts. In Chapter 5 we propose a novel ZnO-based photocatalytic film which

45

possesses long life-time as well as large surface area. The performance and advantages of the film
in serving as the buoyant photocatalysts will be discussed.
Table 2.9. Recent approaches with respect to ZnO buoyant photocatalysts
Type
Morphology
Pollutant
Zno/Polystyrene
particle
MB
n-Zno/p-Ag2O/Nylon Film
Film+particle
MB
Ni/ZnO/Glass
Film+particle Methyl green (MG)
Ir/ZnO/Zn
film
MO
Polypyrrole (ppy)/ZnO/Flyash
particle
MO
Cenosphere (Fac)
Redot/Zno/Fac
particle
MO
Zno/Oak Charcoal
particle
ammonia

Light
UV
Visible
Visible
UV

Reference

Visible

255

Visible
UV

256

251
252
253
254

257

2.2.3 Micromotor system
Micromotor is a device whose dimension is in micro/nano scale. In general, micromotors
have capability to swim in solution (e.g. water) with or without the presence of external stimuli
and sometimes own specific functionality. Recently, upon the combination of various fabrication
techniques and diverse geometry, micromotors have realized multiple functions such as
photocatalysis, cargo delivery, antibacteria, objective capture and so on, which could perform
complex tasks in biomedicines and environmental remediation [258–260]. The recent studies in
photocatalytic degradation have implied the capability of micromotors in rapidly degrading a
variety of pollutants thanks to their advantages such as particular functionalities, suspension
working ways and build-in hybrid nanostructures [261]. Table 10 shows the comparison of three
immobilized systems in terms of some key factors. In detail, when working as photocatalysts,
owing to uniform suspension and large quantity in wastewater, micromotors are sometimes
regarded as a slurry system. But differ from typical photocatalysts in slurry, some of micromotors
have photocatalysts immobilized on surface meanwhile have other functional materials as body.
In this manner, in comparison to abovementioned immobilized systems micromotors possess
46

higher photocatalytic surface area to reactor volume ratio and desired mass transfer thereby
elevates degradation rate [262–265]. Moreover, the integrated functionality on micromotors also
renders milder situation in agglomeration and recovery to the suspended micromotors in solution
as comparing to the similar size of the suspended photocatalysts. More specifically, both the selfpropulsion based on physical and chemical cues and the remote motion-control realized by external
stimuli have been proven to counter the gravitation as well as mass transfer limitation between
micromotors, relaxing drawbacks in slurry system. Additionally the remote motion-control are
able to promote the separation of micromotors after the degradation hence become costcompetitive. Furthermore, the attempts on the hybrid composites and diverse morphology give a
way to further improve the photocatalytic performance of micromotors. Up to date a large number
of approaches regarding water remediation have been reported, in which TiO2 and its hybrid
structures were commonly employed in the construction of micromotors due to outstanding
stability and desired synergy effect in charge transportation [266–268]. It worthwhile notices that
TiO2-based micromotors are built on solid and rigid substrates. These approaches successfully
integrated various materials onto small-size micromotors whereas increased the probability of
corrosion and damage in wastewater environment, which would also have potential to become
extra pollutants as failed recovery. As an alternative, soft micromotors fabricated by biocompatible
and flexible materials could retain shape for a long time span. Nevertheless, some flaws such as
larger dimension and fewer building block candidates impede widespread applications [269]. Thus,
combining advantages of different types of micromotors is one pathway to pursue practical-used
micromotors.
Our literature research indicated that progress of ZnO-based micromotors were mostly on
the manageable movement [270,271] instead of the function of water cleaning was scarcely reported.

47

Taking account the comparable photocatalytic performance, mild fabrication condition and other
advantages, we believe that ZnO-based micromotors for photocatalytic degradation will interest
more researchers in the future. In Chapter 6 we introduce a novel micromotor design which has
embedded ZnO NW array on flexible substrate [272]. The impacts of particular motion and
functionality with respect to degradation of organic dye will be discussed.
Table 2.10. Comparison of three immobilized systems

Light utilization
Mass transfer
Lifetime
Active surface area
Recovery/separation

Immobilized
Photoreactors
Good
Good
Good
Good
Excellent

Buoyant
Photocatalysts
Excellent
Good
Good
Good
Excellent

Micromotor System
Good
Excellent
Excellent
Excellent
Good

Table 2.11. Comparison of slurry and immobilized systems

Advantages

Disadvantages

Slurry System
High catalyst surface area to
reactor volume
Low mass transfer limitation
Serious agglomeration at high
catalyst loading
Difficult catalyst separation

Immobilized System
Continuous degradation process
Easy catalyst separation
Potential high mass transfer
limitation
Potential contamination from
residue catalyst

2.2.4 Summary and outlook
ZnO nanostructures have been proven as one efficient photocatalyst in the past studies. In
this paper we have tried to list the recent progresses regarding ZnO-based photocatalysts since
ZnO has been demonstrated possessing a number of particular advantages that are superior to other
photocatalysts in practical application. For instance, the production cost of ZnO is approximately
one quarter of TiO2 which is much favored by industrial for saving total cost in wastewater

48

treatment. Furthermore, the milder synthesis environment and diverse geometry allow ZnO
photocatalysts having compatibility in coupling with many other materials than that of TiO2
whereby possessing high capability in countering complicated environmental conditions. In
addition, the remarkable performances of detoxification and antimicrobial activity of ZnO have
been investigated, whose applications in biology and biomedical fields implies the potential
multifunctionality of ZnO when using as photocatalysts.
As non-toxic photocatalysts, the improvement of ZnO in terms of degradability have been
extensively explored, where the form of multiple-composites has exhibited outstanding
performance. With the assistance of foreign materials, many aspects of ZnO, including defect
level, energy potential, stability and charge transportation, could be significantly modified. At
meantime, some special properties of foreign materials brought additional functionality which
enhanced the capability of ZnO as photocatalysts. On the other hand, as a well-documented
semiconductor, the fundamental study of the elevated performance towards ZnO and coupled
foreign materials could result a win-win situation as introduced to other applications.
From the literature research we found that considerable efforts in last few years were
devoted to develop novel ZnO-based photocatalysts while the methodology in using them was
rarely improved. Table 2.11 summarizes the advantages and disadvantages of slurry and
immobilized systems in photocatalytic degradation process [273,274]. Recently a number of weak
points that associated with the performance of photocatalysts, such as low-efficiency and nonrecovery, have been overcome by using specific photocatalyst system. However, the flaws are still
existing in those developed systems. More progresses are desired in this field as considering the
challenges in large-scale application. For instance, the slurry system is efficient but there is no
economic way to scale up. The majority of the slurry photoreactors focused more on the highest

49

degradation rate but neglected the post-process for recovery. As the promising alternative,
immobilized system has some practical issues such as demands of external power sources and
rapid reduction of photocatalytic activity, which shorted the lifetime hence increased the entire
cost. It is worth to pay more attention to the ways of properly using photocatalysts in future
research.

50

REFERENCES

51

REFERENCES

1. Abo, R., Kummer, N.-A. & Merkel, B. J. Optimized photodegradation of Bisphenol A in
water using ZnO, TiO2 and SnO2 photocatalysts under UV radiation as a decontamination
procedure. Drink. Water Eng. Sci. 9, 27–35 (2016).
2. Miranda, C. et al. Degradation of Organoarsenicals by Heterogeneous Photocatalysis using
ZnO, TiO2 and UVA. J. Adv. Oxid. Technol. 19, 276–283 (2016).
3. Gnanaprakasam, A., Sivakumar, V. M., Sivayogavalli, P. L. & Thirumarimurugan, M.
Characterization of TiO2 and ZnO nanoparticles and their applications in photocatalytic
degradation of azodyes. Ecotoxicol. Environ. Saf. 121, 121–125 (2015).
4. Souza, R. P. et al. Photocatalytic activity of TiO2, ZnO and Nb2O5 applied to degradation of
textile wastewater. J. Photochem. Photobiol. Chem. 329, 9–17 (2016).
5. Udom, I., Ram, M. K., Stefanakos, E. K., Hepp, A. F. & Goswami, D. Y. One dimensionalZnO nanostructures: Synthesis, properties and environmental applications. Mater. Sci.
Semicond. Process. 16, 2070–2083 (2013).
6. Zhou, H., Qu, Y., Zeid, T. & Duan, X. Towards highly efficient photocatalysts using
semiconductor nanoarchitectures. Energy Environ. Sci. 5, 6732–6743 (2012).
7. Jagadeesh Babu, V., Vempati, S., Uyar, T. & Ramakrishna, S. Review of one-dimensional
and two-dimensional nanostructured materials for hydrogen generation. Phys. Chem. Chem.
Phys. 17, 2960–2986 (2015).
8. Cacciato, G., Zimbone, M., Ruffino, F. & Grimaldi, M. G. TiO2 Nanostructures and
Nanocomposites for Sustainable Photocatalytic Water Purification. (2016).
doi:10.5772/62620
9. Wang, R. et al. Stable ZnO@TiO2 core/shell nanorod arrays with exposed high energy
facets for self-cleaning coatings with anti-reflective properties. J. Mater. Chem. A 2, 7313–
7318 (2014).
10. S. Selinsky, R., Ding, Q., S. Faber, M., C. Wright, J. & Jin, S. Quantum dot nanoscale
heterostructures for solar energy conversion. Chem. Soc. Rev. 42, 2963–2985 (2013).
11. Chabri, S. et al. Mesoporous CuO–ZnO p–n heterojunction based nanocomposites with high
specific surface area for enhanced photocatalysis and electrochemical sensing. Catal. Sci.
Technol. 6, 3238–3252 (2016).
12. Morales Flores, N., Pal, U., Galeazzi, R. & Sandoval, A. Effects of morphology, surface
area, and defect content on the photocatalytic dye degradation performance of ZnO
nanostructures. RSC Adv. 4, 41099–41110 (2014).

52

13. Wan, Q., Wang, T. H. & Zhao, J. C. Enhanced photocatalytic activity of ZnO nanotetrapods.
Appl. Phys. Lett. 87, 083105 (2005).
14. Lai, Y., Meng, M. & Yu, Y. One-step synthesis, characterizations and mechanistic study of
nanosheets-constructed fluffy ZnO and Ag/ZnO spheres used for Rhodamine B
photodegradation. Appl. Catal. B Environ. 100, 491–501 (2010).
15. Zhang, X. et al. Effect of aspect ratio and surface defects on the photocatalytic activity of
ZnO nanorods. Sci. Rep. 4, (2014).
16. Shifu, C. & Gengyu, C. Photocatalytic degradation of organophosphorus pesticides using
floating photocatalyst TiO2 · SiO2/beads by sunlight. Sol. Energy 79, 1–9 (2005).
17. Evgenidou, E., Fytianos, K. & Poulios, I. Semiconductor-sensitized photodegradation of
dichlorvos in water using TiO2 and ZnO as catalysts. Appl. Catal. B Environ. 59, 81–89
(2005).
18. Rajbongshi, B. M. & Samdarshi, S. K. Cobalt-doped zincblende–wurtzite mixed-phase ZnO
photocatalyst nanoparticles with high activity in visible spectrum. Appl. Catal. B Environ.
144, 435–441 (2014).
19. Ge, M. et al. A review of one-dimensional TiO2 nanostructured materials for environmental
and energy applications. J. Mater. Chem. A 4, 6772–6801 (2016).
20. Nair, R. G., Paul, S. & Samdarshi, S. K. High UV/visible light activity of mixed phase
titania: A generic mechanism. Sol. Energy Mater. Sol. Cells 95, 1901–1907 (2011).
21. Rakkesh, R. A. & Balakumar, S. Facile Synthesis of ZnO/TiO2 Core–Shell Nanostructures
and Their Photocatalytic Activities. J. Nanosci. Nanotechnol. 13, 370–376 (2013).
22. Asl, S. K., Rad, M. K. & Sadrnezhaad, S. K. Hydrothermal Preparation of TiO2‐ZnO Nano
Core‐Shell Structure with Quantum Size Effect. in AIP Conference Proceedings 1400, 425–
428 (AIP Publishing, 2011).
23. Sartori, A. et al. Preparation of tetrapod-like ZnO/TiO2 core-shell nanostructures as
photocatalytic powder. Cryst. Res. Technol. 46, 885–890 (2011).
24. Karunakaran, C., Vinayagamoorthy, P. & Jayabharathi, J. Electrical, optical and
photocatalytic properties of polyethylene glycol-assisted sol–gel synthesized Mn-doped
TiO2/ZnO core–shell nanoparticles. Superlattices Microstruct. 64, 569–580 (2013).
25. RamĂ­rez-Ortega, D., MelĂŠndez, A. M., Acevedo-PeĂąa, P., GonzĂĄlez, I. & Arroyo, R.
Semiconducting properties of ZnO/TiO2 composites by electrochemical measurements and
their relationship with photocatalytic activity. Electrochimica Acta 140, 541–549 (2014).
26. Jonidi-Jafari, A., Shirzad-Siboni, M., Yang, J.-K., Naimi-Joubani, M. & Farrokhi, M.
Photocatalytic degradation of diazinon with illuminated ZnO–TiO2 composite. J. Taiwan
Inst. Chem. Eng. 50, 100–107 (2015).
53

27. Wang, L. et al. Piezotronic Effect Enhanced Photocatalysis in Strained Anisotropic
ZnO/TiO2 Nanoplatelets via Thermal Stress. ACS Nano 10, 2636–2643 (2016).
28. Bel Hadjltaief, H., Ben Zina, M., Galvez, M. E. & Da Costa, P. Photocatalytic degradation of
methyl green dye in aqueous solution over natural clay-supported ZnO–TiO2 catalysts. J.
Photochem. Photobiol. Chem. 315, 25–33 (2016).
29. Kaleji, B., Mousaei, M., Halakouie, H. & Ahmadi, A. Sol-gel Synthesis of ZnO
Nanoparticles and ZnO-TiO2-SiO2 Nanocomposites and Their Photo-catalyst Investigation
in Methylene Blue Degradation. J. Nanostructures 5, 219–225 (2015).
30. Pan, L. et al. TiO2–ZnO Composite Sphere Decorated with ZnO Clusters for Effective
Charge Isolation in Photocatalysis. Ind. Eng. Chem. Res. 54, 7226–7232 (2015).
31. Rajbongshi, B. M., Samdarshi, S. K. & Boro, B. Multiphasic bi-component TiO2–ZnO
nanocomposite: synthesis, characterization and investigation of photocatalytic activity under
different wavelengths of light irradiation. J. Mater. Sci. Mater. Electron. 26, 377–384
(2015).
32. Kalpana, K. & Selvaraj, V. A novel approach for the synthesis of highly active
ZnO/TiO2/Ag2O nanocomposite and its photocatalytic applications. Ceram. Int. 41, 9671–
9679 (2015).
33. Da Dalt, S. et al. Preparation and Performance of TiO2-ZnO/CNT Hetero-Nanostructures
Applied to Photodegradation of Organic Dye. Mater. Res. 19, 1372–1375 (2016).
34. Karunakaran, C., Abiramasundari, G., Gomathisankar, P., Manikandan, G. & Anandi, V.
Preparation and characterization of ZnO–TiO2 nanocomposite for photocatalytic disinfection
of bacteria and detoxification of cyanide under visible light. Mater. Res. Bull. 46, 1586–1592
(2011).
35. Topkaya, E., Konyar, M., Yatmaz, H. C. & Öztürk, K. Pure ZnO and composite ZnO/TiO2
catalyst plates: A comparative study for the degradation of azo dye, pesticide and antibiotic
in aqueous solutions. J. Colloid Interface Sci. 430, 6–11 (2014).
36. Wu, L. et al. Fabrication of Regular ZnO/TiO2 Heterojunctions with Enhanced
Photocatalytic Properties. Chem. – Eur. J. 19, 8393–8396 (2013).
37. Gao, J. et al. Preparation of Er3+:YAlO3/Fe-doped TiO2–ZnO and its application in
photocatalytic degradation of dyes under solar light irradiation. Desalination 268, 68–75
(2011).
38. Guo, J., Li, J., Yin, A., Fan, K. & Dai, W. Photodegradation of Rhodamine B on Sulfur
Doped ZnO/TiO2 Nanocomposite Photocatalyst under Visible-light Irradiation. Chin. J.
Chem. 28, 2144–2150 (2010).
39. Nabid, M. R., Sedghi, R., Gholami, S., Oskooie, H. A. & Heravi, M. M. Preparation of New
Magnetic Nanocatalysts Based on TiO2 and ZnO and Their Application in Improved
54

Photocatalytic Degradation of Dye Pollutant Under Visible Light. Photochem. Photobiol. 89,
24–32 (2013).
40. Lei, J. F. et al. Fabrication of Ordered ZnO/TiO2 Heterostructures via a Templating
Technique. Langmuir 29, 13975–13981 (2013).
41. Li, L. et al. Microwave-assisted synthesis of nanocomposite Ag/ZnO–TiO2 and
photocatalytic degradation Rhodamine B with different modes. Colloids Surf. Physicochem.
Eng. Asp. 457, 134–141 (2014).
42. Pozan, G. S. & Kambur, A. Significant enhancement of photocatalytic activity over
bifunctional ZnO–TiO2 catalysts for 4-chlorophenol degradation. Chemosphere 105, 152–
159 (2014).
43. Wang, J. et al. Investigation on solar photocatalytic degradation of various dyes in the
presence of Er3+:YAlO3/ZnO–TiO2 composite. J. Environ. Manage. 91, 677–684 (2010).
44. Zhang, M. et al. Preparation of a high-activity ZnO/TiO2 photocatalyst via homogeneous
hydrolysis method with low temperature crystallization. Mater. Lett. 64, 1883–1886 (2010).
45. Habib, M. A., Shahadat, M. T., Bahadur, N. M., Ismail, I. M. I. & Mahmood, A. J. Synthesis
and characterization of ZnO-TiO2 nanocomposites and their application as photocatalysts.
Int. Nano Lett. 3, 5 (2013).
46. Zhang, D. Effectiveness of photodecomposition of rhodamine B and malachite green upon
coupled tricomponent TiO2(Anatase-Rutile)/ZnO nanocomposite. Acta Chim. Slovaca 6,
245–255 (2013).
47. Nuengmatcha, P., Chanthai, S., Mahachai, R. & Oh, W.-C. Visible light-driven
photocatalytic degradation of rhodamine B and industrial dyes (texbrite BAC-L and texbrite
NFW-L) by ZnO-graphene-TiO2 composite. J. Environ. Chem. Eng. 4, 2170–2177 (2016).
48. Wang, Y. et al. Visible-Light-Responsive TiO 2 -Coated ZnO:I Nanorod Array Films with
Enhanced Photoelectrochemical and Photocatalytic Performance. ACS Appl. Mater.
Interfaces 7, 6093–6101 (2015).
49. Yan, X., Zou, C., Gao, X. & Gao, W. ZnO/TiO2 core–brush nanostructure: processing,
microstructure and enhanced photocatalytic activity. J. Mater. Chem. 22, 5629–5640 (2012).
50. Xiao, F.-X. Construction of Highly Ordered ZnO–TiO2 Nanotube Arrays (ZnO/TNTs)
Heterostructure for Photocatalytic Application. ACS Appl. Mater. Interfaces 4, 7055–7063
(2012).
51. Zhang, A.-Y., Wang, W.-K., Pei, D.-N. & Yu, H.-Q. Degradation of refractory pollutants
under solar light irradiation by a robust and self-protected ZnO/CdS/TiO2 hybrid
photocatalyst. Water Res. 92, 78–86 (2016).

55

52. Bai, H., Zan, X., Zhang, L. & Sun, D. D. Multi-functional CNT/ZnO/TiO2 nanocomposite
membrane for concurrent filtration and photocatalytic degradation. Sep. Purif. Technol. 156,
Part 3, 922–930 (2015).
53. Kanjwal, M. A., Barakat, N. A. M., Sheikh, F. A., Park, S. J. & Kim, H. Y. Photocatalytic
activity of ZnO-TiO2 hierarchical nanostructure prepared by combined electrospinning and
hydrothermal techniques. Macromol. Res. 18, 233–240 (2010).
54. Cheng, C. et al. Enhanced photocatalytic performance of TiO2-ZnO hybrid nanostructures.
Sci. Rep. 4, 4181 (2014).
55. Lin, L. et al. A highly efficient TiO 2 @ZnO n–p–n heterojunction nanorod photocatalyst.
Nanoscale 5, 588–593 (2013).
56. Dhanalakshmi, R., Pandikumar, A., Sujatha, K. & Gunasekaran, P. Photocatalytic and
antimicrobial activities of functionalized silicate sol–gel embedded ZnO–TiO2
nanocomposite materials. Mater. Express 3, 291–300 (2013).
57. Wang, Y. et al. Visible-Light-Responsive TiO2-Coated ZnO:I Nanorod Array Films with
Enhanced Photoelectrochemical and Photocatalytic Performance. ACS Appl. Mater.
Interfaces 7, 6093–6101 (2015).
58. Long, H., Zeng, W. & Zhang, H. Synthesis of WO3 and its gas sensing: a review. J. Mater.
Sci. Mater. Electron. 26, 4698–4707 (2015).
59. Zheng, H. et al. Nanostructured Tungsten Oxide – Properties, Synthesis, and Applications.
Adv. Funct. Mater. 21, 2175–2196 (2011).
60. Dong, P., Hou, G., Xi, X., Shao, R. & Dong, F. WO 3 -based photocatalysts: morphology
control, activity enhancement and multifunctional applications. Environ. Sci. Nano 4, 539–
557 (2017).
61. SzilĂĄgyi, I. M. et al. WO3 photocatalysts: Influence of structure and composition. J. Catal.
294, 119–127 (2012).
62. Adhikari, S. & Sarkar, D. Metal oxide semiconductors for dye degradation. Mater. Res. Bull.
72, 220–228 (2015).
63. Adhikari, S., Sarkar, D. & Madras, G. Highly efficient WO3–ZnO mixed oxides for
photocatalysis. RSC Adv. 5, 11895–11904 (2015).
64. Wang, Y., Cai, L., Li, Y., Tang, Y. & Xie, C. Structural and photoelectrocatalytic
characteristic of ZnO/ZnWO4/WO3 nanocomposites with double heterojunctions. Phys. E
Low-Dimens. Syst. Nanostructures 43, 503–509 (2010).
65. Xie, J. et al. Simple preparation of WO3–ZnO composites with UV–Vis photocatalytic
activity and energy storage ability. Ceram. Int. 40, 12519–12524 (2014).

56

66. Subash, B., Krishnakumar, B., Pandiyan, V., Swaminathan, M. & Shanthi, M. Synthesis and
characterization of novel WO3 loaded Ag–ZnO and its photocatalytic activity. Mater. Res.
Bull. 48, 63–69 (2013).
67. Subash, B., Krishnakumar, B., Sreedhar, B., Swaminathan, M. & Shanthi, M. Highly active
WO3–Ag–ZnO photocatalyst driven by day light illumination. Superlattices Microstruct. 54,
155–171 (2013).
68. YU, C. et al. Preparation of WO3/ZnO Composite Photocatalyst and Its Photocatalytic
Performance. Chin. J. Catal. 32, 555–565 (2011).
69. Subash, B., Senthilraja, A., Dhatshanamurthi, P., Swaminathan, M. & Shanthi, M. Solar
active photocatalyst for effective degradation of RR 120 with dye sensitized mechanism.
Spectrochim. Acta. A. Mol. Biomol. Spectrosc. 115, 175–182 (2013).
70. Subash, B., Krishnakumar, B., Swaminathan, M. & Shanthi, M. Enhanced photocatalytic
performance of WO3 loaded Ag–ZnO for Acid Black 1 degradation by UV-A light. J. Mol.
Catal. Chem. 366, 54–63 (2013).
71. Lam, S.-M., Sin, J.-C., Abdullah, A. Z. & Mohamed, A. R. Sunlight responsive WO3/ZnO
nanorods for photocatalytic degradation and mineralization of chlorinated phenoxyacetic
acid herbicides in water. J. Colloid Interface Sci. 450, 34–44 (2015).
72. Lam, S.-M., Sin, J.-C., Abdullah, A. Z. & Mohamed, A. R. ZnO nanorods surface-decorated
by WO3 nanoparticles for photocatalytic degradation of endocrine disruptors under a
compact fluorescent lamp. Ceram. Int. 39, 2343–2352 (2013).
73. Lam, S.-M., Sin, J.-C., Abdullah, A. Z. & Mohamed, A. R. Transition metal oxide loaded
ZnO nanorods: Preparation, characterization and their UV–vis photocatalytic activities. Sep.
Purif. Technol. 132, 378–387 (2014).
74. Singh, S. & Madras, G. Photocatalytic degradation with combustion synthesized WO3 and
WO3TiO2 mixed oxides under UV and visible light. Sep. Purif. Technol. 105, 79–89 (2013).
75. Shokrollahi, H. A review of the magnetic properties, synthesis methods and applications of
maghemite. J. Magn. Magn. Mater. 426, 74–81 (2017).
76. Mishra, M. & Chun, D.-M. Îą-Fe2O3 as a photocatalytic material: A review. Appl. Catal.
Gen. 498, 126–141 (2015).
77. Li, G. & Mao, L. Magnetically separable Fe3O4–Ag3PO4 sub-micrometre composite: facile
synthesis, high visible light-driven photocatalytic efficiency, and good recyclability. RSC
Adv. 2, 5108–5111 (2012).
78. Li, J., Liu, Z. & Zhu, Z. Magnetically separable ZnFe 2 O 4 , Fe 2 O 3 /ZnFe 2 O 4 and
ZnO/ZnFe 2 O 4 hollow nanospheres with enhanced visible photocatalytic properties. RSC
Adv. 4, 51302–51308 (2014).

57

79. Liu, Y. et al. A magnetically separable photocatalyst based on nest-like Îł-Fe 2 O 3 /ZnO
double-shelled hollow structures with enhanced photocatalytic activity. Nanoscale 4, 183–
187 (2012).
80. Achouri, F. et al. Aqueous synthesis and enhanced photocatalytic activity of ZnO/Fe2O3
heterostructures. J. Phys. Chem. Solids 75, 1081–1087 (2014).
81. Wu, J. et al. Fabrication of an Ag/Fe 2 O 3 /ZnO ternary composite with enhanced
photocatalytic performance. RSC Adv. 6, 4145–4150 (2016).
82. Lee, K. M., Abdullah, A. H., Hussein, M. Z. & Zainal, Z. Synthesis and photocatalysis of
ZnO/Fe2O3 nanocomposite in degrading herbicide 2,4-dichlorophenoxyacetic acid. Sains
Malays. 43, 437–441 (2014).
83. L. Maya-TreviĂąo, M., Villanueva-RodrĂ­guez, M., L. GuzmĂĄn-Mar, J., Hinojosa-Reyes, L. &
Hernández-Ramírez, A. Comparison of the solar photocatalytic activity of ZnO–Fe 2 O 3
and ZnO–Fe 0 on 2,4-D degradation in a CPC reactor. Photochem. Photobiol. Sci. 14, 543–
549 (2015).
84. Yan, W., Fan, H. & Yang, C. Ultra-fast synthesis and enhanced photocatalytic properties of
alpha-Fe2O3/ZnO core-shell structure. Mater. Lett. 65, 1595–1597 (2011).
85. Wu, W. et al. Controllable Synthesis, Magnetic Properties, and Enhanced Photocatalytic
Activity of Spindlelike Mesoporous α-Fe2O3/ZnO Core–Shell Heterostructures. ACS Appl.
Mater. Interfaces 4, 3602–3609 (2012).
86. Maya-Treviño, M. L. et al. Activity of the ZnO–Fe2O3 catalyst on the degradation of
Dicamba and 2,4-D herbicides using simulated solar light. Ceram. Int. 40, 8701–8708
(2014).
87. Xie, J. et al. Îą-Fe2O3 modified ZnO flower-like microstructures with enhanced
photocatalytic performance for pentachlorophenol degradation. Ceram. Int. 41, 9420–9425
(2015).
88. Abdullah, A. H., Mun, L. K., Zainal, Z. & Hussein, M. Z. Photodegradation of
Chlorophenoxyacetic Acids by ZnO/r-Fe2O3 Nanocatalysts: A Comparative Study. Int. J.
Chem. 5, 56 (2013).
89. Xie, J. et al. Synthesis of Îą-Fe2O3/ZnO composites for photocatalytic degradation of
pentachlorophenol under UV–vis light irradiation. Ceram. Int. 41, 2622–2625 (2015).
90. Lee, K. M. & Abdullah, A. H. Synthesis and characterization of zinc oxide/maghemite
nanocomposites: Influence of heat treatment on photocatalytic degradation of 2,4dichlorophenoxyacetic acid. Mater. Sci. Semicond. Process. 30, 298–306 (2015).
91. Qamar, M. T., Aslam, M., Ismail, I. M. I., Salah, N. & Hameed, A. The assessment of the
photocatalytic activity of magnetically retrievable ZnO coated Îł-Fe2O3 in sunlight exposure.
Chem. Eng. J. 283, 656–667 (2016).
58

92. Yin-Fang, C., Cong, W., Su-Juan, W., Yu, L. & Tian-Min, W. Preparation and
Photocatalytic Activity of ZnO/Fe 2 O 3 Nanorod Arrays and ZnO/NiO Nanotube Arrays.
Chin. Phys. Lett. 29, 037201 (2012).
93. Cleuziou, J.-P., Wernsdorfer, W., Bouchiat, V., Ondarçuhu, T. & Monthioux, M. Carbon
nanotube superconducting quantum interference device. Nat. Nanotechnol. 1, nnano.2006.54
(2006).
94. Appenzeller, J., Lin, Y.-M., Knoch, J. & Avouris, P. Band-to-Band Tunneling in Carbon
Nanotube Field-Effect Transistors. Phys. Rev. Lett. 93, 196805 (2004).
95. Jin, Z., Sun, X., Xu, G., Goh, S. H. & Ji, W. Nonlinear optical properties of some
polymer/multi-walled carbon nanotube composites. Chem. Phys. Lett. 318, 505–510 (2000).
96. Garibaldi, S., Brunelli, C., Bavastrello, V., Ghigliotti, G. & Nicolini, C. Carbon nanotube
biocompatibility with cardiac muscle cells. Nanotechnology 17, 391 (2006).
97. Jitianu, A. et al. Synthesis and characterization of carbon nanotubes–TiO2 nanocomposites.
Carbon 42, 1147–1151 (2004).
98. Upadhyayula, V. K. K., Deng, S., Mitchell, M. C. & Smith, G. B. Application of carbon
nanotube technology for removal of contaminants in drinking water: A review. Sci. Total
Environ. 408, 1–13 (2009).
99. Ren, X., Chen, C., Nagatsu, M. & Wang, X. Carbon nanotubes as adsorbents in
environmental pollution management: A review. Chem. Eng. J. 170, 395–410 (2011).
100. Eder, D. Carbon Nanotube−Inorganic Hybrids. Chem. Rev. 110, 1348–1385 (2010).
101. Mallakpour, S. & Khadem, E. Carbon nanotube–metal oxide nanocomposites: Fabrication,
properties and applications. Chem. Eng. J. 302, 344–367 (2016).
102. Chen, C. S. et al. Multi-walled carbon nanotube-supported metal-doped ZnO nanoparticles
and their photocatalytic property. J. Nanoparticle Res. 15, 1295 (2013).
103. Kou, H., Jia, L. & Wang, C. Electrochemical deposition of flower-like ZnO nanoparticles
on a silver-modified carbon nanotube/polyimide membrane to improve its photoelectric
activity and photocatalytic performance. Carbon 50, 3522–3529 (2012).
104. Xiang, Q., Yu, J. & Jaroniec, M. Graphene -based semiconductor photocatalysts. Chem.
Soc. Rev. 41, 782–796 (2012).
105. Ranjith, K. S., Manivel, P., Rajendrakumar, R. T. & Uyar, T. Multifunctional ZnO nanorodreduced graphene oxide hybrids nanocomposites for effective water remediation: Effective
sunlight driven degradation of organic dyes and rapid heavy metal adsorption. Chem. Eng.
J. 325, 588–600 (2017).

59

106. Stoller, M. D., Park, S., Zhu, Y., An, J. & Ruoff, R. S. Graphene-Based Ultracapacitors.
Nano Lett. 8, 3498–3502 (2008).
107. Ahmad, M. et al. A facile one-step approach to synthesizing ZnO/graphene composites for
enhanced degradation of methylene blue under visible light. Appl. Surf. Sci. 274, 273–281
(2013).
108. Azarang, M., Shuhaimi, A., Yousefi, R. & Pilban Jahromi, S. One-pot sol–gel synthesis of
reduced graphene oxide uniformly decorated zinc oxide nanoparticles in starch environment
for highly efficient photodegradation of Methylene Blue. RSC Adv. 5, 21888–21896 (2015).
109. Dou, P. et al. One-step microwave-assisted synthesis of Ag/ZnO/graphene nanocomposites
with enhanced photocatalytic activity. J. Photochem. Photobiol. Chem. 302, 17–22 (2015).
110. Jo, W.-K. & Clament Sagaya Selvam, N. Enhanced visible light-driven photocatalytic
performance of ZnO–g-C3N4 coupled with graphene oxide as a novel ternary
nanocomposite. J. Hazard. Mater. 299, 462–470 (2015).
111. Mu, J. et al. High Photocatalytic Activity of ZnO−Carbon Nanofiber Heteroarchitectures.
ACS Appl. Mater. Interfaces 3, 590–596 (2011).
112. Bozetine, H. et al. Green chemistry approach for the synthesis of ZnO–carbon dots
nanocomposites with good photocatalytic properties under visible light. J. Colloid Interface
Sci. 465, 286–294 (2016).
113. Zhao, X. et al. Facile synthesis of the flower-like ternary heterostructure of Ag/ZnO
encapsulating carbon spheres with enhanced photocatalytic performance. Appl. Surf. Sci.
406, 254–264 (2017).
114. Sharma, S., Mehta, S. K. & Kansal, S. K. N doped ZnO/C-dots nanoflowers as visible light
driven photocatalyst for the degradation of malachite green dye in aqueous phase. J. Alloys
Compd. 699, 323–333 (2017).
115. Ahmad, M. et al. Photocatalytic, sonocatalytic and sonophotocatalytic degradation of
Rhodamine B using ZnO/CNTs composites photocatalysts. Ultrason. Sonochem. 21, 761–
773 (2014).
116. Saleh, T. A., Gondal, M. A. & Drmosh, Q. A. Preparation of a MWCNT/ZnO
nanocomposite and its photocatalytic activity for the removal of cyanide from water using a
laser. Nanotechnology 21, 495705 (2010).
117. Shao, D. et al. Flexible, thorn-like ZnO-multiwalled carbon nanotube hybrid paper for
efficient ultraviolet sensing and photocatalyst applications. Nanoscale 6, 13630–13636
(2014).
118. Ahmad, M. et al. Enhancement in visible light-responsive photocatalytic activity by
embedding Cu-doped ZnO nanoparticles on multi-walled carbon nanotubes. Appl. Surf. Sci.
285, Part B, 702–712 (2013).
60

119. Liu, P. et al. Enhanced photocatalytic performance of ZnO/multi-walled carbon nanotube
nanocomposites for dye degradation. Ceram. Int. 40, 5629–5633 (2014).
120. Dai, K., Dawson, G., Yang, S., Chen, Z. & Lu, L. Large scale preparing carbon
nanotube/zinc oxide hybrid and its application for highly reusable photocatalyst. Chem.
Eng. J. 191, 571–578 (2012).
121. Lv, T. et al. Enhanced photocatalytic degradation of methylene blue by ZnO-reduced
graphene oxide composite synthesized via microwave-assisted reaction. J. Alloys Compd.
509, 10086–10091 (2011).
122. Zhu, G. et al. A facile synthesis of ZnO/CNT hierarchical microsphere composites with
enhanced photocatalytic degradation of methylene blue. RSC Adv. 5, 72476–72481 (2015).
123. Sampaio, M. J. et al. Synergistic effect between carbon nanomaterials and ZnO for
photocatalytic water decontamination. J. Catal. 331, 172–180 (2015).
124. Mu, J. et al. High Photocatalytic Activity of ZnO−Carbon Nanofiber Heteroarchitectures.
ACS Appl. Mater. Interfaces 3, 590–596 (2011).
125. Moussa, H. et al. ZnO rods/reduced graphene oxide composites prepared via a solvothermal
reaction for efficient sunlight-driven photocatalysis. Appl. Catal. B Environ. 185, 11–21
(2016).
126. Xu, T., Zhang, L., Cheng, H. & Zhu, Y. Significantly enhanced photocatalytic performance
of ZnO via graphene hybridization and the mechanism study. Appl. Catal. B Environ. 101,
382–387 (2011).
127. Kavitha, T., Gopalan, A. I., Lee, K.-P. & Park, S.-Y. Glucose sensing, photocatalytic and
antibacterial properties of graphene–ZnO nanoparticle hybrids. Carbon 50, 2994–3000
(2012).
128. Ahmad, M. et al. A facile one-step approach to synthesizing ZnO/graphene composites for
enhanced degradation of methylene blue under visible light. Appl. Surf. Sci. 274, 273–281
(2013).
129. Atchudan, R., Edison, T. N. J. I., Perumal, S., Karthikeyan, D. & Lee, Y. R. Facile synthesis
of zinc oxide nanoparticles decorated graphene oxide composite via simple solvothermal
route and their photocatalytic activity on methylene blue degradation. J. Photochem.
Photobiol. B 162, 500–510 (2016).
130. Kavitha, M. K., Pillai, S. C., Gopinath, P. & John, H. Hydrothermal synthesis of ZnO
decorated reduced graphene oxide: Understanding the mechanism of photocatalysis. J.
Environ. Chem. Eng. 3, 1194–1199 (2015).
131. Cai, R. et al. 3D graphene/ZnO composite with enhanced photocatalytic activity. Mater.
Des. 90, 839–844 (2016).

61

132. Nipane, S. V., Korake, P. V. & Gokavi, G. S. Graphene-zinc oxide nanorod nanocomposite
as photocatalyst for enhanced degradation of dyes under UV light irradiation. Ceram. Int.
41, 4549–4557 (2015).
133. Hossain, M. M., Ku, B.-C. & Hahn, J. R. Synthesis of an efficient white-light photocatalyst
composite of graphene and ZnO nanoparticles: Application to methylene blue dye
decomposition. Appl. Surf. Sci. 354, 55–65 (2015).
134. Tayyebi, A., outokesh, M., Tayebi, M., Shafikhani, A. & Şengör, S. S. ZnO quantum dotsgraphene composites: Formation mechanism and enhanced photocatalytic activity for
degradation of methyl orange dye. J. Alloys Compd. 663, 738–749 (2016).
135. Kale, D. & Thakur, P. Highly efficient photocatalytic degradation and mineralization of 4nitrophenol by graphene decorated ZnO. J. Porous Mater. 22, 797–806 (2015).
136. Chen, D. et al. Graphene-wrapped ZnO nanospheres as a photocatalyst for high
performance photocatalysis. Thin Solid Films 574, 1–9 (2015).
137. Bera, S., Pal, M., Naskar, A. & Jana, S. Hierarchically structured ZnO-graphene hollow
microspheres towards effective reusable adsorbent for organic pollutant via
photodegradation process. J. Alloys Compd. 669, 177–186 (2016).
138. Posa, V. R., Annavaram, V., Koduru, J. R., Ammireddy, V. R. & Somala, A. R. GrapheneZnO nanocomposite for highly efficient photocatalytic degradation of methyl orange dye
under solar light irradiation. Korean J. Chem. Eng. 33, 456–464 (2016).
139. Worajittiphon, P., Pingmuang, K., Inceesungvorn, B., Wetchakun, N. & Phanichphant, S.
Enhancing the photocatalytic activity of ZnO nanoparticles for efficient rhodamine B
degradation by functionalised graphene nanoplatelets. Ceram. Int. 41, 1885–1889 (2015).
140. Rokhsat, E. & Akhavan, O. Improving the photocatalytic activity of graphene oxide/ZnO
nanorod films by UV irradiation. Appl. Surf. Sci. 371, 590–595 (2016).
141. Nuengmatcha, P., Chanthai, S., Mahachai, R. & Oh, W.-C. Sonocatalytic performance of
ZnO/graphene/TiO2 nanocomposite for degradation of dye pollutants (methylene blue,
texbrite BAC-L, texbrite BBU-L and texbrite NFW-L) under ultrasonic irradiation. Dyes
Pigments 134, 487–497 (2016).
142. Men, X. et al. Three-dimensional free-standing ZnO/graphene composite foam for
photocurrent generation and photocatalytic activity. Appl. Catal. B Environ. 187, 367–374
(2016).
143. Reddy, D. A., Ma, R. & Kim, T. K. Efficient photocatalytic degradation of methylene blue
by heterostructured ZnO–RGO/RuO2 nanocomposite under the simulated sunlight
irradiation. Ceram. Int. 41, 6999–7009 (2015).
144. Xu, S. et al. Preparation of ZnO flower/reduced graphene oxide composite with enhanced
photocatalytic performance under sunlight. Ceram. Int. 41, 4007–4013 (2015).
62

145. Ashkarran, A. A. & Mohammadi, B. ZnO nanoparticles decorated on graphene sheets
through liquid arc discharge approach with enhanced photocatalytic performance under
visible-light. Appl. Surf. Sci. 342, 112–119 (2015).
146. Raghavan, N., Thangavel, S. & Venugopal, G. Enhanced photocatalytic degradation of
methylene blue by reduced graphene-oxide/titanium dioxide/zinc oxide ternary
nanocomposites. Mater. Sci. Semicond. Process. 30, 321–329 (2015).
147. Qin, J. et al. Ag/ZnO/graphene oxide heterostructure for the removal of rhodamine B by the
synergistic adsorption–degradation effects. Ceram. Int. 41, 4231–4237 (2015).
148. Xie, H. et al. CuAu–ZnO–graphene nanocomposite: A novel graphene-based bimetallic
alloy-semiconductor catalyst with its enhanced photocatalytic degradation performance. J.
Alloys Compd. 636, 40–47 (2015).
149. Wu, X. et al. Fabrication of ZnO/graphene flake-like photocatalyst with enhanced
photoreactivity. Appl. Surf. Sci. 358, 130–136 (2015).
150. Chen, Y.-C. et al. ZnO–graphene composites as practical photocatalysts for gaseous
acetaldehyde degradation and electrolytic water oxidation. Appl. Catal. Gen. 490, 1–9
(2015).
151. Anirudhan, T. S. & Deepa, J. R. Nano-zinc oxide incorporated graphene
oxide/nanocellulose composite for the adsorption and photo catalytic degradation of
ciprofloxacin hydrochloride from aqueous solutions. J. Colloid Interface Sci. 490, 343–356
(2017).
152. Jabeen, M. et al. UV-Assisted Photocatalytic Synthesis of ZnO-Reduced Graphene Oxide
Nanocomposites with Enhanced Photocatalytic Performance in Degradation of Methylene
Blue. ECS J. Solid State Sci. Technol. 6, M36–M43 (2017).
153. Qin, J. et al. ZnO microspheres-reduced graphene oxide nanocomposite for photocatalytic
degradation of methylene blue dye. Appl. Surf. Sci. 392, 196–203 (2017).
154. Hosseini, S. A., Babaei, S., Hosseini, S. A. & Babaei, S. Graphene Oxide/Zinc Oxide
(GO/ZnO) Nanocomposite as a Superior Photocatalyst for Degradation of Methylene Blue
(MB)-Process Modeling by Response Surface Methodology (RSM). J. Braz. Chem. Soc.
28, 299–307 (2017).
155. Surendran, D. K., Xavier, M. M., Viswanathan, V. P. & Mathew, S. Synthesis of a ternary
Ag/RGO/ZnO nanocomposite via microwave irradiation and its application for the
degradation of Rhodamine B under visible light. Environ. Sci. Pollut. Res. 24, 15360–
15368 (2017).
156. Bai, X. et al. Photocatalytic degradation of deoxynivalenol using graphene/ZnO hybrids in
aqueous suspension. Appl. Catal. B Environ. 204, 11–20 (2017).

63

157. Ma, X. et al. Comparison of photocatalytic reaction-induced selective corrosion with
photocorrosion: Impact on morphology and stability of Ag-ZnO. Appl. Catal. B Environ.
201, 348–358 (2017).
158. She, P. et al. Investigating the size effect of Au nanospheres on the photocatalytic activity
of Au-modified ZnO nanorods. J. Colloid Interface Sci. 499, 76–82 (2017).
159. Muñoz-Fernandez, L., Sierra-Fernandez, A., Milošević, O. & Rabanal, M. E. Solvothermal
synthesis of Ag/ZnO and Pt/ZnO nanocomposites and comparison of their photocatalytic
behaviors on dyes degradation. Adv. Powder Technol. 27, 983–993 (2016).
160. Dhatshanamurthi, P. & Shanthi, M. Enhanced photocatalytic degradation of azo dye in
aqueous solutions using Ba@Ag@ZnO nanocomposite for self-sensitized under sunshine
irradiation. Int. J. Hydrog. Energy 42, 5523–5536 (2017).
161. Pirhashemi, M. & Habibi-Yangjeh, A. Ultrasonic-assisted preparation of plasmonic
ZnO/Ag/Ag2WO4 nanocomposites with high visible-light photocatalytic performance for
degradation of organic pollutants. J. Colloid Interface Sci. 491, 216–229 (2017).
162. Shao, X., Li, B., Zhang, B., Shao, L. & Wu, Y. Au@ZnO core–shell nanostructures with
plasmon-induced visible-light photocatalytic and photoelectrochemical properties. Inorg.
Chem. Front. 3, 934–943 (2016).
163. Jiang, T., Qin, X., Sun, Y. & Yu, M. UV photocatalytic activity of Au@ZnO core–shell
nanostructure with enhanced UV emission. RSC Adv. 5, 65595–65599 (2015).
164. Sun, Y. et al. Complete Au@ZnO core–shell nanoparticles with enhanced plasmonic
absorption enabling significantly improved photocatalysis. Nanoscale 8, 10774–10782
(2016).
165. Kuriakose, S. et al. Facile synthesis of Au-ZnO plasmonic nanohybrids for highly efficient
photocatalytic degradation of methylene blue. Opt. Mater. 64, 47–52 (2017).
166. Andrade, G. R. S., Nascimento, C. C., Silva JĂşnior, E. C., Mendes, D. T. S. L. & Gimenez,
I. F. ZnO/Au nanocatalysts for enhanced decolorization of an azo dye under solar, UV-A
and dark conditions. J. Alloys Compd. 710, 557–566 (2017).
167. Chauhan, P. S., Rai, A., Gupta, A. & Bhattacharya, S. Enhanced photocatalytic performance
of vertically grown ZnO nanorods decorated with metals (Al, Ag, Au, and Au–Pd) for
degradation of industrial dye. Mater. Res. Express 4, 055004 (2017).
168. She, P. et al. A self-standing macroporous Au/ZnO/reduced graphene oxide foam for
recyclable photocatalysis and photocurrent generation. Electrochimica Acta 246, 35–42
(2017).
169. She, P. et al. Controlled preparation and visible light photocatalytic activities of corn coblike Au–ZnO nanorods. J. Mater. Sci. 52, 3478–3489 (2017).

64

170. Lin, F. et al. Intermediate selectivity in the oxidation of phenols using plasmonic Au/ZnO
photocatalysts. Nanoscale 9, 9359–9364 (2017).
171. Wu, S., Chen, Z., Wang, T. & Ji, X. A facile approach for the fabrication of Au/ZnOhollow-sphere-monolayer thin films and their photocatalytic properties. Appl. Surf. Sci.
412, 69–76 (2017).
172. Wu, J., Luo, C., Li, D., Fu, Q. & Pan, C. Preparation of Au nanoparticle-decorated
ZnO/NiO heterostructure via nonsolvent method for high-performance photocatalysis. J.
Mater. Sci. 52, 1285–1295 (2017).
173. Yu, C. et al. Hierarchical nanoflowers assembled with Au nanoparticles decorated ZnO
nanosheets toward enhanced photocatalytic properties. Mater. Lett. 190, 185–187 (2017).
174. Aguirre, M. E., RodrĂ­guez, H. B., San RomĂĄn, E., Feldhoff, A. & Grela, M. A. Ag@ZnO
Core–Shell Nanoparticles Formed by the Timely Reduction of Ag+ Ions and Zinc Acetate
Hydrolysis in N,N-Dimethylformamide: Mechanism of Growth and Photocatalytic
Properties. J. Phys. Chem. C 115, 24967–24974 (2011).
175. Liu, H. R. et al. Worm-Like Ag/ZnO Core–Shell Heterostructural Composites: Fabrication,
Characterization, and Photocatalysis. J. Phys. Chem. C 116, 16182–16190 (2012).
176. Xu, Y. et al. Ag nanoparticle-functionalized ZnO micro-flowers for enhanced
photodegradation of herbicide derivatives. Chem. Phys. Lett. 679, 119–126 (2017).
177. Andrade, G. R. S. et al. Star-shaped ZnO/Ag hybrid nanostructures for enhanced
photocatalysis and antibacterial activity. Appl. Surf. Sci. 399, 573–582 (2017).
178. Huang, Q. & Li, J. Enhanced photocatalytic and SERS properties of ZnO/Ag hierarchical
multipods-shaped nanocomposites. Mater. Lett. 204, 85–88 (2017).
179. Sardar, D., Maity, J., Ghosalya, M. K., Gopinath, C. S. & Bala, T. Facile synthesis of ZnO–
Ag nanocomposite and its photocatalytic activity. Mater. Res. Express 4, 055011 (2017).
180. Rokesh, K., Mohan, S. C., Karuppuchamy, S. & Jothivenkatachalam, K. Photo-assisted
advanced oxidation processes for Rhodamine B degradation using ZnO–Ag nanocomposite
materials. J. Environ. Chem. Eng. (2017). doi:10.1016/j.jece.2017.01.023
181. Kaur, A., Ibhadon, A. O. & Kansal, S. K. Photocatalytic degradation of ketorolac
tromethamine (KTC) using Ag-doped ZnO microplates. J. Mater. Sci. 52, 5256–5267
(2017).
182. Ravichandran, K., Uma, R., Sriram, S. & Balamurgan, D. Fabrication of ZnO:Ag/GO
composite thin films for enhanced photocatalytic activity. Ceram. Int. 43, 10041–10051
(2017).

65

183. Praveen, R., Chandreshia, C. T. B. & Ramaraj, R. Silicate sol–gel matrix stabilized ZnO–
Ag nanocomposites materials and their environmental remediation applications. J. Environ.
Chem. Eng. (2017). doi:10.1016/j.jece.2017.01.048
184. Liu, H. et al. Fabrication of In 2 O 3 /ZnO@Ag nanowire ternary composites with enhanced
visible light photocatalytic activity. RSC Adv. 7, 37220–37229 (2017).
185. Jaramillo-PĂĄez, C., NavĂ­o, J. A., Hidalgo, M. C. & MacĂ­as, M. High UV-photocatalytic
activity of ZnO and Ag/ZnO synthesized by a facile method. Catal. Today 284, 121–128
(2017).
186. Lv, J. et al. Enhanced photocurrent and photocatalytic properties of porous ZnO thin film
by Ag nanoparticles. J. Phys. Chem. Solids 111, 104–109 (2017).
187. Guo, W. & Liu, H. Preparation of ZnO/Ag heterostructure material by one-pot method for
enhanced photocatalytic performance. Chem. Res. Chin. Univ. 33, 129–134 (2017).
188. S, D. K., Mary Xavier, M., V, V. P., N, R. V. & Mathew, S. A quaternary TiO 2
/ZnO/RGO/Ag nanocomposite with enhanced visible light photocatalytic performance. New
J. Chem. 41, 6445–6454 (2017).
189. Moradi, M., Haghighi, M. & Allahyari, S. Precipitation dispersion of Ag–ZnO nanocatalyst
over functionalized multiwall carbon nanotube used in degradation of Acid Orange from
wastewater. Process Saf. Environ. Prot. 107, 414–427 (2017).
190. Zhang, P. et al. Pt/ZnO@C Nanocable with Dual-Enhanced Photocatalytic Performance and
Superior Photostability. Langmuir 33, 4452–4460 (2017).
191. Hu, X. et al. Synthesis and photocatalytic activity of Pt–ZnO hybrid nanocomposite by
solution plasma technology. Nanotechnology 28, 045604 (2017).
192. Chang, Y.-C. & Guo, J.-Y. ZnO/Pt core-shell nanorods on the cotton threads with high
enhanced photocatalytic properties. Mater. Chem. Phys. 180, 9–13 (2016).
193. Zhang, W. et al. Ultralong ZnO/Pt hierarchical structures for continuous-flow catalytic
reactions. Mater. Des. 109, 492–502 (2016).
194. Mohamed, R. M., McKinney, D., Kadi, M. W., Mkhalid, I. A. & Sigmund, W.
Platinum/zinc oxide nanoparticles: Enhanced photocatalysts degrade malachite green dye
under visible light conditions. Ceram. Int. 42, 9375–9381 (2016).
195. Ong, C. B., Ng, L. Y. & Mohammad, A. W. A review of ZnO nanoparticles as solar
photocatalysts: Synthesis, mechanisms and applications. Renew. Sustain. Energy Rev. 81,
536–551 (2018).
196. Fatin, S. O., Lim, H. N., Tan, W. T. & Huang, N. M. Comparison of photocatalytic activity
and cyclic voltammetry of zinc oxide and titanium dioxide nanoparticles toward
degradation of methylene blue. Int. J. Electrochem. Sci. 7, 9074–9084 (2012).
66

197. Guillard, C. et al. Influence of chemical structure of dyes, of pH and of inorganic salts on
their photocatalytic degradation by TiO2 comparison of the efficiency of powder and
supported TiO2. J. Photochem. Photobiol. Chem. 158, 27–36 (2003).
198. Lee, J. H. et al. Design of two types of fluidized photo reactors and their photo-catalytic
performances for degradation of methyl orange. Appl. Catal. Gen. 244, 49–57 (2003).
199. Andronic, L. & Duta, A. The influence of TiO2 powder and film on the photodegradation of
methyl orange. Mater. Chem. Phys. 112, 1078–1082 (2008).
200. Mehrizadeh, H., Niaei, A., Tseng, H.-H., Salari, D. & Khataee, A. Synthesis of ZnFe2O4
nanoparticles for photocatalytic removal of toluene from gas phase in the annular reactor. J.
Photochem. Photobiol. Chem. 332, 188–195 (2017).
201. Bojer, C. et al. Clinical wastewater treatment: Photochemical removal of an anionic
antibiotic (ciprofloxacin) by mesostructured high aspect ratio ZnO nanotubes. Appl. Catal.
B Environ. 204, 561–565 (2017).
202. Wu, C. Facile one-step synthesis of N-doped ZnO micropolyhedrons for efficient
photocatalytic degradation of formaldehyde under visible-light irradiation. Appl. Surf. Sci.
319, 237–243 (2014).
203. Souza, R. P. et al. Photocatalytic activity of TiO2, ZnO and Nb2O5 applied to degradation
of textile wastewater. J. Photochem. Photobiol. Chem. 329, 9–17 (2016).
204. Jassby, D., Farner Budarz, J. & Wiesner, M. Impact of Aggregate Size and Structure on the
Photocatalytic Properties of TiO2 and ZnO Nanoparticles. Environ. Sci. Technol. 46, 6934–
6941 (2012).
205. Banić, N. D. et al. Efficiency of neonicotinoids photocatalytic degradation by using annular
slurry reactor. Chem. Eng. J. 286, 184–190 (2016).
206. Plakas, K. V., Sarasidis, V. C., Patsios, S. I., Lambropoulou, D. A. & Karabelas, A. J. Novel
pilot scale continuous photocatalytic membrane reactor for removal of organic
micropollutants from water. Chem. Eng. J. 304, 335–343 (2016).
207. Laohaprapanon, S., Matahum, J., Tayo, L. & You, S.-J. Photodegradation of Reactive Black
5 in a ZnO/UV slurry membrane reactor. J. Taiwan Inst. Chem. Eng. 49, 136–141 (2015).
208. Khan, W. Z., Najeeb, I., Tuiyebayeva, M. & Makhtayeva, Z. Refinery wastewater
degradation with titanium dioxide, zinc oxide, and hydrogen peroxide in a photocatalytic
reactor. Process Saf. Environ. Prot. 94, 479–486 (2015).
209. Saoud, K. et al. Synthesis of supported silver nano-spheres on zinc oxide nanorods for
visible light photocatalytic applications. Mater. Res. Bull. 63, 134–140 (2015).
210. Saravanan, R. et al. ZnO/Ag/CdO nanocomposite for visible light-induced photocatalytic
degradation of industrial textile effluents. J. Colloid Interface Sci. 452, 126–133 (2015).
67

211. Sanoop, P. K. et al. Synthesis of yttrium doped nanocrystalline ZnO and its photocatalytic
activity in methylene blue degradation. Arab. J. Chem. 9, S1618–S1626 (2016).
212. Feng, X., Guo, H., Patel, K., Zhou, H. & Lou, X. High performance, recoverable
Fe3O4ZnO nanoparticles for enhanced photocatalytic degradation of phenol. Chem. Eng. J.
244, 327–334 (2014).
213. Saikia, L. et al. Photocatalytic performance of ZnO nanomaterials for self sensitized
degradation of malachite green dye under solar light. Appl. Catal. Gen. 490, 42–49 (2015).
214. Samad, A., Furukawa, M., Katsumata, H., Suzuki, T. & Kaneco, S. Photocatalytic oxidation
and simultaneous removal of arsenite with CuO/ZnO photocatalyst. J. Photochem.
Photobiol. Chem. 325, 97–103 (2016).
215. Khataee, A. et al. Synthesis and Characterization of Dysprosium-Doped ZnO Nanoparticles
for Photocatalysis of a Textile Dye under Visible Light Irradiation. Ind. Eng. Chem. Res.
53, 1924–1932 (2014).
216. Yu, J. et al. Photogenerated electron reservoir in hetero-p–n CuO–ZnO nanocomposite
device for visible-light-driven photocatalytic reduction of aqueous Cr( vi ). J. Mater. Chem.
A 3, 1199–1207 (2015).
217. Golzad-Nonakaran, B. & Habibi-Yangjeh, A. Photosensitization of ZnO with Ag3VO4 and
AgI nanoparticles: Novel ternary visible-light-driven photocatalysts with highly enhanced
activity. Adv. Powder Technol. 4, 1427–1437 (2016).
218. Shekofteh-Gohari, M. & Habibi-Yangjeh, A. Facile preparation of Fe3O4@AgBr–ZnO
nanocomposites as novel magnetically separable visible-light-driven photocatalysts. Ceram.
Int. 41, 1467–1476 (2015).
219. Moradi, S., Aberoomand-Azar, P., Raeis-Farshid, S., Abedini-Khorrami, S. & Givianrad,
M. H. The effect of different molar ratios of ZnO on characterization and photocatalytic
activity of TiO2/ZnO nanocomposite. J. Saudi Chem. Soc. 20, 373–378 (2016).
220. Raza, W., Haque, M. M. & Muneer, M. Synthesis of visible light driven ZnO:
Characterization and photocatalytic performance. Appl. Surf. Sci. 322, 215–224 (2014).
221. Zhu, H.-Y. et al. Novel multifunctional NiFe2O4/ZnO hybrids for dye removal by
adsorption, photocatalysis and magnetic separation. Appl. Surf. Sci. 369, 1–10 (2016).
222. Shekofteh-Gohari, M. & Habibi-Yangjeh, A. Novel magnetically separable ZnO/AgBr/Fe 3
O 4 /Ag 3 VO 4 nanocomposites with tandem n–n heterojunctions as highly efficient
visible-light-driven photocatalysts. RSC Adv. 6, 2402–2413 (2016).
223. Shirzad-Siboni, M., Farrokhi, M., Darvishi Cheshmeh Soltani, R., Khataee, A. & Tajassosi,
S. Photocatalytic Reduction of Hexavalent Chromium over ZnO Nanorods Immobilized on
Kaolin. Ind. Eng. Chem. Res. 53, 1079–1087 (2014).

68

224. Liu, Y. et al. Enhanced visible-light photocatalytic activity of Z-scheme graphitic carbon
nitride/oxygen vacancy-rich zinc oxide hybrid photocatalysts. Chin. J. Catal. 36, 2135–
2144 (2015).
225. Dai, K. et al. Graphene oxide modified ZnO nanorods hybrid with high reusable
photocatalytic activity under UV-LED irradiation. Mater. Chem. Phys. 143, 1410–1416
(2014).
226. Chen, Y., Zhao, H., Liu, B. & Yang, H. Charge separation between wurtzite ZnO polar
{001} surfaces and their enhanced photocatalytic activity. Appl. Catal. B Environ. 163,
189–197 (2015).
227. Nezamzadeh-Ejhieh, A. & Khorsandi, S. Photocatalytic degradation of 4-nitrophenol with
ZnO supported nano-clinoptilolite zeolite. J. Ind. Eng. Chem. 20, 937–946 (2014).
228. Shu, H.-Y., Chang, M.-C. & Tseng, T.-H. Solar and Visible Light Illumination on
Immobilized Nano Zinc Oxide for the Degradation and Mineralization of Orange G in
Wastewater. Catalysts 7, 164 (2017).
229. Zhao, P., Qin, N., Wen, J. Z. & Ren, C. L. Photocatalytic performances of ZnO nanoparticle
film and vertically aligned nanorods in chamber-based microfluidic reactors: Reaction
kinetics and flow effects. Appl. Catal. B Environ. 209, 468–475 (2017).
230. Deveci, E. Ü., Dizge, N., Yatmaz, H. C. & Aytepe, Y. Integrated process of fungal
membrane bioreactor and photocatalytic membrane reactor for the treatment of industrial
textile wastewater. Biochem. Eng. J. 105, 420–427 (2016).
231. Chen, Y. L. et al. ZnO nanorod optical disk photocatalytic reactor for photodegradation of
methyl orange. Opt. Express 21, 7240–7249 (2013).
232. Brooms, T. J., Onyango, M. S. & Ochieng, A. Photodegradation of phenol using TiO2, ZnO
and TiO2/ZnO catalysts in an annular reactor. J. Water Chem. Technol. 39, 155–160
(2017).
233. Ä°kizler, B. & Peker, S. M. Synthesis of TiO2 coated ZnO nanorod arrays and their stability
in photocatalytic flow reactors. Thin Solid Films 605, 232–242 (2016).
234. Hsu, M.-H. & Chang, C.-J. Ag-doped ZnO nanorods coated metal wire meshes as
hierarchical photocatalysts with high visible-light driven photoactivity and photostability. J.
Hazard. Mater. 278, 444–453 (2014).
235. Sudrajat, H. & Babel, S. A new, cost-effective solar photoactive system N-ZnO@polyester
fabric for degradation of recalcitrant compound in a continuous flow reactor. Mater. Res.
Bull. 83, 369–378 (2016).
236. Ghaly, M. Y. et al. ZnO/spiral-shaped glass for solar photocatalytic oxidation of Reactive
Red 120. Arab. J. Chem. 10, S3501–S3507 (2017).

69

237. Amiri, H., Ayati, B. & Ganjidoust, H. Textile Dye Removal Using Photocatalytic Cascade
Disk Reactor Coated by ZnO Nanoparticles. J. Mater. Sci. Chem. Eng. 04, 29 (2016).
238. Maleki, A. et al. Photocatalytic degradation of humic substances in the presence of ZnO
nanoparticles immobilized on glass plates under ultraviolet irradiation. Sep. Sci. Technol.
51, 2484–2489 (2016).
239. Hairom, N. H. H., Mohammad, A. W. & Kadhum, A. A. H. Effect of various zinc oxide
nanoparticles in membrane photocatalytic reactor for Congo red dye treatment. Sep. Purif.
Technol. 137, 74–81 (2014).
240. RĂ­os-Valdovinos, E. et al. Theoretical and experimental study of the photocatalytic activity
of ZnO coated tubular reactor. Mater. Sci. Eng. B 179, 41–47 (2014).
241. Mohapatra, D. P. et al. Photocatalytic degradation of carbamazepine in wastewater by using
a new class of whey-stabilized nanocrystalline TiO2 and ZnO. Sci. Total Environ. 485,
263–269 (2014).
242. Adolfson, J. & Berghage, T. Perception and performance under water. (Wiley, 1974).
243. Tu, W., Lin, Y.-P. & Bai, R. Removal of phenol in aqueous solutions by novel buoyant
Composite photocatalysts and the kinetics. Sep. Purif. Technol. 115, 180–189 (2013).
244. Singh, S., Singh, P. K. & Mahalingam, H. Novel Floating Ag+-Doped TiO2/Polystyrene
Photocatalysts for the Treatment of Dye Wastewater. Ind. Eng. Chem. Res. 53, 16332–
16340 (2014).
245. Ni, L. et al. Novel floating photocatalysts based on polyurethane composite foams modified
with silver/titanium dioxide/graphene ternary nanoparticles for the visible-light-mediated
remediation of diesel-polluted surface water. J. Appl. Polym. Sci. 133, n/a-n/a (2016).
246. Wang, X. et al. Insight into visible light-driven photocatalytic degradation of diesel oil by
doped TiO2-PS floating composites. Environ. Sci. Pollut. Res. 23, 18145–18153 (2016).
247. Sboui, M., Nsib, M. F., Rayes, A., Swaminathan, M. & Houas, A. TiO2–PANI/Cork
composite: A new floating photocatalyst for the treatment of organic pollutants under
sunlight irradiation. J. Environ. Sci. (2017). doi:10.1016/j.jes.2016.11.024
248. Wang, X. et al. Synthesis of expanded graphite C/C composites (EGC) based Ni-N-TiO2
floating photocatalysts for in situ adsorption synergistic photocatalytic degradation of diesel
oil. J. Photochem. Photobiol. Chem. 347, 105–115 (2017).
249. Suave, J., Amorim, S. M. & Moreira, R. F. P. M. TiO2-graphene nanocomposite supported
on floating autoclaved cellular concrete for photocatalytic removal of organic compounds.
J. Environ. Chem. Eng. 5, 3215–3223 (2017).

70

250. Lu, Z. et al. Performance of a novel TiO2 photocatalyst based on the magnetic floating flyash cenospheres for the purpose of treating waste by waste. Chem. Eng. J. 225, 34–42
(2013).
251. AltÄąn, Ä°. & SĂśkmen, M. Buoyant Photocatalyst Based on ZnO Immobilized on Polystyrene
Beads for Pollutants Treatment. CLEAN – Soil Air Water 43, 1025–1030 (2015).
252. Abdullah, H., Kuo, D.-H., Kuo, Y.-R., Yu, F.-A. & Cheng, K.-B. Facile Synthesis and
Recyclability of Thin Nylon Film-Supported n-Type ZnO/p-Type Ag2O Nano Composite
for Visible Light Photocatalytic Degradation of Organic Dye. J. Phys. Chem. C 120, 7144–
7154 (2016).
253. Abdel-wahab, M. S., Jilani, A., Yahia, I. S. & Al-Ghamdi, A. A. Enhanced the
photocatalytic activity of Ni-doped ZnO thin films: Morphological, optical and XPS
analysis. Superlattices Microstruct. 94, 108–118 (2016).
254. Wu, B. et al. Fabrication of stable Ir-ZnO/Zn superhydrophobic surface on zinc substrate
for its properties and application. J. Alloys Compd. 699, 489–497 (2017).
255. Dagar, A. & Narula, A. K. Effect of nitrogen-doping on photo-catalytic activity of
polypyrrole/zinc oxide/flyash cenosphere (PPY/ZnO/FAC) composite under visible light. J.
Mater. Sci. Mater. Electron. 28, 8643–8654 (2017).
256. Dagar, A. & Narula, A. K. Effect of ternary PEDOT/ZnO/Flyash-cenosphere photocatalyst
on photo-degradation of methyl orange under visible light. J. Mater. Sci. Mater. Electron.
27, 12777–12785 (2016).
257. Tafreshi, N., Sharifnia, S. & Moradi Dehaghi, S. Box–Behnken experimental design for
optimization of ammonia photocatalytic degradation by ZnO/Oak charcoal composite.
Process Saf. Environ. Prot. 106, 203–210 (2017).
258. Wang, H. & Pumera, M. Fabrication of Micro/Nanoscale Motors. Chem. Rev. 115, 8704–
8735 (2015).
259. Safdar, M., Simmchen, J. & Jänis, J. Light-driven micro- and nanomotors for environmental
remediation. Environ. Sci. Nano 4, 1602–1616 (2017).
260. Xu, L., Mou, F., Gong, H., Luo, M. & Guan, J. Light-driven micro/nanomotors: from
fundamentals to applications. Chem. Soc. Rev. (2017). doi:10.1039/C7CS00516D
261. Eskandarloo, H., Kierulf, A. & Abbaspourrad, A. Nano- and micromotors for cleaning
polluted waters: focused review on pollutant removal mechanisms. Nanoscale 9, 13850–
13863 (2017).
262. Mushtaq, F. et al. Magnetically driven Bi 2 O 3 /BiOCl-based hybrid microrobots for
photocatalytic water remediation. J. Mater. Chem. A 3, 23670–23676 (2015).

71

263. Zhang, Q. et al. Light-Driven Au-WO3@C Janus Micromotors for Rapid Photodegradation
of Dye Pollutants. ACS Appl. Mater. Interfaces 9, 4674–4683 (2017).
264. Zhang, Z., Zhao, A., Wang, F., Ren, J. & Qu, X. Design of a plasmonic micromotor for
enhanced photo-remediation of polluted anaerobic stagnant waters. Chem Commun 52,
5550–5553 (2016).
265. Srivastava, S. K., Guix, M. & Schmidt, O. G. Wastewater Mediated Activation of
Micromotors for Efficient Water Cleaning. Nano Lett. (2015).
doi:10.1021/acs.nanolett.5b05032
266. Wu, Y., Dong, R., Zhang, Q. & Ren, B. Dye-Enhanced Self-Electrophoretic Propulsion of
Light-Driven TiO2–Au Janus Micromotors. Nano-Micro Lett. 9, 30 (2017).
267. Li, J. et al. Water-Driven Micromotors for Rapid Photocatalytic Degradation of Biological
and Chemical Warfare Agents. ACS Nano 8, 11118–11125 (2014).
268. Mou, F. et al. Light-controlled propulsion, aggregation and separation of water-fuelled TiO
2 /Pt Janus submicromotors and their ‘on-the-fly’ photocatalytic activities. Nanoscale 8,
4976–4983 (2016).
269. Wang, H., Gu, X. & Wang, C. Self-Propelling Hydrogel/Emulsion-Hydrogel Soft Motors
for Water Purification. ACS Appl. Mater. Interfaces 8, 9413–9422 (2016).
270. Dong, R. et al. ZnO-based microrockets with light-enhanced propulsion. Nanoscale (2017).
doi:10.1039/C7NR05168A
271. Chen, C. et al. Transient Micromotors That Disappear When No Longer Needed. ACS Nano
10, 10389–10396 (2016).
272. Zhang, Y. & Yeom, J. Pollutant-degrading multifunctional micromotors. in 2017 IEEE 12th
International Conference on Nano/Micro Engineered and Molecular Systems (NEMS) 16–
20 (2017). doi:10.1109/NEMS.2017.8016964
273. Srikanth, B. et al. Recent advancements in supporting materials for immobilised
photocatalytic applications in waste water treatment. J. Environ. Manage. 200, 60–78
(2017).
274. Ong, C. B., Ng, L. Y. & Mohammad, A. W. A review of ZnO nanoparticles as solar
photocatalysts: Synthesis, mechanisms and applications. Renew. Sustain. Energy Rev. 81,
536–551 (2018).

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CHAPTER 3
SYNTHESIS AND APPLICATION OF SINGLE 1D SEMICONDUCTOR
NANOSTRUCTURES
3.1 Synthesis and application of 1D TiO2 nanostructures
3.1.1 Hydrothermal synthesis of TiO2 nanowires
The TiO2 NWs in anatase-phase and rutile-phase have been demonstrated possessing
remarkable performance including photocatalysis [1–4], water-splitting [5–8], solar cells [9–11], and
lithium-ion batteries [12–14]. As an efficient synthesis way, the hydrothermal method is usually
conducted in a sealed chamber with high pressure and temperature. It is known that the
morphology and quality of TiO2 NWs are subject to temperature, reaction duration and solute
concentration [15]. For the TiO2 NWs of a large aspect ratio, an optimized condition is required.
Figure 3.1 illustrated the correlation of factors we obtained from the TiO2 NWs growth via
hydrothermal method. It is shown that the dimension of the as-prepared TiO2 NWs is more related
to the concentration of the solute, more specifically, the molarity of NaOH. In out experiment the
thicker NWs was obtained under the high concentrated NaOH solution. On the other hand, the
NWs in large length could be obtained after longer reaction time.

Figure 3.1. The correlation parameter towards TiO2 NW growth, (a) wire dimension as a function
of molarity of the solute, (b) wire length as a function of growth time. Scale bar: 1 Âľm.

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The as-prepared TiO2 NWs in high aspect ratio is shown in Figure 3.2a. All reagents used
in this experiment were commercially available without further purification. The detail process
was as follows: 0.6 g TiO2 nanopowder (32 nm, Dow Co.) was dispersed in 50 mL 10 M NaOH
solution, which was prepared by NaOH particles (Fisher Scientific) and deionized (DI) water.
After stirred for 5mins at room temperature, the mixed solution was sonicated for another 10 mins
for the further uniform distribution. Then the mixed solution moved to a 200 mL Teflon-lined
autoclave was heated 30 hrs at 180 °C, and centrifuged for the separation of the sediments at 10000
rpm with multiple times after it cooled down at room temperature. Subsequently, the sediments
were rinsed with DI water thoroughly and dried with N2. Figure 3.2b exhibits the XRD pattern of
the as-prepared TiO2 NWs, where the anatase and rutile characteristic peaks of TiO2 could be
identified and marked.

Figure 3.2. (a) A SEM image of the TiO2 NWs via the hydrothermal method. (b) XRD pattern of
the as-prepared TiO2 NWs. Scale bar: 1 Îźm.

As reported, the anatase-TiO2 is a high efficient catalyst in comparison with other
crystalline phases. In order to investigate the performance of the as-prepared TiO2 NWs, the
photocatalysis was carried out in the aqueous solution with Methyl Blue (MB) as the model
pollutant (Figure 3.3a). 10 mg TiO2 NWs was uniformly dispersed in 40 mL of a 10 mM MB
aqueous solution, which was initially kept in dark for 30 mins to reach an equilibrium status. After

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starting the UV light irradiation, a certain volume of the solution was extracted at a constant
interval, which then centrifuged at 10000 rpm for the catalyst removal. The purified solution were
characterized via a UV-Vis spectrometer (see Figure 3.3b). The normalized optical absorbance
recorded at 667 nm from the decontaminated solution and the stock solution as a control was shown
as a function of degradation time. It is seen that the MB dye was decomposed rapidly in the
presence of TiO2 NWs. It is worth noting that the agitated stirrer was employed during the reaction
to improve the mass transfer, which also prevented the sedimentation of the TiO2 NWs.

Figure 3.3. Photocatalytic degradation of MB dye at room temperature. (a) the UV-Visible
absorbance spectra of a MB dye with dispersed TiO2 NWs. (b) normalized MB dye concentration
measured at 665nm from (a) with and without TiO2 NWs as a function of time.

As discussed in Chapter 2, the fixed NWs may be a proper catalyst due to the obvious
advantages in recyclability. In this regard, it is essential to introduce a method to grow TiO2 NWs
on the substrate. So far there are many methods against the direct growth of rutile-TiO2 NWs on
the glass and anatase-TiO2 on the Ti film [1,3,15], whereas only a small number of groups reported
the appropriate method for the anatase-TiO2 NW growth on the glass or flexible substrate [16]. In
this chapter a novel fabrication process was developed regarding the TiO2 NWs growth. Generally,
a colloidal solution combined with the TiO2-coated film was used for the direct growth of the NWs
on the substrate [16–18], which rendered the uniform NWs with the superior electrical property.
Large-scale production remains an issue, however. Here we scaled up this technique by replacing
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the TiO2 film by the anatase-TiO2 nanopowders and successfully obtained the high quality TiO2
NWs on both glass and polymer substrates.

Figure 3.4. The schematic of the fabrication of the TiO2 NWs on the glass substrate.

As depicted in Figure 3.4, the TiO2 nanoparticles dispersed in ethanol were initially sprayed
on the substrate. A uniformly thin TiO2 layer was formed after the few-hour annealing at the
temperature varying from 150°C to 300°C (which is the substrate temperature). The density of the
TiO2 layer was subject to the concentration of the TiO2 nanoparticles while the adhesion force
between TiO2 and the substrate was mainly derived from the annealing. Before transferred to a
Teflon-lined autoclave for the NWs growth, a solution consisting of 0.002 mol K2TiO(C2O4)2, 30
mL diethylene glycol (DEG) and 10 mL DI water was sonicated for 30 mins. Subsequently the
TiO2-coated substrate was entirely immersed into the solution and followed by the consistent
heating at 180 °C for 6 hrs. When the solution was cooled down to the room temperature, the asprepared sample was rinsed by DI water and dried at 150°C for 1 hour in ambient air.
Consequently, the TiO2 NWs were grown on the substrate.

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Figure 3.5. SEM images of the TiO2 NWs via the hydrothermal method grown. (a) on a glass
substrate, (b) on a polyimide film. Scale bar: (a) 100 Îźm on a large view and 10 Îźm in the inset,
(b) 20 Îźm on alarge view and 5 Îźm in the inset.
The TiO2 NWs on the two different substrates are displayed in Figure 3.5. For the glass
substrate (Figure 3.5a), TiO2 NWs have the uniform distribution, where the random cracks can be
attributed to the volume expansion of the NWs during the growth. Compared to the free NWs
(Figure 3.2a), the fixed NWs on the glass have a smaller aspect ratio due to the different growth
mechanism and the mass-transport limitation, which requires the further optimization. Figure 3.5b
reveals TiO2 in hierarchical structures on a polyimide (PI) film. The hierarchical structures are
composed of hemispherical structures of approximately ~2 Îźm in diameter and the numerous short
NWs spiking out from the dome surface, which is larger than the size of the initial TiO2 powder
(~ 32 nm in diameter). One possibility of this unique structure may ascribe to the agglomeration
of the powder during the reaction, preventing the inner nanoparticles from contacting the solution
and thus limiting the further growth. Besides, the numerous gaps and defects observed on the entire
substrate may be caused by the bending of the substrate and its weak adhesion to the nanoparticles,
leading the numbers of structures falling off the surface. The glass possesses the higher adhesion
force to the TiO2 NWs than that of the PI film, which is mainly due to the discrepancy of the
surface energy. A proper surface treatment against the polyimide film is necessary to increase the
surface energy glass.
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Figure 3.6. Photocatalytic degradation of TiO2 NWs grown on the glass substrate at room
temperature. (a) the UV-Visible absorbance spectra change of MB dye with dispersed TiO2 NWs.
(b) degradation of the MB dye with and without TiO2 NWs catalysts measured by the normalized
peak height of (a) at 665 nm as a function of time.
The photocatalytic degradation of the TiO2 NWs fixed on the glass substrate was
implemented at the room temperature (Figure 3.6). A 1 in. by 1 in. glass substrate was immersed
into 20 mL of a MB aqueous solution without stirring. Evidently, the MB dye was decomposed in
the presence of the TiO2 substrate. Compared to the suspended NWs, the degradation rate of the
TiO2 NWs fixed on the glass substrate is twice times slower for the first hour, Lower photocatalytic
performance of the immobilized catalysts can be attributed to the smaller contact area of the
catalyst to the pollutant and the severe mass transport limitation.

However the consistent

degradability of the immobilized NW catalyst implies its stability in a long-term usage, which is
important for developing reliable and highly efficient photocatalysts in the future work.

3.1.2 Electrochemical synthesis of TiO2 nanotubes
Aside from the TiO2 NWs, vertically-aligned TiO2 nanotubes (NTs) are another candidate
for immobilized photocatalysts. TiO2 NTs has been synthesized from the Ti foil or Ti-coated layer
and their high surface area render outstanding photocatalytic activity in the aqueous solution [ 19],
significant improvement in solar cells [20] and high performance in gas sensors [21]. In general,
those particular NT structures are grown differ from TiO2 NWs, which are achieved by top-down
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anisotropic etching rather than bottom-up oriented deposition. Most commonly, the
electrochemical anodization method is used to fabricate the tubular structures, where an electrolyte
concentration and an anodic voltage are the key parameters to affect its geometry.
Here we used the two-step anodization process at room temperature to produce high quality
TiO2 NT arrays. The electrolyte for the anodization was composed of 0.3 wt% NH4F, 99.7% wt%
ethylene glycol and 2 vol% H2O. After 15 mins vigorous stirring, the electrolyte was kept in dark
for an hour to achieve the equilibrium status. A piece of Ti foil cleaned by acetone, isopropanol
and DI water was immersed in the electrolyte serving as a working electrode while a Pt plate as
the counter electrode was placed in the same container around ~8 cm away from the Ti foil. The
Ti foil was firstly anodized for 30 mins at 20V and sonicated in 1M HCl to remove the produced
irregular TiO2 NTs, leaving the hexagonally arranged grooves on the Ti surface. The Ti foil was
subsequently anodized for another 20-30 hours with the anodic voltage varied from 5 V to 30 V.
After rinsed by DI water and dehydrated at 60°C, the vertically-aligned TiO2 NT array could be
obtained on the Ti foil surface.

Figure 3.7. SEM images of the TiO2 NTs via the electrochemical anodization method. (a) one-step
anodization, (b) two-step anodization. Scale bar: (a) 1 Îźm on the large view and 100 nm in the
inset, (b) 5 Îźm on the large view and 500 nm in the inset.
As displayed in Figure 3.7, the irregular TiO2 NTs are observed from the one-step
anodization, which may be caused by the defects on the original surface, hindering or accelerating
79

the local reaction. When this layer was removed, a uniform surface was exposed for the second
anodization, leading to a more regular TiO2 NT array. Furthermore, it is clearly seen that the NT
arrays in both cases are free of cracks, implying their superior mechanical stability. According to
our experiment, the NT length increases with a higher anodic voltage and/or a longer reaction
time, which has good agreement with the previous reports[17,22,23], although the increasing rate of
the TiO2 NTs varies in different electrolytes.

Figure 3.8. SEM images of the TiO2 NT array. (a) cross-sectional view, (b) bottom view. Scale
bar: 200 nm in (a) and 100 nm in (b).
On the basis of the anodization mechanism [17], the TiO2 NTs should exhibit a uniform
radical dimension along its tubular axis till ended with a round convex structure. This characteristic
TiO2 NT structure can be confirmed in Figure 2.8, where a small flake was scratched off from the
anodized substrate (anodized for 34 hrs at 20 V). As depicted in Figure 2.8a, the majority of NTs
stand parallel with the similar length, demonstrating that the NTs have high uniformity in
anodization. The convex structures of the NTs (Figure 2.8b) further indicate that the NTs possess
the similar lengths and dimensions. Moreover, the intact convex ends were obtained thanks to their
weaker adhesion to the substrate, which have thicker tube walls than that of the body [17].
Recently the free-standing TiO2 NT array films have raised more attention as a transparent
photochemical electrode [24]. Therefore significant efforts have been devoted to fabricate a
membrane with an array of through-hole TiO2 NTs. To our knowledge, there are three common
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ways associated with the electrochemical anodization method to fabricate the through-hole NT
array films: (i) thermally-induced self-detachment [25], (ii) electrically-induced in-situ separation
[26] and (iii) vapor-induced etching [19]. We evaluated these techniques and tried to find a reliable
and reproducible method to fabricate large-scale free-standing films with opened TiO2 NT array.
To account for our method, we first tabulated the key step of the three techniques (Table 3.1).
Table 3.1. A brief comparison of the three techniques with respect to the through-out free-standing
TiO2 NT array films.

Thermally-induced
self-detachment

Electrically-induced
in-situ separation

Vapor-induced
etching

First step
Anodization at 60 V
to remove irregular
layer

Second step
Anodization at 60 V
followed by the
thermal treatment

Third step
Anodization at 60 V
to obtain the freestanding film

Anodization at 60 V
to remove irregular
layer

Anodization at 60 V
followed by a shortterm larger anodic
voltage at the end

N/A

Anodization at 60 V

Dissolve the Ti
substrate to obtain the
free-standing semiopen tubular film

Apply HF vapor to
dissolve the closed
bottoms of the freestanding film

Apparently, the free-standing film of the TiO2 NT usually demands a higher anodic voltage
and a proper self-detaching method. In our work, we also noticed that an anodic voltage higher
than 60 V is required to obtain a desired free-standing NT film. In addition, two-step anodization
was employed again to acquire the ordered NT array on the substrate. The key step different from
the above mentioned techniques is the ultra-sonication in DI water after the secondary anodization,
forcing the NT array film to separate from the substrate. As shown in Figure 3.9, the NT film
restrains its free-standing form even after sonication. The opaque yellowish color stems from the
closed bottom of the film. Moreover, the natural bending occurring on the free-standing film can
be attributed to the expansion of the TiO2 NTs during the experiment. Herein, by using the vacuum
81

filtration and the PDMS-assisted transfer printing [27], the flakes of the TiO2 NT films could be
collected as well. These flakes of the TiO2 NTs will be applied to the photocatalytic degradation
test in the future works. The as-prepared TiO2 NT array films detached from the substrate will be
exposed to vaporized HF to open up the closed end for the electrode and nanofluidic applications.

Figure 3.9. Optical images of the as-prepared free-standing film of TiO2 NT array.

Figure 3.10. SEM images of the as-prepared free-standing TiO2 NT film. (a) a Ti substrate after
detachment, (b) the detached TiO2 NT film. Scale bar: (a) 10 Îźm on the large view and 1 Îźm in the
inset, (b) 10 Îźm on the large view and 2 Îźm in the inset.
The detached side of the TiO2 NT film has the convex nanostructures, which should create
the corresponding concave nanostructures on the remaining Ti substrate. In Figure 3.10a, the

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periodical concave nanostructure is observed as the NT film gets peeled off, and shows the smaller
dimension, which is in agreement with the growth mechanism [17]. The angled view in Figure
3.10b displays the uniform NT array fabricated after detached from the substrate, where the parallel
tubular structures with a large length is observed.

Figure 3.11. SEM images of the as-prepared free-standing TiO2 NT film and its corresponding.
Scale bar: 1Âľm.
The crystallinity of the free-standing TiO2 NT film is shown in Figure 3.11. It clearly
exhibits the anatase-TiO2 NTs staying on the Ti substrate. As discussed above, anatase-TiO2 is
considered as a suitable crystalline phase of the TiO2 for the photocatalysis. A lot of works have
been reported regarding the organic-dye degradation by use of free-standing TiO2 NT film [19,28–
30

]. On the basis of these progress, more studies in fabrication of free-standing TiO2 NT film in

larger area and characterization of its photocatalytic activity in various pollutants are of interest to
researchers. Besides, more investigations are also desired in the modification of free-standing TiO2
NT film using numerous doping elements in order to enhance the light absorption in visible spectra
region. It can be seen that free-standing TiO2 NT film is a promising candidate as photocatalysts.

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3.2 Synthesis and application of 1D ZnO nanostructures
3.2.1 Hydrothermal synthesis of ZnO NWs for photocatalysts
ZnO is a wide bandgap semiconductor, and various ZnO nanostructures have been applied
to a variety of devices, such as light emitting diodes [31–35], solar cells [36–39] and chemical sensors
[40–44]. Among many configurations of ZnO nanostructures, vertically-aligned NW array directly
integrated onto substrates has demonstrated remarkable enhancements in the device performance.
As of now, a number of techniques have been developed to synthesize 1D ZnO nanostructures
(e.g. NWs) and can be roughly classified in two primary categories: gas-phase vs. solution-phase
processes. Gas phase methods such as chemical vapor deposition (CVD), metal organic chemical
vapor deposition (MOCVD) and vapor-liquid-solid (VLS) process provide well-aligned,
polycrystalline ZnO NWs. However, the high-temperature nature of these processes limits the type
of substrates for the NW growth. In contrast, solution-based approaches such as the hydrothermal
method can be used to grow ZnO NWs at relatively low temperatures, but the quality of the NWs
is not as good as the ones grown at high temperatures. In this work, we are interested in growing
ZnO NWs on flexible plastic substrates, necessitating a low-temperature synthesis route. In order
to maximally utilize the beneficial properties of ZnO, we first need to develop a method of growing
ZnO NWs onto these substrates.
ZnO NWs grown on thermally-resistant substrates via gas-phase methods have been welldocumented [45–48], which highly ordered, polycrystalline NW arrays have been obtained. Here,
we mainly focus on the solution-phase hydrothermal method because it is more applicable to
substrates not compatible with high temperatures. Technically, the hydrothermal method can be
carried out when there are nanocrystal seeds existing on the surface and the corresponding ions in
the solution. Since either Zn or ZnO crystals can be the initial seeds, there are a number of ways

84

to synthesize ZnO NW array. In this work we will introduce two processes to grow ZnO NWs on
a Zn foil and glass substrate, respectively. The relevant applications will be discussed with the
integration of other fabrication techniques.
To obtain the direct-growth of the ZnO NWs from a bulk Zn substrate, a high purity Zn
foil (>99%) was firstly rinsed by acetone, isopropanol and DI water, consecutively. The solution
consisting of Zinc nitrates hexahydrate and ammonia hydroxide aqueous solution were prepared
and transferred to a Teflon-lined autoclave. The cleaned Zn foil was immersed into the solution
and followed by 12-hour heating at 95°C. The ZnO NWs were obtained and further analyzed after
rinsed by DI water and dried by N2.

Figure 3.12. (a) SEM image of the ZnO NWs on the Zn foil. (b) XRD pattern of the as-prepared
ZnO NWs. (Scale bar: 200 Îźm on the large view and 300 nm in the inset)
The hexagonal ZnO NWs of an average diameter about 80 nm are observed and uniformly
distributed on the entire Zn foil (Figure 3.12a). As reported previously [49], the aspect ratio of these
NWs can be adjusted through the solute concentration, while the NW length is primarily governed
by the reaction time. The marked peaks in the XRD pattern represents the characteristic of the
wurtzite-ZnO and the intrinsic Zn, with the highest intensity in (002) plane and denoting the
preferable orientation along c-axis (Figure 3.12b). Since the photocatalytic activity of these NWs
have been well studied before [49], we mainly explore its potential application in electrical devices.
One advantage of the proposed synthesis method is its reliable connection between ZnO NWs and
85

Zn foil (i.e. substrate) as NWs directly grown from the foil. This ensures a robust electrical contact
at semiconductor-to-metal interface. Here we desire to establish the top electrode onto NW bed to
build a diode from vertically-aligned ZnO NW array. The top electrode of the diode can be
established combining the blanket transfer (BT) technique [27] followed by isotropic oxygen
plasma etching and contact metal deposition. The details of this process and device performance
will be discussed in the following section.

Figure 3.13. The schematic of the fabrication sequence for the patternable ZnO NW arrays on the
Zn foil. A vertically-aligned ZnO NW array can be obtained as the PDMS cover is removed
afterwards.
In order to extend the compatibility of the direct-growth process of ZnO NWs, a modified
method has been developed for creating the predictable NWs patterns. The key strategy is
illustrated in Figure 3.13. First, liquid prepolymer (PDMA-ODA) of PI 2555 (Dow Co.) was spincoated on the surface of a cleaned thin Zn foil. After a 1.5-hour annealing at a constant temperature
of 150°C, the PI film was fully covered the Zn foil with conformal contact. Then the sample was
entirely embedded in the prepared PDMS solution (a ratio of 10 to 1 for prepolymer and curing
agent) and solidified together at 60°C for 12 hours. Subsequently, the middle portion of the PDMS
layer upon the Zn foil side was removed, exposing the Zn foil surface. The PDMS shell serves a
mask layer to selectively grow ZnO NWs and can be simply removed after the synthesis. Finally,

86

the ZnO NWs with the Zn foil were rinsed by DI water and dried for the further characterization.
The applied PI film on the back surface of the Zn foil served as another masking layer, hindering
the growth of the ZnO NWs as well as the flexible substrate (Figure 3.15).

Figure 3.14. (a) SEM images of the patterned ZnO NWs on the Zn foil, (b) boundary of the
patterned ZnO NWs. Scale bar: (a) 50 Îźm on the large view and 10 Îźm in the inset, (b) 20 Îźm on
the large view and 5 Îźm in the inset
The SEM characterization of the patterned ZnO NWs is illustrated in Figure 3.14.
Evidently, neither a PDMS masking layer nor a PI film affected the growth and quality of the
NWs. Uniformly-distributed, high aspect ratio, vertically-aligned ZnO NWs have been
synthesized on the Zn foil. A distinctive boundary can be observed in Figure 3.14c, indicating the
transition from the ZnO growing region to the PDMS masked region. The PDMS mask layer
effectively impeded the solution from seeping into the blocked regions of the foil. One future work
may be related to improving the resolution and the quality of the patterns so that this modified
method can be utilized to large-scale patterns of ZnO NWs.

Figure 3.15. Schematic diagrams of the fabrication for the differential growth of the ZnO NW
arrays on the Zn foil.
87

Figure 3.16. SEM images of the ZnO NWs grown on the ZnO seeding layer. (a-b) the coated
seeding layer was facing up during the reaction, (c-d) ) the coated seeding layer was facing down
during the reaction. Scale bar: (a) 50 Îźm on the large view and 5 Îźm in the inset, (b) 5 Îźm on the
large view and 300 nm in the inset, (c) 10 Îźm on the large view and 500 nm in the inset, (d) 10
Îźm on the large view and 3 Îźm in the inset.
Another capability in using the modified method is growing ZnO NWs in various lengths.
As illustrated in Figure 3.15, the hydrothermal method under the same condition was implemented
for the as-prepared sample (Figure 3.13d) after a silicone-free blue adhesive film (denoted as blue
film) was attached on the PI film. The blue tape worked as the mask layer to prevent the PI film
from being attacked by the solution. We found that a fully-cured PI film was slowly dissolved in
the solution, and the blue film was more resistant to the solution. The PI film is indispensable since
the blue film alone cannot create a conformal contact to the foil surface and act as a supporting
membrane to hold grown NWs. After a certain period of time, the various length of the NWs was
obtained upon the front surface of the Zn foil. This architecture of ZnO NWs on the PI membrane
can be applied to some flexible devices in the future.

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Despite the achievement of the direct-growth method, the alkaline solution needed for the
hydrothermal synthesis limits the type of the substrates. The substrates not compatible with the
alkaline solution entail other synthesis methods. Recently the in-situ growth technique based on a
ZnO seeding layer gains popularity in fabricating crystalline ZnO NWs because of the costeffectiveness and scalability [45–48,50–53].

Figure 3.17. The schematics of the fabrication sequence for the patternable ZnO NW arrays on
the glass substrate.

To synthesize ZnO NWs on the glass, a ZnO seeding layer should be prepared first. Herein,
the seed solution consisting of zinc acetate dehydrate (Sigma Aldrich) and ethylene alcohol (200
prove) was stirred for 2 hours at 60 °C. Prior to spin-coating on a glass plate which was
consecutively cleaned by acetone, isopropanol and DI water, the mixed seed solution was aged at
room temperature for 1 hour. The glass coated with the ZnO seeding layer was then annealed at
150°C for 2 hours and subsequently soaked in the growth solution composed of zinc nitrate
hexahydrate and hexamethylenetetramine (HMTA). The hydrothermal method entails constant
heating at 90°C for 6 hours in the oven. The as-prepared sample was rinsed by DI water and

89

dehydrated at 60°C in the ambient air afterwards. Note that the coated seeding layer on the glass
is suggested to be placed facing down, avoiding the sediments from settling on the ZnO NW array.

Figure 3.18. (a) A SEM image of the patterned ZnO NWs via the ZnO seeding layer, (b) XRD
pattern of the as-prepared ZnO NWs. Scale bar: 1 mm on the large view and 10 Îźm in the inset.

The effect of a glass placement during the NW synthesis is shown in Figure 3.16. There is
an obvious discrepancy in the morphology of the ZnO NWs. As seen in Figure 3.16(a-b), the NWs
disorganized NWs with various lengths/diameters and orientations are originated from the
sediments during the reaction when the growth substrate is placed facing up at the bottom of the
solution. On the other hand, the vertically-aligned NWs with a tighter dimensional control, as
shown in Figure 3.16(c-d), have been synthesized when the growth substrate is held facing down
during the reaction. These NW array are uniformly distributed across the whole glass substrates.
From the cross-sectional view in Figure 3.16b it clearly shows that the sediments are deposited on
top of the uniformly grown, vertically-aligned NW array. Compared to the NWs in Figure 3.16d,
the smaller aspect ratio of the NWs in Figure 3.16b can be found and attributed to the sediments,
covering the NW array and impeding its further growth. In case of the growth substrate facing
down, much fewer sediments are observed atop the NWs (see Figure 3.16d).
Similar to the ZnO NWs on a Zn foil, the NWs on a Si wafer substrate can also be patterned.
The patterns in the experiment were basically created through photolithography. The key strategy
of patterning NWs is displayed in Figure 3.17. A ZnO seeding layer was initially spin-coated on
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the cleaned glass substrate and annealed at 150°C for 1.5 hours. Afterwards, photolithography was
carried out on the seeding layer by spin-coating, baking, developing and post-baking a photoresist
(PR, S1813) layer, consecutively. The patterned glass substrate was then transferred to the growth
solution for synthesizing NWs under the same condition as previously described.
The SEM image (Figure 2.18a) demonstrates the NWs were successfully patterned on the
substrate. However, the obtained patterns were not as good as expected in term of the resolution
and quality. A possible cause is the dissolution of the PR into the growth solution, resulting in the
unknown side reaction. More work will be geared towards selecting a proper PR to reduce
undesirable reactions and developing a technique for large-scale patterning. Figure 3.18b shows
the XRD pattern of the ZnO NWs on the Si wafer via ZnO seeding layer. As discussed above, the
recognized peaks are assigned to the wurtzite-ZnO, where the peak with the highest intensity
implies the c-axis-preferable growth of the NWs.
With the increasing demands of the electrical devices, the NWs are expected to be
synthesized not only on rigid substrates but also on flexible substrates. Solution-base synthesis
techniques become more popular since the majority of the flexible substrate can only survive in
the mild growth solution at relatively low reaction temperatures. In the following work, a modified
hydrothermal method was employed to fabricate the NWs on the PDMS film, which has a few
important advantages in term of the adhesion and flexibility.

3.2.2 Hydrothermal synthesis of ZnO NWs for electronic devices
ZnO is one of the most attractive metal oxide semiconductors due to its remarkable
properties in electromechanical, optoelectronic and electrochemical, which have been widely
applied to various applications such as LED, solar cell, chemical sensor and flexible and

91

transparent electronic devices [48]. A number of processes have been reported for the synthesis of
the one dimensional ZnO nanostructures, including chemical vapor deposition (CVD),
hydrothermal synthesis, electrochemical growth and vapor-liquid-solid (VLS) process. Among
them the hydrothermal method allows us to grow vertically-aligned ZnO nanowire (NW) array in
multiple crystalline orientations at the low temperature [54]. To fully utilize the inherent ZnO NW
structures, it is our interest to construct large parallel arrays of ZnO NW devices for various
electronic and optoelectronic applications by establishing a porous top electrode. Compared to the
conventional NW device architecture (i.e., individual NWs lying on the substrate between two
electrodes), the vertically aligned NW device could mitigate device unreliability due to variability
in NW dimensions and minimize size-dependent noise sources [55]. One fabrication approach to
1D ZnO NW array device relies on transferring the ZnO NW array to a particular substrate which
may lose certain amount of nanowires as well as the original array uniformity [ 56]. It is still a
challenge to efficiently use the ZnO NW arrays on its predetermined substrate.
In this work we develop a novel method for fabricating a gas sensor device directly on asprepared ZnO NW arrays. This approach is compatible with various nanowire arrays grown on
their original substrates. The ZnO NW arrays in high aspect ratio is synthesized on the zinc
substrate. A robust blanket transfer technique is employed to transfer a thin layer of photoresist
onto the ZnO NW tips. A uniform porous electrode is fabricated onto the tips of ZnO NW arrays.
The ZnO NW arrays with the porous electrode can be utilized as a gas sensor substrate for further
tests.
A scheme of the fabrication of ZnO NW array with the porous electrode is shown in Figure
3.19. Firstly a cleaned zinc foil was soaked in the aqueous solution with zinc nitrite and ammonium
hydroxide. After 10 hours heating at 100°C in a Teflon-lined autoclave, the foil was rinsed with

92

DI water and blow dry. A PDMS stamp, spincoated with a thin layer of photoresist, was brought
into an intimate contact to the foil surface at 120°C. The photoresist layer was transferred onto the
ZnO NW array by slowly peeling off the PDMS stamp, consequently [27]. Oxygen plasma etching
was then used to partially expose the tips of the ZnO NWs. A thin Aluminum film was
subsequently angle-deposited to create the top porous electrode. Afterwards, a secondary oxygen
plasma was applied to remove the rest photoresist underneath the Al layer. The final device only
consist of porous electrode, ZnO NW array and Zn substrate.

Figure 3.19. The scheme of the fabrication for the ZnO NW arrays sensor. Highly ordered ZnO
NWs were grown on the zinc substrate vertically.
The as-prepared vertical-aligned ZnO NW array in high aspect ratio is shown in Figure
3.20a. The gap between neighboring ZnO nanowires is equally, ensuring the even transferring of
the photoresist layer. The continuous ZnO NW array also prevent the direct contact between
photoresist and bare Zn substrate. By using focused ion beam (FIB) a pattern was created on the
substrate (Figure 3.20(c-d)), where a 3 Îźm thick oxidized ZnO NW layer could be identified lying
on the substrate, homogenously. Apparently a number of ZnO NW arrays stack on the Zn substrate,
whose crystallinity gradually increase along with the depth of substrate. A finite annealing could
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change the crystalline but not the morphology of the substrate, which means the ZnO layer would
permanently separate the contact of the Zn substrate and the electrode.

Figure 3.20. SEM of the ZnO NW array on the zinc substrate (a) after 10 hours hydrothermal
treatment. ZnO NWs in high aspect ratio were distributed uniformly on the substrate, (b) after Al
electrode fabricated. Porous is observed along each ZnO nanowires, which size is corresponding
to the diameter of the nanowires. Angled view of the ZnO NW array after FIB patterning. ZnO
NWs at the boundary was partially cleaved by FIB in (a). Multiple ZnO NW network layers are
observed in (b). Scale bar: (a-b) 10 Âľm in the large view and 3Âľm in the inset, (c-d) 1 Âľm.

Due to the non-flat topography of the substrate, after the transferred photoresist layer
uniformly infiltrate and encapsulate the tip of ZnO NWs, the after-etching nanowire density on the
surface is lower (Figure 3.20b). The porosity and porous size of the top electrode film could be
simply managed by etching the exposed tip height and the deposition angle. In this experiment the
porous size of the electrode is fabricated around 300 nm in diameter and the corresponding porosity
is about 10 %-12 %. The Al layer in 80 nm is compatible with this design without any collapse.
With the top electrode the electrical property of the device is obtained, a smooth curve obtained in
Figure 3.21 proves the robust contact between the electrode and the ZnO NWs, demonstrating the
reliability of this fabrication process. This structure is proposed as a potential gas sensor, whose
94

high porosity allows sufficient gas flowing in and contacting to the ZnO NW array, resulting a
sensitive current-voltage change.

Figure 3.21. I-V curve of as-prepared ZnO NW array, which behavior is corresponding to the
typical diode.

95

REFERENCES

96

REFERENCES

1. Liu, C. et al. Hydrothermal synthesis of N-doped TiO2 nanowires and N-doped graphene
heterostructures with enhanced photocatalytic properties. J. Alloys Compd. 656, 24–32
(2016).
2. Arcadipane, E. et al. Single-crystal TiO 2 nanowires by seed assisted thermal oxidation of Ti
foil: synthesis and photocatalytic properties. RSC Adv 6, 55490–55498 (2016).
3. Qin, D.-D. et al. Hydrothermal Growth and Photoelectrochemistry of Highly Oriented,
Crystalline Anatase TiO2 Nanorods on Transparent Conducting Electrodes. Chem. Mater.
27, 4180–4183 (2015).
4. Zhang, Y., Han, C., Nadagouda, M. N. & Dionysiou, D. D. The fabrication of innovative
single crystal N,F-codoped titanium dioxide nanowires with enhanced photocatalytic activity
for degradation of atrazine. Appl. Catal. B Environ. 168–169, 550–558 (2015).
5. Naldoni, A. et al. Enhancing Hot Carrier Collection for Solar Water Splitting with Plasmonic
Titanium Nitride. ArXiv160706595 Cond-Mat (2016).
6. Liu, Q. et al. Partial Ion Exchange Derived 2D Cu–Zn–In–S Nanosheets as Sensitizers of 1D
TiO2 Nanorods for Boosting Solar Water Splitting. ACS Appl. Mater. Interfaces 8, 26235–
26243 (2016).
7. Qin, D.-D. et al. PH3-Treated TiO2 Nanorods with Dual-Doping Effect for
Photoelectrochemical Oxidation of Water. J. Phys. Chem. C 120, 22195–22201 (2016).
8. Yang, W. et al. Ferroelectric Polarization-Enhanced Photoelectrochemical Water Splitting in
TiO2–BaTiO3 Core–Shell Nanowire Photoanodes. Nano Lett. 15, 7574–7580 (2015).
9. Li, H. et al. Ultralong Rutile TiO2 Nanowire Arrays for Highly Efficient Dye-Sensitized
Solar Cells. ACS Appl. Mater. Interfaces 8, 13384–13391 (2016).
10. Li, X. et al. Efficient Perovskite Solar Cells Depending on TiO2 Nanorod Arrays. ACS Appl.
Mater. Interfaces 8, 21358–21365 (2016).
11. Lee, K.-W., Kim, M., Kim, J.-M., Kim, J. J. & Lee, I.-H. Enhanced photovoltaic
performance of back-illuminated dye-sensitized solar cell based on TiO2
nanoparticle/nanowire composite film in cobalt redox system. J. Alloys Compd. 656, 568–
572 (2016).
12. Goriparti, S. et al. Direct Synthesis of Carbon-Doped TiO2–Bronze Nanowires as Anode
Materials for High Performance Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 7,
25139–25146 (2015).

97

13. Kim, S. J., Kargar, A., Wang, D., Graham, G. W. & Pan, X. Lithiation of Rutile TiO2Coated Si NWs Observed by in Situ TEM. Chem. Mater. 27, 6929–6933 (2015).
14. Lu, X., Li, G. & Tong, Y. A review of negative electrode materials for electrochemical
supercapacitors. Sci. China Technol. Sci. 58, 1799–1808 (2015).
15. Wang, X., Li, Z., Shi, J. & Yu, Y. One-Dimensional Titanium Dioxide Nanomaterials:
Nanowires, Nanorods, and Nanobelts. Chem. Rev. 114, 9346–9384 (2014).
16. Wu, W.-Q. et al. Hydrothermal Fabrication of Hierarchically Anatase TiO2 Nanowire arrays
on FTO Glass for Dye-sensitized Solar Cells. Sci. Rep. 3, (2013).
17. Galstyan, V., Comini, E., Faglia, G. & Sberveglieri, G. TiO2 Nanotubes: Recent Advances
in Synthesis and Gas Sensing Properties. Sensors 13, 14813–14838 (2013).
18. Maheswari, D. & Sreenivasan, D. Review of TiO2 nanowires in dye sensitized solar cell.
Appl. Sol. Energy 51, 112–116 (2015).
19. Albu, S. P., Ghicov, A., Macak, J. M., Hahn, R. & Schmuki, P. Self-Organized, FreeStanding TiO2 Nanotube Membrane for Flow-through Photocatalytic Applications. Nano
Lett. 7, 1286–1289 (2007).
20. Lin, J., Chen, J. & Chen, X. High-efficiency dye-sensitized solar cells based on robust and
both-end-open TiO2 nanotube membranes. Nanoscale Res. Lett. 6, 1–5 (2011).
21. Albu, S. P., Ghicov, A., Macak, J. M., Hahn, R. & Schmuki, P. Self-Organized, FreeStanding TiO2 Nanotube Membrane for Flow-through Photocatalytic Applications. Nano
Lett. 7, 1286–1289 (2007).
22. MacĂĄk, J. M., Tsuchiya, H. & Schmuki, P. High-Aspect-Ratio TiO2 Nanotubes by
Anodization of Titanium. Angew. Chem. Int. Ed. 44, 2100–2102 (2005).
23. Macak, J. M., Aldabergerova, S., Ghicov, A. & Schmuki, P. Smooth anodic TiO2 nanotubes:
annealing and structure. Phys. Status Solidi A 203, R67–R69 (2006).
24. Berger, S., Ghicov, A., Nah, Y.-C. & Schmuki, P. Transparent TiO2 Nanotube Electrodes
via Thin Layer Anodization: Fabrication and Use in Electrochromic Devices. Langmuir 25,
4841–4844 (2009).
25. Lin, J., Chen, J. & Chen, X. Facile fabrication of free-standing TiO2 nanotube membranes
with both ends open via self-detaching anodization. Electrochem. Commun. 12, 1062–1065
(2010).
26. Wang, D. & Liu, L. Continuous Fabrication of Free-Standing TiO2 Nanotube Array
Membranes with Controllable Morphology for Depositing Interdigitated Heterojunctions.
Chem. Mater. 22, 6656–6664 (2010).

98

27. Zhang, Y., Han, J.-H., Zhu, L., Shannon, M. A. & Yeom, J. Soft lithographic printing and
transfer of photosensitive polymers: facile fabrication of free-standing structures and
patterning fragile and unconventional substrates. J. Micromechanics Microengineering 24,
115019 (2014).
28. Macak, J. M., Zlamal, M., Krysa, J. & Schmuki, P. Self-Organized TiO2 Nanotube Layers as
Highly Efficient Photocatalysts. Small 3, 300–304 (2007).
29. Grimes, C. A. & Mor, G. K. TiO2 Nanotube Arrays: Synthesis, Properties, and Applications.
(Springer Science & Business Media, 2009).
30. Roy, P., Berger, S. & Schmuki, P. TiO2 Nanotubes: Synthesis and Applications. Angew.
Chem. Int. Ed. 50, 2904–2939 (2011).
31. Ren, X. et al. White Light-Emitting Diode From Sb-Doped p-ZnO Nanowire Arrays/n-GaN
Film. Adv. Funct. Mater. 25, 2182–2188 (2015).
32. Shi, Z.-F. et al. Epitaxial growth of vertically aligned ZnO nanowires for bidirectional directcurrent driven light-emitting diodes applications. CrystEngComm 17, 40–49 (2015).
33. Gao, F. et al. Ultraviolet electroluminescence from Au-ZnO nanowire Schottky type lightemitting diodes. Appl. Phys. Lett. 108, 261103 (2016).
34. Wang, C. et al. Enhanced emission intensity of vertical aligned flexible ZnO nanowire/ppolymer hybridized LED array by piezo-phototronic effect. Nano Energy 14, 364–371
(2015).
35. PauportĂŠ, T. et al. Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT
Study, and Application to Light-Emitting Diode. ACS Appl. Mater. Interfaces 7, 11871–
11880 (2015).
36. Dymshits, A., Iagher, L. & Etgar, L. Parameters Influencing the Growth of ZnO Nanowires
as Efficient Low Temperature Flexible Perovskite-Based Solar Cells. Materials 9, 60 (2016).
37. Lu, M.-Y. et al. Plasmonic enhancement of Au nanoparticle—embedded single-crystalline
ZnO nanowire dye-sensitized solar cells. Nano Energy 20, 264–271 (2016).
38. Yang, Y. et al. Hydrothermal growth of ZnO nanowires scaffolds within mesoporous TiO2
photoanodes for dye-sensitized solar cells with enhanced efficiency. Electrochimica Acta
196, 348–356 (2016).
39. Yang, Q., Duan, J., Yang, P. & Tang, Q. Counter electrodes from platinum alloy nanotube
arrays with ZnO nanorod templates for dye-sensitized solar cells. Electrochimica Acta 190,
648–654 (2016).
40. Roso, S., GĂźell, F., MartĂ­nez-Alanis, P. R., Urakawa, A. & Llobet, E. Synthesis of ZnO
nanowires and impacts of their orientation and defects on their gas sensing properties. Sens.
Actuators B Chem. 230, 109–114 (2016).
99

41. Lin, C., Chang, S.-J., Chen, W.-S. & Hsueh, T.-J. Transparent ZnO-nanowire-based device
for UV light detection and ethanol gas sensing on c-Si solar cell. RSC Adv 6, 11146–11150
(2016).
42. Hong, L.-Y., Ke, H.-W., Tsai, C.-E. & Lin, H.-N. Low concentration NO gas sensing under
ambient environment using Cu2O nanoparticle modified ZnO nanowires. Mater. Lett. 185,
243–246 (2016).
43. Lupan, O. et al. Low temperature preparation of Ag-doped ZnO nanowire arrays for sensor
and light-emitting diode applications. in 9749, 97490U–97490U–8 (2016).
44. Lin, C.-H., Chang, S.-J. & Hsueh, T.-J. A low-temperature ZnO nanowire ethanol gas sensor
prepared on plastic substrate. Mater. Res. Express 3, 095002 (2016).
45. Udom, I., Ram, M. K., Stefanakos, E. K., Hepp, A. F. & Goswami, D. Y. One dimensionalZnO nanostructures: Synthesis, properties and environmental applications. Mater. Sci.
Semicond. Process. 16, 2070–2083 (2013).
46. Zhang, Y. et al. Synthesis, Characterization, and Applications of ZnO Nanowires, Synthesis,
Characterization, and Applications of ZnO Nanowires. J. Nanomater. J. Nanomater. 2012,
2012, e624520 (2012).
47. Kołodziejczak-Radzimska, A. & Jesionowski, T. Zinc Oxide—From Synthesis to
Application: A Review. Materials 7, 2833–2881 (2014).
48. Wang, Z. L. Zinc oxide nanostructures: growth, properties and applications. J. Phys.
Condens. Matter 16, R829 (2004).
49. Zhang, J. et al. Double-nucleation hydrothermal growth of dense and large-scale ZnO
nanorod arrays with high aspect ratio on zinc substrate for stable photocatalytic property.
Mater. Lett. 107, 251–254 (2013).
50. Sheetz, R. M., Ponomareva, I., Richter, E., Andriotis, A. N. & Menon, M. Defect-induced
optical absorption in the visible range in ZnO nanowires. Phys. Rev. B 80, 195314 (2009).
51. Arya, S. K. et al. Recent advances in ZnO nanostructures and thin films for biosensor
applications: Review. Anal. Chim. Acta 737, 1–21 (2012).
52. Greene, L. E., Yuhas, B. D., Law, M., Zitoun, D. & Yang, P. Solution-Grown Zinc Oxide
Nanowires. Inorg. Chem. 45, 7535–7543 (2006).
53. Kumar, R., Al-Dossary, O., Kumar, G. & Umar, A. Zinc Oxide Nanostructures for NO2
Gas–Sensor Applications: A Review. Nano-Micro Lett. 7, 97–120 (2015).
54. Zhang, S. et al. Growth and replication of ordered ZnO nanowire arrays on general flexible
substrates. J. Mater. Chem. 20, 10606–10610 (2010).

100

55. In, H. J., Field, C. R. & Pehrsson, P. E. Periodically porous top electrodes on vertical
nanowire arrays for highly sensitive gas detection. Nanotechnology 22, 355501 (2011).
56. Kim, M. K., Yi, D. K. & Paik, U. Tunable, Flexible Antireflection Layer of ZnO Nanowires
Embedded in PDMS. Langmuir 26, 7552–7554 (2010).

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CHAPTER 4
ZNO NANOWIRES/SI NANOCRYSTALS HETEROSTRUCTURES AS IMMOBILIZED
PHOTOCATALYSTS FOR PHOTODEGRADATION
4.1 Introduction
Various forms of ZnO nanostructures have attracted considerable attention as one of the
most efficient catalysts for the photocatalytic degradation due to their high specific surface-area,
direct bandgap, rapid carrier transport, and strong oxidation ability [1–4]. ZnO is also an abundant,
environmentally friendly material, and it is easy and inexpensive to grow its nanostructures [5].
Under UV light irradiation, organic pollutants in waste water streams can be decontaminated in
the presence of ZnO nanostructures, where the photogenerated electrons and holes travel to the
catalyst surface and produce highly oxidizing species such as hydroxyl radicals and super oxide
ions [6] responsible for degradation reactions. Despite their remarkable photocatalytic activity,
ZnO nanostructures suffer from the rapid recombination of photogenerated electron-hole pairs and
low utilization of the solar spectrum due to their limited spectral response to visible light [6].
Considering the UV region covers a small fraction (4~5%) of the solar spectrum, photocatalysts
with the better optical absorption in the visible spectrum (~ 43% of solar spectrum) will lead to the
increased utilization of solar energy. The key to the improved photodegradation performance under
sunlight is the development of functional materials that extend the absorbance spectrum to the
visible region and enhance photoexcited charge separation for reduced electron-hole
recombination. .
Tremendous efforts have been made to boost the optical response of the wide bandgap ZnO
(~3.2 eV) in the visible region, including metal doping (e.g., Al[7], Ag[8], As[9], Fe[10], Mn[11],
Sb[12], Li[13]), non-metal doping [14] (e.g., N-ZnO [15,16], C-ZnO [17,18], S-ZnO [19]), dye
sensitization [20], noble metal decoration (e.g., ZnO/Au [21], ZnO/Ag [22], ZnO/Cu[23]), and
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coupling with narrow-bandgap semiconductors [5] .Among them, the latter two approaches have
been investigated as promising hybrid materials for photocatalysis because ZnO-metal or ZnOsemiconductor heterojunctions suppress carrier recombination by trapping electrons at the
interface and enhance the optical absorption by effectively narrowing down the bandgap [ 24–28].
However, noble metal nanoparticles are relatively expensive, and the metal loading and deposition
method need to be carefully controlled for optimal performance (especially important for a ZnO
core decorated with metal nanoparticles). Instead, hybrid materials of ZnO and semiconductor
nanostructures with a narrow bandgap exhibit superior charge separation as well as enhanced light
absorption capability since both base and add-on materials can directly absorb light energy [5]. Up
to now, ZnO has been coupled with various narrow-bandgap semiconductors for better
photocatalytic functionality, including CuO/Cu2O [29,30,27], CdS/CdTe/CdSe [31–33], WO3 [28] and
Fe2O3 [34]. Here, a relatively unexplored element that forms a hybrid structure with ZnO will be
investigated in the context of photocatalysis – ZnO/Si heterogeneous nanostructures. Si is highly
advantageous because of its abundance and environmental friendliness, and its nanostructures are
well studied in terms of its fabrication and characterization. In particular, Si nanocrystals (Si NCs)
have been extensively investigated as a non-toxic, chemically-stable, and bandgap tunable
component for solar cells [35,36], light emitting devices [37–40], and other optoelectronic applications
[41,42]. Therefore, Si-NCs are a promising material to form ZnO-based heterostructures for
environmentally-friendly and visible-light active photocatalysts.
Another important factor to be considered in developing photocatalysts for water
decontamination is recyclability [43,44]. Most photocatalysts, which are typically in the form of
spherical nanoparticles, are synthesized and tested in a suspension, requiring an additional process
to remove the photocatalysts from the treated water. Immobilized photocatalysts could eliminate

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this costly separation step [43,44], but their performances are limited due to the reduced surface area
of the photocatalysts that are partially embedded into (or deposited onto) the substrate. One
approach to increasing the effective surface area of the immobilized photocatalysts is to harness
vertically-grown one-dimensional nanostructures (e.g. nanowires/nanorods [45–47] and nanotubes
[48]) for improved photocatalytic performance. In particular, ZnO nanowires (ZnO NWs) are easier
to directly grow on a substrate at a high aspect-ratio compared to other semiconducting
photocatalysts (e.g. TiO2) and exhibit high crystallinity for efficient charge transport. There are
several reports on ZnO-NW-based heterostructures for photocatalytic applications [3,49,50], but to
best of our knowledge, Si NCs in conjunction with ZnO NWs have not yet been investigated for
photodegration and other optoelectronic applications.
In this work, novel hybrid structures of Si nanocrystals/ZnO nanowires (Si-NCs/ZnONWs) were synthesized for the first time as immobilized photocatalysts for pollutant degradation.
The Si-NCs, which possess tunable bandgap (1.2 ~2.8 eV), extend the light absorption of ZnO
NWs into the visible spectrum and facilitate the charge carrier separation. The detailed
morphology of the hybrid nanostructures seemed to have a significant influence on the
photocatalytic performance; and therefore, the systematic study of the Si-NC loading effect on the
light absorption, photoluminescence, and photodegradation was conducted under various
illumination conditions to reveal the possible mechanism of the enhanced photodegradation from
the Si-NCs/ZnO-NWs hybrid structures at the optimal Si-NC loading.

4.2 Experimental
4.2.1 Synthesis of ZnO NWs
All reagents used in this experiment were commercially available without further
purification. ZnO NWs were synthesized via the hydrothermal method [51]. The detailed process
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is given as follows: a ZnO seed solution was prepared by adding 10 mM Zn(CH3COO)2¡2H2O
(99.9% Sigma Aldrich) into ethanol (200 prove, Fisher Scientific) and stirred at 58°C for 2 hours.
The mixed solution was spin-coated onto a cleaned glass or Si substrate (25 mm by 25 mm in size)
multiple times at room temperature. The substrate was then kept at 150°C for 1 hour to promote
the adhesion of the seed particles to the substrate. The growth solution was composed of 25 mM
Zn(NO3)2¡6H2O (Sigma Aldrich) and 25 mM hexamethylenetetramine (HMTA, Sigma Aldrich).
Prior to the synthesis, the growth solution was preheated in a convection oven at 90°C for 1 hour
to achieve a thermal equilibrium. The substrate was then immersed into the solution at 90°C for
6 hours for ZnO NW growth. After removal from the solution, the substrate with ZnO NWs was
thoroughly rinsed by DI water and blown dry. A 3-hour-long dehydration step at 60°C was carried
out for the substrate afterwards.

4.2.2 Synthesis of Si nanocrystals
Silicon Nanocrystals (Si NCs) were synthesized using a non-thermal plasma reactor in a
fully gas phase process. This process was originally developed by Mangolini et al. [52] and has
been well-studied for several other nanoparticles’ synthesis [53–55]. Besides producing high quality
nanoparticles, gas phase synthesis has several other advantages including accurate size control,
adjustable particle structure, lesser degree of agglomeration, and hydride-terminated surfaces. The
flow-through plasma reactor consists of a quartz tube with varying outer diameters of 0.5 inch and
1 inch in the top and bottom parts, respectively. The lengths of the contraction and expansion
region are 7 inch and 5 inch with the total length being 1 ft. Argon (Ar) was used as a background
gas with silane (SiH4, 1% in Ar) as a precursor for Si NCs. The gas mixture was flown through
the quartz tube around which a pair of copper electrodes are wrapped. Hydrogen (H2) was flown

105

through the sidearm into the effluent of the plasma to provide additional hydrogen termination and
reduced dangling bond densities [56]. The gas flowrates were 30 standard cubic centimeters per
minute (sccm) of Ar, 80 sccm of Ar/SiH4, and 50 sccm of H2, and the pressure in the reactor was
kept constant at 2.75 Torr using a slit-shaped orifice. Radiofrequency (rf) power at 25 W was
applied to the copper electrodes using a 13.56-MHz power supply and an impedance matching
network. The Si-NCs synthesized using this recipe were approximately 4-5 nm in diameter.

4.2.3 Surface Functionalization Process for Si NCs
As-produced Si-NCs were surface-functionalized in a thermal hydrosilylation reactor [57].
Si-NCs were collected via diffusion onto a stainless steel mesh for this purpose. A
functionalization solution was prepared by mixing mesitylene and 1-decene in a volume ratio of
5:1, and was dried and degassed using molecular sieves (size 4 Çş) and nitrogen bubbling. This
solution was then mixed with Si-NCs (concentration approximately 1 mg/mL) and sonicated for a
few minutes (until the Si NCs were no longer agglomerated at the bottom of the container). This
final solution was heated for about 2 hours under a nitrogen flow at 170°C in a refluxer to
recondense any evaporated solvents. When the reaction was over, the solution became a clear
colloidal liquid rather than a cloudy dispersion. These surface-passivated Si NCs were then dried
and redispersed in toluene. Quantum yield (QY) measurements were performed right after the
functionalization and found to be 40 ‒ 60%.

4.2.4 Preparation of Hybrid Structure
After the surface functionalization, the Si-NCs of 5 mg/mL were spin-coated on the asprepared ZnO-NW substrate at 500 rpm for various times to control the Si-NC loading. In this

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experiment, three different loadings of Si NCs, i.e., 5 ÂľL, 10 ÂľL and 15 ÂľL, were applied to the
ZnO-NW substrates and named as ZS1, ZS2 and ZS3, respectively. For the comparison, a ZnONW substrate was prepared and denoted as ZS0. The subsequent dehydration step was performed
for these hybrid structures at 40°C in vacuum for 3 hours to improve the adhesion of Si-NCs to
ZnO NWs.

4.2.5 Materials Characterization
The morphology, crystallinity and composition of three Si-NCs/ZnO-NWs hybrid
structures and the bare ZnO NW sample were characterized using micro-Raman spectroscopy, Xray diffraction (XRD, Bruker-AXS), scanning electron microscopy (SEM, JEOL 6610LV, Hitachi
S-4700II), energy-dispersive X-ray spectrometer (EDS, JEOL 6610LV), and transmission electron
microscopy (TEM, JEOL 2200FS). X-ray photoelectron spectrometer (XPS, Perkin Elmer Phi
5600 ESCA), Fourier transform infrared spectroscopy (FTIR, Bruker Alpha). The optical response
of the as-prepared samples was obtained by photoluminescence (PL) spectroscopy with the
excitation wavelength at 337 nm and the emission filtered by a 345 nm long pass filter. The PL
lifetime of the Si NCs was collected using a pulsed nitrogen laser excitation source (𝜆 = 337 𝑛𝑚,
pulse duration = 10 ns, and repetition rate = 20 Hz)_with a photomultiplier tube used for detection.
The signal was filtered by a 345 nm long pass filter and a 675 nm +/- 50 nm band pass filter before
detection to spectrally isolate the emission from the Si NCs. Diffuse reflectance from the samples
was measured with a PerkinElmer LAMBDA 1050 UV/Vis/NIR Spectrophotometer with an
integrating sphere attachment.

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4.2.6 Photodegradation Assessment
Under UV light: The photocatalytic degradation was carried out in an aluminum foil-sealed
case equipped with a magnetic stir plate. Two 9 W florescent tubes (Philips, Îť = 375 nm) were
positioned 13 cm above a dye solution container. Methylene blue (1.5%, Sigma-Aldrich) was
diluted to 10 mM and used as a model pollutant. The substrate (cut into 5 mm by 5 mm) with the
given Si-NCs/ZnO-NWs hybrid structure was immersed in the dye solution (12 mL). Before
irradiation, the dye solution was kept in dark for 30 min to establish the absorption-desorption
equilibrium. Upon irradiation, 1 mL of solution was extracted from the container at a constant time
interval (typically 30 min) and centrifuged at 10,000 rpm for 2 min. The purified solution was
then examined by a UV-vis spectrometer to measure the absorption of the remaining methylene
blue. The experiment was repeated a few times to determine the relative error and compared to
that of the substrate with ZnO NWs.
Under visible light and white light: A 250 W Halogen light was employed as a white light
source. Before irradiation, a solar-filtered transparent film (ENPÂŽ ultra clear 7225) was placed
between the light source and the dye solution. This film rejects 96% and 75% of UV and infrared
lights, respectively, and transmits 72% of visible light. Except for the 30-cm-distance of light
source, the remaining procedures were the same as that of the photodegradation experiment under
the UV light. The temperatures at both sides of the transparent film were 30°C (dye solution side)
and 50°C (light source side) 6 hours after light was turned on. Repeat the experiments under the
same condition in the absence of solar-filtered transparent film for the white light irradiation. The
temperatures of solutions in all experiments were kept below 28°C during 6-hour irradiation.

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4.3 Result and discussion

Figure 4.1. Top-down and cross-sectional SEM images of the as-prepared SiNCs/ZnO-NWs hybrid
structures: (a) and (e) for ZS0, (b) and (f) for ZS1, (c) and (g) for ZS2, (d) and (h) for ZS3. Scale
bar: 10 μm in (a) – (d); 500 nm in the inset of (a) – (d); 2 μm in (e) – (h).

The ZnO NWs were synthesized on a glass or silicon substrate by the standard
hydrothermal method, and the resulting NW structures were consistent with those reported in the
literature [58–61]. The Si NCs were synthesized via a non-thermal low pressure plasma process and
subsequently functionalized to stabilize the nanocrystals and reduce the surface oxidation. The Si
NC diameter was carefully controlled to be around 5 nm. Figure 4.1 shows the top-down and crosssectional scanning electron microscopy (SEM) images of the bare ZnO-NW sample (ZS0) and the
ZnO-NWs/Si-NCs hybrid samples with the three different Si-NC loadings (ZS1, ZS2, and ZS3).
The vertical ZnO NWs of a hexagonal shape with the diameter of 50 to 80 nm and the length of
1.5 to 2 Îźm were uniformly grown on the Si or glass substrate, where the average density of the
ZnO NWs is estimated to be around 10/ Âľm2.The Si-NC loading was adjusted by the number of
times that spin-coating of a given aliquot (5 μL) was applied onto the ZnO–NW substrates.

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Figure 4.2. Top-down and cross-sectional SEM images of the as-prepared SiNCs/ZnO-NW hybrid
structures with heavy Si loading, Scale bar: 10 Îźm in (a); 500 nm in the inset of (a); 2 Îźm in (b).

Figure 4.3. SEM/EDS elemental mapping of bare ZnO-NW and Si NCs/ZnO NWs hybrid samples:
(a) ZS0; (b) ZS1; (c) ZS2; (d) ZS3. Zn in Red, O in blue and Si in yellow. The respective weight
fractions of Zn and Si are: Zn 85.2% in ZS0; Zn 80.1% and Si 1.7% in ZS1; Zn 70.7% and Si 5.6%
in ZS2; Zn 61.2% and Si 8.5% in ZS3. Scale bar: 5 Îźm.
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During the spin-coating process, the Si-NC suspension partially embedded into the voids
between the ZnO NWs, depositing the NCs on the NW surface. As seen from Figure 4.1(a-d), the
gaps between ZnO NWs were gradually filled with Si-NCs with increasing loading. The crosssectional views (Figure 4.1(e-h)) reveal that most Si-NCs remain agglomerated around the NW
tips after the deposition and tent over the ZnO NWs. The sample with the highest Si-NC loading
(ZS3) shows the formation of the continuous layer (dark cloudy layer in Figure 4.1h). If the loading
further increases, the extra Si-NCs fully encapsulate the entire NW array and create a thick overdeposited layer (see Figure 4.2). When this hybrid structure with the heavy Si-NC loading is
irradiated, photons from both UV and visible regions are mostly absorbed by Si-NCs before
reaching ZnO NWs. Therefore, it is difficult to consider it as the hybrid structure because the
optical response is dominantly determined by the thick Si-NCs. This also means that there would
be an appropriate Si-NC thickness (or coverage) which avoids significant blockage of the light
transmitting to the ZnO NWs and yet allows the benefits of the hybrid structures.
Elemental analysis using energy-dispersive x-ray spectroscopy (EDS) was performed on
these four samples (ZS0, ZS1, ZS2, and ZS3) to reveal the relative mass ratio of Zn and Si.
Figure 4.3 in the Supplementary Information shows the top-down SEM views of the four samples
and corresponding EDS elemental mapping spectra for Zn, O, and Si. The elemental ratios of Zn
to Si for ZS1, ZS2 and ZS3 are 47.12, 12.63 and 7.2, respectively, demonstrating the increased
quantity of Si for the higher loading of Si NCs. All three samples show that the signals for Si are
evenly distributed across the images, indicating that Si-NCs was applied onto the substrates in a
homogenous fashion. The similarity of the respective distribution between Zn and Si in ZS1, ZS2
and ZS3 also depicts the intimate attachment of Si NCs to ZnO NWs over the substrate, where the
element concentration of Si fluctuates along the morphology of ZnO NW array.

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Figure 4.4. TEM images of (a) bare ZnO NW sample (ZS0), (b) synthesized and functionalized Si
NCs, (c) Si-NCs/ZnO-NW hybrid structure prepare from ZS2. Scale bar: 5 Îźm in (a) and (c). (d)
XRD patterns of ZS0 and ZS3, (e) FTIR of ZS0.

Figure 4.4a show the TEM image of the bare ZnO NW (ZS0), in which the lattice fringes
(~ 0.26 nm) are in a good agreement with the distance between two (0001) planes of wurizite ZnO
[62]. The TEM image of one of the Si-NCs/ZnO-NWs hybrid nanostructures (ZS2) is shown in
Figure 4.4b. The Si NCs of various crystal orientations were observed continuously surrounding
the ZnO NWs. The lattice fringes indicate that both ZnO NWs and Si NCs in the hybrid
nanostructure maintain the crystalline structures. The XRD patterns of ZS0 and ZS3 were
exhibited in Figure 4.4c. All the peaks appearing on both samples correspond to the wurtzite phase
of ZnO NWs [24,61,63], where the peak (002) with the highest intensity depicts the preferable growth
direction [0001] of ZnO NWs. The absence of Si-specific peaks from ZS3 (red) presumably
ascribes to the low volume fraction of Si NCs. The Fourier Transform Infrared Spectra (FTIR) of

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ZS0 was collected in the range of 500 ~ 4000 cm-1. As displayed in Figure 4.4d, the peaks at
3393 cm-1 and 1413 cm-1 correspond to the stretching vibration mode and bending vibration of the
O-H group, respectively. A strong peak at 1557cm-1 is attributed to the C=O stretching vibration.
The peaks observed at 886 cm-1 and 570 cm-1 indicate Zn-O stretching, while the peaks at 2361 cm1

and 2966 cm-1 can be assigned to C-H stretching [64–66].

Figure 4.5. Raman-scattering spectra of (a) the bare ZnO-NWs (ZS0) and three SiNCs/ZnO-NWs
hybrid structures (ZS1, ZS2, and ZS3), (b) ZS0 showing four Raman peaks of bare ZnO.

Figure 4.5a shows the room-temperature Raman scattering spectra of the bare ZnO NWs
(ZS0) and three Si-NCs/ZnO-NWs hybrid structures (ZS1, ZS2, and ZS3), confirming their
crystallinity. For ZS0, four Raman peaks are shown in Figure 4.5b, where the peak with the highest
intensity at 440 cm-1 is assorted to E2 (high) [67], associated with the characteristic of wurtzite ZnO.
The three other peaks located at 340 cm-1, 378 cm-1, and 584 cm-1 are assigned to A1 (TO), E1
(TO), and E1 (LO), respectively. There is another peak obtained at 101 cm-1 (data not shown) for
E2 (low), matching previous reports [67,68]. The Raman spectra for the hybrid structures show one
additional peak at 515 cm-1, corresponding to TO of Si NCs. The peak position of the Si NCs
detected at 515 cm-1 is different from that of single-crystal Si at 521 cm-1, which is attributed to
the lattice expansion of Si NCs [69]. The observed broadening of the peak with a large FWHM is
due to the ZnO peak at 535 cm-1, which belongs to A1 (LO) mode. Furthermore, the peak with the

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asymmetric shape deviated from the Lorentzian function indicates that the size of Si NCs is down
to sub-micro scale in term of the electromagnetic radiation theory [70,71]. By comparing the Raman
peaks of each ZnO NW with different Si-NC loadings, especially A1 phonons, it can be concluded
that no significant peak shift and broadening are observed, suggesting the stable formation and
reproducible synthesis of the hybrid structures.

Figure 4.6. (a) Room-temperature PL spectra of the bare ZnO NW sample and three Si-NCs/ZnONWs hybrid samples, (b) the same PL spectra with the magnified scale to better show the peaks
related to oxygen defects of ZnO; (c-e) schematic diagrams of ZS0 (c), ZS1 (d), and ZS2/ZS3 (e);
(f) time-resolved PL decay of the SiNCs on the ZnO NWs for the three hybrid samples (signal
filtered at 675 nm +/- 50 nm), (g) a bi-exponential fit of the PL decay.

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Room temperature photoluminescence (PL) spectra (see Figure 4.6a) were measured to
investigate the role of Si NCs on the luminescent properties of the hybrid structures. The narrow
peaks at 380 nm from all four samples can be attributed to the near-band-edge (NBE) emission of
ZnO, mostly related to the recombination of free excitons through an exciton-exciton collision
process [72,73]. Another small and broad peak at 570 nm from ZS0 (see Figure 4.6b) may originate
from the recombination of electrons with oxygen vacancies and photogenerated holes in the
valence band, which is typically related to the defect concentration of ZnO[68,74]. Therefore, the
strong NBE emission and weak green emission in the PL spectrum suggests that the hexagonalshaped ZnO NWs have good crystallinity and optical properties. The magnified plot of Figure 4.6b
shows that the weak defect-related emission peaks for ZS1, ZS2 and ZS3 are mostly overwhelmed
by the strong and broad luminescence peak of Si NCs in the range between 500 and 900 nm [75,76].
It can be observed that the ZnO NBE emission peak was initially suppressed at the lowest
Si-NC loading (ZS1) and was then significantly enhanced at a higher loading (ZS2). But the further
increase in the loading (ZS3) reduced the NBE emission. To account for this irregular trend, the
different morphological models of the hybrid nanostructures are shown in Figure 4.6(c-e) as a
function of Si-NC loading. The model in Figure 4.6c corresponds to the bare ZnO NWs (ZS0)
while two models of two different Si-NC loadings are constructed in Figure 4.6d and e based on
the SEM images (Figure 4.1b and f). When incorporated with a small quantity of Si NCs, the ZnO
NWs in ZS1 was partially covered by the NCs. In this case, both ZnO NWs and SiNCs were
simultaneously activated under light irradiation. The Si/ZnO junction facilitated the separation of
photogenerated charge carriers, resulting in the quenching of NBE emission [77]. In addition, the
surface passivation of the ZnO NWs derived from the Si NCs may have reduced the surface defect
sites (i.e. recombination centers) and led to lower radiative recombination. On the other hand, ZS2

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and ZS3 (Figure 4.1e and h) can be represented as the model in Figure 4.6e, where the tips of ZnO
NWs are completely covered by the Si-NC layer. It has been reported that charge carriers
transportation in ZnO/TiO2 hybrid nanostructures may cause the ZnO conduction band (CB) to
bend upwards due to electron trapping at the interface, which acts as an energetic barrier impeding
further electron transfer from the TiO2 [78]. Likewise, the majority of photons were absorbed by Si
NCs and the excessive electrons generated within the Si NCs accumulated at the Si-NCs/ZnONWs interface, which formed a barrier to hinder the transportation of charge carriers across the
Si/ZnO junction. Consequently, radiative recombination within ZnO NWs was promoted, leading
to the stronger NBE emission of ZnO. A further comparison of the PL intensities at 380 nm for
ZS2 and ZS3 (see Figure 4.6a) indicates that the NBE emission intensity decreases as the Si-NC
loading further increases. Note that only the Si NCs in direct contact with or close proximity to
ZnO NWs are highly effective in surface passivation and charge injection. The reduced PL
intensity from ZnO with a higher Si-NC loading can be attributed to extra Si NCs blocking photons
reaching the ZnO NWs and excitons emitting from ZnO. This “screening” effect undercuts the
gains obtained from the enhanced charge transfer at the hybrid interfaces. For example, SEM
imaging from the sample with the heavy loading (ZS3) shows the thick Si-NC layer deposited on
top of the NWs (see Figure 4.1(g-h)). Incident photons would be first absorbed in that layer and
create the electron-hole pairs within the Si NCs. But the charge carriers that were generated far
away (even a couple of NCs away) from the NWs would be recombined within the Si NC layer
and could not get injected into the ZnO NWs. Now fewer photons are available to excite the ZnO
NWs while the emitted energy has to pass through the Si-NC layer, thereby causing a net negative
effect on NBE emission. Meanwhile, the characteristic PL peaks for Si NCs are observed around

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730 nm (Âą 170 nm) for all hybrid structures. As expected, the sample with the higher Si-NC
loading produced the increased PL intensity (i.e., ZS3 > ZS2 > ZS1).
Figure 4.6f shows the time-resolved PL decay of the Si NCs on the ZnO NWs (signal
filtered at 675 nm Âą 50 nm). Each PL decay is normalized by its maximum intensity for ease of
comparison. The corresponding fast and slow decays of the three hybrid nanostructures were
𝑡

identified through a bi-exponential fit (R2 > 0.99 for all fits) using y = 𝑦0 + 𝐴1 exp (− 𝜏 ) +
1

𝑡

𝐴2 exp (− 𝜏 ). According to Figure 4.6g, the fitted fast PL decay time constants (τ1) are 4.78 μs,
2

5.96 Îźs and 6.32 Îźs for ZS1, ZS2, and ZS3, respectively, while the corresponding slow PL decay
time constants (τ2) are 42.64 μs, 46.06 μs and 55.45 μs. Previous studies from literature have
correlated the fast PL decay in the Si NCs to the average carrier lifetime of a non-radiative
recombination process while the slow one describes the radiative recombination process [79,80].
The latter mechanism is more relevant to the NBE emission, and therefore, an emphasis is placed
on τ2. Note that the average exciton lifetime of the room-temperature PL process in pure ZnO NWs
is reported to be around hundreds of picosecond to few nanosecond [81,82] whilst the average
exciton lifetime of Si NCs is approximately 80 Îźs [83]. In the current system, a shorter lifetime of
τ2 was observed at the lower Si-NC loading possibly because the photogenerated electrons in the
Si NCs, after being injected into ZnO, would undergo the recombination process at a faster rate.
This reduction in PL lifetime for the Si-NCs can be explained as the result of electron transport
from the NCs into the ZnO NWs, as has been observed for quantum-dot-sensitized solar cells
(QDSSCs) and other hybrid NC/receptor materials based on cadmium chalcogenide nanocrystals.
At high Si-NC loading, the PL lifetime is dominated by the long lifetimes that are typical for silicon
nanocrystals. At lower Si-NC loading, the PL lifetime is reduced as a result of exciton dissociation

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and electron transport into the ZnO NWs, driven by the energy band alignment between the two
materials which is favorable for electron transfer to ZnO.
Therefore, the exciton lifetime of the hybrid nanostructure is statistically shorter than that
of the pure Si NCs, and an extent of the lifetime reduction is a function of the percentage of Si
NCs involved in the effective charge transfer from Si to ZnO at their interface. In other words,
ZS1 has the lowest τ2 because the majority of Si NCs in ZS1 is interfaced with ZnO NWs, resulting
in the fastest exciton recombination. τ2 was then steadily recovered to that of the pure Si NCs at a
heavier Si-NC loading. Since the quantum yield of the Si NCs is approximately 40%, the radiative
energy transfer lifetime of ZS1, ZS2 and ZS3 can be estimated to be 11.95 Îźs, 14.9 Îźs and 15.8 Îźs,
respectively [84].

Figure 4.7. UV-Vis diffuse reflectance spectra of the bare ZnO NW sample (ZS0) and three SiNCs/ZnO-NWs hybrid samples (ZS1, ZS2, and ZS3). (a) diffuse reflectance spectra, (b)
corresponding Kubelka-Munk plot as a function of the exciting light energy.

Figure 4.7a displays the diffuse reflectance spectra of the bare ZnO NWs as well as the SiNCs/ZnO-NWs hybrid structures. In the given UV-vis region, the diffuse reflectance of ZS0 is
greater than the hybrid structures (in fact, ZS0 > ZS1 > ZS2 > ZS3), indicating the stronger
absorption for a higher Si-NC loading. The plot of the modified Kubelka-Munk function,
(F(R)hv)1/2, vs. the energy of exciting light (hv), can be employed to calculate the optical band gap
before and after the SiNC decoration [28,85] (see Figure 4.7b). Here, F(R) = (1 –R)2/2R (where R is
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the reflectance), h the Planck constant, and v the frequency of light. The x-intercept values from
the extrapolation of the linear slope to photon energy in Figure 4.7b can be related to band gap
energy. The calculated band gap energies of ZS0, ZS1, ZS2, and ZS3 are 3.22 eV, 3.10 eV,
3.14 eV and 3.07 eV, respectively. The maximum bandgap narrowing of 0.15 eV has been
observed upon the formation of ZnO/Si hybrid structures, which is similar to the other ZnOsemiconductor-based hybrid structures [28]. In this paper, the narrowed band gap of the hybrid
structures is expected to extend the spectrum response of the catalyst and enhance
photodegradability under visible light.

Figure 4.8. Degradation profiles of methylene blue dye solution for the bare ZnO NWs (ZS0) and
SiNCs/ZnO-NWs hybrid samples (ZS1, ZS2, ZS3) under (a) UV light, (b) visible light and (c) white
light. (d) Schematic energy band diagram of charge carrier movements in Si-NCs/ZnO-NWs
hybrid structure.

Figure 4.8 shows the photodegradation performance of bare ZnO NWs and three SiNCs/ZnO-NWs hybrids under UV light, visible light and white light. Here the MB dye solution
was used as a model pollutant which was brought to absorption equilibrium after keeping in dark
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for a few hours. Note that the degradation testing was performed on the SiNCs/ZnO-NW hybrid
structures on a Si substrate that was immersed in the solution. Therefore, the photodegradation
performance from this immobilized catalyst platform is inferior to that of the slurry catalyst system
[86], and it is not our intention to compare results from these two very different platforms. Rather,
the emphasis is placed on the comparison between the bare ZnO-NW sample and hybrid samples
under various lighting conditions. The absorption spectra (i.e. peak height) taken from the dye
after being irradiated for different time intervals represent the degraded concentration of MB. The
degradation profile is thus the concentration (C) at the degradation time normalized by the initial
concentration (C0). Photolysis of a clean MB dye solution was performed without catalyst to
measure a degree of photosensitization of the dye. This control experiment was carried out under
the same degradation conditions and compared to the catalyst-added ones. Less than 8%
degradation was observed for UV and visible light conditions when irradiated for 6 hours. For
white light, 16% of MB was degraded within 6 hours.
As shown in Figure 4.8a, the degraded MB concentration from ZS0 and ZS3 reached the
maximum and minimum absorption of 55% and 35%, respectively, during the first 6 hours. The
bare ZnO NW sample (ZS0) exhibited a degradation rate similar to ZS1 but higher than ZS2 and
ZS3 under UV light. As discussed earlier from the PL data, the thick Si-NC layer atop ZnO NWs
may block the incident UV light for ZS2 and ZS3, in which the ZnO NWs in the hybrids absorbed
fewer photons and generated fewer charge carriers in comparison to the pure ZnO NWs. The
observation of ZS1 and ZS2 having better performance than ZS3 suggests the dependence of SiNC layer thickness on the degradation rate. Another possibility originates from the generation
efficiency of the charge carriers under the UV light irradiation. The UV wavelength at 365 nm
corresponded to ZnO bandgap (3.37 eV), which can generate electron-hole pairs more efficiently

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than that of SiNCs. The enhanced degradation rate will be also highly dependent on the ZnO
surface areas exposed to the dye molecules. Thus, ZS3 is less favorable to serve as a catalyst under
UV light irradiation. The electron accumulation could occur due to the slow charge carrier
transport under UV light irradiation, which will be discussed in the band alignment of the hybrid
structures.
The Si NCs with a narrower band gap broadens the spectrum response of the
photocatalysts, while the ZnO NWs alone, with a wide band gap, limited in response to the UV
region of the spectrum. The photocatalytic degradation under visible light (Figure 4.8b) was
carried out in order to further investigate the mechanism of the hybrid photocatalysts. The major
fraction of UV and IR light were filtered (around 97% blocked) and the intensity of the visible
light was reduced approximately by 30%. The trend of the degradation behavior is opposite to that
of the UV light condition (see Figure 4.8a). The maximum degradation of MB was seen with ZS3
while the minimum one is with ZS0. The straightforward explanation of this trend is that the bare
ZnO sample (ZS0) does not absorb the visible light efficiently due to its large band gap and thus
produce fewer charge carriers and oxidants for photodegradation. The three hybrid structures show
essentially little (or minor) difference in degradability. But the fact that ZS3 shows slightly higher
degradability than ZS1 or ZS2 may suggest that the charge transfer/injection between Si NCs and
ZnO NWs does not play a significant role and that the oxidants generated on the Si-NC surface
are more important.
The photodegradation performance of the bare ZnO NWs and SiNCs/ZnO-NWs hybrids
under white light (IR cut off) exhibits the different trend (see Figure 4.8c) in comparison to those
under UV or visible light only. The overall degradability follows the sequence of ZS1 > ZS2 ≈
ZS0 > ZS3, respectively. Since white light consists of both visible and UV spectral regions, the

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photodegradability under white light exhibits the combined effects from either spectral case. In
essence, the higher the Si NC loading the hybrid structure, the poorer the degradation performance
became for the UV case. But the opposite trend was observed for the visible light case. The net
effect is the results seen in Figure 4.8c, but the detailed explanation will require deeper
understanding in the hybrid structures. Preferential light absorption of ZnO or Si in each spectral
region is compounded by the relative placement of nanostructures (i.e., the incident light is always
absorbed by the Si-NC layer first, thus creating the shadowing effect on the ZnO NWs) and the
overall absorption efficiency. It is interesting to note that the degradation performance of ZS1 was
significantly better (more than 15%) than that of ZS0 or ZS2. The enhanced charge separation and
corresponding photogenerated excitons, possibly related to the low ZnO NBE PL intensity of ZS1,
may be responsible for the improved behavior.
In general, photodegradation activity is governed by the amount of participating oxidants
such as superoxide anions (¡O2-) and hydroxyl radicals (¡OH) available for degradation reactions.
Figure 4.8d shows the possible mechanism of generating highly oxidizing species in Si-NCs/ZnONWs hybrid structure. This schematic may be applicable to the near-optimal hybrid structure of
ZS1, where a very thin layer of SiNCs were covered over the ZnO NWs. Then we can assume that
the recombination of the photogenerated excitons in Si NCs is not significant. Upon irradiation,
the Si NCs absorb the photons prior to ZnO NWs and release photogenerated electrons that either
travel to the Si-NC surface to take part in the reduction reaction – converting oxygen molecules in
water into superoxide anions – or transfer to the hybrid interface and get injected into conduction
band (CB) of ZnO NWs. Since the mobility of free electrons is higher than holes in the
semiconductor, the enhanced electrons will contribute to the effective recombination of electronhole pairs of ZnO NWs. Meanwhile, the valance band of Si NCs adopts additional photogenerated

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holes transferred by ZnO NWs, oxidizing H2O to ¡OH for the further pollutant oxidization. On the
other hand, excess photogenerated electrons of ZnO NWs supplied by Si NCs yield ¡O2- from O2.
Obviously, the facilitated charge carriers producing abundant oxidants to degrade the pollutant,
resulting in improved photocatalysis.
As abovementioned, a barrier caused by the charge carrier transportation impeded further
electron transfer from the Si NCs. This phenomenon tends to emerge seriously at higher
concentrated Si NCs, decreasing the enhancement of the Si NCs towards the photocatalytic
degradation, which may an alternative to explain the trend that heavier Si NCs loading caused slow
degradation rate [78].

4.4 Conclusion
In this work we have described the synthesis and characterization of Si-NCs/ZnO-NWs
hybrid structure as promising sunlight-harnessing photocatalysts. Hydrothermal synthesis and
low-pressure non-thermal plasma growth techniques were utilized to create highly crystalline ZnO
NWs and Si NCs, respectively. The morphological, compositional, and crystalline nature of the
ZnO NWs and the hybrid nanostructures were studied using the SEM, TEM, XRD, and FTIR,
confirming the formation of well-defined hybrid structures on the substrates. The optical properties
and functionality of the hybrid structures were also studied as a function of the Si-NC loading by
means of the microRaman spectroscopy, PL, time-resolved PL, and diffuse reflectance
spectroscopy. The photodegradation performance of the bare ZnO-NW and hybrid samples were
tested on the immobilized platform under different lighting conditions, including UV, visible, and
white light, to investigate the effects of hybrid structures on source light dependent photocatalysis.
The improved photocatalytic activity of the hybrid sample with the optimal Si-NC loading was

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attributed to the presence of the proper amount of Si NCs that decorated ZnO NWs, extending the
light sensitivity of hybrid from the UV to visible region, promoting the effective charge separation
(i.e., suppressing exciton recombination), and increasing the exciton lifetime. The bare ZnO-NW
and Si-NCs/ZnO-NWs hybrid samples were immobilized on a flat substrate that provided a
common platform for measuring both photocatalytic performances and optical characteristics,
which made the comparison and analysis more meaningful. We believe that these novel SiNCs/ZnO-NWs hybrid structures may find their usefulness beyond photodegradation and in fact
even more effective in improving the performance of various optical devices.

124

REFERENCES

125

REFERENCES

1. Lee, K. M., Lai, C. W., Ngai, K. S. & Juan, J. C. Recent developments of zinc oxide based
photocatalyst in water treatment technology: A review. Water Research 88, 428–448 (2016).
2. Sirelkhatim, A. et al. Review on Zinc Oxide Nanoparticles: Antibacterial Activity and
Toxicity Mechanism. Nano-Micro Lett. 7, 219–242 (2015).
3. Mohd, A. M. A., Julkapli, N. M. & Abd, H. S. B. Review on ZnO hybrid photocatalyst:
impact on photocatalytic activities of water pollutant degradation. Reviews in Inorganic
Chemistry 36, 77–104 (2016).
4. Johar, M. A. et al. Photocatalysis and Bandgap Engineering Using ZnO Nanocomposites,
Photocatalysis and Bandgap Engineering Using ZnO Nanocomposites. Advances in
Materials Science and Engineering, Advances in Materials Science and Engineering 2015,
2015, e934587 (2015).
5. Gu, X. et al. ZnO based heterojunctions and their application in environmental
photocatalysis. Nanotechnology 27, 402001 (2016).
6. Mohd, A. M. A., Julkapli, N. M. & Abd, H. S. B. Review on ZnO hybrid photocatalyst:
impact on photocatalytic activities of water pollutant degradation. Reviews in Inorganic
Chemistry 0, (2016).
7. Ahmad, M. et al. Preparation of highly efficient Al-doped ZnO photocatalyst by combustion
synthesis. Current Applied Physics 13, 697–704 (2013).
8. Height, M. J., Pratsinis, S. E., Mekasuwandumrong, O. & Praserthdam, P. Ag-ZnO catalysts
for UV-photodegradation of methylene blue. Applied Catalysis B: Environmental 63, 305–
312 (2006).
9. Jeong, T. S., Han, M. S., Youn, C. J. & Park, Y. S. Raman scattering and photoluminescence
of As ion-implanted ZnO single crystal. Journal of Applied Physics 96, 175–179 (2004).
10. Saleh, R. & Djaja, N. F. UV light photocatalytic degradation of organic dyes with Fe-doped
ZnO nanoparticles. Superlattices and Microstructures 74, 217–233 (2014).
11. Ullah, R. & Dutta, J. Photocatalytic degradation of organic dyes with manganese-doped ZnO
nanoparticles. Journal of Hazardous Materials 156, 194–200 (2008).
12. Kim, S. H. et al. Non-catalytic growth of high-aspect-ratio Sb-doped ZnO nanowires by
simple thermal evaporation process: Structural and optical properties. Journal of Alloys and
Compounds 479, 290–293 (2009).
13. Valentini, A., Quaranta, F., Rossi, M. & Battaglin, G. Preparation and characterization of Li‐
doped ZnO films. Journal of Vacuum Science & Technology A 9, 286–289 (1991).
126

14. Samadi, M., Zirak, M., Naseri, A., Khorashadizade, E. & Moshfegh, A. Z. Recent progress
on doped ZnO nanostructures for visible-light photocatalysis. Thin Solid Films 605, 2–19
(2016).
15. Zong, X. et al. Activation of Photocatalytic Water Oxidation on N-Doped ZnO Bundle-like
Nanoparticles under Visible Light. J. Phys. Chem. C 117, 4937–4942 (2013).
16. Yu, Z. et al. Crystallinity-dependent substitutional nitrogen doping in ZnO and its improved
visible light photocatalytic activity. Journal of Colloid and Interface Science 400, 18–23
(2013).
17. Cho, S., Jang, J.-W., Sung Lee, J. & Lee, K.-H. Carbon -doped ZnO nanostructures
synthesized using vitamin C for visible light photocatalysis. CrystEngComm 12, 3929–3935
(2010).
18. Liu, S., Li, C., Yu, J. & Xiang, Q. Improved visible-light photocatalytic activity of porous
carbon self-doped ZnO nanosheet -assembled flowers. CrystEngComm 13, 2533–2541
(2011).
19. Zhang, H., Tao, Z., Xu, W., Lu, S. & Yuan, F. First-principles study of dopants and defects
in S-doped ZnO and its effect on photocatalytic activity. Computational Materials Science
58, 119–124 (2012).
20. Sarkar, S. et al. Hematoporphyrin–ZnO Nanohybrids: Twin Applications in Efficient
Visible-Light Photocatalysis and Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 4,
7027–7035 (2012).
21. Lin, F. et al. Intermediate selectivity in the oxidation of phenols using plasmonic Au/ZnO
photocatalysts. Nanoscale 9, 9359–9364 (2017).
22. Aguirre, M. E., RodrĂ­guez, H. B., San RomĂĄn, E., Feldhoff, A. & Grela, M. A. Ag@ZnO
Core–Shell Nanoparticles Formed by the Timely Reduction of Ag+ Ions and Zinc Acetate
Hydrolysis in N,N-Dimethylformamide: Mechanism of Growth and Photocatalytic
Properties. J. Phys. Chem. C 115, 24967–24974 (2011).
23. Schumann, J. et al. Promoting Strong Metal Support Interaction: Doping ZnO for Enhanced
Activity of Cu/ZnO:M (M = Al, Ga, Mg) Catalysts. ACS Catal. 5, 3260–3270 (2015).
24. Guidelli, E. J., Baffa, O. & Clarke, D. R. Enhanced UV Emission From Silver/ZnO And
Gold/ZnO Core-Shell Nanoparticles: Photoluminescence, Radioluminescence, And Optically
Stimulated Luminescence. Scientific Reports 5, 14004 (2015).
25. Jiang, T., Qin, X., Sun, Y. & Yu, M. UV photocatalytic activity of Au@ZnO core–shell
nanostructure with enhanced UV emission. RSC Advances 5, 65595–65599 (2015).
26. Prabhakar, R. R., Pramana, S. S., Karthik, K. R. G., Sow, C. H. & Jinesh, K. B. Ultra-thin
conformal deposition of CuInS2 on ZnO nanowires by chemical spray pyrolysis. Journal of
Materials Chemistry 22, 13965 (2012).
127

27. Zou, X., Fan, H., Tian, Y. & Yan, S. Synthesis of Cu2O/ZnO hetero-nanorod arrays with
enhanced visible light-driven photocatalytic activity. CrystEngComm 16, 1149–1156 (2014).
28. Adhikari, S., Sarkar, D. & Madras, G. Highly efficient WO3–ZnO mixed oxides for
photocatalysis. RSC Adv. 5, 11895–11904 (2015).
29. Simon, Q. et al. Vertically oriented CuO/ZnO nanorod arrays: from plasma-assisted
synthesis to photocatalytic H2 production. J. Mater. Chem. 22, 11739–11747 (2012).
30. Deo, M. et al. Cu2O/ZnO hetero-nanobrush: hierarchical assembly, field emission and
photocatalytic properties. J. Mater. Chem. 22, 17055–17062 (2012).
31. Tak, Y., Hong, S. J., Lee, J. S. & Yong, K. Solution-Based Synthesis of a CdS
Nanoparticle/ZnO Nanowire Heterostructure Array. Crystal Growth & Design 9, 2627–2632
(2009).
32. Cao, X., Chen, P. & Guo, Y. Decoration of Textured ZnO Nanowires Array with CdTe
Quantum Dots: Enhanced Light-Trapping Effect and Photogenerated Charge Separation. J.
Phys. Chem. C 112, 20560–20566 (2008).
33. Gomez-Daza, O. et al. Mechanism of Formation of Highly Photosensitive CdSe/ZnO
Composite Coatings Obtained by Sintering CdSe / ZnCl2 Screen Printed Layers. J.
Electrochem. Soc. 148, G330–G335 (2001).
34. Liu, Y. et al. Preparation and photocatalytic activity of ZnO/Fe2O3 nanotube composites.
Materials Science and Engineering: B 194, 9–13 (2015).
35. Dutta, M., Thirugnanam, L., Trinh, P. V. & Fukata, N. High Efficiency Hybrid Solar Cells
Using Nanocrystalline Si Quantum Dots and Si Nanowires. ACS Nano 9, 6891–6899 (2015).
36. Liu, C.-Y., Holman, Z. C. & Kortshagen, U. R. Hybrid Solar Cells from P3HT and Silicon
Nanocrystals. Nano Lett. 9, 449–452 (2009).
37. Cheng, K.-Y., Anthony, R., Kortshagen, U. R. & Holmes, R. J. High-Efficiency Silicon
Nanocrystal Light-Emitting Devices. Nano Lett. 11, 1952–1956 (2011).
38. Cheng, K.-Y., Anthony, R., Kortshagen, U. R. & Holmes, R. J. Hybrid Silicon
Nanocrystal−Organic Light-Emitting Devices for Infrared Electroluminescence. Nano Lett.
10, 1154–1157 (2010).
39. Anthony, R. J., Cheng, K.-Y., Holman, Z. C., Holmes, R. J. & Kortshagen, U. R. An AllGas-Phase Approach for the Fabrication of Silicon Nanocrystal Light-Emitting Devices.
Nano Lett. 12, 2822–2825 (2012).
40. Maier-Flaig, F. et al. Multicolor Silicon Light-Emitting Diodes (SiLEDs). Nano Lett. 13,
475–480 (2013).

128

41. Liu, C.-Y., Holman, Z. C. & Kortshagen, U. R. Hybrid solar cells from P3HT and silicon
nanocrystals. Nano Letters 9, 449–52 (2009).
42. Maier-Flaig, F. et al. Multicolor silicon light-emitting diodes (SiLEDs). Nano letters 13,
475–80 (2013).
43. Malato, S., FernĂĄndez-IbĂĄĂąez, P., Maldonado, M. I., Blanco, J. & Gernjak, W.
Decontamination and disinfection of water by solar photocatalysis: Recent overview and
trends. Catalysis Today 147, 1–59 (2009).
44. Silva, F. V., Lansarin, M. A. & Moro, C. C. A Comparison of slurry and inmobilized TiO2
in the photocatalytic degradation of phenol. Latin American applied research 42, 275–280
(2012).
45. Udom, I. et al. A simple photolytic reactor employing Ag-doped ZnO nanowires for water
purification. Thin Solid Films 564, 258–263 (2014).
46. He, Z., Li, Y., Zhang, Q. & Wang, H. Capillary microchannel-based microreactors with
highly durable ZnO/TiO2 nanorod arrays for rapid, high efficiency and continuous-flow
photocatalysis. Applied Catalysis B: Environmental 93, 376–382 (2010).
47. McPeak, K. M. & Baxter, J. B. ZnO Nanowires Grown by Chemical Bath Deposition in a
Continuous Flow Microreactor. Crystal Growth & Design 9, 4538–4545 (2009).
48. Krivec, M., Žagar, K., Suhadolnik, L., Čeh, M. & Dražić, G. Highly Efficient TiO2-Based
Microreactor for Photocatalytic Applications. ACS Appl. Mater. Interfaces 5, 9088–9094
(2013).
49. Zhang, Y. et al. Synthesis, Characterization, and Applications of ZnO Nanowires, Synthesis,
Characterization, and Applications of ZnO Nanowires. Journal of Nanomaterials, Journal of
Nanomaterials 2012, 2012, e624520 (2012).
50. Kołodziejczak-Radzimska, A. & Jesionowski, T. Zinc Oxide—From Synthesis to
Application: A Review. Materials 7, 2833–2881 (2014).
51. Wei, Y. et al. Wafer-Scale High-Throughput Ordered Growth of Vertically Aligned ZnO
Nanowire Arrays. Nano Lett. 10, 3414–3419 (2010).
52. Mangolini, L., Thimsen, E. & Kortshagen, U. High-Yield Plasma Synthesis of Luminescent
Silicon Nanocrystals. Nano Lett. 5, 655–659 (2005).
53. Anthony, R. & Kortshagen, U. Photoluminescence quantum yields of amorphous and
crystalline silicon nanoparticles. Phys. Rev. B 80, 115407 (2009).
54. Felbier, P. et al. Highly Luminescent ZnO Quantum Dots Made in a Nonthermal Plasma.
Adv. Funct. Mater. 24, 1988–1993 (2014).

129

55. Holman, Z. C. & Kortshagen, U. R. A flexible method for depositing dense nanocrystal thin
films: impaction of germanium nanocrystals. Nanotechnology 21, 335302 (2010).
56. Anthony, R. J., Rowe, D. J., Stein, M., Yang, J. & Kortshagen, U. Routes to Achieving High
Quantum Yield Luminescence from Gas-Phase-Produced Silicon Nanocrystals. Adv. Funct.
Mater. 21, 4042–4046 (2011).
57. Mangolini, L., Jurbergs, D., Rogojina, E. & Kortshagen, U. Plasma synthesis and liquidphase surface passivation of brightly luminescent Si nanocrystals. Journal of Luminescence
121, 327–334 (2006).
58. Wei, Y. et al. Wafer-Scale High-Throughput Ordered Growth of Vertically Aligned ZnO
Nanowire Arrays. Nano Lett. 10, 3414–3419 (2010).
59. Greene, L. E., Yuhas, B. D., Law, M., Zitoun, D. & Yang, P. Solution-Grown Zinc Oxide
Nanowires. Inorg. Chem. 45, 7535–7543 (2006).
60. Kang, B. S., Pearton, S. J. & Ren, F. Low temperature (<100°C) patterned growth of ZnO
nanorod arrays on Si. Applied Physics Letters 90, 083104 (2007).
61. Udom, I., Ram, M. K., Stefanakos, E. K., Hepp, A. F. & Goswami, D. Y. One dimensionalZnO nanostructures: Synthesis, properties and environmental applications. Materials Science
in Semiconductor Processing 16, 2070–2083 (2013).
62. Cheng, B. & T. Samulski, E. Hydrothermal synthesis of one-dimensional ZnO
nanostructures with different aspect ratios. Chemical Communications 0, 986–987 (2004).
63. Zhang, S. et al. Hybrid ZnO/ZnS nanoforests as the electrode materials for high performance
supercapacitor application. Dalton Trans. 44, 2409–2415 (2015).
64. Kumaran, S. M. & Gopalakrishnan, R. Structural, optical and photoluminescence properties
of Zn1−xCexO (x = 0, 0.05 and 0.1) nanoparticles by sol–gel method annealed under Ar
atmosphere. J Sol-Gel Sci Technol 62, 193–200 (2012).
65. Sharma, R., Alam, F., K. Sharma, A., Dutta, V. & K. Dhawan, S. ZnO anchored graphene
hydrophobic nanocomposite-based bulk heterojunction solar cells showing enhanced shortcircuit current. Journal of Materials Chemistry C 2, 8142–8151 (2014).
66. Mitra, S., Patra, P., Chandra, S., Pramanik, P. & Goswami, A. Efficacy of highly waterdispersed fabricated nano ZnO against clinically isolated bacterial strains. Appl Nanosci 2,
231–238 (2012).
67. Damen, T. C., Porto, S. P. S. & Tell, B. Raman Effect in Zinc Oxide. Phys. Rev. 142, 570–
574 (1966).
68. Zhang, R., Yin, P.-G., Wang, N. & Guo, L. Photoluminescence and Raman scattering of
ZnO nanorods. Solid State Sciences 11, 865–869 (2009).

130

69. Furukawa, S. & Miyasato, T. Quantum size effects on the optical band gap of
microcrystalline Si:H. Phys. Rev. B 38, 5726–5729 (1988).
70. Doğan, İ. & Sanden, M. C. M. van de. Direct characterization of nanocrystal size distribution
using Raman spectroscopy. Journal of Applied Physics 114, 134310 (2013).
71. Duan, Y., Kong, J. F. & Shen, W. Z. Raman investigation of silicon nanocrystals: quantum
confinement and laser-induced thermal effects. J. Raman Spectrosc. 43, 756–760 (2012).
72. Kong, Y. C., Yu, D. P., Zhang, B., Fang, W. & Feng, S. Q. Ultraviolet-emitting ZnO
nanowires synthesized by a physical vapor deposition approach. Applied Physics Letters 78,
407–409 (2001).
73. Lyu, S. C. et al. Low temperature growth and photoluminescence of well-aligned zinc oxide
nanowires. Chemical Physics Letters 363, 134–138 (2002).
74. Liao, Z.-M. et al. Surface effects on photoluminescence of single ZnO nanowires. Physics
Letters A 372, 4505–4509 (2008).
75. Mangolini, L., Jurbergs, D., Rogojina, E. & Kortshagen, U. Plasma synthesis and liquidphase surface passivation of brightly luminescent Si nanocrystals. Journal of Luminescence
121, 327–334 (2006).
76. AngÄą, A. et al. Photoluminescence through in-gap states in phenylacetylene functionalized
silicon nanocrystals. Nanoscale 8, 7849–7853 (2016).
77. Shi, Z.-F. et al. Photoluminescence performance enhancement of ZnO/MgO heterostructured
nanowires and their applications in ultraviolet laser diodes. Physical Chemistry Chemical
Physics 17, 13813–13820 (2015).
78. RamĂ­rez-Ortega, D., MelĂŠndez, A. M., Acevedo-PeĂąa, P., GonzĂĄlez, I. & Arroyo, R.
Semiconducting properties of ZnO/TiO2 composites by electrochemical measurements and
their relationship with photocatalytic activity. Electrochimica Acta 140, 541–549 (2014).
79. Lee, S.-K. et al. Long lifetime of free excitons in ZnO tetrapod structures. Applied Physics
Letters 96, 083104 (2010).
80. Kwok, W. M., Djurišić, A. B., Leung, Y. H., Chan, W. K. & Phillips, D. L. Time-resolved
photoluminescence from ZnO nanostructures. Applied Physics Letters 87, 223111 (2005).
81. Nmez, E. & Meral, K. Enhancement of Photoluminescence Lifetime of ZnO Nanorods
Making Use of Thiourea. Journal of Nanomaterials 2012, e957035 (2012).
82. Johnson, J. C. et al. Ultrafast Carrier Dynamics in Single ZnO Nanowire and Nanoribbon
Lasers. Nano Lett. 4, 197–204 (2004).
83. Dan’ko, V., Michailovska, K., Indutnyi, I. & Shepeliavyi, P. Photoluminescence decay rate
of silicon nanoparticles modified with gold nanoislands. Nanoscale Res Lett 9, 165 (2014).
131

84. Dutta, M., Thirugnanam, L., Trinh, P. V. & Fukata, N. High Efficiency Hybrid Solar Cells
Using Nanocrystalline Si Quantum Dots and Si Nanowires. ACS Nano 9, 6891–6899 (2015).
85. Biswal, N., Das, D. P., Martha, S. & Parida, K. M. Efficient hydrogen production by
composite photocatalyst CdS–ZnS/Zirconium–titanium phosphate (ZTP) under visible light
illumination. International Journal of Hydrogen Energy 36, 13452–13460 (2011).
86. Cho, I.-H., Park, J.-H. & Kim, Y.-G. Oxidative Degradation and Toxicity Reduction of
Trichloroethylene (TCE) in Water Using TiO2/Solar Light: Comparative Study of TiO2
Slurry and Immobilized Systems. Journal of Environmental Science and Health, Part A 40,
1033–1044 (2005).

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CHAPTER 5
IMMOBILIZED FLOATING PHOTOCATALYSTS: A NOVEL CONFIGURATION OF
ZNO/PDMS HETEROSTRUCTURES
5.1 Introduction
ZnO nanostructures have attracted considerable attention as active components in
electronic devices [1–3], water splitting [4–6] and environmental remediation [7–9] thanks to their
excellent electrical, optical and chemical properties. Especially, a growing interest in water
decontamination via photocatalytic degradation promotes the further development of ZnO
nanostructures as photocatalysts. The state-of-the-art reports have demonstrated the remarkable
performance of ZnO in decomposing organic pollutants [10–13]. Electrons and holes can be excited
on ZnO nanostructures when absorbing appropriate energies from photons, which reacting with
water and oxygen in an aqueous solution are able to produce hydroxyl radicals (¡OH) and
superoxide anions (O2¡-), respectively. These two chemical groups working as the primary
oxidants are capable of decomposing organic contaminants into water, carbon dioxide and other
inorganic molecules through complex chemical reactions. There are two basic kinds of
photocatalysts can be recognized in term of their configurations: (i) slurry photocatalysts [ 14–16]
and (ii) immobilized photocatalysts, which involves immobilized films/nanocomposites [17–19] and
immobilized vertically-grown NWs [20–22]. Slurry photocatalysts, including particles and 1D
nanostructures, are the most common studied configuration, which is considered possessing high
degradability thanks to their large surface-area-to-volume ratios and high homogeneity in aqueous
solution. With proper modification the superior photocatalytic activities of these photocatalysts
can be achieved which are comparable to that of Degussa P25 [23–25]. However some challenges
such as easy-agglomeration, complicated recovery from liquids and increased absorption along the
penetration depth of UV light hindered these photocatalysts in practical use. As an alternative
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route, immobilized film-like photocatalysts are preferable to practical option. This type of
configurations possess great recyclability, whereas their limited active surface area and masstransfer restraint became additional drawbacks. Recently, photoreactor, especially microchannelbased photoreactor, is suggested as one effective way to improve the performance of immobilized
photocatalysts by increasing the surface-to-volume ratio and mass transfer and reducing the
molecular diffusion distance. These photoreactors serving as tools for the degradation of organic
pollutants have been demonstrated with excellent reproducibility, stability and degradability [ 26–
29

]. Moreover, the modification towards this design, such as employing immobilized NW

photocatalysts, further improves its photodegradation rate owing to the larger active surface area
in comparison with that of film [30,31]. However most of these reactors require external power to
drive polluted solution across the reaction chamber, and specific platform is usually built to bring
it closer to the light source, which inevitably increase the cost in scalable water cleanliness. In this
regards, a configuration possessing immobilized NW-like photocatalysts both autonomously
floating and freely moving atop the water is desired to improve the utilization of sunlight and
release the mass-transfer issue.
In this paper we report a configuration with ZnO NWs vertically grown on the flexible
polymer substrate (ZP film). The thin polymer was transparent and able to freely move on top of
water while floating. The ZnO NWs were securely fixed on the polymer substrate via a two-step
synthesis (hydrothermal growth on rigid substrate and second hydrothermal growth on flexible
substrate), where ZnO NWs had the uniform distribution and ordered orientation on the entire
surface. The photocatalytic activities of the floatable configuration before and after the
secondary growth of ZnO NWs were investigated to reveal the impact of surface area to
photocatalysts. Two different pollutants, MB dye and crude oil, were employed to evaluate ZP

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film. And the durability capability of the floatable configuration towards particular pollutant was
also examined. To date, other than the conventional external stimuli that utilizing photons to
activate photocatalysts, piezoelectric effect has also been demonstrated to be effective to dyedecomposition. In view of the above, numerous photocatalysts basing on the mechanism of
piezocatalysis have been investigated [32–35]. In particular, ZnO as a superior piezoelectric and
photocatalytic material whose piezo-photocatalytic mechanism have been revealed [36–38]. In this
study the piezoelectric and photocatalytic properties of ZP film in MB dye decomposition was
also verified and a proper configuration in using ZP film was also discussed.

5.2 Experimental
5.2.1 Synthesis of ZnO nanowire array
All chemicals were of analytical grade and used without further purification. ZnO NWs
was synthesized via the hydrothermal method as previously described by others. In the first step,
10 mM Zn(CH3COO)2¡2H2O (99.9% Sigma Aldrich) was dissolved into methanol (Fisher
Scientific). Followed by vigorous stirring at 60°C for 2 hours, the mixed solution was spin-coated
onto a glass substrate at 1000rpm for 30 seconds. Prior to the seed deposition, the glass substrate
was cleaned by acetone, isopropanol alcohol, and deionized (DI) water, consecutively. A
subsequent annealing was carried out for the seeded glass substrate at 150°C for 1 hour for
dehydration. The growth solution of 25 mM Zn(NO3)2¡6H2O (Sigma Aldrich) and 25 mM
hexamethylenetetramine (HMTA, Sigma Aldrich) was prepared and preheated in convection oven
at 90°C for 1 hour to achieve the thermal equilibrium. The annealed substrate was then immersed
into the growth solution at 90°C for 6 hours. Consequently, vertically-grown ZnO NWs were
obtained on the glass substrate after thoroughly rinsed by DI water.

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5.2.2 Secondary growth of ZnO nanowire array
In the second step, a certain amount of Polydimethylsiloxane (PDMS) solution (10 parts
of SYLGARD 184 pre-polymer and 1 part of the curing agent, Dow Corning Co.) was fully
covered and spin-coated on the NW-grown glass substrate. The spin-coating speed was varied in
term of the desired thickness of the PDMS layer. As an example, spin-coating of 1000rpm for 90
seconds produced a PDMS film of around 300 Âľm thickness. The PDMS layer was thermally cured
at 60°C for 12 hours and cooled to the room temperature overnight. The PDMS layer was then
detached from the glass substrate. To facilitate the peeling of PDMS, we utilized a polyester (PET)
film as a backing layer. The PET film was brought to intimate contact with the PDMS layer prior
to its detachment. The as-fabricated PET/PDMS stack was subsequently placed atop the ZnO
growth solution with the embedded NWs facing down at 70°C for 12 hours. The sample was
thoroughly rinsed in DI water and completely dried. Finally, the PET film was separated from the
PDMS layer to obtain the vertically-aligned ZnO NW array on the PDMS membrane (ZP film).

5.2.3 Materials characterization
The morphology, crystallinity and composition of ZnO NWs both on glass and PDMS
substrate were characterized by scanning electron microscopy (SEM, JEOL 6610LV, Hitachi S4700II), X-ray diffraction (XRD, Bruker-AXS) energy-dispersive X-ray spectrometer (EDS,
JEOL 6610LV), and photoluminance spectrometer (PL, Ocean Optics, Inc. USB spectrometer
and optical fiber) with the excitation laser at 337 nm.

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5.2.4 Photocatalytic assessment and activities
Under UV light: Photocatalytic degradation was carried out in an aluminum foil-sealed
case with a magnetic stirrer stage inside. Two 9 W florescent tubes (Philips Îť=375 nm) were placed
30cm above the dye solution container. A 10mM methylene blue (MB, 1.5%, Sigma-Aldrich)
aqueous solution was prepared as a model polluted water. Four pieces of ZP film (1” x 1” per one
piece) were utilized afloat on top of 40mL of the dye solution (with the ZnO NW side facing
down). Prior to the irradiation, the dye solution was kept in dark for 30min to establish the
absorption/desorption equilibrium. A 1-mL of solution was extracted from the container in every
30 min as the photodegradation progressed. The extracted solution was centrifuged at 10000 rpm
to remove any impurities and examined for the absorbance in a UV-Vis spectrometer (Lambda).
The absorption peak at 667 nm corresponds to MB and the peak height can be correlated to the
MB concentration. The photodegradation experiment was repeated three more times under the
same working condition to test reusability/recyclability. The same degradation experiment (but
only one 1” x 1” ZP film used for comparison) was carried out to evaluate the role of the secondary
growth of ZnO NWs on its photocatalytic performance. The mechanical stability of ZnO NWs on
the PDMS film allowed us to bend and stretch the film and place it on the surface of a small rod
(8 mm). Experimental setup details for the photodegradation experiment using the overhead mixer.
For testing the vertically-aligned ZnO NWs as piezocatalysis, ultrasonic bath (FisherScientific,
160W, 40KHz) and overhead mixer (Dragon Lab, OS20-S) were used to vibrate ZnO NWs when
1 piece of ZP (1” x 1”) were utilized to treat the dye solution (40 mL). In the test of oil degradation,
each sample had 50mg crude oil (Saudi Arabian Berri crude oil) dispersed onto 2 ml DI water. 1
piece of ZP film (circular, 16 mm-diameter) was utilized floating on top of the oil solution. After

137

degradation, 5ml Petroleum ether (Sigma-Aldrich) was used to dilute the rest crude oil prior to
UV/Vis measurement.

5.2.5 Residue inspection
In the end of the experiment, the remaining MB dye solution was filtrated through a filter
paper (MCE, mixed cellulose ether membrane, hydrophilic, 0.22 Âľm, Millipore Co.) in order to
check if the ZnO NWs were dislodged from the PDMS substrate and re-suspended into the
solution. A custom vacuum filtration setup composed of a filter holder, a solvent trap and a vacuum
pump (Alcatel 2021I) was built. After the filter was positioned on the holder, the dye solution was
poured over the filter membrane. A vacuum pressure was applied until all the solutions were
completely filtrated. The filter membrane was kept overnight for the negative pressure to be fully
diminished and for the membrane to be completely dried. The filter membrane was further
dehydrated at 60°C afterwards. Finally, the filter membrane was imaged under the optical
microscope and SEM with 3nm W deposited to search the NWs.

5.3 Result and discussion
In a typical hydrothermal synthesis process, the substrate is deposited with a seed layer
by various means prior to the growth of vertically-aligned ZnO NWs. Therefore, the structural
stability of NWs onto the substrate is often dependent on the adhesion between the seed layer
and the substrate. When a rigid inorganic substrate (e.g. glass, silicon etc.) is used, the seed layer
can be thermally treated at relatively high temperatures (often over 200°C), which significantly
improves the adhesion. However, this high-temperature treatment cannot be applied for polymer

138

substrates. Therefore, the NWs directly grown on the polymer substrate typically suffer from
weak adhesion and are easy to be detached from the substrate.

Figure 5.1. Schematic of the proposed two-step fabrication process of a vertically-aligned ZnO
NW array on a polymer substrate. (a) A ZnO NW array was vertically grown on the glass slide via
the hydrothermal synthesis, (b) a certain amount of a PDMS solution was spin-casted onto the
ZnO NW array and thermally cured afterwards, (c) a PET film was brought into intimate contact
with the PDMS film, (d) the PDMS/PET stack was detached from the glass slide simultaneously,
(e) the PDMS/PET layer became flat due to the recovery of PET film, (f) the ZnO NW array
embedded in the PDMS film was re-grown to a desired length via the same hydrothermal process,
(g) ZP film was obtained after the PET film was removed from the PDMS film, (h) the photograph
of as-prepared ZP film (1” x 1”).
Our approach to improving NWs’ adhesion to the polymer substrate is to utilize the two
step hydrothermal growth and secure the NWs by partially embedding them into a polymer matrix.
Figure 5.1 shows a schematic of the proposed method to fabricate a robust ZP film via the twostep, hydrothermal synthesis. In the first synthesis step (Figure 5.1a), vertically-aligned ZnO NWs
with reasonable aspect ratios were grown from a seeding layer on a glass substrate [ 39]. A thin
PDMS layer was then uniformly spin-coated onto the ZnO NWs to fully encapsulate the NWs
(Figure 5.1b). Here the thickness of the PDMS layer could vary from 200 to 500 Âľm depending on
a different spin-coating speed. Prior to the detachment of the PDMS layer from the glass substrate,

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a PET film brought into conformal contact on PDMS surface to increase the stiffness of the PDMS
layer (Figure 1(c-d)). The PET film was employed since the PDMS layer was too thin to be
detached from the glass substrate undamaged. While the molded PDMS encapsulated the majority
parts of ZnO NWs, their tips remained exposed and served as a seeding layer for regrowth. The
PDMS/PET stack were placed on top of the same growth solution with the NW side facing down
for the secondary hydrothermal synthesis (Figure 5.1(e-f)). Since PDMS/PET is lighter than water,
the stacked film was afloat during the second growth step. The stand-alone ZP film was obtained
after the separation of the PET film (Figure 5.1g). In the single-step hydrothermal synthesis, the
adhesion strength of NWs to the substrate is fixed by the interfacial strength between the seed layer
and the substrate. Therefore, longer NWs are difficult to be grown as they are easily detached from
the substrate. The parts of the NWs are securely anchored into PDMS, allowing the NWs to grow
much longer than ones generated by the single-step process.

Figure 5.2. SEM images of the ZnO NWs (a) on the glass substrate after first growth, and (b) on
the PDMS before secondary growth, (c) on the PDMS substrate (~300 Âľm in thickness) after
secondary growth and (d) on the PDMS substrate after secondary growth (failed detachment).
Scale bar: (a) and (c) 50 Âľm in the large view and 3 Âľm in the inset, (b) 20 Âľm in the large view
and 5 Âľm in the inset, (d) 50 Âľm in the large view and 2 Âľm in the inset.
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Figure 5.3. (a) XRD patterns of ZnO NWs, (b) photoluminescence spectrum of ZnO NWs.
Figure 5.2 shows the SEM images of the ZnO NW array at each key process. In Figure
5.2a, it is observed that the highly ordered ZnO NWs are uniformly distributed on a glass substrate.
Figure 5.2b is the ZP film before the secondary growth, showing that the tips of broken ZnO NWs
were exposed and could serve as a fresh seeding layer. Moreover, the re-grown NWs on the PDMS
substrate (Figure 5.2c) preserved the NW density similar to that on the glass substrate (Figure
5.2a), demonstrating the successful regrowth of the majority of ZnO NWs. Note that the re-grown
NWs would follow the same crystalline orientation as that of broken NWs. A ZP film in failed
detachment is shown in Figure 5.2d, where the sparse NW array was obtained. The space between
NWs represented the loss of NWs in the detachment of ZP film. The hexagonal shapes revealed
the ZnO NWs with an average diameter of about 80 nm. It is worth noting that the morphology of
the as-prepared ZnO NWs on both the glass substrate and PDMS layer were highly affected by
factors such as reaction duration, temperature and solvent concentration as reported in other
literatures [40–42].
The detailed crystalline quality of ZnO NWs on the glass (after the first growth) and PDMS
(after the secondary growth) substrates was characterized by XRD analysis and photoluminescence
(PL) spectrum. As depicted in Figure 5.3a, the XRD patterns of ZnO NWs were recorded from
25° to 75°. It is clearly seen that each diffraction peak from the glass substrate has the

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corresponding peak from PDMS substrate at the same position, implying that the proposed twostep process offered the coherent crystalline structure of ZnO NWs. Additionally, all the diffraction
peak on both substrates can be indexed to the wurtzite ZnO phase, where the peaks with the strong
intensity indicate the c-axis preferred orientation of ZnO NWs [25,43]. The PL spectrum of ZnO
NWs on both substrates (Figure 5.3b) has a good agreement with the previous reports [20,24], where
the near-band-edge (NBE) emission and defect-related peaks are observed around 390 nm and 490
nm, respectively. It is worth noting that the intensity of the defect-related peak is comparable to
that of the NBE peak, denoting the significant defect level of the NWs. The defective NWs may
be beneficial to their photocatalytic activity owing to the decreased surface-state-related
recombination of photogenerated electro-hole pairs [24]. More interestingly, the ratio of the NBE
peak to defect-related peak on glass is close to that on PDMS, which also reveals the coherent
crystal structure of ZnO NWs after the secondary growth. Both XRD analysis and PL spectrum
confirm that the two-step process serving as an in-situ growth method enables the ZnO NWs to
grow consistently with the predetermined state.
ZnO as photocatlaysts have been intensively studied in last few decades [15,44]. Here in
order to investigate the photocatalytic activity of the as-prepared ZnO NWs on the PDMS film,
MB was chosen as a probe pollutant, and its time-dependent decomposition under UV irradiation
(~375 nm) was measured using the UV/Vis spectrometer. In this experiment, the ZP films were
placed afloat with the ZnO NWs facing down towards the MB solution. It is known that PDMS is
transparent to near-UV, and therefore, the UV light penetrates the PDMS layer and activate the
underneath ZnO NWs spontaneously [45–47]. The dye solution was kept in dark condition for half
an hour to achieve the absorption/desorption equilibrium. The UV light irradiation was
subsequently established in perpendicular and with a certain distance to floated ZP films. During

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the experiment the absorbance of solution was measured in constant intervals (see Figure 5.4).
After 3-hour-long experiment, the ZP films were dehydrated and reused for photodegradation in
another fresh MB solution. This process was repeated for three times, and each corresponding
UV/Vis absorbance was recorded at the same interval. Figure 5.4a shows the degradation profiles
of MB in the presence of the ZP films for four consecutive cycles. C/C0 on the Y axis stands for
the ratio of the degraded concentration to the initial concentration of the pollutants, which was
calculated from the absorbance of the MB solution using Bouguer–Lambert–Beer law [26,48–50]. It
is clearly seen that the degradation percentage of MB after 3 hours are 74.9%, 82.1%, 70.6% and
71.4%, indicating the reasonable reusability of the ZP films. We argue that the anchoring of ZnO
NWs into the PDMS film indeed helped the NWs to be mechanically robust and thus recyclable.
This reliable immobilization of ZnO NWs was also demonstrated by inspecting the remaining
degraded solution. No NWs were found in the degraded solution after being centrifuged or
filtrated.
The degradation kinetic of MB is used to evaluate the performance of photocatalysts.
Figure 4(b) displays the corresponding degradation kinetics of MB dye in four consecutive cycles.
According to the pseudo first-order kinetic model, that is, -ln(C/C0)=k*t, the apparent reaction
rate constant k of the degradation in four consecutive cycles (Figure 5.4c) was 0.008, 0.0091,
0.0071 and 0.007 min-1, respectively [24,51–53]. The correlation coefficient (R2) in the fitting were
all greater than 0.97, suggesting that the photocatalytic activity of the as-prepared ZP films in four
cycles were highly durable. The studies in degradation and kinetics confirm the excellent
reusability of the ZP films. The discrepancy of the degradation rates in four cycles may ascribe to
the measurement errors as well as residual intermediates absorbed on the NW surface, leading to
the loss of active surface area of NWs for the decomposition.

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Figure 5.4. (a) degradation profiles of MB dye (at 667nm) in four consecutive cycles, (b)
photodegradation kinetics of MB dye in four consecutive cycles, (c) constant k.

The recyclability of the immobilized photocatalytic system is governed by the stability and
adhesion of photocatalysts to the immobilized substrate. In the present configuration, the PDMS
substrate is used to hold ZnO NWs in place and keep them afloat. The floating immobilized system
entails the substrate whose density is smaller than water, i.e., plastic or polymeric materials and
the immobilization method to integrate photocatalysts onto those organic substrates. The previous
fabrication approach to the floating immobilized photocatalytic system includes the physical vapor
deposition, electrospinning, and spray/dip coating. But these photocatalysts are likely to fall off
from the substrate during handling and repeated operation due to the weak adhesion. To further

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prove the superior structural stability of the proposed immobilized system, we put the ZP film to
various operating conditions such as bending, stretching, contact, and temperature variation.

Figure 5.5. SEM images of the ZP film (a) before thermal and mechanical test and (b) after thermal
and mechanical test. (c) and (d) are the photographs of ZP films (1”x1”) imaged in (a) and (b),
respectively. Scale bar: 10 Âľm in the large view and 1 Âľm in the inset.
Figure 5.5 (a-b) shows the SEM images of the ZnO NWs on the PDMS film. Upon
receiving photons with energy higher than the bandgap of ZnO, ZnO NWs fixed on PDMS
substrate are capable of generating electron-hole pairs, which convert O2 and H2O into O2-¡ and
¡OH, respectively. As reported, these oxidants are able to decompose organic pollutants (e.g. MB
dye) into clean water and carbon dioxide gas [15,16,54]. Like other semiconductor-based
photocatalysts, the photodegradation rate of ZP film was significantly affected by the working
temperature [55]. For evaluation of ZP film the dye solution was maintained under room
temperature during the entire degradation experiment by placing the UV source far away. On the
other hand, consider the thermal stability ZP film was tested in relatively high temperature
solution. In detail ZP film was characterized before and after soaked in 60°C aqueous solution for

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12 hours. As a result, ZP film retained same NW density as well as shape (see Figure 5.5 (c-d))
thanks to the reliable immobilization, which we believe would well accommodate to nature
environment for practical application.

Figure 5.6. Schematics of ZP film (a) before secondary growth of ZnO NWs, (b) after secondary
growth of ZnO NWs, (c) degradation profiles of MB dye (at 667 nm) before/after secondary
growth, (d) degradation profiles of MB dye with/without magnetic stirring.
Many factors play important roles in photodegradation, including active surface areas of
catalysts and mass transfer rate [56]. These two factors are particularly important in the
immobilized photocatalytic system because catalysts fixed on the substrate present much lower
active surface areas and reactants need to diffuse to the substrate surface (thus higher mass transfer
resistance) compared to the slurry suspension-based catalytic system. First, the effect of the surface
area on the degradability can be evaluated by comparing two immobilized systems with different
wire lengths. The schematics shown in Figure 5.6a and b illustrate the ZP film with two different
active surface areas. It is anticipated that the ZP film with the secondary growth would have higher
surface areas of the ZnO NWs exposed to the environment in comparison to the sample without

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the secondary growth and thus present higher photocatalytic degradability. Figure 5.6c shows the
photodegradation performance for these two immobilized systems with and without the secondary
growth of ZnO NWs. As expected, the ZP film after the secondary growth exhibited a higher
degradation rate. Since the degradability experiments were performed in the identical condition,
the discrepancy in photodegradation performance can be attributed to the different active surface
areas of two films. In other words, the improvement of the degradation rate was made by the longer
length of secondary-grown ZnO NWs, rendering larger active surface areas for light absorption
and pollutant decomposition. Secondarily, another important factor limiting the degradation
performance in immobilized photocatalytic systems is a slow mass transfer rate, i.e., sluggish
transport of reactants and products to and from the catalyst surface. The effect of mass transfer on
the degradability was investigated with the assistance of magnetic stirring (at 700rpm, see Figure
5.6d). The degradation profiles in Figure 5.6d present that the ZP films performed enhanced
degradability. Under the same working condition, this improvement was mainly derived from the
magnetic stirring, which was utilized to relax the mass transfer limitation during the degradation
process. Comparing the results, ZP film with secondary growth in Figure 5.6c and Figure 5.6d, it
can be concluded that the mass transfer rate has a much significant impact on the degradability of
ZP film. The combination of ZP film with secondary growth and magnetic stirring is favored in
our experiment.
The strong adhesion of NWs to the substrate enables us to explore a new application, that
is, an immobilized ZP film as piezocatalysts. It is well known that ZnO is a piezoelectric material,
i.e., electric charge generated and accumulated in response to applied mechanical stresses.
Degradation of organic pollutants by piezocatalysts has been previously demonstrated in a slurry
(or suspension) form. Since ZnO NWs are securely adhered to the substrate, large stresses/strains

147

can be applied to the ZP film, facilitating immobilized piezocatalysts. In detail, The ZnO bend or
stretch and mount it onto a circular rod, which was rotating with the rod and ultrasonicated at the
same time. As comparison, another ZP film was floating atop the polluted water which only has
ultrasonic field applied. Even more interestingly, the immobilized floating catalysts can be used to
degrade pollutant faster. The schematics of ZP films with/without ultrasonication for the
investigation of piezocatalysis are illustrated in Figure 5(a-b), where the bended ZnO NWs was
caused by ultrasonic-induced water pressure [32].

Figure 5.7. Schematics of ZP film (a) without vibration, (b) with vibration, (c) degradation profiles
of MB dye (at 667nm) in dark condition with ultrasonication, (d) degradation profiles of MB dye
(at 667nm) under UV light irradiation with ultrasonication.
The piezocatalytic activity of ZP film with vibration is shown in Figure 5.7c (blue line).
Under the presence of an ultrasonic field, 25% of MB dye was decomposed by a floating ZP film
in a dark condition while the reference solution was kept unchanged (red line), proving the
effectiveness of piezocatalysis to MB dye. The degradability of this ZP film was increased
significantly when it was further assisted by UV illumination (Figure 5.7b, blue line), confirming

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the piezo-photocatalytic mechanism of ZnO NWs as reported [36]. The piezo-photocatalytic
mechanism and potential reactions are illustrated in Figure 5.8. As can be seen that the
piezocatalysis performed two functions in the degradation process, including excitation of
electron-hole pairs and separation of photoinduced charge carriers. Owing to the existence of an
increased number of charge carriers, more reactive oxidants were converted hence elevated the
capability of degradation eventually. It is known that the performance of piezocatalysts is highly
governed by vibration frequency [36].

Figure 5.8. Schematic of pizeo-photocatalytic mechanism of ZP film.

As comparison, a setup was built for a ZP film to further experience enhanced vibration
frequency. In the experiment the ZP film was wrapped around a glass rod which was attached on
a rotator as presented in Figure 5.9. The fixed ZP film in the entire degradation process was
ultrasonicated while being rotated (at 700 rpm) simultaneously. Consequently, the immobilized
ZP film on the rod obtained a higher degradation rate than the floating one, which indicates that
an increment of vibration frequency was applied to ZnO NWs. Likewise, the immobilized ZP film
on the rod performed better degradability than the floating one under UV irradiation which may
be attributed to the enhancement of piezo-photocatalytic efficiency. Notably, the discrepancy of
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degradation percentage between fixed ZP film and floated one in dark condition is slightly larger
than that in UV illumination condition, which may ascribe to the efficiency of light utilization. As
mentioned above, the floated ZP film has optimal configuration to utilization the incident light
while the fixed one has limited area as light irradiating vertically. The efficiency loss derived from
this diversity was diluted by improved piezocatalysis.

Figure 5.9. Schematics of (a) mixer setup and (b) float setup, (a1) and (a2) the photograph of
mixer ZP film used in (a), (b1) and (b2) are the photograph floated ZP film used in (b). All the ZP
films are in 1” x 1”.
As of now the considerable efforts have been devoted to treat pollutants from industrials
and agriculture. In the field of photocatalysis the immobilized photocatlaysts are regarded as
promising candidates in removing floating pollutants (e.g. oil spills). In this study, crude oil was
used as another probe pollutant to evaluate the photodegradation capability of the ZP film, where
150

the decomposition was validated by monitoring the concentration of particular components. Figure
5.10a shows the spectral changes of crude oil measured through UV/Vis spectrometer. As can be
seen that crude oil possessed many characteristic peaks due to its complex composition. All peaks
were weakened as the irradiation time increased, indicating the various organic components were
indeed degraded by the ZP film. To clearly analyze the degradation performance of ZP films,
degradation profile of crude oil was plotted as function of an irradiation time for two peaks at 288
nm and 293 nm, respectively (Figure 5.10b). The concentration of crude oil was observed declining
around 20% in 6 hours in the presence of ZP film. In addition, the impact of UV light alone on
decomposing this crude oil was found insignificant (Figure 5.10b, blue line).

Figure 5.10. (a) spectral changes of crude oil UV light irradiation, (b) degradation profiles of
crude oil at 288nm and 293nm under UV light irradiation.

Figure 5.11. crude oil samples (a) before and (b) after degradation
The corresponding crude oil samples before and after degradation are displayed in Figure
5.11 (a-b). In Figure 5.11a, a circular ZP film was placed on top of crude oil layer, which had
151

approached absorption/desorption equilibrium prior to the experiment. After 6-hour degradation
process, the oil concentration was clearly observed to be reduced (see color gradient in Figure
5.11b) after diluted by the same amount of petroleum ether, suggesting the valid degradation
process by means of ZP films.

5.4 Conclusion
In this paper we fabricated a new type of immobilized photocatalysts for the
photodegradation. The ZnO NWs securely fixed on the flexible substrate has been proposed
possessing some advantages: (i) avoided the NW re-suspension, (ii) increased the active surface
area, (iii) increase the utilization of the incident light and (iv) facilitated the mass transfer. As a
result, the floating ZP film exhibited a considerable degradability and reusability against MB dye.
According to the degradation mechanism, since the redox reaction only occurs at the interface of
the solution and the NW, the longer exposed NW part may result better photocatalytic
performance. In addition, a ZP film with more embedded NWs may also have a greater
degradability. In the experiment we also investigated the piezocatalytic and photocatalytic
properties of ZP film. Meanwhile, a proper setup was proposed to effectively improve
piezocatalytic capability of ZP film. Considering the practical challenge in treating floatable
pollutants, crude oil as a model pollutant was tested by ZP film. The result confirmed a slow but
valid photocatalytic degradation process took place in the presence of ZP film. For better
performance, the high power light source needs to be introduced to accelerate the process. Sunlight
irradiation and other types of pollutants need to be introduced to further evaluate ZP film in future
work.

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REFERENCES

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REFERENCES

1. Shi, Z.-F. et al. Epitaxial growth of vertically aligned ZnO nanowires for bidirectional directcurrent driven light-emitting diodes applications. CrystEngComm 17, 40–49 (2015).
2. PauportĂŠ, T. et al. Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT
Study, and Application to Light-Emitting Diode. ACS Appl. Mater. Interfaces 7, 11871–
11880 (2015).
3. Ren, X. et al. White Light-Emitting Diode From Sb-Doped p-ZnO Nanowire Arrays/n-GaN
Film. Adv. Funct. Mater. 25, 2182–2188 (2015).
4. Joo Lee, H., Shin, S.-H., Tae Nam, K., Nah, J. & Hyung Lee, M. Spontaneously polarized
lithium-doped zinc oxide nanowires as photoanodes for electrical water splitting. J. Mater.
Chem. A 4, 3223–3227 (2016).
5. Xu, Y.-F. et al. In situ formation of zinc ferrite modified Al-doped ZnO nanowire arrays for
solar water splitting. J. Mater. Chem. A 4, 5124–5129 (2016).
6. HernĂĄndez, S. et al. Comparison of photocatalytic and transport properties of TiO 2 and ZnO
nanostructures for solar-driven water splitting. Phys. Chem. Chem. Phys. 17, 7775–7786
(2015).
7. Chang, Y.-H., Chiang, M.-Y., Chang, J.-H. & Lin, H.-N. Enhanced visible light
photocatalysis of cuprous oxide nanoparticle modified zinc oxide nanowires. Mater. Lett.
138, 85–88 (2015).
8. Ko, F.-H., Lo, W.-J., Chang, Y.-C., Guo, J.-Y. & Chen, C.-M. ZnO nanowires coated
stainless steel meshes as hierarchical photocatalysts for catalytic photodegradation of four
kinds of organic pollutants. J. Alloys Compd. 678, 137–146 (2016).
9. Solis-Pomar, F. et al. Rapid synthesis and photocatalytic activity of ZnO nanowires obtained
through microwave-assisted thermal decomposition. Ceram. Int.
doi:10.1016/j.ceramint.2016.08.084
10. Lee, K. M., Lai, C. W., Ngai, K. S. & Juan, J. C. Recent developments of zinc oxide based
photocatalyst in water treatment technology: A review. Water Res. 88, 428–448 (2016).
11. Mirzaei, A., Chen, Z., Haghighat, F. & Yerushalmi, L. Removal of pharmaceuticals and
endocrine disrupting compounds from water by zinc oxide-based photocatalytic degradation:
A review. Sustain. Cities Soc. 27, 407–418 (2016).
12. Lam, S.-M., Sin, J.-C., Abdullah, A. Z. & Mohamed, A. R. Degradation of wastewaters
containing organic dyes photocatalysed by zinc oxide: a review. Desalination Water Treat.
41, 131–169 (2012).

154

13. Liu, F., Leung, Y. H., Djurišić, A. B., Ng, A. M. C. & Chan, W. K. Native Defects in ZnO:
Effect on Dye Adsorption and Photocatalytic Degradation. J. Phys. Chem. C 117, 12218–
12228 (2013).
14. Johar, M. A. et al. Photocatalysis and Bandgap Engineering Using ZnO Nanocomposites,
Photocatalysis and Bandgap Engineering Using ZnO Nanocomposites. Adv. Mater. Sci. Eng.
Adv. Mater. Sci. Eng. 2015, 2015, e934587 (2015).
15. Mohd, A. M. A., Julkapli, N. M. & Abd, H. S. B. Review on ZnO hybrid photocatalyst:
impact on photocatalytic activities of water pollutant degradation. Rev. Inorg. Chem. 0,
(2016).
16. Sirelkhatim, A. et al. Review on Zinc Oxide Nanoparticles: Antibacterial Activity and
Toxicity Mechanism. Nano-Micro Lett. 7, 219–242 (2015).
17. Rashid, J., Barakat, M. A., Salah, N. & Habib, S. S. ZnO-nanoparticles thin films
synthesized by RF sputtering for photocatalytic degradation of 2-chlorophenol in synthetic
wastewater. J. Ind. Eng. Chem. 23, 134–139 (2015).
18. Tinacba, E. J. C., NuĂąez, J. A., Tumlos, R. B. & Ramos, H. J. ZnO/Zn and ZnO film
deposited via microwave atmospheric plasma jet as photo-catalyst for Rhodamine 6G dye
degradation. Thin Solid Films doi:10.1016/j.tsf.2016.07.018
19. Ghosh, A. & Mondal, A. Fabrication of stable, efficient and recyclable p-CuO/n-ZnO thin
film heterojunction for visible light driven photocatalytic degradation of organic dyes.
Mater. Lett. 164, 221–224 (2016).
20. Kuo, T.-J., Lin, C.-N., Kuo, C.-L. & Huang, M. H. Growth of Ultralong ZnO Nanowires on
Silicon Substrates by Vapor Transport and Their Use as Recyclable Photocatalysts. Chem.
Mater. 19, 5143–5147 (2007).
21. Chu, F.-H. et al. Well-aligned ZnO nanowires with excellent field emission and
photocatalytic properties. Nanoscale 4, 1471–1475 (2012).
22. Akhavan, O. Graphene Nanomesh by ZnO Nanorod Photocatalysts. ACS Nano 4, 4174–4180
(2010).
23. Adhikari, S., Sarkar, D. & Madras, G. Highly efficient WO3–ZnO mixed oxides for
photocatalysis. RSC Adv. 5, 11895–11904 (2015).
24. Wan, Q., Wang, T. H. & Zhao, J. C. Enhanced photocatalytic activity of ZnO nanotetrapods.
Appl. Phys. Lett. 87, 083105 (2005).
25. Ma, S., Li, R., Lv, C., Xu, W. & Gou, X. Facile synthesis of ZnO nanorod arrays and
hierarchical nanostructures for photocatalysis and gas sensor applications. J. Hazard. Mater.
192, 730–740 (2011).

155

26. Udom, I. et al. A simple photolytic reactor employing Ag-doped ZnO nanowires for water
purification. Thin Solid Films 564, 258–263 (2014).
27. McPeak, K. M. & Baxter, J. B. ZnO Nanowires Grown by Chemical Bath Deposition in a
Continuous Flow Microreactor. Cryst. Growth Des. 9, 4538–4545 (2009).
28. Molinari, R., Grande, C., Drioli, E., Palmisano, L. & Schiavello, M. Photocatalytic
membrane reactors for degradation of organic pollutants in water. Catal. Today 67, 273–279
(2001).
29. Mozia, S. Photocatalytic membrane reactors (PMRs) in water and wastewater treatment. A
review. Sep. Purif. Technol. 73, 71–91 (2010).
30. He, Z., Li, Y., Zhang, Q. & Wang, H. Capillary microchannel-based microreactors with
highly durable ZnO/TiO2 nanorod arrays for rapid, high efficiency and continuous-flow
photocatalysis. Appl. Catal. B Environ. 93, 376–382 (2010).
31. Zhang, Q., Zhang, Q., Wang, H. & Li, Y. A high efficiency microreactor with Pt/ZnO
nanorod arrays on the inner wall for photodegradation of phenol. J. Hazard. Mater. 254–255,
318–324 (2013).
32. Hong, K.-S., Xu, H., Konishi, H. & Li, X. Piezoelectrochemical Effect: A New Mechanism
for Azo Dye Decolorization in Aqueous Solution through Vibrating Piezoelectric
Microfibers. J. Phys. Chem. C 116, 13045–13051 (2012).
33. Lan, S. et al. Performance and Mechanism of Piezo-Catalytic Degradation of 4Chlorophenol: Finding of Effective Piezo-Dechlorination. Environ. Sci. Technol. 51, 6560–
6569 (2017).
34. Wu, J. M., Chang, W. E., Chang, Y. T. & Chang, C.-K. Piezo-Catalytic Effect on the
Enhancement of the Ultra-High Degradation Activity in the Dark by Single- and Few-Layers
MoS2 Nanoflowers. Adv. Mater. 28, 3718–3725 (2016).
35. Lin, H. et al. Piezoelectrically induced mechano-catalytic effect for degradation of dye
wastewater through vibrating Pb(Zr0.52Ti0.48)O3 fibers. Appl. Phys. Lett. 104, 162907
(2014).
36. Xue, X. et al. Piezo-potential enhanced photocatalytic degradation of organic dye using ZnO
nanowires. Nano Energy 13, 414–422 (2015).
37. Hong, D. et al. High Piezo-photocatalytic Efficiency of CuS/ZnO Nanowires Using Both
Solar and Mechanical Energy for Degrading Organic Dye. ACS Appl. Mater. Interfaces 8,
21302–21314 (2016).
38. Guo, X. et al. High-efficiency sono-solar-induced degradation of organic dye by the
piezophototronic/photocatalytic coupling effect of FeS/ZnO nanoarrays. Nanotechnology 27,
375704 (2016).

156

39. Xu, S. & Wang, Z. L. One-dimensional ZnO nanostructures: Solution growth and functional
properties. Nano Res. 4, 1013–1098 (2011).
40. Cui, J. Zinc oxide nanowires. Mater. Charact. 64, 43–52 (2012).
41. Kołodziejczak-Radzimska, A. & Jesionowski, T. Zinc Oxide—From Synthesis to
Application: A Review. Materials 7, 2833–2881 (2014).
42. Zhang, Y. et al. Synthesis, Characterization, and Applications of ZnO Nanowires, Synthesis,
Characterization, and Applications of ZnO Nanowires. J. Nanomater. J. Nanomater. 2012,
2012, e624520 (2012).
43. Chu, F.-H. et al. Well-aligned ZnO nanowires with excellent field emission and
photocatalytic properties. Nanoscale 4, 1471–1475 (2012).
44. Lee, K. M., Lai, C. W., Ngai, K. S. & Juan, J. C. Recent developments of zinc oxide based
photocatalyst in water treatment technology: A review. Water Res. 88, 428–448 (2016).
45. Ponnamma, D., Sadasivuni, K. K., Cabibihan, J.-J., Yoon, W. J. & Kumar, B. Reduced
graphene oxide filled poly(dimethyl siloxane) based transparent stretchable, and touchresponsive sensors. Appl. Phys. Lett. 108, 171906 (2016).
46. Martin, S. & Bhushan, B. Transparent, wear-resistant, superhydrophobic and
superoleophobic poly(dimethylsiloxane) (PDMS) surfaces. J. Colloid Interface Sci. 488,
118–126 (2017).
47. Cheng, T. et al. High-performance stretchable transparent electrodes based on silver
nanowires synthesized via an eco-friendly halogen-free method. J. Mater. Chem. C 2,
10369–10376 (2014).
48. Ingle, J. D. J. & Crouch, S. R. Spectrochemical analysis. (1988).
49. Di Paola, A., GarcĂ­a-LĂłpez, E., MarcĂŹ, G. & Palmisano, L. A survey of photocatalytic
materials for environmental remediation. J. Hazard. Mater. 211–212, 3–29 (2012).
50. Krivec, M., Žagar, K., Suhadolnik, L., Čeh, M. & Dražić, G. Highly Efficient TiO2-Based
Microreactor for Photocatalytic Applications. ACS Appl. Mater. Interfaces 5, 9088–9094
(2013).
51. Ma, S., Li, R., Lv, C., Xu, W. & Gou, X. Facile synthesis of ZnO nanorod arrays and
hierarchical nanostructures for photocatalysis and gas sensor applications. J. Hazard. Mater.
192, 730–740 (2011).
52. Lin, L. et al. A highly efficient TiO 2 @ZnO n–p–n heterojunction nanorod photocatalyst.
Nanoscale 5, 588–593 (2013).

157

53. Cheng, X., Deng, X., Wang, P. & Liu, H. Coupling TiO2 nanotubes photoelectrode with Pd
nano-particles and reduced graphene oxide for enhanced photocatalytic decomposition of
diclofenac and mechanism insights. Sep. Purif. Technol. 154, 51–59 (2015).
54. Lee, K. M., Lai, C. W., Ngai, K. S. & Juan, J. C. Recent developments of zinc oxide based
photocatalyst in water treatment technology: A review. Water Res. 88, 428–448 (2016).
55. Hu, Q., Liu, B., Zhang, Z., Song, M. & Zhao, X. Temperature effect on the photocatalytic
degradation of methyl orange under UV-vis light irradiation. J. Wuhan Univ. Technol.-Mater
Sci Ed 25, 210–213 (2010).
56. Morales Flores, N., Pal, U., Galeazzi, R. & Sandoval, A. Effects of morphology, surface
area, and defect content on the photocatalytic dye degradation performance of ZnO
nanostructures. RSC Adv. 4, 41099–41110 (2014).

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CHAPTER 6
MULTIFUNCTIONAL SOFT MICROMOTORS FOR PHOTOCATALYTIC
DEGRADATION
6.1 Introduction
Photocatalytic degradation is one promising approach to degrading organic contaminants
in water. Many photocatalysts are made of semiconductor nanostructures [1] and typically
employed in the form of suspended particles, namely a slurry system. Slurry photocatalysts are
considered to be an efficient configuration in photodegradation due to their large active surfacearea-to-volume-ratio and a high mass transfer coefficient. Despite the excellent degradation
performance towards numerous pollutants, these nanostructured photocatalysts suffer from the
drawbacks, such as particle agglomeration and more importantly high cost associated with
suspended catalyst separation/recycling limiting widespread adoption in industry. It has been
generally known that up to 70% of the total cost associated with photodegradation-based
wastewater treatment technology is attributed to the separation and recycling of catalyst particles.
Another photocatalytic platform is an immobilized photocatalyst system that has attracted
much attention thanks to the improved recyclability and potentials for large-scale utilization. One
common embodiment of the immobilized photocatalysts is an immobilized photocatalytic reactor
[2–6], whose walls are transparent and coated with photocatalytic materials. As long as
photocatalysts remain fixed in the walls, no extra step is needed to separate photocatalysts from
decontaminated water, significantly reducing an operation cost. However, the immobilized system
inherently possesses two drawbacks. First, surface areas of nanostructured photocatalysts are
significantly reduced or masked due to the surface immobilization. One-dimensional
nanostructures (e.g. nanowires and nanotubes) increase the active surface area compared to zerodimensional nanoparticles when immobilized on the surface. Second, pollutants in bulk water need
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to be transported to the reactor surface for degradation reactions, posing significant mass transport
limitation. Therefore, in photodegradation reactors, continuous flow and/or stirring are often
implemented to enhance the mass transfer coefficients [7–9] but require an external power, i.e., a
higher cost. More recently, an alternative form of immobilized photocatalytic system has been
introduced, namely buoyant or floating immobilized photocatalysts [10–14]. Basically, transparent
substrates lighter than water and relatively hydrophobic are chosen to keep immobilized
photocatalysts afloat. This configuration allows them to be effective in open surface water (i.e., no
reactor required) and especially capable of decomposing floating pollutants (e.g. oil spills).
Because the catalysts are floating on top of water surface in which incident radiation reaches first,
it maximizes light absorption and photogeneration of charge carriers. In Chapter 5, we have
proposed a novel fabrication technique to immobilize vertically-aligned zinc oxide (ZnO)
nanowires (NWs) on a polymer substrate with strong adhesion. The resulting ZnO NW/polymer
system was tested as a buoyant photocatalytic system, demonstrating the effective
photodegradation of dyes and floating crude oil. However, similar to immobilized photoreactors,
immobilized photocatalysts on buoyant substrates exhibit lower degradation efficiency due to mass
transfer limitation and small surface-area-to-volume-ratio.
One approach to enhancing mass transfer rates of photocatalytic systems is to integrate a
functionality of self-propulsion into photocatalysts [15,16]. The main idea is to partially decorate
the surface of photocatalytic micro/nanoparticles with a noble-metal catalyst layer that generates
gaseous bubbles propelling the particles. For example, hydrogen peroxide (H2O2) in water is
catalytically decomposed into H2 and/or O2 on a Pt or Ag surface, and these bubbles propel the
microparticles in an opposite direction [17–19]. This catalytic micromotors with photocatalysts have
shown excellent degrability for organic-dye degradation [15,16]. It is worth mentioning that for

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catalytic micromotors, light is not only working as stimuli to induce the photocatalytic reaction
but also to promote catalytic reaction for bubble generation and thus accelerating propulsion [15,16].
The geometry is another factor that governs the speed and direction of micromotor movement
[16,20–24]. The Su group [25] recently reported that solid cylindrical micromotors with precise
geometry control fabricated by a template-based method performed multiple functionalities of
photocatalysis and self-propulsion, together improving the overall photodegradability. The Mei
group [26] developed tubular-shaped catalytic micromotors based on a rolled-up technique,
demonstrating high speed movement of tubular micromotors owing to the bubble-recoil
mechanism and providing possibilities to salt concentration-independent approaches [18,22,27].
However the complex top-down fabrication process and critical conversion from 2D to 3D are
challenging. Aside from the catalytic reaction, other propulsion approaches harnessing different
external stimuli (e.g. magnetic [28–30], ultrasonic [31] and electrical field [32]) have also allowed
micromotors to swim under remote control and accomplish multiple tasks such as objective capture
and cargo delivery. However, the aforementioned micromotor-based photocatalytic system still
relies on individual particle, and to our knowledge, self-propelled immobilized photocatalysts on
soft substrates have not been demonstrated. In addition, a material platform to integrate true
multifunctionalities, i.e., three or more functionalities integrated onto a single entity, has been
rarely reported.
Here we present a novel multi-material platform to integrate immobilized ZnO
photocatalysts, a catalyst layer for improved locomotion, and magnetic nanoparticles for motion
control and separation. Vertically-aligned ZnO NWs are grown on an elastomeric
(polydimethylsiloxane (PDMS)) substrate with strong adhesion (see Chapter 5), forming a
photocatalytic layer. This film and another PDMS layer containing various functional materials

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like magnetic nanoparticles or noble metal catalysts are bonded together with strain engineering
to produce 3D-shaped soft micromotors. The predetermined shapes, including semi-circular,
circular and over-rolled shape, can be realized by varying the strain mismatch between two PDMS
membranes. Sub-millimeter-sized, 3D soft structures whose outer surfaces are covered with
vertically-aligned ZnO NWs have been fabricated with the flexibility of incorporating various
functional materials. These integrated soft micromotors (ISMs) exhibit the multiple functionalities
including photodegradation, self-propulsion, and magnetic control. photodegradation experiments
with and without integrated functionalities are compared to prove the utility of the proposed multimaterial platform. Three mechanics models are employed to calculate the out-of-plane
deformation of the ISMs, providing a guideline for the construction of the micromotors with
diverse shape. We believe that the proposed multi-material integration method may open an
opportunity for creating polymer-based multifunctional micromotors towards various applications.

6.2 Experimental
6.2.1 Preparation of ZnO NWs
All reagents used in this experiment were commercially available without further
purification. ZnO NWs were synthesized via a hydrothermal method. The detailed process is
available elsewhere (Chapter 5), but a brief account is given as follows: a ZnO seed solution was
prepared by adding 10mM Zn(CH3COO)2¡2H2O (99.9% Sigma Aldrich) into enthanol (200 prove,
Fisher Scientific) and stirred at 58°C for 2 hours. Once cooled down to room temperature, the
mixed solution was spin-coated onto a cleaned glass substrate with multiple times to achieve a
uniform surface coverage. The substrate was then kept at 150°C for 1 hour to improve the adhesion
of the ZnO seed layer to the substrate. The growth solution is composed of 25 mM Zn(NO3)2¡6H2O

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(Sigma Aldrich) and 25 mM hexamethylenetetramine (HMTA, Sigma Aldrich). Prior to the
synthesis, the growth solution was preheated in a convection oven at 90°C for 1hour to achieve a
thermal equilibrium. The substrate was then immersed into the growth solution at 90°C for various
duration (3~12 hours) to obtain a specific NW length. Subsequently, tge substrate was thoroughly
rinsed by DI water. A 3-hour dehydration step at 60°C was carried out for the substrate afterwards.

6.2.2 Fabrication of ISMs
A Sylgard 184 PDMS elastomer kit (DOW Corning) was applied to prepare a PDMS
solution at a ratio of 10 parts base prepolymer to 1 part curing agent. A thin (200~400 Âľm) PDMS
layer (1st) was spin-casted onto the as-fabricated ZnO NW substrate at 2000 rpm and solidified at
60°C for 6 hours. Another PDMS film (2nd) of 300-500 Οm in thickness was prepared in a Petri
dish under the same mixture and curing condition. For the magnetic response, a certain amount of
Fe particles (30~40 Âľm in diameter) were uniformly mixed into the 2nd PDMS solution with a
weight percentage around 5% prior to the solidification of film. The cured PDMS film (2nd) was
pre-stretched in a custom-made uniaxial or biaxial stretcher to a desired length. Both ZnO
NW/PDMS substrate and 2nd PDMS film were plasma-treated according to the literature method
[33], by Corona plasma treater, and bonded consecutively at 60°C. A laser machining system
(Universal Laser System VLS 4.60) was used to dice the plasma-bonded bilayer on the glass
substrate with the predetermined designs. The constructed ISMs were obtained when the bilayer
sandwich was detached from the glass substrate. The released ISMs were then soaked in the 70percent ethanol solution with 5-10 mins sonication to clean the laser-induced residues and
dehydrated at 60°C. In order to re-grow the ZnO NWs, the as-cleaned ISMs was dispersed in the
growth solution again by following the same growth process of ZnO NWs as 6.2.1.

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6.2.3 Characterization
The morphology, crystallinity and composition of ZnO NWs on ISMs were characterized
by scanning electron microscopy (SEM, JEOL 6610LV, Hitachi S-4700II), X-ray diffraction
(XRD, Bruker-AXS), energy dispersive x-ray spectrometry (EDS, JEOL 6610LV) and
photoluminance spectrometer (PL, Ocean Optics, Inc. USB spectrometer and optical fiber) with
the excitation laser at 337 nm..

6.2.4 Photocatalytic assessment and activities
Under UV light: The photocatalytic degradation was carried out in an aluminum foil-sealed
case with a magnetic stirrer stage inside. Two 9W florescent tubes (Philips Îť=375 nm) were
positioned 30 cm above the dye solution container. Methylene blue (1.5%, Sigma-Aldrich) was
diluted to 10 mM concentrated dye solution as a model pollutant. The ISMs was poured onto the
dye solution (20 mL), subsequently. Before irradiation the dye solution was kept in dark for 30min
to establish the absorption-desorption equilibrium. A small amount of solution was extracted from
the container at the constant intervals (60, 120, 180, 240, 300 mins) and centrifuged at 10000 rpm
for 2 mins to remove any residue from the extracted solution. The purified solution was then
examined using the absorbance in UV-Vis spectrometer (Lambda). The photodegradation
experiment was repeated a few times to determine relative errors and recyclability. In a selfpropulsion test, a 5% H2O2 solution was added to each MB dye solution before experiment. The
free ISMs and Ag-deposited in same quantity were dispersed into a H2O2/MB dye solution (20
mL) under the same working condition as before. In the test of magnetic propulsion, under the
same working condition, free ISMs and magnetic ISMs in the same quantity were dispersed into
MB dye solution. No H2O2 was utilized in magnetic propulsion test.

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6.3 Result and discussion
The fabrication process of the ISM (uniaxial) is shown in Figure 6.1. Two layers of PDMS
were utilized to achieve curved structures using strain engineering. Vertically-aligned ZnO NWs
were incorporated into one of the PDMS layer to increase active surface areas. Initially, an array
of vertically-aligned ZnO NWs was hydrothermally synthesized on a rigid (glass in this study)
substrate (Figure 6.1a). the substrate with the ZnO NWs was spin-coated with a PDMS solution
and cured subsequently (Figure 6.1b). After the PDMS layer was thoroughly solidified (Layer Z),
another pre-fabricated PDMS film (Layer P) was brought into intimate contact with the top of
Layer Z (Figure 6.1c). Prior to sandwiching these teo layers, Layer P was uniaxially stretched at a
given strain. Note that the contact surface on Layer P and Layer Z were treated by oxygen plasma
in advance. Once two surfaces were securely bonded, the stretching force applied to Layer P was
removed. Consequently, an instant shrinkage of Layer P generates a shear stress at the interface of
the bilayer structure, but no deformation appears because the rigid substrate holds both layers in
place. At this point, the bilayer film can be released from the substrate or laser machined prior to
release. In either case, upon detachment from the substrate, the interfacial shear stress will cause
out-of-plane deformation, rolling up or curling the bilayer film. An amount of the stretching force
on Layer P plays an important role in ISM fabrication. For the same geometry, the structures with
a larger curvature could be achieved with an increased stretching force. Besides, a reliable plasma
bonding between Layer Z and Layer P is indispensable. Any un-bonded part would affect the ISM
shaping. For the function of self-propulsion, a layer of Ag nanoparticles were deposited on top of
Layer P prior to the detachment of the as-prepared bilayer structure from the rigid substrate.

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Figure 6.1. Fabrication schematic of ISM under uniaxial SPB and producible shapes. (a) verticalaligned ZnO NWs grow on the substrate, (b) spin-cast thin PDMS layer, (c) pre-stretched one thin
PDMS film and plasma-bond to ZnO-PDMS substrate, (d) pattern shapes and remove residue
edges, (e) peel off the bilayer from the substrate and obtain the micromotors, (f) process for the
semi-circular ISMs, (g) process for the circular ISMs, (h) process for the over-rolled ISMs. Scale
bar: 500 Îźm.
When laser-assisted micromachining was introduced to the bilayer structure to increase the
shape diversity and dimension control of ISMs. In detail, the as-prepared bilayer structure was
machined into the pre-determined geometrical patterns prior to the detachment from the rigid
substrate (Figure 6.1d). The specific geometry was designed in light of potential deformation
which would form into a specific 3D shape as the released bilayer structure rolled up (Figure 6.1e).
In this manner, a variety of 3D shapes can be conceived using different geometrical patterns. Note
that the laser-assisted machining can be substituted by other similar fabrication methods. When
the bilayer structure was detached from the rigid substrate, individual ZnO NWs initially grown
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on Layer Z were broken near the interface between Layer Z and the glass substrate. One part of
the ZnO NWs remained embedded in Layer Z and should serve as a seed layer for the re-growth
of ZnO NWs on Layer Z afterwards (Figure 6.1f1, g1 and h1)). Another part of the ZnO NW array
was left onto the rigid substrate which could be also used as a seed layer for new growth, i.e.,
promoting the recyclability of the rigid substrate. Eventually, these curved sub-millimeter
structures with the embedded ZnO NWs with their tips being exposed at the surface were immersed
into the same hydrothermal growth solution, resulting in the regrowth of ZnO NWs on the ISM
surface on Layer Z (Figure 6.1f2, g2 and h2). We reported this regrowth process in Chapter 5,
named it as a secondary growth process. Apparently, there are numerous advantages of this
secondary-grown ZnO NWs array, including a strong and reliable connection to the PDMS layer
(thanks to the partial embedment of ZnO NWs), a larger catalytic surface area (in comparison to a
ZnO film) and selectively grown on the desired surface (only appearing on the particular surface
of the ISMs). This approach is quite different from the conventional nanomaterial-based hybrid
polymer film in which the nanomaterials are randomly dispersed in a matrix. Here, controllability
of placing the functional materials into the target location would improve the photocatalytic
degradation and versatility of the ISMs. The optical microscopic images (Figure 6.1f3, g3 and h3)
shows three geometries produced by uniaxial stretched PDMS bonding (SPB), where the Layer Z
(in dark) and Layer P (in transparent) are clearly observed.
Figure 6.2a exhibits an over-rolled circular ISM prepared through the uniaxial stretching,
where the thicknesses of Layer P and Layer Z are about 200 Îźm and 67 Îźm, respectively. As shown
in Figure 6.2b, the vertically-aligned ZnO NWs are distributed uniformly along the outer surface
of the ISM. No loose NWs were observed after construction. From the inset of Figure 6.2b it can

167

be seen that approximately 2 Îźm-length ZnO NWs were partially embedded in the substrate. The
length of the embedded portion can be adjusted by the initial growth time.

Figure 6.2. (a-d) SEM images of the ISM produced by uniaxial process, (b) outer surface of the
ISM, (c) boundary of Layer P and Layer Z, (d) side surface of Layer P, (e) Photoluminescence
spectrum of the ZnO NWs on ISM and glass, (f) XRD pattern of the ZnO NW on ISM and glass.
Scale bar: 500 Îźm in a; 30 Îźm in b, 10 Îźm in c and 20 Îźm in d; 5 Îźm in the inset of b.
In comparison with the ZnO NWs array on the independent PDMS film, the ZnO NWs
maintained the similar quality on the curved surface, whereas the large gaps between neighboring
ZnO NWs can be attributed to the substrate bending. A distinctive boundary in Figure 6.2c (same
view angle as Figure 6.2a) splits the ISM into a less conducting region (Layer P) and a more
conducting region (Layer Z), where only few ZnO NWs lying on Layer P whereas bunches of ZnO
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NWs coming out of Layer Z. The parallel striped ripples observed in Figure 6.2d indicate that
oxygen plasma treated Layer P may have undergone a uniaxial stretching process with a large
strain [34]. In the current case, it is worth noticing that no ZnO NWs remained on Layer P, proving
the precise growth of the ZnO NWs on the selective surface. XRD patterns in Figure 6.2e exhibited
the wurtzite crystallites and [0001] preferable growth direction of ZnO NWs on the PDMS film,
where the marked peaks are assigned to the characteristic of ZnO, and the peak at (002) with a
higher intensity indicates the preferable growth of ZnO NWs along the c-axis direction. Figure
6.2f illustrates the PL spectrum of the ISM under the excitation of 328 nm UV-LED irradiation.
The PL peak location presented two respective emission bands at 389.58 nm and 490.63 nm, each
corresponding near-band emission and oxygen-derived defects in ZnO, respectively. These results
match those of the ZnO NWs grown on the rigid substrate [35]. High crystallinity from XRD and
strong PL intensity suggest the high quality of the ZnO NWs grown on the ISM via the two-step
growth process. Consider the scalability of the hydrothermal method for ZnO NW synthesis
reported on 4” Si wafer [36–38] and laser micromachining, thousands of diverse ISMs can be
obtained within few hours, representing an efficient and low-cost process.

Figure 6.3. (a) photocatalytic activity of ISMs in different quantities, (b) numbers of ISMs with
various shapes on the degradation rate of MB. Experiments proceeded at room temperature.
A series of experiments were carried out to investigate the photocatalytic degradability of
ISMs under UV irradiation, where methely blue (MB) was used as a probe pollutant. The
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degradation tests on semi-circular ISMs are shown in Figure 6.3a to elucidate the effect of ZnO
NWs on ISM, of which the degradation efficiency varied from 45% to 55% in 6 hours with an
increased amount of ISMs were applied to the photocatalytic process. The influence of various
shapes of ISMs with respect to degradability was also explored which is shown in Figure 6.3b.
Four different shapes of ISMs were examined under the same working condition. As can be seen
that the over-rolled ISMs acquired an excellent degradation efficiency (90%) with a small quantity
in four hours, while much larger quantities of other types of ISMs should be utilized to approach
the similar result. As a conclusion, this discrepancy is mainly attributed to the difference of the
active surface area in diverse shapes. Note that there is a positive linear correlation between the
active surface area and their shapes. For example, the quantity of quarter-circular ISMs was
doubled as that of semi-circular ones, which indicate that the doubled active surface offered a
higher decomposition rate of MB dye. Additionally it also implies that a near-complete
degradation can be achieved by either extending the reaction time or increasing quantity of ISMs.
Evidently, those data above confirms the stable performance of ISMs in photocatalytic
degradation.

Figure 6.4. Time-lapsed images of a micromotor in the 70% ethanol solution with 5% H2O2. (a)
t = 1s, (b) t = 10s, (c) t = 20s and (d) t = 30s. Scale bar: 2 mm.
To demonstrate the versatility of ISMs, the autonomous movement in an aqueous solution
was explored. Here the recoiling force was employed for the purpose of the self-propelling ISM.
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The recoiling force is known to possess the high speed and the long-span spiral motion to propel
the entire structure [24,25]. According to the relevant reports, the recoiling force is usually realized
by the generation oxygen bubbles in the presence of hydrogen peroxide (H2O2) in solution and
noble metal (e.g. Au, Ag, Pt) layer on micromotors [25]. It is worth noticing that a noble metal
layer serving as catalysts can be placed in specific locations when fabricated onto micromotors
with various shapes. For tubular-like micromotors Ag is placed at the inner surface uniformly
where bubbles can generate and accumulate simultaneously. In other designs such as spherical
micromotors, an asymmetric Ag deposition was employed since it is efficient to power asymmetric
micromotors [15,20]. In our experiment, the ISMs in various shapes were found with low Reynolds
number (0.03-0.05) due to the small characteristic length, and their motions should follow with
recoiling mechanism in term of asymmetric Ag locations. In the fabrication of ISMs, Ag was
deposited uniformly across the entire PDMS surface (an extra step between Figure 6.1d and Figure
6.1e) prior to the secondary growth of ZnO NWs. Figure 6.4 shows the autonomous movement of
the semi-circular ISM in a diluted ethanol solution in the presence of H2O2, where bubbles ejected
from the backside surface of the ISM generated a recoiling force to propel the micromotor forward
with an average speed of 125 Âľm/s. The corresponding video is attached in Support Information
(Video S1), in which a linear trajectory was observed during the experiment, indicating that the
sustainable driving force was achieved. A constant rotation accompanied with the global
movement of the semi-circular ISM can be attributed to the poor-oriented driving forces which
generated asymmetrically on the backside surface. Other than the asymmetric designs, the
propulsion of the tubular ISM was also carried out with the Ag film deposited on its backside
surface uniformly. As reported, for a successful tube-shape micromotor O2 needs to be ejected
only at one-end opening, which acted the propelling force to drive the ISM [39]. In our case, since

171

Ag deposited at the inner wall results a symmetric structure, the tubular ISMs could not realize
directional swimming as the bubbles generated on both ends make micromotor stationary [ 24]
(Video S2). Therefore a preferred asymmetric geometry is essential for a great directional motion
of the ISMs in the future.
To date, multi-functional microrobots have become attractive due to its versatility and
operability, and are useful in many applications [40]. For instance, autonomous movement could
improve the performance of micromotors for drug delivery and photocatalytic degradation [ 16,41].
Apart from the recoiling force, other propulsion strategies based on external fields (e.g., ultrasonic,
electric and magnetic), have been developed in realizing movement of micromotors [40]. Amongst,
the magnetic field are considered as an effective external stimuli to guide the motion of the ISMs
remotely [40]. In this work, magnetic materials were employed to create desired ISMs with the
proper response of the stimuli. The magnetically-controlled movement of the semi-circular ISM
in water is displayed in Figure 6.5 (Video S3). As shown, within 60 seconds the ISM was remotely
controlled to move around the “MSU” letters. The ISM during the experiment performed linear
trajectory and the moving speed of ISM varied in terms of the applied magnetic field. The ISM
could freely swim due to the sufficient magnetic force provided by embedded magnetic materials
in comparison to the drag resistance in water. In current case the remote distance of the ISM was
maximally 2 cm. A stronger magnetic field is required to further increase this distance. It can be
imaged that ISMs coupled with magnetic materials is able to be navigated towards desired
objectives, which is advantageous to drug deliver and photocatalytic degradation. More works
need to be done in term of the geometry effect and sustainable against the long-term navigation.

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Figure 6.5. Time-lapsed images of a micromotor atop water. (a) t = 1s, (b) t = 20s, (c) t = 40s and
(d) t = 60s. Scale bar: 2 mm.
The autonomous movement of Ag-ISMs with respect to their enhanced degradation rate
was evaluated. In Figure 6.6 (a), the same concentration of H2O2 was added in MB dye solution
prior to the experiment, which worked as a catalyst accelerating the decomposition of MB dye for
all three scenarios [16]. Obviously, the function of self-propulsion further promoted the degradation
rate in comparison to free Ag-ISMs thanks to the lower mass transfer resistance (Figure 6.6a).
Moreover, both original ISMs and self-propelled Ag-ISMs performed higher degradability than
that of the reference one, confirming the valid photocatalytic capability of ZnO NWs fixed on
these ISMs. This result validates the synergetic effect of the combined functionality, i.e.,
photodegradation and self-propulsion. The experiment in Figure 6.6b presents the comparison of
magnetic-propelled ISMs to original ISMs. A magnetic field was applied to the solution placed on
a stir plate without a magnetic stirrer inside. Instead the solution was populated with magnetic
ISMs and thus mixed by the movement of ISMs. As a result, the magnetic-propelled ISMs have
the best photocatalytic performance, which, likewise, ascribes to the reduction in the mass transfer
resistance derived from the magnetic movement. The all three scenarios in Figure 6.6b exhibit
lower degradation rate than that in Figure 6.6a due to the absence of H2O2, which has a significant
effect to MB dye degradation. Taking account the toxicity of H2O2, magnetic-propelled ISMs are
more promising as photocatalysts regarding the environmental remediation while self-propelling
ISMs are suitable for other applications.
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Figure 6.6. Photocatalytic activity of ISMs (a) with/without self-propulsion, (b) with/without
magnetic propulsion. Experiments proceeded at room temperature.

Figure 6.7. Schematics of the SPB process. (a) Layer P was stretched at a certain ratio, (b) the
bilayer structure was formed between Layer P and Layer Z, (c) The deformation of bilayer
structure took place right after the releasing of stretching force on Layer P, (d) The deformation
process stopped until bilayer structure was fully relaxed, where a curved bilayer structure was
obtained.

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The ISM was converted from a 2D film to a 3D structure arises from the mismatch strain
of the two PDMS layers. The stretching ratio of Layer P with respect to Layer Z is regarded as one
key parameter in the fabrication process. PDMS is known that can be uniaxial stretched over 200%
[42], whose elastic recovery is the major force in inducing SPB for the fabrication of ISMs. Briefly,
the shrinkage initially occurred on Layer P brings linear compression to Layer Z, resulting the
folding motion of the bilayer structure after it is detached from the glass substrate. We noticed that
the resulting curvature of the micromotors is governed by the initial stretching ratio of Layer P. In
other words, the more Layer P is stretched, the larger curvature will be resulted to the micromotors.
Layer Z consisting of the PDMS layer and ZnO NWs was plasma bonded to pre-stretched Layer
P, which would maintain flat until the bilayer structure is formed and detached from the glass
substrate. Upon the investigation of the free body diagram (FBD) of Layer Z and Layer P, we
found that the bilayer structure should be always folding upwards because of the moments at the
interface. The resulting curvature is calculated using various Stoney formula expressions.
The Stoney formula as one developed theory has been widely applied to calculate the
mechanical properties of the bilayer structures [43–46]. Modified expressions in response to various
assumptions and geometrical parameters can be introduced to describe the curvature of the ISM as
a function of its residual strain. The Stoney formula results from the film-substrate bending model
based on a few important assumptions. : (1) the thickness of both substrate and film are very small
in comparison with their lateral dimensions; (2) the thickness of the film is much smaller than that
of the substrate; (3) both the substrate and the film are homogenous, isotropic and linearly elastic;
(4) the rotations and deformations are infinitesimally small; and (5) edge effects are negligible. In
this work, three models, involving the Stoney formula expression for internal film stress and two
modified expressions based on vary assumptions were applied to our structures. The main

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difference is the suppression of Assumption (2), non-negligible thickness of Layer P in the model.
In addition, the initial stress of Layer Z is negligible in comparison with that of Layer P, which is
the extra assumption in the following analysis for the models.
Model 1,
𝜎𝑝 =

1 𝐸𝑧 ℎ𝑧 𝜅
6 (1 − 𝜈𝑧 ) 𝐻
𝐻=

ℎ𝑝
ℎ𝑧

𝐸𝑝
𝑀𝑝 1 − 𝜈𝑝
𝑚=
=
𝐸𝑧
𝑀𝑧
1 − 𝜈𝑧
Where κ represents the curvature of the ZCM, ξ is the strain of the layer, σ stands for the stress of
the layer, M, E, ν and h are the biaxial elastic modulus, Young’s modulus, Poission’s ratio and
thickness of the PDMS layers, where z and p as the subscripts refer to layer Z and layer P,
respectively. Model 1 is the Stoney formula for the calculation of the internal film stress [43,44].
Model 2,
𝜅=

6𝛿
𝐹(𝑚, 𝐻)
𝐻

(1 + 𝐻)2
𝐹(𝑚, 𝐻) =
𝑚𝐻 3 + 𝑚−1 𝐻 −1 + 4𝐻 2 + 6𝐻 + 4
Where δ is the relative reduction of linear dimension. Model 2 is derived considering Timoshenko
beam theory [46].
Model 3,
𝜅=
𝑍(𝑚, 𝐻) = 𝐻𝑚[

6𝛥𝜀
𝑍(𝑚, 𝐻)
ℎ𝑧

1+𝐻
]
1 + 𝐻𝑚(4 + 6𝐻 + 4𝐻 2 ) + 𝐻 4 𝑚2
176

Where Δε is the mismatch strain between layers. Model 3 is the general expression of Stoney
formula for a given bilayer structure with uniform mismatch strain [47].
The comparison of experimental results and modeling ones is displayed in Figure 6.7,
where the Poisson’s ratio substituted in the models was presumed as 0.5 [49–51] for the both layers
due to the same mixture ratio of PDMS films. In model 2 and model 3 we respectively employed
F(m,H) and Z(m,H) as a dimensionless term to describe the correlation of layer thickness and
Young’s modulus, where the Young’s modulus as a thickness-dependent parameter that was
recorded in term of the respective thickness of Layer P and Layer Z, as found by the reported
experimental results [52]. It is worth noted that all input parameters in the analysis were originated
from the initial status of Layer P and Layer Z before forming into the ISMs. Here consider a bilayer
structure consisting of a thick film (Layer P) with tensile stress and a thin relaxed film (Layer Z)
with an original length of interface (L1), an opposite shear force instantly generates at their
interface when releasing, offering an inward moment and a shrink motion simultaneously, leading
a smaller intermediate length with larger deformation. In other words, L obtained at the final state
is always smaller than L1 arising from the tangential shrink of Layer P, which constitutes the major
part governing the mechanical properties. The intimately plasma-bonded Layer P and Layer Z
have to compress along with the interface, resulting the discrepancy of the real axial strain. In the
modelling study we noticed that this issue was partially released by employing L, Lp and Lz
simultaneously into the model as the replacement of L0, although a undesirable deviation would
occur, which needs to be addressed in future work.
It is known from Figure 6.7 that the stretching ratio of Layer P is one key factor influencing
the geometry of the ISMs. As depicted in Fig 6.8(a-b), both curvatures from the analysis and

177

experiment exhibit a linear proportion to Îľp, demonstrating that apparent deformation took place
at heavy stretched Layer P.

Figure 6.8. (a) the comparison of experimental curvatures and analytical curvatures as a function
of the stretching ratio of Layer P, (b) detail comparison of model 2 and model 3, (c) circular ISM
with L=1500 Âľm, (d) over-rolled ISM with L=2000 Âľm. Scalebar:100 Âľm.
From Figure 6.8a it can be concluded that, model 1 has quite large error to fit experimental
data in comparison to other models, which expresses an increased discrepancy at larger stretching
ratio. This deviation is attributed to the limited assumptions which needs further modification.
Model 2 and Model 3 as two modified Stoney formula perform better result-fitting in the range of
current stretching ratios. However, serious deviation emerges in Model 2 at larger stretching ratio
while excellent result-fitting is found on Model 3 all the time, indicating that Model 3 is the most
suitable model for predicting deformation of ISMs in current situation. The further, we noticed
that all three models are not able to predict ISM at sharp shape (over-rolled ISMs). Figure 6.7e
shows two ISMs with similar H and M, apparently, the shape of the ISM constructed via SPB
correlates highly with L under the same Îş where the circular shape can be obtained only when
178

2π/κ≥L≥π/κ. In practical, the region around the ends of the circular ISMs could not deflect as
much as the other parts due to the unbalanced self-constrain, which became thicker and more flat,
resulting an error of Îş in analysis. Similarly, the Îş of over-rolled ISMs (inner circular) differs from
that of the circular ISMs owing to the extra bending arisen from the overlapped free ends. In view
of the above, some volume-dependent factors should be considered in further modifying model 3
in future.

Figure 6.9. (a-g) schematics of biaxial SPB process. (h-l) hollow and semi-closed ZCM with
various shapes and sizes, Scalebar: (h-j) 500 Âľm, (k,l) 1 mm.
In addition to the circular shapes shown above, the capability of SPB in constructing hollow
(and semi-closed) ISM has been proved. As depicted in Figure 6.9, a number of shapes was
achieved with the assist of biaxial stretchable stage. Similar as uniaxial SPB, Layer Z was prepared
and plasma bonded with stretched Layer P, whose stretching ratio upon two directions varies
according to the geometry design (as shown in Figure 6.9 (a-g)). After detaching from substrate,
179

the bilayer structure deformed by shear force immediately, which shrink along two perpendicular
axis towards the body center. In our experiment, the symmetric shapes (i.e. spheres and cubes)
could be obtained through equally stretching Layer P on two directions. In contrast, asymmetric
shapes were originated from different stretching ratio of Layer P on both directions. Obviously,
the biaxial SPB method is promising to pursue the complex shape of ISM. After the investigation
of those ISMs, we found that some particular complex shapes approached by varied stretching
ratio could be analyzed by employing uniaxial model respectively to the priority shrinking
direction and secondary shrinking direction.
For instance, the semi-sphere ISM in Figure 6.9i was achieved by biaxial SPB method with
the equal stretching ratio of Layer P on both direction. This scenario was found to be very close to
the fabrication of circular ISM using uniaxial SPB method (Figure 6.8b, Îľ p =1). The comparison
of analytical curvature (using Model 3) to experimental curvature shows a similar error as the case
shown in Figure 6.8b (purple line vs. red line, Îľp =1), indicating the accuracy of Model 3.
Nevertheless, Model 3 is failed to match the hollow-sphere ISM in Figure 6.9j. This difference
may be attributed to the constitution of structure. The half-sphere ISM reaches relaxation status
whose fingers have no contact to each other, while the fingers on hollow-sphere ISM suffer the
stress from neighbors which prevents the further deformation. This difference is also applicable to
asymmetric ISMs (i.e. Figure 6.9l vs. 6.9k). In addition, the dimension of complex ISMs is
determined by the smaller stretching ratio in current case.
Other than shape diversity, the functionality is expected to be extended using biaxial SPB.
For the hollow-sphere ISM, the cavity inside the structure is able to be used as a cargo. The
manageable cargo transportation regarding micromotors has been investigated extensively in
recent years which has been rendered a variety of capabilities such as objective capturing and

180

unloading [25,40]. Taking account the developed mechanisms for autonomous movement, the
hollow-sphere ISM is a very promising candidate in the application of cargo transportation.
Furthermore, since the deformation is originated from the elastic recovery of PDMS film, the
fabricated ISMs will memory the shape all the time in ambient condition, which guarantees its
long lifetime and stability. Finally, to our knowledge, ISM is the first micromotor that has 1D ZnO
NWs fixed on its surface. This unique structure not only benefits for the photocatalytic degradation
but also paves a way to immobilize 1D nanostructures to other soft micromotors. More potential
applications will be explored in the future.

6.5 Conclusion
In conclusion we have developed a new type of micromotors that has ZnO NWs array
embedded in the particular surface. Plasma treatment is utilized as a reliable method to combine
PDMS films. The remarkable performance in photocatalytic degradation of ISMs has been
demonstrated and few strategies were introduced to integrate the function of autonomous
movement to ISMs. Besides, few mathematic modelling was developed to analyze the deformation
of ISMs in fabrication. Comparing to other polymer-based micromotors, the flexible construction
regarding numerous shapes and sizes of ISM enables the integration of multiple functions to satisfy
diverse working conditions, which is a promising candidate for both environmental remediation
and drug delivery fields. Other than the current components, many other materials will be
employed as heterogenous structure to further improve the photocatalytic performance of ISMs in
the future. More functions will also be integrated by use of particular structures.

181

REFERENCES

182

REFERENCES

1. Chong, M. N., Jin, B., Chow, C. W. K. & Saint, C. Recent developments in photocatalytic
water treatment technology: A review. Water Res. 44, 2997–3027 (2010).
2. Krivec, M., Žagar, K., Suhadolnik, L., Čeh, M. & Dražić, G. Highly Efficient TiO2-Based
Microreactor for Photocatalytic Applications. ACS Appl. Mater. Interfaces 5, 9088–9094
(2013).
3. Shen, C., Wang, Y. J., Xu, J. H. & Luo, G. S. Glass capillaries with TiO2 supported on inner
wall as microchannel reactors. Chem. Eng. J. 277, 48–55 (2015).
4. Krivec, M. et al. The nature of chlorine-inhibition of photocatalytic degradation of
dichloroacetic acid in a TiO2-based microreactor. Phys. Chem. Chem. Phys. 16, 14867–
14873 (2014).
5. PauportĂŠ, T. & RathouskĂ˝, J. Electrodeposited Mesoporous ZnO Thin Films as Efficient
Photocatalysts for the Degradation of Dye Pollutants. J. Phys. Chem. C 111, 7639–7644
(2007).
6. Kim, S. M. et al. Mussel-Inspired Immobilization of Catalysts for Microchemical
Applications. Adv. Mater. Interfaces 2, n/a-n/a (2015).
7. Zhang, Q., Zhang, Q., Wang, H. & Li, Y. A high efficiency microreactor with Pt/ZnO
nanorod arrays on the inner wall for photodegradation of phenol. J. Hazard. Mater. 254–255,
318–324 (2013).
8. Wang, G. et al. Flow Effects on the Controlled Growth of Nanostructured Networks at
Microcapillary Walls for Applications in Continuous Flow Reactions. ACS Appl. Mater.
Interfaces 7, 21580–21588 (2015).
9. Shen, C., Wang, Y. J., Xu, J. H. & Luo, G. S. Glass capillaries with TiO2 supported on inner
wall as microchannel reactors. Chem. Eng. J. 277, 48–55 (2015).
10. Xing, Z. et al. A floating macro/mesoporous crystalline anatase TiO 2 ceramic with
enhanced photocatalytic performance for recalcitrant wastewater degradation. Dalton Trans.
43, 790–798 (2014).
11. AltÄąn, Ä°. & SĂśkmen, M. Buoyant Photocatalyst Based on ZnO Immobilized on Polystyrene
Beads for Pollutants Treatment. CLEAN – Soil Air Water 43, 1025–1030 (2015).
12. Singh, S., Singh, P. K. & Mahalingam, H. Novel Floating Ag+-Doped TiO2/Polystyrene
Photocatalysts for the Treatment of Dye Wastewater. Ind. Eng. Chem. Res. 53, 16332–16340
(2014).

183

13. Ni, L. et al. Novel floating photocatalysts based on polyurethane composite foams modified
with silver/titanium dioxide/graphene ternary nanoparticles for the visible-light-mediated
remediation of diesel-polluted surface water. J. Appl. Polym. Sci. 133, n/a-n/a (2016).
14. Srikanth, B. et al. Recent advancements in supporting materials for immobilised
photocatalytic applications in waste water treatment. J. Environ. Manage. 200, 60–78
(2017).
15. Li, J. et al. Water-Driven Micromotors for Rapid Photocatalytic Degradation of Biological
and Chemical Warfare Agents. ACS Nano 8, 11118–11125 (2014).
16. Soler, L., Magdanz, V., Fomin, V. M., Sanchez, S. & Schmidt, O. G. Self-Propelled
Micromotors for Cleaning Polluted Water. ACS Nano 7, 9611–9620 (2013).
17. Williams, B. J., Anand, S. V., Rajagopalan, J. & Saif, M. T. A. A self-propelled biohybrid
swimmer at low Reynolds number. Nat. Commun. 5, 3081 (2014).
18. Solovev, A. A., Mei, Y., BermĂşdez UreĂąa, E., Huang, G. & Schmidt, O. G. Catalytic
Microtubular Jet Engines Self-Propelled by Accumulated Gas Bubbles. Small 5, 1688–1692
(2009).
19. Valadares, L. F. et al. Catalytic Nanomotors: Self-Propelled Sphere Dimers. Small 6, 565–
572 (2010).
20. Wang, H. & Pumera, M. Fabrication of Micro/Nanoscale Motors. Chem. Rev. 115, 8704–
8735 (2015).
21. Guix, M. et al. Superhydrophobic Alkanethiol-Coated Microsubmarines for Effective
Removal of Oil. ACS Nano 6, 4445–4451 (2012).
22. Manesh, K. M. et al. Template-Assisted Fabrication of Salt-Independent Catalytic Tubular
Microengines. ACS Nano 4, 1799–1804 (2010).
23. Li, J. et al. Water-Driven Micromotors for Rapid Photocatalytic Degradation of Biological
and Chemical Warfare Agents. ACS Nano 8, 11118–11125 (2014).
24. Mou, F. et al. Single-Component TiO2 Tubular Microengines with Motion Controlled by
Light-Induced Bubbles. Small 11, 2564–2570 (2015).
25. Su, M. et al. Shape-Controlled Fabrication of the Polymer-Based Micromotor Based on the
Polydimethylsiloxane Template. Langmuir 31, 11914–11920 (2015).
26. Mei, Y., Solovev, A. A., Sanchez, S. & Schmidt, O. G. Rolled-up nanotech on polymers:
from basic perception to self-propelled catalytic microengines. Chem. Soc. Rev. 40, 2109–
2119 (2011).
27. Gao, W., Uygun, A. & Wang, J. Hydrogen-Bubble-Propelled Zinc-Based Microrockets in
Strongly Acidic Media. J. Am. Chem. Soc. 134, 897–900 (2012).
184

28. Gao, W., Sattayasamitsathit, S., Manesh, K. M., Weihs, D. & Wang, J. Magnetically
Powered Flexible Metal Nanowire Motors. J. Am. Chem. Soc. 132, 14403–14405 (2010).
29. Kline, T. R., Paxton, W. F., Mallouk, T. E. & Sen, A. Catalytic Nanomotors: RemoteControlled Autonomous Movement of Striped Metallic Nanorods. Angew. Chem. 117, 754–
756 (2005).
30. Tottori, S. et al. Magnetic Helical Micromachines: Fabrication, Controlled Swimming, and
Cargo Transport. Adv. Mater. 24, 811–816 (2012).
31. Wang, W., Castro, L. A., Hoyos, M. & Mallouk, T. E. Autonomous Motion of Metallic
Microrods Propelled by Ultrasound. ACS Nano 6, 6122–6132 (2012).
32. Loget, G. & Kuhn, A. Electric field-induced chemical locomotion of conducting objects.
Nat. Commun. 2, 535 (2011).
33. Haubert, K., Drier, T. & Beebe, D. PDMS bonding by means of a portable, low-cost corona
system. Lab. Chip 6, 1548 (2006).
34. Lin, P.-C. & Yang, S. Spontaneous formation of one-dimensional ripples in transit to highly
ordered two-dimensional herringbone structures through sequential and unequal biaxial
mechanical stretching. Appl. Phys. Lett. 90, 241903 (2007).
35. LĂłpez-Romero, S. & GarcĂ­a-H, M. Photoluminescence and Structural Properties of ZnO
Nanorods Growth by Assisted-Hydrothermal Method. World J. Condens. Matter Phys. 03,
152–157 (2013).
36. Wei, Y. et al. Wafer-Scale High-Throughput Ordered Growth of Vertically Aligned ZnO
Nanowire Arrays. Nano Lett. 10, 3414–3419 (2010).
37. Kołodziejczak-Radzimska, A. & Jesionowski, T. Zinc Oxide—From Synthesis to
Application: A Review. Materials 7, 2833–2881 (2014).
38. Kumar, R., Al-Dossary, O., Kumar, G. & Umar, A. Zinc Oxide Nanostructures for NO2
Gas–Sensor Applications: A Review. Nano-Micro Lett. 7, 97–120 (2015).
39. Magdanz, V., Stoychev, G., Ionov, L., Sanchez, S. & Schmidt, O. G. Stimuli-Responsive
Microjets with Reconfigurable Shape. Angew. Chem. Int. Ed. 53, 2673–2677 (2014).
40. Wang, H. & Pumera, M. Fabrication of Micro/Nanoscale Motors. Chem. Rev. 115, 8704–
8735 (2015).
41. Su, M. et al. Shape-Controlled Fabrication of the Polymer-Based Micromotor Based on the
Polydimethylsiloxane Template. Langmuir 31, 11914–11920 (2015).
42. Li, X. et al. Modulating Two-Dimensional Non-Close-Packed Colloidal Crystal Arrays by
Deformable Soft Lithography. Langmuir 26, 2930–2936 (2010).

185

43. Stoney, G. G. The Tension of Metallic Films Deposited by Electrolysis. Proc. R. Soc. Lond.
Ser. Contain. Pap. Math. Phys. Character 82, 172–175 (1909).
44. Guyot, N., Harmand, Y. & MĂŠzin, A. The role of the sample shape and size on the internal
stress induced curvature of thin-film substrate systems. Int. J. Solids Struct. 41, 5143–5154
(2004).
45. Mézin, A. Coating internal stress measurement through the curvature method: A geometrybased criterion delimiting the relevance of Stoney’s formula. Surf. Coat. Technol. 200,
5259–5267 (2006).
46. Ren, L., Qiu, J. & Wang, S. Thermo-adaptive functionality of
graphene/polydimethylsiloxane nanocomposites. Smart Mater. Struct. 21, 105032 (2012).
47. Timoshenko, S. Analysis of Bi-Metal Thermostats. JOSA 11, 233–255 (1925).
48. Clyne, T. W. & Gill, S. C. Residual Stresses in Thermal Spray Coatings and Their Effect on
Interfacial Adhesion: A Review of Recent Work. J. Therm. Spray Technol. 5, 401–418
49. Johnston, I. D., McCluskey, D. K., Tan, C. K. L. & Tracey, M. C. Mechanical
characterization of bulk Sylgard 184 for microfluidics and microengineering. 24, 35017–
35023 (2014).
50. Armani, D., Liu, C. & Aluru, N. Re-configurable fluid circuits by PDMS elastomer
micromachining. in Twelfth IEEE International Conference on Micro Electro Mechanical
Systems, 1999. MEMS ’99 222–227 (1999). doi:10.1109/MEMSYS.1999.746817
51. Inglis, D. W. A method for reducing pressure-induced deformation in silicone microfluidics.
Biomicrofluidics 4, 026504 (2010).
52. Liu, M., Sun, J., Sun, Y., Bock, C. & Chen, Q. Thickness-dependent mechanical properties
of polydimethylsiloxane membranes. J. Micromechanics Microengineering 19, 035028
(2009).

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CHAPTER 7
RAPID AND LARGE-SCALE ASSEMBLY PROCESS FOR 1D NANOSTRUCTURES
7.1 Introduction
One dimensional (1D) nanostructures (NSs), mainly including nanowires (NWs),
nanotubes (NTs), nanorods, and nanobelts, have been widely considered as important building
blocks for future nanoscale devices [1]. Initial proof-of-concept devices such as transistors/logic
circuits[2–6], sensors [7–9], light emitting diodes [10,11], and solar cells [12–14] were fabricated from
individual 1D NSs using serial and resource-intensive approaches, which are not adequte for largescale production. Apart from the fabrication limitation, individual 1D NSs devices may not meet
operation requirements due to their low driving current, low lumens, low signal-to-noise ratio, and
significant device-to-device performance variation [15]. Instead the networks of 1D NSs with
random orientations have been considered a promising path for realizing high performance thinfilm transistors (TFTs) for macro- and flexible electronic applications. Random networks of 1D
NSs solves the scalability problem, improves the current-driving capability of the resulting devices
due to the large number of conducting materials incorporated into the device channel, and greatly
improves the uniformity of the device performance. The common assembly methods used to
obtain random networks of 1D NSs include spin coating [16,17], dip coating [18,19], vacuum filtration
[20,21], and printing [22,23]. The first three methods, while suitable for large-area fabrication,
typically entail the subsequent photolithographic pattern and selective etching/liftoff. Considering
the size of the active components (on the order of a few percent aerial coverage), most of the NS
areas need to be etched away or lifted-off, causing a significant amount of nanomaterials to be
wasted or needed to be recycled.

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Many attempts have been made to genetrate an array of well-defined, random network
patterns directly from solution-processed 1D NSs with the maximal material utilization. One
approach is a use of an external (e.g. electrical [24–27], magnetic [28,29], and acoustic [30,31]) field for
NS localization. Most, if not all, of NSs are deposited onto the target regions on the substrate.
While these field-based methods often accompany the benefit of NS alignment [32,33], a large-area
patterning may still be time-consuming and costly. On the other hand, the template-based
assembly like solution-based self-assembly [34–36] can be readily implemented in a wafer scale.
However, controlling the NS density and uniformity in the patterns across the entire substrate is
still challenging because the surface-mediated interaction with the functionalized NSs extends only
to the first layer. The deposition of the subsequent layers is less controllable, leading to the greater
variabilities in the network thickness and corresponding film properties. Here we consider
filtration-guided assembly (FGA) as a scalable, inexpensive assembly platform that creates 1D NS
patterns without generating any waste stream of nanomaterials and minimizes the environmental
footprint. FGA is based on a vacuum filtration method that uses filter membranes with nanometersize pores. Since the pionerring work by the Wu’s group [37], vacuum filtration has been
extensively utilized for the preparation of macroscopic random-network films from various 1D or
2D NSs such as carbon nanotubes (CNTs) [37–42], nanowires [20,21,43,44], graphene/graphene oxide
[45–49], organic micro/nanofibers [50–52], and a mixture of these nanoscale building blocks [53–55].
FGA utilizes selective filtration of NSs through the open windows on the nanoporous filter
membrane whose surface is patterned by a polymer mask for guiding the NS deposition. CNTs
[56], organic microfibers [57], and biological samples [58,59] were patterned via FGA and used as
functional elements for electronic devices or bioassay. In all of these previous work, anodized
alumina oxide (AAO) membranes were the choice of the filter membrance.

188

But they are

expensive, fragile, and difficult to handle, and thus not suitable for the large-area fabrication. The
pattern resolution of FGA has been also limited to tens to hundreds of microns (for the reasons to
be discussed later).
The objective of this work is to create a template-like FGA platform on robust nanoporous
membranes with high-resolution (sub-10-micron) patternability and demonstrate the homogeneity
(density control) of the patterned 1D NS network over 20 cm2 area. We will utilize the blanket
transfer (BT) method, a modified soft-lithography technique previously developed by our group
[50], to pattern the surface of the filter membrane. Akin to dry film photoresists (PRs), BT
laminates a PR layer onto various substrates, but at a much smaller thickness than dry film PRs,
facilitating the generation of the patterns as small as 5 Îźm. During filtration, the PR pattern will
serve as a template, guiding the deposition of the nanomaterials into the opened regions of the
filter membrane. We will employ a mixed cellulose ester (MCE) membrane as our FGA platform,
which is much more flexible and inexpensive compared to the AAO membranes. The MCE
membrane has been used to assemble large-area random-network films of nanomaterials in the
past [37,60], but selective filtration of FGA has not been explored with the MCE membrane. The
challenges associated with patterning the MCE membrane surface will be addressed. High-purity
semiconducting single-walled CNTs (SWCNTs) will be used as a model 1D NS to demonstrate
the concept of the proposed FGA. A simple technique to pick up and transfer the SWCNT network
film will be presented for the transistor device applications.

189

7.2 Experimental
7.2.1 Blanket transfer process

Figure 7.1. A schematic diagram of the fabrication sequences showing a versatility of PR blanket
transfer in different settings; (a) a PR film spin-coated and partially cured on a flat PDMS stamp;
and the fabrication of exemplary multilayer or freestanding microstructures using the PR BT
process such as (b1-b4) multilayer polymeric structures, (c1-c4) free-standing polymeric
structures, (d1-d4) PR patterns on a porous membrane.
As mentioned above, BT is a modified soft-lithographic process; that is, transferring a spincoated PR film from a flat elastomeric stamp to a substrate (Figure 7.1). The elastomeric stamp is
highly compliant, bringing the PR film into intimate contact with the substrate and eliminating the
need of large lamination pressure. Upon peeling of the stamp, the film can be transferred to a
variety of substrates; flat, etched, curved, fragile, and porous substrates. In addition, the thickness
(submicron to tens of microns) and tone (both positive and negative) of the PR film can be

190

judiciously adjusted for the target applications. One utility of the PR BT process is to apply a PR
layer on top of porous substrates without damaging or blocking the pores. If the PR solution is
directly spin-coated or sprayed, excessive PR would flow into the pores, making it difficult to be
exposed and developed. Moreover, porous AAO membranes are difficult to be patterned because
of their brittle nature [58]. The PR BT process entails very small lamination (or rolling) pressure,
facilitating the transfer of PR to the fragile AAO substrates. Here the same PR transfer principle
can be applied except that a low surface energy of the alumina surface makes the PR transfer to
the AAO membrane more challenging. Surface modification prior to transfer including a short
deposition of metal (e.g. Cr) film helps to increase the surface energy of the alumina membrane
and improve the transfer yield. In addition, the positive-tone PR (e.g. AZ 1518) is preferably used
over the negative PR because the positive PR does not require the post exposure step and can be
removed relatively at ease in the ensuing processes. Once the PR is transferred, the film can be
patterned using photolithography. We observe that parts of the PR film were pressed into the pores
during the transfer, and a higher exposure dose and a longer development time compared to a film
on a flat, rigid substrate are needed to ensure the openness of the pores after patterning. Figure 7.2a
shows line patterns of PR AZ 1518 with the nominal line width and spacing of 5 Îźm on the AAO
membrane. One of these line patterns is imaged using SEM (see Figure 7.2b) to reveal the openness
of the pores after patterning. Individual pores of average 220 nm are clearly seen in the inset image
without the PR residues. The openness of the pores in the patterned AAO membrane can be further
tested by filtrating a nanomaterial suspension through the filter membrane. If pores in the patterned
windows are not blocked, the suspension solution will flow through these open windows and
nanomaterials will be preferentially collected over them. A SWCNT suspension is vacuumfiltrated through the AAO membrane patterned with an array of square openings. The dark jade

191

regions in Figure 7.2c represent an array of SWCNT mats while the light green areas are the PR
layer on the AAO membrane. The SEM image of a single opening and the inset image in Figure
7.2d also suggest that SWCNTs are deposited only on the patterned window. This selective
deposition of the SWCNT mats imply a scalable approach to deterministically assemble
nanomaterials on the substrate [56,57].

Figure 7.2. (a) An optical micrograph of an array of PR 1518 lines patterned on the AAO
membrane; light jade – PR lines, dark jade – AAO membrane, (b) an SEM image of the PR patterns
in (a) showing the open porous structures; scale bar in inset image is 500 nm, (c) an optical
micrograph of an array of SWNT mats after the vacuum filtration of SWNT suspension through
the PR-patterned AAO membrane; light green – PR pattern, dark jade – SWNT mat on the AAO
membrane, (d) an SEM image of the single square window on which the SWNTs are selectively
deposited to form a CNT mat; scale bar in inset image is 500 nm.
7.2.2 Patterning of the MCE membrane
A 3-mm-thick PDMS stamp was firstly prepared by casting the mixed solution (10 parts
of SYLGARD 184 pre-polymer and 1 part of the curing agent, Dow Corning Co.) on a clean Si
wafer. Photoresist (Shipley 1813, MICROCHEM Co.) was then spin-coated on the PDMS stamp
at 5000 rpm for 1min. After 30 sec softbaking at 60°C, the PR layer on the PDMS stamp was
192

brought into an intimate contact with a MCE membrane (Hydrophilic, 0.22 Âľm, 47 mm in
diameter, Millipore Co.) at 110°C for 3 mins. Prior to the PR transfer, the MCE membrane is prebaked at 110°C for 2 mins and mounted onto a rigid substrate (e.g., a flat, 3-mm-thick aluminum
plate). Sometimes a thin (~20 nm thick) Cr layer (sputtered under 50 mPa background pressure)
can be deposited onto the MCE membrane to enhance the adhesion of the PR. The PDMS stamp
was then slowly (< 0.1cm s-1) peeled away from the MCE membrane, leaving the PR layer onto
the membrane. Before cooling down to room temperature, another 1 min soft-baking at 110°C
was performed to improve the adhesion of the PR film to the membrane.

Standard

photolithography, including the UV exposure (AB-M mask aligner, dose: 180 mJ¡cm-2) through a
photomask and the subsequent development (Shipley 352 developer, Dow Co.), were carried to
create the various patterns on the MCE membrane. A mild sonication was sometimes used to
accelerate the development speed and completely open the pores in the patterned windows. Upon
the development, the patterned membrane was rinsed thoroughly with DI water, dried, and stored
in a dust-free environment.

7.2.3 SWCNT solution preparation and vacuum filtration
Two different SWCNT solutions were used in the experiment. For the general FGA
experiment, unsorted SWCNTs (purified HiPCO, Unidym, 0.8 ~ 1.2 nm diameter, 100 ~1000 nm
length) were suspended in 1% sodium dodecyl sulfate aqueous solution. The typical concentration
ranges from 0.5 to 10 mg¡L-1. For the transistor experiments, a 95% pure semiconducting SWCNT
solution was purchased from Nanointegris Technology Inc. and diluted in an ionic surfactant
solution (1%) for filtration. A vacuum filtration setup was purchased from Fisher Scientific
(47 mm filter holder) and connected to a solvent trap and a vacuum pump (Alcatel 2021I). The

193

configuration for filtration is based on the dead-end flow similar to the popular benchtop chemistry
labware (Figure 7.3).

Figure 7.3. (a) The schematic diagram of the filtration system for depositing SWCNTs. (b) Photo
of the filtration system in the experiment.

The dead-end flow filtration allows the uniformly distributed pressure to be applied over
the filter membrane, leading to higher spatial uniformity. Before the patterned MCE membrane
(the patterned side facing up towards the solution) was positioned on a flat glass frit, the glass frit
was made wet by a few water droplets. Applying a vacuum pressure briefly would bring the
membrane into a conformal contact with the frit and remove any air bubbles that may have been
trapped. The prepared SWCNT solution (20~100 mL) was then poured over the filter membrane.
A vacuum pressure was applied until all the solutions were completely removed. The vacuum
pressure was kept for additional 3-5 mins after the filtration, and the filter membrane was retrieved
after the negative pressure diminished. The membrane was further dried overnight, or oven drying
at elevated temperature (~ 80°C) expedited the drying process.

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7.2.4 SWCNT transfer and transistor fabrication
A commercial PVA film (3M Inc.) was first laminated over the MCE membrane towards
the filtrated side. In order to improve the conformal contact between the PVA film and the
SWCNT patterns, we kept the stack under a low vacuum (~ 100 Pa) about 30 mins. By gently
peeling the PVA film off from the MCE membrane, the SWCNT patterns were picked up along
with the PR layer. Meanwhile, an n++ doped Si wafer (University Wafer, <100>, phosphorous,
0.001~0.005 Ί¡cm) with a 100-nm-thick SiO2 layer was used to pattern the Ti/Au electrodes with
various gaps (5, 10, 20, 50 Îźm) using the standard photolithography followed by metal lift-off. To
improve the adhesion of SWCNTs onto the receiving substrate, the SiO2 surface was
functionalized with 3-(aminopropyl)triethoxysilane (APTES, Sigma-Aldrich) as an amineterminated self-assembled monolayer. Prior to the APTES functionalization, the substrate was
cleaned using the standard cleaning procedure (5:1:1 vol. % of DI water, ammonium hydroxide
and hydrogen peroxide) to terminate the SiO2 surface with hydroxyl groups. After laminating the
SWCNT-PR/PVA stack on the APTES-treated SiO2 substrate under the slight pressure and
elevated temperature (~ 90°C), the stack was transferred to a warm water (~ 60°C) bath until the
PVA film was completely dissolved. A brief acetone soaking was then carried out to remove the
transferred PR layers.

7.2.5 Materials and device characterization
All the optimal images were obtained from the upright microscope (Nikon LV100) under
the reflection mode. The SEM images were collected from Hitach S-4700 Field Emission
Scanning Electron Microscopy. The pattern height measurements were performed using Dektak3

195

Surface Profiler. The s-SWCNT transistor characteristics were measured using an Agilent B1500A
semiconductor parameter analyzer.

7.3 Result and discussion

Figure 7.4. A schematic of the filtration guided assembly (FGA) process; (a) spin-coating of PR
on the flat PDMS stamp, (b) conformal contact between PR/PDMS and the MCE filter membrane,
(c) slow peeling of PDMS transferring the PR layer onto the membrane, (d) patterning PRs using
photolithography, (e-f) vacuum filtration to guide nanomaterials onto the pattern openings.

Creating large-area patterns on the surface of porous substrates with sub-10-micron
resolution proves to be challenging. For example, during spin-coating, the pores in the membrane
can be blocked by PR when a negative pressure is applied to a spinner chuck to hold the porous
substrate. Even with the extended exposure and/or PR development times, the PR layer embedded
in the pores cannot be completely removed, hampering the subsequent FGA step. Dynamic spincoating [56] or a special membrane holder [58] was used to reduce the PR flow into the porous
structures, but the pattern size smaller than 10 Îźm has not been demonstrated on porous substrates.
Another challenge is the extremely brittle nature of some porous substrates like the AAO
membrane, which excludes the option of using dry PR films due to high pressure required for
lamination. On the other hand, spin-coating of PR is not compatible with the MCE membranes
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because the solvents in PR attack the membrane. Below we shall show how we utilize the PR BT
to create high-resolution patterns on the MCE membrane, which will serve as the FGA template.
Figure 7.4 illustrates the sequence of the proposed FGA method. First we transfer a
uniform PR film onto a MCE membrane via BT. A thin (2~5 Îźm) PR layer is spin-coated onto a
flat elastomeric (e.g., polydimethylsiloxane (PDMS)) stamp (Figure1a). A highly compliant
PDMS stamp allows the PR film to be brought into intimate contact with the MCE membrane
without the need of large lamination pressure (Figure 7.4b). The detailed description of the BT
technique has been reported elsewhere [50]. The PR layer transferred onto the MCE membrane is
then subject to the standard photolithography process (Figure 7.4c and 7.4d). Depending on the
BT parameters, the transferred PR layer can be partially embedded into pores in the filter substrate.
In order to construct completely opened patterns in the porous membrane, relatively longer (1.5
times the standard recipe) UV exposure and development time are employed with a gentle
sonication. Next, the patterned MCE membrane is placed on a vacuum filtration setup, and the
suspended SWCNTs of a known volume is filtered through the membrane. Almost all SWCNTs
are deposited only onto the opened patterns while the solvent (e.g. water) gets removed through
the filter membrane (Figure 7.4e and 7.4f). The resulting SWCNT network patterns are to be
transferred to the target substrate for further characterization.
Figure 7.5 shows the optical microscope images of the various PR patterns (Shipley 1813,
1.4 ~ 1.6 Îźm thick) on the MCE membrane before and after the SWCNT filtration. The brighter
portions of each image in the left column of Figure 7.5 are the opened patterns on the MCE
membrane while the darker areas in the right column are the filtrated SWCNT networks. As the
SWCNTs are guided by the water flow and stacked on top of one another, they form a random
network structure inside each pattern. Lines, circles, and squares have been patterned on the MCE

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membrane with the minimum feature size of 5 Îźm, which corresponds to our previous report [50].
FGA combined with BT is capable of producing an arbitrary shape of the nanomaterial network
because the pattern is photolithographically defined.

Figure 7.5. Optical microscope images of the PR-patterned MCE membrane before SWCNT
filtration (a),(c),(e) and after filtration (b),(d),(f). Various SWCNT patterns were formed onto the
opened regions of the MCE membrane. Scale bars are 100 Îźm in the large views and 10 Îźm in
the inset.
The line pattern in Figure 7.5b also demonstrates the high density array, where the
proportion of the opened region to the blocked region is about 1 to 1. An extent to which FGA
maximizes the nanomaterial utilization (i.e., minimizing its wastes) can be investigated by the
careful inspection of the patterned filter membrane and the filtrated solvent waste. First, the PR
patterns on the filter membrane are imaged at high resolution in the optical microscope and
HRSEM, corroborating that no apparent residues of SWCNTs were deposited atop the PR surface
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(see Figure 7.5 and 7.6). This means that SWCNTs were faithfully guided towards the openings
on the filter membrane. Secondly, the wasted solvent from the initial filtration was collected and
re-filtrated over a new MCE membrane.

Figure 7.6. Photographic images of the 47-mm MCE membrane patterned with the PR layer (a)
before filtration and (b) after filtration of the SWCNT solution; SEM images of the individual linearray pattern (c) before SWCNT filtration and (d) after the filtration; (e-g) SEM images of the
SWCNT films deposited on the opened pattern for three different SWCNT loadings – 0.1, 0.25,
0.8 mg/L, respectively. Scale bar: 8 mm in (a) and (b), 100 Îźm in large view of (c) and (d), 20 Îźm
in the inset (top right) of (c) and (d), 200 nm in (e-g).
The MCE membranes after the initial filtration and re-filtration were imaged and compared
to each other (Figure 7.7). Few SWCNTs were observed on the re-filtrated membrane, indicating
that most of SWCNTs, if not all, were filtered onto the membrane during the initial filtration. It
has been also shown that the characteristic Raman peak for SWCNTs is missing in the re-filtrated

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membrane (see Figure 7.7c). This near-100% filtration result represents a route to maximal
nanomaterial utilization in assembling 1D NS networks.

Figure 7.7. Photographs of the 47-mm-diameter MCE membranes (a) after filtration of 40 mL of
highly concentrated (~8 mg/L) SWCNT solution and (b) after filtration of the waste solvent
collected from (a). (c) Raman spectroscopy measurements performed on the bare MCE membrane,
after 1st filtration of the SWCNT solution (a) and after re-filtration of the wasted solvent (b). The
characteristic Raman peak (G band) is missing in the re-filtrated MCE membrane.
To systematically study the FGA process and explore its applicability for device
fabrication, we designed an array of line patterns (16 of 20 Îźm wide, 500 Îźm long as shown in
Figure 7.8) and distributed them over a 47-mm-diameter MCE substrate. Each line array would
serve as a channel layer for SWCNT-based transistors. Figure 7.6a and 7.6b show the photographs
of the PR-patterned MCE membrane before and after SWCNT filtration. It is estimated that the
200

MCE membrane has an open area of approximately 1.2 cm2, which corresponds to 6.9% of the
entire filter membrane. As filtrated with the SWCNT solution, the opened regions of the MCE
membrane became darker. The amount of the SWCNTs deposited onto the membrane can be
estimated from the volume and concentration of the SWCNT solution, which were 40 mL and
0.25 mg/L for the sample images shown in Figure 7.6b. From the high resolution SEM image in
Figure 7.6c, the porous structures of the MCE membrane is observed – the nominal average pore
size is 220 nm.

Figure 7.8. (a) A photograph of the 47-mm MCE membrane patterned with the PR layer before
filtration, (b) an optical microscope image of the individual line array pattern from (a), (c) the
same MCE membrane as (a) but after filtration of the SWCNT solution, (d) the SWCNT line-array
pattern from (c). Scale bar: 8 mm in (a) and (c), 100 Îźm in (b) and (d).
One of the key metrics to determine the pattern quality of the proposed technique is the
relative amount of the SWCNTs deposited atop the PR film vs. ones deposited onto the patterned
windows. From the optical and SEM images of the filter membrane (Figure 7.6d and 7.8), no
SWCNTs (black dots in the optical image) are found outside the patterned windows, demonstrating
201

a high yield of patternability (> 99%). Comparing the top right insets of Figure 7.6c and 7.6d, we
can conclude that a conformal contact between the PR layer and the MCE membrane guarantees
the side-sealing and funneling a solvent flow only onto the openings in the filter membrane.
Figure 7.6(e-g) shows the zoomed-in SEM images of the SWCNT patterns, where a number of
distinctive SWCNT bundles can be readily recognized. The total amount of SWCNTs in each
pattern opening can be controlled by the concentration of the SWCNT suspension and its volume.
On a separate note, one may consider a FGA’s potential to align 1D NSs. In fact, the FGA
method was demonstrated in the past for aligning organic microwires [57]. One of the key
requirements for alignment is that the FGA pattern width should be comparable to the length of
NSs, so that the NSs would be aligned to the pattern direction to minimize the flow resistance
exerted by the filtration-induced flow field. Since the length of commercially available sorted
SWCNTs including ones used in this study is rather short (< 0.5 Îźm), the pattern width on the filter
membrane needs to be submicron to achieve some degree of alignment. Due to the limited pattern
resolution (as mentioned earlier, the best resolution so far attained is 5 Îźm), we think that the
alignment of SWCNTs would be impractical. Instead, one can explore the possibility of using
longer1D NSs for aligning experiment.
Since the SWCNT line patterns are formed simultaneously over the entire membrane
surface within a few to tens of minutes, the FGA platform presents a facile approach to the largearea assembly of 1D NSs. Figure 7.9(a) shows the measured average filtration speed as a function
of the solution concentration. The filtration speed is reduced with the increasing concentration of
the SWCNT solution because the SWCNTs would be deposited at a faster rate for the higher
concentrated solution, slowing down the subsequent filtration. Since the instantaneous filtration
speed is a function of time, measured experimentally is in the range of the predicted values for a

202

given filtration condition. Note that if a constant pressure is applied across the filter membrane,
the filtration speed will be slowed down as the SWCNT filter cake is formed within the open
window. For the dead-end filtration process in this experiment, the modified Darcy’s law equation
can be used to express a filtration speed or a flow rate through the membrane. An inverse of the
filtration flow rate (t/V), where t and V are the filtration time and (total solvent) volume, is given
as [61],
𝑡
𝜇𝑐𝛼
𝜇𝐿
= 2 𝑉+
𝑉 2𝐴 𝛥𝑃
ϗ𝐴𝛥𝑃

(1)

where ϗ, A, L are the intrinsic permeability, open area, and thickness of the filter membrane,
respectively. ΔP is the pressure drop across the membrane, μ is the viscosity of the SWCNT
solution, c is the concentration of SWCNTs by weight, and Îą is the specific cake resistance of the
SWCNT pattern. In Equation (1), the first term on the right hand side indicates the flow resistance
due to the filtered SWCNT pattern while the second term is related to the flow resistance imposed
by the filter membrane. For the MCE membrane (L ≈ 150 μm), ϗ can be estimated from the flow
rate measurement of pure water for a given pressure drop and was found to be 4.21 × 10-11 cm2
(consistent with the data sheet, GSWP04700, https://www.emdmillipore.com/US/en/product/).
The solvent of the SWCNT solution is deionized water whose Ο is around 0.89 mPa¡s at 25°C. As
mentioned above, A is calculated to be 1.2 cm2. During the experiment, the applied vacuum
pressure amounted to 100 Pa, indicating a maximum pressure difference of around 105 Pa across
the filter membrane. V is fixed at 40 mL. Based on these parameters, the specific cake resistance
of SWCNTs, α, can be estimated and ranges from 2.82 × 1013 ~ 5.33 × 1013 m/kg [62].
The MCE membrane is a commercial filter with high porosity and hydrophilicity and
commonly used to separate solutes from solvents like water. Quite a few reports including this
work have employed the MCE membranes for nanomaterial filtration thanks to their high porosity
203

and filtration speed [37,60]. The main disadvantage is that MCE can be easily attacked by alcoholbased or other organic solvents, which makes the FGA process limited to aqueous solvents. If
nanomaterials suspended in non-aqueous solvents are of interest for the FGA process, anodic
aluminum oxide (AAO) or polycarbonate (PC) membranes can be viable options. As AAO is an
inorganic membrane, it is resistant to most of organic solvents. PC is not as resistant as AAO and
yet more stable with alcohol-based solvents than MCE. The AAO membrane has high-density and
straight pores, exhibiting an ultrahigh filtration speed (several folds faster than MCE) [ 63–66].
However, it is extremely fragile and difficult to perform the BT process, resulting in a lower FGA
yield. On the other hand, the PC membrane is highly flexible, easier to handle, and amenable for
recycling, but has a much lower porosity than the other two, i.e., requiring a much longer filtration
time for the same amount of the solvent. This lower solvent flux also undermines the pattern
quality of FGA. The choice of the filter membrane, therefore, depends on the solvent type,
required pattern resolution, and available transfer method. The comparison among these filter
membranes is summarized in Table 7.1. For FGA of SWCNTs, we conclude that the MCE
membrane is the best choice in terms of the ease of handling and patternability.
Once completely dried, the SWCNTs on the filter membrane were transferred onto a target
substrate using a water-dissolvable adhesive tape such as polyvinyl alcohol (PVA). The similar
transfer processes have been reported for micro-/nanoscale structure transferring [67–70]. The main
advantage of the PVA-based transfer is that it can pick up almost all the filtrated SWCNTs.
However, it took a long time (> 60 min) to dissolve PVA after transfer, and sometimes the organic
residues were left on the target substrate.

Alternatively, a viscoelastic stamp made of

polydimethysiloxane (PDMS) was utilized to pick up only the SWCNT patterns, which were then
transferred onto the APTES-treated SiO2 substrate. The use of PDMS leads to a faster transfer

204

without any chemicals; but it is difficult to pick up all the SWCNT patterns in its entirety. It can
be attributed to the fact that the SWCNTs are slightly recessed compared to the protruded PR layer,
hampering intimate contact between the PDMS layer and the SWCNT patterns. This issue
becomes worse as a lower amount of SWCNTs is filtrated (i.e., the smaller patterned height).
Therefore, in this study, we employed the former approach for the pattern transfer.
Table 7.1. Comparison of the MCE, AAO and polycarbonate membrane in SWCNT filtration tests.
MCE

AAO

Polycarbonate

FLEXIBILITY

medium

low

high

POROSITY

high

high

low

low

high

medium

low

low

high

BT COMPATIBILITY

high

medium

low

FILTRATION RATE

medium~high

high

low

REUSABILITY

high

medium

high

COST

low

high

medium

SOLVENT
RESISTANCE
SURFACE
HYDROPHOBICITY

Figure 7.9b shows the topography of SWCNTs patterns transferred onto the SiO2/Si
receiving substrate.

The thickness (or height) of the periodic patterns for the SWCNT

concentration of 0.25mg in 1 L water and a filtration volume of 40 mL is around 400 nm. The
standard deviation of < 10% in the thickness measurements indicates the excellent uniformity of
the pattern thickness, which is one of the desired features. The gap distance between the SWCNTs

205

patterns are also uniform and close to the original gap from the PR pattern on the membrane,
demonstrating that the patterns were not disrupted during the transfer process. The spatial
uniformity of the transferred SWCTN patterns have been also investigated and discussed in
Figure 7.10. In general, the maximum thickness of SWCNT patterns is limited by the thickness
of PR layer. When excessive SWCNTs are filtrated, they would deposit over the PR patterns and
bridge the neighboring patterns, creating a continuous network film. The SWCNT patterns in
Figure 4a were transferred from the filter membrane possessing the 1.5-Îźm-thick PR layer,
indicating a success filtration was accomplished.

Figure 7.9. (a) The average filtration speed measured for the various SWNT loadings with the
fixed filtration volume (40 mL), (b) the surface profilometry scan of SWCNT patterns, (c) the sheet
resistances of the unsorted SWCNT patterns from the five locations indicated in the inset (after
transferred onto the glass substrate), (d) the percolation behavior of unsorted SWCNT pattern.
A set of sheet resistances of the unsorted SWCNT patterns (~ 400 nm thick) from the
different positions across the 47-mm transferred area were measured and plotted in Figure 4c,
manifesting a small resistance deviation along their positions and further proving the spatial
uniformity of the SWCNTs patterns. The assembly homogeneity is guaranteed by the process

206

itself [37,45]. As the SWCNTs accumulate on the filter membrane, the filtered network impedes
the solvent flow rate. Therefore, if SWCNTs are deposited more at a particular region of the
porous membrane, the rate of filtration will decrease, allowing more materials to be collected at
thinner or uncovered regions. This self-regulating feature facilitates highly uniform patterning
over the filtration membrane and is one of the reasons why vacuum filtration is widely used to
deposit thin films of various nanomaterials [45]. The unsorted SWCNT patterns of several loadings
have been used to create the conductance as a function of the SWCNT loading, namely a
percolation curve, as shown in Figure 7.9d. The dependence of the loading on conductance
(exponent ~ 2.01) follows the standard percolation theory, demonstrating the percolation in the 3D
film [63].

Figure 7.10. The surface profilometry scans for 6 SWCNT line patterns at 4 different random
locations. The SWCNT patterns were transferred to the silicon surface before the measurements.
The 6 line patterns at each location have the following averages and standard deviations of the
measured heights: (average = 330 nm, standard deviation = 34 nm), (325 nm, 44 nm), (358 nm,
21 nm), (344 nm, 27 nm)
The SWCNT patterns assembled by FGA and transferred onto the substrate can serve as
channel layers for CNT-based transistors. High-purity (~95%) semiconducting SWCNTs (s207

SWCNTs) were filtrated on the patterned MCE membrane. An array of s-SWCNTs was then
picked up by the PVA film. Prior to the s-SWCNT transfer, Ti/Au source/drain electrodes were
created on a SiO2/Si substrate with different sizes of the electrode gap. The gap distance
corresponds to the transistor channel length. The s-SWCNT pattern array on the PVA film was
manually aligned and transferred onto the electrode surface to form bottom-gate transistor devices.
The transferred s-SWCNT line pattern bridging onto the source and drain electrodes is shown in
the inset image of Figure 7.11a. The cracks (white lines) on the s-SWCNT pattern usually came
from the bending of the PVA film during the transferring, which did not affect the property of the
s-SWCNTs patterns. The proposed FGA platform is particularly cost-effective when the 1D NSs
of interest are expensive and the active pattern area is small. For example, a 95% pure s-SWCNTs
solution is sold at around $300 for 1 mg/mL, and a 99% pure one at over $800 for the same amount.
The FGA method effectively minimizes the nanomaterial waste and maximally utilizes them for
the device application. In the experiment, 40 mL of 0.25mg/L SWCNTs can produce over two
hundreds devices simultaneously by using FGA.
Figure 7.11a shows the source-drain current, Isd, versus the gate voltage, Vg, of the sSWCNT device at different source-drain voltages, Vsd, demonstrating the typical p-type transistor
characteristics. The on/off current ratio (Ion /Ioff ) is measure to be around 102. Figure 7.11b
presents the corresponding plots of Isd versus Vsd on the same device, which also exhibit clear
modulation of the channel conductivity by the applied gate voltage. The histograms for the
distributions of the on/off current ratios and field-effect mobilities are shown in Figure 7.11c and
7.11d, respectively.

208

Figure 7.11. Electrical characteristics of the bottom-gated SWCNT transistors. (a) Transfer
characteristics of a representative device (channel length = 5 Âľm, width = 20 Âľm) measured with
VDS varying from –5 to 0 V (Inset image – the false-colored SEM image of the transferred
SWCNTs patterns bridging on the electrodes; The golden color region represents the Au contacts
and three horizontal strips are the SWCNTs patterns. The dark vertical strip in the middle indicates
the gap space between two electrodes, which corresponds to the transistor channel length; Scale
bar = 20 μm). (b) Output characteristics of the same device with VGS varying from 20 to –20 V.
Histogram of the distributions of transistor performance of the on/off current ratio (c) and fieldeffect mobility (d).
The mobility of those devices was deduced from the transfer curve (e.g. Figure 7.11a). In
detail, the mobility was calculated using the following equation [71–73].
Âľ=

𝐿
𝜕𝐼𝑆𝐷
𝐶𝑔 𝑉𝑆𝐷 𝑊 𝜕𝑉𝑔

(2)

where Âľ is the mobility, Cg is the capacitance. L and W are the length and width of the SWCNTs
channel, respectively. For the given ISD, VSD and Vg, the mobility can be determined. These
statistical data were collected over 30 devices on a single 2” wafer and demonstrates that the
dispersed s-SWCNTs in the channel have relatively good electron mobilities but low on/off ratios.
This low on/off ratios can be attributed to the relatively thick (400 ~ 420 nm) transferred sSWCNT layer in which 5% metallic SWCNTs play an important role on the device performance
209

– dramatically reducing the gate-effect. More work is geared towards using s-SWCNTs with better
purity and improving both FGA and transfer techniques to effectively pick up and transfer
SWCNTs of small thickness (< 50 nm), which will in turn improve the device’s on/off ratio
performance.
It is worth mentioning that quantitative comparison between FGA and other assembly
techniques (e.g. traditional lithography combined with etching or lift-off, surface-mediated
directed assembly, and field-assisted assembly) would be highly desirable based on the key metrics
such as cost, throughput, feature size, waste material disposal, and eventual device performance.
However, it is often not feasible, if not impossible, to quantitatively compare these assembly
methods because the metrics have to vary significantly depending on the type of the nanomaterials,
assembly volume and speed, and the target assembled structures. Therefore, more careful study
can be devised as future work to make direct and quantitative comparison across the multiple
assembling techniques for a given nanomaterial and device structure.

7.4 Conclusion
In this work we investigated a scalable yet inexpensive nanomaterial assembly method,
namely filtration-guided assembly (FGA), which provides an array pattern of homogeneous 1D
NS network films. The BT process allowed us to create the various PR patterns of sub-10-micron
resolution on the nanoporous filter membrane. The choice of the filter membrane is mixed
cellulose ester (MCE), which possesses high permeability for fast filtration as well as flexibility
for easy handling. Selective filtration through the PR-patterned MCE membrane provides an
approach to assembling few hundreds of individual SWCNT networks under 10 minutes.
Combining FGA with the PVA- or PDMS-based transfer process, we created an array of the

210

semiconducting SWCNTs random network onto the SiO2/Si substrate with a minimal material
loss. The thickness and electrical resistivity of the transferred SWCNTs were measured over the
20 cm2 substrate, demonstrating the uniformity with less than 5% variance. These semiconducting
(95%) SWCNT networks were tested as the back-gated field-effect transistors. Good electron
mobility but with relatively low on/off current ratios (< 100) was observed mainly due to the large
thickness of the transferred SWCNT layer. More work should be carried out to improve the FGA
and transfer methods to diverse substrates.

211

REFERENCES

212

REFERENCES

1. Law, M., Goldberger, J. & Yang, P. Semiconductor Nanowires and Nanotubes. Annu. Rev.
Mater. Res. 34, 83–122 (2004).
2. Tans, S. J., Verschueren, A. R. M. & Dekker, C. Room-temperature transistor based on a
single carbon nanotube. Nature 393, 49–52 (1998).
3. Bachtold, A., Hadley, P., Nakanishi, T. & Dekker, C. Logic Circuits with Carbon Nanotube
Transistors. Science 294, 1317–1320 (2001).
4. Cui, Y. & Lieber, C. M. Functional Nanoscale Electronic Devices Assembled Using Silicon
Nanowire Building Blocks. Science 291, 851–853 (2001).
5. Xiang, J. et al. Ge/Si nanowire heterostructures as high-performance field-effect transistors.
Nature 441, 489–493 (2006).
6. Vaurette, F., Leturcq, R., Lepilliet, S., Grandidier, B. & Stiévenard, D. Confinementmodulated junctionless nanowire transistors for logic circuits. Nanoscale 6, 13446–13450
(2014).
7. Kong, J. et al. Nanotube Molecular Wires as Chemical Sensors. Science 287, 622–625
(2000).
8. Wan, Q. et al. Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors.
Appl. Phys. Lett. 84, 3654–3656 (2004).
9. Lee, J. et al. A Highly Responsive Silicon Nanowire/Amplifier MOSFET Hybrid Biosensor.
Sci. Rep. 5, 12286 (2015).
10. Duan, X., Huang, Y., Cui, Y., Wang, J. & Lieber, C. M. Indium phosphide nanowires as
building blocks for nanoscale electronic and optoelectronic devices. Nature 409, 66–69
(2001).
11. Dai, X. et al. Flexible Light-Emitting Diodes Based on Vertical Nitride Nanowires. Nano
Lett. 15, 6958–6964 (2015).
12. Tian, B. et al. Coaxial silicon nanowires as solar cells and nanoelectronic power sources.
Nature 449, 885–889 (2007).
13. Kelzenberg, M. D. et al. Photovoltaic Measurements in Single-Nanowire Silicon Solar Cells.
Nano Lett. 8, 710–714 (2008).
14. Ko, M.-D., Rim, T., Kim, K., Meyyappan, M. & Baek, C.-K. High efficiency silicon solar
cell based on asymmetric nanowire. Sci. Rep. 5, 11646 (2015).

213

15. Qi, P. et al. Toward Large Arrays of Multiplex Functionalized Carbon Nanotube Sensors for
Highly Sensitive and Selective Molecular Detection. Nano Lett. 3, 347–351 (2003).
16. Meitl, M. A. et al. Solution Casting and Transfer Printing Single-Walled Carbon Nanotube
Films. Nano Lett. 4, 1643–1647 (2004).
17. Chen, Y. et al. Highly flexible, transparent, conductive and antibacterial films made of spincoated silver nanowires and a protective ZnO layer. Phys. E Low-Dimens. Syst.
Nanostructures 76, 88–94 (2016).
18. Wang, C. et al. Wafer-Scale Fabrication of Separated Carbon Nanotube Thin-Film
Transistors for Display Applications. Nano Lett. 9, 4285–4291 (2009).
19. Ahn, K., Kim, D., Kim, O. & Nam, J. Analysis of transparent conductive silver nanowire
films from dip coating flow. J. Coat. Technol. Res. 12, 855–862 (2015).
20. Lan, B. et al. Multifunctional Free-Standing Membrane from the Self-assembly of Ultralong
MnO2 Nanowires. ACS Appl. Mater. Interfaces 5, 7458–7464 (2013).
21. Xu, W. et al. Fabrication of Flexible Transparent Conductive Films with Silver Nanowire by
Vacuum Filtration and PET Mold Transfer. J. Mater. Sci. Technol. 32, 158–161 (2016).
22. Duan, X., Huang, Y., Agarwal, R. & Lieber, C. M. Single-nanowire electrically driven
lasers. Nature 421, 241–245 (2003).
23. Cai, L., Zhang, S., Miao, J., Yu, Z. & Wang, C. Fully Printed Foldable Integrated Logic
Gates with Tunable Performance Using Semiconducting Carbon Nanotubes. Adv. Funct.
Mater. 25, 5698–5705 (2015).
24. Smith, P. A. et al. Electric-field assisted assembly and alignment of metallic nanowires.
Appl. Phys. Lett. 77, 1399–1401 (2000).
25. Dong, L. et al. Dielectrophoretically Controlled Fabrication of Single-Crystal Nickel Silicide
Nanowire Interconnects. Nano Lett. 5, 2112–2115 (2005).
26. Englander, O., Christensen, D., Kim, J., Lin, L. & Morris, S. J. S. Electric-Field Assisted
Growth and Self-Assembly of Intrinsic Silicon Nanowires. Nano Lett. 5, 705–708 (2005).
27. GarcĂ­a-SĂĄnchez, P., Arcenegui, J. J., Morgan, H. & Ramos, A. Self-assembly of metal
nanowires induced by alternating current electric fields. Appl. Phys. Lett. 106, 023110
(2015).
28. Tanase, M. et al. Magnetic Alignment of Fluorescent Nanowires. Nano Lett. 1, 155–158
(2001).
29. Trotsenko, O., Tokarev, A., Gruzd, A., Enright, T. & Minko, S. Magnetic field assisted
assembly of highly ordered percolated nanostructures and their application for transparent
conductive thin films. Nanoscale 7, 7155–7161 (2015).
214

30. Chen, Y. et al. Tunable Nanowire Patterning Using Standing Surface Acoustic Waves. ACS
Nano 7, 3306–3314 (2013).
31. Strobl, C. J., Schäflein, C., Beierlein, U., Ebbecke, J. & Wixforth, A. Carbon nanotube
alignment by surface acoustic waves. Appl. Phys. Lett. 85, 1427–1429 (2004).
32. Su, B., Wu, Y. & Jiang, L. The art of aligning one-dimensional (1D) nanostructures. Chem.
Soc. Rev. 41, 7832 (2012).
33. Park, B., Bae, I.-G. & Huh, Y. H. Aligned silver nanowire-based transparent electrodes for
engineering polarisation-selective optoelectronics. Sci. Rep. 6, 19485 (2016).
34. Cha, N.-G., Echegoyen, Y., Kim, T.-H., Park, J.-G. & Busnaina, A. A. Convective Assembly
and Dry Transfer of Nanoparticles Using Hydrophobic/Hydrophilic Monolayer Templates.
Langmuir 25, 11375–11382 (2009).
35. Chen, G.-X., Kim, H.-S., Park, B. H. & Yoon, J.-S. Controlled Functionalization of
Multiwalled Carbon Nanotubes with Various Molecular-Weight Poly(l-lactic acid). J. Phys.
Chem. B 109, 22237–22243 (2005).
36. Mann, S. Self-assembly and transformation of hybrid nano-objects and nanostructures under
equilibrium and non-equilibrium conditions. Nat. Mater. 8, 781–792 (2009).
37. Wu, Z. et al. Transparent, Conductive Carbon Nanotube Films. Science 305, 1273–1276
(2004).
38. Hu, L., Hecht, D. S. & GrĂźner, G. Percolation in Transparent and Conducting Carbon
Nanotube Networks. Nano Lett. 4, 2513–2517 (2004).
39. Zhou, Y., Hu, L. & GrĂźner, G. A method of printing carbon nanotube thin films. Appl. Phys.
Lett. 88, 123109 (2006).
40. Mohseni, P. K. et al. Growth and Characterization of GaAs Nanowires on Carbon Nanotube
Composite Films: Toward Flexible Nanodevices. Nano Lett. 8, 4075–4080 (2008).
41. Chan, W.-F. et al. Zwitterion Functionalized Carbon Nanotube/Polyamide Nanocomposite
Membranes for Water Desalination. ACS Nano 7, 5308–5319 (2013).
42. Kemnade, N. et al. Non-destructive functionalisation for atomic layer deposition of metal
oxides on carbon nanotubes: effect of linking agents and defects. Nanoscale 7, 3028–3034
(2015).
43. Madaria, A. R., Kumar, A., Ishikawa, F. N. & Zhou, C. Uniform, highly conductive, and
patterned transparent films of a percolating silver nanowire network on rigid and flexible
substrates using a dry transfer technique. Nano Res. 3, 564–573 (2010).
44. Serre, P., Ternon, C., Stambouli, V., Periwal, P. & Baron, T. Fabrication of silicon nanowire
networks for biological sensing. Sens. Actuators B Chem. 182, 390–395 (2013).
215

45. Eda, G., Fanchini, G. & Chhowalla, M. Large-area ultrathin films of reduced graphene oxide
as a transparent and flexible electronic material. Nat. Nanotechnol. 3, 270–274 (2008).
46. Li, D., MĂźller, M. B., Gilje, S., Kaner, R. B. & Wallace, G. G. Processable aqueous
dispersions of graphene nanosheets. Nat. Nanotechnol. 3, 101–105 (2008).
47. Lotya, M. et al. Liquid Phase Production of Graphene by Exfoliation of Graphite in
Surfactant/Water Solutions. J. Am. Chem. Soc. 131, 3611–3620 (2009).
48. Hu, W. et al. Graphene-Based Antibacterial Paper. ACS Nano 4, 4317–4323 (2010).
49. Aghigh, A. et al. Recent advances in utilization of graphene for filtration and desalination of
water: A review. Desalination 365, 389–397 (2015).
50. Zhang, Y., Han, J.-H., Zhu, L., Shannon, M. A. & Yeom, J. Soft lithographic printing and
transfer of photosensitive polymers: facile fabrication of free-standing structures and
patterning fragile and unconventional substrates. J. Micromechanics Microengineering 24,
115019 (2014).
51. Oh, J. H. et al. Observation of orientation-dependent photovoltaic behaviors in aligned
organic nanowires. Appl. Phys. Lett. 103, 053304 (2013).
52. Yu, H., Bao, Z. & Oh, J. H. High-Performance Phototransistors Based on Single-Crystalline
n-Channel Organic Nanowires and Photogenerated Charge-Carrier Behaviors. Adv. Funct.
Mater. 23, 629–639 (2013).
53. Wu, Q., Xu, Y., Yao, Z., Liu, A. & Shi, G. Supercapacitors Based on Flexible
Graphene/Polyaniline Nanofiber Composite Films. ACS Nano 4, 1963–1970 (2010).
54. Byon, H. R., Gallant, B. M., Lee, S. W. & Shao-Horn, Y. Role of Oxygen Functional Groups
in Carbon Nanotube/Graphene Freestanding Electrodes for High Performance Lithium
Batteries. Adv. Funct. Mater. 23, 1037–1045 (2013).
55. Liu, Y. et al. CuO nanosheets/rGO hybrid lamellar films with enhanced capacitance.
Nanoscale 5, 9134 (2013).
56. Lim, C., Min, D.-H. & Lee, S.-B. Direct patterning of carbon nanotube network devices by
selective vacuum filtration. Appl. Phys. Lett. 91, 243117 (2007).
57. Oh, J. H. et al. Solution-processed, high-performance n-channel organic microwire
transistors. Proc. Natl. Acad. Sci. 106, 6065–6070 (2009).
58. Kim, J., Voelkerding, K. V. & Gale, B. K. Patterning of a nanoporous membrane for multisample DNA extraction. J. Micromechanics Microengineering 16, 33 (2006).
59. Shi, W. et al. Functionalized anodic aluminum oxide (AAO) membranes for affinity protein
separation. J. Membr. Sci. 325, 801–808 (2008).

216

60. Field, C. R., Yeom, J., Salehi-Khojin, A. & Masel, R. I. Robust fabrication of selective and
reversible polymer coated carbon nanotube-based gas sensors. Sens. Actuators B Chem. 148,
315–322 (2010).
61. Mahdi, F. M. & Holdich, R. G. Laboratory cake filtration testing using constant rate. Chem.
Eng. Res. Des. 91, 1145–1154 (2013).
62. Tanaka, T. Filtration characteristics of carbon nanotubes and preparation of buckypapers.
Desalination Water Treat. 17, 193–198 (2010).
63. Yang, S. Y. et al. Nanoporous Membranes with Ultrahigh Selectivity and Flux for the
Filtration of Viruses. Adv. Mater. 18, 709–712 (2006).
64. Jang, J. H. et al. Correlation between particle deposition and the size ratio of particles to
patterns in nano- and micro-patterned membrane filtration systems. Sep. Purif. Technol. 156,
Part 2, 608–616 (2015).
65. Han, Y., Xu, Z. & Gao, C. Ultrathin Graphene Nanofiltration Membrane for Water
Purification. Adv. Funct. Mater. 23, 3693–3700 (2013).
66. Belwalkar, A., Grasing, E., Van Geertruyden, W., Huang, Z. & Misiolek, W. Z. Effect of
processing parameters on pore structure and thickness of anodic aluminum oxide (AAO)
tubular membranes. J. Membr. Sci. 319, 192–198 (2008).
67. Meitl, M. A. et al. Transfer printing by kinetic control of adhesion to an elastomeric stamp.
Nat. Mater. 5, 33–38 (2006).
68. Onoe, H., Iwase, E., Matsumoto, K. & Shimoyama, I. 3D integration of heterogeneous
MEMS structures by stamping transfer. in IEEE 20th International Conference on Micro
Electro Mechanical Systems, 2007. MEMS 175–178 (2007).
doi:10.1109/MEMSYS.2007.4433045
69. He, X. et al. A highly conductive, flexible, transparent composite electrode based on the
lamination of silver nanowires and polyvinyl alcohol. J. Mater. Chem. C 2, 9737–9745
(2014).
70. Dong, J., Mohieddin Abukhdeir, N. & Goldthorpe, I. A. Simple assembly of long nanowires
through substrate stretching. Nanotechnology 26, 485302 (2015).
71. Sangwan, V. K. et al. Fundamental Performance Limits of Carbon Nanotube Thin-Film
Transistors Achieved Using Hybrid Molecular Dielectrics. ACS Nano 6, 7480–7488 (2012).
72. Xue, W., Liu, Y. & Cui, T. High-mobility transistors based on nanoassembled carbon
nanotube semiconducting layer and SiO2 nanoparticle dielectric layer. Appl. Phys. Lett. 89,
163512 (2006).
73. DĂźrkop, T., Getty, S. A., Cobas, E. & Fuhrer, M. S. Extraordinary Mobility in
Semiconducting Carbon Nanotubes. Nano Lett. 4, 35–39 (2004).
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CHAPTER 8
CONCLUSION AND OUTLOOKS
TiO2 and ZnO attracted considerable attention not only because their superior electrical,
chemical and optical properties but also their high capability with other nanomaterials, which
result the prominent performance in numerous applications. In this chapter, we primarily
introduced the synthesis strategy of TiO2 NWs and ZnO NWs and their application in
photocatalysis. In Chapter 1, we briefly reviewed the mechanism of the photocatalysis and the
well-developed

TiO-based

and

ZnO-based

nanostructures.

One

special

application,

micromotors/micromotors was regarded as a promising platform for the use of TiO2 and ZnO. The
main content in Chapter 2 is the summarization of the recent approaches about ZnO-based
photocatalysts in aspects of foreign materials and photocatalysis systems. As a detail literature
review, we identified the feasibility of a potential foreign material (i.e. Si nanoscrystals) and an
efficient photocatalysis system (i.e. micromotors), which were developed and studied in the
following chapters, respectively. The Chapter 3 mostly focused on our recent approaches in
synthesizing one-dimensional nanostructures of pure TiO2 and ZnO via solution-base method and
their numbers of modifications against the drawbacks. We learned that solution-base method
offered high chance to grow varied geometries of TiO2 and ZnO on most of the substrates, which
has a variety of prospective applications. To verify the enhancement of the photocatalytic
properties of ZnO, a ZnO NWs/Si NCs heterogeneous structure was fabricated in Chapter 4, which
the electronic mechanism was studied to interpret the enhanced photocatalysis phenomenon. In
this chapter we also understood that the obtained improvement may not be only applied to the
pollutant decomposition but also good for other optoelectrical devices with the similar driven
principle. With those knowledge we believed that it is worthwhile to combine those distinguished

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nanostructures with an interesting appliance, micromotors, to further raise its performance in
environmental remediation and extend its functions in other fields. Chapter 5 introduced a novel
type of floating catalysts which consisted of ZnO NW array and transparent polymer film. The
embedded ZnO NWs possessed reliable connection to polymer substrate and exhibited remarkable
degradability to probe pollutant as well as crude oil. As the support, PDMS film was found to be
always floating atop water which was advantageous to maximally receive incident light for
photocatalysts. The pizeocatalysis property of ZnO NWs was also investigated with a proper setup
which suggested a way to further improve the degradability of ZnO NWs. On the basis of the
technique in Chapter 5, we first developed a novel platform to efficiently produce soft micromotors
in Chapter 6, which has extraordinary capability in constructing desired complex shapes and very
harmony with ZnO NWs. As one realized function, ISMs assembled with the ZnO NWs provides
excellent photocatalytic degradation against the model pollutant. As a promising device, the
functionalization mainly in autonomous movement was discussed in Chapter 6. It is clearly seen
that the self-movement as a particular capability lead to a significant improvement of
photocatalytic performance. Therefore, we are expecting that the ISMs will eventfully possess
multiple functions which is adapt to most of cases. The reinforcement of the ISMs as our major
future work will be continued in terms of other applications. Since the inspiration of the wonderful
works in the photocatalysts, we will work on coupling valid and efficient heterogeneous structures
and the functional ISMs in the future. Apart from the photocatalysis, another key application, 1D
nanostructure-based electronic devices was introduced in Chapter 7, where a novel assembly
method, FGA, was developed with the assistance of BT method. In this work, SWCNTs was
fabricated as high-performance transistor arrays on wafer scale. As known, nanostructures,
especially TiO2 and ZnO, possess unique electrochemical properties, which can be utilized as the

219

building blocks for the high performance components in the scalable integrated circuits. Since the
FGA-BT method introduced in Chapter 7 has been demonstrated as a cost-effectiveness and easyto-handle strategy for the assembly of low-dimensional materials onto predetermined substrates,
the next step is to test TiO2 NWs and ZnO NWs as the extension. To enrich the field of our
research, we also discussed the compatibility of FGA for other 1D and 2D nanostructures at the
end.
Presently we have developed few reliable and reproducible solution-base synthesis
methods for both TiO2 and ZnO 1D nanostructures. The ongoing modification process towards the
compatibility for the substrates, manageable diverse geometry and large scale fabrication is
expected to be used not only in micromotors but also in other extended applications such as solar
cells and electrical devices. Moreover, the patternability of those 1D nanostructures is extremely
significant in multi-functional micromotors, which is presumed as an advanced technique to confer
additional heterogeneous materials to extend the functionality with the combination of the
optimized SPB platform. In next we will also work on the materials synthesis and the modification
of the platform to obtain catalyst with better degradability. And we will also test the catalysts in
other harsh pollutants except for MB dye and crude oil. A variety of desired catalysts will be
fabricated with suitable platform in the future.

220