NANOTECHNOLOGY APPLICATIONS FOR BIOMASS PRETREATMENT，
FUNCTIONAL MATERIAL FABRICATION AND SURFACE MODIFICATION
By
Wei Wang

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
Chemical Engineering
2012

ABSTRACT
NANOTECHNOLOGY APPLICATIONS FOR BIOMASS PRETREATMENT，
FUNCTIONAL MATERIAL FABRICATION AND SURFACE MODIFICATION
By
Wei Wang
Nanotechnology has gained its prosperity in the past two decades because of extensive
contributions from interdisciplinary collaboration and a favorable interaction with practical
applications. It covers a huge spectrum of applications. The dissertation hereby, in conjunction
with the three research projects conducted by the author, will make contributions to
nanotechnology applications in the following three topics: biomass pretreatment in biofuel
production, functional material fabrication and surface modification.
First, a fast and efficient nano-scale shear hybrid alkaline (NSHA) pretreatment method
of lignocellulosic biomass was introduced.

In this work, corn stover was pretreated in a

modified Taylor-Couette reactor with sodium hydroxide at room temperature, with a two-minute
retention time and a 12500 s

-1

shear rate. Synergistic effects induced by the NSHA pretreatment

disrupted the naturally-formed recalcitrance of biomass and generated nano-scale polysaccharide
aggregates that are ready to be digested. After the pretreatment, results revealed major removals
of hemicellulose and lignin, leaving an up to 82 % of cellulose content in the remaining solid.
Compared with untreated corn stover, an approximately 4-fold increase in enzymatic cellulose
conversion and a 5-fold increase in hemicellulose conversion were achieved.
Second, a nano-deposition strategy was developed to enhance the energy absorption
capacity of aluminum (Al) open-cell foams. The energy absorption capacity of open cell foams
can be enhanced by a homogeneous thickening of the foam struts. However, the enhancement is
compromised since an increase in the plateau stress without a reduction in densification strain

cannot be achieved. To overcome that problem, a featured non-cyanide nano-crystalline copper
electro-deposition system was setup for the coating of open-cell Al foam, and, the energy
absorption capacity as a function of foam pore size and Cu coating thickness was investigated.
An up to 3-time enhancement was achieved with a 60 m Cu coating on Al foams with an
average strut thickness of 192 m. The compressive stress-strain response of the composite
samples showed no significant reduction of the densification strain compared to the uncoated
foams. With the same overall strut thickness, nano-reinforced foams had superior energy
absorption capacity over plain foams, with almost a 2-time enhancement.
Finally, a facile “dip & rinse” method for nickel (Ni) electroless deposition on
hydrophobic polymer surfaces was developed. The electroless deposition (metallization) usually
incorporates a harsh and/or toxic surface conditioning to activate the substrate. To eliminate the
need for that step, a facile method of electroless Ni deposition on various hydrophobic polymer
substrates was demonstrated, by making use of the hydrophobic interactions between
Poly(allylamine

hydrochloride)

(PAH)

and

polymer

substrates

for

catalyst

adsorption/immobilization. Various hydrophobic polymer surfaces with different geometries and
dimensions, including low density polyethylene (LDPE), hi gh density polyethylene (HDPE),
polypropylene (PP) and polystyrene (PS) thin sheets, and PE pellets were tested and Ni was
successfully deposited onto all these surfaces. A kinetic study on polymer thin sheets examples
showed that with 2 hours of deposition, an approximately 2 m thickness was achieved. A
prove-of-concept study showed that Ni coated polymer thin sheets can be further
electrodeposited with heterogeneous metal (Cu), hence enabling a faster thickness growth over
time.

DEDICATION
This dissertation is dedicated to my family, especially my beloved wife, Kaiyao Ni, my parents,
Jianying Jin and Fengqiang Wang, my parents in law, Shaozhen Lu and Zhi’an Ni, and all other
members.

iv

ACKNOWLEDGEMENTS

It has been a great privilege to spend several years in the Department of Chemical
Engineering and Materials Science at Michigan State University for my PhD study and
completion of the dissertation. The Spartan spirit, which I will carry on with wherever I go, has
been inspirational to me and made me who I become today.
This dissertation would not have been accomplished without help from many people in
many ways. I would like to acknowledge all of them from the bottom of my heart.
First and foremost, with immense gratitude I would like to acknowledge my PhD advisor
and mentor, Professor Ilsoon Lee for his continuous guidance and support throughout the
production of this research and dissertation. He has been patiently providing unique insights and
encouragement for me to proceed through the PhD program. His mentorship in various ways is a
precious treasure to me.
I am also grateful to my dissertation committee members, Professor K. Jayaraman,
Professor David Hodge and Professor Jung-Wuk Hong for the invaluable discussions and help to
my research work. Their lectures on different topics helped me improve my knowledge in the
related areas. Also, their constructive suggestions and inputs make this dissertation better.
Special thanks go to Professor Rigoberto Burgueño for his support and all the helpful
discussions, comments and contributions to the Chapter 3 in this dissertation.
I am also thankful to Dr. Jue Lu at Metna Corporation. She has been incredibly helpful in
providing me with the samples, useful information and feedbacks.
I would like to thank my colleagues and office mates at EB 2522, Shaowen Ji, Ankush
Gokhale, Oishi Sanyal, Tongjun Liu, Xianfeng Ma, Rui Lin, Yangmu Liu, Bhushan Awate, Ying

v

Liu, Lee Alexander, Yi Sun, Jing Yu and Anna Song for their friendship and help all over the
years. It is a great pleasure working with these brilliant people. Their company made my time in
the lab enjoyable.
I am indebted to my family for their unconditional love and support. Being away from
my parents for so many years helped me better realize how important the family means to me.
My beloved wife, Kaiyao Ni made me the luckiest man on earth to have her with me for a
lifetime. I appreciate the love, care and everything she did for me. My parents, Jianying Jin and
Fengqiang Wang have had their love and support for me since the day I was born. Their trust in
me and good examples by themselves gave me strength to overcome the challenges and
difficulties throughout my PhD program. My parents in law, Shaozhen Lu and Zhi’an Ni, who
treat me like their own son, are unconditionally supportive and caring all over the years. I am
also thankful to all my other family members for their love and support.
Last but not least, I want to acknowledge the National Science Foundation, Michigan’s
University Research Corridor and Michigan Initiative for Innovation & Entrepreneurship for the
financial support on my research.

vi

TABLE OF CONTENTS

LIST OF TABLES ....................................................................................................................... ix
LIST OF FIGURES .......................................................................................................................x
Chapter 1 Introduction................................................................................................................. 1
1.1 Overview of the nanotechnology........................................................................................ 1
1.2 Nanotechnology for biofuel production ............................................................................ 5
1.3 Nanotechnology for functional material fabrication ....................................................... 8
1.4 Nanotechnology for surface modification ....................................................................... 11
1.5 Scope of the dissertation ................................................................................................... 14
REFERENCES ........................................................................................................................ 19
Chapter 2 Fast and efficient nanoshear hybrid alkaline pretreatment of corn stover for
biofuel and materials production............................................................................................... 27
2.0 Abstract .............................................................................................................................. 27
2.1 Introduction ....................................................................................................................... 28
2.2 Experimental Sections ...................................................................................................... 29
2.2.1 Materials....................................................................................................................... 29
2.2.2 Modified Taylor-Couette reactor (nanomixer) ............................................................ 30
2.2.3 Nanoshear hybrid alkaline pretreatment ...................................................................... 32
2.2.4 Washing ....................................................................................................................... 32
2.2.5 Drying .......................................................................................................................... 33
2.2.6 Compositional analysis ................................................................................................ 33
2.2.7 Enzymatic hydrolysis ................................................................................................... 33
2.2.8 Scanning electron microscope (SEM) imaging ........................................................... 34
2.3 Results and discussions ..................................................................................................... 34
2.3.1 Compositional analysis ................................................................................................ 34
2.3.2 Chemical and energy recovery..................................................................................... 37
2.3.3 Enzymatic hydrolysis ................................................................................................... 38
2.3.4 Scanning electron microscope (SEM) imaging ........................................................... 44
2.4 Conclusions ........................................................................................................................ 48
REFERENCES ........................................................................................................................ 51
Chapter 3 Nano-deposition on 3-D open-cell aluminum foam materials for improved
energy absorption capacity ........................................................................................................ 55
3.0 Abstract .............................................................................................................................. 55
3.1 Introduction ....................................................................................................................... 56
3.2 Experimental Methods ..................................................................................................... 58
3.2.1 Materials and equipment .............................................................................................. 58
3.2.2 Sample pretreatment .................................................................................................... 59
3.2.3 Electro-deposition ........................................................................................................ 60
3.2.4 Scanning electron microscope (SEM) imaging ........................................................... 62
vii

3.2.5 Crystallite size determination using X-ray diffraction (XRD)..................................... 62
3.2.6 Quasi-static compression test ....................................................................................... 63
3.3 Results and discussions ..................................................................................................... 63
3.3.1 3-D deposition visualization and uniformity ............................................................... 64
3.3.2 Quasi-static compression test and energy absorption calculation................................ 73
3.3.3 Comparison between coated and uncoated foam samples with same strut thickness .. 80
3.4 Conclusions ........................................................................................................................ 83
REFERENCES ........................................................................................................................ 85
Chapter 4 A facile method of nickel electroless deposition on various neutral hydrophobic
polymer surfaces ......................................................................................................................... 89
4.0 Abstract .............................................................................................................................. 89
4.1 Introduction ....................................................................................................................... 90
4.2 Experimental sections ....................................................................................................... 92
4.2.1 Materials and equipment .............................................................................................. 92
4.2.2 Ni electroless deposition .............................................................................................. 93
4.2.3 Copper (Cu) electro-deposition.................................................................................... 94
4.2.4 Scanning electron microscopy (SEM) imaging ........................................................... 95
4.2.5 Kinetic study of Ni deposition on polymers ................................................................ 95
4.2.6 Optical microscopy imaging ........................................................................................ 96
4.3 Results and discussions ..................................................................................................... 96
4.3.1 Visualization and microscopic analysis of polymer thin sheets .................................. 96
4.3.2 Visualization and microscopic analysis of polymer pellets and spheres ................... 107
4.3.2.1 Ni deposition on PE pellets ................................................................................. 110
4.3.2.2 Ni deposition on PS microspheres ...................................................................... 111
4.3.3 Kinetic study of Ni deposition ................................................................................... 114
4.3.3 Cu electro-deposition on Ni coated polymer ............................................................. 116
4.4 Conclusions ...................................................................................................................... 119
REFERENCES ...................................................................................................................... 122
Chapter 5 Conclusions and future work ................................................................................. 127
5.1 Conclusions ...................................................................................................................... 127
5.2 Outlook & future work ................................................................................................... 129

viii

LIST OF TABLES

a

Table 2.1 Major fractions of the pretreated corn stover after pretreatment methods [8, 39]. ..... 36
Table 3.1 Specimen name designations and coating information................................................. 72
Table 3.2 Important parameters and calculations for 40 PPI uncoated and copper coated samples
....................................................................................................................................................... 75
Table 3.3 Important parameters and calculations for 20 PPI uncoated and copper coated samples
....................................................................................................................................................... 76
Table 3.4 Important parameters and calculations for 10 PPI uncoated and copper coated samples
....................................................................................................................................................... 77
Table 3.5 Energy absorption calculations and comparison based on Figure 3.15 ........................ 81

ix

LIST OF FIGURES

Figure 1.1 Diverse applications of nanotechnology. (For interpretation of the references to color
in this and all other figures, the reader is referred to the electronic version of this dissertation.) .. 3
Figure 1.2 The Bottom-up (a) and Top-down (b) approaches in nanotechnology application.
Reproduced with permission from Ref [17]. .................................................................................. 4
Figure 2.1 Schematic illustration of the modified Taylor-Couette reactor (nanomixer). ............. 31
Figure 2.2 Major fractions of the remaining solid of CS before and after pretreatment at different
conditions. ..................................................................................................................................... 35
Figure 2.3 Cellulose and hemicellulose conversion profile of the NSHA pretreated CS with 4 g

L and 40 g L NaOH, with a 5 FPU (g cellulose) enzyme loading, and a 50 C incubation
temperature.................................................................................................................................... 39
-1

-1

-1

Figure 2.4 Cellulose and hemicellulose conversion profile of the NSHA pretreated CS with 4 g

L and 40 g L NaOH, with a 20 FPU (g cellulose) enzyme loading, and a 50 C incubation
temperature.................................................................................................................................... 40
-1

-1

-1

Figure 2.5 Cellulose and hemicellulose conversion profile of the NSHA pretreated CS, with a 60
FPU (g cellulose)

-1

enzyme loading, and a 50 C incubation temperature. ................................. 41

Figure 2.6 From crystalline to disrupted structure – before and after pretreatment at different
conditions. a) Untreated corn stover, area pointed by the arrow is an intact cell wall; b) nanoshear
-1
hybrid water pretreated CS; c) 4 g L NaOH NSHA pretreated CS, area pointed by the arrow is
-1
a discontinued structure with a reduced fiber length (fibrillation); d) 20 g L NaOH NSHA
pretreated CS, area indicated by the circle is where cellulose crystalline structure breaks apart
-1
(another form of fibrillation); e) 40 g L NaOH NSHA pretreated CS, minipores and more
disrupted fibers.............................................................................................................................. 45
-1

Figure 2.7 Fibril aggregates on the surface of the cellulose crystalline structure. a) 40 g L
NaOH NSHA pretreated CS, nano-fibril aggregates retained on the surface of a single cellulose
-1
fiber, diameters are around 300-500 nanometers; b) 40 g L NaOH NSHA pretreated CS, more
fibril aggregates with thinner diameters........................................................................................ 47
Figure 3.1 The electro-deposition system applied in this study. A rare metal rectangular bar mesh
attached with copper sheets was used as the anode in the system. The aluminum foam was
connected to the cathode and centered in the mesh to enable the uniform transfer of electrons and
copper cations. .............................................................................................................................. 61
x

Figure 3.2 Electro-deposition kinetic studies on open-cell Al foams. Straight lines are theoretical
values based on Faraday’s law of electrolysis. Dots in different shapes are experimental data. .. 62
Figure 3.3 SEM (a) and EDX elmental mapping (b) on a Cu coated Al foam. The distribution of
each element is: 97.6 wt % (or 91.8 at %) of Cu, 0.4 wt % (or 0.9 at %) of Al and 2.0 wt % (or
7.3 at %) of O. ............................................................................................................................... 65
Figure 3.4 Uncoated Al foams (a) and Cu coated Al foams (b). Left: 10 PPI; middle: 20 PPI;
right: 40 PPI .................................................................................................................................. 66
Figure 3.5 Top view digital image of Cu coated Al foams at cut area ......................................... 66
Figure 3.6 Middle section of a copper electrodeposited Al foam. a) and b) SEM images of
selected areas................................................................................................................................. 67
Figure 3.7 SEM (a) and EDX elemental mapping (b) at sample cut area, where Al and Cu can be
seen at the same time. The distribution of each element is: 72.0 wt % (or 50.0 at %) of Cu, 24.3
wt % (or 39.7 at %) of Al and 3.7 wt % (or 10.3 at %) of O. ....................................................... 68
Figure 3.8 SEM images of Cu coated Al foams at different scales .............................................. 69
Figure 3.9 XRD pattern of a Cu coated Al foam. Crystallite size was calculated using the Cu
peak at 43.3 ° in this curve. Using the Scherrer formula the crystalline size of deposited Cu was
38 nm. ........................................................................................................................................... 71
Figure 3.10 Cu coated Al foams before (a) and after (b) quasi-static compression tests ............. 74
Figure 3.11 Compressive stress-strain response of 10 PPI open-cell Al foams with different
relative densities (i.e., different strut thicknesses). ....................................................................... 74
Figure 3.12 Compressive stress-strain responses of 40 PPI uncoated and coated foam samples.
The samples were compressed at 0.1 in/min, until 80 % strain was achieved. The embedded
figure represents the enlarged elastic region. Linear trend lines were used to fit the curve of the
elastic region of each sample. ....................................................................................................... 75
Figure 3.13 Compressive stress-strain responses of 20 PPI uncoated and coated foam samples.
The embedded figure represents the enlarged elastic region. Linear trend lines were used to fit
the curve of the elastic region of each sample. ............................................................................. 76
Figure 3.14 Compressive stress-strain responses of 10 PPI uncoated and coated foam samples.
The samples were compressed at 0.1 in/min, until 80 % strain was achieved. The embedded
figure represents the enlarged elastic region. Linear trend lines were used to fit the curve of the
elastic region of each sample. ....................................................................................................... 77
Figure 3.15 Compressive stress-strain responses of 10 PPI samples. “US10-HD” denotes an
uncoated 10 PPI sample, with high density (12 (±1) % relative density), and a strut thickness
xi

around 580 m; “CS10-95” represents a coated 10 PPI sample, starting from a 6 (±1) % relative
density, with an approximately 95 m coating thickness. Al foams with 6 (±1) % relative density
were Cu coated until their strut thickness was equivalent to Al foams with 12 (±1) % relative
density. All samples were tested under quasi-static compression at 0.1 in/min. The embedded
figure represents the enlarged elastic region. Linear trend lines were used to fit the curve of the
elastic region of each sample. ....................................................................................................... 81
Figure 4.1 Overall scheme of Ni electroless deposition on a hydrophobic polymer thin sheet.... 97
Figure 4.2 Visualization and comparison of Ni coating on various hydrophobic polymer thin
sheets (LDPE, HDPE, PP, PS). Non-PAH modified polymer sheets were not able to form Ni
coating. ........................................................................................................................................ 100
Figure 4.3 Visualization of Ni coating on a PP polymer thin sheet. Upper left is a SEM image at
the edge of Ni coating; upper right is the corresponding EDX elemental mapping (60 wt % of C,
11 wt % of Au, 29 wt % of Ni); lower left is a SEM image at the main coating body; lower right
is the corresponding EDX elemental mapping (6 wt % of C, 5 wt % of O, 18 wt % of Au, 71
wt % of Ni).................................................................................................................................. 101
Figure 4.4 Visualization of Ni coating on a PS polymer thin sheet. Upper left is a SEM image at
the edge of Ni coating; upper right is the corresponding EDX elemental mapping (44 wt % of C,
3 wt % of O, 8 wt % of Au, 45 wt % of Ni); lower left is a SEM image at the main coating body;
lower right is the corresponding EDX elemental mapping (4 wt % of O, 12 wt % of Au, 84 wt %
of Ni). .......................................................................................................................................... 102
Figure 4.5 Visualization of Ni coating on a HDPE polymer thin sheet. Upper left is a SEM image
at the edge of Ni coating; upper right is the corresponding EDX elemental mapping (37 wt % of
C, 6 wt % of O, 9 wt % of Au, 48 wt % of Ni); lower left is a SEM i mage at the main coating
body; lower right is the corresponding EDX elemental mapping (3 wt % of C, 4 wt % of O, 9
wt % of Au, 84 wt % of Ni). ....................................................................................................... 103
Figure 4.6 Visualization of Ni coating on a LDPE polymer thin sheet. Upper left is a SEM image
at the edge of Ni coating; upper right is the corresponding EDX elemental mapping (51 wt % of
C, 7 wt % of Au, 42 wt % of Ni); lower left is a SEM image at the main coating body; lower
right is the corresponding EDX elemental mapping (28 wt % of C, 3 wt % of O, 8 wt % of Au,
61 wt % of Ni)............................................................................................................................. 104
Figure 4.7 The evolution of Ni coating morphologies on the LDPE surfaces. ........................... 105
Figure 4.8 Overall scheme of the Ni electroless deposition on the hydrophobic polymer
pellet/sphere. ............................................................................................................................... 108
Figure 4.9 Visualization of Ni deposition on PE pellets. Non-PAH modified PE pellets were not
able to form Ni coating. .............................................................................................................. 109

xii

Figure 4.10 SEM images of a) uncoated PE pellets; b) a coated PE pellet. The corresponded
diagram next to each image shows the element signal of a selected area designated in the SEM
image. .......................................................................................................................................... 110
Figure 4.11 SEM images of a) PS microspheres before coating; b) non-PAH modified PS
microspheres after Ni deposition; c) and d) PAH modified PS microspheres after Ni deposition.
..................................................................................................................................................... 112
Figure 4.12 A morphological change of HDPE thin sheet, during Ni electroless deposition. ... 113
Figure 4.13 A morphological change of PS thin sheet, during the Ni electroless deposition..... 114
Figure 4.14 Kinetic studies and comparison of Ni electroless deposition on HDPE and PS,
respectively. A nominal thickness gain over coating time was plotted. ..................................... 115
Figure 4.15 Visualization of Cu deposition on Ni coated PS thin sheets, after Cu electrodeposition. Uncoated PS sheets were not able to form Cu coating............................................. 117
Figure 4.16 Visualization of Cu coating on a Ni coated PS thin sheet, after Cu electro-deposition.
Upper left is a SEM image at the edge of Cu coating; upper right is the corresponding EDX
elemental mapping (36 wt % of C, 3 wt % of O, 21 wt % of Au, 39 wt % of Ni); lower left is a
SEM image at the main coating body; lower right is the corresponding EDX elemental mapping
(3 wt % of O, 14 wt % of Au, 82 wt % of Ni). ........................................................................... 118

xiii

Chapter 1 Introduction

1.1 Overview of the nanotechnology
The word “nano” is derived from a Greek word for dwarf. It represents one billionth.
“Nanotechnology” represents a characterization, operation or controlling of matters on the nanoscale, for unique or improved properties, products and devices [1]. The concept of
nanotechnology originates from a lecture “There is Plenty of Room at the Bottom”, given by the
Nobel laureate Richard Feynman in 1959 [1, 2]. In his lecture, he was challenging the scientific
community, imagining the entire Encyclopedia Britannica to be written on the head of a pin [2].
Since then, without a significant advance in instrumentation, the nanotechnology did not
receive broad attention until the early 1980s, when the scanning tunneling microscope (STM) [3,
4] and atomic force microscope (AFM) [5, 6] were introduced. At the time they were invented,
the STM had a spatial resolution of 0.1 nanometer (nm), whereas the AFM had a spatial
resolution of 0.05 nm. With the development of these techniques and sophisticated instruments,
substances were able to be investigated and rudimentarily manipulated at the nano-scale,
enabling the realization and implementation of the “nano” concept proposed decades ago.
Another reason why the nanotechnology gained its prosperity in recent decades is its vast
interdisciplinary collaboration and a favorable interaction with practical applications. The
nanotechnology covers a broad range of disciplines, including primarily chemistry, physics,
biology, materials science, electronics, engineering, pharmaceutics, and environmental science.
Because of that, the nanotechnology is a versatile approach to address many engineering
problems, as illustrated in Figure 1.1. Among all these applications, the variety of the approaches
and methodologies in nanotechnology falls into two categories: “bottom-up” and “top-down”. As

1

the name suggests, in the bottom-up approach, substances (atoms, molecules, etc.) are selectively
assembled to build up nanostructures [7]. For example, the layer-by-layer self-assembly of
polyelectrolytes with various interactions (electrostatic interaction, hydrogen bond, etc.) to form
a multilayer system is a typical bottom-up approach [8]. On the other hand, in the top-down
approach, materials or patterns start with a larger scale are tailored into nano-scale for desired
functionalities and/or properties [7]. For example, the lithography methodology for microchip
and electronic device fabrication is a typical top-down approach [9]. Figure 1.2 elucidates the
classification of various techniques and applications with the two categories.
With enormous research attention and efforts, up to now, the nanotechnology has covered
a large and diverse spectrum of engineering applications, from the fabrication of nano-sized
metal catalyst for chemical synthesis [10] to the lithography methodology for electronic devices
manufacturing [11], from the molecular level self-assembly for multilayer polymeric composites
[8] to the fabrication and integration of nano-scale materials for solar cell and hydrogen storage
purposes [12], from the newly discovered carbon-based materials (carbon nanotube, graphene,
graphite, etc.) for new devices and composites with enhanced properties [13] to the nanofabrication of revolutionary devices and materials for drug delivery purposes [14], from the
functionalized nanoparticle coated membranes for waste water treatment [15] to the synthesis
and application of nanoparticle based biosensor for detection of proteins with ultra-sensitivity
[16], so on and so forth.
The aim of the dissertation herein, however, is not to thoroughly discuss the full spectrum
of the nanotechnology application. Rather, the dissertation will mainly focus on the
nanotechnology application in the following three topics: biomass pretreatment in biofuel

2

production, functional material fabrication and surface modification. The topics will be discussed
in detail in the following sections.
Nano-sized particles and thin films
Pharmaceuticals & drugs
Genetic and bioengineering
Functional materials
Biosensors & detectors
Electronic devices
Nano-technology

Surface engineering & coating
Renewable energy
Water treatment
Process investigation & design
Chemical synthesis & catalysis
Cosmetics
Figure 1.1 Diverse applications of nanotechnology. (For interpretation of the references to color
in this and all other figures, the reader is referred to the electronic version of this dissertation.)

3

(a)

Bottom-up

Chemical
synthesis

Self-assembly

Positional
assembly

Special
compounds &
Catalysts

Nanoparticles
, Nanofilms,
Nanowires &
Nanotubes

Atomic or
molecular
devices

Cosmetics,
catalysis

Functional
materials

(b)

Top-down

Lithography

Etching,
Grinding

Electronic
devices,
microchips

Precise surface
engineering

Computer
chips,
nanocomputers

High quality
surface and
coatings

Figure 1.2 The Bottom-up (a) and Top-down (b) approaches in nanotechnology application.
Reproduced with permission from Ref [17].

4

1.2 Nanotechnology for biofuel production
Concerns

about decreasing petroleum supplies

vs. increasing demands, and

environmental consequences of fossil fuels (e.g., greenhouse gas emission) have motivated the
research and development of renewable biofuel production in the recent decade [18]. The
lignocellulosic biomass has been recognized as the sustainable resource to produce biofuels with
environmental benefits (less greenhouse gas emissions) [19-21], and abundant quantities without
reducing food supplies [22, 23].
The lignocellulosic biomass refers to plants or agricultural residues that are majorly
composed of cellulose (38-50 %), hemicellulose (23-32 %) and lignin (15-25 %), as well as other
minor components (e.g., protein, pectins, extractives) [24]. Cellulose is a homopolysaccharide of
glucose, linked by β-1,4 glycosidic bonds. Hydrogen bonds are formed to associate cellulose
within chains (intramolecular) and between chains (intermolecular). Hemicellulose is a
heteropolysaccharide of pentoses (e.g., xylose, arabinose) and hexoses (e.g., glucose, mannose,
galactose). It also contains uronic acids, acetate and phenolic groups. Lignin is a complex
heteropolymer of three different polyphenolic compounds, p-coumaryl, coniferyl and sinapyl
alcohol.
Three primary strategies have been proposed and developed for biofuel production from
lignocellulosic biomass, including gasification and fuel derivation, bio-oil production by fast
pyrolysis or liquefaction, as well as ethanol production by hydrolysis and fermentation [25].
Gasification of biomass is a process in which biomass reacts with steam, oxygen or air to
produce syn-gas (i.e., CO and H2), or producer gas (i.e., N2, CO2, CO, H2) [26]. The syn-gas can
be utilized to produce synthetic diesel, methanol or methanol-derived fuels [26-28]. Pyrolysis is
thermal decomposition of feedstock without oxygen or steam [28-30]. Fast pyrolysis (or flash
5

pyrolysis) is usually applied at a high temperature (400-650 °C) and moderate pressure (1-5 atm)
with an extremely short residence time (less than 2 seconds) for a high liquid yield ( > 50 %).
Liquefaction is a process to liquefy the feedstock under a high pressure (50-200 atm) and a high
temperature (250-325 °C) [28, 31]. Compared with the aforementioned two strategies,
conversion of lignocellulosic biomass to ethanol requires lower temperature and pressure, with
high selectivity. Conversion of lignocellulosic biomass to ethanol is a multistep process, in
which polysaccharides are depolymerized by either acid or specific enzymes to monomeric
sugars (pentoses or hexoses), and the resulting sugars are fermented to ethanol by yeasts [32, 33].
A simultaneous saccharification and fermentation (SSF) process is developed to reduce the
reactor cost and reaction time [34, 35].
During the advances of science and technology in biofuel production, the nanotechnology
has played a significant role, from at least two aspects.
The instrumentation with nano-scale or sub nano-scale resolution facilitated a
comprehensive understanding of the ultrastructure of the cell wall and the microscopic
investigation on cell wall deconstruction and enzymatic mechanisms. This aspect of
nanotechnology has been a long-term contribution to the biofuel production fields and is
continuing to make contributions with the invention of more sophisticated instrumentation and
advances in sample preparation techniques. For example, with the help of AFM, the diameter of
a cellulose fibril is found to be around 3-5 nm [22]. With electron microscopy, AFM and other
characterization methods, a multi-scale study on visualization of lignocellulosic biomass cell
wall deconstruction during pretreatment was carried out [36]. The highly porous cell wall
structure with a pore size from 10 to 1000 nm was formed within the plant cell walls and found
to be beneficial for enzyme accessibility to cellulose. In another study, by applying the tapping

6

mode of AFM, the enzymatic hydrolysis of cellulose was visualized [37]. The observation
significantly enhanced understanding of the mechanism of interactions between the enzyme and
the cellulose fibrils.
The other aspect of the nanotechnology for biofuel production, however, is on the
processing and development of new catalysts/materials at a nano-scale. This aspect of
nanotechnology is more recent and diverse and is gaining more attention in the scientifi c
community. Many solid acid catalysts, including zeolites [38, 39], transition metal oxides [4042], supported carbonaceous solid acid catalysts [43-45] have been applied. Zeolites are widely
used in chemical catalysis because they are non-toxic, easy to recover and reuse. Zhang and
Zhao [38] performed cellulose hydrolysis with an H-zeolite catalyst in ionic liquid in a
microwave reactor, and achieved a 37 % glucose yield in 8 min. Macromolecular Lewis acid
compounds supported by zeolites are named heteropoly compounds [46, 47]. Heteropoly
compounds (e.g., H3PW12O40) may possess acidic strength as strong as sulfuric acid. Therefore,
with the zeolite support, they exhibited catalytic activity in cellulose hydrolysis with thermal
stability. A glucose yield of 30 % was reported under 160 °C for 6 hours of reaction time [47].
Transition-metal oxides have been found with catalytic activity for cellulose hydrolysis, and are
also easy to recover and reuse. Tagusagawa et al. [40] found the mesoporous Nb-W oxides
(Nb:W=3:7) compound a recyclable, highly active solid acid catalyst for cellulose
hydrolysis. Carbonaceous solid acid catalysts are prepared by carbonizing D-glucose or sucrose
at 400 °C under N2 and then sulfonating the materials at 150 °C [43]. The catalyst was used to
catalyze the transesterification of vegetable oil for biodiesel production [44]. Carbonaceous solid
acid catalysts manufactured in the nano-scale (10-100 nm) were also studied for cellulose
hydrolysis [48]. A high catalytic performance and excellent recyclability were observed.
7

Nanoparticle catalysts have also been found interesting in the biofuel production area due
to its high surface area to volume ratio and recyclability. For example, in a recent study, cellulase
was immobilized on polyvinyl alcohol/Fe2O3 magnetic nanoparticles, resulting in a three times
improvement in glucose yield, and a 40 % enzymatic activity after four cycles of reuse [49].
Chang et al. [50] used mesoporous silica nanoparticles as scaffolds for the cellulase
immobilization, the results showed an 80 % cellulose-to-glucose conversion with excellent
stability. Fang et al. [51] synthesized hydrotalcite nanoparticles and activated it with Ca(OH)2 to
hydrolyze cellulose. A maximum yield of 47 % was achieved, with high selectivity (85 %) and
stability.

1.3 Nanotechnology for functional material fabrication
Nanotechnology is also a versatile approach to fabricate functional materials. As
introduced in previous sections, the methodologies are described as “top-down” and “bottom-up”
approaches. The top-down approach, which usually involves patterning with litho graphy and
etching, has gained its tremendous success in microelectronics. However, the research interest in
the scientific community has shifted to the “bottom-up” approach, due to the introduction and
development of nano-scale building blocks [52].
The nano-scale building blocks have to fulfill the requirement that at least one dimension
falls into the nanometer scale. Regardless of the geometries, those nano-sized materials can be
colloidal, crystalline, semi-crystalline, polymeric, metallic or metallic oxide.
Nanowires [52-54], nanorods [55, 56] and nanobelts [57, 58] are typical examples for
one-dimensional (1-D) nano-scale building blocks. They can be concentric, ring-shape, round,
hexagonal, square or triangular in cross section. They are anisotropic, with high aspect ratio.
8

These

1-D

nano-scale

building blocks

can be

used

or

modified

with various

functionalities/applications, including nano-scale electronic devices enabled by electron transport
property [59], nanosensing applications enabled by interactions with DNA [60], charge storage
behavior enabled by a functionalized molecule on the nanowire surface [61], enhanced optical
and electromagnetic property enabled by the nonlocal dielectric function [62], and so on.
Nanosheets [63, 64] and nanolayers [65, 66] are examples for two-dimensional (2-D)
nano-scale building blocks. They usually have high width-to-thickness ratio, and can be modified
with surface properties. For example, Sasaki and Watanabe [67] obtained a nanosheet of quasiTiO2 by delaminating titanate, and found the nanosheet with subnanometer thickness with
enhanced optical properties. In another study, Hosono et al. [68] fabricated and applied
nanocrystalline ZnO sheets in dye-sensitized solar cells, obtained a higher conversion efficiency
for 1.5 AM illumination. Vu et al. [66] dispersed clay nanolayers into synthetic and epoxidized
natural rubber and obtained an enhanced mechanical property. Graphene nanosheet is another
typical kind, which consists of sp2 bonded carbon atoms with one-atomic thickness [64]. The
invention of graphene nanosheet won the Nobel Prize for Physics in 2010 [69]. And it is
attracting a growing amount of research attention because of its unique and superior electronic,
optical, magnetic, thermal and mechanical properties, as well as its facile way of preparation [70].
Because of that, an intensive research effort is to incorporate graphene nanosheets in polymers to
form graphene-polymer composite materials for enhanced electrical and thermal conductivity,
and mechanical strength [71]. The intrinsic chemical structure of the graphene also gives rise to
other properties when chemically modified or paired with other chemical compounds. For
example, an enhanced electrocatalytic activity of platinum (Pt) clusters supported on graphene
nanosheets was observed for methanol oxidation reaction [64].
9

Nanoparticles or nanospheres [72, 73] are typical examples for three-dimensional (3-D)
nano-scale building blocks. This area of research also has received a considerable attention for
decades. The idea to incorporate nanoparticles was developed initially to enhance the mechanical
strength of polymers [74, 75]. While the renewing research interests persist even at present due
to the development of new fabrication methodologies and application of new materials, the
research attention is somewhat branched to the functionalization of the nanoparticle surface and
utilization of these newly developed material/structure for advanced uses. Magnetic
nanoparticles have been used in medical diagnoses, such as application for magnetic resonance
imaging and diagnose for cancer [76]. The mono-dispersed polystyrene nanoparticles with or
without surface functional group have been commercially available for a long time and widely
used in biomedical applications [77]. With the emulsion polymerization method and help of
surfactants, the size of the resultant polystyrene can be precisely controlled with a variation less
than 5 %. Also, the nanoparticles can be surface modified with polymer brushes [78], selfassembly monolayers [79], and polyelectrolyte multilayers [80] for unique characteristics and/or
special purposes.
Although starting from the nano-scale, the nanotechnology can also be useful for
macro/bulk applications and functionalities. For example, in civil infrastructures, smart concrete
(concrete integrated with carbon nanofiber or carbon black) has been used in the past decade [81,
82]. The smart concrete was used to sense strain due to its piezoresistance, and to sense damage
due to the relationship between damage and resistance. Han et al. [83] used the self-sensing
smart concrete pavement for traffic flow detection. Veedu [84] presented the potential for
bridges made of the smart concrete to monitor structural health, integrity and traffic information
over the bridge, as well as the highway made of the same material to track traffic information in

10

situ. In solid mechanics, other than numerous study on mechanical enhancement of carbon based
material fillers (graphene, graphite, carbon nanotube, etc.) in polymer matrix, Qu and Dai [85]
mimicked the lizard’s feet and fabricated carbon nanotube based dry adhesive surfaces, and
showed a 473 g stainless steel was hung on a 4 mm × 4 mm SiO2/Si-wafer-supported single wall
nanotube dry adhesive film.

1.4 Nanotechnology for surface modification
Nanotechnology is a powerful tool for surface modification and engineering. With
designated purposes, surfaces can be deliberately modified by making use of various surface
interactions between homogeneous/heterogeneous materials. The surface interactions to cause
adhesion phenomena generally include the following: covalent bonding, hydrogen bonding,
electrostatic interaction, van der Waals interaction, and hydrophobic interaction. In general, the
orders of strength are as follows: covalent bonding > electrostatic interaction > hydrophobic
interaction > hydrogen bonding > van der Waals interaction.
Covalent bonding is a strong chemical linkage that shares a pair of electrons between
atoms [86]. It forms when the bonded atoms have a total energy that is lower than the separate
ones. In the surface application in nanotechnology, the covalent bond has been applied to
functionalize nanoparticles for various purposes. For example, Yamanoi et al. [87] immobilized
a series of omega-alkene-1-thiol-stablized gold nanoparticles onto silicon surfaces by Si-C
covalent bonds. Zhou et al. [88] implanted functionalized Fe 3O4 nanoparticles onto the
functionalized multi-walled carbon nanotube (MWCNT) and observed a stronger magnetic
performance. In their research, poly (acrylic acid) (PAA) was grafted onto the hydroxyl modified
MWCNT and facilitated the covalent bonding between the amino groups of functionalized
11

Fe3O4 and the carboxyl groups of PAA. He et al. [89] reported a similar methodology to
immobilize Fe3O4 nanoparticles to graphene oxide through covalent bonding, induced by the
amino groups of functionalized nanoparticles and the carboxyl groups of functionalized graphene
oxide.
Electrostatic interaction is another type of strong interaction which represents attractive
(oppositely charged) or repulsive (with the same charge) forces between charged atoms or
molecules [90]. The electrostatic interactions have been extensively used in construction of
multicomposite films of charged substances (mostly polyelectrolytes) through layer -by-layer
assembly from aqueous solution, since the early 1990s [8, 91]. With electrostatic attraction
forces, polycations and polyanions can be alternatively deposited onto designated substrates,
regardless of their size, geometry and topology. The methodology and its mechanism have
received great attention and have led to a wide range of applications. For example, Caruso et al.
[92] reported an electrostatic self-assembly of silica nanoparticles onto positively charged
polystyrene particles, which are surface modified by polyelectrolyte multilayers. Wang et al. [93]
electrostatically assembled hydrolyzed poly[2-(3-thienyl) ethanol butoxy carbonyl-methyl
urethane] onto the surface of cellulose acetate electrospun nanofibrous membranes, and obtained
a high optical sensitivity. Agarwal et al. [94] investigated the polyelectrolyte multilayer coated
drug nanoparticles and found that the coating prevents nanoparticle aggregation, and the drug
release rate could be controlled.
Hydrophobic and hydrophilic interactions [90] are ubiquitous interactions occurring in
aqueous solution. They are moderate interactions, weaker than covalent bonding, stronger t han
hydrogen bonding and van der Waals interaction in aqueous environment. In hydrophobic
interaction, non-polar substances (water insoluble molecules) exhibits a “water-repulsive”
12

behavior and tends to aggregate with each other [90, 95]. On the contrary, hydrophilic interaction
is an interaction between water molecule and other molecules in which they tend to attract to
each other [90]. Hydrophobic interaction is more widely used for many purposes due to these
characteristics. By making use of the hydrophobic interaction, Akagi et al. [96] developed a way
to stabilize polyelectrolyte complex nanoparticles, and expected a great potential to apply those
nanoparticles as multifunctional carriers for pharmaceutical and biomedical applications. Song et
al. [97] studied attachment of modified silver nanoparticles to hydrophobic glass beads, and
suggested that the hydrophobic interaction plays a significant role to increase the attachment
efficiency by at least 2-fold.
Hydrogen bonding is a relatively weak interaction in which a hydrogen atom of one
molecule is attracted to an electronegative atom of another molecule [86]. Hydrogen-bonded
architectures are abundant in biological systems (e.g., hydrogen bonding in cellulose fibrils), and
are found useful in many nanotechnology applications. Russell and Ward [98] exhibited an
interesting study, in which hydrogen bonded molecular crystals are designed and synthesized
with dimension control. Examples included one-dimensional hydrogen bonded wires, twodimensional hydrogen bonded layers and nanoporous hydrogen bonded lattices. In another study,
Zhang et al. [99] investigated the self-assembly of polyaniline nanotubes in the presence of
carboxylic acids. It was found that the hydrogen bonds between the polymer chain and the
hydroxyl group of the carboxylic acids are responsible to form aggregated nanotube dendrites.
Lee and Han [100] studied viscoelastic properties of hydrogen bonded polycarbonate/organoclay
nanocomposites, and found that organoclay platelets are well dispersed in hydrogen bonded
nanocomposites.

13

Van der Waals interaction is a weak interaction that induced by electrical interactions
between atoms or molecules that are close to each other [86]. It is the weakest intermolecular
interaction, but occurs ubiquitously in nature. Despite of its weak strength, it is applied in many
aspects in nanotechnology. For example, Sato and Sano [101] demonstrated a methodology to
deposit single-walled carbon nanotubes on to a substrate surface and develop a 2D network that
is held by van der Waals interaction. Researchers put many efforts into mathematical simulation
to describe the van der Waals interaction with nanomaterial/nanostructures. For example, Blagov
et al. [102] developed Lifshitz-type formulas to describe van der Waals interaction between an
atom/molecule and a single-walled carbon nanotube. Another effort by Gatica et al. [103]
calculated van der Waals forces between spherical, cubic and core-shell nanoparticles in vacuum,
using the Axilrod-Teller-Muto 3-body formulation.

1.5 Scope of the dissertation
The dissertation is committed to comprehensively describing and discussing the three
research projects conducted by the author during his PhD program.
In the second chapter, a fast and efficient nano-scale shear hybrid alkaline (NSHA)
pretreatment method of lignocellulosic biomass is introduced. In this work, corn stover was
pretreated in a modified Taylor-Couette reactor with alkali (sodium hydroxide) at room
temperature, with a two-minute retention time and a 12500 s

-1

shear rate. A nanomixer was

used as the modified Taylor-Couette reactor to perform the NSHA pretreatment process. The
synergistic effects (chemical, physical and thermal effects) induced by the nanomixer aimed to
disrupt the naturally-formed recalcitrance of biomass that prevents itself from chemical or
biological attacks. Results showed that the NSHA pretreatment caused an efficient structural
14

breakdown and substantially enhanced the accessibility of enzymes to polysaccharides. An up to
82 % of high cellulose content in the remaining solid was achieved with the NSHA pretreatment
process. Compared with untreated corn stover, an approximately 4-fold increase in enzymatic
cellulose conversion and a 5-fold increase in hemicellulose conversion were achieved. SEM
images revealed that a severe disruption of biomass structure and exposure of cellulose
microfibril aggregates in NSHA pretreated corn stover. The highly efficient nano-scale hybrid
process has a potential to be economical, and therefore to decrease the overall cost of biomass
conversion which is critical to accomplish an economic biofuel production from biomass.
In the third chapter, a nano-deposition strategy was developed to enhance the energy
absorption capacity of aluminum (Al) open-cell foams. Open cell Al foams have been used as
energy absorbers for decades, due to their unique compressive stress-strain responses. Their
energy absorption capacity can be enhanced by thickening the foam struts, or increasing the
foam’s relative density. However, the enhancement is compromised by the inherent
characteristics of its stress-strain property relationship, whereby upon homogeneous strut
thickening, an increase in the plateau stress without a reduction in densification strain cannot be
achieved. The nano-reinforcement exhibits the potentials of providing superior mechanical
properties over the conventional not only at the micro-scale but also at the macro-scale level.
Unlike homogeneously thickening the foam struts, the heterogeneous nano-reinforcement has
potentials to favor a higher plateau stress and a longer densification strain, which is a promising
feature to enhance the energy absorption capacity of cellular foams. In this chapter, nanocrystalline copper (Cu) was uniformly deposited onto the Al foam and novel 3-D Cu/Al,
heterogeneously thickened, composite foam structured materials were fabricated and tested for
the first time. A featured non-cyanide nano-crystalline copper electro-deposition system was

15

setup for the coating of open-cell Al foam, and, the energy absorption capacity as a function of
foam pore size and Cu coating thickness was investigated. The x-ray diffraction tests confirmed
the crystallinity of the Cu deposition and the crystallite size was calculated to be 38 nm. The
energy absorption capacity of Al foams (with average strut thickness of 192 m) reinforced with
a 60 m Cu coating was 3 times greater than that of plain foams. The compressive stress-strain
response of the composite samples showed no significant reduction of the densification strain
compared to the uncoated foams due to the small change in the foam strut thickness and pore
size. A comparison between thin Cu-coated and uncoated Al foam samples with the same overall
strut thickness (i.e., same effective volume density and porosity) showed that the nano-reinforced
foams had superior energy absorption capacity over plain foams with the same overall thickness,
with almost a 2-time enhancement.
In the fourth chapter, a facile “dip & rinse” method for nickel (Ni) electroless deposition
on hydrophobic polymer surfaces was developed. The electroless deposition is widely used
because of its equipment simplicity and versatility. Electroless metal deposition is a catalytic,
redox reaction of a metal ion in an aqueous solution (with a reducing chemical agent), without
external electrical field being applied. It usually includes three main steps: 1) a surface treatment
or conditioning; 2) application of an appropriate catalyst (typically noble metal catalyst, e.g. tin,
palladium) on the substrate surface; 3) metal electroless deposition. Rinsing is required between
the steps. However, in the first step, in order to modify the functionality of the substrate surface
so that catalyst can be sequentially attached, harsh or/and toxic surface conditioning steps are
usually employed. To eliminate the need for harsh and toxic treatment of the substrate, this
chapter demonstrated a facile method of electroless Ni deposition on various hydrophobic
polymer substrates, by making use of the hydrophobic interactions between Poly(allylamine

16

hydrochloride) (PAH) and polymer substrates for catalyst adsorption/immobilization. Various
hydrophobic polymer surfaces with different geometries and dimensions, including low density
polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP) and polystyrene
(PS) thin sheets, and PE pellets were tested and Ni was successfully deposited onto all these
surfaces. Studies showed that without the PAH, Ni coating was not able to form on any of these
surfaces. Among these polymer thin sheets, with the same experiment condition and procedure,
coverage of Ni coating can be ranked in the following order: PS > PP ≈ HDPE > LDPE. A
kinetic study on polymer thin sheets examples showed that with 2 hours of deposition, an
approximately 2 m thickness was achieved. A prove-of-concept study showed that Ni coated
polymer thin sheets can be further electroplated with heterogeneous metal (Cu), hence enabling a
faster thickness growth over time.

17

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26

Chapter 2 Fast and efficient nanoshear hybrid alkaline pretreatment of corn stover for
biofuel and materials production

2.0 Abstract
In this chapter, a fast and efficient nano-scale shear hybrid alkaline (NSHA) pretreatment method
of lignocellulosic biomass is introduced. In this work, corn stover was pretreated in a modified
Taylor-Couette reactor with alkali (sodium hydroxide) at room temperature for two minutes. An
up to 82 % of cellulose content in the remaining solid was achieved with the novel NSHA
pretreatment process. Compared with untreated corn stover, an approximately 4-fold increase in
enzymatic cellulose conversion and a 5-fold increase in hemicellulose conversion were achieved.
Compositional analysis proved significant removals of both lignin and hemicellulose after the
NSHA pretreatment. SEM images revealed that the synergistic effect of NSHA pretreatment
caused severe disruption of biomass structure and exposure of cellulose microfibril aggregates in
NSHA pretreated corn stover.

27

2.1 Introduction
Alternatives for petroleum are urgently needed since worldwide oil depletion is
approaching. Lignocellulosic biomass, such as corn stover, wheat straw, and switch grass etc. is
believed to be the abundant and sustainable feedstock for biofuel, chemicals, and materials
production [1]. Bioethanol production forms the carbon cycle in a more timely fashion compared
to fossil fuel, which leads to the reduction of the greenhouse gas emission [2, 3].
Production of ethanol from lignocellulosic biomass is a multistep process, in which
pretreatment step other than the power plant investment accounts for the largest cost portion, as
high as 19 % [4]. Typically, lignocellulosic biomass pretreatment refers to an essential step to
disrupt the natural polysaccharide-lignin shield that limits the accessibility of enzymes to
cellulose and hemicellulose during saccharification. Intensive research has been conducted on
this topic [5-8].
Over decades, much effort was continuously made to enable the pretreatment to be costcompetitive itself and beneficial to downstream processing as well. Chemical, physical, or
thermal methods have been employed to enhance the biomass digestibility. Chemical effect is
mainly made by inorganic alkalis and acids [9-14]. Different categories of chemicals, such as
lime, sodium hydroxide, ammonia and sulfuric acid, are commonly used. By adding the alkali or
acid, majority of lignin or hemicellulose is removed. Other chemicals, either added as a major
component or additives, include oxidative chemical compounds [15-17] (e.g. hydrogen peroxide,
ozone, etc.), organic solvents [18, 19] (e.g. ethylene glycol, acetylsalicylic and salicylic acids,
ect.), and ionic liquids [20, 21] (e.g. 1-ethyl-3-methylimidazolium acetate, 1-n-butyl-3methylimidazolium, etc. ). Oxidative chemical compounds remove the hemicellulose and lignin
to increase the accessibility of the cellulose. Organic solvents break the internal lignin and

28

hemicellulose bonds. Ionic liquid is used to extract lignin and reduce the crystallinity of cellulose.
With the heat input to increase the pretreatment temperature, thermal effect takes place to
increase the kinetic constant of the pretreatment, therefore reducing the retention time [22-24].
Physical methods mainly include milling, homogenization and flowthrough. Milling [25, 26]
and homogenization [27] serve to reduce particle size and crystallinity as well as degree of
polymerization of cellulose, which help to increase surface area and pore size. By applying
flowthrough, a large portion of hemicellulose and lignin can be removed [28].

Recently,

researchers have investigated a twin screw extrusion process [29-31] for the pretreatment process.
The friction and high shear rate generated by the extruder enhanced the enzymatic accessibility
of the woody biomass. Different effects are usually combined to make the polysaccharides more
accessible to enzymes.
In this work, a novel NSHA pretreatment method is proposed for the first time. By
applying the high velocity shearing along with synergistic mild chemical and thermal effects in a
modified Taylor-Couette reactor, the high-shear shaft work can primarily be transferred to the
nanostructure of biomass, allowing the efficient elimination of lignin and the exposure of
polysaccharide in a very short time.

2.2 Experimental Sections
2.2.1 Materials
The corn stover (CS) was locally harvested at the cook farms (Okemos, MI, geo graphical
coordinates 42.680164, 84.418455. Or 42° 40’ 48.59” North, 84° 25’ 6.44” West) in October
2009. The corn stover leaves and stalks were collected. After harvesting, the corn stover was
washed with tap water and dried at 50 °C for 48 hrs. The dried corn stover was chopped and then
29

milled in a knife mill (Arthur H. Thomas Co., NJ, US), with a sieve size of 1 mm. The processed
corn stover was stored in a freezer for future use. After the CS was transferred to the lab it was
stored in a cold room at 4 C.
Sodium hydroxide (NaOH, Spectrum Chemical mfg Co., CA, US) solution was prepared
in stock for future use. Deionized (DI) water supplied by a Barnstead Nanopure-UV 4 stage
purifier (Barnstead International Dubuque, Iowa), equipped with a UV source and final 0.2 μm
filter with a resistance ≥18.0 MΩcm was used for aqueous solution preparation and washing.

2.2.2 Modified Taylor-Couette reactor (nanomixer)
T K Filmics nanomixer [32] (model 56-50, Primix Corporation, Japan) was used as a
modified Taylor-Couette reactor to perform the NSHA pretreatment process. The nanomixer has
an energy input of 5.5 kW. A schematic diagram of the nanomixer in batch operation is
illustrated in Figure 2.1. The mixing unit mainly consists of 4 components, a shaft, driven by the
motor, an overflow vessel and two concentric cylinders, a turbine and a vessel. The diameter of
the turbine is 52 mm. The diameter of the vessel is 60 mm. The dimension of both the turbine
and the vessel ensures the non-contact between the two components, and the resulting horizontal
gap thickness between the turbine and vessel is 4 mm. In this study, the vessel volume is 80 mL.
Before operation, the overflow vessel and the turbine are consecutively tightened to the shaft
with hex nuts. Then, the vessel was clamped to the overflow vessel before operation, in order to
avoid splash.

30

Figure 2.1 Schematic illustration of the modified Taylor-Couette reactor (nanomixer).

The mixing unit was connected to a control panel for a shear rate control and a
temperature measurement. The shear rate can be controlled in the range from 2500 s
to 12500 s

-1

-1

(~60 Hz)

(~300 Hz). The shear rate was defined as the peripheral rotating speed divided by

the horizontal gap thickness between the turbine and the vessel [33]. Thus, a 10 m s

-1

(~60 Hz)

-1

peripheral rotating speed is equivalent to a shear rate of 2500 s . Water flow can be introduced
through the jacket inlet and outlet, for cooling or heating purposes. The orientation of the vent is
31

deliberately designed to be reverse to the rotating direction of the turbine, to avoid splash during
operation. A more detailed description of the nanomixer can be found in our previous work [34].

2.2.3 Nanoshear hybrid alkaline pretreatment
First, 2 g of stored CS was added into the vessel, with 50 mL DI water or NaOH solution
at different concentrations. After the nanomixer vessel was tightl y clamped to the overflow
-1

vessel, a 2-minute retention time was selected, with its maximum shear rate 12500 s . No
external removal or addition of heat to the system was provided. The vent on the vessel kept the
chamber pressure at ambient pressure all the time during the pretreatment.

The vessel

temperature could reach up to 100 C under ambient pressure. After the high shear spinning was
completed, cooling water was introduced until the system temperature was brought down to 25

C.

The pretreated samples were collected and then taken to the washing step.

2.2.4 Washing
The washing step was necessary after the pretreatment to remove the soluble components
and to adjust the pH. In each washing cycle, pretreated CS was diluted with DI water using 1 L
beaker, after 10~15 min quiescence, the supernatant was vacuum filtered with a Kontes UltraWare 47 mm microfiltration assembly (Thermo Fisher Scientific Inc., MA, US). Depending on
the NaOH concentration, after 5~7 cycles the pH of homogenized suspension could reach ~7. It
required more cycles for higher concentration of NaOH used.

32

2.2.5 Drying
In order to determine the mass of pretreated solid, the washed samples were air-dried
prior to the next step. It should be noted that the drying before saccharification might have an
impact on the maximum conversions achieved, due to cell wall collapse [35, 36]. The effect is
still less severe as compared with the oven-drying process. However, the moisture content test
was conducted and calculated on an oven-dried basis, as per Laboratory Analytical Procedure
(LAP) 001 provided by National Renewable Energy Laboratory (NREL) [37].

2.2.6 Compositional analysis
The compositional analysis was then performed to determine the cellulose, hemicellulose,
and lignin contents of the remaining solid in pretreated or untreated CS. T he experiment was
conducted according to NREL LAP 002 [38].

2.2.7 Enzymatic hydrolysis
The pretreated samples were enzymatically hydrolyzed using ACCELLERASE

TM

1000

(Danisco US Inc. Genencor Div., NY, US). Pretreated CS was immersed in pH 4.8 citrate buffer
solution, then incubated in a water bath shaker (New Brunswick Scientific Co. Inc., NJ, US) at
2.5 Hz, 50 C up to 168 h. The procedure and calculation were performed as per NREL LAP
013 [36]. At least triplicate tests were performed to produce one data point. The glucose and
xylose concentration were tested using high performance liquid chromatography (HPLC, Agilent
Technologies Inc., CA, US) with a Bio-Rad ameinex HPX-87H HPLC column (Bio-Rad
Laboratories, CA, US).

33

2.2.8 Scanning electron microscope (SEM) imaging
The dried pretreated samples were gold coated (EMSCOPE SC500 Sputter coater,
Ashford, Kent, UK) for 3.5 minutes. In order to prevent structural collapse caused by surface
tension force, higher magnification images were critical-point-dried (Blazers CPD 010 Critical
point dryer, Bal Tec AG, Furstentum, Lichtenstein) prior to gold coating. All SEM images were
taken using JEOL 6400V (Japan Electron Optics Laboratories, JP) with a LaB6 emitter.

2.3 Results and discussions
2.3.1 Compositional analysis
Figure 2.2 shows the compositional fractions of the remaining solid in the NSHA
pretreated CS samples. As the concentration of NaOH increased, a higher cellulose content but
lower lignin and hemicellulose contents were obtained. Up to 82 % cellulose content in the
remaining solid of the NSHA pretreated CS was achieved. The removal of lignin and
hemicellulose significantly enhanced the exposure of cellulose fibers to the environment.
However, no further increase of cellulose content was observed, showing a plateau at the NaOH
-1

concentration of 80 g L . In this work, no removal of lignin or hemicellulose by the single
-1

nano-shearing force (i.e., 0 g L NaOH conc.) was found. Also, the fractional changes were not
-1

significant at the low concentration of NaOH (4 g L ).

34

90
Cellulose

Fractional composition (%)

80

Hemicellulose
70
Lignin

60
50

40
30
20
10
0

untreated

0

4

20

40

80

120

200

NaOH concentration (g L-1)
Figure 2.2 Major fractions of the remaining solid of CS before and after pretreatment at different
conditions.
Note:
-1
a. All samples were pretreated with a shear rate of 12500 s and a retention time of 2 min.
-1
-1
b. NaOH concentrations were represented in g L , 0 g L NaOH represents water.
c. Values of lignin content included acid soluble lignin and acid insoluble residue.

35

a

Table 2.1 Major fractions of the pretreated corn stover after pretreatment methods [8, 39].
Condition

Cellulose
content (%)

Hemicellulose
content (%)

Lignin
content (%)

72.7

9.9

9.7

34.4

22.8

18.0

61.9

17.9

8.8

52.7

16.2

25.2

64.4

2.9

26.4

56.7

26.4

14.6

56.9

11.6

23.8

65.7

12.9

21.0

64.1

24.6

8.6

b

NSHA

c

AFEX
d

ARP

e

Controlled pH
Dilute acid

f

g

Lime
h

SO2

NH4OH/HCl

i

j

NaOH
a
b

Results are calculated on an oven-dried basis.
Referred to as nanoshear hybrid alkaline pretreatment, 1:1 NaOH to biomass, room
-1

temperature nanoshear at a shear rate of 12500 s for 2 min.
c
d
e
f
g
h
i
j

Known as ammonia fiber explosion, 1:1 NH3 to biomass, 90 C, 1.5 ×10 Pa for 5 min.
6

Known as ammonia recycle percolation, 1:3 NH3 to biomass, 170 C, 2.2 ×10 Pa for 20 min.
6

190 C for 15 min.

1:20 H2SO4 to biomass, 160 C for 20 min.

1:2 Ca(OH)2 to biomass, 55 C for 4 weeks.
3 % SO2, 190 C for 5 min.

10 % NH4OH, 26 C for 24 h, then 0.3 mol L HCl, 100-108 C for 1 h.
-1

2 % NaOH, 120 C for 30 min.

36

In Table 2.1, for comparison, the compositional fractions of the remaining solid in the
-1

pretreated CS by NSHA and other well-known pretreatment methods are listed. With 40 g L

NaOH (i.e. 1:1 NaOH to biomass), the NSHA pretreated CS has 72.7 % of cellulose content, 9.9 %
of hemicellulose content and 9.7 % of lignin content in the remaining solid. Compared with other
pretreatment methods, the cellulose content in the present study is significant and unique. The
lignin left in the pretreated solid was low. This result is similar to the effect of most other alkalibased pretreatment methods.

On the other hand, the hemicellulose content in the NSHA

pretreated CS was found to be lower unlike other conventional alkali pretreatment methods.
While most pretreatment methods had the retention time from 5 minutes (AFEX) to several
weeks (Lime), the NSHA pretreatment method greatly shortened the retention time to only 2
minutes, but still showed a higher pretreatment efficiency (the highest cellulose content in the
remaining solid) compared with other conventional methods. The high fraction of cellulose
content implies two major potential applications of the NSHA pretreatment.

First, this

technology can be utilized to pretreat biomass for biofuel production. Second, the NSHA with
-1

higher NaOH concentration (above 80 g L ) can also be applied to pulping or related industry
utilizing lignocellulosic biowastes.

2.3.2 Chemical and energy recovery
The remaining liquor after the 2-minute NSHA pretreatment was similar to the black
liquor in pulping industry, which contained inorganic chemicals (mainly sodium hydroxide) and
combustible compounds. The chemical and energy recovery processes such as the conventional
Kraft recovery boiler [40] and the more recent black liquor gasification [41] have been well-

37

developed. Hence, there is a great potential to integrate an appropriate sodium hydroxide and/or
energy recovery processes into our system.

2.3.3 Enzymatic hydrolysis
All existing and studied pretreatment methods, as listed in Table 2.1, primarily aim to
disrupt the naturally formed resistance that protects plant from biological and chemical attacks,
and therefore to enhance the accessibility of enzymes to cellulose and hemicellulose. In other
words, in ethanol production, the reason to apply pretreatment is to reduce either the amount of
the expensive enzymes or the required time during saccharification while achieving a fast and
high conversion of pretreated biomass to monomeric sugars (glucose and xylose). Therefore,
enzymatic hydrolysis of different batches of pretreated biomass at a certain enzyme loading and
saccharification time was carried out in this study.
Figures 2.3-2.5 report the results of enzymatic conversion of CS after the 2-minute
NSHA pretreatment.

As the curves indicate, kinetics varied with enzyme loadings and

pretreatment conditions. The Filter paper unit (FPU) was applied to describe the cellulase
activity. FPU was defined as the activity to achieve 2 mg of glucose from 50 mg of filter paper in
-1

1 h [42]. At an enzyme loading of 5 FPU (g cellulose) , the cellulose in the pretreated CS with
-1

NSHA process at the NaOH concentration of 40 g L was completely converted to glucose after
a 7 days hydrolysis. The completion point shifted to an earlier time when a higher enzyme
loading was applied. The complete cellulose conversion took 3 days at an enzyme loading of 20
FPU (g cellulose)

-1

-1

and 1 day at that of 60 FPU (g cellulose) . And it was noticed that the
-1

cellulose conversion was 92 % already in 1 day at the enzyme loading of 20 FPU (g cellulose) .

38

A similar trend was found for hemicellulose. After pretreated under the NaOH concentration of
-1

40 g L , the hemicellulose in the NSHA pretreated CS was completely converted to xylose in 3
-1

days, at the enzyme loading of 60 FPU (g cellulose) . However, when the pretreatment was
-1

carried out under the NaOH concentration of 4 g L , the hemicellulose conversion in the NSHA
-1

pretreated CS only reached 91 % in 7 days at the enzyme loading of 60 FPU (g cellulose) .

100

Sugars released (%)

90
80
70
60
50
40
30

4 g Lˉ¹ NaOH, cellulose
4 g Lˉ¹ NaOH, hemicellulose
40 g Lˉ¹ NaOH, cellulose
40 g Lˉ¹ NaOH, hemicellulose

20
10
0
0

20

40

60

80

100

120

140

160

180

Hydrolysis time (h)
Figure 2.3 Cellulose and hemicellulose conversion profile of the NSHA pretreated CS with 4 g
-1

-1

L and 40 g L
temperature.

NaOH, with a 5 FPU (g cellulose)

39

-1

enzyme loading, and a 50 C incubation

100
90

Sugars released (%)

80
70

60
50

40

4 g Lˉ¹ NaOH, cellulose

30

4 g Lˉ¹ NaOH, hemicellulose

20

40 g Lˉ¹ NaOH, cellulose

10

40 g Lˉ¹ NaOH, hemicellulose

0
0

20

40

60

80

100

120

140

160

180

Hydrolysis time (h)
Figure 2.4 Cellulose and hemicellulose conversion profile of the NSHA pretreated CS with 4 g
-1

-1

L and 40 g L NaOH, with a 20 FPU (g cellulose)
temperature.

40

-1

enzyme loading, and a 50 C incubation

100

raw CS, cellulose
raw CS, hemicellulose
water, cellulose
water, hemicellulose
20 g Lˉ¹ NaOH, cellulose
20 g Lˉ¹ NaOH, hemicellulose
40 g Lˉ¹ NaOH, cellulose
40 g Lˉ¹ NaOH, hemicellulose

90

Sugars released (%)

80

70
60

50
40
30
20
10
0

0

20

40

60

80

100

120

140

160

180

Hydrolysis time (h)
Figure 2.5 Cellulose and hemicellulose conversion profile of the NSHA pretreated CS, with a 60
FPU (g cellulose)

-1

enzyme loading, and a 50 C incubation temperature.

As shown in Figure 2.5, when the NSHA pretreatment was applied at the NaOH
-1

concentration of 40 g L

(1:1 NaOH to biomass), the cellulose conversion was nearly 100 % in

1 day. The conversion of cellulose in the untreated CS was 28 % after 7 days saccharifi cation.
Meanwhile, the hemicellulose conversion was approximately 100 % after 3 days, whereas the
conversion of hemicellulose in the untreated CS was only 19 % after 7 days saccharification. A
4-fold increase in the cellulose conversion and a 5-fold increase in the hemicellulose conversion
-1

were exhibited. When the NaOH concentration was decreased to 20 g L

41

(1:2 NaOH to

biomass), the conversions of both components were still remarkable. The cellulose conversion
was 93 % in 1 day and almost 100 % after 36 h, and the hemicellulose conversion was 97 % after
3 days. The NSHA pretreatment without any usage of alkali was not sufficient to obtain a good
enzymatic digestibility.
The high efficiency of the NSHA pretreatment on lignocellulosic biomass came from the
smart and synergistic combination of many positive effects. The mechanical nano-scale shearing
generated by the turbine would cause the twist of corn stover in the perpendicular axis to the
fiber and the degradation of the nano-structured cell wall complex along with the NaOH
chemical effect and generate a thermal effect simultaneously due to the severe friction between
the turbine and heterogeneous biomass/solvent.

All those thermochemical and mechanical

effects are reported to be beneficial for the improvement of enzyme digestibility [8]. However,
the single application of shearing force for a few minutes was found to be insufficient to make
the biomass accessible to enzymes, similarly as the application of single alkali chemical force in
such a short time. The natural shield of biomass formed by hemicelluloses and lignin aiming to
protect plants from biological attacks still existed. When alkali such as NaOH was synergistically
combined into the nano-shearing force, the hybrid pretreatment effect was taking place. Sodium
hydroxide is one of the major chemicals that are used to delignify biomass and to solubilize
hemicelluloses [12, 39]. But it is reported that the reaction itself takes long (30 minutes to
several hours), and may be longer when heat-up is required to reach high temperature and/or
high pressure [43, 44]. With the synergistic smart use of high shearing force under alkaline
conditions, the efficiency of lignin removal was greatly enhanced resulting in the significant
decrease of the reaction time to just a couple of minutes. The reason was that because of high
shear rate of the turbine, the diffusion boundary layer between the solid-liquid interphase was

42

-

remarkably getting thinner so that NaOH (i.e., OH ) could interact with biomass cell wall easily
in a very short time. Meanwhile, the micro- and nanoscale cavitation generated by the nanomixer, combined with high shear force, tended to cause the fibrillation of cell walls [45-48], that
is, structural disruption both perpendicular to the fiber axis and longitudinally along the fiber
axis. Temperature rising up to 100

C

in the vessel also helped increase the delignification

reaction rate simultaneously.
The proposed mechanism indicates positive aspects to incorporate the high shear to the
existing pretreatment methodologies and with many other chemicals, due to the synergistic
effects aroused by the high shear rate. Other than the NaOH that is applied in this work, many
other non-volatile chemical compounds, including potassium hydroxide, sulfuric acid, hydrogen
peroxide and ionic liquids can be incorporated with the high shear to enable a fast and efficient
pretreatment. However, for pretreatment with volatile chemical compounds (e.g. organic solvents,
ammonia, ozone, etc.), the reactor is usually sealed (otherwise the chemicals will be vaporized
and exhausted). Thus the high shear will cause an increasing temperature and pressure at the
same time, leading to a reactor run-away. The safety concerns are relatively lessened when a
short retention time (e.g. a few minutes or less) is selected. Without a temperature control,
calcium hydroxide, which has a decreasing solubility with increasing temperature [9, 10], is not
recommended to be incorporated with the high shear rate. Because the rising temperature caused
by the high shear will result in an even more heterogeneous solid-solid phases system, and
therefore greatly weaken the synergistic effects.

43

2.3.4 Scanning electron microscope (SEM) imaging
To further investigate the micro- and nano-scopic mechanism of the NSHA pretreatment,
SEM images of the untreated and NSHA pretreated CS samples were reported in Figures 2.6 and
2.7. Gradual changes of the microscopic structures as a function of NaOH concentration,
revealed in Figure 2.6, indicated that the NSHA pretreatment could efficiently disintegrate
cellulose microfibril aggregates, and open up mini-pores on the surface in such a short time (i.e.,
2 minutes). The biomass cell walls were significantly disrupted when the NSHA pretreatment
was applied even at a moderate NaOH concentration.
As discussed in the previous section, the high shear rate helped significantly reduce the
-

diffusion boundary layers between solid-liquid phases so that well permeated NaOH (OH )
significantly enhanced lignin removal in a very short time. And thus, the high shearing force
could further extend its impact on the cellulose structure, as shown in Figure 2.7, in which tassel like fibrils represent the cellulose nano-fibril aggregates.

Apparently the exposed fibrils

increased the cellulose surface area and benefited for the enzyme accessibility. Meanwhile, with
the same retention time and shear rate, no such fibrils were found in the solid pretreated by the
high shear without using alkali, as shown in Figure 2.6(b).

44

Figure 2.6 From crystalline to disrupted structure – before and after pretreatment at different
conditions. a) Untreated corn stover, area pointed by the arrow is an intact cell wall; b) nanoshear
-1
hybrid water pretreated CS; c) 4 g L NaOH NSHA pretreated CS, area pointed by the arrow is
-1

a discontinued structure with a reduced fiber length (fibrillation); d) 20 g L NaOH NSHA
pretreated CS, area indicated by the circle is where cellulose crystalline structure breaks apart
-1
(another form of fibrillation); e) 40 g L NaOH NSHA pretreated CS, minipores and more
disrupted fibers.

45

Figure 2.6 (cont’d)

46

Figure 2.6 (cont’d)

-1

Figure 2.7 Fibril aggregates on the surface of the cellulose crystalline structure. a) 40 g L
NaOH NSHA pretreated CS, nano-fibril aggregates retained on the surface of a single cellulose
-1
fiber, diameters are around 300-500 nanometers; b) 40 g L NaOH NSHA pretreated CS, more
fibril aggregates with thinner diameters.

47

Figure 2.7 (cont’d)

2.4 Conclusions
A novel and fast nano-scale hybrid pretreatment named nanoshear hybrid alkaline
pretreatment method was introduced in this work. The macro-scale study (e.g., enzymatic
hydrolysis) demonstrated a great enhancement in digestibility of CS with the NSHA pretreatment.
The saccharification rate and overall conversion of cellulose and hemicellulose were greatly
improved over the untreated CS sample.

At the same time, a near-theoretical maximum

enzymatic conversion of the NSHA pretreated sample was achieved in one day. The NSHA
pretreatment was proved to facilitate a significant lignin and hemicellulose removal and a
disruption of cellulose nanostructure. All these merits could contribute to a better digestibility of
CS, in terms of reduced enzyme loading and saccharification time. Moreover, considering that
in the performed enzymatic hydrolysis no optimization of enzyme cocktail was conducted, a
further enhancement of saccharification rate and reduced enzyme loading could be expected.
In addition, the enhancement of digestibility was explored and explained by micro-scale
study (e.g., SEM imaging), in terms of inherent microstructural changes of the remaining solid in
48

the pretreated biomass. According to the SEM images, the NSHA pretreatment was found to
help greatly disrupt the biomass cell walls, and also disintegrate cellulose microfibril aggregates
in a fast and efficient way.

There are still great potentials for further improvement and

optimization on this newly-introduced method, and extended research on this topic is under
investigation.

49

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50

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54

Chapter 3 Nano-deposition on 3-D open-cell aluminum foam materials for improved
energy absorption capacity

3.0 Abstract
Open cell aluminum (Al) foams have been used as energy absorbers for decades. Their energy
absorption capacity can be enhanced by thickening the foam struts, or increasing the foam’s
relative density. However, the enhancement is compromised by the inherent characteristics of its
stress-strain property relationship, whereby upon homogeneous strut thickening, an increase in
the plateau stress without a reduction in densification strain cannot be achieved. In this chapter,
to overcome this inherent barrier, nano-crystalline copper (Cu) was deposited onto the Al foam
and novel 3-D Cu/Al, heterogeneously thickened, composite foam structured materials were
fabricated and tested for the first time. A non-cyanide nano-crystalline copper electro-deposition
system was setup for the coating of open-cell Al foam, and, the energy absorption capacity as a
function of foam pore size and Cu coating thickness was investigated. A variety of
characterization methods confirmed the nano-crystalline structure of the Cu deposition (38 nm).
The energy absorption capacity of Al foams (with average strut thickness of 192 m) reinforced
with a 60 m Cu coating was 3 times greater than that of plain foams. The compressive stressstrain response of the composite samples showed no significant reduction of the densification
strain compared to the uncoated foams due to the small change in the foam strut thickness and
pore size. A comparison between thin Cu-coated and uncoated Al foam samples with the same
overall strut thickness (i.e., same effective volume density and porosity) showed that the nanoreinforced foams had superior energy absorption capacity over plain foams with the same overall
thickness, (~2 times greater).

55

3.1 Introduction
Metallic foams are cellular metals with pores integrated in their structure [1]. The pores
can be closed-cell in which the solid metal forms the face of the cells, or open-cell in which the
solid metal forms the edge of the cells [2, 3]. The fabrication of metallic foams was first reported
in the 1920s in France [4]. The first reference of the fabrication of metallic foams in the United
States was reported in the 1940s [5, 6]. Since then, the techniques for the preparation of metallic
foams have been developed and advanced. Metallic foams have been commercially used in many
sectors, and different materials have been utilized for various applications. Metals used in foams
include aluminum and its alloys (most common), copper, zinc, bronze, iron, nickel, titanium and
gold [1].
Metallic foams with different morphologies can be manufactured in various ways [2, 3,
7-10]. The different methods can be classified based on the state in which the metal is initially
processed [9]: metal vapor, liquid metal, powdered metal, and metal ions. Among those
processes, one of the typical methodologies is to produce foams from the liquid metal, where the
molten metal is mixed with a thickening agent (e.g., CaO) above the melting point of t he metal.
A blowing agent (e.g.,TiH2) is added afterwards to facilitate direct foaming of the metal [11, 12].
A cooling step is needed to produce a foamed block of metal. Metallic foams have various
applications because of their unique mechanical, thermal, electrical, and acoustic properties.
Applications include, but are not limited to, energy absorbers, heat dissipation media, heat
exchangers, battery electrodes, silencers, and sound damping devices [2, 8, 9, 13-19].
Metallic foams have been recognized as excellent energy absorbers for decades due to
their unique compressive stress-stain behavior and their high stiffness-to-weight ratio [13]. The
compressive constitutive response of metal foams is defined by three major stages: the linear
56

elastic region, the plastic collapse region, and the densification region. Two parameters are
important. The plateau stress is the upmost point of the elastic region and represents the highest
stress that can be sustained without permanent damage. The densification strain is the
deformation after which the stress ramps up sharply as it progressively compacts approaching the
behavior of the solid material. Thus, a foam structure with a longer and a higher stress plateau is
considered to have better energy absorbing capacity.
Efforts have been made to reinforce foams for improved energy absorption capacity [19]
by thickening the struts with the same base material, but success has been limited. The
underlying barrier has been that with increased foam strut thic kness the plateau stress increases
but the densification strain decreases [13, 14, 19]. Thus, by thickening the foam struts with the
same material the improvement of the energy absorption capacity might be compromised.
Self-assembled nano-composites or nano-coatings have been introduced to provide
materials with multifunctional properties or to reinforce substrates for enhancements not only at
the micro-scale but also at the macro-scale level [20-32]. Hybrid nanocrystalline Ni/Al metal
foams were fabricated and identified as composite material system with reinforced mechanical
properties [33, 34]. The strategy exhibits the potential to obtain coatings with mechanical
properties superior to those of conventional materials. With the goal of enhancing the energy
absorption capacity of metal foams self-assembled nano-reinforcement could help increase the
plateau stress with a reduced effect on the densification strain. Recently, Jung et al. [35] reported
the use of nano-crystalline nickel (Ni) depositions to reinforce aluminum foam. With a 250 m
Ni coating thickness the plateau stress was considerably increased. However, the thick coating
notably reduced the densification strain, which significantly compromised the energy absorption
capacity.

57

To overcome the inherent opposing goals of maximizing plateau stress without
significantly affecting the densification strain the coating thickness needs to be limited so that the
original foam porosity is not changed significantly. The hypothesis of this study was that a stiffer
metal coating would limit the foam strut thickness and porosity changes, consequently limiting
the reduction of the densification strain. Copper was used as an illustrative example in this study
based on the following rationale. Copper electro-deposition is well established and developed
over several decades [10, 36]. Copper fulfills the noted stiffness requirement with a Young’s
modulus that is almost twice that of aluminum (69 GPa [37]). Meanwhile, although the stiffness
of copper (123 GPa [37]) is less than nickel (206 GPa [37]), copper is less expensive than nickel.
In the presented study open-cell aluminum foams were reinforced by copper nanocrystalline
coating to obtain an enhanced energy absorption capability. Unlike the recently reported work on
Ni coated Al foam [35], this paper reports on nano-crystalline Cu coatings for enhanced energy
absorption of Al foams, and reports on new 3-D Cu nano-deposition techniques and controls for
foam materials. In addition, a systematic experimental characterization that highlights the unique
behavior of nano-reinforced open-cell metal foams is presented.

3.2 Experimental Methods
3.2.1 Materials and equipment
The chemicals used for pretreatment of the Al foams were sodium carbonate (Na 2CO3, ≥
99.0 %), sodium phosphate (Na 3PO4, 96 %), sodium hydroxide (NaOH, ≥ 97.0 %), nitric acid
(HNO3, ≥ 90.0 %), and ammonium hydrogen difluoride ((NH4)HF2, 95 %), all purchased from
Sigma-Aldrich (St. Louis, MO).

58

The Al open-cell foam (ERG Aerospace Co., Oakland, CA) samples were rectangular
bars (25.4 mm × 25.4 mm × 50.8 mm). Foams with different pore sizes (10, 20 and 40 pores per
inch, PPI) and different relative densities (3.5 (±0.5) %, 6 (±1) %, 12 (±1) %) were acquired.
Relative density is the density of the foam divided by the density of the solid parent material. For
the same sample volume, higher relative density means thicker struts.
Deionized (DI) water supplied by a Barnstead Nanopure-UV 4 stage purifier (Barnstead
International Inc., Dubuque, IA), equipped with a UV source and final 0.2 μm filter with a
resistance ≥ 18.0 MΩcm was used for aqueous solution preparation and washing.
The electro-deposition setup featured the following components. A potentiostat (Allied
Plating Supplied, Inc., Hialeah, FL) with a maximum output of 15 amperes and 12 volts. Noncyanide copper electrolytes (Uyemura International Co., Ontario, CA), which mainly contained
copper pyrophosphate as the copper source. And a Pyrex glass container (World Kitchen LLC,
Greencastle, PA) supported by a stirrer/hot plate model 11-300-49 SHP (ThermoFisher
Scientific, Barrington, IL) with a stirring speed range of 60 to 1200 rpm and temperature control
up to 540 °C.
The anode was niobium mesh plated with platinum (Larry King Co., Rosedale, NY), with
dimensions of 140 mm (L) × 55 mm (W) × 65 mm (H). The copper anode (Mcmaster-Carr,
Santa Fe Springs, CA) was attached to the mesh.

3.2.2 Sample pretreatment
Pretreatment of the Al foam before electro-deposition is important to ensure proper
adhesion of the metals that will be subsequently applied to the base material. Pretreatment is also
critical to have the same boundary conditions for different samples. Aluminum foam samples
59

were pretreated as per ASTM B253-87 [38] in the following order. The sample was degreased in
a carbonate-phosphate cleaning solution (25 g/L Na 2CO3 and 25 g/L Na3PO4) at 60 °C for 30
seconds. Afterwards, it was etched in a 50 g/L NaOH solution at 50 °C for 30 seconds. Finally,
the sample was deoxidized with a solution of 500 mL/L HNO 3 and 30 g/L (NH4)HF2 at room
temperature for 30 seconds. The sample was rinsed thoroughly after each step with DI water,
dried, and immediately subjected to electro-deposition.

3.2.3 Electro-deposition
The copper electro-deposition system was set up as illustrated in Figure 3.1. A glass
container was placed on top of a stirrer/hot plate, with a thermocouple placed into the electrolyte
for temperature control. Obtaining a uniform coating thickness is challenging because of the
geometry and topology of the foam sample. Thus, instead of using a metal sheet as an anode, a
rectangular niobium mesh that mimics the foam geometry was used. The aluminum foam sample
was placed in the middle of the anode mesh without contacting any side of it. In this way, each
side of the foam specimen had the same distance to the anode, enabling a uniform transfer of
electrons and copper cations. Meanwhile, each side of the mesh was attached with a copper
anode. The attachment of the copper anode was critical to maintain the copper concentration and
pH value within the electrolyte. An electrolyte of 40 g/L Cu was used at 65 C and a pH of 7.5.
A stirring bar was applied throughout the entire deposition process at 180 revolutions per minute
2

(rpm). The current density was maintained at 4 mA/cm until the desired amount of deposition
was achieved. The classic Faraday’s law of electrolysis [36] was applied to determine the coating
mass gain as a function of time at various current densities (see Figure 3.2). It can be seen that at
60

2

2

lower current densities (i.e., 1 mA/cm and 2 mA/cm ), the experimental data agreed with the
theoretical calculation very well, up to 6 hours of electro-deposition. However, when a higher
2

current density (i.e., 4 mA/cm ) was applied, the experimental data agreed with the theoretical
calculation for the first 6 hours of electro-deposition, and started to level off afterwards. There
are two possible reasons for this observation. First, the deposition efficiency might be lower after
a thick Cu coating on the Al foam struts. Second, diffusion limitation might start to play a
significant role when a thick coating thickness is gained.

Figure 3.1 The electro-deposition system applied in this study. A rare metal rectangular bar mesh
attached with copper sheets was used as the anode in the system. The aluminum foam was
connected to the cathode and centered in the mesh to enable the uniform transfer of electrons and
copper cations.

61

20

18

1 mA/cm2

Mass gain (g)

16

2 mA/cm2

14

4 mA/cm2

12
10
8
6

4
2

0
0

1

2

3

4
5
Time (hr)

6

7

8

9

Figure 3.2 Electro-deposition kinetic studies on open-cell Al foams. Straight lines are theoretical
values based on Faraday’s law of electrolysis. Dots in different shapes are experimental data.

3.2.4 Scanning electron microscope (SEM) imaging
The coated and uncoated aluminum foam samples were evaluated through scanning
electron microscope (SEM) imaging using a Zeiss EVO LS 25 variable pressure SEM. The
microscope is equipped with an energy dispersive x-ray (EDX) detector to determine atomic
compositions. The dried foams were placed into the chamber without further conditioning and a
high vacuum mode was selected during the imaging. Unless otherwise stated, all SEM images
were taken under a high vacuum mode at a 16 kV accelerating voltage and a 25 mm working
distance. All EDX tests were done at a 16 kV accelerating voltage and a 9 mm working distance.

3.2.5 Crystallite size determination using X-ray diffraction (XRD)
The crystallite size of the nano-Cu coating was assessed through X-ray diffraction (XRD).
XRD patterns were obtained on a Bruker D8 DaVinci diffractometer equipped with Cu X-ray

62

radiation operating at 40 kV and 40 mA. The coated foa ms were sliced before XRD analysis.
Peak intensities were obtained by counting with the Lynxeye detector every 0.01 ° at sweep rates
of 0.5 ° 2θ / min. The sample was placed in a poly(vinyl methacrylate) (PVMA) sample holder.
The sample was rotated at 5 degrees per minute. No background correction was applied to the
raw data. The crystallite size of deposited Cu was reported using a single Cu peak according to
the Scherrer formula [39, 40].

3.2.6 Quasi-static compression test
Quasi-static compression tests of plain (uncoated) and coated foam samples were
performed on a universal testing frame (United Testing Systems model SFM-20). The foam
samples (25.4 mm × 25.4 mm × 50.8 mm) were tested along their long dimension. Aluminum
plates (6 mm thick) were bonded (with DP-110 adhesive from 3M, St. Paul, MN) to the ends of
the foam samples before testing to obtain uniform loading regions. All tests were performed at
0.1 in/min until 80 % strain was achieved. Triplicate samples of each kind were tested.

3.3 Results and discussions
When a voltage is applied, the following anodic and cathodic reactions will occur:
Anode:

( )

(
(
(

Cathode:
(

)
)
)

)

(major);
()

(Equation 3.1)

( )

(minor).

( ) (major);
( )

(Equation 3.2)
(Equation 3.3)

(minor).

(Equation 3.4)

63

The Al foam is thus being deposited with Cu when it is connected to the cathode. It is
known that aluminum is reactive to acid and alkali. Thus a pyrophosphate Cu electrolyte was
selected because its optimum working pH is 7.5. A preliminary study using an acid Cu sulfate
bath was conducted. However, the Al foam structure was partially dissolved, thus severely
damaging its energy absorption capacity. Other researchers have reported applying a thin metal
film before the electro-deposition in order to protect the aluminum from dissolving in harsh pH
solutions [35]. However, using an electrolyte that works at neutral pH eliminates the need for
this step.

3.3.1 3-D deposition visualization and uniformity
Figure 3.3 shows the deposition of copper on an open-cell Al foam sample. The foam
samples were visually reddish-orange after the Cu nanocrystalline deposition. A comparison of
uncoated Al foams and Cu coated composite (Cu/Al) foams with various porosities can be seen
in Figure 3.4. The EDX results of a selected area reveal the full coverage (98 wt %) of Cu on top
of Al. A sample was cut in middle to investigate the coating inside of the foam. Figure 3.5 is an
overall top view image of the sample at the cut area. Figures 3.6 (a) and (b) are the top-view
SEM images of the Cu coated sample at a sample section cut. A color contrast between the foam
core material (Al) and coating (Cu) can be seen at the cross section of the cut struts. The SEM
and EDX elemental mapping shown in Figure 3.7 reveal the aggregation of copper on top of the
aluminum foam struts. A series of SEM images of Cu coated Al foam struts with different
magnifications are provided in Figure 3.8.

64

(a)

(b)

Figure 3.3 SEM (a) and EDX elmental mapping (b) on a Cu coated Al foam. The distribution of
each element is: 97.6 wt % (or 91.8 at %) of Cu, 0.4 wt % (or 0.9 at %) of Al and 2.0 wt % (or
7.3 at %) of O.

65

Figure 3.4 Uncoated Al foams (a) and Cu coated Al foams (b). Left: 10 PPI; middle: 20 PPI;
right: 40 PPI

Figure 3.5 Top view digital image of Cu coated Al foams at cut area

66

(a)

(b)

Figure 3.6 Middle section of a copper electrodeposited Al foam. a) and b) SEM images of
selected areas.

67

(a)

(b)

Figure 3.7 SEM (a) and EDX elemental mapping (b) at sample cut area, where Al and Cu can be
seen at the same time. The distribution of each element is: 72.0 wt % (or 50.0 at %) of Cu, 24.3
wt % (or 39.7 at %) of Al and 3.7 wt % (or 10.3 at %) of O.

68

Figure 3.8 SEM images of Cu coated Al foams at different scales

69

Figure 3.8 (cont’d)

The crystallite size of the Cu coating is of interest because it has a direct impact on the
macro-scale mechanical properties [41]. Crystallite size is a measure of each individual

70

crystallographic phase, which represents the average size of a coherent scattering domain [40].
The principle of crystallite size measurements by X-ray diffraction is based on the fact that the
diffraction peak width is inversely proportional to crystallite size [4, 40]. One of the advantages
of using XRD to determine the crystallite size is that XRD provides bulk sampling. Figure 3.9
shows an XRD diagram of the Cu coated foam struts. Al peaks were not observed. Cu peaks
were observed at 43.3° and 50.4°, in accordance with reference values [42]. Crystalline grain
size was calculated using the Scherrer formula [39, 40], as follows:
(Equation 3.5)
where e is the crystallite size, K is the Scherrer constant,  is the wavelength of the applied
radiation, b is the full width at half maximum (FWHM) in radians, and is the peak position.
Accordingly, using the Cu peak at 43.3 °, the crystalline size of the deposited Cu was calculated
to be 38 nm.

600
Cu coated foam
Intensity/Counts

500

Al peak position

400

Cu peak position

300
200
100
0
35

37

39 41 43 45 47 49 51 53 55 57
2 theta (Coupled Two Theta/Theta) WL=1.54

59

Figure 3.9 XRD pattern of a Cu coated Al foam. Crystallite size was calculated using the Cu
peak at 43.3 ° in this curve. Using the Scherrer formula the crystalline size of deposited Cu was
38 nm.
71

Table 3.1 Specimen name designations and coating information
a

b

Relative density, %

Mass gain , g

Average struts
c
thickness , m ± 

6.08

-

390.9 ± 24.2

CS10-1

6.36

7.36

478.4 ± 17.2

CS10-2

6.51

6.84

462.5 ± 20.5

CS10-3

6.39

3.25

431.6 ± 13.3

US20

5.72

-

264.9 ± 18.2

CS20-1

5.92

19.34

438.4 ± 15.4

CS20-2

6.63

6.23

339.6 ± 14.4

CS20-3

5.77

10.00

364.8 ± 13.6

CS20-4

5.79

13.58

393.8 ± 12.5

US40

6.00

-

192.0 ± 14.3

CS40-1

6.35

4.38

240.0 ± 9.6

CS40-2

6.35

14.31

314.0 ± 11.4

CS40-3

6.30

10.03

293.1 ± 18.8

CS40-4

6.30

14.30

309.3 ± 10.7

Sample ID
US10

d

e

a.

The samples have a relative density of 6 (±1) % unless otherwise specified.
Mass gain = the dry mass after deposition – the dry mass before deposition.
c.
Data provided in this column is the average ± standard deviation.
d.
“US” denotes uncoated sample (plain Al), and the following number is the foam pore size in
pores per inch (PPI), e.g., 10 means 10 PPI foam sample.
b.

e.

“CS” denotes coated sample, the following number is the sample pore size in pores per inch
(PPI) and the last number is the serial number of the sample of the same category.

A statistical evaluation was conducted to determine the uniformity of the coatings. After
a varied amount of Cu was deposited on different PPI samples, the strut thicknesses (i.e., foam
72

ligament cross-sectional dimension) of coated and uncoated samples were compared. The
average strut thickness value (see Table 3.1) was obtained based on at least 30 measurements at
different locations of each sample. Compared with the uncoated samples, the strut thicknesses
deviations of coated samples were almost unchanged, indicating that the Cu deposition did not
introduce further deviation on the strut thickness, i.e., the Cu was uniformly deposited on to the
Al foam struts.

3.3.2 Quasi-static compression test and energy absorption calculation
Uncoated (plain Al foam) and Cu coated Al foams were tested under quasi-static
compression load to investigate the energy absorption capacity of the samples. Images of a series
of Cu coated foams before and after the compression tests are shown in the supplementary data,
SD5. For the tests reported in this section, all of the Cu coated samples were based on Al foams
with the same relative density of 6 (±1) %. Using the corresponding stress-strain curve obtained
from testing, the absorbed energy per volume can be calculated by integrating the area under the
curve from 0 up to the densification strain, as follows [13]:

∫
where σ represents the stress and

(Equation 3.6)

 D represents the densification strain. Many algorithms have

been proposed to calculate the densification strain [8, 13, 16], but most of them cannot be
applied to the composite material system considered here. In this study, the densification strain
was defined as the strain at which the stress was equal to 1.5 times the maximum stress before
50 % strain for all samples [16].

73

(a)

(b)

Figure 3.10 Cu coated Al foams before (a) and after (b) quasi-static compression tests

16
14

ρ=12.5%

Stress (Mpa)

12

ρ=6.78%

10

ρ=4.18%

8
6

4
2
0
0%

20%

40%

60%

80%

100%

Nominal strain
Figure 3.11 Compressive stress-strain response of 10 PPI open-cell Al foams with different
relative densities (i.e., different strut thicknesses).

74

8
3

Elastic
region

7

2

Stress (MPa)

6

1

5

0

4

0%

1%

2%

3
2

US40
CS40-30

1

CS40-60
0
0%

20%

40%

60%

80%

100%

Nominal strain
Figure 3.12 Compressive stress-strain responses of 40 PPI uncoated and coated foam samples.
The samples were compressed at 0.1 in/min, until 80 % strain was achieved. The embedded
figure represents the enlarged elastic region. Linear trend lines were used to fit the curve of the
elastic region of each sample.

Table 3.2 Important parameters and calculations for 40 PPI uncoated and copper coated samples
Sample ID
Elastic
Plastification Densification Energy
Enhancement
modulus, MPa stress, MPa
strain, %
absorbed, J
US40
33.49 ± 10.98 0.64 ± 0.12
62.42 ± 0.04
16.65 ± 2.32
-CS40-30

84.82 ± 7.56

1.47 ± 0.13

67.75 ± 0.37

31.77 ± 1.39

91 %

CS40-60

114.95 ± 14.93 2.13 ± 0.19

72.00 ± 1.28

48.52 ± 6.31

192 %

75

8
3

Elastic
region

7
2

Stress (MPa)

6

1

5

0

4

0%

1%

2%

3
US20

2

CS20-30
1

CS20-60

0
0%

20%

40%

60%

80%

100%

Nominal strain
Figure 3.13 Compressive stress-strain responses of 20 PPI uncoated and coated foam samples.
The embedded figure represents the enlarged elastic region. Linear trend lines were used to fit
the curve of the elastic region of each sample.

Table 3.3 Important parameters and calculations for 20 PPI uncoated and copper coated samples
Sample ID
Elastic
Plastification Densification Energy
Enhancement
modulus, MPa stress, MPa
strain, %
absorbed, J
US20
58.29 ± 8.24
1.08 ± 0.05
62.25 ± 0.68 20.69 ± 1.34 -CS20-30

113.57 ± 26.60

1.67 ± 0.14

66.50 ± 0.70

36.76 ± 0.29

78 %

CS20-60

141.51 ± 15.29

2.46 ± 0.19

70.77 ± 0.60

53.16 ± 3.72

157 %

76

8

5
4
3
2
1
0

7

Stress (MPa)

6
5
4

Elastic
region

0%

1%

2%

3
US10

2

CS10-30

CS10-60

1
0
0%

20%

40%

60%

80%

100%

Nominal strain
Figure 3.14 Compressive stress-strain responses of 10 PPI uncoated and coated foam samples.
The samples were compressed at 0.1 in/min, until 80 % strain was achieved. The embedded
figure represents the enlarged elastic region. Linear trend lines were used to fit the curve of the
elastic region of each sample.

Table 3.4 Important parameters
Sample ID
Elastic
modulus, MPa
US10
99.07 ± 11.78

and calculations for 10 PPI uncoated and copper coated samples
Plastification
Densification Energy
Enhancement
stress, MPa
strain, %
absorbed, J
1.77 ± 0.11
64.58 ± 0.67 35.27 ± 1.39 --

CS10-30

157.07 ± 13.87

2.70 ± 0.07

69.86 ± 0.90

53.57 ± 0.64

52 %

CS10-60

192.87 ± 20.94

3.49 ± 0.39

74.49 ± 0.36

69.17 ± 4.41

96 %

A comparison of the stress-strain response among uncoated Al foam samples with
different relative densities was made to assess the effect of this parameter on energy absorption
capacity. A representative curve of each kind was selected for the comparison. For the same

77

sample volume, higher relative density means thicker foam struts. As expected, the plateau stress
increases with higher strut thickness; but the densification strain decreases as shown in Figure
3.11. Thus, thickening the foam struts with the same material will compromise the improvement
on energy absorption capacity.
Figures 3.12-3.14 show the stress-strain curves of coated and uncoated foam samples
with different pore sizes (i.e., PPI). The embedded figures represent the enlarged elastic regi on of
the curves. Tables 3.2-3.4 list the parameters and energy absorption calculations for each curve
corresponding to the different pore size samples. In all cases, the Cu/Al composite foam samples
had distinct features from the plain Al ones. In the elastic region, the stiffness, or effective
modulus, of the coated samples was higher as indicated by the steeper slopes in the response. As
expected, the stiffness gain increased for samples with thicker coating. The plastic or collapse
stress of the coated samples was higher, indicating a larger flexural plastic capacity of the
reinforced struts. The coated foams, however, showed a sudden drop in capacity upon initiating
the plastic collapse region, a feature that was more pronounced for thicker coatings and for lower
PPI foams This behavior is attributed to the combined effect of failure of the coating material
and then the sudden loss of load-carrying capacity due to the inelastic buckling of the cell struts,
for which the accumulated strain energy prior to instability will increase proportionally with the
coating thickness. This behavior was also observed by Jung et al [22] in the response of Ni
coated open-cell Al foams. In the plastic region, the coated samples exhibited a response with a
sustained higher stress along with strain hardening effect that increased for increased coating
thickness. A similar behavior in the plastic region has been observed on density-graded Al foams
[43, 44]. However, more importantly, with the limited coating thicknesses (30 m and 60 m),
the densification strain of the coated samples remained almost the same as that of the uncoated

78

samples. This is the first experimental data showing this unique behavior. As a result, the noted
positive features of the Cu coated foams led to a higher energy absorption capacity with respect
to their corresponding uncoated foam precursor.
The set of three curves shown in Figures 3.12-3.14, respectively, correspond to uncoated
foam samples with the same relative density (6 ±1 %). The strut thickness of a plain uncoated
foam samples varies with pore size. The US10 (uncoated samples, 10 PPI) have the largest strut
thickness, around 391 m; and the US40 samples have the smallest strut thickness, around 192
m. For this reason, a given coating thickness will have a different level of improvement on
energy absorption capacity for different PPI samples. It should be noted that all the coating
thicknesses are defined as one half of the subtraction of foam ligament cross-sectional dimension
after and before coating. The enhancement in energy absorption capacity for 40, 20 and 10 PPI
samples with coating thicknesses of 30 and 60 m is listed in Tables 3.2-3.4. The enhancement
here is quantitatively defined as the difference of absorbed energy between coated and uncoated
sample, divided by the absorbed energy of uncoated sample. As expected, all samples with
thinner coatings exhibited less enhancement of energy absorption capacity. The CS40-60
samples had the most enhancement of energy absorption capacity over the corresponding plain
foam: 192 % (see Table 3.2). For the same coating thickness, the CS10-60 samples had the least
enhancement in energy absorption capacity (see Table 3.4).
The investigation in this section demonstrated that energy absorption capacity
enhancement of the open-cell Al foams is a strong function of foam topology (i.e., foam pore
size), relative density, and coating thickness. The results indicated beneficial strategies in
improving energy absorption capacity of Al foam materials. That is, to reinforce foam struts with

79

a stiff material so that the impact on the foam porosity and structure would be limited. In this
way a higher plastification will be achieved, without severely reducing the densification strain.

3.3.3 Comparison between coated and uncoated foam samples with same strut thickness
The comparison between coated (Cu/Al composite) and uncoated (plain Al) foam
samples with the same overall strut thickness is of our interest because the coating will be
meaningful only if the composite material system exhibits enhanced performance over an
equivalent plain/homogeneous one. This was made possible with careful control of the 3-D foam
coating technique described in this work. A set of 10 PPI samples with different relative densities
was chosen to make the comparison. With the same pore size and sample volume, different
relative densities result in different strut thicknesses. The strut thickness of 10 PPI Al foam with
a 12 (±1) % relative density is around 580 m, while the strut thickness of 10 PPI Al foam with a
6 (±1) % relative density is about 391 m. With the same aforementioned electro-deposition
setup, foam samples with a 6 (±1) % relative density were coated with Cu until an equivalent
strut thickness to that in a foam with 12 (±1) % relative density was achieved. The quasi-static
compressive stress-strain behavior of the plain and coated samples is shown in Figure 3.15.
CS10-95 designates an Al foam of 6 (±1) % relative density with an approximately 95 m Cu
coating thickness. US10-HD refers to an uncoated Al foam with a 12 (±1) % relative density.
Table 3.5 lists and compares the absorbed energy by these two samples, where each value is the
average of three tests.

80

16
6

14

Elastic
region

5
4

12
Stress (Mpa)

3
10

2

1

8

0

0%

6

1%

2%

4
CS10-95
2

US10-HD

0
0%

20%

40%

60%

80%

100%

Nominal Strain
Figure 3.15 Compressive stress-strain responses of 10 PPI samples. “US10-HD” denotes an
uncoated 10 PPI sample, with high density (12 (±1) % relative density), and a strut thickness
around 580 m; “CS10-95” represents a coated 10 PPI sample, starting from a 6 (±1) % relative
density, with an approximately 95 m coating thickness. Al foams with 6 (±1) % relative density
were Cu coated until their strut thickness was equivalent to Al foams with 12 (±1) % relative
density. All samples were tested under quasi-static compression at 0.1 in/min. The embedded
figure represents the enlarged elastic region. Linear trend lines were used to fit the curve of the
elastic region of each sample.

Table 3.5 Energy absorption calculations and comparison based on Figure 3.15
Sample ID
Energy absorbed, J
Enhancement
a

US10-HD
CS10-95

b

54.36 ± 2.85

--

94.12 ± 7.98

73 %

a.

“US10-HD” denotes an uncoated 10 PPI sample with a high relative density (12 ±1 %). The
strut thickness is around 580 m.
b.
“CS10-95” denotes a coated 10 PPI sample starting from a 6 (±1) % relative density foam.
The coating thickness is approximately 95 m.
81

From Figure 3.15 it can be seen that the CS10-95 sample had higher elastic modulus and
a higher plastic capacity. As previously noted, the coated sample has a steep load drop upon
initiation of the plastic region. As previously discussed, this effect is attributed to the larger
strain energy accumulated by the stiffer coated struts, which upon suffering from inelastic
buckling leads to increased loss of load capacity. Again, a similar behavior has been observed in
Ni-coated Al foams [35]. However, two further points must be noted. First, it is hypothesized
that the sudden loss of capacity of the coated struts may also be related to the sudden failure or
debonding of the coating, which implies that the drop in capacity can be reduced if the coating
material can fail in a more gradual manner or if the interfacial bonding between the Cu coating
and the substrate is improved. This remains to be proven. The second point follows from the
observation that the stress drop is also observed in the uncoated sample. In this case the drop is
related to the larger strut thickness in the lower PPI sample, which leads to more stored strain
energy before collapse. A situation that is analogous to the effect of increasing capacity and
stiffness with added coating.
In the plastic region the coated foams exhibited strain hardening behavior similar to the
coated samples in the previous section and very similar to the uncoated samples. However, the
densification strain in the coated 10 PPI sample is notably higher than for the uncoated sample,
which shows the efficiency of the coating material in improving energy absorption capacity.
Calculations show a 73 % enhancement in terms of energy absorption capacity. The result
indicates that the composite foam system has superior energy absorption capacity to the plain Al
foam; or, that for a target energy absorption capacity, the required strut thickness of the
composite foam system could be smaller.

82

3.4 Conclusions
This study presents a novel approach to enhance the energy absorbing characteristic of
open-cell Al foams by reinforcing them through a 3-D copper nano-crystalline electro-deposition
process. The presented investigation was aimed at demonstrati ng the hypothesis that a stiff nanostructured metal coating on an open-cell foam would lead to enhanced energy absorption under
compressive load by increasing the elastic and plastic stress capacity of the composite foam
material with minimum impact on the strut thickness and porosity, thus avoiding a significant
reduction on the foam densification strain. A non-cyanide 3-D Cu electro-deposition system was
developed and successfully implemented to achieve uniform deposition on complex 3-D opencell Al foam samples. A variety of characterization methods were used to confirm that the
processing setup helped achieve uniform Cu nano-crystalline (i.e., 38 nm) depositions.
Quasi-static compression tests of nano-Cu coated Al foams showed that the
reinforcement strategy helped increase the plateau stress without compromising the densification
strain. The energy absorption capacity enhancement from coated and uncoated foams was
systematically investigated and proved to be a strong function of the foam relative densi ty, cell
topology (pore size, strut thickness, etc.) and the electro-deposition conditions. The advantage of
a composite foam system was investigated by comparing the compressive performance of coated
and uncoated foam samples with the same overall strut thickness, which showed that the Cu
nano-crystalline coated Al foam possessed an elongated plastic region and therefore exhibited
better energy absorbing capability.

83

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aluminum foam. Materials Science and Engineering A 2008;489(1-2): 439-443.

88

Chapter 4 A facile method of nickel electroless deposition on various neutral hydrophobic
polymer surfaces

4.0 Abstract
In this chapter, a facile “dip & rinse” method for nickel (Ni) electroless deposition on
hydrophobic polymer surfaces is reported. The hydrophobic interactions between Poly
(allylamine hydrochloride) (PAH) and polymer substrates help eliminate the need for toxic
and/or harsh surface treatment steps for catalys t adsorption/immobilization. Various hydrophobic
polymer surfaces with different geometries and dimensions, including low density polyethylene
(LDPE), high density polyethylene (HDPE), polypropylene (PP) and polystyrene (PS) thin sheets,
PE pellets, as well as non-functionalized PS microspheres, were tested. In all experiments, Ni
was successfully deposited onto these hydrophobic polymer surfaces. Studies showed that,
without the PAH, Ni coating was not able to form on any of these surfaces. Kinetic studies on
polymer thin sheets examples showed that, with 2 hours of deposition, an approximately 2 m
thickness was achieved. A prove-of-concept study showed that Ni coated polymer thin sheets can
be further electroplated with heterogeneous metal (Cu), hence enabling a faster thickness growth
over time.

89

4.1 Introduction
Metal deposition onto heterogeneous materials (also referred to as “metallization”) is of
researchers’ interest because merits of metals can be added to the substrate. The improved
overall properties are usually ascribed to the properties associated with metals, such as abrasion
resistance, friction reduction, electrical and thermal conductivity, or even mechanical hardening
[1-5]. The complexity of the studies usually exists due to their inherent lack of affinity to each
other, although can be overcome by surface modifications [1, 6-10]. The metal deposition onto
polymer is a typical example.
Various metal deposition strategies have been applied for more than decades, or even a
century. For non-conductive surfaces such as polymers, physical and chemical vapor deposition
(PVD and CVD) [11-13], sputter deposition [3, 14], and electroless deposition [15-20] are all
employed strategies. Among these methodologies, electroless deposition is widely used because
of its equipment simplicity and flexibility. Electroless metal deposition is a catalytic, redox
reaction of a metal ion in an aqueous solution (with a reducing chemical agent), without external
electrical field being applied [21]. It usually includes three main steps [17, 18, 22]: 1) a surface
treatment or conditioning; 2) application of an appropriate catalyst (typically noble metal catalyst,
e.g. tin, palladium) on the substrate surface; 3) metal electroless deposition. Rinsing is required
between the steps. However, in the first step, in order to modify the functionality of the substrate
surface so that catalyst can sequentially be attached, harsh and/or toxic surface conditioning steps
are usually employed. The surface conditioning includes a harsh chemical etching [21, 23] (e.g.
sulfuric and chromic acids), or a plasma treatment [6, 16, 24], or a UV source radiation [7, 25,
26], or a laser induced seeding [27, 28].

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There has been a great deal of efforts to improve or even eliminate that harsh treatment
since the 1990s, through various surface modification strategies. For instance, recently, Garcia et
al. [10] introduced poly (acrylic acid) covalent grafting, with which copper was electroless
plated onto the acrylonitrile-butadiene-styrene (ABS). Tengsuwan and Ohshima [29] introduced
a polypropylene-polyethyleneoxide copolymer to increase the hydrophilicity of polypropylene
(PP) and supercritical carbon dioxide to improve infusion of the catalyst; therefore, Ni metal
layer was successfully formed onto PP substrates. Nakagawa et al. [30] introduced ionic
surfactants, rendering an electrostatic attraction of catalyst onto the substrate; thus a selective Ni P electroless plating was implemented.
Apart from all these modification strategies, hydrophobic interaction is another promising
methodology recognized by researchers [31]. Hydrophobic interaction is the ubiquitous
phenomena that in aqueous solution, non-polar substances exhibits a “water-repulsive” behavior
and tends to aggregate with each other [32, 33]. These non-polar substances usually contain
hydrophobic groups, which are lack of affinity with water, such as long chain of carbons [33].
The adsorption of polyelectrolyte multilayers on polymer surfaces via hydrophobic
interactions was first reported by Delcorte et al. [34]. In their work, poly (styrenesulfonate) and
poly (choline methacrylate) were alternately deposited and formed a multilayer system onto
various noncharged polymer surfaces. They also found the use of hydrophobically modified poly
(choline methacrylate) improved the multilayer quality on hydrophobic substrates. Years later,
Park and Hammond [35] identified the hydrophobic interaction between poly (allylamine
hydrochloride) (PAH) with hydrophobic polymers (Teflon-AF, octadecyltrichlorosilane, and
poly (dimethylsiloxane)). PAH is a hydrophobic weak polyelectrolyte, positively charged when
it is protonated [35]. The hydrophobic nature of the PAH backbone can induce hydrophobic

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interactions with other hydrophobic substances in aqueous solution [36]. Since discovered, the
methodology has been applied in many bio-pharmaceutical aspects, including drug delivery [37,
38], protein adsorption and binding [39], and manipulation of bio-signaling [40].
In this work, we demonstrate a facile method of electroless Ni deposition on various
hydrophobic polymer substrates. By making use of the hydrophobic interaction of PAH, the
methodology eliminates the need for harsh and toxic treatment of the substrate. A thickness up to
~2 m of Ni deposition was formed.

4.2 Experimental sections
4.2.1 Materials and equipment
Various hydrophobic polymer thin sheets, including low density polyethylene (LDPE,
semi-clear white), high density polyethylene (HDPE, semi-clear white), polypropylene (PP,
semi-clear white) and polystyrene (PS, opaque white) are thin sheets (30.5 mm × 30.5 mm ×
0.16 mm, Mcmaster-Carr, Santa Fe Springs, CA) when purchased. The film sheets were merely
rinsed with detergent when received and afterwards dried, stored in a cabinet at room
temperature. No chemical treatment was applied. In the experiment, all polymer sheets were cut
into a rectangular shape (2.54 mm × 50.8 mm). Polyethylene (PE) pellets were kindly provided
by Baker Hughes Inc. (Houston, TX), with a nominal average diameter around 500 μm. These
pellets were used as it is. Non-functionalized PS microspheres (w/v 2.5%) with a monodispersed
diameter of 10 m were purchase from Polysciences, Inc. (Warrington, PA). These samples were
stored in a cold room at 4 °C and diluted 10 times before use.
A weak positively charged polyelectrolyte, Poly(allylamine hydrochloride), or PAH
(average Mw ~58,000, Sigma-Aldrich, St. Louis, MO) solution was prepared at 1 g/L. The pH
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was adjusted to 6.5 using 1 M sodium hydroxide (NaOH, Fisher Scientific, Pittsburgh, PA),
unless otherwise noted.
The electroless nickel (Ni) plating catalyst, sodium tetrachloropalladate (II) (Na 2[PdCl4],
98 %, Sigma-Aldrich) was prepared at 5 mM in DI water. The pH was adjusted to 2 using 1 M
hydrochloric acid (HCl, Fisher Scientific). It will be referred as “the Pd catalyst” in this chapter.
Electroless Ni plating bath contained 4 g nickel sulfate (II) (99 %, Sigma-Aldrich), 2 g sodium
citrate (≥ 97 %, Sigma-Aldrich), 0.2 Dimethylamine borane (DMAB, 97 %, Sigma-Aldrich),
lactic acid (85 %, Sigma-Aldrich) in 100 mL DI water. The pH was adjusted to 6.5 using
ammonium hydroxide (NH4 OH, 28 % - 30 %, Fisher Scientific). A more detailed experimental
description can be found in our previous work [15].
Deionized (DI) water supplied by a Barnstead Nanopure-UV 4 stage purifier (Barnstead
International Inc., Dubuque, Iowa), equipped with a UV source and final 0.2 μm fil ter with a
resistance ≥ 18.0 MΩcm was used for all aqueous solution preparation and washing.

4.2.2 Ni electroless deposition
In this study, polymer thin sheets were sequentially interacted with the PAH solution for
30 min, the Pd catalyst for 15 min, and the Ni electroplating bath for 1 h, with a rinse after each
step with DI water. A clamp was used to fix the sample. However, as for polymer pellets and
spheres, the fixation and collection of samples became more complicated because of their size
3

and properties (e.g., density). For PE pellets, since they have lower density (0.91 - 0.95 g/cm )
than water, they float on the surface of an aqueous solution and can only interact with the
aqueous solution partially. To overcome that problem, for each step, the PE pellets were sent into

93

a 15 mL centrifuge tube with the designated chemical solution and rotated in a tube rotator
(Krackeler Scientific Inc., Albany, NY) at ~30 revolutions per minute (rpm) for the same amount
of time. This procedure can ensure that the PE pellets fully interact with the designated
chemicals and the floating issue can be addressed. After each step, PE pellets were vacuum
filtered and washed on a whatman filter paper #1 (Fisher Scientific, retention particle size ~ 11
3

μm). For PS microspheres, since their density (1.06 – 1.12 g/cm ) is higher than water, the tube
rotator was not used. Amicon Ultra-15 centrifugal tubes (Millipore Co., Billerica, MA) were
used for all steps for high recovery of the samples. A centrifuge and at 6000 rpm for 15 min
followed by washing with DI water was applied after each step. After these three steps (PAH, the
Pd catalyst and Ni electroless plating), all samples were air dried and stored in a desiccator.

4.2.3 Copper (Cu) electro-deposition
The Ni coated PS thin sheets were further coated with Cu using the electro-deposition.
The copper electro-deposition system was set up as follows. A glass container (World Kitchen
LLC, Greencastle, PA) was placed on top of a stirrer/hot plate (model no. 11-300-49SHP,
ThermoFisher Scientific, Barrington, IL), with a thermocouple placed into a non-cyanide
electrolyte (Uyemura International Co., Ontario, CA) for temperature control. To get rid of the
directional effect of the anode sheet, a rectangular niobium mesh (Larry King Co., Rosedale, NY)
was used. The PS thin sheet sample was placed in the middle of the mesh, and also in parallel
with the long dimension of the anode mesh. Meanwhile, each side of the mesh was attached with
a copper anode (Mcmaster-Carr, Santa Fe Springs, CA). An electrolyte of 40 g/L Cu was used at
65

C

and a pH of 7.5. A potentiostat (Allied Plating Supplied, Inc., Hialeah, FL) with a

maximum output of 15 amperes and 12 volts was applied for this study. A stirring bar was
94

applied throughout the entire deposition process at 180 rpm. The current density was maintained
2

at approximately 10 mA/cm until the desired amount of deposition was achieved. A detailed
description and illustration of the Cu electro-deposition system can be found i n the previous
chapter.

4.2.4 Scanning electron microscopy (SEM) imaging
The Ni coated samples were evaluated through scanning electron microscope (SEM)
imaging using a Zeiss EVO LS 25 variable pressure SEM. The microscope is equipped with an
energy dispersive x-ray (EDX) detector to determine atomic compositions. Colors with distinct
contrast were deliberately chosen to label the present element in the designated area. Before
imaging, polymer thin sheets were sputter coated with gold (Au) under vacuum (Leica EM
MED020, Buffalo Grove, IL), until a 5 nm coating thickness was achieved.
The dried samples were sent into the chamber without further conditioning, and a high
vacuum mode (less than 1 × 10

-2

Pa) was selected during the imaging. Unless otherwise stated,

all SEM images were taken under high vacuum mode at a 16 kV accelerating voltage and a 25
mm working distance. The EDX studies were performed at a 16 kV accelerating voltage and a 9
mm working distance.

4.2.5 Kinetic study of Ni deposition on polymers
Ni coated HDPE and PS thin polymer sheets were recorded with the mass and
morphology change, as a function of coating time. At the designated time window, each
specimen was pictured with a digital camera. Before each time the sample was weighed, it was
dried with N2 gas at room temperature.
95

4.2.6 Optical microscopy imaging
Samples were also observed using a Keyence optical microscope VHX-600 (Elmwood
Park, NJ) with magnification ranging from 10× to 1000×.

For all images acquired with the

optical microscope, a reflection mode was selected unless otherwise noted.

4.3 Results and discussions
4.3.1 Visualization and microscopic analysis of polymer thin sheets
In this section, four different neutral hydrophobic polymers were selected as substrates
for Ni electroless deposition. Three main steps were included in the following order: 1) an
immersion in the PAH solution; 2) an immersion in the Pd catalyst solution; 3) an immersion in
the Ni electroless plating bath.

96

Figure 4.1 Overall scheme of Ni electroless deposition on a hydrophobic polymer thin sheet

97

Figure 4.1 shows an illustrative scheme of Ni electroless deposition on neutral
hydrophobic thin polymer sheets. Firstly, the application of PAH induced hydrophobic
interactions with the designated polymer surface, therefore, the PAH was adsorbed onto the
polymer surface. As briefly mentioned in section 4.1, the long carbon chain backbones that exist
in both the PAH and the polymer substrate are hydrophobic, therefore, exhibiting a repulsive
nature to the aqueous solution and tend to assemble each other [32, 33]. It should be noted that
even hydrophobic interaction is not a strong interaction, but stronger than van der Waals
interactions and hydrogen bonds in aqueous environment [32]. A variety of conformation of
PAH on hydrophobic surfaces was investigated in the previous studies [35, 41]. It should be
mentioned that when the PAH chains are fully charged, a stretched conformation will be
obtained due to the electrostatic forces between charged groups on the chains [35]. The weak
polyelectrolyte nature of PAH has a reversible equilibrium of dissociation, which is largely
dependent on its local pH and ionization. At a pH lower than pKa value (pKa of PAH is 8.7 [42]),
PAH is primarily protonated, and therefore, spread on to the substrate surface [35]. An
increasing ionic strength will give rise to a decreased layer thickness because of the spreading of
the PAH chains to the surface [35, 43]. At the same time, because of the ionization the PAH
chains are exhibiting a certain degree of “coiling conformation” [35], which is shown in Figure
4.1. The “coiling conformation” results in a random distribution of charged group, both on the
substrate surface and throughout the thickness of the adsorbed PAH layer. Secondly, a catalyst
deposition was achieved by immersing PAH modified substrate into the Pd catalyst solution,
enabling an electrostatic interaction between the pronated PAH (positively charged) and the Pd
catalyst (negatively charged). The electrostatic interaction between catalysts with different
charges and the corresponding polyelectrolytes has been thoroughly investigated in our previous

98

studies [15, 44, 45]. In this work, because of the distribution of the positive charges, catalyst is
attracted and catalytic sites are created throughout the PAH layer thickness too. Finally, when
the designated polymer thin sheet was submerged into the Ni electroless plating bath, the redox
reaction of Ni cations to nonvalent Ni occurred at the corresponding catalytic sites (where
catalyst is present) and forms a thin layer of Ni coating.
A control experiment was performed with exclusion of the first step, in which PAH was
adsorbed onto the designated polymer surfaces. Figure 4.2 shows a systematic comparison of
designated polymers before and after Ni deposition. Without the inclusion of PAH, all
designated polymers were not deposited with Ni at all. The previous research [45] has shown that,
with the same Ni electroless plating bath, no Ni coating was formed without the Pd catalyst.
Combined with that result, we were able to conclude that Ni coating was not formed on the
polymer surface due to the fact that no catalyst was attached. However, with the inclusion of
PAH, Ni was successfully formed on all the designated polymer surfaces.

99

Figure 4.2 Visualization and comparison of Ni coating on various hydrophobic polymer thin
sheets (LDPE, HDPE, PP, PS). Non-PAH modified polymer sheets were not able to form Ni
coating.
100

Figure 4.3 Visualization of Ni coating on a PP polymer thin sheet. Upper left is a SEM image at
the edge of Ni coating; upper right is the corresponding EDX elemental mapping (60 wt % of C,
11 wt % of Au, 29 wt % of Ni); lower left is a SEM image at the main coating body; lower right
is the corresponding EDX elemental mapping (6 wt % of C, 5 wt % of O, 18 wt % of Au, 71 wt %
of Ni).

101

Figure 4.4 Visualization of Ni coating on a PS polymer thin sheet. Upper left is a SEM image at
the edge of Ni coating; upper right is the corresponding EDX elemental mapping (44 wt % of C,
3 wt % of O, 8 wt % of Au, 45 wt % of Ni); lower left is a SEM ima ge at the main coating body;
lower right is the corresponding EDX elemental mapping (4 wt % of O, 12 wt % of Au, 84 wt %
of Ni).

102

Figure 4.5 Visualization of Ni coating on a HDPE polymer thin sheet. Upper left is a SEM image
at the edge of Ni coating; upper right is the corresponding EDX elemental mapping (37 wt % of
C, 6 wt % of O, 9 wt % of Au, 48 wt % of Ni); lower left is a SEM image at the main coating
body; lower right is the corresponding EDX elemental mapping (3 wt % of C, 4 wt % of O, 9 wt %
of Au, 84 wt % of Ni).

103

Figure 4.6 Visualization of Ni coating on a LDPE polymer thin sheet. Upper left is a SEM image
at the edge of Ni coating; upper right is the corresponding EDX elemental mapping (51 wt % of
C, 7 wt % of Au, 42 wt % of Ni); lower left is a SEM image at the main coating body; lower
right is the corresponding EDX elemental mapping (28 wt % of C, 3 wt % of O, 8 wt % of Au,
61 wt % of Ni).

The morphologies of Ni coating on the different polymer surfaces were observed by SEM.
The Ni depositions on all the designated polymer thin sheets were achieved. As shown in Figures
4.3-4.6, representative images at the coating/polymer edge and at the main coating body are
shown, for each designated polymer thin sheet. An EDX elemental mapping investigation was
performed and presented next to the corresponded SEM image. Results showed different
morphologies of Ni coatings on different polymer substrates. The Ni coating on PP appeared
smooth, with minimal defects among all the substrates. The Ni coating on PS exhibited a rough
surface, with an overlapped Ni flake-like morphology. The excessive nucleation and growth of
Ni coating caused a full coverage on the polymer surface. The Ni coatings on HDPE showed a
104

similar flake-like morphology, but with less overlap. The coating had a few delaminated areas
both at the coating edge and the coating main body. The Ni coating on LDPE showed inferior
quality, with a large portion of delamination (interfacial fracture) on the surface. As a result, a
fair amount of the LDPE was uncovered with Ni.

Figure 4.7 The evolution of Ni coating morphologies on the LDPE surfaces.

To further investigate the delamination phenomena, as shown in Figure 4.7, the
morphological study of the Ni coating on LDPE as a function of time was conducted. In order to
eliminate the effect of drying, the Ni coated LDPE samples were subjected to optical microscopy

105

imaging immediately after the coating. As suggested by the Figure 4.7, Ni coating formed a
connected thin layer up to 10 min of coating. The morphology at 15 min showed that Ni coating
was partially disconnected, indicating that the delamination of coating occurred between 10 min
and 15 min of coating. The morphology at 20 min exhibited even more delamination. It should
be noted that the delamination occurs widely in the coating industry [46-49]. Although many
mechanisms have been proposed, generally it is considered that the delamination is due to the
internal mechanical/ thermal stress buildup during the coating [49, 50]. Therefore, to prevent
delamination, a stress relieve step is needed. One of the efficient ways to relieve stress, as
previously reported by our group, is to incorporate nanoparticles [49, 51] at the interface to
deflect and break up the internal stresses.
The delamination could be partially due to different mechanisms of Ni formation on the
designate polymer surfaces, and the weak adhesion between the coating film and the
corresponding substrate. The adhesion between the Ni coating and polymer substrates is not
meant to be strong because the adsorption of PAH on to polymer surfaces is not a strong
interaction. It was reported that the hydrophobic interaction of PAH was used to transfer
multilayer from the stamp when it was initially exploited since the hydrophobic interaction was
weaker than the electrostatic forces [52, 53]. Even though, the weak adhesion between coating
and the substrate is considered detrimental in many actual practices, people have applied those
weakly bonded material structures as intermediate products for composite fabrications [49], and
pattern transfer [52] as mentioned earlier. However, if a stronger adhesion is desired, it can be
achieved by introducing chemical bonds (e.g. cross-linking), or a careful optimization of the Ni
electroless plating bath, polyelectrolyte and the coating procedure.

106

The coverage of the Ni coating on polymer thin sheets is also of interest. According to
the EDX mapping, the PP thin sheets exhibited minimal uncovered area. The PS thin sheet
exhibited a superior Ni coating coverage that the polymer was no longer detected (0 % of C).
The HDPE thin sheets exhibited a similar coverage of Ni coating to PP. The Ni on the LDPE
exhibited a large portion of the uncoated area in the main coating body. By comparing the weight
percentage of Ni and C (polymer), the percentage of Ni coverage on those four polymer thin
sheets can be ranked in the descending order as follows: PS > PP ≈ HDPE > LDPE.

4.3.2 Visualization and microscopic analysis of polymer pellets and spheres
In this section, all the substrates subjected to the Ni coating follows the same process
route as the polymer thin sheets. Figure 4.8 shows the illustrative scheme of the Ni electroless
deposition on the neutral hydrophobic polymer pellets and spheres. The mechanism for the
formation of the Ni coating is the same, other than the geometry and dimension of the substrate.

107

Figure 4.8 Overall scheme of the Ni electroless deposition on the hydrophobic polymer
pellet/sphere.

108

Figure 4.9 Visualization of Ni deposition on PE pellets. Non-PAH modified PE pellets were not
able to form Ni coating.
109

4.3.2.1 Ni deposition on PE pellets
Figure 4.9 exhibits a set of studies of PE pellets before and after the Ni deposition. A
control experiment was also performed with the exclusion of step 1 (PAH dipping). Without the
inclusion of PAH, PE pellets remained uncoated. With the inclusion of PAH, PE pellets were
successfully deposited with Ni, even though the Ni coverage is not perfect on some of the pel lets.

Figure 4.10 SEM images of a) uncoated PE pellets; b) a coated PE pellet. The corresponded
diagram next to each image shows the element signal of a selected area designated in the SEM
image.

The morphologies of the Ni coated PE pellets were observed by SEM (see Figure 4.10).
The coating looks similar to that on the polymer thin sheets. Overlapped flake-like Ni coatings
were formed on the PE pellets. A comparison before and after the Ni electroless plating was

110

made with EDX. With the help of EDX, the area scanning proved formation of Ni on the coated
sample (Figure 4.10 (b)); whereas no Ni peak was observed in the uncoated one (Figure 4.10 (a)).

4.3.2.2 Ni deposition on PS microspheres
PS microspheres are commercially provided with specific surface charge functionalities
[54], ideally negative (e.g. carboxylate-modified PS), positive (e.g. amine-modified PS) and
neutral (e.g. plain PS), but in reality, they can be deviated because of the fabrication
methodology. Emulsion polymerization [55, 56] process is usually employed for the fabrication
of monodispersed PS, since this method can precisely control the particle si ze with a narrow
polydispersity. This methodology includes: 1) the formation of micelles from surfactant
molecules; 2) the addition of monomers (styrene), entering of monomers into micelles; 3) the
addition of an initiator to induce polymerization; 4) the polymerization termination by sulfate
ions from the initiator which remain at the sphere surface. This mechanism gives rise to the
aggregation of anions at the surface, making the surface with charges (negative), even without
functional group. Excessive amount of surfactant will largely increase surface charges of the
sample. Actually the surface charge of the plain PS purchased from polysciences (Warrington,
PA) was tested and gave the value of -20.24 ± 1.09. The variation may exist among different
batches, but still, that value is considered mildly negative.
With that in mind, polystyrene microspheres were also Ni electroless plated in this work.
The same coating strategy was used. A control study showed no Ni deposition was observed
when PAH-step was excluded. Because there is no hydrophobic interaction, also PS surface and
the Pd catalyst are both negatively charged, making the Pd catalyst impossible to be attached.
However, if the PS has a positive surface charge, the Pd catalyst will be adsorbed by electrostatic

111

interactions. A similar work has been done by previous researchers [15]. But when the PAH-step
was included, the Ni coating was formed (see Figure 4.11). However, the morphology of Ni
coated PS microspheres looks different from the coating formed on other samples (polymer thin
sheets, PE pellets). Instead of Ni thin films, small size Ni grains were formed on the PS
microspheres. It could be ascribed to the fact that the electrostatic and hydrophobic interactions
play together, attracting PAH in different conformations and therefore forming Ni deposition in a
different way.

Figure 4.11 SEM images of a) PS microspheres before coating; b) non-PAH modified PS
microspheres after Ni deposition; c) and d) PAH modified PS microspheres after Ni deposition.

112

Figure 4.12 A morphological change of HDPE thin sheet, during Ni electroless deposition.

113

Figure 4.13 A morphological change of PS thin sheet, during the Ni electroless deposition.

4.3.3 Kinetic study of Ni deposition
Figures 4.12 and 4.13 demonstrate gradual morphology changes along with coating time
on the HDPE thin sheet and the PS thin sheet, respectively. The depositions of Ni on the Pd
catalyst-seeded HDPE and PS thin sheets are almost instantaneous. Both substrates showed the
114

Ni coating at the 1st min of coating. In the first 10 min of coating, both substrates showed a
severe morphological change, due to the Ni coating formation. It was also noted that for both
polymers, after 30 min of coating, the morphology of them remain almost unchanged. It is
probably due to the fact that the horizontal coverage of Ni reaches plateau in that time frame, and
the vertical thickness growth became dominant, which will not result in any change in its outlook.

Nominal coating thickness, nm

1900
1700
1500
1300
1100
with PAH, HDPE

900

with PAH, PS

700

no PAH

500
300
100
-100
-20

0

20

40

60

80

100

120

140

Coating time, min
Figure 4.14 Kinetic studies and comparison of Ni electroless deposition on HDPE and PS,
respectively. A nominal thickness gain over coating time was plotted.

The thickness gain over time provided more details. The nominal thickness gains of two
polymer thin sheets were plotted against coating time as shown in Figure 4.14. The thickness
gains for non-PAH modified HDPE and PS remained zero for 2 hours of coating. The nominal
thickness gain was calculated by dividing Ni volume gain by the surface area of designated
coating area as demonstrated in Equation 4.1.

115

[

(

)

(Equation 4.1)

]

where, T denotes the coating thickness, Δm denotes mass gain in each designated time window, ρ
-3

denotes the density of the coated material (for Ni it is 8.91 g cm ), Ld denotes the long
dimension of designated coated area, w denotes the width of the polymer sheet, which equals
25.4 mm, h denotes the thickness of the polymer sheet, which equals 1.6 mm. From the curve,
over 2 hours of Ni electro-deposition, both substrates gained an approximately 2 m coating
thickness. However, the HDPE substrate showed slower Ni mass gain after 1hour of coating;
whereas the PS substrate exhibited a decreasing trend in terms of thickness gain over time after 1
hour, the thickness gain rate was still faster than that of HDPE in that time frame.

4.3.3 Cu electro-deposition on Ni coated polymer
One of the disadvantages of electroless deposition is that, it can only achieve a few
microns or even submicron size thickness, even in hours of processing. This limitation can be
overcome by an electro-deposition method in which an applied electro-field forces a current flow
through an electrochemical cell to cause chemical changes [57]. The electro-deposition can
achieve more than a hundred microns coating thickness in hours. As a matter of fact, in practical,
in order to electroplate a substrate which is non-conductive, a thin layer of metal induced by
electroless plating is usually applied to reinforce the conductivity, allowing the substrate to be
electroplated with either homogeneous or heterogeneous materials afterwards [21, 57]. To
address the aforementioned issues and to demonstrate the feasibility, Ni coated polymer sheets
were electroplated with Cu.

116

Figure 4.15 Visualization of Cu deposition on Ni coated PS thin sheets, after Cu electrodeposition. Uncoated PS sheets were not able to form Cu coating.

117

Figure 4.16 Visualization of Cu coating on a Ni coated PS thin sheet, after Cu electro-deposition.
Upper left is a SEM image at the edge of Cu coating; upper right is the corresponding EDX
elemental mapping (36 wt % of C, 3 wt % of O, 21 wt % of Au, 39 wt % of Ni); lower left is a
SEM image at the main coating body; lower right is the corresponding EDX elemental mapping
(3 wt % of O, 14 wt % of Au, 82 wt % of Ni).

Again a control experiment was conducted, in which an uncoated PS sheet was
electroplated in the same electro-deposition system. As expected, no Cu deposition was achieved,
simply because of the non-conductive nature of the PS. On the contrary, a Ni coated PS thin
sheet was electroplated in the same system, and Cu deposition was successfully achieved (see
Figure 4.15). The Cu deposition was visually reddish-orange. It should be noted that although the
whole sheet was immersed in the electrolyte and subjected to electro-deposition, only the Ni
coated portion was electrodeposited. Similarly, SEM images and corresponding EDX
investigations on the edge of the coating and the main coating body were conducted, as shown in

118

Figure 4.16. A full coverage of Cu on to the Ni coated PS sheet was observed. Upon certain
thickness of Cu deposition, the designated polymer and Ni were not able to be detected.
The following methodology was applied to calculate the nominal coating thickness gain
over time. The mass gain fulfills the classic Faraday’s law of electrolysis as a function of time, at
a constant current [57],
(Equation 4.2)

where, I denotes the applied current, M denotes the molar mass of deposited metal (for Cu it is
-1

64 g mol ), F denotes Faraday’s constant, z denotes the valency number of deposited element
(for Cu it is 2), t denotes time (in second(s)). Equation 4.1 was used to calculate the thickness
gain of deposition, except for the denotation of mass gain. Here it represents the mass gain in
electro-deposition. If substitute Equation 4.1 with Equation 4.2,

[

(

)

]

(Equation 4.3)

thus, a nominal thickness evaluation of Cu electro-deposition as a function of time was obtained.
Based on the calculation, an approximately 15 m coating thickness was gained in 1 hour.

4.4 Conclusions
In this work, a facile methodology for Ni electroless deposition on hydrophobic polymer
surfaces was proposed. The hydrophobic interaction between PAH and a large variety of
hydrophobic polymer surfaces helped eliminate the need for harsh and/or toxic surface treatment
for the catalyst adsorption/immobilization. Various hydrophobic polymers with different
geometries and dimension were tested, and Ni was successfully deposited onto all of these

119

polymer surfaces. Comparisons showed without the PAH, Ni coating was not able to form on
any of these hydrophobic polymer surfaces. A thickness of ~2 m of Ni deposition was obtained
in 2 hours. The coating thickness limitation can be addressed by applying a further electr odeposition, even with a heterogeneous metal. The electro-deposition can expedite the thickness
growth and extend the metallization of non-conductive materials to many aspects.

120

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121

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Chapter 5 Conclusions and future work

5.1 Conclusions
In summary, the nanotechnology based strategies were developed and applied in three
fields: biomass pretreatment in biofuel production, functional material fabrication and surface
modification.
First, a novel and fast nanoshear hybrid alkaline (NSHA) pretreatment method for
lignocellulosic biomass was developed. Corn stover (CS) was pretreated as an illustrative
example in a modified Taylor-Couette reactor with sodium hydroxide at room temperature, with
a two-minute retention time and a 12500 s

-1

shear rate. The NSHA pretreatment substantially

enhanced the digestibility of CS. The saccharification rate and overall conversion of cellulose
and hemicellulose were significantly improved over the untreated CS sample. A complete
enzymatic conversion of the NSHA pretreated sample was achieved in one day. Compared with
untreated corn stover, an approximately 4-fold increase in enzymatic cellulose conversion and a
5-fold increase in hemicellulose conversion were achieved. Also, the NSHA pretreatment
facilitated significant removals of lignin and hemicellulose, leaving an up to 82 % of cellulose
content in the remaining solid. In addition, micro-scale studies revealed inherent microstructural
changes of the remaining solid in the pretreated biomass. The NSHA pretreatment was found to
severely disrupt the biomass cell walls, and also caused fibrillations of cellulose fibers. The
exposure of polysaccharide aggregates were also observed, and believed to be beneficial for
improved accessibility of polysaccharides to enzymes.
Second, to overcome the compromised energy absorption capacity induced by the
inherent characteristics of stress-strain property relationship of open-cell aluminum (Al) foams,
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whereby upon homogeneous strut thickening, an increase in the plateau stress wi thout a
reduction in densification strain cannot be achieved, a nano-deposition strategy was developed.
A featured non-cyanide nano-crystalline copper electro-deposition system was setup for the
coating of open-cell Al foam, with which nano-crystalline copper (Cu) was uniformly deposited
onto the Al foam, and novel 3-D composite foam structured materials were fabricated. The
presented investigation aimed to demonstrate the hypothesis that a stiff nano-structured metal
coating on an open-cell foam would lead to enhanced energy absorption under compressive load
by increasing the elastic and plastic stress capacity of the composite foam material with
minimum impact on the strut thickness and porosity, thus avoiding a significant reduction on the
foam densification strain. The x-ray diffraction tests confirmed the crystallinity of the Cu
deposition and the crystallite size was calculated to be 38 nm. Nano-Cu coated Al foams showed
a quasi-static compressive response with an increased plateau stress, without compromising the
densification strain. The advantage of a composite foam system was investigated by comparing
the compressive performance of coated and uncoated foam samples with the same overall strut
thickness, which showed that the Cu nano-crystalline coated Al foam possessed an elongated
plastic region and, therefore, exhibited better energy absorbing capability.
Finally, a facile “dip & rinse” method for nickel (Ni) electroless deposition on
hydrophobic polymer surfaces was developed. By making use of hydrophobic interactions
between Poly(allylamine hydrochloride) (PAH) and polymer substrates, the proposed
methodology eliminated the need for harsh and/or toxic surface treatment for the catalyst
adsorption/immobilization. Various hydrophobic polymer surfaces with different geometries and
dimensions were tested, and Ni was successfully deposited onto all of the polymer surfaces.
Comparisons showed that, without the PAH, Ni coating was not able to be formed on any of

128

these polymer surfaces. Among these polymer thin sheets, with the same experiment condition
and procedure, coverage of Ni coating can be ranked in the following order: PS > PP ≈ HDPE >
LDPE. The study shows that Ni deposition achieved a thickness of ~2 m in 2 hours. The
coating thickness limitation can be addressed by applying a subsequent electro-deposition, even
with a heterogeneous metal. The electro-deposition can expedite the thickness growth and extend
the metallization of non-conductive materials to many aspects.

5.2 Outlook & future work
The NSHA pretreatment method for lignocellulosic biomass can be extended to at least
two directions. First, the NSHA pretreatment method will be more attractive and competitive
when a cost control is executed since an inexpensive pretreatment step is highly desired for an
economic biofuel production from biomass. As mentioned in Chapter 2, the pretreatment
accounts for the second largest portion in the entire cost of bioethanol production. A favorable
pretreatment should be cost-efficient itself, and also benign to the downstream processes. For
example, dilute acid pretreatment normally leads to the formation of degradation products (e.g.,
furfural), which are toxic to the next step - fermentation. Thus after the pretreatment, the biomass
is subjected to a conditioning and separation step, which raises the cost both in equipment and
operation. The cost control of the NSHA pretreatment can be done by “synergistically”
optimizing the retention time, alkaline concentration, biomass loading, and shear rate. All these
parameters are cost-sensitive. A quick solution will be elongating the retention time by a small
margin while keeping other conditions the same since the small extension will not bring a severe
safety concern. However, that small extension is a high shear process at an elevated temperature
(100

°C)

with an elevated alkaline concentration (water evaporation), which will make

129

considerable differences. At the same time, it would be interesting to further investigate the
effect of NSHA pretreatment on many other biomass species. Comparison can be made to see if
the NSHA pretreatment would be advantageous over other pretreat methods on certain types of
biomass.
Second, from a research standpoint, many other chemicals can be tested and/or co mbined,
other than those which involve safety issues. As previously discussed in Chapter 2, volatile
chemical compounds and calcium hydroxide are not suggested to be incorporated with the high
shear system. Other chemicals may substitute or add to sodium hydroxide for higher efficiencies.
Promising candidates include sodium peroxide (Na 2O2), hydrogen peroxide (H2O2), and ionic
liquids. Water and most acids are less promising, since without the help of high shear, they are
usually combined with high temperature (no less than 160 °C) for an effective breakdown of
biomass.
The work of nano-crystalline copper deposition on to open-cell Al foams for a better
energy absorption capacity can be extended in many perspectives. First, other mechanical tests,
including tension test, dynamic Hopkinson bar test of coated Al foams can be done to investigate
other mechanical insights of the nano-crystalline copper deposition. That part of work is
currently under investigation by our collaborators. Second, a further study on the steep stress
reduction upon initiation of the plastic region can be done, and possible improvements can be
made to reduce or even eliminate the stress drop. The stress drop is mainly due to the
inhomogeneity between Cu and Al, giving rise to the imperfect adhesion at the interface.
Different solutions (e.g., introduce chemical bond) to improve the adhesion may lead to different
overall mechanical responses of coated Al foams. Finally, different materials with higher

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stiffness can be coated on to the Al foams for enhanced mechanical properties. The candidates
include iron (Fe), Ni, metal composites, graphene, graphite and so on.
The work of Ni electroless deposition on hydrophobic polymer surfaces is at the
preliminary stage and can be extended. The potential application can be diverse. Studies can be
done to enhance the interfacial bonding between metal (Ni) and various polymers, for a
composite core-shell structure with higher stability. When combined Ni, because it has better
mechanical strength, electrical and thermal conductivity than polymer, a better interfacial
bonding always helps enhance the desired properties. On the other hand, the deposition strategy
can be applied in the composite fabrication methodology when the interfacial bonding between
metal and polymer is weak.

131