MODIFICATION OF THIN-FILM HEMATITE PREPARED
BY ATOMIC LAYER DEPOSITION FOR IMPROVED SOLAR WATER OXIDATION
By
Kelley Marie H Young

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
Chemistry- Doctor of Philosophy
2015

ABSTRACT
MODIFICATION OF THIN-FILM HEMATITE PREPARED BY ATOMIC LAYER DEPOSITION
FOR IMPROVED SOLAR WATER OXIDATION
By
Kelley Marie H Young
The stimulus for my research stems from the immediate need to reduce our planet’s
dependence of carbon-based fuel in order to minimize the potential detrimental effects of
climate change. It is known that the sun irradiates the surface of our planet with enough
energy in one hour to meet our ever growing energy needs; with the potential to
completely supplement our dependence on carbon based fuels. Since a majority of energy
uses necessitate the need for liquid fuels, developing materials that have the ability to store
energy in chemical bonds are of great interest.
This work has focused on the fabrication, modification, characterization, and analysis of
semiconductor metal oxides for photoanode materials, primarily hematite (α-Fe2O3).
Hematite has ideal photoanode characteristics such as good light absorption, stable in
contact with neutral and basic aqueous electrolytes, and has a low enough valence band
energy to drive water oxidation via photogenerated holes. In addition, hematite is
abundant, making it a cost effective material for potential scalability.
Although hematite has many desirable characteristics as a photoanode material, its
performance has been less than desirable. Water oxidation efficiency is controlled by three
processes of the photoanode: light harvesting by the material, the transport of
photogenerated holes to the solution interface, and hole collection via water oxidation at
the electrode surface.

Specifically, this work aimed to reduce the detrimental recombination of
photogenerated holes on the surface of hematite before they are able to facilitate water
oxidation by treating the surface with a known water oxidation catalyst, Ni(OH)2 using ALD.
In my work, I have shown that mitigating this recombination has had a drastic effect of the
performance of hematite as a photoanode, moving it closer to being a viable photoanode
material.
Once the recipe for Ni(OH)2 was established and reproducible, this was deposited onto well
characterized thin films of Fe2O3 and was fabricated as a photoanode for
photoelectrochemical studies. I was able to develop an electrochemical conditioning
method

for

the

electrodes

that

yielded

stable,

reproducible

results.

Using

photoelectrochemical measurements such as cyclic voltammetry, transient spectroscopy,
and impedance spectroscopy I was able to determine that the addition of Ni(OH)2 to the
surface of Fe2O3 did in fact inhibit detrimental recombination of photogenerated holes.
Ni(OH)2 acted as a charge storage medium (akin to that of a battery) that collected
photogenerated holes from Fe2O3, which in turn oxidized Ni2+ to Ni3+, which then oxidized
water at its surface. This result showed the greatest onset for water oxidation with a
catalyst at the surface of hematite (with a shift in photovoltage on approximately 300 mV),
which is vastly important for improving the efficiency for Fe2O3 as a photoanode material.

Mom and Dad, this is for you.

iv

AKNOWLEDGEMENTS

First I would like to thank my adviser, Tom Hamann. He has demonstrated incredible
wisdom, insight, and patience through my PhD. I was fortunate to have an advisor was
patient with me as I came into my own as a researcher. I greatly admire Tom’s integrity and
work ethic; it is something I hope to carry with me through my academic career. Thank
you.
Secondly, I would like to thank my lab mates, both former and present, for helping to make
my graduate experience so enjoyable. Ben and Jesse, your guidance in my first years was
invaluable, and I am thankful that you challenged me and made the lab an enjoyable place
to be. I am privileged to have had you as colleagues and to continue to call you friends.
Yuling, I am glad that I was able to have to as a lab mate- you were especially good at
balancing out my mean safety notes to the rest of the group. Omid- you are a great
colleague, and though I don’t think I had to mentor you much, I enjoyed our discussions and
am thankful for your insight. To the other young-ones in the lab, I enjoyed mentoring you
and watching you develop into good researchers. You also made lab a lot of fun, so thank
you Josh, Dan, and Yuan- I wish you all the best of luck.
I have been very fortunate in my graduate career to generate friendships with colleagues
whom I truly respect and admire. To my ‘happy hour’ friends, who turned into family,
thank you for keeping me sane (that’s you, Nick). Andrew, thank you for being such a great
work husband as well as a sounding board for all things chemistry and not. Jennie- thank

v

you for your patience, quick wit, and unwavering support. I don’t know what I would have
done without the writing -monster, Friday afternoon talks, and coffee breaks.
To my family- I am truly blessed to have such a strong support system. Knowing that I
always had people rooting for me and caring for me was incredibly reassuring. To my
parents and my brother, thank you for always pushing me to be my best, and encouraging
me to do what makes me happy. Your love and support means the world to me.
Lastly, to my husband and best friend- your love, patience, and encouragement are more
than I could have ever dreamed of. You are the best. Thank you for supporting me always.

vi

TABLE OF CONTENTS
LIST OF FIGURES ........................................................................................................................................ ix
CHAPTER 1: Introduction ........................................................................................................................ 1
1.1 Motivation ................................................................................................................................. 2
1.2 Semiconductor-Liquid Junctions .................................................................................. 4
1.3 Hematite..................................................................................................................................... 5
REFERENCES .................................................................................................................................. 11
CHAPTER 2: Enhanced Photocatalytic Water Oxidation Efficiency with Ni(OH)2
Catalysts Deposited onto α-Fe2O3 via
Atomic Layer Deposition ........................................................................................ 16
2.1 Introduction ......................................................................................................................... .17
2.2 Experimental ........................................................................................................................ 18
2.2.1 Electrode Fabrication ........................................................................................ 18
2.2.2 Photoelectrochemical Measurements ........................................................... 20
2.2.3 Absorptance Measurements ............................................................................. 21
2.2.4 XPS ............................................................................................................................ 21
2.3 Results and Discussion ................................................................................................... 21
2.3.1 Absorbtance ........................................................................................................... 21
2.3.2 Photoelectrochemical Conditioning .............................................................. 22
2.3.3 XPS ............................................................................................................................ 25
2.3.4 Photoelectrochemical Measurements ........................................................... 27
2.4 Conclusions ........................................................................................................................... 33
REFERENCES .................................................................................................................................. 35
CHAPTER 3: Electrochemical Impedance Spectroscopy of Ni(OH)2-coated α-Fe2O3:
Investigation into the Improvement of
Water Oxidation with α-Fe2O3 ........................................................................................ 38
3.1 Introduction ......................................................................................................................... 39
3.2 Experimental ....................................................................................................................... 40
3.2.1 Electrode Fabrication ......................................................................................... 40
3.2.2 Electrochemical Measurements ..................................................................... 42
3.2.3 Electrochemical Conditioning of Ni(OH)2-coated Fe2O3 ......................... 42
3.2.4 J-V and Impedance Spectroscopy ..................................................................... 43
3.3 Results and Discussion ................................................................................................... 43
3.3.1 Conditioning Difference in 100 Ni(OH)2-coated Fe2O3 Electrodes ..... 43
3.3.2 Consilience for 100Ni(OH)2-coated Fe2O3 Electrodes .............................. 54
3.3.3 1 and 100 Ni(OH)2-coated Fe2O3 Electrodes ............................................... 55
3.4 Conclusion ............................................................................................................................. 63
APPENDIX ....................................................................................................................................... 68
REFERENCES ................................................................................................................................. 73
vii

CHAPTER 4: Fabrication of ITO-Inverse Opals for High Aspect Ratio Substrate for
Deposition of α-Fe2O3 by Atomic Layer Deposition ............................................... 76
4.1 Introduction ......................................................................................................................... 77
4.2 Experimental ....................................................................................................................... 82
4.2.1 Glass Preparation ................................................................................................ 82
4.2.2 Synthesis of ITO Inverse Opals ........................................................................ 83
4.2.3 SEM .......................................................................................................................... 84
4.2.4 ALD of Hematite ................................................................................................... 84
4.2.5 UV-Vis ....................................................................................................................... 85
4.2.6 Photoelectrochemical Measurements ........................................................... 85
4.3 Results & Discussion ........................................................................................................ 86
4.3.1 SEM ........................................................................................................................... 86
4.3.2 Absorbtance ........................................................................................................... 88
4.3.3 Photoelectrochemical Measurements ........................................................... 90
4.4 Conclusions and Future Work .................................................................................... 94
REFERENCES ................................................................................................................................. 96

viii

LIST OF FIGURES

Figure 1-1. AM 1.5 Solar Irradiance (black line) and the required thickness of hematite to
absorb 95% of the light, 3/α (orange line) ........................................................................... 7
Figure 1-2. Schematic SCLJ energetics of water oxidation on hematite photoanode. The
green arrows represent favorable charge transfer pathways and the red arrows
represent recombination pathways ........................................................................................ 8
Figure 2-1. Calculated absorptance from transmittance and reflectance measurements for
bare Fe2O3 (black) and NiOx-coated (red) electrodes ................................................... 22
Figure 2-2. Plots of J-V curves for bare Fe2O3 (black) and Ni(OH)2-coated Fe2O3 electrodes
for as-deposited (red), and following 1 hour (yellow), 2 hours (green), and 3 hours
(blue) of conditioning at 1.42 V vs RHE under 1 sun illumination ........................... 23
Figure 2-3. Dark J-V plots for bare (gray) and initial (red), 1 hour (yellow), 2 hours (green)
and 3 hours of conditioning (blue) for NiOx-coated Fe2O3 ......................................... 24
Figure 2-4. XPS spectra of as deposited (black), annealed (green) and conditioned (red)
NiOx-coated Fe2O3 showing the a)Ni 2p and b) the O 1s region. Curve fits for c)
annealed and d) conditioned NiOx-coated Fe2O3 are
shown for the O 1s region. ....................................................................................................... 27
Figure 2-5. Bare Fe2O3 (black) and Ni(OH)2-coated Fe2O3 (red) anodic (light on) transient
measurements at 1.17 V vs RHE ............................................................................................ 29
Figure 2-6. J-V plots under illumination and in the dark with superimposed Faradaic
photocurrent measurements (open symbols) for Ni(OH)2-coated Fe2O3 and bare
Fe2O3. ................................................................................................................................................ 31
Figure 2-7. Cathodic (light off) transient measurements for bare Fe2O3 (black) and
Ni(OH)2-coated Fe2O3 (red) at 1.05 and 0.800 V vs RHE, respectively ................... 32
Figure 3-1. J-V plots under 1 sun illumination for three different batches of conditioned
100 Ni(OH)2-coated Fe2O3 Batch A, Batch B, and Batch C in green, blue, and purple,
respectively. .................................................................................................................................... 44
Figure 3-2. Nyquist plots measured at 900 mV vs RHE for A, B, and C 100Ni(OH)2-coated
Fe2O3 electrodes in green, blue, and purple, respectively ........................................... 46
Figure 3-3. Equivalent circuits used to fit Nyquist plots for a) one semi-circle present and
b) when two semi-circles were present. ............................................................................. 47
Figure 3-4. Bulk capacitance values for 100Ni(OH)2-coated Fe2O3 electrodes A (green
triangles), B (blue diamonds), and C (purple crosses). ................................................. 49

ix

Figure 3-5. Mott-Schottky plots for 100Ni(OH)2-coated Fe2O3 electrodes A (green
triangles), B (blue diamonds), and C (purple crosses). ................................................. 50
Figure 3-6. Bulk resistance values for 100Ni(OH)2-coated Fe2O3 for electrodes A (green
triangles), B (blue diamonds), and C (purple crosses). ................................................. 52
Figure 3-7. Capacitance values for 100Ni(OH)2-coated Fe2O3 electrodes A (green triangles),
B (blue diamonds), and C (purple crosses) ....................................................................... 53
Figure 3-8. Resistance attributed to Ni(OH)2 100Ni(OH)2-coated Fe2O3 electrodes A (green
triangles), B (blue diamonds), and C (purple crosses). ................................................. 54
Figure 3-9. J-V plots under 1 sun illumination and in the dark for bare and 1 and 100
Ni(OH)2-coated Fe2O3 in black, blue, and red, respectively. ....................................... 57
Figure 3-10. Bulk capacitance values for bare Fe2O3 (black circles), 1 Ni(OH)2-coated Fe2O3
(blue triangles), and 100 Ni(OH)2-coated Fe2O3 (red squares). Measurements
performed in the dark are in solid shapes, those conducted under 1 sun illumination
are in the open shapes. .............................................................................................................. 59
Figure 3-11. Bulk resistance values for bare Fe2O3 (black circles), 1 Ni(OH)2-coated Fe2O3
(blue triangles), and 100 Ni(OH)2-coated Fe2O3 (red squares) measured under 1 sun
illumination. ................................................................................................................................... 60
Figure 3-12. Charge-transfer capacitance from surface states values for bare Fe2O3 (black
circles) and charge-transfer capacitance from of Ni(OH)2 for 1 Ni(OH)2-coated Fe2O3
(blue triangles) and 100 Ni(OH)2-coated Fe2O3 (red squares) measured under 1 sun
illumination. ................................................................................................................................... 62
Figure 3-13. Charge-transfer resistance from the surface-states values for bare Fe2O3
(black circles) and charge-transfer resistance of Ni(OH)2 for 1 Ni(OH)2-coated Fe2O3
(blue triangles) and 100 Ni(OH)2-coated Fe2O3 (red squares) measured under 1 sun
illumination. .................................................................................................................................... 63
Figure A-1. Cyclic voltammograms of varying thicknesses of conditioned Ni(OH)2 on FTO
............................................................................................................................................................. 69
Figure A-2. Zoomed view of cyclic voltammograms of varying thicknesses of conditioned
Ni(OH)2 on FTO. ........................................................................................................................... 70
Figure A-3. Capacitance values for varying thicknesses of conditioned Ni(OH)2 on
FTO .................................................................................................................................................... 71
Figure A-4. Resistance values for varying thicknesses of conditioned Ni(OH)2 on
FTO .................................................................................................................................................. 72
Figure 4-1. Schematic of light absorption depth compared to charge collection length in
hematite. .......................................................................................................................................... 77
x

Figure 4-2. AM 1.5 Solar irradiance (black line) and the thickness required for hematite to
absorb 95% of the light, 3/α (orange line). ....................................................................... 78
Figure 4-3. Schematic of planar Fe2O3 on a transparent conducting oxide (TCO) backcontact and thin-film Fe2O3 on a nanostructured TCO. The nanostructured TCO
allows for greater light absorption in the z-direction while keeping charge collection
short in the x-direction. ............................................................................................................. 79
Figure 4-4. Band-bending diagram for a semiconductor liquid junction showing favorable
charge transfer pathways in green and unfavorable recombination pathways in red.
Jbr, Jdr, and Jss indicate photocurrent, J, loss to bulk recombination, depletion region,
and surface state recombination, respectively. Jet is photocurrent lost to back
electron transfer. ......................................................................................................................... 81
Figure 4-5. Schematic of ITO-IO fabrication process .................................................................. 84
Figure 4-6. SEM images of a) 350 nm polystyrene nanospheres dispersed on FTO, b) top
view of multi layer ITO-IO, and c) View of large area approximately single layer ITOIO. ....................................................................................................................................................... 87
Figure 4-7. Calculated percent absorptance for bare FTO (black dash-dot), ITO-IO on FTO
(red dash-dot), 500 cycles of Fe2O3 on planar FTO (black solid), and 500 cycles of
Fe2O3 on ITO-IO (red solid). ..................................................................................................... 89
Figure 4-8. Percent absorptance for Fe2O3-coated planer FTO (black dashed line) and
Fe2O3-coated ITO-IO (red dashed line) corrected for substrate
absorptance. ................................................................................................................................. 90
Figure 4-9. Current density versus voltage curves for planer Fe2O3-coated FTO (black
trace) and Fe2O3 ITO-IO substrates (black trace) in the light (bright colors) and in
the dark (muted colors). ........................................................................................................... 92
Figure 4-10. Nyquist plots for bare FTO (red trace, large plot) and for bare ITO-IO
electrodes (blue trace, inset plot) measured at 800 mV vs RHE. .............................. 93
Figure 4-11. Randal's circuit used for fitting Nyquist plot. ...................................................... 94

xi

Chapter 1: Introduction

Reproduced from ChemistryCatalysis Science and Technology 2013, 3, 1660-1671 with
permission from The Royal Society Chemistry.

1

1.1 Motivation
There is an eminent need to reduce our dependence on carbon-based fuels in order to
minimize the worst potential outcomes associated with climate change.1–6 The impacts of
climate change are widespread and can have cascading effects across multiple sectors such
as agriculture, water, energy, and transportation. What’s more, climate change can have an
eventual direct effect on human health and well-being.5 With the world’s population
estimated to exceed 7 billion and growing, the world’s energy demands will continue to
increase as well, which is projected to reach 31 TW by 2040.

6

Currently, approximately

82% of supplied power is derived from fossil fuels in 2012. The value is projected to be
reduced to 80% by 2040.

6

It is well accepted that the release of carbon dioxide form the

combustion of fossil fuels is a major contributor to global climate change and the negative
effects that will surely come with it. For this reason, it is imperative to develop cost
effective, carbon neural sources of energy in order to minimize these consequences. Of the
renewable sources of energy available ( wind, geothermal, biofuels, hydro, and solar), the
Sun irradiates the Earth’s surface with an average of ~1 kW m-2, making it the largest
resource available and is thus uniquely capable of completely supplanting our dependence
on carbon-based fuels.7
Efficient solar-to-electricity conversion has been available for decades; single crystal
silicon and gallium arsenide photovoltaic cells have achieved power conversion efficiencies
of ~25% and ~29%, respectively.8 One of the primary drawbacks which limits widespread
use of these cells is their high cost.9,10 In addition, since sunlight is diurnal and intermittent,
effective means of storing solar-generated electricity must be developed. Further, some of
2

our major energy uses (e.g. transportation) require liquid fuels since they have a higher
energy density than direct solar illumination can provide. Therefore, the developments of
materials that are able to convert solar energy into chemical fuels are of great interest.
Nature provides a great example of using solar energy to produce fuels in the process of
photosynthesis. In photosynthesis, solar energy is used to drive a series of charge-transfer
reactions which ultimately oxidize water to release O2 and reduce CO2 to produce
hydrocarbon materials. The overall solar energy to stored fuel conversion efficiency of
photosynthesis is estimated to be only ~1%,11 however, which prevents it from being a
practical solution to supplying the world’s fuel demands. Taking a page from Nature, there
has been an ongoing effort to develop artificial photosynthesis systems capable of
converting solar energy into high energy density chemical fuels.12,13 Most interesting to our
group, is the reaction of generating solar fuel via the water splitting my generating O2 gas
and the usable fuel H2 gas. This process is described by the two reactions below:
(1-1)
(1-2)
The challenge at hand is to adapt photovoltaic systems capable of achieving high
efficiencies to drive the water splitting reactions in a scalable fashion. Several artificial
photosynthetic schemes have been devised to address this challenge. The most
straightforward approach is to directly link a photovoltaic device to an electrolyzer, or to
couple water oxidation and reduction catalysts on the surface of a buried photovoltaic
device. For example, Nocera’s group recently utilized a buried triple junction amorphous
silicon photovoltaic device coated with a cobalt-phosphate water oxidation catalyst and a
NiMoZn water reduction catalyst, termed the ‘Artificial Leaf’.14 This is a promising system
3

in that it utilizes only earth abundant materials, is stable, and evolves H 2 in a variety of
aqueous environments.14,15 The efficiency of this cell is primarily limited by use of the
amorphous silicon photovoltaic driving the reactions. An alternative example is a dyesensitized photoelectrochemical cell, which is a close analog to the dye-sensitized solar cell
except that the oxidized sensitizer formed following charge injection into TiO2
nanoparticles drives water oxidation as opposed to oxidation of a redox shuttle. 16–18 While
these types of systems can offer tunability as a result of separate oxidation and reduction
components used to evolve hydrogen and oxygen, they currently rely on rare metal centers
for catalyst materials as well as sensitizers that may not offer long term stability.
1.2 Semiconductor-Liquid Junctions
The first and most widely studied method of solar energy conversion is semiconductorliquid junctions (SCLJ).19 Using SCLJs for solar energy initialized in 1972 with the seminal
report by Fujishima and Honda in which they demonstrated photo-assisted water splitting
on single crystal TiO2 electrodes.20 Since that report, other materials have been used for
solar energy conversion resulting in high photon-to-electrical energy conversion efficiency
demonstrating the promise of utilizing such a system.21,22 For a single material to achieve
efficient unassisted water splitting driven by visible light, a combination of several
challenging material requirements must be met: absorption of a sufficient portion of the
visible solar spectrum, band edge positions capable of generating H2 and O2, and electrode
stability in aqueous electrolytes over a range of pH conditions. Further, since it needs to be
scalable, the semiconductors must be comprised of earth abundant materials. These
requirements make up a ‘tall order’ and to this date no single semiconductor composed of
earth abundant materials has been able to produce sustained, efficient water splitting.
4

There are several alternative configurations of SCLJs that relax some of the constraints
placed on a single absorbing material which are also capable of achieving efficient photodriven water splitting. In an ideal example, the two redox half reactions can be relegated to
separate materials – a photocathode which absorbs low energy photons for the proton
reduction and a photoanode which absorbs high energy photons for water oxidation. Not
only is this configuration capable of absorbing a greater fraction of the solar spectrum, it
also allows for both photoelectrode materials to be investigated and optimized for the
proton reduction and water oxidation reactions independently and in parallel. The
different material configurations in water splitting schemes has been discussed
elsewhere.19 There is a paucity of feasible photoanode materials, however. In addition,
photocatalytic water oxidation at the photoanode surface is generally the rate limiting
process in the overall water splitting reaction. For this dissertation, the one of the most
promising photoanode materials for water oxidation is discussed.
1.3 Hematite
One of the most promising photoanode materials for water oxidation is hematite (αFe2O3). Hematite possesses a corundum structure with FeIII cations in a slightly distorted
octahedral environment. The conduction band is comprised of Fe 3d orbitals and the
valence band originates from Fe 3d(2t2g) and O 2p orbitals.23 The energy difference
between the bands produces a bandgap of 2.1 eV, which allows hematite to absorb photons
out to 590 nm. Integration of the 2.1 eV bandgap with the AM 1.5 solar spectrum results in
a maximum possible photocurrent density of 12.5 mA cm−2.24,25 The theoretical solar-tohydrogen efficiency, ηSTH, can be calculated from the following equation:
(1-3)
5

where Jop is the optimum operating current density of the cell, 1.23 V is the thermodynamic
potential required for the conversion of one molecule of H2O to H2 and ½O2 under standard
conditions per one electron transferred,

is any additional overpotential which needs to be

applied, and Pin is in the incident light power density (for one sun, 100 mWcm-2).19 Using
hematite paired with a material with the ability to match hematite’s maximum current
density through absorption of longer wavelength light transmitted through hematite, and
which is capable of hydrogen evolution and supplying any additional overpotential to
hematite, the maximum theoretical solar-to-hydrogen efficiency using hematite is ~15%. In
order to achieve such efficiency, hematite must be able to absorb a significant portion of
the visible spectrum. The light harvesting efficiency, LHE, of incident photons by a
semiconductor is dependent on the material absorption coefficient, α, and the thickness, l,
according to

. Any given semiconductor needs to be 3/ α thick in order to

absorb 95% of the light at a given wavelength illustrated in Figure 1-1. Therefore hematite,
with

at 550 nm, needs to be 400 nm thick to absorb 95% of the light at

this wavelength.26

6

Figure 1-1. AM 1.5 Solar Irradiance (black line) and the required thickness of
hematite to absorb 95% of the light, 3/α (orange line).26
In addition to good light absorption, hematite is stable in contact with neutral and basic
aqueous electrolytes. The combination of visible light absorption and stability is largely
what sets hematite apart from other materials. In addition, the hematite valence band
energy is low enough, EVB/q = 2.71 V versus RHE, to allow photogenerated holes to drive
the water oxidation reaction (Figure 1-2). Hematite’s conduction band is energetically too
low to reduce water to H2, however as noted above. Application of an external bias through
an additional photovoltaic device or implementation of hematite as the photoanode
material in a tandem system can facilitate efficient overall photocatalytic water splitting.

7

Figure 1-2. Schematic SCLJ energetics of water oxidation on hematite photoanode.
The green arrows represent favorable charge transfer pathways and the red arrows
represent recombination pathways.27
Despite these favorable characteristics, the overall solar energy conversion
efficiency with hematite electrodes has been poor. The efficiency of solar driven water
oxidation on a semiconductor photoanode is generally controlled by the efficiency of three
processes: light harvesting, the transport of photogenerated holes to the solution interface
(hole transport efficiency) and hole collection via water oxidation at the electrode surface
(water oxidation efficiency). The relatively long visible light absorption depth (hundreds of
nanometers) combined with a very short minority charge collection length (~10 nm)
prevent realizing both a high LHE and hole transport efficiency necessary to achieve high
overall solar-to-fuel conversion efficiencies.26,28–33 Much of the recent work on hematite has
therefore focused on nanostructuring and doping strategies in order to overcome these
drawbacks.23,26,34–39

8

While nanostructuring and doping can improve the efficiency of getting
photogenerated holes to the electrode–electrolyte interface, these holes must still be
collected efficiently. The water oxidation reaction at the hematite electrode surface is
generally reported to be sluggish which allows for increased recombination at the
electrode surface with a concomitant loss in efficiency.38,40–44 Previous work in our group
was able to demonstrate this by comparing the current density versus applied potential (JV) response of a hematite electrode in contact with an aqueous electrolyte to that of an
electrolyte when a fast hole collector is present under identical conditions. It was found
that that there is a significant difference in photocurrent response of H2O oxidation
compared to oxidation of the fast hole collector, [Fe(CN)6]4−. From these experiments it was
determined that a planar hematite electrode in contact with a fast, one-electron redox
couple [Fe(CN)6]3−/4− is only limited by the fraction of holes that reach the
semiconductor/liquid interface.32,33,44,45 In other words, the surface–hole collection
efficiency by [Fe(CN)6]4− is essentially unity and the surface-hole collection efficiency for
H2O oxidation is less than unity. We note that similar conclusions were reached by Dotan
et. al. using a H2O2 as a hole collector.46 Since the water oxidation efficiency is determined
by the fraction of holes that oxidize water rather than recombine with electrons at the
electrode surface, improving the surface environment of hematite is necessary in order to
increase efficiency of water oxidation with hematite electrodes.
There appears to be general agreement that surface states play an important role in
determining the water oxidation efficiency. Specifically, recombination at these surface
states competes with the forward water oxidation reaction and it is this kinetic rivalry that
controls the overall efficiency there is wide agreement that recombination at surface states
9

of hematite limits the photocurrent onset potential and thus the solar-to-hydrogen
conversion efficiency.44,47–50 It is therefore reasonable to modify the electrode surface in
order to improve the water oxidation efficiency.
For the majority of this dissertation, we focus on the fact that water oxidation efficiency
is determined by the fraction of holes at the surface that participate in the water oxidation
reaction rather than recombine with conduction band electrons. Thus, the efficiency should
be improved by either accelerating the rate of water oxidation or reducing the rate of
surface-state recombination. To achieve this, the addition of a water oxidation catalyst to
the hematite surface should accelerate the water oxidation kinetics and thus improve the
water oxidation efficiency. Passivation of the surface states facilitating recombination
should minimize this reaction and also improve the overall water oxidation efficiency.
There are many examples in the literature of materials being added to the surface of
hematite which have been shown to shift the photocurrent onset potential cathodically (to
a more positive potential), thereby increasing the photocatalytic water oxidation efficiency.
Examples of surface treatments to hematite’s surface include IrOx, Co-Pi, Co(OH)2/Co3O4,
Ni(OH)2, and NiFeOx.35,40,47,51–60 Since both strategies – catalysis and surface passivation –
would produce the same macroscopic effect of shifting the photocurrent onset,
identification of the actual mechanism of improvement is not straightforward to
unambiguously determine. It is the primary goal of this dissertation to understand the
effects of Ni(OH)2 to the surface of hematite on water oxidation efficiency.

10

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11

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6408.

15

Chapter 2: Enhanced Photocatalytic Water Oxidation Efficiency
with Ni(OH)2 Catalysts Deposited onto α-Fe2O3 via Atomic Layer
Deposition

Reproduced from K. M. H. Young and T. W. Hamann, Chem. Commun., 2014, 50, 8727 with
permission from The Royal Society of Chemistry.

16

2.1 Introduction
The development of materials capable of converting solar energy to chemical fuels is of
great interest since this process offers the possibility of reducing our dependence on
carbon based fuels.1–3 One method to generate solar fuels is to use a semiconductor to split
water and produce O2 and the useable fuel H2. Ideally, the generation of H2 and O2 could be
performed on separate photoelectrodes allowing for independent photocathode (H2
generation) and photoanode (O2 generation) optimization. One of the most promising
photoanode materials for water oxidation is hematite (α-Fe2O3), which has desirable
material characteristics, such as high elemental abundance, good light absorption into the
visible region, stability in neutral and basic aqueous solutions, and a favourable valence
band position to facilitate water oxidation.4 However, hematite suffers from a short hole
collection length compared to its light absorbance depth and slow water oxidation kinetics
at the surface, limiting its overall solar-to-fuel conversion efficiency.5–16 In order to lower
the activation barrier for water oxidation and reduce or bypass recombination of
photogenerated holes, catalysts such as IrOx, Co-Pi, Co(OH)2/Co3O4, Ni(OH)2, and NiFeOx
have been added to hematite’s surface.17–24 While these catalysts have shown an
improvement for the onset of water oxidation ranging from approximately 200-400 mV
cathodic shifts, some are unstable, are not deposited in a conformal manner or have
competing light absorbance in the visible region which impedes the ability to be used
effectively in a nanostructured device. In addition, there is a disagreement in the
mechanism to which the improvement in water oxidation onset occurs.

17

Ni(OH)2 is an especially interesting water oxidation catalyst as it is composed of earth
abundant elements, has minimal competitive light absorption, are stable in neutral and
basic solutions, and has shown good electrocatalytic activity for water oxidation.25,26
Ni(OH)2 was previously coated on Fe2O3 nanowires via dipcoating, which resulted in an
initial cathodic shift of the photocurrent density; however the photocurrent decayed by
~90% – to values lower than the bare hematite – over 30 seconds. This behaviour was
attributed to fast oxidation of Ni2+ to Ni3+, followed by a rate limiting step of further
oxidation to a Ni4+ species, thus resulting in the NiOOH film storing charge but not
producing a sustained enhancement of water oxidation.17 Ni(OH)2/NiOOH has been
recently been shown to produce an adaptive junction when paired with a TiO2 single
crystal semiconductor electrode which resulted in excellent stable water oxidation
performance compared to IrOx.26,27 This discrepancy in the effectiveness and behaviour of
Ni(OH)2 as a water oxidation catalyst on semiconductor surfaces motivated this research.
Herein we report using atomic layer deposition (ALD) to deposit thin films of NiOx onto
the surface of planer Fe2O3 electrodes. ALD allows for self-limiting film growth which
ensures reproducible conformal films with tuneable thickness. Since it is not a line-of-sight
technique, these films can also be deposited uniformly on high surface area hematite films.
In addition, thin films of NiOx are transparent, so there will be no competition for light
absorption in the visible region.
2.2 Experimental
2.2.1 Electrode Fabrication
Working electrodes were fabricated by atomic layer deposition (ALD) (Savannah 100
[Ga2O3, Fe2O3] and Savannah 200 [NiO], Cambridge Nanotech) onto fluorine-doped tin
18

oxide (FTO) coated glass substrates (Hartford Glass, 12Ω cm-2). Prior to deposition of
Ga2O3, FTO glass was sonicated in detergent and DI water, rinsed, sonicated in clean DI
water, and then finally rinsed and sonicated in isopropyl alcohol (IPA). Substrates were
blown dry with N2 directly before being placed in the ALD chamber. Approximately 2nm
(18 ALD cycles) of Ga2O3 was deposited onto cleaned FTO substrates via ALD using
tris(dimethylamido)gallium (III) (Ga2(NMe2)6) (Strem Chemicals Inc.) as Ga precursor and
H2O as an oxidant following a modified version of a previously reported procedure. 28 The
thickness of Ga2O3 was confirmed with ellipsometry on witness Si wafers in the ALD
chamber during deposition. The substrate was held at 200°C and the Ga precursor was
heated to 150 °C, pulsed for 0.2 s, held under exposure mode for 8 s, followed by 12 s N 2
purge. Subsequently, a 0.015 s pulse of H2O was introduced under the same exposurepurge time conditions to oxidize the Ga precursor. As deposited Ga2O3 on FTO substrate
was subsequently coated with ~20 nm (300 ALD cycles) of Fe2O3 in the same chamber.
Thickness of the hematite was confirmed using absorbtance measurements and BeerLambert calculations. Briefly, the substrate remained at 200 °C ferrocene (sublimed from
Sigma Aldrich) held at 70°C was pulsed for 20s, purged with N2 for 5s followed by a 0.015s
pulse of H2O, immediately followed by a 1s pulse of ozone (Yanco Industries ozone
generator). The H2O and O3 pulse was repeated ten times for a single oxidation macrocycle
in order to oxidize ferrocene.29 Fe2O3 with the Ga2O3 underlayer will be referred to simply
as Fe2O3. Following deposition, Fe2O3 was annealed to 500°C in an oxygen environment at
a rate of 17°C per minute, sintered at 500°C for 30 minutes and allowed to cool slowly to
110°C over 2 hours. Subsequently, approximately 10nm (100 ALD cycles) of NiOx was
deposited onto the annealed Fe2O3 by ALD (Savannah 200, Cambridge Nanotech) following
19

a modified procedure.30 Nickel alkyl amidinate (Ni-amd) (AccudepTM Nickel Precursor,
Dow) was used as the nickel precursor and H2O as the oxidant. The substrate was heated to
150°C and the Ni-amd cylinder was heated to 130°C. The Ni(amd) was pulsed for 0.2s and
held under exposure mode for 20 seconds, purged with N2 for 30 s, followed by a water
pulse of 0.015 s under exposure mode for 20 s. This cycle was repeated until the desired
thickness was obtained. A Growth rate of 1.01 Å/cycle was determined by ellipsometery
(Horiba Smart SE) measurements on Si wafers for varying cycles of NiOx from 1-300. NiOx
coated Fe2O3 was annealed to 300 °C in an oxygen environment at a rate of 5°C per minute
and sintered for 1 hour. NiOx coated electrodes were allowed to cool to 80°C over 4 hours
and stored in an 80°C oven when not in use.
2.2.2 Photoelectrochemical Measurements
Electrochemical measurements were measured in a custom glass 3-electrode cell with a
quartz window to allow illumination from the solution side, SS, (striking the NiOx-Fe2O3
first) with a Pt mesh counter electrode, homemade Ag/AgCl reference electrode (calibrated
by referencing to a pristine SCE reference electrode), and a Fe2O3 or NiOx-coated Fe2O3
working electrode clamped to the cell. All electrodes were measured in 1M KOH (pH 14
measured by Fischer Scientific Accumet pH meter) with 0.2M KCl as the supporting
electrolyte. Current-voltage curves were measured using Eco Chemie Mini Autolab
interfaced with Nova 1.9 software at a scan rate of 20 mV/s. Cells were illuminated with a
xenon arc lamp with a Horiba- Jobin monochrometer and fitted with an A.M. 1.5 filter
(Sciencetech Inc). All cells were measured under 1 sun (100W m-2) illumination.

20

2.2.3 Absorptance Measurements
Transmittance and reflectance values were measured for bare FTO, uncoated Fe2O3, and
NiOx-coated Fe2O3 using a labsphere RSA-PE-20 integrating sphere housed in a Lambda 35
UV-Vis Spectrometer.
2.2.4 XPS
XPS measurements were taken at the Composite Materials and Structures Center in the
College of Engineering at Michigan State University for bare and conditioned and nonconditioned NiOx-coated Fe2O3 using a Perkin Elmer Phi 5600 ESCA system with a
magnesium Ka X-ray source at a takeoff angle of 45°.
2.3 Results and Discussion
2.3.1 Absorbtance
Using measured percent transmittance and reflectance measurements for bare and NiOxcoated Fe2O3; absorptance was calculated using equation 2-1 and plotted versus
wavelength In Figure 2-1. The addition of NiOx to Fe2O3 slightly decreases the percent
absorptance for measured wavelengths between 375 and 550 nm, but is essentially
negligible. This result shows that the addition of NiOx should not compete for visible light
absorption of the underlying Fe2O3, making it an ideal catalyst pairing for solution side
illumination.
(2-1)

21

Figure 2-1. Calculated absorptance from transmittance and
measurements for bare Fe2O3 (black) and NiOx-coated (red) electrodes.

reflectance

2.3.2 Photoelectrochemical Conditioning
Working electrodes consist of a 20 nm Fe2O3 (300 ALD cycles) film deposited on top of 2
nm Ga2O3 (18 ALD cycles) coated fluorine-doped tin oxide (FTO) substrate via ALD
following a previously reported procedure.16,28,31 An additional ~10 nm NiOx film was
deposited on some Fe2O3 electrodes via ALD. The NiOx-coated hematite was annealed and
then stored at 80°C when not in use.
Current density versus applied voltage (J-V) measurements of bare and NiOx-coated
Fe2O3 electrodes were measured in the dark and under illumination. Figure 2-2 shows the
typical J-V curves under 1 sun illumination (black curve) of bare Fe2O3 electrodes with a
22

photocurrent onset of ~1.1 V vs RHE, an approximate 100mV improvement over our
previous reports without a Ga2O3 underlayer.15,19,32,33 Electrodes with freshly annealed
NiOx-coated films, shown by the red curve in Figure 2-, exhibit a small cathodic shift in the
photocurrent onset and a small decrease in saturated photocurrent as compared to bare
Fe2O3.

Figure 2-2. Plots of J-V curves for bare Fe2O3 (black) and Ni(OH)2-coated Fe2O3
electrodes for as-deposited (red), and following 1 hour (yellow), 2 hours (green),
and 3 hours (blue) of conditioning at 1.42 V vs RHE under 1 sun illumination.
Dark J-V plots were measured for bare (gray) and initial (red), 1 hour (yellow), 2 hours
(green) and 3 hours of conditioning (blue) for NiOx coated Fe2O3 are shown in Figure 2-3.

23

Figure 2-3. Dark J-V plots for bare (gray) and initial (red), 1 hour (yellow), 2 hours
(green) and 3 hours of conditioning (blue) for NiOx-coated Fe2O3.
The Boettcher group reported a conditioning procedure in which anodic current density of
10 mAcm-2 was applied for 6 hours to NiOx electrodes prepared by spin-coating.26 Their
initial cyclic voltammetry (CV) scans showed a slight redox wave, but subsequent CV scans
taken at 1 hour intervals indicated an increase in the redox wave attributed to the
Ni(OH)2/NiOOH redox couple; a concomitant cathodic shift in the oxygen evolution
reaction (OER) was observed. It was determined that NiOx underwent a structural change
following electrochemical conditioning in which NiOx transforms from a rock salt to a
layered, porous hydroxide/oxyhydroxide structure. The resulting NiOOH structure in these
studies was determined to be the active catalyst for the OER.26 We therefore adapted a
similar conditioning procedure in which NiOx-coated Fe2O3 electrodes were held at a
constant voltage of 1.42 vs RHE under illumination for 1 hour increments. After one hour of

24

conditioning there was an additional 100 mV cathodic shift (Figure 2-2 yellow) in the
photocurrent onset potential as compared to the initial scan (red). In addition, a redox
wave developed, which can be attributed to the Ni(OH)2/NiOOH redox reaction.26
Successive hours of conditioning were measured to monitor electrode evolution, indicated
by the arrows in Figure 2-2. In addition to the magnitude of the onset shift, the redox wave
continued to increase with conditioning. This result correlates the increased magnitude of
the Ni2+/3+ redox wave with an apparent cathodic shift in the onset for water oxidation. This
correlation of improved water oxidation onset is likely due to the increased number of
available Ni3+ sites as the film becomes more porous during conditioning, as shown in
previous studies.26 This procedure was repeated for three hours at which point no change
in the redox wave or J-V was observed. All electrodes used for photoelectrochemical
measurements were therefore conditioned by this method for 3 hours.
2.3.3 XPS
X-ray photoelectron spectroscopy (XPS) measurements were performed on NiO-coated
Fe2O3 before and after photo- electrochemical measurements to determine the nature of
any structural change during the conditioning process. After the spectra were corrected for
charging, Shirley background subtraction was performed. In Figure 2-4a and b, spectra are
shown for as deposited NiOx-coated Fe2O3 (solid line) and for conditioned NiOx-coated
Fe2O3 (dashed line) for the Ni 2p3/2 region and the O1s region, respectively. Fitting in only
the Ni 2p3/2 region to determine Ni2+ or Ni3+ oxides can be complicated, as the binding
energies for each oxide can overlap or only show a slight broadening. For this reason, the
Ni 2p3/2 envelope was curve fit using relative peak positions for both Ni2+ and Ni3+ oxides

25

as well as the O1s peaks.34,35 For the Ni 2p3/2 in Figure 2-4a, it can be observed that some
change did occur upon conditioning, showing a decrease in signal intensity for peaks at
approximately 855.5 and 861 eV as well as a slight broadening of the peak at 861 eV as
compared to as deposited and annealed samples. However, these peaks are still nominally
similar to one another and do not allow for unambiguous identification of the oxidation
state. O 1s peaks for annealed and conditioned samples were also fit, shown in Figure 2-3b,
with curve fits shown in Figure 2-4c and d, respectively.35 Here the difference between
annealed and conditioned samples is more apparent. According to literature ,NiO has a
prominent O1s peak at 529eV and second peak at approximately 532eV due to oxygen
vacancies, Ni(OH)2 has a prominent O1s peak at 531eV with only a small shoulder at
533eV, and β-NiOOH has two O1s peaks, a more intense peak at 531eV and a prominent
shoulder at 529.5 eV.35 From these fits it was determined that annealed samples were
primarily comprised of NiO, with contribution from oxygen deficiencies contributing to the
shoulder seen at 531.5 eV, this is consistent with previous reports of NiO deposited under
similar conditions via ALD.30 Conditioned samples are considered to be primarily Ni(OH)2
according to peak position, though due to the prominent shoulder and fits of both the
Ni2p3/2 and O1s regions, Ni3+ is also likely present. This is entirely plausible, as upon
conditioning a layered oxyhydroxide structure has previously been shown to form,
meaning contributions for both Ni2+ and Ni3+ are to be expected.26 We postulate that
Ni(OH)2 is the primary resting state of the metal oxide, and that upon electrochemically
active conditions, NiO and/or NiOOH is present. From the resulting fits, it was determined
that for both freshly annealed and conditioned NiOx-coated Fe2O3 films that NiO, Ni2+, was
the predominant oxide.34,36 Since there is no obvious change in the oxidation state of Ni
26

which persists, it is probable that Ni(OH)2 exists as a resting state, and the Ni2+/3+ redox
reaction occurs under applied bias. Thus, we attribute the conditioning process to the
structural change from cubic NiO to the layered Ni(OH)2, a well known ion-permeable
water oxidation catalyst. The redox wave that develops in the J–V curves shown in Figure
2-2 is therefore attributed to the NiOOH/ Ni(OH)2 redox couple.

Figure 2-4. XPS spectra of as deposited (black), annealed (green) and conditioned
(red) NiOx-coated Fe2O3 showing the a)Ni 2p and b) the O 1s region. Curve fits for c)
annealed and d) conditioned NiOx-coated Fe2O3 are shown for the O 1s region.
2.3.4 Photoelectrochemical Measurements
27

As shown in Figure 2-2, the onset for water oxidation for Fe2O3 is approximately 1.1 V vs
RHE with a photocurrent density of ~0.18 mA cm-2 at 1.23 V vs RHE. The conditioned
Ni(OH)2-coated Fe2O3 results in a cathodic shift for the onset for water oxidation, though
the precise magnitude cannot be accurately determined since the large Ni(OH)2/NiOOH
capacitive redox wave obscures the Faradaic current voltage behavior. In order to separate
the capacitive and Faradaic currents and indentify the true onset potential, anodic current
transients were measured. Figure 2-5 shows the current response to turning on the light at
a constant potential of 1.17 V vs RHE. For the bare hematite electrode, there is a short spike
of photocurrent which is attributed to trapping of photogenerated holes in surfaces states,
which quickly decays to a very low steady state photocurrent density.15

28

Figure 2-5. Bare Fe2O3 (black) and Ni(OH)2-coated Fe2O3 (red) anodic (light on)
transient measurements at 1.17 V vs RHE.
The Ni(OH)2-coated Fe2O3 electrode also exhibited an initial spike, however in this case it
was followed by a relatively slow (4 second) multi-exponential decay to a steady-state
Faradaic current. The photocurrent measured over the last 170 seconds was averaged to
determine the steady state Faradaic water oxidation current. We further note that a
previous report for Ni(OH)2-coated Fe2O3 indicated instability of the catalyst, with
photocurrents dropping to very low values over 30 seconds.17 For these measurements,
once a plateau current was reached, the performance did not diminish rapidly; there is
however, a slight decrease in photocurrent density over time, which is due to bubbles
forming on the photoanode active area. When electrodes are cleared of bubbles and
29

measured again, the original photocurrent is observed, indicating no degradation of
electrode performance. We tentatively assign the greatly improved stability demonstrated
here, compared to previous reports, to the Ni(OH)2 deposition method, however more
work is required to confirm this.
Analogous anodic transient measurements were performed at varying applied
potentials were the steady state current was averaged over the last 170 seconds of the
measurement and are shown as open symbols superimposed on the J-V scans in Figure 2-.
From this measurement, it can be seen that the onset for water oxidation for Ni(OH) 2coated Fe2O3 is indicated by the forward sweep of the J-V at approximately 800 mV vs RHE,
which represents an approximately 300 mV cathodic shift for the onset of water oxidation
as compared to bare Fe2O3. This result also indicates that the redox wave present for
Ni(OH)2-coated Fe2O3 does not contribute to Faradaic current but is instead a result of the
catalyst charging. We note that similar catalyst charging behaviour was observed for Co-Pi
on hematite, however the resulting cathodic shift of the J-V curve was lower with a
photocurrent onset of ~1 V vs RHE.19 Herein we observe the steady state photocurrent is
approximately 0.4 mA cm-2 at 1.23V vs RHE, which is more than double that of the bare
electrode.

30

Figure 2-6. J-V plots under illumination and in the dark with superimposed Faradaic
photocurrent measurements (open symbols) for Ni(OH)2-coated Fe2O3 and bare
Fe2O3.
Cathodic current transients were also measured in response to turning off the light at
several applied potentials. Figure 2-7 shows cathodic transient currents measured at
potentials negative of the onset of water oxidation, 1.05 V and 0.800V vs RHE for bare
(black trace) and Ni(OH)2-coated Fe2O3 (red trace), respectively. For the bare Fe2O3
electrode, the transient spike is attributed to the de-trapping of holes, or the reduction of
surface states.15 The cathodic transient for Ni(OH)2-coated Fe2O3, however, is attributed to
the reduction of Ni3+ back to Ni2+. This cathodic transient has an initial spike slightly larger
than untreated Fe2O3 and takes a much longer time to decay back to essentially zero
current density, indicating a greater amount of charge passed.
31

Figure 2-7. Cathodic (light off) transient measurements for bare Fe2O3 (black) and
Ni(OH)2-coated Fe2O3 (red) at 1.05 and 0.800 V vs RHE, respectively.
This difference can be quantified by integrating the current transients. This integration
produced a charge passed of approximately 1.6 x1014 electrons per cm2 for bare Fe2O3
electrodes and 1.6 x 1016 electrons per cm2 passed for Ni(OH)2-coated Fe2O3 electrodes.
Assuming just the 0001 crystal face is exposed, the total density of iron atoms at the surface
is 3.9 x1014 cm-2, thus 1.6 x1014 cm-2 is consistent with a surface species. This integration
shows that ten times more charge is stored in the 10 nm Ni(OH)2 film as compared to the
surface species on Fe2O3. In order to pass this much charge while still having nominally the
same geometric surface area, this indicates that the Ni(OH)2-coated Fe2O3 must be a porous
or layered structure in order to have enough available active Ni2+/3+ sites. The assumption

32

of a layered structure is consistent with the large redox wave in the J-V curves in Figure 2-6
and in previous literature reports.26
2.4 Conclusions
We have shown that using by using ALD we were able to deposit thin films of NiO on
Fe2O3 photoelectrodes. XPS measurements indicated that freshly annealed NiO films
are composed primarily of NiO. These Ni(OH) 2 films resulted in a slight
improvement in the water oxidation J-V response in comparison to bare Fe2O3
electrodes. Photoelectrochemical conditioning of the NiO films produced a structural
change to a layered Ni(OH)2, as determined by XPS measurements. The structural
change was monitored through the growth of a large NiOOH/Ni(OH) 2 redox wave in
conjunction with further improvement in the water oxidation onset potential. The
correlation of the increased stored charge (magnitude of the redox wave in the J-V
curve) upon conditioning with the improvement of the J-V curve indicates that the
primary attribute of the Ni(OH)2 film is the ability to separate charge. This charge
separation allows holes stored by Ni(OH)2, as NiOOH, to oxidize water competitively
with recombination of conduction band electrons. This behaviour is consistent with
previous results of Co-Pi on Fe2O3 and Ni(OH)2 on TiO2 electrodes.19,20,27 These
results provide important insight into the design of water oxidation catalysts in
contact with semiconductor electrodes. The most important parameter appears to
be ion-permeability which allows charge separation, rather than intrinsically faster
water oxidation kinetics of an electrocatalyst. Finally, the results reported herein are
in contrast to previous reports of Ni(OH)2 on Fe2O3, however the cause of this

33

discrepancy is still not clear. Further investigations are ongoing in our lab in order to
better understand the mechanistic details of these exciting results.

34

REFERENCES

35

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(17) Wang, G.; Ling, Y.; Lu, X.; Zhai, T.; Qian, F.; Tong, Y.; Li, Y. Nanoscale 2013, 5, 4129–
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37

Chapter 3: Electrochemical Impedance Spectroscopy of
Ni(OH)2-coated α-Fe2O3: Investigation into the Improvement of
Water Oxidation with α-Fe2O3

38

3.1 Introduction
Hematite is an n-type semiconductor and possesses a band-gap of approximately 2.1 eV, is
stable in neutral and basic conditions, and has a low enough valence band energy (E VB/q =
2.71V vs RHE) to facilitate water oxidation by photogenerated holes. Even with these
favorable water oxidation characteristics, efficient solar energy conversion is still lacking.
Hematite possesses a relatively long light absorption depth (hundreds of nanometers)
compared to a relatively short charge collection length (< 10 nm), meaning that a high light
harvesting efficiency results in low hole transport efficiency, with the consequence being a
low solar-to-fuel conversion; the converse is also true.1–7 In addition, water oxidation at
hematite’s surface is considered to be slow, allowing for increased recombination, reducing
the overall efficiency.8–13 The efficiency of water oxidation with hematite electrodes should
be improved by either accelerating the rate of water oxidation or reducing the rate of
surface-state recombination. Addition of a water oxidation catalyst to the hematite surface
should accelerate the water oxidation kinetics and thus improve the water oxidation
efficiency. Passivation of the surface states facilitating recombination should minimize this
reaction and also improve the overall water oxidation efficiency. There are many examples
in the literature of materials being added to the surface of hematite which have been
shown to shift the photocurrent onset potential cathodically, thereby increasing the
photocatalytic water oxidation efficiency by the addition of materials such as IrO x, Co-Pi,
and Co(OH)2/Co3O48,13–27 While the general mechanism for this enhancement is not entirely
agreed upon for these systems, all studies would agree that the cathodic shift is at least
partially due to a reduction in recombination of electrons to iron based surface states.

39

Of recent interest, has been the family of Ni(OH)2/NiOOH ion-permeable redox active
materials.28,29 These porous materials have shown an improvement in photoanode
performance when paired with TiO2 compared to more dense electrocatalyst materials
such as IrOx.28 We have shown previously that the conversion of Ni(OH)2 to a more ionpermeable material by a conditioning process at the surface of hematite has led to a nearly
300 mV cathodic shift in the onset of water oxidation and doubling of the photocurrent at
1.23V vs RHE.

It was also determined through the integration of cathodic transient

(turning light on) measurements that the addition of 10 nm of Ni(OH)2 lead to a 100 times
increase in the charge stored in the Ni(OH)2 film as compared to untreated Fe2O3
electrodes.30 However the mechanism as to the improvement of the porous Ni(OH)2 was
not definitely determined, though it was determined that the ion-permeable, porous
material Ni(OH)2 likely collected holes from Fe2O3 and stored them in NiOOH as Ni3+, which
then oxidizes water competitively with recombination of conduction band electrons.
For this work, we aim to understand the mechanism in which Ni(OH)2 improves water
oxidation properties of hematite. We report the deposition of Ni(OH)2 onto hematite, both
deposited by Atomic Layer Deposition (ALD) to utilize thin-film model electrodes to
uncomplicate charge transfer processes present in highly porous and nano-features
employed for other hematite systems. We investigate the mechanism to which Ni(OH)2coated

Fe2O3 cathodically shifts the onset of water oxidation and increases the

photocurrent at 1.23 V vs RHE compared to uncoated Fe2O3 using electrochemical
impedance spectroscopy.
3.2 Experimental
3.2.1 Electrode Fabrication
40

Working electrodes were fabricated by atomic layer deposition (ALD) (Savannah 100
[Ga2O3, Fe2O3] and Savannah 200 [NiO], Cambridge Nanotech) onto fluorine-doped tin
oxide (FTO) coated glass substrates (Hartford Glass, 12Ω cm-2). Prior to deposition of
Ga2O3, FTO glass was sonicated in detergent and DI water, rinsed, sonicated in clean DI
water, and then finally rinsed and sonicated in isopropyl alcohol (IPA). Substrates were
blown dry with N2 directly before being placed in the ALD chamber. Approximately 2nm
(18 ALD cycles) of Ga2O3 was deposited onto cleaned FTO substrates via ALD using
tris(dimethylamido)gallium (III) (Ga2(NMe2)6) (Strem Chemicals Inc.) as Ga precursor and
H2O as an oxidant following a modified version of a previously reported procedure. 31 The
thickness of Ga2O3 was confirmed with ellipsometry on witness Si wafers in the ALD
chamber during deposition. The substrate was held at 200°C and the Ga precursor was
heated to 150 °C, pulsed for 0.2 s, held under exposure mode for 8 s, followed by 12 s N 2
purge. Subsequently, a 0.015 s pulse of H2O was introduced under the same exposurepurge time conditions to oxidize the Ga precursor. As deposited Ga2O3 on FTO substrate
was subsequently coated with ~20 nm (300 ALD cycles) of Fe2O3 in the same chamber.
Thickness of the hematite was confirmed using absorbtance measurements and BeerLambert calculations. Briefly, the substrate remained at 200 °C ferrocene (sublimed from
Sigma Aldrich) held at 70°C was pulsed for 20s, purged with N2 for 5s followed by a 0.015s
pulse of H2O, immediately followed by a 1s pulse of ozone (Yanco Industries ozone
generator). The H2O and O3 pulse was repeated ten times for a single oxidation macrocycle
in order to oxidize ferrocene.32 Fe2O3 with the Ga2O3 underlayer will be referred to simply
as Fe2O3. Following deposition, Fe2O3 was annealed to 500°C in an oxygen environment at
a rate of 17°C per minute, sintered at 500°C for 30 minutes and allowed to cool slowly to
41

110°C over 2 hours. Subsequently, approximately 10nm (100 ALD cycles) of NiOx was
deposited onto the annealed Fe2O3 by ALD (Savannah 200, Cambridge Nanotech) following
a modified procedure.33 Nickel alkyl amidinate (Ni-amd) (AccudepTM Nickel Precursor,
Dow) was used as the nickel precursor and H2O as the oxidant. The substrate was heated to
150°C and the Ni-amd cylinder was heated to 130°C. The Ni(amd) was pulsed for 0.2s and
held under exposure mode for 20 seconds, purged with N2 for 30 s, followed by a water
pulse of 0.015 s under exposure mode for 20 s. This cycle was repeated until the desired
thickness was obtained. A Growth rate of 1.01 Å/cycle was determined by ellipsometry
(Horiba Smart SE) measurements on Si wafers for varying cycles of NiOx from 1-300. NiOx
coated Fe2O3 was annealed to 300 °C in an oxygen environment at a rate of 5°C per minute
and sintered for 1 hour. NiOx coated electrodes were allowed to cool to 80°C over 4 hours
and stored in an 80°C oven when not in use.
3.2.2 Electrochemical Measurements
Electrochemical measurements were measured in a custom glass 3-electrode cell with a
quartz window to allow illumination from the solution side, SS, (striking the Ni(OH)2-Fe2O3
first) with a Pt mesh counter electrode, homemade Ag/AgCl reference electrode referenced
to a commercials SCE and a Fe2O3 or Ni(OH)2-Fe2O3 working electrode clamped to the cell.
All cells were measured in 1M KOH (pH 14 measured by Fischer Scientific Accumet pH
meter) with 0.2M KCl as the supporting electrolyte.
3.2.3 Electrochemical Conditioning of Ni(OH)2-coated Fe2O3
NiO/Ni(OH)2 has been shown to undergo structural changes following an electrochemical
conditioning process in which the structure changes from a rock salt formation to a layered
42

hydroxide/oxyhydroxide structure. This structure change resulted in a reduction of oxygen
evolution reaction (OER) overpotential as compared to the non-conditioned NiO
electrodes, resulting from an increase in the number of exposed Ni active sites. 29 Following
a previously reported procedure, Ni(OH)2-coated Fe2O3 electrodes were conditioned in
order to achieve optimal electrode performance.30 Briefly, Ni(OH)2-coated Fe2O3 electrodes
were conditioned under illumination for 3 hours. Electrodes were held at approximately
1.4 V vs RHE for 1 hour increments, with a current voltage curve (J-V) measured after each
hour to monitor electrode evolution. After three hours, and no change in the J-V plots was
observed, electrodes were deemed conditioned and used for subsequent electrochemical
measurements.30
3.2.4 J-V and Impedance Spectroscopy
Current-voltage (J-V) curves were measured using Eco Chemie Mini Autolab interfaced
with Nova 1.9 software at a scan rate of 20 mV/s. Electrochemical impedance spectroscopy
was measured with a 10 mV amplitude perturbation for frequencies in the range of 10,000
to 0.02 Hz. Impedance data was fit with appropriate equivalent circuits using Zview
software (Scribner Associates). Cells were illuminated with a xenon arc lamp with a
Horiba- Jobin monochrometer and fitted with an A.M. 1.5 filter (Sciencetech Inc). All cells
were measured under 1 sun (100W m-2) illumination.
3.3 Results and Discussion
3.3.1 Conditioning Difference in 100 Ni(OH)2-coated Fe2O3 Electrodes

43

Several batches of Ni(OH)2-coated Fe2O3 were fabricated over the span of a year and a half
in which Fe2O3 and 100 cycles of Ni(OH)2 was deposited under the same conditions. The
electrodes were all conditioned for 3 hours under illumination with the same applied
potential, however, different electrode performance was observed. Shown in Figure 3-1 are
three representative 100 Ni(OH)2-coated Fe2O3 electrodes that were fabricated at different
times (May 2013- green, October 2013-blue, and September 2014-purple). For ease of
discussion, these batches will be referred to as A, B, and C, respectively. It can be observed
in Figure 3-1 that the best performing electrode has the largest redox wave, and the worst
performing electrode has the smallest redox wave, though only slightly as compared to the
mid-performance electrode.

Figure 3-1. J-V plots under 1 sun illumination for three different batches of
conditioned 100 Ni(OH)2-coated Fe2O3 Batch A, Batch B, and Batch C in green, blue,
and purple, respectively.
44

A, B, and C electrodes have a water oxidation onset potential of 840 mV, 900 mV, and
approximately 1 V, respectively. Their photocurrent density at 1.23V vs RHE also differs
with the A, B, and C electrodes measuring in at 0.5, 0.4, and 0.25 mA cm-2, respectively. This
difference in performance while not ideal for complete reproducibility of the electrode
performances, can be useful to help determine the characteristics of a good Ni(OH)2-coated
electrode, and what effect the Ni(OH)2 has on Fe2O3 to improve water oxidation efficiency.
For these varying batches, the bare (control) electrodes were consistent with typical
electrode performance for Fe2O3. In order to examine the effect of Ni(OH)2 on Fe2O3, we
will first discuss the differences between the good, mid, and relatively poor performing 100
cycle Ni(OH)2-coated Fe2O3 electrodes.
In order to investigate how the addition of Ni(OH)2 to Fe2O3 reduces the onset of
water oxidation and increases photocurrent density as compared to uncoated Fe2O3 ,
electrochemical impedance spectroscopy was employed. Impedance spectroscopy is
measured by probing an electrode by applying at some potential and oscillating that
applied potential at varying frequencies and measuring the current response. This data is
plotted as real and imaginary impedance in a Nyquist plot and information such as
capacitance and resistance values can be fit from the data using appropriate equivalent
circuits. The use of impedance spectroscopy allows for fine details of charge transfer
processes in the bulk of the electrode and at surface/solution interface to be extracted.
Representative Nyquist plots for impedance measurements taken under illumination at
approximately 900 mV vs RHE are shown in Figure 3-2. At this potential, A electrode
(green) is under water oxidation conditions, B (blue) is nearly at the onset of water
oxidation, and C (purple) is approximately 100 mV prior to the onset of water oxidation.
45

Figure 3-2. Nyquist plots measured at 900 mV vs RHE for A, B, and C 100Ni(OH)2coated Fe2O3 electrodes in green, blue, and purple, respectively.
Two semicircles are observed for all of the electrodes, the semi-circle measured at
high frequencies (low Z’) is attributed to the bulk Fe2O3 and the semicircle measured at low
frequencies (high Z’) is attributed to capacitance and charge transfer from the Ni(OH)2.34
For electrode A in green, it is clear that the position of the first semicircle is shifted to
higher impedance values (the real value Z’). This position indicates a higher series
resistance of the electrode, as indicated by the highest frequency measurement.
In order to obtain relevant capacitance and resistance information from the Nyquist
plots, equivalent circuits are used for fitting, as shown in Figure 3-3a and b. In Figure 3-3a
the Randal circuit and is used for fitting if only one semicircle is present in the Nyquist plot,

46

which accounts for the bulk capacitance, Cbulk, bulk resistance also called the charge
transfer resistance, Rbulk, and the series resistance of the electrode Rs.

Figure 3-3. Equivalent circuits used to fit Nyquist plots for a) one semi-circle present
and b) when two semi-circles were present.
Figure 3-3b shows the equivalent circuit used when two semicircles are present, which was
previously developed to model bare Fe2O3 electrodes as well as CoPi-coated Fe2O3
electrodes.13,34 For this model, Cbulk, Rbulk, and Rs, are still present, representing the high
frequency semicircle. For the lower frequency semicircle variable for charge transfer from
surface state for bare Fe2O3 or Ni(OH)2 capacitance for Ni(OH)2-coated Fe2O3, Css/Ni(OH)2 and
charge transfer resistance from the surface states for bare Fe2O3 and from Ni(OH)2 for
Ni(OH)2-coated Fe2O3, Rss/Ni(OH)2.
Three 100Ni(OH)2-coated Fe2O3 electrodes were measured using potential scan
impedance spectroscopy and the data was fit using the equivalent circuit in Figure3-3b
where two semicircles were present (typically in lower potentials before a plateau water
oxidation current is reached) and using the equivalent circuit in Figure3-3a if only one
semicircle was present in the Nyquist plots. From these fits for Cbulk shown Figure 3-4 it
was observed that Cbulk tracked with J-V performance: the higher bulk capacitance
47

correlated with better electrode performance, and a lower Cbulk correlated with relatively
poor electrode performance. In addition, the electrode fabricated electrode C showed a
potential dependence on Cbulk beginning at approximately 900 mV vs RHE and had
relatively constant Cbulk until 1.4 V vs RHE. Electrodes A and B exhibited a less drastic
potential dependence at more negative potentials and maintained a relatively constant Cbulk
across the measured potential ranges until 1.4 V vs RHE. At 1.4V, the bulk of the electrode
is fully depleted and no potential dependence should persist.
From the fitting of Cbulk, it is observed that a higher Cbulk correlates with better photoanode
performance. In addition, fits for electrodes A and B in Figure 3-4 have similar values of
Cbulk across a wide potential range, approximately 2 and 1.7 µF, respectively where as
electrode C has lower Cbulk, approximately 1.4 µF for the potential range from 0.8 to 1.4 V vs
RHE. This difference in the bulk capacitance of the electrode across potential ranges just
previous to and during the onset for water oxidation may play a key role in electrode
performance. A lower bulk capacitance is a result of greater band banding in the bulk of the
electrode, driving holes to the surface and shuttling electrons to the back contact. A greater
capacitance, results in a greater driving force for the favorable processes to happen,
reducing detrimental charge recombination.

48

Figure 3-4. Bulk capacitance values for 100Ni(OH)2-coated Fe2O3 electrodes A (green
triangles), B (blue diamonds), and C (purple crosses).
Mott-Schottky plots were constructed from fitted Cbulk in order to determine the flatband, Efb, position for the 100Ni(OH)2-coated Fe2O3 electrodes, shown in Figure 3-5. MottSchottky plots can give information about the Efb, if the band edges are pinned or not
dopant density, and where the electrode is fully depleted.

49

Figure 3-5. Mott-Schottky plots for 100Ni(OH)2-coated Fe2O3 electrodes A (green
triangles), B (blue diamonds), and C (purple crosses).
For electrodes A and B, there is only a slight potential dependence on Cbulk-2, which can be a
result of three things: either the band edges are pinned at some more positive potential, a
result of some state at the interface with Ni(OH)2, the electrodes are fully depleted across
the measured potential range, or some combination of both. In any of these cases, band
edge pinning, full-depletion, or some combination of the two would result in band edge
positions would result in greater band-bending, and a greater driving force for charge
separation in the bulk of the electrode. For the three selected 100Ni(OH)2-coated Fe2O3
electrodes, there is a correlation with photoanode performance where band-edge pinning
or a fully-depleted electrode is advantageous. This result is different than previous surface
coatings on Fe2O3 studied in our group and others, and the bulk capacitance properties are
affected by the addition of a known catalyst to the surface. 34

50

Consistent with bulk properties observed for the capacitance values, there is a
correlation of electrode performance and the bulk resistance values, Rbulk, shown in Figure
3-5. All three electrodes begin to show an increase in the Rbulk near their respective onsets
for water oxidation, 850 mV, 950 mV, and 1.05 V for electrodes A, B, and C, respectively.
The Rbulk values reach a peak value and begin to decrease in the case of electrode B and
electrode C, and level off for the potential range measured for electrode A. There is also a
correlation with photoanode performance and the Rbulk values; electrode A has the best
onset for water oxidation, and sees an increase near the onset of water oxidation and also
reaches the highest Rbulk values for water oxidation potentials. This observation is
consistent with previous reports with the correlation of the onset of water oxidation and
the increase in Rbulk.13 The worst performing electrode, electrode C, has the most positive
potential at which Rbulk increases as well as the lowest peak for the values of Rbulk shown in
Figure 3-5.

51

Figure 3-6. Bulk resistance values for 100Ni(OH)2-coated Fe2O3 for electrodes A
(green triangles), B (blue diamonds), and C (purple crosses).
The fitting of the low frequency semicircle in the Nyquist plots yields capacitance
and resistance information about the surface states or surface covering of the Fe 2O3
electrode. In the Nyquist plots there were clearly two observable semicircles, and in the
case of the 10 nm of Ni(OH)2 on the surface of Fe2O3, the capacitance was determined to be
the chemical capacitance of Ni(OH)2, CNi(OH)2, shown in Figure 3-7.
For all three electrodes, the CNi(OH)2 reaches a peak capacitance value before each
respective onset for water oxidation. Electrode A, in green, has the highest CNi(OH)2 peak
value at approximately 1. 58 mF, and also has the highest redox wave current density.

52

Figure 3-7. Capacitance values for 100Ni(OH)2-coated Fe2O3 electrodes A (green
triangles), B (blue diamonds), and C (purple crosses).
Keeping with the trend, the peak CNi(OH)2 values for electrode B and electrode C are the next
highest and lowest values, respectively, just as the current density of the redox waves are
the next highest and lowest for each electrode in Figure 3-1. This trend indicates that the
magnitude of the CNi(OH)2 and in turn the magnitude of the redox wave in the J-V plots
correlates with photoanode performance. A large CNi(OH)2 peak signifies a greater capability
of charge storage directly before the onset of water oxidation.
Using the equivalent circuit in Figure 3-3b, the resistance fit results pertaining to the
Ni(OH)2 coating are shown in Figure 3-8. Of the previous fits discussed, the difference
between the three electrodes is not at drastic, but there is still an observable trend.

53

Figure 3-8. Resistance attributed to Ni(OH)2 100Ni(OH)2-coated Fe2O3 electrodes A
(green triangles), B (blue diamonds), and C (purple crosses).
Prior to the onset of water oxidation, the resistance attributed to Ni(OH)2 has a maximum
value and then drops steadily, reaching the minimum value around 100 mV after the onset
of water oxidation for electrode, with A, B, and C reaching minimums at approximately 1.0
V, 1.1 V, and 1.8 V, respectively shown in Figure 3-8. The correlation of reaching a
minimum resistance value near the onset of water oxidation also agrees with previous
studies from our group.13,34 Reaching this minimum value at a more negative potential
correlates with a more positive onset of water oxidation. Therefore, having a lower
resistance due to charge transfer from Ni(OH)2 is imperative for a lower onset for water
oxidation.
3.3.2 Consilience for 100Ni(OH)2-coated Fe2O3 Electrodes

54

Three representative electrodes were selected in order to illustrate the variation that can
exist between different batches of 100Ni(OH)2-coated Fe2O3 photoanodes. The variations
were explored using J-V curves paired with electrochemical impedance spectroscopy in
order to determine what behavior resulted in desirable electrode performance. From
impedance fits, it was determined that the best performing electrode, electrode A, had the
highest constant Cbulk, and there for exhibited the lowest extent of band-edge pinning or
fully depleted behavior also observable in the Mott-Schottky plots in Figure 3-5. Electrode
A also had the highest Rbulk values, which rose sharply near the onset of water oxidation. In
addition, electrode A had the highest peak values for CNi(OH)2, coincidence with the onset of
water oxidation and reach a minimum for RNi(OH)2 at the most negative potential of the three
electrodes. A performance trend was identified as the worst performing photoanode,
electrode C, had a potential dependent Cbulk, with no observable band edge pinning, had the
lowest Rbulk values, lowest CNi(OH)2, and the RNi(OH)2 reached its minimum at the most positive
potential vs RHE. True to the trend, the median performing electrode has values for Cbulk,
Rbulk, CNi(OH)2, and RNi(OH)2 between that of the best performing electrode A and the worst
performing electrode C. From these trends it was determined that a quality performing
100Ni(OH)2-coated Fe2O3 electrode can be recognized by fits from impedance
spectroscopy.
3.3.3 1 and 100 Ni(OH)2-coated Fe2O3 Electrodes
Two thicknesses of Ni(OH)2- 1 and 100 cycles (~0.1 and 10 nm respectively) were
deposited onto ~ 22 nm of annealed Fe2O3 and were conditioned for three hours. J-V curves
were measured in the dark and under 1 sun illumination through the solution side (SS) as

55

the Ni(OH)2 has a negligible effect on light absorption of the underlying Fe2O3.30 Resultant
J-V curves are shown in Figure 3-9 and are of the same electrodes used for previous
conditioning and transient studies.30 As shown in Figure 3-9, the addition of only one cycle
of Ni(OH)2, less than a monolayer of coverage, has a minimal effect on a cathodic shift of the
onset of water oxidation, but does show an improved fill factor . In addition, there have
been several examples of catalysts comprised of Ni and Fe, and it is possible that the
addition of Ni(OH)2 lends to some synergistic activity of neighboring Ni and Fe atoms.
However, these improvements in fill factor lie within typical batch variation for a bare
electrode, and do not show a significant improvement for water oxidation performance
when illuminated form the solution side. 100 Ni(OH)2-coated Fe2O3 shows a large redox
wave that grows in during the conditioning process as seen by the red trace in Figure 3-9.
This redox wave, and the participation of Faradaic current, was discussed previously.30
Briefly, it was determined through transient measurements that the true onset of water
oxidation is in the forward sweep where the current density is zero mA, at approximately
840 mV. This result shows that the addition of 100 cycles (~10 nm) of Ni(OH)2 to Fe2O3 has
a significant effect on the performance of the photoanode with an over 250 mV cathodic
shift for the onset of water oxidation shift as compared and double the photocurrent
density at 1.23 V vs RHE to bare Fe2O3 .

56

Figure 3-9. J-V plots under 1 sun illumination and in the dark for bare and 1 and 100
Ni(OH)2-coated Fe2O3 in black, blue, and red, respectively.
This above result has proven to be the best photoanode of cells fabricated, and there were
three batches fabricated over a year-and-a-half period that had very similar behavior. From
the previous results and discussion of batch variation of 100 Ni(OH)2-coated Fe2O3, the best
electrode was chosen in order to understand what function the Ni(OH)2 has on water
oxidation properties of Fe2O3 as a photoanode. In order to determine the function of
Ni(OH)2 on the surface, we again employed impedance spectroscopy.
Bulk capacitance values for fits performed with equivalent circuits in Figure 3-3a
and b are shown in Figure 3-10 for bare (black circles), 1 Ni(OH)2 coated (blue triangles),
and 100 Ni(OH)2 coated Fe2O3 under illumination (open shapes) and in the dark (closed
57

shapes). For bare Fe2O3, the bulk capacitance in the light and in the dark shows a potential
dependence across the measured potential range, with a decrease in the Cbulk values
nearing the potential at the onset of water oxidation. Fits in the dark show that there is
some Fermi-level pinning present as exhibited by other reports from our group as well. 35
The addition of one cycle of Ni(OH)2 to the surface of hematite also shows a potential
dependence with potential vs RHE as the bare Fe2O3 does, observed in Figure 3-10. It can
be seen that 1 Ni(OH)2 under illumination behaves much the same as bare Fe2O3 under
illumination, which is to be expected due to the similarities in J-V performance in Figure 3-.
However, in the dark, 1 cycle of Ni(OH)2 has a greater affect on the Fermi-level pinning
with an overall lower Cbulk across the measured potential range. This increase in pinning is
likely a result of the bands being pinned to some Ni(OH)2 state in which water oxidation is
not proceeding. The addition of 10 nm of Ni(OH)2 has the greatest affect on the bulk
capacitance properties of Fe2O3 as to be expected by the J-V plots in Figure 3-9. Compared
to bare Fe2O3, 100 Ni(OH)2-coated Fe2O3 exhibits a relatively constant capacitance across
the measured potential range, in both the light and in the dark. In addition, the Cbulk values
are lower than bare Fe2O3 across the measured potential ranges, as much as nearly half the
value at low potentials.

58

Figure 3-10. Bulk capacitance values for bare Fe2O3 (black circles), 1 Ni(OH)2-coated
Fe2O3 (blue triangles), and 100 Ni(OH)2-coated Fe2O3 (red squares). Measurements
performed in the dark are in solid shapes, those conducted under 1 sun illumination
are in the open shapes.
The lack of potential dependence, and the observation that the Cbulk values are essentially
the same in the light and in the dark for 100 Ni(OH)2-coated Fe2O3 suggests that the Fe2O3
band are pinned to some state in the Ni(OH)2, and that this pinning is essential to the
improved water oxidation properties observed in the J-V plots.
Correlation of bulk properties and the J-V performance continues with the
observation of Rbulk values. At potentials before the onset of water oxidation
(approximately 900 mV – 1 V), both bare and 1 Ni(OH)2-coated Fe2O3 reach a peak value
before reaching a minimum near the onset of water oxidation and then begin to increase
again as the electrode becomes fully depleted.
59

Figure 3-11. Bulk resistance values for bare Fe2O3 (black circles), 1 Ni(OH)2-coated
Fe2O3 (blue triangles), and 100 Ni(OH)2-coated Fe2O3 (red squares) measured under
1 sun illumination.
On the contrary, there is no observable peak at the lower measured potentials for 100
Ni(OH)2-coated Fe2O. As shown in Figure 3-11, there is a plateau until approximately the
onset of water oxidation (850 mV) where the values for Rbulk increase before reaching a
plateau at approximately 1.25 V, where the bands are fully depleted. From these values, it is
clear that Rbulk reaches a minimum near the onset of water oxidation, and that a higher Rbulk
before the onset of water oxidation, correlates to a larger resistance to charge transfer to
facilitate water oxidation. Furthermore, the dependence of Rbulk confirms that the addition
of 10 nm of Ni(OH)2 does have an effect on the bulk properties of the electrode.
The low frequency semicircle from the Nyquist plots was fit as the charge transfer
capacitance from the surface states for bare Fe2O3 and as the chemical capacitance of the
60

Ni(OH)2 for the 1 and 100 cycles Ni(OH)2-coated Fe2O3 shown in Figure 3-11. At low
potentials (600-850 mV vs RHE) the bare Css has a lower value than and 1 Ni(OH)2-coated
Fe2O3 and beyond 850 mV 1 Ni(OH)2-coated Fe2O3 has a slightly higher capacitance- which
makes sense as there is a less than monolayer coverage of Ni(OH)2 on the surface, which
contributes to the surface state capacitance of the Fe2O3. 100 Ni(OH)2-coated Fe2O3 exhibits
a much higher CNi(OH)2 than both bare and 1 Ni(OH)2-coated Fe2O3, which is expected as
there is approximately 10 nm of Ni(OH)2 on the surface. To confirm that the measured
capacitance was a result of Ni(OH)2 on the surface, a thickness dependence of Ni(OH)2 on
fluorine-doped tin oxide (FTO) was deposited by ALD for 1, 3, 5, 10, 25, 50, and 100 cycles
of Ni(OH)2. Each of the electrodes was conditioned in pH 14 KOH and 200 mM KCl by cyclic
voltammetry (600 cycles) until the redox wave no longer grew in (J-V plots Appendix A and
B). Impedance spectroscopy was conducted in order to measure the capacitance and
resistance of these anodes and resulting Nyquist plots showed a single semicircle and were
therefore fit with the equivalent circuit in Figure 3-3a. The resultant plots for CNi(OH)2 and
RNi(OH)2 are Appendix C and D, respectively. Using the information obtained from the Cbulk
for the 100 Ni(OH)2 on FTO, the peak values are the same order of magnitude of that of 100
Ni(OH)2-coated Fe2O3. In addition, the capacitance values and resistance values for the
varying thicknesses of Ni(OH)2 scale with thickness. This result confirms that the fit
capacitance values in Figure 3-7 are a result of the Ni(OH)2 and not of a bulk property.

61

Figure 3-12. Charge-transfer capacitance from surface states values for bare Fe2O3
(black circles) and charge-transfer capacitance from of Ni(OH)2 for 1 Ni(OH)2-coated
Fe2O3 (blue triangles) and 100 Ni(OH)2-coated Fe2O3 (red squares) measured under
1 sun illumination.
It is also observed, that CNi(OH)2 reaches a peak value coincident with the onset of water
oxidation. The same is true for bare Fe2O3, more visible on a non-log scale. This result is in
agreement with previous studies of the surface state and capacitance from the Co-Pi
coinciding with the onset of water oxidation.34 This suggests that the water oxidation then
occurs at the surface of Fe2O3 in the case of bare electrodes and from the surface of Ni(OH)2
for coated electrodes.
Values resulting from fitting the low frequency semicircle are attributed to the
resitance of surface states for bare electrodes and to Ni(OH)2 for 1 and 100 Ni(OH)2-coated
Fe2O3 are shown in Figure 3-13.

62

Figure 3-13. Charge-transfer resistance from the surface-states values for bare Fe2O3
(black circles) and charge-transfer resistance of Ni(OH)2 for 1 Ni(OH)2-coated Fe2O3
(blue triangles) and 100 Ni(OH)2-coated Fe2O3 (red squares) measured under 1 sun
illumination.
Bare Fe2O3 has the highest Rss, which is very close in value to 1 Ni(OH)2-coated Fe2O3,
except at values more positive of 1.0 V vs RHE, which are slightly lower. 100 Ni(OH)2coated Fe2O3 has a significant decrease in charge transfer resistance from Ni(OH)2 as
compared to bare and 1 Ni(OH)2-coated Fe2O3. For all measured electrodes, the Rss/Ni(OH)2
reaches a minimum near the onset of water oxidation. The reduced Rss/Ni(OH)2 for 100
Ni(OH)2-coated Fe2O3 signifies the reduced charge transfer resistance for water oxidation
to occur at the surface. Therefore, a reduced Rss/Ni(OH)2 is necessary for improved water
oxidation performance.
3.4 Conclusion

63

Herein we have reported the impedance investigation of 1 and 100 cycles of Ni(OH)2
deposited onto the surface of Fe2O3 via atomic layer deposition. Variation in the deposition
of Ni(OH)2 onto Fe2O3 was investigated in order to determine what charge transfer
characteristics were present for good and bad photoanodes. It was determined that the
best performing 100 Ni(OH)2-coated Fe2O3 exhibited the largest redox wave in the J-V plot
compared to the other 100 Ni(OH)2-coated Fe2O3 photoanodes. From impedance
spectroscopy, it was determined that the best performing electrode had the highest Cbulk
and was also constant across measured potentials. In addition, the best performing 100
Ni(OH)2-coated Fe2O3 exhibited the highest peak Rbulk values after the onset of water
oxidation and the highest CNi(OH)2 and the lowest RNi(OH)2 values prior to the onset of water
oxidation, signifying a reduced resistance charge transfer from Ni(OH)2 for water oxidation.
For 1 and 100 cycles of Ni(OH)2-coated Fe2O3, minimal improvement in J-V behavior
was observed for 1 cycle and a nearly 300 mV cathodic shift for the onset of water
oxidation and double the photocurrent at 1.23 V vs RHE was observed for 100 cycles of
Ni(OH)2 on Fe2O3 compared to uncoated Fe2O3. Impedance spectroscopy was employed to
determine the mechanism to this improvement, specifically in the case of 100 Ni(OH)2coated Fe2O3. For 1 Ni(OH)2-coated Fe2O3, slight differences occurred in all of the
impedance fitting parameters, with the largest differences apparent in the bulk properties.
For Cbulk, 1 Ni(OH)2-coated Fe2O3 showed a decrease across potential ranges before the
onset of water oxidation compared to bare Fe2O3. In addition, Rbulk was lower for 1 Ni(OH)2coated Fe2O3 than bare Fe2O3 prior to the onset of water oxidation. This indicates a lower
bulk resistance for charge transfer in the bulk of the material. The presence of 1 Ni(OH)2 is
also observable for fits from the low frequency feature in the Nyquist plots for Css/Ni(OH)2 and
64

Rss/Ni(OH)2 but the affect is less drastic except at low measured potentials. This result makes
sense as there is less than a monolayer of Ni(OH)2 on the surface of Fe2O3 and therefore
differences in the behavior of the Ni active sites and the bare Fe2O3 would be minimal.
From this result it is not clear if Ni(OH)2 is acting as a catalyst, but it does seem to have
slight effect on the bulk properties of the electrode, where the bands may experience some
pinning to Ni(OH)2 states. In addition, at very low measured potentials (< 800 mV) 1 cycle
of Ni(OH)2 on Fe2O3 has a higher charge transfer capacitance from the surface
states/Ni(OH)2 and a slightly lower charge transfer resistance. This would suggest the Ni
active sites on the surface may be assisting in the passivation of surface states, and that
holes are collected by the Ni2+ active sites before they can recombine with trap states or
bulk electrons.
The effect of 100 cycles of Ni(OH)2 is more drastic as compared to 1 cycle of Ni(OH)2
on Fe2O3. From the J-V plots and previous transient data, it is clear that 10 nm of Ni(OH)2
affects the water oxidation properties of Fe2O3 in a positive manner. Impedance data shows
the charge transfer properties of Fe2O3 are altered significantly with the addition of
Ni(OH)2. Cbulk for 100 Ni(OH)2-coated Fe2O3 is nearly two times lower at negative potentials
and does not show a potential dependence over the measure potential range as compared
to bare Fe2O3. This lack of potential dependence indicates that the addition of 10 nm of
Ni(OH)2 is likely pinning the bands at some potential associated with the Ni(OH)2 species
on the surface, though it is not exactly clear what that exact state is. However, if the bands
are pinned at a more positive potential, this increases band bending, facilitating holes to
the Ni(OH)2 at the interface and electrons to the back contact before they can recombine.
Rbulk for bare Fe2O3 is nearly half an order of magnitude higher than 100 Ni(OH)2-coated
65

Fe2O3 prior to the onset of water oxidation, indicating a greater resistance to charge
transfer, this resistance reaches a minimum for both at their respective onsets for water
oxidation. The reduction in the Rbulk correlates with the cathodic shift in the onset of water
oxidation for 100 Ni(OH)2-coated Fe2O3 compared to bare electrodes.
In addition, 100 Ni(OH)2-coated Fe2O3 has a substantial effect on the surface
behaviors of the electrode. The lower frequency semicircle in the impedance data for
Ni(OH)2-coated Fe2O3 was confirmed to be due to the chemical capacitance and charge
transfer resistance for the Ni(OH)2 by correlation with varying thicknesses of Ni(OH)2
deposited onto FTO glass. At its greatest difference, the Css/Ni(OH)2 values differed by two
orders of magnitude for 100 Ni(OH)2-coated Fe2O3 compared to the bare Fe2O3. This large
capacitance agrees with previous transient studies that Ni(OH)2 is able to store a greater
amount of charge in the material compared to the surface states of Fe2O3. This charge
storage capability signifies that Ni(OH)2 can effectively collect holes from the underlying
Fe2O3 and store them as Ni2+/3+ species competing for detrimental recombination. Rss/Ni(OH)2
values compliment this hypothesis, as 100 Ni(OH)2-coated Fe2O3 shows a reduced charge
transfer resistance across all measured potentials, indicating the relative ease at which
charge can be transferred from Ni(OH)2, with the ability to oxidize water at more cathodic
potentials.
Due to the fact that 1 cycle of Ni(OH)2 has slight affects on both the bulk and surface
properties of Fe2O3 and that 100 cycles of Ni(OH)2 has a more drastic affect on both bulk
and surface properties of Fe2O3 for water oxidation, we propose that the addition of
Ni(OH)2 has a dual affect: 1. Ni(OH)2 pins the band edges of the hematite, resulting in

66

greater band bending, and a greater driving force for photogenerated charges to be
separated, competing positively with recombination and 2. That 1 cycle of Ni(OH)2, because
it is less than a monolayer, can passivate surface states and that 100 cycles of Ni(OH) 2 acts
as a charge storage medium in which holes are collected from the hematite and oxidize Ni2+
to Ni3+, which then in turn oxidizes water. We note that these results vary from previous
surface coatings for Fe2O3 as both bulk and surface properties are affected. However, from
this exciting result, it suggests that a porous material capable of charge storage and can
oxidize water to compete with detrimental recombination and also effects the band edge
positions is ideal for a surface coating to improve water oxidation efficiency with hematite
electrodes. This result lends insights in to desirable surface coating material properties and
their effects on Fe2O3 for solar water oxidation.

67

APPENDIX

68

Figure A-1. Cyclic voltammograms of varying thicknesses of conditioned Ni(OH)2 on
FTO.

69

Figure A-2. Zoomed view of cyclic voltammograms of varying thicknesses of
conditioned Ni(OH)2 on FTO.

70

Figure A-3. Capacitance values for varying thicknesses of conditioned Ni(OH)2 on
FTO.

71

Figure A-4. Resistance values for varying thicknesses of conditioned Ni(OH)2 on FTO.

72

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73

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75

Chapter 4: Fabrication of ITO-Inverse Opals for High Aspect
Ratio Substrate for Deposition of α-Fe2O3 by Atomic Layer
Deposition

76

4.1 Introduction
Hematite is a promising material for use in a system in which direct photoelectrochemical
conversion of sunlight into solar fuels is performed. Hematite is a promising material for
solar water oxidation as it has a band gap of approximately 2.1 eV, resulting in light
adsorption to 600 nm. In addition, hematite is stable in neutral and basic conditions, is
earth abundant, and has a suitable valence band energy to oxidize water.1–6 However, the
widespread implementation of hematite has not yet been realized due to a combination of
detrimental characteristics, including a relatively long light absorption depth compared to
a short minority carrier charge collection length as well as a sluggish water oxidation
reaction at hematite’s surface.2,7–13 In order for hematite to be realized as an effective
photoanode material, these drawbacks need to be addressed. In this chapter, we will focus
on addressing the light absorption depth versus minority charge collection length.

Figure 4-1. Schematic of light absorption depth compared to charge collection length
in hematite.
The disparity of light absorption depth as compared to the charge collection length is
illustrated in Figure 4-1.14,15 The charge collection length is defined by the distance that the
photo-induced charges can be separated and collected by either the back contact (for
77

hematite, an n-type semiconductor, this is electrons) or collected by the solution or
interface (for hematite this is the minority carrier, holes). This short minority charge
mobility is a result of the chemical structure of hematite and therefore the short collection
length cannot be avoided.16,17 The material absorption coefficient, α determines the
wavelengths, f, to be absorbed according to a thickness, l according to Beers Law:
. Any given semiconductor must be 3/α thick in order to absorb 95% of the light at a
certain wavelength. Hematite has an

cm-1 at 550 nm, requiring a thickness of

375 nm to absorb 95% of the light at this wavelength. This is illustrated in Figure 4-2.1

Figure 4-2. AM 1.5 Solar irradiance (black line) and the thickness required for
hematite to absorb 95% of the light, 3/α (orange line).
Photogenerated holes in the semiconductor are driven to the semiconductor-solution
interface by means of the space-charge region, which is on the same order or slightly
greater than the minority carrier charge collection length. Any holes that are generated
78

beyond this space-charge region will surely recombine and are rendered useless for water
oxidation. To this end, our group has shown that thin-films, where the thickness of the films
is the same, or only slightly larger than, the depletion region, show absorbed photon
collection efficiencies (APCE, also referred to as external quantum efficiency) that are the
same or better than thick films that exhibit greater light harvesting.5,6,18 Due to these
materials

characteristics,

much

of

the

work

with

hematite

has

focused

on

nanostructuring.3,19–27 The purpose of nanostructuring is to maximize the light absorption
in the z-direction, which is necessary to accommodate for the optical thickness required for
efficient light-harvesting, and short charge collection length in the x-direction, which is
necessary to circumvent recombination of generated charge carriers, shown in Figure 4-3.

Figure 4-3. Schematic of planar Fe2O3 on a transparent conducting oxide (TCO) backcontact and thin-film Fe2O3 on a nanostructured TCO. The nanostructured TCO
allows for greater light absorption in the z-direction while keeping charge collection
short in the x-direction.
Methods of nanostructuring have included mesoporous films3, nanorods2,23, nanotubes27,
nanonets19, and nanocauliflower26. Of the most exciting has been nanocauliflower,

79

achieving 3 mAcm-2 current density at 1.23 V vs RHE as a result of improved light collection
of optically thicker films. In addition, these structures were porous or had relatively small
grain sizes, minimizing the distance in which the photogenerated holes need to travel
before they can be collected at the semiconductor-electrolyte interface.26 While these
methods of nanostructuring show improved device performance compared to films with
similar thicknesses, they still suffer from recombination within the nanostructured grains
or features due to the discrepancy in the depletion region and the nanostructure feature
size. The design and synthesis of such nanostructured features cannot be easily controlled
and the direct explanation of device improvement is unclear; this makes it difficult to
improve the device or mitigate detrimental characteristics of such nanostructures. The
nanonet structure by the Dunwei Wang group is exciting as it is the first reported use of
thin-film hematite on a high aspect ratio conductive substrate. This approach to
nanostructuring, and perhaps the most controllable, is to use a high aspect ratio conductive
and ideally transparent scaffold in which hematite can be deposited. Utilizing a transparent
conductive oxide (TCO) scaffold for thin films of hematite permit photogenerated holes to
be collected at the interface, and electrons shuttled to the scaffold, mitigating
recombination within the electrode material. Figure 4-4 shows a band-bending diagram for
a semiconductor-liquid junction under illumination. Favorable charge transfer pathways
are indicated by green arrows, in which an electron-hole pair is generated, separated, and
electrons are collected by the TCO and holes are collected by the redox (A/A-) couple in
solution. Recombination pathways are indicated by red arrows in which photo generated
electron-hole pairs recombine in the bulk, depletion region, surface states, and back
electron transfer labeled as Jbr, Jdr, Jss, and Jet, respectively.
80

Figure 4-4. Band-bending diagram for a semiconductor liquid junction showing
favorable charge transfer pathways in green and unfavorable recombination
pathways in red. Jbr, Jdr, and Jss indicate photocurrent, J, loss to bulk recombination,
depletion region, and surface state recombination, respectively. Jet is photocurrent
lost to back electron transfer.
We have shown previously that we can effectively deposit thin films of hematite (~20nm)
by atomic layer deposition (ALD), and doing so would eliminate the need for very thick
films by depositing them on a high aspect ratio scaffold.5,6,18 The requirements for such a
scaffold to be effective are that it is transparent, conductive, and has features in a highenough aspect ratio that allows for adequate light-absorption of the hematite as well as a
short minority carrier charge collection length. In addition, the scaffold must have feature
sizes that still allow for the solution reactants and products to reach all of the active
hematite, as to not have any dormant material. The scaffold should be compatible with
hematite for deposition purposes and be able to withstand basic conditions and the water81

oxidation environment. Lastly, since one of the main draws to using hematite is its
abundance and cost, a scaffold material that is abundant and environmentally benign is
also desirable.
Herein, the focus of this project was to develop a TCO scaffold in which thin-film hematite
could be deposited using ALD. The scaffolds where comprised of indium-doped tin oxide
(ITO) inverse opals (IO). Inverse opals, also called photonic crystals, are structures with a
repeating, ordered scaffold that have been used in displays, lasers, and photovoltaic
devices.28–33 They exhibit behaviors such as multiple light scattering which leads to
enhanced light collection of the material as compared to planer films.32 We coated ITO-IO
structures with hematite by ALD which allowed for conformal coverage of the IOs. Inverse
opals provide for a high degree of tunability, by easily altering the pore size or the
thickness/layers of IOs. This feature, paired with the uniform, conformal coverage provided
by the use of ALD, make tuning the properties of the scaffold and material relatively easy,
with the added benefit of being optimized independently from one another. In this work we
have shown that the use of ITO-IO does increase the absorptance of the photoanodes as
compared to planer films. However, this increase in absorptance did not correlate to an
increase in photocurrent density as expected, but did result in a cathodic shift for the onset
of water oxidation. In the time that this work was being conducted, just such an approach,
using inverse opals and hematite deposited by ALD was published.34
4.2 Experimental
4.2.1 Glass Preparation

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Tech 15 fluorine doped tin oxide (FTO) was purchased from (Hartford Glass) and used as
the planer substrate as well as the substrate for ITO-IOs. FTO substrate was cut into 2 x 2
cm pieces and was sonicated in aqueous detergent for 15 minutes and rinsed with copious
amounts of water. FTO pieces were sonicated in a 0.1 M hydrochloric acid in ethanol
solution for 10 minutes followed by rinsing with copious amounts of water and were dried
in air.
4.2.2 Synthesis of ITO Inverse Opals
Inverse opals (IO) were fabricated loosely following a previously reported procedures. 35,36
A 15 mmol solution of indium acetylacetonate (In(acac)3) dissolved in a 1:1 mixture of
methanol and acetone under gentle heating to 50°C in a round bottom flask fitted in a hot
water bath. After the In (acac)3 dissolved, the solution was allowed to cool slightly and then
5 mmol (10 mol%) of tin(IV) chloride was added drop-wise. The resulting clear solution
was used for drop-casting over nano-spheres. Any remaining solution was stored in a
closed vial for subsequent batches.
Drop casting was performed in a solvent environment in order to reduce cracking of
the film. Cleaned FTO glass was placed on a stage inside an evaporating dish with ~ 0.5
inches of 1:1 methanol to acetone. Polystyrene nanospheres of varying size were
purchased from Polysciences Inc. Nanospheres were 2.6% v/v (aq) and were sonicated for
15 minutes before use to reduce aggregation. A micropipettor was used to drop 5 µL of
dispersed nanospheres on the center of the glass slide. The nanospheres were allowed to
rest for half an hour with a lid over the evaporating dish. Upon resting, four aliquots of 5 µL
of the ITO solution were gently dropped onto the nanospheres, with 15 minutes of drying
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allowed between each aliquot. After all of the aliquots were delivered and allowed to rest
in the evaporating dish for half an hour, the samples were removed from the evaporating
dish and placed in an oven. Calcination was slow in order to remove the polystyrene
nanospheres with little to no cracking of the ITO. Slides were then heated in air using a
programmable furnace that was heated from room temperature to 250 °C at 0.5 °C/min
held for 5 hours, then heated to 500 °C at 1.0 °C/min and held for 12 hours and then
naturally allowed to cool to room temperature. The scheme for ITO-IO fabrication is
illustrated in Figure 4-5.

Figure 4-5. Schematic of ITO-IO fabrication process.
4.2.3 SEM
Scanning electron microscopy (SEM) was measured at the Composite Materials and
Structures Center in the College of Engineering at Michigan State University using a JSM
840 scanning electron microscope. Films were gold coated in order to increase conductivity
and reduce charging affects.
4.2.4 ALD of Hematite

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Calcined ITO inverse opals (ITO-IO) were coated with hematite (Fe2O3) by atomic layer
deposition (ALD) (Savannah 100, Cambridge Nanotech) according to a previously reported
procedure.37 In addition to ITO-IO, planar FTO glass was also coated with Fe2O2 as witness
slides. Prior to deposition of Fe2O3, FTO glass was sonicated in detergent and DI water,
rinsed, sonicated in clean DI water, and then finally rinsed and sonicated in isopropyl
alcohol (IPA). Substrates were blown dry with N2 directly before being placed in the ALD
chamber. Briefly, the substrate remained at 200 °C ferrocene (sublimed from Sigma
Aldrich) held at 70°C was pulsed for 20s, purged with N2 for 5s followed by a 0.015s pulse
of H2O, immediately followed by a 1s pulse of ozone (Yanco Industries ozone generator).
The H2O and O3 pulse was repeated ten times for a single oxidation macrocycle in order to
oxidize ferrocene.37 Following deposition, Fe2O3 was annealed to 500°C in an oxygen
environment at a rate of 17°C per minute, sintered at 500°C for 30 minutes and allowed to
cool slowly to 110°C over 2 hours. Thickness of the hematite was confirmed using
absorbtance measurements and Beer-Lambert calculations.
4.2.5 UV-Vis
Transmittance and reflectance values were measured for bare FTO, ITO-IO on FTO, planar
Fe2O3, and ITO-IO Fe2O3 using a labsphere RSA-PE-20 integrating sphere housed in a
Lambda 35 UV-Vis Spectrometer. Samples were measured striking the backside of the
substrate first to correlate to how photoelectrochemical measurements will be taken.
4.2.6 Photoelectrochemical Measurements
Working electrodes were fabricated from α-Fe2O3 coated ITO inverse opal films by affixing a
bare copper wire (24 gauge) with silver epoxy (MG Chemicals, Canada) to the α-Fe2O3 coated
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slide and completed by masking an active with Solaronix surlyn film (60 µm) (Aubonne,
Switzerland). The working electrodes were clamped into a custom glass 3 neck cell fitted with a
homemade Ag/AgCl reference electrode and a Pt mesh counter electrode. The electrolyte
solution comprised 200 mM KCl in ultrapure DI water (Milli Q 18 MΩ cm-2) and was buffered
to pH 7 with 50mM K3/K4PO4. Photoelectrochemical measurements were made with a Gamry
Instruments Reference 600 potentiostat (Warminster, PA) with a Xe arc lamp interfaced with a
Horiba Jobin Yvon MicroHR monochromator (Kyoto, Japan) as the light source. An AM 1.5
solar filter and neutral density filters (Thorlabs Newton, NJ) were used to simulate sunlight at
100 mW/cm2. Electrochemical impedance spectroscopy was performed with an Autolab
potentiostat (Utrecht, The Netherlands) interfaced with Nova Software (Utrecht, The
Netherlands).
4.3 Results & Discussion
4.3.1 SEM
Achieving uniform, continuous distribution of ITO-IO was variable. Small variations in
concentration, drying time, height of drop-casting, and so on can lead to cracking, islands,
and piled structures. SEM was used in order to observe films through all states; dropcasting of polystyrene nanospheres, ITO solution coating of nanospheres, and finally, ITOIO structures, shown in Figure 4-6a, b, and c, respectively.

86

Figure 4-6. SEM images of a) 350 nm polystyrene nanospheres dispersed on FTO, b)
top view of multi layer ITO-IO, and c) View of large area approximately single layer
ITO-IO.
In Figure 4-6a, it can be seen that polystyrene nanospheres generally order themselves in a
closed packing mode. This closed packing mode is ideal as it creates a well ordered
template for ITO solution to be drop-cast. For these studies, achieving a single layer of
polystyrene nanospheres was desirable for systematic studies on their enhancement when
Fe2O3 is deposited. In this manner, additional layers of IO could be fabricated and coated
with Fe2O3, measuring their performance for water oxidation and determining an optimal
thickness Overall, achieving a single layer of polystyrene nanospheres could be achieved
for relatively large areas (~ 20 x 20 µm), though there was some presence of 2 to 3 layers
observable when scanning the surface with SEM. In Figure 4-6b is an example of multilayer ITO-IO. These structures are still well ordered but do show cracks or breaks in the
structures. This was true throughout most of the multilayered regions. Despite regions of
multilayered structures, achieving uniform, single layer ITO-IOs was achievable by the
method reported herein, shown in Figure 4-6c. It can be seen that ITO-IO are formed when
87

the ITO solution fills in the regions of the closed-packing polystyrene nanospheres. The
resultant ITO-IOs are three-dimensional with a structure feature height roughly two-thirds
the height of the polystyrene nanosphere. The structures are likely not as tall as the
original polystyrene spheres to form the template due to solvent evaporation resulting in
the formation of the ITO solid. Therefore, from this SEM images, it can be concluded that
the ITO inverse opal structures did form, constructing a nanostructure template for Fe2O3
to be deposited on.
4.3.2 Absorbtance
In utilizing a high aspect ratio substrate, it is important to keep it as transparent as possible
in order for it to be utilized from either the solution side or the backside for illumination. If
the substrate is a strong absorber, less photons that can be utilized for water oxidation.
Percent absorbtance was calculated from average percent reflectance and percent
transmittance values measured with the integrating sphere using the following
equation:

In

Figure

4-7,

absorptance values are plotted versus wavelength for bare FTO, ITO-IO on FTO, Fe2O3coated planer FTO, and Fe2O3-coated ITO-IO; black lines plots are a result of planer
substrates and red for ITO-IO- dashes indicate un-coated substrates.

88

Figure 4-7. Calculated percent absorptance for bare FTO (black dash-dot), ITO-IO on
FTO (red dash-dot), 500 cycles of Fe2O3 on planar FTO (black solid), and 500 cycles of
Fe2O3 on ITO-IO (red solid).
For uncoated FTO and ITO-IO substrates (black dot-dashed line and red dot-dashed line,
respectively in Figure 4-7, there is an increase in absorptance for ITO-IO substrates
between 390 and 445nm, with a maximum difference (nearly double) at 410 nm. This
increase in absorptance is expected as there is a greater amount of material on the FTO
glass substrate, reducing the transmittance of the electrode. However, the substrate is still
relatively transparent and suitable for use in a photoanode. For Fe2O3-coated planer FTO
and Fe2O3-coated ITO-IO (black solid line and red solid line, respectively in Figure 4-7)
there is an increase in absorbtance of approximately 10 percent across measured
wavelengths, with the increase tapering to <5% at wavelengths beyond 525 nm.

89

Figure 4-8. Percent absorptance for Fe2O3-coated planer FTO (black dashed line) and
Fe2O3-coated ITO-IO (red dashed line) corrected for substrate absorptance.
To determine the absorptance due to Fe2O3 alone, Fe2O3-coated substrates were corrected
for substrate absorptance, shown in Figure 4-8. Though the ITO-IO substrate had slightly
greater absorptance for wavelengths between 390 and 445 nm, the Fe2O3-coated ITO-IO
still showed greater absorptance, approximately 25 percent, across all measured
wavelengths, even though the same cycles of Fe2O3 were used for each electrode. This
illustrates increasing the thickness of Fe2O3 in the z-direction, while keeping conformal,
relatively thin coatings in the x-direction. The resulting height (z-direction) however, is
roughly 250 nm, which is still less than the 3/α value of 375 nm needed to absorb 95
percent of the light at 550 nm.
4.3.3 Photoelectrochemical Measurements

90

Photoelectrochemical measurements in pH 7 K3/K4PO4 will 200 mM of KCl as a supporting
electrolyte were measured in the dark and under 1 sun illumination to measure water
oxidation performance. Current density versus voltage curves (J –V) curves are shown in
Figure 4-10 for planer Fe2O3 and ITO-IO coated Fe2O3 in the light and in the dark. The Fe2O3
coated ITO-IO curve in red, is expected to have a higher photocurrent density, J, than the
planer Fe2O3 electrodes do to the increase in light absorptance discussed previously.
However, instead there is a nearly 100 mV cathodic shift in the onset of water oxidation
and no increase in J for the measured potential range. This result was unexpected as the
increase in absorptance does not translate to an increase in photocurrent density. One
reason for this may be the form or curvature of the ITO-IO substrate. With the height of the
IO being approximately ~250 nm at their highest point, only a small portion of the incident
light may actually be traveling through all 250 nm, and the height of the Fe 2O3 will change
as the IO slopes to the bottom. This paired with only a modest increase in absorptance may
lead to less than ideal performance. In addition, is it is possible that not all of the IO is
active, there could be breaks or discontinuities in the films that would lead to greater light
absorption, but not necessary translate to increase photocurrent density. The shift in the
photocurrent onset could be due to several factors, including substrate composition. It has
been shown previously that the substrate or underlayer that hematite is deposited on can
have a great effect on electrode performance.38,39

91

Figure 4-9. Current density versus voltage curves for planer Fe2O3-coated FTO (black
trace) and Fe2O3 ITO-IO substrates (black trace) in the light (bright colors) and in the
dark (muted colors).
To determine the cause if the cathodic shift in the onset of water oxidation, electrochemical
impedance spectroscopy was measured on un-coated planer FTO and ITO-IO electrodes.
Impedance was measured at 250 mV vs Ag/AgCl in order to avoid the diffusion region of
the CV taken in pH1 phosphate buffer with 200mM KCl over a frequency range of
1,000,000-0.1 Hz. The resulting Nyquist plots are shown in Figure 4-10 (planer FTO inset,
red trace and ITO-IO blue trace) and upon inspection, it is clear that there is a large
difference in the resistance and capacitance of the two electrodes. In order to quantify the
resistance and capacitance values of the electrodes, Nyquist plots were fit with Z-View
software using the Randal’s circuit shown in Figure 4-11. Rs is the resistance of the cell
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(connections, solutions, etc), Rct is the charge transfer resistance from the electrode to the
solution, and Cct is the double layer or Helmholtz capacitance. The Rs for the cells are
expected to be nominally similar as the electrodes are fabricated in the same manner and
measured under identical conditions. From the fits it was determined that ITO-IO
electrodes had an Rs, Rct, and Cct value of 14.2 Ω, 14.8 Ω, and 26 µF, respectively. For planer
FTO electrodes it was determined that Rs, Rct, and Cct values were 16.2 Ω, 812 Ω, and 2.2 µF,
respectively. These fits illustrate the reduced charge transfer resistance and increased
capacitance for ITO-IO electrodes as compared to planer FTO electrodes. The concurrent
reduction in resistance and increase in capacitance is likely the reason for the cathodic shift
in the onset of water oxidation shown in Figure 4-.

Figure 4-10. Nyquist plots for bare FTO (red trace, large plot) and for bare ITO-IO
electrodes (blue trace, inset plot) measured at 800 mV vs RHE.

93

Figure 4-11. Randal's circuit used for fitting Nyquist plot.
4.4 Conclusions and Future Work
Herein we report the fabrication of ITO-IO by a drop-casting technique for use as a
transparent conducting substrate for the deposition of Fe2O3 by atomic layer deposition for
use as a photoanode. Using UV-vis spectroscopy equipped with an integrating sphere, the
absorptance of Fe2O3-coated ITO-IO showed a 25 percent increase for measured
wavelengths up to 550nm as compared to Fe2O3-coated planer FTO glass. This increase in
absorptance did not lead to an increase in photocurrent density as expected but did lead to
a nearly 100 mV cathodic shift for the onset of water oxidation. The cathodic shift for the
onset of water oxidation was attributed to the substrate, as ITO-IO electrodes had 500
times lower charge transfer resistance and an order of magnitude higher capacitance than
planer FTO electrodes measured under the same conditions.
Future work for this project included optimizing the polystyrene nanosphere size in order
to optimize the inverse opal structures. In addition, ALD methods were being explored for
coatings of the ITO-IO spheres in order to have more continuity for the IO, hopefully
leading to an increase in photocurrent density. In addition, work was being done in order
to ALD TCO materials directly onto polystyrene nanospheres at low temperatures in order
94

to have more controllable, conformal coverage of the nanospheres. However, in the time
that this work was being conducted, another research group published a research article
with the same idea and procedure of using IO structures for substrates for the deposition of
Fe2O3 for use as a photoanode.34

95

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