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Amorphous Aluminium Oxide Overlayer on Hydrothermally Grown Hematite Nanowires

for Photoelectrochemical Water Splitting

Bryan Zhishan Yong

A0125285ESupervisor: Associate Professor Wang Xuesen

Department of Physics

National University of Singapore

Bryan Zhishan YongA0125285E

AcknowledgementsFirst and foremost, I wish to express my heartfelt gratitude for my project supervisor,

Associate Professor Wang Xuesen. Without his passionate support and guidance, this project

would not have come to fruition. The experience of working under him this past year in the

Surface Science Laboratory and the Surface & Nano-Structure Laboratory has been

extremely educational and useful in preparing me for my post-education life.

I also wish to thank NUS Laboratory Technologists Mr Chen Gin Seng (Thinfilms Lab), Mr

Wong How Kwong (Surface Science Lab), and Mr Ho Kok Wen (SSLS), for maintaining and

assisting my operation of various experimental apparatus. This project, and indeed many

others, would not have been possible if not for their efforts.

I also wish to extend my gratitude towards Dr Linda Sellou for her assistance and advice in

the chemical synthesis aspect of this project. She has shown me that scientific pursuit is an

interdisciplinary field and that every member of the scientific community has a part to play

in the grand pursuit of knowledge.

Last but certainly not least, I wish to extend my appreciation to colleagues Mr Wang Zeli and

Mr Tan Wei Zhi for their invaluable assistance at numerous junctures throughout this

project. Their help has been constant and consistent in helping me understand about various

experimental techniques and apparatus, and have enabled me to complete this project.

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Abstract

Hematite, or α-Fe2O3, is a promising photoanode material for hydrogen production in

photoelectrochemical water-splitting due to its abundance, stability, and non-toxicity. As

such, this paper examines a dual approach of morphological control and surface

modification to improve the performance of hematite photoanodes. Hydrothermal

synthesis of nanowire arrays was optimised as a form of morphological control, and

optimum conditions were found to be 24 hour synthesis in autoclave system at 100 oC,

followed by 1 hour thermal annealing at 450 oC. A variation of Le Formal et al.’s surface

modification using atomic layer deposition of highly-ordered α-Al2O3 was applied and

characterised. Surface modification of amorphous aluminium oxide deposited by

atmospheric oxidation of sputtered aluminium metal was found to improve photocurrent

generation by ~70 %, and reduce required applied voltage for water oxidation by ~0.05 V. A further modification by ‘maturing’ samples through electrolysis to remove excess

sputtered aluminium was found to further reduce its required applied voltage for water

oxidation, and managed to replicate improvements to performance noted by Le Formal et

al. to within 20 %. The sample produced by this ‘maturation’ technique was able to

generate photocurrents of 0.781 mA cm-2 with applied voltage of 0.95 V vs normal

hydrogen electrode (NRE), and 1.96 mA cm-2 with applied voltage of 1.23 V vs NRE, and

had a reduction of 0.05 V for onset of water oxidation.

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Table of ContentsAcknowledgements..............................................................................................................................2

Abstract.................................................................................................................................................3

1. Introduction..................................................................................................................................6

1.1 Photoelectrochemical Water Splitting..................................................................................7

1.3 Suitability of Hematite (α-Fe2O3) as Photoanode in PEC Water-Splitting...........................11

2. Experimental Overview..............................................................................................................14

2.1 Hydrothermal Synthesis of Hematite Nanowire Arrays.....................................................14

2.2 Surface Modification of Hematite Nanowires with Alumina Overlayer............................15

2.3 Project Scope.......................................................................................................................16

3. Experimental Methodology........................................................................................................17

3.1 Measurement Techniques Employed..................................................................................17

3.1.1 Measuring Photocurrent Efficiency Using Photoelectrochemical Water-Splitting Cell17

3.1.2 Characterising Techniques for Analysis of Samples....................................................19

3.2 Hydrothermal Synthesis of Hematite Nanowire Arrays.....................................................19

3.3 Sputtering Aluminium Metal on Hematite Nanowires for Oxidation to Aluminium Oxide Overlayer.........................................................................................................................................21

3.4 Optimising Duration of Hydrothermal Synthesis................................................................21

3.5 Optimising Temperature for Thermal Annealing of FeOOH into Hematite........................22

3.6 Optimising Duration of Thermal Annealing of FeOOH into Hematite................................22

3.7 Optimising Sputtered Aluminium for Aluminium Oxide Overlayer Surface Modification on Hematite Nanowires.......................................................................................................................23

4. Results and Discussion................................................................................................................24

4.1 Characterising Effects of Hydrothermal Synthesis Parameters on Hematite Nanowires...24

4.1.1 Effect of Duration of Hydrothermal Synthesis on Nanowire Morphology.................24

4.1.2 Effect on Thermal Annealing Temperature on Hematite Nanowire Photocurrent Generation..................................................................................................................................28

4.1.3 Effect of Duration of Thermal Annealing Duration on Hematite Nanowire Photocurrent Generation............................................................................................................30

4.1.4 Using XPS and XRD to Confirm Chemical Composition and Structural Phase of Nanowires...................................................................................................................................31

4.2 Characterising the Effect of Amorphous Aluminium Oxide Surface Modification on Hematite Nanowire Photocatalysts................................................................................................33

4.2.1 Effect of Amorphous Aluminium Oxide Surface Modification on Photocurrent Generation of Hematite Nanowires............................................................................................33

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4.2.2 Effect of Amorphous Aluminium Oxide Surface Modification on Required Voltage for Onset of Water Oxidation for Hematite Nanowires...................................................................35

4.2.3 Further Modification of Aluminium Oxide Overlayer Through Electrolysis; ‘Maturing’ Aluminium Oxide Surface Modification......................................................................................37

4.3 Future Works.......................................................................................................................39

5. Conclusion...................................................................................................................................41

References..........................................................................................................................................42

Appendix.............................................................................................................................................43

A. Amperometric I-t Curve Shape...............................................................................................43

B. Determining Required Applied Voltage for Onset of Water Oxidation Photocurrent...........44

C. X-Ray Photoelectron Spectroscopy (XPS)...............................................................................46

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1. Introduction

Climate change amid humanity’s insatiable hunger for energy has spurred interest in clean

and green energy sources. The burning of fossil fuels, which has powered humanity’s

industrialisation for more than a century, is unsustainable as oil reserves and natural gas

veins are being depleted.1 Further, fossil fuel burning has led to increased carbon dioxide

levels in our atmosphere and is a key driving force in human-caused climate change. 2 These

factors has resulted in a race to discover and develop clean and renewable energy sources to

fuel humanity’s growth into the 21st century. Of the many alternatives being pursued, the

most promising energy source is the sun. It provides large amounts of radiative energy to

Earth daily, and will continue to do so for billions of years.

With such potential, harvesting of solar energy into usable forms has received much

scientific interest. Thus far, 3 methods have been developed for the harnessing of solar

power:3

1) Regenerative solar cells which convert solar energy directly into electrical

energy, also known as photovoltaic cells.

2) Photosynthetic fuel cells which convert solar energy into chemical energy,

also known as artificial photosynthesis.

3) Photocatalytic fuel cells which photocatalytically degrade organic

compounds to produce electrical energy, largely in degrading pollutants.

On top of harvesting solar energy, storage of such energy remains an important

consideration. Sunlight has large variations due to variations in cloud cover, weather

patterns, and diurnal changes. Thus artificial photosynthesis offers the most elegant

supplemental solution due to direct conversion into a storable form of chemical energy, and

will be the focus of this paper.

Fujishima and Honda first demonstrated the potential of photoelectrochemical cells (PEC) in

artificial photosynthetic harvesting of solar radiation into usable energy in 1972 using Rutile

TiO2.4 Since then PECs have been heavily studied and various other photocatalysts identified


and optimised. This project aims to add to this large body of research by optimising a

potential photocatalyst material, hematite, using methods that are cheap and scalable for

industrial applications.

1.1 Photoelectrochemical Water SplittingPEC cells utilise semiconducting photocatalysts to absorb solar radiation for oxidation and

reduction reactions of the electrolyte. Figure 1 illustrates the components comprising a PEC

cell set-up. Typically, a PEC cell comprises two electrodes, a photoanode and a metal

cathode as a counter electrode, immersed in an electrolyte, with an applied bias voltage

across the electrodes as needed.5 The photoanode comprises nanostructured photocatalyst

for light absorption, while the cathode comprises nanostructured electrocatalyst to facilitate

reduction reactions. Incident photons are absorbed by the photoanode, consisting of a

nanostructured photocatalyst, to generate an electrons and holes. The photogenerated

holes and electrons are then separated by the applied bias voltage and internal band

bending. Photogenerated holes are gathered on the surface of the anode for oxidation

reactions at the anode-electrolyte interface. Photogenerated electrons are gathered and

transported via an external circuit to the cathode surface for reduction reactions at the

cathode-electrolyte interface. Thus redox half-reactions occur at each electrode and are

balanced by the counter electrode.

Figure 1. Schematic of a Photoelectrochemical Cell. PECs comprise a

(transparent) photoanode containing nanostructured photocatalyst (i),

connected to a cathode (counter electrode) containing nanostructured

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electrocatalyst (ii), with electrolyte (iii). Photogenerated electrons are

conducted by external circuit and carry out reduction reactions at cathode,

while photogenerated holes carry out oxidation reactions at photoanode.

The choice of electrolyte and electrode materials for PEC cells depend on the intended use.

The most appealing electrolyte for artificial photosynthesis is water due to its abundance on

earth.6 Hydrogen gas can be harvested through PEC water-splitting with only oxygen gas as a

by-product. Harvested hydrogen gas can then be oxidised to produce energy, with water as a

by-product. Thus both the forward and backward reactions of water-splitting involve no

carbon, making water-split hydrogen a carbon-neutral fuel source which can alleviate

humanity’s impact on Earth’s climate.

In PEC water-splitting, photogenerated electrons and holes are used for redox reactions in

water, similar to electrolysis.5 At the photoanode, oxidation of water to form oxygen gas with

the following oxygen evolution reaction, or OER, with a redox potential of +1.23 eV relative to the normal hydrogen electrode (NHE) at pH =0:

2H 2O ( l )+4h+¿→O 2 (g )+4 H +¿ (aq ) ¿ ¿

While at the cathode, reduction of hydrogen gas from water occurs with the following

hydrogen evolution reaction, or HER, with a redox potential of 0 V vs NHE at pH =0:

2H+¿ (aq)+2e2−¿→H (g ) ¿¿

Thus the complete redox reaction across both electrodes is the following reaction:

2H2O (l )+4 e−¿+4h+¿→O2 ( g) +2H 2( g) ¿¿

Or, since the electrons and holes are photogenerated:

2H 2O ( l ) 4hv→O2 (g )+2H 2 (g )

The photogenerated electrons and holes have to be able to perform their respective redox

half-reactions for water-splitting to occur. This places constrains on the suitability of

materials chosen as photocatalysts.3 First, photogenerated electrons and holes by the

prospective photocatalyst have to be sufficiently energetic, which requires photon energy hv > 1.23 eV, or equivalently λ < 1100 nm. Second, the reducing and oxidising

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potentials of the photogenerated electrons and holes have to be greater than the respective

redox half-reactions.

1.2 Photocatalysts’ Function In Photoelectrochemical

Water-Splitting

Photocatalysts are generally semiconductor materials which have band structure comprising

of conduction and valance bands (CB and VB respectively) separated by a band gap energy, Eg. Photogeneration of electron and holes occur when a photocatalyst absorbs incident

photons with energy hv > Eg through excitation of an electron in the valence band to the

conduction band3. Hence photocatalysts with smaller Eg are able to absorb a wider range of

photon energies as less energy is required to excite the electron to the CB and thus more

favourable. Figure 2 illustrates this semiconductor band structure and absorption

mechanism.

Figure 2. Energy Diagram of Semiconductor Photocatalyst Used In PEC

Water-Splitting. Photogenerated electron and holes are used to power

reduction and oxidation half-reactions of water to produce hydrogen and

oxygen respectively.

When sufficient external bias voltage is applied to semiconducting photoanode placed in

water, a space-charge (depletion) layer is formed at the semiconductor/liquid junction

(SCLJ).5 The photogenerated electrons and holes are separated due to this depletion layer

where the holes are carried to the SCLJ to perform oxidation half-reaction while electrons

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are conducted to the counter electrode (cathode) by external bias voltage for reduction half-

reaction.

Semiconductors can be checked for suitability as photocatalysts by analysing whether their

energy band structures can satisfy the constraints of PEC water-splitting. Eg limits the energy

of the photogenerated electrons and holes. As such, photocatalysts for PEC water-splitting

require Eg > 1.23 eV to ensure photogenerated electrons and holes are sufficiently

energetic for water-splitting, while still possessing as small a Eg as possible for absorbing

more light3. The CB and VB edges in the energy scale relative to normal hydrogen electrode

(NHE) represent the reducing and oxidising potential of photogenerated electrons and holes

respectively. Given the redox potentials of reduction (0.0 eV vs NHE) and oxidation (+1.23 eV vs NHE) of water, the CB edge should be more negative than 0.0eV and the VB edge

should be more positive than 1.23 eV vs NHE.

Figure 3. Diagram of Various Semiconductors and Their Band Edge

Positions. Band edge positions are relative to NHE. Band edge positions are

compared to the potentials for water-splitting redox reactions. Image

adapted from 3.

After photogeneration of the electrons and holes, these charge carriers have to be

transported to their electrode surfaces for reaction with water. In the case where the

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cathode is a metal with good electrocatalyst, the electrode potential is approximately equal

to the reduction redox potential of 0.0 eV vs NHE.5 Photoanode electrode potential is

approximately the potential of the CB edge. Thus for water-splitting, the internal force

driving electrons to the cathode is analogous to a potential approximately equivalent to that

of the potential of the CB edge relative to NHE. As such charge transport could theoretically

occur without an externally applied bias voltage, provided the CB edge potential is more

negative than 0.0 eV vs NHE.

1.3 Suitability of Hematite (α-Fe2O3) as Photoanode in

PEC Water-SplittingHematite, α-Fe2O3, has been identified early as a potential candidate for use in photoanodes

as a photocatalyst. It possesses a band gap of Eg ≈ 1.9 – 2.2 eV, corresponding to λ ≈ 650 – 550 nm which is sufficient for PEC water-splitting while still able to absorb ~40% of solar radiation.6,7 It also has CB edge of V ≈ 0.50 eV vs NHE, and a VB edge of V ≈ 2.6 eV. Hematite is a promising photoanode material due to relatively small Eg as compared to

other candidate photocatalysts like TiO2 (Eg ≈ 3.2 eV) and WO3 (Eg ≈ 2.9 eV), while

also possessing a sufficiently positive VB edge to drive oxygen gas production, with a

theoretical solar to hydrogen efficiency of 12.9 %.8Other than its semiconductor properties, hematite is a popular photoanode candidate also

due to its stability under photocatalytic conditions at both acidic (pH < 4.0) and alkali (pH ≈ 14.0).6,9 Further, hematite is non-toxic, and easy to synthesise. Hematite is also cheap

due to its abundance – iron is the fourth most abundant material in the Earth’s crust behind

oxygen, silicon and aluminium, with hematite being the most stable among iron oxides.

However, hematite also suffers from various drawbacks from being an excellent

photocatalyst. Hematite possesses poor charge carrier properties including5,6:

i. Low conductivity of σ < 1×10-6 Ω-1 cm-1.

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ii. Short hole diffusion length of 2 – 4 nm, as compared to ~100 μm for

TiO2 and ~100 nm for WO3, thus requiring holes to be photogenerated

within 2-4 nm of the depletion layer at SCLJ.iii. Low electron mobility ≈ 10-2 cm2 V-1 s-1 and short excited state lifetime

leads to energy loss via electronic de-excitation. iv. Poor light absorption, requiring 400 – 500 nm films for complete light

absorption. This leads to photogenerated holes being formed far from the

SCLJ and hence have high recombination rates.

These drawbacks prevent hematite from achieving theoretical efficiency. In order to address

them, Sivula, Le Formal and Grätzel proposed a dual approach of morphology control, and

surface modification.6 The effect of this approach on photacatalytic efficiency of hematite is

summarised in Figure 4.

Figure 4. Two Part Strategy for Improving Hematite Performance as

Photoanode. Utilising both morphological control and surface catalysis

strategies can significantly improve hematite performance as photoanode.

Image adapted from Sivula, Le Formal and Grätzel.6

Morphology control involves the use of hematite nanostructures to ensure photogeneration

of holes occur close enough to the depletion layer at SCLJ (~10 nm). 1D nanowire

structures, or nanowire arrays in particular, are suitable for application in photocatalysts for

multiple reasons10. Primarily, nanowire arrays decouple the light penetration path and hole

diffusion path for photoanodes. This effect is illustrated in Figure 5. The decoupling allows

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for longer light penetration paths to alleviate hematite’s poor absorbance, while still having

hole generation to occur close to the SCLJ. Nanowire arrays can even extend light

penetration path past film thickness due to reflection and scattering between nanowires.

Nanowires also increase the contact area between the photocatalyst and the electrolyte

allowing for more active sites for redox half-reactions. Thus nanowire arrays are extremely

effective modification in improving photocatalytic capabilities of hematite.

Figure 5. Illustration of Different Light Absorption and Charge

Transportation in Dense Film, Nanoparticle Film, and Nanowire Arrays

Film. In dense film, light penetration path is the same as charge

transportation path leading to optimisations sacrificing one or the other. In

nanoparticle films, charge carrier paths are shortened to nanoparticle size,

but charge carriers are scattered at nanoparticle boundaries. In nanowire

arrays, there is vectorial charge transfer with different electron and hole

paths. Image adapted from Hoang and Gao.10

Surface catalysts are used to reduce the required bias voltage for oxygen gas generation to

occur. Hematite possesses poor oxygen reaction kinetics, which results in oxygen gas

generation only occurring with external bias voltage VB > 1.03 eV.11 Le Formal et al. have

demonstrated that application of a very thin overlayer of alumina, α-Al2O3, through atomic

layer deposition can improve photocatalytic efficiency of hematite nanostructures11. They

reported an improvement of photocurrent generation by a factor of 3.5 at 1.0 V vs RHE

(regular hydrogen electrode), and a reduction of required bias voltage by 100 mV. Failure

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to reproduce similar increases in photocatalytic efficiency when TiO2 was similarly modified

prompted the conclusion that alumina passivates hole-trapping surface sites on hematite.

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2.Experimental Overview

I intend to optimise hydrothermally grown hematite on fluorine-doped tin oxide (FTO)

coated glass with aluminium oxide as surface catalyst for photocatalytic water-splitting. I will

be employing the hydrothermal growth method outlined by Vayssieres et al.12 for synthesis

of hydrothermal nanowire arrays due to its potential applications in wide-scale production. I

will also be investigating if modifications of thin aluminium metal overlayers which then

oxidise to form amorphous aluminium oxide will reproduce similar results obtained by Le

Formal et al.11 I will be employing both methods in order to improve hematite’s

photocatalytic efficiency, with analysis focusing on these two factors:

i. Magnitude and stability of generated photocurrents when hematite photoanode is

connected to a PEC water-splitting cell.

ii. Required external bias voltage, VB, before onset of water oxidation.

2.1 Hydrothermal Synthesis of Hematite Nanowire

Arrays

Vayssieres et al. developed a hydrothermal synthesis method for hematite nanowire arrays

that can be used to directly grow the nanowire arrays onto chosen crystalline substrates like

FTO glass.12 Their synthesis method utilised a novel concept called “purpose-built materials”

and relies on controlling the surface free energy of the system. By minimising the surface

free energy through low pH conditions, solubility of more stable phases of iron will increase

and hence there will be nucleation of a metastable phase instead. This is due to Ostwald’s

step rule which states that it is not the most stable state, but rather the least stable state

which is initially obtained. Thus if there is sufficient high concentration of precursor ions,

there will be epitaxial crystal growth of the metastable FeOOH along the easy direction of

crystallisation away from the substrate. This would lead to a precursor layer of FeOOH

nanowires which can later then be thermally annealed to obtain pure hematite.


Vayssieres et al. produced well-aligned, single-crystalline hematite nanorods around 100 nm long and 5 nm in diameter, which self-assembled to form bundles of around 50nm in

diameter after 1 hour of hydrothermal synthesis at 100 oC and 1 hour of thermal annealing

at 390 oC. The nanowire arrays were fairly consistent in dimensions and arranged in

uniform arrays. They stated that dimensions of the nanowires can be tailored by adjusting

the hydrothermal synthesis conditions such as temperature and duration.

2.2 Surface Modification of Hematite Nanowires with

Alumina Overlayer

Neufield, Yatom and Toroker applied ab initio computations to support Le Formal et al.’s

conclusions that an alumina overlayer improves hematite’s photocatalytic efficiency through

passivation of surface states.13 They utilised density functional theory + U (DFT+U) for their

computations. They simulated alumina overlayers of varying coverage and thickness on

hematite (0001) surface, then employed first principal methods to determine the reaction

kinetics. They found that oxidation of water was not carried out on alumina surfaces, but

rather exposed hematite surfaces. They have also observed that optimum application of

alumina overlayer should be partial coverage of as few molecular layers as possible.

Applying the above findings to this project, I hope to investigate if application of aluminium

metal onto the surface will be sufficient for the improvements in photocatalytic efficiency of

hematite observed by Le Formal et al. Since partial coverage of extremely thin layers of

alumina is optimal, a sufficiently thin layer of deposited aluminium which then oxidises in

atmosphere to form amorphous aluminium oxide may be sufficient to reproduce similar

improvements in photocatalytic efficiency of hematite nanowires. If successful, this

represents a far easier and cheaper synthesis method for aluminium oxide overlayer on

hematite nanowires without resorting to methods to produce highly-ordered structural

phase of α-Al2O3.

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2.3 Project Scope

In this project, I intend to optimise growth conditions and modification of hematite

nanowire arrays as a photocatalyst in photoanode for PEC water-splitting. The hydrothermal

synthesis outlined by Vayssieres et al. has multiple parameters that can be characterised for

photocatalytic current generation. Due to time and safety constraints, I selected the

following hydrothermal synthesis parameters for optimisation of photocurrent generation:

i. Duration of hydrothermal synthesis.

ii. Temperature of thermal annealing.

iii. Duration of thermal annealing.

For the aluminium oxide overlayer modification, I have decided to investigate if amorphous

aluminium oxide formed from oxidation of deposited aluminium thin films in ambient

conditions will be sufficient to passivate the hematite surface states. If successful, this would

represent a far easier synthesis method of applying an aluminium oxide overlayer as there is

no need for high temperature thermal annealing at T = 1150 oC to obtain crystalline

alumina.14 I will be investigating the relation between amount of aluminium deposited and

its effects on both photocurrent generated as well as the required external bias voltage at

the onset of oxygen gas evolution.

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3.Experimental Method ology

As my synthesis of hematite nanowires with aluminium oxide overlayer involves multiple

steps, individual steps in overall synthesis were optimised before proceeding to optimisation

in the next step. Thus hematite nanowire arrays were produced with variations in a single

parameter while keeping all other parameters constant. These produced nanowire arrays

were then connected into a PEC water-splitting cell to measure photocurrent efficiency. The

results are then analysed to determine the optimum setting for this one parameter which

will then be applied as the standard setting for future optimisation procedures. Finally, the

various parameters and their influence on photocatalytic efficiency were analysed.

3.1 Measurement Techniques Employed

3.1.1 Measuring Photocurrent Efficiency Using Photoelectrochemical

Water-Splitting Cell

The efficiency of hematite nanowire samples were measured in 2 different methods. Both

methods utilised the same experimental set-up shown in Figure 6. A Teflon chamber with

glass side window was used as the PEC reaction chamber. A Xe lamp is set up facing the glass

side window of the PEC reaction chamber such that it will illuminate into the PEC reaction

chamber. The lamp is connected to a power source set at 12 W. An AM 1.5 solar filter was

placed between the lamp and the reaction cell to simulate solar radiation. 2 electrodes and

a reference electrode are connected to a CHI Instruments photoelectrochemical

workstation. The hematite nanowire array sample is turned into a photoanode by using

sandpaper to expose part of the FTO layer for connection to the anode electrode. A piece of

platinum foil is used as the counter electrode. 0.1 M dm-3 Na2SO4 was used as a pH neutral

electrolyte. The workstation will apply a driving voltage across the electrodes, with the

platinum as a metal cathode and the sample as a photoanode, as well as record

photocurrent.


Figure 6. Schematic Diagram of PEC Cell for PEC Water Splitting.

The first method of determining photocatalytic efficiency is through amperometric I-t measurements. This is done via applying a fixed external voltage of 0.4 V vs RHE (reversible

hydrogen electrode) while periodically exposing the hematite photoanode to the filtered

lamp light for 20s and then shielding it from light for 20s. The photocurrent (I) variation

across time (t) will be recorded using the PEC workstation throughout the periodic

exposure/shielding. The plotted I-t graph will give us amount of photocurrent generated at

chosen voltage, as well as provide information regarding the kinetics of electron-hole

recombination (Appendix A).

The second method of determining the photocatalytic efficiency is through linear sweep

voltammetry measurements. This is done by keeping the light on and then using the CHI

instruments workstation to vary the applied driving voltage (measured against RHE) across

the circuit, while recording the generated photocurrent (I) as a function of the applied

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driving voltage (V). The plotted I against V (vs RHE) graph will give us information about

photocurrent generated by the sample at a given applied bias voltage, as well as the

required VB for the onset of water oxidation photocurrent generation (Appendix B).

3.1.2 Characterising Techniques for Analysis of SamplesFor characterising hematite nanowire arrays that are being optimised using morphological

control and surface modification, the following parameters have to be parameterised:

i. Chemical composition of samples.

ii. Crystal phase of samples.

iii. Dimensions of nanowire arrays in samples.

iv. Relative abundance of various chemical states

In order to measure and hence parameterise the above parameters, various experimental

techniques were employed including:

a) Energy dispersive x-ray spectroscopy for i.

b) X-ray photoelectron spectroscopy (XPS) for I and iv.

c) X-ray diffraction (XRD) for ii.

d) Scanning electron microscopy (SEM) for iii.

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3.2 Hydrothermal Synthesis of Hematite Nanowire

ArraysVayssieres et al.’s hydrothermal synthesis involved placing FTO glass substrate into stoppered

conical flask with an aqueous solution at temperature T = 100 oC. Their aqueous solution

consisted of 0.15 M ferric chloride (FeCl3 ⋅ 6H2O) and 1 M sodium nitrate (NaNO3) dissolved

in de-ionised water (18.2 M Ω cm-2) and set at pH 1.5 using hydrochloric acid (HCl).12 I

adapted his method by immersing FTO glass substrates in the above solution and heated

them in an autoclave system comprising a Teflon reaction cell placed into a stainless steel

jacket (to maintain pressure) and heated in an oven pre-heated to 100 oC. Scaling his

recipe for two 100 ml reaction cells, I obtained the following recipe which was used for all

synthesis:

i. 170 ml de-ionised water

ii. 14.45 g NaNO3

iii. 6.89 g FeCl3 ⋅ 6H2O

iv. 655.0 μl of 25% HCl

The solution was then divided into two 100 ml Teflon reaction cells, each containing two

FTO glass substrates with FTO layer facing downwards to prevent settling of particulates

which led incomplete coverage (Figure 7c). Thus four samples could be produced each time,

and comparisons were always done with reference to samples produced within the same

batch.

After heating in oven for desired duration, the reaction cells (steel jackets left on) were then

left in a fumehood to be cooled for 4 hours. If cooling was not done slowly, and reaction cell

rapidly cooled under running tap instead, deposited layer on sample was fragile and flaked

off even with gentle blowing (Figure 7d). After cooling, the samples were removed from the

solution and washed with de-ionised water before being blown dry with N2 gas. At this stage,

samples consisted of a uniform yellow film deposited on the FTO layer (Figure 7a). The

sample will then be placed into pre-heated chemical vapour deposition tube furnace system

for thermal annealing in air to obtain hematite nanowires (Figure 7b).

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Figure 7. Photos of FeOOH Precursor and Thermally Annealed Hematite on

Deposited On FTO Glass. a) Photo of FeOOH precursor deposited on FTO

glass substrate after 24 h hydrothermal synthesis at 100 oC. b) Photo of

hematite nanowire array on FTO glass substrate after thermal annealing of

sample shown in a. at 400 oC for 1 hour. c) Photo showing sample of

hematite nanowire array annealed from sample which had FTO layer of

substrate facing upwards during hydrothermal growth. This led to incomplete

coverage of hematite nanowire arrays on substrate. d) Photo showing sample

which was rapidly cooled in reaction cell under running tap instead of over 4

hours in fumehood. Deposited film very fragile and flaked off when blown

with N2 gas.

3.3 Sputtering Aluminium Metal on Hematite Nanowires

for Oxidation to Aluminium Oxide OverlayerThe surface modification of aluminium oxide overlayer was done through sputtering of

aluminium metal on my hematite nanowire photocatalysts. A radio-frequency (RF)

magnetron sputtering system (Denton Discovery 18 system) was employed to sputter

aluminium from a 99% pure aluminium target onto prepared samples. Prepared samples

were placed into a high vacuum chamber with pressure P < 5 × 10-6 Torr at room

22


temperature with injection of Argon gas at 12 cm3 min-1, and aluminium was sputtered

onto samples with an RF power of 150 W. Samples were then removed from the vacuum

chamber for the sputtered aluminium to oxidise in atmosphere to form amorphous

aluminium oxide.

3.4 Optimising Duration of Hydrothermal SynthesisIn order to determine the relationship between duration of hydrothermal synthesis and

photocurrent generation of thermally annealed hematite nanowires, I prepared samples

that were left in autoclave system set at 100oC for varying durations of t = 1 h, 5 h, 16 h, 20.5 h, and 24 h. After cooling for 4 hours, samples were removed, washed, and

blown dry with N2 gas. SEM imaging and XPS was done to confirm the deposition of FeOOH

nanowires onto FTO glass substrate (Appendix C). These samples were then thermally

annealed at 400 oC for 1 hour. These samples were then analysed in PEC water-splitting

cell and respective amperometric I-t curves obtained for analysis on relationship between

duration of hydrothermal synthesis and photocurrent generation of hematite photoanodes.

It was noted that the sample produced from 24 hours hydrothermal synthesis in autoclave

system produced the sample with the largest photocurrent generation.

3.5 Optimising Temperature for Thermal Annealing of

FeOOH into HematiteAfter FeOOH precursor layer was deposited onto FTO glass substrate, it needs to be

thermally annealed to obtain hematite nanowire arrays. In order to determine the

relationship between temperature of thermal annealing and photocurrent generation of

resultant hematite photoanodes, samples with FeOOH layer deposited were thermally

annealed at temperatures T = 400 oC, 425 oC, 450 oC, and 475 oC for 1 hour.

Samples were then imaged using SEM to obtain morphological data of hematite nanowires.

These samples were then connected to the PEC water-splitting cell as photoanodes and

respective amperometric I-t curves obtained. These I-t curves were analysed for

23


relationship between temperature of thermal annealing and photocurrent generation of

hematite photoanodes. It was noted that the sample annealed at T = 450 oC produced

the photoanode with largest photocurrent generation.

3.6 Optimising Duration of Thermal Annealing of FeOOH

into HematiteTo determine the relationship between duration of thermal annealing of FeOOH and the

photocurrent generation of resultant hematite photoanodes, samples with FeOOH layer

deposited were thermally annealed at T = 400 oC for durations t = 0.5 h, 1 h, 2 h, and 4 h. These samples were then imaged using SEM to obtain morphological data of

hematite nanowires. These samples were then connected to the PEC water-splitting cell as

photoanodes and respective amperometric I-t curves obtained. These I-t curves were

analysed for relationship between temperature of thermal annealing and photocatalytic

efficiency of hematite photoanodes. It was noted that the sample for t = 1h at

temperature T = 450 oC produced the photoanode with largest photocurrent generation.

Overall, the sample that was produced with hydrothermal synthesis in autoclave system for

24 hours before being thermally annealed for 1 hour at 450 oC generated the largest

photocurrent. This sample was then set for XPS and XRD to confirm identity of hematite

nanowires, as well as check for contamination from hydrothermal synthesis process. These

parameters were then used as the standard for optimisation of surface modification of

aluminium oxide overlayer.

3.7 Optimising Sputtered Aluminium for Aluminium Oxide

Overlayer Surface Modification on Hematite Nanowires

Samples of prepared hematite nanowire arrays were coated with overlayers of aluminium

oxide of varying thickness by sputtering aluminium for time t = 0.5 min, 1 min, 1.5 min, and 5 min. These samples were then removed from the sputtering machine and the

24


sputtered aluminium allowed to oxidise in atmosphere for at least 24 hours before

characterisation. The coverage of aluminium oxide on each sample was determined using

XPS (Appendix C). The samples were then connected into the PEC water-splitting cell and

amperometric I-t curve and linear sweep voltammetry measurements were conducted to

determine the photocatalytic efficiency of each sample. The results were then analysed to

determine the relationship between coverage of aluminium oxide and the photocatalytic

efficiency of modified hematite nanowire photoanodes.

25

4.Results and Discussion

4.1 Characterising Effects of Hydrothermal Synthesis

Parameters on Hematite Nanowires

4.1.1 Effect of Duration of Hydrothermal Synthesis on Nanowire

Morphology

Unfortunately, I was unable to reproduce Vayssieres et al’s results with his parameters of

hydrothermal synthesis at 100 oC for 1 hour. Instead, there was no deposition of FeOOH at

all on the FTO glass substrate (Figure 8). Similarly, the samples that underwent hydrothermal

synthesis for 5 hours displayed only partial coverage of FeOOH on FTO substrate (Figure 8).

This could be due to the differences in our hydrothermal synthesis systems: their synthesis

was done in a stoppered flask while my synthesis was done in an autoclave system and

heated from room temperature. The autoclave system has to heat up to 100 oC before the

reaction could begin whereas the stoppered flask solution could be pre-heated which would

account for the discrepancy in results.

Figure 8. Photos of Samples That Underwent Hydrothermal Synthesis for

24 Hours (left), 1 Hour (middle), and 5 Hours (right). Samples were

prepared with hydrothermal synthesis at 100 oC.


Morphology of thermally annealed hematite nanowires were imaged using SEM (JEOL JSM-

6700F) and analysed for nanowire bundle diameter, length and density. Images were

obtained with accelerating voltage V = 5 kV and emission current I = 10 μA, and shown

in Figure 9.

Figure 9. Low Magnification SEM Images for Hematite Nanowire Array

Samples Thermally Annealed from Samples Grown Via Hydrothermal

Synthesis for Durations of 16, 20.5, and 24 Hours. Samples were prepared

via hydrothermal synthesis at 100 oC, followed by thermal annealing at 400oC for 1 hour.

From analysis of the SEM images (Figure 9), it is observed that increasing duration of

hydrothermal synthesis corresponds to decrease in nanowire bundle diameter, while

individual nanowires that comprise the bundle have relatively consistent diameter of

~50nm. The density of individual nanowires seem to be constant, hence thicker bundles

corresponds to fewer density of bundles. Duration of hydrothermal synthesis also seems to

correspond with an increase in nanowire length, but with an apparent saturation in

nanowire length. These results are summarised in Figure 10.

27


Figure 10. Plot of Hematite Nanowire Array Dimensions Against Duration

of Hydrothermal Growth. Samples were thermally annealed at 400 oC for

1 hour. The error bars represent difference between average and

smallest/largest measurements to highlight variance in sample dimensions.

Larger lengths and smaller diameters should result in better photocurrent generation

performance for hematite nanowire arrays. Thus from comparing the various morphological

parameters obtained, we should expect to see better photocurrent generation from samples

that underwent longer hydrothermal synthesis. To confirm this, the above samples were

connected to the PEC water-splitting cell and amperometric I-t measurements taken to

determine photocatalytic efficiency. These results are summarised in Figure 11, with an

example amperometric I-t curve given in Figure 12.

28


Figure 11. Plot for Photocurrent Generated by Sample in PEC Water-

Splitting Cell Under AM 1.5 Illumination Against Duration of

Hydrothermal Growth. Error bars denote largest difference between

measured photocurrent and average photocurrent values.

Figure 12. Amperometric I-t Curve Measured from PEC for 20.5 Hour

Hydrothermal Synthesis Sample. Sample was thermally annealed at 400 oC for 1 hour. PEC was conducted under AM 1.5 illumination at 0.4 V vs

RHE.

Comparing obtained amperometric I-t measurements against morphological data, we can

see that the nanowire diameter is the more significant factor in photocatalytic efficiency.

This could be because our hematite nanowire samples had lengths that were sufficiently

29


long such that absorption of light was saturated and will no longer increase with further

increases in length.

4.1.2 Effect on Thermal Annealing Temperature on Hematite

Nanowire Photocurrent Generation.

After samples were produced and annealed at different temperatures, they were imaged

using SEM to determine effect of annealing temperature on hematite nanowire morphology.

These images are shown in Figure 13.

Figure 13. SEM Images of Hematite Nanowire Arrays Thermally Annealed

at Various Temperatures for 1 Hour. Samples underwent hydrothermal

synthesis for 24 hours at 100oC before thermal annealing. a) Sample that

was annealed at 425 oC for 1 hour. b) Sample that was annealed at 450 30


oC for 1 hour. c) Side view of sample that was annealed at 450 oC for 1

hour d) Sample that was annealed at 475 oC for 1 hour.

As seen in Figure 13, there seems to be a breakdown of nanowire self-assembly into bundles

from 425 oC. This resulted in a loss of uniformity of orientation. Thus samples annealed at 450 oC and 475 oC had nanowires that were far more disordered. This resulted in inability

to measure nanowire morphology as the thin, un-bundled nanowires were too difficult to

image.

To determine their photocatalytic efficiency, these samples were then connected to the PEC

water-splitting cell and amperometric I-t measurements taken under AM 1.5 illumination.

The results of these measurements are summarised below (Figure 14):

Figure 14. Plot of Photocurrent Generated During Amperometric I-t Measurements Against Temperature of Thermal Annealing. The samples

were prepared using hydrothermal synthesis for 24 hours, and were

thermally annealed for 1 hour. Amperometric I-t measurements were

done under AM 1.5 illumination. Error bars represent largest difference

between measured photocurrent and average value.

Thus we can determine that thermal annealing temperature of T = 450 oC produces the

hematite nanowire array that has the greatest photocatalytic efficiency when used as a

photoanode. Unfortunately, as morphology of the nanowires seem to be relatively

unchanged beyond 400 oC, we can only speculate as to the reasons for differing

31


photocurrent generation when annealed at different temperatures. One possibility is that

the annealing conditions would result in different chemical composition or structural phases

of the sample. Thus the identity of samples should be confirmed after thermal annealing

parameters are chosen.

4.1.3 Effect of Duration of Thermal Annealing Duration on Hematite

Nanowire Photocurrent Generation

After samples were annealed at 400 oC for various durations, the samples were connected

in PEC water-splitting cell and amperometric I-t measurements (at AM 1.5 illumination)

done to determine their photocatalytic efficiencies. The results are summarised in Figure 15.

Figure 15. Plot of Photocurrent Generated During Amperometric I-t

Measurements Against Temperature of Thermal Annealing. The samples

were prepared using hydrothermal synthesis for 24 hours and were

thermally annealed at 450oC. Amperometric I-t measurements were

done under AM 1.5 illumination with applied voltage of 0.4 V vs RHE.

Error bars represent largest difference between measured photocurrent

and average photocurrent.

32


From the results, we can determine that thermal annealing at 450oC for 1 hour produces

the hematite nanowire array that has the greatest photocatalytic efficiency when used as a

photoanode. Amperometric I-t curve of this sample is shown in Figure 16.

Figure 16. Amperometric I-t Curve of Sample Prepared with 24 Hour

Hydrothermal Synthesis and Thermal Annealing at 450 oC for 1 Hour.

Amperometric I-t curve was obtained under AM 1.5 illumination at 0.4V vs RHE.

It should be noted that the optimum thermal annealing conditions were determined by first

assuming a value of one and then varying the other. If the effects of annealing temperature

and duration on photocurrent generation are not independent, then all that was done was

to show that thermal annealing at 450 oC for 1 hour would produce one set of optimum

annealing conditions. Thus further work testing various combinations of annealing

conditions should be done to confirm that these conditions would maximise photocurrent

generation using this synthesis method.

4.1.4 Using XPS and XRD to Confirm Chemical Composition and

Structural Phase of Nanowires

After determining the optimum synthesis conditions of 24 hours hydrothermal synthesis

followed by thermal annealing at 450 oC for 1 hour, the chemical composition of the

33


samples were confirmed using XPS analysis. XPS (ESCA, Electron Spectroscopy for Chemical

Analysis) was employed using a magnesium Kα source (1253.6 eV) with emission current

of 2.0 mA and anode voltage of 15 keV. An Omicron 7-channel analyser with step size of 0.05 eV and a constant analyser energy (CAE) of 50 eV was used for analysis. Information

about XPS binding energies was obtained from NIST database.15 The obtained XPS spectrum

for Fe 2p is shown below (Figure 17):

Figure 17. Plot of XPS Fe 2p Spectrum. Sample underwent hydrothermal

synthesis at 100oC for 24 hours and thermal annealing at 450 oC for 1

hour. Peak positions were determined using a 80% Lorentzian-Gaussian fit

using XPSPeak4.1 software.

Analysis of the Fe 2p (700 eV – 740 eV) peaks reveals a 2p3/2 satellite peak at 720.1eV

which is indicative of Fe2O3.16 Thus the chemical composition of the nanowire arrays

produced by our hydrothermal method has been confirmed to be Fe2O3.

After confirmation of sample chemical composition, the structural phase of nanowire

sample was determined using XRD (Brucker D8 Advance). XRD data was analysed using the

JADE6 software and its associated database. The XRD data had its background removed

before peaks were compared against the JADE6 software database for potential structures.

XRD data is shown below (Figure 18).

34


Figure 18. XRD Data With Hematite and Tin Oxide Peaks Identified.

Hematite peaks identified with H, while tin oxide peaks identified with T.

XRD data of sample produced via hydrothermal synthesis at 100 oC for 24

hours followed by thermal annealing at 450 oC for 1 hour.

Analysis of the XRD data showed that the sample contained both hematite and tin oxide.

The presence of tin oxide could have been due to FTO layer being scratched as the hematite

thin film was scratched for XRD analysis. Thus we have confirmed the hydrothermal method

does produce hematite nanowire arrays.

4.2 Characterising the Effect of Amorphous Aluminium

Oxide Surface Modification on Hematite Nanowire

Photocatalysts

4.2.1 Effect of Amorphous Aluminium Oxide Surface Modification on

Photocurrent Generation of Hematite Nanowires

After samples were produced and modified with aluminium oxide overlayer, they were

analysed for proportion of aluminium oxide on surface by using XPS. The proportion of

aluminium oxide to hematite on sample surface was determined by analysing the O 1s peaks

of the XPS spectrum (Appendix C). The summary of percentage aluminium oxide coverage

due to sputtering duration is summarised in Table 1:

35


Table 1. Sputtering Duration and Resultant Percentage Coverage of Aluminium Oxide.

Aluminium Oxide was oxidised in atmosphere from sputtered aluminium metal.

Sputtering Duration (min) Coverage of Aluminium Oxide (%)

0.5 65.2

1.0 74.3

1.5 76.9

5.0 94.6

Photocatalytic efficiency of the modified samples were determined with amperometric I-t measurements, with example shown in Figure 19. The photocurrents generated were then

normalised against an unmodified sample of hematite nanowires that was produced in the

same batch. This was done to determine how much aluminium oxide overlayer improved the

photocurrent generation of the samples. Summary of improvements in photocurrent

generation is summarised in Figure 20.

Figure 19. Amperometric I-t Curve of Sample Prepared With 24 Hour

Hydrothermal Synthesis and Thermal Annealing at 450oC for 1 Hour

with Aluminium Sputtered for 1.5 Minutes. Amperometric I-t curve was

obtained under AM 1.5 illumination at 0.4 V vs RHE.

36


Figure 20. Plot of Photocurrent Generated Against Coverage of

Aluminium Oxide Surface Modification. Samples were produced via

hydrothermal synthesis for 24 hours before being thermally annealed at 450 oC for 1 hour. Aluminium was then sputtered onto samples and

allowed to oxidise in atmosphere.

Analysis of the I-t curves show that modification using sputtered aluminium improved

photocurrent generation by 45 % – 440 %. The sample that was sputtered with

aluminium for 5 minutes had the largest increase in photocurrent generation. Generally,

greater aluminium oxide coverage results in greater photocurrent generated. However, this

increase in photocurrent generation should not be solely due to the effect proposed by

Neufield, Yatom and Toroker as their mechanism involves oxygen generation only occurring

on exposed hematite surfaces. Thus the sample with close to 100 % coverage should have

poor oxygen evolution due to having close to 0 % of the surface being exposed hematite.

4.2.2 Effect of Amorphous Aluminium Oxide Surface Modification on

Required Voltage for Onset of Water Oxidation for Hematite

Nanowires.

The effect of aluminium oxide surface modification on required applied voltage was also

analysed using linear sweep voltammetry. The onset voltage for water oxygen was

37


noticed to have dropped across all samples by 0.1 V, 0.03 V and 0.05 V for

sputtering duration of t = 0.5, 1, and 1.5 minutes respectively. The exception is the

sample that had aluminium sputtered for 5 minutes (Figure 21d). This adds veracity to the

suggestion that passivation of hematite surface states is not the only mechanism that

resulted in these samples possessing greater photocurrent generation. Instead, some

sacrificial reaction, where aluminium is oxidised instead of water, could be the cause of

the large generated photocurrent.

Figure 21. Plots of Linear Sweep Voltammetry Measurements for Samples

with Aluminium Oxide Surface Modification. a) Sample sputtered with

aluminium for 0.5 min. b) Sample sputtered with aluminium for 1 min. c)

Sample sputtered with aluminium for 1.5 min. d) Sample sputtered with

aluminium for 5 min. Linear sweep voltammetry Measurements taken

under AM 1.5 illumination.

38


Further, analysis of the Fe p XPS spectrum showed that the Fe 2p3/2 satellite peak at 720.1eV was no longer present in every sample (Figure 22). Thus the increased

photocurrent generation observed in samples with aluminium oxide surface modification

does not seem to be solely due to passivation of hematite surface states as suggested by

Le Formal et al.

Figure 22. Plot of XPS Fe 2p Spectrum for Sample Sputtered with

Aluminium for 30s. Fe 2p3/2 satellite peak which indicates Fe2O3 no longer

observed under XPS.

The lack of the satellite peak indicates a significant reduction in hematite at the surface. This

could possibly be due to overly thick layers of aluminium being deposited during the

sputtering process. Thus samples were further modified in an attempt to remove excess

aluminium and aluminium oxide and expose more hematite surface for photocatalysis.

4.2.3 Further Modification of Aluminium Oxide Overlayer Through

Electrolysis; ‘Maturing’ Aluminium Oxide Surface Modification

The sample with aluminium sputtered for 5 minutes seems to have too much aluminium

sputtered onto it. Thus the sputtered aluminium layer should consist of a film of aluminium

metal protected from oxidation from aluminium oxide surface. As such, I attempted to

further modify this sample through electrolysis to see if this would improve photocatalytic

39


efficiency, particularly the required voltage for onset of water oxidation. This modification

consisted of connecting the sample to the PEC cell and performing electrolysis by applying

an external voltage of 1.4 V vs RHE for 12 hours. After electrolysis, the sample was

observed to be closer in appearance to an unmodified hematite sample – more reddish and

transparent (Figure 23).

Figure 23. Photos Illustrating Difference in Sample After Electrolysis for 12

hours. Both samples had aluminium sputtered for 5 minutes. Sample on

right was placed in electrolysis for 12 hours at 1.4 V vs RHE

The sample was then analysed for surface chemical composition using XPS, and aluminium

oxide coverage was determined to be 73.8 %. Interestingly, the Fe 2p3/2 satellite peak at 720.1 eV which denotes Fe2O3 was observed again where before it had disappeared, even

for sputtering duration of only 30 seconds. This could be due to areas with different

thickness of aluminium oxide being preferentially removed from the hematite surface. These

results are summarised in Figure 24.

40


Figure 24. XPS Spectrums of Sample with Aluminium Oxide Surface

Modification Post-Electrolysis. a) Plot of O 1s spectrum with fitted

theoretical peaks to determine coverage of aluminium oxide using

XPSPeak4.1 (Appendix C). b) Plot of Fe 2p spectrum. Fe 2p3/2 satellite peak

at 720.1 eV was observed.

After confirming the chemical composition of the sample surface, the sample’s

photocatalytic efficiency was measured using both amperometric I-t and linear sweep

voltammetry measurements. Analysing the results, we obtain an improvement in

photocurrent generation of 1.81 × at 0.4 V vs RHE, while there is an approximate

decrease in required voltage of VB ≈ 0.05V for water oxidation. This sample also

generated a photocurrent of 0.781 mA cm-2 at 1.03 V vs RHE and 1.96 mA cm-2 at 1.23 V vs RHE. Comparison between this sample and that produced by Le Formal et al. is

summarised in Table 2.

Table 2. Summary of Modified Hematite Photocatalyst Performance.

Parameter Le Formal et al.’s Results

Results from ‘matured’ sample

Percentage Difference (%)

Photocurrent, VB = 1.03 V vs NRE 0.95 0.781 mA cm-2 17.8Photocurrent, VB = 1.23 V vs NRE 2.5 mA cm-2 1.96 mA cm-2 21.6

Reduction in Onset Potential 0.1 V 0.05 V 50

41


These results confirm that deposition of highly ordered Al2O3 followed by thermal annealing

is indeed the most effective approach to modifying hematite nanowire surface with

aluminium oxide. Still, these results do represent a promising avenue for further studies as a

cheaper alternative to modifying hematite surface: deposition of very thin layers of

aluminium followed by oxidation in atmosphere followed by a ‘maturation’ period during

which excess aluminium is removed during photocatalytic activity.

4.3 Future Works

While this paper focuses on hydrothermal synthesis nanowire arrays of hematite, there are

other synthesis methods and other nanostructure morphologies available. A comparison can

be done between the results obtained here and other synthesis methods or nanostructure

morphologies. This project was conducted concurrently with colleague Tan’s Final Year

Project on the study of photocatalytic efficiency of nanostructured hematite grown on iron

substrate using the ‘hotplate’ technique.17 Tan’s hotplate technique is a very scalable and

cheap method for producing nanostructured hematite with blade-like morphology. Tan’s

samples managed to produce photocurrent densities on order of 10-4 mA cm-2 at VB = 0.4 V vs NHE, a full two orders of magnitude above those presented here. A comparison

between different synthesis techniques will allow for better understanding of hematite and

its properties as a photocatalyst.

The modification of ‘maturing’ samples with oxidised, sputtered aluminium was not well

explored due to time constraints. Further work can be done to characterise the effects of

initial aluminium oxide coverage and electrolysis duration on the samples’ photocatalytic

efficiency. Tests can also be done on stability of the remaining aluminium oxide overlayer. If

this technique’s parameters are well understood, then it can also be applied to other

overlayer modifications; If specific exposure of photocatalyst surface under overlayer is

desired, this technique can be applied to tune photocatalyst exposure, as well as overlayer

covrage and thickness. Thus this ‘maturing’ technique for modifying overlayers show

promise for further development.

42

5.Conclusion

Overall, this study has examined the hydrothermal synthesis of hematite developed by

Vayssieres et al., as well as a simple surface modification of aluminium oxide oxidised in

atmosphere from sputtered aluminium. The conditions for these methods were analaysed

for their effects on hematite nanowire’s performance in photocurrent generation when

used as a photoanode in a PEC water-splitting cell.

Synthesis parameters were optimised to determine that growth parameters of 24 hours

hydrothermal synthesis in autoclave system followed by 1 hour of thermal annealing at 450 oC would produce highest performance photoanode. There seems to be an increasing trend

for diameter and length of nanowires with increasing duration of hydrothermal synthesis.

While bundling of nanowires stopped occurring at temperatures above 425 oC, there

seemed to be no other effect on nanowire morphology. Instead, annealing at 450 oC for 1

hour produced relatively pure and highly-ordered hematite, which was confirmed using XPS

and XRD.

Attempts to replicate Le Formal et al.’s results via modifying hematite nanowires with

amorphous aluminium oxide via atmospheric oxidation of sputtered aluminium have

resulted in only some success. Onset potential for water oxidation was reduced by 0.1 – 0.03 V, while photocurrent generation was only improved by ~70 %. Attempts to

remove excess aluminium from the surface via electrolysis has resulted in further

improvement in photoanode performance, with the final sample having an ~81 % improvement in photocurrent generation, and performing at half the performance of that

obtained by Le Formal et al. This removal of excess surface catalyst via electrolysis, or

‘maturation’ possesses potential as a future method for tuning surface catalyst composition

and should be explored further.


References1 S. Shafiee and E. Topal, Energy Policy 37, 181 (2009).2 D.J. Wuebbles and A.K. Jain, Fuel Process. Technol. 71, 99 (2001).3 A. Kudo and Y. Miseki, Chem. Soc. Rev. 38, 253 (2009).4 W. Li, S.W. Sheehan, D. He, Y. He, X. Yao, R.L. Grimm, G.W. Brudvig, and D. Wang, Angew. Chemie -

Int. Ed. 54, 11428 (2015).5 P. Lianos, Appl. Catal. B Environ. 210, 235 (2017).6 K. Sivula, F. Le Formal, and M. Grätzel, ChemSusChem 4, 432 (2011).7 Y. Ling, D.A. Wheeler, Jin Zhong Zhang, and Y. Li, in One Dimensional Nanostructures Princ. Appl.‐ ,

edited by T. Zhai and J. Yao (John Wiley & Sons, Inc., 2013), pp. 167–184.8 A.B. Murphy, P.R.F. Barnes, L.K. Randeniya, I.C. Plumb, I.E. Grey, M.D. Horne, and J.A. Glasscock,

Int. J. Hydrogen Energy 31, 1999 (2006).9 K.L. Hardee and A.J. Bard, J. Electrochem. Soc. 123, 1024 (1976).10 S. Hoang and P.X. Gao, Adv. Energy Mater. 6, (2016).11 F. Le Formal, N. Tétreault, M. Cornuz, T. Moehl, M. Grätzel, and K. Sivula, Chem. Sci. 2, 737 (2011).12 L. Vayssieres, N. Beermann, S.E. Lindquist, and A. Hagfeldt, Chem. Mater. 13, 233 (2001).13 O. Neufeld, N. Yatom, and M. Caspary Toroker, ACS Catal. 5, 7237 (2015).14 L. Zhang, H.C. Jiang, C. Liu, J.W. Dong, and P. Chow, J. Phys. D. Appl. Phys. 40, 3707 (2007).15 NIST X-ray Photoelectron Spectroscopy Database, NIST Standard Reference Database Number 20,

National Institute of Standards and Technology, Gaithersburg MD, 20899 (2000),

doi:10.18434/T4T88K, (retrieved 05 April 2018)16 M. Aronniemi, J. Lahtinen, and P. Hautojärvi, Surf. Interface Anal. 36, 1004 (2004).17 Tan W. Z. Unpublished Honours Thesis. (2018)18 K.G. Upul Wijayantha, S. Saremi-Yarahmadi, and L.M. Peter, Phys. Chem. Chem. Phys. 13, 5264

(2011).19 Wang. X. S. Surface Physics Lecture, NUS. (2017)

44

http://dx.doi.org/10.18434/T4T88K


Appendix

A. Amperometric I-t Curve Shape

The I-t curves obtained from amperometric I-t measurements have a distinctive shape. This

is due to the processes which occur when samples are illuminated. Upon illumination, there

is an immediate steady hole current generated from the photocatalyst to the SCLJ due to the

depletion region formed by band bending. Some of these holes become captured and

trapped in surface states. This results in an electron current due to electrons recombining

with trapped holes. Thus the measured photocurrent in external circuit falls from its initial

value as the electron current approaches stability. The inverse is observed when illumination

is observed due to a reversal of these processes. Hole current stops immediately and the

remaining trapped holes recombine with electrons forming a current of opposite sign. These

processes give rise to the sharp drop and overshot of photocurrent as shown in Figure 25.

Figure 25. Illustration of Origin of Shape of Photocurrent Response to Sudden Illumination. Figure obtained from Peter, Wijayantha and Tahir.18

45

B. Determining Required Applied Voltage for Onset of

Water Oxidation Photocurrent

Due to the slow oxygen evolution reaction kinetics on hematite surface, it is often observed

that oxidation of water to produce oxygen gas requires VB > 0.4 V. This increase in

required VB above theoretical value is called the overpotential. Thus in order for

photocatalysts to attain theoretical performance in photocurrent generation, there is a need

reduce the required overpotential of hematite.

When VB is sufficiently large to overcome the overpotential, there will be an increase in

photocurrent generation due to oxidation half-reaction being carried out. The value for VB when this occurs is the onset potential for water oxidation. As a lower onset potential for

water oxidation would both reduce required VB as well as increase photocurrent generation

at lower VB, it is favourable to reduce onset potential as much as possible. The onset

potential for water oxidation is a parameter which can be reduced by surface modifications.

Thus it is important to identify the extent to which the onset potential for water oxidation is

changed by the surface modification.

The onset potential for water oxidation was determined by performing linear voltammetry

measurement, and plotting the obtained I-V graph. Then we draw two tangent lines. The

first line is drawn tangent to the low current values close to 0 when V is low. The second

line is drawn tangent to when I has both increased markedly and achieved a stable rate of

increase. The value for voltage at the intercept of these two tangent lines is taken to be the

onset potential for water oxidation.


Figure 26. Determining the Onset Potential for Water Oxidation. Black line represents

linear sweep voltammetry measurements, while green lines represent drawn tangents

to determine onset potential for sample modified with ‘maturing’ technique. Red and

blue lines represent the same but for an unmodified hematite sample.

47

C. X-Ray Photoelectron Spectroscopy (XPS)

XPS Analysis Background and Methodology

X-ray photoelectron spectroscopy is a spectroscopy technique which utilises high energy X-

rays to probe the surface composition of a sample. Monochromatic X-rays generated from a

source impinges on the surface of the sample ejecting photoelectrons. These ejected

photoelectrons are then collected and their kinetic energy, Ek, is measured with an electron

energy analyser. Energy levels of electron orbitals are quantised as such:19

EB(Z, n, L, J) = −m2 ( Z e2

4π ϵ0 nℏ )2

[1+Z2α2

n2 ( n

j+12

−34)]

As such, each element will have unique energy levels for each electron orbital

corresponding to various permutations of Z, n, l, & j (Table 3). Given that the generated

X-rays have energy E=hv, we can obtain the relation between measured energy of ejected

photoelectrons and binding energy:

hv =Φ + Ek + EBOr: EB = hv –Ek – Φ

As such, we can obtain information about the binding energy, Eb, via analysis of the kinetic

energy of the liberated photoelectron, given that the work function, Φ, of an element is a

unique constant. Given that EB of an element is sensitive to its chemical state, XPS

measurements allow us to determine not only the elemental composition of a sample, but

also the chemical composition of said element. Further, the probability of an electron from

an element atom in a specific chemical state being ejected by X-rays are directly related to

the relative abundance of said element. Thus, XPS allows us to determine the relative

abundance of an element in a specific chemical state.


Table 3: Nomenclature of XPS Peaks.

n l j Index XPS Notation

1 0 ½ 1 1s1/2

2 1 ½ 1 2s1/2

2 1 ½ 2 2p1/2

2 1 3/2 2 2p3/2

I have utilised XPS extensively to determine 2 things:

1) Presence and relative abundance of alumina as compared to other states

and phases of aluminium oxide.

2) Relative abundance of alumina as compared to hematite on surface of

samples.

This was done via the following methodology using the XPSPeak41 software:

a) Spectrum of carbon 1s binding energy was obtained and as calibration for

peak binding energy and FWHM of peak curve.

b) Spectra of relevant peaks of interest were obtained for various elements.

c) Spectra of relevant peaks were fitted with multiple theoretical peaks (80%

Lorentzian-Gaussian) :

i. FWHM of theoretical peaks are fixed according to the carbon 1s data.

ii. These theoretical peaks have peak Eg corresponding to various

chemical states of the element in question, corrected according to

carbon 1s data.

iii. Obtained Eg data is used to determine chemical state composition of

element via comparison with NIST XPS database.15

d) Relative abundance of atoms in specific chemical states are computed using

relative areas of the peaks (Thus obtaining results for 2).

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