Charge Carrier Dynamics in Hematite Photoanodes for Solar ...€¦ · for water oxidation to occur on hematite. Improved understanding of the role of applied bias and the processes

Charge Carrier Dynamics in

Hematite Photoanodes for

Solar Water Oxidation

Stephanie R Pendlebury

Department of Chemistry

Imperial College London

Supervisors: Prof J R Durrant and Dr J Tang

Submitted for Degree of Doctor of Philosophy

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Except where specific reference is made, the material contained in this thesis is the result of my

own work. This dissertation has not been submitted in whole or in part of a degree at this or any

other university, and does not exceed 100 000 words in length.

S R Pendlebury

May 2012

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Abstract

Although the field of solar water splitting is now forty years old, in recent years there has been

an upsurge of research in this area, with the aim of using sunlight to produce hydrogen cheaply

and efficiently. Hematite (α-Fe2O3) is of particular interest as a photoanode material for solar water

splitting, due to its optimum band gap (2.0-2.2 eV) and visible light absorption and stability.

Various modifications – including nanostructuring and doping – have been investigated as routes

to improved efficiencies, thought to be limited by long visible light absorption depths, low charge

carrier mobilities and slow hole-transfer kinetics. Additionally, an anodic applied bias is required

for water oxidation to occur on hematite. Improved understanding of the role of applied bias and

the processes limiting the performance of hematite photoanodes will lead to more directed routes

to photoanode architectures with increased efficiencies.

This Thesis describes the results of transient absorption spectroscopy studies, in conjunction

with photoelectrochemical measurements, of hematite photoanodes. Transient absorption

spectroscopy on microsecond-second timescales allows direct monitoring of the recombination,

trapping and reaction of photogenerated holes, both in isolated hematite films, and in photoanodes

in a fully functional photoelectrochemical cell. Transient photocurrent measurements probe

electron extraction from the photoanode on microsecond-millisecond timescales.

The charge carrier dynamics are found to be strongly dependent on the electron density, which

is controlled by applied electrical bias. The photocurrent generated is found to correlate with the

population of long-lived holes, determined by the kinetics of electron-hole recombination.

Generally, effects which lower electron density result in retarded electron-hole recombination

kinetics, increasing the population of long-lived holes and hence increasing the photocurrent.

Following an introduction and review of the literature, the first results chapter reports that the

effect of a positive applied bias is to retard the otherwise dominant electron-hole recombination,

increasing the lifetime of photogenerated holes such that water oxidation can occur. The relative

timescales of recombination, electron extraction and water oxidation as a function of applied bias

are discussed in the following chapter, in conjunction with the results of excitation density studies.

The third results chapter compares the charge carrier dynamics in photoanodes with different

nanomorphologies. The fourth results chapter discusses the effect of an energetic trap state on

charge carrier dynamics, while the effects of surface treatment with cobalt, which is shown to

retard recombination at low applied bias, is reported in the final results chapter. Overall

conclusions are drawn and the implications of these for photoelectrode design are discussed.

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Acknowledgements

First and foremost my thanks must go to Prof James Durrant, for giving me the opportunity to

work in this field, and for providing guidance, support and a ready smile over the duration. Thanks

also to Dr Junwang Tang for his enthusiasm and copious suggestions. I am always grateful to past

and present members of the Durrant, O’Regan, Klug, Haque and de Mello groups for their advice

and help in the lab, general camaraderie and good humour. Tea, cake and crosswords were much

appreciated. Thanks also to Dr Piers Barnes and Dr Steven Dennison for introducing me to the

theory, the literature and for many helpful discussions, and to Dr Xiaoe Li for sharing her extensive

knowledge and equally extensive collection of lab books. Particular thanks are due to Dr Monica

Barroso and Dr Alex Cowan for their help with just about everything, and for our many, many

discussions over scribbly bits of paper - I think we might have fitted together the edge pieces of the

Fe2O3 puzzle.

This project was entirely dependent on those who provided me with samples to measure: Prof

Michael Grätzel, Dr Kevin Sivula and the rest of the EPFL team; Dr Monica Barroso; Dr Steven

Dennison, Chin Kin Ong et al from Chemical Engineering; Dr Junwang Tang and Prof Jinhua Ye –

many thanks to you all.

I am forever grateful to my parents, who have always encouraged me in everything I have

embarked upon.

And to Richard, for everything.

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Table of Contents

Abstract ........................................................................................................................................................ 2

Acknowledgements ..................................................................................................................................... 4

Table of Contents ........................................................................................................................................ 5

List of Figures .............................................................................................................................................. 7

List of Symbols and Abbreviations ......................................................................................................... 13

List of Publications ................................................................................................................................... 14

Chapter I: Introduction ............................................................................................................................. 16

1.1 Background and Introduction to Solar Water Splitting ............................................ 16

1.2 Theory of Photoelectrochemical Water Splitting .................................................... 17

1.3 Literature Review: Hematite Photoanodes for Water Oxidation ............................. 21

1.3.1 Semiconductor properties of Fe2O3 ................................................................ 21

1.3.2 Kinetic Studies of Water Oxidation on Fe2O3 ................................................. 23

1.4 Project objectives .................................................................................................. 29

Chapter II .................................................................................................................................................... 32

Materials & Methods............................................................................................................................ 32

2.1 Materials: Fe2O3 photoanodes .............................................................................. 33

2.1.1 Undoped and Si-doped APCVD hematite ...................................................... 33

2.1.2 Undoped and doped USP hematite ............................................................... 33

2.1.3 Thick solid PLD hematite ............................................................................... 34

2.1.4 Thin solid ALD hematite ................................................................................ 34

2.1.5 Porous microwave heated hematite .............................................................. 35

2.1.6 Thick solid SP Si-doped hematite .................................................................. 35

2.1.7 Colloidal Ti-doped hematite ........................................................................... 35

2.2 Methods ................................................................................................................ 36

2.2.1 PEC............................................................................................................... 36

2.2.2 TAS ............................................................................................................... 38

2.2.3 TPC ............................................................................................................... 40

Chapter III ................................................................................................................................................... 42

Identification of Photogenerated Hole Absorption in Hematite Photoanodes ........................... 42

3.1 Introduction ............................................................................................................ 43

3.2 Experimental .......................................................................................................... 44

3.3 Charge carrier dynamics of isolated hematite films ................................................ 46

3.4 Charge carrier dynamics of hematite under applied bias ....................................... 55

3.5 Discussion ............................................................................................................. 58

3.6 Conclusions ........................................................................................................... 61

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Chapter IV ................................................................................................................................................... 62

Correlation of Photocurrent with Long-Lived Hole Population as a Function of Applied Bias ....................................................................................................................................................... 62

4.1 Introduction ............................................................................................................ 63

4.2 Experimental .......................................................................................................... 64

4.3 Transient absorption studies of photogenerated holes ........................................... 64

4.4 Transient photocurrent studies of photogenerated electrons .................................. 68

4.5 Excitation density studies ....................................................................................... 70

4.6 Discussion ............................................................................................................. 71

4.7 Conclusions ........................................................................................................... 77

Chapter V .................................................................................................................................................... 78

Comparison of Solid and Mesoporous Hematite Photoanodes ................................................... 78

5.1 Introduction ............................................................................................................ 79

5.2 Experimental .......................................................................................................... 80

5.3 Comparison of carrier dynamics in solid and mesoporous hematite ....................... 81

5.4 UV versus visible excitation ................................................................................... 89

5.5 Conclusions ........................................................................................................... 94

Chapter VI ................................................................................................................................................... 96

Influence of Trap States on Charge Carrier Dynamics ................................................................... 96

6.1 Introduction ............................................................................................................ 97

6.2 Experimental .......................................................................................................... 98

6.3 Spectroscopic Study of Trap State ......................................................................... 98

6.4 Discussion ........................................................................................................... 103

6.5 Conclusions ......................................................................................................... 108

Chapter VII ................................................................................................................................................ 110

Effect of Co-Based Catalysts on Hematite Charge Carrier Dynamics: Comparison of Co2+

and Co-Pi ............................................................................................................................................ 110

7.1 Introduction .......................................................................................................... 111

7.2 Experimental ........................................................................................................ 113

7.3 Effect of Co-adsorption on charge carrier dynamics ............................................. 114

7.4 Discussion ........................................................................................................... 119

7.5 Conclusions ......................................................................................................... 122

Chapter VIII: Concluding Remarks ....................................................................................................... 124

IX: References .......................................................................................................................................... 128

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List of Figures

Scheme 1.1 Semiconductor-electrolyte junction: ECB and EVB are the potentials of the conduction and

valence band edge, EF is the Fermi level, Eg is the band gap, ω is the width of the space-charge (depletion)

layer, VOC is open-circuit potential under illumination, EFn and EFp are the electron and hole quasi-Fermi

levels, respectively, under illumination, and uredox is the redox potential of the electrolyte. ............................ 18

Scheme 1.2 Three-electrode photoelectrochemical cell, with nanostructured Fe2O3 photoanode (working

electrode), reference electrode and metal counter electrode. ......................................................................... 19

Scheme 1.3 Crystal structure of hematite (from reference 29). Left: unit cell showing pairs of face-

sharing octahedral aligned along the c-axis. Right: FeO9 dimer. ................................................................... 22

Scheme 1.4 (a) Typical UV-vis spectrum of hematite; (b) schematic of proposed hematite band structure;

(c) updated band structure of hematite, showing strong Fe 3d/O 2p VB hybridisation. .................................. 23

Scheme 1.5 Proposed reaction scheme for water oxidation on hematite photoanodes (adapted from

references 8 and 17). ...................................................................................................................................... 25

Fig 1.1 SEM images of Si-doped (A and B) and undoped (C) APCVD hematite photoanodes. A: side-

view; B and C: top-down view. From reference 13. ........................................................................................ 33

Fig 1.2 Top-down SEM images of undoped (left) and Nb-doped (right) USP hematite photoanodes.

Images courtesy of Monica Barroso. ............................................................................................................... 34

Fig 1.3 Top-down (c) and side-view (d) SEM images of undoped ALD hematite photoanodes. From

reference 14. .................................................................................................................................................... 34

Fig 1.4 Top-down SEM image of undoped MH hematite photoanode. Image courtesy of Junwang Tang.

......................................................................................................................................................................... 35

Fig 1.5 Top-down SEM image of colloidal Ti-doped hematite photoanode. Insert: before encapsulation.

Image from reference 73. ................................................................................................................................ 35

Scheme 2.1 Photoelectrochemical system and three-electrode cell used for current/voltage,

chronoamperometry and IPCE measurements. See text for details. ............................................................. 36

Fig 2.1 Example of current/voltage curve of a hematite photoanode in the dark (grey) and under white

light illumination (blue). The dark current onset potential is ~0.65 VAg/AgCl, while the photocurrent onset

potential is ~0 VAg/AgCl. Nanostructured Si-doped CVD hematite photoanode under EE (“front-side”)

illumination at approximately 1 Sun intensity, in 0.1 M NaOH......................................................................... 37

Scheme 2.2 General schematic of the transient absorption systems employed - see text for details. ..... 39

Fig 3.1 UV-vis spectra of various hematite photoanodes employed in this study: undoped and Si-doped

nanostructured hematite deposited by atmospheric pressure chemical vapour deposition (CVD; blue lines);

mesoporous undoped hematite deposited by ultrasonic spray pyrolysis (USP, red line) and by microwave

heating (MH, green line). No correction has been made for reflection. .......................................................... 45

Fig 3.2 Transient absorption (TA) spectra of undoped CVD hematite in an argon atmosphere and inset: in

a methanol-saturated argon atmosphere, using 337 nm SE excitation (0.20 mJ.cm-2

at Fe2O3 surface).

Spectra shown were measured 5 and 80 μs after the laser pulse. ................................................................. 46

Fig 3.3 (a) Transient absorption decays of three different hematite films in an argon atmosphere; inset:

the same transient decays normalised and shown on log-log axes, exhibiting power-law-like decay kinetics.

TA decays probed at 600nm with 337 nm excitation (~0.2 mJ.cm-2

). (b) IPCE spectra of the same hematite

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films in a three-electrode cell with 0.1 M NaOH electrolyte (pH ~12.8), at 0.4 VAg/AgCl and under SE

illumination. CVD: dendritic nanostructured Fe2O3 (undoped); USP: mesoporous “platelet” Fe2O3; MH:

mesoporous Fe2O3 deposited by microwave heating. ..................................................................................... 47

Fig 3.4 Comparison of TA decays of undoped (pale blue/orange) and Si-doped (dark blue/brown) CVD

hematite in an argon atmosphere, probed at (a) 600 nm and (b) 900 nm (EE 180 μJ.cm-2

, 337 nm excitation).

Inset: normalised log-log plots. ........................................................................................................................ 49

Fig 3.5 TA spectrum (at 5 μs after the laser pulse) of undoped CVD hematite in an argon atmosphere, at

excitation intensities of 200, 100 and 50 μJ.cm-2

(EE 337 nm excitation). ...................................................... 50

Fig 3.6 Excitation intensity behaviour of undoped CVD hematite in an argon atmosphere, probed at 580

nm (left), 650 and 900 nm (right). Excitation intensities were varied between 27 and 500 μJ.cm-2

(337 nm SE

excitation). The TA amplitude at 1 μs is plotted in (a) probed at 580 nm, and (b) probed at 600 nm (green

triangles) and 900 nm (red rhombuses). The same behaviour is observed at 1-80 μs. The TA decay kinetics

are shown in (c) probed at 580 nm and (d) probed at 650 nm (900nm decays are very similar to those

probed at 650 nm); inset: the same decays normalised. ................................................................................. 51

Fig 3.7 TA decays (probed at 580 nm; SE 337nm, 190 μJ.cm-2

excitation) of undoped CVD hematite in an

argon atmosphere, in 0.1M NaOH, and with hole scavengers including methanol (~0.75 M in 0.1M NaOH)

and iodide (2 mM). Decay dynamics are also essentially identical in the presence of thiocyanate and iso-

propanol. Inset: comparison of SE and EE TA decays of hematite in aqueous KI. ...................................... 52

Fig 3.8 TA decays of an isolated undoped CVD hematite film in water (black) and aqueous AgNO3

solution (2 mM; blue/green), probed at 580 nm (left) and 650 nm (right). Charge carrier dynamics probed at

900 nm are similar to those probed at 650 nm. EE 337nm, 90 μJ.cm-2

excitation. ....................................... 53

Fig 3.9 TA spectrum of an isolated USP Si-doped hematite film in aqueous hydrogen peroxide solution

(~0.38 M). EE 337nm, 0.13 mJ.cm-2

excitation. ............................................................................................ 54

Fig 3.10 Comparison of TA decays of an isolated USP Si-doped hematite film in water (pale blue/orange)

and in aqueous hydrogen peroxide solution (~0.38 M; dark blue/brown) probed at 580 nm and 900nm (EE

337nm, 0.13 mJ.cm-2

excitation). Hydrogen peroxide causes bleaching of the 580 nm signal on timescales

>10 μs, and significantly increases the amplitude of long-lived signals probed at ≥650 nm. .......................... 54

Fig 3.11 Photocurrent/voltage curves for CVD undoped hematite under white light illumination (~1 Sun,

SE) in 0.1M NaOH without (black curve) and with (red curve) ca. 0.2 mM methanol. The dark current is

negligible in the potential region shown........................................................................................................... 56

Fig 3.12 TA decays of CVD undoped hematite in a three-electrode cell under applied bias, probed at (a)

580 nm and (b) 900 nm. In 0.1M NaOH under applied bias of -0.1 VAg/AgCl (blue/orange curve) and +0.4

VAg/AgCl (black curves). SE pulsed (0.33 Hz) 355 nm excitation; excitation densities are matched to those in

Figure 3.7. ........................................................................................................................................................ 56

Fig 3.13 TA decays (probed at 580 nm) of undoped nanostructured hematite in a three-electrode cell

under applied bias. (a) In 0.1M NaOH under applied bias of -0.1 VAg/AgCl (blue curve) and +0.4 VAg/AgCl (black

curve). Upon the addition of methanol in the positive bias condition (red curve), the faster decay indicates the

more facile oxidation of methanol by photogenerated holes. The decay of an isolated hematite film (no bias)

in 0.1M NaOH (green curve) is similar to that under negative applied bias. (b) Comparison of TA decays

with (red) and without (black) methanol at -0.1VAg/AgCl. ................................................................................... 57

Scheme 3.1 Representation of the effect of applied positive electrical bias on the Fermi level of a

nanostructured hematite photoanode. Applied positive bias decreases the background electron density

9

relative to open-circuit conditions, reducing the rate of electron-hole recombination and increasing the

lifetime of photogenerated holes, allowing water oxidation to occur. .............................................................. 59

Fig 4.1 Transient absorption spectra in Si-Fe2O3 CVD at (a) -0.2 VAg/AgCl and (b) +0.4VAg/AgCl at 10 ms,

100 ms, 500 ms and 1 s after the excitation pulse (EE, 355 nm). At early timescales there is a strong bleach

(negative absorption) at wavelengths <625 nm. The spectrum at +0.4VAg/AgCl is essentially the spectrum of

the photogenerated holes. ............................................................................................................................... 65

Fig 4.2 Transient absorption and photocurrent density data for a Si-doped CVD Fe2O3 film as a function

of applied electrical bias. (a) Transient absorption signals (1 μs to 2 s, EE 355 nm excitation, probed at 650

nm) at various applied bias (in 0.1 V increments: pale grey -0.4 VAg/AgCl, brown +0.6 VAg/AgCl). The arrow

indicates increasing number of long-lived holes with increasing positive bias at water-splitting timescales. (b)

Correlation of long-lived photogenerated hole signal amplitude at 100 ms (red diamonds) with photocurrent

(blue line; under 355 nm EE illumination (ca. 550 μW.cm-2

, giving ~54 μA.cm-2

photocurrent at 1.23 VRHE)). 66

Fig 4.3 Median lifetime (t50%)* of the fast decay phase of the transient absorption signal for

photogenerated holes from Figure 4.2a, versus applied bias. ........................................................................ 67

Fig 4.4 TPC decays (measuring extracted electrons) overlaid on corresponding transient absorption

decays (measuring photo-holes) of a Si-doped CVD hematite photoanode, EE excitation at 355 nm, at -0.2,

+0.2, +0.4 and +0.6VAg/AgCl. The TPC axis is shifted upwards and scaled to maximise overlap with the TA

decay. .............................................................................................................................................................. 69

Fig 4.5 Steady-state photocurrent and transient absorption data for a Si-doped CVD Fe2O3 photoanode

as a function of excitation intensity at +0.4 VAg/AgCl. (a) Variation of steady-state photocurrent amplitude

(under 355 nm EE illumination); the red line is the best fit to the data. (b) Variation of transient absorption

photogenerated hole signal (probed at 650 nm, EE 355 nm excitation from 23 μJ.cm-2

(dark green) to 2.21

mJ.cm-2

(brown); the laser intensity used for the majority of the measurements described herein is 200

μJ.cm-2

). (c) normalised slow TA phase at 125 ms - the timescale of water oxidation (2.1 s) is independent

of excitation intensity. Inset: normalised at 10 μs - the fast phase decays more rapidly with increasing

excitation density. (d) Ratio of amplitude of fast and slow decay phases of transient absorption as a function

of excitation intensity; inset: variation of amplitudes with excitation intensity. At the very lowest excitation

intensities (<200 μJ.cm-2

) we approach pseudo-first-order recombination behaviour (i.e. within the small

perturbation regime). ....................................................................................................................................... 70

Table 4.1 Estimated values of potential drop (ΔφSC) across the radius of spherical undoped hematite

nanoparticles of various sizes for two different Debye lengths (LD). ............................................................... 75

Fig 5.1 UV-vis spectra of various types of hematite photoanodes. Spectra taken of the same area of the

photoanodes as used for TAS and PEC measurements. Vertical black lines indicate 355 and 525 nm. ...... 80

Fig 5.2 Current/voltage curves (in 0.1M NaOH, pH ~12.8, white light illumination (ca. 1 Sun) intensity), 10

mV.s-1

) from different types of hematite photoanodes: solid Fe2O3 30 nm (pale blue) and 57 nm (dark blue)

thick (ALD); colloidal Ti-Fe2O3 (green); thick (1 μm) solid SP Sn-Fe2O3 (red). ............................................... 81

Fig 5.3 Transient absorption (TA) decays of holes (probed at 700 nm) in 30 nm thick ALD Fe2O3

photoanodes as a function of applied bias, at 0 (blue), +0.3 (green) and +0.6 VAg/AgCl (brown).

Measurements were made using a three electrode cell with 0.1 M NaOH electrolyte; EE 355 nm excitation

(25 μJ.cm-2

, corresponding to an approximate initial photogenerated hole density of 8x1018

holes.cm-3

). ..... 82

Fig 5.4 Correlation of long-lived photo-hole population (as measured by the amplitude of the TA decay at

200 ms) with photocurrent at +0.4VAg/AgCl, both under 355 nm illumination for various hematite photoanodes

10

under EE (front-side) and SE (back-side) illumination. The best-fit straight line has an intercept of 0.001(4)

mΔOD and gradient of 0.19(4). ....................................................................................................................... 82

Fig 5.5 (a) TA decays of holes in 30 nm thick solid ALD (probed at 700 nm; blue) and ~500 nm thick

nanostructured CVD Fe2O3 photoanodes (probed at 600 nm; EE brown, SE orange). (b) The same TA

decays normalised at 3 ms to show the relative timescales of water oxidation. Measurements were made at

potentials were the photocurrent was almost saturated: 0.4 VAg/AgCl for CVD and 0.6 VAg/AgCl for ALD

photoanodes. ALD measurements used EE 355 nm excitation (25 μJ.cm-2

, corresponding to ~2.4x1013

holes.cm-2

); CVD measurements used EE 355 nm excitation (190 μJ.cm-2


holes.cm-2

, assuming a roughness factor of 20), SE excitation densities were matched to this. .................... 84

Fig 5.6 (a) TA decays of holes (probed at 700 nm) in 30 nm thick ALD Fe2O3 photoanodes at +0.6 VAg/AgCl

as a function of excitation density. EE 355 nm excitation at 5 (grey), 25 (black), 50 (blue), 100 (purple) and

250 μJ.cm-2

(pink). (b) The same TA decays normalised at 3 ms, showing that the kinetics of water oxidation

are independent of excitation density. ............................................................................................................. 84

Fig 5.7 TA decays of holes in colloidal Ti-doped (green), thick (~1 μm) solid Si-doped (purple), 57 and 30

nm thick solid ALD (dark and pale blue, respectively) Fe2O3 photoanodes at positive applied bias where

photocurrent is approximately saturated. EE 355 nm excitation; average excitation density ca 2x1018

-3x1019

photogenerated holes.cm-3

. ............................................................................................................................. 85

Fig 5.8 TA decays of holes (probed at 650 nm) in colloidal Ti-doped Fe2O3 photoanodes at 0.25 (just

anodic of the photocurrent onset potential), 0.4 and 0.6 VAg/AgCl. EE 355 nm excitation at 50 μJ.cm-2

. The

fast decay phase (on the microsecond to hundreds of milliseconds timescale) is significantly longer-lived

than in other hematite photoanodes studied. .................................................................................................. 86

Fig 5.9 Transient photocurrent (TPC) from pulsed light (EE 355 nm; the same excitation densities are

employed as for TAS measurements) excitation of colloidal Ti-doped (green), thick solid SP Si-doped

(purple) and thin solid ALD (blue) hematite photoanodes. The photoanodes were held at positive applied

bias where photocurrent is approximately saturated. Photocurrent transients are normalised for ease of

comparison. ..................................................................................................................................................... 88

Fig 5.10 Comparing EE (pale colours) and SE (dark colours) TPC from colloidal Ti-doped (green, left)

and thick solid SP Si-doped (purple, right) hematite photoanodes (355 nm pulsed excitation). The

photoanodes were held at positive applied bias where photocurrent is approximately saturated.

Photocurrent transients are normalised for ease of comparison. .................................................................... 88

Fig 5.11 TPC from solid SP Si-doped hematite photoanodes (~1 μm thick) under 355 nm (purple) and

525 nm (grey) excitation, illuminated SE (left) and EE (right). The photoanodes were held at positive applied

bias (0.5 VAg/AgCl) where photocurrent is approximately saturated; similar decays are observed at potentials

just anodic of the photocurrent onset. The number of photons absorbed was ~3.0x1018

cm-3

in each

measurement. Photocurrent transients are normalised for ease of comparison; inset: data before

normalisation. .................................................................................................................................................. 89

Fig 5.12 TA spectra of nanostructured CVD Si-doped hematite photoanodes at +0.4 VAg/AgCl (i.e. the

spectra of photogenerated holes) under 355 nm and 525 nm SE excitation. Spectra under EE excitation are

very similar, especially at long timescales. ...................................................................................................... 91

Fig 5.13 TA decays of holes (probed at 650 nm) photogenerated by 525 nm excitation (SE, 0.16 mJ.cm-2

)

in nanostructured CVD Si-doped hematite photoanodes as a function of applied bias, from -0.3 VAg/AgCl

(grey) to +0.4 VAg/AgCl (red). Dynamics under EE excitation are similar. ......................................................... 91

11

Fig 5.14 TA decays of holes (probed at 650 nm) photogenerated by 525 nm (red curves) and 355 nm

(blue curves) excitation (SE, number of photons absorbed matched) in nanostructured CVD Si-doped

hematite photoanodes at 0 VAg/AgCl (left), and +0.4 VAg/AgCl (right). ................................................................ 92

Fig 5.15 TA decays of holes (probed at 650 nm) photogenerated by 525 nm (red curves) and 355 nm

(blue curves) excitation (SE, number of photons absorbed matched) in thick solid SP Si-doped hematite

photoanodes at 0 VAg/AgCl (left), and +0.5 VAg/AgCl (right). ............................................................................... 93

Fig 6.1 (a) Current/voltage curves from nanostructured Si-Fe2O3 photoanodes (in 0.1M NaOH, pH ~12.8,

white light illumination, 10 mV.s-1

). (b) Chopped light photocurrent transients from nanostructured Si-Fe2O3

photoanodes at +0.2 VAg/AgCl (355 nm illumination). ........................................................................................ 98

Fig 6.2 TA decay dynamics of Si-Fe2O3 CVD photoanodes under EE 355 nm pulsed excitation (0.20

mJ.cm-2

) probed at 650 nm (positive signal) and 575 nm (negative signal) at +0.1 VAg/AgCl (green) and +0.4

VAg/AgCl (orange). The “fast decay phase” probed at 650 nm and the bleach probed at 575 nm occur on the

same timescale (1 μs to ~20 ms). ................................................................................................................... 99

Fig 6.3 Transient absorption spectra of Si-Fe2O3 photoanodes at (a) -0.7 VAg/AgCl and (b) +0.4 VAg/Agcl at

10 μs, 100 μs, 1 ms, 10 ms, 100 ms and 1 s (black through blue to grey) after the excitation pulse. EE 355

nm pulsed excitation (0.20 mJ.cm-2

). ............................................................................................................... 99

Fig 6.4 Decay dynamics of Si-Fe2O3 photoanodes probed at 575 nm as a function of applied bias, (a)

from -0.7 to +0.4 VAg/AgCl (black through blue to brown); (b) focusing on -0.7 to -0.3 VAg/AgCl, showing that the

decay dynamics are identical cathodic of -0.4 VAg/AgCl. .................................................................................. 100

Fig 6.5 Overlay of long-lived hole population (given by the amplitude of the transient decay at 100 ms

probed at 650 nm), magnitude of the bleach (probed at 10 μs at 575 nm, inverted and multiplied by 0.1 for

ease of comparison) on the photocurrent density curve (355 nm EE excitation). ......................................... 101

Fig 6.6 Inverted TPC decays (grey, black) overlaid on transient absorption decays (green, orange)

probed at 575 nm under applied bias at +0.1 and +0.4 VAg/AgCl. Si-Fe2O3 CVD photoanodes under 355 nm

EE pulsed excitation. ..................................................................................................................................... 102

Scheme 6.1 Effect of applied bias on trap state and transient absorption bleach (probed at 575 nm). At

negative applied bias, the mid-bandgap state is occupied by electrons, so acts as a hole trap (recombination

centre); a positive transient absorption signal is observed. At positive applied bias, the Fermi level lies below

the trap state, which acts as an electron trap; a negative transient absorption signal (bleach) is observed.

Detrapping of electrons and extraction to the external circuit results in the recovery of the bleach. ............ 105

Scheme 6.2 Effect of applied bias on occupancy of the trap state probed at 575 nm. When the Fermi

level lies above the mid-bandgap state, this state is occupied by electrons (reduced), so acts as a hole trap

(recombination centre). When the Fermi level lies below the trap state is oxidised and acts as an electron

trap. Positive bias increases the width of the space charge layer (lowers the Fermi level in nanoparticulate

films), so more trap states are oxidised. ........................................................................................................ 106

Fig 7.1 Current/voltage curves (in 0.1M NaOH, pH ~12.8, white light illumination, 10 mV.s-1

) of Si-Fe2O3

APCVD photoanodes (dark grey), after Co-treatment with Co(NO3)2 (blue), and after repeated Co-treatment

(pale blue) for SE (dashed; “back-side”) and EE (solid; “front-side”) illumination. Inset: expansion of the

photocurrent onset region. ............................................................................................................................. 114

Fig 7.2 Chopped light photocurrent transients from nanostructured Si-Fe2O3 photoanodes before and

after Co-treatment, at +0.2 VAg/AgCl (355nm EE illumination). SE illumination gives similar results but with

lower photocurrent densities. ......................................................................................................................... 115

12

Fig 7.3 Transient absorption decays of isolated Si-Fe2O3 photoanodes before (black) and after (coloured)

Co2+

-adsorption (355nm 0.20 mJ.cm-2

EE excitation, 0.1M NaOH, no applied bias), probed at (a) 575 nm (b)

650 nm and (c) 900 nm. ................................................................................................................................ 115

Fig 7.4 Charge carrier dynamics of photogenerated holes in Si-Fe2O3 photoanodes before (black) and

after (coloured) Co-treatment (355nm 0.20 mJ.cm-2

EE excitation, 0.1M NaOH) probed at 650 nm under

applied bias: (a) -0.1 VAg/AgCl (b) +0.2 VAg/AgCl and (c) +0.4 VAg/AgCl. ............................................................. 116

Fig 7.5 TPC decays probing electron extraction from Si-Fe2O3 photoanodes before (dark colours) and

after Co-treatment (pale colours) at -0.1 VAg/AgCl (green) and +0.4 VAg/AgCl (orange). Pulsed 355nm 0.20

mJ.cm-2

EE excitation, 0.1M NaOH electrolyte. ............................................................................................ 117

Fig 7.6 Transient absorption spectra of Si-Fe2O3 photoanodes before (left) and after Co2+

-adsorption

(right) at 0 VAg/AgCl (top) and +0.4 VAg/AgCl (bottom), at 10 μs, 100 μs, 1 ms, 10 ms, 100 ms and 1 s (black

through blue to grey) after the excitation pulse. There is a striking similarity between Si-Fe2O3 at +0.4 VAg/AgCl

and Co/Si-Fe2O3 at 0 VAg/AgCl. ........................................................................................................................ 118

Fig 7.7 Transient absorption decays probed at 575 nm under applied bias at (a) 0 VAg/AgCl (just cathodic of

the photocurrent onset potential in the absence of cobalt), and (b) +0.4 VAg/AgCl, (where significant

photocurrent is generated even in the absence of cobalt). Before (black) and after (coloured) Co2+

-

adsorption; cobalt increases the magnitude of the bleach, particularly at low positive applied bias............. 119

Scheme 7.1 Proposed effect of Co2+

-adsorption/Co-Pi deposition on hematite photoanodes................ 121

13

List of Symbols and Abbreviations

a absorption coefficient

A absorbance (of light) (a.u.)

ALD atomic layer deposition

(AP)CVD (atmospheric pressure) chemical vapour deposition

APCE absorbed photon to current conversion efficiency (internal quantum efficiency)

CB conduction band

e- electron

e0 electronic charge (1.602x10-19 C)

ECB position of the conduction band edge (V)

EF Fermi level (V)

EFB flatband potential

Eg bandgap (eV)

EVB position of the valence band edge (V)

ε Dielectric constant (permittivity; C2.J-1.m-1)

ε0 Permittivity of free space (8.854x10-5 C2.J-1.m-1)

EE electrolyte-electrode illumination (i.e. from the front)

FTO fluorine-doped tin oxide

h+ hole

i current density (A.cm-2)

IPCE incident photon to current conversion efficiency (external quantum efficiency)

λ wavelength (nm)

LD Debye length (nm)

MH microwave heated

ND donor density (cm-3)

OER oxygen evolution reaction

ΔOD change in optical density (absorbance) (a.u.)

P power density of illumination (W.cm-2)

PEC Photoelectrochemical

RHE reversible hydrogen electrode

SCLJ semiconductor-liquid (electrolyte) junction

SE substrate-electrode illumination (i.e. from the back)

SP spray pyrolysis

T transmission (of light)

t time (s)

t50% lifetime (s)

TAS transient absorption spectroscopy

TPC transient photocurrent

USP ultrasonic spray pyrolysis

Von photocurrent onset potential

V potential (V)

VB valence band

14

List of Publications

1. “Dynamics of Photogenerated Holes in Nanocrystalline α-Fe2O3 Electrodes for water

Oxidation Probed by Transient Absorption Spectroscopy”

S. R. Pendlebury, M. Barroso, A. J. Cowan, K. Sivula, J. Tang, M. Grätzel, D. Klug,

J. R. Durrant

Chemical Communications 2011, 47, 716-718

2. “Activation Energies for the Rate-Limiting Step in Water Photo-oxidation by

Nanostructured α-Fe2O3 and TiO2”

A. J. Cowan, C. J. Barnett, S. R. Pendlebury, M. Barroso, K. Sivula, M. Grätzel,

J. R. Durrant, D. R. Klug

Journal of the American Chemical Society 2011, 133, 10134-10140

3. “The Role of Cobalt-Phosphate in Enhancing the Photocatalytic Activity of α-Fe2O3

towards Water Oxidation”

M. Barroso, A. J. Cowan, S. R. Pendlebury, M. Grätzel, D. R. Klug, J. R. Durrant

Journal of the American Chemical Society 2011, 133, 14868-14871

4. “Correlating Long-Lived Photogenerated Hole Populations with Photocurrent Densities in

Hematite Water Oxidation Photoanodes”

S. R. Pendlebury, A. J. Cowan, M. Barroso, K. Sivula, J. Ye, M. Grätzel, D. R. Klug,

J. R. Durrant

Energy and Environmental Science 2012, 5, 6304-6312

5. “Dynamics of photogenerated holes in surface modified α-Fe2O3 photoanodes for solar

water splitting”

M. Barroso, C. Mesa, S. R. Pendlebury, A. J. Cowan, T. Hisatomi, K. Sivula,

M. Grätzel, D. R. Klug, J. R. Durrant

Proceedings of the National Academy of Sciences 2012, in press (Early Edition Article)

15

Chapter I: Introduction 16

Chapter I: Introduction

1.1 Background and Introduction to Solar Water Splitting

With rapidly increasing concentrations of carbon dioxide and other greenhouse gases in

the atmosphere and fluctuating oil prices, it is imperative that affordable renewable, carbon-

free forms of energy are developed and commercialised within the next few years.

Producing hydrogen by splitting water using sunlight could be one solution. Such “solar-

hydrogen” could also be used as a chemical feedstock when reacted with carbon dioxide or

nitrogen,1 replacing feedstocks currently derived from fossil fuels.

Although hydrogen can be produced by the conventional electrolysis of water, this is an

energy intensive process. Using sunlight to photogenerate charge carriers in a

semiconductor electrode, which then electrochemically dissociate water is a more

environmentally friendly and potentially lower cost alternative. The advantage of solar water

splitting over using photovoltaics to drive conventional electrolysis is that the photon energy

is converted directly in to chemical energy, simplifying the device so potentially reducing

costs and increasing efficiency.

Many semiconductor materials have been investigated for use as photoelectrodes,

including metal oxide semiconductors such as TiO2, WO3, SrTiO3, Fe2O3, and small band-

gap semiconductors such as GaAs, CdSe and CdS. Although small band-gap

semiconductors absorb more of the solar spectrum, and so are potentially more efficient,

their band energies may be unsuitable for the evolution of both O2 and H2. Non-oxide

semiconductors are often severely corroded or photocorroded under water dissociation

conditions. The practicality of large-scale photoelectrochemical cells for hydrogen

production based on III-V semiconductors is also limited by the high cost of these

materials.2-4

Photodissociation of water by a semiconductor was first discovered using TiO2;5

subsequently this material has been extensively studied. However, the band gap for

anatase TiO2 is 3.2 eV (equivalent to 388 nm), so the maximum theoretical photoconversion

efficiency is only 2.2%.6 Because irradiance changes rapidly with wavelength in the UV

region of the solar spectrum, a small decrease in the size of the band gap can lead to a large

increase in the maximum theoretical efficiency. With a band gap of 2.70 eV, equivalent to

459 nm, WO3 has a maximum possible photoconversion efficiency of 4.8%. Both TiO2 and

WO3 are stable to oxygen evolution.

Unlike TiO2 and WO3, iron oxide (Fe2O3) can absorb light in the visible region of the solar

spectrum. The band gap is usually reported as between 2.0 and 2.2 eV, hence Fe2O3 can

absorb sunlight of wavelengths up to ~600nm – approximately 38% of the solar spectrum7 –

17 Chapter II: Materials & Methods

resulting in a maximum possible photoconversion efficiency of 12.9%.6 This band gap also

lies within the 2.0 – 2.25 eV range for maximum photon to chemical conversion efficiency.8

Fe2O3 is one of the smallest-bandgap semiconductors that is stable to oxygen evolution; it is

stable in neutral and basic solutions,9 and has also been reported as somewhat stable at

acidic pH.10, 11 Although Fe2O3 is an intrinsic n-type semiconductor (due to oxygen

vacancies), it can be doped to produce p-type behaviour.1, 10

In addition to its relatively small band gap and stability under water photolysis conditions,

Fe2O3 is non-toxic and formed of highly abundant elements. Photoanodes can be prepared

by a variety of techniques, including – but not limited to – spray pyrolysis,12 chemical vapour

deposition13 and atomic layer deposition.14 However, several factors limit the water photo-

oxidation efficiency, including a somewhat long absorption depth for visible light15 coupled

with a short hole diffusion length.16, 17 Hole transfer kinetics at the semiconductor-electrolyte

junction have also been reported to be relatively slow, potentially limiting water oxidation

efficiency.17-20 An anodic applied bias is necessary for water photo-oxidation to occur.

These factors are discussed in detail in Section 1.3.

1.2 Theory of Photoelectrochemical Water Splitting

The semiconductor-liquid junction (SCLJ) is analogous to the semiconductor-metal

Schottky barrier.2, 21, 22 When a semiconductor surface is brought in contact with an

electrolyte, charge is transferred between the semiconductor and the electrolyte until the

system has reached equilibrium, i.e. the Fermi levels are at the same energy. Because

there is no formal energy of states in the semiconductor band gap (although defect states

may exist there), the change in Fermi level position will be much greater for the

semiconductor than for the solution. The following descriptions are for n-type

semiconductors (such as Fe2O3), in which electrons are the majority charge carrier and the

Fermi level lies just below the conduction band edge. This results in a build-up of negatively

charged ions in solution (Helmholtz and Gouy layers) at the interface and depletion of

electrons from the near-surface region of the semiconductor. This depletion (or space-

charge) layer is part of the electric double layer at the interface. The extent of the space-

charge layer in to the semiconductor bulk depends on the semiconductor donor density (ND),

i.e. the level of doping, as shown in Equation 1.1, where ω is the width of the space-charge

layer, ΔφSC is the potential drop across the space-charge layer; other symbols have their

usual meaning. A high donor (electron) density results in a narrow space-charge layer, while

a lower donor density results in a wider space-charge layer with a weaker electric field

across it.


Δ

(1.1)

The separation of electrons and holes at the junction causes an electric field, resulting in

band bending across the space-charge layer, so electrons have higher energy at the surface

than in the bulk, as shown in Scheme 1.1. The potential drop in the space-charge layer is

determined by the difference between the Fermi levels of the solution and the semiconductor

when it is free from excess charge (i.e. no band bending). It should be noted that in

nanostructured or particulate semiconductors, no band bending will occur if the particle size

is smaller than the width of the space-charge layer.

A photon with energy greater than that of the semiconductor band-gap energy (Eg), may

excite an electron from the valence band (VB) of the semiconductor in to the conduction

band (CB), leaving a positively charged hole in the valence band. Where band bending is

present, the photogenerated charge carriers migrate (drift) in opposite directions; the

minority charge carriers (holes in an n-type semiconductor) migrate to the surface. Where

there is no band bending, charge carriers migrate by diffusion. Charge separation induces

an electric field which counteracts band bending, and raises the Fermi level. The

thermodynamic upper limit for the energy that can be extracted from the separated

photogenerated charge carriers is the difference between the Fermi levels of the

semiconductor and the solution.

In photoelectrochemical (PEC) water splitting cells, the semiconductor electrode is the

working electrode, while the counter electrode is usually platinum, as shown in Scheme 1.2.

It may be advantageous to employ semiconductor photo-electrodes for both the cathode (p-

Scheme 1.1 Semiconductor-electrolyte junction: ECB and EVB are the potentials of the conduction

and valence band edge, EF is the Fermi level, Eg is the band gap, ω is the width of the space-

charge (depletion) layer, VOC is open-circuit potential under illumination, EFn and EFp are the

electron and hole quasi-Fermi levels, respectively, under illumination, and uredox is the redox

potential of the electrolyte.

n-type

semiconductor electrolyte

Before equilibrium (dark)

ECB

EF

EVB

uredox

EF uredox

ω

ECB

EVB

After equilibrium (dark)

EFn

uredox

ω

EFp

VOC

ECB

EVB

Steady-state illumination


type) and anode (n-type). However, for research purposes a three-electrode system is

usually employed, where an electrical bias (which controls the semiconductor Fermi level) is

applied between the semiconductor working and reference electrodes, while the current

flows between the working and counter electrodes. Saturated calomel electrodes (SCE) or

Ag/AgCl (SSC) are commonly used. For comparability, potentials can be converted to those

versus the reversible hydrogen electrode (RHE) using the Nernst equation:

(1.2)

where E°ref is the standard potential of the reference electrode (approximately +0.2 VRHE

for both SCE and SSC), and E is the potential applied versus the reference electrode used.

The applied bias changes the position of the semiconductor Fermi level, resulting in a

difference between the positions of the Fermi levels of the semiconductor and the solution,

i.e. the system is no longer in equilibrium.

The flatband potential (VFB) is the position of the Fermi level at zero band bending, in

which situation charge carriers readily recombine, so there is essentially no photocurrent.

Thus the flatband potential is sometimes approximated as the onset potential of the

photocurrent (Von), assuming that VFB is high enough (see below). However, this is not

usually a valid approximation as an overpotential is often required to drive the reaction. The

flatband potentials of the valence band and conduction band are determined by the nature of

the material but also shift with pH (Equation 1.2), due to changes in the extent to which the

Scheme 1.2 Three-electrode photoelectrochemical cell, with nanostructured Fe2O3 photoanode

(working electrode), reference electrode and metal counter electrode.

sunlight

En

erg

y

CB

VB

EF

nanostructured

Fe2O3 anode

e–

h+

metal

cathode

A

electrolyte

H+

H2

H2O

O2

1.23 V

V

reference

electrode


surface is protonated. For a heavily-doped n-type semiconductor (such as Fe2O3), the Fermi

level lies close to the conduction band edge.

Conduction band electrons at the surface of the semiconductor can reduce redox couples

with a redox potential more positive than the flatband potential, i.e. ECB is a measure of the

reduction potential of photogenerated electrons. Likewise, valence band holes are capable

of oxidising couples with a redox potential more negative than the valence band flatband

potential; EVB is a measure of photogenerated holes’ oxidation potential. The largest

possible photopotential is the difference between the semiconductor Fermi level in the dark

and the flatband potential.

The overall reaction for water splitting is H2O → H2 + ½O2 (1.3)

In basic solution: anode reaction 2h+ + 2OH- → ½O2 + H2O (1.4a)

cathode reaction 2e- + 2H2O → H2 + 2OH- (1.5a)

In acidic solution: anode reaction 2h+ + H2O → ½O2 +2H+ (1.4b)

cathode reaction 2e- + 2H+ → H2 (1.5b)

There are generally considered to be three criteria to be met by a semiconductor

photoelectrode for overall water splitting. 1. The band gap must be larger than 1.229 eV (the

potential corresponding to the Gibbs free energy change for water dissociation: H2O → H+ +

OH-), but small enough to efficiently absorb sunlight. 2. The redox potentials for H+/H2 and

O2/OH- must lie within the band gap. 3. The semiconductor must be stable under

photoelectrolysis conditions. 4. The material should be abundant and cheap. Concerning

point 1, it has been estimated that a bandgap greater than ~2.5 eV is necessary for water

photolysis by an oxide semiconductor without an applied bias, in order to overcome losses.23

Additionally, Equation 1.4 suggests that water oxidation occurs via a concerted 4-hole

transfer process, however, the actual mechanism may proceed along a series of single-hole

transfer steps. The potentials for such single-hole oxidation (reduction) reactions are

significantly more positive (negative) than the equilibrium four-hole oxidation of water

(+1.229 VRHE).24 This also has some significance for the minimum band gap necessary for

water photolysis to occur without an applied bias.

If the Fermi level of the semiconductor is anodic of (lies below) the H+/H2 redox potential,

an external voltage must be applied to raise the Fermi level of the semiconductor such that

photogenerated electrons reduce protons at the counter electrode. The term photoassisted

water splitting should be used when an applied bias is necessary for the reaction to

proceed.25 The term photocatalytic water slitting is often applied to systems in which no

applied bias is necessary. Photocatalytic systems often consist of a suspension of small

semiconductor particles in solution, in which each particle simultaneously evolves hydrogen

and oxygen. The disadvantage of this is the difficulty in separating the evolved gases, which

are potentially explosive.


The activity of a photoelectrode is usually assessed by measuring the current/voltage (i/V)

curve in the dark and in the light, and by determining the IPCE (incident photon to current

conversion efficiency, i.e. external quantum yield) as a function of wavelength at a particular

potential (see Section 2.2.1). IPCE does not take in to consideration any applied bias.

There are several other methods for calculating the efficiency of water photolysis, which

have been reviewed previously6, 8 and are not considered here. The influence of the choice

of light source on measured photoconversion efficiencies for semiconductor electrodes has

been investigated by Murphy et al.6 Although artificial light sources are more convenient and

practical for measuring photoconversion efficiencies than using solar radiation, their spectra

do not accurately replicate the solar spectrum at the Earth’s surface. Efficiencies calculated

using artificial light sources are often higher than those thermodynamically possible from the

global AM1.5 spectrum (the standard solar reference spectrum). The various light sources

employed in studies reported in the literature means that direct comparison of published

results is not always possible.

Although many different semiconductor materials have been tested for water photolysis

activity, none have yet fulfilled all four of the criteria outlined above for water photolysis

without applied bias.26 An applied bias is almost always necessary for water oxidation to

occur, however this could be overcome using a tandem cell arrangement, with either a

photocathode and photoanode, or a photovoltaic solar cell.10, 27 Since the water oxidation

half-reaction involves four holes for each molecule of O2 produced, this is generally

considered to be more difficult to achieve than the proton reduction half-reaction (two

electrons per H2 molecule). Consequently, much research effort has concentrated on

photoanode materials for water photo-oxidation, i.e. n-type semiconductors such as TiO2,

WO3 and Fe2O3. Metal oxides are favoured due to their stability under water photolysis

conditions. Fe2O3 has a band-gap of ~2 eV, making it a particularly popular choice.

1.3 Literature Review: Hematite Photoanodes for Water Oxidation

1.3.1 Semiconductor properties of Fe2O3

There are several polymorphs of Fe2O3, the structural and magnetic properties of which

have previously been reviewed.28 Hematite (α-Fe2O3) has a corundum-type crystal structure

and is the most thermodynamically stable polymorph. As such, hematite is the Fe2O3

polymorph most commonly employed as a photoanode material. The crystal structure of

hematite is shown in Scheme 1.3.29 Pairs of face-sharing octahedra (Fe2O9 dimers) are

aligned along the c-axis ([001] direction). Maghemite (γ-Fe2O3; inverse spinel structure) is

metastable, transforming to hematite above 400 °C, and has also attracted some interest as

a water photo-oxidation material.


Hematite is a popular photoanode material due to its stability under water photolysis

conditions, and apparently optimal band-gap of 2.0-2.2 eV, allowing the absorption of

wavelengths up to ~600 nm. A typical hematite UV-vis spectrum is shown in Scheme 1.4a.

It was initially assumed that the bottom of the α-Fe2O3 valence band (VB) had mainly O 2p

character, while the top of the VB was mainly Fe 3d. The absorption peak at ~2.4 eV was

attributed to an Fe 3d→Fe 3d transition, while the peak at ~3.2 eV was attributed to an O

2p→Fe 3d transition.30 While the latter is a charge transfer transition so absorbs strongly,

the d→d transition absorbs more weakly (this forbidden transition is phonon-assisted;

magnetic coupling between adjacent Fe cations and hybridisation of Fe 3d/O 2p orbitals are

also likely to break the octahedral symmetry31). This results in a relatively long absorption

depth for visible light; ~100 nm for 500 nm light.15 More recent soft X-ray spectroscopy and

density functional theory (DFT) studies have indicated that the valence band consists of

strongly hybridised Fe-d and O-p orbitals.31, 32 While spectroscopic studies have suggested

that the top of the VB is strongly hybridised, DFT calculations have suggested that the band

gap is primarily between Fe-d states, with the oxygen density of states being largely >1.5 eV

below the top of the VB (Scheme 1.4).

Unfortunately the water oxidation efficiency of hematite under visible light is severely

limited by this long absorption depth coupled with a short hole diffusion length. The hole

diffusion length has been reported as 2-4 nm and ~20 nm.16, 17 This is an indicator of low

mobility and/or rapid electron-hole recombination (discussed further below). The electron

mobility in hematite is also thought to be low (0.01-0.1 cm2.V-1.s-1).33, 34 Conductivity is highly

anisotropic;35 hematite films oriented with the (001) basal plane perpendicular to the

substrate have been reported to facilitate the collection of photogenerated electrons.13

Although the valence band edge lies below (positive of) the O2/H2O redox potential and as

such is suitable for water oxidation, the conduction band edge lies ~0.4 V positive of the

H2/H+ potential; a positive applied bias is necessary for proton reduction to occur.17, 36 The

Scheme 1.3 Crystal structure of

hematite (from reference 29). Left:

unit cell showing pairs of face-sharing

octahedral aligned along the c-axis.

Right: FeO9 dimer.


kinetics of water oxidation on hematite are also thought to be sluggish (discussed in the

following section).

Nevertheless, advances in the fabrication of more complex hematite-based photoanodes

have led to significant improvements in water photo-oxidation activity (usually reported as

photocurrent density at 1.23 VRHE under simulated AM 1.5 illumination). Nanostructured

porous hematite12, 37-39 allows the use of thick films to harvest visible light whilst holes are

photogenerated close to the SCLJ. Doping is often found to increase efficiencies; it is often

suggested this is due to improved electron transport properties.13, 40, 41 Heterojunction

photoanodes have also been fabricated, employing materials with good electron transport

properties to aid electron extraction.42, 43 Various surface modifications have been used,

including thin overlayers of Al2O3 and Ga2O3, thought to reduce electron-hole recombination

by relieving lattice strain.44 A thin overlayer of p-type Fe2O3 has also been shown to increase

the water photo-oxidation activity of hematite.45 Additionally, surface modification with

various materials thought to catalyse water oxidation has led to efficiency gains.13, 46, 47

Although in all cases an applied bias was necessary for water oxidation to occur, this could

be overcome using a tandem cell arrangement.10, 27 For a more detailed review of advances

in water oxidation by hematite photoanodes, the reader is referred to a recent publication.48

1.3.2 Kinetic Studies of Water Oxidation on Fe2O3

Significant improvements in water oxidation properties of hematite photoanodes reported

in recent years have been complimented by increasingly detailed understanding of the

mechanisms and kinetics of the various processes occurring, including recombination,

electron collection, water oxidation etc. Several different photoelectrochemical and transient

optical techniques have been employed in these kinetic studies.

The actual detailed mechanism of water photo-oxidation on hematite is currently unknown.

A recent theoretical study investigated the stability of various Fe2O3 surface terminations

Scheme 1.4 (a) Typical UV-vis spectrum of hematite; (b) schematic of proposed hematite band

structure; (c) updated band structure of hematite, showing strong Fe 3d/O 2p VB hybridisation.

400 500 600 700 800 9000.0

0.5

1.0

1.5

2.0a

bs /

a.u

.

wavelength / nm

3 eV

2 eV

(a) Fe 3d

Fe 3d ?

(c)

Fe 3d-O 2p

Fe 3d

Fe 3d

O 2p

CB

VB

2 eV3 eV

(b)


under photoelectrochemical conditions.49 The ability of different surface terminations to

photo-oxidise water was assessed by calculating the free energies of reaction intermediates

in reaction mechanisms involving a series of one-electron transfer steps. Results indicated

that thermodynamically spontaneous water oxidation is only possible on certain surface

terminations; the driving force for water oxidation by valence band holes is extremely small.

No definite conclusions could be drawn regarding which electron transfer step is the rate-

determining step.

Early photoelectrochemical and impedance studies of hematite photoanodes reported low

faradaic rate constants for water oxidation (0.1-1.0 cm.s-1, cf. 103-104 cm.s-1 for TiO2 and

WO3; it should be noted that rate constants may also have units of s-1).17, 50 It was

suggested that this could be the result of an “energy mismatch” between the Fe2O3 d- and

oxygen p-orbitals (the valence band in TiO2 and WO3 is O 2p in character). Slow charge

transfer kinetics at the hematite surface are likely to result in accumulation of holes at the

Fe2O3 surface. This hole accumulation will affect band-bending and hence electron-hole

recombination.50, 51

Photocurrent transients under chopped light excitation of hematite photoanodes are often

observed.3, 17-19, 40, 43, 45, 50-58 It is generally accepted that these are associated with surface

recombination of conduction band electrons with either of surface-accumulated holes and/or

surface-bound oxidation intermediates (effectively a particular type of surface-bound hole).

The relatively long lifetime of these transients in the absence of hole scavengers (on the

order of 1 s) suggests that the electron-capture cross-section of these surface-bound

holes/intermediates is small, indicating a negatively charged ion, such as OH-, bound to an

Fe ion.51 Efficient hole scavengers, such as H2O2 and [Fe(CN)6]4-, “capture” holes at the

surface faster than H2O, so have been used to investigate the limiting processes in water

oxidation on hematite photoanodes in three-electrode PEC cells.18, 19 Assuming unity hole-

transfer efficiency to the scavenger, this technique has been used to try to separate

recombination and charge transfer limitations. Such studies provide some evidence that

slow hole transfer kinetics may limit the efficiency of water oxidation, but also that bulk

electron-hole recombination and recombination via surface states may be problematic.

Photocurrent measurements and impedance modelling of nanostructured hematite in the

presence and absence of H2O2 have been employed to try to separate the effects of bulk

and surface recombination.19 These revealed a hole injection barrier at the hematite/water

interface, which is not evident is the presence of H2O2. Water oxidation was suggested to be

limited by surface recombination at low applied bias. Surface recombination rapidly

decreased at potentials anodic of the photocurrent onset, attributed to deactivation of

surface traps. Bulk electron-hole recombination was shown to gradually decrease with

increasing positive bias, in proportion with the increasing width of the space-charge layer.


Smaller hematite particles appeared to result in reduced bulk recombination, i.e. improved

charge separation.

The effect of several fast, one-electron redox shuttles (hole scavengers) and variation in

electrolyte pH on steady-state photocurrent densities were investigated using thin, solid

hematite photoanodes.18 Similarly, this study indicated that hole transfer at the SCLJ is the

rate limiting step of water oxidation, while back electron transfer from the conduction band

and/or surface states to oxidised surface species (i.e. surface recombination) is likely to be

the dominant loss pathway which limits the open circuit photovoltage.

Several other frequency-domain studies of water oxidation on hematite have been

published during the course of the research reported in this Thesis. Techniques include

(photo)electrochemical impedance spectroscopy ((P)EIS),53, 55, 59, 60 intensity-modulated

photocurrent spectroscopy (IMPS),51 and potential-modulated and light-modulated

absorption spectroscopies (PMAS and LMAS, respectively).53 Further evidence for slow

hole transfer kinetics on hematite has been provided by these frequency-domain analyses.

A number of such studies have employed a model whereby both hole transfer to the

electrolyte and surface electron-hole recombination occur via the same surface states, as

shown in Scheme 1.5.51, 59, 60 The sluggish hole transfer kinetics at the semiconductor

surface (reported rate constants range from approximately 0.1 to 100 s-1, depending on

applied bias and light intensity51, 60) were reported to result in hole accumulation at the

surface. It has been suggested that these accumulated holes (likely to be surface-bound

water oxidation intermediates such as M-OHx) cause partial Fermi-level pinning.51, 59, 60

PMAS and LMAS have been used to compare the kinetics and intermediates of the

oxygen evolution reaction (OER) on hematite in the dark (PMAS) and in the light (LMAS).53

Very similar spectra of a surface-hole intermediate were obtained, indicating that the

reaction mechanism proceeds via the same intermediate in electrochemical (dark) and

photoelectrochemical water oxidation. The lifetime of this intermediate (consumed by the

Scheme 1.5 Proposed reaction scheme for water

oxidation on hematite photoanodes (adapted from

references 14 and 51).

hν

e-

h+

hole

transfer

recombination

hole flux tosurface

semiconductor electrolyte


OER or recombination with electrons) was found to be on the order of 50 ms. Under solar

light intensities, this is thought to result in a surface hole concentration on the order of 1012

cm-2, approximately 1% of a monolayer. It was suggested that the hole transfer kinetics are

at least partially limited by the low mobility of surface-trapped holes, and that the diffusion of

holes across the hematite surface is necessary in order to produce high-valent Fe-species to

drive water oxidation.

Frequency-domain analyses have also been employed to elucidate the effects of surface

modifications to hematite photoanodes. For example, EIS was used to investigate why a

thin Al2O3 overlayer results in increased water oxidation activity.55 This overlayer was shown

to reduce the resistance associated with charge transfer at the SCLJ and increase the

capacitance of the space-charge layer. Together with chopped-light photocurrent

measurements, these results indicate that the Al2O3 layer does not act as a catalyst, but

instead reduces recombination by “passivating” surface states.

IMPS has also been used to clarify the effect of Co2+ surface treatment of hematite.51

Such cobalt treatments are thought to result in the deposition of a cobalt oxide/hydroxide

species, which are known to be electrocatalysts.61 Hence the improved photocurrents

obtained from Co-treated hematite photoanodes are generally assumed to be the result of

an increase in the water oxidation kinetics. Cobalt is thought to act as a “hole reservoir” for

the four oxidising equivalents required for the production of each O2 molecule.13, 62 However,

the IMPS study demonstrated that the effect of Co-treatment is to suppress surface electron-

hole recombination; no evidence of accelerated water oxidation kinetics was found.51 This is

discussed further in Chapter VI.

Impedance-based measurements allow sample characterisation under working conditions,

and fitting of the frequency-domain response to an appropriate electrical model can provide

information about charge transport, trapping and transfer at the SCLJ, in addition to the

flatband potential and donor density (from the Mott-Schottky relation). However, such

techniques rely upon monitoring electrical outputs, and therefore cannot directly monitor

water oxidation by the minority charge carriers (holes). Additionally, impedance-based

measurements rely on fitting empirical data to a model equivalent circuit, with the results

being dependent on the model chosen.

Time-resolved surface photo-voltage measurements have recently been employed to

investigate the timescale of charge separation at the surface of a nanostructured hematite in

air.63 The accumulation of holes at the Fe2O3 surface was observed, however, this

technique is in its infancy for use in studying hematite photoanodes, and as such current

studies are extremely limited. This type of measurement has the potential to provide useful

information about the dynamics of hole accumulation and consumption at the semiconductor

surface, when applied to photoelectrodes under working conditions.


Transient Absorption Spectroscopy

Transient absorption spectroscopy (TAS) is a pump-probe technique which monitors the

change in optical transmission of a sample due to the absorption of light by photogenerated

charge carriers (electrons and holes). Absorption by charge carriers generated by the pump

beam modulates the transmission of the probe beam. The change in absorption of the

sample is thus a measure of the change in concentration (population) of photogenerated

charge carriers as a function of time after the pump pulse (see Section 2.2). Hence TAS

allows – in theory, at least – measurement of the generation, relaxation, trapping,

recombination, reaction etc of photogenerated charge carriers, depending on the timescale

of the measurement. It is thus a potentially extremely useful tool for studying charge carrier

dynamics in semiconductor photoelectrodes. Although TAS is commonly used to study dye-

sensitised solar cells (usually based on TiO2),64 until the publication of the investigations

described in this Thesis, literature reports of TAS of Fe2O3 were limited. A handful of studies

were published, however the timescales probed were in the femto- to nano-second region

and hence unlikely to be relevant to the timescale of water oxidation. Excitation densities

were typically high (thus not representative of solar irradiance) and the effect of electrical

bias had not been investigated. Nevertheless, these studies provide some information about

the charge carrier dynamics in hematite at timescales prior to those employed in these thesis

studies.

Several time-resolved optical studies of UV-excited charge carrier dynamics in Fe2O3

nanocrystal (on the order of 2-20 nm) suspensions have been reported.65-67 Generally,

decays with picosecond lifetimes were found to be independent of pump or probe

wavelength, hematite/maghemite phase, dopant, pH, or surface adsorbates. It should be

noted that changes to the surface environment are unlikely to affect charge carrier dynamics

on such short timescales. Differences in decay kinetics with excitation density and surface

adsorbates were observed by measurements on longer timescales (up to 3.5 μs) and with

greater excitation densities.67 It is often assumed that thermalised electrons in hematite

absorb in the “red” region of the spectrum while holes absorb in the “blue” region, with little

evidence to support this and somewhat arbitrary divisions between red/blue regions.

However, trapped electrons (attributed to FeII) have been shown to absorb broadly across

500-900 nm in a study of electron injection into the γ-Fe2O3 CB by pulse radiolysis; no

evidence of free CB electrons was observed.68

Transient optical measurements on femto- and pico-second timescales have also been

made of 100 nm thick hematite films on α-Al2O3/α-Cr2O3 substrates, using 407 nm excitation

to avoid absorption by the substrate.69 Differences in initial decay kinetics probed in the blue

(<560 nm) and red regions of the spectrum were attributed to hot hole and electron

relaxation, respectively. Hot CB electron relaxation to the band edge was found to occur on


timescales of hundreds of femtoseconds. Rapid electron-hole recombination and electron

trapping occurred within 5 ps. The lifetime of trapped electrons, probed at 560 and 680 nm,

was reported to be on the order of hundreds of picoseconds to nanoseconds. These results

are in contrast to those reported for nanocrystals of hematite, where trapped electrons,

probed at similar wavelengths (650-850 nm), were found to recombine on a timescale of

tens of picoseconds with no trapped charges observable beyond 100 ps.65 This extremely

rapid recombination was attributed to non-radiative recombination facilitated by a high

density of intrinsic trap states. The very different results of these two studies could be due to

stronger electron-phonon coupling in the nanocrystals,69 although differences in excitation

density may also be responsible.

Charge carrier dynamics in hematite and in α-Fe2O3/α-Cr2O3 core-shell nanoparticles

have been compared using femtosecond TAS.70 Since the CB and VB in α-Cr2O3 lie above

those in α-Fe2O3, photogenerated holes are expected to accumulate in α-Cr2O3, while

electrons should accumulate in α-Fe2O3. This improved charge separation should result in

reduced charge carrier recombination and thus longer transient lifetimes. However, very

little difference in ps decay kinetics was observed for the two systems. Similarly to the α-

Fe2O3/α-Cr2O3 films,69 electron relaxation occurred within 5 ps, and trapping on a timescale

of ~10 ps. This lack of evidence for charge separation at the interface was attributed to fast

recombination and trapping, resulting in very short free charge-carrier lifetimes.70 However,

if charge transport in these materials occurs via a “trap-detrap” mechanism (as in TiO2

nanoparticles64) rather than by free carriers, it is unlikely that such short-timescale

measurements would be able to probe differences in charge separation.

Since these thesis studies began, two further ultrafast TAS studies of hematite for water

oxidation have been reported. Hematite nanorods with a thin coating of WO3 nanoparticles

have been shown to produce greater photocurrent densities than bare hematite nanorods.43

Transient absorption decay dynamics of the bare and WO3-coated nanorods were very

similar on <5 ps timescales. The decay lifetime on 10-100 ps timescales was slightly faster

for the heterojunction when probed at 580 nm, but identical decay dynamics were observed

when probing at 675 nm. These results were interpreted as evidence that WO3 promotes

the extraction of holes from Fe2O3, although it is unclear why no change in decay dynamics

is observed at 675 nm.

Picosecond charge carrier dynamics in Sn-doped hematite nanowires with different

dopant concentrations and morphologies have been compared.39 Although varying these

factors resulted in significant differences in water oxidation efficiency, transient decay

dynamics were essentially unchanged; decay lifetimes were similar to those reported for

hematite nanoparticles.65, 66 These results indicate that picosecond timescale


measurements may not provide useful information about charge carrier dynamics pertinent

to water oxidation.

Prior to the studies described in this thesis, time-resolved optical measurements of

hematite were restricted to sub-microsecond timescales. As described above, these studies

have provided information about the timescales of hot electron relaxation to the CB edge

(hundreds of femtoseconds), and rapid electron-hole recombination and trapping (5-10 ps).

Generally charge transfer to adsorbates or heterojunction materials is not observable on

these sub-microsecond timescales.

Whilst no studies on microsecond or longer timescales had been reported for hematite,

transient absorption studies of water oxidation on nanostructured TiO2 had been published.

Charge carrier dynamics in a nanoporous TiO2 film have been investigated by using

chemical hole (methanol) and electron scavengers (Ag+ or Pt).71 In an argon atmosphere,

the transient absorption decay kinetics of photogenerated electrons and holes (probed at

800 and 460 nm, respectively) were found to be identical, indicating the absence of

quenching mechanisms other than electron-hole recombination. At low excitation densities,

the lifetime of the power-law decay decreased with increasing excitation intensity, according

to the trap-detrap model of electron transport64 (discussed further in Chapter III). In an

aqueous environment (neutral pH), the hole signal was found to decay with a half-life of 0.27

s, indicating that water oxidation occurs on a timescale of hundreds of milliseconds at neutral

pH.

1.4 Project objectives

As discussed in the previous section, although hematite has attracted considerable

research interest as a photoanode material for solar water oxidation, prior to these thesis

studies the mechanism and kinetics of water photo-oxidation on hematite were poorly

understood. The purpose of this project was to investigate the physical processes which

limit water photo-oxidation efficiencies of hematite photoanodes. Transient absorption

spectroscopy (TAS) is used to monitor the photogenerated charge-carrier dynamics in

hematite on the microsecond-seconds timescale. The timescale of these measurements is

likely to be pertinent to the timescale of water oxidation, and is significantly longer than those

of time-resolved optical studies previously reported in the literature. In conjunction with

measurements of the photocurrent response, these studies allow the comparison of charge-

carrier dynamics with water oxidation activity. Specifically, the objective of these

investigations was to explain why some types of hematite photoanode exhibit greater water

photo-oxidation activity than others, i.e. to develop structure-function relationships.


This objective can be broken down into a series of aims:

1) Identification of the transient absorption signal associated with

photogenerated holes. The decay kinetics of photogenerated holes are of greatest

interest, since the holes are responsible for water oxidation. In order to determine

the timescale of water oxidation, it is necessary to find an appropriate probe

wavelength to probe holes (preferably one where the hole signal is not overlapped

with the electron signal), which - on the timescale of these measurements - are likely

to be trapped rather than free carriers. This information is not available from the

literature, since previous studies were conducted on significantly shorter timescales,

potentially prior to hole relaxation/trapping.

2) Determination of the timescale of water oxidation on hematite. Previous

electrochemical studies of hematite photoanodes have indicated that the faradaic

rate constant for water oxidation is low. Photogenerated holes can be directly

monitored with TAS, allowing the timescales of hole recombination and transfer to the

electrolyte to be determined. The competition between the oxygen evolution reaction

and electron-hole recombination/electron-oxidation intermediate recombination is

likely to be one of the major factors limiting water oxidation efficiency.

3) Elucidation of the role of positive applied bias. An anodic applied bias is

necessary for water oxidation to occur on hematite, since the CB edge lies below the

H+/H2 redox potential. However, changing the Fermi level is also likely to affect

electron transport and electron-hole recombination. Additionally, changing the

applied bias may change the rate of water oxidation.

4) Comparison of charge carrier dynamics in various hematite photoanodes with

different morphologies, dopants etc. The relative timescales of electron

extraction, recombination and water oxidation will influence the water oxidation

efficiency of a given photoanode. TAS is used in conjunction with

photoelectrochemical measurements to investigate how differences in

nanomorphology and doping affect the relative timescales of these processes.

5) Investigation of the effect of a water oxidation catalyst on the charge carrier

dynamics and kinetics of water oxidation. Deposition of electrochemical water

oxidation catalysts on the semiconductor surface has been shown to markedly

improve the photocurrent/voltage characteristics of hematite photoanodes. However,

the mechanism of this improvement is unclear. Photogenerated hole kinetics are

monitored with TAS in order to determine whether hole transfer to the catalyst

occurs, and/or the timescale of water oxidation is reduced, or whether the catalyst

increases water oxidation efficiency by some other mechanism.


Although these studies are centred upon hematite photoanodes for water oxidation, the

techniques and methodologies outlined herein are transferrable to other semiconductor

photoelectrodes. The structure-function relationships summarised in Chapter VIII are also

likely to be applicable to many photoelectrode materials other than hematite.

Chapter II: Materials & Methods 32

Chapter II

Materials & Methods

In this section, details of the various hematite photoanodes investigated in the studies

reported in this thesis are given. The main measurement techniques used to examine these

photoanodes – photoelectrochemistry (PEC), transient absorption spectroscopy (TAS), and

transient photocurrent (TPC) – are also described.


2.1 Materials: Fe2O3 photoanodes

Most hematite photoanodes described in this section were deposited on fluorine-doped tin

oxide (FTO) glass substrates, apart from PLD films, which were deposited on indium-doped

tin oxide (ITO) glass.

2.1.1 Undoped and Si-doped APCVD hematite

Nanostructured hematite photoanodes deposited by atmospheric pressure chemical

vapour deposition (APCVD) were supplied by Michael Grätzel’s research group at EPFL.

The preparation method for the undoped and Si-doped APCVD photoanodes has been

reported in detail in the literature,13 but is described briefly here. Precursor solutions of

Fe(CO)5 and tetraethoxysilane (TEOS) are bubbled with Ar gas to create two vapour

streams, which are mixed with air and directed vertically onto an FTO glass substrate heated

to 450 °C. This forms an approximately circular spot of hematite which is thickest in the

centre (ca. 500 nm). The concentration of Si in the doped films is ca. 1.5%. Undoped

hematite is deposited from Fe(CO)5 alone. This deposition method produces nanoporous,

dendritic “cauliflower-like” nanostructured photoanodes, with a roughness factor of ~20

(Figure 1.1).

2.1.2 Undoped and doped USP hematite

Hematite photoanodes deposited by ultrasonic spray pyrolysis (USP) have a platelet-like

mesoporous structure ca. 200m thick, consisting of “leaflets” aligned perpendicular to the

FTO substrate, 5-10 nm thick and 50-100 nm in length (Figure 1.2). The USP preparation

method has been reported in detail in the literature,12 but is described briefly here. Small

droplets of the Fe(III) acetylacetonate precursor solution are generated by pumping the

solution through an ultrasonic spray nozzle. Compressed air is used to carry the precursor

droplets to a tubular oven, where they are oxidised on the surface of the heated substrate.

Nb-doping is achieved by adding 0.5% niobium ethoxide to the precursor solution. Si-doping

Fig 1.1 SEM images of Si-doped (A and B)

and undoped (C) APCVD hematite

photoanodes. A: side-view; B and C: top-down

view. From reference 13.


is achieved in a similar way. These photoanodes were made at EPFL in Michael Grätzel’s

lab, and were supplied by Dr Monica Barroso.

2.1.3 Thick solid PLD hematite

Solid (dense) hematite films approximately 600 nm thick were deposited by pulsed laser

deposition (PLD) onto ITO glass substrates heated to 550 °C. A sintered pressed-hematite

disc is used as the target, which is placed in an O2-filled chamber and irradiated with a 355

nm laser (65 mJ.pulse-1, 10 kHz). After deposition, photoanodes are sintered for 30 minutes

by maintaining the substrate temperature.72 These photoanodes were supplied by Prof

Jinhua Ye, NIMS, Japan.

2.1.4 Thin solid ALD hematite

Atomic layer deposition of hematite results in thin solid (dense) films. The thickness of the

films is carefully controlled since this method results in the deposition of a conformal layer of

hematite per ALD cycle (Figure 1.3). Nitrogen carrier gas is used to transport the ozone

and ferrocene precursors to a heated chamber where hematite is deposited on the heated

FTO glass substrate. After deposition, photoanodes are sintered at 500 °C for 30 minutes.14

These photoanodes were supplied by Prof Thomas Hamman and Ben Klahr, Michigan State

University.

Fig 1.2 Top-down SEM images of undoped (left)

and Nb-doped (right) USP hematite photoanodes.

Images courtesy of Monica Barroso.

Fig 1.3 Top-down (c) and side-view (d) SEM

images of undoped ALD hematite photoanodes.

From reference 14.


2.1.5 Porous microwave heated hematite

These photoanodes are prepared by dissolving iron nitrate and urea precursors in de-

ionised water in an autoclave at room temperature, then heating in a microwave oven at 200

°C for 20 minutes with a piece of clean FTO glass in the solution, resulting in the deposition

of red Fe2O3 powder onto the FTO. Photoanodes are then sintered at

500 °C for 30 minutes. The deposited MH iron oxide film consists of a

porous network of roughly spherical nanoparticles ca. 100-300 nm in

diameter (Figure 1.5). These photoanodes were provided by Dr

Junwang Tang, UCL.

2.1.6 Thick solid SP Si-doped hematite

Relatively dense films of Si-doped hematite approximately 1 μm thick are deposited by

spray pyrolysis from a precursor solution of 50 mM Fe(III) acetylacetonate and 1 mM

tetraethoxysilane (TEOS; Si4+ source). This technique is similar to that described in Section

2.1.2 above, but without the use of an ultrasonic spray nozzle. These photoanodes were

provided by Dr Steven Dennison and Chin Kin Ong, Chemical Engineering Department,

Imperial College.

2.1.7 Colloidal Ti-doped hematite

Porous Ti-doped hematite photoanodes consisting of a nanoporous network of 30-40 nm

particles are produced by doctor blading a colloidal film of hematite nanoparticles with a Ti4+

source (titanium isopropoxide at 5 atom % with respect to iron) onto FTO glass. This is first

annealed at 500 °C to remove organics and sinter the nanoparticles, then coated with a thin

layer of mesoporous silica as a confinement scaffold to encapsulate the nanoparticles, and

annealed briefly at 800 °C to activate the hematite. The confinement scaffold is removed

after annealing.73 These photoanodes were also provided by

Michael Grätzel’s research group at EPFL.

Fig 1.4 Top-down SEM image of undoped MH hematite photoanode.

Image courtesy of Junwang Tang.

Fig 1.5 Top-down SEM image of colloidal Ti-doped hematite

photoanode. Insert: before encapsulation. Image from

reference 73.


2.2 Methods

Hematite photoanodes were typically heat-treated before PEC, TAS or TPC

measurement, except during Co-treatment studies since heat-treatment of the Co-treated

films caused a significant and irreversible reduction in photocurrent. Photoanodes were

heated at 400 °C for ca 30 minutes in order to remove contaminants on the Fe2O3 surface.

2.2.1 PEC

A three-electrode configuration in a home-made PTFE cell with quartz windows with an

Autolab potentiostat (PGSTAT12) controlled by Nova v1.6 software was used for

current/voltage (i/V) and chronoamperometry (including IPCE) measurements. Electrolyte

solutions of 0.1 M NaOH, typically pH 12.8, were prepared from NaOH (reagent grade, used

as received from Sigma-Aldrich) and Milli-Q-water (Millipore Corp., 18.2 MΩ cm at 25 °C).

Initially, electrolyte solutions were de-aerated for ca 30 minutes prior to measurement using

nitrogen gas (BOC) however no significant differences were observed between

measurements before and after de-aeration. In later studies, the electrolyte was not de-

aerated prior to measurements. A Pt-gauze counter electrode was used, while the

Ag/AgCl/3 M NaCl reference electrode (Bioanalytical Systems Inc.) was protected from

degradation by the alkaline electrolyte by the use of a home-made double junction

configuration with a 0.5 M NaClO4 junction electrolyte (pH ~6) and a 3 Å molecular sieve

porous junction frit. Potentials are primarily reported versus the Ag/AgCl/3 M NaCl (referred

to as “Ag/AgCl”) reference electrode; these were converted to those versus the reversible

hydrogen electrode (RHE) using the Nernst equation: ERHE = E°Ag/AgCl + EAg/AgCl + 0.059pH,

where E°Ag/AgCl is the standard potential of the Ag/AgCl reference (ca. 0.21 VRHE at 25 °C),

and EAg/AgCl is the potential versus Ag/AgCl.

Scheme 2.1 Photoelectrochemical system and three-electrode cell used for current/voltage,

chronoamperometry and IPCE measurements. See text for details.

potentiostat

monochromator

photoelectrode(WE)

RE CE

Xe lamp


A 75 W ozone-free Xe lamp (Hamamatsu Photonics) was used as the light source, either

monochromated (Optical Building Blocks Corp.) or “open” (0 nm). The lamp output was

adjusted with neutral density filters such that the white light incident on the cell was

equivalent to ~1 Sun in intensity, although there are some differences between the spectral

distributions of the Xe lamp and the solar spectrum.6 The illuminated area of the Fe2O3

working electrodes was typically 0.12-0.25 cm2; approximately the same part of the Fe2O3

film was illuminated as probed during TAS/TPC measurements. For EE (“electrolyte-

electrode”, i.e. front-side illumination) measurements, a piece of blank substrate was used

as a filter to ensure that the same light intensity was incident on the Fe2O3 for both EE and

SE (“substrate-electrode”, i.e. back-side illumination) measurements. Neutral density filters

were used to modify the output of the lamp for light intensity studies. The cell was allowed to

equilibrate for 10-30 minutes before measurements. A diagram of the system used for PEC

measurements is shown in Scheme 2.1, and example i/V curves of a hematite photoanode

in the dark and under illumination are shown in Figure 2.1.

IPCE (incident photon to current conversion efficiencies) were calculated from steady-

state photocurrent measurements under monochromatic light. For each wavelength, the

dark current was allowed to stabilise before the photoanode was illuminated (typically

around 60 s). The stabilised steady-state photocurrent (typically 60-120 s after illumination

began) was used for IPCE calculations. The absolute light intensity of the incident light was

measured using a calibrated photodiode (Thorlabs S120 UV-sensor). IPCE values were

calculated as a function of wavelength using Equation 2.1, where h is Planck’s constant

(6.626x10-34 m2.kg.s-1), c is the speed of light in a vacuum (2.998x108 m.s-1), e is the

Fig 2.1 Example of current/voltage curve of a hematite photoanode in the dark (dark grey) and

under white light illumination (light grey). The dark current onset potential is ~0.65 VAg/AgCl, while

the photocurrent onset potential is ~0 VAg/AgCl. Nanostructured Si-doped CVD hematite

photoanode under EE (“front-side”) illumination at approximately 1 Sun intensity, in 0.1 M NaOH.

-0.2 0.0 0.2 0.4 0.6

0

1

2

3

4

cu

rre

nt

den

sity /

mA

.cm

-2

bias / V vs Ag/AgCl

white light

dark


electronic charge (1.602x10-19 C), F is the incident photon flux at wavelength λ (nm), I is the

intensity (mW.cm-2) of incident light, and iph is the photocurrent density (mA.cm-2). The

intensity of the incident light was corrected for absorption by the quartz window of the PEC

cell.

(2.1)

2.2.2 TAS

Transient absorption spectroscopy is a pump-probe technique which monitors the change

in optical transmission of a sample due to the absorption of light by photogenerated charge

carriers (electrons and holes). A short, relatively intense pulse of light (the “pump”, typically

from a laser) is used to excite electrons across the band gap of the semiconductor sample.

The transmission of a second, weaker “probe” beam (typically a tungsten or xenon lamp,

often monochromated) through the sample is monitored by a photodiode detector linked via

an oscilloscope and/or data acquisition (DAQ) card to a computer. Absorption by charge

carriers generated by the pump beam modulates the transmission of the probe beam. The

change in absorption (ΔOD) of the sample is thus a measure of the change in concentration

(population) of photogenerated charge carriers as a function of time after the pump pulse.

Hence transient absorption spectroscopy allows – in theory, at least – the generation,

relaxation, trapping, recombination, reaction etc of photogenerated charge carriers to be

monitored, depending on the timescale of the measurement.

The change in optical density (absorption) of a sample is determined by measuring the

current generated by the photodiode as a function of time, which is proportional to the

amount of light transmitted through the sample:

(2.2)

where V is the voltage measured by the photodiode and T is the transmission of the

sample at time t after excitation by the laser pulse. Since T = 10-A (where A is the absorption

of the sample):

(2.3)

For very small value of ΔOD (the difference in absorption between the ground and excited

states), we can approximate this to

(2.4)


Hence

(2.5)

Two different TAS systems were used to acquire the data discussed in this Thesis. A

general schematic of the TAS systems is shown in Scheme 2.2. The first is a microsecond-

millisecond system employing a nitrogen laser (GL-3300, Photon Technology International

Corp.; 337 nm, 1 ns pulse width) and 100 W tungsten lamp (Bentham 1IL) as the pump and

probe, respectively. The pump illumination was directed to the sample via a liquid light

guide. Wavelength selection was achieved using monochromators before and after the

sample. The photocurrent from the detectors was processed by an AC-coupled pre-amplifier

to extract the transient signals, which were magnified by a home-built amplifier-filter system.

The signals were recorded with a digital oscilloscope (Tektronix TDS220) and transferred to

a computer for analysis. The DC offset of the photocurrent from the detector was subtracted

using the pre-amplifier, thus small absorbance changes (<10-5) could be measured. The

output from the light guide was measured using a Coherent energy meter; the intensity of

the pump illumination on the sample was modified by placing neutral density filters placed

between the light guide and the sample.

The second TAS system was used for studies of hematite photoanodes in a complete

photoelectrochemical (PEC) cell on microsecond-second timescales. Bandgap excitation

was achieved using the third harmonic of a Nd:YAG laser (Surelite I-10, Continuum; 355nm,

6 ns pulse duration) transmitted through a liquid light guide. The output from the light guide

was measured using a Newport power meter; and was varied using an iris before the light

guide and by adjusting the Q-switch timing on the laser. A 75 W Xe lamp (Hamamatsu

Photonics) was employed as the probe beam, with monochromators before and after the

sample. In order to measure on the seconds timescale, a home-built feedback loop was

used to correct the variation in the Xe lamp output at these timescales. A home-built pre-

Scheme 2.2 General schematic of the transient absorption systems employed - see text for details.

probeXe lamp

quartz cuvette/ PEC cell

monochromator monochromator detector

oscilloscope/ DAQ card

computer

1E-6 1E-5 1E-4 1E-3 0.01 0.1 10.00

0.25

0.50

0.75

1.00

1.25

m

OD

time / s


amplifier was used to process the photocurrent from the silicon PIN photodiode detector

(Hamamatsu Photonics). The microsecond timescale signal was recorded with a digital

oscilloscope (Tektronix TDS220) and the millisecond-second timescale signal was

processed by a DAQ card (National instruments, NI-6221). A three-electrode cell controlled

using a Ministat 251 (Thompson Electrochemical) was used for measurements with bias

applied to the photoanode. A borosilicate glass cell was used with the same reference and

counter electrode configuration as described above; the reference electrode was masked to

prevent damage by UV-irradiation. The i/V curves in the dark and light were checked

immediately before TAS measurements commenced.

Laser intensities were relatively low (typically ca. 200 μJ.cm-2 incident on the cell, except

for excitation intensity studies) in order to be more comparable to solar irradiation. Laser

repetition rates (0.25-2.0 Hz) were chosen such that the transient absorption signal decayed

to zero before the next laser pulse. Data were collected on the timescale of 1 μs to 2s. TA

decays were measured by averaging over 300-1000 laser pulses, allowing signals on the

order of 10-5 ΔOD to be acquired.

2.2.3 TPC

Transient photocurrent (TPC) measurements were made using the same microsecond-

second TAS system and three-electrode cell as described above, but with the probe beam

blocked. An oscilloscope probe (Tektronix TekP6139A) and digital oscilloscope (Tektronix

TDS220) were used to measure the voltage drop (as a function of time after each laser shot)

across an external measurement resistor (typically 47 Ω) in series with the counter electrode.

In order to acquire the full rise and decay of the signal with good time resolution, each TPC

curve was compiled of 2-3 measurements at different timescales. Each timescale was

measured by averaging over 100-750 laser pulses, such that the conditions used were

comparable to those for TAS. The raw data were converted from volts to current using

Ohm’s Law to give photocurrent as a function of time after the laser pulse.

It should be noted that the time constants associated with both the photocurrent rise and

decay (at early timescales) are likely to be limited by the measurements resistor. The time

constant of the photocurrent decay is given by

(2.6)

where CSCL is the capacitance of the space-charge layer, and the bracketed terms give the

total resistance of the system, typically consisting of the resistance of the measurement

resistor, the electrolyte and the semiconductor. The early timescale (e.g. <100 μs)

photocurrent signal may also be limited by the potentiostat response.

TPC measurements of thin hematite photoanodes employing measurement resistors from

100Ω to 1 kΩ are shown in Figure 2.2. It is clear that although the TPC response at


timescales <30 μs is dependent on the resistance of the measurement resistor, the

photocurrent decays are independent of Rmeasure.

These results suggest that the TPC decay lifetime on timescales greater than a few tens

of microseconds is limited by the resistance(s) associated with the hematite photoanode (i.e.

not RC-limited by Rmeasure). The measurement resistors employed in these thesis studies are

47 to 220 Ω. These resistances are significantly smaller than those reported for hematite

photoanodes from impedance studies. Klahr et al have reported the total resistivities (due to

photoanode and cell) for 60 nm thick ALD hematite photoanodes in pH 13.3 KOH, which

correspond to resistances of approximately 2x105 to 6x103 Ω at applied potentials of 0 to 0.6

VAg/AgCl. These values were obtained under one sun intensity illumination; resistivities

increased with decreasing light intensity.59 Le Formal et al employed impedance

spectroscopy to determine resistances and capacitances associated with the semiconductor

“bulk” and charge transfer at the SCLJ for APCVD Si-doped hematite photoanodes in pH

13.6 NaOH.55 The approximate capacitance and resistance values reported are tabulated

together with the RC time constants calculated from these in Table 2.1. Again, the

resistances associated with the photoanode are significantly larger than those of Rmeasure

employed in these thesis studies.

Potential

/ V vs RHE RSC / Ω CSC / F τSC / s RCT / Ω CCT / F τCT / s

0.8 103 2.5x10-4 0.25 105 2.5x10-4 2.5

1.4 103 1.5x10-4 0.15 105 1.5x10-4 1.5

1E-7 1E-6 1E-5 1E-4 1E-3 0.01

0.0

0.2

0.4

0.6

0.8

1.0

1.2cu

rre

nt

/ m

A

time / s

100

1 k

+0.2 VAg/AgCl

1E-7 1E-6 1E-5 1E-4 1E-3 0.01

0.0

0.5

1.0

1.5

2.0

2.5

cu

rre

nt / m

A

time / s

100

220

1 k

+0.6 VAg/AgCl

Fig 2.2: Transient photocurrent (TPC) decays of a 57 nm thick ALD hematite photoanode, with

various measurement resistors: 100 Ω (black), 220 Ω (blue), and 1 kΩ (red) at 0.2 and 0.6 VAg/AgCl in

0.1 M NaOH (pH ~12.8). Pulsed EE excitation (355 nm, 0.20 mJ.cm-2

, 0.25 Hz).

Table 2.1: Approximate reported resistances (R) and capacitances (C) of the semiconductor (SC

subscript) and charge-transfer at the SCLJ (CT subscript) from impedance measurements of Si-

doped hematite APCVD photoanodes (from reference 55), with calculated time constants (τ).

Chapter III: Identification of Photogenerated Hole Absorption 42

Chapter III

Identification of

Photogenerated Hole Absorption

in Hematite Photoanodes

In which the initial results of transient absorption studies of hematite on the microsecond

to second timescale are discussed. The effect of various chemical scavengers on an

isolated hematite film, and the effect of applied bias on a hematite photoanode in a complete

photoelectrochemical cell are investigated. This allows the identification of the transient

absorption signals due to the long-lived holes that are responsible for water oxidation.

43 Chapter III: Identification of Photogenerated Hole Absorption

3.1 Introduction

As previously discussed in detail in Section 1.3.1, nanocrystalline, mesoporous hematite

(α-Fe2O3) films are considered some of the most promising candidates for the photoanode

material of a photoelectrochemical (PEC) water oxidation device. However, a positive

electrical bias is required for water oxidation to occur on hematite, generally considered

necessary to increase the reduction potential of electrons for proton reduction. A thorough

understanding of charge carrier dynamics and the effect of applied electrical bias are crucial

to devising strategies for enhancing the performance of such photoanode materials.

Photoelectrochemical and impedance methods are typically based on measurements of

electrons extracted from the photoanode to the external circuit (in the case of n-type

semiconductors, which normally have poor hole conductivities). As such, monitoring the

hole dynamics critical for water oxidation presents a significant challenge. Transient

absorption spectroscopy (TAS) is an alternative technique which allows monitoring of – at

least in principle – the recombination, trapping and reaction of both photogenerated

electrons and holes. As such it is an extremely useful tool for probing the charge carrier

dynamics of photoelectrode materials. Until recently TAS was rarely used to study

photoelectrode materials for PEC cells, thus the dynamics of charge separation, transport

and recombination in hematite were poorly characterised prior to these thesis studies.

Transient absorption studies of water oxidation by TiO2 have previously been conducted

by members of our group. The transient absorption spectra of the photogenerated electrons

and holes in TiO2 were identified using Pt and methanol as electron and hole scavengers

respectively.71 Photogenerated hole absorption peaks at 460 nm, while electron absorption

increases with increasing wavelength up to 1000 nm. On the microsecond to second

timescales measured, the photogenerated charge carriers are expected to reside in trap

states. High excitation densities resulted in more rapid electron-hole recombination and

suppressed quantum yield for oxygen production, consistent with the trap-detrap model of

electron transport.64 Recently, TAS has also been used to study the mechanism of water

oxidation on TiO2 in a complete PEC cell for the first time.74 Water oxidation was found to

occur on a timescale of hundreds of milliseconds under positive electrical bias. Comparison

of the electron collection efficiency – from transient photocurrent measurements – and the

quantum efficiency for water oxidation – from dissolved oxygen measurements – showed

that the conversion efficiency of long-lived holes to O2 is approximately 100 %. This implies

that the dominant loss mechanism in water oxidation on TiO2 is charge carrier

recombination.

This chapter describes the first studies of the competition between charge carrier

recombination and water oxidation on Fe2O3, including the first transient absorption


measurements of hematite at timescales greater than nanoseconds, and under applied

electrical bias. Transient absorption spectroscopy was employed in conjunction with

photocurrent/voltage measurements of nanostructured undoped hematite photoanodes.

Relatively low excitation density conditions were employed. Initially, isolated hematite films

(i.e. not connected to a photoelectrochemical cell) were studied by TAS on microsecond-

second timescales, likely to correspond to the timescale of water oxidation. The effect of

various chemical scavengers on the charge carrier dynamics was investigated. This was

followed by TAS and photoelectrochemical measurements of hematite photoanodes in a

three-electrode cell. Evidence for two distinct photogenerated species was found, although

only one species was evident at long (seconds) timescales. Charge carrier dynamics were

found to be strongly dependent on applied bias, while water oxidation was observed to occur

on a timescale of seconds. It is suggested that positive applied bias is necessary to reduce

electron-hole recombination by lowering the background electron density, thus increasing

the lifetime of photogenerated holes such that water photo-oxidation occurs.

3.2 Experimental

Most results discussed in this chapter were obtained using undoped α-Fe2O3

photoanodes, which are prepared by atmospheric pressure chemical vapour deposition

(APCVD) and have a dendritic nanoporous structure.37 Other types of photoanodes studied

include Si-doped APCVD, and undoped and Si-doped hematite deposited by ultrasonic

spray pyrolysis (USP), which have a mesoporous “leaflet” structure.12 Undoped hematite

photoanodes prepared by microwave heating were also used as a comparison, which

consist of a porous assembly of roughly spherical particles with diameters on the order of

100 nm. The UV-vis spectra of some of these photoanodes are shown in Figure 3.1 below.

These show the typical hematite absorption spectrum, with strong absorption in the UV

region (O 2p→Fe 3d) but significantly weaker absorption in the visible (>450 nm), due to the

forbidden nature of this d-d transition.30 The absorption edge is around 600 nm; apparent

absorption at longer wavelengths indicates light scattering and/or the presence of mid-

bandgap states. This is particularly notable for the microwave deposited hematite

photoanodes.

Transient absorption measurements on the µs-s time scale were obtained using band-gap

excitation at 337 nm or 355 nm (~0.2 mJ.cm-2 after absorption by cell, 0.33-2.0 Hz), and a

monochromatic probe beam, as described in detail in the Methods section. Dark- and photo-

current/voltage curves were recorded prior to TAS measurements with applied bias, with a

150 W ozone-free Xe lamp light source (approximately equivalent to 1 Sun intensity,


although there are some differences between the spectral output of the lamp and that of the

solar spectrum6).

IPCE measurements were made as described in detail in the Methods section. Hematite

photoanodes were illuminated from the SE side (i.e. through the substrate) in a three-

electrode configuration, with 0.1M NaOH (pH ~12.8) electrolyte degassed with nitrogen.

Steady-state photocurrents under monochromatic illumination were measured at +0.4

VAg/AgCl (~1.35 VRHE), which is significantly anodic of the photocurrent onset potential.

Fig 3.1 UV-vis spectra of various hematite photoanodes employed in this study: undoped and Si-

doped nanostructured hematite deposited by atmospheric pressure chemical vapour deposition

(CVD; blue lines); mesoporous undoped hematite deposited by ultrasonic spray pyrolysis (USP,

red line) and by microwave heating (MH, green line). No correction has been made for reflection.

400 500 600 700 800 9000.0

0.5

1.0

1.5

2.0

abs /

a.u

.

wavelength / nm

CVD Fe2O

3

CVD Si-Fe2O

3

USP Fe2O

3

MH Fe2O

3


3.3 Charge carrier dynamics of isolated hematite films

The transient absorption (TA) spectrum of undoped nanoporous hematite films measured

in an argon atmosphere is shown in Figure 3.2. The TA spectrum is characterised by a long-

lived (µs-ms) absorption peak at ~575 nm and a broad tail that extends from ~650 nm to the

near IR. Thus the spectrum is clearly divided into two regions either side of 650 nm, which is

indicative of two distinct species. The transient species probed at ~575 nm are also

observed to decay faster than transients probed at wavelengths greater than 625 nm,

providing further evidence for the existence of two distinct species, as discussed further

below. Charge carrier trapping in Fe2O3 has been reported to occur on a picosecond

timescale.65, 69 The spectral features observed here (>1 μs timescales) are likely to

correspond to trapped holes and/or electrons, rather than free carriers. Typical transient

decays in the absence of applied bias (Figures 3.3a) generally show dispersive power-law-

like decay dynamics, as evidenced by the almost linear decays when shown on log-log axes.

Such power law decay kinetics are typical of bimolecular recombination in the presence of

charge trapping, as has been reported previously for nanocrystalline TiO2 films.71, 74 This

behaviour is described by the trap-detrap model, which invokes dispersive electron transport

through a disordered semiconductor.64 In this model, charge carriers become trapped in

localised states, such that charge transport occurs via the thermal excitation of carriers from

these trap states (carriers do not experience long-range forces). Thus the kinetics of charge

transport are dominated by the time constants of release from these localised states. This

Fig 3.2 Transient absorption (TA) spectra of undoped CVD hematite in an argon atmosphere and

inset: in a methanol-saturated argon atmosphere, using 337 nm SE excitation (0.20 mJ.cm-2

at

Fe2O3 surface). Spectra shown were measured 5 and 80 μs after the laser pulse.

600 700 800 900 10000.0

0.2

0.4

0.6

0.8

mO

D

wavelength / nm

5 s Ar

80 s Ar

600 700 800 900 10000.0

0.2

0.4

0.6

0.8

5 s Ar-MeOH

80 s Ar-MeOH


has been successfully modelled by a “continuous time random walk” (CTRW), in which

carriers diffuse with a step time taken from a power-law waiting-time distribution. This model

can be applied to the kinetics of scavenging and bimolecular recombination in

semiconductors. This results in the decay of electrons and holes according to Equation 3.1

at the timescales considered herein, where 0 < α < 1.

(3.1)

Fig 3.3 (a) Transient absorption decays of three different hematite films in an argon atmosphere;

inset: the same transient decays normalised and shown on log-log axes, exhibiting power-law-

like decay kinetics. TA decays probed at 600nm with 337 nm excitation (~0.2 mJ.cm-2

). (b)

IPCE spectra of the same hematite films in a three-electrode cell with 0.1 M NaOH electrolyte

(pH ~12.8), at 0.4 VAg/AgCl and under SE illumination. CVD: dendritic nanostructured Fe2O3

(undoped); USP: mesoporous “platelet” Fe2O3; MH: mesoporous Fe2O3 deposited by microwave

heating.

1E-6 1E-5 1E-4 1E-30.0

0.1

0.2

0.3

0.4

0.5

0.6 CVD

USP

MH

mO

D

time / s

(a)

1E-6 1E-5 1E-4 1E-30.01

0.1

1

350 400 450 500 550 600

0

1

2

3

4

5

6

7

CVD

USP

MH

SE

IP

CE

(%

)

wavelength / nm

(b)


Fitting a power law function to the TA decays of charge carriers in isolated Fe2O3 films,

such as those in Figure 3.3a, gives α in the range 0.42-0.50. This is consistent with

bimolecular recombination as described by the CTRW model. Although the TA decays are

well described by a power-law function over microsecond timescales, at there is some

divergence after ~500 μs. This suggests that the CTRW model does not fully describe

charge carrier recombination in hematite, particularly at long timescales. The power-law

waiting time distribution is thought to arise from the thermal activation of an exponential

distribution of trap states.64 Some deviation from this model may occur if the distribution of

trap states in hematite is not exponential. For example, several studies have suggested that

a high density of localised trap states occurs in hematite, resulting in partial Fermi level

pinning.12, 59, 75 Additionally, the CTRW model of bimolecular recombination assumes that

one of the recombining species is stationary while the other is fixed, which may not be a

valid assumption in this case.

Comparison of the transient absorption decays and IPCE (external quantum yield) spectra

shown in Figure 3.3 suggests that there is some correlation between the initial amplitude and

lifetime of the TA decays and the water oxidation efficiency. Although all three types of

photoanode are formed of undoped, porous hematite, there are significant differences in

IPCE and TA decay dynamics. The CVD Fe2O3, which has a dendritic nanostructure, has

the greatest IPCE of the three films, and also has TA decays with the highest initial

amplitude and longest lifetime. The MH Fe2O3, in contrast, has very poor activity for water

oxidation, as evidenced by extremely low IPCE values. MH Fe2O3 has TA decays with an

initial amplitude approximately one quarter that of the CVD film, and also significantly shorter

decay lifetimes. USP Fe2O3 produces IPCE values between those of the CVD and MH

Fe2O3, and while the USP TA signal intensity is significantly greater than that of the MH film,

USP photoanodes exhibit the same faster decay kinetics as the MH hematite. Greater TA

signal intensity and slower decay kinetics are both indicative of slower charge carrier

recombination. These results suggest that the greater efficiency of the CVD photoanodes is

due to slower electron-hole recombination. Although the poorer performance of the USP

and MH photoanodes may in part be due to their slightly lower light absorption, it is likely that

their performance is limited by electron-hole recombination. This is particularly true for MH

Fe2O3, which is produced by a low-temperature method so is likely to be less crystalline than

the CVD and USP photoanodes. Recombination at grain boundaries is also likely to be

more significant in the MH Fe2O3, which consists of a porous assembly of roughly spherical

particles with diameters on the order of 100 nm.

Although various ultrafast TAS studies have previously indicated that recombination

occurring on sub-nanosecond timescales limits the efficiency of Fe2O3 for water oxidation,39,

65, 69, 70 the results discussed above demonstrate that recombination on microsecond-


millisecond timescales is also important. Figure 3.4 compares the TA decays of isolated

undoped and Si-doped CVD films. Si-doping is widely reported to improve the efficiency of

hematite photoanodes; this is often attributed to improved electron transport to the back

contact.37 It is evident from Figure 3.4a that, in isolated hematite with no external circuit for

electron extraction, Si-doping does not result in significantly more intense transient

absorption signals. This indicates that approximately the same number of photogenerated

charge carriers remain at 1 μs without and with Si-doping. Indeed, Figure 3.4b shows that

Si-doping actually increases the rate of decay (but only at wavelengths >625 nm), i.e. Si-

doping causes faster charge carrier recombination in isolated hematite. This can be

explained by considering that Si acts as an electron donor, by substituting Fe3+ with Si4+.76

Fig 3.4 Comparison of TA decays of undoped (blue/red) and Si-doped (dark grey) CVD hematite

in an argon atmosphere, probed at (a) 600 nm and (b) 900 nm (EE 180 μJ.cm-2

, 337 nm

excitation). Inset: normalised log-log plots.

1E-6 1E-5 1E-4 1E-3 0.010.0

0.2

0.4

0.6

0.8

m

OD

time / s

Si-Fe2O3 600nm

Fe2O3 600nm

(a)

1E-6 1E-5 1E-4 1E-3

0.1

1

1E-6 1E-5 1E-4 1E-30.00

0.05

0.10

mO

D

time / s

Si-Fe2O3 900nm

Fe2O3 900nm

(b)

1E-6 1E-5 1E-4 1E-3

0.1

1


Increasing the background electron density is likely to increase the rate of electron-hole

recombination, thus increasing the decay rate of photoinduced transients. However, in a

complete photoelectrochemical cell, where electrons are extracted to an external circuit, Si-

doping results in significantly larger transient absorption signals, shown in Chapter IV.

There are several indications that two distinct species are probed by transient absorption

spectroscopy in the 550-1000 nm region of the spectrum. The microsecond-millisecond TA

spectrum has a large, sharp peak at ~575 nm, but a broad, less intense absorption

extending from ~625 nm to the NIR. Transients probed at ~575 nm exhibit different

behaviour to those probed at wavelengths greater than 625 nm. In addition to the difference

in effect of Si-doping on decay kinetics, transients probed at ~575 nm generally decay faster

than those probed at long wavelengths. There are also differences in excitation density

behaviour and in the effect of hydrogen peroxide as a chemical scavenger, as discussed

below.

The excitation density dependence of charge carrier dynamics in isolated undoped CVD

photoanodes is shown in Figures 3.5 and 3.6. Again, there is a clear difference in behaviour

between the ~575 nm peak and the broad absorption at wavelengths greater than 625 nm.

In the intensity range studied (27-500 μJ.cm-2), a linear excitation dependency is observed

when probing at 580 nm (Figure 3.6a). However, the number of photogenerated charge

carriers remaining tends towards saturation when probed at 650 and 900 nm (Figure 3.6b).

Additionally, while identical decay kinetics are observed over this excitation intensity range

when probing at 580 nm, higher laser intensities result in marginally faster decay kinetics

when probing at 650 nm (inset Figure 3.6c and d, respectively). A similar, although much

Fig 3.5 TA spectrum (at 5 μs after the laser pulse) of undoped CVD hematite in an argon

atmosphere, at excitation intensities of 200, 100 and 50 μJ.cm-2

(EE 337 nm excitation).

600 700 800 900 10000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

m

OD

wavelength / nm

200 J.cm-2

100 J.cm-2

50 J.cm-2


stronger, effect has been reported for nanocrystalline TiO2, which was attributed to faster

electron-hole recombination at higher laser intensities in accordance with the trap-detrap

model.71

The excitation density dependence of the charge carrier dynamics can be partially

interpreted by considering the relative numbers of states available for the two

photogenerated species to occupy. The concentration of the charge carrier (change in

absorption) probed at 580 nm is linearly dependent on the excitation density (laser intensity).

This suggests that there are many states available for this species (photogenerated hole or

electron) to occupy, so the signal does not saturate. Saturation behaviour is, however,

observed when probing at 650-900 nm, suggesting that there are a limited number of states

available for this species to occupy, such that at sufficiently high laser intensities all the

states are occupied.

Fig 3.6 Excitation intensity behaviour of undoped CVD hematite in an argon atmosphere, probed at

580 nm (left), 650 and 900 nm (right). Excitation intensities were varied between 27 and 500 μJ.cm-2

(337 nm SE excitation). The TA amplitude at 1 μs is plotted in (a) probed at 580 nm, and (b) probed

at 600 nm (green triangles) and 900 nm (red rhombuses). The same behaviour is observed at 1-80

μs. The TA decay kinetics are shown in (c) probed at 580 nm and (d) probed at 650 nm (900nm

decays are very similar to those probed at 650 nm); inset: the same decays normalised.

0.0 0.1 0.2 0.3 0.4 0.50.0

0.2

0.4

0.6

0.8

1.0

580 nm (1 s)

linear fit

mO

D

laser intensity / mJ.cm-2

(a)

0.0 0.1 0.2 0.3 0.4 0.50.00

0.05

0.10

0.15 650 nm (1 s)

900 nm (1 s)

mO

D

laser intensity / mJ cm-2

(b)

1E-6 1E-5 1E-4 1E-3 0.010.0

0.2

0.4

0.6

0.8

1.0

1E-6 1E-5 1E-4 1E-3 0.010.0

0.5

1.0

m

OD

time / s

500 J.cm-2

330 J.cm-2

160 J.cm-2

100 J.cm-2

50 J.cm-2

27 J.cm-2

(c)

1E-6 1E-5 1E-4 1E-3 0.010.00

0.05

0.10

0.15

1E-6 1E-5 1E-4 1E-3 0.011E-3

0.01

0.1

m

OD

time / s

(d)


Chemical scavengers, such as methanol (a hole scavenger) or Ag+ (an electron

scavenger) are commonly employed in order to identify transient absorption signals due to

photogenerated electrons and holes.71 However, the TA decay kinetics of hematite

immersed in argon, 0.1 M NaOH electrolyte, and various hole scavengers – including

methanol and iodide (Figure 3.7), thiocyanate and iso-propanol (not shown) – are almost

identical. The TA spectrum is unchanged in the presence of methanol, as shown in Figure

3.2. The yield and decay dynamics of photogenerated charge carriers monitored by the μs-

ms transient absorption signals at 580 nm and 900nm are essentially insensitive to chemical

scavengers. This behaviour is in stark contrast to that of TiO2, which can photo-oxidise

methanol on a timescale of nanoseconds.71 The differences in decay dynamics between

Fe2O3 and TiO2 is considered in detail in the Discussion section.

A comparison of SE (“back-side”) versus EE (“front-side”) excitation is shown for hematite

in the presence of iodide in the inset of Figure 3.7. Similar decay dynamics are observed in

the presence of other hole scavengers, including 0.1M NaOH, and 0.1M NaOH with added

methanol. In the presence of hole scavengers, the SE and EE decay dynamics are almost

identical, however, at millisecond timescales the EE curve exhibits marginally faster decay

dynamics. This is consistent with the interpretation that under EE illumination charge

carriers are generated closer to the semiconductor-liquid junction than under SE illumination

(the absorption depth of 337 nm light is ~30 nm 15), and thus have a shorter distance to

diffuse to reach the SCLJ. The small degree of scavenging observed on the millisecond

timescale suggests that the TA signal observed at this timescale may be attributed to

Fig 3.7 TA decays (probed at 580 nm; SE 337nm, 190 μJ.cm-2

excitation) of undoped CVD hematite

in an argon atmosphere, in 0.1M NaOH, and with hole scavengers including methanol (~0.75 M in

0.1M NaOH) and iodide (2 mM). Decay dynamics are also essentially identical in the presence of

thiocyanate and iso-propanol. Inset: comparison of SE and EE TA decays of hematite in aqueous KI.

10-6

10-5

10-4

10-3

10-2

0.00

0.05

0.10

0.15

0.20

0.25

m

OD

time / s

Ar(g)

NaOH(aq)

NaOH/CH3OH(aq)

KI(aq)

1E-6 1E-5 1E-4 1E-3 0.01

0.01

0.1

SE KI

EE KI


photogenerated holes. Although reproducible, this scavenging effect is almost negligible,

indicating that only an essentially insignificant fraction of photogenerated holes are

scavenged, even under EE excitation (in the absence of applied electrical bias). In an inert

Ar atmosphere, SE and EE decay dynamics are identical (not shown), since under these

conditions no scavenging occurs.

It appears that, although holes at the top of the Fe2O3 valence band (ca. +2 V versus

RHE) are thermodynamically able to oxidise iodide (E0 = +1.35 VRHE),77 thiocyanate (+1.64

VRHE)77 and methanol (+0.02 VRHE),78 in the absence of applied bias, the photogenerated

holes observed do not have a long enough lifetime to oxidise these species. When

evaluating the lack of methanol and water oxidation on the µs-ms timescales by Fe2O3 holes

it is also important to consider the thermodynamics of the one-electron intermediates in

these oxidation reactions which would lead to a considerably lower thermodynamic driving

force.24

The charge carrier dynamics in hematite in the presence of an electron scavenger were

investigated by employing silver ions under weak UV excitation (to minimise deposits of

silver metal onto the hematite surface). No deposition of silver onto the hematite surface

was observed during TA measurements of hematite in the presence of Ag+, in contrast to the

behaviour of TiO2, which turns black due to Ag-deposition after only a few tens of laser

pulses in the presence of silver ions.71 The charge carrier dynamics of the species probed at

580 nm are unchanged in the presence of silver ions, as shown in Figure 3.8. However,

when probing at wavelengths ≥650 nm, the addition of silver ions causes an increase in the

TA decay lifetime and initial signal intensity. These results indicate that photogenerated

holes are most probably probed at ~650-1000 nm, confirmed by studies under positive

applied bias (discussed below).

Fig 3.8 TA decays of an isolated undoped CVD hematite film in water (black) and aqueous AgNO3

solution (2 mM; blue/green), probed at 580 nm (left) and 650 nm (right). Charge carrier dynamics

probed at 900 nm are similar to those probed at 650 nm. EE 337nm, 90 μJ.cm-2

excitation.

1E-6 1E-5 1E-4 1E-3 0.010.00

0.05

0.10

0.15

1E-6 1E-5 1E-4 1E-3m

OD

time / s

Ag+

(aq)

H2O

650 nm

1E-6 1E-5 1E-4 1E-30.0

0.1

0.2

1E-6 1E-5 1E-4 1E-3

m

OD

time / s

Ag+

(aq)

H2O

580 nm


Using hydrogen peroxide as a scavenger results in substantial changes to the transient

absorption decay dynamics across the 550-1000 nm spectral range, as shown in Figures 3.9

and 3.10. The spectrum in the presence of H2O2 has several similarities to the spectrum

under positive applied bias (see Chapter IV), including a bleach around 575 nm and

increased long-lived signal at ~625-900 nm. The decay kinetics of signals probed ≥650 nm

are significantly retarded in the presence on H2O2, similar behaviour to decay kinetics in the

presence of Ag+. When probing around 575 nm, decay kinetics are initially identical in water

and H2O2, but at ~10 μs H2O2 causes an abrupt increase in the decay kinetics, followed by

bleaching of the signal.

Fig 3.9 TA spectrum of an isolated USP Si-doped hematite film in aqueous hydrogen peroxide

solution (~0.38 M). EE 337nm, 0.13 mJ.cm-2

excitation.

Fig 3.10 Comparison of TA decays of an isolated USP Si-doped hematite film in water (black) and in

aqueous hydrogen peroxide solution (~0.38 M; blue/red) probed at 580 nm and 900nm (EE 337nm,

0.13 mJ.cm-2

excitation). Hydrogen peroxide causes bleaching of the 580 nm signal on timescales

>10 μs, and significantly increases the amplitude of long-lived signals probed at ≥650 nm.

550 600 650 700 750 800 850 900-0.05

0.00

0.05

0.10

0.15

0.20

mO

D

wavelength / nm

1 s

10 s

100 s

800 s

1E-6 1E-5 1E-4 1E-3 0.01-0.05

0.00

0.05

0.10

0.15

0.20

m

OD

time / s

H2O

2

H2O

580 nm

1E-6 1E-5 1E-4 1E-3 0.01

1E-3

0.01

0.1

1

1E-6 1E-5 1E-4 1E-3 0.01

0.00

0.02

0.04

0.06

0.08

0.10

m

OD

time / s

H2O

2

H2O

900 nm

1E-6 1E-5 1E-4 1E-3 0.010.01

0.1

1


Hydrogen peroxide is likely to act as an electron-scavenger via the photo-Fenton reaction,

which has been shown to occur under alkaline conditions.79, 80 The photo-Fenton reaction

occurs when photo-generated Fe(II) species react with hydrogen peroxide to form hydroxide

radicals: Fe(II) + H2O2 → ∙OH + Fe(III). In this case, Fe(II) is equivalent to a photogenerated

electron in the hematite photoanode, likely to be trapped in states below the conduction

band edge. The hydroxyl radical can also be formed by the photolysis of H2O2. The

hydroxyl radical is a strong oxidising agent and as such is also likely to act as an electron

scavenger. However, hydrogen peroxide has previously been employed as an efficient hole

scavenger for hematite photoanodes under applied bias.19 The Fe2O3/H2O2 system is

complex and it is possible that electron scavenging may occur initially, followed by hole

scavenging at long timescales, for example. Different scavenging behaviour may also occur

in isolated hematite films and in photoanodes under applied bias.

Together with the similarity of the TA spectra of hematite in the presence of H2O2 and at

positive applied bias, the change in decay dynamics in the presence of H2O2 at 625-1000 nm

on microsecond-second timescales indicate that these transient measurements are probing

photogenerated holes. It is apparent that the photo-oxidation of H2O2 by these holes occurs

significantly faster than that of other hole scavengers investigated (Figure 3.7), as evidenced

by the significant change in initial signal intensity in the presence of H2O2 (Figure 3.10).

The behaviour probed at ~575 nm is more complex. If this signal were associated with

photogenerated electrons, the transient lifetime and possibly also the initial amplitude would

be expected to decrease in the presence of H2O2, concomitant with the increase in signal

intensity and lifetime observed at longer wavelengths. However, this is not the case. The

initial signal intensity and decay kinetics are essentially unchanged by H2O2, but at >10 μs

the decay kinetics abruptly accelerate and a bleach is observed. This bleach is similar to

that observed at ~575 nm under positive applied bias. The effect of applied bias on the

transient absorption dynamics of hematite photoanodes is now addressed.

3.4 Charge carrier dynamics of hematite under applied bias

In order to investigate the hole kinetics of hematite in a fully-functional PEC cell, a three-

electrode cell with Ag/AgCl/saturated KCl reference electrode, Pt gauze counter electrode

and de-aerated 0.1M NaOH electrolyte with/without ca. 0.2 mM methanol was used. Typical

current/voltage data obtained in the dark and under white light irradiation are shown in

Figure 3.11. As expected, significant photocurrent is only observed with the application of

positive potential to the hematite photoanode, assigned to water photo-oxidation. The

presence of methanol results in a shift in the photocurrent onset potential and significant

enhancement of photocurrent, assigned to methanol photo-oxidation.


Figure 3.12 shows the transient absorption decays probed at 580 and 900 nm, collected

using a complete PEC cell under 355 nm pulsed light irradiation, measured under applied

biases versus Ag/AgCl of -0.1 V (corresponding to approximately open circuit) and +0.4 V

(corresponding to significant photocurrent generation). At -0.1 V, the transient absorption

exhibits microsecond decay dynamics, very similar to those observed for isolated films (as

shown in Figure 3.7). An applied positive bias of +0.4 V results in the appearance of a much

longer lived decay phase (Figure 3.12) at both 580 and 900 nm. At early (ca 1 μs-1 ms)

timescales, a bleach (negative signal) is observed under positive applied bias at 580 nm.

This bleach is similar to that observed in the presence of H2O2, and is effectively the

inversion of the strong positive peak observed in the TA spectrum in the absence of applied

bias. This behaviour is assigned herein to the photo-oxidation and reduction of a trap state

located just below the conduction band edge, and is investigated in detail in Chapter VI.

However, at long (1 ms-1 s) timescales, TA decays probed at 580 and 900 nm exhibit very

Fig 3.11 Photocurrent/voltage curves for

CVD undoped hematite under white light

illumination (~1 Sun, SE) in 0.1M NaOH

without (black curve) and with (red curve)

ca. 0.2 mM methanol. The dark current is

negligible in the potential region shown.

Fig 3.12 TA decays of CVD undoped hematite in a three-electrode cell under applied bias,

probed at (a) 580 nm and (b) 900 nm. In 0.1M NaOH under applied bias of -0.1 VAg/AgCl

(blue/orange curve) and +0.4 VAg/AgCl (black curves). SE pulsed (0.33 Hz) 355 nm excitation;

excitation densities are matched to those in Figure 3.7.

-0.2 0.0 0.2 0.4 0.60.00

0.05

0.10

0.15

0.20

with methanol

without methanol

cu

rre

nt d

en

sity / m

A c

m-2

potential / V vs Ag/AgCl

0.0 0.5 1.0 1.5 2.0

0.000

0.005

0.010

0.015

0.020

m

OD

time / s

+0.4 VAg/AgCl

-0.1 VAg/AgCl

(b) 900 nm

1E-5 1E-4 1E-3 0.01 0.1 1

0.00

0.02

0.04

0.06

0.08

0.0 0.5 1.0 1.5 2.0

0.00

0.01

0.02

0.03

0.04

0.05

m

OD

time / s

+0.4 VAg/AgCl

-0.1 VAg/AgCl

(a) 580 nm

1E-5 1E-4 1E-3 0.01 0.1 1-0.05

0.00

0.05

0.10

0.15


similar behaviour under applied bias, indicating that in fact the same species is probed at

these timescales across the 550-1000 nm region, in contrast to the behaviour observed at

microsecond timescales. The observed increase in lifetime of these signals under applied

positive bias – effectively an electron scavenger – indicates that these long-timescale (>1

ms) TA signals are probing photogenerated holes.

Addition of methanol (a hole scavenger) to the electrolyte solution cathodically shifts the

onset potential and increases the photocurrent density (Figure 3.11). The effect of methanol

on the TA decay probed at 580 nm is shown in Figure 3.13. Under positive applied bias,

addition of methanol to the electrolyte results in a significant reduction in the lifetime of the

TA decay. The slow decay phase probed at 580 nm under +0.4VAg/AgCl can be fitted with a

stretched exponential function with a lifetime of 3±1 s, decreasing to 400±100 ms in

presence of methanol.

The appearance of a long lived transient signal under positive applied bias is strongly

indicative of the formation of long-lived holes which have avoided rapid recombination. The

faster decay of this long-lived signal in the presence of methanol indicates that the transient

lifetime is determined by surface oxidation kinetics, confirming the assignment of this signal

to surface active holes. In this context, the faster kinetics in the presence of methanol are

consistent with the relatively facile oxidation of methanol compared to water. Additional

evidence for this interpretation is given by comparing the TA decays in the presence and

absence of methanol at -0.1 VAg/AgCl (Figure 3.13b), where essentially no photocurrent is

observed. Although addition of methanol results in increased photocurrent at this potential,

the photocurrent density is extremely small (ca. 10 μA.cm-2) even in the presence of the hole

Fig 3.13 TA decays (probed at 580 nm) of undoped nanostructured hematite in a three-electrode

cell under applied bias. (a) In 0.1M NaOH under applied bias of -0.1 VAg/AgCl (blue curve) and +0.4

VAg/AgCl (black curve). Upon the addition of methanol in the positive bias condition (red curve), the

faster decay indicates the more facile oxidation of methanol by photogenerated holes. The decay of

an isolated hematite film (no bias) in 0.1M NaOH (green curve) is similar to that under negative

applied bias. (b) Comparison of TA decays with (red) and without (black) methanol at -0.1VAg/AgCl.

0.0 0.5 1.0 1.5 2.0

0.00

0.01

0.02

0.03

0.04m

OD

time / s

+0.4V

+0.4V with methanol

-0.1V

OC

(a)

0.00 0.25 0.50 0.75 1.00

0.00

0.01

0.02

0.03

m

OD

time / s

NaOH/methanol

NaOH

(b)


scavenger. The TA decays at this potential are essentially identical, with almost no long-

lived signal remaining on the seconds timescale.

It is noted that the amplitude of this long lived hole signal is only ~10% of the initial

photoinduced hole signal at 1 μs (Figure 3.7), indicating that even under positive applied

bias, the majority of photogenerated holes still undergo rapid electron-hole recombination on

the microsecond timescale. The application of a positive bias is clearly necessary even for

the oxidation of methanol, since TA decay dynamics are essentially unchanged by the

addition of this hole-scavenger in the absence of applied bias (Figures 3.2 and 3.7). The

chemical identity of these long-lived holes is not currently known, but could be for example

Fe ions bound to surface hydroxyl radicals.

3.5 Discussion

The results reported above indicate the presence of two distinct photogenerated species,

one with a narrow but intense absorption centred around 575 nm, and one that has a broad

absorption from ~625 nm to the near-IR. The bleaching behaviour of the former in the

presence of hydrogen peroxide and at positive potentials suggests that this feature is

associated with the photo-oxidation and -reduction of a trap state located just below the

conduction band edge, and is investigated in detail in Chapter VI. However, at long (1 ms-1

s) timescales under applied bias, decays probed at 580 and 900 nm exhibit very similar

behaviour, indicating that the same species is probed at these timescales across the 550-

1000 nm region. The observed increase in lifetime of these signals under applied positive

bias, and reduction in lifetime on the addition of methanol, indicate that these long-timescale

(>1 ms) signals are probing photogenerated holes. The existence of long-lived (hundreds of

milliseconds to seconds lifetime) holes is apparently necessary not only for water oxidation

to occur, but also for the oxidation of more easily oxidised hole scavengers such as

methanol. In the absence of a positive applied bias, photogenerated holes do not have a

long enough lifetime for oxidation to occur.

These results could be interpreted in terms of rapid bulk recombination, or slow hole

transfer kinetics at the semiconductor-electrolyte junction resulting in recombination of

surface-trapped holes. It is often considered that the low intrinsic faradaic rate constant for

water oxidation on Fe2O3 limits the performance of this material.17 On the other hand, fast

bulk recombination (indicated by the short hole diffusion length16, 17) is thought to prevent

holes generated in the semiconductor bulk from reaching the surface. The observation that a

positive bias is necessary to generate long lived holes capable of driving surface oxidation

reactions is consistent with the perception that rapid recombination may be the key loss

process in hematite photoelectrodes. According to Gerischer’s electric double-layer theory of


the semiconductor-electrolyte junction,2 a positive bias will increase the width of the

depletion layer and facilitate removal of electrons. However, it has been suggested that this

band bending model does not apply to the nanoporous structures investigated here.37 In the

latter case, the effect of positive bias can be interpreted as a decrease of the background

electron density throughout the film (Scheme 3.1). Under either interpretation, the overall

effect will be a decrease in electron-hole recombination, increasing the yield of longer-lived

holes, as observed in this work. This provides an additional explanation for the requirement

of applied positive bias for water photo-oxidation by iron oxide, specifically that decreased

electron density is necessary to reduce recombination and allow long-lived photoholes to

diffuse to the surface and oxidise water.13 Scheme 3.1 illustrates a system where

recombination is assumed to be mediated by free conduction band electrons, but the results

observed are also consistent with trapped-electron mediated recombination.

Electron-hole recombination has been shown to be the key loss process limiting water

photolysis by TiO2 photoelectrodes.74 Titania (specifically anatase) appears to differ from the

hematite photoanodes studied herein in that scavenging of photogenerated charge carriers –

e.g. by methanol or Ag+ – can be competitive with recombination, even in the absence of

applied bias.71 It is clear that oxidation of scavengers by photogenerated holes in hematite

is not competitive with charge carrier recombination in the absence of applied electrical bias.

Indeed, significant and rapid electron-hole recombination occurs prior to the timescale of

water oxidation, even under positive applied bias. The hole diffusion length is an indicator of

Scheme 3.1 Representation of the effect of applied positive electrical bias on the Fermi level of a

nanostructured hematite photoanode. Applied positive bias decreases the background electron

density relative to open-circuit conditions, reducing the rate of electron-hole recombination and

increasing the lifetime of photogenerated holes, allowing water oxidation to occur.


the relative significance of electron-hole recombination: more dominant recombination

results in a shorter diffusion length. The hole diffusion length in hematite is notoriously short

(reported as 2-4 nm or 20 nm,16, 17) but is significantly longer in titania.81 Recombination may

be more dominant in hematite due to some or all of the following factors. (i) Greater electron

(donor) density in hematite, resulting in faster electron-hole recombination kinetics. The

donor density in undoped hematite is typically 1017-1018 cm-3,36, 37 while anatase TiO2 varies

between 1016 to 1019 cm-3.82 Values for the dielectric constant of hematite are varied,83 but

are the same order of magnitude as those for TiO2 (anatase or rutile), so the degree of

electronic shielding is likely to be similar. (ii) A greater distance for photogenerated holes to

travel to reach the semiconductor surface, due to a longer absorption depth for incident light

in hematite. At the UV excitation wavelengths used in this study (337 and 355 nm), the

absorption coefficient (α, the reciprocal of which gives the light penetration depth) is similar

in hematite and titania.15, 83-85 (iii) Lower charge carrier mobilities in hematite. The hole

mobility in titania has been reported as 16 cm2.V-1.s-1 (at room temperature),86 while the hole

mobility in hematite is thought to be <0.1 cm2.V-1.s-1.87 Electron mobilities in hematite and

titania have been reported as 0.01-0.1 cm2.V-1.s-1 and 0.4 cm2.V-1.s-1, respectively.33, 34, 86

Theoretical calculations have also indicated that hematite hole mobilities are lower than

electron mobilities,35 in contrast to TiO2.86 However, these theoretically determined hematite

electron mobilities are several orders of magnitude smaller than those determined

experimentally.33-35 (iv) Additional relaxation pathways in hematite resulting in faster

recombination, as suggested by ultrafast TAS studies.39, 65 (v) Lower driving force for

oxidation of scavengers on hematite (since the hematite valence band is less positive than

that of anatase or rutile TiO2), or greater activation energy barrier to oxidation. (vi)

Differences in particle size: TAS studies of TiO2 (anatase) on comparable timescales to

those described herein employed nanoporous titania photoanodes with an average particle

diameter of 15 nm.71, 74 However, the hematite photoanodes employed in the studies

described herein generally have significantly larger particle sizes, on the order of hundreds

of nanometres. Significantly, the CVD hematite photoanodes have a dendritic

nanomorphology in which the smallest particles have a diameter of ca 10-20 nm. These

photoanodes give the highest efficiencies and largest transient absorption signals of the

various hematite photoanodes studied. Points (i) and (ii) are unlikely to contribute

significantly to the difference in charge carrier dynamics observed between hematite and

titania. There is little evidence in the literature for point and (iv), although this is still a valid

hypothesis. Point (v) is discussed in more detail in the following chapter. This suggests that

developing nanostructured photoanodes consisting of particles with diameters <20 nm is key

to efficient water photo-oxidation on hematite.


3.6 Conclusions

Transient absorption studies of hematite indicate that photogenerated charge carriers are

relatively long-lived, with lifetimes on the microsecond timescale, even in the absence of

applied bias or chemical scavengers. Trapped photogenerated holes absorb broadly in the

visible-NIR region; a sharp peak in the transient absorption spectrum is observed at ~580

nm. Unlike TiO2, the charge carrier dynamics of Fe2O3 are unchanged in the presence of

hole scavengers, suggesting that charge carrier dynamics are dominated by electron-hole

recombination. However under applied positive electrical bias, at which significant

photocurrent is observed, the lifetime of photogenerated holes is significantly increased,

allowing the oxidation of methanol and water to occur. Water oxidation on undoped

nanostructured hematite occurs on a timescale of seconds; very long-lived holes are

necessary for water oxidation. These results indicate that the role of a positive applied bias

is more complex than previously thought. Positive electrical bias not only increases the

reduction potential of electrons at the cathode, but also reduces the background electron

density in hematite. This results in decreased electron-hole recombination and thus

increased hole lifetime, such that photogenerated holes can diffuse to the semiconductor-

electrolyte junction and oxidise water.

Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population 62

Chapter IV

Correlation of Photocurrent with

Long-Lived Hole Population

as a Function of Applied Bias

In which the charge carrier dynamics in Si-doped nanostructured hematite photoanodes

as a function of applied electrical bias are presented. Transient absorption spectroscopy is

used to probe the photogenerated hole dynamics, while transient photocurrent

measurements follow the dynamics of electron extraction. The effect of excitation intensity

on the kinetics of recombination and water oxidation are also discussed.

63 Chapter IV: Correlation of Photocurrent with Long-Lived Hole Population

4.1 Introduction

In the previous chapter, it was demonstrated that application of a positive (anodic)

electrical bias to a hematite photoanode retards electron-hole recombination. This increases

the lifetime of photogenerated holes by several orders of magnitude, allowing water

oxidation to occur (on a timescale of seconds). This result qualitatively suggests that

photocurrent generation is dependent on the generation of long-lived holes. It is widely

recognised that electron-hole recombination is a major limitation of metal oxide – including

Fe2O3, TiO2 and WO3 – photoanodes for water photo-oxidation. Direct measurements of

such recombination losses and correlation of these losses with the photocurrent as a

function of voltage have been limited to date.

Recently, members of this group used microsecond-second TAS as a function of

temperature to determine the activation barrier to water oxidation on nanostructured

hematite and mesoporous titania photoanodes.20 There was no observable change in hole

decay dynamics in TiO2, in the temperature range 26-54 °C, indicating that any activation

barrier to water oxidation on titania is too small to be measured by this method.

Photogenerated hole dynamics in hematite, however, clearly exhibited a strong temperature

dependence. The activation energy to water photo-oxidation on hematite was determined to

be 0.45 eV, independent of the applied bias at potentials anodic of the photocurrent onset

potential. This work demonstrated that the slower water oxidation kinetics on hematite are

associated with a thermal barrier to water oxidation.

As described in section 1.3, hematite has several advantageous qualities as a photoanode

material for solar water-splitting photoelectrochemical (PEC) cells.48 However, several

factors limit the water photo-oxidation efficiency, including a somewhat long absorption

depth for visible light (ca. 100 nm for λ = 500 nm 15) coupled with a short hole diffusion

length (2-20 nm 16, 17). Hole transfer kinetics at this junction have also been reported to be

relatively slow, potentially limiting water oxidation efficiency.17-20

The highest performing hematite photoanodes reported to date (Si-doped APCVD-

deposited films with IrO2 surface-treatment) achieve incident photon to current efficiencies

(IPCEs) of ~20% at 500 nm and 50% at 300 nm.48 However, such high photo-oxidative

quantum efficiencies are typically only achieved under strong anodic bias conditions,

typically at least 1.23 V versus the reversible hydrogen electrode (RHE), which is equivalent

to the equilibrium dark water-oxidation potential. A key challenge for enhancing the

thermodynamic efficiency of water photo-oxidation by such photoelectrodes is to reduce this

requirement for anodic bias.

This chapter addresses the requirement for an anodic bias via a quantitative analysis of

the correlation between applied bias and the charge carrier dynamics within hematite


photoanodes. Transient absorption spectroscopy is used to follow the dynamics of

photogenerated holes as a function of applied electrical bias. These optical measurements

are complimented by transient photocurrent studies of electron collection by the external

circuit. This allows direct monitoring of the potential dependence of electron-hole

recombination in addition to the dynamics of water oxidation. These transient studies enable

a quantitative consideration of parameters influencing the efficiency of water oxidation by

hematite photoelectrodes. The relationship between the photogenerated hole decay

dynamics and the photocurrent density in a variety of hematite photoanodes is investigated.

Elucidation of the relative timescales of electron extraction and electron-hole recombination

lead to a greater understanding of the processes determining the efficiency of water photo-

oxidation by hematite photoelectrodes, and their dependence upon applied electrical bias.

Additionally, excitation density studies provide some insight in to the mechanism of water

oxidation on hematite.

4.2 Experimental

All results presented in this chapter were obtained using Si-doped hematite photoanodes

deposited by atmospheric pressure chemical vapour deposition (APCVD - referred to as

“CVD” herein), as described in the literature.13 For a typical UV-vis spectrum of these

photoanodes, see Figure 3.1. These photoanodes produce significantly larger photocurrent

densities and transient signals than the undoped hematite CVD photoanodes discussed in

the previous chapter. Measurements were made close to the centre of the film, at a point

where the hematite is ca. 400 nm thick.

Transient absorption spectroscopy with applied bias (on the microsecond to seconds

timescale), transient photocurrent and photoelectrochemical measurements were made as

described in the Methods section. TAS (on the µs-s time scale) and TPC measurements

were obtained using pulsed band-gap excitation at 355 nm (typically ~0.2 mJ.cm-2 after

absorption by cell, 0.25-33 Hz).

4.3 Transient absorption studies of photogenerated holes

Photogenerated holes in hematite photoelectrodes exhibit a broad photoinduced

absorption in the visible/near infrared (see Figure 4.1).20, 88 Transient signals for probe

wavelengths below 625 nm are complicated at early times by the presence of narrow,

intense optical signals from charge carriers trapped in localised intraband states, discussed

further in Chapter VI. To simplify the data reported herein, the transient absorption observed

at 650 nm is employed as a probe of photogenerated hole dynamics in such films. Similar

dynamics were however observed for all probe wavelengths between 625 and 900 nm.


The variation of the photo-hole transient absorption signal with applied electrical bias in

CVD Si-doped hematite, probed at 650 nm, is shown in Figure 4.2a. We observe

qualitatively similar behaviour for all the photoanodes that we have examined, including

doped, undoped, nanostructured and solid hematite. On the timescales of these

measurements (microseconds-seconds), the transient absorption decays exhibit two phases.

A fast phase is observed between 1 μs and ca. 20 ms, with a bias-dependent median

lifetime (t50%), assigned to electron-hole recombination, supported by evidence discussed

below. This fast phase is followed by a slower phase with a lifetime on the hundreds of

milliseconds to seconds timescale that exhibits a bias-dependent amplitude. (It should be

noted that there is some variation in the timescale of the slow transient absorption decay

phase between individual photoanodes of the same type.) We have previously shown that

the decay time of this slow phase is accelerated in the presence of methanol (consistent with

the relatively facile oxidation of methanol compared to water) indicating that this slow phase

results from a surface oxidation reaction, specifically (for aqueous electrolytes in the

absence of chemical hole scavengers) to water photo-oxidation.

Here the yield of long-lived holes as a function of applied bias is considered, before

returning to examine the fast phase decay, assigned to electron-hole recombination, in more

detail. The increasing amplitude of the slow phase of the transient absorption decay with

applied bias is indicated by the arrow in Figure 4.2a. At low applied bias (-0.4 to -0.2

VAg/AgCl) there are very few photo-holes remaining on the 100 ms timescale. As an

increasingly positive bias is applied, the amplitude of this photogenerated hole signal

increases until it saturates at ~0.4 VAg/AgCl, after which it becomes approximately constant

with bias, as illustrated in Figure 4.2b (red diamonds, measured at 100 ms). Also shown in

Fig 4.1 Transient absorption spectra in Si-Fe2O3 CVD at (a) -0.2 VAg/AgCl and (b) +0.4VAg/AgCl at

10 ms, 100 ms, 500 ms and 1 s after the excitation pulse (EE, 355 nm). At early timescales there

is a strong bleach (negative absorption) at wavelengths <625 nm. The spectrum at +0.4VAg/AgCl is

essentially the spectrum of the photogenerated holes.

600 700 800 900

-0.2

-0.1

0.0

0.1

10 ms

100 ms

500 ms

m

OD

wavelength / nm

(a) -0.2VAg/AgCl

600 700 800 900

-0.2

-0.1

0.0

0.1

10 ms

100 ms

500 ms

1 s

m

OD

wavelength / nm

(b) +0.4 VAg/AgCl


Figure 4.2b is the photocurrent density/voltage curve measured for the same photoelectrode.

It is apparent there is an excellent, quantitative correlation between the amplitude of the

long-lived hole signal (measured at 100 ms) and the photocurrent, assigned to water

oxidation. It should be noted that the small increase in photocurrent between -0.3 and 0

VAg/AgCl is scan speed dependent, and most probably does not correspond to water photo-

oxidation but rather to accumulation of holes at the surface/oxidation of trap states.55 This is

examined further in the Discussion section.

Fig 4.2 Transient absorption and photocurrent density data for a Si-doped CVD Fe2O3 film as a

function of applied electrical bias. (a) Transient absorption signals (1 μs to 2 s, EE 355 nm

excitation, probed at 650 nm) at various applied bias (in 0.1 V increments: pale grey -0.4 VAg/AgCl,

brown +0.6 VAg/AgCl). The arrow indicates increasing number of long-lived holes with increasing

positive bias at water-splitting timescales. (b) Correlation of long-lived photogenerated hole signal

amplitude at 100 ms (red diamonds) with photocurrent (blue line; under 355 nm EE illumination

(ca. 550 μW.cm-2

, giving ~54 μA.cm-2

photocurrent at 1.23 VRHE)).

-0.4 -0.2 0.0 0.2 0.4 0.6

0.000

0.025

0.050

0.075

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.6 0.8 1.0 1.2 1.4 1.6

ph

oto

cu

rre

nt

de

nsity /

mA

/cm

2

bias / V vs Ag/AgCl

h+ a

mp

litu

de

at

10

0 m

s /

mO

D bias / V vs RHE

(b)

1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

0.00

0.05

0.10

0.15

0.20

0.25

m

OD

time / s

-0.4 - +0.6 VAg/AgCl

(a)


The quantitative correlation between the amplitude of the long-lived hole signal at various

applied bias and the photocurrent/voltage curve was observed for all the semiconductor

photoanodes examined, including Si-doped and undoped nanostructured CVD films, Nb-

doped mesoporous films fabricated by ultrasonic spray pyrolysis and dense undoped films

deposited by pulsed laser deposition, shown in Chapter V. This correlation provides further

evidence for the importance of long-lived holes in driving water photo-oxidation, and

indicates that this correlation is generic to a broad range of hematite photoelectrodes.

The processes associated with the faster decay phase observed in the transient

absorption data (1 μs-20 ms, Figure 4.2a) are now considered. The median lifetime (t50%)* 89

of this fast phase is plotted versus applied bias in Figure 4.3. It is apparent that this decay

half-time increases by three orders of magnitude as the bias voltage is increased anodically,

reaching a plateau at a lifetime of ~3ms. This behaviour is analogous to that reported

previously for the bias dependence of TiO2 electron/dye-cation recombination in dye

sensitised titania films.90 As for these dye-sensitised titania films, the increase in

recombination time with increasing positive bias is due to a reduction in electron density in

the film induced by the positive bias. Similarly the plateau observed under positive bias is

assigned to the regime where the density of electrons photogenerated by the laser pulse

exceeds those present in the dark under positive bias conditions, such that further

reductions in dark electron density by more positive bias do not result in further retardation of

* The value of t50% was approximated as the time t at which the ratio [ΔOD(t, V)/(ΔOD(1 ms, V)-

ΔOD(30 ms, V))] equals 0.5.

Fig 4.3 Median lifetime (t50%)* of the fast decay phase of the transient absorption signal for

photogenerated holes from Figure 4.2a, versus applied bias.

-0.4 -0.2 0.0 0.2 0.4 0.61E-3

0.01

0.1

1

10

t 50% (

from

1 u

s)

/ m

s

bias / V vs Ag/AgCl


the observed recombination dynamics. Further support for these assignments comes from

the excitation density and photocurrent transient data reported below.

The time resolution of these TA measurements does not allow us to monitor

recombination dynamics faster than ~1 μs. There have been several reports of faster

(picosecond-nanosecond) recombination dynamics in hematite photoelectrodes,39, 65, 67, 69

although such studies were typically undertaken with much higher peak laser powers (and

therefore charge carrier densities) than those employed herein. Indeed it is already

apparent from Figure 4.2a that, at negative potentials, a significant fraction of charge

recombination occurs prior to 1 μs. It is likely that picosecond timescale measurements

probe, at least in part, geminate electron-hole recombination, whereas at the longer

timescales (>1 μs) reported herein only non-geminate recombination occurs.

4.4 Transient photocurrent studies of electron extraction

The transient absorption data reported above are assigned to the absorption of

photogenerated holes within hematite photoanodes. A transient absorption signal in the

550-950 nm region clearly assignable to photogenerated electrons has not been observed.

Instead, transient photocurrent (TPC) measurements are employed to probe the timescale of

electron extraction from hematite photoanodes, measured under the same transient

excitation conditions used for transient absorption measurements. Photocurrents measured

through the back FTO contact primarily correspond to extracted electrons, although there

may be a small cathodic contribution from back-reaction of electrons with surface trapped

holes.91 The early timescale TPC rise is likely to be limited by the measurement resistor;

only the TPC decay at longer timescales (ca. >10 μs) is considered here as this does not

appear to be limited by the measurement resistor (see section 2.2.3).

Figure 4.4 shows transient absorption decays (measuring photogenerated holes) at -0.2,

+0.2, +0.4 and +0.6 VAg/AgCl overlaid with the corresponding TPC decays (measuring

extracted electrons). The TPC signals are strikingly similar to the “fast phase” of the

transient absorption decay curves for all bias conditions, although the TPC decay tracks the

fast phase of the transient absorption decay more closely at low applied bias. Very similar

results are achieved with SE excitation. It is of particular note that at +0.4 VAg/AgCl, bias

conditions which result in the generation of the slow transient absorption decay phase

assigned to the long lived holes driving water oxidation, the TPC curve decays to zero by ca

20 ms. It is thus apparent that under bias conditions where water oxidation is observed,

electron extraction occurs on a timescale much faster than the kinetics of water oxidation.


These transient photocurrent decays can be most easily understood as monitoring the

recovery of electron density towards dark equilibrium following the pulsed excitation. This

recovery in electron density will result from both electron-hole recombination and electron

extraction by the external circuit. Electron-hole recombination will also be dependent upon

this excess electron density. The strong similarity between the TPC and TA decay kinetics

at low bias can thus be explained as follows. The “fast phase” TA signal (monitoring holes)

decays as recombination decreases the number of holes in the film. At potentials cathodic

of significant photocurrent, recombination dominates over electron extraction, so

recombination is also the main process by which the electron population decreases. Since

both the TPC and TA decays are dominated by the same process, their decay kinetics will

be similar. However, at potentials with significant photocurrent, electron extraction and

water oxidation become significant in comparison to charge recombination, resulting in a

small but notable difference between the TPC and TA decay kinetics. An additional factor

likely to impact upon the kinetics of the TPC decays may be the time taken for electrons to

move through the hematite film to the back contact, although very little difference in TPC

decay kinetics is observed between SE and EE excitation. It is noted that the TPC decay

kinetics more closely resemble the recovery of the TA bleach at ~575 nm, associated with

the photo-reduction of a particular trap state. This is discussed in detail in Chapter VI.

Fig 4.4 TPC decays (measuring extracted electrons) overlaid on corresponding transient

absorption decays (measuring photo-holes) of a Si-doped CVD hematite photoanode, EE

excitation at 355 nm, at -0.2, +0.2, +0.4 and +0.6VAg/AgCl. The TPC axis is shifted upwards and

scaled to maximise overlap with the TA decay.

-0.2 VAg/AgCl

1E-5 1E-4 1E-3 0.01 0.1 1

0.00

0.05

0.10

0.15

0.20

m

OD

time/s

0.0

0.1

0.2

0.3

TP

C / m

A

1E-5 1E-4 1E-3 0.01 0.1 10.00

0.05

0.10

0.15

0.20

0.25

m

OD

time/s

0.0

0.5

1.0

1.5

TP

C /

mA

+0.2 VAg/AgCl

1E-5 1E-4 1E-3 0.01 0.1 10.00

0.05

0.10

0.15

0.20

m

OD

time/s

-1

0

1

2

TP

C / m

A

+0.4 VAg/AgCl

1E-5 1E-4 1E-3 0.01 0.1 10.00

0.05

0.10

0.15

0.20

m

OD

time/s

-2

-1

0

1

2

3

TP

C / m

A

+0.6 VAg/AgCl


4.5 Excitation density studies

Steady state photocurrent densities measured at 0.4 VAg/AgCl anodic bias for the same

photoanode as a function of steady state light intensity are shown in Figure 4.5a. It should

be noted that the excitation density range for the transient (laser) measurements is much

greater than for the steady-state (continuous wave) measurements; only the very lowest

laser intensities are likely to correspond to the charge density regime of the steady-state

photocurrent measurements. Figure 4.5b shows transient absorption decays measured

under the same bias as a function of excitation density from 23 μJ.cm-2 to 2.21 mJ.cm-2

Fig 4.5 Steady-state photocurrent and transient absorption data for a Si-doped CVD Fe2O3

photoanode as a function of excitation intensity at +0.4 VAg/AgCl. (a) Variation of steady-state

photocurrent amplitude (under 355 nm EE illumination); the red line is the best fit to the data. (b)

Variation of transient absorption photogenerated hole signal (probed at 650 nm, EE 355 nm

excitation from 23 μJ.cm-2

(dark green) to 2.21 mJ.cm-2

(brown); the laser intensity used for the

majority of the measurements described herein is 200 μJ.cm-2

). (c) normalised slow TA phase at

125 ms - the timescale of water oxidation (2.1 s) is independent of excitation intensity. Inset:

normalised at 10 μs - the fast phase decays more rapidly with increasing excitation density. (d)

Ratio of amplitude of fast and slow decay phases of transient absorption as a function of

excitation intensity; inset: variation of amplitudes with excitation intensity. At the very lowest

excitation intensities (<200 μJ.cm-2

) we approach pseudo-first-order recombination behaviour (i.e.

within the small perturbation regime).

0.01 0.1 10.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

m

OD

time / s

(c)

1E-5 1E-4 1E-3 0.01 0.1 10.0

0.2

0.4

0.6

0.8

1.0

0 500 1000 1500 20000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

ratio fast:slow phase amplitudes

ratio

fa

st:slo

w p

ha

se

am

plit

ud

es

excitation intensity (355 nm) / J.cm-2

(d)0 1000 2000

0.00

0.25

0.50

0.75

1.00

1.25

slow decay

phase amplitude m

OD

fast decay phase amplitude

0 50 100 150 2000.00

0.25

0.50

0.75

1.00

cu

rre

nt d

en

sity / m

A.c

m-2

Light Intensity (%)

(a)

1E-6 1E-5 1E-4 1E-3 0.01 0.1 10.00

0.25

0.50

0.75

1.00

1.25

1.50

m

OD

time / s

(b)


(other data in this chapter being collected at 200 μJ.cm-2). It is apparent that as the

excitation density is increased, the fast transient absorption decay phase becomes faster

and increasingly dominant, consistent with our assignment of this decay phase primarily to

bimolecular electron-hole recombination. Of particular interest is the observation that the half

time of the slow decay phase, assigned to water oxidation by photogenerated holes, is

independent of excitation (and therefore hole) density, as shown in Figure 4.5c. This

observation is discussed further below. The linear dependence of the steady-state

photocurrent density on light intensity under these anodic bias conditions (Figure 4.5a) is

also discussed below. This again has important implications for the processes limiting

photocurrent generation under these bias conditions.

4.6 Discussion

Before discussing in detail the results of these studies, it is important to appreciate the

advantages and limitations of using transient absorption spectroscopy to monitor the yields

and dynamics of photogenerated holes. The core advantage of this technique is that it

allows the presence of photogenerated holes to be directly monitored by the observation of

their optical absorption. This differs from alternative electrochemical based techniques (such

as impendence spectroscopies and photocurrent measurements)12, 13, 17-19, 36, 37, 46, 55, 56, 58, 72,

75, 92, 93 which rely upon monitoring electrical outputs, and therefore cannot directly monitor

hole transfer at the SCLJ. This ability to directly monitor photogenerated holes is particularly

important for photoanodes as it is the photogenerated holes which drive the key electrode

function of water oxidation. However, transient absorption spectroscopy has a significant

disadvantage in that it has a limited sensitivity and therefore requires the use of relatively

intense excitation pulses to generate sufficiently high hole density to be detectable. The

transient absorption system employed for the studies described here has a particularly high

sensitivity, and the excitation densities significantly below those in other transient absorption

studies of hematite photoelectrodes. Nevertheless, our 200 μJ.cm-2 excitation pulses

correspond to absorbed photon densities of ~1019 cm-3. This charge density, while

equivalent to the background electron density of undoped (and below that of Si-doped)

hematite,37 is most probably greater than the charge carrier density generated under

continuous solar irradiation. It is important to appreciate this limitation during the following

discussion.

Using transient absorption spectroscopy to monitor photogenerated holes, and transient

photocurrent to monitor the extraction of photogenerated electrons, the charge carrier

dynamics in hematite photoanodes were observed to be strongly dependent upon applied

potential. The decay of photogenerated holes exhibits two phases, on timescales of ca 1 μs


to 20 ms and 20 ms to >2 s. The fast decay phase is assigned to electron-hole

recombination. This electron-hole recombination becomes slower as the steady state

electron density in the film is reduced under positive electrical bias. This dependence is very

strong, with a 400 mV anodic shift in applied bias resulting in a 1000-fold retardation of the

electron-hole recombination rate (Figure 4.3). The lifetime (t50%) of this fast decay phase

reaches a plateau value of ~3 ms at approximately the same potential as the photocurrent

onset occurs. Although this is likely to correspond to the regime where the density of

electrons photogenerated by the laser pulse exceeds those present in the dark under

positive bias conditions, it should be noted that this lifetime is calculated from the observed

ΔOD at 1 μs. However, recombination is also likely to occur on the nanosecond timescale,

(i.e. prior to the timescale of these transient measurements), which will also partially

determine the population of long-lived holes remaining after ~20 ms. A more accurate

assessment of t50% could be achieved by considering the initial hole signal on the

nanosecond timescale, however these data are not currently available. This may also

contribute to the plateau in t50% under the same bias conditions where the population of long-

lived holes is still increasing. Additionally, there may be some effect of the early-timescale

bleach observed at ~575 nm on the TA decays probed at 650 nm.

The transient photocurrent measurements indicate that electron collection by the external

circuit is also complete by ca. 20 ms, with kinetics similar to those of the fast transient

absorption decay phase. The slow transient absorption decay phase is assigned to residual

photogenerated holes which have not recombined with electrons. There is a quantitative

correlation between the amplitude of this slow decay phase (i.e. the population of long-lived

holes) and the electrode photocurrent monitored as a function of applied bias. The onset of

photocurrent generation at positive bias is attributed to the pronounced retardation in

electron-hole recombination with applied positive potential, allowing electron extraction to the

external circuit and leading to the generation of long-lived holes. These long-lived holes

then go on to drive water oxidation on a timescale of hundreds of milliseconds to seconds.

This analysis is consistent with a recent electrochemical impedance analysis of analogous

silicon doped hematite photoanodes,55 wherein a two-phase response was also reported; the

low frequency element was assigned to charge transfer processes at the semiconductor-

liquid junction, and the high frequency element to processes occurring within the

semiconductor, including charge transport.

The activation energies and rate constant for water oxidation on nanostructured undoped

hematite have previously been studied by this group.20 This rate constant and activation

energy for water oxidation were observed to be independent of applied bias. A similar

conclusion regarding the rate constant is also apparent for the silicon doped films employed

herein from the bias independence of the slow decay phase lifetime apparent in Figure 4.2a.


These observations indicate that the reactivity of photogenerated long-lived holes is

independent of applied bias. This conclusion is consistent with the above analysis, i.e. the

photocurrent is dependent only upon the yield of long lived holes, with this yield being

determined by kinetics of electron-hole recombination on faster timescales.

The long-lived photogenerated hole signal observed for hematite photoelectrodes was

previously assigned to holes localised at or near the Fe2O3 surface, prior to charge transfer

to surface-bound or electrolyte species.20 This charge due to holes accumulated at/near the

Fe2O3 surface after electron extraction is most probably balanced by negative ions in the

electrolyte, forming the Helmholtz layer. Under approximately 1 Sun illumination conditions,

the photocurrent density at 0.4 VAg/AgCl is ~2.1 mA.cm-2 (for these nanostructured, Si-doped

hematite photoanodes). Taking the hole lifetime (water oxidation timescale) to be 1 s, and

assuming a surface roughness of 20,13 this results in a surface hole density on the order of

1015 holes.cm-2. This interpretation is consistent with recent reports of photo-hole

accumulation at/near the Fe2O3 surface under water-photolysis conditions.18, 19, 53

It should be noted that the best correlation between the population of long-lived holes (as

determined from the amplitude of the TA signal at 100 ms) and photocurrent density is best

with an i/V scan rate of 20 mV.s-1, i.e. not the steady-state photocurrent. As the scan rate is

reduced, the amplitude of the current cathodic of the photocurrent onset potential

(associated with recombination via surface states) decreases, and the photocurrent rise

becomes more abrupt. These results suggest that the amplitude of the long-lived hole signal

actually measures the population of holes that are initially transferred to water/surface-bound

water states, and does not take account of the back-reaction of electrons with this oxidation

intermediate (evidenced by photocurrent transients in chopped-light measurements55). It is

likely that this back-reaction occurs on timescales longer than those probed by the TPC

measurements reported herein, i.e. >100 ms.

A quantitative correlation between long-lived photo-hole population and photocurrent

amplitude appears to be a common feature of several metal oxide photoanodes for water

oxidation. A similar association between long-lived holes and water oxidation has also been

observed for nanoporous TiO2 photoanodes, both in the presence of a chemical electron

scavenger71 and under anodic bias,74 and for a nanoporous WO3 film.94 The results reported

herein for different hematite photoanodes indicate that there is a general requirement for

long-lived holes in order for water oxidation to occur. This can be attributed to the slow

timescale for the reaction of these holes with water. The reaction timescale of these holes is

accelerated when oxidising chemical hole-scavengers such as methanol.88 The slow

timescale of water oxidation on hematite can be explained, at least in part, by the

observation of a significant (45 kJ.mol-1) activation barrier for this reaction.20 The activation


barrier to methanol oxidation is considerably lower, confirming that the water oxidation

timescale is limited by slow kinetics at the semiconductor-liquid junction.

It should be noted that the actual hole lifetime required for water oxidation depends on the

timescale of the oxidation reaction on a given material. For instance, the hole lifetime

required for water oxidation on nanoporous TiO2 is approximately two orders of magnitude

shorter than for nanostructured undoped α-Fe2O3 under similar conditions.74, 88 However, it

is clear from the transient measurements reported herein that the timescale of water

oxidation on nanostructured Si-doped hematite is approximately two orders of magnitude

slower than the timescale of electron extraction to the external circuit, and that significant

electron-hole recombination occurs on even faster timescales. The requirement for an

enhancement of the water oxidation rate by two orders of magnitude is a daunting task.

Hence strategies to increase the yield of long-lived holes and therefore to optimise the

performance of hematite photoanodes should focus upon decreasing the level of electron-

hole recombination, either through material design to directly retard the kinetics of electron-

hole recombination, or by accelerating the kinetics of electron extraction by the external

circuit. Indeed, electron mobility in hematite is known to be low,50 and the short hole

diffusion length16, 17 is indicative of rapid electron-hole recombination.

It is striking that, after recovery of the electron density in the film to its equilibrium value

(corresponding to when the photocurrent transient has decayed to zero at ~20 ms), the

lifetime of the remaining long-lived holes is independent of the applied potential. Given that

this applied potential is likely to modulate the average electron density in the film, this

observation strongly suggests that these long-lived holes are localised in a depletion region

(space-charge layer) generated by band bending at the semiconductor-electrolyte interface.

Although several electrochemical impedance studies of hematite in the literature have

provided information about the flatband potential and donor density of various types of

hematite photoanodes,17, 36, 37, 50, 55, 93 the quantitative extent of the depletion region through a

nanostructured photoanode is at present unclear. In semiconductor particles with one

dimension greater than the width of the space-charge layer, some degree of band bending is

likely to exist. The CVD hematite photoanodes studied herein have a dendritic

nanostructure in which the smallest feature size is ca 10-20 nm but which consist of larger

features closer to the substrate, on the order of hundreds of nanometres.13 Following the

methodology outlined by O’Regan et al,95 Equation 4.1 is used to estimate the voltage

difference between the centre of a particle and the semiconductor-liquid junction for an

undoped hematite photoanode.

(4.1)


where

is the Debye length. The radius of the particle (which is

approximated as spherical) is r, while the voltage drop across the radius of the particle is

ΔφSC. Undoped hematite has a donor (electron) density, ND ~1017-1018 cm-3 and dielectric

constant, ε ~30-60.36, 57, 83 This gives a Debye length of 4.6-21 nm. Nanostructured

undoped hematite photoanodes deposited by CVD have feature sizes in the range ca. 20-

250 nm.13 The estimates of ΔφSC for different feature sizes are shown in Table 4.1 below.

The voltage difference between the centre of a particle and the semiconductor-liquid junction

is in the region of ca. 1-20 mV for a 10 nm radius particle, or ca. 25-500 mV for a 100 nm

radius particle. These values indicate that some degree of band-bending is likely to be

present in undoped CVD hematite photoanodes; Si-doping increases the donor density to

1020 cm-3,37 thus decreasing the width of the space-charge layer (although the dielectric

constant will also change).2 Hence it is very likely that a degree of band-bending occurs in

both undoped and Si-doped hematite nanoporous CVD photoanodes.

particle radius / nm ΔφSC for LD = 4.6 nm / mV ΔφSC for LD = 21 nm / mV

10 20 1

50 500 24

125 3.2 x 104 150

The effect of applied bias on these photoanodes is most likely a combination of the classic

band-bending model of the semiconductor-electrode interface2 and that of a nanoparticulate

film with no band-bending. In particles large enough for band-bending to occur (nearer the

substrate), increasing positive applied bias increases the width of the space-charge layer so

sweeps more photogenerated holes to the SCLJ, reducing the rate of recombination and

increasing the population of long-lived holes. In smaller particles, charge carrier transport is

by diffusion only, so positive bias lowers the Fermi level, resulting in diminished background

electron density, leading to decreased recombination and longer hole lifetimes, allowing

more holes to reach the SCLJ. The UV excitation light employed here is absorbed within

~30 nm of solid hematite.15 Hence we are likely measuring a combination of the band-

bending and nanoparticulate regimes in nanoporous photoanodes, particularly with EE

(“front-side”) illumination.

As increasing positive bias is applied, the width of the space-charge layer increases, so

more holes are photogenerated within the space charge layer. The flat-band potential (i.e.

the potential at which no band-bending occurs) of hematite is reported to be ca. 0.4 VRHE,37

equivalent to ca. -0.6 VAg/AgCl under the conditions used in this study. Hence the lowest

Table 4.1 Estimated values of potential drop (ΔφSC) across the radius of spherical undoped

hematite nanoparticles of various sizes for two different Debye lengths (LD).


potential used in this study (-0.4 VAg/AgCl) is close to flat-band, where very few holes are

photogenerated in the space charge layer. At this potential, the transient absorption signal

of photogenerated holes decays rapidly; almost no photogenerated holes remain by ~200 μs

and most recombination occurs prior to the timescale of our transient absorption

measurement (Figure 4.2a). This suggests that the photogenerated holes measured on the

timescale of our transient absorption studies (microseconds-seconds) are mainly those

within the space charge layer and nanoparticulate regions of the photoanode.

Finally, the light intensity dependence of the transient and steady-state measurements is

considered. In general, water photo-oxidation by hematite photoelectrodes depends upon

two processes which might be expected to behave non-linearly with charge carrier densities,

and therefore light intensity: electron-hole recombination and water oxidation. The multi-

electron nature of water oxidation is of particular interest, with four holes being required to

drive the oxidation of two molecules of water, releasing one molecule of oxygen. There are

currently extensive efforts to develop “multi-hole” catalysts for this reaction, motivated in part

by recent advances in our understanding of the structure and function of the manganese-

based water oxidation centre of Photosystem II in higher plants.96 In this regard, it is

particularly striking that the photo-oxidation current observed under positive (anodic) bias for

these films scales linearly with light intensity. There are currently few studies reported in the

literature of the excitation intensity dependence of photocurrent densities for hematite.16, 50, 75,

97

Electron-hole recombination is expected to be a bimolecular process dependent upon

both electron and hole densities. This is supported by both the bias and excitation density

dependence of the fast transient absorption decay phase shown herein, where the rate of

recombination increases with increasing electron density. However, we note that under

steady state conditions, the electron density in the film is likely to be dominated by the dark

or resting electron density. † In this limit, the electron density in the film becomes

independent of light intensity, and electron-hole recombination can be expected to be

linearly dependent on light intensity (effectively corresponding to pseudo-first order

recombination behaviour). We note that the fast phase decay time and relative amplitude

(compared to the slow decay phase amplitude) of our transient absorption data are almost

identical for the two lowest excitation conditions, shown in Figure 4.5. This behaviour

suggests that we are approaching the pseudo-first-order limit in our transient measurements

at the lowest excitation intensities (≤200 μJ.cm-2).

Of equal, or greater, interest is the independence of the half-time of the slow decay phase

upon excitation density and therefore photogenerated hole density (estimated as ca. 1015

† This is particularly true for the steady-state (continuous wave) study reported herein where

excitation density range is likely much lower than for the transient (laser) measurements


holes.cm-2 under solar illumination – see above). The population of long-lived holes is

observed to increase almost ten-fold over the range of excitation densities employed, whilst

the decay time of these long-lived holes remains constant. This behaviour strongly suggests

that the rate determining step in the reaction of these surface trapped holes with

water/surface-bound water species is first order in hole density, and is therefore a single-

hole oxidation. This is consistent with our previous report on the thermal activation

measurements for this reaction (and also single laser shot analysis of water oxidation on

TiO2), which are indicative of water oxidation proceeding via the single-hole oxidation of, for

example, surface-bound hydroxyls (OH-).20, 24 The potentials for such single-hole oxidation

reactions are significantly more positive than the equilibrium four-hole oxidation of water

(+1.23 V versus RHE). However, it appears that the highly oxidising nature of

photogenerated holes in hematite is sufficient to drive water oxidation via a series of single-

hole oxidation steps, without a significant requirement for the accumulation of multiple holes

to drive a concerted multi-hole oxidation reaction, in agreement with a recent theoretical

study of water oxidation on Fe2O3.49

4.7 Conclusions

Transient absorption and transient photocurrent measurements have been employed to

monitor photogenerated holes and electrons, respectively, in hematite photoanodes as a

function of applied bias. A quantitative correlation between the yield of long-lived

photogenerated holes (on the order of hundreds of milliseconds) and photocurrent density

was demonstrated. The rate of water oxidation by these long-lived holes is independent of

hole density, indicating that water oxidation proceeds via a rate-determining single-hole

transfer step. The bias dependence of the yield of long-lived holes is caused by the strong

bias dependence of electron-hole recombination on the micro- to milli-second timescale.

This rapid recombination phase competes with electron collection by the external circuit.

The key factor limiting the efficiency of water photo-oxidation by these hematite

photoelectrodes, and leading to the requirement of thermodynamically undesirable anodic

electrical biases to enable this photo-oxidation, is not the (albeit slow) timescale of water

oxidation by photogenerated holes but rather the rapid electron-hole recombination which

can prevent efficient electron collection. This suggests that strategies to optimise the

performance of such photoelectrodes should focus not so much upon the acceleration of

water oxidation by the addition of co-catalysts, but rather upon either retarding this electron-

hole recombination or accelerating the kinetics of electron extraction by the external circuit.

Chapter V: Comparison of Sold and Mesoporous Hematite 78

Chapter V

Comparison of

Solid and Mesoporous

Hematite Photoanodes

In which the dynamics of photogenerated holes, probed using transient absorption

spectroscopy, and electrons, probed using transient photocurrent measurements, in solid

and nanostructured hematite photoanodes are compared. The differences in efficiency

under UV and visible excitation are also considered.

79 Chapter V: Comparison of Sold and Mesoporous Hematite

5.1 Introduction

Although one of hematite’s most advantageous properties for solar energy conversion is

its strong absorption of visible light, the internal quantum efficiency (APCE) is significantly

lower under visible excitation than UV.14, 37, 40 This is generally accepted to result from the

combination of the relatively long absorption depth for visible light in hematite (ca. 100 nm

absorption depth for 500 nm light15) together with the short hole diffusion length (2-4 nm;16

20 nm 17). Hematite also has a relatively narrow space-charge layer due to a high donor

density. Consequently, under visible excitation holes are photogenerated several times

further from the semiconductor-liquid junction (SCLJ) than their diffusion length. Under SE

(“back-side”) visible excitation, electrons are generated further from the back contact, so are

also more likely to recombine than under UV excitation. However, there is some evidence

that charge carriers generated by visible light are inherently less active for water oxidation; it

has been suggested that higher APCE values under UV excitation are a consequence of

more energetic holes having larger mobilities.14

The main method of mitigating these losses is to employ porous, nanostructured

photoanodes. This type of morphology allows the use of thick films which absorb a large

fraction of incident light, but in which holes have only a short distance to diffuse to the SCLJ.

This is thought to be why Si-doped APCVD hematite photoanodes, which have a

“cauliflower-like” dendritic nanostructure, generate relatively high photocurrents under one

Sun illumination.37 Similarly, arrays of narrow hematite nanorods or nanotubes aligned

perpendicularly to the conducting substrate surface would provide a direct, grain-boundary-

free path for electron transport to the back contact whilst minimising the distance holes must

diffuse to the SCLJ. However, development of such photoanodes has not so far resulted in

higher photocurrents than those generated by APCVD hematite.38, 97, 98 This is likely to be

due to the width of the nanorods/tubes (on the order of 10-25 nm or greater), which could be

significantly larger than the hole diffusion length. Bulk and/or surface defects may also limit

the photocurrent.99 An alternative suggestion for overcoming the long light absorption depth

and short hole diffusion length is to stack several ultra-thin solid photoanodes, in which

photogenerated holes are swept to the semiconductor surface by the electric field (band

bending).100

The two previous chapters in this Thesis focussed on the charge carrier dynamics of

nanostructured hematite photoanodes as a function of applied bias. The population of long-

lived holes responsible for water oxidation was shown to be determined by electron-hole

recombination on sub-millisecond timescales. It was demonstrated that increasing positive

applied bias reduces recombination, likely due to a greater proportion of holes being

generated in the increasingly wide space-charge layer. In this chapter, these analyses are


extended to various hematite photoanodes with different nanomorphologies and

thicknesses. Photogenerated electron and hole dynamics in thick and thin solid

photoanodes, and in nanostructured and dendritic photoanodes are compared. Additionally,

charge carrier dynamics under visible and UV excitation in hematite photoanodes on

microsecond to second timescales are examined. Transient absorption (TA) measurements

are used to probe photogenerated holes, while transient photocurrent (TPC) measurements

probe the dynamics of electron extraction. Current/voltage and steady-state photocurrent

measurements are also employed. TPC measurements clearly show greater efficiencies

under UV excitation. TA measurements are employed to elucidate the loss mechanisms

under visible excitation, however, the limited sensitivity of this optical technique combined

with the low efficiencies of the solid photoanodes employed in these studies limit the

conclusions that can be drawn. Although the timescale of water oxidation is shown to be the

same on solid and nanostructured films, the timescale of electron extraction to the external

circuit is found to be highly dependent on the morphology of the photoanode. Since more

rapid electron extraction reduces losses by electron-hole recombination, these results are

useful for developing structure-function relationships of photoanodes for water oxidation.

5.2 Experimental

Several different types of hematite photoanode were investigated and are described

briefly here; more details are given in Section 2.1 and the references indicated. Very thin (57

and 30 nm thick) undoped solid (non-porous) hematite deposited by atomic layer deposition

(ALD),14 and undoped solid hematite deposited by pulsed laser deposition (PLD),

approximately 600 nm thick.72 Thick (~1 μm) Si-doped hematite photoanodes deposited by

spray pyrolysis, which are relatively non-porous. Colloidal, porous Ti-doped hematite

Fig 5.1 UV-vis spectra of

various types of hematite

photoanodes. Spectra

taken of the same area of

the photoanodes as used

for TAS and PEC

measurements. Vertical

black lines indicate 355

and 525 nm.

400 500 600 700 800 9000

1

2

3

abs /

a.u

.

wavelength / nm

PLD Fe2O

3

SP Si-Fe2O

3

colloidal Ti-Fe2O

3

USP Nb-Fe2O

3

ALD 57 nm thick Fe2O

3


3


photoanodes are produced using a scaffold to encapsulate nanoparticles, which is removed

after annealing, and consist of a nanoporous network of 30-40 nm particles.73 Nb-doped

hematite approximately 200 nm thick with mesoporous “leaflet” nanostructure, deposited by

ultrasonic spray pyrolysis (USP) using a method similar to that already described in the

literature,12 with 0.5% Nb precursor. The UV-vis absorption spectra of these photoanodes

are shown in Figure 5.1. Undoped and Si-doped CVD photoanodes, employed in studies

discussed in previous chapters, were also used (UV-vis spectra shown in Figure 3.1).



described in the Methods section. TAS and TPC measurements were obtained using pulsed

band-gap excitation at 355 nm (typically ~0.2 mJ.cm-2 after absorption by cell, 0.25-33 Hz).

Photocurrent/voltage curves for colloidal Ti-Fe2O3, SP Si-Fe2O3 and ALD Fe2O3 (30 and 57

nm thick) are shown in Figure 5.2.

5.3 Comparison of carrier dynamics in solid and mesoporous hematite

The transient absorption (TA) signal of photogenerated holes (probed at 700 nm) in thin

solid hematite is shown as a function of applied bias in Figure 5.3. The decay of the long-

lived hole signal over a timescale of tens of milliseconds to seconds is attributed to water

oxidation (Chapter IV). These very thin (~30 nm thick) hematite photoanodes produce

significantly smaller TA signals compared to those for thicker CVD hematite discussed

previously, due to the greater transparency of these thin films. As previously observed for

nanostructured hematite photoanodes, the amplitude of the TA signal, probed on

Fig 5.2 Current/voltage curves (in 0.1M NaOH, pH ~12.8, white light illumination (ca. 1 Sun)

intensity), 10 mV.s-1

) from different types of hematite photoanodes: solid Fe2O3 30 nm (pale blue)

and 57 nm (dark blue) thick (ALD); colloidal Ti-Fe2O3 (green); thick (1 μm) solid SP Sn-Fe2O3 (red).

0.0 0.2 0.4 0.6

0.0

0.2

0.4

0.6

0.8

1.0

curr

ent

density /

mA

.cm

-2

bias / V vs Ag/AgCl

colloidal Ti-Fe2O

3

57 nm thick ALD Fe2O

3

30 nm thick ALS Fe2O

3

SP Si-Fe2O

3


milliseconds to seconds timescales, increases with increasing positive applied bias. This is

attributed to greater long-lived hole populations as increasing positive bias decreases the

electron density and thus reduces electron-hole recombination.

As demonstrated in the previous chapter, there is a quantitative correlation between the

amplitude of the TA hole signal on the hundreds of milliseconds timescale (i.e. population of

long-lived holes responsible for water oxidation) and the photocurrent density as a function

Fig 5.3 Transient absorption (TA) decays of holes (probed at 700 nm) in 30 nm thick ALD

Fe2O3 photoanodes as a function of applied bias, at 0 (blue), +0.3 (green) and +0.6 VAg/AgCl

(brown). Measurements were made using a three electrode cell with 0.1 M NaOH electrolyte;

EE 355 nm excitation (25 μJ.cm-2

, corresponding to an approximate initial photogenerated hole

density of 8x1018

holes.cm-3

).

Fig 5.4 Correlation of long-lived photo-hole population (as measured by the amplitude of the TA

decay at 200 ms) with photocurrent at +0.4VAg/AgCl, both under 355 nm illumination for various

hematite photoanodes under EE (front-side) and SE (back-side) illumination. The best-fit straight

line has an intercept of 0.001(4) mΔOD and gradient of 0.19(4).

1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

0.00

0.02

0.04

m

OD

time / s

0 VAg/AgCl

+0.3 VAg/AgCl

+0.6 VAg/AgCl

0.00 0.05 0.10 0.15 0.200.00

0.01

0.02

0.03

0.04

Nb-Fe2O3 USP EE

Fe2O3 CVD SE

Si-Fe2O3 CVD EE

Nb-Fe2O3 USP SE

Fe2O3 PLD EE

Fe2O3 PLD SE

TA

am

plit

ud

e a

t 0

.2s /

mO

D

photocurrent at 0.4V / mA.cm-2


of applied bias. This quantitative correlation is observed for all hematite photoanodes

examined, including Si-doped and undoped nanostructured CVD films, Nb-doped

mesoporous films fabricated by ultrasonic spray pyrolysis and dense undoped films

deposited by pulsed laser deposition. Figure 5.4 plots the amplitude of this long-lived hole

signal (monitored at 200 ms and 575 nm) against the photocurrent measured at +0.4 VAg/AgCl

for these different Fe2O3 photoanodes. There is a clear common correlation between the

amplitude of the long-lived photogenerated hole signal and the photocurrent density. This

correlation provides further evidence for the importance of long-lived holes in driving water

photo-oxidation, and indicates that this correlation is generic to a broad range of hematite

photoelectrodes.

Comparison of the long-lived hole decay signals of thin (30 nm) solid and thicker

nanostructured hematite photoanodes (Figure 5.5) also shows a strong similarity in the water

oxidation kinetics. However, there is a small difference in the kinetics of water oxidation

(decay of the long-lived hole signal on the milliseconds-seconds timescale). Water oxidation

appears to be marginally faster on the solid ALD photoanodes than on the nanostructured

CVD photoanodes. This may be due to a greater surface hole density on the solid ALD

photoanodes, or due to differences in surface structure on the atomic scale that lead to a

more catalytic surface. This first explanation is unlikely, since previous excitation density

studies on CVD photoanodes (see Chapter IV) showed that the timescale of this decay is

independent of excitation density. Additionally, the surface density of photogenerated holes

is approximately the same for nanostructured CVD and solid PLD photoanodes, assuming

that the same proportion of photogenerated holes reaches the surface. The latter point

depends on the relative rates of electron-hole recombination in the different photoanodes,

which are unlikely to be identical.

An excitation density study of the long-lived hole dynamics in solid PLD photoanodes was

conducted in order to aid the interpretation of these results (Figure 5.6). As the excitation

density is increased, the amplitude of the long-lived hole signal initially increases, then

saturates. In the previous excitation density study of CVD photoanodes, the long-lived hole

signal was also shown to saturate (Figure 4.6b). This saturation behaviour may occur due to

increasingly rapid electron-hole recombination which negates any gain in initial number of

photogenerated holes. It may also be due to the saturation (filling) of the states available

for this long-lived hole to occupy. The nature of the long-lived hole probed by these TA

studies is currently unknown. It is plausible, however, that this hole is a surface-bound

species, such as a high-valent Fe ion bound to a hydroxide radical. Given the long lifetime

of this hole (ca. 300 ms on ALD hematite; ca. 2 s on Si-doped CVD photoanodes), it is

possible that these states become blocked with unreacted holes, resulting in saturation of

the signal.


The timescale of water oxidation on these solid ALD photoanodes (~300 ms) is clearly

independent of the excitation (and therefore hole) density. This result is very similar to that

obtained using nanostructured photoanodes, as discussed in Chapter IV. This provides

further evidence that the rate determining step in the reaction of these surface trapped holes

with water/surface-bound water species is first order in hole density, and is therefore a

single-hole oxidation.

Fig 5.5 (a) TA decays of holes in 30 nm thick solid ALD (probed at 700 nm; blue) and ~500 nm

thick nanostructured CVD Fe2O3 photoanodes (probed at 600 nm; EE brown, SE orange). (b) The

same TA decays normalised at 3 ms to show the relative timescales of water oxidation.

Measurements were made at potentials were the photocurrent was almost saturated: 0.4 VAg/AgCl

for CVD and 0.6 VAg/AgCl for ALD photoanodes. ALD measurements used EE 355 nm excitation

(25 μJ.cm-2


holes.cm-2

); CVD measurements used EE 355 nm

excitation (190 μJ.cm-2


holes.cm-2

, assuming a roughness factor of

20), SE excitation densities were matched to this.

Fig 5.6 (a) TA decays of holes (probed at 700 nm) in 30 nm thick ALD Fe2O3 photoanodes at

+0.6 VAg/AgCl as a function of excitation density. EE 355 nm excitation at 5 (grey), 25 (black), 50

(blue), 100 (purple) and 250 μJ.cm-2

(pink). (b) The same TA decays normalised at 3 ms,

showing that the kinetics of water oxidation are independent of excitation density.

1E-4 1E-3 0.01 0.1 1

0.00

0.02

0.04

0.06m

OD

time / s

CVD Fe2O

3 SE

ALD Fe2O

3 EE

CVD Fe2O

3 EE

(a)

1E-4 1E-3 0.01 0.1 1

0

1

2

O

D

time / s

CVD Fe2O

3 EE

CVD Fe2O

3 SE

ALD Fe2O

3 EE

(b)

1E-4 1E-3 0.01 0.1 1

0.0

0.5

1.0

1.5 (b)

m

OD

time / s

1E-6 1E-5 1E-4 1E-3 0.01 0.1 1-0.01

0.00

0.01

0.02

0.03

0.04

0.05

0.06

m

OD

time / s

(a)5-250 μJ.cm-2


It is unclear why the water oxidation kinetics on the 30 nm thick solid ALD photoanode are

approximately one order of magnitude faster than on nanostructured photoanodes. It is

possible that the conformal and relatively smooth surface of the solid ALD hematite is less

disordered or more appropriately structured for binding water or water oxidation

intermediates in such a way as to catalyse the single-hole oxidation reaction probed by

these TA measurements. However, it is notable that the water oxidation kinetics on slightly

thicker (57 nm thick) ALD hematite are the same as on nanostructured hematite, as shown

in Figure 5.7. This suggests that the faster water oxidation kinetics observed for the very

thin ALD films are related to the thickness of hematite, rather than the surface properties

(which are likely to be very similar for different thicknesses of hematite deposited by ALD).

It is also apparent from Figure 5.7 that the TA decays probing photogenerated holes in

these solid photoanodes do not have the same biphasic decay observed for nanostructured

photoanodes (Chapter IV). The “fast decay phase”, associated with electron-hole

recombination, is observed on the microsecond-millisecond timescale in nanostructured

CVD photoanodes. However, this phase is not observed in thin (30 or 57 nm thick) ALD

photoanodes. Although observable in thicker solid photoanodes, the fast decay phase has a

significantly shorter lifetime. Also shown in Figure 5.7 is the TA decay of photogenerated

holes in colloidal Ti-doped hematite photoanodes. These colloidal photoanodes consist of a

nanoporous network of 30-40 nm particles. This small particle size mans that more holes

are generated close to the semiconductor liquid junction compared to photoanodes

consisting of larger particles, resulting in increased peak photocurrents. However, a serious

Fig 5.7 TA decays of holes in colloidal Ti-doped (green), thick (~1 μm) solid Si-doped (purple),

57 and 30 nm thick solid ALD (dark and pale blue, respectively) Fe2O3 photoanodes at positive

applied bias where photocurrent is approximately saturated. EE 355 nm excitation; average

excitation density ca 2x1018

-3x1019

photogenerated holes.cm-3

.

1E-6 1E-5 1E-4 1E-3 0.01 0.1 10.0

0.1

0.2

0.3

m

OD

time / s

colloidal Ti-Fe2O

3

SP 1 m thick Sn-Fe2O

3


3


3


disadvantage of these photoanodes is that the photocurrent onset potential is anodically

shifted compared to CVD hematite photoanodes. This has been attributed to recombination

via surface states at grain boundaries between the colloidal particles, which limits the

efficiency of electron transport to the back contact.73 It has also been suggested that

multiple connections between semiconductor particles in a three-dimensional network result

in slower electron transport to the back contact compared to morphologies with fewer

pathways available to electrons.101

It can be seen from Figures 5.7 and 5.8 that the fast TA decay phase of the colloidal

photoanodes extends over a significantly longer timescale than for nanostructured CVD or

solid photoanodes. While the fast decay phase is complete by ~20 ms in CVD photoanodes

at +0.4 VAg/AgC, the fast decay phase extends to ~400 ms for the colloidal photoanodes at the

same bias (Figure 5.8). The timescale of this fast decay phase was previously found to be

related to the timescale of the transient photocurrent (TPC) decay, and is associated with

electron-hole recombination (Chapter IV). Indeed, the lifetime of this decay phase is

reduced at higher excitation densities (not shown), due to faster electron-hole recombination.

The relative amplitude of the fast decay phase compared to the amplitude of the slow phase

is also much greater for colloidal photoanodes than for CVD photoanodes. These TA results

suggest that electron-hole recombination on micro- to milli-second timescales is a more

significant loss process in colloidal photoanodes than for CVD photoanodes.

Since no transient absorption signal in the 550-950 nm region is clearly assignable to

photogenerated electrons, TPC measurements are employed to qualitatively probe electron

Fig 5.8 TA decays of holes (probed at 650 nm) in colloidal Ti-doped Fe2O3 photoanodes at

0.25 (just anodic of the photocurrent onset potential), 0.4 and 0.6 VAg/AgCl. EE 355 nm excitation

at 50 μJ.cm-2

. The fast decay phase (on the microsecond to hundreds of milliseconds

timescale) is significantly longer-lived than in other hematite photoanodes studied.

1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

0.0

0.1

0.2

0.3

mO

D

time / s

+0.6 VAg/AgCl

+0.4 VAg/AgCl

+0.25 VAg/AgCl


extraction, as demonstrated in Chapter IV. Although the early timescale TPC response is

likely to be limited by the measurement resistor, the TPC from ca. 10 μs shown in the figures

below does not appear to be limited by the measurement resistor (see Section 2.2.3) and

hence provides information about the timescale of electron transport through the

semiconductor. These measurements are made under the same pulsed excitation

conditions used for transient absorption measurements. The TPC signal monitors the

recovery of electron density towards dark equilibrium following the pulsed excitation. This

recovery in electron density will result from electron-hole recombination and electron

extraction by the external circuit, both of which are dependent on electron transport through

the hematite photoanode. Faster electron extraction to the external circuit is expected to

result in reduced electron-hole recombination and thus higher photocurrent densities and/or

a more cathodic photocurrent onset potential.

The TPC signals from colloidal hematite photoanodes are compared with those from thick

(~1 μm) and thin (57 nm) solid hematite photoanodes in Figure 5.9 at positive applied bias

where the photocurrent is approximately saturated. A double peak in the photocurrent

transients is often observed. The first peak could be attributed to charge reorganisation after

laser pulse excitation;102 the charge extracted during this initial peak is only a small fraction

of the total. The second current peak is qualitatively interpreted as the collection of

photogenerated electrons that have diffused to the back contact; given the relatively late

timescales at which this peak occurs (50 μs-10 ms), this is not thought to be limited by the

measurement resistor. The time at which the (second) photocurrent peak occurs is related

to the distance the photogenerated charge must travel to reach the back contact, known as

“time of flight”.

It is clear from Figure 5.9 that electron extraction occurs significantly faster from the thin

solid photoanode (blue curve) than from the thick solid and colloidal photoanodes. Rapid

electron extraction from the thin film is expected since photogenerated electrons only have a

very short distance to diffuse and/or drift (depending on extent of band bending) to reach the

back contact. With this in mind, it might be expected that electron extraction would be

significantly faster from the colloidal photoanode, than from the solid SP photoanode (~1 μm

thick). However, the TPC from the SP photoanode has an initial peak at ca. 10 μs followed

by a second peak at 3 ms, while colloidal TPC only peaks at 5 ms. Similar behaviour is

observed at potentials just anodic of the photocurrent onset.

Together with the behaviour of the fast TA decay phase (Figure 5.8), these results indicate

that electron transport through colloidal hematite photoanodes is significantly slower than

through solid (or nanostructured CVD) hematite. It could be assumed electron transport is

aided in the solid hematite by band-bending, and that this does not occur in the colloidal

hematite photoanodes. However, it is likely that band bending does occur to some extent in


the colloidal photoanodes (as discussed Section 4.6), since the hematite particles are 30-40

nm in diameter. Additionally, the effect of band-bending (thought to occur on a length scale

of at most tens of nanometres) on electron transport in the ~1 μm thick solid photoanodes is

unlikely to be significant compared to the thickness of the hematite film. Instead of diffusing

directly to the back contact as in solid photoanodes, electrons in colloidal photoanodes must

“hop” between particles, as in a dye-sensitised solar cell. As discussed above, this is likely

to be a slower process than diffusion through a solid semiconductor.101 Recombination via

surface states at grain boundaries73 is also likely to be a more important loss pathway in

these high-surface area photoanodes than in solid or less porous photoanodes.

Fig 5.9 Transient photocurrent (TPC) from pulsed light (EE 355 nm; the same excitation

densities are employed as for TAS measurements) excitation of colloidal Ti-doped (green), thick

solid SP Si-doped (purple) and thin solid ALD (blue) hematite photoanodes. The photoanodes

were held at positive applied bias where photocurrent is approximately saturated. Photocurrent

transients are normalised for ease of comparison.

Fig 5.10 Comparing EE (green/purple) and SE (black) TPC from colloidal Ti-doped (left) and

thick solid SP Si-doped (right) hematite photoanodes (355 nm pulsed excitation). The

photoanodes were held at positive applied bias where photocurrent is approximately saturated.

Photocurrent transients are normalised for ease of comparison.

1E-5 1E-4 1E-3 0.01 0.1 10.0

0.2

0.4

0.6

0.8

1.0

no

rma

lise

d c

urr

en

t /

A

time / s

SP Si-Fe2O

3

colloidal Ti-Fe2O

3

57 nm thick ALD Fe2O

3

1E-5 1E-4 1E-3 0.01 0.1 10.0

0.2

0.4

0.6

0.8

1.0

no

rma

lise

d c

urr

en

t /

A

time / s

SE

EE

1E-5 1E-4 1E-3 0.01 0.1 1

0.0

0.2

0.4

0.6

0.8

1.0

no

rma

lise

d c

urr

en

t /

A

time / s

SE

EE


Transient photocurrents from colloidal and thick solid hematite photoanodes illuminated

SE (“back side”) and EE (“front side”) are shown in Figure 5.10. Electron extraction is

significantly faster under SE illumination, since electrons are generated close to the back

contact (the absorption depth (α-1) is ca. 30 nm for 355 nm light15). Consequently, the SE

TPC decay kinetics are very similar for colloidal and thick solid photoanodes, despite the

differences under EE illumination.

5.4 UV versus visible excitation

By changing the wavelength of the excitation light pulse, the absorption depth of the light

is also changed. One of the advantages of hematite for water photo-oxidation is its strong

absorption of visible light (Figure 5.1). However, it is widely known that the efficiency of

water photo-oxidation on hematite is much lower under visible light than UV.14, 37, 40 This has

been linked to the relatively long absorption depth for visible light in hematite (ca 100 nm

absorption depth for 500 nm light15) together with the short hole diffusion length (2-4 nm;16

20 nm 17). Consequently, under visible excitation holes are photogenerated several times

further from the semiconductor-liquid junction than their diffusion length.

This difference in light absorption depth is evident in the TPC from solid SP hematite

photoanodes, shown in Figure 5.11. For these measurements, the excitation (laser pulse)

energies were carefully controlled such that the excitation densities were matched EE/SE

and visible/UV, i.e. the same number of photons were absorbed. Under SE illumination,

electrons generated from UV (355 nm) excitation are extracted from the photoanode faster

Fig 5.11 TPC from solid SP Si-doped hematite photoanodes (~1 μm thick) under 355 nm

(purple) and 525 nm (grey) excitation, illuminated SE (left) and EE (right). The photoanodes

were held at positive applied bias (0.5 VAg/AgCl) where photocurrent is approximately saturated;

similar decays are observed at potentials just anodic of the photocurrent onset. The number of

photons absorbed was ~3.0x1018

cm-3

in each measurement. Photocurrent transients are

normalised for ease of comparison; inset: data before normalisation.

1E-5 1E-4 1E-3 0.01 0.1 1

0.0

0.2

0.4

0.6

0.8

1.0

no

rma

lise

d c

urr

en

t /

A

time / s

SE 525 nm

SE 355 nm

1E-5 1E-4 1E-3 0.01 0.1 1

0.0

0.2

0.4

0.6

0.8

1.0

no

rma

lise

d c

urr

en

t /

A

time / s

EE 525 nm

EE 355 nm


than those generated by visible (525 nm) excitation, due to the generation of charges closer

to the back contact under UV excitation. Accordingly, under EE illumination the main

photocurrent peak occurs at slightly earlier timescales for visible excitation (800 μs and 3 ms

for visible and UV, respectively, at +0.5 VAg/AgCl). It is also evident from the non-normalised

data (inset in Figure 5.11) that a greater number of charges are extracted under UV

excitation than under visible excitation. Since these measurements were conducted under

the same applied potential, and with equal numbers of photons absorbed, a greater number

of electrons extracted is equivalent to larger APCE (absorbed photon to current conversion

efficiency). These results indicate that the lower IPCE (incident photon to current efficiency)

values typically reported for hematite in the visible region, compared to UV (Figure 3.3), are

not only due to the smaller number of photons absorbed.

It is generally accepted that the long absorption depth of visible light in hematite15 is

responsible for the poor efficiency values in this region of the spectrum. Under both SE and

EE visible illumination, carriers are generated far from the SCLJ, so holes are likely to

recombine before reaching the surface. However, there is an alternative argument: UV

excitation excites electrons from the O 2p orbital (charge transfer transition, ~3 eV), while

visible excitation excites electrons from the Fe 3d orbital (d-d transition, ~2 eV). It has been

suggested that in this way two distinct types of holes are initially generated, but holes

generated in the O 2p band would relax to the Fe 3d band, which is thought to have very low

Faradaic efficiencies for water oxidation.17

Using transient absorption spectroscopy to probe holes in hematite photoanodes

generated by UV (355 nm; 3.5 eV) and by visible (525 nm; 2.4 eV) light, it should be possible

to demonstrate that visible light generates a lower yield of the long-lived holes responsible

for water oxidation, compared to UV excitation. Since ultrafast TAS studies have indicated

that electron relaxation and trapping occur on a picosecond timescale,65, 69 photogenerated

holes are almost certainly trapped (e.g. at the top of the valence band, and/or in mid-

bandgap states) on the timescale of the TA measurements reported herein (1 μs-2 s). As

such it is unlikely that microsecond-second TA measurements would differentiate between

holes generated in the O 2p band and those generated in the Fe 3d valence band. Indeed,

as Figure 5.12 shows, the TA spectra of hematite photoanodes at positive applied bias (i.e.

the spectrum of photogenerated holes) are almost identical under UV and visible excitation.

These results indicate that the same type of hole is probed under UV and visible excitation

on microsecond-second timescales. The main difference is in the intensity of the early

timescale bleach at ~575 nm, which is associated with a particular trap state that lies just

below the conduction band edge, and is discussed in detail in the following chapter.

Consequently, the same probe wavelength can be use to probe holes generated by visible

excitation as by UV excitation (usually 650 nm).


The visible light photogenerated hole dynamics as a function of applied bias are also

found to be similar to the dynamics under UV excitation (Chapter IV), as shown in Figure

5.13. As for UV excitation, under visible excitation the decay time (t50%) of the fast decay

phase (which occurs on microsecond-millisecond timescales) increases with increasing

positive applied bias, indicating that electron-hole recombination is reduced as electron

density is lowered. The amplitude of the TA signal on ~10 ms-2s timescales, associated

with the long-lived holes responsible for water oxidation, also increases with increasing

positive applied bias. These long-lived holes, which have avoided recombination, drive

water oxidation on a timescale of hundreds of milliseconds to seconds.

Fig 5.12 TA spectra of nanostructured CVD Si-doped hematite photoanodes at +0.4 VAg/AgCl

(i.e. the spectra of photogenerated holes) under 355 nm and 525 nm SE excitation. Spectra

under EE excitation are very similar, especially at long timescales.

Fig 5.13 TA decays of holes (probed at 650 nm) photogenerated by 525 nm excitation (SE,

0.16 mJ.cm-2

) in nanostructured CVD Si-doped hematite photoanodes as a function of applied

bias, from -0.3 VAg/AgCl (grey) to +0.4 VAg/AgCl (red). Dynamics under EE excitation are similar.

600 700 800 900

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

m

OD

wavelength / nm

355nm 10 s

355nm 100 s

355nm 1 ms

355nm 10 ms

355nm 100 ms

355nm 1 s

600 700 800 900

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

m

OD

wavelength / nm

525 nm 10 s

525 nm 100 s

525 nm 1 ms

525 nm 10 ms

525 nm 100 ms

525 nm 1 s

1E-5 1E-4 1E-3 0.01 0.1 1

0.00

0.05

0.10

0.15

0.20

mO

D

time / s

+0.4 V

+0.3 V

+0.2 V

+0.1 V

0 V

-0.1 V

-0.2 V

-0.3 V


The TA studies of photogenerated hole dynamics discussed so far have shown that holes

generated by visible excitation behave in a qualitatively similar manner to those generated

by UV excitation. Considering literature reports and the IPCE and TPC studies discussed

above, a quantitative comparison would be expected to demonstrate that the population of

long-lived holes (i.e. the amplitude of the TA signal on tens of milliseconds to seconds

timescales) is reduced under visible excitation, compared to UV excitation. However, this is

not the case. Figure 5.14 shows a quantitative comparison of the decay dynamics of holes

(probed at 650 nm) generated by visible and UV excitation in nanostructured hematite

photoanodes. The excitation densities were carefully controlled such that the number of

photons absorbed (i.e. the number of charge carriers photogenerated) was the same at each

wavelength (3.0x1014 holes.cm-2 (geometric)). This is to avoid complications caused by

different rates of electron-hole recombination at different excitation densities, as discussed in

Section 4.5. Although there are some differences in the fast decay phase, associated with

electron-hole recombination, it is clear that the long-lived hole signal, associated with water

oxidation, is almost identical in initial amplitude and lifetime. TA decays are shown for

biases just anodic of the onset potential and at positive applied bias where the photocurrent

is almost saturated (0 and +0.4 VAg/AgCl respectively); the same behaviour is observed at

every potential studied. This result apparently indicates that the same number of long-lived

holes is generated under UV and visible excitation, and that the timescale of water oxidation

is also identical. The latter is to be expected, according to the results discussed above

demonstrating that charge carriers generated by UV and visible excitation relax to form the

same species at timescales faster than those employed in these studies. However, the

former is unexpected, as this suggests that UV and visible illumination are equally efficient at

generating long-lived holes that oxidise water.

Fig 5.14 TA decays of holes (probed at 650 nm) photogenerated by 525 nm (blue curves) and

355 nm (black curves) excitation (SE, number of photons absorbed matched) in nanostructured

CVD Si-doped hematite photoanodes at 0 VAg/AgCl (left), and +0.4 VAg/AgCl (right).

1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

0.00

0.05

0.10

0.15

0.20

0.25

m

OD

time / s

355 nm excitation

525 nm excitation

0 VAg/AgCl

1E-5 1E-4 1E-3 0.01 0.1 10.00

0.05

0.10

0.15

0.20

0.25

0.30

m

OD

time / s

355 nm excitation

525 nm excitation

+0.4 VAg/AgCl


As discussed above, the absorption depth of light in hematite is strongly dependent on the

wavelength. The CVD photoanodes examined above (Figures 5.12-5.14) have a dendritic

“cauliflower-like” nanostructure, in which the smallest feature size is ca 10-20 nm but which

consist of larger features closer to the substrate, on the order of hundreds of nanometres.13

Consequently, changing the wavelength of the excitation pulse not only changes the

absorption depth but also the semiconductor morphology under investigation. This

complicates the interpretation of such results, as the morphology is likely to affect charge

separation efficiencies and recombination rates. In order to avoid such issues, it is

necessary to employ a photoanode in which the nanomorphology is consistent throughout

the hematite thickness. Although the colloidal Ti-doped hematite photoanodes discussed

above fulfil this requirement, the unusually long lifetime of the TA fast decay phase partially

obscures the long-lived hole signal associated with water oxidation. Solid hematite

photoanodes, however, have a more clearly identifiable long-lived hole signal. The ALD

photoanodes discussed above are too thin to give TA signals with sufficient signal-to-noise

ratio under visible excitation. Instead, the thick SP Si-doped photoanodes were employed.

The TA signals of holes (probed at 650 nm) generated by UV and visible excitation in

these solid photoanodes are shown in Figure 5.15. As before, excitation densities were

carefully controlled such that the number of photons absorbed (i.e. the number of

photogenerated charge carriers) was the same at each wavelength. Despite this, and the

homogeneous morphology of these solid photoanodes, as before there is no difference in

hole decay dynamics between UV and visible generation. The amplitude and lifetime of the

long-lived hole signals are almost identical whether UV or visible excitation was employed.

Fig 5.15 TA decays of holes (probed at 650 nm) photogenerated by 525 nm (blue curves) and

355 nm (black curves) excitation (SE, number of photons absorbed matched) in thick solid SP

Si-doped hematite photoanodes at 0 VAg/AgCl (left), and +0.5 VAg/AgCl (right).

1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

0.00

0.05

0.10

0.15

0.20

m

OD

time / s

525 nm excitation

355 nm excitation

0 VAg/AgCl

1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

0.00

0.05

0.10

0.15

0.20

+0.5 VAg/AgCl

m

OD

time / s

525 nm excitation

355 nm excitation


These results from transient absorption studies (measuring photogenerated holes) are in

contrast to those from corresponding transient photocurrent studies (measuring electrons

extracted to the external circuit) discussed above (Figure 5.11). Although the TPC and TAS

measurements were made under near-identical conditions (pulsed excitation with the same

excitation densities, repetition rate etc), the TPC studies clearly show that photocurrent

generation (and hence oxygen evolution) is more efficient under UV than visible excitation.

This is also found for nanostructured CVD hematite.

As discussed in the previous chapter, although the TAS system employed for these

studies has particularly high sensitivity, such optical measurements are significantly less

sensitive than corresponding electrical measurements such as TPC. This difference in

sensitivity is evident from the TPC and TA decays shown above: although TPC traces were

averaged over only 100 laser shots (compared to 300-1000 shots for TAS), they generally

have a noticeably cleaner signals than the TA decays. It may be that the TA measurements

employed for this study are not sensitive enough to differentiate between the populations of

long-lived holes generated by UV and visible excitation. The thick solid photoanodes

employed produce only very low photocurrent densities, even under UV illumination.

Consequently, the observation of small differences in the amplitude of the TA long-lived hole

signal (i.e. the population of holes that go on to oxidise water) is very difficult. Additionally,

although the total number of absorbed photons was constant under UV and visible

excitation, the number of photons absorbed within the space charge layer was not.

5.5 Conclusions

The charge carrier dynamics in solid and nanostructured hematite photoanodes were

investigated using transient absorption spectroscopy and transient photocurrent

measurements to follow photogenerated holes and electrons, respectively. Water oxidation

kinetics were shown to be very similar on solid and nanostructured hematite, occurring on a

timescale of hundreds of milliseconds to seconds. Although water oxidation appears to be

slightly faster on thin (30 nm thick) solid ALD photoanodes than on nanostructured or thicker

(57 nm thick) solid hematite, excitation density studies demonstrated that water oxidation

kinetics are independent of excitation (hole) densities. Transient photocurrent

measurements demonstrate that the timescale of electron collection (i.e. electron transport)

is highly dependent on the nanomorphology of the photoanode. Both transient photocurrent

and transient absorption measurements indicate that electron transport is significantly slower

in colloidal nanoparticulate photoanodes than in solid or nanostructured CVD photoanodes.

Photoanodes in which the morphology is consistent throughout the thickness of the

hematite layer, such as the colloidal and solid hematite photoanodes, allow direct


comparison of the dynamics of charge carriers generated by UV and visible excitation to be

made. Both steady-state and pulsed excitation transient photocurrent measurements clearly

demonstrated higher photoconversion efficiencies under UV (355 nm) than visible (525 nm)

excitation in thick solid hematite photoanodes. However, there was no observable difference

in the transient absorption decay dynamics. This is most likely because such optical

measurements are significantly less sensitive than corresponding electrical (TPC)

measurements. Observation of larger long-lived hole populations under UV excitation,

compared to those under visible excitation, may be achievable with photoanodes that

produce significantly larger photocurrent densities than those employed in this study.

Chapter VI: Influence of Trap State 96

Chapter VI

Influence of Trap States

on Charge Carrier Dynamics

In which evidence for the existence of a particular hematite trap state a few hundred

millivolts below the conduction band edge is presented. This trap state manifests as a

strong transient absorption signal observed at ~575 nm, which is positive in undoped

hematite in the absence of anodic bias, but negative (bleached) under anodic bias and in

doped hematite.

97 Chapter VI: Influence of Trap State

6.1 Introduction

In Chapter III, evidence was presented of a photogenerated species probed at ~575 nm

which exhibits distinctly different behaviour to that of photogenerated holes (probed at 650-

900 nm). Although a strong positive transient absorption signal is observed at this

wavelength for undoped hematite in the absence of applied bias, when a positive potential is

applied a bleach is observed on microsecond to millisecond timescales. In the presence of

hydrogen peroxide or Ag+ ions, the decay kinetics of the TA signal probed at 575 nm are

accelerated, suggesting that electrons are probed at this wavelength. However, the

bleaching behaviour observed at positive bias and with H2O2 indicates that this interpretation

is over-simplistic. Evidence that these TA signals are associated with a particular energetic

(“trap”) state, situated a few hundred millivolts below the conduction band edge, is presented

in this chapter.

As discussed in previous chapters, charge carrier dynamics in hematite are dominated

by electron-hole recombination. Positive bias lowers the electron density, which reduces

electron-hole recombination, allowing sufficient hole lifetimes for water oxidation to occur

(with a timescale of hundreds of milliseconds to seconds). Ultrafast transient optical studies

have also indicated that rapid, efficient electron hole recombination occurs on pico- to nano-

second timescales, thought to be mediated by a high density of intra-bandgap states.39, 65

Several studies employing Mott-Schottky analysis have provided evidence for intra-bandgap

trap states positioned close to the CB edge and approximately 0.5-0.7 eV below the

conduction band edge.36, 50, 52, 103 Additionally, photocurrent transients under chopped light

excitation of hematite photoanodes are often observed.3, 17-19, 40, 43, 45, 50-58 These are

associated with surface recombination of conduction band electrons with either surface-

accumulated holes and/or surface-bound oxidation intermediates, as discussed in Section

1.3. The magnitude of these photocurrent transients is generally diminished in the presence

of chemical hole scavengers, which prevent the accumulation of holes at the hematite

surface. Surface treatments with thin layers of Al2O3 and Ga2O3 (which are isomorphic with

hematite) have been shown to reduce the amplitude of these transients. It has been

suggested that such overlayers relax lattice strain at the hematite surface, thus reducing the

density of trap states through which surface recombination is thought to occur (“passivation”

of surface states).44 A number of frequency-domain studies have employed a model

whereby both hole transfer to the electrolyte and surface electron-hole recombination occur

via the same surface states.51, 59, 60 However, it is not clear that these numerous different

studies are probing the same type of trap state.

Herein evidence is provided for a particular energetic state, situated a few hundred

millivolts below the conduction band edge. Transient absorption spectroscopy is used in


conjunction with transient photocurrent measurements to elucidate the timescales of electron

trapping, electron extraction to the external circuit and electron-hole recombination involving

this trap state.

6.2 Experimental

Si-doped nanostructured hematite films deposited by APCVD were used in this study.13



outlined in the Methods section. Current/voltage measurements were made at a scan rate of

10 mV.s-1. For chronoamperometry measurements, the dark current was allowed to reach a

constant value before the photoanode was illuminated. TAS and TPC measurements both

employed 355 nm, 0.20 mJ.cm-2, 0.25-0.33 Hz EE (“front-side”) pulsed excitation.

6.3 Spectroscopic Study of Trap State

Typical chopped light photocurrent transients from hematite photoanodes are shown in

Figure 6.1b. As discussed above, such photocurrent transient spikes are attributed to

electron recombination with surface-accumulated holes/surface-bound water oxidation

intermediates.55 These transient decays are approximately exponential, with time constants

of ~300 ms. This recombination timescale is significantly longer than that obtained from the

lifetime of the TA fast decay phase (see Chapter IV), which is ~3 ms under the same applied

bias. Given the different conditions employed for these two types of measurements (6 ns

pulse width illumination for TAS, and illumination for tens of seconds for chopped light

photocurrent), the two recombination timescales are not entirely comparable. However, the

large difference in decay lifetimes suggests that different recombination processes are

Fig 6.1 (a) Current/voltage curves from nanostructured Si-Fe2O3 photoanodes (in 0.1M NaOH, pH

~12.8, white light illumination, 10 mV.s-1

). (b) Chopped light photocurrent transients from

nanostructured Si-Fe2O3 photoanodes at +0.2 VAg/AgCl (355 nm illumination).

0.0 0.2 0.4 0.6

0

1

2

3

4(a)

cu

rre

nt d

en

sity /

mA

.cm

-2

bias (V vs Ag/AgCl)

white EE

white SE

dark

80 85 90 95

0.00

0.02

0.04

0.06

0.08

0.10

0.12

cu

rre

nt

de

nsity /

mA

.cm

-2

time / s

+0.2 VAg/AgCl

EE

+0.2 VAg/AgCl

SE

(b)


probed with TAS and chopped light photocurrent transients.

TA decays of the photogenerated hole signal (probed at 650 nm) in nanostructured, Si-

doped hematite under positive applied bias are shown in Figure 6.2. As discussed in

Chapter IV, the positive hole signal consists of two decay phases: the fast decay phase (1

μs to ~20 ms) is associated with electron-hole recombination, while the slow decay phase

(>20 ms) is attributed to water oxidation. Also shown in Figure 6.2 are the decays probed at

575 nm, which exhibit an intense bleach (negative absorption) on microsecond to

millisecond timescales under these positive bias conditions. It is evident that the fast decay

phase (probed at 650 nm) and the bleach (probed at 575 nm) have essentially the same

lifetime. This is discussed further below.

Fig 6.3 Transient absorption spectra of Si-Fe2O3 photoanodes at (a) -0.7 VAg/AgCl and (b) +0.4 VAg/Agcl

at 10 μs, 100 μs, 1 ms, 10 ms, 100 ms and 1 s (black through blue to grey) after the excitation pulse.

EE 355 nm pulsed excitation (0.20 mJ.cm-2

).

Fig 6.2 TA decay dynamics of Si-Fe2O3 CVD photoanodes under EE 355 nm pulsed excitation

(0.20 mJ.cm-2

) probed at 650 nm (positive signal) and 575 nm (negative signal) at +0.1 VAg/AgCl

(green) and +0.4 VAg/AgCl (orange). The “fast decay phase” probed at 650 nm and the bleach

probed at 575 nm occur on the same timescale (1 μs to ~20 ms).

1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

-4

-3

-2

-1

0

1

h+ (650 nm) +0.4 V

h+ (650 nm) +0.1 V

575 nm +0.1 V

575 nm +0.4 V

m

OD

time / s

600 700 800 900-0.02

0.00

0.02

0.04

0.06 10 s

1 ms

10 ms

100 ms

1 s

m

OD

wavelength / nm

(a) -0.7 VAg/AgCl

500 600 700 800 900

-3

-2

-1

0

1

10 s

100 s

1 ms

10 ms

100 ms

1 s

m

OD

wavelength / nm

(b) +0.4 VAg/AgCl


TA spectra of a hematite photoanode at strong cathodic and anodic applied bias are

shown in Figure 6.3 (similar spectra are obtained with EE and SE excitation). This bleach

feature at ca. 525-625 nm is an inversion of the strong positive feature observed in the

spectrum of undoped hematite in the absence of applied positive bias (Figure 3.2). A similar

positive transient absorption feature is observed for Si-doped hematite at strongly negative

applied bias (Figure 6.3a). Although this feature has a positive absorption in undoped

hematite without applied bias, this feature generally has a negative absorption (bleach) in

doped hematite even in the absence of anodic bias. It is generally observed that hematite

photoanodes with greater activity for water oxidation (nanostructured, doped etc) produce a

more intense bleach, i.e. cathodically shift the onset of the bleach. Surface treatment with

cobalt is also observed to shift the onset of this bleach, discussed in the following chapter.

Fig 6.4 Decay dynamics of Si-Fe2O3 photoanodes probed at 575 nm as a function of applied

bias, (a) from -0.7 to +0.4 VAg/AgCl (black through blue to brown); (b) focusing on -0.7 to -0.3

VAg/AgCl, showing that the decay dynamics are identical cathodic of -0.4 VAg/AgCl.

1E-5 1E-4 1E-3 0.01 0.1-0.10

-0.05

0.00

0.05

0.10

(b)

mO

D

time / s

1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0 -0.7 V

-0.6 V

-0.5 V

-0.4 V

-0.3 V

-0.2 V

-0.1 V

0 V

+0.1 V

+0.2 V

+0.3 V

+0.4 V

mO

D

time / s

(a)


Given the relatively narrow wavelength region occupied by this spectral feature, and its

inversion with positive/negative applied bias, it is attributed to an intra-bandgap energetic

state that can be either oxidised or reduced (see below).

The TA bleach dynamics as a function of applied bias, probed at 575 nm, are shown in

Figure 6.4. The depth of the bleach clearly increases with increasing positive bias. The

bleach recovers by ~20 ms; at longer timescales the amplitude of the long-lived signal

increases with increasing positive bias, bearing a strong resemblance to the slow decay

phase of photogenerated holes probed at 650 nm (see Chapter IV). These results signify

that the slow part of the decay phase (>20 ms) of the 575 nm signal probes the same kind of

charge carrier as that probed at longer wavelengths, i.e. long-lived holes that are responsible

for water oxidation on the hundreds of milliseconds to seconds timescale, as previously

indicated in Chapter III.

As more negative bias is applied, the depth of the bleach is decreased until, at -0.4 VAg/AgCl

(approximately equivalent to 0.56 VRHE), a positive decay signal is observed. The initial

(positive) amplitude of this signal increases slightly with more negative applied bias,

however, the decay dynamics at -0.5 to -0.7 VAg/AgCl are identical (Figure 6.4b). One possible

explanation for this behaviour is that at potentials cathodic of the flatband potential (0.4 VRHE

for these Si-doped photoanodes;37 -0.56 VAg/AgCl under these conditions), where no band-

bending occurs, the rate of electron-hole recombination is no longer modulated by changing

the applied bias.

The 575 nm bleach signal first appears at potentials ~200 mV cathodic of the

photocurrent onset potential (Figure 6.5). The increasing magnitude of the (inverted) bleach

Fig 6.5 Overlay of long-lived hole population (given by the amplitude of the transient decay at 100

ms probed at 650 nm), magnitude of the bleach (probed at 10 μs at 575 nm, inverted and multiplied

by 0.1 for ease of comparison) on the photocurrent density curve (355 nm EE excitation).

-0.4 -0.2 0.0 0.2 0.4 0.6

0.000

0.025

0.050

0.075

0.100

0.125

0.00

0.02

0.04

0.06

0.08

0.10

0.12

photo

curr

ent

density /

mA

/cm

2

bias / V vs Ag/AgCl

575 nm x -1/10 (10 us)

650 nm (100 ms)

mO

D


with increasing positive bias follows the shape of the photocurrent/voltage curve, but shifted

cathodically. Although both the photocurrent density and the depth of the bleach increase

with increasing positive bias, there is no quantitative correlation between the magnitude of

the bleach and the photocurrent. This is in contrast to the behaviour of the long-lived hole

signal, as probed at 650 nm on the hundreds of milliseconds timescale (see Chapter IV).

Additionally, the bleach recovers on a timescale shorter than that of water oxidation. This

behaviour suggests the bleach does not probe the species directly responsible for water

oxidation, but follows a species which becomes oxidised at potentials cathodic of the onset

of water oxidation. These results are discussed further in the following section.

Finally, the timescale of electron extraction to the external circuit, as measured by TPC,

and the decay kinetics of the transient absorption bleach are compared in Figure 6.6, where

the inverted TPC decay is overlaid on the transient absorption bleach (probed at 575 nm).

As previously, only the TPC decay (>10 μs) is considered. It was shown in Chapter IV that

the TPC decay and the fast transient absorption decay phase (probed at 650 nm) have very

similar decay kinetics, particularly at low applied bias (for nanostructured, Si-doped

hematite). This was attributed to the domination of both TPC and TAS decays by rapid

electron-hole recombination on the micro- to milli-second timescale. At more positive

applied bias, anodic of the photocurrent onset potential, there were significant differences

between the TPC and TA fast phase decay kinetics. However, it is clear from Figure 6.6 that

even at +0.4VAg/AgCl, ~400 mV anodic of the photocurrent onset potential, the TA bleach and

TPC decay kinetics are extremely similar. Indeed, the lifetimes (from fitting with single

Fig 6.6 Inverted TPC decays (grey, black) overlaid on transient absorption decays (green,

orange) probed at 575 nm under applied bias at +0.1 and +0.4 VAg/AgCl. Si-Fe2O3 CVD

photoanodes under 355 nm EE pulsed excitation.

1E-4 1E-3 0.01 0.1 1

-4

-3

-2

-1

0

mO

D

time / s

TAS, 575 nm, +0.1 V

TAS, 575 nm, +0.4 V

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

inverted TPC, +0.1 V

inverted TPC, +0.4 V

TP

C /

mA


exponential decays) of the TPC decay and the recovery of the TA bleach are almost

identical (0.014 s and ~0.013 s at +0.4VAg/AgCl, respectively). These results suggest that the

recovery of the bleach and the TPC decay are probing the same process, i.e. electron

detrapping and extraction to the external circuit. This is discussed in detail below.

6.4 Discussion

The proximity of the strong absorption/bleach signal at 525-625 nm to the conduction

band edge initially suggests that it may be associated with Moss-Burstein type behaviour of

the hematite band edge.104 This occurs when the lower levels of the conduction band

become populated with electrons, resulting in a blue-shift of the absorption edge. However,

the Moss-Burstein phenomenon is relatively rare, only occurring in semiconductor materials

in which the conduction band has a well-defined edge and a small effective density of states,

such that the Fermi level may shift into the conduction band. This is not the case in

hematite, in which the Fermi level is thought to be ~0.3 eV below the conduction band

edge.36 Additionally, it is likely that Fe2O3 has an exponential distribution of states at the

bottom of the CB, similar to that in TiO2 and other disordered, nanocrystalline metal oxides.64

Moreover, the narrowness of the TA feature at ~575 nm and its inversion with positive

applied bias (Figures 3.1 and 6.3) strongly suggest that it is associated with a intra-bandgap

“trap” state with narrow energetic distribution.

Trap states are generally thought to mediate electron-hole recombination. If the intensity

(depth) of the bleach signal at ~575 nm (on the microseconds timescale) is directly

correlated with the degree of electron-hole recombination, we would expect more intense

bleaching to be associated with lower photocurrent yields. However, the opposite is true;

there is a general correspondence between the intensity of the bleach and the activity of a

given photoanode. For example, both Si-doping and Co2+-adsorption increase the

photocurrent at a given anodic bias,13 and both of these treatments result in a deeper bleach

at a given bias (see the following chapter), i.e. cathodic shift of the bleach intensity. Yet the

presence/depth of the bleach does not directly correlate with the photocurrent onset/density

(Figure 6.5). Instead, the bleach occurs at potentials cathodic of the photocurrent onset

potential and the appearance of the long-lived hole signal. Thus the increasing magnitude of

the (inverted) bleach with increasing positive bias roughly tracks the shape of the

photocurrent/voltage curve, but shifted cathodically by ~200 mV. The bleach signal is

observed to recover (the TA signal becomes positive) by ~50 ms, i.e. faster than the TA slow

decay phase assigned to water oxidation, which has a lifetime on the order of 1 s. These

results suggest that although the 575 nm bleach signal is an indicator of water oxidation


activity, the processes associated with this signal are not directly responsible for water

oxidation.

The recovery of the bleach occurs with an almost identical lifetime to that for the TPC

decay lifetime (Figure 6.6), i.e. the timescale of extraction of electrons from the photoanode

(as discussed in Chapters IV and V; TPC decays at such long timescales are apparently not

limited by the measurement resistor, see section 2.2.3). This strongly indicates that the

recovery of the bleach results from electron de-trapping and extraction to the external circuit.

A similar relationship between the TPC decay and bleach recovery was observed for several

other types of hematite photoanode, suggesting that it is general. Although previous

comparisons of the TPC and TAS (probed at 650 nm) decays showed a similarity between

the TPC and the fast decay phase of the photogenerated hole, there were notable

differences with increasing anodic bias (Chapter IV). The similarity between the timescales

of the TPC and fast phase 650 nm TAS decays was attributed to domination of both electron

and hole decay processes by electron-hole recombination. Figure 6.6 clearly shows that

both the transient absorption fast decay phase (as probed at 650 nm) and the recovery of

the bleach (as probed at 575 nm) occur on similar timescales.‡ As such, it is unsurprising

that there is also a similarity between the timescales of the TPC decay and the bleach

recovery.

The interpretation of these results is outlined in Scheme 6.1. At negative applied bias, the

Fermi level lies close to the conduction band edge and the trap state is occupied by

electrons. There is no ground state absorption from the valence band to the trap state, and

no transient absorption bleach is observed. When a positive bias is applied, width of the

space-charge layer increases (in a solid photoanode, the Fermi level is lowered in a film

consisting of nanoparticles). When the Fermi level lies below the trap state, the trap state is

not occupied by electrons, allowing a ground state absorption from the valence band. This

ground state absorption is lost when electrons that are excited across the band gap by the

laser pulse then relax into the trap state, resulting in a transient bleach (negative absorption).

The bleach recovers (the ground state absorption is regained) as electrons detrap from the

trap state and are extracted to the external circuit. Finally, a positive long-lived transient

signal is regained, which has similar characteristics to the long-lived hole signal (probed at

≥650 nm) that is associated with water oxidation. The relative timescales of electron

detrapping (oxidation of the trap state and bleach recovery; 10-4-10-2 s) and water oxidation

(0.1->2 s) could indicate that the trap state must be oxidised before water oxidation can

occur. This has been suggested by previous studies, which concluded that recombination

‡ This is observed for Si-doped nanostructured hematite photoanodes, but not for solid hematite

photoanodes, in which the fast decay phase (probed at 650nm) has a shorter lifetime than that of the bleach (probed at 575 nm) - see Chapter V. However, solid hematite photoanodes also produce TPC decays with essentially identical kinetics to those of the corresponding TA bleach recovery.


via intra-bandgap trap states in the space-charge layer prevents holes from oxidising water;

water oxidation occurs once the trap state is oxidised.103 Although electron-hole

recombination via the 575 nm trap state is likely to affect the long-lived hole population, it is

probably unsafe to assume that the trap state must be oxidised before water oxidation can

occur.

The intra-bandgap state associated with the transient absorption feature at ~575 nm can

evidently act as either an electron or a hole trap, depending on whether the state is pre-

oxidised (unoccupied by an electron) or pre-reduced, respectively. The occupancy of the

trap is determined by the position of the Fermi level, i.e. by the applied bias. The narrow

spectral region involved (~100 nm wide) implies a narrow energetic distribution of the trap

state. The appearance of the bleach at -0.3 VAg/AgCl, ca. 0.3 V anodic of the flatband

potential, indicates that the trap state is positioned a few hundred millivolts below the

conduction band edge.

This trap state is unlikely to occur only at the hematite surface on the atomic scale, since

the bleach is not observed to saturate, even at strong anodic bias (where the photocurrent is

essentially saturated) on solid hematite photoanodes, which have much lower surface area

Scheme 6.1 Effect of applied bias on trap state and transient absorption bleach (probed at 575

nm). At negative applied bias, the mid-bandgap state is occupied by electrons, so acts as a hole

trap (recombination centre); a positive transient absorption signal is observed. At positive applied

bias, the Fermi level lies below the trap state, which acts as an electron trap; a negative transient

absorption signal (bleach) is observed. Detrapping of electrons and extraction to the external

circuit results in the recovery of the bleach.

positive bias

EF

ECB

EVB

trap

recombination

hν(355 nm)

e-

e-

h+

EF

ECB

EVB

loss ofground state absorption(575 nm)

X

e-

EF

ECB

EVB

hν(355 nm)

e-

h+ e-

ground state absorption(575 nm)

550 600 650 700-1.0

-0.5

0.0

0.5

1.0

mO

D

wavelength / nm

550 600 650 700

-3

-2

-1

0

1

mO

D

wavelength / nm

ECB

EVB

hν(575 nm)

e-

EF


than nanostructured photoanodes. This indicates that a large number of these trap states

are available, and are not fully occupied (saturated) even under significant band-bending.

These results suggest that the trap state is not localised at the surface, but extends

throughout the depth of the hematite film, as shown in Scheme 6.2. As discussed in Chapter

IV, due to the high donor (electron) density of hematite, it is likely that some band bending

occurs even in nanostructured photoanodes. As such, the increasing bleach intensity with

increasing positive bias can be understood by considering the variation in the width of the

space-charge layer/position of the Fermi level. As the width of the space-charge layer

increases/Fermi level moves down with positive bias, further trap states are oxidised (no

longer occupied by electrons), so there is a greater loss of ground state absorption resulting

in a more intense transient bleach. As discussed in previous chapters, as the width of the

space-charge layer increases, the rate of electron-hole recombination decreases resulting in

a larger population of long-lived holes and greater photocurrent density. Consequently, the

intensity of the bleach signal provides an indirect indicator of water oxidation activity. Doped

hematite photoanodes are observed to produce more intense bleach signals than undoped

hematite at a given bias. Doped hematite has a higher electron density, hence a narrower

space-charge layer/higher Fermi level (at a given potential), and thus would be expected to

produce a less intense bleach. However, doping is likely to increase the density of intra-

bandgap trap states, which may explain the more intense bleach signals observed in doped

photoanodes.

The model invoked in Scheme 6.2 is similar to that described by Horowitz, where

impedance and photocurrent measurements provided evidence for a deep donor level 0.7-

0.9 eV below and parallel to the CB edge, and a surface state 0.55 eV below the CB

Scheme 6.2 Effect of applied bias on occupancy of the trap state probed at 575 nm. When the

Fermi level lies above the mid-bandgap state, this state is occupied by electrons (reduced), so

acts as a hole trap (recombination centre). When the Fermi level lies below the trap state is

oxidised and acts as an electron trap. Positive bias increases the width of the space charge layer

(lowers the Fermi level in nanoparticulate films), so more trap states are oxidised.

positive bias

EF

ω

ECB

EVB

+

EF

ω

ECB

EVB

++

++


edge.103 Other studies have also reported deep donor levels 0.5-0.7 eV below the CB edge,

in addition to shallow levels closer to the CB edge.36, 50, 52 It is unclear what the chemical

nature of these trap states may be, although Fe2+ states have been postulated as deep

donors. The shallow levels close to the CB edge may be the same trap state that gives rise

to the 575 nm TA bleach. It seems unlikely that the trap state probed at 575 nm in the time-

resolved studies reported herein is equivalent to the deep donor levels 0.5-0.7 eV below the

CB edge, since the 575 nm state is apparently positioned significantly closer to the CB edge.

Several frequency-domain studies of hematite photoanodes have been published

recently.51, 53, 55, 59, 60 One study into the effect of Al2O3 surface layers on recombination

concluded that such overlayers reduce surface recombination by decreasing the number of

surface states through which recombination occurs.55 However, other studies have

employed a model whereby both hole transfer to the electrolyte and surface electron-hole

recombination occur via the same surface states (Scheme 1.3), which could be

intermediates in the water oxidation mechanism.51, 59, 60 Charging of these states is reported

to cause Fermi-level pinning, where a change in applied bias results mainly in a change in

the voltage drop across the Helmholtz layer. Generally, studies based on the type of model

shown in Scheme 1.3 suggest that the photocurrent onset is coincident with hole

accumulation in these surface states. As discussed in Chapter IV, the TA studies reported

herein indicate that the photocurrent onset correlates with the appearance of a signal

associated with long-lived holes responsible for water oxidation. This suggests that TA

measurements monitoring photogenerated holes (probed at 650-1000 nm) on ca. 10 ms to 2

s timescales may be probing the same species as the surface-state accumulated holes

monitored by frequency domain measurements. The TA hole decay kinetics (i.e. rate of

water oxidation) are independent of applied potential and excitation density (Figures 4.2a

and 4.5c). EIS and IMPS studies have indicated that the hole transfer rate constant

increases with increasing anodic bias and increasing light intensity, varying from

approximately 0 to1 s-1 or 1-10 s-1 at potentials cathodic of the photocurrent onset, to ca. 1-4

s-1 or 10-50 s-1 anodic of Von.51, 60 Additionally, a different EIS study reported surface-trapped

hole lifetimes of 100-10 ms at 0.7-0.8 VSCE respectively (in the dark).17 Consequently, it is

unclear whether TA and frequency domain studies are probing the same hole transfer

process. However, the results of TA studies reported herein indicate that water oxidation

kinetics on hematite are extremely slow, which is likely to result in surface hole

accumulation, in agreement with several frequency domain investigations and studies

employing rapid hole scavengers.18, 19, 51, 53, 59, 60

Optical transmission spectra of these surface trapped holes obtained from potential- and

light-modulated (frequency domain) absorption spectroscopies of hematite photoanodes

have recently been reported.53 Under anodic bias conditions, the PMAS and LMAS spectra


exhibit strong absorption around 500 nm, which decreases into the near IR. These spectra

appear similar to the TA spectra of undoped hematite in the absence of applied bias

reported herein (Figure 3.2). However, under anodic bias the TA hematite spectrum has a

strong negative absorption (bleach) around 575 nm. Clearly further experiments are

required in order to rationalise the different behaviour observed using time-resolved and

frequency-resolved absorption spectroscopies.

Frequency domain studies have reported recombination rate constants that typically

decrease with anodic applied bias, in a similar manner to the increase in charge carrier

lifetimes with anodic bias indicated by TA studies reported herein. However, the rate

constants for surface-state recombination obtained from EIS and IMPS studies are typically

on the order of 1-100 s-1,51, 60 i.e. equivalent to lifetimes of 0.01-1 s. This is significantly

slower than those obtained from TA studies, where the fast decay phase (on timescales of

~1 μs-10 ms) was associated with electron-hole recombination. Instead, recombination rate

constants from frequency domain studies, which may be associated with back electron

transfer to surface-bound water oxidation intermediates, suggest that this recombination

process occurs on timescales equivalent to the beginning of the slow TA decay phase, which

is associated with water oxidation. It is likely that the recombination process probed by TAS

on <10 ms timescales is a different recombination process, i.e. not back-reaction of water

oxidation intermediates.

TAS and TPC studies reported herein have provided evidence for a particular trap state,

positioned a few hundred millivolts below the CB edge. Water oxidation would not be

thermodynamically possible from this trap state, as it lies above the H2O/O2 redox potential.

Hence it is unlikely that this is the surface state probed by the EIS and IMPS studies

discussed above.

6.5 Conclusions

Transient absorption studies of hematite photoanodes have provided evidence of a

particular energetic trap state, positioned a few hundred millivolts below the conduction band

edge. The spectral feature associated with this trap state is a narrow but intense absorption

centred around 575 nm in the absence of anodic bias, but inverts under anodic bias

conditions to form an intense bleach. When the Fermi level lies above the trap state, this

state is occupied by an electron, so acts as a recombination centre (hole trap), resulting in a

positive TA signal. When positive bias shifts the Fermi level below the trap state,

photogenerated electrons can relax into the trap state from the conduction band (electron

trapping). This results in a TA bleach, which recovers as the electron is detrapped and

extracted to the external circuit. Electron trapping occurs on a timescale of <10 μs, electron


detrapping and extraction to the external circuit occur on the milliseconds timescale,

significantly faster than the timescale of water oxidation (hundreds of milliseconds to

seconds). The increasing magnitude of the (inverted) bleach with increasing positive bias

roughly tracks the shape of the photocurrent/voltage curve, but shifted cathodically.

However, no evidence is found to link this trap state to the mechanism of water oxidation.

Chapter VII: Effect of Co-Based Catalysts 110

Chapter VII

Effect of Co-Based Catalysts

on Hematite Charge Carrier Dynamics:

Comparison of Co2+ and Co-Pi

In which the effect on charge carrier dynamics of the addition of cobalt-oxide “catalysts” to

the surface of hematite photoanodes is investigated. Photogenerated hole dynamics are

studied in isolated hematite and in photoanodes in a photoelectrochemical cell before and

after the adsorption of Co2+ to the hematite surface. The results of these studies are

compared to those from studies of hematite/Co-Pi composite photoanodes. Elucidation of

the effect of the cobalt-oxide “catalyst” on the charge carrier dynamics is aided by

understanding of the trap state discussed in the previous chapter.

111 Chapter VII: Effect of Co-Based Catalysts

7.1 Introduction

Despite hematite’s many advantageous properties as a photoanode material for photo-

assisted water splitting, the water oxidation efficiency is limited by rapid electron-hole

recombination.88, 105 The rate-determining step of water oxidation on hematite occurs on a

timescale of hundreds of milliseconds to seconds, while electron extraction to the external

circuit occurs on a timescale of milliseconds. As discussed in previous chapters, this implies

that hole transfer to water or surface-bound water species occurs on a timescale at least two

orders of magnitude slower than charge carrier recombination.105 Several studies have

provided evidence of sluggish charge-transfer kinetics at the hematite-electrolyte interface.17-

20 As demonstrated in Chapter V, changing the morphology of the photoanode affects the

electron transport through the semiconductor film, but does not significantly change the

timescale of water oxidation. However, modifying the hematite surface with water-oxidation

catalysts is expected to increase the rate of water oxidation. Significant increases in

efficiency would be expected for a catalyst that could accelerate water oxidation such that

the timescale of hole transfer from hematite becomes commensurate with electron-hole

recombination. The requirement of an applied electrical bias for water oxidation on hematite

is also a severe limitation of such photoanodes. As such, modifications that reduce this

requirement for applied bias are the subject of intense research effort.

Several different electro-catalysts have been investigated for use with hematite

photoanodes, including RuO2, IrO2 and cobalt-oxide based catalysts. Modifying the

photoanode surface with RuO2 has lead to mixed results, and has not been investigated

extensively.106, 107 Deposition of IrO2 nanoparticles on the surface of nanostructured Si-

doped hematite photoanodes results in shifting the onset potential cathodically by 200 mV

and achieving a photocurrent of >3 mA.cm-2 at 1.23 VRHE under simulated sunlight.46

However, the IrO2 nanoparticles detach from the hematite surface after relatively short

periods of time.

Much interest has focussed on cobalt-oxide based electro-catalysts for water oxidation.

The absorption of a monolayer of cobalt ions from Co2+ solution on to hematite photoanodes

results in ~100 mV cathodic shift in photocurrent potential and increased IPCE.13 The Co-

treatment was achieved simply by soaking the photoanode in a 10 mM aqueous solution of

Co(NO3)2 for a few minutes, which is then rinsed off. This is thought to result in the

deposition of approximately one monolayer of cobalt ions on the hematite surface. The

increased efficiency was attributed to accumulation of photogenerated holes at cobalt

centres with oxo/hydroxo-type ligands, allowing a catalytic cycle for water oxidation involving

Co II/III and II/IV couples similar to that which operates in Photosystem II.


A self-healing cobalt phosphate (“Co-Pi”) amorphous electro-catalyst has recently been

developed.108 This catalyst is thought to have a Co-oxo/hydroxo structure with molecular

dimensions, consisting of edge-sharing CoO6 octahedra.61 At potentials at which oxygen is

evolved, the average valency of the cobalt ions is ≥3. The mechanism of oxygen evolution

on such Co-Pi catalysts is thought to involve a CoIII to CoIV proton-coupled electron-transfer

step prior to the turnover-limiting process.109 Initial studies investigating the effect of Co-Pi

on hematite photoanodes electrodeposited thick (~200 nm) catalyst layers, by applying a

positive bias to a hematite photoanode submerged in a buffer solution of 0.1 M potassium

phosphate (pH 7) containing 0.5 mM Co(NO3)2 for one hour.110 This Co-Pi treatment

cathodically shifted the photocurrent onset by 350 mV. However, the efficiency of these

composite photoanodes was restricted by non-productive light absorption by the catalyst

(which absorbs in the 350-600 nm region, overlapping with the hematite absorption

spectrum), and kinetically limited due to poor mobility of the proton-accepting electrolyte

through the catalyst layer.62 Thin Co-Pi films photo-electrodeposited (under AM 1.5

simulated irradiation, with anodic bias applied for only 200-500 s) on hematite overcome

these limitations, resulting in greater improvements in photocurrent densities and onset

potentials than electrodeposited Co-Pi or absorbed Co2+ catalyst layers.58 It is thought that

photo-assisted electrodeposition results in deposition only where visible light generates

oxidising equivalents, providing a more uniform distribution of Co-Pi onto the semiconductor

surface than obtained by electrodeposition.

Co-Pi and absorbed-Co2+ catalyst layers are deposited on hematite by very different

methods ((photo)-electrodeposition under positive bias, and adsorption from aqueous

solution, respectively).13, 58 However, these result in similar improvements to the PEC

characteristics of hematite photoanodes, namely a cathodic shift in photocurrent onset

potential by typically 100-200 mV and increased photocurrent densities. Such improvements

are often attributed to “catalysed” water oxidation kinetics, with little experimental evidence

to support faster hole transfer kinetics. Due to the high number of redox states available to

Co, it is thought that clusters of Co ions may act as “hole reservoirs” for the four “oxidising

equivalents” (holes) required for the evolution of each O2 molecule, thus catalysing hole

transfer to water. Although several studies characterising the structure of Co-Pi and

possible electro-catalytic mechanisms for water oxidation have been published recently, as

outlined above, there has been almost no attempt to characterise the chemical nature or

structure of the cobalt-based species deposited by Co2+ treatment.

Despite progress in understanding the mechanism of water oxidation on Co-Pi electro-

catalysts in the dark,109 little is known about the charge carrier dynamics of hematite

photoanodes with such cobalt-oxide type surface catalysts. Surface catalysts are generally

assumed to accelerate water oxidation kinetics. Efficient interfacial hole transfer between


hematite and the cobalt-oxide catalyst could increase the electron gradient within the

underlying hematite, aiding charge separation and thus reducing electron-hole

recombination. It has also been tentatively suggested that Fe2O3 surface states, thought to

mediate recombination, may be “passivated” (although the physical meaning of this is rarely

defined) by Co-Pi type catalysts.51, 110 Transient absorption spectroscopy allows the

dynamics of photogenerated holes in hematite photoanodes to be probed as a function of

applied bias.20, 105 Thus the effects of cobalt-oxide type catalysts on charge carrier dynamics

– potentially including transfer of photogenerated holes from Fe2O3 to cobalt ions – in

hematite photoanodes in a working PEC cell can be investigated. An improved

understanding of how these catalysts work could lead to new design rules for more efficient

composite hematite/catalyst photoanodes.

For the studies reported herein, the Co2+ absorption method was employed together with

nanostructured, Si-doped hematite photoanodes to allow direct comparison to literature

studies;13 these results are compared to those obtained in a parallel study of Co-Pi/Fe2O3

composite photoanodes recently published by this group.111 The dynamics of

photogenerated holes in hematite are probed in isolated films and as a function of applied

bias in a complete PEC cell using transient absorption spectroscopy. Transient photocurrent

measurements are used to investigate the dynamics of electrons extracted to the external

circuit. These techniques allow some understanding of the mechanisms by which cobalt-

oxide type materials improve the efficiency of hematite photoanodes for water oxidation.

Elucidation of the effect of the cobalt-oxide “catalyst” on the charge carrier dynamics is aided

by understanding the processes contributing to the transient absorption bleach observed at

~575 nm under positive applied bias, associated with a particular trap state as discussed in

the previous chapter. It is demonstrated that charge carrier dynamics in hematite

photoanodes that have been treated with Co2+ or Co-Pi are almost indistinguishable, despite

the very different methods employed to deposit these two types of cobalt-oxo/hydroxo

layers. No evidence is found for hole transfer from hematite to cobalt, nor for increased

water oxidation kinetics at low to moderate anodic bias. Instead, the increased water

oxidation activity is attributable to a reduction in electron-hole recombination.

7.2 Experimental

Si-doped nanostructured hematite films deposited by APCVD were used in this study. Co-

treatment of the photoanodes was achieved by soaking the photoanode in an aqueous

solution of ~2mM Co(NO3)2 for approximately 30 minutes, then rinsing briefly with de-ionised

water and drying in a nitrogen stream.13 Where photoanodes were treated with Co a second

time, the treatment was repeated as previously. The photoanodes were not heat-treated


before measurements as this is known to destroy the activity of Co-treated hematite.



outlined in the Methods section. For chronoamperometry (chopped light photocurrent)

measurements, the dark current was allowed to reach a constant value before the

photoanode was illuminated. TAS and TPC measurements both employed 355 nm, 0.20

mJ.cm-2, 0.25-0.33 Hz EE (“front-side”) pulsed excitation. The electrolyte was 0.1 M NaOH

(pH ~12.8), not degassed.

7.3 Effect of Co-adsorption on charge carrier dynamics

Current/voltage measurements of nanostructured Si-Fe2O3 photoanodes before and after

Co2+-adsorption are shown in Figure 7.1. The photocurrent density is increased and the

photocurrent and dark current onset cathodically shifted with each application of Co2+. After

two applications, the photocurrent onset is shifted cathodically by approximately 100 mV

(from ca +0.06 to -0.04 VAg/AgCl), and the photocurrent density increased from 1.2 mA.cm-2 to

2.0 mA.cm-2 at +0.27 VAg/AgCl (equivalent to ~1.23 VRHE). These results are comparable to

those already reported in the literature for Co2+ treatment,13 and also very similar to the

cathodic shift in photocurrent onset potential and increase in photocurrent density observed

with Co-Pi deposition onto hematite photoanodes.58

Although initial studies tentatively suggested that the Co-oxide based treatments may

“passivate” hematite surface states,110 Figure 7.2 shows that chronoamperometry transient

photocurrent spikes increase in magnitude after Co2+-adsorption. Such spikes are attributed

Fig 7.1 Current/voltage curves (in 0.1M NaOH, pH ~12.8, white light illumination, 10 mV.s-1

) of

Si-Fe2O3 APCVD photoanodes (dark grey), after Co-treatment with Co(NO3)2 (blue), and after

repeated Co-treatment (pale blue) for SE (dashed; “back-side”) and EE (solid; “front-side”)

illumination. Inset: expansion of the photocurrent onset region.

-0.2 0.0 0.2 0.4 0.6

0

1

2

3

4

curr

ent

density /

mA

.cm

-2

bias (V vs Ag/AgCl)

0.0 0.1 0.2

0.0

0.5

1.0

1.5

Co


to recombination via surface states or back-reaction of surface intermediates55 (although

some of the initial photocurrent may be attributed to cobalt oxidation, particularly at low

anodic bias62) on relatively slow (hundreds of millisecond to second) timescales; see

Chapter VI for further discussion of these transients. Consequently, these results suggest

that the Co-treatment does not passivate surface states responsible for this slow

recombination. Similar photocurrent transients have been reported for both Co-Pi and Co2+-

treated hematite photoanodes.58 Although some studies have reported a decrease in the

amplitude of such photocurrent spikes after Co-treatment, inspection of those results

suggests that the transient response may be shifted cathodically, rather than reduced across

the whole potential range.47, 58, 62

In order to understand how the Co2+-adsorption increases the photocurrent and shifts the

onset, TAS and TPC (microsecond to millisecond timescales) were used to monitor the

dynamics of photogenerated holes and electron extraction, respectively. Transient

absorption decays of isolated hematite films in 0.1M NaOH (i.e. with no applied bias) before

Fig 7.2 Chopped light photocurrent transients from nanostructured Si-Fe2O3 photoanodes

before and after Co-treatment, at +0.2 VAg/AgCl (355nm EE illumination). SE illumination gives

similar results but with lower photocurrent densities.

Fig 7.3 Transient absorption decays of isolated Si-Fe2O3 photoanodes before (black) and after

(coloured) Co2+

-adsorption (355nm 0.20 mJ.cm-2

EE excitation, 0.1M NaOH, no applied bias),

probed at (a) 575 nm (b) 650 nm and (c) 900 nm.

0 20 40 60 80 100

0.0

0.1

0.2

0.3

0.4

0.5

curr

ent

/ m

A.c

m-2

time / s

extra Co/Si-Fe2O

3

Co/Si-Fe2O

3

Si-Fe2O

3

1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

0.0

0.1

0.2

0.3

0.4

0.5

m

OD

time / s

(b)

1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

0.00

0.05

0.10

0.15

0.20

m

OD

time / s

(c)

1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

-1.00

-0.75

-0.50

-0.25

0.00

m

OD

time / s

(a)


and after a single cobalt treatment are shown in Figure 7.3. It is clear that the lifetime of the

charge carriers probed at these wavelengths is increased by several orders of magnitude in

the presence of cobalt, even in the absence of applied bias. As discussed in Chapters III

and IV, long-lived holes, with lifetimes of hundreds of microseconds to seconds, are

necessary for water oxidation to occur on hematite.

The dynamics of photogenerated holes in hematite photoanodes in a complete

photoelectrochemical cell (i.e. under applied bias) were investigated by probing their

transient absorption at 650 nm (Figure 7.4). In the absence of cobalt, the fast phase of this

decay (ca. 1 μs - 20 ms) is associated with electron-hole recombination, while the slow

decay phase (>20 ms) is attributed to water oxidation (see Chapter IV). Although the effect

of Co2+-adsorption is less dramatic than in the absence of applied bias, it results in

significant differences in the decay dynamics. At low applied bias (<0.4 VAg/AgCl,

approximately equivalent to <1.36 VRHE), the population of long-lived holes, as evidenced by

the amplitude of the TA signal at >20 ms, is greater after Co2+-adsorption. The kinetics of

the fast decay phase, associated with charge carrier recombination, are retarded in the

presence of cobalt, particularly at -0.1 V (Figure 7.4a, close to the photocurrent onset

Fig 7.4 Charge carrier dynamics of

photogenerated holes in Si-Fe2O3

photoanodes before (black) and after

(coloured) Co-treatment (355nm 0.20

mJ.cm-2

EE excitation, 0.1M NaOH)

probed at 650 nm under applied bias:

(a) -0.1 VAg/AgCl (b) +0.2 VAg/AgCl and

(c) +0.4 VAg/AgCl. Insets: decays

shown on linear time scales.

1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

m

OD

time / s

Co/Si-Fe2O

3

Si-Fe2O

3

(a) -0.1 VAg/AgCl

0.0 0.5 1.0 1.5 2.0

0.0

0.1

0.2

1E-6 1E-5 1E-4 1E-3 0.01 0.1 10.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

mO

D

time / s

Co/Si-Fe2O

3

Si-Fe2O

3

(c) +0.4 VAg/AgCl

0.0 0.5 1.0 1.5 2.00.0

0.1

0.2

0.3

0.4

0.5

1E-6 1E-5 1E-4 1E-3 0.01 0.1 10.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

m

OD

time / s

Co/Si-Fe2O

3

Si-Fe2O

3

(b) +0.2 VAg/AgCl

0.0 0.5 1.0 1.5 2.0

0.0

0.1

0.2

0.3

0.4


potential after Co-treatment). Faster hole transfer (to either the catalyst or to water/surface-

bound water species) would manifest as more rapid decay of the TA hole signal to zero,

which is not observed in this potential range. There is no evidence for accelerated water

oxidation kinetics, nor is there evidence for hole transfer from hematite to the “catalyst” - in

other words, there is no evidence that cobalt ions act as “hole reservoirs”. These results are

consistent with water oxidation occurring via a series of single hole transfer steps, as

discussed in Chapter IV. These results are essentially identical to those obtained from

mesoporous Nb-doped hematite photoanodes with a thin photo-deposited layer of Co-Pi

(CoOx).111

At more positive applied bias (+0.4 VAg/AgCl, Figure 7.4c) where significant photocurrent is

generated even without cobalt, the initial population of long-lived holes is unchanged after

Co2+-adsorption. However, the decay kinetics of the long-lived holes – attributed to water

oxidation – are slightly faster after Co2+-adsorption. Although this provides very tentative

evidence for increased water-oxidation kinetics in the presence of a Co-oxide type material,

it is emphasised that this only occurs under very high anodic bias conditions, equivalent to

ca. +1.36 VRHE.

Transient photocurrent measurements are employed to qualitatively probe electron

extraction, as demonstrated in Chapter IV. Although the early timescale TPC response is

likely to be limited by the measurement resistor, the TPC from ca. 10 μs shown in the figures

below does not appear to be limited by the measurement resistor (see Section 2.2.3) and

hence provides provide information about the timescale of electron extraction to the external

circuit. TPC decay dynamics of hematite photoanodes before and after Co2+-adsorption

Fig 7.5 TPC decays probing electron extraction from Si-Fe2O3 photoanodes before (black/grey)

and after Co-treatment (pale colours) at -0.1 VAg/AgCl (green) and +0.4 VAg/AgCl (orange). Pulsed

355nm 0.20 mJ.cm-2

EE excitation, 0.1M NaOH electrolyte.

1E-4 1E-3 0.01 0.1

0.00

0.25

0.50

0.75

1.00

cu

rre

nt

/ m

A

time / s

Co/Si-Fe2O

3 +0.4 V

Ag/AgCl

Si-Fe2O

3 +0.4 V

Ag/AgCl

Co/Si-Fe2O

3 +0.1 V

Ag/AgCl

Si-Fe2O

3 +0.1 V

Ag/AgCl


were investigated at -0.1 and +0.4 VAg/AgCl (Figure 7.5). The first potential corresponds to the

photocurrent onset potential after Co-treatment, but at which there is very little photocurrent

in the absence of cobalt. At +0.4 VAg/AgCl, significant photocurrent is generated even in the

absence of cobalt. The TPC rise is not considered as this is likely to be RC-limited;112

instead, only the decays (from ~100 μs) are discussed. Although there is a small increase in

the total charge extracted from the hematite photoanodes after Co2+-adsorption (to be

expected given the increased photocurrent densities observed in Figures 7.1 and 7.2), it is

evident that the decay kinetics are identical before and after Co2+-adsorption. These results

indicate that Co2+-adsorption does not affect the electron transport properties of hematite,

which is unsurprising since this treatment is likely only to modify the hematite surface and

not the bulk.

The effect of Co2+-adsorption on hematite photoanodes under applied bias is further

investigated by considering the transient absorption spectra, shown in Figure 7.6. Spectra of

Si-Fe2O3 at 0 and +0.4 VAg/AgCl before and after Co2+-adsorption are shown. Generally,

application of positive bias or cobalt treatment increases the depth of the intense early

Fig 7.6 Transient absorption spectra of Si-Fe2O3 photoanodes before (left) and after Co2+

-adsorption (right)

at 0 VAg/AgCl (top) and +0.4 VAg/AgCl (bottom), at 10 μs, 100 μs, 1 ms, 10 ms, 100 ms and 1 s (black through

blue to grey) after the excitation pulse. There is a striking similarity between Si-Fe2O3 at +0.4 VAg/AgCl and

Co/Si-Fe2O3 at 0 VAg/AgCl.

500 600 700 800 900-4

-3

-2

-1

0

1

m

OD

wavelength / nm

Si-Fe2O

3 0 V

Ag/AgCl

500 600 700 800 900-4

-3

-2

-1

0

1

Co/Si-Fe2O

3 0 V

Ag/AgCl

m

OD

wavelength / nm

500 600 700 800 900-4

-3

-2

-1

0

1

Co/Si-Fe2O

3 +0.4 V

Ag/AgCl

10 us

100 us

1 ms

10 ms

100 ms

1 s

m

OD

wavelength / nm

500 600 700 800 900-4

-3

-2

-1

0

1

Si-Fe2O

3 +0.4 V

Ag/AgCl

m

OD

wavelength / nm


timescale bleach at ~575 nm (discussed below), and increases the population of long-lived

holes (indicated by the pale blue and grey colours). The similarity between the spectra of

the Co2+/Fe2O3 photoanode at 0 VAg/AgCl and the bare photoanode at +0.4VAg/AgCl is especially

striking, particularly in the 500-650 nm region at 10-100 ms (mid-pale blue) where a weak

negative absorption feature is observed at +0.4 VAg/AgCl without the catalyst, but at 0 VAg/AgCl

with the catalyst. This suggests that the result of Co2+-adsorption is equivalent to an

application of more positive applied electrical bias. Additionally, the similarities between the

spectra before and after Co2+-adsorption indicate that there is no hole transfer from hematite

to the catalyst. These results are also essentially identical to those obtained from Co-

Pi/Fe2O3 composite photoanodes.111

As discussed in Chapter VI, the early timescale bleach at ~575 nm is associated with

photo-induced electron trapping by a particular trap state positioned a few hundred millivolts

below the CB edge. The intensity of this bleach signal is an indicator of the degree of

electron depletion of the hematite. Increased bleach intensity is indicative of increased

space-charge layer width or lowering of the Fermi level (depending on the semiconductor

particle size), as would occur with application of a positive bias. However, Co2+-adsorption

also increases the intensity of this bleach, as shown in the TA spectra (Figure 7.6) and the

dynamics of the TA decays probed at 575 nm, shown in Figure 7.7. This effect is greatest at

more modest potentials, comparable to the difference in long-lived hole dynamics before and

after Co2+-adsorption when probed at 650 nm (Figure 7.4). Both the increased intensity of

the bleach and greater long-lived hole population after Co2+-adsorption are equivalent to the

change in charge carrier dynamics observed upon application of approximately 200 mV

anodic bias. This is comparable to the cathodic shift in onset potential observed after Co2+-

adsorption (Figure 7.1).

Fig 7.7 Transient absorption decays probed at 575 nm under applied bias at (a) 0 VAg/AgCl (just

cathodic of the photocurrent onset potential in the absence of cobalt), and (b) +0.4 VAg/AgCl,

(where significant photocurrent is generated even in the absence of cobalt). Before (black) and

after (coloured) Co2+

-adsorption; cobalt increases the magnitude of the bleach, particularly at low

positive applied bias.

1E-6 1E-5 1E-4 1E-3 0.01 0.1 1-2.0

-1.5

-1.0

-0.5

0.0

Co/Si-Fe2O

3

m

OD

time / s

(a)

Si-Fe2O

3

1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

-4

-3

-2

-1

0

Co/Si-Fe2O

3

m

OD

time / s

(b)

Si-Fe2O

3


7.4 Discussion

As demonstrated in previous chapters, long-lived holes are necessary for water oxidation

on hematite, which occurs on the timescale of hundreds of milliseconds to seconds. It is

clear that Co2+-adsorption causes the long-lived hole population to increase, particularly at

low applied bias and in isolated hematite films. The increased long-lived hole signal (probed

at 650 nm, >10 ms) is accompanied by increased bleaching (probed at 575 nm). The

transient absorption spectra of a hematite photoanode without cobalt-oxide at +0.4 VAg/AgCl

and with cobalt-oxide at 0 VAg/AgCl are strikingly similar. Co2+-adsorption is also observed to

reduce the rate of electron-hole recombination, as evidenced by the longer lifetime of the

fast transient absorption decay phase (<10 ms, probed at 650 nm). The effects of Co2+-

adsorption on hematite charge carrier dynamics are almost identical to those observed on

the application of positive bias, suggesting that Co2+-adsorption results in lowering the

hematite Fermi level and/or increasing the width of the space-charge layer. This

interpretation is consistent with the cathodic shift of the photocurrent onset potential

observed after Co2+-adsorption. As discussed in previous chapters, the application of a

positive applied bias to hematite photoanodes reduces electron-hole recombination and

increases the population of long-lived holes.

Except at high positive applied bias, there is no evidence from the TA studies reported

herein that Co2+-adsorption increases the rate of water oxidation on hematite. At +0.4

VAg/AgCl (~1.36 VRHE), the decay of the long-lived hole signal, attributed to water oxidation, is

slightly faster after Co2+-adsorption, as shown in Figure 7.4c. This provides some tentative

evidence for faster water oxidation, but only at potentials significantly above the

thermodynamic water oxidation potential (1.23 VRHE). Additionally, the similarities in the TA

spectra and the kinetics of long-lived holes also indicate that hole transfer from hematite to

adsorbed cobalt does not occur.

The effect of Co2+-adsorption on the charge carrier dynamics in hematite photoanodes is

nearly identical to that of Co-Pi, studied in parallel within this group.111 The large increase in

charge carrier lifetime in isolated hematite after Co2+-adsorption (Figure 7.3) is almost

indistinguishable from that observed for Co-Pi/Fe2O3 composite photoanodes. The effect of

Co2+ and Co-Pi on charge carrier dynamics in hematite photoanodes under applied bias is

also strikingly similar. Both Co2+-adsorption and Co-Pi deposition have essentially the same

effect as applying a more positive electrical bias: increased amplitude of the long-lived hole

signal, reduced electron-hole recombination rate, and increased 575 nm bleach depth. The

results obtained from transient optical measurements reported herein strongly suggest that

Co2+-adsorption and Co-Pi deposition on hematite result in the formation of the same cobalt-

based material, i.e. a cobalt-oxo/hydroxo structure, as reported for Co-Pi (in which an X-ray


spectroscopy study found little evidence for Co-P bonding).61 This is somewhat surprising,

given the very different deposition techniques employed for each: Co-Pi is deposited under

anodic bias conditions, while no bias is applied for Co2+-adsorption. Structural

characterisation of the Co-oxide type material deposited by Co2+-adsorption, similar to

studies already reported in the literature for Co-Pi, could confirm this hypothesis; such

studies are beyond the scope of this work. However, a recent comparison of the effect of

Co2+-adsorption, electrodeposited Co-Pi, and photo-electrodeposited Co-Pi on hematite

photoanodes also reported strikingly similar effects on the current/voltage characteristics of

the photoanodes.58

The results of these transient spectroscopy studies, namely that Co2+-adsorption and Co-

Pi deposition are not observed to enhance water oxidation kinetics, nor is hole transfer to

cobalt observed, contradict previous assumptions that cobalt-oxide type materials “catalyse”

water oxidation on hematite. Instead, these results indicate that deposition of cobalt-oxide

acts in the same manner as a few hundred millivolts’ anodic bias. This lowers the Fermi

level and/or increases band-bending at the hematite/cobalt-oxide interface. As such, the

cathodically shifted photocurrent onset and increased photocurrent densities are attributed

entirely to decreased electron-hole recombination, leading to more long-lived holes, at least

at potentials cathodic of 1.36 VRHE. This interpretation is summarised in Scheme 7.1. The

results reported herein are also in broad agreement with the conclusions of a recent

photoelectrochemical impedance spectroscopy study, in which Co2+-adsorption was found to

almost completely suppress surface recombination.60 It is possible that a Schottky-type

Co2+ treatment

EF

ω

Fe2O3

h+

e-

hν

recombination

ECB

EVB

H2O

O2

EF

ω

Fe2O3 CoOx

O2

H2O

h+

e-

Scheme 7.1 Proposed effect of Co2+

-adsorption/Co-Pi deposition on hematite photoanodes. It

is also possible that the CoOx layer removes (“passivates”) Fe2O3 surface trap states through

which recombination occurs, however the increase in magnitude of chopped light transient

photocurrent spikes after Co2+

-treatment is not consistent with this explanation.


inorganic heterojunction is formed between the hematite and cobalt-oxide, resulting in

increased band-bending at the hematite surface. A similar junction has been reported for n-

BiVO4/p-Co3O4 composite photoanodes, resulting in reduced electron-hole recombination

due to enhanced charge separation.113 A recent investigation into the nucleation and growth

of Co-Pi has suggested that this material is formed of an array of CoO(OH) clusters,

consisting of edge-sharing CoO6 octahedra.114 It is possible that Co2+/Co-Pi deposition

forms a cobalt oxide structure similar to p-Co3O4. Hematite photoanodes with a thin surface

overlayer of p-type (Mg-doped) hematite have recently been reported to exhibit enhanced

photoelectrochemical properties.45 According to the models of hematite charge carrier

dynamics developed herein, it is possible that such p/n junctions could reduce electron-hole

recombination, and hence enhance charge separation, at the hematite surface. It has been

suggested that cobalt ions may act as “hole reservoirs”, since several oxidation states are

available to cobalt. However, it is possible that a cobalt-oxo/hydroxo layer acts as an

electron reservoir, depleting electrons from the hematite surface and thus reducing the rate

of electron-hole recombination. This effect could be investigated by comparison of the

flatband potential before and after cobalt oxide deposition, however this lies beyond the

scope of these thesis studies.

7.5 Conclusions

The charge carrier dynamics in hematite photoanodes before and after Co2+-adsorption

were studied as a function of applied bias, and the results compared to those obtained from

Co-Pi/Fe2O3 composite photoanodes. The effect of Co2+-adsorption on the charge carrier

dynamics in hematite photoanodes is nearly identical to that of Co-Pi. This indicates that

Co2+-adsorption and Co-Pi deposition on hematite result in the formation of the same cobalt-

based material, i.e. a cobalt-oxo/hydroxo structure. Contrary to previous assumptions, no

evidence was found for hole transfer to the cobalt-oxide, nor for increased water oxidation

kinetics at moderate applied bias. Both Co2+-adsorption and Co-Pi deposition have

essentially the same effect as applying a more positive electrical bias: reduced electron-hole

recombination rate, more intense 575 nm bleach, and increased population of the long-lived

holes responsible for water oxidation. These results are interpreted as evidence that

Co2+/Co-Pi induce increased band-bending at the hematite surface. This increased band-

bending reduces electron-hole recombination, resulting in a greater yield of long-lived holes

– particularly at low applied bias – and hence a cathodic shift in photocurrent onset and

greater photocurrent densities.


Chapter VIII: Concluding Remarks 124

Chapter VIII: Concluding Remarks

The primary focus of the PhD studies reported herein was to investigate the physical

processes which limit water photo-oxidation efficiencies of hematite photoanodes.

Specifically, the objective of these investigations was to explain why some types of hematite

photoanode exhibit greater water photo-oxidation activity than others. Transient absorption

spectroscopy (TAS) was used to monitor the photogenerated charge-carrier dynamics,

primarily of holes, in hematite photoanodes on the microsecond-seconds timescale.

Transient photocurrent (TPC) measurements on timescales of microseconds to tens of

milliseconds were employed to probe the kinetics of electron extraction to the external

circuit. TAS and TPC allowed the timescales of electron-hole recombination, electron

trapping/detrapping, water oxidation and electron extraction to be determined. These

techniques were used in conjunction with measurements of photocurrent response as a

function of voltage, allowing comparison of charge-carrier dynamics with water oxidation

activity.

It was shown in Chapters III and IV that charge carrier dynamics in hematite are

dominated by electron-hole recombination on microsecond to millisecond timescales.

Transient absorption studies of hematite photoanodes in a working PEC cell indicated that

the role of a positive applied bias is more complex than previously thought. Positive

electrical bias not only increases the reduction potential of photogenerated electrons, but

also reduces the background electron density in hematite. This was found to result in

decreased electron-hole recombination and thus increased hole lifetime, such that

photogenerated holes can diffuse/drift to the semiconductor-electrolyte junction and oxidise

water. The timescale of water oxidation was found to be on the order of hundreds of

milliseconds to seconds, hence extremely long-lived holes are required for water oxidation to

occur. A quantitative correlation between the yield of long-lived photogenerated holes and

photocurrent density was demonstrated. The rate constant of water oxidation by these long-

lived holes was independent of hole density, indicating that water oxidation proceeds via a

rate-determining single-hole transfer step. These results support proposed water oxidation

mechanisms consisting of multiple sing-hole transfer steps.

The charge carrier dynamics in hematite photoanodes with various different morphologies,

including thick and thin solid (non-porous), nanoparticulate colloidal, and nanocrystalline

dendritic nanostructured hematite films were compared in Chapter V. Water oxidation

kinetics were shown to be very similar on solid and nanostructured hematite. Transient

photocurrent measurements demonstrate that the timescale of electron collection (i.e.

electron transport) and electron-hole recombination losses are highly dependent on the

125 Chapter VIII: Concluding Remarks

nanomorphology of the photoanode. Since hole transfer (water oxidation) is a slow process,

more rapid electron extraction is likely to reduce losses by electron-hole recombination.

Thus hematite photoanodes should be engineered to maximise rapid electron transport to

the back contact. This may be achieved by employing heterostructures with a material with

high electron mobility between hematite and the back contact, or by using deposition

techniques which minimise grain boundaries.

In Chapter III, results of transient absorption studies with chemical scavengers and

applied bias suggested two distinct photogenerated species: trapped holes and a second

species, with a narrow but intense absorption (in undoped hematite with no applied bias)

centred around 575 nm. The bleaching behaviour of this signal at positive potentials

suggests that this feature is associated with the photo-oxidation and -reduction of a trap

state located just below the conduction band edge; further evidence for this interpretation

was discussed in Chapter VI. TAS and TPC were employed to probe the electron

trapping/detrapping and recombination associated with this trap state. Although trap state

oxidation was found to occur prior to hole transfer to water/surface-bound water species, no

direct evidence was found to suggest that this trap state is directly involved in the water

oxidation mechanism. Instead, the magnitude of the TA bleach was modelled as an

indicator of the degree of electron depletion of the hematite film.

Finally, the effect of Co2+-adsorption on the charge carrier dynamics in hematite

photoanodes was investigated and compared with that of Co-Pi in Chapter VII. These two

cobalt surface treatments, deposited under very different conditions, were found to result in

essentially identical charge carrier dynamics. This indicated that Co2+-adsorption and Co-Pi

deposition on hematite result in the formation of the same cobalt-based material, i.e. a

cobalt-oxo/hydroxo structure. No evidence was found for hole transfer to the cobalt-oxide,

nor for increased water oxidation kinetics at moderate anodic bias. Cobalt treatments were

demonstrated to have essentially the same effect as applying a more positive electrical bias.

These results were interpreted as evidence that Co2+/Co-Pi induce increased band-bending

at the hematite surface. This increased band-bending reduces electron-hole recombination,

resulting in a greater yield of long-lived holes – particularly at low applied bias – and hence a

cathodic shift in photocurrent onset and greater photocurrent densities. The results of these

investigations imply that materials that are thought to be electrocatalysts for water oxidation

may not actually catalyse water photo-oxidation on semiconductor materials. The term

“catalyst” should be used with caution.

These studies have elucidated the relative timescales of electron-hole recombination,

electron trapping/detrapping, water oxidation and electron extraction to the external circuit in

hematite photoanodes with a variety of different nanomorphologies. Generally, effects which

Chapter VIII: Concluding Remarks 126

lower electron density result in retarded electron-hole recombination kinetics, increasing the

population of long-lived holes and hence increasing the photocurrent.

Future Work

Although the primary objectives of this project have been achieved, further studies could

advance our understanding of the physical processes occurring in hematite photoanodes,

and test the models of charge carrier dynamics constructed during these thesis studies.

Some potential areas of investigation are outlined below.

Ultrafast TAS: The time resolution of the TA measurements reported herein does not

allow charge carrier dynamics faster than ~1 μs to be monitored. It is evident that, even

under anodic applied bias, significant electron-hole recombination occurs on sub-

microsecond timescales. Significant differences in recombination dynamics on microsecond

were observed between solid and nanostructured hematite; does the “fast decay phase”

occur on sub-microsecond timescales in solid hematite? Additionally, what processes

associated with the 575 nm trap state occur on <1 μs timescales?

IR-probe TAS: No transient absorption signal clearly assignable to photogenerated

electrons in hematite has yet been observed. It is possible that photogenerated electrons in

hematite absorb at wavelengths greater than 1000 nm. Direct optical measurement of

electrons would allow testing of the models of electron-hole recombination and electron

trapping/detrapping and extraction developed during these PhD studies.

TAS and TPC under white light bias: The TAS and TPC studies reported in this thesis

were conducted in the “dark”, i.e. with no white light incident on the photoanode.

Consequently, these measurements were not made under working conditions typical for

photoelectrochemical cells. Future studies should compare charge carrier dynamics under

white light bias to those reported herein.

Heterojunction photoanodes: As discussed above, heterojunctions of hematite with a

layer of high-electron-mobility material may reduce electron-hole recombination losses by

rapidly extracting electrons to the external circuit. TAS and TPC studies of such

heterojunction photoanodes could be employed to determine whether more rapid electron

extraction from the hematite does occur, and may also elucidate the effects of potential

electron-hole recombination at the junction.

According to the models of hematite charge carrier dynamics developed during these

thesis studies, it is possible that thin surface overlayers of p-type materials could reduce

electron-hole recombination by enhancing band-bending at the hematite surface. Transient

absorption studies of hematite photoanodes with thin overlayers materials such as Co3O4 or

Mg-doped Fe2O3 could aid the understanding of how such p/n junctions enhance the

photoelectrochemical properties of hematite photoanodes.

127 Chapter VIII: Concluding Remarks

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Charge Carrier Dynamics in Hematite Photoanodes for Solar ...€¦ · for water oxidation to occur on hematite. Improved understanding of the role of applied bias and the processes