Publications

Journal article

Pinto F, Wilson A, Moss B, Kafizas Aet al., 2022,

Systematic exploration of WO3/TiO2 heterojunction phase space for applications in photoelectrochemical water splitting

, The Journal of Physical Chemistry C: Energy Conversion and Storage, Optical and Electronic Devices, Interfaces, Nanomaterials, and Hard Matter, Vol: 126, Pages: 871-884, ISSN: 1932-7447

Recent work has shown that heterojunction photoelectrodes can achieve synergistically higher water splitting activity than their parent materials. To optimize the performance in such layered systems, it is necessary to develop new methods capable of assessing heterojunction phase space. Herein, we explore WO3/TiO2 heterojunction phase space as a model system. Using chemical vapor deposition, 71 unique photoanodes were grown (15 single-layer; 56 heterojunctions). The materials were physically characterized using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy analysis, and ultraviolet–visible transmission spectroscopy. Various key performance indicators were measured. Within this WO3/TiO2 heterojunction phase space, the onset potentials ranged from ∼0.45 to ∼0.81 VRHE; the incident-photon-to-current efficiencies at 350, 375, and 400 nm ranged from ∼0.6 to ∼50.9, ∼0.1 to ∼30.0, and ∼0 to ∼15.6%, respectively; and the theoretical solar photocurrents ranged from ∼0.01 to ∼0.94 mA cm–2. Contour plots allowed us to identify regions of heterojunction phase space with high activity and establish trends. We identified an electronic barrier to charge transfer between the heterojunction layers that required a sufficiently high applied potential (≥1.0 VRHE) to be surpassed for synergetic improvements in activity to be observed. We recommend that the methods developed herein, for assessing the performance of sample libraries of heterojunction photoelectrodes, be used alongside combinatorial synthesis methods and high-throughput photoelectrochemical measurements to optimize promising heterojunction systems more rigorously and rapidly.

Journal article

Eisner F, Tam B, Belova V, Ow W, Yan J, Azzouzi M, Kafizas A, Campoy-Quiles M, Hankin A, Nelson Jet al., 2021,

Color-tunable hybrid heterojunctions as semi-transparent photovoltaic windows for photoelectrochemical water splitting

, Cell Reports Physical Science, Vol: 2, Pages: 1-16, ISSN: 2666-3864

The strong but narrow-bandwidth absorption spectra of organic semiconductors make them excellent candidates for semi-transparent solar cell applications in which color specificity is important. In this study, using a hybrid heterojunction combining the transparent inorganic semiconductor copper thiocyanate (CuSCN) with organic semiconductors (C70, PC70BM, C60, ITIC, IT-4F, or Y6), we show that simple color-tunable solar cells can be fabricated in which the transmission spectrum is determined solely by choice of the organic semiconductor. Using a joint electrical-optical model, we show that it is possible to combine the unique attributes of high photovoltage and color tunability to use these heterojunctions as photovoltaic windows in tandem photoelectrochemical (PEC)-photovoltaic (PV) cells. We demonstrate that this configuration can lead to a reduction in the parasitic absorption losses in the PEC-PV cells and, thus, to solar-to-hydrogen efficiencies (>3%) that are higher than that predicted using the traditionally used architecture in which the PV is placed behind the PEC.

Journal article

Moss B, Babacan O, Kafizas A, Hankin Aet al., 2021,

A review of inorganic photoelectrode developments and reactor scale-up challenges for solar hydrogen production

, Advanced Energy Materials, Vol: 11, Pages: 1-43, ISSN: 1614-6832

Green hydrogen, produced using solar energy, is a promising means of reducing greenhouse gas emissions. Photoelectrochemical (PEC) water splitting devices can produce hydrogen using sunlight and integrate the distinct functions of photovoltaics and electrolyzers in a single device. There is flexibility in the degree of integration between these electrical and chemical energy generating components, and so a plethora of archetypal PEC device designs has emerged. Although some materials have effectively been ruled out for use in commercial PEC devices, many principles of material design and synthesis have been learned. Here, the fundamental requirements of PEC materials, the top performances of the most widely studied inorganic photoelectrode materials, and reactor structures reported for unassisted solar water splitting are revisited. The main phenomena limiting the performance of up‐scaled PEC devices are discussed, showing that engineering must be considered in parallel with material development for the future piloting of PEC water splitting systems. To establish the future commercial viability of this technology, more accurate techno‐economic analyses should be carried out using data from larger scale demonstrations, and hence more durable and efficient PEC systems need to be developed that meet the challenges imposed from both material and engineering perspectives.

Journal article

Moss B, Wang Q, Butler K, Grau-Crespo R, Selim S, Regoutz A, Hisatomi T, Godin R, Payne D, Kafizas A, Domen K, Steier L, Durrant Jet al., 2021,

Linking in situ charge accumulation to electronic structure in doped SrTiO3 reveals design principles for hydrogen-evolving photocatalysts

, Nature Materials, Vol: 20, Pages: 511-517, ISSN: 1476-1122

Recently, high solar-to-hydrogen efficiencies were demonstrated using La and Rh co-doped SrTiO3 (La,Rh:SrTiO3) incorporated into a low-cost and scalable Z-scheme device, known as a photocatalyst sheet. However, the unique properties that enable La,Rh:SrTiO3 to support this impressive performance are not fully understood. Combining in situ spectroelectrochemical measurements with density functional theory and photoelectron spectroscopy produces a depletion model of Rh:SrTiO3 and La,Rh:SrTiO3 photocatalyst sheets. This reveals remarkable properties, such as deep flatband potentials (+2 V versus the reversible hydrogen electrode) and a Rh oxidation state dependent reorganization of the electronic structure, involving the loss of a vacant Rh 4d mid-gap state. This reorganization enables Rh:SrTiO3 to be reduced by co-doping without compromising the p-type character. In situ time-resolved spectroscopies show that the electronic structure reorganization induced by Rh reduction controls the electron lifetime in photocatalyst sheets. In Rh:SrTiO3, enhanced lifetimes can only be obtained at negative applied potentials, where the complete Z-scheme operates inefficiently. La co-doping fixes Rh in the 3+ state, which results in long-lived photogenerated electrons even at very positive potentials (+1 V versus the reversible hydrogen electrode), in which both components of the complete device operate effectively. This understanding of the role of co-dopants provides a new insight into the design principles for water-splitting devices based on bandgap-engineered metal oxides.

Journal article

Alotaibi AM, Promdet P, Hwang GB, Li J, Nair SP, Sathasivam S, Kafizas A, Carmalt CJ, Parkin IPet al., 2021,

Zn and N codoped TiO2 thin films: photocatalytic and bactericidal activity.

, ACS Applied Materials and Interfaces, Vol: 13, Pages: 10480-10489, ISSN: 1944-8244

We explore a series of Zn and N codoped TiO2 thin films grown using chemical vapor deposition. Films were prepared with various concentrations of Zn (0.4-2.9 at. % Zn vs Ti), and their impact on superoxide formation, photocatalytic activity, and bactericidal properties were determined. Superoxide (O2•-) formation was assessed using a 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium sodium salt (XTT) as an indicator, photocatalytic activity was determined from the degradation of stearic acid under UVA light, and bactericidal activity was assessed using a Gram-negative bacterium E. coli under both UVA and fluorescent light (similar to what is found in a clinical environment). The 0.4% Zn,N:TiO2 thin film demonstrated the highest formal quantum efficiency in degrading stearic acid (3.3 × 10-5 molecules·photon-1), while the 1.0% Zn,N:TiO2 film showed the highest bactericidal activity under both UVA and fluorescent light conditions (>3 log kill). The enhanced efficiency of the films was correlated with increased charge carrier lifetime, supported by transient absorption spectroscopy (TAS) measurements.

Journal article

Iqbal A, Kafizas A, Sotelo-Vazquez C, Wilson R, Ling M, Taylor A, Blackman C, Bevan K, Parkin I, Quesada-Cabrera Ret al., 2021,

Charge transport phenomena in heterojunction photocatalysts: the WO3/TiO2 system as an archetypical model.

, ACS Applied Materials and Interfaces, Vol: 13, Pages: 9781-9793, ISSN: 1944-8244

Recent studies have demonstrated the high efficiency through which nanostructured core-shell WO3/TiO2 (WT) heterojunctions can photocatalytically degrade model organic pollutants (stearic acid, QE ≈ 18% @ λ = 365 nm), and as such, has varied potential environmental and antimicrobial applications. The key motivation herein is to connect theoretical calculations of charge transport phenomena, with experimental measures of charge carrier behavior using transient absorption spectroscopy (TAS), to develop a fundamental understanding of how such WT heterojunctions achieve high photocatalytic efficiency (in comparison to standalone WO3 and TiO2 photocatalysts). This work reveals an order of magnitude enhancement in electron and hole recombination lifetimes, respectively located in the TiO2 and WO3 sides, when an optimally designed WT heterojunction photocatalyst operates under UV excitation. This observation is further supported by our computationally captured details of conduction band and valence band processes, identified as (i) dominant electron transfer from WO3 to TiO2 via the diffusion of excess electrons; and (ii) dominant hole transfer from TiO2 to WO3 via thermionic emission over the valence band edge. Simultaneously, our combined theoretical and experimental study offers a time-resolved understanding of what occurs on the micro- to milliseconds (μs-ms) time scale in this archetypical photocatalytic heterojunction. At the microsecond time scale, a portion of the accumulated holes in WO3 contribute to the depopulation of W5+ polaronic states, whereas the remaining accumulated holes in WO3 are separated from adjacent electrons in TiO2 up to 3 ms after photoexcitation. The presence of these exceptionally long-lived photogenerated carriers, dynamically separated by the WT heterojunction, is the origin of the superior photocatalytic efficiency displayed by this system (in the degradation of stearic acid). Consequently, our combined computational and ex

Journal article

Wilson AA, Corby S, Francas L, Durrant JR, Kafizas Aet al., 2021,

The effect of nanoparticulate PdO co-catalysts on the faradaic and light conversion efficiency of WO3 photoanodes for water oxidation

, Physical Chemistry Chemical Physics, Vol: 23, Pages: 1285-1291, ISSN: 1463-9076

WO3 photoanodes offer rare stability in acidic media, but are limited by their selectivity for oxygen evolution over parasitic side reactions, when employed in photoelectrochemical (PEC) water splitting. Herein, this is remedied via the modification of nanostructured WO3 photoanodes with surface decorated PdO as an oxygen evolution co-catalyst (OEC). The photoanodes and co-catalyst particles are grown using an up-scalable aerosol assisted chemical vapour deposition (AA-CVD) route, and their physical properties characterised by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and UV-vis absorption spectroscopy. Subsequent PEC and transient photocurrent (TPC) measurements showed that the use of a PdO co-catalyst dramatically increases the faradaic efficiency (FE) of water oxidation from 52% to 92%, whilst simultaneously enhancing the photocurrent generation and charge extraction rate. The Pd oxidation state was found to be critical in achieving these notable improvements to the photoanode performance, which is primarily attributed to the higher selectivity towards oxygen evolution when PdO is used as an OEC and the formation of a favourable junction between WO3 and PdO, that drives band bending and charge separation.

Journal article

Vernardou D, Drosos C, Kafizas A, Pemble ME, Koudoumas Eet al., 2020,

Towards High Performance Chemical Vapour Deposition V2O5 Cathodes for Batteries Employing Aqueous Media

, Molecules, Vol: 25, ISSN: 1420-3049

The need for clean and efficient energy storage has become the center of attention due to the eminent global energy crisis and growing ecological concerns. A key component in this effort is the ultra-high performance battery, which will play a major role in the energy industry. To meet the demands in portable electronic devices, electric vehicles, and large-scale energy storage systems, it is necessary to prepare advanced batteries with high safety, fast charge ratios, and discharge capabilities at a low cost. Cathode materials play a significant role in determining the performance of batteries. Among the possible electrode materials is vanadium pentoxide, which will be discussed in this review, due to its low cost and high theoretical capacity. Additionally, aqueous electrolytes, which are environmentally safe, provide an alternative approach compared to organic media for safe, cost-effective, and scalable energy storage. In this review, we will reveal the industrial potential of competitive methods to grow cathodes with excellent stability and enhanced electrochemical performance in aqueous media and lay the foundation for the large-scale production of electrode materials.

Journal article

Bullen J, Kenney J, Fearn S, Kafizas A, Skinner S, Weiss Det al., 2020,

Improved accuracy in multicomponent surface complexation models using surface-sensitive analytical techniques: adsorption of arsenic onto a TiO2/Fe2O3 multifunctional sorbent

, Journal of Colloid and Interface Science, Vol: 580, Pages: 834-849, ISSN: 0021-9797

Many novel composite materials have been recently developed for water treatment applications, with the aim of achieving multifunctional behaviour, e.g. combining adsorption with light-driven remediation. The application of surface complexation models (SCM) is important to understand how adsorption changes as a function of pH, ionic strength and the presence of competitor ions. Component additive (CA) models describe composite sorbents using a combination of single-phase reference materials. However, predictive adsorption modelling using the CA-SCM approach remains unreliable, due to challenges in the quantitative determination of surface composition. In this study, we test the hypothesis that characterisation of the outermost surface using low energy ion scattering (LEIS) improves CA-SCM accuracy. We consider the TiO2/Fe2O3 photocatalyst-sorbents that are increasingly investigated for arsenic remediation. Due to an iron oxide surface coating that was not captured by bulk analysis, LEIS significantly improves the accuracy of our component additive predictions for monolayer surface processes: adsorption of arsenic(V) and surface acidity. We also demonstrate non-component additivity in multilayer arsenic(III) adsorption, due to changes in surface morphology/porosity. Our results demonstrate how surface-sensitive analytical techniques will improve adsorption modelling for the next generation of composite sorbents.

Journal article

Alqahtani M, Kafizas A, Sathasivam S, Ebaid M, Cui F, Alyamani A, Jeong H-H, Chun Lee T, Fischer P, Parkin I, Grätzel M, Wu Jet al., 2020,

A hierarchical 3D TiO2 /Ni nanostructure as an efficient hole-extraction and protection layer for GaAs photoanodes

, ChemSusChem: chemistry and sustainability, energy and materials, Vol: 13, Pages: 6028-6036, ISSN: 1864-5631

Photoelectrochemical (PEC) water splitting is a promising clean route to hydrogen fuel. The best-performing materials (III/V semiconductors) require surface passivation, as they are liable to corrosion, and a surface co-catalyst to facilitate water splitting. At present, optimal design combining photoelectrodes with oxygen evolution catalysts remains a significant materials challenge. Here, we demonstrate that nickel-coated amorphous three-dimensional (3D) TiO2 core-shell nanorods on a TiO2 thin film function as an efficient hole-extraction layer and serve as a protection layer for the GaAs photoanode. Transient-absorption spectroscopy (TAS) demonstrated the role of nickel-coated (3D) TiO2 core-shell nanorods in prolonging photogenerated charge lifetimes in GaAs, resulting in a higher catalytic activity. This strategy may open the potential of utilizing this low-cost (3D) nanostructured catalyst for decorating narrow-band-gap semiconductor photoanodes for PEC water splitting devices.

Journal article

Moss B, Le H, Corby S, Morita K, Selim S, Sotelo-Vazquez C, Chen Y, Borthwick A, Wilson A, Blackman C, Durrant JR, Walsh A, Kafizas Aet al., 2020,

Anisotropic electron transport limits performance of Bi2WO6 photoanodes

, The Journal of Physical Chemistry C, Vol: 124, Pages: 18859-18867, ISSN: 1932-7447

Bi2WO6 is one of the simplest members of the versatile Aurivillius oxide family of materials. As an intriguing model system for Aurivillius oxides, BiVO4 exhibits low water oxidation onset potentials (∼0.5–0.6 VRHE) for driven solar water oxidation. Despite this, Bi2WO6 also produces low photocurrents in comparison to other metal oxides. Due to a lack of in situ studies, the reasons for such poor performance are not understood. In this study, Bi2WO6 photoanodes are synthesized by aerosol-assisted chemical vapor deposition. The charge carrier dynamics of Bi2WO6 are studied in situ under water oxidation conditions, and the rate of both bulk recombination and water oxidation is found to be comparable to other metal oxide photoanodes. However, the rate of electron extraction is at least 10 times slower than the slowest kinetics previously reported in an oxide photoanode. First-principles analysis indicates that the slow electron extraction kinetics are linked to a strong anisotropy in the conduction band. Preferred or epitaxial growth along the conductive axes is a strategy to overcome slow electron transport and low photocurrent densities in layered materials such as Bi2WO6.

Journal article

Li J, McColl K, Lu X, Sathasivam S, Dong H, Kang L, Li Z, Zhao S, Kafizas AG, Wang R, Brett DJL, Shearing PR, Cora F, He G, Carmalt CJ, Parkin IPet al., 2020,

Multi-scale investigations of delta-Ni0.25V2O5 center dot nH(2)O cathode materials in aqueous Zinc-Ion batteries

, Advanced Energy Materials, Vol: 10, Pages: 1-14, ISSN: 1614-6832

Cost‐effective and environment‐friendly aqueous zinc‐ion batteries (AZIBs) exhibit tremendous potential for application in grid‐scale energy storage systems but are limited by suitable cathode materials. Hydrated vanadium bronzes have gained significant attention for AZIBs and can be produced with a range of different pre‐intercalated ions, allowing their properties to be optimized. However, gaining a detailed understanding of the energy storage mechanisms within these cathode materials remains a great challenge due to their complex crystallographic frameworks, limiting rational design from the perspective of enhanced Zn2+ diffusion over multiple length scales. Herein, a new class of hydrated porous δ‐Ni0.25V2O5.nH2O nanoribbons for use as an AZIB cathode is reported. The cathode delivers reversibility showing 402 mAh g−1 at 0.2 A g−1 and a capacity retention of 98% over 1200 cycles at 5 A g−1. A detailed investigation using experimental and computational approaches reveal that the host “δ” vanadate lattice has favorable Zn2+ diffusion properties, arising from the atomic‐level structure of the well‐defined lattice channels. Furthermore, the microstructure of the as‐prepared cathodes is examined using multi‐length scale X‐ray computed tomography for the first time in AZIBs and the effective diffusion coefficient is obtained by image‐based modeling, illustrating favorable porosity and satisfactory tortuosity.

Journal article

Alotaibi AM, Williamson BAD, Sathasivam SS, Kafizas A, Alqahtani M, Sotelo-Vazquez C, Buckeridge J, Wu J, Nair SP, Scanlon DO, Parkin IPet al., 2020,

Enhanced photocatalytic and antibacterial ability of Cu-doped anatase TiO2 thin films: theory and experiment.

, ACS Applied Materials and Interfaces, Vol: 12, Pages: 15348-15361, ISSN: 1944-8244

Multifunctional thin films which can display both photocatalytic and antibacterial activity are of great interest industrially. Here, for the first time, we have used aerosol assisted chemical vapour deposition (AACVD) to deposit highly photoactive thin films of Cu-doped anatase TiO2 on glass substrates. The films displayed much enhanced photocatalytic activity relative to pure anatase, and showed excellent antibacterial (vs S.Aureus and E.Coli) ability. Using a combination of transient absorption spectroscopy (TAS), photoluminescence (PL) measurements and hybrid density functional theory calculations, we have gained nanoscopic insights into the improved properties of the Cu-doped TiO2 films. Our analysis has highlighted that the interactions between substitutional and interstitial Cu in the anatase lattice can explain the extended exciton lifetimes observed in the doped samples, and the enhanced UV/visible light photoactivities observed.

Journal article

Hwang GB, Huang H, Wu G, Shin J, Kafizas A, Karu K, Toit HD, Alotaibi AM, Mohammad-Hadi L, Allan E, MacRobert AJ, Gavriilidis A, Parkin IPet al., 2020,

Photobactericidal activity activated by thiolated gold nanoclusters at low flux levels of white light

, Nature Communications, Vol: 11, ISSN: 2041-1723

The emergence of antibiotic resistant bacteria is a major threat to the practice of modern medicine. Photobactericidal agents have obtained significant attention as promising candidates to kill bacteria, and they have been extensively studied. However, to obtain photobactericidal activity, an intense white light source or UV-activation is usually required. Here we report a photobactericidal polymer containing crystal violet (CV) and thiolated gold nanocluster ([Au25(Cys)18]) activated at a low flux levels of white light. It was shown that the polymer encapsulated with CV do not have photobactericidal activity under white light illumination of an average 312 lux. However, encapsulation of [Au25(Cys)18] and CV into the polymer activates potent photobactericidal activity. The study of the photobactericidal mechanism shows that additional encapsulation of [Au25(Cys)18] into the CV treated polymer promotes redox reactions through generation of alternative electron transfer pathways, while it reduces photochemical reaction type-ІІ pathways resulting in promotion of hydrogen peroxide (H2O2) production.

Journal article

Drosos C, Moss B, Kafizas A, Vernardou Det al., 2020,

V2O5 as magnesium cathode material with extended cyclic stability

, Journal of Electrochemical Science and Engineering, Vol: 10, Pages: 257-262, ISSN: 1847-9286

In this work, the electrochemical performance of aerosol-assisted chemical vapour depositedvanadium pentoxide cathodes at 600 °C, is presented. The as-grown oxides indicate specificdischarge capacity of 300 mA h g-1 with capacity retention of 92 % after 10000 scans,coulombic efficiency of 100 %, noble structural stability and high reversibility. The presentstudy shows the possibility to grow large-area magnesium cathode material with extendedcycle stability via utilization of an aqueous electrolyte under a corrosive environment. Thisenhanced performance may be a combination of electrode morphology and adherence, whencompared to previous work employing electrode growth temperature at 500 °C.

Journal article

Corby S, Francàs L, Kafizas A, Durrant JRet al., 2020,

Determining the role of oxygen vacancies in the photoelectrocatalytic performance of WO3 for water oxidation

, Chemical Science, Vol: 11, Pages: 2907-2914, ISSN: 2041-6520

Oxygen vacancies are common to most metal oxides, whether intentionally incorporated or otherwise, and the study of these defects is of increasing interest for solar water splitting. In this work, we examine nanostructured WO3 photoanodes of varying oxygen content to determine how the concentration of bulk oxygen-vacancy states affects the photocatalytic performance for water oxidation. Using transient optical spectroscopy, we follow the charge carrier recombination kinetics in these samples, from picoseconds to seconds, and examine how differing oxygen vacancy concentrations impact upon these kinetics. We find that samples with an intermediate concentration of vacancies (∼2% of oxygen atoms) afford the greatest photoinduced charge carrier densities, and the slowest recombination kinetics across all timescales studied. This increased yield of photogenerated charges correlates with improved photocurrent densities under simulated sunlight, with both greater and lesser oxygen vacancy concentrations resulting in enhanced recombination losses and poorer J–V performances. Our conclusion, that an optimal – neither too high nor too low – concentration of oxygen vacancies is required for optimum photoelectrochemical performance, is discussed in terms of the competing beneficial and detrimental impact these defects have on charge separation and transport, as well as the implications held for other highly doped materials for photoelectrochemical water oxidation.

Journal article

Mesa Zamora C, Francas Forcada L, Yang KR, Garrido-Barros P, Pastor Hernandez E, Ma Y, Kafizas A, Rosser TE, Mayer MT, Reisner E, Grätzel M, Batista VS, Durrant Jet al., 2020,

Multihole water oxidation catalysis on hematite photoanodes revealed by operando spectroelectrochemistry and DFT

, Nature Chemistry, Vol: 12, Pages: 82-89, ISSN: 1755-4330

Water oxidation is the key kinetic bottle neck of photoelectrochemical devices for fuel synthesis. Despite advances in the identification of intermediates, elucidating the catalytic mechanism of this multi-redox reactionon metal-oxidephotoanodes remains a significant experimental and theoretical challenge. Here we report an experimental analysis of water oxidation kinetics on four widely studied metal oxides, focusing particularly upon hematite.We observe that hematite is able toaccess a reaction mechanism third order in surface hole density, assigned to equilibration between three surface holes and M(OH)-O-M(OH) sites. This reaction exhibits a remarkably low activation energy (Ea~ 60 meV). Density functional theory is employedto determine the energetics of charge accumulation and O-O bond formation on a modelhematite 110 surface. The proposed mechanism shows parallels with the function of oxygen evolving complex of photosystem II,and provides new insights to the mechanism of heterogeneous water oxidation on a metal oxide surface.

Journal article

Selim S, Pastor E, García-Tecedor M, Morris MR, Francas L, Sachs M, Moss B, Corby S, Mesa CA, Gimenez S, Kafizas A, Bakulin AA, Durrant JRet al., 2019,

Impact of oxygen vacancy occupancy on charge carrier dynamics in BiVO4 photoanodes

, Journal of the American Chemical Society, Vol: 141, Pages: 18791-18798, ISSN: 0002-7863

Oxygen vacancies are ubiquitous in metal oxides and critical to performance, yet the impact of these states upon charge carrier dynamics important for photoelectrochemical and photocatalytic applications, remains contentious and poorly understood. A key challenge is the unambiguous identification of spectroscopic fingerprints which can be used to track their function. Herein, we employ five complementary techniques to modulate the electronic occupancy of states associated with oxygen vacancies in situ in BiVO4 photoanodes, allowing us to identify a spectral signature for the ionisation of these states. We obtain an activation energy of ̴ 0.2 eV for this ionisation process, with thermally activated electron de-trapping from these states determining the kinetics of electron extraction, consistent with improved photoelectrochemical performance at higher temperatures. Bulk, un-ionised states however, function as deep hole traps, with such trapped holes being energetically unable to drive water oxidation. These observations help address recent controversies in the literature over oxygen vacancy function, providing new insights into their impact upon photoelectrochemical performance.

Book chapter

Kafizas A, 2019,

Photocatalytic approaches for converting CO<inf>2</inf> into fuels and feedstocks

, Carbon Dioxide Utilisation: Transformations, Pages: 635-656

Cite

Journal article

Corby S, Pastor E, Dong Y, Zheng X, Francàs L, Sachs M, Selim S, Kafizas A, Bakulin AA, Durrant JRet al., 2019,

Charge separation, band-bending, and recombination in WO3 photoanodes

, Journal of Physical Chemistry Letters, Vol: 10, Pages: 5395-5401, ISSN: 1948-7185

In metal oxide-based photoelectrochemical devices, the spatial separation of photogenerated electrons and holes is typically attributed to band-bending at the oxide/electrolyte interface. However, direct evidence of such band-bending impacting upon charge carrier lifetimes has been very limited to date. Herein we use ultrafast spectroscopy to track the rapid relaxation of holes in the space-charge layer and their recombination with trapped electrons in WO3 photoanodes. We observe that applied bias can significantly increase carrier lifetimes on all time scales from picoseconds to seconds and attribute this to enhanced band-bending correlated with changes in oxygen vacancy state occupancy. We show that analogous enhancements in carrier lifetimes can be obtained by changes in electrolyte composition, even in the absence of applied bias, highlighting routes to improve photoconversion yields/performance, through changes in band-bending. This study thus demonstrates the direct connection between carrier lifetime enhancement, increased band-bending, and oxygen vacancy defect state occupancy.

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