Imperial College London

DrAndreasKafizas

Faculty of Natural SciencesThe Grantham Institute for Climate Change

Lecturer in Climate Change and the Environment
 
 
 
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Contact

 

+44 (0)20 7594 6752a.kafizas Website

 
 
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Location

 

110KMolecular Sciences Research HubWhite City Campus

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Summary

 

Publications

Publication Type
Year
to

84 results found

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

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

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., Linking in-situ charge accumulation to electronic structure in doped SrTiO3 reveals design principles for hydrogen evolving photocatalysts, Nature Materials, ISSN: 1476-1122

Recently, record solar to hydrogen efficiencies have been demonstrated using La,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 VRHE) and a Rh oxidation state dependent reorganisation of the electronic structure, involving the loss of a vacant Rh 4d mid gap state. This reorganisation enables Rh:SrTiO3to be reduced by co-doping without compromising p-type character. In-situ time resolved spectroscopies show the electronic structure reorganisation induced by Rh reduction controls 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, resulting in long-lived photogenerated electrons even at very positive potentials (+1 VRHE), where both components of the complete device operate effectively. This understanding of role of co-dopants provides new insight into the design principles for water splitting devices based on bandgap engineered metal oxides.

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 density functional theory, 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.

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.

Journal article

Jiamprasertboon A, Kafizas A, Sachs M, Ling M, Alotaibi AM, Lu Y, Sirithanon T, Parkin IP, Carmalt CJet al., 2019, Heterojunction α-Fe2O3/ZnO films with enhanced photocatalytic properties grown by aerosol-assisted chemical vapour deposition., Chemistry - A European Journal, Vol: 25, Pages: 11337-11345, ISSN: 0947-6539

Type-I heterojunction films of α-Fe2O3/ZnO are reported herein as a non-titania based photocatalyst that shows remarkable enhancement in the photocatalytic properties towards stearic acid degradation under UVA light exposure (λ = 365 nm), with a quantum efficiency of ξ = 4.42 ± 1.54 × 10-4 molecules degraded/photon, which was about 16 times greater than that of α-Fe2O3, and 2.5 times greater than that of ZnO. As the degradation of stearic acid requires 104 electron transfers for each molecule, this represents an overall quantum efficiency of 4.60% for the α-Fe2O3/ZnO heterojunction. Time-resolved transient absorption spectroscopy (TAS) revealed the charge carrier behavior responsible for this increase in activity. Photogenerated electrons, formed in the ZnO layer, were transferred into the α-Fe2O3 layer on the pre-µs timescale, which reduced electron-hole recombination. This increased the lifetime of photogenerated holes formed in ZnO that oxidise stearic acid. The heterojunction α-Fe2O3/ZnO films grown herein show potential environmental applications as coatings for self-cleaning windows and surfaces.

Journal article

Sachs M, Park JS, Pastor E, Kafizas A, Wilson AA, Francàs L, Gul S, Ling M, Blackman C, Yano J, Walsh A, Durrant JRet al., 2019, Effect of oxygen deficiency on the excited state kinetics of WO3 and implications for photocatalysis, Chemical Science, Vol: 10, Pages: 5667-5677, ISSN: 2041-6520

Oxygen vacancies are widely used to tune the light absorption of semiconducting metal oxides, but a photophysical framework describing the impact of such point defects on the dynamics of photogenerated charges, and ultimately on catalysis, is still missing. We herein use WO3 as a model material and investigate the impact of significantly different degrees of oxygen deficiency on its excited state kinetics. For highly oxygen-deficient films, photoelectron spectroscopy shows an over 2 eV broad distribution of oxygen vacancy states within the bandgap which gives rise to extended visible light absorption. We examine the nature of this distribution using first-principles defect calculations and find that defects aggregate to form clusters rather than isolated vacancy sites. Using transient absorption spectroscopy, we observe trapping of photogenerated holes within 200 fs after excitation at high degrees of oxygen deficiency, which increases their lifetime at the expense of oxidative driving force. This loss in driving force limits the use of metal oxides with significant degrees of sub-stoichiometry to photocatalytic reactions that require low oxidation power such as pollutant degradation, and highlights the need to fine-tune vacancy state distributions for specific target reactions.

Journal article

Crake A, Christoforidis K, Gregg A, Moss B, Kafizas A, Petit Cet al., 2019, The effect of materials architecture in TiO2/MOF composites on CO2 photoreduction and charge transfer, Small, Vol: 15, ISSN: 1613-6810

CO2 photoreduction to C1/C1+ energized molecules is a key reaction of solar fuel technologies. Building heterojunctions can enhance photocatalysts performance, by facilitating charge transfer between two heterojunction phases. The material parameters that control this charge transfer remain unclear. Here, it is hypothesized that governing factors for CO2 photoreduction in gas phase are: i) a large porosity to accumulate CO2 molecules close to catalytic sites and ii) a high number of “points of contact” between the heterojunction components to enhance charge transfer. The former requirement can be met by using porous materials; the latter requirement by controlling the morphology of the heterojunction components. Hence, composites of titanium oxide or titanate and metal–organic framework (MOF), a highly porous material, are built. TiO2 or titanate nanofibers are synthesized and MOF particles are grown on the fibers. All composites produce CO under UV–vis light, using H2 as reducing agent. They are more active than their component materials, e.g., ≈9 times more active than titanate. The controlled composites morphology is confirmed and transient absorption spectroscopy highlights charge transfer between the composite components. It is demonstrated that electrons transfer from TiO2 into the MOF, and holes from the MOF into TiO2, as the MOF induces band bending in TiO2.

Journal article

Selim S, Francàs L, García-Tecedor M, Corby S, Blackman C, Gimenez S, Durrant JR, Kafizas Aet al., 2019, WO3/BiVO4: impact of charge separation at the timescale of water oxidation, Chemical Science, Vol: 10, Pages: 2643-2652, ISSN: 2041-6520

The four hole oxidation of water has long been considered the kinetic bottleneck for overall solar-driven water splitting, and thus requires the formation of long-lived photogenerated holes to overcome this kinetic barrier. However, photogenerated charges are prone to recombination unless they can be spatially separated. This can be achieved by coupling materials with staggered conduction and valence band positions, providing a thermodynamic driving force for charge separation. This has most aptly been demonstrated in the WO3/BiVO4 junction, in which quantum efficiencies for the water oxidation reaction can approach near unity. However, the charge carrier dynamics in this system remain elusive over timescales relevant to water oxidation (μs–s). In this work, the effect of charge separation on carrier lifetime, and the voltage dependence of this process, is probed using transient absorption spectroscopy and transient photocurrent measurements, revealing sub-μs electron transfer from BiVO4 to WO3. The interface formed between BiVO4 and WO3 is shown to overcome the “dead-layer effect” encountered in BiVO4 alone. Moreover, our study sheds light on the role of the WO3/BiVO4 junction in enhancing the efficiency of the water oxidation reaction, where charge separation across the WO3/BiVO4 junction improves both the yield and lifetime of holes present in the BiVO4 layer over timescales relevant to water oxidation.

Journal article

Ai C, Xie P, Zhang X, Zheng X, Li J, Kafizas A, Lin Set al., 2019, Explaining the Enhanced Photoelectrochemical Behavior of Highly Ordered TiO2 Nanotube Arrays: Anatase/Rutile Phase Junction, ACS SUSTAINABLE CHEMISTRY & ENGINEERING, Vol: 7, Pages: 5274-5282, ISSN: 2168-0485

Journal article

Crake A, Christoforidis K, Godin R, Moss B, Kafizas A, Zafeiratos S, Durrant J, Petit Cet al., 2019, Titanium dioxide/carbon nitride nanosheet nanocomposites for gas phase CO2 photoreduction under UV-visible irradiation, Applied Catalysis B: Environmental, Vol: 242, Pages: 369-378, ISSN: 0926-3373

In the field of photocatalysis and particularly that of CO2 photoreduction, the formulation of nanocomposites provids avenues to design a material platform with a unique set of structural, optoelectronic and chemical features thereby addressing shortcomings of single-phase materials and allowing synergistic effects. In this work, inorganic/organic composite photocatalysts for CO2 reduction comprised of titanium dioxide (TiO2) and carbon nitride nanosheets (CNNS) were synthesized using a hydrothermal in-situ growth method. Specifically, pre-formed CNNS were used to synthesize TiO2/CNNS heterostructures with control over the TiO2 facet formation. This synthesis approach improved the catalytic properties by increasing CO2 adsorption capacity and facilitating charge transfer. The materials were characterised by various spectroscopic, imaging, and analytical techniques to investigate their structural (from nano- to macroscale), chemical, and optical properties. TiO2 nanoparticles were efficiently grown on the CNNS. The CO2 adsorption capacity of the composites was measured, and they were tested for CO2 photoreduction under UV-Vis illumination with hydrogen as the reducing agent in a heterogeneous gas-solid system to combine CO2 capture and conversion into a single-step process. Catalytic tests were performed without adding any precious metal co-catalyst. The composites exhibited enhanced CO2 adsorption capacity and photocatalytic CO2 conversion compared to their constituent materials (> ten-fold increase) and outperformed the TiO2 P25 benchmark material. The TiO2/CNNS composite with more {001} TiO2 facets was the most catalytically active. Further investigations using transient absorption spectroscopy (TAS) revealed the control of facet formation improved interfacial transfer at the TiO2/CNNS junction. A photocatalytic mechanism was proposed based on the spectroscopic analyses as well as the CO2 adsorption, and CO2 conversion results.

Journal article

Kafizas A, Xing X, Selim S, Mesa CA, Ma Y, Burgess C, McLachlan MA, Durrant JRet al., 2019, Ultra-thin Al<inf>2</inf>O<inf>3</inf>coatings on BiVO<inf>4</inf>photoanodes: Impact on performance and charge carrier dynamics, Catalysis Today, Vol: 321-322, Pages: 59-66, ISSN: 0920-5861

Bismuth vanadate (BiVO 4 ) has emerged as one of the most promising photoanode materials for oxidising water due to its visible light activity and low cost. Recent studies have shown that the performance of BiVO 4 photoanodes can be remarkably improved when coated with ultra-thin passivation layers. In this article we investigate the use of ultra-thin Al 2 O 3 layers grown using atomic layer deposition (ALD). At an optimum thickness (~0.33nm, 3 ALD cycles), the Al 2 O 3 layer favourably shifted the onset potential by ~200mV and increased photocatalytic currents for the water oxidation reaction. When held at 1.23V RHE , we observe a remarkable increase in the theoretical solar photocurrent; from ~0.47mAcm -2 in uncoated BiVO 4 to ~3.0mAcm -2 in Al 2 O 3 -coated BiVO 4 . Using transient photocurrent (TPC) and transient absorption spectroscopy (TAS) the charge carrier dynamics in Al 2 O 3 -coated BiVO 4 photoanodes were examined for the first time. TPC showed that photogenerated electrons in the BiVO 4 layer were extracted within ~1ms. TAS showed that the remaining holes oxidised water from ~100ms to 1s. Ultra-thin Al 2 O 3 coatings did not improve the reaction kinetics towards water oxidation, but rather, suppressed bi-molecular recombination on the μs-ms timescale in BiVO 4 , and increased the yield of long-lived holes on the ms-s timescale required to oxidise water. This is attributed to an inhibition of surface recombination on BiVO 4 by Al 2 O 3 , which inhibited the early timescale recombination of charge carriers formed within the space charge layer.

Journal article

Rafols i Belles C, Selim S, Harrison NM, Ahmad EA, Kafizas Aet al., 2019, Beyond band bending in the WO3/BiVO4 heterojunction: insight from DFT and experiment, Sustainable Energy and Fuels, Vol: 3, Pages: 264-271, ISSN: 2398-4902

Heterojunction photocatalysts can significantly enhance the efficiency of photocatalytic water splitting. It is well known that the key to such improvements lies at the interfacial region where charge separation occurs. Understanding the origins of this interfacial enhancement can enable the design of better performing water splitting devices. Therefore, in this work, a novel theoretical–experimental approach is developed for the study of photocatalytic heterojunctions using the model system – WO3/BiVO4, where it has been shown that the quantum efficiency of water splitting can approach unity at certain wavelengths. Our photoelectrochemical measurements of this heterojunction show a significantly enhanced performance over its separate components when illuminated through the BiVO4 side but not the WO3 side. This is indicative of more efficient electron transfer (i.e. from BiVO4 to WO3) than hole transfer (i.e. from WO3 to BiVO4) across the junction. Our classical band bending model of this junction predicts noticeable interfacial barriers, but could not explain the reduced performance under back illumination. Our atomistic model was used to investigate the effect of interfacial reconstructions and chemical interactions on the electronic structure of the system. The model reveals a non-staggered valence band, in contrast to the staggered conduction band, due to strong hybridization of valence band orbitals in both materials across the interface. This non-staggered valence band does not provide an energetic driving force for charge separation for hole transfer (i.e. from WO3 to BiVO4 under back illumination). Hence, a significant improvement in performance is only observed under front illumination. This combined approach, using both experiment and theory, results in a more complete understanding of a heterojunction photocatalyst system and provides unique insight into the interfacial effects that arise when two semiconductor materials are brought together

Journal article

Corby S, Francàs L, Selim S, Sachs M, Blackman C, Kafizas A, Durrant JRet al., 2018, Water oxidation and electron extraction kinetics in nanostructured tungsten trioxide photoanodes, Journal of the American Chemical Society, Vol: 140, Pages: 16168-16177, ISSN: 1520-5126

A thorough understanding of the kinetic competition between desired water oxidation/electron extraction processes and any detrimental surface recombination is required to achieve high water oxidation efficiencies in transition-metal oxide systems. The kinetics of these processes in high Faradaic efficiency tungsten trioxide (WO3) photoanodes (>85%) are monitored herein by transient diffuse reflectance spectroscopy and correlated with transient photocurrent data for electron extraction. Under anodic bias, efficient hole transfer to the aqueous electrolyte is observed within a millisecond. In contrast, electron extraction is found to be comparatively slow (∼10 ms), increasing in duration with nanoneedle length. The relative rates of these water oxidation and electron extraction kinetics are shown to be reversed in comparison to other commonly examined metal oxides (e.g., TiO2, α-Fe2O3, and BiVO4). Studies conducted as a function of applied bias and film processing to modulate oxygen vacancy density indicate that slow electron extraction kinetics result from electron trapping in shallow WO3 trap states associated with oxygen vacancies. Despite these slow electron extraction kinetics, charge recombination losses on the microsecond to second time scales are observed to be modest compared to other oxides studied. We propose that the relative absence of such recombination losses, and the observation of a photocurrent onset potential close to flat-band, result directly from the faster water oxidation kinetics of WO3. We attribute these fast water oxidation kinetics to the highly oxidizing valence band position of WO3, thus highlighting the potential importance of thermodynamic driving force for catalysis in outcompeting detrimental surface recombination processes.

Journal article

He G, Han X, Moss B, Weng Z, Gadipelli S, Lai F, Kafizas AG, Brett DJL, Guo ZX, Wang H, Parkin IPet al., 2018, Solid solution nitride/carbon nanotube hybrids enhance electrocatalysis of oxygen in zinc-air batteries, Energy Storage Materials, Vol: 15, Pages: 380-387, ISSN: 2405-8297

Bi-functional electrocatalysts capable of both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are highly desirable for a variety of renewable energy storage and conversion technologies. To develop noble metal alternatives for catalysis, non-noble metal compounds have been tremendously pursued but remain non-ideal to issues relating to stability and population of the number of exposed active sites. Inspired by Engel-Brewer valence bond theory, strongly coupled nickel-cobalt-nitride solid-solution/carbon nanotube hybrids were developed by tuning their bifunctionalities from an atomistic scale. The as-synthesized catalysts demonstrate superior catalytic properties to commercial noble-metal based counterparts, i.e. platinum on a carbon support for ORR and iridium oxide for OER, also with much enhanced stability. First-principle calculations and structural analysis show that the optimized structures potentially possess multiple active sites, both bulk-surface response and separated surface charge distribution from optimization of Ni/Co nitrides could contribute to synergistic effects for improved catalytic performances. This study provides not only unique theoretical insights but also a design concept for producing effective bi-functional catalysts with balanced-ORR/OER active sites for this class of transition metal nitride hybrid system and paves the way for exploring other metal nitrides for similar purposes.

Journal article

Drosos C, Jia C, Mathew S, Palgrave RG, Moss B, Kafizas A, Vernardou Det al., 2018, Aerosol-assisted chemical vapor deposition of V2O5 cathodes with high rate capabilities for magnesium-ion batteries, JOURNAL OF POWER SOURCES, Vol: 384, Pages: 355-359, ISSN: 0378-7753

The growth of orthorhombic vanadium pentoxide nanostructures was accomplished using an aerosol-assisted chemical vapor deposition process. These materials showed excellent electrochemical performance for magnesium-ion storage in an aqueous electrolyte; showing specific discharge capacities of up to 427 mAh g−1 with a capacity retention of 82% after 2000 scans under a high specific current of 5.9 A g−1. The high rate capability suggested good structural stability and high reversibility. We believe the development of low-cost and large-area coating methods, such as the technique used herein, will be essential for the upscalable fabrication of next-generation rechargeable battery technologies.

Journal article

Alotaibi AM, Sathasivam S, Williamson BAD, Kafizas A, Sotelo-Vazquez C, Taylor A, Scanlon DO, Parkin IPet al., 2018, Chemical vapor deposition of photocatalytically active pure brookite TiO2 thin films, Chemistry of Materials, Vol: 30, Pages: 1353-1361, ISSN: 0897-4756

Brookite is the least investigated phase of TiO2 due to the synthetic difficulty of obtaining the pure phase. Here, we present the first ever chemical vapor deposition synthesis of pure brookite TiO2 thin films. The films were highly crystalline and phase pure as determined by X-ray diffraction and Raman spectroscopy studies. Scanning electron microscopy studies showed the films to have a structured morphology consisting of pyramidal features. The photocatalytic properties of the brookite film, tested using stearic acid under UVA (365 nm) irradiation, were superior to both an anatase film grown under similar conditions and NSG Activ glass. Transient absorption spectroscopy showed that the innate electron–hole recombination dynamics are similar in brookite and anatase, akin to previous reports. The superior activity of the brookite film is hence attributed to the higher surface area compared to anatase.

Journal article

Quesada-Gonzalez M, Williamson BAD, Sotelo-Vazquez C, Kafizas A, Boscher ND, Quesada-Cabrera R, Scanlon DO, Carmalt CJ, Parkin IPet al., 2018, Deeper Understanding of Interstitial Boron-Doped Anatase Thin Films as A Multifunctional Layer Through Theory and Experiment, Journal of Physical Chemistry C, Vol: 122, Pages: 714-726, ISSN: 1932-7447

Thin films of interstitial boron-doped anatase TiO2, with varying B concentrations, were deposited via one-step atmospheric pressure chemical vapor deposition (APCVD) on float glass substrates. The doped films showed a remarkable morphology and enhanced photoactivity when compared to their undoped analogues. The TiO2:B films also presented enhanced conductivity and electron mobility as measured by a Hall effect probe as well as a high adherence to the substrate, stability and extended lifetime. The structure and composition of the different samples of TiO2:B films were studied by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and dynamic secondary ion mass spectrometry (D-SIMS). Hybrid density functional theory was used to explore the defect chemistry of B-doped anatase and to understand the experimental results.

Journal article

Gardecka AJ, Bishop C, Lee D, Corby S, Parkin IP, Kafizas A, Krumdieck Set al., 2017, High efficiency water splitting photoanodes composed of nano-structured anatase-rutile TiO2 heterojunctions by pulsed-pressure MOCVD, Applied Catalysis B: Environmental, Vol: 224, Pages: 904-911, ISSN: 0926-3373

In this article, thin solid films are processed via pulsed-pressure metal organic chemical vapour deposition (PP-MOCVD) on FTO substrates over a range of processing times to produce a range of thicknesses and microstructures. The films are highly nanostructured anatase-rutile TiO2 composite films with unique single crystal dendrites. After annealing, carbon was removed, and materials showed improved water splitting activity; with IPCEs above 80% in the UV, photocurrents of ∼1.2 mA cm−2 at 1.23 VRHE at 1 sun irradiance and an extension of photoactivity into the visible range. The annealed material exhibits minimal recombination losses and IPCEs amongst the highest reported in the literature; attributed to the formation of a high surface area nanostructured material and synergetic interactions between the anatase and rutile phases.

Journal article

Ling M, Blackman CS, Palgrave RG, Sotelo-Vazquez C, Kafizas A, Parkin IPet al., 2017, Correlation of Optical Properties, Electronic Structure, and Photocatalytic Activity in Nanostructured Tungsten Oxide, ADVANCED MATERIALS INTERFACES, Vol: 4, ISSN: 2196-7350

Journal article

Crake A, Christoforidis KC, Kafizas A, Zafeiratos S, Petit Cet al., 2017, CO2 capture and photocatalytic reduction using bifunctional TiO2/MOF nanocomposites under UV-vis irradiation, Applied Catalysis B: Environmental, Vol: 210, Pages: 131-140, ISSN: 0926-3373

TiO2 nanosheets and metal-organic framework (NH2-UiO-66) were effectively coupled via an in‐situ growth strategy to form bifunctional materials for the combined capture and photocatalytic reduction of CO2 under UV–vis light irradiation. This was done to take advantage of the high CO2 adsorption capacity of the MOF and the photocatalytic properties of pre-formed TiO2 nanosheets in a single material. The prepared materials were thoroughly characterized using a variety of techniques. They were subsequently tested for CO2 adsorption and CO2 photocatalytic reduction using a heterogeneous gas/solid set-up to imitate both CO2 capture and fixation in a single process. The adopted synthesis process allowed the development of a tight interaction between TiO2 and NH2-UiO-66 forming a heterojunction, while maintaining both the high CO2 uptake and porosity of NH2-UiO-66. The nanocomposites were proven durable and significantly more efficient in reducing CO2 to CO than their single components. Photocatalytic activity was greatly affected by the nanocomposites composition with the optimum TiO2 content doubling the CO evolution rate compared with the pure TiO2. The improved photoactivity was assigned to the enhanced abundance of long lived charge carriers, as revealed by transient absorption spectroscopy (TAS). This most likely occurred due to the effective charge transfer via interface. A possible mechanism is discussed on the basis of the combined catalytic, spectroscopic and CO2 adsorption results.

Journal article

Mesa Zamora CA, Kafizas A, Francàs L, Pendlebury S, Pastor E, Ma Y, Le Formal F, Mayer M, Grätzel M, Durrant Jet al., 2017, Kinetics of Photoelectrochemical Oxidation of Methanol on Hematite Photoanodes, Journal of the American Chemical Society, Vol: 139, Pages: 11537-11543, ISSN: 1520-5126

The kinetics of photoelectrochemical (PEC) oxidation of methanol, as a model organic substrate, on α-Fe2O3 photoanodes are studied using photoinduced absorption spectroscopy and transient photocurrent measurements. Methanol is oxidized on α-Fe2O3 to formaldehyde with near unity Faradaic efficiency. A rate law analysis under quasi-steady-state conditions of PEC methanol oxidation indicates that rate of reaction is second order in the density of surface holes on hematite and independent of the applied potential. Analogous data on anatase TiO2 photoanodes indicate similar second-order kinetics for methanol oxidation with a second-order rate constant 2 orders of magnitude higher than that on α-Fe2O3. Kinetic isotope effect studies determine that the rate constant for methanol oxidation on α-Fe2O3 is retarded ∼20-fold by H/D substitution. Employing these data, we propose a mechanism for methanol oxidation under 1 sun irradiation on these metal oxide surfaces and discuss the implications for the efficient PEC methanol oxidation to formaldehyde and concomitant hydrogen evolution.

Journal article

Kafizas A, Ma Y, Pastor E, Pendlebury SR, Mesa C, Francas L, Le Formal F, Noor N, Ling M, Sotelo-Vazquez C, Carmalt CJ, Parkin IP, Durrant JRet al., 2017, Water Oxidation Kinetics of Accumulated Holes on the Surface of a TiO2 Photoanode: A Rate Law Analysis, ACS CATALYSIS, Vol: 7, Pages: 4896-4903, ISSN: 2155-5435

It has been more than 40 years since Fujishima and Honda demonstrated water splitting using TiO2, yet there is still no clear mechanism by which surface holes on TiO2 oxidize water. In this paper, we use a range of complementary techniques to study this reaction that provide a unique insight into the reaction mechanism. Using transient photocurrent and transient absorption spectroscopy, we measure both the kinetics of electron extraction (t50% ≈ 200 μs, 1.5VRHE) and the kinetics of hole oxidation of water (t50% ≈ 100 ms, 1.5VRHE) as a function of applied potential, demonstrating the water oxidation by TiO2 holes is the kinetic bottleneck in this water-splitting system. Photoinduced absorption spectroscopy measurements under 5 s LED irradiation are used to monitor the accumulation of surface TiO2 holes under conditions of photoelectrochemical water oxidation. Under these conditions, we find that the surface density of these holes increases nonlinearly with photocurrent density. In alkali (pH 13.6), this corresponded to a rate law for water oxidation that is third order with respect to surface hole density, with a rate constant kWO = 22 ± 2 nm4·s–1. Under neutral (pH = 6.7) and acidic (pH = 0.6) conditions, the rate law was second order with respect to surface hole density, indicative of a change in reaction mechanism. Although a change in reaction order was observed, the rate of reaction did not change significantly over the wide pH range examined (with TOFs per surface hole in the region of 20–25 s–1 at ∼1 sun irradiance). This showed that the rate-limiting step does not involve OH– nucleophilic attack and demonstrated the versatility of TiO2 as an active water oxidation photocatalyst over a wide range of pH.

Journal article

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