Imperial College London

DrAndreasKafizas

Faculty of Natural SciencesDepartment of Chemistry

Senior Lecturer
 
 
 
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Contact

 

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

 
 
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Location

 

301GMolecular Sciences Research HubWhite City Campus

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Summary

 

Publications

Publication Type
Year
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94 results found

Schukraft GEM, Moss B, Kafizas AG, Petit Cet al., 2022, Effect of band bending in photoactive MOF-based heterojunctions., ACS Applied Materials and Interfaces, Vol: 14, Pages: 19342-19352, ISSN: 1944-8244

Semiconductor/metal-organic framework (MOF) heterojunctions have demonstrated promising performance for the photoconversion of CO2 into value-added chemicals. To further improve performance, we must understand better the factors which govern charge transfer across the heterojunction interface. However, the effects of interfacial electric fields, which can drive or hinder electron flow, are not commonly investigated in MOF-based heterojunctions. In this study, we highlight the importance of interfacial band bending using two carbon nitride/MOF heterojunctions with either Co-ZIF-L or Ti-MIL-125-NH2. Direct measurement of the electronic structures using X-ray photoelectron spectroscopy (XPS), work function, valence band, and band gap measurements led to the construction of a simple band model at the heterojunction interface. This model, based on the heterojunction components and band bending, enabled us to rationalize the photocatalytic enhancements and losses observed in MOF-based heterojunctions. Using the insight gained from a promising band bending diagram, we developed a Type II carbon nitride/MOF heterojunction with a 2-fold enhanced CO2 photoreduction activity compared to the physical mixture.

Journal article

Bullen JC, Heiba HF, Kafizas A, Weiss DJet al., 2022, Parasitic light absorption, rate laws and heterojunctions in the photocatalytic oxidation of arsenic(III) using composite TiO2/Fe2O3, Chemistry: A European Journal, Vol: 28, ISSN: 0947-6539

Composite photocatalyst-adsorbents such as TiO2/Fe2O3 are promising materials for the one-step treatment of arsenite contaminated water. However, no previous study has investigated how coupling TiO2 with Fe2O3 influences the photocatalytic oxidation of arsenic(III). Herein, we develop new hybrid experiment/modelling approaches to study light absorption, charge carrier behaviour and changes in the rate law of the TiO2/Fe2O3 system, using UV-Vis spectroscopy, transient absorption spectroscopy (TAS), and kinetic analysis. Whilst coupling TiO2 with Fe2O3 improves total arsenic removal by adsorption, oxidation rates significantly decrease (up to a factor of 60), primarily due to the parasitic absorption of light by Fe2O3 (88% of photons at 368 nm) and secondly due to changes in the rate law from disguised zero-order kinetics to first-order kinetics. Charge transfer across this TiO2-Fe2O3 heterojunction is not observed. Our study demonstrates the first application of a multi-adsorbate surface complexation model (SCM) towards describing As(III) oxidation kinetics which, unlike Langmuir-Hinshelwood kinetics, includes the competitive adsorption of As(V), and we further highlight the importance of parasitic light absorption and catalyst fouling when designing heterogeneous photocatalysts for As(III) remediation.

Journal article

Heiba HF, Bullen JC, Kafizas A, Petit C, Skinner SJ, Weiss Det al., 2022, The determination of oxidation rates and quantum yields during the photocatalytic oxidation of As(III) over TiO2, Journal of Photochemistry and Photobiology A: Chemistry, Vol: 424, Pages: 113628-113628, ISSN: 1010-6030

The determination of reaction rates for the photocatalytic oxidation (PCO) of arsenite (As(III)) using TiO2 under UV radiation is challenging due to the numerous experimental processes. This includes chemical processes running simultaneously with PCO (e.g. adsorption of arsenic species, direct UV photolysis of As(III)) and the analytical approach used (e.g. whether As(III) or As(V) are measured and used in the calculation of the PCO rate). The various experimental approaches used to date have led to oxidation rates and rate constants which vary by orders of magnitude and contradicting information on rate laws. Here we present the results of a critical examination of possible controls affecting the experimental determination of PCO rates. First, we demonstrate that the choice of analytical technique is not critical, provided that the rate constants are calculated based on the depletion of As(III) after correction of the directly adsorbed As(III). Second, we show the correction of the directly adsorbed As(III) at each time interval is best done by running two parallel experiments (one under UV and the other in dark) instead of running sequential experiment (i.e. running the experiment in the dark then turning on the UV lamp). These findings are supported by XPS analysis of the oxidation state of TiO2-sorbed As. Third, we demonstrate that photolysis by the light source itself, as well as the chemical composition of the solution (i.e. the effect of HEPES and the ionic strength), can significantly increase As(III) oxidation rates and need to be corrected. Finally, to determine the quantum yield of As(III) oxidation, we measured the photon absorption by the TiO2 photocatalyst. Our results showed that the quantum yield (Ø) for this oxidation reaction was low, and in the region of 0.1 to 0.2 %.

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

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

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

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.

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, Pages: 1-12, 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

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