Publications

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.

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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

Moss B, Lim KK, Beltram A, Moniz S, Tang J, Fornasiero P, Barnes P, Durrant J, Kafizas AGet al., 2017,

Comparing photoelectrochemical water oxidation, recombination kinetics and charge trapping in the three polymorphs of TiO2

, Scientific Reports, Vol: 7, ISSN: 2045-2322

In this article we present the first comparative study of the transient decay dynamics of photo-generated charges for the three polymorphs of TiO2. To our knowledge, this is the first such study of the brookite phase of TiO2 over timescales relevant to the kinetics of water splitting. We find that the behavior of brookite, both in the dynamics of relaxation of photo-generated charges and in energetic distribution, is similar to the anatase phase of TiO2. Moreover, links between the rate of recombination of charge carriers, their energetic distribution and the mode of transport are made in light of our findings and used to account for the differences in water splitting efficiency observed across the three polymorphs.

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Journal article

Godin R, Kafizas A, Durrant JR, 2017,

Electron transfer dynamics in fuel producing photosystems

, Current Opinion in Electrochemistry, Vol: 2, Pages: 136-143, ISSN: 2451-9103

An often overlooked aspect of solar fuel production is the inherent mismatch between bulk charge carrier lifetimes and rates of charge transfer reactions. Considering water oxidation, interfacial charge transfer occurs on the millisecond to second timescales while bulk charge carrier lifetimes of metal oxides are typically in the fast picosecond–nanosecond regime. For charge transfer to efficiently compete with charge recombination, strategies that substantially increase the charge carrier lifetime need to be applied. In this chapter, we discuss the magnitude of the kinetic mismatch, overview common effective charge separation strategies that address this mismatch and highlight recent developments in our understanding of these processes. We also touch upon recent advances in determining the chemical nature of key reaction intermediates.

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Journal article

McCafferty L, O'Rourke C, Mills A, Kafizas A, Parkin IP, Darr JAet al., 2017,

Light-driven generation of chlorine and hydrogen from brine using highly selective Ru/Ti oxide redox catalysts

, Sustainable Energy & Fuels, Vol: 1, Pages: 254-257, ISSN: 2398-4902

Ultrafine ruthenium–titanium oxide catalysts were directly produced using a continuous hydrothermal flow synthesis process and assessed as chloride oxidation catalysts. Selectivity towards chlorine (over oxygen) evolution was shown to generally increase with decreasing ruthenium content. The optimum catalyst was then used to make an anode for a light-driven brine-splitting demonstrator device to produce hydrogen and chlorine gases.

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Journal article

Sotelo-Vazquez C, Quesada-Cabrera R, Ling M, Scanlon DO, Kafizas A, Thakur PK, Lee T-L, Taylor A, Watson GW, Palgrave RG, Durrant JR, Blackman CS, Parkin IPet al., 2017,

Evidence and Effect of Photogenerated Charge Transfer for Enhanced Photocatalysis in WO3/TiO2 Heterojunction Films: A Computational and Experimental Study

, ADVANCED FUNCTIONAL MATERIALS, Vol: 27, ISSN: 1616-301X

Semiconductor heterojunctions are used in a wide range of applications including catalysis, sensors, and solar-to-chemical energy conversion devices. These materials can spatially separate photogenerated charge across the heterojunction boundary, inhibiting recombination processes and synergistically enhancing their performance beyond the individual components. In this work, the WO3/TiO2 heterojunction grown by chemical vapor deposition is investigated. This consists of a highly nanostructured WO3 layer of vertically aligned nanorods that is then coated with a conformal layer of TiO2. This heterojunction shows an unusual electron transfer process, where photogenerated electrons move from the WO3 layer into TiO2. State-of-the-art hybrid density functional theory and hard X-ray photoelectron spectroscopy are used to elucidate the electronic interaction at the WO3/TiO2 interface. Transient absorption spectroscopy shows that recombination is substantially reduced, extending both the lifetime and population of photogenerated charges into timescales relevant to most photocatalytic processes. This increases the photocatalytic efficiency of the material, which is among the highest ever reported for a thin film. In allying computational and experimental methods, this is believed to be an ideal strategy for determining the band alignment in metal oxide heterojunction systems.

Journal article

Kafizas A, Francas L, Sotelo-Vazquez C, Ling M, Li Y, Glover E, McCafferty L, Blackman C, Darr J, Parkin Iet al., 2017,

Optimizing the Activity of Nanoneedle Structured WO3 Photoanodes for Solar Water Splitting: Direct Synthesis via Chemical Vapor Deposition

, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 121, Pages: 5983-5993, ISSN: 1932-7447

Solar water splitting is a promising solution for the renewable production of hydrogen as an energy vector. To date, complex or patterned photoelectrodes have shown the highest water splitting efficiencies, but lack scalable routes for commercial scale-up. In this article, we report a direct and scalable chemical vapor deposition (CVD) route at atmospheric pressure, for a single step fabrication of complex nanoneedle structured WO3 photoanodes. Using a systematic approach, the nanostructure was engineered to find the conditions that result in optimal water splitting. The nanostructured materials adopted a monoclinic γ-WO3 structure and were highly oriented in the (002) plane, with the nanoneedle structures stacking perpendicular to the FTO substrate. The WO3 photoanode that showed the highest water splitting activity was composed of a ∼300 nm seed layer of flat WO3 with a ∼5 μm thick top layer of WO3 nanoneedles. At 1.23 VRHE, this material showed incident photon-to-current efficiencies in the range ∼35–45% in the UV region (250–375 nm) and an overall solar predicted photocurrent of 1.24 mA·cm–2 (∼25% of the theoretical maximum for WO3). When coupled in tandem with a photovoltaic device containing a methylammonium lead iodide perovskite, a solar-to-hydrogen efficiency of ca. 1% for a complete unassisted water splitting device is predicted.

Journal article

Chadwick NP, Kafizas A, Quesada-Cabrera R, Sotelo-Vazquez C, Bawaked SM, Mokhtar M, Al Thabaiti SA, Obaid AY, Basahel SN, Durrant JR, Carmalt CJ, Parkin IPet al., 2017,

Ultraviolet Radiation Induced Dopant Loss in a TiO2 Photocatalyst

, ACS CATALYSIS, Vol: 7, Pages: 1485-1490, ISSN: 2155-5435

Book chapter

Kafizas A, Godin R, Durrant JR, 2017,

Charge Carrier Dynamics in Metal Oxide Photoelectrodes for Water Oxidation

, SEMICONDUCTORS FOR PHOTOCATALYSIS, Editors: Mi, Wang, Jagadish, Publisher: ELSEVIER ACADEMIC PRESS INC, Pages: 3-46

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Citations: 21

Journal article

Sathasivam S, Williamson BAD, Kafizas A, Althabaiti SA, Obaid AY, Basahel SN, Scanlon DO, Carmalt CJ, Parkin IPet al., 2016,

Computational and experimental study of Ta2O5 thin films

, Journal of Physical Chemistry C, Vol: 121, Pages: 202-210, ISSN: 1932-7447

This paper reports the novel synthesis of amorphous Ta2O5 and the subsequent isolation of the orthorhombic (β) crystallographic phase, using aerosol-assisted chemical vapor deposition. Hybrid density functional theory was used to obtain the calculated optical band gap (3.83 eV) for the first time, which closely matches our experimental findings (3.85 eV). The films were highly transparent in the visible and near-IR region of the electromagnetic spectrum. The refractive indexes, calculated using the Swanepoel method, showed good agreement with literature findings. The photocatalytic properties of the films, determined through the photominerilization of stearic acid under 254 nm radiation showed the amorphous sample to be an order of magnitude superior over crystalline β-Ta2O5.

Journal article

Sachs M, Pastor E, Kafizas A, Durrant JRet al., 2016,

Evaluation of Surface State Mediated Charge Recombination in Anatase and Rutile TiO2

, Journal of Physical Chemistry Letters, Vol: 7, Pages: 3742-3746, ISSN: 1948-7185

In nanostructured thin films, photogeneratedcharge carriers can access the surface more easily than indense films and thus react more readily. However, the highsurface area of these films has also been associated withenhanced recombination losses via surface states. We hereinuse transient absorption spectroscopy to compare the ultrafastcharge carrier kinetics in dense and nanostructured TiO2films for its two most widely used polymorphs: anatase andrutile. We find that nanostructuring does not enhance recombinationrates on ultrafast timescales, indicating thatsurface state mediated recombination is not a key loss pathwayfor either TiO2 polymorph. Rutile shows faster, and lessintensity-dependent recombination than anatase, which weassign to its higher doping density. For both polymorphs, weconclude that bulk rather than surface recombination is theprimary determinant of charge carrier lifetime.

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