Publications

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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Journal article

Ma Y, Mesa CA, Pastor E, Kafizas A, Francas L, Le Formal F, Pendlebury SR, Durrant JRet al., 2016,

Rate law analysis of water oxidation and hole scavenging on a BiVO4 photoanode

, ACS Energy Letters, Vol: 1, Pages: 618-623, ISSN: 2380-8195

Spectroelectrochemical studies employing pulsed LED irradiation are used to investigate the kinetics of water oxidation on undoped dense bismuth vanadate (BiVO4) photoanodes under conditions of photoelectrochemical water oxidation and compare to those obtained for oxidation of a simple redox couple. These measurements are employed to determine the quasi-steady-state densities of surface-accumulated holes, ps, and correlate these with photocurrent density as a function of light intensity, allowing a rate law analysis of the water oxidation mechanism. The reaction order in surface hole density is found to be first order for ps < 1 nm–2 and third order for ps > 1 nm–2. The effective turnover frequency of each surface hole is estimated to be 14 s–1 at AM 1.5 conditions. Using a single-electron redox couple, potassium ferrocyanide, as the hole scavenger, only the first-order reaction is observed, with a higher rate constant than that for water oxidation. These results are discussed in terms of catalysis by BiVO4 and implications for material design strategies for efficient water oxidation.

Journal article

Ma Y, Kafizas A, Pendlebury SR, Le Formal F, Durrant JRet al., 2016,

Photoinduced Absorption Spectroscopy of CoPi on BiVO4: The Function of CoPi during Water Oxidation

, Advanced Functional Materials, Vol: 26, Pages: 4951-4960, ISSN: 1616-301X

This paper employs photoinduced absorption and electrochemical techniques to analyze the charge carrier dynamics that drive photoelectrochemical water oxidation on bismuth vanadate (BiVO4), both with and without cobalt phosphate (CoPi) co-catalyst. These results are correlated with spectroelectrochemical measurements of CoII oxidation to CoIII in a CoPi/FTO (fluorine doped tin oxide) electrode during dark electrocatalytic water oxidation. Electrocatalytic water oxidation exhibits a non-linear dependence on CoIII density, with a sharp onset at 1 × 1017 CoIII cm−2. These results are compared quantitatively with the degree of CoPi oxidation observed under conditions of photoinduced water oxidation on CoPi–BiVO4 photoanodes. For the CoPi–BiVO4 photoanodes studied herein, ≤5% of water oxidation proceeds from CoPi sites, making the BiVO4 surface the predominant water oxidation site. This study highlights two key factors that limit the ability of CoPi to improve the catalytic performance of BiVO4: 1) the kinetics of hole transfer from the BiVO4 to the CoPi layer are too slow to effectively compete with direct water oxidation from BiVO4; 2) the slow water oxidation kinetics of CoPi result in a large accumulation of CoIII states, causing an increase in recombination. Addressing these factors will be essential for improving the performance of CoPi on photoanodes for solar-driven water oxidation.

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Journal article

Kafizas A, Wang X, Pendlebury SR, Barnes P, Ling M, Sotelo-Vazquez C, Quesada-Cabrera R, Li C, Parkin IP, Durrantt JRet al., 2016,

Where Do Photogenerated Holes Go in Anatase:Rutile TiO<sub>2</sub>? A Transient Absorption Spectroscopy Study of Charge Transfer and Lifetime

, JOURNAL OF PHYSICAL CHEMISTRY A, Vol: 120, Pages: 715-723, ISSN: 1089-5639

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Citations: 119

Journal article

Ma Y, Le Formal F, Kafizas A, Pendlebury S, Durrant Jet al., 2015,

Efficient suppression of back electron/hole recombination in cobalt phosphate surface-modified undoped bismuth vanadate photoanodes

, Journal of Materials Chemistry A, Vol: 3, Pages: 20649-20657, ISSN: 2050-7496

In this paper, we compared for the first time the dynamics of photogenerated holes in BiVO4 photoanodes with and without CoPi surface modification, employing transient absorption and photocurrent measurements on microsecond to second timescales. CoPi surface modification is known to cathodically shift the water oxidation onset potential; however, the reason for this improvement has not until now been fully understood. The transient absorption and photocurrent data were analyzed using a simple kinetic model, which allows quantification of the competition between electron/hole recombination and water oxidation. The results of this model are shown to be in excellent agreement with the measured photocurrent data. We demonstrate that the origin of the improvement of photocurrent onset resulting from CoPi treatment is primarily due to retardation of back electron/hole recombination across the space charge layer; no evidence of catalytic water oxidation via CoPi was observed.

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Journal article

Wang X, Kafizas A, Li X, Moniz SJA, Reardon PJT, Tang J, Parkin IP, Durrant JRet al., 2015,

Transient Absorption Spectroscopy of Anatase and Rutile: The Impact of Morphology and Phase on Photocatalytic Activity

, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 119, Pages: 10439-10447, ISSN: 1932-7447

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Citations: 122

Journal article

Kafizas AG, Durrant JR, 2015,

Transient Absorption Spectroscopy of Anatase and Rutile: the Impact of Morphology and Phase on Photocatalytic Activity

, The Journal of Physical Chemistry C, ISSN: 1932-7447

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Journal article

Sotelo-Vazquez C, Noor N, Kafizas A, Quesada-Cabrera R, Scanlon DO, Taylor A, Durrant JR, Parkin IPet al., 2015,

Multifunctional P-Doped TiO2 films: a new approach to self-cleaning, transparent conducting oxide materials

, Chemistry of Materials, Vol: 27, Pages: 3234-3242, ISSN: 1520-5002

Multifunctional P-doped TiO2 thin films were synthesized by atmospheric pressure chemical vapor deposition (APCVD). This is the first example of P-doped TiO2 films with both P5+ and P3– states, with the relative proportion being determined by synthesis conditions. This technique to control the oxidation state of the impurities presents a new approach to achieve films with both self-cleaning and TCO properties. The origin of electrical conductivity in these materials was correlated to the incorporation of P5+ species, as suggested by Hall Effect probe measurements. The photocatalytic performance of the films was investigated using the model organic pollutant, stearic acid, with films containing predominately P3– states found to be vastly inferior photocatalysts compared to undoped TiO2 films. Transient absorption spectroscopy studies also showed that charge carrier concentrations increased by several orders of magnitude in films containing P5+ species only, whereas photogenerated carrier lifetimes—and thus photocatalytic activity—were severely reduced upon incorporation of P3– species. The results presented here provide important insights on the influence of dopant nature and location within a semiconductor structure. These new P-doped TiO2 films are a breakthrough in the development of multifunctional advanced materials with tuned properties for a wide range of applications.

Journal article

Pendlebury SR, Wang X, Le Formal F, Cornuz M, Kafizas A, Tilley SD, Graetzel M, Durrant JRet al., 2014,

Ultrafast charge carrier recombination and trapping in hematite photoanodes under applied bias

, Journal of the American Chemical Society, Vol: 136, Pages: 9854-9857, ISSN: 0002-7863

Transient absorption spectroscopy on subpicosecond to second time scales is used to investigate photogenerated charge carrier recombination in Si-doped nanostructured hematite (α-Fe2O3) photoanodes as a function of applied bias. For unbiased hematite, this recombination exhibits a 50% decay time of ∼6 ps, ∼103 times faster than that of TiO2 under comparable conditions. Anodic bias significantly retards hematite recombination dynamics, and causes the appearance of electron trapping on ps−μs time scales. These ultrafast recombination dynamics, their retardation by applied bias, and the associated electron trapping are discussed in terms of their implications for efficient water oxidation.

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