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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.
  • 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
  • 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.
  • 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.
  • 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
  • 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.
  • 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
  • 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
  • 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.
  • Journal article
    Knapp CE, Manzi JA, Kafizas A, Parkin IP, Carmalt CJet al., 2014,

    Aerosol-Assisted Chemical Vapour Deposition of Transparent Zinc Gallate Films

    , CHEMPLUSCHEM, Vol: 79, Pages: 1024-1029, ISSN: 2192-6506
  • Journal article
    Chadwick N, Sathasivam S, Kafizas A, Bawaked SM, Obaid AY, Al-Thabaiti S, Basahel SN, Parkin IP, Carmalt CJet al., 2014,

    Combinatorial aerosol assisted chemical vapour deposition of a photocatalytic mixed SnO2/TiO2 thin film

    , JOURNAL OF MATERIALS CHEMISTRY A, Vol: 2, Pages: 5108-5116, ISSN: 2050-7488
  • Journal article
    Sathasivam S, Kafizas A, Ponja S, Chadwick N, Bhachu DS, Bawaked SM, Obaid AY, Al-Thabaiti S, Basahel SN, Carmalt CJ, Parkin IPet al., 2014,

    Combinatorial Atmospheric Pressure CVD of a Composite TiO<sub>2</sub>/SnO<sub>2</sub> Thin Film

    , CHEMICAL VAPOR DEPOSITION, Vol: 20, Pages: 69-79, ISSN: 0948-1907
  • Journal article
    Kafizas A, Noor N, Carmichael P, Scanlon DO, Carmalt CJ, Parkin IPet al., 2014,

    Combinatorial Atmospheric Pressure Chemical Vapor Deposition of F: TiO 2; the Relationship between Photocatalysis and Transparent Conducting Oxide Properties

    , ADVANCED FUNCTIONAL MATERIALS, Vol: 24, Pages: 1758-1771, ISSN: 1616-301X
  • Journal article
    Kafizas A, Carmalt CJ, Parkin IP, 2013,

    CVD and precursor chemistry of transition metal nitrides

    , COORDINATION CHEMISTRY REVIEWS, Vol: 257, Pages: 2073-2119, ISSN: 0010-8545

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Dr. Andreas Kafizas
Leader - Solar Coatings Group

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