19 results found
Mesa Zamora CA, Kafizas A, Francàs L, et 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.
Kafizas A, Ma Y, Pastor E, et 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.
Ma Y, Mesa CA, Pastor E, et 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.
Morris MR, Pendlebury S, Hong J, et al., 2016, Effect of internal electric fields on charge carrier dynamics in a ferroelectric material for solar energy conversion, Advanced Materials, Vol: 28, Pages: 7123-7128, ISSN: 0935-9648
Spontaneous polarization is shown to enhance the lifetimes of photogenerated species in BaTiO3. This is attributed to polarization-induced surface band bending acting as a thermal barrier to electron/hole recombination. The study indicates that the efficiencies of solar cells and solar fuels devices can be enhanced by the use of ferroelectric materials.
Ma Y, Kafizas A, Pendlebury SR, et 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.
Kafizas A, Wang X, Pendlebury SR, et 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
Feckl JM, Dunn HK, Zehetmaier PM, et al., 2015, Ultrasmall Co3O4 Nanocrystals Strongly Enhance Solar Water Splitting on Mesoporous Hematite, Advanced Materials Interfaces, Vol: 2, ISSN: 2196-7350
The synthesis of crystalline, nonagglomerated, and perfectly dispersible Co3O4 nanoparticles with an average size of 3–7 nm using a solvothermal reaction in tert-butanol is reported. The very small size and high dispersibility of the Co3O4 nanoparticles allow for their homogeneous deposition on mesoporous hematite layers serving as the photoactive absorber in the light-driven water splitting reaction. This surface treatment leads to a striking photocurrent increase. While the enhancement of hematite photoanode performance by cobalt oxides is known, the preformation and subsequent application of well-defined cobalt oxide nanoparticles are novel and allow for the treatment of arbitrarily complex hematite morphologies. Photoelectrochemical and transient absorption spectroscopy studies show that this enhanced performance is due to the suppression of surface electron–hole recombination on time scales of milliseconds to seconds.
Ma Y, Le Formal F, Kafizas A, et 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.
Le Formal F, Pastor E, Tilley SD, et al., 2015, Rate law analysis of water oxidation on a hematite surface, Journal of the American Chemical Society, Vol: 137, Pages: 6629-6637, ISSN: 1520-5126
Water oxidation is a key chemical reaction, central to both biological photosynthesis and artificial solar fuel synthesis strategies. Despite recent progress on the structure of the natural catalytic site, and on inorganic catalyst function, determining the mechanistic details of this multiredox reaction remains a significant challenge. We report herein a rate law analysis of the order of water oxidation as a function of surface hole density on a hematite photoanode employing photoinduced absorption spectroscopy. Our study reveals a transition from a slow, first order reaction at low accumulated hole density to a faster, third order mechanism once the surface hole density is sufficient to enable the oxidation of nearest neighbor metal atoms. This study thus provides direct evidence for the multihole catalysis of water oxidation by hematite, and demonstrates the hole accumulation level required to achieve this, leading to key insights both for reaction mechanism and strategies to enhance function.
Pendlebury SR, Wang X, Le Formal F, et 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.
Le Formal F, Pendlebury SR, Cornuz M, et al., 2014, Back electron-hole recombination in hematite photoanodes for water splitting, Journal of the American Chemical Society, Vol: 136, Pages: 2564-2574, ISSN: 0002-7863
The kinetic competition between electron–hole recombination and water oxidation is a key consideration for the development of efficient photoanodes for solar driven water splitting. In this study, we employed three complementary techniques, transient absorption spectroscopy (TAS), transient photocurrent spectroscopy (TPC), and electrochemical impedance spectroscopy (EIS), to address this issue for one of the most widely studied photoanode systems: nanostructured hematite thin films. For the first time, we show a quantitative agreement between all three techniques. In particular, all three methods show the presence of a recombination process on the 10 ms to 1 s time scale, with the time scale and yield of this loss process being dependent upon applied bias. From comparison of data between these techniques, we are able to assign this recombination phase to recombination of bulk hematite electrons with long-lived holes accumulated at the semiconductor/electrolyte interface. The data from all three techniques are shown to be consistent with a simple kinetic model based on competition between this, bias dependent, recombination pathway and water oxidation by these long-lived holes. Contrary to most existing models, this simple model does not require the consideration of surface states located energetically inside the band gap. These data suggest two distinct roles for the space charge layer developed at the semiconductor/electrolyte interface under anodic bias. Under modest anodic bias (just anodic of flatband), this space charge layer enables the spatial separation of initially generated electrons and holes following photon absorption, generating relatively long-lived holes (milliseconds) at the semiconductor surface. However, under such modest bias conditions, the energetic barrier generated by the space charge layer field is insufficient to prevent the subsequent recombination of these holes with electrons in the semiconductor bulk on a time scale faster than wa
Ma Y, Pendlebury SR, Reynal A, et al., 2014, Dynamics of photogenerated holes in undoped BiVO4 photoanodes for solar water oxidation, Chemical Science, Vol: 5, Pages: 2964-2973, ISSN: 2041-6520
The dynamics of photogenerated holes in undoped BiVO4 photoanodes for water splitting were studied using transient absorption spectroscopy, correlated with photoelectrochemical and transient photocurrent data. Transient absorption signals of photogenerated holes were identified using electron/hole scavengers and applied electrical bias in a complete photoelectrochemical cell. The yield of long-lived (0.1-1 s) photogenerated holes is observed to correlate as a function of applied electrical bias with the width of the space charge layer, as determined by electrochemical impedance spectroscopy. The transient absorption decay time constant of these long-lived holes is also observed to be dependent upon the applied bias, assigned to kinetic competition between water oxidation and recombination of these surface accumulated holes with bulk electrons across the space charge layer. The time constant for this slow recombination measured with transient absorption spectroscopy is shown to match the time constant of back electron transfer from the external circuit determined from chopped light transient photocurrent measurements, thus providing strong evidence for these assignments. The yield of water oxidation determined from these measurements, including consideration of both the yield of long-lived holes, and the fraction of these holes which are lost due to back electron/hole recombination, is observed to be in good agreement with the photocurrent density measured for BiVO4 photoanodes as a function of bias under continuous irradiation. Overall these results indicate two distinct recombination processes which limit photocurrent generation in BiVO4 photoanodes: firstly rapid ([less-than-or-equal]microseconds) electron/hole recombination, and secondly recombination of surface-accumulated holes with bulk BiVO4 electrons. This second 'back electron transfer' recombination occurs on the milliseconds-seconds timescale, and is only avoided at strong anodic biases where the potentia
Barroso M, Pendlebury SR, Cowan AJ, et al., 2013, Charge carrier trapping, recombination and transfer in hematite (α-Fe<sub>2</sub>O<sub>3</sub>) water splitting photoanodes, CHEMICAL SCIENCE, Vol: 4, Pages: 2724-2734, ISSN: 2041-6520
Barroso M, Mesa CA, Pendlebury SR, et al., 2012, Dynamics of photogenerated holes in surface modified α-Fe<sub>2</sub>O<sub>3</sub> photoanodes for solar water splitting, PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, Vol: 109, Pages: 15640-15645, ISSN: 0027-8424
Pendlebury SR, Cowan AJ, Barroso M, et al., 2012, Correlating long-lived photogenerated hole populations with photocurrent densities in hematite water oxidation photoanod, Energy and Environmental Science
Barroso M, Cowan AJ, Pendlebury SR, et al., 2011, The Role of Cobalt Phosphate in Enhancing the Photocatalytic Activity of alpha-Fe(2)O(3) toward Water Oxidation, ISSN: 0002-7863
Cowan AJ, Barnett CJ, Pendlebury SR, et al., 2011, Activation Energies for the Rate-Limiting Step in Water Photooxidation by Nanostructured alpha-Fe(2)O(3) and TiO(2), ISSN: 0002-7863
Cowan AJ, Pendlebury SR, Barroso M, et al., 2011, Charge carrier dynamics in metal oxide water splitting photoelectrodes, 242nd ACS National Meeting
Pendlebury SR, Barroso M, Cowan AJ, et al., 2010, Dynamics of photogenerated holes in nanocrystalline alpha-Fe(2)O(3) electrodes for water oxidation probed by transient absorption spectroscopy, ISSN: 1359-7345
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