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<jats:p>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 impact of these defects on charge separation and transport, as well as the implications held for other highly doped materials for photoelectrochemical water oxidation.</jats:p>

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Bozal-Ginesta C, Mesa CA, Eisenschmidt A, Shankar R, Francàs L, Antón-García D, Warnan J, Willkomm J, Reynal A, Reisner E, Durrant JRet al.,

Charge Accumulation Kinetics in Multi-redox Molecular Catalysts Immobilised on TiO2

<jats:p>Multi-redox catalysis requires the transfer of more than one charge carrier and is crucial for solar energy conversion into fuels and valuable chemicals. In photo(electro)chemical systems, however, the necessary accumulation of multiple, long-lived charges is challenged by recombination with their counterparts. Herein, we investigate charge accumulation in two model multi-redox molecular catalysts for proton and CO<jats:sub>2</jats:sub> reduction attached onto mesoporous TiO<jats:sub>2</jats:sub> electrodes. Transient absorption spectroscopy and spectroelectrochemical techniques have been employed to study the kinetics of photoinduced electron transfer from the TiO<jats:sub>2</jats:sub> to the molecular catalysts in acetonitrile, with triethanolamine as the hole scavenger. At high light intensities, we detect charge accumulation in the millisecond timescale in the form of multi-reduced species. The redox potentials of the catalysts and the capacity of TiO<jats:sub>2</jats:sub> to accumulate electrons play an essential role in the charge accumulation process at the molecular catalyst. Recombination of reduced species with valence band holes in TiO<jats:sub>2</jats:sub> is observed to be faster than microseconds, while electron transfer from multi-reduced species to the conduction band or the electrolyte occurs in the millisecond timescale. Finally, under light irradiation, we show how charge accumulation on the catalyst is regulated as a function of the applied bias and the excitation light intensity.</jats:p>

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Bozal-Ginesta C, Rao RR, Mesa CA, Liu X, Hillman SAJ, Stephens IEL, Durrant JRet al.,

Operando spectroelectrochemistry of redox state kinetics in water-oxidation IrOx electrocatalysts

<jats:p>Hydrous iridium oxides (IrOx) are the best oxygen evolution electrocatalysts available for operation in acidic environments. In this study, we employ time-resolved operando spectroelectrochemistry to investigate the redox states kinetics of IrOx electrocatalyst films for both water and hydrogen peroxide oxidation. Three different redox species involving Ir3+, Ir4+ and Ir4.x are identified spectroscopically and their concentrations are quantified as a function of applied potential. The generation of Ir4.x+ states is found to be the potential determining step for catalytic water oxidation, whilst H2O2 oxidation is observed to be driven by the generation of Ir4+ states. The reaction kinetics for water oxidation, determined from the optical signal decays at open circuit, accelerate from ~ 20 s to < 0.5 s with increasing applied potential above 1.3V vs. RHE (i.e. TOFs per active Ir state increasing from 0.05 to 2 s-1). In contrast, the reaction kinetics for H2O2 are found to be almost independent of the applied potential (increasing from 0.1-0.3 s-1 over a wider potential window), indicative of a first order reaction mechanism. These spectroelectrochemical data quantify the increase of both the density of active Ir4.x+ states and the TOFs of these states with applied positive potential, resulting in the observed sharp turn on of catalytic water oxidation current. We reconcile these data with the broader literature while providing a new kinetic insight into IrOx electrocatalytic reaction mechanisms, indicating a first order reaction mechanism for H2O2 oxidation driven by Ir4+ states, and a higher order reaction mechanism involving the co-operative interaction of multiple Ir4.x+ states for water oxidation.</jats:p>

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Adler C, Selim S, Krivtsov I, Li C, Mitoraj D, Dietzek B, Durrant JR, Beranek Ret al.,

Photodoping and Fast Charge Extraction in Ionic Carbon Nitride Photoanodes

<jats:p>Ionic carbon nitrides based on poly(heptazineimides) (PHI) represent a vigorously studied class of materials with possibleapplications in photocatalysis and energy storage. Herein, we study, for thefirst time, the photogenerated charge dynamics in highly stable and binder-freePHI photoanodes using in operando transientphotocurrents and spectroelectrochemical photoinduced absorption measurements.We discover that light-induced accumulation of long-lived trapped electronswithin the PHI film leads to effective photodoping of the PHI film, resultingin a significant improvement of photocurrent response due to more efficientelectron transport. Whilephotodoping has been previously reported for various semiconductors,it has never been shown before for carbon nitride materials. Furthermore, we find that the extraction kinetics ofuntrapped electrons are remarkably fast in these PHI photoanodes, with electronextraction times (ms) comparableto those measured for commonly employed metal oxide semiconductors. These results shed lighton the excellent performance of PHI photoanodes in alcohol photoreforming,including very negative photocurrent onset, outstanding fill factor, and thepossibility to operate under zero-bias conditions. More generally, the herereported photodoping effect and fast electron extraction in PHI photoanodes establisha strong rationale for the use of PHI films in various applications, such asbias-free photoelectrochemistry or photobatteries. </jats:p>

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Adler C, Selim S, Krivtsov I, Li C, Mitoraj D, Dietzek B, Durrant JR, Beranek Ret al.,

Photodoping and Fast Charge Extraction in Ionic Carbon Nitride Photoanodes

<jats:p>Ionic carbon nitrides based on poly(heptazine imides) (PHI) represent one of the most vigorously studied class of low-cost, tunable and stable polymeric materials with possible applications in photocatalysis and energy storage. However, the fundamental photophysical properties and processes that govern the performance of these materials are still poorly understood and have been studied mostly in form of particle suspensions. We study, for the first time, the photogenerated charge dynamics in highly stable and binder-free PHI photoanodes with excellent performance in photoelectrocatalytic alcohol conversions using <jats:italic>in operando</jats:italic> transient photocurrents (TPC) and spectroelectrochemical photoinduced absorption (PIA) measurements. Interestingly, we discover that light-induced accumulation of long-lived trapped electrons within the PHI film leads to effective photodoping of the PHI film, resulting in a significant improvement of photocurrent response due to more efficient electron transport. While photodoping has been previously reported for various inorganic and organic semiconductors, the here reported beneficial photodoping effect has never been shown before for carbon nitride materials. Furthermore, we find that the extraction kinetics of untrapped electrons are remarkably fast in these PHI photoanodes, with electron extraction times (ms) comparable to those measured for commonly employed metal oxide semiconductors (<jats:italic>e.g.</jats:italic>, TiO<jats:sub>2</jats:sub>, WO<jats:sub>3</jats:sub>, BiVO<jats:sub>4</jats:sub>). These results shed light on the excellent performance of PHI photoanodes in alcohol photoreforming, including very negative photocurrent onset, outstanding fill factor, and the possibility to carry out photoreforming under zero-bias conditions. More generally, the here reported photodoping effect and fast electron extraction in PHI photoanodes re

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Francàs L, Selim S, Corby S, Lee D, Mesa C, Pastor E, Choi K-S, Durrant JRet al.,

Water Oxidation Kinetics of Nanoporous BiVO4 Photoanodes Functionalised with Nickel/iron Oxyhydroxide Electrocatalysts

<jats:p>In this work, spectroelectrochemical techniques are employed to analyse the catalytic water oxidation performance of a series of three nickel/iron oxyhydroxide electrocatalysts deposited on FTO and BiVO4, at neutral pH. Similar electrochemical water oxidation performance is observed for each of the FeOOH, Ni(Fe)OOH and FeOOHNiOOH electrocatalysts studied, which is found to result from a balance between degree of charge accumulation and rate of water oxidation. Once added onto BiVO4 photoanodes, a large enhancement in the water oxidation photoelectrochemical performance is observed in comparison to the un-modified BiVO4. To understand the origin of this enhancement, the films were evaluated through time-resolved optical spectroscopic techniques, allowing comparisons between electrochemical and photoelectrochemical water oxidation. For all three catalysts, fast hole transfer from BiVO4 to the catalyst is observed in the transient absorption data. Using operando photoinduced absorption measurements, we find that water oxidation is driven by oxidised states within the catalyst layer, following hole transfer from BiVO4. This charge transfer is correlated with a suppression of recombination losses which result in remarkably enhanced water oxidation performance relative to un-modified BiVO4. Moreover, despite similar electrocatalytic performance of all three electrocatalysts, we show that variations in water oxidation performance observed among the BiVO4/MOOH photoanodes stem from differences in photoelectrochemical and electrochemical charge accumulation in the catalyst layers. Under illumination, the amount of accumulated charge in the catalyst is driven by the injection of photogenerated holes from BiVO4&

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Imperial College London

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Determining the Role of Oxygen Vacancies in the Photocatalytic Performance of WO3 for Water Oxidation

Charge Accumulation Kinetics in Multi-redox Molecular Catalysts Immobilised on TiO2

Operando spectroelectrochemistry of redox state kinetics in water-oxidation IrOx electrocatalysts

Photodoping and Fast Charge Extraction in Ionic Carbon Nitride Photoanodes

Photodoping and Fast Charge Extraction in Ionic Carbon Nitride Photoanodes

Water Oxidation Kinetics of Nanoporous BiVO4 Photoanodes Functionalised with Nickel/iron Oxyhydroxide Electrocatalysts

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PRIMARY RADICAL PAIR FORMATION IN ISOLATED PHOTOSYSTEM 2 REACTION CENTERS

FEMTOSECOND SPECTROSCOPY OF ENERGY AND ELECTRON-TRANSFER IN ISOLATED PHOTOSYSTEM 2 REACTION CENTERS