Oxygen vacancies are ubiquitous in metal oxides and critical to performance, yet the impact of these states upon charge carrier dynamics important for photoelectrochemical and photocatalytic applications, remains contentious and poorly understood. A key challenge is the unambiguous identification of spectroscopic fingerprints which can be used to track their function. Herein, we employ five complementary techniques to modulate the electronic occupancy of states associated with oxygen vacancies in situ in BiVO4 photoanodes, allowing us to identify a spectral signature for the ionisation of these states. We obtain an activation energy of ̴ 0.2 eV for this ionisation process, with thermally activated electron de-trapping from these states determining the kinetics of electron extraction, consistent with improved photoelectrochemical performance at higher temperatures. Bulk, un-ionised states however, function as deep hole traps, with such trapped holes being energetically unable to drive water oxidation. These observations help address recent controversies in the literature over oxygen vacancy function, providing new insights into their impact upon photoelectrochemical performance.

Ni/Fe oxyhydroxides are the best performing Earth-abundant electrocatalysts for water oxidation. However, the origin of their remarkable performance is not well understood. Herein, we employ spectroelectrochemical techniques to analyse the kinetics of water oxidation on a series of Ni/Fe oxyhydroxide films: FeOOH, FeOOHNiOOH, and Ni(Fe)OOH (5% Fe). The concentrations and reaction rates of the oxidised states accumulated during catalysis are determined. Ni(Fe)OOH is found to exhibit the fastest reaction kinetics but accumulates fewer states, resulting in a similar performance to FeOOHNiOOH. The later catalytic onset in FeOOH is attributed to an anodic shift in the accumulation of oxidised states. Rate law analyses reveal that the rate limiting step for each catalyst involves the accumulation of four oxidised states, Ni-centred for Ni(Fe)OOH but Fe-centred for FeOOH and FeOOHNiOOH. We conclude by highlighting the importance of equilibria between these accumulated species and reactive intermediates in determining the activity of these materials.

Generating charge carriers with lifetimes long enough to drive catalysis is a critical aspect for both photoelectrochemical and photocatalytic systems and a key determinant of their efficiency. This review addresses the charge carrier dynamics underlying the performance of metal oxides as photoanodes and their ability to drive photoelectrochemical water oxidation, alongside wider comparison with metal oxide function in photocatalytic and electrocatalytic systems. We start by highlighting the disparity between the ps -ns lifetimes of electron and holes photoexcited in bulk metal oxides versus the ms -s timescale of water oxidation catalysis. We go on to review recent literature of the dominant kinetic processes determining photoanode performance, namely charge generation, polaron formation and charge trapping, bulk and surface recombination, charge separation and extraction, and finally the kinetics of water oxidation catalysis. With each topic, we review current understanding and note areas of remaining uncertainty or controversy. We discuss the potential for material selection and examine approaches such as doping, nanostructuring, junction formation and/or co-catalyst deposition to enhance performance. Critically, we examine how such performance enhancements can be understood from analyses of carrier dynamics and propose design guidelines for further material or device optimisation.

Semiconducting polymers are versatile materials for solar energy conversion and have gained popularity as photocatalysts for sunlight-driven hydrogen production. Organic polymers often contain residual metal impurities such as palladium (Pd) clusters that are formed during the polymerization reaction, and there is increasing evidence for a catalytic role of such metal clusters in polymer photocatalysts. Using transient and operando optical spectroscopy on nanoparticles of F8BT, P3HT, and the dibenzo[ b,d ]thiophene sulfone homopolymer P10, we demonstrate how differences in the time scale of electron transfer to Pd clusters translate into hydrogen evolution activity optima at different residual Pd concentrations. For F8BT nanoparticles with common Pd concentrations of >1000 ppm (>0.1 wt %), we find that residual Pd clusters quench photogenerated excitons via energy and electron transfer on the femto-nanosecond time scale, thus outcompeting reductive quenching. We spectroscopically identify reduced Pd clusters in our F8BT nanoparticles from the microsecond time scale onward and show that the predominant location of long-lived electrons gradually shifts to the F8BT polymer when the Pd content is lowered. While a low yield of long-lived electrons limits the hydrogen evolution activity of F8BT, P10 exhibits a substantially higher hydrogen evolution activity, which we demonstrate results from higher yields of long-lived electrons due to more efficient reductive quenching. Surprisingly, and despite the higher performance of P10, long-lived electrons reside on the P10 polymer rather than on the Pd clusters in P10 particles, even at very high Pd concentrations of 27000 ppm (2.7 wt %). In contrast, long-lived electrons in F8BT already reside on Pd clusters before the typical time scale of hydrogen evolution. This comparison shows that P10 exhibits efficient reductive quenching but slow electron transfer to residual Pd clusters, whereas the opposite is the case for F8BT. These findings suggest that the development of even more efficient polymer photocatalysts must target materials that combine both rapid reductive quenching and rapid charge transfer to a metal-based cocatalyst.

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 timescales 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 oxidising valence band position of WO3, thus highlighting the potential importance of thermodynamic driving force for catalysis in outcompeting detrimental surface recombination processes.

A medium concentration of oxygen vacancies (VO ≈ 2%) is critical to the performance of WO3 photoanodes for solar water oxidation, enhancing charge separation and reducing recombination across all timescales examined.

Catalyst modification of metal oxide photoanodes can result in markedly improved water oxidation efficiency. However, the reasons for improvement are often subtle and controversial. Upon depositing a CoFe Prussian blue (CoFe-PB) water oxidation catalyst on BiVO 4 , a large photocurrent increase and onset potential shift (up to 0.8 V) are observed, resulting in a substantially more efficient system with high stability. To elucidate the origin of this enhancement, we used time-resolved spectroscopies to compare the dynamics of photogenerated holes in modified and unmodified BiVO 4 films. Even in the absence of strong positive bias, a fast (pre-ms), largely irreversible hole transfer from BiVO 4 to CoFe-PB is observed. This process retards recombination, enabling holes to accumulate in the catalyst. Holes in CoFe-PB remain reactive, oxidizing water at a similar rate to holes in pristine BiVO 4 . CoFe-PB therefore enhances performance by presenting a favorable interface for efficient hole transfer, combined with the catalytic function necessary to drive water oxidation.

Metal oxides and oxyhydroxides exhibit state-of-the-art activity for the oxygen evolution reaction (OER); however, their reaction mechanism, particularly the relationship between charging of the oxide and OER kinetics, remains elusive. Here, we investigate a series of Mn-, Co-, Fe-, and Zn-doped nickel oxides using operando UV–vis spectroscopy coupled with time-resolved stepped potential spectroelectrochemistry. The Ni2+/Ni3+ redox peak potential is found to shift anodically from Mn- < Co- < Fe- < Zn-doped samples, suggesting a decrease in oxygen binding energetics from Mn- to Zn-doped samples. At OER-relevant potentials, using optical absorption spectroscopy, we quantitatively detect the subsequent oxidation of these redox centers. The OER kinetics was found to have a second-order dependence on the density of these oxidized species, suggesting a chemical rate-determining step involving coupling of two oxo species. The intrinsic turnover frequency per oxidized species exhibits a volcano trend with the binding energy of oxygen on the Ni site, having a maximum activity of ∼0.05 s–1 at 300 mV overpotential for the Fe-doped sample. Consequently, we propose that for Ni centers that bind oxygen too strongly (Mn- and Co-doped oxides), OER kinetics is limited by O–O coupling and oxygen desorption, while for Ni centers that bind oxygen too weakly (Zn-doped oxides), OER kinetics is limited by the formation of oxo groups. This study not only experimentally demonstrates the relation between electroadsorption free energy and intrinsic kinetics for OER on this class of materials but also highlights the critical role of oxidized species in facilitating OER kinetics.