Water Oxidation and Electron Extraction Kinetics in Nanostructured Tungsten Trioxide Photoanodes

Abstract

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