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.

A medium concentration of oxygen vacancies (V _O ≈ 2%) is critical to the performance of WO ₃ photoanodes for solar water oxidation, enhancing charge separation and reducing recombination across all timescales examined.

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 Co^II oxidation to Co^III in a CoPi/FTO (fluorine doped tin oxide) electrode during dark electrocatalytic water oxidation. Electrocatalytic water oxidation exhibits a non‐linear dependence on Co^III density, with a sharp onset at 1 × 10¹⁷ Co^III 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 Co^III 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.