Due to their potential long term stability, ease of synthesis, and low production cost, semiconducting metal oxide materials have received much attention for use as photoanode materials in photoelectrochemical water splitting devices. To date, most research has focused on binary semiconducting oxide materials. Since no binary oxide material has currently met all the criteria listed above, researchers have expanded the materials search database to include more complex multinary oxides. Among the multinary oxides investigated, bismuth vanadate, BiVO₄, is the highest performing material. However, charge carrier recombination at the BiVO₄/electrolyte interface remains a limitation. Reactions at the BiVO₄/electrolyte interface may give rise to surface states that can act as relay sites for charge injection into the electrolyte, but also as electron and hole traps that can enhance recombination rates. A detailed understanding of the chemical composition at the BiVO₄/electrolyte interface and its dependence on specific conditions (applied potential and illumination) would provide valuable input for strategies to suppress surface recombination and to further optimize BiVO₄-based photoanode materials.

We have used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to gain a molecular-level understanding of the BiVO₄/aqueous electrolyte interface. With soft X-rays water adsorption from the gas phase at pressures up to a few Torr can be studied providing information about the early stages of solid/electrolyte interface formation. The tender X-ray form (AP-HAXPES) can be used to directly interrogate a solid surface under a bulk-like electrolyte film that is tens of nanometers thick. Our AP-XPS measurements on the BiVO₄(010) single crystal surface indicate that the surface is significantly hydroxylated by ~0.5 Torr. Surface hydroxylation is accompanied by reduced vanadium at the surface which leads to occupied states above the valence band maximum. These states may act as recombination centers on BiVO₄-based photoanodes. Using AP-HAXPES we have studied the open-circuit behaviour of the thin-film BiVO₄/potassium phosphate electrolyte interface when illuminated with a solar simulator. Upon illumination we observe spectral changes consistent with the formation of a thin bismuth phosphate layer and significant restructuring of the electrolyte near the interface. Bismuth phosphate formation under illumination may quench surface states that have been observed in capacitance versus voltage scans. Surface state suppression by bismuth phosphate formation is a also potential explanation for the increase in performance of BiVO₄ photoanodes that have undergone light soaking. In general, these results provide fundamental information about the complex chemical behaviour of semiconductor/electrolyte interfaces in water splitting devices.