Photocharging has recently been demonstrated as a powerful method to improve the photoelectrochemical water splitting performance of different metal oxide photoanodes, including BiVO₄. In this work, we use ambient-pressure X-ray Raman scattering (XRS) spectroscopy to study the surface electronic structure of photocharged BiVO₄. The O K edge spectrum was simulated using the finite difference method near-edge structure program package, which revealed a change in electron confinement and occupancy in the conduction band. These insights, combined with ultraviolet-visible spectroscopy and X-ray photoelectron spectroscopy analyses, reveal that a surface layer formed during photocharging creates a heterojunction with BiVO₄, leading to favorable band bending and strongly reduced surface recombination. The XRS images presented in this work exhibit good agreement with soft X-ray absorption near-edge structure spectra from the literature, demonstrating that XRS is a powerful tool to study the electronic and structural properties of light elements in semiconductors. Our findings provide direct evidence of the electronic modification of a metal oxide photoanode surface as a result of the adsorption of electrolyte anionic species under operating conditions. This work highlights that the surface adsorption of these electrolyte anionic species is likely present in most studies on metal oxide photoanodes and has serious implications for the photoelectrochemical performance analysis and fundamental understanding of these materials.

Electrocatalysis of carbon dioxide can provide a valuable pathway towards the sustainable production of chemicals and fuels from renewable electricity sources. One of the main challenges to enable this technology is to find suitable electrodes that can act as efficient, stable and selective CO₂ reduction catalysts. Modified silver catalysts and in particular, catalysts electrochemically derived from silver-oxides, have shown great promise in this regard. Here, we use operando EXAFS analysis to study the differences in surface composition between a pure silver film and oxide-derived silver catalysts-a nanostructured catalyst with improved CO₂ reduction performance. The EXAFS analysis reveals the presence of trace amounts of oxygen in the oxide-derived silver samples, with the measured oxygen content correlating well with experimental studies showing an increase in CO₂ reduction reactivity towards carbon monoxide. The selectivity towards CO production also partially scales with the increased surface area, showing that the morphology, local composition and electronic structure all play important roles in the improved activity and selectivity of oxide-derived silver electrocatalysts. Earlier studies based on X-ray photoelectron spectroscopy (XPS) were not able to identify this oxygen, most likely because in ultra-high vacuum conditions, silver can self-reduce to Ag⁰, removing existing oxygen species. This operando EXAFS study shows the potential for in situ and operando techniques to probe catalyst surfaces during electrolysis and aid in the overall understanding of electrochemical systems.

The electrocatalytic reduction of CO₂ to chemical fuels has attracted significant attention in recent years. Among transition metals, silver shows one of the highest faradaic efficiencies for CO formation as the main reaction product; however, the exact mechanism for this conversion is not fully understood. In this work, we study the reaction mechanism of silver as a CO₂ reduction catalyst using in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) during electrochemical cycling. Using ATR-FTIR, it is possible to observe the reaction intermediates on the surface of Ag thin films formed during the CO₂ electroreduction reaction. At a moderate overpotential, a proton coupled electron transfer reaction mechanism is confirmed to be the dominant CO₂ reduction pathway. However, at a more negative applied potential, both the COO^- and the COOH intermediates are detected using ATR-FTIR, which indicates that individual proton and electron transfer steps occur, offering a different pathway than at lower potentials. These results indicate that the CO₂ reduction reaction mechanism can be potential dependent and not always involving a concerted proton coupled electron transfer, opening alternative pathways to optimize efficient and selective catalysts for desired product formation. (Graph Presented).