Oxidation State of GaP Photoelectrode Surfaces under Electrochemical Conditions for Photocatalytic CO₂Reduction

Illuminated GaP electrodes selectively reduce CO2 to CH3OH in aqueous solution. To understand the photoelectrocatalytic mechanism, knowledge of the GaP surface atomic structure in contact with water under relevant electrochemical conditions is essential. However, there remains a debate about the oxidation state of GaP, i.e., whether oxide species are present at the surface. To address this issue, we use density functional theory to investigate the adsorption of oxide species on GaP(110), a stable and active facet for CO2 reduction. We predict that GaP(110) indeed could be oxidized at the standard reduction potential for CO2 to CH3OH. However, we find that unoxidized GaP(110) is stable under illumination, as it corresponds to a highly reducing condition induced by photoexcited electrons. We conclude that an oxidized GaP electrode is very likely unstable thermodynamically under photoelectrochemical conditions for CO2 reduction, and therefore, the relevant GaP/water interface for catalysis is indeed the unoxidized one.