Observation of Surface-Bound Negatively Charged Hydride and Hydroxide on GaP(110) in H₂O Environments

Surface-bound species on GaP(110) formed upon interaction with water were investigated through experiment and theory. These studies are motivated by and discussed in the context of electrocatalytic and photoelectrocatalytic schemes for solar fuel production, including especially observations of selective CO₂ reduction to methanol in acidified aqueous solutions of CO₂ and nitrogen-containing heteroaromatics. Experimentally, surface-bound species over 10 orders of magnitude of pressure were spectroscopically identified in situ using synchrotron-based ambient pressure photoelectron spectroscopy. Ga 3d and O 1s core-level spectra indicate that the interaction of GaP(110) with H₂O induces formation of a partially dissociated adlayer, characterized by the presence of both Ga-OH and molecular H₂O species. Measurements of the P 2p core level indicate formation of a negatively charged hydride that irreversibly bonds to surface P in vacuum. The surface densities of the hydroxide and hydride species increase with increasing pressure (surface coverage) of water. Periodic slab calculations using density functional theory were used to study several relevant water configurations at 298 K on this surface. Consistent with earlier theoretical predictions at 0 K, the calculations confirm that Ga-OH, molecular H₂O, and P-H species are thermodynamically stable on the GaP(110) surface under experimental conditions. Isobaric measurements at elevated pressures were used to probe the thermal stabilities of adsorbed species as well as the oxidation of surface Ga and P. The observation of stable surface hydride formation induced by interaction with water is especially notable given the critical role of hydride transfer to catalysts and CO₂ during chemical fuel synthesis reactions in aqueous environments. It is hypothesized that the observed high stability of the hydride on GaP may contribute to its associated remarkable near-100% faradaic efficiency for methanol generation by solar-driven CO₂ reduction in acidified aqueous pyridine solutions [ J. Am. Chem. Soc. 2008, 130, 6342 ] because such stability is known to yield high overpotentials for the competing hydrogen evolution reaction.