Proton-Promoted Electron Transfer in Photocatalysis: Key Step for Photocatalytic Hydrogen Evolution on Metal/Titania Composites

Metal cocatalysts are widely utilized for enhancing photocatalytic conversion. In TiO2-based photocatalysts, a wide range of metals dispersed on TiO2 surfaces were observed to be effective for photocatalytic hydrogen production. To clarify the metal/oxide synergistic effect in photocatalysis and the insensitivity of photoactivity on metal types, here we investigate the mechanism of electron transfer from semiconductor to the cocatalyst by using ab initio molecular dynamics and hybrid density functional theory calculations. By determining the optimal geometry of a Pt-13 subnano cluster on anatase TiO2(101) and quantifying the electron transfer energetics, we find that the electron transfer from oxide to the metal cluster is significantly boosted (exothermic by more than 0.3 eV) by the adsorption of proton on the metal cluster, which is otherwise endothermic without the presence of proton. This cooperative effect between oxide, subnano metal cluster, and adsorbed proton is rationalized from electronic structure analyses. We show that the proton-promoted electron transfer phenomenon in photocatalysis appears to be universally present, as evidenced from theoretical calculations by replacing Pt with other metals, including Co, Ni, Cu, Pd, and Rh. This mechanism differs fundamentally from the proton-coupled electron transfer frequently quoted in electrocatalysis and may assist the photocatalyst design toward highly efficient solar fuel production.