Ultrafast proton-coupled electron transfer in heterogeneous photocatalysis

At metal-oxide/protic-solvent interfaces, partially hydrated or "wet electron" states represent the lowest energy pathway for electron transfer. Here we study the photoinduced charge transfer at the H ₂O/TiO ₂(110) interface by means of time-resolved two-photon photoemission spectroscopy and electronic structure theory. At ∼1 monolayer coverage of H ₂O on partially hydroxylated TiO ₂ surfaces we find an unoccupied electronic state 2.4±0.1 eV above the Fermi level. Density functional theory shows this to be a two-dimensional "wet electron" state, which is distinct from hydrated electrons observed on water-covered metal surfaces. The decay of electrons from the wet electron state by the resonant charge transfer to the conduction band of TiO ₂ occurs in ≤15 femtoseconds. Similar unoccupied electronic structure is observed for CH ₃OH covered TiO ₂(110) surfaces; however, the electron dynamics are considerably more complex. The wet electron state dynamics of CH ₃OH/TiO ₂ exhibit both energy and population decay. The excited state lifetime is strongly coverage dependent increasing to >100 fs range above 1 ML CH ₃OH coverage. Significantly, a pronounced deuterium isotope effect (CH ₃OD) indicates a strong correlation between the interfacial electron transfer and the motion of protons in the molecular overlayer.