Water film-mediated photocatalytic oxidation of oxalate on TiO2

Water films on minerals under humid environment can be photocatalytic hotspots when exposed to sunlight or artificial sources of ultraviolet (and visible) radiation. In this study, we resolved the water film-mediated photocatalysis on a model system of oxalate and TiO2 by in situ infrared spectroscopy. We find that 0.5 to 4 monolayer- (ML) thick water films enhanced photodecomposition rates of oxalate under 21 kPa O2 through the combination of direct hole transfer, ligand-to-metal-charge transfer and the production of hydroxyl radical and reactive oxygen species. Rates were, however, substantially slower in the absence of O2 because charge recombination, together with water film-mediated charge localization, disrupted hole transfer and hydroxyl radical production. Our work adds insight into the impact of humidity on controlling important photocatalytic processes in nature (drying soils, atmospheric aerosols), and technology (water and air treatment).