Mixed Molecular and Dissociative Water Adsorption on Hydroxylated TiO2(110): An Infrared Spectroscopy and Ab Initio Molecular Dynamics Study

We have investigated the structure and dynamics of water (D2O) adsorbed on TiO2(110) for coverages between 0 and 1 monolayer (ML) with infrared reflection absorption spectroscopy and ab initio molecular dynamics (AIMD) simulations. For D2O coverages as low as 0.4 ML on a hydroxylated surface, IR spectra typical of hydrogen-bonded chains of water molecules are observed. However, for D2O coverages ≥0.3 ML, a sharp, high-frequency peak is also observed in the p-polarized spectra that is red-shifted relative to the bridging hydroxyl peak. This new peak is not observed for water adsorbed on an oxidized surface. Based on the AIMD simulations and comparisons with previous IR spectra for TiO2 nanoparticles, the new peak is assigned to terminal hydroxyl groups produced by dissociative adsorption of some of the water on TiO2(110). The simulations indicate that water dissociation is related to the presence of defect electrons in the system, but not due to direct interactions between adsorbed water and bridging hydroxyls.