A DFT study of water adsorption on rutile TiO₂ (110) surface: The effects of surface steps

The associative and dissociative adsorption of water molecules at low-coverage situations on rutile TiO₂ (110) surface with step defects was investigated by the density functional theory calculations. Structural optimization of the hydroxylated/hydrated configurations at step edges along the 111 crystal directions and the dynamic process of water dissociation were discussed to get a better description of the water/TiO₂ interface. Our results indicate that steps on the TiO₂ (110) surface could be an active site for water dissociation. The results of geometry optimization suggest that the stability of hydroxylated configuration is largely dependent on the locations of the H species and the recombination of water molecules from hydroxyls is observed in the fully hydroxylated condition. However, these hydroxyls can be stabilized by the associatively absorbed water nearby by forming competitive intermolecular hydrogen bonds. The dynamics of water dissociation and hydrogen diffusion were studied by the first principles molecular dynamics simulation and our results suggest that the hydrogen released by water dissociation can be transferred among the adsorbates, such as the unsaturated oxygen atoms-H₂O-hydroxyl (TiO-H₂O-OH) complex at step edges, or gradually diffuses to the bulk water system in the form of hydronium (H₃O⁺) at higher water coverage.