Adsorption of water on reconstructed rutile TiO₂(011)- (2×1): Ti=O double bonds and surface reactivity

Recent combined experimental and theoretical studies (Beck et al., Phys. Rev. Lett. 2004, 93, 036104) have provided evidence for Ti=O double-bonded titanyl groups on the reconstructed rutile TiO₂-(011)-(2×1) surface. The adsorption of water on the same surface is now investigated to further probe the properties of these groups, as well as to confirm their existence. Ultraviolet photoemission experiments show that water is adsorbed in molecular form at a sample temperature of 110 K. At the same time, the presence of a 3σ state in the photoemission spectra and work function measurements indicate a significant amount of hydroxyls within the first monolayer of water. At room temperature, scanning tunneling microscopy (STM) suggests that dissociated water is present, and about 30% of the surface active sites are hydroxylated. These findings are well explained by total energy density functional theory calculations and Car-Parrinello molecular dynamics simulations for water adsorption on the titanyl model of TiO₂(011)-(2×1). The theoretical results show that a mixed molecular/dissociative layer is the most stable configuration in the monolayer regime at low temperatures, while complete dissociation takes place at 250 K. The arrangement of the protonated mono-coordinated oxygens in the mixed molecular/dissociated layer is consistent with the observed short-range order of the hydroxyls in the STM images.