The adsorption of water on a reduced TiO 2 anatase (101) surface is investigated with scanning tunneling microscopy and density functional theory calculations. The presence of subsurface defects, which are prevalent on reduced anatase (101), leads to a higher desorption temperature of adsorbed water, indicating an enhanced binding due to the defects. Theoretical calculations of water adsorption on anatase (101) surfaces containing subsurface oxygen vacancies or titanium interstitials show a strong selectivity for water binding to sites in the vicinity of the subsurface defects. Moreover, the water adsorption energy at these sites is considerably higher than that on the stoichiometric surface, thus giving an explanation for the experimental observations. The calculations also predict facile water dissociation at these sites, confirming the important role of defects in the surface chemistry of TiO 2 .

Adsorbate-induced band gap states in semiconductors are of particular interest due to the potential of increased light absorption and photoreactivity. A combined theoretical and experimental (STM, photoemission) study of the molecular-scale factors involved in the formation of gap states in TiO(2) is presented. Using the organic catechol on rutile TiO(2)(110) as a model system, it is found that the bonding geometry strongly affects the molecular electronic structure. At saturation catechol forms an ordered 4 x 1 overlayer. This structure is attributed to catechol adsorbed on rows of surface Ti atoms with the molecular plane tilted from the surface normal in an alternating fashion. In the computed lowest-energy structure, one of the two terminal OH groups at each catechol dissociates and the O binds to a surface Ti atom in a monodentate configuration, whereas the other OH group forms an H-bond to the next catechol neighbor. Through proton exchange with the surface, this structure can easily transform into one where both OH groups dissociate and the catechol is bound to two surface Ti in a bidentate configuration. Only bidendate catechol introduces states in the band gap of TiO(2).