Water Adsorption on Different TiO2 Polymorphs Grown as Ultrathin Films on Pt(111)

By using reactive Ti evaporation, we have grown on Pt(111) stoichiometric TiO2 ultrathin (UT) films of increasing thickness, presenting different polymorphic structures and surface terminations, that is, the thickness-limited lepidocrocite-like nanosheet, TiO2(B) (001) and rutile-TiO2, (100), respectively. This allowed us to study comparatively for the first time the reactivity toward water of different polymorph surfaces in the form of ultrathin films by adopting several in situ surface science tools. There is no evidence for water interaction with lepidocrocite-TiO2 nanosheets, in agreement with theoretical predictions found in the literature. For the first time evidence for some water dissociation on TiO2(B) surfaces is reported. A higher water interaction is observed on the high coverage films, and the observed behavior is in line with the previous literature data obtained on the rutile (100) bulk surface: our data confirm the common interpretation that rutile-TiO2 (100) surface termination is highly reactive toward water dissociation. In this paper we demonstrate that this ability is maintained also in the case of UT films.