Electron spectroscopy studies of the chemisorption of O 2, H 2 and H 2O on the TiO 2(100) surfaces with varied stoichiometry: Evidence for the photogeneration of Ti +3 and for its importance in chemisorption

Abstract

Deposition of a monolayer of titanium on the TiO 2(100)-( 1 × 3) single crystal surface produces an oxygen deficient surface which has the same chemical composition, electronic structure and chemical reactivity as the Ar ion sputtered TiO 2(100) surface. From correlations with studies of oxidation of the Ti(0001) single crystal surface, we provide firm evidence for the existence of Ti +3 species on the clean oxygen deficient TiO 2 surface. The chemisorption of O 2, H 2 and H 2O has been studied by ultraviolet photoemission spectroscopy (UPS), electron energy loss spectroscopy (ELS), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and thermal desorption on the stoichiometric and Ti +3-rich TiO 2(100) surfaces. The presence of surface Ti +3 species causes distinct differences in the nature of chemical bonding between the adsorbed molecule and the substrate surface. From the combination of these studies, we conclude that water is adsorbed associatively on the stoichiometric TiO 2(100)−( 1 × 3) surface while it tends to dissociate into surface hydroxyl groups on the Ti +3 rich surface. UPS and ELS show that while adsorption of H 2O changes the Ti +3 to a different oxidation state, illumination by band gap energy photons can regenerate Ti +3 species by the reaction Ti +4 + e − → Ti +3. An attempt has been made to correlate these results on well characterized surfaces with studies using TiO 2 as a photoanode in the photo-electrochemical cell.