Importance of O vacancies in the behavior of oxide surfaces:: Adsorption of sulfur on TiO2(110) -: art. no. 235414

Synchrotron-based high-resolution photoemission, thermal desorption mass spectroscopy, and first-principles density functional calculations were used to study the adsorption and reaction of sulfur with TiO2(110). At 100-300 K, S atoms bond much more strongly to O vacancy sites than to atoms in the Ti rows of a perfect oxide surface. The electronic states associated with Ti3+ sites favor bonding to S, but there is not a substantial oxide-->adsorbate charge transfer. In general, the bond between S and the Ti cations is best described as covalent, with a small degree of ionic character. For dosing of S at high temperatures (>500 K) a layer of TiSx is formed on TiO2(110). The O signal disappears in photoemission and Auger spectroscopy, and the Ti 2p core levels show a complete TiO2-->TiSx transformation. The O<---->S exchange does not involve the production of SO or SO2 species. Instead, the formation of TiSx involves the migration of O vacancies from the bulk to the surface. The S/TiO2(110) system illustrates how important can be surface and subsurface defects in the behavior of an oxide surface. The exchange of O vacancies between the bulk and surface can lead to unexpected chemical transformations.