Importance of O vacancies in the behavior of oxide surfaces: Adsorption of sulfur onTiO2(110)

Abstract

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 ${\mathrm{TiO}}_2\mathrm{}\left(110\right).$ 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 ${\mathrm{Ti}}^{3+}$ sites favor bonding to S, but there is not a substantial $\mathrm{oxid}\overrightarrow{e}\mathrm{adsorbate}$ 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 ${\mathrm{TiS}}_x$ is formed on ${\mathrm{TiO}}_2\mathrm{}\left(110\right).$ The O signal disappears in photoemission and Auger spectroscopy, and the Ti $2p$ core levels show a complete ${\mathrm{TiO}}_2\to {\mathrm{TiS}}_x$ transformation. The $\mathrm{O}\leftrightarrow \mathrm{S}$ exchange does not involve the production of SO or ${\mathrm{SO}}_2$ species. Instead, the formation of ${\mathrm{TiS}}_x$ involves the migration of O vacancies from the bulk to the surface. The $\mathrm{S}/{\mathrm{TiO}}_2\mathrm{}\left(110\right)\mathrm{}$ 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.