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 ${\mathrm{TiO}}_{2}(110).$ 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 ${\mathrm{Ti}}^{3+}$ sites favor bonding to S, but there is not a substantial $\mathrm{oxid}\stackrel{\ensuremath{\rightarrow}}{e}\mathrm{adsorbate}$ 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 (g500 K) a layer of ${\mathrm{TiS}}_{x}$ is formed on ${\mathrm{TiO}}_{2}(110).$ The O signal disappears in photoemission and Auger spectroscopy, and the Ti $2p$ core levels show a complete ${\mathrm{TiO}}_{2}\ensuremath{\rightarrow}{\mathrm{TiS}}_{x}$ transformation. The $\mathrm{O}\ensuremath{\leftrightarrow}\mathrm{S}$ 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 $\mathrm{S}/{\mathrm{TiO}}_{2}(110)$ 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.
Published Version
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