Sulfur onTiO2(110)studied with resonant photoemission

Abstract

Adsorption of sulfur on ${\mathrm{TiO}}_{2}(110)$ at room temperature (RT) and $350\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ has been studied with ultraviolet photoelectron spectroscopy. A ${\mathrm{TiO}}_{2}(110)$ $(1\ifmmode\times\else\texttimes\fi{}1)$ surface with a small amount of oxygen vacancies was prepared by sputtering and annealing in ultrahigh vacuum. Oxygen vacancies induce a defect state that pins the Fermi level just below the conduction-band minimum. Sulfur adsorption at room temperature leads to the disappearance of this vacancy-related band-gap state, indicating that the surface oxygen vacancies are filled by sulfur. Sulfur-induced valence-band features are identified at binding energies of 3.4 and 8 eV. Adsorption of S at $350\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ forms a $(4\ifmmode\times\else\texttimes\fi{}1)$ superstructure at high coverages $[\ensuremath{\approx}0.9$ monolayer (ML)] that is visible with low-energy electron diffraction. In a previously proposed model for this superstructure, sulfur replaces half of the in-plane oxygen atoms and all the bridging oxygen atoms are removed. In agreement with this model, the oxygen $2s$ peak is decreased significantly and the defect state is increased. Two additional valence features are observed: one at 2.7 eV and one at 3.9 eV. Due to those features the band gap vanishes. In resonant photoemission, these features show a similar, but weaker, resonance profile than the vacancy-related defect state. Hybridized Ti-derived states extend across the whole valence-band region. Generally, a higher resonant photon energy is found for valence-band states with lower binding energies, indicating mainly $3p\ensuremath{-}4s$ transitions in the upper valence band. Adsorption of sulfur reduces the strength of the resonances.