Sulfur on TiO2(110) studied with resonant photoemission - art. no. 115418

Adsorption of sulfur on TiO2(110) at room temperature (RT) and 350 degreesC has been studied with ultraviolet photoelectron spectroscopy. A TiO2(110) (1 x 1) surface with a small amount of oxygen vacancies was prepared by spu ttering and annealing in ultrahigh vacuum. Oxygen vacancies induce a defect state that pins the Fermi level just below the conduction-band minimum. Su lfur adsorption at room temperature leads to the disappearance of this vaca ncy-related band-gap state, indicating that the surface oxygen vacancies ar e filled by sulfur. Sulfur-induced valence-band features are identified at binding energies of 3.4 and 8 eV. Adsorption of S at 350 degreesC forms a ( 4 x 1) superstructure at high coverages [approximate to 0.9 monolayer (ML)] that is visible with low-energy electron diffraction. In a previously prop osed model for this superstructure. Sulfur replaces half of the in-plane ox ygen atoms and all the bridging oxygen atoms are removed. In agreement with this model, the oxygen 2s peak is decreased significantly and the defect s tate 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 reso nant photoemission. these features show a similar, but weaker, resonance pr ofile than the vacancy-related defect state. Hybridized Ti-derived states e xtend across the whole valence-band region. Generally. a higher resonant ph oton energy is found for valence-band states with lower binding energies, i ndicating mainly 3p-4s transitions in the upper valence band. Adsorption of sulfur reduces the strength of the resonances.