Structures of sulfur on TiO2(110) determined by scanning tunneling microscopy, X-ray photoelectron spectroscopy and low-energy electron diffraction

The temperature dependent adsorption of sulfur on TiO2(1 1 0) has been studied with X-ray photoelectron spectroscopy (XPS). scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). Sulfur adsorbs dissociatively at room temperature and binds to fivefold coordinated Ti atoms. Upon heating to similar to 120 degreesC, 80% of the sulfur desorbs and the S 2p peak position changes from 164.3 +/- 0.1 to 162.5 +/- 0.1 eV. This peak shift corresponds to a change of the adsorption site to the position of the bridging oxygen atoms of TiO2(1 1 0). Further heating causes little change in S coverage and XPS binding energies, up to a temperature of similar to 430 degreesC where most of the S desorbs and the S 2p peak shifts back to higher binding energy. Sulfur adsorption at 150 degreesC, 200 degreesC, and 300 degreesC leads to a rich variety of structures and adsorption sites as observed with LEED and STM. At low coverages, sulfur occupies the position of the bridging oxygen atoms. At 200 degreesC these S atoms arrange in a (3 x 1) superstructure. For adsorption between 300 degreesC and 400 degreesC a (3 x 3) and (4 x 1) LEED pattern is observed for intermediate and saturation coverage, respectively. Adsorption at elevated temperature reduces the substrate as indicated by a strong Ti3+ shoulder in the XPS Ti 2p(3) (2) peak, with up to 15.6% of the total peak area for the (4 x 1) structure. STM of different coverages adsorbed at 400 degreesC indicates structural features consisting of two single S atoms placed next to each other along the [0 0 1] direction at the position of the in-plane oxygen atoms. The (3 x 3) and the (4 x 1) structure are formed by different arrangements of these S pairs. (C) 2001 Elsevier Science B.V. All rights reserved.