Monitoring the interaction of sulfur dioxide with a TiO2(1 1 0) surface at 300 K by scanning tunneling microscopy

Scanning tunneling microscopy (STM), Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED) were used to study the interaction of sulfur dioxide with the bulk-terminated (1×1) and the reconstructed (1×2) structure of the TiO2(1 1 0) surface at 300 K. During exposure new features appear first on the added rows of the reconstructed (1×2) surface structure. In general these new features are randomly distributed, i.e., no ordered adsorbate phase builds up. However, in some areas locally ordered linear structures can be observed along the (1×2) rows with a periodicity two or three times higher than the substrate. Similar to these observations, randomly distributed spots also appear on single added rows of the (1×2) surface structure that are located on top of the (1×1) structure of the TiO2(1 1 0) surface. In contrast, additional features on the (1×1) structure itself can be seen only after higher SO2 exposure. Here, an ordered adsorbate phase with a (2×1) structure is formed. The new features of this (2×1) structure are located on top of the (1×1) rows along the [0 0 1] direction, which previously have been shown to represent the fivefold coordinated Ti surface cations. Subsequent AE spectra display a single peak in the sulfur region at 151.3 eV, confirming the predominant formation of Ti–S bonds during SO2 exposures. In LEED a diffuse (1×2) structure with faint (1×2) half-order reflections can be seen, demonstrating the overall disorder on the surface. Point defects on the stoichiometric (1×1) surface structure as well as on the reconstructed (1×2) surface structure are unaffected by SO2 exposure.