WO₃ Monomers Supported on Anatase TiO₂(101), −(001), and Rutile TiO₂(110): A Comparative STM and XPS Study

We combined scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) to study the molecular and electronic structure of submonolayer tungsten oxide supported on anatase TiO 2 (101), −(001), and rutile TiO 2 (110) surfaces. We found that monomeric tungsten oxide species form on all three TiO 2 surfaces upon mild annealing at 400 K, with a geometry depending on the supporting facet. At ∼600 K, surface diffusion of the monomers sets in, but the monomers remain on the surface without diffusing into the bulk even at higher annealing temperatures. As-deposited tungsten oxide at monolayer coverage is stronger oxidized than thick layers. At elevated temperatures (400–900 K), significant reduction is observed, strongly dependent on the TiO 2 facet employed and bulk defects within the substrate. Among the TiO 2 surfaces studied, the weakest reduction by vacuum annealing was found for tungsten oxide supported on anatase TiO 2 (001).