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 TiO2(101), −(001), and rutile TiO2(110) surfaces. We found that monomeric tungsten oxide species form on all three TiO2 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 TiO2 facet employed and bulk defects within the substrate. Among the TiO2 surfaces studied, the weakest reduction by vacuum annealing was found for tungsten oxide supported on anatase TiO2(001).