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

Abstract

We combined scanning tunneling microscopy\n(STM) and X-ray photoelectron\nspectroscopy (XPS) to study the molecular and electronic structure\nof submonolayer tungsten oxide supported on anatase TiO₂(101), −(001), and rutile TiO₂(110) surfaces. We\nfound that monomeric tungsten oxide species form on all three TiO₂ surfaces upon mild annealing at 400 K, with a geometry depending\non the supporting facet. At ∼600 K, surface diffusion of the\nmonomers sets in, but the monomers remain on the surface without diffusing\ninto the bulk even at higher annealing temperatures. As-deposited\ntungsten oxide at monolayer coverage is stronger oxidized than thick\nlayers. At elevated temperatures (400–900 K), significant reduction\nis observed, strongly dependent on the TiO₂ facet employed\nand bulk defects within the substrate. Among the TiO₂ surfaces\nstudied, the weakest reduction by vacuum annealing was found for tungsten\noxide supported on anatase TiO₂(001).