Structural and electronic properties of tungsten trioxides: from cluster to solid surface

Geometries and electronic structures of WO3(001) surface and a series of stoichiometric (WO3) (n) clusters (n = 1-6) have been systematically investigated using first-principles density functional calculations. Six possible reconstructured models of WO3(001) surface with cubic phase are explored, and the most stable configuration is the (root 2 x root 2)R45 degrees surface. The main feature of WO3(001) surface is that the top of valence band is dominated by the 2p states of the bridging oxygen atom, rather than the top terminal oxygen. By comparing the geometrical parameters, from the structural point of view, the W3O9 cluster can be used as the smallest molecular prototype of the WO3 surface. However, in terms of the electronic structure, only until W6O18, the cluster begins to appear the electronic feature of the WO3(001) surface. This may be due to the reason that the W6O18 cluster and the top layer of WO3(001) surface show similar stoichiometry if we treat two kinds of oxygen atoms as different elements. In addition, for the chemical reactivity, using BH3 as a probe molecule, the W6O18 cluster also bears general resemblance to the WO3(001) surface, and the bridging oxygen atoms in two systems are the preferred sites for the nucleophilic reaction. Therefore, our results indicate that the W6O18 cluster with a spherical buckyball structure can be viewed as the smallest molecular model to understand the properties such as catalytic activity of WO3(001) surface.