Driving force for the WO3(0 0 1) surface relaxation

The optimized structure of the WO3(0 0 1) surface with various types of termination ((1 × 1)O, (1 × 1)WO2, and c(2 × 2)O) has been simulated using density functional theory with the Perdew-Wang 91 gradient corrected exchange-correlation functional. While the energy of bulk WO3 depends weakly on the distortions and tilting of the WO6 octahedra, relaxation of the (0 0 1) surface results in a significant decrease of surface energy (from 10.2 × 10−2 eV/Å2 for the cubic ReO3-like, c(2 × 2)O-terminated surface to 2.2 × 10−2 eV/Å2 for the relaxed surface). This feature illustrates a potential role of surface relaxation in formation of crystalline nano-size clusters of WO3. The surface relaxation is accompanied by a dramatic redistribution of the density of states near the Fermi level, in particular a transformation of surface electronic states. This redistribution is responsible for the decrease of electronic energy and therefore is suggested to be the driving force for surface relaxation of the WO3(0 0 1) surface and, presumably, similar surfaces of other transition metal oxides.