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.