Discovering the root of the stability of hexagonal WO3 surfaces from a periodic DFT perspective

Abstract

DFT periodic calculations were carried out in order to gain more insight in relation with the stability of the (0 0 1) and (1 0 0) surfaces of the hexagonal WO3. These surfaces display an uncommon instability which could be attributed to their high polarity and the intrinsic instability problems of the hexagonal phase. Indifferently of the slab thickness, the internal structure of the slabs suffers distortion even at levels of a total loss of the hexagonal phase structure. Stoichiometric and non-stoichiometric O reconstructions do not seem to be practical mechanisms to solve the instability problem of these surfaces, leaving only the partial frozen slabs an efficient way to model the hexagonal (0 0 1) and (1 0 0) surfaces. However, the surface energy of these surfaces is dependent of the optimized layers instead of the frozen ones. Results obtained for the CO adsorption on the (0 0 1) surface showed that a model with six layers and two internal layers frozen to the atomic position of the bulk structure is enough to reach the structural and energetic stability of the CO adsorption.