H2O Adsorption on WO3 and WO3-x (001) Surfaces.

The nature of the interaction of water with the WO3 surface is of crucial importance for the use of this semiconductor oxide in photocatalysis. In this work, we investigate water adsorption and dissociation on both clean and O-deficient (001) WO3 surfaces by means of an accurate DFT approach. The O vacancy formation energy (computed with respect to O2) has been evaluated for all possible surface configurations, and the removal of the terminal O atom along the c axis is found to be preferred, costing about half the corresponding energy in the bulk. The presence of oxygen vacancies leads to a semiconductor to metal transition, confirming the experimental evidence of n-type conductivity in defective WO3 films. H2O preferably adsorbs on WO3 in a molecular undissociated form, due to the presence of W ions at the surface that act as Lewis acid sites. This interaction, about -1 eV per H2O molecule, is not very strong. Contrary to what is usually expected, the presence of oxygen vacancies does not significantly affect H2O adsorption. Finally, we investigated the H2O desorption from a hydroxylated surface. This suggests that the exposure of WO3 to H2 directly results in a hydroxylated surface and the corresponding H2O desorption turns out to be a very efficient mechanism to generate a reduced oxide surface, with important consequences on the electronic structure of this oxide.