H₂O Adsorption on WO₃ and WO_3-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.