Periodic DFT study of water adsorption on m-WO3(001), m-WO3(100), h-WO3(001) and h-WO3(100). Role of hydroxyl groups on the stability of polar hexagonal surfaces

Abstract

Water adsorption on the (001) and (100) surfaces of monoclinic and hexagonal WO3 was studied using a periodic DFT approach. The results showed that dissociation of water on these surfaces is less energetically favourable than molecular adsorption and such phenomenon is a consequence of surface deformations induced by OH groups. Water interacted stronger with the five-fold tungsten atoms of the hexagonal surfaces than with those atoms of the monoclinic surfaces. Nevertheless, adsorption energies are almost the same between both hexagonal and monoclinic surfaces when all available active surface sites were occupied by water molecules, that is, at the maximum water coverage. On monoclinic surfaces, water adsorption gradually became less stable as the coverage increased, but the opposite behaviour was observed with dissociative adsorption. On the (001) surfaces of the hexagonal structures, both molecular and dissociative adsorption became less stable as water coverage increased. On the other hand, on the (100) surface, dissociation was only observed at the maximum coverage. All surfaces remain hydrated at low temperatures; whereas at higher temperatures the most stable structures are the clean surfaces. Lastly, the formation of a monolayer of OH groups on hexagonal surfaces helps to avoid distortion, which provides an explanation of why hexagonal surfaces are observed experimentally despite of being intrinsically unstable.