Authors: Matti Hellström, Daniel Spångberg, Peter Broqvist, and Kersti Hermansson
The interaction between water molecules and small Cu clusters (up to a size of four atoms) adsorbed on the nonpolar ZnO(101̅0) surface has been studied using hybrid density functional theory. We find that the water molecules can give rise to different scenarios: (i) In contrast to water adsorption on the clean ZnO(101̅0) surface, which occurs molecularly, the first water molecule often preferentially dissociates upon adsorption on the Cu cluster, which may be a key step in the water–gas shift reaction. (ii) While the adsorption of the first water molecule on the adsorbed Cu clusters is always more favorable than the adsorption on the bare ZnO surface, the opposite is true for the second molecule. (iii) As a water molecule adsorbs on the adsorbed Cu atom, it induces charge transfer between the Cu and the ZnO, so that an electron from the Cu atom populates the ZnO conduction band (giving an oxidized Cu species). (iv) Water molecule adsorption on the adsorbed Cu trimer results in a spontaneous dissociation of the Cu trimer into an adsorbed dimer and an adsorbed atom, after which the water molecule adsorbs on the atom, again resulting in the Cu–ZnO charge transfer. We also show that the use of a hybrid density functional gives qualitatively different results as compared to a semilocal density functional for this system, and we explain this in terms of the underestimation of the ZnO band gap obtained with the semilocal functional.
J. Phys. Chem. C, 2015, 119 (3), pp 1382–1390