Cooperative Effects in Water Binding to Cuprous Oxide Surfaces

Abstract

Water adsorption on solid surfaces plays a part in a variety of processes, including renewable energy applications. Water adsorption can occur either dissociatively, monomolecularly, or as clusters. In contrast to metal surfaces, the compositional and structural complexity of metal oxide surfaces has inhibited atomic-scale understanding of their interactions with water. Cu2O is a promising photocatalyst and (photo)electrochemical catalyst. Here, we investigate water adsorption on its (111) surface, using density functional theory + U with dispersion corrections. A number of monomolecular and dissociated adsorbate geometries are considered on the two most stable surface terminations. H2O is found to adsorb most strongly when datively coordinated to an unsaturated surface Cu cation; dissociative adsorption is not as favorable as this dative bonding mode of molecular chemisorption. If these Cu cations are not present, H2O binds via hydrogen bonding and electrostatic interactions in surface cavities. We also examine a large variety of mixed modes of coadsorption. Mixtures of datively bonded and hydrogen-bonded water molecules adsorb most strongly, exhibiting a strong lateral interaction. The resulting water clusters can adapt to the underlying adsorption site template and maintain significant water–surface and water–water interactions at the same time. This is possible through the proximity of the unsaturated cationic and anionic adsorption sites. The combination of dative and hydrogen bonding to the surface enables water clustering even at low temperatures, probably due to rapid surface diffusion of the more weakly bonded monomers. The strong dative and lateral interactions keep water clusters adsorbed up to unusually high temperatures. The water hexamer can still be observed at room temperature under ultrahigh vacuum conditions; we predict that datively bound monomers, hexamers, and other similarly constructed clusters will remain bound to the stoichiometric surface up to quite high temperatures under conditions of high relative humidity. We suggest that metal oxides with similar surface compositions should show similar properties. Finally, we predict vibrational frequencies for the adsorbed water molecules and distinguish between water–water and water–surface vibrations for comparison with future experimental studies.