Water Adsorption on Nonpolar ZnO(10(1)over-bar0) Surface: A Microscopic Understanding

We report on first-principles density functional calculations about the adsorption of water molecules on the nonpolar ZnO(10 (1) over bar0) surface, as a function of the molecular coverage. Our results allow us to unravel the reaction mechanisms that drive the partial dissociation of water molecules at saturation coverage: Although not a favored event, concurrent adsorption/dissociation may occur as a compromise between steric repulsion and covalent and hydrogen bond formation with both the substrate and the impinging molecules. The scenario is altered by the presence of defects. We discuss the role of the most common point and extended defects at the outermost layer: these systems exhibit remarkably different electronic properties leading to peculiar and unexpected characteristics for the wet defective surface. Enhanced reactivity of edged nanostructures is predicted, while the catalytic role of oxygen vacancies is questioned. The effects of metallicity induced by hydrogen adsorption on the interaction with water are finally analyzed. We suggest also experimental probes to identify the various adsorption geometries and fully characterize the water vapor/oxide interface.