Structure and energetics of water adsorbed on the ZnO(10(1)over-bar0) surface

We present first-principles total-energy calculations of water adsorption on the ZnO(1010) surface. We find that on ideal ZnO(1010) surfaces, the fully molecular chemisorption is most energetically favorable at coverage <= 0.5 ML, whereas at 1 ML, the mixed adsorption with a (2x1) superstructure is most energetically favorable. However, the mixed adsorption with a (2x2) superstructure is only 0.07 eV higher per H2O molecule than the mixed adsorption with a (2x1) superstructure. Therefore, we expect that domains with mixed (2x1) and (2x2) structures may also exist at elevated temperatures. Moreover, we find that oxygen vacancies in the surface significantly affect the adsorption behavior. For an oxygen vacancy density <= 25%, the dissociative adsorption is energetically preferred at water coverage <= 0.25 ML and the mixed adsorption is energetically favorable at water coverage >= 0.5 ML. The further increase of oxygen vacancy density promotes the dissociative adsorption.