Oxygen Vacancy in Wurtzite ZnO and Metal-Supported ZnO/M(111) Bilayer Films (M = Cu, Ag and Au)

Using spin-polarized DFT+U calculations we have studied the nature of the O vacancy in graphitic-like ZnO bilayer films supported on Cu, Ag, and Au (111) surfaces and compared it with the same defect center formed on free-standing ZnO bilayers and on the ZnO wurtzite (10 (1) over bar0) surface. The formation energy of the oxygen vacancy is similar in bulk ZnO wurtzite and in free-standing ZnO bilayers, about 4.3 eV, while it is about 1 eV smaller on the wurtzite (10 (1) over bar0) surface. The analysis of the density of states, electron density, and charge distribution, shows that the two excess electrons associated with the vacancy are localized at the vacancy site in all these systems. The situation is more complex on the bilayer films on metal. Removing oxygen from the top layer of ZnO/Cu(111) and ZnO/Ag(111) results in charge delocalization over the entire ZnO film, no charge transfer to the support, and the formation energy remains high, as for the unsupported layer, about 4.2 eV. In the case of ZnO/Au(111), however, due to the higher work function of Au, electrons are transferred from the oxide top layer to the metal, and the cost to remove oxygen is strongly reduced by 1.7 eV. For all ZnO/metal supported films, the formation energy of the vacancy is reduced at the metal/oxide interface, showing the important role that metal/oxide interfaces have in determining the reducibility of an oxide. Beside electronic effects, the local structural distortions of the ZnO thin films and the metal support also contribute to reduce the oxygen vacancy's formation energy.