The Structure and Energy of Oxygen Vacancy Formation in Clean and Doped, Very Thin Films of ZnO

We use density functional calculations to study the structure and the energy of formation of the surface oxygen vacancy (V O ) in a free-standing, very-thin film (VTF) of ZnO and substitutionally doped ZnO VTF. We are interested in this system because the walls of very high area oxides used in catalysis contain VTFs. We find that the structure of the ZnO(0001̄) VTF is strongly dependent on the film thickness. Decreasing the thickness of the ZnO(0001̄) VTF increases the distances between the atomic planes along the (0001̄) direction. At a film thickness of five layers ( L ), the energy of the VTF with a wurtzite structure is the same as the energy of the VTF with a hexagonal structure. For thickness <5 layers, the hexagonal VTF is more stable than the wurtzite VTF. The formation of V O is exothermic and leads to a decrease in the interplanar distance along the (0001̄) direction in the entire film, making the structure close to that of wurtzite. The exothermic nature of V O formation suggests that the ZnO(0001̄) VTF with vacancies is thermodynamically stable at low oxygen pressures in the gas phase. The structure of the ZnO(101̄0) VTF does not depend strongly on the film thickness. The formation of a V O at the surface of the ZnO(101̄0) VTF is endothermic and affects the film structure only in the vicinity of the V O . Replacement of some of the Zn atoms at the surface of the stoichiometric ZnO(0001̄) VTF by a dopant (Au, Ag, Cu, Ga, or Al) decreases the interplanar distance along the (0001̄) direction in the entire film, making its structure close to that of wurtzite. The formation of a V O at the surface of a doped VTF is endothermic and, therefore, the presence of the dopant stabilizes the stoichiometric wurtzite structure. Doping the surface of the ZnO(101̄0) VTF with Au, Ag, or Cu lowers the energy of V O formation at the surface and doping with Ga or Al increases it. We also find that Au, Ag, and Cu, which facilitate V O formation, segregate to the surface of the ZnO (101̄0) VTF and Ga and Al, which hinder V O formation, segregate to the bulk.