DFT periodic study of adsorption of glycine on the (0Â 0Â 0Â 1) surface of zinc terminated ZnO

We found that glycine adsorbs on Zn-ZnO surface at low coverage preferentially in the anionic form with dissociation, in a parallel orientation to the surface. The carboxylate moiety forms a Zn-OCO-ZnO ring two surface Zn atoms, and an additional Zn-N bond is formed between the amine function and a surface Zn atom; the proton, product of dissociation, forms a hydride like Zn-H at the surface. The formation of OH groups with oxygens of the underlying layer was also considered, but is less stable. At higher coverage, (2.6 G/nm2) the dissociation of glycine into a glycinate ion is again the most stable configuration. An organized glycinate layer is formed, in which the glycine molecules adopt the ZnO honeycomb structure. Still increasing the coverage leads to the formation of a Zn-glycinate bilayer as shown by ab initio molecular dynamics simulations. In the first layer glycine molecules adopt the honeycomb ZnO lattice. The second layer consists in a Zn-glycinate complex, with a Zn atom extracted from the surface.