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

Abstract

Bio-inorganic interfaces attract attention due to numerous possible applications in industry. Following this interest, we performed a molecular modelling of glycine adsorption on Zincite (ZnO). Adsorption of neutral, zwitterionic and anionic glycine on zinc-terminated polar (0 0 0 1)-ZnO surface, was studied using a periodic Density Functional Theory based method. Several coverages were considered, from the limit of the isolated molecule to the monolayer and bilayer coverage. The interaction of each glycine function (carboxylate and amine) with the Zn–ZnO surface is investigated separately, and then the adsorption through both functions is studied. 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/nm 2) 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.