Glycine Adsorption on (101̅0) ZnO Surfaces

The structural and electronic properties of glycine adsorbed on nonpolar (101̅0) ZnO surfaces have been investigated by using the self-consistent-charge density-functional-based tight-binding (SCC-DFTB) method. A coverage range from 0.25 to 1 monolayer (ML) was considered. We found that the molecule prefers to adsorb dissociatively for all coverages and adsorption modes. For 1 ML coverage, the molecule binds to the surface through either the carboxyl or the amine group with similar adsorption energies. In particular, adsorption through the amine group has a strong influence on the surface electronic properties. For coverages of 0.5 and 0.25 ML, glycine binds to the surface through both functional groups. Additionally, we studied the adsorption of glycine on oxygen-reduced surfaces. Our results show that the presence of oxygen vacancies slightly enhances the molecular adsorption energy compared to the adsorption on the defect-free surface.