Water adsorption on hydroxylated alpha-quartz (0001) surfaces: From monomer to flat bilayer

Water adsorption on the hydroxylated alpha-quartz (0001) surface at various coverages was studied using ab initio total energy calculations and molecular dynamics simulations within density functional theory. The relaxed geometry of the clean surface is characterized with zigzag hydrogen-bond (H-bond) chains of vicinal surface hydroxyls, with strong and weak H-bond interactions appearing alternatively along them. Upon water adsorption, the weak H-bonds on the surface are broken and the corresponding hydroxyls reform H-bonds with the adsorbed molecules, e.g., two H bonds for an isolated water monomer/dimer. Increasing the water adsorption to one monolayer, we find an ordered hexagonal water layer on the oxide surface with a flat bilayer structure, compared with the basal plane of ice Ih. The H-down bilayer configuration is more favored than the H-up bilayer from both the energetic and dynamic points of view. In addition, two kinds of H-bonds with different strengths are identified in the bilayer from the vibrational spectra of the OH stretch modes. The origin of these two kinds of H-bonds is different in nature from those reported in the recent studies of a tessellation ice on the hydroxylated beta-cristobalite (100) surface and a bilayer structure on metal Pt (111) surface.