Stabilization of ultrathin zinc oxide films on metals: Reconstruction versus hydroxylation

Thin (0001)-oriented films of ZnO on metals may exhibit interlayer relaxations, resulting in the hexagonal boron nitride-like crystal structure. The driving force for such reconstruction is the polar instability of either Zn- or O- terminated surfaces of ZnO(0001). Here, we examined surface hydroxylation as another possible stabilization mechanism. Zinc oxide films grown on Pt(111) were studied by infrared reflection-absorption spectroscopy (IRAS) as a function of film thickness and morphology as imaged by scanning tunneling microscopy. Despite prepared in pure oxygen ambient, the "as grown" films on Pt(111) expose hydroxyl groups. In contrast, the bilayer films on Ag(111) do not exhibit OH species, not even upon dosing of hydrogen or water. The results show that hydrogen may efficiently be provided by a Pt support, even for the multilayer films, via hydrogen dissociation and subsequent diffusion of H atoms through the film. Thermal stability of the OH-terminated surfaces depends on the film thickness, with a monolayer film being the least stable. Removal of OH species from a monolayer film proceeds through water desorption and may be accompanied by hydrogen spillover onto more stable multilayer structures. Stabilization of the polar ZnO surface in the metal-supported films seems to be a delicate balance between interlayer relaxation and hydroxylation and depends on the metal support.