posted on 2015-12-17, 07:52authored byBo-Hong Liu, J. Anibal Boscoboinik, Yi Cui, Shamil Shaikhutdinov, Hans-Joachim Freund
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.