Abstract
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, wi...
Highlights
Thermal stability of the OH-terminated surfaces depends on the film thickness, with a monolayer film being the least stable
For the Zn(0001) surface, scanning tunneling microscopy (STM) and density functional theory (DFT) studies showed that the surface is stabilized by the spontaneous formation of Zndeficient triangular pits, one layer in depth, with step edges terminated by undercoordinated O atoms.[5]
The conclusion appears counterintuitive since the clean ZnO(0001)-Zn surfaces are commonly prepared by annealing in UHV at elevated temperatures (∼900 K), whereas our films on Pt(111) were prepared by annealing in 10−6 mbar of O2 at 600 K, favoring the O-terminated surfaces. These “thick” films were further studied by infrared reflection− absorption spectroscopy (IRAS), which revealed considerable amounts of hydroxyls as evidenced by a sharp peak at 3620 cm−1 corresponding to the OH stretching vibrations (Figure 1b)
Summary
Stabilization of polar surfaces in metal oxide systems remains an intriguing fundamental issue in surface science.[1−3] On the basis of studies primarily performed on single crystal surfaces, several possible scenarios are commonly considered: reconstruction via faceting, e.g. the octopolar reconstruction; formation of surface ion vacancies resulting in substoichiometric compositions of the topmost layers; and adsorption of charged adspecies, e.g., by the reaction with ambient gases, in particular of hydrogen and water leading to surface hydroxylation.In this respect, basal faces of ZnO have been studied quite extensively as classical examples of polar surfaces according to Tasker’s classification.[2,4] ZnO crystallizes in hexagonal wurtzite structure, such that the surface perpendicular to the (0001) axis exposes either Zn or O ions in the topmost layer, resulting in Zn-terminated, ZnO(0001) and O-terminated, ZnO(0001) surfaces, respectively, both being polar unstable. Stabilization of polar surfaces in metal oxide systems remains an intriguing fundamental issue in surface science.[1−3] On the basis of studies primarily performed on single crystal surfaces, several possible scenarios are commonly considered: reconstruction via faceting, e.g. the octopolar reconstruction; formation of surface ion vacancies resulting in substoichiometric compositions of the topmost layers; and adsorption of charged adspecies, e.g., by the reaction with ambient gases, in particular of hydrogen and water leading to surface hydroxylation. Many studies point out that the resulted surface structures critically depend on surface preparation
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