Abstract
The growth of ZnO clusters supported by ZnO‐bilayers on Ag(111) and the interaction of these oxide nanostructures with water have been studied by a multi‐technique approach combining temperature‐dependent infrared reflection absorption spectroscopy (IRRAS), grazing‐emission X‐ray photoelectron spectroscopy, and density functional theory calculations. Our results reveal that the ZnO bilayers exhibiting graphite‐like structure are chemically inactive for water dissociation, whereas small ZnO clusters formed on top of these well‐defined, yet chemically passive supports show extremely high reactivity ‐ water is dissociated without an apparent activation barrier. Systematic isotopic substitution experiments using H2 16O/D2 16O/D2 18O allow identification of various types of acidic hydroxyl groups. We demonstrate that a reliable characterization of these OH‐species is possible via co‐adsorption of CO, which leads to a red shift of the OD frequency due to the weak interaction via hydrogen bonding. The theoretical results provide atomic‐level insight into the surface structure and chemical activity of the supported ZnO clusters and allow identification of the presence of under‐coordinated Zn and O atoms at the edges and corners of the ZnO clusters as the active sites for H2O dissociation.
Highlights
Ag(111) and the interaction of these oxide nanostructures with water have been studied by a multi-technique approach combining temperature-dependent infrared reflection absorption spectroscopy (IRRAS), grazing-emission X-ray photoelectron spectroscopy, and density functional theory calculations
Observation of one single, sharp CO band at 2138 cm 1 while the Ag-bonded CO vibration at 2121 cm 1 completely disappeared. This conclusion was further supported by the thickness analysis of ZnO (3.4 Å) based on X-ray photoelectron spectroscopy (XPS) measurements and by a thorough theoretical study using density functional theory (DFT)
Using DFT, we modeled the adsorption of a water monomer on the unsupported ZnO bilayer
Summary
Ag(111) and the interaction of these oxide nanostructures with water have been studied by a multi-technique approach combining temperature-dependent infrared reflection absorption spectroscopy (IRRAS), grazing-emission X-ray photoelectron spectroscopy, and density functional theory calculations. Our results reveal that the ZnO bilayers exhibiting graphite-like structure are chemically inactive for water dissociation, whereas small ZnO clusters formed on top of these well-defined, yet chemically passive supports show extremely high reactivity water is dissociated without an apparent activation barrier. OH-species is possible via co-adsorption of CO, which leads to a red shift of the OD frequency due to the weak interaction via hydrogen bonding. The theoretical results provide atomic-level insight into the surface structure and chemical activity of the supported ZnO clusters and allow identification of the presence of under-coordinated Zn and O atoms at the edges and corners of the ZnO clusters as the active sites for H2O dissociation. Metal-supported ZnO thin films are known to feature tunable structural and chemical properties which differ substantially from those of the bulk wurtzite ZnO.[17,18,19,20,21,22,23,24,25,26,27,28] To date, experimental and theoretical works have focused predominantly on well-defined two-dimensional ZnO films, many of them of bilayer thickness, while much less information is available about small ZnO clusters (or islands) supported on such ZnO bilayers
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