CO adsorption on graphite-like ZnO bilayers supported on Cu(111), Ag(111) and Au(111) surfaces
Graphitic-like ZnO bilayer films deposited on coinage metals, Cu(111), Ag(111), and Au(111) have been studied by density functional theory calculations including dispersion corrections. The scope is to compare on an equal footing the properties of the three systems and in particular the nature of the metal/oxide interface. To this end we have considered the adsorption of a CO probe molecule and the vibrational shifts induced by adsorption on ZnO/Cu(111), ZnO/Ag(111), and ZnO/Au(111) compared to adsorption on the unsupported ZnO bilayer and on the wurtzite ZnO surface. We find that while the interaction of ZnO with Ag and Au supports is dominated by dispersion interactions with little or no charge transfer at the interface, in the case of Cu a moderate electron transfer occurs towards the ZnO bilayer. As a consequence, while the stretching frequency of CO on ZnO/Au is blue-shifted, that on ZnO/Cu is red- shifted compared to free CO. CO on ZnO/Ag is intermediate. In all three cases, however, the ZnO bilayer is almost flat, with a modest rumpling found in the case of Cu as a consequence of the stronger chemical interaction. The results fully explain the CO vibrational shifts of CO on ZnO/Cu(111) [Schott, V. et al. Angew. Chem. Int. Ed. 2013, 52, 1-6] without implying major distortions in the supported film.