Evidence of Charge Transfer to Atomic and Molecular Adsorbates on ZnO/X(111) (X = Cu, Ag, Au) Ultrathin Films. Relevance for Cu/ZnO Catalysts

ZnO bilayers grown on Cu(111), Ag(111), and Au(111) surfaces belong to the class of two-dimensional materials. We show, by means of density functional theory (DFT) calculations including dispersion, that Au, NO2, and O2 species adsorbed on ZnO/Cu(111) induce a spontaneous net charge transfer (CT) via electron tunneling from the Cu support through the insulating ZnO film, resulting in the formation of negatively charged atomic, Au-, or molecular, NO2- and O2-, adsorbates. We show for the case of gold that the CT is found also for ZnO/Ag(111) and ZnO/Au(111) interfaces. The stabilization of the anionic species is accompanied by a polaronic distortion of the ZnO lattice. Other molecules with low electron affinity such as NO and CO2, on the contrary, do not induce the CT. However, charge transfer and activation of two CO2 molecules to form an oxalate species, [C2O4]2-, is promoted by Au1 or Au2 supported anions, and similar catalytic activity is expected for the negatively charged Au clusters. By comparing the properties of the metal-supported ZnO films with those of the free-standing ZnO bilayer, we demonstrate the key role of the metal/oxide interface. These results are relevant in the field of methanol synthesis based on ZnO/Cu catalysts where ultrathin layers of ZnO supported on Cu are formed under reaction conditions.