Evidence of charge transfer to atomic and molecular adsorbates on graphitic-like ZnO/X(111) (X = Cu, Ag, Au) ultrathin films

ZnO bilayers grown on Cu(111), Ag(111), and Au(111) surfaces form graphitic-like flat structures. They 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<sup>-</sup>, or molecular, NO<sub>2</sub><sup>-</sup> and O<sub>2</sub><sup>-</sup>, 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 CO<sub>2</sub>, on the contrary, do not induce the CT. However, charge transfer and activation of two CO<sub>2</sub> molecules to form an oxalate species, [C<sub>2</sub>O<sub>4</sub>]<sup>2-</sup>, is promoted by a single Au anion, 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.