Adlayer growth vs spontaneous (near-) surface alloy formation: Zn growth on Au(111).

As part of an extensive effort to explore the function of Au/ZnO catalysts in the synthesis of methanol from CO and H, we have systematically investigated the temperature dependent growth, structure formation, and surface intermixing of Zn on the herringbone reconstructed Au(111) surface and the thermal stability of the resulting surfaces by scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS). After Zn deposition at low temperatures, at about 105 K (STM) or below (XPS), we observed nucleation and two-dimensional growth of Zn islands mainly at the elbow sites of the Au(111) herringbone reconstruction. This results in local perturbations of the reconstruction pattern of the Au(111) substrate, which can create additional nucleation sites. XPS data indicate that Zn dissolution into deeper layers is kinetically hindered under these conditions, while local exchange with the Au surface layer, in particular at the elbow sites during nucleation, cannot be excluded. Zn deposition at room temperature, in contrast, results in near-surface alloy formation with a strongly distorted pattern of the herringbone reconstruction and condensation of the Zn and exchanged Au adatoms at ascending steps, together with some loss of Zn into deeper layers. Upon annealing, Zn atoms diffuse to lower layers and eventually to the Au bulk, and the surface successively regains its original Au(111) herringbone structure, which is almost reached after 500 K annealing. Compared with previous reports on the growth of other metals on Au(111), Zn shows a rather high tendency for intermixing and near-surface alloy formation.