Comparison Study of Structural Properties and CO Adsorption on the Cu/Au(111) and Au/Cu(111) Thin Films

Supported metal films are often found to display extraordinary properties compared to their bulk counterparts. In this work, we investigate the atomic and electronic structures of both the Au/Cu(111) and Cu/Au(111) thin films with low-temperature scanning tunneling microscopy (STM) and synchrotron radiation photoelectron spectroscopy (SRPES). The high-resolution STM images revealed that the Au films evaporated on Cu(111) at room temperature grow with compressed lattices, which gradually evolve and restore to that of the bulk gold from the fourth layer. Both STM and SRPES evidenced that there are considerable Cu atoms incorporated into each layer of the Au films, whose concentrations decrease stepwise along with the film thickening. As a reversed system, the growth of Cu films on Au(111) starts with agglomerating at the subsurface and adopting a (1 x 1) lattice within a submonolayer coverage. The lattice quickly shrinks to that of bulk Cu(111) from the second layer, yet the electronic property is restored slowly until the third layer. In each Cu film, there were also intermixed Au atoms coming from the substrate, and their concentrations also decrease along with the film thickening. On both the Au/Cu(111) and Cu/Au(111) submonolayer films, CO adsorption was investigated and found to be significantly higher than bare Au(111) but still weaker than the Cu(111) surface. The adsorbed CO molecules were apparently connected to the incorporated Cu atoms in the surface layers, yet the enhanced CO bindings were closely related to the electronic properties of the films. These findings are believed to shed new light on the atomic details of the Cu/Au bimetallic systems, thus deepening the understanding of the specific catalytic activities of the Cu/Au alloys.