CO adsorption studies on pure and Ni-covered Cu(111) surfaces

Abstract

The adsorption of CO on pure and Ni-covered Cu(111) surfaces has been studied by means of LEED, TDS, UPS and work function measurements during adsorption and desorption. Different Ni-coverages between 0.1 and 2 monolayers were obtained by Ni-evaporation controlled by a quartz micro balance and by AES. Near room temperature Ni grows in a layer-by-layer mode on Cu(111). The island structure of the surfaces with submonolayer Ni-coverages is clearly demonstrated by TDS und LEED results obtained after CO adsorption. As with surfaces of bulk Cu-Ni alloys CO adsorption on Cu(111) with submonolayer Ni-coverage is dominated by a site effect. Cu-, Ni-, and mixed adsorption sites can be distinguished. The CO induced work function changes for Ni- and Cu-site adsorption show the same sign as observed with the pure metals. Mixed site adsorption has only a minor influence on the work function. A “ligand effect” observed only for the Ni-site adsorption, and only at small Ni-coverages is discussed in detail. Studies on the adsorption kinetics reveal that the Cu-sites may serve as precursor sites for Ni-site adsorption. Detailed UPS studies demonstrate that the CO-induced emission maxima observed on Cu surfaces with submonolayer Ni-coverages can be interpreted as a superposition of the respective adsorption features observed with the pure metals, roughly separated by their work function difference.