Chemical and electronic properties of ultrathin metal films: The Pd/Re(0001) and Pd/Ru(0001) systems

Abstract

The nature of the electronic and chemical properties of ultrathin Pd films on Re(0001) and Ru(0001) has been studied using x-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and CO chemisorption. The results indicate that the Pd(3${\mathit{d}}_{5/2}$) binding energy for a monolayer (ML) of Pd on Re(0001) and Ru(0001) is perturbed by +0.60 and +0.30 eV, respectively, from that of the surface atoms of Pd(100). These electronic perturbations induce large changes in the chemical properties of the Pd films. TPD results indicate that the desorption temperature of CO from 1 ML of Pd on Re(0001) and Ru(0001) is ∼120 K lower than the corresponding desorption temperature from Pd(100). The XPS and CO-TPD data indicate that Pd transfers charge to the Re and Ru substrates, becoming electron deficient and less efficient at π backdonation toward CO. By comparison of these results with those reported previously for Pd, Ni, and Cu adlayers, a correlation is observed among the electronic perturbations of the adlayers, the cohesive metal-substrate bond strength, the ability of the film to chemisorb CO, and the CO-induced shift in the metal core-level binding energy. In general, the results indicate that the formation of a metal-metal bond at a surface leads to a gain of electron density by the element initially having the greater fraction of empty states in its valence band. This behavior is completely contrary to that seen in bulk alloys, likely a consequence of the anisotropic character of a surface that changes the relative electronegativities of the metal atoms. On the basis of these results, a qualitative scale of surface electronegativities is developed, showing trends that are very different from those found in three-dimensional bulk alloys.