Growth and electronic structure of Nb and Ta films on Pd and their interaction with CO

Abstract

The growth of Nb and Ta films on Pd(111) and their modification of CO chemisorption are studied using ultraviolet photoelectron spectroscopy (UPS), low-energy electron diffraction, and low-energy ion scattering (LEIS). At 300 K, LEIS shows significant amounts of Pd in the top layer even after more than a monolayer of Nb or Ta is deposited. This is in contrast to Pd on Nb or Ta(110) where a Pd monolayer completely covers the substrate. UPS shows that the surface electronic structure at monolayer Nb or Ta coverage resembles that of a Pd monolayer on Nb or Ta(110) and is characterized by strong Pd 4d emission 1–4 eV below Ef and a low density of states at Ef. Molecular CO adsorption is observed at submonolayer coverage; there is little CO adsorption at monolayer coverage and dissociative CO adsorption starts at greater than monolayer coverage. It is concluded that the deposited Nb or Ta monolayers intermix with the Pd(111) substrate and it is also shown that this behavior can be understood by examining the energetics of the system.