Electronic properties of thin Zn layers on Pd(111) during growth and alloying

The growth and alloying of thin Zn layers on Pd(111) was investigated using X-ray and ultraviolet photoelectron spectroscopy as well as low energy electron diffraction and correlated with density functional calculations. At 105 K, the formation of a pseudomorphic Zn monolayer is observed. Upon heating this layer to 550 K or upon deposition of 1 ML at 550 K, an ordered p(2 x 1) PdZn surface alloy with a Pd:Zn ratio of similar to 1:1 is formed, with a characteristic Pd 3d(5/2) peak at a binding energy of similar to 335.6 eV. For deposition of 3 ML Zn at 550 K or by heating 3 ML, deposited at low temperature, to 500 or 600 K, a PdZn alloy with a Pd:Zn ratio of again similar to 1:1 is found in the surface region, with a Pd 3d(5/2) peak at similar to 335.9 eV; the direct preparation at 550 K leads to a more homogeneous and better ordered alloy. The valence band spectrum of this alloy with a low density of states at the Fermi level and pronounced maxima due to the "Pd 4d" band at similar to 2.4 and 3.9 eV closely resembles the spectrum of Cu(111), in good agreement with the calculated density of states for a PdZn alloy of 1:1 stoichiometry. The shift of the "Pd 4d" band to higher binding energies as compared to Pd(111) indicates a charge transfer from Zn to the Pd 4d levels. Overall, the similarity between the ultraviolet photoelectron spectra for the PdZn alloy and for Cu(111) is taken as explanation for the similar chemical activity of both systems in methanol steam reforming. (c) 2005 Elsevier B.V. All rights reserved.