Initial stages of Pd deposition on Au(hkl) part II: Pd on Au(100)

The deposition of Pd onto unreconstructed Au(100) was studied by in situ sc anning tunnelling microscopy and cyclic voltammetry for both an island-free surface and one covered with islands that originate from lifting of the (h ex)-reconstruction. On the bare Au(100) surfaces, a (3 x root 10)-structure of adsorbed [PdCl4](2-) with atomically resolved images of the ligand atom s was observed. An equivalent of about one Pd monolayer is deposited at und erpotentials on Au(100). On top of the resulting layer a distorted-hexagon chloride adlayer is formed. The growth mechanism for Pd on island-covered A u(100) is markedly different from that for the island-free surface. In the overpotential region, Pd films grow epitaxially with a slightly increased s tep density at higher coverage. These thicker Pd overlayers behave very sim ilar to surfaces of massive Pd(100) single crystals, whereas the first Pd l ayers on Au(100) have a clearly different electrochemical behaviour. The pr esence of small Au islands after dissolution of Pd points towards surface a lloy formation in the underpotential region. (C) 2000 Elsevier Science B.V. All rights reserved.