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 scanning tunnelling microscopy and cyclic voltammetry for both an island-free surface and one covered with islands that originate from lifting of the (hex)-reconstruction. On the bare Au(100) surfaces, a (3 x root 10)-structure of adsorbed [PdCl4](2-) with atomically resolved images of the ligand atoms was observed. An equivalent of about one Pd monolayer is deposited at underpotentials on Au(100). On top of the resulting layer a distorted-hexagon chloride adlayer is formed. The growth mechanism for Pd on island-covered Au(100) is markedly different from that for the island-free surface. In the overpotential region, Pd films grow epitaxially with a slightly increased step density at higher coverage. These thicker Pd overlayers behave very similar to surfaces of massive Pd(100) single crystals, whereas the first Pd layers on Au(100) have a clearly different electrochemical behaviour. The presence of small Au islands after dissolution of Pd points towards surface alloy formation in the underpotential region. (C) 2000 Elsevier Science B.V. All rights reserved.