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 (3×10)-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.