Initial stages of Pd deposition on Au(hkl) Part I: Pd on Au(111)

The electrochemical deposition of palladium onto Au(111) from chloride-containing solutions was investigated by in-situ STM and cyclic voltammetry. These methods are complementary by yielding structural and thermodynamic information. For [PdCl 4] 2− adsorbed on Au(111), a distorted hexagonal structure was imaged, and a phase transition within this adsorbed layer was observed. This palladate anion was found to play a crucial role in Pd deposition and dissolution. Pd deposition starts by forming a pseudomorphic layer in the underpotential region, followed by the formation of a second Pd monolayer at overpotentials. By using stepped Au(111) surfaces, an assignment of voltammetric peaks to nucleation of Pd at steps and on terraces was achieved. It is generally observed that Pd nucleates at monoatomic high steps and grows two-dimensionally. The morphology of the overlayers changes with increasing Pd coverage from flat and well-ordered for the first two monolayers to increasingly rough and defect-rich, but retaining Pd(111) characteristics in cyclic voltammetry up to at least 10 monolayers.