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

The electrochemical deposition of palladium onto Au(111) from chloride-cont aining solutions was investigated by in-situ STM and cyclic voltammetry. Th ese methods are complementary by yielding structural and thermodynamic info rmation. For [PdCl4](2-) adsorbed on Au(111), a distorted hexagonal structu re was imaged, and a phase transition within this adsorbed layer was observ ed. This palladate anion was found to play a crucial role in Pd deposition and dissolution. Pd deposition starts by forming a pseudomorphic layer in t he underpotential region, followed by the formation of a second Pd monolaye r at overpotentials. By using stepped Au(111) surfaces, an assignment of vo ltammetric peaks to nucleation of Pd at steps and on terraces was achieved. It is generally observed that Pd nucleates at monoatomic high steps and gr ows two-dimensionally. The morphology of the overlayers changes with increa sing 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. (C) 1999 Elsevier Scien ce B.V. All rights reserved.