In situ STM study of the electrodeposition and anodic dissolution of ultrathin epitaxial Ni films on Au(111)

A detailed in sitcl STM study of the electrodeposition and electrochemical dissolution of Ni on reconstructed Au(lll) electrode surfaces in various electrolytes is presented, demonstrating the electrochemical formation of well-defined, ultrathin, epitaxial Ni films. Formation of Ni nuclei starts below the Ni-0/Ni2+ Nernst potential via place exchange of h-Ti with Au atoms at the elbows of the herringbone reconstruction, followed by nucleation of Ni islands on top of these substitutional Ni atoms at overvoltages eta greater than or equal to 80 mV, and by nucleation at step edges of the Au substrate at eta greater than or equal to 100 mV. At submonolayer coverages islands with two different growth morphologies, compact, triangularly shaped and highly anisotropic, needlelike islands, are observed. Upon further growth these islands coalesce and an almost perfect two-dimensional Ni monolayer is formed. Multilayer growth was studied up to coverages of 5 ML; it exhibits a similar layer-by-layer growth, resulting in very smooth Ni films. In atomic-scale observations a hexagonal Ni lattice with a lattice spacing of 2.5 Angstrom is resolved, similar to the (111) orientation in bulk Ni, and with the same orientation as the Au lattice. The order in the highly defective first Ni layer is considerably improved by second-layer deposition. The significant differences to vapor-deposited Ni on Au(lll) indicate a structure-decisive role of coadsorbates in the electrochemical environment. Dissolution of the Ni films at potentials positive of the Nernst potential proceeds via formation of etch pits and step-flow etching.