In situ STM study of electrodeposition and anodic dissolution of Ni on Ag(111)

Abstract

A study of the electrodeposition and electrochemical dissolution of ultrathin Ni films on Ag(111) electrodes in Watts electrolyte by in situ scanning tunneling microscopy (STM), electrochemical quartz microbalance (EQCM), and cyclic voltammetry (CV) is presented. Ni deposition starts at potentials negative of − 0.72 V s. SCE (i.e., overpotentials η160 mV), where an incommensurate, (111)-oriented film with an in-plane lattice rotation of 0.5° relative to the Ag substrate lattice is formed. The lateral nearest neighbor spacing is as in bulk Ni (2.49 A) for a film thickness 3 ML and expanded for monolayer (2.54 A) and bilayer (2.52 A) films. Depending on the deposition potential, three growth regimes, resulting in different deposit morphologies, are observed: At low overpotentials (160⩽η/mV⩽200) a smooth Ni film is formed ia a 2D step-flow growth process, commencing at steps of the Ag substrate. At medium overpotentials (200η/mV300) a transformation from 2D step-flow to 3D growth occurs, resulting in the selective formation of Ni multilayer islands along the Ag steps. At even higher overpotentials (η300 mV) 3D islands are formed at the steps and on the substrate terraces. The size of the Ni multilayer islands is independent of the terrace widths, indicating that Ni growth proceeds ia direct discharge at step sites (“direct deposition”). The transition from 2D to 3D growth as well as the change in island shape with overpotential can be rationalized by a different potential dependence of the various microscopic nucleation and growth processes. The multilayer growth at steps is attributed to next-layer nucleation at the structural defect induced by the Ag–Ni boundary and can be described quantitatively as a function of deposition time by a simple 2D model. In addition, place-exchange of Ni with Ag surface atoms and encapsulation of Ni islands by Ag is observed. Dissolution of the electrodeposited multilayer Ni films proceeds ia step-flow etching, with a higher dissolution rate for the Ni monolayer as compared to higher Ni layers.