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

A study of the electrodeposition and electrochemical dissolution of ultrath in 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 n egative of -0.72 V vs. SCE (i.e., overpotentials eta greater than or equal to 160 mV), where an incommensurate, (111)-oriented film with an in-plane l attice rotation of 0.5 degrees relative to the Ag substrate lattice is form ed. The lateral nearest neighbor spacing is as in bulk Ni (2.49 Angstrom) f or a film thickness greater than or equal to 3 ML and expanded for monolaye r (2.54 Angstrom) and bilayer (2.52 Angstrom) films. Depending on the depos ition potential, three growth regimes, resulting in different deposit morph ologies, are observed: At low overpotentials (160 less than or equal to eta /mV less than or equal to 200) a smooth Ni film is formed via a 2D step-fl ow growth process, commencing at steps of the Ag substrate. At medium overp otentials (200 greater than or equal to eta /mV greater than or equal to 30 0) 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 h igher overpotentials (eta greater than or equal to 300 mV) 3D islands are f ormed at the steps and on the substrate terraces. The size of the Ni multil ayer islands is independent of the terrace widths, indicating that Ni growt h proceeds via direct discharge at step sites ("direct deposition"). The tr ansition from 2D to 3D growth as well as the change in island shape with ov erpotential 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 in duced by the Ag-Ni boundary and can be described quantitatively as a functi on 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 via step- flow etching, with a higher dissolution rate for the Ni monolayer as compar ed to higher Ni layers.