Structure, dissolution, and passivation of Ni(111) electrodes in sulfuric acid solution: an in situ STM, X-ray scattering, and electrochemical study

Results of a detailed study of Ni(111) surfaces in air and in sulfuric acid solution (pH 1.0-2.7) by a combination of STM, surface X-ray scattering using synchrotron radiation, and electrochemical techniques are presented. Ni(111) samples, prepared via annealing in H2 and exposure to air at room temperature, are covered by a smooth three to four layers thick NiO(111) film with parallel (NiO[11-bar 0] parallel Ni[11-bar 0]) and anti-parallel (NiO[11-bar 0] parallel Ni[1-bar 10]) in-plane orientation. Electrochemical reduction at potentials ≤-0.40 VAg/AgCl results in the formation of a well-defined, oxide-free surface with large terraces, a low surface mobility, and a (1x1) lattice on the atomic scale. X-ray reflectivity data indicate vertical lattice expansion for the topmost Ni layer and a strongly bound sulfate or oxygen species. Active Ni dissolution commences at potentials ≥-0.25 VAg/AgCl by a step-flow mechanism, followed by the rapid formation of large three-dimensional etch pits, leading to considerable surface roughening. In situ STM observations of the passive film formation show at potentials ≥-0.10 VAg/AgCl the nucleation and growth of an initial 'grainy' phase, which is attributed to a Ni hydroxide, followed by a slower restructuring process. According to our combined STM and SXS data, the resulting steady-state passive film exhibits a duplex structure, with a crystalline, inner NiO(111) layer, consisting of exclusively anti-parallel oriented grains (NiO[11-bar 0] parallel Ni[1-bar 10]) which are slightly tilted relative to the substrate lattice, and a porous, probably amorphous hydroxide phase on top. The thickness of the crystalline NiO film increases with potential by 14-17 A V-1. In addition, structural changes of the oxide film during immersion of Ni samples into the sulfuric acid solution at potentials in the passive range and after emersion from the electrolyte were observed, which indicate the slow conversion of the air-formed into the passive oxide and the (partial) reformation of the air-formed oxide, respectively