In situ scanning tunnelling microscopic study of the initial stages of growth and of the structure of the passive film on Ni(111) in 1 mM NaOH(aq)

In situ electrochemical scanning tunnelling microscopic (STM) measurements of the growth mechanism of the passive film on Ni(111) single-crystal surfaces in an alkaline 1 mM NaOH aqueous solution are reported. Stepping the potential from -1,050 mV/SHE, where the terrace topography of the bare Ni(111) surface is observed, to -800 mV generates a continuous step flow resulting from a slow dissolution of the surface localized at step edges of the surface. Islands are observed on the terraces that possibly correspond to the specific adsorption of hydroxide anions, without formation of an ordered structure. The nucleation of the passivating oxide is preferentially located at the step edges at this potential. The ongoing dissolution at the nearby step edges not yet passivated leads to the formation of isolated 2D islands of the passivating oxide. At -780 mV, the formation of extended islands constituted of 2D nanocrystals (similar to 2 nm) is observed on the substrate terraces and blocks the dissolution at the step edges. The nanocrystals have an hexagonal lattice assigned to the formation of a monolayer of Ni(OH)(2)(0001) in strained epitaxy on Ni(111). At E >=-630 mV, the formation of a facetted topography is observed as a result of the 3D growth of the passive film in tilted epitaxy on the substrate. Its structure is in excellent agreement with that expected for an unstrained 3D beta-Ni(OH)(2)(0001) layer. This 3D layer of b-Ni(OH)(2) is not reduced by a cathodic sweep of the potential down to -1,500 mV whereas the 2D islands are reduced under similar conditions. Ageing of the passive film shifts cathodically the potential of non-reversibility.