A SCANNING-TUNNELING-MICROSCOPY STUDY OF THE STRUCTURE OF THIN OXIDE-FILMS GROWN ON NI(111) SINGLE-CRYSTAL SURFACES BY ANODIC POLARIZATION IN ACID ELECTROLYTE

Scanning tunneling microscopy has been used to study ex situ (in air) the thin oxide films (passive films) grown on Ni(111) by anodic polari zation at +550, +650 and +750 mV/SHE in 0.05 M H2SO4. Atomic resolutio n imaging demonstrates the crystalline character of the oxide film and the epitaxy with the substrate. Two levels of roughening with respect to the non-polarized surfaces are observed: on a mesoscopic scale and on the atomic scale. The roughening on the mesoscopic scale increases with higher polarization potentials. The observed roughness is attrib uted to the result of the competition between metal dissolution and nu cleation and growth of the oxide film. The roughening on the atomic sc ale is independent of the polarization potential. It is evidenced by t he formation of a stepped crystalline lattice whose parameters fit tho se of a (111)-oriented NiO surface. The presence of steps indicates a tilt of (8 +/- 5)degrees of the surface of the film with respect to th e (111) orientation. The possible epitaxial relationships resulting fr om the surface tilt are discussed. Local variations of the film thickn ess at the step edges are likely to result from the surface tilt and m ay constitute preferential sites for the local breakdown of passivity.