In situ X-ray scattering study of the passive film on Ni(111) in sulfuric acid solution

Results of an in situ X-ray scattering study of the passive film formed on Ni(111) electrodes by passivation in 0.05 M H2SO4 (pH 1.0) at 0.50 V-Ag/AgC l are reported and compared with results on the film formed by oxidation in air at room temperature. Tn both cases, ultrathin, (111)-oriented NiO film s are observed, which are aligned with the Ni substrate lattice and slightl y expanded along the surface normal with respect to bulk NiO. However, two major structural differences are found: (i) while on the air-formed oxide p arallel (NiO[1 (1) over bar 0] \\ Ni[1 (1) over bar 0]) and antiparallel (N iO[1 (1) over bar 0] \\ Ni[(1) over bar 10]) oriented domains coexist; the passive film exhibits. a well-defined antiparallel orientation and (ii) the lattice of the passive film is, in contrast to that of the air-fomed oxide , tilted relative to the substrate with a broad angular dispersion of the t ilt angle centered at about 3.3 degrees.