Local Electronic Properties of the Passive Film on Nickel Studied by Scanning Tunneling Spectroscopy

Scanning tunneling spectroscopy has been applied to characterize the electronic properties of the granular and intergranular sites of the passive film grown on Ni(111) single-crystal surfaces in acid solution. The passive film, essentially consisting of NiO, shows p-type grains (valence band edge at 0.54 +/- 0.03 eV below Fermi level) with unoccupied Ni 3d surface states at 0.5-1.5 eV above Fermi level in the bandgap. At grain boundaries, the p-type semiconductivity is attenuated as evidenced by a downward shift of 0.12 +/- 0.06 eV of the valence band edge, and the density of surface states is markedly reduced, both effects indicating the presence of oxygen vacancies in these sites. The dominant point defect in the film is metallic vacancies but oxygen vacancies attenuate locally the resulting p-type property at the grain boundaries. The implications of these local electronic properties on oxidative and reductive electronic transfer and on the corrosion resistance are discussed. The oxygen vacancies at intergranular boundaries imply local changes of the properties of anionic mass transport through the passive film and enhancement of Cl- adsorption and/or penetration, which is consistent with the view of passive film grain boundaries playing a key role in passivity breakdown. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.010208jes] All rights reserved.