Spectroscopic ellipsometry study of nickel oxidation in alkaline solution

Abstract

Spectroscopic ellipsometry and cyclic voltammetry were used to study the oxidation of nickel in alkaline solutions. Electrode surface films were formed by air exposure and by electrochemical oxidation at various potentials during the first voltammetric scan. Using a self-compensating ellipsometer and a previously developed data fitting program, it was possible to characterize the surface films formed in situ. We found that oxide layers formed during the polishing of nickel in air have a significant optical effect, and normal polishing conditions can produce NiO of 10 Å thickness. During the positive-going sweep at potentials between −0.9 V and −0.4 V ( vs Hg/HgO), a compact film of β-Ni(OH) 2 is formed. The surface layer that appears after the second anodic peak at +0.5 V contains a mixture of Ni(OH) 2 and NiOOH with an overlayer of β-NiOOH. This overlayer is completely reduced after the cathodic peak at +0.2 V is reached during the negative-going sweep, and the layer remaining at this potential consists of a mixture of Ni(OH) 2 and NiOOH. At very cathodic potentials (−0.9 V) NiOOH is still found on the electrode surface, and some electrode roughness appears. During a potentiostatic hold at this potential, NiOOH is reduced to Ni(OH) 2, and the Ni(OH) 2 is very slowly reduced to Ni with a decrease in electrode roughness.