SPECTROSCOPIC ELLIPSOMETRY STUDY OF NICKEL OXIDATION IN ALKALINE-SOLUTION

Spectroscopic ellipsometry and cyclic voltammetry were used to study t he oxidation of nickel in alkaline solutions. Electrode surface films were formed by air exposure and by electrochemical oxidation at variou s potentials during the first voltammetric scan. Using a self-compensa ting ellipsometer and a previously developed data fitting program, it was possible to characterize the surface films formed in situ. We foun d that oxide layers formed during the polishing of nickel in air have a significant optical effect, and normal polishing conditions can prod uce NiO of 10 Angstrom thickness. During the positive-going sweep at p otentials between -0.9 V and -0.4 V (vs Hg/HgO), a compact film of bet a-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 beta-NiOOH. This overlayer is completely reduced after the cathodic peak at +0.2 V is reached during the negative-going swee p, 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 sti ll found on the electrode surface, and some electrode roughness appear s. 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 decr ease in electrode roughness. (C) 1997 Elsevier Science Ltd. All rights reserved.