An ellipsometric investigation of nickel oxide films in alkaline solution: The multilayer film approach

Abstract

Abstract The properties of anodic oxide films formed on polycrystalline nickel electrodes in alkaline solution under potentiodynamic conditions were studied by means of cyclic voltammetry and combined ellipsometry and reflectometry. The oxide film grown on a Ni surface during the potential scan between hydrogen and oxygen evolution was revealed to be optically inhomogeneous. In this paper a reliable model for two- or three-layer oxide films with different refractive indices is demonstrated. It has been found that Ni(II) oxides form a bottom thin solid state film (final thickness of about 3 nm) of n 3 =2.50(±0.15) -0i (the mean value for λ=632.8 nm) composed of NiO and Ni(OH)2 phases. At higher anodic potentials a further oxidation of Ni(II) oxides and the formation of thick layers of Ni(III) and Ni(IV) phases on top of the passive film occurs, most probably, due to a defect mechanism. The obtained values of the refractive indices are: n 2 =1.41(±0.01)−0.073(±0.005)i and n 1 =1.38(±0.01)−0.06(±0.01)i for oxide film layers containing socalled β-NiOOH and γ-NiOOH hydrous nickel oxide species, respectively.