Ellipso-reflectometry at nickel in sulphuric acid solutions

Abstract

A method was established to derive the optical properties of a substrate and of a thin film on its surface by measuring the intensities at three directions of polarization after reflection of incident monochromatic, linearly polarized light and at different angles of incidence. Results concerning the optical properties of the film were compared to those obtained from a second method in which the reflectivity changes at different angles of incidence with incident light polarized in parallel and perpendicular to the plane of incidence were measured. Measurements were performed with nickel in acid sulphate solutions at different pH values. At λ = 500 nm, the optical constants of nickel were determined and the dependences of optical constants and thicknesses for steady state films evaluated as functions of electrode potcntial and pH value. The state of oxidation increases continuously as the potential becomes morc positive. The state of hydration grows as the potential becomes more positive and the pH value becomes largcr. At negative potentials. thc kinetics of activation is characterized by a fast reduction and hydration of the non-stoichiometric oxide formcd at positive potentials, followed by a slow potential-independent dissolution of the resulting nickel(II) hydroxide. The spectrum of the optical constants in the region 1.4 ⩽ hv ⩽ 4.8 eV for a steady-state film is characteristic of a partly hydrated semiconducting nickel oxide. Optical properties of the film derived from the first method are independent of the angle of incidence within the experimental accuracy. There is a small systematic difference between the mean values from the first method and the values from the second method. The systematic difference is close to the mean scatter of the measurements at different angles of incidence using the first method. Thus, there is no indication of an optical anisotropy of the film.