In situ spectroscopic ellipsometry during electrochemical treatment of zinc in alkaline carbonate electrolyte

Abstract

In situ spectroscopic ellipsometry (SE, 297–820nm, 4.4–1.5eV) during electrochemical oxidation/reduction of zinc in alkaline carbonate electrolyte was performed using a specially designed optical–electrochemical cell. The growth and shrinkage of the interfacial layer on Zn was analysed on the basis of Lekner's first order perturbation theory. For non-absorbing thin films on metal surfaces, an algorithm has been derived to extract the thickness of a surface film directly from ellipsometric data without the need of optical modelling. During cyclic voltammetry (CV), a rapid decrease/increase in the layer thickness in the reduction/oxidation peak has been found. In the potential regime where the surface is oxide-covered, the layer thickness increases/decreases linearly with potential in anodic/cathodic scans. The density of the interfacial region in this regime is constant. During chronoamperometric experiments, a fast correlating jump in thickness was found after potential jumps. An ageing of the films is observed in the absorption spectrum (from the ellipsometric parameter Ψ), which shows changes until ≈30min after potential jumps. Analysis of the current transients points to continuous dissolution of Zn.