Nucleation and growth of cobalt onto different substrates: Part II. The upd-opd transition onto a gold electrode

Abstract

A study of the electrochemical deposition of cobalt was carried out using electrochemical techniques. The formation kinetics and growth of cobalt nuclei onto a polycrystalline gold electrode were studied employing an aqueous 10 −2 M CoCl 2 1 M NH 4Cl solution (pH 9.5). We obtain the potentiostatic j– t plots when the potential step jumps from a potential value in the underpotential deposition zone to a final potential in the opd region. It was found that these current density transients can be described through a kinetic mechanism that involves three different contributions: (a) a Langmuir type adsorption process, (b) 2D diffusion-controlled instantaneous nucleation and (c) 3D nucleation limited by a mass transfer reaction. In order to describe the contribution due to 3D growth we test two different approaches by Scharifker and Mostany (J. Electroanal. Chem. 177 (1984) 13 ) and Heerman and Tarallo (J. Electroanal. Chem. 470 (1997) 70), the values of the experimental parameters A (nucleation rate constant), N 0 (number of active nucleation sites) and D (diffusivity of the depositing ions) obtained in the two cases are quite close, however if the influence of the adsorption and 2D nucleation and growth processes is not considered, A and N 0 are overestimated and underestimated, respectively.