Electromicrogravimetric study of underpotential deposition of Co on textured gold electrode in ammonia medium

Abstract

Formation of a layer of a metal M on a foreign metal substrate, S, at potentials positive to its reversible potential, E_r, (underpotential deposition, UPD) is a phenomenon that appears only in some systems. In the case of the UPD process of Co on a gold substrate, there is still a controversy about its existence, for which reason, in this study, voltammetry and chronoamperometry were coupled with a quartz microbalance in ammonium solution to better understand its formation mechanism. The results obtained show that the Co forms only one layer on the substrate before the bulk deposition of cobalt occurs. During the UPD process, the Co atom completely transfers its two electrons to the electrode and is adsorbed as a discharged species. The monolayer charge density, determined by electromicrogravimetry, is 448 μC cm⁻² and probably corresponds to a commensurate overlayer on the gold substrate. In addition, the UPD process is controlled by adsorption as shown by voltammetric experiments. Finally, the analysis of the process of Co monolayer destruction (desorption) shows a mechanism different from that in the formation process, possibly due to H adsorption on the substrate produced during the process of adsorption of Co.