Electrochemical Bistability Induced by Surface Area Changes

Nanostructured metallic multilayers in the electrodeposition of copper can be spontaneously synthetized when the system is kept far from thermodynamic equilibrium. We have focused on the understanding of how blocking agents, mainly induced by strong adsorption, can tune the thickness of these self-organized layers, therefore, altering the total surface area. For this purpose, we perturbed the electrodeposition of copper under potentiostatic control with small amounts of o-phenanthroline (phen). Indeed, the addition of phen gives rise to a distinct current-potential profile. A negative differential resistance (NDR) and hysteresis are clearly observed in the cyclic voltammograms (Figure 1 a-g). The appearance of the NDR is attributed to the adsorption of the reduced form of [Cu(ll)(o-phen)2]2+ complex in lower potentials which suppresses the copper electrodeposition. The hysteresis, in its turn, indicates that the system has two stable steady states (SSI and SSII) for the same set of controlling parameters, the so-called bistability (BS). By adding increasing values of external resistance (Rext) between the working and reference electrodes, the hysteresis potential intervals become larger. We have monitored the bistability domain by mapping the bifurcation diagram Rext vs. E obtained from the experimental data (Figure 1 h). Our results suggest that strong adsorption of copper complexes formed in the electrolyte can favor the appearance of bistability. This condition is important once it shows the presence of an autocatalytic process - a necessary step to generate electrochemical oscillations.