Theoretical and Experimental Study of Cobalt Nucleation and Growth onto Gold Substrate with Different Crystallinity

Cobalt electrodeposition onto both polycrystalline and monocrystalline (111) gold electrodes was studied using electrochemical techniques. Current density transients were recorded in these systems starting with the applied potential in the underpotential region and then jumping to different potential values in the overpotential zone. From the analysis of the experimental J-t curves, it was found that the process of cobalt deposition involves a 2D-3D nucleation growth transition in both cases. We also found that a mechanism comprising the simultaneous presence of three different contributions namely, Langmuir-type adsorption processes and 2D and 3D nucleation, both mass-transfer-controlled nucleation processes, can adequately describe the experimental evidence. This model predicts a greater nucleation rate and larger number of active nucleation sites for polycrystalline gold substrate compared with the Au(111) substrate for each applied potential. Moreover, from a theoretical quantum study, we analyzed the reactivity of the bare Au(111) surface determining that there exists a distribution of active reduction sites inherent to this substrate. The existence of these sites can be considered as one of several previous steps required for the formation of active nucleation sites. We found a ratio of 2.5×104 active reduction sites for each active nucleation site obtained from experimental results.