Single Atom Nucleation of Cobalt over Graphene Oxide: Theory and Experimental Study

Nucleation and growth mechanism of Co electrodeposition over graphene oxide (GO) was studied using cyclic voltammetry (CV) and chronoamperometry techniques. The studies were performed over an aqueous electrolyte containing 10 mM CoSO₄·7H₂O maintained at an initial pH of 2.5. CV studies established that the deposition mechanism was diffusion-controlled and irreversible. Chronoamperometry studies revealed the presence of three concurrent processes: an initial adsorption current, which indicated adatom layer formation, 3D nucleation and growth of Co islands over GO, and hydrogen evolution over the deposited Co nanoclusters. It was observed that the nucleation rate increased with increasing the overpotential (η) for deposition (from 2.71 × 10⁴ cm^-2 s^-1 at η = 0.35 V to 3.62 × 10⁶ cm^-2 s^-1 at η = 0.90 V). Application of the classical theory of nucleation over the chronoamperometry results suggested that the free energy of formation of the critical nucleus was lower than room temperature thermal energy. This indicated that the nucleation and growth process was not activation-controlled but rather a kinetically controlled process. Application of Milchev's atomistic theory revealed that every single atom of Co deposited over the GO sheet was a supercritical nucleus that could grow into a cluster irreversibly.