The mechanism of electrolytic metal deposition

Abstract

Possible mechanisms of electrolytic metal deposition from aqueous solutions of cupric, nickelous and silver ions are examined by deriving and comparing the relevant potential energy profiles with the purpose of indicating the rate-determining step. The following steps in the overall reaction are considered: ionic transfer from the solution to surface sites; surface diffusion of adsorbed ions; successive dehydration of the adions at lattice building sites. The formation of adions and adatoms in intermediate steps is distinguished and it is shown that neutral adatom formation is unlikely. The heat of activation (∆H 0‡ ) for transfer of ions from the solution to the metal surface, depends upon the site to which transfer occurs, that to a planar site being significantly less than that to other sites (e. g. edges, kinks, etc.). Transfer to form adatoms has prohibitively high ∆H 0‡ values. Lattice building is accomplished by surface diffusion of transferred adions to edges or kinks. At each of the consecutive stages of the deposition process, a change in hydration of the ion or adion occurs and has important effects on the mechanism and kinging of deposition. Direct deposition of divalent ions on to surface sites is shown to be associated with high heats of activation and low electrochemical rate constants as found experimentally for Ni 2+ , Fe 2+ and Co 2+ deposition. When the deposition reaction can occur through an intermediate redox step involving a stable ion as, e. g. in the case of copper, it is shown that lower free energies of activation can result than if the deposition of the divalent ion occurs in one step. This is consistent with the experimental behaviour found for copper deposition. At low overpotentials, it is shown that both the steps of ionic transfer and surface diffusion would have comparable rate constants in the case of copper and silver deposition whilst at higher overpotentials, the reduction of Cu 2+ ions becomes rate determining at copper, whilst ionic transfer of Ag + ions becomes rate determining at silver. This is in agreement with the behaviour observed experimentally.