Copper (II)-Lactate Complexes in Cu₂O Electrodeposition Baths: Stability of the Complexes and Aging of the Baths

The electrodeposition of Cu₂O, represented by the reaction 2Cu(II) + 2OH− +2e → Cu₂O + H₂O in alkaline baths containing concentrated lactic acid (approximately 3 mol dm⁻³) as ligands, has been widely studied. In a previous study, such baths were found to generate the complexes [Cu(H₋₁L)L]⁻ and [Cu(H₋₁L)₂]²⁻ (where L⁻ = CH₃CH(OH)COO⁻ and H₋₁L⁻ = CH3CH(O⁻)COO⁻) in a manner dependent on the bath pH in the alkaline region. This work first assessed the stability of these Cu(II)-lactate complexes based on computational chemistry. The formation of these complexes typically requires approximately one day to reach thermodynamic equilibrium, suggesting that other kinetically favorable, metastable complexes may form prior to the equilibrium complexes. Based on titration curves, [CuL₂(HL)]⁰ was identified as the most likely such intermediate and the effect of this complex on deposition orientation was examined. Electrodeposited Cu₂O obtained from a bath not yet at equilibrium with a pH of approximately 9.5 was randomly oriented whereas deposits from an equilibrated bath showed a preferential 〈100〉 orientation. These findings suggest that bath aging is an essential step in achieving reproducible Cu₂O electrodeposition. Additionally, modifying the bath preparation method was found to prevent the formation of metastable complexes.