Electrodeposition from citrate-based baths with higher concentration of Ni²⁺ and lower concentration of Cu²⁺ is used for both plating of Ni-rich Ni_1-xCu_x alloys and deposition of Cu/Ni thin-film multilayer structures from a single bath. In the latter case, sequential layers of Cu and Ni(Cu) thin films are formed by alternatively switching the deposition potentials. When the concentration of Cu²⁺ is considerably smaller than that of Ni²⁺ in solution (e.g., a few tens of millimolar versus 1M), it is possible to deposit pure copper under diffusion-limited conditions at low overpotentials and then polarise to beyond the onset for Ni deposition to deposit Ni and Cu together. The fraction of Cu in the Ni(Cu) layer can be minimised by having the partial current for Ni deposition several times larger than the diffusion-limited current for Cu deposition. As such, Cu/Ni multilayers have been deposited from boric acid[1] or citric acid[2]-based deposition baths, where the Cu deposition was considered diffusion-limited. However, when designing the citrate-based bath for electroplating in 3D AAO templates, we observed peculiar morphologies, which triggered an in-depth study of the partial copper electrodeposition current. It was found that the passivation film on top of the copper determines the deposition kinetics, with significant effects on copper purity and morphology.

            In this work, a Ni-Cu solution with 0.25 M Na₃Cit from the literature was used as a baseline.[2] The metal ion concentration was maintained, and the concentrations of Na₃Cit were varied. Adding Na₃Cit to a Ni-Cu solution resulted in a significant shift of the Cu²⁺ cathodic reduction peak in a linear sweep voltammetric scan, transitioning from a clear diffusion-limited peak for copper deposition to a peak-less plateau . Furthermore, we investigated the deposition kinetics of Cu²⁺ ions in the solution using a rotating disk electrode setup. This experiment shows faster kinetics for Cu²⁺ reduction in the bath without Na₃Cit compared to the bath that contains Na₃Cit.  The composition of the film was analysed using Secondary Ion Mass Spectrometry (SIMS) and Elastic Recoil Detection (ERD) depth profiling techniques. Based on electrochemical analysis and thin film properties, the proposed deposition mechanism for Cu in a citrate electrolyte system involves the formation of a gel layer near the electrode. This gel layer leads to cathodic passivation, which slows down the kinetics of Cu²⁺ reduction. Moreover, we systematically studied how varying concentrations of Na₃Cit influence the complexation and kinetics of Cu²⁺ electrodeposition and ultimately modelled a mechanism for this single-bath system. Lastly, we developed a 3D interconnected Cu-Ni(Cu) multilayer nanostructure using the 3D AAO template, known as nanomesh. The sequential layers of Cu and Ni(Cu) were deposited inside the AAO template by alternatively switching the deposition potentials.