An all-electrochemical approach for the synthesis of fine-structured nanoporous copper (np-Cu) films has been developed which involves the initial electrodeposition of Cu-containing alloys followed by a selective dissolution or dealloying of the less noble metal via anodization. This process leaves a nanoporous, three-dimensional spongy structure. Among these alloys are Cu-Zn and Cu-Mn that are commonly used as protective and decorative coatings. These types of coatings are typically rich in Cu; however, precursor alloys for dealloying purposes must contain high atomic percentages (at%) of the less noble metal. Here, we present the controlled electrodeposition of Cu-Zn and Cu-Mn alloys with Zn and Mn contents of ≥ 60 at% and ≥ 70 at%, respectively, as precursors for np-Cu synthesis.

The co-electrodeposition of Cu-Zn and Cu-Mn alloys were achieved in baths containing pyrophosphate and ammonium sulfate, respectively. These electrolytes serve as a source of ligands that form complexes with Cu (i.e. [Cu(P₂O₇)]^2- and [Cu(NH₃)_n]²⁺). The use of the complexing agent is necessary to shift the reduction potential of Cu²⁺ negatively towards that of Zn²⁺ or Mn²⁺, thereby decreasing the large deposition potential difference between the two respective metals. The complexation of Cu in either bath was confirmed by voltammetric and spectroscopic studies.

A systematic potentiostatic electrodeposition study revealed that the optimal plating potential for Cu-Zn is at -2.0 V vs. Hg/HgSO₄ with an efficiency of 63–73%. At this potential, the alloy composition is tunable by adjusting the respective percent compositions of the Cu²⁺ and Zn²⁺ salts in the deposition bath, thereby producing highly crystalline, Zn-rich alloys that are suitable for dealloying purposes (1). For Cu-Mn, galvanostatic deposition at current densities of 100–200 mA⋅cm^-2 gave crystalline deposits with low plating efficiencies (<30%) due to the concurrent hydrogen evolution reaction. Nonetheless, Mn-rich alloys were obtained by also varying the concentration ratio of the Cu²⁺ and Mn²⁺ salts in the ammine bath (2). The optimization of the Cu-Zn and Cu-Mn alloy plating parameters and conditions including the structural and compositional characterization will be discussed in detail.

References:

1. Castillo and N. Dimitrov, “Electrodeposition of Zn-Rich Cu_xZn_(1-x) Films with Controlled Composition and Morphology” J. Electrochem. Soc., 2021, 168, 062513.
2. Castillo and N. Dimitrov, “Electrodeposition of Cu-Mn Films as Precursor Alloys for Nanoporous Cu Synthesis” Electrochem, 2021, 2 (3), 520-533.