Dealloying NiCo and NiCoCu Alloy Thin Films Using Linear Sweep Voltammetry

When electrodeposited into thin films, metals have unique and well-known electrochemical potentials at which they will be removed from the film. Theoretically, these potential differences can be utilized to re-oxidize only certain metals in an alloy, thus altering the film’s structure and composition. This dealloying process is understood relatively poorly in the case of nickel-cobalt and nickel-cobalt-copper thin films. Here we discuss these films’ response to linear sweep voltammetry as a means of electrochemical dealloying. A three-electrode electrochemical cell was used for both deposition and dealloying. To perform linear sweep voltammetry on a sample, it was immersed in a sodium sulfate solution in the electrochemical cell and a steadily increasing potential was placed between the working and reference electrodes. For each of four different metal ratios, films were dealloyed to various potentials in order to gain insight into the evolution of the film over the course of the linear sweep. Capacitance, topography, and composition were examined for each sample before and after linear sweep voltammetry was performed. For nickel-cobalt films with high percentages of nickel, dealloying resulted in almost no change in composition, but did result in an increased capacitance, with greater increases occurring at higher linear sweep potentials. Dealloying also resulted in the appearance of large (100-1000 nm) pores on the surface of the film. For nickel-cobalt-copper films with high percentages of nickel, copper was almost completely removed from the film at linear sweep potentials greater than 500 mV. Preliminary data suggested that the linear sweep first removes larger copper-rich dendrites from the film’s surface before creating numerous nano-pores, resulting in a net increase in area