Halide-Dependent Dealloying of Cu_x/Au_y Core/Shell Nanoparticles for Composition Analysis by Anodic Stripping Voltammetry

Here, we describe the composition analysis of 5–7 nm diameter citrate-stabilized Cu/Au core/shell nanoparticles with composition ratios of Cu₁/Au_x (x = 0.001–2) attached to amine-functionalized glass/indium-tin-oxide electrodes using anodic stripping voltammetry (ASV) in acidic KBr or KCl electrolyte solutions. One peak typically appears for Au oxidation, which is at more positive potential than the peaks for Cu. There are typically two peaks for Cu oxidation due to different bonding environments for Cu (Cu–Cu and Cu–Au bonds). The Au and Cu peak positions and size in the ASVs at different compositions are consistent with the formation of a Cu/Au core/shell structure. For low quantities of Au in the alloy (x < 1), the Au oxidation peak included the oxidation of trapped Cu in KBr solution, but there was no trapped Cu in the Au oxidation peak in KCl solution, making composition analysis straightforward in KCl by simply integrating the peaks for Cu and Au separately. Atomic rearrangement of Au due to strong adsorption of Br^– is likely responsible for the stabilized Cu in KBr. For larger amounts of Au (x ≥ 2), most of the Cu is trapped inside the outer Au shell and therefore oxidizes along with the Au in both KCl and KBr solutions, making direct composition analysis impossible in both halides for high Au content.