Electrochemical nucleation and growth of Pd–Ag alloy nanoparticles onto a bare and a conducting polymer-modified glassy carbon electrode from ethaline deep eutectic solvent

Abstract

This work investigates, for the first time, the mechanisms and kinetics of the electrochemical formation of Pd–Ag alloy nanoparticles (Pd–Ag ANPs) on both, bare glassy carbon electrodes (GCE) and polypyrrole-modified GCEs (GCE/Ppy), using a precursor solution dissolved in a eutectic mixture of choline chloride and ethylene glycol, also known as ethaline, deep eutectic solvent (DES). The study demonstrates that the ethaline DES serves as a suitable electrolyte for the synthesis of both Pd–Ag ANPs and polypyrrole. Furthermore, it was found that the potentiostatic electrodeposition of Pd–Ag ANPs onto GCE and GCE/Ppy involves multiple three-dimensional (3D) nucleation events and mass transfer–limited growth of bimetallic clusters. These initial stages are accompanied by adsorption phenomena and electrochemical double-layer charging. The presence of polypyrrole (Ppy) on the GCE surface enhanced significantly the Pd–Ag nucleation frequency, approximately 100 times, compared with the bare GCE under the same applied potential. The simultaneous deposition of Pd–Ag nuclei and the Ppy layer formation on the GCE surface were occurrences experimentally confirmed and the materials characterized using a combination of surface analysis techniques. SEM images revealed Pd–Ag alloy nanoparticles (ANPs) with diameters of approximately 60 nm, along with some larger nanostructured quasispheroidal aggregates about 600 nm in diameter. On the GCE/Ppy electrode, solely Pd–Ag ANPs with diameters of around 50 nm were uniformly distributed across the surface; however, no aggregates were formed. In both cases, EDX elemental mapping showed a strong spatial overlap of Pd and Ag signals, indicating formation of an alloy compositionally homogeneous, which was further corroborated by the XRD analysis. XPS measurements confirmed the elemental composition of the deposits, revealing weight percentages of 25% Pd and 75% Ag, closely matching the composition of the electrolytic bath.