Integrated electrochemical, microscopic, and structural analysis of silver electrodeposition from a cyanide-free thiosulfate electrolyte

Silver electroplating typically employs electrolytes containing silver cyanide complexes with an excess of potassium or sodium cyanide. Handling such solutions poses serious risks to both users and the environment, leading to strict regulatory restrictions. Although cyanide-free silver electroplating baths are safer, their use remains limited due to poorer aesthetic outcomes, reduced chemical stability, and significantly higher costs compared to their cyanide-based counterparts. A possible alternative to the commercially available cyanide-based galvanic baths could be represented by the silver thiosulphate complex [Ag(S2O3)2]3−, which is the focus of this paper. The electrochemical properties of the [Ag(S2O3)2]3−solution were investigated through cyclic voltammetry (CV) at various temperatures, pH levels, and concentrations of the thiosulphate complexing agent. Additionally, potentiostatic deposition experiments on glassy carbon were carried out and monitored by scanning electron microscopy to highlight differences in nanoparticle nucleation and growth behavior as a function of the applied potential. At the first cathodic peak (E1 = −0.583 V vs. SCE), sparse nanoparticles nucleate atop existing ones, forming submicron-sized clusters. In contrast, at a more negative potential (E2 = −1.162 V vs. SCE), rapid nucleation across the substrate results in an uneven surface layer with a bimodal particle size distribution. X-ray photoelectron spectroscopy analysis detected sulfur traces on the coatings' top surface. The presence of sulfur is attributed to the molecular precursor or possibly to adventitious sources in the laboratory environment. Given sulfur's high affinity for silver, its incorporation becomes nearly unavoidable. Eventually, the silver electroplating process was tested in a Hull cell on brass plates at different temperatures, pH, and applied currents to determine the ideal current density for obtaining a silver film of good aesthetic quality. Silver layers of superior aesthetic (rms ≈ 20 nm) quality—smoother and more homogeneous—were obtained at a temperature of 25 °C by applying current densities between 0.25 A dm−2 and 0.50 A dm−2 for both solutions of [Ag(S2O3)2]3− prepared at the two different alkaline pH levels, while maintaining the mass ratio of reagents as AgNO3:K2S2O5:Na2S2O3 = 1:1:5.