Electroplating behavior of a phosphorous-based cyanide-free silver electrolyte in an acidic environment

Abstract

This study entailed the development of a novel acidic silver-plating solution using phosphorus-based compounds. A systematic investigation was conducted to optimize the effects of the complexing agents tris(hydroxypropyl)phosphine (THPP), pyrophosphoric acid (PPA), and phosphoric acid (PA) electrolytes on silver reduction and solution stability. Linear sweep voltammetry demonstrated the stabilizing effect of THPP on Ag ions, revealing cathodic polarization of the Ag reduction potential at [Ag⁺]:[THPP] ratios ranging from 1:1 to 1:5. Notably, the 1:4 ratio exhibited the lowest reduction potential, indicating that the [Ag(THPP)₄]⁺ complex formed under these conditions was the most stable, a conclusion supported by density functional theory (DFT) calculations. Furthermore, the optimal concentration of the plating solution was determined by analyzing current density variations with respect to the concentrations of PPA and PA. The Scharifker–Hills model, energy level analysis using DFT, and cyclic voltammetry elucidated the impact of electrolyte–proton interactions in influencing hydrogen evolution, thereby revealing distinct nucleation mechanisms for Ag reduction in each electrolyte. This study establishes a THPP-based acidic plating solution capable of achieving stable Ag deposition under acidic conditions by optimizing the conditions for complexing agents and electrolytes. These findings suggest the possibility of substituting cyanide-based plating solutions in applications requiring silver plating in acidic environments, potentially enhancing precise plating processes such as pattern plating.