Theoretical and Experimental Study of Functional Additives in Au(I)-DMH Cyanide-Free Gold Electroplating System

Developing a cyanide-free gold electrodeposition system is significant for replacing the cyanide electroplating gold plating system, which is highly harmful to the environment and toxic to human health. In our study, a cyanide-free system for electrodepositing gold (Au) coatings was proposed with AuCl as the main salt and 5,5-dimethyl hydantoin (DMH) as the Au (I) complexing agent. Through theoretical calculations and electrochemical test screening, 4,6-dimethyl-2-mercaptopyrimidine (DMP) is suitable for DMH-based gold electrodeposition and was selected as an electroplating additive. The role of DMP in gold electrodeposition was further studied. In particular, the adsorption–coordination mechanism was proposed to elucidate the role of DMP, based on calculations of the Bader charge density difference for DMP adsorbed on the gold surface and the interaction between DMP and Au⁺. Electrochemical tests such as cyclic voltammetry and chronoamperometry were utilized to study the impact of varying DMP concentrations on gold electrodeposition behavior. Chronoamperometric analysis indicates that Au electrodeposition obeys a 3D diffusion-controlled progressive nucleation process, independent of DMP concentration and applied potential, while the presence of DMP markedly suppresses grain growth. The addition of DMP significantly improved the surface morphology and brightness of the gold coating, resulting in a smooth and dense layer with a golden-yellow appearance. XRD results showed that the addition of DMP reduced the grain size of the Au coating from 22.1 to 15.7 nm, with the gold coating primarily growing along the Au (111) plane. The XPS test results show that the purity of the gold plating layer is high, and there is no inclusion of additives in the coating.