Codeposition mechanism in Sn/Ag nanoparticle composite plating

Mechanism of the Ag nanoparticle codeposition in Sn/Ag–nanoparticle composite plating was studied using a rotating disk electrode. The bath chemistry of the Ag–nanoparticle/Sn2+–pyrophosphate colloidal solutions and the kinetics of the Sn matrix deposition were also discussed. The dialysis experiments of the plating baths showed that the composition of the Ag nanoparticles were stoichiometric, AgSn0.54+(NP)AgSn0.54+(NP), and that their structure was Ag core/Sn4+–pyrophosphate complex shell type. The current density–potential relationships of the Sn matrix deposition showed a two-step features. The first current plateau at around −1.4 V vs. Ag/AgCl was related to the diffusion limited current of SnP2O72−. The Ag codeposition rate was controlled by Tafel kinetics and reached the diffusion limited values, showing that the codeposition rate of the AgSn0.54+(NP)AgSn0.54+(NP) is controlled by electrochemical reactions. The electrochemical kinetic parameters and the diffusion coefficient of the AgSn0.54+(NP)AgSn0.54+(NP) were estimated by fitting the experimental data to the Tafel equation taking into account the concentration of the electroactive species at the cathode. The diameter of the AgSn0.54+(NP)AgSn0.54+(NP) was calculated to be 4.2 nm by substituting the diffusion coefficient estimated here into the Einstein–Stokes equation. The diameter was agreed with the results from the transmission electron microscope examination and the small angle X-ray scattering analysis.

► Concentration ratio of Sn2+ complexes in Sn2+–P2O74− solutions was determined.
► The 1st plateau in 2-step I–E curves of Sn deposition is related to SnP2O72− reduction.
► Ag nanoparticles have AgSn0.5 stoichiometric composition and Sn4+–pyrophosphate shell.
► Ag–nanoparticle codeposition rate showed an electrochemical nature.
► Diameter of Ag nanoparticles was appropriately estimated from diffusion coefficient.