Nucleation of Sn and Sn–Cu alloys on Pt during electrodeposition from Sn–citrate and Sn–Cu–citrate solutions

The initial stages of Sn and Sn–Cu electrodeposition from Sn–citrate and Sn–Cu–citrate solutions on Pt were studied using both current-controlled and potential-controlled electrochemical techniques. For both Sn–citrate and Sn–Cu–citrate solutions, when the current density is controlled to lower than 15 mA/cm2, potentials remain almost constant which is appropriate to plate dense and uniform films. When the current density is controlled to between 25 and 35 mA/cm2, potentials drop quickly initially, followed by a gradual increase to a constant value. When current density is controlled to higher than 50 mA/cm2, potential oscillation happens, and significant hydrogen evolution prevents the formation of dense and continuous Sn and Sn–Cu films. A constant transition time constant indicates a diffusion-controlled process. The diffusion coefficient calculated from the Sand equation is about 3.8 × 10−6 cm2/s for the Sn–citrate solution and 4.1 × 10−6 cm2/s for the Sn–Cu–citrate solution. The morphology of both Sn and Sn–Cu deposits plated under different potentials was examined by atomic force microscopy (AFM) and the distribution of each element were analyzed using Auger imaging. Analysis of both the electrochemical results at −0.72, −1.1 and −1.5 V and AFM images for both Sn and Sn–Cu deposits at −1.1 and −1.5 V suggested progressive nucleation controlled by diffusion for both Sn and Sn–Cu electrodeposition. Tin reacted with Pt to form PtSn4, and co-deposited with Cu to form Cu6Sn5 during nucleation, with more Sn forming at higher applied potentials.