Kinetics and Mechanism of Electroless Silver Deposition on a Polyurethane Substrate at Moderate to High Concentration of Metal Precursor and Reductant

The electrochemical kinetics and mechanism of the electroless deposition of silver were studied in varying concentrations of [Ag(NH₃)₂]⁺ and [N₂H₄]—within moderate to high. The partial cathodic and anodic polarisation curves obtained from the steady state dual cell as well as under the potentiodynamic conditions were superimposed to generate the mixed potentials and the mixed currents. The real rate of silver deposition was determined gravimetrically and then compared with the mixed current obtained under the same bath conditions. The current values of mA/cm² obtained from the mass deposited in gravimetric study and those of the mixed current were close to each other. At moderate [Ag⁺] of 0.001 M and [N₂H₄] of 0.1 M, the position of the mixed potential was in the Tafel region of both cathodic and anodic polarization curves. The mechanism of the reaction was therefore indicated to be under a mixed control. At higher [Ag⁺] of 0.025 M and [N₂H₄] of 1 M, the mixed potential was located in the cathodic Tafel and anodic limiting current regions. The mechanism was therefore under the anodic diffusion control. The analysis of the partial polarization plots and the values of the mixed potential confirmed that irrespective of [Ag⁺] the cathodic reaction was under an activation control, and the anodic reaction was shifted from the activation to the diffusion control mechanism by varying moderate to high [N₂H₄]. The deposits were characterised by the X-ray diffraction and scanning electron microscopy to confirm that there was the formation of agglomerated Ag particles with crystal faces of (111), (200), and (311) planes and a uniform distribution of the nano spherical clusters of silver deposit. The agglomerated particles were of a relatively higher size in the electroless bath containing very high levels of [Ag⁺] and [N₂H₄] than when both ions were at a moderate level of concentration.