Instantaneous-Progressive nucleation and growth of palladium during electrodeposition

Abstract

Potentiostatic electrodeposition of palladium (Pd) was performed at −0.380, −0.280, −0.230, −0.210 and −0.180 V overpotentials in 56 mM PdCl2+0.1 M HCl on Au substrate. The current density(j)–time(t) transients approach a maximum current density ( jmax) at a time tmax. The current density–time transients are also analyzed using the Scharifker–Hills model for instantaneous and progressive nucleation through (j/jmax)2-t/tmax plots. At times less than tmax, the experimental Scharifker–Hills plots are well within the Scharifker–Hills​ guidelines as prescribed by the model for both instantaneous and progressive nucleation. Prior to tmax Pd deposition is majorly due to instantaneous nucleation. The estimated charge density and the surface coverage by Pd nanostructures up to tmax, as validated by Scanning Electron Microscopy analysis, decrease with overpotential. All the deposits exhibit only partial coverage (20%–30% at −0.380 V; and 50%–60% at the other potentials) up to tmax. Beyond tmax, the deposition proceeds through progressive nucleation on the uncovered surface and through 3D and planar diffusional growth. The experimental Scharifker–Hills plots deviate from the corresponding guidelines beyond tmax. Moreover, the results from Electrochemical Impedance Spectroscopy performed at the deposition potentials exhibit mixed charge and mass transport contribution at the electrolyte–substrate interface. The overpotential leading to the mixed charge and mass transport is attributed as the reason for the deviation from the Scharifker–Hills model.