Instantaneous-Progressive nucleation and growth of palladium during electrodeposition

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 2 +0.1 M HCl on Au substrate. The current density(j)-time(t) j )-time( t ) transients approach a maximum current density ( j max ) at a time t max . The current density-time transients are also analyzed using the Scharifker-Hills model for instantaneous and progressive nucleation through (j/jmax)2-t/tmax j / j max ) 2- t / t max plots. At times less than t max , 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 t max Pd deposition is majorly due to instantaneous nucleation. The estimated charge density and the surface coverage by Pd nanostructures up to t max , 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 t max . Beyond t max , 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 t max . 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.