Influence of surface condition on the current densities rendering nucleation loop during cyclic voltammetry for electrodeposition of Pd thin films

Abstract

ABSTRACT Cyclic voltammetry (CV) for Pd electrodeposition on Au from PdCl2+HCl solution between +1.100 and –0.400 V vs Ag/AgCl (3M NaCl) was performed. The CVs exhibit nucleation loop (NL), which can be characterized by higher cathodic current density (j) in the reverse scan rather than that in the forward scan. The reasons for the appearance of NL are investigated by making use of the surface condition of electrode and overpotential (η). Since it is difficult to estimate η during CV, it was obtained by potentiostatic electrodepositions (ED) on the substrates with similar surface conditions as those within NL. These substrates were produced by a potential sweep in (i) forward (FS) and (ii) forward+reverse (RS) scans from open circuit potential up to a representative potential (+0.330 V) within NL. These FS and RS substrates possess partial and significant Pd coverage, respectively. ED at +0.330 V on these substrates shows similar j trends in the initial stages as in NL. η during ED is estimated from the equilibrium potentials and the potential drop due to solution resistance (Rs). Rs is obtained from electrochemical impedance spectroscopy at +0.330 V. The estimated η is higher on RS, supporting higher j during electrodeposition on RS and reverse scan of CV within NL. Electrochemical and morphological analyses suggest that Pd deposition on the FS is nucleation driven whereas that on the RS is growth driven with the surface condition playing an important role in the appearance of NL during CV.