Partial Applicability of Mixed Potential Theory in Unsteady Regime for Cu Electroless Deposition on Pt

Abstract

Electroless deposition (ELD) has been practiced over many decades, but its underlying mechanism is incompletely understood. We explore the applicability of Mixed Potential Theory (MPT) in the unsteady regime during the electroless copper deposition (ELCD) on Pt surface pretreated by dipping Pt in SnCl2 and PdCl2. We demonstrate that ELCD occurs in the low and high potential zone (LPZ & HPZ) at two different reaction rates due to the under-potential deposition of Cu on pretreated Pt in LPZ. The Nyquist plot in HPZ shows one semicircle, while LPZ shows two semicircles. Single semicircle in HPZ suggests dependency only on catalytic potential available on the surface over which ELCD occurs. The two semicircles in LPZ indicate that besides the surface catalytic potential, the dependency is on the reaction intermediate of formaldehyde oxidation, limiting the relevance of MPT. MPT posits that reduction and oxidation in Cu ELD are independent of each other and the role of the surface is only in establishing the catalytic potential. To check the pertinence of MPT, we obtained the polarization resistance (RP) from the Nyquist plot of (a) ELCD solution, (b) CuSO4 with Tartrate and without Formaldehyde i.e. electrochemical copper deposition (ECCD). The RP so obtained shows a significant difference only in LPZ but not in HPZ because of which we argue that MPT is applicable only in the HPZ.