Solvent effects in metal underpotential deposition on single crystal metal surfaces

The underpotential deposition (upd) of Pb and Cd on Ag(111) and Au(111) single crystals grown on freshly cleaved mica has been examined in propylene carbonate (PC), water, tetrahydrofuran (THF), and dimethoxyethane (DME) by cyclic voltammetric techniques. Insight into the cleanliness of the single crystal metal electrode/non-aqueous electrolyte interfaces was gained from a comparison of the potential (E) dependence of the differential capacity (C) in PC solutions with that reported in aqueous media. The cyclic voltammetric features for the Pb Ag(111) and Pb Au(111) upd systems in PC were found to be very similar to those obtained in aqueous media. This behavior was unlike that observed for the upd of Cd on these single crystal substrates in the same two solvents, for which the differences in the voltammetric features, especially in the case of Ag(111), were quite substantial. This latter effect has been ascribed, in part, to the ability of Cd ad-ions to undergo partial discharge on the electrode surface, a factor that changes the solvation characteristics of the adsorbed species and thus modifies the energetics and kinetics of the upd process. For solvents of low dielectric constant, ie THF and DME, the same four upd/substrate systems yielded highly distorted cyclic voltammetry curves. This has been tentatively attributed to extensive ion pairing both at the interface and in the bulk electrolyte. The presence of ion-pairs in the solution phase was evidenced by the lifting of the degeneracy of the v₄ mode of the perchlorate ion (as monitored by transmission FTIR measurements), for which the detailed nature of the band splitting was found to be metal-ion specific.