Surface-enhanced Raman scattering and electrochemical characterization of metal electrode-electrolyte interfaces in nonaqueous systems

摘要

Many important and interesting processes, which are essential components of electrochemical technologies, occur at the electrode/electrolyte interface. Using spectroscopic and electrochemical methods, this research elucidates the interfacial structure in nonaqueous solvents existing at metal electrodes. The behavior of trace interfacial H₂O and Li⁺ and Br⁻ ions at Ag electrodes in the normal alcoholic solvents methanol, ethanol, propanol, butanol and pentanol is examined using surface-enhanced Raman scattering (SERS) and differential capacitance measurements. SERS spectra in the ν(O-H) region exhibit four bands from interfacial H₂O molecules which are a sensitive function of the nature of the solvent, the residual H₂O concentration, and the electrode potential. The results of this study indicate that in the interface, H₂O molecules cluster around Li⁺ and Br⁻ ions and are not homogeneously dispersed. Br⁻ specific adsorption and interfacial solvent behavior in the series of alcohol solvents are interpreted quantitatively through differential capacitance and SERS experiments. Br⁻ surface coverage is calculated using the "Hurwitz-Parsons" analysis of differential capacitance-potential data in these media. These results indicate that almost a monolayer of Br⁻ adsorb on Ag electrodes, but much less Br⁻ ions adsorbs on Au electrodes at positive potentials. The potential of zero charge (pzc) is also quantitatively estimated from these measurements. The technique of "emersion" of an electrochemical interface from electrolyte has opened the door to new possibilities for fundamental electrochemical studies. The relationship between in-situ and emersed electrochemical systems is probed through the study of the interfacial structure of dimethyl sulfoxide (DMSO) on Ag electrodes. The results of this study indicate that pronounced restructuring of the DMSO in the interface is found in the vicinity of the pzc. The solvent orientation is generally retained upon emersion, but a negative emersion-induced potential shift in the interface is observed due to the reorientation of DMSO molecules at gas/solution boundary phase. These data represent the first spectroscopic evidence for the existence of this emersion-induced potential shift.