A single spectroscopic probe for in situ analysis of electronic and vibrational information at both sides of electrode/electrolyte interfaces using surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) at electrode/electrolyte interfaces includes inelastic light scattering not only by molecular vibrations in the electrolyte phase but also by conduction electrons in the metal electrode phase. While the former, i.e., vibrational SERS (VSERS), is widely used to obtain chemical information on electrode surfaces, the latter, i.e., electronic SERS (ESERS), is still under discussion as a possible origin of the SERS background. Given that electronic Raman scattering is essentially sensitive to the surface charge density of a metal, we conducted a thorough comparison of electrochemical potential dependence of SERS signals in both acidic and alkaline media. Significant intensity changes in the SERS background were observed close to the respective potentials of zero charge in acidic and alkaline media, supporting the contention that the generation of the SERS background can be explained by the ESERS mechanism. Moreover, the ESERS intensities, as the SERS background, were reversibly varied by anion adsorption/desorption at the electrochemical interfaces in conjunction with VSERS features originated from surface-adsorbate vibrations. The sensitivity to the surface charge was much higher in this method than in the conventional combined method of reflectance and SERS. In situ monitoring of both chemical and electronic structures at electrode/electrolyte interfaces using a single spectroscopic probe can avoid various experimental uncertainties caused by combined application of different spectroscopic methods leading to facilitation of our deeper understanding of electrode processes.