In surface-enhanced Raman spectra, vibrational peaks are superimposed on a background continuum, which is known as one major experimental anomaly. This is problematic in assessing vibrational information especially in the...

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.

The exceptional properties of liquid water such as thermodynamic, physical, and dielectric anomalies originate mostly from hydrogen-bond networks of water molecules. The structural and dynamic properties of the hydrogen-bond networks...

Surface-enhanced Raman scattering (SERS) is a vibrational scattering spectroscopy that enables selective and sensitive observation of chemical species existing near a metal surface. In particular, SERS is capable of detecting chemical information with high sensitivity at a buried interface such as an electrode/electrolyte interface. Compared to vibrational absorption spectroscopies, SERS can detect low-frequency vibrations in the THz region in addition to high-frequency vibrations in the so-called fingerprint region. Therefore, one can expect to obtain useful chemical information on electrode/electrolyte interfaces from extramolecular vibrations in such a low frequency region [1]. Moreover, we have recently reported that SERS background continuum is ascribed to surface-enhanced electronic Raman scattering, i.e., electronic SERS. This suggests that the background signals can provide electronic information on the metal electrode [2]. In this study, we measured both electronic and vibrational SERS spectra of electrode/electrolyte interfaces under electrochemical potential applications in various pH conditions. A gold electrode was roughened by applying oxidation and reduction cycles in 0.1 M KCl aqueous solution. In-situ SERS spectra for the gold electrode surface were measured in Ar-bubbled aqueous solutions with three different pH conditions under application of electrochemical potentials. A He-Ne laser radiation of 632.8 nm was used to measure SERS signals. Figure shows SERS spectra for the gold surface measured at -0.6, +0.2, and +0.6 V vs. Ag/AgCl in 0.1 M KOH solution. (The measured signal intensities were converted to the susceptibility [2].) In this pH condition, OH⁻ adsorption and surface oxidation occur at around -0.4 V and +0.3 V, respectively, during the positive-going potential scan. For vibrational peaks, the stretching modes of Au-OH⁻ and Au-O clearly showed the potential dependent behaviors, as expected; among these three potentials, νAu-OH- and νAu-O were observed only at +0.2 V and +0.6 V, respectively. Moreover, we found vibrational features in the THz region, which can be attributed to water cluster vibrations. These features decreased in intensity at +0.6 V. On the other hand, for the electronic background continuum, the signal intensity decreased at the potentials of OH⁻ adsorption and surface oxidation during the positive-going scan. This result suggests that surface adsorption-induced electronic change can be detected using electronic SERS. In the poster, we will present a detailed analysis of pH-dependence or anion-dependence of in-situ electronic and vibrational SERS spectra of gold surface. [1] M. Inagaki, K. Motobayashi, K. Ikeda, Electrochemical THz-SERS Observation of Thiol Monolayers on Au(111) and (100) Using Nanoparticle assisted Gap-Mode Plasmon Excitation. J. Phys. Chem. Lett. 8, 4236-4240 (2017) [2] M. Inagaki, T. Isogai, K. Motobayashi, K.-Q. Lin, B. Ren, K. Ikeda, submitted. Figure. SERS spectra of a roughened Au electrode in Ar-bubbled 0.1 M KOH solution under applied potentials of -0.6 V, +0.2 V, and +0.6 V vs Ag/AgCl. Figure 1