In-Situ Electronic and Vibrational SERS Observation of Gold Electrodes Under Various pH Conditions

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