In-Situ Low-Frequency SERS Observation at Various Electrochemical Interfaces
Authors: Ikeda Katsuyoshi 
Published: September 2023

Surface-selective vibrational spectroscopy, which is a powerful tool to study electrochemical interfaces, is typically operated in the mid-infrared region; chemical information obtained can give deep insights into electrochemical reactions at the molecular level. However, when the vibrational frequency range is extended into far-infrared or terahertz (THz) region, one can expect to obtain additional information about the interfaces because external vibrations such as surface-adsorbate vibrations and intermolecular vibrations appear in this range. Unfortunately, most of vibrational methods using THz radiation are not suitable for surface-selective observation of buried electrochemical interfaces. On the other hand, surface-enhanced Raman scattering (SERS) is potentially capable of detecting low-frequency vibrations at interfaces. Recently, the detectable frequency range of SERS has been extended into the low-frequency region down to 10 cm-1 [1], and then a vibrational analysis procedure for such low-frequency region has also been developed using a density of states format of SERS spectrum [2-4]. Using this method, for example, we have simultaneously observed both chemical information and molecular mass of surface adsorbates at interfaces [5]. In this talk, we focus on intermolecular interactions such as hydrogen bonding and anion-cation interactions at electrochemical interfaces. We present in-situ low-frequency SERS observation at various electrochemical interfaces under potential control [6].
To obtain low-frequency SERS spectra, volume Bragg grating filters were used for both monochromatization of He-Ne laser radiation of 632.8 nm and removal of Rayleigh scattered light. SERS-active Au electrodes were prepared by electrochemical surface roughening in 0.1M KCl solution. For SERS-active Pt electrodes, monoatomic Pt overlayers were formed on the roughened Au surfaces. SERS spectra for both aqueous electrolyte solutions and ionic liquid electrolyte solutions were measured on these electrode surfaces under application of electrochemical potentials. The measured spectra were converted to density states of formats by reducing the Purcell factor, Bose-Einstein thermal factor, and frequency factor.
Fig. 1 shows measured and reduced SERS spectra for 0.1M H2SO4/Au interface in the low-frequency region covering both Stokes and anti-Stokes branches. In the measured spectrum, vibrational features are not recognized because the huge background continuum dominates in the spectrum. This background continuum, caused by plasmon-enhanced electronic Raman scattering in the conduction band of Au, can be reduced in the density states of format, leading to considerable improvement in analyzability of vibrational features. Indeed, THz vibrational features are clearly unveiled in the reduced spectrum. Incidentally, the symmetry of the response between the Stokes and anti-Stokes branches in the reduced spectrum indicates that the SERS spectrum is properly converted to the reduced form because the Stokes and anti-Stokes Raman processes are connected due to the time reversal symmetry. Potential-dependence of these THz vibrational features will be discussed in the talk.
[1] M. Inagaki, K. Motobayashi, K. Ikeda, J. Phys. Chem. Lett., 8, 4236-4240 (2017).
[2] M. Inagaki, T. Isogai, K. Motobayashi, K. -Q. Lin, B. Ren, K. Ikeda, Chem. Sci., 11, 9807-9717 (2020).
[3] M. Inagaki, K. Motobayashi, K. Ikeda, Nanoscale, 12, 22988-22994 (2020).
[4] R. Kamimura, T. Kondo, K. Motobayashi, K. Ikeda, Phys. Status Solidi B, 259, 2100589 (2022).
[5] T. Kondo, M. Inagaki, K. Motobayashi, K. Ikeda, Catal. Sci. Technol., 12, 2670-2676 (2022).
[6] T. Isogai, M. Inagaki, M. Uranagase, K. Motobayashi, S. Ogata, K. Ikeda, submitted.

Figure 1