Probing Dielectric Relaxation of Water at Electrochemical Interfaces with Low-Frequency SERS Spectroscopy
Authors: Ikeda Katsuyoshi 
Published: February 2024

Dynamic and structural properties of hydrogen-bonded water molecules in liquid phase have a significant impact on many chemical and biological processes in aqueous systems. Therefore, the bulk behavior of water is quite actively studied from both experimental and theoretical point of view. In the frequency domain, collective motions of water molecules appear in the terahertz (THz) range. For example, the central frequency of Debye-type dielectric relaxation of water exists at around 1 cm-1, and that of the so-called fast Debye relaxation is found at around 10 cm-1. There are several spectroscopic means to obtain such low-frequency information in the bulk liquid of water. However, for interfacial water molecules interacting with a solid surface, their dynamic behavior of hydrogen-bond networks remains unexplored in the experimental research field, despite of the importance of such behavior to molecular-level understanding of heterogeneous electrochemical processes. The reason for lack of such information is due to lack of adequate surface-selective spectroscopic means. In this talk, in-situ observation of the relaxational and vibrational dynamics of water at electrochemical interfaces is demonstrated using low-frequency surface-enhanced Raman spectroscopy (SERS). In this spectroscopic method, not only intermolecular dynamics but also intramolecular vibrational behaviors of water can be simultaneously monitored due to the intrinsic nature of the inelastic scattering technique accessible in the wide frequency range covering both THz and mid-IR regions, which helps us to understand the relation between microscopic and macroscopic behaviors of water molecules.
To obtain low-frequency SERS spectrum, volume Bragg grating filters were used for both monochromatization of He-Ne laser radiation of 632.8 nm and removal of Rayleigh scattered light [1]. SERS-active Au electrodes were prepared by electrochemical surface roughening of Au surface in 0.1M KCl solution. For SERS-active Pt electrodes, monoatomic Pt overlayers were formed on the roughened Au surfaces. SERS spectra for interfacial water 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, which can unveil vibrational features in low-frequency range [2-5].
The measured spectra clearly showed potential-dependent features in the THz region. According to the previously reported spectra for bulk water, these features were assigned to the fast Debye relaxation component and two intermolecular vibration modes of interfacial water layer. By analyzing the observed spectral features by the assistance of classical molecular dynamics (MD) simulations, we found that the potential-induced change of water orientation causes formation of local defects in hydrogen-bonded networks of interfacial water layer and the lateral mobility of such defects on negatively charged surface is larger than that on positively charged surface [6]. Figure shows the dynamical behavior of potential-induced defects in hydrogen-bonded networks of interfacial water molecules on negatively and positively charge surfaces. The details will be discussed in the presentation.
[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