Surface-Enhanced Electronic and Vibrational Raman Spectroscopy of Ionic Liquid [BMI][PF6] on Au Electrode
Authors: Ueno Ryo , Motobayashi Kenta , Ikeda Katsuyoshi 
Published: December 2024

Room-temperature ionic liquids have gathered great attention across various fields in recent years due to their fascinating physical and chemical properties. These liquids are, for example, expected as a next-generation solvent-free electrolyte. Their structure and dynamics at an electrode interface, where an electrical double layer is formed, should differ from those for conventional dilute electrolyte solutions. Because the performance of ionic liquids as electrolytes is significantly influenced by their behavior in the electrical double layer, many vibrational spectroscopic studies have been conducted to understand their properties at the molecular level. However, the interpretation of vibrational spectrum often suffers from its complicated spectral features. In this study, we observed not only the interfacial structure of ionic liquid on Au electrode but also the surface charge density of Au electrode at the same time using surface-enhanced electronic and vibrational Raman scattering (SERS) spectroscopy in a wide frequency range between 10 and over 2000 cm-1, which can provide a complete picture of the electrical double layer at the ionic liquid/Au interface.
In this study, an imidazolium-based ionic liquid, 1-n-butyl-3-methylimidazoium hexafluorophosphate [BMI][PF6], with a residual water concentration below 5 ppm was used as an electrolyte. A SERS-active Au electrode was prepared by drop-casting a colloidal solution of Au nanoparticles with an average diameter of 100 nm onto the surface of a smooth Au electrode, followed by an electrochemical surface cleaning and vacuum-drying at 120°C for 1 hour. Then, the SERS-active Au electrode and the ionic liquid were sealed in a homemade three-electrode electrochemical Raman cell in an Ar-filled glovebox where H2O were below 0.1 ppm. Prior to SERS measurement, oxidation-reduction cycles were applied within the electrochemical potential window of [BMI][PF6] for further cleaning of the Au electrode surface. SERS spectra were measured with a 632.8 nm He-Ne laser under applied potential. The measured spectra were converted to the SERS susceptibility (χ”SERS) spectra using a conversion method described in previous reports1,2.
Fig. 1 shows a typical SERS spectrum of [BMI][PF6] on Au electrode at open circuit potential, which includes both vibrational sharp peaks (vibrational SERS, vSERS) and a broad electronic background continuum (electronic SERS, eSERS). The observed peaks in the range of 1000 cm-1 to 1600 cm-1 are assigned to vibrational modes of BMI cation. The peak at 740 cm-1 is attributed to the symmetric stretching of PF6 anion. There is also a peak at around 180 cm-1, which can be assigned to the external mode of PF6 anion adsorbed on Au. In the THz region (10 cm-1 to 100 cm-1), there exists a broad feature, which is related to anion-cation interactions. When the applied potential was scanned to the negative direction, significant spectral changes were observed in both vSERS peaks and the eSERS background, which can be interpreted as desorption of PF6 anion, replacement of cation/anion layering structures, increase in the surface charge on Au, etc. These changes were in good agreement in capacitive current responses observed in the cyclic voltammogram.
Reference
(1) M. Inagaki, T. Isogai, K. Motobayashi, K. -Q. Lin, B. Ren, K. Ikeda, Chem. Sci., 11, 9807 (2020)
(2) R. Kamimura, T. Kondo, K. Motobayashi, K. Ikeda, Phys. Status Solidi B, 259, 9, 2100589 (2022)

Figure 1