Structures and terahertz dynamics of an imidazolium-based ionic liquid on a gold electrode studied using surface-enhanced Raman scattering†

Room-temperature ionic liquids (RTILs) have been extensively studied for decades as a potential electrolyte alternative. The structural and dynamic properties of RTILs in an electrical double layer (EDL), formed at the electrode/electrolyte interface, differ from those in their bulk phase. Despite the importance of the interfacial properties of RTILs in electrical devices, such behaviours of denser ion structuring in EDL are poorly understood owing to the delicate anion–cation interplay among various intermolecular interactions such as Coulombic interaction, hydrogen bonding interaction, and π-type interaction. Herein, an advanced spectroscopy technique based on surface-enhanced Raman scattering (SERS) is used to achieve simultaneous observation of electronic, chemical, and terahertz-dynamic behaviours of an imidazolium-based ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMI]PF₆), on a gold electrode. Physisorption and desorption of PF₆ anions on the Au surface are of particular importance in understanding the hysteresis behaviour of potential-induced anion–cation replacement in the EDL. It is also highlighted that the restructuring dynamics of the EDL is correlated with the enhancement of translational terahertz motions among cations and anions on a charged surface.