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

Abstract

Room-temperature ionic liquids (RTILs) have been the focus of extensive studies as a potential electrolyte alternative in decades. Structural and dynamic properties of RTILs differ in an electrical double layer (EDL), formed at electrode/electrolyte interfaces, from those in their bulk phase. Despite the importance of the interfacial properties of RTILs in electrical devices, such denser ion structuring behaviours are poorly understood due to the delicate anion-cation interplay among various intermolecular interactions such as Coulombic interaction, hydrogen bonding interaction, π-type interaction, etc. Here, an advanced spectroscopy 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 Au surface are of particular importance in undestanding the hysteresis behaviour of potential-induced anion-cation replacement in the EDL. It is also highlighted that the restructuring dynamics of EDL is correlated with enhancement of translational terahertz motions among cations and anions on a charged surface.