Potential-induced restructuring dynamics of ionic liquids on a gold electrode: Steric effect of constituent ions studied by surface-enhanced infrared absorption spectroscopy

Abstract

The structure of the interface between room temperature ionic liquids (RTILs) and an Au electrode changes with applied potential to compensate the charge on the electrode surface. The dynamics of the restructuring of RTILs triggered by the change in potential has been investigated by using surface-enhanced infrared absorption spectroscopy (SEIRAS). To elucidate the effect of steric hindrance of constituent ions on the potential-dependent dynamics, RTILs composed of differently sized ions are examined: imidazolium cations and bis(perfluoroalkylsulfonyl)amide anions with different (perfluoro)alkyl chain length, or tetrafluoroborate anion. The high interface selectivity and sensitivity of SEIRAS enables in-situ, real time observation of dynamic behaviors of both anions and cations during potential scans or steps. During potential scans for RTILs composed of sufficiently large ions (all the RTILs examined in the present study except for that includes BF4−), a fast, barrier-less change in local ion concentration occurs in the overlayers first, and then a slow cation-anion replacement occurs in the first ionic layer directly contacting with the electrode surface when an overpotential exceeding a critical value is applied. SEIRAS reveals hysteretic changes of the interfacial structure arising from the ion replacement in the first ionic layer. Potential-dependent SEIRAS measurements for RTILs with different ion sizes highlight the significance of steric hindrance of ions for the ion replacement in the first layer.