In-situ STM and AFM Studies on Electrochemical Interfaces in imidazolium-based ionic liquids

Abstract

Voltammetric responses, adsorption behaviors and layered structures of 1-ethyl-2, 3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide (EMMITFSI) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) at Au (111) have been investigated by cyclic voltammetry, in-situ STM and in-situ AFM force curve measurements, respectively. Cyclic voltammetry measurements pointed out that two pairs of sharp redox peaks are located at around 0.1 and 0.7 V in EMMITFSI, but not in EMITFSI. In-situ STM studies revealed no etching of the Au (111) surface in the investigated potential region in EMMITFSI, but an etched Au (111) surface was observed in EMITFSI in the same potential region, indicating that the interaction between EMITFSI and the Au (111) surface is much stronger than that between EMMITFSI and the Au (111) surface. In-situ AFM force curve measurements showed three layered structures at Au (111)/EMITFSI interface, but no force curves with saw-toothed feature can be obtained when the potential is higher than +0.5 V, which implies the disappearance of layered structures. In EMMITFSI, force curves with saw-toothed feature can be obtained even at +1.2 V. The fact that layered structures can form in a broader potential region indicates that a stronger interaction exists between the EMMI cation and the TFSI anion. It is inferred that the methylation of the hydrogen position provides a configuration for EMMI cations with an increased hydrophobic interaction, which maintains the layered structure in wider potential region. As a consequence, the electrode potential may induce structural transition of the layered structures, which is responsible for the origin of the voltammetric features.