Vibrational Spectroscopy of Ionic Liquids Electrochemically Intercalated into Multilayer Graphene

We report insights into the reversible electrochemical intercalation of ionic liquids into multilayer graphene (MLG) using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. The studied device is comprised of an MLG/alumina membrane/copper stack, where the nanoporous alumina membrane is filled with ionic liquid [DEME⁺][TFSI^-], forming a compact electrochemical cell. Upon application of a positive voltage, [TFSI^-] anions intercalate into the interlayer regions of the MLG, despite the anion's diameter (0.9 nm) being nearly 3 times the typical graphene interlayer spacing (0.355 nm). Pronounced spectral changes accompany this poorly understood intercalation. We observe a blue-shift of up to 21.2 cm^-1 in several [TFSI^-] vibrational modes, attributed to mechanical compression within the confined graphene layers and/or ion-ion interactions. Additionally, an infrared peak emerges at 1384 cm^-1, corresponding to the symmetric bending mode of methyl (-CH₃) groups, whose appearance suggests that symmetry breaking within the confined electrochemical environment activates otherwise forbidden transitions in the [DEME⁺] cation. These findings reveal the nanoscale structural and electronic perturbations induced by ionic liquid intercalation, identifying spectroscopic signatures to track intercalation dynamics in layered materials. Raman shifts observed in the graphene indicate doping levels on the order of 10²¹ cm^-1, corresponding to a roughly 100-fold increase in free carrier concentration, thus providing evidence consistent with intercalation. However, these observations challenge our previous interpretation of the complete intercalation of ionic liquids into graphene. We additionally used density-functional tight-binding (DFTB) simulations to qualitatively determine the behavior of the [TFSI^-] anion sandwiched between two graphene sheets for different separation distances from 7 to 10 Å with a 0.5 Å increment. The resulting frequency shifts at smaller separation distances exhibit qualitative agreement with experimental observations, and in the case of greater separation, the peak shifts diminish and plateau, transitioning toward the bulk anion.