Probing dynamics and ion structuring of imidazolium ionic liquid confined at charged graphene surfaces using graphene colloid probe AFM

Driven by the potential applications of ionic liquids (ILs) flow on charging graphene-based surfaces in many emerging technologies, recent research efforts have been directed at understanding the ion dynamics and structuring at IL–graphene interfaces. Here, graphene colloid probe AFM was used to probe the dynamics and ion structuring of 1-butyl-3-methylimidazolium tetrafluoroborate at graphene surfaces under varying biased voltages. In particular, the AFM-measured nanofriction provides a good measure of the dynamic properties for the IL at graphene surfaces. Compared with the IL at unbiased graphene surface (0 V), the charged graphene surfaces with either negative (–1, –2 V) or positive (+1, +2 V) voltages favor the reduction of friction coefficient by the IL. A higher magnitude of the biased voltage applied on the graphene with either sign (–2 or +2 V) results in a smaller value of friction coefficient than that at –1 and +1 V. In combination of AFM-probed contact stiffness, adhesion forces, ion structuring force curves with the ion orientational distribution by molecular dynamics simulation, we discovered that the unbiased graphene surface (0 V) possesses randomly structured IL ions, and the graphene colloid probe is more likely to get stuck, yielding more energy dissipation to contribute to a larger friction coefficient. Biasing of the graphene surface under either negative or positive voltages resulted in uniformly arranged ions, uniformly arranged ions would be resulted, producing a more ordered ion structure and thus a smoother sliding plane to reduce the friction coefficient. Electrochemical impedance spectroscopy for the IL with graphene as an electrode demonstrated a higher ionic conductivity in the IL paired with the biased graphene than the unbiased one, implying a faster ion movement at the charged graphene, which is beneficial in reduction of friction coefficient.