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 liquid (IL) flow for charging graphene-based surfaces in many emerging technologies, recent research efforts have focused on understanding ion dynamics and structuring at IL–graphene interfaces. Here, graphene colloid probe (GrP) atomic force microscopy (AFM) was used to probe the dynamics and ion structuring of 1-butyl-3-methylimidazolium tetrafluoroborate at graphene surfaces under various bias voltages. In particular, the AFM-measured nanofriction provides a good measure of the dynamic properties of the ILs at graphene surfaces. Compared with the IL at the unbiased graphene surface (0 V), the charged graphene surfaces with either negative (–1, –2 V) or positive (+1, +2 V) voltages favor a reduction in the friction coefficient by the IL. A higher magnitude of the bias voltage applied on the graphene surface with either sign (–2 or +2 V) results in a smaller friction coefficient than that at –1 and +1 V. In combination with the AFM-probed contact stiffness, adhesion forces, and ion structuring force curves with an ion orientational distribution according to molecular dynamics (MD) simulations, we discovered that the unbiased graphene surface (0 V) possesses randomly structured IL ions and that the graphene colloid probe is more likely to become stuck, resulting in 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, which produced a more ordered ion structure and, thus, a smoother sliding plane to reduce the friction coefficient. Electrochemical impedance spectroscopy (EIS) for the IL with graphene as an electrode demonstrated a greater ionic conductivity in the IL paired with the biased graphene than in the unbiased one, implying faster ion movement at the charged graphene, which is beneficial for reducing the friction coefficient.