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.

In situ changes in the nanofriction and microstructures of ionic liquids (ILs) on uncharged and charged surfaces have been investigated using colloid probe atomic force microscopy (AFM) and molecular dynamic (MD) simulations. Two representative ILs, [BMIM][BF₄] (BB) and [BMIM][PF₆] (BP), containing a common cation, were selected for this study. The torsional resonance frequency was captured simultaneously when the nanoscale friction force was measured at a specified normal load; and it was regarded as a measure of the contact stiffness, reflecting in situ changes in the IL microstructures. A higher nanoscale friction force was observed on uncharged mica and highly oriented pyrolytic graphite (HOPG) surfaces when the normal load increased; additionally, a higher torsional resonance frequency was detected, revealing a higher contact stiffness and a more ordered IL layer. The nanofriction of ILs increased at charged HOPG surfaces as the bias voltage varied from 0 to 8 V or from 0 to −8 V. The simultaneously recorded torsional resonance frequency in the ILs increased with the positive or negative bias voltage, implying a stiffer IL layer and possibly more ordered ILs under these conditions. MD simulation reveals that the [BMIM]⁺ imidazolium ring lies parallel to the uncharged surfaces preferentially, resulting in a compact and ordered IL layer. This parallel "sleeping" structure is more pronounced with the surface charging of either sign, indicating more ordered ILs, thereby substantiating the AFM-detected stiffer IL layering on the charged surfaces. Our in situ observations of the changes in nanofriction and microstructures near the uncharged and charged surfaces may facilitate the development of IL-based applications, such as lubrication and electrochemical energy storage devices, including supercapacitors and batteries.

The nanofrictional behavior of non-halogentated phosphonium-based ionic liquids (ILs) mixed with diethylene glycol dibutyl ether in the molar ratios of 1:10 and 1:70 was investigated on the titanium (Ti) substrate using atomic force microscopy (AFM). A significant reduction is observed in the friction coefficient μ for the IL-oil mixtures with a higher IL concentration (1:10, μ ~ 0.05), compared to that for the lower concentration 1:70 (μ ~ 0.1). AFM approaching force-distance curves and number density profiles for IL-oil mixtures with a higher concentration revealed that the IL preferred to accumulate at the surface forming IL-rich layered structures. The ordered IL-rich layers formed on the titanium surface facilitated the reduction of the nanoscale friction by preventing direct surface-to-surface contact. However, the ordered IL layers disappeared in the case of lower concentration, resulting in an incomplete boundary layers, because the ions were displaced by molecules of the oil during sliding and revealed to be less efficient in friction reduction.