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.

Rong AN;Yudi WEI;Xiuhua QIU;Zhongyang DAI;Muqiu WU;Enrico GNECCO;Faiz Ullah SHAH;Wenling ZHANG

Herbert Gleiter Institute of Nanoscience,School of Materials Science and Engineering,Nanjing University of Science and Technology,Nanjing 210094,China;High Performance Computing Department,National Supercomputing Center in Shenzhen,Shenzhen 518055,China;Otto Schott Institute of Materials Research(OSIM),Friedrich Schiller University Jena,Jena 07743,Germany;Chemistry of Interfaces,Lule? University of Technology,Lule? 97187,Sweden;School of Mechanical Engineering,Nanjing University of Science and Technology,Nanjing 210094,China

摩擦（英文）

[1]Rong AN;Yudi WEI;Xiuhua QIU;Zhongyang DAI;Muqiu WU;Enrico GNECCO;Faiz Ullah SHAH;Wenling ZHANG-.Ionic liquids on uncharged and charged surfaces:In situ microstructures and nanofriction)[J].摩擦（英文）,2022(11):1893-1912

uncharged,nanofriction,stiffer,+imidazolium,substantiating

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