Identifying Physical and Chemical Contributions to Friction: A Comparative Study of Chemically Inert and Active Graphene Step Edges
Identifying Physical and Chemical Contributions to Friction: A Comparative Study of Chemically Inert and Active Graphene Step Edges
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DOI：
10.1021/acsami.0c08121
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发表时间：
2020-07-01
影响因子：
9.5
通讯作者：
Kim, Seong H.
中科院分区：
文献类型：
--
作者：
Chen, Zhe;Khajeh, Arash;Kim, Seong H.
摩擦有物理和化学两方面的原因。为了区分这些起源和理解它们的综合效应，我们研究了摩擦在石墨烯台阶边缘具有相同的高度和不同的终止化学部分，使用原子力显微镜（AFM）和反应分子动力学（MD）模拟。通过石墨层在环境空气中的物理剥离产生的台阶边缘用羟基（OH）基团封端。在干燥的氮气中，在这个暴露的台阶边缘处用二氧化硅对表面进行测量提供了一个参考，其中物理形貌效应和化学氢键（H-键合）相互作用都是显著的。氢键合，然后抑制在酒精蒸汽环境中进行的AFM实验中，其中在步骤边缘的OH基团被覆盖与物理吸附的酒精分子。最后，埋在另一个石墨烯层下的台阶边缘提供了具有相同高度的化学惰性地形特征。这些系统是由反应MD模拟的OH-终止的步骤边缘上的滑动，烷氧基终止的步骤边缘，或掩埋的步骤边缘。AFM实验和MD模拟的结果表明，在所有情况下，除了掩埋的步骤边缘的步进与降压过程中测量的摩擦滞后。这种滞后的起源被证明是在暴露的步骤边缘，这取决于它们的化学功能的终端组的各向异性偏转。这些发现解释了为什么原子波纹和化学活性表面上的摩擦力很高，这为更广泛地实现超润滑性提供了所需的见解。
Friction has both physical and chemical origins. To differentiate these origins and understand their combined effects, we study friction at graphene step edges with the same height and different terminating chemical moieties using atomic force microscopy (AFM) and reactive molecular dynamics (MD) simulations. A step edge produced by physical exfoliation of graphite layers in ambient air is terminated with hydroxyl (OH) groups. Measurements with a silica countersurface at this exposed step edge in dry nitrogen provide a reference where both physical topography effects and chemical hydrogen-bonding (H-bonding) interactions are significant. H-bonding is then suppressed in AFM experiments performed in alcohol vapor environments, where the OH groups at the step edge are covered with physisorbed alcohol molecules. Finally, a step edge buried under another graphene layer provides a chemically inert topographic feature with the same height. These systems are modeled by reactive MD simulations of sliding on an OH-terminated step edge, a step edge with alkoxide group termination, or a buried step edge. Results from AFM experiments and MD simulations demonstrate hysteresis in friction measured during the step-up versus step-down processes in all cases except the buried step edge. The origin of this hysteresis is shown to be the anisotropic deflection of terminal groups at the exposed step edge, which varies depending on their chemical functionality. The findings explain why friction is high on atomically corrugated and chemically active surfaces, which provides the insight needed to achieve superlubricity more broadly.