School of Materials Science and Engineering/Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology, Nanjing 210094, China.
Department of Engineering and Design, School of Engineering and Information, University of Sussex, Brighton BN1 9RH, U.K.
Langmuir. 2023 Jun 20;39(24):8463-8474. doi: 10.1021/acs.langmuir.3c00710. Epub 2023 Jun 8.
Surface chemistry plays a critical role in the ion structuring of ionic liquids (ILs) at the interfaces of electrodes and controls the overall energy storage performance of the system. Herein, we functionalized the gold (Au) colloid probe of an atomic force microscope with -COOH and -NH groups to explore the effect of different surface chemical properties on the ion structuring of an IL. Aided by colloid-probe atomic force microscopy (AFM), the ion structuring of an imidazolium IL, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF], abbreviated as BP hereafter), on the Au electrode surface and the ion response to the change in the surface chemistry are investigated. AFM morphologies, contact angles, and approaching force-distance curves of the BP IL on the functionalized Au surfaces exhibited that the IL forms a more obvious layering structure on the -COOH-terminated Au surface (Au-COOH), while it forms heterogeneous and aggregating droplets on the -NH surface (Au-NH). The formed uniform and aggregation-free ion layers in the vicinity of the Au-COOH surface are due to the π-π stacking interaction between the delocalized π electrons from the imidazolium ring in the IL [BMIM] cation and the localized π electrons from the sp carbon on the -COOH group. The observation of nano-friction and torsional resonance frequency at the IL-electrode interfaces further demonstrated the ion structuring of the IL at Au-COOH, which results in a more sensitive electrochemical response associated with a faster capacitive process.
表面化学在离子液体（ILs）在电极界面的离子结构中起着关键作用，并控制着系统的整体储能性能。在此，我们通过 -COOH 和 -NH 基团功能化原子力显微镜（AFM）的金（Au）胶体探针，探索不同表面化学性质对 IL 离子结构的影响。借助胶体探针原子力显微镜（AFM），研究了离子液体 1-丁基-3-甲基咪唑六氟磷酸盐（[BMIM][PF6]，简称 BP）在 Au 电极表面的离子结构以及离子对表面化学性质变化的响应。BP IL 在功能化 Au 表面的 AFM 形貌、接触角和接近力-距离曲线表明，IL 在 -COOH 端接的 Au 表面（Au-COOH）上形成更明显的分层结构，而在 -NH 表面（Au-NH）上形成不均匀和聚集的液滴。在 Au-COOH 表面附近形成的均匀且无聚集的离子层归因于 IL [BMIM]+阳离子中离域的π 电子与 -COOH 基团上的 sp 碳的局域π 电子之间的π-π 堆积相互作用。在 IL-电极界面处观察到纳米摩擦和扭转共振频率进一步证明了 IL 在 Au-COOH 上的离子结构，这导致与更快的电容过程相关的更敏感的电化学响应。
