Department of Applied Physics, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, China.
MIIT Key Laboratory of Semiconductor Microstructure and Quantum Sensing, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, China.
J Phys Chem B. 2021 Oct 14;125(40):11232-11241. doi: 10.1021/acs.jpcb.1c05255. Epub 2021 Oct 1.
Rectification phenomena occurring in asymmetric channels are essential for the design of novel nanofluidic devices such as nanodiodes. Previous studies mostly focus on ion current rectification, while its correlations with water dynamics are rarely explored. In this work, we analyze the transport of water and ions through asymmetric graphene channels under the drive of electric fields using molecular dynamics simulations. A key observation is that the water flux also exists in the rectification phenomenon that follows the ion flux behaviors because of their dynamical coupling relation in electric fields, and both their rectification ratios exhibit maximum behaviors with the change of the channel opening ratio. This is because the ion dehydration is highly asymmetric for small opening ratios. In addition, the cations and anions have distinct rectification ratios that are strongly dependent on the field strength, where the values for anions can even be 1-2 orders larger. This can be attributed to their different hydration shell and dehydration processes in the graphene channel. The translocation time of ions displays a power law relation with the field strength, in agreement with the prediction by Langevin dynamics. Due to the exclude-volume effect, the occupancy of water and ions shows a clear competition and thus changes in an opposite trend with the field strength. Our results demonstrate the rectification correlations between water and ions, and tuning the geometry of graphene channels provides a simple and robust new route to achieve high rectification ratios.
在设计新型纳流道器件（如纳米二极管）时，不对称通道中的整流现象至关重要。先前的研究主要集中在离子电流整流上，而很少探讨其与水动力之间的关系。在这项工作中，我们使用分子动力学模拟分析了在电场驱动下通过不对称石墨烯通道的水和离子传输。一个关键的观察结果是，由于水和离子在电场中存在动力学耦合关系，因此水通量也存在于遵循离子通量行为的整流现象中，并且它们的整流比都表现出随着通道开口比的变化而出现最大值的行为。这是因为对于小的开口比，离子的脱水具有高度的不对称性。此外，阳离子和阴离子具有明显不同的整流比，这强烈依赖于电场强度，其中阴离子的比值甚至可以大 1-2 个数量级。这可以归因于它们在石墨烯通道中不同的水合壳和脱水过程。离子的迁移时间与电场强度呈幂律关系，与 Langevin 动力学的预测一致。由于排斥体积效应，水和离子的占有率表现出明显的竞争，因此随着电场强度的变化呈现相反的趋势。我们的结果表明了水和离子之间的整流相关性，并且通过调整石墨烯通道的几何形状，提供了一种实现高整流比的简单而强大的新途径。
