Sharma Bharat Bhushan, Govind Rajan Ananth
Department of Chemical Engineering, Indian Institute of Science, Bengaluru, Karnataka 560012, India.
J Phys Chem B. 2022 Feb 17;126(6):1284-1300. doi: 10.1021/acs.jpcb.1c09287. Epub 2022 Feb 4.
To fulfill the increasing demand for drinking water, researchers are currently exploring nanoporous two-dimensional materials, such as hexagonal boron nitride (hBN), as potential desalination membranes. A prominent, yet unsolved challenge is to understand how such membranes will perform in the presence of defects or surface charge in the membrane material. In this work, we study the effect of grain boundaries (GBs) and interfacial electrostatic interactions on the desalination performance of bicrystalline nanoporous hBN using classical molecular dynamics simulations supported by quantum-mechanical density functional theory (DFT) calculations. We investigate three different nanoporous bicrystalline hBN configurations, with symmetric tilt GBs having misorientation angles of 13.2, 21.8, and 32.2°. Using lattice dynamics calculations, we find that grain boundaries alter the areas and shapes of nanopores in bicrystalline hBN, as compared to the nanopores in monocrystalline hBN. We observe that, although bicrystalline nanoporous hBN with a misorientation angle of 13.2° shows an improved water flow rate by ∼30%, it demonstrates reduced Na ion rejection by ∼6%, as compared to monocrystalline hBN. We also uncover the role of the nanopore shape in water desalination, finding that more elongated pores with smaller sizes (in 21.8- and 32.2°-misoriented bicrystalline hBN) can match water permeation through less elongated pores of slightly larger sizes, with a concomitant ∼3-4% decrease in Na rejection. Simulations also predict that the water flow rate is significantly affected by interfacial electrostatic interactions. Indeed, the water flow rate is the highest when altered partial charges on B and N atoms were determined using DFT calculations, as compared to when no partial charges or bulk partial charges (i.e., charged hBN) were considered. Overall, our work on water/ion transport through nanopores in bicrystalline hBN indicates that the presence of GBs and surface charge can lead, respectively, to a decrease in the ion rejection and water permeation performance of hBN membranes.
为满足日益增长的饮用水需求，研究人员目前正在探索纳米多孔二维材料，如六方氮化硼（hBN），作为潜在的脱盐膜。一个突出但尚未解决的挑战是了解这种膜在膜材料存在缺陷或表面电荷的情况下将如何表现。在这项工作中，我们使用量子力学密度泛函理论（DFT）计算支持的经典分子动力学模拟，研究了晶界（GBs）和界面静电相互作用对双晶纳米多孔hBN脱盐性能的影响。我们研究了三种不同的纳米多孔双晶hBN构型，对称倾斜晶界的取向差角分别为13.2°、21.8°和32.2°。通过晶格动力学计算，我们发现与单晶hBN中的纳米孔相比，晶界改变了双晶hBN中纳米孔 的面积和形状。我们观察到，尽管取向差角为13.2°的双晶纳米多孔hBN的水通量提高了约30%，但与单晶hBN相比，其对钠离子的截留率降低了约6%。我们还揭示了纳米孔形状在水脱盐中的作用，发现尺寸较小的更细长的孔（在取向差为21.8°和32.2°的双晶hBN中）可以与尺寸稍大的不太细长的孔的水渗透相匹配，同时钠离子截留率降低约3-4%。模拟还预测，水通量受界面静电相互作用的显著影响。事实上，与不考虑部分电荷或体相部分电荷（即带电hBN）时相比，当使用DFT计算确定B和N原子上改变的部分电荷时，水通量最高。总体而言，我们关于水/离子通过双晶hBN中纳米孔传输的研究表明，晶界和表面电荷的存在分别会导致hBN膜的离子截留率和水渗透性能下降。
