How Grain Boundaries and Interfacial Electrostatic Interactions Affect Water and Ion Transport Through Nanoporous Hexagonal Boron Nitride

To fulfil the increasing demand for drinking water, researchers are currently exploring nanoporous two-dimensional (2D) materials as potential desalination membranes. Hexagonal boron nitride (hBN), the inorganic analogue of graphene, is one of the promising 2D materials being investigated. A prominent, yet unsolved challenge is to understand how such membranes will perform in the presence of defects in the hBN layer. Indeed, large-area 2D materials are required for membrane separations, which would likely contain grain boundaries (GBs) as inadvertently induced geometrical defects. Additionally, although hBN is a heteropolar 2D material, previous studies have not investigated the role played by electrostatic interactions in its desalination performance. In this work, we study the effect of GBs on and the role played by interfacial interactions in modulating the desalination performance of bicrystalline nanoporous hBN. To this end, we use classical molecular dynamics simulations supported by quantum-mechanical density functional theory (DFT) calculations to 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 bicrystalline nanoporous hBN with the lowest considered misorientation angle of 13.2° shows improved water permeability by ~47% 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 can match the water permeation and ion rejection through less elongated pores of slightly larger sizes. Simulations also predict that the water permeability is significantly affected by interfacial electrostatic interactions. Indeed, the water permeability 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 the role played by GBs and interfacial electrostatic interactions in water/ion transport through nanopores informs the use of large-area, bicrystalline hBN membranes in seawater desalination applications.