Functionalization of Nanoporous Hexagonal Boron Nitride in Water—What Sticks at Nanopore Edges and Its Implications for Selective Desalination

Heteropolar two-dimensional (2D) materials, such as hexagonal boron nitride (hBN), are promising candidates for seawater desalination and osmotic power harvesting. The significant electrostatic interactions that are operative in their vicinity allow the modulation of interfacial properties and nanoscale transport phenomena. Although nanopores would be terminated by various functional groups in aqueous environments, previous studies have considered bare nanopores in hBN. Here, we conduct accurate ab initio molecular dynamics studies of hBN nanopores surrounded by water molecules. Our simulations highlight a high propensity for H and OH functionalization at boron edges, while nitrogen edge atoms are primarily functionalized with H and occasionally with O, highlighting a potential route to selective membranes. We demonstrate the role of the Grotthus mechanism during the functionalization of hBN edges in water and show that functionalized pore atoms are inert for further reactions. We use this knowledge to develop a high-fidelity force field for B-H, B-OH, N-H, and N-O functionalization of hBN nanopores by combining quantum-mechanical calculations for structural optimization, charge determination, and potential energy surface fitting. We show that N-H terminated nanopores are excellent for the rejection of boron, a difficult-to-remove contaminant found in seawater. Additionally, the introduction of N-O groups allows a counter-intuitive increase in both water flux and ion rejection. We also establish a link between the magnitude of the charge difference in the edge atoms and higher salt salt rejection. Overall, our work provides rich mechanistic insights into nanopore functionalization, advances highly accurate models to simulate functional groups at nanopore edges, and demonstrates surprising new findings for boron and salt rejection from seawater, thereby significantly advancing the use of nanoporous 2D materials for membrane separation applications.