Molecular dynamics simulations of functionalized hBN nanopores in water: <i>Ab initio</i> force field and implications for water desalination

Abstract

Heteropolar two-dimensional materials, including hexagonal boron nitride (hBN), are promising candidates for seawater desalination and osmotic power harvesting, but previous simulation studies have considered bare, unterminated nanopores in molecular dynamics (MD) simulations. There is presently a lack of force fields to describe functionalized nanoporous hBN in aqueous media. To address this gap, we conduct density functional theory (DFT)-based ab initio MD simulations of hBN nanopores surrounded by water molecules. The results reveal a high propensity for hydrogen (H) and hydroxyl (OH) functionalization at boron edges, while nitrogen edges are functionalized with H and occasionally with oxygen (O), highlighting a route to tune membranes. We demonstrate the role of the Grotthuss mechanism during the functionalization of hBN edges in water. We develop high-fidelity force fields for H- and OH-functionalized hBN nanopores using potential energy surface fitting based on DFT calculations. The nonbonded parameters for H functionalization are obtained by training a force field for borazine (B3N3H6). We find that the proposed force field enables stable MD simulations of water/ion transport through B- and N-terminated hBN nanopores. Our results also indicate that previous studies that considered bare nanopores without functional groups overestimated the water flux and underestimated the ionic rejection of nanoporous hBN. Overall, our work is expected to enable the realistic modeling of edge-functionalized hBN in aqueous media for various application areas.