Water in nanoporous hexagonal boron nitride nanosheets: a first-principles study

Nanoporous membranes are being explored as efficient materials for water filtration and desalination applications. In this study, we analyzed the behavior of pores within a freestanding hexagonal boron nitride (h-BN) monolayer in contact with water molecules. Our investigation revealed that triangular and rhombic pores induce wrinkles in non-deposited h-BN monolayers because of the repulsion between hydrogen orbitals at their 60° vertices. We found that the addition of N-H or B-H pairs at 60° vertices mitigates these out-of-plane deformations. Surprisingly, the stability of triangular pores with N-H termination is significantly increased by the addition of B-H pairs, whereas the stability of triangular pores with B-H termination is decreased by the addition of N-H pairs, indicating a distinct evolution of the pore shape based on the initial edge termination. Furthermore, we observed that the combination of H and OH is more effective in stabilizing the edges of rhombic pores compared to using only H atoms, whereas the opposite is true for small triangular pores that cannot accommodate the ·OH radical. Our findings also demonstrate that rhombic pores have a high affinity for water absorption, much higher than that of triangular pores. This suggests that the type of pore can significantly alter the hydrophobicity of h-BN and influence water flow through the membrane. Additionally, we observed that water molecules tend to form hydrogen bonds with the N-H-terminated surface in rhombic pores, but not with the B-N-terminated surface, potentially leading to asymmetries in water flow through the pore area. Overall, our study provides valuable insights into the interaction between nanoporous h-BN membranes and water.