Molecular dynamics simulation-directed rational design of nanoporous graphitic carbon nitride membranes for water desalination

Abstract

As a novel two-dimensional material, graphitic carbon nitride (g-C3N4) has proved to be promising for separation membrane. However, the assessment of nanoporous g-C3N4 membrane for efficient desalination process has not been systematically investigated yet. In this work, we designed three g-C3N4 nanopores with different shapes based on the membrane unit structure. The non-equilibrium molecular dynamics simulations were used to study the molecular mechanism of saline water permeation through the nanoporous g-C3N4 membranes. The relationship between water flow rate and external pressure was in good agreement with the classical hydrodynamics. Meanwhile, we observed that the local water diffusion coefficient increased as the enlargement of nanopore sizes, indicating that the nano-effect may also rule the water permeation. For desalination purposes, the designed g-C3N4 nanoporous membrane entirely rejected ions while maintaining a considerable water permeability of 14.9 L/cm2/day/MPa. By analyzing the trajectories of ions passing through the nanopores, we found that the configurational shape could divide one nanopore into several sub-regions, and specific ones may determine the desalination performance of the nanoporous membrane. This work unveils the molecular insights into the desalination process of nanoporous g-C3N4 membrane and further provides useful guidelines for the design of next generation desalination membranes with complex nanopore structures.