Fast water desalination by carbon-doped boron nitride monolayer: transport assisted by water clustering at pores

How Grain Boundaries and Interfacial Electrostatic Interactions Modulate Water Desalination via Nanoporous Hexagonal Boron Nitride

To fulfill the increasing demand for drinking water, researchers are currently exploring nanoporous two-dimensional materials, such as hexagonal boron nitride (hBN), as potential desalination membranes. A prominent, yet unsolved challenge is to understand how such membranes will perform in the presence of defects or surface charge in the membrane material. In this work, we study the effect of grain boundaries (GBs) and interfacial electrostatic interactions on the desalination performance of bicrystalline nanoporous hBN using classical molecular dynamics simulations supported by quantum-mechanical density functional theory (DFT) calculations. We 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, although bicrystalline nanoporous hBN with a misorientation angle of 13.2° shows an improved water flow rate by ∼30%, it demonstrates reduced Na⁺ ion rejection by ∼6%, 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 (in 21.8- and 32.2°-misoriented bicrystalline hBN) can match water permeation through less elongated pores of slightly larger sizes, with a concomitant ∼3-4% decrease in Na⁺ rejection. Simulations also predict that the water flow rate is significantly affected by interfacial electrostatic interactions. Indeed, the water flow rate 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 water/ion transport through nanopores in bicrystalline hBN indicates that the presence of GBs and surface charge can lead, respectively, to a decrease in the ion rejection and water permeation performance of hBN membranes.

Novel adjustable monolayer carbon nitride membranes for high-performance saline water desalination

In this study, via molecular dynamic simulations, we showed that the latest described graphene-like carbon nitride membranes, such as g-C₄N₃, g-C₆N₆, and g-C₃N₄ single-layers, can be used as high-performance membranes for water desalination. In addition to having inherent nanopores and extraordinary mechanical properties, the carbon nitride membranes have high water permeability and strong ion rejection (IR) capability. The important point about carbon nitride membranes is that the open or closed state of the pores can be changed by applying tensile stress and creating a positive strain on the membrane. The effect of the imposed pressure, the tensile strain, the ion concentration, and the effective pore size of the membranes are reported. It is demonstrated that, with the applied tensile strain of 12%, the g-C₆N₆ membrane is the best purification membrane, with a water permeability of 54.16 l cm^-2 d^-1 MPa^-1 and the IR of 100%. Its water permeability is one order of magnitude greater than other one-atom-thick membranes.

Mechanical strength of a nanoporous bicrystalline h-BN nanomembrane in a water submerged state

Due to superior water permeability, structural stability, and adsorption capability, h-BN nanosheets are emerging as an efficient membrane for water desalination. In order to cater to the demand for potable water, large size membranes are required to maintain a high desalination rate from water purification systems. These large size membranes usually contain polycrystals with an offset in their mechanical properties from pristine h-BN nanosheets. In this article, molecular dynamics based simulations were performed in conjunction with a hybrid interatomic potential (reactive force field, TIP3P, and Lennard Jones) to simulate the mechanical strength of nanoporous single and bicrystalline h-BN nanosheets under water submerged conditions. The interaction between the atomic configuration of grain boundary atoms and nanopores in the presence of water molecules helps in investigating the viability of defective h-BN nanomembranes for underwater applications. Higher dislocation density enhances the mechanical strength of nanoporous bicrystalline h-BN nanosheets containing twin nanopores, which makes them a better substitute for water submerged applications as compared to the pristine nanosheets. The mechanical strength of nanoporous single crystalline h-BN nanosheets deteriorates with an increase in the number of nanopores, whereas a contrasting trend was observed with bicrystalline h-BN nanosheets.

The effect of temperature on water desalination through two-dimensional nanopores

Two-dimensional (2D) materials such as graphene, molybdenum sulfide, and hexagonal boron nitride are widely studied for separation applications such as water desalination. Desalination across such 2D nanoporous membranes is largely influenced by the bulk transport properties of water, which are, in turn, sensitive to the operating temperature. However, there have been no studies on the effect of temperature on desalination through 2D nanopores. We investigated water desalination through hydrogen functionalized graphene nanopores of varying pore areas at temperatures 275.0 K, 300.0 K, 325.0 K, and 350.0 K. The water flux showed a direct relation with the diffusion coefficient and an inverse relation with the hydrogen-bond lifetime. As a direct consequence, the water flux was found to be related to the temperature as per the Arrhenius equation, similar to an activated process. The results from the present study improve the understanding on water and ion permeation across nanoporous 2D materials at different temperatures. Furthermore, the present investigation suggests a kinetic model, which can predict the water and ion permeation based on the characteristics of the nanopore.