Water desalination with a single-layer MoS2 nanopore

Abstract

Efficient desalination of water continues to be a problem facing the society. Advances in nanotechnology have led to the development of a variety of nanoporous membranes for water purification. Here we show, by performing molecular dynamics simulations, that a nanopore in a single-layer molybdenum disulfide can effectively reject ions and allow transport of water at a high rate. More than 88% of ions are rejected by membranes having pore areas ranging from 20 to 60 Å(2). Water flux is found to be two to five orders of magnitude greater than that of other known nanoporous membranes. Pore chemistry is shown to play a significant role in modulating the water flux. Pores with only molybdenum atoms on their edges lead to higher fluxes, which are ∼ 70% greater than that of graphene nanopores. These observations are explained by permeation coefficients, energy barriers, water density and velocity distributions in the pores.

Figure 1. Simulation box and different pore architectures.

(a) Schematic of the simulation box consisting of a MoS₂ sheet (molybdenum in blue and sulfur in yellow), water (transparent blue), ions (in red and green) and a graphene sheet (in gray). (b) Left: Mo only pore type. Right: S only pore type. Bottom: mixed pore type.

Figure 2. Water permeation and salt rejection.

(a) Water flux as a function of the applied pressure for mixed, Mo only, S only and graphene nanopores with similar pore areas. (b) Percentage of ion rejection by various pores as a function of the applied pressure. Pores with different edge chemistries as well as various pore areas (denoted by A) are considered. (c) Number of water molecules (#) filtered through Mo only pores as a function of simulation time for different pore areas at a fixed pressure of 250 MPa.

Figure 3. Water density and velocity profiles.

(a) Water density distribution in the radial direction in the mixed, Mo only and S only pores with equivalent pore sizes (mixed, A=55.45 Ų ; Mo only, A=56.42 Ų; S only, A=57.38 Ų) at a fixed pressure of 250 MPa. (b) Density map of water distribution in Mo only (i) and S only (ii) pores. Blue denotes a zero probability of finding a water molecule and red indicates the highest probability of observing a water molecule. (c) Axial velocity of water molecules in the radial direction for mixed, Mo only and S only nanopores.

Figure 4. Effect of pore type on water permeation and salt rejection.

(a) Axial velocity of water molecules in the radial direction at the location of S and Mo atom layers in the Mo only nanopore of A=56.42 Ų at 250 MPa. (b) Axial velocity of water molecules in the radial direction at the location of S and Mo atom layers in the S only nanopore of A=57.38 Ų at 250 MPa. (c) Cartoon representation of the pore architecture for Mo only, S only and graphene nanopore. (d) Performance of various membranes in terms of their ion rejection and water permeation rate. Water permeation rate is expressed per unit area of the membrane and per unit pressure as l cm⁻² per day per MPa.