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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9616 ARTICLE a cd 16 14 12 10 8 6 4 2 0 Mo only b S only Mo site S site 02468 02468 Distance from the center of the pore (Å) Distance from the center of the pore (Å) 100 90 80 70 1E–5 1E–4 1E–3 0.01 0.1 Water permeation rate (l cm–2 per day per MPa) Mo only MoS2 S only Graphene Hydrophobic edges simulations, the porosity (the ratio of the pore area to the membrane area) is smaller, which decreases the permeation rate per unit area of the membrane. In this work, the comparison of MoS2 and graphene is performed by keeping all conditions identical in the simulations. Thus, MoS2 is potentially an efficient membrane for water desalination. We have also investigated the potential performance of other transition metal dichalcogenide (MoSe2, MoTe2, WS2, WSe2 and so on) membranes. It was found that the transition metal atom plays a more important role than the chalcogen atom in desalination. More specifically, varying the Lennard-Jones (LJ) parameters of the chalcogen atom does not lead to a significant change in the ion rejection and water permeation (Supplementary Fig. 3). In conclusion, we have shown that MoS2 membranes are promising for water purification and salt rejection. Mo only pores perform the best among all possible MoS2 pore architectures. MoS2 nanopores with water accessible pore areas ranging from 20 to 60Å2 strongly reject ions allowing o12% of the ions (depending on pore areas) to pass through the porous membranes even at theoretically high pressures of 350MPa. The water permeation rates associated with these MoS2 porous membranes are found to be two to five orders of magnitude greater than that of currently used membrane materials (MFI-type zeolite, commercial polymeric seawater RO, brackish RO, nanofiltration and high-flux RO) and 70% better than the graphene nanopore. The fish-bone, hourglass architecture of Mo only pore with special arrangement of hydrophobic edges and hydrophilic centre within 1-nm length enhances water permeation to a large extent compared with its other counterparts. Methods Molecular dynamics (MD) simulations were performed using the LAMMPS package54. The graphene sheet, which acts as a rigid piston to exert external pressure on saline water, along with the MoS2 sheet, water molecules and ions were created by the Visual Molecular Dynamics55. The saline water box was placed between the graphene and MoS2 sheet and pure water was added on the other side of the MoS2 sheet as shown in Fig. 1. A nanopore was drilled in MoS2 by removing the desired atoms. The accessible pore areas considered range from 20 to 60 Å2 (Supplementary Fig. 4 for details on pore area calculations). The system dimensions are 4 4 13 nm in x, y and z, respectively. The box contains B16,000 atoms and the ions (sodium and chloride) have a molarity of B1.0, which is higher than the usual salinity of seawater (0.599 M) because of the computational cost associated with low-salinity solutions. The extended simple point charge water model was used and the SHAKE algorithm was employed to maintain the rigidity of the water molecule. For non- bonded interactions, the mixing rule was used to obtain the LJ parameters except for carbon–water interactions, which were modelled by the force-field parameters given in ref. 50. The LJ parameters are tabulated in Supplementary Table 1. The LJ cutoff distance was 12 Å. The long-range electrostatic interactions were calculated by the Particle Particle Particle Mesh56. Periodic boundary conditions were applied in all the three directions. For each simulation, first, the energy of the system was minimized for 10,000 steps. Next, the system was equilibrated in constant number of particles, pressure and temperature (NPT) ensemble for 1 ns at a pressure of 1 atm and a temperature of 300 K. Graphene and MoS2 atoms were held fixed in space during equilibration and the NPT simulations allow water to reach its equilibrium density (1 g cm 3). Then, an additional constant number of particles, volume and temperature (NVT) simulation was performed for 2 ns to further equilibrate the system. Temperature was maintained at 300 K using the Nose ́-Hoover thermostat with a time constant of 0.1 ps (refs 57,58). Finally, the production non-equilibrium simulations were carried out in NVT ensemble for 10 ns where different external pressures were applied on the rigid graphene sheet (no longer frozen in space) to characterize the water filtration through the MoS2 nanopores (Supplementary Movie 1). In the production runs, the MoS2 atoms were again held fixed in space to study solely the water transport and ion rejection properties of MoS2. To accelerate the MD simulations and gather enough statistics in the 10-ns simulations, high external pressures ranging from 50 to 350 MPa were considered in this work. Trajectories of atoms were collected every S site Mo site 12 10 8 6 4 2 0 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 1⁄4 56.42 Å2 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 1⁄4 57.38 Å2 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. NATURE COMMUNICATIONS | 6:8616 | DOI: 10.1038/ncomms9616 | www.nature.com/naturecommunications 5 & 2015 Macmillan Publishers Limited. All rights reserved. MFI zeolite Seawater RO Brackish RO Nanofiltrartion High-flux RO Graphene Mo only MoS2 1 10 S site Mo site Percentage of ion rejection Velocity (m s–1) Velocity (m s–1) 0.34 nm 0.5 nmPDF Image | Water desalination with a single-layer MoS2 nanopore
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