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ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9616 Producing fresh water is currently a great challenge facing the society1–4. High capital costs and low efficiency of current desalination technology motivate the need for advances in desalination technology5,6. Approximately, half of the current desalination plants use reverse osmosis (RO) technologies2,5. RO based on traditional polymeric membranes faces several challenges including slow water transport7,8. Advances in nanotechnology open up opportunities to design energy-efficient membranes for water desalination9,10. Nanopores with diameters ranging from a few Angstroms to several nanometres can be drilled in membranes to fabricate molecular sieves11–13. As the diameter of the nanopore approaches the size of the hydrated ions, various types of ions can be rejected by nanoporous membranes promising efficient water desalination. Among nanoscale materials, graphene and carbon nanotubes were extensively studied for both water transport and desalination14–18. Graphene, a single-atom-thick membrane (0.34nm) was demonstrated to have several orders of magnitude higher flux rates compared with conventional zeolite membranes6,11,15,16,19,20. Since water flux through a membrane scales inversely with the membrane’s thickness11, graphene is attractive over most other materials due to its single-atom thickness12,16. It has been shown that chemical functionalization of a graphene nanopore (for example, adding hydroxyl groups) can enhance its permeability19,20, but reduces desalination efficiency19. Hydroxyl groups provide hydrophilic sites at the edge of the pore, which give rise to the attraction of water molecules and enhanced flux due to denser packing of water inside the pore19. Adding precise functional groups to the edge of nanopores requires complex fabrication21; therefore, identifying a single-atom-thick membrane with hydrophilic sites can lead to further advances in water desalination technology. Recently, a nanopore in a single-layer molybdenum disulfide (MoS2) has been investigated for DNA sequencing and has been shown to provide better results compared with graphene nanopores9,22. Compared with graphene, a MoS2 single layer has two types of atoms, that is, molybdenum (Mo) and sulfur (S). A single-layer MoS2 has a thickness of B1.0 nm (ref. 23) and is a mechanically strong material with an effective Young’s modulus of 270±100GPa, that is comparable to that of steel24. The possibility to craft the pore edge with Mo, S or both provides flexibility to design the nanopore with desired functionality. Recently, it has been shown that a nozzle-like structure of protein channels and other nanoscale membranes enhances water permeation25. The fish-bone structure of MoS2 makes it amenable for a nozzle-like sub-nanometer pore for fast water permeation25. Although theoretical studies of membrane efficiency are important in desalination technology, there are other aspects concerning fabrication and manufacturability of membranes such as large-area synthesis with defect-free, well-defined sealed membranes and precise pore generation that need to be addressed. Using a highly focused electron beam, and transmis- sion electron microscope, versatile nanopores with diameters ranging from 1 to 10nm were sculpted successfully in MoS2 membranes9. Waduge et al.26 reported that a large-area, well-sealed membrane with nanopores as tiny as 2.8 nm can be fabricated. Compared with graphene, the contamination of these membranes can be lower as carbon atoms in graphene are more susceptible to contamination during chemical vapour deposition (CVD) growth. Feng et al.27 also achieved high-quality scalable fabrication of nanopores in a single-layer MoS2 with sub- nanometre precision using electrochemical reaction. Several other studies have been performed on the synthesis of large- area MoS2 monolayers28–37. Recently, a few groups29,34,37 have successfully used CVD to produce highly crystalline MoS2 of centimetre dimensions. In another study36, a refined CVD method was proposed to create high-quality monolayer MoS2 crystals in which the grain boundaries of MoS were faceted 2 NATURE COMMUNICATIONS | 6:8616 | DOI: 10.1038/ncomms9616 | www.nature.com/naturecommunications & 2015 Macmillan Publishers Limited. All rights reserved. 2 more strongly than that of graphene resulting in mechanically more stable MoS2 monolayers. Membrane sealing also plays an essential role in the synthesis of large-area membranes required in desalination. Waduge et al.26 showed that their CVD approach resulted in almost fully sealed MoS2 membranes. Combination of these results9,13,26–37 and the recent focus on a single-layer MoS2 fabrication is promising for the large-scale manufacturing of a single-layer MoS2. Here we demonstrate that a single-layer MoS2 can effectively separate ions from water. Using molecular dynamics simulations, we investigate water desalination in MoS2 as a function of pore size, chemistry, geometry and applied hydrostatic pressure. Results Water fluxes. A typical simulation box consists of a single-layer MoS2, a graphene sheet (acting as a rigid piston to apply the external pressure), water and ions (Fig. 1a). Here three pore edge types for MoS2 are considered to study the effect of terminating atoms and pore chemistry on the rate of water permeation and ion rejection. The first type of pore, which is labelled as mixed in this study, is a combination of molybdenum and sulfur atoms. The other two pore types are labelled as Mo only and S only, as these are terminated by molybdenum and sulfur atoms, respec- tively (Fig. 1b). Water fluxes through various MoS2 nanopores as a function of the applied pressure gradient are presented in Fig. 2a. Three MoS2 pore types (mixed, Mo only and S only) were studied to explore their rejection rate and flux. To investigate the relative performance of MoS2 over other two-dimensional materials, a graphene nanopore, which has been shown to be a Rigid piston Salt water MoS2 nanopore S only Mixed 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. b Fresh water Mo onlyPDF Image | Water desalination with a single-layer MoS2 nanopore
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