Rational Design and Strain Engineering of Nanoporous Boron Nitride

Subscriber access provided by UNIV OF CAMBRIDGE

Article

Rational Design and Strain Engineering of Nanoporous Boron Nitride Nanosheet Membranes for Water Desalination Haiqi Gao, Qi Shi, Dewei Rao, Yadong Zhang, Jiaye Su, Yuzhen Liu, Yun-hui Wang, Kaiming Deng, and Rui-Feng Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06480 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Rational Design and Strain Engineering of Nanoporous

Boron

Nitride

Nanosheet

Membranes for Water Desalination Haiqi Gao,†,║ Qi Shi,†,║ Dewei Rao,§ Yadong Zhang,† Jiaye Su,† Yuzhen Liu,† Yunhui Wang,*,‡ Kaiming Deng,† and Ruifeng Lu*,†, †

Department of Applied Physics, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China

‡

Information Physic Research Center, School of Science, Nanjing University of Posts and Telecommunications 210023, People’s Republic of China §

School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, People’s Republic of China



State Key Lab of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China

ABSTRACT：Through systematic molecular dynamics simulations, we theoretically investigate the potential applications of hexagonal boron nitride (h-BN) for seawater desalination. Our results indicate that the rationally designed h-BN membranes have great permeability, selectivity and controllability for water desalination. The size and chemistry of the pores are shown to play an important role in regulating the water flux and salt rejection. Pores with only nitrogen atoms on the edges have higher fluxes than the boron-lined pores. In particular, two-dimensional h-BN with medium-sized N4 pores shows 100% salt rejection with outstanding water permeability, which is several orders of magnitude higher than that of conventional reverse osmosis membranes. Furthermore, we study the mechanical strain effect on the desalination performance of monolayer h-BN with relatively small N3 pores, suggesting that water flux and salt rejection can be precisely tuned by tensile strain. The findings in the present work unambiguously propose that porous boron nitride nanosheets are quite promising as new functional membranes for water desalination.

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

I. INTRODUCTION With population growth, accelerated industrialization and global climate change, the water crisis has become a major global problem in our daily life.1,2 On the earth’s surface, approximately 71% of the total area is oceans and seas, which contain more than 97% of the world’s water. Thus, water desalination will play an important role in supplying fresh water in this century.3,4 Compared with existing commercial techniques, the reverse osmosis (RO) method is more energy efficient and environmentally friendly.5 However, current RO membranes have their own drawbacks, such as low water permeability and membrane fouling.6–8 To solve these issues, ultrathin nanoporous materials have been widely studied, and novel properties, compared with those of traditional RO membranes, have been found.9 With suitable pore sizes, membrane materials can efficiently reject ions from passing through while possessing very fast water penetration.10 Recently, carbon materials and their analogues as well as two-dimensional (2D) transition metal dichalcogenides, such as carbon nanotubes,11–13 boron nitride nanotubes,14 graphene10,15–25 and molybdenum disulfide,26,27 have been extensively explored as promising membranes for water desalination by theoretical simulations and some have even been realized in experiments.20,21,28,29 Based on graphene, one-atomthick membranes have been proven to show higher flux rates than traditional membranes.10 It is well known that flux across a membrane scales inversely with its thickness, and 2D porous materials are highly desired in gaseous or liquid phase separation. In this regard, the development of new single-layer membrane materials 2


Page 2 of 29

Page 3 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with both high water permeability and ion rejection has become a hot topic in the desalination field. Hexagonal boron nitride (h-BN), so-called “white graphene”, is a monolayer material and can be synthesized via various methods.30–33 h-BN, which is as thin as graphene, is composed of alternating boron and nitrogen atoms in a honeycomb structure and has analogous properties to graphene, such as high thermal stability, low dielectric constant, and high mechanical strength.32–34 In addition, h-BN shows unique properties compared to graphene, such as a wide energy gap, electrical insulation, and chemical inertness.35–37 Because of its remarkable mechanical properties, h-BN exfoliated in liquids can be used as a molecular sieve.33,38 Many studies have already discovered potential applications of nanoporous h-BN in DNA sequencing,39,40 water purification,41 and gas separation.42 Recently, an enlightening work43 shed light on the physics behind water transport through h-BN and graphene with the same particular pore shape, demonstrating that a decrease in the water surface tension on h-BN monolayers compared to on graphene is responsible for larger water permeation through h-BN. Nevertheless, the role of h-BN as a membrane for water desalination has not been reported yet. Because h-BN nanosheets can be recycled many times due to their resistance to oxidation, h-BN nanosheets are suitable for a wide range of applications, including water desalination. According to previous experiments, in h-BN, pores with dangling N atoms along the rim are preferentially formed as triangle shapes,37,44 which can be fabricated via either electron-beam punching or chemical etching techniques, similar to those employed with graphene. Based on the 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

experimental findings, in this work, we designed triangular nanopores in h-BN with edges characterized by N or B protrusions and systematically studied their properties for seawater desalination using detailed molecular dynamics (MD) simulations. The MD results clearly show that 2D h-BN can effectively separate ions from water and that water desalination is dependent on the intrinsic pore size, edge chemistry, and applied pressure. Moreover, for porous h-BN membranes with smaller pore areas, which have poor water permeability though salt is completely rejected, the water flux can be significantly enhanced while maintaining 100% ion rejection if we rationally apply moderate tensile strain on the h-BN membrane. II. COMPUTATIONAL MODEL AND METHODS In our porous h-BN model, six equilateral triangular nanopores with two types of pore edges were constructed, as shown in Figure 1. For clarity, the h-BN membranes with a certain number (i) of N or B atoms per side of the triangle are denoted Ni or Bi, respectively. It is clear that the designed monolayer membranes are different in terms of pore size and edge atoms. The pore areas of these membranes are in the range of 42.1–97.7 Å2 and have the sequence N3≈B3 < N4≈B4 < N5≈B5. The simulation system for the N4 membrane is illustrated in Figure 1g, in which the h-BN membrane (34.70 × 35.10 Å2) was placed parallel to the xy plane in the center of the simulation box and surrounded by two water boxes on both sides with a thickness of 30 Å along the z direction. The simulation box was filled with 2170 water molecules and 18 Na+ and Cl– where the water and ions were uniformly distributed in both sides of the membrane, resulting in a salt concentration of 27 g/L, which is close to the salinity (~35 g/L) of 4


Page 4 of 29

Page 5 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


seawater. All simulations were performed using the Gromacs 4.5.5 software program.55 Each system was initially subjected to energy minimization using the steepest descent method with a maximum step size of 0.1 Å and a force tolerance of 10 kJ mol–1 nm–1, and then, velocities were assigned according to the Maxwell–Boltzmann distribution at 300 K. Finally, the non-equilibration MD simulation was conducted at 298 K. For all runs, the h-BN membrane was flexible, except for the atoms at the boundary of the hBN sheet fixed in order to keep the h-BN membrane in the center of the box, while other atoms of the h-BN membrane are allowed to move freely during the MD simulations. It’s means that the areas of the h-BN membrane can freely deform according to the forces acted on by the water molecules and salt ions.66 A cutoff of 14 Å was used to calculate the Lennard-Jones (LJ) interactions, and the particle–mesh Ewald67 method was used to evaluate the electrostatic interactions with a grid spacing of 1.2 Å and realspace cutoff of 14 Å, which is a well-established method for full periodic systems. In our model, the simulation box is full with water and the whole system keeps neutrality, so the periodicity breaking at z direction has very little influence on water desalination, like the previous works68,69 which used the same model and method, however, it needs to note that Grzybowski and co-workers70 have developed Poisson summation formula to calculate the effective interaction of charged particles in a three-dimensional system with periodicity in two dimensions. The MD simulations were implemented in the NVT canonical ensemble with a total time of 30 ns, and periodic boundary conditions were imposed in all three directions (x, y and z). Moreover, the hydrostatic pressure was 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

obtained by including an additional acceleration on every water molecule along the z direction.45 The applied constant force on one molecule is given by F = ΔP·A/n, where ΔP is the chosen pressure, A is the membrane area, and n is the total number of water molecules in the box. In our simulations, the pressure was selected in the range of 10 to 200 MPa. It should be emphasized that the applied pressure used here is obviously higher than practical values (a few MPa); however, it is very common in nonequilibrium MD simulations to apply a higher pressure to reduce thermal noise and enhance the signal/noise ratio within a nanosecond time scale.10,22,24,26,27 Because the time scales for water flux scale linearly with applied pressure, our results can extend to the low-pressure range and remain valid. At a given hydrostatic pressure for the system, the water molecules and ions move along the z direction and possibly pass through the nanoporous h-BN membrane. We used the visual molecular dynamics program (VMD) 1.9.263 to visualize and analyze the data. Moreover, the potential of mean force (PMF) of ion was calculated by using umbrella sampling,71 when ion (Na+ or Cl–) is forced to pass through the center of the pore (z = 0) along the reaction coordinate. The width of umbrella windows was 1 Å for the reaction coordinates varying from –10 Å (left bulk region ) to 10 Å (right bulk region), resulting in 21 windows. Each umbrella window was simulated in 10 ns, where a harmonic force with a constant of 1000 kJ mol–1 nm–2 was applied to the test ions. The last results were analyzed using the weighted histogram analysis method.72 On the other hand, the Boltzmann sampling method73-75 was also used to calculate the PMF of one water molecule passing through the h-BN membrane. Mathematically, the energetic 6


Page 6 of 29

Page 7 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


barrier is defined by

, where R is the gas constant, T is the

temperature and ρ(r) is the density at position r. In this work, water was modeled using the TIP3P potential.56 As for the desalination76 or water transport77 in nanochannels, previous experimental results were consistent with the MD simulations based on TIP3P water model. This model was popularly used in other study desalination27,51,78,79 and water transport studies.80,81 We should keep in mind that the water flow obtained from the TIP3P model in this work is larger than that from SPC/E or TIP4P model, and also it is inferior to SPC/E, TIP4P, TIP4P/2005, and TIP5P models for a range of thermodynamic properties.82 The potential energy of the intermolecular interactions was characterized by the sum of the LJ and Coulomb potentials, and the Lorentz-Berthelot rules was used for LJ interactions. The LJ parameters for the boron and nitride atoms were taken from the literature14 (σBB

= 3.453 Å, εB-B = 0.3971 kJ/mol, σN-N = 3.365 Å, εN-N = 0.6060 kJ/mol). These

parameters have been extensively used in water transport and water desalination through boron nitride nanotubes60–62 and water transport and separation through porous h-BN nanosheets39,43 and thus can properly describe the interaction between h-BN and water. We used the Universal Force Field (UFF)83 parameters for the bonds, bond angles, and dihedrals of h-BN membrane, which have been approved by very good agreements with experiment, ab initio and other MD methods in terms of mechanical properties, tensile rigidity, poisson’s ratio and shear rigidity of h-BN nanosheets.84 Moreover, the LJ parameters for Na+ and Cl– were taken from the AMBER0357 force field. The atomic charges of these h-BN nanosheets were obtained from density functional theory 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

calculations using the generalized gradient approximation with the Perdew–Burke– Ernzerhof functional, as implemented in the Dmol3 software.58 The calculated Hirshfeld charges22 around the nanopores of the h-BN membranes are given in Figure S1 in the Supporting Information. Here, we should note that Aluru and coworkers have worked to further develop accurate force fields for MD simulations. For example, they recently used the random phase approximation to derive better force field parameters and then compared their calculations with the diffusion Monte Carlo method. The proposed force fields were validated by successfully predicting the water contact angle on h-BN, which agrees well with experimental measurements64. III. RESULTS AND DISCUSSION Figure 2 shows the number of net transferred water molecules (Nw) passing through different h-BN membranes under external pressures ranging from 10 to 200 MPa. It is clear that Nw for water molecules across the fine, designed porous membranes scales linearly with simulation time. The Nw linear curves are quite smooth for the membranes with larger pores (B4, N4, B5, and N5), indicating that water molecules permeate the membrane at a relatively constant rate. Moreover, the profiles show that the net transferred water molecules increase with applied pressure and pore size. For the same membranes, the higher the pressure, the greater the net transferred water. Among the hBN membranes with the same terminal atoms around the pores, the smallest pore (N3 or B3) translocates the least water even at high hydrostatic pressures up to 200 MPa. Under the same pressure conditions, the bigger the pore size, the more water molecules pass through the membrane. Therefore, it is quite intuitive that the large-pore N5 8


Page 8 of 29

Page 9 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


membrane has a high net transfer for water even at low pressure. If we compare the results of different pore terminals for membranes with similar pore sizes, we find that N-edged pores lead to more transferred water than B-edged pores, which becomes more obvious with increased pressure. Based on several initial sets of independent simulations for each h-BN membrane, the water flux across the membranes can be extracted from the slopes of the Nw lines, similar to Figure 2. The water fluxes through various h-BN nanopores in Figure 3a show a linear relationship as a function of applied pressure, i.e., a higher pressure applied to the system leads to a larger water flux. Under the same pressure, the water flux across the N5 membrane is significantly higher than that across other aperture membranes. For large-pore membranes at high pressure, water flux through N-type pores is clearly higher than that through B-type pores, which demonstrates that pores with N edges are better for water transport. Aluru and coworkers14 have shown that the water molecules-nitride atoms van der Waals (vdW) attractions are stronger than waterboron vdW attractions, leading nitride pore to have a higher water flux than born pore. It should be emphasized that although the pore chemistry plays a significant role in the permeation ability of water, the pore size is the main factor influencing water permeation. For example, at an external pressure of 200 MPa, water flux through the B5 pores reaches 200/ns, while that through the N3 pores is only 11/ns. Aluru and coworkers proved that one-dimensional ordered chains of water molecules pass through small pores by forming hydrogen-bonding networks; meanwhile, the breakage of hydrogen bonding frequently leads to low water flux through graphene.18 Unlike 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

graphene membranes, the B and N atoms of porous h-BN membranes have the opposite charge (Figure S1). The boron atoms, which have a positive charge, will attract the oxygen atoms of nearby water molecules, and on the other hand, the nitrogen atoms will attract the hydrogen atoms of water.59 This different charge distribution leads to the imparity of the water flux through the N-edged and B-edged pores. For the N-edged pores, the hydrogen atoms of water molecules close to the edge have a suitable affinity for nitrogen, decreasing hydrogen bonding around the pore and leading the water molecules to concentrate on the center of the pore (Figure S4). For the B-edged pore, the water density close to the pore center is smaller than that for the N-edged pore, indicating that the attraction between the O atom and the B atom is strong while it is repulsive between O and N, which leads to more water molecules around the pore edge (Figure S4). As a result, a higher water flux is observed in the N-edged pores compared with the B-edged pores. At a pressure of 100 MPa, the water flux across the N5 membrane is ~112/ns. Under the conditions of equivalent pore size, the flux is 12 times and 2 times greater than that of pristine graphene pores18 and that of N-doped graphene pores, respectively,22 which indicates that nanoporous h-BN membranes possess superb penetration ability for water flow. Notably, it is meaningless in water desalination to merely improve the water flux to a high level (as stated above, this can be done by using large-enough pores) if the ability of the membrane to reject ions is not satisfied. Usually, fast water permeation contradicts efficient ion rejection. To evaluate the overall performance of a membrane material for desalination applications, a tradeoff between salt rejection and water 10


Page 10 of 29

Page 11 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


permeability is key. Before calculating the salt rejection rate, we examined Na+ and Cl– ion translocation events across these six h-BN membranes. No Na+ and Cl– ions were found to crossover the N3, B3, N4 or B4 pores at all chosen pressures due to the small pore sizes. Nevertheless, both ions are able to pass through the larger N5 and B5 pores at medium and high pressures, as seen from the numbers of Na+ (

) and Cl– (



)

ions across the N5 and B5 pores of the h-BN membranes in Figure S2. Roughly similar to the water permeation in Figure 2,

and



increase for the N5 and B5

membranes with increasing applied pressure. Note that the diameter of a water molecule is approximately 2.8 Å,59 whereas the corresponding value of a Na+ ion is approximately 2.0 Å. However, the numbers of ions passing through the pores are much smaller than the amount of water. This is expected because in bulk water, there are an average of 5.2 and 7.1 water molecules in the first hydration shell around the Na+ and Cl– ions, respectively.46 The diameters of the hydration structures of ions are thus larger than the pore sizes of some membranes and the diameter of water. Therefore, for small pores, such as N3, B3, N4 and B4, only water molecules can pass through the membrane. The number of Cl– ions passing through the B5 pores is close to that of Na+ under various pressures (Figure S2). However, a lower rejection of Na+ than Cl– is observed for the N5 membrane. On one hand, it is understandable that the hydration structure of the Cl– ion is larger in diameter than that of the hydrated Na+ ion. On the other hand, previous work has proved that the atomic charge surrounding the pore has an important influence on water desalination, which controls not only the water flow but also ion translocation through the membrane.10,23,65 The N atoms on a bare pore edge are 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 29

negatively charged, and the B atoms are positively charged (see Figure S1), which is the largest difference from the nanoporous graphene membrane. Thus, by considering the charge effect of the pore edge, Na+ ions can pass through the N5 pores more smoothly than Cl– ions. The positively charged B5 pores favor the passage of Cl anions and meanwhile limit Na cations, and thus, the phenomenon that

is equal to



through the B5 pore is reasonable on the basis of repulsive (attractive) interactions between the B-type pore and Na+ (Cl–), as well as the different size of the hydrated ions. We therefore clarify that the electrostatic and steric repulsions work together to effectively reject ions from penetrating the porous h-BN membrane. The calculated percentages of salt rejection by the nanoporous h-BN membranes are illustrated in Figure 3b. Over the entire pressure range of 10 ~ 200 MPa, 100% salt rejection of NaCl is obtained with the N3, B3, N4, and B4 membranes, which is consistent with the fact that no single ion translocation downstream was observed during the simulation period. For the larger N5 and B5 pores, absolute salt rejection (percentage of 100) is obtained under low-pressure conditions. The water/salt selectivity decreases with applied pressure, and this behavior is in agreement with the other reported works.10,26 With larger pore sizes, ions escape through the pores, reducing the rejection efficiency. As analyzed before, charged pore edges terminated with N or B (pore chemistry) also have a notable effect on salt rejection. At a given pressure (≥ 10 MPa), salt rejection by the N5 pores is less than that by the B5 pores. In general, salt rejection is mainly dependent on the pore size, and the pore chemistry plays a secondary role. In our simulations, nanoporous h-BN membranes with water12


Page 13 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


accessible pores strongly reject salt (>72%) from passing through even at a high pressure of 200 MPa. The effective water permeability can be estimated based on the water flux per pore with respect to the applied pressure. Among the calculated results for the six porous hBN membranes in Table 1, the water permeability of the N5 membrane is the highest, whereas that of the B3 membrane is the lowest. In particular, the N4 membrane exhibits perfect salt rejection (100%) with a very remarkable water permeability of 1.00 kg m–2 s–1 MPa–1, which is several orders of magnitude higher than those of current commercial RO membranes and better than that of a graphene filter (~0.678 kg m–2 s–1 MPa–1 measured in experiment).21 To the best of our knowledge, the performance of commercial RO membranes is usually on the order of 0.1 L cm–2 day–1 MPa–1 (0.011 kg m–2 s–1 MPa–1).10 This implies that the h-BN membranes with appropriate pore sizes have the potential to filter salt solution and produce fresh water. Figure 4a represents the water density distribution in the simulated N4 system along the z direction at different pressures. Around z = 30 Å, there are minima in the density profile that correspond to the position of the h-BN layer. The non-zero values of these minima are simply due to water aggregation inside the pore. A large number of water molecules remain on the left side of h-BN membrane under induced pressure, and therefore, the density distribution curve of water on the left side is higher than that on the right side. Upon increasing the pressure from 10 to 200 MPa, water molecules move increasingly faster along the z-direction, and thus, the density on the left side is increased with increasing pressure, and the opposite trend can be observed on the right 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

side because the water molecules that passed through move away from the membrane quickly under induced pressure. As seen from the inset of Figure 4a, the water density on the left side of the N4 membrane increases significantly with increasing pressure. The change in water density becomes weak from low to high pressure, because a larger water density implies stronger repulsion among molecules, and thus, it is more difficult to tightly aggregate water molecules. In addition, the positions of water density peaks appear at about the z = 26.4 Å where around 3.6 Å distant from the h-BN membrane, because the vdW interaction between water and the h-BN membrane gather the water near the membrane. To further understand the mechanism of the water permeation process, the potential of mean force (PMF), which describes the free energy profile of water passing through the membrane, was calculated using Boltzmann sampling under equilibrium conditions. The PMFs at 100 MPa for the six h-BN membranes are provided in Figure 4b. The peaks in the middle of each simulation cell (z = 30 Å) demonstrate that water has to overcome an energy barrier to pass through the membranes. From the PMFs, we know that water will overcome a smaller energy barrier to penetrate a larger pore with the same type of edge. The Nw and water flux decrease in the sequences of N5 > N4 > N3 and B5 > B4 > B3. Moreover, the energy barrier of the PMF for the N-type membrane is slightly lower than that for the B-type membrane if we compare the same-sized pores (Figure 4b). The energy barrier for the same membrane system has almost no relationship to the applied pressure (Figure S3). The tiny discrepancy could result from that a higher pressure will induce larger force on the water, and it seems that water 14


Page 14 of 29

Page 15 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


molecules pass through the porous membrane more easily; however, the intrinsic energy barrier has not been affected. Therefore, the PMF analyses well explain our findings in Figure 2 and Figure 3a. The PMF data in Figure 4c calculated by the umbrella sampling show that the free energy barrier for ions across N5 membrane is higher than that for water molecules (Figure 4b), so the water can pass through the pore more easily than the ions. From Figure 4c, we can also observe that the PMF of Cl− is higher than that of Na+ because of electrostatic and steric repulsions which can explain why Na+ ions pass through the N5 pores more smoothly than Cl– ions. In addition, the free energy barriers for ions via B5 membrane are also higher than that for passing water (Figure S5). In current molecular-sieve membrane studies, accurate control and discrimination of the pores has attracted much attention.47 Through MD simulations, we also confirm that the pore size plays an important role in salt rejection and that N-edged pores in the h-BN membrane show better water permeability than B-edged pores. Next, we mechanically tuned the designed pores by means of tensile strain on the h-BN membrane. Here, we choose the N3 pore because of its perfect salt rejection, and we attempt to improve its water permeability while retaining its 100% salt rejection. Fortunately, monolayer h-BN has good mechanical strength,32,33 and its Young’s modulus (~811 GPa) is slightly smaller than that of graphene (~1 TPa).48 Perforated hBN should be amenable under mechanical strain,49,50 and thus, the pore size of the nanoporous h-BN membrane could be regulated to obtain flexible properties. In some pioneering studies, tensile strain is suggested to effectively modulate the permeation of 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

water through a nanofilter.27,51 In our work, tensile strains up to 9% were rationally chosen to guarantee the stability of the nanoporous N3 membrane, and the strain on the membrane was adjusted by increasing and fixing its cross-sectional area in the x−y plane. With mechanical stretching of the N3 membrane, the bond length of B-N increased, and the pore shape approached oval (Figure S1 and inset of Figure 5b). Specifically, the average B-N bond length near the pore edge changed from 1.446 Å to 1.481 Å (1.529 Å, 1.581 Å) by applying 3% (6%, 9%) strain on the N3 membrane. As a result, the pore area monotonously increased. The estimated values of the pore area are also shown in Figure 5b. Additionally, it is noted that the partial charges of the edge atoms slightly change, as shown in Figure S1. We first tested the water transparency of the strained N3 membrane at a pressure of 100 MPa (Figure 5a). By comparing the cumulative numbers of water molecules, we found that the N3 filter becomes more transparent as the strain is further increased. The water flux at 100 MPa through different strained filters is summarized in Figure 5b. A quasi-linear relationship vs. tensile strain was found; in other words, the N3 filter becomes more transparent to water as the applied strain increases. Under the same pressure, Nw and the water flux achieved by the strained N3 membrane are far larger than those of the unstrained N3 membrane (also see Figure 2a and Figure 3a). As seen from the calculated PMF curves of the strained and unstrained membranes in Figure 5c, the mechanical strain can effectively reduce the energy barrier for water passing through the N3 membrane. It is well known that pore size plays an important role in water transport, and thus, tensile strain, which makes the pore larger, leads to a reduction in the energy barrier for water to pass through 16


Page 16 of 29

Page 17 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the pore. Hence, water molecules can penetrate the enlarged nanopores more easily, leading to high water fluxes. In our MD simulations, Na+ ions start translocating through the 6%-strained N3type h-BN membrane at 60 MPa, as we observed one Na+ ion pass through the N3 membrane. Meanwhile, except for 9% strain at 200 MPa, no translocation of Cl– ions was observed throughout all the simulations at higher strains and pressures. The ion rejection by the N3 membrane under small strain (≤ 5%) was found to be 100%. Even under a large strain of 9%, the salt rejection reached 80% at a high pressure of 200 MPa. As shown in Figure 5d, the percentage of salt rejection for the 6%- and 9%-strained membranes drops with an increase in the pressure. We also estimated the water permeability of the filter in the inset of Figure 5d. For the N3 filter under a strain of 5%, the permeability was 1.321 kg m–2 s–1 MPa–1 with 100% salt rejection. This value is larger than that of the unstrained N4 membrane (1.00 kg m–2 s–1 MPa–1), and it is fairly superior to that of a MoS2 filter (~0.987 kg m–2 s–1 MPa–1, simulated)27 under similar conditions. In the laboratory, there are several strategies to apply strain to layered materials,52–54 which allow for the realization of strain on h-BN nanosheets. IV. CONCLUSIONS In conclusion, by means of detailed molecular dynamics simulations and analyses, we have shown that h-BN membranes with experimentally produced triangular pores can be highly efficient for seawater desalination. In all of the porous h-BN models, N-edged pores outperform B-terminated pores in water desalination. It was proven that larger pores in the perforated h-BN membrane lead to a higher net transferred water flux along 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with a lower salt rejection. In addition, the pore size of the membrane, diameter of the hydrated ions, and electrostatic interactions between the pore edge and the ions are responsible for ion translocation. Furthermore, for the N3 membrane with small pores, mechanical stretching of the filter effectively changes the size of its nanopores. As a result, the water permeability is remarkably improved, reaching 1.321 kg m–2 s–1 MPa– 1

with 100% salt rejection under a tensile strain of ≤ 5%, which is higher than that of

other 2D membranes. By examining of the properties of the unstrained N4 membrane (water permeability: 1.00 kg m–2 s–1 MPa–1, salt rejection: 100%), it was established that the rationally designed N3 and N4 h-BN membranes are good candidates for fast and efficient water desalination. In addition to the high water permeability and ion rejection of the proposed h-BN membrane, its resistance to oxidation and corrosion allow such membranes to be recycled many times. In experiments, porous h-BN membranes are fabricated via thermal treatment. With the development of industrial synthesis technology, h-BN membranes could be produced to be large enough for realistic applications. We expect this work will be of broad interest and will guide relevant theoretical and experimental efforts in developing next-generation RO membranes for seawater desalination. ASSOCIATED CONTENT Supporting Information. More figures and data regarding the partial charges of the edge atoms, ion translocation events and PMF of water for the N4 membrane. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION 18


Page 18 of 29

Page 19 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Corresponding Author * E-mail: [email protected] (R.F.L.). * E-mail: [email protected] (Y.H.W.).

Author Contributions ║

These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was partially supported by NSF of China Grant (21373113, 11374160, 21403111, 11574151), Fundamental Research Funds for the Central Universities (30920140111008, 30916011105), Natural Science Foundation of Jiangsu Province (Grants No. BK20140526), and China Postdoctoral Science Foundation funded project with Grant No. 2014M561576. Q. Shi also acknowledges the support from the program of China Scholarship Council (CSC).

REFERENCES (1) Eliasson, J. The Rising Pressure of Global Water Shortages. Nature 2015, 517, 6. (2) Hillie, T.; Hlophe, M. Nanotechnology and the Challenge of Clean Water. Nat. Nanotechnol. 2007, 2, 663–664. (3) Schiermeier, Q. Purification with a Pinch of Salt. Nature 2008, 452, 260–261. (4) Elimelech, M.; Phillip, W. A. The Future of Seawater Desalination: Energy, Technology, and the Environment. Science 2011, 333, 712–717. (5) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas, B. J.; Mayes, A. M. Science and Technology for Water Purification in the Coming Decades. Nature 2008, 452, 301–310. 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(6) Pendergast, M. M.; Hoek, E. M. V. A Review of Water Treatment Membrane Nanotechnologies. Energy Environ. Sci. 2011, 4, 1946–1971. (7) Spiegler, K. S.; El-Sayed, Y. M. The Energetics of Desalination Processes. Desalination 2001, 134, 109–128. (8) Gupta, K. M.; Qiao, Z.; Zhang, K.; Jiang, J. Seawater Pervaporation through Zeolitic Imidazolate Framework Membranes: Atomistic Simulation Study. ACS Appl. Mater. Interfaces 2016, 8, 13392– 13399. (9) Humplik, T.; Lee, J.; O'Hern, S. C.; Fellman, B. A.; Baig, M. A.; Hassan, S. F.; Atieh, M. A.; Rahman, F.; Laoui, T.; Karnik, R.; et al. N. Nanostructured Materials for Water Desalination. Nanotechnology 2011, 22, 292001. (10) Cohen-Tanugi, D.; Grossman, J. C. Water Desalination across Nanoporous Graphene. Nano Lett. 2012, 12, 3602–3608. (11) Zhao, K. W.; Wu, H. Y. Fast Water Thermo-pumping Flow across Nanotube Membranes for Desalination. Nano Lett. 2015, 15, 3664–3668. (12) Hummer, G.; Rasaiah, J. C.; Noworyta, J. P. Water Conduction through the Hydrophobic Channel of a Carbon Nanotube. Nature 2001, 414, 188–190. (13) Holt, J. K.; Park, H. G.; Wang, Y. M.; Stadermann, M.; Artyukhin, A. B.; Grigoropoulos, C. P.; Noy, A.; Bakajin, O. Fast Mass Transport through Sub-2-nanometer Carbon Canotubes. Science 2006, 312, 1034–1037. (14) Won, C. Y.; Aluru, N. R. Water Permeation through a Subnanometer Boron Nitride Nanotube. J. Am. Chem. Soc. 2007, 129, 2748–2749. (15) Cohen-Tanugi, D.; Lin, L. C.; Grossman, J. C. Multilayer Nanoporous Graphene Membranes for Water Desalination. Nano Lett. 2016, 16, 1027–1033. (16) O'Hern, S. C.; Jang, D.; Bose, S.; Idrobo, J. C.; Song, Y.; Laoui, T.; Kong, J.; Karnik, R. Nanofiltration across Defect-Sealed Nanoporous Monolayer Graphene. Nano Lett. 2015, 15, 3254–3260. (17) Koenig, S. P.; Wang, L.; Pellegrino, J.; Bunch, J. S. Selective Molecular Sieving through Porous Graphene. Nat. Nanotechnol. 2012, 7, 728–732. (18) Suk, M. E.; Aluru, N. R. Water Transport through Ultrathin Graphene. J. Phys. Chem. Lett. 2010, 1, 1590–1594. (19) Cohen-Tanugi, D.; Grossman, J. C. Mechanical Strength of Nanoporous Graphene as a Desalination Membrane. Nano Lett. 2014, 14, 6171–6178. (20) Radha, B.; Esfandiar, A.; Wang, F. C.; Rooney, A. P.; Gopinadhan, K.; Keerthi, A.; Mishchenko, A.; Janardanan, A.; Blake, P.; Fumagalli, L.; et al. Molecular Transport through Capillaries Made with Atomic-scale Precision. Nature 2016, 538, 222–225. (21) Surwade, S. P.; Smirnov, S. N.; Vlassiouk, I. V.; Unocic, R. R.; Veith, G. M.; Dai, S.; Mahurin, S. M. Water Desalination Using Nanoporous Single-layer Graphene. Nat. Nanotechnol. 2015, 10, 459–464. (22) Chen, Q.; Yang, X. Pyridinic Nitrogen Doped Nanoporous Graphene as Desalination Membrane: Molecular Simulation Study. J. Membr. Sci 2015, 496, 108–117. (23) Sint, K.; Wang, B.; Kral, P. Selective Ion Passage through Functionalized Graphene Nanopores. J. Am. Chem. Soc. 2008, 130, 16448–16449. (24) Muscatello, J.; Jaeger, F.; Matar, O. K.; Muller, E. A. Optimizing Water Transport through Graphene-Based Membranes: Insights from Nonequilibrium Molecular Dynamics. ACS Appl. Mater. Interfaces 2016, 19, 12330–12336. (25) Mi, B. Graphene Oxide Membranes for Ionic and Molecular Sieving. Science 2014, 343, 740-742. 20


Page 20 of 29

Page 21 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26) Heiranian, M.; Farimani, A. B.; Aluru, N. R. Water Desalination with a Single-layer MoS2 nanopore. Nat. Commun. 2015, 6, 8616. (27) Li, W.; Yang, Y.; Weber, J. K.; Zhang, G.; Zhou, R. Tunable, Strain-Controlled Nanoporous MoS2 Filter for Water Desalination. ACS Nano 2016, 10, 1829–1835. (28) Celebi, K.; Buchheim, J.; Wyss, R. M.; Droudian, A.; Gasser, P.; Shorubalko, I.; Kye, J. I.; Lee, C.; Park, H. G. Ultimate Permeation across Atomically Thin Porous Graphene. Science 2014, 344, 289–292. (29) Joshi, R. K.; Carbone, P.; Wang, F. C.; Kravets, V. G.; Su, Y.; Grigorieva, I. V.; Wu, H. A.; Geim, A. K.; Nair, R. R. Precise and Ultrafast Molecular Sieving through Graphene Oxide Membranes. Science 2014, 343, 752–754. (30) Lin, Y.; Williams, T. V.; Connell, J. W. Soluble, Exfoliated Hexagonal Boron Nitride Nanosheets. J. Phys. Chem. Lett. 2010, 1, 277–283. (31) Corso, M.; Auwarter, W.; Muntwiler, M.; Tamai, A.; Greber, T.; Osterwalder, J. Boron Nitride Nanomesh. Science 2004, 303, 217–220. (32) Golberg, D.; Bando, Y.; Huang, Y.; Terao, T.; Mitome, M.; Tang, C.; Zhi, C. Boron Nitride Nanotubes and Nanosheets. ACS Nano 2010, 4, 2979–2993. (33) Coleman, J. N.; Lotya, M.; O'Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; et al. Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568–571. (34) Wang, X.; Zhi, C.; Li, L.; Zeng, H.; Li, C.; Mitome, M.; Golberg, D.; Bando, Y. "Chemical Blowing" of Thin-Walled Bubbles: High-Throughput Fabrication of Large-Area, Few-Layered BN and C-x-BN Nanosheets. Adv. Mater. 2011, 23, 4072–4076. (35) Pakdel, A.; Zhi, C. Y.; Bando, Y.; Golberg, D. Low-dimensional Boron Nitride Nanomaterials. Mater. Today 2012, 15, 256–265. (36) Lee, C.; Li, Q.; Kalb, W.; Liu, X. Z.; Berger, H.; Carpick, R. W.; Hone, J. Frictional Characteristics of Atomically Thin Sheets. Science 2010, 328, 76–80. (37) Jin, C.; Lin, F.; Suenaga, K.; Iijima, S. Fabrication of a Freestanding Boron Nitride Single Layer and Its Defect Assignments. Phys. Rev. Lett. 2009, 102, 195505. (38) Nicolosi, V.; Chhowalla, M.; Kanatzidis, M. G.; Strano, M. S.; Coleman, J. N. Liquid Exfoliation of Layered Materials. Science 2013, 340, 1226419. (39) Gu, Z.; Zhang, Y.; Luan, B.; Zhou, R. DNA Translocation through Single-layer Boron Nitride Nanopores. Soft Matter 2016, 12, 817–823. (40) Liu, S.; Lu, B.; Zhao, Q.; Li, J.; Gao, T.; Chen, Y.; Zhang, Y.; Liu, Z.; Fan, Z.; Yang, F.; et al. Boron Nitride Nanopores: Highly Sensitive DNA Single-Molecule Detectors. Adv. Mater. 2013, 25, 4549–4554. (41) Lei, W.; Portehault, D.; Liu, D.; Qin, S.; Chen, Y. Porous Boron Nitride Nanosheets for Effective Water Cleaning. Nat. Commun. 2013, 4, 1777. (42) Zhang, Y.; Shi, Q.; Liu, Y.; Wang, Y.; Meng, Z.; Xiao, C.; Deng, K.; Rao, D.; Lu, R. Hexagonal Boron Nitride with Designed Nanopores as a High-Efficiency Membrane for Separating Gaseous Hydrogen from Methane. J. Phys. Chem. C 2015, 119, 19826–19831. (43) Garnier, L.; Szymczyk, A.; Malfreyt, P.; Ghoufi, A. Physics behind Water Transport through Nanoporous Boron Nitride and Graphene. J. Phys. Chem. Lett. 2016, 7, 3371–3376. (44) Kotakoski, J.; Jin, C. H.; Lehtinen, O.; Suenaga, K.; Krasheninnikov, A. V. Electron Knock-on Damage in Hexagonal Boron Nitride Monolayers. Phys. Rev. B 2010, 82, 113404. (45) Zhu, F. Q.; Tajkhorshid, E.; Schulten, K. Pressure-induced Water Transport in Membrane Channels 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Studied by Molecular Dynamics. Biophys. J. 2002, 83, 154–160. (46) Kou, J.; Zhou, X.; Lu, H.; Wu, F.; Fan, J. Graphyne as the Membrane for Water Desalination. Nanoscale 2014, 6, 1865–1870. (47) Won, J. G.; Seo, J. S.; Kim, J. H.; Kim, H. S.; Kang, Y. S.; Kim, S. J.; Kim, Y. M.; Jegal, J. G. Coordination Compound Molecular Sieve Membranes. Adv. Mater. 2005, 17, 80–84. (48) Bosak, A.; Serrano, J.; Krisch, M.; Watanabe, K.; Taniguchi, T.; Kanda, H. Elasticity of Hexagonal Boron Nitride: Inelastic X-ray Scattering Measurements. Phys. Rev. B 2006, 73, 041402. (49) Zhao, S.; Xue, J. Mechanical Properties of Hybrid Graphene and Hexagonal Boron Nitride Sheets as Revealed by Molecular Dynamic Simulations. J. Phys. D: Appl. Phys. 2013, 46, 135303. (50) Han, T.; Luo, Y.; Wang, C. Effects of Temperature and Strain Rate on the Mechanical Properties of Hexagonal Boron Nitride Nanosheets. J. Phys. D: Appl. Phys. 2014, 47, 025303. (51) Yang, Y.; Li, W.; Zhou, H.; Zhang, X.; Zhao, M. Tunable C2N Membrane for High Efficient Water Desalination. Sci. Rep. 2016, 6, 29218. (52) Plechinger, G.; Castellanos-Gomez, A.; Buscema, M.; van der Zant, H. S. J.; Steele, G. A.; Kuc, A.; Heine, T.; Schueller, C.; Korn, T. Control of Biaxial Strain In Single-layer Molybdenite Using Local Thermal Expansion of the Substrate. 2D Mater. 2015, 2, 015006. (53) Stava, E.; Yu, M.; Shin, H. C.; Shin, H.; Kreft, D. J.; Blick, R. H. Rapid Fabrication and Piezoelectric Tuning of Micro- and Nanopores in Single Crystal Quartz. Lab Chip 2013, 13, 156–160. (54) He, K.; Poole, C.; Mak, K. F.; Shan, J. Experimental Demonstration of Continuous Electronic Structure Tuning via Strain in Atomically Thin MoS2. Nano Lett. 2013, 13, 2931–2936. (55) Hess, B.; Kutzner, C.; van der Spoel, D.; Lindahl, E. GROMACS 4: Algorithms for Highly Efficient, Load-balanced, and Scalable Molecular Simulation. J. Chem. Theory Comput. 2008, 4, 435–447. (56) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. Comparison of Simple Potential Functions for Simulating Liquid Water. J. Chem. Phys. 1983, 79, 926–935. (57) MacKerell Jr, A. D.; Bashford, D.; Bellott, M.; Dunbrack Jr, R. L.; Evanseck, J. D.; Field, M. J.; Fischer, S.; Gao, J.; Guo, H.; Ha, S. All-atom Empirical Potential for Molecular Modeling and Dynamics Studies of Proteins. J. Phys. Chem. B 1998, 102, 3586–3616. (58) Delley, B. From Molecules to Solids with the DMol3 approach. J. Chem. Phys. 2000, 113, 7756– 7764. (59) Li, H.; Zeng, X.C. Wetting and Interfacial Properties of Water Nanodroplets in Contact with Graphene and Monolayer Boron–Nitride Sheets, ACS Nano 2012, 6, 2401–2409. (60) Won, Y. C.; Aluru. N. R. Water Phase Transition Induced by a Stone-Wales Defect in a Boron Nitride Nanotube. J. Am. Chem. Soc. 2008, 130, 13649–13652. (61) Hilder, T. A.; Gordon, D.; Chung, S. H. Salt Rejection and Water Transport through Boron Nitride Nanotubes. Small 2009, 5, 2183–2190. (62) Azamat, J.; Khataee, A.; Joo, S. W. Removal of Heavy Metals from Water through Armchair Carbon and Boron Nitride Nanotubes: A Computer Simulation Study. RSC Adv. 2015, 5, 25097–25104. (63) Humphrey, W.; Dalke, A.; Schulten, K. VMD: Visual Molecular Dynamics. J. Mol. Graphics 1996, 14, 33–38. (64) Wu, Y.; Wagner, L. K.; Aluru, N. R. Hexagonal Boron Nitride and Water Interaction Parameters. J. Chem. Phys. 2016, 144, 164118. (65) Walker, M. I.; Ubych, K.; Saraswat, V.; Chalklen, E. A.; Braeuninger-Weimer, P.; Caneva, S.; Weatherup, R. S.; Hofmann, S.; Keyser, U. F. Extrinsic Cation Selectivity of 2D Membranes. ACS nano 2017, 11, 1340–1346. 22


Page 22 of 29

Page 23 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(66) Ang, E. Y. M.; Ng, T. Y.; Yeo, J.; Liu, Z.; Geethalakshmi, K. R. Free-Standing Graphene Slit Membrane for Enhanced Desalination. Carbon 2016, 110, 350–355. (67) Darden, T.; York, D.; Pedersen, L. Particle Mesh Ewald: An N⋅log(N) Method for Ewald Sums in Large Systems. J. Chem. Phys. 1993, 98, 10089–10092. (68) Kou, J.; Zhou, X.; Lu, H.; Wu, F.; Fan, J. Graphyne as the Membrane for Water Desalination. Nanoscale 2014, 6, 1865–70. (69) Suk, M. E.; Aluru, N. R. Molecular and Continuum Hydrodynamics in Graphene Nanopores. RSC Adv. 2013, 3, 9365. (70) Grzybowski, A.; Gwóźdź, E.; Bródka, A. Ewald Summation of Electrostatic Interactions in Molecular Dynamics of a Three-Dimensional System with Periodicity in Two Directions. Phys. Rev. B 2000, 61, 6706–6712. (71) Torrie, G. M.; Valleau, J. P. Monte Carlo Free Energy Estimates Using Non-Boltzmann Sampling: Application to the Sub-Critical Lennard-Jones Fluid. Chem. Phys. Lett. 1974, 28, 578–581. (72) Roux, B. The Calculation of the Potential of Mean Force Using Computer Simulations. Comput. Phys. Commun. 1995, 91, 275–282. (73) Cohen-Tanugi, D.; Lin, L. C.; Grossman, J. C. Multilayer Nanoporous Graphene Membranes for Water Desalination. Nano lett. 2016, 16, 1027–1033; (74) Zhang, L.; Wu, G.; Jiang, J. Adsorption and Diffusion of CO2 and CH4 in Zeolitic Imidazolate Framework-8: Effect of Structural Flexibility. J. Phys. Chem. C 2014, 118, 8788–8794. (75) Corry, B. Designing Carbon Nanotube Membranes for Efficient Water Desalination. J. Phys. Chem. B 2008, 112, 1427–1434. (76) Zhu, Y.; Gupta, K. M.; Liu, Q.; Jiang, J.; Caro, J.; Huang, A. Synthesis and Seawater Desalination of Molecular Sieving Zeolitic Imidazolate Framework Membranes. Desalination 2016, 385, 75–82. (77) Shen, Y. X.; Si, W.; Erbakan, M.; Decker, K.; De Zorzi, R.; Saboe, P. O.; Kang, Y. J.; Majd, S.; Butler, P. J.; Walz, T. Highly Permeable Artificial Water Channels that can Self-assemble into TwoDimensional Arrays. Proc. Int. Acad. Ecol. Environ. Sci. 2015, 112, 9810–9815. (78) Thomas, M.; Corry, B. A Computational Assessment of the Permeability and Salt Rejection of Carbon Nanotube Membranes and Their Application to Water Desalination. Phil. Trans. R. Soc. A 2016, 374, 20150020. (79) Shen, M.; Keten, S.; Lueptow, R. M. Rejection Mechanisms for Contaminants in Polyamide Reverse Osmosis Membranes. J. Membr. Sci. 2016, 509, 36–47. (80) Liu, B.; Wu, R.; Baimova, J. A.; Wu, H.; Law, A. W. K.; Dmitriev, S. V.; Zhou, K. Molecular Dynamics Study of Pressure-Driven Water Transport through Graphene Bilayers. Phys. Chem. Chem. Phys. 2016, 18, 1886–1896. (81) Sisan, T. B.; Lichter, S. Solitons Transport Water through Narrow Carbon Nanotubes. Phys. Rev. Lett. 2014, 112 (4), 044501. (82) Vega, C.; Abascal, J. L. Simulating Water with Rigid Non-polarizable Models: A General Perspective. Phys. Chem. Chem. Phys. 2011, 13, 19663–19688. (83) Rappé, A. K.; Casewit, C. J.; Colwell, K.; Goddard III, W.; Skiff, W. UFF, A Full Periodic Table Force Field for Molecular Mechanics and Molecular Dynamics Simulations. J. Am. Chem. Soc. 1992, 114, 10024–10035. (84) Boldrin, L.; Scarpa, F.; Chowdhury, R.; Adhikari, S. Effective Mechanical Properties of Hexagonal Boron Nitride Nanosheets. Nanotechnology 2011, 22 , 505702.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

Table 1 Water permeability (kg m–2 s–1 MPa–1) in six h-BN membranes.

permeability

N3

B3

N4

B4

N5

B5

0.132

0.103

1.00

0.363

2.752

2.354

Figure 1. Top views and diameters of the experiment-based (a) N3, (b) N4, (c) N5, (d) B3, (e) B4, and (f) B5 pores of the h-BN membranes. (g) Schematic diagram of a simulation cell for the N4type membrane, in which the blue shade represents the water molecules.

24


Page 25 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. The number of water molecules passing through the (a) N3 and B3, (b) N4 and B4, and (c) N5 and B5 membranes at pressures of 10, 50, 100 and 200 MPa.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. (a) Water flux and (b) salt rejection with respect to pressure for the six h-BN membranes.

26


Page 26 of 29

Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) Density profile of water along the z direction of the system for the N4 membrane at pressures of 10, 50, 100, 150, and 200 MPa. (b) PMF of water for the six h-BN membranes at 100 MPa. (c) PMFs for Na+ (blue) and Cl– (red) across N5 membrane, where the distance is chosen as the position of Na+ or Cl– ions along the pore axis, with zero at the middle of the membrane. .

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. (a) The number of water molecules passing through the strained N3 membrane at 100 MPa. (b) Water flux vs. strain at 100 MPa. The inset shows the enlarged pore, with a shadow indicating the pore surface area (S), and the values of S are provided in parentheses when tensile strain is applied. (c) PMF of water across the N3 membrane without strain (0%) and with a strain of 3%, 6%, and 9%. (d) Salt rejection vs. pressure by the N3 membrane under different tensile strains, and the inset plots show the water permeability under 3 ~ 6% strain.

28


Page 28 of 29

Page 29 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic

29


Rational Design and Strain Engineering of Nanoporous Boron Nitride

Recommend Documents