Letter pubs.acs.org/JPCL
Physics behind Water Transport through Nanoporous Boron Nitride and Graphene Ludovic Garnier,† Anthony Szymczyk,‡ Patrice Malfreyt,¶ and Aziz Ghoufi*,† †
Institut de Physique de Rennes, IPR, UMR CNRS 6251, 263 Avenue du Général Leclerc, 35042 Rennes, France Institut des Sciences Chimiques de Rennes, CNRS, UMR 6226, Université de Rennes 1, 263 Avenue du Général Leclerc, 35042 Rennes, France ¶ Institut de Chimie de Clermont-Ferrand, ICCF, UMR CNRS 6296, BP 10448, F-63000 Clermont-Ferrand, France ‡
S Supporting Information *
ABSTRACT: In this work, molecular dynamics simulations were used to determine the surface tension profile of water on graphene and boron nitride (BN) multilayers and to predict water permeation through nanoporous graphene and BN membranes. For both graphene and BN multilayers, a decrease in surface tension (γ) was evidenced as the number of layers increased. This lessening in γ was shown to result from a negative surface tension contribution due to long-range wetting of water, which also contributes to lower water permeation through a two-layer membrane with respect to permeation through a monolayer. We also showed that a decrease in water surface tension on a BN monolayer with regards to graphene was at the origin of an increase in water permeation through BN. Our findings suggest that nanoporous BN membranes could be attractive candidates for desalination applications.
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nanoporous materials, carbon nanotube (CNT) arrays have been examined for desalination.4 Nevertheless, although both experiments and molecular dynamics (MD) simulations have demonstrated that CNTs can allow fast water flow,5−8 the relatively low ion rejection and the difficulty of producing highquality CNT arrays seriously limit their application at the industrial scale. More recently, it has been shown that the use of nanoporous graphene,9−14 graphyne,15,16 and MoS217 might open new avenues for water treatment. In various processes ranging from the removal of industrial and agricultural contaminants from wastewater to seawater desalination, membranes made with these materials might provide alternatives to conventional water treatment technologies, such as polymeric membrane-based filtration and ion exchange. Indeed, these membranes have very well-defined nanopores through which water molecules could flow while ion passage can be blocked given their larger hydrated size.9 Since the work of Cohen-Tanugi and Grossman (2012)9 highlighting improved water permeability through a graphene nanoporous membrane, several groups have tackled the molecular understanding of microscopic mechanisms driving water and ion transport through these materials. While these works focused on the nanopore design (pore size and chemical functionalization of the pore surface) to improve water permeability, only few works attempted to connect interfacial
ater is ubiquitous in all sectors of society, from drinking to agriculture and from energy supply to industrial manufacturing. With shortages of conventional water sources, new technologies for water supply have a crucial role to play in addressing the world’s clean water needs in the 21st century. Desalination is in many regards the most promising approach to long-term water supply. Indeed, the stock of salted water, which represents about 97% of the world’s water, gives desalination the advantage of a virtually unlimited supply.1,2 Membrane separation processes constitute a basic element in chemical engineering. The potential benefits of these techniques are that they are energy efficient (no phase change), environmentally friendly (in the sense that they require no or limited chemicals addition), modular, and compact. The significant improvements performed in research and development of synthetic polymeric membranes over the past two decades have made membrane processes good candidates to deal with fresh water shortage issues. This class of green separation processes makes use of membranes that act as selective barriers between two phases that are not in thermodynamic equilibrium. Reverse osmosis (RO) is one of these membrane processes. Seawater desalination using RO membranes has become a common method for countries with direct access to the sea. While this technology has proved to be efficient, it remains, however, relatively costly because of the use of high-pressure pumps and the small permeation rate of RO membranes.1−3 Thus, only wealthy countries are able to produce drinking water from RO. One route to overcome the current limitations of RO consists of using graphitic membranes. Among popular © 2016 American Chemical Society
Received: June 20, 2016 Accepted: August 9, 2016 Published: August 9, 2016 3371
DOI: 10.1021/acs.jpclett.6b01365 J. Phys. Chem. Lett. 2016, 7, 3371−3376
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Figure 1. (a) Illustration of water on graphene where red, white, blue, pink, and gray atoms represent the oxygen, hydrogen, nitrogen, boron, and carbon atoms, respectively and scheme allowing the identification of the opposite water reservoir related to a graphene layer. Blue regions correspond to interfacial water. (b) Total surface tension of water on boron nitride (BN) and graphene surfaces. (c) Profile of water surface tension close to a single layer (n = 1) of graphene and boron nitride at 300 K and 1 bar. (d) Enlargement between z = −1 Å and z = 5 Å.
properties, such as surface tension and friction,18 to water and ion transport through graphitic monolayers. Indeed, interfacial properties are fundamental to understand and rationalize transport mechanisms as well as to improve separation performance. The experimental determination of contact angle depends on the impurities and defects on the surface, which may lead to scattering of contact angle values. Because the material surface is critical for the compatibility with the surrounding environment, a number of molecular simulations have been performed to describe the interfacial region at the atomistic scale. Some of these atomistic simulations have been used to predict the contact angle of water on different surfaces.19,20 The contact angle value depends on the chemical nature of the surface and other parameters such as roughness and chemical heterogeneity, which have been much less investigated from this theoretical approach. Another way for estimating solid−liquid interactions is to compute the solid− liquid interfacial tension. We propose here to apply the thermodynamic definitions of the interfacial tension to compute the solid−liquid interfacial tension for graphene− water and boron nitride−water systems. Furthermore, to date only graphene and graphyne membranes have been investigated while boron nitride (BN) monolayers have never been studied as potential nanofilters. Compared with CNTs, BN nanotubes exhibit improved electronic properties, high chemical stability, improved biocompatibility, and high resistance to oxidation. Furthermore,
BN nanotubes have shown superior water permeation properties compared to CNTs of similar diameter and length.21 By using molecular dynamics (MD) simulations, the surface tension profile of water close to graphene and hexagonal BN multilayers was managed and a correlation between water transport through nanoporous membrane and surface tension was established. This connection between transport and surface tension was explored by pressure-driven MD simulations of water through nanoporous graphene and BN multilayers.The main aims of this work were (i) to predict water surface tension on graphene and BN surfaces and (ii) to connect water transport through nanoporous graphene and BN membranes with surface tension. Force fields, computational details, and surface tension calculations are detailed in the Supporting Information. Surface Tension. We report in Figure 1a the surface tension of water on both graphene and boron nitride as a function of number of layers (n). Let us mention that the quantitative agreement between experiment22,23 and calculation of water surface tension on a single graphene layer makes us confident in the quality of the model used in this work. It can be noted that the experimental surface tension (91.5 mN m−1) was obtained by measuring the contact angle on a graphene monolayer. Figure 1a shows (i) a higher surface tension of water on a single graphene layer than on a single boron nitride layer, which is in good agreement with experiment,20 and (ii) a decrease in surface tension as n increases. For both graphene and BN 3372
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Figure 2. Local surface tension of water close to graphene systems with different layer numbers (from n = 1 to n = 6). Red dashed lines correspond to the location of graphene layers.
systems, the surface tension tends to be constant from n = 2. It should be stressed that the flexibility and chirality (zigzag configuration) of the graphitic surface do not impact the surface tension profile (see Figure S3 of the Supporting Information). Moreover, we showed that the surface tension of water on a BN monolayer is mainly due to van der Waals interactions. Indeed, by using a full Lennard-Jones model without electrostatic interactions a surface tension of 76 mN m−1 was computed while a value of 78 mN m−1 was obtained by considering the electrostatic contribution. Thus, the difference between graphene and BN behaviors was mainly due to short-range interactions. To unravel the difference between graphene (γg) and BN BN (γ ), surface tension profiles along the normal of the interface (i.e., the z-axis) were calculated. As shown in Figure 1b, three main peaks were put in evidence on the central graphene layer and on the first interfacial water layers (see Figure 1a). Figure 1b highlights a difference between the intensity of central peaks between BN and graphene, with γg > γBN, which indicates a higher tension on graphene and a better wetting of water on BN. Although the contribution of surface tension located on the first water layer was higher with BN, the total surface tension of water on graphene was greater. As shown in Figure 1c, the increase in the local surface tension of water on BN with respect to graphene was compensated by a negative peak of higher intensity beyond the interface. As shown in Figure S6a, this tension can be attributed to the layered structure induced by excluded volume effects, and this surface tension contribution is due to the difference in density between both water layers.
This increase in wetting with BN is connected to the preferential interactions between surface and water molecules. Although radial distribution functions (RDFs) between atoms of both surfaces and water molecules have similar shapes (see Figure S7 of the Supporting Information) and a minimum located at the same position, the difference in RDF intensity highlights more favorable interactions between BN and water molecules. This can also explain the increase in water density in the vicinity of the BN surface compared with graphene (Figure S6a) as well as the increase in the number of hydrogen bonds close to the BN/water interface (Figure S6b). This was corroborated by the calculation of the total energy between solid surfaces and water molecules, −49.8 kJ mol−1 and −34.9 kJ mol−1 for BN and graphene, respectively. First we focus on the decrease in surface tension when n increases from 1 to 2. We report in Figure 2 the profile of water surface tension on graphene membranes from n = 1 to n = 6. For n = 1 and n = 2, it can be seen (i) that the intensity of the local surface tension of water close to the interface is similar in both cases and (ii) a dramatic decrease in γ(z) on the graphene is obtained for n = 2 with respect to n = 1. This lessening establishes a negative contribution occurring for n = 2. To highlight this negative contribution we performed MD simulations of water for n = 1 and n = 2 with a single water reservoir. As shown in Figure S8 of the Supporting Information, the negative contribution of surface tension was well evidenced. This behavior can be understood if we consider both water reservoirs’ contributions on one graphene layer. As the contributions of water layers are similar in both n = 1 and n = 2 cases, the decrease in γ for n = 2 with respect to n = 1 and 3373
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polymeric membranes for water desalination.9−14 Thus, many works have been carried out to predict and understand water permeability and salt rejection across these NPG membranes. However, no work has been attempted to explore the interplay between surface tension and permeability. Therefore, we performed pressure-driven MD simulations of water through NPG and nanoporous boron nitride (NPBN). First, a nanopore with a surface area of 67.2 Å2 was carved in 2D graphene and BN materials (see Figure S2 of the Supporting Information). A pore diameter of 7 Å was chosen to be in line with the work of Cohen-Tanugi and Grossman9 who showed the highest water permeation with this pore size. Transport was modeled by means of a pressure difference imposed by a graphitic piston. Details of MD simulations and systems are given in the Supporting Information. In order to distinguish water penetration and steady flow regime, pre-equilibration in the NpAT ensemble was conducted at 300 K and 0.1 MPa, where N is the number of molecules, p the pressure, A the surface area, and T the temperature. As shown in Figure S11a, the number of filtered water molecules increased linearly as a function of time, which indicates a steady-state flow. Additionally, to focus on water transport, the initial configuration consisted of two water reservoirs contacting the membrane instead of a single water reservoir on one side of the membrane and an empty box on the other side. This route allowed us to avoid considering the gas−liquid−solid contact line. As shown in Figure 3, water permeability through a NPBN monolayer is higher than through NPG. This result could result
then the negative contribution clearly result from interactions with water in the opposite reservoir (see Figure 1a for an illustration). From a physical point of view this negative contribution in surface tension can be understood as an increase of wetting on water on the opposite graphene layer. Indeed, the range of interactions between graphene and water molecules located at the opposite reservoir is 6.5 Å and corresponds to weak interactions, which explains the increased wetting of the water in the opposite reservoir (long-range wetting). Because the negative contribution is related to the distance between the surface and water in the opposite reservoir, this effect can be accentuated by increasing the number of layers. As shown in Figure 2 for n = 3, a negative contribution is evidenced in the central region. For n = 4, two negative peaks are observed, the sum of which is similar to the negative contribution obtained for n = 3. For n = 3, the distance between interfacial water layers and the central graphene layer is around 6.5 Å. For n = 2, the distance between the graphitic surface and the interfacial water layer at the opposite reservoir is also 6.5 Å. As shown in Figure 2 for n = 3, the positive contributions on two graphene layers do not compensate the negative contribution of the central layer. We found that the negative contribution for graphene and water located at 6.5 Å is −16.1 mN m−1. For n = 3, the distance between the graphene layer with positive contribution in surface tension and water at the opposite reservoir is 9.7 Å. This leads to a lower negative contribution than for the central layer, which decreases the surface tension contribution on the graphitic surface (12.7 mN m−1) with respect to γ(z) for n = 1 (19.7 mN m−1). This effect is emphasized as n increases. Indeed, for n = 4 the distance between graphene layer and water in the opposite reservoir is 12.9 Å, thus leading to a weak negative contribution and a surface tension contribution of 14.9 mN m−1. From n = 4, the surface tension contribution of the interfacial water layers and the closest graphene layer becomes constant because the distance between this layer and water molecules located in the opposite water reservoir is too high, thus leading to negligible interactions. As shown in Figure 2, this tendency was corroborated for n = 5 and n = 6. In summary, the negative surface tension contribution and the so-associated long-range wetting result from the long-range interactions between interfacial layers and opposite water reservoirs. Interestingly, while the difference in surface tension between both graphene and BN monolayers (n = 1) is 16.4 mN m−1, it increases up to 33.8 mN m−1 for n = 2. The lessening in surface tension between n = 1 and n = 2 is higher with BN because of a higher negative contribution in surface tension from n = 2 (see Figure S9 of the Supporting Information). Indeed, for n = 2, the total negative contribution (prepeak of tension linked to the first water layer and the central zone) with BN is estimated to −37.9 mN m−1 against −28.1 mN m−1 for graphene (see Figure S10 of the Supporting Information). This shows an increase in the long-range wetting of water close to the BN surface with respect to graphene. As previously shown for n = 1, the longrange wetting of water for boron nitride compared with graphene is connected to the preferential interactions between surface and water molecules. Water Permeation through Nanoporous Membranes. Because surface tension rules fluid transport through an interface, we explored the impact of long-range wetting on water permeation. Recently, it was shown that two-dimensional (2D) nanoporous graphene (NPG) could be a powerful alternative to usual
Figure 3. Number of water molecules crossing NPG and NPBN membranes as a function of the transmembrane pressure difference.
from the lower surface tension of water on NPBN compared with NPG. We report in Figure 4a,b the 2D contour plots of water density along the zx plane (between y = −2 Å and y = +2 Å) for NPG and NPBN monolayers. Figure 4 shows that water molecules are slightly filtered at the NPG pore center; water molecules slip on the hydrogenated edges. On the other hand, as shown in Figure 4b, more water molecules are filtered at the NPBN pore center while other molecules also slip on the hydrogenated edges. Interestingly, Figure 4b highlights two water overdensities at both sides of the pore center (at z = −2 and 2 Å) in the case of NPBN (with respect to NPG), which is in line with the higher water permeation through the NPBN monolayer. Let us note that the local increase in water density is also evidenced through the density profile along the z-axis (see Figure S11b of the Supporting Information). To 3374
DOI: 10.1021/acs.jpclett.6b01365 J. Phys. Chem. Lett. 2016, 7, 3371−3376
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Figure 4. Two-dimensional density contour plots of water density according to the xz-plane for nanoporous graphene (a) and nanoporous BN (b).
Figure 5. (a) Local surface tension of water on nanoporous graphene and BN monolayers. (b) Local surface tension of water on nanoporous graphene and BN bilayers.
their work, because the interaction between water and BN is stronger than that between water and graphene, a higher water permeation through NPBN monolayer should be expected, which is line with our results. As shown in Figure 3, water permeability through the bilayer membrane (n = 2) was smaller than through monolayers, in agreement with results reported by Cohen-Tanugi et al.25 Interestingly, the difference in water permeability between monolayers and bilayers was more pronounced for BN. In an attempt to understand these findings, the water surface tension close to both bilayers was determined. For the graphene bilayer the decrease in surface tension is due to long-range wetting (negative surface tension), which decreases water molecule sliding through the nanoporous membrane. For the BN bilayer a surface tension gradient is also observed, but as shown in Figure 5b, a peak is evidenced between the two BN layers (Figure 5b). As observed in Figure 5b, this high tension between both BN sheets is due to an enhancement of water density at this position (see inset of Figure 5b). Indeed, water molecules are strongly bounded to both BN surfaces by hydrogen bonds given the short distance between the two BN layers (3.3 Å). To overcome this effect it is necessary to move away the BN layers. An additional simulation was performed by setting the distance between the BN layers to 8 Å to avoid the formation of bridges between interlayer water molecules and the two BN surfaces.The increase in the separation distance of BN layers led to a substantial increase in water permeability by
understand the so-observed difference between both systems, the local surface tension of water on both nanoporous membranes was computed. Figure 5a shows an obvious difference in γ(z) between NPBN and NPG membranes. Indeed, the local surface tension on the graphene membrane was found to be 40 mN m−1, while it was around 7 mN m−1 on the BN membrane. Additionally, while only one peak with an intensity of 40 mN m−1 was evidenced for graphene, two peaks with an intensity of 20 mN m−1 were found on both sides of the BN membrane. These surface tension profiles are in line with the density profiles reported in Figure 4. This difference in the local surface tension is probably at the origin of the enhanced water permeation through the NPBN monolayer with respect to the graphene one because the high local surface tension of water on graphene limits pressure-driven water permeation. In the case of the NPBN monolayer the two peaks located on each side of membrane lead to a surface tension gradient that favors water permeation through the membrane (this is similar to the Marangoni effect). The surface tension is a force that acts on a water molecule crossing through the nanopore. Whatever the force related to the applied pressure, an interfacial resistance remains, thus impacting water permeation. This can be clearly observed by the difference in water permeation between both BN and graphene membranes that differ only from the interfacial interactions. Recently, Zhu et al. showed that water flow through nanopores was impacted by the interaction between water and the edge of the nanopore.24 On the basis of 3375
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(6) Kalra, A.; Garde, S.; Hummer, G. Osmotic water transport through carbon nanotube membranes. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 10175−10180. (7) Holt, J. Fast Mass Transport Through Sub-2-Nanometer Carbon Nanotubes. Science 2006, 312, 1034−1037. (8) Falk, K.; Sedlmeier, F.; Joly, L.; Netz, R.; Bocquet, L. Molecular Origin of Fast Water Transport in Carbon Nanotube Membranes: Superlubricity versus Curvature Dependent Friction. Nano Lett. 2010, 10, 4067−4073. (9) Cohen-Tanugi, D.; Grossman, J. Water Desalination across Nanoporous Graphene. Nano Lett. 2012, 12, 3602−3608. (10) Nair, R.; Wu, H.; Jayaram, P.; Grigorieva, I.; Geim, A. Unimpeded Permeation of Water Through Helium−Leak−Tight Graphene-Based Membranes. Science 2012, 335, 442−444. (11) Wang, E.; Karnik, R. Water desalination: graphene cleans up water. Nat. Nanotechnol. 2012, 7, 552−554. (12) Cohen-Tanugi, D.; Grossman, J. Mechanical Strength of Nanoporous Graphene as a Desalination Membrane. Nano Lett. 2014, 14, 6171−6178. (13) Surwade, S.; Smirnov, S.; Vlassiouk, I.; Unocic, R.; Veith, G.; Dai, S.; Mahurin, S. Water desalination using nanoporous single-layer graphene. Nat. Nanotechnol. 2015, 10, 459−464. (14) Koh, D.-Y.; Lively, R. Nanoporous graphene: Membranes at the limit. Nat. Nanotechnol. 2015, 10, 385−386. (15) Zhu, C.; Li, H.; Zeng, X. C.; Wang, E.; Meng, S. Quantized Water Transport: Ideal Desalination through Graphyne-4 Membrane. Sci. Rep. 2013, 3, 3163. (16) Kou, J.; Zhou, X.; Lu, H.; Wu, F.; Fan, J. Graphyne as the membrane for water desalination. Nanoscale 2014, 6, 1865−1870. (17) Heiranian, M.; Farimani, A.; Aluru, N. Water desalination with a single-layer MoS2 nanopore. Nat. Commun. 2015, 6, 8616−8622. (18) Tocci, G.; Joly, L.; Michaelides, A. Friction of Water on Graphene and Hexagonal Boron Nitride from Ab Initio Methods: Very Different Slippage Despite Very Similar Interface Structures. Nano Lett. 2014, 14, 6872−6877. (19) Werder, T.; Walther, J.; Jaffe, R. L.; Halicioglu, T.; Koumoutsakos, P. On the water-carbon interaction for use in molecular dynamics simulations of graphite and carbon nanotubes. J. Phys. Chem. B 2003, 107, 1345−1352. (20) Li, H.; Zeng, C. Wetting and Interfacial Properties of Water Nanodroplets in Contact with Graphene and Monolayer BoronNitride Sheets. ACS Nano 2012, 6, 2401−2409. (21) Won, C.; Aluru, N. Water Permeation through a Subnanometer Boron Nitride Nanotube. J. Am. Chem. Soc. 2007, 129, 2748−2749. (22) Wang, S.; Zhang, Y.; Abidi, N.; Cabrales, L. Wettability and Surface Free Energy of Graphene Films. Langmuir 2009, 25, 11078− 11081. (23) Rafiee, J.; Mi, X.; Gullapalli, H.; Thomas, A.; Yavari, F.; Shi, Y.; Ajayan, P.; Koratkar, N. Wetting transparency of graphene. Nat. Mater. 2012, 11, 217−222. (24) Zhu, C.; Li, H.; Meng, S. Transport behavior of water molecules through two-dimensional nanopores. J. Chem. Phys. 2014, 141, 18C528−18C533. (25) Cohen-Tanugi, D.; Lin, L.-C.; Grossman, J. Multilayer Nanoporous Graphene Membranes for Water Desalination. Nano Lett. 2016, 16, 1027−1033.
about 150% (from 6.4 molecules/ns to 15.6 molecules/ns). In contrast with BN, water permeation through nanoporous graphene decreased when the separation distance was increased (45.1 molecules/ns and 15.3 molecules/ns for separation distances of 3.3 and 8 Å, respectively). In summary, molecular dynamics simulations were performed to predict surface tension of water on BN and graphene surfaces. Surface tension was found to be smaller on BN than on graphene because of more favorable interactions between B and N atoms and water molecules. This difference in surface tension directly impacted water permeability through nanoporous membranes because a greater wetting increases water molecule sliding along the nanopore. Furthermore, the surface tension of water on both BN and graphene multilayers was found to decrease by increasing the number of layers from n = 1 to n = 2 and then became almost constant for n > 2. This decrease in surface tension resulted from long-range wetting of the interfacial water located beyond 6.5 Å from the surface. This long-range wetting is at the origin of a negative surface tension contribution that contributes to lower water permeability through BN and graphene bilayer membranes with respect to monolayer membranes. Finally, this work allowed us to show negative surface tension contribution and the so-associated long-range wetting to determine the interplay between surface tension and water permeability and to highlight higher water permeability through boron nitride monolayers than through graphene monolayers.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01365. Force field, computational procedure, details about surface tension calculations, and density and surface tension profiles and snapshots (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: aziz.ghoufi@univ-rennes1.fr. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the financial support from the program Champlain 65.102.
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REFERENCES
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