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Atomic insight into water and ion transport in 2D interlayer nanochannels of graphene oxide membranes: Implication for desalination
Journal Pre-proof Atomic insight into water and ion transport in 2D interlayer nanochannels of graphene oxide membranes: Implication for desalination Wen Li, Lei Zhang, Xinyu Zhang, Mutian Zhang, Tengfei Liu, Shougang Chen PII:
S0376-7388(19)33119-9
DOI:
https://doi.org/10.1016/j.memsci.2019.117744
Reference:
MEMSCI 117744
To appear in:
Journal of Membrane Science
Received Date: 8 October 2019 Revised Date:
20 November 2019
Accepted Date: 9 December 2019
Please cite this article as: W. Li, L. Zhang, X. Zhang, M. Zhang, T. Liu, S. Chen, Atomic insight into water and ion transport in 2D interlayer nanochannels of graphene oxide membranes: Implication for desalination, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2019.117744. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Atomic Insight into Water and Ion transport in 2D Interlayer Nanochannels of Graphene Oxide Membranes: Implication for Desalination Wen Lia, Lei Zhanga, Xinyu Zhanga, Mutian Zhanga, Tengfei Liua, Shougang Chena*
a
School of Materials Science & Engineering, Ocean University of China, Qingdao 266100 PR China
Corresponding author: Shougang Chen E-mail address: [email protected]
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Abstract Currently, graphene oxide (GO) membrane is becoming a new-generation material for desalination application with high water permeability and ion rejection rate. The unusual 2D interlayer nanostructure in GO membrane provides a reliable and precise molecular sieving function for fast water transport and ion rejection. However, the studies on molecular mechanisms for water and ion transport in the 2D interlayer nanochannels of GO membrane are far from adequate, and the trade-off between selectivity and permeability is another problem significantly restraining the further development of GO membrane for desalination. In this work, molecular dynamics simulations were conducted to investigate the atomic mechanism of water and ion transport in 2D nanochannels of GO membrane, and the influences of interlayer space and nanostructure on water and ion transport were also revealed. We find that there are strong electrostatic, vdW and hydrogen bond interactions between water/ion and the oxygen-containing groups in GO nanosheets, which largely impedes their transport. The hydration interaction also plays an important role in ion adsorption in the 2D nanochannels. The fast water and ion transport in GO membrane mainly occurs in the non-oxidized region of neighboring GO nanosheets. By combing the influences of interlayer space and nanostructure on water and ion transport, a new design principle for high-efficient GO desalination membrane is proposed. The findings in this work expand our understanding of water and ion transport in GO membrane and may also greatly promote the development of desalination membrane prepared by other 2D materials. Keywords: Graphene oxide; Desalination; 2D nanochannel; Molecular transport; Molecular dynamics simulations
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1. Introduction Nowadays, fresh water shortage has become a worldwide problem because of water pollution and waste[1, 2]. Seawater, which covers 71% of the surface area of our earth, provides an adequate fresh water supply if advanced desalination method is developed. Therefore, in the past decades, many studies have been triggered to develop new methods for seawater desalination[3, 4]. Among which, desalination through reverse osmosis (RO) membrane has become a widely applied technology due to its high-energy efficiency[5-7]. Currently, preparing advanced RO membranes with robust mechanical property, high separation performance (i.e. high water flux and high ion rejection rate), and low operation cost and energy consumption are the main developmental direction for seawater desalination technology[8, 9]. Recently, nanofiltration RO membranes prepared from two-dimensional (2D) materials have presented great potential for desalination application[10-13]. Especially for graphene oxide (GO), the interlayer gallery assembled from atomic-thin GO nanoflakes offers unique 2D nanochannels for fast water transport meanwhile rejecting the transport of other small molecules and ions[14-18]. Up to now, many theoretical[19-21] and experimental[22-24] studies have shown that GO membrane presents superior desalination performance. Overall, previous studies mainly focus on the regulation of interlayer spaces of GO membrane (e.g. chemically functional group modification and physically embedding nanoparticles) to pursue more efficient desalination. Although the increased interlayer space can enhance water permeability, the corresponding ion rejection rate will decrease. Typically, these regulating methods for interlayer space of GO membrane are not easy to realize, meanwhile the trade-off relationship between water permeability and ion rejection rate is another problem needing to be reconsidered. 4
GO is an oxidized form of graphene (GN) including epoxide, hydroxyl, and carboxylic acid groups on it[25-27]. Normally, the epoxide and hydroxyl groups mainly distribute on the base plane of GN, whereas carboxylic acid groups distribute on the edges. However, the distribution of epoxide and hydroxyl groups is not uniform on GN plane[25, 28]. Yan et al. had proved that epoxide and hydroxyl groups tended to aggregate on GN plane[29]. In other word, GO nanosheet can be divided into oxidation and non-oxidation region, and therefore, the oxidation region presents island-like distribution. Considering the nonuniform distribution of oxygen-containing groups on GN sheet, the stacking ways of neighboring GO nanosheets (i.e. oxygen-containing groups facing each other, oxygen-containing groups with pristine GN patches facing each other, and pristine GN patches facing each other) during membrane preparation (e.g. vacuum filtration[30, 31]) can influence the interlayer nanostructures. Meanwhile, the interlayer nanostructures govern the transport of water and ion in the 2D nanochannels between neighboring GO nanosheets. Therefore, the desalination performance of GO membrane can be improved through controlling the assembly structure of neighboring GO nanosheets. However, hardly any studies have been designed to investigate the influence of stacking ways of GO nanosheets on the corresponding desalination performance. In this work, water and ion transport behavior in 2D nanochannels between neighboring GO nanosheets was studied by using atomic non-equilibrium molecular dynamics (MD) simulations. Specially, we considered the influences of the stacking ways of heterogeneously oxidized GO nanosheets on water and ion transport. The complicated cross-interactions between stacking ways and interlayer spaces of GO membrane were revealed. This work also provides a clear design principle to prepare GO membrane for high-efficient desalination by controlling the interlayer space and interlayer nanostructure of GO membrane. 5
2. Computational method 2.1 Models The studied model system is shown in Figure 1A, where a 2D GO nanochannel is held between two fixed He slabs with cracks matching the interlayer space of the nanochannel. The He slab is selected because it has a weak interaction with water and ion, and therefore, the influence of slab adsorption effect on water and ion transport is ignorable. Actually, other fixed slabs can also be used like graphene slabs. However, there are strong nonbonding (vdW) interactions between water molecules and graphene slabs. This will generate stable adsorption layers of water molecules around the slabs[32, 33], which will in turn influence the water and ion getting in and out of the channel. In this work, we only intend to study the transport behavior of water and ion in the 2D interlayer nanochannels of GO membrane and reveal its influence on desalination performance of GO membrane. Therefore, the influence of adsorption effects on water and ion transport originated from the rigid slabs should be excluded. Another two rigid GN slabs are put at the rightmost and leftmost of the system to keep a constant pressure difference between the left and right regions. During simulation, the carbon skeleton of the GO nanosheets was fixed. Constant High pressure (P1, 100 MPa) was applied on the left GN slab to push the salt water (0.5 mol/L) flow from left to right region, and atmospheric pressure (P2, 0.1 MPa) was applied on the right GN slab. Indeed, for GO membrane, more realistic models have been studied before, which includes important features such as imperfect stacking of GO flakes, swelling and collapse of GO membrane, and a broad distribution of interlayer spaces[34-37]. However, GO nanosheets with fixed carbon skeleton were used in this work. This is because that the interlayer space between neighboring GO sheets is a key factor to influence water and ion transport. To study the influence of interlayer space on water and ion transport, accurate 6
controlment of interlayer space is needed, and this just could be realized through fixed carbon skeleton of GO sheets. To offset the flexible influence of GO sheets on water and ion transport, the oxygen-containing groups in the GO sheets are not fixed, which can move freely. To study the effects of stacking ways of neighboring GO nanosheets on the desalination performance of GO membrane, GO nanosheets with different distribution of oxygen-containing groups are built. For simplification, carboxylic acid groups are not considered, and epoxide and hydroxyl groups are evenly distributed in GO nanosheet with C/O ratio of ~3/1. By using these built GO nanosheets, three kinds of 2D GO nanochannels with different interlayer nanostructures are constructed. Figure 1B shows the perspective view of F typed 2D GO nanochannel (F-channel), which is constructed by using two neighboring GO nanosheets with uniform distribution of oxygen-containing groups. When the GO nanosheets are replaced by GO nanosheets with stripe-like distribution of oxygen-containing groups, S and D-types of 2D GO nanochannels are formed. For the S-channel in Figure 1C, stripe-like non-oxidized region only exists in the bottom GO nanosheet, and for the D-channel in Figure 1D, both the nanosheets are GO with stripe-like non-oxidized regions in the middle of the sheets. For the GO nanosheet with stripe-like distribution of oxygen-containing groups, the width of the non-oxidized region is set as 6.15 Å. The influence of interlayer space of the 2D GO nanochannel on its desalination performance was also studied. According to reported work[38], the interlayer space for GO membrane is about 7-13 Å based on ambient humidity. Therefore, the systems with interlayer spaces of 8.3, 9.3, and 10.3 Å were simulated, where the interlayer space was determined based on the perpendicular distance of the carbon skeleton of the neighboring GO nanosheets.
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Figure 1. (A) The constructed system used in this work; (B-D) Perspective views of the 2D nanochannels between neighboring GO nanosheets from z direction. Blue and green balls represent Na+ and Cl- ions, respectively, and water is shown as transparent cyan.
2.2 Simulation setup All the non-equilibrium MD simulations were conducted by using LAMMPS package[39]. All-atom optimized potential for liquid simulations (OPLS-AA) force filed was adopted for GO, Na+, and Clions. According to our previous works[40, 41] and reported results[42, 43], this force field can describe GO accurately, and TIP3P model was used for water. Furthermore, the SHAKE algorithm was used to constrain the bonds and angles of water molecules[44]. The nonbonded force field parameters were obtained from literatures[45, 46][39, 40]. The pairwise Lennard-Jones (L-J) terms were calculated by using Lorentz-Berthelot combining rules. Periodic boundary conditions were applied in X and Y directions. All the studied systems were simulated for 20 ns with a time step of 1 fs. The temperature was controlled at 298 K by using a Nosé-Hoover thermostat with coupling time constant of 1 ps. The cut-off distance for van der Waals (vdW) interaction was set as 12 Å, and particle-particle-particle-mesh (PPPM) method was used to calculate the long-rang electrostatic 8
interaction. The obtained data were collected every 1 ps. The first 4 ns was used to form stable water flow, and the last 16 ns was used for analysis.
3. Results and discussion 3.1 Water flux.
Figure 2. Time evolution of the number of passed water molecules through the (A) F, (B) S, and (C) D-channels with different interlayer spaces; (D) Water flux of the F, S, and D-channel with different interlayer spaces.
Figure 2A-C shows the time evolution of the number of water molecules passed through the studied 2D GO nanochannels with different interlayer spaces, respectively. These curves present a nearly linear relationship between the passed water molecules and time. Therefore, water flux can be calculated through linear fitting of the curves in Figure 2A-C, and the results are shown in Figure 2D. It shows that the water flux increases with the increase of interlayer spaces. As the increase of interlayer spaces, on one hand, it will be easier for water or ion entering and exiting the 2D interlayer 9
nanochannel due to the enlarged entrance and exit sizes; on the other hand, the wider 2D channel has larger volume to accommodate more water molecules in it (Figure 4), which weakens the interfacial viscous resistance for water and ion transport. In this work, we only conducted 20 ns simulation for each system. For a longer time simulation, we think that this linear dependence between water flow and time in Figure 2A-C cannot be maintained because that compared with ions, the fast water transport in the 2D nanochannel will lead to the increase of ion concentration in the left reservoir. Therefore, water flux will decrease after long time simulation. It should be also noticed that the transport efficiency varies with each other in the 2D nanochannel with different interlayer nanostructures. In Figure 2D, at same interlayer space, water flux in the studied 2D GO nanochannels follow the order of D-channel>S-channel>F-channel except for the S-channel with interlayer space of 10.3 Å. In addition, compared with interlayer space, the influence of interlayer nanostructure on water flux is milder. The influences of interlayer spaces and interlayer nanostructures of the 2D GO nanochannels on the transport behavior of water molecules are also investigated by calculating the averaged 2D relative density maps of water molecules in the studied GO nanochannels, and the results are shown in Figure 3. From this figure, the following points can be concluded: (1) From the density maps in XZ planes, water content in the 2D GO nanochannels increases with interlayer spaces (e.g. increased dark red regions from A1, A3, to A5), and the water molecules in the nanochannels are not uniformly distributed (different colors in same figure); (2) From the density maps of XY planes, the structure of water molecules in the nanochannels turns from one layer to double layers when the interlayer space increases from 8.3 to 9.3 Å (e.g. double layers of dark red in A4); (3) For the S and D-channels with interlayer spaces of 9.3 and 10.3 Å, there are one obvious strip-like dark red region (between the 10
dotted black lines in B3-B6 and C3-C4), which suggests a higher water density. For point (1), the increased interlayer space will generate more free volume for molecular adsorption and diffusion, which will lead to a higher water content and the corresponding higher water flux. For the heterogeneous distribution of water molecules, it is caused by the strong dipolar interactions between polar oxygen-containing groups in GO and water molecules, which results in the accumulation of water molecules around the oxygen-containing groups. Although the oxygen-containing groups in the GO nanosheets are uniformly distributed, the oxidation degree is only about 30%. Therefore, the intervals among the oxygen-containing groups are relatively large. As a result, heterogeneous distribution of water molecules in the 2D nanochannels of GO is observed. For point (2), previous studies[37, 41] have proved that water structure in confined 2D nanochannel changes with the corresponding interlayer spaces. Layered water structure can be formed due to interfacial induction from the neighboring GO nanosheets. For the system with interlayer space of 8.3 Å, it can only accommodate one water layer in it due to its limited interlayer space. Once the interlayer space increases to 9.3 Å, the neighboring GO nanosheets can bound one water layer, respectively, and as a result, double water layers are formed. For point (3), we marked the coordinates of the dark red regions, and found that these regions just fall in the non-oxidized regions of the neighboring GO nanosheets in the S and D-channels (Figure 1C and D). The non-oxidized regions have large free volume, which can accommodate more water molecules in them. For the S and D-channels with interlayer space of 8.3 Å, the accumulation of water molecules in the non-oxidized regions is not obvious. This is because that although the free volume for water adsorption at the non-oxidized region increases compared with oxidized region, the enlarged volume is very limited due to the finite interlayer space. There is also only one ordered water layer at 11
non-oxidized regions.
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Figure 3. The 2D density maps of water molecules inside the GO nanochannels. The density maps in XZ (A1, B1, and C1) and XY (A2, B2, and C2) planes of the F, S, D-channels with interlayer distance of 8.3 Å, respectively; The density maps in XZ (A3, B3, and C3) and XY (A4, B4, and C4) planes of the F, S, D-channels with interlayer distance of 9.3 Å, respectively; The density maps in XZ (A5, B5, and C5) and XY (A6, B6, and C6) planes of the F, S, D-channels with interlayer distance of 10.3 Å, respectively.
In Figure 4, we also calculated the time evolution of the total number of hydrogen bonds (H-bonds) between GO nanosheets and water molecules to evaluate their interfacial interactions. In this work, the H-bonds are defined according to a geometrical criterion. The GO and water molecules are hydrogen-bonded if the following two conditions are fulfilled: (1) the distance between an oxygen atom with a hydrogen bonded to it (the donor, D) and another oxygen atom (the acceptor, A) is less than 3.5 Å; (2) The bond angle of the D-H-A is smaller than 30°. Overall, Figure 4A-C shows that as simulation proceeding, the number of H-bonds fluctuates at a relatively stable region. Therefore, to compare them quantitatively, the average number of H-bonds between water molecules and GO nanosheets is calculated and shown in Figure 4D. From Figure 4D, two points can be concluded: (1) At same interlayer space, the total number of H-bonds between water molecules and GO nanosheets follows the order of F-channel>S-channel>D-channel; (2) At same channel nanostructure, the channel with interlayer space of 9.3 Å has a larger average number of H-bonds followed by the 13
channels with interlayer spaces of 8.3 and 10.3 Å. For point (1), the lacking oxygen-containing group in the non-oxidized region of the GO nanosheets (Figure 1C and D) results in the smaller number of H-bonds between GO and water molecules. For point (2), there is only one water layer in the GO nanochannel when the interlayer space is 8.3 Å (e.g. Figure 3A1-A2), which leads to its small number of H-bonds. However, when the interlayer space increases to 9.3 Å, double water layers are formed (e.g. Figure 3C3-C4). This is to say that more water molecules can accumulate in the channel (Figure 3C3-C4) and form H-bonds with the oxygen-containing groups in GO nanosheets.
Figure 4. Time dependent the number of H-bonds between water molecules and GO nanosheets in F, S, and D-channels with interlayer spaces of (A) 8.3, (B) 9.3, and (C) 10.3 Å; (D) The averaged number of H-bonds in the studied channels.
3.2 Ion transport. To investigate ion transport behavior in the studied channels, we firstly observe ion migration in 14
F-channel with interlayer space of 8.3 Å. Figure 5A-C shows the selected snapshots for ion transport at different time. The purple and orange balls are the hydrogen and oxygen atoms in the hydration shell of Na+ and Cl-. At 5.324 ns (Figure 5A), one Cl- ion has adopted in the channel, and one Na+ ion is approaching the channel. At 5.408 ns (Figure 5B), the Cl- ion stays around its original position, and the Na+ ion locates at the entrance of the channel with fewer molecules in its hydration shell. However, at 5.462 ns (Figure 5C), the Cl- ion still stays around its original position, while the Na+ ion has gone back to the bulk salt solution. The dynamic trajectory in Figure 5A-C can be found in Movie S1 in supporting information (SI). These results indicate that it is extremely hard for ion entry and migration in the F-channel with interlayer space of 8.3 Å. According to previous work[48], the naked radii of Na+ and Cl- ions are 1.16 and 1.81, respectively. We also obtained the hydration radii for Na+ ion (3.35 Å) and Cl- ion (3.95 Å) by calculating the ion-oxygen radial distribution functions (Figure S1). For the channel with interlayer space of 8.3 Å, the effective space for ion diffusion at non-oxidized region is about 4.9 Å (approximately equal to interlayer space, 8.3 Å, minus the vdW diameter of carbon atoms, 3.4 Å). At the oxidized region, this effective space is much smaller because of the existence of hydroxyl and epoxy groups. It can be seen that the effective space in the channel with interlayer space of 8.3 Å is less than the diameters of hydrated Na+ and Cl- ions. As a result, most of the hydrated water molecules must be dehydrated before ion entering the channel, and this will generate a large energy barrier for ion entry in the channel (e.g. the Na+ ion in Figure 5A-C). Even ion can enter the channel, there is a strong energy barrier for ion migration in the channel (e.g. the Cl- ion in Figure 5A-C). This is because that the hydroxyl groups in GO nanosheets can also participate in ion hydration (blue dotted lines in Figure 5A-C). As the increase of interlayer spaces, the effective space for ion entry 15
and diffusion is also increased, which makes it easier for ion entering the channel. As a result, multi ions can enter the channel (Figure 5D and E), which can also move in the channel more freely. The dynamic trajectory for ion transport in F-channels with interlayer space of 10.3 Å is shown in Movie S2 in SI. For the effects of interlayer nanostructures on ion transport, the heterostructure of the GO nanosheets will expand the effective space for ion entry and migration at the non-oxidized region of GO nanosheet because of the larger free volume and lower energy barrier. Ion prefers to enter the 2D channel from the non-oxidized region of the D-channel (Figure 5F and Movie S3-S4). It should be also noted that the influences of carboxylic acid functionalities at the edges of GO nanosheets on water and ion transport are not considered in this work. Actually, previous study has proved that carboxylic acid groups will deprotonate across a broad pH range [49]. This will play a significant role in water and ion transport. Especially for ion transport, the negatively charged carboxylic acid groups will repel Cl- and attract Na+. This will lead to a selective transport of positively charged ions.
Figure 5. The snapshots for ion transport in F-channel with interlayer space of 8.3 Å at (A) 5.324, (B) 5.408, and (C) 5.64 ns; Top views of the final ion distribution in F-channel with interlayer spaces of (D) 9.3 and (E) 10.3 Å; (E) Snapshot of hydrated Cl ion in D-channel with interlayer space of 10.3 Å.
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3.3 Effects of interlayer spaces and nanostructures on desalination performance of GO membrane. Figure 6A shows the schematic diagram of the desalination process of GO membrane. The membrane is composed of multilayer GO nanosheets, and in general, the desalination performance of GO membrane is determined by the transport behavior of molecules and ions in the 2D nanochannels of neighboring GO nanosheets. Therefore, the detailed interlayer nanostructure of neighboring GO nanosheets during desalination is shown in Figure 6B. According our studied systems, no matter what the interlayer space is, water flow through the D-channel is faster than other channels. For the F, and S channels, the strong interfacial interactions between water molecules and oxygen-containing groups including electrostatic, vdW, and H-bond interaction result in their slow water transport rate (GO-GO region in Figure 6B). At the nonoxidizing region (pristine GN region in Figure 6B), fast water transport is observed due to the almost frictionless flow of water molecules in pristine GN channels[47]. Therefore, for a GO membrane, regulating the stacking ways of neighboring GO nanosheets to form more D-channels are favorable to improve water permeability. However, at the boundary of oxidized and non-oxidized regions (GO-GN region in Figure 6B), the water flow is also restrained. There is a strong viscous resistance from the interfacial interactions between water molecules and oxygen-containing groups. Therefore, the width of the non-oxidized region is another factor needing to be considered, and a larger width is expected. Similarly, ion transport though the F-channel is also largely restricted due to the polarization effect of the oxygen-containing groups caused by the charged ions and the resulting hydration interactions. The energy barrier for ion transport in the non-oxidized regions of D-channel is small, which is averse to desalination. However, it should be noticed that this kind of channel is typically 17
disconnected in GO membrane. Therefore, high ion rejection rate can still be obtained. The ion rejection rate is mostly determined by the interlayer space. Overall, our results suggest that the desalination performance of GO membrane can be enhanced by regulating the stacking ways of neighboring GO nanosheets, and D-channel is expected. Compared with interlayer nanostructures (i.e. stacking ways), the improvement of water and ion transport in 2D GO nanochannels is more obvious by increasing the interlayer spaces (Figure 2D). However, the ion transport will lower the ion rejection rate. Moreover, the influence of interlayer nanostructure on water flow takes effect only at a limited ranges of interlayer space. At larger interlayer space, its influence on water flow is largely weakened (the F and S channels in Figure 2D). Therefore, from our results, highly efficient desalination membrane can be prepared from GO nanosheets according to the following two steps. Firstly, the interlayer space should be regulated to obtain higher water flux and required ion rejection rate. Secondly, the stacking ways of neighboring GO nanosheets should be controlled to further increase the water permeation. Up to now, many methods have been proposed to regulate the interlayer space of GO membrane. For the stacking ways, we are working on it by regulating their assembly process during membrane preparation. This will be our nest investigation.
Figure 6. Schematic diagrams for (a) desalination process of GO membrane and (b) water and ion transport in 2D GO nanochannel.
4. Conclusions 18
In this work, non-equilibrium MD simulations were preformed to investigate water and ion transport in 2D GO nanochannels with different interlayer space and nanostructure, intending to further improve the desalination performance of GO membrane by regulating the membrane structures. Our results reveal that the fast water and ion transport mainly occurs in the 2D GO nanochannel surrounded by non-oxidized region of GO nanosheets. At oxidized region, the strong electrostatic, vdW, H-bond, and polarization interactions between water/ion and the oxygen-containing groups in GO impede their transport. Therefore, the desalination performance of GO membrane can be obtained through regulating the assemble structure of neighboring GO nanosheets with non-oxidized region facing each other. In addition, compared with interlayer nanostructure, increasing interlayer space is more obvious to enhance water and ion transport. However, for desalination, the enhanced ion transport will reduce the ion rejection rate. Based on our results, we proposed a design principle to obtain GO membrane with highly efficient desalination performance. Firstly, regulating the interlayer space to obtain higher water permeability and required ion rejection rate; Secondly, controlling the stacking ways of neighboring GO nanosheets with non-oxidized region facing each other to further increase the water permeability. This work opens a new direction to improve desalination performance of membranes prepared from 2D materials by controlling interlayer nanostructures.
Acknowledgements This work is financially supported by the Fundamental Research Funds for the Central Universities (201813020, 201964009), China Postdoctoral Science Foundation (2018M640658, 2019T120611), Shandong Provincial Natural Science Foundation, China (ZR2019BB012), and National Natural Science Foundation of China (U1806223). 19
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Graphical abstract
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Highlights Transport behavior of water and ion in 2D nanochannels of GO membrane is studied. Effects of interlayer space and structure on water and ion transport are discussed. Interfacing interactions between GO and ion/water are revealed. A new method is proposed to obtain GO membrane with excellent desalination performance.
Author Contribution Statement Wen Li: Investigation, conceptualization, Visualization, Writing-Original draft; Lei Zhang: Formal Analysis, Data Curation; Xinyu Zhang: Conceptualization, Methodology; Mutian Zhang: Methodology, Conceptualization; Tengfei Liu: Conceptualization, Methodology; Shougang Chen: Supervision, Writing - Review & Editing, Funding acquisition.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S0376-7388(19)33119-9
DOI:
https://doi.org/10.1016/j.memsci.2019.117744
Reference:
MEMSCI 117744
To appear in:
Journal of Membrane Science
Received Date: 8 October 2019 Revised Date:
20 November 2019
Accepted Date: 9 December 2019
Please cite this article as: W. Li, L. Zhang, X. Zhang, M. Zhang, T. Liu, S. Chen, Atomic insight into water and ion transport in 2D interlayer nanochannels of graphene oxide membranes: Implication for desalination, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2019.117744. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Atomic Insight into Water and Ion transport in 2D Interlayer Nanochannels of Graphene Oxide Membranes: Implication for Desalination Wen Lia, Lei Zhanga, Xinyu Zhanga, Mutian Zhanga, Tengfei Liua, Shougang Chena*
a
School of Materials Science & Engineering, Ocean University of China, Qingdao 266100 PR China
Corresponding author: Shougang Chen E-mail address: [email protected]
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Abstract Currently, graphene oxide (GO) membrane is becoming a new-generation material for desalination application with high water permeability and ion rejection rate. The unusual 2D interlayer nanostructure in GO membrane provides a reliable and precise molecular sieving function for fast water transport and ion rejection. However, the studies on molecular mechanisms for water and ion transport in the 2D interlayer nanochannels of GO membrane are far from adequate, and the trade-off between selectivity and permeability is another problem significantly restraining the further development of GO membrane for desalination. In this work, molecular dynamics simulations were conducted to investigate the atomic mechanism of water and ion transport in 2D nanochannels of GO membrane, and the influences of interlayer space and nanostructure on water and ion transport were also revealed. We find that there are strong electrostatic, vdW and hydrogen bond interactions between water/ion and the oxygen-containing groups in GO nanosheets, which largely impedes their transport. The hydration interaction also plays an important role in ion adsorption in the 2D nanochannels. The fast water and ion transport in GO membrane mainly occurs in the non-oxidized region of neighboring GO nanosheets. By combing the influences of interlayer space and nanostructure on water and ion transport, a new design principle for high-efficient GO desalination membrane is proposed. The findings in this work expand our understanding of water and ion transport in GO membrane and may also greatly promote the development of desalination membrane prepared by other 2D materials. Keywords: Graphene oxide; Desalination; 2D nanochannel; Molecular transport; Molecular dynamics simulations
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1. Introduction Nowadays, fresh water shortage has become a worldwide problem because of water pollution and waste[1, 2]. Seawater, which covers 71% of the surface area of our earth, provides an adequate fresh water supply if advanced desalination method is developed. Therefore, in the past decades, many studies have been triggered to develop new methods for seawater desalination[3, 4]. Among which, desalination through reverse osmosis (RO) membrane has become a widely applied technology due to its high-energy efficiency[5-7]. Currently, preparing advanced RO membranes with robust mechanical property, high separation performance (i.e. high water flux and high ion rejection rate), and low operation cost and energy consumption are the main developmental direction for seawater desalination technology[8, 9]. Recently, nanofiltration RO membranes prepared from two-dimensional (2D) materials have presented great potential for desalination application[10-13]. Especially for graphene oxide (GO), the interlayer gallery assembled from atomic-thin GO nanoflakes offers unique 2D nanochannels for fast water transport meanwhile rejecting the transport of other small molecules and ions[14-18]. Up to now, many theoretical[19-21] and experimental[22-24] studies have shown that GO membrane presents superior desalination performance. Overall, previous studies mainly focus on the regulation of interlayer spaces of GO membrane (e.g. chemically functional group modification and physically embedding nanoparticles) to pursue more efficient desalination. Although the increased interlayer space can enhance water permeability, the corresponding ion rejection rate will decrease. Typically, these regulating methods for interlayer space of GO membrane are not easy to realize, meanwhile the trade-off relationship between water permeability and ion rejection rate is another problem needing to be reconsidered. 4
GO is an oxidized form of graphene (GN) including epoxide, hydroxyl, and carboxylic acid groups on it[25-27]. Normally, the epoxide and hydroxyl groups mainly distribute on the base plane of GN, whereas carboxylic acid groups distribute on the edges. However, the distribution of epoxide and hydroxyl groups is not uniform on GN plane[25, 28]. Yan et al. had proved that epoxide and hydroxyl groups tended to aggregate on GN plane[29]. In other word, GO nanosheet can be divided into oxidation and non-oxidation region, and therefore, the oxidation region presents island-like distribution. Considering the nonuniform distribution of oxygen-containing groups on GN sheet, the stacking ways of neighboring GO nanosheets (i.e. oxygen-containing groups facing each other, oxygen-containing groups with pristine GN patches facing each other, and pristine GN patches facing each other) during membrane preparation (e.g. vacuum filtration[30, 31]) can influence the interlayer nanostructures. Meanwhile, the interlayer nanostructures govern the transport of water and ion in the 2D nanochannels between neighboring GO nanosheets. Therefore, the desalination performance of GO membrane can be improved through controlling the assembly structure of neighboring GO nanosheets. However, hardly any studies have been designed to investigate the influence of stacking ways of GO nanosheets on the corresponding desalination performance. In this work, water and ion transport behavior in 2D nanochannels between neighboring GO nanosheets was studied by using atomic non-equilibrium molecular dynamics (MD) simulations. Specially, we considered the influences of the stacking ways of heterogeneously oxidized GO nanosheets on water and ion transport. The complicated cross-interactions between stacking ways and interlayer spaces of GO membrane were revealed. This work also provides a clear design principle to prepare GO membrane for high-efficient desalination by controlling the interlayer space and interlayer nanostructure of GO membrane. 5
2. Computational method 2.1 Models The studied model system is shown in Figure 1A, where a 2D GO nanochannel is held between two fixed He slabs with cracks matching the interlayer space of the nanochannel. The He slab is selected because it has a weak interaction with water and ion, and therefore, the influence of slab adsorption effect on water and ion transport is ignorable. Actually, other fixed slabs can also be used like graphene slabs. However, there are strong nonbonding (vdW) interactions between water molecules and graphene slabs. This will generate stable adsorption layers of water molecules around the slabs[32, 33], which will in turn influence the water and ion getting in and out of the channel. In this work, we only intend to study the transport behavior of water and ion in the 2D interlayer nanochannels of GO membrane and reveal its influence on desalination performance of GO membrane. Therefore, the influence of adsorption effects on water and ion transport originated from the rigid slabs should be excluded. Another two rigid GN slabs are put at the rightmost and leftmost of the system to keep a constant pressure difference between the left and right regions. During simulation, the carbon skeleton of the GO nanosheets was fixed. Constant High pressure (P1, 100 MPa) was applied on the left GN slab to push the salt water (0.5 mol/L) flow from left to right region, and atmospheric pressure (P2, 0.1 MPa) was applied on the right GN slab. Indeed, for GO membrane, more realistic models have been studied before, which includes important features such as imperfect stacking of GO flakes, swelling and collapse of GO membrane, and a broad distribution of interlayer spaces[34-37]. However, GO nanosheets with fixed carbon skeleton were used in this work. This is because that the interlayer space between neighboring GO sheets is a key factor to influence water and ion transport. To study the influence of interlayer space on water and ion transport, accurate 6
controlment of interlayer space is needed, and this just could be realized through fixed carbon skeleton of GO sheets. To offset the flexible influence of GO sheets on water and ion transport, the oxygen-containing groups in the GO sheets are not fixed, which can move freely. To study the effects of stacking ways of neighboring GO nanosheets on the desalination performance of GO membrane, GO nanosheets with different distribution of oxygen-containing groups are built. For simplification, carboxylic acid groups are not considered, and epoxide and hydroxyl groups are evenly distributed in GO nanosheet with C/O ratio of ~3/1. By using these built GO nanosheets, three kinds of 2D GO nanochannels with different interlayer nanostructures are constructed. Figure 1B shows the perspective view of F typed 2D GO nanochannel (F-channel), which is constructed by using two neighboring GO nanosheets with uniform distribution of oxygen-containing groups. When the GO nanosheets are replaced by GO nanosheets with stripe-like distribution of oxygen-containing groups, S and D-types of 2D GO nanochannels are formed. For the S-channel in Figure 1C, stripe-like non-oxidized region only exists in the bottom GO nanosheet, and for the D-channel in Figure 1D, both the nanosheets are GO with stripe-like non-oxidized regions in the middle of the sheets. For the GO nanosheet with stripe-like distribution of oxygen-containing groups, the width of the non-oxidized region is set as 6.15 Å. The influence of interlayer space of the 2D GO nanochannel on its desalination performance was also studied. According to reported work[38], the interlayer space for GO membrane is about 7-13 Å based on ambient humidity. Therefore, the systems with interlayer spaces of 8.3, 9.3, and 10.3 Å were simulated, where the interlayer space was determined based on the perpendicular distance of the carbon skeleton of the neighboring GO nanosheets.
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Figure 1. (A) The constructed system used in this work; (B-D) Perspective views of the 2D nanochannels between neighboring GO nanosheets from z direction. Blue and green balls represent Na+ and Cl- ions, respectively, and water is shown as transparent cyan.
2.2 Simulation setup All the non-equilibrium MD simulations were conducted by using LAMMPS package[39]. All-atom optimized potential for liquid simulations (OPLS-AA) force filed was adopted for GO, Na+, and Clions. According to our previous works[40, 41] and reported results[42, 43], this force field can describe GO accurately, and TIP3P model was used for water. Furthermore, the SHAKE algorithm was used to constrain the bonds and angles of water molecules[44]. The nonbonded force field parameters were obtained from literatures[45, 46][39, 40]. The pairwise Lennard-Jones (L-J) terms were calculated by using Lorentz-Berthelot combining rules. Periodic boundary conditions were applied in X and Y directions. All the studied systems were simulated for 20 ns with a time step of 1 fs. The temperature was controlled at 298 K by using a Nosé-Hoover thermostat with coupling time constant of 1 ps. The cut-off distance for van der Waals (vdW) interaction was set as 12 Å, and particle-particle-particle-mesh (PPPM) method was used to calculate the long-rang electrostatic 8
interaction. The obtained data were collected every 1 ps. The first 4 ns was used to form stable water flow, and the last 16 ns was used for analysis.
3. Results and discussion 3.1 Water flux.
Figure 2. Time evolution of the number of passed water molecules through the (A) F, (B) S, and (C) D-channels with different interlayer spaces; (D) Water flux of the F, S, and D-channel with different interlayer spaces.
Figure 2A-C shows the time evolution of the number of water molecules passed through the studied 2D GO nanochannels with different interlayer spaces, respectively. These curves present a nearly linear relationship between the passed water molecules and time. Therefore, water flux can be calculated through linear fitting of the curves in Figure 2A-C, and the results are shown in Figure 2D. It shows that the water flux increases with the increase of interlayer spaces. As the increase of interlayer spaces, on one hand, it will be easier for water or ion entering and exiting the 2D interlayer 9
nanochannel due to the enlarged entrance and exit sizes; on the other hand, the wider 2D channel has larger volume to accommodate more water molecules in it (Figure 4), which weakens the interfacial viscous resistance for water and ion transport. In this work, we only conducted 20 ns simulation for each system. For a longer time simulation, we think that this linear dependence between water flow and time in Figure 2A-C cannot be maintained because that compared with ions, the fast water transport in the 2D nanochannel will lead to the increase of ion concentration in the left reservoir. Therefore, water flux will decrease after long time simulation. It should be also noticed that the transport efficiency varies with each other in the 2D nanochannel with different interlayer nanostructures. In Figure 2D, at same interlayer space, water flux in the studied 2D GO nanochannels follow the order of D-channel>S-channel>F-channel except for the S-channel with interlayer space of 10.3 Å. In addition, compared with interlayer space, the influence of interlayer nanostructure on water flux is milder. The influences of interlayer spaces and interlayer nanostructures of the 2D GO nanochannels on the transport behavior of water molecules are also investigated by calculating the averaged 2D relative density maps of water molecules in the studied GO nanochannels, and the results are shown in Figure 3. From this figure, the following points can be concluded: (1) From the density maps in XZ planes, water content in the 2D GO nanochannels increases with interlayer spaces (e.g. increased dark red regions from A1, A3, to A5), and the water molecules in the nanochannels are not uniformly distributed (different colors in same figure); (2) From the density maps of XY planes, the structure of water molecules in the nanochannels turns from one layer to double layers when the interlayer space increases from 8.3 to 9.3 Å (e.g. double layers of dark red in A4); (3) For the S and D-channels with interlayer spaces of 9.3 and 10.3 Å, there are one obvious strip-like dark red region (between the 10
dotted black lines in B3-B6 and C3-C4), which suggests a higher water density. For point (1), the increased interlayer space will generate more free volume for molecular adsorption and diffusion, which will lead to a higher water content and the corresponding higher water flux. For the heterogeneous distribution of water molecules, it is caused by the strong dipolar interactions between polar oxygen-containing groups in GO and water molecules, which results in the accumulation of water molecules around the oxygen-containing groups. Although the oxygen-containing groups in the GO nanosheets are uniformly distributed, the oxidation degree is only about 30%. Therefore, the intervals among the oxygen-containing groups are relatively large. As a result, heterogeneous distribution of water molecules in the 2D nanochannels of GO is observed. For point (2), previous studies[37, 41] have proved that water structure in confined 2D nanochannel changes with the corresponding interlayer spaces. Layered water structure can be formed due to interfacial induction from the neighboring GO nanosheets. For the system with interlayer space of 8.3 Å, it can only accommodate one water layer in it due to its limited interlayer space. Once the interlayer space increases to 9.3 Å, the neighboring GO nanosheets can bound one water layer, respectively, and as a result, double water layers are formed. For point (3), we marked the coordinates of the dark red regions, and found that these regions just fall in the non-oxidized regions of the neighboring GO nanosheets in the S and D-channels (Figure 1C and D). The non-oxidized regions have large free volume, which can accommodate more water molecules in them. For the S and D-channels with interlayer space of 8.3 Å, the accumulation of water molecules in the non-oxidized regions is not obvious. This is because that although the free volume for water adsorption at the non-oxidized region increases compared with oxidized region, the enlarged volume is very limited due to the finite interlayer space. There is also only one ordered water layer at 11
non-oxidized regions.
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Figure 3. The 2D density maps of water molecules inside the GO nanochannels. The density maps in XZ (A1, B1, and C1) and XY (A2, B2, and C2) planes of the F, S, D-channels with interlayer distance of 8.3 Å, respectively; The density maps in XZ (A3, B3, and C3) and XY (A4, B4, and C4) planes of the F, S, D-channels with interlayer distance of 9.3 Å, respectively; The density maps in XZ (A5, B5, and C5) and XY (A6, B6, and C6) planes of the F, S, D-channels with interlayer distance of 10.3 Å, respectively.
In Figure 4, we also calculated the time evolution of the total number of hydrogen bonds (H-bonds) between GO nanosheets and water molecules to evaluate their interfacial interactions. In this work, the H-bonds are defined according to a geometrical criterion. The GO and water molecules are hydrogen-bonded if the following two conditions are fulfilled: (1) the distance between an oxygen atom with a hydrogen bonded to it (the donor, D) and another oxygen atom (the acceptor, A) is less than 3.5 Å; (2) The bond angle of the D-H-A is smaller than 30°. Overall, Figure 4A-C shows that as simulation proceeding, the number of H-bonds fluctuates at a relatively stable region. Therefore, to compare them quantitatively, the average number of H-bonds between water molecules and GO nanosheets is calculated and shown in Figure 4D. From Figure 4D, two points can be concluded: (1) At same interlayer space, the total number of H-bonds between water molecules and GO nanosheets follows the order of F-channel>S-channel>D-channel; (2) At same channel nanostructure, the channel with interlayer space of 9.3 Å has a larger average number of H-bonds followed by the 13
channels with interlayer spaces of 8.3 and 10.3 Å. For point (1), the lacking oxygen-containing group in the non-oxidized region of the GO nanosheets (Figure 1C and D) results in the smaller number of H-bonds between GO and water molecules. For point (2), there is only one water layer in the GO nanochannel when the interlayer space is 8.3 Å (e.g. Figure 3A1-A2), which leads to its small number of H-bonds. However, when the interlayer space increases to 9.3 Å, double water layers are formed (e.g. Figure 3C3-C4). This is to say that more water molecules can accumulate in the channel (Figure 3C3-C4) and form H-bonds with the oxygen-containing groups in GO nanosheets.
Figure 4. Time dependent the number of H-bonds between water molecules and GO nanosheets in F, S, and D-channels with interlayer spaces of (A) 8.3, (B) 9.3, and (C) 10.3 Å; (D) The averaged number of H-bonds in the studied channels.
3.2 Ion transport. To investigate ion transport behavior in the studied channels, we firstly observe ion migration in 14
F-channel with interlayer space of 8.3 Å. Figure 5A-C shows the selected snapshots for ion transport at different time. The purple and orange balls are the hydrogen and oxygen atoms in the hydration shell of Na+ and Cl-. At 5.324 ns (Figure 5A), one Cl- ion has adopted in the channel, and one Na+ ion is approaching the channel. At 5.408 ns (Figure 5B), the Cl- ion stays around its original position, and the Na+ ion locates at the entrance of the channel with fewer molecules in its hydration shell. However, at 5.462 ns (Figure 5C), the Cl- ion still stays around its original position, while the Na+ ion has gone back to the bulk salt solution. The dynamic trajectory in Figure 5A-C can be found in Movie S1 in supporting information (SI). These results indicate that it is extremely hard for ion entry and migration in the F-channel with interlayer space of 8.3 Å. According to previous work[48], the naked radii of Na+ and Cl- ions are 1.16 and 1.81, respectively. We also obtained the hydration radii for Na+ ion (3.35 Å) and Cl- ion (3.95 Å) by calculating the ion-oxygen radial distribution functions (Figure S1). For the channel with interlayer space of 8.3 Å, the effective space for ion diffusion at non-oxidized region is about 4.9 Å (approximately equal to interlayer space, 8.3 Å, minus the vdW diameter of carbon atoms, 3.4 Å). At the oxidized region, this effective space is much smaller because of the existence of hydroxyl and epoxy groups. It can be seen that the effective space in the channel with interlayer space of 8.3 Å is less than the diameters of hydrated Na+ and Cl- ions. As a result, most of the hydrated water molecules must be dehydrated before ion entering the channel, and this will generate a large energy barrier for ion entry in the channel (e.g. the Na+ ion in Figure 5A-C). Even ion can enter the channel, there is a strong energy barrier for ion migration in the channel (e.g. the Cl- ion in Figure 5A-C). This is because that the hydroxyl groups in GO nanosheets can also participate in ion hydration (blue dotted lines in Figure 5A-C). As the increase of interlayer spaces, the effective space for ion entry 15
and diffusion is also increased, which makes it easier for ion entering the channel. As a result, multi ions can enter the channel (Figure 5D and E), which can also move in the channel more freely. The dynamic trajectory for ion transport in F-channels with interlayer space of 10.3 Å is shown in Movie S2 in SI. For the effects of interlayer nanostructures on ion transport, the heterostructure of the GO nanosheets will expand the effective space for ion entry and migration at the non-oxidized region of GO nanosheet because of the larger free volume and lower energy barrier. Ion prefers to enter the 2D channel from the non-oxidized region of the D-channel (Figure 5F and Movie S3-S4). It should be also noted that the influences of carboxylic acid functionalities at the edges of GO nanosheets on water and ion transport are not considered in this work. Actually, previous study has proved that carboxylic acid groups will deprotonate across a broad pH range [49]. This will play a significant role in water and ion transport. Especially for ion transport, the negatively charged carboxylic acid groups will repel Cl- and attract Na+. This will lead to a selective transport of positively charged ions.
Figure 5. The snapshots for ion transport in F-channel with interlayer space of 8.3 Å at (A) 5.324, (B) 5.408, and (C) 5.64 ns; Top views of the final ion distribution in F-channel with interlayer spaces of (D) 9.3 and (E) 10.3 Å; (E) Snapshot of hydrated Cl ion in D-channel with interlayer space of 10.3 Å.
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3.3 Effects of interlayer spaces and nanostructures on desalination performance of GO membrane. Figure 6A shows the schematic diagram of the desalination process of GO membrane. The membrane is composed of multilayer GO nanosheets, and in general, the desalination performance of GO membrane is determined by the transport behavior of molecules and ions in the 2D nanochannels of neighboring GO nanosheets. Therefore, the detailed interlayer nanostructure of neighboring GO nanosheets during desalination is shown in Figure 6B. According our studied systems, no matter what the interlayer space is, water flow through the D-channel is faster than other channels. For the F, and S channels, the strong interfacial interactions between water molecules and oxygen-containing groups including electrostatic, vdW, and H-bond interaction result in their slow water transport rate (GO-GO region in Figure 6B). At the nonoxidizing region (pristine GN region in Figure 6B), fast water transport is observed due to the almost frictionless flow of water molecules in pristine GN channels[47]. Therefore, for a GO membrane, regulating the stacking ways of neighboring GO nanosheets to form more D-channels are favorable to improve water permeability. However, at the boundary of oxidized and non-oxidized regions (GO-GN region in Figure 6B), the water flow is also restrained. There is a strong viscous resistance from the interfacial interactions between water molecules and oxygen-containing groups. Therefore, the width of the non-oxidized region is another factor needing to be considered, and a larger width is expected. Similarly, ion transport though the F-channel is also largely restricted due to the polarization effect of the oxygen-containing groups caused by the charged ions and the resulting hydration interactions. The energy barrier for ion transport in the non-oxidized regions of D-channel is small, which is averse to desalination. However, it should be noticed that this kind of channel is typically 17
disconnected in GO membrane. Therefore, high ion rejection rate can still be obtained. The ion rejection rate is mostly determined by the interlayer space. Overall, our results suggest that the desalination performance of GO membrane can be enhanced by regulating the stacking ways of neighboring GO nanosheets, and D-channel is expected. Compared with interlayer nanostructures (i.e. stacking ways), the improvement of water and ion transport in 2D GO nanochannels is more obvious by increasing the interlayer spaces (Figure 2D). However, the ion transport will lower the ion rejection rate. Moreover, the influence of interlayer nanostructure on water flow takes effect only at a limited ranges of interlayer space. At larger interlayer space, its influence on water flow is largely weakened (the F and S channels in Figure 2D). Therefore, from our results, highly efficient desalination membrane can be prepared from GO nanosheets according to the following two steps. Firstly, the interlayer space should be regulated to obtain higher water flux and required ion rejection rate. Secondly, the stacking ways of neighboring GO nanosheets should be controlled to further increase the water permeation. Up to now, many methods have been proposed to regulate the interlayer space of GO membrane. For the stacking ways, we are working on it by regulating their assembly process during membrane preparation. This will be our nest investigation.
Figure 6. Schematic diagrams for (a) desalination process of GO membrane and (b) water and ion transport in 2D GO nanochannel.
4. Conclusions 18
In this work, non-equilibrium MD simulations were preformed to investigate water and ion transport in 2D GO nanochannels with different interlayer space and nanostructure, intending to further improve the desalination performance of GO membrane by regulating the membrane structures. Our results reveal that the fast water and ion transport mainly occurs in the 2D GO nanochannel surrounded by non-oxidized region of GO nanosheets. At oxidized region, the strong electrostatic, vdW, H-bond, and polarization interactions between water/ion and the oxygen-containing groups in GO impede their transport. Therefore, the desalination performance of GO membrane can be obtained through regulating the assemble structure of neighboring GO nanosheets with non-oxidized region facing each other. In addition, compared with interlayer nanostructure, increasing interlayer space is more obvious to enhance water and ion transport. However, for desalination, the enhanced ion transport will reduce the ion rejection rate. Based on our results, we proposed a design principle to obtain GO membrane with highly efficient desalination performance. Firstly, regulating the interlayer space to obtain higher water permeability and required ion rejection rate; Secondly, controlling the stacking ways of neighboring GO nanosheets with non-oxidized region facing each other to further increase the water permeability. This work opens a new direction to improve desalination performance of membranes prepared from 2D materials by controlling interlayer nanostructures.
Acknowledgements This work is financially supported by the Fundamental Research Funds for the Central Universities (201813020, 201964009), China Postdoctoral Science Foundation (2018M640658, 2019T120611), Shandong Provincial Natural Science Foundation, China (ZR2019BB012), and National Natural Science Foundation of China (U1806223). 19
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Graphical abstract
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Highlights Transport behavior of water and ion in 2D nanochannels of GO membrane is studied. Effects of interlayer space and structure on water and ion transport are discussed. Interfacing interactions between GO and ion/water are revealed. A new method is proposed to obtain GO membrane with excellent desalination performance.
Author Contribution Statement Wen Li: Investigation, conceptualization, Visualization, Writing-Original draft; Lei Zhang: Formal Analysis, Data Curation; Xinyu Zhang: Conceptualization, Methodology; Mutian Zhang: Methodology, Conceptualization; Tengfei Liu: Conceptualization, Methodology; Shougang Chen: Supervision, Writing - Review & Editing, Funding acquisition.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.