Molecular Insight into Water Transport through Heterogeneous GO

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Surfaces, Interfaces, and Applications

Molecular Insight into Water Transport through Heterogeneous GO-based Two-Dimensional Nanocapillary Tongfei Xu, Ming Zhang, Zhijun Xu, and Xiaoning Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09330 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 16, 2019

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ACS Applied Materials & Interfaces

Molecular Insight into Water Transport through Heterogeneous GO-based Two-Dimensional Nanocapillary Tongfei Xu, Ming Zhang, Zhijun Xu* and Xiaoning Yang* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, China

*: Corresponding authors: [email protected], [email protected]

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ABSTRACT: Nanofluidics in two-dimensional (2-D) heterogeneous layered materials with hybrid overlapping structures exhibit promising potential in filtration and separation applications. However, molecular transport across the heterogeneous interlayer galleries remains largely unexplored, in particular, there exists disputation in the function and performance of hybrid graphene oxide (GO)-based laminate membrane for the water transport. Herein, heterogeneous 2-D GO-based nanochannels were employed as a typical platform to investigate the water flow by non-equilibrium MD simulation. It is demonstrated that both heterogeneous and homogeneous GO nanochannels exhibit similar reduced water flow behavior, even if one surface of the 2-D channel is the pristine graphene. In particular, the flow rate in the hybrid GO/pristine graphene nanochannels does not lie between those of the oxidized and the pristine regions, and the high-friction GO surface suppresses the water transport and controls the entire flow performance. This result is qualitatively consistent with the recent experimental observation. By comparing with the MD simulation, a hydrodynamic model was developed to describe the flow rate for 2-D heterogeneous nanochannels. The reduced water transport has been revealed as the distinct vertical dragging effect, arising from the synergistic effect between the interfacial affinity from GO surfaces and the interlayer molecular interaction. Our results provide novel physical pictures for the molecular transport inside heterogeneous 2-D nanochannels. KEYWORDS: Graphene oxide (GO), Heterostructure, Nanocapillary, Molecular simulation, Water transport.


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INTRODUCTION Two-dimensional (2-D) layered materials with unique laminate structure and intriguing advantages have shown potential applications in extensive fields1-3. In the formed laminate structure, the subnanometer 2-D interlayer channel is the critical route for molecular transport and permeation. In parallel with the progress of 2-D materials, the emerging hetero/hybrid structures in the 2-D channels have drawn increasing attention over the past few years and have been identified as an important direction4-14. Such heterostructures can be made by mixing and stacking different types of 2-D crystals, for example, Graphene oxide (GO)-MoS211 and TitaniaGO13, usually serving to overcome the inherent limitations of each material and exhibiting novel properties6. These formed hybrid 2-D nanochannels open up the possibility to explore emerging innovative applications by matching different 2-D materials. GO-based layered materials with 2-D nanochannels can provide a favorable pathway for molecular sieving and water permeation, which exhibited promising potential in membrane separation15-18. Due to the inherent non-uniform distribution nature of functional groups19,20, there inevitably exist numerous heterogeneous or mixed interlayer structures overlapped by pristine and oxidized regions in various GO-based membranes. Considering the completely different surface properties for the two regions, the pristine/oxidized graphene hybrid structure could act as a typical system to offer a versatile platform for investigation of the nanofluidic behavior through heterogeneous hydrophilic/hydrophobic 2-D channels. More importantly, this oxidized/pristine heterostructure has been fabricated and highlighted in recent emerging newtype GO-based hybrid membranes21-24. For example, the layered graphene/GO mixed hybrid membranes have been prepared in the recent experiments21,22, and show exceptionally highefficient ion rejection than pure GO membranes. Xi et al.24 have also fabricated a novel GO


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composite membrane by doping with reduced graphene oxide sheets, which constructed the unique heterogeneous capillary microstructures and exhibited extraordinary stability in water and better separation performance. Understanding the molecular transport across the heterogeneous GO interlayers not only provide direct guidance on the design of novel GO-based membranes, but also explore the unique nanofluidic process inside extensive 2-D heterostructures. Recently, MD simulations were conducted to study the water equilibrium diffusion and nonequilibrium transport in various GO channels25. The simulation results indicated the heterogeneous mixed region constructed by oxidized and pristine graphene (PG) sheets could play a prominent role in the rapid transport of water across GO laminate membranes. However, the conflicting phenomenon was observed in recent experiment and simulation26, in which Xu et al. found that the mismatches between functionalized and pristine graphene patches on neighboring GO layers can lead to drastically retarded water permeation, which implicitly indicated the hybrid/mixed regions in GO nanochannels could increase the transport resistance. However, this previous work did not explicitly concern with the flow behavior of hybrid/mixed GO channels. The fundamental understanding of molecular transport is still vague, in particular, how the asymmetrical interfaces in the hybrid GO-based nanochannels affect and control the water transport remain unclear. Generally, molecular transport in 2-D nanochannels is primarily governed by surface properties, which can be conveniently controlled by modifying the starting sheets. In view of the asymmetric surfaces in 2-D hybrid nanochannels, it is very likely to produce some specific transport phenomena beyond the conventional cognition. Besides, the impact of the heterogeneous surface feature on nanofluidic transport, as well as the individual effect of the two channel walls is not known. Therefore, figuring out the behavior and mechanism for confined


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transport in hybrid nanochannels is a fundamental and essential work. Currently, the hydrodynamic theory of nanofluidic flow in 2-D heterogeneous channels faces huge challenge due to the high inhomogeneity of density and viscosity variation along the confined direction. The slip-based Poiseuille equation with approximately quadratic velocity distribution has been established to describe the flow behavior within homogeneous 2-D nanopores27-29. However, the present hydrodynamics model has not been applied for hybrid/mixed 2-D channels. With the above in mind, in this work, a series of comprehensive non-equilibrium molecular dynamics (MD) simulations were conducted to study water transport through various types of 2-D GO hybrid channels, which were constructed via overlapping and matching different GO sheets (pristine graphene sheet or functionalized GO sheets with different degree of oxidations). A modified Poiseuille theoretical equation was developed to correlate and predict the nonequilibrium MD data. This theoretical model is expected to provide a reasonable description of molecular flow through 2-D heterogeneous nanochannels. Our simulation suggests that hybrid or mixed GO channels are not able to afford rapid water transport across GO laminate membranes. The underlying mechanism of how heterogeneous nanostructure controls the interlayer water transport has been revealed. Our result presents a new understanding of the design and fabrication of 2-D layered materials. METHODS Simulation Models and Procedure: To construct a series of heterogeneous GO nanochannels, PG sheet and different GO sheets (47.8 Å×68.2 Å) were considered in this work, as shown in the upper panel of Figure S1, in which PG, GO1, GO2, GO4 denote pristine graphene, oxidized graphene with oxidization concentration c = 10%, 20%, and 40%, respectively. In those GO structures, hydroxyl and epoxy groups randomly distributed on the two both planes based on the


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Lerf-Klinowski model20,30, while other groups, such as carboxylic acid groups are not considered in our models due to its negligible quantities20 and general location along the edges30. Nanochannels were constructed by overlapping two sheets and the initial system configuration was created with water intercalated in the interlayer galleries, while keeping the density of water molecules fixed at about 1 g/cm3, as displayed in the bottom panel of Figure S1. The two slabs with the varying interlayer distance of 10Å~20Å were placed along the xy plane as a nanochannel in the center of simulation box (41.8 Å×68.2 Å×50 Å), where water transports along the y-direction. The period boundary conditions were used along the x and y directions. All the sp2 carbon atoms in GO were constrained and treated as neutral Lennard-Jones (L-J) spheres using the parameters proposed by Cheng and Steele31, while the surface functional groups were allowed to freely move, for which the all-atom optimized potential for liquid simulations (OPLS-AA) force filed was used. This force field has been widely applied in the system of GO in aqueous solutions and it shows excellent consistency with the experimental results32,33. Water molecules were simulated with the SPC/E model34 by following the previous studies32,35. For all pairwise L-J terms, the Lorentz-Berthelot mixing rule was applied. The vdW interactions are truncated at 10 Å, and the long-range electrostatic interactions were calculated by utilizing the particle−particle particle-mesh (PPPM) algorithm. Classical molecular dynamics (MD) simulations are performed in the canonical ensemble (NVT) using the large-scale atomic/molecular massively parallel simulator (LAMMPS)36. The simulation temperature of 298K was controlled by Nose-Hoover thermostat. During the nonequilibrium MD simulations, the thermostat was only applied to the vertical direction of water flow. Pressure-driven water flow was performed by directly adding forces to water molecules to achieve the steady state of water flow in the nanochannels. In order to generate the desired


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pressure difference (ΔP), the applied force (f) was exerted on individual atom based on the equation f = ΔP·A/n, where A is the area of the Cross-sectional area and n refers to the total number of water molecules. This method has been widely used to simulate Poiseuille fluid flow27,29,32,35. Due to the different flow resistance of each nanochannel, the different pressures ranging from 2~8MPa were applied for the PG-PG system, 50~200 MPa for PG-GO1, PG-GO2 and PG-GO4 pores, while 200~800 MPa for the complete GO channels. The high external pressures in this simulation were used to reduce thermal noise and enhance signal/noise ratio37,38 as well as to obtain precise data for water flux in finite simulation time. In the NEMD process, a total of 10ns simulation was conducted for each flow system and the last 6ns is regarded as a stable flow state for data analysis, such as permeability and velocity profiles. In order to confirm the reasonability of our simulation time, additional 20-ns non-equilibrium MD simulation was also run. We can observe that there is no difference in the simulated flow rates between the two simulation runs. We can be sure that the 10 ns NEMD simulation data are sufficient and reliable. Hydrodynamic model: Interlayer water transport through 2-D slit channel can be considered as the incompressible stable viscous flow between two parallel plates. Herein the flow is fully developed with only considering the flow velocity distribution in the confined direction. The Navier-Stokes equation can be simplified as:

  2  P  2   z  L

(1)

where  is the viscosity, v is the flow velocity, and P is the pressure. For the heterogeneous 2-D capillary channel in our work, the asymmetric slip boundary conditions are as follows:

v dv  z 0 , dz z 0 La

v dv   z h dz z  h Lb


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(2)


where La , Lb denote the two slip lengths. v

z 0

and v

z h

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denote the interfacial slip velocity at

two wall surfaces. For nanopores, the flow behavior is mainly determined by the friction interaction with the solid surface39, and meanwhile, it is well known that surface friction can be characterized by the surface slip lengths. Therefore, it is expected that the introduction of two surface slip parameters in the hydrodynamics model is able to capture the flow performance of the hybrid GO nanochannels. It is further assumed that the shear viscosity has no dependence on z and any state variables. By integrating Equation (1) with the above boundary conditions, we can obtain the velocity profile as follows,

h 2  2hLb h 2  2hLb 1 P 2 v= ( )( z zLa ) 2 L h  La  Lb h  La  Lb

(3)

The further integration of Equation (3) will give us the volumetric flow rate ( Qhetero ), h

Qhetero   vdz  W  0

 3  hLa  hLb  4 La Lb   P Wh3 1   12 L h  h  La  Lb   

(4)

where  , L ,W, h , La , Lb denote the viscosity of water, the length of channel, the width of channel, the interlayer distance of channel, and the slip lengths of two opposite surfaces, respectively. RESULTS AND DISCUSSION Non-equilibrium MD simulations: We firstly explored the pressure-driven water flow through various hybrid 2-D GO channels which were overlapped with different sheets (Figure 1(a) and Figure S1). Figure S2 presents the simulated water volumetric flux (Q) for each hybrid channel as a function of the applied pressures (∆P). The detailed computational procedure of Q was given in the Supporting Information (see Figure S2). The linear relations demonstrate a stable pressuredriven flow mode, which is in good agreement with the observations in previous studies for different 2-D nanochannels27,40. The linearity of volumetric flux scaling with driving pressure


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suggests that the results obtained at high pressure can be extrapolated to low-pressure conditions in actual processes. According to the slope of the linear flux-pressure curves, the ratio of Q/∆P is usually used as the permeance factor, which is presented in Figure 1(b) for different 2-D channels. Apparently, the water permeance factor display an increased performance with a larger interlayer gallery. Besides, the permeance factor is reduced with the rising degree of oxidation of one GO sheet for each mixed channel, which is probably ascribed to the increased flow resistance from the surface functional groups.

Figure 1. (a) Schematic of simulation system for water flow through heterogeneous GO slit pore, which was constructed via overlapping different graphene bilayers. Color code: C in the GO sheet: cyan and yellow; O in epoxy: orange; O in the hydroxyl and water: red; H in hydroxyl of water: white. (b) The permeance factor (Q/∆P) versus the interlayer distances for various heterogeneous pores, the solid lines represent the predictive results through the model equation, Equation (5).

For comparison, the flow behavior of four symmetrical GO nanochannels with different widths was shown in Figure 2. It was observed that the homogeneous GO pores generally exhibit


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comparable permeance factor with the above heterogeneous GO pores (in Figure 1). However, as expected, the PG-PG pore displays obviously high flow rate, as compared with the GO pores. Accordingly, Figure 3 shows the further comparison of simulated permeability (in the unit of L/cm2/day/MPa) for individual 2-D channel. The permeability was calculated by dividing the flow rate (Q) with the pressure and flowing crossing area.

Figure 2. (a) Schematic of simulation system for water flow through homogeneous GO slit pore. Color coding is the same as Figure 1(a). (b) The permeance factor (Q/∆P) versus interlayer distance for the four homogeneous pore models. The solid lines represent the fitting results through Equation 6.

It is observed that both mixed and uniform GO channels display the analogous water permeability in the order of magnitude, which is higher than those from commercial membranes41. However, compared to the PG channel, both of them show a dramatically reduced interlayer permeability, even though one side of the mixed channel is the PG surface. The hybrid


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heterogeneous GO pores exhibit a similar water permeability with the homogeneous GO channels. This result implies that the surface heterogeneity is not a deciding aspect in reducing the water flow rate for these GO nanochannels. The presence of GO surface in the GO nanochannels might be the critical factor suppressing and controlling the flow rate in the GObased 2-D nanochannels.

Figure 3. A comparison of the permeabilities for different types of GO nanochannels.

Theoretical model analysis: According to Equation (4), for interlayer water transport through 2D mixed GO channel, the ratio of Q/∆P can be expressed as, Qhetero Wh 3  3( hLa  hLb  4 La Lb   1   P 12L  h( h  La  Lb ) 

(5)

In the case of homogeneous nanochannel, Equation (5) can be simplified to the classical Poiseuille relation29,32:


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Qhomo 

P  6L  Wh3 1  s  12 L h  

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(6)

Herein, Ls is the slip length for symmetrical GO pores, which is supposed to be intrinsic to the relevant fluid-solid systems in different nanochannels35,42. Thus, the slip length is considered as a model parameter of the system, which can be determined by fitting the simulated Q/∆P data for homogeneous GO nanochannel based on Equation (6). The fitting results were shown in Figure 2(b) and the obtained slip lengths for different GO surfaces were listed in Table 1. It is noticed that the agreement between the fitting curves and simulation values is generally good, which designates that the Poiseuille model is able to represent the water flow in GO homogeneous nanochannels. It should be noted that a constant viscosity of bulk water ( =0.729 mPa  s for the SPC/E model)43 was approximately used in the theoretical representation, wherein the confinement impact was not defined. The fitted slip length is generally smaller than previous result29,32, and meanwhile, for GO2 and GO4 surfaces, the negative slip lengths were observed in the fitted parameters. The underestimated or negative slip lengths in the GO channels suggest that although the theoretical model ignores the impact of viscosity change of confined water, the fitted parameters of fluid-solid slip length might inherently include the inhomogeneous confinement effect. Thus, the fitted slip length can be considered as the effective model parameter including all the factors. We additionally made a sensitivity analysis of the slip length parameter (see Figure S3). It is observed that, although the slip length is very small, a significant difference in the flow rate can be caused if it is neglected. Therefore, the slip length parameter is very important to describe the flow behavior and the noslip boundary condition with zero slip length cannot be applied in our GO systems.


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Table 1. Fitting parameters of slip length (Ls) for various surfaces C LS/Å

PG 374.00

GO1 1.55

GO2 -0.20

GO4 -1.00

The unique negative slip length is not new and has been extensively reported in previous experimental and theoretical studies44-49. In general, the slip at fluid-solid boundaries can be quantified by Navier boundary condition50, and the definition of slip length is based on the fluid velocity profile at the solid-fluid boundary. Thus, the slip length is positive if the zero-velocity point is outside the channel, and it is possibly negative if inside47,49. In fact, a positive slip length will generally produce a higher flow rate; a negative slip length implies a lower flow rate, which is possible when the fluid-solid interaction is very strong and molecules near solid surfaces would have less mobility. In our results, there exists negative slip lengths for the GO nanochannels with the high oxidization degree. This means that the interfacial water molecules feel enhanced friction interaction with the surface functional groups of GOs and the molecules exhibit nearly immobilized near the GO surfaces, demonstrating a significant suppression of water flow in the GO pores. The relevant flow velocity profiles, for the homogeneous GO2-GO2 and GO4-GO4 channels, were shown in Figure S4. One can observe that a parabolic stable velocity distribution, as expected from the classical hydrodynamics model, is obtained in the center portion of pores. We can evidently observe that the zero-velocity point is located inside the pores (Figure S4). The velocity profiles confirm the negative slip lengths obtained for the GO2 and GO4 surfaces in the fitting results. Figure 1(b) presents the theoretical prediction of the flow permeance factor for mixed GO pores based on Equation (5) using the obtained slip lengths (in Table 1). The theoretical predictive results fairly agree well with the simulated data, indicating that our hydrodynamics model with reasonable slip length parameters is able to describe and predict the flow behavior


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across heterogeneous nanocapillaries. Nevertheless, there exist some deviations between the simulated data and the theoretical prediction, wherein the prediction values are always a little higher than those from the simulation. The reason is not very clear in the present situation. This might be attributed to the approximation using bulk water viscosity without considering the confinement effect. The enhanced inhomogeneity of the relevant density and viscosity along the confined direction needs to be carefully considered in hydrodynamics model development. In addition, the use of width-independent surface slip length could also cause the difference between the simulated and theoretical results. Further investigation is necessary to include these aspects. However, the reasonable consistency between the theoretical prediction and simulation data might suggest that the hydrodynamics equation with reasonable slip length parameters can be acted as a phenomenological model to describe the actual flow behavior of mixed hybrid channels. The flow enhancement factor (ε) in the mixed GO pores was calculated as

  Qsimulated / Qno  slip , where Qno  slip  Wh3  P 12 L is obtained from the classical Poiseuille equation32,51. As seen in Figure 4(a), the PG-PG channel exhibits a significant high flow enhancement (100~400), and the homogeneous GO channels with high oxidation degree yield the smallest flow enhancement factors. These results are in good agreement with previous theoretical reports29,35. However, for mixed GO channels, the enhancement factor is also dramatically reduced (4). This indicates that the presence of functional groups in GO sheet might destroy the internal water-water HB structures, and meanwhile, the surface groups could facilitate to form HBs with interfacial water molecules,


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leading to the disordered interfacial water structures in OL (Figure 6(b)). The conclusion can also be supported by the different HB density distributions (in the unity of 1/Å3) along the Y-Z direction between graphene and GO interfacial layers (Figure S8). The above result also denotes that when transport in the OL, water molecules have to continuously break the local stable HB structures and form new HBs, thus causing a significant energetic penalty and impeding the molecular transport of interfacial water layer near the oxidized surface.

Figure 6. (a) The density profiles of water along the confinement direction (z) for PG-GO2 pore with different interlayer distances. (b) Two-dimensional surface density distributions of water molecules at the first contact layer near the wall surface for PG-GO2 with interlayer distance of 10Å, PL and OL represent the interfacial water layers near PG and GO sheet, respectively.


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Accor ding to the above analysis, we provide a physical description of the significantly reduced water flow near the functionalized sheet for heterogeneous GO nanochannels. However, what is the cause of the dragged flow for the heterogeneous GO pores? We suspect that the HB interaction between different hydrolayers might play a leading role in the dragging force. Hence, we calculated the spatial distribution of the number of HBs of each water molecule along zdirection, including the intralayer and interlayer contributions (each layer is defined as the thicknesses of 1.0Å). The water-water HB profiles for GO2-PG pore with the pore sizes of 10Å and 20Å are presented in Figures 7(c) and 7(d), respectively. Apparently, the intralayer HB numbers are higher than those of interlayer at the solid-liquid interface, and the water layer near PG sheet formed more intralayer HBs compared to GO2 sheet, implicating water molecules prefer to lie in parallel to the basal plane at pristine graphene surface. However, from the HB profiles, it can be also found that a considerable proportion of interlayer HBs exist in the whole channel regions. Therefore, different from the collective movement of water confined in PG channel without the need to destroy the interlayer HBs, for hybrid GO channel, the functional groups on oxidized sheet not only directly break down the nearby interfacial water layer, but also indirectly provide adhesion dragging action to the whole water molecules in the nanopore due to the strong water-water interaction stemmed from abundant interlayer HB interaction.


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Figure 7. The distributions of the average number of HBs per water molecule in the PL(left) and OL(right) for the GO2-PG pore with interlayer distances of 10Å (a) and 20Å (b). The spatial distributions of the number of water-water hydrogen bonds, which were categorized as the intralayer and interlayer contributions of each water molecule confined in the GO2-PG pore with interlayer distance of 10Å (c) and 20Å (d).

In short, the observed vertical dragging transport phenomenon is evidently attributed to the synergistic effect between the interfacial affinity of oxidized sheet and the strong water-water interlayer interaction. However, this strong interlayer dragging interaction originated from GO surfaces has not exposed in the previous work25. This dragging effect revealed in our work also provides an additional support to the side-pinning effect arising from the neighboring GO region, which has been revealed in the previous study32. Finally, in order to provide auxiliary proof of the flowing breakdown in the heterogeneous GO channels, we conducted an additional MD simulation for the more practical GO slit pores, which include different regions. As shown in Figure 8(a), the GO channel was equally divided into three different domains (PG-PG, GO-GO, and PG-GO) along the y-axis and the water transport is simulated along the x-direction. Figure 8(b) shows the corresponding velocity profile


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(red circle) and the flow velocity field (vector arrow) for stable-state flow. It is observed that, as compared with PG-PG region, both GO-GO and PG-GO regions exhibit obviously reduced flow rate, suggesting the heterogeneous PG-GO region could enormously suppress the water fast transport, consistent with the preceding result. The flow velocity profiles for the PG-GO or GOGO region in Figure 8(b) showed a concave shape, which is different from the convex shape in the single 2-D GO channel (Figure 5(a)). The different velocity distributions are due to their different flow capillaries. In Figure 8, for the 2-D combined channel structure, both the overlapping hybrid surfaces and the side-pinning effect

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will cooperatively disturb the water

flow. Furthermore, according to the vector arrow of flow direction, we can notice that water inside PG-PG domain shows an ordered and regulated flow direction, while for PG-GO or GO-GO region, the movement of water molecules displays a disorganized pattern, which corresponds to the breakdown of fast water transport. The observed vertical dragging transport phenomenon is evidently attributed to the synergistic effect between the interfacial affinity of oxidized sheet and the strong water-water interlayer interaction. This result confirms that only PG-PG region might serve as the most favorable passage for water transport through GO interlayers, whereas the PGGO mixed region still provides the low flow rate for molecular permeation.


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Figure 8. (a) Schematic of the simulation system for the 2-D hybrid pore model with three various coexisted channels (PG-PG, PG-GO and GO-GO); water flow along X direction. (b) The corresponding velocity profile (red circle) and the flow field (vector arrow) for the hybrid flow system.

CONCLUSIONS In our work, a combination of comprehensive MD simulation and theoretical analysis was conducted to investigate the water transport behavior through a series of 2-D hybrid/mixed GO nanochannels. It is found that water flow through the heterogeneous 2-D galleries exhibits the significantly reduced permeability compared to the ultrafast flow between PG sheets, suggesting that heterogeneous GO nanochannels in laminated GO membranes could not provide a rapid water transport. This result is consistent with the previous experimental observation, but different


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from the available simulation result. Meanwhile, the unique semi-parabolic velocity profiles in heterogeneous pores reveal that there exists a prominent vertical dragging effect. The presence of the GO surface in the hybrid channels could significantly suppress the water flow and provide the controlling effect. Based on the Navier-Stokes equation with asymmetric boundary conditions, a hydrodynamic model was developed. The consistency between theoretical and simulated results indicates that the theoretical model could effectively describe the flow behavior for general 2-D hybrid channels, which is expected to be applied for the actual prediction and design of 2-D nanofluidic channels. According to interfacial microstructure and energetic analysis, this unusual dragging effect is ascribed to the synergistic impact between the high interfacial friction of oxidized graphene sheet and strong interlayer water-water interaction. The affinity between GO and water induces a stable sticky surface hydrolayer, and the interaction between neighboring water molecules further provide an adhesion force to impede the entire flow rate. Overall, our result presented new understandings of the water permeation across GO-based laminate membranes with heterogeneous structures. ASSOCIATED CONTENT Supporting Information Simulation configuration and additional simulated results for the flow rate, velocity profiles and interfacial structure and energetic analysis. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS


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