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Functionalized Graphene Nanosheet as a Membrane for Water Desalination Using Applied Electric Fields: Insights from Molecular Dynamics Simulations Jafar Azamat J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 06 Oct 2016 Downloaded from http://pubs.acs.org on October 6, 2016

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The Journal of Physical Chemistry

The progress of ion separation through the functionalized pore of graphene nanosheet was simulated using MD simulations. At the beginning, the saltwater are uniformly distributed in the central box as can be seen in initial configuration. First, we apllied various external electric fields to the simulated system. As a result of this work, ions overcome the barrier and permeate through the left and right pores embedded in the graphene. As the simulation time increases, more and more ions permeated through these pores and finaly clean water obtained in central box. 50x50mm (300 x 300 DPI)

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Functionalized Graphene Nanosheet as a Membrane for Water Desalination Using Applied Electric Fields: Insights from Molecular Dynamics Simulations

Jafar Azamat* *

Corresponding author: Department of Chemical Engineering, Ahar Branch, Islamic Azad University, Ahar, Iran; Tel.: +984144232163; Fax: +984144232163; E-mail: [email protected]

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ABSTRACT Desalination is a favorable method employed to supply clean water in recent years. Various contaminants entering water resources must be removed from water by novel structures like nanostructure membranes. Accordingly, molecular dynamics simulations were performed to study the ions removal from the water using a graphene nanosheet (GNS) based on the permeability and selectivity of graphene. The studied system consisted of two functionalized GNSs immersed in the aqueous ionic solution of NaCl. The GNSs had one pore each, both being approximately of the same size. For the ions removal from water using these GNS, an external electric field was applied to the system. For the preferential permeation of cation or anion across the graphene, the pore of GNS was functionalized by passivating each carbon atom at the edge of pore by fluoride (F-pore), negatively charged, and hydrogen atoms (H-pore), which were positively charged. The results showed that by using of the electric field, the F-pore and the H-pore of GNS were preferential selective to Na+ and Cl-, respectively; also, the higher the electric field, the faster the movement of the ions from the salty water. The calculations of the potential of mean force for ions showed that sodium and chloride ions encountered an energy barrier and thus, cation and anion failed to permit across the H-pore and F-pore of GNS, respectively. Based on the results of this research, the functionalized GNS, as a membrane, can be suggested as a device in the field of water desalination.



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1. INTRODUCTION Water demand is in many domains such as human consumption and the use in agriculture and industry; with population growth, water use has also grown. In arid regions, the scarcity of freshwater resources is serious, tuning in to an important topic. Many arid areas do not have freshwater resources in the form of rivers and lakes. Therefore, saltwater is considered as the substitute water resource. Oceans and seawaters are the main resources of saltwater. Saltwater is desalinated to produce appropriate freshwater for consumption.1 Today, new technologies providing cost-effective ways of providing freshwater are used in the water desalination industry. One of these technologies is the use of the nanoporous membrane technology. Nanopores membranes can be used as separation filters so that small molecules or ions can pass across them while large molecules cannot. The requirement for desalination membranes could not be easily met. These membranes should be cheap, mechanically stable, highly flux, salt rejecting, and resistant to fouling. One of these membranes is graphene nanosheet (GNS),2 in which pores can be created with the unsaturated carbon atoms passivated by various chemical functional groups. Since its recent isolation, 3 GNS membrane has attracted scientiﬁc attention in many exciting experimental 4 and theoretical studies

5

due to its individual properties like the high intrinsic mobility,

electrical conductivity,

7

optical transmittance

8

6

and the large specific surface area volume

ratio.9 These properties have attracted much attention for the potential applications in many areas.10 Because of ultimate thinness

11

of graphene, it offers a wide range of opportunities

for membrane applications. GNS is very promising as a membrane for water desalination with high selectivity and high permeability, due to precise sieving properties and its small thickness, respectively. Molecular separation by means of GNS can be done using pores created in it.

12

The

perfect GNS does not have pores and is impermeable to molecules or ions. 13 This is because 3 ACS Paragon Plus Environment

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the electron density of GNS rings is substantial to repel ions trying to pass across them. Thus, in order to do ion separation using GNS, drilling pores is required. After drilling pores, these pores should be functionalized by passivating each carbon atom by means of functional groups such as hydrogen or fluoride. These chemically functionalized pores of graphene4b, 14 can have unique properties and applications.

15

Recently, ion separation via GNS using

molecular dynamics (MD) simulation method has been carried out.16 Single layer GNS with functionalized pores in its center can be competently employed for ion and molecular separation.

17

He and et al. designed nanopores in GNS that could distinguish between

sodium and potassium ions. Their results showed that under an electric filed, a GNS with carbonyl groups conducted potassium over sodium ion. A GNS functionalized by carboxylate could selectively bind sodium ion, but transported potassium over sodium ion. 18 In another study, Suk et al. studied ion transport and dynamics in GNS with and without the application of an external voltage drop. In the equilibrium condition, ion concentration in GNSs was dropped sharply from the bulk concentration when the radius of pore was smaller than 9 Å.19 Konatham et al. investigated the passage of water and ions across pores of GNS. Diameters of GNS pore were extended from 7.5Å to 14.5Å. The results indicated that ion exclusion could be obtained via pristine pores of the diameter 7.5Å, while the ions could simply permeate the pristine pores of the diameters 10.5Å and 14.5Å. They showed that carboxyl functional groups could enhance ion exclusion, but the effect became less pronounced as both the diameter of pore and the ion concentration were increased.20 Filtering salt from water using functionalized GNS was investigated by Cohen-Tanugi and Grossman. They reported the desalination performance as a function of applied pressure pore size, and chemical functionalization. Their results indicated that the GNS ability to ban the ion passage depended on the diameter of pore.21Also, other nanostructured membranes such as MoS2 22 nanotubes, 23 and zeolites 24 could be used for water desalination. 4 ACS Paragon Plus Environment


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In this research, two kinds of single-layer GNSs with a functionalized pore in their center were used to show their ability for the separation of salt from an aqueous solution in water desalination devices. The MD simulations method was used for this purpose. We studied ion separation mechanism by means of functionalized GNSs based on the external electric field applied to the system. In spite of abundant studies of MD simulations addressing the water desalination through functionalized GNSs,20, 25 their abilities for ion separation has not yet been studied to examine their capability for removing cations and anions from aqueous solutions simultaneously. Our results could be used for the design of the energy-efficient nanostructure membranes for water desalination. In the following sections, first, we clarify the procedure of MD simulations and then demonstrate the results of our investigations.

2. COMPUTATIONAL METHOD AND DETAILS In this work, in order to carry out water desalination through the functionalized pores of GNS, MD simulations method was used. The size of the simulation box was 30×30×90 Å3. The MD box involved 1600 water molecules with 0.5 M NaCl at the initial step; the molecules were placed between two GNS membranes with different functionalized pores (see Figure 1). This concentration of ions in the water was about the same salinity of seawater. The use of higher concentrations of this content was not reasonable. It should be noted that to remove high concentrations of ions, more simulation time was needed. The geometric optimization of a GNS membrane with a functionalized pore in its centre was done using the density functional theory (DFT); more specifically, this was done by means of GAMESS

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in the B3LYP level of theory with the 6-31G basis set implementation to

obtain the optimized structures and their atomic charges. In this method, graphene was allowed to move in two dimensions and its size was selected to be big enough, so that there would be no need to use boundary conditions or the periodic code in DFT calculations. In 5 ACS Paragon Plus Environment

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comparison with the systems of recent molecular simulation research in this field,

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we

believe that this method, as well as size system used in this study is reasonable and good results can be obtained from it. The results obtained from DFT for the studied system are represented in Table 1. The partial charges for graphene atoms were calculated by the CHelpG scheme developed by Breneman and Wiberg. 28 The sizes of the GNS were 30×30 Å2. There were 377 C atoms and 9 F atoms in the left GNS and 9 H atoms in the right GNS. These two kinds of membrane are shown in Figure 2 with accurate details. During the simulations, to avoid the vertical displacement of the entire GNS membranes, they were restrained via a harmonic constraint of 40 kcal/mol.Å2 during the simulations and an external electric field was applied perpendicularly to the simulation cell. The applied electric field consisted of internal units of NAMD package.

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The electric field

range from 0 to 35 V was used in this study. GNS with an intrinsic tensile strength of 130 GPa and a Young's modulus of 1 TPa

30

is one of the strongest materials. Therefore, we

believe that the use of high electric field doesn’t affect in GNS. For the investigation of ion removal from water, we used only the electric field. EOF (Electro osmotic flow) is defined as the motion of molecules across thin channels caused by an applied electric field. When the electric field is applied to the system, the anions are moving to the anode. This ions movement can encourage the movement of water molecules. The EOF method is a useful means for transporting analytes. Unlike pressure-driven flows in which the pressure drop is needed to maintain a certain fixed mean, and the flow speed is increased inversely as the square of the capillary radius, in EOF, the voltage needed is independent of the capillary radius. Thus, EOF is very capable for transferring the infinitesimal volumes of fluid across narrow tubes. 31 It can be very interesting to investigate the ion separation process, particularly the influence of pore size of nanosheets on the permeation of dissimilar ions. The diameter of 6 ACS Paragon Plus Environment


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the pores in the GNS was about 6 Å and the pore density was 0.11 nm-2. The rim of pores can greatly influence the ion separation properties. A good separating membrane should have a controllable pore size and a stable structure. In this study, we designed variant pores with different sizes and shapes along with different rim terminations in GNS. But some of them didn’t have the favorite selectivity. As well as the F-pore and H-pore, other atom types were examined for the functionalization of pores such as the Si atom and the hydroxyl group. Finally, the best choice with high separation ability was the F-pore and the H-pore. Also, if large functions such as -NH3+, or -COOH were used for functionalization, the ions would not pass through them. Also, when we used pores larger than 6 Å, selective separation of ions was not observed; in the case of pores smaller than 6 Å, ions passing through pores did not happen. This showed that ion permeation was hindered when the size of pore was smaller than the hydrated radius of ions. At the beginning, the energy of simulation box was minimized for 1 ns in zerotemperature, and its temperature was increased to 298 K and maintained at this temperature with a Langevin thermostat. Finally, simulations were carried out for 6 ns under applied electric field and at constant volume, using the NAMD 2.10,32 with 1 fs time step as done in previous researches,

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at a constant pressure (1 bar) with a hybrid Nose-Hoover Langevin

piston. For each choice of electric field, the MD simulations were run 5–10 sets with different initial distributions, in order to produce more accurate averages. The force field parameters for GNS and sodium and chloride ions were obtained from references

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and for water molecules, the TIP4P water model

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was used to exactly

reproduce the entropic and hydrogen bonding behavior of water. 36 A value of 12 Å was used to cut off the van der Waals interactions. For long-range electrostatic interactions, the PME (Particle Mesh Ewald) method

37

was used. All analyses were done using visual molecular

dynamics.38 The cross interaction Lennard-Jones parameters between species could be 7 ACS Paragon Plus Environment

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obtained by the Lorenz-Berthelot mixing rules. 39 The ion permeation phenomenon through pores was clarified by calculating the PMF (potential of the mean force).40 For calculating the PMF experienced by an ion when moving across a pore, the simulation box was divided into windows of equal length along the z-axis of the system and then, an ion was placed at several positions along the reaction coordinate and the force experienced by the ion was sampled. Each sampling window was simulated for 1 ns. The ion PMF was obtained by means of the umbrella sampling technique 41 with an ion harmonically restrained in 0.1 Å steps, and 15 kcal/mol.Å2 force constant when the system was in equilibrium. Collective analysis of PMF was done using the WHAM (weighted histogram analysis) method. 42

3. RESULTS AND DISCUSSION As stated, we used MD simulations method to investigate the water desalination by means of applying electric field. The simulated system consisted of the 0.5 M NaCl aqueous solution and two GNSs with different functionalized pores. For ion separation through GNS pores, an external electric field was applied to the system. Under the influence of this electric field, Na+ and Cl- ions permeated through the embedded pores in GNS. The use of the F-pore and the H-pore in this work was quite interesting. The Cl- ions permeated from the H-pore entered the right box of system and Na+ ions permeated from the F-pore entered the left box of system. Certainly, if we switch the position of the F-pore and H-pore membranes (or switch the direction of the electric field), ion separation does not happen. This is because, although chloride ions (with the negative charge) tend to move towards the positive pole of system, the F-pore (with the negative charge) will prevent it; in the case of sodium ions (with the positive charge), they tend to move towards the negative pole of system, but the H-pore (with the positive charge) will prevent it. In this research, for 8 ACS Paragon Plus Environment


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the study of system, many parameters including ion-water radial distribution functions, water flux, hydrogen bonds and ion retention time were investigated.

3.1. PMF calculations Although the functionalized pores in two GNSs had a large enough radius to enter the two kinds of Na+ and Cl- ions, the results showed that the ions permeated across these pores selectively, after applying the electric field. The transportation directions of Na+ and Clwere not the same, and they were transported in the opposite direction of the simulation box because the electric field was just in one direction. Chloride and sodium ions were permeated from the central box to the right and left boxes, respectively. The pore of the right GNS (H-pore) was functionalized by passivating each carbon atom with hydrogen atoms having the positive charge; and in the left GNS, its pore (F-pore) was functionalized with fluoride with the negative charge. This preferred passage of ion through functionalized pores of GNS could be justified by the potential of the mean force; i.e., free energy profiles for a selective ion as it was moved along the system. The results showed that Cl- ions were able to go through the H-pore. In contrast, Na+ ions were not able to permeate through this pore. In the case of the F-pore, the opposite phenomenon occurred and only Na+ ions were permeated from it. Figure 3 shows PMF for the considered ions. As can be seen, there was a large energy barrier in the H-pore for Na+ which inhibited the Na+ permeation. In the F-pore, large energy barrier was for chloride ions. In both membranes, by approaching the GNS, PMF was increased, reaching its maximum value in the pore center.

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This was because of the

interactions between the functionalized group of pores and ions. In the case of the H-pore, due to the positive charge of hydrogen atoms in the edge of pore, chloride ions with the negative charge tended to permeate through this pore, but sodium ions with the positive


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charge were not reluctant to pass through it. In the F-pore with the negative charge on the fluoride, the opposite phenomenon occurred. It could be concluded that ions with high free energy barrier could not permeate the functionalized pore.

3.2. Water structure and hydrogen bonds The fluid structure of water molecules could be determined with the density profile. During the system simulations, waters had a structure different from that its bulk water. Figure 4 displays the density profile of water molecules in the left, right and central boxes. In this system, water molecules concentrated in the region of ±0.35 nm around the left and right GNSs. This behaviour of water molecules presented two sharp peaks near each GNS. The layered structure of water with high density was observed in the region close to the GNS, but in the region far away from the GNS, the water density was 1 g.cm-3. This behaviour of water molecules has also been observed in other works. 20, 44 The special structure of waters, in the region close to the GNS, is due to their non-bonded interactions with it. The GNS operated as a barrier in front of the waters and so, they were concentrated at a certain distance from it. Also, with applying an electric field, the water molecules achieved regular structure due to the permanent electric dipole moment. Indeed, the electric field could align the dipole of waters in the electric field direction. 45 Variations of the average number of hydrogen bonds of water molecules relative to electric fields could be investigated. Figure 5 demonstrations the average number of water hydrogen bonds under various applied electric fields. As can be seen, with the increase of the electric field, the average number of hydrogen bonds of water molecule was increased. The water structure was similar to an ice-like structure with increasing the electric fields. Indeed, the spatial orientation of water molecule could be influenced by the electric field, and their rotation was caused by the electric field changing the hydrogen atoms site. Consequently, the



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average number of hydrogen bonds was increased with the increase of electric field, and the water structure had a well-organized structure. 46 Hydrogen bonds autocorrelation function (C(t)) is used to describe the pair dynamic of the hydrogen bonds.47 C(t) parameter is defined using equation (1):

C (t ) =

H (0) H (t ) H

(1)

In this equation, H(t) is a population operator for the hydrogen bond, which is 1 if a hydrogen bond is existent at time t and 0 otherwise. C(t) is the possibility of hydrogen bonded at time 0 to time t. Thus, it specifies how fast hydrogen bonds are relaxed. The faster decrease of C(t) suggests that the hydrogen bonds formation in the system is weak and breaks frequently. As can be seen in Figure 6, over time, this parameter was reduced and its reduction was intensified with a reduction in the electric field; this showed that the hydrogen bonds breaking could have been increased if the applied electric field has been decreased. This was because the average number of hydrogen bonds of water molecules was increased with raising the electric field (see Figure 5); it was concluded that the rates of hydrogen bond relaxation depended on the strength of the electric field. It can be concluded that the hydrogen bonds of water depends on the electric field; so that the structure of hydrogen bond is enhanced with increasing the electric field.

3.3. Ion and water permeation Salt rejection and water permeation of membrane are the two significant aspects describing the water desalination performance. In most scientific studies, for efficient water desalination, the best performance has been obtained with higher water permeability and higher salt rejection. 22b, 25d, 48 All of these systems are designed so that ions do not pass and waters just permit the pore (the reverse osmosis phenomenon) and clean water is obtained 11 ACS Paragon Plus Environment

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through this. But, in our system, clean water was obtained by means of removing ions from water, not by rejecting them. Thus, optimum condition consisted of more ions and less water passing through pore. Figure 7a displays the total number of sodium and chloride ions passing across the GNS pores. With raising electric field, the number of ions passing was increased. It showed that the ion removal was increased at the higher voltage, so that the high voltage induced the high force on the ions. Therefore, the removal of ions with increasing the applied electric field was well done. As can be seen in this Figure, after 30 V, 100% ions permeated from pores and clean water was obtained. It should be noted that the complete removal of ions from the central box to left and right boxes in the three electric fields 30, 32.5 and 35 V did not happen at the same time. By increasing the applied electric field, the time required to remove ions was decreased (see Figure 7b). Also, the time required for one ion passing through the pores (retention time) can be one of the effective parameters, because by reducing this time, desalination could be done more quickly. The retention times of ions are presented in Figure 8 as a function of the applied electric field. As can be seen, with increasing the electric field, the ion retention time was reduced. Also, the retention time of chloride ion in the H-pore was larger than that of sodium ion in the F-pore. It could be due to high energy barrier for Cl- in the H-pore, relative to the energy barrier of Na+ in the F-pore (see Figure 3). This was why the retention time of chloride ion was larger than that of sodium ion. It should be emphasized that chloride ions needed more time for passing across the GNS pores. To investigate the water permeability through the pores of GNSs, we calculated the water fluxes (Φ) 49 through pores as a function of the electric field. The water flux was calculated as Φ = V.υ.ρ, where V was the average rate of waters per unit time, υ was the volume per water, and ρ was the nanopore density in the GNS.

25d

Water molecules permeated through



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the GNS at a constant rate, and that the water flux was decreased with applied electric fields (see Figure 9), but ions showed an increasing trend with applied electric fields (see Figure 7). In other works, water flux through various pores of GNS as a function of the applied pressure has been investigated.

21, 25d, 50

In these works, In these works, it has been shown

that with enhancing the applied pressure to the system, water flux is also increased. So it can be said that if applied pressure is used for water desalination, GNS with a pore in it centre can be appropriate for water desalination with the passing water and the non-passing ion through them.

17

In the event that in our system, the opposite phenomenon occurs, ions are

permeated and waters have very low flux, remaining in the central box as the pure water. To investigate the influence the distance between two GNS membranes (layer spacing) on the ion separation under the specific applied electric field, we performed new runs with various layer spacing’s. It was observed that when the layers were sufficiently spaced (≥ 10 Å), the ion separation was independent of layer spacing. This continued to hold at very large layer separations, suggesting that the ion separation was governed by the energy barriers of the separate layers in ≤10 Å. In the small layer spacing, the energy barriers of two membranes influenced the ion separation; so, ion passing was severely reduced.

3.4. Radial distribution function (RDF) Impey et al.,

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showed that an ion is surrounded by water molecules and these water

molecules are oriented by the ion, and they tend to be carried by ion as the it passages in solution. Displacement of the ions changes its chemical potential during the simulations; because of the applied external electric field, it could be useful in explaining why water would move.52 The above phenomenon, which is called ionic hydration, can be seen and proved by the radial distribution function (RDF). Experiments 51

53

and molecular simulations

have confirmed this ion-water shell structure. That is due to the interaction between ion


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and water. RDF of ion-water is calculated through the trajectories files of simulations saved during the time. Figure 10 shows the RDF ions-waters in different electric fields; Figure 10(a) displays Na+-water RDF's and Figure 10(b) represents Cl--water RDF's. At a short distance, RDFs were zero, because of the strong repulsive forces between atoms. In each RDF, the location and intensity of peaks were two main parameters. As can be seen, the location of the first maximum peaks was similar in all electric fields, but in the case of sodium and chloride ions, it was not similar. Also, in each electric field, the intensity of this peak was dissimilar, showing that the hydration number of ions was different. The positions of the first peaks in two RDFs was in a good agreement via the its values in the bulk system and the simulation results of others value for Na+-water distance.

53b

54

corresponded to the experimental

The orientation of the surrounding water can also be

affected by the electric field,46, 55 that it leads to different interaction energy between the ion and its surrounding water molecules. This energy is correlated with the waters orientation distributions. The special water molecules orientation in the hydration shell of ions is determined by the balance of the van der Waals and electrostatic interactions of waters around the ion.

4. CONCLUSION For water desalinating, MD simulations were performed to study the ion permeation across the functionalized pores of GNS. The MD results showed that by means of an external electric field, desalination was realized by the GNS pores. These functionalized pores operated selectively, so that sodium and chloride ions were permeated from the F-pore and H-pore, respectively. The opposite phenomenon was never eventuated. This preferred passage of ion through the functionalized pores of GNS was justified by the PMF calculations. In the H-pore, due to the positive charge of the hydrogen atoms, Cl- tended to 14 ACS Paragon Plus Environment


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permeate through it, but Na+ was not reluctant to pass through it. In the F-pore, with the negative charge on the fluoride atoms, the opposite phenomenon occurred. It could be concluded that the external electric field had an influence on the ions separation, hydrogen bonds of system, the retention time of ions and RDF. Finally, the results highlighted the capability of the GNSs with the functionalized pores in its centre, with respect to water desalination technology.

NOTES The author declares no competing financial interest.

ACKNOWLEDGMENTS Author thanks the Ahar Branch Islamic Azad University and Iranian Nanotechnology Initiative Council for their support.


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Tables.

Table1. Partial charges of GNS atoms calculated by DFT method. Type of atom

Partial charge

Partial charge of carbon atoms in the H-pore

-0.115 e

Partial charge of carbon atoms in the F-pore

+0.290 e

Partial charge of hydrogen atoms in the H-pore

+0.115 e

Partial charge of fluoride atoms in the F-pore

-0.290 e



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FIGURES.

Figure 1. Schematic representation of the simulation system. Ions placed between two GNSs with a functionalized pore in their centers in saltwater. The whole system is immersed in water. Pore of left graphene was functionalized with the fluoride (F-pore) and pore of right graphene was functionalized with the hydrogen atoms (H-pore). The direction of the applied electric field was shown also. (yellow: Na+; green: Cl-; red: oxygen; white: hydrogen; black: carbon).


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Figure 2. Functionalized pore of GNSs; left: functionalized pore with the fluoride (F-pore) and right: functionalized pore with the hydrogen atoms (H-pore).



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Figure 3. The PMF for sodium and chloride ions in the: (a) H-pore, and (b) F-pore. The results display that chloride ion was able to go across the H-pore due to a small energy barrier. In contrast, sodium ion wasn’t able to permeate across this pore. In the case of F-pore, the opposite phenomenon occurred.


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Figure 4. Density profile of water molecules (Dash lines indicate the GNSs). Water molecules show tendency to concentrate around the left and right membranes. This behavior of water molecules is presented two sharp peaks near each GNS.



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Figure 5. The average number of hydrogen bonds of water molecules in the simulation box under various electric fields.


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Figure 6. Autocorrelation function of hydrogen bonds in some applied electric filed.



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Figure 7. (a) The total number of ion passing across the H-pores (Cl-) and the F-pore (Na+). Each data point characterizes the average of 5-10 sets run. After 30 V, 100% ions permeated from pores and clean water was obtained. (b) Time required for the removal of ions in different electric fields.


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Figure 8. Retention time for chloride ion in the H-pore and sodium ion in the F-pore at the applied electric fields. With increasing applied voltage, the retention time of ions reduced. Also, the retention time of chloride ion in the H-pore is larger than that of sodium ion in the Fpore.



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Figure 9. The water flux in various applied electric fields. Lines were achieved from a linear regression. Each data point characterizes the average of 5-10 sets of simulations with their error bars.


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Figure 10. Ions structure in the bulk system. RDF for sodium and chloride ions in the simulation box under various electrical fields: (a) RDF Na+-water, (b) RDF Cl--water.


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