Electric field assisted desalination of water using B- and N-doped-graphene sheets: A non-equilibrium molecular dynamics study

Journal Pre-proof Electric field assisted desalination of water using B- and N-dopedgraphene sheets: A non-equilibrium molecular dynamics study

Maryam Kamal Kandezi, Muhammad Shadman Lakmehsari, Chérif F. Matta PII:

S0167-7322(19)33295-7

DOI:

https://doi.org/10.1016/j.molliq.2020.112574

Reference:

MOLLIQ 112574

To appear in:

Journal of Molecular Liquids

Received date:

11 June 2019

Revised date:

22 January 2020

Accepted date:

24 January 2020

Please cite this article as: M.K. Kandezi, M.S. Lakmehsari and C.F. Matta, Electric field assisted desalination of water using B- and N-doped-graphene sheets: A non-equilibrium molecular dynamics study, Journal of Molecular Liquids(2018), https://doi.org/10.1016/ j.molliq.2020.112574

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© 2018 Published by Elsevier.

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Electric Field Assisted Desalination of Water Using B- and N-DopedGraphene Sheets: A Non-Equilibrium Molecular Dynamics Study Maryam Kamal Kandezia, Muhammad Shadman Lakmehsaria*, Chérif F. Mattab a

Department of Chemistry, Faculty of Science, University of Zanjan, P.O. Box 45195-313,

Zanjan, Iran. b Department of Chemistry and Physics, Mount Saint Vincent University, Halifax,

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Nova Scotia B3M 2J6, Canada

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* Correspondence: [email protected]; [email protected]

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ABSTRACT

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The possibility and mechanism of water desalination using newly designed doped graphene sheets is reported. It is hereby demonstrated, through molecular dynamics (MD)

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simulations, that an electric field-assisted selective ion separation is possible using boronand nitrogen-doped nano-porous graphene sheets. The sheets are shown to behave as

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nano-sponges, with potential future applications in water desalination by electrodialysis, for example. Ion separation is found to increase as the extent of ion hydration decreases

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under increasing field strengths. Boron- and nitrogen-doped graphene sheets are found to exhibit acceptable mechanical properties and, simultaneously, good ion selectivities and

processes.

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may be candidates for future technological applications in desalination and other refining

KEYWORDS: Desalination technology; functionalized graphene; molecular dynamics simulations; applied external electric fields; ions separation.

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1.

INTRODUCTION

The United Nations projects serious Worldwide potable and irrigation water shortages by the year 2050 that may affect up to five billion people. This shortage reflects the fastgrowing global population, the fast urbanization, warmer climate, and increase in demands for irrigation to feed the growing population. Current tensions between Egypt and Ethiopia sparked by the construction of the Renaissance Dam on the Blue Nile exemplify the urgency of finding alternative sources for fresh water. Several countries including

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those of the Gulf Cooperation Council (GCC) rely already on desalination as their primary source of potable water with a total of 1.7 billion m3 of desalinated water produced by

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trend of increase reliance on desalination plants.

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these six gulf states during 1997 [1]. Many more countries are expected to join in this

Membrane-based ion separation is the basis for several important technological

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applications in desalination and demineralization. Membranes characteristics such as pore size [2-5], functionality [6-9], and doping [10-13] can be optimized for the removal of

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salts and heavy metals with minimal effects on the water flow. Ion separation by membranes rests on a number of different principles such as electro-dialysis, membrane

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distillation, filtration, and reverse osmosis [14, 15]. Clearly, desalination and demineralization by membrane technologies can be improved by fine-tuning membrane

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structural and mechanical properties [16-22], which is the motivation of this work. Graphene, as a nono-porous membrane, is a candidate for technological

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applications in water desalination [8-10, 17] given its significantly higher efficiency of ion separation compared to membranes made from other materials. The thin (one-layer) structure and relatively large surface area [23, 24] makes graphene-based membranes particularly advantageous, in principle, since the water flux is inversely related to the thickness of the membrane. Rational design of better performing membrane, the subject of this work, is accomplished by altering the chemical structure of graphene nano-sheets to increase ion selectivity while maintaining their high structural strength. Elemental doping of graphene nano-sheets is probed computationally here using molecular dynamics (MD) simulations as a possible means to improve ion selectivity. This work precedes the eventual fabrication and experimental testing of the proposed membranes to the best

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knowledge of the authors. Boron and nitrogen atoms, because of their similar covalent atomic radius to carbon (0.80 Å for B, 0.74Å for N, and 0.77Å for C [25]), are good candidates for the doping of carbon material.

Nitrogen-doped nano-porous graphene has the desirable

mechanical strength and thermal stability for a candidate membrane material [23]. Furthermore, experimental methods to fabricate nitrogen-doped porous graphene materials are well documented [26-28].

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In 2015 a novel nitrogen-doped graphene sponge (NGS) with high surface area and pore volume has been fabricated [26]. This nitrogen doping, by changing the

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hydrophilicity of surface, improves the “wettability” of the surface by the solute [29].

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These exciting newly synthetized surfaces have focused primarily on NaCl sorption. It is the purpose of the present work to complement and go beyond the present experimental

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state-of-the-art by predicting the performance of doubly-doped graphene sheets as molecular separators under the effect of applied electric fields.

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The performance of functionalized nano-porous graphene oxide membranes in the desalination of water is known surpass graphene membranes, but the reason(s) for this

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empirical observation remains not fully understood [30]. Further, it appears that the selective ion transport in functionalized carbon nanotube can be enhanced by the

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application of an external electric field that can alter ionic currents [31]. In other words, it is possible to manipulate the time ions spend inside or outside a carbon nanotube by

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adjusting the external field polarity. These finding can lead to applications such as nanosensors, confined chemical reactions, and microfluidics and can be a basis for desalination using doped graphene nanosheet [31]. In nitrogen-doped carbon nanomaterials, the nitrogen atom is negatively charged being more electronegative than carbon (with electronegativities of 3.04 and 2.55, respectively, on Pauling’s scale [32]). Meanwhile, boron-doping makes graphene electrophilic [33] and increase its electron acceptor ability from Cl-. Considering that boron (with electronegativity of 2.04 [32]) is less electronegative than carbon, borondoped regions exhibits positively charged carbon atoms [33]. Molecular dynamics simulations of ions and water passage through a carbon

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nanotube have helped understand the coupling of the transport of water and ions, an understanding that is crucial for the design of desalination systems and ion separator devices [34]. While electro-catalysis, that is, the modulation of the energetics of chemical reaction with external fields [35-43], is a well-developed area, little work has been done in the area of MD simulations under external fields in the context of desalination technologies using carbon-based nanomaterials. One of the few salient studies that fills this gap in the literature is one by Lohrasebi et al. who studied water desalination in the

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presence of an electric field by a system of two graphene membranes opposed to two external electric fields on opposite sides [44]. A basic question emerges from this

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discussion, and that is: Can one envision a substantial improvement in adsorption by the

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modulation of the doping of graphene sheets, particularly if doped sheets are used in conjunction with applied external static electric fields? This work will demonstrate that

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this question has a positive answer. A mechanism for ion separation by these B-N doped porous graphene membranes under an electric field, not known to the best of the authors

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knowledge at the time of writing, is also proposed. These doped graphene membranes appear promising as nano-sponge for removing ions from salt-water. COMPUTATIONAL METHODS

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Visual Molecular Dynamics (VMD) [45] was used to construct all initial guesses while

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MD simulations were performed using the LAMMPS code [46]. The salt-water solution was composed of 12,000 water molecules, 112 Na+, and 112 Cl− ions (corresponding to a

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salt concentration of 1.0 mol/L). A higher salinity than seawater (∼0.7mol/L) was chosen in order to increase the occurrence of ion and graphene doped sheet interactions and to increase the precision of the results for a given system size and simulation time. The dimensions are nanometers and the graphene doped sheet has no pores. The system is minimized and optimized to check the density of water before performing the NVT. The box size is 4×4×8 Å3. The box along the z-axis was considered to be two times as larger than other directions to minimize the interaction between the parallel sheets. A 4×4 graphene sheet was chosen. A 10% doping of boron and nitrogen was created on two regions in the left and right side of a graphene sheet by replacing carbon atoms that are randomly selected by nitrogen and boron atoms. Periodic boundary conditions (PBC) were

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applied in the three dimensions. Uniform electric fields were applied in the z-direction (perpendicular to graphene plane). Intramolecular forces on C, B and N atoms in the doped graphene was described with the Tersoff potential [47] and the forces on water molecules were described with the TIP3P model which has been shown to accurately reproduce the behavior of liquid water [48]. Interactions between water-ion and C, B and N atoms were modeled with LennardJones (LJ) parameters using the Lorentz-Berthelot mixing rules. The LJ cut-off was set to

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8Å. The long-range electrostatic interactions were computed with the Particle Mesh Ewald (PME) method. The Coulombic cut-off was set to 10Å and desired relative error in forces

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set to 1.0×10-4 (A precision value of 1.0×10-4 means one part in 10000. This setting is

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used in conjunction with the pairwise cutoff to determine the number of K-space vectors for style Ewald.)

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The equations of motion were integrated via a velocity Verlet method with the time-step of 1fs. The salt-water and B-N doped graphene sheets were first subjected to

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minimization (the variation of total energy of system during equilibration (Figure S1), the velocity auto correlation function of system during equilibration (Figure S2), the kinetic,

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potential and total energy of system during equilibration (Figure S3), the radial distribution function (RDF) between Na+ and Cl- ions and carbon of graphene sheet in

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absence of elemental doping (Figure S4), axial velocity profile of water molecules in the z-direction of simulation cell vertical to graphene sheet (Figure S5), the RDF of Na+ and

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Cl- ions and N and B (doping element) in absence of external electrical field (Figure S6). The MD simulations were carried out by two successive steps. Firstly, the equilibration without electric field was achieved using the NPT ensemble at 300K and 1MPa for 1ns. Then, all NEMD simulations after the imposition of the electric field were performed in NVT ensemble at 300K during 2ns simulation time by using the Nose– Hoover thermostat (the temperature “start to stop” is 300K while the temperature “damp” is 0.01ps). In addition to field-free calculations, electric fields of three strengths oriented along the z-direction (perpendicular to the doped graphene sheet) were studied. The strength of the imposed fields are 1.5, 1.0, and 0.5V/Å.

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3.

RESULT AND DISCUSSION

Figure 1 illustrates the simulation system, showing a doped graphene membrane and water ion surrounded. In the absence of the field, ions are surrounded by their full hydration shells. As a result, most of these ions cannot pass through the graphene sheet since the diameter of the graphene pores is almost equal to their hydrated diameter. Only a few of the ions that are dehydrated can pass through the pores of the graphene membrane. Ion dehydration involves the separation of water molecules from the ions, an

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endergonic and hence non-spontaneous process, prohibiting most of the ions from crossing the pores of the membrane – since their effective size is much larger due to the

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hydration shell [49-51]. Electric fields, through electrostatic interaction with the ions, can

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provide the energy necessary to overcome this thermodynamic barrier. Therefore, while no significant ion separation occurs in the absence of an external electric field [44], the

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separation can be induced through the application of an external electric field of sufficient

the ions through the membrane.

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strength to strip the solvation shells from the ions in the direction favoring the passage of

These physical principles are applied here to design sheets capable of segregating

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ions in salt water, namely, Na+ and Cl– ions, under externally-applied electric fields. A schematic view of the proposed system composed of a boron-nitrogen-doped graphene

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sheet immersed in salt water is displayed in Figure 2. To investigate the effect of an external electric field on the ion separation process and water flow through the B and N

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doped graphene sheet, three external electric fields of different intensities (1.5, 1.0, and 0.5V/Å) were applied along the positive z-direction (vertical on sheet). In the presence of an electric field, Na+ and Cl– ions sustain oppositely directed forces along the z-direction. Exposing the system to an external electric field imposes electrostatic forces on the ions, which with proper choice of the field direction can lead to direct the motion of the ions toward the sheet leading to water-ion separation. The Cl– ions move toward the right side of the sheet (in Figure 3). On the other hand, Na+ ions stay in left side of the membrane with a diminished probability of Na+ ions crossing the membrane. As a result, with the passage of time, Na+ ions will remain at the left side. As can be seen in Figure 3, the sheet of B-N doped graphene separates the ions almost

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completely after 2ns simulation at 300K under an imposed electric field. Finally, the literature suggests that regional doping can enhance the efficiency of a selective ion separation [33, 52]. The physical basis for such an enhanced selectivity is that Na+ can interact with the (negatively charged) nitrogen atoms more effectively while boron may have intrinsic tendency to interact with Cl– ion. 3.1

The Differential Interactions of Ions and Water with the Nano-Sheet as a

Basis for Membrane Selectivity

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To investigate the strength of interaction between the ions and boron- and nitrogen-doped regions, the potential interaction between two ions and doping element were investigated.

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Figure S7 illustrates the interaction potential between ions pairs and doped region. As can be seen in Figure S7, the interaction of Na+ with N-doped regions is stronger than B-doped

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regions.This stable interaction may be behind the better adsorption of Na+ ions near N-

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doped regions. While Cl- ions tend to interact with B-doped region. Figure S8 shows that increasing the electrical field intensity from 1 to 1.5V/Å decreases the interaction between

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Na+ and both B and N doped regions. Upper curves (1.5V/Å) exhibit a higher interaction

membrane.

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potential, i.e., it destabilizes the interaction and enhances ionic movement across the

The effect of the electrical forces to which the ions are subjected on adsorption near of the

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graphene sheet is now investigated. Figure 4-A shows the two dimensional contours of the force on imported Na+ ion along z-direction and Figure 4-B shows the change in the force

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along the same direction. The force applied on a Na+ ion moving from left to right (toward the graphene sheet) increases as can be seen from the figure (the force contours are mostly in the orange to red regions). The interaction of Na+ ion with N-doped region is depicted in light red and orange (i.e., higher energy regions – of the plot), while the interaction of Na+ ion with B-doped region is in blue and green (lower force region). Figure 5-A shows the force acting on Cl- ions moving along z-direction of simulation box and Figure 5-B also shows the change in the force along te z-direction. As expected, the maximum interaction occurs where the graphene sheet located. The interaction of Cl- ions with B-doped region located in green area (middle energy regions) while the Fz of Cl- in N-doped region is located in blue area (lower energy

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area), indicating that Cl- tend to move and adsorb near the B-doped region. The magnitude of interaction of Cl- ions with B-doped region is considerably smaller than that of Na+ ions with N-doped region as can be seen from Figures 4 and 5. These observations indicate that doped grahene sheet is a “nano-sponge” that can selectively desalinate water due to the assymetry in the affinity to Na+. Figure S9 shows the force vectors acting on the ions in presence of E-field (electric field) of 1.0V/Å. The direction of the arrows shows that the applied E-field make the ions

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move in opposite directions on the basis of their respective charges which greately enghances ion separation.

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The influence of doping on ion movement is also studied by following the forces

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to which the ions are subjected as well as their velocities near the graphene sheet (in absence and presence of doping atoms). As seen in Figure 6, ions moving from left to

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right are exposed to a monotonous force. This indicates no selective adsorption on graphene sheet (without doped atoms) and lends support to our proposal about the

3.2

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selective ion separation based on doped graphene sheets. Radial Distribution Function (RDF) as a Diagnostic of Interactions

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The radial distribution function (RDF) [53], g(r), also known as the pair correlation functions describes (in this study) how Na+ and Cl- as ions and atoms in water are spaced

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apart as a function of distance from a reference. The RDF is a reliable measure of the size of the hydration shell surrounding the ions [53].

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The RDF representing the interaction between the ions and the oxygen atoms of the solvent water is displayed in Figure 7 while the RDF between ions and water atoms with carbon atoms of the doped nano-sponge is plotted in Figure 8. From Figure 7, the RDF curves of Cl-/O at E-field 1.0V/A exhibit two distinct peaks, while that of Na+/O exhibit shoulder-like peaks. The first peak of RDF with the solvent’s oxygen atoms lies at approximately 3.15Å for Na+ and 1.8Å for Cl- with corresponding calculated hydrated radii of 1.15Å and 1.21Å, respectively. Since among ions of the same charge, the hydration energy is decreasing function of the ion’s radius, increasing in the hydration energy will also increase the number of water molecule surrounding the ions. The more the hydration of an ion the lower this ion’s mobility and its ability to cross the pores of the

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membrane [52, 54-55]. This is why Cl- ion passage is harder to the right side of the studied sheet. The interactions of B-N doped graphene sheet with the ions and the atoms of the water are now explored to determine the hydrophobicity/hydrophilicity of the sheets. As seen from Figure 8, there exists a sharp peak at around 2Å and a second peak at around 3Å due to a strong interaction between Na+ ion and C from the graphene sheet. In contrast, there is no significant peaks of H and O which indicates the absence of a significant

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interaction between water and the graphene sheet. These comparisons reveal that the C atom of graphene sheet has, unsurprisingly, a hydrophobic character, while the sharp

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peaks between ions and the sheet imply that the graphene sheet can be a good candidate for of nano-sponge ion separator.

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Figure 9-A and 9-B illustrate the RDF between N and B doped atoms and Na+

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and Cl- ions. Figure 9-A shows that there is a sharp peak for B-Cl- at around 2.7Å suggesting their mutual interaction. Furthermore, there are several peaks that show the

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dispersion of the neighbouring shells of Na+ ion at different intervals to the B doped atom, which implies a lack of proper interaction between B and Na+ ion. A similar pattern

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appears also in Figure 9-B and which reveals a sharp peak around 2.2Å and broad peaks at about 3-4Å. Thus, Na+ ion and N-doped sheets interact favourably and Na+ and N are well

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structured around one another. Clearly, the probability of finding Na+ ions around an Ndoped region and Cl- ions around a B-doped region is high which implies that N and B can

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effectively adsorb Na+ and Cl- ions, respectively. For N-Na+ and B-Cl- interactions, results confirm that the magnitude of external electric field is correlated with ion selectivity especially in the strongest studied field of 1.5V/Å (Figure S10). Also, ion selectivity decreases with the decrease in the magnitude of the field. The RDF of N-Na+ and B-Cl- at a field with strength of 1.0V/Å is presented in Figure S11. Meanwhile, the RDF of N-Na+ and B-Cl- at a field of 0.5V/Å is plotted in Figure S12. These latter curves show that the strongest field enhances Na+ ions’ interactions with N-doped regions of the graphene sheet. These findings suggest that a better ion selectivity (of the N-Na+ and B-Cl- types) can be achieved with doped graphene sheets.

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3.3 Electric Fields as Enhancers of Diffusion and Selectivity through Membranes The effect of the interaction strength between water, ions, and the doping B and N atoms on atom displacements in the precence of electric fields is now elucidated using the mean square displacement (MSD). The mobility of water molecules and ions near the sheet can be obtianed from the following equation MSD (t )  (r (t ) - r (0)) 2  (1/ N ) i 1 (ri (t ) - ri (0)) 2 , N

(1)

where N is the number of particles to be averaged, r (0) is the reference position of each

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particle, r (t) is the position of each particle in determined time t.

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Figure 10 shows the MSD for water and ions in absence of electric fields. The figure shows that in the absence of fields, water molecules can move more than the ions

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and, among the ions, the one with the smallest radius (Na+) is sorrounded with more water

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molecules which hinders its free motion more than in the case of Cl- (the ion with the largest radius). This qualitative picture is reflected as a larger mean square displacement

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for Cl- than for Na+ [56].

Figure 11 displays the mean square displacement of water and ions as a function

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of the externally applied electric fields. This figure confirms that the species and doped graphene sheet collision have the effect on the mobility of molecules along the z-axis

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which are observable in the MSD. These results go beyond reports: Ref. [40, 56]. We note in passing that the MSD in the z-direction increases first before it eventually plateaus with

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time. It is also noteworthy that all MSDs have similar trends that differ primarily in their slopes. This figure also suggests that intensifying the field is associated with an increase in the MSD of Na+ and Cl- ion but has no effect on the displacement of the water molecules since these carry no net electric charge. The MSD plots in Figure 11 also show that the magnitude of displacement of atoms approach an assymptote which results from the particles collisions with the doped graphene sheet and the establishment of an effective interaction between the ions and the sheet. 3.4

Electric Fields Significantly Alter the Diffusion Coefficients

The diffusion coefficient of a species can be computed from the slope of MSD of its center of mass [53]:

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D

slope of MSD , 2d

(2)

where d denotes the dimensional factor (d = 2, 4 and 6 for 1D, 2D and 3D diffusion, respectively), and D is the diffusion coefficient [53]. Figure 12 shows the diffusion coefficients of species under the different field strengths. A larger slope of MSD curve indicates a larger diffusion coefficient which implies that molecules under these conditions diffuse at a faster rate [57]. As can be gleaned from the figure, in the absence of fields, water molecules move rapidly while the

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heavily solvated Na+ and Cl- ions are slow, their ionic mobility being hindered by their

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cortege of solvation molecules. One also notes that Na+ ions exhibit a lower displacement than both Cl- ions and water molecules. Since the hydration energy of Na+ ion is more

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than Cl- ion, the number of water accumulated around the Na+ ion is more than Cl-.

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Therefore the solvated Na+ ions are less mobile than Cl- ion [44, 52, 58]. Externally-applied electric fields tend to dehydrate ions in aqueous media.

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Hence, one can note a dramatic change in the behavior of Na+, the ion with the lowest mobility in the absence of external fields, that becomes the most mobile after stripping

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water molecules from its solvation shell under the effect of the various studied field strengths. That is to say that Na+ ions have more mobility than Cl- and water as a result of

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smaller radius of this species than can only be manifested after removing much of its solvation shell structure. As a consequence, Na+ has the largest diffusion coefficient than

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all other species at all field strengths. The diffusion coefficients of all species increase with the field strength up to the maximal studied value of 1.0V/Å but to a lesser extent than Na+. These results can be understood in the following way. Under weaker fields, the hydrated ions are still cluttered by the surrounding water of solvation which hinders their diffusion (smaller diffusion coefficients) while at large field magnitudes the diffusing species are much smaller and hence the field effect on their diffusion becomes more significant. So, the field effect on diffusion is not and cannot be a linear physical process, it is inherently non-linear as can be seen in Figure 12. Also, increasing the field strength to 1.5V/Å results in decreasing the diffusion

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coefficient of Na+ and waters. It means that at higher field strength, Na+ settling in near the sheet. The slight increase in diffusion coefficient of Cl- ions and also its higher movement to the right side will result in better Cl- separation. At higher field strength Clis surrounded by more waters molecules and consequently is more inactive than Na+ in 1.5V/Å that shows Cl- more movements, while Na+ has the most movement in 1.0V/Å. Therefore, Na+ ions movements decreases. The effect of the sheet on ion separation is further explored through the study of

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the velocity of species in the system on the two sides of the nanosheet. Figure 13-A shows that the species’ velocities decrease through simulation time, which is the result of

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collision between the ions and the graphene sheet. In Figure 13-B, the velocity of Na+ ion along the z-direction is presented. From a glance at A, B and C parts of Figure 13, Na+

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ions moving from right to left accelerate near the sheet while Cl- ions, moving from left to

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right, accelerate toward the sheet consistently with external field direction. The speed of Cl- is smaller than that of Na+ ion which is attributed to the larger ionic radius of the

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former and its heavier atomic weight (Cl = 35.45 g.mol-1, Na = 22.99g.mol-1). Mechanical Strength of Doped Graphene is Comparable to Pristine Graphene

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Young's modulus of elasticity is the tensile-curve gradient in the linear region (elastic), is

x 

x E

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defined by the following equation: ,

(3)

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where σx, is the stress on the object along the x-axis, εx is the strain (length variation) due to the stress introduced into the x-axis, and the E is the Young's modulus [52]. In simulations of Young’s modulus, different parameters such as the thickness of a single layer of graphene, type of loading and boundary conditions, effects of interactions of non-neighboring atoms, and the size of the graphene sheet and doped atoms can influence the results. Young’s modulus of B- and N-doped graphene differs from that of pristine graphene since the presence of defects decreases the structural resistance of sheets. In Table 1, the forces which overcome the structural resistance of pure and doped graphene sheet are listed. The magnitude of changes in Young’s modulus are not

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substantial upon doping which means that the proposed structures have a relatively good mechanical resistance – comparable to pristine graphene – and, hence, are good candidates for water refinery applications. 4.

CONCLUSIONS

The desalination and selective ion separation performance of monolayer graphene nanosheets with B- and N-doped regions under various external electric fields have been simulated by MD. The simulation results suggest that B- and N- doped graphene can

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exhibit better ion separation than pristine graphene while conserving the latter’s mechanical strength. Furthermore, the imposition of external electric fields perpendicular

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to the membrane significantly improves ion separation and selectivity within all studied

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field strengths. The diameter of the hydrated ions, dehydration due to the imposed external field, and the electrostatic interactions between doping atoms and ions in solutions are

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together responsible for the efficiency of ion translocation and separation. The interactions between the N and B doped pores and Na+ and Cl– respectively is found to be crucial in

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determining the selective permeability of the doped graphene nanosheet. Further, the calculated Young’s Modulus for doped and undoped sheets indicates that the doping has

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minimal effects on the structural strength of the graphene sheet, making the doped graphene an attractive target for future explorations as a base for an efficient membrane

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desalination technology especially if assisted by external fields.

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Table 1. Young’s Modulus of pristine and BN doped graphene

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pristine

0.088Tpa

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BN doped

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graphene

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Figure 1. An initial simulation cell consisting BN-doped graphene sheet and ions and

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water on both sides. (Green = Cl-, purple = Na+, pink = B, blue = N, gray = C).

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Figure 2. BN-doped graphene sheet in water-ion environment. (Green = Cl-, purple = Na+,

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pink = B, blue = N, gray = C).

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Figure 3. Schematic view of BN-doped graphene sheet after 2ns simulation at 300K with

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the electric field vector directed from left-to-right. (Green = Cl-, purple = Na+, gray = C).

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A

B N doped

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B doped

Figure 4. (A) Fz contour of Na+ ions on left side of BN-doped graphene sheet at 1.0V/A,

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and (B) Fz near the sheet in form of curve. The orange sheet represents a BN-doped

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graphene sheet.

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B

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A

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Figure 5. (A) Fz contour of Cl- ions on left side of graphene doped sheet at 1.0V/A, and

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(B) Fz near the BN-doped graphene sheet in form of curve.

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A

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Figure 6. (A) The force contours on Na+, and (B) the velocity Na+ gains while moving

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represents a doped graphene sheet.

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from left toward doped graphene sheet at field strength of 1.0V/Å. The blue stripe

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Figure 7. The RDF between oxygen atoms of water and ions at electric field of strength =

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1.0V/Å at 300K.

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Figure 8. The RDF between carbon atoms of the BN-doped graphene and the ions and

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water atoms at electric field strength of 1.0V/Å and 300K.

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B

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electric field strength of 1.0V/Å at 300K.

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Figure 9. The RDFs between (A) boron atom and ions, and (B) nitrogen atom and ions at

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Figure 10. The MSD for water and ions in absence of electrical fields at 300K.

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Figure 11. The MSD (Å2) in z-direction of water and ions as a function of electric field

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strength in V/Å at 300K.

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Figure 12. Diffusion coefficient (Å2/s) of water and ions as a function of electric field

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strength in V/Å at 300K.

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A

C

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B

Figure 13. (A) Velocity graph of water and ions, (B) velocity contours of Na+ ion, and (C) Cl- ion through z-direction at electric field strength of 1.0V/Å at 300K.

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CRediT author statement

Maryam Kamal Kandezi

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Muhammad Shadman Lakmehsari

Conceptualization Methodology Software Investigation Writing - Original Draft Visualization Conceptualization Methodology Software Validation Investigation Writing - Review & Editing Supervision Project administration Methodology Validation Investigation Writing - Review & Editing

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Chérif F. Matta

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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.

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☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Highlight 

Water desalination through NEMD simulations for electrodialysis process is studied.



Newly designed B and N doped graphene sheet is investigated for salt rejection.



Doped graphene sheets exhibit acceptable Young’s Modulus and mechanical

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properties. Force, velocity and RDF of species under external electrical field are calculated.



It is shown doped graphene demonstrate well candidate for desalination

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

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technology.

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