⇦Chinese Physics Letters, 2020, Vol. 37, No. 11, Article code 116101 Tuning the Water Desalination Performance of Graphenic Layered Nanomaterials by Element Doping and Inter-Layer Spacing Fuxin Wang (王福鑫)1, Chao Zhang (张超)2, Yanmei Yang (杨燕美)3, Yuanyuan Qu (屈媛媛)1*, Yong-Qiang Li (李永强)1, Baoyuan Man (满宝元)2, and Weifeng Li (李伟峰)1* Affiliations 1School of Physics and State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China 2Collaborative Innovation Center of Light Manipulations and Applications, Shandong Normal University, Jinan 250358, China 3College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes (Ministry of Education), Shandong Normal University, Jinan 250014, China Received 21 July 2020; accepted 18 September 2020; published online 8 November 2020 Supported by the National Natural Science Foundation of China (Grant No. 11874238), the Basic Research Project of Natural Science Foundation of Shandong Province (Grant No. ZR2018MA034), and Collaborative Innovation Funds of Shandong Normal University.
*Corresponding authors. Email: lwf@sdu.edu.cn; quyuanyuan@sdu.edu.cn Citation Text: Wang F X, Zhang C, Yang Y M, Qu Y Y and Li Y Q et al. 2020 Chin. Phys. Lett. 37 116101    Abstract Through atomic molecular dynamics simulations, we investigate the performance of two graphenic materials, boron (BC$_{3}$) and nitrogen doped graphene (C$_{3}$N), for seawater desalination and salt rejection, and take pristine graphene as a control. Effects of inter-layer separation have been explored. When water is filtered along the transverse directions of three-layered nanomaterials, the optimal inter-layer separation is 0.7–0.9 nm, which results in high water permeability and salt obstruction capability. The water permeability is considerably higher than porous graphene filter, and is about two orders of magnitude higher than commercial reverse osmosis (RO) membrane. By changing the inter-layer spacing, the water permeability of three graphenic layered nanomaterials follows an order of C$_{3}$N $\ge$ GRA $>$ BC$_{3}$ under the same working conditions. Amongst three nanomaterials, BC$_{3}$ is more sensitive to inter-layer separation which offers a possibility to control the water desalination speed by mechanically changing the membrane thickness. This is caused by the intrinsic charge transfer inside BC$_{3}$ that results in periodic distributed water clusters around the layer surface. Our present results reveal the high potentiality of multi-layered graphenic materials for controlled water desalination. It is hopeful that the present work can guide design and fabrication of highly efficient and tunable desalination architectures. DOI:10.1088/0256-307X/37/11/116101 PACS:61.20.Ja, 31.15.xv, 81.05.ue © 2020 Chinese Physics Society Article Text Due to the confluence of population growth, rapid urbanization, industrialization and agricultural production, the global freshwater crisis is becoming one of the most serious challenges that scientists and policymakers currently face.[1,2] Seawater accounts for more than 97% of the world's water resources, therefore, desalination of seawater to obtain fresh water is believed to be able to greatly alleviate much of the stress that presently plagues fresh water supplies.[3] In recent decades, desalination methods have improved via water filtering architectures based on porous filter materials in reverse osmosis (RO) technique.[4] In RO, a water-permeable membrane is placed at the interface between seawater and pure water. Pressure is applied at the seawater side to facilitate the flow of water through the ion-exclusive membrane. Generally, the water permeability of commercial membrane is low, about $0.1$ L$\cdot$cm$^{-2}$day$^{-1}$MPa$^{-1}$.[5–8] With the rapid development of nanotechnology, two-dimensional (2D) nanomaterials have demonstrated great potentialities as filtering materials due to their well-recognized physical and mechanical properties.[9] As the prototype 2D material, graphene was first reported in 2004 and may be the best studied nanomaterial which becomes an outstanding candidate in seawater desalination.[10–12] The milestone work of graphene as a desalination filter was conducted by Cohen-Tanugi and Grossman in 2012[13] from a molecular dynamics (MD) simulation perspective, which has attracted a great deal of attention.[14,15] Later, experimental evidences of using porous graphene in desalination have been realized.[16,17] By precisely controlling sizes of holes in a graphene layer through a chemical approach, selective filtration of water or certain ions can be realized.[18] In addition, mechanical stretching of MoS$_{2}$[19] and C$_{2}$N[20] monolayers can effectively change the sizes of their nanopores and control the “open” and “closed” states of water flux. Generally, the introduction of nano-sized pores to graphene membranes is complicated, which is achieved by perforating techniques (plasma etching, ion irradiation, chemical treatment, and so on).[17,21–23] Despite the high performance of porous graphene filters, including both the high salt rejection capability and the high water flux, fabrication of large graphene sheets with precise size holes remains of challenge.[24] In addition, the introduction of holes in the graphene layer will significantly reduce the mechanical strength of graphene (which is also true for other 2D nanomaterials) and make it difficult for these filters to work in a high-pressure water filtration environment.[25] So far, nanomaterials with regular nano-porous apertures have been studied for desalination, such as graphyne,[26,27] FePc,[28,29] and nanotubes.[30,31] Besides the approach of water desalination by porous filters, water transport through inter-layer space of 2D materials is an alternative method because graphene is intrinsically multi-layered in nature. In 2012, Nair et al. reported that the advantages of low-friction and capillary channels facilitate water molecules to pass through the stacked graphene in a quick manner.[32] Later, Kim et al.[33] also reported that for multilayer graphene membranes prepared by pre-oxidation and KOH activation of graphite, the water flux can reach 0.89 L$\cdot $cm$^{-2}$day$^{-1}$MPa$^{-1}$. Unfortunately, the salt rejection rate is low, around 20%. In addition, Radha et al. investigated the water transport of multilayer graphene by fabricating a nano-sized capillary channel between graphene layers. They found that under the pressure of 100 MPa, the water permeability was about 2–3 orders of magnitude higher than the commercial RO technique.[34] Until 2017, a breakthrough of graphene filters was made by Fang et al., who reported the precise control of inter-layer spacing of multi-layered graphene by hydrated cation and achieved precise ion sieving.[35] In addition, the fabrication of the desalination filter with multilayer graphene is technically easier than making holes in single-layer graphene from experimental perspective.[36] In addition to graphene, numerous multi-layered 2D materials have been created. For example, the lamellar graphene oxide (GO) membranes are capable to filter water and reject ions, which is confirmed by both experiment[37] and simulation.[38] Recently, two derivative structures of graphene, BC$_{3}$ and C$_{3}$N, have been successfully synthesized and demonstrated to have unique mechanical, electronic and optical properties that have encouraged their applications in arrays of nano-technologies.[39,40] Benefited from the $sp^{2}$-hybridized topology of the structures as depicted in Fig. 1(a), BC$_{3}$ and C$_{3}$N exhibit high structure stability and excellent mechanical properties.[40–42] Due to the differences of electronegativity of boron, carbon and nitrogen atoms in the B–C and C–N bonds, there are intrinsic electron transfers in the BC$_{3}$ and C$_{3}$N layers. Compared to graphene, the redistribution of electrons is expected to engender distinctive properties that are absent in a pristine graphene filter. Based on these perspectives, we investigate the water filtration properties of two graphenic nanomaterials of BC$_{3}$ and C$_{3}$N, taking pristine graphene as a control and using molecular dynamic simulations. The inter-layer distance of the multi-layers is explored in a range of 0.7–0.9 nm, which results in the balance of high-water permeability and robust salt rejection capability. A separation of 0.7 nm is the lower limit of inter-layer spacing of multi-layer graphenic filters for efficient water permeability, while 0.9 nm is the upper limit for efficient ion rejection. It is found that the water flux of three inter-layer filtration models is considerably higher than the porous graphene filter, and is two order of magnitude higher than the commercial RO membranes. In comparison to graphene and C$_{3}$N, we can find that BC$_{3}$ filters demonstrate an inter-layer spacing sensitive water permeability, which could be utilized as mechanically controllable filtration nano-devices for seawater desalination. Model and Simulation Methods. All molecular dynamics (MD) simulations were performed using the GROMACS 2018.4 package.[43] The AMBER99SB force field[44] was used for water and salt ions. The BC$_{3}$ with B doping rate of 25% and C$_{3}$N with N doping rate of 25% were adopted as representative models from previous studies.[45] In detail, the atomic charges for BC$_{3}$ and C$_{3}$N were calculated by quantum mechanical calculations implemented in Gaussian09 at HF/6-31g$^*$ level and parameterized by the RESP method. The SPC/E water model[46] was used. All bonds involving hydrogen atoms were constrained by the LINCS method.[47] The particle mesh Ewald (PME)[48,49] method was used to treat the long-range electrostatic interaction. While for the van der Waals (vdW) interaction, a cutoff distance of 1.2 nm was adopted for calculations. The update frequency of the nonbonded interaction pair list was 10 fs. Canonical sampling was conducted by the velocity rescaling method under the constant temperature of 300 K. A movement integration step of 1 fs was used in all the simulations. As shown in Fig. 1(b), the simulation box contains 14534 water molecules, 44 Na$^{+}$, 44 Cl$^{-}$ in the BC$_{3}$ system, and 13870 water molecules, 42 Na$^{+}$ and 42 Cl$^{-}$ in C$_{3}$N and graphene systems, respectively, resulting in a molar concentration of 0.4 mol/L of NaCl. A graphene monolayer was used as a piston to encourage the water flow from seawater region toward the pure water compartment, through the filter. A force in the $z$ direction was applied to the piston at 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 MPa pressure equivalents. Each system was first equilibrated for 10 ns, followed by 100 ns productive simulation for the data collection.
cpl-37-11-116101-fig1.png
Fig. 1. (a) Top view of the BC$_{3}$, C$_{3}$N and graphene layer, respectively. The red rhombus indicates the primitive cell. (b) Illustration of the simulation model. (c) Water flow at various pressures in the BC$_{3}$ system.
Results and Discussion.—The Desalination Capacity of Graphenic Filters and Pressure Effect. The main objective of this work is to compare the water permeable performance of three graphenic filters (graphene, BC$_{3}$ and C$_{3}$N). Therefore, we first accessed the desalination capacity of three multilayer filters with an inter-layer separation of 0.8 nm as the representative structure as presented in Fig. 1. The water box is divided into two regions by the nano-filter, pure water and salty water as depicted in Fig. 1(b). Taking the multi-layered BC$_{3}$ filter as representative, the water flux (in units of filtered water molecules per nanosecond) with respect to external pressure is illustrated in Fig. 1(c). At a simulated inter-layer distance of 0.8 nm, a linear dependence is found for water flux to the applied pressure. In the range of 10–100 MPa, the water flow (expressed as the filtered number of water molecules per nanosecond) follows a dependence of flow $f= 0.632P - 1.075$, where $P$ denotes the pressure of piston. This is also true for the other filters, C$_{3}$N and graphene (data is not shown) and is consistent with our previous studies of MoS$_{2}$ and C$_{2}$N water filers.[19,20] Cohentanugi et al.[50] also reported the linear relationship of water flux to applied pressure. Based on this phenomenon, the linear relationship makes it reasonable to evaluate the water transparency of simulated filters by the dynamic quantities which are derived down to the operating condition of typically several MPa in RO plants. In addition, we use a pressure stimulus of 100 MPa for enhanced generation of simulation trajectories for the following analyses. The Water Permeability with Respect to Inter-Layer Separations. Because of the small size of water and ion atoms, permeability of the nano-filters is sensitive to the inter-layer separations, which is the key factor that affects the filtration performance. More importantly, the precise control of the inter-layer spacing of graphene and graphene oxide layers has been realized by filling the inter-layer space with hydrated cations of different sizes,[51,52] applying external pressure regulation (EPR)[53] and making nanoscale capillaries through van der Waals assembly.[34] Thus, a systematical understanding of the influence of inter-layer separation on water permeability is essential for seeking the optimized separating conditions. Considering this, we then explored the water permeability of three nano-films and considered three inter-layer separations of 0.7 nm, 0.8 nm and 0.9 nm. From the results shown in Figs. 2(a)–2(c), a common feature is that the flow of water finally reaches a plateau for all simulations in 100 ns, indicating that water molecules in the seawater region is fully depleted and proving that our simulation time is sufficient to reach good statistics. This also reveals that the three graphenic layered structures are effective filtering materials. It is worth mentioning that a value of 0.7 nm, which is the lower limit of the separation, is comparable to the size of a water molecule because of simulations with smaller inter-layer separation of 0.6 nm, and no water translocation has been observed. Moreover, by comparing the three nanostructures, a clear difference was observed. For BC$_{3}$, when inter-layer space increases from 0.7 nm to 0.9 nm [Fig. 2(a)], the slope of the curve (water flux) changes more dramatically, indicating a relatively stronger dependence of water permeability on BC$_{3}$ spacing. While for C$_{3}$N [Fig. 2(b)] and graphene [Fig. 2(c)] filters, the effect of separation on water flux is weaker, especially for 0.8 and 0.9 nm separated graphene filters. From these results, it can be concluded that the boron-doped graphene filter is sensitive to inter-layer separation and thus is more tunable by controlling separation. To quantitatively evaluate the water desalination performance of three nanomaterials, we further calculate the water permeability in units of L$\cdot $cm$^{-2}$day$^{-1}$MPa$^{-1}$. As shown in Fig. 2(d), the water permeability gradually increases with the increasing inter-layer separation. The tunability of BC$_{3}$ is relatively larger, which increases from 12.4 L$\cdot $cm$^{-2}\cdot $day$^{-1}\cdot $MPa$^{-1}$ to 16.8 L$\cdot$cm$^{-2}$day$^{-1}$MPa$^{-1}$. For C$_{3}$N and graphene, the tunability is smaller, which increases from 15.4 L$\cdot$cm$^{-2}$day$^{-1}$MPa$^{-1}$ to 17.6 L$\cdot$cm$^{-2}$day$^{-1}$MPa$^{-1}$ for C$_{3}$N and from 15.0 L$\cdot$cm$^{-2}$day$^{-1}$MPa$^{-1}$ to 17.4 L$\cdot$cm$^{-2}$day$^{-1}$MPa$^{-1}$ for graphene. Comparing these three materials, we can find that elemental doping plays a more important role at smaller inter-layer distances (0.7 nm for instance). While at larger separation (0.9 nm), the regulation of different element doping on water permeability becomes weaker. These multilayer filters achieve a water permeability of two orders of magnitude higher than the commercially used reverse osmotic (RO) membrane (typically $\sim$0.1 L$\cdot $cm$^{-2}$day$^{-1}$MPa$^{-1}$).[5,6] For porous single-layer graphene filter, Surwade et al.[54] reported the highest water permeability of $\sim$6 L$\cdot$cm$^{-2}$day$^{-1}$MPa$^{-1}$, which is around 1/3 of the present three graphenic filters working at the inter-layer filtering mode. A comparable work was conducted by Shi et al. to study the water transport in bilayer graphene with ripples and obtained water flux from 1.13 to 24.48 L$\cdot$cm$^{-2}$day$^{-1}$MPa$^{-1}$.[55] Generally, our present results are principally consistent with Ref. [55] though curvature of graphene hinders water transport to a certain extent. In addition, we also find that although the BC$_{3}$ filter is more sensitive to inter-layer separation, its permeability is systematically smaller than that of C$_{3}$N and graphene at the same separation. This is mainly attributed to the different water distributions at the water-nanomaterial interface, which will be discussed in the following.
cpl-37-11-116101-fig2.png
Fig. 2. Time evolutions of the numbers of water molecules (left tick labels) and ions (right tick labels) filtered by (a) BC$_{3}$, (b) C$_{3}$N and (c) graphene filters. (d) Water permeability of the three filters at different inter-layer spacings.
The Salt Rejection Capability. Performance of a desalination filter should be determined by a trade-off between water transparency and rejection of salt ions. In particular, due to the small size of the ions, the salt rejection may be decreased at larger separations of the filter layers. Thus, it is essential to evaluate the salt rejection capability of three graphenic filters at various inter-layer separations. As demonstrated by the dotted lines in Figs. 2(a) (BC$_{3}$) and 2(b) (C$_{3}$N), in the entire simulations, no event of ion passage through the boron- and nitrogen-doped graphene filters is detected for inter-layer separations of 0.7 and 0.8 nm. For 0.9 nm separated C$_{3}$N layers, ion passage events occurs after 65 ns at a rate of 0.18 ions/ns [Fig. 2(b)] when the water molecules in the seawater region has fully depleted. The ultrahigh concentration of salt ions is believed to cause severe acceleration to ion passage through the filter. However, such an ion passage is not expected to happen under the seawater conditions, which is typically around 35 g/L. Meanwhile, a few ions are observed to pass through the graphene filter with 0.8 and 0.9 nm separations [Fig. 2(c)]. This reveals a fact that the 0.9 nm separation can be treated as the upper limit of the inter-layer separation because salt rejection performance of the three models becomes relatively poorer after this criterion although the water flux is high. From these analyses of water permeability and salt resistance performance, we can conclude that the multilayer filter models with 0.7 nm to 0.8 nm inter-layer separations have optimum performance of both high-water permeability and strong salt rejection capability. Furthermore, the water permeability of the BC$_{3}$ filter is more tunable by changing the inter-layer separation, although has a lower salt rejection capability than the C$_{3}$N and graphene filters at 0.9 nm separation. The Free Energy Significance of Water and Ions When Passing Through Filters. To probe the physical mechanism, i.e., reasons why three filters demonstrate different ability to filter salt ions and water molecules, we quantitatively evaluate the changes of free energy of water, Na$^{+}$ and Cl$^{-}$ during the filtering processes. Without loss of validity, we adopt an identical structure for the three filters with an inter-layer separation of 0.8 nm as the representative system and calculate the potential of mean force (PMF) of three solution components moving from the seawater region into the filter as illustrated in Fig. 3(a). The PMF profiles are given in Figs. 3(b)–3(d), where dotted lines represent the filter-seawater interface. The free energy values in seawater region and far from the filter are taken as the zero point for each case (free energy in seawater reservoir is treated as 0 kcal/mol).
cpl-37-11-116101-fig3.png
Fig. 3. (a) The illustration of model for potential of mean force (PMF) calculations. (b)–(d) The PMF of Na$^{+}$, Cl$^{-}$ and water from seawater region to pure water region. The inter-layer spacing is 0.8 nm. (e) The PMF of water from seawater region to pure water region in three nanomaterials with 0.7 nm inter-layer spacing. The vertical dashed line in each figure indicates the filter-seawater boundary.
For the BC$_{3}$ filter as depicted in Fig. 3(b), the PMF profiles clearly reveal that both Na$^{+}$ and Cl$^{-}$ translocations need to overcome significant free energy barriers of around 3 kcal/mol (measured from the first peak from seawater into filter region). Similarly, in the C$_{3}$N [Fig. 3(c)] and graphene [Fig. 3(d)] filters, the values reach around 3–4 kcal/mol, although the specific values differ for cations and anions. In detail, for C$_{3}$N, Cl$^{-}$ has a higher energy barrier of 4.2 kcal/mol than Na$^{+}$. While for graphene, it demonstrates a higher barrier of 4.4 kcal/mol for the cation of Na$^{+}$ instead of Cl$^{-}$ ions. Generally, high barriers of 3–4 kcal/mol can effectively block ions to translocate into filter interior, thus being blocked. This is consistent with normal MD simulations which reveals the high salt rejection performance of three nanomaterials. Meanwhile, the free energy barriers to transmit through the three filters are very low for water, around 0.4–0.5 kcal/mol, which allows the quick transmission of water and results in the high-water permeability of three filters. When we further examine the free energy profiles of water inside the filter regions, it is found that a small energy barrier of $\sim $0.128 kcal/mol exists for the BC$_{3}$ filter [Fig. 3(b)], which is absent in C$_{3}$N and graphene. This reveals that BC$_{3}$ has a relatively rougher surface than C$_{3}$N and graphene. This is more obvious when quantitatively calculated the PMF profiles for three filters at a more intimate inter-layer distance of 0.7 nm. As summarized in Fig. 3(e), in seawater region, the shapes of PMF are in close proximity to each other for three nanomaterials, indicating the similar dynamic character for water to enter into three filters. However, a periodic barrier is only found inside the BC$_{3}$ filter, which is around 0.35 kcal/mol. Meanwhile, the periodicity of the barrier is comparable to the crystal lattice of the BC$_{3}$. In contrast, the barriers for C$_{3}$N and graphene is only 0.05 kcal/mol. From these results, it can be safely interpreted that the water transmission inside BC$_{3}$ is obviously hindered by the rough surface, resulting in lower diffusive velocity and relatively smaller net flux. This is fully consistent with the systematically smaller water flux of the BC$_{3}$ filter, compared to other two filters at similar inter-layer separation as shown in Fig. 2(d).
cpl-37-11-116101-fig4.png
Fig. 4. The two-dimensional density map of water molecules on the surface of three nanomaterials.
To probe the origin of periodic PMF barriers of water inside BC$_{3}$, we intuitively monitor the distribution of water molecules near the nanomaterial surface. The behavior of these interfacial water and, particularly, the direct contacting water determine the water flux inside the nano-filters. Technically, we monitor the first water solvation shell around the nanomaterial which exists within 0.34 nm from the surface and calculate the density distribution in the transverse directions of the surface. Figure 4(a) depicts the water density on the BC$_{3}$ surface, where clear water density peaks are identified. Especially interesting is that each peak represents a water density of $\sim $140 water/nm$^{-3}$, which is about 4-fold of that of bulk water (with a density of $\sim $34 water/nm$^{-3}$). It is also found that the distribution is comparable to the lattice of the BC$_{3}$ crystal where each peak locates near a C atom. Thus, during the seawater filtering process, the C atoms act like attractive energy wells for water. Compared to BC$_{3}$, the periodic distribution of water molecules is not found in C$_{3}$N [Fig. 4(b)] and graphene [Fig. 4(c)], which behave to be more smoothly for the adsorbed water. In light of the above analyses, it can be interpreted that although boron atom doping to graphene effectively enhances the hydrophilicity of the surface by inducing in-plane dipole, this also enhances the columbic attraction to water, which effectively hinders the water transmission process. Despite the lower water permeability, the salt rejection capability of BC$_{3}$ is also relatively lower at the same inter-plane separation. This is attributed to the hydrophilic nature of the surface which is less robust to block the hydrated ions. In summary, we have constructed the water desalination models based on multi-layer BC$_{3}$, C$_{3}$N and pristine graphene. The water permeability and salt rejection capabilities of these three filters are explored, taking the inter-layer spacing into consideration and using MD simulations. Our results reveal that inter-layer desalination demonstrates higher performance than a porous graphene filter and is around two orders of magnitude higher than the commercial RO. For the three graphenic materials, the salt rejection rate is close to 100% at an inter-layer spacing of 0.7–0.8 nm. Amongst three filters, the water permeability of the BC$_{3}$ filter is slightly lower than C$_{3}$N and graphene. The different water transparency performance is mainly attributed to the electron transfer inside the layered structure, which results in distinct water mobility on the material surface. By comparing BC$_{3}$ with the graphene filter, it is found that enhanced hydrophilicity of BC$_{3}$ does not necessarily improve the water permeability performance. Moreover, change of inter-layer spacing can effectively regulate the water permeability of the filters, especially for BC$_{3}$, which could be utilized as mechanically controllable filtration nano-devices for seawater desalination. From the experimental point of view, BC$_{3}$ and C$_{3}$N have been synthesized and they show high structure stability. Thus, they are adopted as the representative doping models. Our present results are principally applicable to these models with a similar doping ratio of 25%. Further efforts are required for other types of graphenic monolayers with different doping ratios. Generally, our present findings demonstrate the high promise of mechanically controllable desalination filters, wherein the water permeability can be precisely regulated by mechanical stress. These findings can also support designs and fabrications of tunable nano-devices for filtration and related applications. 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