Elsevier

Carbon

Volume 153, November 2019, Pages 481-494
Carbon

Review article
Molecular simulations on graphene-based membranes

https://doi.org/10.1016/j.carbon.2019.07.052Get rights and content

Abstract

The graphene-based membranes, including nanoporous graphene and graphene oxide, have been demonstrated to be very promising both in water purification and gas separation. Molecular simulation method is a very effective tool to study the permeation processes of the graphene-based membranes involving the molecular transport in the nano-confined spaces. In this perspective review, we summarize the recent advanced molecular simulation works on the graphene-based membranes and point out the main problems existing at present. The representative works focusing on the discussion of the separation mechanisms of water purification and gas separation are especially highlighted. Currently, the simulation works almost all focus on the ideal structures of graphene membranes and rarely aim at the graphene-based membranes with complicated structures of interlaced pores and channels and with complicated chemical compositions. This review is expected to make people understand the research progress on the graphene-based membranes and stimulate more related molecular simulation works especially on the graphene oxide membranes. Coupling with the advanced membrane fabrication technologies, the large-scale and all-atomic molecular simulations on the graphene-based membranes considering the realistic structures and compositions can definitely promote the development of these state-of-the-art separation membranes and others involving two-dimensional materials.

Introduction

Membrane separation technology is very crucial for many industrial processes, such as water purification and gas separation. Permeance, the mole number of permeated molecules per unit membrane area in per unit time at per unit driving pressure, is an essential parameter for characterizing the permeation abilities of permeating species through separation membranes, such as molecules and ions. For the separation membranes, high permeance is always very important for improving the separation efficiency. It is generally deemed that the membrane permeance is inversely proportional to the membrane thickness. Graphene is a two-dimensional carbon material with a thickness of only one-atom-layer and composed of sp2-bonded and honeycomb-packed atoms. Thus, graphene and its derivatives [[1], [2], [3], [4], [5], [6], [7]], which have the thinnest thickness known to science, are becoming an easy choice as promising separation membranes. Since the born of graphene, scientists put forward ongoing efforts to realize the graphene-based separation membranes; up to now, two totally different concepts of graphene-based membranes have been proposed. For the pristine graphene, any molecules or ions as small as helium cannot permeate through at room temperature, because the graphene sheet is packed so tightly with carbon atoms that only tiny spaces are left among the carbon atoms. In order to be employed as a separation membrane, graphene must be artificially introduced into selective pores to sieve the molecules based on the size-sieving effects. This kind of graphene with introduced selective nanopores, called as nanoporous graphene (NPG), is the first concept of graphene-based membranes. It has been widely demonstrated that the NPG membranes are very effective for the water purification and gas separation. The separation mechanism of NPG membranes is basically the molecular size-sieving effects of the nanopores in graphene sheets, as shown in Fig. 1. Graphene oxide (GO) is a derivative of graphene presenting layered structures and substantial oxidation groups (hydroxyl, carboxyl, epoxy, carbonyl etc.) asymmetrically functionalized on the graphene sheets and edges. GO sheets are the second proposed concept of graphene-based membranes and were also demonstrated to be competent for the water purification and gas separation. The molecular and ionic separation mechanisms of the GO membranes are basically the selective transport through the spacing channels between GO laminate sheets and through the defects and wrinkles in GO sheets (see Fig. 1).

Although the NPG membranes and GO membranes both have the characteristics of good chemical stability [[8], [9], [10], [11], [12]] and strong mechanical strength [[13], [14], [15], [16], [17], [18], [19], [20]] etc., they have their own advantages and disadvantages. From the permeance point of view, the permeances of NPG membranes with an atomic thickness are several orders of magnitude higher than those of GO membranes; from the membrane fabrication point of view, the fabrication of industrial-scale GO membranes is very easy both for the synthesis and scaling up, while the fabrication of NPG membranes is really challenging. For the NPG membranes, the top-down fabrication method which synthesizes the nanostructures by removing crystal planes from the original structures on the substrates includes the complicated processes of graphene transfer and pore generation; while the bottom-up method, which synthesizes the nanostructures on the substrates by stacking atoms, is more concise but very skillful. Because of their distinctive advantages, the NPG and GO membranes have different development paces. The GO membranes seem more promising currently for their easy fabrication; however, the NPG membranes are the ultimate aim of people owing to their ultrahigh permeance brought by the thinnest thickness known to science.

The NPG membranes are proposed early by Jiang et al. [2] in 2009, who firstly theoretically demonstrated that the NPG can be efficiently employed as the gas separation membranes. Afterward, several similar theoretical works [[21], [22], [23], [24]] further showed the possibility of NPG as efficient gas separation membranes. Then, the first experimental measurement on the gas transport through NPG membranes was performed by Koenig et al. [25] in 2012; they measured the transport rates of several different gases through the NPGs with micrometre areas by using the mechanical resonance and pressurized blister method. More excitedly, in 2014 Celebi et al. [26] measured the transport rates of gases across a double-layer NPG with an area of square millimeters. These earlier works coupling with the subsequent outstanding works [[27], [28], [29]] promoted the NPG gas separation membranes from concept to reality step by step. As for the water purification by NPG membranes, they follow closely with the NPG gas separation membranes; in 2012 Cohen-Tanugi et al. [3] firstly showed that the NPG can be applied as the water purification membranes to effectively filter NaCl salt from water. Then, several experimental works were successively conducted; in 2014 O'Hern et al. [30] measured the transport rates of different ions through the NPG membranes; in 2015 Surwade et al. [31] measured the salt rejection rate of the NPG membranes and showed a nearly 100% rejection rate and a high water permeation rate. Namely, the NPG water purification membranes are also developing rapidly although they were studied relatively late. Currently, the NPG membranes are really coming as many advanced experimental works [[32], [33], [34], [35], [36], [37], [38]] are conducted both for water purification and gas separation.

GO-based membranes for water purification and gas separation [[39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49]] were extensively tested, after Nair et al. [50] observed the unimpeded water permeation through the GO laminate sheets with a sub-micrometre thickness. For water purification, in 2013 Hu and Mi [44] showed that the GO membranes can allow the water permeation by flowing through the spaces among the GO sheets while reject the impurities by the mechanisms of size exclusion and charge effects. More detailed investigations were performed subsequently; Joshi et al. [49] reported that the transport rates of small ions through GO sheets were greatly higher than the theoretical values by thousands of times and that the GO laminate sheets can block the solutes with a hydrated radii larger than 4.5 Å; Huang et al. investigated [42] the water permeation rates through the GO sheets, with considering the effects of pH value, salt concentration and water pressure. While for the gas separation, Li et al. [48] showed that the GO membranes prepared by the filtration method presented a high selectivity up to 3400 and 900, respectively, for the separation of gaseous H2/CO2 and H2/N2 mixtures. Since then, many researchers explored the applications of GO-based membranes in other gas separation fields, e.g. CO2 capture [41]. Until now, many experimental works [[51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63]] are conducted to show that the GO membranes are applicable both for water purification and gas separation.

Although many challenges are still faced currently in the device development [64], a great progress has been achieved for the graphene-based membranes. The recent developments in the graphene-based membranes were properly summarized in several excellent review papers [[65], [66], [67], [68], [69]]. For the graphene-based membranes, experimental studies are usually difficult to conduct, but the simulations are always effective. Molecular simulations are the methods probing the transport phenomena from the molecular levels by considering the motion trajectories of various molecules. Hence, molecular simulation is a very effective tool to study the graphene-based membranes involving molecular separation processes in nanospaces. The molecular permeation mechanisms and the effects of various tedious factors, such as chemical functionalization and structural parameter, are especially expected to be revealed by molecular simulations. In this review, the recent advances in the molecular simulations on both NPG-based membranes and GO-based membranes for the applications in water purification and gas separations are summarized. The representative works are highlighted, accompanying with the discussions of the separation mechanisms, optimization measures and so on. It is expected that these highlights can promote people to understand from the molecular insights the current challenges in the applications of graphene-based membranes for water purification and gas separation. Meanwhile, the understanding of these cutting-edge studies can promote the development of other graphene-related carbon-based separation membranes [[70], [71], [72], [73]] and stimulate the applications of two-dimensional atomically thick membranes, especially the graphene-based membranes, in other fields such as isotope separation [[74], [75], [76], [77]], hydrogen storage and production [5,[78], [79], [80], [81], [82], [83]], and DNA sequencing [[84], [85], [86], [87], [88], [89]].

Section snippets

Water purification

The molecular simulation works were conducted considerably on the NPG membranes for water purification. Currently, the molecular simulations on graphene-based membranes are almost all based on the classical molecular dynamics (MD) methods without considering the quantum effects by quantum mechanical calculations. The classical MD method not only is competent for simulating the separation processes of graphene membranes in most cases, but also requires less computational resources comparing to

Water purification

As a derivative of graphene, in the GO membranes the edges and planes of graphene sheets are asymmetrically modified by oxygen-containing chemical functional groups, including hydroxyl, epoxy, carboxyl, carbonyl, phenol etc. The structures of GO membranes are more complex comparing to the NPG membranes, but the large-scale GO membranes can be easily prepared by some skillful processes, such as the filtration process [48,49], the spray-coating process [50] etc. Therefore, plenty of experimental

Conclusions and outlook

NPG and GO are two kinds of graphene-based two-dimensional highly-permeable membranes, presenting great potentials in the applications of water purification and gas separation. Molecular simulation is an effective tool for studying the molecular transport through the graphene-based membranes with nanoscale pores and channels, especially for revealing the underlying mechanisms for water purification and gas separation. In this perspective review, we summarize the recent molecular simulation

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgement

The financial supports from National Natural Science Foundation of China for general program (No. 51876169) and Distinguished Young Scientists (No. 51425603) are highly acknowledged.

References (193)

  • Q. Chen et al.

    Pyridinic nitrogen doped nanoporous graphene as desalination membrane: molecular simulation study

    J. Membr. Sci.

    (2015)
  • M. Hosseini et al.

    Improving the performance of water desalination through ultra-permeable functionalized nanoporous graphene oxide membrane

    Appl. Surf. Sci.

    (2018)
  • M. Hosseini et al.

    Water desalination through fluorine-functionalized nanoporous graphene oxide membranes

    Mater. Chem. Phys.

    (2019)
  • M.J. Allen et al.

    Honeycomb carbon: a review of graphene

    Chem. Rev.

    (2010)
  • D.E. Jiang et al.

    Porous graphene as the ultimate membrane for gas separation

    Nano Lett.

    (2009)
  • D. Cohen-Tanugi et al.

    Water desalination across nanoporous graphene

    Nano Lett.

    (2012)
  • S. Hu et al.

    Proton transport through one-atom-thick crystals

    Nature

    (2014)
  • H.W. Kim et al.

    Selective gas transport through few-layered graphene and graphene oxide membranes

    Science

    (2013)
  • C.O. Girit et al.

    Graphene at the edge: stability and dynamics

    Science

    (2009)
  • A. Fasolino et al.

    Intrinsic ripples in graphene

    Nat. Mater.

    (2007)
  • S. Zhang et al.

    Structure, stability, and property modulations of stoichiometric graphene oxide

    J. Phys. Chem. C

    (2013)
  • L. Wang et al.

    Stability of graphene oxide phases from first-principles calculations

    Phys. Rev. B

    (2010)
  • C. Lee et al.

    Measurement of the elastic properties and intrinsic strength of monolayer graphene

    Science

    (2008)
  • C. Gómez-Navarro et al.

    Elastic properties of chemically derived single graphene sheets

    Nano Lett.

    (2008)
  • Q. Fang et al.

    Freestanding bacterial cellulose-graphene oxide composite membranes with high mechanical strength for selective ion permeation

    Sci. Rep.

    (2016)
  • C. Bora et al.

    Preparation of polyester resin/graphene oxide nanocomposite with improved mechanical strength

    J. Appl. Polym. Sci.

    (2013)
  • L. Wang et al.

    Single-layer graphene membranes withstand ultrahigh applied pressure

    Nano Lett.

    (2017)
  • H.L. Lee et al.

    Effect of porosity on the mechanical properties of a nanoporous graphene membrane using the atomic-scale finite element method

    Acta Mech.

    (2017)
  • D. Cohen-Tanugi et al.

    Mechanical strength of nanoporous graphene as a desalination membrane

    Nano Lett.

    (2014)
  • S. Blankenburg et al.

    Porous graphene as an atmospheric nanofilter

    Small

    (2010)
  • J. Schrier

    Helium separation using porous graphene membranes

    J. Phys. Chem. Lett.

    (2010)
  • H.L. Du et al.

    Separation of hydrogen and nitrogen gases with porous graphene membrane

    J. Phys. Chem. C

    (2011)
  • Q. Xue et al.

    N-doped porous graphene for carbon dioxide separation: a molecular dynamics study

    Chin. Sci. Bull.

    (2014)
  • S.P. Koenig et al.

    Selective molecular sieving through porous graphene

    Nat. Nanotechnol.

    (2012)
  • K. Celebi et al.

    Ultimate permeation across atomically thin porous graphene

    Science

    (2014)
  • J. Zhao et al.

    Etching gas-sieving nanopores in single-layer graphene with an angstrom precision for high-performance gas mixture separation

    Sci. Adv.

    (2019)
  • S. Huang et al.

    Single-layer graphene membranes by crack-free transfer for gas mixture separation

    Nat. Commun.

    (2018)
  • M.S.H. Boutilier et al.

    Molecular sieving across centimeter-scale single-layer nanoporous graphene membranes

    ACS Nano

    (2017)
  • S.C. O'Hern et al.

    Selective ionic transport through tunable subnanometer pores in single-layer graphene membranes

    Nano Lett.

    (2014)
  • S.P. Surwade et al.

    Water desalination using nanoporous single-layer graphene

    Nat. Nanotechnol.

    (2015)
  • Z. Yuan et al.

    Stable, temperature-dependent gas mixture permeation and separation through suspended nanoporous single-layer graphene membranes

    Nano Lett.

    (2018)
  • J.H. Song et al.

    Tunable ion sieving of graphene membranes through the control of nitrogen-bonding configuration

    Nano Lett.

    (2018)
  • T.A. Tabish et al.

    A facile synthesis of porous graphene for efficient water and wastewater treatment

    Sci. Rep.

    (2018)
  • M.S.H. Boutilier et al.

    Knudsen effusion through polymer-coated three-layer porous graphene membranes

    Nanotechnology

    (2017)
  • S.C. O'Hern et al.

    Nanofiltration across defect-sealed nanoporous monolayer graphene

    Nano Lett.

    (2015)
  • P.R. Kidambi et al.

    Nanoporous atomically thin graphene membranes for desalting and dialysis applications

    Adv. Mater.

    (2017)
  • D. Jang et al.

    Water and solute transport governed by tunable pore size distributions in nanoporous graphene membranes

    ACS Nano

    (2017)
  • K. Huang et al.

    A graphene oxide membrane with highly selective molecular separation of aqueous organic solution

    Angew. Chem. Int. Ed.

    (2014)
  • H. Huang et al.

    Salt concentration, pH and pressure controlled separation of small molecules through lamellar graphene oxide membranes

    Chem. Commun.

    (2013)
  • J. Shen et al.

    Membranes with fast and selective gas-transport channels of laminar graphene oxide for efficient CO2 capture

    Angew. Chem. Int. Ed.

    (2015)
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