German Edition: DOI: 10.1002/ange.201600438Separation MembranesInternational Edition: DOI: 10.1002/anie.201600438

Two-Dimensional-Material Membranes: A New Familyof High-Performance Separation MembranesGongping Liu, Wanqin Jin,* and Nanping Xu

graphene · membranes · separation ·two-dimensional materials

1. Introduction

The discovery of graphene triggered a great deal ofattention toward two-dimensional (2D) materials in con-densed matter physics, materials science, and chemistry.[1–5]

Owing to their unique atomic thickness and micrometerlateral dimensions, 2D materials have been explored increas-ingly as a fundamental platform to develop separationtechnologies.[6–14] Synthetic membranes are used widely inmany separation processes, from industrial-scale processessuch as removing salt from seawater and separating atmos-pheric gases, to smaller-scale processes in chemical synthesisand purification.[15] The membranes function by forminga barrier between the two phases, restricting the movement ofsome molecules while letting others through. In principle,membranes made from 2D materials can be as thin as a singleatom to enable minimum transport resistance and maximumpermeation flux. Nano- or sub-nanometer apertures derivedfrom intrinsic porous structures, accurate perforation, or thecontrollable assembly of 2D materials allow for highlyselective transport of liquids, gases, ions, and other speciesthrough membranes. In recent years, various 2D materials,

including the graphene-family,[2, 5] ex-foliated dichalcogenides and layeredoxides,[3] zeolites,[14] and metal–organicframework (MOF) nanosheets[13] have

been demonstrated to be superb building blocks for high-performance membranes.

Based on their atomic structure, 2D materials are eitherporous or nonporous. They can therefore be fabricated asseparation membranes with two basic forms: nanosheet andlaminar membranes (Figure 1). In general, the former consistsof a monolayer or a few layers of 2D material with intrinsi-cally uniformly sized pores (for example, zeolite, MOFs) ordrilled nanopores (graphene) for selective permeation. Thelatter is formed by assembling 2D material nanosheets (suchas graphene oxide, GO) into laminates with interlayergalleries used to provide molecular passages. By tailoringthe in- and out-of-plane nanostructures, these membranesderived from 2D materials exhibit extraordinary molecularseparation properties in various membrane processes, such asultrafiltration, nanofiltration, reverse osmosis, forward osmo-sis, pervaporation, and gas separation.

Does recent research indicate that 2D-material mem-branes are a temporary curiosity or a new family of high-performance separation membranes? In this Minireview, weaim to answer this question by analyzing recent progress inthe design and fabrication of nanosheets and laminar 2D-material membranes. We focus on the summary and discus-sion of different approaches for the formation of 2D-materialmembranes and the role of the starting materials andtransport channels, and we highlight the nanostructures andphysicochemical properties that are essential for high-effi-ciency membrane separation. Fundamental and technicalchallenges for advancing 2D-material membranes for liquidand gas separations are also discussed.

Two-dimensional (2D) materials of atomic thickness have emerged asnano-building blocks to develop high-performance separationmembranes that feature unique nanopores and/or nanochannels.These 2D-material membranes exhibit extraordinary permeationproperties, opening a new avenue to ultra-fast and highly selectivemembranes for water and gas separation. Summarized in this Mini-review are the latest ground-breaking studies in 2D-materialmembranes as nanosheet and laminar membranes, with a focus onstarting materials, nanostructures, and transport properties. Challengesand future directions of 2D-material membranes for wide imple-mentation are discussed briefly.

[*] Dr. G. Liu, Prof. W. Jin, Prof. N. XuState Key Laboratory of Materials-Oriented Chemical EngineeringJiangsu National Synergetic Innovation Center forAdvanced MaterialsNanjing Tech University (former Nanjing University of Technology)5 Xinmofan Road, Nanjing 210009 (China)E-mail: wqjin@njtech.edu.cn

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2. Two-Dimensional-Material NanosheetMembranes

Nanosheets of 2D materials with atomic thickness can beused as ultimate membranes for separation. Membrane poresare provided either by an intrinsic porous structure such as inzeolites and MOFs, or by drilled pores such as in graphene-like materials. The permeation flux and selectivity of nano-sheet membranes are determined by the membrane thicknessand pore structures. Important key points to produce high-performance nanosheet membranes include the high-qualityfabrication of nanosheets and effective approaches for trans-lating nanosheets into membranes. Reliable measurements ofmolecular permeation through 2D-material membranes thatare a few nanometers thick remain an experimental challenge.This section focuses on 2D-material nanosheet membraneswith regard to the issues mentioned above.

2.1. Zeolite Membranes

Zeolites are crystalline silica-based microporous materialswith pore sizes that range from 0.25 to more than 1 nm.[14]

Two-dimensional zeolites, nanosheets with thickness compa-rable to the unit-cell-dimensions of the corresponding struc-ture, offer exciting opportunities for the development ofultra-thin molecular sieve membranes.[16] Fabrication of 2Dzeolite membranes relies on:[17] 1) an availability of suspen-sions that contain zeolite nanosheets with a uniform thicknessand high aspect ratio, colloidal stability, and free of amor-phous and non-exfoliated contaminants; and 2) the develop-ment of deposition techniques for the quantitative transfer of

suspended zeolite nanosheets to porous substrates to formoriented thin membranes.

In recent years, the Tsapatsis group realized high-purityexfoliated MFI nanosheet (1.5-unit-cell thick, 300-nm lateralsize) suspensions by developing a polymer-melt-compound-ing exfoliation technique combined with a density gradientcentrifugation purification method (Figure 2a–c).[18, 19] The2D MFI framework contains 10-member ring pores that runinside and through the layers; these are of extreme impor-tance because they provide shape-selective diffusion insidethe layers while reducing the characteristic diffusion length toca. 1 nm.[14] Filtration of suspensions through the poroussupports is used to deposit oriented nanosheet multilayerswith thickness of sub-100 nm.[20] However, the filtered MFInanosheet film contains non-selective gaps, and thus is unableto exhibit separation selectivity. These gaps were reduced bymild secondary solvothermal growth of the nanosheet filmand the established orientation is preserved by using appro-priate structure-directing agents such as tetrapropyl ammo-

Gongping Liu was born in Jiangxi, China in1988. After receiving his Ph.D. under thesupervision of Professor Wanqin Jin fromNanjing University of Technology in 2013,he joined Nanjing Tech University as anassistant professor. His current researchinterest focuses on the rational design andengineering of advanced membranes basedon organic–inorganic composite materialsand two-dimensional materials, withapplications in molecular separations includ-ing CO2 capture, organic compounds recov-ery, and purification.

Wanqin Jin is a professor of ChemicalEngineering at Nanjing Tech University. Hereceived his Ph.D. from Nanjing Universityof Technology in 1999. He was a researchassociate at Institute of Materials Research& Engineering of Singapore (2001), anAlexander von Humboldt Research Fellow(2001–2013), and visiting professors atArizona State University (2007) and Hirosh-ima University (2011, JSPS invitation fel-lowship). His current research focuses onmembrane materials and processes. He haspublished over 200 publications and 60patents.

Nanping Xu is a professor of ChemicalEngineering at Nanjing Tech University, anda member of the Chinese Academy ofEngineering. He received his PhD fromNanjing University of Technology in 1989.He was a visiting professor at University ofPittsburg (1994), Oak Ridge National Labo-ratory (1996–1997), and University of Na-goya (2002, JSPS visiting scholar). He hasbeen engaged in the fields of chemicalengineering and membrane processes, andhas published 150 refereed journal papersand 5 books in the area of inorganicmembranes.

Figure 1. Main types of 2D materials and corresponding membranes.

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nium cations. The thickness of the current state-of-the-art 2Dzeolite membranes is � 100 nm (Figure 2d,e),[14] which is 10-times thinner than typical three-dimensional (3D) zeolitemembranes, although it is much thicker than emergingnanosheet graphene-based membranes. The intergrown MFInanosheets were preferentially b-oriented, as confirmed byX-ray diffraction, which indicates that the 5.5 � straight poreMFI channels were normal to the membrane surface. Asa result, these 2D MFI membranes exhibited a high flux andselectivity for industrially important separations (for exam-ple, xylene and butane isomers). For instance,[20] the p-xylenepermeance was 1.3–3.6 � 10�7 mol m�2 s�1 Pa�1 and the sepa-ration factor was 185–1050, with an equimolar feed of p- ando-xylene at 150 8C.

To improve the membrane throughput, Tsapatsis pointedout that the thickness of 2D zeolite membrane can be reducedto 10 nm if sufficiently compact layers can be deposited.[14]

This enables them to be competitive nanosheet membranes.Future efforts could focus on increasing the aspect ratio andits uniformity in the zeolite nanosheet. However, broadimplementation of 2D zeolite membranes is restricted by thelimited number of recognized 2D zeolite forms. Furthermore,the current synthesis is costly and produces a low yield ofhigh-quality 2D zeolite nanosheets. Alternatively, incorporat-ing 2D zeolite nanosheets into a polymer matrix to formmixed matrix nanostructures would be a good compromisewith regard to the engineering aspect.[21] Sufficient utilizationof zeolite molecular sieving properties is also stronglydependent on whether the zeolite nanosheets are fullyexfoliated with a thin and intact structure.

2.2. MOF Membranes

As a new family of nanoporous molecular sieves, MOFswith an abundant porous structure and functional surfacegroups have been demonstrated to be an excellent candidatefor use as molecular sieve membranes with superior perfor-mance.[22] Layered crystalline structures have been found ina large number of MOFs,[23] which highlights the greatpotential of MOF-based 2D membranes.[13] Despite challeng-es in pursuing MOF nanosheets with preserved morpholog-ical and structural integrity, two successful cases wereachieved recently based on the top-down[24] and bottom-up[25] synthesis of MOF nanosheets. In the top-down strategy,the Yang group[24] treated a layered MOF precursor, Zn2-(bim)4 (bim = benzimidazole), using low-energy wet ballmilling followed by ultrasonication in methanol/propanolmixtures. This exfoliation approach created 1-nm-thick Zn2-(bim)4 monolayers with � 600 nm lateral size and highcrystallinity (Figure 3a,b). The bottom-up case[25] was accom-plished by diffusion-mediated modulation of the MOF growthkinetics. Dispersible copper 1,4-benzenedicarboxylate(CuBDC) MOF nanosheets were formed in the contact zone(solvent spacer layer) of a linker solution (benzene 1,4-dicarboxylic acid) and a metal solution (Cu2+ ions). CuBDC

Figure 2. Zeolite nanosheet membrane:[14, 18] a) TEM images of b-oriented MFI nanosheets. b) Atomic force microscopy (AFM) (tappingmode) topographical images of MFI nanosheets. c) Relaxed surfacestructures of MFI nanosheets obtained by structure optimization of1.5-unit-cell-thick MFI structures with Car–Parrinello molecular dynam-ics. Si, O, and H atoms are yellow, red, and white, respectively. Viewedalong the c (upper) and b (bottom) axes, SEM top (d) and cross-sectional (e) images of 100-nm-thick MFI nanosheet membrane, madeby secondary growth of deposited nanosheets. Reproduced withpermissions. Copyright 2011 the American Association for the Ad-vancement of Science. Copyright 2014, John Wiley and Sons.

Figure 3. MOF nanosheet membrane:[24] a) Tapping-mode AFM topo-graphical image of Zn2(bim)4 nanosheets on silicon wafer. The heightprofile of the nanosheets along the black lines was marked in theimage. b) Selected area electron diffraction pattern (white circle)shows the diffraction from (hk0) planes within a few-layered nano-sheet. A simulated selected area electron diffraction pattern of Zn2-(bim)4 nanosheet down the c axis is also shown, SEM top view (c) andcross-sectional view (d) of a Zn2(bim)4 nanosheet layer on a-Al2O3

support. Reproduced with permission. Copyright 2014 the AmericanAssociation for the Advancement of Science.

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lamellae of 5–25 nm thickness and 0.5-4 mm lateral dimensionwere produced by this three-layer approach.

Similar to the 2D zeolites, two general approaches existfor making 2D MOFs as separation membranes: 1) deposi-tion of MOF nanosheets on the porous substrate surface toform a pure 2D MOF membrane, and 2) incorporation ofMOF nanosheets into polymers to make a 2D MOF-basednanocomposite membrane. Yang and co-workers[24] reportedpure 2D MOF membranes by assembling the abovemen-tioned Zn2(bim)4 nanosheets on ceramic substrates by a hotdrop coating method. Owing to the single-unit-cell thicknessof MOF nanosheets, the assembled 2D Zn2(bim)4 membraneswere only several nanometers thick (Figure 3c,d). The porousceramic substrate is used to provide substantial mechanicalstrength that enables the actual permeation measurement ofsuch thin membranes (gas mixture feed up to 150 8C), but nopressure drop was applied across the membranes to preventdistortion of the MSN layers. The Zn4(bim)4 unit has anestimated aperture size of � 0.21 nm and pores in thenanosheets are constructed with four flat bim molecules,which offers an exciting opportunity for sieving gases rapidly.The effective pore size should be slightly larger because of thestructural flexibility of the nanosheets. The 2D Zn2(bim)4

membranes exhibited superb separation for H2 (0.29 nm)over CO2 (0.33 nm): H2 permeance of up to several thousandGPUs (1 GPU = 1 � 10�6 cm3 cm�2 s�1 cmHg�1 at STP) witha H2/CO2 selectivity higher than 200. It is interesting to findthat the suppression of the ordered lamellar stacking of MOFnanosheets could achieve a simultaneous increase in perme-ance and selectivity. It was explained that the lamellarordering of the nanosheets would block the permeationpathway for H2 but have only a small effect on CO2 leakage.In another study, Gascon and co-workers[25] incorporatedCuBDC nanosheets into polyimide (Matrimid 5218) matricesto fabricate CuBDC/polyimide nanocomposite membranes.This led to a remarkable improvement in CO2/CH4 separationperformance, together with an unusual but desired increase inthe selectivity with feed pressure. An improved efficiency ofthe molecular discrimination and elimination of unselectivepermeation pathways by the MOF nanosheets were regardedas the main contributions. These two breakthroughs openedthe door to the rational design and fabrication of 2Dmembranes based on MOF nanosheets, which led to a highlyimproved separation performance compared with the 3DMOF membranes.

2.3. Nanoporous Graphene Membranes

Graphene, the monolayer of graphite, is a well-known 2Dmaterial;[26] however, its perfect single-layer is impermeableto molecules, including the smallest gas, helium.[27] This isbecause the p-orbitals of graphene form a dense, delocalizedcloud that blocks the gap within its aromatic rings.[28] No gapexists in electron density around the aromatic rings throughwhich molecules may pass. This barrier property, and thecarbon atomic thickness, preeminent mechanical strength,and chemical inertness of graphene, motivated intense studies

in drilling holes in graphene nanosheets for developingnanoporous graphene membranes.[6]

Nanoporous graphene with a high density and uniformsubnanometer pores is predicted to allow ultrafast highpermeance and high selectivity because the atomic thinnessprovides little resistance to flow, yet blocks the passage ofspecies that are larger than the pores. Calculations by theGrossman group[29] highlighted the potential of nanoporoussingle-layered graphene for water desalination. Their resultsindicate that the nanopores created in graphene nanosheetsexhibited full salt rejection and water passage (23.1 �2

hydrogenated pore and 16.3 �2 hydroxylated pore) and thewater permeability was as high as 39–66 L cm�2 day�1 MPa�1.The water permeability was 2 to 3 orders of magnitude greaterthan that of conventional reverse osmosis membranes witha similar salt rejection of 99%. Arulu and co-workers[30] foundthat a graphene with a pore larger than 0.8 nm would allowfaster water transport through the membrane compared withthin carbon nanotube (CNT) membranes. The feasibility ofusing nanoporous graphene nanosheets for ion and gasseparation has also been demonstrated by molecular dynam-ics simulation studies.[31, 32]

Continuous experimental efforts have been devoted toexplore the potential of nanoporous graphene membranesusing various perforation techniques, including focusedelectron beam ablation,[33] focused ion beam irradiation,[34]

ultraviolet-induced oxidative etching,[35] ion bombardmentand chemical oxidative etching,[36] and oxygen plasma etch-ing.[37] Well-defined pore sizes can be created by focusedelectron beam irradiation of the graphene monolayer, but thisapproach is limited to a small membrane area, and the poresize ranging from 3.5 to 100 nm is unsuitable for effectivemolecular sieving separation. The oxidative etching routeeffectively improved the scalability and reduced the pore sizeto the sub-nanometer scale. However, from a practicalengineering standpoint, the pore density is still insufficientand the pore size distribution requires improvement. Itremains technically challenging to perform precise, large-area, high-density perforation in graphene nanosheets.[7, 9]

Nevertheless, recent experimental advances have demon-strated that single-layer nanoporous graphene can be used asnanofiltration and desalination membranes. Surwade et al.[37]

generated tunable nanometer-sized pores (0.5–1 nm) ina graphene monolayer using an oxygen plasma-etchingprocess (Figure 4 a,b). The resulting membranes exhibitednearly 100 % rejection for dissolved monovalent ions (K+,Na+, Li+, Cl�) with water fluxes of up to 106 gm�2 s�1.

Nanoporous graphene membranes must also confront thechallenge of intrinsic defects from the growth processes (forexample, defects with diameter of 1–15 nm during chemicalvapor deposition of graphene on copper) and extrinsic defectsfrom graphene transfer (� 100–200 nm in diameter). Toaddress this critical problem, the Karnik group[38] recentlydeveloped a multiscale leakage-sealing process that exploitsthe nonpolar nature and impermeability of pristine grapheneto block defects selectively (Figure 4c–e). A centimeter-scaledefect-sealed monolayer graphene nanosheet membrane wasproduced. Subsequently, they introduced pores in the mem-brane by high-energy gallium-ion bombardment to nucleate

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isolated defects followed by etching of the defects intosubnanometer pores. The pore size ranged from 0.162 to0.5 nm, with a pore density of 1012–1013 cm�2. The membranerejected multivalent ions and small molecules, and exhibiteda water flux that was consistent with prior molecular dynamicssimulation.

Recently, findings from the Geim group[39, 40] appear torefute the well-demonstrated atomic impermeability ofsingle-layer graphene. They reported that graphene andboron nitride (hBN) are excellent conductors of protonsacross their 2D structure, so protons can pass throughgraphene or hBN nanosheets.[39] They explain this observation

by the different electron densities of electron clouds that canbe pierced by incident protons in the 2D materials. Protontransport is a thermally activated process, with energy barriersE of� 0.8 and� 0.3 eV for monolayers of graphene and hBN,respectively. This unexpected finding may make nanosheetmembranes promising candidates for use in hydrogen-basedtechnologies (such as fuel cells).[41] Recently, they also showedthat monolayers of graphene and hBN can be used to separatehydrogen ion isotopes.[40] Graphene and hBN monolayerswere exfoliated mechanically and suspended over micro-meter-sized holes etched in silicon wafers. As shown inFigure 5, a proton-conducting polymer, Nafion, and Pdelectrodes were used as coatings to facilitate the permeationmeasurements. Based on two complementary approaches,electrical measurements and gas-flow detection by massspectrometry, they demonstrated that deuterons (D) perme-ated through as-prepared nanosheet membranes much slowerthan protons (H), which results in a H/D separation factor of� 10 at room temperature. The highly selective permeation ofisotopes is attributed to the different activation barriers posedby 2D materials that result from a difference of � 60 meVbetween zero-point energies of incident protons and deuter-ons. Furthermore, the scalability was shown by using CVD-grown graphene nanosheets to build centimeter-sized mem-branes, which effectively pumped out hydrogen from a mix-ture of deuterium and hydrogen. The energy cost of thisapproach was estimated at 0.3 kWh per kilogram of feedwater, which is much lower than the cost of existing enrich-ment processes (for example, cryogenic distillation). Theirresults may give rise to new research directions for nanosheetmembranes.

2.4. Other Nanosheet Membranes

In addition to graphene, the potential of other 2Dnanosheet membrane materials with an assumed impervious-ness to atoms is being investigated, although most studiesremain at a molecular simulation stage. Heiranian et al.[42]

achieved water desalination with a single-layer molybdenumdisulfide (MoS2). The optimized pores lead to a 70% greaterflux than that of graphene nanopores. This behavior wasattributed to the unique structures and chemistry of Mo-onlypores. The potential advantage of fabricating high-qualitymonolayers with large area makes MoS2 a promising candi-date for developing new kinds of 2D-nanosheet membranes.Recently, porphyrin-based 2D polymers, with uniform micro-pores and close to atom-thin thicknesses, were explored forgas separation by designing expanded porphyrins computa-tionally with different pore sizes.[43] The ideal selectivity thatwas evaluated from passing-through barriers was of the orderof 106 for CO2/N2 separation, and a permeance of 104–105

GPU for CO2 was determined based on classical moleculardynamics simulations. This result indicated that 2D poly-mers[44] may offer a bottom-up platform for 2D nanosheetmembranes.

Broadly speaking, self-assembled monolayers (SAMs),namely monomolecular films formed by the spatially con-trolled organization of molecules on solid surfaces, could be

Figure 4. Nanoporous graphene membrane: a) Schematic and SEMimage of single-layer graphene suspended on a 5-mm-diameter hole.Several approaches have been used for nanoporous graphene fabrica-tion: bombardment by ions, by electrons, and by oxygen plasmatreatment. b) Aberration-corrected scanning transmission electron mi-croscopy (STEM) images of graphene after 1.5-s exposure to oxygenplasma.[37] c) Graphene membrane fabrication and defect-sealing pro-cedure: (c-1) chemical vapor deposition (CVD) graphene on a coppersubstrate, containing grain boundaries and pinhole defects, is trans-ferred to a polycarbonate track etch (PCTE) membrane (c-2) by firstpressing the graphene-on-copper onto the PCTE membrane andsubsequently etching away the copper. After transfer, the graphenemembrane contains the intrinsic defects and grain boundaries, as wellas fabrication defects such as tears unintentionally introduced duringtransfer. (c-3) To seal nanoscale defects and leaks, �3.5 nm of hafniais deposited onto the graphene surface using atomic layer deposition(ALD) to selectively cover grain boundaries and seal nanometer-sizedintrinsic defects. (c-4) Next, nylon-6,6 is formed in the pores of thePCTE substrate underlying the graphene layer by interfacial polymeri-zation (IP). (d) Photograph of final membrane with the graphene layervisible at the center of the PCTE membrane. Scale bar is 0.5 cm.e) Aberration-corrected STEM images of pores created in graphenemembrane through ion bombardment followed by chemical oxidation.Scale bar is 0.5 nm.[38] Reproduced with permissions. Copyright 2015Nature Publishing Group. Copyright 2015 American Chemical Society.

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considered as a special kind of 2D nanosheet membrane.[45]

Persistent interests in SAMs lay in their providing a straight-forward approach to coat and functionalize solid surfaces withmolecule-thick nanolayers. A typical example of SAMmembranes are the so-called layer-by-layer (LBL) mem-branes,[46] with well-defined molecular composition andthicknesses from ten to several hundreds of nanometers.The introduction of graphene stimulated the pursuit of carbonnanomembranes (CNMs), which are derived from variousaromatic self-assembled monolayers, combining the atomicthickness and excellent toughness of graphene with the

chemical functionality of SAMs. In general,CNMs are fabricated by generating a func-tional molecular monolayer on a solid sur-face and then crosslinking this layer to forma molecularly thin membrane that is able tobe released from the surface. Recently,Schrettl et al.[47] developed an approach formaking 2D CNMs with extended lateraldimensions based on the self-assembly andsubsequent carbonization of the n-hexaneamphiphile at the air/water interface. Theydemonstrated that CNMs are mechanicallystable and can be functionalized, which issimilar to reduced GO, with a molecularlydefined thickness of 1.9 nm and lateraldimensions on the order of centimeters. Itis presumed that these dimensions arelimited by dimensions of the self-assembly(Langmuir) trough. Fabricating approachesand the mechanical, optical, and electricalproperties of these functional CNMs havebeen studied widely, which is instructive fordesigning 2D-material membranes.[45]

3. Two-Dimensional-MaterialLaminar Membranes

Although extraordinary separationproperties have been predicted and ach-ieved in nanosheet membranes, the chal-lenge of fabricating large-area and integrat-ed nanosheets with uniform nanoporesremains.[7, 9] Laminar membranes assembledfrom 2D materials provide a more practicalapproach for using 2D materials for separa-tion.[12] Two-dimensional interlayer chan-nels between nanosheets could yield fastand selective transport of small molecules.To date, various 2D materials have beenfabricated as laminar membranes, includingGO, transition metal dichalcogenides andcarbides, and layered double hydroxides(LDHs). Laminar structures have also beenintroduced into existing membrane materi-als to form hybrid membranes. Issues re-lated to laminar membranes are discussed inthis section.

3.1. Graphene Oxide Membranes

As one of the most important derivatives of graphene,GO[48] is an essential 2D building block for separationmembranes. Similar to graphene, GO nanosheets are alsosingle-atom thick with lateral dimensions as high as tens ofmicrometers. Moreover, they contain plenty of oxygen-containing groups, such as hydroxyl, carboxyl, carbonyl, andepoxy groups, on the edges and basal planes of the GO

Figure 5. 2D materials for sieving hydrogen isotopes:[40] a,b) proton versus deuteronconductivities of 2D crystals. a) Examples of proton current I vs. bias voltage V (I—V)characteristics for hydron transport through monolayers of hBN (main panel) and graphene(lower inset).Top inset: Experimental setup. Pd electrodes supply protons or deuterons intoH- or D-Nafion; 2D crystals serve as barriers for hydrons. b) Proton and deuteronconductivities (shaded and solid bars, respectively) for the most hydron-conductive 2Dcrystals. Each bar (solid or shaded) corresponds to a different device (nearly 30 are shown).The dashed lines mark the average conductivities for the six sets of devices, and the shadedareas around them show the standard errors. Insets: Charge density (in electrons per �2)integrated along the direction perpendicular to graphene (left) and monolayer hBN(right).[39] c–e): Isotope separation by electrochemical pumping of hydrons through gra-phene. c) Mass spectrometry setup. d) Fraction of H atoms at the output for different [H+]inputs. Inset: Energy barrier presented by a 2D crystal for proton and deuteron transfer. Theblack and blue horizontal lines indicate zero-point states of protons and deuterons,respectively, in Nafion and water. The solid red curve shows the separation dependenceexpected for the known difference E(D)�E(H) =60 meV, with no fitting parameters.e) Schematic and optical photo of CVD-graphene device used for mass spectrometrymeasurements. CVD graphene is transferred onto a Nafion film that is in turn glued toa gasket using epoxy. Reproduced with permissions. Copyright 2016 the AmericanAssociation for the Advancement of Science. Copyright 2014 Nature Publishing Group

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nanosheet. In addition, GO nanosheets can be mass-producedcost-effectively through chemical oxidization and ultrasonicexfoliation of graphite. These exceptional features ensure GOis a versatile platform for constructing well-defined nano-structures. For instance, the stacking of GO nanosheets intolaminar (layered) structures has been considered as a power-ful and scalable approach to fabricate graphene-based 2Dmembranes.[6] In contrast to nanosheet membranes where in-plane nanopores provide transport channels, the 2D channelsbetween stacked GO nanosheets allow molecules to passthrough. The oxygen-containing functional groups on GOnanosheets, besides making GO dispersible in water, alsoprovide convenient sites for enhancing specific interactions(for example, hydrogen bonds, electrostatic) with transportcomponents (water, CO2, and ions).

With the advantages of a high aspect ratio structure andwater-dispersible property, GO nanosheets can be assembledreadily into laminar membranes by filtration,[49–53] drop-casting,[54] spin-coating,[55,56] LBL deposition[57] and othermethods. Vacuum filtration is the most common and straight-forward route to fabricate GO membranes on substrates witharbitrary configuration (for example, flat, tubular, and hollowfibers). The membrane thickness can be controlled readily bythe amount of GO in the introduced suspension.[53] Drop-casting is usually used to fabricate free-standing GO mem-branes, which shows a drawback in the composite systembecause of the insufficient interfacial adhesion between theGO layer and the substrate. The inherently weak adhesioncan be improved by surface modification of the substrate.[58]

The advantages of spin-coating come from the rapid assemblyprocess and potential in-plane ordered arrangement of GOnanosheets. The LBL strategy is able to tailor the GO layerthickness precisely at molecular level. Ideally, as few as twostacked GO nanosheets are adequate to create a sievingchannel, with the prerequisite of no defects during thedeposition.[12] A detailed summary and discussion of fabrica-tion methods for GO mem-branes can be found else-where.[6]

During the assembly ofGO nanosheets into lami-nar membranes, two kindsof transport channels arecreated: 1) plane-to-planeinterlayer galleries formedby face-to-face nanosheetinteractions, and 2) in-plane structural defectsand/or slit-like poresformed by edge-to-edgenanosheet interactions. Mo-lecular transport throughGO-assembled laminarmembrane occurs in in-plane slit-like pores andthen plane-to-plane inter-galleries.[52] Geim and co-workers[56] first observedthe unimpeded permeation

of water through free-standing 0.1–10 mm-thick GO laminarmembranes. Water molecules permeate through the 2Dinterlayer channels formed between GO nanosheets andfollow a tortuous path primarily over the hydrophobicnonoxidized surface rather than the hydrophilic oxidizedGO region. They claimed that the nearly frictionless surfaceof the non-oxidized GO nanosheets facilitates the ultra-fastflow of water molecules. Subsequent to this pioneering work,many studies have demonstrated that the intergalleriesbetween GO nanosheets play a vital role in the fast andselective transport of water, ions, and gases. This also accountsfor the significant potential of GO nanosheets in water andgas separation membranes. Therefore, precise tuning of thenanostructures in the intergalleries becomes a fundamentalissue to achieve high-performance GO laminar membranes. Italso generates common interest in communities for studyingmolecular transport through 2D channels. Mi summarizedseveral methods to obtain tunable interlayer galleries andtheir different applications for separation (Figure 6).[12]

The face-to-face interactions, oxygen-containing func-tional groups, and corrugation of GO nanosheets generateinterlayer galleries in GO laminar membranes.[59] Accordingto studies by Geim and co-workers,[50] the typical emptyinterlayer spacing of GO laminar membrane was estimated at� 0.5 nm. When the GO membrane was immersed in anaqueous solution, the empty spacing could be increased tomore than � 0.9 nm because of the hydration of oxygen-containing groups on the GO nanosheets. The resulting 2Dinterlayer channels exhibited a sharp cut-off for species withradii larger than 0.45 nm. Solvation phenomena in ethanolwere also used to tune the interlayer spacing of GO laminarmembranes as they are applied in organic solvent nano-filtration.[60] In contrast, intergalleries can be narrowed by thereduction of oxygen-containing groups on GO nanosheets, byeither thermal or chemical routes. Su et al.[61] showed thathydroiodic- or ascorbic-acid-reduced GO laminates could

Figure 6. Schematic of tuning of interlayer galleries of laminar membranes:[12] a) Separation capability of GOmembrane is tunable by adjusting the nanochannel size. b) Several methods to synthesize GO membraneshave been reported or envisioned; GO nanosheets can be packed physically by vacuum filtration (options1 to 3), or they can be stabilized by covalent bonds, electrostatic forces, or both (options 4–6) during layer-by-layer assembly. Reproduced with permission. Copyright 2014 the American Association for the Advancementof Science.

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exhibit excellent barrier properties, highly impermeable to allgases and liquids because of the significantly decreasedinterlayer spacing. One challenge in the reduction treatmenton GO membranes is how to maintain the structural integrityof GO nanosheets and the entire laminate.

Intercalation chemistry is thought to be a controllableapproach for modulating the intergalleries of GO laminarmembranes. Various nanomaterials and molecules have beenintercalated into GO nanosheets to form hybrid structures.When intercalating nanomaterials, one should consider theircompatibility with GO. Carbon-based materials such ascarbon dots,[62] single-walled carbon nanotubes (SWCNTs),[63]

and multi-walled carbon nanotubes[64] are preferred in theliterature. Owing to the expanded interlayer space betweenneighboring GO nanosheets, the intercalated GO laminarmembranes exhibited 1–2-fold water flux improvements atthe same rejection rate.[62–64] By using a similar intercalationroute, nanomaterial was also used as a sacrificial template tocreate more 2D interlayer channels in the GO ultrafiltrationmembranes, which lead to a 10-fold enhancement in waterflux while maintaining the same rejection rate.[51] In general,molecular intercalation involves a crosslinking reactionbetween intercalated molecules (for example, diamine,[65]

1,3,5-benzenetricarbonyl trichloride[57]) and GO nanosheets.Thus, the interlayer spacing was finely tuned and the stabilityof the GO laminar structure could be improved. It has beendemonstrated that the intergallery size is closely related to thedimensions of the intercalated molecules or nanomaterials,which enables intercalation as a flexible approach to controlthe nanostructures of 2D materials laminar membranes.

Regardless of the above achievements in GO membraneswith well-tuned interlayer spacing and high water flux, therejection rates of state-of-the-art GO membranes are rarelybetter than those of commercial nanofiltration (NF) mem-branes.[64] GO membranes must still confront the challenge ofregulating subnanometer-sized intergalleries that are neces-sary for precise sieving of monovalent ions (water desalina-tion) and small gases (gas separation). Very recently, weproposed the rational design of external forces to manipulatethe nano-architecture of GO-assembled 2D channels pre-cisely with an empty interlayer height of � 0.4 nm for fasttransport and selective sieving of gases.[66] As shown inFigure 7, the designed external forces are applied bothoutside and inside the GO laminate. They overcome theintrinsic repulsive electrostatic interactions between GOlayers collaboratively, with the aim to eliminate non-selectivestacking defects and stimulate a highly ordered assembly ofGO nanosheets. This approach enabled the development ofhighly permeable 2D GO membranes with remarkablemolecular-sieving properties, and offered a 2- to 3-orders ofmagnitude higher H2 permeability and 3-fold enhancement inH2/CO2 selectivity compared to commercial membranes. Thiswork contributed a notable step to push 2D-material mem-branes toward implementation for precise molecular separa-tion. The extension of this approach to water desalinationmembranes is underway.

GO laminates possess unparalleled rapid water-transportchannels; however, the majority of GO membranes fail toachieve the theoretically predicted results in real separation

processes. With the premise of well-controlled 2D intergal-leries between GO nanosheets, the full utilization of theserapid GO laminate transport channels is an emergingchallenge. For instance, the separation process of GOmembranes for solvent dehydration is governed by a prefer-ential sorption–diffusivity mechanism.[49] Water molecules arefirst adsorbed into the membrane surface and are thentransported through the GO intergalleries across the mem-brane. Our recent study[49] found that the water sorptionprocess can be regarded as the control step. This means thatfast 2D channels are most likely not utilized fully during theseparation if the surface adsorption is insufficient. Therefore,we designed a bio-inspired membrane that coupled anultrathin surface water-capturing polymeric layer (< 10 nm)with GO laminates.[67] As shown in Figure 8, the hydrophilicpolymer (chitosan) acts as a surface layer to capture a largenumber of water molecules preferentially from feed mixtures,and results in an increase in driving force across the GOlaminates. Two-dimensional water channels within the inter-layers of GO nanosheets could be used fully to achieve fastand selective water permeation through the integratedmembrane. The prepared membrane exhibited highly selec-tive water permeation with an excellent water flux of over10000 gm�2 h�1, which exceeds the performance of state-of-the-art membranes for butanol dehydration.[68]

Although it has received less attention, the critical role ofin-plane structural defects and/or slit-like pores that partic-ipate in the separation should not be ignored, particularly forthe ultra-thin membranes with only a few interlayer galleries.Li et al.[53] used a facile filtration process to fabricate GOmembranes as thin as 1.8 nm, showing surprisingly highselectivity for H2/CO2 (3400) and H2/N2 (900) mixtures. Theultra-thin GO layer requires extra care to prevent potentialdamage when conducting gas permeation experiments. Theauthors attached heat-resistant Kapton tape with a hole onthe GO membranes to expose the desired membrane area forgas permeation. No pressure drop was applied across themembranes to prevent the thin supports from breaking ordeforming. These GO membranes, after reduction, showeda three-orders of magnitude lower water flux; however, thegas permeance remained almost unchanged. They proposedthat the main transport pathway and permeation of gasmolecules was attributed to selective structural defects ratherthan interlayer spacing. Other groups who have studied GOlaminar membranes also attributed shortcuts provided bydefects in GO nanosheets to a higher water flux than thecalculated value based on Poiseuille�s law.[51, 52] Inspired by thephenomena, in-plane mesoporous GO sheets were preparedby re-oxidation and subsequent assembly into laminarmembranes.[69] The introduced in-plane pores increased theeffective path, and shortened the mass transport pathsignificantly, which resulted in a 2- to 3-times higher waterpermeance than that of the pristine GO membrane, whilepreserving the rejection for small molecules. Nevertheless,similar to the situation in nanoporous graphene membranes,the precise generation and introduction of selective defects inGO laminar membranes still requires much effort.

In addition to the transport channels either from plane-to-plane intergalleries or in-plane defects/pores, the functional

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groups on GO nanosheets could influence the moleculartransport at least in two manners:[6] 1) oxygen-containinggroups on the basal plane and edge of GO could formhydrogen bonding with water or gases to achieve preferentialadsorptions in the GO membrane. For instance, a muchhigher water sorption ability than organic compounds (meth-anol, butanol, and dimethyl carbonate) was found in quartzcrystal microbalance measurements.[49,67] Adsorption testsindicated that the gas adsorption of GO powders andresultant membranes followed the order of CO2>CH4>

N2>H2.[53,85] These strong interactions between permeate

species and GO may promote or prohibit the diffusionthrough GO membranes, which was found to be dependent onthe GO laminar nanostructures.[55] 2) Negatively charged

groups on GO nanosheetscould afford electrostatic in-teractions with the chargedmolecules or ions in nanofil-tration and salt rejection.[52]

According to the exclusiontheory, the potential at theinterphase of solution andmembrane tends to excludeco-ions, and counter ions arerejected to maintain the solu-tion electro-neutrality on eachside of the membrane. Zhuand co-workers[54, 71] carriedout systematic studies on se-lective ion transport throughGO laminar membranes inwhich diverse interactionswere formed between differ-ent ions and GO nanosheets.

The exciting nanostruc-tures and transport propertiesof GO laminar membranes ledthem to great success in thehigh-efficiency purification ofliquids and gases by pressurefiltration, pervaporation, andgas separation. A detailedsummary of separation perfor-mance can be found else-where.[6] Nowadays, engineer-ing of GO membranes hasreceived increasing attention.The feasibility of large-scalefabrication and operationalstability is particularly signifi-cant currently.[9] We developeda type of hollow fiber GOmembrane using the facile fil-tration method.[49] Hollow fi-bers are an industrially pre-ferred membrane form be-cause of its advantages inhigh-packing density, cost-ef-fectiveness, and self-support-

ing structure. GO nanosheets can be stacked easily on thecurved surface of hollow fibers despite their high curvatureand elongated shape. The hollow fiber GO membraneexhibited selective water permeation and showed greatpotential in solvent dehydration applications.[49, 67] The viabil-ity of GO hollow fiber membranes in wastewater treatmentand organic solvent nanofiltration has been demonstratedrecently.[72–74] Furthermore, the instability of the interlayernanochannels formed by stacking GO sheets representsa major challenge when using GO laminar membranes forwater purification. The crosslinking of GO nanosheets usingchemical groups could offer stable, covalently bound GOlaminates with well-defined interlayer spacing.[9,12] These canbe produced by mixing GO sheets and linking agents and

Figure 7. External force-manipulated subnanometer 2D channels for ultra-fast gas sieving :[66] (a) Externalforce-driven assembly (EFDA) approach for fabricating 2D channels, which involves 3D external forces in x,y, and z axes. Enlarged schematic shows force analysis for one 2D channel unit consisting of GOnanosheets and polymer chains. Three main forces are included: intrinsic force, outer external forces(compressive force, centrifugal force and shear force), which are applied outside the 2D channel unit, andinner external force (GO-polymer molecular interactions), which are applied inside the 2D channel unit.b) Hypothetical evolution of surface and cross-section of GO-assembled 2D channels from intrinsic force-induced disordered structure (left) to highly-ordered laminar structures (right) driven by introducedsynergistic external forces. c) Cross-sectional SEM and TEM images of EFDA-GO membrane. Reproducedwith permission. Copyright 2016 American Chemical Society.

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filtering the suspension on a support layer, or by using theLBL assembly approach.

3.2. Transition Metal Dichalcogenide and Carbide Membranes

After the discovery of the first 2D material, graphene, 2Datomic crystals such as transition metal dichalcogenides andcarbides have received increased attention in recent yearsbecause of their unique physics and chemistry.[5] However,only a few studies have reported the use of these types of 2Dmaterials for membrane separation. This may be related tothe low aspect ratio of the exfoliated nanosheets that make itdifficult to form robust and defect-free membranes. Recently,monolayers of molybdenum disulfide (MoS2),[75, 76] tungstendisulfide (WS2),[70] and MXene[77] that had been exfoliatedchemically with a micrometer lateral size, were fabricated aslaminar membranes by a filtration method (Figure 9). Thedifferent surface chemistries of these 2D atomic crystalsprovide these membranes with several new features com-pared to graphene-based membranes.

Peng and co-workers[75] reported a laminar MoS2 mem-brane with almost the same thickness (� 1.8 mm) and channelsize (� 3 nm) as that of the GO membrane, which exhibitsa 3- to 5-times higher water flux for the filtration of Evans

blue (EB) molecules. They attributed this flux improvementto the exposure of the sulfur atoms in the MoS2 monolayersheets that provide a higher hydrophilicity and more passagechannels for water molecules. Recently, they reported thinnerWS2 laminar membranes (� 0.5 mm; Figure 9a, b) with a twotimes higher water flux (450 L m�2 h�1 bar�1) than that of MoS2

membranes with a similar EB rejection (89 %).[70] The waterflux was improved by duplicated nanochannels using ultra-thin nanostrands as templates without loss of rejectionefficiency. They found that 0.3–0.4 MPa transmembranepressure-induced nano-cracks could generate new fluidicnanochannels that result from increased porosity and de-creased transport path, and led to a much higher water flux inthe channelled WS2 membrane. It is worth noting that both

Figure 8. High-efficiency water-transport channels using the synergisticeffect of a hydrophilic polymer and GO laminates:[67] a) Schematic ofthe water–organic (for example, n-butanol) separation process usingthe synergistic effect of hydrophilic polymer and GO laminates.b) Digital photograph of prepared CS@GO membrane. c) Comparisonof GO membranes and CS@GO membrane with state-of-the-artmembranes for water/butanol dehydration. Reproduced with permis-sion. Copyright 2015, John Wiley and Sons.

Figure 9. Several kinds of 2D laminar membranes. a,b) WS2 mem-brane:[70] a) AFM image of WS2 nanosheets, and b) cross-sectionalSEM image of 500-nm-thick WS2 membrane. c, d) MoS2 membrane:[76]

c) AFM image of single-layered MoS2 nanosheet, and d) cross-sec-tional SEM image of 35-nm-thick MoS2 membrane on anodic alumi-num oxide support. e, f) MXene membrane:[77] e) photograph andf) cross-sectional SEM image of Ti3C2Tx membrane. g,h) LDH mem-brane:[78] g) top and h) cross-sectional image of NiAl LDH membrane.Reproduced with permissions. Copyright 2014–2015 American Chem-ical Society. Copyright 2014–2015 RSC.

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the MoS2 and WS2 laminar membranes exhibited at least a 1-week stability for water filtration,[70,75] which is rarelyreported for GO membranes. The absence of oxygen-containing groups may contribute to the structural stabilityof laminates when immersed in water. The possibility of using2D dichalcogenide membranes for gas separation was alsoexplored by Jin and co-workers.[76] Ultrathin (17–60-nm-thick) laminate-stacked MoS2 membranes were developed forH2 separation (Figure 9 c,d). Although high gas permeanceresulted, the Knudsen diffusion selectivity suggested theexistence of excessively large gas channels in the MoS2

membranes.In 2011, the Gogotsi group reported a family of 2D early

transition metal carbides termed MXenes, which wereproduced by etching out A layers from the layered Mn+1AXn

phases.[79] MXenes have shown promising application inelectrochemical energy storage owing to ion interactionsbetween the atomically thin layers.[80] For instance, a widerange of cations with varying sizes and charges (Na+, K+,NH4

+, Mg2+, and Al3+) can be intercalated into the Ti3C2Tx

interlayer, which results in structural expansion or contrac-tion. Moreover, Ti3C2Tx is an efficient adsorbent for cationicdyes and heavy metal ions. Until recently, this Ti3C2Tx MXene2D material was used in ion separation membranes based oncharge- and size-selective permeation (Figure 9e, f).[77] Sim-ilar to GO laminar membranes, the hydrophilic nature ofTi3C2Tx with water within the intergalleries promoted waterflow. The MXene membrane rejected cations with hydrationradii larger than the interlayer spacing (� 0.6 nm). Moreover,cations with a larger charge showed an order of magnitudeslower permeation compared with single-charged cations.

3.3. Layered Double Hydroxides Membranes

Layered double hydroxides (LDHs) are typically layeredcompounds that consist of regularly arranged, positivelycharged brucite-like 2D layers and charge-compensatinganions located in interlayer galleries.[81] They have the generalformula [M1�x

2+Mx3+(OH)2][A

n-]x/n·zH2O (M2+, M3+, An�, andH2O represent di- and tri-valent metal ions, n-valent anions,and the interlayer water, respectively). By varying the metalions and charge-compensating anions, the gallery height canbe tuned from nano- to subnanometer scale. More impor-tantly, in contrast to the typical exfoliation-assembly proce-dure for most 2D membranes, the formation of 2D sheets,self-assembly into crystallites and their deposition on a sub-strate can be completed in one step under hydrothermalconditions.[78] This smart membrane formation process offersan exciting opportunity for cost-effective fabrication of 2D-material membranes.

The Caro group have started to explore the potential ofLDHs in 2D-material membranes.[78, 82] In contrast to the top-down synthesis route, an in situ growth method was used toprepare well-intergrown NiAl-CO3 LDH membranes onporous alumina substrates (Figure 9g,h).[78] A 2D intergalleryheight of 0.31 nm was formed in this kind of carbonate-intercalated laminar membrane, which fulfils the requirementof size-discrimination-based gas separation (that is, molec-

ular-sieving separation). As expected, the prepared LDHmembranes exhibited remarkable molecular sieve properties(for example, selectivity for H2/CH4 mixtures of � 80), whichmakes them attractive in H2 purification. They furtherdemonstrated that CO2 dissolved in the precursor solutioncould be used to control the preferred orientation andthickness of the LDH layers.[82] Trace CO2 induced ab-oriented membranes, whereas saturated CO2 produced ran-dom-oriented membranes. In principle, the ab-oriented LDHmembrane was expected to yield a higher gas performancebecause its interlayer gallery was arranged perpendicular tothe substrate, which minimizes the mass transfer resistance.However, the high aspect ratio of LDH crystallites and theinsufficient supply of CO2 may have caused the formation ofnon-selective defects within the LDH layer. This led to theformation of an ab-oriented membrane with lower H2

selectivity compared with the compact random-orientedone. In addition, the in situ growth method was alsoapplicable for fabricating ZnAl-NO3 LDH membranes. Inthis case, the intergallery height of 0.41 nm was larger thanthat of the NiAl-CO3 LDH membrane, because of theintercalation of different anions. The rich intercalationchemistry places LDH membranes with a widespread appli-cation in molecular separation.

3.4. Hybrid Membranes

The versatile physicochemical properties of 2D materialscan extend the spectrum of laminar membranes significantlyfrom pure to hybrid nanostructures formed by 2D laminatescombined with another polymeric or inorganic membranematerial. Most hybrid laminar membranes are mixed matrixmembranes (MMMs) that are fabricated mainly by incorpo-rating nanosheets into a polymer matrix.[83] The mainadvantage of the mixed-matrix strategy is the facile combi-nation of the excellent processability of polymer and theunique characteristics of the 2D materials. Moreover, com-pared with the commonly used aqueous environment for purelaminar membranes, the polymer matrix with various func-tional groups could provide 2D materials with a more flexibleassembly environment. This would offer a variety of oppor-tunities for constructing delicate 2D laminate-based nano-structures. The well-established polymeric membrane tech-nology could be applied readily for 2D material MMMs,which represent the most realistic approach for pushing 2D-material membranes into practical implementation.[9]

Like the pure laminar membranes, 2D material MMMshave been well-studied for water and gas separation and ion/proton exchange.[10, 11] The hydrophilicity of GO nanosheets isused to enhance the water permeation properties of poly-meric membranes through the effective mixed-matrix strat-egy. Biological fouling can also be reduced by the introduc-tion of GO into existing membranes for water treatment byblending GO into the polymer matrix or membrane surfacefunctionalization of GO. Graphene nanomaterials possessintrinsic antimicrobial properties,[84] which induces inactiva-tion of bacterial cells upon direct contact by physical andoxidative damage to cell membranes. Furthermore, the high

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specific surface area of graphene makes it an ideal scaffoldmaterial for anchoring different types of antimicrobial com-pounds, with silver being the most extensively studiedmaterial thus far. Additional details can be found in a recentreview paper.[9]

We reported GO/polyether block amide (PEBA) MMMsfor selective CO2 separation.[85] Induced by the hydrogenbonding between GO and PEBA, GO nanosheets wereassembled into several-layered GO laminates with molecular-sieving interlayer spacing and straight diffusion pathways(Figure 10 a). Besides the improved selective diffusion, thepreferential CO2 adsorption of GO nanosheets enhanced theadsorption selectivity. Fast and selective gas transport chan-nels existed in the uniformly dispersed GO laminates, whichprovides the GO MMMs with excellent preferential CO2

permeation performance (PCO2: 100 Barrer [1 Barrer =

10�10 cm3 (STP) cmcm�2 s�1 cmHg�1)], a(CO2/N2): 91) andextraordinary operational stability (over 6000 min). Furtherstudies demonstrated that the gas permeation propertiescould be finely manipulated by controlling the lateral size andoxidation degree of GO nanosheets.

With the aid of a polymeric matrix, several 2D materialsthat face challenges of forming pure laminar membranescould start as MMMs. For instance, graphitic carbon nitride(g-C3N4, CNs), an emerging graphene-analogue material thatpossesses regularly distributed triangular nanopores (esti-mated as 0.31 nm) throughout the entire laminar structure,was incorporated into a sodium alginate matrix to determineits effect in membrane separation (Figure 10 b).[86] Thehorizontally aligned laminar structure of CNs embedded inMMMs could afford ordered channels for water transport,whereas the nanoporous structure of CNs could providea molecular sieving effect. Thus, the water flux and ethanol/water separation factor improved significantly by increasingthe CN loading by 3 wt %. In addition, a reduced GO-modified CNs was used as a 2D filler to enhance the substrateproperties of porous polyethersulfone for thin compositeforward osmosis membranes.[88]

In addition to MMMs, 2D materials were used recently asa platform to advance the fabrication techniques of existingmembranes. Current studies focused mainly on the hybrids ofGO and MOF. Hu et al.[87] proposed 2D GO nanosheets forMOF nanocrystal growth, and obtained MOF/GO hybridnanosheets as the seeding layer to synthesize ultrathin MOFmolecular sieving membranes (Figure 10c). The 2D seedinglayer acted as a barrier between two different synthesissolutions, which self-limited crystal growth and eliminateddefects during the contra-diffusion process. The resultingultrathin ZIF-8/GO membranes (� 100 nm thickness)showed excellent molecular sieving gas separation properties,with a high selectivity of 7.0 for CO2/N2 and 405 for H2/C3H8.Huang et al.[89] used the intrinsic barrier property of graphenemonolayers to seal the MOF crystal gaps to improve the MOFmembrane gas selectivity. Bicontinuous ZIF-8@GO mem-branes were fabricated by LBL deposition of GO on thesemicontinuous ZIF-8 layer. By patching the non-selectivecrystal gaps with a GO layer, gases permeated through theZIF-8 micropores (0.34 nm), which resulted in an improvedhydrogen selectivity for H2/CO2, H2/N2, H2/CH4, and H2/C3H8

mixtures. Caro and co-workers[90] demonstrated the conceptof “partial conversion of LDH laminates” for designing andfabricating high-performance MOF gas separation mem-branes.

4. Summary and Outlook

The rapid growth in interest and the large number ofachievements in recent years suggests that 2D-materialmembranes are becoming a family of high-performanceseparation membranes. The unique atomic thickness of 2Dmaterials stimulates the continuous pursuit of ultrathin andselective membranes for separation, regardless of fundamen-tal and engineering studies. Recent breakthroughs in exfolia-tion and perforation of monolayers promoted the throughputand accuracy of nanosheet membranes for molecular separa-tion. Although they were initiated from GO, and GO still

Figure 10. Two-dimensional-material hybrid membranes: a) Schematicrepresentation of the assembly of GO nanosheets in a polymericenvironment based on hydrogen bonds formed between differentgroups on GO and the PEBA chain.[85] b) Highly water-selective hybridmembrane by incorporating g-C3N4 nanosheets into a polymer ma-trix.[86] c) Illustration of synthesis of ultrathin ZIF-8/GO membrane:coating of flexible ZIF-8/GO nanosheets on a porous support, such asanodic aluminum oxide, and subsequent secondary growth by contra-diffusion method.[87] Reproduced with permissions. Copyright 2015–2016, John Wiley and Sons. Copyright 2015, Elsevier.

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remains a focus, laminar membranes have been explored inseveral other graphene-family 2D atomic crystals and layeredoxides. Their fabrication approaches have been well-estab-lished and the precise manipulation of intergallery dimen-sions for efficient separation is of great importance. Two-dimensional-material membranes with well-defined transportchannels and ultrathin thickness have demonstrated extra-ordinary performance for water and gas separations.

Current challenges that restrict the wide implementationof 2D-material membranes include the limited availabletechniques for exfoliating the high aspect ratio and intactnanoporous monolayers from bulk crystals, and drillinguniform, high-density, large-area, subnanosized pores ingraphene nanosheets, together with how to scale such atom-thick membranes into applicable separation devices. Apply-ing these ultra-thin membranes in practical separationremains a technical challenge in this emerging field. More-over, the performance of state-of-the-art laminar membranesis far from the theoretical predicted values. This may be theresult of an ambiguous understanding of structure–propertyrelationships for the complicated nanochannels. To addressthese challenges, future directions may focus on exploringemerging 2D-material membrane platforms by including newkinds of 2D materials that have already shown success inother related fields. Theoretical models should be updated todescribe the particularly confined transport behavior through2D-material membranes accurately, accompanied by the in-depth characterization of transport passages. Significanteffort should be applied to develop robust membranes withstable performance under realistic operating conditions. Moreresearch is also required to address specific requirementsconcerning various exciting, yet challenging, applications suchas desalination and fuel cells.

Acknowledgements

We acknowledge the National Natural Science Foundation ofChina (grant Nos. 21406107, 21476107, 21490585), the Inno-vative Research Team Program by the Ministry of Educationof China (grant No. IRT13070), the Natural Science Founda-tion of Jiangsu Province (grant No. BK20140930) and theProject of Priority Academic Program Development ofJiangsu Higher Education Institutions (PAPD) for financialsupport.

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Received: January 14, 2016Revised: March 17, 2016Published online: && &&, &&&&

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Minireviews

Separation Membranes

G. Liu, W. Jin,* N. Xu &&&&—&&&&

Two-Dimensional-Material Membranes:A New Family of High-PerformanceSeparation Membranes

Separation goes small : Two-dimensionalmaterials of atomic thickness haveemerged as high-performance separationmembranes. The latest advances in the

design and fabrication of 2D-materialmembranes are reviewed, along witha discussion about the challenges forfuture applications.

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