Zhuang Liu ,Wei Wang ,2,Xiaojie Ju ,2,Rui Xie ,2,Liangyin Chu ,2,3,*

1 School of Chemical Engineering,Sichuan University,Chengdu 610065,China

2 State Key Laboratory of Polymer Materials Engineering,Sichuan University,Chengdu 610065,China

3 Jiangsu National Synergetic Innovation Center for Advanced Materials(SICAM),Nanjing 211816,China

1.Introduction

In recent decades,membrane technologies show great importance for global sustainable development in myriad fields such as solving resource shortage and environmental pollution[1-3].Particularly,to improve the decontamination of water,as well as to increase water supplies through the safe re-use of wastewater and efficient desalination of sea and brackish water,membranes and membrane technologies are playing paramount roles[4],because separations based on membranes show fantastic features including no phase change,no additives and low energy consumption in comparison to conventional separation methods[1].However,most of currently commercial membranes are subject to a trade-off between productivity(i.e.,permeability)and efficiency(i.e.,selectivity),known as the Robeson's upper bound[2,5].To address this dilemma,state-of-the-art membrane materials are developed with fantastic ability to distinguish molecules based on size and shape by thickness-reduction approach to maximize their permeability[6,7].

Graphene is a single atomic plane of graphite[8],which presents excellent mechanical strength,high electrical conductivity,superior thermal conductivity and other fantastic properties because of its sp2hybridized C atoms arranging in a honeycomb lattice.Rationally,the single-atom-thick graphene is considerable for acting as membrane barrier,but it is con firmed being impermeable to atoms,molecules and ions such as helium[9].Fortunately,as the derivative of graphene,graphene oxide(GO)contains pristine regions,oxidized regions,and a small fraction of holes,and has drawn great attention for assembling separation membranes with fantastic separation capabilities due to the distinctive transfer channels[10-16].The passages of the graphene-based membranes for mass transfer are generally built by two approaches.One is stacking the graphene-based sheets to form interlayer spaces,which could connect and form networks of nanocapillaries between the stacked sheets,providing passages for mass transfer.Molecules and ions could diffuse in the direction parallel in the interlayer space from the edge of the stacked sheets.The water molecules can exhibit“hyperlubricity”in the well-de fined channels of the graphene-based membranes,similar to the water-permeation mechanism as previously described for small-diameter carbon nanotubes and hydrophobic nanopores[17].The other one is creating defects on the basal plane of the graphene sheets by ion and electron irradiation[18-21]or chemical routes[10,22-24].The intrinsic defects in graphene form nanopores,which exhibit selective ionic and water transport due to ion hydration and steric effects that are sensitive to the pore size and functional groups lining the pore,analogous to biological ion channels[25-28].Conceivably,graphene-based membranes have a great deal to offer over existing technologies for desalination,ionic or organic molecular separation in the aqueous environments due to their well-defined channels with an assistance of oxygen-containing groups.

In this review,we will highlight the recent developments of graphene-based membranes for molecular and ionic separations in aqueous environments.The mass-transfer mechanisms of graphenebased membranes in the aqueous solutions and the potential applications for desalination,organic molecular rejection and specific ionic separation are discussed.The discussion is divided into three parts.It is started with the introduction of stacked graphene-based membranes for molecular and ionic sieving and separation.Then,one-atom-thick graphene-based membranes for ionic and molecular separations are introduced.Lastly,an outlook is provided to describe the future challenges in the development and industrial use of graphene-based membranes.

2.Stacked Graphene-based Membranes for Molecular and Ionic Separation and Sieving

2.1.The fabrication approaches and mass-transfer mechanisms

In general,to achieve the stacked structure,the graphene-based membranes could be fabricated by three approaches.One is making the stacked graphene-based membranes by filtration of the graphene sheets through membrane filters.The stacked membranes can be dried in air or vacuum environments,and peeled from the filters[29,30].This filtration method is not only simple and highly reproducible but also allows precise control over the thickness of stacked membranes.Another method to produce the stacked structure is by spray or spin-coating of graphene-based sheets suspensions in water due to the intrinsic repulsive edge-to-edge GO sheet interactions and face-to face attractive capillary forces created by the spin-operation[31].Furthermore,large-area stacked membranes could be prepared using a gravure printing machine with high concentration of graphenebased sheets(e.g.,about 20-60 mg·ml-1of GO sheets),which demonstrates a continuous approach for membrane production[32].

The stacked inter-layer spacing has been proven to play a significant role in molecular transport.The stacked sheets are usually GO or chemically converted/reduced graphene.For the GO sheets,the pristine region has the similar structure as graphene,while the oxidized region has plentiful of oxygen-containing functional groups such as hydroxyl,epoxy and carboxyl groups[33].When a membrane is stacked with GO[Fig.1(a)],the oxidized groups that tend to cluster and leave large,percolating pristine regions support interlayer spaces considering as nanocapillaries ofca.0.6-0.7 nm under dry conditions[34-36].The oxidized groups could strongly hydrate with water molecules.Accordingly,when the GO membranes are immersed in aquous solution or high humidity environment,the intercalating water molecules prop up a large interlayer space to 1.2-1.3 nm,which has been con firmed by X-ray diffraction(XRD)or neutron scattering[36,37].Considering the GO sheets with a thickness of 0.34 nm,the interlayer space can be considered an empty space of highth of 0.8-1.0 nm[36].Water molecules with～0.3 nm in size can accommodate to form a monolayer or multilayer between pristine-graphene sheets due to the increased space(＞0.6 nm)[Fig.1(b)],which are expected to be highly mobile[36,38].While due to hydrogen bonding and a narrower space available for diffusion,water is expected to be less mobile within the oxidized regions,which are unlikely to contribute to water permeation.Because of the network of nanocapillaries that enables to open up in the hydrated state and accept only species that fit in,GO membrane enables to separate the molecules and ions according to their hydrated radius in the aqueous environment.The intercalating water blocks the nanocapillaries,as a result,small species with a hydrated radius of＜0.45 nm,such as Na+,K+or Mg2+,could permeate through the membranes with fast rates accompanied by the assistance of intercalating water layer.Whereas,large ions and organic molecules,such as acetone,hexane,ethanol,and propanol,exhibit no detectable permeation[36,39].Thus,the GO membranes presenta kind of valuable material candidates for separation and filtration technologies,e.g.,desalination,organic compounds-containing wastewater treatment,or dewatering.

With partly or mildly reduced graphene stacking a membrane[40],the mass-transfer mechanism of such membrane is analogous to the GO membrane beacuese of the existence of the oxidized groups on the sheets[36,40].When the GO sheets is reduced under violent condition,the chemically converted graphene(CCG)is corrugated due to the presence of some sp3hybridized carbon atoms and topological defects[41].Thus,stacking of CCG sheets would result in the formation of fuzzy nanochannels through the membrane,making the membrane permeable[Fig.1(c)].The amplitude of corrugation of CCG sheets can be controlled,and they become more corrugated in water simply by hydrothermal treatment at elevated temperatures[41].The water permeation rate increases with increasing the corrugation degree[41,42].

2.2.The stability in aqueous solutions and space adjustment

Fig.1.Schematic diagram for the structures of graphene-based membranes stacked with GO sheets(a,b)and CCG sheets(c)[36,41].

The stacked GO membranes may suffer from instability in the water due to the high hydrophilicity of GO sheets,and even could disintegrate in water.Recently,Yehet al.[43]investigated the origin of the structural stability of GO membranes in water.They fabricated freestanding GO membranesviavacuum-assisted filtering GO dispersions on anodized aluminum oxide(AAO) filter discs and Teflon micro filters respectively.The stability of the both GO membrane in water is completely different[Fig.2(a)],because the oxidized groups on the GO sheets form hydration with water molecules.When immersed in water,the hydrogen bonding between water and hydroxyl/epoxy groups can provide strong repulsive hydration forces to separate the GO sheets from each other[33,44,45].Furthermore,the carboxyl groups become negatively charged on hydration and could afford electrostatic repulsion among GO sheets to make the GO(Teflon)membranes disintegrated in water.While,with AAO filter disk to prepare GO membranes,a lot of Al3+released from the AAO filters during the filtration can effectively crosslink the GO sheets and strengthen the stability of resultant GO membranes in water[43].Multivalent ions enable to bind readily to oxygen functional groups on GO sheets[Fig.2(b)].For example,Mg2+,Ca2+,Al3+and borate anions could chemically crosslink the GO sheets to enhance the stability of stacked GO membranes[43,46,47].However,these crosslinked Go membranes with metal ions may suffer from instability in acid or base solutions[45].Alternatively,some other molecules such as glutaraldehyde[48],alkylamines[49],1,3,5-benzenetricarbonyl trichloride[50],dopamine[51],benzimidazole[52],diamine[53],polyallylamine[54],poly(vinyl alcohol)and poly(methyl methacrylate)[55],could covalently crosslink adjacent GO nanosheets in the GO membranes[Fig.2(c)].Building interlayer cross-linking between the GO sheetsviamolecules not only enhences the mechanical strength and the water-based stablity but also adjusts the GO spacing through sandwiching appropriately sized spacers between GO nanosheets[56].

Fig.2.(a)Photographs showing the stability of GO membranes vacuum- filtrated through Te flon and AAO micro filters in water,in which the GO(Teflon)membrane rapidly disintegrates in water,while the GO(AAO)membrane remains intact[43].(b)Scheme of the GO membrane with metalionic bonding,in which connections between GO sheets and metalions enhance the mechanical properties and stability of GO membrane with metal ion binding[46].(c)Scheme of building interlayer cross-linking between the GO sheets with molecules such as 1,3,5-benzenetricarbonyl trichloride[50].

The approach of increasingπ-π interactions between GO sheets also could significantly improve the stability of stacked GO membranes in aquesous environments[45].By constructing orderly lamellar nanostructure of GO membranes doped with partially reduced graphene oxide(prGO)sheets,the GO membranes are with both extraordinary stabilities in water,acid,and base solutions as well as regularly controllable lamellar spacings[45].The enhencement of stability of GO membranes will expand their application scope and provide better performances in their applications with aqueous solution environments.

2.3.Applications for separations

The stacked GO-based membrane is demonstrated as an efficient separation membrane with promise for applications in molecular and ionic separations in aqueous environments.Joshiet al.[39]investigated selective mass transport through the stacked GO membranes preparedviavacuum filtration.Using a U-shaped tube,which was divided into feed and permeation compartments by the GO membrane[Fig.3(a)],they measured permeation rates of different molecules including glycerol,toluene,ethanol,benzene and dimethyl sulfoxide,and no permeation could be detected over a period of many weeks by monitoring liquid levels and using chemical analysis.Then,the feed compartment is filled with various solutions including ions and organic molecules to determine whether any of the solutes could permeate into the deionized water on the other side of the GO membrane.Several salt solutions with different ion sizes are employed to examine the ion permeability through GO membranes as a function of ionic size.As shown in Fig.3(b),the stacked GO membrane blocks all solutes with hydrated radii larger than 0.45 nm.Smaller ions permeate through the membranes at rates thousands of times faster than what is expected for simple diffusion[39].This behavior is caused by a network of nanocapillaries that open up in the hydrated state and accept only species that fit in,which enable precise,superfast sieving of ions and molecules by size,as well as desalination.

Fig.3.(a)Photograph of a piece of GO membrane covering a 1-cm aperture in copper foil and the setup for molecular sieving experiments.(b)Permeation rates of various solutes through 5 μm-thick GO membranes with 1 mol·L-1 feed solution as a function of the hydrated radii.No permeation could be detected for the solutes in the gray area[39].

Fig.4.(a)Schematic of hollow fiber GO membrane for selective water permeation of aqueous dimethyl carbonate solution(Inset:SEM image of cross-section of the hollow fiber GO membrane).(b)The water content in permeate showing the separation performance of the GO membrane for dehydration of water-containing dimethyl carbonate solution[57].

Huang and Jinet al.[57]prepared GO membrane supported on a ceramic hollow fiber by a vacuum suction method to separate water from dimethyl carbonate(DMC)/water mixtures by using a pervaporation process[Fig.4(a)].In this work,three different concentrations of DMC/water mixtures were studied,i.e.,1%,2%,and 2.6%feed water content by mass.The separation performance is improved under higher feed water content.As shown in Fig.4(b),at 25°C and 2.6 wt%feed water content,the permeate water content reaches 95.2 wt%with a high permeation flux(1702 g·m-2·h-1).This GO membrane exhibited excellent water permeation for dimethyl carbonate/water mixtures due to the typical solution-diffusion mechanism.Because of the presence of oxidized groups for hydrogen bonding with water,GO can combine water more preferentially than DMC,and also the diffusivity of water is much faster than that of other molecules.Thus,selective and fast permeation of water is resulted from contributions of both enhanced diffusivity and solubility,making the membranes promising candidates for dehydration applications.

Hanet al.[40]fabricated ultrathin chemically converted graphene nanofiltration membranes on microporous substrates for water treatment of rejecting organic pollutant such as methyl blue and direct red.Differing from reduced GO synthesized using a strong reduction agent such as hydrazine,the base-re fluxing reduced GO(bRGO)was partially reduced by adding base solution such as NaOH or KOH.The membranes prepared by bRGO showed high retention(＞99%)for organic dyes and moderate retention(ca.20%-60%)for ion salts due to the physical sieving and electrostatic interaction.Moreover,oxygen moieties in hydrophilic domains that impede the transport of water due to the strong interaction are decreased,leaving more domains without any oxygen functional groups for the rapid water transport.

3.Atomically Thick Graphene-based Membranes for Ionic Separation

3.1.Desalination

Different from that in the stacked graphene-based membranes,the mass transport routes in the atomically thick graphene membranes are the etched nanometer-sized pores.A number of theoretical studies have suggested that the selectivity and permeability of such membranes could be vastly superior to the polymer-based filtration membranes because of its one-atom thickness[58-63].

Cohen-TanugiandGrossmaninsimulated the transport by classical molecular dynamics and reported that the nanometer-scale pores in a single-layer freestanding graphene can effectively separate salt from waterforuse in desalination systems[Fig.5(a)][60].The computational results indicate that the desalination performance of such single-layer membranes as a function of pore size,chemical functionalization and applied pressure,and the nanoporous graphene membranes'ability to prevent the salt passage depends critically on the pore diameter and properly sized pores allow for flowing water while blocking ions.The hydroxyl groups bonded to the edges of graphene pores can roughly double the water flux due to their hydrophilic characters[Fig.5(b)].The increase in water flux comes at the expense of less consistent salt rejection performance,which is attribute to the ability of hydroxylfunctional groups to substitute for water molecules in the hydration shell of the ions.Compared with the existing reverse osmosis(RO)membranes,the functionalized nanoporous graphene membranes exhibit full salt rejection in theory.As shown in the Fig.5(c),the calculated water permeability of 66 L·cm-2·d-1·MPa-1through the nanoporous graphene is 2-3 orders of magnitude higher than the conventional RO membranes.Theoretical studies indicated that the performance of such nanoporous graphene membranes could be superior to current polymer-based filtration membranes for water purification.

Fig.5.(a)Hydrogenated(top left)and hydroxylated(top right)graphene pores,and side view(bottom)of the computational system of Cohen-Tanugi and Grossmanin.(b)Computed water permeability for nanoporous graphene functionalized with hydrogen and hydroxyl groups for various pore sizes,in which water permeability scales roughly linearly with the area of hydroxylated or hydrogenated pores.(c)Performance chart for functionalized nanoporous graphene versus existing technologies[60].

Recently,experimental works has been carried out on the usage of nanoporous graphenes for water desalination[64,65].Surwadeet al.[65]created nanoscale pores in a layer of graphene by oxygen plasma etching in order to validate the effectiveness of graphene-based desalination of water,and the results exhibited both high salt rejection and exceptionally rapid water transport properties.By exposing to oxygen plasma with only 0.5 s,the D peak intensity rose to one-third of that for the G peak indicated by the Raman spectra(ID/IG≈0.333),in which theID/IGratio has been extensively used as a measure of the integrity of graphene for measuring the defects.After a longer exposure of 6 s,the 2D peak completely disappeared and the D and G peaks broadened to resemble those ofa disordered carbon material,indicating significant defect formation[Fig.6(a)].Using aberration corrected scanning transmission electron microscopy(STEM)imaging[Fig.6(b)],the pore density and size of the graphene membrane are con firmed,which correlate the trans-membrane transport properties.The plasma-etched graphene membranes show rapid water passage,with the rate of water transport dependent on the plasma etching time,i.e.,the density and size of the nanopores[Fig.6(c)].Membranes with larger pore densities do not show the asymmetric water flux,pointing to their non-selective nature.It can be rationalized by binding of ions to the nanopores,which effectively block the water flux.While the salt rejection of the low-defect-density membranes is exceptionally high.The selectivity of the porous graphene membrane withID/IG≤0.5 is 105,which corresponds to nearly 100%salt rejection[Fig.6(d)].With the optimum pore size,the membranes exhibit a salt rejection rate of nearly 100%and rapid water transport.In particular,water fluxes of up to 106g·m-2·s-1at40°C are measured using pressure difference as a driving force,while water fluxes measured using osmotic pressure as a driving force does not exceed 700 g·m-2·s-1·MPa-1.To date,both theoretical and experimental works represent the proofs of the effectiveness and potential of atomically thick graphene-based membranes as next-generation materials for desalination applications[66].

3.2.Specific separations

Several works have reported that the graphene nanopores could be tuned to selectively transport cations or anions by chemical modifications[59,67].By simulation,the F-N-pore in graphene is terminated by negatively charged nitrogens and fluorines,favoring the passage of cations.The H-pore is terminated by positively charged hydrogens,favoring the passage of anions[59].Rollingset al.performed the cation selectivity of graphene nanopores by experiments[67].The single graphene nanopores with 20 nm in diameter created by electrical pulse method preferentially permit the passage of K+cations over Clanions with selectivity ratios of over 100 and conduct monovalent cations up to 5 times more rapidly than divalent cations.The K+/Clselectivity can be explained by elevated concentrations of mobile cations near the graphene surface.

Compared with the separation of cations and anions,the discrimination of the ions with the same charge,similar hydration properties and ionic radii such as Na+and K+is more challenging[68,69].Inspired by the biological protein channels,which have many remarkable properties such as gating,high permeability and selectivity,Heand Corryet al.designed a biomimetic Na+or K+-selective graphene nanopore using molecular dynamics simulations by altering the pore size and the chemical nature of the pore rim[67].Under a transmembrane voltage bias,a nanopore containing four carbonyl groups preferentially conducts K+over Na+due to an ～2.9 kJ·mol-1higher energy barrier of Na+than K+.While the nanopore functionalized by four negatively charged carboxylate groups selectively binds Na+but transports K+over Na+,because of its stronger affinity(by 4.3 kJ·mol-1)for Na+.Interestingly,when the nanopore contains three carboxylate groups,it shows a voltage-dependent selectivity,which can be tuned by changing the magnitude of the applied voltage bias.For such nanopore,under lower voltage bias,it transports ions in a single- file mannerand exhibits Na+selectivity,dictated by the knock-on ion conduction and selective blockage by Na+.While under higher voltage bias,the nanopore is K+-selective,as Na+no longer blocks the pore.Guo and Leeet al.calculated the maximum electrostatic potential of the crown ether embedded in graphene[70].The images of the atomic con figurations of what they calculate to be the most stable oxygen atoms incorporated in graphene are directly obtained using aberration-corrected scanning transmission electron.The calculations indicate that the crown ether con figurations in graphene should exhibit selectivity for different cations depending on the crown ether ring size,a key property of individual crown ether molecules,but have the added property of being rigid and planar,prized features sought in pre-organized receptors for selective ion binding.However,these calculation results still need experimental verifications.

Fig.6.(a)Raman spectra(514 nm excitation)of suspended graphene after different exposure times to oxygen plasma.(b)Aberration-corrected STEM images of graphene after 1.5 s exposure to oxygen plasma,in which pores with characteristic dimensions of～1 nm are clearly seen.(c)Water loss after 24 h and ionic conductivity through the same porous graphene membranes etched at various exposure times.(d)Water/salt selectivity as a function of I D/I G ratio showing exceptionally high salt rejection rate for optimum etching time[65].

The experimental performances of both stacked graphene-based membranes and atomically thick graphene-based membranes for ionic separations are summarized in Table 1.The graphene-based membranes show extraordinary permeability and selectivity.However,compared with the calculated results,the experimental performances of the graphene-based membranes for ionic separation still have enormous potential to be enhanced,particularly in the separation of ions with the same charge,and similar hydration properties and ionic radii.

Table 1Experimental performances of graphene-based membranes for ionic separations

4.Conclusions and Outlook

In summary,membrane-based separations have been increasingly applied to address the global challenges such water scarcity and the pollution of aquatic environments.High permeability and selectivity of separations are the keys to membranes suitable for applications.At this point,graphene and its functionalized analogue,GO,CCG,prGO are attractive materials for next-generation membranes due to the atomic thickness,mechanical strength and chemical stability.By fantastic creation of the channel for mass transfer,i.e.,stacking the graphene-based sheets to form interlayer spaces,or generating defects on the basal plane of the graphene sheets,the graphene-based membranes can separate molecules and ions with high precision in aqueous environments.However,there are still several important issues need to be solved before large-scale applications.(1)One of the key challenges in the development of graphene-based membranes is how to achieve controllable and industrial-scale productions.For the stacked graphene-based membranes,although some technologies such as filtration method,coating approach and printing strategy have been developed,industrial manufacture techniques are still challenging.The fabricating equipment for graphene-based membranes is different from existing ones,and thus should be re-developed.For the atomically thick graphene-based membranes,the difficulty lies in the fabrication of pores with precise dimensions and featured functional group.(2)Moreover,the separation mechanism of the atomically thick graphene-based membranes has to be studied more intensely.Although simulation is used to explore the mechanism of molecular permeation and various theories have been put forward,certain discrepancies still exist between theoretical and experimental results,and some theoretical works should be verified by experimental results[68-70].(3)Further studies to understand the evolution of membrane performance for long-term use in water under some harsh operation conditions should also be carried out before practical applications.

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