INORGANIC CHEMISTRYFRONTIERS

REVIEW

Cite this: DOI: 10.1039/c4qi00230j

Received 15th December 2014,Accepted 3rd March 2015

DOI: 10.1039/c4qi00230j

rsc.li/frontiers-inorganic

Graphene and graphene oxide: advancedmembranes for gas separation and waterpurication

Quan Xu,*a Hong Xu,a Jiarui Chen,b Yunzu Lv,a Chenbo Dongc andTheruvakkattil Sreenivasan Sreeprasadc

Advanced membrane systems with excellent permeance are important for controllable separation pro-

cesses, such as gas separation and water purication. The ideal candidate materials should be very thin to

provide high permeance, be sti enough to withstand working under high applied pressure, with a large

surface area and micro- or nano-pore structure for excellent selectivity. Graphene oxide (GO) nanosheets

are graphene with oxygen-containing functional groups, obtained by treating graphite with strong oxidi-

zers. Graphene-based materials, by virtue of their high mechanical strength, large surface area, single-

atom-thick unique two-dimensional honeycomb lattice structure, and narrow pore distribution, provide

exciting opportunities to assemble novel types of advanced, ultra-thin, high-eciency membrane

devices. In this contribution, we discuss the progress made in the direction of using graphene oxide as

high-eciency membranes for gas separation and water purication. The primary focus will be on intro-

ducing the fabrication processes, exceptional properties, and innovative membrane applications of two-

dimensional graphene oxide materials for controllable separation processes. This state-of-the-art review

will provide a platform for understanding the intricate details of gas and water molecular transport

through laminar graphene oxide membranes, as well as a summary of the latest process in the eld.

Introduction

Separation is a critical industrial process that finds a broadrange of applications in natural gas purification, hydrogen pro-duction, and sea-water desalination.1,2 The low ecacy of theprevailing technologies necessitates the formulation ofadvanced separation techniques to solve urgent environmental

Quan Xu

Quan Xu received his Ph.D.degree from the Department ofMaterials Science and Engineer-ing, University of North Texas in2013, with Prof. Zhenhai Xia. Hejoined the faculty of the Instituteof New Energy, China Universityof Petroleum (Beijing) in 2014 asan assistant professor. Hisresearch interests focus on thedevelopment of membrane cata-lysis technology, and synthesis ofnanomaterials for environmentalapplications as well asbiotechnology.

Hong Xu

Hong Xu received her B.S. degreefrom Tongji University in 2012and her M.S. degree from theInstitute of New Energy, ChinaUniversity of Petroleum (Beijing).She is currently a Ph.D. studentat the State Key Lab of Heavy OilProcessing and her researchfocuses on separation processengineering.

aState Key Laboratory of Heavy Oil Processing, Institute of New Energy, China

University of Petroleum (Beijing), Beijing, 102249, China.

E-mail: xuquan@cup.edu.cnbDepartment of Chemical Engineering, Xian Jiaotong University, Xian, Shaanxi

710049, ChinacDepartment of Civil and Environmental Engineering, Rice University, Houston,

Texas 77005, USA

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and energy issues. In comparison to traditional distillation,3

absorption,4 adsorption5,6 or sublimation,7 membrane-basedtechnology exhibits considerable promise. The key advantagesof membrane-based strategies for small-to-medium scale gasseparation and water filtration applications are (i) overalleconomics, (ii) safety, (iii) comparatively environmentallyfriendly nature, and (iv) easy recycling.8,9 Currently, mem-branes made from polymers, zeolite and silicon are being suc-cessfully applied in industry.10 However, permeability andselectivity are two inherent trade-os for conventional mem-brane materials that limit their usage.

Recently, considerable interest has been aroused by carbon-based nanomaterials because of their easy accessibility, excellentmechanical properties, biocompatibility, and environmentalfriendliness.11,12 One-dimensional carbon nanomaterials (e.g.carbon nanotubes, CNTs) were initially believed to be promisingcandidate materials for membrane assembly due to their uniquehollow structure with open ends and extremely strong mechan-ical properties. However, there is a technical barrier in theassembly of vertically-aligned nanotube arrays with high densityon a large scale,13 and the fabrication of CNT-based separationmembranes remains a theoretical possibility. Another carbon-based nanomaterial with great mechanical properties, carbondiamond composed of amorphous sp3 hybridized carbon atoms,was also investigated for filtration applications. However, thelarge energy consumption, due to the low utilization eciency ofcarbon diamond material-based membranes devices, inhibitedtheir large scale industrial application.14 Thus, the developmentof more applicable and economical carbon nanomaterial-basedseparation membranes is urgently needed.

Graphene, a single-atom-thick sheet of sp2 hybridizedcarbon atoms arrayed in a honeycomb pattern, has opened thedoor for assembling separation membranes with improvedseparation capabilities.15 The concept and the preparation ofgraphene-based materials have been explored for over 100years. As early as in 1859, Brodie prepared graphitic oxide witha strong oxidizing mixture.16 In 1958, Hummers reported aredox method to prepare graphitic oxide.17 Since its first iso-lation in 2004, graphene has drawn increasing attention indiverse branches of science and technology owing to its excep-

tional physicochemical properties, including mono-atomicthickness, high mechanical strength, and significant chemicalinertness.18 With its ubiquitous properties, graphene has dis-played great potential for atomic permeation, water transpor-tation, and gas separation.1921 For mass production, user-controllable pore distribution, and application under high-pressure conditions, graphenes oxygen-containing analogue,graphene oxide (GO), is being considered for filtration/mem-brane fabrication and utilization. The possibility of facile andlarge scale production of GO and its unique properties hasopened up new opportunities for the production of advancedmembrane separation technology. GO-based membranes havebeen used in water treatment and liquid organic molecularseparation.22,23 It should be noted that the physicochemicalstatus of the oxygen-containing groups in GO sheets plays asignificant role in determining their separation performance.This oers several straightforward strategies to control poresize in the tiny channels of GO via tuning the types and formsof oxygen-containing groups.

In this mini-review paper, the application of graphene andGO under consideration for separation membrane applicationsis discussed. The discussion is divided into three parts. Thefirst part describes the application of graphene and its deriva-tives in gas separation, while the second part focuses on theirunique applications in ion sieving, as well as their role inwater purification with dierent mechanisms. Lastly, the finalpart of the review gives a summary and outlook of graphene-based separation membranes, the existing challenges and newdirections in this emerging research field. This critical reviewoers a comprehensive discussion of graphene-based mem-branes from the perspective of broad separation applications,as well as the advanced progress of such membranes withimproved separation performance.

Graphene and GO for gas and watermolecule separation

Compared to polymer, zeolite, and silica-based membranes,layered graphene and GO membranes display distinguishing

Jiarui Chen

Jiarui Chen is currently a seniorstudent at the Department ofChemical Engineering, XianJiaotong University. His researchinterest is nanomaterial manu-facture and characteristics.

Yunzu Lv

Yunzu Lv received his B.S. fromthe College of Science, ChinaUniversity of Petroleum(Huadong). He is currently agraduate student at the Instituteof New Energy, China Universityof Petroleum (Beijing). Hisresearch focuses on manufactur-ing membranes for water purifi-cation.

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characteristics including excellent stiness, capacity for large-scale production, and hydrophobic properties. Recent studieshave demonstrated that graphene-based membranes exhibit sig-nificant potential for gas2426 and water molecule separation.27

Graphene and GO for gaseousselectivity

A mono-layer of graphene is well known to resist the per-meation of small gaseous molecules (e.g. He),28 as the electrondensity of its hexagonal rings will repel the atoms and mole-cules trying to pass through them. Based on this, an atomiclayer of pristine graphene can be considered as a perfect gasbarrier material. Although graphene is impermeable to gases,nanoporous graphene (NPG), due to its high surface porosity,can eciently select gases. Hence, graphene, with its mona-tomic thickness, can potentially surpass the permeability andselectivity limits of conventional membranes and is consideredas the ideal membrane material. Theoretical computationalstudies revealed that NPG had higher selectivity and flow ratescompared to traditional polymer and silica-based membranes.For example, Jiang et al.29 (Fig. 1(a)) investigated the per-meability and selectivity of nitrogen-doped porous single-layergraphene and found that the graphene layer exhibited highselectivity (up to 108) toward H2CH4 gas flow after designingand adjusting the subnanometer-sized pores. In addition,Schrier30 simulated NPG with dierent functionalized poresfor separating He, Ne and CH4 and found the possibility ofhigh helium selectivity using a porous graphene membrane.Also, Hauser et al.31 pointed out that NPG with dierent func-tionalized graphene pores can eciently select 3He over 4He.Du et al.,32 via a simulated separation process for H2 and N2,reported that the H2 flow rate through NPG was linear inrelation to the pore size of graphene, while N2 flow rate wasindependent of it. Trinh et al.,33 with the help of the calculatedbonding energy between the molecule and graphene, analysedthe selectivity and self-diusion of CO2 and H2 on few-layergraphene (Fig. 1(b)) at dierent temperatures. It was suggested

that the selectivity of CO2 over H2 is five times greater at lowertemperatures compared to that at higher temperatures. Thethickness of the graphene also was found to regulate the self-diusion and flow rate of gases. The pore size, the nature ofthe functional groups at the edges of the pores, thickness,temperature, local defects and wrinkling (Fig. 1(c))34 are con-sidered as the key factors aecting the selectivity of pristinegraphene. Other mechanisms, such as the pore translocation-limited mechanism, surface adsorption mechanism andrearrangement reaction mechanism, are also intensively evalu-ated and discussed in the literature.3538

Since theoretical results illustrated the tremendous promiseof graphene and GO as next generation membranes (Fig. 2(b)),39

Chenbo Dong

Chenbo Dong received his B.S.from Beijing Forestry Universityin 2007 and M.S. from ChinaUniversity of Petroleum (Beijing)in 2010. He obtained his Ph.D.in 2014 from West Virginia Uni-versity. Now he works as a post-doctoral research associate atRice University. His researchfocuses on the development offunctional materials for biomedi-cal and environmental appli-cations. T. Sreenivasan Sreeprasad

T. S. Sreeprasad received hisPh.D. degree from the IndianInstitute of Technology Madras,India in 2011. He is an author of34 research articles and threebook chapters. He is currentlyworking as a postdoctoralresearch fellow at Rice Univer-sity. His research interests arecentered on low-dimensionalnanomaterials and quantumstructures, and their applicationsin sensing, energy, and environ-ment-related applications.

Fig. 1 (a) Creation of a nitrogen-functionalized pore within a graphenesheet. Color code: C, black; N, green; H, cyan.29 (b) Typical snapshot ofa gas mixture of CO2 and H2 in equilibrium with a graphite surface. Thetemperature is T = 300 K and the number of particles is N = 700. Thegreen, red and white colors represent carbon, oxygen, and hydrogenatoms, respectively.33 (c) Schematic illustration of possible gas transportthrough a few-layered, defective graphene membrane.34 (d) Large areaGO can be bent over 90 degrees.47 (e) Atomic force microscopy (AFM)image of a graphene/PTMSP membrane surface; wrinkles and defectsare clearly seen in the surface.34 (f ) Manufactured graphene nano-mesh;40 the pore size of the graphene sheet can be controlled.52

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several strategies have been developed to synthesize NPG toachieve a functional separation membrane. The most impor-tant methods include electron beam40,41 and ultraviolet-induced oxidative42 etching of holes on graphene surfaces, cre-ation of porous two-dimensional sheets from molecular build-ing blocks,43 and sandwiching/tethering of graphene withcarbon nanotubes to create 3D structures.44 The progress inexperimental processes allowed the successful transfer of largearea GO sheets onto dierent substrates under suitable con-ditions.45 Further, the ability to precisely control the thicknessand chemical functionalization of GO46 resulted in the devel-opment of facile approaches for fabricating GO-based mem-branes. Compared to graphite foils, the unique interlocking-tile arrangement of the nanoscale GO showed a higher fractureenergy (350 kJ m3) and high tensile strength (42 2 GPa).47

Although GO has high tensile strength and bending strength,it can still be folded over 90 degrees (Fig. 1(d)), and this sti-yet-flexible property of GO makes it a promising membrane.Moreover, various routes have been successfully reported forthe production of large area graphene48,49 and GO50 with accu-rate evaluation of its mechanical properties.51 For example,Kim et al.34 successfully transferred a large area of grapheneand GO onto a polymer substrate and measured the transportand selectivity of O2N2 and CO2N2 on graphene and itsderivative-based membrane (Fig. 1(e)). Selective gas diusioncan be achieved by controlling the gas flow channels andpores via dierent stacking processes. Heating the system andapplying pressure could also result in a better selectivity of thesystem. During the flux process, gases interact with both poresand interlayers. Thus, functional groups inserted at the poreedges or between layers can further enhance the selectivity ofthe system. Other eective methods are also reported for theformation of structured pores in graphene membranes. Russoet al.41 successfully constructed NPG with pore radii as smallas 3 with atomic precision using ion beam exposure. Baiet al.52 employed block copolymer lithography to prepare NPGwith neck widths as low as 5 nm. Li et al.53 created GO with athickness approaching 1.8 nm, supported on porous anodicaluminium oxide (AAO). These membranes showed mixture

separation selectivities (as high as 3400 and 900) for H2CO2and H2N2 mixtures. Bai et al.

52 made a graphene nanomeshand could precisely control the pore size and pattern after aseries of processes (Fig. 1(f )).

Traditional membranes, such as silica, polymer and zeolite,follow the rule that selectivity is a product of diusivity andsolubility.10 Numerical attempts have been developed forincreasing those two parameters to obtain the desired per-formance. However, this law is no longer valid for grapheneand GO. For single-layer graphene, the selectivity is closelyaected by the pore size. The traditional concept of thicknessis no longer linearly related to the diusion speed and needsto be reconsidered. Although graphene is one of the strongestmaterials in the world, due to its unique 2D ultrathin structureit has excellent flexibility and a high diusion coecient. Forexample, Celebi et al.54 deposited pre-synthesized bi-layers ofgraphene on a SiNx frame, punctured with 49 pores each of4 um in diameter, forming graphene with a calculated thicknessof 1 nm. The H2 separation permeance of the porous graphenemembrane is much higher than those of membranes made ofpolymer, silicon or composite materials (Fig. 2(a)). Defects andpore size are considered to be two critical parameters whichdetermine the selectivity. Pressure and temperature, which cancontrol the eusion rate, will also aect the permeation.33

Graphene and GO as membranes forwater purication

Water flux and the rejection of dissolved impurities are twomain aspects governing the productivity and separation per-formance of graphene and GO-based membranes in waterpurification processes. When the pore diameter is larger than0.8 nm, graphene membranes show higher water flux thanCNT membranes due to higher velocity in the centre region.55

An interesting study indicated that graphene nanopores areable to block salt ions with 23 orders of magnitude greaterwater permeability than commercial reverse osmosis mem-branes.56 However, the helium-impermeable GO membraneallows unimpeded water permeation.57 Since unusual mole-cular behaviour occurs on nanometer-length scales, variousmechanisms of water permeation through the layer-by-layerand porous microstructure of the GO membrane are proposedusing molecular dynamics (MD) simulations. Nair et al.57

attributed the anomalous water permeation to low frictionbetween the water monolayer and pristine graphene regionsand proposed a capillary-driven flow mechanism. Based on thework of Nair et al., Boukhvalov et al.58 suggested that interlayerspaces determine the formation of a hexagonal ice bilayerbetween flakes in pristine regions and the melting transitionof ice at the edges, and are responsible for perfect water per-meation and the water flux. Since monolayer graphene sheetsare hydrophobic with a water contact angle of 95100,59 Weiet al.60 investigated the wetting properties of GO and foundthat the patterns of the oxidized region and the morphologicalcorrugation of the sheet critically influence the spreading of

Fig. 2 (a) Comparison of H2CO2 separation performances of porousgraphene membranes (7.6 nm pore diameter, with 4.0% porosity) andother membranes: graphene oxide (GO),3453 poly(1-trimethylsilyl-1-propyne) (PTMSP), polyetherimide (PEI), carbon molecular sieve (CMS),zeolite, silica, metalorganic framework (MOF), and SiC.54 (b) Simulationresult of CO2 ow between GO layers.

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water droplets (Fig. 3(a)). Huang et al.61 recently developed anadvanced filter membrane with a superior separation perform-ance by constructing nanostrand-channelled GO ultrafiltrationmembranes with a narrow pore size distribution (35 nm). Theenhanced viscous water flow through the channels was attribu-ted to the intricate porous structure and significantly reducedchannel length in the device. Wei and co-authors also attribu-ted the enhanced water flow in graphenic membranes to theirunique porous microstructures and pointed out that the side-pinning eect caused by H-bonds between water moleculesand oxidized regions could inhibit fast water transport in gra-phene channels, and enhanced water flow was prominentlyattributed to the porous microstructure.62 Therefore, in orderto improve water permeability, a small sheet size, high densityof inter-edge spaces, and wide nanochannels are suggested inGO membrane fabrication.62 The interlayer channel size canbe tuned by intercalating dierent-sized cross-linkers.63 Fur-thermore, Xu et al.64 quantified the nature of interlayer flowand concluded that the chemical functionalization and relax-ation of nano-confinement in GO can cause the breakdown offlow enhancement. On the other hand, the eect of membranestructure and operation conditions on water flux in grapheneand GO-based membranes have been explored both via simu-lation and experiments. By using computational methods,David et al.65 proposed that water flux through monolayer gra-phene has a linear relationship with pore area. Moreover, itwas understood that hydrophilic pores increased water per-meation due to H-bonding between water molecules and poreedges, and quantitatively the hydrophilic edges of the porescontribute more to high water flux compared to the pore areaand size of graphenic membranes. For traditional polymericmembranes, flux is inversely proportional to the thickness.However, the same argument seems invalid for GO mem-

branes. Meng Hu et al.66 synthesized GO membranes withdierent numbers of layers and tested the water separationperformance and found no obvious correlation between waterflux and the number of layers. Yi Han et al.,67 via an investi-gation on base-reflux-induced reduced GO, concluded thatwater flux increased linearly with pressure under 7 bar anddecreased exponentially with GO loading. The above twostudies showed contradictory conclusions on the relationshipbetween GO thickness and water flux, and reiterated that thereis a need to comprehensively investigate this aspect.

The rejection of ions and organic dye molecules by gra-phene and GO membranes is also intensively studied. Theselectivity of this process is primarily attributed to size exclu-sion and interactions (including chemical and electrostaticones) with functional groups. Grossman et al.65 proposed thehigh performance of monolayer graphene to filter out NaClfrom water via their computational investigation. Small pores,low pressure, and hydrophobic pores reject salts more ecien-tly due to direct size exclusion, while the larger eectivevolume of ions and the lack of H-bonding can lead to a higherenergy barrier to ionic passage. Hu et al.66 reported that GOmembranes display significant rejection of monovalent anddivalent salts and moderate or high rejection of organic dyes.Similar results are also observed in the work of Han et al.67

The high rejection for organic dyes is ascribed to size exclusionand electrostatic interaction, and the rejection of ions toDonnans exclusion. Huang et al.68 investigated the eects ofsalt concentration, pH, and pressure on the controlled separ-ation of small molecules through GO membranes. All thesefactors modulated the separation performance due to theirrole in changing the interlayer spacing. Specially, low pH (

through a pervaporation process. The separation process wasbelieved to follow a sorptiondiusion mechanism. Tanget al.74 also used a pervaporation process for dehydration ofethanol using GO thin films. The selectivity obtained from thebinary-component test was much lower than the ideal valuecalculated from single-component feed analysis. This can beattributed to the increased ethanol flux through the thin filmscaused by the enlarged interlayer space due to the H-bondingbetween water molecules and the functional groups on the GOsurface.

Due to their excellent intrinsic mechanical strength,75 highchemical stability,76 high antibacterial activity,77 and exquisiteantifouling properties,47 GO membranes demonstrated signifi-cant promise for applications in water purification in variousstudies. By adjusting the pore size and nanochannels viainduction and modification of surface oxygen-containinggroups, GO membranes can be oriented towards a narrow poresize distribution, which is advantageous for precise sievingbased on the size exclusion mechanism.

Conclusions and outlook

In summary, membranes based on two-dimensional mono-layer graphene and its functionalized analogue, GO, with theirultrafast permeance, outstanding mechanical properties aswell as exceptional energy-eciency, are increasingly comingto the fore as promising candidates for precise and selectivegas separation and water purification processes (Fig. 4).

Although graphene and GO demonstrate excellent selecti-vity and superior performance against multiple gas mixtures,several challenges still exist in their path to industrial appli-cation. Firstly, the grain boundary, defects, impurities,polymer, or molecular residues can block the pores on gra-phene or GO sheets. This will result in the dramatic reductionof the flow rate. Hence, the mixture needs to be carefully puri-fied before entering graphene membrane systems. The humid-ity is also reported to aect the selectivity. In addition, forgraphene or GO-based gas separation membranes, the selecti-vity for H2H2S, CO2CH4, or mixtures of three or more gases

is still not well characterized. Thus a comprehensive analysisof the parameters that influence the process is necessarybefore predicting the mechanism of selectivity. These studiesare expected to attract a great deal of interest from industry,especially for natural gas and shale gas industries, due to theimportance of the separation process in their sustainability.Secondly, the possibility of finding a large number of inherentdefects in the products hurts the chance for large scale indus-trial application of graphene and GO. Although a single layerof graphene has a Youngs modulus of up to 1 TPa, local bondbreaking and bond rotation at the crack tip of the graphene50

could lead to dramatic loss of mechanical properties in pro-ducts. This can render the membranes less robust and morefragile, and even create unexpected large pores during use,and result in the failure of the separation system. Since poresize is the most important parameter in controlling the per-formance of graphenic membrane systems, GO can selectivelyseparate gas mixtures though modulation of its layer-to-layerdistance and the chemical groups on the surfaces and layerinterfaces as well. The third challenge is the cost. Previously,the high cost of production of GO was retarding its large scaleapplications. However, the advent of newer technologies hasdecreased the cost of graphene and GO and more and moremethods are being developed to fabricate defect-free, largerarea graphene with controllable pore structure/size and inter-layer distance. The authors envisage that this particular fieldwill attract increased interest from academia and industry inthe coming years.

As for water purification, although considerable progresshas been achieved in the fundamental understanding andspecialized industrial applications of graphene and GO-basedmembranes, there are still several promising topics that areworth exploring. For example, the precise design of user-con-trolled GO membranes, including the pattern of oxidizedregions, the interlayer space, the pore size, modification ofpore chemistry, and the number of graphene layers, is still notentirely successful. The lessons learned about the parametersthat determine the small molecule permeation and separationperformance can be transplanted to fabricate solid-state ionicdevices or microfluidic devices. Likewise, detailed investi-gations on the eect of the interlayer space or surface defecton water flux, and whether size exclusion or interaction con-tributes more to the selectivity, are also essential. A compro-mise between water flux and selectivity may be made todetermine the optimal membrane fabrication parameters. Inorder to achieve higher water flux without sacrificing selecti-vity, modifying the membrane with dierent atoms and func-tional groups to adjust water anity could be employed. Inaddition, the swelling of GO in aqueous solutions needs to beregulated to achieve the expected selectivity. Moreover, forindustrial processes, the operating pressure is an importantfactor which needs to be considered. Operating without apply-ing a dierential pressure will be highly beneficial. Further-more, if high purity of the product (e.g. to produce high purityacid from a high-concentration FeCl3 solution) is required,pre-treatment or post-treatment will be needed, whichFig. 4 The outlook of the research on graphene and GO membranes.

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increases energy consumption and total cost. Therefore,additional eorts are needed for minimizing the dierencebetween theoretical and experimental results on water/iontransport mechanisms across GO nanosheets in water. Thereis also a need to consider and solve the contamination issuefor graphene or GO-based membranes. The eects of adsorp-tion of metals and organic dyes, ionic clogging inside thecapillaries, and coordination occupation on the surface of oxi-dized regions needs further investigation and optimization. Tofurther improve the outstanding antifouling capability of GO-based membranes, it is crucial to tune the balance betweenthe fouling rejection and flux decline. Improving the antimi-crobial eciency and mechanical strength of the freestandingmembranes can be the key to the successful balancing act.

Compared to GO, graphene is simpler to design, more toxicto microbacteria,78 and more resistant to swelling in water,ionic clogging inside the capillaries and coordination occu-pation on the surface of oxidized regions. Thus, it is morecompetitive compared to GO. For large scale productivity, andindustrial use, novel reduction methods need to be developed.A suitable substrate is imperative to avoid dispersion in theliquid phase and to ensure water transportation. Experimentsillustrated that the water flux is approximately linear inrelation to the applied pressure, and so graphene membranescannot be used without energy input. Study also showed thateven at low pressure (

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