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Jian et al., Sci. Adv. 2020; 6 : eaay3998 5 June 2020

SCIENCE ADVANCES | RESEARCH ARTICLE

1 of 9

MATERIALS SCIENCE

Ultrathin water-stable metal-organic framework

membranes for ion separation

Meipeng Jian1, Ruosang Qiu1, Yun Xia1, Jun Lu1, Yu Chen2, Qinfen Gu3*, Ruiping Liu4,5,

Chengzhi Hu4, Jiuhui Qu4,5, Huanting Wang1, Xiwang Zhang1*

Owing to the rich porosity and uniform pore size, metal-organic frameworks (MOFs) offer substantial advantages

over other materials for the precise and fast membrane separation. However, achieving ultrathin water-stable

MOF membranes remains a great challenge. Here, we first report the successful exfoliation of two-dimensional

(2D) monolayer aluminum tetra-(4-carboxyphenyl) porphyrin framework (termed Al-MOF) nanosheets. Ultrathin

water-stable Al-MOF membranes are assembled by using the exfoliated nanosheets as building blocks. While

achieving a water flux of up to 2.2 mol m−2 hour−1 bar−1, the obtained 2D Al-MOF laminar membranes exhibit re-

jection rates of nearly 100% on investigated inorganic ions. The simulation results confirm that intrinsic nanopores

of the Al-MOF nanosheets domain the ion/water separation, and the vertically aligned aperture channels are the

main transport pathways for water molecules.

INTRODUCTION

Ion separation with energy-efficient and environment-friendly mem-

branes is essential in water environmental fields, e.g., wastewater

recycling and seawater and brackish water desalination (1). Polymers

are, by far, the most widespread membrane materials, largely owing

to their easy processability and low cost (2). However, traditional

polymeric membranes for ion separation from water are usually with

a dense-selective layer, leading to the insurmountable permeability-

selectivity trade-off, governed by the solution-diffusion model (3).

In contrast, nanoporous membranes where nanopores act as the

sieving role may overcome the limitation (4,5). In this regard, re-

cent advances in nanoporous membranes, such as porous polymers,

nanotube, zeolite, and aquaporin-based membranes, have witnessed

substantial progress (6–8). The advancements collectively recognized

the potential of nanoporous membranes in enhancing ion sieving

capacity if the separation channels are properly designed. Neverthe-

less, most nanoporous membranes are usually thick at micrometer

scale and are formed by discrete channels, hampering membrane

permeability (9,10). Recently, two-dimensional (2D) materials, such

as graphene oxide (GO), reduced GO, MoS2, etc., have recently

emerged as building blocks for membrane synthesis (11–13). These

2D nanosheets have constructed a new class of membranes with an

ultrathin thickness, in which the interlayer space between adjacent

nanosheets acts as selective nanochannels for ion sieving (14). De-

spite the thin architecture and special transport channels of these 2D

laminar membranes, there are still deficiencies in separation perform-

ance, such as high transport tortuosity and insecure/improper

interlayer distance (15,16).

Metal-organic frameworks (MOFs) are a scientifically compelling

and evolving class of highly porous materials (17). Thus, MOFs are

expected to be one of the most promising materials for separation

membranes (18,19). In particular, the use of 2D MOF nanosheet-

based membranes for gas separation holds the promise of making a

breakthrough in achieving a simultaneous increase of both permea-

bility and selectivity (20). However, it remains a daunting challenge

to fabricate ultrathin MOF membranes (less than 100 nm) for water-

related processing, since most reported MOF membranes are typically

thick because of 3D crystal constitution and suffer from insufficient

hydrolytic stability (21,22).

Here, we report the preparation of water-stable monolayer alumi-

num tetra-(4-carboxyphenyl)porphyrin framework (termed Al-MOF)

nanosheets and demonstrate their excellence as building materials

for membranes in ion separation from water. Exfoliated Al-MOF

nanosheets exhibit a long-term structural robustness in aqueous

environment and can form a laminar membrane via a facile vacuum

filtration on porous substrates. The resulting 2D Al-MOF laminar

membrane exhibits an extremely low permeability to tested ions

(~3.3×10−6mol m−2hour−1bar−1) but achieves water fluxes of up

to 2.2 mol m−2hour−1bar−1. Overall, the 2D MOF membranes out-

perform the most reported 2D laminar membranes on the water/ion

selectivity. In addition, the interlayer distance in the Al-MOF lami-

nar membrane is self-locked via parallel - interactions, leading to

a steady performance for more than 750 hours.

RESULTS AND DISCUSSION

Bulk-type Al-MOF crystals were obtained through a modified solvent-

thermal method (23). The corresponding scanning electron micro-

scopy (SEM) and atomic force microscopy (AFM) images (Fig.1A

and fig. S1, A and B) show a layered crystalline structure. Consider-

ing the weak interlayer bonding in the [0k0] direction of the bulk-

type Al-MOF crystals, a facile sonication approach was used to

successfully exfoliate them into 2D nanosheets (Method section and fig.

S1C). Impressively, the convenient exfoliation route can reach a high

nanosheet yield of approximately 90% (fig. S1D). The 2D ultrathin

morphology of exfoliated Al-MOF nanosheets is revealed by trans-

mission electron microscopy (TEM) images (Fig.1B and fig. S2A).

More than 80% of the Al-MOF nanosheets have a lateral size between

200nm and 2 m (fig. S2B). After the exfoliation, the Al-MOF

1Department of Chemical Engineering, Monash University, Clayton, Victoria 3800,

Australia. 2Monash Centre for Electron Microscopy, Monash University, Clayton, Victoria

3800, Australia. 3Australian Synchrotron (ANSTO), Clayton, Victoria, 3168, Australia.

4State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-

Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China. 5Center

for Water and Ecology, State Key Joint Laboratory of Environment Simulation and

Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China.

*Corresponding author. Email: xiwang.zhang@monash.edu (X.Z.); qinfeng@ansto.

gov.au (Q.G.)

The Authors, some

rights reserved;

exclusive licensee

American Association

for the Advancement

of Science. No claim to

original U.S. Government

Works. Distributed

under a Creative

Commons Attribution

NonCommercial

License 4.0 (CC BY-NC).

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Jian et al., Sci. Adv. 2020; 6 : eaay3998 5 June 2020

SCIENCE ADVANCES | RESEARCH ARTICLE

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nanosheet suspension shows an excellent dispersibility, and the col-

loidal solution can be reserved for more than 2 months (fig. S2C).

The thickness of Al-MOF nanosheets was measured by AFM to be

around 1.9nm (Fig.1C), which is close to the theoretical height

(~1.35 nm) of a monolayer Al-MOF nanosheet (Fig.1D).

The crystallinity of Al-MOF nanosheets was examined by syn-

chrotron x-ray powder diffraction (XRD). The crystal structures of

Al-MOF crystals viewed down the [001] direction and the mono-

layer Al-MOF nanosheet viewed from the [010] direction are illus-

trated in Fig.1(DandE,respectively). The observed XRD pattern

of exfoliated Al-MOF nanosheets fit well with the calculated pat-

terns of monolayer Al-MOF nanosheets (Fig.1F), confirming their

inherent structural features of Al-MOF crystal. In addition, a selected-

area electron diffraction pattern gives individual diffraction spots,

demonstrating the single-crystal nature of the exfoliated Al-MOF

nanosheets (fig. S3). Compared with Al-MOF bulks, XRD peaks of

Al-MOF nanosheets are weak, and a few peaks even disappear (fig. S4),

largely owing to the loss of diffraction signals in the out-of-plane

direction and the nonplanar shape of the nanosheets (24). X-ray

photoelectron spectroscopy (XPS), energy-dispersive spectroscopy

(EDS), ultraviolet-visible spectra (UV-Vis), attenuated total reflec-

tance Fourier transform infrared spectroscopy (ATR-FTIR), and

thermogravimetric analysis (TGA) characterizations in figs. S5 to

S9 further reveal that the exfoliated nanosheets preserve the struc-

tural integrity.

Exfoliated Al-MOF nanosheets also feature a microporous struc-

ture (type I isotherm) and give a specific surface area of 602m2 g−1

(fig. S10A). Meanwhile, it displays authentic angstrom-size pores

from experimental isotherm analyses in Fig.1G. However, the pore

distribution plot of Al-MOF nanosheets shows a slight difference

from that of its bulk counterpart, which could be caused by the ex-

foliation effect and inevitable restacking of dried nanosheets along

the [0k0] direction (fig. S10B). To assess the water stability, Al-MOF

nanosheets were soaked in water for a month. The water-treated

Al-MOF nanosheets exhibited identical XRD patterns to their ini-

tial status (fig. S11). Furthermore, the N2 adsorption-desorption

Fig. 1. Synthesis and structure of Al-MOF nanosheets. (A) SEM image of the representative Al-MOF bulk crystals. (B) TEM image of exfoliated Al-MOF nanosheets. (C) AFM

topographical image of Al-MOF nanosheets on a silicon wafer. Inset is the corresponding height profile. (D) Single monolayer Al-MOF nanosheet viewed from the [001]

direction. (E) Crystal structure of Al-MOF viewed down the [010] direction. The Al coordination polyhedra are depicted in blue, whereas nitrogen, oxygen, and carbon

atoms are shown in purple, red, and gray, respectively. H atoms are omitted for clarity. (F) Rietveld refinement of the synchrotron XRD data of Al-MOF nanosheets. a.u.,

arbitrary units. (G) Pore-size distribution of Al-MOF nanosheets from N2 adsorption-desorption measurement.

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Jian et al., Sci. Adv. 2020; 6 : eaay3998 5 June 2020

SCIENCE ADVANCES | RESEARCH ARTICLE

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isotherm and pore distribution of the Al-MOF nanosheets, after

being immerged in water, were both similar to those of the pristine

sample (fig. S12). Likewise, porphyrin ligand and Al3+ were not

present in the filtrate after 1-month water stability test (fig. S13).

These results confirm the unchanged crystallinity of Al-MOF nano-

sheets after prolonged immersion in water. In another aspect, Al-MOF

nanosheets kept their characteristic diffraction peaks after being

exchanged with NaCl (fig. S14A), which indicates that Al-MOF

nanosheets could withstand exposure to inorganic ions, having a

desired chemical endurance. In situ high-temperature synchrotron

XRD characterization was also conducted on Al-MOF nanosheets

from 50° to 190°C, and no obvious variations were observed on the

phase transformations and crystalline lattices with the increase in

temperature, which elucidates the pore rigidity in Al-MOF nano-

sheets (fig. S14, B to D).

A 2D Al-MOF laminar membrane (Fig.2A) was assembled by

vacuum filtration of a diluted Al-MOF nanosheet suspension (fig.

S15) using anodic aluminum oxide (AAO) support with a pore size

of 100nm. In an apparent contrast to bare AAO support (Fig.2B),

a top-view SEM image of a~10-nm-thick membrane shows a uni-

form coverage of Al-MOF nanosheets on the surface of AAO sup-

port, and no visible defects were observed (Fig.2C). The continuous

and flat Al-MOF laminar membrane was visualized by AFM and

cross-sectional SEM characterizations (Fig.2D and fig. S16). In ad-

dition, the membrane exhibits a hydrophilic character, demonstrated

by a water contact angle of 44° (fig. S17). By controlling the loading

of Al-MOF nanosheets, the thickness of the membranes can be pre-

cisely tuned from a few nanometers to micrometers (Fig.2E and

figs. S18 and S19). Notably, a typical homogeneous laminar struc-

ture is seen when the membrane thickness reaches 500nm (Fig.2E).

Fig. 2. Characterizations of Al-MOF membranes. (A) Digital photo of an as-prepared 100-nm-thick Al-MOF laminar membrane on AAO substrate. (B) SEM image of a

bare AAO substrate. (C) SEM image of a sub–10-nm-thick Al-MOF laminar membrane on AAO substrate. The visibility of substrate background elucidates the ultrathin

coverage. (D) Cross-sectional overview of a 100-nm-thick Al-MOF laminar membrane on AAO substrate. (E) Magnified cross-sectional views of 2D Al-MOF membranes

with different thicknesses. Membranes less than 100 nm (green and gold) show a compact stacking, whereas the membrane at a thickness of 500 nm (purple) apparently

shows typical laminar structure. Scale bars, 500 nm. (F) Cross-sectional TEM image of the 2D Al-MOF laminar membrane. (G) GIXRD pattern of the Al-MOF laminar mem-

brane. The pattern was acquired from a thick membrane (~20 m) due to the detection limit. The sharp (0k0) phase peak at 2 = 7.6° indicates an average value of 6.0 Å.

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Jianetal.,Sci.Adv.2020;6:eaay39985June2020SCIENCEADVANCESRESEARCHARTICLE1of9MATERIALSSCIENCEUltrathinwater-stablemetal-organicframeworkmembranesforionseparationMeipengJian1,RuosangQiu1,YunXia1,JunLu1,YuChen2,QinfenGu3,RuipingLiu4,5,ChengzhiHu4,JiuhuiQu4,5,HuantingWang1,XiwangZhang1Owingtotherichporosityanduniformporesize,metal-organicframeworks(MOFs)offersubstantialadvantagesoverothermaterialsforthepreciseandfastmembraneseparation.However,achievingultrathinwater-stableMOFmembranesremainsagreatchallenge.Here,wefirstreportthesuccessfulexfoliationoftwo-dimensional(2D)monolayeraluminumtetra-(4-carboxyphenyl)porphyrinframework(termedAl-MOF)nanosheets.Ultrathinwater-stableAl-MOFmembranesareassembledbyusingtheexfoliatednanosheetsasbuildingblocks.Whileachievingawaterfluxofupto2.2molm−2hour−1bar−1,theobtained2DAl-MOFlaminarmembranesexhibitre-jectionratesofnearly100%oninvestigatedinorganicions.ThesimulationresultsconfirmthatintrinsicnanoporesoftheAl-MOFnanosheetsdomaintheion/waterseparation,andtheverticallyalignedaperturechannelsarethemaintransportpathwaysforwatermolecules.INTRODUCTIONIonseparationwithenergy-efficientandenvironment-friendlymem-branesisessentialinwaterenvironmentalfields,e.g.,wastewaterrecyclingandseawaterandbrackishwaterdesalination(1).Polymersare,byfar,themostwidespreadmembranematerials,largelyowingtotheireasyprocessabilityandlowcost(2).However,traditionalpolymericmembranesforionseparationfromwaterareusuallywithadense-selectivelayer,leadingtotheinsurmountablepermeability-selectivitytrade-off,governedbythesolution-diffusionmodel(3).Incontrast,nanoporousmembraneswherenanoporesactasthesievingrolemayovercomethelimitation(4,5).Inthisregard,re-centadvancesinnanoporousmembranes,suchasporouspolymers,nanotube,zeolite,andaquaporin-basedmembranes,havewitnessedsubstantialprogress(6–8).Theadvancementscollectivelyrecognizedthepotentialofnanoporousmembranesinenhancingionsievingcapacityiftheseparationchannelsareproperlydesigned.Neverthe-less,mostnanoporousmembranesareusuallythickatmicrometerscaleandareformedbydiscretechannels,hamperingmembranepermeability(9,10).Recently,two-dimensional(2D)materials,suchasgrapheneoxide(GO),reducedGO,MoS2,etc.,haverecentlyemergedasbuildingblocksformembranesynthesis(11–13).These2Dnanosheetshaveconstructedanewclassofmembraneswithanultrathinthickness,inwhichtheinterlayerspacebetweenadjacentnanosheetsactsasselectivenanochannelsforionsieving(14).De-spitethethinarchitectureandspecialtransportchannelsofthese2Dlaminarmembranes,therearestilldeficienciesinseparationperformance,suchashightransporttortuosityandinsecure/improperinterlayerdistance(15,16).Metal-organicframeworks(MOFs)areascientificallycompellingandevolvingclassofhighlyporousmaterials(17).Thus,MOFsareexpectedtobeoneofthemostpromisingmaterialsforseparationmembranes(18,19).Inparticular,theuseof2DMOFnanosheet-basedmembranesforgasseparationholdsthepromiseofmakingabreakthroughinachievingasimultaneousincreaseofbothpermea-bilityandselectivity(20).However,itremainsadauntingchallengetofabricateultrathinMOFmembranes(lessthan100nm)forwater-relatedprocessing,sincemostreportedMOFmembranesaretypicallythickbecauseof3Dcrystalconstitutionandsufferfrominsufficienthydrolyticstability(21,22).Here,wereportthepreparationofwater-stablemonolayeralumi-numtetra-(4-carboxyphenyl)porphyrinframework(termedAl-MOF)nanosheetsanddemonstratetheirexcellenceasbuildingmaterialsformembranesinionseparationfromwater.ExfoliatedAl-MOFnanosheetsexhibitalong-termstructuralrobustnessinaqueousenvironmentandcanformalaminarmembraneviaafacilevacuumfiltrationonporoussubstrates.Theresulting2DAl-MOFlaminarmembraneexhibitsanextremelylowpermeabilitytotestedions(~3.3×10−6molm−2hour−1bar−1)butachieveswaterfluxesofupto2.2molm−2hour−1bar−1.Overall,the2DMOFmembranesout-performthemostreported2Dlaminarmembranesonthewater/ionselectivity.Inaddition,theinterlayerdistanceintheAl-MOFlami-narmembraneisself-lockedviaparallel-interactions,leadingtoasteadyperformanceformorethan750hours.RESULTSANDDISCUSSIONBulk-typeAl-MOFcrystalswereobtainedthroughamodifiedsolvent-thermalmethod(23).Thecorrespondingscanningelectronmicroscopy(SEM)andatomicforcemicroscopy(AFM)images(Fig.1Aandfig.S1,AandB)showalayeredcrystallinestructure.Consider-ingtheweakinterlayerbondinginthe[0k0]directionofthebulk-typeAl-MOFcrystals,afacilesonicationapproachwasusedtosuccessfullyexfoliatetheminto2Dnanosheets(Methodsectionandfig.S1C).Impressively,theconvenientexfoliationroutecanreachahighnanosheetyieldofapproximately90%(fig.S1D).The2DultrathinmorphologyofexfoliatedAl-MOFnanosheetsisrevealedbytrans-missionelectronmicroscopy(TEM)images(Fig.1Bandfig.S2A).Morethan80%oftheAl-MOFnanosheetshavealateralsizebetween200nmand2m(fig.S2B).Aftertheexfoliation,theAl-MOF1DepartmentofChemicalEngineering,MonashUniversity,Clayton,Victoria3800,Australia.2MonashCentreforElectronMicroscopy,MonashUniversity,Clayton,Victoria3800,Australia.3AustralianSynchrotron(ANSTO),Clayton,Victoria,3168,Australia.4StateKeyLaboratoryofEnvironmentalAquaticChemistry,ResearchCenterforEco-EnvironmentalSciences,ChineseAcademyofSciences,Beijing100085,China.5CenterforWaterandEcology,StateKeyJointLaboratoryofEnvironmentSimulationandPollutionControl,SchoolofEnvironment,TsinghuaUniversity,Beijing100084,China.Correspondingauthor.Email:xiwang.zhang@monash.edu(X.Z.);qinfeng@ansto.gov.au(Q.G.)Copyright©2020TheAuthors,somerightsreserved;exclusivelicenseeAmericanAssociationfortheAdvancementofScience.NoclaimtooriginalU.S.GovernmentWorks.DistributedunderaCreativeCommonsAttributionNonCommercialLicense4.0(CCBY-NC).Downloadedfromhttps://www.science.orgonOctober18,2021Jianetal.,Sci.Adv.2020;6:eaay39985June2020SCIENCEADVANCESRESEARCHARTICLE2of9nanosheetsuspensionshowsanexcellentdispersibility,andthecol-loidalsolutioncanbereservedformorethan2months(fig.S2C).ThethicknessofAl-MOFnanosheetswasmeasuredbyAFMtobearound1.9nm(Fig.1C),whichisclosetothetheoreticalheight(~1.35nm)ofamonolayerAl-MOFnanosheet(Fig.1D).ThecrystallinityofAl-MOFnanosheetswasexaminedbysyn-chrotronx-raypowderdiffraction(XRD).ThecrystalstructuresofAl-MOFcrystalsvieweddownthe[001]directionandthemono-layerAl-MOFnanosheetviewedfromthe[010]directionareillus-tratedinFig.1(DandE,respectively).TheobservedXRDpatternofexfoliatedAl-MOFnanosheetsfitwellwiththecalculatedpat-ternsofmonolayerAl-MOFnanosheets(Fig.1F),confirmingtheirinherentstructuralfeaturesofAl-MOFcrystal.Inaddition,aselected-areaelectrondiffractionpatterngivesindividualdiffractionspots,demonstratingthesingle-crystalnatureoftheexfoliatedAl-MOFnanosheets(fig.S3).ComparedwithAl-MOFbulks,XRDpeaksofAl-MOFnanosheetsareweak,andafewpeaksevendisappear(fig.S4),largelyowingtothelossofdiffractionsignalsintheout-of-planedirectionandthenonplanarshapeofthenanosheets(24).X-rayphotoelectronspectroscopy(XPS),energy-dispersivespectroscopy(EDS),ultraviolet-visiblespectra(UV-Vis),attenuatedtotalreflec-tanceFouriertransforminfraredspectroscopy(ATR-FTIR),andthermogravimetricanalysis(TGA)characterizationsinfigs.S5toS9furtherrevealthattheexfoliatednanosheetspreservethestruc-turalintegrity.ExfoliatedAl-MOFnanosheetsalsofeatureamicroporousstruc-ture(typeIisotherm)andgiveaspecificsurfaceareaof602m2g−1(fig.S10A).Meanwhile,itdisplaysauthenticangstrom-sizeporesfromexperimentalisothermanalysesinFig.1G.However,theporedistributionplotofAl-MOFnanosheetsshowsaslightdifferencefromthatofitsbulkcounterpart,whichcouldbecausedbytheex-foliationeffectandinevitablerestackingofdriednanosheetsalongthe[0k0]direction(fig.S10B).Toassessthewaterstability,Al-MOFnanosheetsweresoakedinwaterforamonth.Thewater-treatedAl-MOFnanosheetsexhibitedidenticalXRDpatternstotheirini-tialstatus(fig.S11).Furthermore,theN2adsorption-desorptionFig.1.SynthesisandstructureofAl-MOFnanosheets.(A)SEMimageoftherepresentativeAl-MOFbulkcrystals.(B)TEMimageofexfoliatedAl-MOFnanosheets.(C)AFMtopographicalimageofAl-MOFnanosheetsonasiliconwafer.Insetisthecorrespondingheightprofile.(D)SinglemonolayerAl-MOFnanosheetviewedfromthe[001]direction.(E)CrystalstructureofAl-MOFvieweddownthe[010]direction.TheAlcoordinationpolyhedraaredepictedinblue,whereasnitrogen,oxygen,andcarbonatomsareshowninpurple,red,andgray,respectively.Hatomsareomittedforclarity.(F)RietveldrefinementofthesynchrotronXRDdataofAl-MOFnanosheets.a.u.,arbitraryunits.(G)Pore-sizedistributionofAl-MOFnanosheetsfromN2adsorption-desorptionmeasurement.Downloadedfromhttps://www.science.orgonOctober18,2021Jianetal.,Sci.Adv.2020;6:eaay39985June2020SCIENCEADVANCESRESEARCHARTICLE3of9isothermandporedistributionoftheAl-MOFnanosheets,afterbeingimmergedinwater,werebothsimilartothoseofthepristinesample(fig.S12).Likewise,porphyrinligandandAl3+werenotpresentinthefiltrateafter1-monthwaterstabilitytest(fig.S13).TheseresultsconfirmtheunchangedcrystallinityofAl-MOFnanosheetsafterprolongedimmersioninwater.Inanotheraspect,Al-MOFnanosheetskepttheircharacteristicdiffractionpeaksafterbeingexchangedwithNaCl(fig.S14A),whichindicatesthatAl-MOFnanosheetscouldwithstandexposuretoinorganicions,havingadesiredchemicalendurance.Insituhigh-temperaturesynchrotronXRDcharacterizationwasalsoconductedonAl-MOFnanosheetsfrom50°to190°C,andnoobviousvariationswereobservedonthephasetransformationsandcrystallinelatticeswiththeincreaseintemperature,whichelucidatestheporerigidityinAl-MOFnanosheets(fig.S14,BtoD).A2DAl-MOFlaminarmembrane(Fig.2A)wasassembledbyvacuumfiltrationofadilutedAl-MOFnanosheetsuspension(fig.S15)usinganodicaluminumoxide(AAO)supportwithaporesizeof100nm.InanapparentcontrasttobareAAOsupport(Fig.2B),atop-viewSEMimageofa~10-nm-thickmembraneshowsauni-formcoverageofAl-MOFnanosheetsonthesurfaceofAAOsup-port,andnovisibledefectswereobserved(Fig.2C).ThecontinuousandflatAl-MOFlaminarmembranewasvisualizedbyAFMandcross-sectionalSEMcharacterizations(Fig.2Dandfig.S16).Inad-dition,themembraneexhibitsahydrophiliccharacter,demonstratedbyawatercontactangleof44°(fig.S17).BycontrollingtheloadingofAl-MOFnanosheets,thethicknessofthemembranescanbepre-ciselytunedfromafewnanometerstomicrometers(Fig.2Eandfigs.S18andS19).Notably,atypicalhomogeneouslaminarstruc-tureisseenwhenthemembranethicknessreaches500nm(Fig.2E).Fig.2.CharacterizationsofAl-MOFmembranes.(A)Digitalphotoofanas-prepared100-nm-thickAl-MOFlaminarmembraneonAAOsubstrate.(B)SEMimageofabareAAOsubstrate.(C)SEMimageofasub–10-nm-thickAl-MOFlaminarmembraneonAAOsubstrate.Thevisibilityofsubstratebackgroundelucidatestheultrathincoverage.(D)Cross-sectionaloverviewofa100-nm-thickAl-MOFlaminarmembraneonAAOsubstrate.(E)Magnifiedcross-sectionalviewsof2DAl-MOFmembraneswithdifferentthicknesses.Membraneslessthan100nm(greenandgold)showacompactstacking,whereasthemembraneatathicknessof500nm(purple)apparentlyshowstypicallaminarstructure.Scalebars,500nm.(F)Cross-sectionalTEMimageofthe2DAl-MOFlaminarmembrane.(G)GIXRDpatternoftheAl-MOFlaminarmem-brane.Thepatternwasacquiredfromathickmembrane(~20m)duetothedetectionlimit.Thesharp(0k0)phasepeakat2=7.6°indicatesanaveragevalueof6.0Å.Downloadedfromhttps://www.science.orgonOctober18,2021Jianetal.,Sci.Adv.2020;6:eaay39985June2020SCIENCEADVANCESRESEARCHARTICLE4of9Thelaminarstructurewasalsorevealedbyacross-sectionalTEMcharacterization(Fig.2F).Furthermore,thesynchrotrongrazingincidencex-raydiffraction(GIXRD)analysisobservedaprominent(0k0)peakat2=7.6°,showingthedvalueof~6.0Å.Thismani-feststhattheslitwidth(h)isclosetothesizeoftherectangularporeinonelayer(δ=6.1Å;Fig.2Gandinset).ThewaterpermeationacrosstheAl-MOFlaminarmembranewasfirstexaminedbymeasuringtheweightlossofacontainercov-eredbya100-nm-thickAl-MOFmembrane(fig.S20).Figure3Ashowsthatthewaterevaporationrateofthesealedcontainerisclosetothatoftheopenaperture(intheabsenceoftheAl-MOFmem-brane).ThisdemonstratestheunimpededwatervaporflowthroughtheAl-MOFmembrane.Afterward,wefurtherinvestigatedtheper-meationoftheAl-MOFmembraneforliquidwaterinadiffusioncellusingdeionizedwaterand0.5MCoCl2asfeedanddrawsolu-tions,respectively(fig.S21).Thevolumeofthedrawsolutiongrad-uallyincreasedwithtime,owingtowatertransportfromthefeedsidedrivenbytheosmoticpressuredifference(Fig.3B).ThetwoexperimentscollectivelyshowthattheAl-MOFlaminarmembraneispermeabletowatermolecules.Inthediffusiontest,itisworthnotingthatthewaterpermeanceisdependentonthesaltinthedrawsolutions.HighwaterpermeancewasachievedbyAlCl3andCoCl2solutions,whereasrelativelylowwaterpermeancewasachievedforNaCl,KCl,MgCl2,andCaCl2solutions(Fig.3C).ThisanomalousresultmightbecausedbythedifferenceintheaffinityofthesecationsontoAl-MOFsheets,whichwasverifiedbyanadsorptionexperiment(fig.S22).FurtherXPScharacterizationshowsthatalu-minumhydroxylgroupsfromAl-MOFnanosheetsplayavitalroleinadsorbingtheseions(fig.S23).Consideringthattheadsorbedionssuppresswatertransporttosomeextent,surfacemodificationtoinhibitadsorptionofionscouldbeastrategyinfuturestudiestoimprovethewaterpermeanceoftheAl-MOFmembranes.ThepermeationratesoftheseionsintheAl-MOFlaminarmem-branewerealsoevaluatedinthediffusioncell,using0.5MNaCl,KCl,MgCl2,CaCl2,AlCl3,andCoCl2,respectively.Theyareallultralow,lessthan3.3×10−6molm−2hour−1bar−1(Fig.3C),whichisgenerallyconsideredimpermeable(nearly100%rejection)(11).Comparedwithotherreported2Dlaminarmembranes,theAl-MOFmembranehasalowerionpermeance(tableS1).AfterthediffusiontestingusingNaCl,thesurfaceandunderneathlayersoftheusedmembranewerecharacterizedbyXPS.Exceptforthemembranesurface,NaClwashardlydetectedinsidethemembrane(fig.S24).ThisfurtherverifiesthehindranceofAl-MOFporesonthehydratedsalts.Inaddition,anionspecieshavenoapparentimpactonsaltpermeation(fig.S25).Becauseoftheaffinityofsomesalts(NaCl,KCl,MgCl2,andCaCl2)onAl-MOFactivesites,thewaterpermeanceFig.3.PerformancesofAl-MOFmembranes.(A)WaterevaporationthroughtheAl-MOFmembrane.Thefiguredepictstheweightlossofwaterfromacontainersealedwitha100-nm-thickAl-MOFmembrane.Insetisaschematicsetupforthewaterevaporationprocess.(B)Liquidvolumechangeofthedrawsolutionwithtimeduringthediffusionprocess.FeedsideisDIwater,whereasdrawsideis0.5MCoCl2aqueoussolution.InsetisaschematicU-shapedsetupforthediffusionprocess.(C)Waterfluxthrougha100-nm-thickAl-MOFmembraneusingdifferentdrawsolutions(0.5M)andthecorrespondingionpermeationrates.(D)Correlationbetweenwaterfluxandwater/saltselectivityofAl-MOFmembranesandotherrepresentative2Dlaminarmembranesondifferentsupports.ThedetaileddataarelistedintableS1.Eachsetofsymbolsrepresentsadifferentsalt.Downloadedfromhttps://www.science.orgonOctober18,2021Jianetal.,Sci.Adv.2020;6:eaay39985June2020SCIENCEADVANCESRESEARCHARTICLE5of9ofAl-MOFlaminarmembranesiscomparablylow.Despitethat,thewaterpermeanceoftheAl-MOFmembraneisstillcomparabletothoseofthestate-of-the-art2DlaminarmembraneswhenAlCl3andCoCl2areusedasdrawsolutions(Fig.3D).Notably,thewaterpermeanceoftheAl-MOFmembranecanbeexponentiallyincreasedto2.22molm−2hour−1bar−1byreducingthethicknessdownto20nm,whilehighsaltrejectionremains(fig.S26).Owingtotheultralowsaltpermeation,thewater/ionselectivityoftheAl-MOFlaminarmembranereachesupto5.00×105(Fig.3DandtableS1),whichoutperformsmost2Dlaminarmembranesondifferentsub-stratesreportedsofar.Thelong-termintegrityoftheAl-MOFmembranewasexaminedbyassessingtheNa+permeationrateandwaterfluxinacontinuoustesting.Asshowninfig.S27,thesteadyplotofNa+concentrationatthefeedsideandaconstantwaterfluxofthemembraneover30dayswereobserved,confirmingthelong-lastingstability.ThesuperiorstabilityofAl-MOFmembranesshouldbeattributedtothelockingeffectofadjacentnanosheetsbymeansofparallel-interaction(25,26).Meanwhile,fig.S28givestheunchangedreflection(0k0)peakat2=7.6°oftheAl-MOFmembraneafter1-monthcontinuoustesting.Furthermore,theantiswellingabilityofAl-MOFmembraneswerevisuallyexamined(fig.S29).InadditiontotheAAOsubstrate,Al-MOFlaminarmembraneswithsimilarperformancewerealsosuccessfullysynthesizedonlow-costpolymersubstratessuchaspolycarbonateandpolyethersulfone(fig.S30).Moleculardynamics(MD)simulationswereconductedtogaininsightsintosaltrejectionandwatertransportintheAl-MOFmembrane.First,thekineticbehaviorofwaterandiontransportFig.4.WatertransportbehaviorthroughAl-MOFmembranes.(A)CrystallineillustrationofAl-MOFmembraneconstructedwithtwo-layernanosheetsunderABstackingsequence(viewedalongthe[010]direction).Dashedlinespresenttwodifferentincisionpositionsforcross-sectionalmembranegeometries(markedwithcut1andcut2,respectively).Belowfigureisthecorrespondingwaterdensitymap.Thebluecolorcorrespondstonowaterexistenceandtheredcorrespondstothemaximumwaterdensity.(B)Linearwaterdensityprofilecollectedfromtheupperwaterdensitymap.Whitelinesrepresentthewaterdensityinthethreearrangedpores(9.3ÅintheXaxis×3.7ÅintheYaxis),andredlinesrefertothatinthetwoarrangedpores(3.7ÅintheXaxis×9.3ÅintheYaxis),asillustratedinwhiteandreddashedrectangles,respectively.(C)Sideviewofthemembraneatcut1sectionisviewedalongthe[001]direction,whichgivestheinterlayerdistanceof6.1Å.Belowfigureisthecorrespond-ingwaterdensitymap.(D)Sideviewofthemembraneatcut2sectionisviewedalongthe[100]direction.Whitedashedrectanglesstandforthelow-waterdensitiesinsidetheinterlayerspace.Downloadedfromhttps://www.science.orgonOctober18,2021Jianetal.,Sci.Adv.2020;6:eaay39985June2020SCIENCEADVANCESRESEARCHARTICLE6of9acrosstheporeapertureofAl-MOFmembraneswassimulated.Inaccordancewiththeexperimentalresults,allexaminedionsareef-fectivelyblockedbytheAl-MOFnanopores,whereaswatermole-culesareallowedtopenetrateonthebasisofsizeexclusion(tableS2andfig.S31).AlthoughbothAAandABstackingregimesaretheo-reticallypossiblewhenAl-MOFnanosheetsareassembledintomembranes,densityfunctionaltheory(DFT)calculationrevealsthatABstackingismorelikelythanAAstackingduetoalowerDFTenergy(fig.S32).TheDFTcalculationalsoshowsthattheinterlayerspaceoftheAl-MOFlaminarmembraneformedviaABstackingis6.1Å,whichisconsistentwiththeGIXRDcharacterization(Fig.2Gandfig.S32).Therefore,onthebasisofABstacking,anMDmodelfortheAl-MOFlaminarmembranewasbuilt,whichconsistsoftwo-layerAl-MOFnanosheets(Fig.4),tocomputetheprobabilitydistributionofwatermoleculesinsidethemembrane.Thewaterdensitymap(Fig.4A)revealsthatthewaterflowishighlylocalizedtotheintrinsicporesofAl-MOFnanosheets.Fur-thermore,thecorrespondinglineargradientprofile(Fig.4B)indi-catesthatwatermoleculesalignsidebysidewhenflowingthroughthesepores(asillustratedinthewaterdensitymap)duetoconfinedspace.ThesideviewsofwatertransportchannelsoftheABstackedmembranearepresentedbyusingtwoincisionpositions(Fig.4,CandD).Mostofthewatermoleculesareobservedinthestraightchannels(theverticallyalignedintrinsicporesofneighboringAl-MOFnanosheets),whereasasmallamountofwaterisintheinter-layerspaces,asindicatedwithwhitedashedrectangles(Fig.4,CandD).Thisisinagreementwiththewatertrajectoryresults,whichshowthatmostwatermoleculesflowthroughtheAl-MOFmembraneviatheverticallyalignedaperturechannels(straightflow),andonly17.08%ofwatermoleculesshiftfromonechanneltoanotherviatheinterlayerspace(shiftflow)whenpassingthroughthemembrane(fig.S33).SimilarwaterdynamicbehaviorwasobservedinAAstackingmodel(fig.S34),althoughtheshiftflowismuchmorelikelytooccurbecauseoftherelativelylargerinterlayerspaces(6.2and12.5Å).Furthermore,theMDsimulationsonNaCldiffusionrevealthatsaltspeciescannotpermeatethroughtheslitchannelsbetweenthenanosheetsinbothABandAAstackedmembranes(fig.S35).CONCLUSIONOurfindingsdemonstratedthefabricationofultrathinnanoporousmembranesassembledby2DMOFnanosheetsforionseparationfromwater.Theobtainedlaminarmembraneexhibitedanexcellentlong-termstabilityinwater,againsttheintractableswellingfor2D-basedmembranes.Allthetestedionshadultralowpermeationrates,whichwereattributedtotheAl-MOFporehindrance.WatertransportmainlyoccursintheverticallyalignedaperturechannelsformedbytheintrinsicporesofAl-MOFnanosheets.Thismembraneopensupthepossibilitytoexploreemergingnanoporous-basedmembranesandmeetsthecriticalneedforincreasedselectivityfordesalinationmembranes(27).However,weenvisagethatfullunder-standingofthemembranerequiresfurthereffortsintermsofapertureshape,channelcharge,transportfriction,poredensity,etc.MATERIALSANDMETHODSMaterialsAl(NO3)3·9H2O,pyrazine,andp-xylenewereallpurchasedfromSigma-Aldrich.Tetrakis(4-carboxyphenyl)porphyrin(H2TCPP,97%)waspurchasedfromTokyoChemicalIndustryCo.Ltd.Allsaltpowders,N,N-dimethylformamide(DMF),andethanolwerepurchasedfromMerck.Allthechemicalswereofanalyticalgradeandwereusedasre-ceivedwithoutfurtherpurification.Deionizedwaterusedinallexper-imentswasfromaMilli-Qsystem(AdvantageA10,MerckMillipore,USA).AAOdiscfilters(100-nmpore,13-mmdiameter)werepurchasedfromGEHealthcareWhatman.Polyethersulfone(PES;30-nmpore,13-mmdiameter)andpolycarbonatetracketch(PCTE;100-nmpore,25-mmdiameter)membranefilterswerepurchasedfromSteritech.MethodsSynthesisofAl-MOFbulksBulk-typeAl-MOFwaspreparedfollowingamodifiedmethodinourpreviousstudy(28).First,93.23mgofAl(NO3)3·9H2O,14mgpyrazine,150mlofN,N-dimethylformamide(DMF),and50mlofethanolweremixedina250-mlSchottDuranbottleandsonicated30mintodissolvecompletelyatroomtemperature.ThisistheAlprecursorsolutionforAl-MOFproduction.Second,200mgoftetrakis(4-carboxyphenyl)porphyrin(H2TCPP)wasdissolvedin200mlofDMFwithanassistanceof30-minsonication.ThisistheligandsolutionforAl-MOFsynthesis.Al-MOFbulksweresynthe-sizedinatypicalprocedurebypipetting8mlofAlprecursorsolu-tionand4mloftheligandsolutionina20-mlglassvial,respectively.Theglassvialwasthencapped,andthemixturewasstirredonanorbitalvortexshaker(Labco)for1min.Afterward,thecappedvialswereheatedto120°Cfor16hoursinanoilbath.Last,theresultingpurpleprecipitatewascollectedbycentrifugationandwashedthreetimeswith40mlofabsoluteethanol.SonicationexfoliationofAl-MOFbulkstomonolayernanosheetsLikewise,Al-MOFbulksfromfourglassvialswereobtainedafterthesolvent-thermalreaction.Thepurpleresultantwaswashedwithabsolute40mlofDMFtwiceand40mlofethanolonce,respectively.Thefinaldispersioninethanolwasthenbath-sonicatedfor3hoursusingaUnisonicsFXP12Msonicbath(40kHz,100W).Tocombattheconsiderablewaterheatingcausedbyconsecutivesonication,thebathwaterwasrenewedperiodicallyevery30min.Aftersonica-tion,thesuspensionbecomeshighlydispersed.Toremovetheun-exfoliatedbulks,thedispersionwascentrifuged(Sigma2-16P)at8500rpmfor30min.Theretainedsupernatantwasthususedformembraneassembling.TheexfoliationyieldratewascalculatedthroughUV-Vismonitor,asshowninfig.S2(DandE).ToobtaindriedsamplesofAl-MOFnanosheetsandbulksforfuturecharac-terizations,weusedafreeze-dryingprocess,whichwasperformedinafreezedryer(FreeZone2.5liters,LabconcoCorporation,USA).Forthesakeofeliminatingwaterinterference,theas-synthesizedAl-MOFnanosheetsandbulksweredriedbyfreezingtheircolloidalsuspensionsinp-xyleneandremovingthesolventviafreeze-dryingfor3days.Al-MOFnanosheetsafterthewaterstabilitytestweredriedfollowingthesameprocessesasdescribedabove,exceptthatwaterwasusedasthesolvent.PreparationofAl-MOFnanosheetaqueoussuspension(1mg/liter)OriginalAl-MOFnanosheetdispersioninethanolwascalibratedat2000mg/literbyaUV-Visspectrophotometerbasedonthepre-determinedstandardcurve(fig.S2,DandE).Wethendilutedtheabovedispersionto1mg/literwithdeionized(DI)waterformem-branepreparation,asshowninfig.S14.MembranefabricationAl-MOFmembraneswerefabricatedbyavacuumfiltration(Welch,2511WOB-LPump)ofthedilutedAl-MOFnanosheetaqueousDownloadedfromhttps://www.science.orgonOctober18,2021Jianetal.,Sci.Adv.2020;6:eaay39985June2020SCIENCEADVANCESRESEARCHARTICLE7of9suspension(1mg/liter)ontheprescribedporoussubstrates(seemoreinformationofthesesubstratesin“Materials”section).TheAl-MOFnanosheetloading(g/mm2)ormembranethickness(nm)wascon-trolledbyvaryingthevolumeoftheAl-MOFsuspensiontobefil-tered.Forconsistency,themembranesdescribedinthisworkwereall100nmthick,unlessspecifiedotherwise.Theresultantmem-branesweredriedinanovenat60°Covernightandthenstoredinavacuumdesiccatorbeforetesting.MembranesealingPleaserefertofig.S21fordetailedsteps.CharacterizationX-raypowderdiffraction.Becauseofthedetectionlimitoflabora-toryx-raysources,thestructureandphasetransformationofAl-MOFsamplesweredeterminedbyusingsynchrotronXRD.Al-MOFnanosheetsandbulksampleswereloadedin0.5-mmIDKaptoncapillaries,whichweresealedatbothendswithaLoctiteadhesive.Thepatternswerecollectedatanenergyof18keV(=0.812Å)andaMYTHEN-IIcapillarydetector(d=250mm)onthehigh-resolutionpowderdiffractionbeamlineatAustralianSynchrotron(ANSTO).Withregardtothemembranespecimens,GIXRDisapowerfulmethodtoprovideinformationoftheinterlayerdistanced(nm).However,owingtothex-raydiffractiondetectionlimitforthinmembranes,weelaboratelypreparedathickfreestandingmem-brane(~20m)toharvestthesignals.ThethickfreestandingAl-MOFmembranewasfirstfabricatedbyvacuumfiltrationofAl-MOFnanosheetsonaPCTEsupportandthenslowlytransferredinDMFsolvent(membranesidefaceup).Afterapproximately5min,PCTEsubstratewascompletelydissolvedinDMF,andthethickfreestandingAl-MOFmembranewouldbefloatedonthesurfaceofDMF.Afterward,asquaretransparentmicroscopeglassslide(35cmby35cm,1-mmthickness,ThermoFisherScientific)wasusedtomaketheisolatedmembraneintegralseatedonthesurface.Intheend,thethickmembraneontheslidewasdriedfor2daysintheovenat80°C.Field-emissionscanningelectronmicroscope.Themorphologyofthesampleswasdetectedviaafield-emissionscanningelectronmi-croscope(FESEM;FEIMagellan400XHR)equippedwithEDS.ForexaminingmorphologiesofAl-MOFnanosheetsandbulks,onedrop(1-cm-diameterpipettetip)oftheirethanolsuspensions(2mg/liter)wasdepositedonafresh1cmby1cmsquareofsiliconwaferandthendriedinair.CrosssectionofthesupportedmembraneswaspreparedbybreakingthemembranesdepositedonAAOsubstratesusingafinetweezer.AllSEMspecimenswerecoatedwithiridium(1.5to2.0nmthick)toeliminatechargingeffect.Transmissionelectronmicroscope.AFEITecnaiG2T20TWINoperatedat200kVwasusedforTEMstudies.Allimageswerere-cordedusingacharge-coupleddevicecamera.SamplesofAl-MOFnanosheetsandbulkswerepreparedbyaddingonedrop(1-cm-diameterpipettetip)oftheirethanoldispersion(2mg/liter)onholeycarbongrids(230meshCu,EMC)andwereair-dried.Cross-sectionalTEMexaminationformembranespecimenswascarriedoutusingaLeicaUltraCutSultramicrotomewithadiamondknife.Thesectionswerethenmountedonholeycarbonfilmcoppergrids(400meshCu,Pelco),andthepreparedsampleswereair-driedovernightforfurthermicroscopeobservations.Atomicforcemicroscopy.AFMimageswerecollectedusingaBrukerDimensionIcon.Nanosheetandbulkspecimenswerepre-paredbyplacingadropoftheirethanolsuspensionsonafresh1cmby1cmsquareofsiliconwaferfollowedbyair-drying.AFMcanti-levertipsfromRTESPA(MPP-11120-10)wereused.Theanalysiswasperformedinatappingmodeunderair.TheimageanalysiswasperformedwiththesoftwareNanoScopeAnalysisversion1.5.Thermogravimetricanalysis.TGAwasperformedonaPerkin-ElmerSTA6000.Driedsamplesofnanosheetsandbulkswerebothheatedfrom25°to800°Catarateof10°C/mininair.N2adsorption-desorptionisotherms.N2adsorption-desorptionisothermsofthesesamplesweremeasuredbyaMicromeriticsASAP2020volumetricadsorptionanalyzeratliquidnitrogentem-perature(77K).Samplesofnanosheetsandbulkswerebothde-gassedat150°Cfor12hoursundervacuumbeforemeasurement.AttenuatedtotalreflectanceFouriertransforminfraredspectra.FunctionalgroupsofnanosheetsandbulkswereidentifiedbyaPerkinElmerATR-FTIRspectrometerwithadiamondcrystalandresolutionof4cm−1byaveragingthemeasurementsover16scans.Anaverageof10adjacentpointsfromthediamondcrystaldetectorwassmoothlyappliedtothedriednanosheetsandbulksamples.Zetapotentialmeasurement.Zetasizer(MalvernNanoZS,UK)wasusedtodeterminethesurfacezetapotentialofAl-MOFnanosheets.Eachsamplescanwasrepeatedthreetimes.Al-MOFnanosheetwatersuspension(5mg/liter)atdifferentpHswaspreparedforthemeasurement.X-rayphotoelectronspectra.X-rayphotoelectronspectraweredeterminedbyusinganAXISUltraspectrometer(KratosAnalytical,Manchester,UK)withanAlKanode(1486.6-eVphotonenergy,0.05-eVphotonenergyresolution,300W).Ultraviolet-visiblespectra.UV-VisspectrawererecordedonaUVspectrophotometer(ShimadzuUV-2401PC).BothAl-MOFnanosheetsandbulksuspensionswerepreparedat5mg/literforthemeasurement.Inductivelycoupledplasmaopticalemissionspectroscopy.Induc-tivelycoupledplasmaopticalemissionspectroscopy(ICP-OES)fromPerkinElmer(Optima7000DV)wasusedtoquantifythecon-centrationofionsintheionseparationexperiment.Contactangle.ThestaticcontactangleoftheAl-MOFmembranewasmeasuredbyplacingadropletofwater(2l)onthemembraneusingacapillarywithadiameterof0.7mm(OCA15EC,DataPhysics,Germany).Theequippeddigitalcamerawasusedtomonitortheshapeofthedropletimmediatelyafterthedropletdeposition.Theaveragevalueofthecontactanglewasdeterminedfromthemea-surementsofthecontactanglesatsevendifferentlocationsonmembranes.IonseparationtestsoftheAl-MOFmembranesThesealingstepsofAl-MOFmembranesintodiffusioncellsrefertofig.S20A.ConsideringthepotentialdeformityofthethinAl-MOFmembranescausedbyanexternalpressure,theseparationperfor-mancewasevaluatedusingaself-madediffusioncellasshowninfig.S20B,inwhichthepermeationprocesswasdrivenbyanosmoticpressure.Al-MOFmembranesfacingdrawsolutionweretightlyfixedinthemiddlebyclamps.Differentconcentrations(0.02,0.05,0.10,0.20,0.50,and1.00M)ofsaltsolutions(NaCl,NaNO3,Na2SO4,KCl,MgCl2,CaCl2,CoCl2,andAlCl3,respectively)wereusedasthedrawsolutions.Deionizedwaterwasusedasthefeedsolution.Mag-neticstirringwasappliedinbothdrawandfeedsidestoalleviatetheexternalconcentrationpolarizationeffect.Themasschangeofthedrawsolutionwasmeasuredbymonitoringtheheightincreaseofliquidlevelatthedrawsolutionside(thedraw-sidecompartmentwasreformedwith1.5-mminnerdiameteratthetop;fig.S20B).ThesaltleakageintothefeedsolutionwasmonitoredbyaconductivityDownloadedfromhttps://www.science.orgonOctober18,2021Jianetal.,Sci.Adv.2020;6:eaay39985June2020SCIENCEADVANCESRESEARCHARTICLE8of9meter(labCHEM-CP)andICP-OES.Thesystemtemperaturewasmaintainedat25°±0.5°Cthroughouttheexperiment.Thewaterflux(Jw;molm−2h−1)andsaltionpermeationrate(Js;molm−2hour−1)ofAl-MOFmembraneswerecalculatedasfollows(29)Jw=∆V─A∙∆tJs=(Ct∙Vt)−(C0∙V0)───────────A∙∆t∙Mwwhere∆Visthevolumechange(l=5.6×10−5mol)ofthedrawsolutionoverarunningtimeinterval∆t(hours)ineachexperiment.AistheeffectiveareaoftheAl-MOFmembrane(7mm2).C0andV0denotetheinitialsaltconcentration(M)andfeedsolutionvolume(ml),whileCtandVtaretheirrespondingvaluesatagiventimet.Mwisthemolecularweightofsalts(g/mol).Thewater/saltselectivityJw_Jsisdefinedastheratioofthewaterfluxtosaltionpermeationrate(30–32).WaterevaporationtestoftheAl-MOFmembranesForevaporationtest,30mlofwaterwasfilledina150-mlflaskineachrun.Arubberwith8-mmstraightporewaspluggedinthetopneck,andwaterproofgluesurroundstheedgetoavoidanyleakage.SealedAl-MOFmembranes(7mm2)werestronglygluedonthetoprubberoftheflaskbythecarbontabs(ProSciTech).Theentireap-paratuswasunderadarkenvironmentandrunningatastabletem-peratureof25°±0.5°C.Theweightlosswasconstantlymonitoredusingadigitalcomputer-controlledbalance(ANDFX-3000i).SaltsadsorptioncapacityonAl-MOFnanosheetsAdsorptionisothermexperimentswereconductedin20-mlplasticvialscontainingsaltsolution(10ml)andAl-MOFnanosheetpow-der(10mg).Theinitialconcentrationsofthesesaltswere0.02,0.05,0.10,0.20,0.50,and1.0M,respectively.Thevialswereshakenat200rpmfor24hoursat25°C.Beforetheadsorptionexperiments,themixturesuspensionsweresonicatedfor10mintoalleviateaggregation.Solutionsafteradsorptionwerefilteredwithsyringeprefilters(PTFEMillipore;poresize,0.2m).Theconcentrationchangeofeachsaltsolutionbeforeandafteradsorptionwasdeter-minedbymeasuringtheconductivity.Theadsorptioncapacityq(mmol/g)wascalculatedasfollows(33)q=∆C∙V─mwhere∆C(M)istheconcentrationdifferenceofsaltsolutionbeforeandaftersaltadsorption,V(ml)isthevolumeofsaltsolution,andm(mg)istheadsorbentweight.DFTcalculationsDFTcalculationswereperformedusingtheViennaAbinitioSimulationPackage5.4.4codeonAustralianSynchrotronComputeInfrastruc-ture.ThegeneralizedgradientapproximationwithaPerdew-Burke-Ernzerhofexchangecorrelationfunctionwasused.Theinteractionsbetweentheioniccoresandthevalenceelectronsweretreatedbyultrasoftpseudopotentialswithatomicpseudopotentialscorrespond-ingtoNa3s1,K3s23p64s1,Mg3s2,Ca3s23p64s2,Co3d84s1,andAl3s23p1.Thezero-dampingDFT-D3dispersioncorrectionmethodofGrimmewasusedtoaccountfortheimportanceofvanderWaalsinteractionsoftheadsorptionofionsandinterlayersofAl-MOFinthesystem.Inallcalculations,thecutoffenergyoftheplanewavewassetat400eV,andMonkhorstPackk-pointwasusedtoensurethetotalenergyvalueconvergencewithin1meVperatom.DFTgeometryoptimizationwasconductedtorevealthemoststablestackingconfigurationofAl-MOFnanosheet-assembledmem-branes.FigureS32showsthetypicalstackingconfigurationsofthestackedmembranemodelsfromtwo-layerAl-MOFnanosheets.Fromourcalculations,itshowsthatthebindingenergyoftwo-layerAl-MOFnanosheetsinABstackingarethelowest,revealingthemoststablestructure.MDsimulationsAll-atommolecularstructuresofAl-MOFmembraneswerebuiltonthebasisoftheresultsofDFTsimulationsandchargedviatheGasteigermethod(34).Solvatingthefixedmembranesaccordingtotherequirementsof(i)soakingand(ii)diffusionprocessesisfur-therdescribedbelow.Onefree-vibratingimpermeablesheetwasplacedateachendofthereservoir,whichwasfarawayfromthesimulationboxboundary,tomaintainthesystemequilibrium.Lateron,weextendedthesystemwithperiodicboundaryconditioninalldirections.ThesimulationswereundertakenbyNotAnotherMolecularDynamics(NAMD)program(Git-2018-09-13Linux-x86_64-multicore)witha2-fstimestepunder300KthatwasperformedintheNVTensemble.NonbondedinteractionwascalculatedwithCHARMMGeneralForceFieldandMultivalentIonForceField(35),whichwereappliedwithLorentz-Berthelotmixingrules.Itscutoffradiuswasusedwith12Å.ParticlemeshEwaldwasusedforlong-rangeelectrostaticinteraction,andSHAKEAlgorithmwasappliedonTIP3Pwatermolecules.1)Soakingprocess:Solvatingfixedtwo-layerAAorABstackedAl-MOFnanosheets(thecorrespondinginterlayerdistanceisbasedonDFTresults)withwatermolecules;averagingoutthenumberofwatermoleculesinsidemembranesover4nsformembranewaterdistributionandtransporttrajectory.2)Diffusionprocess:Solvatingfixedone-layerAl-MOFnanosheetswithwatermoleculesandionizedonesidewith0.5MAlCl3,CoCl2,CaCl2,MgCl2,NaCl,andKCl,respectively;collectingthetotalnumberofwatermoleculesattheionizedsideover30nsforcalculatingtransmembranewaterflux.3)NaClrejectionperformancebytheslitchannel:Two-layerAl-MOFnanosheetswerebuiltforillustration.ThemiddlepartofthefirstAl-MOFnanosheetcrystallinewassubtracted,whilethesec-ondAl-MOFnanosheetcrystallinewasashortnanosheettocoverthedefectandformtherequiredslitchannelwiththefirstlayer;solvatingthemembranewithwatermoleculesandionizedonesidewith0.5MNaCl.ThecorrespondingNaClconcentrationwasaver-agedfroma2-nsfiltrationprocess.SUPPLEMENTARYMATERIALSSupplementarymaterialforthisarticleisavailableathttp://advances.sciencemag.org/cgi/content/full/6/23/eaay3998/DC1REFERENCESANDNOTES1.M.R.Chowdhury,J.Steffes,B.D.Huey,J.R.McCutcheon,3Dprintedpolyamidemembranesfordesalination.Science361,682–686(2018).2.W.J.Koros,C.Zhang,Materialsfornext-generationmolecularlyselectivesyntheticmembranes.Nat.Mater.16,289–297(2017).3.J.R.Werber,C.O.Osuji,M.Elimelech,Materialsfornext-generationdesalinationandwaterpurificationmembranes.Nat.Rev.Mater.1,16018(2016).4.D.L.Gin,R.D.Noble,Designingthenextgenerationofchemicalseparationmembranes.Science332,674–676(2011).5.H.B.Park,J.Kamcev,L.M.Robeson,M.Elimelech,B.D.Freeman,Maximizingtherightstuff:Thetrade-offbetweenmembranepermeabilityandselectivity.Science356,eaab0530(2017).Downloadedfromhttps://www.science.orgonOctober18,2021Jianetal.,Sci.Adv.2020;6:eaay39985June2020SCIENCEADVANCESRESEARCHA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