German Edition: DOI: 10.1002/ange.201609306MembranesInternational Edition: DOI: 10.1002/anie.201609306

A Two-Dimensional Lamellar Membrane: MXene Nanosheet StacksLi Ding, Yanying Wei,* Yanjie Wang, Hongbin Chen, Jgrgen Caro,* and Haihui Wang*

Abstract: Two-dimensional (2D) materials are promisingcandidates for advanced water purification membranes. Anew kind of lamellar membrane is based on a stack of 2DMXene nanosheets. Starting from compact Ti3AlC2, delami-nated nanosheets of the composition Ti3C2Tx with the func-tional groups T (O, OH, and/or F) can be produced by etchingand ultrasonication and stapled on a porous support byvacuum filtration. The MXene membrane supported on anodicaluminum oxide (AAO) substrate shows excellent waterpermeance (more than 1000 Lm@2 h@1 bar@1) and favorablerejection rate (over 90 %) for molecules with sizes larger than2.5 nm. The water permeance through the MXene membrane ismuch higher than that of the most membranes with similarrejections. Long-time operation also reveals the outstandingstability of the MXene membrane for water purification.

The newly fashioned two-dimensional (2D) materials, suchas graphene and graphene oxide (GO),[1] exfoliated nano-sheets of metal–organic frameworks (MOFs)[2] and zeolitenanosheets,[3] and the transition metal dichalcogenides(TMDs),[4, 5] have attracted increasing attention owing totheir outstanding mechanical properties, excellent thermalstability, and superior flexibility. Nowadays, a novel kind of2D layered material named MXenes, a family of earlytransition metal carbides, has received increasing attention,which was first reported by BarsoumQs group.[6] Until now, themost studied MXene has been Ti3C2TX, which was delami-nated successfully in 2011.[7] Ti3C2TX is normally producedfrom Ti3AlC2 through a HF etching process. The Ti3C2TX isterminated by TX, where Trepresents O, OH, and/or F groups,while x is the number of terminating groups.[8–13] Owing to itsflexibility, superior structural stability, high electrical con-ductivity, and hydrophilic surfaces, Ti3C2TX has been widelyused in super capacitors,[9] lithium-ion batteries,[10] oxygen-evolution reaction,[11] and heavy metal adsorption.[12, 13]

2D materials are promising potential candidates for futurefunctional separation membranes. For example, Li et al.reported an ultrathin GO membrane with good hydrogenseparation selectivity.[14] Nanoporous 2D graphene mem-

branes have also been applied in desalination and nano-filtration.[15, 16] Subsequently, Peng and co-workers assembledthe chemically exfoliated MoS2 and WS2 nanosheets into size-selective separation membranes.[17] Recently, two reports onthe synthesis of MOF nanosheets for MOF-based mixedmatrix membranes (MMM) appeared. In a bottom-up con-cept, single exfoliated MOF layers are formed in the contactzone of a linker and a metal solution followed by sedimenta-tion and used subsequently in MMM.[18] In a top-downstrategy, a 2D MOF is exfoliated by first wet ball-millingfollowed by exfoliation in a solvent under ultrasonication, andthen membranes were prepared as stacked sheets.[2] Thepioneering breakthrough works on MOF nanosheets arebased on TsapatsisQ work on dispersible exfoliated zeolitenanosheets and their capabilities as selective membranes.[3,19]

The same concept is followed when GO nanosheets arestacked and form a thin gas selective layer.[14, 20] Consequently,the 2D MXene materials are also expected to be applied inmembranes for gas separation and water purification. How-ever, there is so far no report on the inorganic MXene-basedmembranes until now, except the paper by Gogotsi et al. forion sieving.[21]

Herein, we propose a kind of 2D lamellar membrane withTi3C2TX MXene nanosheets and its application in waterpurification. The MXene membrane with an extremely shorttransport pathway and large amounts of nanochannels showsexcellent water permeance (more than 1000 L m@2 h@1 bar@1)and favorable rejection rate (over 90%) for molecules withsizes around 2.5 nm. This water permeance is much higherthan that of the mostly studied membranes with similarrejections.

The preparation of the MXene membrane is shown inFigure 1 and the Supporting Information, Scheme S1. Ti3AlC2

particles were first etched by HF solution to generate Ti3C2TX

powder. By extracting Al as AlF3, the interaction between thelayers is weakened. The MXene nanosheets can be obtainedby sonication-assisted exfoliation. The positively chargedFe(OH)3 colloidal solution was chosen to intercalate thenegatively charged MXene nanosheets to create expandednanochannels. Subsequently, after a simple vacuum filtrationprocess and hydrochloric acid solution (HCl) treatment toremove the Fe(OH)3 nanoparticles, the ultimate MXenemembrane can be obtained.

To achieve a high quality MXene membrane, the prepa-ration of small flakes of MXene nanosheets, which can be alsocalled MXene nanofragments, is important. The shift of (002)peak to lower angles and the disappearance of the mostintense diffraction peak of Ti3AlC2 at 3988 (2q) in the X-raydiffraction (XRD) patterns indicate that the Ti3AlC2 issuccessfully converted into Ti3C2TX (Figure 2a).[7] Asobserved in the scanning electron microscopy (SEM)images in Figure 2b and the Supporting Information, Fig-

[*] L. Ding, Dr. Y. Wei, Y. Wang, H. Chen, Prof. Dr. H. H. WangSchool of Chemistry and Chemical EngineeringSouth China University of Technology510640 Guangzhou (China)E-mail: ceyywei@scut.edu.cn

hhwang@scut.edu.cn

Prof. Dr. J. CaroInstitute of Physical Chemistry and ElectrochemistryLeibniz University of HannoverCallinstrasse 3A, 30167 Hannover (Germany)E-mail: Juergen.caro@pci.uni-hannover.de

Supporting information for this article can be found under:http://dx.doi.org/10.1002/anie.201609306.

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ure S1, Ti3AlC2 has changed into a loosely stacked structureafter HF etching. After sonication, the MXene nanosheetswith the thickness of around 2 nm were obtained, as shown inthe atomic force microscopy (AFM) image (Figure 2c). Atransmission electron microscopy (TEM) image (SupportingInformation, Figure S2a) shows the exfoliated MXene nano-sheets to be quite thin. The corresponding lattice fringes ofthe MXene nanosheets can be clearly observed by HRTEM(Supporting Information, Figure S2b). The size distribution ofthe MXene nanosheets (Figure 2d) stemming from a large-scale AFM image (Supporting Information, Figure S3) indi-cates a relatively uniform lateral size of around 100–400 nm.

The 2D lamellar MXene membranes can be prepared withthe as-synthesized MXene nanosheets filtered on a porousAAO substrate (Supporting Information, Figure S4). Tocreate more transport channels for water, nanowires or

nanoparticles are usually used as pore former. Here, thepositively charged Fe(OH)3 (23.25 mV of zeta potential)nanoparticles with diameter around 4–5 nm (SupportingInformation, Figure S5) were chosen to form the nanochan-nels. The Fe(OH)3 nanoparticles can be bound to thenegatively charged MXene nanosheets (zeta potential is@34.75 mV) via electrostatic interaction. It can be found thatthe Fe(OH)3 nanoparticles disperse homogeneously with theMXene nanosheets, as shown in energy-dispersive X-rayspectroscopy (EDX) elemental maps (Supporting Informa-tion, Figure S6). For comparison, the membrane directlyfiltrated by MXene nanosheets without channeling by Fe-(OH)3 nanoparticles (named M1) and the composite mem-brane containing MXene and Fe(OH)3 (named M2) are alsoprepared with the same amount of MXene nanosheets(Supporting Information, Figure S7). After removing theFe(OH)3 nanoparticles of M2 by HCl, the ultimate MXenemembrane is obtained, which exhibits a more rough surfacemorphology (Figure 3a) compared with M1 (SupportingInformation, Figure S7a). The cross-sectional SEM image inFigure 3b shows that the MXene membrane possessesa typical lamellar structure. Elemental maps in Figure 3c–h

show that all elements distribute homogeneously and noobvious signal of Fe remained, which confirms that theFe(OH)3 nanoparticles have been almost completelyremoved by HCl dissolution. From Fourier transform infrared(FTIR) spectroscopy and X-ray photoelectron spectroscopy(XPS), as shown in the Supporting Information, Figures S8and S9, the MXene surfaces are terminated by O, OH, and/orF groups, which is in accordance with the previous report.[7]

After successful preparation, the MXene membrane wasapplied in water purification and it was firstly evaluated withthe Evans blue (EB, 1.2 nm X 3.1 nm) solutions at roomtemperature. It has to be noted that the vacant AAO support

Figure 1. MXene membrane preparation.

Figure 2. a) XRD patterns of Ti3AlC2 and Ti3C2TX powder. b) SEMimage of Ti3C2TX powder. c) AFM image of MXene nanosheets depos-ited on a mica plate. d) Particle size distribution of the MXenenanosheets.

Figure 3. a) SEM image (inset: macroscopic photograph) of theMXene membrane surface. b) High magnification of SEM image of thecross-sectional view of the MXene membrane supported on AAO(inset: representation of the layered structure). c) Low-magnificationSEM image of the cross-sectional view of MXene membrane supportedon AAO and corresponding elemental maps of d) aluminum,e) oxygen, f) titanium, g) carbon, and h) iron from the same area withsame the scale bar.

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with uniform pore size around 200 nm gives a water perme-ance of 4500 L m@2 h@1 bar@1 and no rejections for EB mole-cules. From Figure 4a, it can be found that the MXenemembrane with thickness of 400 nm exhibits a water perme-ance of 1084 L m@2 h@1 bar@1 and a high rejection rate of 90%

for EB molecules. The water permeance of the MXenemembrane is about 5 and 10 times higher than those of the M1and M2 membrane, respectively. The improved water per-meance can be attributed to the additional nanochannelsformed in the MXene membrane, as confirmed by SEMimages (Supporting Information, Figure S6d–f). It can be seenthat the thickness of the MXene membrane is smaller thanthat of the M2 membrane but larger than that of M1membrane, which indicates that the MXene membraneexhibits a more loosely lamellar structure, equivalent to theenlarged interspace between the MXene nanosheets. Addi-tionally, the change of the interspace could also be confirmedby the XRD patterns. As shown in the Supporting Informa-tion, Figure S10, compared to M1 membrane, the (002) peakin the MXene membrane appears at a lower angle, whichindicates that the interspace between the MXene nanosheetshas been enlarged.[17, 21, 22] Therefore, these additional nano-channels provide additional transport fluidic channels forwater.

Dependence of the separation performance of the MXenemembrane on the membrane thickness has also been studiedusing EB, Cytochrome (Cyt. c, 2.5 X 3.7 nm) molecules, andgold nanoparticles (diameter of 5 nm) solutions. As shown inthe Supporting Information, Figure S11, when the MXenemembrane thickness is smaller than 0.8 mm, the rejectionincreases with increasing thickness. For a thin MXene

membrane, which contains some defects, water prefers to gothrough the larger defects. When the membrane thickness isbigger than 0.8 mm, the rejection reaches almost 100 %. Thereason is that a thicker membrane leads to fewer defects inthe selective layer, and thus the water flows through the gapsbetween nanosheets.

To evaluate the pore size of the MXene membrane,a series ions or molecules with different sizes have beenseparated (Figure 4b; Supporting Information, Table S1)through the 400 nm-thick MXene membrane. It can beconcluded that the MXene membrane excludes nearly100 % of bovine serum albumin (BSA), nearly 100% ofgold nanoparticles (5 nm), 97% Cyt. c, 93% 5,10,15,20-tetrakis-(N-methyl-4-pyridyl)-21,23-H-porphyrintetratosy-late (TMPyP, 1.7 X 1.7 nm2), 90% EB, and 85 % rhodamine B(RB, 1.8 X 1.4 nm2).[17, 21–23] But for molecules with sizes lessthan 1 nm, the membrane cannot effectively separate themfrom the solution, such as K3[Fe(CN)6] (0.9 X 0.9 nm2) (withthe rejection of 32 %). These results indicate that the pore sizeof the MXene membrane is around 2–5 nm. Moreover, themembrane exhibits excellent water permeance for all of themolecule solutions (around 1000 L m@2 h@1 bar@1). The perfectseparation of the proteins (BSA) further verifies the favor-able applications of this nanoporous 2D lamellar membrane.The UV/Vis absorption spectra of the retentate feed andpermeate solutions is summarized in the Supporting Infor-mation, Figure S12. Furthermore, the rhodamine B concen-tration in the retentate side increased gradually with time(Supporting Information, Figure S13). It is clear to see thatthe concentration of the solution on the retentate side isobviously higher than that of the original feed solution.Additionally, the total amount of molecules from both thepermeate and retentate sides is very close to the original feedamount of the molecules, which implies that the molecules aremostly rejected by the MXene membrane rather than beingabsorbed or reacted with the membrane. Moreover, the UV/Vis measurements of the solution after membrane immersion(Supporting Information, Figure S14) and the XPS analysis ofeach spent membrane (Supporting Information, Figure S15)demonstrate that any physical adsorption or chemical reac-tion between the Mxene membrane and the molecular speciescan be ignored. Therefore, the possible separation mechanismof the MXene membrane would be molecule sieving due tothe different sizes of the membrane pores and the feedmolecules.

Moreover, the pressure dependence of the MXenemembrane on the separation performance was measuredusing the device shown in the Supporting Information,Figure S16. As demonstrated in the Supporting Information,Figure S17 for a 400 nm-thick MXene membrane, withincreasing feed pressure from 0.1 MPa to 0.6 MPa, therejection rate for EB molecules decreases from 90% to82.5% (data left of the dashed line). The rejection decreaseswith increasing feed pressure suggests that there is a highercontribution of convective transport of the solute throughdefects at higher pressure. When the pressure was reduced tothe starting value of 0.1 MPa (data right of the dashed line)after the pressure loading test, no serious decline of therejection and water permeance can be observed compared

Figure 4. a) Comparison of the performance of the M1, M2, andMXene membranes for the separation of EB molecules at roomtemperature. b) Separation performance of the MXene membranes fordifferent molecules with different sizes.

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with the initial data, which demonstrates that the MXenemembrane is relatively stable in the pressure loading-unloading cycling.

Ideally, an efficient membrane used for industrial filtra-tion and separation processes should not only have well-defined channel size with excellent selectivity and high waterpermeance, but should also be robust.[16] In contrast to GOmembranes, the MXene membrane remains intact andunchanged even after immersion in water for more than onemonth (Supporting Information, Figure S18). Moreover, afterthe long-term water immersion treatment, the MXenemembrane still exhibits favorable rejection rate and waterpermeance with solution of 5 nm gold nanoparticles. Fur-thermore, the MXene membranes are hydrophilic, witha water contact angle of 4588 (Supporting Information, Fig-ure S19), which is beneficial for water separation. Its hydro-philicity and water stability augur well for the utilization ofthe MXene membrane in water purification. For going a stepfurther for practical applications, the MXene membrane wasapplied to filter gold nanoparticles (5 nm) over a long periodusing a home-made cross-flow filtration device (SupportingInformation, Figure S20). In a 28 h filtration operation (Fig-ure 5a), the rejection efficiency and the water permeancealmost maintained at a constant level, which further demon-strates the good stability of the MXene membrane.[9]

Compared with other 2D membranes, including the GO,MoS2, WS2, and other nanostructured membranes, theMXene membrane prepared in this work exhibits excellentseparation performance under similar experimentalconditions (Figure 5b; Supporting Information, Table S2).

Although several membranes show higher rejection rate(more than 98%), their corresponding water permeanceremains much lower than 100 Lm@2 h@1 bar@1. However, the0.4 mm-thick MXene membrane not only shows a goodrejection rate (90% for EB and 97 % for Cyt. c), but alsoholds an extremely high water permeance(1084 L m@2 h@1 bar@1 and 1056 Lm@2 h@1 bar@1, respectively)in comparison with other water treatment membranes. Suchhigh water permeance of the MXene membrane can beexplained from the following two aspects. Firstly, MXenenanofragments (nanosized MXene sheets) are used instead ofthe traditional microsized MXene sheets to obtain short andabundant transport pathways, which is beneficial for watertransport (Supporting Information, Figure S21). Our resultsare in accordance with the finding of Zhu et al., who foundthat the water permeation rate through the GO membranescan be enhanced by decreasing the flake size of the nano-sheets and/or creating more nanochannels between thenanosheets.[23] Secondly, intercalated nanoparticles are usedas the former of the distance of slit pores between MXenenanofragments, to achieve larger interlayer distance andcreate more nanochannels after their removal (SupportingInformation, Figure S22). Huang et al. also gave someevidence that the water permeance of the nanostrand-channeled GO membranes is 10-fold enhanced compared tothat of the GO membranes without sacrificing the rejectionrate.[22] Therefore, considering the shorter transport pathwayand more nanochannels formed in the MXene membraneresulted from the above two structural advantages, theMXene membrane exhibits excellent performance for watertransport. Compared with the commercial polymeric ultra-filtration membranes (30 kDa and 50 kDa polyethersulfone(PES) ultrafiltration membranes, Sepro Company), ourMXene membrane exhibits much better water permeance,as well as also higher rejections for each probe molecules/ionsunder study (Supporting Information, Table S3).

In summary, a new kind of a 2D lamellar membrane basedon stacks of Mxene nanosheets are prepared successfully byfiltration deposition on AAO substrates. During the on-filtration, colloidal Fe(OH)3 has been used as distance holderfollowed by HCl dissolution. The MXene membrane exhibitsan excellent water permeance (more than1000 Lm@2 h@1 bar@1) and a high rejection rate (90 %) formolecules with sizes larger than 2.5 nm when applied in waterpurification. To the best of our knowledge, the MXenemembrane prepared in this work shows the highest waterpermeance with proper rejection among various 2D mem-branes supported on porous substrate. Moreover, also theexcellent stability recommends the 2D lamellar MXenemembranes for applications in water purification. Mxenes,as a new kind of 2D materials, opens a door for thedevelopment of highly efficient membranes for water treat-ment and other applications.

Acknowledgements

We gratefully acknowledge the funding from by the Sino-German center for Science Promotion (GZ 911), the Natural

Figure 5. a) Separation performance versus filtering time for filtrationof gold nanoparticles (5 nm) solution using the 1 mm thick MXenemembrane. b) Comparison of the separation performance of theMXene membrane and various previously reported membranes, aswell as the commercial PES membrane (*= EB, ?=Cyt. c solu-tions).[17, 22, 24–26] For detailed experimental conditions, see the Support-ing Information, Table S2.

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Science Foundation of China for Distinguished YoungScholars of China (no. 21225625), Natural Science Founda-tion of China (21536005, 51621001), and the Natural ScienceFoundation of the Guangdong Province (2014A030312007).

Conflict of interest

The authors declare no conflict of interest.

Keywords: membranes · MXenes · separation · Ti3C2TX ·two-dimensional nanosheets

How to cite: Angew. Chem. Int. Ed. 2017, 56, 1825–1829Angew. Chem. 2017, 129, 1851–1855

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Manuscript received: September 23, 2016Revised: November 15, 2016Final Article published: January 10, 2017

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