Communications
Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201609306
German Edition:
DOI: 10.1002/ange.201609306
Membranes
A Two-Dimensional Lamellar Membrane: MXene Nanosheet Stacks
Li Ding, Yanying Wei,* Yanjie Wang, Hongbin Chen, Jgrgen Caro,* and Haihui Wang*
Abstract: Two-dimensional (2D) materials are promising
candidates for advanced water purification membranes. A
new kind of lamellar membrane is based on a stack of 2D
MXene nanosheets. Starting from compact Ti3AlC2, delaminated nanosheets of the composition Ti3C2Tx with the functional groups T (O, OH, and/or F) can be produced by etching
and ultrasonication and stapled on a porous support by
vacuum filtration. The MXene membrane supported on anodic
aluminum oxide (AAO) substrate shows excellent water
permeance (more than 1000 L m@2 h@1 bar@1) and favorable
rejection rate (over 90 %) for molecules with sizes larger than
2.5 nm. The water permeance through the MXene membrane is
much higher than that of the most membranes with similar
rejections. Long-time operation also reveals the outstanding
stability of the MXene membrane for water purification.
The newly fashioned two-dimensional (2D) materials, such
as graphene and graphene oxide (GO),[1] exfoliated nanosheets of metal–organic frameworks (MOFs)[2] and zeolite
nanosheets,[3] and the transition metal dichalcogenides
(TMDs),[4, 5] have attracted increasing attention owing to
their outstanding mechanical properties, excellent thermal
stability, and superior flexibility. Nowadays, a novel kind of
2D layered material named MXenes, a family of early
transition metal carbides, has received increasing attention,
which was first reported by BarsoumQs group.[6] Until now, the
most studied MXene has been Ti3C2TX, which was delaminated successfully in 2011.[7] Ti3C2TX is normally produced
from Ti3AlC2 through a HF etching process. The Ti3C2TX is
terminated by TX, where T represents O, OH, and/or F groups,
while x is the number of terminating groups.[8–13] Owing to its
flexibility, superior structural stability, high electrical conductivity, and hydrophilic surfaces, Ti3C2TX has been widely
used in super capacitors,[9] lithium-ion batteries,[10] oxygenevolution reaction,[11] and heavy metal adsorption.[12, 13]
2D materials are promising potential candidates for future
functional separation membranes. For example, Li et al.
reported an ultrathin GO membrane with good hydrogen
separation selectivity.[14] Nanoporous 2D graphene mem[*] L. Ding, Dr. Y. Wei, Y. Wang, H. Chen, Prof. Dr. H. H. Wang
School of Chemistry and Chemical Engineering
South China University of Technology
510640 Guangzhou (China)
E-mail: [email protected]
[email protected]
Prof. Dr. J. Caro
Institute of Physical Chemistry and Electrochemistry
Leibniz University of Hannover
Callinstrasse 3A, 30167 Hannover (Germany)
E-mail: [email protected]
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201609306.
Angew. Chem. Int. Ed. 2017, 56, 1825 –1829
branes have also been applied in desalination and nanofiltration.[15, 16] Subsequently, Peng and co-workers assembled
the chemically exfoliated MoS2 and WS2 nanosheets into sizeselective separation membranes.[17] Recently, two reports on
the synthesis of MOF nanosheets for MOF-based mixed
matrix membranes (MMM) appeared. In a bottom-up concept, single exfoliated MOF layers are formed in the contact
zone of a linker and a metal solution followed by sedimentation and used subsequently in MMM.[18] In a top-down
strategy, a 2D MOF is exfoliated by first wet ball-milling
followed by exfoliation in a solvent under ultrasonication, and
then membranes were prepared as stacked sheets.[2] The
pioneering breakthrough works on MOF nanosheets are
based on TsapatsisQ work on dispersible exfoliated zeolite
nanosheets and their capabilities as selective membranes.[3, 19]
The same concept is followed when GO nanosheets are
stacked and form a thin gas selective layer.[14, 20] Consequently,
the 2D MXene materials are also expected to be applied in
membranes for gas separation and water purification. However, there is so far no report on the inorganic MXene-based
membranes until now, except the paper by Gogotsi et al. for
ion sieving.[21]
Herein, we propose a kind of 2D lamellar membrane with
Ti3C2TX MXene nanosheets and its application in water
purification. The MXene membrane with an extremely short
transport pathway and large amounts of nanochannels shows
excellent water permeance (more than 1000 L m@2 h@1 bar@1)
and favorable rejection rate (over 90 %) for molecules with
sizes around 2.5 nm. This water permeance is much higher
than that of the mostly studied membranes with similar
rejections.
The preparation of the MXene membrane is shown in
Figure 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 the
layers is weakened. The MXene nanosheets can be obtained
by sonication-assisted exfoliation. The positively charged
Fe(OH)3 colloidal solution was chosen to intercalate the
negatively charged MXene nanosheets to create expanded
nanochannels. Subsequently, after a simple vacuum filtration
process and hydrochloric acid solution (HCl) treatment to
remove the Fe(OH)3 nanoparticles, the ultimate MXene
membrane can be obtained.
To achieve a high quality MXene membrane, the preparation of small flakes of MXene nanosheets, which can be also
called MXene nanofragments, is important. The shift of (002)
peak to lower angles and the disappearance of the most
intense diffraction peak of Ti3AlC2 at 3988 (2q) in the X-ray
diffraction (XRD) patterns indicate that the Ti3AlC2 is
successfully converted into Ti3C2TX (Figure 2 a).[7] As
observed in the scanning electron microscopy (SEM)
images in Figure 2 b and the Supporting Information, Fig-
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Figure 2. a) XRD patterns of Ti3AlC2 and Ti3C2TX powder. b) SEM
image of Ti3C2TX powder. c) AFM image of MXene nanosheets deposited on a mica plate. d) Particle size distribution of the MXene
nanosheets.
ure S1, Ti3AlC2 has changed into a loosely stacked structure
after HF etching. After sonication, the MXene nanosheets
with the thickness of around 2 nm were obtained, as shown in
the atomic force microscopy (AFM) image (Figure 2 c). A
transmission electron microscopy (TEM) image (Supporting
Information, Figure S2a) shows the exfoliated MXene nanosheets to be quite thin. The corresponding lattice fringes of
the MXene nanosheets can be clearly observed by HRTEM
(Supporting Information, Figure S2b). The size distribution of
the MXene nanosheets (Figure 2 d) stemming from a largescale AFM image (Supporting Information, Figure S3) indicates a relatively uniform lateral size of around 100–400 nm.
The 2D lamellar MXene membranes can be prepared with
the as-synthesized MXene nanosheets filtered on a porous
AAO substrate (Supporting Information, Figure S4). To
create more transport channels for water, nanowires or
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nanoparticles are usually used as pore former. Here, the
positively charged Fe(OH)3 (23.25 mV of zeta potential)
nanoparticles with diameter around 4–5 nm (Supporting
Information, Figure S5) were chosen to form the nanochannels. The Fe(OH)3 nanoparticles can be bound to the
negatively charged MXene nanosheets (zeta potential is
@34.75 mV) via electrostatic interaction. It can be found that
the Fe(OH)3 nanoparticles disperse homogeneously with the
MXene nanosheets, as shown in energy-dispersive X-ray
spectroscopy (EDX) elemental maps (Supporting Information, Figure S6). For comparison, the membrane directly
filtrated by MXene nanosheets without channeling by Fe(OH)3 nanoparticles (named M1) and the composite membrane containing MXene and Fe(OH)3 (named M2) are also
prepared with the same amount of MXene nanosheets
(Supporting Information, Figure S7). After removing the
Fe(OH)3 nanoparticles of M2 by HCl, the ultimate MXene
membrane is obtained, which exhibits a more rough surface
morphology (Figure 3 a) compared with M1 (Supporting
Information, Figure S7a). The cross-sectional SEM image in
Figure 3 b shows that the MXene membrane possesses
a typical lamellar structure. Elemental maps in Figure 3 c–h
Figure 1. MXene membrane preparation.
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Figure 3. a) SEM image (inset: macroscopic photograph) of the
MXene membrane surface. b) High magnification of SEM image of the
cross-sectional view of the MXene membrane supported on AAO
(inset: representation of the layered structure). c) Low-magnification
SEM image of the cross-sectional view of MXene membrane supported
on AAO and corresponding elemental maps of d) aluminum,
e) oxygen, f) titanium, g) carbon, and h) iron from the same area with
same the scale bar.
show that all elements distribute homogeneously and no
obvious signal of Fe remained, which confirms that the
Fe(OH)3 nanoparticles have been almost completely
removed by HCl dissolution. From Fourier transform infrared
(FTIR) spectroscopy and X-ray photoelectron spectroscopy
(XPS), as shown in the Supporting Information, Figures S8
and S9, the MXene surfaces are terminated by O, OH, and/or
F groups, which is in accordance with the previous report.[7]
After successful preparation, the MXene membrane was
applied in water purification and it was firstly evaluated with
the Evans blue (EB, 1.2 nm X 3.1 nm) solutions at room
temperature. It has to be noted that the vacant AAO support
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Communications
with uniform pore size around 200 nm gives a water permeance of 4500 L m@2 h@1 bar@1 and no rejections for EB molecules. From Figure 4 a, it can be found that the MXene
membrane with thickness of 400 nm exhibits a water permeance of 1084 L m@2 h@1 bar@1 and a high rejection rate of 90 %
Figure 4. a) Comparison of the performance of the M1, M2, and
MXene membranes for the separation of EB molecules at room
temperature. b) Separation performance of the MXene membranes for
different molecules with different sizes.
for EB molecules. The water permeance of the MXene
membrane is about 5 and 10 times higher than those of the M1
and M2 membrane, respectively. The improved water permeance can be attributed to the additional nanochannels
formed in the MXene membrane, as confirmed by SEM
images (Supporting Information, Figure S6d–f). It can be seen
that the thickness of the MXene membrane is smaller than
that of the M2 membrane but larger than that of M1
membrane, which indicates that the MXene membrane
exhibits a more loosely lamellar structure, equivalent to the
enlarged interspace between the MXene nanosheets. Additionally, the change of the interspace could also be confirmed
by the XRD patterns. As shown in the Supporting Information, Figure S10, compared to M1 membrane, the (002) peak
in the MXene membrane appears at a lower angle, which
indicates that the interspace between the MXene nanosheets
has been enlarged.[17, 21, 22] Therefore, these additional nanochannels provide additional transport fluidic channels for
water.
Dependence of the separation performance of the MXene
membrane on the membrane thickness has also been studied
using EB, Cytochrome (Cyt. c, 2.5 X 3.7 nm) molecules, and
gold nanoparticles (diameter of 5 nm) solutions. As shown in
the Supporting Information, Figure S11, when the MXene
membrane thickness is smaller than 0.8 mm, the rejection
increases with increasing thickness. For a thin MXene
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membrane, which contains some defects, water prefers to go
through the larger defects. When the membrane thickness is
bigger than 0.8 mm, the rejection reaches almost 100 %. The
reason is that a thicker membrane leads to fewer defects in
the selective layer, and thus the water flows through the gaps
between nanosheets.
To evaluate the pore size of the MXene membrane,
a series ions or molecules with different sizes have been
separated (Figure 4 b; Supporting Information, Table S1)
through the 400 nm-thick MXene membrane. It can be
concluded that the MXene membrane excludes nearly
100 % of bovine serum albumin (BSA), nearly 100 % of
gold nanoparticles (5 nm), 97 % Cyt. c, 93 % 5,10,15,20tetrakis-(N-methyl-4-pyridyl)-21,23-H-porphyrintetratosylate (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 less
than 1 nm, the membrane cannot effectively separate them
from the solution, such as K3[Fe(CN)6] (0.9 X 0.9 nm2) (with
the rejection of 32 %). These results indicate that the pore size
of the MXene membrane is around 2–5 nm. Moreover, the
membrane exhibits excellent water permeance for all of the
molecule solutions (around 1000 L m@2 h@1 bar@1). The perfect
separation of the proteins (BSA) further verifies the favorable applications of this nanoporous 2D lamellar membrane.
The UV/Vis absorption spectra of the retentate feed and
permeate solutions is summarized in the Supporting Information, Figure S12. Furthermore, the rhodamine B concentration in the retentate side increased gradually with time
(Supporting Information, Figure S13). It is clear to see that
the concentration of the solution on the retentate side is
obviously higher than that of the original feed solution.
Additionally, the total amount of molecules from both the
permeate and retentate sides is very close to the original feed
amount of the molecules, which implies that the molecules are
mostly rejected by the MXene membrane rather than being
absorbed or reacted with the membrane. Moreover, the UV/
Vis measurements of the solution after membrane immersion
(Supporting Information, Figure S14) and the XPS analysis of
each spent membrane (Supporting Information, Figure S15)
demonstrate that any physical adsorption or chemical reaction between the Mxene membrane and the molecular species
can be ignored. Therefore, the possible separation mechanism
of the MXene membrane would be molecule sieving due to
the different sizes of the membrane pores and the feed
molecules.
Moreover, the pressure dependence of the MXene
membrane on the separation performance was measured
using the device shown in the Supporting Information,
Figure S16. As demonstrated in the Supporting Information,
Figure S17 for a 400 nm-thick MXene membrane, with
increasing feed pressure from 0.1 MPa to 0.6 MPa, the
rejection rate for EB molecules decreases from 90 % to
82.5 % (data left of the dashed line). The rejection decreases
with increasing feed pressure suggests that there is a higher
contribution of convective transport of the solute through
defects at higher pressure. When the pressure was reduced to
the starting value of 0.1 MPa (data right of the dashed line)
after the pressure loading test, no serious decline of the
rejection and water permeance can be observed compared
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Communications
with the initial data, which demonstrates that the MXene
membrane is relatively stable in the pressure loadingunloading cycling.
Ideally, an efficient membrane used for industrial filtration and separation processes should not only have welldefined channel size with excellent selectivity and high water
permeance, but should also be robust.[16] In contrast to GO
membranes, the MXene membrane remains intact and
unchanged even after immersion in water for more than one
month (Supporting Information, Figure S18). Moreover, after
the long-term water immersion treatment, the MXene
membrane still exhibits favorable rejection rate and water
permeance with solution of 5 nm gold nanoparticles. Furthermore, the MXene membranes are hydrophilic, with
a water contact angle of 4588 (Supporting Information, Figure S19), which is beneficial for water separation. Its hydrophilicity and water stability augur well for the utilization of
the MXene membrane in water purification. For going a step
further for practical applications, the MXene membrane was
applied to filter gold nanoparticles (5 nm) over a long period
using a home-made cross-flow filtration device (Supporting
Information, Figure S20). In a 28 h filtration operation (Figure 5 a), the rejection efficiency and the water permeance
almost maintained at a constant level, which further demonstrates the good stability of the MXene membrane.[9]
Compared with other 2D membranes, including the GO,
MoS2, WS2, and other nanostructured membranes, the
MXene membrane prepared in this work exhibits excellent
separation performance under similar experimental
conditions (Figure 5 b; Supporting Information, Table S2).
Figure 5. a) Separation performance versus filtering time for filtration
of gold nanoparticles (5 nm) solution using the 1 mm thick MXene
membrane. b) Comparison of the separation performance of the
MXene membrane and various previously reported membranes, as
well as the commercial PES membrane (* = EB, ? = Cyt. c solutions).[17, 22, 24–26] For detailed experimental conditions, see the Supporting Information, Table S2.
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Although several membranes show higher rejection rate
(more than 98 %), their corresponding water permeance
remains much lower than 100 L m@2 h@1 bar@1. However, the
0.4 mm-thick MXene membrane not only shows a good
rejection rate (90 % for EB and 97 % for Cyt. c), but also
holds
an
extremely
high
water
permeance
(1084 L m@2 h@1 bar@1 and 1056 L m@2 h@1 bar@1, respectively)
in comparison with other water treatment membranes. Such
high water permeance of the MXene membrane can be
explained from the following two aspects. Firstly, MXene
nanofragments (nanosized MXene sheets) are used instead of
the traditional microsized MXene sheets to obtain short and
abundant transport pathways, which is beneficial for water
transport (Supporting Information, Figure S21). Our results
are in accordance with the finding of Zhu et al., who found
that the water permeation rate through the GO membranes
can be enhanced by decreasing the flake size of the nanosheets and/or creating more nanochannels between the
nanosheets.[23] Secondly, intercalated nanoparticles are used
as the former of the distance of slit pores between MXene
nanofragments, to achieve larger interlayer distance and
create more nanochannels after their removal (Supporting
Information, Figure S22). Huang et al. also gave some
evidence that the water permeance of the nanostrandchanneled GO membranes is 10-fold enhanced compared to
that of the GO membranes without sacrificing the rejection
rate.[22] Therefore, considering the shorter transport pathway
and more nanochannels formed in the MXene membrane
resulted from the above two structural advantages, the
MXene membrane exhibits excellent performance for water
transport. Compared with the commercial polymeric ultrafiltration membranes (30 kDa and 50 kDa polyethersulfone
(PES) ultrafiltration membranes, Sepro Company), our
MXene membrane exhibits much better water permeance,
as well as also higher rejections for each probe molecules/ions
under study (Supporting Information, Table S3).
In summary, a new kind of a 2D lamellar membrane based
on stacks of Mxene nanosheets are prepared successfully by
filtration deposition on AAO substrates. During the onfiltration, colloidal Fe(OH)3 has been used as distance holder
followed by HCl dissolution. The MXene membrane exhibits
an
excellent
water
permeance
(more
than
1000 L m@2 h@1 bar@1) and a high rejection rate (90 %) for
molecules with sizes larger than 2.5 nm when applied in water
purification. To the best of our knowledge, the MXene
membrane prepared in this work shows the highest water
permeance with proper rejection among various 2D membranes supported on porous substrate. Moreover, also the
excellent stability recommends the 2D lamellar MXene
membranes for applications in water purification. Mxenes,
as a new kind of 2D materials, opens a door for the
development of highly efficient membranes for water treatment and other applications.
Acknowledgements
We gratefully acknowledge the funding from by the SinoGerman center for Science Promotion (GZ 911), the Natural
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Communications
Science Foundation of China for Distinguished Young
Scholars of China (no. 21225625), Natural Science Foundation of China (21536005, 51621001), and the Natural Science
Foundation 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 – 1829
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Manuscript received: September 23, 2016
Revised: November 15, 2016
Final Article published: January 10, 2017
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