Self-Crosslinked MXene (Ti3C2Tx) Membranes with Good Anti

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Self-Crosslinked MXene (Ti3C2Tx) Membranes with Good Anti-Swelling Property for Monovalent Metal Ions Exclusion Zong Lu, Yanying Wei, Junjie Deng, Li Ding, Zhong-Kun Li, and Haihui Wang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b04612 • Publication Date (Web): 03 Sep 2019 Downloaded from pubs.acs.org on September 3, 2019

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ACS Nano

Self-Crosslinked MXene (Ti3C2Tx) Membranes with Good Anti-Swelling Property for Monovalent Metal Ions Exclusion Zong Lu, Yanying Wei,* Junjie Deng, Li Ding, Zhong-Kun Li and Haihui Wang*

School of Chemistry and Chemical Engineering, South China University of Technology, 510640 Guangzhou, China. Correspondence and requests for materials should be addressed to Yanying Wei ([email protected]) and Haihui Wang ([email protected]).

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ABSTRACT Two-dimensional (2D) membrane-based separation technic has been increasingly applied to solve the problem of fresh water shortage via ion rejection. However, these 2D membranes often suffer from the notorious swelling problem when immersed in solution, resulting poor rejection for monovalent metal ion. Design of the anti-swelling 2D lamellar membranes has been proved to be a big challenge for highly efficient desalination. Here, a kind of self-crosslinked MXene membrane is proposed for ion rejection with obviously suppressed swelling property, which takes the advantage of the hydroxyl-terminal groups on the MXene nanosheets by forming Ti-O-Ti bonds between the neighboring nanosheets via the self-crosslinking reaction (-OH + -OH = O- + H2O) through facile thermal treatment. The permeation rates of the monovalent metal ions through the self-crosslinked MXene membrane are about two orders of magnitude lower than that through the pristine MXene membrane, which indicates the obviously improved performance on the ion exclusion by self-crosslinking between the MXene lamellas. Moreover, the excellent stability of the self-crosslinked MXene membrane during the 70 h-long-term ion separation also demonstrates its promising anti-swelling property. Such a facile and efficient self-crosslinking strategy not only gives the MXene membrane good anti-swelling property for metal ions rejection, which is also suitable for many other 2D materials with tunable surface functional groups during membrane assembling.

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KEYWORDS membrane separation, two-dimensional, MXene membrane, ion sieving, swelling

Membrane-based separation technology has aroused increasing attention for water purification owing to its low energy consumption and easy operation.1-8 The burgeoning two-dimensional (2D) membranes have attracted strong attention due to their adjustable molecule/ion sieving capability, good thermostability, easy preparation and good mechanical property, such as the transition metal dichalcogenides (TMDs),9,

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graphene and graphene oxide (GO).11-15 These 2D membranes have been applied in small molecule sieving (dye, gas, solvent, etc.) due to their abundant nanochannels for mass transport and excellent sieving property with tunable pore size.16, 17 However, it is still a challenge to apply these 2D membranes in the field of ion rejection due to the notorious swelling problem. When immersed in aqueous solution, most 2D membranes tend to absorb water molecules into the interlayer space between neighboring 2D nanosheets, which results in swelling and weakened ion exclusion ability.18-21 It is pointed out that inhibiting swelling is significant to provide possibilities for 2D membranes to improve ion rejection performance.20 Therefore, several strategies have been developed to solve such swelling problem for their applications in ion rejection. One way is the physical confinement, for example, Nair et al.20 inhibited the swelling by physical means of encapsulation with epoxy, although it was complicated for industrial application. Another method is the chemical approach, such as covalent crosslinking by polymer molecules or cross-linking agents,16, 22-25 reduction of 2D nanosheets.26 Jin and the co-workers27 found that the GO 3


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membranes modified by potassium ions could effectively fix the interval spacing at about 11 Å by strong cation-π between the ions and nanosheets. Liu et al.26 found that the effective channel heights of the membrane can be fixed to ~8 Å by controlling the content of the oxygen-containing functional groups, which gave effective sieving performance for multi-valent ions. However, on the other hand, the GO membranes crosslinked with 1, 3, 5-benzenetricarbonyl trichloride (TMC) showed high water flux but low rejection of monovalent salts (only around 6~46%), due to the size effect of the cross-linking agent itself.24 In other words, because the cross-linking agent was intercalated in the interlayer of the 2D membrane, which occupied some space of the molecular size itself, leading to an increase on the d-spacing and consequent weakened ion rejection. Similarly, another kind of GO membrane crosslinked with ethylenediamine and diamine monomers showed suppressed over-expansion of interlayer spacing in aqueous solution, but the rejection of monovalent metal ions was only 30~40%.25 Although these crosslinking agents can fix the interlayer spacing to suppress the swelling of 2D membranes, but they also make the rejection of small ions (especial for the monovalent metal ions) sacrificed to some extent due to the size effect of themselves. Moreover, most crosslinking processes are always relatively complicated, which also hinder their further applications. Therefore, up to now, it is still difficult to fabricate the 2D membranes with highly anti-swelling property and excellent monovalent ions exclusion performance.18, 28, 29 Here, we propose a self-crosslinking strategy to prepare the 2D lamellar membranes with anti-swelling property based on MXene (Ti3C2Tx). MXene is a kind of early 4


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transition metal carbides and nitrides, which possesses abundant of surface functional groups with good flexibility and hydrophilic surfaces.30-39 The well-known dehydroxylation is usually applied in aluminosilicates with rich hydrophilic functional groups (hydroxyl), where the bond of Si-O-Si can be formed to change its hydrophilicity or realize functionalization.40-42 Considering that there are some hydroxyl groups on the surface of MXene nanosheets, the formation of Ti-O-Ti bonds is also expected by dehydroxylation between the neighboring MXene nanosheets. Once the bond of Ti-O-Ti is formed, the swelling behavior of the MXene membrane would be suppressed due to the relatively fixed interlayer spacing in solutions. Therefore, the dehydroxylated MXene membrane could has the ability to block monovalent metal ions benefited from the controlled stable interlayer spacing. In this work, series of selfcrosslinked MXene membranes (SCMMs) have been fabricated successfully via the self-crosslinking reaction (-OH + -OH = -O- + H2O) between the neighboring MXene nanosheets by facile thermal treatment of the pristine MXene membranes (PMMs), as shown in Figure 1. Because the stoichiometric ratio of hydroxyl groups on the surface of MXene nanosheet is usually no more than 26% and the surface terminal groups are randomly distributed on the MXene nanosheets, as shown in our previous work and literaures,37, 43 which indicates that the OH groups are not vicinal. In other words, the intralayer condensation can be ignored because the OH groups do not present in high enough concentration, while interlayer condensation will be the main reaction during treatment. The as-synthesized SCMMs exhibit obvious suppressed swelling property and excellent monovalent ions exclusion performance with good long-term stability 5


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compared with that of the PMMs before self-crosslinking due to the formed Ti-O-Ti bonds between the MXene nanosheets.

Figure 1. The self-crosslinking process of the MXene membranes via -OH + -OH = -O- + H2O between –OH on neighboring MXene nanosheets, i.e. the conversion from PMM to SCMM.

RESULTS AND DISCUSSION Synthesis and Characterizations of the MXene nanosheets: The MXene nanosheets solution, the PMMs and SCMMs were prepared as shown in Figure S1 and Note S1. The MAX (Ti3AlC2) particles shown in Figure 1a and Figure S2 were etched by a mixed solution of LiF and HCL to get MXenes (Ti3C2Tx). The concentration of the MXene nanosheets solution can be calculated as Note S2. The PMMs were fabricated by vacuum-assisted filtration on the porous polyamide substrate. In order to promote the self-crosslinking between the neighboring MXene nanosheets in the PMMs via -OH + -OH = -O- + H2O, thermal treatment at vacuum is followed to prepared the SCMMs. The images of the scanning electron microscopy (SEM, Figure 2b and c) and 6


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the atomic force microscopy (AFM, Figure 2d-e and Figure S3) indicate that the MXene nanosheets exhibit an average thickness of ~1.4 nm with lateral size in the range of 0.5~1 μm, and Figure 2e shows the size distribution of the MXene nanosheets. Considering the theoretical thickness of a monolayer MXene is ~1 nm, combining the thickness contribution of the surface functional groups on the nanosheets, the assynthesized 1.4 nm-thick MXene should be monolayer nanosheets.44 The X-ray diffraction (XRD) patterns in Figure 2f show the disappearance of intense diffraction peak of MAX at 39o (2θ) and the shift of (002) peak to lower angles, suggesting that the bulky MAX has been successfully exfoliated to MXene nanosheets.

Figure 2. (a) The SEM image of the Ti3AlC2 power. (b) and (c) SEM images of the MXene nanosheets deposited on the AAO substrate. (d) AFM image of a monolayer MXene nanosheet. (e) Size distribution of the as-synthesized MXene nanosheets. (f) XRD patterns of the Ti3AlC2 power and the PMM assembled with MXene nanosheets.

Characterizations of the PMMs and SCMMs: After thermal self-crosslinking

treatment at different temperatures, series of SCMMs can be obtained, which are named as SCMM-80, SCMM-120 and SCMM-180, where the thermal treatment temperature 7


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is labeled in the end. The SEM (Figure 3a-d) and AFM images (Figure S4) show that the surface of the MXene membranes become more and more rough with the increase of self-crosslinking temperature, and the roughness parameters of Rq (root mean square roughness) and Ra (arithmetic average roughness) increase from 127 nm and 97 nm to 191nm and 154 nm, respectively, as shown in Table S1. In addition, due to the dehydroxylation during self-crosslinking between the neighboring MXene nanosheets via -OH + -OH = -O- + H2O, Figure S5 showed that the water contact angle of SCMMs increased slightly with the increasing of treatment temperature. Furthermore, the energy-dispersive X-ray spectroscopy (EDXS) mapping images of the elemental distribution (Figure 3i-l) indicate the homogeneous distribution of Ti, C, O and F in the PMM and SCMMs even after thermal self-crosslinking. In addition, The atomic ratio of Al in the PMM and SCMMs shown in Table S2 is 0.43%, 0.15%, 0.23%, 0.17%, respectively, which indicates that Al atoms have been almost etched completely.45

Figure 3. The SEM and AFM images of the PMM and SCMMs. The SEM images of the membrane surface (a-d) and cross-section (e-h), inset images are the AFM images of membrane surface. (i-l) The elemental mapping analysis of the cross-section of the PMM and SCMMs. The dashed area indicates the selected area for EDXS mapping. (a, e, i) PMM; (b, f, j) SCMM8


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80; (c, g, k) SCMM-120; (d, h, l) SCMM-180.

As we can see from the Fourier transform infrared spectroscopy (FTIR) results of the PMM and SCMMs shown in Figure 4a, it is obvious that the adsorption peak strength of the bound water (3450 cm-1) and hydroxyl (1637 cm-1) gradually reduces with the increasing temperature of self-crosslinking. In addition, compared with the FTIR spectrum result of the PMM (Figure 4b), two obvious peaks at wavenumber of ~780 cm-1 and ~870 cm-1 can be found in that of SCMMs, which are attributed to the stretching vibration of Ti-O-Ti bond, indicating that the formation of bonds between the MXene nanosheets in the SCMMs.46 On the other hand, the Raman results of the PMM and SCMMs shown in Figure 4c prove the presence of -F, -O and -OH functional groups on the surface of all these MXene membranes, where the strong peak position at 200 cm-1 and 727 cm-1 corresponds to the A1g modes of Ti3C2O2, while the peak at 284 cm-1 is assigned to the Eg modes of Ti3C2(OH)2, and the two peaks at 124, 615 cm-1 correspond to the Eg modes of Ti3C2F2.47 Compared with that of the PMM, the Raman results of the SCMMs indicate that the bands corresponding to the A1g modes of Ti3C2O2 gradually shifted to lower wavenumber, from 727 cm-1 to 701 cm-1, which is resulted from the anharmonic frequency shift and quasiharmonic lattice expansion during the thermal self-crosslinking process.48 In order to identify the bonding reaction temperature, the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the MXene membrane are conducted. As shown in Figure 4d, there are two main regions of the weight loss from room temperature and 200 oC: from room temperature to 100 oC (Region I) and 100 9


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oC~200 oC (Region II). The weight loss in Region I is ~7.6 %, which could be associated

with the losing of free water between interlayer, while the weight loss in Region II is ~1.77 %, which should be related to the losing of combined water between interlayer and water produced by the bond reaction between the surface groups of MXene nanosheets. It can be noted that there is a tiny platform around 120 oC, which could be attributed to the occurrence of bonding reaction.40

Figure 4. (a) The FTIR spectrum of the PMM and SCMMs. (b) FTIR spectrum of the PMM and SCMMs with wavenumber ranging from 600~1500 cm-1. (c) The Raman spectra of the PMM and SCMMs, inset image is the partial enlargement in the range of 700-765 cm-1. (d) Thermogravimetric analysis of the MXene membrane from room temperature to 500 oC at vacuum with heating rate of 10 oC min-1. The inset curve shows the magnification of the temperature range from 50 oC~200 oC.

In order to analyze the elemental state inside of the MXene membranes, the etching XPS analysis was also conducted on the PMM and SCMMs. Figure 5a and b show that the chemical nature of oxygen in inner PMM and SCMM-180 after Ar+ sputtering for

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300 s using the 3kV Ar+ beam raster of 2.5 × 2.5 μm2 over the probed area. The O 1s region of the PMM shown in Figure 5a and Table S3 exhibit that the fractions of C-TiOx (moiety Ⅰ) and C-Ti-(OH)x (moiety Ⅱ) are 26% and 43%, respectively. After thermal self-crosslinking process, the C-Ti-Ox fraction in the SCMM-180 increases to 47%, while the fraction of C-Ti-(OH)x decreases to 11% (Figure 5b). In addition, the high resolution XPS spectra of the PMM and SCMM-180 in Ti 2p (Figure S6) and the full XRD patterns of PMM and SCMMs (Figure S7) demonstrates that there is no obvious change in the ratio of TiO2, where the proportion of TiO2 in the SCMM-180 can be calculated to be only 0.08% (Table S4) by the area ratio of XPS spectra (Figure S6), indicating that the membranes are not oxidized.49 As is known, there are only several existence form of O in the membranes: surface groups (including -OH, =O, O-) on the MXene nanosheets and TiO2 (if be oxidized). Considering that the amount of stable =O will not change during thermal treatment, thus, according to the law of conservation of O, the increased ratio of =O/-O- from 26% to 47% can be attributed to the increased amount of -O- , which indicates the gradually decreased -OH and formed -O- via the reaction of -OH + -OH = -O- + H2O during such thermal treatment. As shown in Figure 5c and d, when the etching depth reaches deeper than 15 nm, the proportion of elements inside of the MXene membranes tends to be constant, indicating a homogeneous state of the elements inside of the PMM and SCMM, while the atomic ratio change of Ti, C and F with etching depth smaller than 15 nm is due to the surface pollutants or slight oxidation.45 Furthermore, the atomic ratio of the PMM and SCMM180 below the etching depth of 15 nm shown in Table S5 indicates that the ratio of Ti/O 11


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increases from 1.66 to 2.10, and the ratio of C/O increases from 1.14 to 1.60, respectively. These results confirm the conversion of the Ti-OH groups to Ti-O groups between the MXene nanosheets after the self-crosslinking process. The high resolution XPS spectra of the PMM and SCMM-180 for C, and F with etching depth of 60 nm are showed in Figures S8-10 and the detailed XPS analysis of Ti, C and F are given in Note S3.

Figure 5. (a) Component peak-fitting of the XPS spectra for O1s with etching depth of 60 nm in the PMM and (b) SCMM-180. (c) The atomic concentration with XPS etching depth in the PMM and (d) SCMM-180. The etching rate is 0.2 nm/s, the total time is 300 s.

Furthermore, besides the above characterizations, the self-crosslinking reaction in the SCMMs has also been proved by XRD analysis. As is known, the d-spacing of the 2D membranes is a key factor for ion rejection. For the dry PMM and SCMMs as shown in Figure 6a, the d-spacing calculated from the Bragg’s equation gradually decreases from 13.6 Å to 12.8 Å, which indicates that the height of the 2D nanochannels decrease with the increasing thermal self-crosslinking temperature via -OH + -OH = -O- + H2O 12


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and dehydration. When immersed in solution during ion separation, the d-spacing of PMM expands to 16.6 Å due to the property of easy swelling, in contrast, the d-spacing of SCMMs could maintained at 15.4 Å owing to the suppressed swelling resulted from self-crosslinking reaction, and the swollen change decreases slightly with the increasing thermal self-crosslinking temperature (Figure 6b). The XRD results shown in Figure 6c suggest that the swelling of the MXene membranes has been suppressed by the selfcrosslinking reaction between the neighboring MXene nanosheets. During the thermal treatment of PMM, if only the dehydration occurred instead of self-crosslinking between the interlayer, the water molecules could be absorbed back reversibly when the membrane was immersed in water again. Therefore, in order to further verify that the irreversible self-crosslinking reaction indeed occurs between the nanosheets rather than the removal of interlayer water molecules, the SCMM-80, SCMM-120, SCMM180 were immersed in water for 6 h again followed with drying at room temperature for 12 h, named as “wet-to-dry” SCMMs. Comparing the d-spacings of the assynthesized SCMMs and corresponding “wet-to-dry” SCMMs shown in Figure 6d, it can be found that they exhibit similar d-spacings, demonstrating that the water molecules could not reversibly enter into the interlayer of the SCMMs after selfcrosslinking. The SCMMs exhibit stable 2D nanochannel structure due to selfcrosslinking between adjacent MXene nanosheets rather than only dehydration.

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Figure 6. (a) D-spacing of the PMM and SCMMs in dry state. (b) D-spacing of PMM and SCMMs in wet state. (c) The XRD patterns of the PMM and SCMMs in dry and wet states. (d) Comparison of the XRD patterns of the as-synthesized SCMMs and the wet-to-dry SCMMs (the one immersed in water again followed by drying) in dry states.

Ion exclusion and anti-swelling performance of the PMMs and SCMMs: In order to

investigate the effect of the self-crosslinking on the ion rejection and anti-swelling performance of the MXene membranes, the permeation rates of various monovalent ions through the PMM and SCMMs are measured in a home-made U-shape instrument, as shown in Figure S11. Figure 7 exhibits the relatively high monovalent ions permeation rates through the PMM compared with that through the SCMMs, where the permeation rates of K+ (hydrated diameter of 6.62 Å), Na+ (hydrated diameter of 7.16 Å) and Li+ (hydrated diameter of 7.64 Å) are 0.232 mol h-1 m-2, 0.222 mol h-1 m-2 and 0.283 mol h-1 m-2, respectively. Based on the previous XRD results shown in Figure 6c, the effective nanochannel height for mass transport between two adjacent MXene

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nanosheets can be calculated by the d-spacing subtracting the thickness of a monolayer MXene nanosheets (~1 nm).47, 50 Therefore, the effect empty spacing of wet PMM and SCMM-80, SCMM-120 and SCMM-180 immersed in solution are 6.6 Å, 5.6 Å, 5.5 Å and 5.4 Å, respectively. It can be found that the swollen PMMs with effect empty spacing of 6.6 Å has no obvious exclusion performance for the monovalent ions due to the partial dehydration of the dehydrated ions.20 To the contrary, the permeation rates of K+, Na+ and Li+ through the SCMM-180 are 3.37×10-3 mol h-1 m-2, 6.66×10-3 mol h1 m-2

and 8.8×10-3 mol h-1 m-2 (Figure 7 a and b), which are about 70, 33 and 32 times

lower than that through the PMM, respectively. This is because after self-crosslinking, the swollen empty spacing of the SCMM-180 is reduced to 5.4 Å due to the formation of bonds between two adjacent MXene nanosheets. In order to study the mechanism of ion permeation through our membranes, Na+ permeation through the PMM and SCMM-180 at different temperatures are carried out, as shown in Figure 7c. The permeation rate of Na+ follows the Arrhenius equation, exp (-E/kBT), where E is the activation energy and kB is the Boltzmann constant. It can be calculated that the activation energy for Na+ permeation through the PMM and SCMM180 with d-spacing of 6.6 and 5.4 Å are 2.34 kJ mol-1 (EPMM) and 7.66 kJ mol-1 (ESCMM180),

respectively, which indicates that it will be much easier for Na+ passing through

the PMM than passing through the SCMM-180. In addition, it can be found that the ion permeation rate varied by two to three orders of magnitude when the activation energy only changed three times.20 And it is reasonable that the ion permeation rate of Na+ passing through the PMM (0.222 mol h-1 m-2) is around 33 times higher than that 15


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passing through the SCMM-180 (6.66 ×10-3 mol h-1 m-2) due to the relatively smaller activation energy. It is known that the hydrated diameters are in order of K+ (6.62 Å) < Na+ (7.16 Å) < Li+ (7.64 Å), but all of them are larger than the empty spacing of the wet SCMMs immersing in solutions (5.4~5.6 Å, calculated as the d-spacing of membrane (15.4~15.6 Å) minus the thickness of monolayer MXene nanosheets (~10 Å)). In other words, all the hydrated monovalent metal ions would be dehydrated partially when entering the nanochannels of the membrane, which is in accordance with the literatures.20 Therefore, the transport of hydration ions through membrane with a narrower empty spacing not only depends on the size of hydrated ions, but also on the energy barrier associated with dehydration. That is why the permeation rates of K+, Na+, Li+ do not show the tendency with the order of their hydration diameters. Due to exposing to vacuum and the low H2O vapor pressure atmosphere, the -OH bound with Ti on the nanosheets has a tendency to decompose to -O- and H2O through the equilibrium driven chemical reaction -OH + -OH = -O- + H2O during selfcrosslinking process.51, 52 Subsequently, the produced water molecule starts to desorb from the surface of nanosheets, and the residual O atom is indicated to be bound to Ti atoms in the fcc position on the upper and lower nanosheets.43 Thus, the SCMMs exhibit much better ion exclusion performance with the ion permeation rates of around 10-3 orders of magnitude. Furthermore, it can be found that the water permeance through the SCMMs (Figure 7d) obtained by forward osmosis process (Figure S12) are in the range of 0.05~0.06 L 16


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h-1 m-2 bar-1, only a little bit lower than that through the PMM, indicating that the selfcrosslinking treatment does not make obvious sacrifice on the water flux through the MXene membranes. On the other hand, it is obvious that the salt rejection of SCMMs are much higher than that of the PMM owing to the decreased interlayer spacing. Taking NaCl as example, the salt rejection rate of 98.6% could be achieved by SCMM180, while only 55.4% is obtained by the PMM.

Figure 7. (a, b) The permeation rates of K+, Na+, Li+ through the PMM and SCMMs. (c)　 Temperature dependence of Na+ permeation rate passing through the PMM and SCMM-180. Dotted lines are best fits to the Arrhenius behaviour. (d) The water permeance and NaCl rejection through the PMM and SCMMs.

The long-term test of ion exclusion with PMM and SCMMs for more than 70 h are shown in Figure 8, which indicates that the self-crosslinking treatment gives the MXene membranes good anti-swelling property and stable monovalent ion rejection performance. It is obvious that the ion concentration in the permeate side through the 17


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PMM increases sharply with time with a high slope compared with that through the SCMMs, as shown in Figure 8a-c. In order to further analysis the slight change of the ion permeation rates through the PMM and SCMMs with time, the first derivatives of ion concentration in the permeate side versus time are shown in Figure 8d-f. The first derivative curve of the ion concentration in the permeate side of the PMM can be divided to several stages. In the first stage (0-10 h), the PMM became wet gradually during this process, and water molecules were adsorbed on the two-dimensional nanosheets by hydrogen bonding interaction.19 In the second stage (10-40 h), the nanochannels were in a relatively stable state due to the weak wan der Waals force between MXene layers, and the ion permeation rates tend to be constant. But after 40 h-operation, serious swelling occurred, MXene nanosheets tend to repel from each other in solution owing to the presence of hydrophilic functional groups, resulting in weak interaction force between layers to resist the entry of hydrated ions. After swelling, more monovalent ions entered into the interlayer and gave increasing ion permeation rates. In contrast, the SCMM-80, SCMM-120 and SCMM-180 almost exhibit horizontal lines of ion concentration with much slow growth slopes, and the corresponding first derivatives of ion concentration in the permeate side versus time (named as the instant concentration change rate) also seem to be horizontal lines, which indicates that the SCMMs give noteworthy low and stable monovalent ions permeation rates compared with the PMM, demonstrating the beneficial effect of the selfcrosslinking treatment on the anti-swelling property of 2D membranes. The selfcrosslinking process can effectively suppress the swelling problem of the MXene 18


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membranes, leading to better monovalent ion exclusion performance and long-term life. It should be noted that the monovalent ion permeation rates through the bare porous polyamide substrate (fresh one and the ones treated at various thermal self-crosslinking temperature) are about 10-1 mol h-1 m-2, as shown in Figure S13, indicating that the substrate gives no contribution on the ion exclusion. Moreover, the stability of the SCMMs under acid and base conditions is also studied. As shown in Figure S14a, there is no obvious change in the characteristic peak at wavenumber of 763~780 cm-1 in the FTIR spectrum of the SCMM-180 after immersion in acid and base solutions, which is attributed to the stretching vibration of Ti-O-Ti bond, indicating the Ti-O-Ti bond is stable in acid and base solutions.46 Furthermore, the full XRD patterns of the SCMM-180 after treatment with acid and base are shown in Figure S14b. It can be noted that there is no significant change on the phase structure and the d-spacing of SCMM-180 after treatment with acid/ neutral/ base solutions are 13.0 Å, 13.2 Å and 13.2 Å, respectively, which presents no obvious change compared with that of the wet-to-dry SCMM-180 (13.2 Å), demonstrating that the SCMM-180 is stable in acid/base solution. Additionally, it is also proved that the Ti-O-Ti still maintained a strong NMR signal after hydrolysis treatment, indicating the good stability of the condensed groups (Ti-O-Ti).53 Therefore, compared with polymer membranes, the SCMMs exhibit good stability in acid/base conditions, which is promising for desalination of salt solutions. As shown in Table S6, the salt rejection and water permeance of the SCMMs have 19


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been compared with that of many other two-dimensional membranes and some commercial polymer membranes applied in desalination or ions exclusion. It can be found that the ion rejections of the SCMMs (>97%) in this work are much higher than that of most other membranes, although the water permeance needs to be improved.

Figure 8. (a-c) The ion concentrations in the permeate side of the PMM and SCMMs with permeation time. (d-f) The first derivatives of the ion concentration in the permeate side of the PMM and SCMMs versus time.

CONCLUSIONS We propose a kind of self-crosslinked MXene membranes with good anti-swelling property and promising monovalent ion exclusion performance by simple thermal selfcrosslinking treatment taking advantages of the functional groups on the MXene nanosheets. The swelling of MXene membrane is obviously suppressed by self20


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crosslinking due to the formed -O- bridge between adjacent nanosheets, which gives a good long-term stability and monovalent ions rejection performance simultaneously. The FTIR, high resolution of etching XPS and Raman characterizations demonstrate the release of hydroxyl functional groups and the formation of Ti-O-Ti. Compared with the PMM, the permeation rate of the monovalent ions (K+, Na+ and Li+) through the SCMMs are reduced from the order of 10-1 mol h-1 m-2 to 10-3 mol h-1 m-2, i.e. the ion exclusion performance of the SCMMs have been improved by 30 to 70 times after selfcrosslinking treatment. In addition, owing to the formation of the strong bond between two adjacent MXene nanosheets during self-crosslinking, the SCMMs exhibit wonderful long-term stability for monovalent ions for more than 70 h. The instant concentration change rate of K+, Na+ and Li+ are 4.51×10-7 mol L-1 s-1, 4.18×10-7 mol L-1 s-1 and 5.25×10-7 mol L-1 s-1, respectively, which indicates that the SCMMs can effectively hold the nanochannels with good anti-swelling property after selfcrosslinking, even in harsh acid/base solutions. More importantly, such a facile thermal self-crosslinking strategy brings an idea to prepare 2D membranes with enhanced antiswelling property in the fields of seawater desalination and monovalent ions rejection.

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MATERIALS AND METHODS Preparation of the Ti3C2Tx (MXene) nanosheets. The 2D MXene nanosheets solution was obtained as follows:37 2 g of LiF (purchased from Aladdin) was dissolved in a mixture of 24 ml HCl (12 M, purchased from sinopharm Chemical Reagent Co., Ltd.) and 10 ml deionized (DI) water in a 250 ml Teflon beaker. Then 2 g Ti3AlC2 (purchased from Laizhou Kai Kai Ceramic Materials Co., Ltd.) was added to the solution with magnetic stirring at 35 oC for 24 h. Subsequently, the obtained suspension was diluted and washed by DI water, then centrifuged at 3500~5000 rpm for several times until the pH of the supernatant reached 6~7. The solution containing the singlelayer MXene nanosheets was obtained after ultra-sonication for several hours. Then the suspension was centrifuged at 3500 rpm for 30 min to remove the unexfoliated precursors, and the supernatant was the aimed 2D MXene nanosheets solution. The concentration of the obtained MXene solution was about 0.154 mg mL-1. Preparation of the PMMs. After a simple vacuum-assisted filtration (VAF) of the MXene solution on porous polyamide substrate (diameter of 0.45 mm, pore size of 0.22 μm, purchased from Jinteng Co., Ltd) shown in the third step in Figure S1, the pristine MXene membranes (PMMs) were obtained. The thickness of PMM was adjusted by controlling the amount of deposition MXene nanosheets solution on substrates. All MXene membranes were dried at room temperature for 12 h. Preparation of the SCMMs. The SCMM-80, SCMM-120, SCMM-180 were prepared from the PMMs via thermal treating at 80 oC, 120 oC and 180 oC for 24 h in a vacuum drying chamber for self-crosslinking, respectively. Subsequently, the series of 22


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SCMMs were cooled down to the room temperature. Measurement of the ions permeation. The membranes were sealed in the middle of a home-made U-shaped device (Figure S11a). The feed cabin was filled with monovalent ions solution (0.2 M), while the permeation cabins was loaded with DI water. The concentration polarization effect nearby the MXene membranes was avoided by magnetic stirring. The ionic conductivity was detected by the ion conductivity meter shown in Figure S11b, and the ion permeation rates were obtained by simple calculation based on the ionic conductivity (details can be found in Note S4). Calculation details of the water permeance and ion rejection can be found in Note S5. Characterizations. The SEM images were obtained from the Hitachi SU8220 device. The elemental mapping analysis was achieved from the EDX (Oxford EDS, with INCA software). The XRD analysis was conducted by the Bruker D8 Advance with filtered Cu-Kα radiation (40 kv and 40 mA, λ=0.154 nm), where the step was 0.02o, the step time was 2 s, the 2θ range was 2-10o or 2-90o. The FTIR characterization was conducted by Bruker VERTEX 33 units in the wavenumber range of 400-4000 cm-1. The XPS analysis was obtained using the ESCALAB 250 spectrometer (Thermo Fisher Scientific) with monochromatic Al-Kα radiation (1486.6 eV) under a pressure of 2 ×10-9 torr. The AFM images were obtained using a Bruker Multi Mode 8 scanning probe microscope (SPM, VEECO) in trapping mode. Raman spectroscopy was performed on the LabRAM Aramis Raman microscope with 633 nm laser excitation. Water contact angle was conducted by Dataphysics OCA40 Micro. The TGA and DSC were performed on the NETZSCH STA449F5. 23


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ASSOCIATED CONTENT Supporting Information: Supporting information is available free of charge from ACS Nano home page (https://pubs.acs.org/journal/ancac3). Additional supporting notes, figures and tables in supporting information.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Yanying Wei: 0000-0001-7481-8962 Haihui Wang: 0000-0002-2917-4739 Notes The authors declare no competing financial interest. Author Contributions Z.L conducted the experiments. Z.L., Y.W. and H.W. conceived the idea and designed the experiments. Z.L., J.D., Y.W. and H.W. analyzed the data and interpreted the results. Y.W. and H.W. supervised the project. Z.L. wrote the manuscript. The manuscript was written based on the contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT We gratefully acknowledge the funding from the Natural Science Foundation of China (21606086, 21536005 and 51621001), Ministry of Science and Technology of the People's Republic of China, Guangdong Natural Science Funds for Distinguished Young Scholar (2017A030306002), the Guangzhou Technology Project (no.

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201707010317) and Fundamental Research Funds for the Central Universities.

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