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Artificial Water Channels-MXene Membranes for
Enhanced Desalination

Fan Yi, Li-Bo Huang, Li Kong, Yeji Chen, Jie Gao, Xianghao Meng, Zexuan

He, Mihail Barboiu, Zhongyi Jiang

► To cite this version:

Fan Yi, Li-Bo Huang, Li Kong, Yeji Chen, Jie Gao, et al. Artificial Water Channels-MXene Membranes for Enhanced Desalination. CCS Chemistry, 2025, pp.1-13. 10.31635/ccschem.025.202405207. hal-05225358

HAL Id: hal-05225358

https://hal.science/hal-05225358v1

Submitted on 27 Aug 2025

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Distributed under a Creative Commons Attribution 4.0 International License
# Artificial Water Channels-MXene Membranes for Enhanced Desalination

Fan Yi¹, Li-Bo Huang¹*, Li Kong², Yeji Chen¹, Jie Gao¹, Xianghao Meng¹, Zexuan He¹, Mihail Barboiu³* & Zhongyi Jiang¹,⁴*

¹School of Chemistry and Chemical Engineering, Hainan University, Haikou 570228, ²Department of Chemistry, Hong Kong University of Science and Technology, Hong Kong 999077, ³Institut Européen des Membranes, Adaptive Supramolecular Nanosystems Group, University of Montpellier, ENSCM-CNRS, Montpellier 34095, ⁴Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072

*Corresponding authors: lhuang@hainanu.edu.cn; mihail-dumitru.barboiu@umontpellier.fr; zhyjiang@tju.edu.cn

**Cite this:** CCS Chem. **2025**, Just Published. DOI: 10.31635/ccschem.025.202405207

Biological aquaporins possessing unimolecular directional pores provide exceptional transport performances for water translocation. Inspired by biological counterparts, artificial water channels (AWCs) could be used to overcome the permeability/selectivity trade-off of traditional desalination membranes. Here, we demonstrate that the directional incorporation of AWCs in composite MXene membranes, provided the first example of biomimetic two-dimensional (2D) membranes, enabling Å-scale separations with intrinsic water-to-salt permselectivity on the operational reverse osmosis (RO) desalination conditions. Compared with the pristine MXene membrane, the synergy of size sieving and electrostatic repulsion of the AWCs-MXene membranes provided doubled permeance (4-6 L m⁻² h⁻¹ bar⁻¹), and salt rejection (87% (NaCl), 93% (MgCl₂), 97% (Na₂SO₄ and MgSO₄)). Additionally, the AWCs-MXene membranes showed significantly enhanced anti-swelling and anti-fouling

abilities, demonstrating high stability with almost no performance decline after 7-day filtration. Thus, the rational design of 2D AWCs-MXene membranes combined the adjustable interlayer spacing with adaptive AWCs pathways, dedicated to selective water transport under molecular control.

<figure><img src="image_3.png" /></figure>

**Keywords:** artificial water channel, sub-nanochannel, MXene, 2D membrane, desalination

## Introduction

Water scarcity is an important challenge due to the global increase in population, industrialization, and climate change.¹² To mitigate it, desalination by reverse osmosis (RO) relying on membranes that can translocate water and remove small solutes or ions has been more

and more adopted.³,⁴ Currently, the material used in RO processes is predominately polyamide-based thin-film composite (TFC) membranes limited by the permeance and selectivity trade-off.⁵

De novo design and construction of nano- and sub-nanochannel membranes significantly impact the performance of selective water permeation and have attracted
<figure><img src="image_4.png" /></figure>

widespread attention in the fields of materials science and biomimetics.⁶⁻⁸ Two-dimensional (2D) lamellar membranes formed by the face-to-face stacking of 2D materials, typically possess regularly spaced interlayer channels with dimensions of 1 nm or less, which hold promising applications in many separation scenarios by allowing controllable mass transport.⁹⁻¹⁵ Generally, mass transfer occurs within the physical interlayer gaps, referred to as interlayer spacing.¹⁶,¹⁷ Therefore, designing regularly spaced and sufficiently narrow interlayer water selective channels can enable the membranes to exhibit high permselectivity for solutes with similar physicochemical properties. Currently, widely used 2D materials include graphene derivatives, metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and transition metal carbides/nitrides (MXenes).¹⁸⁻²² These materials exhibit channel sizes ranging from 0.3 to 2 nm and demonstrate excellent performance in certain separation scenarios such as water reuse, carbon dioxide separation, and alcohol-water separation.²³⁻²⁵ Generally, narrower interlayer channels have the capability to selectively retain smaller solutes, albeit at the expense of reduced water flux.²⁶

MXenes are a novel family of 2D materials with an extremely high aspect ratio, making them easily fabricated into defect-free 2D lamellar membranes.²⁷⁻²⁹ Consequently, MXenes are frequently employed in pressure-driven membrane-based water treatment and purification technologies, offering promise for enhancing energy efficiency and lowering costs.²⁵,²⁹⁻³² However, the intrinsic hydrophilicity and high swelling tendency of MXenes membrane result in a high instability of interlayer spacing.³³ During the water purification process, the MXene membrane suffers from an expansion of interlayer spacing due to the intercalation of water molecules, leading to a significant performance decline in the separation applications.³⁴ To address this concern, studies have revealed that inducing self-crosslinking among MXene nanosheets can effectively curb swelling during water purification.³³ Similarly, by introducing intercalators (TiO₂, ethanol, pillar[n]arene, etc.), we can functionalize the interlayer spacing of the membranes and minimize swelling during application thereby demonstrating enhanced stability in water treatment.¹⁸,²⁰,³³,³⁵,³⁶ Nevertheless, the MXene membranes often exhibit inferior pressure-driven desalination performance, particularly with low salt rejection.

Inspired by aquaporins (AQPs), which enable highly selective transport of water molecules across cell membranes, artificial water channels (AWCs) possess higher levels of physical and chemical stability, which allows for their potential integration with most synthetic materials.³⁷⁻³⁹ Similar to AQPs, AWCs stabilize water channel superstructures that prevent their disruption and explain the selective salts/proton rejection.³⁹,⁴⁰ Therefore, AWCs have a tremendous potential to enhance the desalination performances of composite membranes embedding such

water channel dedicated superstructures embedded in polymeric host matrices.⁴¹⁻⁴⁵ The supramolecular self-assembly of AWC monomers is heavily influenced by the intrinsic properties of host matrixes. Depending on polymerization conditions, solid percolated multi-channels are most of the time disordered within macroscopic solid polyamide membranes.⁴⁶⁻⁴⁹ Although AWCs have made some progress in polymeric membranes, their feasibility in 2D membranes has not yet been investigated.

We postulated that multichannel AWCs embedded in 2D lamellar membranes might become oriented and exhibit similar high water permselectivity, as observed for single-channel AWCs in bilayer membranes. In the present work, we demonstrate an efficient strategy to integrate the 2D nanochannels of Ti₃C₂Tₓ MXene membrane with interlayer AWCs dedicated to the selective transport of water under molecular control. With this facile one-step strategy, we fabricated the highly precise and controllable AWCs-MXene membranes. The Ti₃C₂Tₓ (Tₓ = -OH, =O and -F) nanosheets are perfect building/host nano-units featuring abundant functional groups on their surface, thus facilitating facile functionalization and directional insertion of lamellar AWCs filling 2D interlayer space of Ti₃C₂Tₓ membrane. The intercalation of AWCs into MXene membranes significantly enhanced the long-term stability. In contrast to graphite oxide or other 2D membranes suffering from substantial permeance decline during operation,¹⁸,⁵⁰ the AWCs-MXene membranes displayed stable performance throughout 7-day filtration, underscoring their potential for practical, long-term applications in membrane-based separations.

## Experimental Methods

### Preparation of single-layer Ti₃C₂Tₓ nanosheets

The Al layer in Ti₃AlC₂ powder was etched by HF generated in situ by a reaction between HCl and LiF. Firstly, 1 g of LiF powder was slowly added to 20 mL of concentrated HCl while being stirred until completely dissolved. Next, 1 g of Ti₃AlC₂ (200 mesh) powder was slowly added to the mixture of LiF and HCl etching solution, stirred uniformly, and then heated to 45 °C and stirred for 36 h. It should be noted that a small slit or vent should be reserved in the Teflon reactor for the release of hydrogen, generated during the reaction. After the reaction was completed, the precipitate was washed repeatedly with deionized water. The resulting solution was divided equally into centrifuge tubes and the supernatant was poured off at the end of centrifugation (4000 rpm for 20 min at room temperature). During the washing process, the centrifuge speed was gradually increased from 4,000 to 10,000 rpm. Washing was repeated several times until the supernatant pH was > 6. Next, the solution was centrifuged at 1500 rpm for 15 min at room
temperature, and the black supernatant poured out was the stripped solution of MXene nanosheets. The MXene nanosheets possess the characteristics of high surface area and facile functionalization. Compared with the ultrasonic method, this approach yielded MXene nanosheets with larger lateral size, higher concentration, and better oxidation resistance. After freeze-drying, it was determined that the concentration of the prepared Ti₃C₂Tx solution was 4.5 mg mL⁻¹.

## Synthesis of H4

The chemical reaction is depicted in the Supporting Information Scheme S1. 10 mmol of amino propanediol was mixed with an equal molar amount of octyl isocyanate uniformly under sonication. Then the mixture was dissolved in 20 mL of a mixture solvent consisting of tetrahydrofuran (THF), ethyl acetate (EA), and acetonitrile (CH₃CN) in a volume ratio of 2:1:1, while being stirred until complete dissolution. Subsequently, the reaction system was heated to 80 °C and stirred for 2 h under reflux. Then the resulting solution was dried by vacuum rotary evaporation to obtain a white solid. The obtained solid was washed with methanol and hexane and then collected as the solid precipitate by centrifugation. The remaining solvents were dried in a vacuum, and the resulting white powder was the product.

## Synthesis of HC8

The chemical reaction is depicted in the Supporting Information Scheme S2. 10 mmol of octyl isocyanate was dissolved in 25 mL of a mixed solvent consisting of THF, EA, and CH₃CN in a volume ratio of 2:1:2 under stirring. Then an equimolar amount of histamine was added, and the reaction mixture was heated to 80 °C and refluxed for 3 h with stirring, after that histamine was gradually dissolved in the solution. Upon completion of the reaction, a white precipitate appeared in the flask. After removing the solvents using vacuum rotary evaporation, the resulting solid was dissolved in a small amount of ethanol and a large amount of hexane was added with stirring, causing a gradual precipitation of white solid. The liquid was separated using centrifugation, and the solvent was completely removed from the solid under vacuum drying. The resulting solid was the product.

## Preparation of H4-Ti₃C₂Tx and HC8-Ti₃C₂Tx membranes

First, the prepared Ti₃C₂Tx solution was diluted ten times with water and then used for membrane preparation. Next, 5 mg of **H4** or **HC8** powder was dissolved in a mixture solvent of 30 mL water and ethanol in a 1:1 volume ratio. The solution was ultrasonicated for 30 min to ensure uniform dispersion. Then a certain volume (600 µL) of Ti₃C₂Tx nanosheets solution was added to the **H4** or

**HC8** solution and stirred to disperse evenly. Subsequently, vacuum-assisted self-assembly was employed to filter the dispersion liquid onto a polyethersulfone (PES) support. In cross-flow filtration applications, the actual usable diameter of the membranes was 20 mm.

## Desalination performances of Ti₃C₂Tx, H4-Ti₃C₂Tx, and HC8-Ti₃C₂Tx membranes

In cross-flow filtration experiments, a volume of 1000 mL of salt solution with a concentration of 1000 ppm was used each time. The membrane was sealed in the cross-flow cell and subjected to a transmembrane pressure of 4 Bar. Ion chromatography was employed to determine the salt concentration of the collected permeate. The permeance (P, L m⁻² h⁻¹ bar⁻¹) was calculated by the equation 1:

$$P = \frac{\Delta V}{A \times t \times \Delta P} \qquad (1)$$

where ΔV (L) is the corresponding volume on the permeant side; A (m²) is the effective area of the membranes; t (h) is the filtration time; and ΔP (bar) is the operational transmembrane pressure. The salt rejection was calculated as equation 2:

$$R = \frac{C_f - C_p}{C_f} \times 100\% \qquad (2)$$

where C_f and C_p are the salt concentrations in the feed solution and on the permeant side measured by the ion chromatography, respectively.

## Results and Discussion

### Synthesis of AWCs and fabrication of AWCs-MXene membranes

Two types of previously reported alkylureido-monomers **H4** and **HC8** (Figure 1a and Supporting Information Figures S1 and S2) have been used to self-assemble into Hydroxy and I-quartet AWCs, respectively.⁵¹⁻⁵³ The urea, hydroxyl, and imidazole groups exhibited strong H-bonding interactions with the Ti₃C₂Tx surface enabling strong interaction of AWCs lamellar phases with compatible 2D MXene host matrix. Subsequently, the vacuum-assisted self-assembly method was utilized to intercalate **H4** and **HC8** AWCs into the interlayer space between Ti₃C₂Tx nanosheets, resulting in the formation of the functionalized membranes **H4**-Ti₃C₂Tx and **HC8**-Ti₃C₂Tx (Figure 1b), respectively. Experimental findings indicated that, in comparison to the pristine Ti₃C₂Tx membrane, the functionalized membranes exhibited enhanced water permeance, salt rejection, and long-term stability, mainly attributed to the rapid and high water permselectivity of AWCs. Based on our previous
research, Hydroxy channels and I-quartet channels could be obtained through adaptive self-assembly of alkyureido-polyol **H4** and alkyureido-imidazole **HC8**, respectively.³⁷,⁴⁷ During the drying process, AWCs tended to form polycrystalline structures, as shown in Figure 1a.⁵² In channels composed of hydroxyls or imidazoles, H-bonding occurred with water molecules, encapsulating water wires or clusters that facilitated the rapid transport of water molecules within AWCs. This represented a straightforward and efficient approach for integrating high-density AWCs with nanoscale directionality useful for the improvement of membrane separation performances, displaying potentially inspiring broader applications.

## Characterization of Ti₃C₂Tₓ nanosheets

Ti₃C₂Tₓ nanosheets were fabricated using an etching method, where the reaction between HCl and LiF generated HF in situ to selectively remove the Al atoms layer from Ti₃AlC₂ MAX (M, early transition metal; A,

Aluminium; and X, carbon or/and nitrogen) powder.⁵⁴ The typical Tyndall effect (Supporting Information Figure S3) and Zeta potential (Supporting Information Figure S16) indicated that Ti₃C₂Tₓ nanosheets exhibited good dispersibility and stability in water. Through optimization of the etching conditions, Ti₃C₂Tₓ nanosheets with larger lateral dimensions were obtained, which contributed to the fabrication of regular stacked and defect-free Ti₃C₂Tₓ and AWCs-MXene membranes. As shown in Figure 2a, the scanning electron microscopy (SEM) of Ti₃C₂Tₓ nanosheets revealed their ultrathin thickness and translucent morphology, with the lateral size of Ti₃C₂Tₓ nanosheets being approximately 5.0 µm (Figure 2b and Supporting Information Figure S4). Transmission electron microscopy (TEM) further confirmed the ultrathin 2D structure of nanosheets, as shown in Figure 2c.

The corresponding energy-dispersive X-ray spectroscopy (EDXS) revealed a homogeneous Ti, C, O, and F elemental distribution within Ti₃C₂Tₓ nanosheets (Figure 2d). The high-resolution TEM (HRTEM) images

<figure><img src="image_5.png" /><figcaption>**Figure 1** | AWCs monomers and schematic illustration of AWCs-MXene membranes. (a) Chemical structures of **H4** and **HC8** monomers and their adaptive self-assembled Hydroxy and I-quartet channels, in which octyl and partial ureido group have been omitted for clarity. (b) Schematic representation of AWCs-MXene membranes obtained from the combination of Ti₃C₂Tₓ nanosheets and interlayer inserted AWCs, resulting in the formation of combined interlayer nanochannels with directional sub-nanometric water permselectivity pathways within hierarchical pore functionalized biomimetic 2D membranes.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 2** | Characterization of the as-synthesized Ti₃C₂Tₓ nanosheets. (a) SEM image of Ti₃C₂Tₓ nanosheets. (b) TEM image of Ti₃C₂Tₓ nanosheets and (c) the corresponding elemental mapping. (d) Lateral size distribution of Ti₃C₂Tₓ nanosheets. (e) AFM image of one Ti₃C₂Tₓ nanosheet and (f) the corresponding height profile of the Ti₃C₂Tₓ nanosheets on cleaved mica.</figcaption></figure>

revealed distinct lattice fringes (Supporting Information Figure S5), indicating the high crystallinity and structural integrity of the prepared Ti₃C₂Tₓ nanosheets. Moreover, the selected area electron diffraction (SAED) pattern (Supporting Information Figure S6) of the Ti atom layer within the Ti₃C₂Tₓ nanosheets clearly exhibited its hexagonal symmetry.

The atomic force microscopy (AFM) showed that the thickness of Ti₃C₂Tₓ nanosheets was about 1.9 nm (Figure 2e,f), consistent with the theoretical monolayer thickness.⁵⁵ Comparing the X-ray photoelectron spectroscopy (XPS) spectrum of MAX powder and Ti₃C₂Tₓ nanosheets (Supporting Information Figure S7) revealed the disappearance of Al element in the Ti₃C₂Tₓ nanosheets spectrum, indicating that the Al element was effectively removed during the etching process. Further, Supporting Information Figure S8 shows the X-ray diffraction (XRD) patterns of MAX powder and Ti₃C₂Tₓ nanosheets. In comparison to the diffraction peaks of MAX powder, the intense diffraction peak at around 39° (2θ) disappears in Ti₃C₂Tₓ nanosheets, and the (002) peak shifts to lower angles, confirming the successful exfoliation of Ti₃C₂Tₓ nanosheets from the MAX powder.

The Ti₃C₂Tₓ, **H4**-Ti₃C₂Tₓ, and **HC8**-Ti₃C₂Tₓ membranes were well prepared by layer-by-layer stacking of nanosheets through vacuum-assisted self-assembly on PES support and formation of an oriented layered structure. As shown in Supporting Information Figure S9, 600 µL of Ti₃C₂Tₓ nanosheets solution was uniformly dispersed into the dispersion of **H4** and **HC8** colloidal

aggregates to prepare **H4**-Ti₃C₂Tₓ and **HC8**-Ti₃C₂Tₓ membranes through the vacuum-assisted self-assembly process. Previous dynamic light scattering (DLS) experiments revealed that a mixture of ethanol:water at a 1:1 ratio, **H4** and **HC8** compounds self-assembled into colloidal aggregates with a hydrodynamic diameter (Dh) at approximately 600 nm.<sup>41,51,52</sup> After vacuum drying preparation of the Ti₃C₂Tₓ, **H4**-Ti₃C₂Tₓ and **HC8**-Ti₃C₂Tₓ membranes (Supporting Information Figure S10), the **H4** and **HC8** AWCs were firmly intercalated in the interlayer spacing of Ti₃C₂Tₓ nanosheets.

## Characterization of Ti₃C₂Tₓ, H4-Ti₃C₂Tₓ and HC8-Ti₃C₂Tₓ membranes

To further illustrate the interlayer inclusion of **H4** and **HC8** compounds, XPS was used for characterizing the reference Ti₃C₂Tₓ and functionalized **H4**-Ti₃C₂Tₓ, **HC8**-Ti₃C₂Tₓ, membranes, which revealed the presence of nitrogen characteristic peaks from **H4** and **HC8** compounds in both spectra (Supporting Information Figure S11). The C1s deconvolution spectra of Ti₃C₂Tₓ, **H4**-Ti₃C₂Tₓ, and **HC8**-Ti₃C₂Tₓ membranes are shown as Supporting Information Figure S12. After the interaction of **H4** and **HC8**, the emergence of C-N (285.7 eV) peaks could be observed, with respective fractions of 13.64% (**H4**-Ti₃C₂Tₓ) and 29.94% (**HC8**-Ti₃C₂Tₓ). The corresponding intensity of the N-C=O (288.2 eV) peak showed a significant increase. Additionally, Supporting Information Figure S13 shows the O 1s deconvolution
<figure><img src="image_7.png" /></figure>

spectra of the membranes. The O=C (532.4 eV) peak fraction increased significantly from 13.99% of Ti₃C₂Tₓ to 25.01% of **H4**-Ti₃C₂Tₓ and 29.55% of **HC8**-Ti₃C₂Tₓ. Specifically, in the **H4**-Ti₃C₂Tₓ membrane, the intensity of the C-OH (531.3 eV) peak exhibited a significant enhancement, primarily due to the **H4** compound containing two hydrophilic hydroxyl groups. These data indicated the successful intercalation of **H4** and **HC8** into the Ti₃C₂Tₓ membrane.

Furthermore, attenuated total reflection Fourier-transform infrared (ATR-FT-IR) spectroscopy was used to confirm the structure of Ti₃C₂Tₓ, **H4**-Ti₃C₂Tₓ, and **HC8**-Ti₃C₂Tₓ membranes (*Supporting Information Figure S14*). The FT-IR spectrum of the Ti₃C₂Tₓ membrane (black curve) showed peaks for stretching vibration peak of C-C (1290 cm⁻¹), C-F (1390 cm⁻¹), and O-H (1637 cm⁻¹) bonds. The appearance of the stretching vibration peaks of C=O at 1630 cm⁻¹, the bending and stretching vibration peaks of N-H at 1570 and 3311 cm⁻¹, and the stretching vibration peaks of C-N at 1240 cm⁻¹, all of which were attributed to the presence of the urea groups in **H4** and **HC8** compounds observed from the FT-IR spectra of **H4**-Ti₃C₂Tₓ and **HC8**-Ti₃C₂Tₓ (red and blue curves). The asymmetric and symmetric stretching vibration peaks of methylene (-CH₂-) and methyl groups (-CH₃) in alkyl chains were observed at 2926 and 2850 cm⁻¹ and at 1450 and 1380 cm⁻¹, respectively. The presence of a characteristic absorption band of organic compounds in the functionalized membrane indicated the successful intercalation of **H4** and **HC8** AWCs into the Ti₃C₂Tₓ membrane. Similarly, *Supporting Information Figure S15* shows that the XRD of Ti₃C₂Tₓ, **H4**-Ti₃C₂Tₓ, and **HC8**-Ti₃C₂Tₓ membranes provided empirical proof for the AWCs embedding, as mentioned above. After the interaction of AWCs, the characteristic diffraction peaks of **H4** and **HC8** molecules appeared in the XRD patterns. The patterns revealed a shift in the **H4**-Ti₃C₂Tₓ and **HC8**-Ti₃C₂Tₓ membranes' peak, potentially attributed to alterations in the crystal arrangement of **H4** and **HC8** AWCS within the confined interlayer spacing.

As shown in *Supporting Information Figure S16*, the Zeta potential of Ti₃C₂Tₓ membrane is -14.93 mV, while that for **H4**-Ti₃C₂Tₓ and **HC8**-Ti₃C₂Tₓ membranes were -11.11 and -9.12 mV, respectively. The surface charge of Ti₃C₂Tₓ was modulated after the interaction of **H4** and **HC8** AWCs, mainly resulting from their hydrogen-bond interactions with anionic Ti₃C₂Tₓ nanosheets. Meanwhile, the surfaces of both **H4**-Ti₃C₂Tₓ and **HC8**-Ti₃C₂Tₓ membranes still maintained a negative charge. In addition, *Supporting Information Figure S17* exhibited the FT-IR spectra of **H4** and **HC8** powder. In **H4**-Ti₃C₂Tₓ and **HC8**-Ti₃C₂Tₓ membranes, the infrared peaks of O-H, N-H, and C-H bonds shifted to a lower wavenumber, which mainly arose from the fact that the formation of hydrogen bonds caused an increase in the original chemical bond length, resulting in a decreased vibration frequency.

Additionally, SEM and AFM were used to observe the surfaces of Ti₃C₂Tₓ, **H4**-Ti₃C₂Tₓ, and **HC8**-Ti₃C₂Tₓ membranes. The SEM image in Figure 3a depicts the surface of the Ti₃C₂Tₓ membrane, with C, Ti, O, and F elements uniformly distributed on the membrane surface (*Supporting Information Figure S18*). After intercalating **H4** and **HC8** AWCs, the surface of both membranes exhibited more grooves, as shown in Figure 3d,g, with the N element from **H4** and **HC8** evenly distributed on the surface (*Supporting Information Figures S19 and S20*). The AFM analysis (*Supporting Information Figure S21*) indicated that the arithmetic average roughness (*R*<sub>a</sub>) and root-mean-square roughness (*R*<sub>q</sub>) of the Ti₃C₂Tₓ membrane are 50.5 and 62.9 nm (*Supporting Information Table S1*), respectively. These values increase to *R*<sub>a</sub> = 54.4 nm and *R*<sub>q</sub> = 67.6 nm, or to *R*<sub>a</sub> = 68.8 nm and *R*<sub>q</sub> = 88.2 nm after intercalating **H4** and **HC8** molecules, respectively. These results suggested that the surface of the Ti₃C₂Tₓ membrane was relatively smooth, and the surface roughness increased after the intercalation of AWCs, leading to an increased surface area. This was expected to result in higher water permeance of the **H4**-Ti₃C₂Tₓ and **HC8**-Ti₃C₂Tₓ membranes during the cross-flow filtration.

Next, the wetting properties of the membrane surfaces were characterized through water contact angle measurements (*Supporting Information Figure S22*). Although Ti₃C₂Tₓ nanosheets exhibited high hydrophilicity, a reduction in hydrophilic behavior on the nanosheet surface was observed after vacuum drying, which led to a water contact angle of 99.1° ± 0.3° for the dry Ti₃C₂Tₓ membrane. Following the intercalation of **H4** and **HC8** AWCs, the water contact angles decreased to 92.0° ± 0.1° and 86.9° ± 0.1°, respectively. These results indicated greater hydrophilicity on the surfaces of the **H4**-Ti₃C₂Tₓ and **HC8**-Ti₃C₂Tₓ membranes, suggesting that the interaction with **H4** and **HC8** positively affected water affinity. This enhancement was expected to contribute to higher water permeance of the AWCs-MXene membranes.

TEM was also employed to investigate the interlayer spacing structure of the membranes. The cross-sectional TEM images of Ti₃C₂Tₓ, **H4**-Ti₃C₂Tₓ, and **HC8**-Ti₃C₂Tₓ membranes were presented in Figure 3b,e,h, and their corresponding inverse fast Fourier transform (IFFT) pattern profiles of the blue line (Figure 3c,f,i) were obtained. Due to the relatively large lateral dimensions of Ti₃C₂Tₓ nanosheets, they are stacked in a face-to-face ordered manner, forming well-defined interlayer nanochannel structures in all the membranes. The interlayer spacing of the Ti₃C₂Tₓ membrane was approximately 1.24 nm, as determined by TEM, and EDXS analysis confirmed the uniform distribution of C, Ti, O, and F elements in the corresponding regions (*Supporting Information Figure S23*). After the intercalation of **H4** and **HC8**, the interlayer spacing slightly increased to 1.37 nm (**H4**-Ti₃C₂Tₓ
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 3** | Morphology of the Ti₃C₂Tₓ, **H4**-Ti₃C₂Tₓ, and **HC8**-Ti₃C₂Tₓ membranes. The surface SEM images, cross-sectional TEM images, and the corresponding IFFT patterns of the blue line area. (a-c) Ti₃C₂Tₓ membrane, (d-f) **H4**-Ti₃C₂Tₓ membranes, and (g-i) **HC8**-Ti₃C₂Tₓ membranes.</figcaption></figure>

membrane) and 1.34 nm (**HC8**-Ti₃C₂Tₓ membrane), respectively. EDXS analysis revealed a uniform distribution of N elements introduced by **H4** and **HC8** molecules throughout the interlayer, indicating successful intercalation and homogeneous dispersion of **H4** and **HC8** monomers (Supporting Information Figures S24 and S25). The larger interlayer channels were reminiscent of intercalated AWCs having the ability to selectively and rapidly transport water molecules. Consequently, the **H4**-Ti₃C₂Tₓ and **HC8**-Ti₃C₂Tₓ membranes were expected to exhibit enhanced separation performance in salt rejection applications.

## Membrane separation performances

The water permeance and salt rejection of these membranes were evaluated under cross-flow filtration conditions (Supporting Information Figure S26). Subsequently, a series of salt solutions (NaCl, Na₂SO₄, MgCl₂, and MgSO₄) was employed to evaluate the separation efficacy, encompassing salts containing various types of ions and hydrated cations diameter (Supporting Information

Figure S27 and Table S2). As shown in Supporting Information Figure S28, the surface and cross-sectional SEM images of the PES membrane revealed that it exhibited nearly complete ion rejection when treated with salt solutions (Supporting Information Figure S29). First, the optimal composition of the membranes was evaluated. As shown in Figure 4a, varying Ti₃C₂Tₓ nanosheet loadings in membranes were evaluated for their separation performance with NaCl solution. Higher loading of Ti₃C₂Tₓ nanosheets led to thicker membranes (Supporting Information Figure S30), typically exhibiting fewer defects and longer mass transfer pathways, thereby enhancing the salt rejection while having a significant negative impact on membrane permeance. As previously observed in graphene and other types of 2D membranes.⁵⁶⁻⁵⁸ It was observed that as the thickness of the Ti₃C₂Tₓ membrane increased, the permeance continuously decreased while the rejection increased. The rejection reached its peak at a Ti₃C₂Tₓ membrane thickness of 90 nm beyond which no significant improvement in rejection was noted, but the permeance continued to decrease. Therefore, when the
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 4** | Separation performances of Ti₃C₂Tₓ, **H4-Ti₃C₂Tₓ** and **HC8-Ti₃C₂Tₓ** membranes. (a) Ti₃C₂Tₓ membrane thickness dependent separation performance of NaCl solution. (b) Permeance and rejection performances of Ti₃C₂Tₓ, **H4-Ti₃C₂Tₓ**, and **HC8-Ti₃C₂Tₓ** membranes for 1000 ppm NaCl solution at 0.4 MPa. Permeance and rejection performance of **H4-Ti₃C₂Tₓ** (c) and **HC8-Ti₃C₂Tₓ** membranes (d) for 1000 ppm NaCl MgCl₂, Na₂SO₄, MgSO₄ solution. Time-dependent changes of permeance (e) and Na₂SO₄ rejection (f) of Ti₃C₂Tₓ, **H4-Ti₃C₂Tₓ**, and **HC8-Ti₃C₂Tₓ** membranes during the 7-day filtration.</figcaption></figure>

Ti₃C₂Tₓ membrane thickness was at 90 nm, the Ti₃C₂Tₓ membrane exhibited the optimal separation performance (P = 2.87 ± 0.08 L m⁻² h⁻¹ bar⁻¹, R(NaCl) = 46.38% ± 0.93%). Subsequent experiments were conducted using Ti₃C₂Tₓ nanosheets with the optimal thickness and equivalent amount.

In the subsequent preparation, varying amounts of **H4** and **HC8** were individually incorporated to obtain the corresponding **H4-Ti₃C₂Tₓ** and **HC8-Ti₃C₂Tₓ** membranes. As depicted in **Supporting Information Figure S31**, the performance of these membranes was improved with the increasing amounts of AWCs resulting in a progressive enhancement of both permeance and NaCl rejection performances. Optimal separation was achieved for a loading of 5 mg of **H4** or **HC8**, whereas an increase in permeance was observed, accompanied by a decrease in rejection at a loading of 10 mg. Consequently, the optimal addition amount of **H4** or **HC8** was determined to be 5 mg. Thermogravimetric analysis (TGA) was used to further determine the specific amount of water channel molecules added to the **H4** and **HC8** membranes. As shown in **Supporting Information Figure S32**, the TGA curves of **H4**, **HC8**, Ti₃C₂Tₓ nanosheets, **H4-Ti₃C₂Tₓ** membrane, and **HC8-Ti₃C₂Tₓ** membrane revealed that **H4** and **HC8** molecules decomposed significantly at 150 and 180 °C, respectively, while the Ti₃C₂Tₓ nanosheets showed minimal mass change throughout the heating process. TGA analysis

revealed that the actual loading masses of H4 and HC8 were 4.76 and 4.70 mg, respectively, and there was less loss of AWCs during the vacuum-assisted self-assembly process. As illustrated in Figure 4b, a comparative analysis of the separation performance demonstrates a 1.5-fold and 2-fold increase of the NaCl solution permeabilities of **H4-Ti₃C₂Tₓ** and **HC8-Ti₃C₂Tₓ** membranes, respectively for (5.95 ± 0.18 L m⁻² h⁻¹ bar⁻¹) and rejection (87.02 ± 1.74%), when compared with the reference Ti₃C₂Tₓ membrane. The intercalation of H4 and HC8 channels facilitated faster and more selective water transport.

The performances of **H4-Ti₃C₂Tₓ** and **HC8-Ti₃C₂Tₓ** membranes were evaluated with a series of salt solutions as shown in Figure 4c,d. The **H4-Ti₃C₂Tₓ** membrane exhibited a permeance of 4.57 ± 0.14 L m⁻² h⁻¹ bar⁻¹ and rejection of 87.02% ± 1.74% when processing a NaCl solution. When processing a MgCl₂ solution, the permeance increased to 5.55 ± 0.17 L m⁻² h⁻¹ bar⁻¹, and the rejection also increased to 92.80% ± 1.85%. Furthermore, when handling a Na₂SO₄ solution, the permeance further increased to 6.08 ± 0.18 L m⁻² h⁻¹ bar⁻¹, with the rejection increased to 97.03% ± 1.94%. Similarly, the **HC8-Ti₃C₂Tₓ** membrane showed a similar trend of permeance and rejection. These cations have different hydrated diameters, with Na⁺ being 7.16 Å and Mg²⁺ being 8.56 Å.²³,⁵⁹ The **H4-Ti₃C₂Tₓ** and **HC8-Ti₃C₂Tₓ** membranes demonstrated significant size-sieving effects. As the
ion-hydrated diameters increased, both permeance and rejection showed noticeable improvements. The interlayer channel size of the Ti₃C₂Tₓ reference membrane was larger than the cations' hydrated diameters, which resulted in low rejection of the membrane. However, after AWCs intercalated into the interlayer spacing, the functionalized membranes exhibited higher separation performances due to the reduction in mass transfer space and the selective and fast transfer of water clusters featuring hydroxy and I-quartet channels. The order of rejection of the salts was R(Na₂SO₄) > R(MgSO₄) > R(MgCl₂) > R(NaCl), which was in accordance with the Donnan exclusion theory, indicating that electrostatic repulsion played an important role in the salt rejection. The **H4-Ti₃C₂Tₓ** and **HC8-Ti₃C₂Tₓ** membranes were negatively charged, which could repel co-ions, and the repulsion of counter-ions to maintain the electrical neutrality of the feed solution could also affect the salt rejection. Additionally, the charged membranes were quite suitable for rejection of the high-valent co-ions salts such as MgCl₂, and Na₂SO₄, due to the electrostatic repulsion between the membrane and high-valent co-ions being much higher than the electrostatic attraction with the low-valent counter-ions. Therefore, the main separation mechanisms of the **H4-Ti₃C₂Tₓ** and **HC8-Ti₃C₂Tₓ** membranes were size sieving effect and electrostatic repulsion.

During the salt rejection process, as water molecules continuously permeated through the membrane, their intercalation into the nanosheet interlayer spacing caused the membrane swelling, leading to an increase in interlayer spacing. This expansion allowed salt ions to enter and block the interlayer channels, leading to a significant decrease in permeance and rejection, which greatly affected the long-term separation performance of the membranes. Hence, XRD analysis verified the change in *d*-spacing (calculated by Bragg's equation) of Ti₃C₂Tₓ, **H4-Ti₃C₂Tₓ**, and **HC8-Ti₃C₂Tₓ** membranes in the dry and completely wet states after immersion in solution overnight. As shown in *Supporting Information Figure S33*, the XRD of these membranes in dry and wet states revealed that the *d*-spacing of the Ti₃C₂Tₓ membrane increased by 1.54 Å, that is, changed from 13.58 Å in the dry state to 15.12 Å in the wet state, primarily due to its tendency to swell. In contrast, for the **H4-Ti₃C₂Tₓ** and **HC8-Ti₃C₂Tₓ** membranes, the change in *d*-spacing between dry and wet states was only around 0.3 Å, indicating that the intercalation of AWCs significantly enhanced the anti-swelling properties of the functionalized membranes. The high anti-swelling ability could greatly contribute to the long-term stability of the membranes' separation performance during salt rejection.

As shown in Figure 4e,f, the permeance and rejection of the Ti₃C₂Tₓ, **H4-Ti₃C₂Tₓ**, and **HC8-Ti₃C₂Tₓ** membranes were monitored over 7 days period while processing with Na₂SO₄ solution. We observed that after 48 h, the

separation performance of the Ti₃C₂Tₓ membrane began to decline, with a 56.43% decrease in permeance and a 26.10% decrease in rejection after 168 h. In contrast, the **H4-Ti₃C₂Tₓ** and **HC8-Ti₃C₂Tₓ** membranes did not exhibit significant deterioration in both permeance and rejection and maintained high stability over the same period. As shown in *Supporting Information Figure S34*, the TGA curves of the **H4** and **HC8** membranes after 7-day filtration showed that the loading amounts of AWCs in both **H4-Ti₃C₂Tₓ** and **HC8-Ti₃C₂Tₓ** membranes had barely changed. The mass loss of **H4** and **HC8** molecules was 1.74% and 0.68%, respectively, which suggested that the **H4-Ti₃C₂Tₓ** and **HC8-Ti₃C₂Tₓ** membranes exhibited high stability during the filtration. This indicated that the intercalation of AWCs significantly enhanced the membranes' fouling resistance and separation stability; thus, this method holds quite a promise for extension to other types of AWCs or materials.

To further understand the separation mechanism of these membranes for salt rejection, the zeta potential of the Ti₃C₂Tₓ, **H4-Ti₃C₂Tₓ**, and **HC8-Ti₃C₂Tₓ** membranes was measured under varying pH conditions (*Supporting Information Figure S35*). The isoelectric point (IEP) of the Ti₃C₂Tₓ membrane was around pH 2; therefore, the Ti₃C₂Tₓ membrane was positively charged at pH < 2 and carried a negative charge at pH > 2. Similarly, the IEP of **H4-Ti₃C₂Tₓ** and **HC8-Ti₃C₂Tₓ** membranes is at around pH 2.5 and 3.5, respectively. As shown in Figure 5a,b, the **H4-Ti₃C₂Tₓ** and **HC8-Ti₃C₂Tₓ** membranes were treated with Na₂SO₄ solution at different pH values, respectively. When the **H4-Ti₃C₂Tₓ** membrane was exposed to a Na₂SO₄ solution of pH 2.5, its permeance remained relatively unchanged compared with solutions of pH 1.0 and 6.0, but the rejection showed a slight decrease of 88.63% ± 1.77%. Similarly, when the **HC8-Ti₃C₂Tₓ** membrane was exposed to a solution with a pH of 3.5, the rejection also showed a slight decrease to 83.42% ± 2.09%. This indicated that the charge properties of the membranes influenced separation performance, attributed to the electrostatic repulsion between the membrane and ions, contributing to enhanced salt rejection. As shown in *Supporting Information Figure S36*, the XRD patterns of the fresh Ti₃C₂Tₓ membrane and the Ti₃C₂Tₓ membrane stored for three months after filtration exhibited minimal changes to peak shape. Both XRD patterns displayed a slightly enlarged (002) peak, indicating that the membrane structure had not undergone significant alterations. This suggested that the Ti₃C₂Tₓ nanosheets maintained high stability and oxidation resistance after being stacked to form the membrane.

As shown in Figure 5c, the AWCs-MXene membranes exhibited enhanced desalination performance. Compared with the other reported and our reference MXene membranes, they showed significant improvements in both permeance and salt rejection. Additionally, their separation performance reached a high level compared
<figure><img src="image_12.png" /><figcaption>**Figure 5** | *The influence of membrane surface charge on the performance and comparison with literature.* (a) The separation performance of **H4-Ti₃C₂Tₓ** membrane in Na₂SO₄ solution at pH = 1.5, 2.5 and 6.3. (b) The separation performance of **HC8-Ti₃C₂Tₓ** membrane in Na₂SO₄ solution at pH = 1.5, 3.5 and 6.3. (c) Rejection vs Permeance of **H4-Ti₃C₂Tₓ** membranes compared with other reported membranes in literature, see *Supporting Information Table S3* for detailed comparison.</figcaption></figure>

with graphene oxide or other materials. These enhance-
ments were mainly attributed to the parallel and orderly
interlayer spacing formed by the large lateral size of
Ti₃C₂Tₓ nanosheets, stabilized via AWCs intercalation.
Selective water translocation events through AWCs were
induced by size-sieving effects related to water cluster-
ing and pore flow convection effects, along with com-
bined electrostatic repulsion effects.⁵¹,⁶⁰,⁶¹ Together they
led to a significant enhancement in desalination. Thereby,
this method of introducing AWCs into Ti₃C₂Tₓ mem-
branes showed promising universality, and applying this
approach might introduce various types of AWCs into
other types of membrane materials (such as COFs, MOFs,
and polymers), potentially enhancing their separation
performance.

Conclusion

In summary, large lateral size Ti₃C₂Tₓ nanosheets were
synthesized and intercalated with **H4** and **HC8** mono-
mers forming Hydroxy and I-quartet AWCs within the
interlayer spacing of Ti₃C₂Tₓ membranes through vacu-
um-assisted self-assembly. The optimal separation per-
formance was achieved by controlling the amount of
Ti₃C₂Tₓ nanosheets and **H4** or **HC8** AWCs molecular
monomers. The water permeance and salt rejection of
the AWCs-MXene membranes were evaluated for NaCl,
MgCl₂, Na₂SO₄, and MgSO₄ salt solutions under cross-
flow filtration conditions. **H4-Ti₃C₂Tₓ** and **HC8-Ti₃C₂Tₓ**
membranes exhibited significantly enhanced permeance
and salt rejection compared with the pristine Ti₃C₂Tₓ
membranes. For instance, the permeance of the **H4-**
Ti₃C₂Tₓ membrane was 6.08 ± 0.18 L m⁻² h⁻¹ bar⁻¹ and
Na₂SO₄ rejection was 97.03% ± 1.94%, exhibiting almost
no decline (permeance 5.65 L m⁻² h⁻¹ bar⁻¹ and Na₂SO₄
rejection 95.59%) during filtration for up to 7 days, which
is far superior to the pristine Ti₃C₂Tₓ membrane (per-
meance 1.44 L m⁻² h⁻¹ bar⁻¹ and Na₂SO₄ rejection 42.21%).

Separation mechanistic studies revealed that the parallel
and regular interlayer channel structure of the funcio-
nalized membranes exhibited a size-sieving effect; due to
the charged nature of the composite membranes, the
electrostatic repulsion between the membrane surface
and ions also contributes to the enhancement of salt
rejection. The selective and water pore-flow transport
facilitated by AWCs contributed significantly to the in-
crease of both water permeance and salt rejection. Ad-
ditionally, the intercalation of Hydroxy and I-quartet
channels greatly improved the anti-swelling and anti-
fouling abilities of the AWCs-MXene membranes. This
study validates the feasibility of applying AWCs in MXene
membranes, thereby opening up new avenues for the
applications of AWCs and providing some valuable guid-
ance on the design of 2D membranes.

Supporting Information

Supporting Information is available and includes general information on materials, methods, and instruments; synthesis and characterization of the membranes; diffraction data; SEM, AFM, and TEM images; XPS data, FT-IR, zeta potential, water contact angle measurements and TGA; cross-flow filtration details; filtration data and comparison of filtration performances with literature data.

Author Contributions

Z. Jiang and L.B. Huang conceived the idea and designed
the experiment. F. Yi carried out the experiments and
analyzed the data. L. Kong, J. Gao, Y. Chen, X. Meng,
and Z. He contributed to the synthesis of Ti₃C₂Tₓ
nanosheets. M. Barboiu provided constructive sugges-
tions for membrane fabrication and characterization.
L.B. Huang, M. Barboiu, and Z. Jiang co-wrote the manu-
script. All authors participated in the discussion.
## Conflict of Interest

The authors declare no competing interests.

## Funding Information

The authors gratefully acknowledge financial support from the National Key Research and Development Program of China (grant no. 2022YFB3805204), the Startup Research Foundation of Hainan University (grant no. KYQD(ZR)-22061), and the M-ERA NET 2021 project (grant no. ANR-22-MER3-0002-01, COFFEE).

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