MXene Membranes Research Articles

Triple-enhanced Raman scattering sensors from flexible MXene/Au nanocubes platform via attenuating the coffee ring effect

Developing substrates that combine sensitivity and signal stability is a major challenge in surface enhanced Raman scattering (SERS) research. Herein, we present a flexible triple-enhanced Raman Scattering MXene/Au nanocubes (AuNCs) sensor fabricated by selective filtration of Ti3C2Tx MXene/AuNCs hybrid on the Ti3C2Tx MXene membrane and subsequent treatment with 1H,1H,2H,2H-perfluoro-octyltriethoxysilane (FOTS). The resultant superhydrophobic MXene/AuNCs-FOTS membrane not only provides the SERS substrate with environmental stability, but also imparts analyte enrichment to enhance the sensitivity (LOD = 1 × 10−14 M) and reliability (RSD = 6.41%) for Rhodamine 6G (R6G) molecules owing to the attenuation of the coffee ring effect. Moreover, the triple enhancement mechanism of combining plasmonic coupling enhancement from plasmonic coupling (EM) of nearby AuNCs at lateral and longitudinal direction of MXene/AuNCs-FOTS membrane, charge transfer (CT) from Ti3C2Tx MXene and target molecules and analyte enrichment function provides the substrate with excellent SERS performance (EF = 3.19 × 109), and allows efficient quantification of biomarkers in urine. This work could provide new insights into MXenes as building blocks for high-performance substrates and fill existing gaps in SERS techniques.

Nanoconfinement enabled non-covalently decorated MXene membranes for ion-sieving

Covalent modification is commonly used to tune the channel size and functionality of 2D membranes. However, common synthesis strategies used to produce such modifications are known to disrupt the structure of the membranes. Herein, we report less intrusive yet equally effective non-covalent modifications on Ti3C2Tx MXene membranes by a solvent treatment, where the channels are robustly decorated by protic solvents via hydrogen bond network. The densely functionalized (-O, -F, -OH) Ti3C2Tx channel allows multiple hydrogen bond establishment and its sub-1-nm size induces a nanoconfinement effect to greatly strengthen these interactions by maintaining solvent-MXene distance and solvent orientation. In sub-1-nm ion sieving and separation, as-decorated membranes exhibit stable ion rejection, and proton-cation (H+/Mn+) selectivity that is up to 50 times and 30 times, respectively, higher than that of pristine membranes. It demonstrates the feasibility of non-covalent methods as a broad modification alternative for nanochannels integrated in energy-, resource- and environment-related applications.

Visible-light-driven photocatalytic ZnO@Ti3C2Tx MXene nanofiltration membranes for enhanced organic dyes removal

Two-dimensional (2D) Ti3C2Tx MXene nanosheets have been identified as promising for constructing ultrathin laminar nanostructures with nanoscale water transport channels for water purification. Herein, ZnO@Ti3C2Tx MXene nanofiltration (NF) membranes were prepared via coupling ZnO nanoflowers to the Ti3C2Tx MXene-based laminar nanostructure, which exhibited photocatalytic self-cleaning properties driven by visible light for enhanced organic dye removal. The physicochemical properties and NF performance with and without photocatalytic treatment were systematically characterized. The prepared ZnO@Ti3C2Tx MXene NF membrane could reject 95.44% Congo red (CR) and demonstrated an excellent water permeability of 138.81 L·m−2·h−1·bar−1 (LMH/bar). In comparison to the Ti3C2Tx MXene membrane and the same amount of ZnO powders, the ZnO@Ti3C2Tx MXene membrane demonstrated higher photodegradation performance to 10 ppm MB. Impressively, the membrane showed an enhanced separation performance for 6 cycles with a RhB rejection > 88.48% and a permeability of 138.69 LMH/bar. Lastly, the synergistic photocatalytic mechanism of ZnO@Ti3C2Tx MXene composite nanostructure was investigated. Such results may offer a novel insight for the fabrication of laminar structured Ti3C2Tx MXene-based NF membranes with photocatalytic induced anti-fouling performance for enhanced dye water purification.

Roll‐To‐Roll Fabricating MXene Membranes with Ordered Interlayer Distances For Molecule And Ion Separation

AbstractApplication‐oriented assembly of two‐dimensional nanosheets with uniform nanochannels is critical for fabricating sophisticated, high‐performance membranes for water treatment. However, fabricating the desired membranes by a simple, fast, and effective method is a challenge as most of the previous methods are based on batch processes rather than a continuous roll‐to‐roll process. Here, a simple Meyer rod‐coating approach to continuously fabricate large‐size and flat MXene membranes at a scale up to 5 m is introduced. This study demonstrated that a high MXene concentration, above 10 mg mL−1, is critical in processability due to the desired viscosity, surface tension, and viscoelastic properties. The as‐made MXene membranes show that shearing and solutal‐Marangoni flow can considerably improve the ordering of the stacked MXene nanoflakes. Thus, the rod‐coated MXene membranes demonstrate a smaller surface roughness and interlayer distance compared to the MXene membranes fabricated by the most commonly vacuum‐assisted filtration. The rod‐costed MXene membranes show superior performance in dye and mono/divalent cation separation. The proposed roll‐to‐roll Meyer rod‐coating method can also be used to fabricate MXene‐based composites, such as MXene/carbon nanotubes and MXene/polymer, using the inks containing high concentration MXene and other desired compositions. This roll‐to‐roll method will promote an industry‐level fabrication and application of MXene‐based membranes.

Membrane Reactor Supported by MXene (Ti3C2TX) for Hydrogen Production by Ammonia Decomposition

Hydrogen (H2) production by ammonia (NH3) decomposition offers a solution to the energy problem. In this work, an MXene (titanium carbide, Ti3C2TX) membrane was successfully synthesized by depositing it onto the anodic aluminum oxide (AAO) substrate using vacuum-assisted filtration. Then, a membrane reactor supported by MXene was developed for H2 production by NH3 decomposition. The MXene membrane reactor could achieve an H2 permeance of 2.85 × 10–7 mol m–2 s–1 Pa–1 and an H2/N2 selectivity of 20 at 773.15 K. Next, a Ni-La/γ-Al2O3 catalyst prepared by impregnation and sintering was added to catalyze the NH3 decomposition process. The experimental results showed that the reactor could achieve a 99% NH3 conversion at 773.15 K, i.e., 9% higher than that of the packed catalytic reactor (without membrane). A mathematical model of the membrane reactor was developed to explain the experimental results. The calculated activation energies (Ead) of the MXene membrane for H2 and N2 permeances were 64.95 and 69.53 kJ mol–1, respectively. The activation energy (Ear) of NH3 decomposition was 147.8 kJ mol–1. Both the experimental and modeling results showed that the membrane reactor could enhance NH3 conversion and promote the production of high-purity H2.

Flexible Self-Powered Friction Piezoelectric Sensor Based on Structured PVDF-Based Composite Nanofiber Membranes.

With the rapid development of the economy and technology, intelligent wearable devices have gradually entered public life. Flexible sensors, as the main component of wearable devices, have been widely concerned. However, traditional flexible sensors need an external power supply, lacking flexibility and sustainable power supply. In this study, structured poly(vinylidene fluoride) (PVDF)-based composite nanofiber membranes doped with different mass fractions of MXene and zinc oxide (ZnO) were prepared by electrospinning and were then assembled to flexible self-powered friction piezoelectric sensors. The addition of MXene and ZnO endowed PVDF nanofiber membranes with better piezoelectric properties. The structured PVDF/MXene-PVDF/ZnO (PM/PZ) nanofiber membranes with a double-layer structure, interpenetrating structure, or core-shell structure could further enhance the piezoelectric properties of PVDF-based nanofiber membranes through the synergistic effects of filler doping and structural design. In particular, the output voltage of the self-powered friction piezoelectric sensor made of a core-shell PM/PZ nanofiber membrane showed a good linear relationship with the applied pressure and could produce a good piezoelectric response to the bending deformation caused by human motion.

Improvement of desalination performance by adjusting the arrangement of lamellar MXene membrane

Due to the large surface area, rich termination groups, and good electrical properties, MXene is regarded as one of the most potential 2D materials in seawater desalination. Exploring the optimization strategy of MXene membranes has been a current research hotspot. A new method for optimizing membranes is proposed in this paper. Firstly, the arrangement of the homogeneous MXene membrane is investigated by using molecular dynamics simulation. The simulation results showed that the strong electrostatic interactions sides at the entrance enhanced the ion sieving performance, especially for the two-SEI sides, but the water permeation is not high. A series of heterogeneous membranes is proposed to improve water permeation. The result shows that the water permeation of F-O membrane improves the maximum, the ion rejection of the heterogenous membrane is higher than the homogeneous membrane. Our work reported the arrangement of heterogenous MXene membrane improve the desalination performance, which provides a new membrane strategy for better desalination performance.

Hybrid membranes of polyphenylsulfone/functionalized MXene nanosheets for the pervaporation dehydration of acetone and isopropyl alcohol

AbstractThe resistance of pervaporation (PV) dehydration membranes to organic solvents is of great importance. Polyphenylsulfone (PPSU) is known to resist most organic media; however, its acetone dehydration performance has not been evaluated yet. In the present study, MXene hydrophilic nanosheets with hydroxyl functional groups were synthesized and then incorporated in the PPSU membrane to prepare the PPSU/MXene PV membrane for the first time. The prepared membranes were used in the dehydration of acetone/water and isopropyl alcohol (IPA)/water. By comparing the attenuated total reflectance Fourier transform infrared spectroscopy spectra of the pure membrane and the nanocomposite membrane, the incorporation of hydroxyl functional groups into PPSU via MXene loading was confirmed. MXene improved the hydrophilicity and tensile properties of the membrane. The separation factor in acetone dehydration dropped with MXene incorporation up to 1 wt% and then increased with further MXene loading; however, it stayed almost unchanged in IPA dehydration with adding 0.5, 1, and 1.5 wt% MXene to the membrane. Incorporating 1 wt% MXene, the pervaporation separation index (PSI) was improved by 5.8 times in acetone dehydration and 9.7 times in IPA dehydration. The PPSU/1 wt% MXene membrane provided a remarkably higher separation factor (261 vs. 136) and PSI (99 vs. 58 kg/m2h) in acetone dehydration than in IPA dehydration; however, total flux was slightly lower in acetone dehydration (0.38 vs. 0.43 kg/m2h). This study implies that PPSU/MXene is a promising membrane material for the PV dehydration of organics.Highlights MXene hydrophilic nanosheets were synthesized. The synthesized MXene was loaded into the polyphenylsulfone membrane. PPSU/MXene membranes were used in pervaporation dehydration. MXene improved the hydrophilicity and tensile properties of the membrane. PPSU/MXene is a promising membrane material for PV dehydration of organics.

Transversal nanochannel-enabled MXene laminated membranes for superior oil-water separation: A fluid mosaic cytomembrane inspired approach

Transition metal carbide and nitride materials (MXenes) have been greatly appreciated for their facilely exfoliated and hydrophilic two-dimensional (2D) layered structure to fabricate laminated membranes for oil separation. However, the denser slits and longer interlayer transport of traditional MXene membranes usually result in the poor permeability and antifouling performance. Herein, a novel approach was developed by synthesizing ultra-thin MXene and ferroferric oxide doped molybdenum disulfide (FM) and subsequently embedding FM into MXene laminates. This method yielded additional transversal nanochannels that enable lateral transport in a manner akin to that of a mosaic transport protein. Additionally, the incorporation of FM resulted in the creation of extremely irregular surfaces that form extensive membrane structures, reminiscent of tightly packed phospholipid bilayers. The incorporation of flower-like FM composites with MXene laminates endowed the resultant membrane with improved interface hydrophilicity, intensive surface roughness and enhanced permeability. These effects synergistically contributed to an exceptionally high pure water flux of the resultant membrane (3.12 × 104 L m−2 h−1), surpassing that of the control membrane (6.15 × 103 L m−2 h−1) without FM by over 5-fold. Furthermore, the optimum membrane exhibited enhanced permeance to oil mixtures (3.75 × 103 L m−2 h−1) and emulsions (4.25 × 102 L m−2 h−1), with separation of over 99% remaining. Impressively, the permeability of severely fouled membranes could be restored by 99% under visible light, highlighting the admirable photocatalytic self-cleaning ability enabled by MXene and FM. The underlying mechanisms that account for the superior antifouling performance and self-cleaning ability of the laminated membrane were also elucidated by the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. The facile fabrication strategy employing transversal transport laminates provides a remarkable approach for water purification and other fluidic separation applications.

ZIF-67/Ti3C2 MXene intercalated membrane with enhanced physical/chemical coupling light-to-heat conversion for laser ignition

Laser induced high temperature pulse (HTP) generation has been widely used in some high-immediacy applications requiring dramatic temperature changes. Recent years researchers put interest in light-to-heat (include photothermal, light induced exothermic reaction and combination of both) materials to reduce the initiating energy of the laser ignition. However, the laser energy for realizing sufficient light-to-heat conversion is still high. MXene exhibits excellent light absorption, high light-to-heat conversion efficiency and has abundant metastable atoms which trend to undergo exothermic chemical reactions, but its incomplete oxidation affects its total heat release when the laser power is relatively high. Herein, we have prepared a ZIF-67/Ti3C2 MXene intercalated membrane. It is found that porous ZIF-67 exhibits a synergistic effect on MXene and can significantly improve its physical/chemical coupled HTP by enhancing its light absorption and subsequent energy release during its oxidation. ε 2,4,6,8,10,12-(hexanitrohexaaza) cyclododecane (ε-CL-20) laser ignition experiments prove that ZIF-67/Ti3C2 MXene intercalated membrane exhibits better ignition performance than pure MXene membrane by reducing both ignition threshold (151.2 mJ) and ignition delay (21 ms) at a low laser power intensity (59.7 W/cm2). This work can provide an insight for the structural design of new light-to-heat membranes.

Tubular MXene/SS membranes for highly efficient H2/CO2 separation

AbstractAccurately constructing membranes based on two‐dimensional (2D) materials on commercial porous substrates remains a significant challenge for H2 purification. In this work, a series of tubular 2D MXene membranes are prepared on commercial porous stainless steel substrates via fast electrophoretic deposition. Compared with other methods, such as filtration or drop coating, and so on. such preparation route shows the advantages of simple operation, high efficiency for membrane assembly (within 5 min) with attractive reproducibility, and ease for scale‐up. The tubular MXene membranes present excellent gas separation performance with hydrogen permeance of 1290 GPU and H2/CO2 selectivity of 55. Furthermore, the membrane displays extremely stable performance during the long‐term test for more than 1250 h, and about 93% of the membranes from one batch have exceeded the DOE target for CO2 capture. Most importantly, this work provides valuable referential significance for other 2D materials‐based membranes for future application development.

MXene/ZIF-L co-stacking membranes with high water permeation for solute-tailored selectivity

Herein, we design a composite MXene membrane with high permeability and stability by introducing highly porous two-dimensional ZIF (zeolitic imidazolate framework)-L sheets. On the one hand, the ZIF-L sheets provide additional nanochannels in the vertical direction, which shorten and enrich the mass transport pathways of the MXene membrane. On the other hand, a moderate increase in interlayer spacing of Ti3C2TX nanosheets is detected after introducing ZIF-L. Additionally, the Ti-O-Zn/Ti-F-Zn chemical bonds between Ti3C2Tx and ZIF-L nanosheets provide structural stability to the composite membrane. When applied in nanofiltration, the optimal co-stacking membrane with a ZIF-L mass fraction of 9.10 % exhibits an ultrahigh pure water flux of 287.29 L m-2h−1 bar−1, which is ∼ 2.41 times higher than the pristine MXene membrane (124.98 L m-2h−1 bar−1). Meanwhile, the composite MXene/ZIF-L membrane shows an excellent dye/salt separation efficiency, as indicated by the high Congo Red rejection (99.2 %) and the low MgCl2 rejection (1.58 %) in a mixing dye/salt system. This work provides a promising strategy for constructing a two-dimensional membrane with high permeation for dye/salt separation.

Fabrication of a new composite membrane consisting of MXene/PES /PEI for biofuel dehydration via pervaporation

A novel two-dimensional transition metal carbide with great potential for diverse applications is MXenes, which are synthesized and characterized in this paper. The study focuses on developing polyethersulfone (PES) and polyetherimide (PEI) mixed membranes and their modification with MXene nanoparticles for ethanol dehydration. X-ray diffraction (XRD), and Fourier transform infrared (FTIR) techniques confirmed that MXene and MAX were appropriately synthesized. The morphology and characteristics of the membranes were investigated by FTIR, field emission scanning electron microscopy (FE-SEM) imaging, and EDX map analysis. the result of contact angle analysis shows that the hydrophilicity of the MXene membrane is modified. Also the membrane's pervaporation ability evaluated for ethanol dehydration. The modified membrane, containing 5 wt% MXene, demonstrates superior performance with a high flux rate (>81.7 g/m2h) and a reasonable separation coefficient (112.67).

Molecular simulation study of 2D MXene membranes for organic solvent nanofiltration

MXene membranes has emerged as promising membrane materials due to their rigid lamellar structure and easily functionalized terminal groups. Understanding the mechanism of solvent transportation through MXene membrane at the microscopic level is significant for the development of new MXene based membranes. In this study, MXene membranes with different interlayer d-spacings (e.g., 0.9 nm, 1.45 nm, 2.0 nm, and 2.5 nm) are investigated for organic solvent nanofiltration (OSN) of four solvents (e.g., acetone, acetonitrile, methanol, and ethanol) and a model solute (Nile red). In membrane with the d-spacing of 0.9 nm, the solvent fluxes are regulated by the arrangements of solvents in the membrane. An ordered orientation and arrangement of solvent would result in a high flux. At this time, the solvent arrangement is dominated by the interaction energies between the solvent and membrane surface. When the d-spacing is larger than 1.45 nm, the solvent property (e.g., viscosity) plays the dominant role in flux determination. Compared with reported membranes, a substantially higher solvent flux can be achieved through the 2D MXene lamellar membranes. The solute rejections are controlled by a size-sieving mechanism, and 100% rejection of Nile red could be obtained in MXene membranes with 0.9 and 1.45 nm d-spacings. From the bottom-up, this work reveals the key governing factors that dominate the solvent permeation and solute rejection of 2D MXene lamellar membranes, and would also promote the development of new OSN membranes.

Paper‐Mill Waste Reinforced Nanofluidic Membrane as High‐Performance Osmotic Energy Generators

AbstractNanofluidic membranes consisting of 2D materials and polymers are considered promising candidates for harvesting osmotic energy from river estuaries owing to their unique ion channels. However, micron‐scale polymer chains agglomerate in the nanochannels, resulting in steric hindrance and affection ion transport. Herein, a nanofluidic membrane is designed from MXene and xylan nanoparticles that are derived from paper‐mill waste. The demonstrated membrane reinforced by paper‐mill waste has the characteristics of green, low‐cost, and outstanding performance in mechanical properties and surface‐charge‐governed ionic transport. The MXene/carboxmethyl xylan (CMX) membrane demonstrates a high surface charge (ζ‐potential of −44.3 mV) and 12 times higher strength (284.96 MPa) than the pristine MXene membrane. The resulting membrane shows intriguing features of high surface charge, high ion selectivity, and reduced steric hindrance, enabling it high osmotic energy generation performance. A potential of the nanofluidic membrane is ≈109 mV, the corresponding current of up to 2.73 µA, and the output power density of 14.52 mW m−2 are obtained under a 1000‐fold salt concentration gradient. As the electrolyte pH increases, the power density reaches 56.54 mW m−2. This works demonstrate that CMX nanoparticles can effectively enhance the properties of the nanofluidic membrane and provide a promising strategy to design high‐performance nanofluidic devices.

Rational Design of MXene Hollow Fiber Membranes for Gas Separations.

One scalable and facile dip-coating approach was utilized to construct a thin CO2-selection layer of Pebax/PEGDA-MXene on a hollow fiber PVDF substrate. An interlayer spacing of 3.59 Å was rationally designed and precisely controlled for the MXene stacks in the coated layer, allowing efficient separation of the CO2 (3.3 Å) from N2 (3.6 Å) and CH4 (3.8 Å). In addition, CO2-philic nanodomains in the separation layer were constructed by grafting PEGDA into MXene interlayers, which enhanced the CO2 affinity through the MXene interlayers, while non-CO2-philic nanodomains could promote CO2 transport due to the low resistance. The membrane could exhibit optimal separation performance with a CO2 permeance of 765.5 GPU, a CO2/N2 selectivity of 54.5, and a CO2/CH4 selectivity of 66.2, overcoming the 2008 Robeson upper bounds limitation. Overall, this facile approach endows a precise controlled molecular sieving MXene membrane for superior CO2 separation, which could be applied for interlayer spacing control of other 2D materials during membrane construction.

Enhanced Yield of Large-Sized Ti3C2Tx MXene Polymers Nanosheets via Cyclic Ultrasonic-Centrifugal Separation.

Water pollution has spurred the development of membrane separation technology as a potential means of solving the issue. In contrast to the irregular and asymmetric holes that are easily made during the fabrication of organic polymer membranes, forming regular transport channels is essential. This necessitates the use of large-size, two-dimensional materials that can enhance membrane separation performance. However, some limitations regarding yield are associated with preparing large-sized MXene polymer-based nanosheets, which restrict their large-scale application. Here, we propose a combination of wet etching and cyclic ultrasonic-centrifugal separation to meet the needs of the large-scale production of MXene polymers nanosheets. It was found that the yield of large-sized Ti3C2Tx MXene polymers nanosheets reached 71.37%, which was 2.14 times and 1.77 times higher than that prepared with continuous ultrasonication for 10 min and 60 min, respectively. The size of the Ti3C2Tx MXene polymers nanosheets was maintained at the micron level with the help of the cyclic ultrasonic-centrifugal separation technology. In addition, certain advantages of water purification were evident due to the possibility of attaining the pure water flux of 36.5 kg m-2 h-1 bar-1 for the Ti3C2Tx MXene membrane prepared with cyclic ultrasonic-centrifugal separation. This simple method provided a convenient way for the scale-up production of Ti3C2Tx MXene polymers nanosheets.

Layer-by-layer repaired lamellar membrane for low stacking defect of MXene nanosheets and efficient separation performance in water purification

Two-dimensional (2D) materials have been widely considered to be used in advanced membranes for their excellent separation efficiencies. The filtration performance of lamellar membrane will be further improved after overcoming the stacking defects of 2D materials in membrane fabrication. Herein, a novel layer-by-layer repair method was proposed to fabricate MXene lamellar membranes with few defects via serial layer-by-layer compaction, crosslinking, glass rod rolling treatment and self-crosslinking. The method reduced the irregular arrangement of MXene nanosheets, repaired the wrinkled surface of every few-layer MXene nanosheets and enhanced the link between adjacent MXene nanosheets. The thickness of the layer-by-layer repaired MXene membranes was 50 nm. The layer-by-layer repaired MXene membranes possessed the rejection rates of NaCl, MgCl2 and methylene blue of 72.6%, 93.8% and 99.8% respectively associating with water flux of 5.9 L·m−2·h−1, 7.4 L·m−2·h−1 and 9.8 L·m−2·h−1 due to their regular nanochannels. They represented high stability in nanofiltration for 120 h. The swelling of MXene nanosheets was weakened in the filtration. The repair strategy is potentially applied in fabrication of other 2D lamellar membranes and their enhancement of filtration performance.

Prominently improved CO2/N2 separation efficiency by ultrathin-ionic-liquid-covered MXene membrane

MXene-based membranes exhibit great potential in the gas separation but still suffer from the poor selectivity for gases with similar dynamic diameters, such as CO2 and N2. Here, we propose a prototype of composite membrane in atomistic scale theoretically, which is constructed by covering MXene with an ultrathin layer of ionic liquid (IL) and potentially has very good performance on the separation of CO2/N2 mixtures. A series of molecular dynamics simulations were carried out to investigate the influences of the MXene-slit width and the IL-film thickness on CO2/N2 separation performance. The gas separation performance is significantly enhanced after covering the IL film, and the IL-film thickness acts as a major factor to determine the performance. With the amounts of the IL increased from one to two layers, the CO2 permeance decreases from 2.3 × 104 to 2.6 × 103 GPU, while the N2 permeance declines more sharply, resulting in an increased selectivity. Compared to the IL-film thickness, the MXene-slit width has less influence on the gas permeance. However, the CO2/N2 selectivity decreases as the slit width enlarges. Finally, we concluded that the composite membrane composed of the MXene with 8 Å slit and 40 pairs of IL (about one and a half layers) exhibits the best CO2/N2 separation performance, with the CO2 permeance up to 104 GPU, and no N2 passing through the membrane during the simulations. Furthermore, the interaction of gas-membrane and the penetration process of gas were also investigated. The excellent CO2/N2 separation performance is ascribed to the synergistic effect of IL on the surface of the MXene and anions in the slit.

Biomimetic MXene membranes with negatively thermo-responsive switchable 2D nanochannels for graded molecular sieving

Negatively thermo-responsive 2D membranes, which mimic the stomatal opening/closing of plants, have drawn substantial interest for tunable molecular separation processes. However, these membranes are still restricted significantly on account of low water permeability and poor dynamic tunability of 2D nanochannels under temperature stimulation. Here, we present a biomimetic negatively thermo-responsive MXene membrane by covalently grafting poly (N-isopropylacrylamide) (PNIPAm) onto MXene nanosheets. The uniformly grafted PNIPAm polymer chains can enlarge the interlayer spacings for increasing water permeability while also allowing more tunability of 2D nanochannels for enhancing the capability of gradually separating multiple molecules of different sizes. As expected, the constructed membrane exhibits ultrahigh water permeance of 95.6 L m−2 h−1 bar−1 at 25 °C, which is eight-fold higher than the state-of-the-art negatively thermo-responsive 2D membranes. Moreover, the highly temperature-tunable 2D nanochannels enable the constructed membrane to perform excellent graded molecular sieving for dye- and antibiotic-based ternary mixtures. This strategy provides new perspectives in engineering smart 2D membrane and expands the scope of temperature-responsive membranes, showing promising applications in micro/nanofluidics and molecular separation.