MXene Membranes Research Articles

Optimal cross-linking of MXene-based membranes for high rejection and low adsorption with long-term stability for water treatment

The unique layered structure and abundant 2D nanochannels of MXene membranes have demonstrated outstanding permeability, indicating their great potential in the field of water treatment. However, their unstable interlayer spacing and irregular layer structure have limited their ability to separate small molecular pollutants effectively. In this study, we successfully prepared composite MXene membranes with high rejection and low adsorption properties by exploiting the differences in the surface functional groups and chain lengths of dopamine (DA) and polyethylene glycol (PEG) through an optimal cross-linking approach. These composite membranes exhibited excellent stability in terms of interlayer spacing and mitigated the adverse effects of stacking defects. Compared to MXene membranes, the composite membranes achieved a 1.76-fold increase in dye rejection efficiency, reaching 98.7% at most, with an adsorption ratio of only 1.8%. Moreover, outstanding performances were also exhibited in pressure and concentration load tests. Furthermore, in the context of forward osmosis applications, the composite membranes demonstrated excellent Na+ rejection of 99.9%. More importantly, these membranes showed no performance deterioration throughout 50 filtration cycles, with only a slight increase in permeance of 19.3% over a 38-day testing period. Thus, this work presents a novel solution for fabricating MXene composite membranes with low swelling, minimal defects, and high stability, enhancing its potential in the field of water and wastewater treatment.

Functionalized Ti3C2 MXene nanoporous as a reverse osmosis membrane: Insights from the atomistic level

Two-dimensional materials are highly efficient in membrane-based separation applications because of their small thickness, which is generally of the same order of magnitude as that of the chemical compounds to be separated. MXenes show an important efficiency in the desalination process, and functionalized Ti3C2 pores have not yet been investigated. We performed a molecular dynamics simulation to analyze the effect of pore area and chemical functionalization of the Ti3C2 MXene membrane on reverse osmosis desalination performance. We studied two configurations: hydrogenated (-H) pores and hydroxylated (-OH) pores. Our findings suggest the hydrophobic group (H) pores have the highest water permeability, and the hydrophilic group (OH) has an important salt rejection. The molecular dynamics results show that nanoporous Ti3C2 with an area equal to 100 Ų and a hydrogenated group linked to a carbon atom has the highest water permeability compared to other nanoporous Ti3C2 investigated with a salt rejection of more than 95 % for low pressure. In addition, the nanoporous Ti3C2 MXene membrane can be useful in the process of water desalination.

Marangoni‐Driven Self‐Assembly MXene As Functional Membrane Enables Dendrite‐Free and Flexible Zinc–Iodine Pouch Cells

AbstractZn metal is a promising anode material in aqueous batteries, but the direct use of Zn foil encounters severe issues of dendrite formation and side reactions, causing short cycle life. Conventional thick and rigid insulating protection layers may impede Zn ion diffusion and detach during mechanical deformation of the battery. Herein, a dendrite‐free zinc anode is demonstrated by grafting a thin (≈10 nm) Ti3C2Tx MXene functional membrane which is formed via Marangoni‐driven self‐assembly. The thin MXene membrane initiates uniform nucleation and promotes deposition of (002)‐oriented Zn in a lateral growth mode. Meanwhile, the membrane functions as a soft, stress‐adaptive, and protective layer to the underneath active zinc. This functional membrane renders Zn anode with improved cycling stability without notable dendrite formation or side‐reaction products. Flexible Zn─I2 pouch cells fabricated from the MXene‐covered Zn foil (20 µm thick) anode and a modified ZnI2 cathode demonstrate stable capacity up to 1500 cycles and unchanged voltage upon various mechanical deformations. This work affords new insights into designing functional soft membranes for constructing flexible Zn‐based energy devices.

Bio‐inspired Two‐dimensional Nanofluidic Ionic Transistor for Neuromorphic Signal Processing

AbstractVoltage‐gated ion channels prevalent in neurons play important roles in generating action potential and information transmission by responding to transmembrane potential. Fabricating bio‐inspired ionic transistors with ions as charge carriers will be crucial for realizing neuro‐inspired devices and brain‐liking computing. Here, we reported a two‐dimensional nanofluidic ionic transistor based on a MXene membrane with sub‐1 nm interlayer channels. By applying a gating voltage on the MXene nanofluidic, a transmembrane potential will be generated to active the ionic transistor, which is similar to the transmembrane potential of neuron cells and can be effectively regulated by changing membrane parameters, e.g., thickness, composition, and interlayer spacing. For the symmetric MXene nanofluidic, a high on/off ratio of ~2000 can be achieved by forming an ionic depletion or accumulation zone, contingent on the sign of the gating potential. An asymmetric PET/MXene‐composited nanofluidic transitioned the ionic transistor from ambipolar to unipolar, resulting in a more sensitive gate voltage characteristic with a low subthreshold swing of 560 mV/decade. Furthermore, ionic logic gate circuits, including the “NOT”, “NAND”, and “NOR” gate, were implemented for neuromorphic signal processing successfully, which provides a promising pathway towards highly parallel, low energy consumption, and ion‐based brain‐like computing.

Bio-inspired Two-dimensional Nanofluidic Ionic Transistor for Neuromorphic Signal Processing.

Voltage-gated ion channels prevalent in neurons play important roles in generating action potential and information transmission by responding to transmembrane potential. Fabricating bio-inspired ionic transistors with ions as charge carriers will be crucial for realizing neuro-inspired devices and brain-liking computing. Here, we reported a two-dimensional nanofluidic ionic transistor based on a MXene membrane with sub-1 nm interlayer channels. By applying a gating voltage on the MXene nanofluidic, a transmembrane potential will be generated to active the ionic transistor, which is similar to the transmembrane potential of neuron cells and can be effectively regulated by changing membrane parameters, e.g., thickness, composition, and interlayer spacing. For the symmetric MXene nanofluidic, a high on/off ratio of ~2000 can be achieved by forming an ionic depletion or accumulation zone, contingent on the sign of the gating potential. An asymmetric PET/MXene-composited nanofluidic transitioned the ionic transistor from ambipolar to unipolar, resulting in a more sensitive gate voltage characteristic with a low subthreshold swing of 560 mV/decade. Furthermore, ionic logic gate circuits, including the "NOT", "NAND", and "NOR" gate, were implemented for neuromorphic signal processing successfully, which provides a promising pathway towards highly parallel, low energy consumption, and ion-based brain-like computing.

A flexible ammonia sensor based on MXene membrane with high sensitivity

AbstractAmmonia (NH3) of high concentration will pose a threat to ecological environment or human health, and exhaled NH3 is significant in disease monitoring and diagnosis. Thus, developing a highly sensitive gas sensor is significant to monitor NH3 concentration in complex environments. However, traditional NH3 sensors either need high working temperature, or face the challenge of poor conductivity/ sensitivity. In this work, NH3 sensors based on self‐assembled MXene membrane have been fabricated. As‐prepared sensors show a high sensitivity of 2.10 ppm−1 towards extremely low concentrations of NH3 (ppb level) at room temperature, attributed to large surface area and high conductivity. In addition, the sensors also display low detection limit (50 ppb), fast response time (41 s), good recoverability, long‐term stability (15 days) and excellent flexibility (1000 bending cycles) towards NH3. The results provide insights into the development of highly sensitive NH3 sensors for industrial or biomedical applications.

Hybrid functional membranes through layer-by-layer assembly of Ti3C2Tx MXene and gelatin-stabilized calcium phosphate nanospheres

MXene-based membranes exhibit promising properties for various applications; however, pure MXene membranes lack sufficient environmental stability and mechanical strength. This work demonstrates that incorporating gelatin-stabilized amorphous calcium phosphate (Gel-ACP) nanospheres into MXene membranes via layer-by-layer assembly significantly enhances flexibility and mechanical strength while retaining electrical conductivity. A pressure-assisted stacking technique was introduced to efficiently fabricate membranes with tunable thickness not achievable through single filtration steps. The Gel-ACP nanospheres, synthesized by co-precipitation of calcium phosphate within gelatin solution, provided a ductile and cytocompatible reinforcement that augmented the properties of the MXene matrix. Direct mixing of the components led to particle aggregation and non-uniform dispersion. In contrast, controlled layer-by-layer nanostructuring maintained membrane conductivity around 102 Scm−1 while dramatically improving mechanical integrity. The optimized hybrid membranes exhibited specific electromagnetic shielding effectiveness of ∼21,000 dBcm2g−1 and withstood over 90 kPa vacuum pressure without rupture, six times higher than pure MXene membranes. Cytocompatibility was confirmed by the proliferation of human mesenchymal stem cells on the membranes. Moreover, the layered membrane exhibited excellent adhesion to bone-mimicking structures, indicating potential utility for bone tissue engineering. Overall, this work provides new design principles for engineering hybrid membranes through controlled multicomponent assembly to overcome intrinsic limitations and impart multifunctionality.

Alkaline-oxidized MXene composite membrane of ultra high flux and advanced rejection performance for water purification

MXene is a metal-carbon/nitrogen compound material with a two-dimensional layered structure. As a new form of two-dimensional material, it has great potential in the field of water purification because of its properties of high permeability and hydrophilicity. In this study, a MXene composite membrane was prepared by in-situ oxidation of titanium dioxide on the MXene surface by an alkaline oxidation method. A series of characterizations of the prepared MXene composite membrane was carried out. Scanning electron microscopy showed that with an increasing degree of oxidation, the extent of agglomeration of MXene nanosheets increased, thus generating more water channels that are conducive to the transport of water molecules, and an increasing water flux. The penetration performance, interception performance, and stability of the MXene composite membrane were tested. The effect of different concentrations of KOH on the properties of the composite membrane was investigated. It was found that the flux of the composite membrane increased with alkali concentration by 4.6–28.8 times the flux without exposure to alkali. It was found that the rejection rate of the composite membrane for pollutants (dyes, humic acid, bovine serum albumin protein) decreased only slightly with increasing alkali concentration, and was ≥94% of the pure MXene membrane rejection rate in all cases. The rejection performance of the composite membrane was related to the size and charge properties of the pollutant molecules. The results of the stability tests showed that the composite membranes had a high degree of physical and flux stability in a range of aqueous solutions.

Ultrathin H-MXM as An "Ion Freeway" for High-Performance Osmotic Energy Conversion.

Nanofluidic membranes are currently being explored as potential candidates for osmotic energy harvesting. However, the development of high-performance nanofluidic membranes remains a challenge. In this study, the ultrathin MXene membrane (H-MXM) is prepared by ultrathin slicing and realize the ion horizontal transportation. The H-MXM membrane, with a thickness of only 3 µm and straight subnanometer channels, exhibits ultrafast ion transport capabilities resembling an "ion freeway". By mixing artificial seawater and river water, a power output of 93.6 Wm-2 is obtained. Just as cell membranes have an ultrathin thickness that allows for excellent penetration, this straight nanofluidic membrane also possesses an ultrathin structure. This unique feature helps to shorten the ion transport path, leading to an increased ion transport rate and improveS performance in osmotic energy conversion.

Shearing Liquid-Crystalline MXene into Lamellar Membranes with Super-Aligned Nanochannels for Ion Sieving.

Ion-selective membranes are crucial in various chemical and physiological processes. Numerous studies have demonstrated progress in separating monovalent/multivalent ions, but efficient monovalent/monovalent ion sieving remains a great challenge due to their same valence and similar radii. Here, this work reports a two-dimensional (2D) MXene membrane with super-aligned slit-shaped nanochannels with ultrahigh monovalent ion selectivity. The MXene membrane is prepared by applying shear forces to a liquid-crystalline (LC) MXene dispersion, which is conducive to the highly-ordered stacking of the MXene nanosheets. The obtained LC MXene membrane (LCMM) exhibits ultrahigh selectivities toward Li+ /Na+ , Li+ /K+ , and Li+ /Rb+ separation (≈45, ≈49, and ≈59), combined with a fast Li+ transport with a permeation rate of ≈0.35 mol m-2 h-1 , outperforming the state-of-the-art membranes. Theoretical calculations indicate that in MXene nanochannels, the hydrated Li+ with a tetrahedral shape has the smallest diameter among the monovalent ions, contributing to the highest mobility. Besides, the weakest interaction is found between hydrated Li+ and MXene channels which also contributes to the ultrafast permeation of Li+ through the super-aligned MXene channels. This work demonstrates the capability of MXene membranes in monovalent ion separation, which also provides a facile and general strategy to fabricate lamellar membranes in a large scale.

F-free etching and ingenious construction of hydrogel layer-prepared MXene membranes for oily wastewater separation.

A strategy for the preparation of hydrogel layer MXene membranes by an F-free method was proposed. It maintained high permeance (2686.1 L m-2 h-1 bar-1) and separation efficiency (99.99%) even after 300 min of emulsion separation. The membrane resisted harsh chemical and microbiological environments and efficiently treated actual oily wastewater.

Effect of surface functionalization on DNA sequencing using MXene-based nanopores.

As one of the most promising types of label-free nanopores has great potential for DNA sequencing via fast detection of different DNA bases. As one of the most promising types of label-free nanopores, two-dimensional nanopore materials have been developed over the past two decades. However, how to detect different DNA bases efficiently and accurately is still a challenging problem. In the present work, the translocation of four homogeneous DNA strands (i.e., poly(A)20, poly(C)20, poly(G)20, and poly(T)20) through two-dimensional transition-metal carbide (MXene) membrane nanopores with different surface terminal groups is investigated via all-atom molecular dynamics simulations. Interestingly, it is found that the four types of bases can be distinguished by different ion currents and dwell times when they are transported through the Ti3C2(OH)2 nanopore. This is mainly attributed to the different orientation and position distributions of the bases, the hydrogen bonding inside the MXene nanopore, and the interaction of the ssDNA with the nanopore. The present study enhances the understanding of the interaction between DNA strands and MXene nanopores with different functional groups, which may provide useful guidelines for the design of MXene-based devices for DNA sequencing in the future.

Spontaneous and rapid electrostatic solvent nanofiltration based on a conductive layered membrane

An electrostatic solvent nanofiltration (ESN) method was developed for separating water from a variety of miscible solvents with high flux and negligible energy consumption by using an electrically conductive layered MXene membrane.

Understanding the swelling behavior of Ti3C2Tx MXene membranes in aqueous media

Two-dimensional (2D) lamellar MXene membranes have demonstrated ultrafast water permeance and outstanding ion rejection performance, thus showing great potential for water purification.

Kelp Nanofiber‐Based Composite Membranes for Highly Efficient Osmotic Energy Conversion

AbstractThe blue osmotic energy harvesting based on the nanofiber‐based layered membranes has received tremendous attention. However, the most nanofiber‐based ion‐selective membranes are suffering from uneconomical, environmentally unfriendly characteristics and a long growth cycle, as well as insufficient power density. In this work, kelp cellulose nanofibers (KCNFs) from green and inexpensive kelp with short growth cycle are compounded with hydrophilic MXene nanosheets to prepare KCNF/MXene nanofluidic membranes. Notably, the KCNF/MXene‐40 (40 is the mass fraction of KCNF) membrane with an aligned nacre‐like nanostructure achieves 3.03‐fold increase in mechanical strength compared to the pure MXene membrane. The ingenious combination of the surface charge of the MXene nanosheets and the space charge formed by negatively charged KCNFs endows the KCNF/MXene‐40 membrane with enhanced cation selectivity (≈0.85), high energy conversion efficiency of 24.04% and high power density of 66.23 W m−2 in a 500‐fold concentration gradient, which has exceeded most reported state‐of‐the‐art 2D nanofluidic membranes. More importantly, the membrane exhibits the exceptional long‐term stability with relatively unchanged power density within 6000 s. Under the conditions of the natural seawater and simulated river water, the KCNF/MXene‐based generator deliveries an output power density of ≈10.56 W m−2.

The Impact of Surface Chemistry and Synthesis Conditions on the Adsorption of Antibiotics onto MXene Membranes.

MXene, a two-dimensional (2D) nanomaterial with diverse applications, has gained significant attention due to its 2D lamellar structure, abundance of surface groups, and conductivity. Despite various established synthesis methods since its discovery in 2011, MXenes produced through different approaches exhibit variations in structural and physicochemical characteristics, impacting their suitability for environmental application. This study delves into the effect of synthesis conditions on MXene properties and its adsorption capabilities for four commonly prescribed antibiotics. We utilized material characterization techniques to differentiate MXenes synthesized using three prevalent etchants: hydrofluoric acid (HF), mixed acids (HCl/HF), and fluoride salts (LiF/HCl). Our investigation of adsorption performance included isotherm and kinetic analysis, complemented by density functional theory calculations. The results of this research pinpointed LiF/HCl as an efficient etchant, yielding MXene with favorable morphology and surface chemistry. Electrostatic interactions and hydrogen bonding between MXene surface terminations and ionizable moieties of the antibiotic molecules emerge as pivotal factors in adsorption. Specifically, a higher presence of oxygen terminations increases the binding affinities. These findings provide valuable guidance for etchant selection in environmental applications and underscore the potential to tailor MXenes through synthesis conditions to design membranes capable of selectively removing antibiotics and other targeted substances.

Amino acid-bonded Ti3C2Tx MXene nanofiltration membranes with superior antifouling property for enhanced water purification

MXene nanosheets have been identified as promising building blocks for high-performance nanofiltration (NF) membranes in water purification. Herein, we report a series of amino acid-bonded Ti3C2Tx MXene membranes with enhanced NF performance and antifouling properties via glycine (Gly), l-glutamic acid (L-Glu), l-lysine (L-Lys) or l-cysteine (L-Cys) modification. The physicochemical properties, separation performances and anti-fouling behaviors of the pure Ti3C2Tx MXene membrane, Gly-, L-Glu-, L-Lys- and L-Cys-bonded Ti3C2Tx MXene membranes were comparatively investigated. The stable Ti–N bonds were found between the amino acids and the MXene nanosheets and different amino acids lead to different properties of the corresponding membranes. While maintaining ultra-high rejections to various organic dyes, all the amino acid functionalized membranes exhibited enhanced water permeability (7.17–13.11 L·m−2·h−1·bar−1) compared with that of the pure MXene membrane. Importantly, the Gly-bonded membrane showed the optimal separation performance with high salt rejections of 86.28%, 81.36%, 72.88% and 66.82% to MgSO4, MgCl2, Na2SO4 and NaCl, respectively, and the membrane also exhibited superior anti-fouling property with a flux recovery rate of 80.59% to bovine serum albumin (BSA). The separation mechanism was further revealed which may offer a simple yet efficient way for enhancing 2D MXene-based NF membranes.

Light-Augmented Multi-ion Interaction in MXene Membrane for Simultaneous Water Treatment and Osmotic Power Generation.

The mixing of wastewater and natural water releases abundant osmotic energy. Harvesting this energy could significantly reduce the energy and economic cost of water treatment, leading to sustainable wastewater treatment technology. Yet, such energy harvesting is highly challenging because it requires a material that is highly permeable to nontoxic ions while rejecting toxic ions in wastewater to reach high power density and prevent environmental pollution. In this work, we demonstrate that a light-augmented biomimetic multi-ion interaction in an MXene membrane can simultaneously realize high permeability of Na+ ions for enhanced osmotic power generation and high selectivity to heavy metal ions up to a ratio of 2050 for wastewater treatment. The Na+ permeability is enhanced by the photothermal effect of the MXene membrane. The transport of heavy metal ions, however, is suppressed because, under angstrom-confinement, heavy metal ions are strongly electrostatically repelled by the increased number of permeating Na+ ions. As a result, the membrane can stably generate osmotic power from simulated industrial wastewater, and the power density can be enhanced by 4 times under light illumination of approximate 1 sun intensity. This work highlights the importance of multi-ion interaction for the transport properties of ionic materials, which remains rarely investigated and poorly understood in previous studies.

In situ bridges of nano-titanium dioxide constructed in MXene self-cleaning loose nanofiltration membranes for dye wastewater treatment

It has been a longstanding problem to combat the oxidation process of MXene membranes. Here, we propose a strategy to construct nano bridge defect repair loose nanofiltration membrane with the help of MXene oxidation. The oxidation kinetics of Ti3C2Tx was investigated in both model and experimental aspects. The acid solution leads to aggregation and smaller d-spacing. However, the oxidation of MXene was accelerated in alkaline solution via reaction with hydroxyl anions resulting in a hydrophilic MXene membrane surface. The water permeability of the pH = 12-Ti3C2Tx membrane increased to a maximum value of 26.5 L·m−2·h−1·bar−1 together with a high rejection (98 %) of Janus green B (JGB). Titanium dioxide, which was in situ formed by oxidation of MXene, gives the membrane self-cleaning properties under UV light. The water recovery ratio was as high as 96.6 %. To further study the mechanism of in situ oxidation of MXene to form nano-titanium dioxide, the oxidation stages were analyzed which revealed that the in-situ oxidation process involved three different oxidations stages. This research offers a new perspective on MXene membrane design, paving the way for a new method of in situ nano bridge defect repair loose MXene nanofiltration membrane.

A High-Performance N2-Selective MXene Membrane with Double Selectivity Mechanism for N2/CH4 Separation.

Membrane-based separation process for unconventional natural gas purification (mainly N2/CH4 separation) has attracted more attention due to its considerable economic benefits. However, the majority of separation membranes at this stage, particularly N2-selective membranes, achieve the desired separation target by mainly relying on the diffusivity-selectivity mechanism. To overcome the limitation of a single mechanism, 2D lamellar MXene membranes with a double selectivity mechanism are prepared to enhance N2 permeance and N2/CH4 selectivity via introducing unsaturated metal sites into MXene, which can form specific interactions with N2 molecules and enhance N2 permeation. The resulting membranes exhibit an inspiring N2/CH4 separation performance with an N2 permeance of 344 GPU and N2/CH4 selectivity of 13.76. The collaboration of the double selectivity mechanism provides a new idea for the development of a novel N2-selective membrane for N2 removal and CH4 purification, which further broadens the application prospects of membrane separation technology in the field of unconventional natural gas purification.