MXene Membranes Research Articles

The Impact of Metal Ions on MXene Membranes: Critical Role of Titanium Vacancies.

Two-dimensional transition metal carbides and nitrides (MXenes) and MXene-based membranes hold promise for applications including water purification and seawater desalination; however, their environmental behavior and fate in these matrices remain unknown. In this study, we systematically assessed the reaction efficiencies of Ti3C2Tx at varying important environmental conditions. Our experiments revealed that copper and iron ions accelerated the oxidation rate of Ti3C2Tx 55.4 and 33.4 times, respectively. TiO2 and amorphous carbon were identified as the primary solid products. Based on in situ water-phase atomic force microscopy, atomic high-angle annular dark-field scanning transmission electron microscopy, and theoretical results, we postulate that metal ions enhance Ti3C2Tx oxidation by spontaneously migrating and anchoring at Ti vacancies, which then become active sites for this reaction. This process increases the adsorption of H2O and oxygen, making the Ti vacancy-rich surface convex area the most vulnerable site to attack. The findings in this study provide useful information for a comprehensive understanding of the interaction between MXene structural defects and metal ions as well as for the design and modification of MXene membranes resistant to metal ion impact.

EGCG mildly reduced single atom Co-MXene catalytic membrane: Catalytic and antioxidant properties

As a 2D nanomaterial with high hydrophilic and co-catalytic properties, MXene has been widely studied in the field of membrane separation and advanced oxidation. However, its susceptibility to oxidation in catalytic environments poses a significant challenge to its long-term stability and performance. Therefore, constructing an oxidation-resistant MXene catalytic membrane technology and ensuring its excellent flux and catalytic performance are important for the practical application of MXene membranes. Herein, we prepared a cobalt monoatomic intercalated M/E0.1-Co catalytic separation membrane by mild reduction of epigallocatechin-3-gallate (EGCG). EGCG not only connects the MXene lamellae during filtration through strong hydrogen bonding, but also protects the chemical stability of the MXene substrate through its antioxidant ability. Furthermore, the trace amount of EGCG in the confined domain space also promotes the rate-limiting step of the transition from the high-valent state to the low-valent state in Co single-atom catalysis. The stability of the catalytic separation layer was ensured along with a 100% degradation performance of sulfamethoxazole. This study establishes a bridge between the catalytic oxidation and chemical stability of a wide range of MXene-based materials nowadays and provides a viable proposal for the practical application of long-term utilized MXene materials in catalysis.

Tuning Nanochannel Microenvironments of a Thermoresponsive MXene Membrane for Mixed Molecule Gradient Separation.

Drawing inspiration from dynamic biological ion channels, researchers have developed various artificial membranes featuring responsive nanochannels. Typically, these membranes modify mass transport behaviors by manipulating the responsive layer on the inner surfaces of the intrinsic layer. In this study, we build two-dimensional lamellar membranes composed of titanium carbide MXene and poly(N-isopropylacrylamide), endowed with dual-level regulatable nanochannels, achieved through adjustments of nanochannel microenvironments. The size of these two-dimensional nanochannels can be altered by both the thermoresponsive polymer layer and the intrinsic MXene layer that could undergo spontaneous oxidation. The multilevel regulation strategy substantially enhances the molecular selectivity of MXene separation membranes, which is further applied for precise gradient separation toward multiple molecules. This advancement showcases the versatility and transformative capabilities of responsive nanochannel technology, setting the stage for innovative developments in diverse fields.

Bio-inspired MXene membranes for enhanced separation and anti-fouling in oil-in-water emulsions: SHAP explainability ML

Optimizing membrane performance for efficient water treatment is crucial for sustainable development and environmental protection, aligning with UN SDGs. This study involves experimental design, statistical reliability of small data, and explainable machine learning (ML) using SHAP (Shapley Additive Explanations). The research uses ML models and statistical tests to ensure data reliability and stationarity and investigate various membranes’ fouling and separation efficiency (MX-CM, PDMX-CM, and SPDMX-CM). Stationarity tests, including the Augmented Dickey–Fuller (ADF) and Phillips–Perron (PP) tests, revealed that MX-CM is stationary at level (I(0)), while PDMX-CM and SPDMX-CM required first differencing (I(1)) to achieve stationarity. SHAP analysis showed that in the fouling study, higher values of PDMX-CM and MX-CM positively influenced model predictions, with SHAP values of +0.09 for Cycle, −0.06 for PDMX-CM, and −0.06 for MX-CM. In the separation efficiency study, Cycle had a neutral impact (0.00), PDMX-CM had a slight positive effect, and MX-CM had a slight negative impact. These findings highlight the importance of ensuring data stationarity and utilizing SHAP for model explainability in predicting membrane performance. Accurate preprocessing and model interpretation enhance decision-making and optimization in membrane fouling and separation efficiency studies, ensuring robust and reliable ML models.

Regulating molecules/ions sieving channels of MXene-based membranes by edge-capping strategy

Regulating molecules/ions transport within two-dimensional (2D) sub-nanometer channels to achieve a higher separation efficiency is of considerable interest for MXene membranes in sustainable water treatment. Herein, sodium tripolyphosphate (STPP) was introduced into Ti3C2Tx nanosheets to construct a series of modified STPP-MXene membranes based on the edge-capping strategy. The critical role of edge-capping in modulating selective molecules/ions separation within 2D sub-nanometer channels was highlighted. The edge-capping method was demonstrated to moderately narrow the shoulder spacing between adjacent Ti3C2Tx nanosheets and impaired their positive electric field intensity. In addition, the DFT results showed that the cations exhibited a much lower relative energy with STPP-Ti3C2Tx (-0.0024 eV) compared to Ti3C2Tx nanosheets (-0.2092 eV), causing cations to permeate more readily through the STPP-MXene membrane. As a result, the appropriately designed and prepared STPP-MXene membrane demonstrated an excellent molecules/ions selectivity (CR/NaCl selectivity = 52.6, CR/Na2SO4 selectivity = 30.3), much higher than that of the pristine MXene membrane. Moreover, by protecting the Ti at the edges of Ti3C2Tx nanosheets, the STPP-MXene membrane retained its original two-dimensional structure to maintain a stable separation performance, even when exposed to a water environment for up to 60 days. In contrast, the pristine MXene membrane was completely oxidized due to its weak stability. This study provides a theoretical basis for the feasibility of edge-capping strategy as a broad modification alternative to design high-efficiency molecular/ionic sieving MXene membranes.

CdS quantum dots modified two-dimensional MXene photocatalytic membrane for efficient removal of organic dyes from aquatic systems

MXene, a novel two-dimensional (2D) material, has been applied for the removal of dye molecules from wastewater; owing to its ability to stack into unique interlayer permeation channels. However, the narrow interlayer permeation channels of MXene limit its high permeability, and the dye contaminants tend to block the channels on the membrane surface as well as between the laminae. In view of this, CdS quantum dots (QDs) were incorporated with photocatalytic ability into the MXene channels, and the fabricated CdS QDs@MXene/PVDF photocatalytic composite membrane had high separation efficiency by using PVDF polymer as the support layer. The results manifested that; the CdS QDs were homogeneously dispersed in the MXene separation layer, and the introduction of CdS QDs optimized the interlayer permeation channels of MXene, with permeate flux of 502.6 L·m−2·h−1. The composite membrane exhibited an excellent removal of Congo red and Rhodamine B (CR and Rh B) dyes under the coupling of membrane separation and photocatalytic process, with the optimal removal ratio of 97.3 % for CR and 98.5 % for Rh B, respectively. Moreover, the CdS QDs endowed the membrane with self-cleaning ability, superior permeability and selectivity under five consecutive photocatalytic cycles, demonstrating the great in-situ resistance to contamination. After five intermittent cycles, the dye retention rate still exceeded 90 % and the permeate flux reached 388.0 L·m−2·h−1. This study provides an exceptional reference for quantum dots modified two-dimensional MXene membranes.

Atomistic understanding of Ti3C2 MXene membrane performance for separation of nitrate ions from aqueous solutions

Water desalination, which is a reliable method for providing drinking water and a suitable solution, as well as the membrane filtration method in wastewater treatment, has increased significantly in recent years. In this research, the separation of nitrite and nitrate ions from aqueous solutions was done using the MXene membrane of the Ti3C2 type using molecular dynamics simulation. In this study, various parameters, such as pore size MXene structure, characteristics of cavities, applied pressure, and flux were investigated. To investigate the removal of toxic pollutants from water, water flux, potential mean force, distribution of water molecules, and density were investigated. The results showed that the amount of penetration through the membrane increased with the increase in pressure. It was observed that by applying pressure to the system, the number of water molecules accumulated in front of the membrane decreases because they quickly pass through the membrane, which indicates the positive effect of increasing pressure on the separation rate of molecules. The permeability of this membrane was several times higher than the existing membranes in the industry. So that Mexene membranes, which consist of at least two layers, can repel ions with 100% success.

Novel ultrastable 2D MOF/MXene nanofluidic membrane with ultralow resistance for highly efficient osmotic power harvesting

Two-dimensional (2D) materials have shown great potential in harvesting osmotic power due to their high membrane selectivity, but the high resistance from tortuous pathways of 2D nanofluidic membranes still impedes the further improvement in output performance. Here, we report an innovative 2D MXCT (MXene/Cu-TCPP) lamellar membrane with ultralow resistance for highly efficient osmotic power generation. The incorporation of 2D Ti3C2Tx MXene with rich functional groups not only resolves the water-stability issue of 2D metal-organic framework (MOF) Cu-TCPP, but provides large surface charges for selective ion transport. The orderly sub-2 nm framework channels of Cu-TCPP provide much shorter permeation pathways for fast ion transport, thus endowing the MXCT membrane with ultralow resistance. Consequently, the MXCT membrane reaches an ultrahigh power output of ∼8.29 W/m2 by mixing seawater and river water, which is ∼275 % higher than that of the pristine MXene membrane. Additionally, it outperforms all the reported single-layer 2D nanosheet-based osmotic power generators under the same experimental conditions in terms of output power and internal resistance (9 kΩ). This work presents a reliable strategy for stabilizing 2D Cu-TCPP MOF in electrolytes, opening new avenues for designing promising 2D nanofluidic membranes for efficient blue energy harvesting and ionic devices.

Determination of low carbon aldehydes and ketones in cigarette smoke by MXene membrane enrichment-liquid chromatography

Low carbon aldehydes and ketones are typical substances harmful to human body produced during cigarette smoking. Their contents in cigarette smoke are important indicators for evaluating its toxicity and the filtration effect of cigarette filter tips, which provides important guidance for its rational design. In this work, MXene membrane with unique lamellar structure was synthesized and loaded onto glass fiber filters to achieve effective enrichment of low carbon aldehydes and ketones. Compared to commercial Cambridge filters, the MXene-loaded filters exhibited higher extraction efficiency towards low-carbon aldehydes and ketones, making viable the detection of butyraldehyde, which was not detected by that enriched with Cambridge filters. Therefore, a MXene-based membrane enrichment-HPLC method was developed for the determination of low-carbon aldehydes and ketones in cigarette smoke with detection limits ranging from 0.133 μg/mL to 0.285 μg/mL. The applicability of the method was verified by analyzing three different types of filter cigarettes with the concentration in the range of 0.5–140 μg/branch for all the analytes, which were in good agreement with the manufacturer's results. The method is accurate and sensitive, and can be used for the quantitative determination of low carbon aldehydes and ketones in cigarette smoke.

Bioinspired MXene Membranes with pH-Responsive Channels for High-Performance Water Purification

Bioinspired MXene Membranes with pH-Responsive Channels for High-Performance Water Purification

A Janus, robust, biodegradable bacterial cellulose/Ti3C2Tx MXene bilayer membranes for guided bone regeneration

Guided bone regeneration (GBR) stands as an essential modality for craniomaxillofacial bone defect repair, yet challenges like mechanical weakness, inappropriate degradability, limited bioactivity, and intricate manufacturing of GBR membranes hindered the clinical efficacy. Herein, we developed a Janus bacterial cellulose(BC)/MXene membrane through a facile vacuum filtration and etching strategy. This Janus membrane displayed an asymmetric bilayer structure with interfacial compatibility, where the dense layer impeded cell invasion and the porous layer maintained stable space for osteogenesis. Incorporating BC with Ti3C2Tx MXene significantly enhanced the mechanical robustness and flexibility of the material, enabling clinical operability and lasting GBR membrane supports. It also contributed to a suitable biodegradation rate, which aligned with the long-term bone repair period. After demonstrating the desirable biocompatibility, barrier role, and osteogenic capability in vitro, the membrane's regenerative potential was also confirmed in a rat cranial defect model. The excellent bone repair performance could be attributed to the osteogenic capability of MXene nanosheets, the morphological cues of the porous layer, as well as the long-lasting, stable regeneration space provided by the GBR membrane. Thus, our work presented a facile, robust, long-lasting, and biodegradable BC/MXene GBR membrane, offering a practical solution to craniomaxillofacial bone defect repair.

Covalently bridged MXene/COF hybrid membrane toward efficient dye separation

Two-dimensional (2D) MXene-based membranes combine unique atomic thickness, well-arranged interlayer channel, and have received growing attention in molecular/ion separation and transport. The low permeability and instability of MXene-based membranes in water purification enable it difficult to be utilized in practical processing. Here, we present a high water-permeability and long-lived MXene membrane by assembling 2D heterostructured MXene/COF flakes on macroporous polymeric support. The intercalation of COFs into the interlayer of Ti3C2Tx nanosheets not only increases the adjacent interlayer space, but also provides abundant molecular sieving pores, thereby enhancing the permeability of MXene membrane. While being applied in removal of organic dyes, the MXene/COF hybrid membrane exhibits a superior water permeance (up to ∼ 300 L m−2 h−1 bar−1), and several satisfactorily rejections for different dyes (98.2% for congo red, 98.7% for chrome black T, 96.4% for methyl blue, and 97.2% for methyl orange) are simultaneously achieved. Additionally, the covalent anchoring of COFs on MXene surface can greatly improve the stability. The reduction of hydrophilicity of MXene surface mitigates water molecular absorption on the fluid channel wall and endow the membrane with well swell tolerance in aqueous environment. Such high performance MXene/COF hybrid membrane is mainly caused by the physical sieving mechanism supported by the repulsive interaction, as well as adsorption capability originating from porous COFs. This work provides an alternative approach for the fabrication of high performance MXene-based membranes used for highly effective dye wastewater treatment, and extends the application of 2D membranes in desalination, pollutant treatments, and resource recovery.

Bendable, lamellar MXene membrane electrodes with diverse diffusion pathways for potential application in miniaturized capacitive deionization devices

Flexible devices have shown great application potential in portable and miniaturized devices. However, work on flexible electrodes for capacitive deionization (CDI) has not been well developed due to the issues of desalination property, processability, and availability under actual bending conditions. Herein, we constructed a kind of flexible lamellar MXene membrane by assembling Ti3C2Tx sheets with both crumpled and perforated structures and studied their desalination performance in conventional planar and bent configurations. The crumpled and perforated features in Ti3C2Tx (C-P-TC) nanosheets could optimize the in-plane and out-of-plane diffusion channels of the stacked samples, respectively, effectively improving the accessibility of membranes. As a result, the desalination performance of such C-P-TC electrodes was superior to that of those assembled from crumpled or perforated sheets alone. Significantly, this flexible C-P-TC electrode still showed excellent performance at real bend deformation, which possessed not only almost the same CDI capacity as the traditional planar configuration but also good cycling stability. This work not merely proposes an alternative strategy to improve the CDI performance of lamellar MXene electrodes by regulating the nanostructures of channels but also confirms their effective deionization ability under real bent state, greatly facilitating the potential use of flexible electrodes in miniaturized CDI devices.

All in one doubly pillared MXene membrane for excellent oil/water separation, pollutant removal, and anti-fouling performance

Given the diversity and complexity of coexisting oil/dyes/heavy metal ions/microorganisms in wastewater and volatile organic compounds (VOCs) in the air, developing separation materials featured in higher separation efficiency and lower energy consumption for oil and water separation, pollutant removal, and anti-fouling is urgently needed, but it remains a major challenge till now. Herein, a multifunctional Ti3C2 MXene membrane with unique double pillar support was proposed by liquid phase ultrasonication and vacuum filtration to overcome the above challenge. Introducing cetyl-trimethyl ammonium bromide (CTAB) and calcium chloride/sodium alginate (CaCl2/SA) to the MXene membrane as crossed double pillars and superhydrophilic surface increases the tolerance and wettability of the membrane. The fabricated doubly pillared MXene (d-Ti3C2) membrane exhibits superior oil/water (O/W) separation efficiency (99.76%) with flux (1.284 L m−2 h−1) for canola oil and organic dye removing efficiency for methyl blue (MB) 99.85%, malachite green (MG) 100%, and methyl violet (MV) 99.72%, respectively, which is 1.05, 1.44, 1.22, and 1.28 fold compared with pre-pillared Ti3C2 (p-Ti3C2). The superior anti-oil/dye/fouling is attributed to lower oil conglutination, high hydrophily, and antibacterial activity. The versatile MXene membrane also shows distinguished separation of VOCs (η > 99%) from polluted air. The experimental and molecular dynamics (MD) computational simulation results illustrate that the superior separation efficiency of the Ti3C2 MXene membrane is ascribed to the unique doubly pillared space channel. This study paves a new road to further research on one step integration strategy for complex O/W separation, wastewater and VOCs removal, and anti-fouling via tuning nano/macro architecture.

Thermal self-crosslink after etching for regulated preparation of Ti3C2 type MXene membrane and its preliminary gas separation

We present an innovative methodology for the synthesis of MXene membranes through a dual-stage process involving etching and subsequent thermal self-crosslinking. A molar ratio of 1 (Al3+):9 (F−) using HCl/LiF was employed to convert raw Ti3AlC2 (MAX phase) into MXene within 48 h at 40 °C. This procedure predominantly yielded monolayers distinguished by diameters exceeding 500 nm, elevated crystallinity and a high overall yield. Advanced characterization techniques, including FESEM, TEM, HRTEM, AFM, XPS, and FTIR, were utilized. Instrumental analysis confirmed the formation of MXene exhibiting a single-flake morphology with diameters exceeding 500 nm. These monolayers were intact and continuous, with smooth peripheries and a uniform thickness of 2.1 nm. The surfaces were predominantly composed of carbon (C), oxygen (O), and titanium (Ti) atoms, interconnected by chemical bonds such as C–Ti–O, C–Ti–OH, C–C, C–O, and Ti–O. In the subsequent phase, vacuum filtration facilitated the assembly of a self-supporting MXene membrane. Thermal treatment at 170 °C for 30 h resulted in the reinforcement of C–Ti–O bonds within the nanosheets, increasing their prevalence to 43.14 % and 19.47 %, respectively. This thermal regulation reduced the interlayer d-spacing from 4.33 to 3.54 Å, which significantly improved the gas separation efficiency beyond the Knudsen diffusion limit, as demonstrated by the αH2/CF4 value exceeding 23.0.

Dynamic covalent interface engineering for 2D MXene membrane with interchangeable surface groups and tunable layer spacing for water treatment

Two-dimensional (2D) MXene-based materials are promising for water purification due to their nanometer interlayers, however, achieving adaptable and engineerable nanochannel surface properties in the 2D MXene membrane remains challenging. We propose a pioneering approach, called dynamic covalent interface engineering (DCIE), to address this issue. A poly (4-vinylphenylboric acid)-grafted MXene membrane was prepared, binding interchangeable diols via dynamic boronic ester chemistry. Alternative diols enhanced interlayer distancing to 1.49–1.60 nm. The interlayer-anchored diols transition from anionic to cationic and from hydrophilic to hydrophobic states through reversible covalent bond formation and cleavage, with no residue effect of previous diols. As a result, the membrane with swiftly tailored nanochannel properties exhibits exceptional switchable permeability and selectivity (98 % for Congo Red, 97 % for Gentian Violet) for various water treatment scenarios. This innovative method presents a promising strategy for fabricating customizable and switchable 2D MXene-based membranes with enhanced molecular transport and sieving effects, thus advancing water purification applications.

Water desalination across nanoporous Ti3C2 MXene

Nanostructured materials have a modest thickness, which is comparable in scale to the chemical substances to be separated making them highly efficient in membrane-based separation applications. We performed a molecular dynamics simulation to investigate the effect of the functionalization of the nanoporous Ti3C2 MXene membrane on desalination efficiency. Hydrogenated (–H) and hydroxylated (–OH) pores are the two configurations that we looked at. The calculated water permeability, in the case of hydroxylated (–OH) groups linked to carbon atoms, is equal to 100 L.m−2.h−1.bar−1, and the salt rejection is equal to 100 %.

Designing desalination MXene membranes by machine learning and global optimization algorithm

The development of energy-efficient and low-cost desalination techniques is crucial. Among these techniques, reverse osmosis (RO) is considered one of the most promising solutions for addressing the global water crisis. It has been widely implemented for both large-scale and distributed water desalination. One such promising desalination membrane is MXene with nanopores, thanks to its unique properties. However, accurately predicting the performance of the desalination process or designing new materials is challenging due to the complexity of the process and the various tunable properties of the membranes and nanopores themselves. The combination of machine learning (ML) and global optimization algorithms offers a superior approach to material design from multiple perspectives. Our study demonstrates that Particle Swarm Optimization (PSO) exhibits faster convergence speed and stability compared to Genetic Algorithms. Ti3C2O2 with a specific charge on the nanopore mouth is an excellent candidate, as determined by the particle swarm optimization algorithm. By analyzing water density and ion density along the nanopore, we gain a deep understanding of how pore charge and functionalized groups affect salt rejection and water permeation. The integration of ML and global optimization algorithms can facilitate the design of materials with outstanding desalination performance.

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.