Bi-functional super-hydrophilic/underwater super-oleophobic 2D lamellar Ti3C2Tx MXene/poly (arylene ether nitrile) fibrous composite membrane for the fast purification of emulsified oil and photodegradation of hazardous organics

Recent Advancement in Emerging MXene-Based Photocatalytic Membrane for Revolutionizing Wastewater Treatment

MXene-based photocatalytic membranes provide significant benefits for wastewater treatment by effectively combining membrane separation and photocatalytic degradation processes. MXene represents a pioneering 2D photocatalyst with a variable elemental composition, substantial surface area, abundant surface terminations, and exceptional photoelectric performance, offering significant advantages in producing high-performance photocatalytic membranes. In this review, an in-depth overview of the latest scientific progress in MXene-based photocatalytic membranes is provided. Initially, a brief introduction to the structure and photocatalytic capabilities of MXene is provided, highlighting their pivotal role in promoting the photocatalytic process. Subsequently, in pursuit of the optimal MXene-based photocatalytic membrane, critical factors such as the morphology, hydrophilicity, and stability of MXenes are meticulously taken into account. Various preparation strategies for MXene-based photocatalytic membranes, including blending, vacuum filtration, and dip coating, are also discussed. Furthermore, the application and mechanism of MXene-based photocatalytic membranes in micropollutant removal, oil-water separation, and antibacterial are examined. Lastly, the challenges in the development and practical application of MXene-based photocatalytic membranes, as well as their future research direction are delineated.

MOF@MXene nanocomposite as a novel modifier to extend the application of PES mixed-matrix nanofiltration membranes for water treatment

MXene-based membranes, as a type of modified membrane, have unique structures that attract attention for water treatment but suffer from low water flux. To address this, MXene was manipulated with UiO-66-NH2 nanoparticles to create UiO-66-NH2@MXene 2D-nanocomposites for the modification of the PES membrane. Herein, we synthesized a novel modified MXene-based PES membrane. The MXene, UiO-66-NH2, and UiO-66-NH2@MXene were assessed using the Fourier transform infrared, X-ray diffraction pattern, X-ray photoelectron spectroscopy, and zeta potential analysis. Field emission scanning electron microscopy was used to evaluate the MXene-based materials and prepared membranes, and the surface topography of the fabricated membranes was studied using atomic force microscopy. The membrane modified by 0.25 wt% of modifier was able to not only remove 72% and 81% of methylene blue and crystal violet cationic dyes, but also recorded more than 91% rejections for methyl blue, methyl orange, acid fusion, and Congo red anionic dyes. Using the same membrane, salt rejections of 91%, 87%, 79%, and 62% were achieved for Na2SO4, MgSO4, MgCl2, and NaCl, respectively. Water flux was also increased by more than 4 times in the membrane modified with 0.25 wt% of the novel nanocomposite modifier, and the water contact angle of the membrane with 0.5 wt% decreased from 65° to 38° compared to the pristine PES membrane. Besides, the anti-fouling properties were exceptionally improved in the membranes modified by the introduced UiO-66-NH2@MXene nanocomposite modifier.

Recent advances of MXene@MOF composites for catalytic water splitting and wastewater treatment approaches

MXenes are a group of 2D material which have been derived from the layered transition metal nitrides and carbides and have the characteristics like electrical conductivity, high surface area and variable surface chemical composition. Self-assembly of clusters/metal ions and organic linkers forms metal organic framework (MOF). Their advantages of ultrahigh porosity, highly exposed active sites and many pore architectures have garnered them a lot of attention. But poor conductivity and instability plague several conventional MOF. To address the issue, MOF can be linked with MXenes that have rich surface functional groups and excellent electrical conductivity. In this review, different etching methods for exfoliation of MXene along with the synthesis methods of MXene/MOF composites are reviewed, including hydrothermal method, solvothermal method, in-situ growth method, and self-assembly method. Moreover, application of these MXene/MOF composites for catalytic water splitting and wastewater treatment were also discussed in details. In addition to increasing a single MOF conductivity and stability, MXenes can add a variety of new features, such the template effect. Due to these benefits, MXene/MOF composites can be effectively used in several applications, including photocatalytic/electrocatalytic water splitting, adsorption and degradation of pollutants from wastewater. Finally, the authors explored the current challenges and the future opportunities to improve the efficiency of MXene/MOF composites.

Multifunctional Super-Hydrophilic MXene/Biomass Composite Aerogel Evaporator for Efficient Solar-Driven Desalination and Wastewater Treatment

Solar-driven interfacial evaporation is recognized as a sustainable and effective strategy for desalination to mitigate the freshwater scarcity issue. Nevertheless, the challenges of oil contamination, salt accumulation, and poor long-term stability of the solar desalination process limit its applications. Herein, a 3D biomass-based multifunctional solar aerogel evaporator is developed for water production with fabricated chitosan/lignin (CSL) aerogel as the skeleton, encapsulated with carbonized lignin (CL) particles and Ti3C2TiX (MXene) nanosheets as light-absorbing materials. Benefitting from its super-hydrophilic wettability, interconnected macropore structure, and high broadband light absorption (ca. 95.50%), the prepared CSL-C@MXene-20 mg evaporator exhibited a high and stable water evaporation flux of 2.351 kg m-2 h-1 with an energy conversion efficiency of 88.22% under 1 Sun (1 kW m-2) illumination. The CSL-C@MXene-20 mg evaporator performed excellent salt tolerance and long-term solar vapor generation in a 3.5 wt.% NaCl solution. Also, its super-hydrophilicity and oleophobicity resulted in superior salt resistance and anti-fouling performance in high salinity brine (20 wt.% NaCl) and oily wastewater. This work offers new insight into the manufacture of porous and eco-friendly biomass-based photothermal aerogels for advanced solar-powered seawater desalination and wastewater purification.

Photocatalytic PAN Nanofibrous Membrane through Anchoring a Nanoflower-Branched CoAl-LDH@PANI Heterojunction for Organic Hazards Degradation and Oil-Containing Emulsified Wastewater Separation

The synergistic treatment of oily wastewater containing organic hazards and emulsified oils remains a big challenge for membrane separation technology. Herein, the photocatalytic membrane, which combined the physical barrier and catalytic oxidation-driven degradation functionality, was fabricated via anchoring a nanoflower-branched CoAl-LDH@PANI Z-scheme heterojunction onto a porous polyacrylonitrile mat and using tannic acid as an adhesive. The assembly of such a Z-scheme heterojunction offered the superior photocatalytic degradation performance of soluble dyes and tetracycline (up to 94.3%) to the membrane with the improved photocatalytic activity of 2.33 times compared with the CoAl-LDH@pPAN membrane. Quenching experiments suggested that the •O2- was the most reactive oxygen species in the catalytic reaction system of the composite membrane. The greatly enhanced photocatalytic activity was attributed to the effective inhibition of photogenerated hole-electron combination using PANI as a carrier, with charge transferring from LDH to PANI. The possible photocatalytic degradation mechanism was proposed based on VB-XPS, electron spin resonance spectroscopy, and DRS technologies, which was confirmed by density functional theory calculation. Meanwhile, benefiting from the superhydrophilic/oleophobic feature and low oil adhesion, the membrane exhibited high permeability for isooctane emulsion (3990.39 L·m-2·h-1), high structure stability, and satisfactory cycling performance. This work provided a strategy to develop superwetting and photocatalytic composite membranes for treating complex multicomponent pollutants in the chemical industry.

Superhydrophilic Photothermal-Responsive CuO@MXene Nanofibrous Membrane with Inherent Biofouling Resistance for Treating Complex Oily Wastewater

MXene, a recently emerged 2D material, has garnered substantial attention for a myriad of applications. Despite the growing interest, there remains a noticeable gap in exploring MXene-based membranes for the simultaneous achievement of photomodulated oil/water separation, bacterial resistance, and the removal of pollutants in the treatment of oily wastewater. In this work, we have successfully synthesized a novel multifunctional CuO@MXene-PAN nanofibrous membrane (NFM) featuring unique nanograin-like structures. Benefitting from these unique structures, the resultant membrane shows excellent superwetting properties, significantly enhancing its performance in oil/water separation. In addition, the membrane's photothermal property boosts its permeance by 40% under visible light illumination within 30 min. Furthermore, the resultant membrane shows decent dye removal efficiency in an aqueous solution, e.g., Rhodamine B (RhB), promoting efficient degradation with high reusability under visible light. Most remarkably, the resultant membrane exhibits superior anti-biofouling capability and consistently resists the adhesion of microorganisms such as cyanobacteria over a 14 day period. Thus, the combined effect of superior superwetting properties, photothermal responsivity, photocatalytic activity, and the antibacterial effect in CuO@MXene-PAN NFM contributes to the efficient treatment of intricate oily wastewater. This synergistic combination of superior properties in the membrane could be an appealing strategy for the broad development of multifunctional materials to prevent fouling during actual separation performance.

Robust PVA/GO@MOF membrane with fast photothermal self-cleaning property for oily wastewater purification

The poor mechanical durability and weak fouling resistance of oil/water separation membranes severely restrict their applications in industry. Herein, a robust PVA/GO@MOF membrane with fast photothermal self-cleaning capability was developed through facile chemical crosslinking and suction-filtration strategies. Attributed to the powerful underwater superoleophobicity, the PVA/GO@MOF membrane exhibited extraordinary anti-oil adhesion even for high-viscosity crude oil and continuous crude oil emulsion purification capability with stable flux (1020 L m-2 h-1 bar-1) and exceptional efficiency (> 99.3%) even after 60 min. Most importantly, in comparison to reported photocatalytic self-cleaning oil/water separation membranes, the PVA/GO@MOF membrane can degrade organic contaminants more rapidly with a higher degradation rate (99.9%) in 50 min due to the superior photothermal conversion capacity. The synergistic photothermal and photocatalytic effects significantly enhanced photodegradation efficiency, which created opportunities for in-depth treatment of complex oily wastewater. Besides, the obtained membrane displayed excellent chemical and mechanical durability with underwater oil contact angle (UWOCA) above 150° even in harsh environments, such as corrosive solutions, UV irradiation, ultrasound treatment, abrasion experiment and bending test. Therefore, the developed PVA/GO@MOF membrane with robust durability and fast photocatalytic self-cleaning property is highly expected to purify oily wastewater and degrade organic pollutants.

Antibacterial Hydrophilic ZnO Microstructure Film with Underwater Oleophobic and Self-Cleaning Antifouling Properties

Super-hydrophilic and oleophobic functional materials can prevent pollution or adsorption by repelling oil, and have good circulation. However, traditional strategies for preparing these functional materials either use expensive fabrication machines or contain possibly toxic organic polymers, which may prohibit the practical application. The research of multifunctional ZnO microstructures or nanoarrays thin films with super-hydrophilic, antifouling, and antibacterial properties has not been reported yet. Moreover, the exploration of underwater oleophobic and self-cleaning antifouling properties in ZnO micro/nanostructures is still in its infancy. Here, we prepared ZnO microstructured films on fluorine-doped tin oxide substrates (F-ZMF) for the development of advanced self-cleaning type super-hydrophilic and oleophobic materials. With the increase of the accelerators, the average size of the F-ZMF microstructures decreased. The F-ZMF shows excellent self-cleaning performance and hydrophilic (water contact angle ≤ 10°) and oleophobic characteristics in the underwater antifouling experiment. Under a dark condition, F-ZMF-4 showed good antibacterial effects against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) with inhibition rates of 99.1% and 99.9%, respectively. This study broadens the application scope of ZnO-based material and provides a novel prospect for the development of self-cleaning super-hydrophilic and oleophobic materials.

Design and assembly of Ag-decorated Bi2O3 @ 3D MXene Schottky heterojunction for the highly permeable and multiple-antifouling of fibrous membrane in the purification of complex emulsified oil pollutants

Membrane separation technology has potential for purifying emulsified oily wastewater. However, the oils, soluble organic substances, and microorganisms can cause complex membrane fouling problems, thereby reducing the separation efficiency and service life. Herein, a highly permeable and multiple-antifouling composite membrane was prepared using porous PAN fibrous mat as support backbone for the assembly of Ag-decorated Bi₂O₃ @ 3D MXene Schottky heterojunction and hydrophilic TA as the adhesive. The unique arrangement of 3D MXene heterojunction and hydrophilic functionalization effectively broke through the limitation of separation flux and synergistically enhanced the anti-fouling performance of membrane. Such fibrous composite membrane achieved an exceedingly high permeability (2717-3328 L·m^-2·h^-1) for various emulsified oils, while ensuring excellent oil/water emulsion retention rate (99.59%) and good cycle stability. Meanwhile, the composite membrane displayed favorable photocatalytic degradation performance toward degrading MeB (96.1%) and antibacterial ability. Furthermore, the MD simulation and free radical trapping experiments were carried out to unravel the molecular interactions during the separation process and the photocatalytic mechanism of composite membrane, respectively. Overall, the combination of photocatalytic self-cleaning, anti-oil adhesion, and antibacterial effect renders the membrane high permeability and multiple-antifouling performance, which provides a new strategy for dealing with complex oily wastewater in petrochemical industry.

C,N co-doped TiO2 hollow nanofibers coated stainless steel meshes for oil/water separation and visible light-driven degradation of pollutants

Complex pollutants are discharging and accumulating in rivers and oceans, requiring a coupled strategy to resolve pollutants efficiently. A novel method is proposed to treat multiple pollutants with C,N co-doped TiO₂ hollow nanofibers coated stainless steel meshes which can realize efficient oil/water separation and visible light-drove dyes photodegradation. The poly(divinylbenzene-co-vinylbenzene chloride), P(DVB-co-VBC), nanofibers are generated by precipitate cationic polymerization on the mesh framework, following with quaternization by triethylamine for N doping. Then, TiO₂ is coated on the polymeric nanofibers via in-situ sol-gel process of tetrabutyl titanate. The functional mesh coated with C,N co-doped TiO₂ hollow nanofibers is obtained after calcination under nitrogen atmosphere. The resultant mesh demonstrates superhydrophilic/underwater superoleophobic property which is promising in oil/water separation. More importantly, the C,N co-doped TiO₂ hollow nanofibers endow the mesh with high photodegradation ability to dyes under visible light. This work draws an affordable but high-performance multifunctional mesh for potential applications in wastewater treatment.

Fabrication of highly efficient Rh-doped cobalt-nickel-layered double hydroxide/MXene-based electrocatalyst with rich oxygen vacancies for hydrogen evolution

The development of nonprecious metal catalysts for producing hydrogen from economical alkaline water electrolysis that is both stable and efficient is crucial but remains challenging. In this study, Rh-doped cobalt-nickel-layered double hydroxide (CoNi LDH) nanosheet arrays with abundant oxygen vacancies (Ov) in-situ grown on Ti₃C₂T_x MXene nanosheets (Rh-CoNi LDH/MXene) were successfully fabricated. The synthesized Rh-CoNi LDH/MXene exhibited excellent long-term stability and a low overpotential of 74.6 ± 0.4 mV at -10 mA cm^-2 for hydrogen evolution reaction (HER) owing to its optimized electronic structure. Experimental results and density functional theory calculations revealed that the incorporation of Rh dopant and Ov into CoNi LDH and the coupling interface between Rh-CoNi LDH and MXene optimized the hydrogen adsorption energy, which accelerated the hydrogen evolution kinetics, thereby accelerating the overall alkaline HER process. This work presents a promising strategy for designing and synthesizing highly efficient electrocatalysts for electrochemical energy conversion devices.

Preparation of 2D Materials and Their Application in Oil-Water Separation

The problems of environmental pollution are increasingly severe. Among them, industrial wastewater is one of the primary sources of pollution, so it is essential to deal with wastewater, especially oil and water mixtures. At present, biomimetic materials with special wettability have been proven to be effective in oil-water separation. Compared with three-dimensional (3D) materials, two-dimensional (2D) materials show unique advantages in the preparation of special wettable materials due to their high specific surface area, high porosity, controlled structure, and rich functional group rich on the surface. In this review, we first introduce oil-water mixtures and the common oil-water separation mechanism. Then, the research progress of 2D materials in oil-water separation is presented, including but not limited to their structure, types, preparation principles, and methods. In addition, it is still impossible to prepare 2D materials with large sizes because they are powder-like, which greatly limits the application in oil-water separation. Therefore, we provide here a review of several ways to transform 2D materials into 3D materials. In the end, the challenges encountered by 2D materials in separating oil-water are also clarified to promote future applications.

Poly(arylene ether nitrile)/lamellar MXene nanosheet composite films fabricated via bio-inspired dopamine surface chemistry

2D lamellar MXene nanosheets have shown the promising candidate for preparing dielectric polymer composites due to their excellent electrical and mechanical properties. However, the high dielectric loss and low temperature resistance restrict their further application, which are still big challenges. In this work, MXene nanosheets were modified by dopamine mediated chemical crosslinking with polyethylenimine, which was further incorporated into the temperature-resistant poly (arylene ether nitrile) (PEN) matrix via a simple solution-casting method to prepare the dielectric MXene/PEN composite film. Specially, the insulating layer originated from polyethylenimine and polydopamine not only enhanced the interface polarization and the uniform dispersion of MXene in the polymer matrix, but also prevented the formation of conductive network. As a result, the MXene/PEN composite film achieved the high dielectric constant of 13.3 (1 kHz) when filling content was 7 wt%, and the dielectric loss was suppressed to 0.042. As the filling content reached 5 wt%, the MXene/PEN composite film had the maximum tensile strength and tensile modulus of 70.9 MPa and 3042.6 MPa, respectively, while maintaining a high elongation at break larger than 6.5%. In addition, the composite film retained the thermal decomposition temperature (T10%) of 460–521°C and the glass transition temperature higher than 149°C. Therefore, this work provides an alternative way to prepare thermally stable and dielectric polymer composite film with high mechanical strength and low dielectric loss, which is essential to the modern electronic applications.

Potential of MXene-based membranes in water treatment and desalination: A critical review

MXenes have emerged as wonderful materials that earned enormous attention in the last decade for applications in various fields. The potential of MXenes in the development of novel membranes has been explored recently by many researchers. This review critically assessed the recent advances in applications of MXene-based materials for the development of novel membranes. The synthesis routes of the MXene-based membranes are discussed, and the applications of developed membranes in water treatment and desalination are elaborated in detail. MXene-based membranes have demonstrated excellent potential in water treatment and desalination for the removal of dyes, metal ions, and salts from water. These membranes have unveiled exceptional antifouling potential and were proven to be a good choice to be employed in oil/water (O/W) separation. Besides impressive progress, numerous barriers restrict the practical applications of these membranes. The challenges related to synthesis routes of MXenes and MXene-based membranes, their stability and reusability potential, and the development of membranes on large scale are highlighted. Finally, recommendations for future work are suggested to overcome these limitations in future.

Element-Doped Mxenes: Mechanism, Synthesis, and Applications

Heteroatom doping can endow MXenes with various new or improved electromagnetic, physicochemical, optical, and structural properties. This greatly extends the arsenal of MXenes materials and their potential for a spectrum of applications. This article comprehensively and critically discusses the syntheses, properties, and emerging applications of the growing family of heteroatom-doped MXenes materials. First, the doping strategies, synthesis methods, and theoretical simulations of high-performance MXenes materials are summarized. In order to achieve high-performance MXenes materials, the mechanism of atomic element doping from three aspects of lattice optimization, functional substitution, and interface modification is analyzed and summarized, aiming to provide clues for developing new and controllable synthetic routes. The mechanisms underlying their advantageous uses for energy storage, catalysis, sensors, environmental purification and biomedicine are highlighted. Finally, future opportunities and challenges for the study and application of multifunctional high-performance MXenes are presented. This work could open up new prospects for the development of high-performance MXenes.