Ultrathin two-dimensional membranes by assembling graphene and MXene nanosheets for high-performance precise separation

Abstract

Membrane technology has attracted significant attention in the field of separation and purification due to its high efficiency and low energy consumption. In this case, to overcome the “trade-off” limitation between high permeability and selectivity, tremendous efforts have been devoted to designing and exploiting ultrathin membranes. Consequently, graphenes and MXenes, which have the advantages of single-atom layer thickness, large specific surface area, ease of fabrication, and controllable layer-by-layer assembly, have emerged as ideal candidates for the fabrication of ultrathin membranes. Compared to conventional polymeric membrane materials, 2D separation membranes assembled using graphene and MXene nanosheets, leveraging their unique laminar structure and ultrathin thickness, significantly shorten the transport path of guest molecules, minimizing the mass transfer resistance while maintaining or even enhancing separation selectivity. Furthermore, precise sieving of guest molecules can be achieved by tuning the interlayer spacing or pore sizes of these 2D nanosheets. This review comprehensively summarizes the state-of-the-art advancements in ultrathin 2D membranes assembled by graphene and MXene nanosheets, including the synthesis of nanosheets, the fabrication principles, structure design, and mass transfer mechanism of ultrathin 2D membranes, and their related precise separation applications. We focus on advanced ultrathin 2D membranes that simultaneously exhibit high permeability and selectivity and discuss the suitable strategies and mechanisms to develop ultrathin 2D membranes that can overcome the inherent “trade-off” effect in traditional separation membranes. Finally, a critical consideration of the opportunities and challenges associated with ultrathin 2D membranes assembled by graphene and MXene nanosheets towards high-performance precise separation is also presented.

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1 Introduction

With the acceleration of industrialization, environmental and energy issues have become increasingly severe, and thus, the application of separation membranes has gradually become widespread. Separation membranes are special thin-layer materials with selective permeability, which allow one or several substances (i.e. molecules and ions) in the fluid/gas to pass through while blocking others, thus achieving the purpose of concentration, separation, and purification.¹ With the advent of membrane technology, microfiltration membranes, ion exchange membranes, reverse osmosis membranes, ultrafiltration membranes, and gas membrane separation have been widely developed.^2,3 These membranes possess an excellent ability to selectively separate substances while maintaining the original biological system environment, efficiently concentrate and enrich products, and effectively remove impurities. They are also associated with convenient operation, compact structure, low energy consumption, simplified process, no secondary pollution generation, and no requirement for chemical addition. Thus, these membranes are gradually becoming a basic unit operation process in the petroleum and chemical industry and in the agriculture, food and medicine fields. Since the 1960s, membrane technology has been widely applied in separation fields such as gas separation, water purification, and seawater desalination.^2–4 In recent years, precise separation has become urgent and plays an increasingly crucial role in modern technology, given that it can not only enhance the utilization efficiency of limited water resources but also promote the recovery of key materials and the production of high-value chemicals and drugs. With continuous improvement of these technologies, their effectiveness is gradually approaching the inherent physical separation limits of existing membranes.^1,5

Currently, most commercial membranes are based on amorphous polymers such as polysulfone (PSf), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polydimethylsiloxane, and highly cross-linked polyamide materials. These materials dominate the separation membrane market due to their excellent comprehensive separation performance, low cost, and good processability. However, polymer membranes struggle to balance high permeability and high selectivity, given that they often exhibit a “trade-off” relationship, where one increases at the cost of the other,⁶ leading to the Robeson upper bound. Additionally, their poor chemical and thermal stability limits their practical applications, while plasticization and aging effects pose significant challenges. However, although the incorporation of nanoparticles has somewhat addressed these issues, the poor interfacial compatibility between inorganic nanofillers and polymers can lead to defects, which form non-selective channels, hindering the achievement of high selectivity. Thus, to achieve high-performance, precise separation, it is crucial to design selective permeation layers as thin as possible while developing interfacial support scaffolds with hierarchical pore structures to significantly enhance the membrane permeability and selectivity.⁴ Furthermore, chemical stability in organic solvents and resistance to membrane fouling are vital considerations to ensure structural integrity, good chemical and thermal stability during the separation process, and improved adaptability to various application scenarios. In this regard, the fabrication of separation membranes by assembling two-dimensional (2D) nanosheets with ultrathin nanoscale thickness and micrometer-scale lateral dimensions results in unique nanopore and nanochannel structures, offering extremely short transport paths. This enables the minimization of the mass transfer resistance while maintaining high separation selectivity. Furthermore, by regulating the interlayer spacing or pore size of 2D nanosheets, precise sieving of guest molecules can be achieved in ultrathin 2D membranes. Consequently, ultrathin membranes fabricated by assembling 2D nanosheets are promising candidates to simultaneously achieve high flux and high selectivity, thus addressing the limitations of traditional separation membranes.^3–5 They are considered as highly potential candidates as efficient precise separation membranes to break the inherent “trade-off” issue in traditional separation membranes.

Since Geim successfully exfoliated single-layer graphene from graphite in 2004,⁷ the family of 2D materials has become extremely attractive candidates for membrane separation due to their fascinating physical and chemical properties. Subsequently, researchers have successfully synthesized a series of novel atomically thin 2D nanosheets, including graphene oxide (GO),⁸ MXenes,¹ boron nitride, and transition metal dichalcogenides (TMDs).⁹ The rapid development of these novel materials has further broadened the application prospects of 2D nanosheets in the field of precise membrane separation.¹⁰ 2D nanosheets with a unique laminar structure have been considered as appealing alternatives in advanced separation compared with traditional polymeric materials. Due to their ultrathin atomic thickness and controllable layer-by-layer assembly with adjustable interlayer structure, 2D nanosheets have been increasingly explored as a foundational platform for precise separation technologies. In the case of 2D membranes used for separation, the disordered stacking of nanosheets and the irregular distribution of the interlayer channels can lead to non-selective defects such as pinholes and voids, which reduce the separation selectivity. Therefore, constructing 2D membranes with highly ordered and regularly arranged interlayer channels helps to suppress the formation of defects and enhance the separation selectivity. Depending on their atomic structure, 2D nanosheets are either porous or non-porous. Therefore, they can be fabricated into separation membranes in two basic forms, porous and laminar membranes. Typically, the former consists of single or several layers of porous nanosheets, with intrinsic uniformly sized pores (such as zeolites and MOFs) or drilled nanopores for selective permeation (graphene). For example, Liu et al.¹¹ reported the fabrication of thin film nanocomposite (TFN) membranes for desalination by embedding 2D Ni-MOF nanosheets in a polyamide selective layer. According to the results, the addition of 0.015 wt% of 2D MOF nanosheets to TFN membranes resulted in 2.5-times higher water permeability with well-maintained salt rejection compared to the pristine membranes. Jian et al.¹² fabricated an ultrathin 2D Al-MOF membrane via vacuum filtration, in which nearly 100% salt ions were rejected in a forward osmosis process. The latter was formed by assembling 2D nanosheets (such as graphene, GO, and MXene) into laminates with an interlayer space, which provided molecular transport nanochannels (Fig. 1). By regulating the in-plane and out-of-plane nanostructures, these ultrathin membranes assembled by 2D nanosheets exhibited remarkable molecular separation characteristics in various separation processes such as ultrafiltration, nanofiltration, reverse osmosis, forward osmosis, osmosis, and gas separation.^13,14

Fig. 1 Ultrathin 2D laminar membranes for selective molecule and ion transport.

Graphene^15,16 and MXene,¹⁷ as outstanding representatives of 2D materials, exhibit unique physicochemical properties and extensive application potential. Graphene, with its hexagonal honeycomb lattice composed of sp² hybridized carbon atoms, stands out in the field of filtration materials due to its ultrathin structure,¹⁸ strong mechanical properties,^19,20 and adjustable nanoscale pores. Its derivative, graphene oxide, further enhances the flexibility of modification and functionalization through its rich oxygen-containing functional groups. MXene, composed of 2D transition metal carbides/nitrides/carbonitrides, offers a high surface area, biocompatibility, hydrophilicity, and excellent electrical conductivity due to its abundant surface functional groups. Its layered structure provides rapid transport channels for gases and small molecules, further enhancing the membrane separation performance.²¹ Both materials share unique advantages due to their 2D characteristics, indicating their broad application outlook in the field of membrane separation.^22–24

Over the past few years, significant progress has been made in the fabrication of ultrathin 2D membranes by assembling graphene and MXene nanosheets (Fig. 2), and we believe that it is time to systematically highlight the recent progress and future trends in high-performance precise separation. To gain deeper insight into the significance of 2D nanosheets to develop advanced separation membrane technology to overcome the “trade-off” limitation, this review comprehensively summarizes the state-of-the-art advancements in the emerging ultrathin 2D membranes assembled by graphene and MXene nanosheets. We present the advances in both the theoretical and experimental chemical science and engineering of ultrathin 2D membranes based on graphene and MXenes, including their principles, structure design, fabrication, and precise separation application. Special attention is given to advanced ultrathin 2D membranes, which simultaneously exhibit high permeability and selectivity. Moreover, we provide critical views on understanding the strategies and mechanisms for the assembly of graphene/MXene nanosheets to develop ultrathin 2D membranes to overcome the “trade-off” effect and discuss the transport behaviors and separation mechanisms of molecules and ions within ultrathin 2D membranes. Finally, critical consideration of the future prospective of emerging ultrathin 2D membranes by assembling graphene and MXene nanosheets towards high-performance precise separation is presented.

Fig. 2 Ultrathin 2D membrane by assembling graphene and MXene nanosheets for high-performance separation to overcome the “trade-off” limitation.

2 Graphene-based membranes for separation

2.1 Synthesis of graphene

It is worth noting that the key points to obtain high-performance ultrathin 2D membranes not only depends on the efficient assembly of nanosheets into laminar membranes with controllable inter-layer nanochannels, but also high-quality ultrathin nanosheets. Specifically, high-quality nanosheets typically have a uniform thickness, good crystallinity, and fewer defects. These characteristics are crucial for both the performance of the nanosheets and the laminar membrane assembled from them. For instance, a uniform thickness ensures consistent nanoscale channel dimensions, enhancing the separation precision of the membrane. Alternatively, good crystallinity improves the mechanical strength and chemical stability of the nanosheets, allowing the laminar membrane to maintain a stable performance even in harsh environments. Furthermore, the presence of minimal defects reduce the non-specific adsorption and leakage during material transfer, improving the separation efficiency and selectivity of the membrane. Hence, the extensive adoption of graphene nanosheets heavily depends on the development of high-yield and cost-effective preparation technologies. Consequently, a multitude of scholars have devised various effective approaches including bottom-to-up and up-to-bottom fabrication strategies to fabricate graphene nanosheets, among which the most prevalent are mechanical exfoliation, epitaxial growth, liquid-phase exfoliation, oxidation–reduction, and chemical vapor deposition. Table 1 shows the different methods for the synthesis of graphene and their advantages and disadvantages. It is worth noting that porous membranes are mainly based on the bottom-to-up strategy, while layer-by-layer assembly membranes are mostly based on the solution-processable up-to-bottom approach (Fig. 3). This mainly due to the fact that the bottom-to-up approach enables nanopores to be drilled in the nanosheets, while the up-to-bottom strategy can produce a uniformly dispersed graphene solution for efficient layer-by-layer assembly.

Table 1 Different methods for the synthesis of graphene and their advantages and disadvantages

Synthesis method	Advantages	Disadvantages	Ref.
Mechanical exfoliation	· High quality	· Low yield	7
	· Simple and practical	· Difficult to control
	· Cost effective	· High material requirements
Epitaxial growth	· Large-area high quality	· Higher cost	25
	· Strong controllability	· Stringent growth conditions
	· Variety of substrate choices	· Difficulty in transfer
		· Presence of lattice defects
Liquid-phase exfoliation	· Low cost	· Limited solvent selection	26
	· Environmentally friendly	· Graphene hardening post-exfoliation
	· Easy to control
	· High yield	· Difficulty in solvent removal
	· Solvent diversity
Oxidation–reduction	· Large-scale preparation capability	· Lower quality	27
	· Low cost	· Environmental pollution issues
	· Mature process	· Difficult process control
	· Ease of modification	· Complex subsequent processing
Chemical vapor deposition	· High quality	· High equipment costs	28
	· Large area production	· Complex process
	· Clear growth mechanism	· Harsh growth conditions
	· Compatibility with other materials	· Lattice defects

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Fig. 3 (a) Bottom-to-up and (b) up-to-bottom approaches for the fabrication of graphene nanosheets for membrane separation applications.

2.1.1 Mechanical exfoliation. Traditional mechanical exfoliation methods involve peeling graphene from graphite through techniques such as tape adhesion.⁷ In 2004, Geim and Williams, together others, prepared a small amount of monolayer graphene by repeatedly adhering graphite with tape. They etched grooves on the graphite and transferred it to a glass substrate, followed by repeated tape adhesion and ultrasonic treatment. By immersing silicon wafers coated with SiO₂ films into a solution, the van der Waals force facilitated the adsorption of graphene nanosheets with a thickness of less than 10 nm on the silicon wafers, achieving the separation of the graphene layers.²⁹ This method has the advantages of simple process flow and low production costs. However, the resulting graphene is has a small size, is difficult to control, and obtained in low yield. Thus, it can only be used for small-scale batch production, making it challenging to achieve the industrialization and large-scale production of graphene to fabricate ultrathin membranes for practical separation applications.

2.1.2 Epitaxial growth. The epitaxial growth methods include silicon carbide (SiC) epitaxial growth and metal-catalyzed epitaxial growth. SiC epitaxial growth refers to the process of heating an SiC single crystal at high temperatures, evaporating the Si atoms on the SiC surface and leaving the C atoms behind, which reconstruct through self-assembly, ultimately yielding graphene based on the SiC substrate.²⁵ Alternatively, metal-catalyzed epitaxial growth involves introducing hydrocarbons under ultra-high vacuum conditions on the surface of a catalytically active transition metal substrate, such as Pt, Ir, Ru, and Cu. By heating, the adsorbed gases undergo catalytic dehydrogenation to produce graphene. During the adsorption process, the gases can cover the entire metal substrate, and the growth process is self-limiting, meaning that the substrate does not repeatedly adsorb gases after the initial adsorption. Consequently, the resulting graphene is mostly monolayer and can be produced uniformly over large areas. This method is similar to chemical vapor deposition (CVD), but metal-catalyzed epitaxial growth requires higher temperatures and consumes more energy.

2.1.3 Liquid-phase exfoliation. Liquid-phase exfoliation involves immersing graphite in tailored solvents or surfactants. Through ultrasonic agitation, this method efficiently isolates single- or multi-layer graphene from the graphite surface, which is the typical up-to-bottom synthesis strategy. The liquid-phase exfoliated dispersion preserves the inherent morphology and properties of graphene, facilitating its assembly onto various environments and substrates.²⁶ The underlying mechanism for liquid-phase exfoliation can be understood as follows: graphite consists of layers of graphene stacked together and bound by van der Waals forces. When immersed in suitable liquids, the interaction between the solvent and graphene nanosheets provides the energy required for exfoliation. Subsequently, ultrasonic waves generate shear forces and cavitation effects, which disrupt the van der Waals interactions between the graphene layers, leading to the effective exfoliation of well-dispersed graphene nanosheets.³⁰

2.1.4 Oxidation–reduction. The preparation of graphene through the reduction of graphite oxide involves mixing natural graphite with strong acids and strong oxidizing agents to produce an aqueous solution of GO. Subsequently, this solution is subjected to ultrasonic dispersion to form graphene oxide (single-layer graphite oxide). Finally, a reducing agent is added to remove the oxygen-containing groups, such as carboxyl, epoxy, and hydroxyl groups, from the surface of the GO, resulting in the production of graphene.^27,31,32 Given that the single-layer graphene obtained through this method still contains a very small amount of oxygen-containing groups, and its conjugated structure also exhibits a certain degree of defects, it is generally referred to as chemically reduced graphene oxide (CRG) to distinguish it from graphene in a strict sense. There are three primary methods for oxidizing graphite, where the first is the Hummers' method,³³ the second is the Brodie method,³⁴ and the third is the Staudenmaier method.³⁵ Among them, the Hummers' method boasts high safety and produces a wrinkled layered structure of graphite oxide with abundant oxygen-containing functional groups. It exhibits excellent dispersion in aqueous solutions and is frequently employed in the process of preparing graphene through the reduction of GO. Currently, the reduction of GO is the most commonly used method for the synthesis of graphene due to its high efficiency and low cost, enabling large-scale industrial production. However, a significant drawback of this method is the potential for wastewater pollution arising from large-scale production. Additionally, strong oxidants can severely disrupt the electronic structure and crystal integrity of graphene, impacting its electronic properties and generating defects during the graphene production process.

2.1.5 Chemical vapor deposition (CVD). Chemical vapor deposition (CVD) is a technique that involves the decomposition of carbon-containing compounds through chemical reactions at relatively high temperatures, resulting in the growth of graphene on a substrate.²⁸ The specific process involves placing a planar substrate (such as metal films or metal single crystals) in an atmosphere with decomposable precursors (e.g., methane and ethylene) at high temperature. Through high-temperature annealing, carbon atoms are deposited on the surface of the substrate to form multi-layer or single-layer graphene. Subsequently, the metal substrate is removed via chemical etching, yielding free-standing graphene nanosheets. The growth of graphene can be controlled based on parameters such as the type of substrate, growth temperature, and precursor flow rate, allowing the adjustment of the properties such as growth rate, thickness, and area of 2D graphene nanosheets.^36,37

2.2 Graphene-based separation membranes

Graphene as a 2D nanosheet features a hexagonal honeycomb lattice structure, where carbon atoms are arranged in sp²-hybridized orbitals.^15,16 Essentially, it is a monolayer of graphite,³⁸ boasting a thickness equivalent to a single carbon atom. This unique atomic configuration endows graphene with exceptional mechanical properties,³⁹ chemical stability,^19,20 and thermal stability,⁴⁰ positioning it as a promising ultrathin nanosheet for the fabrication of separation membranes. Investigations revealed that the flawless single-layer graphene, devoid of atomic defects between its densely packed carbon atoms, possesses a dense electron cloud structure that even the helium molecule, with its minute atomic radius, cannot penetrate freely. This quality enables graphene to effectively impede the permeation of gases, liquid molecules, and ions.⁴¹ Researchers have further discovered that introducing precisely sized nanopores onto the surface of the pristine sp²-hybridized atomic lattice of graphene or strategically designing stacked structures to reconstruct the 2D graphene nanochannels can aid the production of ultra-thin selective barriers, offering both high flux and desired separation capabilities.^42,43

2.2.1 Nanoporous graphene separation membranes.
2.2.1.1 Fabrication of nanoporous graphene separation membranes. By strategically introducing suitable defects into graphene nanosheets, graphene membranes can be fabricated with pores that boast diverse shapes, precise dimensions, and versatile functionalities on their surfaces. This endows them with remarkable selectivity in various applications, including gas mixture separation and seawater desalination.⁴⁴ Notably, porous graphene separation membranes, with atomically thin film thickness and vertically aligned pore architecture, offer the most direct and efficient route for molecular transportation. This signifies immense potential and promising prospects for enhancing the permeability efficiency of separation membranes. Hence, intense efforts have been devoted to drilling holes in graphene nanosheets to develop nanoporous graphene membranes. The common techniques to create pores in graphene membranes include ion bombardment, electron beam etching, plasma etching, and template etching.⁴⁵

Among these techniques, ion bombardment^45,46 uses focused ion beams (He⁺, Ar⁺, and Ga⁺) as the energy source to fabricate nanopores in graphene nanosheets. By controlling the energy and angle of incident ions, a series of nanopores with controllable pore diameters and taper angles can be obtained according to the requirements of specific applications (Fig. 4a). Alternatively, electron beam etching^47,50 primarily utilizes a strong electron beam generated by a field emission transmission electron microscope, as depicted in Fig. 4b and c. This approach allows the rapid fabrication of single nanopores on graphene membranes with precisely controllable pore diameters. Furthermore, plasma etching technology can create uniformly distributed defective nanopores on the surface of graphene membranes.⁴⁸ During the pore formation process, differences in the chemical functional groups at the pore edges may arise due to variations in the reaction atmosphere. The pore size can be controlled by adjusting the etching time (Fig. 4d). Lastly, the template etching approach,⁴⁹ as shown in Fig. 4e, overcomes the high equipment requirements of ion beam and electron beam etching. It is cost-effective, suitable for batch operations and it is easy to achieve pore arrays to fabricate nanoporous graphene with a high density and uniform subnanometer pores, allowing ultrafast high-permeance and high selectivity.

Fig. 4 (a) Process of creating controlled pores in graphene membranes through ion bombardment techniques.⁴⁵ Copyright 2014, ACS. (b) Schematic diagram of the process for the fabrication of graphene nanopore using electron beam and (c) TEM image of a fabricated nanopore in a graphene membrane.⁴⁷ Copyright 2010, ACS. (d) Schematic diagram of the process for the fabrication of graphene nanopore by plasma sputtering defects.⁴⁸ Copyright 2015, Springer Nature. (e) Schematic diagram of the process for the fabrication of graphene nanopore array via template etching approach.⁴⁹ Copyright 2010, Springer Nature.

2.2.1.2 Mass transfer mechanism of nanoporous graphene separation membranes. Based on graphene with a single layer of carbon atoms, the construction of porous structures on its layers results in the formation of single-layer porous graphene membranes (SGMs). In 2012, Cohen-Tanugi et al.⁵¹ demonstrated through theoretical calculations the immense potential of monolayer graphene with nanopores as an efficient salt filtration medium. They further explored the impact of hydrogenated and hydroxylated nanopores on the performance of SGMs using classical molecular dynamics simulations (Fig. 5a). Their study found that the ion separation performance of SGMs is primarily influenced by two key factors, the pore size and the chemical functional groups at the pore edges. The ion separation ability of SGMs decreases with an increase in pore size. Specifically, when the pore size is larger than 4.5 Å, most ions can pass through easily. Therefore, when the pore size of SGMs is smaller than this threshold, they can effectively retain Na⁺ and Cl⁻ from an aqueous solution, achieving efficient separation. Besides the pore size, the functional groups and charge distribution at the nanopore edges also significantly impact the separation performance of SGMs. As shown in Fig. 5b and c, the structures terminated with hydrophilic functional groups (such as oxygen-containing functional groups) exhibit a higher water flux due to their strong interaction with water molecules compared to porous graphene membranes terminated with hydrophobic functional groups (such as hydrogen or fluorine atoms). As depicted in Fig. 5d, the transport of water molecules in nanoporous graphene membranes mainly follows a size sieving mechanism, while the type of functional groups at the pore edges and their hydrogen bonding strength with water molecules also significantly affect the water transport rate.⁵² Meanwhile, during the transport of hydrated ions through porous graphene membranes terminated with hydrophilic functional groups (such as oxygen-containing functional groups), the hydrogen bonding interaction and dehydration effect between the hydration shell and the oxygen-containing functional groups reduce the transmembrane energy barrier for ions, leading to the improved transmembrane transport of hydrated ions and reduced retention efficiency.

Fig. 5 (a) Schematic of hydrogenated and hydroxylated graphene nanopore and desalination process.⁵¹ Copyright 2012, ACS. (b) Water permeability as a function of pore size and (c) salt rejection as a function of applied pressure for the hydrogenated and hydroxylated nanopore graphene membranes.⁵¹ Copyright 2012, ACS. (d) Illustration of the transport model and mechanism of steric/activated mass transport of graphene nanopore.⁵² Copyright 2017, Springer Nature.

In summary, the mass transfer of molecules and ions through graphene nanopores in liquid environments is influenced by a complex array of factors, such as spatial hindrance, chemical affinity, and electrostatic interactions among different solutes. Current research efforts are primarily directed towards fabricating large-area graphene films via CVD methods²⁸ and artificially introducing pores to investigate their mass transfer performance. However, these graphene products are predominantly polycrystalline and possess limited strength, making them inadequate for high-pressure separation applications. Consequently, the CVD synthesis and transfer of high-quality, uniform graphene films continue to pose significant challenges. The intrinsic defects, grain boundaries, and inconsistent layer thicknesses of graphene films can all hinder the precise control of the pore size and density during pore creation, thereby compromising the separation efficiency of porous graphene membranes. Additionally, traditional pore-forming techniques exhibit several shortcomings, including uneven pore sizes and a concentration of pore diameters within a narrow range, which further compromises the strength of nanoporous graphene membranes. Therefore, although porous graphene membranes hold immense promise in separation applications such as water treatment and seawater desalination, their widespread commercialization remains fraught with numerous difficulties and challenges in practical applications.

2.3 Graphene oxide-based separation membranes

2.3.1 Graphene oxide-based laminar separation membranes. Graphene oxide (GO), which is produced through the enhanced Hummers' method,³³ stands out as an exceptional graphene derivative with vast practical applications, primarily due to its scalability. Similar to graphene, GO nanosheets exhibit a single-atom layer thickness, spanning tens of micrometers in lateral dimensions. As a unique amphiphilic 2D carbon nanomaterial, GO boasts an abundance of hydrophilic functional groups such as carboxyl, hydroxyl, and epoxy, facilitating its exceptional dispersibility in water.⁵³ Moreover, its structural resemblance to aquaporins, which are proteins well-known for their ion recognition abilities, hints at potential applications in filtration, separation, and biomimetic ion transport. Due to its distinctive high aspect ratio and water dispersibility, GO can be seamlessly transformed into layered membranes via various liquid-phase film formation techniques, including vacuum filtration, drop casting, blade coating, spin coating, spraying, and layer-by-layer self-assembly. Distinguishing itself from the in-plane nanopores in nanosheet membranes, the 2D channels between stacked GO nanosheets permit the permeation of molecules. The oxygen-containing functional groups on GO nanosheets not only bolster their water dispersibility but also offer convenient sites for enhanced specific interactions (e.g., hydrogen bonding and electrostatic interactions) with transport components such as water, carbon dioxide, and ions. The robust hydrogen bonds between GO layers significantly contribute to their excellent mechanical properties, enabling them to exist independently without a supporting substrate. Owing to these structural peculiarities, the mass transfer of gases, water molecules, and ions in solution through GO layered films has garnered widespread research interest.
2.3.1.1 Fabrication of graphene oxide-based laminar separation membranes. Currently, vacuum filtration⁵⁴ is the most commonly used strategy to fabricate laminar graphene oxide (GO) membranes. In this process, the vacuum pressure drives the GO nanosheets to uniformly spread on the surface of a porous substrate, with water molecules being pressed out through the interlayer gaps. The GO nanosheets self-assemble into a layered structure on the substrate surface through van der Waals forces, as shown in Fig. 6a.⁵⁵ The microstructure of the separation membranes prepared by this method is largely influenced by the pressure and deposition rate. Huang et al.⁶³ prepared GO composite membranes on ceramic fibers using vacuum filtration, which exhibited an excellent separation performance for organic (dimethyl carbonate)/water mixtures at 25 °C, with a rejection rate of 95.2%. The vacuum filtration method stands out for its simplicity and diverse substrate choice. By fine-tuning the filtrate volume and GO concentration, precise control of the thickness of the GO membrane is achievable. Moreover, the GO nanosheets, characterized by their highly aligned layered structure and rich oxygen-containing functional groups, stack neatly in a perpendicular orientation. This stacking pattern structure, together with the strong hydrogen bonding and π–π interactions between layers, contributes to the superior mechanical properties of the membranes. Nevertheless, their preparation process often necessitates the use of a supporting substrate, and the membranes tend to be fragile and challenging to maintain their integrity when detached from the base membrane.

Fig. 6 (a) Schematic illustration of the preparation of GO membrane by vacuum/pressure-assisted filtration.⁵⁵ Copyright 2015, Elsevier. (b) Schematic diagram of the preparation of GO membrane by spin coating.⁵⁶ Copyright 2008, ACS. (c) Schematic diagram of thermal spraying method to prepare a GO membrane on an aluminium substrate.⁵⁷ Copyright 2017, Elsevier. (d) Schematic illustration of the preparation of large-area GO membrane by rod-coating technique.⁵⁸ Copyright 2021, Wiley. (e) Schematic diagram of layer-by-layer assembly method to prepare GO membrane.⁵⁹ Copyright 2015, AAAS. Other techniques for fabricating GO membrane: (f) electrospinning.⁶⁰ Copyright 2021 Elsevier. (g) Continuous centrifugal casting.⁶¹ Copyright 2021, Elsevier. (h) Electrodeposition.⁶² Copyright 2014, ACS.

Surface coating is a process for the preparation of GO membranes through the natural evaporation, concentration, and drying of solvents. Spin coating⁵⁶ mainly involves adjusting the rotation speed to evenly disperse the solution on the substrate, followed by drying to obtain an ultrathin membrane. Fig. 6b presents a schematic diagram of the spin coating method for the preparation of GO membranes. Zhao et al.⁶⁴ prepared GO/PAM membranes by successively coating polyacrylamide (PAM) and GO on a base membrane using the spin coating method. During the spin coating assembly process, GO stacks on the membrane surface, enhancing the rejection performance of the separation membrane for Mg²⁺ and Ca²⁺. Spray coating involves spraying the liquid to be coated through a spray gun and nitrogen atomization, evenly distributing it on a heated substrate and allowing the atomized droplets to evaporate rapidly on the substrate to prepare membranes.⁶⁵ Shen et al.⁵⁷ developed a method for fabricating GO membranes by employing spray coating and solvent evaporation. This method leverages ultra-small sprayed droplets to ensure coating uniformity and enhance the evaporation efficiency. Moreover, by adjusting the number of coating layers and the duration of heat exposure for solvent evaporation, as shown in Fig. 6c, precise control of the GO membrane structure is achieved. Fig. 6d illustrates a scalable technique reported by Professor John H. Xin's team⁵⁸ for the preparation of ultra-thin and uniform GO membranes. This technique incorporates a continuous process, combining Mayer bar coating with short-time, high-power UV reduction, to conveniently and uniformly produce large-area (30 × 80 cm²) GO membranes with nanometer-scale thickness. This approach effectively reduces and adjusts the interlayer spacing of the GO membranes, leading to enhanced desalination efficiency. The resulting membranes demonstrated excellent water permeability, surpassing 60.0 kg m⁻² h⁻¹, and high separation efficiency, achieving greater than 96.0% efficiency for sodium sulfate (Na₂SO₄) solutions. Furthermore, they maintained superior mechanical stability under diverse harsh cross-flow conditions. However, although surface coating methods offer simplicity in equipment, controllability in conditions, and adjustable membrane area and thickness, they are associated with low utilization efficiency and may not always yield membranes with optimal uniformity.

The layer-by-layer (LBL) assembly technique involves the sequential deposition and self-organization of multiple layers through various interactions, including hydrogen bonding, electrostatic attraction, and covalent bonding. As demonstrated in Fig. 6e,⁵⁹ by adjusting the number of assembled layers, the thickness of GO membranes can be precisely controlled. The abundance of functional groups on the surface of GO, together with its exceptional water dispersibility, positions it as an ideal candidate for constructing composite membranes through the LBL approach. Hu et al.⁶⁶ were pioneers in utilizing polydopamine-modified polysulfone membranes as a support layer and trimellitic acid chloride (TMC) as a cross-linking agent to facilitate the layer-by-layer self-assembly of GO into multilayered GO membranes. This method harnesses covalent crosslinking to construct a stable graphene-based separation membrane structure that retains its integrity even under rigorous conditions such as repeated scrubbing and ultrasonic exposure. Notably, the water flux of these membranes can exceed 4 to 10 times that of traditional nanofiltration membranes. However, despite the excellent structural stability provided by the strong electrostatic forces between GO nanosheets, achieving the continuous and large-scale fabrication of GO membranes remains a challenge using the LBL method.

In addition to the above-mentioned methods, researchers have also developed electrostatic spinning (Fig. 6f),⁶⁰ continuous centrifugal casting (Fig. 6g),⁶¹ and electrodeposition (Fig. 6h)⁶² techniques to assemble GO nanosheets to fabricate high-performance graphene-based membranes for precise separation.

Electrospinning technology is a simple and cost-effective method that can continuously produce fibers with diameters at the nano/micro scale. In the electrospinning process, graphene nanosheets are typically blended with polymer spinning solutions and transformed into graphene-based separation membranes with unique fiber structures through high-voltage driving. Parameters such as the concentration of the spinning solution, distance between the collector and the needle tip, flow rate, voltage, and duration can all influence the transport and separation performance of the graphene-based separation membranes.⁶⁷ In summary, electrospinning technology not only enables precise control of the porosity and microstructure of graphene-based fiber membranes, but also endows them with large specific surface areas and excellent mechanical properties, giving them broad application potential in numerous precise separation fields.⁶⁸

The continuous centrifugal casting technique involves pouring or spraying a graphene-based dispersion onto the inner surface of a rapidly rotating drum. As the drum rotates at a different speed from the dispersion, the resulting shear force aligns the graphene-based nanosheets neatly on the inner wall of the drum. Meanwhile, the centrifugal force facilitates the formation of a densely packed structure within the graphene-based separation membrane.⁶⁹ This approach offers high preparation efficiency, scalability for large-scale production, and mechanical flexibility in adjusting the interlayer spacing of the membrane by varying the rotation speed.⁶²

Electrodeposition is a method for preparing graphene-based separation membranes by utilizing an external electric field to drive the slow migration and assembly of suspended charged graphene-based nanosheets onto the surface of an electrode substrate. Due to the ease of the deprotonation of the functional groups in the GO structure, these nanosheets can migrate from the solution to the electrode surface under an electric field. Electrodeposition has advantages such as high deposition rate, controllable membrane thickness, good uniformity, and strong adhesion. However, partial reduction of GO often occurs during the deposition process.^70–72 In summary, various methods for the preparation of graphene-based separation membranes have been developed recently, both on a laboratory scale and industrial scale. The selection of the appropriate preparation technique is crucial for controlling the microstructure of graphene-based separation membranes, which further affects their transport and separation performance in molecule/ion separation.

2.3.1.2 Mass transfer mechanism of graphene oxide-based separation membranes.
2.3.1.2.1 Transmission of gas and water molecules. During the assembly process of GO nanosheets into layered membranes, two types of transport channels are formed, as follows: (1) interlayer channels between the nanosheets formed by face-to-face interactions and (2) in-plane structural defects and/or slit-like pores created by edge-to-edge interactions between the nanosheets. Molecular transport through the layered membranes assembled by GO nanosheets first occurs in the in-plane slit-like pores, and then in the interlayer channels between the nanosheets.⁷³

In 2012, Nair et al.⁷⁴ pioneered the exploration of the mass transfer phenomenon in GO membranes. Through spin coating, they fabricated submicron-thick GO membranes, as illustrated in Fig. 7a and b. Their findings revealed that these membranes possess remarkable barrier properties against gases (such as helium, hydrogen, and nitrogen) and organic solvent vapors, successfully hindering their diffusion, as depicted in Fig. 7c. In contrast, for water molecules, the GO membranes exhibited unhindered permeability, as shown in Fig. 7d. Water molecules permeate through the 2D interlayer channels formed between GO nanosheets, traversing the curved paths that predominantly traverse hydrophobic non-oxidized surfaces rather than hydrophilic graphene oxide regions. The researchers attributed this exceptional permeability to the nearly friction-free nature of the graphene oxide nanosheet surfaces in the non-oxidized regions, which significantly facilitates the ultrahigh-speed flow of water molecules. Since this groundbreaking study, numerous subsequent investigations have highlighted the pivotal role of the channels between graphene oxide nanosheets for the rapid and selective transport of water, ions, and gases. Extensive research⁷⁵ has illuminated the distinctive underlying mechanism the mass transfer process in GO membranes. The oxygen-containing functional groups play the central role in their mechanism, which act as supportive scaffolds, effectively expanding the originally condensed graphite structure to provide ample space for water molecules to traverse and permitting the smooth passage of 1 to 2 layers of water molecules (as depicted in Fig. 7e and f). Additionally, the interconnected graphitized regions on the surfaces of GO nanosheets create intricate nanocapillary networks. These networks interact with water molecules with extremely low friction, thereby significantly enhancing the efficiency of water molecule transmission between layers. As a result, the interlayer spacing of GO membranes functions as an efficient 2D nanopore channel, precisely enabling the permeation and retention of hydrated ions with varying particle sizes. Furthermore, this GO membrane, owing to its exceptional hydrophilicity, can swiftly adsorb water molecules into its pores, resulting in remarkable permeation rates, as evidenced in Fig. 7g and h.

Fig. 7 (a) Photograph of a 1 μm thick GO membrane peeled off from Cu foil. (b) Electron micrograph of the cross section of the membrane. (c) Schematic view of the possible permeation through the laminates. (d) Examples of He-leak measurements for a freestanding submicron-thick GO membrane and a reference PET film (1 mbar = 100 Pa).⁷⁴ Copyright 2012, AAAS. (e) Weight loss for a container sealed with a GO membrane. (f) Permeability of GO membrane to water and various small molecules. (g) (top) Photograph of GO and cross-sectional SEM images of GO membrane. (bottom) Schematic diagram of stacked GO structure and nano-structural parameters characterizing an infiltrated network. (h) (top) Schematic diagram of lateral nanoconfined flow interlaminated between two sheets. (bottom) Schematic diagram of the vertical flow at the open edges.⁷⁵ Copyright 2014, AAAS. (i) Schematic drawing of the labelling of 0.1 mol L⁻¹ MgCl₂ source solution by 30 wt% D₂O tracer. (j) Schematic diagram of the labelling of drain solutions by 30 wt% D₂O tracer when dissolving 0.1 mol L⁻¹ MgCl₂ in the source solution. (k) Water permeation through GOCM membranes in both directions when dissolving ions in source. (l) Water permeation rates through GOCM membranes in both directions in the presence of source ions.⁷⁶ Copyright 2015, RSC.

By strategically assembling a few layered GO microsheets (with thickness in the range of 3 to 10 nm) and precisely controlling the membrane channels through diverse stacking techniques, remarkable gas selectivity can be achieved. Specifically, under the conditions of higher relative humidity, well-interconnected GO membranes exhibit efficient CO₂/N₂ separation capabilities.⁷⁷ Furthermore, Li et al.⁷⁸ employed the vacuum filtration method to fabricate ultrathin GO membranes, boasting a thickness of approximately 1.8 nm and incorporating specific structural defects. These membranes possessed remarkable selectivity, achieving values of up to 3400 and 900 for H₂/CO₂ and H₂/N₂ mixed gases, respectively. These values significantly exceed the performance of the most advanced microporous membranes by 1 to 2 orders of magnitude. Consequently, the application of ultrathin GO membranes for the separation of H₂ from mixed gases holds immense promise.

The research findings indicate that GO membranes possess gas selectivity, which initially appears to differ with the observations reported by Nair et al. However, this apparent contradiction is merely superficial. The discrepancy is attributed to the thickness of the GO membranes. In the earlier studies, the membranes were fabricated to be just a few nanometers thick, at which point a continuous network of nanocapillary channels had not yet materialized, permitting gas permeation through existing defects. In contrast, in the latter study, the GO membranes were significantly thicker, reaching the micrometer in scale, and possessed a continuous network of GO capillary channels with minimal defects, effectively blocking gas permeation.

All the above-mentioned research focused on the permeability of gas and water vapor. The permeation phenomenon of liquid water is much more complicated than that of gas because there is no distinguishable difference between the source liquid and the filtrate separated by the GO film, both on the macro- and microscale. Sun et al.⁷⁶ solved this problem by using the element tracing method to study the performance of liquid water permeating through a GO film on a microporous filter membrane substrate under no pressure. A certain amount of heavy water (deuterium oxide, D₂O) was used as a tracer to label liquid water, and the permeation characteristics of the liquid water were studied. MgCl₂ was added to the source liquid, and the water in the source liquid and the filtrate was tracked separately, as shown in Fig. 7i and j, respectively. It was found that the permeation coefficient of ions and water was in the same order of magnitude, as shown in Fig. 7k. The permeability coefficient was 4–5 orders of magnitude higher than that of polymer channels with pore size less than microns, and the ion transport mainly depended on the water flow. The slight contact between ions and nanocapillaries had a certain effect on the rapid ion transport, as shown in Fig. 7l. This phenomenon indicates that the GO membrane is semi-permeable, and water molecules tend to transport in the direction of inverse ion gradient when passing through the GO membrane. The above-mentioned research clarifies the mechanism of water molecules passing through the GO membrane at a high speed, and the ultra-thin GO membrane is selective to gas. Next, we focus on the permeation behavior of ions in GO membranes for precious water separation.

2.3.1.2.2 Transmission of ions in solution. All the above-mentioned research focused on the mass transfer phenomena of gases and water molecules through GO membranes. However, given the current severe freshwater crisis, the precise separation of ultrathin GO membranes in seawater desalination primarily aims to remove salt ions from seawater. Therefore, investigating the ion transfer behavior of GO membranes holds significant practical separation importance.

Sun et al.⁷⁹ prepared free-standing GO membranes with micrometer-level thickness using the drop-casting method and studied the permeation behavior of different ions in solution, as shown in Fig. 8a and b. When equal volumes of salt solution and deionized water were added to the two ends of the GO membrane, it was found that sodium salts permeated quickly, while heavy metal salts permeated slower, and copper salts and organic compounds were effectively blocked by the GO membrane. This result indicates that sodium salts can be effectively separated from copper salts and organic compounds. Further research on the mechanism of ion permeation through GO membranes was conducted, testing several salts with a hydration radius following the order of Mn²⁺ > Cd²⁺ > Cu²⁺ > Na⁺. The permeation results showed the order of Na⁺ > Mn²⁺ > Cd²⁺ > Cu²⁺. This suggests that ion permeation is not directly related to the size of the hydration radius, but rather due to the different interactions between metal salt ions and the functional groups on GO sheets. When testing the permeation of several sodium salt solutions with the same concentration, the permeation results showed the order of NaOH > NaHSO₄ > NaCl > NaHCO₃, indicating that the anions in the salt solution also affect the permeation results. Subsequently, the permeation mechanisms of different salt ions were studied,^80,81 as shown in Fig. 8c and d. The results showed that transition metal cations tended to interact with oxygen-containing functional groups on GO sheets, while main group metal cations were more inclined to interact with sp² groups through cation–π bonds. The different interactions of ions led to different ion selectivity by the GO membranes. When the GO film was modified with amino acid-targeting functional groups, the permeation performance of ions was weakened. This suggests that the ion transport rate can be modulated by modifying the targeting functional groups on GO films. These studies not only clarify the selective ion permeation mechanism of GO films, but also lay the foundation for research on modifying the targeting functional groups on GO films to separate target ions.

Fig. 8 (a) Penetration processes of different ionic compounds through GO membranes. (b) Initial stages of the penetration processes.⁷⁹ Copyright 2013, Wiley. (c) Cu²⁺ and (d) Mg²⁺ permeation rates through blank microfilters, GO, G-OH, G-COOH, and G-NH₂ membranes with the cellulose microfilters underneath.⁸⁰ Copyright 2014, ACS.

2.3.2 Graphene-based composite separation membranes. Single separation membranes often undergo swelling due to their inherent properties, such as general hydrophilicity, which leads to performance degradation, increased energy consumption, and reduced lifespan. Consequently, developing high-performance composite membranes has become a key focus in the field of membrane separation. Composite membranes consist of a mechanically supportive base membrane and a skin layer. The common base membrane materials include polysulfone, polyethersulfone, cellulose acetate, and polyvinylidene fluoride (PVDF). Optimizing the materials and structure of the skin layer can significantly enhance the performance of composite membranes. The common skin layer materials, such as carbon nanotubes and SiO₂, offer high metal ion rejection, good antimicrobial properties, and functionalization capabilities but often face issues with dispersion and stability. GO, a 2D nanocarbon material with a large specific surface area, is rich in hydroxyl and carboxyl groups, providing strong hydrophilicity, excellent dispersion, superior anti-fouling properties, and ease of functionalization, making it an ideal skin layer material. Assembling GO nanosheets hold promising applications in water treatment, but the separation membranes prepared from pristine GO face issues such as low permeation flux and poor stability. In the case of layered GO membranes, there are mainly two types of material transport channels, as follows: (1) the interlayer structure between adjacent GO nanosheets and (2) the narrow slits formed by pore defects and edges within the GO. Adjusting the transport nanochannels of GO membranes has a significant impact on optimizing the transport of substances within the membranes, which can significantly improve their separation performance. Thus, researchers have employed appropriate physical and chemical modification techniques to regulate the assembly of GO nanosheets, optimizing the pore structure of their 2D nanochannels and the chemical state of the membrane surface. This results in GO-based separation membranes with high water flux, excellent rejection selectivity, and good structural and chemical stability. Currently, the main methods to regulate the interlayer spacing and stability of layered GO-based membranes include weak reduction, small molecule crosslinking, and macromolecule intercalation.
2.3.2.1 Fabrication of graphene oxide-based composite separation membranes.
2.3.2.1.1 Weak reduction. Reducing GO nanosheets can effectively diminish the concentration of oxygen-containing functional groups on their surfaces, inhibiting their dissociation in solution and enhancing the stability of the membrane. However, excessive heat treatment or chemical reduction of the GO membrane eliminates most of these functional groups, intensifying the π–π interaction between sheets and reducing the interlayer spacing to 0.36 nm. This results in the collapse of the nanopores and disruption of the water transport nanochannels, making the graphene membrane impermeable to water and gases.⁸² On the contrary, a moderate level of weak reduction can boost the number of sp² regions in the GO nanosheets, improving the water flux while also significantly enhancing the structural stability of the membrane in solution.

As shown in Fig. 9a and b, Gao et al.⁷³ prepared a weakly reduced, dry, and robust ultrathin graphene nanofiltration membrane through an alkaline reflux method. The membrane exhibited a high pure water flux of up to 21.8 L m⁻² h⁻¹ bar⁻¹ under pressure-driven conditions and displayed a high rejection rate for organic dyes and salts. Jose et al.⁸⁹ adopted a relatively gentle hydrothermal reduction method to control the degree of GO reduction by adjusting the hydrothermal treatment temperature or time for the preparation of the GO nanosheets. Studies have shown that weak reduction can maintain good dispersibility and hydrophilicity of GO nanosheets. Moreover, weak reduction not only increases the number of sp² regions in the GO nanosheets but also maintains a comparable interlayer spacing to the original GO membrane in most regions. The water flux of the weakly reduced GO membrane reached 56.3 L m⁻² h⁻¹ bar⁻¹, which was 4 times and 10⁴ times higher than that of the original GO membrane and the highly reduced GO membrane, respectively. It also achieved a rejection rate of over 95% for various dye molecules. Additionally, the weakly reduced GO membrane exhibited better structural stability and superior separation performance compared to the original GO membrane in acidic and alkaline environments. In summary, the appropriate weak reduction of GO can lower the transport energy barrier of water molecules and introduce more defects into the interlayer, thereby improving the water flux of the separation membrane.

Fig. 9 (a) Digital photo of uGNM coated on an AAO disk (left) and twisted uGNM coated on a PVDF membrane (right).⁷³ Copyright 2013, Wiley. (b) Schematic representation of brGO: graphene sheet with a certain amount of holes, where most of the oxidized groups are located on the edges and the periphery of the holes on it.⁷³ Copyright 2013, Wiley. (c) SEM images of the typical corrugated layered structure of CrGO and the multilayered structure of HrGO.⁸³ Copyright 2018, Elsevier. (d) Schematic illustration of the formation mechanism of the GO membranes incorporated with multivalent metal cations.⁸⁴ Copyright 2020, ACS. (e) Typical cross-sectional SEM images of GO and rGO-TH membranes.⁸⁵ Copyright 2020, Springer Nature. (f) FESEM images with inset showing magnified images of GO membrane and GO-CHNs membrane with 9 wt% CHNs.⁸⁶ Copyright 2020, Elsevier. (g) Schematic illustration of intercalating soft polyacrylonitrile gel particles (PAN GPs) to fabricate the GO nanofiltration membranes.⁸⁷ Copyright 2020, ACS. (h) Schematic diagram illustrating the procedures for the synthesis of the multi-nanochannel PP-intercalated GO membrane.⁸⁸ Copyright 2024, ACS.

2.3.2.1.2 Small molecule crosslinking. To prevent the undesirable swelling and dissociation of ultrathin GO membranes in solution, functionalization strategies are also employed to reinforce the cross-linking among GO nanosheets, thus enhancing their structural integrity. There are two key approaches for achieving this cross-linking, i.e., covalent cross-linking, where chemical bonds are formed to solidify the interlayer connections, and non-covalent cross-linking methods, which harness intermolecular forces such as electrostatic and π–π interactions to foster stable binding between the nanosheets.

As shown in Fig. 9d, Xie et al.⁸⁴ proposed a method to improve the structural stability of GO membranes by in situ cross-linking using high-valent metal cations. Specifically, a metal foil was placed under a porous cellulose nitrate-acetate (CN-CA) substrate, and the acidic GO dispersion etched the metal foil to release high-valent metal cations, which cross-linked the membrane during the natural deposition of GO. The high-valent cations entered the interlayer spaces of the GO membrane in situ, resulting in a significant improvement in the structural stability of the membrane in aqueous environments. Rajesh et al.⁹⁰ prepared a robust graphene oxide framework (GOF) membrane by reducing the oxidized functional groups in GO and alternately depositing two cross-linking agents, branched polyethyleneimine (BPEI) and thiourea (TU). The GOF membrane achieved a high rejection rate of 99.5% for both anionic dye methyl orange and cationic dye rhodamine B. The prepared GOF membrane exhibited good stability in aqueous solutions under acidic and alkaline conditions, with an Na₂SO₄ rejection rate of 94%. Besides amine-containing molecular polymers, organic small molecules such as glutaraldehyde and theanine (TH) can also be used for the cross-linking of GO membranes. Ren et al.⁸⁵ found that tannic acid (TA) and theanine (TH) molecules, both extracted from tea, not only weakly reduced GO but also acted as intercalating agents to increase the interlayer spacing in the GO membrane and cross-linked with the GO nanosheets (Fig. 9e). This significantly improved the water flux and stability of the GO membrane, demonstrating excellent rejection capabilities for dye molecules with different molecular sizes and charge types.

2.3.2.1.3 Macromolecule intercalation. The high specific surface area and abundant functional group structures of GO nanosheets enable them to have good compatibility with guest molecules (such as polymers and nanosheets). Mixing or layer-by-layer assembly of GO nanosheets with spacers of a certain size, such as nanoparticles or macromolecular polymers, can orderly separate the adjacent GO nanosheets in ultrathin 2D GO membranes, creating suitable lateral nanochannels between the assembled layers, which improves the solvent permeation rate.

As shown in Fig. 9f, Hao et al.⁸⁶ utilized copper hydroxide nanorods as an interlayer in GO to obtain a GO nanofiltration membrane with high permeation flux. The use of nanoscale copper hydroxide as a spacer uniformly increased the free volume between the GO nanosheets, thereby significantly enhancing the permeation capacity of the GO membrane. Zhang et al.⁸⁷ prepared polyacrylonitrile gel soft particles (PAN GPs, 1–8 nm) as nanoscale intercalating agents for GO membranes. By controlling the deformation, swelling, and PAN GPs-π interactions between the GO membrane layers, precise regulation of the sub-nanometer interlayer spacing was achieved, while the regular layered structure of the GO membrane was fully preserved (Fig. 9g). Furthermore, alkaline treatment could generate charges and dual hydrophobic/hydrophilic properties on the surface of PAN GPs, organically integrating water channel retention with rapid water permeation, successfully constructing a structure similar to water/protein channels. The results of this study showed that this GO composite membrane could retain over 96% of heavy metal complex ions (Cu-NTA, Cu-CA, Cu-EDTA, Ni-EDTA, and Cr-EDTA), and its water flux was 4–13 times higher than that of similar membranes reported in the literature. Fig. 9h shows the multichannel polymer-intercalated ultrathin GO membrane successfully developed by Zhu et al.,⁸⁸ utilizing a simple assembly of GO nanosheets, polyvinyl alcohol, and polyacrylic acid for the efficient separation of water, salts, and volatile organic compounds during the membrane distillation process.

2.4 Application of graphene-based separation membranes

2.4.1 Gas separation. Graphene, which is composed of closely packed carbon atoms, does not allow the passage of gas molecules. Thus, to achieve selectivity and permeability for mixed gases, researchers have proposed two strategies, including creating nanopores on graphene nanosheets and utilizing the interlayer space by assembling graphene nanosheets to construct selective gas transport nanochannels.⁹¹ In 2009, Jiang et al.⁹² proposed that nanoporous graphene with specific pore sizes and geometric structures would be an effective gas separation membrane. They designed two different atom-modified porous graphene membranes based on first principles and found (Fig. 10a and b) that the selectivity for H₂/CH₄ mixtures could reach up to 10⁸ orders of magnitude when separated through N-functionalized pores, while the selectivity through the fully H-passivated pores was extremely high, reaching up to 10²³ orders of magnitude, with high H₂ permeability. This theoretically confirms the gas separation properties of the nanoporous graphene membrane. Du et al.⁹³ drilled a series of porous ultrathin 2D membranes with different pore sizes on a single-layer graphene nanosheet, as shown in Fig. 10c. They used molecular dynamics simulations (MD) to investigate the effect of pore size on the permeability of various gases and found that the efficient selective separation of H₂ and N₂ can be achieved by precisely controlling the shape and size of the nanopores.

Fig. 10 (a) Creation of a nitrogen-functionalized pore within a graphene sheet.⁹² Copyright 2009, ACS. (b) Hexagonally ordered porous graphene. The dotted lines indicate the unit cell of the porous graphene. Color code: C, black; N, green; and H, cyan.⁹² Copyright 2009, ACS. (c) Structure of porous graphene models.⁹³ Copyright 2011, ACS. (d) Measuring leak rates in porous graphene membranes.⁹⁴ Copyright 2012, Springer Nature. (e) 50 nm-wideapertures FIB-drilled on the freestanding graphene (GaFIB) (scale bar, 500 nm), and 7.6 nm wide apertures perforated in a similar way (HeFIB) (scale bar, 100 nm).⁹⁵ Copyright 2014, AAAS. (f) Membrane figures of merit and comparisons.⁹⁵ Copyright 2014, AAAS. (g) H₂/CO₂ separation performance of EFDA–GO membranes compared with state-of-the-art gas separation membranes.⁹⁶ Copyright 2013, AAAS. (h) Schematic representation showing the separation and killing capability of MDR pathogens using nisin-conjugated porous graphene oxide membrane.⁹⁷ Copyright 2015, RSC. (i) Schematic illustration of the mechanically strong, large-area GNM/SWNT hybrid membrane for efficient water desalination.⁹⁸ Copyright 2019, AAAS.

The aforementioned discoveries stemmed from simulation results, which researchers subsequently translated into practice by fabricating a range of porous graphene membranes with varying pore sizes. Koenig et al.⁹⁴ pioneered the experimental validation of the remarkable gas separation capabilities of micrometer-sized nanoporous graphene membranes. By employing ultraviolet oxidation etching, they created nanopores within the graphene nanosheets and assessed the permeation rates of gases such as H₂, CO₂, Ar, N₂, and CH₄ through pressurized bubble tests and mechanical resonance measurements. Their study revealed that the molecular diffusion rate decreased with an increase in molecular size, aligning well with the theoretical predictions (Fig. 10d). Nonetheless, their tests were confined to micrometer-sized graphene membranes, significantly highlighting their potential for industrial separation applications. Additionally, porous graphene membranes crafted by drilling holes in single-layer graphene possess limited mechanical durability, making them prone to damage during operation. Thus, to address this issue, researchers have focused on fabricating composite membranes to bolster the stability and mechanical resilience of graphene-based membranes. For instance, Celebi et al.⁹⁵ utilized focused ion beam bombardment to introduce uniformly sized nanopores into CVD bilayer graphene. Their research showed that porous graphene with a pore size of 7.6 nm exhibited a hydrogen permeability coefficient of several orders of magnitude higher than that of other gas separation membranes while maintaining a comparable H₂/CO₂ selectivity (Fig. 10e and f).

Fabricating nanopores with precise diameters in graphene is a challenging task. Randomly assembled graphene nanosheets have a relatively small average interlayer spacing (0.355 nm), making the passage of gas molecules unfavorable. In contrast, assembled GO membranes (0.6–1.2 nm) have a much larger interlayer spacing than graphene, which depends on the humidity, making them a better candidate for gas separation applications. Kim et al.⁹⁶ reported the preparation of GO membranes on polyethersulfone (PES) supports using a spin-coating method. The fabricated membranes exhibited a superior gas separation performance compared to the existing upper limits of gas separation membranes (Fig. 10g). Kim et al. also investigated the gas separation properties of multilayer GO membranes and found that by assembling GO nanosheets in different ways to control the nanochannels for airflow, gas-selective separation capabilities could be achieved. Additionally, Jin et al.⁹⁹ proposed the ordered assembly of GO nanosheets through the synergistic control of mechanical and molecular forces. This allowed the precise regulation of the rapid “gas nanochannels” in assembled GO membranes at the sub-nanometer scale, achieving the highly efficient screening of hydrogen molecules from gas mixtures and breaking the performance “trade-off” limitation of traditional membrane materials for separation. Wu et al.¹⁰⁰ developed hybrid membranes for CO₂/CH₄ and CO₂/N₂ separation by incorporating one-dimensional carbon nanotubes (CNTs) and 2D GO nanosheets as dual fillers into a Matrimid 5218 matrix. This approach leveraged the synergistic effect of the different nano-filler morphologies within the membranes. By fine-tuning the ratio of CNTs and GO during the assembly process, they could precisely control the physical, chemical, and topological properties of the membranes. This allowed them to uncover the inherent relationship between the filler morphology and gas permeation selectivity, ultimately enhancing the CO₂ diffusion selection mechanism. The resulting membranes exhibited a remarkable CO₂ separation performance. Specifically, CNTs and GO fillers demonstrated excellent compatibility with the polymer matrix, ensuring their uniform distribution. The sheet-like structure of the laminar GO provided robust steric hindrance, which prevented the agglomeration of CNTs, while CNTs interspersed between the GO nanosheets discouraged GO restacking. Furthermore, a method was devised to manipulate the membrane structure, encouraging CNTs to align vertically and GO to align parallel to the membrane surface. The smooth inner walls of CNTs functioned as efficient gas transmission nanochannels, enhancing the CO₂ permeability, while the layered structure and surface hydroxyl and carboxyl groups of GO boosted the separation selectivity. This approach significantly enhanced the CO₂ separation process.

Graphene without defects can significantly extend the path of gas molecules passing through membranes, effectively blocking gas permeation. Thus, by utilizing this characteristic and incorporating graphene into polymers, the resulting membrane material significantly outperforms the gas barrier properties achieved by adding carbon materials such as CNTs, carbon fibers, and carbon black.¹⁰¹ Based on the excellent performance of graphene composite membranes, highly efficient gas barrier films can be developed for applications in food packaging and corrosion protection of metal structures, thereby extending the service life of products. This technology exhibits broad application prospects in industries such as shipbuilding, coatings, food, and pharmaceuticals.

2.4.2 Water purification. In recent years, with the development of the modern chemical industry, an increasing number of new types of water environmental pollutants, such as heavy metal ions and various stubborn organic pollutants, have attracted attention and pose a serious threat to human health.¹⁰² These pollutants are often difficult to remove effectively in conventional wastewater treatment processes, making it urgent to develop efficient and environmentally friendly new water treatment technologies. In this case, graphene-based membranes can be used to separate different substances in water by adjusting the pore size and interlayer spacing, which can be applied to remove antibiotics, dyes, pathogenic bacteria, heavy metals, and other contaminants from water.

Liu et al.¹⁰³ utilized HI vapor to reduce GO membranes and produce rGO membranes. After reduction, the hydrophobicity of the membranes increased, with the water contact angle increasing from 38° to 78°. Compared to the original GO membranes, the rGO membranes exhibited significantly improved salt rejection rates, perfectly retaining Cu²⁺ and Acid Orange 7. Kanchanapally et al.⁹⁷ discovered that GO membranes modified with specific peptides can effectively identify, separate, and remove drug-resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA). This is because the pore size of the GO membranes is approximately 300 nm, which can completely retain MRSA with a diameter of 1000 nm (Fig. 10h). Jin et al.¹⁰⁴ proposed a novel membrane structure with nanoparticles grown in situ on graphene nanosheets. This design significantly increased the number of fast “water channels” within the graphene stacks while improving the pressure resistance and resistance to cross-flow. Additionally, nanoparticle@graphene membranes were effectively deposited on the inner surface of tubular ceramic support layers, achieving a water flux that is 1–2 orders of magnitude higher than commercial membranes while effectively retaining dye molecules and heavy metal ions from wastewater. The graphene membrane and its preparation method have significant potential for industrial scale-up. As shown in Fig. 10i, Yang et al.⁹⁸ reported the preparation of a large-area graphene-nanomesh/single-walled carbon nanotube (GNM/SWNT) hybrid membrane with excellent mechanical strength while fully utilizing the advantages of atomic thickness of ultrathin membranes. The single-layer GNM possessed a high density of sub-nanometer pores, effectively transporting water molecules while blocking solute ions or molecules to achieve size-selective separation. Zhang et al.¹⁰⁵ synthesized graphene-doped nanocomposite membranes (NCMs) by uniformly dispersing a novel graphene-based additive, GO-PEI (polyethylenimine), onto the bulk polymer polyethersulfone through π–π interactions. The experiments on bovine serum albumin (BSA) filtration demonstrated that the NCM membranes exhibited a retention rate of over 90% for BSA. However, after 2 h of filtration, the NCM membranes without GO-PEI addition were severely fouled, resulting in a decrease in the permeate flux to approximately 30%. In contrast, the NCM membranes with the addition of GO-PEI exhibited increased porosity and the formation of a dense hydrated layer on their surface due to the synergistic effect of the hydrophilic and charged functional groups of GO-PEI. This not only increased the permeate flux but also effectively avoided membrane fouling, thus preventing membrane pollution and delaying the decline in the membrane flux. Additionally, under π–π interactions, NCMs containing a high proportion of GO can selectively adsorb and remove aromatic hydrocarbons without using sieving, thereby maintaining the high permeability of the NCM ultrathin membrane.

2.4.3 Seawater desalination. Approximately 70% of the Earth's surface is covered with water, but the scarcity of clean water resources is one of the most severe global challenges. Accordingly, researchers have devoted significant efforts to developing water treatment technologies, including seawater desalination and purification techniques such as reverse osmosis (RO), including multi-stage and closed-loop RO, electrodialysis, and thermally driven processes. However, their high energy consumption and material costs, safety concerns from high pressure, and mineral depletion remain serious issues. Filtration techniques are commonly used to remove insoluble particles and molecules from aqueous solutions, but filtering dissolved salt ions for desalination is a challenging task. Thus, to address water scarcity, the introduction of nanopores on graphene and their size regulation can achieve selective permeability for different molecules and ions.

Cohen-Tanugi et al.⁵¹ utilized molecular dynamics simulations to compare the seawater desalination processes of two types of nanopores (H-modified nanopores and OH-modified nanopores) in monolayer graphene. They found that the H-modified nanopores exhibited superior selectivity, while the OH-modified nanopores increased the water flux. This difference depends on the hydrophilic properties of the two nanopores and their ability to displace water molecules in the ion hydration shell. The salt rejection rate of the nanoporous monolayer graphene was 99%, and the hydrophilic termini enabled a water flux of up to 2750 L m⁻² h⁻¹ bar⁻¹, which is 2–3 orders of magnitude higher than commercial reverse osmosis membranes. The Amir Barati Farimani team¹⁰⁶ conducted extensive molecular dynamics simulations and statistical analyses to study bilayer MoS₂-graphene and compare it with graphene and MoS₂ (Fig. 11a and b). By optimizing the heterostructure membrane, they achieved an increased water flux while maintaining a high ion rejection rate. Additionally, the layer-by-layer assembly of GO can be utilized to form selective nanochannels, creating a selective permeation effect for salt ions, which is applicable for seawater desalination. Shi et al.¹⁰⁷ constructed a thin-layer GO membrane with Na⁺ self-rejection capability, high permeability, and multi-stage filtration strategy to obtain freshwater from salt solutions under 1 atmosphere of pressure (Fig. 11c and d). After five and eleven multi-stage filtration cycles, the Na⁺ concentration decreased from 0.6 mol L⁻¹ to 0.123 mol L⁻¹ (below physiological concentration) and 0.015 mol L⁻¹ (freshwater), respectively. Compared to commercial RO membranes, the energy consumption was only 10% with a 10-fold increase in water flux. Interestingly, the energy consumption of this multi-stage filtration strategy was close to the theoretical minimum. Theoretical calculations suggest that this Na⁺ self-rejection effect is attributed to the low transport rate between the hydrated cations and water molecules within the GO membrane due to the interaction between the hydrated cations and π bonds. Quan et al.¹⁰⁹ proposed a strategy of electrostatically induced ion-restricted partitioning in rGO membranes to break the correlation between anions and cations, inhibiting anion–cation co-transport, and thereby significantly improving the seawater desalination performance (Fig. 11f–i). The developed membrane achieved an NaCl rejection rate of 95.5% and a water permeability of 48.6 L m⁻² h⁻¹ bar⁻¹. Under pressure-driven conditions, it also exhibited a desalination rate of 99.7% and a water flux of 47.0 L m⁻² h⁻¹, outperforming the reported graphene-based membranes under osmotic driving conditions.

Fig. 11 (a) Water desalination system featuring a heterogeneous nanoporous membrane simulated using molecular dynamics (MD).¹⁰⁶ Copyright 2024, ACS. (b) Number of filtered water molecules versus time (10 ns) in seven different nanoporous membranes under an external pressure of 100 MPa.¹⁰⁶ Copyright 2024, ACS. (c) Schematic drawing of multistage filtration system for seawater desalination.¹⁰⁷ Copyright 2023, ACS. (d) Digital photograph of a mixed cellulose ester substrate-supported GO membrane and cross-sectional scanning electron microscopy image.¹⁰⁷ Copyright 2024, ACS. (e) Schematic of our mass spectrometry setup.¹⁰⁸ Copyright 2016, AAAS. (f) Schematic diagrams of the preparation procedure and the wrinkle structures of rGO and ArGO-PSSNa membranes.¹⁰⁹ Copyright 2024, Springer Nature. (g) Digital photographs of rGO and ArGO-PSSNa membranes.¹⁰⁹ Copyright 2024, Springer Nature. (h) Top SEM images of rGO membrane.¹⁰⁹ Copyright 2024, Springer Nature. The inset of (h) shows a schematic diagram of the wrinkle structures of the rGO membrane. (i) Top SEM images of ArGO-PSSNa membrane.¹⁰⁹ Copyright 2024, Springer Nature. The inset of (i) shows a schematic diagram of the wrinkle structures of the ArGO-PSSNa membrane.

In summary, graphene-based membranes, characterized by their unique 2D structure and adjustable physicochemical properties, exhibit precise sieving capabilities and are widely applied in fields such as water purification and seawater desalination. However, stability issues have limited their further use in aqueous solutions, particularly due to the swelling, redispersion, and peeling phenomena of GO membranes in water, which degrade their performance. Through molecular dynamics simulations and density functional theory calculations, Jiang et al.¹¹⁰ discovered the critical role of ionic bridges in controlling the interlayer spacing in GO membranes, thereby enhancing their stability in water. Jin et al.¹¹¹ proposed a molecular bridge strategy that reinforces GO layers with interlayer short-chain molecular bridges and adheres GO layers to porous substrates using long-chain molecular bridges, aiming to improve the mechanical strength of the membranes. The rational construction and adjustment of the molecular bridges allow GO membranes to exhibit remarkable durability under harsh operating conditions, presenting new opportunities for the application of 2D laminar films in aqueous environments.

Table 2 summarizes some of the experimental studies on the preparation and performance of graphene-based separation membranes.

Table 2 The preparation and performance of graphene-based membranes^a

Membrane types	Preparation method	Permeability/flux	Salt rejection (%)/retention (%)	Ref.
a GOF: GO framework; GOM: GO membrane; GNM: graphene-nanomesh; SWNT: single-walled carbon nanotubes; PEI: polyethylenimine; and PES: poly(ether sulfone).
Nanoporous single-layer graphene	Oxygen plasma etching	106 g m^−2 h^−1	∼100% (monovalent ions) retention	112
Ultrathin graphene nanofiltration membrane	Filtration-assisted assembly and alkaline reflux	21.8 L m^−2 h^−1 bar^−1	>99% organic dyes) retention, 20–60% (ion salts) retention	73
Ultrathin GO nanofiltration membrane	Mayer bar coating and UV reduction	60 L m^−2 h^−1	96% (Na2SO4) rejection	58
GOF membrane	Layer-by-layer assembly and small molecule crosslinking	24 L m^−2 h^−1	99.5% (anionic dye methyl orange and cationic dye rhodamine B) rejection, 94% (Na2SO4) rejection	90
ATPP@GOM membrane	Vacuum filtration and macromolecule intercalation	15.2 ± 0.9 L m^−2 h^−1	97.3% ± 1.5% (Cu-EDTA) rejection	87
Ultrathin rGO membrane	Vacuum filtration and weak reduction	57.0 L m^−2 h^−1	—	103
GNM/SWNT hybrid membrane	Oxygen plasma etching	252 L m^−2 h^−1 bar^−1	86% (NaCl) rejection	98
GO-PEI PES nanocomposite membrane	Non solvent-induced phase inversion	322.8 L m^−2 h^−1	>90% (BSA) retention, 55–70% (LYZ) retention, ∼100% (E. coli) retention	105

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2.4.4 Isotope removal. The rapid development of the nuclear industry has generated significant nuclear waste, and radioactive substances pose a serious threat to human survival. Radioactive pollution is difficult to eliminate and can only decay over time, making it one of the urgent environmental issues that needs to be addressed.

GO possesses a high specific surface area and abundant carboxyl and hydroxyl groups. Due to the immobilization effect of the negatively charged oxygen-containing functional groups on metal ions, it can be utilized in the treatment process of nuclear waste. Chen et al.¹¹³ performed hydrothermal treatment on a mixture of GO and TiO₂ to obtain a titanate/GO composite membrane. This composite membrane could adsorb radioactive cobalt (⁶⁰Co), other radionuclides, and heavy metals. Their study showed that the adsorption of Co(II) on the particle surface is affected by multiple mechanisms. At pH < 8, the adsorption of ⁶⁰Co by the composite membrane was mainly influenced by pH and ionic strength; however, at pH > 8, the adsorption of ⁶⁰Co by the composite membrane is not affected by ionic strength. The research conducted by the Lozada-Hidalgo team at the University of Manchester, UK¹⁰⁸ showed that monolayer graphene and h-BN can be used for hydrogen isotope separation, with the transmembrane transport rate of deuterium being significantly lower than that of protons. The separation constant between the two isotopes at room temperature was as high as 10. This isotope effect is attributed to the difference in zero-point energy (∼60 milli-electron volts) between protons and deuterons, which translates into a difference in activation energy barriers during transport, thus providing an effective method for enriching hydrogen isotopes (Fig. 11e). However, the preparation of micrometer-scale single-layer graphene membranes by mechanical exfoliation is cumbersome, with extremely low yield, limiting their large-scale applications and making them only suitable for basic research. A. K. Geim et al.¹¹⁴ prepared large-area graphene membranes through the CVD strategy, which could effectively separate protons and deuterons. Although the graphene membranes prepared by CVD had defects and pores, their separation constant at room temperature was still as high as 8, ensuring separation selectivity while achieving a 100% faradaic current efficiency for current-driven hydrogen production.

The high screening efficiency of graphene membranes for isotopes can help simplify the production of heavy water and nuclear waste cleanup processes. They have a wide range of applications in the analytical and chemical tracing fields and are expected to become a new generation of super “filters”.

3 MXene-based membranes for separation

Besides graphene, as a new type 2D nanosheet similar to graphene, MXene nanosheets also hold great potential in precise separation. Their graphene-like layered structure enables layer-by-layer assembly to form ultrathin laminar membranes with regulable selective transport nanochannels. Simultaneously, the emergence of 2D MXene nanosheets further adds new possibilities to overcome the inherent “trade-off” limitations between high permeability and selectivity in traditional polymeric membranes, showing broad precise separation application prospects. Although the development of ultrathin MXene-based membranes is still in its infancy, they have already been utilized in high-performance precise water/gas separation applications. The excellent separation performance of ultrathin MXene membranes in gas separation is mainly due to their 2D laminar morphology, but this is not a unique advantage of MXene nanosheets. For example, graphene, COFs, and MOFs can all form 2D layered membranes. However, it worth noting that in the field of water treatment, MXene nanosheets highlight their unique superiority as separation membranes due to their high surface charge, hydrophilicity, ion adsorption capacity, mechanical strength, and chemical tunability compared with other ultrathin 2D nanosheets.^115–117

3.1 Synthesis of MXene nanosheets

MXenes are a new type of 2D carbonitride nanosheets,¹¹⁵ with the chemical formula M_n+1X_nT_x (n = 1–3),¹¹⁶ where M stands for a transition metal, X represents carbon or nitrogen, and T_x refers to functional groups such as –OH, =O, and –F. MXenes can be obtained by etching the A layer of MAX phases, such as Ti₂AlC, Ti₃AlC₂, Ti₄AlC₃,¹¹⁷ where A represents a group of elements (e.g., Al, Si, and Sn). Since the first MXene was reported in 2011, the name “MXene” has been used to emphasize the loss of the A element in MAX phases and its 2D structural features are similar to that of ultrathin graphene nanosheets.¹¹⁸

To date, over 30 types of MXenes have been reported.¹¹⁹ Based on the different transition metal compositions, MXenes can be categorized into three groups including single transition metal type, solid solution type, and ordered bimetallic type. However, despite the vast variety of MXenes, more than 70% of current research is focused on Ti₃C₂T_x. MXenes have a high specific surface area, good hydrophilicity, mechanical properties, conductivity, and low diffusion resistance.¹²⁰ Meanwhile, they have rich functional groups on their surfaces, making it easy to adjust the interlayer spacing. They also possess high chemical and mechanical stability,¹²¹ which is conducive to water purification. Furthermore, MXenes have high antibacterial efficiency against both Gram-negative Escherichia coli and Gram-positive Bacillus subtilis. Thus, due to the unique layered structure and various excellent properties of MXenes, they are widely used in the field of precise separation.¹²²

The preparation of MXenes typically involves the selective etching of the A-layer elements from the MAX phases.¹²³ This process can be achieved by using acids/fluoride salts as etchants. After etching, the MXene material retains its 2D layered structure and may possess some surface functional groups, such as –OH, =O, and –F. The presence of these functional groups can influence the chemical properties and application performance of ultrathin MXene membranes. Therefore, during the preparation process, the etching conditions and subsequent treatments can be controlled to optimize the properties and structure of MXene nanosheets.

The quality of MXene nanosheets is significantly influenced by their synthesis method. Different synthesis methods yield varying effects on the size, morphology, crystallinity, surface functional groups, and interlayer structure of the nanosheets, which impact the overall performance and application potential of MXene nanosheets. The two main strategies used for the fabrication of MXenes are top-down and bottom-up. Selecting the appropriate method is crucial for fine-tuning the overall physicochemical properties of the membranes, such as the nanosheet morphology, functionality, and size, for specific purposes or applications.¹²⁴

3.1.1 Top-down synthesis. The top-down synthesis method is the most commonly used approach for the preparation of Ti₃C₂T_x nanosheets. Based on different etching reagents and methods, this approach can be further divided into solution etching and non-aqueous etching methods. The solution etching method is the earliest established method for preparing MXenes. It mainly includes HF etching, acid/fluoride salt etching, electrochemical etching, and alkaline solution etching. The non-aqueous etching method mainly includes molten salt etching and halogen etching, which further enrich the preparation methods and properties of MXenes.¹²⁵
3.1.1.1 Etching.
3.1.1.1.1 Hydrofluoric acid (HF) etching. In 2011, for the first time, Naguib et al. successfully prepared MXene nanosheets by etching ternary MAX phases using hydrofluoric acid (Fig. 12a). They mixed HF solution with Ti₃AlC₂ powder and this mixture was stirred and left to react for 2 h to prepare the MXene. Currently, the use of hydrogen fluoride to etch MAX phases is relatively widespread. This method produces MXene phases with a clear layered structure and relatively uniform interlayer spacing.¹¹⁸

Fig. 12 (a) Schematic diagram of the use of HF etching on MAX to prepare MXene.¹¹⁸ Copyright 2011, Wiley. (b) Schematic diagram of the preparation of Ti₂C₂T_x by etching Ti₃AlC₂ with a mixture of HCl and LiF.¹²⁶ Copyright 2014, Springer Nature. (c) Schematic diagram of the preparation of Ti₂CT_x (MXene) by electrochemical etching method.¹²⁷ Copyright 2017, RSC. (d) Schematic diagram of the preparation of fluorine-free Ti₃C₂T_x-MXene by etching Ti₃AlC₂ with concentrated alkali method.¹²⁸ Copyright 2018, Wiley. (e) Schematic diagram of the preparation of Ti₄N₃-MXene by molten salt etching method.¹²⁹ Copyright 2016, RSC. (f) Schematic diagram of the preparation of MXene with –Cl terminal groups by etching MXA phase using molten ZnCl₂.¹³⁰ Copyright 2019, ACS. (g) Schematic diagram of the preparation of MXene by halogen etching method.¹³¹ Copyright 2021, ACS. (h) Schematic diagram of the preparation of Mo₂C MXene on Mo/Cu alloy using chemical vapor deposition CVD technique.¹³² Copyright 2022, AIP.

3.1.1.1.2 Etching with a mixture of hydrochloric acid (HCl) and lithium fluoride (LiF). Due to the high danger and corrosive nature of HF, its excessive use can easily cause significant harm to the human body and significant pollution to the environment. Therefore, it is essential to select an etching agent that is less harmful to the human body and environmentally friendly. In 2014, Ghidiu et al. first successfully etched Ti₃AlC₂ using a mixture of HCl and LiF, obtaining clay-like Ti₂CT_x, as shown in Fig. 12b.¹²⁶ The principle is to use a strong acid and fluoride salt to react in situ to generate HF, which reacts with the A atomic layer to achieve etching. In addition to LiF, similar fluoride salts include NH₄HF₂ and NH₄F. Halim mixed NH₄HF₂ solution with Ti₃AlC₂ powder and used a gentle method for slow etching. Over time, the A in the MAX phase was gradually etched, resulting in MXene with a uniform structure (Fig. 12c). This method employs conditions, has safer experimental operation, and has been successfully extended to many MAX compounds.¹³³
3.1.1.1.3 Electrochemical etching. To further regulate the structure of MXene and the types of T elements, and improve the electrochemical performance of MXenes, an electrochemical etching method has been proposed. Sun et al. successfully synthesized Ti₂CT_x (MXene) using electrochemical etching in a diluted HCl aqueous solution, where the prepared MXene was not terminated with –F groups (Fig. 12c). The chemical etching method not only improves the environmental safety during the preparation of MXenes, but also provides a faster method.¹²⁷
3.1.1.1.4 Concentrated alkaline etching. The concentrated alkaline etching method refers to the process of using concentrated alkali to react with aluminum (Al) when the A layer in the MAX phase is aluminum, generating soluble aluminate ions to achieve the effect of etching the A layer. Li et al. utilized the amphoteric property of Al to prepare high-purity fluorine-free Ti₃C₂T_x-MXene through the hydrothermal etching of Ti₃AlC₂ in a 27.5 mol L⁻¹ NaOH solution at 270 °C, enhancing the electrochemical performance of MXenes (Fig. 12d).¹²⁸
3.1.1.1.5 Molten salt etching. The molten salt etching method refers to the preparation of MXenes using molten fluoride salts, Lewis acid salts, or Lewis base salts (such as LiF, NaF, and ZnCl₂) at high temperatures. By selecting different compositions of molten salts, the functional terminations on the surface of MXenes can be adjusted. Urbankowski et al. mixed the Ti₄AlN₃ precursor with various fluoride salts and heated it under argon to 550 °C for 30 min.¹²⁹ At high temperatures, the fluoride etched the Al element in the MAX phase, resulting in the formation of Ti₄N₃-MXene (Fig. 12e). Furthermore, the molten salt etching method has also been utilized to prepare fluorine-free MXenes. Li et al. utilized molten ZnCl₂ to etch the MAX phase, resulting in MXenes with –Cl terminal groups (such as Ti₃C₂Cl₂ and Ti₂CCl₂) (Fig. 12f).¹³⁰ This marked the first time that MXenes fully terminated with –Cl were successfully prepared. MXenes terminated with –Cl exhibit better stability and stronger electrochemical performance compared to that terminated with –F. This provides a green and feasible route for the preparation of MXenes without using HF-based chemical methods.
3.1.1.1.6 Halogen etching. The halogen etching method utilizes the reaction between halogens and the Al element in the MAX phase to etch the Al element, thereby obtaining MXene. Jawaid et al. synthesized MXene from Ti₃AlC₂ using halogens (Br₂, I₂, ICl, and IBr) (Fig. 12g) in an anhydrous medium.¹³¹ The research results indicated that this method can provide a mild and efficient new approach for the preparation of halogen-terminated MXene-based materials.
3.1.1.2 Exfoliation. The 2D properties of materials often manifest only after they are exfoliated into single-atom layers or thin sheets of several atomic layers. Therefore, exfoliation is a crucial step in the preparation of 2D nanosheets. After etching, a large number of MXene layers is still stacked, connected by hydrogen bonds.¹¹⁷ According to research findings, some organic molecules or metal cations can spontaneously insert between the stacked MXene layers, making the MXene stack easier to exfoliate. They are usually introduced through solutions.¹³⁴ For example, when HCl and LiF are used as etching agents, Li⁺ inserts between the MXene layers during the etching process. After ultrasonic exfoliation, a large number of MXene sheets can be obtained without the need for additional molecular or ion intercalation.¹³⁵ The exfoliated MXene sheets are uniformly dispersed in the solution, forming a stable colloidal solution.

3.1.2 Bottom-up synthesis. As the top-down methods for the preparation of MXene materials gradually mature, researchers have also explored methods to grow high-quality MXenes in a controlled manner. The basic idea behind the bottom-up synthesis for preparing MXenes is to introduce organic compounds into metals as carbon or nitrogen sources and directly grow metal carbides or nitrides (MXenes) on the surface of the substrate under high-temperature conditions. The bottom-up approach primarily consists of methods such as chemical vapor deposition (CVD),¹³² plasma processing,¹³⁶ and templating method.¹³⁷ Among them, CVD has attracted widespread attention due to its advantages in terms of controllability, uniformity, and scalability. Öper et al. successfully prepared Mo₂C MXene on Mo/Cu alloy using CVD technology with methane as the carbon source and investigated the influence of the carrier gas flow rate on the structure of the Mo₂C MXene (Fig. 12d). They found that by controlling the carrier gas flow rate, the thickness of the Mo₂C MXene (ranging from 7 to 145 nm) and the coverage rate on the substrate surface (11% to 68%) could be controlled.¹³² CVD can be used to prepare large-area, high-quality 2D ultra-thin MXene crystals. Compared to the top-down synthesis, the MXene materials prepared by bottom-up synthesis exhibit higher crystallinity and uniform layer thickness. They also do not contain T_x terminations, leading to higher conductivity. However, the bottom-up synthesis has a smaller production scale and requires specialized equipment compared to the top-down synthesis. Table 3 shows the different methods for the synthesis of MXenes and their advantages and disadvantages.

Table 3 Different methods for the synthesis of graphene and their advantages and disadvantages

Synthesis method	Advantages	Disadvantages	Refs
HF etching	· Universality	· Hazard	118
	· Efficiency
	· Clear layered structure of MXenes
Etching with a mixture of HCl and LiF	· Mild method	· Required reaction condition control	126 and 133
	· Improved safety	· Increased cost
	· Uniform structure of MXenes
Electrochemical etching	· Mild reaction conditions	· Potentially longer reaction time	127
	· High controllability
Concentrated alkaline etching	· High safety	· Harsh reaction conditions	128
	· Good surface reactivity	· Potential for disrupting MXene structure
Molten salt etching	· Unique reaction mechanism	· High reaction temperature requirement	129 and 130
	· Production of fluorine-free MXenes	· Possible phase transition
Halogen etching	· Controllable surface chemistry	· Complex control of reaction conditions	131
	· Enhanced electrochemical performance	· Possible introduction of impurities
	· Expanded application fields of MXenes	· Possibly low preparation efficiency
Chemical vapor deposition (CVD)	· High production efficiency	· High investment and technical difficulty	132
	· Controllable surface functional groups
	· MXene large-area, uniform coverage	· Material limitations
	· Broad application prospects

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3.2 MXene-based separation membranes

Ultrathin 2D MXene nanosheets have outstanding intrinsic properties such as hydrophilicity, abundant surface terminations, and structural and chemical tunability, making them an ideal material to assemble laminar membranes for efficient ion or molecular separation. Furthermore, by tuning the layer-by-layer assembly process of MXene nanosheets and their physicochemical properties, highly selective ultrathin MXene-based membranes can be easily achieved. MXenes are primarily utilized for the preparation of layered membranes, and can also be employed to fill polymer matrices to form organic–inorganic hybrid matrices. MXene layered membranes are constructed by stacking adjacent MXene nanosheets, where the nanoscale channels between the layers offer pathways for mass transfer and separation. MXene layered membranes can be primarily classified as follows: (1) single-component MXene layered membranes and (2) MXene layered composite membranes.

3.2.1 Single-component MXene layered membrane. Ren et al. attempted to assemble ultrathin 2D Ti₃C₂T_x (MXene) sheets into standalone or supported membranes (Fig. 13a).¹³⁸ The 2D MXene membranes possessed a controllable thickness ranging from hundreds of nanometers to several micrometers, exhibiting excellent mechanical flexibility, high mechanical strength, hydrophilic surfaces, and electrical conductivity. They were used for precise charge-selective and size-selective rejection of ions and molecules. This opens a door for the assembly of 2D MXene nanosheets into highly efficient and selective separation membranes. As shown in Fig. 13b, Ding et al. assembled Ti₃C₂T_x MXene membranes on a porous support by vacuum filtration onto an anodic aluminum oxide (AAO) substrate.¹³⁹ The precisely assembled Ti₃C₂T_x MXene nanosheets were peeled off from the substrate to form a self-supported Ti₃C₂T_x MXene membrane. The MXene membrane exhibited excellent water permeability (greater than 1000 L m⁻² h⁻¹ bar⁻¹) and a good rejection rate (exceeding 90%) for molecules with a size greater than 2.5 nm. Shen et al. assembled 2D MXene nanosheets into 20 nm-thick membranes for gas separation.¹⁴⁰ The well-stacked pristine MXene laminar membranes with adjustable selective nanochannels displayed an outstanding molecular sieving performance, enabling preferential H₂ transport (Fig. 13c).

Fig. 13 (a) Preparation, structure, and image of the membrane by assembling ultrathin MXene-Ti₃C₂T_x nanosheets.¹³⁸ Copyright 2015, ACS. (b) Schematic diagram of the assembly and mass transfer process of the MXene membrane.¹³⁹ Copyright 2017, Wiley. (c) Schematic illustration of the mass transfer and H₂ transport through MXene membranes.¹⁴⁰ Copyright 2018, Wiley. (d) Separation and mass transfer mechanism of ultrathin MXene-derived membrane supported on an α-Al₂O₃ tubular support.¹⁴¹ Copyright 2019, Elsevier. (e) Ion sieving and mass transfer process of LMM and SMM.¹⁴² Copyright 2024, ACS.

The mass transfer in ultrathin MXene membranes is primarily achieved through the interlayer spacing between the MXene nanosheets and the formation of a long transport pathway, enabling efficient molecular separation. Thus, by adjusting the interlayer spacing between 2D MXene nanosheets, well-defined transport nanochannels can be established, facilitating the separation of specific ions or molecules. In addition to the interlayer distance, the lateral size of MXene nanosheets can also be altered by adjusting their synthesis conditions.¹⁴³ Sun et al. designed ultrathin MXene-derived membranes (thickness: 100 nm) supported on α-Al₂O₃ tubular supports and fine-tuned the interlayer spacing between adjacent nanosheets by adjusting the sintering temperature.¹⁴¹ As shown in Fig. 13d, water loss and defunctionalization (–OH) within the MXene membranes occurred at elevated temperatures. Below 400 °C, the spacing decreased from 3.71 Å (at 60 °C) to 2.68 Å (at 400 °C), and the prepared MXene-derived membrane (T400) exhibited good ion rejection. Fan et al. developed a 2D MXene membrane that can control the spacing and surface functionalization of MXene nanosheets through heat treatment, thereby influencing the gas diffusion mechanism and enhancing the H₂/CO₂ selectivity.¹⁴⁴ The interlayer spacing of the MXene membrane decreased from 3.4 Å (at 25 °C) to 2.7 Å (at 500 °C), enhancing the molecular sieving performance. This membrane exhibited an H₂ permeability of 2.05 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ and good selectivity for H₂/N₂ mixtures at 320 °C. Due to the excellent chemical stability of the MXene membrane, no degradation was observed during the 200 h permeation and separation test. In addition, the separation performance of MXene membranes can also be adjusted by adjusting the lateral size of MXene nanosheets to change the transport pathways. As shown in Fig. 13e, Huang et al. prepared large lateral-sized MXene membranes (LMM) to alter the transport pathways, and LMM exhibited the preferential transport of monovalent ions (Li⁺, Na⁺, and K⁺) while blocking the permeation of multivalent ions (Ca²⁺, Mg²⁺, and Al³⁺). Compared to MXene-based membranes assembled by small lateral-size nanosheets, the membranes assembled by LMMs exhibited higher selectivity for precise separation.¹⁴²

Ultrathin 2D MXene membranes exhibit significant potential in molecular separation due to their unique properties, including hydrophilicity, flexibility, and excellent mechanical strength. However, the high hydrophilicity of the MXene surface makes it easier for water molecules to be absorbed into the interlayer spacing between adjacent MXene sheets, leading to membrane expansion and affecting the separation performance. Meanwhile, Ti₃C₂T_x nanosheets often form a dense structure through a face-to-face stacking pattern. The inherent repulsion between Ti₃C₂T_x nanosheets creates non-selective defects, limiting the exposed surface area and reducing water permeability. Thus, to address these issues, researchers have conducted a series of studies to enhance the permeability, stability, and strength of MXene membranes, ultimately improving their separation performance. Combining MXenes with other materials to form composite membranes can better optimize the interlayer structure of the membrane and regulate the interlayer spacing of MXenes. Furthermore, MXenes can be used as a filler in polymer matrices to form mixed-matrix membranes. The introduction of MXenes can reduce the mass transfer resistance, restrict the interfacial channels, and adjust the membrane properties. Additionally, MXenes can serve as 2D scaffolds, assisting in the membrane preparation process and further enhancing the separation and mass transfer performance of the membrane.¹⁴⁵

3.2.2 MXene layered composite membrane. Single-component MXene layered membranes suffer from issues such as severe nanosheet stacking and irregular interlayer nanoscale channels. Thus, to further improve the separation performance of MXenes, composite membranes can be formed through intercalation or crosslinking methods. Compared to the single-component MXene layered membranes, MXene composite membranes can enhance the mass transfer efficiency by adjusting the transport channels.
3.2.2.1 Intercalation. The strong van der Waals forces present between MXene nanosheets during their assembling process tends to induce their aggregation and restacking, which can negatively affect their separation performance. Accordingly, an efficient design principle to address this issue is to introduce external components between the ultrathin 2D MXene nanosheets.¹⁴⁶ The commonly used external components include ions, small molecules, nanomaterials, and polymers. The intercalated components can adjust the interlayer spacing between MXene nanosheets, further enhancing the separation performance of MXene-based separation membranes. Fan et al. prepared molecular sieving membranes (MSM) supported on Al₂O₃ hollow fibers by embedding Ni²⁺ among MXene nanosheets (Fig. 14a).¹⁴⁷ The prepared membrane exhibited an excellent H₂/CO₂ separation performance. Moreover, Ni²⁺ can regulate the interlayer spacing of MSMs. During the 200 h test, the obtained membrane maintained an excellent gas separation performance without any substantial decline. Wu et al. inserted cetyltrimethylammonium bromide-modified halloysite nanotubes (C-HNTs) between MXene nanosheets to assemble a relatively loose MXene/C-HNT composite forward osmosis (FO) membrane (Fig. 14b). The insertion of C-HNTs effectively increased the interlayer spacing among the MXene nanosheets and enhanced the compressive resistance of the composite membrane. The assembled membrane possessed excellent water permeability and dye desalination performance.¹⁴⁸ Sun et al. prepared a series of inorganic–organic hybrid nanosheets based on pillararene-intercalated MXene nanosheets (Fig. 14c).¹⁴⁹ Compared with pristine MXene nanosheets, these hybrid nanosheets exhibited larger lateral dimensions. Membranes were formed through vacuum-assisted filtration. The as-prepared membranes exhibited a relatively high water permeance, rejection, and stability for treating water containing antibiotics under dead-end filtration and cross-flow filtration conditions. Furthermore, graphene, a 2D material, has been widely used in the preparation of separation membranes. The composite of 2D nanomaterial graphene with MXene nanosheets can not only effectively regulate the interlayer spacing of MXene membranes but also leverage the distinct advantages of both 2D materials. This allows the production of MXene composite membranes with superior selectivity and permeability. Liu et al. developed a novel composite membrane based on GO and MXene (Fig. 14d).¹⁵⁰ The composite membrane with a GO/MXene mass ratio of 1/4 exhibited a higher water flux (71.9 L m⁻² h⁻¹ bar⁻¹) compared to the GO membrane. It also demonstrated a removal rate exceeding 99.5% for common small-molecule organic dyes (NR, MB, CV, and BB).

Fig. 14 (a) Schematic illustration of Ni²⁺-embedded MXene composite membrane.¹⁴⁷ Copyright 2021, HEP. (b) Schematic illustration of C-HNT-embedded MXene composite membrane.¹⁴⁸ Copyright 2022, Elsevier. (c) Schematic illustration of pillararene-intercalated MXene composite membrane.¹⁴⁹ Copyright 2022, Wiley. (d) Schematic illustration of GO/MXene composite membrane.¹⁵⁰ Copyright 2020, Elsevier. (e) Schematic illustration of the process for preparing APTES/MXene-ACNT composite membrane through crosslinking of ACNTs with MXene.¹⁵¹ Copyright 2021, Elsevier. (f) Schematic illustration of CMC/MXene composite membrane.¹⁵² Copyright 2024, Elsevier.

3.2.2.2 Cross-linking. The abundant functional groups on the surface of MXene nanosheets make them easy to be modified or functionalized. Physico-chemically functionalized MXene nanosheets can be employed to assemble laminar membranes, significantly improving the compatibility and separation performance. The formation of MXene composite membranes through crosslinking technology can significantly enhance their compatibility and separation performance. To date, various chemical crosslinking techniques have been employed to form composite membranes, including self-crosslinking, molecular crosslinking, and ionic crosslinking. Lin et al. crosslinked MXene 2D nanosheets with polydopamine (PDA) to create a series of novel composite membranes.¹⁵³ The assembled PDA@MXene nanosheets supported on cellulose acetate (PDA@MXene/CA) membranes by vacuum filtration exhibited excellent hydrophilicity, with a pure water flux of 271.2 L m⁻² h⁻¹ bar⁻¹, which was 277% higher than that of the unmodified MXene membrane. Additionally, the assembled PDA@MXene/CA membranes also exhibited enhanced dye separation capabilities. Furthermore, the ultrathin 2D PDA@MXene/CA membranes possessed strong antifouling properties and good antibacterial capabilities. As shown in Fig. 14e, Zhang et al. crosslinked MXene nanosheets using 3-aminopropyltriethoxysilane (APTES). Through the co-condensation reaction of APTES, the interfacial tension between MXene and water or dissolved oxygen could be reduced, thereby improving the stability of the MXene membranes. Compared to the pure MXene membrane (500.7 L m⁻² h⁻¹ bar⁻¹), the optimal modified membrane, APTES/MXene-ACNTs/CA, achieved a pure water flux of 2892.8 L m⁻² h⁻¹ bar⁻¹. Moreover, the modified membrane exhibited good antifouling and anti-swelling properties, which enhanced the separation stability and reliability for precisely separation.¹⁵¹ Recently, Yang et al. incorporated sodium carboxymethyl cellulose (CMC) into the interlayer of MXene and fabricated CMC/MXene composite membranes through a crosslinking method (Fig. 14f).¹⁵² The results demonstrated that when the content of CMC/MXene membrane was 20%, the selectivity of the CMC/MXene membrane towards Li/Mg and Li/Ca was approximately 11.86 and 10.89, which was 3.47 and 3.84 times higher than that of the pristine MXene membrane, respectively. The preparation of CMC/MXene composite membranes offers an effective solution for the selective separation of monovalent and divalent ions.

3.2.3 MXene-based mixed matrix membranes. MXene-based mixed matrix membranes provide a more practical membrane design principle for practical precise separation applications. MXene nanosheets can be used as fillers in polymer matrices and the presence of MXene can reduce the mass transfer resistance, limit interfacial channels, and adjust the nanochannels of the membrane.¹⁵⁴ Furthermore, the abundant functional groups on the surface of nanosheets endow MXenes with advantages such as high surface area, biocompatibility, hydrophilicity, low diffusion resistance, activated metal hydroxide sites, and excellent conductivity, making MXenes as one of the most popular inorganic fillers.¹⁵⁵ The strategy of introducing MXenes into polymer membranes has been proven to be an effective method to improve their separation performance.¹⁵⁶

For example, Heydari et al. assembled MXene nanosheets into UV-curable acrylate polyvinyl alcohol (APVA) to create a novel APVA-MXene mixed matrix membrane (MMM).¹⁵⁷ Due to the layered structure of MXenes with 2D interlayer transport nanochannels, good compatibility with the polymer matrix, and high surface hydrophilicity. As shown in Fig. 15a, when the MXene loading reached 3 wt%, the permeation flux reached 942 g m⁻² h⁻¹ (an increase of 106% compared to the original APVA membrane) and the separation factor was 294 (an increase of 79% compared to the original APVA membrane). The incorporation of MXene nanosheets significantly improved the permeation flux and separation factor of the APVA-MXene MMM. Nabeeh enhanced the performance of ultrafiltration membranes by assembling Ti₃C₂T_x MXene nanosheets into a polyethersulfone (PES) matrix to create a mixed matrix membrane (MMM).¹⁶¹ The incorporation of MXene nanosheets improved the morphology, hydrophilicity, and mechanical properties of the membrane. Additionally, the assembled PES-MXene MMM exhibited an ultra-high water flux of 2280 LMH/bar, 98% oil rejection rate, and water vapor permeability (WVP) of 18 100 GPU.

Fig. 15 (a) Schematic diagram of the preparation of mixed matrix membranes by assembling MXene nanosheets into an APVA matrix, together with their permeability and separation factors.¹⁵⁷ Copyright 2023, Elsevier. (b) Schematic diagram of the preparation of mixed matrix membranes by assembling MXene nanomaterials into a polyethersulfone (PES) matrix.¹⁵⁸ Copyright 2020, Wiley. (c) Schematic diagram of the preparation of mixed membranes by assembling MXene nanosheets and UiO-66 nanoparticles into a Pepax-1657 matrix.¹⁵⁹ Copyright 2024, Elsevier. (d) Effect of MXene nanosheets on preventing defects in TiO₂ mesoporous membranes.¹⁶⁰ Copyright 2018, Elsevier.

Due to the relatively random assembling pattern of 2D nanosheets in membranes, which will lead to disadvantageous permeability caused by tortuous diffusion pathways, a new strategy using pre-constructed materials with uniform nanochannels as fillers has been proposed. Based on this, Guan et al. etched Ti₃AlC₂ to prepare multilayer MXene (m-MXene), where the channels aggregate as a whole. Compared to traditional single sheets, this ensures that ineffective filler assembling is impossible, greatly promoting selective the permeation performance. Subsequently, m-MXene/poly(amide-6-b-ethylene oxide) (Pebax) MMMs were successfully assembled.¹⁵⁸ The mixed matrix membrane exhibited a CO₂ permeability of 86.22 Barrer and CO₂/N₂ selectivity of 104.85, surpassing the Robeson upper bound (2008). The m-MXene/Pebax MMM demonstrated a significant enhancement in CO₂ separation (Fig. 15b). Utilizing an ion-intercalated MXene as a filler to prepare mixed matrix membranes can further enhance the separation performance of the membranes. As shown in Fig. 15c, Ajebe et al. used MXene (Ti₃C₂T_x) nanosheets and UiO-66 nanoparticles as fillers and combined them with the Pepax-1657 matrix to synthesize MMMs for CO₂ gas separation.¹⁵⁹ The MXene nanosheets served as selective channels, achieving high selectivity through the polar surface groups at the termini of the MXene nanosheets. The combination of UiO-66 and MXene not only enhanced the selectivity but also increased the solubility and diffusivity through selective CO₂ adsorption and the tortuous pathways provided by the MXene nanosheets.

Furthermore, MXene nanosheets, which possess high mechanical strength, can be used to assist in membrane assembly. Xu et al. introduced MXene nanosheets into a titanium dioxide (TiO₂) sol to prepare mesoporous membranes.¹⁶⁰ Due to the capillary force generated by the porous substrate, sol penetration and membrane cracking are inevitable. However, assembling MXene nanosheets into a sol can effectively inhibit sol penetration (Fig. 15d). The filtration experiments showed that the resulting membranes, especially the assembled hollow fiber TiO₂-MXene membranes, exhibited a narrow pore size distribution, ideal dextran rejection properties, and high-water permeation flux.

3.3 Fabrication of MXene-based membranes

3.3.1 Vacuum-assisted assembly. Vacuum-assisted assembly is a simple and cost-effective technique to fabricate self-assembled membranes by continuously removing the solvent. This method has the advantages of simplicity and ease of operation for the preparation of ultrathin membranes by assembling 2D nanosheets. Under the action of a pressure difference, the 2D nanosheets closely stacked into a macroscopic assembly, and gradually assemble into ultrathin 2D membranes.

Vacuum-assisted assembly is the most commonly used method for preparing MXene laminar membranes.¹⁴⁰ In 2015, the first self-supporting MXene membrane assembled by vacuum-assisted filtration was reported.¹³⁸ Ren et al. assembled a binder-free and flexible titanium carbide membrane by filtering a colloidal titanium carbide solution onto a hydrophilic polyvinylidene fluoride (PVDF) substrate. Additionally, vacuum-assisted assembly can also be used for the preparation of composite membranes. Recently, Zhuo et al. prepared robust PAA/MXene hybrid membranes using a vacuum-assisted filtration method (Fig. 16a).¹⁶² Moreover, vacuum-assisted assembly can be used to vary the thickness of MXene membranes by altering the volume and concentration of the solution. Wahid Mohammadi and colleagues utilized the vacuum-assisted filtration assembly technique to produce Ti₃C₂/PANI hybrid membranes with different thicknesses in the range of 4–90 μm on Celgard membrane filters.¹⁶⁸

Fig. 16 (a) Schematic diagram and SEM image of the assembly of PAA/MXene membranes using vacuum-assisted filtration.¹⁶² Copyright 2024, ACS. (b) Schematic diagram and physical image of the roll-to-roll assembly of MXene membranes via Meyer rod coating.¹⁶³ Copyright 2023, Wiley. (c) Schematic diagram of the preparation of NF membranes using layer-by-layer assembly.¹⁶⁴ Copyright 2023, Elsevier. (d) Schematic diagram of the assembly process of tubular MXene/SS membranes supported on stainless steel.¹⁶⁵ Copyright 2023, Wiley. (e) Schematic diagram of the assembly of ZIF-8@TiCT_x membranes using interfacial polymerization.¹⁶⁶ Copyright 2024, Elsevier. (f) Schematic diagram of the assembly of MXene/PVDF membranes using electrospinning technology.¹⁶⁷ Copyright 2024, Elsevier.

As the most commonly assembled strategy for fabricating 2D laminar membranes, vacuum-assisted filtration is almost applicable to all 2D nanosheets with suitable liquid dispersion systems. However, this method requires a large amount of solvent to disperse the 2D nanosheets. Also, the preparation of thicker layered membranes can take tens of hours or even several days, and irregularities may occur during large-scale filtration preparation. Therefore, it is more suitable for the small-batch preparation of thinner laminar membranes.

3.3.2 Surface coating assembly. In addition to the aforementioned vacuum-assisted filtration assembly strategy, coating 2D MXene suspensions on porous substrates is a common, simple, and scalable membrane assembling approach. The performance of ultrathin 2D membranes can be adjusted by selecting the appropriate coating method. Li et al. continuously produced large-sized and flat MXene membranes up to 5 meters in size through a simple Meyer rod coating method (Fig. 16b).¹⁶³ Compared to the most common 2D MXene laminar membranes assembled by vacuum-assisted filtration, the rod-coated MXene membranes exhibited a smaller surface roughness and interlayer spacing, producing high-performance membranes with well-defined nanochannels for precise separation.

They exhibited excellent performances during the separation process of dyes and monovalent/divalent cations. The proposed roll-to-roll Meyer rod coating method can also be used to assemble MXene-based ultrathin composite membranes using inks containing high concentrations of MXene and other desired components, such as MXene/carbon nanotubes and MXene/polymers.

3.3.3 LBL assembly. LBL assembly involves the layer-by-layer deposition of oppositely charged polyelectrolytes onto a charged substrate through hydrogen bonds or electrostatic forces. It enables the formation of membranes coated on substrates with various surface topographies, such as planar surfaces, spherical particles, pores, and fabrics. By coordinating various assembly parameters, including the concentration and type of polyelectrolytes, inorganic salts in the solution, pH value, number of bilayers, and the first and last layers, the thickness, surface charge, and pore size of the coating can be easily controlled, thus adjusting the selectivity and permeability of the membrane. For instance, as shown in Fig. 16c, Hu et al. used a plant polyphenolic tannic acid coating formed on the substrate surface as a mediator to conduct LBL self-assembly of negatively charged 2D MXene nanosheets and positively charged polyethyleneimine (PEI), resulting in a well-structured nanofiltration (NF) membrane.¹⁶⁴ The assembled membrane exhibited an ultra-high dye/salt separation factor of 758.1, surpassing most of the previously reported data.

The membrane structure formed by LBL assembly is similar to that assembled by vacuum filtration, given that the ultrathin 2D MXene nanosheets are deposited onto the substrate. However, the key difference is the strong binding force between the nanosheets formed through LBL layer-by-layer assembly, which significantly enhances the mechanical stability of the membrane.

3.3.4 Electrophoretic deposition (EPD) assembly. Electrophoretic deposition assembly is a technique that utilizes an external electric field in a colloidal solution to move 2D nanosheets with surface charges towards electrodes with an opposite charge.¹⁶⁹ Subsequently, nanosheets are deposited onto the electrode surface. After a certain period of time, a complete thin membrane can be formed on the electrode surface. Electrophoretic deposition has been applied in various fields such as electroplating, component separation, and preparation of separation membranes. This method boasts advantages such as rapid preparation and the ability to assemble membranes with uniform thickness and no defects, making it promising for the large-scale production of separation membranes. Luo et al. assembled a series of tubular 2D MXene membranes on commercial porous stainless steel substrates through a rapid EPD strategy (Fig. 16d).¹⁶⁵ Compared to other methods, this preparation route offers advantages such as simple operation, high membrane assembly efficiency (within 5 min), good reproducibility, and scalability. The assembled tubular MXene membranes exhibited an excellent gas separation performance, with a hydrogen permeability of 1290 GPU and H₂/CO₂ selectivity of 55. Additionally, these assembled membranes demonstrated an extremely stable performance during a long-term separation process exceeding 1250 h. Zhang et al. prepared sandwich-structured MXene/COF membranes using layer-by-layer electrophoretic deposition (LBL EPD) technology, where COF nanospheres (COFs NPs) served as the middle layer.¹⁷⁰ During the EPD process, controlling the EPD time could conveniently adjust the thickness of each layer, enabling the rapid production of defect-free MXene/COF membranes. The ultrathin MXene layer provided excellent membrane-forming properties for the COF NPs, and under the optimal conditions, the MXene/COF membrane achieved a high permeance of 200.2 L m⁻² h⁻¹ bar⁻¹ with a rejection rate of 98.9% for Congo Red (CR). This exceptional performance remained stable even within the pH range of 7–11 and during long-term separation.

3.3.5 Other assembly methods. In addition to the aforementioned assembly methods, other techniques have been developed to assemble ultrathin MXene-based membranes, such as interfacial polymerization and electrospinning. Interfacial polymerization is the core method for assembling thin-film nanocomposite (TFN) membranes. Wei et al. successfully synthesized a TFN nanofiltration membrane of ZIF-8@TiCT nanocomposite membrane using interfacial polymerization (Fig. 16e).¹⁶⁶ The results showed that the ZIF-8@TiCT composite was uniformly dispersed in the polymer, effectively enhancing the pore structure and polarity of the NF membrane, and thus improving its permeability. The electronegative nature of ZIF-8@TiCT also strengthened the negative charge on the polymer membrane surface, significantly improving the its ability to reject ReO₄ while maintaining a low Na retention rate. This is helpful for the efficient and selective recovery of ReO₄ from high-salt wastewater, which has great potential for applications in seawater desalination.

Electrospinning is an emerging method for preparing MXene-based membranes. Electrospun nanofibers possess advantages such as low cost, high specific surface area, ease of surface modification and functionalization of polymer chains, and adjustable thermomechanical properties. Ma et al. integrated MXene (Ti₃C₂T_x) nanosheets with polyvinylidene fluoride (PVDF) nanofiber membranes through electrospinning assembly technology (Fig. 16f).¹⁶⁷ The resulting MXene/PVDF nanofiber membranes exhibited exceptional hydrophobicity and strong optical light absorption across the entire solar spectrum, with high oil flux and effective rejection ratios. Moreover, its photothermal and permeability properties remained significant under extreme conditions, making it useful for oil/water separation and distillation in complex environments.

3.4 Application of MXene-based separation membranes

The ultrathin 2D laminar structure, high hydrophilicity, adjustable interlayer spacing, and excellent mechanical strength of MXene nanosheets endow the assembled membranes with a high separation performance.¹⁷¹ Thus far, ultrathin 2D MXene-based membranes have been widely used for the separation and purification of gases, water, and organic solvents, holding great promise for high-performance precise separation.

3.4.1 Gas separation.
3.4.1.1 Fundamental for gas separation in assembled MXene membranes. MXene nanosheets can self-assemble due to the strong interaction of hydrogen bonds and van der Waals forces. The gas separation mechanism of MXene membranes is similar to other 2D layered membranes, primarily consisting of molecular sieving, Knudsen diffusion, viscous flow, and capillary condensation.¹⁷²
3.4.1.1.1 Molecular sieving. Molecular sieving occurs when the pore size (or interlayer spacing) of the gas separation membrane falls between the kinetic diameters of different gas molecules. When a mixture of gases comes into contact with the membrane, the gas molecules with larger kinetic diameters are retained on the upper surface of the membrane, while the smaller molecules can more easily pass through the ultrathin 2D MXene assembled membrane, thereby achieving precise gas separation.
3.4.1.1.2 Knudsen diffusion. Knudsen diffusion dominates gas separation when the mean free path of gas molecules is larger than their transport channels in MXene assembled membranes. The molecular weight of the gas determines the selectivity of separation. The gas flow rate decreases with an increase in molecular weight, which is inversely proportional to the square of the molecular weight. Therefore, the most effective way to increase the gas flow rate is to construct gas separation membranes with materials of the smallest molecular weight, as shown in eqn (1).¹⁵⁵where J represents the flux through the MXene assembled membranes, n denotes the molar concentration of the gas, r is the pore radius, Δp represents the transmembrane pressure, R is the gas constant, T is the temperature, τ refers to the tortuosity of the pore, l represents the diffusion length, and D_k is the Knudsen diffusion coefficient, which is defined as shown in eqn (2).¹⁵⁵where M_w represents the molecular weight of the permeating gas.
3.4.1.1.3 Viscous flow. Viscous flow occurs in larger nanochannels, where the nanochannel size of MXene-based membranes exceeds the mean free path of the gas molecules (the mean free path of a gas molecule is the arithmetic average of the lengths of the free paths that a molecule may travel between successive collisions). In this case, gas molecules are transported through the bulk flow of the fluid in ultrathin 2D MXene assembled membranes.¹⁷³
3.4.1.1.4 Capillary condensation. The capillary condensation mechanism achieves separation when one of the gases in a mixture partially condenses as it passes through the nanochannels in MXene-assembled membranes at low temperatures. Separation through capillary condensation in MXene-based membranes exhibits high selectivity, where the extent of removal of a gas species from the feed gas is limited by the condensation pressure of the gas at the operating temperature, pore size, and membrane assembly structure.

In summary, the gas separation in MXene-assembled membranes can operate through various transport mechanisms, but in actual membrane separation processes, it is often a synergistic effect of multiple separation mechanisms.

3.4.1.2 Gas separation applications.
3.4.1.2.1 Capture and separation of CO₂. Due to the rapid industrial development, CO₂ poses a severe threat to the global environment. Thus, discovering effective CO₂ capture methods is crucial for mitigating the greenhouse effect. Currently, most studies have shown that MXene mixed matrix membranes can achieve good CO₂ capture and separation. Recently, Liu et al. incorporated MXene into a polyether-block-amide copolymer (Pebax) matrix to prepare a mixed matrix membrane (MMM) for CO₂ capture (Fig. 17a). The introduction of MXene nanosheets provided additional molecular transport channels while enhancing the CO₂ adsorption capacity, thereby improving both the CO₂ permeability and CO₂/N₂ selectivity.¹⁷⁴ When the MXene loading was 0.15 wt%, the MXene/Pebax membrane exhibited a high separation performance, with a CO₂ permeability of 21.6 GPU and CO₂/N₂ selectivity of 72.5. Furthermore, the obtained membrane showed good stability during continuous operation for up to 120 h. Wang et al. incorporated MXene nanosheets into cross-linked polyether block amide (Pebax)/poly(ethylene glycol) methyl ether acrylate.¹⁷⁵ The resulting mixed matrix (Fig. 17b) exhibited an excellent CO₂/N₂ and CO₂/H₂ separation performance. The MXene-Ti₃C₂T_x nanosheets expanded the free volume within the mixed matrix membrane (MMM), leading to a significant enhancement in CO₂ permeability. Additionally, these MXene-incorporated MMMs exhibited preferential adsorption capabilities for CO₂ over light gases, contributing to outstanding CO₂/N₂ and CO₂/H₂ selectivity (64.3 and 19.2, respectively), enabling ultra-efficient CO₂ capture. Luo et al. prepared high-performance CO₂-philic MXene/PEG mixed matrix membranes through a simple vacuum filtration process.¹⁸⁰ The prepared MXene/PEG-based MMM exhibited excellent CO₂ permeability and selectivity when separating CO₂/N₂ and CO₂/CH₄. When the loading of MXene nanosheets in the CMC@MXene filler was fixed at 1.5 mg mL⁻¹, the self-supporting Pebax/CMC@MXene-1.5 MMM possessed the maximum CO₂ permeance of 521 GPU in separation of CO₂/N₂ and 444 GPU in the separation of CO₂/CH₄ at 1 bar and 25 °C. Additionally, the fabricated self-supporting Pebax/CMC@MXene MMMs exhibited excellent thermal stability, durability, and mechanical tensile properties.

Fig. 17 (a) Schematic diagram of Pebax-based mixed matrix membrane and MXene nanosheets.¹⁷⁴ Copyright 2020, Wiley. (b) Structure of mixed matrix membranes (MMMs) and the mechanism of carbon dioxide separation.¹⁷⁵ Copyright 2024, ACS. (c) Schematic diagram of NH₃ decomposition and H₂ permeation in MXene membrane reactors.¹⁷⁶ Copyright 2023, ACS. (d) Schematic diagram of two nanotunnel models for gas diffusion through Pd-MXene membranes.¹⁷⁷ Copyright 2022, Elsevier. (e) Schematic diagram of transport channels in vertically stacked MXene/rGO composite membranes.¹⁷⁸ Copyright 2024, RSC. (f) Structure of ZIF-67 and the separation mechanism for helium gas.¹⁷⁹ Copyright 2022, Elsevier.

3.4.1.2.2 Separation and extraction of H₂. Hydrogen energy is considered the most promising clean energy in the 21st century. The combustion product of hydrogen is only pure water. Unlike most fossil fuels, hydrogen is not a primary energy source that can be obtained directly; rather, it is a secondary energy source that must be produced from another energy source. Membrane-in this case, assisted separation is expected to produce high-purity H₂. In the separation process, the smaller-sized H₂ molecules transmit faster compared to larger-sized gas molecules such as CO₂. Moreover, the strong interaction between CO₂ and the oxygen-containing functional groups of titanium carbide results in a capture effect, subsequently leading to a significant reduction in the permeation rate of CO₂. Consequently, it can be stated that layered MXene membranes are selective membranes for H₂. Ding et al. prepared MXene free-standing membranes through vacuum filtration, which exhibited ultrahigh H₂ permeability and H₂/CO₂ selectivity.¹³⁹ Wu et al. successfully assembled ultrathin MXene membranes by depositing MXene nanosheets onto an anodic alumina oxide (AAO) substrate using vacuum-assisted filtration assembly.¹⁷⁶ Subsequently, they developed an MXene-supported membrane reactor that can enhance NH₃ conversion and promote the production of high-purity H₂ (Fig. 17c).

Furthermore, the selectivity of MXene membranes can be enhanced by adjusting the interlayer spacing and transport channels between the MXene nanosheets. Wang et al. expanded the interlayer spacing of MXene nanosheets (from 2.29 to 3.26 Å) by intercalating Pd into the layered MXene membrane (Fig. 17d). The resulting Pd-MXene membrane exhibited an excellent H₂ permeability of 2.66 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ and high H₂/CO₂ selectivity of 242.¹⁷⁷ Li et al. grew MOF-801 crystals with a particle size of about 20 nm uniformly on MXene nanosheets.¹⁸¹ Then, they assembled ultrathin 2D membranes by vacuum-filtering the synthesized MOF-801@MXene nanosheets onto the surface of a porous organic substrate. The resulting 2D membrane exhibited an excellent gas separation performance, with an H₂ permeability of 2200 GPU. Dong et al. prepared a composite membrane with ultra-short transport channels by vertically stacking MXene and rGO.¹⁷⁸ As shown in Fig. 17e, the vertical channels enabled ultra-fast transport and high permeation flux. Compared to horizontally stacked MXene/rGO (H-MXene/rGO), the vertically stacked MXene/rGO (V-MXene/rGO) exhibited an improvement of one order of magnitude in H₂ permeation flux.

3.4.1.2.3 Extraction of helium. Furthermore, helium is a strategically valuable gas. To separate and recover helium from natural gas, researchers have mainly focused on the separation of He/CH₄ and He/N₂.¹⁸² Zhao et al. prepared a well-intergrown ZIF-67 membrane with the help of layered assembled MXene nanosheets. Despite being only 90 nm thick, the ultrathin 2D MXene nanosheets could carry a large amount of Co(II) ions. The single gas and mixed gas permeation experiments showed that the ZIF-67 membrane assembled with 2D MXene nanosheets, exhibited an excellent gas separation performance, together with long-term chemical (to CO₂ and H₂O) and thermal stability, a permeability greater than 6.6 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹, and selectivity for mixed gases of He/N₂ and He/CH₄ higher than 13, outperforming the most advanced MOF membranes (Fig. 17f).¹⁷⁹

3.4.2 Water treatment.
3.4.2.1 Fundamental for water treatment of MXene assembled membranes. Ultrathin 2D MXene assembled membranes utilized for the separation and purification of water and organic solvents primarily rely on three mechanisms including size exclusion, Donnan exclusion, and adsorption.¹⁸³
3.4.2.1.1 Size exclusion. Size exclusion, also known as molecular sieving, is the primary mechanism in MXene-assembled membranes, which relies heavily on the interlayer spacing between the MXene nanosheets. Solutes larger than the interlayer spacing of the MXene nanosheets are effectively screened or retained, while smaller solutes are allowed to pass through the interlayer space of MXene-based membranes.¹⁸⁴
3.4.2.1.2 Donnan exclusion. In the Donnan exclusion principle, ions carrying the same charge as the membrane are electrostatically repelled. Due to this electrostatic repulsion of charged solutes, the nanoslits and interlayer spacing in the assembled MXene nanosheets can be larger than the size of the solutes to be separated.¹⁸⁵
3.4.2.1.3 Adsorption. Ultrathin MXene nanosheets, with their large specific surface area and specific active functional groups on their surface, serve as excellent adsorbents. During the filtration process on ultrathin 2D MXene membranes, solutes such as heavy metals and dyes present in the feed solution are adsorbed onto the surface functional groups of the MXene nanosheets as they pass through.
3.4.2.2 Water treatment applications. Ultrathin 2D MXene-based membranes have been widely used in water purification technologies due to their adjustable interlayer spacing, good antifouling and antibacterial properties, and high photothermal efficiency.
3.4.2.2.1 Oil–water separation. Due to the increase in the discharge of multi-component harmful oily wastewater, oil–water separation has become a serious global concern. In this case, the adoption of membrane separation technology to treat oily wastewater can significantly reduce its harm to the surrounding environment and human health. Given that the interlayer spacing of ultrathin 2D MXene membranes is relatively small, while the droplet sizes of oil–water emulsions or suspensions usually range from tens of nanometers to several micrometers, the interlayer sieving effect of MXene-assembled membranes can significantly reduce the oil content in wastewater. Moreover, the structure of 2D MXene membranes contains abundant hydrophilic groups, which can prevent the adsorption and aggregation of oil pollution, thus improving the anti-fouling ability of the membrane. Recently, Zhang et al. developed a super hydrophilic and underwater super oleophobic stainless steel mesh (SSM) with ultra-high flux using tannic acid (TA)-modified ZIF-8 nanoparticles (TZIF-8) and 2D MXene nanosheets (Fig. 18a).¹⁸⁶ TZIF-8 increased the interlayer space of the MXene nanosheets, enhancing the flux permeability and rejection rate of the ultrathin MXene-based composite membrane. The TZIF-8@MXene/SSM membrane maintained its underwater super oleophobic properties in various pH solutions and organic solvents. Additionally, after treatment under harsh conditions, the assembled TZIF-8@MXene/SSM membrane exhibited an oil-water separation efficiency over 99%. It possessed excellent antifouling, stability, durability, and recyclability characteristics. Zhang et al. embedded sodium-based bentonite into MXene nanosheets, and then assembled a series of sodium-based bentonite@MXene (NBM) composite membranes through a one-step hydrothermal pretreatment and vacuum self-assembly on the surface of polyvinylidene fluoride membranes.¹⁹² The sandwiched assembly structure composed of MXene nanosheets and sodium-based bentonite not only had a positive effect on the stability of the composite membrane, but also formed a special micro-nano structure on the membrane surface, enabling the ultrathin 2D membrane to have switchable wettability. This allowed the effective separation of different components in oil/water emulsions.

Fig. 18 (a) Schematic diagram of the separation mechanism of TZIF-8@MXene/SSM for oil–water mixtures, together with the separation flux and efficiency for different oil–water mixtures, and the separation flux and efficiency after immersion in acid, base, and salt solutions.¹⁸⁶ Copyright 2024, Elsevier. (b) Schematic diagram of the assembled polyethersulfone-supported MXene/TiO₂NT (PMT) membrane.¹⁸⁷ Copyright 2023, MDPI. (c) Schematic diagram of the SC-MXene membrane and its nanofiltration performance for different salt separations.¹⁸⁸ Copyright 2021, Elsevier. (d) Schematic diagram of solar-driven seawater desalination and emulsion separation using MXene-based Janus porous fiber membranes.¹⁸⁹ Copyright 2022, Elsevier. (e) Schematic diagram of CNF-intercalated MXene composite membrane.¹⁹⁰ Copyright 2023, Elsevier. (f) Schematic diagram of Al³⁺ cross-linked MXene composite membrane.¹⁹¹ Copyright 2020, Springer Nature.

Although ultrathin 2D MXene-based membranes have attracted increasing attention in the field of separation and purification, the issue of MXene-assembled membranes being easily contaminated by oil still limits their further application in oil–water separation.¹⁹³ Based on this, Zeng et al. inserted titanium dioxide nanotubes (TiONT) with an excellent aspect ratio into the interlayer space of 2D MXene-based membranes to prepare polyethersulfone-loaded MXene/TiONT (PMT) membranes (Fig. 18b). The incorporation of TiONT not only increased the interaction energy between the PMT membrane and pollutants, but also enhanced the antifouling performance of the membrane.¹⁸⁷ The assembled PMT membrane exhibited excellent antifouling and self-cleaning properties, with flux recovery rates reaching 83% and 100% for bovine serum albumin and humic acid as contaminants under visible light irradiation, respectively. Amari et al. assembled a tungsten oxide (WO₃)/MXene composite membrane on a polyarylene ether sulfone (PAES) substrate.¹⁹⁴ The prepared composite membrane possessed superhydrophilic (SH) and superoleophobic (SO) properties, enabling the high-performance treatment of oily wastewater. Additionally, it could degrade organic pollutants using ultraviolet light in a short period of time, achieving self-cleaning and antifouling properties.

3.4.2.2.2 Seawater desalination. Seawater desalination is open-source and incremental technology for water resource utilization, and it is also an urgent and important field requiring rapid development worldwide. MXene-assembled laminar membranes have easily adjustable interlayer spacings, which can be designed to retain hydrated ions while allowing water molecules to pass through rapidly, making them suitable for size sieving requirements. Usman et al. developed a sulfonated TFN (STFN) nanofiltration membrane by assembling MXene nanosheets.¹⁹⁵ The assembled ultrathin MXene membrane exhibited a thin-layered laminar structure, binding well on the sulfonated TFN membrane. The assembled MXene nanosheets significantly inhibited the movement of amine monomers, thereby promoting the formation of a thin polyamide layer and improving the desalination efficiency. The hydrophilicity of the assembled STFN membrane was significantly enhanced, with the lower limit of the contact angle reduced to 24°. The filtration performance showed that both the water flux and desalination rate of the sulfonated thin-film composite STFN membrane were improved. The rejection rates of the assembled STFN membrane for Na₂SO₄ and MgSO₄ were as high as >93%. The antifouling and chlorine resistance of the STFN membranes were also improved. This research can provide a reference for the development of novel assembled STFN membranes for high-performance seawater desalination applications.

In addition, the negatively charged groups on the surface of MXenes can achieve the selective sieving of ions with different valence states through the Donnan effect via electrostatic interactions. As shown in Fig. 18c, Meng et al. utilized the conductivity of MXene nanosheets to assemble a novel laminar surface-charged MXene membrane (SC-MXene).¹⁸⁸ By adjusting the assembly loading of MXene nanosheets, the thickness of the membrane could be well controlled. More importantly, the surface charge of the MXene membrane could be controlled by coating a layer of polyethyleneimine (PEI) through electrostatic interactions between the ultrathin negatively charged MXene nanosheets and the positively charged PEI. The assembled SC-MXene membrane exhibited high water permeability and high salt rejection during nanofiltration or forward osmosis, demonstrating significant potential in applications such as water desalination and water ion-selective transport.

Furthermore, considering the photothermal properties of MXene nanosheets, combining solar distillation with membrane technology to produce photothermal membrane desalination/distillation is an ideal method for supplying freshwater from seawater. Li et al. developed a Janus photothermal porous fiber membrane composed of a hydrophobic MXene/poly(dimethylsiloxane) (PDMS) coating and a hydrophilic polylactic acid/TiO₂ nanofluid porous fiber membrane.¹⁸⁹ The assembled MXene/PDMS coating served as the photothermal layer and generated localized heat at the water–vapor interface when exposed to light (Fig. 18d). The Janus porous fiber membrane exhibited a stable photothermal conversion efficiency of 60% under 1 sun illumination, with a freshwater production rate of 1 kg m⁻² h⁻¹, rather low ion concentration (≤1 ppm), and an extremely high desalination rate of up to 99.95%, with its antibacterial activity reaching 100%.

However, biological membrane fouling has become a significant challenge during the precise membrane separation process for efficient water treatment. Microorganisms adhere to and grow on the surface of membranes, secreting extracellular polymers to form biofilms. The aging of biofilms causes them to decompose, producing substances such as proteins and polysaccharides, which promote the adsorption of other microorganisms and lead to the deposition of pollutants on the membrane surface, ultimately blocking the membrane pores. This results in a reduction in the separation and permeation performance of the membrane. Thus, the development of antibacterial membranes with high permeability, selectivity, and stability is of great significance for advancing membrane water treatment technology. Fortunately, MXene nanosheets can be applied in the development of high-performance antibacterial membranes.¹⁹⁶ Cheng et al. assembled three MXene-based 2D composite membranes (M2-M4) using polyethersulfone (PES) as the substrate, namely GO@MXene, O-g-C₃N₄@MXene, and BiOCl@MXene composite membranes. M2-M4 further improved the antibacterial activity of the membrane against Escherichia coli and Staphylococcus aureus, especially for M4, achieving antibacterial rates of 50% and 82.4%, respectively.¹⁹⁷ Furthermore, Yang et al. developed a 2D nanofiltration membrane with high selectivity, permeability, and antibacterial properties by incorporating the polycation of polydiallyldimethylammonium chloride (PDDA) into the Ti₃C₂T_x MXene laminar architecture through an electrostatic assembly strategy. The resulting Ti₃C₂T_x/PDDA composite membrane not only exhibited high water permeability but also demonstrated excellent anti-adhesion and antibacterial activities against both Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus) with inhibition rates of 90% and 95%, respectively.¹⁹⁸

High stability is a crucial characteristic of separation membrane materials. However, MXenes are prone to oxidation and degradation in aqueous environments, which can compromise their performance and application effects. To utilize MXenes in liquid separation, it is imperative to address the issue of insufficient stability during their synthesis and membrane preparation processes. Firstly, during the synthesis of MXene nanosheets, given that MXenes are susceptible to oxidation, strategies such as modifying the MAX phase, adjusting the degassing atmosphere, using organic solvents, and incorporating antioxidants can be employed to delay the oxidation of MXenes, thereby enhancing their oxidation stability. In addition, studies have shown that inorganic salts such as NaCl, LiCl, and CaCl₂ can be used to stabilize MXenes in aqueous systems by reducing the water activity, which can extend the shelf life of MXenes to 400 days.¹⁹⁹ Furthermore, Zhang et al. showed that MXene nanosheets with smaller sizes exhibit poorer stability.²⁰⁰ Thus, separating larger MXene nanosheets from smaller ones can not only extend their shelf life but also enhance the performance of MXenes. Additionally, MXene membranes encounter certain difficulties when used for water treatment. Mainly, when exposed to deionized water or various salt solutions, the interlayer spacing of the MXene membrane increases, leading to the issue of swelling. This can lead to an increase in the interlayer distance and a decrease in the membrane separation performance. Thus, to address this issue, methods such as intercalation and crosslinking can be employed to stabilize the interlayer spacing of MXene membranes, thereby preventing their swelling and maintaining a stable performance. Zhang et al. inserted flexible and hydrophilic carboxylated cellulose nanofibers (CNFs) into the layered structure of MXenes.¹⁹⁰ As shown in Fig. 18e, the insertion of CNF effectively stabilized the interlayer distance and improved the anti-swelling property of the Ti₃C₂T_x membrane. This composite membrane demonstrated an exceptional anti-swelling performance (76 h) in an aqueous environment and possessed high stability. Zheng et al. embedded COF between the layers of TiCT nanosheets, which not only increased their interlayer spacing but also provided abundant molecular sieve pores, enhancing the permeability of the MXene membrane.²⁰¹ The resulting MXene/COF composite membrane exhibited excellent water permeability (up to ∼300 L m⁻² h⁻¹ bar⁻¹), and also good anti-swelling properties and stability. Additionally, through crosslinking reactions, the connections between adjacent nanosheets can be strengthened to fix the interlayer spacing of the membrane, thereby improving the anti-swelling property of MXene membranes and enhancing their separation performance. Lu et al. developed a self-crosslinked MXene membrane, which exhibited remarkable stability during a long-term ion separation process of 70 h, demonstrating excellent resistance to swelling.²⁰² This indicated that the self-crosslinking between MXene layers significantly improved the ion rejection performance and enhanced the stability. Zhang et al. stabilized and controlled the interlayer spacing of MXene membranes by anchoring cucurbiturils onto the surface of MXene nanosheets.²⁰³ The resulting MXene layered membrane maintained a stable separation performance over 30 filtration cycles, demonstrating excellent stability and resistance to swelling. Ding et al. utilized the principle of concentration diffusion to prepare an Al³⁺-crosslinked MXene-based membrane (Fig. 18f).¹⁹¹ This composite membrane exhibited outstanding non-swelling stability in aqueous solutions for up to 400 h. This study demonstrated that the intercalation and crosslinking of Al³⁺ ions effectively suppressed the swelling of the MXene structure in aqueous solutions, enabling the MXene membrane to maintain excellent structural stability in water.

3.4.3 Organic solvent processing. Organic solvents are a class of organic compounds that are liquid at room temperature and have relatively small molecular weights. They are diverse in types, including alkanes, alcohols, aldehydes, ketones, esters, ethers, amines, benzenes, hydrogenated hydrocarbons, olefins, halogenated hydrocarbons, terpenes, nitrogen-containing compounds, sulfur-containing compounds, and heterocyclic compounds. In this case, functionalized MXene-based membranes can achieve more effective organic solvent separation.²⁰⁴ As shown in Fig. 19a, Xing et al. wrinkled flat Ti₃C₂T_x MXene nanosheets through a simple freeze-drying method. After assembling the wrinkled Ti₃C₂T_x (c-Ti₃C₂T_x) nanosheets into a laminar membrane, the c-Ti₃C₂T_x MXene membrane possessed numerous interfacial voids with enlarged interlayer nanochannels. Consequently, the permeation of both water and organic solvents was significantly enhanced. The assembled c-Ti₃C₂T_x MXene membrane achieved ultra-fast permeation rates of 5460 and 3745 L m⁻² h⁻¹ bar⁻¹ for water and acetone, respectively.²⁰⁵ Wei et al. fabricated a 2D GO/MXene (GM) composite layered membrane through a simple vacuum filtration assembly method (Fig. 19b).²⁰⁶ It enabled ultrafast water and organic solvent permeation and molecular separation. The assembled GM membrane exhibited ultrahigh fluxes (21.02, 48.32, 25.03, 10.76, and 6.18 L m⁻² h⁻¹ for water, acetone, methanol, ethanol, and IPA, respectively) for pure solvents and excellent dye molecule separation performance (over 90%) in both aqueous and organic solutions. Li et al. achieved the co-assembly of 2D organic components, Dopa thin-walled microcapsules, and typical inorganic nanosheets, MXene. As shown in Fig. 19c, a novel organic–inorganic hybrid ultrathin 2D membrane was designed and prepared. The introduction of MXene nanosheets not only increased the distance between adjacent Dopa microcapsules, enhancing the solvent flux of the membrane, but also improved the structural stability by adjusting the chelation strength between the MXene nanosheets and Dopa microcapsules.²⁰⁷ The assembled optimal Dopa/MXene composite membrane exhibited an excellent performance in the field of organic solvent nanofiltration (OSN), achieving a flux of 723 L m⁻² h⁻¹ bar⁻¹ and rejection rate of over 90% for methanol and reactive black.

Fig. 19 (a) Schematic diagram of the separation mechanism of ultrathin membrane assembled by wrinkled 2D MXene nanosheets.²⁰⁵ Copyright 2020, ACS. (b) Schematic diagram of the separation mechanism of GO/MXene (GM) composite laminar membranes.²⁰⁶ Copyright 2019, Elsevier. (c) Schematic diagram of the assembly of Dopa/MXene membranes along with the SEM images.²⁰⁷ Copyright 2024, Elsevier.

Moreover, the interaction between organic solvents and membranes usually leads to the swelling of the polymer, ultimately reducing the selectivity of the membrane. The development of membranes with high solvent resistance can overcome the bottleneck of traditional polymer membranes in the chemical industry.²⁰⁸ The application of MXene assemblies in the preparation of organic solvent-resistant membranes is promising given that their terminal groups can enhance the internal interactions within the membrane. Additionally, ultrathin 2D MXene membranes exhibit excellent hydrophilicity and selective permeability. Han et al. introduced MXene nanosheets into a P84 polymer matrix, which was cross-linked with triethylenetetramine (TETA).²⁰⁹ This not only improved the selective permeability of the polyimide membrane but also enhanced its solvent resistance. The ultrathin 2D membrane assembled with 18% P84 and 1% MXene nanosheets exhibited a high flux (268 L m⁻² h⁻¹) and high rejection rate (100%) for gentian violet (408) at 0.1 MPa and ambient temperature. The MXene nanosheets endowed the membrane with a large number of transport nanochannels and a denser functional layer, significantly enhancing its precise separation performance. After being cross-linked with TETA, the ultrathin 2D membrane assembled by MXene nanosheets still exhibited good solvent resistance against dimethylformamide (DMF), acetone, and methanol over a soaking period of 18 days.

Table 4 summarizes some of the experimental studies on the preparation and performance of MXene-based separation membranes.

Table 4 The preparation and performance of MXene-based membranes^a

Membrane types	Preparation method	Permeability/flux	Stability	Ref.
a CNFs: Carboxylated cellulose nanofibers; APTES: 3-aminopropyltriethoxysilane; ACNTs: amine-functionalized carbon nanotubes; Pebax: polyether–polyamide block copolymer; CMC: branched carboxymethyl cellulose; and CA: cellulose acetate.
Ti3C2Tx MXene membrane	Vacuum-assisted filtration	1000 L m^−2 h^−1 bar^−1	28 h	139
MXene membrane	Vacuum-assisted filtration	(H2) 2.05 × 10^−7 mol m^−2 s^−1 Pa^−1	200 h	144
Tubular MXene membrane	Electrophoretic deposition method	(H2) 1290 GPU	1250 h	165
Ni^2+/MXene composite membrane	Vacuum-assisted filtration	(H2) 8.35 × 10^−8 mol m^−2 s^−1 Pa^−1	200 h	147
AL^3+/MXene composite membrane	Vacuum-assisted filtration	1.1–8.5 L m^−2 h^−1 bar^−1	400 h	191
CNFs/MXene composite membrane	Vacuum-assisted filtration	26.0 L m^−2 h^−1 bar^−1	76 h	190
GO/MXene composite membrane	Filtration	71.9 L m^−2 h^−1 bar^−1	One month	150
APTES/MXene-ACNTs/CA composite membrane	Vacuum-assisted filtration	2892.8 L m^−2 h^−1 bar^−1	7 days	151
Pebax/CMC@MXene MMM	Solution casting method	(CO2) 521 GPU	60 h	164
MXene/Pebax MMM	Spin coating method	(CO2) 21.6 GPU	120 h	174
Crumpled MXene lamellar Membrane	Vacuum-assisted filtration	5460 L m^−2 h^−1 bar^−1	One month	184

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4 The similarities and differences between graphene-based and MXene-based separation membranes

As outstanding representatives of 2D materials, both graphene and MXene exhibit unique physicochemical properties and broad application potential. The unique 2D structure, adjustable pore size, and interlayer channels of graphene make it stand out in the field of separation membranes. The oxygen functional groups of graphene oxide further enhance its flexibility in terms of modification and functionalization. Alternatively, MXenes demonstrate highly adjustable interlayer channels due to their unique layered structure and abundant surface functional groups, enabling the efficient screening of specific molecules or ions. Thus, the 2D characteristics of both materials give them significant advantages in the field of membrane separation, indicating their broad application prospect.

Structure determines characteristics. In this case, graphene, a 2D honeycomb lattice material formed by a single layer of carbon atoms arranged in sp²-hybridized orbitals, exhibits unique physical and chemical properties. Graphene oxide is obtained by introducing oxygen-containing functional groups such as hydroxyl and carboxyl groups into graphene, which significantly change its surface chemical environment and constructs a novel pore structure. This promotes the orderly stacking of GO nanosheets, forming an efficient nanoscale pore separation membrane. MXenes are 2D materials obtained by processing MAX phase materials, and their structure is similar to graphene but with richer surface termination groups. MXene nanosheets also demonstrate the ability to construct nanoscale pore separation membranes, and their interlayer spacing and pore structure can be highly customized through precise regulation of their surface functional groups and interlayer intercalation materials.

Given the unique 2D layered structure and adjustable nanochannel characteristics of graphene and MXenes, the size and morphology of the interlayer channels in separation membranes based on these two materials can be adjusted through specific preparation processes. This gives them the ability to efficiently and selectively screen specific molecules or ions. Table 5 systematically summarizes and compares the similarities and differences in the key performance indicators such as permeability, separability, and stability of graphene-based separation membranes and MXene-based separation membranes. The following is a detailed elaboration of the above-mentioned core characteristics.

Table 5 Comparison of the performances of graphene-based and MXene-based separation membranes

2DNM membranes properties	Graphene-based membranes	MXene-based membranes
Permeability	· Permeability is mainly improved by punching holes or compounding with other materials	· Efficient penetration is achieved by utilizing its own nanochannel structure
	· High permeability can also be achieved under certain conditions, such as the construction of short straight water channels, but often requires more complex preparation processes	· Exhibit higher permeability in gas separation, especially in hydrogen permeability and H2/CO2 selectivity
Selectivity	· Their selectivity primarily stems from their unique 2D structure, pore size, adjustability of interlayer channels, and potential for surface functionalization	· Their selectivity mainly depends on their unique layered structure, abundant surface termination groups, and surface charge distribution
Stability	· Excellent, chemically and thermally stable	· Good, but may be affected by superficial functional groups
Processability	· A variety of preparation methods, suitable for large-scale production	· Their preparation process is relatively complex, but can be customized by adjusting the parameters
Molecular sieving capability	· Depends on the size and shape of the defect or interlayer channel	· Wide, including a wide range of molecules such as gases, liquids, and ions
Scope of application	· With their high permeability and stability, they show great potential in the fields of gas separation and liquid filtration	· With their abundant surface functional groups, regular and controllable transport channels and excellent electrochemical properties, they have unique advantages in the fields of electrochemical separation, energy conversion and molecular sieving

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4.1 Permeability

Due to their unique 2D layered structural design, both graphene-based and MXene-based separation membranes exhibit excellent permeability. Their nanoscale interlayer channels provide a smooth pathway for molecules or ions, thus achieving the advantage of high permeation flux. However, there are differences in permeability between them in specific aspects.

4.1.1 Water molecule permeability. In the case of graphene-based separation membranes, taking GO as an example, their properly regulated interlayer channels allow water molecules to pass through quickly while effectively blocking other larger solutes. This characteristic gives them broad application prospects in fields such as seawater desalination and wastewater treatment. Additionally, these membranes possess good hydrophilicity and stability, maintaining efficient permeability for a long time.

In contrast, although MXene-based separation membranes also demonstrate good permeability, they may be slightly inferior to optimized graphene-based membranes in terms of water molecule permeability. However, in the field of gas separation, MXene-based membranes exhibit higher permeability, especially for small molecule gases such as H₂, which have a higher permeation rate. This is due to the rich surface groups and sub-nanometer channel structure of MXene nanosheets, which reduce the transmission resistance of gas molecules within the membrane.

4.1.2 Transmission pathways and resistance. In the case of graphene-based separation membranes, their interlayer channels are relatively short and direct, minimizing the resistance during water molecule transmission, and thus achieving high water permeation flux. Furthermore, by adjusting the stacking pattern and interlayer distance of GO sheets, the permeation performance can be further optimized.

In contrast, MXene-based separation membranes may possess longer and more tortuous transmission pathways between layers, which somewhat increases the resistance to water molecule transmission. However, strategies such as intermediate layering, layer-by-layer assembly, and ternary blending can be employed to shorten the transmission channels or enlarge the interlayer spacing of MXene membranes, thereby enhancing their water permeability. For instance, constructing ultrathin MXene membranes with short water transmission channels or multilayer MXene composite membranes with low channel tortuosity through alternating deposition significantly boosts their permeation performance.

4.2 Selectivity

Graphene-based and MXene-based separation membranes both exhibit high selectivity, but their separation mechanisms and application areas differ. Graphene-based membranes primarily achieve molecular separation through size effects and surface chemistry, making them widely used in seawater desalination and wastewater treatment. In contrast, MXene-based membranes rely on a combination of size effects and surface termination groups for selective separation, particularly suitable for gas separation.

4.2.1 Molecular separation mechanism. The separation mechanism of graphene-based membranes depends on the size effect of interlayer channels and surface chemistry. By adjusting the stacking and interlayer distance of GO sheets, nanochannels with specific pore size distributions are formed, enabling the separation of molecules of different sizes. The oxygen-containing functional groups on the surface of GO membranes also affect their hydrophilicity and molecular adsorption capacity, further enhancing their selectivity. Alternatively, the separation mechanism of MXene-based membranes is more complex, involving not only size effects but also the role of surface termination groups. These termination groups interact with specific molecules or ions through electrostatic interactions, hydrogen bonding, or chemical adsorption, enabling selective separation. Additionally, the selectivity of MXene membranes can be further enhanced by forming composites with other materials, such as polymers and inorganic nanoparticles.

4.2.2 Application areas. Graphene-based separation membranes are widely used in seawater desalination, wastewater treatment, and osmotic evaporation due to their exceptional molecular sieving capability and stability. These membranes effectively remove salts from seawater, harmful substances from wastewater, and solute molecules from solutions, achieving high purity and recovery rates in separation processes. MXene-based separation membranes, with their unique physicochemical properties and tunable components, exhibit significant potential across various fields including gas separation, water treatment, seawater desalination, and organic solvent nanofiltration. Notably, in the gas separation sector, MXene-based membranes efficiently separate and purify small molecules such as hydrogen gas, offering substantial industrial value.

4.3 Stability

Both materials exhibit high stability due to the superior physico-chemical properties conferred by their unique 2D structures. However, they differ in terms of oxidation stability, swelling stability, and chemical resistance.

4.3.1 Oxidation stability. Graphene has exceptional chemical stability and resistance to oxidation. However, when graphene is oxidized to form GO for membrane fabrication, although the oxygen-containing functional groups (e.g., hydroxyl and carboxyl) on its surface can increase the hydrophilicity of the membrane, they may also reduce its oxidation resistance. Thus, GO-based membranes can be affected by oxidation over extended exposure to oxidative environments. MXenes, with their exposed metal atoms and abundant surface termination groups, are prone to reacting with oxygen and oxygen-containing functional groups, resulting in relatively poor oxidation stability. Therefore, special care is needed during the preparation and application of MXene-based membranes to prevent oxidation. Researchers often enhance the oxidation stability of MXenes through surface modification, intercalation treatments, or use of composite materials to reduce exposure to oxygen and reaction activity.

4.3.2 Swelling stability. Typically, graphene-based membranes exhibit good swelling stability, maintaining structural and performance stability in aqueous solutions. This is attributed to the tight stacking and strong interactions between the graphene layers. In contrast, MXene-based membranes are more susceptible to swelling in aqueous solutions due to the larger interlayer spacing and weaker interactions between MXene nanosheets, which allows water molecules to penetrate and expand the membrane. To mitigate swelling, researchers often use ion intercalation, cross-linking, or composite structures to enhance their interlayer interactions and stability.

4.3.3 Chemical corrosion resistance. Both materials show certain chemical corrosion resistance, but the specific performance may vary depending on the preparation method and application scenarios. Generally, graphene-based membranes have better chemical corrosion resistance due to the high chemical stability of graphene itself, while MXene-based membranes may exhibit different chemical corrosion resistance due to their surface termination groups and interlayer structure.

4.4 Processability

Both graphene and MXenes, being 2D nanomaterials, exhibit high processability. However, there are specific differences in aspects such as liquid-phase dispersion, curlability and flexibility, and compatibility with other materials.

4.4.1 Liquid-phase dispersion. The dispersion of graphene in liquid phases is relatively poor due to the strong π–π interactions between graphene layers, which tend to aggregate. Therefore, special dispersants or dispersion methods are often required to enhance its liquid-phase dispersion when preparing graphene-based membranes. Alternatively, MXenes have good dispersion in liquid phases due to their surface functional groups (e.g., hydroxyl and fluorine groups) and larger interlayer spacing. This excellent liquid-phase dispersion simplifies and improves the efficiency of MXene-based membrane preparation.

4.4.2 Curlability and flexibility. Despite the high mechanical strength and flexibility of graphene, it may suffer from damage or deformation during curling and folding processes. Thus, care must be taken to protect the structural integrity of graphene-based membranes when preparing complex shapes or structures. Alternatively, MXenes, with their unique 2D structure and good curlability, can be easily fabricated into various shapes and structures. Additionally, MXene-based membranes exhibit excellent flexibility, maintaining structural and performance stability under different stress conditions.

4.4.3 Compatibility with other materials. Both materials can be combined with other substances (such as polymers and inorganic compounds) to create composite materials with enhanced properties or new characteristics. However, their performance in this regard differs. Graphene, with its high surface energy and chemical inertness, may require special surface treatments to improve its compatibility and bonding with other materials. MXenes, with their surface functional groups and good liquid-phase dispersion, more readily form uniform composites with other materials.

4.5 Molecular sieving capability

Both exhibit significant molecular sieving capabilities, primarily based on size exclusion principles. However, there are differences in their pore structure, precise size control, sieving mechanisms, and specific performance. These unique differences offer a range of choices and optimization strategies for diverse molecular sieving applications.

4.5.1 Channel type and size. The channel type and size of graphene-based separation membranes can be controlled through various preparation methods (such as chemical etching and ion bombardment), allowing for precise molecular sieving. In contrast, MXene-based separation membranes mainly feature an interlayer spacing between nanosheets, with a relatively fixed size and distribution, although it can be adjusted by altering the stacking and interlayer distance of MXene nanosheets.

4.5.2 Sieving mechanism. In addition to size exclusion, MXene-based separation membranes may also influence the molecular sieving process through adsorption. Due to the abundance of functional groups on the surface of MXenes, they may have strong interactions with specific gas molecules (such as CO₂), altering the diffusion rate and selectivity of gas molecules. In comparison, graphene-based separation membranes primarily rely on the size exclusion mechanism for sieving.

4.5.3 Sieving performance. Current research indicates that MXene-based membranes show excellent sieving performances in some gas separation applications, such as H₂/CO₂ separation. However, graphene-based membranes may achieve further enhancements in sieving performance through optimized fabrication processes and conditions due to their greater flexibility in pore control.

5 Conclusions and outlook

Graphene and MXene, both as 2D membrane materials, possess a single atomic layer thickness. Separation membranes made from these materials are characterized by an ultra-thin structure, stable physicochemical properties, and adjustable nanotransport channels, exhibiting unique advantages in the field of membrane separation. Currently, significant achievements have been made in the research on building nanoscale transport channels and optimizing the separation performance based on these 2D nanosheets. Breakthroughs have also been achieved in the study of the transport mechanisms, effectively addressing issues such as the susceptibility to contamination and the difficulty of balancing selectivity and permeability in commercial membranes. However, their precise regulation still faces many challenges. Thus, to further enhance the performance of these membranes, future research should focus on the development and application of precise regulation technologies to achieve more efficient and accurate separation effects.

(1) Long-term stability is a crucial characteristic of separation membranes. However, graphene-based and MXene-based membranes often undergo structural deformation in solution. This is due to their layered structure being maintained solely by relatively weak van der Waals forces, and prolonged soaking in solutions typically leads to the dissociation of their 2D layers. Additionally, the hydrophilic surface of GO membranes and the high hydration of MXenes contribute to their tendency to swell in aqueous solutions. To advance the further application of these two membrane materials, there is an urgent need to enhance their chemical and structural stability.

(2) The second issue focuses on simplified scaling-up technology. The difficulties encountered by 2D material membranes in industrial scale-up technology are the main reason why it is difficult for them they to surpass polymer membranes. Numerous efforts have been made regarding this important aspect to date. For example, Li et al.¹⁶³ used the Mayer rod coating method to prepare MXene membranes exceeding 5 meters in length, with adjustable thicknesses ranging from nanometers to micrometers and good permeability. However, this method requires a high concentration of MXene dispersion and lacks effective adjustment of the membrane structure. Deng et al.²¹⁰ adopted electrophoretic deposition, rapidly depositing negatively charged MXene nanosheets onto a positively charged substrate driven by electric field force, thus forming a large-area MXene membrane exceeding 500 cm². Nevertheless, this approach is limited by the conductive substrate. Therefore, to overcome these challenges, further exploration of simpler and cost-effective preparation methods for 2D material membranes is needed to scale up and transform these atomic-level thin films into practical separation equipment.

(3) In addition to the aforementioned issues, it is equally important to conduct in-depth research on the mass transport mechanisms within the confined nano/sub-nano channels of graphene-based and MXene-based membranes. This requires integrating practical cases and simulation models to explore deviations from classical fluid dynamic diffusion laws and the complex interactions between molecules/ions and the nanochannel walls. Interpreting the mass transport behavior in these membranes at the atomic level can not only reveal the microscopic mechanisms of molecular/ionic transport in confined channels but also provide theoretical support for the design of novel 2D material layered membrane structures. These theoretical breakthroughs will lay the foundation for the innovation and advancement of membrane technology.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Natural Science Foundation of China (No. 52203061, 52303060), the Natural Science Foundation of Shandong Province (No. ZR2023QB046), the Natural Science Foundation of Qingdao (23-2-1-91-zyyd-jch), Young Talent of Lifting engineering for Science and Technology in Shandong Province (SDAST2024QTA066), and the University Synergy Innovation Program of Anhui Province (No. GXXT-2023-096).

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Footnote

† These two authors contribute equally to this work.

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