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  1. Abstract

    Water pollution is a major global challenge, as conventional polymeric membranes are not adequate for water treatment anymore. Among emerging materials for water treatment, composite membranes are promising, as they have simultaneously improved water permeation and ions rejection. Recently, a new family of 2D materials called MXenes has attracted considerable attention due to their appealing properties and wide applications. MXenes can be incorporated into many polymeric materials due to their high compatibility. MXenes/polymer composite membranes have been found to have appealing electrical, thermal, mechanical, and transport properties, because of strong interactions between polymer chains and surface functional groups of MXenes and the selective nanochannels that are created. This article reviews advances made in the area of ion‐selective MXene‐based membranes for water purification. It puts the advances into perspective and provides prospects. MXenes’ properties and synthesis methods are briefly described. Strategies for the preparation of MXene‐based membranes including mixed‐matrix membranes, thin‐film nanocomposite membranes, and laminated membranes are reviewed. Recent advances in ion‐separation and water‐desalination MXene‐based membranes are elucidated. The dependence of ion‐separation performance of the membranes on fabrication techniques, MXene's interlayer spacing, and MXene's various surface terminations are elucidated. Finally, opportunities and challenges in ion‐selective MXene‐based membranes are discussed.

     
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  2. Abstract

    MXenes, two‐dimensional (2D) transition metal carbides and/or nitrides, possess surface termination groups such as hydroxyl, oxygen, and fluorine, which are available for surface functionalization. Their surface chemistry is critical in many applications. This article reports amine functionalization of Ti3C2TxMXene surface with [3‐(2‐aminoethylamino)‐propyl]trimethoxysilane (AEAPTMS). Characterization techniques such as X‐ray photoelectron spectroscopy verify the success of the surface functionalization and confirm that the silane coupling agent bonds to Ti3C2Txsurface both physically and chemically. The functionalization changes the MXene surface charge from −35 to +25 mV at neutral pH, which allows for in situ preparation of self‐assembled films. Further, surface charge measurements of the functionalized MXene at different pH values show that the functionalized MXene has an isoelectric point at a pH around 10.7, and the highest reported positive surface charge of +62 mV at a pH of 2.58. Furthermore, the existence of a mixture of different orientations of AEAPTMS and the simultaneous presence of protonated and free amine groups on the surface of Ti3C2Txare demonstrated. The availability of free amine groups on the surface potentially permits the fabrication of crosslinked electrically conductive MXene/epoxy composites, dye adsorbents, high‐performance membranes, and drug carriers. Surface modifications of this type are applicable to many other MXenes.

     
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  3. Gas separation membranes incorporating two-dimensional (2D) materials have received considerable attention in recent years, as these membranes have shown outstanding physical, structural, and thermal properties and high permeability- selectivity. The reduced thickness and diversity of the gas transport mechanisms through in-plane pores (intrinsic defects), in-plane slitlike pores, or plane-to-plane interlayer galleries provide the membranes with a significant sieving ability for energy-efficient gas separation. The discovery of 2D transition metal carbides/nitrides materials, MXenes, has provided new opportunities in the gas separation membrane area because of their hydrophilicity, rich chemistry, high flexibility, and mechanical strength. This Review puts into perspective recent advances in 2D-material-based gas separation membranes. It discusses research opportunities mainly in MXene-based gas membranes, highlights modification approaches for tuning the in-plane and plane-to-plane nanoslits, explains governing mechanisms of transport through these membranes, and compares their advantages and disadvantages with those of other 2D materials. It also discusses current challenges and provides prospects in this area. 
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    MXene/polymer nanocomposites simultaneously benefit from the attractive properties of MXenes and the flexibility and facile processability of polymers. These composites have shown superior properties such as high light-to-heat conversion, excellent electromagnetic interference shielding, and high charge storage, compared to other nanocomposites. They have applications in chemical, materials, electrical, environmental, mechanical, and biomedical engineering as well as medicine. This property-based review on MXene/polymer nanocomposites critically describes findings and achievements in these areas and puts future research directions into perspective. It surveys novel reported applications of MXene-based polymeric nanocomposites. It also covers surface modification approaches that expand the applications of MXenes in nanocomposites. 
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