Ion transport is crucial for biological systems and membrane-based technology. Atomic-thick two-dimensional materials, especially graphene oxide (GO), have emerged as ideal building blocks for developing synthetic membranes for ion transport. However, the exclusion of small ions in a pressured filtration process remains a challenge for GO membranes. Here we report manipulation of membrane surface charge to control ion transport through GO membranes. The highly charged GO membrane surface repels high-valent co-ions owing to its high interaction energy barrier while concomitantly restraining permeation of electrostatically attracted low-valent counter-ions based on balancing overall solution charge. The deliberately regulated surface-charged GO membranes demonstrate remarkable enhancement of ion rejection with intrinsically high water permeance that exceeds the performance limits of state-of-the-art nanofiltration membranes. This facile and scalable surface charge control approach opens opportunities in selective ion transport for the fields of water transport, biomimetic ion channels and biosensors, ion batteries and energy conversions.

2D materials' membranes with well-defined nanochannels are promising for precise molecular separation. Herein, the design and engineering of atomically thin 2D MXene flacks into nanofilms with a thickness of 20 nm for gas separation are reported. Well-stacked pristine MXene nanofilms are proven to show outstanding molecular sieving property for H 2 preferential transport. Chemical tuning of the MXene nanochannels is also rationally designed for selective permeating CO 2 . Borate and polyethylenimine (PEI) molecules are well interlocked into MXene layers, realizing the delicate regulation of stacking behaviors and interlayer spacing of MXene nanosheets. The MXene nanofilms with either H 2 -or CO 2 -selective transport channels exhibit excellent gas separation performance beyond the limits for state-of-the-art membranes. The mechanisms within these nanoconfined MXene layers are discussed, revealing the transformation from "diffusion-controlled" to "solution-controlled" channels after chemical tuning. This work of precisely tailoring the 2D nanostructure may inspire the exploring of nanofluidics in 2D confined space with applications in many other fields like catalysis and energy conversion processes.fabrication. Hence, MXene is considered to be a novel potential candidate for developing separative 2D-material membranes. However, there are a very few reports on MXene separation membranes. Gogotsi and co-workers first reported MXene membranes for rejection of trivalent cation in solution using nonpressure diffusion. [12] A high water permeance was obtained by applying MXene stacks in the pressure-filtration process, [13] but the membrane can only rejected matters with a size larger than 2.5 nm. Very recently, Wang and co-workers [14] reported the manufacturing of MXene membranes with highly ordered nanochannel structures for high-performance separation of H 2 /CO 2 , which opens the door of applying MXene membranes for molecular separation. It is considered that rationally regulating the nanostructure of 2D channels may, thereby, enlighten the exploring of MXene materials for sub-nanoscale separation with versatile functionalities.Herein, we report the design and engineering of MXene nanofilms with tunable transport channels for gas separation. Ultrathin pristine MXene nanofilms with a thickness down to 20 nm were fabricated by horizontally aligning the exfoliated MXene nanosheets on porous substrates. Molecular sieving channels within pristine MXene nanofilm can be formed to show highly selective H 2 permeation, as shown in Figure 1. Interestingly, these MXene laminates, as functionalized by borate and amine, exhibit different stacking behaviors and tunable interlayer spacing, allowing preferential CO 2 permeation. The resulting separation performance of either H 2 -or CO 2selective MXene nanofilms is beyond the performance limits for state-of-the-art membranes.

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Graphene oxide (GO)-polyether block amide (PEBA) mixed matrix membranes were fabricated and the effects of GO lateral size on membranes morphologies, microstructures, physicochemical properties, and gas separation performances were systematically investigated. By varying the GO lateral sizes (100-200 nm, 1-2 lm, and 5-10 lm), the polymer chains mobility, as well as the length of the gas channels could be effectively manipulated. Among the as-prepared membranes, a GO-PEBA mixed matrix membrane (GO-M-PEBA) containing 0.1 wt % medium-lateral sized (1-2 lm) GO sheets showed the highest CO 2 permeation performance (CO 2 permeability of 110 Barrer and CO 2 /N 2 mixed gas selectivity of 80), which transcends the Robeson upper bound. Also, this GO-PEBA mixed matrix membrane exhibited high stability during long-term operation testing. Optimized by GO lateral size, the developed GO-PEBA mixed matrix membrane shows promising potential for industrial implementation of efficient CO 2 capture.

Developing advanced membranes with high separation performance and robust mechanical properties is critical to the current water crisis. Herein, a general and scalable fabrication of nanoparticles (NPs)@reduced graphene oxide (rGO) membranes with significantly expanded nanochannels meanwhile ordered laminar structures using in situ synthesized NPs@rGO nanosheets as building blocks is reported. Size‐ and density‐controllable NPs were uniformly grown on the regularly stacked rGO nanosheets through coordination, followed by filtration‐deposition on inner surface of porous ceramic tubes. The NPs bonded rGO building blocks enabled the as‐prepared membranes 1–2 orders of magnitudes higher water permeance than the counterparts while keeping excellent rejections for various organic matters and ions. Moreover, the industrially preferred GO‐based tubular membrane exhibited an extraordinary structural stability under high‐pressure and cross‐flow process of water purification, which is considered as a notable step toward realizing scalable GO‐based membranes. © 2017 American Institute of Chemical Engineers AIChE J, 2017

As a subclass of metal-organic framework materials, zeolitic imidazolate frameworks (ZIFs) have exhibited great potential for numerous applications because of their special three-dimensional structure. Up to now, utilizing ZIF membranes for liquid separations is still limited because it is very difficult to select suitable materials and to fabricate integrated membranes. In this work, a modified contra-diffusion method was carried out to prepare ZIF-71 hollow fiber membranes. The metals Zn(2+) and the organic links imidazole would meet and react on the interface of ceramic hollow fiber through diffusion. The as-prepared ZIF-71 membrane exhibits good performance in separation of ethanol-water mixtures.

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Graphene oxide (GO), as a representative two-dimensional material, has shown great prospect in developing high-performance separation membranes via forming ordered and tunable nanochannels. However, for aqueous molecular separations, the implementation of an excellent separation performance remains a critical challenge due to the membrane swelling phenomenon and the trade-off effect between permeation flux and separation factor. Herein, a facile and tunable approach is presented for introducing water transport promoters into GO interlayer channels to construct water transport highways. The combination of covalently cross-linked channel structure, facilitated water-selective sorption, and expedited water-preferential diffusion overcome the trade-off effect, achieving a superior performance from an ultrathin GO membrane with a flux of 5.94 kg/m 2 •h and a water/butanol separation factor of 3,965, which exceeds the performance of state-of-the-art membranes for water/ butanol separation. The strategy proposed here is straightforward, holding great potential to produce high-efficiency GO and other two-dimensional (2D)-material membranes for precise aqueous molecular separations.