Lamellar carbon nitride membrane for enhanced ion sieving and water desalination

Chemistry for water treatment under nanoconfinement

The global freshwater crisis, exacerbated by escalating pollution, poses a significant threat to human health. Addressing this challenge required innovative strategies to develop highly efficient and process-adaptable materials for water decontamination. In this regard, nanomaterials with confinement structures have emerged as a promising solution, outperforming traditional nanomaterials in terms of efficiency, selectivity, stability, and process adaptability, thereby serving as an ideal platform for designing novel functional materials for sustainable water treatment. This Review focuses on recent advancements and employment of nanoconfinement effects in various water treatment processes, emphasizing the fundamental chemistry underlying nanoconfinement effects. Also, the existing knowledge gaps related to nanoconfinement effects and future prospects for expanding their applications in diverse water treatment scenarios are discussed.

Biomimetic ion channels with subnanometer sizes for ion sieving: a mini-review

The remarkable ion selectivity of biological systems has inspired the development of artificial ion channels with Ångström-scale precision, expanding their potential applications in ion separation, energy conversion, and water purification. This mini-review systematically examines fundamental ion-sieving mechanisms operating at the subnanoscale, highlighting advanced fabrication strategies involving synthetic ion channels on lipid bilayers and solid-state ion channels. We further explore membrane material innovations spanning zero-dimensional nanopores to three-dimensional crystalline frameworks, emphasizing structure-function relationships in channel design. The discussion concludes with critical perspectives on scalability challenges and future research directions, outlining pathways toward next-generation sustainable ion sieving technologies.

Nature-Inspired Engineering Separation Materials and Devices

Separation is a fundamental process in natural living systems. Their separation capabilities have inspired the design of various separation materials and devices. Despite some progress having been made, a comprehensive overview is still lacking. In this Perspective, we first review the development of separation technologies. We then summarize some typical living systems exhibiting superior separation capabilities from compositions and microstructures to separation mechanisms. Next, we highlight key advancements in nature-inspired separation materials and integrated devices. Finally, we propose future research directions and opportunities, emphasizing the importance of physical and chemical design and internal and external stimulus regulation. These nature-inspired materials and devices show great potential in biomedicine, environmental remediation, energy conversion, food safety, and analysis testing.

Facile orientation control of MOF-303 hollow fiber membranes by a dual-source seeding method

Metal‒organic frameworks (MOFs) are nanoporous crystalline materials with enormous potential for further development into a new class of high-performance membranes. However, the preparation of defect-free and water-stable MOF membranes with high permselectivity and good structural integrity remains a challenge. Herein, we demonstrate a dual-source seeding (DS) approach to produce high-performance, water-stable MOF-303 membranes with hollow fiber (HF) geometry and preferentially tailored crystallographic orientation. By controlling the nucleation site density during secondary growth, MOF-303 membranes with a preferred crystallographic orientation (CPO) on the (011) plane were fabricated. The MOF-303 membrane with CPO on (011) provides straight one-dimensional permeation channels with a superior water flux of 18 kg m-2 h-1 in pervaporative water/ethanol separation, which is higher than that of most of the reported zeolite membranes and 1-2 orders of magnitude greater than that of previously reported MOF membranes. The straight water permeation channels also offer a promising water permeance of 15 L m-2 h-1 bar-1 and a molecular weight cut-off (MWCO ≈ 269) for dye nanofiltration. These results provide a concept for developing ultrapermeable MOF membranes with good selectivity and structural integrity for pervaporation and nanofiltration.

Effectively Sieving Alkali Metal Ions Using Functionalized Graphene Oxide Membranes by Exploiting Water-Repellent Interactions

Sieving membranes capable of discerning different alkali metal ions are important for many technologies, such as energy, environment, and life science. Recently, two-dimensional (2D) materials have been extensively explored for the creation of sieving membranes with angstrom-scale channels. However, because of the same charge and similar hydrated sizes, mostly laminated membranes typically show low selectivity (<10). Herein, we report a facile and scalable method for functionalizing graphene oxide (GO) laminates by dually grafting cations and water-repellent dimethylsiloxane (DMDMS) molecules to achieve high selectivities of ∼50 and ∼20 toward the transport of Cs+/Li+ and K+/Li+ ion pairs, surpassing many of the state-of-the-art laminated membranes. The enhanced selectivity for alkali metal ions can be credited to a dual impact: (i) strong hydrophobic interactions between the incident cations' hydration shells and the water-repellent DMDMS; (ii) the efficient screening of electrostatic interactions that hamper selectivity.

Polyoxometalate Clusters Confined in Reduced Graphene Oxide Membranes for Effective Ion Sieving and Desalination

Efficient 2D membranes play a critical role in water purification and desalination. However, most 2D membranes, such as graphene oxide (GO) membranes, tend to swell or disintegrate in liquid, making precise ionic sieving a tough challenge. Herein, the fabrication of the polyoxometalate clusters (PW12) intercalated reduced graphene oxide (rGO) membrane (rGO-PW12) is reported through a polyoxometalate-assisted in situ photoreduction strategy. The intercalated PW12 result in the interlayer spacing in the sub-nanometer scale and induce a nanoconfinement effect to repel the ions in various salt solutions. The permeation rate of rGO-PW12 membranes are about two orders of magnitude lower than those through the GO membrane. The confinement of nanochannels also generate the excellent non-swelling stability of rGO-PW12 membranes in aqueous solutions up to 400 h. Moreover, when applied in forward osmosis, the rGO-PW12 membranes with a thickness of 90 nm not only exhibit a high-water permeance of up to 0.11790 L m-2 h-1 bar-1 and high NaCl rejection (98.3%), but also reveal an ultrahigh water/salt selectivity of 4740. Such significantly improved ion-exclusion ability and high-water flux benefit from the multi-interactions and nanoconfinement effect between PW12 and rGO nanosheets, which afford a well-interlinked lamellar structure via hydrogen bonding and van der Waals interactions.

Electrostatic-induced ion-confined partitioning in graphene nanolaminate membrane for breaking anion-cation co-transport to enhance desalination

Constructing nanolaminate membranes made of two-dimensional graphene oxide nanosheets has gained enormous interest in recent decades. However, a key challenge facing current graphene-based membranes is their poor rejection for monovalent salts due to the swelling-induced weak nanoconfinement and the transmembrane co-transport of anions and cations. Herein, we propose a strategy of electrostatic-induced ion-confined partitioning in a reduced graphene oxide membrane for breaking the correlation of anions and cations to suppress anion-cation co-transport, substantially improving the desalination performance. The membrane demonstrates a rejection of 95.5% for NaCl with a water permeance of 48.6 L m-2 h-1 bar-1 in pressure-driven process, and it also exhibits a salt rejection of 99.7% and a water flux of 47.0 L m-2 h-1 under osmosis-driven condition, outperforming the performance of reported graphene-based membranes. The simulation and calculation results unveil that the strong electrostatic attraction of membrane forces the hydrated Na+ to undergo dehydration and be exclusively confined in the nanochannels, strengthening the intra-nanochannel anion/cation partitioning, which refrains from the dynamical anion-cation correlations and thereby prevents anions and cations from co-transporting through the membrane. This study provides guidance for designing advanced desalination membranes and inspires the future development of membrane-based separation technologies.

Bio-Inspired Subnanofluidics: Advanced Fabrication and Functionalization

Biological ion channels can realize high-speed and high-selective ion transport through the protein filter with the sub-1-nanometer channel. Inspired by biological ion channels, various kinds of artificial subnanopores, subnanochannels, and subnanoslits with improved ion selectivity and permeability are recently developed for efficient separation, energy conversion, and biosensing. This review article discusses the advanced fabrication and functionalization methods for constructing subnanofluidic pores, channels, tubes, and slits, which have shown great potential for various applications. Novel fabrication methods for producing subnanofluidics, including top-down techniques such as electron beam etching, ion irradiation, and electrochemical etching, as well as bottom-up approaches starting from advanced microporous frameworks, microporous polymers, lipid bilayer embedded subnanochannels, and stacked 2D materials are well summarized. Meanwhile, the functionalization methods of subnanochannels are discussed based on the introduction of functional groups, which are classified into direct synthesis, covalent bond modifications, and functional molecule fillings. These methods have enabled the construction of subnanochannels with precise control of structure, size, and functionality. The current progress, challenges, and future directions in the field of subnanofluidic are also discussed.

Regulating ion affinity and dehydration of metal-organic framework sub-nanochannels for high-precision ion separation

Membrane consisting of ordered sub-nanochannels has been pursued in ion separation technology to achieve applications including desalination, environment management, and energy conversion. However, high-precision ion separation has not yet been achieved owing to the lack of deep understanding of ion transport mechanism in confined environments. Biological ion channels can conduct ions with ultrahigh permeability and selectivity, which is inseparable from the important role of channel size and "ion-channel" interaction. Here, inspired by the biological systems, we report the high-precision separation of monovalent and divalent cations in functionalized metal-organic framework (MOF) membranes (UiO-66-(X)2, X = NH2, SH, OH and OCH3). We find that the functional group (X) and size of the MOF sub-nanochannel synergistically regulate the ion binding affinity and dehydration process, which is the key in enlarging the transport activation energy difference between target and interference ions to improve the separation performance. The K+/Mg2+ selectivity of the UiO-66-(OCH3)2 membrane reaches as high as 1567.8. This work provides a gateway to the understanding of ion transport mechanism and development of high-precision ion separation membranes.

Two-Dimensional Sulfonate-Functionalized Metal-Organic Framework Membranes for Efficient Lithium-Ion Sieving

Two-dimensional (2D) membranes have shown promising potential for ion-selective separation but often suffer from the trade-off between permeability and selectivity. Herein, we report an ultrathin 2D sulfonate-functionalized metal-organic framework (MOF) membrane for efficient lithium-ion sieving. The narrow pores with angstrom precision in the MOF assist hydrated ions to partially remove the hydration shell, according to different hydration energies. The abundant sulfonate groups in the MOF channels serve as hopping sites for fast lithium-ion transport, contributing to a high Li-ion permeability. Then, the difference in affinity of the Li+, Na+, K+, and Mg2+ ions to the terminal sulfonate groups further enhances the Li-ion selectivity. The reported ultrathin MOF membrane overcomes the trade-off between permeability and selectivity and opens up a new avenue for highly permselective membranes.

Polymer carbon nitride nanosheet-based lamellar membranes inspired by "couple hardness with softness" for ultrafast molecular separation in organic solvents

Membranes with ultrafast molecular separation ability in organic solvents can offer unprecedented opportunities for efficient and low-cost solvent recovery in industry. Herein, a graphene-like polymer carbon nitride nanosheet (PCNN) with a low-friction surface was applied as the main membrane building block to boost the ultrafast transport of the solvent. Meanwhile, inspired by the concept of "couple hardness with softness", soft and flexible graphene oxide (GO) was chosen to fix the random stack of the rigid PCNN and tailor the lamellar structure of the PCNN membrane. The optimal PCNN/GO lamellar membrane shows a remarkable methanol permeance of 435.5 L m-2 h-1 bar-1 (four times higher than that of the GO membrane) while maintaining a high rejection for reactive black (RB, 98.9% in ethanol). Molecular dynamics simulations were conducted to elucidate the ultrafast transport mechanism of the PCNN/GO membrane. This study reveals that PCNN is a promising building block for lamellar membranes and may open up new avenues for high-performance molecular separation membranes.

Selective Ion Transport in Two-Dimensional Lamellar Nanochannel Membranes

Precise and ultrafast ion sieving is highly desirable for many applications in environment-, energy-, and resource-related fields. The development of a permselective lamellar membrane constructed from parallel stacked two-dimensional (2D) nanosheets opened a new avenue for the development of next-generation separation technology because of the unprecedented diversity of the designable interior nanochannels. In this Review, we first discuss the construction of homo- and heterolaminar nanoarchitectures from the starting materials to the emerging preparation strategies. We then explore the property-performance relationships, with a particular emphasis on the effects of physical structural features, chemical properties, and external environment stimuli on ion transport behavior under nanoconfinement. We also present existing and potential applications of 2D membranes in desalination, ion recovery, and energy conversion. Finally, we discuss the challenges and outline research directions in this promising field.