New 2D Metal-Organic Monoacid Framework (MOmAF): Realization of Extreme Water Repellence.

Metal-organic frameworks (MOFs) are normally moisture-sensitive and unstable in aqueous environments, which has considerably limited their practical applications because water/moisture is ubiquitous in many industrial processes. New materials with superior water stability are, therefore, in great demand and vital to their practical applications. Here, a novel oil/water interfacial assembly strategy is demonstrated for the synthesis of a new class of metal-organic monoacid framework (MOmAF) with exceptional water stability and chemical stability. Superhydrophobic 2D sheets are synthesized at room temperature, while 1D nanotubes are obtained via the self-scrolling of their 2D sheets for the first time. In addition, a simple sequential drop-casting method is developed to coat as-synthesized MOmAF structures onto porous membranes. This can potentially open up new avenues in the design of superhydrophobic self-cleaning MOmAF materials without tedious post-synthetic modifications and usher in a new class of materials meeting industrial needs.

Tuning the morphology of Cu-MOFs nanostructures for sensitive ascorbic acid sensing.

Metal organic frameworks are an attractive platform to develop fascinating electrocatalysts for the oxidation of ascorbic acid (AA), and their different morphologies have been hinted in literature to impact their sensing performance. In this work, by varying the reaction medium of metal source and organic ligand, copper 2-hydroxybenzene-1,4-dicarboxylate (CuBDC-OH) nanosheets (NSs), nanorods (NRs) and bulk were generated. Thereinto, CuBDC-OH-NSs displayed the highest sensitivity of 151.99µA mM-1cm-2in the linear range of 12-1074µM, which is 1.5 times greater than that of CuBDC-OH NRs and 3.5 times greater than that of CuBDC-OH bulk. The electrochemical analyzes manifested that the superiority of nanosheets originated from higher oxidative current, larger electrochemical active surface area and lower charge transfer resistance, which enabling the efficient electro-oxidation of AA. Additionally, satisfactory selectivity, stability and reproducibility were obtained.

Interfacial Synthesis of an Ultrathin Two-Dimensional Polymer Film via [2 + 2] Photocycloaddition.

A carbon-carbon-linked, ultrathin, two-dimensional (2D) polymer film was prepared at the air/water interface through photochemically triggered [2 + 2] cycloaddition. The preorganization of the monomers on the water surface and the subsequent photo-polymerization led to the successful preparation of the ultrathin 2D polymer film. The obtained film is continuous, free standing, and has a large area (over 50 µm2). Transmission electron microscopy (TEM) and atomic force microscopy (AFM) give clear evidence of the ultrathin film morphology. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) indicate successful photo-induced [2 + 2] polymerization.

Preparation of a Large Amount of Ultrathin Graphdiyne.

We report a facile strategy to prepare a large amount of ultrathin graphdiyne (GDY) with good crystallinity in the mixture of oil-water systems. By simply mixing the solution of GDY monomer in CHCl3 and the copper acetate solution as a catalyst in water and stirring at room temperature, ultrathin GDY films with a thickness of ∼4 nm were obtained in a yield of 95 %. This work provides a feasible path for the substantial preparation of GDY films and may pave the way to the development of substantial preparation of the GDY materials.

An Efficient Interfacial Synthesis of Two-Dimensional Metal-Organic Framework Nanosheets for Electrochemical Hydrogen Peroxide Production.

Two-dimensional (2D) metal-organic framework nanosheets (MOF NSs) play a vital role in catalysis, but the most preparation is ultrasonication or solvothermal. Herein, a liquid-liquid interfacial synthesis method has been developed for the efficient fabrication of a series of 2D Ni MOF NSs. The active sites could be modulated by readily tuning the ratios of metal precursors and organic linkers (RM/L ). The Ni MOF NSs display highly RM/L dependent activities towards 2e oxygen reduction reaction (ORR) to hydrogen peroxide (H2 O2 ), where the Ni MOF NSs with the RM/L of 6 exhibit the optimal near-zero overpotential, ca. 98 % H2 O2 selectivity and production rate of ca. 80 mmol gcat -1 h-1 in 0.1 M KOH. As evidenced by X-ray absorption fine structure spectroscopy, the coordination environment of active sites changed from saturation to unsaturation, and the partially unsaturated metal atoms are crucial to create optimal sites for enhancing the electrocatalysis.

A Two-Dimensional Polyimide-Graphene Heterostructure with Ultra-fast Interlayer Charge Transfer.

Two-dimensional polymers (2DPs) are a class of atomically/molecularly thin crystalline organic 2D materials. They are intriguing candidates for the development of unprecedented organic-inorganic 2D van der Waals heterostructures (vdWHs) with exotic physicochemical properties. In this work, we demonstrate the on-water surface synthesis of large-area (cm2 ), monolayer 2D polyimide (2DPI) with 3.1-nm lattice. Such 2DPI comprises metal-free porphyrin and perylene units linked by imide bonds. We further achieve a scalable synthesis of 2DPI-graphene (2DPI-G) vdWHs via a face-to-face co-assembly of graphene and 2DPI on the water surface. Remarkably, femtosecond transient absorption spectroscopy reveals an ultra-fast interlayer charge transfer (ca. 60 fs) in the resultant 2DPI-G vdWH upon protonation by acid, which is equivalent to that of the fastest reports among inorganic 2D vdWHs. Such large interlayer electronic coupling is ascribed to the interlayer cation-π interaction between 2DP and graphene.

Highly Crystalline and Semiconducting Imine-Based Two-Dimensional Polymers Enabled by Interfacial Synthesis.

Single-layer and multi-layer 2D polyimine films have been achieved through interfacial synthesis methods. However, it remains a great challenge to achieve the maximum degree of crystallinity in the 2D polyimines, which largely limits the long-range transport properties. Here we employ a surfactant-monolayer-assisted interfacial synthesis (SMAIS) method for the successful preparation of porphyrin and triazine containing polyimine-based 2D polymer (PI-2DP) films with square and hexagonal lattices, respectively. The synthetic PI-2DP films are featured with polycrystalline multilayers with tunable thickness from 6 to 200 nm and large crystalline domains (100-150 nm in size). Intrigued by high crystallinity and the presence of electroactive porphyrin moieties, the optoelectronic properties of PI-2DP are investigated by time-resolved terahertz spectroscopy. Typically, the porphyrin-based PI-2DP 1 film exhibits a p-type semiconductor behavior with a band gap of 1.38 eV and hole mobility as high as 0.01 cm2 V-1 s-1 , superior to the previously reported polyimine based materials.

Two-Dimensional Boronate Ester Covalent Organic Framework Thin Films with Large Single Crystalline Domains for a Neuromorphic Memory Device.

Despite the recent progress in the synthesis of crystalline boronate ester covalent organic frameworks (BECOFs) in powder and thin-film through solvothermal method and on-solid-surface synthesis, respectively, their applications in electronics, remain less explored due to the challenges in thin-film processability and device integration associated with the control of film thickness, layer orientation, stability and crystallinity. Moreover, although the crystalline domain sizes of the powder samples can reach micrometer scale (up to ≈1.5 µm), the reported thin-film samples have so far rather small crystalline domains up to 100 nm. Here we demonstrate a general and efficient synthesis of crystalline two-dimensional (2D) BECOF films composed of porphyrin macrocycles and phenyl or naphthyl linkers (named as 2D BECOF-PP or 2D BECOF-PN) by employing a surfactant-monolayer-assisted interfacial synthesis (SMAIS) on the water surface. The achieved 2D BECOF-PP is featured as free-standing thin film with large single-crystalline domains up to ≈60 µm2 and tunable thickness from 6 to 16 nm. A hybrid memory device composed of 2D BECOF-PP film on silicon nanowire-based field-effect transistor is demonstrated as a bio-inspired system to mimic neuronal synapses, displaying a learning-erasing-forgetting memory process.

Highly Transparent, Flexible, and Mechanically Strong Nanopapers of Cellulose Nanofibers @Metal-Organic Frameworks.

Freestanding nanopapers were fabricated by the assembly of metal-organic frameworks (MOFs) onto cellulose nanofibers (CNFs). The CNFs are wrapped by continuously nucleated MOF layers (CNF@MOF) by interfacial synthesis, with the charge density on the surface of the CNFs and the dosage of the surfactant polyvinylpyrrolidone (PVP) being carefully adjusted. The obtained CNF@MOF nanofibers with long-range, continuous, hybrid nanostructures were very different to the composites formed by aggregation of MOF nanoparticles on the substrates. Four typical MOFs (HKUST-1, Al-MIL-53, Zn-MOF-74, ZIF-CO3 -1) were successfully grown onto CNFs in aqueous solutions and further fabricated into freestanding nanopapers. Because of their unique nanostructures and morphologies, the corresponding flexible nanopapers exhibit hierarchical meso-micropores, high optical transparency, high thermal stability, and high mechanical strength. A proof-of-concept study shows that the CNF@MOF nanopapers can be used as efficient filters to separate volatile organic compounds (VOCs) from the air. This work provides a new path for structuring MOF materials that may boost their practical application.

Controlled Growth of CH3 NH3 PbBr3 Perovskite Nanocrystals via a Water-Oil Interfacial Synthesis Method.

Fundamental insights into the reaction kinetics of organic-inorganic lead halide perovskite nanocrystals (LHP NCs) are still limited due to their ultrafast formation rate. Herein, we develop a water-oil interfacial synthesis of MAPbBr3 NCs (MA=CH3 NH3 + ), which prolongs the reaction time to tens of minutes. This method makes it possible to monitor in situ the formation process of MAPbBr3 NCs and observe successive spectral evolutions from 438 to 534 nm in a single reaction by extending reaction time. The implementation of this method depends on reducing the formation rate of PbBr6 4- octahedra and the diffusion rate of MA. The formation of PbBr6 4- is a rate-determining step, and the biphasic system offers a favorable reaction condition to control the mass transfer of MA. The effects of temperature and concentration of precursor and ligand are investigated in detail.

Towards Macroscopic Crystalline 2D Polymers.

Periodic and nanoporous monolayer polymers, the structures of which can be viewed as molecular fishing nets, have been classified as 2D polymers. They have been previously synthesized under mild photoirradiation conditions in the interior of layered single crystals of well-designed monomers, followed by a liquid-phase exfoliation. While these mild conditions allow for full structure control, the size of 2D polymers obtained cannot exceed that of the crystals from which they are prepared. In this Review, we discuss different concepts currently pursued to prepare macroscopically sized 2D polymers, focusing on syntheses at the air-water and liquid-liquid interfaces. While these interfaces are larger reaction loci than single crystals, sheet-like polymers obtained at them pose complex and time-consuming analytical challenges. Some of these challenges are concretely discussed and indicators are provided for identifying the promising cases, enabling to concentrate on them in the future research. Additionally, this Review discusses three representative examples of 2D polymers to provide a state-of-the-art picture of this emerging field of polymer and materials science. Finally, we sketch the range of applications, such as nanomembranes, electronics, optoelectronics, and electrocatalysts for water splitting, that are relevant for these novel organic 2D materials.

Metal-Organic Framework (MOF) Nanorods, Nanotubes, and Nanowires.

New mechanisms for the controlled growth of one-dimensional (1D) metal-organic framework (MOF) nano- and superstructures under size-confinement and surface-directing effects have been discovered. Through applying interfacial synthesis templated by track-etched polycarbonate (PCTE) membranes, congruent polycrystalline zeolitic imidazolate framework-8 (ZIF-8) solid nanorods and hollow nanotubes were found to form within 100 nm membrane pores, while single crystalline ZIF-8 nanowires grew inside 30 nm pores, all of which possess large aspect ratios up to 60 and show preferential crystal orientation with the {100} planes aligned parallel to the long axis of the pore. Our findings provide a generalizable method for controlling size, morphology, and lattice orientation of MOF nanomaterials.

In Situ Synthesis of a Supramolecular Hydrogelator at an Oil/Water Interface for Stabilization and Stimuli-Induced Fusion of Microdroplets.

Supramolecular hydrogels are expected to have applications as novel soft materials in various fields owing to their designable functional properties. Herein, we developed an in situ synthesis of supramolecular hydrogelators, which can trigger gelation of an aqueous solution without the need for temperature change. This was achieved by mixing two precursors, which induced the synthesis of a supramolecular gelator and its instantaneous self-assembly into nanofibers. We then performed the in situ synthesis of this supramolecular gelator at an oil/water interface to produce nanofibers that covered the surfaces of the oil droplets (nanofiber-stabilized oil droplets). External stimuli induced fusion of the droplets owing to disassembly of the gelator molecules. Finally, we demonstrated that this stimuli-induced droplet fusion triggered a synthetic reaction within the droplets. This means that the confined nanofiber-stabilized droplets can be utilized as stimuli-responsive microreactors.

Direct Fabrication of Free-Standing MOF Superstructures with Desired Shapes by Micro-Confined Interfacial Synthesis.

Recently, metal-organic frameworks (MOFs) with multifunctional pore chemistry have been intensively investigated for positioning the desired morphology at specific locations onto substrates for manufacturing devices. Herein, we develop a micro-confined interfacial synthesis (MIS) approach for fabrication of a variety of free-standing MOF superstructures with desired shapes. This approach for engineering MOFs provides three key features: 1) in situ synthesis of various free-standing MOF superstructures with controlled compositions, shape, and thickness using a mold membrane; 2) adding magnetic functionality into MOF superstructures by loading with Fe3 O4 nanoparticles; 3) transferring the synthesized MOF superstructural array on to flat or curved surface of various substrates. The MIS route with versatile potential opens the door for a number of new perspectives in various applications.

Solar Carboxylation Using CO2: Interfacially Synthesized Flexible Covalent Organic Frameworks Film as a Photocatalyst for Highly Selective Solar Carboxylation of Arylamines with CO2.

Carbon dioxide (CO2) conversion into value-added chemicals/fuels by utilizing solar energy is a sustainable way to mitigate our dependence on fossil fuels and stimulate a carbon-neutral economy. However, the efficient and affordable conversion of CO2 is still an ongoing challenge. Here, we report an interfacially synthesized visible-light-active Ni(II)-integrated covalent organic frameworks (TaTpBpy-Ni COFs) film as a photocatalyst for efficient CO2 conversion into carboxylic acid under ambient conditions. Notably, the TaTpBpy-Ni COFs film showed excellent photocatalytic activity for the carboxylation of various arylamines with CO2 to the corresponding arylcarboxylic acid via C-N bond activation under solar-light irradiation. Moreover, this carboxylation protocol exhibits mild reaction conditions and good functional group tolerance without the necessity of using stoichiometric metallic reductants. This work shows a benchmark example of not only the interfacially synthesized COFs film used as a photocatalyst for solar-light energy utilization but also the selective solar chemical production system of arylcarboxylic acid directly from CO2.

Interfacial nanoarchitectonics for ZIF-8 membranes with enhanced gas separation.

Metal-organic framework (MOF) membranes are potentially useful in gas separation applications. Conventional methods of MOF membrane preparation require multiple steps and high-pressure conditions. In this study, a reliable one-step interfacial synthesis method under atmospheric pressure has been developed to prepare zeolitic imidazolate framework-8 (ZIF-8) membranes supported on porous α-Al2O3 disks. To obtain optimal ZIF-8 membranes, three reaction parameters were investigated, namely, reaction temperature, reaction time, and concentration of the organic linker (i.e., 2-methylimidazole). The growth of ZIF-8 membranes under various parameters was evaluated by field-emission scanning electron microscopy, and the optimal synthesis conditions were determined (i.e., 80 °C for 12 h in 50 mM of 2-methylimidazole). The as-synthesized ZIF-8 membranes were then applied to CO2/N2 gas separation, which exhibited a maximum separation factor of 5.49 and CO2 gas permeance of 0.47 × 10-7 mol·m-2·s-1·Pa-1.

Interfacial Synthesis of a Monolayered Fluorescent Two-Dimensional Polymer through Dynamic Imine Chemistry.

A fluorescent monolayered two-dimensional polymer (2DP) containing both tetraphenylethylene (TPE) and imine linkages is synthesized at air-water interface using the Langmuir-Blodgett method. We designed TPE-based monomers with long distances between the TPE and the imine linkages to avoid the charge transfer and therefore keep the fluorescence. A monolayered 2DP provided with more than 104 µm2 in domain size and around 0.8 nm thickness was obtained through a successive Schiff base reaction at air-water interface. The nanostructures and fluorescent property of 2DP films were characterized by optical microscopy, SEM, TEM, AFM and fluorescence spectrum. Most importantly, the tip-enhanced Raman spectroscopy (TERS) was utilized here to confirm the success of the polycondensation of monolayered 2DP.

Resistive Switching Memory Performance of Two-Dimensional Polyimide Covalent Organic Framework Films.

Two-dimensional polyimide covalent organic framework (2D PI-NT COF) films were constructed on indium tin oxide-coated glass substrates to fabricate two-terminal sandwiched resistive memory devices. The 2D PI-NT COF films condensated from the reaction between 4,4',4″-triaminotriphenylamine and naphthalene-1,4,5,8-tetracarboxylic dianhydride under solvothermal conditions demonstrated high crystallinity, good orientation preference, tunable thickness, and low surface roughness. The well-aligned electron-donor (triphenylamine unit) and -acceptor (naphthalene diimide unit) arrays rendered the 2D PI-NT COF films a promising candidate for electronic applications. The memory devices based on 2D PI-NT COF films exhibited a typical write-once-read-many-time resistive switching behavior under an operating voltage of +2.30 V on the positive scan and -2.64 V on the negative scan. A high ON/OFF current ratio (>106 for the positive scan and 104-106 for the negative scan) and long-term retention time indicated the high fidelity, low error, and high stability of the resistive memory devices. The memory behavior was attributed to an electric field-induced intramolecular charge transfer in an ordered donor-acceptor system, which provided the effective charge-transfer channels for injected charge carriers. This work represents the first example that explores the resistive memory properties of 2D PI-COF films, shedding light on the potential application of 2D COFs as information storage media.

Preparation of N-Graphdiyne Nanosheets at Liquid/Liquid Interface for Photocatalytic NADH Regeneration.

Two-dimensional (2D) N-graphdiyne (N-GDY) nanosheets containing different number of N were synthesized by polymerization of triazine, pyrazine, and pyridine-based monomers at liquid/liquid interface. The configurations and nanostructures of N-GDY were well-characterized. The wettability changed to more hydrophilic as the N contents increased. The collected N-GDY was further employed as metal-free photocatalyst for NADH regeneration. The catalytic performance was related with the N content in the graphdiyne. The N3-GDY demonstrated the best activity. This strategy provided a new promising platform of designing unique 2D N-GDY with tunable performance in biorelated catalysis.

Sulfur-rich Graphdiyne-Containing Electrochemical Active Tetrathiafulvalene for Highly Efficient Lithium Storage Application.

A sulfur-rich graphdiyne (TTF-GDY) material containing electrochemically active unit of tetrathiafulvalene was successfully synthesized at the liquid/liquid interface under mild conditions. The extended two-dimensional (2D) π-conjugated framework contains large in-plane cavities to accommodate defined sulfur elements and precise sulfur content. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) characterizations demonstrated that the TTF-GDY adopted the layered structures. X-ray photoelectron spectroscopy (XPS), Raman spectrum, selected area electron diffraction (SAED), crystal modeling, and molecular-mechanics-based calculation further confirmed the proposed framework structure and the expected chemical compositions. Considering the unique structure and sulfur-rich characteristic, we then applied this material as an electrode in lithium battery applications. Due to the unique 2D framework structure, the TTF-GDY exhibited a high specific capacity of 837.6 mAh/g, particularly having a robust long-term stability under the high current density. These results demonstrated that such a sulfur-rich TTF-GDY, as a novel electrochemical material would have great application potential in energy fields.