Free-standing Metal-organic Frameworks Research Articles

Design and Synthesis of Transferrable Macro-Sized Continuous Free-Standing Metal-Organic Framework Films for Biosensor Device.

Metal organic framework (MOF) films have attracted abundant attention due to their unique characters compared with MOF particles. But the high-temperature reaction and solvent corrosion limit the preparation of MOF films on fragile substrates, hindering further applications. Fabricating macro-sized continuous free-standing MOF films and transferring them onto fragile substrates are a promising alternative but still challenging. Here, a universal strategy to prepare transferrable macro-sized continuous free-standing MOF films with the assistance of oxide nanomembranes prepared by atomic layer deposition and studied the growth mechanism is developed. The oxide nanomembranes serve not only as reactant, but also as interfacial layer to maintain the integrality of the free-standing structure as the stacked MOF particles are supported by the oxide nanomembrane. The centimeter-scale free-standing MOF films can be transferred onto fragile substrates, and all in one device for glucose sensing is assembled. Due to the strong adsorption toward glucose molecules, the obtained devices exhibit outstanding performance in terms of high sensitivity, low limit of detection, and long durability. This work opens a new window toward the preparation of MOF films and MOF film-based biosensor chip for advantageous applications in post-Moore law period.

MOF-Based Membranes for Remediated Application of Water Pollution.

Membrane separation plays a crucial role in the current increasingly complex energy environment. Membranes prepared by metal-organic framework (MOF) materials usually possess unique advantages in common, such as uniform pore size, ultra-high porosity, enhanced selectivity and throughput, and excellent adsorption property, which have been contributed to the separation fields. In this comprehensive review, we summarize various designs and synthesized strategies of free-standing MOF and composite MOF-based membranes for water treatment. Special emphases are given not only on the effects of MOF on membrane performance, removal efficiencies, and elimination mechanisms, but also on the importance of MOF-based membranes for the applications of oily and micro-pollutant removal, adsorption, separation, and catalysis. The challenges and opportunities in the future for the industrial implementation of MOF-based membranes are also discussed.

Free-standing conductive nickel metal-organic framework nanowires as bifunctional electrodes for wearable pressure sensors and Ni-Zn batteries

Free-standing metal-organic frameworks (MOFs) with controllable structure and good stability are emerging as promising materials for applications in flexible pressure sensors and energy-storage devices. However, the inherent low electrical conductivity of MOF-based materials requires complex preparation processes that involve high-temperature carbonization. This work presents a simple method to grow conductive nickel MOF nanowire arrays on carbon cloth (Ni-CAT@CC) and use Ni-CAT@CC as the functional electrodes for flexible piezoresistive sensor. The resulting sensor is able to monitor human activity, including elbow bending, knee bending, and wrist bending. Besides, the soft-packaged aqueous Ni-Zn battery is assembled with Ni-CAT@CC, a piece of glass microfiber filters, and Zn foil acting as cathode, separator, and anode, respectively. The Ni-Zn battery can be used as a power source for finger pressure monitoring. This work demonstrates free-standing MOF-based nanowires as bifunctional fabric electrodes for wearable electronics.

Freestanding MOF-Derived Honeycomb-Shape Porous MnOC@CC as an Electrocatalyst for Reversible LiOH Chemistry in Li-O2 Batteries

In rechargeable Li-O2 batteries, the electrolyte and the electrode are prone to be attacked by aggressive intermediates (O2- and LiO2) and products (Li2O2), resulting in low energy efficiency. It has been reported that in the presence of water, the formation of low-activity LiOH is more stable for electrolyte and electrode, effectively reducing the production of parasitic products. However, the reversible formation and decomposition of LiOH catalyzed by solid catalysts is still a challenge. Here, a freestanding metal-organic framework (MOF)-derived honeycomb-shape porous MnOC@CC cathode was prepared for Li-O2 batteries by in situ growth of urchin-like Mn-MOFs on carbon cloth (CC) and carbonization. The battery with the MnOC@CC cathode exhibits an ultrahigh practical discharge specific capacity of 22,838 mAh g-1 at 200 mA g-1, high-rate capability, and more stable cycling, which is superior to the MnOC powder cathode. X-ray diffraction and Fourier transform infrared results identify that the discharge product of the batteries is LiOH rather than highly active Li2O2, and no parasitic products were found during operation. The MnOC@CC cathode can induce the formation of flower-like LiOH in the presence of water due to its unique porous structure and directional alignment of Mn-O centers. This work achieves the reversible formation and decomposition of LiOH in the presence of water, offering some insights into the practical application of semiopen Li-O2 batteries.

Free-Standing Metal–Organic Framework Membranes Made by Solvent-Free Space-Confined Conversion for Efficient H2/CO2 Separation

Metal-organic frameworks (MOFs) are promising candidates for the advanced membrane materials based on their diverse structures, modifiable pore environment, precise pore sizes, etc. Nevertheless, the use of supports and large amounts of solvents in traditional solvothermal synthesis of MOF membranes is considered inefficient, costly, and environmentally problematic, coupled with challenges in their scalable manufacturing. In this work, we report a solvent-free space-confined conversion (SFSC) approach for the fabrication of a series of free-standing MOF (ZIF-8, Zn(EtIm)2, and Zn2(BIm)4) membranes. This approach excludes the employment of solvents and supports that require tedious pretreatment and, thus, makes the process more environment-friendly and highly efficient. The free-standing membranes feature a robust and unique architecture, which comprise dense surface layers and highly porous interlayer with large amounts of irregular-shaped micron-scale pore cavities, inducing satisfactory H2/CO2 selectivities and exceptional H2 permeances. The ZIF-8 membrane affords a considerable H2 permeance of 2653.7 GPU with a competitive H2/CO2 selectivity of 17.1, and the Zn(EtIm)2 membrane exhibits a high H2/CO2 selectivity of 22.1 with an excellent H2 permeance (6268.7 GPU). The SFSC approach potentially provides a new pathway for preparing free-standing MOF membranes under solvent-free conditions, rendering it feasible for scale-up production of membrane materials for gas separation.

Photoswitchable Nanoporous Metal–Organic Framework Monolayer Film for Light-Gated Ion Nanochannel

Two-dimensional metal–organic framework (MOF) monolayer films, which can be considered as large-area, periodically ordered thin films with atom- or single-unit thickness, have attracted extensive attention due to their special properties and wide range of potential applications. However, the preparation of MOF monolayer films is still a great challenge, and the introduction of functional groups into MOF monolayer films is seldom reported. Here, we report the first centimeter-scale, photo-switchable, free-standing MOF monolayer film formed by self-assembly of poly(methyl methacrylate-co-acrylamidoazobenzene)-coated MOF nanoparticles at the water–air interface. This self-assembled MOF monolayer film is extremely thin (∼220 or 50 nm) and has a high MOF loading (85 wt %), while exhibiting remarkable reversible photoswitching properties under alternating exposure to 365 nm UV and white light. Inspired by this, a light-gated ion nanochannel device is fabricated, which enables reversible ″OFF–ON″ changes of the ionic flux and has promising applications in the field of photoelectric conversion. This provides a modular route to design large-area, free-standing functional MOF monolayer films.

General Synthesis and Solution Processing of Metal-Organic Framework Nanofibers.

By virtue of their extraordinarily high surface areas, ordered pore structures, various compositions, and rich functionality, metal-organic frameworks (MOFs) are of great interest in diverse fields such as gas separation, sensing, catalysis, energy, environment science, and biomedicine. However, the difficulty in processing MOF crystals and controlling the MOF superstructure is emerging as a critical issue in their application. Herein, it is reported that a robust template, i.e., nanofibrillated cellulose (NFC), can be used for the synthesis of MOF materials with 1D nanofiber morphology. NFC@MOF core-shell nanofibers with a uniform network structure and high aspect ratios can be prepared by use of this template. The small crystal size, flexibility, and good dispersity of the NFC@MOF nanofibers make it convenient for the macroscale assembly and solution processing of MOF materials. A proof-of-concept study is demonstrated wherein freestanding MOF nanofiber membranes represent good performance in applications of water treatment and heterogeneous catalysis reaction. This general synthesis and solution-processing strategy may herald a new era in promoting the industrial application of MOFs.

Freestanding Metal–Organic Frameworks and Their Derivatives: An Emerging Platform for Electrochemical Energy Storage and Conversion

Freestanding Metal–Organic Frameworks and Their Derivatives: An Emerging Platform for Electrochemical Energy Storage and Conversion

Freestanding Metal-Organic Frameworks and Their Derivatives: An Emerging Platform for Electrochemical Energy Storage and Conversion.

Metal–organic frameworks (MOFs) have recently emerged as ideal electrode materials and precursors for electrochemical energy storage and conversion (EESC) owing to their large specific surface areas, highly tunable porosities, abundant active sites, and diversified choices of metal nodes and organic linkers. Both MOF-based and MOF-derived materials in powder form have been widely investigated in relation to their synthesis methods, structure and morphology controls, and performance advantages in targeted applications. However, to engage them for energy applications, both binders and additives would be required to form postprocessed electrodes, fundamentally eliminating some of the active sites and thus degrading the superior effects of the MOF-based/derived materials. The advancement of freestanding electrodes provides a new promising platform for MOF-based/derived materials in EESC thanks to their apparent merits, including fast electron/charge transmission and seamless contact between active materials and current collectors. Benefiting from the synergistic effect of freestanding structures and MOF-based/derived materials, outstanding electrochemical performance in EESC can be achieved, stimulating the increasing enthusiasm in recent years. This review provides a timely and comprehensive overview on the structural features and fabrication techniques of freestanding MOF-based/derived electrodes. Then, the latest advances in freestanding MOF-based/derived electrodes are summarized from electrochemical energy storage devices to electrocatalysis. Finally, insights into the currently faced challenges and further perspectives on these feasible solutions of freestanding MOF-based/derived electrodes for EESC are discussed, aiming at providing a new set of guidance to promote their further development in scale-up production and commercial applications.

Free-standing metal-organic framework (MOF) monolayers by self-assembly of polymer-grafted nanoparticles.

We report a general method for the synthesis of free-standing, self-assembled MOF monolayers (SAMMs) at an air–water interface using polymer-brush coated MOF nanoparticles. UiO-66, UiO-66-NH2, and MIL-88B-NH2 were functionalized with a catechol-bound chain-transfer agent (CTA) to graft poly(methyl methacrylate) (PMMA) from the surface of the MOF using reversible addition-fragmentation chain transfer polymerization (RAFT). The polymer-coated MOFs were self-assembled at the air–water interface into monolayer films ∼250 nm thick and capable of self-supporting at a total area of 40 mm2. Mixed-particle films were prepared through the assembly of MOF mixtures, while multilayer films were achieved through sequential transfer of the monolayers to a glass slide substrate. This method offers a modular and generalizable route to fabricate thin-films with inherent porosity and sub-micron thickness composed of a variety of MOF particles and functionalities.

Fabrication of a freestanding metal organic framework predominant hollow fiber mat and its potential applications in gas separation and catalysis

Freestanding MOF predominant hollow fiber mats are fabricatedviaan oxide-to-MOF conversion approach. The unique hollow fiber mats show promising potential in CO2adsorption, CO2/N2separation, and VOC abatement applications.

Nanopatterned metal–organic framework electrodes with improved capacitive deionization properties for highly efficient water desalination

Metal–organic frameworks (MOFs) are a family of highly porous materials with great potential to replace the dominance of porous carbon materials for capacitive deionization (CDI) application. However, MOF particles cannot be directly applied to fabricate CDI electrodes without the support of polymeric binders, which would consequently lead to the formation of a pyknotic electrode structure with poorer electrical conductivity, and lower accessible surface area, limiting their further CDI application. Herein we report the preparation of free-standing MOF electrodes by patterning MOFs nanocrystals directly on carbon cloth scaffold. Through such binder-free preparation procedure, the nanopatterned MOF electrode showcases an improved electrical conductivity and accessible surface area, giving rise to a maximum desalination capacity of 21.3 mg g−1, which is higher than those of the reported MOF electrodes and many carbon-based electrodes. This work highlights the significance of binder-dree preparation procedure on the CDI performance of MOF-based electrodes, and holds great potential for practical CDI application.

Ultrathin Metal-Organic Framework Nanosheets with Ultrahigh Loading of Single Pt Atoms for Efficient Visible-Light-Driven Photocatalytic H2 Evolution.

A surfactant-stabilized coordination strategy is used to make two-dimensional (2D) single-atom catalysts (SACs) with an ultrahigh Pt loading of 12.0 wt %, by assembly of pre-formed single Pt atom coordinated porphyrin precursors into free-standing metal-organic framework (MOF) nanosheets with an ultrathin thickness of 2.4±0.9 nm. This is the first example of 2D MOF-based SACs. Remarkably, the 2D SACs exhibit a record-high photocatalytic H2 evolution rate of 11 320 μmol g-1 h-1 via water splitting under visible light irradiation (λ>420 nm) compared with those of reported MOF-based photocatalysts. Moreover, the MOF nanosheets can be readily drop-casted onto solid substrates, forming thin films while still retaining their photocatalytic activity, which is highly desirable for practical solar H2 production.

Fabrication of MOF Thin Films at Miscible Liquid-Liquid Interface by Spray Method.

Metal-organic frameworks (MOFs) are an intriguing class of porous crystalline inorganic-organic hybrid materials. The fabrication of oriented, crystalline thin film of MOFs is expected to open novel avenues to traditional applications and to serve myriad advanced technologies. Here, a facile spray-assisted miscible liquid-liquid interface (MLLI) synthetic strategy is carried out and reported under mild condition that utilizes miscible interface for the rapid and controllable fabrication of large-area free-standing MOF thin films. The methodology can employ various metal nodes and organic ligands to yield various high quality lamellar/granulous MOF thin films at MLLI, which indicates the universality of the MLLI strategy.

Controlled Intercalation and Chemical Exfoliation of Layered Metal-Organic Frameworks Using a Chemically Labile Intercalating Agent.

Creating ordered two-dimensional (2D) metal-organic framework (MOF) nanosheets has attracted extensive interest. However, it still remains a great challenge to synthesize ultrathin 2D MOF nanosheets with controlled thickness in high yields. In this work, we demonstrate a novel intercalation and chemical exfoliation approach to obtain MOF nanosheets from intrinsically layered MOF crystals. This approach involves two steps: first, layered porphyrinic MOF crystals are intercalated with 4,4'-dipyridyl disulfide through coordination bonding with the metal nodes; subsequently, selective cleavage of the disulfide bond induces exfoliation of the intercalated MOF crystals, leading to individual freestanding MOF nanosheets. This chemical exfoliation process can proceed efficiently at room temperature to produce ultrathin (∼1 nm) 2D MOF nanosheets in ∼57% overall yield. The obtained ultrathin nanosheets exhibit efficient and far superior heterogeneous photocatalysis performance compared with the corresponding bulk MOF.

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.

Constructing free standing metal organic framework MIL-53 membrane based on anodized aluminum oxide precursor.

Metal organic framework (MOF) materials have attracted great attention due to their well-ordered and controllable pores possessing of prominent potentials for gas molecule sorption and separation performances. Organizing the MOF crystals to a continuous membrane with a certain scale will better exhibit their prominent potentials. Reports in recent years concentrate on well grown MOF membranes on specific substrates. Free standing MOF membranes could have more important applications since they are independent from the substrates. However, the method to prepare such a membrane has been a great challenge because good mechanical properties and stabilities are highly required. Here, we demonstrate a novel and facile technique for preparing the free standing membrane with a size as large as centimeter scale. The substrate we use proved itself not only a good skeleton but also an excellent precursor to fulfill the reaction. This kind of membrane owns a strong mechanical strength, based on the fact that it is much thinner than the composite membranes grown on substrates and it could exhibit good property of gas separation.

Polymer‐Supported and Free‐Standing Metal–Organic Framework Membrane

Thrive on a polymer: A polymer-supported method is developed and used to prepare well-intergrown and free-standing metal–organic framework (MOF) membranes. By using this method, a stainless steel net/PMMA–PMAA-supported HKUST-1 membrane was fabricated and applied in gas separation. Results indicated that such a membrane has a high separation factor to H2/N2, H2/CH4, H2/CO2 gas mixtures (see figure).

Fabrication of free‐standing ultrathin films of porous metal‐organic frameworks by liquid‐phase epitaxy and subsequent delamination

AbstractIn this work we describe an approach to fabricate MOF (metal‐organic framework) platelets with predefined shapes and well‐defined thicknesses. The process is based on growing thin, highly oriented and ordered MOF films by selective liquid‐phase epitaxy on prestructured organic substrates. In a second step, the MOF platelets are delaminated employing a lift‐off process. The freestanding MOF platelets were characterized with TEM and SEM, which showed no evidence of fracture or deformation of the delaminated shapes. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)