Pristine Metal-organic Frameworks Research Articles

Transforming an insulating metal–organic framework into electrically conducting metal–organic framework⊃conducting polymer composites

Owing to their diverse potentials to help advance modern electronics and energy technologies, electrically conducting metal–organic frameworks (MOFs) have emerged as one of the most coveted functional materials within the past decade. The key to developing electrically conducting MOFs is to equip them with mobile charge carriers and facilitate long-range charge movement. Circumventing the challenges and unpredictability associated with the construction of intrinsically conducting MOFs, herein, we have converted a structurally robust and porous but intrinsically insulating Zn-dpzNDI MOF based on an electron deficient dipyrazolate-naphthalenediimide (dpzNDI) ligand into electrically conducting MOF⊃conducting-polymer (MOF⊃CP) composites via oxidative polymerization of preloaded redox-active 3,4-ethylenedioxythiophene (EDOT) and pyrrole (Py) monomers to corresponding PEDOT and PPy polymers, which are well-known hole-transporters. After monomer loading and in-situ polymerization, the resulting MOF⊃CP composites remained crystalline but became less porous, suggesting that the amorphous CP chains were mostly confined to the MOF cavities. The presence of CPs was confirmed by infra-red (IR), diffuse-reflectance UV–Vis–NIR and STEM-EDX analyses. Whereas the pristine MOF had immeasurably low conductivity (σ < 10−12 S/m), the MOF⊃PEDOT and MOF⊃PPy composites displayed significantly higher electrical conductivity: 1.8 × 10−5 and 2.5 × 10−3 S/m, respectively. Thus, we have transformed an intrinsically insulating MOF into electrically conducting MOF⊃CP via in-situ oxidative polymerization of electron-rich monomers, a versatile strategy that could be adopted to engineer this much coveted but elusive electronic property practically in any porous MOFs.

Charge Separation by Creating Band Bending in Metal-Organic Frameworks for Improved Photocatalytic Hydrogen Evolution.

Metal-organic frameworks (MOFs) have been intensively studied as a class of semiconductor-like materials in photocatalysis. However, band bending, which plays a crucial role in semiconductor photocatalysis, has not yet been demonstrated in MOF photocatalysts. Herein, a representative MOF, MIL-125-NH2 , is integrated with the metal oxides (MoO3 and V2 O5 ) that feature appropriate work functions and energy levels to afford the corresponding MOF composites. Surface photovoltage results demonstrate band bending in the MOF composites, which gives rise to the built-in electric field of MIL-125-NH2 , boosting the charge separation. As a result, the MOF composites present 56 and 42 times higher activities, respectively, compared to the pristine MOF for photocatalytic H2 production. Upon depositing Pt onto the MOF, ∼6 times higher activity is achieved. This work illustrates band bending of MOFs for the first time, supporting their semiconductor-like nature, which would greatly promote MOF photocatalysis.

Configuration of hetero-framework via integrating MOF and triazine-containing COF for charge-transfer promotion in photocatalytic CO2 reduction

Photocatalytic CO2 reduction into high value-added chemicals is of considerable prospect for tackling quandaries in energy scarcity. One potent strategy to raise up its conversion efficacy is engineering heterostructure by virtue of tunable substituent in organic structure to facilitate the charge delivery within the photocatalyst. Herein, three metal organic frameworks (MOFs) built by the same ligand but different metal ions, are in-situ grown onto a triazine-containing covalent framework (COF), TP-TA, resulting in three hetero-frameworks, namely, In-MOF@TP-TA, Zr-MOF@TP-TA and Fe-MOF@TP-TA (In-MOF, Zr-MOF and Fe-MOF are abbreviations for NH2-MIL-68(In), NH2-UiO-66(Zr) and NH2-MIL-101(Fe) MOFs, respectively). Owning to the configured type-II heterojunctions that apparently suppress the charge recombination, the light-driven CO2 reduction reaction (CO2RR) manipulated by the three hybrids behave higher catalytic performance by contrast to the pristine MOFs or COF. Amid the three heterostructure, In-MOF@TP-TA provides the optimal catalytic activity, giving the CO and CH4 production rate as 25 and 11.67 μmol·g−1·h−1, respectively. In addition, their photocatalytic activities follow the order as In-MOF@TP-TA > Zr-MOF@TP-TA > Fe-MOF@TP-TA, consisting with the CB potentials of the MOF components (from negative to positive) and in turn certifying the charge flowing orientation within the type-II heterostructure. Our work broadens the rational design for the covalently integrated heterostructures as well as their melioration in photocatalytic CO2RR.

Homologous MXene‐Derived Electrodes for Potassium‐Ion Full Batteries

AbstractThe development of potassium‐ion battery (PIB) electrode materials is critical for promoting their use in next‐generation energy storage systems. Although metal–organic frameworks (MOFs) are appealing electrode materials, their performance in PIBs remains unsatisfactory. The low K+ adsorption energy (ΔEa) on the saturated coordination of MOFs can explain the limited capacity. Herein, MXene‐derived MOF nodes (NMD‐MOF) are unlocked and used as anodes in PIB. The NMD‐MOF anode exhibits substantially increased capacity (250 mA h g−1 at 0.05 A g−1), as well as good rate‐performance and excellent capacity retention. Density functional theory calculations reveal that the ΔEa at unlocked node sites is significantly higher than at intact node sites of the pristine MOF. Furthermore, the NMD‐MOF anode and homologous MXene‐derived K+‐intercalated vanadium oxide (MD‐KVO) cathode combine to assemble a PIB, which delivers an encouraging capacity of 63 mA h g−1 at 50 mA g−1 and high energy (143 Wh kg−1) and power density (440 W kg−1). The fabrication of MXene‐derived electrode materials and the unlocking node strategy for binding site activation may spur further research into highly active electrode materials for energy storage devices.

The role of MOF based nanocomposites in the detection of phenolic compounds for environmental remediation- A review

Phenolic compounds would be the emerging pollutant by 2050, because of their wide spread applicability in daily life and therefore the adoption of suitable detection methods in which identification and separation of isomers is highly desirable. Owing to the fascinating features, Metal-organic framework (MOF), a class of reticular materials holds a large surface area with tunable shape and adjustable porosity will provide strong interaction with analytes through abundant functional groups resulting in high selectivity towards electrochemical determination of phenolic isomers. Nevertheless, the sensing performance can still be further improved by building MOF network (intrinsic resistance) with functional (conducting) materials, resulting in MOF based nanocomposite. Herein, this review provides the summary of MOF based nanocomposites for electrochemical sensing of phenolic compounds developed from 2015. In this review, we discussed the demerits of pristine MOF as electrode materials, and the requirement of new class of MOF with functional materials such as nanomaterials, carbon nanotubes, graphene and MXene. The history and evolution of MOF nanocomposite-based materials are discussed and also featured the impressive physical and chemical properties. Besides this review discusses the factors influencing the conducting pathway and mass transport of MOF based nanocomposite for enhanced sensing performance of phenolic compounds with suitable mechanistic illustrations. Finally, the major challenges governing the determination of phenolic compounds and the future advancements required for the development of MOF based electrodes for various applications are highlighted.

Utilizing Metal-Organic Frameworks to Achieve High-Efficiency CO2 Electroreduction

Electrochemical CO2 reduction reaction (CO2RR) is the key part of clean energy generation and utilization, which has great potential to help the world to reach the carbon-neutral energy cycle in the future. In line with the development of metal-organic frameworks (MOFs) with the large specific area and considerable porosity in the past two decades, some of the MOF-based electrocatalysts have shown superior ability to accelerate CO2RR. However, regarding such a significant CO2RR process, some critical disadvantages, including inferior robustness, low yield and selectivity, and idealistic working environment, are still required to be concentrated on. Herein, a comprehensive outline of the reaction mechanism of CO2 conversion and rational synthesis of the state-of-the-art pristine MOFs is given. Further, recent progress of pristine MOF-based electrocatalysts in CO2RR is systematically summarized. Lastly, the major limitations and future opportunities in MOF electrocatalysis for CO2RR are presented.

Indium based metal-organic framework/carbon nanotubes composite as a template for In2O3 porous hexagonal prisms/carbon nanotubes hybrid structure and their application as promising super-capacitive electrodes

The development of porous nano/microsctructural materials, has a major focus to provide impressive electrochemical features in energy storage technology. Metal-organic frameworks (MOFs) are emerging as talented materials in supercapacitors due to their intrinsic porosity properties, which can be well-controlled through molecular engineering. As smart pathways, utilizing MOFs as templates for generating carbon, metal oxide, and hydroxides, etc. materials, or hybridized MOFs have shown to be quite beneficial. MOF Hybridizing with carbon nanotubes helps considerably eliminate the defects of pristine MOFs and enlargement the capacity to a certain extent which accounts for the increment of conductivity and electron/ion transports capability. Furthermore, adding hierarchy to the MOFs structure using incorporating of carbon nanotubes can improve composite stability. Herein we synthesized interweaved MIL 68 (In) with multi wall carbon nanotubes (MIL 68 (In)-CNTs) composite and also we employ it as a template for In2O3 porous hexagonal prisms hybridized with carbon nanotubes (In2O3 PHP-CNTs). Facile solvothermal method and simple annealing treatment are used for these approaches. We investigate their electrochemical performance using a three-electrode cell system. Using cyclic voltammetry (CV) technique, MIL 68 (In)-CNTs electrode modifier exhibit the superior capacity of 601 Fg−1 at the speed rate of 5 mVs−1. Besides, we assemble the symmetric supercapacitor (SSC) device via the MIL 68 (In)-CNTs nano platform. The synergistic effect between MIL 68 (In) and CNTs brings forth to an improved electrochemical performance with high energy and power density values of 13.3 Whkg−1 and 300 Wkg−1, respectively. This device shows high specific capacitance of 314 Fg−1 at 0.5 Ag−1, good rate capability and excellent cycling stability of 95% after 4000 cycles.

High Protonic Conductivity of Three Highly Stable Nanoscale Hafnium(IV) Metal-Organic Frameworks and Their Imidazole-Loaded Products.

Attracted by the exceptional structural rigidity and inherent porous structures of the Hf-based metal-organic frameworks (MOFs), we adopted a rapid synthesis approach to preparing three nanoscale MOFs, Hf-UiO-66 (1), Hf-UiO-66-(OH)2 (2), and Hf-UiO-66-NH2 (3), and systematically explored the water-assisted proton conductivities of the original ones and the post-modified products. Interestingly, the proton conductivities (σ) of all three MOFs exhibit significant temperature and humidity dependence. At 98% RH and 100 °C, their optimal σ values can reach up to 10-3 S·cm-1. Consequently, imidazole units are loaded into 1-3 to obtain related MOFs, Im@1, Im@2, and Im@3, and the σ values of the imidazole-loaded products are boosted to 10-2 S·cm-1. Note that these modifications not only do not change the frameworks of the pristine MOFs but also do not affect their high chemical and water stability. The proton-conductive mechanisms of these MOFs before and after modification have been thoroughly discussed based on structural analyses, N2 and H2O vapor adsorptions, and activation energy values. The excellent structural stability as well as the durability and stability of their proton conduction ability indicate that these MOFs can be used in the field of fuel cells and so on.

Bromine Vapor Induced Continuous p- to n-Type Conversion of a Semiconductive Metal-Organic Framework Cu[Cu(pdt)2

Guest-promoted modulation of the electronic states in metal-organic frameworks (MOFs) has brought about a new field of interdisciplinary research, including host-guest chemistry and solid-state physics. Although there are dozens of studies on guest-promoted enhancement of the electrical conductivity properties, including stoichiometry, conductive carriers and structure-property relationships have been scarcely studied in detail. Herein, we studied the effects of continuous and controlled bromine vapor doping on structural, optical, thermoelectric, and semiconducting properties of Cu[Cu(pdt)2] (pdt = 2,3-pyrazinedithiolate) as a function of bromine stoichiometry. We demonstrated that the same material could act as both p- and n-type semiconductors by tuning the stoichiometry of Br doped in Brx@Cu[Cu(pdt)2], and a change in the charge-carrier type from holes in pristine MOF to electrons upon bromine vapor doping was observed. Bromine molecules acted as an oxidant, causing the selective oxidation of [CuII(pdt)2] in the host framework. In addition, a redox hopping pathway between the partially oxidized CuII/CuIII center contributed to the enhancement of the electrical conductivity of the MOF.

Recent progress of functional metal–organic framework materials for water treatment using sulfate radicals

Human health is being threatened by the ever-increasing water pollution. Sulfate radical (SO4•−)-based advanced oxidation processes (SR-AOPs) are rapidly being developed and gaining considerable attention due to their high oxidation potential and selectivity as a way to purify water by degrading organic contaminants in it. Among the catalytic materials that can activate the precursor to generate SO4•−, metal–organic frameworks (MOFs) are the most promising heterogeneous catalytic material in SR-AOPs because of their various structure possibilities, large surface area, ordered porous structure, and regular activation sites. Herein, an in-depth overview of MOFs and their derivatives for water purification with SR-AOPs is provided. The latest studies on pristine MOFs, MOF composites, and MOF derivatives (metal oxides, metal–carbon hybrids, and carbon materials) are summarized. The mechanisms of decomposition of pollutants in water via radical and non-radical pathways are also discussed. This review suggests future research directions for water purification through MOF-based SR-AOP.

The Decisive Role of Spin States and Spin Coupling in Dictating Selective O2 Adsorption in Chromium(II) Metal-Organic Frameworks.

The coordinatively unsaturated chromium(II)-based Cr3 [(Cr4 Cl)3 (BTT)8 ]2 (Cr-BTT; BTT3- =1,3,5-benzenetristetrazolate) metal-organic framework (MOF) has been shown to exhibit exceptional selectivity towards adsorption of O2 over N2 /H2 . Using periodic density functional theory (DFT) calculations, we attempted to decipher the origin of this puzzling selectivity. By computing and analyzing the magnetic exchange coupling, binding energies, the partial density of states (pDOS), and adsorption isotherms for the pristine and gas-bound MOFs [(Cr4 (X)4 Cl)3 (BTT)8 ]3- (X=O2 , N2 , and H2 ), we unequivocally established the role of spin states and spin coupling in controlling the gas selectivity. The computed geometries and gas adsorption isotherms are consistent with the earlier experiments. The binding of O2 to the MOF follows an electron-transfer mechanism resulting in a CrIII superoxo species (O2 .- ) with a very strong antiferromagnetic coupling between the two centers, whereas N2 /H2 are found to weakly interact with the metal center and hence only slightly perturb the associated coupling constants. Although the gas-bound and unbound MOFs have an S=0 ground state (GS), the nature of spin the configurations and the associated magnetic exchanges are dramatically different. The binding energy and the number of oxygen molecules that can favorably bind to the Cr center were found to vary with respect to the spin state, with a significant energy margin (47.6 kJ mol-1 ). This study offers a hitherto unknown strategy of using spin state/spin couplings to control gas adsorption selectivity in MOFs.

Nickel-Based Metal-Organic Frameworks as Electrocatalysts for the Oxygen Evolution Reaction (OER).

The exploration of earth-abundant electrocatalysts with high performance for the oxygen evolution reaction (OER) is eminently desirable and remains a significant challenge. The composite of the metal-organic framework (MOF) Ni10Co-BTC (BTC = 1,3,5-benzenetricarboxylate) and the highly conductive carbon material ketjenblack (KB) could be easily obtained from the MOF synthesis in the presence of KB in a one-step solvothermal reaction. The composite and the pristine MOF perform better than commercially available Ni/NiO nanoparticles under the same conditions for the OER. Activation of the nickel-cobalt clusters from the MOF can be seen under the applied anodic potential, which steadily boosts the OER performance. Ni10Co-BTC and Ni10Co-BTC/KB are used as sacrificial agents and undergo structural changes during electrochemical measurements, the stabilized materials show good OER performances.

Metal-organic-framework based catalyst for hydrogen production: Progress and perspectives

Fossil fuel shortage and global warming have inspired scientists to search for alternative energy sources which are green, renewable, and sustainable. Hydrogen formed from water splitting has been considered as one of the most promising candidates to replace traditional fuels due to its low production cost and zero-emission. Metal-organic frameworks (MOFs) have been considered as potential catalysts for hydrogen production from water splitting account for their flexible structure, ultra-large surface area, and chemical component diversification. This paper reviews different kinds of MOF-related electrocatalysts, involving metals, metal oxides, single atoms, metal phosphides, metal nitrides, and metal dichalcogenides for hydrogen production. Also, MOF-based photocatalysts consisting of pristine MOFs, MOFs as supporters, and MOF-derived heterojunction architectures are reviewed. The finding of MOF-based catalysts for hydrogen generation is summarized. The pros and cons of different MOF-based materials as catalysts for water splitting are discussed. Finally, current challenges and the potential developments of these unique materials as catalysts are also provided.

Pyrrolic N wrapping strategy to maximize the number of single-atomic Fe-Nx sites for oxygen reduction reaction

Iron-nitrogen-carbon (Fe–N–C) catalysts with a representative single-atomic structure are promising platinum group metal-free catalysts for the oxygen reduction reaction (ORR) as they exhibit comparable activity to commercial catalysts. To enhance the ORR activity of Fe–N–C catalysts, the number of single Fe atoms coordinated N (Fe-Nx) should be maximized. In this study, a strategy is devised to increase the number of Fe-Nx sites using electrostatic interactions between electronegative pyrrolic-N and electropositive Fe ions. Pyrrolic N-rich carbon (pNC) is dispersed on the surface of the metal-organic framework (MOF) to form composite supports ([email protected]). Owing to the well-dispersed pNC and electrostatic interactions, the number of Fe-Nx sites on the [email protected] hollow carbon framework (Fe/[email protected]) increases dramatically compared to that on the pristine MOF (Fe/HCF). The original shape of the Fe-absorbed MOF is maintained by the conversion of pNC into carbon layer within the framework by pyrolysis at 1000 °C even though pure Fe-absorbed MOF collapses. An anion exchange membrane fuel cell (AEMFC) with Fe/[email protected] is fabricated, and it shows a high current density of 437 mA cm−2 at 0.6 V and a power density of 343 mW cm−2. This performance suggests that the synthesized catalysts are excellent potential cathodic catalysts for AEMFCs.

Cerium-based metal–organic framework as an electrocatalyst for the reductive detection of dopamine

Thin films of a chemically stable and electrochemically active cerium(IV)-based metal–organic framework (MOF), Ce-MOF-808, are capable of electrocatalyzing the reduction of dopaquinone (DAQ) generated by the electrochemical pre-oxidation of dopamine (DA) present in 3-(N-morpholino)propanesulfonic acid buffer solutions. As the first study reporting the use of pristine cerium-based MOFs in electrochemical DA detection, the stability of the MOF during electrochemical operations, optimized thin-film loading, and the electrocatalytic mechanism are investigated. By utilizing the cathodic Faradaic responses for the reduction of the electrochemically pre-generated DAQ as a probe to detect DA in the electrolyte, the interference signals from the commonly coexisting ascorbic acid and uric acid can be effectively reduced.

Metal organic framework/polyelectrolyte composites for water vapor sorption applications.

Metal-organic framework (MOF) core particles of MIL-101(Cr), aluminum fumarate (Basolite® A520), MIL-53-TDC, zirconium fumarate, and UiO-66 were modified by adsorption of thin polyelectrolyte (PE)-based shells without deterioration of their crystal structure. By applying different PEs and depositing a single layer (MOF/PE) or one to three layer-by-layer assembled bilayers (MOF/LbL), the mass percent of shell material in the composite was varied from 0.6-2.5% to 50%. Under a constant relative pressure of water vapor, the moisture uptake by a MOF/PE and a MOF/LbL is rather comparable with its S-shaped curvature to that of pristine MOFs. The relevant differences, such as a shift of the ascending adsorption part to lower/higher relative pressure or an increase/decrease in water uptake in selected regions, are associated with the core-shell structure and related to the morphological changes of the MOF powders. The hydrophilic surface promotes the formation of liquid menisci at the points of contact between particles and accelerates the moisture uptake and loss. A decrease in water sorption under an atmosphere with high humidity by some composites can be associated with the inhibition of liquid water condensation by the more hydrophobic shells.

Metal-organic frameworks for advanced transducer based gas sensors: review and perspectives.

The development of gas sensing devices to detect environmentally toxic, hazardous, and volatile organic compounds (VOCs) has witnessed a surge of immense interest over the past few decades, motivated mainly by the significant progress in technological advancements in the gas sensing field. A great deal of research has been dedicated to developing robust, cost-effective, and miniaturized gas sensing platforms with high efficiency. Compared to conventional metal-oxide based gas sensing materials, metal–organic frameworks (MOFs) have garnered tremendous attention in a variety of fields, including the gas sensing field, due to their fascinating features such as high adsorption sites for gas molecules, high porosity, tunable morphologies, structural diversities, and ability of room temperature (RT) sensing. This review summarizes the current advancement in various pristine MOF materials and their composites for different electrical transducer-based gas sensing applications. The review begins with a discussion on the overview of gas sensors, the significance of MOFs, and their scope in the gas sensing field. Next, gas sensing applications are divided into four categories based on different advanced transducers: chemiresistive, capacitive, quartz crystal microbalance (QCM), and organic field-effect transistor (OFET) based gas sensors. Their fundamental concepts, gas sensing ability towards various gases, sensing mechanisms, and their advantages and disadvantages are discussed. Finally, this review is concluded with a summary, existing challenges, and future perspectives.

Synthesis of Tostadas‐Shaped Metal‐Organic Frameworks for Remitting Capacity Fading of Li‐Ion Batteries

AbstractElectrode design strategies that aim to increase the electrochemical performance of Li‐ion batteries (LIBs) play a key role in tapping into the power of the energy transformations involved. Metal‐organic frameworks (MOFs) have attracted scientific interest as electrode materials for LIBs, while the utilization of pristine MOFs is hindered by limited conductivity and stability, partly due to their lack of hierarchically structured pores. Herein a hydrothermal‐mechanical synthesis is reported by combining the one‐pot chemical fabrication of Ni3(2,3,6,7,10,11‐hexaiminotriphenylene)2 sheets and particles, and the mechanical assembly of these building blocks to improve electrical conductivity is also described. The as‐prepared ensemble (denoted as NHM) exhibits a Tostadas‐shaped structure with enriched ultramicropores and micropores. The charge‐discharge profile of NHM gives a superior reversible capacity of 1280 mA h g−1 after 100 cycles at the rate of 0.1 A g−1, surpassing the state‐of‐art pristine MOFs‐based anodes. Moreover, NHM is capable of maintaining 392 mA h g−1 at 1 A g−1 after 1000 cycles, the completion of a stability test in coin cell‐powered light emitting diodes further visualizes the remitted capacity fading of NHM. This work breaks through the limitation of capacity for pristine MOFs, providing a new pathway for achieving better energy conversion and storage.

Applications of Metal-Organic Frameworks in Water Treatment: A Review.

The ever-expanding scale of industry and agriculture has led to the gradual increase of pollutants (e.g., heavy metal ions, synthetic dyes, and antibiotics) in water resources, and the ecology and wastewater are grave problems that need to be solved urgently and has attracted widespread attention from the research community and industry in recent years. Metal-organic frameworks (MOFs) are a type of organic-inorganic hybrid material with a distinctive 3D network crystal structure. Lately, MOFs have made striking progress in the fields of adsorption, catalytic degradation, and biomedicine on account of their large specific surface and well-developed pore structure. This review summarizes the latest research achievements in the preparation of pristine MOFs, MOF composites, and MOF derivatives for various applications including the removal of heavy metal ions, organic dyes, and other harmful substances in sewage. Furthermore, the working mechanisms of utilizing adsorption, photocatalytic degradation, and membrane separation technologies are also briefly described for specific pollutants removal from sewage. It is expected that this review will provide inspiration and references for the synthesis of pristine MOFs as well as their composites and derivatives with excellent water treatment performance.

Molecular Stabilization of Sub-Nanometer Cu Clusters for Selective CO2 Electromethanation.

Electrochemical CO2 methanation powered by renewable electricity provides a promising approach to utilizing CO2 in the form of a high-energy-density, clean fuel. Cu nanoclusters have been predicted by theoretical calculations to improve methane selectivity. Direct electrochemical reduction of Cu-based metal-organic frameworks (MOFs) results in large-size Cu nanoparticles which favor multi-carbon products. This study concerns an electrochemical oxidation-reduction method to prepare Cu clusters from MOFs. The derived Cu clusters exhibit a faradaic efficiency of 51.2 % for CH4 with a partial current density of >150 mA cm-2 . High-resolution microscopy, in situ X-ray absorption spectroscopy, in situ Raman spectroscopy, and a range of ex situ spectroscopies indicate that the distinctive CH4 selectivity is due to the sub-nanometer size of the derived materials, as well as stabilization of the clusters by residual ligands of the pristine MOF. This work offers a new insight into steering product selectivity of Cu by an electrochemical processing method.