Bulk Metal-organic Frameworks Research Articles

Electric‐Field Assisted In Situ Hydrolysis of Bulk Metal–Organic Frameworks (MOFs) into Ultrathin Metal Oxyhydroxide Nanosheets for Efficient Oxygen Evolution

AbstractFacile preparation of low‐cost electrocatalysts for efficient oxygen evolution reaction (OER) remains a big challenge. Herein, a novel strategy for ultrafast (20 s) transformation of bulk metal–organic frameworks (MOFs) into ultrathin metal oxyhydroxide nanosheets for efficient OER has been developed. For two isomeric MOFs (FJI‐H25Fe and FJI‐H25FeCo), only the metastable FJI‐H25FeCo bulk can immediately transform into FeCo‐oxyhydroxides nanosheets through electric‐field assisted hydrolysis. The potential evolution process from MOF bulk to FeCo‐oxyhydroxides nanosheets has been investigated in detail. The as‐made nanosheets exhibit excellent OER performances, showing an extremely low overpotential of 231 mV at the current density of 10 mA cm−2, a relatively small Tafel slope of 42 mV dec−1, and long‐term durability of at least 30 h. This work not only provides a novel strategy for facile preparation of low‐cost and efficient OER electrocatalysts, but also represents a new way for preparation of metal oxyhydroxides nanosheets with good crystallinity and morphology, and a fresh method for mild synthesis of nanosized derivatives from MOF materials.

Electric-Field Assisted In Situ Hydrolysis of Bulk Metal-Organic Frameworks (MOFs) into Ultrathin Metal Oxyhydroxide Nanosheets for Efficient Oxygen Evolution.

Facile preparation of low-cost electrocatalysts for efficient oxygen evolution reaction (OER) remains a big challenge. Herein, a novel strategy for ultrafast (20 s) transformation of bulk metal-organic frameworks (MOFs) into ultrathin metal oxyhydroxide nanosheets for efficient OER has been developed. For two isomeric MOFs (FJI-H25Fe and FJI-H25FeCo), only the metastable FJI-H25FeCo bulk can immediately transform into FeCo-oxyhydroxides nanosheets through electric-field assisted hydrolysis. The potential evolution process from MOF bulk to FeCo-oxyhydroxides nanosheets has been investigated in detail. The as-made nanosheets exhibit excellent OER performances, showing an extremely low overpotential of 231 mV at the current density of 10 mA cm-2 , a relatively small Tafel slope of 42 mV dec-1 , and long-term durability of at least 30 h. This work not only provides a novel strategy for facile preparation of low-cost and efficient OER electrocatalysts, but also represents a new way for preparation of metal oxyhydroxides nanosheets with good crystallinity and morphology, and a fresh method for mild synthesis of nanosized derivatives from MOF materials.

Synthesis of Zinc-Terephthalate Metal Organic Frameworksand Their Application to Redox Batteries

Metal-organic frameworks (MOFs) made of inorganic ions and organic linkers are attracting attention for their unique properties because of their widely open porous crystal structure. We have recently succeeded in microwave-assisted hydrothermal synthesis of Zn-Terephthalate (TPA) MOF particles [1]. Several different types such as Zn3(OH)4(TPA)･6H2O (Type I) and Zn4(OH)6(TPA) (Type II) in layered structures were obtained depending on pH of the precursor solution. They exhibited proton-selective reversible redox reactions to indicate possibilities for membrane-free redox batteries [2]. However, still little is known about their electrochemical behavior (such as redox potential, coulombic capacity, cycle stability, and their difference between I and II). In this study, we have undertaken quantitative electrochemical analysis at the Zn-TPA MOF electrodes to evaluate their applicability to batteries. The precursor solution containing 0.1 M zinc acetate and 0.05 M TPA was basified to pH 7.0 and 5.9 to yield Types I and II, respectively. White precipitates after hydrothermal reaction at 150℃ and 30 min. under microwave irradiation were centrifugally collected. 35wt% of MOF pastes were prepared in 2-butanol containing acetylacetone, coated onto an FTO glass substrate to fabricate porous electrodes with a projected area of 2 cm2. While the film thickness for both Types I and II was about 10 µm, the density was 0.25 and 0.45 g cm-3, corresponding to porosity of 90 and 82%, respectively, assuming the density of bulk MOFs of Types I and II to be the same as that of Zn3(OH)4(TPA) reported earlier (2.538 g cm-3) [3]. The redox potentials of Types I and II were -1.16 and -1.08 V vs. Ag/AgCl in a neutral KCl aq. as judged from redox peaks in cyclic voltammograms (CVs, Fig. 1). Although Type II initially exhibited the higher redox activity than Type I, it gradually decreased over the repetition of CVs, whereas coulombic reversibility always was around 100% (Fig. 1). Thus, the redox active fraction calculated by assuming redox reactions as, Zn3(OH)4(TPA)･6H2O + 2H+ + 2e- ⇌ Zn3(OH)4(TPAH2)･6H2O (1) Zn4(OH)6(TPA) + 2H+ + 2e- ⇌ Zn4(OH)6(TPAH2) (2)was about 0.22% for Type I and stayed unchanged, whereas it dropped from 0.89% (initial) to 0.50% (100th cycle) for Type II. In fact, the XRD pattern of Type II electrode after 100 CV cycles revealed its conversion into Type I, described as, Zn4(OH)6(TPA) + 2H+ + 2e- + 6H2O → Zn3(OH)4(TPAH2)･6H2O + Zn(OH)2 (3)Although Type I appears to be the better electrode material for its more negative potential and higher stability, the absolute redox capacity is so small because of its small redox active fraction. That may be caused by poor necking of the particles as suggested from its extremely high porosity. Therefore, control of particle size, conditions of paste preparation and film fabrication need to be reviewed to improve its redox activity.[1] Y. Hirai et al., Microsys. Technol., 24, 699–708 (2017)[2] Y. Hirai et al., ECS Trans., 88(1):259-268 (2018)[3] A. Carton et al., Solid State Sciences., 8 (2006) 958–963 Figure 1

Insights into the binding mechanism of 2D copper-tetrakis-(4-carboxyphenyl)-porphyrin metal-organic framework nanosheets with Rhodamine B: Spectroscopic and thermodynamics studies

Metal-organic frameworks (MOFs) of copper-tetrakis-(4-carboxyphenyl)-porphyrin (CuTCPP) were prepared by a solvothermal route. The two-dimensional (2D) ultrathin nanosheets with larger specific surface area show higher adsorption capacity for rhodamine B (RhB), compared with the bulk MOF of CuTCPP. The binding mechanism of the 2D nanosheets with RhB was investigated based on spectroscopic methods. The results reveal that the fluorescence of RhB is efficiently quenched by 2D CuTCPP nanosheets, and the quenching mechanism is totally the static quenching by forming a ground state RhB-CuTCPP nanosheets complex with an apparent association constant of 3.953 × 103 M−1. The interaction between RhB and CuTCPP nanosheets leads to the H-aggregate of RhB in the complex. Higher-order aggregate may also be involved in the complexation. Thermodynamic studies demonstrate that RhB-CuTCPP nanosheets interaction is a spontaneous process and driven by electrostatic and hydrophobic forces. The 2D CuTCPP nanosheets can be regenerated from the complex and used repeatedly.

Methanol and Humidity Capacitive Sensors Based on Thin Films of MOF Nanoparticles.

The successful development of modern gas sensing technologies requires high sensitivity and selectivity coupled to cost effectiveness, which implies the necessity to miniaturize devices while reducing the amount of sensing material. The appealing alternative of integrating nanoparticles of a porous metal-organic framework (MOF) onto capacitive sensors based on interdigitated electrode (IDE) chips is presented. We report the deposition of MIL-96(Al) MOF thin films via the Langmuir-Blodgett (LB) method on the IDE chips, which allowed the study of their gas/vapor sensing properties. First, sorption studies of several organic vapors like methanol, toluene, chloroform, etc. were conducted on bulk MOF. The sorption data revealed that MIL-96(Al) presents high affinity toward water and methanol. Later on, ordered LB monolayer films of MIL-96(Al) particles of ∼200 nm were successfully deposited onto IDE chips with homogeneous coverage of the surface in comparison to conventional thin film fabrication techniques such as drop-casting. The sensing tests showed that MOF LB films were selective for water and methanol, and short response/recovery times were achieved. Finally, chemical vapor deposition (CVD) of a porous thin film of Parylene C (thickness ∼250-300 nm) was performed on top of the MOF LB films to fabricate a thin selective layer. The sensing results showed an increase in the water selectivity and sensitivity, while those of methanol showed a huge decrease. These results prove the feasibility of the LB technique for the fabrication of ordered MOF thin films onto IDE chips using very small MOF quantities.

Defect Creation in Surface-Mounted Metal-Organic Framework Thin Films.

Defect engineering is a strategy for tailoring the properties of metal-organic frameworks (MOFs). Plenty of efforts have been devoted to study the defect chemistry and structures of bulk MOFs; however, the reported example of a defect-engineered surface-mounted MOF (SURMOF) thin film is rare. In this work, defects were incorporated in SURMOF thin films by using defect-generating linkers and taking advantage of the liquid-phase stepwise epitaxial layer-by-layer growth (LBL). Two methods based on the LBL, named mixing method and alternating method, are proposed for incorporating defects in the prototypical SURMOF HKUST-1 by partially substituting the parent H3btc (benzene-1,3,5-tricarboxylic acid) linker with a set of defect-generating linkers H2ip (isophthalic acid), H2OH-ip (5-hydroxyisophthalic acid), and H2pydc (3,5-pyridinedicarboxylic acid). The crystallinity and phase purity of the obtained "defected" SURMOFs were confirmed by X-ray diffraction, infrared reflection absorption spectroscopy, and Raman spectroscopy. The incorporation of the defect-generating linkers and the types of induced defects were characterized by ultraviolet-visible spectroscopy, time-of-flight secondary ion mass spectrometry, methanol adsorption, scanning electron microscopy, and 1H nuclear magnetic resonance spectroscopy (after digestion of the samples). These two methods provide avenues for controlling the defect formation in MOF thin films.

MXene Derived Metal-Organic Frameworks.

Synthesis of nanoscale metal-organic frameworks (MOFs) is a highly challenging task because conventional soluble metal salt precursors are not easy to manipulate spatially, thus normally leading to bulk MOFs. In the present work, V2CTx MXene is demonstrated for the first time as a metal precursor to fabricate two-dimensional (2D) MOF nanosheets, whose thickness (6 to 18 nm) can be tuned by varying the reaction temperature. The highly electronegative surface atoms of MXene and sufficient accessible attacking sites for ligands are responsible for the evolution of 2D MOF nanosheets. Moreover, highly oriented and smooth MOF thin films have been grown based on these nanosheets using a convenient spin coating process. With the impregnation of nonvolatile H3PO4, the MOF thin film exhibits a proton-conducting property. This study demonstrates that high-quality 2D MOF sheets and thin films are enabled by 2D MXene precursors. We believe that the high-quality MOF films prepared in this study pave the way for many device applications.

Ultra-thin metal-organic framework nanoribbons.

Structure engineering of metal-organic frameworks (MOFs) at the nanometer scale is attracting increasing interest due to their unique properties and new functions that normally cannot be achieved in bulk MOF crystals. Here, we report the preparation of ultra-thin MOF nanoribbons (NRBs) by using metal-hydroxide nanostructures as the precursors. Importantly, this general method can be used to synthesize various kinds of ultra-thin MOF NRBs, such as MBDC (M = Co, Ni; BDC = 1,4-benzenedicarboxylate), NiCoBDC, CoTCPP (TCPP = tetrakis(4-carboxyphenyl)porphyrin) and MIL-53(Al) NRBs. As a proof-of-concept application, the as-prepared ultra-thin CoBDC NRBs have been successfully used as a fluorescent sensing platform for DNA detection, which exhibited excellent sensitivity and selectivity. The present strategy might open an avenue to prepare MOF nanomaterials with new structures and unique properties for various promising applications.

N-Doped Carbon Aerogel Derived from a Metal-Organic Framework Foam as an Efficient Electrocatalyst for Oxygen Reduction.

Metal-organic frameworks (MOFs) are promising alternative precursors for the fabrication of heteroatom-doped carbon materials for energy storage and conversion. However, the direct pyrolysis of bulk MOFs usually gives microporous carbonaceous materials, which significantly hinder the mass transportation and the accessibility of active sites. Herein, N-doped carbon aerogels with hierarchical micro-, meso-, and macropores were fabricated through one-step pyrolysis of zeolitic imidazolate framework-8/carboxymethylcellulose composite gel. Owing to the hierarchical porosity, high specific surface area, favorable conductivity, excellent thermal and chemical stability, the as-prepared N-doped carbon aerogel exhibits excellent oxygen reduction reaction (ORR) activity, long-term durability, and good methanol tolerance in alkaline medium. This work thus provides a new way to fabricate new types of MOF-derived carbon aerogels for various applications.

Broad Mid-Infrared Luminescence in a Metal-Organic Framework Glass.

Metal–organic framework (MOF) glasses are a newly discovered family of melt-quenched glasses. Despite considerable progress in understanding the nature of MOF glasses, their photonic functionalities have not been found so far. Here, we report on the first breakthrough regarding the photonic functionalities of MOF glasses, that is, finding of the luminescence in melt-quenched MOF glasses. The finding was achieved on a zeolitic imidazolate framework (ZIF) series, that is, the ZIF-62 series: Zn1–xCox(Im)1.7(bIm)0.3, x = 0, 0.1, and 0.5, where Co substitutes Zn in ZIF-62 forming single-phased solid solutions. Remarkably, we observed broadband mid-infrared (Mid-IR) luminescence (in the wavelength range of 1.5–4.8 μm) in both the crystalline and amorphous solid solutions. The intensity of the luminescence in ZIF glass is gradually enhanced by increasing the level of Co concentration. The observed Mid-IR emission originates from d–d transition of Co ions. The discovery of the luminescence in ZIF-62 glass may pave the way toward new photonic applications of bulk MOF glasses.

Two-Dimensional Metal-Organic Framework/Enzyme Hybrid Nanocatalyst as a Benign and Self-Activated Cascade Reagent for in Vivo Wound Healing.

Metal-organic frameworks (MOFs)-based peroxidase mimics have been seldom applied in the biomedical field, especially in vivo. One of the main reasons is their optimum reactions occur in strong acidic environments with a pH of 3-4, severely limiting their applications in living systems where neutral pH is usually required. Other types of peroxidase mimics also suffer such a fatal defect. Additionally, the direct introduction of the relatively high concentrated and toxic reaction reagent H2O2 would induce undesired damage to normal tissues. Herein, a MOF-based hybrid nanocatalyst as a benign and self-activated cascade reagent has been constructed. Owing to better catalytic performance compared with three-dimensional bulk MOF, an ultrathin two-dimensional (2D) MOF (2D Cu-TCPP(Fe)) nanosheet is chosen as a model of peroxidase mimic to physically adsorb glucose oxidase (GOx) for fabricating such a hybrid nanocatalyst. Nontoxic glucose can be continuously converted into abundant gluconic acid and H2O2 by GOx, avoiding the direct use of relatively high concentrated and toxic H2O2 and minimizing the harmful side effects. The generated gluconic acid can decrease the pH value from 7 to 3-4, dramatically activating the peroxidase-like activity of 2D Cu-TCPP(Fe) nanosheets. Meanwhile, the produced H2O2 is used for subsequent catalysis of activated 2D Cu-TCPP(Fe) nanosheets, leading to efficient generation of an extremely toxic hydroxyl radial and antibacterial capacity. In vitro and in vivo results illustrate the designed benign and self-activated cascade reagent possesses a robust antibacterial effect with negligible biotoxicity.

Optical properties of a melt-quenched metal-organic framework glass.

Metal-organic framework (MOF) glasses are characterized by the possession of both inorganic and organic components, linked in a continuous network structure by coordination bonds. To the best of our knowledge, the optical properties of MOF glasses have not been reported until now. In this work, we prepared a transparent bubble-free bulk MOF glass, namely, the ZIF-62 glass (ZnIm2-xbImx), using our newly developed hot-pressing technique, and measured its optical properties. The ZIF-62 glass has a high transmittance (up to 90%) in the visible and near-infrared wavelength ranges, which is comparable to that of many oxide glasses. Using the Becke line nD method, we found that the ZIF-62 glass exhibits a refractive index (1.56) similar to most inorganic glasses, though a lower Abbe number (∼31).

Hierarchical Hybrid Metal-Organic Frameworks: Tuning the Visible/Near-Infrared Optical Properties by a Combination of Porphyrin and Its Isomer Units.

Hybrid metal-organic frameworks (MOFs) with core/shell-like hierarchical structure comprised of zirconium metal and porphyrin (e.g., TPP) and its isomer, N-confused porphyrin (NCP), were synthesized through a seed-mediated reaction. The hierarchical structures of hybrid MOFs were characterized by the microscopic image analyses (e.g., scanning electron microscope (SEM), energy dispersive X-ray (EDX) spectrometry, and confocal laser scanning microscope (CLSM)). Taking advantage of the intrinsic light-harvesting properties of the porphyrin dye and the N-confused isomer, changing the core/shell layer structures of hybrid MOFs allows for tuning of the visible-to-near-infrared (NIR) absorption/emission characters, excited-state energy migrations, and photosensitization capabilities. The Förster energy transfer event occurring in the bulk MOF samples by photoexcitation enabled us to control the photoinduced singlet oxygen generation through the comprehensive light-harvesting ability of these hybrid porphyrinic MOFs. Therefore, implementation of a precisely designed porphyrin "substitute" into the MOF-based materials indeed provides a new mimic of the photosynthetic pigment system and should be potentially applicable for solar-light-driven devices.

A highly sensitive self-enhanced aptasensor based on a stable ultrathin 2D metal-organic layer with outstanding electrochemiluminescence property.

3D bulk metal-organic frameworks (MOFs) have received growing interest in electrochemiluminescence (ECL) assays because they can provide a high specific surface for loading a large quantity of ECL luminophores, but the ECL efficiency of bulk MOFs is still low since some interior luminophores are difficult to be excited. Herein, an ultrathin 2D metal-organic layer (MOL) for grafting self-enhanced ruthenium complexes (Ru-l-Lys) was first synthesized to greatly increase the utilization ratio of luminophores. Compared with 3D bulk MOFs, ultrathin 2D MOL could provide more accessible postmodification sites for grafting the Ru-l-Lys complexes; the self-enhanced Ru-l-Lys complexes on MOL were easily excited by electrons due to the shortened ion/electron-transport distance and the removal of diffusion barriers. Furthermore, the electron transfer path between the Ru(ii) luminophore and coreactant (l-Lys) was shortened and the energy loss of the luminophores decreased, which significantly improved the ECL efficiency. As expected, our work manifested that the Zr-MOL's loading amount of Ru-l-Lys was about 1.23-fold higher than that of a 3D bulk Zr-MOF, and the ECL intensity and efficiency of Ru-l-Lys-Zr-MOL were around 93.45-fold and 1.64-fold higher than those of control Ru-l-Lys-Zr-MOF, respectively. Considering all of these merits, in this work, we utilized the prepared Ru-l-Lys-Zr-MOL as a highly efficient ECL indicator for the first time to fabricate a highly sensitive self-enhanced aptasensor for mucin 1 (MUC1) determination. The proposed aptasensor showed high sensitivity with a linear range from 1 fg mL-1 to 100 pg mL-1 with a detection limit of 0.72 fg mL-1; it also exhibited excellent specificity and stability. It is noteworthy that this work not only provides a new strategy to design and synthesize high-performance ECL materials, but also opens a new way to develop ultrasensitive ECL sensors for bioanalysis.

Mechanical Properties and Processing Techniques of Bulk Metal-Organic Framework Glasses.

Melt quenched metal-organic framework (MOF) glasses define a new category of glass, distinct from metallic, organic, and inorganic glasses, owing to the dominant role of metal-ligand coordination bonding. The mechanical properties of glasses in general are important given their application in protective coatings and display technologies, though little is known about MOF glasses in this respect. The experimental elucidation of key properties such as their scratch resistance has been limited by the lack of processing methodologies capable of producing bulk glass samples. Here, nanoindentation was used to investigate the Young's modulus and hardness of four melt-quenched glasses formed from zeolitic imidazolate frameworks (ZIF): agZIF-4, agZIF-62, agZIF-76, and agZIF-76-mbIm. The creep resistance of the melt-quenched glasses was studied via strain-rate jump (SRJ) tests and through constant load and hold (CLH) indentation creep experiments. Values for the strain-rate sensitivity were found to be close to those for other glassy polymers and Se-rich GeSe chalcogenide glasses. Vacuum hot-pressing of agZIF-62 resulted in an inhomogeneous bulk sample containing the glass and amorphous non-melt-quenched aZIF-62. Remelting and annealing, however, resulted in the fabrication of a transparent, bubble-free bulk specimen, which allowed the first scratch testing experiments to be performed on an MOF glass.

Influence of Tcnq Loading on the Mechanical Properties of Metal-Organic Framework Films

Bulk Metal-organic-framework (MOF) materials are designed scaffold-like compounds that consist of metal ions connected by organic ligands, forming highly ordered porous structures. Bulk MOFs yield typically powders with micron sized crystallites, which limit their applications. Many practical applications require instead suitably uniform MOF films. In recent years, efforts to grow highly oriented Surface Anchored Metal-Organic Framework (SURMOF) films of Cu3(BTC)2 (BTC: benzenetricarboxylic acid), known as HKUST-1 MOF, have been successful by utilizing a liquid phase epitaxy (LPE) spray method on surface functionalized gold coated silicon substrates. Originally MOFs were intended for gas storage due to high storage capacity inside the porous MOF bulk material. New potential device applications in thermoelectrics and microelectronics [1,2] were explored based on the premise that the electrical properties of the otherwise insulating MOF can be custom tailored by loading the porous MOF structure with metal clusters. For modulating the electrical conductivity of MOF films the most effective method is to infiltrate guest molecules inside the porous framework to transport ions, and hence to modulate the electrical properties of the host framework. Early work focused on the mechanical properties of pristine SURMOF films [3,4]. In this study, we have investigated the influence of loading pristine highly oriented MOF films with TCNQ on the mechanical properties by utilizing nanoindentation methods to measure the hardness and modulus of SURMOF films with and without TCNQ loading. Our findings reveal that undoped pristine SURMOF samples are relatively harder with an intrinsic hardness of ~ 1.6 GPa in the upper third of the MOF film. The hardness of the pristine SURMOF samples is steadily increasing as a function of the penetration depth. The hardness is approaching the substrate Si hardness as the indenter tip approaches the substrate. The measured modulus values of the pristine SURMOF samples are also steadily increasing as a function of the indentation depth. However, the TCNQ loaded SURMOF samples differ markedly and do not show a substrate effect. Interestingly the loaded SURMOF films are much softer and exhibit a constant hardness as a function of depth of ~ 0.5 GPa. As the indenter approaches the Si substrate there is no measurable change in the constant hardness and an absence of a substrate effect. The modulus values of TCNQ loaded SURMOF films is also significantly lower compared with the pristine sample and remains constant as a function of indenter penetration depth. Thus, loading the porous scaffolding of SURMOF films with TCNQ did not only alter the electrical conductivity but triggers marked changes in the mechanical properties, i.e., softening effect. This softening effect in TCNQ infiltrated MOF films can be accounted for by a widening of the lattice constant und the concomitant introduction of lattice defects that weaken the crystal and lead to a much lower hardness.

Ultrathin Metal-organic Framework Membranes Used for Industrial Separation

Recently, separation and purification identified as a huge and stringent challenge in industrial processes and environmental protection. Being high efficient and advanced technology, metal-organic framework (MOF) membranes and membrane processes have presented tremendous potentials in application of gas separation. Compared with conventional bulk MOF membranes, the dense and polycrystalline MOF ultrathin membranes with nanometre-thick layers have shown outstanding gas separation performances due to minimized mass-transfer barrier. In this paper, three typical synthetic strategies of MOF ultrathin molecule-sieving membranes are introduced, and their performances in gas separation are compared.

Enhanced Electrocatalytic Oxygen Evolution by Exfoliation of a Metal–Organic Framework Containing Cationic One-Dimensional [Co4(OH)2]6+ Chains

Metal–organic frameworks (MOFs) are an emerging class of heterogeneous electrocatalyst, though the focus for the vast majority of them has been on employing them as a precursor for carbon materials or as a host material for encapsulating catalytically active species. Herein, we report the preparation of a metal–organic nanosheet with one-dimensional (1-D) [Co4(OH)2]6+ chains via delamination of a 3-D MOF. The resultant atomically thin nanosheets are highly active, robust, and recyclable oxygen evolution reaction (OER) electrocatalysts with a low overpotential 318 mV (without iR compensation) to achieve a current density of 10 mA cm–2. These values along with the small Tafel slope (54 mV dec–1) exhibit a superior performance to the bulk MOF precursor and the benchmark RuO2 catalyst under the identical condition. The electrochemical studies ascribe the excellent OER activity to the high surface area, accessible CoII sites, and good charge transfer of the nanosheets.

Nanocrystalline Metal–Organic Frameworks/Carbon Nanotubes Composites As Catalytic Cathode Materials for Lithium–Oxygen Batteries with Long Cycle Life

To meet the needs of emerging technologies such as all-electric vehicles with a long drive range, there are now strong motivations for the development of rechargeable batteries with high specific energy, so called “beyond Li-ion batteries.” Among these beyond Li-ion batteries, lithium–air (Li–O2) batteries have received much attention owing to their five times higher theoretic energy density (ca. 3500 W h/kg) than that of lithium-ion batteries. However, Li–O2 batteries still suffer from several challenges that hinder their practical applications, including low energy efficiency, short cycle life, and poor rate capability. These problems are mainly caused by the sluggish oxygen reduction/evolution reaction (ORR/OER), low stability of electrolyte against O2 −•, and the poor conductivity of discharge product (Li2O2).1 To improve the performance of Li–O2 batteries, much effort has been devoted to searching for new catalytic cathode materials to promote ORR/OER kinetics, which is crucial to lowering overpotential and improving the reversibility.2 Metal–organic frameworks (MOFs), a novel type of crystalline porous materials, are built with metal nodes or clusters (secondary building units) and organic linkers. For the last two decades, MOFs have received considerable attentions for gas storage, catalysis, and energy storage, due to their high surface area and chemically unsaturated metal sites.3 It has been also demonstrated that bulk MOFs could be good electrocatalysts for Li–O2 batteries.4 However, most of MOFs have poor electrical conductivity, which limits their applications in electrocatalysis. Herein, we report nanocrystalline MOFs/CNTs composites as catalytic cathode materials for Li–O2 batteries to further enhance the catalytic performance of MOFs. The direct growth of MOFs on CNTs (MOFs@CNTs) can not only enhance the electronic conductivity but also mitigate the agglomeration issues of MOFs nanoparticles, making them suitable in the applications of electrocatalysis. The as-prepared MOF-74@CNTs hybrid cathodes showed the improved cycling performance of Li-O2 batteries. We have also investigated the effects of chemically unsaturated and various redox-active metal sites on the catalytic performance in Li–O2 batteries. The fundamental mechanism of how MOFs enhance the performance of Li-O2 batteries will also be discussed. Reference 1 Aurbach, D., McCloskey, B. D., Nazar, L. F. & Bruce, P. G. Advances in understanding mechanisms underpinning lithium–air batteries. Nature Energy 1, 16128, doi:10.1038/nenergy.2016.128 (2016). 2 Ma, Z. et al. A review of cathode materials and structures for rechargeable lithium–air batteries. Energy & Environmental Science 8, 2144-2198 (2015). 3 Furukawa, H., Cordova, K. E., O’Keeffe, M. & Yaghi, O. M. The chemistry and applications of metal-organic frameworks. Science 341, 1230444 (2013). 4 Wu, D. et al. Metal–organic frameworks as cathode materials for Li–O2 batteries. Adv Mater 26, 3258-3262, doi:10.1002/adma.201305492 (2014).

A Highly Solvent-Stable Metal-Organic Framework Nanosheet: Morphology Control, Exfoliation, and Luminescent Property.

Compared to bulk metal-organic framework (MOF), 2D MOF nanosheets have gained intensive research attention due to their ultrathin thickness and large surface area with highly accessible active sites. However, structural deterioration and morphological damage have impeded producing high-quality MOF nanosheets during exfoliation. Here, first a new layered bulk MOF ZSB-1 is synthesized and several solvents such as isopropanol, methanol, n-hexyl alcohol, and N,N-dimethylformamide are surveyed to examine their performance for the exfoliation of layered ZSB-1. As a result, a highly solvent-stable metal-organic framework rectangular nanosheet retaining undamaged morphology is obtained by the soft-physical method in n-hexyl alcohol. Theoretical simulations reveal that the strong interaction energy between n-hexyl alcohol and MOF layers is responsible for the best exfoliation performance of making the bulk MOF into nanosheets. In addition, ZSB-1 shows a tunable fluorescence peak position, fluorescent lifetime, and quantum yield by simply changing the solvent and morphology. Besides, the ZSB-1 was selected as a fluorescence sensor to detect metal ions, and ZSB-1 nanosheet exhibits excellent sensing ability for Fe3+ . It is worth noting that the ZSB-1 nanosheet has better detection limit performance of 0.054 × 10-6 m than that of its bulk counterpart.