Thin Film Growth of 3D Sr-based Metal-Organic Framework on Conductive Glass via Electrochemical Deposition

Integration of metal-organic frameworks (MOFs) as components of advanced electronic devices is at a very early phase of development and the fundamental issues related to their crystal growth on conductive substrate need to be addressed. Herein, we report on the structural characterization of a newly synthesized Sr-based MOF {[Sr(2,5-Pzdc)(H₂O)₂] ⋅ 3 H₂O}_n ( 1 ) and the uniform crystal growth of compound 1 on a conducting glass (fluorine doped tin oxide (FTO)) substrate using electrochemical deposition techniques. The Sr-based MOF 1 was synthesized by the reaction of Sr(NO₃)₂ with 2,5-pyrazinedicarboxylic acid dihydrate (2,5-Pzdc) under solvothermal conditions. A single-crystal X-ray diffraction analysis revealed that 1 has a 3D structure and crystallizes in the triclinic P space group. In addition, the uniform crystal growth of this MOF on a conducting glass (FTO) substrate was successfully achieved using electrochemical deposition techniques. Only a handful of MOFs have been reposed to grown on conductive surfaces, which makes this study an important focal point for future research on the applications of MOF-based devices in microelectronics.     

金属有机骨架(MOFs)结构的透射电子显微分析

金属有机骨架(Metal organic framework,MOF)配位聚合物作为一类重要的多孔材料具有诸多独特的性能.新型MOF材料的结构表征与确定一直是该研究领域的关键性研究问题.由于单晶X-射线衍射等结构测定方法对晶体尺寸有一定限制,小尺寸MOF新材料的晶体结构确定一直是亟待解决的科学难题.透射电子显微分析方法(Transmission electron microscopy,TEM)作为纳米尺寸晶体材料最有力的结构表征手段之一,已经被逐渐应用于MOF新材料领域,展现出了巨大的应用潜力.本文以几个国内外有代表性的工作为例,浅析TEM在MOF材料领域的发展现状.     

Block Copolymer‐Templated Approach to Nanopatterned Metal–Organic Framework Films

The fabrication of patterned metal–organic framework (MOF) films with precisely controlled nanoscale resolution has been a fundamental challenge in nanoscience and nanotechnology. In this study, nanopatterned MOF films were fabricated using a layer‐by‐layer (LBL) growth method on functional templates (such as a bicontinuous nanoporous membrane or a structure with highly long‐range‐ordered nanoscopic channels parallel to the underlying substrate) generated by the microphase separation of polystyrene‐b ‐poly(2‐vinylpyridine) (PS‐b ‐P2VP) block copolymers. HKUST‐1 can be directly deposited on the templates without any chemical modification because the pyridine groups in P2VP interact with metal ions via metal‐BCP complexes. As a result, nanopatterned HKUST‐1 films with feature sizes below 50 nm and controllable thicknesses can be fabricated by controlling the number of LBL growth cycles. The proposed fabrication method further extends the applications of MOFs in various fields.     

Self‐Generation of Surface Roughness by Low‐Surface‐Energy Alkyl Chains for Highly Stable Superhydrophobic/Superoleophilic MOFs with Multiple Functionalities

We transformed the hydrophilic metal–organic framework (MOF) UiO‐67 into hydrophobic UiO‐67‐R s (R=alkyl) by introducing alkyl chains into organic linkers, which not only protected hydrophilic Zr₆O₈ clusters to make the MOF interspace superoleophilic, but also led to a rough crystal surface beneficial for superhydrophobicity. The UiO‐67‐R s displayed high acid, base, and water stability, and long alkyl chains offered better hydrophobicity. Good hydrophobicity/oleophilicity were also possible with mixed‐ligand MOFs containing metal‐binding ligands. Thus, a (super)hydrophobic MOF catalyst loaded with Pd centers efficiently catalyzed Sonogashira reactions in water at ambient temperature. Studies of the hydrophobic effects of the coordination interspace and the outer surface suggest a simple de novo strategy for the synthesis of superhydrophobic MOFs that combine surface roughness and low surface energy. Such MOFs have potential for environmentally friendly catalysis and water purification.     

Modification of Extended Open Frameworks with Fluorescent Tags for Sensing Explosives: Competition between Size Selectivity and Electron Deficiency

Three new electron‐rich metal–organic frameworks ( MOF‐1 – MOF‐3 ) have been synthesized by employing ligands bearing aromatic tags. The key role of the chosen aromatic tags is to enhance the π‐electron density of the luminescent MOFs. Single‐crystal X‐ray structures have revealed that these MOFs form three‐dimensional porous networks with the aromatic tags projecting inwardly into the pores. These highly luminescent electron‐rich MOFs have been successfully utilized for the detection of explosive nitroaromatic compounds (NACs) on the basis of fluorescence quenching. Although all of the prepared MOFs can serve as sensors for NACs, MOF‐1 and MOF‐2 exhibit superior sensitivity towards 4‐nitrotoluene (4‐NT) and 2,4‐dinitrotoluene (DNT) compared to 2,4,6‐trinitrotoluene (TNT) and 1,3,5‐trinitrobenzene (TNB). MOF‐3 , on the other hand, shows an order of sensitivity in accordance with the electron deficiencies of the substrates. To understand such anomalous behavior, we have thoroughly analyzed both the steady‐state and time‐resolved fluorescence quenching associated with these interactions. Determination of static Stern–Volmer constants (K_S) as well as collisional constants (K_C) has revealed that MOF‐1 and MOF‐2 have higher K_S values with 4‐NT than with TNT, whereas for MOF‐3 the reverse order is observed. This apparently anomalous phenomenon was well corroborated by theoretical calculations. Moreover, recyclability and sensitivity studies have revealed that these MOFs can be reused several times and that their sensitivities towards TNT solution are at the parts per billion (ppb) level.     

Revelation of the molecular assembly of the nanoporous metal organic framework ZIF-8

Crystalline nanoporous materials are one of the most important families of complex functional material. Many questions pertaining to the molecular assembly mechanism of the framework of these materials remain unanswered. Only recently has it become possible to answer definitively some of these questions by observation of growing nanoscopic surface features on metal organic frameworks (MOFs) through use of in situ atomic force microscopy (AFM). Here we reveal that a growth process of a MOF, zeolitic imidazolate framework ZIF-8, occurs through the nucleation and spreading of successive metastable unenclosed substeps to eventually form stable surface steps of the enclosed framework structure and that this process is reliant on the presence of nonframework species to bridge the developing pores during growth. The experiments also enable identification of some of the fundamental units in the growth process and the stable crystal surface plane. The former findings will be applicable to numerous nanoporous materials and support efforts to synthesize and design new frameworks and to control the crystal properties of these materials.     

Localized Crystallization of Enantiomeric Organic Compounds on Chiral Micro‐patterns from Various Organic Solutions

The controlled crystallization of enantiomers of an organic compound (a cyclic phosphoric acid derivative) on templated micro‐patterned functionalised surfaces is demonstrated. Areas where a complementary chiral thiol has been located were effective heterogeneous nucleation centres when a solution of the compound is evaporated slowly. Various organic solvents were employed, which present a challenge with respect to other examples when water is used. The solvent and the crystallization method have an important influence on the crystal growth of these compounds. When chloroform was employed, well‐defined crystals grow away from the surface, whereas crystals grow in the plane from solutions in isopropanol. In both cases, nucleation is confined to the polar patterned regions of the surface, and for isopropanol growth is largely limited within the pattern, which shows the importance of surface chemistry for nucleation and growth. The apparent dependence on the enantiomer used in the latter case could imply stereo‐differentiation as a result of short‐range interactions (the templating monolayer is disordered, even at the nanometre scale). The size of the pattern of chiral monolayer also determines the outcome of the crystallization; 5 μm dots are most effective. Despite the low surface tension of the samples (relative to the high surface tension of water), differential solvation of the polar and hydrophobic layers of the solvents allows crystallization in the polar regions of the monolayer, therefore the polarity of the regions in which heterogeneous nucleation takes place is indeed very important. Despite the complex nature of the crystallization process, these results are an important step towards to the use of patterned surfaces for heterogeneous selective nucleation of enantiomers.     

Influence of temperature on metal-organic frameworks

Reaction temperature is one of the key parameters in the synthesis of metal-organic frameworks （MOFs）. Though there is no convergence with regard to the various experimental parameters, reaction temperature has been found to have remarkable influence on the formation and structure of MOFs, especially toward the control of topology and dimensionality of the MOF structures. Theoretically, the reaction temperature affects directly the reaction energy barrier in reaction thermodynamics and the reaction rate in the reaction kinetics. This review aims to show the influence of reaction temperature on crystal growth/assembly, structural modulation and transformation of MOFs, and to provide primary information and insights into the design and assernblv of desired MOFs.     

A Heterometallic MOF based on Monofunctional Linker by “One-pot” Solvothermal Method for Highly Selective Gas Adsorption

Heterometallic MOFs, as an important branch of MOF materials, open up a new way to design and synthesize innovative MOF materials. Hence, it has positive significance for the development of MOF materials. By utilizing monofunctional linker CBDA [5,5'-(carbonylbis(azanediyl))-diisophthalic acid], a heterometallic MOF In/Gd-CBDA was obtained under “one-pot“ solvothermal method. The three-dimensional framework exhibits (3,4,6)-connected network belonging to a new topology. In addition, the synthetic conditions that affect the growth of crystal have been studied in detail. Numerous experiments led to the finding that the type and composition of solvents are very important for inducing the recognition of carboxylic acid groups at different positions coordination to diverse metals. Furthermore, the mechanism of this kind of heterometallic MOFs assembled by monofunctional linker under “one-pot” solvothermal method has been clarified, which is mainly related to the steric hindrance of functional groups and the properties of heterometals. Based on ideal adsorbed solution theory (IAST), In/Gd-CBDA shows the efficient selective gas adsorption, especially for equimolar mixtures of CO₂/CH₄ (13) and C₃H₈/CH₄ (321). This work of heterometallic MOF material assembled by monofunctional linker under “one-pot” solvothermal method provides a new platform for the development of MOF materials in terms of enriching the topological family and broadening possibilities in advanced applications.     

Surface‐Specific Functionalization of Nanoscale Metal–Organic Frameworks

A method for modifying the external surfaces of a series of nanoscale metal–organic frameworks (MOFs) with 1,2‐dioleoyl‐sn‐glycero‐3‐phosphate (DOPA) is presented. A series of zirconium‐based nanoMOFs of the same topology (UiO‐66, UiO‐67, and BUT‐30) were synthesized, isolated as aggregates, and then conjugated with DOPA to create stably dispersed colloids. BET surface area analysis revealed that these structures maintain their porosity after surface functionalization, providing evidence that DOPA functionalization only occurs on the external surface. Additionally, dye‐labeled ligand loading studies revealed that the density of DOPA on the surface of the nanoscale MOF correlates to the density of metal nodes on the surface of each MOF. Importantly, the surface modification strategy described will allow for the general and divergent synthesis and study of a wide variety of nanoscale MOFs as stable colloidal materials.     

Surface chemistry of metal-organic frameworks at the liquid-solid interface

Metal-organic frameworks (MOFs) are a fascinating class of novel inorganic-organic hybrid materials. They are essentially based on classic coordination chemistry and hold much promise for unique applications ranging from gas storage and separation to chemical sensing, catalysis, and drug release. The evolution of the full innovative potential of MOFs, in particular for nanotechnology and device integration, however requires a fundamental understanding of the formation process of MOFs. Also necessary is the ability to control the growth of thin MOF films and the positioning of size- and shape-selected crystals as well as MOF heterostructures on a given surface in a well-defined and oriented fashion. MOFs are solid-state materials typically formed by solvothermal reactions and their crystallization from the liquid phase involves the surface chemistry of their building blocks. This Review brings together various key aspects of the surface chemistry of MOFs.     

Evaluation of defect induced surface heterogeneity in Metal-Organic Framework materials with alkali dopants employing adsorption isotherm modelling

In this article, an assessment of surface structural heterogeneity in porous metal organic framework (MOF) structure has been demonstrated by employing the methane and carbon-dioxide adsorption isotherms data. The virgin MIL-101-(Cr) MOF was synthesized by the hydrothermal method and defects were induced in the MOF structure by doping with various alkali (K, Na, Li) cations. The synthesized MOFs were characterized by XRD, SEM, EDX and BET measurement techniques. In order to understand the defect induced surface heterogeneity by alkali cation dopants, the surface energy distributions for CH₄ and CO₂ adsorptions on MOFs were measured by Dubinin – Astakhov model equation. The surface heterogeneity is mainly controlled by the limiting uptakes of adsorbates, the polarizability of adsorbates and the adsorbate-adsorbent interaction energy.     

Multi‐shelled Hollow Metal–Organic Frameworks

Hollow metal–organic frameworks (MOFs) are promising materials with sophisticated structures, such as multiple shells, that cannot only enhance the properties of MOFs but also endow them with new functions. Herein, we show a rational strategy to fabricate multi‐shelled hollow chromium (III) terephthalate MOFs (MIL‐101) with single‐crystalline shells through step‐by‐step crystal growth and subsequent etching processes. This strategy relies on the creation of inhomogeneous MOF crystals in which the outer layer is chemically more robust than the inner layer and can be selectively etched by acetic acid. The regulation of MOF nucleation and crystallization allows the tailoring of the cavity size and shell thickness of each layer. The resultant multi‐shelled hollow MIL‐101 crystals show significantly enhanced catalytic activity during styrene oxidation. The insight gained from this systematic study will aid in the rational design and synthesis of other multi‐shelled hollow structures and the further expansion of their applications.     

Multi-shelled Hollow Metal–Organic Frameworks

Hollow metal–organic frameworks (MOFs) are promising materials with sophisticated structures, such as multiple shells, that cannot only enhance the properties of MOFs but also endow them with new functions. Herein, we show a rational strategy to fabricate multi-shelled hollow chromium (III) terephthalate MOFs (MIL-101) with single-crystalline shells through step-by-step crystal growth and subsequent etching processes. This strategy relies on the creation of inhomogeneous MOF crystals in which the outer layer is chemically more robust than the inner layer and can be selectively etched by acetic acid. The regulation of MOF nucleation and crystallization allows the tailoring of the cavity size and shell thickness of each layer. The resultant multi-shelled hollow MIL-101 crystals show significantly enhanced catalytic activity during styrene oxidation. The insight gained from this systematic study will aid in the rational design and synthesis of other multi-shelled hollow structures and the further expansion of their applications.     

A Metal Chelating Porous Polymeric Support: The Missing Link for a Defect‐Free Metal–Organic Framework Composite Membrane

Since the discovery of size‐selective metal–organic frameworks (MOFs), researchers have tried to incorporate these materials into gas separation membranes. Impressive gas selectivities were found, but these MOF membranes were mostly made on inorganic supports, which are generally too bulky and expensive for industrial gas separation. Forming MOF layers on porous polymer supports is industrially attractive but technically challenging. Two features to overcome these problems are described: 1) a metal chelating support polymer to bind the MOF layer, and 2) control of MOF crystal growth by contra‐diffusion, aiming at a very thin nanocrystalline MOF layer. Using a metal chelating poly‐thiosemicarbazide (PTSC) support and adjusting the metal and organic ligand concentrations carefully, a very compact ZIF‐8 (ZIF=zeolitic imidazolate framework) layer was produced that displayed interference colors because of its smooth surface and extreme thinness—within the range of visible light. High performances were measured in terms of hydrogen/propane (8350) and propylene/propane (150) selectivity.     

Crystal Growth Mechanisms and Morphological Control of the Prototypical Metal–Organic Framework MOF‐5 Revealed by Atomic Force Microscopy

Crystal growth of the metal–organic framework MOF‐5 was studied by atomic force microscopy (AFM) for the first time. Growth under low supersaturation conditions was found to occur by a two‐dimensional or spiral crystal growth mechanism. Observation of developing nuclei during the former reveals growth occurs through a process of nucleation and spreading of metastable and stable sub‐layers revealing that MOFs may be considered as dense phase structures in terms of crystal growth, even though they contain sub‐layers consisting of ordered framework and disordered non‐framework components. These results also support the notion this may be a general mechanism of surface crystal growth at low supersaturation applicable to crystalline nanoporous materials. The crystal growth mechanism at the atomistic level was also seen to vary as a function of the growth solution Zn/H₂bdc ratio producing square terraces with steps parallel to the <100> direction or rhombus‐shaped terraces with steps parallel to the <110> direction when the Zn/H₂bdc ratio was >1 or about 1, respectively. The change in relative growth rates can be explained in terms of changes in the solution species concentrations and their influence on growth at different terrace growth sites. These results were successfully applied to the growth of as‐synthesized cube‐shaped crystals to increase expression of the {111} faces and to grow octahedral crystals of suitable quality to image using AFM. This modulator‐free route to control the crystal morphology of MOF‐5 crystals should be applicable to a wide variety of MOFs to achieve the desired morphological control for performance enhancement in applications.     

Design and Synthesis of a Water‐Stable Anionic Uranium‐Based Metal–Organic Framework (MOF) with Ultra Large Pores

Ionic metal–organic frameworks (MOFs) are a subclass of porous materials that have the ability to incorporate different charged species in confined nanospace by ion‐exchange. To date, however, very few examples combining mesoporosity and water stability have been realized in ionic MOF chemistry. Herein, we report the rational design and synthesis of a water‐stable anionic mesoporous MOF based on uranium and featuring tbo‐type topology. The resulting tbo MOF exhibits exceptionally large open cavities (3.9 nm) exceeding those of all known anionic MOFs. By supercritical CO₂ activation, a record‐high Brunauer‐Emmett‐Teller (BET) surface area (2100 m² g^?1) for actinide‐based MOFs has been obtained. Most importantly, however, this new uranium‐based MOF is water‐stable and able to absorb positively charged ions selectively over negatively charged ones, enabling the efficient separation of organic dyes and biomolecules.     

金属有机框架材料对Cr(Ⅵ)离子的吸附去除研究进展

重金属离子污染问题一直备受关注.开发利用多孔材料吸附去除水中重金属离子一直是材料、环境等相关学科领域的研究热点之一.金属有机框架材料(metal-organic frameworks,MOFs)是一类新型的多孔材料,具有结构多样、比表面积大、孔径可调、孔表面特征易设计调控等特点,在气体分离、催化、传感等领域表现出极大的...     

Metal-organic framework membranes: From synthesis to electrocatalytic applications

Metal-organic frameworks (MOFs), as an emerging family of porous inorganic-organic crystal materials, exhibit widely applications in gas storage and separation, drug release, sensing, and catalysis, owing to easily adjustable pore sizes, uniformly distributed metal centers, high surface areas, and tunable functionalities. However, MOF crystal powders are usually difficult to be directly applied into specific devices because of their brittleness, insolubility and low compatibility. Therefore, to expand versatile MOF membranes with robustness and operational flexibility is urgent to satisfy practical applications. Although numerous reports have reviewed the synthesis and applications of MOF membranes, relatively few reports the electrocatalytic properties based on MOF membranes. Herein, this mini-review provides an overview of preparation of MOF membranes, including directed synthesis, secondary growth and electrochemical deposition method. Meanwhile, fabrication of ultrathin 2D MOF nanosheets those can be also defined as a kind of nanoscale MOF membranes is also mentioned. Electrocatalytic performance of oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and CO₂ reduction reaction (CO₂RR) for diverse MOF membranes/nanosheets and their derivatives are introduced.     

Controllable Modular Growth of Hierarchical MOF‐on‐MOF Architectures

Fabrication of hybrid MOF‐on‐MOF heteroarchitectures can create novel and multifunctional platforms to achieve desired properties. However, only MOFs with similar crystallographic parameters can be hybridized by the classical epitaxial growth method (EGM), which largely suppressed its applications. A general strategy, called internal extended growth method (IEGM), is demonstrated for the feasible assembly of MOFs with distinct crystallographic parameters in an MOF matrix. Various MOFs with diverse functions could be introduced in a modular MOF matrix to form 3D core–satellite pluralistic hybrid system. The number of different MOF crystals interspersed could be varied on demand. More importantly, the different MOF crystals distributed in individual domains could be used to further incorporate functional units or enhance target functions.