Metal Organic Framework Single Crystals Research Articles

Isotope Effect-Enabled Crystal Enlargement in Metal-Organic Frameworks.

Synthesizing large metal-organic framework (MOF) single crystals has garnered significant research interest, although it is hindered by the fast nucleation kinetics that gives rise to numerous small nuclei. Given the different chemical origins inherent in various types of MOFs, the development of a general approach to enhancing their crystal sizes presents a formidable challenge. Here, we propose a simple isotopic substitution strategy to promote size growth in MOFs by inhibiting nucleation, resulting in a substantial increase in the crystal volume ranging from 1.7- to 165-fold. Impressively, the crystals prepared under optimized conditions by normal approaches can be further enlarged by the isotope effect, yielding the largest MOF single crystal (2.9 cm × 0.48 cm × 0.23 cm) among the one-pot synthesis method. Detailed in situ characterizations reveal that the isotope effect can retard crystallization kinetics, establish a higher nucleation energy barrier, and consequently generate fewer nuclei that eventually grow larger. Compared with the smaller crystals, the isotope effect-enlarged crystal shows 33% improvement in the X-ray dose rate detection limit. This work enriches the understanding of the isotope effect on regulating the crystallization process and provides inspiration for exploring potential applications of large MOF single crystals.

A facile synthesis of monolithic MOF single crystal for gas separation via a supersaturation strategy

Large-area metal–organic framework (MOF) single crystal membrane materials hold great appeal in gas separation field due to their outstanding separation selectivity and permeability. Nevertheless, it is currently difficult to prepare MOF single crystals large enough for gas separation tests owing to the limited manufacturing processes. In this work, we fabricate a self-supporting MOF single crystal (Co-btec SC) membrane with 0.3 × 0.1 cm2 dimension via a facile supersaturation synthesis strategy. Benefiting from the one-dimensional (1D) transmission pathway and three-dimensional (3D) connection network, the obtained Co-btec SC has ultra-high permeability of 47,647 Barrer. Besides, the H2O molecules firmly filled in the pore channels greatly improve the affinity and solubility for CO2, and thus Co-btec SC yields CO2 preferential permeation in CO2/H2 mixture with superior CO2/H2 selectivity of 18.79. In addition, the Co-btec SC shows dramatic high-temperature and high-humidity separation stabilities. To demonstrate the versatility of this strategy, we also successfully fabricated hydrogen-bonded organic framework single crystals (HOF SCs) with the size of about 0.4 cm. This supersaturation strategy opens a new way to fabricate the large-area MOF single crystal membranes.

Nonlinear Metal–Organic Framework Crystals for Efficient Multicolor Coherent Optical Emission

AbstractNon‐centrosymmetric metal–organic frameworks (MOFs) have recently emerged as a promising class of coordination polymers for nonlinear optics and light conversion. Nevertheless, achieving high endurance and efficiency for the light conversion within the whole visible range by a single crystal of MOF remains a challenge. This article reports on the design of two non‐centrosymmetric MOFs based on a 1,3,5‐benzenetricarboxylic acid ligand and Er/Co ions, providing an efficient and simultaneous multiple second and third optical harmonic generations (SHG, THG) from 400 to 750 nm with a high endurance (over 100 cycles or 3 h of continuous operation). Optical experiments and quantum chemical modeling of the MOF single crystals confirm the polarization and chemistry‐dependent efficiency of SHG and THG, associated with the varied MOFs' symmetry and dipole moment. The observed coherent light conversion within the whole visible range by MOF single crystals at ambient conditions makes it possible to achieve the multicolor (up to three) coherent emission required for modern laser technologies, making MOFs highly competitive and sustainable materials for nonlinear optics.

Hollow Mesoporous Metal Organic Framework Single Crystals Enabled by Growth Kinetics Control for Enhanced Photocatalysis

AbstractThe tailored design of hollow mesoporous metal−organic framework (MOF) single crystals to realize the unimpeded mass transport and long‐range carrier migration for advanced photoelectric applications is attractive while being a major challenge. Here, a kinetically mediated micelle assembly strategy is presented to synthesize hollow UiO‐66(Ce) single crystals in 1,3,5‐trimethylbenzene (TMB)‐H2O emulsion system with Pluronic F127/P123 as dual surfactants. This synthesis features the employment of modulator acetic acid, which can coordinate growth kinetics of MOFs, allowing the nucleation of MOF on block polymer micelles and transforming the self‐assembly of block polymer/MOF composite monomicelles from water to TMB/H2O emulsion interface and the growth of MOF from aggregation to oriented attachment, and thus ensuring the formation of hollow mesoporous UiO‐66(Ce) single crystals. Moreover, a precise control in cavity diameter from ≈0 to ≈600 nm, mesopore structure from close to radial and dendritic can be achieved by tuning the amount of TMB and the ratio of F127/P123. As a result, the hollow mesoporous UiO‐66(Ce) single crystals with large radial mesopores (≈25 nm) and high surface area (≈1061 m2 g) exhibit excellent photocatalytic performance for H2 generation owing to enhanced permeability and suppressed electron‐hole combination.

Anisotropic Band-Edge Absorption of Millimeter-Sized Zn(3-ptz)2 Single-Crystal Metal-Organic Frameworks.

Metal–organic frameworks (MOFs) have emerged as promising tailor-designed materials for developing next-generation solid-state devices with applications in linear and nonlinear coherent optics. However, the implementation of functional devices is challenged by the notoriously difficult process of growing large MOF single crystals of high optical quality. By controlling the solvothermal synthesis conditions, we succeeded in producing large individual single crystals of the noncentrosymmetric MOF Zn(3-ptz)2 (MIRO-101) with a deformed octahedron habit and surface areas of up to 37 mm2. We measured the UV–vis absorption spectrum of individual Zn(3-ptz)2 single crystals across different lateral incidence planes. Millimeter-sized single crystals have a band gap of Eg = 3.32 eV and exhibit anisotropic absorption in the band-edge region near 350 nm, whereas polycrystalline samples are fully transparent in the same frequency range. Using solid-state density functional theory (DFT), the observed size dependence in the optical anisotropy is correlated with the preferred orientation adopted by pyridyl groups under conditions of slow crystal self-assembly. Our work thus paves the way for the development of optical polarization switches based on metal–organic frameworks.

Single‐Crystal Capacitive Sensors with Micropatterned Electrodes via Space‐Confined Growth of the Metal–Organic Framework HKUST‐1

AbstractThe polyhedral shape and fragile nature of single crystals of metal–organic frameworks (MOFs) hinder their processing using standard microfabrication techniques. As a result, the fabrication of surface patterns and microelectrodes on MOF single crystals is challenging, which limits both the direct electrical interrogation of crystals in research and their integration into optical and electrical devices. Herein, a new strategy to readily fabricate surface‐embossed micropatterns and surface‐embedded gold electrodes on MOF single crystals using a space‐confined, one‐step solvothermal synthesis is demonstrated. This method is based on the confined growth of the MOF crystal around prefabricated metal patterns with different adhesion strengths to the growth surface, producing features as small as 5 µm. As a proof of concept, micropatterns are created on flat single crystals of the MOF HKUST‐1, exhibiting the function as a diffraction grating. Additionally, a single‐crystal HKUST‐1 capacitive gas sensor with surface‐embedded electrodes that serves as a stable platform for the investigation of ethanol diffusion in the MOF pores is fabricated. The space‐confined fabrication strategy is promising for the integration of fragile MOF single crystals in various optical and electrical devices.

Growth Pattern Control and Nanoarchitecture Engineering of Metal-Organic Framework Single Crystals by Confined Space Synthesis.

The nanoarchitecture engineering of metal–organic frameworks (MOFs) is a fascinating but intellectually challenging concept that opens up avenues for both tailoring the properties of MOFs and expanding their applications. Herein, we report the confined growth of ZIF-8 single crystals in a three-dimensionally ordered (3DO) macroporous polystyrene replica and reveal that their growth patterns, morphologies, and nanoarchitectures can be highly engineered using the concentration of the precursor. Impressively, the favorable in situ confined growth enables the successful fabrication of 3DO sphere-assembled ZIF-8 single crystals or 3DO single-crystalline ZIF-8 sphere arrays when a low- or high-concentration precursor solution, respectively, is used as the feedstock. Furthermore, our strategy can be extended to the preparation of other 3DO MOF single crystals, including ZIF-67 and HKUST-1, with similar controllable hierarchical nanoarchitectures. With the successful preparation of a series of diameter-tunable ZIF-8 single-crystalline spheres, we further unravel their interesting size–performance relationship in the Knoevenagle reaction between benzaldehyde and malononitrile, wherein the smallest spheres show the fastest first-order reaction kinetics. This study not only develops a general strategy for engineering the nanoarchitectures of MOF single crystals but also provides fundamental knowledge of the mechanism for the growth of hierarchical single crystals under confined spaces.

Determining factors in the growth of MOF single crystals unveiled by in situ interface imaging

<h2>Summary</h2> We trace the evolution of crystal interface during the solvothermal synthesis of metal-organic frameworks (MOFs) in flow cell by <i>in situ</i> dark-field microscope. Precise measurement of the growth rate at specific crystal facets allows for the extraction of the key determining factors for the crystal growth of five typical MOFs, i.e., Cu-MOF-74 (1D), Co-ZIF, Cu-MOF-2-BDC, Cu-MOF-2-NDC (2D), and HKUST-1 (3D). Rather than strongly correlating to the corresponding chemical formulas, the compositional organic linker and metal ion play vital but separate roles for different MOFs, giving insight into the crystal growth mechanism. Moreover, upon interruption, retreat and recovery of the crystal interface are observed, unveiling the influence from interface to MOF crystal growth. This also leads to the discovery of a transition layer that mediates the linkage of molecular building blocks to form MOF single crystals at the interface.

Precise Design and Deliberate Tuning of Turn-On Fluorescence in Tetraphenylpyrazine-Based Metal-Organic Frameworks.

The manipulation on turn-on fluorescence in solid state materials attracts increasing interests owing to their widespread applications. Herein we report how the nonradiative pathways of tetraphenylpyrazine (TPP) units in metal-organic frameworks (MOFs) systems could be hindered through a topological design approach. Two MOFs single crystals of different topology were constructed via the solvothermal reaction of a TPP-based 4,4',4″,4‴-(pyrazine-2,3,5,6-tetrayl) tetrabenzoic acid (H4TCPP) ligand and metal cations, and their mechanisms of formation have been explored. Compared with the innate low-frequency vibrational modes of flu net Tb-TCPP-1, such as phenyl ring torsions and pyrazine twists, Tb-TCPP-2 adopts a shp net, so the dihedral angle of pyrazine ring and phenyl arms is larger, and the center pyrazine ring in TPP unit is coplanar, which hinders the radiationless decay of TPP moieties in Tb-TCPP-2. Thereby Tb-TCPP-2 exhibits a larger blue-shifted fluorescence and a higher fluorescence quantum yield than Tb-TCPP-1, which is consistent with the reduced nonradiative pathways. Furthermore, Density functional theory (DFT) studies also confirmed aforementioned tunable turn-on fluorescence mechanism. Our work constructed TPP-type MOFs based on a deliberately topological design approach, and the precise design of turn-on fluorescence holds promise as a strategy for controlling nonradiative pathways.

Micro-/nano-sized multifunctional heterochiral metal–organic frameworks for high-performance visible–blind UV photodetectors

Multifunctional heterochiral micro-/nano-sized metal–organic framework single crystals are developed for high-performance visible–blind UV photodetection.

Growing and Shaping Metal-Organic Framework Single Crystals at the Millimeter Scale.

Controlling and understanding the mechanisms that harness crystallization processes is of utmost importance in contemporary materials science and, in particular, in the realm of reticular solids where it still remains a great challenge. In this work, we show that environments mimicking microgravity conditions can harness the size and shape of functional biogenic crystals such as peptide-based metal-organic frameworks (MOFs). In particular, we demonstrate formation of the largest single crystals with controlled nonequilibrium shapes of peptide-based MOFs reported to date (e.g., those featuring curved crystal habits), as opposed to the typical polyhedral microcrystals obtained under bulk crystallization conditions. Such unique nonequilibrium morphologies arise from the interplay between the diffusion-controlled supply of precursors in simulated microgravity environments and the physical constraints imposed during crystal growth. In fact, our method mimics two main strategies of morphogenesis in biomineralization, i.e., spatial and morphological control, both being largely unexplored in the field of self-assembled functional materials. The presented results may open new opportunities to study and understand fundamental questions of relevance to materials science, such as how the size and shape of artificial crystals can influence their properties and functions while providing a strategy to tailor the size and shape of peptide-based MOF single crystals to specific applications.

Local Structure Evolvement in MOF Single Crystals Unveiled by Scanning Transmission Electron Microscopy

The response of metal–organic frameworks (MOFs) toward external stimuli, such as light, heat, and electrons, usually involves a local change in their structures. Although diffraction methods allow for monitoring of the overall dynamics of the crystal structure, the local environment on each crystal and specific position within the crystal remains largely unknown. Here, the collective change in the pores of MOF single crystals was visualized by scanning transmission electron microscopy (STEM) with an interpretable resolution of 4.7 Å to give a clear observation of each individual pore. The local evolvement occurred before the deterioration of image resolution; therefore, the maintenance of overall image resolution does not guarantee the preservation of structure integrity. The local evolvement in response to electron irradiation was quantitatively analyzed and was found to be dependent on both the lattice plane and specific position in the crystal. The crystal shrunk gradually as the electron dose increased. This was likely attributed to the displacement of the molecular building blocks in the MOF backbone. The displacements were inhomogeneous across the individual crystal, corroborating with the local twisting and rotation of the crystal. This STEM method provided an example for the spatial and temporal study of the local structure evolvement of MOFs under stimuli.

Ordered Macro-Microporous Metal-Organic Framework Single Crystals and Their Derivatives for Rechargeable Aluminum-Ion Batteries.

Constructing ordered hierarchical porous structures while maintaining their overall crystalline order is highly desirable but remains an arduous challenge. Herein, we successfully achieve the growth of single-crystalline metal-organic frameworks (MOFs) in three-dimensional (3D) ordered macroporous template voids by a saturated solution-based double-solvent-assisted strategy with precise control over the nucleation process. The as-prepared single-crystalline ordered macro-microporous Co-based MOFs (SOM ZIF-67) exhibit an ordered macro-microporous structure with robust single-crystalline nature. Moreover, SOM ZIF-67 can serve as a precursor to derive 3D-ordered macroporous cobalt diselenide@carbon (3DOM CoSe2@C) through a facile carbonization-selenization treatment. The as-derived 3DOM CoSe2@C can well preserve the 3D-ordered macroporous structure of the precursor. More importantly, CoSe2 nanoparticles could be uniformly confined in the conductive ordered macroporous carbon framework, affording regularly interconnectedmacroporous channels and large surface area. As a result, when evaluated as a cathode material for aluminum-ion batteries, the ordered macroporous structure could not only effectively facilitate the diffusion of large-sized chloroaluminate anions but also increase the contact area with electrolyte and provide more exposed active sites, thereby exhibiting superior reversible rate capacity (86 mA h g-1 at 5.0 A g-1) and remarkable cycling performance (125 mA h g-1 after 1000 cycles at 2.0 A g-1).

Modulating Guest Uptake in Core-Shell MOFs with Visible Light.

A two-component core-shell UiO-68 type metal-organic framework (MOF) with a nonfunctionalized interior for efficient guest uptake and storage and a thin light-responsive outer shell was prepared by initial solvothermal MOF synthesis followed by solvent-assisted linker exchange. The bulky shell linker features two tetra-ortho-fluorinated azobenzene moieties to exploit their advantageous photoisomerization properties. The obtained perfect octahedral MOF single crystals can be switched repeatedly and with an unprecedented efficiency between E- and Z-rich states using visible light only. Due to the high photoswitch density per pore of the shell layer, its steric demand and thus molecular uptake (and release) can be conveniently modulated upon green and blue light irradiation. Therefore, the "smart" shell acts as a light-controlled kinetic barrier or "gate" for the diffusion of cargo molecules in and out of the MOF crystals.

Dual-band simultaneous lasing in MOFs single crystals with Fabry-Perot microcavities

Multi-band microlasers based on single microcrystalline materials with Fabry-Perot (F-P) cavities are critically and technologically essential. Here, we demonstrate simultaneous dual-band lasing output (615 and 685 nm) in metal-organic frameworks (MOFs) and organic dyes hybrid single crystals, which support F-P resonances. Through a two-step assembly strategy, two different types of cationic pyridinium hemicyanine dye molecules can be encapsulated into the channel pores of anionic bio-MOF-1-2Me successfully. In addition, the employment of the host-guest system significantly increases the dye loading, enhances luminescent efficiency, and diminishes the aggregation-caused quenching (ACQ) effect in the resultant MOFs/dye composites. This finding not only combines the characteristic of MOFs materials with excellent luminescent properties of organic dyes, but also points out a simple and promising strategy to design multi-band microlasers based on F-P mechanism, opening a low-cost avenue for the rational design of miniaturized lasers in the future.

Size control over metal-organic framework porous nanocrystals.

Porous nanocrystals of metal-organic frameworks (MOFs) offer greater bioavailability, higher surface-to-volume ratios, superior control over MOF membrane fabrication, and enhanced guest-sorption kinetics compared to analogous bulk phases, but reliable synthesis of uniformly sized particles remains an outstanding challenge. Here, we identify the smallest and most probable sizes of known MOF nanocrystals and present an exhaustive comparative summary of nano- versus bulk-MOF syntheses. Based on critical analysis of reported size data and experimental conditions, an alternate to the LaMer model is proposed that describes nanocrystal formation as a kinetic competition between acid-base and metal-ligand reactivity. Particle growth terminates when ligands outcompete metal-ion diffusion, thereby arresting polymerization to produce kinetically trapped particle sizes. This model reconciles disparate trends in the literature and postulates that minimum particle sizes can be achieved by minimizing the relative ratios of metal-to-linker local concentrations. By identifying conditions that disfavor small nanocrystal sizes, this model also provides routes towards macroscopic MOF single crystals. A universal "seesaw" relationship between nanocrystal sizes and the concentrations of acidic surface-capping ligands provides a roadmap for achieving precise synthetic control. Best practices in synthesis, characterization, and data presentation are recommended for future investigations so that MOF nanocrystals may achieve their full potential as advanced nanomaterials.

Anisotropic Water-Mediated Proton Conductivity in Large Iron(II) Metal–Organic Framework Single Crystals for Proton-Exchange Membrane Fuel Cells

Herein we present a new proton-conducting iron(II) metal–organic framework (MOF) of an unusual structure formed by chains of alternating bistriazolate-p-benzoquinone anions and iron(II) cations with four axially coordinated water molecules. These chains assemble via π–π stacking between the aromatic units to form a three-dimensional grid-like network with channel pores filled with water molecules. The material was structurally characterized by single-crystal XRD analysis, and its water and thermal stability was investigated. The proton conductivity was studied by impedance measurements on needle-like single crystals. A simple but efficient measurement setup consisting of interdigital electrodes was used. The influence of the crystal orientation, temperature, and humidity was investigated. The iron(II)-MOF showed the highest proton conductivity of 3.3·10–3 S cm–1 at 22 °C and 94% relative humidity. Contrary to most known structures, the conductivity in this material is controlled by chemical properties of ...

Photopatterning of fluorescent host-guest carriers through pore activation of metal-organic framework single crystals.

Encoded fluorescent particles are fabricated through the selective uptake of dyes in photopatterned metal-organic framework single crystals. The concept is based on spatially controlled photochemical cleavage of pore-blocking pendant groups. Because of the crystalline and porous nature of the host, this approach enables guest uptake that is tunable and can be triggered though controlled irradiation.

Tuning metal–carboxylate coordination in crystalline metal–organic frameworks through surfactant media

Although it has been widely demonstrated that surfactants can efficiently control the size, shape and surface properties of micro/nanocrystals of metal–organic frameworks (MOFs) due to the strong interactions between surfactants and crystal facets of MOFs, the use of surfactants as reaction media to grow MOF single crystals is unprecedented. In addition, compared with ionic liquids, surfactants are much cheaper and can have multifunctional properties such as acidic, basic, neutral, cationic, anionic, or even block. These factors strongly motivate us to develop a new synthetic strategy: growing crystalline MOFs in surfactants. In this report, eight new two-dimensional (2D) or three-dimensional (3D) MOFs have been successfully synthesized in an industrially-abundant and environmentally-friendly surfactant: polyethylene glycol-200 (PEG-200). Eight different coordination modes of carboxylates, ranging from monodentate η1 mode to tetra-donor coordination µ3-η1:η2:η1 mode, have been founded in our research. The magnetic properties of Co-based MOFs were investigated and MOF NTU-Z6b showed a phase transition with a Curie temperature (Tc) at 5 K. Our strategy of growing crystalline MOFs in surfactant could offer exciting opportunities for preparing novel MOFs with diverse structures and interesting properties.

Metal–Organic Framework Single Crystals as Photoactive Matrices for the Generation of Metallic Microstructures

The photocatalytic properties of MOF-5, a highly porous metal–organic framework (MOF) material, can be exploited to generate metallic patterns by direct laser writing. The strict confinement of photoreduction to the focal volume of the laser beam, where irradiation intensity is highest, allows the generation of 3D metallic structures inside a single MOF-5 crystal.