Amorphous Metal-organic Frameworks Research Articles

A general flame aerosol route to kinetically stabilized metal-organic frameworks

Metal-organic frameworks (MOFs) are highly attractive porous materials with applications spanning the fields of chemistry, physics, biology, and engineering. Their exceptional porosity and structural flexibility have led to widespread use in catalysis, separation, biomedicine, and electrochemistry. Currently, most MOFs are synthesized under equilibrium liquid-phase reaction conditions. Here we show a general and versatile non-equilibrium flame aerosol synthesis of MOFs, in which rapid kinetics of MOF formation yields two distinct classes of MOFs, nano-crystalline MOFs and amorphous MOFs. A key advantage of this far-from-equilibrium synthesis is integration of different metal cations within a single MOF phase, even when this is thermodynamically unfavorable. This can, for example, produce single-atom catalysts and bimetallic MOFs of arbitrary metal pairs. Moreover, we demonstrate that dopant metals (e.g., Pt, Pd) can be exsolved from the MOF framework by reduction, forming nanoclusters anchored on the MOF. A prototypical example of such a material exhibited outstanding performance as a CO oxidation catalyst. This general synthesis route opens new opportunities in MOF design and applications across diverse fields and is inherently scalable for continuous production at industrial scales.

Decoding the Biomimetic Mineralization of Metal-Organic Frameworks in Water.

This study unveils the "green" metal-organic framework (MOF) structuring mechanism by decoding proton transfer in water during ZIF-8 synthesis. Combining in situ small- to wide-angle X-ray scattering, multiscale simulations, and quantum calculations, we reveal that the ZIF-8 early-stage nucleation and crystallization process in aqueous solution unfolds in three distinct stages. In stage I, imidazole ligands replace water in zinc-water cages, triggering an "acidity flip" that promotes proton transfer. This leads to the assembly of structures from single zinc ions to 3D amorphous cluster nuclei. In stage II, amorphous nuclei undergo a critical transformation, evolving into crystalline nuclei and subsequently forming mesoscale-ordered structures and crystallites. The process proceeds until the amorphous precursors are completely consumed, with the transformation kinetics governed by an energy barrier that determines the rate-limiting step. In stage III, stable crystallite nanoparticles form in solution, characterized by a temperature-dependent thermal equilibrium of molecular interactions at the crystal-solution interface. Beyond these core advancements, we explore the influence of encapsulated pepsin and nonencapsulated lysozyme on ZIF-8 formation, finding that their amino acid proton transfer capacity and concentration influence the resulting biomolecule-MOF composite's shape and encapsulation efficiency. The findings contribute to understanding the molecular mechanisms behind biomimetic mineralization and have potential implications for engineering proteins within amorphous MOF nuclei as protein embryo growth sites.

Exploring Order/Disorder Molecular Networks on Metal Surfaces

Materials featuring disordered structures often exhibit unique properties. Metal-organic frameworks (MOFs) are a class of hybrid materials composed of metal nodes and coordinating organic linkers[1]. Recently, there has been growing interests in MOFs with structural disorder, and the investigations of amorphous structures on surfaces[2]. Herein, we demonstrate a bottom-up method to construct molecular networks on metal surfaces with order-disorder transition by preparing two-component samples with different stoichiometric ratios of two organic molecule linkers sharing the same symmetry but differing in size[3]. We investigate the topological and electronic structures of the order/disorder molecular networks by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) under ultra-high vacuum (UHV). The results may advance our understanding of the formation mechanism and electronic behaviors characteristic of monolayer molecular networks with structural disorders, holding the potential to catalyze advancements in the design and application of new amorphous MOF materials with intriguing properties.[1] L. Dong, et al., Progress in Surface Science, 91, 101–135 (2016).[2] J. Fonseca, et al., Journal of Materials Chemistry A, 9.17, 10562-10611 (2021).[3] J. Lu, et al., ACS Nano, 17, 20, 20194–20202 (2023).

Optimizing post-treatment strategies for enhanced oxygen reduction/evolution activity in Co–N–C electrocatalyst

Uncovering the origin of bifunctional oxygen reduction/evolution reaction (ORR/OER) activity of M-N-C type electrocatalysts is essential for their extensive usage in fuel cells and metal-air batteries. The electrochemical properties of heterogeneous M-N-C catalysts depend on the surface structure, which can be modified via different synthetic methods. Herein, we investigate the effect of structure on the activity of Co–N–C electrocatalyst material derived from an amorphous metal-organic framework TAL-2. Specifically, TAL-2 served as the sole precursor for the synthesis of a range of Co–N–C catalysts via various carbonization and acid-leaching modes. Co–N–C catalysts were prepared by carbonizing TAL-2 at different temperatures (700, 800, 900, and 1000 °C) and subsequent acid-leaching using different acids, namely HCl, H2SO4, and HNO3, either individually or in various combinations. This resulted in the generation of 36 distinct catalyst samples, each exhibiting unique morphological characteristics. By systematically varying these parameters, we aimed to unravel the intricate relationship between post-synthetic treatment and the resulting electrocatalytic properties, shedding light on the rational design of Co–N–C catalysts for enhanced ORR/OER.

Stöber method to amorphous metal-organic frameworks and coordination polymers

The Stöber method is a widely-used sol-gel route for synthesizing amorphous SiO2 colloids and conformal coatings. However, the material systems compatible with this method are still limited. Herein, we have extended the approach to metal-organic frameworks (MOFs) and coordination polymers (CPs) by mimicking the Stöber method. We introduce a general synthesis route to amorphous MOFs or CPs by making use of a base-vapor diffusion method, which allows to precisely control the growth kinetics. Twenty-four different amorphous CPs colloids were successfully synthesized by selecting 12 metal ions and 17 organic ligands. Moreover, by introducing functional nanoparticles (NPs), a conformal amorphous MOFs coating with controllable thickness can be grown on NPs to form core-shell colloids. The versatility of this amorphous coating technology was demonstrated by synthesizing over 100 core-shell composites from 20 amorphous CPs shells and over 30 different NPs. Besides, various multifunctional nanostructures, such as conformal yolk-amorphous MOF shell, core@metal oxides, and core@carbon, can be obtained through one-step transformation of the core@amorphous MOFs. This work significantly enriches the Stöber method and introduces a platform, enabling the systematic design of colloids exhibiting different level of functionality and complexity.

In Situ Reconstruction of Scalable Amorphous Indium-Based Metal-Organic Framework for CO2 Electroreduction to Formate over an Ultrawide Potential Window.

Amorphous metal-organic frameworks (aMOFs) are highly attractive for electrocatalytic applications due to their exceptional conductivity and abundant defect sites, but harsh preparation conditions of "top-down" strategy have hindered their widespread use. Herein, the scalable production of aMIL-68(In)-NH2 was successfully achieved through a facile "bottom-up" strategy involving ligand competition with 2-methylimidazole. Multiple in situ and ex situ characterizations reveal that aMIL-68(In)-NH2 evolutes into In/In2O3-x as the genuine active sites during the CO2 electrocatalytic reduction (CO2RR) process. Moreover, the retained amino groups could enhance the CO2 adsorption. As expected, the reconstructed catalyst demonstrates high formate Faradaic efficiency values (>90%) over a wide potential range of 800 mV in a flow cell, surpassing most top-ranking electrocatalysts. Density functional theory calculations reveal that the abundant oxygen vacancies in aMIL-68(In)-NH2 induce more local charges around electroactive sites, thereby promoting the formation of HCOO* intermediates. Furthermore, 16 g of samples can be readily prepared in one batch and exhibit almost identical CO2RR performances. This work offers a feasible batch-scale strategy to design amorphous MOFs for the highly efficient electrolytic CO2RR.

Amorphization of MOFs with rich active sites and high electronic conductivity for hydrazine oxidation

The applications of metal-organic frameworks (MOFs) in electrocatalysis are limited by the perfect crystalline nature with insufficient coordinatively unsaturated sites and low electronic conductivity. Here, we report an electrosynthesis method to induce rapid assembly of amorphous MOFs on carbon cloth (denoted as aMnFc’/CC), achieving the amorphization of MOFs with rich active sites and high electronic conductivity. The long-range disordered structure of aMnFc’/CC forms a large number of oxygen vacancies to modify the coordination and electronic structures of Mn sites. In the electrocatalytic hydrazine oxidation reaction (HzOR), the aMnFc’/CC electrocatalyst exhibits superior activity to its crystalline MOF counterparts. Density functional theory calculations reveal that the unsaturated Mn centers present in aMnFc’/CC facilitate efficient electron transfer while simultaneously lowering the free energy of the rate-determining step in the HzOR process.

Orchestrating apoptosis and ferroptosis through enhanced sonodynamic therapy using amorphous UIO-66-CoOx

The development of efficient and multifunctional sonosensitizers is crucial for enhancing the efficacy of sonodynamic therapy (SDT). Herein, we have successfully constructed a CoOx-loaded amorphous metal-organic framework (MOF) UIO-66 (A-UIO-66-CoOx) sonosensitizer with excellent catalase (CAT)- and glutathione-oxidase (GSH-OXD)-like activities. The A-UIO-66-CoOx exhibits a 2.6-fold increase in singlet oxygen (1O2) generation under ultrasound (US) exposure compared to crystalline UIO-66 sonosensitizer, which is attributed to its superior charge transfer efficiency and consistent oxygen (O2) supply. Additionally, the A-UIO-66-CoOx composite reduces the expression of glutathione peroxidase (GPX4) by depleting glutathione (GSH) through Co3+ and Co2+ valence changes. The high levels of highly cytotoxic 1O2 and deactivation of GPX4 can lead to lethal lipid peroxidation, resulting in concurrent apoptosis and ferroptosis. Both in vitro and vivo tumor models comprehensively confirmed the enhanced SDT antitumor effect using A-UIO-66-CoOx sonosensitizer. Overall, this study emphasizes the possibility of utilizing amorphization engineering to improve the effectiveness of MOFs-based sonosensitizers for combined cancer therapies.

Amorphous metal-organic frameworks loaded on BiVO4 photoanodes with unique internal metal-like structure for promoting photoelectrochemical water splitting

Although ferrocene (Fc) based Metal-Organic Frameworks (MOFs) can act as oxygen evolution co-catalysts (OECs) for improvement of catalytic reactivity, the poor conductivity and lack of highly active metal sites limit its further application in photoelectrochemical (PEC) water splitting. Herein, the amorphous NiFc-MOF was grafted on BiVO4 photoanode (BiVO4@aNiFc-MOFs) for efficient PEC water splitting. This novel BiVO4@aNiFc-MOFs exhibits high current density of 4.34 mA cm−2 at 1.23 VRHE and relative low onset potential of 0.223 VRHE. The subsequent characterizations demonstrate that Ni species with metal-like state in bulk of aNiFc-MOFs form strong metal-support interaction with BiVO4, thereby promoting the interfacial charge transfer. Moreover, the surface of aNiFc-MOFs is short-range ordered with abundant coordinatively unsaturated Ni sites, creating a more favorable pathway for oxygen evolution reaction from thermodynamics. This work provides a simple method to design photoanodes with efficient OECs of amorphous MOFs for feasible PEC water splitting application.

Iodine Adsorption-Desorption-Induced Structural Transformation and Improved Ag+ Turn-On Luminescent Sensing Performance of a Nonporous Eu(III) Metal-Organic Framework.

Posttreatment of pristine metal-organic frameworks (MOFs) with suitable vapor may be an effective way to regulate their structures and properties but has been less explored. Herein, we report an interesting example in which a crystalline nonporous Eu(III)-MOF was transferred to a porous amorphous MOF (aMOF) via iodine vapor adsorption-desorption posttreatment, and the resulting aMOF showed improved turn-on sensing properties with respect to Ag+ ions. The crystalline Eu-MOF, namely, Eu-IPDA, was assembled from Eu(III) and 4,4'-{4-[4-(1H-imidazol-1-yl)phenyl]pyridine-2,6-diyl}dibenzoic acid (H2IPDA) and exhibited a two-dimensional (2D) coordination network based on one-dimensional secondary building blocks. The close packing of the 2D networks gives rise to a three-dimensional supramolecular framework without any significant pores. Interestingly, the nonporous Eu-IPDA could absorb iodine molecules when Eu-IPDA crystals were placed in iodine vapor at 85 °C, and the adsorption capacity was 1.90 g/g, which is comparable to those of many MOFs with large BET surfaces. The adsorption of iodine is attributed to the strong interactions among the iodine molecule, the carboxy group, and the N-containing group and leads to the amorphization of the framework. After immersion of the iodine-loaded Eu-IPDA in EtOH, approximately 89.7% of the iodine was removed, resulting in a porous amorphous MOF, denoted as a-Eu-IPDA. In addition, the remaining iodine in the a-Eu-IPDA framework causes strong luminescent quenching in the fluorescence emission region of the Eu(III) center when compared with that in Eu-IPDA. The luminescence intensity of a-Eu-IPDA in water suspensions was significantly enhanced when Ag+ ions were added, with a detection limit of 4.76 × 10-6 M, which is 1000 times that of pristine Eu-IPDA. It also showed strong anti-interference ability over many common competitive metal ions and has the potential to sense Ag+ in natural water bodies and traditional Chinese medicine preparations. A mechanistic study showed that the interactions between Ag+ and the absorbed iodine, the carboxylate group, and the N atoms all contribute to the sensing performance of a-Eu-IPDA.

Amorphous Engineering of Scalable Metal-Organic Framework-Derived Electrocatalyst for Highly Efficient Oxygen Evolution Reaction.

The engineering of amorphous metal-organic frameworks (MOFs) offers potential opportunities for the construction of electrocatalysts for efficient oxygen evolution reaction (OER). Herein, highly efficient OER performance and durability in alkaline electrolyte are discovered for MOF-derived amorphous and porous electrocatalysts, which are synthesized in a brief procedure and can be facilely produced in scalable quantities. The structural inheritance of MOF amorphous catalysts is significant for the retention of catalytic sites and the diffusion of electrolytes, and the presence of Fe sites can change the electronic structure and effectively control the adsorption behavior of important intermediates, accelerating reaction kinetics. The obtained amorphous A-FeNi can be transformed from FeNi-MOF effortlessly and instantly, and it only needs low overpotentials of 152 and 232mV at 10 and 100mA cm-2 with a Tafel slope of 17mV dec-1 in 1m KOH for OER. Moreover, A-FeNi possesses high corrosion resistance and durability, therefore A-FeNi can work continually for at least 400h at 100mA cm-2. This work may pave a new avenue for the design of MOFs-related amorphous electrocatalyst.

Adsorption of perfluoroalkyl substances on deep eutectic solvent-based amorphous metal-organic framework: Structure and mechanism

Perfluoroalkyl substances (PFASs) are a class of emerging organic pollutants characterized by high toxicity, environmental persistence, and widespread detection in water sources. The removal of PFASs from water is a matter of global concern, given their detrimental impact on both the environment and public health. Many commonly used PFAS adsorbents demonstrate limited adsorption capacities and/or slow adsorption kinetics. Therefore, there is an urgent need for the development of efficient adsorbents. For the first time, this work systematically investigated the performance of a deep eutectic solvent (DES)-based amorphous metal-organic framework (MOF) for the adsorption of PFASs with different carbon-chain lengths under the state of the mixture in aquatic environments. The adsorption mechanism was probed by a suite of adsorption kinetics studies, adsorption isotherm profiling, spectral characterization, and ab initio molecular dynamics (AIMD) simulations, revealing that PFAS adsorption is driven by synergistic capturing effects including acid/base coordination, CF-π (carbon-fluorine-π), hydrogen bonding, and hydrophobic interactions. Furthermore, the adsorption processes of short-chain and long-chain targets were found to involve different rate-controlling steps and interaction sites. Hydrophobic interactions facilitated the swift arrival of long-chain PFASs at the coordinatively interacting sites between carboxyl termini and Lewis acid Zr unsaturated sites, thanks to their lower reaction barriers. On the other hand, the adsorption of short-chain PFASs primarily relied on a Zr hydroxyl-based ligand exchange force, which would take place at Brønsted acid sites. The existence of massive structural disorder in amorphous UiO-66 led to the development of larger pores, thus improving the accessibility of abundant adsorption sites and facilitating adsorption and diffusion. The presence of multiple types of interactions and flexible structure in defect-rich amorphous UiO-66 significantly increased the exposure of functional groups to the adsorbates. Additionally, this material possessed outstanding regeneration efficiency and outperformed other MOF-based adsorbents with high affinity for targets. It enhances our understanding of the adsorption performances and mechanisms of amorphous materials toward PFASs, thereby paving the way for designing more efficient PFAS adsorbents.

Enhanced light driven CO2 conversion based on silver bismuth sulfide hollow octahedrons coated with amorphous metal–organic frameworks

AgBiS2 hollow octahedrons coated with ZIF-67 outer layers formed a core–shell heterostructure with more favorable charge transfer kinetics, which conduced to electron accumulation and photosensitizer adsorption for CO2 photoreduction.

Transient intermediate in the formation of an amorphous metal-organic framework.

Amorphous metal-organic frameworks are rarely formed via direct synthesis. Our limited understanding of their atomic assembly in solution prevents full exploitation of their unique structural complexity. Here, we use in situ synchrotron X-ray absorption spectroscopy with sub-second time resolution to probe the formation of the amorphous Fe-BTC framework. Using a combination of spectral fingerprinting, linear combination analysis, and principal component analysis coupled with kinetic analyses, we reveal a multi-stage formation mechanism that, crucially, proceeds via the generation of a transient intermediate species.

Synthetic and analytical considerations for the preparation of amorphous metal-organic frameworks.

Metal-organic frameworks (MOFs) are hybrid porous materials presenting several tuneable properties, allowing them to be utilised for a wide range of applications. To date, focus has been on the preparation of novel crystalline MOFs for specific applications. Recently, interest in amorphous MOFs (aMOFs), defined by their lack of correlated long-range order, is growing. This is due to their potential favourable properties compared to their crystalline equivalents, including increased defect concentration, improved processability and gas separation ability. Direct synthesis of these disordered materials presents an alternative method of preparation to post-synthetic amorphisation of a crystalline framework, potentially allowing for the preparation of aMOFs with varying compositions and structures, and very different properties to crystalline MOFs. This perspective summarises current literature on directly synthesised aMOFs, and proposes methods that could be utilised to modify existing syntheses for crystalline MOFs to form their amorphous counterparts. It outlines parameters that could discourage the ordering of crystalline MOFs, before examining the potential properties that could emerge. Methodologies of structural characterisation are discussed, in addition to the necessary analyses required to define a topologically amorphous structure.

Machine learning interatomic potentials for amorphous zeolitic imidazolate frameworks

Accurate microscopic models of amorphous metal–organic frameworks (MOFs) are difficult to create. Machine learning potentials based on data from ab initio molecular dynamics offer a novel way to achieve this goal.

Hierarchical porous amorphous metal-organic frameworks constructed from ZnO/MOF glass composites.

For the first time, hierarchical porous amorphous metal-organic frameworks (HP-aMOFs) containing ultramicropores, micropores, and mesopores were synthesized by etching a composite of MOF glass (agZIF-76) and ZnO using ammonia. These materials show potential applications in the adsorption of C2 hydrocarbons.

Co‐Based Amorphous Layered Metal–Organic Frameworks: An Efficient Bifunctional Catalyst for Water Electrolysis

The pursuit of efficient and cost‐effective electrocatalysts for hydrogen production via water splitting remains a significant challenge. Herein, a unique layered amorphous metal–organic frameworks, designated as Co‐HAB, is introduced. When utilized in oxygen evolution reaction (OER) and hydrogen evolution reaction, this Co‐HAB‐based electrocatalyst demonstrates superior activity and stability. Employing a mixture of Co‐HAB and carbon black (Co‐HAB‐C) as both cathode and anode electrocatalysts in a 1 m potassium hydroxide electrolyte, the cell exhibits a voltage of merely 1.552 V and attains a current density of 10 mA cm−2, which is suitable for hydrogen generation. Furthermore, density functional theory computations indicate that the modest OH* adsorption capability at the catalytic site plays a pivotal role in enhancing the OER catalytic efficiency, as confirmed by alcohol molecular probe tests.

Order-Disorder Transition of Two-Dimensional Molecular Networks through a Stoichiometric Design.

Materials with disordered structures may exhibit interesting properties. Metal-organic frameworks (MOFs) are a class of hybrid materials composed of metal nodes and coordinating organic linkers. Recently, there has been growing interest in MOFs with structural disorder and the investigations of amorphous structures on surfaces. Herein, we demonstrate a bottom-up method to construct disordered molecular networks on metal surfaces by selecting two organic molecule linkers with the same symmetry but different sizes for preparing two-component samples with different stoichiometric ratios. The amorphous networks are directly imaged by scanning tunneling microscopy under ultrahigh vacuum with a submolecular resolution, allowing us to quantify its degree of disorder and other structural properties. Furthermore, we resort to molecular dynamics simulations to understand the formation of the amorphous metal-organic networks. The results may advance our understanding of the mechanism of formation of monolayer molecular networks with structural disorders, facilitating the design and exploration of amorphous MOF materials with intriguing properties.

Continuous Amorphous Metal-Organic Frameworks Layer Boosts the Performance of Metal Anodes.

Employing metal anodes can greatly increase the volumetric/gravimetric energy density versus a conventional ion-insertion anode. However, metal anodes are plagued by dendrites, corrosion, and interfacial side reaction issues. Herein, a continuous and flexible amorphous MOF layer was successfully synthesized and used as a protective layer on metal anodes. Compared with the crystalline MOF layer, the continuous amorphous MOF layer can inhibit dendrite growth at the grain boundary and eliminate ion migration near the grain boundary, showing high interfacial adhesion and a large ion migration number (tZn2+ = 0.75). In addition, the continuous amorphous MOF layer can effectively solve several key challenges, e.g., corrosion of the zinc anode, hydrogen evolution reaction, and dendrite growth on the zinc surface. The prepared Zn anode with the continuous amorphous MOF (A-MOF) layer exhibited an ultralong cycling life (around one year, more than 7900 h) and a low overpotential (<40 mV), which is 12 times higher than that of the crystalline MOF protective layer. Even at 10 mA cm-2, it still showed high stability for more than 5500 cycles (1200 h). The enhanced performance is realized for full cells paired with a MnO2 cathode. In addition, a flexible symmetrical battery with the Zn@A-ZIF-8 anode exhibited good cyclability under different bending angles (0°, 90°, and 180°). More importantly, various metal substrates were successfully coated with compact A-ZIF-8. The A-ZIF-8 layer can obviously improve the stability of other metal anodes, including those of Mg and Al. These results not only demonstrate the high potential of amorphous MOF-decorated Zn anodes for AZIBs but also propose a type of protective layer for metal anodes.