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.     

Metal–Organic Frameworks for Oxygen Storage

We present a systematic study of metal–organic frameworks (MOFs) for the storage of oxygen. The study starts with grand canonical Monte Carlo simulations on a suite of 10 000 MOFs for the adsorption of oxygen. From these data, the MOFs were down selected to the prime candidates of HKUST‐1 (Cu‐BTC) and NU‐125, both with coordinatively unsaturated Cu sites. Oxygen isotherms up to 30 bar were measured at multiple temperatures to determine the isosteric heat of adsorption for oxygen on each MOF by fitting to a Toth isotherm model. High pressure (up to 140 bar) oxygen isotherms were measured for HKUST‐1 and NU‐125 to determine the working capacity of each MOF. Compared to the zeolite NaX and Norit activated carbon, NU‐125 has an increased excess capacity for oxygen of 237 % and 98 %, respectively. These materials could ultimately prove useful for oxygen storage in medical, military, and aerospace applications.     

Ultrathin Chiral Metal–Organic‐Framework Nanosheets for Efficient Enantioselective Separation

Chiral metal–organic framework (CMOF) nanosheets only a few layers thick remain a virgin land waiting for exploration. Herein, the first examples of ultrathin CMOF nanosheets are prepared by the confinement growth of two‐dimensional (2D) chiral layers, which are assembled by helical metal–organic chains within microemulsion. This convenient and easily scaled up inverse microemulsion method gives a series of 2D CMOF nanosheets composed of variable metal nodes or chiral ligands. More significantly, thanks to the exceptionally large number of chiral sites exposed on surfaces, the as‐obtained CMOF nanosheets exhibit much higher enantioselectivity in chiral separation compared with their bulk counterparts.     

Synthetic Strategies for Constructing Two‐Dimensional Metal‐Organic Layers (MOLs): A Tutorial Review

Two‐dimensional (2D) metal‐organic layers (MOLs) are the 2D version of metal‐organic frameworks (MOFs) with nanometer thickness in one dimension. MOLs are also known as 2D‐MOFs, 2D coordination polymers, ultrathin MOF nanosheets (UMOFNs) or coordination nanosheets in literature. This new category of 2D materials has attracted a lot of interests because of the opportunity in combining molecular chemistry, surface/interface chemistry and material chemistry of low dimensional materials in these systems. Several synthetic strategies have been developed for the construction of 2D MOLs, but the general synthesis of MOLs still presents a challenge. This tutorial level review summarizes the recent progress in the fabrication of novel 2D MOLs and aims to highlight challenges in this field.     

A Generalized Strategy for the Synthesis of Large‐Size Ultrathin Two‐Dimensional Metal Oxide Nanosheets

Two‐dimensional (2D) nanomaterials show unique electrical, mechanical, and catalytic performance owing to their ultrahigh surface‐to‐volume ratio and quantum confinement effects. However, ways to simply synthesize 2D metal oxide nanosheets through a general and facile method is still a big challenge. Herein, we report a generalized and facile strategy to synthesize large‐size ultrathin 2D metal oxide nanosheets by using graphene oxide (GO) as a template in a wet‐chemical system. Notably, the novel strategy mainly relies on accurately controlling the balance between heterogeneous growth and nucleation of metal oxides on the surface of GO, which is independent on the individual character of the metal elements. Therefore, ultrathin nanosheets of various metal oxides, including those from both main‐group and transition elements, can be synthesized with large size. The ultrathin 2D metal oxide nanosheets also show controllable thickness and unique surface chemical state.     

On‐Chip Tailorability of Capacitive Gas Sensors Integrated with Metal–Organic Framework Films

Gas sensing technologies for smart cities require miniaturization, cost‐effectiveness, low power consumption, and outstanding sensitivity and selectivity. On‐chip, tailorable capacitive sensors integrated with metal–organic framework (MOF) films are presented, in which abundant coordinatively unsaturated metal sites are available for gas detection. The in situ growth of homogeneous Mg‐MOF‐74 films is realized with an appropriate metal‐to‐ligand ratio. The resultant sensors exhibit selective detection for benzene vapor and carbon dioxide (CO₂) at room temperature. Postsynthetic modification of Mg‐MOF‐74 films with ethylenediamine decreases sensitivity toward benzene but increases selectivity to CO₂. The reduced porosity and blocked open metal sites caused by amine coordination account for a deterioration in the sensing performance for benzene (by ca. 60 %). The enhanced sensitivity for CO₂ (by ca. 25 %) stems from a tailored amine–CO₂ interaction. This study demonstrates the feasibility of tuning gas sensing properties by adjusting MOF–analyte interactions, thereby offering new perspectives for the development of MOF‐based sensors.     

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.     

Large‐Scale,Bottom‐Up Synthesis of Binary Metal–Organic Framework Nanosheets for Efficient Water Oxidation

Ultrathin metal–organic framework (MOF) nanosheets (NSs) offer potential for many applications, but the synthetic strategies are largely limited to top‐down, low‐yield exfoliation methods. Herein, Ni–M–MOF (M=Fe, Al, Co, Mn, Zn, and Cd) NSs are reported with a thickness of only several atomic layers, prepared by a large‐scale, bottom‐up solvothermal method. The solvent mixture of N,N‐dimethylacetamide and water plays key role in controlling the formation of these two‐dimensional MOF NSs. The MOF NSs can be directly used as efficient electrocatalysts for the oxygen evolution reaction, in which the Ni–Fe–MOF NSs deliver a current density of 10 mA cm^?2 at a low overpotential of 221 mV with a small Tafel slope of 56.0 mV dec^?1, and exhibit excellent stability for at least 20 h without obvious activity decay. Density functional theory calculations on the energy barriers for OER occurring at different metal sites confirm that Fe is the active site for OER at Ni–Fe–MOF NSs.     

Large‐Scale,Bottom‐Up Synthesis of Binary Metal–Organic Framework Nanosheets for Efficient Water Oxidation

Ultrathin metal–organic framework (MOF) nanosheets (NSs) offer potential for many applications, but the synthetic strategies are largely limited to top‐down, low‐yield exfoliation methods. Herein, Ni–M–MOF (M=Fe, Al, Co, Mn, Zn, and Cd) NSs are reported with a thickness of only several atomic layers, prepared by a large‐scale, bottom‐up solvothermal method. The solvent mixture of N,N‐dimethylacetamide and water plays key role in controlling the formation of these two‐dimensional MOF NSs. The MOF NSs can be directly used as efficient electrocatalysts for the oxygen evolution reaction, in which the Ni–Fe–MOF NSs deliver a current density of 10 mA cm^?2 at a low overpotential of 221 mV with a small Tafel slope of 56.0 mV dec^?1, and exhibit excellent stability for at least 20 h without obvious activity decay. Density functional theory calculations on the energy barriers for OER occurring at different metal sites confirm that Fe is the active site for OER at Ni–Fe–MOF NSs.     

Two‐Dimensional Co@N‐Carbon Nanocomposites Facilely Derived from Metal–Organic Framework Nanosheets for Efficient Bifunctional Electrocatalysis

Metal–organic frameworks (MOFs) and MOF‐derived nanomaterials have recently attracted great interest as highly efficient, non‐noble‐metal catalysts. In particular, two‐dimensional MOF nanosheet materials possess the advantages of both 2D layered nanomaterials and MOFs and are considered to be promising nanomaterials. Herein, we report a facile and scalable in situ hydrothermal synthesis of Co–hypoxanthine (HPA) MOF nanosheets, which were then directly carbonized to prepare uniform Co@N‐Carbon nanosheets for efficient bifunctional electrocatalytic hydrogen‐evolution reactions (HERs) and oxygen‐evolution reactions (OERs). The Co embedded in N‐doped carbon shows excellent and stable catalytic performance for bifunctional electrocatalytic OERs and HERs. For OERs, the overpotential of Co@N‐Carbon at 10 mA cm^?2 was 400 mV (vs. reversible hydrogen electrode, RHE). The current density of Co@N‐Carbon reached 100 mA cm^?2 at an overpotential of 560 mV, which showed much better performance than RuO₂; the largest current density of RuO₂ that could be reached was only 44 mA cm^?2. The Tafel slope of Co@N‐Carbon was 61 mV dec^?1, which is comparable to that of commercial RuO₂ (58 mV dec^?1). The excellent electrocatalytic properties can be attributed to the nanosheet structure and well‐dispersed carbon‐encapsulated Co, CoN nanoparticles, and N‐dopant sites, which provided high conductivity and a large number of accessible active sites. The results highlight the great potential of utilizing MOF nanosheet materials as promising templates for the preparation of 2D Co@N‐Carbon materials for electrocatalysis and will pave the way to the development of more efficient 2D nanomaterials for various catalytic applications.     

Controllable Synthesis of Porphyrin‐Based 2D Lanthanide Metal–Organic Frameworks with Thickness‐ and Metal‐Node‐Dependent Photocatalytic Performance

Synthesizing 2D metal–organic frameworks (2D MOFs) in high yields and rational tailoring of the properties in a predictable manner for specific applications is extremely challenging. Now, a series of porphyrin‐based 2D lanthanide MOFs (Ln‐TCPP, Ln=Ce, Sm, Eu, Tb, Yb, TCPP=tetrakis(4‐carboxyphenyl) porphyrin) with different thickness were successfully prepared in a household microwave oven. The as‐prepared 2D Ln‐TCPP nanosheets showed thickness‐dependent photocatalytic performances towards photooxidation of 1,5‐dihydroxynaphthalene (1,5‐DHN) to synthesize juglone. Particularly, the Yb‐TCPP displayed outstanding photodynamic activity to generate O₂^? and ¹O₂. This work not only provides fundamental insights into structure designing and property tailoring of 2D MOFs nanosheets, but also pave a new way to improve the photocatalytic performance.     

Efficient Visible‐Light‐Driven Carbon Dioxide Reduction by a Single‐Atom Implanted Metal–Organic Framework

Modular optimization of metal–organic frameworks (MOFs) was realized by incorporation of coordinatively unsaturated single atoms in a MOF matrix. The newly developed MOF can selectively capture and photoreduce CO₂ with high efficiency under visible‐light irradiation. Mechanistic investigation reveals that the presence of single Co atoms in the MOF can greatly boost the electron–hole separation efficiency in porphyrin units. Directional migration of photogenerated excitons from porphyrin to catalytic Co centers was witnessed, thereby achieving supply of long‐lived electrons for the reduction of CO₂ molecules adsorbed on Co centers. As a direct result, porphyrin MOF comprising atomically dispersed catalytic centers exhibits significantly enhanced photocatalytic conversion of CO₂, which is equivalent to a 3.13‐fold improvement in CO evolution rate (200.6 μmol g^?1 h^?1) and a 5.93‐fold enhancement in CH₄ generation rate (36.67 μmol g^?1 h^?1) compared to the parent MOF.     

Atomically Dispersed Metal Sites in MOF‐Based Materials for Electrocatalytic and Photocatalytic Energy Conversion

Metal sites play an essential role in both electrocatalytic and photocatalytic energy conversion. The highly ordered arrangements of the organic linkers and metal nodes as well as the well‐defined pore structures of metal‐organic frameworks (MOFs) make them ideal substrates to support atomically dispersed metal sites (ADMSs) located in their metal nodes, linkers, and pores. Porous carbon materials doped with ADMSs can be derived from these ADMS‐incorporating MOF precursors through controlled treatments. These ADMSs incorporated in pristine MOFs and MOF‐derived carbon materials possess unique advantages over molecular or bulk metal‐based catalysts and bridge the gap between homogeneous and heterogeneous catalysts for energy‐conversion applications. This Review presents recent progress in the design and incorporation of ADMSs in MOFs and MOF‐derived materials for energy‐conversion applications.     

Bimetallic Metal‐Organic Framework‐Derived Nanosheet‐Assembled Nanoflower Electrocatalysts for Efficient Oxygen Evolution Reaction

Non‐noble metal‐based metal–organic framework (MOF)‐derived electrocatalysts have recently attracted great interest in the oxygen evolution reaction (OER). Here we report a facile synthesis of nickel‐based bimetallic electrocatalysts derived from 2D nanosheet‐assembled nanoflower‐like MOFs. The optimized morphologies and large Brunauer–Emmett–Teller (BET) surface area endow FeNi@CNF with efficient OER performance, where the aligned nanosheets can expose abundant active sites and benefit electron transfer. The complex nanoflower morphologies together with the synergistic effects between two metals attributed to the OER activity of the Ni‐based bimetallic catalysts. The optimized FeNi@CNF afforded an overpotential of 356 mV at a current density of 10 mA cm^?2 with a Tafel slope of 62.6 mV dec^?1, and also exhibited superior durability with only slightly degradation after 24 hours of continuous operation. The results may inspire the use of complex nanosheet‐assembled nanostructures to explore highly active catalysts for various applications.     

Ultrathin Metal–Organic Framework Nanosheets with Ultrahigh Loading of Single Pt Atoms for Efficient Visible‐Light‐Driven Photocatalytic H2 Evolution

A surfactant‐stabilized coordination strategy is used to make two‐dimensional (2D) single‐atom catalysts (SACs) with an ultrahigh Pt loading of 12.0 wt %, by assembly of pre‐formed single Pt atom coordinated porphyrin precursors into free‐standing metal–organic framework (MOF) nanosheets with an ultrathin thickness of 2.4±0.9 nm. This is the first example of 2D MOF‐based SACs. Remarkably, the 2D SACs exhibit a record‐high photocatalytic H₂ evolution rate of 11 320 μmol g^?1 h^?1 via water splitting under visible light irradiation (λ>420 nm) compared with those of reported MOF‐based photocatalysts. Moreover, the MOF nanosheets can be readily drop‐casted onto solid substrates, forming thin films while still retaining their photocatalytic activity, which is highly desirable for practical solar H₂ production.     

Atypical Hybrid Metal–Organic Frameworks (MOFs): A Combinative Process for MOF‐on‐MOF Growth,Etching, and Structure Transformation

The structural, compositional, and morphological features of metal–organic frameworks (MOFs) govern their properties and applications. Construction of hybrid MOFs with complicated structures, components, or morphologies is significant for the development of well‐organized MOFs. An advanced route is reported for construction of atypical hybrid MOFs with unique morphologies and complicated components: 1) MOF‐on‐MOF growth of a 3D zeolitic imidazolate framework (ZIF) on a ZIF‐L template, 2) etching of a part of the 2D ZIF‐L template, and 3) structural transformation of 2D ZIF‐L into 3D ZIF. The formation of core–shell‐type MOF rings and plates is controlled by regulating the three processes. The formation route for the core–shell‐type MOF rings and plates was monitored by tracking changes in morphology, structure, and composition. Carbon materials prepared from the pyrolysis of the core–shell‐type hybrid MOFs displayed enhanced oxygen reduction reaction activities compared to their monomeric counterparts.     

Template‐Directed Approach Towards the Realization of Ordered Heterogeneity in Bimetallic Metal–Organic Frameworks

Controlling the arrangement of different metal ions to achieve ordered heterogeneity in metal–organic frameworks (MOFs) has been a great challenge. Herein, we introduce a template‐directed approach, in which a 1D metal–organic polymer incorporating well‐defined binding pockets for the secondary metal ions used as a structural template and starting material for the preparation of well‐ordered bimetallic MOF‐74s under heterogeneous‐phase hydrothermal reaction conditions in the presence of secondary metal ions such as Ni²⁺ and Mg²⁺ in 3 h. The resulting bimetallic MOF‐74s were found to possess a nearly 1:1 metal ratio regardless of their initial stoichiometry in the reaction mixture, thus demonstrating the possibility of controlling the arrangement of metal ions within the secondary building blocks in MOFs to tune their intrinsic properties such as gas affinity.     

A Solvent‐Free Hot‐Pressing Method for Preparing Metal–Organic‐Framework Coatings

Metal–organic frameworks (MOFs), with their well‐defined pores and rich structural diversity and functionality, have drawn a great deal of attention from across the scientific community. However, industrial applications are hampered by their intrinsic fragility and poor processability. Stable and resilient MOF devices with tunable flexibility are highly desirable. Herein, we present a solvent‐ and binder‐free approach for producing stable MOF coatings by a unique hot‐pressing (HoP) method, in which temperature and pressure are applied simultaneously to facilitate the rapid growth of MOF nanocrystals onto desired substrates. This strategy was proven to be applicable to carboxylate‐based, imidazolate‐based, and mixed‐metal MOFs. We further successfully obtained superhydrophobic and “Janus” MOF films through layer‐by‐layer pressing. This HoP method can be scaled up in the form of roll‐to‐roll production and may push MOFs into unexplored industrial applications.     

Template‐Induced Diverse Metal–Organic Materials as Catalysts for the Tandem Acylation–Nazarov Cyclization

In our continuing quest to develop a metal–organic framework (MOF)‐catalyzed tandem pyrrole acylation–Nazarov cyclization reaction with α,β‐unsaturated carboxylic acids for the synthesis of cyclopentenone[b]pyrroles, which are key intermediates in the synthesis of natural product (±)‐roseophilin, a series of template‐induced Zn‐based ( 1–3 ) metal‐organic frameworks (MOFs) have been solvothermally synthesized and characterized. Structural conversions from non‐porous MOF 1 to porous MOF 2 , and back to non‐porous MOF 3 arising from the different concentrations of template guest have been observed. The anion–π interactions between the template guests and ligands could affect the configuration of ligands and further tailor the frameworks of 1–3 . Futhermore, MOFs 1–3 have shown to be effective heterogeneous catalysts for the tandem acylation–Nazarov cyclization reaction. In particular, the unique structural features of 2 , including accessible catalytic sites and suitable channel size and shape, endow 2 with all of the desired features for the MOF‐catalyzed tandem acylation–Nazarov cyclization reaction, including heterogeneous catalyst, high catalytic activity, robustness, and excellent selectivity. A plausible mechanism for the catalytic reaction has been proposed and the structure–reactivity relationship has been further clarified. Making use of 2 as a heterogeneous catalyst for the reaction could greatly increase the yield of total synthesis of (±)‐roseophilin.     

Exfoliation of Metal–Organic Frameworks to Give 2D MOF Nanosheets for the Electrocatalytic Oxygen Evolution Reaction

The structure and properties of materials are determined by a diverse range of chemical bond formation and breaking mechanisms, which greatly motivates the development of selectively controlling the chemical bonds in order to achieve materials with specific characteristics. Here, an orientational intervening bond-breaking strategy is demonstrated for synthesizing ultrathin metal–organic framework (MOF) nanosheets through balancing the process of thermal decomposition and liquid nitrogen exfoliation. In such approach, proper thermal treatment can weaken the interlayer bond while maintaining the stability of the intralayer bond in the layered MOFs. And the following liquid nitrogen treatment results in significant deformation and stress in the layered MOFs’ structure due to the instant temperature drop and drastic expansion of liquid N₂, leading to the curling, detachment, and separation of the MOF layers. The produced MOF nanosheets with five cycles of treatment are primarily composed of nanosheets that are less than 10 nm in thickness. The MOF nanosheets exhibit enhanced catalytic performance in oxygen evolution reactions owing to the ultrathin thickness without capping agents which provide improved charge transfer efficiency and dense exposed active sites. This strategy underscores the significance of orientational intervention in chemical bonds to engineer innovative materials.