Hollow Metal–Organic‐Framework Micro/Nanostructures and their Derivatives: Emerging Multifunctional Materials

Hollow metal–organic framework (MOF) micro/nanostructures and their derivatives are attracting a great amount of research interest in recent years because their hierarchical porous structures not only provide abundant, easily accessed metal sites but also endow 3D channels for rapid mass transport. As a result, they demonstrate significant advantages in many applications including catalysis, gas sensors, batteries, supercapacitors, and so on. Nevertheless, studies on hollow MOFs and their derivatives are still at the beginning of this field, and the relationship between their structures and application performances is not yet reviewed comprehensively. Herein, the synthetic strategies and practical applications of hollow micro/nanostructured MOFs and their derivatives are summarized, and their corresponding prospects are also discussed.     

Applications of Metal–Organic‐Framework‐Derived Carbon Materials

Carbon materials derived from metal–organic frameworks (MOFs) have attracted much attention in the field of scientific research in recent years because of their advantages of excellent electron conductivity, high porosity, and diverse applications. Tremendous efforts are devoted to improving their chemical and physical properties, including optimizing the morphology and structure of the carbon materials, compositing them with other materials, and so on. Here, many kinds of carbon materials derived from metal–organic frameworks are introduced with a particular focus on their promising applications in batteries (lithium‐ion batteries, lithium–sulfur batteries, and sodium‐ion batteries), supercapacitors (metal oxide/carbon and metal sulfide/carbon), electrocatalytic reactions (oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction), water treatment (MOF‐derived carbon and other techniques), and other possible fields. To close, some existing problem and corresponding possible solutions are proposed based on academic knowledge from the reported literature, along with a great deal of experimental experience.     

Hollow Functional Materials Derived from Metal–Organic Frameworks: Synthetic Strategies,Conversion Mechanisms,and Electrochemical Applications

Hollow materials derived from metal–organic frameworks (MOFs), by virtue of their controllable configuration, composition, porosity, and specific surface area, have shown fascinating physicochemical properties and widespread applications, especially in electrochemical energy storage and conversion. Here, the recent advances in the controllable synthesis are discussed, mainly focusing on the conversion mechanisms from MOFs to hollow‐structured materials. The synthetic strategies of MOF‐derived hollow‐structured materials are broadly sorted into two categories: the controllable synthesis of hollow MOFs and subsequent pyrolysis into functional materials, and the controllable conversion of solid MOFs with predesigned composition and morphology into hollow structures. Based on the formation processes of hollow MOFs and the conversion processes of solid MOFs, the synthetic strategies are further conceptually grouped into six categories: template‐mediated assembly, stepped dissolution–regrowth, selective chemical etching, interfacial ion exchange, heterogeneous contraction, and self‐catalytic pyrolysis. By analyzing and discussing 14 types of reaction processes in detail, a systematic mechanism of conversion from MOFs to hollow‐structured materials is exhibited. Afterward, the applications of these hollow structures as electrode materials for lithium‐ion batteries, hybrid supercapacitors, and electrocatalysis are presented. Finally, an outlook on the emergent challenges and future developments in terms of their controllable fabrications and electrochemical applications is further discussed.     

Enhanced Chemisorption and Catalytic Effects toward Polysulfides by Modulating Hollow Nanoarchitectures for Long‐Life Lithium–Sulfur Batteries

Hollow nanostructures with intricate interior and catalytic effects hold great promise for the construction of advanced lithium–sulfur batteries. Herein, a double‐shelled hollow polyhedron with inlaid cobalt nanoparticles encapsulated by nitrogen‐doped carbon (Co/NC) nanodots (Co‐NC@Co₉S₈/NPC) is reported, which is acquired by using imidazolium‐based ionic‐polymer‐encapsulated zeolitic imidazolate framework‐67 as a core‐shelled precursor. The Co/NC nanodots promote redox kinetics and chemical adsorbability toward polysulfides, while the interconnected double shells serve as a nanoscale electrochemical reaction chamber, which effectively suppresses the polysulfide shuttling and accelerates ion/electron transport. Benefiting from structural engineering and reaction kinetics modulation, the Co‐NC@Co₉S₈/NPC‐S electrode exhibits high cycling stability with a low capacity decay of 0.011% per cycle within 2000 cycles at 2 C. The electrode still shows high rate performance and cyclability over 500 cycles even in the case of high sulfur loading of 4.5 mg cm^?2 and 75 wt% sulfur content. This work provides one type of new hollow nanoarchitecture for the development of advanced Li–S batteries and other energy storage systems.     

Coordination‐Induced Interlinked Covalent‐ and Metal–Organic‐Framework Hybrids for Enhanced Lithium Storage

Covalent organic frameworks (COF) or metal–organic frameworks have attracted significant attention for various applications due to their intriguing tunable micro/mesopores and composition/functionality control. Herein, a coordination‐induced interlinked hybrid of imine‐based covalent organic frameworks and Mn‐based metal–organic frameworks (COF/Mn‐MOF) based on the Mn? N bond is reported. The effective molecular‐level coordination‐induced compositing of COF and MOF endows the hybrid with unique flower‐like microsphere morphology and superior lithium‐storage performances that originate from activated Mn centers and the aromatic benzene ring. In addition, hollow or core–shell MnS trapped in N and S codoped carbon (MnS@NS‐C‐g and MnS@NS‐C‐l) are also derived from the COF/Mn‐MOF hybrid and they exhibit good lithium‐storage properties. The design strategy of COF–MOF hybrid can shed light on the promising hybridization on porous organic framework composites with molecular‐level structural adjustment, nano/microsized morphology design, and property optimization.     

Metal–Organic‐Framework‐Based Catalysts for Photoreduction of CO2

Photoreduction of CO₂ into reusable carbon forms is considered as a promising approach to address the crisis of energy from fossil fuels and reduce excessive CO₂ emission. Recently, metal–organic frameworks (MOFs) have attracted much attention as CO₂ photoreduction‐related catalysts, owing to their unique electronic band structures, excellent CO₂ adsorption capacities, and tailorable light‐absorption abilities. Recent advances on the design, synthesis, and CO₂ reduction applications of MOF‐based photocatalysts are discussed here, beginning with the introduction of the characteristics of high‐efficiency photocatalysts and structural advantages of MOFs. The roles of MOFs in CO₂ photoreduction systems as photocatalysts, photocatalytic hosts, and cocatalysts are analyzed. Detailed discussions focus on two constituents of pure MOFs (metal clusters such as Ti–O, Zr–O, and Fe–O clusters and functional organic linkers such as amino‐modified, photosensitizer‐functionalized, and electron‐rich conjugated linkers) and three types of MOF‐based composites (metal–MOF, semiconductor–MOF, and photosensitizer–MOF composites). The constituents, CO₂ adsorption capacities, absorption edges, and photocatalytic activities of these photocatalysts are highlighted to provide fundamental guidance to rational design of efficient MOF‐based photocatalyst materials for CO₂ reduction. A perspective of future research directions, critical challenges to be met, and potential solutions in this research field concludes the discussion.     

Multishelled NixCo3–xO4 Hollow Microspheres Derived from Bimetal–Organic Frameworks as Anode Materials for High‐Performance Lithium‐Ion Batteries

Metal–organic frameworks (MOFs) featuring versatile topological architectures are considered to be efficient self‐sacrificial templates to achieve mesoporous nanostructured materials. A facile and cost‐efficient strategy is developed to scalably fabricate binary metal oxides with complex hollow interior structures and tunable compositions. Bimetal–organic frameworks of Ni‐Co‐BTC solid microspheres with diverse Ni/Co ratios are readily prepared by solvothermal method to induce the Ni _x Co_{3? x} O₄ multishelled hollow microspheres through a morphology‐inherited annealing treatment. The obtained mixed metal oxides are demonstrated to be composed of nanometer‐sized subunits in the shells and large void spaces left between adjacent shells. When evaluated as anode materials for lithium‐ion batteries, Ni _x Co_{3? x} O₄‐0.1 multishelled hollow microspheres deliver a high reversible capacity of 1109.8 mAh g^?1 after 100 cycles at a current density of 100 mA g^?1 with an excellent high‐rate capability. Appropriate capacities of 832 and 673 mAh g^?1 could also be retained after 300 cycles at large currents of 1 and 2 A g^?1, respectively. These prominent electrochemical properties raise a concept of synthesizing MOFs‐derived mixed metal oxides with multishelled hollow structures for progressive lithium‐ion batteries.     

Complex Hollow Nanostructures: Synthesis and Energy‐Related Applications

Hollow nanostructures offer promising potential for advanced energy storage and conversion applications. In the past decade, considerable research efforts have been devoted to the design and synthesis of hollow nanostructures with high complexity by manipulating their geometric morphology, chemical composition, and building block and interior architecture to boost their electrochemical performance, fulfilling the increasing global demand for renewable and sustainable energy sources. In this Review, we present a comprehensive overview of the synthesis and energy‐related applications of complex hollow nanostructures. After a brief classification, the design and synthesis of complex hollow nanostructures are described in detail, which include hierarchical hollow spheres, hierarchical tubular structures, hollow polyhedra, and multi‐shelled hollow structures, as well as their hybrids with nanocarbon materials. Thereafter, we discuss their niche applications as electrode materials for lithium‐ion batteries and hybrid supercapacitors, sulfur hosts for lithium–sulfur batteries, and electrocatalysts for oxygen‐ and hydrogen‐involving energy conversion reactions. The potential superiorities of complex hollow nanostructures for these applications are particularly highlighted. Finally, we conclude this Review with urgent challenges and further research directions of complex hollow nanostructures for energy‐related applications.     

A Metal–Organic‐Framework‐Based Electrolyte with Nanowetted Interfaces for High‐Energy‐Density Solid‐State Lithium Battery

Solid‐state batteries (SSBs) are promising for safer energy storage, but their active loading and energy density have been limited by large interfacial impedance caused by the poor Li⁺ transport kinetics between the solid‐state electrolyte and the electrode materials. To address the interfacial issue and achieve higher energy density, herein, a novel solid‐like electrolyte (SLE) based on ionic‐liquid‐impregnated metal–organic framework nanocrystals (Li‐IL@MOF) is reported, which demonstrates excellent electrochemical properties, including a high room‐temperature ionic conductivity of 3.0 × 10^‐4 S cm^‐1, an improved Li⁺ transference number of 0.36, and good compatibilities against both Li metal and active electrodes with low interfacial resistances. The Li‐IL@MOF SLE is further integrated into a rechargeable Li|LiFePO₄ SSB with an unprecedented active loading of 25 mg cm^‐2, and the battery exhibits remarkable performance over a wide temperature range from ?20 up to 150 °C. Besides the intrinsically high ionic conductivity of Li‐IL@MOF, the unique interfacial contact between the SLE and the active electrodes owing to an interfacial wettability effect of the nanoconfined Li‐IL guests, which creates an effective 3D Li⁺ conductive network throughout the whole battery, is considered to be the key factor for the excellent performance of the SSB.     

Ternary Alloys Encapsulated within Different MOFs via a Self‐Sacrificing Template Process: A Potential Platform for the Investigation of Size‐Selective Catalytic Performances

Functional nanoparticles encapsulated within metal–organic frameworks (MOFs) as an emerging class of composite materials attract increasing attention owing to their enhanced or even novel properties caused by the synergistic effect between the two functional materials. However, there is still no ideal composite structure as platform to systematically analyze and evaluate the relation between the enhanced catalytic performance of composites and the structure of MOF shells. In this work, taking RhCoNi ternary alloy nanoflowers, for example, first the RhCoNi@MOF composite catalysts sheathed with different structured MOFs via a facile self‐sacrificing template process are successfully fabricated. The structure type of MOF shells is easily adjustable by using different organic molecules as etchant and coordination reagent (e.g., 2,5‐dihydroxyterephthalic acid or 2‐methylimidazole), which can dissolve out the Co or Ni element in the alloy template in a targeted manner, thereby producing ZIF‐67(Co) or MOF‐74(Ni) shells accordingly. With the difference between the two MOF shells in the aperture sizes, the as‐prepared two RhCoNi@MOF composites preform distinct size selectivity during the alkene hydrogenation. This work would help us to get more comprehensive understanding of the intrinsic role of MOFs behind the enhanced catalytic performance of nanoparticle@MOF composites.     

Shape‐Defined Hollow Structural Co‐MOF‐74 and Metal Nanoparticles@Co‐MOF‐74 Composite through a Transformation Strategy for Enhanced Photocatalysis Performance

In recent years, metal–organic frameworks (MOFs) have received extensive interest because of the diversity of their composition, structure, and function. To promote the MOFs' function and performance, the construction of hollow structural metal–organic frameworks and nanoparticle–MOF composites is significantly effective but remains a considerable challenge. In this article, a transformation strategy is developed to synthesize hollow structural Co‐MOF‐74 by solvothermal transformation of ZIF‐67. These Co‐MOF‐74 particles exhibit a double‐layer hollow shell structure without remarkable shape change compared to original ZIF‐67 particles. The formation of hollow structure stemmed from the density difference of Co between ZIF‐67 and Co‐MOF‐74. By this strategy, hollow structural Co‐MOF‐74 with different sizes and shapes are obtained from corresponding ZIF‐67, and metal nanoparticles@Co‐MOF‐74 is synthesized by corresponding nanoparticles@Co‐ZIF‐67. To verify the structural advantages of hollow structural Co‐MOF‐74 and Ag nanoparticles@Co‐MOF‐74, photocatalytic CO₂ reduction is used as a model reaction. Conventionally synthesized Co‐MOF‐74 (MOF‐74‐C), hollow structural Co‐MOF‐74 synthesized by transformation method (MOF‐74‐T) and Ag nanoparticles@Co‐MOF‐74 (AgNPs@MOF‐74) are used as cocatalysts in this reaction. As a result, the cocatalytic activity of MOF‐74‐T and AgNPs@MOF‐74 is 1.8 times and 3.8 times that of MOF‐74‐C, respectively.     

MOF‐Based Hierarchical Structures for Solar‐Thermal Clean Water Production

Solar‐thermal water evaporation, as a promising method for clean water production, has attracted increasing attention. However, solar water evaporators that exhibit both high water vapor generation ability and anti‐oil‐fouling ability have not been reported. Here, a unique metal–organic‐framework‐based hierarchical structure, referred to as MOF‐based hierarchical structure (MHS), is rationally designed and prepared, which simultaneously displays a high solar absorption and a superhydrophilic and underwater superoleophobic surface property. As a proof‐of‐concept application, a device prepared from the MHS can achieve a high solar‐thermal water evaporation rate of 1.50 kg m^?2 h^?1 under 1 sun illumination. Importantly, the MHS also possesses an excellent anti‐oil‐fouling property, ensuring its superior water evaporation performance even in oil‐contaminated water. The high solar‐thermal water evaporation rate and anti‐oil‐fouling property make the MHS a promising material for the solar‐thermal water production.     

2D Metal–Organic Frameworks (MOFs) for High‐Performance BatCap Hybrid Devices

Two identical layered metal–organic frameworks (MOFs) (CoFRS and NiFRS) are constructed by using flexible 1,10‐bis(1,2,4‐triazol‐1‐yl)decane as pillars and 1,4‐benzenedicarboxylic acid as rigid linkers. The single‐crystal structure analysis indicates that the as‐synthesized MOFs possess fluctuant 2D networks with large interlayer lattices. Serving as active electrode elements in supercapacitors, both MOFs deliver excellent rate capabilities, high capacities, and longstanding endurances. Moreover, the new intermediates in two electrodes before and after long‐lifespan cycling are also examined, which cannot be identified as metal hydroxides in the peer reports. After assembled into battery‐supercapacitor (BatCap) hybrid devices, the NiFRS//activated carbon (AC) device displays better electrochemical results in terms of gravimetric capacitance and cycling performance than CoFRS//AC devices, and a higher energy‐density value of 28.7 Wh kg^?1 compared to other peer references with MOFs‐based electrodes. Furthermore, the possible factors to support the distinct performances are discussed and analyzed.     

Three‐Layered Hollow Nanospheres Based Coatings with Ultrahigh‐Performance of Energy‐Saving,Antireflection, and Self‐Cleaning for Smart Windows

In this study, a well‐controlled interfacial engineering method for the synthesis of SiO₂/TiO₂/VO₂ three‐layered hollow nanospheres (TLHNs) and TLHNs‐based multifunctional coatings is reported. The as‐prepared coatings allow for an outstanding integration of thermochromism from the outer VO₂(M) layer, photocatalytic self‐cleaning capability from the middle TiO₂(A) layer, and antireflective property from internal SiO₂ HNs. The TLHNs coatings exhibit excellent optical performance with ultrahigh luminous transmittance (T_lum‐l = 74%) and an improved solar modulation ability (ΔT_sol = 12%). To the best knowledge, this integrated optical performance is the highest ever reported for TiO₂/VO₂‐based thermochromic coatings. An ingenious computation model is proposed, which allows the n_eff of nanostructured coatings to be rapidly obtained. The experimental and calculated results reveal that the unique three‐layered structure significantly reduces the refractive index (from 2.25 to 1.33 at 600 nm) and reflectance (Rave, from 22.3 to 5.3%) in the visible region as compared with dense coatings. Infrared thermal imaging characterization and self‐cleaning tests provide valid evidence of SiO₂/TiO₂/VO₂ TLHNs coatings' potential for energy‐saving and self‐cleaning smart windows. The exciting inexpensive and universal fabrication process for well‐defined structures may inspire various developments in processable and multifunctional devices.     

Metal–Organic‐Framework‐Derived Hybrid Carbon Nanocages as a Bifunctional Electrocatalyst for Oxygen Reduction and Evolution

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are cornerstone reactions for many renewable energy technologies. Developing cheap yet durable substitutes of precious‐metal catalysts, especially the bifunctional electrocatalysts with high activity for both ORR and OER reactions and their streamlined coupling process, are highly desirable to reduce the processing cost and complexity of renewable energy systems. Here, a facile strategy is reported for synthesizing double‐shelled hybrid nanocages with outer shells of Co‐N‐doped graphitic carbon (Co‐NGC) and inner shells of N‐doped microporous carbon (NC) by templating against core–shell metal–organic frameworks. The double‐shelled NC@Co‐NGC nanocages well integrate the high activity of Co‐NGC shells into the robust NC hollow framework with enhanced diffusion kinetics, exhibiting superior electrocatalytic properties to Pt and RuO₂ as a bifunctional electrocatalyst for ORR and OER, and hold a promise as efficient air electrode catalysts in Zn–air batteries. First‐principles calculations reveal that the high catalytic activities of Co‐NGC shells are due to the synergistic electron transfer and redistribution between the Co nanoparticles, the graphitic carbon, and the doped N species. Strong yet favorable adsorption of an OOH* intermediate on the high density of uncoordinated hollow‐site C atoms with respect to the Co lattice in the Co‐NGC structure is a vital rate‐determining step to achieve excellent bifunctional electrocatalytic activity.     

Alkylated Meso‐Macroporous Metal–Organic Framework Hollow Tubes as Nanocontainers of Octadecane for Energy Storage and Thermal Regulation

Alkylated meso‐macroporous Cr‐MIL‐101 hollow tubes (ACHT) are synthesized through the coordination of Cr³⁺ with alkylated bridged ligands. The alkylated ligands work as both bridging ligands and structure‐directing agents, which presents a new strategy for synthesizing meso‐macroporous metal–organic frameworks (MOFs). This is the first example of 1D meso‐macroporous MOF hollow tubes with large pore diameter (5–136 nm). Furthermore, the octadecane@ACHT form‐stable phase change materials (FSPCMs) are prepared using ACHT as nanocontainers for octadecane. The nanocavity of ACHT possesses enough storage space for octadecane, and the large number of alkyl chains on the ACHT framework induces octadecane molecules to crystallize from α‐ to β‐crystals. Considering the comfortable phase transition temperature (27.7 °C) and the relatively large thermal storage capacity (187 J g^?1), the octadecane@ACHT FSPCMs can be used in interior wall of building to improve the indoor thermal comfort or in the field of thermal‐regulated textiles and clothing.     

Highly Water‐Stable Lanthanide–Oxalate MOFs with Remarkable Proton Conductivity and Tunable Luminescence

Although proton conductors derived from metal–organic frameworks (MOFs) are highly anticipated for various applications including solid‐state electrolytes, H₂ sensors, and ammonia synthesis, they are facing serious challenges such as poor water stability, fastidious working conditions, and low proton conductivity. Herein, we report two lanthanide–oxalate MOFs that are highly water stable, with so far the highest room‐temperature proton conductivity (3.42 × 10^?3 S cm^?1) under 100% relative humidity (RH) among lanthanide‐based MOFs and, most importantly, luminescent. Moreover, the simultaneous response of both the proton conductivity and luminescence intensity to RH allows the linkage of proton conductivity with luminescence intensity. This way, the electric signal of proton conductivity variation versus RH will be readily translated to optical signal of luminescence intensity, which can be directly visualized by the naked eye. If proper lanthanide ions or even transition‐metal ions are used, the working wavelengths of luminescence emissions can be further extended from visible to near infrared light for even wider‐range applications.