Nanostructured Conductive Metal–Organic Frameworks: Synthesis and Applications

Conductive metal–organic frameworks (c‐MOFs) have aroused extensive attention due to their excellent properties by integrating the features of both conductive and porous crystalline materials. Recent investigations in miniaturization of c‐MOFs into nanostructured conductive MOFs (nc‐MOFs) endow them with additional advantages such as reduced diffusion distance, accelerated mass/electron transfer, enhanced active site exposure, and accessibility, further widening their applications with reinforced performances compared to the conventional bulk counterparts. Herein, a timely review on the latest achievements in the field of nc‐MOFs is provided. First, we systematically introduce the synthetic strategies for producing nc‐MOFs with controllable morphologies and compositions. Second, the enhanced performance of nc‐MOFs in electrocatalysis, supercapacitors, batteries, sensors, and photocatalysis is highlighted. Finally, a discussion on the challenges and opportunities of the synthesis and applications of nc‐MOFs is presented. It is expected that this review will shed light on the future development of nc‐MOFs with promising application potential.

40][41] However, a dedicated review on the recent breakthroughs in nanostructure engineering and broad applications of nc-MOFs is still rare.
Here, we present the first review from the angle of nc-MOFs with their latest progress in the synthesis and applications, as summarized in Scheme 1. First, the synthetic strategies of nc-MOFs, including direct growth, modulation method, interfacial synthesis, and sacrificial template method, and their resultant nanostructures will be presented.Subsequently, the applications of nc-MOFs in electrocatalysis, supercapacitors, batteries, sensors, and photocatalysis will be introduced with the structure-performance relationship elucidated.Finally, we will discuss the future challenges and prospects in this research field.

Synthesis and Structures of nc-MOFs
The synthesis of c-MOF with controllable morphology and size is crucial for tuning their properties in applications.Over the past few years, various fabrication strategies have been developed for the synthesis of nc-MOFs, including direct growth, modulation method, interfacial synthesis, and sacrificial template method.On this basis, nc-MOFs with diverse geometric morphologies, ranging from 0D nanoparticle, 1D nanorod or nanotube, 2D nanosheet to 3D hierarchical assemble, or even complex multishell hollow structure, have been successfully fabricated.The detailed process of each strategy and resultant nc-MOFs will be summarized in the following sections.

Direct Synthesis
As one of the most effective and versatile strategies for preparing nc-MOFs, direct synthesis is generally performed by dissolving the metal sources and organic ligands in selected solvents followed by a hydro/solvothermal or room-temperature coprecipitation process.The structure of resultant nc-MOFs is affected by the reaction parameters including solvent type, precursor concentration, pH, reaction time, and temperature.And their composition can also be facilely tuned by changing the kinds of metal source and/or organic ligand.
Different from the traditional nanorods with solid feature, Liu et al. reported the preparation of Cu-benzenehexathiol (Cu-BHT) hollow nanotubes with high specific area and abundant crystal defects (66.6%). [55]The synthesis was performed by reacting BHT with Cu(NO 3 ) 2 in ethanol solution at À9 °C for 30 min (Figure 2a), which was facile and time-saving.The SEM, TEM, and powder X-Ray diffraction (XRD) results (Figure 2b-d [42] Copyright 2012, American Chemical Society.e) Scheme for the preparation of bimetallic NiCo-CAT; f ) SEM image and g) high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and corresponding elemental mapping images of NiCo-CAT.Reproduced with permission. [48]opyright 2019, Wiley-VCH.h) Diagram of the synthesis process of M 3 (HHTQ) 2 ; i) SEM and j) HRTEM images of Cu 3 (HHTQ) 2 .Reproduced with permission. [52]Copyright 2022, Wiley-VCH.
Cu 3 C 6 S 6 structure.The shell thickness and hollow cavity size were measured to be ≈6 and 78 nm, respectively.
Compared to the currently dominated 1D rod-like morphology, design of nc-MOFs with 2D nanosheet structure may grant them unique properties of high surface-to-volume atom ratios, rapid mass transfer, and highly exposed active sites.Encouragingly, Xin et al. developed a room-temperature coprecipitation route for the synthesis of Co 3 (HITP) 2 (HITP = 2,3,6,7,10,11-hexaaminotriphenylene) nanosheets. [57]By mixing CoCl 2 aqueous solution, HITP and concentrated aqueous ammonia with continuous stirring at room temperature for 3 h, Co 3 (HITP) 2 nanosheets with wrinkled surface were generated (Figure 3a,b).In the HRTEM image, clear patterns with arrangement of hexagonal pores were observed, showing the characteristic feature of conjugated c-MOFs (Figure 3c,d).More recently, Cheng et al. reported the synthesis of Co-DABDT (DABDT = 2,5-diamino-1,4-benzenedithiol) with rectangular nanosheet structure. [58]The thickness and length were determined to be 40 and 80 nm, respectively.Besides the direct use of ligands, Anderson et al. designed a stannylated triphenylenehexathiolate precursor (SnTHT, THT = 2,3,6,7,10,11hexa(bis-dibutyltintyltriphenylene) as a protected benzene hexathiolate. [59]By further mixing SnTHT with FeCl 2 and thiophenol, crystalline 2D Fe 4 (THT) 2 nanoflakes were obtained.As another typical work, Fu et al. reported the construction of 3D c-MOF assembles composed of ultra-thin nanosheets as subunits. [60]n their synthesis, Co 2þ and hexaaminobenzene (HAB) were used as metal nodes and organic linkers, which were reacted in a DMF/H 2 O/NH 3 •H 2 O mixture solution at 75 °C for 2 h.The resultant Co-HAB exhibited a hierarchical flower-like structure with a diameter of ≈500 nm.The thickness of the nanosheets was measured to be about 4.5 nm.When changing the reaction temperature and time into 100 °C and 12 h or using H 2 O only as solvent at room temperature, Co-HAB with the forms of nanoparticle and bulk material was obtained, showing the vital role of reaction conditions in tuning the structure of c-MOFs.
Also using HAB as the ligands, bimetal c-MOFs with multishelled hollow structure could be fabricated via a two-step route. [61]The first step underwent sequential amination, protonation, and oxidation reactions by using manganese (II) nitrate, iron (II) nitrate, and HAB as precursors, leading to the generation of Mn/Fe-HAB nanospheres with solid structure (Figure 4a).Through a second-step thermal treatment under nitrogen atmosphere at 300 °C for 1 h, the solid spheres were transformed into an intricated hollow multishelled structure (HMSS).As shown in Figure 4d, the Mn/Fe-HAB HMSS possessed quintuple-shelled hollow spherical morphology with an average diameter from 100 to 300 nm and shell thickness of about 30 nm (Figure 4b,d).The Pawley-refined XRD pattern showed a hexagonal crystal structure (Figure 4c), well matched with previously reported HAB-based c-MOFs.In the element mapping images (Figure 4e), the elements of Fe, Mn, C, and N were found to be evenly distributed in each shell in Mn/Fe-HAB HMSS.

Modulation Method
Modulation method refers to the synthesis process using modulators to control the structures (e.g., shape, size, defect) of  and c,d) HRTEM images of Co 3 (HITP) 2 .Reproduced with permission. [57]Copyright 2020, Elsevier B.V.
MOFs by regulating the nucleation kinetics or inducing the direction-specific growth.Different from the wide application in traditional MOFs, only few works have been reported for the synthesis of c-MOFs by modulation method.As a representative example, Xiao et al. reported the synthesis of Cu 3 (HHTP) 2 NCs with distinct rod, block, and flake-like morphologies via an organic quinone-mediated synthesis. [62]The organic quinones as the chemical oxidant and modulator could change the oxidation degree of HHTP ligand and thus regulate the morphology of Cu 3 (HHTP) 2 NCs.During the synthesis, 5-dichloro-1,4benzoquinone was first used to treat the HHTP ligand for preoxidation.Afterward, CuSO 4 was further added with slow introduction of additional oxidant (2,6-dimethy-1,4-lbenzoquinone), generating the Cu 3 (HHTP) 2 (Figure 5a).By simply tuning the equivalents from 0, 0.3 to 0.5, Cu 3 (HHTP) 2 NCs that exhibited nanorods (Figure 5b) with a length of 1.15 AE 0.80 μm and diameter of 0.13 AE 0.05 μm, block-like particles (Figure 5c) with a length of 0.55 AE 0.33 μm and diameter of 0.33 AE 0.18 μm, and nanoflakes (Figure 5d) with a length of 0.06 AE 0.02 μm and diameter of 0.45 AE 0.19 μm were fabricated respectively, resulting in an over 60-fold decrease in the particle aspect ratio, from 8.85 to 0.13.
Except for quinone, CH 3 ONa was also employed as modulator to control the deprotonation level of ligand, which further altered the coordination kinetics and thus affected the morphology of c-MOFs NCs (Cu-BHT) (Figure 5e). [63]Without CH 3 ONa, the reaction between CuCl 2 and BHT led to the formation of Cu-BHT crystals with nanorod morphology.By adding CH 3 ONa (3 mM) with other conditions unchanged, Cu-BHT nanoparticles with amorphous nature were obtained.

Interfacial Synthesis
Compared with the direct synthesis and modulation method that produce powder products, the interfacial synthesis is conducive for fabricating c-MOF-based large-area films or arrays assembled by confining the reaction at the biphasic interface.To date, interfacial growth methods based on liquid-liquid, liquid-solid, and gas-solid interfaces have been developed for nc-MOFs synthesis.

Liquid-Liquid Interface
For the liquid-liquid process, the growth of nc-MOFs generally takes place at the interface between two kinds of solvents with one solvent containing the metal and the other containing the ligand.The metals and ligands diffuse toward the interface, mainly resulting in c-MOF nanosheets and their assembled films.For instance, by spreading an HHTP-dissolved ethyl acetate solution onto a Cu(OAc) 2 aqueous solution, a bluish film with dark color was formed at the interphase at room temperature after a few minutes (Figure 6a,b). [64]The internal structure of the film was investigated by TEM, showing interconnected nanosheets with sharp edges, length of 20-200 nm and hexagonally arranged channels (Figure 6c).The as-made film was then transferred onto a SiO 2 substrate.However, partial cracks were generated in the films with inhomogeneous thickness ranging from 10 to 70 nm.As another typical example of liquid-liquid synthesis, Nishihara et al. reported the preparation of Ni-BTT (BTT = 1,2,4,5-benzenetetrathiol) film at the interface between Reproduced with permission. [61]Copyright 2019, Royal Society of Chemistry.
Ni(OAc) 2 /NaBr aqueous solution and BTT dissolved dichloromethane. [65]ore recently, Chu et al. developed a liquidÀliquid interfacial self-assembly system for the preparation of metal-BHT nanosheets, which could be directly transferred onto the substrate as homogeneous film. [66]Using Cu-BHT as a typical example, the assembly between Cu 2þ and BHT occurred at the interface between H 2 O and C 6 H 5 Cl (Figure 6d-e).The Cu-BHT nanosheets exhibited janus surfaces including a rough surface (facing the organic solvent phase) composed of crystal slabs and a flat surface (facing the H 2 O phase) with nanoscale holes (Figure 6g,h).The formation of rough surface indicated that the Cu 2þ ions with small radius and fast diffusion rate may pass across the interface, resulting in the growth of Cu-BHT nanoparticles in the organic solvent phase.The formed nanosheets were parallelly arranged into film, whose thickness could be controlled by tuning the dosage of reactants, reaction time, and interfacial confinement.In addition, by simply changing the pH of the H 2 O phase, the content of Cu vacancies could be efficiently altered (Figure 6f ).Concretely, the content of Cu vacancies gradually increased by raising the pH from 0, 1 to 2 (Figure 6i).Overall, such a synthesis route realized the control of c-MOF structures at both nano and atomic length scales, which could be even extended to the fabrication of Ni-BHT and Ag-BHT nanosheets.

Liquid-Solid Interface
Apart from liquid-liquid system, liquid-solid interface-assisted routes have emerged to fabricate c-MOF-based composites or self-standing nanoarrays by interfacial growth of nc-MOFs on the surface of preformed substrates with nano or macroscopic sizes.During the growth process, the substrates were general stable without significant structure changes.For example, Xu et al. reported the synthesis of c-MOF layers on cellulose nanofibers (CNFs) with the formation of core-shell CNF@c-MOF structures by interfacial growth strategy (Figure 7a). [67]The cellulose CNFs were extracted from Cladophora algae with a modification by TEMPO oxidation (TEMPO = 2,2,6,6-tetramethylpiperidin-1-yloxyl).Abundant carboxyl groups were thus introduced on the surface of CNFs as anchoring sites for coordination with Ni 2þ .With the addition of Ni(NO 3 ) 2 •6H 2 O and organic ligand HHTP or HITP, the heterogeneous nucleation and growth of Ni-HHTP or Ni-HITP were induced on the surface of CNFs.The highly crystalline c-MOF layer with thickness of ≈5 nm was uniformly coated on CNFs, resulting in the core-shell CNF@c-MOF  and d) nanoflakes.Reproduced with permission. [62]Copyright 2022, Royal Society of Chemistry.e) Scheme for the synthesis of Cu-BHT with tunable morphologies.Reproduced with permission. [63]Copyright 2017, American Chemical Society.
Except for the hybridization of c-MOF with non-MOF materials, traditional MOFs can also serve as the substrate for c-MOF growth toward the construction of MOF-on-MOF heterostructures.Recently in 2022, UiO-66-NH 2 (UiO = University of Oslo)@Ni-HHTP heterostructures were successfully synthesized by Moon et al. [70] UiO-66-NH 2 nanocrystals with octahedral shape were firstly fabricated by a hydrothermal method (Figure 7f ), which were then immersed in a HHTP solution for surface fuctionalization.Subsequently, Ni 2þ ions as metal sources were added for the framework synthesis of Ni-HHTP (Figure 7e).As shown in Figure 7g, Ni-HHTP nanorods with length of ≈50-100 nm were evenly adhered on the surface of UiO-66-NH 2 with diameter of ≈400 nm, forming a core-satellite structure (Figure 7h).Such a synthesis is also applicable to macroscopic substrates for constructing c-MOF-based self-standing arrays, which is more conducive for the practical applications.In 2017, Xu et al. employed carbon fiber paper (CFP) as the substrate for the growth of Cu-HHTP nanowire arrays (Figure 8a,b). [71]The SEM images (Figure 8c-e) showed that the smooth surface of CFP was homogeneously covered by high-density nanowires with uniform hexagonal-prism shape and hexagonal top facet after Cu-HHTP growth.The solid-liquid route is highly versatile for different substrates and c-MOFs with various components and shapes.For example, Ni-HITP nanosheet array [72] and bimetal c-MOF (CoCu-CAT) nanorod array [73] could also be fabricated on CFP and carbon paper (CC), respectively, using the similar method.Despite of CFP, Cu-HHTP nanowire arrays can also be produced on the flexible polypyrrole membranes. [74]gure 6.a) Scheme for the interfacial synthesis of c-MOF films; b) photograph of the film formed at the liquid-liquid interface; and c) HRTEM images of c-MOF films.Reproduced with permission. [64]Copyright 2018, Wiley-VCH.d) Chemical reaction for synthesizing Cu-BHT; e) interfacial growth process; f ) illustration of metal vacancy engineering via precise pH regulation (yellow spheres, protons; black spheres, C atoms; gold spheres, S atoms; green spheres, Cu atoms; red circles, Cu vacancies); g) upside surface facing the aqueous phase and h) downside surface facing the organic phase of Cu-BHT; and i) XPS spectra of Cu 2p of Cu-BHT.Reproduced with permission. [66]Copyright 2022, American Chemical Society.

Gas-Solid Interface
Gas-solid interfacial synthesis is usually performed by using the chemical vapor deposition (CVD) method, mainly producing c-MOF thin films on selected substrate materials.For example, Park et al. synthesized a Cu 3 (C 6 O 6 ) 2 thin film by vapor-phase ligand substitution reaction between bis(2,4-pentanedionato) copper (II) (Cu(acac) 2 ) and THQ on the SiO 2 /Si substrate. [75]n their synthesis, two inner tubes containing Cu(acac) 2 and THQ precursors were placed in a large quartz tube face by face.By heating at 230 °C, a vapor-phase reaction was initiated for the growth of c-MOF, where the film thickness could be finely controlled by simply changing the Cu(acac) 2 /THQ mass ratios.

Sacrificial Template Method
The general process of the sacrificial template method uses presynthesized metal-containing materials as sacrificial metal precursors to react with the ligand solution for the fabrication of nc-MOFs.Different from the interfacial growth process, the substrates/templates are requested to be unstable during the reaction, which is partially or completely consumed to supply metal ions.The structure of resultant nc-MOFs is mainly determined by the shape and composition of the templates.
In 2021, Lou et al. reported the fabrication of Fe(OH) x @Cu-MOF hollow nanoboxes (NBs) through a successive solvothermal reaction/etching strategy (Figure 9a). [76]Cu 2 O NBs, as the sacrificial metal resources, were first reacted with HHTP ligands for the growth of c-MOFs layer on the surface of Cu 2 O NBs via a hydrothermal treatment.During this process, the Cu 2 O NBs were partially consumed to release Cu 2þ ions for the nucleation of Cu-HHTP on the surface by coordination with HHTP, resulting in core-shell Cu 2 O@CuHHTP NBs.Afterward, the inner Cu 2 O core was selectively removed by Fe 3þ ions with the generation of a thin Fe(OH) x shell underneath the surface of Cu-HHTP shell, forming Fe(OH) x @Cu-MOF NBs.As shown in Figure 9b-e, the Fe(OH) x @Cu-HHTP hybrid materials  and d) TEM images of CNF@c-MOF nanofibers.Reproduced with permission. [67]Copyright 2019, American Chemical Society.e) Scheme of the synthesis process of the UiO-66-NH 2 @Ni-HHTP heterostructures; SEM images of f ) pure UiO-66-NH 2 ; g) pure Ni-HHTP and h) UiO-66-NH 2 @Ni-HHTP heterostructures.Reproduced with permission. [70]Copyright 2022, Elsevier B.V.  and d,e) the Cu-HHTP nanowires arrays growing on CFP.Reproduced with permission. [71]Copyright 2017, Wiley-VCH.(d,e) TEM, f ) HAADF-STEM and corresponding elemental mapping images of Fe(OH) x @Cu-MOF NB.Reproduced with permission. [76]Copyright 2021, American Association for the Advancement of Science.exhibited a uniform hollow NB morphology with a shell thickness of ≈20 nm.The energy-dispersive X-ray mapping results (Figure 9f ) indicated the Fe(OH) x layer was tightly adhered on the internal surface of Cu-MOF outer shell.
During the same period, traditional nonconductive MOFs were used as the sacrificial template for the synthesis of hollow bimetallic c-MOF NBs (CoCu-MOF NBs) by the same group. [77]o-based zeolitic framework nanocubes (ZIF-67 NCs) as the starting material were first converted into hollow tannic acid-Co complex (TA-Co) NBs via a chemical etching process using TA as the etching reagent (Step I).Subsequently, the TA-Co NBs were transformed to TA-CoCu NBs through a cationexchange treatment in a Cu 2þ ion solution, during which part of the Co atoms was substituted by Cu (Step II).Finally, CoCu-MOF NBs were fabricated by immersion of the TA-CoCu NBs into a HHTP solution (Step III), where the TA linkers were completely replaced by HHTP due to its stronger chelating ability with metal ions (Figure 10a).The SEM and TEM images (Figure 10d,e) showed that CoCu-MOF NBs possessed uniform hollow NBs with a shell thickness of ≈150 nm and evenly distributed Co, Cu elements in the shell (Figure 10f ).
Apart from the materials at nanoscale, macroscopic substrates have also been widely used as sacrificial templates for synthesizing c-MOFs arrays.A typical example is the construction of c-MOF/layer double hydroxide (LDH) nanotree (CoNiRu-NT) heterostructure on a Ni foam (NF) substrate reported by Zhai et al. [78] The synthesis process involved two steps: 1) the deposition of ternary CoNiRu-LDH nanotrunk on the NF substrate and 2) the controllable growth of c-MOF nanobranches on nanotrunk via partial conversion of LDH into c-MOF nanorods (Figure 11a).The CoNiRu-LDH nanotrunk exhibited vertical and slender nanorods with a diameter of ≈100 nm on the NF substrate (Figure 11b-top).By reaction with HHTP, trimetal c-MOF branches with ≈20 nm in diameter were grafted onto the surface of CoNiRu-LDH nanotrunk to form CoNiRu-NT with a dendritic nanostructure (Figure 11b-bottom, c,d).The XRD patterns (Figure 11e) showed the coexistence of the diffraction peaks assigned to LDH and c-MOF, verifying the successful graft of c-MOF.In addition to nanorod array, Zheng et al. synthesized a nickel-based c-MOF (Ni-BDT, BDT = 1,4-benzenedithiol) nanosheet array by using Ni(OH) 2 nanosheets deposited CC as the sacrificial template in 2017. [79]In their synthesis, the Ni(OH) 2 nanosheets were first vertically grown on CC.Through a subsequent hydrothermal reaction with BDT, the Ni(OH) 2 was transformed into Ni-BDT.
Different from the framework synthesis of c-MOF via hydrothermal processes in above works, Liu et al. developed a dual temperature zone-based CVD strategy for the in situ growth of conductive Cu 3 (HHTP) 2 on the surface of Cu foil that served as sacrificial substrate. [81]First, the ligand was sublimated in the higher temperature zone.The ligand vapor was then transported to the lower temperature zone with the subsequent deposition on the Cu foil, resulting in the growth of Cu 3 (HHTP) 2 nanowire arrays via a solid-solid reaction (Figure 13a,b).After the reaction, the color of Cu foil turned from bright yellow to blue-black.From a microscopic perspective, oriented Cu 3 (HHTP) 2 nanowires were uniformly and densely arranged on the surface of Cu foil (Figure 13c,d).Most of the nanowires possessed a hexagonal rod morphology (Figure 13e).
Except for Cu foil, CuO can also serve as the sacrificial template to construct Cu-BHT thin films. [82]In a typical synthesis, the CuO layer was first deposited on glass wafer and then converted into Cu-BHT under BHT vapor.The resultant Cu-BHT film exhibited relative smooth surface and controlled thickness from 20 to 85 nm.
Collectively, four main classes of synthetic strategies including direct growth, modulation method, interfacial synthesis, and sacrificial template method have been developed for the fabrication of nc-MOF.Direct synthesis represents the most effective and versatile strategy for preparing nc-MOFs, which is sometimes difficult to be well controlled.The morphologies of the obtained nc-MOF are mainly limited in 1D nanorod or 2D nanosheet.In comparison, the modulation method can efficiently regulate the shape and size of nc-MOF by modulating the nucleation and growth of parent MOFs.To avoid the negative impact of modulators on the properties of nc-MOF, their complete removal is necessary but not so straightforward in most cases.Moreover, the choice of modulators mainly relies on empirical knowledge, preventing the rapid development of this promising strategy.Different from direct growth and modulation method for fabricating nc-MOF usually in the form of nanoparticles, interfacial synthesis is a feasible strategy to fabricate nc-MOF nanofilms or nanoarrays at macroscopic scale.The major challenge faced by this method is to enable a stable and homogeneous reaction interface in large reaction vessel or equipment.Using sacrificial template method, the structure of nc-MOFs can be customized by using selected templates ranging from nanoscale to macroscopic scale.The limitation is the complex procedure involving presynthesis of template, growth of nc-MOF and removal of template.Additionally, the relatively harsh condition for template removal may damage the structure of nc-MOF.

Applications
The breakthroughs in the design and synthesis of nc-MOFs endow them with distinctive structural features and properties for various emerging applications including electrocatalysis,  [78] Copyright 2022, Wiley-VCH.
supercapacitors, batteries, sensors, and photocatalysis.The following sections will introduce the related progresses with detailed examples.

Hydrogen Evolution Reaction/Oxygen Evolution Reaction (HER/OER)/Water Splitting
Hydrogen fuel is recognized as the cleanest energy with high energy density and environmental friendliness.85] However, the two half-reactions including HER and OER with sluggish kinetics seriously limit the energy conversion efficiency.To address the above issues, Chen et al. fabricated a binder-free electrode by growing bimetallic bimetal CoCu-CAT nanorod arrays on CC for HER. [73]An overpotential of only 52 mV was required for CuCo-CAT/CC to achieve a current density of 10 mA cm À2 , comparable to the commercial 20 wt% Pt/CC (42 mV) and much lower than that of monometallic Cu-CAT/CC (121 mV) and Co(OH) x -CAT/CC (147 mV) (Figure 14a).The density functional theory (DFT) calculations revealed the vital role of the synergism between Co and Cu sites as follows: Cu sites as intrinsic active site have lower Gibbs free energy of H* (ΔG H* ) and the Co sites with more negative ΔE H 2 O (Figure 14b,c) acted as synergetic sites for promoting the H 2 O adsorption and thus the H 2 production.Together with the facilitated electron and mass transfer by three-dimension interconnected structure of self-standing arrays, the activity and stability for HER were enhanced.
In addition to the HER, accelerating the kinetics of OER is another important direction.In this regard, Lou et al. reported the synthesis of CoCu-MOF hollow NBs as efficient OER electrocatalysts. [77]As shown in Figure 14d-e Very recently, the application of nc-MOFs in overall water splitting with simultaneously boosted HER and OER activity was further demonstrated by Chen's et al. [86] In their work, a bifunctional electrocatalyst was designed by deposition of Ru-doped Co-CAT (RuCo-CAT) nanoarrays on CC (Figure 15a), which required overpotentials of only 38 and 200 mV at 10 mA cm À2 for HER and OER, respectively (Figure 15b,c), superior than Pt/C and other control samples.Moreover, the water-splitting electrolyzer based on RuCo-CAT nanorod arrays required a cell voltage of as low as 1.47 V to deliver a current density of 10 mA cm À2 .The origin of the remarkable performance was ascribed to the optimized adsorption energy of hydrogen and H 2 O by the construction of Co-Ru bimetal sites.

Oxygen Reduction Reaction (ORR)
Four-electron ORR (4e-ORR) is an important half-reaction in metal-air cell and fuel cell for energy conversion, where the development of high-performance electrocatalysts to overcome the slow kinetics is also the crucial step. [87,88]To this end, Peng et al. reported the use of Co-HITP as efficient ORR electrocatalysts. [89]Compared to Ni-HITP and bimetal CoNi-HITP, Co-HITP exhibited better ORR performances (Figure 16a-c) with an onset potential of 0.91 V, a half-wave potential of 0.80 V, a diffusion-limited current density of 5.52 mA cm À2 (at 0.50 V) and a Tafel slope of 89 mV dec À1 , close to the commercial 20% Pt/C.The remarkable electrocatalytic activity of Co-HITP in ORR was attributed to the enhanced oxygen adsorption on the active metal sites with unpaired 3d electrons.In contrast, the 3d orbitals of Ni sites were fully filled by paired electron in Ni-HITP, leading to relatively weak oxygen adsorption.
As the competitive process of 4e-ORR, the 2e-ORR pathway is a green technology for production of hydrogen peroxide, which is an important chemical that has been widely applied in paper manufacturing, waste-water treatment, and chemical synthesis. [90,91]In 2022, Liu et al. compared the 2e-ORR performances of three c-MOFs including Cu-HHTP, Cu-HITP, and Ni-HITP. [92]mong these samples, Cu-HHTP was demonstrated to exhibit the best 2e-ORR performance with a H 2 O 2 selectivity of 95%, H 2 O 2 production rate of 792.7 mmol g cat À1 h À1 and a Faraday efficiency of ≈85.4% (Figure 16d-f ).The authors employed the theoretical calculations to elucidate the underlying reasons of enhanced performance of Cu-HHTP.Accordingly, the electronic redistribution of Cu-O-C centers (between Cu metal sites and organic ligands) promoted the generation of *OOH intermediate over C site during the reaction process, favoring the 2e-pathway (Figure 16g).on Cu foil.Reproduced with permission. [81]Copyright 2020, Royal Society of Chemistry.

Carbon Dioxide Reduction Reaction (CO 2 RR), Nitrogen Reduction Reaction (NRR), and NRA
To mitigate the negative impacts of climate change, electrochemical CO 2 RR is of great significance. [93,94]The products from CO 2 RR such as C 2 H 4 and CH 4 are also promising alternatives for fossil fuels.Nevertheless, the low selectivity and sluggish kinetics of electrochemical CO 2 RR caused by the competitive reaction HER hindered the practical application.Recently, nc-MOFs have been reported as outstanding CO 2 RR electrocatalysts.As a typical example, Lan et al. developed a cupper-based c-MOF (Cu-DBC, DBC═dibenzo-[g,p]chrysene-2,3,6,7,10,11,14,15-octaol) with Cu-O 4 sites as efficient electrocatalysts for CO 2 RR. [95]Traditional Cu-HHTP, Cu-TTCOF with porphyrin Cu-N 4 sites, and Cu-PPCOF with phthalocyanine Cu-N 4 sites was also fabricated as comparison (Figure 17a).Benefiting from the favorable thermodynamics of Cu-O 4 sites for the adsorption of *H (Figure 17d), Cu-DBC presented the largest shape area in the radar chart with best performances among all samples as indicated by the highest Faraday efficiency of CO 2 RR and CH 4 , largest total and partial CH 4 current density and biggest turnover frequency values (Figure 17b,c).
Ammonia (NH 3 ), as an important raw material for chemical synthesis, is mainly produced by industrial Haber-Bosch process, which however consumes huge energy and releases  [73] Copyright 2021, Wiley-VCH.d) LSV curves; e) the overpotentials at 10 mA cm À2 ; f ) Tafel slopes; g) k 2 χ(k) oscillation curves and h) Fourier transform plots of Co K-edge extended X-ray absorption fine structure spectra; i) free energy diagram of OER process.Reproduced with permission. [77]opyright 2021, Wiley-VCH.and c) OER.Reproduced with permission. [86]Copyright 2023, Wiley-VCH.massive amount of CO 2 .As a promising alternative route, electrocatalytic NRR that is conducted under mild conditions with zero CO 2 emission has recently received extensive attention. [96,97]ery recently, Du et al. reported the synthesis of a Cu 3 (HITP) 2 / hexagonal boron nitride nanosheet (h-BN) (Cu 3 (HITP) 2 @h-BN) heterojunction for electrocatalytic NRR. [98]Due to the high porosity, abundant oxygen vacancies, elaborately designed Cu-N/B-N dual active sites with reduced free energy of rate-determining step (*N 2 to *NNH), and accelerated electron transfer of n-n heterojunction (Figure 17g), the optimized Cu 3 (HITP) 2 @h-BN composite material showed superior NRR performance with a Faraday efficiency of 42.5% and NH 3 production of 146.2 μg h À1 g cat À1 (Figure 17e,f ).
Electroreduction of NO 3 À to ammonia (NRA) represents a prospective route for both high-value-added NH 3 synthesis and wastewater treatment, where the design of high-efficiency electrocatalysts to drive the complex electron-proton-coupled process is crucial. [99,100]Very recently, two kinds of nc-MOFs, Ni-HHTP (denoted as Ni-O 4 -CCP) and Ni-HITP (Ni-N 4 -CCP) were applied as electrocatalysts to investigate the impact of coordination environment on NRA performance. [101]The electrochemical results showed that Ni-O 4 -CCP exhibited a higher NH 3 yield rate of 1.83 mmol h À1 mg À1 (Figure 18a) and Faraday efficiency of 94.7% (Figure 18b).The theoretical calculation results indicated the regulated electronic structure of Ni sites in Ni-O 4 -CCP could reduce the energy barrier of the rate-limiting step (NO* to HNO*) and promote the proton migration for hydrogenation, resulting in enhanced NRA activity (Figure 18c).

Supercapacitors
As a promising energy storage technology, supercapacitors have gained great passions due to their high-power density, rapid charge/discharge capability, and long-time stability. [102,103]Recently, nc-MOFs have been used as advanced electrode materials to improve the performance of supercapacitors.For instance, CNF@c-MOF papers were fabricated as electrode materials for flexible supercapacitors.Thanks to the facilitated mass transportation and charge transfer (Figure 19a) originated from the hierarchical porosity and high-conductive framework, CNF@c-MOF-based double-layer supercapacitors delivered a high capacitance of 96 mF cm À2 at 0.2 mA cm À2 within 0-1.0 V (Figure 19b) in poly(vinyl alcohol) in aqueous KCl, which could even power a red light-emitting diode (LED) under different bending/folding deformations (Figure 19c). [67]ater in 2020, Xia et al. reported the construction of all-solidstate flexible supercapacitors by using a hybrid architecture of Cu-CAT nanowires arrays integrated on polypyrrole (PPy)  and c) Tafel plots.Reproduced with permission. [89]Copyright 2020, Wiley-VCH.d) The disk and ring currents; e) H 2 O 2 selectivity (%) and electron transfer number (n); f ) H 2 O 2 Faraday efficiency; and g) scheme of 2e À ORR mechanism for Cu-HHTP.Reproduced with permission. [92]Copyright 2022, Elsevier B.V. membrane (Cu-CAT-NWAs/PPy) as the electrode materials (Figure 19d). [74]In such a design, the Cu-CAT provided the high conductivity and abundant active surface area, and the PPy membranes afforded high mechanical flexibility and excellent charge transfer skeleton.Consequently, the Cu-CAT-NWAs/PPy showed an areal capacitance of 252.1 mF cm À2 (Figure 19e), an energy density of 22.4 μWh cm À2 , and a power density of 1.1 mW cm À2 , outperforming most reported MOFs-based electrode materials.In addition, a superior cycle capability and mechanical flexibility were achieved over a wide range of working temperatures (Figure 19f ).

Batteries
Due to the high energy density and specific capacitance, long cycle life and lightweight, battery technologies have shown great commercial application potentials in many areas such as portable electronic products. [104]The unprecedented advantages of c-MOFs NCs also make them excellent candidates as electrode materials for batteries such as lithium-ion batteries (LIBs), lithium-sulfur batteries (LSBs), and zinc batteries (ZBs).
As the most commercialized battery, the investigation on the electrode materials for LIBs has been one of the most important directions. [105,106]Inspiringly, Winter synthesized flakeand rodlike Cu-based c-MOFs (Cu-HHTP) for LIBs with a focus on the investigation of morphology-dependent remaining diffusion limitations. [107]Their results demonstrated that the flake-like particles could offer large number of open pores and vertical pore directions for shortening the diffusion ways and overcoming a Li þ diffusion limitation.However, the diffusion of Li þ in rod-like particles was seriously limited along the growth direction with low pore accessibility (Figure 20a).Copyright 2021, Springer Nature.e) LSV curves in air and N 2 -saturated electrolytes; f ) NH 3 yield and Faraday efficiencies; and g) calculated Gibbs free energies of the NRR process on Cu 3 (HITP) 2 @h-BN.Reproduced with permission. [98]Copyright 2023, Wiley-VCH.c) photographs of LED powered by the devices.Reproduced with permission. [67]Copyright 2019, American Chemical Society.d) Scheme of the Cu-CAT-NWAs/PPy-based flexible supercapacitors; e) galvanostatic charging/discharging curves under different current densities; and f ) different temperatures of repeated heating/cooling.Reproduced with permission. [74]Copyright 2020, Wiley-VCH.
Compared to LIBs, LSBs have higher theoretical capacities of Li (3840 mA h g À1 ) and S (1675 mA h g À1 ), and thus have received rapidly increasing attention in the past decades. [108,109]c-MOFs with high conductivity and porous framework for confinement of sulfur have recently been proved to be highly suitable for LSBs by Lou et al. [110] In their work, bimetallic ZnCo-MOF hollow NBs were employed as cathode materials for LSBs, exhibiting a high reversible capacity of 1076 mAh g À1 (Figure 20c), a long-time stability with a little capacity decrease of 0.048% per cycle over 300 cycles at 0.5 C and stable Coulombic efficiency higher than 98% (Figure 20d).DFT calculations (Figure 20e,f ) indicated that the Co-O 4 sites exhibited a strong adsorption of lithium polysulfides (LiPSs) and accelerated redox kinetics of LiPSs.Combination with the confinement effect of sulfur and of LiPSs by the hollow cavity, the remarkable LSBs performance was achieved.
Even promising, the large-scale application of lithium-based batteries is inhibited by the high material costs and safety concerns.ZBs with high theoretical capacity, low toxicity, high safety, and relatively low cost of zinc have recently emerged. [111,112]For commercializing ZBs, the development of new cathode materials with high performance is the key.Within this context, Stoddart et al. utilized the Cu 3 (HHTP) 2 as the cathode material for rechargeable aqueous ZBs. [113]The high electrical conductivity and large pores of Cu 3 (HHTP) 2 could facilitate the transport of electrons and Zn 2þ ions to active sites.Besides, the quinoid units in HHTP with good redox activity were conducive to the Zn 2þ ion insertion.Benefiting from these structural merits, Cu 3 (HHTP) 2 showed a high reversible capacity of 228 mAh g À1 at 50 mA g À1 and high cyclability with only 25.0% of capacity decline after 500 cycles at an extremely high current density of 400 mA h À1 (Figure 21a,b).Furthermore, the slight changes of the O1s and Cu 2p speaks of charged Cu 3 (HHTP) 2 after cycles compared with the pristine cathode verified the highly reversible redox reaction during the electrochemical process (Figure 21c,d).

Sensors
As another important electrochemical application, electrochemical sensors have gained rapid development due to their widespread applications in life and industry. [114,115]Benefiting from the high conductivity, strong interaction with analyte  [107] Copyright 2023, Wiley-VCH.c) Rate capabilities; d) cycling performance at 0.5 C; e) adsorption models of Li 2 S 6 , and f ) energy profiles of the Li 2 S decomposition on ZnCo-MOF and Zn-MOF.Reproduced with permission. [110]Copyright 2021, Wiley-VCH.22b). [116]c-MOFs nanorods were densely attached on the surface of textile texture (Figure 22a), producing flexible devices with excellent toxic gas sensing performances.Ni-HITP-and Ni-HHTP-based devices showed low limit of detection of 0.16 and 1.4 ppm for NO and 0.52 and 0.23 ppm for H 2 S (Figure 22c), respectively, among the best of reported MOF-based chemiresistors for NO and H 2 S detection.Apart from the gas analytes, Luo et al. demonstrated the superiority of defective c-MOFs film in electrochemical detection of liquid H 2 O 2 . [66]The Cu vacancies in Cu-BHT films were revealed to serve as high-efficiency active sites for adsorption and reduction of H 2 O 2 , and optimize the electronic structure, resulting in a low detection limit of 16.5 nM, high repeatability, and long-term stability.

Photocatalysis
Photocatalysis is regarded as the most promising technology to address the global environmental and energy issues by converting solar energy into chemical energy. [117,118]To achieve high-performance photocatalysis reactions, the design of efficient photocatalysts is the core.By virtue of the high electron transfer capability, strong light absorption, and high activity, c-MOFs have recently been developed as alternative photocatalysts for several emerging reactions.For example, Zhu reported hybrid photocatalytic system for visible light-driven photocatalytic CO 2 reduction by using Ni 3 (HITP) 2 as the cocatalyst, triethanolamine as the electron donor and [Ru(bpy) 3 ] 2þ (bpy = 2,2 0 -bipyridine) as the photosensitizer (Figure 23a). [119]The photocatalytic results displayed a CO production rate of 34.5 mmol g À1 h À1 and a selectivity of 97% with negligible generation of H 2 (Figure 23b).The photocatalytic system also demonstrated superior stability with almost no activity decay after 6 repeated catalytic cycles (Figure 23c).Later on, Kang et al. constructed a C 3 N 4 / Ni-HHTP composite photocatalyst for H 2 O 2 production via 2e-ORR. [120]During the reaction process, the charge carrier recombination of Ni-CAT as the main active component for ORR was significantly inhibited with the assistance of C 3 N 4 , resulting in a H 2 O 2 yield of 1801 μmol g À1 h À1 in pure water.

Conclusion and Outlook
In summary, we provide an overview of the recent advances in nc-MOFs with respect to their synthetic strategies, achieved structures, and functional applications.Despite these fantastic Reproduced with permission. [113]Copyright 2019, Springer Nature.
progresses, this research field is still in its infancy stage with significant room for improvement and further investigation, especially compared to the rapid development of traditional MOF NCs [121,122] and other nanomaterials (e.g., noble metals [123,124] ).Future efforts are recommended to be contributed to the following aspects: 1) Understanding the underlying formation mechanism of nc-MOFs is of great significance for guiding the synthesis, while mainly relies on postulations.Scientific explanations of the mechanisms of initial nucleation and subsequent particle growth, and how reaction factors such as precursor, pH, temperature and solvent affect these two key steps are highly desired.To this end, using advanced in situ characterization techniques such as in situ TEM, XRD, Fourier transform infrared spectroscopy, and small-angle X-Ray scattering to monitor the nucleation and growth behaviors of c-MOFs NCs at both molecular and nanoscale represent a promising route.Besides, the structural characterization of nc-MOFs is mainly performed by using XRD, traditional TEM, SEM, N 2 sorption, etc., through which visualization of the local structural features of nc-MOFs at atomic level (e.g., defects, distribution of different metals) is hardly realized.To address this issue, low-dose TEM and cryo-TEM techniques with little structural damage of c-MOFs may serve as efficient tools.2) The controllable synthesis of nc-MOFs with well-defined morphologies and tunable compositions is an important and long-term task.From the morphological perspective, most of the reported nc-MOFs were nanorods and nanosheets.The structural diversity is much less than traditional MOF nanocrystals.nc-MOFs with other common morphologies such as nanocubes, nanopolyhedrons, and nanospindles have been rarely fabricated.Except that, construction of nc-MOF with more complex structures (e.g., yolk-shell structure, 3D macroporous architecture, hollow structure, and even asymmetric structure) is expected to produce new properties, while there is lack of efficient synthetic strategies.From the aspect of composition, the current research attention is predominately focused on the design of new ligands with several other valuable directions overlooked.One typical example is the introduction of ligand and/or metal defects, which has been demonstrated to be a promising strategy for tuning the electronic structures for traditional MOFs.Besides, fabrication of mixed-metal or mixed-ligand nc-MOFs is extremely attractive.To further integrate multifunctionalities, combination of nc-MOFs with other functional materials (e.g., noble metal nanoparticles, metal oxides/sulfides/nitrides, polymers, and biomolecules) toward higher-order composite materials may provide new opportunities.Another inspiration from traditional MOF [125,126] is utilization of nc-MOFs as precursors for preparing derived materials (e.g., metal sulfides, metal phosphides, carbon-based materials, and so on), which remains largely unexplored.Additionally, the currently reported nc-MOFs mainly exhibit 2D layer-stacked structures.Those with 1D or 3D conjugated frameworks are scarce but may deliver unique properties with respect to electron transfer, mass diffusion, etc.To create nc-MOFs with distinctive stacking structures, developing new ligands and ligand-metal coordination modes is required.3) From the application side, the structural features of nc-MOFs endow them with unique superiorities in electrochemistry-related applications such as electrocatalysis, supercapacitors, batteries, and sensors.Even so, considerable opportunities still exist for further investigation.For instance, employment of c-MOFs NCs as electrode materials for electrochemical nitrate reduction reaction, sodium and potassium ion batteries are seldomly reported.The narrow band gap with strong light-harvesting capability and excellent electron transfer ability makes c-MOFs NCs promising candidates as photocatalysts.However, except for the few cases for CO 2 reduction and H 2 O 2 production, design of efficient photocatalytic systems based on nc-MOFs for H 2 evolution, N 2 fixation, and organic pollutant degradation has been rarely demonstrated.Apart from electro and photocatalysis, the applications of nc-MOFs in other important fields such as heterogeneous catalysis, disease diagnosis, and drug delivery have just started.
Overall, this review has highlighted the achievements in the emerging field of nc-MOFs.Our personal perspectives on challenges and opportunities are expected to inspire further developments in both synthesis and applications of nc-MOFs.Through persistent research efforts, massive exciting outcomes will be reported in the near future.Reproduced with permission. [116]Copyright 2017, American Chemical Society.
Scheme 1. Illustration of the morphologies, synthesis strategies, and applications of nc-MOFs.

Figure 1 .
Figure 1.a) View of the M-CAT structure along the c axis; b) SEM, c) TEM, and d) HRTEM images of Ni-CAT.Reproduced with permission.[42]Copyright 2012, American Chemical Society.e) Scheme for the preparation of bimetallic NiCo-CAT; f ) SEM image and g) high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and corresponding elemental mapping images of NiCo-CAT.Reproduced with permission.[48]Copyright 2019, Wiley-VCH.h) Diagram of the synthesis process of M 3 (HHTQ) 2 ; i) SEM and j) HRTEM images of Cu 3 (HHTQ) 2 .Reproduced with permission.[52]Copyright 2021, Wiley-VCH.

Figure 8 .
Figure 8. a) Scheme of the synthetic process of Cu-HHTP nanowires arrays on CFP fibers; b) crystal structure of Cu-CAT viewed along the c-axis; SEM and photographic image (inset) of c) the CFP,and d,e) the Cu-HHTP nanowires arrays growing on CFP.Reproduced with permission.[71]Copyright 2017, Wiley-VCH.

Figure 9 .
Figure 9. a) Illustration of the formation procedure of Fe(OH) x @Cu-MOF NB; b,c) SEM,(d,e) TEM, f ) HAADF-STEM and corresponding elemental mapping images of Fe(OH) x @Cu-MOF NB.Reproduced with permission.[76]Copyright 2021, American Association for the Advancement of Science.
Figure 12. a) Schematic overview of the transformation of ZIF film to c-MOF films; b) scheme of the conversion mechanism from ZIF-8 to Zn-HHTP-H; c) phase diagram that correlates the solvent composition and reaction temperature with structures; d-g) SEM,and h-k) TEM images of ZIF-8, Zn-HHTP-H, Zn-HHTP-HS, and Zn-HHTP-NW.Reproduced with permission.[80]Copyright 2023, Springer Nature.

Figure 17 .
Figure 17.a) The formular diagrams of four c-MOFs; b) overall CO 2 RR performance; c) Faraday efficiency for CO 2 RR and HER; and d) calculated *H adsorption.Reproduced with permission.[94]Copyright 2021, Springer Nature.e) LSV curves in air and N 2 -saturated electrolytes; f ) NH 3 yield and Faraday efficiencies; and g) calculated Gibbs free energies of the NRR process on Cu 3 (HITP) 2 @h-BN.Reproduced with permission.[98]Copyright 2023, Wiley-VCH.

Figure 19 .
Figure19.a) Schematic illustration of the fastened mass and charge transfer in CNF@c-MOF; b) calculated areal capacitances of CNF@c-MOF; and c) photographs of LED powered by the devices.Reproduced with permission.[67]Copyright 2019, American Chemical Society.d) Scheme of the Cu-CAT-NWAs/PPy-based flexible supercapacitors; e) galvanostatic charging/discharging curves under different current densities; and f ) different temperatures of repeated heating/cooling.Reproduced with permission.[74]Copyright 2020, Wiley-VCH.

Figure 20 .
Figure 20.a) Scheme of assumed Li ion diffusion path over the Cu-HHTP with two different morphologies; b) specific delithiation capacity.Reproduced with permission.[107]Copyright 2023, Wiley-VCH.c) Rate capabilities; d) cycling performance at 0.5 C; e) adsorption models of Li 2 S 6 , and f ) energy profiles of the Li 2 S decomposition on ZnCo-MOF and Zn-MOF.Reproduced with permission.[110]Copyright 2021, Wiley-VCH.
molecules, and highly tunable chemical and physical properties, c-MOFs have been recently demonstrated to be important platforms for electrochemical sensors.For instance, Mirica et al. developed a direct solution-phase self-assembly strategy for fabricating flexible textile-based devices by integrating c-MOFs (Ni-HITP and Ni-HHTP) into fabrics (Figure

Figure 22 .
Figure 22. a) Photograph and SEM images of c-MOFs on textile; b) diagrams for dosing sensors with analytes; and c) representative responses of NO and H 2 S for Ni 3 HITP 2 (blue) and Ni 3 HHTP 2 .Reproduced with permission.[116]Copyright 2017, American Chemical Society.