Recent progress in strategies for preparation of metal-organic frameworks and their hybrids with different dimensions

Hard template method

The hard template method induces the nucleation and growth of MOFs nanostructures on the surface of template materials, such as graphene oxide, Te nanowires and ZnO nanorods, to form MOFs-based one-dimensional hybrid materials^[65-69]. The morphology of the material and the pore size is modulated with templates. Back in 2010, Jahan et al. had successfully synthesized unusual MOFs nanowires by using benzoic acid functionalized graphene oxide (BFG) as a structurally oriented hard template to induce nucleation of MOF-5 on its surface and growth along the [220] crystallographic plane direction^[70]. In order to synthesize MOF/BFG complexes, they tried adding different amounts of BFG (1,4 and 5 wt%) as the precursors and eventually found that the diameter of the nanowires closely correlated with the lateral size of the functionalized graphene. In 2012, Pachfule et al. speculated that the nano-hollow structure might have special encapsulation capabilities due to its constraint^[65]. Subsequently, highly crystalline MOFs were grown in the inner cavities of one-dimensional carbon nanofibers (CNF) using pre-synthesized CNF as templates with reasonable experimental parameters. The analysis of the powder X-ray diffraction data indicated that the MOF phase was the thermodynamically unstable MOF-2 [Zn(bdc)], rather than the expected MOF-5 [Zn₄O(bdc)₃], and high-resolution transmission electron microscopy of the MOF-2@CNF composite also clearly showed MOF-2 nanocrystals stacked in the inner cavities of the one-dimensional carbon nanofibers. To further explore the crystal growth, they functionalized the CNF, introduced functional groups and then performed MOF synthesis, and found that the MOF-2 crystals grew on the outer surface of the CNF, indicating the importance of surface functionalization of carbon templates. Commercial trace-etched polycarbonate (PCTE) membranes have also been widely used as hard templates for the synthesis of inorganic nanowires. Caddeo et al. then used polycarbonate (PC) templates to deposit Cu nanowires, which were then converted to Cu₃(BTC)₂ MOF nanowires by electrochemical oxidation by injecting an organic linker solution into the template^[71]. Since the MOF is formed inside the template, the shape is consistent with the host nanochannel and MOF nanowires with diameters between 80 and 260 nm can be obtained. In 2018, Arbulu et al. used PCTE to synthesize continuous 1D ZIF-8^[72]. ZIF-8 nanomorphology is related to Zn metal salt concentration and PCTE membrane pore size [Figure 2A-C].

Figure 2. (A-C) TEM images of different 1D ZIF8-100 nanostructures. (D) Schematic diagram of the NFC template for the synthesis of MOF nanofibers. (E) Mechanistic scheme summarizing the principal phases of the formation of MIL-69(Al) NWs. (A-C) Adapted with permission^[72]. Copyright 2018, John Wiley and Sons. (D) Adapted with permission^[73]. Copyright 2022, John Wiley and Sons. (E) Adapted with permission^[76]. Copyright 2020, John Wiley and Sons. MOFs: Metal-organic frameworks; 1D: one-dimensional; NFC: nanofibrillated cellulose; NWs: nanowires; NPs: nanoparticles.

In general, the development of applications for MOFs is strongly influenced by the superstructure. With the aid of templates, MOFs can be assembled into superstructures in solvents, but templates for highly homogeneous 1D nanomaterials are not available and the synthesis of superstructures is limited as a result. Recently, Li et al. reported the synthesis of MOF materials with 1D nanofiber morphology using nanofibrillated cellulose (NFC) as a template^[73]. The synthesized core-shell nanofibrous materials have the advantages of a high aspect ratio, great flexibility, good dispersion and ease of large-scale assembly. The prepared ZIF-8, UiO-66 and MIL-53 (Al) nanofibers have high aspect ratios of over 100 and can be used to assemble into a variety of macroscopic materials [Figure 2D]. NFC is considered to be a good amphiphilic material due to the presence of both hydrophilic and hydrophobic alkane fractions, and can be dispersed in both polar aprotic organic solvents and non-polar solvents alone. This unique property makes NFC an ideal template for the synthesis of 1D flexible MOF nanofibers. The synthetic strategy that uses solution treatment is expected to ensure the production of MOF nanofibers at scale and promote the industrialization of MOFs.

It is well known that low-symmetry nanocrystals (NCs) are important in areas such as catalysis, sensing and electronics^[74,75]. While the majority of MOF nanoparticles (NPs) are obtained as polycrystalline powders or spherical nanocrystals (NCs), and only a very few one-dimensional anisotropic MOF nanostructures were identified. In 2020, Muschi et al. formulated an original approach to synthesize MIL-69 (Al) nanowires (NWs) grown along the [001] crystal plane direction by using graphene oxide as a template modifier^[76]. Graphene oxide nanowires are one-dimensional carbon materials formed by rolling graphene oxide sheets from one side or edge into Archimedean-type helices^[77]. Inspired by this, they exploited the potential rolling of graphene oxide and the following anisotropic growth of MIL-69 (Al) in the inner cavities of graphene oxide nanoscrolls to contain the formation of single-crystal MOFs [Figure 2E]. The anisotropic growth is motivated by the interaction forces between the MIL-69 (Al) NPs and the hydroxyl and carboxyl functional groups of the graphene oxide.

Sacrificial template method

The sacrificial template method is more attractive than the hard template method, which can easily remove the template but hardly triggers the collapse of the 1D MOF structures^[78]. This is a process in which a number of 1D metal oxides or hydroxides are used as templates for their own in-situ dissolved metal ions properties, ultimately inducing the formation of 1D MOFs. In recent years, metal nanoparticles (MNPs) doped in MOFs have been widely used in the field of multiphase catalysis^[79-81]. However, the key to the catalytic efficiency of MNPs@MOFs lies in how to shorten the distance between the reactants and the active sites to achieve this process, which means that the spatial distribution of MNPs in MOFs needs to be precisely tuned. In 2017, Yang et al. employed spherical metal oxides both as a support for MNPs and as a sacrificial template for growing MOFs to achieve precise encapsulation of controlled positioning of MNPs in 1D MOFs [Figure 3A]^[82]. Initially, the MNPs were first loaded onto the metal oxides, and the organic ligands in solution were coordinated with the metal ions in oxides, and the MNPs were then encapsulated into the MOF crystals [Figure 3B-E]. The concentration of the organic ligand regulates the spatial position of the MNPs in the MOFs. When the concentration of organic ligands is high, the nucleation of MOFs follows a “dissolution precipitation mechanism”^[83,84]. As increasing the concentration of Zn(Hmim)₄²⁺, ZIF-8 crystals were slowly deposited on the metal oxide surface to form a polycrystalline layer, while MNPs were encapsulated between the interface of ZnO and ZIF-8 as they remained on the metal oxide surface. When the concentration of organic ligands is low, the conversion follows a “local conversion mechanism” and the MNPs are immobilized on the surface of the MOFs^[85]. It was found that the method was applicable to a wide range of MNPs and MOFs, and that the synthesized MNPs@MOFs had significantly enhanced catalytic activity. In 2018, Li et al. also used metal oxides encapsulated into ZIF-8 to achieve good catalytic performance^[86].

Figure 3. (A) Spatial localization of MNPs in MOF crystals regulated by template-sacrifice method. (B-E) TEM images of ZnO NRs (B), ZnO NRs@Au (C) and (D and E) ZnO NRs@Au@ZIF-8 composites with Au NPs at different locations: (D) inside and (E) near to the surface of ZIF-8. (F) Formation of the hierarchical CNT/Co₃O₄ microtubes. (G-I) FESEM of the synthesized ZIF-67 microtubes. (A-E) Adapted with permission^[82]. Copyright 2017, Springer Nature. (F-I) Adapted with permission^[96]. Copyright 2016, John Wiley and Sons. MOFs: Metal-organic frameworks; MNPs: metal nanoparticles; NPs: nanoparticles; FESEM: field emission scanning electron microscopy.

Among all nanostructured materials, nanotubes have attracted a particular attraction due to the characteristic application properties shown in adsorption, catalysis and sensing due to the fact that these fascinating tubular nanostructures provide three different contact regions, i.e., the interior, the exterior surface as well as the ends^[87-90]. Nanotube-structured electrode materials are capable of enhancing electrochemical properties not possible for conventional bulk materials, thanks to higher surface areas and shorter diffusion paths that enhance the attraction for free electrons^[91-95]. Among the various structural designs, hierarchical tubular structures (HTSs) have received considerable attention for their advantages such as pore volume for expanding the electrode/electrolyte contact area. In 2016, Chen et al. devised a multi-step strategy to use zeolite imidazolate framework (ZIF-67) as a single source of carbon and cobalt in the resultant composites through a controlled chemical transformation, and a two-step annealing process to obtain a layered CNT/Co₃O₄ hollow tubular structure, which has excellent as an anode material for Li-ion batteries^[96]. They selected electrospun polyacrylonitrile (PAN)-cobalt acetate [Co(Ac)₂] composite nanofibers as a bifunctional template and made use of the strong coordination of 2-methylimidazole and the cobalt source on the template, resulting in the growth of uniform ZIF-67 nanocrystal shells on the nanofibers [Figure 3F-I]. After DMF treatment, the nanofibers were removed to form tubular structures. In the final step, carbon nanotubes are retained after annealing treatment, yielding layered CNT/Co₃O₄ microtubes. The transformation from cobalt solid nanoparticles to Co₃O₄ hollow particles can be attributed to the Kirkendall effect during the annealing process^[97]. In the same year, Liu et al. used Cu₂O as a template and successfully converted pristine nanowires into layered HKUST-1 nanotubes by immersing Cu₂O nanowires in a mixture of acetic acid, ethanol and H₃BTC/DMA using the characteristics of acetic acid to dissolve Cu₂O^[98]. They have experimentally found that the solvent group is critical to the growth of nanomaterials and that the solvent plays an important role in the balance between the dissolution rate, nucleation rate and crystal growth of copper ions.

Electrospinning

Electrospinning has been widely used as a method for preparing nanowire fibers due to the mature synthetic technology and low cost^[99,100]. A simple electrostatic spinning device consists of three main components [Figure 4A], a high-voltage power supply, a capillary/syringe with a needle and a grounded collector^[101]. The choice of precursor solution is also one of the keys, with the viscosity of the precursor determining the size and morphology of the fiber materials^[102,103]. The mechanism of electrospinning is predicated on the ejection and elongation of a molten viscous polymer under a high-voltage electric field, followed by solidification at the collector in the form of a stream of charged liquid. There are two main routes to forming MOF-polymer nanofiber structures via electrostatic spinning, namely “direct electrostatic spinning” and “surface modification”, both of which can be converted into MOF nanofiber derivatives by post-treatment^[104]. In the direct electrostatic spinning method, a slurry containing MOF particles is mixed with polymers and directly electrospun into nanofiber complexes^[105-107]. However, this direct electrostatic spinning method is not suitable for applications pursuing a material pore perspective, due to the fact that the slurry of mixed polymers can clog the pores inside the MOFs. The second method of generating complexes is reached by growing MOF particles in situ on the surface of nanofibers^[108,109]. This in situ growth mode maintains the advantages of the porosity and high specific surface area of MOFs and can also have good applications in the field of adsorption. The “surface modification” method has become the preferred method for electrostatic spinning in recent years, despite its high experimental requirements. Researchers can design the composition and morphology of the material by doping it with metals or other small molecules after the electrostatic spinning process, thereby expanding the diversity of electrostatically spun nanofibers. In terms of energy materials, the high structural stability of electrostatically spin-synthesized 1D MOFs effectively prevents structural expansion and collapse of the electrode material. The 1D structure also improves the material’s distance for charge transport and ion diffusion during discharge. Electrospinning techniques effectively inhibit the aggregation of MOF particles and improve the utilization of active substances by building a network structure; therefore, they are also considered an effective way to expand the range of practical applications of MOFs^[110-112].

Figure 4. (A) Schematic diagram of the electrostatic spinning unit. (B) The synthesis of hollow particle-based N-doped carbon nanofibers (HPCNFs-N). (C and D) FESEM and TEM images of the PAN/ZIF-8 composite nanofibers. (E and F) FESEM and TEM images of the HPCNFs-N sample. (G) Diagrammatic representation of the different synthetic routes for the structuring of HKUST-1 into MOF NFs. (A) Adapted with permission^[101]. Copyright 2007, American Chemical Society. (B-F) Adapted with permission^[113]. Copyright 2017, The Royal Society of Chemistry. (G) Adapted with permission^[115]. Copyright 2021, Springer Nature. MOFs: Metal-organic frameworks; FESEM: field emission scanning electron microscopy; PAN: polyacrylonitrile; NFs: nanofibers.

In 2017, Chen et al. generated hollow particle-based nitrogen-doped carbon nanofibers (HPCNFs-N) [Figure 4B] by encapsulating ultra-fine ZIF-8 particles into electrospun polyacrylonitrile (PAN) and subsequently carbonizing them, which were found to have strong specific capacitance as supercapacitor materials^[113]. Field emission scanning electron microscopy (FESEM) images revealed that the PAN/ZIF-8 composite nanofibers were directly homogeneous [Figure 4C and D] and after nitrogen heat treatment, the HPCNFs-N samples maintained their previous morphology without significant changes [Figure 4E and F]. In 2020, Huang et al. also synthesized polyacrylonitrile-covered ZIF-8@PAN composites directly using electrospinning^[114]. The yolk-shell Sb@C nanoboxes embedded in carbon nanofibers (Sb@CNFs) were prepared after a combination of ion exchange and thermal reduction treatments, and the electrochemical mechanism of the Sb@CNFs electrodes was investigated using in-situ TEM characterization techniques. The direct electrospinning method inevitably blocks the pores of MOFs by polymers, while the phase conversion method bypasses these problems. Dou et al. then combined the electrostatic spinning and phase conversion methods to fabricate highly structured MOF nanofibers for methane storage applications^[115]. The surfaces of polyacrylonitrile nanofibers (PAN NFs) were pretreated using Cu(OH)₂, followed by phase conversion to HKUST-1 crystals [Figure 4G]. Compared to direct electrospinning, this conversion method results in a higher loading of MOFs on PAN NFs substrates.

Modifiers-induced growth method

For a long time, scientists have been working on reducing the size of MOFs crystals to the nanoscale without changing the properties of the MOFs crystals themselves and investigating the correlation between the porous properties of the nanocrystals and the interfacial structure. The direct synthesis of 1D MOFs via metal precursors and organic ligands is more difficult and most are usually obtained in alcoholic solvents or ethanol/water mixtures^[116,117]. Over the years, ligand-modulated synthesis of MOFs via monocarboxylic acids, organic bases, dopamine and amphiphilic copolymers as modulators has also been reported^[118-124]. It is important to understand the crystal growth of metal framework materials, and the modulation method involves the use of molecules that are structurally similar to the ligand and a weaker coordination affinity to the metal clusters to compete with the ligand and thus regulate the coordination equilibrium of the crystal morphology growth of MOFs. When the coordination equilibrium between the metal ion and the organic linker is altered, the crystal size, morphology and crystallinity of the MOFs can be controlled^[125-127]. The relatively weak interactions between ligand bonds dominate the assembly process of MOFs and also lead to the formation of crystals. In 2009, Tsuruoka et al. used a simple and straightforward method to modulate the coordination equilibrium by adding a modulator with the same chemical function as the linker to the reaction system to enhance the one-dimensional anisotropic fusion of nanocrystals by impeding the coordination interactions between the metal ions and the organic linker^[128]. They synthesized one-dimensional [Cu₂(ndc)₂-(dabco)] nanorods (ndc = 1,4-naphthalene dicarboxylate; dabco = 1,4-diazabicyclo [2.2.2] octane) using acetic acid as a modifier. It was found that the increase in acetic acid concentration significantly inhibited the growth of Cu-NDC in the [100] direction, prompting eventual growth along the [001] direction to form nanorods. In 2016, Pachfule et al. successfully synthesized MOF-74-Rod at room temperature using a unique modulator-assisted approach to stabilize the active metal sites on the surface of MOF crystals by introducing salicylic acid modulators into the system to guide the growth of MOF in rod-like morphology [Figure 5A]^[129]. They then used KOH-assisted sonication, where K⁺ was inserted into the carbon nanorods in a disordered stack during solution-based sonication, and again during subsequent heat treatment, where the K⁺ containing nanorods were unfolded to form a nanoribbon structure. Further acoustic treatment and thermal activation of the rod-like MOF could open up new avenues for the efficient production of one- and two-dimensional carbon materials. Conventional acoustic treatment is ultrasonication, which relies primarily on the transmission of acoustic waves through a medium such as a solution, causing oscillation of the medium particles. In contrast, non-traditional surface acoustic waves (SAWs) are electromechanical waves with nano- or sub-nano-amplitudes that are accompanied by changes in the electric field along the propagation path^[130]. SAWs combine acoustic and electric fields and offer advantages in controlling the thickness of stripped nanosheets. By 2019, they had also synthesized chestnut-shell spherical superstructure metal-organic framework nanorods (SS-MOFNR) by hydrothermal transformation using urea as a modulator^[131]. In control experiments without the addition of urea, the coexistence of unassembled MOF nanorods and partially spherical superstructures was observed using SEM. In contrast, the addition of urea resulted in the formation of morphologically homogeneous and highly crystalline SS-MOFNRs [Figure 5B-G]. By investigating the time-dependent process, it was found that with increasing time, Zn-MOF-74 NPs were first transformed into Zn-MOF-HT nanorods, and finally, micro-sized SS-MOFNRs were slowly assembled, a process that was transformed gradually. The screening of the solvent revealed that water as a solvent was favorable to the formation of 1D MOFs. SS-MOGNR also maintained its original morphology after carbonization with argon, and its performance as a carrier for the Pd immobilization catalyst was improved.

Figure 5. (A) Synthesis scheme for MOF-74-Rod. (B-D) SEM images of SS-MOFNR composed of 1D MOF nanorods. (E) SEM image of a fractured SS-MOFNR showing its internal hollow structure. (F and G) TEM images of SS-MOFNR. (A) Adapted with permission^[129]. Copyright 2016, Springer Nature. (B-G) Adapted with permission^[131]. Copyright 2019, John Wiley and Sons. MOFs: Metal-organic frameworks; SS-MOFNR: spherical superstructure metal-organic framework nanorods; 1D: one-dimensional.

Phase transition method

The single-crystal-to-single-crystal (SCSC) conversion is an intriguing and essential process in chemistry and material science and is an important feature in the preparation of coordination polymers^[132-139]. Over the last few decades, much research demonstrated that SCSC conversions of MOFs could be triggered by external stimuli, including physical and chemical processes, such as temperature, pressure, gas adsorption, etc.^[140-145]. Through these stimuli, the number of ligands and dimensions can be changed, such as 0D to 1D/2D and 1D to 2D/3D, leading to the synthesis of MOFs^[146]. In 2019, Han et al. achieved the transition from QUST-81 to QUST-82 through thermodynamically controlled crystal phase transitions in physical stimuli^[147]. Synthesis of QUST-81 crystals at 100 ^oC, followed by high-temperature heating to 120 ^oC in the mother liquor again, enables the formation of new stable QUST-82 crystals [Figure 6A and B]. In contrast, there is no dissolution or recrystallization of the single crystal during this process. Single crystal X-ray diffraction (SCXRD) showed that the coordination environment of QUST-82 changed during the crystalline phase transformation, with new coordination bonds forming. The formation of new bonds is also indirectly evidenced by the color of the reactants, with QUST-81 crystals showing a color change from dark green to colorless back to green, indicating the presence of a reduction of Cu²⁺ to Cu⁺ followed by re-oxidation to Cu²⁺ in air. The heterometallic MOF QUST-82 serves as an excellent adsorbent for the elimination of organosulfur. However, there are not many studies on chemical stimuli with the same metal but different coordination environments causing the SCSC transition of MOFs. In particular, there are few discussions on the crystalline transformation of 1D MOFs. Chen et al. did advance research^[148]. Starting with 0D Cu nanoparticles [MOP-1, Cu₂₄(m-BDC)₂₄(DMF)₁₄(H₂O)₁₀], they began to explore the effect of SCSC conversions induced by different organic ligands on the structure. They first synthesized 0D rhombic MOP-1 crystals, and SXRD data showed them to be connected by a Cu₂(COO^-)₄ paddlewheel unit and two isophthalic acids (m-BDC). Two terminal ligands are attached to each unit, pointing to the exterior and center of the crystal, respectively. Compared to oxygen atoms, nitrogen atoms are less electronegative and give more electrons, making it easier to give electrons to form coordination bonds with metal salts. They then introduced 1-methylimidazole as a second ligand in an attempt to break the original coordination structure and transform the 0D structure. SXRD confirmed that the introduction of 1-methylimidazole resulted in the coordination of a Cu atom to two oxygen atoms in two m-BDCs and three nitrogen atoms in three 1-methylimidazoles to form a 1D chain structure. In 2017, Yang et al. synthesized 1D-NCPs (nanoscale coordination polymers) containing gambogic acid (GA) through a phase transfer strategy^[149]. PEGylated 1D-NCPs were constructed by mixing Mn ions and indocyanine green (ICG, a NIR dye) along with poly-l-histidine-PEG (pHis-PEG). First, metal ions and organic ligands self-assemble through coordination interactions in solution methanol to form a three-dimensional porous structure. It is then transferred into an aqueous solution, where the highly polar water molecules coordinate with the metal ions to form a one-dimensional nanofiber-like structure [Figure 6C]. An interesting phenomenon occurred when the reaction phase was transformed from an organic solvent to an aqueous solution, and the granular structure of the Mn-ICG@pHis-PEG NCPs nanoparticles slowly transformed into a fibrous one-dimensional structure over time. The authors found that the 3D porous structure becomes unstable due to the larger sulphonic acid anions of ICG. In strongly polar aqueous solutions, the Mn²⁺ coordination environment was changed and the hydrogen binding led to the formation of a 1D chain-like structure. For the change of the nanostructure from 3D to 1D, they performed molecular dynamics fitting computational simulations on Mn²⁺ according to the universal force field^[150]. The research found that in methanol, Mn²⁺ is coordinated to six sulfonate anions of organic ligand ICG. Concurrently, Mn²⁺ could also coordinate with the imidazole ring of poly-histidine-PEG. Poly-histidine-PEG relying on imidazole groups can strongly bind metal ions, but poly-histidine-PEG has only one imidazole ring and can only coordinate with one Mn²⁺. Therefore, when Mn²⁺ is coordinated with poly-histidine-PEG and the growth of MOF stops, 3D porous MOFs are formed. After the introduction of highly polar water molecules, the hydroxyl group can connect with the sulfonate anion to form a hydrogen bond to form a 1D structure. This simple crystal phase transfer strategy, which can be extended to various kinds of metal ions, is expected to open up new opportunities for the synthesis of MOFs.

Figure 6. (A) SEM images of QUST-81-SC, QUST-82-SC, and QUST-82-P (SC = single crystal and P = powder). (B) The SC-SC phase transition process between QUST-81 and QUST-82. (C) Scheme to illustrate one-step synthesis of 1D PEGylated NCPs. (A and B) Adapted with permission^[147]. Copyright 2019, John Wiley and Sons. (C) Adapted with permission^[149]. Copyright 2017, John Wiley and Sons. NCPs: Nanoscale coordination polymers; 1D: one-dimensional.

Recrystallization

The anisotropic 1D MOF structures were prepared only by the template method and surfactant-assisted method, but the single-crystal nano MOFs or nano MOFs with high aspect ratios could not be prepared^[70,151]. Taking graphene oxide templates as an example, graphene oxide flakes are thinner and more brittle. The surface is modified with functional groups such as hydroxyl groups, which are easily chelated with metal ions, while the carboxyl groups are mainly distributed on the sides and edges of the flakes. It is therefore difficult to form MOFs with high aspect ratios using it as a structural guide for molecular assembly. Surfactant-assisted methods are often used to modulate the morphology and size of MOFs and are generally used as auxiliary agents. Moreover, surfactants are often classified as anionic, cationic, amphiphilic and polymeric, and there is some skill in choosing which surfactant to use. Utilizing metal oxides as metal source precursors, the synthesis of MOFs phases by secondary crystallization has been reported previously^[152-154]. However, the synthesis of 1D MOF nanostructures with both single crystal and ultra-long morphologies is still a great challenge. Commonly, the kinetic process of crystallization influences the morphology of the crystals. Different precursors have different solubilities in specific solvents. Lower crystallization rates facilitate the formation of high aspect ratio, anisotropic MOFs. By 2018, Zou et al. had successfully fabricated the first high aspect ratio single-crystal Co-MOF nanotubes via an amorphous MOF-mediated recrystallization method (AMMRA)^[155]. To achieve anisotropic, ultra-long single crystals. They first prepared Co-MOF-74 nanoparticles (Co-MOF-74-NP) in the amorphous phase, exploiting the property that the amorphous Co-MOF-74-NP dissolves slightly in water and later recrystallized the Co-MOF-74-NP in aqueous solution (pH = 7.15) to produce ultra-long single crystals of Co-MOF-74 nanotubes (Co-MOF-74-NT) [Figure 7]. Compared to the low aspect ratio Co-MOF-74 micro-rod (Co-MOF-74-MR) (3-30 μm in length, 1-5 μm in diameter), the secondary crystallization method demonstrates the great advantage of nanosize control to achieve a homogeneous morphology of high aspect ratio nanotubes.

Figure 7. Synthesis protocols for Co-MOF-74-NT and Co-MOF-74-MR. Adapted with permission^[155]. Copyright 2018, American Chemical Society.

Micromechanical exfoliation

In 2004, Novoselov et al. succeeded in obtaining the first graphene with a perfect hexagonal honeycomb lattice by repeatedly peeling off highly oriented pyrolytic graphite using transparent tape, inspired by the use of tape to remove impurities from the surface of graphite while working on metal field effect transistors^[156]. Subsequently, this micromechanical peeling using tape was widely used in preparing 2D materials. Although 2D materials are mostly structures with weak van der Waals force interactions between layers stacked on top of each other, there are strong covalent bonds in each sheet and 2D materials have been gaining popularity in the field of covalent functionalization in recent years. Traditionally, surface modification is carried out after obtaining the ultrathin 2D material. However, due to the poor reactivity of the 2D MOFs, defects and incomplete functionalization were often performed after modification^[164]. In 2018, Lopez-Cabrelles et al. designed pre-synthetic ligand functionalization followed by micromechanical exfoliation to successfully prepare defect-free functionalized 2D MOF [Figure 9A and B]^[165]. The crystallinity and integrity of the exfoliated flakes were confirmed by projection electron microscopy and regional electron diffraction patterns [Figure 9C]. Although micromechanical peeling is easy to handle, it is less efficient. There is also a risk of disrupting the quantum structure of the material during stripping, resulting in stripping losses and affecting large-scale stripping.

Figure 9. (A) A common strategy for the functionalization of 2D (inorganic) materials, consisting of two steps. (B) Pre-synthetic functionalization, where the functionalized block is first obtained prior to mechanical stripping, resulting in a quasi-perfect array of unaltered functional groups attached to the surface. (C) (i) Low magnification TEM image of a thin section, where the darker grey areas are associated with thicker sections; (ii-iv) The experimental SAED pattern corresponding to the [0 0 1] area axis (iii) was obtained for the selected area [surrounded by the green circle on the TEM image in (ii)] and fitted very well with the simulated SAED pattern (iv). (A-C) Adapted with permission^[165]. Copyright 2018, Springer Nature. 2D: Two-dimensional.

Ultrasonic stripping

As a new member of the 2D material family, 2D MOF materials, although still in the early stages of research, have larger lateral dimensions and ultra-thin thicknesses than other inorganic nanosheet materials, giving them a larger surface area than other inorganic nanosheet materials^[50,166,167]. For 2D MOF crystals, the different layers are attached by weak interactions, including hydrogen bonding and van der Waals forces. If the weak interactions are broken, monolayers of 2D-MOFs can be obtained. This idea has been implemented by a simple ultrasonic peeling method. Ultrasound waves are originated from acoustic cavitation within collapsing bubbles. After the solution has been processed by ultrasound, ultrasound energy accumulates in the oscillating bubbles and can generate high pressures and local temperatures^[168]. Small bubbles in a solution periodically undergo a process of creation, expansion and bursting. As the bubbles burst, energy is applied to the stripped precursor with a shock wave and tensile stresses are generated within the components of the precursor until the nanosheets are stripped. Ultrathin MOF nanosheets (UMOF NSs) can be prepared using the ultrasonic stripping strategy. UMOF NSs are characterized by the following features: nanoscale thickness allowing fast transport, superior electron transfer, ligand-unsaturated metal sites and exposed catalytically active surfaces^[169-171]. In 2016, Zhao et al. then used ultrasonic exfoliation to prepare a series of UMOF NSs with ligand-unsaturated sites and high electrocatalytic activity towards OER at room temperature^[54]. It showed that the coordination-unsaturated metal atoms are the main active centers and this type of UMOF NSs is expected to be a strategy for synthesizing atomically accurate active catalysts.

At present, few systematic studies have been carried out on the exfoliation of bulk materials into ultrathin 2D nanosheets. For this reason, the discovery of new methods to facilitate the exfoliation of freestanding nanosheets is a necessary crystallographic challenge. Previously, the effect of the length of the suspended chains located in the MOF pores on the exfoliation has often been neglected. However, in 2019, Ashworth et al. attempted to increase the length of hydrophobic alkyl chains in functionalized ligands to explore their effect on ultrasonically exfoliated MOF materials [Figure 10A]^[172]. To assess the impact of chain length on the particle size of the exfoliated nanosheets, they carried out an analysis using atomic force microscopy (AFM). It was found that as the alkyl chain length increased by two methylene units, the thickness of the nanosheets became thinner but the lateral dimensions increased. The longer alkyl chain length resulted in a lower concentration of suspended material, indicating poorer solvent-nanosheet interaction, directly causing a lower average particle size dispersion for nanosheets that have longer alkyl chains [Figure 10B-G]. Nevertheless, longer alkyl additionally increases interlayer distances and diminishes other interlayer interactions and makes it easier to separate the layers during sonication, while at the same time, the solvent is instrumental in offsetting the energy loss to create new interfaces during the peeling process^[173,174]. In the same year, Tan et al. used metal clusters constructed from bulky hydrophobic ligands to prepare extremely thin, anisotropic UMOF NSs under ultrasonic exfoliation^[175]. Ultrasonic exfoliation method can well separate 3D porous layered hydrophobic TBSC-capped Zn₄ clusters (H₄TBSC = p-tert-butylsulfonyl calix[4]arenes, Figure 11A-F). This study provides a new strategy for inducing stereoselectivity in organocatalysis, offering value for the development of UMOF NSs with applications in a variety of enantioselective applications. In 2020, Jian et al. reported the preparation of water-resistant monolayer porphyrin framework nanosheets using simple sonication and constructed water-stable Al-MOF films using ultrathin nanosheets^[176]. As shown in Figure 11G, the measured thickness of the Al-MOF nanosheets was approximately 1.9 nm, which is close to the theoretical thickness of 1.35 nm. Strictly speaking, increasing the ultrasonic time can improve the peel yield; regulating the power can affect the peel results. The ultrasonic stripping method ensures both morphology and property integrity, but it is inconvenient to control nanosheet thickness.

Figure 10. (A) General reaction scheme for the targeted synthesis of MOFs [Cu(n)(DMF)]_n, where n = 1-5. Defines the R groups in the dicarboxylic acid ligand precursors H₂1 to H₂5. (B-D) AFM images of Cu(3), Cu(4) and Cu(5) nanosheets following ultrasonic stripping and centrifugation (1500 rpm for 1 h) in MeCN. (E-G) Scatterplot of correlation for MON dimensions. (A-G) Adapted with permission^[172]. Copyright 2019, American Chemical Society. MOFs: Metal-organic frameworks; AFM: atomic force microscopy; MON: MOF nanosheets.

Figure 11. (A) Schematic diagram of exfoliation of MOF 1 blocks into UMOF NSs. (B) SEM image of MOF 1. (C) TEM image of MON 1. (D) MON 1 Tyndall effect of suspensions in i-PrOH. (E) SEM image of the Al-MOF bulk crystals. (F) TEM image of exfoliated Al-MOF nanosheets. (G) The AFM image of Al-MOF nanosheets on a silicon wafer, the inset shows the corresponding height profile. (A-D) Adapted with permission^[175]. Copyright 2019, American Chemical Society. (E-G) Adapted with permission^[176]. Copyright 2020, American Association for the Advancement of Science. MOFs: Metal-organic frameworks; UMOF NSs: ultrathin MOF nanosheets; MON: MOF nanosheets; AFM: atomic force microscopy.

Chemical exfoliation

As well as micro-mechanical exfoliation and ultrasonication, chemical exfoliation with the selection of organic small molecules as insertion agents is commonly used for preparing UMOF NSs. In 2017, Ding et al. used a novel intercalation and chemical exfoliation method to obtain UMOF NSs (~1 nm) 2D MOF nanosheets from layered MOF crystals^[177]. The obtained UMOF NSs exhibit highly efficient multiphase photocatalytic performance compared to the corresponding layered MOFs bulk. They first designed new MOFs by inserting a chemically unstable bipyridyl ligand, 4,4'-dipyridyl disulfide (DPDS), into the layered MOF crystals and showed that the interlayer distance in the new MOF crystals expanded after inserting DPDS. Then, the nature of the trimethylphosphine (TMP) compound to reduce disulfide bonds was used to sever the DPDS, causing the interlayer forces in the volume-expanded layered MOF crystals to weaken and be exfoliated into ultra-thin nanosheets [Figure 12A]. This method is expected to be of great use in the synthesis of a variety of ultrathin 2D MOF nanosheets with specific structures and properties^[178]. In 2018, Huang et al. reported a strategy based on electrochemical/chemical exfoliation of columnar MOFs to selectively break interlayer coordination bonds and to prepare 2D MOFs that could facilitate the oxygen evolution reaction^[179]. Moreover, some functional groups interacting with H⁺ ions, such as hydroxyl and ester groups, are present in bulk 2D MOFs. In 2021, Liu et al. first protonated bulk Zn-MOFs under acidic conditions and then used a negatively charged Congo red (CR) dye as a “tape” to attach to the Zn-MOF surface using electrostatic interactions to further break the weak van der Waals forces [Figure 12B], which further breaks the weak van der Waals forces, resulting in an ultra-thin monolayers (3.4 nm) 2D nanosheet^[180]. While for the in-situ exfoliating process, the adsorption of CR molecules prevented the re-accumulation of the exfoliated nanosheets, providing excellent adsorption properties. The chemical exfoliation, though ingratiating, has a certain threshold, usually with a selective link between the insertion agent and the peeled object, and is therefore not widely applicable. Both insertion and stripping processes are involved, and often, the insertion step is the decisive step in the stripping process and the stripping rate is not easily controlled.

Figure 12. (A) Schematic diagram of the entire formation of 2D MOF nanosheets by chemical exfoliation. Adapted with permission^[177]. Copyright 2017, American Chemical Society. (B) Schematic representation of the synthesizing 2D Zn-MOF nanosheets. Adapted with permission^[180]. Copyright 2021, Royal Society of Chemistry. MOFs: Metal-organic frameworks; 2D: two-dimensional.

Solution stripping

With the further exploration of 2D materials, a number of chemical strategies have been developed and applied to the preparation of UMOF NSs. In 2013, Gallego et al. completely exfoliated the laminate by solvent interaction alone, before which solvent exfoliation (without the application of other external forces) of 3D laminates had almost not been reported^[181]. They achieve complete separation of the crystals by immersing the layered MOF [Cu(µ-pym₂S₂)(µ-Cl)]_n · nY (where Y can be MeOH, H₂O or 1/2EtOH) in water. The interlayer cavities in the crystal structures can be filled by solvent molecules in a simplistic and repetitive manner. This solvent exchange does not affect the crystallinity of the single crystals. The structural features of the compounds show that the solvent in the interlayer cavities prohibits strong layer-to-layer interactions. This method is very soft in terms of energy due to solvent-assisted procedures, avoiding the potential complications of exfoliation in laminar crystals or solid matrices. In 2017, Wang et al. used the freeze-thaw method to exfoliate a kind of bulk MOFs, MAMS-1 [Ni₈(5-bbdc)₆(µ-OH)₄], into large, defect-free 2D nanosheets [Figure 13A-D]^[182]. The hexane solution undergoes a transformation between a solid and a liquid state under the influence of temperature, which generates a shear force that leads to the exfoliation of nanosheets. These nanosheets are also used as molecular sieve membrane structures to facilitate gas separation. The Aida team used SMe-H₂ip ligands to synthesize a porous kagome lattice of bulk MOF^[183]. When immersed in a non-plasmonic polar solvent, employing N,N-dimethylformamide (DMF) or tetrahydrofuran (THF), kgm^SMe was discovered to swell rapidly like an accordion, gradually exfoliating the monolayer nanosheets. The solvent stripping is similar to the chemical exfoliation where the solvent is used as an insert to strip the material. The disadvantages are slow peel rates, low yields and a narrow range of applications.

Figure 13. (A) MAMS-1 crystals freeze-thawed and exfoliated into dispersed nanosheets. (B) FE-SEM image of a laminated MAMS-1 crystal. (C) Crystal structure of MAMS-1. (D) AFM image of purified MAMS-1 nanosheets. (A-D) Adapted with permission^[182]. Copyright 2017, Springer Nature. MOFs: Metal-organic frameworks; FE-SEM: field-emission scanning electron microscopy; AFM: atomic force microscopy.

Interface synthesis

Restricting the growth of MOF crystals to the interface of two dissimilar phases is a common method for synthesizing 2D MOF nanosheets (MONs)^[187,188]. Interfacial synthesis confines metal ions and organic ligands at the interface of two phases to form a thin sheet of MOFs. With the aid of non-destructive optical methods, it is possible to monitor the growth of the crystal interface and indicate the presence of a reversible transition layer at the growth interface, which provides guidance for the synthesis of MOFs^[189,190]. Coordination reactions that typically occur at the interface of two phases have the advantage of large area, good size ratio and easy transfer to the device^[191]. By considering the type of interface, interfacial synthesis can be divided into three categories: liquid/air interface, liquid/solid interface and liquid/liquid interface.

Liquid/air interface: accompanied by the evaporation process of the organic solvent at the interface and then the introduction of surface pressure with the Langmuir-Blodgett (LB) technique, the grown monolayer MOFs can be comfortably transferred to the substrate. In 2015, Sakamoto et al. used the liquid/air interface reaction to synthesize monolithic nanosheets with a lateral size larger than 10 µm in size^[192]. They added a solution of dichloromethane containing a very small amount of three-way dipyrrin ligand molecule (L1) slowly and dropwise to an aqueous phase containing zinc (II) acetate. Along with the rapid volatilization of dichloromethane, an immediate coordination between L1 and zinc (II) ions is achieved, generating a monolayer of MOF (N1) at the interface. Ying et al. fabricated flexible Cu(dhbc)₂(bpy) · H₂O MONs at the air/water interface and dispersed them in an isopropanol solution with a significant Tyndall effect [Figure 14A-D]^[193]. The thickness of the nanosheets shown in Figure 14C can be clearly observed as about 4.4 nm. Subsequently, they combined MONs with graphene oxide nanosheets (GONs) to prepare ultra-thin hybrid films (~100 nm) that can be used for CO₂ separation. Zhang et al. synthesized the conductive MOF film [Co₃(HOB)₂]_n (HOB = 1,2,3,4,5,6-benzenehexaol) using (Langmuir-Blodgett) LB technology^[194]. Interestingly, this 2D film has good electrical conductivity and detects hydrogen peroxide well with an ultra-low detection rate of 1.05 × 10^-8 wt%.

Figure 14. (A) Scheme for the preparation of layered less Cu(dhbc)₂(bpy) · H₂O MONs at the air/water interface. (B) AFM image of Cu(dhbc)₂(bpy) · H₂O MONs. Inset: height profiles of MONs corresponding to the three labeled lines. (C and D) TEM and HRTEM image of Cu(dhbc)₂(bpy) · H₂O MONs. (E) Schematic illustration of SDA strategy for the synthesis of CoNi-MOFNA, (F) SEM image. (G) TEM image. (H) AFM image. (I) Formation of isolated, anisotropically shaped DUT-134(Cu) ·DMF microcrystals. (J) Spontaneous self-assembly of the microcrystals into face-to-face or edge-to-face arrangements in fresh DMF solution. (K and L) Freestanding carpets and spontaneous roll-ups resulting in lengths up to ca. 100 µm in diameter and up to 20 mm or longer. Bottom: (I) SEM image of a DUT-134(Cu) flake. (J) surface of the carpet. (K) macrostructure of the dried carpet. Photos: (L) Translucent DUT-134 carpet in DMF (left) and large “multi-walled” macro-tubes in DMF (right). (A-D) Adapted with permission^[193]. Copyright 2021, John Wiley and Sons. (E-H) Adapted with permission^[199]. Copyright 2019, Elsevier. (I-L) Adapted with permission^[207]. Copyright 2022, John Wiley and Sons. MOFs: Metal-organic frameworks; MONs: MOF nanosheets; AFM: atomic force microscopy; SDA: self-dissociative assembly; DMF: dimethylformamide.

Liquid/solid interface: the reactants frequently constructed MONs by self-assembly at the two-phase interface between a metal or carbon substrate and a solution. Commonly utilized substrates are nickel foam (NF), graphite-carbon ensemble graphene, etc.^[195]. In recent years, inorganic salts with cheap prices, large crystallite planes, and high stability have gradually become common substrates for the synthesis of large-area 2D materials^[196,197]. Huang et al. then utilized sodium chloride templates to in-situ prepare high-quality ultrathin ZIF-67 nanosheets^[198]. The limited solvent and the appropriate amount of sodium chloride were key factors in the synthetic process. The growth of ZIF-67 was confined to the narrow space of sodium chloride microcrystals, ensuring that the ZIF-67 nanoparticles extended along the microcrystalline planes. In 2020, this team proposed a self-dissociative assembly (SDA) strategy to synthesize ultrathin nanosheets that served as highly active oxidative evolution reaction (OER) catalysts^[199]. Firstly, they picked a cobalt-nickel alloy as the template and mixed the template with a solvent that could dissolve the metal source and ligand. In a hydrothermal process, benzenedicarboxylic acid (BDC) oxidized the CoNi alloy template and dissociated Co²⁺ and Ni²⁺ on the template surface. Co²⁺ and Ni²⁺ were, in turn, coordinated with BDC and assembled into ultra-thin CONi-MOF nanosheet arrays (CoNi-MOFNA) on the alloy substrate surface [Figure 14E-H]. During this dissociation-assembly process, the nucleation of MOFs occurs at the surface of alloy, which results in restricted growth sites for locating the metal source and guaranteeing the homogeneity of the nanoarrays.

Liquid/liquid interface synthesis: two kinds of solutions with different densities are frequently utilized to dissolve metal salts and ligands, respectively, and the nanosheets are formed in the limited space at the interface of the two phases. Most MOFs are considered to be electrical insulators due to their poor electrical conductivity, but the emergence of new synthetic methods over the last decade or so has revealed some examples of conductive MOFs that allow them to be used as optical electronic components^[200-205]. Arora et al. have bridged the gap between materials synthesis and technological applications by synthesizing the first two-dimensional MOFs for photoelectric detection through a liquid/liquid interface strategy^[206]. They took advantage of the difference in density between 2,3,6,7,10,11-triphenylenehexathiol (THT) and water to synthesize Fe₃(THT)₂(NH₄)₃ films at the THT/water interface. They were also able to achieve controlled growth of the film thickness depending on the reaction time. Structurally, the monolayer Fe₃(THT)₂(NH₄)₃ film exhibits 2D mesh structures with many planar hexagonal connections, and Fe atoms were coordinated with THT organic ligands to form honeycomb structures with a pore size of about 1.9 nm. This is the first comprehensive study of the performance of MOF material components in terms of optical response, temperature and wavelength, and these materials will be promising candidates for optoelectronic applications. In 2021, Schwotzer et al. focused on the assembly of 2D-MOF nanosheets into complex superstructure crystals, successfully assembling small DUT-134 platelets into giant “carpet” structures by using a liquid-liquid interface approach [Figure 14I-L]^[207]. In a quasi-interfacial synthesis strategy, DMF plays a special role in the process, acting as a binder between the nanoparticles and the “carpet” structure, with the support of DMF leading to the synthesis of thin films. The translucent “carpet” enhances the accessibility of the Cu²⁺ sites after the removal of the solvent, improving the adsorption of H₂S and providing a reference for air filtration. The Cánovas team succeeded in synthesizing large multilayer Fe₃(THT)₂(NH₄)₃ films with a CHCl₃/water interface, and adjusting the reaction time allowed the film thickness to be controlled^[208]. Li et al. developed the synthesis of bimetallic Ni-M-MOF (M = Al, Mn, Zn, Co and Cd) nanosheets using the mixed solvent N,N-dimethylacetamide (DMAC) and water^[209]. This ultrathin heterometal can be used as an efficient electrocatalyst for oxygen evolution reactions.

Layer-by-layer assembly

The layer-by-layer (LBL) synthesis method is a sub-method of the liquid phase epitaxy (LPE) procedure. The LBL process is defined as the cyclic immersion of functionalized substrates such as gold and silicon in a solution containing metal ions and a solution containing organic linkers, resulting in highly crystalline surface-anchored MOFs (SURMOFs)^[210-213]. Between each immersion, the samples need to be washed with a pure solvent to remove excess scaffolds^[214]. The thickness of the film can be regulated by the number of repeated immersions. A few years earlier, Sakaida et al. reported the first 2D tetracyanonickelate-based MOF by drawing on their previous method of synthesizing 3D MOF^[215]. Since the orientation of the crystals heavily influenced the deposition process, they tried to form self-assembled monolayer (SAM) by first immersing the gold substrates in an ethanolic solution of 4-mercaptopyridine overnight. The substrates were then alternately immersed in the two ethanol solutions for 20 cycles, resulting in the synthesis of highly crystalline films [Figure 15A]. Xiao et al. constructed ultrathin TA-Zn²⁺ layers based on tannic acid (TA) and Zn²⁺ on polyethersulfone substrates using the LBL method, and subsequently converted the TA-Zn²⁺ layers into ZIF-8 films by immersing them in the 2-methylimidazole (Hmim) solution as a zinc source^[216]. The LBL method has also been applied to heteroepitaxy and Liu et al. have reported the use of the LBL method to fabricate a photoactive MOF-on-MOF heterostructure^[217]. Conductive MOF films have great applications in various fields, such as energy storage and optical field, due to their thin thickness, conductivity and large area size. In 2021, the preparation of 2D Cu₃(HHTP)₂ (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) films were achieved by Zhao et al. through the layer-by-layer assembly method^[218]. Controlled thickness growth was easily achieved by controlling the growth period [Figure 15B-D]. The synthesized Cu₃(HHTP)₂ pores have a hexagonal structure with a pore diameter of only 1.8 nm, which can improve the transport efficiency of electrolyte ions and greatly encourage the development of energy memory devices.

Figure 15. (A) Diagram of LbL film fabrication. (B) Schematic synthesis of Cu₃(HHTP)₂. (C) Diagram of layer-by-layer growth of Cu₃(HHTP)₂ films on ITO/PET substrates by repeated cyclic growth. (D) Top view of Cu₃(HHTP)₂ growth on the surface of an ITO/PET substrate. (A) Adapted with permission^[215]. Copyright 2017, American Chemical Society. (B-D) Adapted with permission^[218]. Copyright 2021, John Wiley and Sons.

Surfactant-assisted strategy

The synthesis of 2D MOF nanosheets or nanofilms using interfacial synthesis and LBL methods usually requires specific substrates and relatively complex preparation steps, but the direct synthesis of 2D MOF nanosheets has not been reported. Common surfactants, such as polyvinylpyrrolidone (PVP) and cetyltrimethylammonium bromide (CTAB), not only limit the growth of MOFs along the stacking direction but also assist in stabilizing the MOFs precursors. In 2015, Cao et al. firstly reported a surfactant-mediated strategy for the direct production of ultrathin, homogeneous, high-quality 2D MOF nanosheets^[50]. They selected PVP as a surfactant to control the various anisotropic growth of Zn-TCPP crystals to form ultrathin MOF nanosheets with a thickness of less than 10 nm [Figure 16A-D]. Later, they investigated the generality of different metals for forming 2D nanosheets and found that 2D nanosheets could be constructed when using Zn, Cu, Cd or Co salts as metal precursors. Subsequently, in 2016, they prepared 2D porphyrin paddlewheel framework-3 (PPF-3) MOF nanosheets using a surfactant-mediated method and obtained 2D composites for super electrode materials by sulfidation and carbonization^[219]. It has been shown that PVP molecules can selectively attach to the surface of MOFs, reducing interlayer contact and stacking, resulting in the growth of MOFs in a two-dimensional direction and eventually forming MONs^[220]. Hang et al. used PVP to prepare 2D Zn-TCPP MOF for controlled photodynamic therapy (PDT) materials for the treatment of specific cancers^[221].

Figure 16. (A) Routine and surfactant-assisted synthesis of MOF. (B) STEM image of Zn-TCPP nanosheets. (C) TEM image of single Zn-TCPP nanosheets. (D) AFM images of Zn-TCPP nanosheets, Scale bar: 2 µm. (E) Scheme for the synthesis of ultrathin HHB-Cu nanosheets using a surfactant-assisted method. (F) Scheme for the synthesis of HHB-Cu. (G) SEM images of HHB-Cu NSs with lamellar morphology and layered structures. (H) AFM image and height profile along the white line marked on the HHB-Cu NSs. (A-D) Adapted with permission^[50]. Copyright 2015, John Wiley and Sons. (E-H) Adapted with permission^[222]. Copyright 2020, Royal Society of Chemistry. MOFs: Metal-organic frameworks; AFM: atomic force microscopy; NSs: nanosheets.

In 2020, Wang et al. employed the anionic surfactant sodium dodecyl sulfate (SDS) to prepare homogeneous 2D conjugated MOFs (2D c-MOFs)^[222]. In this process, SDS molecules acted as structural modifiers, and the negatively charged hydrophilic groups in the molecule preferentially anchored on the surface of MOFs under electrostatic interactions, alleviating the buildup between layers and facilitating the formation of ultrathin nanosheets [Figure 16E]. HHB-Cu was first constructed and subsequently stripped of HHB-Cu NSs under sonication. The prepared single crystal HHB-Cu NSs are characterized by a thickness of 4-5 nm (~8-10 atomic layers) [Figure 16F-H].

Zirconium-based MOFs have received much attention compared to other metal-based MOFs due to their higher thermal stability and rich structural types^[223,224]. Reports on Zr-MOF nanosheets are limited when compared to many other MOF nanosheets, due to the strong metal coordination bonds^[49]. In contrast, in 2019, Zhang et al. demonstrated a simple strategy for the direct synthesis of 2D Zr-BDC MOF nanosheets^[225]. Considering the defective linker in UIO-66, they chose a bio-based surfactant (SAAS-Cm) as a ligand competitor to attach to the defect in Zr-BDC MOFs, leading to anisotropic growth of MOFs. Pseudo-assembly into bulk MOFs occurs during the growth of the crystals, and the interaction forces between the layers of the bulk MOFs are weakened by the interaction of the surfactant hydrophobic chains, which in turn disintegrate into ultrathin nanosheets (3-4 nm). Furthermore, they found that changing the number of alkyl groups on the hydrophobic chains can simply modulate the thickness of 2D nanosheets (3-60 nm).

MOFs/Metal oxides

The doping of metal oxides is a way to stably modify MOFs and improve their performance. One of the keys to research on the sensing aspects of trace gases is the detection of acetone gas^[301]. Long-term exposure to acetone can cause irreversible damage to human health, and MOFs have attracted a lot of attention as new functional nanomaterials for gas sensing. Zhou et al. designed and synthesized a ZnO@ZIF-71(Co) composite sensor in order to achieve the detection of acetone^[302]. As is known, the element Co activates oxygen in air to generate reactive oxygen, and Co in the form of oxide or metal has been widely used to study the detection of acetone^[303,304]. While introducing Co into porous MOFs and constructing co-doped such probes is relatively rare. The response amplitude of the complex they prepared was approximately 100 times higher than that of the undoped sensor, and the Co site in ZIF-71 (Co) catalyzes the activation of oxygen and promotes the decomposition of acetone, ultimately improving the sensitivity to trace amounts of acetone (50 ppb). Water pollution has been a hot topic of concern for environmentalists and scientists over the years, and the adsorption of heavy metal ions such as Hg (II), Pb (II) and Cr (V) is a particularly high priority. Porous and tunable MOFs are highly studied in metal adsorption, but their poor water stability limits their application development. Zhang et al. used a two-step process to prepare 3D MoO₃@ZIF-8 core-shell nanorod structures for the reduction of highly toxic Cr (V) to non-toxic Cr (III)^[305]. Characterization revealed that new bonds were formed between MoO₃ and ZIF-8 in the complexes. Subsequent analysis of their photoluminescence spectra and transient photocurrent density maps confirmed the generation and transfer of holes. The hybrid heterojunction formed improves the charge separation efficiency. The 3D core-shell structure not only has a large specific surface area but is also much more stable for aqueous environments. The high surface area of ZIF-8 also provides more active sites for electrons and protons. The formation of new bonds enhances the “synergistic effect” of both, as well as the electron capture capacity and photocatalytic activity of the hybrid material. Abdollahi et al. developed Fe₃O₄@TMU-32 complexes through a surface charge modulation strategy^[306]. This composite exhibited high adsorption capacity for Hg (II) and Pb (II) metal ions.

In recent years, our group has done a lot of research on MOFs/metal oxides. In 2020, we used a simple hydrothermal method to prepare highly alkaline stable Co₃O₄@Co-MOF complexes^[307]. Co₃O₄ on the surface of Co-MOF effectively improves the alkaline stability, allowing this composite to remain structurally stable in 3.0 M KOH solution for as long as 15 days [Figure 24A and B]. The material has a high specific capacity of 1020 F g^-1 at a current density of 0.5 A g^-1 and the maximum energy density of the constructed flexible device is 21.6 mWh cm^-3, offering significant advantages in terms of improved durability and capacitance. When targeting the disadvantage that polyoxometalate (POM) lacks sufficient active sites as a water oxidation catalyst, we propose to compound it with ZIF-67 with sufficient surface area to prepare a yolk/shell ZIF-67@POM hybrid, which can be used as a sustainable heterogeneous water oxidation catalyst [Figure 24C]^[308]. This unique yolk/shell structure has a potential synergy between ZIF-67 and POM, with an overpotential of only 287 mV at a current density of 10 mA cm^-2, resulting in excellent electrocatalytic activity toward OER. We also prepared a series of MOF@PBA with multi-level structures by compounding ultrathin MOF nanobelts with PBA through an in-situ cation exchange strategy [Figure 24D]^[309]. The multi-level nanostructures exposed more active sites, which promoted charge transfer, and the resulting complexes exhibited efficient catalytic oxygen precipitation under strongly basic conditions.

Figure 24. (A) Synthesis of Co₃O₄@Co-MOF hybrids. (B) Optical and SEM images of Co₃O₄@Co-MOF hybrids after different reaction times. (C) Schematic diagram of the synthesis of yolk/shell ZIF-67@POM hybrid material. (D) Formation process of Ni-MIL-77@PBA hybrid structure. (E) Schematic of the fabrication of TMO/ZIF-67 hybrids. (“···” stands for different MOFs). (F and G) SEM (F) and TEM images of ZIF-67 sample after annealing. (H) SEM image of Co₃O₄/ZIF-67. (I) TEM image of Co₃O₄/ZIF-67. (A and B) Adapted with permission^[307]. Copyright 2019, Oxford University Press. (C) Adapted with permissFElectrochemical sensors for the identification ofion^[308]. Copyright 2019, American Chemical Society. (D) Adapted with permission^[309]. Copyright 2019, Royal Society of Chemistry. (E-I) Adapted with permission^[310]. Copyright 2021, Elsevier. MOFs: Metal-organic frameworks; PBA: Prussian blue analogues.

MOF hybrids have applications not only in catalysis but also in energy storage. The poor electrical conductivity of pure MOFs has been affecting their promotion in the field of energy storage. In 2021, we tried a seed-mediated growth method to prepare Co₃O₄/ZIF-67 composites [Figure 24E]^[310]. The Co₃O₄/ZIF-67 composites retain the morphology of ZIF-67 with the edges of Co₃O₄ partially exposed, as shown in Figure 24F-I. The prepared hybrids obtained the semiconductor properties of transition metal oxide (TMOs) and the high specific surface area of ZIF-67, which enhanced the lithium storage performance when the Co₃O₄/ZIF-67 blend was used as an anode for lithium-ion batteries.

The hybridization of MOFs and metal oxides is not complicated, but it is difficult to combine them to achieve better properties and applications considering their respective characteristics. For now, the semiconductor properties of most metal oxides, combined with the large specific surface area and porosity of MOFs, hybridization is mainly used in the field of catalysis, and future expansion for other applications is necessary. For example, the relatively good electrical conductivity of metal oxides, the large theoretical specific capacitance and the framework properties of MOFs. Given their characteristics, the prospects of the complexes in energy storage can be envisaged, but the drawbacks of volumetric expansion of the metal oxides and the structural instability of the MOFs are frequently inevitable during charging and discharging. Therefore, adequately combining the advantages and improving the disadvantages is a great challenge for the hybridization.

MOFs/Carbides

Carbon-based materials mainly include graphene, carbon nanotubes and activated carbon, which have been widely used in the field of materials due to their low price. The combination of MOFs and carbon-based materials produce mixtures that make up for the shortcomings of pure MOFs, enhancing the strength, electrical conductivity and stability of the MOFs, which are of great application value.

It is well known that graphene, as a star representative of 2D materials, is widely sought after because of its large specific surface area and high electrical conductivity^[311]. In 2020, Venkadesh et al. prepared Cu-MOF/graphene hybrids using solvothermal and electrochemical methods^[312]. The fabricated Cu-MOF/graphene composites showed a porous flower shape with good electrical conductivity, high surface area to enhance the electron transfer with caffeine, and good electrochemical properties for the electrochemical oxidation of caffeine. The developed sensor can accurately determine the concentration of caffeine in coffee and tea with good recovery. 3D graphene (3DG) networks also provide high surface area and high electrical conductivity for charge transport due to their porous nature. 3DG was synthesized by chemical vapor deposition (CVD) growth by Khiarak et al. and then needle-like CoNi MOFs were grown on the surface of the 3DG network by in-situ solvothermal process^[313]. 3DG structures provide highly active sites and accelerate the diffusion of ions from the electrolyte to the active sites. Using the synergistic effect of the metal components, the directional growth of bimetallic MOFs on 3DG facilitates charge transfer^[314]. This material provides a good activity for catalytic cracking of water.

Carbon nanotubes (CNTs) have been investigated for their extremely high aspect ratio and functionalization. Huang et al. used a mixed H₂O₂/NaOH solution to post-treat the Ce-MOF/CNTs mixture to induce the oxidation of Ce atoms from Ce³⁺ to Ce^4+[315]. Electrochemical sensors for the identification of hydroquinone (HQ) and catechol (CC) were prepared. The modified electrodes exhibited two distinct oxidation peaks and higher oxidation currents compared to the untreated composites. Quan et al. synthesized CNT by pyrolysis of ZIF-67 and compounded it with aminated MIL-53(Al) to obtain a heterogeneous structure called N-MIL@CNT [Figure 25A]^[316]. It was found that the - attraction forces, hydrogen bonding and interfacial synergistic effects induced by this heterostructure contributed to the adsorption of organic compounds. Zeolite imidazolium frameworks (ZIFs), a subset of MOFs, exhibit some chemical and thermal stability in aqueous solutions, making them promising candidates for the adsorption of pollutants^[317]. ZIF-67@C-MOF-74 hybrid was prepared by Rio et al. by carbonizing MOF-74 as a precursor^[318]. ZIF-67@C-MOF-74 is easily regenerated and has good extraction capacity for phenolic compounds and can be used as an adsorbent for toxic pollutants.

Figure 25. (A) Schematic representation of the synthesis of N-MIL@CNT and corresponding morphological characterizations. (B) Illustration of the synthetic route for the preparation of Pt single-atom coordinated ultra-thin MOF nanosheets (PtSA-MNSs) and corresponding morphological characterizations. (C) Schematic representation of the production of a core-shell type hybrid MOF (Fe-MIL-88B@Fe-MIL-88C) by anisotropic MOF-on-MOF growth, (C₁-C₉) SEM images of Fe-MIL-88B@Fe-MIL-88C during the formation process to monitor MOF-on-MOF growth. (D) Schematic representation of the proposed anisotropic MOF-on-MOF growth process from the tip to the middle. (A) Adapted with permission^[316]. Copyright 2020, American Chemical Society. (B) Adapted with permission^[328]. Copyright 2019, John Wiley and Sons. (C and D) Adapted with permission^[354]. Copyright 2020, American Chemical Society. MOFs: Metal-organic frameworks.

MOFs/Metal single-atom or nanoparticles

Metal nanoparticles (MNPs), especially noble metals such as Pt, Pd, Ag and Au, are excellent co-catalysts as electron acceptors with low Fermi energy levels^[319,320]. In contrast, porous MOFs with high periodicity and tunable porosity are ideal platforms for loading nanoscale metal particles. Utilizing the closed nature and regular cavities of MOFs can effectively limit the aggregation and fusion of MNPs, resulting in a stable and homogeneous structure that facilitates diffusion and transport^[321].

Single-atom catalysts (SACs), which have caught fire in recent years, have become the new frontier in multiphase catalysis due to their maximum atom utilization efficiency and excellent catalytic performance^[322-324]. Although many innovative synthetic strategies have been explored just to obtain stable SACs, the problems of low metal loading and atom aggregation have not yet been solved. The research team found that MOFs can serve as excellent supports for single metal atoms catalysis^[325-327]. Then Zuo et al. prepared ultrathin 2D MOFs (PtSA-MNSs) coordinated with high concentrations of Pt single atoms with the assistance of PVP^[328]. The Pt atoms coordinated within the porphyrin ring are well dispersed after the formation of the MOF. The thickness of the resulting nanosheets is approximately 2.4 ± 0.9 nm [Figure 25B]. The ultrathin structure suggests that more active sites are exposed and also minimizes the transport distance of photon-generated carriers from the interior of the material to the surface, thereby suppressing undesired electron-hole complexation and allowing efficient visible-light-driven H₂ evolution. Generally, MOFs functionalized with noble metal nanoparticles are very effective materials for photocatalytic reactions, especially when the metal particles have matching crystal faces with the MOFs. Qiu et al. prepared noble metal-doped Zr-MOF catalysts using a facile post-synthesis method and investigated the photocatalytic oxidation process of aromatic alcohols^[329]. They found that Au NPs promoted the oxidation of aromatic alcohols while Pt NPs inhibited the oxidation, and that the low conversion of aromatic alcohols over Pt/Zr-MOFs was due to the exposure of the Pt (200) crystal plane, which resulted in low yields during the production of O₂^-. By studying the mechanism, this work will improve the reference for the development of new photocatalytic materials. The bimetallic-doped MOFs also tend to exhibit notable performance. Among them are Pt-Ni nanoparticles with hollow open frameworks that exhibit superior catalytic behavior. Previous studies have focused on metal nanoparticles, rarely combining them with a second material, let alone encapsulating them as a core structure in a second material. In 2020, Chen et al. reported the encapsulation of Pt-Ni nanocentric structures in ZIF-8 alone to achieve core-shell and yolk-shell structures^[330]. They used the surfactant hexadecyltrimethylammonium bromide (CTAB) to encapsulate and control the Pt-Ni nanoframe. The surface-functionalized Pt-Ni nanoframe could be well dispersed in aqueous solution and subsequently added to ZIF-8 solution to prepare single core-shell complexes. 3D tomography confirmed the hollow nanoframe ground embedding.

While a wide variety of complexes of MOFs and metal nanoparticles are available, current methods are far from adequate for practical applications. The influence of both MOFs and MNPs on the catalytic properties of the complexes limits the complete understanding of the catalytic mechanism process. In the future, the development of in-situ characterization techniques will help us to understand the formation process and catalytic mechanism of the composites, and the completeness of the characterization tools can reasonably guide us to design specific composite catalysts for specific catalytic reactions.

MOFs/Polymers

With the development of polymer materials, the flexible nature of polymers, which have a softer structure than MOFs, can accommodate MOFs and regulate the growth of MOFs^[331,332]. Given these advantages, the hybridization of MOFs and polymers also creates great synergies, allowing the hybrids to have more complex structures and exhibit specific properties that are functionally superior to the individual components.

Previously, it has been found that MOF/polymers can strengthen the properties of MOFs by maintaining framework stability or enhancing the absorption of analytes^[333]. Cohen utilized a post-synthesis modified polymerization method to composite nylon and MOF via an interfacial polymerization technique^[334]. Compared to non-covalent hybrids, the covalent hybrids significantly increased the rate of decomposition of the simulant by nearly an order of magnitude. This material is expected to be designed as a potential textile. Previously, Hwang et al. developed a new nanoscale mesocrystal using a double-hydrophilic block polymer (DHBC) as a crystal modifier, as opposed to an amphiphilic block polymer^[118]. DHBC grows selectively and anisotropically attached to specific surfaces of MOFs, generating vertically aligned hexagonal rod mesocrystals. This work demonstrates the ability of polymers to modulate the growth of MOFs, providing a new means of building a complex and robust morphological platform.

The hybridization of MOFs as carriers with various polymers provides new insights into the future exploration of complex nanostructures. Multifunctional MOF-polymer hybrids are of great potential value in areas such as biomedicine and gas separation^[335]. Nevertheless, the hybrids still have limitations in some aspects (e.g., selectivity and long-term stability) which remain to be improved in the future.

MOF-on-MOF

MOF-on-MOF hybrid frameworks are generally constructed using epitaxial growth. Epitaxial growth is the process of depositing crystalline material on specific crystalline planes of the same orientation and similar lattice spacing of the crystal substrate, which often generates core-shell shaped or layered structures^[336,337]. The key to growth is the selection of suitable host and guest candidate MOFs for the epitaxy of MOF-on-MOF materials. Normally, secondary MOFs with well-matched lattice constants will grow homogeneously in all directions of the original MOF substrate, generating a core-shell structure. In contrast, when the lattice constants of the host and guest are significantly different, the original MOF substrate will change, which may cause the secondary MOF layer to stop growing and the inherent elasticity of the MOF may offset the interfacial energy, thus continuing to produce epitaxial growth^[338]. Therefore, epitaxial growth methods can generally be divided into isotropic growth MOF-on-MOF and anisotropic growth MOF-on-MOF.

Isotropic growth on MOF substrates is achieved by matching lattice parameters in the a, b and c directions, leading to more flexible core-shell nanostructures^[339-341]. Metal sources and organic ligands are key to triggering isotropic growth. In 2016, Choi et al. then successfully synthesized the isotropically growing core-shell MIL-68@MIL-68-Br using MIL-68 as a template and MIL-68-Br as a secondary MOF^[342]. The same metal source, with little difference in ligands, led to almost identical structures for MIL-68 (Orthorhombic, a = 21.77 Å, b = 37.68 Å, c = 7.23 Å) and MIL-68-Br (Orthorhombic, a = 21.73 Å, b = 37.61 Å, c = 7.22 Å), resulting in isotropic growth. The length and thickness of the hexagonal rod structure increased with time. EDX spectral scans confirmed that the Br atoms were distributed as components of the shell structure at the edges of the hexagonal rod structure. Small differences in the ligands can lead to isotropic growth. Similarly, metal ion size can cause changes in cell parameters, but two similarly sized metal sources may be able to ignore differences in cell parameters. In 2018, Pan et al. used epitaxial growth to prepare core-shell ZIF-8 (a = b = c = 16.9910 Å) and ZIF-67 (a = b = c = 16.9589 Å) with similar topology and unitary cellular parameters ZIF-8@ZIF-67^[343]. Due to the relatively similar ion sizes of Zn²⁺ (74.0 pm) and Co²⁺ (74.5 pm), it is easy to form isotropically grown lattice-matched core-shell MOFs. The use of ZIF-67 or ZIF-8 as precursors for the fabrication of isotropic structures has been previously reported by Zhan et al.^[344].

Lattice mismatch secondary MOF growth tends to result in abnormally shaped particles that tend to grow anisotropically only on the specific surface of the MOF substrate; hence, the construction of MOF heterostructures has attracted much attention^[345-347]. Recently, anisotropic MOF-on-MOF growth has been shown to be an effective means of achieving MOF heterostructures, controlling the growth rate and orientation of secondary MOFs, etc.^[348-353]. The structure of anisotropically grown MOF-on-MOFs is usually sandwich or tubular in shape, but Lee et al. found that even if the lattice of secondary MOFs and MOF basal cells do not match, core-shell hybrid MOFs can be generated under anisotropic growth^[354]. The cell parameters show that MIL-88B and MIL-88C have a large mismatched cell lattice in the c-direction, with Fe-MIL-88C (a = 10.22 Å, c = 23.60 Å, V = 2020 ų) able to grow anisotropically on the MIL-88B (a = 11.05 Å, c = 18.99 Å, V = 1980 ų) template, resulting in a Fe-MIL-88B @Fe-MIL-88C shell-type hybrid MOF [Figure 25C and D]. By monitoring the reaction time, the growth of Fe-MIL-88C first starts at the ends of the template and then propagates towards the center of the template in the c direction, resulting in an increase in the overall size of the template and the formation of a well-defined core-shell hybrid. Gu et al. used an anisotropic MOF-on-MOF strategy to prepare two molecules with crystal surface differences into multilayer MOF heterostructures^[355]. A series of intercalated ZIF-L heterostructures were constructed by stacking ZIF-L-Zn and ZIF-L-Co in alternating three-, five-, seven- and multilayer structures, which are expected to be precursors for derivatives with tunable magnetic and catalytic properties.