1 Introduction
Metal-organic framework (MOF) compounds are a kind of crystalline porous materials with periodic network structure, which are formed by self-assembly of metal ions or metal clusters as nodes and bridged organic ligands. MOF usually has many properties, such as porous, large specific surface area and multi-metal sites, so it has a wide range of applications in gas storage, molecular separation, catalysis, drug delivery and energy storage
[1⇓⇓⇓⇓⇓⇓~8]. In recent years, the research on the structure, synthesis, properties and applications of MOF is in the ascendant, and its related research is also one of the fastest growing research fields in the field of chemistry and materials
[7,9,10].
The coordination unsaturated metal sites in MOF materials are atomically dispersed, and these metal sites can be used as highly active reaction sites in catalysis, separation and sensing reactions
[4,11]. However, due to the large amount of resistance in the microscopic pores of the micron-scale bulk MOF, it is difficult to completely remove the residual solvent molecules, so it is very challenging to maximize the exposure of the metal sites of the MOF to provide more catalytic active centers. By designing and preparing the MOF material into a two-dimensional sheet, the advantages of the MOF can be combined with the inherent excellent physical properties of an ultra-thin two-dimensional material, so that the activity of the MOF in a plurality of chemical reactions can be effectively improved
[12]. The thickness of the ultrathin two-dimensional MOF nanomaterials is usually less than 5 nm, and the ultrathin two-dimensional MOF nanomaterials have more abundant catalytic active sites, higher density of defects, exchangeable coordination sites and stronger electron transfer ability, which can better improve the shortcomings of poor conductivity, less surface active sites and low catalytic activity of the bulk MOF
[13]. The ultra-thin two-dimensional MOF materials are formed by metal-organic coordination bonds and only have the thickness of one or more atoms, so they have better and more unique optical, electrical and mechanical properties, and have more promising application prospects in the fields of catalysis, separation, sensing and microelectronic devices
[14,15]. Most of the reported ultrathin two-dimensional MOF nanomaterials have a layered structure. However, there are still many challenges and innovative possibilities in how to synthesize ultra-thin two-dimensional MOF nanomaterials efficiently, controllably and in large quantities. To be sure, as a new member of the two-dimensional nanomaterial family, ultra-thin two-dimensional MOF nanomaterials provide broad opportunities for basic research and application. With the deepening understanding of the characteristics of ultra-thin two-dimensional nanomaterials and their advantages over MOF materials, the preparation methods of two-dimensional MOF nanomaterials have been a hot research topic for researchers, but there is still much room for improvement and innovation in the preparation of ultra-thin two-dimensional MOF nanomaterials with a thickness of less than 5 nm on the basis of ensuring the performance.
Figure 1 shows the development history and important breakthroughs of the preparation methods of ultra-thin two-dimensional MOF nanomaterials in the past decade. These methods include exfoliation method, interface (gas-liquid interface, liquid-liquid interface) preparation and template method. In this paper, the latest research progress in the field of ultra-thin two-dimensional MOF nanomaterials is reviewed, and the preparation methods of ultra-thin two-dimensional MOF nanomaterials with a thickness of less than 5 nm are emphatically introduced.The applications of the ultra-thin two-dimensional MOF nanomaterials in different fields are discussed, and the future development direction of this promising research field is prospected.
2 Method for preparing ultrathin two-dimensional MOF nano material
The preparation of ultra-thin two-dimensional MOF nanomaterials requires that the longitudinal growth of MOF crystals should be limited to a thickness of less than 5 nm without affecting their lateral growth. At present, that report preparation methods can be roughly divide into a top-down method, a bottom-up method and a two-dimensional oxide template sacrificial method
[21⇓⇓⇓~25]. The top-down method is to peel off the bulk MOF structure into ultra-thin two-dimensional MOF nanomaterials through physical or chemical effects, while the bottom-up method is to directly synthesize ultra-thin two-dimensional MOF nanomaterials from metal nodes and organic ligands by limiting the layer stacking in the vertical direction of MOF. The two-dimensional oxide template sacrificial method is a novel method for converting metal oxide flakes into ultra-thin two-dimensional MOF nanomaterials.
2.1 Top-down method
The MOF with layered structure has stronger intralayer coordination bonds and weaker interlayer van der Waals forces or hydrogen bonds. The top-down method can overcome the weak interaction between layers, so as to peel off the MOF crystal with a layered structure and form an ultra-thin two-dimensional MOF nanosheet material. The advantage of the top-down approach to the preparation of clean and highly crystalline ultrathin two-dimensional MOF nanosheets is that it is simple and efficient. Top-down methods include ultrasonic exfoliation, freeze-thaw exfoliation, and intercalation/chemical exfoliation. Among them, the ultrasonic peeling method is the most common and classical method, and the freeze-thaw method is relatively milder to peel MOF crystals.
2.1.1 Ultrasonic stripping method
The ultrasonic exfoliation method is essentially to exfoliate the laminated phase MOF material by the external force of ultrasonic waves to prepare ultra-thin two-dimensional MOF nanomaterials. Since Nielsen et al reported the preparation of ultra-thin Zn(C
12H
14O
4) by ultrasonic exfoliation in 2008, the ultrasonic exfoliation method with simple operation has been widely used, and it is still the most common method to prepare ultra-thin two-dimensional MOF nanomaterials
[26]. With the help of ultrasonic energy, on the one hand, it causes shear force and vibration effect between layers, on the other hand, it causes the collapse of bubbles formed after cavitation to produce micro-jet and impact, which makes Van der Waals force or hydrogen bond between layers easier to break, thus improving the stripping efficiency. The ultrasonic stripping method can roughly control the thickness and transverse dimension of the two-dimensional MOF nanosheet by controlling the frequency, energy density, amplitude, power and duration of the ultrasonic wave on the basis of selecting an appropriate solvent.
The ultrasonic exfoliation method is generally divided into three steps, including the selection of a suitable solvent, the optimization of ultrasonic parameters, and centrifugal deposition. Zamora et al. report exfoliation of layered MOF material [Cu
2Br(IN)
2]
n(IN=isonicotinato, isonicotinic acid] by sonication
[27]. In the [Cu
2Br(IN)
2]
n, the two-dimensional layered structural units are stacked together through π-π interactions between isonicotinic acid aromatic rings to form the final MOF crystal structure. The π-π interaction between layers can be overcome by ultrasonic technology.After ultrasonic treatment for 30 min, the bulk [Cu
2Br(IN)
2]
n crystals were successfully peeled off and dispersed in water, and the ultra-thin two-dimensional layered MOF products were prepared by centrifugal deposition.
In recent years, a variety of layered MOF structure crystals have been exfoliated into two-dimensional MOF nanosheets by ultrasonic treatment. For example, Luo et al. Prepared monolayer two-dimensional [gadolinium pyrazine] nanosheets by ultrasonic exfoliation
[28]. Yang et al. Successfully exfoliated the layered MOF crystal Zn
2(bim)
4(bim=benzimidazole, benzimidazole) in a mixed solvent of methanol and n-propanol with a solvent volume ratio of 1:1 by combining ultrasonic exfoliation with wet ball milling
[19]. Morsali et al. Reported that in the ultrasonic preparation of MOF material TMU-34, when the ultrasonic time was extended from 100 to 160 min, TMU-34 could change from a block to a uniform lamellar layer. It was also reported that the ultrasonic time for synthesizing Lu-MOF and Y-MOF was 80 min, and when the ultrasonic power was increased from 70 W to 100 W, the size of the product decreased from 5 μm to 1 μm
[29][30]. In general, although the time required for ultrasonic stripping of different MOF materials is different, increasing the ultrasonic power and prolonging the time can effectively reduce the agglomeration of MOF lamellae, but also make the size of the lamellae smaller.
The application of ultrasonic technology to exfoliate MOF to prepare ultra-thin two-dimensional MOF nanomaterials is very common. It has been reported that the solvent plays an important role in the ultrasonic exfoliation of layered MOF. The selection of suitable solvent will promote the effective stripping of MOF thin layer, make it more stable and improve the yield. In order to further improve the stripping efficiency and obtain a stable ultrathin two-dimensional MOF suspension, a mixed solvent may be used. Wherein one solvent is used for highly stripping the MOF, and the other solvent is used for highly stably stripping the obtained ultrathin two-dimensional MOF nanosheet
[31⇓~33]. At the same time, other mechanical external forces such as vibration stirring can also be used to prepare ultra-thin two-dimensional MOF nanomaterials, and vibration, stirring and ultrasonic technology can be used in combination. In addition, how to improve the solvent to better avoid the fragmentation of ultra-thin two-dimensional MOF nanosheets is still a problem. The systematic study of solvent effects will help to achieve efficient exfoliation and stabilization of ultra-thin two-dimensional MOF nanosheets.
2.1.2 Freeze-thaw stripping method
Although the ultrasonic stripping method is easy to operate, it is easy to cause a large number of fragments of the prepared ultra-thin two-dimensional MOF nanomaterials under the action of external force, which affects its application. Therefore, in addition to the ultrasonic/mechanical exfoliation method, other top-down methods have also been used to prepare ultra-thin two-dimensional MOF nanomaterials. Previously, exfoliation of hydrogenated boron nitride (HBN) bulk materials by thermal expansion of the solvent due to temperature changes has been reported
[34]. And that solvent molecule can weaken and destroy the interaction force between the layered MOF crystal. The choice of solvent is very important. The surface tension, dipole moment, relative dielectric constant and boiling point of the solvent will affect the expansion and exfoliation of the layered MOF crystal. Among them, the surface tension of the solvent is the most important factor. Based on this, Zhao et al. Reported a strategy of using n-hexane solvent with low surface tension and low boiling point (69.degree. C.) to assist the stripping
[35]. A MOF crystal, MAMS-1, was gently exfoliated by thermal expansion of the n-hexane solvent volume. In the preparation process, MOF crystal powder was dispersed in the n-hexane solvent and frozen in liquid nitrogen (-196 ° C), followed by thawing in a water bath at 80 ° C. During the repeated freezing and thawing cycles, the shear force generated by the volume change of n-hexane between the solid and liquid phases acts on the suspended MOF crystals, resulting in the exfoliation of the MOF crystals to form discrete and independent ultra-thin two-dimensional MOF nanosheets.
2.1.3 Intercalation/chemical exfoliation method
It is an effective way to prepare ultrathin two-dimensional nanomaterials in large quantities by stripping three-dimensional structural materials by physical or chemical methods. At present, ultrathin two-dimensional MOF nanomaterials are mainly prepared by ultrasonic mechanical peeling of two-dimensional MOF layered crystals. However, this method is difficult to control and has low efficiency, and the obtained two-dimensional nanomaterials also have the problems of large thickness and inhomogeneity. At the same time, due to the weakening of the interaction between MOF crystal layers, the mechanical or solvent-mediated exfoliation process is not well controlled, which often leads to different thicknesses and low yields (usually less than 15%) of ultra-thin two-dimensional MOF nanosheets
[36]. Therefore, it is a key and worthy research direction to develop new exfoliation techniques without affecting the weak coordination bonds between metal-organic ligands. Jiang Hailong of the University of Science and Technology of China has developed a controllable and efficient intercalation/chemical exfoliation method for the preparation of ultra-thin two-dimensional MOF nanosheet Zn
2(PdTCPP)
[20]. Based on the exploration of two-dimensional polymer materials, the advantages of disulfide bond functional groups with certain stability and bond-breaking properties are used to find a suitable layered MOF crystal structure, insert dipyridine ligands with disulfide bonds into the crystal, and use chemical reduction to break disulfide bonds, so as to strip the layered MOF to obtain ultra-thin two-dimensional MOF nanomaterials (Figure 2). The method is different from the traditional method for preparing the ultrathin two-dimensional MOF nanomaterial by ultrasonically and mechanically peeling off a layered MOF crystal, and dipyridine (DPDS) ligands containing disulfide bonds are inserted into the interlayer of the MOF crystal through coordination, so that a novel interlayer insertion type Zn
2(PdTCPP)(DPDS)
2 crystal is obtained. Then, through the chemical reduction of disulfide bonds to regulate the interlayer interaction force, the synergistic effect of DPDS and metal nodes weakens the interlayer interaction of MOF crystals, so that the layered MOF is chemically peeled off to obtain ultra-thin two-dimensional MOF nanosheets. The experimental results show that the yield of this method is about 57% higher than that of the ultrasonic mechanical exfoliation method under mild conditions, and the thickness of the ultrathin two-dimensional MOF nanomaterials is about 1 nm.
Meanwhile, the ultrathin two-dimensional MOF nanosheet obtained by the preparation method has a larger area, and metal organic framework nanosheets with different thicknesses can be obtained by controlling the reduction degree of disulfide bonds in bipyridine ligands,It successfully overcomes the difficulty of uncontrollable size and thickness in the preparation of ultra-thin two-dimensional MOF nanomaterials by ultrasonic/mechanical exfoliation.
Among the above top-down methods, ultrasonic stripping is still the most commonly used method because of its simple operation. Aiming at the problem that the exfoliated product is easy to fragment, the mixed solvent is also used to improve it, but the selection and proportion of the mixed solution are still difficult. Similar to the ultrasonic exfoliation method with the help of external force, the micromechanical method can also be used to prepare ultra-thin two-dimensional MOF nanomaterials. Tape-assisted micromachining has previously been a good application in the preparation of single-layer graphene
[37]. However, the practical application of the micromechanical lift-off method is limited by its low yield, manual dependence, lack of precision and repeatability, and difficulty in controlling the size, thickness and shape of ultra-thin two-dimensional MOF nanomaterials. Compared with the ultrasonic peeling method, the freeze-thaw method is a mild and feasible method. The intercalation/chemical exfoliation method is efficient and controllable, and has obvious advantages. While there are advantages to the top-down approach, there are still many challenges and opportunities. For example, it is necessary to continue to study how to keep the exfoliated ultra-thin two-dimensional MOF nanosheets intact and stable so as to prevent them from accumulating again. For practical applications, the yield and production speed also need to be improved.
2.2 Bottom-up method
Non-layered MOF nanocrystals, such as UiO-67 (Zr-BPDC) nanosheets, (OX-1((HNEt
3)
2[Zn
3BDC
4]) nanosheets, ZIF-8(Zn(MIM)
2) nanosheets and ZIF-67(Co(MIM)
2) nanosheets, are difficult to be exfoliated by top-down method, but can be prepared by bottom-up method
[38][39][40][41]. In fact, almost all two-dimensional MOF nanomaterials with non-layered structure are synthesized by bottom-up method.
Bottom-up methods include ultrasonic synthesis, interfacial synthesis, surfactant-assisted synthesis, microdroplet synthesis and salt microcrystal template-constrained synthesis. Ultrasonic technology can be used not only to exfoliate and prepare ultra-thin two-dimensional MOF nanosheets, but also to synthesize them directly. Ultrasonic synthesis of ultrathin two-dimensional MOF nanomaterials is a rapid, energy-saving and environmentally friendly method. The interface synthesis method utilizes a liquid-liquid interface, a liquid-gas interface or a liquid-solid interface to prepare the ultrathin two-dimensional MOF nano material; In the surfactant-assisted synthesis method, the addition of the surfactant restricts the vertical growth of the MOF and is beneficial to dispersing the synthesized ultrathin two-dimensional MOF nanomaterials. The micro-droplet method is a new method for continuous preparation of ultra-thin two-dimensional MOF nanomaterials, and the prepared ultra-thin two-dimensional MOF nanomaterials have the advantages of high dispersion, high crystallinity, high yield and uniform thickness
[26].
2.2.1 Ultrasonic synthesis method
In recent years, ultrasound has also been used in the direct synthesis of ultra-thin two-dimensional MOF nanomaterials, which is different from the principle of ultrasonic exfoliation in top-down method. Ultrasonic synthesis of ultra-thin two-dimensional MOF nanomaterials is a fast, energy-saving and environmentally friendly method because ultrasonic can quickly generate high temperature (up to 5000 K) and high pressure (up to 1000 bar), and quickly heat or cool. In 2016, Tang et al. Prepared ultra-thin two-dimensional MOF nanosheets NiCo-UMOFNs by ultrasonic synthesis
[42]. They dissolved Ni ions, Co ions and terephthalic acid (BDC) ligands in DMF, water and ethanol, and then added TMA (TMA = trimethylamine). Ultrathin two-dimensional NiCo-UMOFN nanosheets were prepared by stirring for 5 min at room temperature to form a colloidal suspension, which was then sonicated continuously for 8 H (40 kHz) under airtight conditions. Co and Ni are connected to each other on the (200) plane to form a two-dimensional bimetallic layer, which is separated by BDC ligands to form the crystal structure of NiCo-MOF.
Similarly, the ultrasonic synthesis method can also be used to prepare other ultra-thin two-dimensional MOF nanosheets with different metal nodes and ligands, such as Mn-UMOFN nanosheets, Ni-MOF nanosheets, [Zn(BDC)(H
2O)]
n nanosheets, CoTDA nanosheets (TDA = 2,5-mercaptodicarboxylate) and MOF hybrid nanosheets based on different metals
[43][44][45][46][47].
2.2.2 Interfacial synthesis
In the interfacial synthesis method, organic ligands and metal nodes react at the interface of two phases. Therefore, MOF only grows in a limited two-dimensional interface region, resulting in the formation of ultra-thin two-dimensional nanosheets
[20]. The two-dimensional MOF nanosheet mainly adopts three interfaces: liquid-liquid interface, liquid-gas interface, and liquid-solid interface. In addition, ultra-thin two-dimensional MOF nanomaterials can also be prepared by chemical vapor deposition at the solid-gas interface. Liquid-liquid interface synthesis uses two immiscible liquids, such as water/dichloromethane and water/ethyl acetate, to dissolve metal ions and organic ligands and react them at the interface. The synthesis of MOF nanosheets Cu-BHT (BHT = benzene hexathiol) reported by Zhu et al. Is a typical example of liquid-liquid interface synthesis
[17]. BHT was dissolved in methylene chloride and water was added to the methylene chloride solution to form an oil/water interface. And then adde that copper ion aqueous solution into the aqueous solution, and react the metal copper ions and the organic ligand BHT at the water/dichloromethane interface to form the Cu-BHT nanosheet. Similarly, Marinescu et al. reported the synthesis of Co-BHT and Co-THT (THT = triphenylenehexathiol) nanosheets via the liquid-liquid interface
[48,49]. The results show that although the thickness of 2D MOF nanosheets can be adjusted by changing the concentration of organic ligands, the thickness of the prepared 2D MOF nanosheets is often too large, usually much more than 100 nm, which is far from the expected thickness of ultra-thin 2D MOF nanomaterials less than 5 nm.
It is more effective to synthesize ultra-thin two-dimensional MOF nanomaterials by liquid-gas interface synthesis, and even single-layer two-dimensional MOF nanosheets can be obtained. By using the flatness of the liquid poured into the container, metal ions and organic ligands grow on the surface of the planar liquid in direct contact to form ultrathin two-dimensional MOF nanosheets without pretreatment. In the early years, Kitagawa et al. reported the synthesis of a series of MOF nanosheets at the water/air interface
[50,51]. A very small amount of organic solvent is added to the surface of water, and after the organic solvent is evaporated, a water/air liquid-gas interface is formed, and metal ions and organic ligands react on the interface to synthesize the multilayer MOF nanosheet. Taking the synthesis of CoTCPP-py-Cu (TCPP = tetra (4-carboxyphenyl) porphyrin) as an example, as shown in fig. 3, a mixed solution of chloroform and methanol (volume ratio: 3:1) containing CoTCPP ligand and pyridine was dispersed on the surface of a Cu ion solution
[16]. After chloroform and methanol were evaporated, the ligand on the surface was compressed by LB film method to form a dense monolayer of ligand. Finally, the copper ion is combined with the CoTCPP ligand to form a two-dimensional monolayer CoTCPP-py-Cu nanosheet with good crystallinity at the water/air interface. At the same time, a two-dimensional MOF thin layer material with any thickness and layered growth can be obtained through a plurality of times of cyclic operation.
In addition, the liquid-gas interface synthesis method has also been widely used to prepare hexagonal layered ultrathin two-dimensional MOF nanomaterials composed of triangular organic ligands and square planar metal nodes. For example, Nishihara et al. Reported the synthesis of monolayer and multilayer Ni-BHT nanosheets nano-1 at the water/air interface
[52]. Feng et al. Also reported the preparation of two-dimensional Ni-THT nanosheets with large area and thickness of about 0.7 nm by a similar method
[53]. Recently, Qiao Zhihua and Zhong Chongli of Tianjin University of Technology reported a new MOF membrane
[54]. Ultra-thin low crystallinity (LC) MOF films were synthesized on a porous flexible polymer support layer by a simple and general method of interface layer polarization induction. The prepared ultra-thin LC-MOF membrane creates abundant open metal sites, improves the convenient transportation characteristic of olefin molecules, and improves the separation performance of olefin/alkane. These experimental results clearly demonstrate that multilayer or even monolayer ultra-thin two-dimensional MOF nanomaterials can be successfully prepared by liquid-gas interface synthesis, and the transfer of MOF nanomaterials can be carried out without destroying their structure.
The third interface of the interfacial synthesis method is the liquid-solid interface. For example, Otsubo et al. reported layer-by-layer growth through a liquid-solid interface to synthesize two-dimensional MOF nanosheets Fe(py)
2[Pt(CN)
4](py=pyridine, pyridine), Fe(py)
2[Ni(CN)
4], and Fe(pz)
2[Pt(CN)
4](pz=pyrazine, pyrazine)
[55][56][57]. Like other two-dimensional materials, there are early reports on the preparation of two-dimensional MOF nanosheets by chemical vapor deposition (CVD) at the solid-gas interface. Chemical vapor deposition requires the coordination reaction of metal precursors and organic ligands on the metal surface under ultra-high vacuum conditions. The deposited samples were annealed at high temperature for several minutes to increase the mobility and activity of the MOF precursors, and ordered ultrathin two-dimensional MOF nanosheets were obtained after annealing. In 2003, Lin et al. Used this method to prepare two-dimensional MOF nanosheets on a clean and flat Cu (100) surface
[58,59]. It has also been reported that Cu and 2HTPyP (2HTPyP = 5,10,15,20-tetra (4-pyridyl) porphyrin) ligands were coordinated on Au (111) surface to prepare two-dimensional MOF nanosheets
[60]. The method can realize the growth of monomolecular coordination nanosheets with high aspect ratio, but requires annealing treatment. Unfortunately, this approach rarely involves how to remove the nanoflakes from the metal surface to make free-standing nanoflakes. How to maintain the crystallinity of MOF materials without additional defects is a problem to be solved in the process of exfoliation and transfer of two-dimensional MOF nanosheets.
In a word, the interfacial synthesis method can produce two-dimensional MOF nanosheets with large lateral dimensions, and its chemical properties and structure can be adjusted by ligand design and controlling the synthesis conditions such as solvent concentration and substrate. The thickness of 2D MOF nanosheets synthesized at the liquid-liquid interface is larger than that synthesized at the interface. The effect of liquid-solid interface is not good, and there are few reports. Therefore, the interface that can ideally prepare the expected monolayer or multilayer ultrathin two-dimensional MOF nanosheets is mostly the liquid-gas interface. Because the method is simple and effective, the prepared ultrathin MOF nano material is compact in structure and easy to transfer
[52].
2.2.3 Surfactant-assisted synthesis
Surfactants such as cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and polyvinylpyrrolidone (PVP) are often used in the preparation of highly stable two-dimensional MOF nanosheets. The adsorption of the surfactant on the surface of the MOF nanomaterial can provide a two-dimensional confinement region for the coordination reaction and restrict the growth of the MOF crystal along the vertical direction; On the other hand, it is helpful to reduce the surface energy and prevent the prepared ultrathin two-dimensional MOF nano material from gathering again.
The ultrathin two-dimensional MOF nanosheets Zn-TCPP can be synthesized with the assistance of surfactant PVP
[36]. As shown in fig. 4, in the traditional synthesis method, the metal nodes in the Zn
2(COO)
4 are coordinately connected with the TCPP ligands, and Zn-TCPP sheets are forced to stack into massive layered MOF crystals under the action of high surface energy. However, by adding the surfactant PVP, these surfactant molecules can selectively interact with the metal nodes, thus significantly weakening the interlayer interaction. Meanwhile, due to the strong coordination capacity between the organic ligand and the metal node, the lateral growth of the organic ligand and the metal node is not affected, and a stable ultrathin two-dimensional Zn-TCPP nanosheet is finally prepared. All of these nanosheets exhibit micron-scale aspect ratios and large lateral dimensions.
2.2.4 Microdroplet synthesis method
Although ultra-thin two-dimensional MOF nanomaterials with complete morphology and controllable thickness can be directly synthesized by surfactant-assisted synthesis, the inevitable residues of surfactant auxiliary substances used may lead to insufficient availability of active sites on the surface of ultra-thin two-dimensional MOF nanomaterials, thus reducing their catalytic performance
[61,62].
Zhao et al. Reported a novel bottom-up method for the preparation of ultrathin two-dimensional MOF nanosheets Zr-BTB (Zr = zirconium, BTB = benzene tribenzoate) with a thickness of 3 nm
[63]. Firstly, ZrCl
4 and 1,3,5-tris (4-carboxyphenyl) benzoic acid (HBTB) were dissolved in formic acid and DMF, respectively. The resulting solution was then charged into a micromixer with a magnetic stirring device to form a homogeneous solution (fig. 5). The shearing force caused by the velocity difference between the two immiscible fluids is used to disperse the reactant solution to form well-dispersed microdroplets. The droplets then passed along a 2 mm diameter spiral channel and entered a heated bath reaction tube at 130 ° C. Cooling, filtering, washing and vacuum drying to obtain the ultrathin two-dimensional MOF nano material Zr-BTB. The dynamic mixing reaction dominated by diffusion and forced convection in the micro-droplet flow control system inhibits the longitudinal growth of the two-dimensional Zr-BTB, blocks the weak interaction force between the layers of the Zr-BTB crystal, and prevents the ultra-thin two-dimensional MOF nanosheets from stacking again. At the same time, diffusion and strong convection also promote the mixing of reactants in the microreactor, thus significantly shortening the synthesis time. The method can selectively control the growth direction of the MOF crystal, and the prepared ultrathin two-dimensional MOF nanosheet has the characteristics of high dispersibility, high crystallinity, high yield and uniform thickness.
The micro-droplet method significantly changes the growth of ultra-thin two-dimensional MOF nanomaterials by using the force in the continuous flow reaction process. The forces are generated in the process of droplet flow, mainly the diffusion force and the forced convection force between the droplet and the liquid. At the same time, the surface tension, interfacial tension, viscous force and elastic force in the droplet will also affect the MOF growth. The method effectively inhibits or even blocks the stacking effect between the layers of the two-dimensional MOF material, so that ultrathin two-dimensional MOF nanosheets Zr-BTB with the thickness of 3 nm can be prepared in batches. The ultrathin two-dimensional MOF nanomaterial has nanometer-level thickness and micrometer-level transverse size, and the specific surface area is increased by 2 times of that of the original block-like MOF crystal. Another advantage of this method is that the thickness of ultra-thin two-dimensional MOF nanomaterials can be controllably adjusted. The thickness of the ultrathin two-dimensional MOF nanomaterial can be controlled by adjusting the residence time of the reactants in the microchannel. Compared with the Zr-BTB crystal with a multi-stacking structure, the ultrathin two-dimensional Zr-BTB nanosheet has higher surface area and better gas adsorption performance. Other chiral MOF nanosheets can also be synthesized by using the confinement effect of the microdroplet growth strategy.
2.2.5 Salt microcrystal template constraint synthesis method
Inspired by the growth and synthesis of ultra-thin two-dimensional MOF nanosheets in a closed system by microdroplet method, Gogotsi et al. Established a strategy to prepare ultra-thin two-dimensional MOF nanomaterials in the presence of a large number of salt microcrystalline templates
[64]. Furthermore, Dong et al. selected a typical zeolitic imidazolium salt framework ZIF-67, which is coordinated by Co
2+ and 2-methylimidazole (MeIM), and used this strategy to confine the reactants in a very thin space between the salt crystallites to prepare ultra-thin two-dimensional ZIF-67 nanosheets with a thickness of 4.5 nm (Fig. 6)
[41].
In that method, the salt microcrystal serve as a reaction substrate template, and a reactant precursor solution is diffused on the salt microcrystal template aft being mixed. As a result, that reactant solution is confine to a very thin thickness region between the salt crystallites, effectively control the growth direction of ZIF-67 (fig. 6). The ratio of MOF precursors to salt crystallites affects the thickness of the confinement region, so the thickness of the prepared ultra-thin two-dimensional MOF nanomaterials is also closely related to the ratio of MOF precursors to salt crystallites.
The salt microcrystal template constrained synthesis method is a very economical bottom-up method, and the cheap salt microcrystal template can be easily washed away by deionized water, so the production scale of the nanosheet can be expanded only by increasing the amount of salt. The method is economical and high in yield, and has great application prospect.
2.3 Two-dimensional oxide template sacrificial method
In 2017, Zhao et al. Directly prepared two-dimensional NUS-8 nano-MOF materials by a hydrothermal method without surfactant modulation
[65]. However, its thickness is 10 ~ 20 nm, which is far from the ideal thickness of less than 5 nm. Zhu et al. Of the University of Queensland also reported a surfactant-free two-dimensional oxide template sacrificial method to obtain ultra-thin two-dimensional MOF-74 nanomaterials with a thickness as low as 2 nm
[18]. In this method, highly active amorphous metal oxide nanosheets (M/M1M2-ONS) such as ultrathin Fe-Co oxide nanosheets (FeCo-ONS) with a thickness of 1.2 nm were used as sacrificial templates (Fig. 7) to synthesize ultrathin two-dimensional MOF-74 nanosheets (M/M1M2-MNS) through restricted ligand coordination. Under hydrothermal conditions, M/M1M2-ONS leached metal ions in an acidic ligand solution, and the metal ions were enriched on the surface and coordinated with the ligand material. The metal ions near the surface of the metal oxide are increased, thereby inducing the constrained growth of the MOF crystal into a two-dimensional thin layer structure. Experiments show that the method is flexible and universal. The flexibility is reflected in the flexible combination of metal sites. Meanwhile, a series of ultrathin two-dimensional monometallic (such as M: Co, Ni, Cu) and bimetallic (such as M1M2: FeCo, NiFe, CoCu) MOF-74 nanosheets can be prepared, and ultrathin two-dimensional BTC MOF (BTC = benzenetricarboxylic acid) nanosheets can also be synthesized. Ultrathin two-dimensional Ni-BTC MOF and NiFe-BTC MOF nanosheets were synthesized using H
3BTC as ligand. The thickness of the ultrathin two-dimensional FeCo MOF-74 nanosheet (FeCo-MNS) is 2.6 nm. The thickness of the ultrathin two-dimensional Ni-BTC MOF and NiFe-BTC MOF nanosheets is as low as 1.5 nm.
In a word, the preparation of ultrathin two-dimensional MOF nanomaterials by two-dimensional oxide template sacrifice method does not need to use structural regulators, and the surface contains a large number of coordination unsaturated metal active sites.Therefore, its catalytic activity is also significantly higher than that of bulk MOF particles, which avoids the problem of performance decline caused by the masking of active sites caused by surfactant-assisted method. The method is beneficial to the shape control and mass preparation of the ultrathin two-dimensional MOF nano material in actual production.
3 Application of ultrathin two-dimensional MOF nanomaterials
Ultrathin two-dimensional MOF nanomaterials have unique physical and chemical properties and broad application prospects, and the application research of this material has become a hot spot in recent years. So far, ultrathin two-dimensional MOF nanomaterials have shown good performance in many fields, such as gas separation, catalysis, energy storage conversion, sensing platform and biomedicine.
3.1 Gas separation and adsorption
Gas separation technology has important application value in clean energy and environmental sustainable development. Compared with traditional distillation and extraction, membrane-based gas separation has the advantages of simple operation, energy saving, smaller space requirement and higher separation efficiency. Ultrathin two-dimensional MOF nanomaterials not only have ultrathin thickness and good pore structure, but also have good permeability and high selectivity, which are expected to achieve high permeability and high selectivity, and have great potential as membrane separation materials.
The gas membrane separation application of the ultra-thin two-dimensional MOF nanosheet is shown in fig. 8
[66]. The ultrathin two-dimensional MOF nanoplate-based membrane has two kinds of permeation channels, one is a short channel (channel I, green arrow) composed of nanoplate-based pore arrays, and molecules of different sizes can selectively permeate the membrane; The other is a long and tortuous interlayer gap (channel II, red arrow). For gases containing molecules of different sizes, accurate separation and purification can be achieved by designing appropriate pore sizes to allow small molecules to pass through channel I quickly while allowing larger molecules to pass through the longer channel II. At the same time, in some cases, target molecules can permeate along the membrane channel, while other molecules are completely blocked outside the membrane, and separation can also be achieved.
Yang et al. Reported the separation of H
2/CO
2 using ultrathin two-dimensional Zn
2(bim)
4 nanometer-based molecular sieves as separation membranes
[67]. A separation membrane with a thickness of several nanometers can be easily prepared by thermally dropping a single layer of ultra-thin two-dimensional Zn
2(bim)
4 nanosheets on a porous ceramic (α-Al
2O
3) substrate. The ultrathin two-dimensional Zn
2(bim)
4 nanosheet has the characteristics of high density, small pore diameter (0.21 nm) and ultrathinness, and the separation membrane shows excellent separation performance for H
2/CO
2. Its selectivity to H
2 is very high, and the maximum permeability of H
2 can reach 3760 GPUs, which far exceeds the upper limit of H
2/CO
2 binary gas separation. At the same time, due to the high thermal and hydrothermal stability of the ultrathin two-dimensional Zn
2(bim)
4 nanosheets, the two-dimensional separation membrane can still show efficient performance under different cold and hot conditions for up to 400 H. After this work, Yang et al. Synthesized another ultrathin two-dimensional MOF nanosheet membrane Zn
2(benzimidazole)
3(OH)(H
2O), which also has excellent performance in H
2/CO
2 separation
[68].
In addition to the pure ultrathin two-dimensional MOF nanosheets, there are also reports on the preparation of two-dimensional MOF nanocomposite membranes by mixing ultrathin two-dimensional MOF-199 and MOF-5 nanosheets with polyimide (PI) to form polymer matrices, which are used for the study of CO
2/CH
4 gas separation
[69⇓~71]. The high selectivity of the ultrathin two-dimensional MOF nanosheet composite membrane fully demonstrates its great potential in gas separation.
Compared with the application of ultra-thin two-dimensional MOF nanomaterials in gas separation, there are few reports on the application of MOF nanomaterials in gas adsorption. In 2013, Wang et al. Reported the synthesis of two-dimensional ZIF-L nanosheets at room temperature by controlling the ratio of metal nodes and organic ligands in aqueous solution
[72]. The two-dimensional ZIF-L nanosheets with buffer pores have excellent CO
2 adsorption capacity and CO
2/CH
4 adsorption selectivity.
3.2 Catalytic field
Like other porous materials, MOF has a large specific surface area and high porosity, and has a wide range of applications in the field of heterogeneous catalysis. Compared with the bulk MOF, the ultra-thin two-dimensional MOF nanomaterials with nano-scale thickness are beneficial to substrate diffusion and have a large number of highly active sites on the surface, so they show excellent performance in many catalytic reactions. The abundance of active sites and the unique electronic properties of ultrathin two-dimensional MOF nanomaterials open the door to their application in the field of electrocatalysis, and they have been widely used as electrocatalysts in hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and other important electrocatalytic reactions
[73⇓~75].
Feng et al. Used liquid-gas interface synthesis method to prepare single-layer ultra-thin two-dimensional Ni-THT MOF nanosheets using water/air interface
[53]. Its transverse dimension is up to several square millimeters, and its thickness is only 0.7 ~ 0.9 nm. When the monolayer ultrathin two-dimensional MOF nanosheet Ni-THT was used as a catalyst for hydrogen evolution reaction, it showed an initial overpotential of 110 mV, and the overpotential was only 333 mV at a current density of 10 mA/cm
2. In addition, ultra-thin two-dimensional MOF nanomaterials as catalysts for oxygen evolution reaction are also very active. For example, ultra-thin two-dimensional MOF nanosheets NiCo-UMOFNs prepared by Tang et al. Through ultrasonic synthesis can be used as OER catalysts for oxygen evolution reaction
[42]. The onset potential of the ultrathin two-dimensional NiCo-UMOFNs nanosheets at 10 mA/cm
2 is 250 mV, which is lower than that of Ni-UMOFNs nanosheets (321 mV), Co-UMOFNs nanosheets (371 mV), bulk NiCo-UMOFNs crystals (317 mV) and commercial RuO
2(279 mV). This significantly enhanced activity can be attributed to the coupling effect between the unsaturated metal atoms and the cobalt and nickel ions. The ultrathin two-dimensional MOF nanomaterials synthesized by the two-dimensional oxide template sacrificial method mentioned above also show high catalytic activity for oxygen evolution reaction
[18]. Compared with the original MOF crystal, the OER activity of the ultrathin two-dimensional FeCo-MNS nanosheets is significantly enhanced, and the ultrathin two-dimensional MOF nanosheets FeCo-MNS-1.0 has the best catalytic activity, which is attributed to its large number of coordination-unsaturated active sites. Under the overpotential of 300 mV, after 1000 s of oxygen evolution reaction in 0. 1 mol/L KOH solution, the crystal structure of the catalyst can still maintain good, and the catalytic activity is very stable.
Ultrathin two-dimensional MOF nanomaterials have also been reported as catalysts for oxygen reduction reactions. Dinca et al. Prepared two-dimensional layered Ni
3(HITP)
2(HITP=2,3,6,7,19,11-hexaiminotriphenylene)MOF nanosheets and tested the catalytic activity of MOF films for oxygen reduction reaction in alkaline medium
[76]. In 0. 1 mol/L KOH-water solution saturated with oxygen, the onset potential of the MOF film material is 0. 82 V, which is comparable to that of the most active non-platinum metal catalyst. At the same time, 88% of the initial current density can be maintained for more than 8 H at 0. 77 V of the reversible hydrogen electrode (RHE), showing good stability.
With high density of catalytic sites and rich and tunable pore structure, MOFs have developed rapidly as a new photocatalyst in recent years. Due to the unique atomic structure, ultra-thin two-dimensional MOF nanomaterials have obvious advantages over bulk materials: (1) ultra-thin thickness makes these materials have a large specific surface area, which significantly promotes light absorption, material transfer and the exposure of abundant surface active sites; (2) the ultrathin two-dimensional MOF nano-material reduces the distance of charge migration on the surface and improves the degree of charge separation; And (3) that ultrathin two-dimensional MOF nano material can provide a good platform for the reasonable design of the multifunctional photocatalyst and meet the requirement of different photocatalytic application fields
[77]. Lin et al. Realized ultrathin two-dimensional Ni-MOF nanosheets as a photocatalyst for the photoreduction of CO
2[78]. Ultrathin two-dimensional Ni-MOF nanosheets were prepared by the traditional ultrasonic exfoliation method with BDC and TEA as organic ligands and nickel ions as metal nodes, and water-assisted freezing was added to prevent aggregation. The average thickness of the prepared two-dimensional Ni-MOF nanosheets is about 1. 0 nm, which is close to the thickness of the MOF monolayer (about 1. 1 nm). Thanks to this ultra-thin layer structure, a large number of coordination unsaturated metal sites are exposed on the surface, which is conducive to the adsorption of CO
2 molecules and the stabilization of the initial Ni-CO
2 adduct, promoting the conversion of CO
2 to CO. At the same time, the weak H
2O affinity of the ultra-thin two-dimensional Ni-MOF nanosheets inhibits the proton transfer and reduces the production of H
2 by-products. Therefore, the ultrathin two-dimensional Ni-MOF nanosheets show high activity and selectivity for the photoreduction of CO
2 in both pure CO
2 and diluted CO
2.
3.3 Sensing platform
The practical application of sensing technology requires sensors with high sensitivity, high selectivity and high stability, which also promotes the development of new nano-sensing materials. Due to its two-dimensional characteristics and structural diversity, ultrathin two-dimensional MOF nanomaterials have the advantages of high conductivity, high specific surface area, high sensitivity and outstanding fluorescence suppression (quenching) ability. Therefore, ultrathin two-dimensional MOF nanomaterials are widely used to construct new sensing platforms for different sensing systems to detect a variety of guest substances, such as DNA. At present, the applications of ultra-thin two-dimensional MOF nanomaterials in sensing platforms include fluorescence sensors, electrochemical sensors and optical sensors. Zhang et al. Reported a fluorescence sensor based on ultra-thin two-dimensional MOF nanosheets for the detection of DNA
[36]. Compared with bulk MOF nanocrystals, ultrathin two-dimensional Zn-TCPP and Co-TCPP nanosheets, the ultrathin two-dimensional Cu-TCPP nanosheets have better fluorescence quenching efficiency. This enhanced fluorescence quenching ability from bulk materials to nanosheets can be attributed to the increased surface area of the nanosheets, exposing more active sites to facilitate energy/electron transfer from the fluorescent molecules to the quencher. Fluorescence enhancement was used to quantitatively detect target DNA at a detection limit of 20 pmol/L. More importantly, the fluorescence sensor based on the ultra-thin two-dimensional Cu-TCPP nanosheet can simultaneously detect multiple DNAs by using ssDNA labeled with different fluorescent dyes as probes. Therefore, fluorescence sensors based on ultra-thin two-dimensional MOF nanosheets have good application prospects in the analysis of biological and chemical samples, and have important value in the fields of medical diagnosis, disease prevention and environmental monitoring.
In addition to fluorescence sensors, ultra-thin two-dimensional MOF nanosheets can also be used as basic materials to construct electrochemical sensors. The structure of the MOF material is adjustable and flexible, and ultrathin two-dimensional MOF nanosheets with high electrochemical activity can be designed by using redox or catalytically active organic ligands or metal ions as building blocks
[62]. Electrochemical sensors based on ultra-thin two-dimensional MOF nanosheets can be used to detect H
2O
2, proteins, glucose and nitrite
[62][79][80][81]. In addition, the combination of ultra-thin two-dimensional MOF nanosheets with other nanomaterials is an effective way to improve the analytical performance of two-dimensional MOF nanosheet electrochemical sensors. For example, gold nanoclusters embedded in two-dimensional Zr-MOF nanosheets as electrode materials can be used to detect cocaine
[82]. The strong affinity for target molecules, high specific surface area, and good electrochemical conductivity of the nanocomposites endow the sensor with excellent sensitivity and selectivity.
3.4 Energy Storage
Ultra-thin two-dimensional MOF nanomaterials have also shown excellent performance in the field of energy storage and conversion, such as lithium-ion batteries and supercapacitors. Ning et al. Reported that the electrochemical performance of two-dimensional Co-MOF nanosheets with a thickness of less than 10 nm is much better than that of bulk MOF
[46]. Li et al. Prepared ultrathin two-dimensional Mn-UMOFNs nanosheets and Ni-UMOFNs nanosheets, which showed good capacity and cycle performance as anode materials for lithium ion batteries
[43]. At present, the research in the field of supercapacitors requires that capacitor devices have both energy density and power density, especially high power density. According to the surface charge storage mechanism, the specific surface area and conductivity of the material have a great impact on the performance of supercapacitors. In 2018, Feng et al. Reported the application of two-dimensional MOF material PTC-Fe (PTC = perthiolated coronene, hexabenzene) in the field of semiconductor magnetic devices
[83]. Ultra-thin two-dimensional MOF nanomaterials have large planar size, ultra-thin thickness and high specific surface area, which meet the requirements of supercapacitor electrode materials and have broad development prospects.
3.5 Other application
In addition to the above applications, ultrathin two-dimensional MOF nanomaterials have also been applied in other fields such as chiral separation and biomedicine. In 2018, Tang et al. Reported the separation of chiral compounds using chiral ultra-thin two-dimensional MOF nanomaterials
[84]. They prepared chiral ultrathin two-dimensional MOF nanosheets L (D) -Thr-Zn with a thickness of 3.7 nm by reacting chiral N- (4-pyridylmethyl) -L (D) threonine ligand with Zn(CH
3COO)
2 by microdrop method. In the chiral separation, it showed better enantioselectivity than the bulk MOF. The ultrathin two-dimensional MOF nanosheet has adjustable framework functional groups, a pore structure and excellent biodegradability, and can introduce drug molecules as a linker, so that the ultrathin two-dimensional MOF nanosheet can be used as a nano-carrier for delivering and releasing chemotherapeutic drugs, nucleic acids and drugs
[85]. Lin et al. reported that two-dimensional UiO MOF nanosheets simultaneously efficiently delivered cisplatin and aggregated small interfering RNAs (siRNAs) to drug-resistant ovarian cancer cells
[86]. In addition to nanocarriers, ultrathin two-dimensional MOF nanosheets can also be used as photosensitizers for photodynamic therapy (PDT), a highly efficient technique that causes apoptosis and necrosis in the presence of photosensitizers, light, and tissue oxygen by generating cytotoxic reactive oxygen species
[87]. These studies have opened up a new way for the application of ultra-thin two-dimensional MOF nanomaterials in the medical field.
4 Conclusion and prospect
In a word, two-dimensional MOF thin layer nanomaterials have been deeply studied in many application fields due to their rich and tunable skeleton functional groups, pore structure, surface active sites and ultra-thin two-dimensional structure. In this paper, the unique structure and classification of ultra-thin two-dimensional MOF nanomaterials are introduced, and the top-down and bottom-up preparation methods of ultra-thin two-dimensional MOF nanomaterials are reviewed, and the advantages and limitations of each preparation method are analyzed. At the same time, the application potential and prospect of ultra-thin two-dimensional MOF nanomaterials in the fields of gas adsorption and separation, optical/electrocatalysis and sensing platform, energy storage and conversion are discussed in detail. Among them, gas separation, electrocatalysis and sensing technology are the most widely used fields of ultra-thin two-dimensional MOF nanomaterials. In view of the rapid development of electronic devices and energy fields, ultra-thin two-dimensional MOF nanomaterials are also promising for applications in this field, such as rechargeable batteries, supercapacitors and photocatalysts.
Despite the rapid development of this research field, many achievements have been made, but the research time of ultra-thin two-dimensional MOF nanomaterials is relatively short, and it is still in its infancy, and there is still much room for development. On the one hand, the preparation of ultra-thin two-dimensional MOF nanomaterials does not have a general method that can stably prepare ultra-thin two-dimensional MOF nanomaterials with high quality, excellent performance, simple operation, high efficiency, high yield and universality, and there is still a long way to go for commercial application. Secondly, the growth mechanism of ultra-thin two-dimensional MOF nanomaterials is complex, and there are many controlled factors, and the characterization methods for ultra-thin structure are relatively lacking. Finally, from the application field, due to the poor preparation efficiency and high cost of ultra-thin two-dimensional MOF nanomaterials, some MOF structures are very sensitive to the actual environment such as water, acid and alkali, which limits their application scope to a certain extent. These challenges and shortcomings require researchers to work together to solve, but also provide new opportunities.
Science and Technology Projects of State Grid Corporation of China(5419-201999542A-0-0-00)
PDF
