A two-dimensional porous metal-organic framework nanosheet material, its preparation method and application

Abstract

The present invention belongs to the technical field of nanomaterial synthesis, and discloses a two-dimensional porous metal-organic framework nanosheet material, a preparation method thereof and an application thereof. The two-dimensional porous metal-organic framework nanosheet material is a two-dimensional porous MOF nanosheet material prepared by solution mixing and standing under the action of an anionic surfactant, using a zinc-containing metal salt as a zinc source and 2-methylimidazole as an organic ligand. The single crystal chemical formula of the two-dimensional porous MOF nanosheet material is Zn(C4H6N2)2, and the two-dimensional porous MOF nanosheet belongs to a two-dimensional planar rectangular structure. The present invention utilizes the electrostatic shearing force guided by an anionic surfactant, and at the same time constructs pore defects as a soft template agent to directionally limit the self-assembly growth direction of the material, thereby effectively preparing a two-dimensional porous MOF nanosheet material with a novel structure and stable morphology, and using it as a positive electrode material for supercapacitors.

Description

Technical Field

[0001] The present invention belongs to the technical field of nanomaterial synthesis, and particularly relates to a two-dimensional porous metal-organic framework nanosheet material, a preparation method thereof, and an application thereof. Background Art

[0002] With the development of the international trend of energy conservation and emission reduction, the development of renewable clean energy has become a top priority. Solar energy, tidal energy, wind energy, etc. are all clean energies that have been studied more, but they generally have limitations such as high cost and geographical restrictions. Therefore, they cannot continuously provide energy and are difficult to be widely applied to real life on a large scale. Therefore, the research on electrochemical energy conversion and storage systems is extremely important. Among them, supercapacitors have shown potential application prospects in portable electronic products, backup power storage, electric vehicles, etc. due to their superior power density, fast charge and discharge rates, and long cycle life, and have attracted extensive attention from the academic and industrial circles in the past few decades. The performance of the electrode material almost determines the performance of the supercapacitor. However, most traditional electrode materials such as carbon-based materials, metal oxide materials, and conductive polymers have problems such as low voltage window, small specific capacitance, and poor cycle stability. With the continuous in-depth research, researchers in this field have found that metal-organic framework materials (Metal-organic framework, MOF) are inorganic-organic hybrid porous materials formed by assembling organic ligands with metal ion nodes or clusters, with high porosity, adjustable pore size distribution, convenient synthesis, and structural designability. Compared with traditional materials, it has controllable pore size and high chemical surface, so it has broad research prospects in applications such as energy storage, electrocatalysis, sensors, and biomedicine.

[0003] Among them, two-dimensional MOF nanosheets have received increasing attention in the fields of energy storage, information storage, etc. The inherent advantages of the two-dimensional structure include: allowing rapid mass transfer and superior electron transfer; enhancing the clear interface between the electrolyte and the electrode, ensuring rapid charge transfer along the 2D basal plane, thereby improving the carrier transport performance; the highly open structure enables a large proportion of exposed surface metal sites to achieve high energy storage activity, etc. In addition, nanoporous engineering of nanomaterials can be used to direct and adjust chemical interactions by constructing pore defects and controlling pore size, shape, continuity, volume, chemical environment, etc., thereby preparing energy storage devices with excellent performance. In addition, the porous structure can enable ions or electrons to be transported into the electrode more quickly, greatly shortening the ion or electron transport path, effectively promoting the penetration of the electrolyte into the bulk of the electrode, and alleviating the volume change of the electrode material and releasing the structural strain during the charge and discharge process. Therefore, two-dimensional porous materials combine the unique properties of two-dimensional and porous structures, representing a new research direction in materials science. The introduction of ordered mesopores can not only effectively regulate the electronic structure, but also open up in-plane transport channels and increase the accessible surface sites of two-dimensional materials. The introduction of the pore defect structure endows two-dimensional materials with smaller diffusion resistance and larger specific surface area, thereby exposing more metal active sites, and has great application potential. So far, there has been no report on the in-situ synthesis method of two-dimensional porous MOF nanosheets. For the research of other two-dimensional porous materials, most of them focus on methods such as redox etching, high-energy particle bombardment, pyrolysis calcination, etc. See the records in the literature: Su Shihao and Wang Xinwei and Xue Jianming. Nanopores in two-dimensional materials: accurate fabrication. [J]. Materials horizons, 2021, 8(5) : 1390-1408. Among them, the steps of the redox etching method are complicated and the etched pore sites are random, and it is difficult to control the position of the final nanopores; the high-energy particle bombardment method relies on high-precision instrument equipment, with high costs and cannot be prepared in large quantities; the pyrolysis calcination method is complex to operate, and the porous products are often carbonized derivatives and it is difficult to obtain the in-situ target products. In addition, due to the low rigidity and crystal stability of two-dimensional MOF materials, the above methods are difficult to apply and there is no report yet. Therefore, there is currently a greater lack of an effective in-situ synthesis method for two-dimensional porous MOF nanosheets, which greatly restricts the application exploration of in-situ two-dimensional MOF in flexible energy storage, sensing, electrocatalysis and other fields. As electrode materials, most MOF materials have poor carrier mobility and low conductivity, which more seriously restricts the application of two-dimensional MOF materials in the field of supercapacitors. Summary of the Invention

[0004] The object of the present invention is to solve the problems existing in the prior art, and to provide a two-dimensional porous metal-organic framework nanosheet material and its preparation and application. By using the electrostatic shearing force guided by an anionic surfactant, while constructing pore defects as a soft template agent, the self-assembly growth direction of the material is directionally restricted, so as to effectively prepare a novel two-dimensional porous MOF nanosheet material with a novel structure and stable morphology.

[0005] The technical solution disclosed by the present invention is as follows:

[0006] In the first aspect, the present invention provides a two-dimensional porous metal-organic framework nanosheet material, and the two-dimensional porous metal-organic framework nanosheet material is a two-dimensional porous MOF nanosheet material prepared by mixing and standing a zinc-containing metal salt as a zinc source and 2-methylimidazole as an organic ligand under the action of an anionic surfactant; the single crystal chemical formula of the two-dimensional porous MOF nanosheet material prepared by the present invention is Zn(C4H6N2)2, and the two-dimensional porous MOF nanosheet belongs to a two-dimensional planar rectangular structure, and the length of any side of the two-dimensional porous MOF nanosheet is in the range of 1.2-2 μm, and 3-10 macropore defects with a diameter range of 100-300 nm are distributed on its surface, and the single sheet thickness is 30-50 nm.

[0007] It should be noted that the morphology of the two-dimensional porous MOF nanosheet is a uniformly dispersed rectangular sheet structure, which is smooth and regular as a whole, and the shape of the macropore defects on the surface is circular, oval or other irregular shapes.

[0008] Preferably, the zinc source is zinc acetate or zinc nitrate;

[0009] Preferably, the anionic surfactant is sodium dodecyl sulfate (SDS).

[0010] In the second aspect, the present invention also provides a preferred preparation method for the above two-dimensional porous metal-organic framework nanosheet material, and the specific preparation method includes the following steps:

[0011] S1. Dissolve the zinc-containing metal salt hydrate, 2-methylimidazole and sodium dodecyl sulfate in deionized water respectively, and ultrasonically disperse them evenly;

[0012] S2. Inject the sodium dodecyl sulfate dispersion into the zinc-containing metal salt dispersion, stir and mix well, and then quickly inject the 2-methylimidazole dispersion into the above mixture, and stir and mix with a glass rod;

[0013] S3. Transfer the mixture into a culture bottle for sealing, and let it stand and react in an incubator;

[0014] S4. Wash the substance obtained in S3 with methanol and centrifuge and separate it 3-4 times to obtain the two-dimensional porous metal-organic framework nanosheet material.

[0015] In step S1, the molar ratio of the zinc-containing metal salt hydrate, 2-methylimidazole, and dodecyl sulfuric acid is 1:(3 - 3.5):0.5.

[0016] Preferably, in step S1, the zinc-containing metal salt hydrate is zinc acetate dihydrate Zn(CH3COO)2·2H2O, i.e., ZnAC2·2H2O, and the concentration of ZnAC2·2H2O is 9.98 mg·mL -1 , the concentration of 2-methylimidazole is 11.19 - 11.96 mg·mL -1 , and the concentration of sodium dodecyl sulfate is 6.58 mg·mL -1 .

[0017] Preferably, in step S3, the rapid stirring time is 15 - 20 s.

[0018] Preferably, in step S3, the temperature for static reaction in the incubator is 26 °C, and the reaction time is 24 h.

[0019] In a third aspect, the present invention also provides the application of the above two-dimensional porous metal-organic framework nanosheet material as a positive electrode material for supercapacitors. And the present invention proves through experiments that: using the above two-dimensional porous metal-organic framework nanosheet material as the positive electrode material to perform three-electrode system electrochemical characterization test on its alternating current impedance spectrum EIS, and analyzing its behavior curve, the electron / ion conductivity is significantly higher than that of the non-porous nanosheet, and the structure of pore defects plays a positive role in improving the conductivity.

[0020] The beneficial effects of the present invention are as follows:

[0021] 1. This application adopts a soft template method based on anionic surfactant orientation. The negative charges on the anionic surfactant molecules can be pre-assembled with metal ions through electrostatic interactions and are mainly adsorbed on the crystal surface rich in metal ions, thereby restricting the rectangular growth of the crystal; in addition, as Figure 7 shown, the concentration effect of the high-concentration reaction system causes excessive surfactant SDS to remain / attach to the middle rectangular site, and then plays an electrostatic shearing guiding role from the inside out, that is, forming original pore defects. The rectangular, monodisperse, flat and uniform two-dimensional porous metal-organic framework nanosheet material prepared by the present invention has high chemical stability and thermal stability. The obtained material is rich in large pore structures. Compared with the non-porous structure nanosheet, it expands the carrier transport channels and can be used as electrode materials for clean energy, electrocatalytic materials, information storage dielectric layers, etc. The scientific research and market prospects are broad, providing a new synthesis idea for constructing two-dimensional porous MOF materials.

[0022] 2. The present application uses a surfactant-assisted template method to in-situ synthesize two-dimensional porous MOF nanosheet materials in one step. Compared with the redox etching method in the prior art, the advantages are that no additional reactants and reagents need to be added, and the pore-forming position can be predicted; compared with the high-energy particle bombardment method in the prior art, the advantages are that it does not rely on high-precision instruments and can be prepared in large quantities; compared with the pyrolysis calcination method in the prior art, the advantages are that the steps are simple and the cost is low, and the integrity of the product can be guaranteed, etc. In addition, compared with the above traditional methods for preparing porous MOF, the method of synthesizing structural defects by the in-situ soft template method is more stable and convenient, has good controllability, and does not require additional experimental operations for one-step synthesis, which is beneficial to the uniform distribution of the product morphology. The product prepared in the present application has a good two-dimensional porous nanosheet morphology, and there are in-situ macroporous defects with a size of 100-300 nm on the surface. When applied as the positive electrode material of a supercapacitor, it shows better electron and ion conductivity than the non-porous nanosheets, indicating that the constructed macroporous structure is more conducive to the penetration and transport of electrolyte ions, which helps to improve the electron / ion conductivity of the material.

[0023] 3. The present application uses a soft template method based on anionic surfactant orientation to prepare two-dimensional porous MOF nanosheets. The anionic surfactant is used as a soft template to restrict the crystal growth direction. Under the electrostatic shearing force from the inside out and from the outside in, a macroporous defect morphology is successfully constructed. The in-situ one-step aqueous phase synthesis does not require any secondary addition and auxiliary instruments, and can avoid or minimize the use of experimental resources such as acid-base organic solvents and high-power ultrasonic machines commonly used in the synthesis of conventional porous MOF, realizing an environmentally friendly synthesis strategy;

[0024] 4. Using the two-dimensional porous MOF nanosheet material designed in the present application to prepare a working electrode shows better electron / ion conductivity than non-porous nanosheets; the preparation of this material provides a new synthesis method for constructing two-dimensional porous MOF, and can also be applied to the field of supercapacitors. As the electrode material of a supercapacitor, it has high practical application value and provides ideas for the simple preparation of clean energy materials;

[0025] 5. The present application prepares two-dimensional porous MOF nanosheet materials in a short time through a soft template method based on anionic surfactant orientation. The preparation process is simpler, saving preparation time, and the cost is low, which is conducive to large-scale commercial promotion. Description of the Drawings

[0026] Figure 1 It is a high-magnification SEM electron micrograph of the single-piece morphology of the two-dimensional porous MOF nanosheet material prepared in Example 1;

[0027] Figure 2 It is a low-magnification SEM electron micrograph of the overall morphology of the two-dimensional porous MOF nanosheet material prepared in Example 1;

[0028] Figure 3 is the X-ray diffraction of the two-dimensional porous MOF nanosheet material prepared in Example 1 Figure X XRD;

[0029] Figure 4 is the high-resolution transmission electron microscopy (HRTEM) image and elemental distribution map of the two-dimensional porous MOF nanosheet material prepared in Example 1;

[0030] Figure 5 is the N2 adsorption-desorption isotherm of the two-dimensional porous MOF nanosheet material prepared in Example 1;

[0031] Figure 6 is the electrochemical impedance spectroscopy (EIS) of the two-dimensional porous MOF nanosheet material prepared in Example 1;

[0032] Figure 7 is the schematic diagram of the pore size formation principle of the two-dimensional porous MOF nanosheet material of the present invention;

[0033] Figure 8 is the pore size illustration of the two-dimensional porous MOF nanosheet material prepared in Example 1;

[0034] Figure 9 is the SEM image of the two-dimensional porous MOF nanosheet material prepared in Example 2;

[0035] Figure 10 is the SEM image of the two-dimensional porous MOF nanosheet material prepared in Example 3. Detailed implementation manners

[0036] The following examples further illustrate the content of the present invention, but should not be construed as limiting the present invention. Without departing from the essence of the present invention, modifications and substitutions made to the methods, steps or conditions of the present invention all fall within the scope of the present invention.

[0037] S1. Dissolve 0.03 g of ZnAC2·2H2O in 1 mL of deionized water, dissolve 0.02 g of sodium dodecyl sulfate in 1 mL of deionized water, dissolve 0.0336 g of 2-methylimidazole in 1 mL of deionized water, and obtain a uniform dispersion after ultrasonic dissolution and dispersion for 2 min, then cool it at room temperature;

[0038] S2. Inject the sodium dodecyl sulfate dispersion into the ZnAC2·2H2O dispersion at room temperature, and then quickly inject the 2-methylimidazole dispersion into the above mixture, and stir quickly with a glass rod for 15 - 20 s for mixing;

[0039] S3. Seal the mixture in a culture bottle, set the temperature of the incubator to 26°C, and let it stand for 24 h for reaction;

[0040] S4. Centrifuge the reaction product 4 times with methanol and deionized water at room temperature, remove the surface impurities of the reaction product after ultrasonic washing, and then place the washed reaction product in a vacuum drying oven at 60 °C for drying treatment to finally obtain a two-dimensional porous MOF nanosheet material;

[0041] S5. When preparing the electrode material, compound the prepared two-dimensional porous MOF nanosheet material with a conductive agent, superconducting carbon black, and a binder, polyvinylidene fluoride, in a mass ratio of 80:15:5, grind them, and coat them on a nickel foam current collector as a working electrode.

[0042] Figure 1 is the SEM image of the two-dimensional porous MOF nanosheet material prepared in this example. It can be seen from the figure that the morphology of the obtained product is a two-dimensional planar rectangular structure with side lengths of 1.2 - 2 μm, mesoporous defects of 100 - 300 nm are distributed on the surface, and the thickness of a single sheet is 30 - 50 nm.

[0043] Figure 2 is the SEM image of the two-dimensional porous MOF nanosheet material prepared in this example. It can be seen from the figure that the obtained material is a uniformly dispersed rectangular sheet-like structure, the product morphology is uniform, and there are 3 - 10 irregular macroporous defects of 100 - 300 nm distributed on the surface.

[0044] Figure 3 is the X-ray diffraction Figure X RD of the two-dimensional porous MOF nanosheet material prepared in this example. It can be seen from the figure that the crystal plane peaks given by the crystal structure of the obtained material are completely consistent with the crystal plane peaks of the ZIF-8 single crystal XRD, indicating the successful preparation of the ZIF-8 type MOF with the assistance of SDS.

[0045] Figure 4 is the high-resolution transmission electron microscopy image HRTEM and element distribution of the two-dimensional porous MOF nanosheet material prepared in this example. It can be seen from the figure that the presence of three elements, Zn, C, and N, in the nanosheets indicates the successful preparation of the ZIF-8 nanosheets, and the presence of Na and S elements proves the existence and distribution of the surfactant SDS at the pore defects.

[0046] Figure 5 is the N2 adsorption - desorption isotherm of the two-dimensional porous MOF nanosheet material prepared in this example. It can be seen from the figure that it shows a typical type IV Langmuir isotherm with H3 hysteresis, and the Brunauer - Emmett - Teller (BET) specific surface area is 852.8 m 2 g -1 , and the Langmuir specific surface area is 1029.8 m 2 g-1

[0047] Figure 6 This is the electrochemical impedance spectroscopy (EIS) of the two-dimensional porous MOF nanosheet material prepared in this example. In the figure, imporous NS represents non-porous nanosheets, and porous NS represents porous nanosheets. From Figure 6 it can be seen that the arc radius in the high-frequency region of the porous nanosheets is significantly smaller than that of the non-porous nanosheets, with a smaller Rct, a larger charge transfer rate, and strong electronic conductivity; the line slope in the low-frequency region of the porous nanosheets is significantly larger than that of the non-porous nanosheets, with a smaller Zw, a larger ion diffusion rate, and strong ionic conductivity.

[0048] Figure 8 This is an illustration of the pore size of the two-dimensional porous MOF nanosheet material prepared in this example, with irregular macropore defects having an average diameter of 100 - 300 nm.

[0049] The sample prepared in this example has better conductivity than non-porous nanosheets. As an electrode material for supercapacitors, the arc radius in the high-frequency region of the EIS impedance spectrum of the product is significantly smaller than that of non-porous nanosheets, and the slope in the low-frequency region is much larger than that of non-porous zinc-based zeolitic imidazolate framework metal-organic framework nanosheets.

[0050] The difference between Example 2 and Example 1 is only that Zn(NO3)2·6H2O is used instead of ZnAC2·2H2O in step S1, and the rest of the steps are the same. The specific process of step S1 is as follows:

[0051] S1. Weigh 0.041 g of Zn(NO3)2·6H2O and dissolve it in 1 mL of deionized water. Mix 0.02 g of sodium dodecyl sulfate and dissolve it in 1 mL of deionized water. Dissolve 0.0336 g of 2-methylimidazole in 1 mL of deionized water. After ultrasonic dissolution and dispersion for 2 min, a uniform dispersion is obtained and cooled at room temperature.

[0052] Figure 9 is the SEM image of the two-dimensional porous MOF nanosheet material prepared in this example. It can be seen from the figure that the morphology of the obtained product is a two-dimensional planar rectangular structure with side lengths of 1.2 - 2 μm, macropore defects with a size of 100 - 300 nm distributed on the surface, and a single sheet thickness of 30 - 50 nm.

[0053] The sample prepared in this example is a two-dimensional planar rectangular structure with side lengths of 1.2 - 2 μm, having 3 - 6 macropore defects, and a single sheet thickness of 30 - 50 nm. The single sheet thickness is 30 - 50 nm.

[0054] Example 3 is different from Example 1 only in that in step S1, the ratio of zinc source: 2-methylimidazole: SDS is adjusted to 1:3.5:0.5, and the concentration of 2-methylimidazole is adjusted to 11.96 mg·mL -1 , that is, the dosage is 0.0651 g, and the rest of the steps are the same. The specific process of step S1 is as follows:

[0055] S1. Dissolve 0.03 g of ZnAC2·2H2O in 1 mL of deionized water, dissolve 0.02 g of sodium dodecyl sulfate in 1 mL of deionized water, dissolve 0.0651 g of 2-methylimidazole in 1 mL of deionized water, ultrasonically dissolve and disperse for 2 min to obtain a uniform dispersion, and cool at room temperature;

[0056] Figure 10 is the SEM image of the two-dimensional porous MOF nanosheet material prepared in this example. It can be seen from the figure that the morphology of the obtained product is a two-dimensional planar rectangular structure with side lengths of 1.2 - 2 μm, 6 macropore defects with sizes of 100 - 200 nm are distributed on the surface, and the thickness of a single sheet is 30 - 50 nm.

[0057] Through experiments, it is known that the two-dimensional porous MOF nanosheet material is prepared by a soft template method guided by the surfactant sodium dodecyl sulfate (SDS). The preparation process is simple, the time-consuming is short, and the electron / ion conductivity is good. It can be applied to fields such as supercapacitor energy storage, and has potential electrochemical performance after being prepared into electrode materials.

[0058] The above shows and describes the basic principles, main features and advantages of the present invention. However, the above are only specific embodiments of the present invention, and the technical features of the present invention are not limited thereto. Any other embodiments obtained by those skilled in the art without departing from the technical solution of the present invention should be covered within the patent scope of the present invention.

Claims

1. A preparation method of a two-dimensional porous metal-organic framework nanosheet material, characterized in that The two-dimensional porous metal-organic framework nanosheet material is a two-dimensional porous MOF nanosheet material prepared by solution mixing and standing under the action of an anionic surfactant, using a zinc-containing metal salt as the zinc source and 2-methylimidazole as the organic ligand. The single-crystal chemical formula of the two-dimensional porous MOF nanosheet material is Zn(C4H6N2)2, and the two-dimensional porous MOF nanosheet belongs to a two-dimensional planar rectangular structure. The length of any side of the two-dimensional porous MOF nanosheet is in the range of 1.2 - 2 μm, and there are 3 - 10 macropore defects with a diameter range of 100 - 300 nm distributed on its surface, and the single sheet thickness is 30 - 50 nm; The zinc source is zinc acetate or zinc nitrate; The anionic surfactant is sodium dodecyl sulfate SDS; The specific preparation method of the two-dimensional porous metal-organic framework nanosheet material includes the following steps: S1. Dissolve the zinc-containing metal salt hydrate, 2-methylimidazole, and sodium dodecyl sulfate in deionized water respectively, and disperse them evenly by ultrasonic treatment; S2. Inject the sodium dodecyl sulfate dispersion into the zinc-containing metal salt dispersion, stir and mix well, and then quickly inject the 2-methylimidazole dispersion into the mixture, and stir and mix with a glass rod; S3. Transfer the substance obtained in S2 into a culture flask for sealing, and stand and react in an incubator; S4. Wash the substance obtained in S3 with methanol and centrifuge and separate it 3 - 4 times to obtain the two-dimensional porous metal-organic framework nanosheet material; In step S1, the molar ratio of the zinc-containing metal salt hydrate, 2-methylimidazole, and sodium dodecyl sulfate is 1:(3 - 3.5):0.5; In step S1, the zinc-containing metal hydrate is zinc acetate dihydrate Zn(CH3COO)2·2H2O or Zn(NO3)2·6H2O. Zn(CH3COO)2·2H2O is ZnAC2·2H2O. And in step S2, the concentration of ZnAC2·2H2O is 9.98 mg·mL -1 , and the concentration of Zn(NO3)2·6H2O is 13.67 mg·mL -1 ; the concentration of 2-methylimidazole is 11.19 - 11.96 mg·mL -1 , and the concentration of sodium dodecyl sulfate is 6.58 mg·mL -1 .

2. The preparation method of a two-dimensional porous metal-organic framework nanosheet material according to claim 1, characterized in that, In step S3, the rapid stirring time is 15 - 20 s.

3. The preparation method of a two-dimensional porous metal-organic framework nanosheet material according to claim 1, characterized in that, In step S3, the temperature for standing and reacting in the incubator is 26 °C, and the reaction time is 24 h.

4. Application of the two-dimensional porous metal-organic framework nanosheet material prepared by the preparation method described in claim 1 as a positive electrode material for supercapacitors.

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