A MOFs superstructure controllable synthesis ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheet and a preparation method thereof

Abstract

The application belongs to the technical field of nanomaterials, and particularly relates to a metal organic framework (MOFs) superstructure controllable synthesis ultra-thin two-dimensional boron-nitrogen co-doped carbon nanosheet and a preparation method thereof. The preparation method comprises the following steps: 1) synthesizing a MOFs precursor by using a solvothermal method; 2) calcining the MOFs precursor to obtain a three-dimensional boron-nitrogen co-doped carbon material; 3) mixing and stirring the three-dimensional boron-nitrogen co-doped carbon material with a solvent, water-bath heating and condensation refluxing, then vacuum filtering by using a vacuum filtering device, and drying by using an oven to obtain the ultra-thin two-dimensional boron-nitrogen co-doped carbon nanosheet. The preparation method is simple, efficient, low in cost and easy for industrial production. The two-dimensional boron-nitrogen co-doped carbon nanosheet has the advantages of being easy to modify by introducing metal and other heteroatoms, having a large specific surface area and pore volume, and exhibiting excellent catalytic reaction activity in electrocatalytic reactions, and is an excellent porous and multifunctional carbon material.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of nanomaterials, and particularly relates to a MOFs superstructure controllable synthesis of ultra-thin two-dimensional boron-nitrogen co-doped carbon nanosheet and a preparation method thereof. BACKGROUND

[0002] In recent years, due to the characteristics of abundant source, low price, high electrical conductivity and surface designability, carbon-based materials have been widely used in energy storage and conversion fields. Among them, two-dimensional (2D) carbon nanomaterials have high-density surface active sites, excellent photoelectric and mechanical properties, and are easy to interface transmission and have shorter diffusion paths, and are widely used in hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction and nitrogen reduction reaction. The traditional methods for obtaining two-dimensional nanosheets mainly include mechanical exfoliation method, ultrasonic exfoliation method, ion exchange exfoliation method, chemical vapor phase exfoliation method, two-dimensional template method and low-dimensional nanocrystal self-assembly method. The mechanical exfoliation method has low yield and difficult size control; the ultrasonic exfoliation method has difficult exfoliation effect control and is easy to form defects; the ion exchange exfoliation method has difficult reaction control and high requirements for reaction conditions; the chemical vapor phase exfoliation method has harsh reaction conditions, complex preparation conditions and high cost; and the two-dimensional template method and the low-dimensional nanocrystal self-assembly method have relatively small lateral scale. Therefore, the development and design of two-dimensional nanosheets with excellent stability, multifunctionality and high activity have important research significance and practical value for solving the problems existing in energy storage and conversion technology.

[0003] Generally, two-dimensional nanosheets are designed for specific functional applications, and the development of multifunctional nanosheets is an effective way to reduce components and cost, but it is still difficult to integrate multiple functions into nanosheet catalysts at present. In addition, the industrial application of two-dimensional nanosheets based on heteroatom uniform doping is very limited, and the main reason is that the industrialization cost is very high, and each functional nanosheet needs a specific production line, so the preparation of multifunctional two-dimensional nanosheets has important practical significance, and the regulation of the structure and composition of two-dimensional nanosheets becomes very critical. As known, metal-organic frameworks (MOFs) are a new type of porous crystalline material formed by the connection of metal ions and organic ligands through coordination bonds, and have good structure modification characteristics. MOFs derivatives not only can well maintain the high specific surface area and porosity characteristics of the precursor, but also can realize heteroatom doping, defect construction, 2D / 1D construction and carbon coating through high-temperature carbonization or chemical treatment of the precursor MOFs. Therefore, MOFs derivatives have very broad application prospects in electrode materials, catalyst supports, supercapacitors and other aspects, and it has important significance to use MOFs to dope B and N at the same time to prepare two-dimensional B / N uniformly co-doped MOFs derivative carbon materials.

[0004] Chinese patent CN112537770A discloses a method for preparing nitrogen-doped two-dimensional carbon nanosheets. The method involves ultrasonically dissolving glucose, urea, and sodium dodecylbenzenesulfonate (SDBS) in deionized water, mixing the solutions in an alkaline aqueous solution, drying them, and then pyrolyzing them at high temperature to obtain nitrogen-doped two-dimensional carbon nanosheet materials. This process produces unique morphologies and exhibits excellent electrochemical performance, making it suitable as an electrode material for novel secondary batteries. However, the uneven nitrogen doping and random distribution of active sites are detrimental to catalyst activity and can affect the catalyst's electrochemical stability.

[0005] Chinese patent CN110127661A discloses a method for preparing two-dimensional ordered mesoporous nanosheets through inorganic salt interface-induced assembly. Using a soluble inorganic salt as a substrate and an amphiphilic block copolymer as a template agent, the method employs a vacuum filtration, solvent evaporation induction, and gradient-controlled Oswald curing process to induce material formation. Subsequently, the template agent is removed under N2 protection to obtain highly ordered monolayer two-dimensional mesoporous nanosheet materials. This method utilizes readily available raw materials and produces materials with large pore sizes, regular spherical channels, and an ordered arrangement. However, the process is complex, the material's structure and composition cannot be controlled, and it cannot integrate multifunctional carbon materials with different internal structures.

[0006] Therefore, there is a need to design a multifunctional two-dimensional porous material that is simple to prepare, has a controllable structure and composition, and exhibits good selectivity for reactants. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for the controllable synthesis of ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures.

[0008] This invention utilizes metal-organic frameworks (MOFs) as precursors for two-dimensional nanosheets. In this invention, MOFs are used to obtain boron-nitrogen uniformly doped ultrathin two-dimensional nanosheets through a simple liquid exfoliation method. These nanosheets exhibit excellent catalytic activity in electrocatalytic reactions and are an excellent porous and multifunctional carbon material.

[0009] The preparation method of this invention provides a simple, efficient, low-cost, and easily industrialized method for preparing nitrogen-doped ultrathin two-dimensional nanosheets. By controlling the composition and structure of MOFs, multifunctional carbon materials with different internal structures and heteroatom doping can be prepared. The prepared carbon materials have large specific surface areas and pore volumes. Furthermore, by utilizing the flexibility and diversity of organic ligands in MOFs, boron and nitrogen doping can be uniformly introduced into the carbon materials, forming a uniform carbon network framework and metal catalytic centers. Their synergistic effect is beneficial for electrochemical catalysis, hydrodesulfurization, selective hydrogenation, and other hydrogenation reactions, showing broad application prospects.

[0010] The technical solution of this invention is implemented as follows:

[0011] A method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures includes the following steps:

[0012] 1) Preparation of MOF precursors:

[0013] A certain amount of organic ligand, metal salt and boric acid were weighed and dissolved in a solvent and stirred thoroughly; then MOF precursors were synthesized by a solvothermal method.

[0014] 2) Place the MOF precursor prepared in step 1) in a corundum ceramic boat, then place it in a tube furnace and calcine it at 600-1100℃ for 0.5-12 hours under a certain inert atmosphere to obtain a three-dimensional flower-like boron-nitrogen co-doped carbon material.

[0015] 3) Place the three-dimensional flower-shaped boron-nitrogen co-doped carbon material prepared in step 2) into a round-bottom flask containing solvent, then heat it in a water bath at 60-90°C and reflux for 12-36 hours. Use a vacuum filtration device to filter the refluxed suspension and place it in an oven to dry at 80-120°C for 12-36 hours to obtain two-dimensional boron-nitrogen co-doped carbon nanosheets.

[0016] Preferably, the synthesis method for preparing the MOF precursor in step 1) is a solvothermal method.

[0017] Preferably, the organic ligand in step 1) is selected from one or more combinations of imidazole, 2-methylimidazolium, 4,4'-bipyridine, 2,2'-bipyridine, o-phenanthroline, and pyrazine.

[0018] Preferably, in step 1), the metal salt is selected from one or more combinations of zinc, nickel, cobalt, or iron metal salts; the metal salt is selected from one or more combinations of sulfate hydrate, nitrate hydrate, chloride hydrate, or acetate hydrate metal salts.

[0019] Preferably, the amount of boric acid used in step 1) is 2.7g to 2.9g.

[0020] Preferably, the molar ratio of the organic ligand to the metal salt in step 1) is (1:10) to (10:1).

[0021] Preferably, the solvent in step 1) is one or more of methanol, ethanol, N,N-dimethylformamide, or water.

[0022] Preferably, the solvent in step 3) is one or a combination of N,N-dimethylformamide, water, isopropanol, dimethyl sulfoxide, and N-methylpyrrolidone.

[0023] Preferably, the reaction conditions for the solvothermal method used in step 1) to prepare the MOF precursor are as follows: the mixed solution of organic ligand and metal salt dissolved in solvent is transferred to a polytetrafluoroethylene-lined reactor and heated to 100-150°C for 6-72 hours. After centrifugation and washing, the MOF precursor is obtained.

[0024] Preferably, the inert atmosphere in step 3) is any one of H2 / Ar, argon, nitrogen or helium; the heating rate of the inert atmosphere is 1 to 20 °C / min, from room temperature to 600 to 1100 °C; the gas flow rate of the inert atmosphere is 1 to 100 mL / min.

[0025] Based on the same inventive concept, this invention also provides a method for controllable synthesis of ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets from MOF superstructures, which are prepared using the above-described preparation method.

[0026] Beneficial effects

[0027] This invention discloses a method for controllable synthesis of ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures. The beneficial effects of this invention are:

[0028] 1. This invention utilizes MOFs with different structures and compositions as precursors to obtain uniformly doped ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets with controllable elemental content through simple heat treatment. Compared with traditional carbon materials, the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets obtained by this invention have a more uniform activity distribution, significant advantages, a larger specific surface area and pore volume, and the diversity of MOF structures and compositions allows for various modifications and functionalizations of carbon materials, which is highly beneficial for the promotion and development of carbon materials.

[0029] 2. The ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets of the present invention can improve the mass-charge transfer efficiency in the catalytic process, and through the synergistic effect with the metal active centers derived from MOFs, the catalytic effect of the catalyst can be greatly improved.

[0030] 3. In addition, the preparation process of this invention has low requirements for reaction equipment, no harmful gases or organic substances are involved, the process is simple, and it is suitable for large-scale industrial production. Attached Figure Description

[0031] Figure 1 The image shows the X-ray powder diffraction (XRD) pattern of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in Example 1.

[0032] Figure 2 SEM images of the precursor MOFs (MBON-1);

[0033] Figure 3SEM image of a three-dimensional flower-like boron-nitrogen-doped carbon material (B / NC NF);

[0034] Figure 4 SEM image of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets (B / NC NS) prepared in Example 1;

[0035] Figure 5 This is a transmission electron microscope (TEM) image of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in Example 1;

[0036] Figure 6 This is a scanned elemental distribution map of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in Example 1;

[0037] Figure 7 Atomic force microscopy (AFM) testing of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in Example 1 Figure 1 ;

[0038] Figure 8 Atomic force microscopy (AFM) testing of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in Example 1 Figure 2 ;

[0039] Figure 9 X-ray photoelectron spectroscopy (XPS) of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in Example 1;

[0040] Figure 10 The nitrogen physical adsorption curves are those of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in Example 1.

[0041] Figure 11 This is a pore size distribution diagram of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in Example 1;

[0042] Figure 12 The linear sweep voltammetry curves are for the catalytic electrochemical nitrogen reduction reaction (NRR) of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in Example 1 in 0.1 M KOH.

[0043] Figure 13 The linear sweep voltammetry curves are for the catalytic electrochemical nitrogen reduction reaction (NRR) in 0.1 M Na2SO4 saturated with ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in Example 1.

[0044] Figure 14 The linear sweep voltammetry curves of the catalytic electrochemical nitrogen reduction reaction (NRR) in 0.1M HCl for the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in Example 1 are shown.

[0045] Figure 15These are the chronocurrent curves of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in Example 1 at different voltages;

[0046] Figure 16 The yield and Faraday efficiency of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in Example 1 at different voltages;

[0047] Figure 17 This is a stability test diagram of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in Example 1. Detailed Implementation

[0048] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0049] Unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. The term "and / or" as used in this specification includes any and all combinations of one or more of the associated listed items.

[0050] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0051] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0052] A method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures includes the following steps:

[0053] 1) Preparation of MOF precursors:

[0054] A certain amount of organic ligand, metal salt and boric acid were weighed and dissolved in a solvent and stirred thoroughly; then MOF precursors were synthesized by a solvothermal method.

[0055] 2) A certain amount of the MOF precursor prepared in step 1) is placed in a corundum ceramic boat, and then placed in a tube furnace. The furnace is heated to 600-1100℃ and calcined for 0.5-12 hours under a certain inert atmosphere to obtain a three-dimensional flower-like boron-nitrogen co-doped carbon material.

[0056] 3) Place the three-dimensional flower-shaped boron-nitrogen co-doped carbon material prepared in step 2) into a round-bottom flask containing solvent, then heat it in a water bath at 60-90°C and reflux for 12-36 hours. Use a vacuum filtration device to filter the refluxed suspension and place it in an oven to dry at 80-120°C for 12-36 hours to obtain two-dimensional boron-nitrogen co-doped carbon nanosheets.

[0057] In this invention, different MOF materials were synthesized using a conventional solvothermal method. These materials were then subjected to simple heat treatment to obtain uniformly doped three-dimensional porous carbon materials with controllable elemental content. Subsequently, the three-dimensional carbon materials were exfoliated into two-dimensional ultrathin nanosheets using liquid exfoliation. In this invention, MOFs not only serve as raw materials for generating two-dimensional nanosheets but also as spacers to prevent their aggregation, which significantly increases the specific surface area of ​​the material. More importantly, the abundant metal active centers and uniform distribution of heteroatoms such as nitrogen at the molecular level in MOFs allow for uniform metal active centers and boron-nitrogen doping on the two-dimensional nanosheets, both of which enhance the catalytic activity and functionality of the composite film. This invention utilizes the pyrolysis of MOF precursors to prepare three-dimensional porous carbon materials, followed by liquid exfoliation to obtain ultrathin two-dimensional boron-nitrogen co-doped carbon materials. The process is simple and conducive to large-scale production.

[0058] In this invention, there are no special restrictions on organic ligands and metal salts, all of which are commercially available products.

[0059] There are no special restrictions on the vacuum filtration method in step 2) of this invention; any vacuum filtration method known to those skilled in the art is acceptable.

[0060] Preferably, the synthesis method for preparing the MOF precursor in step 1) is a solvothermal method.

[0061] Preferably, the organic ligand in step 1) is selected from any one of imidazole, 2-methylimidazolium, 4,4'-bipyridine, 2,2'-bipyridine, o-phenanthroline, and pyrazine. Among these, imidazole, 2-methylimidazolium, 4,4'-bipyridine, 2,2'-bipyridine, o-phenanthroline, and pyrazine are nitrogen-containing organic ligands; more preferably, the nitrogen-containing organic ligand is selected from imidazole or 2-methylimidazolium; most preferably, the nitrogen-containing organic ligand is selected from 2-methylimidazolium, the use of 2-methylimidazolium resulting in a better MOF precursor structure prepared in step 1).

[0062] Preferably, the metal salt in step 1) is selected from any one of zinc, nickel, cobalt, or iron metal salts; the metal salt in step 1) is selected from any one of sulfate hydrate, nitrate hydrate, chloride hydrate, or acetate hydrate metal salts; more preferably, the metal salt is selected from zinc or cobalt metal salts, and the metal salt is selected from nitrate hydrate; most preferably, the metal salt is selected from zinc nitrate hexahydrate, which can obtain a variety of MOFs, and the effect of liquid-phase exfoliation of MOFs to form carbon nanosheets is the best.

[0063] Preferably, in step 1), the molar ratio of the organic ligand to the metal salt is (1:10) to (10:1), and at this molar ratio, the concentration of the organic ligand in step 1) is 0.1 to 1 mol / L, and the concentration of the metal salt is 0.05 to 0.5 mol / L. More preferably, in step 1), the molar ratio of the organic ligand to the metal salt is (1:1) to (5:1), and at this molar ratio, the concentration of the organic ligand in step 1) is 0.1 to 0.7 mol / L, and the concentration of the metal salt is 0.13 mol / L. Most preferably, in step 1), the molar ratio of the organic ligand to the metal salt is 4:1, and at this molar ratio, the concentration of the organic ligand is 0.5 mol / L, and the concentration of the metal salt is 0.13 mol / L. At this molar ratio and concentration of the organic ligand and the metal salt, a better MOF structure can be obtained.

[0064] Preferably, the solvent in step 1) is any one of methanol, ethanol, N,N-dimethylformamide, or water. It is understood in this invention that the solvent used in the solvothermal synthesis of MOF precursors is methanol, ethanol, or N,N-dimethylformamide. Most preferably, the solvent in step 1) is methanol, as methanol is inexpensive, readily available, and has good compatibility, making it suitable for various MOF precursor structures.

[0065] Preferably, the reaction conditions for the solvothermal method used in step 1) to prepare the MOF precursor are as follows: a mixed solution of organic ligand, metal salt and boric acid dissolved in a solvent is transferred to a polytetrafluoroethylene-lined reactor and heated to 100-150°C for 6-72 hours. The product is then centrifuged and washed to obtain the MOF precursor. More preferably, the temperature is raised to 120-150°C and reacted for 24 hours. Most preferably, the temperature is raised to 150°C and reacted for 12 hours.

[0066] Preferably, the inert atmosphere in step 2) is any one of H2 / Ar, argon, nitrogen, or helium, and most preferably, the inert atmosphere is nitrogen; the heating rate of the inert atmosphere is 1-20℃ / min, from room temperature to 600-1100℃, more preferably, the heating rate is 5-10℃ / min, and most preferably, the heating rate is 5℃ / min; the gas flow rate of the inert atmosphere is 1-100mL / min, more preferably, the gas flow rate of the inert atmosphere is 20-100mL / min. If the purge gas flow rate is too high (greater than 100mL / min), it will cause disturbance at high temperature; if the gas flow rate is too low (below 20mL / min), it will affect the reaction on the surface of the bulk material during the thermal reaction. Most preferably, the gas flow rate of the inert atmosphere is 40mL / min.

[0067] Preferably, the heat treatment temperature in step 3) is 600–1100°C, and the heat treatment time is 0.5–12 hours; more preferably, the heat treatment temperature is 700–1000°C. If the temperature is too low (below 400°C), MOFs cannot form carbon nanotubes; if the temperature is too high (above 1000°C), it will increase the degree of graphitization and the degree of aggregation of metal centers, reduce the number of active sites, and thus affect the catalytic activity. When the heat treatment temperature is 700–1000°C, the heat treatment time is 6–8 hours; most preferably, the heat treatment temperature is 900°C, and the heat treatment time is 6 hours.

[0068] Based on the same inventive concept, this invention also provides a two-dimensional boron-nitrogen co-doped carbon nanosheet, which can introduce heteroatoms such as metals to modify the carbon nanosheet, and has a large specific surface area and pore volume, forming a carbon network framework with uniform boron-nitrogen doping and metal catalytic centers.

[0069] The phase composition of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared by this invention was determined by X-ray powder diffraction pattern using a Bruker D8 X-ray diffractometer.

[0070] The morphology of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in this invention was obtained by field emission scanning electron microscopy (SEM) using a Zeiss Gemini 500 field emission scanning electron microscope.

[0071] The internal morphology and elemental distribution of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in this invention were obtained by transmission electron microscopy (TEM) images and EDS elemental distribution maps using a JEOL JEM2100F transmission electron microscope from Japan.

[0072] The specific surface area of ​​the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared by this invention was displayed by low-temperature nitrogen adsorption-desorption curves using an Autosorb-iQ2 fully automated specific surface area and pore size distribution analyzer from the American company Quanta Computer.

[0073] To further understand this application, the following detailed description, in conjunction with embodiments, of a method for controllable synthesis of ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures and its preparation.

[0074] Example 1

[0075] A method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures, comprising the following steps:

[0076] 1) Preparation of MBON-1 MOF precursor: Weigh 1.116 g of zinc nitrate hexahydrate and 1.232 g of 2-methylimidazole and dissolve them in 30 mL of methanol. After stirring thoroughly, transfer the solution to a high-pressure reactor with a polytetrafluoroethylene liner and react at 150 °C for 12 hours. Centrifuge the product to separate the supernatant and precipitate. Wash the precipitate three times with methanol to obtain the MBON-1 precursor.

[0077] 2) Weigh 700mg of the MBON-1 precursor from step 1) and place it in a corundum ceramic boat. Put the ceramic boat into a tube furnace, purge the air from the tube furnace, and introduce nitrogen at a rate of 40mL / min as a protective atmosphere. Heat to 900℃ at a heating rate of 5℃ / min and hold at this temperature for 6 hours (during which nitrogen is continuously purged at a rate of 40mL / min) to obtain the three-dimensional boron-nitrogen co-doped carbon material.

[0078] 3) Take 1g of the three-dimensional boron-nitrogen co-doped carbon material obtained in step 2) and place it in a vacuum filtration device to filter the refluxed suspension (filter membrane diameter is 5cm). Then place it in an oven and dry it at 80-120℃ for 12-36 hours to obtain two-dimensional boron-nitrogen co-doped carbon nanosheets.

[0079] Figure 1 This is the X-ray powder diffraction (XRD) pattern of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in this embodiment. From... Figure 1 It can be seen that B / NC NS has diffraction peaks at 2θ values ​​of 25.8° and 40°~45°, which can correspond to the (002) and (100) crystal planes of graphite carbon.

[0080] Figures 2-4 These are scanning electron microscope (SEM) images of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in this embodiment, wherein... Figure 2 SEM images of the precursor MOFs (MBON-1); Figure 3 SEM image of a three-dimensional flower-like boron-nitrogen-doped carbon material (B / NC NF); Figure 4 SEM image of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets (B / NC NS) prepared in this embodiment. Figures 2-4It can be seen that the B / NC NF obtained by high-temperature calcination retains the morphology of the precursor (MBON-1), has a three-dimensional nanoflower superstructure, and is composed of many nanosheets with a thickness of 45 nm, such as Figure 2 and 3 As shown; B / NC NS obtained by water bath exfoliation exhibits a two-dimensional nanosheet morphology in SEM, such as... Figure 4 As shown.

[0081] Figure 5 This is a transmission electron microscope (TEM) image of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in this embodiment. Figure 5 As can be seen, B / NC NS exhibits a two-dimensional nanosheet morphology.

[0082] Figure 6 This is the elemental distribution map of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in this embodiment. Figure 6 It can be seen that B, C and N are uniformly distributed in B / NC NS.

[0083] Figures 7-8 This is an atomic force microscope image of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in this embodiment. Figure 7 AFM for B / NC NS Figure 1 , Figure 8 AFM for B / NC NS Figure 2 The illustration shows the thickness of the nanosheets. From... Figures 7-8 It can be seen that the nanosheets after water bath exfoliation are an ultrathin two-dimensional material.

[0084] Figure 9 This is the X-ray photoelectron spectrum of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in this embodiment. Figure 9 In the image, (a) is the B1s high-resolution image of the B / NC NS; (b) is the C1s high-resolution image of the B / NC NS; and (c) is the N1s high-resolution image of the B / NC NS. Figure 9 It can be seen that the characteristic peaks at 190.6 eV and 192.5 eV belong to the BC and BO bonds, respectively, while the characteristic peak at 191.8 eV belongs to the BN bond; the characteristic peaks at -283.8 eV, -284.8 eV, and -286.8 eV belong to the CB, CC, and CN bonds, respectively; and the characteristic peaks at 397.6, 398.6, and 400.5 eV belong to the NB, NC, and Graphitic N bonds, respectively.

[0085] Figures 10-11 This is the nitrogen physical adsorption curve of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in this embodiment. Figure 10 The attached diagram shows the nitrogen adsorption and desorption process of B / NC NS. Figure 11This is a pore size distribution diagram for B / NC NS. (Through...) Figure 10 The nitrogen physical adsorption curve and pore size distribution diagram show that the obtained nanosheets have a high specific surface area, with a specific surface area and pore volume of 172 m². 2 / g and 0.17cm 3 / g.

[0086] Figures 12-14 This is the linear sweep voltammetry curve of the electrochemical nitrogen reduction reaction (NRR) catalyzed by the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in this embodiment. Figure 12 The LSV curves of B / NC NS in 0.1M KOH saturated with N2 or Ar are shown. Figure 13 The LSV curves of B / NC NS in 0.1M Na2SO4 saturated with N2 or Ar are shown. Figure 14 The LSV curves of B / NC NS in 0.1M HCl saturated with N2 or Ar are shown. The graph reveals significant differences in current density for B / NC NS in 0.1M KOH, 0.1M Na2SO4, and 0.1M HCl saturated with N2 or Ar. The difference in current density between N2 and Ar is most pronounced in the 0.1M KOH electrolyte, with the difference increasing progressively from -0.1V (vs. RHE). In 0.1M Na2SO4, when the voltage exceeds -1V (vs. RHE), the current density under N2 atmosphere almost completely overlaps with that under Ar atmosphere, indicating that nitrogen selectivity is not significant at this point. In 0.1M HCl, when the voltage exceeds -0.36V (vs. RHE), the current density under N2 atmosphere is lower than that under Ar atmosphere, indicating that the hydrogen evolution reaction dominates at this point, and B / NC NS is more favorable for the hydrogen evolution reaction after this potential.

[0087] Figure 15 These are the chronoamperometry curves of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in this embodiment at different voltages. From... Figure 15 As can be seen, within the potential range of -0.1V to -0.7V (vs RHE), the current density of the B / NC NS chronocurrent curve did not change significantly within 2 hours. The B / NC NF showed significant fluctuations at -0.6V (vs RHE), which is because the hydrogen evolution reaction caused a large number of bubbles to appear on the catalyst-coated carbon paper, resulting in significant fluctuations.

[0088] Figure 16The figures show the yield and Faradaic efficiency of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in this embodiment at different voltages. The B / NC NS achieves the maximum Faradaic efficiency (FE) at -0.2V (vs. RHE), while the ammonia yield reaches its maximum at -0.4V (vs. RHE). Above -0.4V (vs. RHE), the Faradaic efficiency of the B / NC NS decreases. This is because, in addition to the nitrogen reduction to ammonia synthesis reaction, the entire cathode reaction is accompanied by a hydrogen evolution reaction. Increasing the potential promotes both the nitrogen reduction to ammonia synthesis reaction and the hydrogen evolution reaction. When the hydrogen evolution reaction dominates, more electrons are reduced to hydrogen by hydrogen ions, leading to a decrease in the Faradaic efficiency of nitrogen to ammonia synthesis.

[0089] Figure 17 This is a stability test of the ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets prepared in this embodiment. From... Figure 17 As can be seen, the B / NC NS remained generally stable during the 48-hour chronocurrent test, with occasional small fluctuations due to air bubbles adhering to the carbon paper. The slight decrease in B / NC NS after the 48-hour chronocurrent test is likely due to material peeling off the carbon paper.

[0090] Example 2

[0091] A method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures, comprising the following steps:

[0092] 1) Preparation of MBON-1 MOF precursor: Weigh 1.116 g of zinc nitrate hexahydrate and 1.232 g of 2-methylimidazole and dissolve them in 30 mL of methanol. After stirring thoroughly, transfer the solution to a high-pressure reactor with a polytetrafluoroethylene liner and react at 150 °C for 12 hours. Centrifuge the product to separate the supernatant and precipitate. Wash the precipitate three times with methanol to obtain the MBON-1 precursor.

[0093] 2) Weigh 800mg of the MBON-1 precursor from step 1) and place it in a corundum ceramic boat. Put the ceramic boat into a tube furnace, purge the air from the tube furnace, and introduce nitrogen at a rate of 40mL / min as a protective atmosphere. Heat to 900℃ at a heating rate of 5℃ / min and hold at this temperature for 6 hours (during which nitrogen is continuously purged at a rate of 40mL / min) to obtain the three-dimensional boron-nitrogen co-doped carbon material.

[0094] 3) Take 1g of the three-dimensional boron-nitrogen co-doped carbon material obtained in step 2) and place it in a round-bottom flask containing solvent. Then, heat it in a water bath at 60-90°C and reflux for 12-36 hours. Use a vacuum filtration device to filter the refluxed suspension (filter membrane diameter is 5cm) and put it in an oven to dry at 80-120°C for 12-36 hours to obtain two-dimensional boron-nitrogen co-doped carbon nanosheets.

[0095] Example 3

[0096] A method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures, comprising the following steps:

[0097] 1) Preparation of MBON-1 MOF precursor: Weigh 1.120 g of zinc nitrate hexahydrate and 1.232 g of 2-methylimidazole and dissolve them in 30 mL of methanol. After stirring thoroughly, transfer the solution to a high-pressure reactor with a polytetrafluoroethylene liner and react at 150 °C for 12 hours. Centrifuge the product to separate the supernatant and precipitate. Wash the precipitate three times with methanol to obtain the MBON-1 precursor.

[0098] 2) Weigh 700mg of the MBON-1 precursor from step 1) and place it in a corundum ceramic boat. Put the ceramic boat into a tube furnace, purge the air from the tube furnace, and introduce nitrogen at a rate of 40mL / min as a protective atmosphere. Heat to 900℃ at a heating rate of 5℃ / min and hold at this temperature for 6 hours (during which nitrogen is continuously purged at a rate of 40mL / min) to obtain the three-dimensional boron-nitrogen co-doped carbon material.

[0099] 3) Take 1g of the three-dimensional boron-nitrogen co-doped carbon material obtained in step 2) and place it in a round-bottom flask containing solvent. Then, heat it in a water bath at 60-90°C and reflux for 12-36 hours. Use a vacuum filtration device to filter the refluxed suspension (filter membrane diameter is 5cm) and put it in an oven to dry at 80-120°C for 12-36 hours to obtain two-dimensional boron-nitrogen co-doped carbon nanosheets.

[0100] Example 4

[0101] A method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures, comprising the following steps:

[0102] 1) Preparation of MBON-1 MOF precursor: Weigh 1.116 g of zinc nitrate hexahydrate, 1.240 g of 2-methylimidazole and 2.7834 g of boric acid and dissolve them in 30 mL of methanol. After stirring thoroughly, transfer the solution to a high-pressure reactor with a polytetrafluoroethylene liner and react at 150 °C for 12 hours. Centrifuge the product to separate the supernatant and precipitate. Wash the precipitate three times with methanol to obtain the MBON-1 precursor.

[0103] 2) Weigh 700mg of the MBON-1 precursor from step 1) and place it in a corundum ceramic boat. Put the ceramic boat into a tube furnace, purge the air from the tube furnace, and introduce nitrogen at a rate of 40mL / min as a protective atmosphere. Heat to 900℃ at a heating rate of 5℃ / min and hold at this temperature for 6 hours (during which nitrogen is continuously purged at a rate of 40mL / min) to obtain the three-dimensional boron-nitrogen co-doped carbon material.

[0104] 3) Take 1g of the three-dimensional boron-nitrogen co-doped carbon material obtained in step 2) and place it in a round-bottom flask containing solvent. Then, heat it in a water bath at 60-90°C and reflux for 12-36 hours. Use a vacuum filtration device to filter the refluxed suspension (filter membrane diameter is 5cm) and put it in an oven to dry at 80-120°C for 12-36 hours to obtain two-dimensional boron-nitrogen co-doped carbon nanosheets.

[0105] Example 5

[0106] A method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures, comprising the following steps:

[0107] 1) Preparation of MBON-1 MOF precursor: Weigh 1.116 g of zinc nitrate hexahydrate, 1.232 g of 2-methylimidazole and 2.7834 g of boric acid and dissolve them in 30 mL of methanol. After stirring thoroughly, transfer the solution to a high-pressure reactor with a polytetrafluoroethylene liner and react at 150 °C for 12 hours. Centrifuge the product to separate the supernatant and precipitate. Wash the precipitate three times with methanol to obtain the MBON-1 precursor.

[0108] 2) Weigh 700mg of the MBON-1 precursor from step 1) and place it in a corundum ceramic boat. Put the ceramic boat into a tube furnace, purge the air from the tube furnace, and introduce nitrogen at a rate of 40mL / min as a protective atmosphere. Heat to 900℃ at a heating rate of 5℃ / min and hold at this temperature for 4 hours (during which nitrogen is continuously purged at a rate of 40mL / min) to obtain the three-dimensional boron-nitrogen co-doped carbon material.

[0109] 3) Take 1g of the three-dimensional boron-nitrogen co-doped carbon material obtained in step 2) and place it in a round-bottom flask containing solvent. Then, heat it in a water bath at 60-90°C and reflux for 12-36 hours. Use a vacuum filtration device to filter the refluxed suspension (filter membrane diameter is 5cm) and put it in an oven to dry at 80-120°C for 12-36 hours to obtain two-dimensional boron-nitrogen co-doped carbon nanosheets.

[0110] Example 6

[0111] A method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures, comprising the following steps:

[0112] 1) Preparation of MBON-1 MOF precursor: Weigh 1.116 g of zinc nitrate hexahydrate, 1.232 g of 2-methylimidazole and 2.7834 g of boric acid and dissolve them in 30 mL of methanol. After stirring thoroughly, transfer the solution to a high-pressure reactor with a polytetrafluoroethylene liner and react at 150 °C for 12 hours. Centrifuge the product to separate the supernatant and precipitate. Wash the precipitate three times with methanol to obtain the MBON-1 precursor.

[0113] 2) Weigh 700mg of the MBON-1 precursor from step 1) and place it in a corundum ceramic boat. Put the ceramic boat into a tube furnace, purge the air from the tube furnace, and introduce nitrogen at a rate of 40mL / min as a protective atmosphere. Heat to 900℃ at a heating rate of 5℃ / min and hold at this temperature for 5 hours (during which nitrogen is continuously purged at a rate of 40mL / min) to obtain the three-dimensional boron-nitrogen co-doped carbon material.

[0114] 3) Take 2g of the three-dimensional boron-nitrogen co-doped carbon material obtained in step 2) and place it in a round-bottom flask containing solvent. Then, heat it in a water bath at 60-90°C and reflux for 12-36 hours. Use a vacuum filtration device to filter the refluxed suspension (filter membrane diameter is 5cm) and put it in an oven to dry at 80-120°C for 12-36 hours to obtain two-dimensional boron-nitrogen co-doped carbon nanosheets.

[0115] Example 7

[0116] A method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures, comprising the following steps:

[0117] 1) Preparation of MBON-1 MOF precursor: Weigh 1.116 g of zinc nitrate hexahydrate, 1.232 g of 2-methylimidazole and 2.7900 g of boric acid and dissolve them in 30 mL of methanol. After stirring thoroughly, transfer the solution to a high-pressure reactor with a polytetrafluoroethylene liner and react at 150 °C for 12 hours. Centrifuge the product to separate the supernatant and precipitate. Wash the precipitate three times with methanol to obtain the MBON-1 precursor.

[0118] 2) Weigh 700mg of the MBON-1 precursor from step 1) and place it in a corundum ceramic boat. Put the ceramic boat into a tube furnace, purge the air from the tube furnace, and introduce nitrogen at a rate of 40mL / min as a protective atmosphere. Heat to 900℃ at a heating rate of 5℃ / min and hold at this temperature for 6 hours (during which nitrogen is continuously purged at a rate of 40mL / min) to obtain the three-dimensional boron-nitrogen co-doped carbon material.

[0119] 3) Take 1g of the three-dimensional boron-nitrogen co-doped carbon material obtained in step 2) and place it in a round-bottom flask containing solvent. Then, heat it in a water bath at 60-90°C and reflux for 12-36 hours. Use a vacuum filtration device to filter the refluxed suspension (filter membrane diameter is 5cm) and put it in an oven to dry at 80-120°C for 12-36 hours to obtain two-dimensional boron-nitrogen co-doped carbon nanosheets.

[0120] Example 8

[0121] A method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures, comprising the following steps:

[0122] 1) Preparation of MBON-1 MOF precursor: Weigh 1.116 g of zinc nitrate hexahydrate, 1.232 g of 2-methylimidazole and 2.7834 g of boric acid and dissolve them in 30 mL of methanol. After stirring thoroughly, transfer the solution to a high-pressure reactor with a polytetrafluoroethylene liner and react at 150 °C for 12 hours. Centrifuge the product to separate the supernatant and precipitate. Wash the precipitate three times with methanol to obtain the MBON-1 precursor.

[0123] 2) Weigh 700mg of the MBON-1 precursor from step 1) and place it in a corundum ceramic boat. Put the ceramic boat into a tube furnace, purge the air from the tube furnace, and introduce nitrogen at a rate of 40mL / min as a protective atmosphere. Heat to 900℃ at a heating rate of 5℃ / min and hold at this temperature for 8 hours (during which nitrogen is continuously purged at a rate of 40mL / min) to obtain the three-dimensional boron-nitrogen co-doped carbon material.

[0124] 3) Take 1g of the three-dimensional boron-nitrogen co-doped carbon material obtained in step 2) and place it in a round-bottom flask containing solvent. Then, heat it in a water bath at 60-90°C and reflux for 12-36 hours. Use a vacuum filtration device to filter the refluxed suspension (filter membrane diameter is 5cm) and put it in an oven to dry at 80-120°C for 12-36 hours to obtain two-dimensional boron-nitrogen co-doped carbon nanosheets.

[0125] Example 9

[0126] A method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures, comprising the following steps:

[0127] 1) Preparation of MBON-1 MOF precursor: Weigh 1.116 g zinc nitrate hexahydrate, 1.240 g 2-methylimidazole and 2.7900 g boric acid and dissolve them in 30 mL methanol. After stirring thoroughly, transfer the solution to a high-pressure reactor with a polytetrafluoroethylene liner and react at 150 °C for 12 hours. Centrifuge the product to separate the supernatant and precipitate. Wash the precipitate three times with methanol to obtain the MBON-1 precursor.

[0128] 2) Weigh 700mg of the MBON-1 precursor from step 1) and place it in a corundum ceramic boat. Put the ceramic boat into a tube furnace, purge the air from the tube furnace, and introduce nitrogen at a rate of 40mL / min as a protective atmosphere. Heat to 900℃ at a heating rate of 5℃ / min and hold at this temperature for 5 hours (during which nitrogen is continuously purged at a rate of 40mL / min) to obtain the three-dimensional boron-nitrogen co-doped carbon material.

[0129] 3) Take 1g of the three-dimensional boron-nitrogen co-doped carbon material obtained in step 2) and place it in a round-bottom flask containing solvent. Then, heat it in a water bath at 60-90°C and reflux for 12-36 hours. Use a vacuum filtration device to filter the refluxed suspension (filter membrane diameter is 5cm) and put it in an oven to dry at 80-120°C for 12-36 hours to obtain two-dimensional boron-nitrogen co-doped carbon nanosheets.

[0130] Example 10

[0131] A method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures, comprising the following steps:

[0132] 1) Preparation of MBON-1 MOF precursor: Weigh 1.120 g zinc nitrate hexahydrate, 1.240 g 2-methylimidazole and 2.7834 g boric acid and dissolve them in 30 mL methanol. After stirring thoroughly, transfer the solution to a high-pressure reactor with a polytetrafluoroethylene liner and react at 150 °C for 14 hours. Centrifuge the product to separate the supernatant and precipitate. Wash the precipitate three times with methanol to obtain the MBON-1 precursor.

[0133] 2) Weigh 700mg of the MBON-1 precursor from step 1) and place it in a corundum ceramic boat. Put the ceramic boat into a tube furnace, purge the air from the tube furnace, and introduce nitrogen at a rate of 40mL / min as a protective atmosphere. Heat to 900℃ at a heating rate of 5℃ / min and hold at this temperature for 6 hours (during which nitrogen is continuously purged at a rate of 40mL / min) to obtain the three-dimensional boron-nitrogen co-doped carbon material.

[0134] 3) Take 1g of the three-dimensional boron-nitrogen co-doped carbon material obtained in step 2) and place it in a round-bottom flask containing solvent. Then, heat it in a water bath at 60-90°C and reflux for 12-36 hours. Use a vacuum filtration device to filter the refluxed suspension (filter membrane diameter is 5cm) and put it in an oven to dry at 80-120°C for 12-36 hours to obtain two-dimensional boron-nitrogen co-doped carbon nanosheets.

[0135] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions claimed by the present invention.

Claims

1. A method for the controllable synthesis of ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures, characterized in that, Includes the following steps: (1) Preparation of MOF precursors: A certain amount of organic ligands, metal salts and boric acid were weighed and dissolved in a solvent and stirred thoroughly; then MOF precursors were synthesized by solvothermal method. (2) The MOF precursor prepared in step (1) was calcined in an inert atmosphere to obtain a three-dimensional flower-like boron-nitrogen co-doped carbon material. (3) The three-dimensional flower cluster boron nitrogen co-doped carbon material prepared in step (2) is mixed and stirred with solvent, heated and refluxed, and the suspension obtained by reflux is filtered. The carbon material obtained after filtration is dried to obtain the two-dimensional boron nitrogen co-doped carbon nanosheets. The heating, condensation, and reflux conditions are: water bath heating and condensation reflux at 60–90°C for 12–36 hours; the drying conditions are: drying at 80–120°C for 12–36 hours; the metal salt is selected from zinc metal salt.

2. The method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures according to claim 1, characterized in that, In step (1), the organic ligand is selected from one or more combinations of imidazole, 2-methylimidazolium, 4,4'-bipyridine, 2,2'-bipyridine, o-phenanthroline, and pyrazine; the metal salt is selected from one or more combinations of sulfate hydrate, nitrate hydrate, chloride hydrate, or acetate hydrate metal salt; and the solvent is one or more combinations of methanol, ethanol, N,N-dimethylformamide, or water.

3. The method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures according to claim 2, characterized in that, In step (1), the amount of boric acid used is 2.7g to 2.9g, and the molar ratio of the organic ligand to the metal salt is (1:10) to (10:1).

4. The method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures according to claim 1, characterized in that, In step (1), MOF precursors are synthesized using a solvothermal method. The reaction conditions for the solvothermal method are as follows: a mixed solution of organic ligands, metal salts, and boric acid dissolved in a solvent is transferred to a polytetrafluoroethylene-lined reactor, heated to 100-150°C and reacted for 6-72 hours. The product is then centrifuged and washed to obtain the MOF precursor.

5. The method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures according to claim 1, characterized in that, In step (2), the MOF precursor is placed in a corundum ceramic boat and then placed in a tube furnace and calcined at 600-1100℃ for 0.5-12 hours under a certain inert atmosphere.

6. The method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures according to claim 1, characterized in that, In step (3), the solvent is one or a combination of N,N-dimethylformamide, water, isopropanol, dimethyl sulfoxide, and N-methylpyrrolidone.

7. The method for controllably synthesizing ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures according to claim 1, characterized in that, In step (2), the inert atmosphere is any one of H2 / Ar, argon, nitrogen or helium; the heating rate of the inert atmosphere is 1 to 20 °C / min, from room temperature to 600 to 1100 °C; the gas flow rate of the inert atmosphere is 1 to 100 mL / min.

8. A method for controllable synthesis of ultrathin two-dimensional boron-nitrogen co-doped carbon nanosheets using MOF superstructures, characterized in that... The two-dimensional boron-nitrogen co-doped carbon nanosheets are prepared using any one of the preparation methods described in claims 1-7.

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