A method for preparing two-dimensional metal nitrides using bulk metal oxide powders

Abstract

The application discloses a method for preparing two-dimensional metal nitride by using bulk metal oxide powder, which comprises the following steps: 1) uniformly mixing and grinding metal oxide powder with alkali metal carbonate or alkali metal hydroxide; adding neutral alkali metal or alkali earth metal halide salt, and uniformly mixing and grinding again; 2) performing heat preservation reaction on the mixture obtained in the step 1) under ammonia atmosphere, and cooling; 3) performing acid washing on the reaction product obtained in the step 2), washing to neutral, performing suction filtration and centrifugation, and drying, so as to obtain two-dimensional metal nitride in nanosheet shape. The application firstly proposes a method for preparing two-dimensional metal nitride nanosheet by using alkaline-neutral mixed double salt as an auxiliary, and the alkali metal carbonate or alkali metal hydroxide is used as a reactant to synthesize an intermediate of alkali metal oxide with two-dimensional layer-like structure, and the neutral alkali metal or alkali earth metal halide salt is used as a molten salt auxiliary agent to promote the nitridation conversion of the alkali metal oxide. The method is suitable for preparing two-dimensional niobium nitride by using bulk niobium oxide, two-dimensional tungsten nitride by using bulk tungsten oxide, and two-dimensional molybdenum nitride by using bulk molybdenum oxide, and is related to a simple process and low cost.

Description

Technical Field

[0001] This invention belongs to the field of two-dimensional material preparation technology, specifically relating to a method for preparing two-dimensional metal nitrides using bulk metal oxide powder. Background Technology

[0002] Two-dimensional transition metal nitrides (TMNs), as a member of the emerging family of two-dimensional materials, possess excellent intrinsic conductivity, electrochemical stability, and good mechanical flexibility, with minimal obstruction to electron / ion transport. They represent a novel high-performance electrode material with promising applications in electrochemical energy storage and conversion. However, unlike other layered compounds, metal nitrides are non-layered materials, with atoms connected by strong three-dimensional chemical bonds. This means that it is difficult to prepare TMNs using conventional bottom-up exfoliation methods for layered two-dimensional materials. Furthermore, the high surface activity of transition metal nitrides due to their rich unsaturated dangling bonds leads to island-like growth during in-situ processes. Currently, research on TMNs remains significantly limited by their preparation methods. Therefore, developing a simple, green, efficient, and controllable method for preparing TMNs is an urgent problem to be solved.

[0003] Currently, the main methods for synthesizing two-dimensional transition metal nitrides both domestically and internationally include: 1) Exfoliation: Exfoliation was one of the earliest methods used to prepare single-layer or few-layer two-dimensional materials. It primarily utilizes external forces to overcome the interlayer van der Waals forces of layered materials, causing the exfoliated thickness to continuously decrease, ultimately obtaining a single-layer or few-layer material. However, exfoliation is only suitable for exfoliating two-dimensional layered materials and not for two-dimensional non-layered materials. 2) Chemical vapor deposition (CVD): CVD is also an indispensable and effective method for preparing two-dimensional materials and is relatively mature. Currently, CVD has been successfully used to grow two-dimensional materials of various types. For example, in the literature "Wang YQ, Jian CY, Hong WT, Wei L, Nonlayered 2D ultrathin molybdenum nitride synthesized through the ammonolysis of 2D molybdenum dioxide[J]. Chemical Communications, 2021, 57(2): 223-226," MoO3 was used as the molybdenum source. MoO2 nanosheets were deposited on a SiO2 / Si substrate by CVD deposition using high-temperature evaporation of MoO3, followed by nitridation to obtain MoN nanosheets. Although this method can obtain high-quality, large-diameter MoN two-dimensional sheets, the yield is low and it is not suitable for industrial production. 3) Selective etching method. Selective etching method is relatively limited and can only be used to prepare certain two-dimensional materials. Usually, two-dimensional transition metal nitrides are prepared by selectively etching the A layer atoms in MAX materials. For example, in "Venkateshalu S, Cherusseri J, Karnan M, et al. New method for the synthesis of 2D vanadiumnitride (MXene) and its application as a supercapacitor electrode[J].ACS omega,2020,5(29):17983-17992.", pure two-dimensional V2NT was obtained by selectively etching away the Al layer in V2AlN using layered V2AlN as a precursor and LiF-HCl as an etchant. xPowder. This method is mostly used for the synthesis of carbide Mxenes. For the preparation of nitride Mxenes, it is limited to V2N, Mo2N, Ti2N, and Ti4N3. There are no reports on the synthesis of Nb-based nitride Mxenes. Selective etching methods often use large amounts of corrosive HF / LiF-HCl solutions as etching agents, which does not conform to the concept of green development. 4) Hydrothermal (solvothermal) method. The hydrothermal (solvothermal) method is beneficial to enhance the reaction kinetics due to the reaction in a high-pressure sealed environment. It can obtain uniform two-dimensional nanosheets at a lower reaction temperature and has low requirements for experimental conditions. However, due to the limitations of reaction temperature and equipment, it is difficult to achieve high crystallinity in the two-dimensional materials obtained by the hydrothermal method. 5) Molten salt-assisted exfoliation method, such as the patent "CN201810826139.X", uses metal sulfides as raw materials and alkali metal salts to assist in the preparation of two-dimensional molybdenum nitride and two-dimensional tungsten nitride. However, this method is only suitable for the large-scale preparation of two-dimensional nitrides from some inexpensive sulfides. For expensive sulfides such as NbS2, it is not feasible to achieve large-scale industrial production. At the same time, this method is limited to the preparation of nitrides that can exist stably in high-temperature and strong alkaline environments, such as molybdenum nitride and tungsten nitride. However, it is not suitable for nitrides such as niobium nitride that are unstable in high-temperature and strong alkaline environments.

[0004] In summary, current methods for preparing two-dimensional transition metal nitrides all have certain limitations. Therefore, further exploration of efficient, low-cost, and easy-to-operate preparation processes for two-dimensional transition metal nitrides is of significant research and application value. Summary of the Invention

[0005] The main objective of this invention is to address the problems and shortcomings of existing technologies by providing a method for preparing two-dimensional metal nitride nanosheets using bulk metal oxide powder, thereby achieving efficient and selective conversion of bulk metal oxide materials into two-dimensional nitride nanosheets. Furthermore, the preparation method is simple, convenient to operate, and suitable for industrial application.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing two-dimensional metal nitride nanosheets using (micrometer-scale) metal oxides includes the following steps:

[0008] 1) Grind and mix the metal oxide powder with alkali metal carbonate or alkali metal hydroxide until uniform; then add neutral alkali metal or alkaline earth metal halide salt, and grind and mix again until uniform;

[0009] 2) The mixture obtained in step 1) is heated under an ammonia atmosphere to carry out a heat preservation reaction, and then cooled;

[0010] 3) The mixture obtained in step 2) is acid-washed, washed with water until neutral, filtered, centrifuged, and dried to obtain nanosheet-like two-dimensional metal nitrides.

[0011] In the above scheme, the metal oxide powder is commercially available powdered niobium pentoxide, molybdenum trioxide or tungsten trioxide, with a size of 0.5-80μm.

[0012] Preferably, the ammonia atmosphere is a mixture of nitrogen and ammonia or argon and ammonia or pure ammonia, and the ammonia content is 5-100%.

[0013] Preferably, when the metal oxide is niobium pentoxide powder, the alkali metal carbonate or alkali metal hydroxide is potassium carbonate, and the alkali metal halide salt is potassium chloride, the molar ratio of niobium pentoxide to potassium carbonate is 1:(1.0-6.0); and the molar ratio of potassium chloride to niobium pentoxide is (4-50):1.

[0014] Preferably, when the metal oxide is tungsten trioxide, the alkali metal carbonate or alkali metal hydroxide is potassium carbonate, and the alkali metal halide salt is potassium chloride, the molar ratio of tungsten trioxide to potassium carbonate is 1:(1.0-6.5); and the molar ratio of potassium chloride to tungsten trioxide is (5-50):1.

[0015] Preferably, when the metal oxide is molybdenum trioxide, the alkali metal carbonate or alkali metal hydroxide is potassium carbonate, and the alkali metal halide salt is potassium chloride, the molar ratio of molybdenum trioxide to potassium carbonate is 1:(1.0-6.5); and the molar ratio of potassium chloride to molybdenum trioxide is (5-50):1.

[0016] Preferably, when niobium pentoxide is selected as the metal oxide, the heat preservation reaction temperature in step 2) is 820-950℃, and the heat preservation time is 3-8h.

[0017] Preferably, when tungsten trioxide is selected as the metal oxide, the heat preservation reaction temperature in step 2) is 660-950℃, and the heat preservation time is 1-7h.

[0018] Preferably, when molybdenum trioxide is selected as the metal oxide, the heat preservation reaction temperature in step 2) is 550-950℃, and the heat preservation time is 1-7h.

[0019] In the above scheme, the heating rate used in step 2) is 1-15℃ / min.

[0020] In the above scheme, the pickling step uses a mixed acid composed of one or more of HCl, H2SO4, acetic acid, or formic acid. When niobium pentoxide is selected as the metal oxide, the acid concentration is 0.5-6 mol / L; when tungsten trioxide is selected as the metal oxide, the acid concentration is 0.5-5 mol / L; and when molybdenum trioxide is selected as the metal oxide, the acid concentration is 0.5-4 mol / L.

[0021] In the above scheme, the drying step can be achieved by freeze drying.

[0022] The principle of this invention is as follows:

[0023] This invention utilizes low-cost alkali metal carbonates, such as K₂CO₃, and neutral alkali metal or alkaline earth metal halide salts, such as KCl or MgCl₂, as auxiliary agents to achieve efficient conversion from metal oxides to two-dimensional metal nitride nanosheets. Simultaneously, the preparation of two-dimensional nitrides is achieved by controlling the reaction time, gas flow rate, and reaction temperature: during the heating process, K₂CO₃ decomposes in situ into K₂O and CO₂ gases; K₂O reacts with the metal oxide to generate the two-dimensional intermediate KMO. X (M = Nb, Mo, W), intermediate product KMO X (M = Nb, Mo, W) preferentially forms a low-melting-point eutectic with potassium chloride (or MgCl2). In the molten state, this facilitates diffusion during nitridation, prevents agglomeration, and promotes two-dimensional nucleation. Simultaneously, potassium chloride lowers the pH of the reaction environment, preventing the nitridation products from melting in a strongly alkaline environment and ensuring a mild reaction environment. Under an ammonia atmosphere, N atoms slowly diffuse into KMO. X Between the (M = Nb, Mo, W) lattice elements, the structure can be supported during the substitution of nitride atoms, preventing structural collapse during the reaction and thus further promoting the formation of two-dimensional morphologies of metal nitrides. Simultaneously, the gas generated in situ at high temperature acts as an auxiliary dispersant, preventing nanosheet stacking and further promoting the formation of dispersed metal nitride nanosheets.

[0024] This invention uses low-cost K2CO3 and KCl (or MgCl2) as salt auxiliaries, resulting in two-dimensional nitrides with good dispersibility, high crystallinity, and high purity. Furthermore, the synthesis process involved is simple and efficient, which is conducive to large-scale preparation.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] 1) This invention uses low-cost, commercially available metal oxides (Nb2O5, WO3, MoO3) as raw materials, and alkali metal carbonates or alkali metal hydroxides (K2CO3, Na2CO3, Li2CO3, KOH, NaOH, LiOH) as auxiliary agents, with neutral alkali metal halides (KCl, NaCl, LiCl) or alkaline earth metal halides (MgCl2, CaCl2) as synergistic agents. It does not require the use of two-dimensional metal oxides as precursors, resulting in low cost. Using low-cost, commercially available bulk Nb2O5, WO3, and MoO3 powders as raw materials, this invention is the first to propose an alkaline-neutral double-salt assisted method for synthesizing two-dimensional metal nitride nanosheets, which has a simple preparation process and low cost.

[0027] 2) It is the first time that K2CO3 can be used as a reactant and simultaneously as an auxiliary agent during the heating reaction to synergistically promote the formation of two-dimensional nitrides with KCl, thereby obtaining two-dimensional nitride nanosheets with good dispersibility and relatively uniform size and morphology.

[0028] 3) By controlling the reaction time and temperature, the precise and controllable preparation of two-dimensional nitrides can be achieved;

[0029] 4) The synthesis process involved is simple and efficient, and the by-product salt after the reaction is easy to remove. It is a green and industrially feasible process route for the preparation of two-dimensional materials.

[0030] 5) The two-dimensional nitrides obtained by this invention have fewer defects, are easy to disperse, have high purity and high crystallinity, and involve low synthesis costs and are environmentally friendly, which can provide a new approach for the low-cost and mass production of two-dimensional nitrides. Attached Figure Description

[0031] Figure 1 A two-dimensional Nb provided in the embodiments of the present invention x N y Preparation process;

[0032] Figure 2 The image shows the XRD pattern of the commercial bulk Nb2O5 powder used in this embodiment of the invention.

[0033] Figure 3 The images are scanning electron microscope (SEM) images of commercial bulk Nb2O5 powder used in the embodiments of the present invention at magnifications of (a) 5000x and (b) 10000x.

[0034] Figure 4 The XRD pattern of the product prepared in Example 1 of this invention;

[0035] Figure 5 The two-dimensional Nb prepared in Example 1 of this invention x N y Morphology of nanosheets at (a) 20000x and (b) 15000x magnification;

[0036] Figure 6 The electrochemical (a) CV curve and (b) GCD curve of the two-dimensional product obtained in Example 1 of this invention in an acidic electrolyte;

[0037] Figure 7 Rate performance curve of the product obtained in Example 1 of this invention in acidic electrolyte;

[0038] Figure 8The long-cycle performance curve of the product obtained in Example 1 of this invention in acidic electrolyte;

[0039] Figure 9 The images show (a) a morphology diagram (15000x) and (b) an XRD pattern of the product prepared by the process described in Comparative Example 1 of this invention.

[0040] Figure 10 This is a morphological image of the product prepared by the process described in Comparative Example 2 of the present invention;

[0041] Figure 11 (a) Morphology image at 15000x and (b) XRD pattern of the product prepared by appropriately increasing the nitriding temperature in Example 2 of the present invention;

[0042] Figure 12 (a) is a morphological image of the product obtained in Example 3 of the present invention; 12(b) is a morphological image of the product obtained in Example 4 of the present invention;

[0043] Figure 13 This is a morphology diagram of the product obtained when potassium hydroxide and potassium chloride mixed salt were added in Example 5 of the present invention.

[0044] Figure 14 This is a morphology diagram of the product obtained when a mixed salt of potassium carbonate and magnesium chloride was added in Example 6 of the present invention.

[0045] Figure 15 These are XRD patterns of nitridation at different times in Example 7 of the present invention;

[0046] Figure 16 The images shown are (a) SEM images and (b) XRD images of the rapidly cooled product obtained in Example 7 of this invention.

[0047] Figure 17 This is a morphology diagram of the product obtained in Comparative Example 3 of the present invention;

[0048] Figure 18 This is a morphology diagram of the product obtained in Example 8 of the present invention;

[0049] Figure 19 Here are (a) a morphology image and (b) an XRD pattern of the product obtained in Comparative Example 4 of this invention;

[0050] Figure 20 Here are (a) a morphology image and (b) an XRD pattern of the product obtained in Comparative Example 5 of this invention;

[0051] Figure 21 The topographic image 15000x (a) obtained in Example 9 of the present invention and the topographic image 15000x (b) obtained in Example 10 are shown.

[0052] Figure 22The images shown are (a) morphology diagram 15000x and (b) XRD pattern obtained in Embodiment 11 of the present invention.

[0053] Figure 23 (a) is a morphological image 15000x obtained in Embodiment 12 of the present invention; (b) is a morphological image 10000x obtained in Embodiment 13 of the present invention;

[0054] Figure 24 (a) is a morphological image 15000x obtained in Embodiment 14 of the present invention; (b) is a morphological image 15000x obtained in Embodiment 15 of the present invention; (c) is a morphological image 15000x obtained in Embodiment 16 of the present invention;

[0055] Figure 25 (a) is a morphological image of 15000x obtained in Comparative Example 6 of the present invention; (b) is a morphological image of 10000x obtained in Comparative Example 7 of the present invention;

[0056] Figure 26 The images shown are (a) morphology image 15000x and (b) XRD pattern obtained in Embodiment 17 of the present invention.

[0057] Figure 27 The morphology diagram 10000x obtained in Embodiment 19 of the present invention; Detailed Implementation

[0058] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0059] In the following embodiments, the niobium pentoxide, tungsten trioxide, and molybdenum trioxide used are commercially available niobium pentoxide powder, commercially available tungsten trioxide powder, and commercially available molybdenum trioxide powder, with a size of 0.5 to 80 μm;

[0060] Example 1

[0061] A method for preparing two-dimensional niobium nitride nanosheets using commercial niobium pentoxide includes the following steps:

[0062] 1) Weigh out niobium pentoxide powder and potassium carbonate powder in a molar ratio of 1:3 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to niobium pentoxide of 16:1, add it, grind and mix it evenly, and transfer it to a porcelain boat.

[0063] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 850°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95% and hold it for 3 hours, then cool it to room temperature.

[0064] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, and filter.

[0065] 4) The reaction product obtained in step 3) is centrifuged and then vacuum dried for 12 hours to obtain a well-dispersed two-dimensional Nb. x N y Nanosheets.

[0066] 5) The two-dimensional Nb obtained in step 4) x N y Electrochemical tests were performed on the nanosheet samples in 1 mol / L H2SO4.

[0067] In this embodiment, the niobium pentoxide powder used is commercially available niobium pentoxide powder, and its XRD pattern and microstructure image are shown in [reference needed]. Figure 2 and Figure 3 The results showed that the diffraction peaks of the commercially available niobium pentoxide powder were basically consistent with the positions of the diffraction peaks of the standard card Nb2O5PDF#27-1003, indicating that its purity was high. Figure 3 The scanning electron microscopy characterization results showed that the commercially available niobium pentoxide powder used was composed of aggregated particles with an overall average size in the micrometer range.

[0068] The product obtained in this embodiment was subjected to X-ray diffraction analysis, and the results are shown in the figure. Figure 4 The results showed that the diffraction peak positions of the obtained product were consistent with those of the standard card Nb5N6 PDF#74-0607 and NbN .801 The results are largely consistent with PDF#74-0780, with no other redundant diffraction peaks, indicating that the final product obtained after molten salt reaction, acid washing, and centrifugal drying is pure two-dimensional Nb. x N y .

[0069] Figure 5 The image shown is a scanning electron microscope (SEM) image of the final product of this embodiment. It can be seen that the two-dimensional Nb obtained using the method described above is... x N y The nanosheets have virtually no overlap, exhibit good dispersion, and are relatively thin, with a size generally around 1-2 μm.

[0070] Figure 6 (a) and 6(b) are the final product Nb of this embodiment, respectively. x N y From the CV and GCD curves, it can be seen that the two-dimensional Nb obtained by the method described above in this invention... x N y The nanosheets exhibit a high specific capacity of 224 F / g at a current density of 1 A / g. Figure 7The rate performance curve and Figure 8 The long cycling curves also show that the capacity retention rate is as high as 65% at a high current density of 10 A / g, and it can still retain 98.2% of the capacity after 10,000 cycles at a current density of 5 A / g. This indicates that the two-dimensional niobium nitride prepared by this method has excellent rate performance and good stability, and is a highly promising electrode material for supercapacitors.

[0071] Comparative Example 1

[0072] A method for preparing two-dimensional niobium nitride nanosheets using commercial niobium pentoxide includes the following steps:

[0073] 1) Weigh out niobium pentoxide powder and anhydrous potassium carbonate in a molar ratio of 1:3 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to niobium pentoxide of 16:1, add it, grind and mix it evenly, and transfer it to a porcelain boat.

[0074] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 800°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95%, hold it at that temperature for 3 hours, and then cool it to room temperature.

[0075] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, and filter.

[0076] 4) Centrifuge the reaction product obtained in step 3), take the upper layer, and finally vacuum dry for 12 hours.

[0077] Figure 9 (a) is a scanning electron microscope image of the final product of this comparative example. It can be seen that the reaction temperature is too low to form a complete two-dimensional structure.

[0078] The product obtained in this comparative example was subjected to X-ray diffraction analysis, and the results are shown below. Figure 9 (b) The results show that a small amount of intermediate product KNbO3 was not completely converted in the product obtained at 800℃.

[0079] Comparative Example 2

[0080] A method for preparing two-dimensional niobium nitride nanosheets using commercial niobium pentoxide includes the following steps:

[0081] 1) Weigh out niobium pentoxide powder and potassium carbonate powder in a molar ratio of 1:3 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to niobium pentoxide of 16:1, add it, grind and mix it evenly, and transfer it to a porcelain boat.

[0082] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 1000℃ at a heating rate of 5℃ / min under an argon-ammonia atmosphere with an ammonia content of 95%, hold it at that temperature for 3 hours, and then cool it to room temperature.

[0083] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, and filter.

[0084] 4) The reaction product obtained in step 3) is centrifuged and then vacuum dried for 12 hours.

[0085] Figure 10 The image shown is a scanning electron microscope image of the final product of this comparative example. It can be seen that the two-dimensional morphology is destroyed when the temperature is too high.

[0086] Example 2

[0087] A method for preparing two-dimensional niobium nitride nanosheets using commercial niobium pentoxide includes the following steps:

[0088] 1) Weigh out niobium pentoxide powder and anhydrous potassium carbonate in a molar ratio of 1:3 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to niobium pentoxide of 16:1, add it, grind and mix it evenly, and transfer it to a porcelain boat.

[0089] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 950°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95%, hold it at that temperature for 3 hours, and then cool it to room temperature.

[0090] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, and filter.

[0091] 4) Centrifuge the reaction product obtained in step 3), take the upper layer, and finally vacuum dry for 12 hours.

[0092] The products obtained in this embodiment were analyzed by SEM and X-ray diffraction, and the results are shown in the figures below. Figure 11 (a), 11(b), the results show that the diffraction peak positions of the obtained product are consistent with those of the standard card Nb5N6 PDF#74-0607 and NbN .801 The two-dimensional Nb#74-0780 is basically consistent with the original, with no other redundant diffraction peaks, and it can still maintain a two-dimensional morphology, indicating that appropriately increasing the temperature can still yield two-dimensional Nb with good morphology. x N y .

[0093] Example 3

[0094] A method for preparing two-dimensional niobium nitride nanosheets using commercial niobium pentoxide includes the following steps:

[0095] 1) Weigh out niobium pentoxide powder and potassium carbonate powder in a molar ratio of 1:1 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to niobium pentoxide of 4:1, add it, grind and mix it evenly, and transfer it to a porcelain boat;

[0096] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 850°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95% and hold it for 3 hours, then cool it to room temperature.

[0097] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, and filter.

[0098] 4) The reaction product obtained in step 3) is centrifuged and then vacuum dried for 12 hours.

[0099] Figure 12 (a) is a scanning electron microscope image of the final product of this embodiment. It can be seen that appropriately reducing the ratio of potassium carbonate, niobium pentoxide and potassium chloride can also yield niobium nitride with a two-dimensional morphology.

[0100] Example 4

[0101] A method for preparing two-dimensional niobium nitride nanosheets using commercial niobium pentoxide includes the following steps:

[0102] 1) Weigh out niobium pentoxide powder and potassium carbonate powder in a molar ratio of 1:3 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to niobium pentoxide of 16:1, add it, grind and mix it evenly, and transfer it to a porcelain boat.

[0103] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 850°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 5%, hold it at that temperature for 3 hours, and then cool it to room temperature.

[0104] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, and filter.

[0105] 4) The reaction product obtained in step 3) is centrifuged and then vacuum dried for 12 hours.

[0106] Figure 12 (b) is a scanning electron microscope image of the final product of this embodiment. It can be seen that even with a reaction atmosphere of low ammonia concentration, niobium nitride with a two-dimensional morphology can still be obtained.

[0107] Example 5

[0108] A method for preparing two-dimensional niobium nitride nanosheets using commercial niobium pentoxide includes the following steps:

[0109] 1) Weigh out niobium pentoxide powder and potassium hydroxide powder in a molar ratio of 1:6 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to niobium pentoxide of 16:1, add it, grind and mix it evenly, and transfer it to a porcelain boat.

[0110] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 850°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95% and hold it for 3 hours, then cool it to room temperature.

[0111] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, and filter.

[0112] 4) The reaction product obtained in step 3) is centrifuged and then vacuum dried for 12 hours.

[0113] Figure 13 The image shown is a scanning electron microscope image of the final product of this embodiment. It can be seen that alkali metal hydroxides can play the same role as alkali metal carbonates, and can also produce two-dimensional niobium nitride with good morphology.

[0114] Example 6

[0115] A method for preparing two-dimensional niobium nitride nanosheets using commercial niobium pentoxide includes the following steps:

[0116] 1) Weigh out niobium pentoxide powder and potassium carbonate powder in a molar ratio of 1:3 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of magnesium chloride to niobium pentoxide of 16:1, add it, grind and mix it evenly, and transfer it to a porcelain boat.

[0117] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 850°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95% and hold it for 3 hours, then cool it to room temperature.

[0118] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, and filter.

[0119] 4) The reaction product obtained in step 3) is centrifuged and then vacuum dried for 12 hours.

[0120] Figure 14 The image shown is a scanning electron microscope image of the final product of this embodiment. It can be seen that when the metal halide is an alkaline earth metal halide, niobium nitride with a two-dimensional morphology can also be obtained.

[0121] Example 7

[0122] A method for preparing two-dimensional niobium nitride nanosheets using commercial niobium pentoxide includes the following steps:

[0123] 1) Weigh out niobium pentoxide powder and potassium carbonate powder in a molar ratio of 1:3 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to niobium pentoxide of 16:1, add it, grind and mix it evenly, and transfer it to a porcelain boat.

[0124] 2) Place the ceramic boat from step 1) into a tube furnace and heat it to 850°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95%. Hold the temperature for 0 min, 30 min, 60 min, 120 min, and 180 min respectively, and then cool it to room temperature.

[0125] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, and filter.

[0126] 4) The reaction product obtained in step 3) is centrifuged and then vacuum dried for 12 hours.

[0127] Figure 15 The XRD pattern of the final product in this embodiment shows that the formation process of niobium nitride is a process in which niobium pentoxide, which has Lewis acidity, preferentially reacts with potassium oxide, which is decomposed from potassium carbonate, to generate an anisotropic intermediate product KNbO3, followed by a gradual nitridation transformation of KNbO3.

[0128] Figure 16 (a) is the XRD pattern of the rapidly cooled product in this embodiment. Figure 16 (b) is a scanning electron microscope image of the rapidly cooled product in this embodiment, illustrating that niobium pentoxide preferentially reacts with alkaline potassium carbonate to form potassium niobate during the reaction. The niobium source for niobium nitride comes entirely from the intermediate product potassium niobate, which has a three-dimensional cubic morphology. After 180 min of reaction, potassium niobate is completely nitrided to obtain pure two-dimensional Nb. x N y .

[0129] Comparative Example 3

[0130] 1) Weigh out KNbO3 and potassium chloride in a molar ratio of 1:16, grind them thoroughly in a mortar, and then transfer them to a porcelain boat;

[0131] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 850°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95% and hold it for 3 hours, then cool it to room temperature.

[0132] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, filter, and finally vacuum dry for 12 hours.

[0133] Figure 17 The image shows a scanning electron microscope (SEM) image of the final product of this comparative example, illustrating that direct thermal nitriding of the KNbO3 and KCl mixture cannot lead to the evolution of a two-dimensional morphology.

[0134] Example 8

[0135] 1) Weigh out KNbO3 and potassium chloride in a molar ratio of 1:16, grind them thoroughly in a mortar, and then transfer them to a porcelain boat;

[0136] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 850°C in an Ar protective atmosphere at a heating rate of 5°C / min and hold it for 1 hour, then switch to ammonia gas and hold for 3 hours, and finally cool it to room temperature.

[0137] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, filter, and finally vacuum dry for 12 hours.

[0138] Figure 18 The image shown is a scanning electron microscope image of the final product in this embodiment. When KNbO3 and KCl are first eutectic and then thermally nitrided, niobium nitride with a two-dimensional morphology can be generated, indicating that the eutectic state of KNbO3 and KCl is an important basis for the two-dimensional growth of niobium nitride.

[0139] Comparative Example 4

[0140] 1) Weigh out niobium pentoxide powder and potassium carbonate powder in a molar ratio of 1:3, grind them thoroughly in a mortar, and then transfer them to a porcelain boat;

[0141] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 850°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95%, hold it at that temperature for 3 hours, and then cool it to room temperature.

[0142] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, and filter.

[0143] 4) The reaction product obtained in step 3) is centrifuged and then vacuum dried for 12 hours.

[0144] Figure 19 (a) is a scanning electron microscope image of the final product of this comparative example. Figure 19 (b) shows the XRD pattern of the final product of this comparative example, which shows that two-dimensional niobium nitride cannot be generated when potassium carbonate is used as an auxiliary agent.

[0145] Comparative Example 5

[0146] 1) Take niobium pentoxide powder and potassium chloride powder at a molar ratio of 1:16, grind them thoroughly in a mortar, and then transfer them to a porcelain boat;

[0147] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 850°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95% and hold it for 3 hours, then cool it to room temperature.

[0148] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, and filter.

[0149] 4) The reaction product obtained in step 3) is centrifuged and then vacuum dried for 12 hours.

[0150] Figure 20 (a) is a scanning electron microscope image of the final product of this comparative example. Figure 20 (b) is the XRD pattern of the final product of this comparative example. It can be seen that two-dimensional niobium nitride cannot be generated when potassium chloride is used as an auxiliary agent.

[0151] Example 9

[0152] 1) Weigh out niobium pentoxide powder and potassium carbonate powder in a molar ratio of 1:6 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to niobium pentoxide of 50:1, add it, grind and mix it evenly, and transfer it to a porcelain boat;

[0153] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 850°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95% and hold it for 3 hours, then cool it to room temperature.

[0154] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, and filter.

[0155] 4) The reaction product obtained in step 3) is centrifuged and then vacuum dried for 12 hours.

[0156] Figure 21 (a) is a scanning electron microscope image of the final product of this embodiment. By appropriately increasing the proportion of potassium chloride, two-dimensional niobium nitride is still generated.

[0157] Example 10

[0158] 1) Weigh out niobium pentoxide powder and potassium carbonate powder in a molar ratio of 1:6 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to niobium pentoxide of 50:1, add it, grind and mix it evenly, and transfer it to a porcelain boat;

[0159] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 820°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95%, hold it at that temperature for 8 hours, and then cool it to room temperature.

[0160] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, and filter.

[0161] 4) The reaction product obtained in step 3) is centrifuged and then vacuum dried for 12 hours.

[0162] Figure 21 (b) is a scanning electron microscope image of the final product of this embodiment. By appropriately extending the reaction time, two-dimensional niobium nitride is still generated.

[0163] Example 11

[0164] A method for preparing two-dimensional tungsten nitride nanosheets using commercial tungsten trioxide includes the following steps:

[0165] 1) Weigh out tungsten trioxide powder and potassium carbonate powder in a molar ratio of 1:3 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to tungsten trioxide of 16:1, add it, grind and mix it evenly, and transfer it to a porcelain boat.

[0166] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 800°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95%, hold it at that temperature for 3 hours, and then cool it to room temperature.

[0167] 3) The reaction product obtained in step 2) was added to 1M HCl solution and washed and stirred for 2 hours. Then, it was repeatedly washed with deionized water until the pH reached 7. After filtration and centrifugation, it was finally vacuum dried for 12 hours to obtain a well-dispersed two-dimensional W. 4.6 N4 nanosheets.

[0168] The product obtained in this embodiment was analyzed by scanning electron microscopy, and the results are shown in the figure. Figure 22 (a) It can be seen that the two-dimensional W can be obtained by using the method described above in this invention. 4.6 N4 nanosheets. Figure 22 (b) X-ray diffraction analysis of the final product of this embodiment shows that the final product is a pure two-dimensional W 4.6 N4.

[0169] Example 12

[0170] A method for preparing two-dimensional tungsten nitride nanosheets using commercial tungsten trioxide includes the following steps:

[0171] 1) Weigh out tungsten trioxide powder and potassium carbonate powder in a molar ratio of 1:1 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to tungsten trioxide of 50:1, add it, grind and mix it evenly, and transfer it to a porcelain boat.

[0172] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 950°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95%, hold it at that temperature for 3 hours, and then cool it to room temperature.

[0173] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, filter, centrifuge, and finally vacuum dry for 12 hours.

[0174] Figure 23 (a) is a scanning electron microscope image of the final product of this embodiment. It can be seen that by appropriately adjusting the ratio of potassium carbonate to tungsten trioxide and the nitriding temperature, two-dimensional tungsten nitride nanosheets can still be obtained.

[0175] Example 13

[0176] A method for preparing two-dimensional tungsten nitride nanosheets using commercial tungsten trioxide includes the following steps:

[0177] 1) Weigh out tungsten trioxide powder and potassium carbonate powder in a molar ratio of 1:6.5 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to tungsten trioxide of 50:1, add it, grind and mix it evenly, and transfer it to a porcelain boat.

[0178] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 680°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95%, hold it at that temperature for 3 hours, and then cool it to room temperature.

[0179] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, filter, centrifuge, and finally vacuum dry for 12 hours.

[0180] Figure 23 (b) is a scanning electron microscope image of the final product of this embodiment. It can be seen that by appropriately increasing the proportion of potassium carbonate and lowering the nitriding temperature, two-dimensional tungsten nitride nanosheets can still be obtained.

[0181] Example 14

[0182] A method for preparing two-dimensional tungsten nitride nanosheets using commercial tungsten trioxide includes the following steps:

[0183] 1) Weigh out tungsten trioxide powder and potassium carbonate powder in a molar ratio of 1:1 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to tungsten trioxide of 10:1, add it, grind and mix it evenly, and transfer it to a porcelain boat;

[0184] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 850°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95%, hold it at that temperature for 7 hours, and then cool it to room temperature.

[0185] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, filter, centrifuge, and finally vacuum dry for 12 hours.

[0186] Figure 24 (a) is a scanning electron microscope image of the final product of this embodiment. It can be seen that by appropriately changing the salt ratio and extending the nitriding time, two-dimensional tungsten nitride nanosheets can still be obtained.

[0187] Example 15

[0188] A method for preparing two-dimensional tungsten nitride nanosheets using commercial tungsten trioxide includes the following steps:

[0189] 1) Weigh out tungsten trioxide powder and potassium hydroxide powder in a molar ratio of 1:3 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to tungsten trioxide of 16:1, add it, grind and mix it evenly, and transfer it to a porcelain boat;

[0190] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 800°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95%, hold it at that temperature for 3 hours, and then cool it to room temperature.

[0191] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, filter, centrifuge, and finally vacuum dry for 12 hours.

[0192] Figure 24 (b) is a scanning electron microscope image of the final product of this embodiment. It can be seen that alkali metal hydroxides and alkali metal carbonates have the same effect and can both promote the formation of two-dimensional tungsten nitride nanosheets.

[0193] Example 16

[0194] A method for preparing two-dimensional tungsten nitride nanosheets using commercial tungsten trioxide includes the following steps:

[0195] 1) Weigh out tungsten trioxide powder and potassium carbonate powder in a molar ratio of 1:3 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of magnesium chloride to tungsten trioxide of 16:1, add it, grind and mix it evenly, and transfer it to a porcelain boat.

[0196] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 800°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95%, hold it at that temperature for 3 hours, and then cool it to room temperature.

[0197] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, filter, centrifuge, and finally vacuum dry for 12 hours.

[0198] Figure 24 (c) is a scanning electron microscope image of the final product of this embodiment. It can be seen that both alkali metal halides and alkaline earth metal halides can promote the formation of two-dimensional tungsten nitride nanosheets.

[0199] Comparative Example 6

[0200] A method for preparing two-dimensional tungsten nitride nanosheets using commercial tungsten trioxide includes the following steps:

[0201] 1) Weigh out tungsten trioxide powder and potassium carbonate powder in a molar ratio of 2:1 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to tungsten trioxide of 16:1, add it, grind and mix it evenly, and transfer it to a porcelain boat.

[0202] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 800°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95%, hold it at that temperature for 3 hours, and then cool it to room temperature.

[0203] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, filter, centrifuge, and finally vacuum dry for 12 hours.

[0204] Figure 25 (a) is a scanning electron microscope image of the final product of this comparative example. It can be seen that when the amount of potassium carbonate added is too low, two-dimensional tungsten nitride cannot be generated.

[0205] Comparative Example 7

[0206] A method for preparing two-dimensional tungsten nitride nanosheets using commercial tungsten trioxide includes the following steps:

[0207] 1) Weigh out tungsten trioxide powder and potassium carbonate powder in a molar ratio of 1:3 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to tungsten trioxide of 16:1, add it, grind and mix it evenly, and transfer it to a porcelain boat.

[0208] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 980°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95%, hold it at that temperature for 3 hours, and then cool it to room temperature.

[0209] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, filter, centrifuge, and finally vacuum dry for 12 hours.

[0210] Figure 25 (b) is a scanning electron microscope image of the final product of this comparative example, which shows that the two-dimensional tungsten nitride nanosheets are destroyed when the nitriding temperature is too high.

[0211] Example 17

[0212] A method for preparing two-dimensional molybdenum nitride nanosheets using commercial molybdenum trioxide includes the following steps:

[0213] 1) Weigh molybdenum trioxide powder and potassium carbonate powder in a molar ratio of 1:3 and grind them thoroughly in a mortar; then weigh potassium chloride according to the requirement of a molar ratio of potassium chloride to molybdenum trioxide of 16:1, add it, grind and mix it evenly, and transfer it to a porcelain boat.

[0214] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 750°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95%, hold it at that temperature for 3 hours, and then cool it to room temperature.

[0215] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, and filter.

[0216] 4) The reaction product obtained in step 3) is centrifuged and then vacuum dried for 12 hours to obtain two-dimensional molybdenum nitride nanosheets with good dispersibility.

[0217] Figure 26 (a) is a scanning electron microscope image of the final product of this embodiment. It can be seen that the method described above can also obtain two-dimensional MoN nanosheets. X-ray diffraction analysis of the product obtained in this embodiment is shown in the figure. Figure 26 (b) The results show that the final product obtained after acid washing and centrifugal drying via molten salt reaction is pure two-dimensional MoN.

[0218] Example 18

[0219] A method for preparing two-dimensional molybdenum nitride nanosheets using commercial molybdenum trioxide includes the following steps:

[0220] 1) Weigh out molybdenum trioxide powder and potassium carbonate powder in a molar ratio of 1:1 and grind them thoroughly in a mortar; then weigh out potassium chloride according to the requirement of a molar ratio of potassium chloride to molybdenum trioxide of 50:1, add it, grind and mix it evenly, and transfer it to a porcelain boat;

[0221] 2) Place the ceramic boat from step 1) into a tube furnace, heat it to 600°C at a heating rate of 5°C / min under an argon-ammonia atmosphere with an ammonia content of 95%, hold it at that temperature for 3 hours, and then cool it to room temperature.

[0222] 3) Add the reaction product obtained in step 2) to 1M HCl solution for washing and stirring for 2 hours, then wash repeatedly with deionized water until the pH is 7, and filter.

[0223] 4) The reaction product obtained in step 3) is centrifuged and then vacuum dried for 12 hours.

[0224] Figure 27 The image shown is a scanning electron microscope image of the final product of this embodiment. It can be seen that by appropriately adjusting the salt ratio and nitriding temperature, two-dimensional molybdenum nitride nanosheets can still be obtained.

[0225] The above embodiments are merely illustrative examples and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations; therefore, any obvious variations or modifications derived therefrom remain within the scope of protection of this invention.

Claims

1. A method for preparing two-dimensional metal nitrides using bulk metal oxide powder, characterized in that, Includes the following steps: 1) Grind and mix the metal oxide powder with alkali metal carbonate or alkali metal hydroxide until uniform; then add neutral alkali metal or alkaline earth metal halide salt, and grind and mix again until uniform; 2) The mixture obtained in step 1) is heated under an ammonia atmosphere to carry out a heat-holding reaction, and then cooled; 3) The reaction product obtained in step 2) was acid-washed, water-washed until neutral, filtered, centrifuged, and dried to obtain nanosheet-like two-dimensional metal nitrides. The metal oxide powder is niobium pentoxide, molybdenum trioxide, or tungsten trioxide. The alkali metal carbonate or alkali metal hydroxide is selected from K2CO3; The neutral alkali metal or alkaline earth metal halide salt is KCl; When the metal oxide is niobium pentoxide, the molar ratio of niobium pentoxide to K2CO3 is 1:(1.0-6.0), the molar ratio of KCl to niobium pentoxide is (4-50):1, the holding temperature is 820-950 ℃, and the holding time is 3-8 h; When the metal oxide is tungsten trioxide, the molar ratio of tungsten trioxide to K2CO3 is 1:(1.0-6.5), the molar ratio of KCl to tungsten trioxide is (5-50):1, the holding temperature is 660-950 ℃, and the holding time is 1-7 h; When the metal oxide is molybdenum trioxide, the molar ratio of molybdenum trioxide to K2CO3 is 1:(1.0-6.5), the molar ratio of KCl to molybdenum trioxide is (5-50):1, the reaction temperature is 550-950 ℃, and the holding time is 1-7 h.

2. The method according to claim 1, characterized in that, The metal oxide powder has a size of 0.5-80 µm.

3. The method according to claim 1, characterized in that, The ammonia atmosphere is a mixture of nitrogen and ammonia or argon and ammonia or pure ammonia, with an ammonia content of 5-100%.

4. The method according to claim 1, characterized in that, The pickling step uses a mixture of one or more of the following acids: HCl, H2SO4, acetic acid, or formic acid.

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