A method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion

Abstract

This invention belongs to the field of nanolayered materials technology, specifically a method for preparing monolayer or few-layer MXene materials using organic-assisted perchloric acid explosion. It combines acid etching and explosion to achieve exfoliation, using organic-assisted perchloric acid to generate an explosive reaction, allowing for layered exfoliation under the impact of the explosion while simultaneously undergoing acid etching. This method for preparing monolayer or few-layer MXene materials not only features short reaction time, high etching efficiency, and low cost, but also produces high yields of monolayer or few-layer MXene materials with excellent properties. The operation is simple, requiring no complex equipment, and is conducive to large-scale production.

Description

Technical Field

[0001] This invention relates to the field of nanolayered materials technology, specifically to a method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion. Background Technology

[0002] MXenes, a novel two-dimensional material discovered at Drexel University in 2011, are two-dimensional layered materials. Like graphene, MXenes are structurally two-dimensional and layered, possessing not only the high specific surface area and high electrical conductivity of graphene but also advantages such as tunable nanolayer thickness and the combination of electrical conductivity and hydrophilicity. Generally, MXenes are prepared using either a top-down or bottom-up approach. The currently common method is the top-down strategy, which forms the layered structure of MXenes by etching the "A" element of the precursor MAX phase. The precursor MAX phase is a three-dimensional layered carbide or nitride with the general chemical formula M. n+1 AX n In this context, M is a pre-transition metal (such as Sc, V, Ti, Zr, and Cr), A is a main group element (such as Al, Si, or Ga), X is carbon, nitrogen, or carbon-nitrogen, and the value of n determines the number of M layers. The chemical formula of the etched two-dimensional layered MXenes material is M0. n+1 X n T x (n = 1, 2, or 3), where T is a surface group (such as O, OH, or F), depending on the reaction solution. The MAX phase belongs to a close-packed hexagonal crystal system, in which A-layer atoms exist in alternating layers formed by M and X in a manner similar to intercalation. MA bonds are weaker than MX bonds because MX bonds possess strong bonds such as covalent, metallic, and ionic bonds, while MA bonds are weaker bonds with only metallic properties. Therefore, this provides a theoretical basis for etching away A-layer atoms. In recent years, MXenes materials have developed rapidly in various fields, such as energy storage, photoelectrocatalysis, sensors, and electromagnetic shielding, and the preparation processes have become increasingly diverse. From the HF solution used when it was first discovered to the various methods and processes used to improve the performance of related applications through molten salts, in-situ etching, high-temperature alkaline solutions, Lewis acid etching, chemical vapor deposition (CVD), and electrochemical methods.

[0003] Multilayer MXenes obtained after selective etching can be transformed into single-layer or few-layer MXene materials through molecular intercalation mechanical exfoliation. However, the intercalating agents commonly used are toxic organic compounds, such as dimethyl sulfoxide (Nano-Micro Letters, 14(1), 213), hydrazine hydrate (ACS Nano 2021, 15, 4, 6420–6429), and tetrabutylammonium hydroxide (Angew. Chem. Int. Ed. 2016, 55, 1-7). Furthermore, DMSO exhibits some selectivity for layered materials; for example, DMSO is selective for materials other than Ti3C2T. x Other MXenes did not show significant intercalation effects (ACS Nano 2015, 9, 10, 9507–9516).

[0004] Monolayer or few-layer MXene materials, with their larger specific surface area and better hydrophilicity, and their potential applications in photoelectrocatalysis and adsorption, have attracted widespread attention from researchers. However, conventional chemical and physical methods are insufficient for the direct preparation of monolayer or few-layer MXene materials. Therefore, developing a simple, efficient, low-cost, low-requirement, and reproducible method for directly obtaining monolayer or few-layer MXene materials remains a current research challenge. Summary of the Invention

[0005] To address the problems existing in the prior art, the main objective of this invention is to propose a method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion.

[0006] To address the aforementioned technical problems, according to one aspect of the present invention, the present invention provides the following technical solution:

[0007] A method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion includes the following steps:

[0008] S1. Mix the MAX phase material powder with perchloric acid to obtain a mixed slurry, and add it to a high-pressure reactor;

[0009] S2. Add organic auxiliary agent to the mixed slurry in the high-pressure reactor, then evacuate the high-pressure reactor and introduce argon gas, and heat it to 125-135℃ to cause an explosive reaction.

[0010] S3. After cooling the explosion reaction products to room temperature, freeze-dry them to obtain single-layer or few-layer MXene materials.

[0011] As a preferred embodiment of the method for preparing single-layer or few-layer MXene materials by organic-assisted perchloric acid explosion according to the present invention, in step S1, the MAX phase material is one of Ti3AlC2, Ti2AlC, Nb2AlC, V2AlC, Cr2AlC, and Ta4AlC3.

[0012] In a preferred embodiment of the method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion according to the present invention, the concentration of perchloric acid in step S1 is 50-60 wt%.

[0013] As a preferred embodiment of the method for preparing single-layer or few-layer MXene materials by organic-assisted perchloric acid explosion according to the present invention, in step S1, the MAX phase material is ground and passed through a 200-mesh sieve to obtain MAX phase material powder.

[0014] As a preferred embodiment of the method for preparing single-layer or few-layer MXene materials by organic-assisted perchloric acid explosion according to the present invention, in step S1, the mass ratio of MAX phase material powder to perchloric acid is 1:(0.5~1).

[0015] As a preferred embodiment of the method for preparing single-layer or few-layer MXene materials by organic-assisted perchloric acid explosion according to the present invention, in step S2, the organic auxiliary is one of the combustible organic compounds including ethanol, acetone, and toluene.

[0016] In a preferred embodiment of the method for preparing single-layer or few-layer MXene materials by organic-assisted perchloric acid explosion according to the present invention, in step S2, the mass ratio of the mixed slurry to the organic auxiliary is 1:(5-10).

[0017] As a preferred embodiment of the method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion according to the present invention, the centrifugation speed is 2500-3000 rpm and the centrifugation time is 5-10 min.

[0018] As a preferred embodiment of the method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion according to the present invention, the freeze-drying is carried out in a vacuum environment with a vacuum pressure ≤10Pa.

[0019] As a preferred embodiment of the method for preparing single-layer or few-layer MXene materials by organic-assisted perchloric acid explosion according to the present invention, the freeze-drying temperature is -90 to -100°C and the freeze-drying time is 24 to 48 hours.

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

[0021] This invention proposes a method for preparing monolayer or few-layer MXene materials using organic-assisted perchloric acid explosion. The method combines acid etching and explosion to achieve exfoliation. Organic-assisted perchloric acid is used to generate an explosive reaction, allowing for layered exfoliation under the impact of the explosion during acid etching. This method for preparing monolayer or few-layer MXene materials not only features short reaction time, high etching efficiency, and low cost, but also produces high yields of monolayer or few-layer MXene materials with excellent properties. The operation is simple, requires no complex equipment, and is conducive to large-scale production. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0023] Figure 1 This is a SEM image of the MAX phase Ta4AlC3.

[0024] Figure 2 SEM images of single-layer or few-layer MXene materials prepared in Example 1 of this invention.

[0025] Figure 3 This is a TEM image of a single-layer or few-layer MXene material prepared in Example 1 of the present invention.

[0026] Figure 4 The image shows the Raman diagram of the monolayer or few-layer MXene material prepared in Example 1 of this invention.

[0027] Figure 5 This is a SEM image of the material prepared in Comparative Example 1 of this invention.

[0028] Figure 6 This is a SEM image of the material prepared in Comparative Example 2 of this invention.

[0029] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0030] The technical solutions described below in conjunction with the embodiments will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the 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.

[0031] According to one aspect of the present invention, the present invention provides the following technical solution:

[0032] A method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion includes the following steps:

[0033] S1. Mix the MAX phase material powder with perchloric acid to obtain a mixed slurry, and add it to a high-pressure reactor;

[0034] S2. Add organic auxiliary agent to the mixed slurry in the high-pressure reactor, then evacuate the high-pressure reactor and introduce argon gas, and heat it to 125-135℃ to cause an explosive reaction.

[0035] S3. After cooling the explosion reaction products to room temperature, freeze-dry them to obtain single-layer or few-layer MXene materials.

[0036] Preferably, in step S1, the MAX phase material is one of Ti3AlC2, Ti2AlC, Nb2AlC, V2AlC, Cr2AlC, and Ta4AlC3.

[0037] Preferably, in step S1, the concentration of perchloric acid is 50-60 wt%. Specifically, the concentration of perchloric acid can be any one or a range between any two of, for example, 50 wt%, 51 wt%, 52 wt%, 53 wt%, 54 wt%, 55 wt%, 56 wt%, 57 wt%, 58 wt%, 59 wt%, and 60 wt%.

[0038] Preferably, in step S1, the MAX phase material is ground and passed through a 200-mesh sieve to obtain MAX phase material powder.

[0039] Preferably, in step S1, the mass ratio of MAX phase material powder to perchloric acid is 1:(0.5-1). Specifically, the mass ratio of MAX phase material powder to perchloric acid can be any one of, or any combination thereof, such as 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1.

[0040] Preferably, in step S2, the organic auxiliary is one of the combustible organic compounds including ethanol, acetone, and toluene.

[0041] Preferably, in step S2, the mass ratio of the mixed slurry to the organic additive is 1:(5-10). Specifically, the mass ratio of the mixed slurry to the organic additive can be any one of, for example, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, or any range between two of them.

[0042] Preferably, the centrifugation speed is 2500–3000 rpm, and the centrifugation time is 5–10 min. Specifically, the centrifugation speed can be any one or a range between any two of, for example, 2500 rpm, 2600 rpm, 2700 rpm, 2800 rpm, 2900 rpm, and 3000 rpm; the centrifugation time can be any one or a range between any two of, for example, 5 min, 6 min, 7 min, 8 min, 9 min, and 10 min.

[0043] Preferably, in step S2, liquid nitrogen is used for quick freezing before freeze drying.

[0044] Preferably, in step S2, the freeze-drying is carried out under vacuum, with a vacuum pressure ≤10Pa.

[0045] Preferably, in step S2, the freeze-drying temperature is -90 to -100°C, and the freeze-drying time is 24 to 48 hours. Specifically, the freeze-drying temperature can be any one or a range between any two of, for example, -90°C, -92°C, -94°C, -96°C, -98°C, and -100°C, and the freeze-drying time can be any one or a range between any two of, for example, 24 hours, 30 hours, 36 hours, 42 hours, and 48 hours.

[0046] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0047] Example 1

[0048] A method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion includes the following steps:

[0049] S1. Take 20g of MAX phase Ta4AlC3 material powder (its SEM image is shown below). Figure 1 (As shown) and 50wt% perchloric acid are mixed at a mass ratio of 1:1 and stirred continuously for 1 hour to obtain a mixed slurry, which is then added to a high-pressure reactor;

[0050] S2. Add a 90wt% ethanol solution to the mixed slurry in the high-pressure reactor at a mass ratio of 1:10. Then, evacuate the high-pressure reactor and introduce argon gas. Heat the reactor to 130°C to cause an explosive reaction.

[0051] S3. After cooling the explosion reaction products to room temperature, remove them, wash until neutral, and then centrifuge multiple times at 3000 rpm for a total of 30 min. The supernatant after centrifugation is first rapidly frozen with liquid nitrogen, then freeze-dried at -95°C under a vacuum of 1 Pa for 36 h to obtain a monolayer or few-layer Ta4C3T. xMXenes material.

[0052] The monolayer or few-layer Ta4C3T prepared in this embodiment x SEM images of MXenes materials are shown below. Figure 2 As shown, the TEM image is as follows Figure 3 As shown, the Raman diagram is as follows: Figure 4 As shown. By Figure 2 It can be seen that the Ta4C3T reaction after the use of organic matter to assist the perchloric acid explosion reaction... x MXene materials exhibit a few-layer thin-film structure; by Figure 3 It can be seen that the Ta4C3T reaction after the use of organic matter to assist the perchloric acid explosion reaction... x The nanosheets of MXene material are relatively thin; Figure 4 It can be seen that the Ta4C3T reaction after the use of organic matter to assist the perchloric acid explosion reaction... x MXene nanosheets include Ta4C3T x The characteristic peak of MXene has a shift difference of 50, indicating a few-layer structure.

[0053] Example 2

[0054] A method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion includes the following steps:

[0055] S1. Mix 20g of MAX phase Ta4AlC3 material powder with 50wt% perchloric acid at a mass ratio of 1:0.8, stir continuously for 1 hour to obtain a mixed slurry, and add it to a high-pressure reactor.

[0056] S2. Add a 90wt% ethanol solution to the mixed slurry in the high-pressure reactor at a mass ratio of 1:5. Then, evacuate the high-pressure reactor and introduce argon gas. Heat the reactor to 130°C to cause an explosive reaction.

[0057] S3. After cooling the explosion reaction products to room temperature, remove them, wash until neutral, and then centrifuge multiple times at 3000 rpm for a total of 30 min. The supernatant after centrifugation is first rapidly frozen with liquid nitrogen, then freeze-dried at -94°C under a vacuum of 1 Pa for 42 h to obtain a monolayer or few-layer Ta4C3T. x MXenes material.

[0058] Example 3

[0059] A method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion includes the following steps:

[0060] S1. Mix 20g of MAX phase Ta4AlC3 material powder with 60wt% perchloric acid at a mass ratio of 1:0.5, stir continuously for 1h to obtain a mixed slurry, and add it to a high-pressure reactor.

[0061] S2. Add a 90wt% ethanol solution to the mixed slurry in the high-pressure reactor at a mass ratio of 1:8. Then, evacuate the high-pressure reactor and introduce argon gas. Heat the reactor to 125°C to induce an explosive reaction.

[0062] S3. After cooling the explosion reaction products to room temperature, remove them, wash until neutral, and then centrifuge multiple times at 3000 rpm for a total of 30 min. The supernatant after centrifugation is first rapidly frozen with liquid nitrogen, then freeze-dried at -95°C under a vacuum of 1 Pa for 48 h to obtain a monolayer or few-layer Ta4C3T. x MXenes material.

[0063] Example 4

[0064] A method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion includes the following steps:

[0065] S1. Mix 20g of MAX phase Ta4AlC3 material powder with 60wt% perchloric acid at a mass ratio of 1:0.5, stir continuously for 1h to obtain a mixed slurry, and add it to a high-pressure reactor.

[0066] S2. Add a 90wt% ethanol solution to the mixed slurry in the high-pressure reactor at a mass ratio of 1:5. Then, evacuate the high-pressure reactor and introduce argon gas. Heat the reactor to 135°C to induce an explosive reaction.

[0067] S3. After cooling the explosion reaction products to room temperature, remove them, wash until neutral, and then centrifuge multiple times at 3000 rpm for a total of 30 min. The supernatant after centrifugation is first rapidly frozen with liquid nitrogen, then freeze-dried at -96°C under a vacuum of 1 Pa for 32 h to obtain a monolayer or few-layer Ta4C3T. x MXenes material.

[0068] Example 5

[0069] A method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion includes the following steps:

[0070] S1. Mix 20g of MAX phase Ti3AlC2 material powder with 50wt% perchloric acid at a mass ratio of 1:1, stir continuously for 1h to obtain a mixed slurry, and add it into a high-pressure reactor.

[0071] S2. Add acetone solution with a concentration of 30wt% to the mixed slurry in the high-pressure reactor at a mass ratio of 1:5. Then, evacuate the high-pressure reactor and introduce argon gas, and heat it to 128°C to cause an explosive reaction.

[0072] S3. After cooling the explosion reaction products to room temperature, remove them, wash until neutral, and then centrifuge multiple times at 3000 rpm for a total of 48 min. The supernatant after centrifugation is first rapidly frozen with liquid nitrogen, then freeze-dried at -90°C under a vacuum of 1 Pa for 28 h to obtain monolayer or few-layer Ti3C2T. x MXenes material.

[0073] Example 6

[0074] A method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion includes the following steps:

[0075] S1. Mix 30g of MAX phase Ti2AlC material powder with 60wt% perchloric acid at a mass ratio of 1:0.8, stir continuously for 1 hour to obtain a mixed slurry, and add it to a high-pressure reactor.

[0076] S2. Add a 50wt% toluene solution to the mixed slurry in the high-pressure reactor at a mass ratio of 1:8. Then, evacuate the high-pressure reactor and introduce argon gas, and heat it to 130°C to cause an explosive reaction.

[0077] S3. After cooling the explosion reaction products to room temperature, remove them, wash until neutral, and then centrifuge multiple times at 3000 rpm for a total of 24 min. The supernatant after centrifugation is first rapidly frozen with liquid nitrogen, then freeze-dried at -92°C under a vacuum of 1 Pa for 24 h to obtain monolayer or few-layer Ti2CT. x MXenes material.

[0078] Example 7

[0079] A method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion includes the following steps:

[0080] S1. Mix 20g of MAX phase Nb2AlC material powder with 60wt% perchloric acid at a mass ratio of 1:0.5, stir continuously for 1h to obtain a mixed slurry, and add it to a high-pressure reactor.

[0081] S2. Add a 90wt% ethanol solution to the mixed slurry in the high-pressure reactor at a mass ratio of 1:10. Then, evacuate the high-pressure reactor and introduce argon gas. Heat the reactor to 132°C to cause an explosive reaction.

[0082] S3. After cooling the explosion reaction products to room temperature, remove them, wash until neutral, and then centrifuge multiple times at 3000 rpm for a total of 30 min. The supernatant after centrifugation is first rapidly frozen with liquid nitrogen, then freeze-dried at -95°C under a vacuum of 1 Pa for 36 h to obtain monolayer or few-layer Nb2CT. x MXenes material.

[0083] Example 8

[0084] A method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion includes the following steps:

[0085] S1. Mix 20g of MAX phase V2AlC material powder with 60wt% perchloric acid at a mass ratio of 1:0.5, stir continuously for 1h to obtain a mixed slurry, and add it to a high-pressure reactor.

[0086] S2. Add a 90wt% ethanol solution to the mixed slurry in the high-pressure reactor at a mass ratio of 1:10. Then, evacuate the high-pressure reactor and introduce argon gas. Heat the reactor to 130°C to cause an explosive reaction.

[0087] S3. After cooling the explosion reaction products to room temperature, remove them, wash until neutral, and then centrifuge multiple times at 3000 rpm for a total of 30 min. The supernatant after centrifugation is first rapidly frozen with liquid nitrogen, then freeze-dried at -95°C under a vacuum of 1 Pa for 30 h to obtain monolayer or few-layer V2CT. x MXenes material.

[0088] Example 9

[0089] A method for preparing monolayer or few-layer MXene materials by organic-assisted perchloric acid explosion includes the following steps:

[0090] S1. Mix 20g of MAX phase Cr2AlC material powder with 60wt% perchloric acid at a mass ratio of 1:0.5, stir continuously for 1h to obtain a mixed slurry, and add it to a high-pressure reactor.

[0091] S2. Add a 90wt% ethanol solution to the mixed slurry in the high-pressure reactor at a mass ratio of 1:10. Then, evacuate the high-pressure reactor and introduce argon gas. Heat the reactor to 131°C to cause an explosive reaction.

[0092] S3. After cooling the explosion reaction products to room temperature, remove them, wash until neutral, and then centrifuge multiple times at 3000 rpm for a total of 30 min. The supernatant after centrifugation is first rapidly frozen with liquid nitrogen, then freeze-dried at -100°C under a vacuum of 1 Pa for 35 h to obtain monolayer or few-layer Cr2CT. x MXenes material.

[0093] Comparative Example 1

[0094] The difference from Example 1 is that 20g of MAX phase Ta4AlC3 material powder was mixed with 50% perchloric acid at a mass ratio of 1:0.2.

[0095] SEM images of the materials prepared in this comparative example are shown below. Figure 5 As shown, from Figure 5 It can be clearly observed that: the material prepared in Comparative Example 1 and the MAX phase Ta4AlC3 powder that did not undergo the etching reaction (such as...) Figure 1 As shown, they are almost indistinguishable, both exhibiting a tightly stacked morphology.

[0096] Comparative Example 2

[0097] The difference from Example 1 is that a 90wt% ethanol solution was added to the mixed slurry in the high-pressure reactor at a mass ratio of 1:4.

[0098] SEM images of the materials prepared in this comparative example are shown below. Figure 6 As shown, from Figure 6 It can be clearly observed that the material prepared in Comparative Example 2 has a cracked morphology, but it is different from the MAX phase Ta4AlC3 powder that did not undergo an etching reaction (such as...). Figure 1 (As shown) there is almost no difference.

[0099] As can be seen from the above examples and comparative examples, this invention combines acid etching and an explosive method to achieve exfoliation. Organic substances are used to assist perchloric acid in generating an explosive reaction, allowing for layered exfoliation under the impact of the explosion while acid etching is being performed. This method for preparing single-layer or few-layer MXene materials not only has a short reaction time, high etching efficiency, and low cost, but also produces high yields of single-layer or few-layer MXene materials with excellent properties. The operation is simple, requires no complex equipment, and is conducive to large-scale production.

[0100] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A method of organic assisted high-chlorate explosive preparation of single or few layer MXene material, characterized in that, The method comprises the following steps: S1, mixing MAX phase material powder with perchloric acid to obtain a mixed slurry, and adding the mixed slurry into a high-pressure reaction kettle; the concentration of the perchloric acid is 50-60 wt%, and the mass ratio of the MAX phase material powder to the perchloric acid is 1:(0.5-1); S2, adding an organic auxiliary agent into the mixed slurry in the high-pressure reaction kettle, then vacuumizing the high-pressure reaction kettle and introducing argon gas, and heating to make the high-pressure reaction kettle have an explosion reaction at 125-135 ℃; the organic auxiliary agent is one of combustible organic substances including ethanol, acetone and toluene; the mass ratio of the mixed slurry to the organic auxiliary agent is 1:(5-10); S3, freezing and drying the explosion reaction product after being cooled to room temperature to obtain a single-layer or few-layer MXene material.

2. The method of claim 1, wherein the method is assisted by an organic material. In the step S1, the MAX phase material is one of Ti3AlC2, Ti2AlC, Nb2AlC, V2AlC, Cr2AlC and Ta4AlC3.

3. The method of claim 1, wherein the method is assisted by organic matter. In the step S1, the MAX phase material is ground, and the MAX phase material powder is obtained after being sieved through a 200-mesh sieve.

4. The method of claim 1, wherein the method is assisted by organic matter. The freezing and drying is performed in a vacuum environment, and the vacuum pressure is ≤10 Pa.

5. The method of claim 1, wherein the method is assisted by organic matter. The freezing and drying temperature is -90 to -100 ℃, and the freezing and drying time is 24-48 h.

Images