A preparation method of MXene nanosheets

Abstract

The present invention relates to a method for preparing MXene nanosheets, which rapidly exfoliates through buffer-assisted ball milling and combines an acid / alkali solution etching strategy to prepare MXene nanosheets with high efficiency, high yield, and on a large scale. The method comprises the following steps: (1) mixing a ternary MAX-phase precursor material and an acid or alkali etching solution to obtain a mixed dispersion A; (2) mixing the dispersion A, a buffer-assisted grinding material, and ball milling balls, and performing ball milling to obtain a ball-milled dispersion B; (3) centrifugally washing the ball-milled dispersion B with deionized water to obtain a mixed precipitate C; (4) dispersing the mixed precipitate C in a dispersion solvent, performing ultrasonic or vortex oscillation, and centrifugally separating to obtain a MXene nanosheet dispersion. The preparation method realizes the simultaneous exfoliation and etching of the ternary MAX-phase material, achieving high-efficiency, high-yield, and large-scale preparation of large-size single-layer MXene nanosheets. The highest preparation yield reaches 96%, the single-layer rate exceeds 90%, and the preparation time is less than 24 hours.

Description

Technical Field

[0001] The present invention belongs to the technical field of nanomaterials, and particularly relates to a preparation method of MXene nanosheets; the preparation method is a high-yield preparation method of MXene nanosheets. Background Art

[0002] Two-dimensional transition metal carbides / nitrides and carbonitrides (MXenes) are a new type of two-dimensional nanomaterials following two-dimensional nanomaterials such as graphene, hexagonal boron nitride, and transition metal dichalcogenides. MXene is generally prepared by selectively etching the A layer in the ternary layered MAX phase, and more than 30 MXene materials with different components and configurations have been successfully prepared. The chemical general formula of MXene materials is M n+1 X n T x (n = 1 - 4), M is a transition metal element (such as Ti, Sc, Zr, Nb, etc.), X represents carbon, nitrogen or carbonitrogen elements, and T x represents the functional groups (=O, -OH, -F, -Cl, etc.) attached to its surface during the preparation process (Science, 2021, 372, 1165.). MXene (such as Ti3C2T x ) has good mechanical properties (fracture strength of 17.3 GPa), excellent electrical properties (electrical conductivity exceeding 24000 S cm -1 ), and rich surface functional groups, and has broad application prospects in the fields of energy storage, catalysis, water purification, photothermal conversion, flexible sensing, electromagnetic interference shielding, etc. The surface and edges of MXene materials contain rich oxygen-containing functional groups such as hydroxyl and epoxy, making MXene nanosheets have good hydrophilicity and can be highly concentrated and dispersed in polar solvents such as water. Through a simple solution processing process, a stable and uniform MXene dispersion can be quickly assembled into multifunctional MXene macroscopic structures, such as flexible conductive fibers, conductive and tough films, ultra-light elastic aerogels, and conductive artificial shell bulk materials. The rich oxygen-containing functional groups on the surface make MXene easy to functionalize and have strong interactions with other materials, making MXene materials a chemically tunable platform to prepare high-performance MXene nanocomposites. MXene nanocomposites have important application prospects in application fields such as flexible multifunctional wearable fabrics, human-computer interaction, and EMI shielding. As the basic elemental material for constructing multifunctional MXene nanocomposites, MXene nanomaterials, therefore, developing efficient, high-yield, and large-scale preparation technologies for MXene elemental materials is the key to the wide application of MXene nanocomposites.

[0003] The first type of MXene material (Ti3C2T x) was prepared and named in 2011 by researchers such as Yury Gogotsi and Michel W. Barsoum by selectively etching the Al layer in the ternary layered carbide Ti3AlC2 (MAX) with hydrofluoric acid, and it is the most widely studied MXene material at present (Adv. Mater., 2011, 23, 4248.). Ti3C2T x The key points for the preparation of MXene nanosheets are the atomic etching of the Al layer and monolayer exfoliation, generally including two preparation processes. The chemical stability of the amphoteric Al atomic layer is weaker than that of the Ti3C2 layer. By using etching reagents such as acids, alkalis or molten salts, the Al atomic layer can be selectively removed from the MAX phase to obtain accordion-like multi-layer MXene (m-MXene). However, there are interactions between the layers in the multi-layer m-MXene (such as van der Waals forces), which prevent their subsequent exfoliation and dispersion. Intercalating agents (dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), isopropylamine, choline hydroxide, n-butylamine, tetrabutylammonium hydroxide (TBAOH), etc.) are needed for intercalation exfoliation. Over the past decade, researchers have developed a variety of etching methods, including HF etching, in-situ synthesis of HF etching, alkali etching, halogen etching, electrochemical etching, molten salt etching, etc. Alkali etching is prone to form insoluble Al oxides on the MXene surface, hindering further etching. The reported yield of halogen etching is currently low. Electrochemical etching can achieve rapid and efficient etching of the Al layer, but there are still huge challenges in realizing low-cost and batch production. Molten salt etching is a method that has received much attention at present, and it can prepare a variety of new MXene materials without fluorine end groups. However, it requires high temperature and subsequent exfoliation processes. Currently, in the field of functional composite materials, HF etching and in-situ synthesis of HF etching are the mainstream methods with the highest yield for preparing single-layer MXene nanosheets. Direct use of high-concentration HF is more harmful to the human body, and the accordion-like multi-layer Ti3C2T x MXene requires the assistance of an intercalating agent for exfoliation and is not easy to exfoliate. The in-situ synthesis of HF etching method uses an aqueous solution of HCl / LiF to in-situ synthesize HF, avoiding the danger of directly using HF, realizing simultaneous etching and ion intercalation, and effectively improving the exfoliation efficiency. In 2014, Ghidiu et al. first prepared Ti3C2T by the in-situ synthesis of HF method using HCl / LiF xMXene (“clay” method), etching time is 45 h, and the yield is 45% (Nature, 2014, 516, 78.). Subsequently, Shahzad et al. improved the “clay” method by increasing the molar concentration of HCl / LiF to develop the minimum-intensity layer delamination (MILD) method. The etching time is 24 h, and then a large-sized MXene nanosheet dispersion (∼1.5 mg / mL) is prepared by manual shaking and exfoliation. However, the yield was not reported in this work (Science, 2016, 353, 1137.). In 2020, Huang et al. developed a cyclic freeze-thaw method during the exfoliation process, using the volume expansion caused by water freezing to promote the exfoliation of MXene layers. The yield is 39%, and the yield can reach 81.4% by combining 1 h of ultrasonic treatment. However, the yield calculation method is the mass ratio of exfoliated MXene nanosheets to the mass of m-MXene after etching (Adv. Funct. Mater. 2020, 30, 1910048.). Zeraati et al. developed an evaporation nitrogen-assisted MILD method with an etching time of 24 h. The yield is 30–40% under manual shaking, and the yield is 60% by combining 10 min of ultrasonic treatment (Nanoscale, 2021, 13, 3572.). In 2022, Zhang et al. carried out exfoliation by a power focusing delamination method with multiple cyclic vortex oscillations during the exfoliation process, and the yield is 61.2% (Adv. Sci. 2022, 2202748.).

[0004] However, the preparation of MXene nanosheets in the prior art still has the following problems: For the preparation method based on in-situ synthesis and HF etching, there are problems such as long time consumption, difficult exfoliation, and low monolayer yield. The main reason is that the etching process is a slow etching under stirring and static conditions, with a slow etching rate (24–45 h). At the same time, the by-products between layers are difficult to exchange quickly, further hindering the etching process. In addition, the H + and by-products in the multi-layer m-MXene interlayer with a stable accordion structure require long-time shaking and washing and exfoliation processes (∼12 h), resulting in long preparation time and low exfoliation yield (<∼70%). Currently, there is a lack of a method for efficiently, highly productively, low-costly, and large-scale preparation of MXene nanosheets; the existence of the above problems makes MXene nanosheets unable to meet the requirements of commercial production.

[0005] Therefore, developing a strategy for efficiently, low-costly, and highly productively large-scale preparation of MXene nanosheets provides a basis for the elementary materials for the wide application of MXene composites, which is of great significance in the large-scale industrial preparation and application of MXene. Summary of the Invention

[0006] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a method for preparing MXene nanosheets, which is a method for preparing MXene nanosheets with high yield. The preparation method can effectively solve the problems of complex preparation process, high cost, low yield, long time consumption and difficulty in large-scale production in the prior art, and obtain MXene nanosheet materials with high yield.

[0007] To achieve the purpose of this invention, the following technical solutions are adopted:

[0008] The present invention provides a method for preparing MXene nanosheets, and the preparation method includes the following steps:

[0009] (1) Stir and mix the ternary MAX phase precursor material and the etching solution to obtain a mixed dispersion A; wherein, the etching solution is an acidic or alkaline etching solution;

[0010] (2) Place the dispersion A obtained in step (1), the buffer grinding aid material and the grinding balls into an acid- and alkali-resistant vacuum ball mill tank for ball milling to obtain a ball-milled dispersion B;

[0011] (3) Centrifuge and wash the ball-milled dispersion B obtained in step (2) with deionized water to obtain a mixed precipitate C;

[0012] (4) Disperse the mixed precipitate C in step (3) in a dispersion solvent, perform ultrasonic or vortex oscillation, and finally centrifuge to separate the buffer grinding aid material to obtain a MXene nanosheet dispersion;

[0013] Preferably, the preparation method further includes step (5) continuously preparing functional fibers, films or bulk macroscopic composite materials from the MXene nanosheet dispersion obtained in step (4) through a solution processing process.

[0014] The preparation method provided by the present invention is a "top-down" method that efficiently combines exfoliation and etching. First, a ternary MAX-phase precursor material, an acidic or alkaline etching solution with a specific molar concentration, a buffer-assisted grinding material, and grinding balls are mixed in a certain proportion and milled in a vacuum ball mill jar. The ball milling has a high-speed transverse shearing and frontal impact on the MAX-phase material, generating a strong shearing force and a positive pressure on the material respectively. The positive pressure crushes the material to reduce its size, while the shearing force is conducive to the exfoliation of the layered material to reduce its thickness. The acidic or alkaline etching solution during the ball milling process can etch the A-layer atoms of the MAX-phase material. At the same time, the exfoliated MAX flakes expose the A-layer atoms, and the acidic or alkaline etching solution can quickly react chemically with the A-layer atoms to accelerate the etching process. In addition, an inorganic van der Waals layered material with a low Mohs hardness is innovatively added to the ball milling as a key buffer substance and shear-assisted grinding substance to assist the ball milling. The van der Waals layered material has an efficient buffering and lubricating effect. The interlayer can effectively absorb the positive pressure impact energy of the grinding balls acting on the MAX-phase material during ball milling, preventing in-plane fracture of the MAX material. It also protects the exfoliated single-layer / few-layer MXene flakes from further fracture during continuous ball milling, thus exfoliating large-sized MXene nanosheets. And it effectively prevents direct collisions between the grinding balls and between the grinding balls and the ball mill jar, preventing damage. Secondly, the inorganic van der Waals layered material with relatively matched size and flaky structure can increase the shear stress points acting on the MAX-phase material during ball milling, improve the yield of ball milling shear exfoliation, and accelerate the exfoliation of the MAX-phase material. Both ball milling exfoliation and chemical etching are rapid processes, realizing a synergistic promotion effect between exfoliation and etching. Under the synergistic action of mechanical force and chemical force, the MAX-phase material is efficiently etched and exfoliated into single-layer / few-layer MXene nanosheets quickly; after ball milling, the obtained mixed dispersion is washed and subjected to simple ultrasonic or vortex oscillation steps, which can further achieve interlayer exfoliation of the few-layer MXene to obtain MXene nanosheets with a high single-layer rate; the obtained MXene nanosheet dispersion can be precisely size-selected and screened by liquid-phase gradient cascade centrifugation, making the size distribution of the MXene nanosheets nearly a single distribution to obtain MXene nanosheets of different sizes. Finally, through a solution processing process, the obtained MXene nanosheet dispersion can be continuously used to prepare multifunctional fibers, films, and bulk macroscopic composite materials.

[0015] The preparation method provided by the present invention has the characteristics of simplicity, low cost, high efficiency, and high yield, and can achieve large-scale industrial production. Finally, monolayer MXene nanosheet materials with a wide range of sizes of 0.01 - 10 μm in lateral dimension are obtained, which fully meet the requirements of MXene nano-based materials in multifunctional and high-performance MXene composites such as fibers, films, and bulk materials, and have important application prospects in application fields such as flexible multifunctional wearable fabrics, human-computer interaction, and electromagnetic interference shielding.

[0016] In the present invention, the ternary MAX phase precursor material in step (1) is a general term for ternary metal carbides, nitrides, and carbonitrides, which includes any one of the compounds with the molecular general formula M n+1 AX n (n = 1, 2, 3). Wherein M represents any one or at least two combinations of elements in the transition metal elements including any one of the elements in Group IVB, VB, VIB, VIIB, or VIII of the periodic table, A is a Group III or Group IV main group element, and X is any one or at least two combinations of carbon, nitrogen, or carbon and nitrogen elements.

[0017] Preferably, the ternary MAX phase precursor material is Ti3AlC2, Ti2AlC, Ti2AlN, Ti3AlCN, V2AlC, Ti3AlCN, Ti3SiC2, Nb2AlC, (Nb,Zr)4AlC3, Mo2Ga2C, and specific compounds satisfying the above molecular general formula. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the compounds included in the above range.

[0018] Preferably, the size of the ternary MAX phase precursor material is 50 - 1000 mesh, such as 100 mesh, 200 mesh, 300 mesh, 400 mesh, 500 mesh, 600 mesh, 700 mesh, 800 mesh, or 900 mesh, and specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0019] In the present invention, for the acid or alkaline etching solution in step (1), the acidic etching solution is a mixed solution of fluoride salt and inorganic acid, and the alkaline solution is an alkali metal solution.

[0020] Preferably, the fluoride salt, such as lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF), cesium fluoride (CsF), or calcium fluoride (CaF), and specific compounds of the above fluorides. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the compounds included in the above range.

[0021] Preferably, in the acidic etching solution, the molar concentration of the fluoride salt is 5 to 15 M, such as 6 M, 8 M, 10 M, 12 M, or 14 M, etc.

[0022] Preferably, the inorganic acid is any one or a mixture of two of, for example, hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3), hydrofluoric acid (HF), etc.

[0023] Preferably, in the acidic etching solution, the molar concentration of the inorganic acid is 3 to 12 M, such as 4 M, 5 M, 7 M, 9 M, or 11 M, etc.

[0024] Preferably, the alkali metal solution is any aqueous solution of, for example, sodium hydroxide or potassium hydroxide, as well as specific compounds of the above alkali metals. Due to space limitations and for the sake of simplicity, the compounds included in the scope of the present invention are not exhaustively listed herein.

[0025] Preferably, the molar concentration of the alkali metal solution is 1 to 10 M, such as 2 M, 4 M, 6 M, 8 M, or 9 M, etc.

[0026] The acid or alkaline etching solution is an acid or alkaline etching solution with a specific molar concentration.

[0027] As a preferred technical solution of the present invention, the mass ratio of the ternary MAX phase precursor material in step (1) to the volume of the acid or alkaline etching solution with a specific molar concentration is: for 1 g of the MAX phase material, the volume of the etching solution is 10 to 200 mL, such as 20 mL, 30 mL, 50 mL, 70 mL, 90 mL, 110 mL, 130 mL, 150 mL, 170 mL, or 190 mL, etc., and more preferably 20 to 60 mL.

[0028] As a preferred technical solution of the present invention, in step (1), the mixing is carried out in an acid and alkali resistant bottle. The acid and alkali resistant bottle is, for example, a bottle made of materials such as polytetrafluoroethylene.

[0029] Preferably, the stirring and mixing in step (1) is magnetic stirring, and the stirring speed is 100 to 1000 rpm, such as 200 rpm, 400 rpm, 600 rpm, 800 rpm, or 900 rpm.

[0030] Preferably, the stirring and mixing time is 10 to 60 min, such as 15 min, 20 min, 30 min, 40 min, 50 min, or 55 min.

[0031] As a preferred technical solution of the present invention, when adding the MAX phase to the acid or alkaline etching solution, a chemical reaction is likely to occur to generate gas. Stirring and mixing in an acid and alkali resistant bottle is conducive to discharging some of the reaction gases, improving the experimental stability, and ensuring the safety of the preparation process.

[0032] In the present invention, the buffering and grinding aid material in step (2) is a solid-phase inorganic van der Waals layered material with low Mohs hardness;

[0033] Preferably, the solid-phase inorganic van der Waals layered material with low Mohs hardness is, for example, any one or a combination of at least two of the layered materials such as flake graphite, hexagonal boron nitride, transition metal dichalcogenides, and graphite-phase carbon nitride;

[0034] As a preferred technical solution of the present invention, the above-mentioned buffering and grinding aid material is easy to separate from the system of MXene nanosheet materials, easy to remove, and will not introduce impurities; and the buffering and grinding aid material has a low Mohs hardness, a lamellar structure and size matching, and can absorb strong impact energy between lamellar layers, thereby producing good lubrication, buffering protection and peeling promotion effects on MAX phase materials.

[0035] Preferably, the lateral size of the van der Waals layered material is 1 to 5000 μm, for example 5 μm, 50 μm, 100 μm, 500 μm, 1000 μm, 1500 μm, 2000 μm, 2500 μm, 3000 μm, 3500 μm, 4000 μm or 4500 μm, more preferably 50 to 1000 μm.

[0036] As a preferred technical solution of the present invention, the mass ratio of the MAX phase precursor material in step (1) to the buffering grinding aid material in step (2) is 1:(0.1-100), for example, 1:3, 1:5, 1:7, 1:9, 1:10, 1:12, 1:15, 1:18, 1:20, 1:22, 1:24, 1:26, 1:28, 1:30, 1:35, 1:40, 1:50, 1:60, 1:70, 1:80, 1:90 or 1:95, etc., and more preferably 1:(0.5-10).

[0037] As a preferred technical solution of the present invention, when the mass ratio of the MAX phase material to the buffering grinding aid material is 1: (1 to 100), the buffering lubrication and auxiliary ball milling effects of the buffering grinding aid material can be fully exerted, while simplifying the process and saving costs. If the buffering grinding aid material is not added or the amount is too small, it is insufficient to play the role of buffering lubrication ball milling, the size of the prepared MXene nanosheets is small, and the ball milling balls and ball milling jars are damaged; if the amount of buffering grinding aid material is too much, the ball milling peeling effect is reduced, the MXene nanosheet preparation yield is reduced, unnecessary waste of resources is caused, and the burden of later impurity removal is increased, which violates the original intention of high efficiency and energy saving.

[0038] Preferably, the material of the ball milling balls in step (2) includes any one or a combination of at least two of zirconia, stainless steel, tempered steel, agate, hard tungsten carbide, silicon nitride or sintered corundum.

[0039] Preferably, the diameter of the ball milling balls in step (2) is 0.5 - 20 mm, such as 0.8 mm, 1 mm, 2 mm, 3 mm, 4 mm, 6 mm, 8 mm, 10 mm, 12 mm, 14 mm, 16 mm, 18 mm or 19 mm, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the specific point values included in the scope of the present invention are not exhaustively listed herein. Any one size or a combination of at least two sizes is acceptable.

[0040] Preferably, the mass ratio of the MAX phase precursor material in step (1) to the ball milling balls in step (2) is 1:(10 - 1000), such as 1:20, 1:30, 1:40, 1:50, 1:60, 1:70, 1:80, 1:90, 1:100, 1:120, 1:150, 1:180, 1:200, 1:300, 1:400, 1:500, 1:700, 1:900 or 1:950, etc., and more preferably 1:(50 - 300).

[0041] As a preferred technical solution of the present invention, the above ball milling is easy to operate, can fully exfoliate the MAX phase material, has a high degree of mechanization, and is easy for large-scale production.

[0042] Preferably, the material of the acid and alkali resistant vacuum ball milling tank in step (2) includes any one of polytetrafluoroethylene, zirconia, stainless steel, tempered steel, agate, hard tungsten carbide, silicon nitride or sintered corundum.

[0043] Preferably, the volume of the acid and alkali resistant vacuum ball milling tank is 0.1 - 100 L, such as 100 mL, 200 mL, 250 mL, 500 mL, 1 L, 5 L, 10 L, 50 L, 70 L or 90 L, etc., and more preferably 0.1 - 1 L.

[0044] Preferably, the acid and alkali resistant vacuum ball milling tank is evacuated and sealed, and the vacuum degree is 0.05 - 0.1 MPa, such as 0.07 MPa, 0.08 MPa, 0.085 MPa, 0.09 MPa or 0.095 MPa, etc.

[0045] In the present invention, the ball milling rotation speed in step (2) is 100 - 1500 r / min, such as 200 r / min, 300 r / min, 400 r / min, 500 r / min, 800 r / min, 1000 r / min, 1200 r / min or 1400 r / min, as well as specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range. Further preferably, it is 500 - 1000 r / min.

[0046] Preferably, the ball milling time is 1 - 100 h, such as 2 h, 4 h, 8 h, 10 h, 12 h, 16 h, 18 h, 20 h, 22 h, 24 h, 28 h, 30 h, 35 h, 40 h, 45 h, 50 h, 60 h, 80 h or 90 h, as well as specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range. Further preferably, it is 8 - 24 h.

[0047] As a preferred technical solution of the present invention, the ball milling time is 1 - 100 h. By controlling the length of the ball milling time, the MAX phase material is peeled and etched to obtain MXene nanosheets. As the ball milling time extends, the MAX phase material is peeled and etched more completely, and the yield of MXene nanosheets is higher. However, as the ball milling time extends, the size of the nanosheets shows a decreasing trend. If the ball milling time is less than 1 h, the ball milling is incomplete, and the peeling and etching effect of the MAX phase material is not obvious; if the ball milling time is higher than 100 h, the peeling yield tends to reach the limit, resulting in waste of energy, which goes against the original intention of high efficiency and energy saving.

[0048] Preferably, step (2) further includes a step of separating the ball milling balls.

[0049] Preferably, the method for separating the ball milling balls is sieve screening or pipette liquid transfer separation.

[0050] In the present invention, the centrifugal washing in step (3) is to add deionized water and shake it manually repeatedly to disperse, and then perform centrifugal precipitation to remove the acidic solution for washing. The centrifugal rotation speed is 1000 - 7000 r / min, such as 1500 r / min, 2000 r / min, 2500 r / min, 3000 r / min, 3500 r / min, 4000 r / min, 4500 r / min, 5000 r / min, 6000 r / min or 6500 r / min, etc. Further preferably, it is 2000 - 4000 r / min;

[0051] Preferably, the centrifugation time is 1 to 60 min, such as 2 min, 3 min, 5 min, 10 min, 15 min, 20 min, 40 min, 50 min or 55 min, etc., and more preferably 2 to 10 min.

[0052] Preferably, in step (3), the dispersion is washed until the pH of the dispersion approaches 6 or exfoliated dark MXene nanosheets are observed in the upper layer.

[0053] In the present invention, the dispersion solvent in step (4) is selected from any one or a combination of at least two of water, ethanol, isopropanol, methanol, formamide, N-methylformamide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, tetrabutylammonium hydroxide, acetone, methyl ethyl ketone, etc.;

[0054] Preferably, the ultrasonic treatment in step (4) is ultrasonic treatment in an ice-water bath, and the ultrasonic power is 30 to 200 W, such as 35 W, 40 W, 50 W, 60 W, 70 W, 80 W, 90 W, 100 W, 120 W, 140 W, 160 W, 180 W or 190 W, etc., and more preferably 50 to 100 W.

[0055] Preferably, the ultrasonic treatment time is 10 to 120 min, such as 15 min, 20 min, 25 min, 30 min, 40 min, 50 min, 60 min, 80 min, 100 min or 110 min, etc., and more preferably 20 to 60 min.

[0056] Preferably, the vortex oscillation in step (4) is carried out using a vortex oscillator, and the rotation speed is 1000 to 2000 r / min, such as 1100 r / min, 1200 r / min, 1400 r / min, 1600 r / min, 1800 r / min or 1900 r / min, etc.

[0057] Preferably, the oscillation time is 2 to 20 r / min, such as 3 min, 4 min, 5 min, 7 min, 10 min, 12 min, 14 min, 16 min, 18 min or 19 min, etc.

[0058] In the present invention, in step (4), the centrifugation is used to separate the buffer-assisted grinding material and the unexfoliated MAX phase material, and the centrifugation speed is 1000 to 5000 r / min, such as 1500 r / min, 2000 r / min, 2500 r / min, 3000 r / min, 3500 r / min, 4000 r / min or 4500 r / min, etc., and more preferably 1500 to 3000 r / min.

[0059] Preferably, the centrifugation time is 5 to 60 minutes, such as 7 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, 30 minutes, 40 minutes, 50 minutes or 55 minutes, etc.

[0060] As a preferred technical solution of the present invention, in the step of separating the buffer grinding aid material and the unpeeled MAX phase material, the centrifugation speed is 1000 to 5000 r / min and the time is 5 to 60 minutes, which can achieve the complete separation of the buffer grinding aid material, the unpeeled MAX phase material and the MXene nanosheet material. If the speed is less than 1000 r / min, the effect of complete impurity removal cannot be achieved; if the centrifugation speed is greater than 5000 r / min, a large amount of MXene nanosheets will be removed simultaneously, affecting the yield; if the centrifugation time is less than 5 minutes, the separation of the buffer grinding aid material is incomplete; if the centrifugation time is greater than 60 minutes, the buffer grinding aid material will completely settle, and the peeled MXene nanosheets will also settle to a certain extent, affecting the yield of MXene nanosheets.

[0061] In the present invention, the process of step (4) can be repeated multiple times until the peeled MXene nanosheets are completely collected. The number of repetitions is 1 to 10 times, such as 2 times, 3 times, 4 times, 5 times, 6 times, 7 times, 8 times or 9 times, etc., and further preferably 3 - 5 times;

[0062] As a preferred technical solution of the present invention, the process of step (4) is repeated multiple times, and the number of repetitions is 1 to 10 times, which can completely collect the peeled MXene nanosheet material. If the number of repetitions is less than or equal to 1 time, the complete collection of MXene nanosheets cannot be achieved, affecting the yield; if the number of repetitions is greater than 10 times, the MXene nanosheets have been completely collected, resulting in a waste of energy, which goes against the original intention of high efficiency and energy saving.

[0063] In the present invention, the obtained MXene nanosheets are a general term for two-dimensional transition metal carbides, nitrides and carbonitrides, which include any one of the compounds with the molecular general formula following the formula M n+1 X n T x (n = 1 - 4), where M is usually any one or at least two combinations of elements in the transition metal elements of groups IVB, VB, VIB, VIIB or VIII in the periodic table, X represents any one or at least two combinations of carbon, nitrogen or carbon-nitrogen elements, and T x represents a functional group (=O, -OH, -F, -Cl, etc.) attached to the surface of MXene.

[0064] Preferably, the obtained MXene is Ti2CT x 、Ti3C2T x, V2CT x , Ti2NT x , Ti3CNT x , (Nb,Zr)4C3T x or Mo2CT x , and specific compounds satisfying the above general formula of MXene molecules. Due to space limitations and for the sake of simplicity, the compounds included in the above range are not exhaustively listed in the present invention.

[0065] Preferably, the concentration of the MXene nanosheet dispersion obtained in step (4) is 1 - 50 mg / mL, such as 2 mg / mL, 5 mg / mL, 8 mg / mL, 10 mg / mL, 13 mg / mL, 15 mg / mL, 18 mg / mL, 20 mg / mL, 25 mg / mL, 30 mg / mL, 35 mg / mL, 40 mg / mL or 45 mg / mL, etc., and more preferably 3 - 20 mg / mL.

[0066] Preferably, step (4) further includes recycling the buffer grinding aid material. By washing, drying and collecting the buffer grinding aid material separated by centrifugation, it can be reused.

[0067] As a preferred technical solution of the present invention, the above buffer grinding aid material has good chemical stability, can be recycled and reused, has simple operation, low preparation cost, and can achieve large-scale production.

[0068] Preferably, step (4) further includes performing liquid-phase gradient cascade centrifugation size selection on the obtained MXene nanosheet dispersion to screen MXene nanosheets of different sizes.

[0069] Preferably, the rotation speed of the liquid-phase gradient cascade centrifugation is 1000 - 20000 r / min, such as 1500 r / min, 2000 r / min, 3000 r / min, 4000 r / min, 6000 r / min, 8000 r / min, 10000 r / min, 12000 r / min, 13000 r / min, 15000 r / min, 17000 r / min or 19000 r / min, as well as specific point values between the above point values. Due to space limitations and for the sake of simplicity, the specific point values included in the above range are not exhaustively listed in the present invention, and more preferably 2000 - 16000 r / min.

[0070] Preferably, the number of stages of the liquid-phase gradient cascade centrifugation is 4 - 40 stages, such as 4 stages, 5 stages, 6 stages, 7 stages, 8 stages, 9 stages, 10 stages, 11 stages, 12 stages, 13 stages, 14 stages, 15 stages, 16 stages, 17 stages, 18 stages, 19 stages, 20 stages, 25 stages, 30 stages, 35 stages or 38 stages, etc.

[0071] Preferably, the centrifugation time for each stage in the liquid-phase gradient cascade centrifugation is independently selected from 10 to 60 min, such as 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min or 55 min, and the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0072] Preferably, the liquid-phase gradient cascade centrifugation is a centrifugation with gradually increasing rotation speed, and the rotation speed difference between adjacent stages in the gradient cascade centrifugation is 500 - 2000 r / min, such as 600 r / min, 700 r / min, 800 r / min, 900 r / min, 1000 r / min, 1100 r / min, 1300 r / min, 1500 r / min, 1700 r / min or 1900 r / min, and the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0073] The liquid-phase gradient cascade centrifugation can perform fine size selection on the collected MXene nanosheet dispersion. When the rotation speed difference between adjacent stages in the gradient cascade centrifugation is less than 1000 r / min, the size differentiation is not obvious and the workload is extremely large, which can be used for fine screening; when the rotation speed difference between adjacent stages in the gradient cascade centrifugation is greater than 2000 r / min, the size distribution of the obtained MXene nanosheets is too large to meet the requirement of nearly single distribution in size, and the size screening accuracy is reduced.

[0074] Preferably, the average lateral size of the MXene nanosheets in step (4) is 0.001 - 40 μm, such as 5 nm, 10 nm, 30 nm, 50 nm, 100 nm, 500 nm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, 3 μm, 5 μm, 7 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm or 35 μm, and the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range, and is further preferably 0.01 - 10 μm.

[0075] Preferably, the thickness of the MXene nanosheets is 1 - 5 layers, such as 1 layer, 2 layers, 3 layers or 4 layers.

[0076] The MXene nanosheet dispersion obtained by the preparation method of the present invention can continuously prepare functional fibers, films and bulk macroscopic composites through a solution processing process. The solution processing process includes techniques such as wet spinning, coating, spraying, vacuum filtration, blade coating, ice template construction and freeze drying, etc., to obtain MXene fibers, films and bulk macroscopic composites.

[0077] The yield of MXene nanosheets exfoliated from ternary MAX phase materials by the preparation method of the present invention is 1-99%, such as 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 95%, etc., and is further preferably 50-96%; the monolayer rate of the MXene nanosheets is 80-100%, such as 85%, 90%, 92%, 94%, 96%, 98% or 99%, etc. As well as the specific point values between the above point values, due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0078] In the present invention, the preparation method specifically includes the following steps:

[0079] (1) Place the ternary MAX phase precursor material and an acid or alkaline etching solution in an acid and alkali resistant bottle in a ratio of 1 g:(20-60 mL), stir and mix, the stirring speed is 100-1000 rpm, and the time is 10-60 min. Obtain a mixed dispersion A;

[0080] (2) Place the dispersion A obtained in step (1), the buffer grinding material and the grinding balls in an acid and alkali resistant vacuum ball mill tank in a ratio of MAX:buffer material:grinding balls = 1:(0.5-10):(50-300), evacuate and seal, the vacuum degree is 0.05-0.1 MPa, and then carry out ball milling, wherein the ball milling speed is 500-1000 r / min, and the ball milling time is 8-24 h, to obtain a ball milled dispersion B;

[0081] (3) Centrifuge and wash the ball milled dispersion B obtained in step (2) with deionized water until the pH = 6, wherein the centrifuge speed is 2000-4000 r / min, and the centrifuge time is 2-10 min, to obtain a mixed precipitate C;

[0082] (4) Disperse the mixed precipitate C obtained in step (3) in a dispersion solvent, and perform ultrasonic treatment in an ice-water bath or vortex oscillation. The ultrasonic power is 50-100 W, and the ultrasonic time is 20-60 min; the rotation speed of the vortex oscillation is 1000-2000 r / min, and the oscillation time is 2-20 r / min; finally, centrifuge to separate the buffer grinding aid material, the centrifuge speed is 1500-3000 r / min, and the centrifuge time is 5-60 min. Collect the MXene nanosheet dispersion, and repeat the collection of the MXene nanosheet dispersion. The number of repetitions is 3-5 times until the exfoliated MXene nanosheets are completely collected; and perform liquid-phase gradient cascade centrifugal size selection on the obtained MXene nanosheet dispersion to screen MXene nanosheets of different sizes.

[0083] (5) Continuously prepare functional fibers, films or bulk macroscopic composites from the MXene nanosheet dispersion obtained in step (4) through a solution processing process, where the solution processing process includes techniques such as wet spinning, coating, spraying, vacuum filtration, doctor blading, ice template construction, and freeze drying.

[0084] Compared with the prior art, the present invention has the following beneficial effects:

[0085] (1) The preparation method of MXene nanosheets provided by the present invention is a "top-down" exfoliation and etching method using a ternary MAX phase precursor material as the raw material. In the preparation method, through the selection of raw materials and the setting of process parameters in the ball milling process, combined with chemical etching with an acidic or alkaline etching solution, the rapid exfoliation and etching of the ternary MAX phase precursor material are realized simultaneously, so as to exfoliate and etch MXene nanosheets with high efficiency; by introducing a key buffer grinding aid substance and selecting parameters, the impact force energy of the ball milling is reduced and the shear stress point is increased, realizing the protection of MXene nanosheets and promoting the exfoliation of MXene nanosheets, and obtaining high-yield and large-size MXene nanosheets.

[0086] (2) The preparation method provided by the present invention has the advantages of high efficiency, high yield, simple operation, low cost, scalable preparation, and controllable MXene size. The yield of the prepared MXene nanosheets can reach 96%. Moreover, the monolayer rate of the MXene nanosheets is high, exceeding 90%. The buffer grinding aid can be recycled and reused, and the preparation cost is low. Finally, the obtained MXene nanosheets have good hydrophilicity and good dispersibility in polar solvents such as water. The concentration of its dispersion exceeds 200 mg / mL. The stable high-concentration MXene dispersion can be assembled into MXene fibers, films, and bulk macroscopic composites through solution processing techniques such as wet spinning, coating, spraying, vacuum filtration, doctor blading, ice templating, and freeze drying. The efficient, high-yield, and scalable preparation method provided by the present invention is of great significance in the large-scale commercial preparation and application of MXene. BRIEF DESCRIPTION OF THE DRAWINGS

[0087] Figure 1 Schematic diagram of the preparation process flow of the MXene nanosheets reported in the present invention;

[0088] Figure 2 Physical and SEM images of the raw materials MAX Ti3AlC2, buffer grinding aid flake graphite, and zirconia milling balls in Example 1;

[0089] Figure 3 AFM image, TEM image, SEM image of the MXene nanosheets obtained in Example 1, and the corresponding size and thickness;

[0090] Figure 4 Dispersion of the MXene nanosheets obtained by large-scale preparation in Example 1;

[0091] Figure 5 XRD spectral characterization of the MXene nanosheets obtained in Example 1;

[0092] Figure 6 XPS spectral characterization of the MXene nanosheets obtained in Example 1;

[0093] Figure 7 Physical and SEM images of the MXene fibers, films, and aerogel materials further assembled from the MXene nanosheets obtained in Example 1;

[0094] Figure 8 Mechanical, electrical properties, and electromagnetic interference shielding properties of the MXene fibers, films, and aerogel materials obtained in Example 1;

[0095] Figure 9 SEM morphology images and corresponding particle size statistics of the MXene nanosheets with different sizes obtained in Example 5. Detailed implementation manners

[0096] The technical solutions of the present invention will be further described below through specific implementation manners in combination with the accompanying drawings. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention. Moreover, through the description of the following embodiments, those skilled in the art can fully implement all the contents of the claims of the present invention.

[0097] As Figure 1 shown, a preparation method of MXene nanosheets includes the following steps:

[0098] (1) Stir and mix a ternary MAX phase precursor material and an etching solution to obtain a mixed dispersion A; wherein, the etching solution is an acidic or alkaline etching solution;

[0099] (2) Place the dispersion A obtained in step (1), a buffer grinding aid material, and grinding balls into an acid- and alkali-resistant vacuum ball mill jar for ball milling to obtain a ball-milled dispersion B;

[0100] (3) Centrifuge and wash the ball-milled dispersion B obtained in step (2) with deionized water to obtain a mixed precipitate C;

[0101] (4) Disperse the mixed precipitate C in step (3) in a dispersion solvent, perform ultrasonic or vortex oscillation, and finally centrifuge to separate the buffer grinding aid material to obtain a MXene nanosheet dispersion;

[0102] (5) Continuously prepare functional fibers, films, or bulk macroscopic composites from the MXene nanosheet dispersion obtained in step (4) through a solution processing process.

[0103] Example 1

[0104] This example provides a preparation method of Ti3C2T x MXene nanosheets, specifically including the following steps:

[0105] (1) Add 4.8 g of LiF to a polytetrafluoroethylene bottle (200 mL) containing 60 mL of HCl (9 M) solution, and stir magnetically at 400 rpm for 10 min. Gradually add 2 g of Ti3AlC2 MAX with a particle size of 400 mesh to the HCl / LiF mixed solution, and stir magnetically at 400 rpm for 20 min to obtain a MAX / HCl / LiF mixed solution.

[0106] (2) Add 200 g of zirconia milling balls (diameter: 3 mm) and 3 g of flake graphite (average particle size: 400 μm) into a 250 mL polytetrafluoroethylene vacuum milling jar, and then transfer the MAX / HCl / LiF mixed solution into the polytetrafluoroethylene vacuum milling jar. Use a diaphragm pump to evacuate (0.085 MPa) and seal the vacuum milling jar. Then, place the vacuum milling jar into a planetary ball mill (model: TJX-450) for ball milling at room temperature (25 °C), with a rotation speed of 1000 rpm and a ball milling time of 12 h. After ball milling, let it stand for 30 min, and then slowly open the air release valve of the vacuum milling jar under the fume hood to obtain the ball-milled mixture.

[0107] (3) Centrifuge the obtained mixture using a centrifuge (model: HC-3518) at 3500 rpm (1329×g) for 5 min to remove the supernatant. Add deionized water to the obtained precipitate and shake it manually for washing, and repeat the above centrifugation process 4 - 5 times until the upper dispersion liquid turns dark green (or the pH value is 6) after centrifugation. Remove the upper dispersion liquid to obtain the mixed precipitate.

[0108] (4) Disperse the precipitate in 480 mL of deionized water and perform simple ice bath ultrasonic treatment for 30 min with an ultrasonic power of 60 W. Subsequently, centrifuge the obtained dispersion liquid at 3000 rpm (977×g) for 15 min to remove the flake graphite and unpeeled MAX phase particles, and obtain the exfoliated single-layer Ti3C2T x MXene nanosheet dispersion liquid. Take 5 mL of the volume (V 抽滤体积 ) of the MXene dispersion liquid and perform vacuum filtration through an organic microporous membrane (0.22 μm) to obtain the MXene film. Dry the film by vacuum heating (35 °C) for 24 h, weigh the film (m 薄膜 ), determine the concentration of the dispersion liquid (calculation formula: dispersion liquid concentration = m 薄膜 / V 抽滤体积 ), and measure that the concentration of the MXene dispersion liquid is 4 mg / mL. Calculate the preparation yield by comparing with the weight of the Ti3AlC2 MAX raw material (calculation formula: yield = total mass of all MXene nanosheets / mass of MAX phase material, m MXene / m MAX × 100%), and measure that the yield of the MXene nanosheets is 87%.

[0109] (5) Centrifuge the obtained MXene nanosheet dispersion liquid to prepare dispersion liquids with different concentrations (such as 30 mg / mL, 100 mg / mL), and finally continuously prepare macroscopic composite materials such as MXene fibers, MXene films, and MXene aerogel blocks through solution processing processes such as wet spinning, blade coating, ice templating combined with vacuum freeze-drying. Specifically, the steps are as follows:

[0110] a. Preparation of MXene fibers by wet spinning method. The prepared MXene dispersion was formulated into a concentration of 100 mg / mL, magnetically stirred for 30 min to make it uniformly dispersed, and then degassed to obtain a paste-like MXene spinning solution. 25 g of NH4Cl, 10 mL of NH4OH solution and 500 mL of deionized water were mixed to prepare a coagulation bath solution. The spinning solution was transferred to a 1 mL syringe, and under the push of a micro-injection pump, the spinning solution was injected into a rotating coagulation bath (rotation speed of 20 rpm) through a spinneret with a diameter of 380 μm. The obtained MXene fibers were soaked in the coagulation bath for 30 min, and then the fibers were washed with a mixed solution of water and ethanol, and the fibers were collected on a bobbin and dried in a vacuum oven at 35 °C for 24 h to obtain MXene fibers with a fiber diameter of about 60 μm.

[0111] b. Preparation of MXene thin films by doctor blade method. The prepared MXene dispersion was formulated into a concentration of 30 mg / mL, magnetically stirred for 30 min to make it uniformly dispersed, and then degassed. The obtained uniformly dispersed solution was coated into a film on a polyvinylidene fluoride film substrate by an automatic film coater (BEVS1811 / 3) at a speed of 150 cm s -1 −1. The tabletop temperature was 40 °C, the doctor blade length was 5 cm, and the gap size was 0.4 mm. Subsequently, it was vacuum dried at 35 °C for 24 h, and a large-area MXene thin film was peeled off from the substrate. The film area was 55 cm 2 2, and the thickness of the MXene thin film was about 3.2 μm.

[0112] c. Preparation of MXene layered framework aerogel blocks by bidirectional freeze-casting method. The prepared MXene dispersion was formulated into a concentration of 30 mg / mL, magnetically stirred for 30 min to make it uniformly dispersed, and then degassed. The obtained uniformly dispersed solution was poured into a self-made polydimethylsiloxane mold (12 mm × 12 mm × 18 mm). The bottom of the mold was in contact with the surface of the steel plate, and one end was in contact with liquid nitrogen. After the solution was completely frozen, the sample was placed in a freeze dryer and sublimated at -60 °C and a vacuum degree of less than 1 Pa for 72 h to obtain MXene layered framework aerogel blocks with a layered framework layer spacing of about 18 μm.

[0113] The thickness of the MXene nanosheets was recorded in intelligent mode by an atomic force microscope (Bruker Multimode8). The surface and cross-sectional morphologies of the MAX phase raw materials, flake graphite, prepared MXene nanosheets, and MXene macroscopic materials used in this example were tested by a scanning electron microscope (JEOL, JSM-7500F); the microscopic morphology of the MXene nanosheets was tested by a transmission electron microscope (FEI TitanThemis G3); and the structure of the MXene nanosheets was characterized by an X-ray diffractometer (Shimadzu XRD-6000) using Cu-Kα radiation and a scanning speed of 4° min -1 The structure of the MXene nanosheets was characterized. Digital cameras were used to take optical images of the MAX phase raw materials, flake graphite, zirconia milling balls, MXene nanosheet dispersions, and MXene macroscopic materials. The mechanical, electrical, and electromagnetic interference shielding properties of the MXene fibers, films, and aerogel blocks were characterized by an electronic universal testing machine (EM3.103-T) and the two-probe method using a Keithley 2400 multi-functional digital source meter and an AV3629 high-performance microwave integrated vector network analyzer. The test results are as Figures 2 to 8 shown.

[0114] Figure 2 These are the physical and scanning electron microscope images of the raw material MAX Ti3AlC2, the buffer grinding aid material flake graphite, and the zirconia milling balls in this example. Optical image a and SEM image b of the MAX Ti3AlC2 raw material; optical image c and SEM image d of the flake graphite raw material; optical images e and f of the zirconia milling balls at different magnifications.

[0115] Figure 3 These are the atomic force microscopy images, transmission electron microscope images, and scanning electron microscope images of the MXene nanosheets obtained in this example, as well as the corresponding dimensions and thicknesses. AFM image a and single-sheet height distribution diagram b of the MXene nanosheets; c is the TEM image of the MXene nanosheets and its SAED diffraction pattern (inset); d is the HRTEM image of the MXene nanosheets in the region and the FFT diagram (inset); SEM image e and particle size statistical histogram f of the MXene nanosheets. It can be seen from Figure 3 this that the thickness of the MXene nanosheets is monolayer, about 1.6 nm, and the average size is about 3.0 μm.

[0116] Figure 4 This is the MXene nanosheet dispersion prepared on a large scale in this example. a is the MXene dispersion produced in batches (concentration: 4 mg / mL). b is the digital image of the MXene / H2O dispersion under laser irradiation (concentration: 0.1 mg / mL). From Figure 4As can be seen, the preparation method reported in the present invention has a high yield and can achieve large-scale preparation of MXene nanosheets.

[0117] Figure 5 This is the XRD spectral characterization of the MXene nanosheets obtained in this example. As can be seen from Figure 5 it that the Al layer elements are completely etched without introducing other impurities, proving that the MXene nanosheets are successfully exfoliated.

[0118] Figure 6 This is the XPS spectral characterization of the MXene nanosheets obtained in this example. a is the high-resolution XPS spectra of Ti3AlC2 MAX raw material and MXene nanosheets for Al 2p; b is the high-resolution XPS spectra of MXene nanosheets for Ti 2p. As can be seen from Figure 6 it that it is further verified that the Al layer elements are completely etched and it is proved that the MXene nanosheets have rich functional groups.

[0119] Figure 7 These are the physical pictures and SEM images of the MXene nanosheets obtained in this example further assembled into MXene fibers, films and aerogel materials. SEM image a of the surface of MXene fibers prepared by wet spinning, SEM image b of the cross-section; optical image c of the surface of large-area MXene films prepared by spin coating, SEM image d of the cross-section; optical image e of the MXene layered framework aerogel block prepared by the two-way freezing ice template technique standing on a foxtail, SEM image f of the cross-section. As can be seen from Figure 7 it that the MXene nanosheets prepared by the preparation method reported in the present invention can be quickly assembled into macroscopic MXene composite materials, which have important application prospects in the field of functional composite materials.

[0120] Figure 8 These are the mechanical, electrical properties and electromagnetic interference shielding properties of the MXene fibers, films and aerogel materials obtained in this example. a is the tensile stress-strain curve of MXene fibers; b is the tensile stress-strain curve of MXene films; c is the compression and rebound stress-strain curve of MXene aerogel blocks; d is the summary histogram of the tensile strength and conductivity of MXene fibers and films; e is the electromagnetic interference shielding performance of MXene films. The thickness of the MXene film is about 3.2 μm.

[0121] Example 2

[0122] The difference between this example and Example 1 is only that in step (2), by solely changing the mass ratio of Ti3AlC2 MAX and flake graphite (1:0.5; 1:1; 1:1.5; 1:2; 1:2.5; 1:3), the preparation yields under different MAX and graphite ratios are obtained.

[0123] Example 3

[0124] The difference between this example and Example 1 is only that in step (2), by solely changing the ball milling time (4 h; 8 h; 12 h; 16 h; 20 h; 24 h; 28 h), the preparation yields under different ball milling times are obtained.

[0125] Example 4

[0126] The difference between this example and Example 1 is only that in step (2), by solely changing the ball milling speed (500 r / min; 600 r / min; 700 r / min; 800 r / min; 900 r / min; 1100 r / min), the preparation yields under different ball milling speeds are obtained.

[0127] Example 5

[0128] The difference between this example and Example 1 is only that in step (4), liquid-phase gradient cascade centrifugation size selection is performed on the obtained MXene nanosheets to screen out MXene nanosheets of different sizes. Among them, the obtained MXene dispersion is centrifuged successively at 4.5 krpm, 6 krpm, 7.5 krpm, and 9 krpm, the centrifugation time is fixed at 20 min, and the precipitates under different centrifugation speeds and the upper-layer dispersion after 9 krpm centrifugation are collected (the sample names are successively: "3-4.5 krpm"; "4.5-6 krpm"; "6-7.5 krpm"; "7.5-9 krpm" and "9 krpm--"). Thus, MXene nanosheets of different sizes are obtained.

[0129] Figure 9 Scanning electron microscope morphology diagrams and corresponding particle size statistics of MXene nanosheets of different sizes obtained in this example. SEM images of MXene nanosheets of different sizes corresponding to samples "3-4.5 krpm" a, "4.5-6 krpm" b, "6-7.5 krpm" c, "7.5-9 krpm" d, and "9 krpm--" e; f histogram summarizing the nanosheet size of different samples. It can be seen from Figure 9 that the preparation method reported in the present invention can achieve the controllable preparation of MXene nanosheets of different sizes.

[0130] Example 6

[0131] The difference between this example and Example 1 is only that in step (1), the MAX phase material is Ti2AlC MAX.

[0132] Example 7

[0133] The difference between this example and Example 1 is only that in step (1), the MAX phase material is Ti2AlN MAX.

[0134] Example 8

[0135] The difference between this example and Example 1 is only that in step (1), the MAX phase material is Ti3AlCN MAX.

[0136] Example 9

[0137] The difference between this example and Example 1 is only that in step (1), the MAX phase material is V2AlC MAX.

[0138] Example 10

[0139] The difference between this example and Example 1 is only that in step (1), the MAX phase material is Ti3AlCN MAX.

[0140] Example 11

[0141] The difference between this example and Example 1 is only that in step (1), the MAX phase material is Ti3SiC2 MAX.

[0142] Example 12

[0143] The difference between this example and Example 1 is only that in step (1), the MAX phase material is Nb2AlC MAX.

[0144] Example 13

[0145] The difference between this example and Example 1 is only that in step (1), the fluoride salt is sodium fluoride (NaF).

[0146] Example 14

[0147] The difference between this example and Example 1 is only that in step (1), the inorganic acid is sulfuric acid (H2SO4).

[0148] Example 15

[0149] The difference between this example and Example 1 is only that in step (1), the etching solution is 60 mL of an aqueous sodium hydroxide solution with a molar concentration of 4 M.

[0150] Comparative Example 1

[0151] The difference between this example and Example 1 is only that in step (2), the buffer grinding aid material flake graphite is not added.

[0152] Comparative Example 2

[0153] 4.8 g of LiF was added to a 200 mL PTFE bottle containing 60 mL of HCl (9 M) solution, and magnetically stirred at 400 rpm for 5 min. 2 g of Ti3AlC2 MAX was gradually added to the HCl / LiF mixed solution and magnetically stirred at 400 rpm for 20 min. Subsequently, magnetic stirring was carried out at room temperature (25 °C) for 12 h at a speed of 1000 rpm. Using the same washing procedure as in step (3) of Example 1, the obtained mixture was centrifuged at 3500 rpm for 5 min by a centrifuge and washed repeatedly to remove the supernatant until the pH value was equal to 6. Then, the precipitate was dispersed in deionized water and ultrasonically treated in a simple ice-water bath for 30 min with an ultrasonic power of 60 W. Finally, the dispersion was centrifuged at 3000 rpm for 15 min, and the upper-layer MXene nanosheet dispersion was collected.

[0154] Comparative Example 3

[0155] The difference between this example and Example 1 is only that in step (1), no acid / alkali etching solution was added, but 60 mL of deionized water solution was added.

[0156] Product performance test:

[0157] (1) Preparation yield and size test of MXene nanosheet products:

[0158] The MXene nanosheets obtained in Examples 1 to 15 and Comparative Examples 1 to 2 were characterized by scanning electron microscopy to observe the size and particle size statistics. The concentrations of the MXene nanosheet dispersions obtained in all examples and comparative examples were measured, and the yields were calculated. The concentration measurement and yield calculation methods were as follows: Take 5 mL of the volume (V 抽滤体积 ) of the MXene dispersion and perform vacuum filtration through an organic microporous membrane (0.22 μm) to obtain a MXene thin film. The thin film was dried in vacuum heating (35 °C) for 24 h, and the weight of the thin film (m 薄膜 ) was weighed to determine the concentration of the dispersion (calculation formula: dispersion concentration = m 薄膜 / V 抽滤体积 ). The corresponding preparation yield was calculated by comparing the weight of the Ti3AlC2 MAX raw material (calculation formula: total yield = total mass of all MXene nanosheets / mass of MAX phase material, m MXene / m MAX ×100%). The results are shown in Table 1.

[0159] Table 1

[0160]

[0161]

[0162] From the data in Table 1, it can be seen that through the preparation methods provided in Examples 1 to 15 of the present invention, large-sized (micrometer-scale) single-layer MXene nanosheets can be efficiently, highly yield, scalably and generally prepared. The lateral size of the MXene nanosheets reaches 3.0 μm, and the thickness is a single layer; the highest yield of the MXene nanosheets reaches 96%, and the preparation time is less than 24 h.

[0163] In Comparative Examples 1 to 2, graphite, a buffer grinding aid, was not added for ball milling, or ball milling was not carried out. Finally, the sizes of the obtained MXene nanosheets were all nanoscale (average size less than 800 nm), and the total yields of the nanosheets were very low. The yield of ball milling without adding graphite was 59% (Comparative Example 1), while the yield of not carrying out ball milling was only 1-2% (Comparative Example 2); in Comparative Example 3, ball milling was carried out without adding an acid / alkaline etching solution, and as a result, MXene nanosheets could not be prepared. It shows that in the process of preparing large-sized MXene nanosheets with high yield by the present invention, the addition of ball milling, buffer grinding aid material and acid / alkaline etching solution are all indispensable.

[0164] According to the above examples and test data, it can be seen that micrometer-scale MXene nanosheets, including Ti2CT x , Ti3C2T x , V2CT x , Ti2NT x , Ti3CNT x or Nb2CT x , have been generally prepared with high yield by the preparation method provided by the present invention, and the total yield can reach more than 82%, and the highest yield can reach 96%. Through the comparative experiments of single-factor variables in Examples 1 to 5, it can be seen that by controlling the ratio of MAX and graphite, ball milling time, ball milling speed and the speed of gradient cascade centrifugation, the corresponding size and yield can be regulated to make the preparation effect and preparation yield reach the optimum. At the same time, the ratio of raw materials and buffer grinding aid materials, as well as ball milling time and ball milling speed, are also the key factors affecting the size and yield of MXene. Under the preferably experimental conditions, the yield and size of the product can be guaranteed to the greatest extent without causing unnecessary waste, while simplifying the process and saving costs.

[0165] (2) Mechanical, electrical and electromagnetic interference shielding performance tests of macroscopic material products:

[0166] Exemplarily, the MXene nanosheet dispersion obtained in Example 1 was continuously prepared into macroscopic composite materials such as MXene fibers, MXene films and MXene aerogel blocks through solution processing processes such as wet spinning, blade coating, ice templating combined with vacuum freeze-drying ( Figure 7 ).

[0167] The mechanical, electrical and electromagnetic interference shielding properties of MXene films and MXene aerogel blocks were characterized by using an electronic universal mechanical testing machine (EM3.103-T), a double probe method on a Keithley 2400 multi-function digital source meter and an AV3629 high-performance microwave integrated vector network analyzer ( Figure 8 ). The test results are as follows:

[0168] The tensile strength of the fiber is 72.3±1.0MPa, the Young's modulus is 17.04±0.21GPa, and the toughness is 0.16±0.01MJ / m 3 ( Figure 8 a), the conductivity reaches 2395.01±108.37S / cm( Figure 8 (d) The results show that the prepared MXene nanosheets have good mechanical and electrical properties, and can be used to continuously prepare high-performance MXene composite fibers, which can be used as an ideal elementary material for multifunctional flexible wearable fabrics.

[0169] The tensile strength of the MXene film is 100.6±2.1MPa, the Young's modulus is 14.69±0.51GPa, and the toughness is 0.52±0.03MJ / m 3 ( Figure 8 b), the conductivity reaches 4865.43±106.45S / cm( Figure 8 The electromagnetic shielding effectiveness of MXene film in the range of 2-18 GHz (full band) is 33.0 dB on average, and in the range of 7.7-17.6 GHz (including the X-band of 8-12 GHz) is 34.5 dB on average, and the electromagnetic shielding effectiveness at 8.8 GHz is 36.3 dB ( Figure 8 e), exceeding the commercial standard of 20dB, showing excellent mechanical and electrical properties and good EMI shielding performance, and has important application prospects in low-cost, large-area, large-scale lightweight EMI coatings.

[0170] MXene aerogel can recover to its original shape after being compressed by more than 60%, showing good compression rebound performance ( Figure 8 (c) Elastic MXene aerogels with porous layered structures have broad application prospects in the fields of thermal insulation and sound absorption and noise reduction.

[0171] The applicant declares that the present invention illustrates a high-yield preparation method of MXene nanosheets through the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent substitution of the raw materials selected for the present invention, the addition of auxiliary components, the selection of specific methods, etc. all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A preparation method of MXene nanosheets, characterized in that, The preparation method includes the following steps: (1) Stir and mix the ternary MAX-phase precursor material and the etching solution to obtain a mixed dispersion A; wherein, the etching solution is an acidic or alkaline etching solution; (2) Place the dispersion A obtained in step (1), the buffer grinding aid material, and the grinding balls into an acid- and alkali-resistant vacuum ball milling tank for ball milling to obtain a ball-milled dispersion B; (3) Centrifuge and wash the ball-milled dispersion B obtained in step (2) with deionized water to obtain a mixed precipitate C; (4) Disperse the mixed precipitate C in step (3) in a dispersion solvent, perform ultrasonic or vortex oscillation, and finally centrifuge to separate the buffer grinding aid material to obtain a MXene nanosheet dispersion; The ternary MAX-phase precursor material in step (1) is Ti3AlC2, Ti2AlC, Ti2AlN, Ti3AlCN, V2AlC, Ti3AlCN, Ti3SiC2, Nb2AlC, (Nb,Zr)4AlC3 or Mo2Ga2C, with a size of 50 - 900 mesh; The acidic or alkaline etching solution in step (1), wherein the acidic etching solution is a mixed solution of a fluoride salt and an inorganic acid, and the alkaline solution is an alkali metal solution; the fluoride salt includes any one of lithium fluoride, sodium fluoride, potassium fluoride, cesium fluoride, and calcium fluoride. In the acidic etching solution, the molar concentration of the fluoride salt is 5 - 15 M; the inorganic acid includes any one or a mixture of two of hydrochloric acid, sulfuric acid, nitric acid, and hydrofluoric acid. In the acidic etching solution, the molar concentration of the inorganic acid is 3 - 12 M; the alkali metal solution includes any one of aqueous sodium hydroxide and aqueous potassium hydroxide, with a molar concentration of 1 - 10 M; The stirring and mixing time in step (1) is 10 - 60 min; the buffer grinding aid material in step (2) is a solid-phase inorganic van der Waals layered material, and the solid-phase inorganic van der Waals layered material includes any one or a combination of at least two of flake graphite, hexagonal boron nitride, transition metal dichalcogenide, and graphite-phase carbon nitride layered material. The two-dimensional planar size of the van der Waals layered material is 1 - 5000 μm; The mass ratio of the MAX-phase precursor material in step (1) to the buffer grinding aid material in step (2) is 1:(0.1 - 100); the material of the grinding balls in step (2) includes any one or a combination of at least two of zirconia, stainless steel, agate, hard tungsten carbide, and silicon nitride; The diameter of the grinding balls in step (2) is any one size or a combination of at least two sizes from 0.5 - 20 mm; The mass ratio of the MAX-phase precursor material in step (1) to the grinding balls in step (2) is 1:(10 - 1000); The acid- and alkali-resistant vacuum ball milling tank in step (2) is evacuated and sealed, with a vacuum degree of 0.05 - 0.1 MPa; The ball milling speed in step (2) is 100 - 1500 r / min; the ball milling time is 1 - 100 h; The average lateral size of the MXene nanosheets in step (4) is 0.001 - 40 μm, and the thickness is 1 - 5 layers.

2. The preparation method according to claim 1, wherein The mass ratio of the ternary MAX phase precursor material described in step (1) to the volume of the acidic or alkaline etching solution is: 1 g of MAX phase material corresponds to 10 - 200 mL of etching solution.

3. The preparation method according to claim 1, characterized in that The mass ratio of the MAX phase precursor material described in step (1) to the buffer grinding aid material in step (2) is 1:(0.5 - 10); The material of the acid- and alkali-resistant vacuum ball milling jar described in step (2) is any one of polytetrafluoroethylene, zirconia, stainless steel, alloy steel, agate, hard tungsten carbide, silicon nitride or sintered corundum; The volume of the acid- and alkali-resistant vacuum ball milling jar is 0.1 - 100 L.

4. The preparation method according to claim 1, wherein Step (2) further includes a step of separating the milling balls, and the method for separating the milling balls is screening with a sieve or pipette pipetting separation.

5. The preparation method according to claim 1, wherein The centrifugal washing in step (3) is to add deionized water, shake and disperse by hand repeatedly, and perform centrifugal precipitation to remove the acidic solution for washing. The centrifugal speed is 1000 - 7000 r / min; The centrifugation time is 1 - 60 min; In step (3), the washing is carried out until the pH of the dispersion liquid reaches 6 or dark MXene nanosheets are observed to be peeled off on the upper layer.

6. The preparation method according to claim 1, characterized in that, The dispersion solvent described in step (4) is selected from any one or a combination of at least two of water, ethanol, N-methylformamide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide and tetrabutylammonium hydroxide; The ultrasonic treatment in step (4) is ultrasonic treatment in an ice-water bath, and the ultrasonic power is 30 - 200 W; The vortex oscillation is carried out by using a vortex oscillator, the rotation speed is 1000 - 2000 r / min, and the oscillation time is 2 - 20 min; In step (4), after centrifugal separation, the buffer grinding aid material and the unpeeled MAX phase material are removed, and the centrifugal speed is 1000 - 5000 r / min; The concentration of the obtained MXene nanosheet dispersion liquid in step (4) is 1 - 50 mg / mL.

7. The preparation method according to claim 1, wherein Step (4) further includes performing liquid-phase gradient cascade centrifugation size selection on the obtained MXene nanosheet dispersion liquid to screen MXene nanosheets of different sizes; The rotation speed of the liquid-phase gradient cascade centrifugation is 1000 - 20000 r / min; the number of stages of the liquid-phase gradient cascade centrifugation is 4 - 40 stages; The centrifugation time of each stage in the liquid-phase gradient cascade centrifugation is independently selected from 10 - 60 min; The liquid-phase gradient cascade centrifugation is centrifugation with a gradually increasing rotation speed, and the rotation speed difference between adjacent stages in the gradient cascade centrifugation is 500 - 2000 r / min.

8. The preparation method according to claim 1, characterized in that, The yield of MXene nanosheets peeled from the ternary MAX phase material is 1 - 99%, and the monolayer ratio is 80 - 100%.

9. An MXene nanosheet obtained by the preparation method according to any one of claims 1 - 8.

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