Methods of removing MXene functional groups and preparing defunctionalized MXenes

Abstract

The application discloses a method for removing MXene functional groups and preparing defunctionalized MXene, wherein the method for removing MXene functional groups comprises the following steps: heating MXene containing functional groups and a thermite or a magnether to a predetermined temperature to ignite the reaction agent to generate an aluminum thermal reaction or a magnesium thermal reaction, stopping heating and cooling to normal temperature. The application aims at the technical problems that the existing method for removing the functional groups of MXene material is complex in operation, not universal, and difficult to effectively remove the surface functional groups such as -F and -Cl which have strong interaction with M atoms on the surface of MXene, and provides a method for quickly removing the surface functional groups of MXene material based on a self-propagating method.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of new materials, and in particular relates to a method for removing MXene functional groups and preparing defunctionalized MXene. BACKGROUND

[0002] MXene is a kind of transition metal carbide or nitride with two-dimensional layered structure. MXenes are mainly prepared by selectively etching A layer elements in MAX phase precursors, wherein M represents early transition metal elements (Sc, Ti, V, Cr, etc.), A is an element mainly from groups 13-16 (Al, Si, etc.), and X is C and / or N. The general formula of MXene can be represented as M n+1 X n T x (n = 1-4), wherein T x represents a surface terminal, which is generally -F, -O, -OH, -Cl, -Br, etc. Due to its unique two-dimensional layered structure, hydrophilic surface and metal conductivity, MXene has wide application prospects in the fields of electrochemical energy storage, electromagnetic shielding, wave absorption, catalysis, biomedicine, sensing, etc.

[0003] Etching to prepare MXene is usually carried out in an aqueous solution of hydrofluoric acid, so the MXene surface is terminated with a mixture of -F, -O and -OH functional groups, which is usually represented as T x Etching MAX phase in Lewis acid molten salt above 500℃ will produce MXene with -Cl or -Br termination. Theoretical calculations show that the type of surface termination and the presence or absence have a significant impact on the physical and chemical properties of MXene. For example, Qing Tang et al. predicted through theory that bare Ti3C2 monolayer exhibits low lithium diffusion barrier and high lithium storage capacity (up to Ti3C2Li2 stoichiometry) as an electrode material for lithium-ion batteries. -F and -OH functional groups hinder the transport of lithium and reduce the storage capacity of lithium.

[0004] In 2020, Vladislav Kamysbayev et al. reacted Ti3C2Br2 and Ti2CBr2 with LiH at 300℃, which first realized the removal of MXene surface functional groups and prepared bare Ti3C2 and Ti2C. However, this method cannot remove -F, -O, -OH and other surface functional groups with strong interaction with M atoms on the surface of MXene, and does not have wide applicability. In addition, this method is complex to operate and requires precise control of reaction conditions, making it difficult to achieve rapid preparation of defunctionalized MXene. At present, there is no method that can quickly remove MXene functional groups.

[0005] The currently reported method for preparing bare MXene monolayers is the reaction of -Br-terminated MXene with LiH at 300 DEG C. This method cannot remove the -F, -O, -OH, -Cl functional groups on the surface of MXene, has great limitations, and is complex to operate and requires precise control of reaction conditions. The reducing agent LiH required for the reaction is chemically unstable, LiH is flammable, and reacts violently with water to generate corrosive lithium hydroxide and hydrogen gas, is easily deliquesced in air to produce hydrogen, and faces safety problems such as flammability and explosiveness. SUMMARY

[0006] The purpose of the present application is to solve the technical problems that the existing methods for removing functional groups of MXene materials are complex to operate, not universal, and difficult to effectively remove -F, -Cl and other surface functional groups with strong interaction with M atoms on the surface of MXene, and to provide a method for rapidly removing surface functional groups of MXene materials based on self-propagating method.

[0007] The first aspect of the present application provides a method for removing functional groups of MXene, comprising the steps of: heating MXene containing functional groups with a thermite or a magnether to a predetermined temperature to ignite the thermite or the magnether to generate an aluminothermic reaction or a magnether reaction, stopping heating and cooling to room temperature to obtain MXene with removed functional groups.

[0008] In some embodiments, the above-mentioned thermite comprises aluminum powder and metal oxide powder.

[0009] In some embodiments, the metal oxide in the above-mentioned thermite is selected from one or more of Fe2O3, Fe3O4, V2O5, Cr2O3, MnO2.

[0010] In some embodiments, the above-mentioned magnether comprises magnesium powder and metal oxide powder.

[0011] In some embodiments, the metal oxide is selected from one or more of Fe2O3, Fe3O4, FeO, WO2, WO3, SiO2.

[0012] In some embodiments, the above-mentioned thermite or magnether contains an ignition agent.

[0013] In some embodiments, the above-mentioned functional groups include one or more of -F, -OH, -Cl, -Br, -I, -S, -Se, -Te, -NH.

[0014] In some embodiments, the above-mentioned MXene material containing functional groups is prepared by liquid etching method, molten salt etching method or gas phase etching method.

[0015] In some embodiments, the above-mentioned predetermined temperature is greater than or equal to 300 DEG C; preferably, the predetermined temperature is between 800 DEG C and 1000 DEG C.

[0016] In some embodiments, the functional group-containing MXene and the thermite or magnether are separately placed in different containers.

[0017] The second aspect of the present application also provides a method for preparing a defunctionalized MXene or MXene-MAX heterojunction, comprising the steps of: heating a MAX phase material, an etchant and a thermite to a first predetermined temperature to melt the etchant; and then heating to a second predetermined temperature to ignite the reaction agent to generate an aluminothermic reaction; the etchant is selected from a metal halide salt, and the melting point of the metal halide salt is less than the ignition temperature of the thermite. In the implementation process, the second predetermined temperature is greater than the ignition temperature of the thermite; and the first predetermined temperature is greater than the melting point of the metal halide salt and less than the ignition temperature of the thermite. That is, the technical concept of the method of the present application is to utilize the temperature difference between the molten salt etching reaction and the ignition aluminothermic reaction, and to realize the etching of the MAX phase and the removal of the functional group through the step-by-step heating process in a tube furnace.

[0018] In some embodiments, the metal halide salt is selected from one or more of zinc chloride, iron chloride, copper chloride and zinc bromide; more preferably, the metal halide salt is zinc chloride.

[0019] In some embodiments, the thermite comprises aluminum powder and metal oxide powder; preferably, the metal oxide is selected from one or more of Fe2O3, Fe3O4, V2O5, Cr2O3 and MnO2.

[0020] In some embodiments, the ignition temperature of the thermite is between 800°C and 1000°C.

[0021] In some embodiments, the A component in the MAX phase material is etched completely or partially. When the etchant etches the A component in the MAX phase material completely, the resulting product is a MXene material; when the etchant etches the A component in the MAX phase material partially, the resulting product has both MXene structure and MAX phase structure, which is a MXene-MAX heterojunction material.

[0022] In some embodiments, the MXene phase material is mixed with the etchant to obtain a mixture, and then the mixture is separately placed with the thermite.

[0023] In some embodiments, the first predetermined temperature is between 300°C and 800°C, and the second predetermined temperature is greater than or equal to 800°C; preferably, the first predetermined temperature is between 400°C and 600°C, and the second predetermined temperature is between 800°C and 1000°C.

[0024] The third aspect of the present application provides a defunctionalized MXene material or MXene-MAX heterojunction material prepared by the above method.

[0025] The fourth aspect of the present application provides an application of the above defunctionalized MXene material or MXene-MAX heterojunction material in the fields of electromagnetic shielding, wave absorption, catalysis and batteries.

[0026] The present application provides a simple and effective removal method for the removal of surface functional groups such as -F and -Cl which have strong interaction with M atoms on the surface of MXene, and the high temperature of aluminum or magnesium thermal reaction is utilized without changing the morphology and crystal structure of MXene, and the removal of functional groups is realized at the same time, which is simple and easy to operate and has industrial practical value. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The XRD spectrum (a) and SEM image (b) of the MXene prepared in Example 1 of the present application.

[0028] Figure 2 The XPS spectrum of the MXene prepared in Example 1 of the present application.

[0029] Figure 3 The XRD spectrum (a) and SEM image (b) of the B-MXene Ti3C2 prepared in Example 1 of the present application.

[0030] Figure 4 The XPS spectrum of the B-MXene Ti3C2 prepared in Example 1 of the present application.

[0031] Figure 5 The XRD spectrum (a) and SEM image (b) of the MXene prepared in Example 2 of the present application.

[0032] Figure 6 The XPS spectrum of the MXene prepared in Example 2 of the present application.

[0033] Figure 7 The XRD spectrum (a) and SEM image (b) of the B-MXene Ti3C2 prepared in Example 2 of the present application.

[0034] Figure 8 The XPS spectrum of the B-MXene Ti3C2 prepared in Example 2 of the present application. DETAILED DESCRIPTION

[0035] The technical solutions of the present application are described below through specific examples. It should be understood that one or more steps mentioned in the present application do not exclude the existence of other methods and steps before and after the combination steps, or other methods and steps can be inserted between these explicitly mentioned steps. It should also be understood that these examples are only used to illustrate the present application and not to limit the scope of the present application. Unless otherwise specified, the numbering of each method step is only for the purpose of identifying each method step, and is not intended to limit the arrangement order of each method or to limit the scope of the implementation of the present application. Changes or adjustments of the relative relationship can also be considered as the scope of the implementation of the present application without substantial technical content changes.

[0036] The raw materials and instruments used in the examples are not specifically limited in source and can be purchased on the market or prepared according to conventional methods well known to those skilled in the art.

[0037] The present application provides a method for removing functional groups from MXene, which comprises: placing functional group-containing MXene and an aluminum or magnesium thermite in the same space under vacuum or inert atmosphere, heating to a first predetermined temperature to ignite the aluminum or magnesium thermite, stopping heating and cooling to room temperature to obtain functional group-removed MXene. In the method of the present application, the aluminum or magnesium thermite is ignited to produce a self-sustaining high temperature by instantaneous aluminum or magnesium thermal reaction, thereby removing the functional groups on the MXene to obtain functional group-removed MXene (Bare MXene, hereinafter abbreviated as B-MXene).

[0038] The types of surface functional groups contained in MXene are generally related to the choice of etchant in the etching method. Common functional groups include: -F, -OH, Cl, -Br, -I, -S, -Se, -Te, -NH. The method of the present application can effectively remove these functional groups and has universality. Functional group-containing MXene can be obtained by liquid phase etching, gas phase etching or molten salt etching. The etchant for liquid phase etching is a mixture of hydrofluoric acid solution or hydrochloric acid solution and metal fluoride salt. The etchant for gas phase etching can be selected from gaseous hydrogen halide (such as HCl, HBr, HI) or halogen element (such as I2). The etchant for molten salt etching can be selected from metal halide salt (such as ZnCl2, CuCl2, AgCl, etc.).

[0039] The thermite of the present application is a mixed powder comprising aluminum powder and metal oxide, which can be selected from, but not limited to, one or more of Fe2O3, Fe3O4, V2O5, Cr2O3, MnO2. In some embodiments, the thermite can further comprise an ignition agent, such as magnesium, for initiating the thermite reaction between the aluminum powder and the metal oxide. The thermite of magnesium is a mixed powder comprising magnesium powder and metal oxide, which can be selected from, but not limited to, one or more of Fe2O3, Fe3O4, FeO, WO2, WO3, SiO2. However, due to the low ignition temperature of the magnesium thermite (400-500°C), it is more difficult to store safely, and in practice, the thermite with a higher ignition temperature (800-1000°C) is preferred.

[0040] The temperature for igniting the thermite reaction in the present application is preferably between 800-1000°C. In some embodiments, in the presence of an ignition agent, the predetermined temperature can be the temperature at which the ignition agent burns, such as in the presence of a magnesium strip, the predetermined temperature is between 300°C and 600°C, which can achieve ignition.

[0041] In a preferred embodiment, MXene and thermite or magnesium thermite are placed in separate containers close to each other to avoid introducing reaction byproducts into MXene.

[0042] The present application also provides a method for directly preparing defunctionalized MXene or MXene-MAX heterojunction, which uses a low-melting-point molten salt (melting point less than 800°C) as an etchant to etch all or part of the A component in the MAX phase material to obtain MXene or MXene-MAX heterojunction; then heating the ignition thermite to achieve defunctionalization of the MXene or MXene-MAX heterojunction. The method of the present application takes advantage of the temperature difference between the molten salt etching reaction and the ignition thermite reaction, and through the step-by-step heating process in the tube furnace, both etching of the MAX phase and removal of the functional group are achieved, simplifying the process flow of defunctionalized MXene and greatly saving energy consumption. Due to the low ignition temperature of the magnesium thermite, it is not suitable for this method.

[0043] In some embodiments, a low-boiling-point (boiling point less than 1000°C) molten salt is also preferred. In the defunctionalization step, the high reaction heat generated by the thermite reaction can also vaporize the low-boiling-point molten salt, which is beneficial for the molten salt etchant to be removed from the reaction system with the gas stream, avoiding or reducing the difficulty of subsequent water washing to remove impurities. The low-boiling-point molten salt can be selected from one or more of zinc chloride, iron chloride, copper chloride, and zinc bromide. The preferred etchant of the present application is zinc chloride, which has the lowest melting point (283°C). The technical features of the present application are further illustrated by the following specific examples:

[0044] Example 1

[0045] This embodiment provides a method for removing Ti3C2T. x The surface functional group method involves preparing MXene using a conventional liquid-phase etching method. The etchant is a mixture of lithium fluoride (LiF) and concentrated hydrochloric acid, and the precursor is the MAX phase Ti3AlC2. More specifically, the process includes: adding 1g of Ti3AlC2 to the etching solution (45ml concentrated HCl, 15ml deionized water, and 4.8g LiF), stirring and etching in a 35℃ oil bath for 24 hours. After etching, the product is subjected to conventional ultrasonic stripping, washed with deionized water, and freeze-dried to obtain two-dimensional sheet-like Ti3C2T. x (or represented as Ti3C2F) x ).

[0046] The obtained Ti3C2T x X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed, and the results are as follows: Figure 1 As shown in a and b, the XRD pattern shows a characteristic (002) diffraction peak of MXene at 2θ = 6.5°, and the SEM image shows a two-dimensional sheet-like structure. Figure 2 Ti3C2T x The X-ray photoelectron spectroscopy (XPS) spectrum showed strong peaks for Ti 2p and C 1s, as well as strong peaks for functional groups such as O 1s and F 1s, with no Al peak observed. This indicates that the product prepared by this method mainly contains -F functional groups.

[0047] The method for removing functional groups according to the present invention includes the following steps: (1) mixing 1 gram of aluminum powder and 3 grams of ferric oxide evenly to prepare an aluminothermic agent, and placing it in a large crucible; (2) taking 500 mg of Ti3C2T x (3) Place the crucible in a small crucible and place the small crucible inside a large crucible containing the thermite. Place the crucible on a porcelain boat, put it into a tube furnace, and pass argon gas for protection at a rate of 0.5 L / min. (4) Heat the tube furnace to 800°C at a heating rate of 3°C / min and hold for 1 min. Ignite the thermite, stop heating and allow it to cool naturally to room temperature. Collect the reaction product (represented as B-MXene or Ti3C2).

[0048] The XRD pattern of this B-MXene shows characteristic (002) diffraction peaks of MXene at around 2θ = 3°, and no TiC peaks are observed. Figure 3 (a) indicates that high temperature did not cause a transformation in the MXene crystal structure; the interlayer spacing of the two-dimensional MXene decreased, and the SEM image showed that the MXene Ti3C2T structure was preserved. x Two-dimensional morphological features ( Figure 3 (b) XPS Atlas ( Figure 4)showed strong peaks of Ti 2p and C 1s, and no strong peaks of F 1s and other functional groups were found, and the O 1s peak was derived from the inevitable contact with oxygen adsorbed oxygen after exposure to air, which proved that the defunctionalized MXene Ti3C2 was successfully prepared by the self-propagating method of the application.

[0049] In other embodiments, the thermite in Example 1 can also be replaced by a magnesium thermite, such as magnesium powder and Fe2O3. The magnesium thermite, similar to the aluminum thermite, can also ignite to produce high temperature instantaneously, achieving the effect of defunctionalization. Considering safety, the application preferably uses an aluminum thermite.

[0050] Example 2

[0051] This example provides an embodiment of a molten salt method for preparing MXene combined with defunctionalization. In this example, the etchant is zinc chloride (ZnCl2), and the precursor MAX phase is Ti3AlC2. The process includes the steps of molten salt etching of the MAX phase and ignition of the aluminum thermite to remove the functional groups.

[0052] More specific implementation steps include: (1) MXene preparation step by molten salt method: 1g of Ti3AlC2 is mixed with 2.1g of ZnCl2 mixture in a small crucible, which is then placed inside a large crucible containing an aluminum thermite (a mixture of 1g of aluminum powder and 3g of Fe2O3 powder), the crucible is placed on a porcelain boat and placed in a tube furnace, and argon gas is introduced at a rate of 0.5L / min; under argon atmosphere, the temperature is raised to 400℃ at a rate of 10℃ / min, and then kept for 48h. In this step, ZnCl2 melts and etches Ti3AlC2 to generate MXene Ti3C2Cl containing Cl functional groups x ; (2) defunctionalization step: the temperature is raised to 800℃ at a rate of 10℃ / min, and then kept for 1min. In this step, the aluminum thermite is ignited, argon gas is introduced into the tube furnace at a rate of 0.5L / min, and the product is taken out after natural cooling to room temperature.

[0053] The MXene product prepared by step (1) molten salt method is characterized by XRD pattern Figure 5 a) shows the characteristic (002) diffraction peak of MXene at 2θ = 7.8°, and SEM image Figure 5 b) shows that it has a typical accordion morphology, indicating that the accordion-shaped MXene Ti3C2Cl x is successfully prepared. XPS spectrum Figure 6 ) shows strong peaks of Ti 2p and C 1s, and strong peaks of Cl 2p and other functional groups, and no Al peak is found. The product obtained in step (2) is characterized by XRD pattern Figure 7a) Similar to MXene, the MXene characteristic (002) diffraction peak appears at 2θ = 7.8°, and there is no TiC diffraction peak, indicating that high temperature does not cause the crystal structure of MXene to change, and the interlayer spacing of the accordion structure of MXene is almost not affected; the SEM image Figure 7 b) shows that it retains the typical accordion morphology of MXene. The XPS spectrum Figure 8 ) shows strong peaks of Ti 2p and C1s, and no strong peak of Cl 2p functional group is found. It shows that the self-propagating method of the application successfully retains the morphology characteristics of MXene, removes the Cl functional group on MXene, and obtains the product of B-MXene Ti3C2 which is removed functional group.

[0054] Example 3

[0055] This embodiment provides an embodiment of a MXene-MAX heterojunction combined with a functional group removal method prepared by a molten salt method, which is similar to Example 2, except that the etching time in the specific implementation step (1) is 12h, and the molten ZnCl2 partially etches the Al atoms in Ti3AlC2. Finally, the functional group-removed Ti3C2-Ti3AlC2 heterojunction material is obtained.

[0056] By controlling the etching time in the step of preparing MXene by a molten salt method, the degree of etching of the MAX phase by the molten salt can be controlled. When the molten salt does not completely etch the A component in the MAX phase, the product obtained is a MXene-MAX heterojunction material containing both MXene and MAX structure.

[0057] In the functional group removal step, the high reaction heat generated by the aluminothermic reaction can also vaporize the low-boiling-point molten salt, which is beneficial to the removal of the gas-phase etchant from the system with the gas flow, avoiding the subsequent water washing step and greatly simplifying the production process flow.

[0058] In other embodiments, the metal halide salt ZnCl2 in Example 2 can also be replaced by other low-boiling-point metal halide salts, such as zinc bromide, iron chloride, etc.

[0059] In other embodiments, the MAX phase material (chemical formula: M n+1 AX n ) in Example 2 can also be replaced by other types of elements, such as M representing a transition metal element, selected from one or more of titanium, tungsten, molybdenum, chromium, niobium, vanadium, and zirconium, A selected from aluminum, silicon, gallium, and tin elements, and X selected from one or more of carbon, nitrogen, or boron elements.

[0060] The foregoing description of specific exemplary embodiments of the application has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the application to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the application be limited not with this detailed description, but rather by the claims appended hereto.

Claims

1. A method of de-functionalizing MXene, characterized by, The steps comprise: The functional group-containing MXene and the thermite or magnethermite are placed separately in different containers, heated to a predetermined temperature to ignite the thermite or magnethermite to produce a self-sustaining high temperature for instantaneously removing the functional groups on the MXene surface, and then the heating is stopped and the temperature is cooled to room temperature to obtain the defunctionalized MXene; the functional groups include one or more of -F, -OH, -Cl, -Br, -I, -S, -Se, -Te, -NH.

2. The method of claim 1, wherein, The thermite includes aluminum powder and metal oxide powder.

3. The method of claim 2, wherein, The metal oxide is selected from one or more of Fe2O3, Fe3O4, V2O5, Cr2O3, MnO2.

4. The method of claim 1, wherein, The magnethermite includes magnesium powder and metal oxide powder.

5. The method of claim 4, wherein, The metal oxide is selected from one or more of Fe2O3, Fe3O4, FeO, WO2, WO3, SiO2.

6. The method of any one of claims 1 to 5, wherein, The thermite or magnethermite contains an ignition agent.

7. The method of claim 1, wherein, The functional group-containing MXene material is prepared by a liquid etching method, a molten salt etching method, or a gas etching method.

8. The method of claim 1, wherein, The predetermined temperature is greater than or equal to 300°C.

9. The method of claim 1, wherein, The predetermined temperature is between 800°C and 1000°C.

10. A method of preparing a defunctionalized MXene or MXene-MAX heterojunction, characterized by the steps The steps comprise: The MAX phase material, the etchant, and the thermite are heated to a first predetermined temperature to melt the etchant, and then heated to a second predetermined temperature to ignite the reaction agent to produce a self-sustaining high temperature for instantaneously removing the functional groups on the MXene surface; the etchant is selected from a metal halide salt, and the melting point of the metal halide salt is less than the ignition temperature of the thermite; The MAX phase material is mixed with the etchant to obtain a mixture, and then placed separately from the thermite.

11. The method of claim 10, wherein, The metal halide salt is selected from one or more of zinc chloride, iron chloride, copper chloride, and zinc bromide.

12. The method of claim 10, wherein, The metal halide salt is zinc chloride.

13. The method of claim 10, wherein, The thermite includes aluminum powder and metal oxide powder.

14. The method of claim 13, wherein, The metal oxide is selected from one or more of Fe2O3, Fe3O4, V2O5, Cr2O3, MnO2. The ignition temperature of the thermite is between 800°C and 1000°C.

15. The method of claim 10, wherein, The A component in the MAX phase material is etched completely or partially.

16. The method of claim 10, wherein, The first predetermined temperature is between 300°C and 800°C, and the second predetermined temperature is greater than or equal to 800°C.

17. The method of claim 16, wherein, The first predetermined temperature is between 400°C and 600°C, and the second predetermined temperature is between 800°C and 1000°C.

18. A defunctionalized MXene material obtained by the method of any one of claims 1 to 9, or a MXene material or MXene-MAX heterojunction material prepared by the method of any one of claims 10 to 17.

19. Use of the MXene material or MXene-MAX heterojunction material of claim 18 in the fields of electromagnetic shielding, wave absorption, catalysis, and batteries.

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