Method for improving conductivity and oxidation resistance of MXene

Abstract

The invention discloses a method for improving the conductivity and oxidation resistance of MXene, and relates to the technical field of MXene materials.The method comprises the following steps that MXene is added into an organic macromolecule intercalation solution for intercalation, and a single-layer MXene nanosheet solution and a few-layer MXene nanosheet solution are obtained through washing, ultrasonic treatment and other treatment; adding the single few-layer MXene nanosheet solution into an alkali metal salt solution, carrying out flocculation and spontaneous ion exchange, and centrifuging to remove liquid, so as to obtain MXene flocculate; and finally, adding the MXene flocculate into an alkali metal salt solution for storage, or washing the MXene flocculate with deionized water until the MXene is not flocculated any more and is in a colloidal state for use. According to the method, organic macromolecules combined with the surface of MXene are removed through ion exchange, the conductivity of the MXene material can be effectively improved, preservation of the MXene material is facilitated, and the method has a high application value.

Description

Technical Field

[0001] The present invention relates to the technical field of MXene materials, and in particular to a method for improving the electrical conductivity and oxidation resistance of MXene. Background Art

[0002] MXenes is a general term for a large class of layered two-dimensional transition metal carbons and / or nitrides, with a general chemical formula of M n+1 X n T x , where M represents transition metal elements such as Ti, V, Nb, Mo, Zr, Hf, Ta, etc., X represents C and / or N elements, and T represents terminal groups such as -F, -O, -OH, etc. MXenes are usually obtained by selectively etching the A atomic layer in the precursor MAX phase. The unique structure and composition give MXenes excellent hydrophilicity, metal-like conductivity, outstanding flexibility, ultra-high electrochemical specific capacity and easy processing. Therefore, MXenes are widely used in many fields such as electrochemical energy storage, smart sensing, electromagnetic shielding, biomedicine and optics. For example, in the field of sensors, when MXenes are used as flexible piezoresistive sensor materials, they can show ultra-high sensitivity and excellent wearable flexibility.

[0003] However, there are several problems with the prior art. First, most of the etching preparation methods of MXene are still HF direct etching, in-situ HF formation or difluoro-based etching agents. However, when the MXene obtained by the above method is further used to obtain a MXene nanosheet colloidal solution, an external intercalation agent is required to obtain a high-concentration solution (when no intercalation agent is used, for example, the single-layer yield after ultrasonic treatment is less than 5%). And for this HF acid or HF-like acid-etched MXene, the intercalation yield of macromolecules such as tetraalkylammonium hydroxide is often the highest, but current studies have ignored the effect of intercalated macromolecules on the performance of MXene. When organic macromolecules are intercalated, the organic macromolecules combine with MXene nanosheets through electrostatic action, resulting in the inability to remove the macromolecules through multiple water washing and centrifugation. In addition, due to the large interaction between MXene and macromolecules, this method will waste a large amount of MXene colloids even after centrifugal washing. In addition, studies have shown that this macromolecular intercalation leads to a large interlayer spacing, and water, oxygen and macromolecules will promote MXene degradation. Therefore, existing MXene colloids often show large interlayer spacing, resulting in lower conductivity (<500S / cm) and faster oxidation (<10 days). Therefore, it is necessary to seek a treatment method to remove MXene intercalated macromolecules and keep them non-oxidized in water for a long time, so that the film conductivity is higher and the storage time is longer. Summary of the invention

[0004] In order to solve the above technical problems, the purpose of the present invention is to provide a method for improving the electrical conductivity and antioxidant properties of MXene. This method removes organic macromolecules bound to the surface of MXene by ion exchange, which can effectively improve the electrical conductivity of the MXene material and is beneficial to the preservation of the MXene material, and has great application value.

[0005] The technical solution of the present invention to solve the above technical problems is as follows: a method for improving the electrical conductivity and oxidation resistance of MXene is provided, comprising the following steps:

[0006] (1) adding MXene to an organic macromolecular intercalation solution, heating and stirring, then centrifuging to discard the supernatant and washing with water, then adding water and ultrasonically treating, and finally centrifuging to collect the supernatant to obtain a single-layer MXene nanosheet solution;

[0007] (2) adding the single few-layer MXene nanosheet solution obtained in step (1) to an alkali metal salt solution, heating and stirring, waiting for automatic flocculation, then standing to spontaneously perform ion exchange, and then centrifuging to remove the liquid to obtain MXene floccules;

[0008] (3) adding the MXene flocs obtained in step (2) to an alkali metal salt solution for storage;

[0009] Alternatively, the MXene flocs obtained in step (2) are washed with deionized water until the MXene no longer flocculates but is in a colloidal state for use.

[0010] The beneficial effects of the technical solution of the present invention are:

[0011] (1) The method of the present invention first uses organic macromolecules to intercalate MXene, and the stirring time is 3-12h. The longer the time, the higher the single-layer yield, which can reach up to 90%. After that, most of the organic macromolecules are removed by washing 1-3 times, and the supernatant turns black. At this time, continuing to wash will result in the loss of a single few-layer MXene nanosheet. Stop washing and collect the upper solution by ultrasonic treatment to obtain a single few-layer MXene nanosheet. Then, add alkali metal salt solution, MXene will quickly combine with alkali metal salt, and due to the negative charge of MXene and the positive charge of cations, it will completely settle within 5 minutes. In addition, the steric effect of macromolecules also makes it easier for alkali metal ions to combine with MXene, and further replaces macromolecules through ion exchange, thereby achieving the removal of organic macromolecules.

[0012] (2) This method can significantly improve the antioxidant properties of MXene materials and extend the shelf life. When inorganic salts are added to the MXene suspension, their hydration not only reduces the free water molecules, but also forms a protective barrier on the MXene surface by forming a hydrated ion layer. This barrier can effectively isolate the direct contact between oxygen molecules and MXene, thereby significantly inhibiting the oxidative attack of oxygen molecules on MXene. In addition, the presence of inorganic salts may also change the ionic strength of the solution, further affecting the solubility and diffusion rate of oxygen molecules, providing additional protection for MXene.

[0013] Furthermore, MXene is Ti 3 C 2 T x 、V 4 C 3 T x 、Ti 3 CN、Mo 2 C.V 2 CT x and Nb 2 CT x At least one of the above. It may also be other types of MXene materials.

[0014] Furthermore, in step (1), the organic macromolecular intercalation solution is at least one of tetramethylammonium hydroxide solution (TMAOH), tetraethylammonium hydroxide solution (TEAOH) and tetrabutylammonium hydroxide solution (TMBOH); and the concentration of the organic macromolecular intercalation solution is 1-3 mol / L.

[0015] Furthermore, in step (1), the mass volume ratio of MXene and organic macromolecular intercalation solution is 1-3 g: 30-50 mL.

[0016] Furthermore, in step (1), heating and stirring are performed at 40-60° C. for 3-12 h.

[0017] Furthermore, in step (2), ultrasonic treatment is performed for 20-40 min.

[0018] Furthermore, in step (2), the alkali metal salt solution is at least one of a sodium chloride solution, a lithium chloride solution and a potassium chloride solution; and the concentration of the alkali metal salt solution is 1-5 mol / L.

[0019] Furthermore, in step (2), the volume ratio of the single few-layer MXene nanosheet solution to the alkali metal salt solution is 0.5-1.5:5.

[0020] Furthermore, in step (2), heating and stirring are performed at 40-60° C. for 20-40 min.

[0021] Further, in step (2), the mixture is allowed to stand for 3-12 hours.

[0022] Furthermore, the centrifugal rotation speed is 3000-4000r / min.

[0023] The present invention has the following beneficial effects:

[0024] 1. The method of the present invention can effectively remove the intercalant organic macromolecules bound to the surface of MXene through ion exchange, so that the conductivity of the MXene stripped of organic macromolecules is increased by at least 10 times.

[0025] 2. The method of the present invention has mild conditions and can avoid the oxidation of MXene. The water reaction of inorganic salts can also reduce the activity and coordination of dissolved oxygen in the aqueous solution.

[0026] 3. In addition, the MXene treated by the method of the present invention can be stored for a long time without being oxidized. When needed, it can also be redispersed into MXene colloid by multiple deionized water washings for continued use. This method has the advantages of low cost, mild reaction conditions and long storage time. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the implementation process in Example 1;

[0028] Figure 2 This is a comparison diagram before and after flocculation in Example 1;

[0029] Figure 3 This is a picture showing the effect of MXene preservation after treatment in Example 2;

[0030] Figure 4 XRD comparison diagram of MXene before and after removing organic macromolecules;

[0031] Figure 5 This is a comparison chart of the conductivity test results before and after removing organic macromolecules from MXene. DETAILED DESCRIPTION

[0032] The principles and features of the present invention are described below, and the examples are only used to explain the present invention and are not used to limit the scope of the present invention. If no specific conditions are specified in the embodiments, they are carried out according to normal conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be purchased commercially.

[0033] Example 1

[0034] A method to improve the electrical conductivity and oxidation resistance of MXene, the process is as follows Figure 1 As shown, the following steps are included:

[0035] (1) 2g MXene (Ti 3 C2 T x ) was added to 40 mL of 3 mol / L tetramethylammonium hydroxide solution (TMAOH), heated and stirred at 60 °C for 12 h, then centrifuged at 3500 r / min to discard the supernatant and washed with water, then added water and ultrasonically treated for 30 min, and finally centrifuged at 3500 r / min to collect the upper solution to obtain a single few-layer MXene nanosheet solution (TMAOH-Ti 3 C 2 T x );

[0036] (2) 20 mL of the single few-layer MXene nanosheet solution obtained in step (1) was added to 100 mL of 5 mol / L lithium chloride solution, heated and stirred at 60 °C for 30 min, and then waited for automatic flocculation. Then, it was allowed to stand for 12 h to spontaneously exchange ions, and then centrifuged at 3500 r / min to remove the liquid to obtain MXene flocs (before and after flocculation, as shown in Figure 2 shown);

[0037] (3) Washing the MXene flocs obtained in step (2) with deionized water until the MXene no longer flocculates but is in a colloidal state for use.

[0038] Example 2

[0039] A method for improving the electrical conductivity and oxidation resistance of MXene, comprising the following steps:

[0040] (1) 2 g MXene (V 4 C 3 T x ) was added to 40 mL of 2 mol / L tetramethylammonium hydroxide solution (TMAOH), heated and stirred at 50 °C for 6 h, then centrifuged to discard the supernatant and washed with water, then added with water and ultrasonically treated for 30 min, and finally centrifuged to collect the upper solution to obtain a single few-layer MXene nanosheet solution;

[0041] (2) adding 20 mL of the single few-layer MXene nanosheet solution obtained in step (1) to 100 mL of 3 mol / L lithium chloride solution, heating and stirring at 50° C. for 30 min to allow for automatic flocculation, then allowing the solution to stand for 6 h to allow spontaneous ion exchange, and then centrifuging to remove the liquid to obtain MXene floccules;

[0042] (3) The MXene flocs obtained in step (2) are added to an alkali metal salt solution for storage.

[0043] Example 2 The treated MXene was stored for two months. Figure 3 As shown, it will not degrade and turn white.

[0044] Example 3

[0045] A method for improving the electrical conductivity and oxidation resistance of MXene, comprising the following steps:

[0046] (1) 1g MXene (Ti 3 CN) was added into 50 mL of 1 mol / L tetraethylammonium hydroxide solution (TEAOH), heated and stirred at 40 °C for 12 h, then centrifuged to discard the supernatant and washed with water, then added with water and ultrasonically treated for 40 min, and finally centrifuged to collect the upper solution to obtain a single few-layer MXene nanosheet solution;

[0047] (2) adding 10 mL of the single few-layer MXene nanosheet solution obtained in step (1) to 100 mL of 1 mol / L sodium chloride solution, heating and stirring at 40° C. for 40 min to allow for automatic flocculation, then allowing the solution to stand for 3 h to allow spontaneous ion exchange, and then centrifuging to remove the liquid to obtain MXene floccules;

[0048] (3) Washing the MXene flocs obtained in step (2) with deionized water until the MXene no longer flocculates but is in a colloidal state for use.

[0049] Example 4

[0050] A method for improving the electrical conductivity and oxidation resistance of MXene, comprising the following steps:

[0051] (1) 3g MXene (V 2 CT x ) was added to 30 mL of 3 mol / L tetrabutylammonium hydroxide solution (TMBOH), heated and stirred at 60 °C for 3 h, then centrifuged to discard the supernatant and washed with water, then added water and ultrasonically treated for 20 min, and finally centrifuged to collect the upper solution to obtain a single few-layer MXene nanosheet solution;

[0052] (2) 30 mL of the single few-layer MXene nanosheet solution obtained in step (1) was added to 100 mL of a 5 mol / L potassium chloride solution, heated and stirred at 60° C. for 20 min, and then waited for automatic flocculation, and then allowed to stand for 12 h to allow spontaneous ion exchange, and then centrifuged to remove the liquid to obtain a MXene flocculent;

[0053] (3) Washing the MXene flocs obtained in step (2) with deionized water until the MXene no longer flocculates but is in a colloidal state for use.

[0054] Test Example 1

[0055] The MXene solution containing organic macromolecules (TMAOH-Ti 3 C 2 T x) and ion-exchanged MXene (LiCl-Ti 3 C 2 T x ) is coated as MXene film.

[0056] (1) XRD test results are as follows Figure 4 As shown in the XRD spectrum, it can be seen that after ion exchange, the interplanar spacing decreases from 1.457nm to 1.246nm, which indicates that the macromolecules are replaced and turned into smaller alkali metal cations intercalated between MXene sheets.

[0057] (2) In addition, the conductivity of MXene was tested by four-probe method. The results are as follows: Figure 5 As shown, the conductivity increased from 200 S / cm to over 3000 S / cm after ion exchange.

[0058] Test Example 2

[0059] Other methods were used to test the removal effect of intercalating agents on organic macromolecules.

[0060] (1) After the organic macromolecules are intercalated, solvents such as isopropanol / n-hexane are added for replacement washing. 30 mL of organic solvent is added to every 10 mL of MXene solution containing organic macromolecules. After shaking for a few minutes, it is found that due to the polar functional groups on the surface of MXene, it cannot be well dissolved in organic solvents, while organic molecules can be dissolved in organic solvents. Therefore, centrifugation at 3500r / min for 1 minute and multiple washings are performed to finally obtain MXene with most of the organic solvent removed. However, after filtration and membrane formation, the tested conductivity is less than 800S / cm, which shows that although the removal of most of the organic molecules has increased the conductivity by 4 times from 200S / cm, it is still far from that of ion exchange. This is because solution replacement only removes the macromolecules in the solution, while the macromolecules bound to the MXene by electrostatic force cannot be removed. It was later found that the MXene would begin to degrade in water within 20 days. Therefore, this method cannot remove the macromolecules bound to the surface of MXene.

[0061] (2) The second method is to degrade organic molecules at high temperature. Through experiments, it was found that MXene intercalated with tetramethylammonium hydroxide can completely remove organic molecules at 400°C. However, this method has a fatal problem. The water solvent and macromolecules are removed at a high temperature of 400°C, resulting in the direct formation of a MXene film. The MXene film cannot be dispersed again in the aqueous solution to form a colloid due to the strong interaction between the monolayer and the monolayer stacking. Therefore, it cannot be processed like colloidal MXene. In addition, the easy oxidation conditions of MXene are water, oxygen and high temperature. The temperature of this method is above 400°C, and the MXene obtained by this method is also oxidized.

[0062] In summary, it is difficult to completely remove the organic macromolecules bound to the surface of MXene by washing with other solvents, and MXene after high-temperature degradation treatment is difficult to process and use, and it is easy to cause oxidation of MXene. The method of the present invention can effectively remove organic macromolecules through ion exchange, which not only greatly improves the electrical conductivity of MXene, but also is beneficial to the preservation of MXene materials.

[0063] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent substitution or improvement made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for improving the electrical conductivity and oxidation resistance of MXene, characterized in that: The following steps are involved: (1) adding MXene to an organic macromolecular intercalation solution, heating and stirring, then centrifuging to discard the supernatant and washing with water, then adding water and ultrasonically treating, and finally centrifuging to collect the supernatant to obtain a single-layer MXene nanosheet solution; (2) adding the single few-layer MXene nanosheet solution obtained in step (1) to an alkali metal salt solution, heating and stirring, waiting for automatic flocculation, then standing to spontaneously perform ion exchange, and then centrifuging to remove the liquid to obtain MXene floccules; (3) adding the MXene flocs obtained in step (2) into the alkali metal salt solution for storage; Alternatively, the MXene flocs obtained in step (2) are washed with deionized water until the MXene no longer flocculates but is in a colloidal state for use.

2. The method for improving the electrical conductivity and oxidation resistance of MXene according to claim 1, characterized in that: In step (1), the organic macromolecular intercalation solution is at least one of tetramethylammonium hydroxide solution, tetraethylammonium hydroxide solution and tetrabutylammonium hydroxide solution; and the concentration of the organic macromolecular intercalation solution is 1-3 mol / L.

3. The method for improving the electrical conductivity and oxidation resistance of MXene according to claim 1, characterized in that: In step (1), the mass volume ratio of the MXene and the organic macromolecular intercalation solution is 1-3 g: 30-50 mL.

4. The method for improving the electrical conductivity and oxidation resistance of MXene according to claim 1, characterized in that: In step (1), heat and stir at 40-60°C for 3-12h.

5. The method for improving the electrical conductivity and oxidation resistance of MXene according to claim 1, characterized in that: In step (2), ultrasonic treatment is performed for 20-40 min.

6. The method for improving the electrical conductivity and oxidation resistance of MXene according to claim 1, characterized in that: In step (2), the alkali metal salt solution is at least one of a sodium chloride solution, a lithium chloride solution and a potassium chloride solution; and the concentration of the alkali metal salt solution is 1-5 mol / L.

7. The method for improving the electrical conductivity and oxidation resistance of MXene according to claim 1, characterized in that: In step (2), the volume ratio of the single few-layer MXene nanosheet solution to the alkali metal salt solution is 0.5-1.5:

5.

8. The method for improving the electrical conductivity and oxidation resistance of MXene according to claim 1, characterized in that: In step (2), heat and stir at 40-60° C. for 20-40 min.

9. The method for improving the electrical conductivity and oxidation resistance of MXene according to claim 1, characterized in that: In step (2), let it stand for 3-12 hours.

10. The method for improving the electrical conductivity and oxidation resistance of MXene according to claim 1, characterized in that: The centrifugal speed is 3000-4000r / min.

Images