Synthesis Method of MXene

MXene is a new type of two-dimensional layered material, and its synthesis methods are diverse, mainly including the following:

Wet chemical etching method: This is one of the most commonly used methods. MXene is prepared by etching the MAX phase with hydrofluoric acid (HF). Wet chemical etching methods mainly include the following methods:

Hydrofluoric acid (HF) etching method: This is the original and most commonly used method. By immersing the MAX phase (such as Ti₃AlC₂) in a hydrofluoric acid solution, the selective permeation of F-ions is used to destroy the M-A metal bond in the MAX phase, thereby achieving the removal of the A layer. For example, Ti₃AlC₂ reacts in an HF solution to generate Ti₃C₂T_x MXene. Although this method is effective, there are environmental and health risks because HF is highly corrosive and volatile.

Combination etching method of fluoride salts and acids: In order to reduce the impact on the environment, researchers have developed a method of etching using a combination of fluoride salts (such as LiF) and strong acids (such as HCl). This method can avoid the direct use of HF and can also effectively remove A-layer atoms. For example, a mixed solution of LiF and HCl can be used to etch Ti₃AlC₂ to prepare Ti₃C₂T_x MXene.

In addition to the above methods, there are also methods such as etching using NaOH aqueous solution, HCl electrolyte, NH₄HF₂ organic solvent, and CuCl₂ molten salt reaction. These methods have their own advantages and disadvantages. For example, the MXene nanosheets obtained by NaOH aqueous solution reaction and HCl electrolyte etching have fewer defects. In addition, some researchers use electrochemical etching, which is considered to be mild and efficient.

High-temperature fluoride melting method: This method involves melting the MAX phase with fluoride at high temperature to achieve etching. This method can effectively avoid the use of HF, thereby improving the safety of the experiment. High-temperature fluoride melting method is a method for preparing MXene. This method involves mixing the MAX phase with a fluorine-containing metal salt and then heating it at high temperature in an inert gas atmosphere to generate MXene. This method utilizes the etching effect of fluoride to remove the Al element in the MAX phase through high-temperature treatment to form a two-dimensional MXene material.

In the high-temperature fluoride melting method, fluorine-containing salts are usually used as reaction media. These salts melt at high temperatures and help prevent the oxidation of reactants during high-temperature synthesis. For example, researchers used a mixed molten salt of KF, LiF and NaF and kept it at 550°C for 30 minutes to successfully achieve the selective etching of the Al layer in Ti₄AlN₃, thereby obtaining two-dimensional MXene.

The advantage of this method is that it can effectively remove the Al element in the MAX phase under high temperature conditions, and the use of molten salt as a reaction medium can improve the efficiency and selectivity of the reaction. However, high-temperature treatment may also bring some challenges, such as the oxidation resistance of the material. Therefore, in practical applications, MXene materials need to be properly post-treated to improve their stability in high-temperature environments.

Bottom-up synthesis method: The bottom-up synthesis method of MXene is a method for preparing two-dimensional materials based on atoms or molecules through a direct synthesis strategy. This method usually involves techniques such as chemical vapor deposition (CVD) and physical vapor deposition (PVD), which can produce large-area, continuous and uniform MXene films. Compared with the traditional top-down method, the bottom-up method has higher controllability and precision, and can better control the composition, size and surface functionalization of MXene.

In the bottom-up synthesis process, high-temperature reaction conditions are usually used, such as at least 1085°C, by reacting carbon or nitrogen sources in methane or ammonia with transition metal atoms to generate high-quality MXene crystals. The advantage of this method is that the nucleation and growth processes can be affected by precisely adjusting the growth conditions, thereby controlling the morphology, thickness and size of the MXene crystals. In addition, this method can also avoid the problem of waste liquid and waste gas generated during etching. Bottom-up methods also include ion sputtering, which bombards the target surface with low-energy heavy ions to produce sputtered atoms, thereby forming a repetitive nanolayer composed of a single phase. Although this method has a low yield and high equipment requirements, it can prepare MXene with no end groups on the surface.

Lewis acid molten salt stripping method: Lewis acid molten salt stripping method is a general method for preparing MXene materials. Different from traditional solution stripping methods (such as using HF acid), this method has higher chemical safety and lower difficulty and cost in waste liquid treatment. This method uses molten inorganic salts at high temperatures as etchants to synthesize MXene materials by selectively etching the A atomic layer in the MAX phase. This method is not only applicable to a variety of MAX phase materials, but also can adjust the functional groups on the MXene surface to obtain MXene materials with different chemical properties.

The advantages of the Lewis acid molten salt exfoliation method are its universality and controllability. For example, researchers can prepare MXene by selecting suitable Lewis acid molten salts and MAX phase systems, and obtain MXene materials with different functional groups on the surface by adjusting the molten salt anion coordination.

Alkali etching method: Alkaline etching of MXene is a method for selectively removing A-layer elements (such as aluminum) from MAX phase precursors to obtain two-dimensional MXene materials. This method usually needs to be carried out at high temperatures and high concentrations of alkaline solutions to ensure sufficient corrosiveness to remove the A-layer elements in the MAX phase. In the alkaline etching process, sodium hydroxide (NaOH) is often used as an etchant. For example, Li et al. developed a method for producing Ti₃C₂T_x MXene using sodium hydroxide, which uses hydroxide anions (OH) to target the aluminum layer, resulting in oxidation of aluminum atoms and the addition of more base to complete the exposed titanium atoms. This method can produce more OH and O termination groups compared to traditional HF etching, thereby improving the performance of MXene. In addition, in order to improve the effect of alkaline etching, it is usually necessary to treat under anaerobic conditions to avoid the interference of dissolved oxygen in the reaction. For example, when using concentrated NaOH for hydrothermal treatment, the dissolved oxygen in DI water must be removed by Ar gas. This method can effectively remove the A-layer elements in the MAX phase and obtain high-quality MXene materials

Electrochemical etching: Electrochemical etching is an advanced technology for preparing MXene, which achieves the selective removal of the A-layer of MAX phase materials by reacting under the action of an electric field. This method is different from traditional chemical etching, which involves two independent reaction processes at the anode and cathode. In the electrochemical etching process, a fluorine-containing electrolyte, such as tetrafluoroboric acid (HBF4), is usually used as an etchant. This method is milder and safer than traditional HF etching, and can achieve higher yields and better material properties. For example, electrochemical etching using HBF4 as an electrolyte can more effectively prepare MXene sheets with larger lateral sizes, mainly due to the suppressed HF decomposition and the formation of larger pores. A significant advantage of electrochemical etching is its environmental friendliness and safety. Compared with chemical etching using toxic HF, electrochemical etching reduces the use of hazardous substances while enabling large-scale production. In addition, this method also allows the etching rate and MXene quality to be optimized by adjusting the electrolyte concentration and temperature. However, electrochemical etching also has some challenges. Since the MAX phase is a dense block with only the surface in contact with the electrolyte, a three-layer structure of MXA phase inner core, MXene middle layer and carbide-derived carbon may be formed over time, which makes MXene difficult to collect and has a low yield. To overcome this problem, intercalants can be added to expand the interlayer spacing, promote the etching of the internal MAX phase and inhibit excessive etching of the outer MXene.

Ultrasonic assisted exfoliation: Ultrasonic assisted exfoliation is a commonly used method in the preparation of MXene. It achieves efficient exfoliation of MXene nanosheets through high-intensity ultrasonic treatment. In the preparation process of MXene, hydrofluoric acid (HF) etching method, such as LiF/HCl mixed solution etching method, is usually used to etch away the metal layer (such as Al) in the MAX phase material to obtain MXene nanosheets. The etched product needs to be further treated to achieve the exfoliation of single-layer or few-layer MXene. Ultrasonic treatment plays a key role in this process. It weakens the hydrogen bonds or van der Waals forces between multilayer MXene by generating microbubbles and cavitation effects, thereby promoting the separation between layers. Studies have shown that under appropriate temperature and ultrasonic power conditions, ultrasonic treatment can significantly improve the exfoliation efficiency of MXene. For example, at 70°C and moderate ultrasonic power conditions, gentle and thorough exfoliation can be achieved. In addition, high-intensity ultrasonic exfoliation (HIUE) environment can efficiently prepare high-yield MXene materials in a short time (such as 3 hours). However, it should be noted that the ultrasonic exfoliation process of MXene is not static, because the size and thickness of the original MAX phase particles are different, resulting in different sizes of multilayer MXene obtained by etching. Therefore, in actual operation, the time and intensity of ultrasonic exfoliation need to be adjusted according to the specific situation to avoid damage to the MXene nanosheets.

These methods have their own advantages and disadvantages, and the selection of a suitable synthesis method depends on the specific application requirements and experimental conditions. For example, although the wet chemical etching method is simple, it requires strict control of the HF concentration to avoid defects; while the high-temperature fluoride melting method can be carried out at a lower temperature, but requires special equipment and conditions. In addition, the research on new low-cost synthesis methods will help promote the application of MXene in fields such as environmental governance.

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