Two-dimensional graphene has excellent optical, electrical, thermal and mechanical properties, and has huge application prospects in many traditional industries and strategic emerging industries, and is known as the next-generation key basic material. However, the bottleneck problem in the industrialization and application of graphene is how to prepare high-quality graphene products with high efficiency, large-scale, low-cost, and environmentally friendly production.

　　Two-dimensional graphene has excellent optical, electrical, thermal and mechanical properties, and has huge application prospects in many traditional industries and strategic emerging industries. It is known as the next-generation key basic material. However, the bottleneck problem in the industrialization and application of graphene is how to prepare high-quality graphene products with high efficiency, large-scale, low-cost, and environmentally friendly production.

　　Graphene is one of the emerging strategic materials that will be developed in this century. It has excellent physical and chemical properties such as stable structure, high conductivity, high toughness, high strength, and super large specific surface area (Table 1), which can greatly improve the composite material. Performance, to realize the important application of composite materials in many traditional fields and strategic emerging industries such as aerospace, military industry, semiconductors and new-generation displays, and is known as the next-generation key basic material.

　　Countries all over the world attach great importance to and raise graphene to an unprecedented height, investing a lot of manpower, material resources and financial resources to seize this strategic high ground. The European Commission listed graphene as one of only two "flagship projects for future emerging technologies." The United States also regards graphene as a strategic industry that is as important as 3D printing technology to support future technological development. China also clearly stated in the "Twelfth Five-Year Development Plan for the New Material Industry" to actively develop graphene materials.

　　1 Application-oriented graphene preparation method

　　Micromechanical peeling method

　　Geim et al. first etched microgrooves with a width of 20 μm to 2 mm and a depth of 5 μm on the surface of highly oriented pyrolytic graphite using an oxygen plasma beam, then glued them to a glass substrate with photoresist for firing, and then repeatedly removed them with transparent glue. The graphite flakes are exfoliated from the graphite, placed in an acetone solution and oscillated ultrasonically, and then the monocrystalline silicon wafer is placed in the acetone solvent. Due to van der Waals force or capillary force, the monolayer graphene will be adsorbed on the silicon wafer, thus successfully preparing Single-layer graphene.

　　This method directly exfoliates a few or single-layer graphene from graphite, which is simple and easy to implement and does not require harsh experimental conditions. The obtained graphene maintains a perfect crystal structure with few defects and high quality. The disadvantage is that the production efficiency of graphene is extremely low, which is limited to basic research in the laboratory.

　　epitaxial growth method

　　This method uses single crystal 6H-SiC as a raw material, and after etching with hydrogen, it is heated under high vacuum by electron bombardment to remove oxides. After confirming that the oxides on the sample surface are completely removed by Auger electron spectroscopy, under ultra-low vacuum (1.333×10? 8Pa), high temperature (1200～1450℃) conditions, constant temperature for 1～20min, thermal decomposition removes the oxide Si, decomposes graphene sheets whose thickness is controlled by temperature on the single crystal (0001) surface.

　　The graphene prepared by this method has high conductivity and is suitable for electronic devices with high electrical performance requirements. The main disadvantage is that this method will produce difficult-to-control defects and multi-domain structure, it is difficult to obtain a long-range ordered structure, and it is difficult to prepare large-area graphene with a single thickness. In addition, the preparation conditions are harsh, the cost is high, and the separation must be carried out under high pressure and vacuum conditions.

　　Graphite intercalation method

　　"This method uses natural flake graphite as a raw material, and alkali metal elements as an intercalating agent, and a graphite intercalation compound is obtained by mixing and reacting the intercalating agent with graphite. The graphite intercalation compound accelerates the exfoliation process of graphite from two aspects. First, the insertion of the intercalant increases the distance between graphite layers and weakens the van der Waals force between graphite layers. Secondly, after lithium, potassium, cesium and other alkali metals are inserted, an electron is input into the graphite crystal lattice to make the crystal face negatively charged and generate electrostatic repulsion, which makes the graphite crystals easy to peel and separate. Finally, the graphene sheet is obtained by ultrasonic and centrifugal treatment.

　　However, the graphene sheets prepared by this method are multi-layered (>10 layers) with a thickness of more than tens of nanometers, and the added intercalation material will destroy the sp2 hybrid structure of graphene, which affects the physical and chemical properties of graphene. .

　　solution stripping method

　　The solvent exfoliation method is to disperse graphite in a solvent to form a low-concentration dispersion, use ultrasound or high-speed shear to weaken the van der Waals force between the graphite layers, insert the solvent between the graphite layers, and perform layer-by-layer peeling to prepare graphene . In 2014, Paton et al. first dispersed graphite in N-methylpyrrolidone (NMP) solvent, used simple high-speed shear to quickly and efficiently exfoliate graphite, and obtained a stable dispersion of graphene with few layers, and proposed a method to realize graphene An effective way of large-scale production.

　　The liquid phase exfoliation method can prepare high-quality graphene. The entire liquid phase exfoliation process does not introduce chemical reactions, avoiding the introduction of structural defects on the graphene surface, which provides high-quality graphene for the application of high-performance electronic devices. The main disadvantage is that the yield is very low, and it is not suitable for large-scale production and commercial applications.

　　Chemical Vapor Deposition (CVD) method

　　This method is the main method for the industrial large-scale preparation of semiconductor thin film materials by chemically reacting the reacting substances in a gaseous state under higher temperature conditions, and annealing to produce solid substances that are deposited on the surface of the metal substrate. The CVD method prepares graphene by heating at a high temperature to decompose the gas into carbon atoms and hydrogen atoms, annealing to deposit carbon atoms on the surface of the substrate to form graphene, and finally removing the metal substrate by chemical etching. In 2009, Hong et al. deposited graphene with a thickness of 6-10 atomic layers on a nickel layer for the first time by CVD. In 2013, Bharathi et al. prepared large-size single-crystal graphene with a diameter of about 1 cm through the CVD method.

　　The CVD method is considered to be the most promising method for producing high-quality, large-area graphene, and the most potential industrial method for producing graphene films. However, this method is not suitable for preparing large-scale graphene macroscopic powder, which limits its application. In addition, the separation of graphene from the substrate is a method of chemically corroding metals, which requires a large amount of acid to be consumed, which will cause huge pollution to the environment, and at the same time make the cost high. Therefore, how to peel off the substrate to obtain intact graphene efficiently and at low cost is the main problem faced by this method.

　　oxidation reduction method

　　The oxidation-reduction method can be simplified into three steps of "oxidation-exfoliation-reduction". Specifically, the graphite is first oxidized with a strong oxidizing agent, and the surface of the graphite is oxidized to form hydrophilic hydroxyl, epoxy, and carboxyl groups containing oxygen groups. This process will expand the interlayer distance of graphite from the original 0.34nm to 0.8nm. The expansion of the interlayer distance can effectively weaken the van der Waals attraction between the layers and is easy to peel off; then the graphite oxide is peeled off by the ultrasonic method. The graphite oxide suspension radiates in dense and dense phases, causing a large number of tiny bubbles to be generated in the liquid. These bubbles form and grow in the negative pressure zone where the ultrasonic wave propagates in the longitudinal direction, and quickly close in the positive pressure zone. This is called " In the process of the cavitation effect, the closed bubble can form an instantaneous high pressure exceeding 1.0×108Pa. The continuously generated high pressure is like a series of small “explosions” that continuously impact the graphite oxide, causing the graphite oxide flakes to rapidly peel off to obtain a single layer. Graphene oxide; Finally, the graphene oxide is reduced at high temperature or in a reducing solution to reduce and remove the hydroxyl, epoxy, and carboxyl groups on the surface of the graphene oxide to restore the perfect two-dimensional sp2 of graphene. Hybrid structure to obtain graphene products.

　　summarized the current main preparation methods of graphene in terms of product quality, cost-effectiveness, environmental friendliness, purity, yield, and industrialization prospects. It can be seen that, compared to other preparation methods with complex operations, high cost or low yield, the oxidation-reduction method can produce high-quality graphene in large quantities and efficiently, and the process is relatively simple, which is the current large-scale preparation of graphene materials. The only effective way.