The preparation methods of graphene mainly include the following methods: mechanical peeling method, redox method, epitaxial method, silicon carbide epitaxy method, Hemer method and chemical vapor deposition method.
Mechanical peeling
The mechanical exfoliation method uses the friction and relative movement between an object and graphene to obtain a thin layer of graphene material. This method is simple to operate, and the obtained graphene usually maintains a complete crystal structure. In 2004, two British scientists used scotch tape to peel off natural graphite layer by layer to obtain graphene. It was also classified as a mechanical peeling method. This method was once considered to have low production efficiency and was unable to industrialize mass production. This method can prepare micron-sized graphene, but its controllability is low, and it is difficult to achieve large-scale synthesis.
In 2016, Chinese scientists invented a simple and efficient green peeling technology to realize the large-scale preparation of few-layer graphene through the soft rolling transfer process between "ball-microbial".
Redox method
The redox method is to oxidize natural graphite by using chemical reagents such as sulfuric acid, nitric acid, and oxidants such as potassium permanganate and hydrogen peroxide to increase the spacing between graphite layers and insert oxides between graphite layers to prepare graphite oxide (Graphite Oxide). Then, the reactant is washed with water, and the washed solid is dried at a low temperature to obtain graphite oxide powder. The graphite oxide powder is peeled off by physical peeling, high-temperature expansion, and other methods to obtain graphene oxide. Finally, the graphene oxide is chemically reduced to obtain graphene (RGO). This method is simple to operate and has a high yield, but the product quality is low. The oxidation-reduction method uses strong acids such as sulfuric acid and nitric acid, which is dangerous and requires a large amount of water for cleaning, which causes great environmental pollution.
Graphene prepared by the redox method contains abundant oxygen-containing functional groups and is easy to modify. However, it is difficult to control the oxygen content of the reduced graphene when the graphene oxide is reduced. At the same time, the graphene oxide will be continuously reduced under the influence of the sun and the high temperature in the carriage during transportation. Therefore, the oxidation-reduction method The quality of graphene produced is often inconsistent from batch to batch, and it is difficult to control the quality.
Oriented epiphysis
The epitaxial method uses the atomic structure of the growth substrate to "seed" graphene. First, carbon atoms are infiltrated into ruthenium at 1150°C and then cooled. After cooling to 850°C, a large number of previously absorbed carbon atoms will float on the surface of ruthenium. The single layer of carbon atoms in the final lens shape will grow into a complete layer of graphene. After the first layer is covered, the second layer begins to grow. The graphene in the bottom layer will interact strongly with ruthenium, and after the second layer, it is almost completely separated from ruthenium, leaving only weak electrical coupling. However, the graphene sheets produced by this method are often uneven in thickness, and the adhesion between the graphene and the matrix will affect the characteristics of the carbon layer.
Silicon carbide epitaxy
The SiC epitaxy method is to sublime silicon atoms away from the material in a high-temperature environment of ultra-high vacuum, and the remaining C atoms are reconstructed through self-assembly to obtain graphene-based on the SiC substrate. This method can obtain high-quality graphene, but this method requires higher equipment.
Hemer Method
The graphite oxide is prepared by the Hummer method; the graphite oxide is placed in water and ultrasonically dispersed to form a uniformly dispersed graphene oxide solution with a mass concentration of 0.25 g/L to 1 g/L, and then the mass is added dropwise to the graphene oxide solution Ammonia with a concentration of 28%; dissolve the reducing agent in water to form an aqueous solution with a mass concentration of 0.25g/L～2g/L; mix the prepared graphene oxide solution and the reducing agent aqueous solution evenly, and place the resulting mixed solution in the oil The mixture is stirred under bath conditions, and after the reaction is completed, the mixture is filtered, washed, and dried to obtain graphene.
Chemical vapor deposition
Chemical vapor deposition (CVD) is a method of using carbon-containing organic gas as a raw material for vapor deposition to produce graphene films. This is the most effective way to produce graphene films. The graphene prepared by this method has the characteristics of large area and high quality, but the cost is relatively high at this stage, and the process conditions need to be further improved. Because the thickness of the graphene film is very thin, a large-area graphene film cannot be used alone and must be attached to macroscopic devices to be of value, such as touch screens, heating devices, and so on.
The low-pressure vapor deposition method is used by some scholars. It generates a single layer of graphene on the surface of Ir. Through further research, it can be known that this graphene structure can cross metal steps, and the continuous and micron-scale single-layer carbon structure is gradually increasing. Ir formed on the surface. [35] Single crystal graphene on the order of millimeters is obtained by the method of surface segregation. Centimeter-level graphene and epitaxial growth of graphene on polycrystalline Ni film were discovered by some scholars. The surface of a 300-nanometer thick Ni film was heated at 1000°C and exposed to a CH4 atmosphere at the same time. After a period of reaction Later, a large area of a few layers of graphene film will be formed on the metal surface.
