1, micromechanical stripping method
In 2004, Geim et al. successfully stripped a single layer of graphene from highly oriented pyrolytic graphite by micromechanical stripping method for the first time and observed it. Geim research group successfully prepared quasi-two-dimensional graphene by this method and observed its morphology, revealing the reasons for the existence of two-dimensional crystal structure of graphene. High-quality graphene can be prepared by micro-mechanical stripping method, but it has the disadvantages of low yield and high cost, and can not meet the requirements of industrialization and large-scale production. At present, it can only be prepared as a small laboratory.
2, chemical vapor deposition method
Chemical vapor deposition (CVD) has made a new breakthrough in the large-scale preparation of graphene for the first time (refer to CVD for preparing high-quality graphene). CVD method refers to the process of preparing solid materials by chemical reaction of reaction substances in gaseous state to generate solid substances and depositing them on the surface of heated solid substrate.
Kong et al. of MIT, Hong et al. of Sungkyunkwan University in Korea and Chen et al. of Purdue University in the United States are preparing graphene by CVD. They used a simple tube deposition furnace with nickel as the substrate. Introducing carbon-containing gas, such as hydrocarbon, and decomposing into carbon atoms at high temperature to deposit on the surface of nickel to form graphene. Graphene film was separated from nickel sheet by slight chemical etching, and graphene film was obtained. When the light transmittance is 80%, the conductivity of the film can reach1.1×106s/m, which is a potential substitute for the current transparent conductive film. High-quality and large-area graphene can be prepared by CVD, but the price of ideal substrate material single crystal nickel is too expensive, which may be an important factor affecting the industrial production of graphene. CVD method can meet the requirements of large-scale preparation of high-quality graphene, but the cost is high and the process is complicated.
3. Redox method
Redox method is the best method to prepare graphene because of its low cost and easy realization. But also can prepare stable graphene suspension, thus solving the problem that graphene is not easy to disperse. Oxidation-reduction method refers to the reaction of natural graphite with strong acid and strong oxidizing substance to generate graphite oxide (GO), which is dispersed by ultrasonic wave to prepare graphene oxide (single layer graphite oxide). Reducing agent is added to remove oxygen-containing groups on the surface of graphite oxide, such as carboxyl, epoxy and hydroxyl, to obtain graphene.
After the redox method was put forward, it became the simplest method to prepare graphene in the laboratory with its simple and easy process, which was favored by the majority of graphene researchers. Ruoff et al. found that graphene can be obtained by adding chemicals such as dimethyl hydrazine, hydroquinone, sodium borohydride (NaBH4) and liquid hydrazine to remove oxygen-containing groups of graphene oxide. Stable graphene suspension can be prepared by redox method, which solves the problem that graphene is difficult to disperse in solvents.
The disadvantage of redox method is that the macroscopic preparation is easy to bring waste liquid pollution, and the prepared graphene has certain defects, such as topological defects such as five-membered rings and seven-membered rings or structural defects with -OH groups, which will lead to the loss of some electrical properties of graphene and limit its application.
4, solvent stripping method
The principle of solvent stripping method is that a small amount of graphite is dispersed in the solvent to form a low concentration dispersion, and van der Waals force between graphite layers is destroyed by ultrasonic wave. At this time, the solvent can be inserted between graphite layers and peeled off layer by layer to prepare graphene. This method will not destroy the structure of graphene like redox method, and can prepare high-quality graphene. Among N- methylpyrrolidone, the yield of graphene is the highest (about 8%), and the conductivity is 6500 s/m. It is found that highly oriented pyrolytic graphite, thermal expansion graphite and microcrystalline artificial graphite are suitable for preparing graphene by solvent stripping method. High quality graphene can be prepared by solvent stripping method, and no defects are introduced into the surface of graphene during the whole liquid phase stripping process, which provides a broad application prospect for its application in microelectronics, multifunctional composite materials and other fields. The disadvantage is that the yield is low.
5, solvothermal method
Solvothermal method is an effective method to prepare materials by heating the reaction system to the critical temperature (or near the critical temperature) in a special closed reactor (autoclave) with organic solvent as the reaction medium.
Solvothermal method solves the problem of large-scale preparation of graphene, but it also brings negative effects of low conductivity. In order to solve this problem, researchers combined solvothermal method and redox method to prepare high-quality graphene. Dai et al. found that the resistance of graphene films prepared by reducing graphene oxide under solvothermal conditions is lower than that prepared under traditional conditions. Solvothermal method has attracted more and more scientists' attention because it can prepare high-quality graphene in a closed system at high temperature and high pressure. The combination of solvothermal method and other preparation methods will become another highlight of graphene preparation.
6. Other methods
The preparation methods of graphene include high-temperature reduction, photoreduction, epitaxial crystal growth, microwave, arc and electrochemical methods. On this basis, the author put forward a new method to prepare nano-graphene microchips by mechanical method, and achieved good results in the mass production of graphene. How to make comprehensive use of the advantages of various graphene preparation methods, learn from each other's strong points, solve the problem of insoluble and unstable graphene and improve its structure and electrical properties is a hot and difficult point in future research, and also opens up a new way for the preparation and synthesis of graphene in the future.
7. Preparation and Prospect of Graphene
Large-scale preparation of high-quality graphene crystal materials is the basis of all applications, and developing simple and controllable chemical preparation methods is the most convenient and feasible way, which requires chemists' long-term unremitting exploration and efforts; Chemical modification of graphene: chemical modification, doping, surface functionalization and synthesis of graphene derivatives, development of graphene and related materials to achieve more functions and applications; Surface chemistry of graphene: Due to the unique atomic and electronic structure of graphene crystal, the interaction between gas molecules and graphene surface will show many unique phenomena, which will provide a unique model surface for surface chemistry, especially surface catalysis research; At the same time, graphene has a perfect two-dimensional periodic plane structure and can be used as an ideal catalyst carrier. Metal/graphene system will provide a brand-new model catalysis research system for surface catalysis research.