Introduction to the preparation of graphene by chemical method

Graphene has unique structure and excellent performance. In recent years, it has attracted extensive research interest in chemistry, physics and materials, and has made a lot of progress in the preparation of graphene. This paper mainly explains the method of preparing graphene by chemical method.

At present, graphene is mainly prepared by chemical methods in the laboratory. The method was first used as a nucleus of benzene rings or other aromatic systems. Six Cs on benzene rings or large aromatic rings were replaced by multi-step coupling reactions, and the cycle was repeated. Make the aromatic system larger, To obtain graphene with a certain size of planar structure. On this basis, people continue to improve, making the graphite oxide reduction method become the most promising and promising method for the synthesis of graphene and its materials. In addition, chemical vapor deposition and crystal epitaxial growth methods can also be used to prepare high-purity graphene on a large scale.

Preparation of graphene by chemical vapor deposition

The principle of chemical vapor deposition is to introduce one or more gaseous substances into a reaction chamber and react chemically to form a new material deposited on the substrate surface. It is one of the most widely used technologies for the large-scale industrialization of semiconductor thin film materials.

Srivastava et al. used microwave enhanced chemical vapor deposition to grow petals of about 20 nm thickness on the Si substrate surrounded by Ni, and studied the influence of microwave power on the morphology of graphite sheets. A graphite sheet with a smaller thickness than the previous preparation method was obtained. The results showed that the larger the microwave power, the smaller the graphite sheet, but the higher the density. The graphite sheet prepared by this method contains more Ni elements.

Kim et al. added a layer of Ni with a thickness of less than 300 nm to the Si substrate, then heated the substance in a mixture of methane, hydrogen, and argon at 1000 °C, and quickly reduced it to room temperature. This process can deposit 6 to 10 layers of graphene in the upper part of the Ni layer. The graphene prepared by this method has high conductivity, good transparency, and high electron mobility(~ 370cm2 /(V · s)), and has a room temperature half-integer quantum Hall effect. Graphical graphene films can be prepared by making Ni-layer graphics. These films can be transferred to different flexible substrates while ensuring quality. This transfer can be achieved by two methods: First, the Ni is corroded with a solvent to float the graphene film on the surface of the solution, and then the graphene is transferred to any required substrate; Another is the use of rubber-stamped technology to transfer thin films.

Chemical vapor deposition method can meet the requirements of large-scale preparation of high-quality, large-area graphene, but at this stage due to its high cost, complex process and precise control conditions, the development of graphene preparation of this method is constrained. Further research.

Preparation of graphene by epitaxial growth method

ClarieBerger et al. used this method to prepare single-layer and multi-layer graphene flakes and studied their properties. By heating, graphene is obtained by removing Si from the Si-formed(00001) surface of a single crystal 6H-SiC. The sample after oxidation or H2 etching of the surface is at a high vacuum(UHV; Basepressure 1.32 × 10-8Pa) Heated to 1000 °C by electron bombardment to remove surface oxides(repeatedly remove oxides to improve surface mass), and after the oxide is completely removed by Auger electron spectroscopy, it is heated up to 1250-1450 °C, Temperature 1-20 min. Graphite flakes on the Si surface grow slowly and cease to grow soon after reaching high temperatures, while graphite sheets on the C surface are not limited and their thickness can reach 5 to 100 layers. The thickness of the graphene sheet formed is determined by the heating temperature. Two types of graphene can be obtained by this method: one is graphene that grows on the Si layer. Due to the contact with the Si layer, the conductive properties of this graphene are greatly affected; The other is graphene, which grows on the C layer and has excellent electrical conductivity. Both are greatly affected by the SiC substrate. This method has harsh conditions(high temperature, high vacuum) and the resulting graphene is not easily separated from the substrate and can not be used for mass manufacturing of graphene.

Graphene prepared by graphite oxide reduction

Graphene is prepared by graphite oxide reduction method by dispersing graphite sheets in strong oxidizing mixed acids such as concentrated nitric acid and concentrated sulfuric acid, and then adding potassium permanganate or potassium chlorate strong and other oxidants to oxidize to obtain graphite oxide(GO) hydrosol. After ultrasonic treatment to obtain graphene oxide, Finally, graphene is obtained by reduction. This is the most commonly used method for preparing graphene.

Graphite itself is a hydrophobic substance. However, the oxidation process has led to the formation of a large number of structural defects. These defects can not be completely eliminated even after annealing at 1100 °C. Therefore, there are a large number of hydroxyl groups, carboxyl groups, and epoxy groups on the surface and edges of GO., It's a hydrophilic substance. Due to the presence of these functional groups, GO easily reacts with other reagents to obtain modified graphene oxide. At the same time, the GO layer spacing(0.7 ~ 1.2 nm) is also larger than the original graphite layer spacing(0.335 nm), which is conducive to the intercalation of other material molecules. There are generally three methods for preparing GO: Standenmaier, Brodie, and Hummers. The basic principle of the preparation is to treat graphite with strong proton acid first to form a graphite interlayer compound, and then add a strong oxidant to oxidize it. GO reduction methods include chemical liquid phase reduction, thermal reduction, plasma method reduction, hydrogen arc discharge stripping, super-boundary water reduction, light reduction, solvent thermal reduction, and microwave reduction.

For the first time, Stankovich et al. oxidized and dispersed scale graphite in water, and then reduced it with hydrazine hydrate. During the reduction process, high molecular weight sodium polystyrene sulfonate (PSS) was used to adsorb and coat the surface of the graphite oxide layer to avoid agglomeration. Due to the strong non-covalent bond between PSS and graphene (π? π stacking force), the aggregation of graphene sheets is prevented, and the composite has good solubility in water (1mg/mL). Thus, a PSS-coated modified oxidized graphite monolith was prepared. On this basis, Stankovich et al. prepared a modified single-layer graphene/polystyrene composite material having a low diafiltration value (about 0.1% by volume fraction) and excellent electrical conductivity (0.1 S/m)..

This method is environmentally friendly, efficient, low-cost, and can be industrialized on a large scale. Its disadvantage is that strong oxidants will seriously damage the electronic structure of graphene and the integrity of crystals, affecting electronic properties, thus limiting its application in the precise field of Microelectronics to a certain extent.

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