Graphenes

Graphene is a single-atom layer two-dimensional planar crystal composed of carbon atoms with a honeycomb structure, which is another new element structure after fullerene and carbon nanotubes. Graphene is a stable material, because theoretically thermodynamics do not allow any two-dimensional crystal to exist at a limited temperature. The discovery of graphene immediately attracted widespread attention in the physical world.

Figure 1. Graphene structure and common morphology.

Applications:

Graphene is both the thinnest and toughest material, whose fracture strength is a hundred times higher than the best steel. It has a high-quality two-dimensional crystal structure, ultra-high electron mobility at room temperature, good mechanical properties and thermal conductivity.

• Heavy metal sewage purification: The theoretical value of the specific surface area of graphene can reach 2630m²/g, and there are a large number of hydroxyl, carboxyl, epoxy and other strong adsorption groups on the surface, making graphene have a strong adsorption effect. With the in-depth study on the adsorption properties of graphene, the surface functional groups can be changed or combined with magnetic materials to avoid agglomeration after metal absorption, so that the graphene can be recycled.
• Electrochemical energy storage: The special two-dimensional structure of graphene can realize point-to-face contact of active materials, and has high conductivity and low conduction threshold. Graphene films, powders, composite materials and inorganic nanoparticles show excellent performance in lithium battery and solar cell applications.
• Sensor: The conjugated two-dimensional structure of graphene makes it have higher electron accommodation, transmission capacity, electrical conductivity and larger specific surface area. This structure makes graphene very sensitive to the surrounding environment. In the electrochemical sensor, the larger surface area of graphene makes heteroatoms doped into graphene produce more active sites, improve and enhance the electrocatalytic activity of the material and electrode sensitivity, and improve the sensing of the electrochemical sensor sensitivity.
• Extraction technology: Because of its unique chemical and physical properties, graphene is not easily contaminated during the extraction process and has good extraction adsorption. Currently, it is mainly used in magnetic solid phase extraction, solid phase micro extraction and solid phase extraction. Traditional solid phase extraction is more mature than the first two, which has been commercialized.
• Textile field: The application of graphene and its oxides in the textile field mainly focuses on the development of new textile fiber materials, fabric function finishing and fabric dyeing, but the application restriction lies in the poor solubility of graphene. At present, wet spinning technology and modified graphene fiber structure are used to expand the application of graphene in textile field.

Production Processes:

• Mechanical peeling method: This method uses mechanical external force to peel off graphene or its nanosheets from highly oriented pyrolytic graphite. The advantage of this method is that the prepared graphene crystal has a complete molecular structure, simple operation, low cost, and the disadvantage is low efficiency, making it not suitable for large-scale production.
• Chemical vapor deposition method: This method is an important approach for preparing graphene on a large scale. It is mainly through the reasonable control of the time, temperature, pressure, gas concentration, catalyst, etc. of the gaseous substance on the metal surface chemical reaction system, so that graphene can be smoothly deposited on the substrate. The graphene prepared by this method has few defects and high quality.
• Epitaxial growth method: This method uses high-temperature heating of silicon carbide to remove silicon in silicon carbide so that carbon atoms can be rearranged and crystallized to prepare graphene sheets. The quality of graphene prepared by epitaxial growth method is good, but it is difficult to prepare graphene with uniform thickness and large area, and the strong force between the substrate and the graphene is not conducive to the separation of graphene and the substrate, so it is not suitable for large-scale production.
• Cut carbon tube method: The preparation of graphene by carbon tube cutting method uses oxidant such as argon gas, sulfuric acid, potassium permanganate and the like to longitudinally cut single-layer or multi-layer coaxial sleeve carbon nanotubes into anisotropic strip-shaped graphene sheets.

References:

1. Sun Yiqing, Wu Qiong, Shi Gaoquan. Graphene based new energy materials [J]. Energy Environ. Sci., 2011, 4, 1113-1132.
2. Chee W. K., Lim H. N., Huang N. M., Harrison I. Nanocomposites of graphene/polymers: a review [J]. RSC Adv., 2015, 5, 68014-68051.
3. Chen Kunfeng, Song Shuyan, Liu Fei, Xue Dongfeng. Structural design of graphene for use in electrochemical energy storage devices [J]. Chem. Soc. Rev., 2015, 44, 6230-6257.