# Research Progress of Gas Sensor Based on Graphene and Its Derivatives: A Review
## 3 Preparation and Properties of Graphene  
### Preparation of Graphene  
Micro-mechanical exfoliation is the earliest method for preparing graphene. [PERSON] et al. (2004), from the University of Manchester, obtained the single-layer, high-purity graphene for the first time by repeatedly peeling flakes of graphite off the platform which were fixed onto a silicon dioxide/silicon (\(\mathrm{SiO_{2}}\)/\(\mathrm{Si}\)) substrate using scotch tapes [22]. Single-layer graphite can be torn down by pasting the graphite tape repeatedly due to the weak van der Waals forces between graphite sheets. With the progress of research on graphene, it is found that the production of graphene by mechanical peeling is low in yield, poor in repeatability and difficult to precisely control, so it can only be used in laboratories [23]. To make graphene practical, how to rapidly prepare large-area, high-quality monolayers and few layers of graphene films has become a subject of extensive research. Thus far, several fabrication routes for the production of graphene have been established, such as chemical vapor deposition, silicon carbide epitaxial growth method, oxidation-reduction method, etc. Chemical vapor deposition (CVD) method is controllable and can be widely used in the preparation of graphene to increase the contact area with the test substance, and thus easier to use in the field of sensors [24; 25]. In high-temperature decomposable gases (CH\({}_{4}\) and C\({}_{2}\)H\({}_{2}\)), carbon atoms are deposited on the surface of the substrate (such as Cu, Pt, Ni, Ru and Ir) by high-temperature annealing to form single-layer graphene [26; 27; 28; 29; 30; 31]. In 2006, [PERSON] et al. first made about 35 layers of graphene using thermal chemical vapor deposition [32]. This method has the advantages of low cost, simple steps, potential for mass production and environmental friendliness, providing a completely new direction for future research. Early in 2009, the [PERSON] research group at Sungkyunkwan University in Korea used a silicon wafer deposited with a polycrystalline Ni film as a substrate to prepare large-area and small-layer graphene, and graphene successfully transferred from the substrate, which set off the upsurge of CVD graphene preparation [33]. Subsequently, [PERSON] et al. from the University of Texas at Austin grew graphene on copper foil by a carbon atom chemical vapor deposition method at a temperature of 1000 \({}^{\circ}\)C using a mixed atmosphere of methane and hydrogen (H\({}_{2}\)), as shown in Figure 2[34]. The CVD method is considered to be the most promising method for the preparation of high quality, large area graphene, but there are still some disadvantages: high cost; complex production process; breakage, wrinkle and pollution of graphene and material waste during transfer; and inability to control the layer number of graphene produced due to the uncontrollable carbon source. Epitaxial growth refers to the growth of a layer of monocrystalline layer with the same orientation as the substrate on a single crystal substrate, as if the original crystal had been extended outward [35]. The raw material used for epitaxial growth was silicon carbide (SiC). The crystal was heated at 1200-1600 \({}^{\circ}\)C, Si atoms evaporate and C atoms remained on the surface, leading to the formation of graphene. Substrate-based epitaxial graphene can be obtained in this way [36]. [PERSON] et al. (2004), from Georgia Institute of Technology, produced a graphene thin layer have similar area as the original SiC on the (0001) crystal plane by heating large area single crystal SiC to high [37]. The epitaxial growth method is highly attractive for the preparation of high-quality graphene. However, there are still some problems in controlling the number of graphene layers and the repeated preparation of large-area, and researchers need to continue deep research. The oxidation-reduction method is currently the most widely applied way to achieve large-scale industrial preparation of graphene.