# Research Progress of Gas Sensor Based on Graphene and Its Derivatives: A Review
## 3 Preparation and Properties of Graphene
### The Property of Graphene  
Graphene, a single, one-atom-thick sheet of carbon atoms arranged in a honeycomb lattice and the two-dimensional building block for carbon materials, as shown in Figure 3. It has attracted great interest for a wide range of applications. It is also the thinnest material in the world [42]. The thickness of graphene is about 0.335 nm, the bond length is 0.142 nm, the bond angle is 120\({}^{\circ}\).  
The unique structure of graphene determines its unique properties. Graphene has high strength, excellent electrical conductivity and high thermal conductivity, and quantum Hall effect can be observed at normal temperature [43]. It also has a huge surface area, good adsorption and transmission, and optical absorption of exactly \(\pi\alpha\approx 2.3\%\) (in the infrared limit, where \(\alpha\) is the fine structure constant), and is almost transparent [44]. The strong interaction between single carbon-carbon bonds leads to its high mechanical strength, the Young's modulus of graphene is up to 1 Tera Pascal (TPa), about five times that of steel [45], and its breaking strength is up to 130 GPa [46].  
Graphene has excellent electrical conductivity. The carrier velocity in graphene reaches \(1/300\) of the speed of light, far exceeding the movement speed of the carrier in the general conductor [47]. Conductivity is up to 106 S/m [48]. In view of the above advantages, graphene is expected to become the core material of the new generation of electronic components in the future.  
Graphene also has excellent thermal conductivity. The thermal conductivity of graphene at room temperature is up to 5300 W/m\(\cdot\)K, which is higher than that of diamond (2000 W/(m\(\cdot\)K)) and carbon nanotube (3500 W/(m\(\cdot\)K)) [49]. Graphene is the material with the highest thermal conductivity known at present. It has a good application prospect in the field of integrated circuits.  
Graphene has huge surface area. The theoretical calculation showed that the surface area of the monolayer graphene was 2630 m\({}^{2}\)/g, three times as much as activated carbon [50]. The transmittance of monolayer graphene is 97.7%, almost transparent [51]. Graphene has an important application  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Method** & **Carbon Source** & **Substrate** & **Temperature (\({}^{\circ}\)C)** & **Reference** \\ \hline Micro-mechanical exfoliation & graphite & SiO\({}_{2}\)/Si & Room temperature & [13] \\ Chemical Vapor Deposition & CH\({}_{4}\)\({}_{\mathrm{C}}\)\({}_{2}\)H\({}_{2}\) & Cu, Pt, Ni, Ru, Ir & \textgreater{}1000 & [15; 16; 17; 18; 19; 20; 21; 22; 23] \\ Epitaxial growth & SiC & SiC & 1200–1600 & [24] \\ The oxidation–reduction method & graphite & – & \textgreater{}500 & [27] \\ \hline \hline \end{tabular}
\end{table}
Table 1: Different methods for graphene synthesis.  
Figure 3: Ideal monolayer graphene structure diagram.  
prospect in different fields due to its excellent properties, such as nanomaterials, transparent conductive films, composite materials and sensors [52].  
The study of the presence of graphene and its derivatives in aqueous environments is critical to the stability of graphene. [PERSON] et al. from Washington University in St. Louis studied aqueous aggregation processes of the GO and crumpled GOs (CGO, with different degrees of thermal reduction, and thus oxygen functionality) [53]. The results showed that the more oxidized CGO materials (with more functional groups such as carboxyl and phenolic) are relatively more stable in water. They also studied the interactions of GO and CGO with natural organic matter (NOM) in aquatic environments. The presence of NOM was proven to considerably increase the critical coagulation concentrations (CCC) for all GO materials evaluated, and the effect on CGO was more obvious. The stability is primarily due to enhanced steric repulsion via adsorbed NOM [54]. The research by [PERSON] et al. provides new information of GO behavior in water and leads people to conduct more in-depth research.