# Research Progress of Gas Sensor Based on Graphene and Its Derivatives: A Review
## 4 Graphene Gas Sensor
### Gas Sensors Based on Graphene/Metal or Metal Oxide Composites
The graphene-loaded 0.1 wt % Ni-doped SnO\({}_{2}\) sensor exhibited notably high acetone response of 169.7-200 ppm acetone with a short response time of \(\sim\)5.4 s at the optimal operating temperature of 350 \({}^{\circ}\)C. The enhanced acetone-sensing performances of Ni-doped SnO\({}_{2}\) nanoparticles loaded with graphene may be attributed to large specific surface area of the composite structure, specifically high interaction rate between acetone vapor and graphene-Ni-doped SnO\({}_{2}\) nanoparticles interfaces and high electronic conductivity of graphene. Therefore, the graphene loaded Ni-doped SnO\({}_{2}\) sensor is a promising candidate for fast, sensitive and selective detection of acetone. In 2015, [PERSON] et al. from University of Electronic Science and Technology of China proposed a patented graphene/metal oxide composite membrane gas sensor [86]. Metal oxide particles, as a catalytic medium, can reduce the chemical barrier of the electron transfer when the gas molecules are in contact with the graphene, so that enhances the gas sensing properties of the single graphene gas sensing film. Experiments showed that the material's response to 0.5 ppm formaldehyde gas increased from 27.5% to nearly 40% compared with a single graphene gas sensor. Gas sensitivity greatly improved. In 2016, [PERSON] et al. from Jilin University proposed a graphene gas sensor based on G/SnO\({}_{2}\)/ZnO composite and a preparation method thereof [87]. The gas sensitive material was a three-dimensional porous structure material made of graphene, tin oxide and zinc oxide. The material had a pore size of 3 to 10 nm and a BET specific surface area of 100 to 230 m\({}^{2}\)/g, and  
Figure 16: The resistance curves of CuO/rGO and pure rGO sensors to 10 ppm of acetone [84]. had good adsorbability to gas molecules. The prepared graphene gas sensor had high and fast response to different concentrations of NO\({}_{2}\), the response time wad less than one minute, and had good repeatability. In 2017, [PERSON] et al. from Huazhong Agricultural University successfully prepared SnO\({}_{2}\) nanoparticles coated with graphene oxide nanoplate by hydrothermal method and tested its response to ethanol, acetone and toluene gas [88]. In Table 2, the prepared GO-SnO\({}_{2}\) composite had more porosity and higher specific surface area than the pure SnO\({}_{2}\). The increase of specific surface area may be due to the aggregation and ablation of GO particles by GO to form pores, favors gas sensing applications and showed relatively high sensitivity and hence better adsorption of gas molecules. The average pore size of the material was about 10 nm, with small variations in the proportion of doped SnO\({}_{2}\). Gas detection showed that GO as a dopant can significantly increase the gas sensitivity to gases. According to the experimental results, 0.3 wt % GO sensor had the best gas sensitivity to ethanol, acetone and formaldehyde. For toluene gas, 1.0 wt % GO-SnO\({}_{2}\) had the greatest sensitivity. Recently, [PERSON] et al. from Gazi University synthesized ammonia gas sensors based on rGOdecorated by Ag, Au and Pt nanoparticles (NPs) using a single-step chemical reduction process in 2017 [89]. The responses for 1 ppm NH\({}_{3}\) were calculated as 6.52%, 2.87% and 0.5% for AgNPs-rGO, PtNPs-rGO and AuNPs-rGO, respectively. Considering sensitivity, response and recovery time, and LOD, it was observed that AgNPs-rGO sensor exhibited a significant sensing performance for ammonia gas with a low detection at room temperature. Microwave (MW)-assisted irradiation is an original way for the synthesis of nanomaterials due to its volumetric heating that can eliminate temperature gradient. In 2017, ZnO/graphene nanocomposites was successfully synthesized by [PERSON] et al. from Hanyang University using MW irradiation [90]. The response of MW irradiated ZnO/graphene nanocomposite sensor to 1 ppm NO\({}_{2}\) were 12.57%, and for 10 ppm NO\({}_{2}\) were 46.42%, which were higher than pristine ZnO sensor and ZnO/graphene nanocomposite sensor.