# 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  
One-dimensional metal oxide nanowires (NWs) or nanorods (NRs), such as ZnO, SnO\({}_{2}\), and Cu\({}_{2}\)O, are widely used in the field of sensing due to their high specific surface area, large aspect ratio, and good flexibility [81]. However, these nanostructures have poor electrical conductivity. The combination between metal oxide and two-dimensional graphene can effectively improve electrical conductivity and improve sensing performance. Doping has been widely used as an effective way to improve the performance of gas sensors. Doping semiconductor oxides with carbon-based materials results in new physical and chemical properties of the resulting composite, while also increasing the sensitivity and selectivity of graphene sensing. At the same time, the metal oxide and graphene doping can also prevent the graphene sheet in the drying process of aggregation, graphene yield and quality will be greatly enhanced. In 2011, [PERSON] from Hanyang University, South Korea employed CVD-graphene sheets along with thin metal layers as the top electrodes of vertically aligned ZnO NRs (ZnO NRs-Gr/M), and studied the performance of this composite sensor [82]. The ZnO NRs-Gr/M hybrid structure could keep sufficient space between the whole nanowires to ensure rapid delivery of gas, as shown in Figure 15. Moving gas molecules could maximize exposure to the NR surface, making highly sensitive gas sensor devices, so that the test results more accurate. In addition, ZnO NRs-Gr/M had unique mechanical properties and good light transmittance in the visible spectrum, making it possible to find new applications in the field of flexible machinery or transparent sensors. The device showed rapid response to different concentrations of ethanol vapor, and the response to 10-50 ppm ethanol vapor could reach 9-90. Figure 14: The flow chart of the preparation of ternary composite. Figure 15: Schematic illustrating of the key steps for fabricating the ZnO NRs-Gr/M hybrid architectures [82]. In 2014, [PERSON] et al. from Jilin University synthesized reduced graphene oxide-zinc oxide (rGO-ZnO) nanoparticle composites by redox method [83]. Gas sensitivity tests showed that the response of rGO-ZnO nanoparticle composites gas sensor to 5 ppm of NO\({}_{2}\) reaching 25.6%, the response time was 165 s and the recovery time was 499 s.  
In 2014, [PERSON] et al. from Nanjing University of Science and Technology synthesis cupric oxide-zinc oxide/reduced graphene oxide (CuO-ZnO/rGO) ternary complex by one-step hydrothermal method [84]. Characterization analysis showed that nanoscale p-n junctions were formed between the CuO and ZnO nanoparticles on the rGO. The obtained CuO-ZnO/rGO ternary composite had excellent acetone response (9.4 to 10 ppm), as shown in Figure 16, almost 1.5 and 2.0 times that of CuO-ZnO and ZnO/rGO, respectively. In addition, ternary composites showed weaker sensing performance on ethanol, which can effectively distinguish between acetone and ethanol. These advantages were attributed to the excellent substrate properties of rGO lamellae and the effective p-n junction of CuO-ZnO. In 2015, [PERSON] et al. from Chiang Mai University prepared nickel-doped stannic oxide/graphene oxide (Ni-doped SnO\({}_{2}\)/GO) acetone gas sensor [85]. Gas detection results showed that the acetone response of the SnO\({}_{2}\) sensing film was found to be substantially improved, up to 54.2 at 200 ppm and 350 \({}^{\circ}\)C with the lowest Ni-doping level of 0.1 wt %, but further increase of Ni-doping concentration resulted in deteriorated acetone response. In contrast, the acetone response tended to be substantially enhanced with increasing the graphene loading level up to 5 wt %.