# Research Progress of Gas Sensor Based on Graphene and Its Derivatives: A Review
## 4 Graphene Gas Sensor
### Gas Sensors Based on Graphene/Polymer Composites  
In 2009, [PERSON] found that photolithography fabrication of graphene devices would inevitably leave polymer photoresist on the graphene surface at a thickness of about 1 nm [75]. The residue has a significant impact on the device's transmission performance and gas sensing. Figure 12 shows the response of the graphene sensor with polymethylmethacrylate (PMMA) residues to gas with a strong electrical response to ppm-level nonanal vapor before and after the device is washed. These polymers were chemically doped with graphene, and could be used to concentrate gas to improve gas sensing performance. The discovery opened the new dimension of graphene gas sensing field. Figure 11: Real-time current behavior of ME-Gr (black line), CVD-Gr (red line) and LPE-Gr (green line) based chemi-resistors upon exposure to sequential NO\({}_{2}\) pulses (blue rectangles) at decreasing concentrations from 1.5 down to 0.12 ppm. Each exposure step lasts 4 min, preceded and followed by 20 min long baseline and recovery phases, respectively, in N\({}_{2}\) atmosphere. The baseline preceding the first pulse lasts 30 min in order to allow the device to better stabilize in the test chamber. The current is normalized at the value I\({}_{0}\) during the gas inlet of the first pulse exposure [74]. Figure 12: Measured sensor responses, before (black) and after (red) sample cleaning, to vapors of octanoic acid [75]. Subsequently, many research groups started the preparation of graphene/polymer composite gas sensors. In 2012, [PERSON] et al. from Tsinghua University prepared a graphene oxide/polypyrene (GO/PPr) polymer and tested gas sensing [76]. The composite exhibited good sensing properties due to its unique structure. The toluene gas showed a fast, linear and reversible response with a sensitivity of \(9.87\times 10^{-4}\). At the same time, [PERSON] et al. from Shanghai Jiao Tong University demonstrated a practical NH\({}_{3}\) gas sensor based on graphene-polyaniline (PANI) hybrids for the first time in 2012 [77]. The combination of graphene and PANI showed positive synergistic effect on ammonia detection that the response was 59.2% to 50 ppm NH\({}_{3}\). The material also showed great response to hydrogen gas, which is reported by [PERSON] et al. from Guilin University of Electronic Technology [78]. A t\({}_{\text{res}}\) of 20 s and t\({}_{\text{rec}}\) 50 s were obtained when the sensor was exposed to 1 vol % hydrogen at 25 \({}^{\circ}\)C. The large specific surface area of the PANI-GO composite and the catalytic activity of the Pd nanoparticles were the key factors of enhancement of the interaction between the hydrogen molecules and sensing surface led to the improvement in hydrogen-gas-sensing performance. In 2013, [PERSON] et al. from Suzhou University put forward a highly sensitive gas sensor based on graphene/polyaniline hybrid materials [79]. Polyaniline is an organic semiconductor molecule with excellent performance. Its preparation cost is low; the film making process is simple; it is easily compatible with other technologies; and it can work at normal temperature. It has become a hot spot in the research of gas sensors. The combination of graphene and polyaniline can exert the advantages of two performance materials and is of great significance for improving the performance of the sensor. Researchers dispersed graphene/polyaniline hybrid materials in organic solvents; the dispersion was added dropwise Pt electrode surface and dried to obtain the gas sensor. It can be seen in Figure 13 that the response of this composite to NH\({}_{3}\) is greatly increased compared to pure graphene, and the resistance change rate can reach 30%. During the preparation of graphene devices by photolithography, polymer photoresists inevitably remain on the surface of graphene. The composite material has been shown to have important synergistic effects on certain special gases because of its unique selectivity and sensitivity due to the electrical properties.