# Research Progress of Gas Sensor Based on Graphene and Its Derivatives: A Review
## 4 Graphene Gas Sensor  
### Gas Sensors Based on Pristine Graphene  
In 2007, [PERSON] et al. used mechanically stripped graphene for the detection of individual gas molecules [55]. In the experiment, the response of graphene to 1 ppm ammonia (NH\({}_{3}\)), carbon monoxide (CO), nitric oxide (NO) and water vapor was measured and the change of resistance was recorded. Figure 4 shows that the resistance of graphene increases after access NH\({}_{3}\) and CO. The electron transferred to the graphene material as these two gas molecules adsorbed on the surface of graphene, resulting in reduced conductivity, which increased resistance. The opposite happened after access to water vapor and nitrogen dioxide (NO\({}_{2}\)). In 2012, [PERSON] et al. from Yonsei University studied the response of graphene to NH\({}_{3}\) with different layer number and length-to-width (L/w) ratio [56]. The graphene was prepared from highly oriented pyrolytic graphite (HOPG) through mechanical cleavage. Figure 5a shows the different layers of graphene have similar responses to NH\({}_{3}\), indicating that the graphene layer (mono-, bi- and tri-layer) has no obvious influence on the sensitivity of gas sensing. Figure 5b indicats that the response time and response intensity obviously changes with the change of L/w. To sum up, the key factor that affects the sensing of NH\({}_{3}\) by graphene is the aspect ratio rather than the layers. Figure 4: Chemical sensitivity of graphene [55]. The micro-mechanical exfoliation method is inefficient, time-consuming, and cannot meet the requirements of wholesale industrialization. Chemical vapor deposition can fill this gap and achieve relatively large area preparation of high quality graphene. In 2011, [PERSON] et al. reported that the surface resistance of monolayer graphene changes significantly after the adsorption of O\({}_{2}\) molecules due to the p-type doping of O\({}_{2}\) molecules to graphene. The detection limit of the simple O\({}_{2}\) sensor was 1.25% in volume ratio [57]. [PERSON] et al. from University of Wisconsin--Milwaukee prepared patterned vertical graphene nanosheets using plasma-enhanced chemical vapor deposition. The conductivity decreased in NH\({}_{3}\) atmosphere, and increased in NO\({}_{2}\) atmosphere [58]. In 2012, [PERSON] et al. from Rensselear Polytechnic Institute detected trace amounts of NO\({}_{2}\) and NH\({}_{3}\) in air at room temperature and atmospheric pressure using graphene films synthesized by CVD [59]. The LOD were 100 parts-per-billion (ppb) for NO\({}_{2}\) and 500 ppb for NH\({}_{3}\), which were markedly superior to commercially available NO\({}_{2}\) and NH\({}_{3}\) detectors. In 2015, [PERSON] et al. from Jadavpur University prepared a graphene based gas sensor with two Pd electrodes for sensing H\({}_{2}\) in air [60]. The lowest response and recovery times obtained in this study were 11.8 s and 23.6 s, respectively, at 130 \({}^{\circ}\)C in air ambient with Pd/graphene/Pd planar sensor device. In 2016, 3D graphene flowers (GF) cluster patterns were successfully prepared by [PERSON] et al. from Nanyang Technological University by inexpensive homebuilt microwave plasma-enhanced chemical vapor deposition (MPCVD) [61]. The GF sensor achieved a high response of 1411% to 10 ppm NO\({}_{2}\) and a response of 101% to 200 ppb NO\({}_{2}\). Furthermore, the sensor achieved a theoretical LOD of 785 ppt. The sensor also had great recovery characteristic that it took as little as 2 s to reach 90% signal recovery and only 20 s to achieve 100% recovery. Small size wearable electronic devices are a major trend in the development of science and technology, and demands for scalable and flexible electronics are increasing. In 2016, [PERSON] et al. from Korea University adhered stretchable patterned graphene gas sensor driven by integrated micro-super capacitor (MSC) array on the same deformable substrate, as shown in Figure 6a [62]. The sensor used a stretchable micro-capacitor as a substrate; patterned graphene was placed thereon as a gas-sensitive material.