## 3 Results and Discussion  
### Structural Characterization  
The samples must be characterized before the measurement of gas sensing performance to identify the microstructure and morphology. The XRD patterns of rGO, SnO\({}_{2}\) and SnO\({}_{2}\)-rGO are presented  
Figure 1: Schematic diagram of the SnO\({}_{2}\)–reduced graphene oxide (rGO) nanocomposite planar gas sensor. Figure 2: Real image and schematic diagram of the gas sensing analysis measurement system. in Figure 3a. It is seen that a broad diffraction peak centered at 20 = 24.5\({}^{\circ}\) with interlayer distance of 3.78 A of rGO was observed, which is close to the typical (002) diffraction peak of graphite at 20 = 26.6\({}^{\circ}\). The peak of GO which might appear in the range of 20 (10\({}^{\circ}\)-12\({}^{\circ}\)) was not observed, which further indicates that the GO had been almost completely reduced to rGO. Meanwhile, several strong peaks appeared at 20 of 26.6\({}^{\circ}\), 33.7\({}^{\circ}\), 37.8\({}^{\circ}\) and 51.6\({}^{\circ}\), in agreement with the (110), (101), (200) and (211) planes of rutile SnO\({}_{2}\) (JCPDS 77-0452). The main diffraction peaks of SnO\({}_{2}\)-rGO were similar with those of SnO\({}_{2}\) and the characteristic peak of rGO is invisible as seen in Figure 3a, probably owing to the fact that the rGO is wrapped by SnO\({}_{2}\) nanocrystals through the hydrothermal treatment, or that the weak peak of rGO is swamped by the presence of the strong peak around 26.6\({}^{\circ}\) at (110) plane of the SnO\({}_{2}\) nanocrystals [27]. Figure 3b shows the Raman spectrum results of SnO\({}_{2}\)-rGO, which can be used to prove the existence of rGO in the SnO\({}_{2}\)-rGO nanocomposite. Two representative peaks were observed around at 1355 cm\({}^{-1}\) and 1586 cm\({}^{-1}\), corresponding with disorder-induced (D) band and tangential (G) band of rGO. Due to the ultrasonic exfoliation and hydrothermal treatment, the GO was reduced and more defects were introduced which enhance the performance of gas adsorption, leading to the D and G band of rGO having some minor peak shifts to 1360 cm\({}^{-1}\) and 1608 cm\({}^{-1}\) of GO. Moreover, the three typical peaks located at round 472 cm\({}^{-1}\), 641 cm\({}^{-1}\) and 781 cm\({}^{-1}\) can be matched with the E\({}_{g}\), A\({}_{g}\) and B\({}_{2g}\) vibration modes of rutile SnO\({}_{2}\)[28]. All these mentioned characteristic peaks appeared in the Raman spectrum of SnO\({}_{2}\)-rGO. It was confirmed that the as-synthesized SnO\({}_{2}\)-rGO nanocomposite was made up of rGO and SnO\({}_{2}\). The characteristics of combustion of SnO\({}_{2}\)-rGO were investigated using thermogravimetric analysis (TGA) and the results are as shown in Figure 3c. The first 2.199% mass loss (approximately 100 \({}^{\circ}\)C) was due to water solvent molecules absorbed into the reduced GO bulk material. The following 5.096% decrease (400 \({}^{\circ}\)C) represents elimination of the remaining functional groups; further decomposition takes place up to 800 \({}^{\circ}\)C. Experimental settings were as follows: temperature scanning rate: 5 \({}^{\circ}\)C/min; temperature range 30-800 \({}^{\circ}\)C, and purging inert gas: N\({}_{2}\). SnO\({}_{2}\)-rGO has good thermal stability in the range of temperatures (100-300 \({}^{\circ}\)C). Figure 3: (a) X-ray diffraction (XRD) patterns; (b) Raman spectra of rGO, SnO\({}_{2}\) and SnO\({}_{2}\)-rGO samples; and (c) thermogravimetric analyses (TGA) results of SnO\({}_{2}\)–rGO nanocomposites. Figure 4a depicts the TEM and high-resolution TEM (HRTEM) micrographs of rGO. The quality of rGO is good because of its excellent transparency and obvious fold boundary. In Figure 4b, the SnO\({}_{2}\) nanoparticles with irregular grain size up to 10 nm in diameter were successfully synthesized. From the TEM image of SnO\({}_{2}\)-rGO in Figure 4c,d, it can be seen that the SnO\({}_{2}\) nanoparticles were uniformly loaded on the rGO nanosheet surface.