## 3 Results and Discussion
### C\({}_{2}\)H\({}_{2}\) Gas Sensing Properties  
C\({}_{2}\)H\({}_{2}\) gas is one of the main fault characteristic gases dissolved in power transformer oil, which can indicate the transformer discharge faults effectively. Thus, we carried out the gas sensing properties of the fabricated gas sensors for C\({}_{2}\)H\({}_{2}\) gas. Figure 6a shows the resistance curve (resistance of the sensor in the air versus temperature) of rGO, SnO\({}_{2}\) and SnO\({}_{2}\)-rGO gas sensors. Then the operating temperatures of rGO, SnO\({}_{2}\) and SnO\({}_{2}\)-rGO gas sensors were investigated at a temperature range of 100 \({}^{\circ}\)C to 300 \({}^{\circ}\)C, with an interval of 20 \({}^{\circ}\)C. Figure 6b shows the relationship between response and temperature for those gas sensors to 50 ppm C\({}_{2}\)H\({}_{2}\) gas. It is obvious that the responses of rGO-SnO\({}_{2}\) first ascended and then descended with the temperature increasing, while those of rGO and SnO\({}_{2}\) increased slowly in the temperature range of measurement. Although the reduced graphene oxide gas sensor shows a good response to gases at room temperature [31], the SnO\({}_{2}\) gas sensor usually works at high temperatures (about 360 \({}^{\circ}\)C) [32]. This behavior might be attributed to the potential  
Figure 5: XPS (X-ray photoelectron spectroscopy) spectra of (**a**) survey of SnO\({}_{2}\)–rGO; (**b**) Sn 3d region of SnO\({}_{2}\)–rGO; (**c**) C 1s region of SnO\({}_{2}\)–rGO; and (**d**) O 1s region of SnO\({}_{2}\)–rGO. barrier formed by the chemisorbed oxygen ions (\({\rm O_{n}}^{-}\)(ads)) on the SnO\({}_{2}\) surface, which prevents the C\({}_{2}\)H\({}_{2}\) molecules from reacting at low temperature. The SnO\({}_{2}\) gas sensor at low temperature possibly underwent a small chemical activation (the adsorption-desorption kinetics of test gas on the sensing materials), and showed a small response. At high temperatures, due to the enhanced chemical activation, the adsorbed gas molecules may escape before their reaction, leading to a response drop. Due the presence of SnO\({}_{2}\) in the SnO\({}_{2}\)-rGO nanocomposite, the optimum operating temperature is between that of rGO and SnO\({}_{2}\). The operating temperature when the response of SnO\({}_{2}\)-rGO reached the maximum value about 12.4 was 180 \({}^{\circ}\)C, which is called the optimum operating temperature and proved that the introduction of rGO was beneficial to reducing the optimum operating temperatures for the SnO\({}_{2}\)-based gas sensor. Figure 6c shows the response-recovery curve of SnO\({}_{2}\)-rGO to different concentrations of C\({}_{2}\)H\({}_{2}\) gas from 0.5 ppm to 50 ppm at its optimum operating temperature (180 \({}^{\circ}\)C). When SnO\({}_{2}\)-rGO gas sensor was exposed to the C\({}_{2}\)H\({}_{2}\) gas, its response clearly increased. In contrast, the response of SnO\({}_{2}\)-rGO gas sensor was decreased backed to the initial state after air was delivered into the chamber. To further study the response-recovery performance, the dynamic sensing transient of SnO\({}_{2}\)-rGO sensor to 1 ppm  
Figure 6: Results of C\({}_{2}\)H\({}_{2}\) gas sensing properties: (**a**) Resistance of the SnO\({}_{2}\), rGO and SnO\({}_{2}\)–rGO gas sensors at different operating temperatures; (**b**) Response of the rGO, SnO\({}_{2}\) and SnO\({}_{2}\)–rGO gas sensors to 50 ppm C\({}_{2}\)H\({}_{2}\) gas at different operating temperatures; (**c**) Response under different gas concentrations from 0.5 ppm to 50 ppm for C\({}_{2}\)H\({}_{2}\) of SnO\({}_{2}\)–rGO at 180 \({}^{\circ}\)C; (**d**) Dynamic sensing transient of SnO\({}_{2}\)–rGO sensor to 1 ppm C\({}_{2}\)H\({}_{2}\) gas. C\({}_{2}\)H\({}_{2}\) gas was shown in Figure 6d. By definition, the response and recovery times of SnO\({}_{2}\)-rGO were estimated to be around 54 s and 23 s, respectively. The response curve of SnO\({}_{2}\)-rGO gas sensor to various concentrations (from 0.5 ppm to 500 ppm) of C\({}_{2}\)H\({}_{2}\) gas at optimum operating temperatures of 180 \({}^{\circ}\)C was plotted in Figure 7a.