## 3 Results and Discussion
### Gas Sensing Mechanism  
According to our experimental results, SnO\({}_{2}\)-rGO nanocomposites have excellent performance with respect to detection of C\({}_{2}\)H\({}_{2}\), which shows potential for application. The possible sensing mechanism is discussed as follows. Commonly, the sensing mechanism of MO gas sensors is explained by the adsorption-desorption model, the resistance changes of the MOs through the interaction between adsorbed oxygen ions (\(O^{2-}\), \(O^{-}\) or \(O_{2}\)\({}^{-}\)) and target gas molecules. As we know, rGO nanosheets prepared by chemical or thermal treatment will show a \(p\)-type semiconductor characteristic [42]. Thus, the heterojunctions (\(n\)-\(n\) or \(n\)-\(p\)) can be formed when rGO is composited with MOs. This will affect the gas sensing performance by impacting on the resistances change of the nanocomposites. In order to explain the gas sensing mechanism in SnO\({}_{2}\)-rGO nanocomposites as thoroughly as possible, several possible mechanisms will be considered, as follows. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & \multirow{2}{*}{**Material**} & **Temperature** & **Measuring** & **Response** & **\%Response (s)** & **\%Recovery (s)** \\  & & (\({}^{\circ}\)C) & **Range (ppm)** & **(S = R\({}_{\alpha}\)/R\({}_{\beta}\))** & **\%Response (s)** & **\%Recovery (s)** \\ \hline
[38] & Pure SnO\({}_{2}\) & 300 & 200–10,000 & 6.3 (10,000 ppm) & 34 & \textgreater{}10 min \\
[39] & Sm\({}_{2}\)O\({}_{3}\)/SnO\({}_{2}\) & 180 & 1–5000 & 63.8 (1000 ppm) & 3 & 17 \\
[40] & Pt/ZnO & 300 & 50–10,000 & 43 (1000 ppm) & 6 & 65 \\
[41] & ZnO/rGO \({}^{1}\) & 250 & 30–1000 & 18.2 (100 ppm) & 100 & 28 \\ Our & SnO\({}_{2}\)\(\rightarrow\)FOO} & 180 & 0.5–500 & 12.4 (50 ppm) & 54 & 23 \\ \hline \hline \end{tabular} \({}^{1}\) rGO: reduced graphene oxide.  
\end{table} 
Table 1: Comparison of previously reported and our work on C\({}_{2}\)H\({}_{2}\) gas sensors. Firstly, the nanostructure of as-synthesized SnO\({}_{2}\) grains are composed of nanoparticles, as shown in Figure 8b. The adsorption-desorption model and boundary barrier model are described in Figure 8a. As the SnO\({}_{2}\) works as a \(n\)-type semiconductor characteristic, when the SnO\({}_{2}\) grain is exposed to the air, the O\({}_{2}\) molecules in air will adsorbed on the surface of the SnO\({}_{2}\) grain to form the adsorbed oxygen ions (\(O^{2-}\), \(O^{-}\) or \(O_{2}{}^{-}\)). As the electrons on the surface of SnO\({}_{2}\) are captured by O\({}_{2}\) molecules, a wide electron depletion layer (DL) is formed. When the SnO\({}_{2}\) which contains adsorbed oxygen ions comes into contact with reducing gases, the trapped electrons will release back to the surface of SnO\({}_{2}\) and the width of the DL will be reduced. For example, the possible interaction of C\({}_{2}\)H\({}_{2}\) has been proposed in the reference [41] as follows:  
\[\mathrm{O_{2}+2}e^{-}\leftrightarrow 2O^{-}(ads) \tag{3}\]  
\[\mathrm{C_{2}H_{2}+O^{-}(ads)\rightarrow\mathrm{CO_{2}+H_{2}O+}e^{-}} \tag{4}\]  
Meanwhile, the electron transfer between different SnO\({}_{2}\) nanoparticles is an important factor for gas sensing behavior. If the electron in the nanoparticle is to transfer to another, the boundary barrier which is formed by space charge must be overcome. The contact resistance in the interface will affect the resistance of the gas sensor. When the SnO\({}_{2}\) is in the air, the electron on the surface of SnO\({}_{2}\) will be trapped and the boundary is raised. In contrast, in the reducing gases the electron is released back to the surface and the barrier is lowered. The increase or decrease of the width of the DL and boundary barrier according to the interaction between adsorbed oxygen ions make an important contribution to the sensing performance of SnO\({}_{2}\).