## 3 Results and Discussion
### Gas Sensing Mechanism
Figure 8: (**a**) Schematic illustration of sensing mechanism of pure SnO\({}_{2}\) nanoparticles; (**b**) structure of SnO\({}_{2}\) nanoparticles and SnO\({}_{2}\)-rGO nanocomposites; and (**c**) schematic illustration of sensing mechanism of SnO\({}_{2}\)–rGO nanocomposites. Secondly, for SnO\({}_{2}\)-rGO nanocomposites possessing \(n\)-type character, electrons acted as electron carriers. The structure of SnO\({}_{2}\)-rGO is shown in Figure 8b and the mechanism is investigated in Figure 8c. When it is exposed to air, three types of electron DL may be formed, as shown in Figure 8c. DL1 is the first electron depletion layer, which is formed on the surface of SnO\({}_{2}\) nanoparticles owing to the adsorbed oxygen ions [43]; DL2 is the second electron depletion layer, which is formed by the electrons and transfers from the surface of SnO\({}_{2}\) to the rGO during the formation of \(p\)-\(n\) heterojunctions; and DL3 is the third electron depletion layer. It appears in the area where SnO\({}_{2}\) is embed in rGO nanosheets and forms the \(p\)-\(n\) heterojunctions [18]. All these electron depletion layers will prevent the migration of electron, leading to the high-resistance state of the nanocomposites. When the reducing gas is introduced, the width of these three types DLs will undergo different changes. The trapped electrons in the DL1 will be released back to the SnO\({}_{2}\), decreasing the width of the DL1; generally, that of DL2 is supposed to be decreased because the reducing gas molecules may be adsorbed on the surface of rGO and donate the electrons to it. However, the work functions of SnO\({}_{2}\) and rGO are 4.55 eV and 4.75 eV, respectively. Because the work function of SnO\({}_{2}\) is smaller than that of rGO, the electrons in SnO\({}_{2}\) will transfer to rGO to equate the Fermi level and form a barrier (0.2 eV), as shown in Figure 8c. While the reducing gas is introduced, the value of (\(E_{\text{C}}\)-\(E_{F}\)) in SnO\({}_{2}\) will decrease the barrier for electron transfer from SnO\({}_{2}\) to rGO. As a result, the number of electrons in SnO\({}_{2}\) will be sufficient, leading to a smaller resistance of SnO\({}_{2}\) and a broadened width of DL2. Through the relative resistance change of SnO\({}_{2}\) is much larger than that of rGO, the width of DL3 in \(p\)-\(n\) heterojunctions is reduced. The specific nanostructure of SnO\({}_{2}\)-rGO nanocomposites which could cause the different types of electron depletion layers has made great contribution to the change of resistance. Thirdly, the formation of \(p\)-\(n\) heterojunctions between SnO\({}_{2}\) and rGO can help to enhance the gas sensing properties, and includes many other factors. As the rGO is obtained by chemical or thermal treatment of graphene oxide, there may be many oxygen functional groups on the surface of rGO, such as hydroxyl (-OH) and carboxyl (-COOH) which will provide more adsorption sites for the formation of adsorbed oxygen ions and target gas molecules [44; 45]. Besides, the introduction of rGO can effectively prevent the aggregation of SnO\({}_{2}\) nanoparticles, because there are many overlaps among the SnO\({}_{2}\) nanoparticles that must be generated, reducing the exposed active surface [18]. Due to these factors, the resistance variation shows a larger change when compared to individual SnO\({}_{2}\) or rGO. According to the proposed possible mechanisms, we conclude that the SnO\({}_{2}\)-rGO gas sensing mechanism combines with the effect of the adsorbed oxygen ions and grain boundary barrier of SnO\({}_{2}\), the structural characteristics of rGO and the formation of local \(p\)-\(n\) heterojunctions between SnO\({}_{2}\) and rGO. All these sources can be considered as the resistance modulation of the SnO\({}_{2}\)-rGO nanocomposites. We suggest that the combination of two sensing mechanisms, being related to rGO-SnO\({}_{2}\)\(p\)-\(n\) heterojunctions and structural characteristics of rGO, is responsible for the enhancement of the gas sensing properties of rGO-loaded SnO\({}_{2}\).