# Highly Sensitive and Selective H\({}_{2}\)S Chemical Sensor Based on ZnO Nanomaterial
## 3 Results and Discussions
### Gas Sensing Properties
In addition, the H\({}_{2}\)S molecules can adsorb in metastable configurations on the surface of ZnO at relatively high operating temperatures [66]. Therefore, the sulfuration and desulfuration reversible reactions can occur between the H\({}_{2}\)S and ZnO (Equations (3) and (4)). These reactions occur simultaneously without creating the stable intermediate product of ZnS. However, the sulfuration reaction influences the capability of oxygen to extract electrons from ZnO. That is to say, the ZnO nanostructure plays a role similar to the catalyst [67, 68]. \[\mathrm{O}_{2}+2\mathrm{e}^{-}\to 2\mathrm{O}^{-} \tag{1}\]  
\[\mathrm{H}_{2}\mathrm{S}+3\mathrm{O}^{-}\to\mathrm{H}_{2}\mathrm{O}+\mathrm{ SO}_{2}+3\mathrm{e}^{-} \tag{2}\]  
\[\mathrm{H}_{2}\mathrm{S}+ZnO\to\mathrm{H}_{2}\mathrm{O}+\mathrm{ZnS} \tag{3}\]  
\[2\mathrm{ZnS}+3\mathrm{O}_{2}\to 2\mathrm{ZnO}+2\mathrm{SO}_{2} \tag{4}\]  
The sensing properties of the prepared ZnO nanomaterial towards H\({}_{2}\)S were compared with that of previously reported H\({}_{2}\)S gas sensors based on ZnO (Table 2). The obtained ZnO material showed a significant improvement in gas sensing response compared with the other structures at their optimal operating temperatures. Moreover, the obtained samples have an advantage in their sensing performance compared to some of the doped and mixed ZnO structures at their optimal working conditions. The higher gas sensing response of the obtained material towards H\({}_{2}\)S was attributed to its morphology, which induced effective diffusion as well as providing a larger effective surface area due to the form and porosity of the structure. In this case, the chains were composed of nanoparticles connected to each other, forming a very high surface area and thereby enhancing the response of the ZnO material. Figure 3: Dynamical response of the ZnO nanomaterial towards 5, 10, 20 and 30 ppm of H\({}_{2}\)S at 400 \({}^{\circ}\)C.  
The selectivity of the fabricated ZnO sensors was studied towards 20 ppm of ammonia (NH\({}_{3}\)), dimethylamine (DMA) and acetone (C\({}_{3}\)H\({}_{6}\)O) at the optimal operating temperature of 400 \({}^{\circ}\)C (Figure 4). The responses of the material towards NH\({}_{3}\) and C\({}_{3}\)H\({}_{6}\)O were 2 and 0.4, respectively. The structure was not sensitive towards DMA at 400 \({}^{\circ}\)C. Instead, the response towards H\({}_{2}\)S was about 6300. The results obtained indicate that the prepared ZnO nanomaterial has a high selectivity towards H\({}_{2}\)S against other interfering gases. This significant difference in the structure sensing response towards H\({}_{2}\)S and other interfering gases can be related to the sulfuration and desulfuration reversible reactions on the surface of the material. Since the aforementioned reactions were not involved in the sensing mechanism of the ZnO towards interfering gases, the material showed a weaker response to them. Thus, the disparity in the sensing mechanism between H\({}_{2}\)S and other reducing gases could be the reason for the better sensing performance of ZnO towards H\({}_{2}\)S.