# Highly Sensitive and Selective H\({}_{2}\)S Chemical Sensor Based on ZnO Nanomaterial
## 3 Results and Discussions
### Gas Sensing Properties  
In order to determine the optimal operating temperature of the obtained ZnO nanomaterials towards H\({}_{2}\)S, we measured their gas sensing response at the operating temperatures of 200, 300, 400 and 500 \({}^{\circ}\)C. Figure 2a reports the relationship between the measured sensing response of the ZnO and its operating temperature. The response of structures increased with the temperature until 400 \({}^{\circ}\)C and decreased at 500 \({}^{\circ}\)C. The gas sensing mechanism of chemiresistive-type sensors is driven by reactions of the gaseous compounds with chemi- and physisorbed surface oxygen species [18; 60; 61]. Furthermore, the charge carriers have insufficient energy to overcome the barrier height energy at low working temperatures of the structure [13]. The sensing response of the oxide material at relatively low operating temperatures is mainly dominated by the physical adsorption process, where the gas molecules are adsorbed on the structure surface due to the van der Waals forces. The chemisorption process of H\({}_{2}\)S is improved with the increase in the operating temperature of ZnO [18; 62; 63]. Thus, the reaction of the H\({}_{2}\)S molecules with the ionosorbed oxygen species on the surface of ZnO was improved due to the increase of the sensor operating temperature, leading to an enhanced gas sensing response of the material. The reduced response at 500 \({}^{\circ}\)C can be attributed to the faster desorption rate of H\({}_{2}\)S on the surface of the ZnO structure at high temperatures [62; 63]. Consequently, the optimal operating temperature of the prepared ZnO nanomaterial towards H\({}_{2}\)S is 400 \({}^{\circ}\)C. The sensitive performance of the ZnO was further investigated by exposing the structure to different concentrations of H\({}_{2}\)S at its optimal operating temperature. The response amplitude of the ZnO sensor increased with the H\({}_{2}\)S concentration (Figure 2b). The sensor showed responses of about 1, 1400, 6300 and 7400 towards 5, 10, 20 and 30 ppm of H\({}_{2}\)S. The obtained response values show that the prepared ZnO nanomaterial has an excellent response towards relatively low concentrations of H\({}_{2}\)S.  
Figure 3 presents the dynamical response of the ZnO structure towards 5, 10, 20 and 30 ppm of H\({}_{2}\)S at 400 \({}^{\circ}\)C. It can be clearly seen that the conductance of the material increased rapidly when H\({}_{2}\)S was introduced to the test chamber. After the H\({}_{2}\)S flow was switched off, the conductance returned \begin{table}
\begin{tabular}{c c} \hline \hline
**Element** & **Atomic \% (\(\pm\)7\%)** \\ \hline Zn & 48 \\ O & 52 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Table of the quantitative analysis of the ZnO nanomaterial obtained by Energy-dispersive X-ray (EDX). Figure 2: (**a**) Gas sensing response versus operating temperature dependence of the ZnO nanomaterial towards 30 ppm of H\({}_{2}\)S. (**b**) Gas sensing response variation of the ZnO nanomaterial depending on the concentration of H\({}_{2}\)S (5, 10, 20 and 30 ppm) at 400 \({}^{\circ}\)C.  
to the baseline, which confirms the reversible electrical response of the prepared ZnO nanomaterial towards H\({}_{2}\)S. The conductance increase of the structure upon interaction with a reducing gas is typical behavior for an n-type oxide material such as ZnO [64]. The aforementioned conduction change mechanism of the ZnO can be explained as follows: The oxygen was ionosorbed on the surface of the ZnO (typically as O\({}^{-}\)) under exposure to air at 400 \({}^{\circ}\)C (Equation (1)). Thus, an electron-depleted layer was formed on the surface of the material, increasing the surface potential [18, 61]. When the sensing structure was exposed to the H\({}_{2}\)S, the depletion layer was narrowed, followed by a decrease in the surface potential caused by the chemical reactions between the ionosorbed oxygen and H\({}_{2}\)S molecules (Equation (2)) [65]. Consequently, the electrical conductance of material was increased.