## 3 Results and Discussion
### CO Analysis
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Temperature** & **CO** & **Response** & **Temperature** & **CO** & **Response** \\ (\({}^{\circ}\)**C**) & **(ppm)** & **(R)** & **(\({}^{\circ}\)**C)** & **(ppm)** & **(R)** \\ \hline \multirow{3}{*}{200} & 100 & 0.87 & \multirow{3}{*}{300} & 100 & 35.52 \\  & 200 & 0.67 & & 200 & 55.50 \\ \cline{1-1}  & 300 & 3.14 & & 300 & 72.67 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Response of NiSb\({}_{2}\)O\({}_{6}\) pellets at different concentrations and operating temperatures. Figure 8: Response of NiSb\({}_{2}\)O\({}_{6}\) nanoparticles obtained at 900 \({}^{\circ}\)C in CO atmospheres as a function of **(a)** CO concentration, **(b)** operating temperature. had a greater response and a better performance in detecting CO at different operating temperatures (100-300 \({}^{\circ}\)C). We also carried out tests in dynamic atmospheres as a function of time, applying direct current (d. c.) signals at 200 \({}^{\circ}\)C. For this, extra dry air flows (21% O\({}_{2}\) + 79% N\({}_{2}\); 400 cm\({}^{3}\)/min for 7 min at the test's start and 300 cm\({}^{3}\)/min for 13 min at the test's end) were supplied to the measuring chamber to stabilize the pellets' surface, while the CO flows were 100, 150, and 200 cm\({}^{3}\)/min. The total time for the dynamic cycles was approximately 20 min. The results are shown in Figure 9.  
It is observed in Figure 9a that, as the CO flow increases, pellets' electrical resistance decreases significantly. The estimated drop in electrical resistance was \(\sim\)1008.59 M\(\Omega\) at 100 cm\({}^{3}\)/min, \(\sim\)1273.27 M\(\Omega\) at 150 cm\({}^{3}\)/min, and \(\sim\)1474.49 M\(\Omega\) at 200 cm\({}^{3}\)/min. The response and recovery times were \(\sim\)2.24 and \(\sim\)1.39 min at 100 cm\({}^{3}\)/min, and \(\sim\)2.77 and \(\sim\)2.00 min at 150 cm\({}^{3}\)/min. At 200 cm\({}^{3}\)/min, the response time was \(\sim\)2.10 min. The response and recovery times were calculated according to reference [47], where 90% of the electrical resistance in the test gas and 10% of the resistance in the air are considered. We observed that the response and recovery times in the dynamic tests (see Figure 9) were short. That was because by introducing the CO into the measurement chamber, it reacted immediately with the material's surface due to the effect of temperature, with the consequent immediate change in electrical resistance (that is, the response time). On the other hand, when the test gas stopped injecting and the air was introduced (during the recovery time), the material tended to return almost immediately to its initial resistance values, which meant that the detection process was reversible. The changes in electrical resistance shown in Figure 9a are typical for n-type semiconductors [11]; however, it is reported in the literature that NiSb\({}_{2}\)O\({}_{6}\) tends to be a p-type semiconductor when exposed to atmospheres (for example, CO\({}_{2}\) and LPG) such as ours (CO) [28]. We also made measurements in large air flows (400 and 300 cm\({}^{3}\)/min), obtaining a supersaturation of oxygen species \(\left(\mathbf{O}^{-}\right)\) on the pellets' surface, originating an oxidation process with the test gas [45]. This caused all the charge carriers (electrons and holes) to interact with each other, allowing competition between them and resulting in a majority flow of electrons on the pellets' surface [48]. Another factor that influenced the trends shown in Figure 9a was the operating temperature [12; 40]. According to the literature, the oxygen species on the pellets' surface are adsorbed by the effect of temperature (in our case at 200 \({}^{\circ}\)C) [11], releasing electrons that are then captured to form ionized species \(\left(\mathbf{O}_{2}^{-},\mathbf{O}^{-},\mathbf{or}\mathbf{O}^{2-}\right)\)[49].