## 3 Results and Discussion
### CO Analysis
This means that, when the pellets' surface comes into contact with the CO molecules at 200 \({}^{\circ}\)C, the latter react with the ionized species, causing an electron  
Figure 9: Sensing tests at 200 \({}^{\circ}\)C and different CO and air flows as a function of time: (**a**) variation of electrical resistance, (**b**) variation of sensitivity percentage. transfer during the adsorption and desorption of the gas molecules on the material's surface [11] and, with it, the decrease in electrical resistance depicted in Figure 9a. Shifts in percent sensitivity as a function of time are shown in Figure 9b. The response (sensitivity) was determined considering the variations in electrical resistance for 100, 150, and 200 cm\({}^{3}\)/min of CO at 200 \({}^{\circ}\)C when the surface of the pellets was stabilized by means of extra-dry air flows, as in the case shown in Figure 9a. To obtain the graphs of Figure 9b, the following formula was considered [50]: S(%) = (G\({}_{\text{G}}\) - G\({}_{\text{O}}\))/G\({}_{\text{O}}\)\(\times\) 100, where G\({}_{\text{G}}\) and G\({}_{\text{O}}\) are the electrical conductances (1/electrical resistance) in CO and air, respectively. From the results, for the flow of 100 cm\({}^{3}\)/min, the estimated sensitivity was \(\sim\)57.10% (first curve). For the 150 cm\({}^{3}\)/min flow, the sensitivity increased to \(\sim\)84.77% (second curve). Finally, for the flow of 200 cm\({}^{3}\)/min, the percentage obtained was \(\sim\)113.75%. As expected, increasing the CO flow increased the pellets' percent sensitivity. With this, it can be corroborated that the increase in time-dependent sensitivity in a material such as NiSb\({}_{2}\)O\({}_{6}\) is closely related to the variation in the test gas concentration [29]. Indeed, it is reported in the literature that as the concentration of the test gases increases, the sensitivity of the material increases [32; 34]. This is due to the fact that the higher the gas concentration value is, the more easily it reacts with the oxygen species (**O\({}^{-}\)**) on the pellets' surface due to the temperature effect (in our case, at 200 \({}^{\circ}\)C), causing an increase in the gas oxidation and, thus, an electron release, causing NiSb\({}_{2}\)O\({}_{6}\)'s sensitivity to rise [20; 34]. Some studies report that the adsorption of oxygen species on a semiconductor's surface due to the operating temperature effect is what causes the high sensitivity of materials such as ours [28; 29; 35; 48; 49; 51]. In particular, our results (displayed in Figure 9) show better thermal stability, high sensitivity, reproducibility, and efficiency in the detection of different CO concentrations compared to those reported in references [14; 28; 29; 31; 34; 35; 42; 46; 49], where tests in static and dynamic CO, CO\({}_{2}\), and LPG atmospheres were performed. A mechanism that explains the interaction between a material such as NiSb\({}_{2}\)O\({}_{6}\) and a gas such as CO is based on the drop of the depletion layer due to oxygen adsorption [51; 52]. When a gas such as CO is adsorbed on the material's surface, it reacts with the chemisorbed oxygen species [52], producing CO\({}_{2}\) molecules and a release of electrons that move to the conduction band [20]. This produces changes in the material's conductance (or electrical resistance) [53] and, therefore, variations in the electrical response in CO atmospheres (see Figure 9a,b). Therefore, since the reaction mechanism between a gas such as CO and the surface of a semiconductor such as NiSb\({}_{2}\)O\({}_{6}\) depends largely on the operating temperature [20; 52], a reaction that may be occurring on the pellets' surface at 200 \({}^{\circ}\)C is as follows [20; 41; 49; 52; 54]:  
\[\textbf{O}_{2}^{-}+2\textbf{CO}\ \rightarrow\ 2\textbf{CO}_{2}+\textbf{e}^{-} \tag{3}\]  
where \(\textbf{O}_{2}^{-}\) is the oxygen ions on the material's surface and \(\textbf{e}^{-}\) is the electrons in the bulk material [20; 49; 54].