## 3 Results and Discussion
### CO Analysis  
The gas response was analyzed with the material synthesized at 900 \({}^{\circ}\)C, where a minor presence of the secondary phase was obtained. To evaluate NiSb\({}_{2}\)O\({}_{6}\)'s detection capacity at different temperatures (100-300 \({}^{\circ}\)C) and CO concentrations (1-300 ppm), tests were conducted in static atmospheres using pellets made with powders of the material calcined at 900 \({}^{\circ}\)C. The results are shown in Figure 8 as a function of the test gas concentration (Figure 8a) and operating temperature (Figure 8b). According to the analysis, the pellets did not show changes in electrical resistance at 100 \({}^{\circ}\)C regardless of the increase in the CO concentration. In contrast, at 200 \({}^{\circ}\)C, a small variation in resistance was recorded, obtaining a sensitivity magnitude of ~3.14 at 300 ppm CO. When increasing the operating temperature to 300 \({}^{\circ}\)C, very significant changes in electrical resistance took place, with a maximum sensitivity magnitude of ~72.67 at 300 ppm of CO. During the experiments, it was observed that the increase in the CO concentration caused the increase in the pellets' response (see Table 1). Figure 7: Morphologies’ distribution of the NiSb\({}_{2}\)O\({}_{6}\) powders calcined at 900 \({}^{\circ}\)C: (**a**) particle size, (**b**) size of the polyhedron-type morphologies. This was due to the fact that when the test gas was in contact with the oxygen on the pellets' surface, it oxidized, causing a transfer of charge carriers on the material's surface [11, 39, 40]. In the literature, it is reported that the increase in response in materials such as NiSb\({}_{2}\)O\({}_{6}\) at temperatures above 150 \({}^{\circ}\)C is due to the **O\({}^{-}\)** and **O\({}^{2-}\)** ionic oxygen species [37, 39], which are very reactive at relatively high temperatures (in our case, at 300 \({}^{\circ}\)C) [11, 39]. At temperatures below 200 \({}^{\circ}\)C, the oxygen species that appear the most are of the **O\({}_{2}^{-}\)** type [11, 39]. Such species are less reactive at temperatures below 100 \({}^{\circ}\)C because the thermal energy supplied is not enough to favor chemical reactions between the oxygen species, the CO, and the surface of the pellets, causing low kinetic interaction between the material nanoparticles and the CO molecules [40, 41, 42]. This resulted in not recording significant increases in the pellets' response at 100 \({}^{\circ}\)C (see Figure 8a,b). It is important to mention that the excellent response and good stability recorded at 300 \({}^{\circ}\)C were mainly associated with the microstructural features obtained, which led to an increase in catalytic sites for the gas reaction with the nanoparticles' surface [43, 44, 45]. The dependence of gas sensing on the operating temperature is due to several facts. One of them is that the type of oxygen species adsorbed on the oxide depends on temperature [28]. As mentioned, atomic species, which are more reactive than molecular species, predominate at temperatures above 150 \({}^{\circ}\)C [13]. Another fact is that the oxidation reaction is an activated process, where its rate increases with temperature. Thermal energy is needed to activate the adsorption of ionized oxygen species and overcome the barriers of the sensing reactions [20]. Additionally, desorption and diffusion processes are also temperature dependent. Thus, the sensitivity can rise appreciably with increasing temperature [24]. Different literature reports have shown that the gas response at low temperatures starts to disappear at around 200 \({}^{\circ}\)C and is replaced by the typical sensitivity response at high temperatures [13, 20]. The optimum temperature depends on the sensing material and the sensed gas. Therefore, as in previous reports [29, 43], a noticeable response variation was observed at 300 \({}^{\circ}\)C. Comparing our results with references [29, 42, 43, 46], where similar experiments were carried out in static gas atmospheres, we found that our pellets