## 3 Results and Discussion
### Propane Analysis  
Figure 10 shows the results of the dynamic response at 0.1 and 1 kHz of NiSb\({}_{2}\)O\({}_{6}\) films exposed to propane at 360 \({}^{\circ}\)C. In Figure 10a,b, the results are shown for films stabilized through a flow of extra dry air (1500 cm\({}^{3}\)/min) for 5 min. Then, without removing the air, 600 ppm of propane was injected into the measuring chamber for 5 min, recording the decrease in sensitivity magnitude \(\mid\!Z\!\mid\) by varying the frequency from 0.1 to 1 kHz. The values of \(\mid\!Z\!\mid\) at 0.1 kHz were in the range of 6.6-7.05 M\(\Omega\), with response and recovery times of 42.96 and 35.56 s, respectively. At 1 kHz, the \(\mid\!Z\!\mid\) values were 1.74-1.75 M\(\Omega\), with response and recovery times of 11.91 and 5.88 s, respectively. The response and recovery  
times were calculated for both frequencies considering 90% of the response in the test gas and 10% of the value when exposed only to air [47]. Although NiSb\({}_{2}\)O\({}_{6}\) is a p-type semiconductor [28], the trend in Figure 10a,b is attributed to the fact that the decrease in \(\mid\)Z\(\mid\) is caused by the continuous flow of oxygen-rich air (21% O\({}_{2}\)), which causes oxidation (**O\({}^{-}\)**) [48; 54; 56] and an oxygen saturation (**O\({}^{-}\)** and **O\({}^{2-}\)**) [29; 35; 43; 54; 57] that reacts with the propane on the films' surface, causing the excellent response shown in Figure 10a-e.  
We observed that the good dynamic response of the NiSb\({}_{2}\)O\({}_{6}\) films in propane atmospheres was caused by the rapid adsorption of the gas that reacted with the film's surface, causing an electron exchange between the oxygen species adsorbed on the surface and the surface itself [48]. It has been reported in the literature that the electrons on the material's  
Figure 10: (**a,b**) Dynamic response at 0.1 and 1 kHz of NiSb\({}_{2}\)O\({}_{6}\) in 600 ppm of C\({}_{3}\)H\({}_{8}\) at 360 \({}^{\circ}\)C, (**c–e**) resistance, reactance, and capacitance as a function of time at 1 kHz in 600 ppm of C\({}_{3}\)H\({}_{8}\) at 360 \({}^{\circ}\)C.  
surface are the most probable cause of the electrical response variation and, therefore, of the improvement in the sensor response [11; 48; 50; 58]. The variations in the impedance magnitude (\(|Z|\)) and in the response and recovery times were due to the chemical reaction between the surface of the thick films and the propane gas. The reaction occurred because there was a high kinetic activity of the gas molecules due to the operating temperature, causing an immediate response (response time) to 600 ppm of the gas. As in the case of CO, when the propane stopped injecting and the air was introduced into the system, the impedance values tended to immediately return to their stable values (recovery time). It was corroborated that at 0.1 and 1 kHz, the response and recovery times decreased significantly compared to other semiconductors tested as gas sensors [14; 28; 34; 58]. The reaction mechanism that could explain the adsorption and desorption of propane gas molecules on a semiconductor's surface, such as the one studied here, is not yet fully established. Some authors report that the propane molecules on the film's surface react with adsorbed oxygenated species, producing CO\({}_{2}\), water vapor, and the release of electrons on the surface [59]. Other authors believe that a reaction that could be occurring between a given semiconductor and propane gas could be the following: C\({}_{3}\)H\({}_{8}\) + 10**O**\({}^{-}\)\(\rightarrow\) 3 CO\({}_{2}\) + 4H\({}_{2}\)O + e\({}^{-}\)[35; 60]. This means that when the propane gas is adsorbed, it dissociates on the material's surface because of the temperature (in our case at 360 \({}^{\circ}\)C), causing changes in the sensor's electrical response (see Figure 10a-e). At this point, it is convenient to address several sensing mechanisms of metal oxide semiconductors in more detail. These mechanisms can be classified into the following two categories: microscopic and macroscopic [61].