## 3 Results and Discussion
### Gas Sensing Investigations  
Sensing temperature is an important parameter of gas sensors. Initially, to determine the optimal sensing temperature of the fabricated gas sensor, it was exposed to 10 ppm H\({}_{2}\) gas at different temperatures (Figure S1). The sensor revealed the highest response to H\({}_{2}\) gas at 300 \({}^{\circ}\)C; thus, it was selected as the optimal sensing temperature. In fact, at low temperatures, the gas did not have sufficient energy to overcome the adsorption barrier and a low response was obtained. At 300 \({}^{\circ}\)C, the adsorption rate and desorption rate were equal, resulting in the highest amount of response. However, with a further increase in sensing temperature to 350 \({}^{\circ}\)C, the response was decreased owing to a higher desorption rate relative to the adsorption rate at high temperatures. Therefore, all remaining sensing tests were measured at 300 \({}^{\circ}\)C. Figure 5a presents transient resistance plots of SnO\({}_{2}\) gas sensors with one, two, and three layers over the substrate when exposed to different amounts (10, 50, 100, 500, and 1000 ppm) of H\({}_{2}\) gas at 300 \({}^{\circ}\)C. The resistance of all the sensors decreased upon the introduction of reducing H\({}_{2}\) gas, revealing the n-type nature of the sensors. In fact, reducing gases react with adsorbed oxygen on the sensor surface, and the electrons are released to the sensor layer. Accordingly, for n-type gas sensors, the resistance decreases thanks to providing more charge carriers (electrons) to the sensor. In addition, the sensors exhibited good reversibility, where upon stoppage of the gas, the resistance returned to its initial value. This is an important feature for practical application of the gas sensor. To check and compare the behavior of gas sensors, the gas responses were calculated, and the  
Figure 4: (a) XPS survey of commercial SnO\({}_{2}\) particles, (b) OIs and (c) Sn 3d XPS core-levels. corresponding sensing calibration curves are presented in Figure 5b to better understand the behaviors of the sensors. Overall, no significant differences were observed among the sensing results of different gas sensors. Therefore, the number of applied layers, from one to three over the substrate had no significant effect on the sensing performance. Based on the obtained results, one layer of sensing material was applied on the substrate for further experiments. In the next step, we studied the behavior of one layer of the sensors applied on the tubular substrate with different numbers of substrate rotations (5, 10, 15, and 20). Figure 6a displays the transient resistance graphs of the SnO\({}_{2}\) gas sensor (one layer) applied to a tubular substrate rotating with different rotations (5, 10, 15, and 20 times) when exposed to various amounts (10, 50, 100, 500, and 1000 ppm) of H\({}_{2}\) gas at 300 \({}^{\circ}\)C. Also, relevant calibration curves are presented in Figure 6b. Based on the obtained results, the sensor applied on the substrate with ten rotations had the highest response at all concentrations, whereas the sensors applied on the substrates with five and twenty rotations had the lowest response to H\({}_{2}\) gas at low (10, 50, and 100 ppm) and high (500 and 1000 rpm) concentrations, respectively. Figure 5: (**a**) Sensing plots of SnO\({}_{2}\) gas sensors with one, two, and three layers over the substrate to different amounts of H\({}_{2}\) at 300 \({}^{\circ}\)C (10, 50, 100, 500, and 1000 ppm); (**b**) sensing calibration plots of sensors with different layers. Figure 6: (**a**) Dynamic sensing curves of SnO\({}_{2}\) gas sensor applied to tubular substrate with different numbers of rotations (5, 10, 15, and 20 times) when exposed to various amounts of H\({}_{2}\) gas at 300 \({}^{\circ}\)C. (**b**) Relevant sensing calibration graphs when the substrate was exposed to various amounts of H\({}_{2}\) under various rotations.