## 3 Results and Discussion
### Gas Sensing Investigations
Subsequently, with the single-layer SnO\({}_{2}\) sensor applied under ten rotations of substrate, we also studied the sensing behavior at different rotation speeds (5, 7, and 12 rpm) in response to various amounts of H\({}_{2}\) at 300 \({}^{\circ}\)C, and then we obtained the relevant calibration graphs (Figure 7a,b). There was a noticeable difference between the sensors' responses under different substrate rotation speeds. The sensor prepared at a substrate rotation of 7 rpm showed the highest response, whereas the sensor prepared at 5 rpm substrate speed exhibited the lowest response. According to the above findings, one layer of the sensing material applying on the substrate with ten rotations at a rotation speed of 7 rpm results in the best sensor efficiency. Since the annealing temperature can affect the particle size and crystallinity of SnO\({}_{2}\) particles, and these factors ultimately can affect the sensor performance, we also studied the effect of annealing temperature on the gas response. After determining the optimal conditions, we determined the optimal annealing temperature for the SnO\({}_{2}\) gas sensors. For this purpose, SnO\({}_{2}\) particles were annealed at different temperatures (400, 500, 600, and 700 \({}^{\circ}\)C) for 1 h in air. Figure 8 reveals the response of the sensors annealed at different temperatures to 10 ppm H\({}_{2}\) gas, and the inset in Figure 8 depicts transient resistance plots of SnO\({}_{2}\) gas sensors applied to tubular substrates under optimized conditions and annealed at 400, 500, 600, and 700 \({}^{\circ}\)C to 10 ppm H\({}_{2}\) at 300 \({}^{\circ}\)C. According to the obtained results, the sensor annealed at 400 \({}^{\circ}\)C exhibited the highest response to H\({}_{2}\) gas, and therefore, it was selected for other experiments. The decrease in the sensor's response annealed at higher temperatures may be related to the growth of particles at higher temperatures, which decreases the overall surface area of the sensor and causes a decrease in the gas response. [PERSON] et al. [39] also investigated the effect of the annealing temperature (450, 500, and 550 \({}^{\circ}\)C) on the gas response of ZnFe\({}_{2}\)O\({}_{4}\) and reported that the best sensing performance was obtained at 450 \({}^{\circ}\)C, due to the low particle size of the sensing temperature at this temperature. [PERSON] et al. [40], investigated the effect of crystallization time on the gas response of ZnO hollow nanofibers and the sensors crystallized for a longer time, which revealed a better response relative to other sensors, which was related to the high crystallinity of the sensor. In the present study, even though at higher annealing temperatures a higher crystallinity is expected, it seems that the role of particle growth is more important than that of improved crystallinity, as pointed out by another study by [PERSON] et al. [41]. Figure 7: (**a**) Sensing plots of SnO\({}_{2}\) gas sensor applied to tubular substrate with different rotation speeds (5, 7, and 12 rpm) to various concentrations of H\({}_{2}\) gas at 300 \({}^{\circ}\)C. (**b**) Corresponding sensing calibration plots. In the next step, the reproducibility of the optimized sensor was tested by fabricating two sensors under optimal fabrication procedures and exposing them to various amounts of H\({}_{2}\) at 300 \({}^{\circ}\)C (Figure S2a,b). Figure S2c presents the calibration curves of the two gas sensors. The sensors exhibited almost the same responses to various levels of H\({}_{2}\) gas, demonstrating their desirable reproducibility. For real applications, especially for H\({}_{2}\) gas detection, the response time is a highly important parameter, so we also calculated the response time of the optimal gas sensor. In Figure 9, the response times of the optimized sensor to 10, 50, 100, 500, and 1000 ppm H\({}_{2}\) at 300 \({}^{\circ}\)C are given. The response times to the mentioned concentrations were 37, 17, 16, 16, and 14 s, respectively.