# Effect of Platinum Doping on the Morphology and Sensing Performance for CuO-Based Gas Sensor
## 3 Results and Discussions
### Gas Sensing Characteristics  
To investigate the optimum operating temperature and additive amount of the Pt-doped CuO based sensor for H\({}_{2}\)S, we examined the responses of the four sensors to 10 ppm H\({}_{2}\)S gas from room temperature to 260 \({}^{\circ}\)C. Figure 7a illustrates that the response of all the doped sensors increased once the operating temperature changed from room temperatures to 40 \({}^{\circ}\)C, while the undoped sensor has its maximum response at 70 \({}^{\circ}\)C. Furthermore, all the Pt-doped sensors exhibit a much higher response than the pristine one. The 1.25 Pt-CuO sensor showed the highest response of 135.1 to 10 ppm of H\({}_{2}\)S gas at 40 \({}^{\circ}\)C, which was 13.1 times higher than that of the pure CuO sensor (R\({}_{\rm g}\)/R\({}_{\rm a}\) = 10.3) at the same temperature. At a high operating temperature (above 240 \({}^{\circ}\)C), the pure CuO and all Pt-doped sensors showed a small response (1.6-2.7). Therefore, the optimized doping concentration is 1.25 wt % Pt for CuO and the operating temperature is 40 \({}^{\circ}\)C. Figure 7b shows the dynamic response curves of the gas sensors, based on the pure CuO and Pt-doped CuO to 10 ppm H\({}_{2}\)S at 40 \({}^{\circ}\)C. It proves that the 1.25 Pt-CuO gas sensor has the highest response. However, it is noteworthy that neither the pure CuO sensor nor Pt-doped CuO sensor could be completely recovered to the baseline. Figure 8 shows the sensing behavior of the sensor, based on 1.25 wt % Pt-doped CuO towards 10 ppm H\({}_{2}\)S gas at 40 \({}^{\circ}\)C. As it is shown in Figure 8a, the resistance of the 1.25 Pt-CuO sensor is 0.1 k\(\Omega\) when exposed to air. After the sensor was exposed to H\({}_{2}\)S gas, the response resistance increases to 13.51 k\(\Omega\), thus the response of the 1.25 Pt-CuO sensor to 10 ppm H\({}_{2}\)S is as high as 135.1 (R\({}_{\rm g}\)/R\({}_{\rm a}\)). Although the Pt-doped CuO sensors were highly sensitive to detect H\({}_{2}\)S gas, the extremely long recovery time was the bottleneck for real practice. Figure 8b shows the dynamic response-recovery curves of the sensor; it took 37 s to reach 90% of the resistance variation. Figure 8b also shows that it took more than 3000 s to recover about 5% of the maximum value after removal of the H\({}_{2}\)S. In fact, the response could not be completely recovered to the baseline, and it is the common issue for gas sensors operating at a low temperature. According to [PERSON] et al. [37] because of molecular affinity, Cu and S could easily form a helical bond at a low operating temperature. Therefore, H\({}_{2}\)S could be present in the form of molecular adsorption on the surface, and the adsorption is intermediate between physisorption and chemisorption. Figure 6: Typical N\({}_{2}\) adsorption-desorption isotherms of different specimens, as follows: (**a**) 0.00 Pt-CuO and (**b**) 1.25 Pt-CuO. In this work, we proposed a solution to solve the problem that the gas sensors could not be completely recovered at a low operating temperature. The principle of this scheme is based on the Joule heating effect, which accelerates the detachment of the H\({}_{2}\)S molecules from the CuO surface by applying a short electric current pulse. As shown in Figure 9a, V\({}_{\rm c}\) is the test voltage; R\({}_{\rm i}\) is a load resistor, and the voltage drop on R\({}_{\rm i}\) is denoted as V\({}_{\rm out}\); V\({}_{\rm h}\) is the heating voltage applied on the the heating wire; and V\({}_{\rm m}\) is the 4.6 V electric current pulse. Figure 9b shows the dynamic response curves of the 1.25 Pt-CuO sensor to 1 ppm H\({}_{2}\)S gas at 40 \({}^{\circ}\)C. Once the H\({}_{2}\)S was removed and the sensor was exposed to air, the response could not be recovered to its original baseline. After applying a 4.6 V (the temperature was 300 \({}^{\circ}\)C) voltage pulse, the 1.25 Pt-CuO sensor returned to its original state within 15 s. Reproducibility is an important indicator to evaluate the performance of the sensor.