# Effect of Platinum Doping on the Morphology and Sensing Performance for CuO-Based Gas Sensor
## 3 Results and Discussions
### Gas Sensing Characteristics
Figure 9b also shows the dynamic response curves of the 1.25 Pt-CuO sensor to 10 ppm H\({}_{2}\)S gas. In the five-cycle continuous test process, the sensors based on the Pt-doped CuO exhibit good stability, and their gas-sensing performance, such as sensitivity, and response/recovery speed did not show obvious resistance attenuation. In addition, such a'recovery heating' process had tiny effect on the response of the sensor, which made the gas sensor highly reusable. Therefore, by applying a short electric current  
Figure 8: (**a**) The resistance variation of the 1.25 Pt-CuO sensor to 10 ppm H\({}_{2}\)S at 40 \({}^{\circ}\)C; (**b**) the corresponding sensing transients of the 1.25 Pt-CuO sensor. The inset of (**b**) is the magnification of sensing transients between 300 s to 700 s.  
Figure 7: (**a**) Response of 0.00 Pt-CuO, 0.75 Pt-CuO, 1.25 Pt-CuO, and 1.50 Pt-CuO sensors to 10 ppm H\({}_{2}\)S as a function of operating temperature; (**b**) the corresponding sensing transients of 0.00 Pt-CuO, 0.75 Pt-CuO, 1.25 Pt-CuO, and 1.50 Pt-CuO sensors to 10 ppm H\({}_{2}\)S at 40 \({}^{\circ}\)C. The inset of (**b**) is the resistance variation of four sensors to 10 ppm H\({}_{2}\)S at 40 \({}^{\circ}\)C.  
pulse, we have developed high performance H\({}_{2}\)S gas sensors based on the Pt-doped CuO with a low operating temperature. Selectivity is one of the key parameters of the gas sensors. To explore the selectivity of the sensors, the dynamic response curves of the 1.25 Pt-CuO based sensor to 1 ppm H\({}_{2}\)S and 50 ppm of other gases were measured at 40 \({}^{\circ}\)C. The results are shown in Figure 10a. Compared with the responses to H\({}_{2}\)S gas, the responses to other reducing gases were negligible. Figure 10b shows the responses of the 1.25 Pt-CuO sensor to various gas samples, including 50 ppm C\({}_{2}\)H\({}_{5}\)OH, NH\({}_{3}\), CH\({}_{3}\)COCH\({}_{3}\), NO\({}_{2}\), Cl\({}_{2}\), H\({}_{2}\), NO, and 1 ppmH\({}_{2}\)S, operated at 40 \({}^{\circ}\)C. The sensitivity of such sensors are 32.9 and 2.4 to 1 ppm H\({}_{2}\)S and 50 ppm NO\({}_{2}\), respectively, while the responses to other gases ranged between 1.1 and 1.2. Therefore, 1.25 wt % Pt doping led to highest selectivity and sensitivity for the detection of H\({}_{2}\)S at low operating temperature (i.e., 40 \({}^{\circ}\)C). Figure 11a displays the dynamic response-recovery transient of the sensor based on 1.25 Pt-CuO, under the H\({}_{2}\)S concentration ranging from 0.1 to 25 ppm, at the operating temperature of 40 \({}^{\circ}\)C. It is apparent that as the concentration of the H\({}_{2}\)S gas increases, the response curve also displays a stepwise distribution. Figure 11b shows the response of the gas sensor to different concentrations of H\({}_{2}\)S gas. Figure 10: (**a**) Dynamic response curves of 1.25 Pt-CuO and 0.00 Pt-CuO based sensor to 1 ppm H\({}_{2}\)S and 50 ppm various interfering gases at 40 \({}^{\circ}\)C; (**b**) gas response (R\({}_{\rm a}\)/R\({}_{\rm g}\) or R\({}_{\rm g}\)/R\({}_{\rm a}\)) of 1.25 Pt-CuO to 50 ppm different gases and 1 ppm H\({}_{2}\)S gas, operated at 40 \({}^{\circ}\)C.  
Figure 9: (**a**) Sensing diagram of the gas sensor with a 4.6 V electric modulation voltage; (**b**) the recovery cycles for 10 ppm H\({}_{2}\)S of the 1.25 Pt-CuO sensor at 40 \({}^{\circ}\)C, followed by a 4.6 V electric modulation voltage. It can be observed that the gas response increased as the concentration increased. Even when the H\({}_{2}\)S concentration is as low as 300 ppb, the response of the sensor is still 3.2. According to the value of response (R\({}_{\rm g}\)/R\({}_{\rm a}\) > 2), as the standard for a valid response [38], it can be said that the gas sensor based on 1.25 Pt-CuO has a detection limit of H\({}_{2}\)S gas as low as 300 ppb. Figure 12 illustrates the long-term stability of the gas sensor based on 1.25 Pt-CuO. The results reveal that the response of the sensor changes by less than 10%, indicating that our sensor is stable and reliable.