## 3 Results and Discussion
TEM images of (**a2**) NiO, high-resolution (HR)-TEM, (**a3**) NiO, (**b3**) 1% CQDs@NiO, (**c3**) 2% CQDs@NiO, and (**d3**) 3% CQDs@NiO. NiO and the composites were probed as sensing materials in CH4 by measuring the resistivity variations, i.e., Rg/Ra (Rg = gas resistance, Ra = air resistance) as a function of operational temperature and gas concentration at an RH of 20% (Figure 3). In Figure 3a the gas response of the probed materials is reported as a function of temperature. The optimal value was 175 \({}^{\circ}\)C for pristine NiO and 150 \({}^{\circ}\)C for the composites. Previous NiO-based sensors for methane detection worked at optimum temperatures of 200 \({}^{\circ}\)C (ultra-thin NiO [17]), 250 \({}^{\circ}\)C (rGO-NiO [28]), and 330 \({}^{\circ}\)C (SnO\({}_{2}\)-NiO [45]), which in all cases are higher than in the current work. This optimum temperature is also lower or, at best, equal to the working temperature of SnO\({}_{2}\)-based sensors [41; 42; 43]. The 1%CQDs@NiO and 2%CQDs@NiO composites also displayed a larger Rg/Ra ratio than pristine NiO, whereas the highest loaded composite 3%CQDs@NiO displayed the worst performance. Further analyses were carried out on the best-performing composite, i.e., the 1%CQDs@NiO. In Figure 3b, the gas response vs. concentration is reported for detection at 150 \({}^{\circ}\)C, in the range 5-50 ppm, showing a reasonably linear response (the \(R^{2}\) coefficient of the linear regression is 0.99). In the inset, the error bars of the measurements are reported. The detection range is very good, since most of the proposed devices detect CH4 in higher concentration ranges, whereas it is desired to be capable of detecting even small gas leaks. From the slope of the linear fit, it is possible to estimate to the limit of detection (LOD) as:  
\[\text{LOD}=3\frac{rms_{noise}}{slope}, \tag{1}\]  
where \(rms_{noise}\) and \(slope\) are the root-mean-square error of the baseline and the slope value of the linear curve [27]. For the 1%CQDs@NiO sensor, we appraised an LOD of 0.1 ppm. Figure 3c demonstrates the dynamic response and recovery curve of the 1%CQDs@NiO to 20 and 30 ppm CH4 at 150 \({}^{\circ}\)C. The sensor response increased dramatically when CH4 gas was injected into the test chamber and was swiftly recovered to the initial resistance when the sensor was exposed to air. The time taken by the sensor to reach 90% of the total resistance change is defined as the response time in the case of gas adsorption, whereas the recovery is the time needed for the specular gas desorption process [54; 55]. The response and recovery times of the NiO sensor toward 30 ppm CH4 were calculated to be about 10 s and 14 s, respectively. The performance of the 1%CQDs@NiO composite-based sensor is compared to previously reported data in Table 2. The reproducibility of the 1%CQDs@NiO-based sensor measurements is reported in Figure S2 (Supplementary Materials). The stability of the 1%CQDs@NiO sensor over time was probed by testing the response to 30 ppm CH4 at 150 \({}^{\circ}\)C in a time span of three weeks, revealing a rather constant behavior (Figure 3d). The effect of the relative humidity on the 1%CQDs@NiO sensors was probed by monitoring the response to 30 ppm CH4 at 150 \({}^{\circ}\)C as a function of increasing RH (Figure 3d). The sensor maintained an excellent CH4-sensing performance with little response loss in a high-RH environment due to the increased oxidation reaction facilitated by the CDQ decoration of the NiO (Figure 3e). Finally, the selectivity of