## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sensing** & **Methane** & **Temp** & **Gas** & **Gas** & **Response** & **Response/Recovery** & **Reference** \\
**Element** & **(ppm)** & **(\({}^{\circ}\)C)** & **Rg/Ra** & **Ra/Rg** & & **Time (s)** & **Reference** \\ \hline
1\%CQDs@NiO & 30 & 150 & 77.3 & & 10/14 & This work \\ UT-NiO & 30 & 200 & \(\sim\)50 & & 15/20 & [17] \\ rGO-NiO & 1000 & 250 & 15 & & 6–18/16–20 & [28] \\ rGO-SnO\({}_{2}\) & 1000 & 150 & & 47.6 & 61/330 & [44] \\ Pd-SnO\({}_{2}\) & 100 & 340 & & 4.38 & 3/5 & [42] \\
2.5-Pt/SnO\({}_{2}\) & 500 & 100 & & 1.98 & – & [43] \\ SnO\({}_{2}\)-NiO & 500 & 330 & 15.2 & & 28/44 & [45] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of methane sensor characteristics in this work to recent studies. UT = ultra-thin. rGO = reduced graphene oxide. The response of p-type sensors is presented as Rg/Ra, while the n-type sensor response is presented as Ra/Rg, where Rg = gas resistance, and Ra = air resistance. the 1%CQDs@NiO sensor was evaluated by investigating the comparative response to four target gases (CH4, CO2, NH3, and acetone) at 30 ppm concentration, operating at 150 \({}^{\circ}\)C. The results are shown in Figure 3f and indicate a marked selectivity toward CH4, with poor responses to the other gases. Figure 3: Gas responses of the sensing elements, defined as Rg/Ra, where Rg is the gas resistance and Ra is the air resistance: (**a**) gas response to 30 ppm CH4 as a function of temperature—red dotted line (-\(\bullet\)-) 1%CQDs@NiO, green dotted line (-\(\blackdot\)-) 2%CQDs@NiO, blue dotted line (-\(\blackdot\)-) 3%CQDs@NiO, black dotted line (-\(\blackdot\)-) NiO; (**b**) gas response of 1%CQDs@NiO operated at 150 \({}^{\circ}\)C and R\({}_{\rm H}\) 20%; in the inset, the same plot is presented with the error bars of the measurement and without symbols; (**c**) response and recovery curves of 1%CQDs@NiO at 20 ppm and 30 ppm, R\({}_{\rm H}\) 20%; (**d**) gas response of 1%CQDs@NiO to 30 ppm CH4 over three weeks, R\({}_{\rm H}\) 20%; (**e**) gas response of 1%CQDs@NiO to 30 ppm CH4 as a function of relative humidity (black dotted line (-\(\blackdot\)-) and percentage of response retention (green line with triangles -\(\dot\)-); (**f**) selectivity of 1%CQDs@NiO, operated at 150 \({}^{\circ}\)C and exposed to 30 ppm of the indicated gases (R\({}_{\rm H}\) = 20%). The detection mechanism is based on the reaction between CH\({}_{4}\) and adsorbed oxygen according to Equation (2). \[\text{CH}_{4}+4\text{O}^{-}\ \rightarrow\ 4\text{H}_{2}\text{O}+\ \text{CO}_{2}+4 \text{e}^{-}. \tag{2}\]  
The first step of the overall process is the adsorption of O\({}_{2}\) by reduction on the surface to O\({}^{-}\), with formation of a hole accumulation layer (HAL) related to electrostatic interaction of opposite charges [56]. In this regard, the morphology plays a role, since the edges of the polygonal-shaped structure of the NiO nanoparticles may be efficient in capturing O\({}_{2}\). The effects of the deposition of the CQDs on NiO can be reconducted to the formation of a hetero-junction potential barrier at the interface between CQDs and NiO, which greatly facilitates the adsorption and diffusion of the methane molecule [28; 57]. More in detail, the deposition of CQDs on the NiO causes an increase in resistance following the electron-hole equilibration at the interface. On the other hand, the interface between CQDs and NiO junctions is a source of defects which are potential gas adsorption sites. Therefore, a larger number of gas molecules may adsorb and react at the interface, resulting in a decreased thickness of the HAL and higher resistance of the CQDs-NiO as compared to pure NiO sensor. By the time the chemical reaction is accomplished, electrons flow from the CQDs to NiO, which results in a further decrease of the hole concentration in NiO and even larger resistance [58; 59]. Furthermore, an electrocatalytic effect may take place [60] through methane activation.