## 4 Conclusions  
In conclusion, the annealing process of \(\text{Co}_{2}\text{SnO}_{4}\) film synthesized by spray pyrolysis method, denoted as \(\text{CTO}_{\text{as}}\), is beneficial for ethanol detection. After annealing, we successfully obtained the \(\text{Co}_{2}\text{SnO}_{4}/\text{Co}_{3}\text{O}_{4}\) composite, denoted as \(\text{CTO}_{\text{ann}}\). The new composite presents an improvement in response under ethanol at the optimal temperature of 200 \({}^{\circ}\)C, due to the increases in its porosity and grain boundary density, which offers a more specific \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sensor Material** & \begin{tabular}{c} **Fabrication** \\ **Approach** \\ \end{tabular} & **Conc. (ppm)** & \begin{tabular}{c} **Response** \\ **“S”** \\ \end{tabular} & \begin{tabular}{c} **Sens Temp.** \\ **(°C)** \\ \end{tabular} & \
\begin{tabular}{c} **\(\tau_{\text{res}}/\tau_{\text{rec}}\)** \\ **(s)** \\ \end{tabular} & **Ref.** \\ \hline \(\text{Co}_{2}\text{SnO}_{4}\) spinel thin film & Spray pyrolysis & 500 & 3.5 & 150 & 20/24 & [37] \\ \hline SnO\({}_{2}\) (triton) & Spin coating & 500 & 3 & RT & 15/720 & [38] \\ \hline \(\text{Zn}_{2}\text{SnO}_{4}\) spinel cube & Hydrothermal & 600 & 5.5 & 325 & 18/45 & [39] \\ \hline \(\text{\alpha}\text{-Fe}_{2}\text{O}_{3}\) nanorods & Hydrothermal & 500 & 8 & 220 & \(-/-\) & [40] \\ \hline \multirow{2}{*}{Porous SiC} & Electrochemical etching &
\begin{tabular}{c} Saturated \\ vapors \\ \end{tabular} & 1.2 & RT & 85/57 & [41] \\ \hline \(\text{NiCo}_{2}\text{O}_{4}\) spinel nanoparticles & Hydrothermal & 500 & 0.6 & RT & 20/26 & [42] \\ \hline \(\text{TiO}_{2}\) NWs & Electrospinning & 500 & 7.5 & 500 & 23/1 & [43] \\ \hline \(\text{Cu}_{2}\text{O}\) nano-columnar & GLAD technique & 500 & 8.12 & 200 & 60/180 & [44] \\ \hline \(\text{Zn}_{2}\text{SnO}_{4}\text{cuboctahedra}\) & Chemical method & 600 & 0.01 & 325 & 18/45 & [45] \\ \hline \(\text{Co}_{2}\text{SnO}_{4}/\text{Co}_{3}\text{O}_{4}\) composite & Spray pyrolysis & 500 & 13.5 & 200 & 35/30 & This work \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparative study of \(\text{Co}_{2}\text{SnO}_{4}/\text{Co}_{3}\text{O}_{4}\) composite (\(\text{CTO}_{\text{ann}}\)) ethanol sensing parameters with other ethanol sensing materials. Figure 8: Stability investigation of samples under 500 ppm of ethanol vapor at the optimum temperatures: (a) \(\text{CTO}_{\text{as}}\) and (b) \(\text{CTO}_{\text{ann}}\). surface for oxygen adsorption. Further, we observed an improvement in the film's stability compared with the non-annealed one. We also noted that after the annealing process the Co\({}_{2}\)SnO\({}_{4}\) film underwent a switching from n- to p-type conductivity under ethanol, due to the p-type Co\({}_{3}\)O\({}_{4}\) phase formation in the Co\({}_{2}\)SnO\({}_{4}\) structure. The sensing mechanism was found to be an oxidation reaction which depends on the working temperature, resulting in negative charges, such as a carbonate anion for CTO\({}_{\text{as}}\) because the oxidation reaction of ethanol occurs with O\({}^{2-}\) at the optimal temperature of 150 \({}^{\circ}\)C, and an electron for CTO\({}_{\text{ann}}\) because the oxidation reaction of ethanol occurs with O\({}^{-}\). at the optimal temperature of 200 \({}^{\circ}\)C. The resulting charges will be moved to the bulk of the samples, which raises the current intensity due to the increase in the electron density in the CTO\({}_{\text{as}}\) layer (n-type conductivity) and decreases the current intensity for CTO\({}_{\text{ann}}\) due to the decrease in hole density after recombination with electrons (n-type conductivity). The improvement in responses was explained by the increase in grain boundary heterojunctions in the CTO\({}_{\text{ann}}\) film, wherein the conductivity was controlled by the energy barrier changes. Finally, the stability of CTO\({}_{\text{as}}\) and CTO\({}_{\text{ann}}\) over a period of three months was investigated; the results show an improvement in stability after the annealing process.