## 3 Results and Discussion
### Co2 SnO4 and Co2 SnO4/Co3O4 Sensing Results and Discussion under Ethanol  
The samples' ethanol sensing tests were performed in the concentration range of ethanol vapor at 100-1000 ppm; the gas response \"S\" is defined as follows:  
\[\text{S}=\frac{\text{I}_{\text{gas}}-\text{I}_{0}}{\text{I}_{0}} \tag{4}\]  
for the CTO\({}_{\text{as}}\) sample, which presented the conductivity behavior of an n-type semiconductor; and  
\[\text{S}=\frac{\text{I}_{\text{gas}}-\text{I}_{0}}{\text{I}_{\text{gas}}} \tag{5}\]  
for the CTO\({}_{\text{ann}}\) sample, which showed the conductivity behavior of a p-type semiconductor. \(\text{I}_{\text{gas}}\) and \(\text{I}_{0}\) are the current intensities of layers under ethanol gas (vapor) and dry air, respectively. Firstly, in order to determine the optimal working temperatures, the concentration of ethanol was fixed at 500 ppm and the temperature was varied from 100 \({}^{\circ}\)C to 250 \({}^{\circ}\)C. The obtained optimal working temperatures were found to be 150 \({}^{\circ}\)C for CTO\({}_{\text{as}}\) and 200 \({}^{\circ}\)C for CTO\({}_{\text{ann}}\), as shown in Figure 5a,b. After optimal working temperature deduction, a dynamic measurement under gas was made in order to deduce the response (\(\tau_{\text{res}}\)) and the recovery (\(\tau_{\text{rec}}\)) times, reported in the last table. The chemical reactions which occurred between the ethanol vapor and the samples' sensitive layers when the samples were exposed to gas were also analyzed, and the most probable detection mechanism was deduced. For these measurements, each film  
Figure 5: CTO\({}_{\text{as}}\) and CTO\({}_{\text{ann}}\) ethanol sensing results: (**a**) optimal working temperature determination of as-prepared film (CTO\({}_{\text{As}}\)) under 500 ppm of ethanol; (**b**) optimal working temperature determination of annealed film (CTO\({}_{\text{Ann}}\)) under 500 ppm of ethanol; (**c**) dynamic current variation of prepared layers at optimum temperatures; (**d**) layers’ response evolutions versus ethanol concentration. was exposed to six different concentrations of ethanol, namely 100, 200, 300, 400, 500 and 1000 ppm, at its optimum temperature. The duration of each cycle was about 10 min under ethanol vapor. In the overall measurements, dry air was used as a carrier gas. CTO\({}_{\text{ann}}\) film showed an improvement in ethanol sensitivity compared with the as-prepared sensor CTO\({}_{\text{as}}\) with a switch from n- to p-type conductivity; this result could be attributed to gas interaction with the p-type Co\({}_{3}\)O\({}_{4}\) phase formation after the annealing process. In addition, we noted an increase in grain boundaries in the CTO\({}_{\text{ann}}\) sensitive layer which became more granular, with small grains, therefore increasing the density of grain boundaries, which are the favored oxygen adsorption regions. The main reason for the observed grain size decrease is the porosity decrease in the grains with the increase in annealing temperature (i.e., the grain become more compact) as reported in the literature [34]. For working temperatures below 200 \({}^{\circ}\)C, the oxygen molecular form O\({}_{2}^{-}\) dominates, and above this temperature the ionic forms O\({}^{-}\) and O\({}^{2-}\) are the predominant adsorbed oxygen species [35].