## 3 Results and Discussion
### Co2 SnO4 and Co2 SnO4/Co3O4 Sensing Results and Discussion under Ethanol
Thus, the possible ethanol interaction mechanisms with the CTO\({}_{\text{as}}\) film at the optimum temperature of 150 \({}^{\circ}\)C could be presented by the following reactions for temperatures below 200 \({}^{\circ}\)C:  
\[\text{O}_{2~{}(\text{gas})}~{}+\text{e}_{(\text{bulk})}^{-}\rightarrow\text{O }_{2(\text{surface})}^{-} \tag{6}\]  
\[\text{C}_{2}\text{H}_{5}\text{OH}_{(gas)}+3\text{O}_{2(\text{surface})}^{-} \leftrightarrow 3\text{H}_{2}\text{O}_{(gas)}+2\text{CO}_{2(gas)}+3\text{e}_{( bulk)}^{-} \tag{7}\]  
The resulting CO\({}_{2}\) and the pre-adsorbed OH\({}^{-}\) will react together as follows:  
\[\text{CO}_{2(\text{gas})}+\text{OH}_{(\text{surface})}^{-}\leftrightarrow \text{HCO}_{3(\text{surface})}^{-} \tag{8}\]  
\[\text{HCO}_{3(\text{surface})}^{-}+\text{OH}_{(\text{surface})}^{-} \leftrightarrow\text{CO}_{3(bulk)}^{2-}+\text{H}_{2}\text{O}_{(\text{gas}/ \text{surface})} \tag{9}\]  
Equation (6) shows that the oxygen molecule is absorbed on the layer's surface without dissociating, by trapping an electron from the bulk, and it becomes negatively charged. After that, the \(\left(\text{O}_{2}^{-}\right)\) adsorbed oxygen species will react with ethanol vapor (C\({}_{2}\)H\({}_{5}\)OH) to produce water vapor and carbon dioxide (CO\({}_{2}\)) and an electron (e\({}^{-}\)), according to Equation (7). The carbon dioxide (CO\({}_{2}\)) will react with preadsorbed hydroxide (OH\({}^{-}\)) according to Equation (8), to provide a reaction product, a bicarbonate ion \(\left(\text{HCO}_{3}^{-}\right)\), which in turn will react with preadsorbed hydroxide OH\({}^{-}\) to produce a carbonate anion \(\left(\text{CO}_{3}^{2-}\right)\) and water as a final product (Equation (9)). The resulting negative charges (carbonate anion and electron) will move to the CTO\({}_{\text{as}}\) bulk which, according to the XRD pattern and TEM image already presented in Figures 3 and 4, respectively, present only a pure phase of Co\({}_{2}\)SnO\({}_{4}\) and thus increase the carrier charge density. This explains the current increases under ethanol, with the film showing n-type conductivity as already illustrated in Figure 5c at 150 \({}^{\circ}\)C. Conversely, for CTO\({}_{\text{ann}}\) film, which presents a phase mixture of Co\({}_{2}\)SnO\({}_{4}\)/Co\({}_{3}\)O\({}_{4}\), we obtained an opposite behavior under ethanol, as also shown in Figure 5c at 200 \({}^{\circ}\)C, showing the p-type conductivity which is dominated by the Co\({}_{3}\)O\({}_{4}\) species, and which explains the n- to p-type switch observed for annealed film (CTO\({}_{\text{ann}}\)) under ethanol vapor. The probable main ethanol reaction at the optimum temperature of 200 \({}^{\circ}\)C for CTO\({}_{\text{ann}}\) is the oxidizing of ethanol given by the following reactions [36]:  
\[\frac{1}{2}\text{O}_{2(\text{gas})}+\text{e}_{(\text{bulk})}^{-}\rightarrow \text{O}_{(\text{surface})}^{-} \tag{10}\]  
\[\text{C}_{2}\text{H}_{5}\text{OH}_{(\text{gas})}+\text{O}_{(\text{surface})}^{- }\rightarrow\text{CH}_{3}\text{CHO}_{(\text{surface})}+\text{H}_{2}\text{O}_ {(\text{gas})}+\text{e}_{(\text{bulk})}^{-} \tag{11}\]  
Equation (10) shows that the oxygen molecule is absorbed on the layer's surface with dissociation by trapping an electron from the bulk and becoming negatively charged. Following this, the (O\({}^{-}\)) adsorbed oxygen species will react with ethanol vapor (C\({}_{2}\)H\({}_{5}\)OH) to give water vapor, an acetaldehyde molecule (CH\({}_{3}\)COH) and an electron (e\({}^{-}\)), according to Equation (11). The electron produced by the previous reaction (Equation (11)) is injected into the bulk, which induces a decrease in the hole concentrations, causing a decrease in current intensity.