## 3 Results and Discussion
### Co2 SnO4 and Co2 SnO4/Co3O4 Sensing Results and Discussion under Ethanol
#### 3.2.1 Detection Mechanism under Ethanol  
The ethanol response improvement of CTO\({}_{\text{ann}}\) is the result of porosity as well as an increase in grain boundaries. The grain boundary regions are the favored oxygen adsorption regions, as already mentioned above. Consequently, when the proportion of grain boundaries increases in the film due to a decrease in grain size, the oxidation reaction rate became more pronounced for this film wherein the current intensity variation increases due to the increases in charge transfer, and thus the response variation increases. In order to confirm the advanced effects of grains and grain boundaries in the proposed interaction mechanism between ethanol and films, an AC investigation of elaborated films, presented in Figure 6, was performed at the optimum temperatures under 500 ppm of ethanol vapor. Figure 6: The CTO\({}_{\text{as}}\) and CTO\({}_{\text{ann}}\) AC experimental results and modeling. The AC analysis evidenced that, under ethanol, the \(\text{CTO}_{\text{as}}\) film impedance could be modelled by a single parallel RC circuit giving one semicircle in a Nyquist plot, attributed to the grain regions. In this case, the \(\text{CTO}_{\text{as}}\) ethanol response is controlled only by the grain regions, whereas for \(\text{CTO}_{\text{ann}}\), we note the apparition of a second semicircle showing that the response is mainly controlled by the grain boundary regions. The RC modeling results are reproduced in Table 1, and fit well with the AC experimental results. The previous results of the AC investigation allow us to affirm that the response enhancement of the \(\text{CTO}_{\text{ann}}\) sensor is mainly controlled by the oxidation reactions with oxygen adsorbed in grain boundary regions formed by \(\text{Co}_{3}\text{O}_{4}/\text{Co}_{2}\text{SnO}_{4}\) p-n heterojunctions. In the other sensor regions there is probably a coexistence of \(\text{Co}_{3}\text{O}_{4}/\text{Co}_{3}\text{O}_{4}\) p-p homojunctions and \(\text{Co}_{2}\text{SnO}_{4}/\text{Co}_{2}\text{SnO}_{4}\) n-n homojunctions, so their electrical (response) behaviors operate against each other. The \(\text{CTO}_{\text{as}}\) sensor, which presents a pure phase of \(\text{Co}_{2}\text{SnO}_{4}\) according to the XRD pattern and TEM analyses (Figures 3 and 4a), was probably formed only by \(\text{Co}_{2}\text{SnO}_{4}/\text{Co}_{2}\text{SnO}_{4}\) n-n homojunctions. The current variation under ethanol is less than that of the \(\text{CTO}_{\text{ann}}\) sensor formed by \(\text{Co}_{3}\text{O}_{4}/\text{Co}_{2}\text{SnO}_{4}\) p-n heterojunctions. In effect, the mechanism could be related to the contact at the junction interface between \(\text{Co}_{3}\text{O}_{4}\) and \(\text{Co}_{2}\text{SnO}_{4}\) in the heterojunctions and between \(\text{Co}_{2}\text{SnO}_{4}\) and \(\text{Co}_{2}\text{SnO}_{4}\) in the homojunctions, so the sensing property variations for the \(\text{CTO}_{\text{as}}\) and \(\text{CTO}_{\text{ann}}\) semiconductors are controlled by the energy barrier changes \"\(qV\)\" at homojunctions or heterojunctions under ethanol. This energy barrier, in turn, depends on work function differences between \(\text{Co}_{3}\text{O}_{4}\) and \(\text{Co}_{2}\text{SnO}_{4}\) given by the following equation:  
\[qV=q(\varnothing_{\text{Co2 SnO4}}-\varnothing_{\text{Co3}\text{O4}}) \tag{14}\]  
where \(\varnothing_{\text{Co3}\text{O4}}\) and \(\varnothing_{\text{Co2 SnO4}}\) are the work functions for \(\text{Co}_{3}\text{O}_{4}\) and \(\text{Co}_{2}\text{SnO}_{4}\), respectively.