## 3 Results and Discussion  
### Characterization of Elaborated Films  
The XRD layer pattern of the as-prepared sample (CTO\({}_{\text{as}}\)) confirmed the synthesis of pure Co\({}_{2}\)SnO\({}_{4}\) with a spinel phase, as shown in Figure 3. All diffraction peaks could be indexed in the JCPDS 29-0514 card for the Co\({}_{2}\)SnO\({}_{4}\) spinel cubic single phase. After annealing, the obtained sample (CTO\({}_{\text{ann}}\)) was identified as a mixture of spinel Co\({}_{2}\)SnO\({}_{4}\) phase and spinel Co\({}_{3}\)O\({}_{4}\) pure phases; the obtained Co\({}_{2}\)SnO\({}_{4}\)/Co\({}_{3}\)O\({}_{4}\) composite was indexed by both standard cards: JCPDS no. 29-0514 for the Co\({}_{2}\)SnO\({}_{4}\) spinel phase and JCPDS 78-1970 for the Co\({}_{3}\)O\({}_{4}\) spinel phase.  
Figure 3: XRD pattern of the as-prepared (CTO\({}_{\text{as}}\)) and annealed (CTO\({}_{\text{ann}}\)) Co\({}_{2}\)SnO\({}_{4}\) films.  
Figure 2: Gas sensing setup.  
The crystallite sizes of the prepared samples were calculated using the following Scherrer formula (Equation (3)):  
\[\mathrm{D_{hkl}}=\frac{0.9\ \lambda}{\beta.\mathrm{Cos\theta}} \tag{3}\]  
where \(\lambda\) is the wavelength of the implemented X-ray diffraction, \(\beta\) is the full width at half maximum (FWHM) of the (hkl) diffraction plane in radians and h is the corresponding incident angle.  
The average crystallite sizes were found to be about 80 nm for the \(\mathrm{CTO_{as}}\) pure \(\mathrm{Co_{2}SnO_{4}}\) (311) phase and 50 nm and 30 nm for the \(\mathrm{Co_{2}SnO_{4}}\) (311) and \(\mathrm{Co_{3}O_{4}}\) (111) directions, respectively. In the polycrystalline \(\mathrm{CTO_{ann}}\) layer, no other phase formation of impurities was observed in the diffractogram. Figure 4a,b show the high-magnification SEM images of the samples obtained before and after the annealing treatment at 800 \({}^{\circ}\)C.  
We note that the high-magnification SEM image shows a random combination of compact grain agglomerations with average sizes ranging from about 50 nm to 150 nm in size. After annealing, the film shows a large number of individual grains with average sizes ranging from about 20 nm to 50 nm in size, leading to an increase in porosity and offering a large active surface area available for oxygen molecule adsorption.  
High surface area is the main factor in a sensor's sensitive layer which enables it to achieve higher sensitivity.  
The polycrystalline nature of the annealed film offered a greater available surface area for atom fraction and oxygen molecule adsorption on the surfaces of grains and at interfaces between grains (grain boundaries), suggesting that the microstructure of the films is suitable for ethanol vapor sensing applications. The variations in the sizes of particles seen in SEM images were comparable to the grain sizes estimated by XRD measurements.  
In addition, the phase transition from \(\mathrm{Co_{2}SnO_{4}}\) (\(\mathrm{CTO_{as}}\)) to \(\mathrm{Co_{2}SnO_{4}}\)/\(\mathrm{Co_{3}O_{4}}\) composite (\(\mathrm{CTO_{ann}}\)) was confirmed by TEM images (Figure 4). Figure 4a shows the \(\mathrm{Co_{2}SnO_{4}}\) (311) monocrystal structure of the as-prepared film with an interplanar spacing of 0.26 nm, whereas Figure 4b shows the multicrystal structure of the annealed film with high crystallinity and purity. The interplanar spacing for \(\mathrm{CTO_{as}}\) and \(\mathrm{CTO_{ann}}\) is calculated as 0.26 nm and 0.46 nm, respectively, matching well with the interplanar spacing of \(\mathrm{Co_{2}SnO_{4}}\) (311) and \(\mathrm{Co_{3}O_{4}}\) (111), and confirming the elaboration of the \(\mathrm{Co_{2}SnO_{4}}\)/\(\mathrm{Co_{3}O_{4}}\) composite.  
Figure 4: TEM images of pure \(\mathrm{Co_{2}SnO_{4}}\) (a) and \(\mathrm{Co_{2}SnO_{4}}\)/\(\mathrm{Co_{3}O_{4}}\) composite (b).