## 3 Results
### Thermal Decomposition under Air Atmosphere  
Figure 13a shows the result of TG analysis in air for Cobalt (II) acetate tetrahydrate. The first decomposition step (\(\Delta\)w = 30%)) was analogous to that obtained in a nitrogen atmosphere and corresponded to the total dehydration of the sample. Above 175 \({}^{\circ}\)C, the acetate groups began to decompose in such a way that at 270 \({}^{\circ}\)C, the exothermic decomposition abruptly ended. The total weight loss nicely agreed with the formation of the Co\({}_{3}\)O\({}_{4}\) oxide. Actually, the XRD pattern of the ATG final product, shown in Figure 13b, perfectly matched that of PDF-00-086-8289, corresponding to the Co\({}_{3}\)O\({}_{4}\) spinel. Figure 11: (**a**) TEM image of Zn-blende CoO nanoparticles and (**b**) corresponding fast Fourier transform (FFT) pattern. (**c**) HRTEM images and (**d**) corresponding FFT pattern of a representative nanocrystal along [001] zone axis. Figure 12: (**a**) TEM image of rocksalt-CoO nanoparticles and (**b**) corresponding fast Fourier transform (FFT) pattern. A 2D contour plot of the thermal evolution of the X-ray diffraction (XRD) maxima intensity of the total decomposition process is shown in Figure S9 in Supporting Information. Figure 14a-h depicts the XRD patterns at some selected temperatures. As in the results obtained in N\({}_{2}\), the sample became amorphous when the hydration water was lost. The cobalt acetate tetrahydrate stayed up to 70 \({}^{\circ}\)C; from this temperature onwards, only the dihydrate acetate (present in the commercial product as impurity phase) remained up to around 100 \({}^{\circ}\)C (Figure 14a-d). No crystalline phase was observed in the 100-200 \({}^{\circ}\)C temperature range (Figure 14d,e). The 270 \({}^{\circ}\)C scan (Figure 14f) signifies the presence of Co\({}_{3}\)O\({}_{4}\) as the only crystalline phase. From this temperature and up to 400 \({}^{\circ}\)C (Figure 14g), no structural change could be observed other than an increase in the crystallinity of the sample. This phase was maintained during cooling until room temperature (Figure 14h). Among the cobalt oxides, the most studied phase is the spinel Co\({}_{3}\)O\({}_{4}\), and special attention has been paid to control its morphology. For example, [PERSON] et al., [10] investigated the effect of the fuel on the microstructure of the obtained product using a combustion method. [PERSON] et al., [7] studied the effects of precursors, solvents, and calcination temperatures on the morphology development of cobalt oxide particles with the sol-gel method. [PERSON] et al. controlled crystal growth in the synthesis of Co\({}_{3}\)O\({}_{4}\) nanostructures with cubic morphologies by using a hydrothermal route [35]. The morphology of the obtained Co\({}_{3}\)O\({}_{4}\) was studied by SEM. Figure 15a displays a low-magnification image showing the characteristic morphology of the raw material with particle agglomeration. The high-magnification image shown in Figure 15b reveals that the nanoparticles had a rounded morphology with diameters ranging from 20 to 100 nm. These morphological characteristics are typical of solids obtained by thermal decomposition processes in which large volumes of gas are not released. Figure 16 shows the HREM images and their corresponding FFT of two representative projections of the sample along the [001] (Figure 16a) and [110] (Figure 16b) zone axes. The lattice fringes of 0.289 nm (Figure 16a) and 0.49 nm (Figure 16b) correspond to the interplanar distances of the (020) and (111) Co\({}_{3}\)O\({}_{4}\) spinel planes, respectively. Figure 13: (**a**) TGA plot for the decomposition of a cobalt (II) acetate tetrahydrate commercial sample under air atmosphere; (**b**) XRD pattern of the final product of the ATG analysis. All the intense maxima correspond to the Co\({}_{3}\)O\({}_{4}\) spinel. Figure 14: X-ray powder diffraction of cobalt acetate tetrahydrate heated under air atmosphere from RT to 400 \({}^{\circ}\)C and afterward cooled down to RT.