## 3 Results
### Morphological and Microstructural Characterization  
We used scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to characterize the morphological and microstructural features of our samples (detailed study is described in Section III of the Supporting Information). Below, we describe the most relevant results.  
Figure 10b shows the morphology of the new cobalt (II) oxyacetate Co\({}_{3}\)O(CH\({}_{3}\)COO)\({}_{4}\) phase described above (fuchsia-pink color). In most of the particles, the elongated plate-like morphology of the starting acetate Co(CH\({}_{3}\)COO)\({}_{2}\cdot\)4H\({}_{2}\)O (Figure 10a) was retained, although the particle size was smaller at close to 3-30 \(\upmu\)m in length. In addition, aggregates  
Figure 9: X-ray powder diffraction of cobalt acetate tetrahydrate heated under N\({}_{2}\) atmosphere at 400 \({}^{\circ}\)C. The sample was kept at 400 \({}^{\circ}\)C while several scans were measured. Starting time difference between the first (**a**) and the last one (**b**) was 27 min. Main peaks of identified phases are marked as follows: black circle for platinum (PDF-01-087-0642), blue square for CoO rocksalt polymorph (PDF 00-048-1719), purple inverted triangle for Co (PDF-01-089-4308), and orange crossed circle for Co\({}_{3}\)O\({}_{4}\) (PDF-01-086-8289).  
of variable size, formed by round-shaped nanoparticles, were also observed (Figures S3 and S4 in Supporting Information).  
The SEM images of the green Zn-blende CoO polymorph (obtained by quenching at 332 \({}^{\circ}\)C/N\({}_{2}\)) show the presence of bundles of chains and sheets formed by nanoparticles of rounded (spherical) shape with a homogeneous particle size distribution within the nanometric scale (average size of 100 nm). These nanoparticles joined each other, forming micrometric planes and chains of variable length (Figure S8 in SI).  
This polymorph retained the morphological features of Co\({}_{3}\)O(CH\({}_{3}\)COO)\({}_{4}\), indicating a certain topotactic relationship between them. After the decomposition process, its plate-like morphology was maintained in the cobalt oxide. This morphology was formed by highly homogeneous nanoparticles maintaining the particle packing of the precursor and leading to a sheet-assembling morphology (Figure 10c,d).  
Finally, the morphological study of the rocksalt CoO polymorph (Figure 10e,f) revealed quite homogeneous particles with rounded shapes and sizes on the nanometric scale (close to 100 nm).  
Figure 11 corresponds to the TEM images of the CoO Zn-blende type. All the particles showed a spherical morphology and an average size between 5 and 10 nm (Figure 11a). Figure 11b shows the corresponding FFT pattern characteristic of a polycrystalline sample that can be indexed to the Zn-blende structure, where the distances of 0.270 and 0.24 nm correspond to 111 and 002 interplanar distances, respectively. Figure 11c,d shows the HREM images and their corresponding FFT of one nanocrystal of the sample along the [001] projection. The distances of 0.24 nm correspond to the (200) crystal planes of the cubic Zn-blende structure.  
Figure 10: SEM micrographs of: (**a**) lightly ground commercial cobalt (II) acetate tetrahydrate sample, (**b**) elongated plates microcrystals of Co(II) oxyacetate Co\({}_{3}\)O(CH\({}_{3}\)COO)\({}_{4}\), (**c**) cubic blende CoO polymorph at low magnification 7000 \(\times\) and (**d**) enlargement of (**c**), and (**e**) rocksalt CoO polymorph at (7500\(\times\)) and (**f**) at high-magnification (50,000 \(\times\)).  
The TEM image of rocksalt CoO shown in Figure 12 shows agglomerates of crystalline faceted nanoparticles with sizes on the nanometer scale (between 5 and 10 nm in length). The corresponding FFT pattern, characteristic of a polycrystalline sample, is shown in the Figure 12b. The strong ring patterns can be well-indexed to the fcc CoO structure, where the distances of 0.243 and 0.211 nm correspond to 111 and 002 interplanar distances, respectively, of rocksalt-CoO oxide.