## 2 Results and Discussion
(**b**) Zoom on the 18\({}^{\circ}\)–30\({}^{\circ}\) 20 range of the same diffractogram and comparison with that of the Co\({}_{3}\)O\({}_{4}\):V 20% at. sample. (**c**) Field-emission scanning electron microscopy (FE-SEM) image of a typical Co\({}_{3}\)O\({}_{4}\):V sample (20% at) showing the micro-wire-like morphology. The inset shows a magnified area where the particle-like sub-units of the micro-wires are evidenced. (with relative lattice planes) corresponding to the desired phase, \(\alpha\)-MoO3, can be seen at 12.8\({}^{\circ}\), 23.4\({}^{\circ}\), 25.8\({}^{\circ}\), 27.4\({}^{\circ}\), 33.8\({}^{\circ}\), and 39\({}^{\circ}\) 20, relative to the planes (0 2 0), (1 1 0), (0 4 0), (0 2 1), (1 0 1) and (0 6 0), respectively. In Figure 3b, two low intensity Bragg peaks at 21.6\({}^{\circ}\) and 24.9\({}^{\circ}\) 20 are clearly visible in the V 20 % at. nominal doped diffractogram. From a search and match procedure, these could be related to the presence of V\({}_{9}\)Mo\({}_{6}\)O\({}_{40}\) (PDF 34-0527) which is also well fitted in the Pawley refinement (Figure S3c). The absence of such a phase in the samples with a lower percentage of nominal doping could be related to the instrumental detection limit, or the polyoxovanadate could be present in an amorphous state. No shift of the reflections is evident (as can be seen in Figure 3b and from the complete set of diffractograms in Figure S1), so the amount of substitutional V\({}^{5+}\) ions for Mo\({}^{6+}\) ions might be negligible. Moreover, the calculated lattice parameters for the MoO\({}_{3}\): V 20% at. sample (Figure S3c) differ by less than 0.01 A with respect to the pure compound, proving that the structural change in the unit cell is very limited. In Figure 3c, an SEM image of a V-doped MoO\({}_{3}\) sample is shown, highlighting the presence of lamellar structures of highly variable dimensions in the order of the \(\upmu\)m, similar to what was obtained elsewhere in the literature [59]: this structural variability can be due to the calcination step, which promotes the coalescence of the smaller objects with the bigger ones, leading to a non-homogenous distribution of size. The hydrothermally synthesized and subsequently calcinated MO samples are optically characterized by diffuse reflectance spectroscopy. The recorded spectra are reported in Figure S2 in the S.I. The spectra of Co\({}_{3}\)O\({}_{4}\):V samples present two characteristic bands at 700 and 400 nm, attributed to a charge transfer transition from oxygen to the metal center, namely from O\({}^{2}\)- to Co\({}^{3+}\) and from the O\({}^{2}\)- to Co\({}^{2+}\), respectively [60]. It is worth to mention that, among other MOs, Co\({}_{3}\)O\({}_{4}\) is particularly sensitive to oxygen due to the simultaneous presence of Co\({}^{2+}\) and Co\({}^{3+}\) ions that affect the oxygen content within the lattice. It has been proved that the amount of oxygen can be tuned by the addition of heteroatoms, by treatment with plasma or with different gases [61; 62; 63]; however, stabilizing the oxygen vacancy in Co\({}_{3}\)O\({}_{4}\) remains challenging and specific advanced structural characterization is required in order to characterize it, such as synchrotron extended X-ray absorption fine structure (EXAFS) or X-ray absorption near edge fine structure (XANES). Figure 3: (**a**) P-XRD diffractogram of hydrothermally synthesized undoped MoO\({}_{3}\). (**b**) Zoom on the 20.5\({}^{\circ}\)–30\({}^{\circ}\) 20 range of the same diffractogram and comparison with that of the MoO\({}_{3}\):V 20% at. sample. (**c**) SEM image of a typical MoO\({}_{3}\):V sample (20% at.) showing the micro-lamellar-like morphology. From the reflectance spectra, a further analysis allows one to determine the band gap energy using the Tauc plot method [64].