## 2 Results and Discussion  
In Figure 1, a scheme of the hydrothermal synthesis used to prepare V-doped MoO\({}_{3}\) and Co\({}_{3}\)O\({}_{4}\) mesostructures is presented (experimental details are provided in the Experimental Section). In the first step, the precursors of cobalt or molybdenum and vanadium are dissolved in water together with the vanadium precursor. During the second step, a precipitating agent is added under solution stirring. Next, a Teflon reactor containing the mixture is sealed in a Teflon-lined stainless-steel reactor and heated at 150 \({}^{\circ}\)C for a set time to perform the hydrothermal reaction in the third step. The separated powders are finally calcined to yield the final V-doped and undoped (for reference) MOs (Co\({}_{3}\)O\({}_{4}\):V and MoO\({}_{3}\):V). The samples obtained by this procedure are characterized by powder X-Ray diffraction (P-XRD) to verify the formation of the desired phase (cubic for Co\({}_{3}\)O\({}_{4}\), space group FD-3m (227), orthorhombic for MoO\({}_{3}\), (\(\alpha\)-MoO\({}_{3}\)), space group Pbnm (62)). Figure 2a shows the diffractogram of undoped Co\({}_{3}\)O\({}_{4}\), confirming the presence of the cubic Co\({}_{3}\)O\({}_{4}\) phase (PDF 74-1657), with a unit cell parameter \(a\) estimated by Pawley refinement (Figure S3a) of 8.0838 \(\pm\) 0.0014 A (Figure S3a). The reflections located at 2\(\theta\) = 19\({}^{\circ}\), 31.2\({}^{\circ}\), 36.8\({}^{\circ}\), 38.6\({}^{\circ}\), 44.8\({}^{\circ}\), 55.7\({}^{\circ}\), 59.5\({}^{\circ}\) and 65.3\({}^{\circ}\) are attributed to the planes (1 1), (2 2 0), (3 1 1), (2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) of Co\({}_{3}\)O\({}_{4}\), respectively [53]. Diffractograms of V-doped samples (Figure S1 in the Supporting Information--S.L) show the cubic Co\({}_{3}\)O\({}_{4}\) pattern and no other phases are detected, nor a shift of the reflections' positions as a consequence of V-doping. An additional phase is detected only at the highest nominal concentration of dopant (i.e., 20% at. ), as shown in Figure 2b, where weak reflections appear at 2\(\theta\) = 20.3\({}^{\circ}\), 26.2\({}^{\circ}\). Specifically, they are ascribable to the lattice planes (0 0 1), (1 1 0) of crystalline orthorhombic V\({}_{2}\)O\({}_{5}\) (PDF 41-1426) [54], indicating the formation of a second and separated MO phase in the synthetized material when a relatively high amount of V-precursor is introduced in the reaction mixture. The textural characterization with scanning electron microscopy (SEM, Figure 2c) discloses the peculiar morphology of this synthesized Co\({}_{3}\)O\({}_{4}\), constituted of particles of some tens of nm fused together to generate individual microwires (cross-section is in the nanoscale). To the best of our knowledge, a similar hierarchical arrangement of Co\({}_{3}\)O\({}_{4}\) nanoparticles (of which many examples exist [55; 56] also in the form of nanowires [57; 58]) into longer wire-like structures has never been reported before. Figure 3 shows the P-XRD patterns of the undoped MoO\({}_{3}\) and MoO\({}_{3}\):V 20% at. samples (respectively, in Figure 3a,b). The reflections correspond to those of the orthorhombic MoO\({}_{3}\)\(\alpha\)-phase (PDF 65-2421) with calculated unit cell parameters of \(a\) = 3.9602 \(\pm\) 0.0003 A, \(b\) = 13.8540 \(\pm\) 0.0011 A, \(c\) = 3.6949 \(\pm\) 0.0002 A estimated by Pawley refinement (Figure S3b). However, this analysis shows that a mixture of orthorhombic (\(\alpha\)-MoO\({}_{3}\)) and monoclinic (\(\beta\)-MoO\({}_{3}\), PDF 85-2405) phases of MoO\({}_{3}\) is present in the sample. Major diffraction peaks  
Figure 1: Schematic representation of the hydrothermal synthesis procedure used in this work to obtain two metal oxides MOs with vanadium guest species, namely Co\({}_{3}\)O\({}_{4}\):V and MoO\({}_{3}\):V.  
Figure 2: (**a**) Powder X-Ray diffraction (P-XRD) diffractogram of hydrothermally synthesized undoped Co\({}_{3}\)O\({}_{4}\) after the calcination step.