## 1 Introduction
Among the different doping elements, specific transition metal ions such as Mn, Fe, Cu, Co, Cr [26; 27; 28] are the most promising ones because of the number of oxidation states available that can in principle tune the electronic properties of the host MO. Our choice in this work was addressed towards an aliovalent vanadium (V) substitution, because this metal can be easily oxidized to its pentavalent form in V\({}_{2}\)O\({}_{5}\), a MO that possesses a work function between 6.4 eV and 7 eV [29]). In this way, other MOs that contains V(V) ions as dopants could be likely p-doped, thus improving their charge mobility, as occurred in some examples in the literature involving vanadium and MoO\({}_{3}\)[30]. In the literature, the doping of Co\({}_{3}\)O\({}_{4}\) has been previously performed using different elements such as Ru, N, La, Sn, Cu, Li, Ag, V and adopting various synthetic/deposition/processing approaches, such as co-precipitation [31], plasma treatment [32], spin coating [33], spray coating [34], dip coating [35], magnetron sputtering [36], thermal decomposition [37], and solvothermal or hydrothermal synthesis [38; 39; 40; 41]. This last synthetic methodology is the most valuable among the others because it can be carried out under green conditions (water-based solutions) at low costs (i.e., with simple wet-chemistry laboratory equipment and easy chemical procedures), and typically pursues the fast crystallization of materials already under low temperature subcritical conditions. Moreover, it allows for a good control over reaction conditions (such as the fine tuning of the dopant concentration), while ensuring high yields. However, only a few syntheses are reported in the literature for the preparation of V-doped Co\({}_{3}\)O\({}_{4}\). Magnetron sputtering [36] is suitable for direct film deposition, but is not usable to obtain materials to be subsequently processed through a solution-based technique. A case of V-doped Co\({}_{3}\)O\({}_{4}\) synthesized hydrothermally is reported in the literature for catalytic purposes [41], but the electronic properties are not described, thus remaining hitherto uncharacterized. On the other hand, starting from a theoretical analysis that predicts the possibility of balancing the oxygen vacancies in the lattice, [42] MoO\({}_{3}\) has been doped using both chemical and physical deposition techniques such as CVD [43], the sol-gel method [44], the solution combusted method [45], magnetron sputtering [46], and also hydrothermal routes [47; 48], similar to Co\({}_{3}\)O\({}_{4}\). This last procedure is widely applied for realizing doped Mo\({}_{3}\)O\({}_{4}\) samples with different elements such as Ni [43; 45], S [49] and a range of other transition metals [50; 51; 52] and in particular, vanadium [30]. In this work, we describe the subcritical hydrothermal synthesis of V-doped Co\({}_{3}\)O\({}_{4}\) micro-wires and MoO\({}_{3}\) micro-lamellae starting from water soluble precursors (followed by a calcination step), with the aim of finely tuning the electronic properties of these MO semiconductors to improve their potential for exploitation in energy-related devices. We also report on the outcomes obtained by implementing a systematic variation in concentration of the introduced dopants (from 1% to 20% at. with respect to the host oxide), which happen to be significantly different for the two investigated MO host matrices, allowing us to draw preliminary conclusions on the arrangement of the vanadium guest species within the two different crystalline lattices. We also show that such a tuning of the doping level in these semiconducting MOs can lead to a parallel tuning of the Fermi level towards the realization of materials with a higher work function compared to the undoped analogues, with interesting perspectives for applications in energy-conversion devices.