## 2 Results and Discussion
This result is in substantial accordance with the trend in E\({}_{\text{g}}\) values determined from the Tauc plots, in which the band gap does not change in Co\({}_{3}\)O\({}_{4}\), while it decreases with increasing doping in MoO\({}_{3}\), for at least the first three samples (see Figure 4 and Table 1). Furthermore, the ICP analysis highlights that a significative amount of dopant is found already at very low V concentrations in MoO\({}_{3}\). However, a shift towards higher diffraction angles of the reflections in the corresponding XRD patterns is not observed, and only at 20 % at. of nominal doping an additional phase is detected (likely V\({}_{9}\)Mo\({}_{6}\)O\({}_{40}\)). Therefore, with the present data, it is still difficult to understand whether vanadium ions are present as substitutional with respect to molybdenum or in other forms within the sample. On the contrary, the Co\({}_{3}\)O\({}_{4}\) samples do not show any relevant vanadium incorporation except for the 20% at. specimen, which most likely is a biphasic material, i.e., a mixed V\({}_{2}\)O\({}_{5}\)-Co\({}_{3}\)O\({}_{4}\) composite, as anticipated from P-XRD (Figure 2b). The reason for this different doping behavior with respect to the vanadium ion dopants can probably be ascribed to the different crystal structures of the Mos, with \(\alpha\)-MoO\({}_{3}\) and V\({}_{2}\)O\({}_{5}\) being both orthorhombic whereas Co\({}_{3}\)O\({}_{4}\) has a cubic lattice. In light of these considerations, it appears reasonable to foresee an easier accommodation of the V(V) cations in the structure of MoO\({}_{3}\) with respect to that of Co\({}_{3}\)O\({}_{4}\). On the other hand, we exclude a pure influence of the ionic radii of the cations  
\begin{table}
\begin{tabular}{c c c c} \hline
**Sample** & **V \% 1** & **E\({}_{\text{g}}\) (eV) 2 (\(\pm\)4\%)** & **WF (eV) 4** \\ \hline Undoped Co\({}_{3}\)O\({}_{4}\) & - & 1.74; 2.24  & 4.8 \(\pm\) 0.11 \\ \hline Co\({}_{3}\)O\({}_{4}\):V (1\% at) & \textless{}1 & 1.50; 2.20 [FOOTNOTE:6]Footnote 6: The \(\textless{on the doping efficiency, because these are comparable for all the cations here considered, with the appropriate coordination number (54 pm for V(V) and 59 pm for Mo(VI) with a coordination number VI, being \(\alpha\)-MoO\({}_{3}\) constituted by MoO\({}_{6}\) distorted octahedra, 58 pm for high spin Co(II) in T\({}_{\rm d}\) coordination and 54.5 pm for high spin Co(III) in O\({}_{\rm h}\) coordination, with Co\({}_{3}\)O\({}_{4}\) having the typical spinel structure Co\({}^{\rm II}\)[Co\({}^{\rm III}\)]\({}_{2}\)O\({}_{4}\)) [68]. KPM analysis provides a value of the work function (WF), which can substantially vary upon effective chemical doping, and thus provides information on the doping character (p- or n-) in the semiconducting materials. The trends in the WF of the MO materials (Figure 5) are highly related to the optical and ICP data: in the Co\({}_{3}\)O\({}_{4}\):V samples series the WF remains practically unchanged (4.7\(\div\)4.8 eV) across most doping values except for the highest one, for which a significative variation takes place (from \(\sim\)4.75 eV up to \(\sim\)5.15 eV). The increased WF being obtained only at the highest V-doping level in Co\({}_{3}\)O\({}_{4}\) can be related to the presence of the second V\({}_{2}\)O\({}_{5}\) phase formed, which is perhaps located on the external surface of the MO mesostructures, thus largely contributing to the measured value (WF of pure V\({}_{2}\)O\({}_{5}\) should be \(\sim\)7 eV [69]). On the contrary, for the MoO\({}_{3}\) samples, a linear increase in the WF (from 5.14 eV to 6.10 eV) is observed with increasing dopant concentration, for the lowest values thereof, while the WF does not increase further only at the highest concentration (with a 6.11 eV WF value obtained when the nominal amount of vanadium in the MO doubles from 10% at. to 20% at. ), thus suggesting also in this case the occurrence of a phase separation within the material as for the case of Co\({}_{3}\)O\({}_{4}\):V 20% (at).