## 2 Results and Discussion
This method is based on the relationship existing between light absorbance and band gap energy, described by the equation \((\alpha hv)^{\frac{1}{7}}=B\big{(}hv-E_{g}\big{)}\)[65], where \(\alpha\) is the absorption coefficient, \(hv\) is the incident photon energy, \(E_{g}\) is the energy of the band gap and \(\
u\) is a constant; \(\gamma\) is equal to 1/2 for direct transitions or 2 for indirect ones. In this case, the reflectance spectra are transformed into the corresponding absorption spectra using the Kubelka-Munk Function \((F(R_{\infty}))\). In the Tauc plots presented in Figure 4a, the product \((F(R_{\infty})hv)^{\frac{1}{7}}\) (with \(\gamma\) assuming the value of 1/2 for a direct band gap in Co\({}_{3}\)O\({}_{4}\)) is plotted as a function of the incident photon energy, resulting in the determination of two band gaps for the Co\({}_{3}\)O\({}_{4}\):V samples. The intersection of the linear fit of the Tauc plot with the x-axis, gives the value of the band gap energy, resulting in two band gaps for the Co\({}_{3}\)O\({}_{4}\):V samples. Reflectance measurements and relative band gap calculations were performed for the MoO\({}_{3}\):V samples as well (reflectance spectra are reported in Figure S2 of the S.I.). In there, a main band around 375 nm (3.3 eV) can be observed only for the undoped sample, which shifts towards higher wavelengths for the doped samples, following an unclear relationship with the doping concentration. Moreover, a narrow and intense peak is found in the near-infrared (NIR) region at approximately 1080 nm (1.1 eV) that was previously attributed to the intervalence charge transfer Co\({}^{2+}\rightarrow\) Co\({}^{3+}\)[66]. The Tauc plot is also built for this second MO series (Figure 4b), with the main difference in the \(\gamma\) value used, which in this case is 2 for an indirect band gap semiconductor [65]. The results of all these linear fittings are reported in Table 1. From this analysis, it appears that the bandgap of Co\({}_{3}\)O\({}_{4}\) remains almost unvaried, with only a slight increase in the sample doped with 20% at. nominal vanadium. For MoO\({}_{3}\) instead a progressive decrease in the E\({}_{g}\) from the undoped material is evident, up to the 10% at. doped sample, after which a plateau is reached as described by the values reported in Table 1, likely as a consequence of a doping saturation effect. Figure 4: Tauc plot analysis for the (**a**) Co\({}_{3}\)O\({}_{4}\):V and (**b**) MoO\({}_{3}\):V samples series. Grey lines indicates the linear fits used to extrapolate the E\({}_{g}\) values. The smooth shape of the Tauc plots at band gap edges reveals the presence of shallow defects within the band gap of the investigated vanadium-doped cobalt oxides (attributed to oxygen vacancies as usually happens in oxides) [67], and a substantial shift of E\({}_{\text{g}}\) for the vanadium-doped molybdenum ones, suggesting that electronic properties of these MO semiconductors are affected by vanadium inclusion. Therefore, a more detailed electronic characterization is required to understand the effect of the ionic dopant on the energetics of these species, which can be obtained by Kelvin probe microscopy (KPM), together with a precise analytical determination of the actual quantity of dopants present in the lattice, which can be obtained by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) analysis. Although similar synthetic conditions were adopted for both the V-doped MOs, we can state with good approximation that only MoO\({}_{3}\) has been successfully incorporated into the matrix, as shown by ICP analysis, whose results are reported in Table 1. In more detail, significantly lower amounts of vanadium are found within the Co\({}_{3}\)O\({}_{4}\)-based samples with respect to the MoO\({}_{3}\)-based ones, despite the nominal doping concentrations being similar.