## 3 Results and discussion
### Crystalline Phase, Spatial Homogeneity, and Crystallinity Degree of the Oxide
The spectrum of sample TZ21 exhibits ten peaks at 143, 177, 231, 266, 344, 391, 444, 468, 614, and 710 cm\({}^{-1}\). They arise from the \(E_{\text{g}}\)(1), \(A_{\text{g}}\)(1), \(E_{\text{g}}\)(2), \(A_{\text{g}}\)(2), \(A_{\text{g}}\)(3), \(E_{\text{g}}\)(3), \(E_{\text{g}}\)(4), \(A_{\text{g}}\)(4), \(E_{\text{g}}\)(5), and \(A_{\text{g}}\)(5) symmetrical phonon modes of ZnTiO\({}_{3}\) (space group R3 No. 148) [55]. The downshift of the higher frequency modes hints at the occurrence of tensile strain within the NFs. Five peaks originating from the \(F_{\text{2g}}\)(1), \(E_{\text{g}}\), \(F_{\text{2g}}\)(2), \(F_{\text{2g}}\)(3), and \(A_{\text{1g}}\) phonon modes of the inverse spinel Zn\({}_{2}\)TiO\({}_{4}\) (space group \(\text{O}_{\text{h}}\)\({}^{7}\)-\(\text{F}\)d\(3m\)) are detected in the spectrum of sample TZ12, at 254, 309, 352, 476, and 722 cm\({}^{-1}\), respectively [49; 55]. Besides, the detection of a peak ascribable to the \(A_{\text{1g}}\) mode of titania with \(r\)-TiO\({}_{2}\) structure at 151 cm\({}^{-1}\)[49] confirms the formation of a secondary phase in this sample. Figure 2: Results of (**a**) XRPD and (**b**) MRS analyses. Effect of the Variation of the Ti:ZniMolar Ratio in the Precursor Solution on the Photocatalytic Activity of the Resulting Electrospun Oxide NFs  
The photocatalytic activity of the set of samples produced from precursor solutions with different Ti:Zn molar ratios toward the degradation of MB was evaluated at RT. This was accomplished by dispersing the as-calcalined oxide NFs (0.33 mg mL\({}^{-1}\)) in an aqueous MB solution with an initial concentration of 15 uM and measuring the absorbance after exposure to 350 nm (3.54 eV) UV radiation for given time intervals. Figure S4 displays the spectrum of the as-prepared aqueous MB solution. Two absorption bands, peculiar to MB [56], are observed at 291 and 664 nm, with two weaker characteristic contributions at 247 and 617 nm. The most intense absorption band arises from n-\(\pi^{\star}\) vibronic transitions, while \(\pi\)-\(\pi^{\star}\) transitions are responsible for the detection of the band at 291 nm [56]. With increasing irradiation time, the intensity of absorption bands progressively weakens, indicating that the cationic dye gradually degrades. Figure 3a-e displays the time evolution of the strongest MB absorption band, as measured in the presence of the investigated photocatalysts. As usual [21; 28], the ratio between absorbance at an irradiation time \(t\) (\(A_{t}\)) and initial absorbance (\(A_{0}\)), both measured at 664 nm wavelength, was utilized to estimate the percentage of dye degradation as \(D\) = 100\(\cdot\)(\(1-A_{t}/A_{0}\)). The obtained results are shown in Figure 3f and compared with the literature ones in Table S1. Many groups have reported that photocatalysts based on Ti- and Zn-oxides exhibit better performance with respect to both TiO\({}_{2}\) and ZnO [26; 28; 30]. Contrary to expectations, all the Ti/Zn-oxide NFs (samples TZ21, TZ11, and TZ12) under evaluation exhibit lower activity compared to the TiO\({}_{2}\) and ZnO NFs (samples TZ10 and TZ01, respectively). At any time, the lowest degradation rate pertains to sample TZ11. After 2 h, only 12% MB has degraded, whereas photocatalysts prepared via hydrothermal method from ZnCl\({}_{2}\) and TiCl\({}_{4}\) in a 1:1 molar ratio exhibit better performance [30]. Samples TZ12 and TZ21 exhibit activities towards the dye solution discoloration similar to each other. Their degradation rates are 41 and 42% after 2 h, respectively. After 3 h, \(D\) rises up to 56 and 59% (for TZ21 and TZ12, respectively), reaching values falling in the range of those obtained, in the same time, with a greatly higher concentration (10 mg mL\({}^{-1}\)) of electrospun Ti-Zn complex oxide NFs produced from different precursors (see entry 4 in Table S1) [27].