## 3 Results and discussion
### Crystalline Phase, Spatial Homogeneity, and Crystallinity Degree of the Oxide
As known [47], the photocatalytic performance strongly depends on the specific surface area: as a general trend, the larger the specific surface area, the greater the number of available sites for the adsorption of the dye molecules and, hence, the higher the catalytic activity. Chemical etching with a strong base is a strategy commonly adopted to increase the surface area of nanomaterials [45; 47]. For this purpose, after calcination, a portion of sample TZ21 was treated with a NaOH solution, as described in Section 2.2. The treatment did not alter either the morphology (Figure S5) or the crystalline phase of the oxide (Figure S6). It resulted in NF thinning (the center of the NF diameter distribution moved from 650 to 550 nm), average crystallite size increase (from 19 to 27 nm) due to the selective removal of defective surface oxide regions and Na-incorporation, as suggested by the downshift of the strongest diffraction peaks (inset of Figure S6) and confirmed by the results of the elemental mapping via SEM/EDX (Figure S5). Although the treatment with NaOH produces a notable increase in the surface area [47], the activity of the chemically etched photocatalyst (coded as TZ21 NaOH) improves only to a limited extent with respect to that of the un-etched one (Figure S7). After 2 h, 51% (against 42%) MB has degraded. This finding indicates that specific surface area is not the factor mostly limiting the photocatalytic activity of the sample towards the dye degradation. ZnO-TiO\({}_{2}\) nanocomposites, prepared by hydrothermal [29; 30] or solvothermal method [57], have been reported to catalyze the MB degradation process under irradiation with VIS light, showing high reaction rate constants (3.00-6.52 \(\times\) 10\({}^{-1}\) min\({}^{-1}\), under irradiation with 862 W cm\({}^{-2}\) sunlight) [29] and better performance with respect to both ZnO and TiO\({}_{2}\) alone (compare entries 12-16 in Table S1) [30; 57]. Hence, the photocatalytic activity of the present Ti/Zn-oxide NFs toward the degradation of MB was evaluated under exposure to 550 nm (2.25 eV) VIS light. Figure S8 shows the results obtained by dispersing 0.33 mg mL\({}^{-1}\) TZ12 and TZ21 NaOH in an aqueous MB solution with a 15 uM initial concentration. Sample TZ12 is practically photocatalytically inactive. The activity of sample TZ12 is very low: after 2 h, \(D\) is <9%. This suggests that VIS light does not have sufficient energy to activate the catalyst  
Figure 3: Effect of the variation of the Ti:Zn molar ratio in the precursor solution on the photocatalytic performance of the resulting oxide NFs. (**a-e**) Time-evolution of the absorbance spectra of an aqueous MB solution with initial concentration of 15 uM and catalyst load of 0.33 mg mL\({}^{-1}\), under exposure to 350 nm UV radiation; the shown spectra refer to photocatalysts (**a**) TZ10, (**b**) TZ21, (**c**) TZ11, (**d**) TZ12, and (**e**) TZ01. (**f**) Corresponding degradation rates as a function of time. by generating the electron-hole pairs necessary for the redox reactions with the species adsorbed on its surface [32; 33; 34]. At underexposure with higher energy radiation (3.54 against 2.25 eV), the activity of the present electrospun NFs toward the degradation of MB improves in the order TZ11 \(<\) TZ12 \(\cong\) TZ21 \(<\) TZ01 \(<\) TZ10 (Figure 3f). In particular, sample TZ10 (_a_-TiO\({}_{2}\)) fully discolors the MB solution in only 40 min (\(D\) = 99%). In the same time interval, 86% MB has degraded by the use of sample TZ01 (ZnO). On the contrary, when samples TZ21 (ZnTiO\({}_{3}\)), TZ12 (chiefly Zn\({}_{2}\)TiO\({}_{4}\)), and TZ11 (highly disordered Zn\({}_{2}\)TiO\({}_{4}\)) are utilized as catalysts, after 40 min, most of the dye (81.7, 82.4 and 96.1%, respectively) is still present in the solution. The better performance of both the binary oxide NFs is in line with the reports by [PERSON] et al. [27] on Ti/Zn oxide NFs prepared via ES by using different precursors.