## 3 Results and discussion
### On the Best Performing Oxide NFs  
The outstanding photocatalytic behavior of sample TZ01 has been previously discussed [21]. It is due to the architecture of the NFs, consisting of interconnected polycrystalline nanoplatelets, with defects (located at the grain boundaries) that, acting as traps for the photogenerated \(e^{-}\)-\(h^{+}\) pairs, reduce the rate for their recombination and make them available for the reduction and oxidation reactions. In this work, we focused on \(a\)-TiO\({}_{2}\) NFs, which at the optimal load (0.33 mg mL\({}^{-1}\)) exhibit higher activity toward the MB degradation (\(k\) = 1.12 \(\times\) 10\({}^{-1}\) min\({}^{-1}\), i.e., 6.72 h\({}^{-1}\)) compared to P25-TiO\({}_{2}\), a mixture of anatase and more active rutile, commonly regarded as the photocatalysis benchmark (e.g., \(k\) = 1.46 h\({}^{-1}\) for P25 NPs [64] and \(k\) = 5.02 \(\times\) 10\({}^{-2}\) min\({}^{-1}\) for P25 NPs immobilized into a thin electrospun polymer mat [65]). A deeper insight into its morphology was achieved by carrying out an AFM analysis. Figure 5a and Figure S12a show representative topographic images of the NPs composing the NFs, as obtained after the dissolution of the NFs in acetone. The distributions of the diameters and heights of the NPs, as resulting from the statistical analysis of the topographic images, are shown in Figure 5b,c, respectively. These results validate those previously obtained on the ZnO NFs [21]. As designed, the architecture of \(a\)-TiO\({}_{2}\) NFs consists of interconnected oxide NPs (compare Figure S12c,d,g with Figure S12e,f,h, respectively). Anatase NPs are polycrystalline in nature (the average NP size exceeds the average crystallite size) and larger in diameter with respect to the ZnO ones (69 \(\pm\) 13 nm against 47 \(\pm\) 6 nm). The bi-modal distribution of the NP heights indicates that the \(a\)-TiO\({}_{2}\) NFs NF is constituted by both thinner and thicker NPs (mainly 8.0 \(\pm\) 0.5 nm and 13.1 \(\pm\) 1.3 nm in height) compared to ZnO platelets (9.3 \(\pm\) 1.3 nm in height) [21]. Additional information about photocatalyst TZ10 is inferred from the high magnification SEM/EDX analysis (Figure S13). The analysis reveals the successful achievement of sub-stoichiometric titania oxide (TiO\({}_{1.9}\)). The oxygen-deficiency of the oxide (5%), which is beneficial for the photocatalytic activity [20,21,66], is further confirmed by the results of the quantitative analysis of the micro-Raman spectra of the sample (Figure S14 and Table S2). As known [67, 68, 69], the lowest frequency \(E_{\text{g}}\) peak of anatase may blue-shift and broaden (with respect to bulk) owing to phonon confinement effects arising from finite size of the oxide grains (<10 nm) or shortening of the correlation length due to the presence of defects and non-stoichiometry. Since the crystallite size resulting from XRPD data (27.6 nm) rules out the occurrence of finite-size effects, the presence of localized defects, i.e., oxygen vacancies and interstitials mainly located at the grain boundaries [20,21], is responsible for the broadening of the peak (13.7 against 7.6 cm\({}^{-1}\)) with respect to oxide nanocrystals having similar size [67]. Last, but not least, we have assessed the stability and reusability of the best performing photocatalyst (TZ10) under optimal conditions (0.33 mg mL\({}^{-1}\) load). Reusability of the catalyst is an important aspect of the heterogeneous catalysis process to which industries are particularly interested. The reuse generally involves filtration and/or centrifugation to separate it from the reaction medium and regeneration via washing with proper liquids or heat treatment to remove from its surface intermediates, often responsible for its deactivation/poisoning [70,71]. These complex and time-consuming procedures impact the costs of the process [70]. In this work, we demonstrate that the catalyst can be reused _without_ separating it from the reaction medium or washing it with water or other liquids.