## 3 Results and discussion
### Crystalline Phase, Spatial Homogeneity, and Crystallinity Degree of the Oxide  
The crystalline phase of the oxide formed upon calcination was assessed via the XRPD analysis (Figure 2a). The diffractogram of sample TZ10 exhibits the peaks typical of the anatase (\(a\)-TiO\({}_{2}\)) tetragonal phase of titania (JCPDS Card No. 21-1272) [28; 30; 47]. The diffraction peaks of sample TZ01 can be indexed to reflections from the crystallographic planes of the wurtzite hexagonal structure of zinctic phase (ZnO, JCPDS card No. 36-1451) [20; 21; 28; 30; 47]. The comparison with the literature [26; 27; 28] proves that the crystalline phase formed in the Ti- and Zn-oxide NFs strongly depends on the Ti:Zn molar ratio in the precursor solution. Most of the peaks appearing in the diffractogram of sample TZ21 are peculiar to the ilmenite-type zinc meta-titanate (ZnTiO\({}_{3}\)) with hexagonal structure (JCPDS Card No. 26-1500) [48]; besides, two weaker peaks compatible with the formation of \(a\)-TiO\({}_{2}\) and rutile (\(r\)-TiO\({}_{2}\), JCPDS Card No. 21-1276) phases of titania are detected. Most of the diffraction peaks of sample TZ12 can be indexed to reflections from the crystallographic planes of the zinc ortho-titanate (Zn\({}_{2}\)TiO\({}_{4}\)) with inverse spinel structure (JCPDS Card No. 19-1483) [49; 50]; in addition, some weak signal compatible with the presence of \(r\)-TiO\({}_{2}\) as a secondary phase is observed. In the diffractogram of sample TZ11, the peaks are substantially located at the same angular positions as in sample TZ12, but they are much broader, indicating lower crystallinity and/or crystal size. The average size of the oxide crystallites was estimated from the most intense peak of each XRPD pattern via the Scherrer's formula, \(d_{\rm C}=K\lambda/\beta\)cos\(\theta\), where \(K\) (0.9) is the Scherrer constant, \(\lambda\) (0.1541 nm) stands for the X-ray wavelength, \(\beta\) indicates the FWHM (full width at half maximum) of the diffraction peak, and \(\theta\) denotes the reflection angle [21; 51]. The \(d_{\rm C}\) values obtained (reported in Figure 2a) range between 6.5 and 27.6 nm; they increase in the order TZ11 \(<\) TZ12 \(<\) TZ21 \(<\) TZ01 \(<\) TZ10. Figure 1: Results of the SEM analysis. (**a**–**e**) SEM micrograph of photocatalysts (**a**) TZ10, (**b**) TZ21, (**c**) TZ11, (**d**) TZ12, and (**e**) TZ01 (insets: NF diameter distributions); (**f**) range and average value of the NF diameter. The spatial homogeneity of the samples and their crystallinity degree were evaluated by MRS (Figure 2b and Figure S3). The comparison among the spectra recorded at different random positions in each sample (Figure S3) reveals the lack of appreciable shifts in the positions and relative intensities of the peaks. This finding indicates that the produced photocatalysts are featured by high spatial uniformity. Figure 2b displays their averaged micro-Raman spectra. In agreement with the indications emerging from the XRPD analysis, the lowest crystallinity degree pertains to sample TZ11, whose spectra exhibit no evident Raman peak and an intense photoluminescence (PL) background (Figure S3c). In accordance with previous studies [21; 46], an intense PL background, originating from point defects and non-stoichiometry, is present also in the spectrum of sample TZ01 (not shown here), where the \(E_{2}\)(high) and \(E_{2}\)(low) modes, arising from the motion of the O atoms and the vibrations of Zn sub-lattice in the hexagonal ZnO structure (space group \(\text{P}6_{3}mc\)), are detected at 441 and 584 cm\({}^{-1}\), respectively [52]. In the spectrum of sample TZ10, the Raman active modes of tetragonal anatase (space group \(\text{I}4_{1}\)/\(\it{amd}\)) are detected at 143 cm\({}^{-1}\) (\(E_{\text{g}}\)), 195 cm\({}^{-1}\) (\(E_{\text{g}}\)), 396 cm\({}^{-1}\) (\(B_{\text{1g}}\)), 517 cm\({}^{-1}\) (\(A_{\text{1g}}\)-\(B_{\text{1g}}\)), and 639 cm\({}^{-1}\) (\(E_{\text{g}}\)) [53; 54].