## 3 Results and Discussion
The maximum of titania absorption located at 320-367 nm could be attributed to isolated Ti\({}^{4+}\) sites in octahedral coordination [24] related to full connectivity of Ti-O-Ti linkages in Figure 3a. In Figure 3b, the maximum of titania absorption located at 336-378 nm was observed, and the UV-VIS DRS spectra of FS-NT (2-5 mL) showed a red-shifted onset of the low-energy edge, possibly due to the influence of a small amount of N. The prepared samples had strong absorption in the region at 400-800 nm due to the presence of Fe\({}_{3}\)O\({}_{4}\)[18]. The DRS were converted to absorption coefficients, F(R), via the Kubelka-Munk function. The energy bandgap (\(E_{g}\)) of the samples was determined according to the Tauc method from the plot of (F(R)\(tv\))\({}^{2}\) versus \(lv\) by Equation (2):  
\[ahv=C\big{(}hv-E_{g}\big{)}^{2} \tag{2}\]  
The bandgaps are shown in Figure 3c,d. The values of bandgaps of FS-NT (0 mL), FS-NT (2 mL), FS-NT (3 mL), FS-NT (4 mL), and FS-NT (5 mL) were 2.92 eV, 2.76 eV, 2.67 eV, and 2.52 eV, respectively. Compared with FS-NT (0 mL), FS-NT (2-5 mL) had a smaller bandgap, and although the light absorption range was in the ultraviolet range, it was still expanded. The reason for this is that N atoms in FS-NT (2-5 mL) replaced some  
Figure 2: TEM micrographs of (**a**) FS-NT (0 mL) and (**b**) FS-NT (3 mL). HRTEM images of (**c**) FS-NT (0 mL) and (**d**) FS-NT (3 mL). SAED of (**e**) FS-NT (0 mL) and (**f**) FS-NT (3 mL). O atoms in the lattice of TiO\({}_{2}\) and formed a O-Ti-N bond, which created an additional energy level and further reduced the bandgap width. This result is consistent with the literature [25; 26]. The chemical state of FS-NT (0 mL) and FS-NT (3 mL) was investigated using XPS analysis, as shown in Figure 4. Figure 4a,b present the N 1s spectra of FS-NT (0 mL) and FS-NT (3 mL), respectively. In Figure 4a,b, binding energy of 401.55 eV was attributed to the surface molecularly chemisorbed dinitrogen [27], while binding energy of 400 eV may be attributed to the presence of NH\({}_{4}\)OH (N-H) [28], possibly due to the presence of NH\({}_{4}\)OH in the Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) residual during the preparation process. In Figure 4b, compared with FS-NT (0 mL), two additional peaks of binding energy were observed at 396.8 eV and 405.4 eV, respectively. N 1s peaks at 396.8 eV were attributed to substitutional nitrogen (O-Ti-N) [27; 29; 30], while a binding energy of 405.4 eV may be attributed to interstitial nitrogen [29; 31; 32]. The results are consistent with those of XRD and SAED. Figure 4c shows the Ti 2p XPS spectra of FS-NT (0 mL). The binding energies at 458.8 and 464.5 eV were associated with Ti\({}^{4+}\) 2p\({}_{3/2}\) and Ti\({}^{4+}\) 2p\({}_{1/2}\), respectively, as confirmed by the spin orbit splitting of 5.7 eV [33]. Therefore, it could be inferred that titanium ion was in the form of Ti\({}^{4+}\), and Ti\({}^{3+}\) did not exist in the FS-NT. Figure 4d shows the binding energies of 458.6 and 464.3 eV corresponding to Ti\({}^{4+}\) 2p\({}_{3/2}\) and Ti\({}^{4+}\) 2p\({}_{1/2}\) for FS-NT (3 mL), respectively. It can be clearly seen that Ti 2p peak values of FS-NT (3 mL) were smaller than those of FS-NT (0 mL), which may be due to N substituting for O, causing a decrease in the binding energy. The derived electronic interaction between Ti and N anions resulted in partial electron transfer from N to Ti. At the same time, the electron density of Ti was also increasing due to the fact that the electronegative energy of N was less than that of O. This revealed that N was incorporated into TiO\({}_{2}\) and replaced O [34]. In the O 1s XPS spectra, the presence of Ti-O and surface-adsorbed O\({}_{2}\) or OH\({}^{-}\) can be confirmed by the deconvoluted peaks at binding energies of 530.2 and 532.7 eV in Figure 4e,f [34; 35], respectively. Figure 3: UV-VIS diffuse reflectance spectra of (**a**) FS-NT (0 mL) and (**b**) FS-NT (2–5 mL). Tauc plots obtained for (**c**) FS-NT (0 mL) and (**d**) FS-NT (2–5 mL).