## 3 Results
### Optical Characterization  
Figure 4a shows photoluminescence spectra of the four nanostructures. The highest emission in the blue band was observed in TiO\({}_{2}\)-ns/TiO\({}_{2}\)-nc, while the lowest was in TiO\({}_{2}\)-nt. In general, the emissions of nanostructures with anatase phase are higher than self-assembled nanostructures with amorphous phase, as reported in our previous work [16]. The main defect states of TiO\({}_{2}\)-based nanomaterials with this characteristic spectrum are self-trapped excitons (STE), single-ionized oxygen vacancies (Vo*), doubly-ionized oxygen vacancies (Vo*), and excited states of Ti\({}^{3+}\) at 416 nm; 443 nm; 531 nm; and 802 nm, respectively [16; 22; 23; 24; 25; 26]. Figure 2: FE-SEM image of the surface of: (**a**) TiO\({}_{2}\)-nt; (**b**) TiO\({}_{2}\)-nc; (**c**) TiO\({}_{2}\)-ns assembled on TiO\({}_{2}\)-nc; and (**d**) TiO\({}_{2}\)-nb assembled on TiO\({}_{2}\)-nc synthesized by electrochemical anodization. Figure 3: AFM image of the cross-section of: (**a**) TiO\({}_{2}\)-nt; (**b**) TiO\({}_{2}\)-nc; (**c**) TiO\({}_{2}\)-ns assembled on TiO\({}_{2}\)-nc; and (**d**) TiO\({}_{2}\)-nb assembled on TiO\({}_{2}\)-nc synthesized by electrochemical anodization. Figure 4b-e shows deconvolutions calculated for each emission spectra correspondingly to TiO\({}_{2}\)-ns/TiO\({}_{2}\)-nc; TiO\({}_{2}\)-nb/TiO\({}_{2}\)-nc; TiO\({}_{2}\)-nc; and TiO\({}_{2}\)-nt, respectively. Basically, the four TiO\({}_{2}\) films had the same defect states but in different areas [24; 26; 27]. The  
Figure 4: (**a**) Photoluminescence spectra of the four TiO\({}_{2}\) nanostructures. Deconvolution of the photoluminescence spectra for: (**b**) TiO\({}_{2}\)-ns/TiO\({}_{2}\)-nc; (**c**) TiO\({}_{2}\)-nb/TiO\({}_{2}\)-nc; (**d**) TiO\({}_{2}\)-nc; and (**e**) TiO\({}_{2}\)-nt. (**f**) Comparison of defect states, length of nanotube, thickness of the film and of the nanostructure between the four nanomaterials. self-assembled nanostructures had the highest amount of defect states: TiO\({}_{2}\)-ns/TiO\({}_{2}\)-nc was the richest in STE and Vo* (the latter being comparable to TiO\({}_{2}\)-nb/TiO\({}_{2}\)-nc), while TiO\({}_{2}\)-nb/TiO\({}_{2}\)-nc had the highest number of Vo** and Ti\({}^{3+}\), in contrast TiO\({}_{2}\)-nt was the poorest nanomaterial in the amount of STE, Vo*, and Ti\({}^{3+}\). Figure 4f shows the relationship between defect states area and thickness of TiO\({}_{2}\) films. Note that the area of Vo* is inversely proportional to the thickness of the film. Therefore, thinner films had a greater number of single-ionized oxygen vacancies. However, we did not find a relationship between STE, Vo** and Ti\({}^{3+}\) with the thickness of the film. The single-ionized oxygen vacancies area of TiO\({}_{2}\)-ns/TiO\({}_{2}\)-nc was 2.64, 1.6643, and 1.0386 times larger than TiO\({}_{2}\)-nt, TiO\({}_{2}\)-nc, and TiO\({}_{2}\)-nb/TiO\({}_{2}\)-nc, respectively. Therefore, we suggest that nanobowls and nanosheets have more defect states due to their geometric shape, added to this, the self-assembled nanostructures form a hybrid nanostructure with exceptional optical properties [28]. From the EDS results, it can be seen self-assembled nanostructures had lower oxygen content, which may be related to defect states, specifically oxygen vacancies due to deficiency of oxygen in the TiO\({}_{2}\) film (see Table S1). The fact that the thickness of the nanosheets is thinner than that of the nanobowls could be a crucial factor in enhancing the emission of self-assembled nanostructures. Another essential characteristic is the length of the tubular structures (nanocavities), the length of TiO\({}_{2}\)-nb/TiO\({}_{2}\)-nc was shorter than the length of the other three films; nevertheless, its emission was higher compared to TiO\({}_{2}\)-nc and TiO\({}_{2}\)-nt (Figure 4f).