## 3 Results and Discussion  
### Electrode Characterization  
The obtainment of Ti electrodes modified with TiO\({}_{2}\) nanotubes (Ti/TiO\({}_{2}\)-NT) was confirmed through scanning electron microscopy analysis (Figure 1A). The surface exhibited continuous coverage with TiO\({}_{2}\) nanotubes, with diameters measuring 93 \(\pm\) 7.5 nm and wall thicknesses of 22 \(\pm\) 2.5 nm. The self-reduced TiO\({}_{2}\) nanotube (Ti/TiO\({}_{2}\)-SR) electrodes showed no significant alterations in morphology post-treatment (Figure 1B). An energy dispersive X-ray analysis of the Ti/TiO\({}_{2}\)-NT electrode showed the presence of Ti and O in a ratio of about 1:2, confirming the predominant structures of the nanotubes as that of TiO\({}_{2}\), while the Ti/TiO\({}_{2}\)-NT electrode showed a decrease in oxygen content (Figure S1, see Supplementary Materials), suggesting the partial removal of oxygen atoms from the TiO\({}_{2}\) lattice, leading to the formation of oxygen vacancies or defects. Furthermore, the chemical composition and oxidation state of titanium in the Ti/TiO\({}_{2}\)-NT and Ti/TiO\({}_{2}\)-SR electrodes were checked through X-ray photoelectron spectroscopy (XPS) analysis. Figure 1C shows the Ti2p XPS core spectra of the samples, in which two broad peaks centered at about 459.6 and 467.3 eV corresponding to the characteristic Ti2p\({}_{1/2}\) and Ti2p\({}_{3/2}\) peaks, attributed to Ti\({}^{4+}\), which is characteristic of titanium dioxide (TiO\({}_{2}\)). As the characteristic Ti2p\({}_{1/2}\) and Ti2p\({}_{3/2}\) peaks of Ti\({}^{3+}\) are located at sites slightly more negative than those of Ti\({}^{4+}\), the peaks of the Ti/TiO\({}_{2}\)-SR exhibit a slightly negative shift in binding energy (0.2 eV) when compared to those of Ti/TiO\({}_{2}\)-NT. This suggests the presence of a Ti\({}^{3+}\) state in the lattices [22; 23]. The XPS spectra of O1s showed a shoulder peak at \(\sim\)532 eV, which belongs to Ti-OH species, and was observed to be slightly higher in the reduced TiO\({}_{2}\) nanotubes. This could indicate that oxygen vacancies are added to the TiO\({}_{2}\) lattice during the cathodic polarization process. The impurity level caused by Ti\({}^{3+}\) self-doping is present in the TiO\({}_{2}\) band gap as [O\({}^{\prime\prime}\) vi\({}^{3+}\)] [24], which could enhance the photocatalytic activity in the visible range. The UV-Vis diffuse reflectance spectra (Figure 1D) facilitated the examination of the optical absorption properties of the obtained electrodes. A subsequent conversion of the data via the Kubelka-Munk function (Figure 1D, inset) enabled the determination of their band gap. The Ti/TiO\({}_{2}\)-NT electrode demonstrated a photo-response within the ultraviolet region (385 nm), a phenomenon attributable to the inherent band gap of TiO\({}_{2}\). Light scattering, pores or cracks in the nanotube arrays, or the presence of oxygen vacancies and Ti\({}^{3+}\) species in the synthesized TiO\({}_{2}\) nanotubes may be liable for the relatively weak absorption of TiO\({}_{2}\) nanotubes in the visible-light spectrum. Despite modern fabrication techniques, previous research [25] has shown that the annealing process of TiO\({}_{2}\) nanotubes is unable to remove oxygen vacancies and Ti\({}^{3+}\) species. Below the TiO\({}_{2}\) conduction band (CB), Ti\({}^{3+}\) species can absorb visible light and hasten the formation of isolated defect energy levels. The creation of photogenerated electrons as a result of this absorption may cause a change from Ti\({}^{3+}\) states to the CB of TiO\({}_{2}\). Upon electrochemical reduction of TiO\({}_{2}\) nanotubes, the introduction of Ti\({}^{3+}\) centers results in additional electronic states below the conduction band. This effectively reduces the band gap from 3.31 eV to 3.21 eV, allowing TiO\({}_{2}\) to absorb lower-energy (longer-wavelength) photons, resulting in a bathochromicshift (393 nm).