## 3 Results and Discussion
### Electrode Characterization
This behavior is beneficial for improving the photocatalytic performance of the Ti/TiO\({}_{2}\)-SR electrode by enhancing electrical conductivity (faster electron transfer) and increasing surface reactivity (better interactions between the sensor surface and heavy metal ions). Figure 1: (**A,B**) Scanning electron microscopy images, (**C**) core-level XPS spectra in the Ti2p and O1s regions, (**D**) UV-Vis diffuse reflectance spectra, (**E,F**) Mott-Schottky and Nyquist plots of the Ti/TiO\({}_{2}\)-NT and Ti/TiO\({}_{2}\)-SR electrodes, respectively. Insets: (**D**) the corresponding Kubelka–Munk transformation of the diffuse reflectivity spectrum; (**F**) EIS curves in the high-frequency region. Potential-dependent capacity measurements were performed at a frequency of 1 kHz on the Ti/TiO\({}_{2}\)-NT and Ti/TiO\({}_{2}\)-SR electrodes in order to clarify the impact of self-doping on the electrical characteristics of TiO\({}_{2}\) nanotubes. The Mott-Schottky (MS) plot is shown in Figure 1E as 1/C\({}^{2}\) vs. capacity. The linear portion of the MS plot curves have positive slopes, which suggest that TiO\({}_{2}\) nanotube n-type semiconductor properties are unaffected by Ti\({}^{3+}\) electrochemically induced self-doping. The estimated flat-band potential for Ti/TiO\({}_{2}\)-NT is \(-\)0.27 V, and the carrier density is 4.87 \(\times\) 10\({}^{19}\) cm\({}^{-3}\). After the self-reducing process, the Ti/TiO\({}_{2}\)-SR electrode showed a decrease in the flat-band potential (E\({}_{\text{fb}}\) = \(-\)0.89 V) and an increase in the carrier density (1.61 \(\times\) 10\({}^{20}\) cm\({}^{-3}\)), being in agreement with previous results [12]. Band bending is either negligible or nonexistent at the E\({}_{\text{fb}}\) or the point in the semiconductor where there is no net electric field. An electron accumulation area is produced when the conduction and valence bands are bent downward at the surface, as indicated by a negative value of E\({}_{\text{fb}}\). This suggests that the semiconductor's surface may have an overabundance of electrons. The surface of reduced TiO\({}_{2}\) becomes less positively charged as Efb drops or may be negatively charged if the reduction is substantial. It is anticipated that the surface will have an excess of negative charge because of the buildup of electrons because our sample was reduced at a potential of \(-\)1.5 V. The positively charged metallic ions from the solution may therefore be more attracted to the negatively charged or electron-rich TiO\({}_{2}\) surface when the E\({}_{\text{fb}}\) is lower. This would facilitate the reduction or deposition of metallic ions on the surface. TiO\({}_{2}\) produces electron-hole pairs when exposed to radiation, and in reduced TiO\({}_{2}\), there are frequently more electrons available for reduction processes. These electrons' enhanced ability to interact with metal ions in solution is made possible by their decreased flat-band potential, which also promotes the reduction of ions and raises the possibility that they will deposit onto the TiO\({}_{2}\) surface. Also, the smaller band gap and lower E\({}_{\text{fb}}\) increase the amount of electrons in the conduction band and lower the barrier to electron migration to the surface, hence increasing the electron transfer efficiency. Electrochemical impedance spectroscopy measurements were performed in order to gain a better understanding of the electrodes' interfacial electrochemical behavior. Nyquist plots for the Ti/TiO\({}_{2}\)-NT and Ti/TiO\({}_{2}\)-SR electrodes are shown in Figure 1F. The diameter of the semicircle, which corresponds to the charge transfer resistance, is indicative of the charge transfer process at high frequencies. In comparison to the Ti/TiO\({}_{2}\)-NT electrode, the Ti/TiO\({}_{2}\)-SR electrode exhibits a smaller semicircle. Additionally, the resistance to electron transfer is lower in the Ti/TiO\({}_{2}\)-SR sample when impedance data are fitted to the equivalent circuit model depicted in Figure S2 (see Supplementary Materials).