## 3 Results and Discussion
### Optical, Electrochemical and Electrical Properties  
The UV-Vis absorption spectra of the synthesized Ti-TiO\({}_{2}\) layers and Ti-TiO\({}_{2}\)-CuMnO\({}_{2}\) heterostructures in the range of 300-800 nm were presented in Figure 6. It was noted that the synthesized Ti-TiO\({}_{2}\)-CuMnO\({}_{2}\) structures have a strong absorption in the UV range, demonstrating that the heterostructures have a strong affinity for UV light. The absorption peak of the Ti-TiO\({}_{2}\) layer was found at around 380 nm, and gradually increased with the chemical treatment time (Figure 6a). The optical band-gap energy of the as-synthesized heterostructures was determined using the Tauc plot [39] shown in Figure 6c. The _Eg_ optical band-gap energy was derived from the intersect of the straight line with the _hv_-axis of the Tauc plot. The estimated band gap of the Ti-TiO\({}_{2}\) is about 2.95 eV (Figure 6b), which corresponds to the rutile form of TiO\({}_{2}\)[40]. The modification of the spectra shapes shown in Figure 4b confirms the interface between TiO\({}_{2}\) and CuMnO\({}_{2}\). After the deposition of the CuMnO\({}_{2}\) thin film, the calculated _Eg_ was about 2.69 eV, 2.79 eV, and 3.07 eV depending on the chemical treatment time (Figure 6d). Figure 6: UV-Vis absorption spectra of the Ti-TiO\({}_{2}\) layers (**a**) and Ti-TiO\({}_{2}\)-CuMnO\({}_{2}\) (**b**) heterostructures; The _ahv_ versus _Eg_ curve for the Ti-TiO\({}_{2}\) (**c**) and the Ti-TiO\({}_{2}\)-CuMnO\({}_{2}\) (**d**). The Mott-Schottky analysis presented in Figure 7a-c shows the electronic energy levels of the Ti-TiO\({}_{2}\) (Ti support) layers and CuMnO\({}_{2}\) (FTO support) film that was investigated by electrochemical impedance measurements; conduction type and carrier concentration were calculated using the Mott-Schottky equation [26]. Figure 7a shows the positive slope for Ti-TiO\({}_{2}\) layers, suggesting the \(n\)-type behavior of the semiconductor, with the \(V_{FB}\) values being determined as \(-0.38\) V vs. NHE (S1), \(-0.36\) V vs. NHE (S2) and \(-0.48\) V vs. NHE (S3), with \(E\)(NHE) = \(E\)(Ag/AgCl) + 0.197 (unit in V). Also, the carrier densities were calculated for S1 (\(1.45\times 10^{17}\) cm\({}^{-3}\)), S2 (\(1.81\times 10^{17}\) cm\({}^{-3}\)) and S3 (\(2.59\times 10^{17}\) cm\({}^{-3}\)). For CuMnO\({}_{2}\), the Mott-Schottky plot illustrated in Figure 7b demonstrates the \(p\)-type characteristic with a \(V_{FB}\) of about 0.98 V vs. NHE, and a calculated carrier density of about 1.74 \(\times\) 10\({}^{18}\) cm\({}^{-3}\). Figure 7c shows the illustrated diagram of the corresponding conduction and valence band edges based on the band-gap energies that were calculated from UV-Vis spectra (Figure 6c,d) and the calculated \(V_{FB}\) values. The current-voltage properties of the as-synthesized oxide \"\(n\)-\(p\)\" heterojunction, which was investigated at room temperature in dark and UV conditions, is presented in Figure 8. The \(n\)-TiO\({}_{2}\)/\(p\)-CuMnO\({}_{2}\) heterojunction with a rectifying behavior was highlighted in all three cases that were investigated in the experimental program, both in dark and UV conditions. In addition, all of the cases (\(I\)-\(V\) measurements made in the interval \(-2\) and \(+2\) V) showed an increased current and a reduced turn-on voltage under UV conditions,  
Figure 7: Mott-Schottky analysis of (**a**) Ti-TiO\({}_{2}\) layers and (**b**) CuMnO\({}_{2}\) film; (**c**) diagram for corresponding conduction and valence band edges. compared to those in the dark state. This may be due to an increase in the charge-carrier concentration due to UV excitation [41]. Figure 8: Current-voltage characteristics of _n_-TiO\({}_{2}\)/_p_-CuMnO\({}_{2}\) heterojunction for (**a**) S1a; (**b**) S2a and (**c**) S3a under dark, UV and Vis irradiation (Inset: ohmic contact measurements); Log from forward and reverse bias for (**d**) S1a; (**e**) S2a and (**f**) S3a, under dark and UV illuminated conditions.