## 3 Results
### X-ray Photoelectron Spectroscopy (XPS)  
The X-ray Photoelectron Spectroscopy (XPS) quantifications were performed to analyze the elemental composition of anatase TiO\({}_{2}\) and Cu-doped samples. Figure 6a provides the survey spectrum of anatase TiO\({}_{2}\) and 0.5% Cu-doped TiO\({}_{2}\) calcined at 650 \({}^{\circ}\)C. It illustrates the presence of titanium, oxygen and additionally copper for the doped sample. A small amount of carbon peak also emerged, which is acquired during the synthesis and calcination processes. An introduction of Cu into TiO\({}_{2}\) lattice resulted in substitution of Ti\({}^{4+}\) by the Cu\({}^{2+}\) ion [74]. Hence, instead of O-Ti-O bonds new bonds of Cu-O are established. Figure 6b provides the high-resolution spectra of Ti 2p, which exhibits very sharp, symmetric and intense peaks, indicating the presence of Ti\({}^{4+}\) state. The deconvoluted Ti 2p peaks of anatase appears at 457.4 eV and 463.06 eV, which is ascribed to Ti 2p\({}_{3/2}\) and Ti 2p\({}_{1/2}\) (Ti-O bond) respectively. In the case of Cu-TiO\({}_{2}\) 650 \({}^{\circ}\)C, a significant shift of around 1.7 eV is observed for both Ti 2p\({}_{3/2}\) and Ti 2p\({}_{1/2}\) (459.06 eV and 464.83 eV) [75; 76]. The shift in the peak is attributed to the introduction of Cu in the lattice. The Ti 2p peaks of Cu-TiO\({}_{2}\) 650 \({}^{\circ}\)C do not show any indication of Ti\({}^{3+}\) or Ti\({}^{2+}\) states. Similarly, Figure 6c illustrates the high-resolution spectra of O 1s, where peaks at 528.59 eV constitutes to the crystal lattice oxygen (O-Ti\({}^{4+}\)) and 530.40 eV for adsorbed hydroxyl or oxygen molecules. Whilst the O 1s spectrum of the doped sample showed a similar shift as observed for Ti 2p spectrum attributed to the introduction of copper atom in the lattice [77]. The Cu 2p\({}^{3}\) spectrum (Figure 6d) on deconvolution exhibits an intense peak at 933.13 eV and a shorter peak at 934.60 eV, ascribed to Cu\({}^{+}\) and Cu\({}^{2+}\) states [75]. Figure 5: Raman spectra of anatase, rutile and 0.5% Cu-TiO\({}_{2}\) 650 \({}^{\circ}\)C.  
Figure 7 illustrates the high-resolution spectra of 0.5% Cu-TiO\({}_{2}\) samples calcined at different temperatures. It is observed that with increase in temperature from 500 \({}^{\circ}\)C to 700 \({}^{\circ}\)C the Cu\({}^{+}\) state observed at 933.1 eV gradually decreases. While the Cu\({}^{2+}\) state at 934.3 eV is only observed at 650 \({}^{\circ}\)C and 700 \({}^{\circ}\)C. Additionally, two satellite peaks of Cu\({}^{2+}\) were also observed at 941.3 eV and 944.2 eV for 700 \({}^{\circ}\)C [75; 78]. The intensity of the peak at 934.3 eV gradually increases with the increase in calcination temperature [75; 79]. Usually, the TiO\({}_{2}\) exhibits stable anatase phase in the range of 500-600 \({}^{\circ}\)C and converts to rutile with increase in temperature. However, the change in oxidation states with increase in calcination temperature in our Cu-doped samples (change in oxygen vacancies) contributes to the high temperature stability of the oxygen-rich anatase phase at 650 \({}^{\circ}\)C [23; 34]. Figure 6: (**a**) Survey spectrum of anatase and 0.5% Cu-TiO\({}_{2}\); (**b**) high resolution Ti 2p spectra of anatase and 0.5% Cu-TiO\({}_{2}\) 650 \({}^{\circ}\)C; (**c**) high resolution O 1s spectra of anatase and 0.5% Cu-TiO\({}_{2}\) 650 \({}^{\circ}\)C.; (**d**) high resolution Cu 2p3 spectra of 0.5% Cu-TiO\({}_{2}\) 650 \({}^{\circ}\)C.  
Figure 7: High-resolution Cu 2p3 spectra of 0.5% Cu-TiO\({}_{2}\) samples calcined at different temperatures. Previous studies have reported that Cu\({}^{2+}\)/Cu\({}^{+}\) replaces Ti\({}^{4+}\) from the TiO\({}_{2}\) lattice resulting in the formation of oxygen vacancies. This leads to the formation of high absorption of a visible light and narrowing of band gap, as observed in Figure 8. The TiO\({}_{6}\) octahedron has Ti in Ti\({}^{4+}\) state with an ionic radius of 0.0605 nm. On the other hand, the ionic radii of Cu\({}^{2+}\) and Cu\({}^{+}\) exist as 0.073 nm and 0.077 nm, respectively [80].