## 3 Results and Discussion
### Characterization of Photocatalyst
The peak at lower BE (284.4 eV) corresponded to the net of interconnected carbon atoms with a combination of sp\({}^{2}\) and sp\({}^{3}\) hybridization, derived from the breakdown of graphite layers [48]. The second peak and the tail were associated with carbon bonded to oxygen in a variety of functional groups, with a higher degree of oxidation as BE increased. This region of the spectrum was fitted using components centered at 286.4 eV, 288.2 eV, and 289.5 eV, corresponding to C-O, C=O, and O-C=O bonds, respectively [48]. In comparison, the rGO spectrum shows an intense peak at 284.4 eV with a smaller shoulder elongated towards higher BE values. The main peak, related to non-oxygenated carbon, was fitted using an asymmetric function according to the higher proportion of carbon sp\({}^{2}\) after the reduction of GO. The left side of the spectrum includes the contribution of different functional groups containing oxygen, just like in GO but with a noticeable decrease of intensity, indicating effective deoxygenation of GO nanosheets [48]. The carbon region of the TiO\({}_{2}\) sample reveals the typical spectrum shape of adventitious carbon contamination. It was fitted using the main peak at 285.3 eV, compatible with hydrocarbons, and two smaller components at 286.7 eV and 289.4 eV, related to the C-O and O-C=O groups. C 1s spectra of the TiO\({}_{2}\)@rGO nanocomposites (5 wt.% and 15 wt.% of rGO) combine components from both rGO and TiO\({}_{2}\) samples. This fact was easily perceptible in the spectrum of TiO\({}_{2}\)@rGO (5 wt.% of rGO), which showed a shape similar to the adventitious carbon spectra but with a shoulder at lower BE, towards the position of rGO. This way, both spectra (5 wt.% and 15 wt.% of rGO) were fitted using four components, the first one related to the C net of rGO, at 284.4 eV; the second one coinciding with the energy of hydrocarbons in TiO\({}_{2}\); and two more components associated to oxygen functionalities. As expected, there is a noticeable increase in the area of the component  
Figure 3: X-ray diffraction patterns. (**A**) pure graphite and synthesized GO (inset shows rGO); (**B**) TiO\({}_{2}\) and their synthesized nanocomposites. associated with rGO in the nanocomposite containing 15 wt.% of rGO with respect to one having 5 wt.%. High-resolution Ti 2p spectra of TiO2 and TiO2@rGO nanocomposites (Figure 4B) showed two symmetric peaks at 459.2 eV and 464.9 eV, corresponding to Ti 2p3/2 and Ti 2p1/2, respectively. Both the BE values and spin-orbit splitting of 5.7 eV were in good agreement with Ti (IV) in a TiO2 chemical environment [44]. Supporting this, Figure 4C shows the O 1s spectra of TiO2 and both TiO2@rGO nanocomposites. All of them had a main peak at 530.4 eV, assigned to oxygen bonded to titanium, and a smaller and wider component around 531.8 eV, commonly ascribed to hydroxyl groups covering the surface oxygen vacancies in the TiO2 structure [49; 50], and also compatible with oxygen in organic compounds. Figure 4: (**A**) C 1s, (**B**) Ti 2p, and (**C**) O 1s high-resolution XPS spectra of TiO2 and TiO2@rGO nanocomposites with different ratios prepared using the same procedure. In the case of C 1s (**A**), GO and rGO spectra were included for comparison and (**D**) full-spectrum scan. The synthesized TiO\({}_{2}\) and TiO\({}_{2}\)@rGO nanocomposites and commercially available P-25 were analyzed by scanning electron microscope (SEM). Representative images of the analyzed powders are shown in Figure 5. The size of P-25 grains was around 30 nm (Figure 5A). The synthesized TiO\({}_{2}\) particles were half smaller and were spherical in shape, just like P-25 (Figure 5B). SEM images of synthesized GO and prepared rGO are displayed in Supplementary Figure S1. Figure 5C,D shows the TiO\({}_{2}\)@rGO nanocomposites with 5 and 15 wt.% of rGO, which aggregated into larger forms. It can be seen that TiO\({}_{2}\) particles had a similar size as pure powder, and they were embedded in the surface of rGO sheets (insets in Figure 5C,D) [51; 52].