## 3 Results and Discussion
### Photocatalytic Activity of TiO\({}_{2}\)@rGO Nanocomposites  
Diffuse reflectance UV-Vis spectroscopy for modulation of the bandgap energy of the synthesized materials was calculated by Kubelka-Munk function, as shown in Figure 6. The bandgap results were obtained, giving \(E_{\text{g}}\) values of 2.71 eV, 3.03 eV, and 3.15 eV for synthesized TiO\({}_{2}\)@rGO nanocomposites and for pure TiO\({}_{2}\) particles. The bandgap energy of reference P-25 material was compared and displayed in the Supplementary Figure S3. The obtained results indicate that the decrease of the bandgap of the synthesized nanocomposites occurs with the introduction of rGO sheets. It can be observed that the photoactivity could be shifted into visible light. Firstly, the photolysis tests (without catalysts) were performed, and it can be reported that the degradation of MB and RhB dyes was negligible under the irradiation. Before the photocatalytic tests, the adsorption properties of synthesized catalysts were evaluated. The obtained results of adsorption tests for MB and RhB dyes in the dark for different catalysts are displayed in Figure 7. The adsorption test is related to adsorption-desorption equilibrium among MB and RhB dyes molecules and catalysts surface. It could be seen that prepared nanocomposites showed higher adsorption of dyes compared to pure TiO\({}_{2}\) particles and reference P-25 material. Indeed, the dyes adsorption for pure TiO\({}_{2}\) particles  
Figure 5: SEM images. (**A**) commercial P-25; (**B**) synthesized TiO\({}_{2}\); (**C**) TiO\({}_{2}\)@rGO nanocomposite with 5 wt.% of rGO; (**D**) TiO\({}_{2}\)@rGO nanocomposite with 15 wt.% of rGO. and reference P-25 material was negligible for both dyes. On the other hand, the RhB and MB adsorption for synthesized nanocomposites was dependent on the amount of rGO. The RhB adsorption for TiO\({}_{2}\)@rGO (5%) was 1.6% which was lower than for the MB at 4.8%. The highest adsorption capacity of dyes was observed for TiO\({}_{2}\)@rGO (15%) nanocomposite with 25.8% for RhB and 13.8% for MB. Figure 6: The absorption threshold energy (inset shows the bandgap energy) of synthesized pure TiO\({}_{2}\) particles and nanocomposites TiO\({}_{2}\)@rGO with 5 wt% and 15 wt.% of rGO thermal treated at 300 \({}^{\circ}\)C.  
Figure 7: The percentage of adsorption for MB (blue) and RhB (pink) dyes after 60 min of stirring in the dark. The photocatalytic performances of TiO\({}_{2}\) and respective nanocomposites were evaluated for dyes degradation under experimental parameters as reported in Supplementary Table S1. The photoactivity of prepared photocatalysts in comparison with reference P-25 material was monitored based on MB (cationic dye) and RhB (zwitterionic dye) decomposition under the irradiation (depicted in Figure 8A,B). It can be seen in Figure 9A,B that the synthesized nanocomposites increased the decomposition of RhB and MB significantly compared to synthesized TiO\({}_{2}\) photocatalyst. Both synthesized nanocomposites have a very efficient MB and RhB decolorization after 120 min, which was very similar to reference P-25 material. The photoactivity of TiO\({}_{2}\)@rGO (5%) was obtained with 99.6% of MB degradation and 99.9% of RhB photodecomposition. The nanocomposite with a higher amount of rGO (TiO\({}_{2}\)@rGO (15%)) showed similar photodegradation results, 98.1% for (MB) and 99.8% for (RhB). Meanwhile, the photoactivity evaluation of synthesized TiO\({}_{2}\) particles was significantly lower than that of nanocomposites and referenced P-25 material. The degradation rate of TiO\({}_{2}\) was 70.9% for RhB and 73.9% for MB. The photodegradation rate of MB and RhB dyes followed the pseudo-first-order kinetics. The linear kinetic rate constant (\(k\)) in detail is reported in Table 2. It could be determined that pseudo-first-order rate constants of TiO\({}_{2}\)@rGO nanocomposites were significantly higher than for pure TiO\({}_{2}\) nanoparticles in the degradation of both dyes. The structure of organic pollutants played an important role in the photocatalytic process.