## 3 Results and Discussion
### Photocatalytic Performance  
The results of the photocatalytic activity of the two different samples are presented next. Comparison of obtained results of adsorption, photolysis, and photocatalysis degradation of MB dye from an aqueous medium (milli-Q water) under simulated solar light irradiation and also under UV-A irradiation are presented in Figure 4A. Photolytic and photocatalytic performance for the degradation of MB over immobilised rGO/TiO\({}_{2}\) nanocomposite, bare TiO\({}_{2}\), and without catalyst under ultraviolet and simulated solar irradiation is presented. To clarify the effect of direct photolysis of MB dye, the photolytic tests were performed under both sources of irradiation. It can be seen from Figure 4B that only \(\sim\)11.5% of MB dye is degraded from an aqueous medium (milli-Q water) after 120 min of simulated solar irradiation without presence of catalyst. There is no photolytic degradation of MB dye under UV-A irradiation. The UV-A irradiation did not provide enough energy to degrade a complex organic pollutant such as MB dye. On the other hand, when the MB  
Figure 3: Diffuse reflectance UV-Vis spectra of (**A**) pure TiO\({}_{2}\) and (**B**) rGO/TiO\({}_{2}\) nanocomposite. Insets are corresponding plots of transformed Kubelka–Munk function versus the energy of the light for as-prepared photocatalysts. Figure 2: SEM images of immobilised (**A**) TiO\({}_{2}\) and (**B**) rGO/TiO\({}_{2}\) on surface of glass substrate before use. dye was exposed to the photolysis with the simulated solar irradiation, it exhibited a slight degradation. The simulated solar irradiation is probably more efficient by providing a higher energy density and is therefore more suitable for the degradation of complex organic pollutants such as MB [37]. It can be seen that the adsorption of MB for immobilised pure TiO\({}_{2}\) photocatalyst was negligible, while slight adsorption capacity of MB on the surface of immobilised rGO/TiO\({}_{2}\) photocatalyst was observed (approximately 10%) after 60 min stirring in the dark. This phenomenon can be attributed to the strong \(\pi\)-\(\pi\) interactions between the benzene rings of MB structure and the aromatic surface of rGO nanosheets, which clearly contribute to the pollutants' adsorption [38]. After 120 min of UV-A irradiation, the photodegradation efficiency of bare TiO\({}_{2}\) and rGO/TiO\({}_{2}\) photocatalysts is about 41.9% and 55.8% for MB dye from an aqueous medium (milli-Q water), whereas the same photocatalyst shows increased photodegradation efficiency under simulated solar irradiation to about 46.6% and 92.7%, respectively. The obtained photodegradation results demonstrate that the immobilised rGO/TiO\({}_{2}\) nanocomposite has superior photocatalytic activity, particularly under simulated solar irradiation. The photodegradation rate constant (\(k\)) can be calculated by extrapolating time-dependent degradation function in a logarithmic relation i.e., of \(-ln\) (\(c/c_{0}\)) versus irradiation time (\(t\)) as shown in Figure 4C. The obtained results for UV-A irradiation were as follows: 0.00452 min\({}^{-1}\) and 0.00677 min\({}^{-1}\) for pure TiO\({}_{2}\) and rGO/TiO\({}_{2}\) nanocomposite, respectively. The obtained linear correlation coefficient (\(R^{2}\)) was 0.9874 and 0.9991 for pure TiO\({}_{2}\) and rGO/TiO\({}_{2}\) nanocomposite, respectively. The obtained results for simulated solar  
Figure 4: (**A**) Adsorption, photolytic and photocatalytic degradation of MB from an aqueous medium (milli-Q water) under ultraviolet-A (UV-A) and simulated solar irradiation (S) as a function of irradiation time. (**B**) photodegradation efficiency of photolytic and photocatalytic degradation of MB after 120 min. (**C**) the \(-\)ln [MB]/[MB]\({}_{0}\) versus reaction time, and (**D**) kinetic rate constants of photocatalytic reactions by immobilised rGO/TiO\({}_{2}\) nanocomposite photocatalyst.