## 3 Results and Discussion
### Photocatalytic Activity of TiO\({}_{2}\)-Fe\({}_{3}\)O\({}_{4}\) Composite systems
The process can be explained as follows: after irradiation of TiO\({}_{2}\) with light (1), photons that have higher energy than the TiO\({}_{2}\) bandgap are absorbed, and the energy is consumed for the transition of TiO\({}_{2}\) unpaired electrons from the valence band to the conduction band. The conduction band electrons interact with surface oxygen leading to the creation of superoxide radicals (2), which then react with Fe\({}_{3}\)O\({}_{4}\) within the composite system leading to the transformation of magnetite to maghemite (3) [3; 24]. \[\text{TiO}_{2}+hv \rightarrow h^{+}+e^{-} \tag{1}\] \[\text{O}_{2}+e^{-} \rightarrow \bullet\text{O}_{2}^{-}\] (2) \[\text{2 Fe}_{3}\text{O}_{4}+\frac{1}{2}\bullet\text{O}_{2}^{-} \to 3(\gamma-\text{Fe}_{2}\text{O}_{3}) \tag{3}\]  
Figure 6 shows the proposed mechanism of the above-mentioned process. Due to the transformation of Fe\({}_{3}\)O\({}_{4}\) to \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) within the composite material during the irra  
Figure 5: Photocatalytic degradation tests of ibuprofen with bare TiO\({}_{2}\) and TiO\({}_{2}\)–Fe\({}_{3}\)O\({}_{4}\) composite material under irradiation with (**A**) UV light (365 nm) and (**B**) green light (523 nm). diation process, the ROS produced in the process are consumed during the oxidative transformation of magnetite to maghemite instead of being used in the photodegradation of model pollutant, i.e., ibuprofen. This leads to lowered photodegradation rates. Table 3 shows the comparison to findings reported in similar studies where TiO2-Fe3O4 was used in photocatalysis. A similar effect could be observed in a different study where the effect of Fe3O5/TiO2 on the photodegradation of CHCl3 was investigated [25]. Although Fe3O5/TiO2 synthesized by hydrothermal method showed similar degradation rates to bare TiO2, Fe3O5/TiO2 prepared by an impregnation method revealed worse rates than bare TiO2. Another study reports that the addition of Fe3O4 to TiO2 reduced the system's efficiency in removing bisphenol A (BPA) under UV-A irradiation in a photocatalytic, non-Fenton system. Based  
Figure 6: Photoc oxidative transformation of Fe3O4 into \(\gamma\)-Fe2O3 in a TiO2–Fe3O4 composite system. The proposed mechanism of the process is as follows: (1) conduction band electrons and electron holes are formed as a consequence of irradiation of the TiO2 layer; (2) the electrons interact with oxygen present in the reaction environment producing superoxide radicals; (3) superoxide radicals interact with Fe3O4 atoms leading to (4) its oxidation into \(\gamma\)-Fe2O3. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Material \({}^{\text{a}}\)** & **Method of Preparation** & **Photocatalytic Substrate** & **Irradiation Wavelength** & **Reaction Time** & **TiO2**–**Fe3O4 Photoefficiency b** & **Reference** \\ \hline TiO2– & Mechano- & & & & & & \\ Fe2O4 & thermal & Ibuprofen & 365, 523 nm & 2 h & 44\%, \(\sim\)5\% & 19\%, \(<\)5\% & This study \\ Fe2O5/TiO2 & Impregnation & Chloroform & 365 nm & 10 h & \(\sim\)60\% & \(\sim\)30\% & [25] \\ Fe3O4/TiO2 & Not indicated & BPA & 365 nm & 2 h & 56\% & 50\% & [26] \\ Fe3O4@TiO2 & Vapor-thermal & Rhodamine B & UV & 1 h & \(\sim\)85\% & \(\sim\)35\% & [27] \\ FeTiNC & Pulsed laser & Cr(VI) ions & UV, Vis \(>\) 380 & 2 h & 24\%, 3\% [28] & 69\%, 19\% & [29] \\  & deposition & & nm & & & & \\ TiO2/Fe3O4 & Ultrasonic & Orange G & Vis & 2 h & \(\sim\)20\% & \(\sim\)60\% & [30] \\ TiO2/Fe3O4 & Ultrasonic & Paracetamol & 254 nm & 5 h & \(\sim\)95\% & \(\sim\)89\% & [31] \\ \hline \hline \end{tabular} \({}^{\text{a}}\) as referred to in the referenced source material, \({}^{\text{b}}\) photoefficiencies are listed in order of irradiation wavelengths as reductions of initial concentration of the substrate, BPA-bisphenol A.  
\end{table} 
Table 3: TiO2–Fe3O4 usage in photocatalysis.