# Application of Spinel and Hexagonal Ferrites in Heterogeneous Photocatalysis
## 4 TiO\({}_{2}\)-Based Magnetic Photocatalysts
### Spinel Ferrites with TiO\({}_{2}\)  
The coupling of spinel ferrites and titania (pristine or modified) in order to obtain a highly photoactive material with magnetic properties has commonly been reported in the recent literature, as summarized in Table 3.  
It should be underlined that MFe\({}_{2}\)O\({}_{4}\) NPs in TiO\({}_{2}\) composites (and in other ferromagnet-photocatalyst materials) can exhibit a dual function. First, as semiconductors with narrower bandgap than UV-active photocatalysts, they could take part in photocatalytic reactions, creating a heterojunction, which might cause both vis activity and efficient charge carriers' separation [100]. At the same time, their magnetic properties allow easy and cheap recycling of photocatalysts after reactions. However, in some cases, the direct connection between magnetic and photocatalytic materials is not worthwhile, especially when the separation is emphasized. For example, environmental conditions (e.g., acidic pH) and enhanced charge carrier migration (e.g., resulting in photocorrosion) could cause the leaching of iron ions from the composite, thus resulting in the destruction of the spinel ferrite structure. The possible reactions of Fe\({}_{3}\)O\({}_{4}\) oxidation/reduction are presented in Equations (1)-(4) and Figure 5.  
\[\text{Fe}^{3+}(\text{s})+\text{e}^{-} \rightarrow \text{Fe}^{2+}(\text{s}) \tag{1}\] \[\text{Fe}_{2}\text{O}_{3}+6\text{H}^{+}+2\text{e}^{-} \rightarrow 2\text{Fe}^{2+}+3\text{H}_{2}\text{O}\] (2) \[\text{Fe}_{3}\text{O}_{4}+8\text{H}^{+}+3\text{e}^{-} \rightarrow 3\text{Fe}^{2+}+4\text{H}_{2}\text{O}\] (3) \[\text{Fe}^{2+}(\text{aq})+\text{h}^{+} \rightarrow \text{Fe}^{3+}(\text{aq}) \tag{4}\] \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{1}{c}{**Photocatalyst**} & **Pollutants** & \multicolumn{1}{c}{**Experimental**} & \multicolumn{1}{c}{**Findings**} & \multicolumn{1}{c}{**Ref.**} \\ \hline \multirow{4}{*}{Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)/TiO\({}_{2}\)-M (M = Pt/Cu)} & \multirow{4}{*}{Ph, 4-NPh} & 300 W Xe light (UV/vis, 60 W\(\cdot\)m\({}^{-2}\)); 2 g-L\({}^{-1}\); 2 g-L\({}^{-1}\); 100\% Ph deg. and 50\% TOC removal} & \multirow{4}{*}{[95]} \\  & & & 20 ppm (Ph) & \\  & & & or 500 \(\upmu\)M (4 NPh); 60 min & & \\  & & & (Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)/TiO\({}_{2}\)-Pt) & \\ \hline \multirow{4}{*}{ZnFe\({}_{2}\)O\({}_{4}\)/SiO\({}_{2}\)/TiO\({}_{2}\)} & \multirow{4}{*}{ETD} & 300 W Xe light (UV/vis or vis & \multirow{4}{*}{100\% deg. (UV/vis); 40\% deg. (vis); 6 cycles} & \multirow{4}{*}{[96]} \\  & & & 30 min & \\ \hline \multirow{4}{*}{TiO\({}_{2}\)-Fe\({}_{3}\)O\({}_{4}\)-M (M = Ag/Au)} & \multirow{4}{*}{Phenols} & 120 W Hg lamp (UV) or 150 W & \(\sim\)0.29 mm\(\cdot\)h\({}^{-1}\) & \\  & & Hal. lamp (UV/vis, & (TiO\({}_{2}\)-Fe\({}_{3}\)O\({}_{4}\)/Ag; UV); 350–700 nm); & & \\  & & & 2 g\(\cdot\)L\({}^{-1}\); 1 mM; 7 h & (TiO\({}_{2}\)-Fe\({}_{3}\)O\({}_{4}\)/Ag; vis) & \\ \hline \multirow{4}{*}{NiFe\({}_{2}\)O\({}_{4}\)@TiO\({}_{2}\) core@shell} & \multirow{4}{*}{MO} & 300 W Xe lamp with cut-off & \multirow{4}{*}{90\% deg. (NiFe\({}_{2}\)O\({}_{4}\)@40\% TiO\({}_{2}\))} & \multirow{4}{*}{[98]} \\  & & & filter (\(\lambda\) \(>\) 400 nm); 0.8 g\(\cdot\)L\({}^{-1}\); 8 ppm; 90 min & & \\ \hline \multirow{4}{*}{Co\({}_{0.5}\)Zn\({}_{0.25}\)Mo\({}_{0.25}\)Fe\({}_{2}\)O\({}_{4}\)/TiO\({}_{2}\)} & \multirow{4}{*}{MO; MB} & 180 W solar simulator; 1 g\(\cdot\)L\({}^{-1}\); 4\(\cdot\)10\% M; 80 min (MB) & 95\% MO deg. (MB) & \\  & & & or 360 min (MO) & (Co\({}_{0.5}\)Zn\({}_{0.25}\)Ni\({}_{0.25}\)Fe\({}_{2}\)O\({}_{4}\)/TiO\({}_{2}\)) & \\ \cline{1-1}  & & & & 99\% MB deg. (MB) & \\ \cline{1-1}  & & & & (Co\({}_{0.5}\)Zn\({}_{0.25}\)Mn\({}_{0.25}\)Fe\({}_{2}\)O\({}_{4}\)/TiO\({}_{2}\)) & \\ \hline \multirow{4}{*}{+NPh—4-nitrophenol; conv.—conversion; deg.—degradation; ETD—etodolac; Hal.—halogen; MB—Methylene Blue; MO—Methyl Orange; Ph—phenol; TOC—total organic carbon.} & \multirow{4}{*}{[99]} \\ \cline{1-1}  & & & & \\ \end{tabular}
\end{table}