# Application of Spinel and Hexagonal Ferrites in Heterogeneous Photocatalysis
## 5 Other Magnetic-Photocatalytic Composites
### Hexerrites Composites
For example, [PERSON] et al. proposed a double-magnetic composite of ZnFe\({}_{2}\)O\({}_{4}\)/SrFe\({}_{12}\)O\({}_{19}\) as a visible-light active photocatalyst towards the degradation of MB [125]. It was proposed that the composite exhibits both improved magnetic and photocatalytic performance, i.e., (i) SrFe\({}_{12}\)O\({}_{19}\) enhances the magnetic properties of ZnFe\({}_{2}\)O\({}_{4}\), which are quite weak for the pure spinel phase (M\({}_{5}\) value of ca. 1.75 and 39 Am\({}^{2}\)-kg\({}^{-1}\) for the pure ZnFe\({}_{2}\)O\({}_{4}\) and the composite, respectively), and (ii) substitution of Fe\({}^{3+}\) by Zn\({}^{2+}\) ions at the 2a sites of the M-ferrite structure results in enhanced photocatalytic activity when compared to pristine ZnFe\({}_{2}\)O\({}_{4}\). This substitution caused a presence of Fe\({}^{2+}\) in the composite, and the resulting Fe\({}^{2+}\)/Fe\({}^{3+}\) interactions could be responsible for an increased e\({}^{-}\) transport and a further increase in photocatalytic activity of the composite. However, reusability tests showed a constant decrease in activity between the first and fifth cycle (from 95% to 70% degradation), connected with possible photocatalysts loss between the cycles as well as surface blocking and partial destruction of the composite mesostructured. Another photocatalyst was designed by [PERSON] et al., who prepared a BaFe\({}_{12}\)O\({}_{19}\)/Sm\({}_{2}\)Ti\({}_{2}\)O\({}_{7}\) structure in its bare and Ag-decorated forms [126]. It was found that Ag has a positive impact on the activity, resulting in 100% degradation of MB during 150 min of vis irradiation (400-770 nm). It should be pointed out that the positive effect of noble metals has been well known and intensively investigated for more than forty years, as they usually work as an electron scavenger, inhibiting charge carriers' recombination [127; 128; 129; 130]. Unfortunately, recycling causes a slight activity decrease (as in many ferrite-based photocatalysts), to 90% degradation after four cycles. Another visible-light active structure was proposed by [PERSON] et al., who prepared a SrFe\({}_{12}\)O\({}_{19}\)/BiOBr 2D-2D nanocomposite for the degradation of Rhodamine B (\(\lambda>420\) nm) [131]. An increase in the activity of BiOBr was noticed for samples with 3 wt% and 5 wt% Sr-ferrite, probably because of increased charge carrier separation, where electrons migrate  
Figure 8: The schematic illustration of the proposed mechanism for the photocatalytic degradation of Rhodamine B on BaFe\({}_{12}\)O\({}_{19}\) ferrite/g-C\({}_{3}\)N\({}_{4}\) composite. Reprinted with permission from [123]. Copyright (2017) Elsevier. to BiOBr and holes to the SrFe\({}_{12}\)O\({}_{19}\). The highest removal efficiency of 97% after 30 min of visible-light irradiation was achieved for the sample with 5 wt% of SrFe\({}_{12}\)O\({}_{19}\). Similar to other ferrite photocatalysts, an activity loss (97% to ca. 90%) was observed during five cycles of degradation. Finally, scavenger-based experiments showed that \({}^{\bullet}\)OH radicals play a less significant role in the examined system than e\({}^{-}\) and \({}^{\bullet}\)O\({}_{2}\)\({}^{-}\), which might suggest that the oxidative potential of h\({}^{+}\) at the ferrite surface might be too low to effectively oxidize water to \({}^{\bullet}\)OH. An interesting approach was suggested by [PERSON] et al., in which BaFe\({}_{12}\)O\({}_{19}\) ferrite was coupled with Ag\({}_{3}\)PO\({}_{4}\) photocatalysts to create an in situ Fenton-like photosystem for degradation of bisphenol A under visible light (\(\lambda\geq 420\) nm) [132]. It is known that Ag\({}_{3}\)PO\({}_{4}\) is able to generate H\({}_{2}\)O\({}_{2}\) due to 2-electron oxygen reduction (photocatalytic reaction); then, H\({}_{2}\)O\({}_{2}\) is responsible for further degradation of organic pollutants [133]. An approach proposed by [PERSON] et al.