# Application of Spinel and Hexagonal Ferrites in Heterogeneous Photocatalysis
## 5 Other Magnetic-Photocatalytic Composites
### Hexerrites Composites
It was proposed that increased photocatalytic activity of the composite could be caused by three possible factors: (i) charge separation between both phases, with electrons accumulating on the Bi\({}_{2}\)O\({}_{3}\) phase, (ii) increased mobility of the charge carriers inside the SrFe\({}_{12}\)O\({}_{19}\) structure due to its reman magnetic field, and (iii) increased light absorption efficiency by SrFe\({}_{12}\)O\({}_{19}\) (when compared to pristine Bi\({}_{2}\)O\({}_{3}\)). Other composites utilizing different Bi-based photocatalysts were also investigated by [PERSON] et al., combining SrFe\({}_{12}\)O\({}_{19}\) ferrite with Bi\({}_{4}\)O\({}_{5}\)Br\({}_{2}\) and Bi\({}_{2}\)O\({}_{4}\)Cl [139; 140]. During both studies, the composites were tested for degradation of Rhodamine B (10 ppm) under UV/vis irradiation (Xe lamp). The synergistic effect was observed between Sr-ferrite and Bi\({}_{4}\)O\({}_{5}\)Br\({}_{2}\) when the composite contained 5 wt% of SrFe\({}_{12}\)O\({}_{19}\). However, there was no synergy between the components in the case of the SrFe\({}_{12}\)O\({}_{19}\)/Bi\({}_{3}\)O\({}_{4}\)Cl photocatalyst. Ultimately, the 5 wt% Sr-ferrite/Bi\({}_{4}\)O\({}_{5}\)Br\({}_{2}\) was found to be the most active sample in both studies. It was proposed that an activity increase was caused by the charge carriers' separation, with electrons accumulating on the ferrite phase. Scavenger-based studies have shown the dominant role of \({}^{\bullet}\)O\({}_{2}\)\({}^{-}\) radicals and an insignificant contribution of \({}^{\bullet}\)OH. The latter might relate to the unsuitable position of the valence band edge for both components. The reusability tests for five consecutive cycles indicated a constant activity decrease (from 100% to 85% of degradation). Although it has been proposed that an activity decrease is caused by the composite loss during the  
Figure 9: The schematic illustration of the proposed mechanism for degradation of 2-mercaptoethanolbenzothiazole over the ternary BaFe\({}_{12}\)O\({}_{19}\)/Ag/AgBr nanocomposite. Reprinted with permission from [136]. Copyright (2020) Elsevier. recovery, the constant change in the reaction rates between the cycles might also suggest that some damage could occur directly during the process. Moreover, a novel structure, based on SrFe\({}_{12}\)O\({}_{19}\) ferrite, carbon quantum dots (CQDs) and CeO\({}_{2}\), was proposed by [PERSON] et al. [141]. The ternary composite allows an increase in both the adsorption efficiency of pollutants (here MB) and photocatalytic efficiency under simulated sunlight (AM1.5), when compared to pristine CeO\({}_{2}\), CeO\({}_{2}\)/CQDs, and SrFe\({}_{12}\)O\({}_{19}\)/CeO\({}_{2}\) materials. It was proposed that the formation of II-type heterojunction between Sr-ferrite and Ce\({}_{2}\)O is responsible for enhanced photocatalytic activity. Ternary composite could decompose 90% of MB during 4 hours of irradiation, but a decrease in activity to 65% was observed after four subsequent cycles. Finally, hexagonal ferrites have also been combined with conjugated polymers, such as polyaniline (PANI) and polythiophene (PTh). For example, [PERSON] et al. prepared two composite systems using Zn,Co,Sn-co-doped BaFe\({}_{12}\)O\({}_{19}\) ferrite with either multiwall carbon nanotubes (MWCNTs) or polyaniline [142]. The composites, tested towards degradation of MB under irradiation with an Hg lamp (\(\lambda>400\) nm), showed increased activity in comparison to the pure ferrite; however, MWCNT-based composite slightly outperformed the PANI-based one. In both cases, a decrease in degradation efficiency was observed after the three degradation cycles. A similar study was also performed by [PERSON] et al., who combined Ce,Nd-co-doped SrFe\({}_{12}\)O\({}_{19}\) ferrite with polythiophene [142]. Again, the combination of ferrite with polymer increased the degradation efficiency (MB) by ca. two times.