# Application of Spinel and Hexagonal Ferrites in Heterogeneous Photocatalysis
## 5 Other Magnetic-Photocatalytic Composites
### Hexerrites Composites
not only utilized BaFe\({}_{12}\)O\({}_{19}\) as a magnetically separable part of the composite, but also enabled further conversion of photogenerated H\({}_{2}\)O\({}_{2}\) to other reactive oxygen species (ROS), such as a \({}^{\bullet}\)OH and \({}^{\bullet}\)O\({}_{2}\)\({}^{-}\). The quantification of generated H\({}_{2}\)O\({}_{2}\) for pristine Ag\({}_{3}\)PO\({}_{4}\) and Ag\({}_{3}\)PO\({}_{4}\)/BaFe\({}_{12}\)O\({}_{19}\) composite, as well as ESR-DMPO experiments for the observation of \({}^{\bullet}\)OH and \({}^{\bullet}\)O\({}_{2}\)\({}^{-}\) radicals, were used for the mechanism clarification. For 10% Ba-ferrite composite, the amount of both radicals was significantly increased, whereas the concentration of H\({}_{2}\)O\({}_{2}\) was ca. three times lower than that of Ag\({}_{3}\)PO\({}_{4}\). Unfortunately, the stability tests showed an efficiency decrease from 80% to ca. 60% after four cycles of 30 min of irradiation. Much worse stability was observed for pristine Ag\({}_{3}\)PO\({}_{4}\), where a decrease in activity from 50% to ca. 5% was noticed (Photocorrosion of Ag\({}_{3}\)PO\({}_{4}\) and Ag\({}_{4}\)P\({}_{2}\)O\({}_{7}\) is a well-known problem [134; 135]). As another example of BaFe\({}_{12}\)O\({}_{19}\) ferrite combined with an Ag-based photocatalyst, [PERSON] et al. proposed a BaFe\({}_{12}\)O\({}_{19}\)/AgBr composite for visible-light degradation of 2-mercaptoethanolobenzothiazacle--MBT (\(\lambda\geq 420\) nm) [136]. Again, the synergistic effect between both phases was observed, with the highest degradation efficiency (100% during 30 min) observed for the photocatalyst containing 10% BaFe\({}_{12}\)O\({}_{19}\) ferrite. However, XPS analysis showed the presence of zero-valent silver (Ag\({}^{0}\)), thus indicating the formation of a BaFe\({}_{12}\)O\({}_{19}\)/AgBr/Ag ternary structure. The charge transfer and separation between these three components was recognized as a main reason behind the increased activity of the composite. Based on the combined Mott-Schottky and ESR-DMPO experiments, a Z-scheme heterojunction was proposed with (i) Ag\({}^{0}\) acting as an \"intermediate\" for electron transfer between BaFe\({}_{12}\)O\({}_{19}\) and AgBr, (ii) oxygen reduction to \({}^{\bullet}\)O\({}_{2}\)\({}^{-}\) on AgBr, and (iii) H\({}_{2}\)O oxidation to \({}^{\bullet}\)OH on BaFe\({}_{12}\)O\({}_{19}\). The proposed mechanism is presented in Figure 9. The slight activity loss was observed during recycling, i.e., 90% degradation after four 30 min cycles (but still almost 100% for a 1 h process). In another study, [PERSON] et al. prepared a SrFe\({}_{12}\)O\({}_{19}\)/MoS\({}_{2}\) heterostructure as the visible-light active photocatalysts for MB and methyl orange (MO) degradation under simulated sunlight [137]. The highest activity was observed for the composite with 1:2 mass ratio of Sr-ferrite:MoS\({}_{2}\), reaching ca. 100% and 70% degradation of MB (120 min) and MO (60 min). A possible charge separation between both phases was suggested, with electron and hole migration to SrFe\({}_{12}\)O\({}_{19}\) and MoS\({}_{2}\), respectively, generating ROS (\({}^{\bullet}\)OH and \({}^{\bullet}\)O\({}_{2}\)\({}^{-}\)). During six subsequent cycles of MB degradation, a decrease in activity to 82% was observed. However, XRD and XPS analyses did not reveal any significant change in the composite structure. Furthermore, SrFe\({}_{12}\)O\({}_{19}\) was also coupled with Bi\({}_{2}\)O\({}_{3}\) for MB degradation under visible-light irradiation (\(\lambda>420\) nm) [138]. It was found that an increase in Sr-ferrite content between 30% and 40% caused a photocatalytic activity increase, with a maximum efficiency obtained for 35% of SrFe\({}_{12}\)O\({}_{19}\). The most important finding was the stable activity (100% degradation of MB during 4 h of irradiation) for three subsequent cycles.