# Application of Spinel and Hexagonal Ferrites in Heterogeneous Photocatalysis
## 5 Other Magnetic-Photocatalytic Composites
### Hexerrites Composites  
Similar to spinel ferrites, hexaferrites have also been combined with different carbon-based materials, especially for possible visible-light response, as summarized in Table 6. For example, [PERSON] et al. investigated the photoactivity of PbFe\({}_{12}\)O\({}_{19}\)/graphene and PbFe\({}_{12}\)O\({}_{19}\)/CNT composites for degradation of methyl orange under UV irradiation and acidic conditions (pH2-3) [121]. An increased activity of the composite in comparison to the simple PbFe\({}_{12}\)O\({}_{19}\) structure, especially for the graphene-based photocatalyst, was explained by the inhibition of charge carriers' recombination, resulting from the separation of the \(\pi\)-\(\pi^{*}\) orbitals of graphene due to the interactions with the 2p oxygen orbitals from the ferrite structure. Ultimately, 75% degradation efficiency was observed for PbFe\({}_{12}\)O\({}_{19}\)/graphene after 70 min of irradiation. A unique approach was proposed by [PERSON] et al., where BaFe\({}_{12}\)O\({}_{19}\)/g-C\({}_{3}\)N\({}_{4}\) photocatalysts were used to reduce Hg(II) ions [122]. Although the photocatalysts contained only 1-4 mass% of Ba-ferrite, a significant activity increase was observed with respect to the sole BaFe\({}_{12}\)O\({}_{19}\) and g-C\({}_{3}\)N\({}_{4}\) materials. It was proposed that enhanced activity could be caused by charge carrier migration and cumulation on different components, i.e., electrons and holes on g-C\({}_{3}\)N\({}_{4}\) and ferrite, respectively. The best activity was obtained for 3%-BaFe\({}_{12}\)O\({}_{19}\) composite under vis irradiation (\(\lambda\) > 420 nm), reaching 100% reduction of Hg\({}^{2+}\) ions within 30 min. Reusability of the composite was also tested, and a slight decrease in activity (from 100% to 90%) after five consecutive cycles was ascribed to be caused by the blocking of the surface active sites (no photocatalyst loss). Furthermore, [PERSON] et al. also investigated BaFe\({}_{12}\)O\({}_{19}\)/g-C\({}_{3}\)N\({}_{4}\) composites, using them as catalysts in the photo-Fenton process of rhodamine B degradation under visible-light irradiation [123]. Again, a synergistic effect was observed, with the highest activity achieved for 16.8 wt% of BaFe\({}_{12}\)O\({}_{19}\). It was also proposed that charge separation between phases is responsible for the activity increase. However, in the contrast to the report by [PERSON] et al. [122], it was proposed that excited electrons react with H\({}_{2}\)O\({}_{2}\) on the surface of BaFe\({}_{12}\)O\({}_{19}\), whereas holes migrate to g-C\({}_{3}\)N\({}_{4}\), as shown in Figure 8. Scavenger-based experiments have confirmed an active role of \({}^{\bullet}\)OH, h\({}^{+}\) and \({}^{\bullet}\)O\({}_{2}\)\({}^{-}\) on the degradation efficiency. Reusability tests during four cycles showed a slight decrease in activity, from 98% to 91% degradation, during 100 min of irradiation. Quaternary composite SrFe\({}_{12}\)O\({}_{19}\)/SiO\({}_{2}\)/TiO\({}_{2}\)/GO was also proposed by [PERSON] et al. towards direct sunlight-driven photocatalytic degradation of 2,4-dichlorophenol  
Figure 7: The schematic illustration of charge carriers’ migration in BiFeO\({}_{3}\)-ZnFe\({}_{2}\)O\({}_{4}\) composite. Reprinted with permission from [119]. Copyright (2018) Creative Commons Attribution. (2_A_-DCP) [124]. An increased activity of 4-phase composite in comparison to pristine TiO\({}_{2}\) and TiO\({}_{2}\)/GO nanostructures was explained by the possible charge transfer between TiO\({}_{2}\) and GO and increased specific surface area of the composite. It was proposed that the ferrite contribution to the photocatalytic mechanism is insignificant because of its coverage with an insulating layer (SiO\({}_{2}\)). Moreover, it was shown that photocatalysts could be efficiently reused, as complete degradation of 2.4-DCP was observed during three cycles within 2.5\(-\)5 h (depending on the light intensity) of sunlight irradiation. Finally, hexagonal ferrites, combined with other photocatalysts, have also been intensively studied.