# Application of Spinel and Hexagonal Ferrites in Heterogeneous Photocatalysis
## 6 Conclusions and Future Perspectives  
The literature review reflects the recent need to search for innovative combinations of semiconductors with ferrite NPs for heterogeneous photocatalysis. Various studies have been carried out at multiple research centers worldwide, being largely focused on the choice of appropriate ferromagnets to produce advanced magnetic photocatalytic systems. Based on the presented literature, it was found that spinel and hexagonal ferrite magnetic NPs, despite their magnetic properties enabling easy separation, have received moderate attention as the main photocatalytic material, probably because of lower activity than well-known photocatalysts (e.g., TiO2) and stability concerns. However, they could be an interesting component of photocatalytic materials, with a possible synergistic effect, resulting in increased activity under UV light and the appearance of vis response. However, their stability (even though, in many cases, it is much better than single-component photocatalysts) should be improved for possible commercialization, as most lose activity during recycling.  
Moreover, though ferrites have proven to be a suitable choice for the separation of the multi-phase photocatalysts, there are still some unknown aspects of their operation in the photocatalytic systems. Above all, the ability of the ferrite materials to generate reactive oxygen species by themselves; as well, further identification of the main ROS and the positions of CB and VB edges of such ferrite materials are usually missing. This leadsto confusing results reported in different studies. For example, opposite charge transfers between the same components was claimed (e.g., BaFe\({}_{12}\)O\({}_{19}\)/g-C\({}_{3}\)N\({}_{4}\)[122; 123]) or the oxidative role of \({}^{\bullet}\)OH radicals in the presence of Ba-ferrite [78; 81] was suggested, whereas other studies excluded such mechanisms [131; 137]. Since water purification from organic pollutants remains the most investigated application of the magnetic photocatalysts, these details, as well as the possible factors that affect them, are considered as crucial for further studies to better understand the interactions between hexaferrites, other photocatalysts, and pollutants. Moreover, the surface structure of the magnetic materials is usually not considered, though many reports on other photocatalysts have proven how important it is for efficient photocatalytic performance [111; 144]. In this regard, surface-based studies of such materials are highly recommended as another direction for further research.  
It is also thought that the mechanism of ferrite-based photocatalysts needs further investigation, as only a few studies discussed the possible mechanism in detail (showing direct proof instead of speculation). Moreover, though the use of dyes is very convenient for fast and easy activity testing (and important for environmental purification), it is not recommended for mechanism discussion due to the additional mechanisms of photocatalyst sensitization (barely discussed in the presented papers).  
Even though there are many unsolved issues and unknown aspects, it is thought that the study of the creation of a new generation of ferromagnetic-photocatalytic systems will grow as a result of evolving strategies for environmental protection, since easy and inexpensive separation of photocatalysts for reuse is highly important.  
Writing--original draft preparation, S.D., A.K., Z.B. and E.K.; Writing--review and editing, S.D., A.K., Z.B. and E.K. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability StatementData are available in a publicly accessible repository.  
The authors declare no conflict of interest.