# Application of Spinel and Hexagonal Ferrites in Heterogeneous Photocatalysis
## 1 Introduction
However, it is obvious that worse photocatalytic activity is expected for immobilized than suspended photocatalysts due to (i) mass transfer limitations, (ii) surface area decrease, and (iii) lower number of active sites for photocatalytic reactions. Additionally, the stability of immobilized photocatalysts is challenging, since photocatalyst leakage (detachment from the support [22; 24; 27]) and the destruction of suspended supports (e.g., microsphere breaking under shear forces inside the photoreceptor [31]) have been observed. Accordingly, two different synthesis approaches have been proposed for efficient photocatalyst recovery: (i) mesoporous photocatalysts of micrometer size, thus allowing efficient separation by simple physical methods, i.e., sedimentation or filtration [37; 38; 39], and (ii) photocatalysts with magnetic properties. The latter is presented and discussed in this review. The application of an external magnetic field, proposed as an alternative method of photocatalyst recovery, means that separated material must be bifunctional, i.e., with photocatalytic and magnetic properties. Ferromagnets have been viewed as the most promising magnetic materials to be used for this purpose. Their various combinations with popular semiconductors have been proposed in different configurations, i.e., from disordered mixtures [40], through more complex structures, such as core-shells [41], to advanced multilayered materials [42]. It should be pointed out that ferromagnet-semiconductor composites could also form efficient heterojunctions, since several magnetic oxides exhibit photocatalytic activity themselves [43], and thus the heterojunction composite might be even more active than the sole photocatalyst forming it. However, this might also result in the instability of the composite, as ferrites are not as photostable as typical photocatalysts (mainly titania), and thus dissolution and leakage of active components could occur due to photocorrosion. Accordingly, an insulator interlayer between the photocatalyst and ferrites has been proposed in more advanced structures. In addition to the above-mentioned issues in the field of photocatalysis, attention should be paid to the fact that scientific knowledge in the subject of ab initio is still insufficient, as pointed out by, among others, [PERSON] et al. [44]. Ab initio is a computational chemistry method based on quantum chemistry, and in the case of a photocatalyst, it enables determination of, among others, exciton lifetimes, stability, the redox position of valence and conduction bands, bandgap, and adsorption energies. For example, [PERSON] et al. [45] explained the correlation between the packing factor and photocatalytic activity, whereas [PERSON] et al. [46] successfully investigated the geometric structures and electronic properties of Ni-doped TiO\({}_{2}\) anatase and rutile. The authors described the calculation and simulation by a plane wave pseudopotential method based on density functional theory (DFT). However, the ab initio calculations in the field of photocatalysis are still challenging due to the current problems in computational chemistry; the vast majority of advanced experimental systems cannot be recreated, even by the supercomputer systems [47]. Therefore, it is necessary to further develop this field with regard to photooxidation processes [48] as well as the aspects of the separation of photocatalysts after processing. This review presents all the aspects, focusing on the newest trends, applied for the use of spinel and hexagonal ferrites (selected groups of ferromagnets) in heterogeneous photocatalysis, as an efficient method for semiconductor recycling. surface area) and simple and cheap synthesis procedure. Recently, they have also been applied for photocatalytic purposes, as discussed in the next sections.