**8. Conclusions and future perspectives**

The Co ferrite continues to attract considerable attention due to its unique and exciting properties and opens new doors towards many potential applications. The properties of Co ferrite can be easily controlled by preparation technique, morphology, dopants type/content and cation distribution between tetrahedral (A) and octahedral (B) sites. There is a high number of studies that reported the physical, chemical, magnetic, electrical and optical properties of undoped and doped Co ferrites. Also, an increasing interest towards the incorporation of newer ions into the Co ferrite lattice in order to tailor its properties was noticed. The excellent properties of divalent transition metal doped Co ferrites, together with the possibility to tailor their particle size, shape, purity and chemical composition became a promising alternative for future generation nanomaterials designed for various industrial, environmental and medical applications.

*Advanced Functional Materials*

and UV-light [64].

and Cu content, respectively [57, 65].

electron transfer rate [69].

**7. Applications of dielectric properties**

The photocatalytic activity of a wide range of doped Co ferrites were tested on rhodamine B (RhB), methyl orange (MO), methylene blue (MB) and congo red (CR), synthetic dyes known to be highly toxic and carcinogenic. Nanocrystalline magnetic ZnxCo1−xFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) with good photocatalytic activity was obtained by reverse micelle technique [63]. The doping with Zn increased the RhB degradation rate and reduced the degradation time, while, the band gap increased with increasing Zn content [63]. The photocatalytic degradation of CR and Evans blue by ZnxCo1−xFe2O4 (x = 0.0, 0.2, 0.4, 0.6) prepared using curd as a fuel through the combustion method was found also to increase with the increase of Zn doping up to x = 0.4, suggesting that Zn doped Co ferrite are better photocatalysts than Co ferrite [43]. The photocatalytic activity of MxCo1−xFe2O4 (M = Zn, Cu, Mn; x = 0.0, 0.25, 0.50, 0.75) NPs synthesized by citrate sol-gel method enhanced with increasing M content, but were lower than that of undoped Co ferrite in case of

M = Cu and Zn and higher in case of M = Mn used for MB degradation [31].

The photocatalytic performance of Co0.6Zn0.4CuxFe2 − xO4 (x = 0.2, 0.4, 0.6, 0.8 and 1.0) obtained by sol-gel auto combustion method was evaluated by MO dye degradation under visible light and presence of hydrogen peroxide. The results showed that the degradation of MO enhances as the content of Cu in Co-Zn ferrites increases, due to the strong preference of Cu2+ ions for the octahedral (B) sites [61]. The photocatalytic degradation of CR by Cu0.5Co0.5Fe1.9Bi0.1O4 NPs obtained by solution combustion technique was found to have around 90% efficiency, the photocatalyst being stable and reusable [39]. High removal percentage of CR and bisphenol A was reported for Co0.5Cu0.5Fe1.95Ce0.05O4 after exposure to both visible

The Zn1−xCoxFe2O4 (x = 0.03, 0.1, 0.2) and CuxCo0.5 − xNi0.5Fe2O4 (x = 0.1, 0.2, 0.3, 0.4) NPs obtained by facile reduction-oxidation route and respectively precipitation method in the presence of oleic acid as a surfactant, were found to be able to photodegrade MB, the degradation efficiency decreasing with the increase of Zn

Despite the high number of applications of transitional metal doped Co ferrites in the photocatalytic decomposition of various organic pollutants, there are only few studies on their use in organic synthesis. The Ni-substituted Co ferrite NPs supported on arginine-modified graphene oxide nanosheets (Ni0.5Co0.5Fe2O4@Arg–GO) were proven to be effective for the one-pot tandem oxidative synthesis of 2-phenylbenzimidazole derivatives [66]. NixCo1−xFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) ferrite NPs obtained by microemulsion method were found to effectively reduce 4-nitrophenol to 4-aminophenol in the presence of NaBH4 as reducing agent [67].

Another important application of magnetic spinel NPs is in the field of renewable energy production and storage as catalysts for driving the water electrolysis by enhancing the hydrogen and oxygen evolution reactions (HER, OER). Ni-Co ferrite (Co0.5Ni0.5Fe2O4) anchored on ultrathin conductive graphene oxide nanosheets acts as a highly active, stable and low-cost electrocatalyst in the water splitting processes, being a low-cost alternative to noble metal-oxides catalyst [68]. The OER and HER catalytic activity of CoxNi1−xFe2O4 (x = 0.0, 0.25, 0.5, 0.75, 1.0) NPs prepared by citric acid assisted sol-gel combustion method was found to be lower than bulk Ni ferrite, the Ni content increase improving the catalytic activity and the

The significant progress in information technology, electronics and wireless communication devices together with a new trend of miniaturization and

**58**
