**6. Applications of catalytic activity**

*Advanced Functional Materials*

and safety [47].

investigated by Maksoud et al. The tested pathogens were gram-positive bacteria (*S. epidermidis, S. aureus, MRSA and E. faecalis, B. subtilis*), gram-negative bacteria (*A. baumannii, E. cloacae, E. coli, K. pneumoniae, P. aeruginosa*) and uni-cellular fungi (*C. albicans*). Zn-Co ferrite NPs displayed a maximum growth inhibition

The antimicrobial activity of co-doped Co0.5M0.5Fe2O4 NPs (M = Cu, Zn, Mn, Ni) obtained by the sol-gel process using citric acid as the chelating agent tested against *E. coli* and *S. aureus* revealed that substituted Co ferrite NPs exhibited the most effective biocidal property, while the substitution of Zn and Cu in Co ferrite

The applications of nanotechnology in various medical areas, especially in drug delivery have been extensively explored lately. Considering the ultra-small (1–100 nm) and controllable size, high surface-to-mass ratio and high reactivity of

The nanosized spinel ferrites and transition metal-substituted ferrites could successfully substitute some antibiotics that are currently used to combat pathogenic bacteria in the gastrointestinal tract of animals, as well as other biomedical applications. Many studies reported the synthesis and characterization of transition metal substituted Co ferrite NPs, but the attention dedicated to their biocompatibility in view of *in vivo* biomedical applications to assure their safe clinical use is still limited. The key criteria for their clinical applications are good biocompatibility

The use of magnetic nanoparticles in biomedical applications demands appropriate shape and size, high magnetization, good ability to deliver the pharmacologically active compounds, non-toxicity and biodegradability. The overall biocompatibility of Co0.5M0.5Fe2O4 (M = Cu, Zn, Mn, Ni) NPs synthetized by the sol-gel process using citric acid as chelating agent decreased in the following order: Co0.5Mn0.5Fe2O4 < Co0.5Cu0.5Fe2O4 < Co0.5Zn0.5Fe2O4 < Co0.5Ni0.5Fe2O4. The biocompatibility of NPs depended on the toxicity of transition metal and the releasing rate of transition metal ions into the cell culture medium [48]. Some possible mechanisms of magnetic NP-based antimicrobial drug delivery to microorganisms could be: (*i*) the NPs fuse with microbial cell wall or membrane and release the carried drugs into the bacteria cell; (*ii*) the NPs bind to cell wall and continuously release the drug, which diffuses into the interior of the microorganisms [49]. Iqbal et al. reported the development of Zn0.5Co0.5Fe2O4 NPs, with the required shape and size, as anti-cancer drug with passive targeting NPs delivering system into cancerous cells by applying photodynamic therapy through controlling the particle size

The applications of ferrites in tissue engineering are limited due to their inertness towards bioactivity and release of some toxic elements into the human body fluid. However, the migration can be controlled by encapsulation of ferrite NPs by glass matrix. The addition of bioglass in the ferrite displays some biodegradability and supports better osteoblasts growth *in vitro*. In this regard, the bioactive glass containing Co0.2Cu0.8Fe2O4 prepared using self-propagating high-temperature synthesis, showed good potential in bone hyperthermia application [50]. In magnetic hyperthermia, the ferrite NPs are used as local heat dissolving agents in external magnetic field. After their introduction into the body through blood, the body's immune system identifies them as foreign substances and the body rejects the material. To overcome this problem, the

against *K. pneumoniae*, *P. aeruginosa* and *C. albicans* [30].

NPs considerably enhanced the antibacterial activity [45].

NPs, they easily interact with biological systems [46].

according to the human body (HepG2) cells [40].

**5. Applications of biological properties**

**56**

The catalysts are important players in numerous chemical processes, especially in organic synthesis or decomposition of persistent pollutants. In the last decade, the use of magnetic NPs as catalysts attracted considerable interest due to the enhancement of the reaction speed, possibility of catalyst separation from the reaction medium by using an external magnet, without any filtration or centrifugation, and its reuse or recycling [53, 54]. In this regard, several conventional synthesis methods were replaced with more eco-friendly options that use magnetic nanosized catalysts. The catalytic processes that use magnetic NPs as catalysts include decomposition of recalcitrant organics, dehydrogenation, oxidation, alkylation and coupling reactions [9, 54].

The catalytic activity is influenced by the particle size, surface area, morphology, nature and concentration of the catalyst [10, 53]. In spinel ferrites, the presence of cations with different charges determines its catalytic properties as it allow internal redox reactions [55]. The distribution of metal ions between tetrahedral (A) and octahedral (B) sites also influences the catalytic activity. Thus, by doping transition metal ions in the ferrite structure, the cationic distribution is changed resulting in modified catalytic activity [10].

One of the main applications of magnetic NPs as catalysts is in the photocatalytic degradation or organics in the presence of visible or UV light. The photocatalytic activity of NPs is based on their capacity to efficiently absorb photons, that excite electrons from the valence band into the conduction band, leaving positively charged vacancy to react with the water molecules and to generate active radicals such are hydroxyl (·OH) or superoxide (·O2) that further react with the pollutants [56]. Beside the ability to absorb photons, the reusability, recyclability, low cost, chemical stability and high corrosion resistance are important factors in the selection of photocatalysts [57]. The crystallite size, surface area, band gap, cations distribution among tetrahedral (A) and octahedral (B) sites and magnetic properties are influenced by the dopant type and amount [10, 43].

In the last decades, a wide range of non-biodegradable organic dyes, inks and pigments were identified in wastewaters from the leather, textile, printing, paper, food and cosmetics industries. These dyes may pose carcinogenic and mutagenic risks and are difficult to treat using conventional water treatment methods. Nanosized Co ferrite is a magnetic material with high *HC* and moderate *MS*, narrow band gap, low toxicity, low price and good catalytic activity [10, 58]. By doping, its structural and catalytic properties may be further enhanced. The doped and codoped Co ferrites are promising catalysts that may decompose recalcitrant organic chemicals from wastewaters or enhance the synthesis of organics [9, 52, 56, 59–61]. The doping of transition metal ions (Cr, Mn, Co, Zn) into the spinel lattice of Co ferrite influences the physicochemical properties and improves their stability [11]. Moreover, the doping favors the formation of mixed or inverse spinel structures and introduces new donor or acceptor levels, which boosts the visible light activated photocatalyst activity [62].

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 and UV-light [64].

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 and Cu content, respectively [57, 65].

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 electron transfer rate [69].

## **7. Applications of dielectric properties**

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

**59**

applications.

*Progress, Challenges and Opportunities in Divalent Transition Metal-Doped Cobalt Ferrites…*

by highly resistive grain boundaries. The resistive grain boundaries are more effective at low frequencies, while the highly conducting grains act at high frequencies [29]. The dielectric constant at low applied frequencies is determined by the space charge polarization favored by the electrons grouped along the resistive grain boundary [6, 29]. In case of doped Co ferrites, the dielectric constant depends on the content of Fe3+, Co2+ and other divalent transition ions present in the spinel structure [6]. Generally, the dielectric properties are strongly influenced by the grain size, porosity, synthesis method and annealing temperature [6, 29].

multifunctionality led to the necessity of new materials with special characteristics. Considering the structural, electric and magnetic properties, the nanosized ferrites may become important candidates for applications in microwave communication systems, electromagnetic devices, resonators and filters for satellites, broadcasting equipment, batteries, supercapacitors and many other microwave devices [70]. The dielectric structure of ferrites consists in well conducting grains separated

Considering its good chemical and thermal stability, high electrical resistivity, magnetic anisotropy, high *HC*, moderate *MS*, superparamagnetism, ferrimagnetism and dielectric structure, the Ni doped Co ferrite is a good candidate for microwave devices and data storage [71]. The Co1−xNixFe2O4 (0.0 ≤ x ≤ 1.0) synthesized using simple, low temperature auto-combustion method showed high resistance. As the Ni content increases, the dielectric constant and loss tangent decrease and remain constant at higher frequencies, while conductivity increases with increasing frequency [71]. The use as supercapacitors of mixed ternary Cu-Co-Ni ferrites obtained by sol-gel synthesis and citric acid as chelating agent was investigated by Bhujun et al. [72]. The cyclic voltammogram profiles showed that the capacitive behavior is close to ideal rectangular shape, confirming the reversibility of the system and the decrease of specific capacitance value with the increase of the cycle numbers. The specific capacitance of Cu0.5Co0.5Fe2O4 (221 Fg−1) was higher than of Ni0.5Co0.5Fe2O4

MnxCo1−xFe2O4 (x = 0.2, 0.4, 0.6, 0.8) NPs synthesized by sol-gel precipitation method have dielectric properties that decrease with the increase of the doping ratio for x = 0.2–0.6 [29]. However, the Mn0.8Co0.2Fe2O4 was found to have the highest dielectric constant (8.38) at 100 Hz, due to the increasing porosity and grain boundaries between the small sized grains. The low *HC* and low dielectric loss between 100 and 100 kHz indicate its potential use as inductor and transformer for switch-mode power supplies [29]. MnCoFeO4 NPs with the average particle size in the range of 30–40 nm synthesized via a simple one-pot co-precipitation method were also proven to be suitable as

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

*DOI: http://dx.doi.org/10.5772/intechopen.93298*

(60 Fg−1) and showed excellent cycling stability [72].

high-performance capacitors for electrical energy storage [73].

**8. Conclusions and future perspectives**

#### *Progress, Challenges and Opportunities in Divalent Transition Metal-Doped Cobalt Ferrites… DOI: http://dx.doi.org/10.5772/intechopen.93298*

multifunctionality led to the necessity of new materials with special characteristics. Considering the structural, electric and magnetic properties, the nanosized ferrites may become important candidates for applications in microwave communication systems, electromagnetic devices, resonators and filters for satellites, broadcasting equipment, batteries, supercapacitors and many other microwave devices [70].

The dielectric structure of ferrites consists in well conducting grains separated by highly resistive grain boundaries. The resistive grain boundaries are more effective at low frequencies, while the highly conducting grains act at high frequencies [29]. The dielectric constant at low applied frequencies is determined by the space charge polarization favored by the electrons grouped along the resistive grain boundary [6, 29]. In case of doped Co ferrites, the dielectric constant depends on the content of Fe3+, Co2+ and other divalent transition ions present in the spinel structure [6]. Generally, the dielectric properties are strongly influenced by the grain size, porosity, synthesis method and annealing temperature [6, 29].

Considering its good chemical and thermal stability, high electrical resistivity, magnetic anisotropy, high *HC*, moderate *MS*, superparamagnetism, ferrimagnetism and dielectric structure, the Ni doped Co ferrite is a good candidate for microwave devices and data storage [71]. The Co1−xNixFe2O4 (0.0 ≤ x ≤ 1.0) synthesized using simple, low temperature auto-combustion method showed high resistance. As the Ni content increases, the dielectric constant and loss tangent decrease and remain constant at higher frequencies, while conductivity increases with increasing frequency [71]. The use as supercapacitors of mixed ternary Cu-Co-Ni ferrites obtained by sol-gel synthesis and citric acid as chelating agent was investigated by Bhujun et al. [72]. The cyclic voltammogram profiles showed that the capacitive behavior is close to ideal rectangular shape, confirming the reversibility of the system and the decrease of specific capacitance value with the increase of the cycle numbers. The specific capacitance of Cu0.5Co0.5Fe2O4 (221 Fg−1) was higher than of Ni0.5Co0.5Fe2O4 (60 Fg−1) and showed excellent cycling stability [72].

MnxCo1−xFe2O4 (x = 0.2, 0.4, 0.6, 0.8) NPs synthesized by sol-gel precipitation method have dielectric properties that decrease with the increase of the doping ratio for x = 0.2–0.6 [29]. However, the Mn0.8Co0.2Fe2O4 was found to have the highest dielectric constant (8.38) at 100 Hz, due to the increasing porosity and grain boundaries between the small sized grains. The low *HC* and low dielectric loss between 100 and 100 kHz indicate its potential use as inductor and transformer for switch-mode power supplies [29]. MnCoFeO4 NPs with the average particle size in the range of 30–40 nm synthesized via a simple one-pot co-precipitation method were also proven to be suitable as high-performance capacitors for electrical energy storage [73].
