1 Introduction

Spinel ferrite nanoparticles (SFNPs) have been widely used several applications such as photodegradation of organic pollutants [1,2,3,4,5,6,7,8], gas sensors [9], high-recurrence gadgets [10], and water-splitting [11,12,13]. SFNPs also are useful in increasing reaction yield, lowering reaction temperatures, and catalyzing certain chemical reactions. Several investigations have been carried out which establish the photocatalytic as well as antibacterial activity of SFNPs [14,15,16,17,18]. Recent research has also proven that Zn (II) substituted cobalt ferrites are more effective in comparison to Mn(II) and Cu(II) substituted cobalt ferrites [19, 20] as antibacterial materials. Due to their low energy band gap, high surface area/volume ratio, and ease of synthesis, SFNPs are viewed as valuable photocatalysts [21,22,23,24,25,26,27,28]. The upcoming section discusses the synthesis techniques employed for designing SFNPs.

1.1 Structure of spinel ferrite nanoparticles (SFNPs)

SFNPs are metal oxides having a spinel structure with the general formula AB2O4, where A and B address different metal cations situated at the tetrahedral (A site) and octahedral (B site). The metal cations are tetrahedrally/octahedrally coordinated to oxygen atoms at both the sites. The physico-chemical properties are significantly impacted by the type, quantity and composition of ferrites [29,30,31]. On the basis of the distribution of cations at the tetrahedral (A) and octahedral (B) sites, SFs have been classified into three categories; normal spinel ferrites, inverse spinel ferrites and mixed spinel ferrites. In normal SFs, the divalent cations M2+occupy tetrahedral (A) sites while the trivalent cations Fe3+ are on octahedral (B) site. The structural formula of such ferrites is [M2+](A)[Fe23+](B)O42−) (for example ZnFe2O4, CdFe2O4) [32]. The structure of normal spinel is depicted in Fig. 1 [33].

Fig. 1
figure 1

Structure of spinel ferrite reproduced from reference no [33]

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Inverse spinel ferrites are those that have the trivalent ions evenly distributed between the A and B sites and the divalent ions only on the B site. The standard formula for inverse spinel structure is ([M2+Fe3+] B [Fe3+] A O4) (for example, FeOFe2O3, NiFe2O4, CuFe2O4 and CoFe2O4). In this structure, half of the Fe3+ ions occupy A-sites while the remaining occupies B-sites [34].

In mixed spinel ferrites, the divalent cations M2+ and the trivalent cations Fe3+ ions are distributed at both the octahedral (B) site and the tetrahedral (A) site (for example MnFe2O4) [35]. The structure of inverse spinels is illustrated in Fig. 2 [36].

Fig. 2
figure 2

Structure of Inverse Spinels derived from reference no [36]

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The basic rule to determine whether the structure is normal spinel or inverse spinel can be understood on the basis of crystal field stabilization energy (CFSE) as: For normal spinel’s CFSE of B+3 ions in Octahedral Field > CFSE of B+3 ions in Tetrahedral Field > CFSE of A+2 ions in Octahedral Field > CFSE of A+2 ions in Tetrahedral Field [i.e. if CFSE of B+3 ions in Octahedral Field is largest of all CFSE]. For inverse spinel’s CFSE of A+2 ions in Octahedral Field > CFSE of A+2 ions in Tetrahedral Field > CFSE of B+3 ions in Octahedral Field > CFSE of B+3 ions in Tetrahedral Field [i.e. if CFSE of A+2 ions in Octahedral Field is largest of all CFSE].

2 Synthesis techniques of spinel ferrites (SFs)

SFs are prepared using a variety of methods, including electrochemical, hydrothermal, micro emulsion, sol–gel, hydrothermal, solid-state, sono-chemical, precipitation and template methods. Each of these processes have advantages and limitations of its own which are discussed in the upcoming sections.

2.1 Sol-gel method

Metal alkoxide are utilized in the sol-gel process, which undergo hydrolysis and buildup a gel at room temperature. The sol-gel mixture strategy enjoys a few benefits such as solid stoichiometric control, and lack of utilization of sophisticated modern instruments. It is one of the most widely adopted techniques for controlling the microstructure. It is possible to blend ferrite materials with a restricted size propagation and controlled shape by changing the given temperature in sol-gel somewhere in the range of 25 °C - 200 °C. The sol-gel approach is utilized to synthesize simple spinel ferrites such as CuFe2O4, ZnFe2O4, NiFe2O4, CdFe2O4 [28]. The sintering temperature, sintering time, and addition of dopants impact the physicochemical properties of spinel ferrites, Fig. 3.

Fig. 3
figure 3

Schematic representation of synthesis of SFs by Sol-Gel method

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2.2 Co-precipitation method

This is one of the oldest methods to obtain spinel ferrites. The arrangement in this approach contains a combination of divalent and trivalent progress metals in a mole proportion of 1:2. As a source of trivalent metal particles, dissolvable salts containing Fe (III) are regularly used, and the test is normally completed in a basic media. To produce high-quality ferrite materials, the synthesis procedure demands precise pH adjustment and which is commonly provided by Ammonium or sodium hydroxide solutions. The solution is then vigorously stirred or agitated in the absence or presence of heat under inert conditions. To obtain phase pure ferrites, the resultant powder must be annealed at various temperatures ranging from 500 to 100 °C, Fig. 4 [37].

Fig. 4
figure 4

Schematic representation of synthesis of SFs by precipitation method

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2.3 Hydrothermal method

The hydrothermal method is one of the ecofriendly and promising synthesis methods available for the synthesis of SFs. In this method an iron salt such as Fe(NO3)3.9H2O/ FeCl3.6H2O, and a salt of the metal is used, commonly M(NO3)2, MSO4, or MCl2. Depending on the type of metal salt being used, salts are dissolved in water or another solvent while being stirred, and the pH is then adjusted to a range between 7 and 12. The mixture is heated for 12 to 24 h in an autoclave before naturally cooling to room temperature. Followed by washing and filtration or centrifugation to obtain the solid, it is then overnight dried at around 85 °C. The precursor chemicals are pulverized collectively in a ball-mill for mechano-thermal treatment, rather than dissolving. The Fe (III) salt is added to a metal oxide (M2O3) seed when using seed-hydrothermal procedures. The identical process as previously described above is followed while heating the two compounds in an autoclave, Fig. 5 [38].

Fig. 5
figure 5

Schematic representation of synthesis of SFs by hydrothermal method

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2.4 Solvo-thermal method

The solvothermal method is utilized to make ferrite materials with enhanced physical and chemical properties, which can be used in biomedical applications. In order to facilitate the interaction of precursors during synthesis, the procedure uses a solvent under moderate to high pressure (usually between 1 atm and 10,000 atm) and temperature (generally between 100 °C and 1000 °C). Fluid or non-watery solvents can be used to deliver ferrite materials with accurate command over size dispersion, shape, and glasslike progressively works in the solvothermal process. Certain trial factors, like reaction temperature, reaction duration, solvent, surfactant, and antecedents, can be changed to differ these actual highlights. Utilizing the solvothermal combination process, the no of ferrite materials and composites have been made, Fig. 6 [39].

Fig. 6
figure 6

Schematic representation of synthesis of SFs by Solvothermal method

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2.5 Micro- emulsion method

In this method small modification to the synthesis conditions easily produces nanoparticles with a restricted size distribution and size control. The three components of this method—water, oil, and surfactant allow for the control of a number of parameters, including particle size, geometry, morphology, homogeneity, and surface area [40]. It usually involves two sub-methods one utilizing oil/water and other water/oil. In both cases surfactants are used always in concentration which is above the critical micelle concentration (CMC).

2.6 Comparison of synthesis methods

There are also large number of methods available that are utilized in the synthesis of SFs, like sono-chemical[41], electro-chemical, [42], and microwave assisted method [43]. The properties of SFs are strongly dependent upon the method utilized for the synthesis, thus preparation method can be selected on the basis of desired application. Every method has its set of advantages and disadvantages. The advantages and disadvantages of various synthesis methods are shown in Table 1.

Table 1 Advantages and disadvantages of different routes of synthesis of SFs
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3 Techniques adopted for characterizing SFNPs

Understanding and surveying the results of SFNPs requires an exhaustive investigation of their physico-chemical features. There are different ways for describing FNPs, each with different set of information and nature of data, for example: X-ray diffraction (XRD), Raman spectroscopy (RS), atomic force microscopy (AFM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), superconducting quantum interference device (SQUID), Fourier transform infrared (FT-IR) spectroscopy, Energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy(XPS), thermogravimetric Analysis (TGA) [44,45,46,47,48].

3.1 Size and shape

FNP's size and shape are the main qualities that decide their strength and attractive properties [49]. Further developed approaches are expected to decide the exact size of SFNPs, particularly those under 20 nm [50]. For deciding the size and state of SFNPs, number of approaches has been explored.

The most well-known procedures are high-resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FE-SEM). The fundamental way of operation of nanoparticles can be understood utilizing these strategies. Scherer's condition is utilized to compute the size of SFNPs from XRD diffraction. TEM results provide size affirmation, Fig. 7 [51]. X-ray photoelectron spectroscopy, Mossbauer spectroscopy, and dynamic light scattering (DLS) are a section of different strategies used to decide mean molecule size.TEM estimations of Fe3O4 and Fe3O4-alpha-olefin sulphonate (AOS) NPs, for instance, uncovered normal sizes of 10 nm -20 nm and 5 nm -10 nm, which were in agreement with the DLS studies, Fig. 8 [52].

Fig. 7
figure 7

(Reproduced from Ref. [51], Copyright 2015, Elsevier)

TEM pictures and XRD examples of Fe3O4 NPs with size 5–15 nm

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Fig. 8
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DLS estimations of: (a) unadulterated Fe3O4 NPs and, (b) Fe3O4-AOS NPs covered with twofold layered AOS. (Ref. [52], Copyright 2015, Elsevier)

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3.2 Elemental and mineral composition

Inductively coupled plasma mass spectrometry (ICP-MS) can be utilized to decide the basic composition of SFNPs with more accuracy and recognition limits [53, 54]. ICP-OES and atomic absorption spectroscopy (AAS) are also utilized for exploring the compositions of SFNPs.

3.3 Structure and bonding

XPS, FT-IR, TGA, Raman spectroscopy (RS), and X-beam absorption spectroscopy are some of the known strategies used to research primary and promising properties. (XAS), XPS and FT-IR are utilized to determine the foundation of metal–oxygen bonds, providing recognizable proof of the presence of spinel structure as well as the presence of other substances adsorbed on the outer layer of particles. The oxidation state, restricting energy, and essential substance of materials on a superficial level can be generally resolved utilizing XPS. Figure 9 shows the XPS spectra of Fe3O4, MnFe2O4 and CoFe2O4 [25]. The zeta potential Fig. 9(a), of CoFe2O4 is similar to that of Fe3O4, but MnFe2O4 showed higher value than Fe3O4. The XRD patterns of MnFe2O4, CoFe2O4 and Fe3O4 were indexed to spinel ferrites, and Fe3O4 to cubic crystalline bulk magnetite. The XPS profiles show that the surface molar ratio of Fe/Mn or Fe/Co for MnFe2O4, CoFe2O4 are 2:1, in accordance with the metal ion ratio taken in solution. The remanence magnetization values of MnFe2O4, CoFe2O4 and Fe3O4 were 3.49, 8.46 and 0.70 emu/g, while maximal saturation magnetization of MnFe2O4, CoFe2O4 and Fe3O4 were 32.02, 46.99 and 55.41 emu/g respectively.

Fig. 9
figure 9

(a) Zeta potential as a function of pH; (b) X-ray diffraction pattern; (c) wide XPS scan; and (d) VSM curves of Fe3O4, MnFe2O4, and CoFe2O4 MNPs. (Ref. [25], Copyright 2010, Elsevier)

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The FT-IR spectra of NiFe2O4, CoFe2O4 and ZnFe2O4 is shown in Fig. 10 [55]. The spectra shows metal–oxygen stretching vibration from the tetrahedral site to be around 584–540 cm−1 range and for the octahedral site to be in the range of 397–389 cm−1.

Fig. 10
figure 10

IR Spectra of NiFe2O4, CoFe2O4 and ZnFe2O4 reproduced from Reference no [55]

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3.4 Surface morphology

AFM, TEM, and SEM could be utilized to operate the surface morphology of SFNPs. These techniques are normally used to imagine and quantify the state of nanoparticles. TEM, specifically, is a more adaptable procedure for getting substance, morphological, and crystallite data on FNPs [56]. It can convey more prominent goal down to the nuclear level. TEM can make nuclear scale basic guides for compositional investigation when utilized related to other spectroscopic instruments like EDS and electron energy loss spectroscopy (EELS), Fig. 11(a-e) [57].

Fig. 11
figure 11

(a) HAADF-STEM images of nanoparticles showing spinel structure in magnetite cores. (b) integrated map of the Mn L2,3 EELS signal, where green intensity shows the Mn distribution. (c) integrated map of the Fe L2,3 EELS signal, where red intensity shows the Fe distribution. (d) Composite image showing Fe-rich core and Mn-rich shell in the nanoparticles

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3.5 Magnetic measurements

EPR, VSM, and SQUID are popular techniques used to examine and measure the magnetic characteristics of SFNPs. EPR is a delicate procedure for detecting and identifying free radicals and paramagnetic centers in chemicals and chemical processes, such as the F-center. When high sensitivity magnetization measurements are necessary, VSM and SQUID are used, especially because VSM's sensitivity limit is around 106 emu [58].

3.6 Properties of Spinel Ferrites

On the basis of distribution and nature of the cations on the tetrahedral (A) and octahedral (B) sites, SFs can be divided into different catogeries as: paramagnetic, diamagnetic, ferromagnetic, ferrimagnetic, antiferromagnetic, and superparamagnetic [59]. Superparaamagnetic nanoparticles are very useful in wastewater treatment. Superparaamagnetism is observed in small ferro and ferri magnetic nanomaterials (3–50 nm) [60].The spinel ferrites usually have low band gap energy values which makes them suitable for processes like photocatalysis [61].Spinel ferrites are widely used at microwave frequencies because they have low electrical conductivities when compared to other magnetic materials. These materials are typically semiconductors with conductivities between 102 and 10–11 Ω−1 cm−1.

4 Photocatalytic applications of SFNPs

The presence of various toxins in modern wastewater release is an overall concern. Harmful natural poisons like colors, phenols, chloro-phenols, and nitro-phenols have for quite some time been an issue to treat and eliminate, particularly when presented to visible light. A few results have shown that these pollutants can be eliminated using SFNPs as photocatalyst, and it is one of most extensively investigated area of research [61,62,63,64]. SFNPs incorporate their capacity to work as a photocatalyst (without adjustment), in composites (surface covered with semiconductors), and within the sight of oxidants like H2O2. Under visible light, an electron (e) is stimulated from the valence band (VB) to the conduction band (CB) of SFNPs leaving a hole (h+). At the CB (reaction 1), the deposited e− is successfully polished off by separated oxygen gas present in the water [65, 66].The hole (h+) created by photolytic exposure joins with water to deliver vigorous hydroxyl free radicals (reaction 3). The contaminants are in closeness to the vigorous radicals because of the great adsorption limit of SFNPs, and are hence immediately gone after by hydroxyl radicals framed on the outer layer of the photocatalyst, (reaction 4) [66]. The degraded fragments are then desorbed from the ferrite surface regions. Figure 12 shows a schematic of the process of photocatalysis.

Fig. 12
figure 12

Schematic outline showing the arrangement of receptive oxygen and hydroxyl extremist under apparent light illumination of semiconductor or ferrite-semiconductor composite, connection with natural contaminations and conceivable corruption items

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$${\mathrm e^-\hspace{0.17em}+\hspace{0.17em}{\mathrm O}_2\hspace{0.17em}\rightarrow\hspace{0.17em}{\mathrm O}_2}^{\bullet-}$$
(1)
$${{{\mathrm{O}}_{2}}^{\bullet -}-\hspace{0.17em}+\hspace{0.17em}{2\mathrm{H}}^{+}\hspace{0.17em}+\hspace{0.17em}{2\mathrm{e}}^{-}\hspace{0.17em}\to \hspace{0.17em}}^{\bullet }\mathrm{OH}\hspace{0.17em}+\hspace{0.17em}{\mathrm{OH}}^{-}$$
(2)
$${{\mathrm{h}}^{+}\hspace{0.17em}+\hspace{0.17em}{\mathrm{H}}_{2}\mathrm{O}\hspace{0.17em}\to }^{\bullet }\mathrm{OH}\hspace{0.17em}+\hspace{0.17em}{\mathrm{H}}^{+}/{\mathrm{OH}}^{\bullet }$$
(3)
$$\dot{}\mathrm{OH}\hspace{0.17em}+\hspace{0.17em}\mathrm{Organic\,contaminant}\,(\mathrm{OC})\hspace{0.17em}\to \hspace{0.17em}{\mathrm{CO}}_{2}(\mathrm{ g})\hspace{0.17em}+{\mathrm{H}}_{2}\mathrm{O }(1)$$
(4)
$${\mathrm{e}}^{-}+\hspace{0.17em}\mathrm{OC}\hspace{0.17em}\to \hspace{0.17em}{\mathrm{OC}}^{-}\to \hspace{0.17em}\mathrm{degradation\,products}$$
(5)
$${\mathrm{h}}^{+}+\hspace{0.17em}+\hspace{0.17em}\mathrm{OC}\hspace{0.17em}\to \hspace{0.17em}\mathrm{OC}+\hspace{0.17em}\to \hspace{0.17em}\mathrm{degradation\,products}$$
(6)

It is reported that Fe3O4@SiO2 NCs, work as a Fenton-like impetus for the breakdown of H2O2 and bring about a high pace of discoloration of MB at neutral pH [67]. In another work, high strength of Fe3O4@TiO2 was seen with close to 100% MB degradation following 5 min of H2O2 dilution without any UV illumination [68]. MnFe2O4/GSC and MnFe2O4/BT NCs were utilized to photo catalytically degrade ampicillin (AMP) and oxy antibiotic medication (OTC). About 96- and 83%-AMP degradation and 99% and 90% OTC degradation was achieved following 60 and 120 min, respectively [69]. The photocatalytic movement of pyrrole engraved CoFe2O4/MWCNTs NCs for the degradation of 2-mercaptobenzothiazole was explored in another review [70]. Xiong et al. [71] coordinated CdSZnFe2O4(0.10) (CdS-with 10% ZnFe2O4) and CdSCoFe2O4(0.05) and assessed their photocatalytic impact on RhB degradation. 80% of RhB was degraded using the SFNPs but only 30% of RhB was disintegrated during the third cycle, showing an impressive drop in photocatalytic action. Figure 13(a), (b) shows the schematic of RhB degradation using SFNP.

Fig. 13
figure 13

(Reproduced from ref. [71])

(a) Recyclability of CdS, CdS-ZnFe2O4(0.10), and CdS-CoFe2O4(0.05) for three cycles, (b) photocatalytic action of the SFNPs

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5 Conclusion

The review highlights the synthesis and photocatalytic applications of SFNPs. On account of their low energy band gap, remarkable magnetic properties, they are utilized in various applications. The diversity in ferrite group nanoparticles can accommodate various elements in different oxidation states which are outstanding and industrially beneficial. There is scope for improvement in the synthesis strategies which can increase the diverse applications of these materials besides photocatalysis. These nanomaterials serve as good research niche for further applications in a variety of organic catalytic processes.