TiO₂ has four polymorphs: brookite (orthorhombic), rutile (tetragonal), anatase (tetragonal) and TiO₂-B. Anatase, rutile, and brookite phases have bandgaps of 3.2, 3.0, and 3.4 eV, respectively. Brookite is less thermodynamically stable, but rutile is stable and anatase is metastable. At temperatures exceeding 600 °C, irreversible conversion of brookite and anatase TiO₂ to rutile occurs [44].

It has been observed that bandgaps of TiO₂ nanoparticles with 5–10 nm particles narrow up to 0.2 eV. Anatase is the most photocatalytically effective phase of TiO₂ [45] and is widely used in semiconducting materials for environmental applications [46]. It has higher mobility of electron–hole pairs and higher surface hydroxyl density. Brookite-phase activity has not been thoroughly examined [47,48]. Rutile TiO₂ is usually investigated in fundamental investigations. Due to the existence of oxygen vacancies, TiO₂ is classed as an n-type semiconductor [49]. Because crystalline TiO₂ has a less photoexcited electron–hole recombination than amorphous TiO₂, it has higher photocatalytic activity [50,51]. Recombination of photogenerated charge carriers is a disadvantage of TiO₂ photocatalysts, as it decreases overall reaction quantum efficiency [52]. During the recombination processes, the photoexcited electrons return to the valence band non-radiatively or radiatively [53,54,55]. An attractive feature of the TiO₂ photocatalyst is its potential to be activated by visible light. To increase photocatalytic activity and decrease recombination of photoexcited charge carriers, several methods have been followed, such as heterojunction formation and doping with non-metals, metals, and nanosized crystals, which can alter the electronic and optical properties of TiO₂ [56,57,58,59,60]. By using dopants in TiO₂, its bandgap reduces [61]. Metal doping has long been known to improve visible light absorbance of TiO₂ [62,63] and increase its photocatalytic activity under UV irradiation [63,64,65,66], but the introduction of metal ion results in thermal instability, which reduces the reuse of TiO₂ photocatalyst and recombination of electron–hole pairs occur at a fast rate. Non-metal doping is more efficient than metal ion doping due to more thermal stability and less formation of recombination centers [67,68,69,70,71].

Nanocomposites of ferrites with TiO₂ have been synthesized using different chemical methods. A summary of methods used to synthesize magnetic nanocomposites has been discussed in this section.

In typical sol–gel synthesis, formation of gel materials with three-dimensional structure occurs by condensation and hydrolysis reaction of metal ions. It is an irreversible process: once gel is formed, it cannot break down. There is covalent interaction between gel particles. In this method, huge precursors are used during conversions and can be performed at or near room temperature. For chemical catalysts, porous ceramic xerogels with large surface area act as support and thin-film deposition is important for material possessing desired optical and magnetic properties [24]. This method uses simple equipment, produces highly homogeneous and pure products at low temperatures, and allows modification of the surface.

Tatarchuk et al. [72] developed magnetic nanocomposites of CoFe₂O₄@TiO₂ using the Pechini sol–gel method with ethylene glycol and citric acid as chelating agents and titanium (IV) polymeric precursor solution. XRD study of CoFe₂O₄@TiO₂ indicated the presence of 46% and 54% of anatase and rutile crystal phase, respectively, in the titanium dioxide (TiO₂) component, and the average size of cobalt ferrite (CoFe₂O₄) and TiO₂ nanoparticles was about 30 and 8 nm, respectively. SEM revealed that because of deposition of TiO₂ on the CoFe₂O₄ surface, particles became large. The IR spectrum of the nanocomposite illustrated the shift of bands, which was the result of isomorphic heterovalent substitution in the octahedral position. Dadfar et al. [73] and Xu et al. [74] fabricated SrFe₁₂O₁₉/TiO₂ and TiO₂/ZnFe₂O₄ nanostructures by the sol–gel method. XRD indicated the presence of impurity phases (�-Fe₂O₃) at pH 2.5 and 4.5, while pure SrFe₁₂O₁₉ and TiO₂ phases were formed at pH 3.5. The production of the rutile phase was prohibited to some extent by highly dispersed ZnFe₂O₄ nanoparticles in TiO₂/ZnFe₂O₄ nanocomposites. TEM confirmed average particle sizes of 15–75 nm of SrFe₁₂O₁₉/TiO₂ and uniform attachment of ZnFe₂O₄ nanoparticles to TiO₂ nanoparticles to produce a coupled semiconductor. The electron diffraction pattern displayed very weak intensity of the electron diffraction pattern for the ZnFe₂O₄ phase, which was attributed to the dispersion of ZnFe₂O₄ nanoparticles amid the TiO₂ nanoparticles. The value of saturation magnetization, remnant magnetization, and coercivity decreased with increasing amounts of TiO₂ in TiO₂/ZnFe₂O₄, due to the contribution of the non-magnetic TiO₂ component to total sample volume.

Lahijani et al. [75] fabricated a PbFe₁₂O₁₉-TiO₂ nanocomposite using the sol–gel method. From XRD analysis, the average size of crystals was found to be 88 nm. FTIR study indicated that absorption bands at 544, 716, 935 and 1401 nm⁻¹ corresponded to stretching modes of Fe-O, Ti-O, Pb-O and C-C bonds, respectively. Heating the compound in the presence of a polyhydroxy alcohol, such as ethylene glycol, promotes polymerization. The metal ions are equally dispersed inside the organic matrix, resulting in a homogeneous resin—the polymeric precursor [76]. The immobilization of metal complexes in such rigid organic polymeric networks can decrease segregation of specific metals, ensuring molecular compositional uniformity. Polymeric precursors can be used to make ferrite–TiO₂ nanocomposites. Mouro et al. [77] used a polymeric precursor technique to make nanometric TiO₂/CoFe₂O₄ composites. X-ray diffraction, Raman spectroscopy, surface area through N₂ physisorption, zeta potential, scanning and high-resolution transmission electron microscopy were used to characterize the as-prepared sample. The lack of a rutile phase in the nanocomposites was confirmed by Raman spectroscopy. XRD patterns revealed that thermal treatment did not cause phase segregation. The synthesized nanocomposites showed an increase in surface area, a change in surface charge in relation to pure TiO₂, and selectivity in the photodegradation. The materials had photocatalytic activity due to the presence of TiO₂ on their surfaces, and CoFe₂O₄ cores were beneficial for separating and recovering photocatalysts after use in an oxidative process.

The ultrasonic technique involves using sound energy to agitate the particles in a solution, resulting in physical and chemical changes. This method is found to be the most promising method for manufacturing nanocomposites such as oxides, carbides, alloys and colloids with almost uniform distribution of nanoparticles [78]. Ultrasound irradiation causes unstable bubbles in liquids to form, grow, and collapse rapidly at temperatures as high as 5000 K, pressures as high as 20 MPa, and cooling speeds of 10¹⁰ Ks⁻¹ [79]. Ultrasonication was used for the synthesis of titania-coated γ-Fe₂O₃ magnetic activated carbon and Fe₃O₄-TiO₂ photocatalyst with saturation magnetization value of 2.21 and 14 emug⁻¹, respectively [80,81]. XRD revealed that the titania coated on the magnetic activated carbon maintained the anatase phase, with the same set of characterization peaks for both Fe₃O₄ and Fe₃O₄-TiO₂, indicating that the ultrasound irradiation had no effect on the crystal structure of the Fe₃O₄ nanoparticles. The tiny crystallite size was shown by the broad nature peak of the deposited TiO₂ on Fe₃O₄-TiO₂. SEM revealed the homogeneous distribution of titania. Superparamagnetic characteristics of Fe₃O₄-TiO₂ photocatalyst at room temperature were confirmed by a magnetization hysteresis loop. HRTEM analysis showed the occurrence of a heterojunction in the Fe₃O₄-TiO₂ nanocomposite. XPS showed that the peaks at 710.5 eV and 458.8 eV originated from Fe 2p and Ti 2p energy levels, respectively, implying that Fe₃O₄ and TiO₂ were largely present as separated phases in Fe₃O₄-TiO₂ composites.

This approach requires the preparation of a mixed aqueous solution of starting chemicals such as chlorides, nitrates, or sulfites of Fe³⁺, as well as of divalent metal ions in the requisite stoichiometric ratio. The precipitates are then generated by treating the solution with NaOH, filtration, washing twice, and drying. This approach entails four steps: nucleation, growth, coarsening, and agglomeration, all occurring at the same time [82]. Nanocomposites of ferrites of good quality and phase purity can be obtained by this method. This process offers various advantages such as high yield, high product purity, lack of necessity to use organic solvent, good reproducibility and low cost. It is a quick and easy way to make ultrafine particles that are disseminated in a variety of media. By carefully monitoring the preparation parameters, it is possible to achieve control over the surface morphology, structure, and chemical composition with this procedure.

Coprecipitation was employed for fabrication of CoFe₂O₄/TiO₂ and ZnFe₂O₄-TiO₂ nanocatalysts [83,84]. The average particle size of the coprecipitated nanocatalysts was 50 nm for CoFe₂O₄ and 150 nm for CoFe₂O₄/TiO₂ according to SEM images, which was consistent with the crystallite size predicted from XRD data. The spherical geometry (9 ± 2 nm) of ZnFe₂O₄ nanoparticles and spherical-like morphology (11 ± 3 nm) of ZnFe₂O₄-TiO₂ nanocomposites and fine dispersion of black particles (CoFe₂O₄) on the gray surface (TiO₂) of CoFe₂O₄/TiO₂ were depicted in the TEM images. Bandgaps of CoFe₂O₄/TiO₂, CoFe₂O₄, ZnFe₂O₄-TiO₂, and ZnFe₂O₄ were found to be 2.8, 1.1, 2.3, and 2.1 eV, respectively, which suggested the formation of a visible light-active photocatalyst. The large bandgap of the composite may be due to the mixing effect of the bandgap and the interfacial coupling effect between ZnFe₂O₄ and TiO₂, CoFe₂O₄ and TiO₂. XRD pattern analysis revealed that in CoFe₂O₄/TiO₂, the conversion of anatase to the rutile phase of TiO₂ happened at a TiO₂ annealing temperature of ≤600 °C, confirming the purity of ZnFe₂O₄ and TiO₂ phase in ZnFe₂O₄-TiO₂, which was in agreement with HRTEM and SAED investigations. Haw et al. [85] found that in comparison to conventional rutile-phase TiO₂ and pure urchin-like TiO₂ (3D TiO₂) microparticles, nanocomposites of CoFe₂O₄-3D TiO₂ showed an increase in photodegradation of methylene blue, and this nanocomposite demonstrated a generally constant photocatalytic efficacy with low degradation. Two sets of lattice fringes were visible at the TiO₂-CoFe₂O₄ interface, with d₁₁₀ = 0.322 nm of rutile phase titania and d₃₃₁ = 0.193 nm of CoFe₂O₄. The CoFe₂O₄ nanoparticles were clearly visible on the 3D urchin-like TiO₂ structure, and each element was uniformly distributed over the surface of the TiO₂ microsphere on scanning transmission electron microscopy. XRD peaks were designated to the rutile phase of 3D urchin-like TiO₂ and broader peaks confirmed the presence of nanosized CoFe₂O₄ in the sample.

The hydrothermal method is a way of crystallizing a chemical, utilizing an aqueous solution at a high vapor pressure as well as temperature [86]. At standard temperatures (100 °C) and pressures (<1 atm), it is generally depicted as crystal formation or crystal synthesis from insoluble chemicals. Autoclaves are used to carry out the process, which is performed at a controlled pressure and temperature. This enables the utilization of temperatures that are higher than the boiling point of water or an organic solution. The concoction reaction that occurs beyond the dissolvable breaking point and at pressures over bar is known as hydrothermal synthesis. The hydrothermal technique has the following advantages: it is simple to acquire nanotube morphology, variations in the synthesis process may be used to improve TiO₂ nanotube characteristics, and it is a method that can be used for a variety of applications.

Hydrothermal deposition of a hard ferromagnetic strontium ferrite (SrFe₁₂O₁₉) over TiO₂ can result in a photocatalyst that is both ferromagnetic and visible light-active. The TiO₂-supported SrFe₁₂O₁₉ nanocomposite has a strong ferromagnetic property and is quite stable when it comes to losing its magnetic property. XRD images of TiO₂-supported SrFe₁₂O₁₉ nanocomposite correspond to mixture of anatase and rutile crystal phases. EDS results confirmed that TiO₂-supported SrFe₁₂O₁₉ consists of Fe, Ti, Sr, C, O and Si. The BET-specific surface area of TiO₂ was 49.5 m²g⁻¹ with a total pore volume of 0.1553 cm³g⁻¹, which compares with 38.3 m²g⁻¹ and 0.0155 cm³g⁻¹ for TiO₂-supported SrFe₁₂O₁₉. Due to the deposition of SrFe₁₂O₁₉ nanoparticles onto the TiO₂, the surface area and pore volume of the TiO₂ nanocomposite reduces. The paramagnetic impact of TiO₂ lowered the coercivity (H_C) and saturation magnetization (M_S) of TiO₂-supported SrFe₁₂O₁₉. The photocatalyst TiO₂-supported SrFe₁₂O₁₉ had H_C of 2125.5 G. The coercivity of TiO₂-supported SrFe₁₂O₁₉ nanocomposite showed that it was a good ferromagnetic material [87]. Nguyen and Doong [88] and Pongwan et al. [89] fabricated ZnFe₂O₄-TiO₂ and CoFe₂O₄/TiO₂ nanostructure photocatalysts by the hydrothermal method. The ZnFe₂O₄-TiO₂ nanocomposite was prepared by coupling 0.2–2 wt % narrow-bandgap material of p-type ZnFe₂O₄ with n-type anatase TiO₂. Transmission electron microscopy (TEM) and high-resolution TEM confirmed average particle sizes of 8–9 nm and 5–35 nm for ZnFe₂O₄-TiO₂ and CoFe₂O₄/TiO₂, respectively. It was observed that ZnFe₂O₄ and TiO₂ were intimately linked, which led to a decrease in electron–hole recombination rate as well as enhanced photocatalytic activity of ZnFe₂O₄-TiO₂ heterostructures under visible light irradiation. When the loading amount of ZnFe₂O₄ increased from 0.5 to 2 wt %, SEM revealed a slight increase in particle size of ZnFe₂O₄-TiO₂ nanocomposites. The N₂ adsorption–desorption isotherms showed that the combination of TiO₂ with ZnFe₂O₄ can increase the specific surface area. XRD analysis indicated that ZnFe₂O₄-TiO₂ can retain the crystallinity of both nanoparticles. Electron diffraction pattern of CoFe₂O₄/TiO₂ displayed the brightness of polymorphic discrete rings of the crystalline particles, suggesting quite a high degree of crystallinity in polycrystals. In this study, the obtained maximum saturation magnetization and coercivity of CoFe₂O₄ and CoFe₂O₄/TiO₂ nanocomposites were found to be 32.58, 29.64 emug⁻¹ and 0.15, 0.05 kOe, respectively.

The solvothermal technique employs a non-aqueous solvent and a considerably higher temperature, allowing high boiling point solvents to be utilized. The solvothermal method outperforms the hydrothermal method in terms of shape, size control, distribution, and crystallinity of nanoparticles [90]. Organic solvents, which have a low relative permittivity and are free of ionic species, are used in the solvothermal process to produce a product devoid of foreign anions and ionic species. The benefits of both the hydrothermal and sol–gel methods are combined in this technique. This approach may be utilized in the ceramics sector to regulate the particle size of synthesized materials and to fabricate tiny particles, such as magnetic titania photocatalyst [91]. Atacan et al. [92] developed ZnFe₂O₄/Ag-TiO₂ by the solvothermal method. The formation, structure and morphology of prepared samples were characterized by X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy and vibrating sample magnetometry. In this study, the obtained value of saturation magnetization of ZnFe₂O₄/Ag-TiO₂ was 5.5 emug⁻¹. XRD patterns indicated that no chemical reaction occurred between ZnFe₂O₄ and Ag-TiO₂, because no peaks related to other impurities were observed. Nguyen and Doong [93] synthesized ZnFe₂O₄/TiO₂ heterostructure by this method.