1. Introduction
TiO
2 has four polymorphs: brookite (orthorhombic), rutile (tetragonal), anatase (tetragonal) and TiO
2-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 TiO
2 to rutile occurs
[44][1].
It has been observed that bandgaps of TiO
2 nanoparticles with 5–10 nm particles narrow up to 0.2 eV. Anatase is the most photocatalytically effective phase of TiO
2 [45][2] and is widely used in semiconducting materials for environmental applications
[46][3]. It has higher mobility of electron–hole pairs and higher surface hydroxyl density. Brookite-phase activity has not been thoroughly examined
[47,48][4][5]. Rutile TiO
2 is usually investigated in fundamental investigations. Due to the existence of oxygen vacancies, TiO
2 is classed as an n-type semiconductor
[49][6]. Because crystalline TiO
2 has a less photoexcited electron–hole recombination than amorphous TiO
2, it has higher photocatalytic activity
[50,51][7][8]. Recombination of photogenerated charge carriers is a disadvantage of TiO
2 photocatalysts, as it decreases overall reaction quantum efficiency
[52][9]. During the recombination processes, the photoexcited electrons return to the valence band non-radiatively or radiatively
[53,54,55][10][11][12]. An attractive feature of the TiO
2 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 TiO
2 [56,57,58,59,60][13][14][15][16][17]. By using dopants in TiO
2, its bandgap reduces
[61][18]. Metal doping has long been known to improve visible light absorbance of TiO
2 [62,63][19][20] and increase its photocatalytic activity under UV irradiation
[63[20][21][22][23],
64,65,66], but the introduction of metal ion results in thermal instability, which reduces the reuse of TiO
2 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][24][25][26][27][28].
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.
2. Sol–Gel Method
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][29]. This method uses simple equipment, produces highly homogeneous and pure products at low temperatures, and allows modification of the surface.
Tatarchuk et al.
[72][30] developed magnetic nanocomposites of CoFe
2O
4@TiO
2 using the Pechini sol–gel method with ethylene glycol and citric acid as chelating agents and titanium (IV) polymeric precursor solution. XRD study of CoFe
2O
4@TiO
2 indicated the presence of 46% and 54% of anatase and rutile crystal phase, respectively, in the titanium dioxide (TiO
2) component, and the average size of cobalt ferrite (CoFe
2O
4) and TiO
2 nanoparticles was about 30 and 8 nm, respectively. SEM revealed that because of deposition of TiO
2 on the CoFe
2O
4 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][31] and Xu et al.
[74][32] fabricated SrFe
12O
19/TiO
2 and TiO
2/ZnFe
2O
4 nanostructures by the sol–gel method. XRD indicated the presence of impurity phases (�-Fe
2O
3) at pH 2.5 and 4.5, while pure SrFe
12O
19 and TiO
2 phases were formed at pH 3.5. The production of the rutile phase was prohibited to some extent by highly dispersed ZnFe
2O
4 nanoparticles in TiO
2/ZnFe
2O
4 nanocomposites. TEM confirmed average particle sizes of 15–75 nm of SrFe
12O
19/TiO
2 and uniform attachment of ZnFe
2O
4 nanoparticles to TiO
2 nanoparticles to produce a coupled semiconductor. The electron diffraction pattern displayed very weak intensity of the electron diffraction pattern for the ZnFe
2O
4 phase, which was attributed to the dispersion of ZnFe
2O
4 nanoparticles amid the TiO
2 nanoparticles. The value of saturation magnetization, remnant magnetization, and coercivity decreased with increasing amounts of TiO
2 in TiO
2/ZnFe
2O
4, due to the contribution of the non-magnetic TiO
2 component to total sample volume.
Lahijani et al.
[75][33] fabricated a PbFe
12O
19-TiO
2 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][34]. 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–TiO
2 nanocomposites. Mouro et al.
[77][35] used a polymeric precursor technique to make nanometric TiO
2/CoFe
2O
4 composites. X-ray diffraction, Raman spectroscopy, surface area through N
2 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 TiO
2, and selectivity in the photodegradation. The materials had photocatalytic activity due to the presence of TiO
2 on their surfaces, and CoFe
2O
4 cores were beneficial for separating and recovering photocatalysts after use in an oxidative process.
3. Ultrasonic Method
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][36]. 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 10
10 Ks
−1 [79][37]. Ultrasonication was used for the synthesis of titania-coated γ-Fe
2O
3 magnetic activated carbon and Fe
3O
4-TiO
2 photocatalyst with saturation magnetization value of 2.21 and 14 emug
−1, respectively
[80,81][38][39]. XRD revealed that the titania coated on the magnetic activated carbon maintained the anatase phase, with the same set of characterization peaks for both Fe
3O
4 and Fe
3O
4-TiO
2, indicating that the ultrasound irradiation had no effect on the crystal structure of the Fe
3O
4 nanoparticles. The tiny crystallite size was shown by the broad nature peak of the deposited TiO
2 on Fe
3O
4-TiO
2. SEM revealed the homogeneous distribution of titania. Superparamagnetic characteristics of Fe
3O
4-TiO
2 photocatalyst at room temperature were confirmed by a magnetization hysteresis loop. HRTEM analysis showed the occurrence of a heterojunction in the Fe
3O
4-TiO
2 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 Fe
3O
4 and TiO
2 were largely present as separated phases in Fe
3O
4-TiO
2 composites.
4. Coprecipitation Method
This approach requires the preparation of a mixed aqueous solution of starting chemicals such as chlorides, nitrates, or sulfites of Fe
3+, 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][40]. 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 CoFe
2O
4/TiO
2 and ZnFe
2O
4-TiO
2 nanocatalysts
[83,84][41][42]. The average particle size of the coprecipitated nanocatalysts was 50 nm for CoFe
2O
4 and 150 nm for CoFe
2O
4/TiO
2 according to SEM images, which was consistent with the crystallite size predicted from XRD data. The spherical geometry (9 ± 2 nm) of ZnFe
2O
4 nanoparticles and spherical-like morphology (11 ± 3 nm) of ZnFe
2O
4-TiO
2 nanocomposites and fine dispersion of black particles (CoFe
2O
4) on the gray surface (TiO
2) of CoFe
2O
4/TiO
2 were depicted in the TEM images. Bandgaps of CoFe
2O
4/TiO
2, CoFe
2O
4, ZnFe
2O
4-TiO
2, and ZnFe
2O
4 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 ZnFe
2O
4 and TiO
2, CoFe
2O
4 and TiO
2. XRD pattern analysis revealed that in CoFe
2O
4/TiO
2, the conversion of anatase to the rutile phase of TiO
2 happened at a TiO
2 annealing temperature of ≤600 °C, confirming the purity of ZnFe
2O
4 and TiO
2 phase in ZnFe
2O
4-TiO
2, which was in agreement with HRTEM and SAED investigations. Haw et al.
[85][43] found that in comparison to conventional rutile-phase TiO
2 and pure urchin-like TiO
2 (3D TiO
2) microparticles, nanocomposites of CoFe
2O
4-3D TiO
2 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 TiO
2-CoFe
2O
4 interface, with d
110 = 0.322 nm of rutile phase titania and d
331 = 0.193 nm of CoFe
2O
4. The CoFe
2O
4 nanoparticles were clearly visible on the 3D urchin-like TiO
2 structure, and each element was uniformly distributed over the surface of the TiO
2 microsphere on scanning transmission electron microscopy. XRD peaks were designated to the rutile phase of 3D urchin-like TiO
2 and broader peaks confirmed the presence of nanosized CoFe
2O
4 in the sample.
5. Hydrothermal Method
The hydrothermal method is a way of crystallizing a chemical, utilizing an aqueous solution at a high vapor pressure as well as temperature
[86][44]. 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 TiO
2 nanotube characteristics, and it is a method that can be used for a variety of applications.
Hydrothermal deposition of a hard ferromagnetic strontium ferrite (SrFe
12O
19) over TiO
2 can result in a photocatalyst that is both ferromagnetic and visible light-active. The TiO
2-supported SrFe
12O
19 nanocomposite has a strong ferromagnetic property and is quite stable when it comes to losing its magnetic property. XRD images of TiO
2-supported SrFe
12O
19 nanocomposite correspond to mixture of anatase and rutile crystal phases. EDS results confirmed that TiO
2-supported SrFe
12O
19 consists of Fe, Ti, Sr, C, O and Si. The BET-specific surface area of TiO
2 was 49.5 m
2g
−1 with a total pore volume of 0.1553 cm
3g
−1, which compares with 38.3 m
2g
−1 and 0.0155 cm
3g
−1 for TiO
2-supported SrFe
12O
19. Due to the deposition of SrFe
12O
19 nanoparticles onto the TiO
2, the surface area and pore volume of the TiO
2 nanocomposite reduces. The paramagnetic impact of TiO
2 lowered the coercivity (H
C) and saturation magnetization (M
S) of TiO
2-supported SrFe
12O
19. The photocatalyst TiO
2-supported SrFe
12O
19 had H
C of 2125.5 G. The coercivity of TiO
2-supported SrFe
12O
19 nanocomposite showed that it was a good ferromagnetic material
[87][45]. Nguyen and Doong
[88][46] and Pongwan et al.
[89][47] fabricated ZnFe
2O
4-TiO
2 and CoFe
2O
4/TiO
2 nanostructure photocatalysts by the hydrothermal method. The ZnFe
2O
4-TiO
2 nanocomposite was prepared by coupling 0.2–2 wt % narrow-bandgap material of p-type ZnFe
2O
4 with n-type anatase TiO
2. Transmission electron microscopy (TEM) and high-resolution TEM confirmed average particle sizes of 8–9 nm and 5–35 nm for ZnFe
2O
4-TiO
2 and CoFe
2O
4/TiO
2, respectively. It was observed that ZnFe
2O
4 and TiO
2 were intimately linked, which led to a decrease in electron–hole recombination rate as well as enhanced photocatalytic activity of ZnFe
2O
4-TiO
2 heterostructures under visible light irradiation. When the loading amount of ZnFe
2O
4 increased from 0.5 to 2 wt %, SEM revealed a slight increase in particle size of ZnFe
2O
4-TiO
2 nanocomposites. The N
2 adsorption–desorption isotherms showed that the combination of TiO
2 with ZnFe
2O
4 can increase the specific surface area. XRD analysis indicated that ZnFe
2O
4-TiO
2 can retain the crystallinity of both nanoparticles. Electron diffraction pattern of CoFe
2O
4/TiO
2 displayed the brightness of polymorphic discrete rings of the crystalline particles, suggesting quite a high degree of crystallinity in polycrystals. In this
res
tudyearch, the obtained maximum saturation magnetization and coercivity of CoFe
2O
4 and CoFe
2O
4/TiO
2 nanocomposites were found to be 32.58, 29.64 emug
−1 and 0.15, 0.05 kOe, respectively.
6. Solvothermal Method
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][48]. 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][49]. Atacan et al.
[92][50] developed ZnFe
2O
4/Ag-TiO
2 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
res
tudyearch, the obtained value of saturation magnetization of ZnFe
2O
4/Ag-TiO
2 was 5.5 emug
−1. XRD patterns indicated that no chemical reaction occurred between ZnFe
2O
4 and Ag-TiO
2, because no peaks related to other impurities were observed. Nguyen and Doong
[93][51] synthesized ZnFe
2O
4/TiO
2 heterostructure by this method.