Recent advances in spinel ferrite-based magnetic photocatalysts for efficient degradation of organic pollutants

In recent years, the heavy consumption of organic compounds, such as organic dyes, phenols, and pharmaceuticals, has resulted in large amounts of debris in the natural aquatic environment, originating from industrial effluents (Shao et al. 2021). The increased discharge of organic pollutants has aroused considerable attention worldwide, due to their threat to human health (Rodriguez-Mozaz et al. 2015). This environmental issue has been considered a tough problem that urgently needs to be addressed since conventional treatment methods can hardly eliminate these pollutants due to their stable molecular structure (Agunbiade & Moodley 2016). Advanced oxidation processes have been reported as an effective route to eliminate these refractory compounds (Michael-Kordatou et al. 2018), and among various processes, photocatalysis has attracted large attention (Xing et al. 2018; Zhu et al. 2022). The main reason is its green and eco-friendly features: no requirement of fossil energy and chemical reagents (only solar energy), the conversion of toxic organic compounds into less toxic intermediates, and eventually inorganic substances (Nair et al. 2021).

Most reported photocatalysts are in powder form, which requires an additional process to collect them from water (Arora et al. 2021). The high cost of recycling photocatalysts greatly restricts its practical application potential. Because of this, spinel ferrite has been introduced to the photocatalytic system due to its magnetic property, which ensures high recycle performance via magnetic separation (Shah et al. 2022). However, its low photocatalytic activity under visible light greatly restricts its practical application.

In order to overcome both restrictions, the construction of a photocatalytic composite with two or more semiconductors has been intensively studied, and the enhanced photocatalytic performance is ascribed to the appropriate band alignment. Figure 1(b)–1(e) presents four different band alignment strategies. Figure 1(c) presents type-I heterojunction (straddling-gap junction) (Jabbar et al. 2023), in which the photo-induced e⁻ and h⁺ from photocatalytic semiconductor II (PS II) all migrate to photocatalytic semiconductor I (PS I). This accumulates the recombination of charge carriers, leading to reduced photocatalytic activity. Figure 1(d) presents a type-II heterojunction (staggered-gap junction), in which photo-induced e⁻ and h⁺ migrate to relatively lower energy bands (Jiang et al. 2022). This hinders the recombination of charge carriers and improves photocatalytic activity, yet reduces the total redox ability since charge carriers end up gathering in bands with low energy potentials. Figure 1(e) presents a type-III heterojunction (broken-gap junction), in which photo-induced charge carriers stay in the original PS bands without interface migration (Li et al. 2022b). This makes no contribution to photocatalytic efficiency since there is no interaction and synergy between PS I and PS II.

Figure 1(b) presents a special case of heterojunction similar to staggered-gap junction (type-II, Figure 1(d)), that is, Z-scheme heterojunction (Liu et al. 2022b). In this scenario, e⁻ in PS II moves toward PS I, meanwhile, h⁺ in PS I moves toward PS II, until both charge carriers recombine, leaving e⁻ with high reductive potential at the CB of PS I and h⁺ with high oxidative potential at the VB of PS II. Z-scheme heterojunction preserves the high redox ability of charge carriers, which is beneficial to the complete degradation of organic pollutants. In conclusion, staggered-gap junction (type-II) and Z-scheme heterojunction are promising strategies for improving photocatalytic activity, especially Z-scheme photocatalytic heterojunction composites.

The general molecular formula of spinel ferrites is MFe₂O₄, where M stands for divalent ions, such as Ni²⁺, Zn²⁺, Mn²⁺ (Dasgupta et al. 2016). There are three types of spinel based on the difference in crystal structure: normal spinel, inverse spinel, and complex spinel (Zhao et al. 2017). As for normal spinel (Figure 1(f)), such as ZnFe₂O₄, Zn²⁺ and Fe³⁺ are located at tetrahedral and octahedral sites, respectively (Li et al. 2019). As for inverse spinel (Figure 1), such as NiFe₂O₄, Ni²⁺ and half of Fe³⁺ occupy octahedral sites, and the other half of Fe³⁺ occupies tetrahedral sites (Nawaz et al. 2020). As for complex spinel (Figure 1), such as MnFe₂O₄, Mn²⁺, and Fe³⁺ randomly occupy tetrahedral and octahedral sites, and 20% of Mn²⁺ ions occupy octahedral sites and 80% of Mn²⁺ ions occupy at tetrahedral sites (Kefeni et al. 2017).

The keyword analysis of reported scientific literature works helps to describe topic(s) in a specific research area. As for all published papers, the authors generally set a list of keywords to describe the main focus of their work, as well as the essential concepts. Sometimes editors and reviewers also added more keywords based on the corresponding research field(s), which ends up with hundreds of keywords related to a certain research field (Padilla et al. 2018). VOSviewer software provides the option to choose keywords from all sources, including authors and publishers.

The terms ‘synthesis’ and ‘structural’ are the most used keywords. Both keywords are highly related to the physicochemical properties, which affect its photocatalytic property. Thus, in this review, the connections between the synthesis method, structural property and photocatalytic activity are discussed.

In recent decades, nano-sized spinel ferrites have been intensively studied, especially their catalytic application in the environmental remediation field (Li et al. 2004). Compared with bulk materials (large-sized), nanoparticles exhibit superior catalytic performance, optical properties, and chemical stability (Du et al. 2016). These advantages make nano-sized spinel ferrite a suitable catalytic material for practical application (Rostami et al. 2013). Especially, nanoscale spinel ferrite exhibits superparamagnetic properties, which enables spinel ferrite-based composite to be easily recycled via external magnet (Nadimi et al. 2019). However, the disadvantages of pristine spinel ferrite restrict its catalytic activity: high agglomeration and rapid charge carrier recombination (Zhu et al. 2019). To address these problems, a great many types of research have been conducted on developing spinel ferrite-based composites with high photocatalytic activity (Chandrasekaran et al. 2018; Qin et al. 2021). This paper aims at providing a timely and systematical review of the design and photocatalytic degradation of spinel ferrite (ZnFe₂O₄, NiFe₂O₄ and MnFe₂O₄)-based composites, offering an inspiring and useful discussion on the relationship between structural property and photocatalytic activity, as well as future research suggestions.

Although spinel ferrite has been considered a promising catalyst, the low photocatalytic activity under visible light greatly restricts its practical application (Gebreslassie et al. 2019; Leonel et al. 2021). One of the reported methods to develop high-performance photocatalysts is the combination of photocatalysts with functional materials (Fouad et al. 2021). Carbon-based materials, such as graphene and graphitic carbon nitride (g-C₃N₄), are described as follows.

As one of the forms of carbon, graphene presents a unique crystal structure, which endows it with different properties. In 2004, Novoselov et al. reported the two-dimensional morphology of graphene, as well as its astonishing electric field effect (Novoselov et al. 2004). In 2010, Geim and Novoselov were awarded the Nobel Prize in Physics, due to their outstanding contribution to the research field of graphene (Li et al. 2022a). Because of its ultrahigh tensile strength and specific surface area, abundant surface functional groups, excellent electronic and thermal conductivity, and superior chemical and thermal stability, graphene has been intensively studied in the fields of electronics, electric batteries, semiconductor and composite (Yu et al. 2020). In recent years, graphene has been used as the carrier of various photocatalysts (Dutta et al. 2019). In this section, the main preparation methods of spinel ferrite and its graphene-based composite are discussed, and the brief information of these papers is summarized in Table 1.

The first one is the hydrothermal method, which uses water as medium and requires high temperature (100–300 °C) at high pressure (1–30 MPa) in a confined container to produce nanocrystals (Soufi et al. 2021). This is currently the most widely used preparation method. At these extreme conditions, all precursors (metal ions) in water acquire high energy, which greatly accelerates the crystallization process (Narang & Pubby 2021).

Another preparation strategy is the sol–gel auto-combustion method, which also uses metal ions as precursors. During the preparation period, metal salts are fully dissolved into water or ethanol by vigorous stirring, under alkaline conditions (Bhandare et al. 2020). After several hours, the obtained gel is heated (dehydrated, 80–90 °C) and calcinated at 450–800 °C (Bhandare et al. 2020).

Luna et al. immobilized NiFe₂O₄ nanoparticles onto graphene oxide (GO) via this method, denoted as NiFe₂O₄@GO, which exhibited superior photocatalytic degradation toward dye methylene blue compared with pure NiFe₂O₄ (Bayantong et al. 2021). This result was also ascribed to the excellent electric conductivity and adsorption ability of GO. As one derivative of graphene, GO also had four functions to enhance the catalytic activity of spinel ferrite. However, this composite only exhibited photocatalytic activity under UV irradiation, only a tiny part of sunlight energy. The substrate GO cannot alter the band structure of NiFe₂O₄, and thus cannot extend its light absorption range. This greatly restricted the practical application of NiFe₂O₄@GO. It is suggested that visible-light-driven component should be introduced to extend the light response range of NiFe₂O₄. The key factor during sol–gel auto-combustion is the calcination temperature, which affects the particle size and crystallinity (Khalid et al. 2021).

The third preparation strategy is the co-precipitation method, which utilizes metal precipitation as the precursor. The precipitation is formed by mixing two or more metal ions in a solution. The precipitation is then calcinated to obtain spinel ferrite (Rabi et al. 2021).

Bidisha et al. decorated MnFe₂O₄ onto reduced GO (MnFe₂O₄/rGO) via this method (Mandal et al. 2020). The sphere-like MnFe₂O₄ nanoparticles were well-dispersed onto rGO nanosheets with reduced agglomeration, which improved the surface condition of MnFe₂O₄/rGO. MnFe₂O₄/rGO presented higher photodegradation efficiency toward methylene blue, much better than that of bare MnFe₂O₄. As another derivative of graphene, rGO also exhibited four functions to enhance the catalytic activity of spinel ferrite. The limitation of this work is also the utilization of UV light, which restricted its practical application. Various factors affect the structural properties (particle size and shape) of spinel ferrite during the co-precipitation process: salt precursor, solution medium, temperature, pH and extra surfactant (Haneef et al. 2021).

Different preparation methods have unique advantages and disadvantages. As for the hydrothermal method, due to the extreme conditions, the obtained product often exhibits controlled particle size, morphology and nanocrystal dispersion. However, this method also has its shortcomings, such as relatively low output amount and safety of production equipment (Zhu et al. 2021). As for the sol–gel auto-combustion method, the product often has good uniformity and purity; however, the fabrication conditions are highly uncontrollable, and unwanted byproducts are formed (Khalid et al. 2021). As for the co-precipitation route, it has been considered a facile and cost-effective method for spinel ferrite fabrication, however, currently, the obtained products exhibit poor crystallinity (Rabi et al. 2021). This method has great potential for the development of large-scale production, once this problem can be addressed (Soylak et al. 2021).

Although graphene and its derivatives have many functions to improve the photocatalytic performance of spinel ferrite, they can't change its light response range, in other words, NiFe₂O₄ or MnFe₂O₄ still can't be activated by visible light after immobilized onto these substrates. These spinel ferrites should combine other semiconductors and form a heterojunction composite to extend their light absorption range and realize high solar light utilization.

Wang et al. reported for the first time that graphitic carbon nitride (g-C₃N₄) exhibited visible-light-driven photocatalytic activity (Wang et al. 2009). Due to its fascinating properties, g-C₃N₄ is a promising n-type semiconductor, with abandgap of 2.7 eV (Balakrishnan & Chinthala 2022). Different from other carbon nitride compounds, g-C₃N₄ exhibits a unique delocalized conjugate molecular structure, which is composed of stacking carbon nitride layers interconnected via amines (Fu et al. 2018). The unique two-dimensional structure, as well as the strong bonding between C and N atoms, result in excellent electronic conductivity, and high chemical and thermal stability (Ismael 2020). However, its practical application is restricted due to low photocatalytic performance, owing to its rapid electron–hole pair recombination, low specific surface area and low adsorption ability (Mishra et al. 2019; Truong et al. 2022). Moreover, the long recovery operation of g-C₃N₄ from water also hinders its commercial application potential (Zhao et al. 2021b). Recently, spinel ferrite/g-C₃N₄ heterojunctions show excellent solar-to-chemical energy conversion rates (Das & Chowdhury 2021). These composites exhibit interesting properties: unique structural properties, extended light absorption range, enhance charge carrier mobility and high redox capability, as well as facile magnetic recycling ability (Keerthana et al. 2022). All these greatly improve the photodegradation performance. The micro-structures of these composites are discussed in this section, such as type-II heterojunction, p–n junction and S-scheme structure. The brief information of these papers is summarized in Table 2.

Wu et al. prepared hierarchical ZnFe₂O₄/g-C₃N₄ heterojunction by solvothermal method (Wu et al. 2018). Typical microspheres were formed by ZnFe₂O₄ nanoparticles, and their diameter was limited due to the confined space effect of g-C₃N₄. Compared with pure ZnFe₂O₄, ZnFe₂O₄/g-C₃N₄ showed promoted photodegradation performance toward organic dyes under visible light. The composite was reported as a type-II heterojunction. The CB edge of g-C₃N₄ was more negative than that of ZnFe₂O₄, while the VB edge of ZnFe₂O₄ was more positive than that of g-C₃N₄. After irradiation, the photo-induced e⁻ migrated from g-C₃N₄ to ZnFe₂O₄, and the photo-induced h⁺ moved in the opposite direction. This migration promoted charge carrier separation, thus prolonging the lifetime and enhancing photocatalytic performance. Above all, the construction of heterojunction should follow the energy band matching principle, as well as close contact between components (Huang et al. 2018). However, ZnFe₂O₄/g-C₃N₄ composite exhibited reduced redox ability, due to the less positive VB position and less negative CB position. As for ZnFe₂O₄/g-C₃N_4, despite its enhanced charge carrier mobility, further, development is needed due to its poor redox ability.

Different from ZnFe₂O₄, NiFe₂O₄ and MnFe₂O₄ are p-type semiconductors (Jiang et al. 2021; Agboola et al. 2022), thus the micro-structure properties of NiFe₂O₄/g-C₃N₄ and MnFe₂O₄/g-C₃N₄ should be attributed as p–n junctions (Kong et al. 2022). When a p–n junction is built, an internal electric field is formed at the interface region due to electron (from an n-type semiconductor) and hole (from a p-type semiconductor) diffusion. Similar to type-II heterojunction, during light irradiation, electrons move to less negative CB while holes move to less positive VB due to the internal electric field, resulting in prolonged charge carrier lifetime and promoted photocatalytic activity. Xie et al. prepared MnFe₂O₄/g-C₃N₄ and evaluated the photocatalytic activity by Rhodamine B photodegradation (Xie et al. 2022). The composite with 30 wt% MnFe₂O₄ exhibited the highest photocatalytic activity under visible light. The reusability tests proved that, after five cycles, the photodegradation rate slightly dropped, implying its excellent stability and convenience in recovery. The interface contact between MnFe₂O₄ and g-C₃N₄, and the formation of an internal electric field greatly enhanced charge carrier mobility due to their energy band positions. The p–n junctions exhibit enhanced photocatalytic activity as well as excellent recyclability and stability, proving their high potential to be applied in wastewater treatment.

Due to the excellent electron conductivity, graphene can also be introduced to g-C₃N₄ to improve its photocatalytic performance. Up to now, plenty of papers on g-C₃N₄/graphene have been published, however, the combination of g-C₃N₄/graphene and spinel ferrite has seldom been reported. It is believed that the combination of heterojunction and graphene can greatly enhance photocatalytic efficiency (Hafeez et al. 2023).

In 2020, Tsvetkov et al. reported the ternary ZnFe₂O₄/rGO/g-C₃N₄ nanocomposite (Tsvetkov et al. 2020), prepared by the solvothermal method. During the photodegradation process, rGO (reduced GO) acted as an electron mediator, and efficiently separated photo-induced charge carriers (e⁻/h⁺), it also enhanced the adsorption ability to target pollutants. Meanwhile, ZnFe₂O₄ and g-C₃N₄ combined as type-II heterojunctions, which also reduced charge carrier recombination. These factors greatly improved the overall photodegradation efficiency. In conclusion, the introduction of the functional substrates such as graphene can further improve the performance of type-II heterojunction.

Above all, if combined with g-C₃N₄ or graphene, spinel ferrite-based photocatalysts can effectively eliminate organic pollutants. However, the preparation of graphene or its derivatives is very sophisticated and expensive. In the following section, several cost-effective carbon materials are introduced as photocatalyst substrates (Chen et al. 2019). The brief information of these papers is summarized in Table 3.

Second, multi-walled carbon nanotubes (MWCNTs). Hazarika et al. decorated MnFe₂O₄ nanoparticles onto MWCNTs (MWCNTs/MnFe₂O₄) via the hydrothermal method (Hazarika et al. 2018), with a particle size of 27–30 nm. The nanocomposites exhibited superior phenol photodegradation efficiency compared with pristine MnFe₂O₄, due to the inhibition of photo-induced e⁻/h⁺ pair recombination by MWCNTs. However, the degradation experiments were carried out under UV light, which means MWCNTs/MnFe₂O₄ only responded to visible light. This greatly hinders its practical application since UV light is only a small proportion of the sunlight spectrum. The production cost for MWCNTs is still relatively high, thus it is suggested that much cheaper carbon materials with similar functions should be developed as a substrate of the photocatalyst.

Above all, the photocatalyst composed of spinel ferrite and carbon material can effectively eliminate various organic pollutants; however, higher demands are proposed nowadays to meet the growing requirements for wastewater treatment. Thus, developing high-performance and easy-recyclable photocatalysts from low-cost materials has been a challenging and urgent task. It is suggested that researchers focus on the development of ternary photocatalytic composite, which is composed by spinel ferrite-based heterojunction and carbon material. Moreover, the development of cost-effective carbon materials, such as biochar, is another important research topic especially to improve the stability of the long-term photodegradation process.

The heterojunctions of spinel ferrite and Bi-based semiconductors have recently been extensively studied in recent years. Typical Bi-based semiconductors are: BiOX (X = Cl, Br, I), Bi₂WO₆, Bi₂O₂CO₃, BiVO₄, Bi₂S₃ and so on.

Due to the unique structure and excellent light absorption ability, Bi-based materials have become a hotspot in the research field of photocatalysis. Among various Bi-based materials, Sillén-type bismuth oxyhalides (BiOX, X = Cl, Br, I) have drawn great interest (Wang et al. 2020b). Due to its narrow bandgap, BiOX possesses a high absorption ability to visible light (Ye et al. 2014). Moreover, their two-dimensional layered structure also facilitates photocatalytic performance. However, several drawbacks restrict their practical application (Mengting et al. 2022), such as high charge carrier recombination, and catalyst recovery limitations. Until now, many methods have been proposed to overcome these problems (Hussain et al. 2022), and among them, the incorporation of spinel ferrite has been considered a promising strategy (Suresh et al. 2021), which not only facilitates photocatalytic performance but also enhances recyclability via magnetic recovery (Ma et al. 2021). This strategy endows BiOX with a high potential for wastewater treatment. The brief information of the following papers is summarized in Table 4.

Yosefi et al. synthesized flower-like p-BiOI/n-ZnFe₂O₄ junction via a solvothermal method (Yosefi et al. 2017). The composite showed higher photodegradation efficiency toward acid orange 7 under visible light, compared with pure ZnFe₂O₄ and BiOI. In the optimal composite, the mass ratio of BiOI/ZnFe₂O₄ was 80/20. The p–n junction facilitated the charge carrier separation and thus enhanced photocatalytic performance. This is similar to the above-mentioned p-BiOCl/n-ZnFe₂O_4. Xia et al. prepared flower-like NiFe₂O₄/BiOI (NFO/BOI) nanocomposites by solvothermal method (Xia et al. 2018). The photoactivity was evaluated by the degradation of organic dyes under visible light. The NiFe₂O₄/BiOI sample with 15 wt% NiFe₂O₄ exhibited the highest photodegradation performance. The quenching tests proved was the primary radical. The formation of type-II heterostructure improved the separation and restricted the recombination of electron–holes pairs. The composite showed no notable decrease in photoactivity after five recycles, implying its good stability. Based on the above literature works, it can be concluded that the coupling of BiOBr and NiFe₂O₄, or the coupling of BiOBr and MnFe₂O₄ is regarded as the optimal combination, compared with BiOCl, BiOI and other spinel ferrites. This is because of their appropriate band matching, leading to the construction of Z-scheme heterojunction. It is suggested that researchers should pay attention to both composites in future work, in order to develop high-performance photocatalysts for wastewater treatment.

Bi₂WO₆ is often regarded the simplest member of the Aurivillius family with one perovskite layer in its molecular structure (Tachibana 2015), its excellent visible light response ability makes itself a good candidate for photocatalytic purposes. However, its high charge carrier recombination greatly reduced its photocatalytic activity, thus greatly restricting its practical application potential. The coupling of Bi₂WO₆ and other semiconductors lead to the formation of composites with varied morphologies, and has been considered an effective strategy to build high-performance photocatalysts. The brief information of the following papers is summarized in Table 5.

Zhong et al. synthesized Bi₂WO₆/ZnFe₂O₄ composite via a two-step solvothermal method (Zhong et al. 2017). The combination of ZnFe₂O₄ greatly enhanced the adsorption and photodegradation ability of Bi₂WO₆, and endowed the catalyst with the magnetic recyclable property. The composite with 0.15 wt% of ZnFe₂O₄ presented the highest photodegradation efficiency toward TC hydrochloride (TCH) under visible light, with aremoval percentage of 96.85% in 90 min. The enhanced photoactivity was ascribed to the effective separation of photo-generated electron–hole pairs at the interface. After five recycling tests, the TCH removal rate dropped to 81.52%, which was attributed to the catalyst loss during recycling processes. The photocatalytic mechanism of Bi₂WO₆/ZnFe₂O₄ was attributed to atype-II heterojunction.

Except for BiOX and Aurivillius Bi-based compounds, other Bi-based photocatalysts, such as BiVO₄ and Bi₂S₃, have also attracted attention. The photocatalytic materials based on these materials and spinel ferrites have been developed. The brief information of the following papers is summarized in Table 6.

Above all, the photocatalysts derived from spinel ferrite and Bi-based materials have been applied in various organic pollutant degradation, which all exhibited excellent photoactivity and recyclability. Future research work should focus on the proper energy band coupling between bismuth semiconductor and spinel ferrite, in order to build a Z-scheme structure for superior photocatalytic activity. As for current knowledge, Bi₂O₂CO₃/ZnFe₂O₄, Bi₂WO₆/NiFe₂O₄ and BiVO₄/MnFe₂O₄ are typical examples. Other issues should also be considered in future work: the catalyst amount during photodegradation should be further reduced through the modification and optimization of the photocatalyst's structure and physicochemical properties. Moreover, the stability and recyclability of photocatalysts should be further improved.

TiO₂ has long been studied as a green semiconductor, especially in the aspect of photocatalytic degradation, due to its advantages such as abundance, low cost, nontoxicity and chemical stability (Nur et al. 2022). However, the wide band gap of pure TiO₂ (3.2 eV) greatly limits its light absorption range (Li et al. 2022d), only responding to UV light, which accounted for 4% of solar energy. Besides, other disadvantages, such as rapid charge carrier recombination, low recyclability and activity regulation, also restrict its practical application (Mohd Adnan et al. 2019). To overcome all these issues and design visible-light-driven photocatalysts, the coupling of TiO₂ and spinel ferrite has been proven effective and promising. The brief information of the following papers is summarized in Table 7.

Wang et al. prepared TiO₂/NiFe₂O₄ heterojunction via the hydrothermal method and used it to detect sub-ppm trimethylamine with the aid of visible light (Wang et al. 2022). Trimethylamine (TMA) is a tertiary amine compound, responsible for the unpleasant smell of decomposing fish, thus it is an effective indicator of seafood freshness (Zhao et al. 2021a). TiO₂/NiFe₂O₄ sensor exhibited enhanced gas-sensing performance, due to the formation of p–n heterojunction, which enhanced charge flow between two components. This work also extended the application potential of TiO₂/NiFe₂O₄ heterojunction. Moreover, the photocatalytic activity of the TiO₂/NiFe₂O₄ composite can be altered after TiO₂ was doped. Kuyumcu et al. prepared TiO₂/NiFe₂O₄ by a complex assisted vapor thermal method (Firtina-Ertis & Kerkez-Kuyumcu 2022), and coupled Ag⁺ by wet-impregnation. This composite was used in photocatalytic hydrogen production, and the sample with 12 wt% of NiFe₂O₄ and 0.5 wt% of Ag⁺ presented the best photoactivity. •OH played a vital role during this process, as well as ⁠. Based on this, S-scheme heterojunction was proposed to interpret the photocatalytic mechanism (Fu et al. 2019). There was an internal electric field from NiFe₂O₄ to TiO₂, the photo-induced e⁻ on the CB of TiO₂ combined with photo-induced h⁺ on the VB of NiFe₂O₄. The h⁺ on the VB of TiO₂ reacted with the methanol via Ag⁺ and generated CO₂, while the e⁻ on the CB of NiFe₂O₄ combined with H⁺ and generated H₂. As long as proper band alignment between components, S-scheme heterojunction can be built between p-type and n-type semiconductors, or between two n-type semiconductors, or two p-type semiconductors (Naderi et al. 2023). This junction exhibits promoted redox ability similar to the Z-scheme, so it can be seen as a promising route to develop a highly efficient photocatalytic composite. The introduction of doping further enhanced photodegradation efficiency by prolonging charge carrier lifetime, providing an extra strategy to enhance photocatalytic application potential.

Gad-Allah et al. fabricated a core double-shell MnFe₂O₄@rGO@TiO₂ composite via a successive hydrothermal and calcination method (Abdel-Wahed et al. 2020). The composite exhibited superior photodegradation of ofloxacin under solar light, compared with single component. This work also revealed that MnFe₂O₄@rGO@TiO₂ of 50-mg rGO (calcined at 400 °C for 2 h) exhibited the highest photodegradation rate at pH = 5.4 of water polluted with ofloxacin. MnFe₂O₄ acted as recombination center due to its energy band position, thus leading to less efficient photoactivity, however, as an interlayer between MnFe₂O₄ and TiO₂, rGO attracted the photo-induced e⁻ from CB of TiO₂ and prolonged its lifetime to enhance the overall photoactivity. This ternary composite conserved its photoactivity after six consecutive runs, proving itself stable for long-term practical application. In conclusion, although the combination of MnFe₂O₄@TiO₂was attributed as type-II heterojunction, the substrate rGO acted as interlayer and thus hindered charge carrier recombination, promoting overall photocatalytic efficiency. It is suggested that, heterojunction components should be loaded onto functional material to further improve the entire photodegradation performance, for example, carbon-based materials.

Above all, the coupling of spinel ferrite with TiO₂ not only improved photocatalytic performance, but also endowed the magnetic recyclable ability. However, there are many types of junction structures as for photocatalytic mechanisms. Thus, it is suggested that more research work should be done to investigate the impact of fabrication methods and conditions on the band alignment of spinel ferrite/TiO₂ composites. Meanwhile, the doping and substrate loading strategies are also important, and related research is needed in future research work.

As one of the extensively studied photocatalysts, ZnO has attracted considerable interest due to its low cost, nontoxicity and high active properties (Goktas & Goktas 2021). However, pure ZnO can only be stimulated by UV light, due to its wide band gap (Hullavarad et al. 2009). This greatly lowers its response to sunlight, since UV light merely accounts for 4% of the sunlight spectrum. Besides, another disadvantage of ZnO is its low stability, since it can be affected by photo-corrosion (Taylor et al. 2019). Both factors greatly restrict its practical application as a photocatalyst. Thus, researchers have been developing various strategies to modify ZnO to improve its quantum efficiency, improving the photocatalytic activity and stability (Tian et al. 2012).

One of the effective methods is the combination of ZnO with other semiconductors (Goktas & Goktas 2021). Kuang et al. synthesized an octahedral-like ZnO/ZnFe₂O₄ composite via the solvothermal method (Kuang et al. 2019), and evaluated its photoactivity via dye photodegradation. The sample with a molar ratio of 3:1 presented the highest photodegradation performance, 82.7 and 91.2% after 120 min of UV-visible light and 140 min of visible light, respectively. Compared with ZnO and ZnFe₂O₄, the heterojunction exhibited higher photocatalytic performance, due to the synergetic effect of mesoporous structure, effective charge carrier separation at the interface, which improved light response ability, adsorption and degradation ability toward organic dye. this composite also exhibited good stability, no obvious decrease in photoactivity can be detected after four cycles. Choudhary et al. also reported the photodegradation using ZnO/ZnFe₂O₄ composite (Choudhary et al. 2022), prepared via the co-precipitation method. The superior photodegradation efficiency of ZnO/ZnFe₂O₄ was ascribed to extended light absorption range, and suppressed charge carrier recombination due to heterostructure. They also reported the outstandingly stable property of ZnO/ZnFe₂O₄. However, due to the band alignment, the reported composites from both papers were attributed to type-II heterojunctions, in which electrons moved to a less negative position while holes moved to a less positive position, leading to reduced redox ability.

Boutra et al. combined tannic acid (TA, a carbon material) with ZnO/MnFe₂O₄ via a two-step solvothermal method (Boutra et al. 2020). The ternary catalyst presented higher dye photodegradation performance under visible light. Both the narrow band gap of MnFe₂O₄ and the electron transfer ability of TA enhanced the photodegradation performance. The ternary composite still remained high photodegradation efficiency after five cycles, proving its high stability and reusability. In conclusion, the introduction of a third material such as TA can promote photodegradation performance, which is similar to the functions of the above-mentioned carbon materials.

The brief information of the above papers is summarized in Table 8. Above all, the introduction of spinel ferrite affects ZnO in several aspects: first, tuning the band gap of ZnO; second, enhancing the photocatalytic activity; third, improving the stability and recyclability. However, as to its practical application potential, the visible light response range and photocatalytic activity of ZnO-based composite should be further extended, such as the construction of a Z-scheme junction via third-party component addition. Thus, to develop novel photocatalysts with high visible-light absorption ability, high quantum efficiency and stability, is still a major research hotspot in the field of photocatalysis.

Recently, quantum dots (QDs) have been intensively studied due to their high specific surface area, low cost, numerous active sites, water insolubility, and excellent thermal stability (Sharma et al. 2019). Thanks to the unique quantum confinement effect derived from ultra-small size (2–10 nm), the accumulated electrons in QDs result in an enhanced light absorption range (Wang et al. 2020a). The combination of semiconductor and QDs has been reported as a cost-effective strategy to build photocatalytic heterojunction, which exhibits enhanced photocatalytic activity due to effective charge carrier separation (Yuan et al. 2021). In this section, some typical QDs are introduced as coupling semiconductor nanoparticles: ZnS, ZnO, Cu–In–S, and Ag–In–S. Although currently there have been no reports on the combination of these QDs with spinel ferrite, it is suggested that this combination is promising for the development of high performance photocatalysts.

Ikram et al. reported the photoelectrochemical conversion of ZnO(QD)/α-Fe₂O₃ thin films (Ikram et al. 2015). This composite showed 5.5 fold enhancement in photocurrent density under sunlight. α-Fe₂O₃ acted as solar energy harnessing material while ZnO QDs promoted charge carrier separation and transportation, since photo-induced holes can be trapped in the oxygen vacancy state, thus hindering the recombination of charge carriers. In conclusion, due to its wide band gap, ZnO(QD) exhibited similar optical properties to that of ZnS(QD), thus more work is suggested to be done to transform ZnO(QD)-based composite into a visible-light-driven photocatalyst.

Cu–In–S (CIS) has been reported as a spectral sensitizer, absorbing light across the entire visible light region (Yue et al. 2018). It can also inject electrons into a wide band gap semiconductor (Yue et al. 2018). Based on this, Raevskaya et al. reported a TiO₂/CIS@ZnS photoanode for photoelectrochemical solar cells (Raevskaya et al. 2016), and the optimal cell exhibited the average conversion efficiency of 8.15%, under the irradiation of a Xe lamp (30 mW cm⁻²). This cell also showed excellent stability. CIS@ZnS was used as a light harvester, presenting enhanced light conversion efficiency compared with CIS, since ZnS suppressed the recombination of charge carriers. In conclusion, the visible light absorption ability of CIS is much better than that of ZnS or ZnO, thus it can be considered as a promising route to combine CIS(QD) with spinel ferrite to build high-performance photocatalyst under the condition of appropriate band alignment.

Shi et al. reported the Ag–In–S QDs for photocatalytic CO₂ reduction to syngas under visible light (Shi et al. 2021). The Ag to In molar ratio affected the activity and selectivity by adjusting the band gap structure and charge carrier lifetime due to structural defects. The addition of Co(bpy)₃²⁺ (bpy = 2′2-bipyridine) can enhance the conversion of CO₂ to CO as well as promote the photo-stability of Ag–In–S QDs. In conclusion, the excellent visible-light-driven catalytic activity of Ag–In–S QDs proved itself a good option to be combined with spinel ferrite for the development of a high-performance photocatalyst. The adjustable band gap structure of Ag–In–S QDs can be a flexible feature to couple Ag–In–S QDs with spinel ferrite with varied band positions to realize proper band alignment.

Due to the excellent photocatalytic activity, stability and magnetic property, spinel ferrite-based composites have been intensively studied in pollutant degradation applications. The related research findings present excellent removal performance of organic pollutants, offering an effective strategy for wastewater treatment, and proving spinel ferrite-based materials as a promising candidate for industrial scale applications. This paper mainly concludes the construction and photodegradation application of spinel ferrite-based materials: spinel ferrite/carbon material, spinel ferrite/Bi-based materials, spinel ferrite/TiO₂ and spinel ferrite/ZnO, in which ZnFe₂O₄, NiFe₂O₄ or MnFe₂O₄ are chosen as spinel ferrites. The review compared three widely used preparation routes: hydrothermal (and solvothermal), co-precipitation and sol–gel auto-combustion methods. Among these, co-precipitation is promising for large-scale production, yet it still has certain drawbacks. This review also discussed the relationship between structural property and photocatalytic activity. Out of various junction structures, Z-scheme and S-scheme can realize both high photocatalytic activity and redox capability, and their loading onto function materials can further improve photodegradation performance, thus all these are considered key topics for future research. Moreover, there is a lack of research on pollutant degradation in real water bodies. All these aspects imply that, as for the practical application of spinel ferrite-based materials, there is still a long way to go, since a myriad of problems need to be fixed, which requires more systematic research work in the future.

The authors sincerely thank the financial support given from the National Natural Science Foundation of China (41977205).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.