Spinel-Ferrite-Decorated Graphene-Based Nanocomposites for Enhanced Photocatalytic Detoxification of Organic Dyes in Aqueous Medium: A Review

Abstract

The contamination of organic dye molecules in aquatic environments caused by the effluents released from vast industrial establishments has been a matter of serious concern in recent years, owing to their strong non-biodegradable nature and acute toxicity. Semiconductor-mediated visible-light-driven photocatalytic-dye detoxification is considered as a sustainable technique because it abundantly utilizes the available solar energy and releases environmentally friendly chemicals such as H₂O as byproducts. Adequate textural and microstructural properties, an extended visible-light response, pronounced isolation and transfer of photoinduced charge carriers, and facile magnetic-separation characteristics make spinel-ferrite-decorated graphene or its analogues’ (GO/rGO) nanocomposites (MFGNs) a versatile photocatalytic system for the efficacious detoxification of dyes. Therefore, this review article emphasizes their exceptional photodegradation performance in terms of systematic studies of the above-mentioned features, after a brief description of the synthesis protocols. The mechanism of the photodetoxification of dyes over MFGNs is precisely demonstrated in three different sections based on their redox abilities. The kinetics of the MFGN-driven photodecomposition of dyes are then highlighted. We discuss the role of different parameters such as pH, temperature, catalyst dose, and dye concentration in augmented photocatalytic-dye-degradation reactions. Finally, the emerging challenges that act as hurdles in achieving superior photocatalytic-dye-detoxification performance are addressed, along with the conclusion. We then propose some possible future research directions in order to overcome these challenges, for impressively accomplishing the photodegradation of organic dyes.

1. Introduction

Water is the most vital and essential component for the survival of living organisms. The use of safe and clean water leads to a healthier life. However, massive establishments in industries such as paper, pigments, textiles, apparel, etc., discharge substantial amount of dye-containing effluents into natural aqueous ecosystems [1,2]. It has been reported that 10–12% of dyes are used in textile industries every year, out of which 20% are released in the form of waste water into nearby water bodies. According to a World Bank report, 17–20% of water pollution is caused by the effluents from the dyeing and textile industries. The Ecological and Toxicological Association of the Dyestuffs Manufacturing Industry (ETAD) reported that 90% of the tested 4000 dyes exhibited a medial lethal dose (LD₅₀) of greater than 2 × 10³ mg kg⁻¹. Being strongly non-biodegradable, these organic molecules freely enter into the human body and pose serious ailments owing to their acute toxicity [3]. Therefore, it has been a challenge for the scientific community to eradicate these noxious substances from aquatic environments. Various techniques including adsorption, membrane separation, ion exchange, coagulation, etc., have been employed to remove these dyes from water and waste water [4,5,6]. Although the adsorption process has been widely adopted for the removal of dyes due to its cost effectiveness and ease of operation, its practical application is greatly restricted by several bottlenecks such as a prolonged desorption period, the generation of secondary pollutants, and the incomplete removal of dyes [7]. In contrast, the photocatalytic degradation of dyes over a semiconductor’s surface under visible light irradiation has been garnering excitement in recent years, as it completely decomposes dye molecules into harmless products such as H₂O and CO₂ [8,9].

Semiconductor-mediated photocatalytic-dye degradation is being extensively investigated after the pioneering work of Frank and Bard on the photocatalytic oxidation of CN^- in water over a series of semiconductors under light irradiation [10]. The fundamental mechanism of semiconductor-based photocatalytic-dye detoxification proceeds through three crucial steps (Scheme 1). Step I involves the absorption of photon fluxes with energy higher than or equal to the band gap of the semiconductor, followed by the production of electron–hole pairs. Step II accounts for the separation and transfer of these photoinduced excitons. In step III, the photon-produced charge carriers undergo redox reactions to release highly reactive species such as superoxide anions (O₂⁻) and hydroxyl free radicals (HO^•), which are responsible for the decomposition of dyes. In some cases, holes (h⁺) also participate in the degradation reaction [11,12]. However, the possibility of the recombination of the photon-produced excitons cannot be neglected. The efficiency of a semiconductor photocatalyst relies on its redox ability and the availability of charge carriers on its surface. Numerous semiconducting photocatalysts, namely TiO₂, CeO₂, ZnO, etc., have been applied for the photodegradation of dyes. Their poor visible-light-absorption capability fails to produce the required number of charge carriers under solar energy irradiation [13,14,15]. Usually, the visible-light-absorption capacity of a semiconductor is the prerequisite to achieve the required number of charge carriers. Subsequently, visible-light-responsive semiconductors such as CdS, g-C₃N₄, BiVO₄, etc., are used to harness solar energy in order to produce a higher number of electron–hole pairs. Nevertheless, the high rate of charge carriers’ recombination led them to exhibit poor photocatalytic performance regarding dye decomposition [16,17]. In addition, difficulties in the separation of these photocatalysts from a treated solution made the downstream processing tedious [18]. In this context, the search for a suitable photocatalytic system with an extended visible-light-absorption ability, a pronounced charge carriers’ isolation and transportation capability, and a good recoverable affinity are highly desirable.

Contrastingly, spinel ferrites with appreciable magnetic properties can facilely be separated from an aqueous environment under the influence of an external magnetic field. Their narrow band gap energy, required band edge positions, facile fabrication techniques, enlarged surface area, reasonable stability, and low cost have persuaded the research community to utilize them in versatile applications in the visible-light-induced photocatalytic-dye-degradation process [19]. Spinel ferrites are categorized as homogenous materials with the general formula MFe₂O₄ (MF). M represents doubly positively charged metal ions, and Fe exists as Fe³⁺. The M²⁺ ions are placed at the tetrahedral crystallographic sites, while the Fe³⁺ ions occupy the octahedral crystallographic sites. The structure of the ferrites mostly depends on the arrangement of the Fe³⁺ and M⁺² ions on both of the crystallographic sites. Spinel ferrite is a closely packed oxide structure where the oxide ions are in an FCC arrangement. In a normal spinel structure, the M²⁺ ions occupy the tetrahedral sites, whereas the octahedral sites are utilized by Fe³⁺ ions. For example, ZnFe₂O₄ crystallizes with a normal spinel structure, in which the Zn²⁺ and Fe³⁺ ions occupy the tetrahedral and octahedral sites, respectively_. In an inverse spinel structure, the Fe³⁺ ions are equally distributed between the tetrahedral and octahedral sites, and the M²⁺ ions occupy the remaining half of the octahedral sites. Ferrites such as NiFe₂O₄, CoFe₂O₄, etc., possess inverse spinel structures [20,21].

Notwithstanding several advantageous features, the photoactivity of MFs lies far away from the expectations, mainly due to the poor charge carriers’ separation and narrow visible-light-responsive window. In order to overcome these impediments, MFs are composited with 2D nanomaterials such as graphene as well as its derivatives GO and rGO (GN). The robust interaction between the two, with a retention of structure and properties, is very much crucial for augmented photocatalytic performance [22,23]. This can be achieved by adopting suitable synthesis protocols and can be confirmed through various characterization techniques. GNs have a π-conjugated electronic structure with an ultrafast electron-transfer ability, which facilitates the isolation and transportation of the photoinduced electrons from the MF surface upon thorough interaction. Concurrently, the recombination of electron and hole pairs is significantly inhibited, and the MF-decorated GN exhibits ameliorated photocatalytic performance [24]. Furthermore, the effective combination of GN with MFs extends the visible-light-absorption range to a longer wavelength region. In addition, the firm adherence of MF materials significantly improves MFGNs’ microstructural and textural properties, which facilitate the effective adsorption of dye molecules for efficient catalytic reactions. Owing to these auspicious advantages, MFGNs have been investigated in diversified fields of photocatalysis including energy generation and environmental remediation. The Wang group synthesized ZnFe₂O₄/graphene composites for the photocatalytic degradation of methylene blue (MB) in 2011. They successfully achieved 90% MB detoxification within 90 min of visible light exposure [25]. Hou et al. reported that 91.4% photodegradation of p-chlorophenol was obtained by ZnFe₂O₄/graphene composites [26]. Behera et al. fabricated a ZnFe₂O₄/rGO system for photocatalytic water splitting and ciprofloxacin degradation. This MFGN system was able to produce 410.32 µmol/h of H₂ via water splitting and perform 73% ciprofloxacin degradation under visible-light irradiation [27]. In another study, 28.8 μmol/(L·cm²) methanol was produced by the photocatalytic conversion of CO₂ over CuFe₂O₄/GO composites. The amount of methanol formed by the composite was 1.9 times greater than that of pristine CuFe₂O₄ [28]. However, extensive research has been carried out in the field of photocatalytic-dye detoxification using MFGNs to date. Therefore, it is of great scientific significance to summarize the photocatalytic performance of MFGN composites regarding the degradation of organic dyes.

In this review, the synthesis protocols for the preparation of MFGN composites are depicted. Further, we address the enhanced photocatalytic performance of these composites in terms of improved textural and microstructural properties, comprehensive charge carriers’ separation and migration, and magnified-light-response affinity. The kinetics and mechanism of the photogradation of dyes over MFGNs are discussed. The factors affecting photocatalytic performance over MFGN composites for dye degradation are briefly illustrated. The enhanced stability and facile recoverability of these classes of composites are depicted thoroughly. We then compare the dye-degradation ability of MFGNs with MFs. In conclusion, some of the advantages of the present system in exhibiting impressive performance regarding the photodestruction of organic dye molecules are depicted. At the end, we highlight the challenges that prevent the expected performance of MFGNs for photocatalytic-dye detoxification. The future research directions are then highlighted, with an objective of overcoming the persisting challenges for enhancing the photocatalytic-dye-detoxification capability of MFGNs.

2. Synthesis Protocols

Synthesis strategies have a great impact on the morphology, structure, and composition of nanocomposites. Thus, a well-organized synthesis protocol can help in achieving nanocomposites with better crystallinity, optimum particle size, and desired microstructure, which eventually leads to peerless photocatalytic activity [29]. Spinel-ferrite-decorated graphene-based nanocomposites (MFGNs) are synthesized via different processes such as solvothermal, hydrothermal, co-precipitation, sol–gel, and ball milling. Fei et al. reported a one-step solvothermal method to synthesize magnetic rGO/ZnFe₂O₄. They used ZnCl₂, FeCl₃·6H₂O, and GO as the precursors, with ethylene glycol as the solvent. The mixture was then autoclaved at 200 °C for 24 h to obtain the desired product. The strong and sharp diffraction peaks, as shown in the XRD pattern (Figure 1a), resembles those of the cubic ZnFe₂O₄ well [30]. This confirms that the zinc ferrite in the composite retains crystallinity. The disappearance of the (002) peak at 2θ = 26.2° corresponding to the graphite (Figure 1a) in the composite, indicated the destruction of the regular stacking of the rGO sheets in the presence of ZnFe₂O₄ nanoparticles. Solvothermal methods were also used to prepare the rGO/CoFe₂O₄ nanohybrid, using FeCl₃·6H₂O and CoCl₂·6H₂O as the precursors in a mixture of ethylene glycol and deionized water. With the addition of GO dispersion, the prepared solution was stirred and autoclaved at 180 °C for 12 h. The obtained composite was analyzed by XPS, as shown in Figure 1b. Intense Co 2P_3/2 peaks with binding energy at 780.5 and 786.4 eV confirmed the presence of Co²⁺ species in the composite. Likewise, Fe 2P level with binding energy at 711.4 eV and 724.5 eV corresponding to Fe 2P_3/2 and Fe 2P_1/2, respectively, confirmed the presence of Fe³⁺ ions in the composite. The presence of C 1s spectrum indicated surface functionalities in the composite. Combining all these pieces of evidence, it was confirmed that the CoFe₂O₄ nanoparticles were anchored firmly on the surface of the graphene nanosheets [31]. The Ai group was manifested from the SEM micrographs (Figure 1c) that solvothermally fabricated the CoFe₂O₄/rGO nanocomposites, which possessed intimate interfacial contact between the CoFe₂O₄ nanoparticles and rGO sheets [32]. Meidinchi and co-authors synthesized ZnFe₂O₄/rGO by a hydrothermal method. The aqueous mixture of Zn(NO₃)₂, Fe(NO₃)₃, and GO powder was subjected to hydrothermal treatment at 180 °C for 12 h to obtain the desired product. The Raman spectra of these samples are shown in Figure 1d. The G band and D band appeared near 1595 cm⁻¹ and at 1353 cm⁻¹, respectively, indicating the presence of GO. The I_g/I_d ratio for GO was found to be 0.77, while that for ZnFe₂O₄/rGO decreased to 0.54. This decrease in the I_g/I_d ratio indicated the presence of chemical bonds between the ZnFe₂O₄ nanoparticles and rGO. This confirmed the successful formation of the ZnFe₂O₄/rGO composite [33]. MnFe₂O₄/GO nanohybrids were reportedly synthesized by the co-precipitation method using FeCl₃, MnSO₄, and an aqueous graphene oxide solution. The mixture solution was subjected to stirring at 80 °C, and then the pH of the mixture was adjusted to 10.5 by adding the required volume of a NaOH solution. The nanohybrids obtained were characterized by Fourier Transform Infra-Red (FTIR) spectra. The absorption peaks at 490 cm⁻¹and 577 cm⁻¹ corresponded to the metal–oxygen bond stretching vibrations, which confirmed the formation of MnFe₂O₄ nanoparticles. Bands at 1044 cm⁻¹, 1236 cm⁻¹, 1415 cm⁻¹, and 1620 cm⁻¹ corresponded to GO (Figure 1e). This information helped in support of the successful formation of a MnFe₂O₄/GO composite [34]. Rostami et al. prepared ZnFe₂O₄/graphene nanohybrids via the sol–gel method using Zn(NO₃)₂·6H₂O, FeCl₃·6H₂O, and citric acid as precursors. The molar ratio of cations to citric acid was maintained at 1:2. SEM studies of a ZnFe₂O₄/graphene composite revealed that ZnFe₂O₄ nanocrystals with an 85–95 nm diameter were decorated with exfoliated graphene nanosheets [35]. The ball milling technique was also used to prepare CoFe₂O₄/rGO nanohybrids. In a typical experiment, a mixture containing a predetermined amount of cobalt and iron precursors along with a given amount of GO colloid was adjusted to pH 10 by adding NH₃ and then was milled with an oscillation of 25 s⁻¹ for 6 h to obtain CoFe₂O₄/rGO. It was observed from the TEM image shown in Figure 1f that CoFe₂O₄ nanoparticles with a 10 to 15 nm size are evenly distributed over exfoliated and wrinkled graphene nanosheets [36].

3. Role of Decoration of Spinel Ferrites on Graphene in MFGN for Enhanced Photocatalytic-Dye Detoxification

MFGNs show enhanced photocatalytic activity mainly due to three factors. Firstly, the π-conjugated electronic structure of GN acts as an efficient electron acceptor and transporter. Photogenerated electrons can readily be transferred to the GN surface owing to its higher work function [37,38]. Thus, the presence of GN in the MFGN composites can readily decrease the electron–hole pair recombination, and the photoinduced charge carriers are distinctly available to carry out redox reactions. Secondly, graphene plays a pivotal role in extending the light-absorption range of MF to the red end of the visible region [39]. As a result, MFGN becomes able to harness a larger amount of solar radiation, which is an indispensable aspect of augmented photoactivity. Moreover, the 2D structure of graphene enables MFGN to possess an enlarged surface area, enhanced pore structure, and adequate morphology, which are advantageous for acquiring abundant active surface sites. Theses active sites adsorb the organic dye molecules effectively for efficient photodegradation. In this section, we describe these aspects in detail.

3.1. Textural and Microstructural Properties of MFGN

The surface properties of MFGNs play an important role in the adsorption of pollutants. A large specific surface area, smaller particle size, and distinct porous structure help in the adsorption of the organic dyes over the MFGN surface. This ultimately leads to ameliorated photocatalytic activity. As shown in the FESEM image (Figure 2a) of the CoFe₂O₄/rGO composite, CoFe₂O₄ nanoparticles were homogenously anchored on the wrinkled rGO surface. The rGO loading led to a decrease in the agglomeration of CoFe₂O₄ along with a reduction in its particle size, from 58 nm to 49 nm. As a result, rGO-loaded CoFe₂O₄ exhibited 1.3 times more Methylene blue (MB) photodegradation under visible-light illumination compared to bare CoFe₂O₄ nanoparticles [40]. Baynosa et al. demonstrated that the improved textural properties of ZnFe₂O₄/rGO nanocomposites are responsible for the boosted photodetoxification of MB. They observed from the BJH-adsorption pore-size distribution curves that the average pore diameter for ZnFe₂O₄ was 12.4 nm (Figure 2b), while that of the ZnFe₂O₄/rGO nanocomposite was only 5.2 nm (Figure 2c). Furthermore, the hybridization of rGO with ZnFe₂O₄ has made a quantum leap in specific surface area, from the 113.8 m² g⁻¹ observed for pristine ZnFe₂O₄ (Figure 2d) to 141.3 m² g⁻¹ (Figure 2e). The improved textural properties of the composite photocatalyst possessed a higher number of active sites, which facilitated the adsorption of pollutants and, hence, promoted the photocatalytic activity of the ZnFe₂O₄/rGO nanocomposite that was observed. In fact, the ZnFe₂O₄/rGO nanocomposite decomposed MB at a rate about 7.6 times higher than the pristine one [41].

3.2. Separation and Migration of Photoinduced Charge Carriers

Photogenerated electron–hole pair’s recombination has been reported as one of the prime reasons for restricting the efficiency of photocatalysts. The recombination rate of an electron–hole pair is quite faster than their lifetime. Thus, they become recombined within a few nanoseconds of their production. This leads to the destruction of maximum charge carriers [42,43] The photocatalytic performance of MFs is greatly reduced owing to the rapid rate of recombination of charge carriers. The anchoring of GN sheets with MFs facilitates the mobility of photoinduced electrons, and, hence, the separation as well as transfer of excitons is improved significantly. This reduces the recombination rate and enables the excitons to be abundantly available at the photocatalyst’s surface for carrying out redox reactions. As a result, the photocatalytic efficiency of MFGNs is improved considerably [44]. The enriched charge carriers’ transfer and separation are assessed with the help of photoluminescence (PL), electrochemical impedance spectroscopy (EIS), and photovoltage–photocurrent response studies. Mokhtar et al. demonstrated the pronounced charge transfer and separation efficiency of a MnFe₂O₄/graphene composite using PL spectra at an excitation wavelength of 310 nm (Figure 3a). The maximum PL emission intensity of pristine MnFe₂O₄ is at 460 nm, indicating a higher rate of recombination, whereas that of the MnFe₂O₄/graphene nanohybrid was much reduced. The diminished PL intensity is indicative of distinct electron and hole separation in the presence of graphene. The different coloured line in the PL spectra represents the emission efficiency of pristine MnFe₂O₄ along with its composite with graphene at different weight ratios [45]. The increased charge-separation efficiency of the prepared composite photocatalyst is due to the robust adherence of graphene with MnFe₂O₄. Electrochemical impedance spectroscopy (EIS) is useful to investigate the charge-transfer resistance in a semiconductor composite. Nazim et al. conducted EIS studies for Co_xZn_1−xFe₂O₄ and its composite with rGO. From the EIS spectra given in Figure 3b, a large semicircle can be observed for pristine ferrite compared to that observed for the ferrite/rGO composite. This indicates a higher interfacial charge-transfer resistance in the case of pristine ferrite. Due to a lower resistance in the MFGN composite, the transfer of electrons to the graphene surface can conveniently occur. This process leads to boosted photocatalytic activity [46]. Fu et al. reported a conspicuous charge transfer and separation in MnFe₂O₄/graphene, with respect to that observed for pristine MnFe₂O₄ and GO. They observed that the photovoltage and photocurrent response of MnFe₂O₄/graphene is five times higher than that of MnFe₂O₄. For pristine MnFe₂O₄, the photocurrent of the electrode was reported to be 0.02 μA, whereas in the case of a MnFe₂O₄/graphene composite the value was increased to 0.098 μA. This indicates the prominent separation of the photogenerated electrons and holes in the composite as a result of the electronic interaction between the graphene sheets and MF [47].

3.3. Light Absorption Range

The band gap energy of a semiconductor plays a crucial role in determining the photocatalytic activity. Although MFs with narrow band gap energies can harness visible light from solar radiation, the efficiency of absorption is poor owing to their contracted absorption window. To achieve significant activity, the MFs should be capable of absorbing visible light in a broad range, so the maximum amount of photon fluxes can be utilized [48]. This can be achieved with a red shift in the visible-light absorption of the MFs. Graphene and its analogues not only succor in charge trapping and migration but also facilitate the broadening of the visible-light-absorption range [49]. Therefore, coupling of graphene or its analogues with MFs shifts the light response to a longer wavelength along with a wider absorption range. This enables MFGN-composite photocatalysts to absorb and utilize solar radiation in a proficient way [50]. The Iqbal group prepared sheet-on-sheet nanocomposites such as CaFe₂O₄/graphene and studied the effects of the addition of graphene on light-absorption ability. The light-absorption range of pristine CaFe₂O₄ was extended to the visible region of a longer wavelength in the presence of graphene (Figure 4a). As evident from the Tauc plots shown in Figure 4b, the band gap energy (BGE) of the pristine MF decreases with the addition of graphene. The BGE for pristine CaFe₂O₄ was determined to be 1.93 eV, whereas that for a CaFe₂O₄/graphene (0.25) composite was found to be 1.91 eV. This decreasing trend in BGE is attributed to the strong interaction between the MF and GN. The XPS spectra of the prepared MFGN shows that the Ca 2p and Fe 2p peaks are shifted slightly toward the lower binding energy, indicating the formation of a Fe-O-C covalent bond. As a result, additional energy states are introduced between the conduction and valence band of CaFe₂O₄, and, thus, the BGE was decreased. The decreased BGE allows for a greater number of photons from the visible region to be absorbed. As a result, the CaFe₂O₄/graphene (0.25) exhibited superior visible-light-responsive photocatalytic performance and degraded 99.4% of the MB in 90 min [51]. The UV–visible diffusive reflectance spectra shown in Figure 4c represent the change in the light-absorption range of pristine ZnFe₂O₄ with the addition of graphene. It was observed that pure ZnFe₂O₄ had a strong peak around <625 nm in the visible region of the solar spectrum. Nevertheless, with the introduction of graphene, the composite showed strong absorption peaks in the 650–800 nm range. The enhancement in the light-absorption range of pristine ZnFe₂O₄ was mainly caused due to the background absorption of rGO [52]. Liang et al. demonstrated that the visible-light response of a NiFe₂O₄/rGO composite was escalated by raising the rGO content from 0.2 to 0.35 g [53]. This could be attributed to the desired quantity of rGO, which reduces the reflection of visible light [24]. Therefore, MFGN with the required amount of GN exhibits pronounced photoactivity under visible-light illumination.

4. Fundamental Mechanisms for Photocatalytic-Dye Detoxification over MFGN

MFGN-based photocatalysis can be used as a breakthrough technology for the degradation of the organic dyes in aquatic systems. Photocatalytic organic dye detoxification takes place through a radical pathway, in which hydroxyl radical (HO^•), holes (h⁺) and superoxide (O₂^−•) radicals are mainly involved. After the generation of electron–hole pairs from the MFs by the interaction of visible light of the suitable wavelength presented in Equation (1), the photoinduced electrons are conveniently transferred through the graphene sheet (Equation (2)), leading to the improved separation of electrons and holes [54].

MFGN + hν → MFGN (h⁺ + e⁻)

(1)

e⁻ + graphene → graphene (e⁻)

(2)

The photoexcited and well-separated e⁻s carry out reduction reactions with surface-adsorbed oxygen molecules to generate highly reactive superoxide (O₂⁻) anions, as shown in Equation (3), only when the conduction band edge level of the MF lies above the potential of the O₂/O₂⁻ redox couple (${\mathrm{E}}_{\mathrm{O}2/\mathrm{O}2•-}^{\mathrm{o}}=-0.33\mathrm{V} \mathrm{vs}.\mathrm{NHE} \mathrm{at} \mathrm{pH} 7.0$). Similarly, strongly reactive ·OH radicals are produced through the water-oxidation reaction by the holes of the MFs with VB positions that are more positive than the redox potential of the H₂O/·OH couple (${\mathrm{E}}_{\mathrm{H}2\mathrm{O}/·\mathrm{OH}}^{\mathrm{o}}=2.29 \mathrm{V} \mathrm{vs}.\mathrm{NHE} \mathrm{at} \mathrm{pH} 7.0$) or that of the OH⁻/·OH couple (${\mathrm{E}}_{\mathrm{OH}-/·\mathrm{OH}}^{\mathrm{o}}=1.99 \mathrm{V} \mathrm{vs}.\mathrm{NHE} \mathrm{at} \mathrm{pH} 7.0$), according to Equations (4) and (5) [55,56]:

O₂ + graphene (e⁻) → O₂⁻

(3)

$${\mathrm{H}}_2\mathrm{O}+h_{VB}^{+} \to ·\mathrm{OH}+{\mathrm{H}}^{+}$$

(4)

$${\mathrm{OH}}^{-}+h_{VB}^{+} \to ·\mathrm{OH}$$

(5)

Therefore, the Conduction Band (CB) and Valence Band (VB) positions of the MFs play a pivotal role in determining the mechanistic pathways for the photodecomposition of organic dyes. Based on the potential band edge values, MFs can be classified into three categories, as presented in Scheme 2. These are (i) MFs with a strong reduction ability, (ii) MFs with a strong oxidation ability, and (iii) MFs with a moderate redox ability. Depending on the redox abilities of the MFs, the photodetoxification of dye molecules over the MFGNs proceeds by following three different routes.

4.1. Mechanism for Photodetoxification of Dyes over MFGNs with Strong Reduction Ability

The photoinduced electrons of MFGNs with a strong reduction ability, owing to the more negative CB levels of MFs such as ZnFe₂O₄, SnFe₂O₄, etc., are comfortably transported to the surface of graphene or its derivatives. These electrons reduce the surface-adsorbed O₂ molecules to O₂⁻ species, which decomposes the dye molecules according to Equation (6). On the other hand, the photogenerated holes at the VB of the MFs with an adequate potential can take part in the process of

O₂⁻+ Dye → CO₂+ H₂O

(6)

MFGN (h⁺) + Dye → Degraded products

(7)

dye detoxification, per Equation (7). However, the possibility of the formation of ·OH radicals through the holes-driven water-oxidation reaction is overruled due to the inadequate VB levels of these MFs. However, the production of ·OH radicals was ascertained with the help of the photo-Fenton reaction mechanism. According to this, the photogenerated electrons on the graphene sheets reduce H₂O₂ into ·OH and OH⁻, per Equation (8) [57].

Graphene (e⁻) +H₂O₂ → OH^. + OH⁻ + Graphene

(8)

$${\mathrm{Fe}}^{3+}+{\mathrm{OH}}^{-}\stackrel{\mathrm{h}\mathsf{\nu }}{\to }{\mathrm{Fe}}^{2+}+{\mathrm{OH}}^{. }$$

(9)

Then, the Fe³⁺ ions of MFGN combine with OH^- ions in the presence of light energy to form Fe²⁺ ions and ·OH radicals (Equation (9)). Similarly, the M²⁺ ions of MFGN undergo a reaction with H₂O₂ to liberate ·OH radicals and form M³⁺ ions, according to Equation (10). The Fe²⁺ ions that are formed are oxidized by H₂O₂, as presented in Equation (11), to produce Fe³⁺ ions as well as ·OH radicals. Additionally, these ions also convert M³⁺ ions into M²⁺ ions, as presented in Equation (12). The strongly reactive ·OH radicals that are formed participate in dye degradation, per Equation (13) [58,59]

M²⁺ + H₂O₂ → M³⁺ + OH⁻ + OH

(10)

Fe²⁺ +H₂O₂ → Fe³⁺ + OH⁻ + OH

(11)

Fe²⁺ + M³⁺ → Fe³⁺ + M²⁺

(12)

OH^. + Dye → H₂O + CO₂

(13)

The overall photodegradation process is presented in Scheme 3. Baynosa et al. reported that the photon-produced electrons at the CB of ZnFe₂O₄ migrate to the rGO surface, where they reduce the adsorbed O₂ to form O₂⁻. The obtained O₂⁻ species undergo degradation of MB. Concurrently, the holes produced at the VB of ZnFe₂O₄ directly decompose MB molecules without producing HO^• radicals, as the VB level of ZnFe₂O₄ is placed above that of the H₂O/·OH couple. They carried out scavenging tests in order to provide evidence in support of the existence of O₂⁻ and h⁺ as the primary reactive species. Figure 5a shows that the decrease in photodegradation was found to be 70.8%, 23.2%, and 4.6% in the presence of p-benzoquinone (pBQ), NaHCO₃, and isopropyl alcohol (IPA), respectively, which are the quenchers for O₂⁻, h⁺, and HO^•, respectively. These experimental pieces of evidence suggest that O₂⁻ and h⁺ are the primary reactive species for the photodegradation of MB by a ZnFe₂O₄/rGO composite [41]. In a similar investigation, the Qu group demonstrated that O₂⁻ and h⁺ are the prime reactive species for the photodegradation of AO7 over CuFe₂O₄/GO composites. The possibility of the formation of HO^• radicals is ruled out, since the VB position of CuFe₂O₄ is more anodic than that required to undergo WOR. On the other hand, the more negative CB level of CuFe₂O₄ enables the photoinduced electrons to reduce O₂ molecules into O₂⁻ species [60]. However, Lu et al. demonstrated that the major reactive species is the HO^• radicals, which are produced by the photoelectrochemical decomposition of H₂O₂ over the ZnFe₂O₄/graphene surface with the help of photon-generated electrons, according to Equation (8). In order to ascertain the formation of the HO^• radicals, Lu et al. carried out fluorescent spectral studies using fluorescent probe terephthalic acid (TA). Upon the interaction of visible light, TA easily reacted with the OH^. radicals to form highly fluorescent 2-hydroxy terephthalic acid (TAOH), with fluorescent spectral peak intensities at 430 nm (Figure 5b) that increased with an increase in time span. This evidences that under visible-light illumination and in the presence of H₂O₂, a ZnFe₂O₄/graphene composite produces OH^. radicals, which play a vital role in the degradation of organic dyes [61]. The h⁺s are reported as the major active species for the photocatalytic Congo red degradation on the ZnFe₂O₄/graphene composite, as only 6.9% of the dye was degraded in the presence of EDTA, which acts as the h⁺ scavenger. In addition, the OH^. radicals and O₂⁻ species are also involved in the degradation process [62]. In another study, Fan and co-authors suggested that the dye decomposition reaction over the ZnFe₂O₄/graphene composite might involve the Fenton and photo-Fenton processes, as presented in Equations (8)–(13) [24].

4.2. Mechanism for Photodetoxification of Dyes over MFGNs with Strong Oxidation Ability

The VB positions of MFs such as NiFe₂O₄, CdFe₂O₄, etc., are relatively more cathodic than the potential needed for water oxidation, as presented in Scheme 2. Therefore, the holes at the VB of MFs acquire strong oxidation abilities to convert H₂O/OH^- into strongly reactive OH^. species, which acts as a strong oxidant for the mineralization of the dyes. Further, the photogenerated electrons are migrated to the graphene surface and reduce the O₂ molecules into O₂⁻ species, which also takes part in the photodegradation process. Moreover, the holes at the VB of MFs receive enough potential to degrade the dyes. The detailed process for photocatalytic-dye degradation is explained in Scheme 4. According to the Chen group, the photogenerated electrons from the CB of NiFe₂O₄ are conveniently transferred to an RGO sheet due, to its excellent electron-transport property. This substantially improved the separation efficiency of the excitons. The electrons reduce the O₂ molecules into O₂⁻ anions at the RGO surface. In the meantime, the holes at the VB of NiFe₂O₄ with a robust oxidation ability react with OH⁻ ions to form OH^. radicals. In addition, a portion of the holes directly react the dye molecules into harmless products. Generally speaking, h⁺, ·OH, and O₂⁻ are involved in the efficient photodegradation of MB [53]. Mandhakini and co-authors proposed the decreasing tendency of different reactive species for the degradation of MB over NiFe₂O₄/rGO composites as ·OH > O₂⁻ > h⁺ > e⁻ [63]. A similar observation was reported by Rahman et al. for the photodecomposition of MB with the help of Ce-substituted NiFe₂O₄/rGO. However, the role of electrons in the dye-degradation process is considered negligible [64].

4.3. Mechanism for Photodegradation of Dyes over MFGNs with Moderate Redox Ability

Spinel ferrites such as CoFe₂O₄, MnFe₂O₄, etc., experience a moderate redox ability, as their VB and CB positions are placed above the potential required for water oxidation and below the potential needed for O₂ reduction, respectively. Since the direct production of ·OH and O₂⁻ from WOR and ORR, respectively, is thermodynamically unreasonable, the dye-degradation pathway proceeds through the photo-Fenton mechanism that was depicted in Section 4.1 and presented in Equations (8)–(13). Wei and co-authors demonstrated that MnFe₂O₄/rGO nanocomposites degraded MB efficiently through the photo-Fenton process [65]. In another set of experiments, ·OH radicals were found as the primary reactive species in the photodecolorization of MO and MB over CoFe₂O₄/rGO composites, and these were generated through the photo-Fenton mechanism [66]. Mazarji et al. conducted scavenging experiments in order to confirm the major reactive species that takes part in the degradation of BR46 over rGO-CoFe₂O₄ by the photo-Fenton process. They used ethanol, DMSO, and EDTA as scavenging agents for OH^., O₂⁻, and h⁺, respectively. Figure 5c shows that the dye-degradation reaction was significantly suppressed in the presence of ethanol, suggesting that ^.OH played the principal role in the pollutant degradation [67].

The major disadvantage of the photo-Fenton process is that it is usually carried out at a pH less than 3.0, as Fe³⁺ becomes precipitated as Fe(OH)₃ at higher pH values [68]. Further, the SO₄^- species generated from peroxymonosulfate (PMS) activation is considered as a potential candidate for the degradation of dyes [69]. The mechanism of dye detoxification through PMS activation is as follows. The metal ions (M²⁺) of MFGN, in the presence of catalytically activated PMS, produce SO₄⁻ and M³⁺, as shown in Equation (14). The M³⁺ and Fe³⁺ ions undergo reduction by MFGN-activated PMS (Equations (15) and (16)) to M²⁺ and Fe²⁺, respectively. The oxidation of Fe²⁺ ions in the presence of PMS, per Equation (17), led to the regeneration of Fe³⁺ along with the formation of SO₄⁻. The release of SO₄⁻ can also occur by the reaction of PMS with electrons provided by rGO, per Equation (18). The SO₄⁻ species that is formed is strongly reactive and decomposes the organic dyes efficiently into harmless products, as shown in Equation (19) [70].

HSO₅⁻ + M²⁺→ M³⁺ + SO₄⁻+ OH⁻

(14)

HSO₅⁻ + M³⁺→ M²⁺ + SO₅⁻ + H⁺

(15)

HSO₅⁻ + Fe³⁺→Fe²⁺ + SO₅⁻ + H⁺

(16)

HSO₅⁻ + Fe²⁺ →Fe³⁺ + SO₄⁻+ OH⁻

(17)

HSO₅⁻ + e_CB⁻ → SO₄⁻ + OH⁻

(18)

Dyes + SO₄⁻ → H₂O + CO₂

(19)

The Qi group used CoFe₂O₄/rGO to degrade organic contaminants such as Carbamazepine (CBZ), Tetracycline (TC), Rhodamine B (RhB), and Congo Red (CR) through PMS activation under LED illumination. They observed a complete degradation of dyes (RhB and CR) within 10 min [71].

5. Reaction Kinetics for Photocatalytic-Dye Degradation over MFGN

Kinetic study is an important strategy to evaluate the performance of a photocatalyst. The photocatalytic degradation of dyes follows the pseudo-first-order rate equation, per the Langmuir–Hinshelwood model [72]. This model can be expressed by Equation (20).

$$ln\frac{C_0}{C_t}=kt$$

(20)

where C₀ represents the initial dye concentration, C_t is the concentration at any time t under solar light, and k is the rate constant. The value of the rate constant is graphically calculated from the slope of the straight line obtained by plotting the values of $ln\frac{C_0}{C_t}$ vs. time (t). Zhang and co-workers reported that the k value for MB degradation over pristine CdFe₂O₄ was increased by 5 times, from 0.00145 to 0.00543 min⁻¹, upon the integration of graphene. This impressive increase in the k value for the CdFe₂O₄/graphene nanocomposites suggested their superior performance over the pristine one [73]. Higher rate constant values were also observed for MnFe₂O₄/rGO (0.019 min⁻¹) in contrast to bare MnFe₂O₄ (0.012 min⁻¹) during PMS-activated Orange II degradation. The composite photocatalyst exhibited a high rate of photodegradation of Orange II in the presence of anions such as Cl⁻, HCO₃⁻, NO₃⁻, and CH₃COO⁻, which are commonly available in waste water. The degradation rate was observed in the order: NO₃⁻ < CH₃COO⁻ < HCO₃⁻ < Cl⁻ [74]. The Zou group also separately studied the effect of Cl^- concentration on the degradation of diclofenac and acetaminophen [75,76,77,78]. The degradation rate constant for Orange II was increased from 0.019 to 0.156 min⁻¹ with an increase in the Cl⁻ concentration from 0 to 0.1 M. The Cl⁻ ions are oxidized to reactive chlorine species such as Cl₂ and HOCl at a higher Cl⁻ concentration, per Equations (21) and (22), respectively. These active chlorine species undergo degradation of Orange II, thereby increasing the degradation rate constant [74].

2Cl⁻ + HSO₅⁻ + H⁺ → SO₄²⁻ + Cl₂ + H₂O

(21)

Cl⁻ + HSO₅⁻ → SO₄²⁻ + HOCl

(22)

the MnFe₂O₄/rGO composite also exhibited an excellent degradation efficiency for the decomposition of methyl violet (MV), MO, MB, and Rhodamine B (RhB) including Orange II [74]. In general, MFGN photocatalysts can be considered as proficient photocatalysts for organic dye degradation.

6. Impact of Reaction Parameters on Photocatalytic-Dye Detoxification by MFGNs

Reaction parameters such as solution pH, catalyst dose, dye concentration, and temperature significantly influence the dye-degradation performance of the photocatalysts [79]. Studies optimized these parameters for the maximum efficiency of MFGNs for the photodetoxification of dyes.

6.1. pH of the Solution

The greater the extent of adsorption of the dye molecules on the MFGN surface, higher is the maximum photodegradation efficiency. Dye adsorption is largely influenced by the nature of the dye and the surface charge that appears on MFGN [80]. The pH of the dye solution strongly influences the surface charge of MFGN and, hence, the extent of dye adsorption. The photocatalytic activity of the NiFe₂O₄/GO nanocomposites were compared at different pH conditions. At a low pH, the surface of the composite photocatalyst becomes positively charged (as pH < pH_pzc). Since MB is a cationic dye, it occupies positive charges, which are repelled from the catalyst surface, acquiring similar charges. As a result, MB molecules could not effectively be adsorbed on the photocatalyst, and a dismal photocatalytic detoxification performance was observed. Conversely, NiFe₂O₄/GO surfaces attain negative charges under alkaline conditions where pH > pH_pzc. This expedites the positively charged MB molecules to become greatly adsorbed on the negatively charged composite surfaces through electrostatic attraction, and the composite photocatalyst exhibited impressive performance for MB degradation. Additionally, the diffusion of surface-generated hydroxyl radicals resulted in the maximum degradation of MB at a higher pH. The composite showed its maximum MB degradation of 90.84% at pH 10 [81]. Zhang et al. also reported similar results for the degradation of MB over a CdFe₂O₄/graphene composite. They achieved more than 94% MB degradation at pH 9–10 [73].

6.2. Temperature

The degradation of organic dye over a photocatalyst is largely influenced by temperature. The collision probability between the dye and the photocatalyst increases with an increase in temperature. This leads to an enhanced photodegradation efficiency, and maximum dye decomposition is observed at a particular temperature [82]. A further increase in temperature causes the desorption of dye molecules from the photocatalyst surface. As a result, the photodegradation rate is substantially decreased. Liang and co-workers studied the effect of temperature on photocatalytic MB degradation over a NiFe₂O₄/rGO composite. The degradation rate constant (k) started increasing with an increase in temperature up to 25 °C. However, the rate constant value was significantly decreased by raising the temperature further (Figure 6a) [53]. Therefore, 25 °C is considered as the optimum temperature for maximum photodetoxification.

6.3. Catalyst Dosage

Catalyst dosage/loading indicates the minimum amount of MFGN photocatalyst required for the maximum photodegradation of a dye. It has a distinct role in photocatalytic-dye detoxification. Kodasama et al. studied the effect of CuFe₂O₄/GO loading on the photocatalytic decomposition of reactive black 5 (RB5). They observed that the degradation of RB5 was increased from 66.7% to 100%, when the catalyst dose was raised from 0.05 g/L to 0.5 g/L. This could be attributed to the increase in the number of active sites and the surface area of the composite photocatalyst, when increasing the catalyst dose’s amount. The availability of an enlarged surface area and a higher number of surface active sites facilitated the adsorption of dye molecules on CuFe₂O₄/GO. Moreover, an increase in the mass-to-volume ratio of the photocatalyst accelerated the light-absorption efficiency. As a result, an improved photodegradation performance was observed [83]. Yuan and co-authors demonstrated, as shown in Figure 6b, that 50 mg (0.5 g/L) of ZnFe₂O₄/rGO displayed the maximum MB degradation. A further increase in the catalyst amount led to a decrease in the photodetoxification, probably due to the blockage of active sites that resulted from the increased extent of agglomeration [84]. In addition, the increased agglomeration prevented the passage of visible light, due to which the dye-degradation rate is decelerated.

6.4. Effect of Dye Concentration

The initial dye concentration has a major role in the photocatalytic-dye-degradation process. With an increase in dye concentration, the active sites on the surface of the MFs are accommodated by the dye molecules and gradually attain saturation. Therefore, the light-absorption efficiency of the photocatalyst is largely affected [85,86]. This caused the inadequate production of photogenerated charge carriers, due to which the photodetoxification efficiency of the MFGNs was greatly lowered. Chen et al. investigated the effect of initial AO7 concentration on its degradation over a CuFe₂O₄/GO surface. They observed that the rate of photodegradation for AO7 was decreased with an increase in its concentration (Figure 6c) [60]. However, Shang and co-authors demonstrated that a CoFe₂O₄/graphene composite took 2 h for the complete decomposition of a 20 mg·L⁻¹ MB solution, while a 40 mg·L⁻¹ MB solution was completely degraded in 3 h. Dye degradation initially takes place slowly at a higher concentration due to the diminished light-absorption capacity. After the degradation of some of the dyes, the light response was improved, which enhanced the degradation rate and complete degradation occurs at longer time span [87].

7. Comparison of Photocatalytic Performance of MFGNs for Dye Detoxification

MFGNs exhibited excellent performance for the photocatalytic degradation of organic dyes with respect to that of pristine MFs. The performances of various MFGNs were compared with those of bare MFs and are listed in

Table 1. Comparison in performance of MFGNs for photocatalytic-dye detoxification with pristine MFs.

SFGNs	Targeted Dye	Catalyst Dosage (gL−1)	Initial Dye Conc. (mgL−1)	Degradation Efficiency (%)	Number of Times Increase in Activity w.r.t. Pristine SF	References
CoFe2O4/rGO	BR46	0.02	20	62	1.45	[67]
CoFe2O4/rGO	BR18	0.02	20	79	2.25	[67]
ZnFe2O4/rGO	MB	0.5	10	98.0	7.6	[41]
MnFe2O4/GO	MB	0.3	10	97	1.15	[89]
MnFe2O4/GO	MB	0.25	20	100	1.2	[47]
CaFe2O4/G	MB	0.2	10	99.4	1.7	[51]
CdFe2O4/G	MB	1	10	89.22	2.2	[73]
CoFe2O4/GO	MB	0.25	10	~100	2.9	[87]
ZnFe2O4/G	Congo red	0.5	15	92.21	2.3	[62]
MnFe2O4/rGO	Orange II	0.05	20	90	1.6	[74]
Ce-doped NiFe2O4/rGO	MB	0.1	10	94.67	1.8	[64]
ZnFe2O4/G	RhB	1	20	100	3.3	[61]
ZnFe2O4/rGO	MB	0.5	50	92.4	1.04	[84]
MnFe2O4/rGO	MB	0.4	-	62	1.7	[65]
MnFe2O4/GO	MB	1	10	100	4	[45]
ZnFe2O4/G	MB	0.2	50	84	1.7	[24]
CoFe2O4/rGO	MB	0.5	5	74	~1.4	[40]
NiFe2O4/rGO	MB	0.25	20	99.1	~90	[53]
CoFe2O4/rGO	MB	0.25	20	100	-	[36]
CoFe2O4/rGO	RhB	0.25	20	72.2	-	[36]
CoFe2O4/rGO	MO	0.25	20	37.5	-	[36]
CoFe2O4/G	MB	0.25	20	100	~10	[72]
CoFe2O4/G	RhB	0.25	20	94	-	[72]
CoFe2O4/G	MO	0.25	20	71	-	[72]
CoFe2O4/G	Active-black BL-G	0.25	20	66	-	[72]
CoFe2O4/G	Active-red RGB	0.25	20	61	-	[72]
CoFe2O4/rGO	MB	1	10	73	~20	[32]
CoFe2O4/rGO	MO	0.5	20	100	-	[54]
CoFe2O4/rGO	MB	0.5	30	100	-	[54]
CoFe2O4/rGO	RhB	0.5	25	100	-	[54]
CoFe2O4/rGO	RhB	0.2	10	~100	~4	[71]
NiFe2O4/rGO	MG	0.2	-	96.5	~1.5	[63]
ZnFe2O4/Graphene	MO	1	15	5	-	[52]
ZnFe2O4/Graphene	MB	1	15	56	-	[52]
ZnFe2O4/Graphene	RhB	1	15	15	-	[52]
NiFe2O4/GO	MB	0.5	-	~40	-	[81]

. Mandal et al. reported that 97% of MB was degraded within 60 min over a MnFe₂O₄/rGO composite, whereas bare MnFe₂O₄ nanoparticles degraded only ~84% in 290 min [88]. Mokhtar et al. also demonstrated that a MnFe₂O₄/graphene oxide composite degraded 100% of MB in 60 min. The degradation efficiency was found to be four times better than that displayed by its pristine counterpart [45]. These results suggest that the incorporation of graphene into MFs facilitated the transport of photoinduced electrons through its large aromatic structure, and, hence, the recombination of the electron–hole pairs was appreciably hindered. This led to an improved photocatalytic performance. Gan et al. fabricated CoFe₂O₄/graphene composites containing 5, 10, 20, and 40 wt % of graphene and compared their photocatalytic performance for MB degradation. Only ∼34% of MB was degraded by pristine CoFe₂O₄ in 2 h. Conversely, the CoFe₂O₄/graphene nanocomposites exhibited a much higher efficiency for dye degradation. The CoFe₂O₄/graphene (5 wt%) required 2 h to degrade 100% of the MB present in the system. When the graphene content was increased to 10 wt%, the photocatalytic activity increased further. In particular, the photocatalyst required only about 1 h to complete the degradation process. However, the photocatalytic activity was adversely affected with a further increase in graphene content. Complete MB degradation was observed after 2 and 3 h, when the composite photocatalyst contained 20 and 40 wt% of graphene, respectively. It was confirmed that 10 wt% of graphene was the adequate amount to form a robust interaction with CoFe₂O₄, due to which the composite exhibited its maximum degradation efficiency. A further increase in the graphene content blocked the surface active sites of CoFe₂O₄ for the dye molecules, and, hence, the degradation efficiency was considerably decreased [87].

8. Stability and Recoverability

The stability of a photocatalytic system under operating conditions is a vital aspect for its practical applications. Photocatalysts with substantial chemical and physical stability can be recovered and reused frequently. This increases their cost-effectiveness and makes them decorous for large-scale production [20]. The presence of graphene in MFGNs acts as a support material, and spinel ferrite nanoparticles are decorated over graphene surface through strong chemical bonds. This anchorage enables MFGNs to be used frequently without any noticeable loss in physicochemical properties. The magnetically recoverable rGO/ZnFe₂O₄ nanocomposite showed no significant decrease in its photocatalytic activity regarding MB detoxification for up to nine consecutive cycles, and the degradation rate was maintained above 90% for each cycle [88]. Rahman and co-workers reported that a Ce-doped Nickel ferrite/rGO composite retained its crystal structure even after five consecutive cycles. They observed no such significant change in the XRD pattern of the sample after being used for five runs (Figure 7a). Any changes in the chemical bonding and structure after reuse can easily be evident from FTIR studies. Figure 7b shows the FTIR data for a Ce-doped NiFe₂O₄/rGO composite before and after use. It was observed from the spectra that the vibrational modes are quite similar for both the samples, except for some disturbances in the peaks around 1250 and 1000 cm⁻¹ for the fresh sample. The deviation might be due to the presence of MB. This suggests that the MB was completely mineralized over the photocatalyst under visible-light irradiation [64]. Yin et al. studied the microstructural changes in a CoFe₂O₄/rGO nanocomposite after six consecutive cycles. They compared the TEM image of the prepared nanocomposite with that obtained after six cycles of dye degradation (Figure 7c). The two images showed no remarkable changes, revealing that the composite photocatalyst retained its morphology even after six cycles of use, and the ferrite nanoparticles were arranged uniformly as before on the exfoliated graphene sheets [31]. These reported pieces of evidence prove the extraordinary stability exhibited by MFGN nanocomposites for the photodetoxification of organic dyes.

The facile recoverability of a photocatalyst from a treated solution is another important criterion for large-scale application. MFGNs contain MFs as magnetic components, which can conveniently be separated from the suspension with the help of an external magnetic field. Mandal et al. studied the magnetic hysteresis of MnFe₂O₄ and a MnFe₂O₄/rGO composite using a vibrating sample magnetometer (VSM). They found that both of them show a super-magnetic nature. The saturation magnetization of bare ferrite was found to be 41 emu g⁻¹. However, Mandal et al. observed that the magnetic behavior of the MnFe₂O₄/rGO nanocomposite is weakened to a small extent with a saturation magnetization of 37 emu g⁻¹ (Figure 7d). As graphene is a magnetically inactive material, it reduces the saturation magnetization value of the composite photocatalyst [89]. Nevertheless, the composite can still be magnetically separated from the applied solution. Figure 7e shows the facile recovery of a NiFe₂O₄/rGO composite from a dye-degradation system using an external magnetic field [90].

9. Conclusions

Dye-containing effluents contribute significantly to elevating the level of water pollution. Among existing techniques, the semiconductor-mediated photocatalytic detoxification of dyes is considered to be an advantageous technology, since it utilizes sustainable solar light as well as abundantly available water resources. Additionally, the formation of harmless H₂O and CO₂ molecules as byproducts makes this technique environmentally friendly. MFGNs exhibit excellent performance for the photodegradation of organic dyes. For instance, a ZnFe₂O₄/rGO composite degraded MB 7.6 times more than pristine ZnFe₂O₄ under solar-light irradiation. The augmented photoactivity was attributed to the impeccable hybridization of GNs with the MFs. This review article depicts various synthesis protocols for the fabrication of MFGNs, focusing on their augmented dye-degradation performance and highlighting the possible photocatalytic-dye-detoxification mechanisms. An overview is presented in Scheme 5.

Synthesis strategies play an important role in modulating photoactivity through the optimization of crystallinity, particle size, and microstructural properties. A solvothermally synthesized ZnFe₂O₄/rGO nanocomposite retained the cubic structure of ZnFe₂O₄ along with an enhanced crystallinity. CoFe₂O₄/rGO nanocomposites prepared by the ball milling technique possessed 10–15 nm particles, which were uniformly distributed over exfoliated graphene sheets. These composite photocatalysts displayed excellent performance in contrast to their pristine counterparts.

The robust decoration of GNs with MFs caused a synergetic effect of the modified textural and microstructural properties, pronounced charge carriers’ separation and migration, and an extended light-absorption range, with the composite photocatalysts displaying an exceptionally high dye-degradation efficiency. The integration of rGO with CoFe₂O₄ reduced the particle size to 49 nm. Similarly, rGO anchoring favored a decrease in the pore diameter of ZnFe₂O₄ from 12.4 nm to 5.2 nm and enhanced its surface area from 113.8 to 141.3 m² g⁻¹. PL, EIS, and photocurrent studies revealed distinct isolation and increased transportation of charge carriers. This elongated the life time of the charge carriers that are predominantly available at the MFGN surface, which improved the photodegradation of dyes. For example, the photocurrent response of MnFe₂O₄/graphene was found to be five times higher than that of the pristine one, as an indicative of the better separation of the photoinduced electron and hole pairs. It was also evident that GNs broaden the light-response range, when combined properly with MFs. A CuFe₂O₄/rGO photocatalyst showed an extended light response up to 800 nm. Moreover, parameters such as pH of solution, temperature, catalyst dose, and dye concentration greatly influence the dye-degradation reaction. The exceptional stability and facile recoverability experienced by MFGNs can be a major advantage in the support of their possible practical applications for dye degradation.

The enhanced photodegradation of dyes over MFGNs could be explained through three different mechanisms, depending on the redox ability of MFs. These are (i) MFGNs with a strong reduction ability, (ii) MFGNs with a strong oxidation ability, and (iii) MFGNs with a moderate redox ability. In the first case, strongly reactive OH free radicals are generated through the photo-Fenton process. The second mechanism proposes the release of OH free radicals directly by h⁺-driven water-oxidation reactions. According to the third mechanism, reactive species such as OH and SO₄⁻are produced through the photo-Fenton and PMS activation processes, respectively. These species with robust oxidizing abilities greatly detoxify the dye molecules.

By and large, MFGNs exhibited excellent photocatalytic activity for the degradation of organic dyes. However, this photocatalytic system faces a number of challenges for its practical applicability. These emerging challenges are addressed in this section, with the hope for possible solutions in the future. Despite prominent charge carriers’ separation and migration owing to the robust integration of GN with MFs, the life span of the excitons on MFGNs are required to further increase in order to achieve desired photoactivity. Though MFGNs exhibited an extended light-absorption range compared to pristine MFs, the light-response window needs to be further widened to Near Infra-Red (NIR) region in order for the complete utilization of the solar spectrum. It is highly desirable to improve MFGNs’ redox ability, which is essentially required for producing enough reactive species with a strong oxidizing ability to carry out the dye-degradation reaction efficiently. The pH of the photodegradation reaction should be neutrally maintained at pH = 7.0, for application in natural water bodies. Efforts may, therefore, be explored in the future to overcome these emerging challenges to enable the MFGNs’ photocatalytic system to become a benchmark in the photodetoxification of dyes.

Future research directions may be shaped as follows, in order to obtain the desired MFGN systems for the efficient photocatalytic detoxification of dyes. Construction of ternary composites by integrating narrow band gap semiconductors with MFGN might be a timely strategy to extend the light-absorption range up to NIR, to harness the complete solar spectrum. Designing Z-scheme/S-scheme heterostructured architectures would enable MFGN systems with a robust redox ability as well as improved charge carriers’ separation and migration. This would boost the efficiency of the present photocatalytic system, thus exhibiting a strong performance for the dye-detoxification process. Although operation of a photocatalytic-dye-decomposition reaction at natural pH is a strenuous task, the proper design of a photocatalytic system might produce a coveted solution for this problem. The emerging challenges and future research directions are depicted in Scheme 6. A cumulative effort from all quarters including scientists, researchers, and engineers will pragmatically assist to move MFGNs to the front line in the process of the complete detoxification of organic dyes through solar-induced photocatalysis. This will certainly shed light onto the survival of aquatic lives and downstream users in the future. We hope this review article will be helpful for researchers in understanding the concepts required to design an effective photocatalytic system for the complete removal of dyes from natural water bodies.