Trends Sci. 2026; 23(4): 11805

Recent Advancements in Metal Ferrite Hybrid Photocatalysts for Wastewater Treatment

Yogesh Kumar¹, Vidushi Karol², Abhishek Awasthi³,

Anshu Sharma^1,*, Vineet Kumar⁴ and Kulvinder Singh⁵

¹Department of Physics, School of Basic and Applied Sciences, Maharaja Agrasen University,

Himachal Pradesh 174103, India

²Department of Physics, Department of Applied Sciences, Chandigarh Engineering College, Punjab, India

³Department of Biotechnology, School of Basic and Applied Sciences, Maharaja Agrasen University,

Baddi 174103, India

⁴Department of Biotechnology, School of Bioengineering and Biosciences, Lovely Professional University,

Punjab India

⁵Department of Chemistry, DAV College, Chandigarh 160011, India

(^*Corresponding author’s e-mail: [email protected])

Received: 6 September 2025, Revised: 23 October 2025, Accepted: 1 November 2025, Published: 25 December 2025

Abstract

Environmental pollution, especially water contamination, presents significant challenges to ecosystems, public health, and sustainability. Ferrite-based nanomaterials, specifically hybrids combining ferrites with several materials, have emerged as promising photocatalysts for wastewater treatment. These hybrids leverage ferrites’ unique structural, optical, and magnetic properties while addressing challenges such as electron-hole recombination, limited visible light absorption, and agglomeration. Recent advancements in synthesis techniques, including sol-gel, co-precipitation, and green methods, have enabled precise control over ferrite hybrid properties, enhancing photocatalytic efficiency and scalability. The integration of ferrites introduces synergistic effects, such as enhanced charge carrier separation, expanded light absorption into the visible spectrum, and enhanced surface area. Innovations like doping, plasmonic enhancement, and Z-scheme heterojunctions have further optimized these materials for efficient pollutant degradation. Studies have demonstrated their potential to degrade dyes, pharmaceuticals, and other emerging contaminants under visible and solar light. Despite their promise, challenges remain in understanding the mechanisms of charge transfer, optimizing synthesis for large-scale production, and ensuring stability in real wastewater conditions. This review highlights the transformative potential of ferrite-based hybrids as scalable, cost-effective, and environmentally friendly solutions for advanced wastewater treatment. Their development marks a crucial step toward mitigating water pollution and achieving global water sustainability goals.

Keywords: Photocatalysis, Wastewater treatment, Metal ferrites, Hybrids, Environmental application

Introduction

Environmental pollution is a pressing concern in contemporary discourse, holding profound implications for ecosystems, public health, and global sustainability [1,2]. Within this intricate landscape, water pollution surfaces as a critical issue exacerbated by the substantial volume of wastewater generated from diverse sources, including domestic, commercial, and industrial activities. This wastewater carries dissolved and suspended matter, including chemicals, soaps, and heavy metals [3]. Particularly, industrial wastewater, with its high pollutant load, poses a significant threat by contaminating water resources, risking human health, and negatively impacting ecosystems [4-7]. The gravity of the issue is underscored by poor water quality being responsible for many diseases, with over 80% of global diseases linked to contaminated water [8,9]. Additionally, the global water crisis is further intensified by the discharge of untreated wastewater from various sources, including kitchens, bathrooms, toilets, industrial effluents, and agricultural runoff [10]. The UN Environment Programme analysis transforms the narrative by redefining wastewater as a transformative solution, emphasizing its potential for sustainable wastewater management and endorsing a circular economy approach. If strategically managed, wastewater has the capacity to provide alternative energy to half a billion people, exceed global desalination capacity tenfold, and offset more than 10% of global fertilizer use [11,12]. However, with only 11% of treated wastewater currently reused globally, there exists an urgent need to harness the potential of wastewater for clean water, sustainable energy, and nutrient resources, envisioning a pivotal solution to global climate challenges and water insecurity [13].

The treatment of industrial wastewater poses several challenges. Industrial wastewater is more complex than domestic sewage and contains numerous organic and inorganic pollutants, making it difficult to meet stringent discharge standards and water reclamation requirements [14]. Industrial wastewater treatment techniques include adsorption, advanced oxidation processes (AOPs), coagulation and flocculation, biological treatment, membrane filtration, precipitation, ion exchange, and dissolved air flotation (DAF) [15-19]. Adsorption is a promising technique for wastewater treatment, but it can lead to major sludge generation and high costs for certain adsorbents [20,21]. Other challenges with adsorption techniques are Low efficiency due to partial adsorbent utilization, challenges in chemical regenerant disposal during bed regeneration [22], issues of low separation efficiency, high operational costs and extended treatment hours in conventional approaches for oily wastewater. Conventional biological treatment technologies alone are often inadequate for meeting these standards [23]. Additionally, there is a risk of secondary pollutant generation associated with various wastewater treatment techniques [24]. However, Researchers are employing diverse strategies to overcome limitations of various methods and optimize wastewater treatment plants for maximum efficiency and sustainability. AOPs offer several advantages over traditional methods for industrial wastewater treatment. AOPs utilize reactive free radical species to degrade persistent compounds and minimize toxicity [25]. These mechanisms, combined with hydrodynamic cavitation, ultrasound, and sono-electrochemical oxidation, result in hybrid AOPs with significant oxidative capacities [26]. AOPs are effective at removing a variety of pollutants, including refractory organic compounds, micropollutants, and color and odor components [27]. They can also achieve high removal percentages, such as 98.43% for oil from oily wastewater [28].

Photocatalysis, a promising advanced oxidation process (AOP), utilizes photocatalysts to expedite light-induced chemical reactions, providing a cost-effective, versatile, and environmentally friendly approach for mitigating diverse harmful pollutants in both air and water [29]. Photocatalysis can effectively remove emerging pollutants like pharmaceuticals [30,31], pesticides [32,33] and organic compounds [34-36] from industrial and municipal wastewater using visible, UV radiation, and solar light. A variety of photoreactors, such as slurry, membrane, annular, closed-loop step, rotating-drum, and micro photoreactors, have been constructed and used for wastewater treatment [37]. These reactors have used heterogeneous, homogeneous, and mixed photocatalyst systems to accomplish effective photocatalysis [38]. Photocatalysis operates at ambient temperatures and pressures, reducing the requirements for reaction facilities and operating costs [39]. However, despite its advantages, photocatalysis for wastewater treatment faces several challenges depending upon the type of photocatalyst used. Metal oxides, such as TiO₂, which is the most studied photocatalyst, have large band gaps (≥ 3 eV), which restricts their absorption of visible light and limits their photocatalytic activity under sunlight, resulting in limited applicability due to the expense of producing ultraviolet light [40]. Photo corrosion of metal oxide-based photocatalysts is also a significant challenge in their practical use. TiO₂, Cu₂O, and ZnO have been found to exhibit increased corrosion rates under illumination, leading to degradation of their performance [41,42]. In the visible light regime, metal chalcogenides like MoS₂, ZnS, CdS, etc. have better solar spectral response due to their narrow band gap. However, Metal chalcogenides, while possessing diverse architectures and narrow band gaps, face several drawbacks in photocatalysis. These include limited synthesis methods, low degradation efficiency, rapid recombination of charge carriers, difficulty in separation and recovery from the reaction mixture, and poor chemical stability leading to the photo corrosion process, and reduced efficiency in pollutant removal [43-46].

Ferric oxide nanotechnology is the manipulation of nanoscale materials to produce certain properties and functionalities, which allow them to be used in a variety of applications [47]. Nanoparticles, which range in size from 1 to 100 nm, have distinct features in terms of form, magnetic, structure, electrical conductivity, catalytic activity, mechanical strength, optical scattering, and physicochemical properties [48-50]. The term “magnetic nanoparticles” encompasses a broad spectrum of nanoscale materials that benefit from magnetic properties, making them versatile for diverse applications. This category includes metallic, bimetallic, and metal oxide nanoparticles, as well as core-shell and Janus-type structures in different configurations [51,52]. Magnetic nanoparticles can be synthesized using various transition metals such as nickel (Ni), iron (Fe), and cobalt (Co), along with their respective compounds. Notably, iron exhibits multiple valence states, ranging from 0 to +3. Haematite (α-Fe₂O₃) is a popular material due to its abundance, low cost, and high photo-stability, making it suitable for many applications [53]. Iron oxides come in several forms, such as maghemite (γ-Fe₂O₃), goethite (FeO(OH)), haematite (α-Fe₂O₃), and magnetite. Haematite (α-Fe₂O₃) has the highest thermodynamic stability when exposed to oxygen. Due to their core-shell structure, iron oxide nanoparticles (IONPs) have characteristics that are similar to both hydrous iron oxides and metallic iron [54,55]. Because of these characteristics, IONPs are positioned as prospective enzyme mimics for a range of uses. Iron oxide systems’ special properties have been extensively studied for applications in biology and technology [56]. IONPs are positioned as prospective enzyme mimics for a range of applications due to their characteristics. Numerous studies have been conducted on the special properties of iron oxide systems for applications in biology and technology [56]. Iron (III) oxides have different magnetic properties from their bulk equivalents, particularly as particle size decreases, due to nanoscale confinement and surface effects. Among the many benefits of magnetic IONPs are their high level of physical and chemical stability, biocompatibility, and affordability [55]. In the nanoscale range, the quantum effect significantly influences their magnetic, electrical, and optical behaviors. Notably, Fe₃O₄ and γ-Fe₂O₃ nanoparticles exhibit superparamagnetic properties at sizes around 10 - 20 nm, enhancing their performance in various applications. Their high surface-to-volume ratio also improves dispersibility in solutions, making them highly suitable for practical applications. Metal nano ferrites have emerged as promising photocatalysts for industrial wastewater treatment [57-61]. The band gap energies of these ferrite nanoparticles range from 2.80 to 3.5 eV, indicating their potential for visible light photocatalysis. These ferrites exhibit unique physical and chemical characteristics that make them effective in eliminating pollutants from wastewater. Their tunable band gaps, surface areas, and magnetic properties enable efficient photodegradation of organic pollutants. The high stability and magnetization of ferrites allow for easy separation and recovery of the photocatalysts from the treated water [62,63] Moreover, enhanced electrical conductivity in transition metal ferrites arises from efficient electron hopping between distinct metal valence states at oxygen sites, facilitating the transfer of charge carriers [64,65]. Owing to their Abundant availability, affordability, ease of synthesis, and excellent biocompatibility, metal nano ferrites have a huge potential for wastewater treatment. This investigation delves into an extensive examination of diverse metal ferrites and hybrid nanostructures based on metal ferrites, assessing their potential for advanced photocatalytic applications in wastewater treatment [66].

The elimination of both organic and inorganic contaminants from wastewater is the main topic of this review. Recent studies on wastewater treatment [67-70], have been published by our research group, with a focus on the application of ferrite materials in this field. With an emphasis on recent developments in their synthesis and the related difficulties, the review will investigate the efficacy of spinel ferrite-derived nanomaterials as proficient photocatalysts for the remediation of wastewater [71-75]. This analysis will explore the latest developments in doped, binary and ternary spinel ferrite nanocomposites, alongside their enhanced properties and photocatalytic applications when integrated with advanced materials such as TiO₂, MXene, and g-C₃N₄. Based on their magnetic coercivity, ferrite materials can be broadly divided into 2 groups: Soft ferrites, which have low coercivity, and hard ferrites, which have high coercivity. Furthermore, ferrites are categorized into spinel, garnet, hexagonal, and orthoferrite based on their crystal forms [71,72]. Among these, spinel ferrites are the most common, with the general chemical formula MFe₂O₄, where M represents a divalent metal (such as Co, Cu, Ni, Zn, etc.), and the structure consists of iron and oxygen in tetrahedral and octahedral coordination sites [73]. With 56 atoms - 32 oxygen anions, 16 trivalent metal cations, and 8 divalent metal cations - the spinel ferrite takes on a face-centred cubic (FCC) lattice. In this configuration, Fe³⁺ and M²⁺ ions are positioned within interstitial sites, with Fe³⁺ occupying octahedral sites and M²⁺ residing in tetrahedral sites, while the oxygen anions are systematically arranged in a cubic close-packed structure [74,75]. The spinel ferrite structure’s physical and chemical characteristics are largely determined by the distribution of M²⁺ and Fe³⁺ ions within it. The qualities of the material are impacted by the cations’ random distribution between the 2 locations in a mixed spinel structure. The final characteristics of the ferrites can also be greatly influenced by the synthesis circumstances, including reaction temperature, pH, and calcination procedure [76].

The straightforward synthesis process, cost-effectiveness, elevated electrical resistivity, limited bandgap energy, magnetic permeability, substantial charge transfer capability, non-toxic nature, recyclability, as well as exceptional thermal and chemical resilience represent merely a selection of the prominent properties associated with spinel ferrites [77,78]. Because of these characteristics, ferrites can be used in a wide range of applications, including gas sensors, medication delivery, electronic devices, biomedical devices, information storage, mobile communication, water splitting, water treatment, and catalysis. Their narrow bandgap (1.5 - 2 eV) allows them to serve as effective photocatalysts under visible light, which is particularly useful for environmental remediation applications, including wastewater treatment. Spinel ferrites exhibit significant promise for the photocatalytic destruction of organic contaminants in wastewater treatment. Because of their tiny bandgaps, visible light can be used efficiently, which helps pollutants mineralize into innocuous byproducts like CO₂ and H₂O. This method reduces waste generation compared to traditional treatment techniques. Additionally, the magnetic properties of spinel ferrites make it easier to separate them from the reaction mixture after the degradation process, thus allowing for easy reuse in continuous catalytic cycles. Numerous approaches have been investigated to improve spinel ferrites’ photocatalytic activity. These include the creation of mixed ferrites, doped ferrites, hybrid heterojunctions, and nanocomposites based on ferrite [79-84]. For example, the photocatalytic degradation of azo dyes under sunlight has been examined for Co-Ni mixed ferrite (Co_0.5Ni_0.5Fe₂O₄). Electrons are excited by light absorption, creating electron-hole (e-h) pairs that take part in redox reactions and release reactive radicals (such O₂⁻•) that break down the dye [85]. Additionally, doping Co-Ni mixed ferrite with Ce has been shown to lower its bandgap, enhancing its visible light absorption and improving photocatalytic efficiency by reducing electron-hole recombination [86].

Another strategy involves hybrid organic-inorganic nanomaterials. For example, Nd-doped Rhodamine B (RhB) has been photo catalytically degraded using NiFeO₄ in conjunction with carbon nanotubes (CNTs). In this system, electrons in the Nd-NiFeO₄/CNTs composite are excited by light irradiation from the valence band (VB) to the conduction band (CB). While the holes oxidize OH⁻ to OH• radicals, these excited electrons convert O₂ to O₂⁻• radicals. CNTs’ enormous surface area aids in these redox processes during photocatalysis, which causes pollutants to degrade [86]. Furthermore, the creation of ferrite nanocomposites has been shown to improve photocatalytic activity. For instance, a CuFe₂O₄/Bi₂O₃ nanocomposite demonstrated enhanced photocatalytic performance for methyl blue (MB) removal compared to pure CuFe₂O₄. By adding BiO₃, the material’s specific surface area is increased, aggregation is decreased, and light absorption is enhanced, all of which lead to more effective electron-hole pair separation [79,87]. The spinel ferrites hold significant promise for wastewater treatment, particularly in photocatalytic applications. Their ability to effectively decompose organic contaminants in the presence of visible light, along with their magnetic characteristics, makes them a compelling and sustainable alternative for environmental remediation.

Scope of review

Magnetic nanoparticles have drawn a lot of interest because of their special qualities and possible uses in the separation and removal of environmental contaminants. Among them, ferrites are particularly promising as photocatalysts, with band gap ranging from 1.1 to 2.3 eV, allowing them to absorb visible light. Spinel ferrites offer the advantage of easy magnetic separation, making them ideal for reuse. Their crystal structure, low bandgap, excellent electrical and magnetic properties, and high chemical and thermal stability position them as top contenders in photocatalytic applications. Developed in the last few decades, photocatalysis has many uses, such as the production of coating materials, the removal of heavy metal ions, wastewater treatment, and the degradation of volatile organic compounds. It plays a crucial role in addressing microbial contamination as the global demand for freshwater increases. Various homogeneous and heterogeneous photocatalytic systems have been developed for wastewater treatment, offering cost-effective solutions that operate at ambient conditions. This review aims to present a comprehensive and critical analysis of ferrite-based photocatalysts and their hybridization with advanced functional materials such as ZnO, TiO₂, MXene, and g-C₃N₄ for environmental remediation applications, particularly the degradation of organic contaminants in wastewater. The discussion encompasses both the fundamental mechanisms and practical performance of these composite systems, drawing on recent advances in materials science and photocatalysis. The key focus areas include how the integration of ferrites with semiconductors and conductive materials influences charge separation efficiency, electron transport, and recombination dynamics, which are central to enhancing photocatalytic activity. The review assesses the optical characteristics of these composites, particularly bandgap engineering and light absorption behavior, supported by UV-Vis. spectroscopy data from recent literature. Additionally, the recyclability and operational stability of these catalysts across multiple degradation cycles are evaluated to determine their practical viability. By synthesizing data across various material combinations and synthesis strategies, this review identifies trends, performance limitations, and emerging opportunities in the design of efficient ferrite-based photocatalysts. It also outlines future directions for research, including hybridization with next-generation materials and improved methods for tuning photocatalytic properties to meet the demands of sustainable water treatment technologies.

Synthesis of metal ferrite nanomaterials

The synthesis of metal oxide nanoparticles can be achieved through a variety of methods, each offering unique advantages in terms of size control, surface properties, and scalability. This section discusses several commonly used techniques for their preparation.

Co-precipitation method

The co-precipitation method is a popular technology for manufacturing high-purity, nanoscale magnetic ferrite nanostructures due to its low complexity and high yield [88]. This approach is inexpensive and provides precise control over particle size and content. It demands low temperatures and produces materials with high crystallinity, homogeneity, and textural qualities [88,89]. However, notable limitations associated with co-precipitation include extensive particle agglomeration, sub-optimal crystallization, heterogeneous particle size distribution, and the necessity for careful pH control [90,91]. Inorganic salts (such as chlorides, sulphates, or nitrates) are first dissolved in water or another solvent. After correcting the pH to between 7 and 12 and continuously stirring, a precipitate form, which is then extracted by filtering or centrifugation, cleaned, and dried. The rate of pH shift has a substantial impact on particle aggregation and crystallization [88]. Nanostructured ferrites are typically synthesized by combining Co²⁺ and Fe³⁺ salts with a strong base [92]. Nanoparticle size, shape, and magnetic characteristics can be adjusted by varying experimental parameters such as reaction temperature, duration, reagent concentration, pH, and drying temperature [93]. CoFe₂O₄ nanoparticles (2 - 47 nm) were synthesized using metal chloride salts and varying quantities of NaOH and NH₄OH solutions, with reaction durations of 2 h and temperatures ranging from 20 - 100 °C [93]. These characteristics influence particle size, with greater temperatures and reaction periods typically producing larger particles [93]. In some cases, an oxidizing agent such as KNO₃ is used to convert Fe²⁺ to Fe³⁺, leading to the formation of both CoFe₂O₄ and magnetite (Fe₃O₄). The crystalline quality and saturation magnetization of the resulting particles depend on the surfactant used and the annealing temperature [94]. Furthermore, reverse co-precipitation is a version of the process in which precursor ions are introduced into the precipitant solution rather than the other way around. This guarantees that the precipitant is supersaturated, resulting in a more thorough precipitation and smaller nanoparticles than in classical co-precipitation [95]. Huixia et al. [88] reported the production of CoFe₂O₄ nanoparticles using reverse co-precipitation.

Hydrothermal/ solvothermal method

The hydrothermal approach is frequently utilized for large-scale ferrite nanoparticle (NP) synthesis due to its excellent yield efficiency. NPs are formed by combining aqueous solutions of divalent metal acetates (Co, Ni, Zn, Mn) using iron nitrate and a carbon-derived nanostructure template at an alkaline pH [88,96]. In an autoclave, the mixture undergoes heating to around 200 °C. The resulting precipitate is subsequently subjected to washing, isolated through centrifugation, and subjected to annealing at approximately 500 °C to remove the carbon template [97]. Surfactants such as ethylenediamine or citric acid are frequently used during the solution phase to refine the composition of the crystals, particle size, and morphology [98]. This method is advantageous as it doesn’t require complex processing or extremely high temperatures. It allows the adjustment of NP properties by varying temperature, pressure, reaction time, and the nano templates used. The hydrothermal method offers several benefits, including excellent control over nucleation, small particle sizes with narrow distributions, regulated morphology, high reaction rates, high yields, and simplicity in the process. There have been reports of magnetic nanosized CoFe₂O₄ being produced via this method without the need for additional calcination steps. However, the impact of synthesis time on CoFe₂O₄ NP size and morphology remains a topic of discussion [99]. CoFe₂O₄ nanorods have also been synthesized using cetyltrimethylammonium bromide (CTAB) as a surfactant. Although porous anodic aluminum oxide templates are employed for one-dimensional CoFe₂O₄ nanostructures, the costs are high, and the yields are relatively low. For other ferrites, NiFe₂O₄ nanocrystals were synthesized using FeCl₃·6H₂O and NiCl₂·6H₂O with CTAB as a surfactant. ZnFe₂O₄ was produced from metallic Zn and FeCl₂ in ammonia solutions [100]. Moreover, the hydrothermal synthesis of various ferrites (MFe₂O₄, where M = Co, Ni, Zn and Mn) utilizing metal acetylacetonates in conjunction with aloe vera extracts resulted in the production of nanoparticles (NPs) that are not only well-crystallized but also exhibit high yield at elevated calcination temperatures [101]. Alzami et al. [102] synthesized Zn_0.5Ni_0.5 Fe₂O₄ by hydrothermal process combined with PPy. The PPy and NFs nanohybrid improved several properties, such as reducing recombination, speeding up charge transfer, and extending visible light absorption. These enhancements led to superior MG dye mineralization compared to the ZNF catalyst. The nanohybrid showed stability and photocatalytic efficiency, with optimal conditions being 15 ppm MG dye, acidic solution, and intense visible light [102].

Sonochemical synthesis

The sonochemical process is recognized as one of the most effective methods for producing ferrite nanoparticles (NPs). This method takes advantage of the phenomenon of acoustic cavitation, which involves the development, growth, and implosion of bubbles in liquid media. Cavitation’s severe circumstances, including high temperatures (5,000 K), pressures (20 MPa), and rapid cooling speeds (10¹⁰ K/s), create particles with distinct characteristics. A step, surfactant-free sonochemical technique produced extremely crystalline monodisperse CoFe₂O₄ NPs with homogeneous spherical morphologies and high saturation magnetization (MS) [103]. This method requires a short synthesis time (approximately 70 min) and eliminates the need for post-synthesis annealing. Key advantages of the sonochemical approach include simplicity, cost-effectiveness, safety, environmental friendliness, uniform size distribution, high surface area, rapid reaction times, and excellent phase purity [103]. However, its limitations include the production of only small concentrations of NPs and the tendency for particle agglomeration [55]. High-energy ultrasonication generates localized heat points, reaching pressures of 1,800 atm and temperatures up to 5,000 K. These extreme conditions cause erupting bubbles to collapse, facilitating the formation of iron oxides. This technique also enables the production of monodispersed nanoparticles with diverse morphologies, making it suitable for large-scale nanoparticle synthesis [104]. Natarajan et al. [105] used the sonochemical approach to create nanoscale amorphous iron particles by dissolved iron carbonyl in decane. When heated over 300 °C, these particles fuse and crystallize.

Microwave-assisted combustion method

The microwave-assisted combustion technique synthesized nanoparticles (NPs) by converting microwave radiation into thermal energy created within the material. Unlike traditional heating, which applies heat externally, this inside heating dramatically saves operating time and energy usage. The approach has various advantages, including simplicity, quick reaction time, high output rates, environmental friendliness, outstanding reproducibility, and low cost of energy [106–108]. It also enables easy handling and precise control over parameters. By allowing molecular-level mixing of reagents through microwave interaction, the process provides excellent control of stoichiometry, homogeneity, purity and, morphology [88,96,109]. Nevertheless, this methodology can be expensive, primarily due to the employment of expensive reactants, including urea, L-alanine, glycine, carbohydrazide, and citric acid, which facilitate the combustion process. It is also less ideal for growing up and effective reaction monitoring [108,110]. The heat provided by microwaves allows for quick, uniform heating, resulting in homogenous nanomaterials with small particle size distribution. For instance, nitrates of zinc, iron, manganese, copper, nickel, and cobalt are solubilized in deionized water, maintained at a pH of 9.4, and enclosed in tetrafluoromethoxyl (TFM) [109,111]. The mixture is subsequently subjected to thermal processing in a microwave oven at a temperature of 160 °C for a duration of 30 min. After drying, polyvinyl alcohol (PVA) is added as a binder, and the mixture is sintered for 30 min [88]. This approach is both faster and more economical, producing fine, uniform NPs [96].

Sol-gel process

The sol-gel technique constitutes a low-temperature procedure that encompasses both hydrolysis and the condensation of metallic precursors, which may comprise salts or alkoxides, aimed at generating a 3-dimensional inorganic framework. In this process, monomers are transformed into a colloidal form of solution (sol), which then solidifies into a gel as particles bind together after the solvent evaporates. The sol-gel process is simple, cost-effective, and environmentally friendly, making it ideal for preparing nanocomposites (NCs) [112]. It facilitates precise regulation of microstructural attributes, particulate dimensions, distribution, structural configuration, and chemical composition through careful alteration of the synthesis parameters [88,113]. This methodology is frequently employed to synthesize fine, dense, homogeneous, single-phase ferrite nanoparticles (NPs). In contrast to conventional techniques, the sol-gel method offers superior stoichiometric precision and facilitates the production of ferrites at reduced thermal conditions [114,115]. These nanoparticles can be manufactured as films or colloidal powders. However, the approach has drawbacks, such as low efficiency and lengthy synthesis durations. As an example, glycolic acid was used as a chelating agent to synthesize NiFe₂O₄ nanostructures with a typical particle size of 27 nm. In the case of CoFe₂O₄@SiO₂, the silica network helps protect the NPs, reducing surface roughness and spin disorder, resulting in significantly higher coercivity (HC) values at room temperature compared to uncoated CoFe₂O₄ [88,116]. Metal nitrates are frequently employed as a source of metal ions, glycols (such as 1,2-ethanediol, 1,3-propanediol, or 1,4-butanediol) serve as chelating agents, tetraethyl orthosilicate (TEOS) functions as a precursor for the matrix, ethanol acts as a solvent, and nitric acid is also utilized. During annealing, the NPs tend to clump together and form larger particles, prompting additional investigation into the impacts of thermal energy on unwanted particle growth [117,118].

The integrative application of sol-gel and auto-combustion methodologies presents a systematic and economically viable approach for the synthesis of high-purity, homogeneous nanopowders at reduced annealing temperatures. This procedure entails the dissolution of nitrate salts in aqueous medium, the incorporation of an organic complexing agent, such as citric acid, the adjustment of the pH to a neutral value of 7, and the subsequent heating of the solution to 70 °C to facilitate the formation of the sol. At a temperature of 110 °C, the extraction of residual water occurs, and the nitrate-fuel gel is subjected to an autocatalytic combustion reaction. Citric acid and glucose are commonly used fuels because of their strong complexing properties and low ignition temperatures. The sol-gel auto-combustion method has successfully synthesized CoFe₂O₄ NPs ranging in size from 11 to 40 nm, with annealing temperatures between 800 and 1,000 °C [119]. The organic precursor approach, like the Pechini process, requires preparing an aqueous solution containing cations, then chelating them with carboxylic acids, heating the solution to produce a precursor, and then annealing the precursors [119]. Carboxylic acids serve as both complex agents and fuel. Using this method, magnetic CoFe₂O₄ NPs (38.0 - 92.6 nm) were prepared with tartaric and oxalic acids as precursors [120].

Combustion method

The combustion process is a straightforward, quick, and inexpensive technology that avoids the need for intermediary decomposition or calcination procedures. The process is predicated upon an exothermic, rapid, and self-sustaining chemical reaction that incorporates metallic salts and organic fuels (including sucrose, urea, glycine, sucrose, citric acid, hydrazine or polyvinyl alcohol), which function as reducing agents [127]. The characteristics of the resultant powders, including crystallite size and surface area, are influenced by the reaction’s enthalpy or combustion temperature, which varies depending on the fuel employed and the ratio of fuel to oxidant. Furthermore, the selection of fuel is instrumental in determining the morphology and phase formation of nanoparticles (NP) [121]. A notable advantage of this method is that the heat required for the reaction is generated internally rather than from an external source. Metal nitrates are commonly used as precursors due to their high-water solubility and ability to combust easily when mixed with an appropriate fuel [122]. Ammonium nitrate is frequently used as an additional oxidant to expand microstructures and enhance NP surface area without affecting the quantities of other reactants. The combustion technique is extensively employed for the fabrication of ceramic substances, composite materials, and ferrimagnetic nanostructures due to its efficacy, minimal preparation duration, economically viable precursors, uncomplicated apparatus, meticulous control of balance, tailored particle size distribution, high-purity outputs, and the stabilization of metastable phases. Nevertheless, its drawbacks include elevated combustion temperatures and suboptimal manufacturing yields [123]. This technique is particularly effective for producing high-quality CoFe₂O₄, NiFe₂O₄, and MnFe₂O₄ NPs, with their size, saturation magnetization (MS), and coercivity (HC) values tailored by adjusting the nitrate-to-fuel ratio [88]. For instance, nanocrystalline NiFe₂O₄ can be prepared by mixing metal nitrates with citrate to form a colloidal sol, heating the xerogel, undergoing an auto-combustion process to yield loose powder, and annealing the powder at 700 °C [96]. This approach produces high-purity, homogenous NPs with minimum waste but necessitates high temperatures. The high purity is due to the removal of contaminants that are volatile species during burning.

Green synthesis method

The biosynthetic approach offers a sustainable alternative to traditional chemical methods for synthesizing nanomaterials, using plant extracts, microorganisms, and enzymes. This eco-friendly route provides simplicity, high yield, and crystalline nanomaterials with desirable properties [124,125]. For example, Aloe vera extract, rich in long-chain polysaccharides, serves as an efficient bio-reducing agent for synthesizing ferrites, facilitating the uniform distribution of ferrite particles. This method is not only cost-effective but also employs non-toxic reducing and gelling agents, making it an environmentally friendly option [126]. However, certain challenges hinder the broader adoption of biosynthetic techniques. These include the challenges of managing nanoparticle (NP) size and characteristics, the requirement for high temperatures to transform precursors into crystalline materials, and the synthesis of polydisperse or surface-capped NPs. Furthermore, repeatability remains a challenge due to incomplete mixing of reactants and undesirable side reactions, both of which have a negative impact on NP characteristics. Alternative synthesis methods have been explored to address these issues. For instance, using Ni and Fe nitrates with freshly extracted egg white (ovalbumin) in aqueous media yields NiFe₂O₄ NPs with ferrimagnetic behavior and saturation magnetization (MS) values ranging from 26.4 - 42.5 emu/g at room temperature [127]. Despite these advances, repeatability and size control remain major issues. Heat and mechanical treatments, while important for certain structural features, can modify crystal structure and reduce photocatalytic activity. The combustion process is the simplest and fastest of the various ways, but it requires exact monitoring of the ratio of organic fuel to nitrates to maintain size and magnetic characteristics. Solid-state methods yield larger particles at higher temperatures, while citrate precursor methods produce smaller NPs at lower temperatures. For optimum, reproducible synthesis, critical processes include complete reactant mixing, automation, and exact control over reaction variables [128]. In summary, sol-gel and chemical co-precipitation methodologies are recognized as the most advantageous approaches for the fabrication of homogeneous, highly pure, and narrowly dispersed nanoferrites, achieving a harmonious equilibrium between the simplicity of synthesis and meticulous regulation of nanoparticle attributes [129].

Principle of photocatalytic degradation

Due to its many benefits and wide range of uses in environmental remediation, especially the removal of contaminants from wastewater, the use of photo catalysts in aqueous suspension has attracted a lot of attention [130]. The process known as photocatalysis occurs when substances like metal oxides, semiconductors, and organic molecules function as catalysts when exposed to light, facilitating chemical reactions that are fueled by light energy. In contrast to traditional catalysis, photocatalysis uses visible or ultraviolet (UV) light to activate a photocatalyst, increasing the pace of a chemical reaction. Because the materials’ catalytic qualities are essential to the process, choosing the right photocatalysts is essential to attaining superior photocatalytic performance. These characteristics include things like the availability of oxidizing agents, band-gap values, and electron-hole recombination rates. To maximize the photocatalytic reaction, factors pertaining to the manufacturing of photocatalysts and the properties of pollutants must also be carefully considered. In order to enhance the photocatalytic properties of a material, various methodologies have been employed, such as surface modification, incorporation of dopants, fabrication of composites, and the development of heterostructures [131]. A potent illumination source, an appropriate photocatalyst (such as MFe₂O₄), and a suitable oxidizing agent are indispensable for the photocatalytic degradation of organic pollutants. The initiation of the process is facilitated by the excitation of electrons from the valence band (VB) into the conduction band (CB) through the absorption of photons possessing energies that exceed the material’s band-gap energy (Eg). In contrast, photons characterized by longer wavelengths or reduced energy typically dissipate energy in the form of heat. According to Figure 1, the photocatalytic reaction stages entail the semiconductor material absorbing light, which causes electrons to go from the VB to the CB and leave a hole in the VB.

The efficiency of the photocatalyst is determined by the band gap, an energy barrier that separates these bands and inhibits electron-hole recombination. Through scavenging, the hydroperoxyl radical that is created during this process prolongs the life of the photogenerated holes. On the surface of the photoexcited photocatalyst, oxidation and reduction activities occur [132]. Electrons and holes recombine when oxygen eliminates the electrons, producing superoxide radicals (O₂⁻), which in turn produce hydrogen peroxide (H₂O₂) and hydroperoxyl radicals (HO₂⁻). Intermediates and final products are the result of additional interactions between these reactive species. Photonic activation of the catalyst is the first step in an effective photocatalytic system [132]. The following describes the next steps in using UV light to activate MFe₂O₄ in order to eliminate organic pollutants [133]. In summary, steps 1 to 5 show how the photocatalytic oxidation cycle produces hydroxyl radicals (OH-). A positive hole and an electron are produced at the VB and CB, as well, when sunlight strikes MFe₂O₄ (Eq. (1)). When positive holes, hydroxide (OH), and water (H₂O) mix, hydroxyl radicals (OH⁻) are created (Eqs. (2) and (3)). The presence of oxygen inhibits electron-hole recombination, and these hydroxyl radicals are the main agents responsible for decomposing organic pollutants. Furthermore, the interaction of oxygen (O₂) with electrons produces superoxide anions (Eq. (4)). Electron build-up in the CB may enhance electron and hole recombination, lowering photocatalytic efficiency, if oxygen reduction and pollutant oxidation are not coordinated. Therefore, improving the effectiveness of photocatalytic degradation requires preventing electron accumulation. Lastly, organic contaminants are broken down into carbon dioxide (CO₂) and water (H₂O) by hydroxyl radicals (OH⁻) binding to them (Eq. (5)).

Figure 1 Schematic diagram illustrating the photocatalytic degradation mechanism of organic pollutants [133].

The bonding of hydroxyl radicals (OH^-) to the contaminants successfully decreased the content of organic pollutants. In conclusion, photocatalysts are essential to the photodegradation of organic contaminants. Because of its low band-gap energy, the perfect photocatalyst can lower the rate at which electrons and positive holes recombine. It should also be non-toxic, photoactive, physiologically and chemically inert, able to absorb visible or near-UV light, and resistant to photo corrosion [134]. Numerous photocatalysts have been thoroughly investigated; each has pros and cons, which has led to a broad investigation of appropriate photocatalysts for various pollutants.

Metal nanoferrites as photocatalysts for wastewater treatment

Metal nanoferrites have attracted growing interest in the field of photocatalysis due to their unique structural, electronic, and magnetic properties [135-138]. These substances exhibit a spinel crystal configuration, defined by a cubic close-packed arrangement of oxygen anions, with metal cations positioned in both tetrahedral (A) and octahedral (B) sites. The distribution of cations between these sites can be tailored by adjusting synthesis conditions or doping, enabling precise control over their band structure, magnetic behavior, and surface activity [139,140]. Nano-structuring further enhances their surface area-to-volume ratio, shortens charge diffusion paths, and increases the density of reactive sites, all of which are beneficial for photocatalytic applications. One of the most compelling features of metal nanoferrites is their ability to absorb visible light efficiently, with bandgap energies typically ranging between 1.4 and 2.4 eV depending on the specific metal ion and synthesis route employed. This makes them suitable for solar-driven photocatalytic processes aimed at degrading persistent organic pollutants such as dyes, pharmaceuticals, pesticides, and phenolic compounds. Additionally, their magnetic characteristics facilitate straightforward post-reaction extraction through the application of external magnetic fields, rendering them especially appealing for recurrent utilization in heterogeneous catalysis, thereby diminishing both the financial expenditure and intricacy associated with wastewater treatment methodologies. Metal nanoferrites have also demonstrated strong potential in activating peroxymonosulfate or peroxydisulfate to generate reactive sulfate radicals (SO₄•⁻), which are known for their high redox potential (2.5 - 3.1 V) and greater selectivity compared to hydroxyl radicals [141,142]. In such AOPs, the transition metal ions (e.g., Fe²⁺, Co²⁺ and Cu²⁺) at the surface of the nanoferrites play a crucial role in facilitating electron transfer reactions, thereby accelerating the decomposition of oxidants and the subsequent generation of reactive species [143,144]. Despite these advantages, several challenges limit the widespread application of metal nanoferrites in photocatalysis. One major concern is the rapid recombination of photoinduced electron-hole pairs, which significantly reduces quantum efficiency. Moreover, their redox potential may be insufficient to fully oxidize certain organic pollutants, especially under mild conditions. Long-term stability under repeated cycling and in harsh environments, such as high ionic strength or varying pH, also remains a critical issue, as some ferrites are prone to leaching or surface passivation.

To overcome these limitations, researchers have explored a wide range of modification strategies, including metal or non-metal doping, formation of heterojunctions with other semiconductors (e.g., TiO₂, ZnO and g-C₃N₄), surface functionalization, and coupling with carbon-based materials like graphene and carbon nanotubes as discussed in the sections ahead. Such approaches aim to improve charge separation, extend light absorption, and enhance the adsorption of pollutants or oxidants at the catalyst surface. Furthermore, various synthesis methodologies, including co-precipitation, sol–gel, combustion, hydrothermal, and microwave-assisted techniques, have undergone optimization to regulate parameters such as particle size, morphology, and crystallinity - factors that significantly impact photocatalytic efficacy.

A systematic investigation into the relationships between structural features, electronic properties, and catalytic performance is thus essential to guide the rational design of next-generation nanoferrite-based photocatalysts. With continued research and development, metal nanoferrites hold significant promise for sustainable, efficient, and scalable solutions to contemporary environmental challenges, particularly in the realm of wastewater treatment and emerging contaminant removal.

Structural parameters affecting photocatalysis

The photocatalytic activity of metal nanoferrites is intrinsically governed by their crystal structure, cation distribution, and nanoscale morphology. All metallic ferrites exhibit crystallization in a cubic spinel configuration (space group Fd3̅m), characterized by a densely packed lattice of oxygen atoms with metallic cations residing in tetrahedral (A) and octahedral (B) coordination sites. The classification of spinels into normal, inverse, or mixed categories depends on the distribution of M²⁺ and Fe³⁺ ions across the respective sites. In the case of normal spinels such as ZnFe₂O₄, the divalent M²⁺ ions are situated at the A-sites, while the Fe³⁺ ions occupy the B-sites; conversely, in inverse spinels such as CoFe₂O₄, the Fe³⁺ ions are distributed between the A and B sites, with M²⁺ ions residing solely at the B-sites. Mixed spinels like NiFe₂O₄ exhibit an intermediate or partially ordered arrangement of cations between the 2 sites, and this distribution is often influenced by synthesis parameters such as pH, calcination temperature, and precursor ratio. This structural arrangement directly affects the electronic band structure, surface energy, and charge separation behavior of ferrites. The electron transfer processes within ferrites are primarily driven by super exchange interactions between cations (Fe³⁺–O²⁻–Fe³⁺ or M²⁺–O²⁻–Fe³⁺), where the strength and directionality of these interactions influence magnetic ordering and electronic conductivity [145,146]. For instance, in zinc ferrite, the optical response is governed by the degree of inversion, which determines the distribution of Zn²⁺ and Fe³⁺ ions between tetrahedral and octahedral sites; higher inversion increases transitions involving the Zn²⁺ 4s energy level (emission at ~464 nm) and reduces transitions from the Fe³⁺ 3d states near the valence band to conduction band levels (emission at ~547 nm). This shift in electronic transitions enhances the lifetime of electrons excited into the Zn²⁺ 4s state, promoting more efficient charge separation and thereby improving the overall photocatalytic activity [147]. In particular, electron hopping between Fe²⁺ and Fe³⁺ at octahedral B-sites enhances local conductivity, a factor that is critical in suppressing recombination of photo-induced charge carriers during photocatalysis [148,149]. Additionally, the magnetic properties arising from these interactions enable easy magnetic separation of catalysts post-treatment, a practical advantage for environmental applications.

The nanoscale size and morphology of ferrite nanoparticles play a critical role in determining their photocatalytic performance. Particle size directly affects surface area, band gap energy, and the density of active sites, all of which are key parameters influencing photocatalytic efficiency. Smaller ferrite nanoparticles generally exhibit enhanced activity due to their higher surface to volume ratio, which increases the availability of active sites for photocatalytic reactions. Zirconium ferrite nanoparticles with an average size of 43 nanometers have demonstrated up to 95% degradation efficiency of rhodamine B under ultraviolet irradiation, surpassing the performance of larger particles. Changes in particle size also alter optical properties; in zirconium ferrite, an increase in particle size resulted in a reduction of the band gap from 2.5 to 2.24 eV, thereby improving the absorption of visible light and enhancing photocatalytic activity [150]. In addition to size, the morphology of ferrite nanoparticles significantly influences their catalytic behavior. Distinct morphologies such as needle like, cube like, plate like, and spherical forms affect surface area, light absorption, and interaction with pollutants. Needle like zinc ferrite structures have exhibited the highest photocatalytic degradation efficiency for methyl orange, attributed to their optimized surface area and crystallinity [151]. Spherical nanoparticles, widely reported in the literature, offer uniform light exposure and enhanced interaction with contaminants due to their homogeneous distribution and large reactive surface area, contributing to improved photocatalytic performance [152]. The synthesis of spinel ferrite nanoparticles using methods like urea-assisted combustion results in nanoscale particles with high crystallinity, which is crucial for their magnetic and electronic properties. The absence of secondary phases in these materials further supports efficient transport [153].

Moreover, defect engineering has become a critical strategy for boosting the photocatalytic efficiency of ferrites, especially for solar driven environmental applications. By deliberately introducing structural imperfections such as oxygen or cation vacancies, this approach enhances light absorption, promotes charge carrier separation, and suppresses recombination through the formation of midgap trap states [154,155]. The concentration of oxygen vacancies in spinel ferrites can be significantly enhanced by doping with lower valent or smaller radius cations and through controlled thermal treatment. In NiFe₂O₄, e.g., oxygen nonstoichiometry (δ) increases markedly with rising sintering temperature, from 0.0014 at 1,150 °C to 0.0355 at 1,400 °C, due to thermally induced oxygen loss under reduced oxygen partial pressures, thereby modifying the defect landscape and electronic structure [156]. These defects not only broaden visible light utilization by tuning the optical bandgap but also increase the density of surface-active sites, thereby accelerating redox reactions and improving overall photocatalytic performance. However, excessive disorder can also introduce non-radiative recombination centers, underscoring the need for optimal defect engineering. The type of spinel arrangement, cation distribution, degree of inversion, crystallinity, and presence of defects collectively influence key performance metrics including light absorption, charge carrier mobility, surface reactivity, and redox capability. Rational control over these structural parameters through judicious selection of synthesis routes and dopants is therefore essential for tailoring nanoferrites for specific photocatalytic applications.

Compositional Parameters affecting photocatalysis

In addition to structural features, compositional modifications through doping or hybridization play a pivotal role in optimizing the photocatalytic potential of metal nanoferrites. Divalent transition metal ions such as Mn²⁺, Cu²⁺, Co²⁺, and Zn²⁺ are widely employed for A-site substitution in spinel ferrites to enhance photocatalytic activity. These dopants modulate the local crystal field environment and introduce defect states that promote efficient charge separation while preserving the structural integrity of the spinel lattice [157,158]. Their ionic radii closely match that of A-site cation, minimizing lattice strain, and their variable oxidation states facilitate the formation of oxygen vacancies and electronic trap states [29], both of which contribute to reduced charge carrier recombination and extended light absorption [159]. The incorporation of such cations fine-tunes the band structure, enhances redox potential, and improves the overall photocatalytic kinetics by fostering better electron mobility and surface reactivity under visible light irradiation [160–162]. The improved photocatalytic activity is attributed to the unique characteristics of Mn²⁺ ions, which facilitate polaron formation and hopping, enhancing electron transport within the spinel lattice [159]. In a similar study, copper-doped NiFe₂O₄ spinel nanoparticles synthesized via a microwave combustion method exhibited a notable band gap reduction from 3.25 to 2.39 eV with increasing Cu content, enhancing visible-light absorption and charge carrier dynamics. The optimized composition, Ni_0.6Cu_0.4Fe₂O₄, achieved a significantly higher photocatalytic degradation efficiency (99.85%) compared to pristine NiFe₂O₄ (88.26%), primarily due to improved separation and reduced recombination of photogenerated electron-hole pairs [163].

Trivalent metal cation doping is generally employed at the B site in spinel ferrites, using ions such as Cr³⁺, Al³⁺, Ga³⁺, Y³⁺, and Sc³⁺. These dopants influence Fe³⁺–O²⁻–Fe³⁺ super exchange interactions, modify cation distribution, and introduce lattice distortions or oxygen vacancies that enhance charge carrier separation and reduce recombination [164,165]. Transition metal ions like Cr³⁺ improve redox activity and enhance electrical conductivity [165,166], while non-transition dopants such as Al³⁺ and Y³⁺ create defect states and strain in the lattice, broadening light absorption and increasing surface reactivity [167]. The type and concentration of dopants in metal nanoferrites significantly influence both photocatalytic efficiency and magnetic properties by modifying the band structure, charge dynamics, and magnetic interactions. For instance, Cr³⁺ doping in Ni-Zn ferrites initially enhances saturation magnetization but reduces it at higher concentrations due to site occupancy effects [145,165]. Al³⁺ substitution in Mn_0.5Cu_0.5Cr_xFe₂₋_xO₄ disrupts magnetic coupling and decreases both saturation magnetization and coercivity, attributed to the non-magnetic nature of Al³⁺ and reduced particle size [164]. Conversely, Ga³⁺ doping in NiCr₀.₂Fe₁.₈O₄ improves magnetic moment and remanence by strengthening magnetic interactions [168]. In a detailed study, rare earth elements (Sm, Dy, Ho, Er and Yb) were doped into manganese ferrite (MF) via the coprecipitation method to enhance both photocatalytic and magnetic properties. UV-Vis. spectra showed broad absorption from 250 - 800 nm, enabling efficient utilization of both UV and visible light. Among the dopants, Sm, Dy, and Ho-doped MF significantly improved charge separation and electrochemical conductivity, attributed to enhanced electron mobility and reduced recombination of photogenerated carriers. Electrochemical tests revealed a trend in electron transfer efficiency (Sm-MF > Dy-MF > Ho-MF > MF > Er-MF > Yb-MF), with Sm-MF exhibiting the most positive corrosion potential (−0.10 V), indicating excellent structural stability and photocatalytic performance. Conversely, Er and Yb dopants negatively impacted the electrochemical behavior, highlighting the importance of dopant selection in optimizing MF-based photocatalysts [169]. In a study on cerium-doped spinel ferrites synthesized via chemical co-precipitation, Ni_0.6Cu_0.4Ce_xFe_2−xO₄ (x = 0.0 - 1.5) demonstrated a clear dependence of photocatalytic performance on the doping level, with the highest degradation efficiency of ∼91% for Congo red (CR) observed at x = 1.5 under 60 mg catalyst dose at pH 6.5. The activity followed the trend: Ni_0.6Cu_0.4Fe₂O₄ < Ni_0.6Cu_0.4Ce_0.5Fe_1.5O₄ < Ni_0.6Cu_0.4CeFeO₄ < Ni_0.6Cu_0.4Ce_1.5Fe_0.5O₄. Structural and optical analyses revealed that increased Ce³⁺ substitution at x = 1.5 resulted in enhanced octahedral cation occupancy, larger particle size, and a narrowed bandgap, facilitating improved light absorption and charge transfer. Additionally, the catalyst retained structural integrity and photocatalytic efficiency upon recycling, highlighting its stability and reusability [170].

Eu³⁺-doped CoFe₂O₄ nanoparticles synthesized via a sol–gel auto-combustion method exhibited enhanced photocatalytic performance, achieving 95.98% degradation of Rhodamine B under visible light compared to 89.77% by the undoped sample. The improvement is attributed to the role of Eu³⁺ ions as charge trapping centers, which effectively reduce electron-hole recombination. Moreover, the internal redox cycling between Eu³⁺ and Eu²⁺ extends charge carrier lifetimes and facilitates the generation of reactive oxygen species (•O₂⁻, •OH), thereby boosting photocatalytic efficiency [171]. Another crucial compositional strategy involves forming heterojunctions between ferrites and other semiconductors, which not only suppresses rapid charge recombination by promoting spatial separation of electrons and holes but also enhances the overall photocatalytic efficiency. The construction of Type II, Z scheme, or Schottky junctions facilitates synergistic charge transfer between semiconductors with aligned band structures, while simultaneously helping the catalysts maintain their structural integrity and catalytic performance over multiple cycles [172-174]. The combination of spinel ferrites with materials such as g-C₃N₄, TiO₂, ZnO, and MXenes is increasingly popular due to their complementary electronic structures and their ability to promote effective charge separation (discussed in detail later). Schottky junctions are formed when a metal with a higher work function is coupled with a semiconductor, creating a potential barrier that facilitates electron flow while minimizing recombination [175]. The use of noble metals like Au and Pt in these junctions is beneficial due to their high work function, which enhances the barrier height and improves charge transfer efficiency [176]. The Schottky junction facilitates the generation and transfer of plasmon-induced hot electrons, which are crucial for photocatalytic reactions. These hot electrons can be efficiently collected and utilized, leading to enhanced photocatalytic activity [177]. The Z-scheme mechanism involves the transfer of electrons from the conduction band of one semiconductor to the valence band of another, maintaining high redox potential for pollutant degradation [178]. Surface functionalization by introducing functional groups, polymers, or nanoparticles improves the photocatalytic performance of metal ferrites by enhancing reactant adsorption, facilitating charge separation, and promoting surface redox reactions. For instance, Surface functionalization of Co_1−xZn_xFe₂O₄ nanodots with polyaniline (PANI) has been achieved via hydrothermal process. The Co₁₋_xZn_xFe₂O₄/PANI nanophotocatalyst exhibited exceptional photocatalytic performance, achieving nearly 98% degradation of DR-1, RO-16, RB-222 and RY-145 dyes within a span of 5 min. The most effective degradation was noted at a dye concentration of 30 mg/L and a nanophotocatalyst concentration of 1 g/L, with optimal results realized at a 1:0.67 ratio of Co₁₋_xZn_xFe₂O₄ to aniline. The nanophotocatalyst exhibited stable photocatalytic performance across a wide range of pH values and temperatures, maintaining approximately 82% efficiency after 7 cycles, demonstrating its excellent reusability and stability [179]. Using Ulva lactuca L. biomass and extracts, green synthesis of biochar/CoFe₂O₄ (BCF) nanocomposites was achieved, yielding a multifunctional material with superior photocatalytic properties. The optimized nanocomposite, characterized by a weight ratio of biochar to CoFe₂O₄ of 10:4, demonstrated a diminished band gap of 1.58 eV and accomplished a 96% degradation of Congo Red under visible light within a time frame of 70 min (k = 0.0394 min⁻¹), markedly exceeding the performance of pure biochar (37%) and unmodified CoFe₂O₄ (58%) [180]. Decorating metal nanoferrites with plasmonic nanoparticles such as Ag or Au induces localized surface plasmon resonance, which amplifies visible light absorption and enhances photocatalytic reactivity through improved charge carrier dynamics. The Au/ZnFe₂O₄@rGO composite achieved 96% degradation of bromophenol blue dye under sunlight, significantly outperforming pure ZnFe₂O₄, due to the enhanced charge separation and electron transfer facilitated by the plasmonic effect of Au NPs [181]. Moreover, activation-induced photocatalysis using metal nanoferrites focus on integrating external stimuli such as persulfate activation [182], photo-Fenton-like reactions [183] and sonophotocatalysis [184] to boost photocatalytic efficiency. These approaches leverage the inherent redox activity and magnetic recoverability of spinel ferrites to generate reactive oxygen species (ROS) more effectively.

Operational parameters

Beyond intrinsic structural and compositional characteristics, the photocatalytic performance of metal nanoferrites is heavily influenced by external operational parameters such as pH, temperature, light intensity, pollutant concentration, and oxidant dosage. These conditions modulate the surface charge of the photocatalyst, the generation and lifetime of reactive species, and the adsorption-desorption equilibrium of pollutant molecules. Temperature plays a significant role in influencing the morphology, crystalline, electronic, and magnetic properties of spinel ferrites, all of which are important determinants of their photocatalytic performance. In the case of SrFe₂O₄, the synthesis temperature has a notable effect on morphology, with a transition observed from nanoribbons to dendritic structures and ultimately to hexagonal nanoplatelets as the temperature increases from 85 to 160 °C. These morphological changes correlate with improved photocatalytic degradation of methylene blue, indicating enhanced performance at higher synthesis temperatures [185]. Temperature also affects cation distribution within the spinel lattice. For instance, in NiFe₂O₄, an increase in calcination temperature leads to a decrease in the occupancy of Ni²⁺ ions at the tetrahedral (A) sites, influencing the material’s magnetic properties [186]. Higher synthesis temperatures generally promote increased crystallinity, which can suppress electron-hole recombination and thereby improve photocatalytic efficiency [187]. Structural parameters such as lattice constants and bond lengths, which are sensitive to temperature, also impact the optical behavior of these materials. In CuFe₂O₄, higher calcination temperatures enhance both saturation magnetization and photocatalytic activity, attributed to changes in the structural and optical characteristics [188]. Similarly, Mg_0.7Zn_0.3Fe₂O₄ nanoparticles exhibit increased dielectric properties and photocatalytic degradation efficiency at elevated temperatures, reflecting improved charge carrier dynamics [189].

Temperature can further influence the electronic band structure of ferrite photocatalysts, potentially enabling absorption of lower-energy photons and enhancing redox capability during photocatalytic reactions [190]. However, while elevated temperatures often improve properties such as crystallinity and charge separation, excessively high temperatures may lead to particle agglomeration, resulting in reduced surface area and fewer active sites, which can adversely affect photocatalytic efficiency [191,192]. Therefore, the optimal synthesis temperature must be carefully tailored to each spinel ferrite composition and synthesis route. Additionally, environmental and economic considerations associated with high-temperature synthesis should be addressed to ensure the sustainable and cost-effective production of spinel ferrites for photocatalytic applications.

The pH level during synthesis significantly affects the crystallite size and lattice parameters of spinel ferrites. For instance, Ni_0.3Cu_0.3Zn_0.4Fe₂O₄ synthesized at different pH levels showed variations in crystallite size from 31.12 to 45.32 nm and lattice dimensions from 8.4175 to 8.4368 Å, indicating structural enhancement with pH variation [193]. In CuFe₂O₄ spinel, pH influenced the distribution of iron and copper ions within the octahedral and tetrahedral sublattices, affecting the overall structure and properties of the ferrite [194]. In MgFe₂O₄, an increase in pH resulted in larger particle sizes and improved crystallinity, which enhanced the magnetic properties [195]. Changes in pH can shift the conduction and valence band edges of the photocatalyst, affecting the generation and separation of charge carriers. This shift can either enhance or reduce the photocatalytic activity depending on the alignment of the band edges with the redox potential of the pollutants [196]. The sol-gel method used for SrFe₂O₄ nanoparticles demonstrated that increasing pH from 1 to 9 leads to a decrease in the band gap, which is associated with changes in crystallite size and ion distribution [197].

The pH of the solution affects the surface charge of the photocatalyst, which can enhance or inhibit the adsorption of pollutants. For instance, at a pH lower than the point of zero charge (pH ZPC), the catalyst surface becomes positively charged, which can enhance the degradation of negatively charged pollutants through electrostatic attraction [198]. Conversely, at a pH higher than the pH ZPC, the surface becomes negatively charged, which may repel negatively charged pollutants as evident from the photo-degradation evofloxacin using Ni-Zn ferrite [199]. At acidic pH levels, the increased concentration of protons can lead to the protonation of surface hydroxyl groups on nanoferrites, which reduces electrostatic repulsion between particles and promotes aggregation [200]. This aggregation can decrease the available surface area for catalytic reactions and potentially lead to sedimentation resulting in decreased photocatalytic efficiency. Moreover, Ferrites may undergo partial dissolution or leaching of metal ions (e.g., Fe³⁺, Co²⁺ and Ni²⁺), especially under strong acidic conditions (pH < 4). This compromises structural integrity and reduces photocatalytic activity over time. Conversely, extreme alkaline conditions may also lead to the formation of metal hydroxide precipitates, which can alter the surface properties and reactivity of the nanoferrites. Therefore, maintaining a near-neutral pH is often considered optimal for the stability of metal nanoferrites in photocatalytic applications. From a process engineering perspective, minimizing the need for chemical pH adjustment post photocatalysis reduces treatment costs and simplifies reactor design. While laboratory studies often demonstrate higher photocatalytic efficiencies under extreme pH values, these conditions are less feasible for large-scale implementation. Therefore, developing photocatalysts that retain high performance under near-neutral pH is crucial for advancing real-world wastewater treatment technologies.

Hybrids of metal ferrite as photocatalysts

Titanium dioxide (TiO₂)/ ferrites

Titanium dioxide (TiO₂) is a well-studied photocatalyst because of its chemical stability, nontoxicity, strong oxidative strength, and low cost. It has been widely employed in environmental cleaning, biocidal applications, and energy-based applications such as solar energy production and photocatalytic water splitting [201,202]. However, TiO₂ has severe limitations, most notably a high band gap that limits its activation to UV light and the quick recombination of photogenerated electron-hole pairs, which decreases photocatalytic efficacy. To overcome these restrictions, a variety of techniques have been used, including metal and non-metal doping to improve charge carrier separation and lower the band gap, as well as dye sensitization to boost visible light absorption and electron lifetime [203]. Incorporating spinel ferrites with TiO₂ presents a promising approach to address these challenges. Spinel ferrites, such as those with the MFe₂O₄ structure (where M is a transition metal), offer unique properties like a smaller band gap suitable for visible light absorption, magnetic properties, and biocompatibility, making them versatile for various applications. The combination of TiO₂ with ferrites can enhance photocatalytic performance by improving charge separation and extending the light absorption range. For instance, TiO₂-doped nanocrystalline Ni_0.4Cu_0.3Zn_0.3Fe₂O₄ ferrites have shown a decrease in band gap energy with increased TiO₂ doping, indicating enhanced semiconducting properties [204]. Additionally, the magnetic properties of ferrites facilitate the separation and recovery of photocatalysts under a magnetic field, addressing the issue of material separation from process suspensions [73]. However, TiO₂ has severe limitations, most notably a high band gap that limits its activation to UV light and the quick recombination of photogenerated electron-hole pairs, which decreases photocatalytic efficacy. To overcome these restrictions, a variety of techniques have been used, including metal and nonmetal doping to improve charge carrier separation and lower the band gap, as well as dye sensitization to boost visible light absorption and electron lifetime. O₂⁻ species

Helmy et al. [205] utilized CuCe-Ferrite/TiO₂ and TiO₂ nanostructures synthesized through a co-precipitation process, followed by heating at 550 °C for 2 h. The author attributed the superior photocatalytic activity of the TiO₂/CuCe ferrite composite compared to bare TiO₂ to the addition of CuCe-Ferrite dopants. These dopants create localized full states and/or oxygen vacancies within the TiO₂ crystals. As a result, these changes lead to the formation of a mid-bandgap, effectively shifting the optical energy band edge of TiO₂ into the visible light range, thus enhancing its photocatalytic performance as shown in Figure 2.

Figure 2 (a) Schematic of the synthesis process for CuCe-Ferrite/TiO₂; (b) Performance of CuCe-Ferrite/TiO₂ NPs in degrading 10 ppm R250 dye over various time intervals; (c) Longevity of CuCe-Ferrite/TiO₂ and TiO₂ NPs after ten usage cycles [205].

Li et al. [206] investigated the photocatalytic potential of ZnFe₂O₄/TiO₂ (ZF-T) nanocomposites, focusing on their performance under visible light. The synthesis involved dissolving Zn(CH₃COO)₂·2H₂O, Fe(NO₃)₃·9H₂O, and TiO₂ in ultrapure water, followed by stirring, ultrasonication, and adjusting the pH to 10. The mixture was autoclaved at 180 °C for 12 h, then dried and heated at 400 °C. Various ZnFe₂O₄/TiO₂ ratios (0%, 1%, 5%, 7%, 10% and 15%) were examined. ZnFe₂O₄ has a band-gap of 1.90 eV, compared to TiO₂’s 3.20 eV, which extends light absorption into the visible region. This doping creates lattice defects in TiO₂, which improves electron-hole pair separation and transport, hence increasing photocatalytic activity. Because of ZnFe₂O₄’s high surface area, ZF-T nanocomposites have a greater specific surface area than pure TiO₂. This increased surface area improves visible light absorption and free radical production, thereby enhancing the photodegradation of malachite green (MG). Under conditions of 10 mg/L malachite green (MG), 1.0 g/L catalyst concentration, pH 7.0, and illumination with a 500 W xenon lamp for 240 min, the 10% ZF-T composite achieved 90.1% MG degradation, significantly outperforming pure TiO₂, which reached 53.5%. ZnFe₂O₄ exhibits a Z-type heterojunction structure that facilitates efficient charge transfer under visible light. Electrons in ZnFe₂O₄’s valence band (VB) may shift to its conduction band (CB) since TiO₂ has a lower CB energy position (−0.29 eV) than ZnFe₂O₄ (−1.54 eV). This energy differential enables easy electron transfer from ZnFe₂O₄ to TiO₂’s CB, resulting in greater separation of electron-hole pairs and lower recombination rates. Consequently, the ZF-T photocatalytic system effectively mitigates the high electron-hole recombination tendency of TiO₂, thereby enhancing interfacial charge transfer efficiency. Furthermore, despite the relatively stable nature of benzene rings with a low delocalization energy of 152 kJ/mol, the energy provided by the free radicals and electronic holes generated in the ZF-T system under visible light is adequate to disrupt the delocalized π bonds. This disruption results in the complete degradation of methyl green (MG), demonstrating the effectiveness of the ZF-T system in photocatalytic applications. The photocatalytic efficiency remained above 80% over 5 consecutive 4-hour cycles, demonstrating good stability and reusability. UV-vis spectroscopy alongside liquid chromatography-mass spectrometry confirmed the successful disintegration of benzene rings and the degradation of heterocyclic aromatic hydrocarbons, thereby affirming the superior efficacy of the ZF-T system.

NiFe₂O₄ was synthesized via sol-gel auto-combustion using glycine as a fuel and combined with TiO₂ through solid dispersion to form a NiFe₂O₄/TiO₂ heterostructure. Electrochemical analysis showed NiFe₂O₄ as p-type and TiO₂ as n-type, with NiFe₂O₄ exhibiting lower charge transfer resistance, enhancing photocatalytic activity. For NiFe₂O₄, the band gap is found to be 1.64 eV, which allows it to absorb visible light. TiO₂ has a band gap of 3.21 eV, making it effective in absorbing UV light. The mixture of wavelengths in the heterostructure attempts to use both visible and UV light for photocatalytic activities. Solar irradiation (hv > Eg) causes electrons to travel from the conduction band (CB) of p-NiFe₂O₄ to the CB of n-TiO₂, while holes move from the valence band (VB) of n-TiO₂ to the VB of p-NiFe₂O₄, preventing charge carrier recombination. Hydroxyl radicals (•OH) generated by water reduction on the n-TiO₂ surface attack Congo red dye molecules. The unique band positions of p-NiFe₂O₄/n-TiO₂ facilitate the reduction of O₂ to superoxide (O₂•⁻), forming a Z-scheme photocatalytic system. This mechanism leads to a 97% degradation efficiency of Congo red dye after 180 min of sunlight irradiation [207].

In a similar study, Ciocarlan et al. [208] synthesized ferrite magnetic nanoparticles (MNPs) with the composition Co_0.5Zn_0.25M_0.25Fe₂O₄ (M = Ni, Cu, Mn and Mg) using a co-precipitation method. This spinel structure, which incorporates various cations, offers a promising solution to the common limitations faced in photocatalytic reactions. The inclusion of Co²⁺ ions impart strong magnetic properties to the ferrite systems, facilitating the separation and reuse of the photocatalysts. Zn²⁺ has a low energy band gap and excellent catalytic capabilities in its oxide form, leading to increased photocatalytic efficiency in the composite. The UV-vis diffuse reflectance spectra (DRS) confirm the activation of these nanocomposites under visible light, which is crucial for solar-driven applications. The presence of magnetic nanoparticles (MNPs) in the composite also contributes to the improved crystallinity of the titania phase, which is beneficial for photocatalytic performance. For methyl orange (MO), Co_0.5Zn_0.25Ni_0.25Fe₂O₄/TiO₂ achieved a 95% degradation rate according to UV-vis spectroscopy and approximately 80% based on TOC measurements after 6 h of solar irradiation. For methylene blue (MB), the most effective degradation was observed with Co_0.5Zn₀.₂₅Ni₀.₂₅Fe₂O₄/TiO₂ and Co_0.5Zn_0.25Mn_0.25Fe₂O₄/TiO₂, each achieving 99% degradation as measured by UV-vis spectroscopy within 80 min of solar light exposure. This type of magnetic composite, similar to previously reported CoFe₂O₄-TiO₂ and Fe₃O₄-TiO₂ systems, not only shows high efficiency in dye photodegradation but also offers the advantage of easy recovery post-process due to its magnetic nature.

Transitioning from photocatalytic oxidation to photoelectrochemical processes significantly enhances wastewater remediation by integrating electron flow with photocatalytic reactions. This approach allows precise control over oxidative processes and improves pollutant degradation by leveraging the synergy of light-driven charge generation and electrochemical reactions. Photoelectrochemical oxidation fits within the photocatalytic regime when semiconductors are employed to facilitate both light-induced charge generation and electrochemical reactions, thereby optimizing performance under various conditions. The integration of Plasmonic NPs like Ag and Au into heterojunctions has demonstrated improved photocatalytic degradation of dyes and antibiotics. This impact is particularly noticeable when the dimensions of the Ag nanostructures is optimized, as proven by Ag nanocubes on TiO₂, which produced 12 times more photocatalytic hydrogen than pristine TiO₂ [208]. The presence of plasmonic metals like Au improves photocatalytic performance by enhancing light absorption through localized surface plasmon resonance and increasing charge carrier separation due to efficient electron transfer. This is supported by studies on Au/TiO₂ systems, where metal-support interactions improved charge separation and migration [209,210]. The combination of plasmonic nanoparticles with magnetic components like Fe₃O₄ in TiO₂ composites creates magneto-plasmonic photocatalysts. These composites benefit from both the magnetic separation capabilities of Fe₃O₄ and the enhanced light absorption due to plasmonic effects, leading to significantly improved photocatalytic degradation of pollutants [211].

The synthesis and characterization of Ag-CoFe₂O₄@TiO₂ hetero-nanostructures were effectively achieved using the sol-gel combustion technique, which facilitated the creation of a heterojunction with enhanced electrochemical and photoelectrochemical properties. This method involved dissolving citric acid in distilled water, followed by the addition of cobalt nitrate and iron nitrate in a 1:2 ratio. The pH was adjusted to 10 - 12 using NaOH, and a black-colored AgO solution was incorporated, resulting in the formation of Ag-CoFe₂O₄ gel. This gel was calcined at 550 °C for 120 min to produce Ag-CoFe₂O₄ powder, which was then washed and dried to obtain the nanocomposites. The incorporation of cobalt into TiO₂ results in a narrowing of the band gap, which is evident from the UV-DRS results. This narrowing is caused by the creation of defect sites, such as oxygen-vacancy sites and Ti-C bond formation, which increase light absorbance and improve TiO₂’s photocatalytic efficiency by shifting the absorption intensity. Specifically, the addition of silver (Ag) to the Co Fe₂O₄@TiO₂ (CFT) structure causes a modest red shift in the absorption edge, which reflects increased photocatalytic efficiency under visible light circumstances. The band gap of the Ag-modified CFT (ACFT) is reduced from 2.0 to 1.9 eV, extending the photocatalysis spectrum into the visible light range and allowing for more efficient solar energy utilization. When •OH radicals react with the isobutyl group in ibuprofen, they form several metabolites, which undergo further degradation through decarboxylation and hydroxylation, eventually leading to ring cleavage and the mineralization of CO₂ and H₂O. Under direct sunlight irradiation, the ACFT heterostructures achieve complete oxidation of ibuprofen at a potential of 0.8 V versus the reversible hydrogen electrode (RHE), showcasing their high performance in photoelectrochemical applications [212].

Following the comprehensive assessment of the photocatalytic performance of ferrite/TiO₂ heterostructures, it is crucial to address the underlying challenges that impact their operational efficiency and effectiveness in wastewater treatment applications. One significant issue is ensuring the phase stability and compatibility of the ferrite and TiO₂ components under operational photocatalytic conditions, as phase separation or degradation could undermine effectiveness. Efficient charge carrier dynamics are essential, necessitating design strategies to enhance charge separation and reduce recombination. The heterostructure must also offer a high surface area and active sites to maximize photocatalytic activity. Given TiO₂’s typical UV absorption and ferrites’ visible light absorption, optimizing the light absorption range to cover a broad spectrum is crucial. Additionally, the stability of the photocatalyst under the harsh conditions of wastewater, including variable pH, temperature, and the presence of diverse contaminants, is a key concern. The scalability and cost-effectiveness of synthesizing these heterostructures for industrial applications, alongside their reusability and regeneration potential, are also important factors that are given in Table 1. Finally, tailoring the photocatalyst for broad-spectrum or targeted contaminant removal remains a challenge, requiring precise optimization of material properties and operational parameters.

Table 1 Photocatalytic performance of various ferrite/TiO₂ composites.

SiO₂/ ferrites

SiO₂, though not an active photocatalyst due to its wide band gap, plays an important role in improving photocatalytic system performance due to its outstanding stability, large surface area, and capacity to scatter active photocatalysts and avoid agglomeration. Ferrite-based materials, while appealing due to their morphological and optical features, frequently suffer restrictions such as quick charge carrier recombination and particle accumulation, which diminish their photocatalytic effectiveness [224,225]. Agglomeration leads to a decrease in the surface area available for photocatalytic reactions, thereby diminishing the overall activity of the catalyst. Forming SiO₂/ferrite composites is advantageous for several reasons, particularly in terms of agglomeration, surface area, and electron-hole recombination. The incorporation of a SiO₂ matrix helps in controlling the size and distribution of ferrite nanoparticles, which significantly reduces agglomeration. For instance, the sol-gel method used to prepare CoFe₂O₄ nanoparticles dispersed in a SiO₂ matrix results in spherical particles that are less agglomerated, as the SiO₂ matrix provides numerous nucleation sites that restrict particle growth and prevent clumping [226]. In addition, the incorporation of Co_0.4Zn_0.4Ni_0.2Fe₂O₄ nanoparticles within SiO₂ at varying ratios demonstrates that the specific surface area is enhanced with an increase in SiO₂ content, which is essential for photocatalytic applications that necessitate materials with elevated surface areas [227].

Ferrite/SiO₂ core-shell and intermediate composites play an important role in increasing photocatalytic efficiency by reducing electron-hole recombination. The SiO₂ layer serves as a protective barrier around the ferrite core, shielding it from corrosion and degradation. This physical barrier also impedes direct electron-hole contact within the ferrite, thereby minimizing recombination rates at the surface [228]. The work by Hariani et al. [229] on NiFe₂O₄/SiO₂/NiO magnetic nanocomposites revealed significant improvements in the photocatalytic degradation of methyl orange dye under UV irradiation. The study involved the characterization of NiFe₂O₄, NiFe₂O₄/SiO₂, and NiFe₂O₄/SiO₂/NiO using X-ray Diffraction (XRD), which revealed specific peaks corresponding to their crystalline structures, indicating successful synthesis and coating processes (Figure 3). The optical characteristics were examined through UV-Vis Diffuse Reflectance Spectroscopy (DRS), revealing that the band gap values for NiFe₂O₄, NiFe₂O₄/SiO₂, and NiFe₂O₄/SiO₂/NiO were 1.81, 2.21, and 2.67 eV, correspondingly. The composite demonstrated a remarkable degradation efficiency of 95.76% for methyl orange dye under optimal conditions, which comprised a pH of 4, a catalyst dosage of 0.50 g/L, and 10 mg/L of initial dye concentration.

Figure 3 (a) Degradation efficiency at different catalyst dosages and (b) Magnetic properties of prepared samples open access [229].

The magnetic properties of NiFe₂O₄/SiO₂/NiO decreased due to the coating of nonmagnetic SiO₂ on NiFe₂O₄, which weakened its magnetic field. Despite this, after being used in the photocatalytic degradation process, the material could still be separated from the solution using a permanent magnet. Even after 5 cycles of recurrent use, NiFe₂O₄/SiO₂/NiO retained good catalytic efficacy, demonstrating that it is an efficient and reusable photocatalyst. The presence of the SiO₂ interlayer improved charge separation, resulting in higher photocatalytic performance compared to composites without the interlayer.

A study conducted by Kumar et al. [230] concentrated on the photocatalytic degradation of 2 azo dyes, Methyl Red (MR) and Methyl Orange (MO), utilizing Fe₃O₄, Fe₃O₄/SiO₂, and Fe₃O₄/SiO₂/Ru hybrid magnetic composites in the presence of visible light. Photoluminescence (PL) spectral analysis was performed to assess the efficacy of charge carrier trapping, migration, transfer, separation, and recombination mechanisms within the nanocatalysts. The Fe₃O₄/SiO₂/Ru HMCs exhibited a reduced PL intensity when compared to pure Fe₃O₄ and Fe₃O₄/SiO₂, signifying a diminished likelihood of recombination and enhanced photocatalytic performance. Vibrating sample magnetometry (VSM) investigations revealed the super-paramagnetic characteristics of the HMCs along with a reduction in magnetization attributed to the diamagnetic contribution from the silica shell and the Ru nanoparticles. Under visible light, Fe₃O₄/SiO₂@Ru generates electron-hole pairs. Water reacts with holes to produce ⋅OH radicals, while electrons reduce oxygen to form ⋅O2−. These radicals degrade azo dyes into CO₂ and H₂O. This is clearly depicted in Figure 4. The optimal conditions for the Fe₃O₄/SiO₂@Ru photocatalyst are a catalyst dosage of 0.8 g/L, 10 mg/L of initial dye concentration, and a pH of 4, achieving degradation percentages of 95% for methyl orange and 91% for methyl red when irradiated for 150 min under visible light.

Figure 4 Mechanism of MO/MR dye degradation through photocatalysis by Fe₃O₄@SiO₂@Ru hybrid magnetic composites open access [230].

The Fe₃O₄/SiO₂/Ru HMCs maintain over 90% degradation efficiency even after 6 cycles of use, indicating their stability and durability for repeated applications, which is crucial for practical and sustainable wastewater treatment.

The plasmonic nature of silver nanoparticles (AgNPs) enhances the photocatalytic activity of Ferrite/SiO₂/ hybrid nanocomposites by supporting localized surface plasmon resonances (LSPRs) that improve light absorption and generate hot electrons [231]. Hariani et al. [232] synthesized core-shell Fe₃O₄/SiO₂/TiO₂ nanoparticles with Ag modification. Fe₃O₄ was coprecipitated and used in the Stöber method to form Fe₃O₄/SiO₂. Fe₃O₄/SiO₂/TiO₂ nanoparticles were prepared by dissolving Fe₃O₄/SiO₂ in ethanol, adding titanium tetrabutoxide (TBOT) and ammonia, agitating at 70 °C, isolating, washing and calcining at 350 °C. Ag was impregnated using AgNO₃ and NaBH₄, followed by washing and vacuum drying to form Fe₃O₄/SiO₂/TiO₂@Ag (Figure 5). The band gap characteristics of Fe₃O₄/SiO₂/TiO₂@Ag were evaluated through the examination of the (αhʋ)² versus hʋ graph, revealing TiO2 to be an indirect band gap semiconductor. The Fe₃O₄/SiO₂/TiO₂@Ag composite displays an extensive absorption peak within the visible spectrum at wavelengths exceeding 400 nm, whereas TiO₂ presents a pronounced absorption peak in the ultraviolet region at 353 nm.

Figure 5 UV-Vis DRS spectra comparison of TiO₂ and Fe₃O₄/SiO₂/TiO₂@Ag open access [232].

The energy band gap of Fe₃O₄/SiO₂/TiO₂@Ag (2.92 eV) is found to be less than to the band gap associated with TiO₂ (3.24 eV). This indicates that Fe₃O₄/SiO₂/TiO₂@Ag exhibits greater efficacy in photocatalytic degradation when subjected to visible light irradiation. The composite exhibits a saturation magnetization of 49.4 emu/g, following the order: Fe₃O₄ > Fe₃O₄/SiO₂ > Fe₃O₄/SiO₂/TiO₂ > Fe₃O₄/SiO₂/TiO₂@Ag, demonstrating the effect of each layer on magnetic properties. Under optimal conditions (pH 4, 0.5 g/L dosage, 10 mg/L dye concentration, 100 min of irradiation), the composite material demonstrated an exceptionally elevated degradation efficiency, reaching an impressive figure of 96.52% when applied to the decolorization of Congo red dye. It maintains stability with only a 4.83% decline in efficiency after 5 cycles, indicating durability for repeated use. Khan et al. [233] found that SiO₂@CuFe₂O₄-AgI composites, particularly those containing 15% AgI, outperform pure and other hybrid composites in terms of photocatalytic performance. During methyl orange (MO) degradation, the composite with 15% AgI loading falls 16 times more rapidly than pristine CuFe₂O₄ and 8 times faster than pure AgI. The degradation rate of methylene blue (MB) is 18.67 and 9.33 times quicker, respectively. The SiO₂@CuFe₂O₄-AgI (15%) composite exhibits an enhanced surface area, a diminished recombination rate of photogenerated electron-hole pairs, expedited separation of photogenerated charge carriers, and an elevated redox capacity, all of which collectively enhance its photocatalytic efficacy.

Electrochemical impedance spectroscopy evaluations were conducted to gain deeper insights into the photoinduced charge transfer mechanisms and capacitance characteristics of various samples. The results indicate that the hybrid ferrite composite, SiO₂@CuFe₂O₄-AgI (15%), exhibits the lowest impedance arc compared to pristine AgI, CuFe₂O₄, and SiO₂@CuFe₂O₄ samples. This implies that the composite exhibits diminished resistance and superior interfacial photoinduced charge transfer efficiencies. Such attributes play a significant role in the augmented photocatalytic efficacy of the photocatalyst. The charge transfer mechanisms of the SiO₂@CuFe₂O₄-AgI (15%) photocatalyst under visible light are guided by an S-scheme process. In this mechanism, the conduction band (ECB) electrons of AgI, with a reduction potential of −0.26 V vs. NHE, migrate to the valence band (EVB) of CuFe₂O₄. Meanwhile, the EVB holes of AgI, with an oxidation potential of +0.25 V vs. NHE, transfer to the ECB of CuFe₂O₄. The conduction band of CuFe₂O₄ has a reduction potential of −1.05 V vs. NHE, allowing efficient reduction of O₂ to superoxide radicals (•O₂⁻), while AgI EVB potential is suitable for oxidizing H₂O to •OH radicals, as shown in Figure 6.

Figure 6 S-Scheme heterojunction mechanism for dye degradation using SiO₂@CuFe₂O₄-AgI composite [233].

This S-scheme mechanism enhances charge separation and photocatalytic efficiency, with SiO₂ acting as an electron transporter, reducing recombination rates and improving the overall photocatalytic performance. Furthermore, the hybrid composite exhibits chemical stability and can be reused, retaining its efficacy after ten cycles, thereby rendering it an economically viable alternative for photocatalytic applications. In conclusion, the inclusion of a SiO₂ interlayer in ferrite/SiO₂ composites provides significant benefits in enhancing photocatalytic activity by improving charge separation and enabling better light penetration. However, optimizing the thickness of the SiO₂ interlayer is crucial. Excessively thick layers can reduce light absorption, increase recombination rates, and introduce diffusion barriers for both the photocatalyst and dye molecules, thereby slowing down the overall photocatalytic process. On the other hand, insufficient thickness may fail to effectively prevent charge recombination, thus potentially decreasing the photocatalytic efficiency of the composite material [234].

Table 2 Photocatalytic performance of various ferrite/SiO₂ composites.

Zinc oxide (ZnO)/ ferrites

Zinc oxide (ZnO) is a widely studied semiconductor material known for its significant photocatalytic potential, particularly in the degradation of organic pollutants. Its high surface area, efficient charge transport, and superior photosensitivity make it an attractive candidate for photocatalysis [246]. ZnO can degrade a wide range of organic pollutants, including dyes, antibiotics, pesticides, and analgesics, and has been well-documented, highlighting its potential in wastewater treatment [247]. The material’s photocatalytic activity is primarily driven by its large band gap of approximately 3.3 eV, which allows it to absorb ultraviolet (UV) light and generate electron-hole pairs necessary for the degradation process [248,249]. However, ZnO’s photocatalytic efficiency is hindered by several limitations. One major drawback is its inability to fully utilize the solar spectrum, as it predominantly absorbs UV light, which constitutes only about 4% of the total solar radiation [250]. Furthermore, ZnO has an increased recombination tendency for photogenerated electron-hole pairs, considerably reducing its photocatalytic efficacy [251]. Several ways have been used to circumvent these restrictions, including doping with metals and nonmetals, producing heterostructures, and merging ZnO with other materials like graphene to promote separation of charge and extend light absorption into the visible range [252,253].

The formation of ZnO-based heterostructures with narrow bandgap materials effectively extends the absorption region from UV to visible light, enhancing photocatalytic performance. A notable example is the spinel ferrite/ZnO composite, which exhibits significant benefits in photocatalysis, making it highly effective for environmental remediation and energy applications. For instance, Saidi et al. [254] synthesized ZnO/ZnFe₂O₄ nanocomposites via a hydrothermal method, producing ZF-600 and ZF-700 samples by annealing at 600 and 700 °C, respectively. The band gap values were determined to be approximately 2.2 eV for ZF-600 and 2.1 eV for ZF-700, both lower than pure ZnO (3.37 eV), indicating enhanced suitability for visible light applications. Absorption spectra revealed effective visible light absorption for both samples, with ZF-600 exhibiting higher absorption due to its smaller crystallite size compared to ZF-700.

Sonia et al. [255] successfully synthesized ZnFe₂O₄-ZnO nanocomposites via a hydrothermal method for photocatalytic degradation of contaminants. These nanocomposites exhibited tunable band gaps (1.73 - 1.88 eV) influenced by ZnO content, promoting visible light absorption. HRTEM confirmed their spherical morphology while VSM analysis revealed magnetic properties for easy post-treatment recovery. The 1:4 ZnFe₂O₄-ZnO (ZZ14) composite demonstrated the highest efficiency (88% RB dye degradation in 90 min) and good reusability over multiple cycles, highlighting their potential for practical and sustainable photocatalytic applications. The p-type ZnFe2O4 and n-type ZnO in ZnFe₂O₄/ZnO nanocomposites form a type-II p-n heterojunction because ZnFe₂O₄ has a more negative conduction band (CB) and valence band (VB) than ZnO. UV irradiation causes both materials to absorb photons, resulting in electron-hole pairs in which electrons are excited from the VB to the CB. The higher energy of ZnFe₂O₄’s CB than that of ZnO enables photoexcited electrons to transfer from ZnFe₂O4’s CB to ZnO’s CB, while holes migrate from ZnO’s VB to ZnFe₂O₄’s VB, resulting in effective charge separation and superior degrading capabilities.

Undoped ZnO is primarily diamagnetic [256,257], but its magnetic properties can be influenced by factors such as particle size, morphology [258-260], and lattice deformation [256,261]. In contrast, ferrite/ZnO composites exhibit significantly enhanced magnetic properties due to the inclusion of inherently magnetic spinel ferrites. These increased magnetic properties allow for the easy recovery and reuse of the photocatalyst using magnetic separation techniques, boosting the efficiency and sustainability of the photocatalytic process. Hamed et al. [262] employed solid-state and sol-gel methods to synthesize ZnO/NiFe₂O₄ hetero nanocomposites with different weight percentages of NF (0%, 5%, 10%, 15%, 20%, and 25%). To obtain hysteresis loops, magnetic characteristics were analyzed with a vibrating sample magnetometer (VSM). The findings indicated ferromagnetic characteristics, with both saturation magnetization (Ms) and coercivity (Hc) varying according to NF content and synthesis method. A positive correlation was observed between NF content and saturation magnetization. Observed values of saturation magnetization are higher for samples prepared via the sol-gel method compared to those prepared via the solid-state method. This enhancement in magnetization is attributed to the migration of Zn ions into the NiFe₂O₄ crystal structure, redistributing Fe³⁺ ions from tetrahedral to octahedral sites, which promotes increased magnetization despite the smaller particle size of the S-G samples.

The ZnO content significantly impacts the photocatalytic properties of ferrite/ZnO composites, enhancing their efficiency in applications such as pollutant degradation. Elshypany et al. [263] synthesized Fe₃O₄/ZnO nanocomposites via a solid-state reaction method with varying ZnO content. The nanocomposites (MZ1, MZ2, MZ3, MZ4, MZ5) were synthesized by mixing Fe₃O₄ and ZnO in ratios of 0.2:1, 0.4:1, 0.6:1, 0.8:1 and 1:1. X-ray diffraction (XRD) and Raman spectroscopy validated the effective synthesis, which produced distinct ZnO and Fe₃O₄ peaks. UV-Vis diffuse reflectance spectroscopy (DRS) revealed a band gap redshift in Fe₃O₄/ZnO compared to pure ZnO, whereas photoluminescence (PL) spectra revealed decreased electron-hole recombination. Photocatalytic efficiency against methylene blue (MB) under visible light showed MZ4 exhibited the highest activity, followed by MZ3, MZ2, MZ5, and MZ1, as determined by pseudo-first-order kinetics.

These studies collectively highlight the potential of ferrite/ZnO composites for high efficiency photocatalysis in environmental applications, attributed to their synergistic properties and structural enhancements. Table 3 illustrates the photocatalytic activity of ferrite/ZnO composites and hybrids against various contaminants.

Table 3 Photocatalytic performance of various ferrite/ZnO composites.

MXenes/ ferrites

MXenes are a class of 2-dimensional nanostructures characterized by the general formula M_n+1X_nT_x, where M refers to an early transition metal (such as Ti, Nb, Mo, V, Zr, Sc, Hf, W), X signifies carbon and/or nitrogen, and Tx represents surface terminations including -O, -OH, and -F. The structure of MXenes is derived from their corresponding MAX (or M_n+1AX_n) phases, which have a layered structure consisting of M-X layers interleaved with A layers (where A is typically aluminum). By means of selective etching of the A (elements from groups 13 or 14, such as Al, Si, Ga) layers, MXenes are synthesized, preserving a layered architecture characterized by transition metal carbide or nitride sheets interspersed with surface terminations [275]. MXenes’ uncommon structure and bonding play a crucial role in their photocatalytic properties. The 2-dimensional structure has a large surface area, which allows for more active sites in photocatalytic reactions. The existence of hydrophilic functional groups (such as OH- and O-) across the MXene surface improves its dispersibility in aqueous solutions and interaction with reactants. Furthermore, MXenes’ high electrical conductivity facilitates efficient charge transport, which is critical for minimizing electron-hole recombination in photocatalysis [276,277].

MXenes can serve as excellent electron acceptors and co-catalysts, promoting effective charge separation and transfer. The surface termination group have been found to provide optimal catalytic sites, enhancing the overall photocatalytic efficiency [278-280]. The formation of heterojunctions with MXenes broadens the light absorption window from visible to near-infrared (NIR) [281], crucial for efficient solar energy utilization, and improves interfacial charge transfer and separation by forming a Schottky junction to enhance charge separation and increase the production of reactive oxygen species [282]. The complimentary coupling of ferrite and MXene in heterostructures improves photocatalytic performance (Table 4) by facilitating charge separation and increasing light absorption, addressing the inherent limits of each material in its pure state. Alsafari et al. [283] compared the photocatalytic performance of CuFe₂O₄/MXene nanohybrids with pristine CuFe₂O₄, demonstrating significantly enhanced activity. Copper ferrite (CuFe₂O₄) was synthesized by mixing iron nitrate (Fe(NO₃)₃) and copper nitrate (Cu(NO₃)₂) in deionized water with citric acid, followed by heating, drying, and annealing. MXene (Ti₃C₂) was prepared by etching Ti₃AlC₂ in hydrofluoric acid, followed by washing, intercalation with DMSO, and ultrasonication. The Ti₃C₂/CuFe₂O₄ nanohybrids were fabricated by mixing and sonication, with subsequent drying. SEM analysis revealed distinct morphologies: the MAX phase had a closely packed surface, while MXene showed an accordion-like structure, and Ti₃C₂/CuFe₂O₄ nanohybrids exhibited copper ferrite intercalation preventing restacking. FTIR confirmed functional groups such as Ti–O, C–F, and C=O. The photocatalytic degradation of methylene blue (MB) was examined under visible light. The CuFe₂O₄/MXene nanohybrids achieved 94% degradation in 60 min, 4.5 times higher than pristine CuFe₂O₄. The enhanced performance is attributed to the direct Z-scheme mechanism, where CuFe₂O₄ generates charge carriers under light and MXene’s narrow bandgap facilitates electron-hole recombination. Electrons from CuFe₂O₄ recombine with MXene’s holes, with MXene serving as a hole reservoir. These electrons react with oxygen to produce superoxide radicals, leading to hydroxyl radicals that degrade MB. Electrochemical impedance spectroscopy (EIS) demonstrated that the nanohybrids have decreased charge transfer resistance, which improves electron-hole separation and hence increases photocatalytic efficiency. The magnetic characteristics of the nanohybrids also helped in separation and reusability. The photocatalytic performance of various Ferrite/MXene Composites is given in Table 4.

Table 4 Photocatalytic performance of various ferrite/MXene composites.