Ahmad Farhan

Chemosphere 310 - Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere Metal ferrites-based nanocomposites and nanohybrids for photocatalytic water treatment and electrocatalytic water splitting Ahmad Farhan a, Javeria Arshad a, Ehsan Ullah Rashid a, Haroon Ahmad a, Shahid Nawaz b, Junaid Munawar c, Jakub Zdarta d, Teofil Jesionowski d, *, Muhammad Bilal d, ** a Department of Chemistry, University of Agriculture Faisalabad, 38040, Faisalabad, Pakistan Department of Chemistry, The University of Lahore, Lahore, Pakistan College of Chemistry, Beijing University of Chemical Technology, 100029, China d Institute of Chemical Technology and Engineering, Faculty of Chemical Technology, Poznan University of Technology, Berdychowo 4, PL-60695, Poznan, Poland b c H I G H L I G H T S G R A P H I C A L A B S T R A C T • Efficiency of Fe2O3-based nano­ composite for photocatalytic water remediation. • Iron oxide structural attributes and synthetic strategies are discussed. • Electrocatalytic potential of Fe2O3based nanocomposites. • Fe2O3-based semiconductor hetero­ junctions in wastewater treatment. • Ongoing issues and prospects to reveal the promise of Fe2O3-based catalysts. A R T I C L E I N F O A B S T R A C T Keywords: Metal ferrites heterostructure Synthesis strategies Photocatalysis Electrolysis Wastewater treatment Hydrogen evolution Water splitting Photocatalytic degradation is one of the most promising technologies available for removing a variety of syn­ thetic and organic pollutants from the environmental matrices because of its high catalytic activity, reduced energy consumption, and low total cost. Due to its acceptable bandgap, broad light-harvesting efficiency, sig­ nificant renewability, and stability, Fe2O3 has emerged as a fascinating material for the degradation of organic contaminants as well as numerous dyes. This study thoroughly reviewed the efficiency of Fe2O3-based nano­ composite and nanomaterials for water remediation. Iron oxide structure and various synthetic methods are briefly discussed. Additionally, the electrocatalytic application of Fe2O3-based nanocomposites, including oxygen evolution reaction, oxygen reduction reaction, hydrogen evolution reaction, and overall water splitting effi­ ciency, was also highlighted to illustrate the great promise of these composites. Finally, the ongoing issues and future prospects are directed to fully reveal the standards of Fe2O3-based catalysts. This review is intended to disseminate knowledge for further research on the possible applications of Fe2O3 as a photocatalyst and electrocatalyst. * Corresponding author. ** Corresponding author. E-mail addresses:-(T. Jesionowski),-(M. Bilal). https://doi.org/10.1016/j.chemosphere- Received 22 July 2022; Received in revised form 18 September 2022; Accepted 7 October 2022 Available online 12 October-/© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). A. Farhan et al. Chemosphere 310 - Fig. 1. Representation of various wastewater treatment technologies. to degrade naturally. Wastewater treatment facilities are now widely built-in metropolitan areas as a critical step in reducing wastewater pollutants and harmful substances. To decompose organic waste and lower pathogen loads, wastewater treatment plants utilize physical, biological, and chemical treatment processes. Using aerobic/anaerobic processes, wastewater contaminants are neutralized and may be safely discarded or reused. Nitrogen, phosphorus, and carbon are removed, at a considerable cost, like energy use and nutritional depletion (Ahmed et al., 2022). Although these methods, such as coagulation, precipitation, and adsorption, help treat wastewater but have limitations, i.e., less efficient, high cost, produce byproducts, and organic pollutants are resistant to these methods (Vikrant et al., 2019). All of the technologies described have certain limitations, i.e., extensive operating and repairing costs, complicated procedures, and generation of harmful sludge (Kubra et al., 2021). Water-pollution treatment modalities include physical, chemical, and biological treatments. Filtration, ion exchange, membrane filtering, and other processes are among them. For efficient water pollution cleanup, the majority of treatment methods are utilized in combination. Therefore, they should be treated carefully, and photocatalytic degra­ dation serves the purpose (Ejraei et al., 2019). Various water treatment technologies are being used, as shown in Fig. 1. As a possible photocatalyst, ferric oxide (Fe2O3) has been widespread use in removing organic pollutants and dyes. However, to the best of our knowledge, very little research has been conducted on the photo­ catalytic reaction for the dyes removal and organic contaminants remediation over Fe2O3, and the accompanying reaction pathways have not been thoroughly investigated. In addition, the results of this research make it clear that Fe2O3 has a diverse selection of photocatalytic po­ tentials and many prospective applications for the foreseeable future. Recently, there has been a lot of interest in using Fe2O3 as a photo­ catalyst, and it is believed that this study throws light on some hitherto unknown features of this exciting field of wastewater treatment. Critical research for clean and sustainable energy systems has been prompted by the growing severity of energy crises and environmental concerns. Water electrolysis is a potentially useful method for producing hydrogen and oxygen on a big scale. Both the high cost of noble metal catalysts (Pt-, Ru-, and Ir-based materials) for the oxygen evolution re­ action (OER) and the hydrogen evolution reaction (HER) encourage the development of non-noble transition metal-based active catalysts with economically feasible rates. Using fossil fuels has caused significant is­ sues with the environment and the availability of energy (Cao et al., 2017; Wang and Astruc, 2019). Many researchers have looked to renewable energy sources, including solar power, geothermal heat wind, 1. Introduction Surface water pollution, evaporation, and a lack of potable water are the most pressing issues facing mankind today (Farhan et al., 2022; Nawaz et al., 2021). Water shortage is also becoming problematic due to pollution, population, and climate change. Water scarcity will be a popular saying in the coming decades, prompting actions ranging from home sales to war unless novel ways to provide safe water are discov­ ered. Although water cleaning and disinfection methodologies can help these but methods are typically chemical and energy-intensive, neces­ sitating massive investments and technical expertise (Khan and Malik, 2019). In this sense, the advancement of water treatment technologies is essential to meet water quality standards for reuse while protecting the environment. Among different types of organic and inorganic pollutants, nutrients and agricultural leftovers, pathogens, suspensions, nuclear waste, and thermal pollution are only a few potential sources of water contamination. These are only a handful of the many causes of water contamination (Chowdhary et al., 2020). It is worth noting that these pollutants can endanger the environment and have mutagenic and carcinogenic effects on humans, aquatic life, and other living organisms (Vickers, 2017). The status of the world’s water sources has deteriorated substantially in recent years due to the discovery of several colored and harmful organic pollutants in soils and aquatic habitats (Munawar et al., 2022). Synthesized organic compounds, fatty acids, and oxygen-demanding waste are all examples of organic pollution. Wastewater treatment plants overflow from cities and homes, as well as from the manufac­ turers of canned goods, distilleries, pulp and paper mills, and other similar enterprises; all include pollutants that need oxygen to decom­ pose. Hydrocarbons like oil are examples of organic contaminants (Figoli and Criscuoli, 2017). Aerobic oxidation of the aforementioned organic pollutants depletes the dissolved oxygen in the water body. This has devastating effects on the aquatic ecosystem of the water body. Historically, it was thought that the textile industry used a massive amount of dyes (about 200 000 tons annually) but that there was no practical way to remove them (Singha et al., 2021). Differentiating between naturally occurring colors and those created by people (artificial colors) is feasible. Natural colors come from parts of the plant, like berries, roots, bark, leaves, lichens, fungi as well as wood, whereas synthetic colors are made from earth minerals, chemicals, and petroleum derivatives. They often have considerable molecular weight and a complex structure; they are also soluble in water and resistant to degradation. Additionally, organic dyes such as azo and fluorescein represent a severe threat to mammalian tissues since they are so difficult 2 A. Farhan et al. Chemosphere 310 - and hydrogen, as potential answers to these issues. Hydrogen’s high energy density and widespread availability have made it an attractive energy option in recent years (Liu et al., 2020a). Electrocatalytic water splitting is potentially useful for producing hydrogen energy (Chen et al., 2021). This is because it has abundant raw resources, uses envi­ ronmentally friendly practices, and has safety equipment. It is calculated that 1.23 V are the theoretical cell voltage for electrocatalytic water splitting. However, the thermodynamic obstacles mean water splitting requires a high voltage, which wastes energy. A low overpotential im­ plies a high degree of activity in this setting, making it an important quality to consider when assessing an electrocatalyst’s performance. As a result, developing an efficient electrocatalyst with a low overpotential is a critical area of research. Among the many potential catalyst/electrode materials for HER iron stands out because of high catalytic activity, its low cost, and adequate theoretical capacity. In the past, researchers have looked at several methods for producing materials based on Fe2O3. For example, Si and co-authors reported synthesizing Fe2O3 nanoparticles by a hydrothermal technique, in which the pH value was controlled during the reaction process, resulting in a highly modest low overpotential of 321 mV (Abraham et al., 2021). ZnO nanorod/-Fe2O3 composites developed by Lin’s team using a hydrothermal method exhibited enhanced catalytic performance. Zhong et al. (Wang et al., 2020a)created an electrode with an excellent specific capacity of 1196 mAh/g by designing a three-dimensional graphene-like matrix composited with nanoscale Fe2O3. The Fe2O3 nanoparticles on MWCNT created by Piao and co­ workers using a simple refluxing and heating procedure have an adequate rate ability and stable long-term cycle performance (Song et al., 2020). Although these advancements are noteworthy, the poor active sites and sluggish charge transfer kinetics of Fe2O3 remain a problem. Slow electron transport and a lack of active sites frequently lead to a rapid fall in rate performance and cycle endurance in catalytically active and theoretically perfect Fe2O3. The incorporation of carbon materials significantly increases the rate performance and cycle durability of Fe2O3. Several treatments have been attempted so far to overcome these significant drawbacks. A common byproduct of this production tech­ nique is a material with a decreased number of intrinsic active sites (Wang et al., 2016). Iron oxide-based composites are prominent composites for produc­ ing hydrogen, which is comprehensibly discussed in this review. In this article, we look at some of the more recent developments in the research into how iron oxide electrocatalysts can be used to split water. Elec­ trochemical devices intended to convert solar energy directly into hydrogen may find usage for these electrocatalysts. We have spent a considerable amount of time discussing the water splitting efficiency of nanocomposites based on iron oxide, including the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR), as well as everything in between. We believe that the conversations that take place in this location will be beneficial not only to those who are new to the subject matter at hand but also to those who are seasoned professionals in the relevant sector (through the use of comparative tables and a focus on the more recent literature). There are other places where more in-depth discussions of electro­ catalytic HER and OER can be obtained; readers interested in learning more can look in those places. Collectively photocatalytic and electro­ catalytic performance of Fe2O3-based nanocomposites is discussed in this review. structure, electric, catalytic, mechanical, optical scattering, and physi­ ochemical properties (Vasantharaj et al., 2019). The phrase “magnetic nanoparticle” refers to an inclusive range of nanoscale constituents benefit from being magnetized, allowing them to be used in a variety of applications. This categorization encompasses a wide variety of nano­ particles, such as metallic, bimetallic or metal oxide, as well as core-shell structures and Janus-type nanoparticles, in various designs (Jishkariani et al., 2017). Various transition metal ions, such as Ni, Fe, Co, and their compounds, can be utilized to create magnetic nanoparticles. Iron is a specific element that can occur in various forms, starting from “0′′ to “3′′ valence. Hematite (α-Fe2O3), one of the forms of iron oxides, because of its high earth abundance, low cost, and high photo-stability, has been recognized as one of the finest materials for the purpose (Shen et al., 2016). Each iron compound has unique properties; some have a mag­ netic while some have ferromagnetic nature. Iron oxides have various forms like Maghemite (γ- Fe2O3), Goethite (FeO(OH), Hematite (αFe2O3), and Magnetite (Fe3O4). Hematite (α- Fe2O3) is a more thermo­ dynamically stable mineral under oxygen than the other iron oxides (Narayanan and Han, 2016). IONPs have properties of both hydrous iron oxides and metallic iron due to their core-shell structure (Kaur and Sidhu, 2021). For their inherent enzyme-like properties, they have recently been named “Nanozyme.” They have catalytic capabilities comparable to those of other oxidases such as catalase, superoxide dismutase, sulfite oxidase, peroxidase, and superoxide dismutase, suggesting that they might be used as enzyme mimics (Gao et al., 2020). The iron oxide systems, which have been extensively investigated for biological and technological uses, are among these structures (Sosa-A­ costa et al., 2020). Because of nanoscale confinement and surface ef­ fects, the magnetic properties of iron (III) oxides display unusual features that differ from those of bulk equivalent materials as particle size decreases (Ji et al., 2018). Magnetic iron oxide nanoparticles (IONPs) offer several benefits, including the fact that they are compar­ atively inexpensive, have good physical and chemical stability, and are biocompatible (Saif et al., 2019). The quantum effect dominates the behavior of IONPs in the nanoscale range, impacting the matter’s magnetic, electric, and optical characteristics. The superparamagnetic characteristics of Fe3O4 and γ-Fe2O3 nanoparticles become visible at a small size of around 10–20 nm, allowing for superior performance in applications. They also have exceptional dispensability in solutions due to their improved surface-to-volume ratio (Arias et al., 2018). 3. Synthesis of ferric oxide Iron nanoparticles are synthesized using a variety of chemical and physical methods. Thermal decomposition (Li et al., 2019), reverse micelles (Tokubuchi et al., 2021), sonochemical reactions (Uma et al., 2020), co-precipitation (Kushwaha and Chauhan, 2021), sol-gel syn­ thesis (Tadic et al., 2019), micro-emulsion technology, hydrothermal synthesis (Luo et al., 2019), electrospray synthesis, flow injection syn­ thesis, and colloidal chemistry-based methods are some of the chemical-based methods used in synthesizing Fe2O3 nanoparticles (Mohamed et al., 2020). Algae, fungus, bacteria, and plants have all been discovered to be cost-effective and environmentally acceptable systems for nanoparticle production. Many researchers have been drawn to the intersection of medicinal plants and bioactive nanoparticles to produce nanomaterials with a variety of uses. Because of their bioactive components and rich phytochemistry, medicinal plants have been frequently used and favored for nanoparticle manufacturing (Ovais et al., 2016). 2. Ferric oxide 3.1. Co-precipitation method Nanotechnology allows the modification of materials at the nano­ scale to accomplish the desired properties and functions. This makes it possible to manage the material and use it in a variety of applications (Jabbar et al., 2022). Nanoparticles are materials with distinct proper­ ties in size (usually ranging from 1 to 100 nm), form, magnetic, Synthesis via the co-precipitation method is a simple and convenient way to obtain iron oxide nanoparticles like Fe3O4 and α-Fe2O3. The morphological manipulation of shape or size and the chemical 3 A. Farhan et al. Chemosphere 310 - Fig. 2. Sonication method for iron oxide nanoparticles synthesis. composition of NPs is determined by the metal precursors used. Azarifar et al. proposed the synthesis method of ferric oxide nanoparticles via mixing ferrous (Fe-II) and ferric (Fe-III) ions in strongly alkaline envi­ ronments. The proportions of the various reagents used in synthesizing the Fe3O4 in an oxygen-rich environment and at ambient temperature using a solution of ferric salt precursors are discussed, and the value of pH, the reaction temperature, as well as the medium’s ionic strength. The precipitation of nanoparticles was caused by mixing components without changing pH, stirring for 15 min at 50 rpm. The black precipi­ tate was separated from the resulting mixture, rinsed with distilled water, and ethanol, and dried in an oven at about 50 ◦ C. When the conditions for co-precipitation were improved, repeatable nanoparticle quality was achieved (Azarifar et al., 2016). Farahmandjou and Soflaee synthesized α-Fe2O3 nanoparticles using a simple co-precipitation method that included FeCl3⋅6H2O (iron chloride hexahydrate) as a precursor and solution of ammonia as a precipitator in 2015. In order to confirm the synthesis of α-Fe2O3 NPs, they utilized different character­ ization techniques. They confirmed that as-synthesized α-Fe2O3 (he­ matite) structure showed nanocrystals. When the samples were calcined at around 500 ◦ C the particle size of the α-Fe2O3 nano-powders was found to be 30 nm with uniform size, according to the XRD pattern, and was also confirmed using other techniques (Farahmandjou and Soflaee, 2015). widely used process for producing Fe3O4 nanoparticles. Without any surfactants, iron precursors in water are subjected to high temperature and pressure in a microwave autoclave. The method yields crystalline structures of iron oxide NPs with diameters smaller than 100 nm. This method can produce a variety of morphologies, including hollow spheres and nanocubes. With increasing hydrothermal temperature, the saturation magnetization, average particle size, and coactivity of Fe3O4 nanoparticles increase. According to several researchers, this approach’s major drawbacks include high temperatures and pressure values during the synthesis process (Jamkhande et al., 2019). Deba-taraja et al. also used this method in 2017. In that technique, the mixture was placed in a Teflon cup at around 140 ◦ C for 1 h. The precipitate was washed out of the solution after being drained from the reactor. The material was washed and dried in an oven (about 80 ◦ C) for 12 h. The major drawback of that approach was the slow kinetics of reaction at that temperature as well as the failure to obtain quality NPs with hydrophilic surface morphology smaller than 10 nm (Debataraja et al., 2017). 3.4. Thermal decomposition method This begins with the disintegration of metal precursor in an organic liquid and then proceeds by oxidation. The method yields monodispersed nanoparticles with very few crystalline defects (Unni et al., 2017). Maghemite or Magnetite nanoparticles relatively smaller can be produced using this strategy, depending on the kind of precursor as well as other oxidizing substances in the fluid. Despite the benefits of this method, like the ability to obtain monodispersed as well as small-sized constituents, the NPs produced are only soluble in solutions that are not polar., restricting their use in the biomedical field. However, con­ trolling the size of the particles of Fe3O4 NPs by the thermal breakdown approach demands the use of significant quantities of harmful and expensive precursors and surfactants. 3.2. Micro-emulsion method The water-in-oil micro-emulsion method utilizes drops of water as nano-reactors in a constant organic part with the involvement of sur­ factant molecules. The process is said to provide greater control over crystal growth. Iron oxides are precipitated out by precursors of iron in the mainly sited aqueous medium within the micelles using this method. Because iron precursors aren’t reactive in the organic phase, iron oxides (IOs) do not precipitate. The size and shape of the NPs can be controlled by the precursors used and by adjusting the size of the droplets of water. Bozkurt G reported the synthesis of α-Fe2O3 by micro-emulsion and confirmed the synthesis using different characterization techniques. TGA analysis revealed that the whole formation of Fe2O3 occurred at around 400 ◦ C. SEM images observed a plate-like structure formation in α-Fe2O3 NPs with a homogeneous distribution. The particle size was calculated to be nearly 13.1 nm from the XRD data using the Scherrer equation. The FTIR spectrum revealed Fe–O vibrations in the tetrahedral and octahedral regions. According to the UV–Vis spectrum, NPs exhibited optical absorption at a wavelength of about 420 nm. Furthermore, the optical band gap of the α-Fe2O3 NPs was measured to be 3.2 eV (BOZKURT, 2020). 3.5. Sonochemical method The sonochemical method is a relatively new development in the formation of metal oxides. High energy ultra-sonication results in localized points of heat with pressures of 1800atm and temperatures of up to 5000 K. The high temperature causes the eruptive bubbles to collapse and iron oxides to form. This approach can also produce monodispersed nanoparticles with a wide range of morphologies. As a result, this method may be used to produce nanoparticles on a massive scale. (Gupta and Srivastava, 2019). Natarajan et al. used sonochemical methodology to create nanoscopic amorphous iron powders by dis­ ◦ solving iron carbonyl in decane. When subjected to heat above 300 C, these powders fuse and crystallize. Fig. 2 depicts iron carbonyl soni­ cation in the availability of a polyvinylpyrrolidone (PVP) stabilizer, which results in well-dispersed iron nanoparticles (Natarajan et al., 2019). 3.3. Hydrothermal method The solvothermal technique, referred to as hydrothermal, is another 4 A. Farhan et al. Chemosphere 310 - Fig. 3. Various structures of iron oxide. 3.6. Pechini method 3.8.1. Comparative analysis among various synthesis techniques Undoubtedly, all the previously mentioned traditional techniques can produce massive amounts of nanoparticles of the desired size and shape. Nonetheless, these methodologies necessitate incredibly high costs and complex and outdated mechanisms. Therefore, the one method which is so far considered a better option is the green synthesis approach (Shrestha et al., 2021). In contrast to traditional physical and chemical methodologies, green synthesis has numerous advantages, including simplicity, a small industrialized process, speedy, cost-effectiveness, and less waste generation (Nikolaidis, 2020). The Pechini method, known as polymeric precursors, is a popular formulation for obtaining oxide nanoparticles. This method has over­ come most of the issues and disadvantages associated with the sol-gel method. This technique produces reproducible, well-ordered, mono­ disperse NPs. Furthermore, this is one of the simplest methods for pro­ ducing NPs. This method has some drawbacks, including particle clumping and the high price of certain reagents. Furthermore, the shape and size of NPs in the steady state are uncontrollable. As a result, the only way to influence the NPs’ sizes is to alter the sintering conditions and the initial metal proportion inside the gel. The process starts with metal salt being dissolved in water at about 70 ◦ C. Then, ethylene glycol and citric acid are added, resulting in an esterification process, after which the resulting oxide NPs can be formed after drying and calcination (Gharibshahian et al., 2017). 4. Structure of ferric oxide Ferrimagnetic IONPs (magnetite or maghemite) have received a great deal of attention for basic study and their potential in a wide range of practical applications. Iron oxides are polymorphic chemical sub­ stances that include Fe3O4 (magnetite), γ- Fe2O3 (maghemite), and he­ matite (α-Fe2O3) as shown in Fig. 3. The most researched ones are γ-Fe2O3 and Fe3O4, which have amazing features in the nano range (such as high specific surface area, superparamagnetic, biocompatible, and so on) because quantum effects impact matter behavior, optical, electrical, and magnetic properties at this size scale. As a result, these materials are suited for surface modification and functionalization in a variety of applications at the nanoscale (Ajinkya et al., 2020). Maghemite has single domains that are 5–20 nm in diameter. The iron oxide γ-Fe2O3 crystal structure is either cubic (P4332 space group) with partial Fe vacancy disorder or tetragonal (P41212 space group) with complete site ordering and c/a≈3. The γ-Fe2O3 and Fe3O4 compounds have highly similar atomic structures because they form in the spinel structure, which has octahedrally and tetrahedrally coordinated metal sites. While Fe3O4 includes divalent Fe2+ and trivalent Fe3+ ionic species, every iron in γ-Fe2O3 is trivalent. In the γ-Fe2O3 spinel structure, the insertion of vacancies at octahedral coordinating cation sites maintains charge neutrality (Andersen et al., 2021). 3.7. Sol-gel method Iron oxide NPs are also synthesized by the sol-gel method. Starting with precursor molecules such alkoxides and metallic or inorganic salts, an oxide skeleton is attained through polymerization reactions and lowtemperature hydrolysis, allowing the formulation of metastable oxide phases (Imran et al., 2020). The sol-gel approach has many advantages, including using less expensive precursors and simple processing condi­ tions that yield nano-sized particles with a uniform size (Parashar et al., 2020). Furthermore, the Sol-gel method yields highly pure and homo­ geneous materials at low temperatures. The major drawback of this approach is utilizing organic solvents; the procedure is also established in phases, making it both tedious and costly (MODAN and PLĂIAȘU, 2020). This method cannot be classified as eco-friendly because of its toxic chemicals. 3.8. Biological/green method 5. Photocatalysis and ferric oxide as an efficient photocatalyst Green (or biological) nanomaterial synthesis is considered the most reliable nanoparticle synthesis because of its non-toxic nature, as it does not require harmful chemicals for its synthesis. It also does not release any harmful byproducts. These NPs do not endanger our surrounding environment as well as human health (Yew et al., 2020). Large numbers of NPs with the required form and size may be produced using the previously outlined processes. As a result of their intricacy and anti­ quated nature, the implementation of these methods is prohibitively expensive. Barzinjy et al. used a biological method to synthesize magnetite nanoparticles from Rhus coriaria extract (Barzinjy et al., 2020). The process was safe, non-toxic, and eco-friendly. Photocatalysis, in its broadest sense, is any process in which a catalyst uses light as a stimulus to initiate or accelerate a chemical re­ action. The term “photocatalytic reaction” describes this mechanism (Hitam et al., 2019; Byrne et al., 2018). Because it uses sustainable solar energy, heterogeneous photocatalysis is an excellent option for treating wastewater that contains colors and organic pollutants using advanced oxidation processes (AOPs). Because of this, it is highly recommended to eliminate organic pollutants and colors (Khaki et al., 2017). The various AOPs technologies are shown in Fig. 1. The primary objective of AOPs is to treat wastewater to an acceptable concentration of harmful and hazardous compounds before discharging it into waterways. Hydrogen 5 A. Farhan et al. Chemosphere 310 - Table 1 Iron oxide-based carbon composites for efficient water treatment. Composites Pollutant Dose rate Removal efficiency Contact time Pollutant dosage Synthesis method Reference (α-Fe2O3-GO) powder α-Fe2O3/reduced graphene oxide (RGO) Fe2O3–TiO2@AC fiber Fe2O3–TiO2@activated carbon fiber Fe2O3/graphene/CuO and (RhB) dye Methylene blue dye 20 mg 50 mg 64% 97% 120 min – 25 mg/L 10 mg/L Phenol Cu(ii) and as(v) ions – – 100% 90% – – – – Simple reaction Simple solvothermal technique Electrospinning method Electrospinning process Khoshnam et al. (2021) Muthukrishnaraj et al. (2015) Li et al. (2016) Han et al. (2017) Methylene blue 50 mg 94.27% 40 min 20 mg/L α- Fe2O3 @GO Methylene blue 100 mg 40% 80 min 40 mg/L Nuengmatcha et al. (2019) Liu et al. (2017) WO3–Fe2O3-rGO Methylene blue and Rhoda mine B Rhodamine B 100 mg 98% and 94% 20 min 30 mg/L A simple solvothermal method Facile hydrolysis method Hydrothermal process 5 mg 81.8% 60 min 20 mg/L Hydrothermal process Fe2O3–TiO2–GA peroxide (H2O2), Fenton’s reagent, ozone (O3), UV light, or a catalyst can all be used to generate highly potential chemical oxidants on-site, which is necessary for AOPs (Ong et al., 2018). Hydroxyl radicals are reactive species that transform contaminants like colors and refractory organic compounds adsorbed on a photocatalyst’s surface. Other bene­ fits of AOPs include their ability to minimize the toxicity of organic compounds via strong photocatalytic effectiveness and moderate reac­ tion conditions and their potential mineralization into safer products (Danish et al., 2021). Despite recent advances in environmental cleanup, water splitting, and photosynthesis using photocatalysis, finding semi­ conductor photocatalysts that efficiently harness solar energy for energy consumption remains challenging. Consequently, many studies and numerous alterations have been conducted to locate the photocatalyst capable of eliminating organic pollutants and dyes in the most efficient and promising method (Du et al., 2019). Iron oxides are regarded as the most beneficial for photocatalytic processes among metal oxide semi­ conductors. This is because of its outstanding stability, recyclability, and earth-abundant availability, in addition to its small Eg (2.3 eV) and proficient light harvesting efficiency (Dai et al., 2019). Furthermore, Fe2O3 has found widespread use as a sensitizer for broad bandgap photocatalysts to progress the light-harvesting by these photocatalysts. Once Fe2O3 absorbs visible light, the produced reactive charge carriers may accelerate chemical reactions by activating nearby chemical com­ ponents (Liu et al., 2015). Because of Fe2O3 magnetic properties, there has also been increasing interest in combining it with other substances to create composites that may streamline the recovery process. For instance, Zhang and colleagues have devised a one-step combustion technique to create the magnetic Fe2O3/TiO2/graphene (GTF) hybrid (Zhang et al., 2013). This catalyst was used in the visible-light-induced photodegradation of MB. The re­ sults of the cycle tests demonstrate the potential stability of this GTF hybrid and its increased visible-light absorption spectrum, decreased charge recombination, and broadened absorption spectra. In addition, this GTF hybrid absorbs more visible light than either of its parent compounds. In addition, the GTF photocatalyst may be easily removed from the reaction solution by applying an external magnetic field. Another study used a magnetic Fe2O3/ZnFeO4@Ti3C2 MXene photo­ catalyst to eliminate Rhodamine B (RhB) and dangerous Cr(VI) in water. The photocatalytic characteristics of MXene were used to achieve this result (Zhang et al., 2020a). The ultrasonic-assisted self-assembly tech­ nique was applied to fulfill the aim of dispersing the Fe2O3/ZnFe2O4 heterojunctions present on the Ti3C2 MXene surface. This catalyst comprises magnetic features, which contribute to its remarkable reus­ ability, in addition to a high conductivity level and the presence of many heterostructure interfaces, both of which contribute to increased visible light harvesting. Priyadharsan et al. (2019) Zhang et al. (2018a) 6. Application of Fe2O3-based carbon composites in photocatalytic wastewater treatment Carbonaceous compounds, such as graphene, graphitic carbon nitride, activated carbon, carbon nanotubes, and carbon quantum dots, are considered more environmentally and physiologically sound solu­ tions when compared to inorganic molecules (Sharma et al., 2019). These materials provide a variety of distinct benefits, including chemical inertness, stability, and the ability to tailor structural and electrical features, to name just a few of those benefits. Because of their high surface area and the ease with which they can transport charge carriers, they have also found utility as a substrate for a broad range of metal oxide nanoparticles (Khalid et al., 2017). Graphene sheets, a valuable carbonaceous substance, may be able to supply more active sites for photocatalytic processes. Additionally, it has been repeatedly estab­ lished that graphene sheets might boost photostability by avoiding the development of aggregates throughout the process. Graphene, a type of carbon, has a variety of useful features, including a high conductivity of both electricity and heat and a large surface area (Tan et al., 2019). Table 1 shows complete detail of various composites showing good photocatalytic activity. With increasing emphasis on effective and cost-effective wastewater decontamination, Fe2O3 and its composites have been considered promising and prosperous materials to remove various contaminants such as heavy metal cations, heavy metal/metalloid complexation or­ ganics, and organics, owing to their advantages of effective adsorption efficiency, excellent visible light catalytic activity, and outstanding Fenton reaction potential (Hitam and Jalil, 2020). Several methods like Coagulation/Flocculation (Aziz et al., 2018), solvent extraction (Crini and Lichtfouse, 2019), ion exchange mechanism (Wu et al., 2021), photocatalytic degradation (Chen et al., 2017), microbial degradation (Levine, 2016), and membrane filtration (Mulungulungu et al., 2021) were used for wastewater treatment. Photocatalytic degradation was offered as an optimal approach because of its great efficiency and low cost. Hematite graphene oxide powder and thin film nanocomposites were produced in one research. Hematite’s photocatalytic activity has been greatly enhanced when combined with other composite materials like graphene and graphene oxide (Zolghadr et al., 2017; Padhi et al., 2017). The photocatalytic activity of two synthesized composites was compared at different concentrations. Powder nanocomposites were shown to be more successful than thin-film composites at removing. RhB dye. α Fe2O3-GO-5% powder nanocomposites eliminated more than 64% of the dye, whereas thin-film nanocomposites removed just under 47% of the dye. The reusability study was conducted on both materials, and α Fe2O3 -GO-5% powder nanocomposites eliminated more dye, almost 63% in 6 cycles (Khoshnam et al., 2021). Researchers successfully synthesized a series of nanorod composites of α-Fe2O3/reduced graphene oxide (RGO) using a simple solvothermal 6 A. Farhan et al. Chemosphere 310 - Table 2 Fe2O3-based polymer composites exhibiting enhanced photocatalytic activity. Composites Pollutant pH Removal efficiency Contact time Synthesis method Pollutant dosage Reference (Fe2O3/PS) 4-CP and 4-NP 4.3 96.2% for 4-CP and 89.2 for 4-NP 60 min γ-Ray irradiation method Wang et al. (2019a) TiO2/polythiophene/ γ- Fe2O3 P3HT and nano αFe2O3 composite Arsenic 2–6 BPA and TC 7 for BPA and 8 for TC 90.1% in As(V) form 99% for BPA and 97% for TC 75 min for BPA and 90 min for TC 20 mg/mL for 4-CP and 10 mg/mL for 4NP A stock solution of 2 g L− 1 but used 50 mL 5 mg/L Zhu et al. (2020) 91% 100 min (50 mg/L, 200 mL Qin et al. (2020) 6 73% 170.28 min 60 mg/L Hasan et al. (2020) 8 96.51% 60 min 4 94.6% for OG and 89.2% for MO 40 min for OG and 60 min for MO PANI/α- Fe2O3/ β-FeOOH PACT@γ- Fe2O3 bio-based chitosan/ Fe2O3/NiFe2O4 Fe2O3@CS RhB dye Malachite green dye Malachite green dye OG and MO within 40 and 60 min technique. Graphene, the best option in carbonaceous materials, is a 2D carbon sheet with unique chemical, mechanical, thermal, and physical properties for synthesizing graphene-based semiconductors (Albero et al., 2019; Kuang et al., 2019). Under visible light irradiation, these composites demonstrate better degradation efficiency and increased photocatalytic efficacy than the pure α- Fe2O3 nanorod. The deminer­ alization of the dyes was observed using UV–visible spectroscopy, which revealed a reduction in absorbance intensity and concentration. The degradation efficiency of the composite towards methylene blue dye was determined to be 97%. The increased photocatalytic activity is due to the lower recombination rate and the greater adsorption rate caused by graphene inclusion (Muthukrishnaraj et al., 2015). Zhao L et al. reported the effective synthesis of a new g-C3N4/C/Fe2O3 photocatalyst by binding g- C3N4 nanosheets over C/Fe2O3 that was created utilizing collagen fiber as the charcoal resource and the Fe tanning process to decrease Cr(VI) under artificial sun irradiation. Under the same cir­ cumstances, a g-C3N4/C/Fe2O3 photocatalyst outperformed g- C3N4, and the reduction efficiency of Cr(VI) enhanced as the Fe concentration in photocatalyst (g-C3N4/C/Fe2O3) increased (Zhao et al., 2021). To decontaminate phenol wastewater, an electrospinning approach was used to create a mesoporous iron–titanium blended oxides@acti­ vated carbon (AC) fiber membrane. Due to the FeO3, TiO2, and carbon combination, DRS demonstrates that the composite has strong photon absorption from visible light and ultraviolet light irradiation. The developed nano Fe2O3–TiO2@AC fiber membrane may operate as an efficient adsorbent and recyclable photocatalyst for 100% removal of phenolic contaminants. This hybrid approach may find widespread use in the remediation of diverse organic waste fluids (Li et al., 2016). A modified electrospinning procedure created a three-dimensional nano Fe2O3–TiO2@activated carbon fiber membrane. According to the water purification results, this composite membrane may be reused more than ten times to eliminate methyl orange from a 50 mL synthetic dye wastewater with a 20 mg/L level. This membrane may also be utilized in synthetic wastewater containing heavy metals to remove As (V) and Cu (II) ions with an efficiency greater than 90%. The hybrid purification approach might be widely employed to treat organic or heavy metal polluted water (Han et al., 2017). A simple solvothermal method was utilized to fabricate Fe2O3/graphene/CuO (FGC) nanocomposites. It was considered an important catalyst for dye removal: methylene blue. It showed removal efficiency of about 94% within 40 min of contact time. The optimum pH was 5.8 (Nuengmatcha et al., 2019). α-Fe2O3@GO was fabricated via the facile hydrolysis method. This method proved to be a good method for methylene blue removal from wastewater. The degradation efficiency was 40% at a contact time of 80 min (Liu et al., 2017). It was anticipated that the charge transfer capabilities of graphene One-pot oxidation polymerization Photocatalytic efficacy One-step in-situ polymerization Free radical polymerization Green chemistry Co-precipitation Liu et al. (2020b) Ansari et al. (2022) Vigneshwaran et al. (2021) sheets would improve for use in photocatalytic applications when two or more types of metal oxides are uniformly spread over the sheets. A simple, low-cost, hydrothermal one-pot approach was used to produce WO3 and Fe2O3 nanoparticles on graphene sheets. The synthesized Fe2O3 nanoparticles decorated on the surface of graphene sheets with WO3 nanoparticles showed strong charge transport capabilities. The final WO3–Fe2O3-rGO (WFG) nanocomposites reported improved pho­ tocatalytic, heavy metal removal, and antibacterial capabilities. It has been demonstrated that subjecting a photocatalytic process to sunshine may effectively remove rhodamine B (94%) and methylene blue (98%) from water. The synergistic effect of iron, tungsten, and reduced gra­ phene oxide resulted in enhanced photocatalytic properties (Priyad­ harsan et al., 2019). Numerous nanocomposites with novel 3D architectures might be fabricated using a single-pot hydrothermal technique followed by freeze-drying. These nanocomposites were known as Fe2O3–TiO2–GA and were co-doped with Fe2O3 and TiO2. The one with 30% Fe2O3 has the best absorptivity (95.0%), while the one with 25% Fe2O3 had the highest overall efficiency (97.7%) for rhoda­ mine B dyes during 1 h over 5.0 mg of the nanocomposites through adsorption and visible-light-driven photocatalytic degradation. 7. Fe2O3-based polymer composites application in photocatalytic wastewater treatment One research used γ-Ray irradiation to create Fe2O3/polystyrene (Fe2O3/PS) composite fibers. In the sixth cycling, the photocatalytic activity of 4-NP (4-nitrophenol) and 4-CP (4-chlorophenol) remained at 80% and 75%, respectively, and the composite fiber showed high recyclability, which offers application development prospects for wastewater treatment (Wang et al., 2020b). Under visible-light irradi­ ation, the photocatalytic capabilities of Fe2O3, PS, and Fe2O3/PS cata­ lysts for the degradation of 4-NP and 4-CP were investigated in an acidic solution (pH 4.3). For 4-CP and 4-NP degradation, Fe2O3/PS composite fiber demonstrated stronger photocatalytic activity than PS and Fe2O3. The 0.3- Fe2O3/PS photocatalyst achieved an outstanding efficiency of 91.3% for 4-CP degradation and an optimal efficacy above 86.2% for 4-NP after 60 min of visible-light irradiation. When the high starting concentrations were 20 mg/mL and 10 mg/mL, the photocatalytic effi­ cacy of 0.3- Fe2O3/PS for 4-NP and 4-CP degradation were increased to 96.2% and 89.2%, respectively (Wang et al., 2019a). TiO2/polythiophene/γ- Fe2O3 was synthesized to eliminate trace As (III) from an aqueous solution using one-pot oxidation polymerization (Liu et al., 2022). The nanocomposite demonstrated remarkable adsorption stability of As(III) in a low-pH zone (2–6). Under visible light irradiation, over 99.1% of As(III) was converted to As(V), and the nanocomposite absorbed 90.1% of the generated As(V). The 7 A. Farhan et al. Chemosphere 310 - photocatalyst dose was 0.5 g L− 1. The standard solution was diluted to the As(III) stock solution to 2 g L− 1 and measured the photocatalytic reaction activity and adsorption capacity in 50 mL of solution. The re­ sults revealed that in the initial pH range of 2–6, the total arsenic reduction by the adsorbent remained (3.00–3.52 mg g− 1) steady (Liu et al., 2020b). Another work used electrophoretic deposition to create a P3HT and nano α- Fe2O3 composite photocatalyst array known as F1P. (P3HT) Poly-3-hexylthiophene is a photocatalytic conjugated polymer with high photocatalytic efficacy (Floresyona et al., 2017). P3HT has the same energy band as α - Fe2O3. The proper matching of these two materials’ energy bands gives a very strong probability of creating the direct Zscheme system. The values of BPA (bisphenol A) and TC (tetracycline) were both 5 mg/L. F1P’s TC degradation rate was 97% in 90 min, while its BPA degradation rate was 99% in 75 min. The TC degradation impact was greatest at pH 8, and the BPA degradation effect was greatest at pH 7 (Zhu et al., 2020). PANI/α- Fe2O3/β-FeOOH composites were made in one study using a simple one-step in-situ polymerization process under acidic conditions to evaluate RhB dye degradation. When the illumi­ nation period is 100 min, the degradation efficacy of PANI/αFe2O3/β-FeOOH to RhB reaches 91%, as shown in Table 2. In a reactor containing an aqueous Rhodamine B dye solution (50 mg/L, 200 mL), 0.1 g of samples were dispersed (Qin et al., 2020). In another experiment, the material PACT@γ-Fe2O3 was created by grafting with chitosan biopolymer and acrylamide monomer’s oxidative free radical polymerization in the presence of γ- Fe2O3 nanoparticles as a filler. The central composite design (CCD) optimum variables were contact duration of 170.28 min, the adsorbent dosage of 0.75 g, pH 6, and Malachite green dye concentration of 60 mgL− 1. The expected percentage removal of dye by PACT@γ- Fe2O3 was estimated to be 77% under these optimised circumstances, which fits well with the experi­ mental result of 73% (Hasan et al., 2020). Modified chitosan with different functional groups has a strong potential as an effective adsor­ bent in water pollution removal. Green chemistry was used to effectively create a novel magnetic adsorbent, bio-based chitosan/Fe2O3/NiFe2O4. After 30 min, the MG dye is completely removed (80%). The best con­ ditions for methyl green (MG) removal were found to be pH = 8, contact period of 60 min, adsorbent dose of 0.2 g, and 25 ◦ C, with an adsorption uptake capacity of 77.22 mg/g and high dye percentage removal of 96.51% (Ansari et al., 2022). One study used the co-precipitation approach to create Fe2O3@CS (Fe2O3 reinforced chitosan) nanocomposite. The chitosan makes its surface available for the helpful creation of the nanocomposite. Bio­ polymers have high stability and are easily reprocessed by cross-linking or mixing with other molecules. As a result, they were considered excellent host resources to manufacture a wide range of materials. These nanocomposites were created with increased catalytic performance to eliminate organic dyes such as Orange Green (OG) and Methyl Orange (MO). The results showed that the greatest degradations for OG and MO within 40 and 60 min were 94.6% and 89.2%, respectively. The degradation of OG and MO dye molecules was greatest at pH–4 (Vigneshwaran et al., 2021). Fig. 4. Photocatalytic degradation of 4-CP using meso-Fe2O3/TiO2 catalyst. (Priyadharsan et al., 2019). A one-pot hydrothermal process combined with freeze-drying is used to create a unique 3D architecture of (Fe2O3–TiO2–GA) Ferric oxide and titanium dioxide co-doped graphene aerogel nanocomposites in one research. The one with 30 wt% ferric oxides has the best absorptivity (95.0%), while the one with 25 wt% ferric oxides has the highest overall efficiency (97.7%) for RhB dyes (20.0 mg L1, 25 mL) over 5.0 mg of the nanocomposites within 1 h (Zhang et al., 2018a). Another article reported that the nanocomposite g-C3N4/ZnO@-ferric oxide was created by sol-gel and direct pyrolysis techniques. Under light illumination, the g-C3N4/ZnO@-ferric oxide nanocomposite decomposed 99.34% of tartrazine in 35 min (Balu et al., 2019). In previous research, a photocatalytic composite of carbon quantum dots, TiO2, and Fe2O3 was created using a multi-step hydrothermal process. Methylene blue dye was effectively degraded by the nano­ composite (Zhang et al., 2019a). A solvothermal precipitation-deposition process was used to create Ag2O/ferric oxide p-n heterojunctions constituted of flower-like ferric oxide and silver oxide nanoparticles. RhB and 4-CP were degraded by the nano­ composite. The efficacy of degradation was 85.3% and 57.9% for RhB (rhodamine B) and 4-CP (4-chlorophenol), respectively (Li et al., 2017). One other research was performed on semiconductor heterojunction composites. Ultrasonic and ex-situ methods were used to design a new iron oxide/GO/WO3 composite, Z-scheme solid-state photocatalyst. Crystal violet (CV), methylene blue dyes, and phenol were all degraded with a removal efficiency of 98% (Mohamed, 2019). The mesoporous Fe3+ integrated TiO2 and mesoporous Ti4+ incor­ porated Fe2O3 were produced using a sol-gel synthesis of mesoporous Fe2O3/TiO2 that took place in the presence of varying loadings of Fe 2O3. Fe3+-incorporated TiO2 could absorb light in the visible area, and its proponents claim that it is a superior catalyst to TiO2 for the photo­ catalytic destruction of 4-chlorophenol under visible light. The presence of free Fe2O3 helped photosensitize TiO2, and as a result, both com­ pounds are critical for the photocatalytic destruction of water contam­ inants by solar light. In this way, the additive effects of visible light absorption and photosensitization of Fe2O3/TiO2 in the degradation of 4-chlorophenol in the presence of sunlight are proven, which is shown in Fig. 4. This catalyst also has the capability of mineralizing various organic contaminants that are found in wastewater. Because the surface of the catalyst contains protons, amine-based pollutants such as dyes are able to be effectively adsorbed and promptly broken down. Detailed discusses Fe2O3 based semiconductor heterojunctions have been made in Table 3. 8. Fe2O3-based semiconductor heterojunctions applications in photocatalyst/wastewater treatment Iron oxides were regarded as one of the most promising metal oxide semiconductors for photocatalytic processes because of their low Eg (2.3 electron volt), outstanding stability recyclability, and high capturing of visible light, and earth-abundant nature. Tungsten oxide and Fe2O3 nanoparticles were produced on graphene sheets using a simple and inexpensive one-pot hydrothermal technique. Hybridized (WFG) Tung­ sten oxide-Fe2O3-rGO Oxide nanocomposites revealed increased pho­ tocatalytic, antibacterial properties and heavy metal removal. Under solar light irradiation, photocatalytic removal efficiencies for methylene blue dyes (98%) and rhodamine B (94%) were shown to be better 8 A. Farhan et al. Chemosphere 310 - Table 3 Fe2O3-based semiconductor heterojunctions for photocatalytic activity. Composites Dose rate Pollutant Removal efficiency Contact time WO3/Fe2O3/rGO 0.1 g/L MB and RhB 20 min Fe2O3–TiO2–GA (25%) g-C3N4/ZnO@αFe2O3 CQDs/TiO2/Fe2O3 (CTF) Ag2O/Fe2O3 5.0 mg/L RhB 98% for MB and 94% for RhB 97.7% 50 mg/ 100 mL – Tartrazine dye 99.3% 35 min Degradation of MB 86.5% – – Degradation of RhB and 4-CP 85.3% for RhB, 57.9% for 4-CP – RhB (5 mg L ) and 4-CP (1 mg L− ) Fe2O3/GO/WO3 – 98% – – Fe2O3/TiO2 – Degradation of MB, crystal violet (CV) dyes, and phenol 4-chlorophenol Solvothermal precipitation-deposition method Ex-situ and ultrasonic – 180min 25 ppm Sol-gel 60 min Pollutant dosage Synthesis method Reference 5mg/25 mL × 20 mg/L – One pot hydrothermal method One-pot hydrothermal method Direct pyrolysis and sol-gel methods Multi-step hydrothermal Priyadharsan et al. (2019) Zhang et al. (2018a) Balu et al. (2019) − Zhang et al. (2019a) Li et al. (2017) Mohamed (2019) Palanisamy et al. (2013) Table 4 Various composites showing effective water remediating capability. Composites Dose rate Pollutant pH Removal efficiency Contact time Synthesis method Pollutant dosage Reference FMM and CTAB modified NCs and MoO3/Fe2O3/rGO 0.5 g/L Congo red dye MB dye 3 500.64 and 664.30 mg/g 99.47% 140 min Co-precipitation method (50 mg/L) – Situ hydrothermal-assisted AB92 6.5 20 min A facile method 10 mg/L <30 min Thermal pyrolysis 5ppm Prajapati and Mondal (2021) Anjaneyulu et al. (2018) Salari and Kohantorabi (2020) Padervand et al. (2021). Pragada and Thalla (2021) Rajendran et al. (2022) Fe2O3/MoO3/AgBr 0.1 g/ 100 mL 300 mg/L Fe3N/Fe2O3/C3N4 0.04 g/L RhB dye – Fe2O3–TiO2/PVP – TCS 10 83.72% 300 min Post-treatment technique g-C3N4/TiO2/α- Fe2O3 – RhB dye – ~95.7% 50 min Facile calcination and hydrothermal process – 9. Fe2O3-based trinary composites application in photocatalyst/ wastewater major source of organic dyes. These pollutants are increasing in our water bodies at alarming levels, which is a severe concern for flora and fauna because they have the capacity to cause fatal diseases even in trace amounts (Guo et al., 2018). Photocatalytic degradation has been proved as a promising technique for removing major contaminants from aquatic mediums (Heidarpour et al., 2020). A novel approach involved an efficient post-treatment protocol for greywater treatment. The ternary film of Fe2O3–TiO2/(PVP) was syn­ thesized to degrade triclosan from wastewater. The synthesized photo­ catalytic composite has a degradation efficacy of about 84% within a contact time of 300 min (Pragada and Thalla, 2021). Facile calcination and hydrothermal method were used to fabricate magnetic ternary photocatalytic composite. g-C3N4/TiO2/α-Fe2O3 showed an excellent degradation capacity of 95.7% of RhB within 50 min under light radi­ ations (Rajendran et al., 2022). Detailed discussions about Fe2O3-based trinary composites have been made in Table 4. Compared to single and binary type materials, tertiary materials like tertiary metal oxides provide many advantages in several ways, such as high surface area, high optical and electrical properties, high area for interaction with other materials, and easy alteration of surface work functions. In one of the studies, the tertiary phase nanocomposites were fabricated using the facile co-precipitation method. The composite was Fe2O3–Mn2O3–Mn3O4 (FMM), whose surface was modified with CTAB, a cationic surfactant, to enhance surface functionalities. Almost 500.64 mg/g and 664.30 mg/g of Congo red dye were degraded by nano­ composites, and CTAB-modified nanocomposites, respectively (Praja­ pati and Mondal, 2021). Another research was conducted on the synthesis of ternary composites in which MoO3/Fe2O3/rGO-5% com­ posites were formed, which showed a maximum degradation capacity of about 99.47% due to its unique surface capabilities (Anjaneyulu et al., 2018). Ternary composites are considered the best materials for decon­ taminating wastewater and in the degradation of many organic dyes due to synergistic effects, charge carrier’s effective separation, and high absorption of visible light. A facile approach was used to synthesize Fe2O3/MoO3/AgBr ternary photocatalyst to degrade acid blue 92 dye at the optimum time, pH, and dose rate (Salari and Kohantorabi, 2020). Another article reported the synthesis of the ternary composite prepared by the thermal pyrolysis approach. Fe3N/Fe2O3/C3N4 photocatalyst was good enough to eradicate harmful pollutants from wastewater. Experi­ mental results showed that only 0.04 g of the photocatalyst could remove 5 ppm of rhodamine B dye solution within 30 min under an acidic medium (Padervand et al., 2021). RhB is an organic dye, and many industries like food, plastic, textile, and cosmetic industries are the 10. Fe2O3-based hybrid composites application in photocatalysis/wastewater treatment In the past, the removal of methylene blue from wastewater was facilitated by using a hybrid composite α- Fe2O3/polyacrylonitrile (PAN). The base composite was chosen to be PAN. The inclusion of αFe2O3 as a nano photocatalyst on the PAN surface created an effective hybrid photocatalytic composite for the degradation of methylene blue. The best MB adsorption parameters for the synthesized hybrid com­ posite were 50 ppm starting MB concentration, 200 min contact dura­ tion, and pH 10 (Mohammadreza Miraboutalebi et al., 2020). A facile chemical synthesis technique was used to fabricate an Au@ Fe2O3 nanostructure on a graphene substrate for photocatalytic substrate-driven dye degradation. The percentage removal of methylene 9 A. Farhan et al. Chemosphere 310 - Table 5 Fe2O3-based hybrid composites for water treatment. Photocatalyst dose rate α- Fe2O3/PAN and 1 g/L Au @α- Fe2O3 α-Fe2O3/ZnSe 0.1 g/L Fe2O3–Mn2O3 Fe2O3–SnO2/BC 21.06 mg/L 2.0 g/L α-Fe2O3@UiO-66 – Fe3O4@SiO2@ZnO–Au core-shell – Synthesis method Facile chemical synthesis Facile hydrothermal method. One-step coprecipitation reaction Absorption & conversion process Multi-step chemical methods Pollutant pH Removal efficiency Contact time Pollutant dosage Reference Methylene blue 10 22.17 mg/g 200 min 50 ppm Methylene blue 95% 60 min Mohammadreza Miraboutalebi et al. (2020) Sahoo et al. (2022) Congo red 98.9% 60 min 100 ppm Khurram et al. (2021) 5.29 ≥99% 30 min Eslami et al. (2018) 6–9 95% 15 min 312.18 μg L− 1. 10 mg− 1 – 100% 50 min – Zhang et al. (2019b) – 93.5% 80min – Wang et al. (2018a) Arsenite (As (III) Methylene blue dye Methylene blue dye Rhodamine B blue after 60 min was 95%. Because of gold nanoparticles’ multi­ functionality in catalytic, antibacterial, and other fields, they have seen a surge in study interest (Qin et al., 2018a). When these nanoparticles are 7–8 nm in size, their activity is at its peak. However, tiny particles in wastewater treatment cause other challenges, such as particle aggrega­ tion and floating on the water’s surface (Tada, 2019). The facile hydrothermal approach created the α- Fe2O3/ZnSe nano­ composite. For a Congo red synthetic solution of 100 ppm, the α- Fe2O3/ ZnSe nanocomposite showed a degradation efficiency of 98.9%. Iron oxide nanoparticles have caught the interest of many researchers due to their advantageous qualities, such as excellent stability at varying pH levels, abundance, and inexpensive cost. After 60 min of irradiation, the synthesized nanocomposite removed 98.9% of the Cr. This demonstrates the nanocomposite’s greatest efficiency (Khurram et al., 2021). The Fe2O3–Mn2O3 hybrid nanocomposite was successfully synthesized in the past. These particles showed almost 99% As(III) oxidation at pH of 5.29 and 21.06 mg/L photocatalyst concentration after 30 min of contact time (Eslami et al., 2018). One study described the production of a nanohybrid composite Fe2O3–SnO2/BC via a one-step co-precipitation process. The composite was tested for Methylene blue dye adsorption from water, and the findings showed that a 2.0 gL-1 quantity of Fe2O3–SnO2/BC was adequate to remove almost 95% dye in 15 min at 6–9 pH (Siddiqui et al., 2019). Table 5 illustrates Fe2O3-based hybrid composites for water treatment. Siddiqui et al. (2019) these nanomaterials will be determined by their commercialization. One problem is finding the correct source for introducing them into the production of a nanocomposite that can increase the overall effective­ ness of adsorption. Combining iron oxide nanoparticles with activated carbon, for example, improves the overall efficacy of these materials as nano adsorbents. Aside from these concerns, they are also highly adaptable, insensitive to harmful pollutants, and able to cope with a broader spectrum of contaminants. Because iron-based nanoparticles have a greater reac­ tivity efficiency than iron-based conventional materials, they are rec­ ommended for environmental applications. According to reports, these particles have been identified as remediating groundwater, soil, and air. Moreover, these particles can effectively react with toxic environmental pollutants. Another additional benefit of IONPs is that they do not necessarily involve a carrier as a catalyst. The use of IONPs for drug delivery to specific areas of the body is extremely beneficial. Several researchers are attempting to increase the usability of these nano­ materials as biosensors. In short, iron oxide nanoparticles have grown dramatically, and their multifunctional applications have the potential to bring about tremendous advances in a variety of fields (Nizamuddin et al., 2019). The rate of greywater generation increases due to popu­ lation development, and new pollutants have become a growing source of worry due to the negative consequences of these chemicals on human health and the environment. Hence, the photocatalyst Fe2O3–TiO2/PVP is an effective candidate for greywater treatment. Future studies in this field should concentrate on the degradability and toxicity of IONPs and their preparation using green chemistry to reduce environmental contamination as much as achievable. Effective implementation in this sector will help advance numerous scientific investigations or com­ mercial uses and improve life quality. 11. Challenges and perspectives of ferric oxide in water treatment Magnetic NMs are now the focus of major research for wastewater treatment. However, some nanoparticles, such as ferric oxide NPs, can create major problems that must be handled. First and foremost, it is critical to determine the ecotoxicity of nanoparticles in aquatic systems to forecast nanomaterial behavior and evaluate exposure routes. Nanomaterials on the verge of commercialization and mass production will permeate the aquatic environment, exposing humans to them through contact with skin, inhalation, and direct consumption of contaminated drinking water. Despite recent advances, producing highquality magnetic IONPs with adjustable shapes and sizes in a regulated way will remain a problem in the future. Furthermore, because the production of iron oxide NPs is a complicated process hence, under­ standing the synthetic pathways in detail is a little difficult (Prajitha et al., 2019). The biggest impediment to successful use is the high cost of manu­ facture. However, researchers are currently striving to minimize the cost of production so that it may be used as a nanoadsorbent for wastewater treatment. Another difficulty is the scarcity of full-scale plant operating outcomes to assess feasibility. No project is available for the bulk manufacturing of iron oxide–based nanomaterials. As a result, the fate of 12. Fe2O3-based nanocomposites for hydrogen evolution reaction Hydrogen is a chemical that can be recycled entirely and almost indefinitely, making it an ideal candidate for use as an alternative en­ ergy source. In addition, hydrogen satisfies all of the requirements often associated with such alternatives. Consequently, it is now being evalu­ ated as a possible future energy carrier in transitioning away from the present economy’s reliance on hydrocarbons (Li et al., 2011). The electrolysis of alkaline water to create hydrogen has been gaining a growing degree of attention since quite some time ago (Dresselhaus and Thomas, 2001). When it comes to producing hydrogen that has a high level of purity, this approach is considered to be good for the environ­ ment (Bard and Fox, 1995). An electrode must possess several charac­ teristics for the electrochemical hydrogen evolution process (HER) to be successful. These characteristics include a low overpotential, physical, active surface area, electrochemical stability, low cost, selectivity, 10 A. Farhan et al. Chemosphere 310 - Table 6 Iron oxide-based composites for hydrogen evolution reaction. Composite Tafel slope Overpotential Current density Double layer capacitance Ni–P–Fe2O3–TiO2 121 mV⋅dec− 76.6 mV⋅dec− 98 mV⋅dec− 66 mV⋅dec− 76 mV⋅dec− 327 mV⋅dec− 67.8 mV⋅dec− 36.2 mV⋅dec− 116 mV 250 mA/ cm2 10 mA/ cm2 10 mA/ cm2 10 mA/ cm2 10 mA/ cm2 10 mA/ cm2 10 mA/ cm2 10 mA/ cm2 4.9 × 10− cm 2 – FeS–NCs rGO/Fe2O3–TiO2 Fe2O3(1)-Co(1 S–Fe2O3/IF Al-amorphized Fe2O3 nanofibrous membranes SA-Fe2O3 (Ov) IrO2–Fe2O3 - 245 mV 96 mV 390 mV 134 mV 325 mV 102.9 mV 78 mV Resistance (ct) Stability Synthetic method References 42.32 Ω 12 h – 48 h thermal decomposition method. simple one-step synthesis. Shibli and Sebeelamol (2013) Jiang et al. (2019) – 4231 Ω cm2 24 h hydrothermal method 9.70 mF/cm 2 – 11 h – – 50 h one-step calcination method. simple one-step synthesis. Sumi et al. (2020a) Fu et al. (2019) 2.38 mF/cm 2 – 10 h 0.28 Ω 24 h 232.7 Ω – 5 8.5 mF/cm electrical conductivity, and ease of use strong (Aal et al., 2008). When the electrical and chemical properties of Fe2O3 and TiO2 are combined, a synergistic catalytic effect may be formed for electro­ chemical processes. Fe2O3–TiO2 was synthesized by a process called thermal decomposition. A major focus was to determine whether adding this mixed oxide composite to Ni–P electrodes, which are commonly used catalytic electrodes for the hydrogen evolution process in alkaline media, would increase their catalytic efficiency. Results depicted that adding Fe2O3–TiO2 mixed oxide to the Ni–P matrix significantly decreased the overpotential (116 mV) during the hydrogen evolution process (HER). To conduct the HER, 32% NaOH solution was used. Tafel and impedance measurements showed that the Ni–P coated electrodes had dramatically increased electrochemical activity. Since the addition of the Fe2O3–TiO2 mixed oxide composite improved the metallurgical and electrochemical properties of the Ni–P matrix, the amount of this incorporation should be optimised. The testing conditions were dy­ namic, yet the electrodes remained stable throughout the investigation (Shibli and Sebeelamol, 2013). Jibo Jiang et al. successfully synthesized Fe2O3 nanocatalysts on Ndoped carbon material by calcining the latter at high temperatures. Due to the rarity of iron oxide and the difficulty in fabricating electro­ catalytic hydrogen evolution materials using transition metal oxides, the Fe2O3 nanocatalysts on N-doped carbon nanomaterial were synthesized and proved to be of great importance in the electrochemistry field, as thoroughly discussed in Table 6. The material calcined at 800 ◦ C exhibited outstanding morphology. Tafel slope of 76.6 mV dec− 1 and overpotential of 245 mV were indicative of good electrochemical ki­ netics and low overpotential in the material Fe2O3–NCs-800. Further, the LSV curve of the as-synthesized electrocatalytic material exhibited little change before and after thousands of cycles of CV scanning. Its remarkable electrochemical stability was shown by the fact that hydrogen evolution persists for 48 h at an overpotential of 350 mV. Since Fe2O3–NCs-800 could be easily synthesized using low-cost, readily available raw materials, it might play a significant role in industrial hydrogen evolution. It provides a crucial baseline for studying the electrocatalytic hydrogen development of metal oxides in basic media (Jiang et al., 2019). This research looked at how rGO addition to Fe2O3–TiO2–NiP might boost its catalytic performance. Decorative rGO flakes containing Fe2O3 and TiO2 were produced by a hydrothermal method. Morphological studies and BET isotherm analysis have shown that the specific surface area of Fe2O3–TiO2/rGO is significantly increased after being trans­ formed into ternary composites. As a result of extensive testing, the coating’s composition was fine-tuned for maximum HER performance. The active surface area is enhanced due to electrons being efficiently conducted from the highly porous and folded surface of rGO to the metal mF/ 2 electrospinning followed by calcination traditional treatment followed by calcination thermal decomposition method Hao et al. (2022) Cui et al. (2020) Wang et al. (2022) Yang et al. (2017) oxides in Fe2O3–TiO2/rGO. The coating’s high Cdl and low Rct values prove that this has improved its efficiency at hydrogen evolution. Fe2O3–TiO2/rGO integrated NiP coating, with the low overpotential of 96 mV and the low Tafel slope of 98 mV/dec, suggested a fast rate of the hydrogen evolution process. Including a Fe2O3–TiO2/rGO composite coating might explain the catalyst’s high catalytic activity by increasing the contact surface and exposing more active sites. Interaction between iron, graphene oxide and titanium resulted in enhanced composite performance for HER (Sumi et al., 2020a). Hydrogen generation by water splitting requires a catalyst with high electrocatalytic activity and stable long-term operation; however, creating such a catalyst has proven difficult. Herein, a simple, green, and cost-effective approach for synthesizing a range of nitrogen-doped gra­ phene materials (Fe2O3–Co NPs-N-GR) with varying proportions of bimetallic atoms added were developed and used to produce materials with high hydrogen evolution reaction efficiencies (HER). In addition, the one-step calcination approach was used to achieve N doping, GO reduction, and to form Fe2O3 NPs, Co NPs. Fe2O3(1)-Co(1) NPs-N-GR shows superior HER performance compared to the other series of cata­ lysts we developed. Significant catalytic activity and high durability for HER across a broad pH range are shown by Fe2O3(1)-Co(1) NPs-N-GR. Fe2O3(1)-Co(1) NPs-N-GR exhibits superior electrocatalytic efficiency toward HER in acidic solution (0.5 M H2SO4) compared to the alkaline solution (1.0 M NaOH), with an initial overpotential of 0.36 V, a Tafel slope of 66 mV/dec, and current densities of 10 mA cm2 at an over­ potential of 0.39 V. Combining Fe2O3–Co nanoparticles with N co-doped graphene results in a material with exceptional compositional and structural features, which in turn leads to remarkable HER catalytic performance (Fu et al., 2019). Researchers developed a simple and cost-effective approach to fabricate a Polyaniline/Fe2O3 composite coating that may serve as an efficient electrode in the electrocatalytic alkaline HER. Tunable surface properties of the PANI/Fe2O3 composite coating (PANI/Fe2O3–2GL) facilitate electron transport from the highly conducting polymer to the metal. This boosts the coating’s active surface area and the density of electrons near its surface. This increases the efficiency with which hydrogen was absorbed by the coating’s surface, enhancing the HER activity. The composite coating’s high exchange current density of 95.32 mA/cm2 and relatively low overpotential of 110 mV indicated higher HER activity. Composite Tafel slope (105 mV⋅dec− 1), double layer capacitance (931 μF), and charge transfer resistance (245 Ohm/ cm2) indicated higher HER activity. On the coated surface, HER un­ dergoes a Volmer–Heyrovsky reaction, the rate of which is controlled by the Heyrovsky step. The composite coating’s resilience at extreme re­ action conditions, even after prolonged HER, substantiates its suitability for use with commercial electrocatalysts (Sumi et al., 2020b). 11 A. Farhan et al. Chemosphere 310 - Research and development of highly active, long-lasting, and affordably priced catalysts for the hydrogen evolution reaction (HER) were of the highest significance since electrolysis of water was one of the most potentially successful processes for a hydrogen-based economy. The HER activity of IrO2 is seldom studied since it had a smaller cathodic current than platinum, even though IrO2 was one of the most active catalysts for OER in a PEM electrolyzer. A composite oxide of IrO2 and Fe2O3 was synthesized by the thermal decomposition process. Hydrogen might be more readily adsorbed on the surface of the IrO2–Fe2O3 elec­ trode, which should result in a considerable increase in the H- under­ potential deposition (H-UPD) redox current. Compared to the CV curves of IrO2, hydrogen was not adsorbed on the surface. When both elec­ trodes were immersed in a solution containing 0.5 M H2SO4, the IrO2–Fe2O3 electrode exhibited more HER activity than the IrO2 elec­ trode (LSV). Synergistic action between Ir and Fe took place in the IrO2–Fe2O3 electrode, as well as a change in the electrical structure of IrO2. The observed slope of the Tafel plot, 36.2 mV dec− 1, further sup­ ports the concept that the composite performed HER proceeds through a Volmer-Heyrovsky mechanism (Yang et al., 2017). Among various composites, IrO2–Fe2O3 and rGO/Fe2O3–TiO2 showed a minimum overpotential of 78 mV and 96 mV IrO2–Fe2O3 showed the best HER performance due to the unique heterojunction formed between Ir and Fe that resulted in enhanced charge transfer with minimum charge resistance. regarded to be modest. A number of investigations have revealed the exceptional electrocatalytic activity of Ni-based oxides due to the pres­ ence of tiny quantities of Fe ions, which is an intriguing observation. Similarly, adding Fe to oxides based on Ni and Co may obviously improve the catalytic activity of these compounds (Burke et al., 2015). It is generally accepted that catalytically active iron plays a crucial role in OER functioning. To develop a Fe-based oxide nanocatalyst, an inves­ tigation of the intrinsic electrocatalytic process from the perspective of Fe atoms in iron oxide is necessary. This is due of the availability of the Fe element on earth, as well as the fact that it is not damaging to the environment. Sustainable energy generation relies on the ongoing search for more effective and less expensive electrocatalysts for the OER. Template-free, straightforward method for synthesizing Ni-doped Fe2O3 nanoclews (denoted Ni–Fe2O3) for an electrocatalyst. In addition to their remark­ able longevity, Ni–Fe2O3 nanoclews showed remarkable electrocatalytic activity toward OER, with a minute Tafel slope of 68 mV/dec and a low overpotential of 277 mV at a current density of 10 mA/cm2. Because of their clew-like shape, which consists of small nanorods with a homo­ geneous distribution of component parts, the nanoclews demonstrated exceptional OER activity with long-lasting structural and morphological stability. Despite the nanoclews’ stable structural and morphological stability, this was nonetheless achieved. The unique structural features of Ni–Fe2O3 nanoclews allowed for the exposure of more catalytically active sites, and the atomic-scale synergistic effect arising from the combination of Ni and Fe contributes to the improved intrinsic catalytic activity of these nanoclews (Samanta et al., 2019). The development of a readily available, inexpensive, and highly active electrocatalyst for a simple OER based on transition metals was a key challenge that must be solved. The creation of sufficient catalysts for an efficient water electrolysis process was further complicated by the unknowns inherent in the water splitting reaction, which comprises both the evolution of hydrogen and the OER. Iron oxide (FeO3) and iron phosphide (FeP) were explained in this article, along with the creation of an iron oxide/iron phosphide (Fe2O3/FeP) heterostructure for use as inexpensive electrocatalysts in the electrolysis of water. It had been shown that the heterostructure Fe2O3/FeP was a more effective elec­ trocatalyst than its counterparts. Remarkably, the oxygen evolution process may commence at a voltage as low as 1.49 V, far lower than the voltage required by the reverse hydrogen electrode (RHE). At an over­ potential of 264 mV for OER in a 1.0 M potassium hydroxide solution, the electrocatalyst may generate a current density of 10 mA/cm2, and the Tafel slope can be computed to be 47 mV dec− 1. This material per­ forms better under all tested circumstances than its complementary electrocatalysts (Fe2O3 and FeP). The results are vast improvements over those previously reported for Fe-based OER electrocatalysts. The longterm stability of the Fe2O3/FeP electrocatalyst has been shown by driving OER at 1.65 V relative to RHE for about 12.5 h (Ahmad et al., 2022). Oxygen evolution is vital in water splitting and metal-air batteries. Without a precious metal catalyst, this reaction requires a large over­ potential. Finding a substitute for rare metal catalysts is a significant challenge for water splitting. Poor electric conductivity and electrolyte permeability limit Fe2O3 water oxidation catalytic activity. This study synthesized a composite of Fe2O3 nanorods and oxidized multi-walled carbon nanotubes using urea-assisted co-precipitation. Hollow Fe2O3 nanorods with poor crystallinity improved electrochemically active surface area and electrolyte diffusion pathways. Carbon nanotubes reduced faradaic resistance. Catalytically, the Fe2O3/carbon nanotube composite outperformed Fe2O3, carbon nanotubes, and their physical combination. This paper presents a simple design of an effective and adaptable Fe2O3-based oxygen evolution catalyst (Bandal et al., 2016). A novel synthesis procedure was designed to synthesize different metal-doped iron oxide parallelepipeds utilizing a low-temperature light-driven bottom-up chemistry approach devoid of any template molecule to be evaluated as an electrocatalyst in a basic environment for 12.1. Fe2O3-based nanocomposites for oxygen evolution reaction As a consequence of the use of fossil fuels over the last several de­ cades, the level of pollution in the environment has become exceedingly problematic. Therefore, it is of the utmost importance to create tech­ nology for the generation of renewable energy that is both clean and green at an affordable price (Yuksel and Kaygusuz, 2011; Liu et al., 2010). Hydrogen energy is one of the technologies commonly thought to be an excellent replacement for existing fossil fuels to solve the severe energy crisis and pollution of the environment (Momirlan and Vezir­ oglu, 2002). The electrochemical splitting of water is a technology that is both effective and inexpensive for producing hydrogen energy (Kudo and Miseki, 2009). In addition, a substantial half-reaction known as the OER is identified as occurring during the electrochemical process of water splitting. On the other hand, it is well known that OER continues to be plagued by sluggish kinetics due to the four-electron transfer mechanism (McCrory et al., 2013). Because of this, there is a pressing need to research and develop efficient electrocatalysts for OER. At the moment, the most common types of electrocatalysts are oxides of ruthenium (Ru) and iridium (Ir) (Qin et al., 2018b). Regrettably, the development of the study, as well as its broad application, are hampered by the high price and the scarcity of the resource. As a result, it is desirable to investigate and create OER catalysts made of non-noble metals because of their lower cost and high activity. In recent years, considerable breakthroughs have been achieved in the electrocatalytic performances of nanomaterials based on transition metals (i.e., Co, Ni, Mn, and Fe). Transition metal oxides, in general, have the advantages of cheap cost and high electrocatalytic activity, and they have been proven to be a potential choice for rare and noble metal catalysts for a substantial length of time (Sun et al., 2017; Osgood et al., 2016; Li et al., 2018). Furthermore, transition metal oxides are often relatively cheap. Fe-based oxide, in particular, is viewed as a potentially viable substitute for OER electrocatalysts owing to the availability of its providing ingredients and the ease of its synthesis (Shibli and Sebeela­ mol, 2013). Fe-based oxide has previously been employed for a range of applications, including electrocatalysis (Wu et al., 2016), photocatalysis (Wang, 2019), and as well as the detection of melamine (Huang et al., 2018), and has also acquired good results in these applications. How­ ever, classic electrocatalysts (i.e., oxides based on Ni and Co) are more often used due to their high catalytic activity (Manjunatha et al., 2019). Consequently, the electrocatalytic potential of this chemical is usually 12 A. Farhan et al. Chemosphere 310 - effectiveness of water electrolysis (OER). Water splitting efficiency may be increased by using low-cost OER electrocatalytic materials. To generate a hierarchical electrode that satisfied the aforementioned criteria, iron foam (IF) was chemically etched using glacial acetic acid to form vertical alignment FeS/Fe2O3 heterogeneous nanosheets on the IF (FeS/Fe2O3/IF). By adjusting the electrical structure and exposing more active sites, nanosheets with heterogeneous interfaces produced in situ on IF greatly enhance catalytic activity and stability. The overpotential of 266.5 mV needed to get 10 mA cm2 for FeS/Fe2O3/IF was relatively low, and the Tafel slope was only 51.17 mV/dec. FeS/Fe2O3/IF operated nonstop for 50 h at high current density, demonstrating its great stability during the OER process. Due to its excellent electrocatalytic stability and activity, FeS/Fe2O3/IF was a promising material for application in in­ dustrial electrocatalytic oxygen evolution reactions (OER) (Guo et al., 2022). Fe2O3–CoSe2@Se/CC composite generated by in-situ hydrothermal growth of a selenium-coated cobalt selenide (CoSe2@Se) catalyst on carbon cloth (CC). The optimised Fe2O3–CoSe2@Se/CC catalyst displays remarkable stability (10 mA cm2@70/h) in the electrocatalytic OER process with a low Tafel slope (50.2 mV dec− 1). Overall, water splitting using Fe2O3–CoSe2@Se/CC at the anode required only 1.58 and 1.69 V to yield 10 and 100 mA cm2, respectively. The improved electrocatalytic efficiency was owing to the three-dimensional coral-like shape, which exposes more active areas and species synergy. A novel strategy for designing nanostructured OER hybrid catalysts was comprehensively discussed (Wan et al., 2022). In a nutshell, it was possible to effectively manufacture a sugar-cubic Fe2O3/nitrogen-doped graphene nanocomposite on nickel foam by combining a hydrothermal procedure with a three-step electrochemical approach. This method deposited reduced graphene on a 3D macro­ porous nickel foam substrate. The substrate had a large surface area and appropriate mechanical properties, which made it possible to simplify the electrodeposition of sugar cubic nanoparticles on the reduced gra­ phene and to keep the sugar nanoparticles stable during mechanical and electrochemical testing. In addition, the well-ordered sugar-cubic Fe2O3 nanoparticles atop graphene offer a high active surface area at the electrode/electrolyte interface and reduce the amount of time it takes for active species to diffuse. Water, a non-hazardous and environmen­ tally friendly solvent, was utilized in the hydrothermal process of pro­ ducing Fe2O3 nanoparticles. This approach was known as the “green” method. As a consequence of this, this process was used as a method for the production of Fe2O3 nanoparticles that is both cost-effective and non-toxic. In addition, the synergistic action of conductive nitrogendoped graphene and sugar cubic iron oxide nanoparticles induced a negative shift in charge transfer resistance, which was measured as RCT = 0.91 Ω. As a result, the sugar-cubic Fe2O3/N-doped graphene nano­ composite that was created demonstrated an exceptional level of elec­ trocatalytic activity for OER in alkaline electrolytes. The FO/NG nanocomposites were immediately synthesized in the natural environ­ ment by an electrochemical technique consisting of three stages. The new aqueous-based electrochemical approach seems to have a great deal of potential to be scaled up since it has advantageous characteristics such as speed and simplicity. In addition to this, when the elemental components were combined to form a homogenous nanocomposite, the characteristics that were inherent to each component altered in a sig­ nificant way. Therefore, nano components synergy led to improved electrochemical performance, making the FO/NGs interesting for developing high-performance applications involving energy conversion (Mousavi and Shahrokhian, 2022). For the electrolysis of water, one of the ongoing challenges in developing non-noble metal electrocatalysts that are exceptionally efficient for the oxygen evolution process (OER) at high current den­ sities. Researchers successfully developed a brand-new three-dimen­ sional ternary hybrid electrocatalyst using a thermal phosphorization technique. This electrocatalyst was made up of Fe2O3 that had devel­ oped a close association with Ni2P/Ni(PO3)2 that was created on Fig. 5. Synthetic scheme for nickel, cobalt and manganese doped iron oxide utilizing low temperature light driven bottom-up approach as efficient elec­ trocatalyst for Oxygen evolution reaction (OER). OER, as shown in Fig. 5. A comprehensive investigation of the morphology proved that the hierarchical 2-d morphology of M − Fe2O3 has evolved due to the assembly of multiple microscopic nanorods. We next examined their electrocatalytic activity toward OER and compared their activities throughout the OER process. The doped M − Fe2O3 demonstrated enhanced catalytic activity compared with the conven­ tional Fe2O3, owing to the synergistic action of the doped metal coupled with Fe. Among all of the as-prepared catalysts, Ni–Fe2O3 displayed outstanding stability and OER activity due to the enhanced electroneg­ ativity of Ni compared to that of Co or Mn. Finally, we illustrated how the doped metal boosted the activity of the catalysts by altering their electronegativity (Samanta and Jana, 2021). An efficient oxygen evolution reaction (OER) catalyst based on a novel nonprecious Fe2O3 nanoparticle decorated nickel oxide nanosheet (Fe2O3 NPs@NiO NSs) composite was prepared using a rapid one-pot electrochemical exfoliation method. Ultrathin NiO nanosheets, which resemble graphene, were used to create the nanocomposite, and the Fe2O3 NPs were uniformly linked to them. Fe2O3 nanoparticles and NiO nanosheets form a nanocomposite that is highly conductive to the charge/mass transfer of electrolyte ions and oxygen due to its large surface area (194.1 m2/g). Fe2O3 NPs@NiO NSs exhibit high catalytic performance, a low overpotential (221 mV), a small Tafel slope (53.4 mV dec–1), and 20 h long-term durability. It was shown that adding Fe2O3 NPs helps speed up charge transfer and enhances the surface area that may be used for electrochemistry (ECSA) (Qiu et al., 2020). Critically and rationally produced electrocatalysts not based on platinum show promise in electrochemical oxygen evolution reaction (OER) due to their dependability and low cost. Active sites, mass charge transfer, and porosity in a catalytic system were all improved using a hydrothermally engineered, oxygen-defective Sm2O3/Fe2O3 composite with a gyroid shape. Many different types of analytical methods were used to characterize the synthetic materials. Compared to pure Sm2O3 and Fe2O3, the hierarchical Sm2O3/Fe2O3 composite in alkaline media exhibits a much-reduced Tafel slope (75 mV/dec) and overpotential (272 mV) to achieve a current density of 10 mA/cm2. Oxygen-defective Sm2O3/Fe2O3 displayed exceptional electrical characteristics due to its porous design, which is mainly responsible for the material’s excellent electrocatalytic intrinsic OER performance. At an applied potential (0.7 V), hierarchical Sm2O3/Fe2O3 displayed remarkable activity and sta­ bility but with a somewhat reduced current density. With its exceptional electrochemical findings, Sm2O3/Fe2O3 was a potential electrocatalyst for electrochemical energy production (Abid et al., 2021). The oxygen evolution reaction, which involves a complicated electron-proton transfer mechanism, severely decreases the 13 A. Farhan et al. Chemosphere 310 - Table 7 Iron oxide-based nanocomposites for oxygen evolution reaction. Composite Tafel slope Overpotential at 10 mA/cm2 Double layer capacitance Resistance (ct) Stability Synthetic method References Ni–Fe2O3 68 mV dec− 1 47 mV dec− 1 68 mV dec− 1 102 mV dec− 1 76 mV dec− 1 62 mV dec− 1 53.4 mV dec− 1 32 mV dec− 1 103 mV dec− 1 75 mV dec− 1 55 mV dec− 1 45 mV dec− 1 33 mV dec− 1 66 mV dec− 1 45 mV dec− 1 51.7 mV dec− 1 34 mV dec− 1 50.2 mV dec− 1 45 mV dec− 1 277 mV 1.7 mF/cm 2 13.4 Ω cm2 10 h Simple reaction 265 mV 2.7 mF/cm 2 – 12.5 h In situ synthesis Samanta et al. (2019) Ahmad et al. (2022) 383 mV 8.3 mF/cm 2 23.7 Ω 12 h urea-assisted co-precipitation Bandal et al. (2016) 351 mV 1.6 mF/cm 22 65 Ω – light-driven hydrothermal process 322 mV 2.57 mF/cm 2 46 Ω – light-driven hydrothermal process 285 mV 3.46 mF/cm 2 28 Ω 16 h light-driven hydrothermal process 221 mV 24.8 mF/cm 2 27.6 Ω 20 h – 120 h one-pot electrochemical exfoliation method hydrothermal process Samanta and Jana (2021) Samanta and Jana (2021) Samanta and Jana (2021) Qiu et al. (2020) Fe2O3/FeP Fe2O3/CNT Mn-Doped Fe2O3 Co–Fe2O3 Ni–Fe2O3 Fe2O3 NPs@NiO NSs Fe2O3/NiFe-LDHs Fe2O3/CoOx-air Sm2O3/Fe2O3 Fe2O3@h-Co9S8@C Fe2O3/NiO Co3O4/Fe2O3@NF Fe2O3–MnO/NF NiCo2S4/Fe2O3 FeS/Fe2O3/IF Fe0.67 Ni⋅33OOH–Fe2O3@NF Fe2O3–CoSe2@Se/CC Fe2O3@CNT 2 220 mV 1.2 mF/cm Li et al. (2022) 316 mV – – 16 h photodeposition Zhu et al. (2018) 272 mV 2.97 mF/cm 2 – 15 h hydrothermal process Abid et al. (2021) 205 mV 15.47 mF/cm 2 – 12 h 182 mV 14.49 mF/cm – 20 h hydrothermal process and annealing Simple reaction Huang et al. (2021) 2 254 mV 0.71 mF/cm 2 – 24 h hydrothermal process Yang et al. (2021) 370 mV – Li et al. (2021) – 20 h Fe2O3–MnO/NF Kim et al. (2019) 23.9 mF/cm 2 6.03 Ω 24 h Solvothermal and sulfurization Fereja et al. (2022) 266 mV 2.13 mF/cm 2 3.55 Ω 50 h solvothermal method Guo et al. (2022) 234 mV 7 mF/cm 2 – 110 h hydrothermal process Wang et al. (2019b) 250 mV 0.7 mF/cm 2 48.6 Ω 70 h hydrothermal process Wan et al. (2022) 270 mV – – 100 h green mechanochemical one-pot method Palem et al. (2022) 190 mV commercial Ni foam. It had been given the chemical formula Fe2O3@­ Ni2P/Ni(PO3)2/NF. On the surface of the three-dimensional nickel foam, these nanoparticles have developed consistently. The synergistic effect between synthesized composite and their strong coupling effect allows the 3D hybrid to achieve a superior electrocatalytic OER performance at ultra-high current densities in alkaline electrolytes. This is because the hybrid possesses both of these effects. This is due to the synergistic impact that prepared composite has on one another. Applying potentials of 1.57 and 1.60 V were required to generate the high catalytic current densities of 500 and 1000 mA/cm2, respectively. These potentials were practically the lowest in all of the previously reported transition metal (Ni and Fe) based phosphide and phosphate electrocatalysts, and they were even better than those of the benchmark Ir/C catalyst (1.64 and > 1.8 V at 100 and 500 mA cm2 respectively) (Cheng et al., 2019). A detailed discussion of the OER of various composites has been made in Table 7. Among various advanced iron oxide nanocomposites for oxygen evolution reaction, Fe2O3/NiO and NiCo2S4/Fe2O3 showed the lowest overpotential, 182 mV and 190 mV Fe2O3/NiO achieved remarkable performance in the OER, a very simple dipping-and-heating technique was used to convert the surface of Ni foam (NF) into an interface-rich FeNi oxide layer which resulted in a unique nanostructure between nickel and iron. 12.2. Fe2O3-based nanocomposites for overall water splitting Air pollution and the hastening of global warming are only two major environmental problems caused by traditional fossil fuel con­ sumption. This redirects research efforts away from conventional fossil fuels and toward renewable energy conversion and storage technologies such as fuel cells, metal-air batteries, and supercapacitors (Choi et al., 2019; Zheng et al., 2018; Shen et al., 2015). Electrochemical water splitting is a practical method of converting renewable energy because it combines OER with HER). Conversely, precious metal catalysts are generally needed at the cathode and anode to properly drive the HER and OER processes. High-performance HER catalysts are Pt-based ma­ terials (Fan et al., 2019), whereas OER materials are widely considered Ir- and Ru-based (Shah et al., 2020). These catalytic materials have a wide range of potential commercial uses but are currently prohibitively Table 8 Fe2O3-based composites for overall water splitting (HER). Composite Tafel slope Overpotential Current density Double layer capacitance Resistance Stability Synthetic method References ZIF-8/TiO2/Fe2O3 102 mV⋅dec− 59 mV⋅dec− 291 mV 10 mA/cm2 – – Carbonization 284 mV 50 mA/cm2 – – robust structural stability Long term Vattikuti et al. (2021) Zhang et al. (2021) Cu2O@Fe2O3@CC500 1 1 14 electrodeposition method A. Farhan et al. Chemosphere 310 - Table 9 Fe2O3 based composites for overall water splitting (OER). Composite Tafel slope Overpotential Current density Double layer capacitance Resistance Stability Synthetic method References ZIF-8/TiO2/Fe2O3 64 mV d ec− 1 290 mV 10 mA/cm2 7.9 mF/cm2 4.9 Ω Carbonization Vattikuti et al. (2021) Cu2O@Fe2O3@CC500 66 mV d ec− 1 296 mV 10 mA/cm2 12.08 mF/cm2 – robust structural stability 20 electrodeposition method Zhang et al. (2021) expensive and scarce. Electrocatalysts that can separate water utilizing earth-abundant, low-cost materials have been the focus of many research and development efforts. The electrocatalytic performance of carbon and nitrogen produced from ZIF-8 implanted in TiO2/Fe2O3 nanostructures was better regarding electrochemical water splitting. According to the findings, the C, N-ZIF/TiFe nanostructure proved to be the most effective catalyst for producing HER and OER compared to ZIF/TiFe and TiFe nanostructures. These findings demonstrated that adding a TiO2/Fe2O3 semiconductor to the highly active carbon and nitrogen found in ZIF-8 increased the total amount of H2O splitting, as shown in Tables 8 and 9. Some groups that help sustain the surface-active site of the catalyst and adsorbent intermediary molecules have been credited as being responsible for a potential OER mechanism. It exhibited exceptional electrocatalytic ac­ tivity and remarkable stability, and it had the potential to accomplish ground-breaking and unique applications in fuel cells (Vattikuti et al., 2021). Here, a simple and scalable thermal process is used to produce Cu2O@Fe2O3@carbon cloth electrodes or Cu2O@Fe2O3@CC. To ach­ ieve a current density of 10 mA/cm2 in an alkaline medium, this pre­ pared catalyst hierarchical assembly required an ultralow overpotential of 296 mV (for oxygen evolution reaction, OER) and 188 mV (for hydrogen evolution reaction, HER). With a cell voltage of just 1.675 V at 10 mA/cm2, the built water electrolyzer employing bifunctional com­ posite as both anode and cathode showed excellent stability (Zhang et al., 2021). ZIF-8/TiO2/Fe2O3 and Cu2O@Fe2O3@CC-500 showed comparable performance for both HER and OER. Both composites showed almost similar overpotential and similar water-splitting performance. MnO2 electrocatalysts by first synthesizing the material hydrothermally and then pyrolyzing it in the air. There is a larger possibility for oxygen reduction with these electrocatalysts (ORR). However, the ORR effec­ tiveness of transition metal oxides for electrocatalysis is much lower than that of catalysts based on noble metals. Due to the absence of conductance and catalytic activity that may capture oxygen species, transition metal oxides cannot play this role (Xue et al., 2019). Due to their abundant supplies, adaptable architecture, high elec­ troconductivity, and tunable pore structures, carbon-based materials have attracted a lot of studies (Wang et al., 2018c). While there are clear advantages, such as those mentioned above, more work has been done to improve their manufacturing techniques, ORR catalytic activity, and stability. Studies relevant to the issue suggest that the catalytic activity and the stability of a carbon matrix may be significantly improved by including transition metal atoms (like as ferrous or copper) or hetero­ atoms (like nitrogen, sulphur, or phosphorus) into the matrix (Li et al., 2020; Lu et al., 2020; Liu et al., 2021). An increase in graphitization degree catalyzed by metals at high temperatures may significantly improve crystallinity and electrical conductivity when metals are uni­ formly doped into carbon-based composites (Zhang et al., 2020b; Aijaz et al., 2016). Adding equal amounts of metal complexes to carbon-based composites throughout the doping process might achieve this goal. By substituting sp2-hybridized carbon atoms with other atoms, heteroatoms may alter the atomic charge distribution, creating possible active surface areas for oxygen adsorption (Hu and Dai, 2019). To do this, sp2 hybrid carbon atoms can be swapped out for other carbon atoms. For instance, Yin and coworkers found a straightforward approach to creating N-doped micro/mesoporous bimetallic compounds using CNTs, which exhibit remarkable ORR performance (Liu et al., 2020c). Yin and co­ workers first described this technique in a paper. Using zeolitic imida­ zolate frameworks (ZIFs), Liu and coworkers presented trifunctional electrocatalysts. Co2P embedded in Co, N, and P multi-doped carbon was one component of these electrocatalysts. Excellent performance was shown in both alkaline and acidic conditions by the carbon-based electrocatalysts for ORR that were synthesized by Wang and col­ leagues (Wang et al., 2017a). To facilitate the rapid exchange and reactivity of the reactant mol­ ecules (for example, O2, OH, and H2O), it is necessary to tailor the structure of the catalyst to provide highly exposed active surfaces at the electrode-electrolyte-O2 triple-phase contacts. Recent studies have shown that electrospinning is a practical and feasible tool for creating 1D structure materials for electrocatalysts. In addition to a high specific surface area and excellent stability, these electrocatalysts also have efficient electron and mass transmission channels (Ji et al., 2019; Jin et al., 2018). To produce fuel cells and zinc–air batteries, oxygen reduction must first occur, and platinum is the best electrocatalyst for this purpose. Platinum atoms distributed singly on a substrate often exhibit low catalytic activity and selectivity owing to the unfavourable adsorption of oxygen. Although there is a reason for optimism, it is important to remember that this approach does not always succeed in reducing the amount of platinum required. The process involves loading platinum onto a-Fe2O3 to produce a catalyst with distributed Pt–Fe pair sites and higher activity. The highly charged electrical interaction be­ tween the Pt and Fe pair sites leads to only partly filled orbitals are –O studied. These sites might adsorb oxygen together, shattering the O– 12.3. Fe2O3-based nanocomposites for the oxygen reduction reaction Recent years have seen a proliferation of research into renewable energy conversion technologies, including fuel cells and metal-air bat­ teries (Li et al., 2020; Yao et al., 2020). When the energy supplement accessible to society is becoming inadequate, these systems are meant to transform renewable energy sources efficiently and environmentally friendly. However, broad and economically viable use has been hin­ dered by the oxygen reduction reaction’s (ORR) sluggish kinetics (Su et al., 2019; Kulkarni et al., 2018; Zhan et al., 2019). Although platinum group metal catalyst is widely regarded as the most cutting-edge catalyst to improve ORR kinetics, its limited reserve, expensive demand, poor stability, and good permeability to poisoning pose serious barriers to its widespread use (Yang et al., 2019; Su et al., 2021). To this end, efforts have been made to find alternatives to Pt-based electrocatalysts, such as those based on metals other than noble metals. Nitrides, oxides, phos­ phides, and sulphides of transition metals, as well as carbon-based compounds, fall into this category (Zhang et al., 2018b; Wang et al., 2018b; Zha et al., 2020). Transition-metal oxides (including FexOy, CoxOy, and MnxOy, among others) have been the focus of extensive study as possible effective electrocatalysts (Zhang et al., 2019c; Browne et al., 2019; Wu et al., 2012). Zha and colleagues used a solvothermal method to create nano-heterojunction Fe3O4/Co3O4 ORR electrocatalysts. Excellent ORR catalytic activity (Wu et al., 2017) and remarkable endurance are two hallmarks of these catalysts. Chen and coworkers produced oxygen-free 15 A. Farhan et al. Chemosphere 310 - Table 10 Fe2O3-based composites for the oxygen reduction reaction. Catalyst Onset Potential Electrolyte Electron Transfer Stability Stability efficiency Resistance Synthesis Method References Pt/Fe2O3 Fe2O3/C α-Fe2O3/N-CNTs Fe2O3@MoS2/NGNS h Fe2O3/N-doped carbon sphere γ-Fe2O3@CNFs-12 Fe2O3/N-MCCS Fe2O3–MoO3/NG Fe2O3–MoO3/NG OMCs–Fe2O3 Fe–Fe2O3@N-doped-C Ov-Fe2O3@FeSA@NC α-Fe2O3/alkalinized C3N4 1.15 V 0.42 V − 0.21 V 0.9476 V − 50mV – 0.965 V – − 0.06 V 0.92v 0.913 V 0.82 V 0.1 M 0.1 M KOH 0.1 M 0.1 M 0.1 M 0.1 M 0.1 M 0.1 M 0.1 M 0.1 M 0.1 M - ~ 3.92 ~4 3.7 48 h – 35000s 30 000 s 70,000s Long term 40000s – – 20000s 20000s 10000s – – – 96.4% 82.6% – 82% 97% – 91.7% 92% 86% 5.6Ω – – ~66 Ω – – – 54.1 Ω – – – – Simple reaction – annealing method Thermal treatment Thermal treatment Pyrolysis calcination Thermal treatment carbonization Pyrolysis Pyrolysis annealing Gao et al. (2021) Arya Gopal et al. (2021) Sun et al. (2015) Chuong et al. (2018) Xiao et al. (2019) Jin et al. (2018) Gan et al. (2022) Maiti et al. (2021) Wang et al. (2017a) Wang et al. (2017b) Ren et al. (2022) Chen et al. (2022) KOH KOH KOH KOH KOH KOH KOH KOH KOH KOH KOH bond if they desorb OH* from the platinum site. The onset and half-wave potentials of the catalyst are 1.15 V and 1.05 V, respectively, under alkaline circumstances (against the reversible hydrogen electrode). A synergetic effect between iron and platinum was responsible for the excellent oxygen reduction reaction performance (Gao et al., 2021). To discover a competitive replacement for platinum and solve the global energy problem, transition metal oxides/carbon substrate hybrids are potential non-precious metal ORR electrocatalysts. Due to the simultaneous creation of the Fe2O3 crystal configuration and nitrogen doping on CNTs, both -Fe2O3/N-carbon nanotubes (N-CNTs) and -Fe2O3/N-CNTs nanocatalysts are created. The ORR electrocatalytic activity of Fe2O3/N-CNTs catalysts was greater than that of their con­ stituents at a lower potential (− 0.21 and − 0.27 V), suggesting a fourelectron-dominant ORR process. Crystal distortions on octahedral Fe2O3 preserved substantial potential for displacement of iron or other ions, operating as active sites and contributing to its increased ORR catalytic capacity (Sun et al., 2015). A successful application of a simple method to synthesize a hierarchical three-dimensional architecture of iron oxide NPs encapsulated in MoS2/nitrogen-doped graphene nano­ sheets as a non-Pt cathodic catalyst for oxygen reduction reaction in fuel cell applications. The unique physicochemical and topological proper­ ties allowed for rapid heterogeneous oxygen breakdown. The catalyst’s remarkable catalytic performance was equal to that of Pt/C. Its opera­ tional stability was much better than the Pt/C product, with 96.1% retention after 30 000 s and remarkable alcohol tolerance (Chuong et al., 2018). Detail discuss of the ORR of various composites has been made in Table 10. ➢ To see whether iron oxide materials with unusual crystal structures behave differently during electrochemical activity, different iron oxide materials must be prepared. ➢ Other factors like heat, light, and so on must be added. ➢ Further research on Iron oxide-based composite for overall water splitting, including HER, OER, overall water splitting, and ORR, should be conducted to get better material for electrochemical activity. 14. Concluding remarks This article summarizes studies on Fe2O3 as a photocatalyst for pollutant degradation and as an effective electrocatalyst for energy application. We have included background materials on photocatalytic degradation and numerous synthesis pathways of different morphol­ ogies of Fe2O3 to help you understand the effects of diverse photo­ catalytic and electrocatalytic systems and preparation procedures on Fe2O3 performance. Support materials heterojunction with Fe2O3 were comprehensively discussed for their water remediation and energy application. Suitable support material will provide a well-dispersed photocatalyst with a large surface area, many active sites, and improved pollutant adsorption. Both heterojunctions are successful if they separate charge carriers more effectively, increasing visible light harvesting. Oxygen vacancies and metal flaws are researched for enhanced Fe2O3 performance. Many studies have looked for defect sites that might enlarge the band gap and improve charge separation, boosting photocatalytic activity. Metal defects and oxygen vacancies generate mid-gap states below the essential bandgap (CB). This en­ hances visible light harvesting and charge carrier separation. Similarly, a detailed discussion has been made about OER, ORR, and HER for advanced energy applications. Fe2O3-based nanocomposites have proved vital electrocatalyst finding their application in energy produc­ tion. Low charge transfer resistance, low Tafel slope, overpotential, and greater stability enable iron oxide nanocomposites to electrocatalyst efficiently. 13. Challenges and future prospects The electrochemical industry has just recently begun researching compounds containing iron oxide, and there are still numerous problems that need to be addressed. In electrochemical investigations, the activity and selectivity of the end-products may be affected by a wide variety of characteristics; some of these parameters include structure, morphology, oxygen vacancy, iron ions, and others; however, it is not yet known which of these aspects plays an essential function. These is­ sues provide a hurdle to application on a broad scale and point to spe­ cific areas where more research is required to build more effective iron oxide catalysts. In light of the recent developments as well as the ongoing challenges, it is suggested that more studies be carried out on the following four topics. Credit authors statement Ahmad Farhan, Javeria Arshad, Muhammad Bilal: Conceptuali­ zation, Data analysis and curation, Validation, Writing - original draft, review & editing. Ehsan Ullah Rashid, Haroon Ahmad, Shahid Nawaz, Junaid Munawar: Methodology, Data analysis and curation, Validation, Writing - review & editing. Jakub Zdarta, Teofil Jesio­ nowski, Muhammad Bilal: Software, Supervision, Project adminis­ tration, Validation, Visualization, Writing - original draft, review and editing. ➢ It is necessary to develop new methods of synthesis to generate iron oxide nanomaterials with a variety of specific morphologies and crystal structures, such as nano-tips, nanoparticles, two-dimensional layered structures, and so on. This will allow for an increase in the catalyst’s overall performance. Declaration of competing interest The authors declare that they have no known competing financial 16 A. Farhan et al. Chemosphere 310 - interests or personal relationships that could have appeared to influence the work reported in this paper. Chen, F.-Y., et al., 2021. Stability challenges of electrocatalytic oxygen evolution reaction: from mechanistic understanding to reactor design. Joule 5 (7),-. Chen, F., et al., 2022. α-Fe2O3/alkalinized C3N4 heterostructure as efficient electrocatalyst for oxygen reduction reaction. J. Mater. Sci. 57 (3),-. Cheng, X., et al., 2019. A strongly coupled 3D ternary Fe 2 O 3@ Ni 2 P/Ni (PO 3) 2 hybrid for enhanced electrocatalytic oxygen evolution at ultra-high current densities. J. Mater. Chem. 7 (3), 965–971. Choi, J., et al., 2019. Au-doped PtCo/C catalyst preventing Co leaching for proton exchange membrane fuel cells. Appl. Catal. B Environ. 247, 142–149. Chowdhary, P., et al., 2020. Role of industries in water scarcity and its adverse effects on environment and human health. In: Environmental Concerns and Sustainable Development. Springer, pp. 235–256. Chuong, N.D., et al., 2018. Hierarchical heterostructures of ultrasmall Fe2O3encapsulated MoS2/N-graphene as an effective catalyst for oxygen reduction reaction. ACS Appl. Mater. Interfaces 10 (29),-. Crini, G., Lichtfouse, E., 2019. Advantages and disadvantages of techniques used for wastewater treatment. Environ. Chem. Lett. 17 (1), 145–155. Cui, F., et al., 2020. Novel synthesis of Al-amorphized, flexible Fe2O3 nanofibrous membranes for enhanced electrocatalytic H2 evolution. Compos. Commun. 22, 100470. Dai, X., et al., 2019. Reversible redox behavior of Fe2O3/TiO2 composites in the gaseous photodegradation process. Ceram. Int. 45 (10),-. Danish, M.S.S., et al., 2021. Photocatalytic applications of metal oxides for sustainable environmental remediation. Metals 11 (1), 80. Debataraja, A., et al., 2017. Investigation of nanostructured SnO2 synthesized with polyol technique for CO gas sensor applications. Procedia Eng. 170, 60–64. Dresselhaus, M.S., Thomas, I., 2001. Alternative energy technologies. Nature 414 (6861), 332–337. Du, M., et al., 2019. Fluorine doped TiO2/mesocellular foams with an efficient photocatalytic activity. Catal. Today 327, 340–346. Ejraei, A., Aroon, M.A., Saravani, A.Z., 2019. Wastewater treatment using a hybrid system combining adsorption, photocatalytic degradation and membrane filtration processes. J. Water Proc. Eng. 28, 45–53. Eslami, H., et al., 2018. Efficient photocatalytic oxidation of arsenite from contaminated water by Fe2O3-Mn2O3 nanocomposite under UVA radiation and process optimization with experimental design. Chemosphere 207, 303–312. Fan, X., et al., 2019. Porous nanocomposite gel polymer electrolyte with high ionic conductivity and superior electrolyte retention capability for long-cycle-life flexible zinc–air batteries. Nano Energy 56, 454–462. Farahmandjou, M., Soflaee, F., 2015. Synthesis and characterization of α-Fe2O3 nanoparticles by simple co-precipitation method. Phys. Chem. Res. 3 (3), 191–196. Farhan, A., et al., 2022. Multifunctional Graphene-Based Nanocomposites and Nanohybrids for the Abatement of Agro-Industrial Pollutants in Aqueous environments–A Review. Environmental Pollution, 119557. Fereja, S.L., et al., 2022. Construction of NiCo2S4/Fe2O3 hybrid nanostructure as a highly efficient electrocatalyst for the oxygen evolution reaction. Electrochim. Acta 405, 139793. Figoli, A., Criscuoli, A., 2017. Sustainable Membrane Technology for Water and Wastewater Treatment. Springer. Fu, M., et al., 2019. Fe2O3 and Co bimetallic decorated nitrogen doped graphene nanomaterial for effective electrochemical water split hydrogen evolution reaction. J. Electroanal. Chem. 849, 113345. Gan, R., et al., 2022. Fe2O3-encapsulated and Fe-Nx-containing hierarchical porous carbon spheres as efficient electrocatalyst for oxygen reduction reaction. Int. J. Hydrogen Energy 47 (4),-. Gao, L., Fan, K., Yan, X., 2020. Iron Oxide Nanozyme: A Multifunctional Enzyme Mimetics for Biomedical Application. Nanozymology, pp. 105–140. Gao, R., et al., 2021. Pt/Fe2O3 with Pt–Fe pair sites as a catalyst for oxygen reduction with ultralow Pt loading. Nat. Energy 6 (6), 614–623. Gharibshahian, M., Mirzaee, O., Nourbakhsh, M., 2017. Evaluation of superparamagnetic and biocompatible properties of mesoporous silica coated cobalt ferrite nanoparticles synthesized via microwave modified Pechini method. J. Magn. Magn Mater. 425, 48–56. Guo, S., et al., 2018. Visible-light-driven photoreduction of CO2 to CH4 over N, O, Pcontaining covalent organic polymer submicrospheres. ACS Catal. 8 (5),-. Guo, W.H., et al., 2022. Semi-sacrificial template growth of FeS/Fe2O3 heterogeneous nanosheets on iron foam for efficient oxygen evolution reaction. J. Alloys Compd. 909, 164670. Gupta, A., Srivastava, R., 2019. Mini submersible pump assisted sonochemical reactors: large-scale synthesis of zinc oxide nanoparticles and nanoleaves for antibacterial and anti-counterfeiting applications. Ultrason. Sonochem. 52, 414–427. Han, C., et al., 2017. Preparation and characterization of 3D nano Fe2O3–TiO2@ activated carbon fiber membrane for waste water treatment. J. Nanosci. Nanotechnol. 17 (8),-. Hao, Q., et al., 2022. Construction of Fe2O3 nanosheet arrays by sulfur doping toward efficient alkaline hydrogen evolution. ACS Appl. Energy Mater. 5 (2),-. Hasan, I., et al., 2020. Removal of malachite green by polyacrylamide-g-chitosan γ-Fe2O3 nanocomposite-an application of central composite design. Groundw. Sustain. Dev. 11, 100378. Heidarpour, H., et al., 2020. Enhanced decolorization of rhodamine B solution through simultaneous photocatalysis and persulfate activation over Fe/C3N4 photocatalyst. Chem. Eng. Res. Des. 153, 709–720. Hitam, C.N., Jalil, A., 2020. A review on exploration of Fe2O3 photocatalyst towards degradation of dyes and organic contaminants. J. Environ. Manag. 258, 110050. Data availability Data will be made available on request. Acknowledgment This research was funded by the National Science Centre, Poland, under the Research Grant number 2020/37/K/ST8/03805. References Aal, A.A., Hassan, H.B., Rahim, M.A., 2008. Nanostructured Ni–P–TiO2 composite coatings for electrocatalytic oxidation of small organic molecules. J. Electroanal. Chem. 619, 17–25. Abid, A.G., et al., 2021. Scalable synthesis of Sm2O3/Fe2O3 hierarchical oxygen vacancy-based gyroid-inspired morphology: with enhanced electrocatalytic activity for oxygen evolution performance. Energy Fuel. 35 (21),-. Abraham, A.M., Ponnurangam, S., Thangadurai, V., 2021. Facet-engineered tungsten disulfide for promoting polysulfide electrocatalysis in lithium–sulfur batteries. Inorg. Chem. 60 (17),-. Ahmad, I., et al., 2022. Design and fabrication of Fe2O3/FeP heterostructure for oxygen evolution reaction electrocatalysis. J. Alloys Compd. 894, 162409. Ahmed, S.F., et al., 2022. Progress and challenges of contaminate removal from wastewater using microalgae biomass. Chemosphere 286, 131656. Aijaz, A., et al., 2016. Co@ Co3O4 encapsulated in carbon nanotube-grafted nitrogendoped carbon polyhedra as an advanced bifunctional oxygen electrode. Angew. Chem. Int. Ed. 55 (12),-. Ajinkya, N., et al., 2020. Magnetic iron oxide nanoparticle (IONP) synthesis to applications: present and future. Materials 13 (20), 4644. Albero, J., Mateo, D., García, H., 2019. Graphene-based materials as efficient photocatalysts for water splitting. Molecules 24 (5), 906. Andersen, H.L., et al., 2021. Local and long-range atomic/magnetic structure of nonstoichiometric spinel iron oxide nanocrystallites. IUCrJ 8 (1), 33–45. Anjaneyulu, R.B., et al., 2018. Visible light enhanced photocatalytic degradation of methylene blue by ternary nanocomposite, MoO3/Fe2O3/rGO. J. Asian Ceram. Soc. 6 (3), 183–195. Ansari, M.J., et al., 2022. Preparation of new bio-based chitosan/Fe2O3/NiFe2O4 as an efficient removal of methyl green from aqueous solution. Int. J. Biol. Macromol. 198, 128–134. Arias, L.S., et al., 2018. Iron oxide nanoparticles for biomedical applications: a perspective on synthesis, drugs, antimicrobial activity, and toxicity. Antibiotics 7 (2), 46. Arya Gopal, S., et al., 2021. Kinetic insights into the mechanism of oxygen reduction reaction on Fe2O3/C composites. ACS Appl. Mater. Interfaces 13 (37),-. Azarifar, D., Badalkhani, O., Abbasi, Y., 2016. Silica-modified magnetite Fe3O4 nanoparticles grafted with sulfamic acid functional groups: an efficient heterogeneous catalyst for the synthesis of 3, 4-dihydropyrimidin-2 (1 H)-one and tetrahydrobenzo [b] pyran derivatives. J. Sulfur Chem. 37 (6), 656–673. Aziz, A., Agamuthu, P., Fauziah, S.H., 2018. Removal of bisphenol A and 2, 4-Di-tertbutylphenol from landfill leachate using plant-based coagulant. Waste Manag. Res. 36 (10), 975–984. Balu, S., et al., 2019. Synthesis of α-Fe2O3 decorated g-C3N4/ZnO ternary Z-scheme photocatalyst for degradation of tartrazine dye in aqueous media. J. Taiwan Inst. Chem. Eng. 99, 258–267. Bandal, H., et al., 2016. Fe2O3 hollow nanorods/CNT composites as an efficient electrocatalyst for oxygen evolution reaction. Electrochim. Acta 222,-. Bard, A.J., Fox, M.A., 1995. Artificial photosynthesis: solar splitting of water to hydrogen and oxygen. Accounts Chem. Res. 28 (3), 141–145. Barzinjy, A.A., et al., 2020. Green synthesis of the magnetite (Fe3O4) nanoparticle using Rhus coriaria extract: a reusable catalyst for efficient synthesis of some new 2naphthol bis-Betti bases. Inorg. Nano-Metal Chem. 50 (8), 620–629. Bozkurt, G., 2020. Synthesis and characterization of α-Fe2O3 nanoparticles by microemulsion method. Erzincan Univ. J. Sci. Technol. 13 (2), 890–897. Browne, M.P., Sofer, Z., Pumera, M., 2019. Layered and two dimensional metal oxides for electrochemical energy conversion. Energy Environ. Sci. 12 (1), 41–58. Burke, M.S., et al., 2015. Cobalt–iron (oxy) hydroxide oxygen evolution electrocatalysts: the role of structure and composition on activity, stability, and mechanism. J. Am. Chem. Soc. 137 (10),-. Byrne, C., Subramanian, G., Pillai, S.C., 2018. Recent advances in photocatalysis for environmental applications. J. Environ. Chem. Eng. 6 (3),-. Cao, X., et al., 2017. Hybrid micro-/nano-structures derived from metal–organic frameworks: preparation and applications in energy storage and conversion. Chem. Soc. Rev. 46 (10),-. Chen, F., et al., 2017. Hierarchical assembly of graphene-bridged Ag3PO4/Ag/BiVO4 (040) Z-scheme photocatalyst: an efficient, sustainable and heterogeneous catalyst with enhanced visible-light photoactivity towards tetracycline degradation under visible light irradiation. Appl. Catal. B Environ. 200, 330–342. 17 A. Farhan et al. Chemosphere 310 - Hitam, C., Jalil, A., Abdulrasheed, A., 2019. A review on recent progression of photocatalytic desulphurization study over decorated photocatalysts. J. Ind. Eng. Chem. 74, 172–186. Hu, C., Dai, L., 2019. Doping of carbon materials for metal-free electrocatalysis. Adv. Mater. 31 (7),-. Huang, W.-C., et al., 2018. Magnetic nanoparticle-based lateral flow immunochromatographic strip as a reporter for rapid detection of melamine. J. Nanosci. Nanotechnol. 18 (10),-. Huang, S., et al., 2021. Encapsulating Fe2O3 nanotubes into carbon-coated Co9S8 nanocages derived from a MOFs-directed strategy for efficient oxygen evolution reactions and Li-ions storage. Small 17 (51),-. Imran, M., et al., 2020. In-vitro hemolytic activity and free radical scavenging by sol-gel synthesized Fe3O4 stabilized ZrO2 nanoparticles. Arab. J. Chem. 13 (11),-. Jabbar, K.Q., Barzinjy, A.A., Hamad, S.M., 2022. Iron oxide nanoparticles: preparation methods, functions, adsorption and coagulation/flocculation in wastewater treatment. Environ. Nanotechnol. Monit. Manag., 100661 Jamkhande, P.G., et al., 2019. Metal nanoparticles synthesis: an overview on methods of preparation, advantages and disadvantages, and applications. J. Drug Deliv. Sci. Technol. 53, 101174. Ji, J., et al., 2018. Synthesis and electromagnetic and microwave absorption properties of monodispersive Fe3O4/α-Fe2O3 composites. ACS Appl. Nano Mater. 1 (8),-. Ji, D., et al., 2019. Atomically transition metals on self-supported porous carbon flake arrays as binder-free air cathode for wearable zinc− air batteries. Adv. Mater. 31 (16),-. Jiang, J., et al., 2019. Fe2O3 nanocatalysts on N-doped carbon nanomaterial for highly efficient electrochemical hydrogen evolution in alkaline. J. Power Sources 426, 74–83. Jin, T., et al., 2018. 1D nanomaterials: design, synthesis, and applications in sodium–ion batteries. Small 14 (2),-. Jishkariani, D., et al., 2017. Preparation and self-assembly of dendronized Janus Fe3O4–pt and Fe3O4–au heterodimers. ACS Nano 11 (8),-. Kaur, K., Sidhu, A.K., 2021. Green synthesis: an eco-friendly route for the synthesis of iron oxide nanoparticles. Front. Nanotechnol. 3, 47. Khaki, M.R.D., et al., 2017. Application of doped photocatalysts for organic pollutant degradation-A review. J. Environ. Manag. 198, 78–94. Khalid, N., et al., 2017. Carbonaceous-TiO2 nanomaterials for photocatalytic degradation of pollutants: a review. Ceram. Int. 43 (17),-. Khan, S.T., Malik, A., 2019. Engineered nanomaterials for water decontamination and purification: from lab to products. J. Hazard Mater. 363, 295–308. Khoshnam, M., et al., 2021. α-Fe2O3/graphene oxide powder and thin film nanocomposites as peculiar photocatalysts for dye removal from wastewater. Sci. Rep. 11 (1), 1–15. Khurram, R., Wang, Z., Ehsan, M.F., 2021. α-Fe2O3-based nanocomposites: synthesis, characterization, and photocatalytic response towards wastewater treatment. Environ. Sci. Pollut. Control Ser. 28 (14),-. Kim, J., et al., 2019. Electrochemical synergies of heterostructured Fe2O3-MnO catalyst for oxygen evolution reaction in alkaline water splitting. Nanomaterials 9 (10), 1486. Kuang, Y., Shang, J., Zhu, T., 2019. Photoactivated graphene oxide to enhance photocatalytic reduction of CO2. ACS Appl. Mater. Interfaces 12 (3),-. Kubra, K.T., Salman, M.S., Hasan, M.N., 2021. Enhanced toxic dye removal from wastewater using biodegradable polymeric natural adsorbent. J. Mol. Liq. 328, 115468. Kudo, A., Miseki, Y., 2009. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 38 (1), 253–278. Kulkarni, A., et al., 2018. Understanding catalytic activity trends in the oxygen reduction reaction. Chem. Rev. 118 (5),-. Kushwaha, P., Chauhan, P., 2021. Synthesis of spherical and Rod-Like EDTA assisted α-Fe2O3 nanoparticles via Co-precipitation method. Mater. Today Proc. 44,-. Levine, R., 2016. Microbial Degradation of Sulfonamide Antibiotics. Li, Y., et al., 2011. MoS2 nanoparticles grown on graphene: an advanced catalyst for the hydrogen evolution reaction. J. Am. Chem. Soc. 133 (19),-. Li, S., et al., 2016. A spirobifluorene and diketopyrrolopyrrole moieties based nonfullerene acceptor for efficient and thermally stable polymer solar cells with high open-circuit voltage. Energy Environ. Sci. 9 (2), 604–610. Li, S., et al., 2017. Excellent visible-light photocatalytic activity of p-type Ag2O coated ntype Fe2O3 microspheres. Mater. Lett. 188, 368–371. Li, F.L., et al., 2018. Nanoscale trimetallic metal–organic frameworks enable efficient oxygen evolution electrocatalysis. Angew. Chem. 130 (7),-. Li, Y., et al., 2019. Facile and efficient synthesis of α-Fe2O3 nanocrystals by glucoseassisted thermal decomposition method and its application in lithium ion batteries. J. Power Sources 416, 62–71. Li, W., et al., 2020. Defect engineering for fuel-cell electrocatalysts. Adv. Mater. 32 (19),-. Li, L., et al., 2021. Fe2O3/NiO interface for the electrochemical oxygen evolution in seawater and domestic sewage. ACS Appl. Mater. Interfaces 13 (31),-. Li, C.-F., et al., 2022. Interfacial electronic modulation by Fe2O3/NiFe-LDHs heterostructures for efficient oxygen evolution at high current density. Appl. Catal. B Environ. 306, 121097. Liu, C., et al., 2010. Advanced materials for energy storage. Adv. Mater. 22 (8), E28–E62. Liu, X., et al., 2015. Facile synthesis of porous Fe2O3 nanorods and their photocatalytic properties. J. Saudi Chem. Soc. 19 (5), 479–484. Liu, Y., et al., 2017. Enhanced catalytic degradation of methylene blue by α-Fe2O3/ graphene oxide via heterogeneous photo-Fenton reactions. Appl. Catal. B Environ. 206, 642–652. Liu, J., et al., 2020a. Achieving delafossite analog by in situ electrochemical selfreconstruction as an oxygen-evolving catalyst. Proc. Natl. Acad. Sci. USA 117 (36),-. Liu, X., et al., 2020b. One-pot synthesis of a magnetic TiO2/PTh/γ-Fe2O3 heterojunction nanocomposite for removing trace arsenite via simultaneous photocatalytic oxidation and adsorption. Ind. Eng. Chem. Res. 60 (1), 528–540. Liu, H., et al., 2020c. Metal–organic-Framework-Derived Co2P nanoparticle/multi-doped porous carbon as a trifunctional electrocatalyst. Adv. Mater. 32 (36),-. Liu, X., et al., 2021. Zinc assisted epitaxial growth of N-doped CNTs-based zeolitic imidazole frameworks derivative for high efficient oxygen reduction reaction in Znair battery. Chem. Eng. J. 414, 127569. Liu, H., et al., 2022. Photocatalytic removal of tetracycline by a Z-scheme heterojunction of bismuth oxyiodide/exfoliated g-C3N4: performance, mechanism, and degradation pathway. Mater. Today Chem. 23, 100729. Lu, X.F., et al., 2020. Metal–organic frameworks based electrocatalysts for the oxygen reduction reaction. Angew. Chem. 132 (12),-. Luo, S.-h., et al., 2019. Hydrothermal synthesis and characterization of α-Fe2O3/C using acid-pickled iron oxide red for Li-ion batteries. J. Hazard Mater. 368, 714–721. Maiti, K., Kim, N.H., Lee, J.H., 2021. Strongly stabilized integrated bimetallic oxide of Fe2O3-MoO3 Nano-crystal entrapped N-doped graphene as a superior oxygen reduction reaction electrocatalyst. Chem. Eng. J. 410, 128358. Manjunatha, R., et al., 2019. Electrochemical ammonia generation directly from nitrogen and air using an iron-oxide/titania-based catalyst at ambient conditions. ACS Appl. Mater. Interfaces 11 (8),-. McCrory, C.C., et al., 2013. Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction. J. Am. Chem. Soc. 135 (45),-. Modan, E.M., Plăiașu, A.G., 2020. Advantages and disadvantages of chemical methods in the elaboration of nanomaterials. Metall. Mater. Sci. 43 (1), 53–60. The Annals of “Dunarea de Jos” University of Galati. Fascicle IX. Mohamed, H.H., 2019. Rationally designed Fe2O3/GO/WO3 Z-Scheme photocatalyst for enhanced solar light photocatalytic water remediation. J. Photochem. Photobiol. Chem. 378, 74–84. Mohamed, H.E.A., et al., 2020. Bio-redox potential of Hyphaene thebaica in biofabrication of ultrafine maghemite phase iron oxide nanoparticles (Fe2O3 NPs) for therapeutic applications. Mater. Sci. Eng. C 112, 110890. Mohammadreza Miraboutalebi, S., et al., 2020. Polyacrylonitrile/α-Fe2O3 hybrid photocatalytic composite adsorbents for enhanced dye removal. Chem. Eng. Technol. 43 (6),-. Momirlan, M., Veziroglu, T., 2002. Current status of hydrogen energy. Renew. Sustain. Energy Rev. 6 (1–2), 141–179. Mousavi, D.S., Shahrokhian, S., 2022. Sugar-cubic Fe2O3/nitrogen-doped graphene nanocomposite as high-performance anode material for oxygen evolution reaction. J. Alloys Compd. 910, 164852. Mulungulungu, G.A., Mao, T., Han, K., 2021. Efficient removal of high-concentration copper ions from wastewater via 2D g-C3N4 photocatalytic membrane filtration. Colloids Surf. A Physicochem. Eng. Asp. 623, 126714. Munawar, J., et al., 2022. Nanostructured materials for water/wastewater remediation. In: Integrated Environmental Technologies for Wastewater Treatment and Sustainable Development. Elsevier, pp. 413–432. Muthukrishnaraj, A., et al., 2015. α-Fe2O3/reduced graphene oxide nanorod as efficient photocatalyst for methylene blue degradation. Mater. Res. Innovat. 19 (4), 258–264. Narayanan, K.B., Han, S.S., 2016. One-pot green synthesis of Hematite (α-Fe2O3) nanoparticles by ultrasonic irradiation and their in vitro cytotoxicity on human keratinocytes CRL-2310. J. Cluster Sci. 27 (5),-. Natarajan, S., et al., 2019. Multifunctional magnetic iron oxide nanoparticles: diverse synthetic approaches, surface modifications, cytotoxicity towards biomedical and industrial applications. BMC Mater. 1 (1), 1–22. Nawaz, S., et al., 2021. Mitigation of environmentally hazardous pollutants by magnetically responsive composite materials. Chemosphere 276, 130241. Nikolaidis, P., 2020. Analysis of Green Methods to Synthesize Nanomaterials. Green synthesis of nanomaterials for bioenergy applications, pp. 125–144. Nizamuddin, S., et al., 2019. Iron oxide nanomaterials for the removal of heavy metals and dyes from wastewater. Nanoscale Mater. Water Purif. 447–472. Nuengmatcha, P., et al., 2019. Enhanced photocatalytic degradation of methylene blue using Fe2O3/graphene/CuO nanocomposites under visible light. J. Environ. Chem. Eng. 7 (6), 103438. Ong, C.B., Ng, L.Y., Mohammad, A.W., 2018. A review of ZnO nanoparticles as solar photocatalysts: synthesis, mechanisms and applications. Renew. Sustain. Energy Rev. 81, 536–551. Osgood, H., et al., 2016. Transition metal (Fe, Co, Ni, and Mn) oxides for oxygen reduction and evolution bifunctional catalysts in alkaline media. Nano Today 11 (5), 601–625. Ovais, M., et al., 2016. Green synthesis of silver nanoparticles via plant extracts: beginning a new era in cancer theranostics. Nanomedicine 12 (23),-. Padervand, M., Rhimi, B., Wang, C., 2021. One-pot synthesis of novel ternary Fe3N/ Fe2O3/C3N4 photocatalyst for efficient removal of rhodamine B and CO2 reduction. J. Alloys Compd. 852, 156955. Padhi, D.K., et al., 2017. Green synthesis of Fe3O4/RGO nanocomposite with enhanced photocatalytic performance for Cr (VI) reduction, phenol degradation, and antibacterial activity. ACS Sustain. Chem. Eng. 5 (11),-. Palanisamy, B., et al., 2013. Sol–gel synthesis of mesoporous mixed Fe2O3/TiO2 photocatalyst: application for degradation of 4-chlorophenol. J. Hazard Mater. 252, 233–242. 18 A. Farhan et al. Chemosphere 310 - Palem, R.R., et al., 2022. Fabrication of Fe2O3 nanostructure on CNT for oxygen evolution reaction. Ceram. Int. 48 (19, Part B),-. Parashar, M., Shukla, V.K., Singh, R., 2020. Metal oxides nanoparticles via sol–gel method: a review on synthesis, characterization and applications. J. Mater. Sci. Mater. Electron. 31 (5),-. Pragada, S.C., Thalla, A.K., 2021. Polymer-based immobilized Fe2O3–TiO2/PVP catalyst preparation method and the degradation of triclosan in treated greywater effluent by solar photocatalysis. J. Environ. Manag. 296, 113305. Prajapati, A.K., Mondal, M.K., 2021. Development of CTAB modified ternary phase α-Fe2O3-Mn2O3-Mn3O4 nanocomposite as innovative super-adsorbent for Congo red dye adsorption. J. Environ. Chem. Eng. 9 (1), 104827. Prajitha, N., Athira, S., Mohanan, P., 2019. Bio-interactions and risks of engineered nanoparticles. Environ. Res. 172, 98–108. Priyadharsan, A., et al., 2019. Crumpled sheet like graphene based WO3-Fe2O3 nanocomposites for enhanced charge transfer and solar photocatalysts for environmental remediation. Appl. Surf. Sci. 470, 114–128. Qin, L., et al., 2018a. Gold rush” in modern science: fabrication strategies and typical advanced applications of gold nanoparticles in sensing. Coord. Chem. Rev. 359, 1–31. Qin, Q., et al., 2018b. Low loading of RhxP and RuP on N, P codoped carbon as two trifunctional electrocatalysts for the oxygen and hydrogen electrode reactions. Adv. Energy Mater. 8 (29),-. Qin, R., Hao, L., Li, J., 2020. Acid-assisted one-step in-situ polymerization synthesis of PANI/α-Fe2O3/β-FeOOH composites and its formation mechanism. J. Inorg. Organomet. Polym. Mater. 30 (11),-. Qiu, F., et al., 2020. Rapid synthesis of large-size Fe2O3 nanoparticle decorated NiO nanosheets via electrochemical exfoliation for enhanced oxygen evolution electrocatalysis. Inorg. Chem. 60 (2), 959–966. Rajendran, R., et al., 2022. Designing of TiO2/α-Fe2O3 coupled g-C3N4 magnetic heterostructure composite for efficient Z-scheme photo-degradation process under visible light exposures. J. Alloys Compd. 894, 162498. Ren, J., et al., 2022. Porphyrin polymer-derived single-atom Fe assisted by Fe2O3 with oxygen vacancy for efficient oxygen reduction reaction. Appl. Surf. Sci. 592, 153301. Sahoo, R.K., et al., 2022. Facile synthesis of Super-paramagnetic Au@ α-Fe2O3 hybrid nanoparticle and its assembly on graphene substrate for visible light Photo-catalysis. Appl. Surf. Sci. 577, 151954. Saif, S., et al., 2019. Polymeric nanocomposites of iron–oxide nanoparticles (IONPs) synthesized using Terminalia chebula leaf extract for enhanced adsorption of arsenic (V) from water. Colloids Interfaces 3 (1), 17. Salari, H., Kohantorabi, M., 2020. Fabrication of novel Fe2O3/MoO3/AgBr nanocomposites with enhanced photocatalytic activity under visible light irradiation for organic pollutant degradation. Adv. Powder Technol. 31 (1), 493–503. Samanta, A., Jana, S., 2021. Ni-, Co-, and Mn-doped Fe2O3 nano-parallelepipeds for oxygen evolution. ACS Appl. Nano Mater. 4 (5),-. Samanta, A., Das, S., Jana, S., 2019. Doping of Ni in α-Fe2O3 nanoclews to boost oxygen evolution electrocatalysis. ACS Sustain. Chem. Eng. 7 (14),-. Shah, S.A., et al., 2020. One step in-situ synthesis of Ni3S2/Fe2O3/N-doped carbon composites on Ni foam as an efficient electrocatalyst for overall water splitting. Appl. Surf. Sci. 527, 146918. Sharma, S., et al., 2019. Carbon quantum dot supported semiconductor photocatalysts for efficient degradation of organic pollutants in water: a review. J. Clean. Prod. 228, 755–769. Shen, L., et al., 2015. NiCo2S4 nanosheets grown on nitrogen-doped carbon foams as an advanced electrode for supercapacitors. Adv. Energy Mater. 5 (3),-. Shen, S., et al., 2016. Hematite heterostructures for photoelectrochemical water splitting: rational materials design and charge carrier dynamics. Energy Environ. Sci. 9 (9),-. Shibli, S., Sebeelamol, J., 2013. Development of Fe2O3–TiO2 mixed oxide incorporated Ni–P coating for electrocatalytic hydrogen evolution reaction. Int. J. Hydrogen Energy 38 (5),-. Shrestha, R., et al., 2021. Technological trends in heavy metals removal from industrial wastewater: a review. J. Environ. Chem. Eng. 9 (4), 105688. Siddiqui, S.I., Zohra, F., Chaudhry, S.A., 2019. Nigella sativa seed based nanohybrid composite-Fe2O3–SnO2/BC: a novel material for enhanced adsorptive removal of methylene blue from water. Environ. Res. 178, 108667. Singha, K., et al., 2021. Harmful environmental effects for textile chemical dyeing practice. In: Green Chemistry for Sustainable Textiles. Elsevier, pp. 153–164. Song, Z., et al., 2020. Tailored multifunctional hybrid cathode substrate configured with carbon nanotube-modified polar Co (PO3) 2/CoP nanoparticles embedded nitrogendoped porous-shell carbon polyhedron for high-performance lithium–sulfur batteries. J. Colloid Interface Sci. 575, 220–230. Sosa-Acosta, J.R., et al., 2020. DNA-Iron Oxide Nanoparticles Conjugates: Functional Magnetic Nanoplatforms in Biomedical Applications. Surface-modified Nanobiomaterials for Electrochemical and Biomedicine Applications, pp. 19–47. Su, H., et al., 2019. Co–Mn spinel supported self-catalysis induced N-doped carbon nanotubes with high efficiency electron transport channels for zinc–air batteries. J. Mater. Chem. 7 (39),-. Su, C., et al., 2021. Defects-rich porous carbon microspheres as green electrocatalysts for efficient and stable oxygen-reduction reaction over a wide range of pH values. Chem. Eng. J. 406, 126883. Sumi, V., et al., 2020a. Effect of rGO on Fe2O3–TiO2 composite incorporated NiP coating for boosting hydrogen evolution reaction in alkaline solution. Int. J. Hydrogen Energy 45 (4),-. Sumi, V., et al., 2020b. Tuning of the electrocatalytic characteristics of PANI/Fe 2 O 3 composite coating for alkaline hydrogen evolution reaction. Dalton Trans. 49 (33),-. Sun, M., et al., 2015. α-and γ-Fe2O3 nanoparticle/nitrogen doped carbon nanotube catalysts for high-performance oxygen reduction reaction. Sci. China Mater. 58 (9), 683–692. Sun, X., et al., 2017. A general approach to synthesise ultrathin NiM (M= Fe, Co, Mn) hydroxide nanosheets as high-performance low-cost electrocatalysts for overall water splitting. J. Mater. Chem. 5 (17),-. Tada, H., 2019. Size, shape and interface control in gold nanoparticle-based plasmonic photocatalysts for solar-to-chemical transformations. Dalton Trans. 48 (19),-. Tadic, M., et al., 2019. Magnetic properties of hematite (α− Fe 2 O 3) nanoparticles synthesized by sol-gel synthesis method: the influence of particle size and particle size distribution. J. Electr. Eng. 70 (7), 71–76. Tan, H.L., et al., 2019. Decorating platinum on nitrogen-doped graphene sheets: control of the platinum particle size distribution for improved photocatalytic H2 generation. Chem. Eng. Sci. 194, 85–93. Tokubuchi, T., et al., 2021. Enhanced photoelectrochemical water splitting efficiency of hematite (α-Fe2O3)-Based photoelectrode by the introduction of maghemite (γ-Fe2O3) nanoparticles. J. Photochem. Photobiol. Chem. 410, 113179. Uma, K., et al., 2020. Enriched silver plasmon resonance activity on the sonochemical synthesis of ZnO flowers with α-Fe2O3 as an efficient catalyst for photo-Fenton reaction and photo-oxidation of ethanol. J. Water Proc. Eng. 34, 101089. Unni, M., et al., 2017. Thermal decomposition synthesis of iron oxide nanoparticles with diminished magnetic dead layer by controlled addition of oxygen. ACS Nano 11 (2),-. Vasantharaj, S., et al., 2019. Biosynthesis of iron oxide nanoparticles using leaf extract of Ruellia tuberosa: antimicrobial properties and their applications in photocatalytic degradation. J. Photochem. Photobiol. B Biol. 192, 74–82. Vattikuti, S.P., et al., 2021. Onion-ring-like carbon and nitrogen from ZIF-8 on TiO2/ Fe2O3 nanostructure for overall electrochemical water splitting. J. Phys. Chem. Lett. 12 (25),-. Vickers, N.J., 2017. Animal communication: when i’m calling you, will you answer too? Curr. Biol. 27 (14), R713–R715. Vigneshwaran, S., et al., 2021. Photocatalytic performance of chitosan tethered magnetic Fe2O3-like (3D/2D) hybrid for the dynamic removal of anionic dyes: degradation and mechanistic pathways. Int. J. Biol. Macromol. 183,-. Vikrant, K., et al., 2019. Nanomaterials for the abatement of cadmium (II) ions from water/wastewater. Nano Res. 12 (7),-. Wan, Z., et al., 2022. Designing coral-like Fe2O3-regulated Se-rich CoSe2 heterostructure as a highly active and stable oxygen evolution electrocatalyst for overall water splitting. J. Electroanal. Chem. 904, 115928. Wang, F., 2019. Magnetically recoverable Fe2O3/N-graphene with enhanced visible photocatalytic performance. Chem. Eng. Res. Des. 142, 189–194. Wang, Q., Astruc, D., 2019. State of the art and prospects in metal–organic framework (MOF)-based and MOF-derived nanocatalysis. Chem. Rev. 120 (2),-. Wang, L., et al., 2016. Three-dimensional macroporous carbon/Fe3O4-doped porous carbon nanorods for high-performance supercapacitor. ACS Sustain. Chem. Eng. 4 (3),-. Wang, H., et al., 2017a. Mesoporous spinel nanofibers and nitrogen-doped carbon nanotubes as high-performance electrocatalyst for oxygen reduction in alkaline and neutral media. Energy Technol. 5 (2), 283–292. Wang, H., et al., 2017b. Metal-organic gel-derived Fe-Fe2O3@ nitrogen-doped-carbon nanoparticles anchored on nitrogen-doped carbon nanotubes as a highly effective catalyst for oxygen reduction reaction. Electrochim. Acta 232, 114–122. Wang, D., et al., 2018a. Controlled preparation of superparamagnetic Fe3O4@ SiO2@ ZnO-Au core-shell photocatalyst with superior activity: RhB degradation and working mechanism. Powder Technol. 327, 489–499. Wang, R., et al., 2018b. MOF-derived bifunctional Cu3P nanoparticles coated by a N, Pcodoped carbon shell for hydrogen evolution and oxygen reduction. Adv. Mater. 30 (6),-. Wang, Y.-J., et al., 2018c. A review of carbon-composited materials as air-electrode bifunctional electrocatalysts for metal–air batteries. Electrochem. Energy Rev. 1 (1), 1–34. Wang, J.-C., et al., 2019a. A novel synthesis of oleophylic Fe2O3/polystyrene fibers by γ-Ray irradiation for the enhanced photocatalysis of 4-chlorophenol and 4-nitro­ phenol degradation. J. Hazard Mater. 379, 120806. Wang, M., et al., 2019b. Increased charge and mass transfer derived-sheet-like Fe0. 67Ni0. 33OOH-Fe2O3@ NF array for robust oxygen evolution reaction. Appl. Surf. Sci. 493, 351–358. Wang, P., et al., 2020a. Graphene-like matrix composites with Fe2O3 and Co3O4 as cathode materials for lithium–sulfur batteries. ACS Appl. Nano Mater. 3 (2),-. Wang, C., et al., 2020b. Self-assembled BiOCl/Ti3C2Tx composites with efficient photoinduced charge separation activity for photocatalytic degradation of p-nitrophenol. Appl. Surf. Sci. 519, 146175. Wang, C., et al., 2022. Oxygen vacancy-engineered Fe2O3 porous microspheres with large specific surface area for hydrogen evolution reaction and lithium-sulfur battery. Colloids Surf. A Physicochem. Eng. Asp., 129476 Wu, Z.-S., et al., 2012. 3D nitrogen-doped graphene aerogel-supported Fe3O4 nanoparticles as efficient electrocatalysts for the oxygen reduction reaction. J. Am. Chem. Soc. 134 (22),-. Wu, M., et al., 2016. Cost-effective carbon supported Fe2O3 nanoparticles as an efficient catalyst for non-aqueous lithium-oxygen batteries. Electrochim. Acta 211, 545–551. Wu, Q., et al., 2017. From carbon-based nanotubes to nanocages for advanced energy conversion and storage. Accounts Chem. Res. 50 (2), 435–444. 19 A. Farhan et al. Chemosphere 310 - Zhang, J.-J., et al., 2018a. Three-dimensional Fe2O3–TiO2–graphene aerogel nanocomposites with enhanced adsorption and visible light-driven photocatalytic performance in the removal of RhB dyes. J. Ind. Eng. Chem. 61, 407–415. Zhang, J., et al., 2018b. Defect and pyridinic nitrogen engineering of carbon-based metal-free nanomaterial toward oxygen reduction. Nano Energy 52, 307–314. Zhang, J., et al., 2019a. Fabrication of carbon quantum dots/TiO2/Fe2O3 composites and enhancement of photocatalytic activity under visible light. Chem. Phys. Lett. 730, 391–398. Zhang, R., et al., 2019b. α-Fe2O3 nanoclusters confined into UiO-66 for efficient visiblelight photodegradation performance. Appl. Surf. Sci. 466, 956–963. Zhang, H.-M., et al., 2019c. Fe3O4 encapsulated in porous carbon nanobowls as efficient oxygen reduction reaction catalyst for Zn-air batteries. Chem. Eng. J. 375, 122058. Zhang, H., et al., 2020a. Preparation of magnetic α-Fe2O3/ZnFe2O4@ Ti3C2 MXene with excellent photocatalytic performance. Ceram. Int. 46 (1), 81–88. Zhang, Z., et al., 2020b. 3D flower-like ZnFe-ZIF derived hierarchical Fe, N-Codoped carbon architecture for enhanced oxygen reduction in both alkaline and acidic media, and zinc-air battery performance. Carbon 161, 502–509. Zhang, N., et al., 2021. Optional construction of Cu2O@ Fe2O3@ CC architecture as a robust multifunctional photoelectronic catalyst for overall water splitting and CO2 reduction. Chem. Eng. J. 426, 131192. Zhao, L., et al., 2021. Novel g-C3N4/C/Fe2O3 composite for efficient photocatalytic reduction of aqueous Cr (VI) under light irradiation. Ind. Eng. Chem. Res. 60 (37),-. Zheng, X., et al., 2018. Controllable synthesis of Ni x Se (0.5≤ x≤ 1) nanocrystals for efficient rechargeable zinc–air batteries and water splitting. ACS Appl. Mater. Interfaces 10 (16),-. Zhu, S., et al., 2018. Investigation of oxygen vacancies in Fe2O3/CoOx composite films for boosting electrocatalytic oxygen evolution performance stably. J. Electroanal. Chem. 827, 42–50. Zhu, H., et al., 2020. A novel immobilized Z-scheme P3HT/α-Fe2O3 photocatalyst array: study on the excellent photocatalytic performance and photocatalytic mechanism. J. Hazard Mater. 389, 122119. Zolghadr, S., Kimiagar, S., Davarpanah, A., 2017. Magnetic property of $\alpha $-Fe2O3–GO nanocomposite. IEEE Trans. Magn. 53 (12), 1–6. Wu, J., et al., 2021. A novel modified method for the efficient removal of Pb and Cd from wastewater by biochar: enhanced the ion exchange and precipitation capacity. Sci. Total Environ. 754, 142150. Xiao, Z., et al., 2019. Confinement of Fe2O3 nanoparticles in the shell of N-doped carbon hollow microsphere for efficient oxygen reduction reaction. Chem. Eng. Sci. 207, 235–246. Xue, X., et al., 2019. N, P-coordinated fullerene-like carbon nanostructures with dual active centers toward highly-efficient multi-functional electrocatalysis for CO 2 RR, ORR and Zn-air battery. J. Mater. Chem. 7 (25),-. Yang, X., et al., 2017. Synthesis and characterization of an IrO 2–Fe 2 O 3 electrocatalyst for the hydrogen evolution reaction in acidic water electrolysis. RSC Adv. 7 (33),-. Yang, L., et al., 2019. Carbon-based metal-free ORR electrocatalysts for fuel cells: past, present, and future. Adv. Mater. 31 (13),-. Yang, Y., et al., 2021. Construction of Co3O4/Fe2O3 nanosheets on nickel foam as efficient electrocatalyst for the oxygen evolution reaction. J. Phys. Chem. Solid. 148, 109680. Yao, X., et al., 2020. Designed synthesis of three-dimensional callistemon-like networks structural multifunctional electrocatalyst: graphitic-carbon-encapsulated Co nanoparticles/N-doped carbon nanotubes@ carbon nanofibers for Zn-air batteries application. Compos. B Eng. 193, 108058. Yew, Y.P., et al., 2020. Green biosynthesis of superparamagnetic magnetite Fe3O4 nanoparticles and biomedical applications in targeted anticancer drug delivery system: a review. Arab. J. Chem. 13 (1),-. Yuksel, I., Kaygusuz, K., 2011. Renewable energy sources for clean and sustainable energy policies in Turkey. Renew. Sustain. Energy Rev. 15 (8),-. Zha, R., et al., 2020. Engineering the surface active sites of actiniae-like hierarchical Fe3O4/Co3O4 nanoheterojunction for efficient oxygen reduction reaction. Dyes Pigments 180, 108439. Zhan, Y., et al., 2019. Templated growth of Fe/N/C catalyst on hierarchically porous carbon for oxygen reduction reaction in proton exchange membrane fuel cells. J. Power Sources 431, 31–39. Zhang, D., et al., 2013. Two-phase hydrothermal synthesis of TiO2–graphene hybrids with improved photocatalytic activity. J. Alloys Compd. 572, 199–204. 20