Recent advances and perspectives on iron-based photocatalysts

Abstract

Global warming caused by the overuse of non-renewable fossil energy has resulted in serious global energy and environmental issues, including resource scarcity, melting glaciers, fires and locust plague. Accordingly, the design and development of high-performance, earth-abundant, non-toxic and cost-effective photocatalysts to realize solar energy conversion, organic pollutant degradation, carbon dioxide reduction and hydrogen production are considered the most effective methods to resolve both the energy and environmental crises. Iron is the second most abundant metal (5.0%) on Earth and has been extensively introduced in photocatalytic reactions to improve the generation, separation and utilization efficiency of charge carriers owing to its suitable band gap, redox position and low redox overpotential. Thus, iron-based photocatalysts have gained prominence as viable candidates owing to their abundance, eco-friendliness and exceptional photocatalytic performance. This critical review delves into the recent advances in the design of iron-based nanomaterials. Initially, we present an overview of the recent advancements in the design of iron-based heterojunctions (type II, Z-scheme, and S-scheme heterojunctions). Subsequently, we thoroughly summarize the application of iron-based photocatalysts in oxygen and hydrogen evolution reactions, carbon dioxide conversion and nitrogen fixation. Finally, we outline the future perspectives for the improvement of next-generation iron-based photocatalysts.

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1 Introduction

The photocatalytic conversion of solar energy is highly desirable to attain alternative energy sources to address the energy crisis and environmental pollution.^1–7 Photocatalysts are a type of catalyst that control the rate of reactions using light in the ultraviolet or visible light region. In photocatalysts, the difference between their valence and conduction bands is termed band gap, where reduction or oxidation reactions can take place via light-excited electrons and holes. Thus, band gap tuning is the most important factor that controls the efficacy of photocatalysts. The discovery of TiO₂ as a photocatalyst, especially for hydrogen production from water, has led to a lot of research on the development of highly competent semiconductor-based photocatalysts.^8–22 Semiconductor-based photocatalysts harvest solar energy for the degradation of environmental pollutants. The photocatalytic reaction involves three steps, namely, generation, separation, and utilization efficiency of charge carriers. Therefore, to achieve greater photo-induced charge carrier generation, photocatalysts should possess a narrow band gap, and separation efficiency could be increased by developing heterostructures, whereas carrier utilization efficiency could be enhanced using low over potential co-catalysts. However, the presence of defects with low light absorption ability, compromised band position, high speed charge carrier recombination and photocorrosion have enhanced the exploration of approaches to narrow band gaps and design more effective semiconductor photocatalysts. Since iron (Fe) is the second most abundant metal on Earth with tremendous photocatalytic performance, it has long been widely used, as shown in Fig. 1(a). With time, iron has been proved to be a promising earth-abundant element in different fields. Iron and iron-based compounds have been proven to be promising materials for the photocatalytic and photo-electrocatalytic utilization of clean solar energy. Thus, extensive efforts have been devoted to exploiting iron and iron-based compound catalysts, and iron oxides, such as Fe₂O₃, Fe₃O₄, FeOOH, LaFeO₃ (LFO) and BiFeO₃ (BFO).^23–33 However, although Fe₂O₃ and BiFeO₃ as iron-based photocatalytic materials have shown excellent activity in different photocatalytic reactions, their relatively positive conduction band, and thus redox potential towards the hydrogen evolution reaction are not favorable conditions for hydrogen production,³⁴ while the OER activity of FeOOH is low because of its too strong interaction with hydroxyl species.³⁵

Fig. 1 (a) Concentration of abundant metals in the earth's crust. (b) Number of annual publications from 2000 to 2023 of iron-based photocatalysts (Google Scholar). (c) Iron-based nanomaterials and their applications as photocatalysts. (d) Band gap energies and band-edge energies of iron-based materials^36–46 (produced by using data from these references).

Within the last decade, photocatalytic reduction/oxidation employing iron-based compounds has attracted significant attention from researchers, and consequently the related number of publications has been increasing annually, as shown in Fig. 1(b). Our literature survey showed the current annual trend of publications on iron-based materials. An upsurge in the number of publications was found in the last year and the data showed that iron-based materials have broad prospects in the field of photocatalysis. However, the positive potential of iron-based materials limits their usage in reduction photocatalytic reactions. In this case, recently, the integration of Fe₂O₃ with other semiconductor heterojunctions (such as COF and CsPbBr₃)^47–49 has been shown to produce appropriate CO₂ reduction and hydrogen production photocatalysts. In particular, it has been reported in past studies that Fe-based catalysts (Fe₂O₃/2D COFs) with strong coordination between iron oxide and COF effectively transferred the photogenerated electrons from the conduction band of iron oxide to the valence of COFs, thus causing the efficient separation of photogenerated charges and good photoactivity.⁵⁰ Moreover, the tilting of the conduction band of BFO through a piezoelectric field makes it active for HER,⁵¹ which enables the use of iron as a reduction reaction (CO₂, H₂, and N₂) catalyst with wide application prospects. In this regard, many investigations and modifications have been performed to develop outstanding and promising photocatalysts for solar energy conversion processes.

The importance of photocatalysis in various applications such as hydrogen generation,^52–55 artificial photosynthesis,^56–59 water treatment,^60–63 and toxic gas removal^64–66 makes it one of the most exciting topics of research. To date, the reviews in the literature mainly focused on iron-based photocatalysts and their applications in photocatalytic reactions. However, none of the previous reviews systematically discussed the studies on iron-based nanocrystals and their potential applicability in different photocatalytic reactions. In this regard, some details are available on iron-based MOFs and iron-based-complexes in previous reviews, but a systematic introduction to different iron-based catalysts and their role especially in tuning the band energies and respective conduction/valence bands and improving the photocatalytic performances of materials is lacking. Thus, in view of the prospects of iron-based photocatalysts in photocatalytic reactions, a systematic description of the role of different types of iron-based materials in photocatalytic reactions is presented in this review. The use of iron-based photocatalysts in different catalytic reactions is presented in Fig. 1(c), revealing that iron-based materials have a broad range of applications.

In addition, the band potentials of various iron-based materials are presented in Fig. 1(d).^36–46 The band structure of these materials indicates their suitability for various photocatalytic processes. For the first time, this review describes the types of iron compounds that are currently researched in different catalytic reactions. Initially, we present the basic background on photocatalytic reactions. Specifically, we focus on the most widely used iron oxides, iron-based MOFs, iron-doping in semiconductor materials, iron-alloys, iron-based complexes and other iron-based materials, such as sulfides, selenides and phosphides (FeS, FeSe, FeP, respectively) with obvious photocatalytic advantages. Then, we summarize the role of iron catalysts as co-catalysts, type II heterostructures, Z-scheme heterostructures and S-scheme structures in improving the photocatalytic activity and the different charge separation mechanisms in iron-based photocatalysts. Subsequently, the recent reports on iron-based catalysts for photocatalytic oxidation and reduction such as treatment of wastewater, hydrogen production, oxygen production, CO₂ reduction, and N₂ reduction are also discussed. Finally, a prospect based on several new research hotspots and existing problems that have not been addressed before is thoroughly discussed.

2 Types of iron-based nanomaterials

Considering the abundant reserves, low cost, excellent redox overpotential and narrow band gap of the iron-based nanomaterial, various iron-based nanomaterial, such as oxides, sulphides, nanocomposites, and iron doped materials has been studied (Fig. 2). Owing to their positive conductive band, most of the iron-based nanomaterials were mainly adopted for photocatalytic degradation reaction, unfortunately, for the energy conversion, the iron-based nanomaterials only was used as co-catalyst or the component for construction of heterostructure. It is noteworthy that conductive band can be altered by variation of pH value and the iron-based nanomaterial could be adopted to form some high efficiency Z scheme hetero-structure. All of these methods have made possible the direct use of iron-based nanomaterial in energy conversion.

Fig. 2 Types of iron-based nanomaterials with the processes involved in photocatalysis.

2.1 Iron oxides

Iron oxides are some of the most widely available metal oxides, and recently magnetic iron oxides have attracted significant attention due to their important contribution to environmental research. Various magnetic iron oxides have been synthesized with different morphologies and sizes^67–69 (Table 1). Three decades ago, one of the abundant oxides in the Earth, iron hematite, α-Fe₂O₃, attracted great attention when used as a reactant to prepare photo anodes for the oxidation of water.⁷⁰ This is attributed to its various promising properties such as non-hazardous nature, narrow band gap, and abundant availability and lost cost.^71,72 Furthermore, it satisfies all the basic requirements for a catalyst to be used on a large industrial scale. However, hematite loses its efficiency due to its limitations of short charge carrier life span, short diffusion lengths, and poor conduction.⁷³ Thus, various techniques have been employed to modify hematite to overcome these drawbacks. To enhance the photocatalytic and the electrocatalytic activity of hematite, modifications were introduced via surface, size, and morphological engineering, doping, alloying, heterojunction synthesis etc. To date, there has been a lot research on the development of various materials using mixed iron oxides, which have all the advantages of hematite but this research did not highlight their drawbacks. Some materials based on iron such as spinel-type MFe₂O₄ (M = metal ion)^74,75 and perovskite-type MFeO₃⁷⁶ have proven to possess the above-mentioned properties. Research has shown that the physicochemical properties of some of these materials are similar to that of hematite. Iron oxides, such as hematite (α-Fe₂O₃), maghemite (γ-Fe₂O₃), and wurtzite (FeO), possess different stoichiometric and crystal structures. The magnetic properties of hematite are not affected by its photoelectrochemical properties, but the spin configuration of iron can affect its carrier transportation properties. In the near-infrared region, hematite can absorb photons because its transition-state electrons weaken its absorption bands, which are located between two energy orbital levels of Fe³⁺ ions due to their division by a crystal field.⁷⁷ Given that α-Fe₂O₃ exhibits an indirect band gap with the direct transition of O (2p) to Fe (3d), and also d to d transition, it means that its bandwidth is above 3.2 eV, with the related composites exhibiting this behavior.^78–80 Iron-based nanomaterials have been proved to be environment-friendly semiconductors, leading to their use in numerous fields such as gas sensors, lithium ion batteries, water treatment, photocatalysis and generation of H₂ by water splitting.^81–83 Many Earth abundant metals show photocatalytic activity, including metal oxides, sulphides, and nitrides. However, most of them operate by absorbing radiation in the UV region, which limits their use in the solar spectrum. In this case, although nitrides and sulphides possess a lower band gap, their low stability in aqueous medium poses a great drawback. One of the oxides of iron hematite (Fe₂O₃) has been recently recognized as one of the most feasible photocatalysts because its bandgap is quite low (1.9 to 2.3 eV). Hematite is also considered suitable for harvesting a good amount of visible light. Furthermore, it maintains stability, can be recycled, and also abundant on Earth, making this semiconductor a highly favorable catalyst for photoreactions.⁸⁴ Iron is also used as a sensitizing agent for improving the performance of other photocatalysts possessing wider band gaps. This indicates that iron not only exhibits a good performance on its own but when used as a sensitizing agent with wide band catalysts, it also improves the visible light-harvesting performance of these catalysts.⁸⁵ Previous research showed that a reactive charge is formed and carried by Fe₂O₃ when it absorbs visible light gets, which is how it activates and improves the performance of adjacent wide band catalysts.⁸⁶ Moreover, it has many other advantages as follows: (1) its light absorption reaches up to 600 nm, utilizing almost 40% of the solar spectrum. (2) It shows high stability in aqueous medium with a high pH. (3) It is cheaper in comparison to other materials. (4) It is easily available. Alternatively, it has also been proven to be a good material for photochemical water splitting. Hematite has a lower valence band in comparison to the oxidation potential of water (due to its narrow band gap), making it a good material as the photo anode for photo-electrochemical water splitting.

Table 1 Iron oxide-based nanoparticles as photocatalysts

Photocatalytic materials	Synthesis methods	Factors affecting photocatalytic property	Application	Crystalline size/morphology	Ref.
α-Fe2O3 nano and bulk materials	Hydrothermal process	Smaller size and increase in specific surface area	Oxygen evolution	Irregular and poly-dispersed	87
α-Fe2O3–Fe2O3 powders	Chemical precipitation	Effect of calcination temperature, catalyst loading, and reaction temperature	Degradation of reactive brilliant blue (X-BR)	Non-crystalline	88
Porous α-Fe2O3 nanorods	Hydrothermal process	Porosity enhances the photocatalytic property	Degradation of MB		89
Porous α-Fe2O3 nanostructures	Ni^2+/surfactant-assisted synthetic method	Porosity and surface area enhance performance	Degradation of methylene blue	Porous morphology with a rough, flue-like surface	90
Micro/nanostructured α-Fe2O3 spheres (MNFSs)	Surfactant- and template-free method	High specific surface area together with special porous structure	Degrade the rhodamine 6G	Micro-nanostructure	83
α-Fe2O3 nanowires	Electrospinning method	High specific surface area together with special porous structure	Decolorization of RhB	Nanowires	91
Cubic and disc α-Fe2O3	Ion-mediated hydrothermal route	Discs exhibited the most desirable photocatalytic behavior	Decolorization of RhB	Cubic and disc	92
Fe2O3/Fe3O4	Green synthesis	High surface area	Congo red dye	Nanoparticles	93
C. papaya fruit extract coated α-Fe2O3 NPs	Green synthesis	High surface area and porosity	Methyl orange	Agglomerated nanoparticles	94
Fe3O4 and α-Fe2O3/γ-Fe2O3	Sol gel	Heterojunction	Rhodamine B	Crystalline nanoparticles	95
Fe3O4	Green synthesis O. tenuiflorum	Methylene blue dose	Methylene blue	Spherical nanoparticles	96
Fe2O3	Green synthesis (Glycyrrhiza glabra)	Root extract	Methylene blue	Irregular nanoparticles	97
Iron oxide nanoparticle-based hydrogel	Co-precipitation	Gel like nature	Tetracycline	Spherical	98
Fe2O3 NPs	Green method	Husk rice like shape and rhombohedral crystal phase with high stability	Rhodamine B dye	Husk rice like	99
TiO2 anchored on Fe oxides@carbon	Co-precipitation	—	4-Chlorophenol	—	100
Superparamagnetic iron oxide nanoparticles	Co-precipitation	Agglomerated nanoparticles	Tetracycline	Round shape	101
Iron oxide nanoparticles	Green method	Spherical nanoparticles	Methyl orange dye	Spherical and amorphous	102
γ-Fe2O3 nanocubes	Hydrothermal & sol–gel methods	Nano flower morphology	Rhodamine dye	Nano-flower	103

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This is because water splitting involves the homogenous production of H₂ and O₂ after the dispersal of the photocatalytic material in pure water.¹⁰⁴ Theoretically, hematite has a solar conversion efficiency of about 15.5% due to its photocurrent density of about 12.6 mA cm⁻² in an ideal cell.^105,106

However, certain factors such as the low diffusion length of holes, poor conductivity, and high electron–hole recombination rate, which lead to a low efficiency together with the requirement of a larger potential for water oxidation, have limited the performance of hematite.^75,107–110 Thus, researchers have devoted their efforts to overcoming these disadvantages posed by the deviating behavior of hematite.^104,111 Besides its use in water splitting-like reduction process and oxidation process, hematite is also used for the filtration of organic compounds during water treatment, removal of heavy metals, and absorbance and degradation of different dyes.

Sundra Murthy et al.¹¹² explained the process involved in the breakdown of Congo red dye using Fe₂O₃ as a photocatalyst. When the charge transfers from the LUMO level of the dye to the conduction band of the photocatalyst, photosensitization of the dye occurs. Following this, a photochemical reaction takes place, where an electron–hole pair is generated by α-Fe₂O₃ upon exposure to light. When an electron moves from the valence band to the conduction band of Fe₂O₃, it leaves a hole in the valence band. Subsequently, superoxide radicals are produced by iron oxide when it reacts with the chemically absorbed oxygen on the surface of the photocatalyst.

The dissociated water (H⁺) together with the super radical forms hydroperoxyl radicals, generating hydrogen peroxide. Holes are generated on the surface of the catalyst together with hydroxyl radicals. The CR dye reacts with the intermediate species and these reactive radicals degrade the dye into non-toxic compounds. Given that the photocatalytic property of a material is determined by its surface area, researchers have focused on minimizing the size of photocatalytic materials and forming hematite nanoparticles to increase their photocatalytic properties. In this case, many methods such as hydrolysis,⁸⁸ coprecipitation,^89,113 hydrothermal methods,^114–116 solvothermal methods,^117,118 ionic liquid-assisted synthesis,¹¹⁹ thermal decomposition,¹²⁰ combustion methods,¹²¹ and combustion reflex condensation and hydrothermal method combined have been used to produce iron oxide in different shapes together with nanocrystalline properties. Furthermore, numerous methods have been developed for the synthesis iron oxide, including hydrothermal, co-preparation, sol gel, solid-state, electro-spinning and ion-mediated methods.

The photocatalytic activity of three different forms of Fe₂O₃ was compared by Townsend et al.,⁸⁷ which included bulk (size 120 nm), ultra-sonicated bulk (size 40 nm) and nano-powder of α-Fe₂O₃ (size 5.4 nm). It was found that as the size became smaller, the rate of oxygen evolution increased, and among them, the rate was the highest for α-Fe₂O₃ (1072 mol h⁻¹). In the case of the nano-powder, the diffusion length of holes and the crystallite size were comparable, making more holes available to react with water. The effect of many factors on the catalytic properties of materials was reported by Dang et al.,⁸⁸ including reaction temperature, amount of catalyst, reaction temperature, and reaction duration.

An increase in calcination and reaction temperature and the amount of catalyst was reported to increase the photocatalytic activity to a certain extent, after which the photocatalytic activity decreased. Besides, numerous studies have been performed, where the authors reported enhanced photocatalytic performances depending on the morphology, temperature, concentration, etc.

Many other major factors also play a significant role in enhancing photocatalytic behavior such as porosity. For example, Zhang et al.⁸⁹ synthesized porous structures of Fe₂O₃, and also analyzed how the process of the degradation of methylene blue (MB) occurs.

Specifically, they analyzed the effect of void fraction and the amount of catalyst. An optimum amount (20 mg) of catalyst was reported to be required to achieve a higher rate of degradation, where an increase or decrease in this amount lowers the photocatalytic activity. Illumination decreases if the amount of catalyst is increased, and the degradation of organic dye decreases with an increase in the amount of catalyst due to the reduction in the number of active sites. Geng et al.⁹⁰ synthesized Fe₂O₃ with a porous structure, where its pores made its upper surface look and feel rough (Fig. 3). It was concluded that the presence of the porous surface increased the performance of Fe₂O₃ due to the enhanced surface area, followed by better degradation of MB.

Fig. 3 (a) and (b) SEM images of iron oxide nanoparticles. (c) TEM image and (d) typical HRTEM image and SAED pattern of alpha-Fe₂O₃ nanomaterial. Reproduced from ref. 90 with permission from The Royal Society of Chemistry.

Gang et al.⁸³ synthesized nano- or microspheres of Fe₂O₃via the combination of hydrothermal synthesis and heat treatment. The micro- and nano-spheres degraded the dye more effectively than the nano-powder. According to the calculations, it was estimated that micro/nanostructured α-Fe₂O₃ spheres have more than double the reactivity rate compared to that of the nano-sized particles. The calculated estimation also suggested that these spherical structures have an impressive reaction rate, which is 12 times more than that of the particles with micron sizes. Enhanced photocatalysis occurs because the porous structure increased the surface area of Fe₂O₃. Numerous methods to synthesize iron oxide with different morphologies have been reported, among which the most used method is the electrospinning method.

Deng et al.⁹¹ synthesized α-Fe₂O₃ nanowires using electrospinning methodology to photo-catalytically decolorize RhB and drive water splitting. Kusior et al.⁹² studied another process to prepare iron oxides. An ion-mediated hydrothermal process was employed to synthesize various shapes of Fe₂O₃ using acetate precursors such as Al³⁺ and Zn²⁺, and the resultant product was applied to decolorize RhB. In the synthesis process, iron nitrate nano hydrate was mixed in deionized water and stirred before adding ammonia and acetate precursor. It was found that in the absence of metal ions, small crystals were formed with an irregular shape. Comparatively, when zinc and aluminum ions were added to the reaction mixture, cubic and disc hematite structures were formed, respectively.

Wang et al.¹²² predicted that larger grain formations were caused by the aggregation and nucleation of small cubic crystallites of iron oxide upon the addition of metal ions. It was observed that an increase time to 32 hours also increased the size of the cubic nanostructures, resulting in an almost idyllic cubic formation.

However, an increase in temperature led to a decrease in the size of the cubes, and the discs went underwent a change in length, while preserving their single-particle thickness. The experiments showed that the effectiveness of Fe₂O₃ as an independent photocatalyst has not been proven to date despite its many advantages. The drawbacks of Fe₂O₃ include the rapid recombination of charges, poor carrier migration, and short distance (2–4 nm) for hole dispersion.^123,124 Therefore, further research is required to synthesize Fe₂O₃-based catalysts exhibiting promising photocatalytic performances.

2.2 Iron-based nanocomposites

Iron-based composites have been explored as photocatalysts for different related applications (Table 2). Amani-Ghadim et al.¹²⁵ reported the synthesis of the α-FeOOH nanomaterial via a one-step hydrothermal method, which produced orthorhombic structured nanoparticles. The photocatalytic activity was evaluated by the photo-degradation of Acid Orange 7 (AO7). Photolysis was performed in the absence of α FeOOH, which resulted in zero activity, while adsorption was negligible in the presence of the FeOOH nanomaterial. Alternatively, αFeOOH possessed a suitable band gap, making it a good photocatalyst (2.5 eV). Similarly, Guo et al.¹²⁶ reported the synthesis of FeOOH(H), FeOOH(P) and FeOOH(O) via hydrothermal, precipitant–hydrolysis and oxidation–hydrolysis, respectively. These products acted as ozonation catalysts for the removal of NO_x (x = 1, 2). Among them, FeOOH(H) showed the best performance, achieving 85.6% removal of NO_x.

Table 2 Iron-based composites as photocatalysts

Photocatalytic materials	Synthesis methods	Factors affecting photocatalytic property	Application	Crystalline size/morphology	Ref.
Rod-like alpha-FeOOH	Facile hydrothermal method	Rod-like structure enhances performance	Decolorization of azo dye	Rod like	125
FeOOH(H), FeOOH(P) FeOOH(O)	Hydrothermal method	Different structure	NOx removal	Nanoparticle	126
Goethite FeOOH	Simple method	Surface energy manifest enhance performance	OER and HER		127
Nanoparticle BiFeO3	Sol–gel method	Cetyltrimethylammonium bromide (CTAB) surfactant concentration	Degradation of methylene blue	Nanoparticle	128
Nano particle BiFeO3	Solution combustion method	Porosity and morphology	Degradation of methylene blue	—	129
Nanoparticle BiFeO3	Microwave-assisted solution consumption	Impurity phases of conventionally	Degradation of methylene blue	Nanoparticle	130
Bulk BiFeO3	Hydrothermal method	Crystal nucleation and crystal growth	HER & degradation of RhB	Bulk material	131
Microsphere BiFeO3	Microwave-assisted hydrothermal	Morphology and porosity	Peroxymonosulfate for the decomposition of diclofenac (DCF)	Microsphere shape	132
Nanoparticle ZnFe2O4	Hydrothermal method	Formic acid presence	Reduction of Cr(VI) to Cr(III)	Nanoparticles	133
Nanotubes ZnFe2O4	Hydrothermal method	Morphology and magnetization enhance	Degradation of chlortetracycline	Nano cubes	134
Fe(CH3COO)2/RuO2/Ru^0	Hydrothermal method	RuO2 and Ru^0 components	Degradation of Congo red dye	Nanoparticles	135
Cu/α-Fe2O3	Co-precipitation	Electron trapping	Methylene blue	Nanoparticles	135
Fe3O4/TiO2	Sol–gel	Molar ratio of Fe/Ti = 1 : 4	Norfloxacin degradation	Nanoparticles	136
Artificial chloroplast-like phosphotungstic acid@Fe2O3-CNTs	Hydrothermal	Keggin unit of phosphotungstic acid	Tetracycline	Nanoparticles	137
Zn/α-Fe2O3	Electrochemical	Zn doping	Methylene blue	Nanoparticles	138
Fe2O3/ZnO	Green method	Size of nanoparticles of 3.78 nm	Methylene blue	Spherical nanocomposites	139
g–C3N4–Zn/FeSe2	Hydrothermal	g-C3N4	Rhodamine B	Nanoflower	140
αFe2O3@polypyrrole	Chemical oxidative polymerization	Heterojunction assembly	Methyl orange	Nanoparticles	141
Fe3O4@SiO2/ZnO–Ag	Co-precipitation	Hierarchically interconnected porous structure	Methylene blue	Core shell nanoparticles	142
Fe2O3–zeolite	Ball milling	—	Methylene blue	Nanoparticles	143
Ag/ZnS/Fe3O4	Co-precipitation/sonochemical	Ag doping	Rhodamine B	Nanoparticles	144
NiFe-LDH/biochar	Co-precipitation	Large surface area and fast carrier transfer ability	Reactive red 120	nanoparticles	145
F/Fe/TiO2/SnO2/SiO	Sol–gel	Dopants (F and Fe)	Allura red, sunset yellow, and tartrazine	Nanoparticles	146
GO/Co/Fe3O4	Co-precipitation	Doping	Methylene blue	Nanoparticles	147
Fe oxide/Fe hydroxide/N-rGO layers	Laser-based method	Doping	Antibiotics	Nanoparticles	148

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This performance was due to the high surface area and high density of the surface hydroxide ions, thus leading to the high production of hydroxide radicals. In another study Huang et al.¹²⁷ reported that FeOOH nanomaterials also can be utilized for efficient photocatalytic oxygen evolution reaction (OER) activities, making FeOOH an appropriate candidate for the water splitting reaction. BiFeO₃ is another type of iron oxide. In this case, Ahmadi et al.¹²⁸ reported the synthesis of BiFeO₃ nanoparticles via a sol–gel auto-combustion method and observed the effect of the addition of cetyltrimethylammonium bromide (CTAB) surfactant on their morphology (Fig. 4). Here, the BiFeO₃ nanoparticles showed 85% degradation for methylene blue (MB) in the presence of visible light. Asefi et al.¹³⁰ prepared BiFeO₃via a microwave-assisted combustion method with a variation in the fuel content (pH = 0.5, 0.75, 1 and 2). BiFeO₃ nanoparticles were observed to form at low fuel contents. It was also observed that the phases related to the impurity in BiFeO₃ disappeared upon calcination at 600 °C. This photocatalyst showed strong adsorption in the visible region due to its narrow band gap in the range of 1.86–2.07 eV.

Fig. 4 FESEM images of bismuth ferrate (a) without CTAB (b) and with CTAB after calcination at 650 °C for 2 h. Reproduced with permission from ref. 128 Copyright©Springer Science + Business Media, LLC, part of Springer Nature. 2018.

In another work, Basith et al.¹³¹ investigated the synthesis of BiFeO₃ nanoparticles with a variation in the reaction temperature from 200 °C to 120 °C via the hydrothermal method during the synthetic scheme (Fig. 5(a)–(d)). The formation of rhombohedral perovskite BiFeO₃ nanoparticles with an average size of 20 nm was observed at 160 °C (Fig. 5(e) and (f)). Subsequently, the BiFeO₃ nanoparticles were assessed for photocatalytic degradation and hydrogen production via water splitting. These nanoparticles showed a band gap of 2.1 eV in the visible range. Further, Bharath Kumar et al.¹⁴⁹ reported the synthesis of nano- and microparticles of bismuth ferrite via electrospinning and sol–gel methods with rod-like and micro-spherical morphology, respectively, which led photo-catalysis. Han et al.¹³² reported the synthesis of BiFeO₃ microspheres via a microwave-assisted hydrothermal method, which were employed to activate peroxymonosulfate to degrade diclofenac. Degradation seemed to be promoted by the production of sulfate and hydroxyl radicals in visible light, which was proved by the detection of sulfate radicals and hydroxyl radicals in the BiFeO₃/peroxymonosulfate system by electron spin resonance (ESR). Besides BiFeO₃, ZnFe₂O₄ has also been proven to be a good photocatalyst. Thus, Islam et al.¹³³ used ZnFe₂O₄ nanoparticles for the photocatalytic reduction of Cr(VI) to Cr(III) in the presence of formic acid. The photocatalytic activity was observed to be only 0.4% in the presence of pure ZnFe₂O₄ but was enhanced to 95% in the presence of formic acid within four hours. It was concluded that formic acid is responsible for capturing the photo-generated holes, thus eventually converting formate (HCOO⁻) ions into carbon dioxide radicals (˙CO₂⁻). Here, the high negative redox potential for ˙CO₂⁻ radicals can lead to the easy reduction of Cr(VI) species to Cr(III) under UV irradiation. In another similar work, Jia et al.¹³⁴ reported the synthesis of ZnFe₂O₄ nanocubes via a hydrothermal method, which was used for the photocatalytic degradation of chlortetracycline (CTC) in a visible light/H₂O₂ system. The degradation efficiency of the ZnFe₂O₄ nanocubes for CTC remained constant (>45%) even after three recycling cycles. Therefore, ZnFe₂O₄ nanotubes can be considered a favorable photocatalyst under visible light irradiation.

Fig. 5 FESEM images of (a) bismuth ferrate bulk material and (b) corresponding size histogram. FESEM images of (c)–(e) nanoparticles of bismuth ferrate at 200 °C, 180 °C and 160 °C, respectively, and size histogram of bismuth nanoparticles prepared at 160 °C. Reproduced from ref. 131 Copyright. ©The Royal Society of Chemistry, 2018.

2.3 Iron-based MOFs

Due to the globally increasing demand for energy, researchers are focusing on the sustainable and economic production of solar energy and its conversion into chemical energy. As members of organic–inorganic hybrid materials family, MOFs have both catalytic centers and light harvesting sites. MOFs as semiconductors exhibit good absorption of UV-visible light irradiation, which is mostly attributed to ligand-to-metal or metal-to-ligand charge transfer (LMCT and MLCT, respectively).^150–154 Environmental studies have extensively focused on Fe-MOF-based photocatalysts in different applications through photocatalytic oxidation or reduction, dye degradation and different organic pollutants (Table 3). The wide range of choice of coordination modes between organic ligands of Fe-MOFs and Fe atoms contributes to improving the absorption band. Ultraviolet or light rays can activate (as previously reported Fe-MOF) various Fe-MOFs, such as MIL-53(Fe),^155,156 MIL-100(Fe),¹⁵⁷ MIL-88B(Fe),¹⁵⁸ MIL-68(Fe),¹⁵⁰ and MIL-101(Fe)¹⁵⁹ to the LMCT state. Consequently, they exhibit photocatalytic activity in the hydrogen production reaction, oxygen evaluation reaction and organic pollutant degradation by reacting with water and other electron acceptors such as persulfate,¹⁵⁵ and H₂O₂¹⁵⁶ by forming reactive radicals. Previous work also showed the degradation of tetracycline through oxidation under visible light irradiation with various Fe-MOFs such as MIL-53(Fe), MIL-100(Fe), MIL-101(Fe),¹⁶⁰ and MIL-53(Fe), MIL-100(Fe), and MIL-101(Fe) exhibited the efficiency of 40.6%, 57.4%, and 96.6%, respectively, due to their excellent light-harvesting ability owing to their large surface area. The previous studies used the decolorization of rhodamine and methylene blue dyes to test the catalytic activity of MOFs.

Table 3 Iron-based MOFs as photocatalysts

Photocatalytic materials	Preparation methods	Experimental details	Reactive species	Application	Photocatalyst performance	Ref.
ZnO/MIL-101(Fe)	Hydrothermal method	RhB solution (10 mg L^−1), 0.5 g L^−1 of catalyst	h^+ and ˙OH	Degradation of RhB	97.1%	161
CdS/MIL-53(Fe)	Solvothermal process	0.075 g of catalyst, 10 mg L^−1 KTC solution (100 mL)	e^−, h^+, and ˙OH	Degradation of Ketorolac tromethamine	80%	162
MIL-53(Fe)/NiSe2 nanosheets	Solvothermal process	0.05 g of photocatalyst, 90 mL of D.I., 10 mL of lactic acid (sacrificial reagent),	H^+/H2 (−0.41 vs. NHE, pH = 7)	Hydrogen production	10.31 mmol h^−1 g^−1	163
MOF/GO	Facial synthesis	50 mg and 5 mL triethylamine	h^+ and ˙OH	Hydrogen production	318.0 μmol h^−1 g^−1	164
Ag/CQDs/MIL-53(Fe)	Photochemical reduction procedure	50 mL of MB solution	˙O^2−- and ˙OH h^+ exerts an enormous function	Degradation of MB	93.05% in 120 min sunlight, 75.75%, 76.42% & 41.48% for MB under UV light, visible light & infrared light	165
Pt/MIL-100(Fe)	Photo-reduction approach	45 mg photocatalyst in a mixture of water/MeOH (v/v = 3 : 1, 22.5 mL) solution,	MeOH is acting as the electron donors	Hydrogen evaluation reaction	109 μmol g^−1 h^−1	157
BFO/MIL-53(Fe)	Facile solvothermal method	30 mg catalyst, 22 μL of benzyl alcohol, 3 mL of acetonitrile	Hydroxyl radical (˙OH), hole (h^+), electron (e^−), superoxide free radicals (O2˙^−)	Partial oxidation of aromatic alcohols under visible light.		165
MIL-101(Fe), MIL-53(Fe), MIL-88B(Fe)–NH2		Me-CN and TEOA solution (60 mL, 5/1 v/v)	e^−, h^+, and ˙OH	CO2 reduction	59 μmol	159
Iron-based MOF containing sulfonamide (MOF-BASU1)	Solvothermal method	One hour, solvents play a vital role, 15 mg catalyst, oxygen presence (10 hours, 80%)	e^−, h^+, and ˙OH	Benzyl alcohol oxidation	97%	166
MIL-101-X(Fe)	Solvothermal method	Dissociation of H2O2 and accelerates the iron cycling process	e^−, h^+, and ˙OH	Antibiotics removal	100%	167
S-MIL-101(Fe)	Solvothermal method		e^−, h^+, and ˙OH	Ciprofloxacin	94.7%	168

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It was observed that MIL-53(Fe) exhibited a high light absorption band in the range of 200–400 nm, with peaks at 240 nm and 290 nm, which can be attributed to stronger Fe–O cluster LMCT adsorption.¹⁵⁶ The RhB concentration of 10 mg L⁻¹ was decomposed completely with 50 min under irradiation in the abovementioned system. Similarly, the MIL-68(Fe) catalyst was used by Jing et al.¹⁵⁰ for the simultaneous reduction of chromium Cr(VI) and oxidation of malachite green dye. This study found that the removal rate of malachite green dye and chromium reached 84.3% and 94.8% with concentrations of 30 ppm/10 ppm, respectively. This study also highlighted that the chromium reduction rate could be enhanced by increasing the concentration of malachite green dye given that it captures the holes, thus hindering the recombination of the photo-generated carriers. The Fe–O cluster of MIL-68(Fe) can produce photo-generated electrons under visible light irradiation. Generally, pure Fe–MOFs as photocatalysts exhibit poor charge transfer and lack of catalytic sites.

An effective method to enhance the photocatalytic performance can be established by introducing various catalysts to create a hybrid catalytic system. In a study, graphene oxide (GO) was mixed with MIL-88(Fe), showing enhanced photocatalytic activity.¹⁶⁹ The mixture of MIL-88(Fe) and GO (about 60 min) completed the removal of rhodamine dye faster than MIL-88(Fe). This enhancement can be attributed to the introduction of GO, which enhanced the light absorption and hindered the recombination of charge carriers. Similarly, Wang and Li¹⁵⁷ studied MIL-100(Fe) by doping Pd nanoparticles and used a double-solvent impregnation method for the photochemical reaction between benzyl alcohol and aniline. MIL-100(Fe) exhibited a conversion rate of up to 88% when loaded with Pd, with a selectivity of 76% for N-benzylamine after 12 h of irradiation, which was much better than that by pristine MIL-100(Fe). When the photo-generated electrons in MIL-100(Fe) moved to the metal Pt with low Fermi levels, it increased the utilization of photo-generated carriers. Given that single iron MOFs has several deficiencies such as lack of absorbance of light and high recombination rate, these drawbacks need to be addressed. In this case, the performance of iron-based MOFs can be enhanced via synthesis with other materials. Amdeha et al.¹⁶¹ studied active nanocomposites under visible light. ZnO was synthesized using a green method, and then loaded using the hydrothermal method on MIL-101(Fe) to synthesize ZnO/MIL-101(Fe).

This material was investigated as a heterogeneous catalyst to degrade Rhodamine B (RhB) dye. The bandgap of 3.2 eV of ZnO decreased to 2.85 eV of ZnO/MIL-101(Fe) by mixing MIL-101(Fe) loaded with ZnO, which was ascribed to the intense activity under visible light. The experiments showed that the (OH) center and h⁺ played a vital role in the degradation of RhB. The authors also focused on testing the reusability of the composite up to three times. The prepared ZnO/MIL-101(Fe) showed promising results for the degradation of RhB (10 mg L⁻¹) of 97.1%, having the reaction rate of k = 0.0339 min⁻¹ using 0.5 mg of the catalyst under visible light irradiation. Organic linkers can enhance the light-harvesting process and activation of semiconductor quantum dots through linker to metal charge transfer (LMCT) under light.

The structure and flexible composition of MOFs help to tune the light absorption capacity to make efficient use of available solar energy and their crystalline structure supports the faster transfer of charges to metal clusters from the photoexcited organic linkers. These properties make MOF materials promising photocatalytic candidates, and thus they have become the focus of research. MOFs can be applied in a variety of fields such as water reduction and oxidation,^170–172 CO₂ reduction,^{157,173–175} organic transformation^176–178 and environmental remediation.^179,180 Chaturvedi et al.¹⁶² studied MIL-53(Fe) and used a solvothermal process to synthesize CdS/MIL-53(Fe) via a simple but complete approach.

The resultant CdS/MIL-53(Fe) was utilized to decompose ketorolac tromethamine (KTC) in a water solution under visible light irradiation for 330 min. The photocatalytic degradation efficiency of CdS/MIL-53(Fe) was almost 80% higher than that of pristine MIL-53(Fe). The heterostructure formation of CdS/MIL-53(Fe) supported the separation of photo-generated carriers and their transfer. Furthermore, the KTC photodegradation process was based on another scavenger study. Poor electric conductivity is one of the main issues that impede photoactivity in metal organic frameworks (MOFs).

Saouma et al.¹⁸¹ utilized the proton-coupled electron transfer (PCET) reactions of the metal–organic framework (MOF) MIL-125 to produce the corresponding phenol with the retrieval of the original oxidized MOF. It was observed that MOF was photo-charged throughout its bulk and not only on its surface. Thus, the phenoxyl reaction must have occurred on its surface. Degradation was also performed, and the proposed route is in Fig. 6. Yao et al.¹⁶⁴ introduced a small amount of graphene oxide in the MOF-templated synthesis of iron oxide. This arrangement resulted in an elevated flat band potential. The produced iron-based MOFs showed high photogenerated charge carrier separation and transportation. It was assessed for photocatalytic hydrogen production with an evolution rate of 318.0 μmol h⁻¹ g⁻¹. In this study, Chen et al.¹⁶³ used a two-step solvothermal method to fabricate MIL-53(Fe) microrods decorated with metal-free NiSe₂ nanosheets for photocatalytic H₂ evolution. The experiment showed that the efficiency of photocatalytic H₂ evolution improved when MIL-53(Fe) was decorated with NiSe₂ nanosheets.

Fig. 6 Photoreduction of MIL-125 and subsequent oxidation. Reproduced from ref. 181. Copyright©2018, the American Chemical Society.

In their study, they explained that 1.0 wt% NiSe₂ decoration provided 10.31 mmol h⁻¹ g⁻¹ production of H₂ under visible light, which was 11.1 times greater than that of pristine MIL-53(Fe) of 0.93 mmol h⁻¹ g⁻¹, and this performance was also better than that of Pt/MIL-53(Fe). Without the use of any precious metal, the quantum efficiency of MIL-53(Fe) decorated with NiSe₂ reached 10.8% at 420 nm. This experiment proved to be a vital step toward producing NiSe₂-based photocatalysts for H₂ evolution without employing precious metals.

Numerous ternary MOF-based catalyst have been reported, e.g. MOF/semiconductor/carbon-based composites with greater efficiency. A ternary MOF based composite was successfully synthesized by Zhang et al.,¹⁶⁵ Ag/CQDs/MIL-53(Fe), for the first time, showing high photocatalytic activity and full-spectrum absorption. Under light irradiation, the methylene blue degradation rate reached up to 93.05% in 120 min for 15-Ag/CQDs/MIL-53(Fe), which is an amazing rate for photocatalytic activity. Additionally, under infrared, UV, and visible light, the MB degradation was 41.48%, 75.75%, and 76.42%, respectively, for 15-Ag/CQDs/MIL-53(Fe). In a recent experiment conducted by Su et al.,¹⁸² they focused on iron-based MOFs having the same organic ligands MIL-53/88B/101(Fe) and their amine derivatives. These composites were employed to oxidize water photocatalytically with Na₂S₂O₈ acting as an electron acceptor and [Ru(bpy)₃]³⁺ as a photosensitizer. The experimenters showed that under different conditions, these MOF composites exhibited oxygen evolution activity. According to this study, MIL-101(Fe) performed the best among MOFs for the visible light photocatalytic oxidation of water. These experiments shed light on the performance of iron-based MOFs as catalysts for water oxidation. However, despite these promising results, iron-based MOFs exhibit low stability. Zhang et al.¹⁶⁵ studied many MOFs, and among them, iron-based MOFs such MIL-53(Fe) exhibited excellent photocatalytic activity under visible light due to the availability of Fe–O clusters. However, the rapid recombination of charges carriers (electron and holes) impeded its photocatalytic activities. Thus, to address this limitation, they fabricated heterojunctions. They partially destroyed the surface of the three-dimensional MIL-53(Fe) bipyramid and added bismuth ferrite nanosheets to its surface. This treatment led to an increase in the surface area and number of exposed active sites in the surface as well as enhanced the light absorption capacity. Due to the presence of FeO bonds in both components of the composite, the MIL-53(Fe) and BFO interfaces improved the charger carrier separation and transfer. This experiment tested the composite to oxidize benzyl alcohol to aldehydes, which showed excellent photocatalytic activity and high stability.

Moreover, Fe-MOF also has photocatalytic activity for other applications, attracting great attention for CO₂ reduction to solve environmental issues. To investigate the photocatalytic reduction of CO₂, Wang et al.¹⁵⁹ studied three iron-based MOFs, MIL-53/88B/101(Fe), and their amino derivatives. Triethanolamine (TEOA) was used to measure the photocatalytic activity of these iron-based MOFs. Due to the presence of UMCs, MIL-101(Fe) exhibited the best performance among the iron-based MOFs with 59 μmol of HCOO⁻ in 8 h.

This study proposed that the excitation of the Fe–O clusters assisted CO₂ reduction through electron transfer. Additionally, the Fe–O clusters can receive extra electrons from the amine groups, and they can also absorb light and provide photo-generated electrons. This study provides a comprehensive view of the role of Fe–MOFs in photocatalytic reduction and highlights key points for developing photocatalysts for CO₂ reduction under visible light. Yuting et al. reported another MOF-based photocatalyst of 1/GO/Fe₃O₄, which was comprised of Ce(BTB)(H₂O) (MOF-1, H3BTB = 1,3,5-benzenetrisbenzoic acid), graphene oxide (GO), and iron oxide (Fe₃O₄) for the photocatalytic degradation of chlortetracycline (CTC).¹⁵³ Q. Wang et al. reported the synthesis of an iron-based MOF named MIL-88A(Fe)/Ti₃C₂ MXene/resorcinol-formaldehyde (MIL-88A(Fe)/Ti₃C₂/RF, MTR), which was tested for its photocatalytic efficiency for the degradation of organics and inactivating bacteria.¹⁵⁴

2.4 Iron-doped composites

Numerous attempts have been made in the past to increase the photocatalytic properties of pristine earth abundant metal iron. This section focuses on the impact of doping on the surface modification of iron. Doping and structure modifications are among the techniques tested by scientists to evaluate the photocatalytic performance of catalysts (Table 4). Some of these attempts doped Fe₂O₃ with TiO₂, WO₃, and ZnO. Ai,¹⁸³ WO₃, Ti, ZnO,¹⁸⁴ and Ga¹⁸⁵ were used as doping agents with Fe₂O₃ to enhance its photocatalytic activity. The purpose of the doping process is to limit the rapid charge carrier recombination and enhance the absorption of visible light by creating defect states in the bandgap of the composites. In one case, defects sites are created by trapping CB electrons or VB holes to enhance the charge carrier transfer and impede recombination. In the second case, under sub-bandgap irradiation, the transfer of electrons to CB from the defect states or to the defect states from VB is allowed in a controlled fashion.

Table 4 Iron-doped composites as photocatalysts

Material photocatalyst	Synthesis method	Dopant material molar ration/condition	Experimental detail	Photocatalytic application	Performance before doping	Photocatalytic performance	Ref.
Fe-doped TiO2	Sol–gel method.	0.5 mol% ratio of Fe : Ti under pH 3	300 W xenon lamp lambda (λ) > 400 nm cut off filter visible light source	Degrade phenol solution	33%	73% increase in efficiency, within 90 min of reaction time	186
Fe-doped DyCrO3	Sol–gel method		20 mg of the photocatalyst, mercury–xenon lamp mercury–xenon lamp	Hydrogen production			187
Ag@Fe–TiO2 nanowires & nanoparticles	Convenient sol–gel and one-pot solvothermal	Mole ratios of Fe dopants to Ti elements 1%, mole ratios of deposited Ag metal to TiO2 (3, 5, 7) NWs, NPs	400 nm cut-off filter visible light source, light intensity 80 mW cm^−1 240 min, 10 vol% methanol as an electron donor	Formaldehyde degradation & hydrogen evolution,		4037 & 3512 μmol cm^−2, (NW and NPs), 5% Ag@Fe–TiO2 NWs is 0.0190 min^−1, which was 1.1 times to that of 5% Ag@Fe–TiO2 NPs	188
Aluminum doped on iron oxide	Simple cost-effective technique (anodization)	0.5% Al to iron oxide	Sunlight different time, using the UV-Vis–NIR spectrophotometer	Methylene blue MB	56% degradation	180 min, 96% degradation	189
Fe-doped CeO2	Co-precipitation technique	0-pre molar ration	0.03 g of photocatalyst, Hg lamp (λ ≥ 400 nm)	Hydrogen production		641 μmol h^−1	190
FCo2P/Fe2P/IF	Phosphidation method	0.5 mol% ratio		Hydrogen production			191
Iron-doped SrMoO4	Solvothermal method	0.01 mol to SRMoO4	60 mg of photocatalyst, a 300 W Xe lamp, ultrapure N2 gas, circulating water (15 °C)	Nitrogen reduction performance	66.7 μM g^−1 h^−1	93.1 μM g^−1 h^−1 for FSMO-1.6	192
Iron/S doped graphene	Ball milling		Material dose and the initial Cr(VI) conc. varied from 0.5–1.5 g L^− 1 and 0–200 mg L^− 1 initial pH was 5.7 without adjustment.	Removal of Cr	17%	70.2 mg g^− 1, 99%	193
Sulfur-doped nano zero-valent iron @biochar (BM-SnZVI@BC)	Ball milling	BC/S^0/Fe^0 = 3 : 1 : 1		Removal of P		25.00 mg P g^−1, 84%	194
Ag/Fe-doped ZnO composites	Hydrothermal method	Ag/Fe = 4 : 1	80-min irradiation in visible light region with 1.2% ratio of Fe to Zn was 1.2%.	Degradation of methyl orange		92%	195
NZVI@BiFeO3/g-C3N4	Hydrothermal and pyrolysis method.		i.e. pH = 9, NZVI@BiFeO3/g-C3N4 = 10 mg/100 mL, (oxidant = 18 mM, time = 120 min)	Rhodamine B	70%	97%	196

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The main categories of doping materials include non-metal ions and metal ions including noble and transition metals. Generally, researchers use a selection of metals that can potentially decrease the energy level of the band gap and transfer electrons. The light source activates the metal ions to generate electron holes in the doped catalyst composites. Consequently, the metal ions improve the interfacial generation of electrons and holes and rate of charge recombination significantly in a photocatalyst matrix, which results in greater photo-reactivity.

The morphology of the most active photocatalysts, such as TiO₂, also changes when doped with metal dopants. Various metal dopants, including iron,¹⁹⁷ zinc,¹⁹⁸ and copper,¹⁹⁹ have been studied to analyze their impact on photocatalytic activity. Many studies have been reported on enhancing their photocatalytic activity by doping iron. For example, Moradi et al.¹⁸⁶ investigated acid treatment to significantly increase the photocatalytic activity of iron-doped TiO₂ nanoparticles. The deposition of iron oxide created layers on the surface of the nanoparticles, which significantly impeded their photocatalytic activity. In this experiment, an acid treatment process using HCl was employed to remove this contamination layer. Consequently, a 73% increase in photocatalytic activity was observed when 500 mg L⁻¹ of Fe-0.5-TiO₂ was used under visible light irradiation in 1 mg L⁻¹ phenyl solution for 90 min, increasing the removal efficiency to 57% from 33%. Moreover, previous research proved that doping has a good effect not only on photocatalytic performance but also on the formation of different structures such as nanowires and nano-rods. Ahsan et al.¹⁸⁷ reported the synthesis of Fe-doped DyCrO₃ nanoparticles via the sol–gel method with a band gap of 2.45 eV compared to that of 2.82 eV of DyCrO₃ nanoparticles. This composite showed three times efficiency in relation to hydrogen production compared to the DyCrO₃ nanoparticles. Similar work was reported by Liu et al.,²⁰⁰ in which a one-pot solvothermal and modified sol–gel method was used to synthesize silver-modified iron-doped TiO₂ nanowires and nanoparticles (Ag@Fe–TiO₂ NWs and NPs). Ag@Fe–TiO₂ showed a better photocatalytic performance in formaldehyde degradation and H₂ evolution, which was attributed to the accelerated charge migration, increased exposed surface area and light-responsive range on the spectrum, and reduced recombination of photo-generated carriers. All the Ag@Fe–TiO₂ composites demonstrated improved reusability for photochemical activity. Mangish et al.¹⁹⁰ used the co-precipitation method with a variation in dopant concentration for the synthesis of Fe-doped CeO₂ nanoparticles. The lattice energy is strongly responsible for the band gap tuning of CeO₂ in the range of 3.0–1.85 eV. Doping is responsible for better photo-redox reaction. Joseph et al.¹⁸⁹ experimented on a very cost-effective technique called electrochemical doping to synthesize metal-doped iron-oxide nanostructures prepared via anodization. The composites were prepared by doping aluminum with iron oxide. The valence edge bond was displaced toward a higher energy when doped with aluminum according to the analysis of the X-ray photoelectron spectra of the valence bond (VB XPS). When the optical data was evaluated combined with the VB XPS analysis to measure the Fermi level shift to the minimum conduction band, it showed the formation of a donor defect level on the doping. The aluminum-doped nanostructures showed enhanced electric conductivity and reduced bandgap to 1.72 eV from 1.96 eV, which enhanced its photocatalytic activity compared to the undoped composites. Iron-based doping not only showed a good performance in different applications such as hydrogen production process and degradation of organic dye pollutant, and it also exhibits an effective photocatalytic performance in the nitrogen reduction process. For example, Luo et al.¹⁹² studied the Haber–Bosch process, which requires a lot of energy, and developed an alternative method for the green synthesis of ammonia via photocatalytic nitrogen reduction. They used a solvothermal method to synthesize iron-doped SrMoO₄ for solar nitrogen reduction (Fig. 7). The experiment showed that iron doping altered the intrinsic band gap of SrMoO₄ and expanded the absorption spectrum from the ultraviolet to visible light region. Iron-doped SrMoO₄ with the optimal concentration exhibited an improved photocatalytic nitrogen reduction performance compared to that of the pristine SrMoO₄. The characterization results revealed that the optimal concentration of iron doping enhanced the interfacial charge transfer as well as impeded the rapid recombination of photo-induced carriers. Iron-doped SrMoO₄ absorbs more solar light due to its narrow intrinsic band gap, while at the same maintains thermodynamic activity with suitable band energies for nitrogen reduction. This type of study presents an alternative strategy to design photocatalysts based on active nitrogen fixation.

Fig. 7 Schematic illustration of the preparation process. Reproduced with permission from ref. 192 ©2019, Elsevier Ltd.

2.5 Iron-based alloy

Iron is one of the common naturally existing elements as well as a transition metal. Different iron compounds show many compositions, crystal structures, and valence states. In the last decade, numerous studies have been conducted on employing iron to form iron-based alloy. In this section, we discuss iron-based alloys and their photocatalytic performances in environmental remediation, for example water splitting oxidation reduction process (Table 5). For example, Zhu et al.²⁰¹ discussed wastewater treatment to alleviate environmental damage and established the highly recognized process of sulfate-radical-based advanced oxidation (SR-AOPs), where an extended soft chemical solution process was used under atmosphere-dependent calcination to prepare a set of Co/Fe-based catalysts.

Table 5 Iron-based alloys as photocatalysts

Photocatalytic material	Synthesis method	Alloy material	Experimental detail	Photocatalytic application	Performance	Ref.
Co–Fe alloy Co–Fe nitride CoFe2O4	Soft-chemical process	Fe : Co is 2 : 1 molar ratio	Catalyst, PMS and orange II, 0.1 g L^−1, 0.5 g L^−1, 50 mg L^−1, 1 mL methanol solution	Activation of PMS & degradation of organic orange II	90% removal within 10 min	201
CoFe@N-GC/FeCo alloy	Heterogeneous advanced oxidation processes (AOPs)		Catalyst 0.05 g L^−1, PMS 0.2 mM, NOF 15 μM, temperature 25 °C, initial pH 6.0.	Degradation of antibiotics norfloxacin (NOF) coupling with PMS	94.4% during the fourth cycle	202
Fe78Si13B9/(FeCoNi)78Si13B9	Melt spinning method		Orange II of 40 mg L^−1, 1.0 g catalyst with UV-light source	Degradation of orange II & azo dyes	Orange II by Fe78Si13B9 amorphous alloy ribbons reaches 98.67% in 70 min, the decolorization of orange II by (FeCoNi)78Si13B9 high-entropy is 33.3%	203
Al–Fe-alloy	Chemical process		4-CP 300 mL, concentration 5 g L^−1, initial pH of the solution pH 2.5	Degradation of 4-chlorophenol (4-CP).	Al–Fe10 alloy of 5 g L^−1 from an initial concentration of 50 mg L^−1 to 2.7 mg L^−1 in 5 h (a removal rate of 95%) at initial pH 2.5	204
FeCu@C/g-C3N4	One-step method	2 : 1, 1 : 1 and 1 : 2	300 W Xe lamp, 10 mg photocatalyst, 15% triethanolamine (TEOA 100 mL)	Hydrogen production	7.22 μmol h^−1	205
CoFe alloy	Annealing method			Degradation of oxytetracycline	90% within 150 min	206

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The process of calcination in the presence of air, ammonia, and nitrogen produced an Fe–Co catalyst with Fe–Co alloy and Fe–Co nitride and cobalt ferrite (CoFe₂O₄), respectively, as the dominant phase. In the process for the activation of peroxymonosulfate (PMS) to degrade the organic Orange II, Fe/Co-based catalysts have higher efficiency. Several catalysts have activation energy following the decreasing order of Co–Fe alloy > Co–Fe nitride > CoFe₂O₄. The major active intermediate species for all the reacting catalysts are sulfate radicals causing dye degradation. A Ca carbon-based catalyst was employed by Ding et al.²⁰² to reduce environmental damage, and nitrogen-doped graphitic carbon (CoFe@N-GC) was reported to capture the FeCo alloy, and the carbonization route was used to convert the cobalt-modified Prussian blue (PB) precursor to the original position. This catalyst was applied to degrade the antibiotic norfloxacin (NOF) as an advance or innovative catalyst by combining with peroxymonosulfate (PMS). The catalysts showed better results for the degradation of organic pollutants than other catalysts, e.g. CoFe₂O₄, CuO, and biochar. It is clear that singlet oxygen and sulfate radicals are the dominant reactive oxidative species (ROS). It was concluded that CoFe@N-GC exhibited better results after 4 consecutive cycles, showing 94.4% NOF degradation in just 20 min. Ji et al.²⁰³ illustrated that amorphous alloy ribbons (Fe₇₈Si₁₃B₉) and high-entropy amorphous alloy ribbons (FeCoNi)₇₈Si₁₃B₉ can be manufactured with the help of the melt spinning method. Later, the performance of decolorization was thoroughly examined (Fig. 8). By using Fe₇₈Si₁₃B₉ amorphous ribbons, an Orange II solution of 40 ppm was degraded fully in almost 70 min together with a 52.4% decline in chemical oxygen demand (COD), exhibiting the considerable effect of the alloy on the degradation of azo dyes.

Fig. 8 Photocatalysis by amorphous alloy ribbons of (FeCoNi)₇₈Si₁₃B₉. Reproduced from ref. 203 with permission. ©2019, Elsevier B.V.

Alternatively, under the same conditions, no chemical degradation of dye was observed with the (FeCoNi)₇₈Si₁₃B₉ ribbons. However, they showed a physical adsorption process. Moreover, (FeCoNi)₇₈Si₁₃B₉ ribbons exhibited a lower rate of decolorization reaction than Fe₇₈Si₁₃B₉ amorphous ribbons, which is k = 0.071 min⁻¹, also changing the dye molecules into environmentally friendly inorganic substances. Furthermore, the pseudo-first-order kinetic model efficiently described the degradation processes. Chlorophenols are highly toxic and intractable substances in the environment. Thus, Wu et al.²⁰⁴ investigated the degradation of 4-chlorophenol (4-CP) by using Al and Fe mixtures, zero-valent aluminum (Al), zero-valent iron (Fe), and Al–Fe alloy. In the absence of oxygen (anoxic condition) at the primary pH level of 2.5 for 10 h, no removal of 4-chlorophenol was observed.

Meanwhile, in the presence of oxygen (aerated conditions), up to 99% 4-chlorophenol was removed. Al–Fe10, Al/Fe10, Al and Fe eliminated the 99%, 78%, 83% and 56% of 4-chlorophenol, respectively. When an Al/Fe mixture was employed to remove 4-chlorophenol, within the first 4 h, mixture was in Fe mode, and then it shifted to the Al mode. It was observed that hydroxyl radicals (˙OH) and reactive oxygen played a pivotal role in the degradation of 4-chlorophenol (4-CP). This reaction was carried out in two steps and two intermediate products 4-chlorocatechol (4-CC) and hydroquinone (HQ) were detected. By using the one-step method, for the first time, bimetallic FeCu nanoparticles coated by graphitic carbon were manufactured by Chen et al.²⁰⁵ To improve the photocatalytic generation of hydrogen under visible-light, g-C₃N₄ was introduced because it possesses better photocatalyst activity (7.22 μmol h⁻¹). Accordingly, the FeCu@C/g-C₃N₄ composite exhibited higher photocatalytic activity due to the synergistic effect between the graphitic carbon layer and binary metals.

The g-C₃N₄-generated photo carriers captured the catalytic activity centers of Fe, resulting in their agglomeration on the surface of the FeCu alloys. Thus, the stability level of FeCu increased by the coating it with a graphitic carbon layer, which also improved the electron transfer rate.

2.6 Iron-based complexes

Iron is an earth-abundant and environmentally benign metal, and thus it has been investigated in complexes as photocatalysts.^207–212 Perutz and Procacci²¹³ studied the release of H₂ by an Fe-hydride complex via photo-induced reductive elimination. Among the iron-based complexes, mainly two classes (monohydride and dihydride) can be employed for photo-induced H₂ release, where the Fe-dihydride complexes are mainly Fe–carbonyl and Fe–phosphine structures. Between them, Fe–phosphine dihydrides show better photo-reactivity. Alternatively, iron porphyrins play a vital role in photocatalysis. Carolyn L et al.²¹⁴ synthesized a series of Fe(III) complexes for reducing protons into hydrogen gas. Here, the iron complexes acted as electrocatalysts, which were incorporated into a photocatalytic system for hydrogen gas production, where fluorescein was used as the chromophore and triethylamine as the sacrificial electron source for hydrogen evolution with iron complexes. Robert et al.²¹⁵ used the phenolic group for the modification of iron porphyrins to reduce CO₂ to CO with a selectivity >90% in the homogenous systems.^216–219 This photocatalyst showed high efficiency, high selectivity, high stability, and easy recycling. Lin et al.²²⁰ reported the selective conversion of carbon dioxide to carbon mono-oxide via the heterogenous catalyst g-C₃N₄/FeTCPP under visible-light irradiation. FeTCPP acted as the catalytic center with the low-cost g-C₃N₄ nanosheets acting as the light absorber. Electron transfer was observed from the g-C₃N₄ nanosheets to FeTCPP, which showed 98% selectivity with the maximum rate of 6.52 mmol g⁻¹ in 6 h for the production of carbon mono-oxide. In another similar work, carbon dots were introduced in g-C₃N₄ and tetra(4-carboxyphenyl)porphyrin iron(III) chloride (FeTCPP) molecular catalyst by Zhang et al.²²¹ The obtained hybrid photocatalyst shows high efficiency for the production of carbon mono-oxide and hydrogen under visible-light irradiation with a yield of 23.1 mmol g⁻¹ and 71.1 mmol g⁻¹ in 6 h, respectively. The trace amount of carbon quantum dots was considered to be responsible for providing a substitute channel for electron transfer. Li et al.²²² reported the functionalization of ethylenediamine with CdS (CdS-EF), followed by their coupling with tetra(4-carboxyphenyl) porphyrin iron(III) chloride (FeTCPP) for CO₂ photoreduction. The ethylene diamine-functionalized hybrid showed better activity compared to the non-functionalized hybrid due to the hydrogen bonding between the amino groups of CdS-EF and carboxyl groups of FeTCPP. This hydrogen bonding provided channels for electron transfer from CdS to FeTCPP. Zhong et al.²²³ reported the synthesis of a heterogeneous Fenton-like photocatalyst for the photodegradation of 4-chlorophenol. This photocatalyst was an acrylic acid or hydroxylamine hydrochloride-modified woo-based complex of iron. Its good activity is due to the strong interaction between iron and hydroxamic acid, thus hindering the leaching of iron. This approach was novel for the synthesis of biopolymer-based photocatalysts.

Tabar et al.²²⁴ reported the synthesis of a graphitic iron complex with a surface area of 575 m² g⁻¹ and average pore size of 21 nm. Its photocatalytic activity was tested for the degradation of Reactive Blue 13 dye. Ellagic acid (EA) based metal–organic complexes (MOCs) were produced by stirring with iron, bismuth and cerium ions as the central metal. This complex was tested for the reduction of Cr(VI), showing complete removal within 20 min.²¹² Wang et al.²²⁵ synthesized an iron-based complex comprised of bis(terpyridine)iron(II) complexes for the selective reduction of CO₂ with 99.4% selectivity and turnover frequency of 127 min⁻¹ in dimethylformamide/H₂O solution. Light irradiation for 2 h produced more than 0.3 mmol CO molecules from CO₂ in the presence of 0.05 μmol catalyst. Carbon dioxide reduction was studied by Kosugi et al.²²⁶ employing an iron-complex-based photocatalyst under visible light.

The catalyst showed a great performance for CO₂ reduction with the selectivity of 99.9%, and apparent quantum yield for CO production of 0.298% at 400 nm with stability for up to 96 h. Semiconductors have been proven to be promising for photocatalytic energy production and environmental remediation. However, inorganic semiconductors exhibit the drawbacks of agglomeration and delayed solar energy conversion.

In this context, Xue et al.²¹² synthesized ellagic acid (EA)-based metal–organic complexes (MOCs via stirring process at room temperature with Fe³⁺, Bi³⁺ and Ce³⁺ as the metal center). The EA–Fe photocatalyst showed excellent photocatalytic activity for Cr(VI) reduction, with its complete removal within 20 min. EA–Fe was 15 and 5 times more effective than bare EA for degradation of tetracycline and rhodamine B dye, respectively. It was observed that EA–Fe can produce superoxide radicals, which can reduce heavy metals and can degrade organic contaminants. This work presents insight for manufacturing multifunctional MOCs with high photocatalytic efficiency.

2.7 Iron-based COFs

Iron-based covalent organic frameworks (COFs) have received significant interest as prospective photocatalysts because of their distinct structural features and possible uses in domains such as environmental remediation, energy conversion, and chemical synthesis. Iron-based COFs are made of organic building blocks containing iron atoms, which are connected by strong covalent bonds, creating a porous and crystalline framework. The integration of iron within the structure confers certain functions and catalytic features to the material. Furthermore, the characteristics of iron-based COFs can be tailored for certain applications due to the potential to control their structure, composition, pore size, and surface areas.^227,228 However, despite their promising features, challenges such as scalability, synthesis stability, and fine control of their active sites need to be addressed. Furthermore, developing new synthetic methods, enhancing photocatalytic activity, and investigating the fundamental reaction mechanisms are vital to optimizing the practical use of iron-based COFs in photocatalysis.²²⁸ For instance, Yao et al.²²⁹ reported the design and fabrication of Fe@COF catalysts, which were utilized as peroxymonosulfate (PMS) activators for organic pollutant degradation. Through electronic structural modification, iron doping preferentially formed effective single-atom Fe–N_x active sites in the carbon framework, resulting in notable catalytic abilities. In situ electron paramagnetic resonance (EPR) spectrometry and quenching experiments demonstrated that the singlet oxygen (¹O₂) produced by the Fe@COF/PMS catalyst was the main contributor to the organic pollutant degradation, instead of sulfate and hydroxyl radicals.

The readily available single-atom Fe–N_x active sites with optimal binding energy were identified to effectively activate PMS to generate ¹O₂, whereas the rich pyrrolic nitrogen could serve as the adsorption site for organic molecules, leading to outstanding catalytic activity in a wide pH range. Iron-based COF composites are also favourable for water splitting reactions. For instance, Xu et al.²³⁰ coupled COFs with BiFeO₃ to produce a Z-scheme core@shell hetero-structure for overall water splitting. The charge carrier separation and utilization performance were significantly enhanced primarily due to the interaction between the polarized electric field and photo-induced charge carriers, together with the precisely controlled modification of the shell thickness. The BiFeO₃@TpPa-1-COF composite photocatalyst outperformed other piezo- and COF-based photocatalysts, producing ∼1416.4 μmol h⁻¹ g⁻¹ of H₂ and 708.2 μmol h⁻¹ g⁻¹ of O₂, respectively, during ultrasonication and light illumination. Likewise, Yan et al. reported a simple one-pot iono-thermal approach for growing ZnFe₂O₄ nanoparticles in situ on iron porphyrin covalent triazine-based frameworks (FeP–CTFs). This method prevented ZnFe₂O₄ from agglomerating and clogging the CTF pores. Experiments revealed that the FeP–CTF anchoring sites with Ru(bpy)₃²⁺ enhanced the crystal-plane ratio and improved the CO₂ reduction performance (∼178 μmol h⁻¹ g⁻¹ for CO) of ZnFe₂O₄/FeP–CTFs under visible light compared to the pure ZnFe₂O₄, FeP–CTFs, and their physical mixtures. The robust interactions between the ZnFe₂O₄ NPs and FeP–CTF support resulted in enhanced charge carrier separation and transfer and improved CO₂ photoreduction, as confirmed by means of various experiments and DFT studies.

In addition to iron-based MOFs, some reports were published on iron-based covalent organic frameworks (COFs). Xu et al. reported the preparation of a ferric acetylacetonate/covalent organic framework (Fe(acac)₃/COF) composite via the interfacial polymerization method. Subsequently, this photocatalyst was used for the oxidation of benzyl alcohol.²³¹ Mei-Ling Xu et al. also studied covalent organic frameworks (COFs) by preparing Fe–MOF-derived α-Fe₂O₃ and FeP–PC, which acted as a dual co-catalyst. It was evaluated for water splitting to activate ketoenamine-based TpPa-1-COF, with both the porous α-Fe₂O₃ and FeP–PC substituted as a framework support to efficiently avoid the agglomeration of TpPa-1-COF during the fabrication and photocatalytic process.²³²

2.8 Other iron-based materials

Although iron has shown photocatalytic activity as iron-based MOFs, iron-doped materials, iron alloys and compounds, numerous other iron-based materials have been reported, including iron sulphides, phosphides and nitrides (FeS, FeSe, FeP, FeC, and FeN), to also exhibit excellent performances as photocatalysts in different applications. In the previous section we discussed iron-based compounds, iron alloys, iron MOFs, iron doping, but numerous other iron-based nanocrystals have good photocatalytic properties such as iron-based sulphides, iron-based phosphides, iron-based carbides and iron-based nitrides. Zhong et al.²³³ reported the preparation of the FeSe/CdS nanomaterial with FeSe as a co-catalyst for the photocatalytic evolution of H₂. This composite showed higher activity compared to pure CdS due to field-derived photo-generated electrons, which then transferred to the surface of FeSe, resulting in electron–hole separation.

Zeng et al.²³⁴ reported the excellent oxidation of p-nitrophenol using pyrite (FeS₂) prepared via the solvothermal method. Its excellent oxidation performance was attributed to the rapid conversion between ferric and ferrous ions under visible light irradiation, thus leading to the excessive production of reactive oxygen species. Metal sulfide photocatalysts, which were considered desirable for water splitting, followed by hydrogen production, are associated with the drawback of rapid photo-generated electron–hole pair recombination. In this context, Zhang et al.²³⁵ reported the preparation of porous transition-metal thiophosphates. This photocatalyst was capable of showing long term hydrogen production with a band gap of 2.04 eV, together with stability for 56 h due to the 7 nm-thick nanosheets of FePS₃. This synthesis paved the way for the synthesis of more dianion-based inorganic nanomaterials for hydrogen production. Zeynali et al.²³⁶ reported the synthesis of a core–shell photocatalyst of FePt–ZnIn₂S₄via the hydrothermal method for hydrogen production. Zeng et al.²³⁷ reported the synthesis of heterojunction nanoparticles of FeP nano-dots/g-C₃N₄. Here, FeP acted as a co-catalyst to facilitate charge separation and provide sites for hydrogen evolution (177.9 μmol h⁻¹ g⁻¹).

Recently, the accumulation of plastic garbage has become an increasingly significant environmental challenge to the United Nations objectives and worldwide issue for sustainable development. Accordingly, the conversion of plastic polymers into high-value compounds such as carbon nanotubes offers a viable method for recycling that does not produce carbon dioxide emissions. In a recent study, Wu et al. reported the transformation of different types of plastic polymers (polycarbonate (PC), high-density polyethylene (HDPE), and polystyrene (PS)) into high-quality single-walled carbon nanotubes (SWNTs) with the assistance of a rationally designed heterogeneous catalyst, i.e., an iron metal catalyst (Fe) supported by porous magnesia (Mg). The exceptional catalytic activity of the Fe/MgO catalyst was ascribed to the robust metal-support contact and the substantial carbon solubility of Fe, enabling it to maintain a reasonably high carbon flux resulting from the self-decomposition of the polymer. Furthermore, this method is also suitable for converting real-world plastic waste and polymer mixtures into single-walled carbon nanotubes (SWNTs). These studies not only provide a profound understanding of the catalyst activation and polymer degradation processes, but also showcase the process of transforming household trash into valuable products, therefore revitalizing real-world discarded plastic. Furthermore, this kind of methodology is also suitable for converting real-world plastic waste and polymer mixtures into single-walled carbon nanotubes (SWNTs).²³⁸

Shen et al. reported the use of an iron-based catalyst for the microwave-assisted pyrolysis (MAP) of plastics with the objective of conversion of high-density polyethylene into hydrogen and carbon nanotubes (CNTs). The iron-based catalysts (FeAlO_x) and modified catalyst supports including activated carbon Fe@AC, silicon carbide Fe@SiC and silicon dioxide Fe@SiO₂ having varied microwave-absorbing properties were explored for valorizing waste plastics towards clean fuels and value-added CNTs. It was found that the microwave-absorbing property of the Fe-based catalysts was important to enable the formation of hot spots on the catalyst surface, which are highly beneficial for the initial thermal cracking of plastics. Also, an increase in the Fe loading, as exemplified by the FeAlO_x catalyst, was desirable to promote the further conversion of intermediates (from thermal cracking) to produce more hydrogen from MAP. Further, it was studied by designing FeAlO_x catalysts having altered iron loadings. An increase in iron content from 7% to 22% enhanced the gas yield from 86.3% to 93.7% but did not affect the morphology of CNTs. The conclusions from this research can lead to the future design and development of microwave-responsive catalysts for the microwave-assisted pyrolysis of plastic wastes towards a circular economy.²³⁹

3 Classification of heterojunctions

In heterojunctions, two composites are coupled, which maintain their original identity. The difference between the potential of the conduction and valence band is the driving force for the formation of a good photocatalyst. In this case, strong redox ability can be achieved if there is a high conduction band and deep valence band positions. However, if the target is maximum solar energy harvesting, then the photocatalyst should possess low conduction band and shallow valence band positions. These two conditions are specific for mono-component photocatalysts, thus motivating researchers to devote their efforts to synthesizing heterojunction photocatalysts (Table 6).

Table 6 Iron-based heterojunction composites as photocatalysts

Photocatalytic material	Synthesis method	Photocatalytic application	Photocatalytic performance	Ref.
Summary of iron type II heterojunction
Fe2O3/TiO2	Sol–gel and self-assembly	Photocatalytic degradation of MB	97%	240
meso-Fe2O3/TiO2	Sol–gel process	4-Chlorophenol (4-CP)	No data	241
γ-Fe2O3/β-TiO2	Metal-ion intervened hydrothermal and high temperature hydrogenation route under normal pressure	CT photodegradation	99.3%	242
CQDs/TiO2/Fe2O3 (CTF)	Multi-step hydrothermal technique	Photodegradation of MB	86.5%	243
g-C3N4–Fe2O3	Polycondensation	Photodegradation of textile effluents (TE) and methyl orange (MO)	4.92 × 10^−2 min^−1, 97%	244
Ag2O/Fe2O3	Solvothermal precipitation-deposition method	Photodegradation of RhB and 4-CP	85.3%, 57.9%	245
ZnFe2O4@CuFe2O4@SiO2	Co-precipitation method	Methylene blue	98.6%	246
2D/2D Z-scheme-based α-Fe2O3 @NGr, 2D α-Fe2O3 (as photocatalyst II) and 2D nitrogen-doped graphene (NGr) as photocatalyst I	Hydrothermal method	Photocatalytic hydrogen evolution	6.4 μmol mgcat^−1 h^−1	247
α-Fe2O3/Cu2O	Hydrothermal method	Oxidation of levofloxacin.	More than 70%	248

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The S-scheme and Z-scheme are the two models used to describe the electron transport chain in photocatalysis, which represent different mechanisms of electron flow and energy conversion. In the S scheme, light energy is absorbed by photosystems I and II (PSI and PSII). PSII absorbs photons, leading to the excitation of electrons, which are then passed through an electron transport chain. The electrons lost from PSII are replaced by the oxidation of water molecules, generating oxygen as the byproduct. Alternatively, the Z-scheme is a more refined version of the S-scheme, which was proposed to explain the observation that the energy levels of electron carriers are arranged in a zigzag pattern. In the Z-scheme, electrons are transferred between PSII and PSI in a series of redox reactions, creating a zigzag pattern when plotted against the midpoint potentials of the carriers. In the present study, the discussion is related to different types of heterojunctions (type II, Z-scheme, and S-scheme) comprised of iron-based material for photocatalysis.

3.1 Type-II heterojunctions

Heterojunction assemblies are very useful to construct efficient photocatalysts, which are composed of two semiconductors, which provide a pathway for the transfer of charge carriers generated via irradiation from one semiconductor to other.²⁴⁹ The main drawbacks of photocatalysts involves the recombination of charge carriers, which can be tackled via heterojunction assembly. Iron-based materials are investigated here as heterojunction-based photocatalysts to investigate solutions to the problem related to the recombination of charge. For this purpose, iron-based nanomaterials have been composited with different materials. Given that iron shows similar characteristics to titania, iron-based nanomaterials have been composited with titania, leading to a reduction in their band gap and better separation of charge carriers.²⁵⁰ Significant research has been conducted on composites of iron oxide and titania to study their photocatalytic abilities. Similarly, Ahmed et al.²⁴⁰ described the synthesis of Fe₂O₃/TiO₂ nanoparticles having a large surface area using the sol–gel method, which were subsequently tested for the degradation of methylene blue dye (MB). In another study, Reichenberger et al.²⁵¹ reported that iron oxide with a content in the range of 1 to 7 wt% is sufficient to maintain the anatase phase. It also prevents conversion to a more stable state, which can be the less active rutile phase. This occurs due to the separation of the iron oxide nanomaterial (amorphous layers) between titania particles, thus stabilizing the anatase phase by hindering the particle growth. Subsequently, it was tested for the photodegradation of MB.²⁵¹ In another similar study, Palanisamy et al.²⁴¹ reported the use of 10–90 wt% iron oxide-based titania mesoporous photocatalysts (meso-Fe₂O₃/TiO₂) for the photodegradation of 4-chlorophenolwere. For comparison, meso iron oxide nanoparticles and meso titania nanoparticles were tested individually for the photodegradation of 4-CP.

It was found that the meso-Fe₂O₃/TiO₂ catalysts exhibited better photodegradation compared to meso iron oxide and meso titania. This was attributed to the inclusion of iron in the titania framework. Meso iron oxide can absorb more visible light compared to the heterojunction assembly but its low band gap (E_g) increases the recombination rate of electrons and holes. In last few years, a representative antibiotic named tetracycline has been widely employed for the treatment of infections in humans and animals.²⁵² However, when this antibiotic is released into water bodies, it can enter the human body, causing resistance to antibiotics.²⁵³ Accordingly, the use of titania as a photocatalyst has been reported for the degradation of many organic pollutants. In this context, Balati et al.²⁵⁴ reported the synthesis of black titania (TiO₂) to utilize light in the visible region and near infrared red region to enhance the photocatalytic performance.

Ren et al.²⁴² reported that iron oxide in a heterojunction (γ-Fe₂O₃/β-TiO₂) was proven to be very efficient for the photodegradation of tetracycline antibiotics. This efficiency was attributed to the narrowed band gap with the comprehensive consumption of visible light. Therefore, these photocatalysts permit the energy- and cost-effective treatment of waste water.²⁵⁵ In recent years, carbon quantum dots (CQDs) have been investigated due to their external surface, poor aqueous solubility, low toxicity, and chemical inertness.^256,257 In photocatalysis, they offer great absorptivity and excellent electron migration properties via the fabrication of heterostructures with other photocatalysts.²⁵⁸ In this case, Zhang et al.²⁴³ reported the synthesis of a CQD/TiO₂/Fe₂O₃ (CTF) composite. Titania can absorb light in the UV region, and thus upon the incorporation of iron oxide, a red shift to a longer wavelength was observed, thus extending its absorption to the visible region. The strong utilization of visible light and high charge carrier migration and separation were attributed to the π–π bonds between CQDs and MB. Subsequently, this photocatalyst was tested for the degradation of MB, showing 86% degradation in 180 min. According to the proposed mechanism, the electrons in iron oxide get excited and transferred to the conduction band of titania. Then, oxygen and hydroxide radicals are produced to degrade MB.²⁵⁹ In a different approach, Babar et al. used a simple calcination process at a temperature of 600 °C to form Fe₂O₃ from waste toner powder followed, by its coupling with g-C₃N₄via the polycondensation method.²⁴⁴ Similarly, Reddy et al. described the photocatalytic properties of g-C₃N₄–Fe₂O₃ in comparison to pure g-C₃N₄ and Fe₂O₃. Charge carriers were found in both g-C₃N₄ and Fe₂O₃ due to their narrow band gap. In the proposed mechanism, the photo-excited electrons in the conduction band of g-C₃N₄ are transferred to the conduction band of Fe₂O₃ with a lower E_CB of +0.33 eV compared to g-C₃N₄ with an E_CB of −1.13 eV. Simultaneously, the transfer of holes occurs from the valence band of iron oxide to that of g-C₃N₄.²⁶⁰ Li et al.²⁴⁵ reported the synthesis of Ag₂O/Fe₂O₃ flower-shaped p–n heterojunctions via the precipitation deposition method involving solvent-thermal precipitation. Subsequently, they were tested for the degradation of RhB and 4-CP in comparison of pure Fe₂O₃ under visible light. This coupling of silver oxide and iron oxide resulted in a downward shift in the energy band of iron oxide and upward shift in the energy band of silver oxide, thus resulting in the formation of a spatial charge region at the junction. Irradiation caused the transfer of photogenerated electrons from the conduction band of silver oxide to iron oxide together with holes, thus improving the separation of charge carriers.²⁶¹ All the above-mentioned heterojunctions belong to type II heterojunction but presented some drawbacks, which can be tackled by Z-scheme heterojunctions.

3.2 Z-scheme heterojunctions

The type II heterojunctions exhibit some drawbacks, which can be overcome using another type of heterojunction called Z-scheme heterojunction. Z-Scheme photocatalysts are constructed based on the theory of photosynthesis, which precisely mimic the process of photosynthesis in plants. An iron-based Z-scheme heterojunction can be created by utilizing two semiconductors with an appropriate intermediate coupling. The material has a staggered structure due to the presence of Fe³⁺/Fe²⁺ ions. In this mechanism, the formation of holes in the valence band of PC I (photocatalyst-I) is potentially attributed to light. Subsequently, these holes undergo reactions with electron donors, leading to the generation of the corresponding electron acceptors. Likewise, the electrons in the valence band of PC II (photocatalyst II) generated by radiation interact with electron acceptors, leading to the involvement of donors. Subsequently, the photogenerated electrons still present in the conduction band of PC I and the holes in the valence band of PC II participate in distinct oxidation and reduction reactions. As a result of these interactions, the entire system has strong capacity to undergo redox reactions at distinct locations.²⁶²

Thus, Z-scheme heterojunctions show the advantages of efficient separation of charge carriers and good redox ability compared to other heterojunction based photocatalyst.²⁶³ There are three types of Z-scheme photocatalysts. The first is the typical Z scheme with a redox ion pair (Fe³⁺/Fe²⁺), which operates as charge carriers. The second is the all-solid-state Z-scheme, which involves the inclusion of a solid electron mediator such as silver and gold nanoparticles to aid in charge carrier transfer. The third is the direct Z scheme heterojunction, which does not require a mediator, and thus the charge transfer, as indicated by its name, happens directly via an internal electric field.²⁶⁴ This scheme inspired many researchers to fabricate different products including iron-based composites. In this context, Huo et al.²⁶⁵ reported the preparation of an iron oxide-coupled MOF (MIL-101 (Cr)) to form a direct α-Fe₂O₃/MIL-101 (Cr) Z scheme. Both have been proved to be efficient photocatalysts, thus making the perfect combination; however, they have the limitations of the large band gap of the MOF and insufficient surface area of iron oxide. This heterojunction showed a better performance compared to its individual components. This composite was prepared via a hydrothermal method for the degradation of carbamazepine (CBZ). In this process, electrons gather in the conduction band of MIL-101 (Cr) having a potential of 1.33 V, while the holes accumulate in the valence band of α-Fe₂O₃ with a potential of 2.39 eV. The negative potential of MIL-101(Cr) compared to O₂/O₂⁻ causes the reaction of the electrons in the conduction band MIL-101(Cr) with O₂ to create oxygen radicals. Alternatively, the formation of hydroxide radicals happens via the holes residing in the valence band of iron oxide with a positive potential as compared to OH⁻/OH and H₂O/OH, leading to a strong reduction and oxidation potential in the Z-scheme.

Balu et al.²⁶⁶ described the synthesis of a ternary Z-scheme (g-C₃N₄/ZnO@α-Fe₂O₃) for the photodegradation of tartrazine (Acid Yellow 23). Less charge recombination was proven for g-C₃N₄/ZnO@α-Fe₂O₃ as compared to the pristine g-C₃N₄via a PL study, where its small band gap of 2.6 eV enabled it to harvest more visible light. This composite showed 99.34% degradation compared to its individual components. All-solid-state Z-schemes require the presence of a mediator with a high negative and positive potential compared to photosystem I and photosystem II.²⁶⁷ In this context, graphene oxide was tested as a mediator due to its stability, morphological flexibility, cost effectiveness and high density of active sites.²⁶⁸ Mohamed et al.²⁶⁹ constructed an all-solid-state Z-scheme heterojunction of GO-mediated Fe₂O₃/GO/WO₃ for the photodegradation of phenolic dyes and phenol. The ternary composite was proven to be more effective compared to the individual and binary composite of GO-modified iron oxide, GO-modified tungsten oxide and tungsten oxide-modified iron oxide. In this composite, the strong oxidation potential of PS-I and strong reduction of PS II were maintained by the migration of photoexcited electrons in the conduction band of tungsten oxide (PS-I) to the valence band of iron oxide (PS-II), thus discouraging the charge recombination. In another Z-scheme, a simple one-pot synthesis was reported by Kang et al.²⁷⁰ to form Fe₂O₃/C-g-C₃N₄. This composite was tested for the degradation of Rhodamine B dye (RhB).

Carbon-modified composites show greater activity compared to non-modified composites, thus indicating that the amorphous carbon layers play a major role in photodegradation. Actually, the Z-scheme mechanism happens in carbon-modified composites. The Z-scheme (carbon modified) showed high hole movement compared to the type II heterojunction (un-modified).²⁷¹ Jiang et al.⁴⁹ reported that an all-solid-state Z-scheme of lead halide perovskite (PVK) was deemed a promising photocatalyst alternative because of its remarkable photoelectrical properties, where the severe charge recombination limits the catalytic activity of catalysts. Zhang et al.⁵⁰ reported the preparation of a Z-scheme heterojunction (Fe₂O₃/TpPa-2-COF) composed of a covalent–organic framework (COF) for the photocatalytic evolution of hydrogen, as shown in Fig. 9. This composite showed 53 times higher hydrogen evolution compared to TpPa-2-COF, which is comparable to that of Pt (co-catalyst). This COF-based Z-scheme heterostructure is an efficient noble metal-free photocatalyst for hydrogen evolution, paving the way for further research on noble metal-free photocatalysts for hydrogen evolution.

Fig. 9 Schematic illustration of the synthesis of a series of Fe₂O₃/TpPa-2-COF hybrid materials. Reproduced from ref. 50 with permission. ©The Royal Society of Chemistry, 2020.

Similarly, Zhang et al.⁴⁷ reported the synthesis of a steady organic–inorganic Z-scheme heterojunction, which involved linking crystalline covalent organic frameworks (COFs) with different semiconductors for artificial photosynthesis, as shown in Fig. 10. In this case, water oxidation was observed to be performed by semiconductors of TiO₂, Bi₂WO₆, and α-Fe₂O₃. Here, CO₂ reduction was performed by COF-316/318, showing a high photocatalytic conversion of CO₂-to-CO with H₂O acting as the electron donor without any photosensitizers or sacrificial agents. This was the first report in the utilization of covalently bonded COF/inorganic–semiconductor systems having Z-scheme for artificial photosynthesis.

Fig. 10 Schematic representation of the preparation of COF-318-SCs via the condensation of COF318 and semiconductor materials. Reproduced from ref. 47 with permission from©2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

3.3 S-scheme heterojunctions

Here, we reviewed type II heterojunctions and Z scheme heterojunctions for photocatalysis in the presence of iron-based nanomaterials but these heterojunctions have some fundamental problems. In the Z-scheme, direct Z-scheme share the shortcomings of the traditional Z-scheme and all-solid-state Z-scheme photocatalysts. Thus, to overcome this, it is better to replace the Z-scheme and type II heterojunction with another scheme that describes the mechanism of photocatalysis more clearly. This has been done in the past and named S-scheme (step-scheme) heterojunction. Depending on the band structure, there are two types of photocatalysts involved in the S-scheme heterojunction, reduction photocatalyst (RP) and oxidation photocatalyst (OP), as shown in Fig. 11(a). The reduction photocatalyst, which has a high conduction band, has been used in the production of solar fuels. In this type of heterojunction, the production of electrons under irradiation is desirable, whereas the non-essential holes are eliminated using a sacrificial reagent. In contrast, oxidation photocatalysts have been used to address issues related to the environment such as pollutant degradation. In oxidation photocatalysts, the photogenerated holes are the key agents. S-scheme heterojunctions have both oxidation and reduction photocatalysts with a staggered band structure. S-Scheme heterojunctions are actually quite similar to type II heterojunctions but have a different charge transfer mechanism (Fig. 11(b) and (c)). Type II heterojunctions have the shortcoming of weak redox ability, whereas S scheme heterojunctions show strong redox ability due to the reserved photogenerated electrons and holes in the conduction band of the oxidation photocatalyst and valence band of the reduction photocatalyst, respectively.²⁷²

Fig. 11 Comparison of charge transfer between type-II and S-Scheme heterojunctions. (a) Band structures of oxidation and reduction photocatalyst, (b) charge transfer in type-II heterojunction and (c) charge transfer in S-scheme heterojunction. Reproduced from ref. 272 with permission of ©2020 Elsevier Inc.

As iron-based nanomaterials, bismuth-doped zinc ferrite nanoparticles with (Bi–ZnFe₂O₄/S-g-C₃N₄) and without sulfur-doped graphitic carbon nitride support were synthesized as photocatalysts via the hydrothermal method and tested for the photodegradation of MB dye via the S-scheme. In this case, 9% Bi–ZnFe₂O₄ showed the maximum activity for the degradation of methyl blue. Subsequently, it was mixed with different amounts of S-g-C₃N₄ to check its effect on photodegradation. Here, the composite with 30% C₃N₄ showed a high performance.²⁷³ In another report on an S-scheme heterojunction, a magnetically recoverable α-Fe₂O₃/g-C₃N₄/SiO₂ photocatalyst was synthesized via a thermal polycondensation and calcination process, respectively, for the removal of RhB dye in the presence of H₂O₂. Here, 97% degradation was reported for the degradation of RhB within 120 min. The mechanistic pathway showed that a series of N-deethylated products was generated, followed by ring-opening during the degradation of RhB, which presented the possibility that photosensitization induced photocatalysis.²⁷⁴ Similarly, Akshhayya, C., et al. fabricated MnO₂ and CuFe₂O₄-based nanocomposites (NCs) via a co-precipitation method towards the photodegradation of methyl blue dye. The degradation rate of the composite material (0.0011 min⁻¹) was 2.75- and 2.2-fold high than that of the single CuFe₂O₄ (0.004 min⁻¹) and MnO₂ (0.005 min⁻¹) photocatalysts, respectively. This research revealed insight into S-scheme heterojunction photocatalysts that maintain e⁻ and h⁺ in the conduction band of RP and the valence band of OP, showing a high redox potential. The efficiency of reusability was found to be 98.91% up to the 4th cycle.²⁷⁵ A unique Fe₂WO₆/SrTiO₃ S-scheme composite was reported using a two-step hydrothermal method. Under visible light exposure, the best performance was demonstrated by 20 wt% Fe₂WO₆/SrTiO₃ for the photodegradation of RhB dye. Fe₂WO₆/SrTiO₃ showed 1.9- and 2.7-fold higher photocatalytic activity due to the S-scheme integration of single Fe₂WO₆ and pristine SrTiO₃, respectively.²⁷⁶ An S-scheme-based heterojunction has also been used for the reduction of H₂S molecules to S and other products. For this purpose, SiO₂ nanospheres were sandwiched between layers of COF modified with iron oxide nanoparticles (Fig. 12). The core–shell SiO₂@Fe₂O₃@COF composite was fabricated by adopting a multistep process, mainly Stöber and double ligand-regulated solvent heating. The S-scheme heterojunction effectively enhanced the photoinduced charge carrier separation according to the energy band structure analysis.²⁷⁷ Photocatalytic water splitting via S-scheme heterojunctions has emerged as a novel topic of research. In this context, lanthanum orthoferrite (LaFeO₃), a perovskite-oxide (ABO₃)-type structure, was prepared because of its higher chemical stability, narrower band energy and low cost as a catalyst; however, it exhibited the drawbacks of high electron–hole pair recombination.

Fig. 12 S-Scheme photocatalysis in the presence of SiO₂@Fe₂O₃@COF for H₂S reduction. Reproduced from ref. 277 with permission. ©2023, Published by Elsevier Inc.

Accordingly, reduced graphene oxide nanosheets were integrated in it for photocatalytic water splitting under visible light illimitation to overcome the limitation of LaFeO₃, which induced charge separation. This nanocomposite demonstrated ∼21 times higher photocurrent density compared to pure LaFeO₃. Furthermore, the integration of RGO with LaFeO₃ caused a prominently increase in the separation/transfer of photoinduced charge carriers, which endowed the LaFeO₃/RGO S-scheme heterojunction with a remarkable H₂ production performance of ∼82 mmol g⁻¹ h⁻¹, which was higher in comparison to that of the bare LaFeO3 of ∼9 mmol g⁻¹ h⁻¹. These studies provide a novel strategy for modulating highly well-organized 1D–2D heterojunctions for both practical and photocatalysis applications.²⁷⁸

4 Applications of iron-based photocatalysts

Iron-based nanocomposites have mainly been adopted as catalysts for the degradation of environmental pollutants. Alternatively, recently, iron-based photocatalysts were also used as photocatalysts for energy conversion, hydrogen generation, CO₂ reduction, nitrogen fixation and pollutant degradation (Fig. 13 and Table 7).

Fig. 13 Photocatalytic applications of iron-based materials and nanomaterials.

Table 7 Application of iron-based composites as photocatalysts

Photocatalyst material	Synthesis method	Application	Photocatalytic performance	Ref.
CoF–Fe2O3	Simple method	Hydrogen evolution reaction	3.3 mmol h^−1 g^−1	50
CoF–TiO2/Bi2WO6/Fe2O3	Condensation method	CO2 reduction	69.67 mmol g^−1 h^−1	47
α-Fe2O3/amine-RGO/CsPbBr3	Solvent evaporation deposition method	CO2 reduction	3.132.46 μmol g^−1	49
Fe2O3-g-C3N4 Z-scheme	Heating Fe–melamine supramolecular framework precursor	Photodegradation of RhB	100%	279
α-Fe2O3/MIL-101 (Cr) Z scheme	Hydrothermally prepared	Carbamazepine (CBZ) photodegradation	100%	265
g-C3N4/ZnO@α-Fe2O3	Direct pyrolysis and sol–gel methods	Photodegradation of tartrazine	99.34%	266
Fe2O3/GO/WO3	Ex situ and ultrasonic	Photodegradation of crystal violet (CV), MB, and phenol	98%	269
MnFe2O4/TA/ZnO	Hydrothermal method	Degradation of CR	84.2%	280
Bi25Fe1−xCrxO40	Hydrothermal process	Photo-degradation	92.5%, 120 min	281
FePc/BiOBr	Facile method	Removal of tetracycline & ciprofloxacin		282
ZrO2/Fe2O3/rGO	In situ-hydrothermal method	Degradation of CR & AP	98.43% in 60 min and 97.76% in 80 min	283
g-C3N4/ZnO@α-Fe2O3/Z-Scheme	Direct pyrolysis & sol–gel method	Photodegradation of tartrazine	99.34%	266
CST/γ-Fe2O3-BT	Green synthesis	BPA photocatalytic degradation	0.00104 min^−1	284
TiO2-N-doped LaFeO3	Wet-chemical synthesis	CO2 reduction	CH4 (∼110 μmol), CO (∼150 μmol)	285
MgO/LaFeO3:Er^3+	Chemical method	CO2 reduction	CO (∼71.52 μmol g^−1 h^−1) CH4 (∼5.54 μmol g^−1 h^−1)	286
Copper phthalocyanine (CuPc)/α-Fe2O3	Wet-chemical synthesis	CO2 reduction	CO (∼17 μmol g^−1) CH4 (∼4 μmol g^−1)	287
α-Fe2O3/g-C3N4	Wet-chemical synthesis	CO2 reduction	CO evolved ∼27.2 μmol g^−1 h^−1	288
Sr-doped LaFeO3/Bi4O5Br2	Chemical method	CO2 reduction	CH4 evolved ∼10.14 μmol g^−1	289
LaFeO3/CdS-x	Sol–gel and solvothermal	Levofloxacin degradation	97.3% in 100 min	290
LaFeO3/montmorillonite	Sol–gel method	RhB degradation	99.34% in 90 min	291
LaFeO3/TiO2	Hydrothermal method	MO degradation	∼95% in 180 min	292
Graphene-supported LaFeO3	Chemical method	Hydrogen production	3388 μmol gcat^−1	293
PANI/MgFe2O4	Simple synthesis	10 mg	Visible light	294
Fe2O3/C-g-C3N4 (F-Cg) Z-scheme	One-step carbonizing process	Photocatalytic degradation of RhB	0.043 min^−1	270
NaFeZr-MOR composite	Hydrothermal method	CO2 reduction	1.3% and 12.2 (mmol gcat^−1 h^−1)	295
Iron doped CeO2	Combustion method	CO2 reduction	9.0 and 7.7 folds than that of CeO2	296
ZnFe2O4/FeP-CTFs	Via ionic liquid (ionothermal method)	CO2 reduction	178 μmol h^−1 g^−1	297
Sb-modified Bi25FeO40	Hydrothermal method	N2 fixation	NH3 yield 2.62 μg h^−1 cm^−2	298
C3N4/ZnFe2O4	Hydrothermal method	N2 fixation	NH3 yield 147.33 μmol L^−1	299
Ferriporphyrin-based metal–organic framework (MOF) PCN-222(Fe)	Green synthesis	N2 fixation	NO3^− yield 110.9 μg h^−1 mgcat^−1	300
Pristine graphitic carbon nitride (PCN, g-C3N4)/cerium ferrite (CeFeO3, CFO) composites	Calcination method	N2 fixation	573.12 μmol l^−1 g^−1	301
FeIn2S4/palygorskite	Microwave hydrothermal method	N2 fixation	583 μmol g^−1	302
Fe/Zr-MOFs	—	N2 fixation	49.8 μmol g(cat)^−1 h^−1	303
TiO2/MIL-88A(Fe)/g-C3N4	Hydrothermal method	N2 Fixation	1084.31 μmol (g h)^−1	304
Vanadium-doped iron polysulfide/C3N5	—	N2 Fixation	1916.5 μmol h^−1 gcat^−1	305
(Fe3C/Fe@PCNF-F)	Electrospinning sol gel method	N2 Fixation	29.2 μg h^−1 mgcat^−1	306

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4.1 Oxygen/hydrogen evolution reaction (OER/HER)

Concerns about the depletion of fossil fuels and rising contamination have heightened the interest in the advancement of clean and sustainable energy approaches.

In this case, Li et al.³⁰⁷ reported the design of noble metal-free cocatalysts, which played a crucial role in facilitating the photocatalytic conversion of solar energy. In this study, photocatalysts having a heterostructure of MoS₂-transition metals (Fe, Co, Ni) on g-C₃N₄ nanosheets were prepared, with a significant improvement in photocatalytic hydrogen production and pollutant degradation. Cheng et al.³⁰⁸ examined precious metal-free cocatalysts, which are very desirable to improve the hydrogen evolution reaction. It was observed that the noble metal-free Fe₂P–Co₂P cocatalyst enhanced the photocatalytic activity of graphitic carbon nitride (g-C₃N₄). In the study by Pan et al.,³⁰⁹ they prepared iron phosphide-based nanoparticles together with cadmium sulphide for hydrogen production.

In this study, ethanol employed as an electron donor at high pH, thus giving the hydrogen evolution of about 200 mmol g⁻¹ h⁻¹. Here, electrons were transferred from the conduction band of cadmium sulphide to the valence band of iron phosphide at the rate of 7.4 × 10⁻⁹ s⁻¹ with a faster rate of hole scavenging compared to electron relaxation in cadmium sulphide. Qi et al.³⁷ described the synthesis of a C₃N₄-modified Fe₂O₃@FeP material by annealing, which involved the phosphidation of a g-C₃N₄/Fe metal–organic framework (MOF). In this study, Fe₂O₃@FeP was also employed as a co-catalyst. This composite showed improved photocatalysis compared to g-C₃N₄, which was ascribed to the improved intensity of visible-light absorption, the type II heterojunction with iron oxide and iron phosphide enhancing the e–h separation and FeP acting as a co-catalyst, thus accelerating the hydrogen ion reduction. Sun et al.³¹⁰ reported the preparation of iron, cobalt and nickel-based phosphides for hydrogen production via photocatalysis. An assembly of M₂P/S-C₃N₄ (M = Ni, Fe, Co) was formed to study the effect of different metal phosphides for photocatalytic H₂ generation, where the metal phosphides acted as co-catalysts. In another study, Zhao et al.²⁵⁵ reported the synthesis of polymeric 2D g-C₃N₄ nanostructures. However, it exhibited the drawbacks of quick recombination of charge carriers together with unsatisfactory solar-to-hydrogen efficiency. This study prompted researchers to investigate the synthesis of cost-effective and efficient co-catalysts to speed up the charge transport. In a similar study reported by Zhu et al.,⁴³ FeP was successfully decorated on the surface of the Zn_xCd_1−xS photocatalyst through an in situ phosphating process. Xu et al.³¹¹ described the construction of cocatalysts while maintain a suitable interface to facilitate the separation of charge carriers for photocatalytic hydrogen production with an apparent quantum yield of 45.8% at 420 nm. Bazri et al.³¹² developed a ternary heterostructure (rGO-α-Fe₂O₃/β-FeOOH) for photoelectrochemical water splitting. It was observed that rGO-α-Fe₂O₃/β-FeOOH exhibited an enhanced OER performance due to its excellent potential to achieve a high rate at low energy consumption. An Fe₃O₄@Fe₂O₃–TiO₂ complex oxide was designed to tune the band gap of Fe₂O₃–TiO₂ for hydrogen evolution. Aniline hydrochloride as a proton source was responsible for the enhanced hydrogen evolution rate (2.366 mmol/0.5 g h⁻¹). The observed mechanism revealed that methanol together with aniline hydrochloride played an important role in efficient hydrogen evolution (98.30%), which was almost 74% under sunlight. This composite had the merits of low cost, easy preparation, high stability and nontoxicity.³¹³ The electrocatalytic activity was improved via an eco-friendly approach by synthesizing FeP nanorod arrays with a polypyrrole (PPy) shell coating on carbon textiles (CTs) (Fig. 14).³¹⁴ Polypyrrole was used in this composite due its high conductivity and it decreased the activation energy barrier, thus enhancing the electrocatalytic activity. In another study, an ohmic/step scheme heterojunction (2D-2D Zn_0.7Cd_0.3S (ZCS)-Fe₂O₃/Ti₃C₂) was synthesized for high HER activity (Fig. 15). Here, the S-scheme is responsible for the effective promotion of charge separation between the ZCS nanosheets and Fe₂O₃ nanosheets with 2D Ti₃C₂ MXene nanosheets as co-catalysts.³¹⁵

Fig. 14 Ice-sugar gourd-like structured FeP@PPy core–shell nanorod arrays on carbon textiles, with effective HER performance. Reproduced from ref. 314. ©2021 Elsevier B.V. All rights reserved.

Fig. 15 Ohmic/step scheme heterojunction of Ti₃C₂ nanosheet-decorated 2D-2D Zn_0.7Cd_0.3S (ZCS)–Fe₂O₃ for HER. Reproduced from ref. 315 with permission from©2021, Elsevier B.V.

In oxygen evolution, a molybdenum nitride/molybdenum oxide (Fe–Mo₅N₆/MoO₃-550) composite-based electrocatalyst showed an excellent OER performance with current densities of 10 and 20 mA cm⁻² at overpotentials of 201 and 216 mV, respectively.³¹⁶ A two-step method (hydrothermal and pyrolysis) was adopted for the synthesis of Fe–Co/NC for hydrogen evolution reaction and oxygen evolution reaction. The improved electrocatalytic activity was attributed to the presence of porous nanocarbon having iron/cobalt. This successful combination paved the way for the design of advanced electrocatalysts for water splitting via doping and interface engineering.³¹⁷

4.1.1 Role of co-catalyst in HER/OER. Issues related to the energy crisis can be solved by photocatalytic/catalytic water splitting for the manufacture of solar fuels. In this case, semiconductors with a narrow band gap are preferred for photocatalytic water splitting to attain the efficient charge separation and migration of charge carriers. Also, they exhibit good thermodynamic feasibility for photo-generated electrons and holes to carry out the water separation reaction but they can recombine in the absence of suitable reactive sites. Therefore, it is imperative to enable surface reactions through catalytic processes. The entire water splitting reaction is a thermodynamic uphill reaction (ΔG° = 237 kJ mol⁻¹ = 1.23 eV), which is comprised of several electron transfers processes.^232,318 In this case, the investigation of the hydrogen evolution and oxygen evolution half-reactions in the presence of sacrificial electron donors and acceptors is necessary to filter the best photocatalysts, thus leading to an effective photocatalytic/catalytic water separation system. Thus, to understand this better, here, we categorize different types of catalysts/photocatalysts, which will be helpful for future researchers to form desirable catalysts for achieving the required results. The co-catalyst is an alternative significant constituent in all water splitting systems (Table 8). The main role of the co-catalyst is to deliver active sites for the redox reaction, which reduce the activation energy to speed up the rate of the reaction. Moreover, co-catalysts can also enhance the sturdiness of the photocatalyst by effectively using photo-induced charge carriers and by protecting the semiconductor surface from oxidation via holes.³¹⁹ Also, the co-catalyst components tend to combine with that of the electrocatalyst. Specific metal/metal oxide-based nanoparticles located on the surface of photocatalysts have the tendency to act as co-catalysts.^51,311,320

Table 8 Summary of iron-based photocatalytic cocatalysts

Photocatalytic materials	Synthesis method	Co-catalyst	Light source	Mass [g]/(solution)/volume [mL]	Application/product	Photocatalytic efficiency	Ref.
g-C3N4/FexP	Two-step hydrothermal and phosphidation method	FeP	300 W Xe lamp, UV cut off filter (λ > 420 nm)	10 mg catalyst 9 mL, 1 mL of triethanolamine (TEOA)	H2 production	166.4 μmol g^−1	321
Fe–Pi/Fe2O3	Simple chemical precipitation	Fe–Pi cocatalyst			OER		322
Fe/Ru oxide/Bi4TaO8Cl	Impregnation method	Fe–Ru oxide	Xe lamp L-420 cut-off filter visible light (400 < λ < 800 nm)	Catalyst 0.2 g, 250 mL of FeCl3 solution (4 mM, pH 2.5 adjusted with HCl)	OER	16 μmol g^−1	323
MoS2/(Fe, Co, Ni)/g-C3N4	Simple hydrothermal method	Fe, Co and Ni		20 mg catalyst 420 light	H2 production	1.7, 4.1 and 5.12 mmol h^−1 g^−1	307
Fe2P–Co2P/g-C3N4	In situ phosphating procedure.	Fe2P–Co2P	300 W Xe lamp 420 nm cut off filter	10 mg catalyst, 20 mL, 10% TEOA	H2 evolution	347 μmol h^−1 g^−1	308
Fe2P/CdS nanosheets	Mechanical mixing method	Fe2P	300 W Xe arc lamp, UV-cut off filter (= 420 nm)	1.5 mg Fe2P, 8.5 mg CdS NSs, 10 mg L-cysteine, (80 mL) ethanol (20 mL) at pH 14.9	H2 production	220 mmol g^−1 h^−1	309
g-C3N4/Fe2O3@FeP	Self-assembly-pyrolysis and phosphidation methods	Fe2O3@FeP	300-W Xe lamp with cut-off filter (λ = 420 nm) visible-light	10 mg catalyst in 100 mL, 10% of triethanolamine (TEOA)	H2 production	12.03 mmol g^−1 h^−1	37
M2P, M = Fe, Co, and Ni/g-C3N4	Co-precipitation method	Fe, Co, Ni	Xe light source 400, 440, 480, or 520 nm	2 mg catalyst 5 mL of 10 vol% TEOA	H2 generation	Ni2P/SeCN 0.41 μmol h^−1 at λ = 400 nm	310
FeP/g-C3N4	Low-temperature phosphidation method	FeP	300 W Xe lamp 420 nm cutoff filter	10 mg catalyst 50 mL triethanolamine (10 vol%)	H2 generation	215 μ mol g^−1 h^−1	255
FeP/Zn0.5Cd0.5S-P	In situ phosphating process		300-W xenon arc lamp 420-nm cut-off visible light.	10 mg catalysts 10 mL, 9 mL water, 1 mL lactic acid (sacrificial agent)	H2 generation	24.45 mmol g^−1 h^−1	43
CN/FeNiP/g-C3N4	Phosphatized	Fe2P/Ni2P	300-W Xe lamp 420 nm cut-off filter visible-light	10 mg catalyst 80 mL of 10% triethanolamine (TEOA) 1 mmol L^−1 of EY as photosensitizer.	H2 generation	13.81 mmol g^−1/sensitization Eosin Y (EY)	311
rGO-α-Fe2O3/β-FeOOH	One-pot hydrothermal-ionic liquid				OER		312
N@FexOy@MoS2	Sol–gel method	FexOy	300-W xenon arc lamp 420-nm cut-off visible light.		OER	99%	324

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Cocatalysts frequently stimulate HER or OER, and therefore named reducing or oxidation cocatalysts, respectively. However, these materials can also increase the oxygen reduction reaction and hydrogen oxidation reaction, which will be favoured over HER/OER on a thermodynamic basis.^325,326 Also, it is necessary for these materials to possess the characteristics of chemical stability under photo irradiation in water and sufficient carrier mobility.^1,327 Some materials even exhibit a high apparent quantum yield (AQY) of several tens of percentages in the ultraviolet region after modification with appropriate cocatalysts,^328–330 which were only responsive to 4% of the solar spectrum (UV region). In this case, a wider range of the spectrum can be utilized by constructing photocatalysts with narrower band gaps. Since 2000, significant efforts have been dedicated to the formation of sensitive and selective photocatalysts for the absorption of light in the visible light region for overall water splitting (OWS) via the synthesis of novel semiconductor materials and modification of existing materials.^{327,331–334}

In recent years, numerous metal composites have been reported to act as co-catalysts, among which iron-based materials are the most abundant to act as cocatalysts.^335,336 For example, Du et al.³³⁷ reported the use of the noble metal-free Fe₂P as an active cocatalyst for photocatalytic H₂ production under visible light. Fu et al.³³⁵ reported the synthesis of an FeP/CdS composite as a photocatalyst with H₂ evolution activity, in which FeP acted as a co-catalyst. Lewis et al.³³⁶ also confirmed the co-catalytic ability of FeP when deposited on the surface of TiO₂ for the prolonged evolution of H₂ under irradiation with UV light.

Zhao et al.³²¹ used iron phosphide as a co-catalyst with carbon nitride (Fe₂P/g-C₃N₄) for hydrogen evolution. Lie et al.³²² used co-precipitation to form Fe–P at the surface of hematite. Argon treatment on Fe–P/Fe₂O₃ introduced oxygen vacancies on phosphorous. In this case, Z-scheme photocatalysis was used for hydrogen evolution. Nakada et al.³²³ overcame the drawback of backward oxidation of Fe²⁺via the modification of the photocatalyst by co-loading Fe/Ru oxide for better water oxidation. The recent advances in water-splitting photocatalysts/catalysts are summarized in detail in the following sections for both reduction and oxidation processes, and further, the role of iron-based materials as heterostructure and Z-scheme heterojunction-based catalysts is described below.

4.2 Pollutant degradation

In the case of pollutant degradation, heterogenous photocatalysis has been found to be best choice due to its natural consumption ability and renewability. This is done to reduce the toxicity of wastewater to a desired concentration before discharging it into water bodies. Recently, the use of carbon-based materials and iron-based materials as main products or as supporting products has become popular for wastewater treatment. Boutra et al.²⁸⁰ presented the coupling of magnetic manganese ferrite (MnFe₂O₄) to non-magnetic zinc oxide (ZnO), enabling the reliable separation of the photocatalyst from wastewater after treatment, thus enhancing its utility. In this work, the hydrothermal method was used to prepare a composite of ZnO/MnFe₂O₄/tannic acid for the degradation of Congo red dye.

Xiong et al.²⁸¹ reported the use of the hydrothermal method for the synthesis of Cr-doped Bi₂₅FeO₄₀ product (Bi₂₅Fe_0.75Cr_0.25O₄₀). Yin et al.²⁸² reported the synthesis of iron phthalocyanine/bismuth oxybromide (FePc/BiOBr) for the photodegradation of tetracycline and ciprofloxacin. Iron phthalocyanine is responsible for enhancing the visible light absorption of bismuth oxybromide. Anjaneyulu et al.²⁸³ reported the synthesis of a ternary composite of ZrO₂/Fe₂O₃/rGO for the photocatalytic degradation of Congo Red and acetophenone via visible light irradiation. In another similar work, Balu, et al.²⁶⁶ reported the synthesis of a Z scheme photocatalyst of g-C₃N₄/ZnO@α-Fe₂O₃via direct pyrolysis and sol gel method for the degradation of tartrazine. This composite showed the degradation of 99% within 35 min.

Cao, et al.²⁸⁴ used green chemistry for the synthesis of sulfur-doped titanium dioxide modified by surfactant loaded on magnetic bentonite (CST/γ-Fe₂O₃-BT) for the degradation of bisphenol A (BPA). An external magnetic field was used to recover this composite from the reaction medium due to the presence of iron oxide. Bashir et al.²⁹⁴ reported the photocatalytic removal of indigo carmine dye by a PANI/MgFe₂O₄ nano-ferrite composite, showing the photo-degradation efficiency of 97.52% compared to nano ferrite composites (94.36%).

An oxygen vacancy (OV)-functionalized iron-based composite composed of iron oxide-carbon-vermiculite (OV-ICV) was prepared using an iron-rich self-heating waste bag and tested for the degradation of micropollutants through the activation of peroxydisulfate (PDS). Briefly, 0.1 g L⁻¹ of OV-ICV was used to remove 95% of 1.0 mg L⁻¹ carbaryl (CB) within 30 min. The oxygen vacancies generated more active sites and localized electrons, thus promoting the charge transfer ability.³³⁸ Similarly, another photocatalyst named sulphur-doped iron composite having Fe–S@CN with a distribution of FeS and Fe₃C over carbon nanosheets was used for the removal of carbamazepine within 30 min. It was found that different radicals such as hydroxyl radicals (OH˙), sulfate radicals (SO₄˙⁻) and total singlet oxygen (¹O₂) and superoxide radicals (O₂˙⁻) showed the contribution of about 8.7%, 19.2% and 72.1%, respectively, for carbamazepine removal.³³⁹ Metal organic frameworks (MOFs) have been used for the degradation of many pollutants, and thus in the present work, an iron-based MOF was used for the removal of tetracycline hydrochloride. Nitrogen-doped carbon dots were composited with g-C₃N₄/α-Fe₂O₃ (CNFO) for the photoelectrochemical (PEC) degradation of trimethoprim and hydrogen evolution (Fig. 16).³⁴⁰

Fig. 16 Trimethoprim removal by N-doped carbon dot/g-C₃N₄/α-Fe₂O₃ composite (CNFO). Reproduced from ref. 340 with permission from©2022, Elsevier B.V.

Among the Fenton catalysts, the rich active sites, good computability and tunable structure of poly(ionic liquids) (PILs) make them desirable entities for coupling with different substances. In the present case, an iron-based PIL/polydopamine (PDA) composite (PDVIm-Fe/PDA) was formed for the removal malachite green. The removal rate of about 99% was attributed to the rich iron content, presence of OH˙ and presence of SO₄˙⁻ radicals. Its good stability and adoptability make it the best candidate for commercial applications in wastewater treatment.¹⁹³

4.3 CO₂ reduction

Presently, the world is facing an energy crisis with acute environmental problems including carbon oxide emission, and thus the conversion of carbon dioxide offers a solution to both issues via an ecofriendly photocatalytic process.³⁴¹ Due to its importance, significant research has been devoted to the utility of iron-based materials for the photo-reduction of CO₂.^308,342,343 In this context, α-Fe₂O₃/g-C₃N₄ (FCN) was reported to be synthesized via a hydrothermal method for the production of CH₃OH via the reduction of CO₂. The exceptional photocatalytic activity of the composite compared to its individual products was attributed to the thinner band gap of the FCN hybrid than bare g-C₃N₄, thus leading to the better absorption of visible light.

Wang et al.³⁴² described the one-pot synthesis of α-Fe₂O₃–ZnO rod/rGO via an electrochemical growth method for the photoreduction of CO₂ to CH₃OH under visible light. In this case, iron oxide as a sensitizer has increased the capability of ZnO to absorb visible light. The rGO nanosheets offer a large surface area and high migration of charge carriers. rGO was used to increase the migration rate of charge carriers at the surface of ZnO. Similarly, Song et al.²²⁵ reported the synthesis of α-Fe₂O₃/LaTiO₂N for carbon dioxide reduction without sacrificial agents. In this case, α-Fe₂O₃/LaTiO₂N produced the yield of 29.0 and 38.0 μmol g⁻¹ of CO and CH₄, respectively, under illumination for 3 h (Fig. 17).

Fig. 17 Photocatalytic conversion of CO₂ by LaTiO₂N-coupled α-Fe₂O₃. Reproduced from ref. 225 with permission from©2021, Elsevier B.V.

To realize carbon dioxide reduction, Wang et al.³⁴⁴ studied the synthesis of g-C₃N₄/α-Fe₂O₃ bridged by Al–O (Z-scheme) nanocomposites with an aqueous solution as the Al–O source. In this case, the electrons in the conduction band of iron oxide with low reduction potential were transferred to the valence band of g-C₃N₄ having a low oxidation potential, thus enhancing the charge separation.

During this process, the strong oxidation holes were preserved in the valence band of iron oxide and strong reduction electrons were preserved in the conduction band of g-C₃N₄. In an another work, Bagvand et al.³⁴⁵ reported the photocatalytic efficiency of BiFeO₃/ZnS, which was synthesized via the hydrothermal method. Yang et al.³⁴⁶ studied the effects of using additives such as Na and Cu for increasing the efficiency of Fe-based catalysts for direct CO₂ reduction to valuable hydrocarbons. However, the selective hydrogenation of CO₂ toward aromatics is still challenging. In this case, hierarchical nanocrystalline HZSM-5 comprised of Fe₂O₃ with promoters of Na and Cu was fabricated for CO₂ hydrogenation. Consequently, 33.26% CO₂ conversion was achieved with the aromatic selectivity of about 57.74% with the amount aromatics in the liquid products reaching up to 94.81%. Addition of Na and Cu in iron oxide activated the iron species and converted the intermediates. Moreover, the mesoporous nature and suitable acidity of HZSM-5 due to the post-treatment modification led to efficient aromatic selectivity and stability.

Sarabia et al.³⁴⁷ used the hydrothermal method to synthesize CuFeO₂ oxide using precursors of Cu₂O and FeOOH with the mineralizer of NaOH. In this case, the highest uptake of carbon dioxide was observed in the sample with the highest ratio of rhombohedral and hexagonal delafossite phases. In another work by Jiang et al.,³⁴⁸ a comparable statement and mechanism were reported for the photo reduction of carbon dioxide employing a direct Z-scheme system having urchin-like α-Fe₂O₃ and g-C₃N₄. Chen et al.³⁴⁹ synthesized a highly selective iron-based composite via the ball milling method for CO₂ hydrogenation. Under the conditions of 1.0 MPa, 320 °C and 9600 mL h⁻¹ g _cat⁻¹ CO₂, the conversion of 32.1% was reported for this catalyst with a selectivity of about 55.4%. This performance was observed to be high compared to other catalysts working under the same conditions. In this catalyst, O–Fe/Mg–O was formed after the incorporation magnesium into iron oxide (Fe₃O₄), which contributed to the adsorption and activation of CO₂ to CO.

4.4 N₂ fixation

Ammonia is widely employed for the manufacturing of fertilizers, pharmaceuticals, and artificial fibers. In this case, the Haber process has been used widely for the synthesis of ammonia on the industrial scale by splitting the strong triple bond of the nitrogen gas molecule at high pressure in the range of 20–50 MPa and temperature in the range of 500–600 °C.³⁵⁰ Numerous methods have been tested for nitrogen fixation, among which photocatalysis is popular because it has high possibility for N₂ fixation due to its green nature and cost-effectiveness. Recently, a study on Fe₂O₃ doping on porous g-C₃N₄ was done for photocatalytic nitrogen fixation.³⁵¹ During this process, artificial light was illuminated for N₂ fixation, leading to strong adsorption and operative carrier separation. This photocatalysis follows the Z-scheme mechanism, in which the close positions of the valence band of g-C₃N₄ and conduction band of Fe₂O₃ led to the rapid recombination of electrons in the conduction band of iron oxide with the holes in the valence band of g-C₃N₄, thus improving the charge separation and resulting in an enhanced photocatalytic performance.

Photocatalysis over semiconductors is a feasible method for nitrogen fixation under mild conditions. Mou et al.³⁵⁰ reported the synthesis of g-C₃N₄/metal oxide composites via a ternary deep eutectic solvent (DES) strategy. The DES was employed as a homogeneous solution to support the g-C₃N₄ nanosheet on the metal oxide after pyrolysis. This process can enhance the number of active sites for photocatalysis, resulting in good catalytic efficiency.

This process led to the maximum efficiency for ammonia generation (4380 mol L⁻¹ h⁻¹). The interfacial interaction between C₃N₄ nanosheets and iron oxide nanoparticles is responsible for the efficient photocatalytic activity. Similarly, in another study, Chen et al.³⁵² introduced low-temperature denitrification for the oxidation of NO into the harmless NO³⁻. In this study, hydrogen peroxide vapor was used to oxidize nitrogen oxide over the titania surface, followed by the absorption of the oxidation products in sodium hydroxide solution. In this study, the effect of the iron content on denitrification was investigated (Fig. 18). Zou et al.³⁵³ reported the formation of rGO-BiFeO₃via a hydrothermal method for the efficient removal of ammonia under visible light. Modification by r-GO increased the surface area to attain 91.20% degradation of NH₃ without an oxidizing agent. The above-mentioned reports are sufficient to describe the potential of Fe-based materials either in liquid or gas reactions. However, it is believed that there are numerous iron-based materials still to be discovered.

Fig. 18 Iron-doped titania (Fe/TiO₂) for denitrification. Reproduced from ref. 352 with permission under (CC-BY).

5 Photocatalytic mechanism and requirements

The process of photocatalysis is initiated by the electron forming a cluster of energy, which is referred to as excitation after photon absorption, causing the electron to move from the valence band to the conduction band as the first step of photocatalysis. In the second step, the formed electron–hole pair may recombine either inside the semiconductor or on its surface, causing a substantial amount of energy to be released, which may be observed in the form of light or heat. This recombination is the process that limits photocatalysis after capturing a photon. The holes and electrons that move on the semiconductor surface and do not recombine instantly take part in various redox reactions with the absorbents, which include water, oxygen, and other organic and inorganic species, and labelled as step three and four, respectively.^354–357

Redox reactions are fundamental in photocatalytic processes and solar fuel production. However, they are limited by the reduction potential of electrons that are excited by photons in the conduction band and in the valence band by the oxidation potential possessed by the photo-generated holes. Therefore, the key design parameter that governs the rate of transfer is the redox potential.^357,358 Among the iron materials, iron oxide is considered a suitable semiconductor because of its cost effectiveness, good stability and easy recovery after the reaction.^359,360 In this case, the design of iron oxide is crucial when employing it as a semiconductor for both elementary and applied research because recycled iron oxides should have a comparatively narrow band gap value. Therefore, goethite and hematite are frequently considered as photocatalysts recently because of their low band gap (2.2 eV). Methods have been reported to advance the photocatalytic performances of iron-based semiconductor systems involving plasmonic structures, composite heterostructures with a narrow/wide band gap, p–n heterojunctions, noble metal loading, graphene loading, etc.^361–363 Overall, an ideal iron-based semiconductor photocatalytic system design should satisfy the following requirements. Firstly, the synthetic scheme and photocatalysis process should be simple with high yield. Secondly, the composite system should show an improved photocatalytic performance compared to bare iron oxide. Thirdly, the composite photocatalyst should be reprocessed by an external magnetic field. Finally, the composite photocatalyst should be stable and have enough resistance to photo corrosion at room temperature. For photocatalysis, semiconductor oxides such as titania, zirconium oxide, zinc oxide, tungsten oxide, molybdenum oxide, tin oxide, iron oxide and semiconductor sulfides such as zinc sulphide, cadmium sulphide, molybdenum sulphide and tungsten sulphide can be used as catalysts for photo-induced chemical reactions due to their intrinsic electronic structure.^{181,364–368}

6 Conclusion and prospects

In conclusion, iron-based photocatalysts have interesting prospective applications in a number of fields, particularly the environmental and energy fields. Their unique electrical structure and surface chemistry enable effective light absorption and charge carrier separation, resulting in increased photocatalytic activity. The recent developments in iron-based photocatalysts have marked an important breakthrough in the field of photocatalysis. The multifaceted features of iron, including its low cost, natural abundance, and environmental friendliness, have driven it to the forefront of research aimed at designing sustainable and highly efficient photocatalytic materials. Extensive research on various iron-based materials, such as iron oxides, iron-based MOFs, iron doped composites, iron-based alloys, and iron-based complexes, has revealed their diverse photocatalytic capabilities, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), carbon dioxide reduction (CO₂) and nitrogen fixation (N₂). This critical review highlighted the basic mechanisms and factors influencing the photocatalytic performance of iron-based photocatalysts. From the design to fabrication of materials, researchers have gained valuable insights into optimizing the photocatalytic activities of these iron-based photocatalysts. Furthermore, the synergy gained by combining iron with other co-catalysts or adjusting its crystal structure provides new opportunities for tailoring the photocatalytic features of these materials. It can be summarized that a catalyst that is well dispersed and possessing a high surface area, along with a high abundance of active sites and considerable improvement in the absorption of pollutants can be achieved using a suitable material for supporting the photocatalyst.

Iron-based photocatalysts have a promising future in environmental remediation and solar energy conversion. However, some key challenges and future perspectives should be considered.

1. Regarding the use of iron-based nanomaterials for water treatment, adsorption and catalysis studies should be performed under realistic conditions regarding the presence of organic matter, co-ions, pH, pollutant concentrations, etc. Also, the functioning of nanosorbents/catalysts must be inspected more critically for potential use in commercial water treatment.

2. In-depth investigation is critical to understand the complex mechanisms that govern iron-based photocatalysis. In situ spectroscopy and computational simulation approaches will improve our understanding of the reaction pathways and allow for the systematic development of highly effective photocatalysts.

3. Further studies are mandatory to achieve insight into the complex charge transfer mechanisms of the Z-scheme heterojunction by using advanced technologies and methods, including time-resolved correlation technology, in situ technology, synchrotron radiation technology, and theoretical calculation. The integration of iron-based photocatalysts with other semiconductor materials, such as titanium dioxide (TiO₂) and graphitic carbon nitride (g-C₃N₄), can lead to synergistic effects and improved photocatalytic performance. Future efforts can focus on developing heterojunctions or composite photocatalysts to harness the strengths of multiple materials for enhanced photocatalytic activity and stability.

4. Besides, the desired stability of iron-based materials in both alkaline and acidic media is still problematic due to the reduction of Fe³⁺ to Fe²⁺ and leaching of iron from the photocatalyst into the electrolyte. Solid solutions in which wide band gap robust perovskites, such as SrTiO₃, or oxides, such as TiO₂, are combined with iron-based compounds can represent a viable approach.

5. Iron-based photocatalysts have shown potential for photocatalytic water splitting to produce hydrogen as a clean and renewable energy source. Future efforts can concentrate on improving the photocatalytic activity and stability of these catalysts, as well as reducing the production costs to make hydrogen generation economically viable and ecologically friendly O₂ extraction from explosive H₂ and O₂ mixtures.

6. Iron-based photocatalysts can be utilized for the photocatalytic reduction of carbon dioxide (CO₂) to produce value-added chemicals and fuels, contributing to carbon capture and utilization efforts. Future research can focus on developing efficient catalysts with high selectivity for specific carbon-based products. Alternatively, iron-based photocatalysts can play a crucial role in the synthesis of solar fuels, such as methanol and hydrocarbons, through the photoreduction of carbon dioxide or water. Future studies should aim to optimize the catalyst design and reaction conditions to enhance the overall efficiency and selectivity of solar fuel production processes.

7. Iron-based photocatalysts can facilitate various organic transformations under mild reaction conditions, enabling the synthesis of valuable organic compounds with a reduced environmental impact. Future research can explore new photocatalytic reactions and reaction mechanisms, as well as the development of novel catalyst architectures to expand the scope of photocatalytic organic synthesis.

8. Bridging the gap between laboratory tests and their practical implementation in real-life scenarios is critical. Researchers should prioritize the development of scalable methods for the synthesis and evaluation of the long-term durability of iron-based photocatalysts for practical applications in environmental remediation and solar fuel production.

Iron-based nanocrystals display promise for energy conversion and environmental remediation. The future directions are promising due to the advances in understanding their photocatalytic properties and the ongoing synthesis of novel materials. For significant outcomes, application-oriented research should carefully examine the technical limitations described earlier. Finally, despite the broad prospect of chances and innovations of iron-based nanosystems for technological and science applications, a comprehensive understanding of the environmental and health impacts of engineered nanoparticles together with their toxicological effects, fate, and bioaccumulation is required. Therefore, iron-based materials should be further studied prior to their safe inclusion into the natural environment.

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22201182, 22071081, 21601063), the Opening Project of Key Laboratory of Inorganic Functional Materials and Devices, Chinese Academy of Sciences (KLIFMD202007) and the Guangdong Basic and Applied Basic Research Foundation (2022A1515010649).

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Footnote

† Nayab Arif and Muhammad Nadeem Zafar are equivalent authors.

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