A Review on Synthesis , Properties , and Environmental Application of Fe-Based Perovskite

Department of Research, Hindustan Institute of Technology and Science, Padur, Chennai 603103, India Department of Chemistry, KCG College of Technology, Chennai 600100, India Department of Chemistry, Hindustan Institute of Technology and Science, Padur, Chennai 603103, India School of Advanced Material Sciences and Engineering, Kumoh National Institute of Technology, Deahak-ro 61, Gumi-si 39177, Gyeongsangbuk-do, Republic of Korea Department of Mechanical Engineering, Institute of Technology, Hawassa University, Hawassa, Ethiopia


Introduction
Recently, in the materials family, Perovskite materials have attracted the attention of researchers due to their technological signi cance. Perovskites possess excellent stability and physicochemical, electrical, dielectric, piezoelectric, and superconducting properties which are structure-dependent [1][2][3]. Hence, they are widely used as catalytic, electric, and magnetic materials [4][5][6]. Generally, perovskite is represented by the formula ABO 3 and new forms of perovskite include A3B2X9, A2BB/X6, A2BX6, and A4BX6, where A and B are cations and X is a halogen or oxygen anion. e cation and anion compositions of perovskite materials can tune di erent structures and properties which lead to wide applications [3,7]. e development of nanomaterials and their novel properties enforced the scientists to downsize the perovskite structures to nanoregime to foster its performance and applications [8]. Nanoperovskites have better catalytic efciency [9] and more enhanced conductivity than bulk due to their large amount of grain boundaries [10], dielectrics [11], and so forth.
presents an overview on chemical and biological method of synthesis of Fe-based Perovskite. Further, the properties enhancement due to size reduction is detailed. Also, the applications of Fe-containing Perovskite in the field of environment are reviewed.

General Synthetic Methods of Fe-
Based Perovskites e synthetic methods of Perovskite play a vital role in designing the structure and properties. Developing an efficient and controllable synthetic method for Fe-based Perovskite is essential to have novel properties with significant applications in environment and so forth. Currently, the research is on the synthesis of Fe-based Perovskite to replace precious metals in the purification process [15].
Various methods of synthesis of Fe-based Perovskite have been reported with different composition and morphologies. Conventional method of synthesis has many disadvantages such as in homogeneity and defects, which makes them unsuitable for various applications. To overcome these disadvantages, new methods have been developed such as wet chemical synthesis, sol-gel, and coprecipitation, hydrothermal, microwave, and biological methods. e synthesis of Fe-based Perovskite is classified into wet chemical and other synthesis routes.

Wet Chemical Synthesis.
Wet chemical synthesis has been extensively used for the synthesis of Fe-based Perovskite nonmaterials of higher surface area at low temperature, including sol-gel, coprecipitation, solvothermal, and hydrothermal. Various researchers made successful attempts and achieved differently structured Perovskite with a large surface area. A few methods generally used for synthesis are overviewed here.

Sol-Gel Synthesis.
e sol-gel method is extensively used for the preparation of Fe-based Perovskite nonmaterial with large surface area. e advantages of this method are inexpensive precursors, simple preparations, and so forth. Among the Fe-based Perovskites, synthesis of BiFeO 3 is difficult due to the volatile nature of Bi 2 O 3 and it requires a higher temperature. Saira Raiz et al. vanquished it and reported BiFeO 3 synthesis using sol-gel method [16]. eingi et al. have synthesized LaFeO 3 nanoparticles by solgel method using citric acid with a lower band gap [17]. Liu et al. have synthesized a series of compounds [CaMnO 3 , CaFeO 3 , and CaMn 1 -xFe x O 3 ] using the sol-gel method. Citric acid was used as a gelling agent and glycol was added to obtain composition homogeneity and to avoid segregation [18]. Aziz et al. studied the synthesis of GdxMn1−xFe1−yCuyO 3 nonmaterials by sol-gel autocombustion route [19]. By thermal decomposition of the gel complex of LaFe-(C6H8O7 H 2 O), Shabbir et al. reported a unique sol-gel procedure for generating nano-sized Perovskite-type LaFeO 3 powder. e scientists discovered that optimizing the gelling conditions leads to the creation of the LaFeO 3 Perovskite phase without the need for an explosion or combustion process, as well as pH control. A pure Perovskite phase with a particle size of about 25 nm was found to be above 600°C [20]. Recently, few researchers worked on synthesis of LaFeO 3 Perovskite powders by sol-gel method for dye-sensitized solar cell applications. ey obtained pure single phase at 850°C by xerogel formation. e flow chart of synthesis is shown in Figure 1.

Coprecipitation Method.
e coprecipitation method is one of the most convenient techniques for synthesizing nano-Perovskite with many components by adding precipitants to get a good degree of homogeneity. Many precipitating agents such as ammonia, NaOH, and amines are used. Among them, ammonia is always preferred because it can be removed easily upon heating. Temperature, pH, reaction time, speed, and source concentration must all be addressed when using coprecipitation technique. Muneeswaran et al. have synthesized BiFeO 3 nanopowder by coprecipitation method using ammonia at pH � 10 [22]. Wang et al. have reported the synthesis of BiFeO 3 by coprecipitation method using composite precipitants. e composite precipitants avoid the aggregation of nanoparticles [23]. Khorasani-Motlagh et al. have used octanoic acid surfactant for the synthesis of sphere like NdFeO 3 nanocrystals by coprecipitation method [24]. On the basis of A-site and B-site ion selection, several precipitating agents have been reported. Haron et al. synthesized nano-LaFeO 3 at 800°C by coprecipitation method using NaOH as precipitating agent.
e photograph of the synthesized LaFeO 3 is shown in the picture (Figure 2). e synthesis cost was low and no waste was observed in this method [25].

Hydrothermal Method.
e hydrothermal method is a solution reaction-based method. In this method, crystalline structure is obtained for nanomaterials without calcinations. Gómez-Cuaspud et al. synthesized pure phase LaFeO 3 Perovskite by the hydrothermal method without calcinations [26]. Single crystal BiFeO 3 microplates were developed by hydrothermal method using C 6 H 10 BiNO 8 as reductant and surface modifier [27].
Many researchers have recently used the hydrothermal approach to make nano-BiFeO 3 Perovskite. e list is as follows: Sazali et al. used biopolymer (chitosan) assisted hydrothermal synthesis of BiFeO 3 nanoparticles and got enhanced magnetic properties [28]. Another group used a hydrogen peroxide assisted hydrothermal approach to make shape-controlled BiFeO 3 microspheres. To establish an oxygen-rich environment, they employed H 2 O 2 [29]. Han et al. also created spindle-like BiFeO 3 powders with a width of 0.6 mm and a thickness of 0.3 mm at pH 14 and 200°C for 72 hours, adding a little amount of H 2 O 2 . Chen and colleagues used an ethanol-assisted hydrothermal technique to make BiFeO 3 nanopowders. e ethanol solvent is used extensively in this process to produce pure phase BiFeO 3 [30]. On increasing the hydrothermal reaction time from 6 h to 15 h, BiFeO 3 microcylinders were obtained by Di et al. [31]. e hydrothermal microwave approach produced BiFeO 3 Perovskite crystals with better uniformity than the solid-state reaction method at low temperature of 180 o C [32]. Researchers recently used a hydrothermal approach to produce nano-BiFeO 3 using different amounts of chitosan biopolymer and KOH as a precipitating agent [28]. Wafer BiFeO 3 was synthesized by hydrothermal method without using mineralizer or precipitant.

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Mesbah et al. employed a hydrothermal technique to synthesize nano-LaFeO 3 using lanthanum and iron salts in a stoichiometric ratio. To obtain nano-sized LaFeO 3 , PVP was used as a capping agent, and alkali (NaOH) was added [33].

Microwave Method.
Microwave method is an alternative to the conventional heating method. Microwave heating has higher heating rates and short processing time, which controls the microstructure and better functional properties compared to conventional synthesis [34] of homogeneous Perovskite. Rafael synthesized Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ Perovskite by microwave method and achieved same densification as conventional method with less processing time. e grain size obtained was nanoscale compared to conventional method. e microwave hydrothermal synthesis of BiFeO 3 crystallites at 467 K for 2 hours was initially reported by Komarneni [35]. Joshi et al. 2.6. Crystallography of Fe-Based Perovskite. Perovskite has an ABO 3 form, where B is a transition metal ion with a short radius, bigger A is an alkali earth metal or lanthanide with a larger radius, and O is the oxygen ion in a 1 : 1 : 3 ratio. Atom B is positioned in the cube corner of the ABO 3 Perovskite cubic unit cell, with oxygen atoms positioned in face-cantered positions. Atom A is positioned in the body centre. Size, a shift in oxidation states, and Jahn-Teller processes are three primary concepts that are typically used to explain the distortions in Perovskites. Electroneutrality and the other ionic radii parameters are two prerequisites for Perovskite formation. In accordance with electroneutrality, the Perovskite formula needs to be neutrally balanced, so when the charges of A and B ions are added, the result should be equal to the total charge of the oxygen ions. e radii of A and B ions should be rA > 0.090 nm and rB > 0.051 nm in accordance with the specifications for ionic radii [38].
Understanding the type of crystallographic defects that regulate the functional characteristics of the Fe-based Perovskite material is crucial for understanding how they affect the material's properties. Marezio and Dernier have explained how the crystal structure of LaFeO 3 , an Fe-based Perovskite, changes depending on temperature and doping. At room temperature and up to 957°C, the structure of undoped LaFeO 3 adopts a Perovskite arrangement that is orthorhombic (space group Pbnm). In the LaFeO 3 crystal structure A � 5.55Å, b � 5.56Å, and c � 7.86Å are the edge lengths of the unit cell at room temperature, which contains four structural units (Z � 4) [39]. Between 960 and 1005°C, LaFeO 3 exhibits a first-order structural phase transition from orthorhombic to rhombohedral. is transition results from the B-site cations' tilting, which changes the magnetic characteristics. Similar to this, a phase change happens when alkaline earth cations dope LaFeO3 at site A. Kotomin and his coworkers have found that though SrFeO 3 and CaFeO 3 are isoelectronic species, their structural geometry differs due to their distinct ionic radii. Since the ionic radius of Ca 2+ is lower than that of Sr, it possesses a monoclinic phase. In contrast, SrFeO 3 has a cubic structure [40]. Wang et al. observed that, at room temperature, BiFeO 3 bulk structure has rhombohedral symmetry with a lattice constant of 5.63Å. e structure and characteristics of BiFeO 3 are changed when sites A and B are substituted. Due to the reduced ionic radii of lanthanide ions relative to Bi3+, when lanthanide ions are substituted to site A of BiFeO 3 , the phase of the material changes from rhombohedral to orthorhombic. Similar to this, the BiFeO 3 structure shifts from rhombohedral to triclinic phase when alkaline earth ions are substituted. Phase change was noticed when site B Fe ions were substituted with ions that had similar ionic radii and electronegativity, such as Ti and Mn [41]. GdFeO 3 and GdFeO 3 doped with Mn were both produced by the sol-gel method by Maity et al. e orthorhombic phase of GdFeO 3 with the Pbnm space group is seen. O type orthorhombic phase, which exhibits decreased unit volume and increased photocatalytic activity, is retained when Fe is replaced with 30 percent Mn [42]. Chandra Sekhar created MnFeO 3 using the combustion process. He achieved cubic structure with the space group Ia3, as well as the lattice constant value of a � 9.40 which is in good agreement with the JCPDS Card No. 010750894 [43]. Sumalin created LaFeO 3 via a polymerized complex technique ( Figure 3). e synthesized sample's lattice parameters, a � 0.5564 nm, b � 0.7855 nm, and c � 0.5556 nm, were in good agreement with those of orthorhombic LaFeO 3 [44]. In order to create Fe-based Perovskite, a variety of synthetic techniques have been employed, including sol-gel, combustion, and polymerized complex. However, the doping of ions merely modifies the phase, lattice structure, and lattice parameters.
Fe-based Perovskite exhibits a variety of interesting properties like ferromagnetic property as in the case of BaFeO 3 and increased ferromagnetism in (Ba/Ca/Sr)FeO 3 . It showed an enhanced magnetic property with increased magnetic moment which meets the needs of spintronic devices [45]. Although Fe-based Perovskite exhibits a range of properties, the focus of our analysis is on ferromagnetic property, electrical conductivity, and catalytic activity. We chose the aforementioned properties since our analysis is based on environmental applications. In addition, several   [46,47]. Similar to the above reports, waferlike BiFeO 3 also shows ferromagnetic property [48]. Doping with Fe enhances ferromagnetic behavior. Few reports are cited here; Luo and his colleagues looked into the effect of Fe doping on the magnetic properties of Perovskite cobaltite [49]. Fe doping enhances ferromagnetism in the systems of Pr 1−y Ca y Co 1−x Fe x O 3 and Gd 0.55 Sr 0.45 Co 1−x Fe x O 3 , while further increasing Fe content suppresses ferromagnetism and results in spin-glass behavior. Alternatively, as long as Fe is doped, ferromagnetism is suppressed in the systems, and no spin-glass behavior is observed in the sample with Fe doping up to 0.3. e phenomenon seen above is thought to be caused by a rivalry between ferromagnetic and antiferromagnetic interactions via intermediate spin. Suresh et al. reported that doping 20% Fe in SmCrO 3 decreases the transition of magnetization and flips the magnetization without changing the direction of the applied magnetic field [50]. Manju et al. discovered that replacing Fe/Co in BaSnO 3 materials improved the material's ferromagnetic properties. e F centre exchange interactions are responsible for the enhancing attribute [51]. Bi 0.5 Na 0.5 TiO 3 materials were recently discovered to have ferromagnetism at room temperature. By adding MgFeO 3 to the material, ferromagnetism was produced in the source, lowering the band gap from 3.09 eV to 2.43 eV [52]. e magnetic characteristics of Fe doped BaZrO 3 were also investigated by Nisar et al. ey discovered that doping Fe in the Zr site improves the material's magnetic moment. e material showed enhanced ferromagnetism due to the arrival of unpaired electrons of Fe 3+ [53]. Rajamani et al. [54] used pulsed-laser deposition to make ferromagnetic Ba(Ti 1-x Fe x )O 3 thin films (0.15 × 0.5) and discovered that the saturation magnetization (MS) increased with the Fe content.

Electrical Conductivity.
In Perovskite-type oxides, electronic conduction is a crucial property. Electronic conduction above R.T. is critical for everyday items, since it assists in the propagation of electrical signals. For solid oxide fuel cells (SOFCs) and solid oxide electrolytic cells, electrode materials with excellent conductivities in both oxidizing and reducing environments are in high demand (SOECs). In the presence of air, many oxide materials have a high conductivity. e key challenge is to find a suitable stable oxide anode material that can conduct well in a reducing environment. To overcome the problem, the researchers are working on Fe-based double Perovskite, and a few literature works are listed below. In a reducing atmosphere, Anikina [57]. Nevertheless, a number of researchers are working on it to expose its features. Using Fe-based Perovskites, photocatalytic water splitting for hydrogen production was researched by many researchers. Vincent and his research team used magnetron sputtering to deposit LaFeO 3 film together with g-C 3 N 4 in order to study its photocatalytic properties. In comparison to pure LaFeO 3 , they saw hydrogen generation of 74% at a rate of 10.8 mol/hr/cm −2 for LaFeO 3 /g-C 3 N 4 [58]. Similar to this, Iervolina et al. synthesized LaFeO 3 catalysts through using solution combustion method with citric acid as the fuel and investigated their photocatalytic properties. For the purpose of generating hydrogen, they investigated into the photocatalytic degradation of glucose solution. ey found that increasing the amount of citric acid by twofold boosts the LaFeO 3 surface area and the photocatalytic property of hydrogen generation [59]. Ibrahim successfully created n-type LaFeO 3 Perovskite using the sol-gel process. He looked into the hydrogengenerating photocatalytic property and developed a lowcost, robust photoelectrochemical cell for solar energy conversion [60]. e photocatalytic degradation of dyes using Fe-based Perovskite is also being researched due to its high stability, nontoxicity, and small band-gap energy. Ismail synthesized LaFeO 3 using the sol-gel method and investigated its photocatalytic ability to degrade 4-chlorophenol (4-CP) and rhodamine B (RhB). He noticed that LaFeO 3 that has been calcined at 700 o C has the highest photocatalytic activity [61]. For the photocatalytic degradation of rhodamine B (RhB) and p-chlorophenol under visible light irradiation, Pirzada et al. developed LaFeO 3 /Ag 2 CO 3 nanocomposites by coprecipitation technique. Under natural sunlight, they achieved degradation efficiencies of 99.5 percent for RhB and 59 percent for p-chlorophenol in less than 45 minutes [62]. To increase the photocatalytic activity of LaFeO 3 , Vijayaraghavan developed a composite made of LaFeO 3 nano-Perovskite-RGO-NiO. By conducting a research study on Congo red dye degradation and hydrogen and oxygen evolution by water splitting, the photocatalytic property of the composite was investigated [63]. e mechanism of the photocatalytic dye degradation and water splitting by LaFeO 3 is depicted in Figure 4.
Microwave-prepared BiFeO 3 and LaFeO 3 were examined for their photoelectrochemical properties. LaFeO 3 showed stronger water splitting than BiFeO 3 because of the Jahn-Teller distortion, which leads to charge separation [64]. Kim and his coworkers investigated the Fe doping in Co-based Perovskite oxide and explored its catalytic activity towards oxygen evolution reaction in alkaline media. ey found that incorporation of Fe in site B doping enhances OER efficiency and stability and showed intrinsic properties too [65]. Because of their intrinsic activity, distinct physicochemical features, and diverse compositions, Fe-based Perovskite oxides have attracted a lot of attention as a potential kind of noble-metal-free candidates for hydrogen evolution reaction (HER) at the cathode [66].

Environmental Applications.
Fe-based Perovskites possess excellent thermal stability and catalytic properties, which make them a potential candidate for environmental applications. In synthetic methods, the substitution of cations in sites A/B is the aspect that induces the catalytic properties in Fe-based Perovskite [67]. Fe-based Perovskites are of low cost and they possess excellent activity in the remediation of pollutants from the environment. Fe-based Perovskites are used for sensing the environmental pollutants in gaseous form. Few examples are cited below.

Fe-Based Perovskite as Sensor.
Detecting and monitoring the toxic gases are important for environment protection. Gas sensors are of low cost and they are better alternative to the existing analytical techniques. e increasing demand of highly selective and sensitive sensors has urged the researchers to focus on Fe-based Perovskite as sensor due to its thermal and chemical stability. Fe-based Perovskites have been used for sensing various gases such as carbon monoxide (CO) and oxygen (O 2 ), and various Febased Perovskite and transition metal substituted Perovskite have been used as a sensor for detecting gases which are shown in Table 2. Lanto et al. have studied the gas sensing property for LaFeO 3 , Sr, and Mg modified LaFeO 3 nanoparticles. ey found that modified LaFeO 3 showed less sensitivity to CO, C 2 H 4 , and CH 4 compared to unmodified LaFeO 3 [68]. Recently, Fabio developed a gas sensor for detecting CO gas using Ti substituted lanthanum ferrite Perovskite (LaFe 0.8 Ti 0.2 O 3 ) [69]. Since it has a low band-gap energy, it does not show any ionic domain and is capable of detecting gases under any reducing/oxidizing pressure range. Bi 5 Ti 3 FeO 15 nanoparticles were synthesized and their gas sensing properties were studied. Jamil found that it was selective towards oxygen when tested in the presence of other gases and alcohols and proposed as a practical oxygen sensor. e gas sensing setup is depicted in Figure 5 [70]. 2% weight Pd doped LaFeO 3 prepared by Xiao Feng et al. showed good response for detecting low concentration of acetone [71]. Wang et al. prepared LaFeO 3 nanocrystalline powders by sol-gel method for sensing carbon dioxide gas [72]. Cao and coworkers reported the ethanol gas sensing property of chlorine doped LaFeO 3 nanocrystals [73]. Ma et al. prepared mesoporous hollow PrFeO 3 (praseodymium ferrite) nanofibers by electrospinning method and studied its sensing property towards acetone [74]. Chen et al. used lotus leaf as biotemplate for synthesizing Ag-LaFeO 3 nanoparticles. ey also found that the synthesized nanoparticle (Ag-LaFeO 3 ) exhibits enhanced xylene gas sensing property [75]. Similarly, Han et al. also prepared SmFeO 3 nanofibers using electrospinning method for the detection of ethylene glycol [76]. Perovskite Ag-LaFeO 3 nanofibers were prepared by Wei and coworkers for sensing HCHO gas which is a toxic VOC [77]. Based on the above research, Yang et al. also developed porous LaFeO 3 for HCHO sensing at125°C [78]. 6 Advances in Materials Science and Engineering Similarly, Queral to et al. [79] synthesized the LaFeO 3 nanofibers by calcination at 600°C for the sensing of sulphur containing gases.

Fe-Based Perovskite as Adsorbent.
Adsorption is one of the effective, economical, and cheap methods of removing pollutant from wastewater. Adsorption depends on various factors such as surface area, porosity, size distribution, density, and surface charge. Growing demand on adsorbents has led the researchers to focus on nanoparticles due to its high surface-to-volume ratio [86,87]. Researchers are doing effective research to develop adsorbents of low cost with high adsorption capacity [88,89]. In view of this, Fe-based Perovskite nanoparticles are considered to be an effective adsorbent because they possess excellent structure and they are employed for the adsorption of pesticides, dyes, heavy metal ions, and volatile organic compounds (Table 3). e adsorption of various dyes by Fe-based Perovskite was also investigated. Shima capped La 0.9 Sr 0.1 FeO 3 nano-  Figure 5: Gas sensing setup of Bi 5 Ti 3 FeO 15 [70].

Advances in Materials Science and Engineering
Perovskite with CTAB and was applied as an adsorbent for the removal of Congo red dyes in aqueous and real samples [90]. He also optimized the adsorption process with various factors such as pH, contact time, dye concentration, and temperature. He proposed that La 0.9 Sr 0.1 FeO 3 showed 10 times higher adsorption capacity than the pure one. Ming explored strontium ferrite nano-Perovskite for the degradation of organic pollutants Bisphenol A and acid orange without any stimulants under dark condition. He achieved efficient results and further stressed that strontium ferrite nano-Perovskite can be an alternative material for low-cost water treatment [91]. Recently the adsorption of rhodamine B by LaFeO 3 -ACF was reported by Deng et al. [94]. ey prepared activated carbon fibers (ACF) from cotton waste and decorated on LaFeO 3 by sol-gel and thermal treatment. e adsorption efficiency of LaFeO 3 -ACF was higher compared to LaFeO 3 due to the electrostatic interaction, hydrogen bonding, π-π stacking, and cation-π interactions. e adsorption of methylene blue dye from water by Gd 0.5 Sr 0.5 FeO 3 Perovskite synthesized from sol-gel method was reported [95]. e adsorption of heavy metals by Febased Perovskite was also explored. e adsorption of As(V) by NdFeO 3 synthesized by the polymeric gel precursor method was reported [92]. Removal of As(III) from the water was faster and more efficient by CuFe 2 O 4 /PMS Perovskite than by CuFe 2 O 4 [93].
Recently, in the field of photocatalysis, Perovskite-based catalysts have grabbed great attention from researchers. Fe-based Perovskite exhibits an excellent visible light-driven photocatalytic property. LaFeO 3 showed an excellent and better photocatalytic property than Fe 2 O 3 [96]. Doping of Mn in LaFeO 3 and its photocatalytic property is investigated. e doping enhances the photocatalytic property [97]. BiFeO 3 was also used as a photocatalyst due to the electron-hole separation. Doping of metals in BiFeO 3 and its influence on photocatalytic property was also investigated. Gd and Ca doped BiFeO 3 showed enhanced photocatalytic degradation of dyes [98,99]. Further studies have to be explored on the enhancement of photocatalytic studies of BiFeO 3 . Similarly, Jaffari et al. also prepared the Pd doped BiFeO 3 microcomposite by hydrothermal technique. e prepared composite has coral-shaped BiFeO 3 surface loaded with spherical Pd nanoparticles, which exhibits more enhanced photoactivity than pure BiFeO 3 . e enhanced photoactivity is due to the Pd dopant, and the composite possesses excellent recyclability with minimum leakage of Pd after six runs. ey also proposed that Pd doped BiFeO 3 microcomposite exhibits as potent antimicrobial agent [100].
Sydorchuk et al. recently have investigated the photocatalytic properties of PrCo 1−x Fe x O 3 Perovskite powders [101].
eir studies were focused on the influence of structure and its composition on photocatalytic properties.
Researchers also reported that GaFeO 3 has good water splitting capacity without any cocatalyst [102]. Another report states that YFeO 3 has four times enhanced photocatalytic activity than TiO 2 :P25 [103]. Fe-based Perovskite is extensively applied as an environmental catalyst due to its magnetic recovery.
irumalai Rajan and his coworkers prepared floral-like LaFeO 3 nanostructures by surfactant assisted hydrothermal method. e prepared floral LaFeO 3 nanostructure was used for degradation of rhodamine B (RhB) and methylene blue (MB) under visible light irradiation [96].

Fe-Based Perovskite as Catalyst.
Advanced oxidation processes (AOPs) have emerged in recent years as efficient and effective wastewater treatment technologies. Photocatalysis has risen to prominence among the AOPs as a promising technology for addressing environmental issues. Perovskite and Perovskite-related materials are third-generation photocatalysts which fit into the photocatalytic characteristics by establishing a stable structure and solid solution with a variety of metal ions to achieve the required band engineering for photoelectrocatalytic applications [67]. Fe-based Perovskite serves as a visible light photocatalytic material owing to its low cost and small band-gap compared to the titanium-based Perovskite [104]. In the field of environmental remediation, ferrite-based Perovskites have proven to be promising materials for photocatalytic and photoelectrocatalytic applications. e magnetic and electrical properties of ferrite-based Perovskite draw interest. Due to a distortion in their crystal structures, they feature an intrinsic electric dipole moment, which enhances the separation of photo-generated charges during the photoexcitation process [105]. e production of electrons and holes on the surface of the catalyst causes photocatalytic degradation of dyes. e adsorbed compounds will undergo redox reaction with the produced electrons and holes on the catalyst [106]. e properties of the nanostructured Perovskite material are influenced not only by the composition but also by the structure, morphology, phase, shape, and size. Keeping in its view, irumulairajan et al. used a hydrothermal process to make LaFeO 3 in three different shapes: nanocubes, nanorods, and nanospheres. e efficacy of the produced nanostructures on photocatalytic degradation of rhodamine dye is being investigated. ey found that LaFeO 3 nanostructures have better photocatalytic activity. Nanospheres, on the other Methylene blue -- [95] 8 Advances in Materials Science and Engineering hand, were shown to be more efficient than TiO 2 [96]. Similarly, the same researchers prepared floral nanostructured LaFeO 3 with a band-gap of 2.10 eV, which showed better photocatalytic efficacy in decomposing rhodamine B and methylene blue when compared to bulk LaFeO 3 [107]. Doping of Fe-based Perovskite at both sites gives higher photocatalytic efficiency by reducing the band-gap. e smaller the band-gap, the more visible light absorption for photocatalytic degradation due to e-hole recombination. For example, Hu et al. investigated the photocatalytic property of Sm doped BFO nanoparticles for the degradation of MO under visible light, finding a reduced energy band-gap of 2.06 eV [108]. In the presence of H 2 O 2 , the visible light photodegradation of MB dye by BFO doped with Ba, Na, and K metal ions was also studied [109]. Jaffari et al. examined the photocatalytic degradation of malachite green dye and phenol from wastewater by doping Pb in BFO nanomaterial. In comparison to bulk BFO and TiO 2 , they discovered that Pb substitution increased photocatalytic efficiency. e increased photoactivity was attributed to the suitable Pb concentration, which increased the trapping capability, which aided in the formation and transmission of the produced e-h+ pairs [110]. Similarly, Dy doping in BFO material resulted in MB degradation of 92 percent [111]. e photocatalytic efficiency of Perovskite is improved by Fe doping. e photocatalytic degradation of RNL azo dye was investigated using Fe doped BaSnO 3 . Fe doping in BaSnO 3 creates intermediate levels in the band-gap, trapping electrons and preventing electron-hole recombination, and increasing photocatalytic efficiency [112].
Compared to photocatalytic degradation of organic pollutants, photoelectrochemical degradation is found to be more advantageous. Fe-based Perovskites are playing a key role in degrading the organic pollutant as photoelectrocatalyst. Reports on Fe-based Perovskite as photoelectrocatalyst are meager. Nkwachukwu et al. synthesized La doped BiFeO 3 by hydrothermal method and studied its photoelectrocatalytic degradation property on dyes (Orange II, Congo red, and methylene blue) and pharmaceutical pollutants (acetaminophen and sulfamethoxazole). 10% La doping has enhanced the photoelectrocatalytic efficiency by reducing the band-gap. e schematic representation of its mechanism and photoelectrocatalytic efficiency is given in Figure 6 [113]. e principal pollutant responsible for ozone depletion in the stratosphere is nitrous oxide. Infrared radiation is also absorbed by N 2 O, which contributes to the greenhouse effect. us, one of the key research areas in environmental catalysis is to control N 2 O emissions from industries [114]. Although there are a few strategies for reducing N 2 O emissions, direct catalytic decomposition is thought to be a simple method for removing the gas. Because of their low cost and high thermal stability, Fe-based Perovskites are applied as a suitable catalyst for N 2 O reduction and decomposition reactions.
By doping BaTiO 3 with Fe, the photoelectrochemical activity is increased. BaTiO3 showed improved photoelectrochemical activity for hydrogen generation after being doped with Fe [115]. Several research groups have produced numerous BiFeO 3 nanostructures and investigated their photoelectrochemical characteristics. Photo-induced water oxidation activity of single-crystalline BiFeO 3 nanocubes was discovered by Joshi et al., implying that BFO could be a promising material for photocatalytic applications [36]. For photoelectrochemical water splitting, BiFeO 3 was produced and employed as a photocatalyst [116].
Fe-based Perovskites have been extensively investigated as oxidation catalysts due to their oxygen-carrying capacity. Larger LaFeO 3 crystals have a smaller band-gap and consequently a weak O-Fe bond strength, leading to increased methane  Figure 6: Schematic representation of photoelectrocatalytic degradation of dyes and pharmaceutical pollutants using BiFeO 3 and 10% La doped BiFeO 3 [113].
conversion activity and a large amount of detachable O [117]. Researchers have discovered LaFeO 3 and La 0.8 Sr 0.2 FeO 3 Perovskites with low oxygen mobility as promising partial oxidation selective catalysts, but La 0.5 Sr 0.5 Fe 1-x Co x O 3 (x � 0, 0.5, 1) Perovskites with high oxygen mobility are well suited to methane total combustion [118]. e optical properties of Perovskite are enhanced by dopants. e capacity of Fe doping in Perovskites to improve the oxygen evolution reaction is explored here. e significance of Fe in improving OER activity and stability has remained a mystery until now.
After replacing Fe 3+ in the LaFeO 3 Perovskite with Cu 2+ and Ni 2+ , the catalytic activity for the N 2 O decomposition reaction enhanced. e catalytic activity was further boosted by increasing the reaction temperature. In the case of Niand Cu-included LaFeO 3 samples, the increased oxygen mobility and minor increase in surface area resulted in the establishment of additional active sites for N 2 O adsorption and then decomposition [119]. Fe-based Perovskites as photocatalysts are commonly associated with separation, regeneration, and recycling challenges, despite their favourable and prospective green environmental photocatalytic uses.

Conclusion and Outlook
It has been proven in recent years that Perovskites offer the ideal combination of properties to use for environmental issues. Fe-based Perovskites have been synthesized and applied for environmental remediation. For use in environmental applications, more works must still be put into developing Fe-based Perovskites with superior efficiency and great stability. To satisfy the demands of large-scale synthesis for industrial manufacturing, a Perovskite synthetic process that is easier to use and more productive is still required. e ability of Fe-based Perovskites to remove new pollutants through photocatalysis is currently of significant interest. A wide variety of synthetic techniques are required to create novel materials that improve photocatalytic activity.
Reports on adsorption and degradation of pollutants showed the efficiency of Fe-based Perovskite. Although many compounds are reported on photocatalytic property, there is no detailed study report. e influence of structure, doping, and composition on the photocatalytic property has to be explored further. Fe-based Perovskite exhibits ferroelectric and ferromagnetic properties, and how it influences photocatalytic property is not investigated. Further understanding is needed for an efficient Fe-based Perovskite photocatalyst.
ere are few reports on the Fe-based Perovskite as sensor, adsorbent, and photocatalyst. Further effort is needed for the synthesis of Fe-based Perovskite as an upcoming future material for environmental applications. e cost-effectiveness of the design of Fe-based Perovskite-based devices is still a concern for the researchers. For the design and development of nano-Perovskite sensors, catalysts, and adsorbents for environmental implications, several scientists are currently working on Fe-based Perovskites.

Data Availability
All data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.