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

Authors: G. Jayanthi; Sowrirajan Sumathi; Karthik Kannan; V. Andal; Sivaraj Murugan
Journal: Advances in Materials Science and Engineering
 (2022)

---

## Abstract

Perovskite has attracted the attention of researchers owing to its intriguing physicochemical properties and wide applications. Recently, Fe-based Perovskite as well as its nanoforms is an extensively studied material due to its photocatalytic activity, multiferroic properties, and chemical stability. Fe-based Perovskite exhibits a range of characteristics that become helpful for different applications such as catalysis, electrochemical sensors, and solar cells. This review summarizes the synthesis, properties, and environmental applications of Fe-based Perovskite. This review highlights and provides an overview of the transition metal substituent in Fe-based Perovskite and its properties and how it influences its application in wastewater treatment and catalysis. This article furnishes an overview on synthesis, properties, and environmental application of Fe-based Perovskite.


---

## Body

## 1. Introduction

Recently, in the materials family, Perovskite materials have attracted the attention of researchers due to their technological significance. Perovskites possess excellent stability and physicochemical, electrical, dielectric, piezoelectric, and superconducting properties which are structure-dependent [1–3]. Hence, they are widely used as catalytic, electric, and magnetic materials [4–6]. Generally, perovskite is represented by the formula ABO3 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. The cation and anion compositions of perovskite materials can tune different structures and properties which lead to wide applications [3,7].The 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 efficiency [9] and more enhanced conductivity than bulk due to their large amount of grain boundaries [10], dielectrics [11], and so forth.Recently, Fe-containing mixed oxides with Perovskite structure, AFeO3(A = Mn, Co, or Fe), have been studied extensively to develop catalysts to replace precious metals and environmental purification catalysts. Fe-based Perovskites possess good material stability, nonstoichiometric properties, and good phase stability and hence are used as oxygen catalyst [12]. The mixed valence of Fe3+ and Fe4+ in Perovskite will create ionic mobility and have an effect on catalytic properties [13]. The valence states of Fe in Perovskite are important on the defects and they influence the transport properties such as electrical conductivity [14].Owing to the interest shown in the Fe-containing Perovskite, this mini-review presents the methods of synthesis, properties, and applications (Scheme1). This paper 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.Scheme 1
Schematic representation of the mini-review.


## 2. General Synthetic Methods of Fe-Based Perovskites

The 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. The synthesis of Fe-based Perovskite is classified into wet chemical and other synthesis routes.


### 2.1. 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.


### 2.2. Sol-Gel Synthesis

The sol-gel method is extensively used for the preparation of Fe-based Perovskite nonmaterial with large surface area. The advantages of this method are inexpensive precursors, simple preparations, and so forth. Among the Fe-based Perovskites, synthesis of BiFeO3 is difficult due to the volatile nature of Bi2O3 and it requires a higher temperature. Saira Raiz et al. vanquished it and reported BiFeO3 synthesis using sol-gel method [16]. Theingi et al. have synthesized LaFeO3 nanoparticles by sol-gel method using citric acid with a lower band gap [17]. Liu et al. have synthesized a series of compounds [CaMnO3, CaFeO3, and CaMn1-xFexO3] 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−yCuyO3 nonmaterials by sol-gel autocombustion route [19]. By thermal decomposition of the gel complex of LaFe–(C6H8O7 H2O), Shabbir et al. reported a unique sol-gel procedure for generating nano-sized Perovskite-type LaFeO3 powder. The scientists discovered that optimizing the gelling conditions leads to the creation of the LaFeO3 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 LaFeO3 Perovskite powders by sol-gel method for dye-sensitized solar cell applications. They obtained pure single phase at 850°C by xerogel formation. The flow chart of synthesis is shown in Figure 1.Figure 1
Flow chart of the synthesis of LaFeO3 [21].


### 2.3. Coprecipitation Method

The 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 BiFeO3 nanopowder by coprecipitation method using ammonia at pH = 10 [22]. Wang et al. have reported the synthesis of BiFeO3 by coprecipitation method using composite precipitants. The composite precipitants avoid the aggregation of nanoparticles [23]. Khorasani-Motlagh et al. have used octanoic acid surfactant for the synthesis of sphere like NdFeO3 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-LaFeO3 at 800°C by coprecipitation method using NaOH as precipitating agent. The photograph of the synthesized LaFeO3 is shown in the picture (Figure 2). The synthesis cost was low and no waste was observed in this method [25].Figure 2
LaFeO3 powder synthesized by coprecipitation method [25].


### 2.4. Hydrothermal Method

The 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 LaFeO3 Perovskite by the hydrothermal method without calcinations [26]. Single crystal BiFeO3 microplates were developed by hydrothermal method using C6H10BiNO8 as reductant and surface modifier [27].Many researchers have recently used the hydrothermal approach to make nano-BiFeO3 Perovskite. The list is as follows: Sazali et al. used biopolymer (chitosan) assisted hydrothermal synthesis of BiFeO3 nanoparticles and got enhanced magnetic properties [28]. Another group used a hydrogen peroxide assisted hydrothermal approach to make shape-controlled BiFeO3 microspheres. To establish an oxygen-rich environment, they employed H2O2 [29]. Han et al. also created spindle-like BiFeO3 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 H2O2. Chen and colleagues used an ethanol-assisted hydrothermal technique to make BiFeO3 nanopowders. The ethanol solvent is used extensively in this process to produce pure phase BiFeO3 [30]. On increasing the hydrothermal reaction time from 6 h to 15 h, BiFeO3 microcylinders were obtained by Di et al. [31]. The hydrothermal microwave approach produced BiFeO3 Perovskite crystals with better uniformity than the solid-state reaction method at low temperature of 180oC [32]. Researchers recently used a hydrothermal approach to produce nano-BiFeO3 using different amounts of chitosan biopolymer and KOH as a precipitating agent [28]. Wafer BiFeO3 was synthesized by hydrothermal method without using mineralizer or precipitant.Mesbah et al. employed a hydrothermal technique to synthesize nano-LaFeO3 using lanthanum and iron salts in a stoichiometric ratio. To obtain nano-sized LaFeO3, PVP was used as a capping agent, and alkali (NaOH) was added [33].


### 2.5. 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 Ba0.5Sr0.5Co0.8Fe0.2O3-δ Perovskite by microwave method and achieved same densification as conventional method with less processing time. The grain size obtained was nanoscale compared to conventional method. The microwave hydrothermal synthesis of BiFeO3 crystallites at 467 K for 2 hours was initially reported by Komarneni [35]. Joshi et al. described a simple microwave process for making well-defined single-crystalline Perovskite BiFeO3 nanocubes [36]. Zhu et al. obtained well-dispersed Perovskite BFO nanocrystals under similar experimental circumstances as Joshi’s technique, with the exception of a longer reaction period (60 min) [37].


### 2.6. Crystallography of Fe-Based Perovskite

Perovskite has an ABO3 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 ABO3 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. The 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 LaFeO3, an Fe-based Perovskite, changes depending on temperature and doping. At room temperature and up to 957°C, the structure of undoped LaFeO3 adopts a Perovskite arrangement that is orthorhombic (space group Pbnm). In the LaFeO3 crystal structure A = 5.55 Å, b = 5.56 Å, and c = 7.86 Å are the edge lengths of the unit cell at room temperature, which contains four structural units (Z = 4) [39]. Between 960 and 1005°C, LaFeO3 exhibits a first-order structural phase transition from orthorhombic to rhombohedral. This 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 SrFeO3 and CaFeO3 are isoelectronic species, their structural geometry differs due to their distinct ionic radii. Since the ionic radius of Ca2+ is lower than that of Sr, it possesses a monoclinic phase. In contrast, SrFeO3 has a cubic structure [40]. Wang et al. observed that, at room temperature, BiFeO3 bulk structure has rhombohedral symmetry with a lattice constant of 5.63 Å. The structure and characteristics of BiFeO3 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 BiFeO3, the phase of the material changes from rhombohedral to orthorhombic. Similar to this, the BiFeO3 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]. GdFeO3 and GdFeO3 doped with Mn were both produced by the sol-gel method by Maity et al. The orthorhombic phase of GdFeO3 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 MnFeO3 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 LaFeO3 via a polymerized complex technique (Figure 3). The 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 LaFeO3 [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.Figure 3
Diagram of the polymeric precursor used in the PC technique to create LaFeO3 nanoparticles [44].Fe-based Perovskite exhibits a variety of interesting properties like ferromagnetic property as in the case of BaFeO3 and increased ferromagnetism in (Ba/Ca/Sr)FeO3. 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 Fe-based Perovskites exhibit intriguing properties which are shown in Table 1.Table 1
Distinctive properties of Fe-based Perovskite.
Distinctive propertyFe-based PerovskiteFerromagnetic propertyBaFeO3,GdFeO3Electrical conductivitySrFeO3Magnetic propertyLaFeO3Catalytic propertySrFeO3, BiFeO3FerroelectricNdFeO3ElectrodeSmFeO3


### 2.7. Ferromagnetic Property

Nano-Fe-based Perovskite has superior ferromagnetic properties when compared to bulk materials. Here are a few examples. Due to its heliomagnetic order of wavelength of 62 nm, hollow BiFeO3 nanoparticles display increased ferromagnetic properties when compared to bulk material [46, 47]. Similar to the above reports, wafer-like BiFeO3 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 Pr1−yCayCo1−xFexO3 and Gd0.55Sr0.45Co1−xFexO3, 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. The 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 SmCrO3 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 BaSnO3 materials improved the material’s ferromagnetic properties. The F centre exchange interactions are responsible for the enhancing attribute [51]. Bi0.5Na0.5TiO3 materials were recently discovered to have ferromagnetism at room temperature. By adding MgFeO3 to the material, ferromagnetism was produced in the source, lowering the band gap from 3.09 eV to 2.43 eV [52]. The magnetic characteristics of Fe doped BaZrO3 were also investigated by Nisar et al. They discovered that doping Fe in the Zr site improves the material’s magnetic moment. The 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(Ti1-xFex)O3 thin films (0.15 × 0.5) and discovered that the saturation magnetization (MS) increased with the Fe content.


### 2.8. 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. The 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 et al. studied the conductivity of SrFe1-xNbxO3, where x = 0.05, 0.1, 0.2, 0.3, 0.4, and discovered that SrFe0.9Nb0.1O3 has the highest conductivity. At temperatures below 800°C, however, the conductivity remains below 1 S/cm [55]. Lan et al. studied the conductivity of SrFe0.9Nb0.1O3 and discovered that it has a conductivity of 30 S/cm in a reducing environment, which is significantly greater than previously reported values. In a reducing environment, it was also shown that partially replacing Fe with Cu in SrFe0.9Nb0.1O3 can increase conductivity even further. At 415°C and 5% H2/Ar, the Perovskite oxide SrFe0.8Cu0.1Nb0.1O3 (SFCN) has conductivities of 63 Scm1 and 60 Scm1, respectively [56].


### 2.9. Catalytic Property

The high surface activity to oxygen reduction ratio or oxygen activation that resulted from the large number of oxygen vacancies in Fe-based Perovskites was partially responsible for their high catalytic activity. Fe-based Perovskites can be used in a variety of catalytic environmental reactions. The doping of Fe in site B of Perovskites enhances the stability and catalytic activity [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 LaFeO3 film together with g-C3N4 in order to study its photocatalytic properties. In comparison to pure LaFeO3, they saw hydrogen generation of 74% at a rate of 10.8 mol/hr/cm−2 for LaFeO3/g-C3N4 [58]. Similar to this, Iervolina et al. synthesized LaFeO3 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. They found that increasing the amount of citric acid by twofold boosts the LaFeO3 surface area and the photocatalytic property of hydrogen generation [59]. Ibrahim successfully created n-type LaFeO3 Perovskite using the sol-gel process. He looked into the hydrogen-generating photocatalytic property and developed a low-cost, robust photoelectrochemical cell for solar energy conversion [60].The 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 LaFeO3 using the sol-gel method and investigated its photocatalytic ability to degrade 4-chlorophenol (4-CP) and rhodamine B (RhB). He noticed that LaFeO3 that has been calcined at 700oC 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 LaFeO3/Ag2CO3 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 LaFeO3, Vijayaraghavan developed a composite made of LaFeO3 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]. The mechanism of the photocatalytic dye degradation and water splitting by LaFeO3 is depicted in Figure 4.Figure 4
LaFeO3’s mechanism for water splitting and dye degradation [63].Microwave-prepared BiFeO3 and LaFeO3 were examined for their photoelectrochemical properties. LaFeO3 showed stronger water splitting than BiFeO3 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. They 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].


### 2.10. 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.


#### 2.10.1. 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. The 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 (O2), and various Fe-based 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 LaFeO3, Sr, and Mg modified LaFeO3 nanoparticles. They found that modified LaFeO3 showed less sensitivity to CO, C2H4, and CH4 compared to unmodified LaFeO3 [68]. Recently, Fabio developed a gas sensor for detecting CO gas using Ti substituted lanthanum ferrite Perovskite (LaFe0.8Ti0.2O3) [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. Bi5Ti3FeO15 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. The gas sensing setup is depicted in Figure 5 [70]. 2% weight Pd doped LaFeO3 prepared by Xiao Feng et al. showed good response for detecting low concentration of acetone [71]. Wang et al. prepared LaFeO3 nanocrystalline powders by sol-gel method for sensing carbon dioxide gas [72]. Cao and coworkers reported the ethanol gas sensing property of chlorine doped LaFeO3 nanocrystals [73]. Ma et al. prepared mesoporous hollow PrFeO3 (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-LaFeO3 nanoparticles. They also found that the synthesized nanoparticle (Ag-LaFeO3) exhibits enhanced xylene gas sensing property [75]. Similarly, Han et al. also prepared SmFeO3 nanofibers using electrospinning method for the detection of ethylene glycol [76]. Perovskite Ag-LaFeO3 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 LaFeO3 for HCHO sensing at125°C [78]. Similarly, Queral to et al. [79] synthesized the LaFeO3 nanofibers by calcination at 600°C for the sensing of sulphur containing gases.Table 2
The use of Fe-based Perovskites as sensors for detecting various environmental pollutants.
Fe-based PerovskiteDetecting pollutantPollutant limit (ppm)Form of PerovskiteReferenceEuFe0.9Co0.1O3AcetonePowder[80]LaFeO3CO22000Powder[72]Chlorine doped LaFeO3EthanolPowder[73]PrFeO3 nanofibersAcetone500Nanofibers[74]Ag-LaFeO3Xylene10Powder[75]SmFeO3Ethylene glycol5Nanofibers[76]Ag-LaFeO3HCHONanofibers[77]LaFeO3HCHOPowder[78]LaFeO3SulphurPowder[79]Ca doped BiFeO3Hydrogen500Thin film[81]BiFeO3Carbon monoxide30Powder[82]BiFeO3SulphurLowPowder[83]SmFeO3Acetylene2–80Thin film[84]LaFeO3NOx5Thin film[85]Figure 5
Gas sensing setup of Bi5Ti3FeO15 [70].


#### 2.10.2. 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).Table 3
The use of Fe-based Perovskite as adsorbent for dyes and heavy metal ions.
Fe-based PerovskiteAdsorbing pollutantContact timeRemovalReferenceCTAB-capped La0.9Sr0.1FeO3Anionic Congo red—97%[90]SrFeO3-δBisphenol A and acid orange 824 hrs83%[91]NdFeO3As(V)—126.58%[92]CuFe2O4/PMSAs(III)—63.9 mg g−1[93]LaFeO3-ACFRhodamine B—182.6 mg g−1[94]Gd0.5Sr0.5FeO3Methylene blue——[95]The adsorption of various dyes by Fe-based Perovskite was also investigated. Shima capped La0.9Sr0.1FeO3 nano-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 La0.9Sr0.1FeO3 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 LaFeO3-ACF was reported by Deng et al. [94]. They prepared activated carbon fibers (ACF) from cotton waste and decorated on LaFeO3 by sol-gel and thermal treatment. The adsorption efficiency of LaFeO3-ACF was higher compared to LaFeO3 due to the electrostatic interaction, hydrogen bonding, π-π stacking, and cation-π interactions. The adsorption of methylene blue dye from water by Gd0.5Sr0.5FeO3 Perovskite synthesized from sol-gel method was reported [95]. The adsorption of heavy metals by Fe-based Perovskite was also explored. The adsorption of As(V) by NdFeO3 synthesized by the polymeric gel precursor method was reported [92]. Removal of As(III) from the water was faster and more efficient by CuFe2O4/PMS Perovskite than by CuFe2O4 [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. LaFeO3 showed an excellent and better photocatalytic property than Fe2O3 [96]. Doping of Mn in LaFeO3 and its photocatalytic property is investigated. The doping enhances the photocatalytic property [97]. BiFeO3 was also used as a photocatalyst due to the electron-hole separation. Doping of metals in BiFeO3 and its influence on photocatalytic property was also investigated. Gd and Ca doped BiFeO3 showed enhanced photocatalytic degradation of dyes [98, 99]. Further studies have to be explored on the enhancement of photocatalytic studies of BiFeO3. Similarly, Jaffari et al. also prepared the Pd doped BiFeO3 microcomposite by hydrothermal technique. The prepared composite has coral-shaped BiFeO3 surface loaded with spherical Pd nanoparticles, which exhibits more enhanced photoactivity than pure BiFeO3. The enhanced photoactivity is due to the Pd dopant, and the composite possesses excellent recyclability with minimum leakage of Pd after six runs. They also proposed that Pd doped BiFeO3 microcomposite exhibits as potent antimicrobial agent [100].Sydorchuk et al. recently have investigated the photocatalytic properties of PrCo1−xFexO3 Perovskite powders [101]. Their studies were focused on the influence of structure and its composition on photocatalytic properties.Researchers also reported that GaFeO3 has good water splitting capacity without any cocatalyst [102]. Another report states that YFeO3 has four times enhanced photocatalytic activity than TiO2:P25 [103]. Fe-based Perovskite is extensively applied as an environmental catalyst due to its magnetic recovery. Thirumalai Rajan and his coworkers prepared floral-like LaFeO3 nanostructures by surfactant assisted hydrothermal method. The prepared floral LaFeO3 nanostructure was used for degradation of rhodamine B (RhB) and methylene blue (MB) under visible light irradiation [96].


#### 2.10.3. 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. The 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].The production of electrons and holes on the surface of the catalyst causes photocatalytic degradation of dyes. The adsorbed compounds will undergo redox reaction with the produced electrons and holes on the catalyst [106]. The 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, Thirumulairajan et al. used a hydrothermal process to make LaFeO3 in three different shapes: nanocubes, nanorods, and nanospheres. The efficacy of the produced nanostructures on photocatalytic degradation of rhodamine dye is being investigated. They found that LaFeO3 nanostructures have better photocatalytic activity. Nanospheres, on the other hand, were shown to be more efficient than TiO2 [96]. Similarly, the same researchers prepared floral nanostructured LaFeO3 with a band-gap of 2.10 eV, which showed better photocatalytic efficacy in decomposing rhodamine B and methylene blue when compared to bulk LaFeO3 [107].Doping of Fe-based Perovskite at both sites gives higher photocatalytic efficiency by reducing the band-gap. The 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 H2O2, 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 TiO2, they discovered that Pb substitution increased photocatalytic efficiency. The 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]. The photocatalytic efficiency of Perovskite is improved by Fe doping. The photocatalytic degradation of RNL azo dye was investigated using Fe doped BaSnO3. Fe doping in BaSnO3 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 BiFeO3 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. The schematic representation of its mechanism and photoelectrocatalytic efficiency is given in Figure 6 [113].Figure 6
Schematic representation of photoelectrocatalytic degradation of dyes and pharmaceutical pollutants using BiFeO3 and 10% La doped BiFeO3 [113].The principal pollutant responsible for ozone depletion in the stratosphere is nitrous oxide. Infrared radiation is also absorbed by N2O, which contributes to the greenhouse effect. Thus, one of the key research areas in environmental catalysis is to control N2O emissions from industries [114]. Although there are a few strategies for reducing N2O 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 N2O reduction and decomposition reactions.By doping BaTiO3 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 BiFeO3 nanostructures and investigated their photoelectrochemical characteristics. Photo-induced water oxidation activity of single-crystalline BiFeO3 nanocubes was discovered by Joshi et al., implying that BFO could be a promising material for photocatalytic applications [36]. For photoelectrochemical water splitting, BiFeO3 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 LaFeO3 crystals have a smaller band-gap and consequently a weak O–Fe bond strength, leading to increased methane conversion activity and a large amount of detachable O [117]. Researchers have discovered LaFeO3 and La0.8Sr0.2FeO3 Perovskites with low oxygen mobility as promising partial oxidation selective catalysts, but La0.5Sr0.5Fe1–xCoxO3 (x = 0, 0.5, 1) Perovskites with high oxygen mobility are well suited to methane total combustion [118].The optical properties of Perovskite are enhanced by dopants. The capacity of Fe doping in Perovskites to improve the oxygen evolution reaction is explored here. The significance of Fe in improving OER activity and stability has remained a mystery until now.After replacing Fe3+ in the LaFeO3 Perovskite with Cu2+ and Ni2+, the catalytic activity for the N2O decomposition reaction enhanced. The catalytic activity was further boosted by increasing the reaction temperature. In the case of Ni- and Cu-included LaFeO3 samples, the increased oxygen mobility and minor increase in surface area resulted in the establishment of additional active sites for N2O 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.


## 2.1. 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.


## 2.2. Sol-Gel Synthesis

The sol-gel method is extensively used for the preparation of Fe-based Perovskite nonmaterial with large surface area. The advantages of this method are inexpensive precursors, simple preparations, and so forth. Among the Fe-based Perovskites, synthesis of BiFeO3 is difficult due to the volatile nature of Bi2O3 and it requires a higher temperature. Saira Raiz et al. vanquished it and reported BiFeO3 synthesis using sol-gel method [16]. Theingi et al. have synthesized LaFeO3 nanoparticles by sol-gel method using citric acid with a lower band gap [17]. Liu et al. have synthesized a series of compounds [CaMnO3, CaFeO3, and CaMn1-xFexO3] 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−yCuyO3 nonmaterials by sol-gel autocombustion route [19]. By thermal decomposition of the gel complex of LaFe–(C6H8O7 H2O), Shabbir et al. reported a unique sol-gel procedure for generating nano-sized Perovskite-type LaFeO3 powder. The scientists discovered that optimizing the gelling conditions leads to the creation of the LaFeO3 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 LaFeO3 Perovskite powders by sol-gel method for dye-sensitized solar cell applications. They obtained pure single phase at 850°C by xerogel formation. The flow chart of synthesis is shown in Figure 1.Figure 1
Flow chart of the synthesis of LaFeO3 [21].


## 2.3. Coprecipitation Method

The 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 BiFeO3 nanopowder by coprecipitation method using ammonia at pH = 10 [22]. Wang et al. have reported the synthesis of BiFeO3 by coprecipitation method using composite precipitants. The composite precipitants avoid the aggregation of nanoparticles [23]. Khorasani-Motlagh et al. have used octanoic acid surfactant for the synthesis of sphere like NdFeO3 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-LaFeO3 at 800°C by coprecipitation method using NaOH as precipitating agent. The photograph of the synthesized LaFeO3 is shown in the picture (Figure 2). The synthesis cost was low and no waste was observed in this method [25].Figure 2
LaFeO3 powder synthesized by coprecipitation method [25].


## 2.4. Hydrothermal Method

The 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 LaFeO3 Perovskite by the hydrothermal method without calcinations [26]. Single crystal BiFeO3 microplates were developed by hydrothermal method using C6H10BiNO8 as reductant and surface modifier [27].Many researchers have recently used the hydrothermal approach to make nano-BiFeO3 Perovskite. The list is as follows: Sazali et al. used biopolymer (chitosan) assisted hydrothermal synthesis of BiFeO3 nanoparticles and got enhanced magnetic properties [28]. Another group used a hydrogen peroxide assisted hydrothermal approach to make shape-controlled BiFeO3 microspheres. To establish an oxygen-rich environment, they employed H2O2 [29]. Han et al. also created spindle-like BiFeO3 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 H2O2. Chen and colleagues used an ethanol-assisted hydrothermal technique to make BiFeO3 nanopowders. The ethanol solvent is used extensively in this process to produce pure phase BiFeO3 [30]. On increasing the hydrothermal reaction time from 6 h to 15 h, BiFeO3 microcylinders were obtained by Di et al. [31]. The hydrothermal microwave approach produced BiFeO3 Perovskite crystals with better uniformity than the solid-state reaction method at low temperature of 180oC [32]. Researchers recently used a hydrothermal approach to produce nano-BiFeO3 using different amounts of chitosan biopolymer and KOH as a precipitating agent [28]. Wafer BiFeO3 was synthesized by hydrothermal method without using mineralizer or precipitant.Mesbah et al. employed a hydrothermal technique to synthesize nano-LaFeO3 using lanthanum and iron salts in a stoichiometric ratio. To obtain nano-sized LaFeO3, PVP was used as a capping agent, and alkali (NaOH) was added [33].


## 2.5. 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 Ba0.5Sr0.5Co0.8Fe0.2O3-δ Perovskite by microwave method and achieved same densification as conventional method with less processing time. The grain size obtained was nanoscale compared to conventional method. The microwave hydrothermal synthesis of BiFeO3 crystallites at 467 K for 2 hours was initially reported by Komarneni [35]. Joshi et al. described a simple microwave process for making well-defined single-crystalline Perovskite BiFeO3 nanocubes [36]. Zhu et al. obtained well-dispersed Perovskite BFO nanocrystals under similar experimental circumstances as Joshi’s technique, with the exception of a longer reaction period (60 min) [37].


## 2.6. Crystallography of Fe-Based Perovskite

Perovskite has an ABO3 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 ABO3 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. The 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 LaFeO3, an Fe-based Perovskite, changes depending on temperature and doping. At room temperature and up to 957°C, the structure of undoped LaFeO3 adopts a Perovskite arrangement that is orthorhombic (space group Pbnm). In the LaFeO3 crystal structure A = 5.55 Å, b = 5.56 Å, and c = 7.86 Å are the edge lengths of the unit cell at room temperature, which contains four structural units (Z = 4) [39]. Between 960 and 1005°C, LaFeO3 exhibits a first-order structural phase transition from orthorhombic to rhombohedral. This 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 SrFeO3 and CaFeO3 are isoelectronic species, their structural geometry differs due to their distinct ionic radii. Since the ionic radius of Ca2+ is lower than that of Sr, it possesses a monoclinic phase. In contrast, SrFeO3 has a cubic structure [40]. Wang et al. observed that, at room temperature, BiFeO3 bulk structure has rhombohedral symmetry with a lattice constant of 5.63 Å. The structure and characteristics of BiFeO3 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 BiFeO3, the phase of the material changes from rhombohedral to orthorhombic. Similar to this, the BiFeO3 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]. GdFeO3 and GdFeO3 doped with Mn were both produced by the sol-gel method by Maity et al. The orthorhombic phase of GdFeO3 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 MnFeO3 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 LaFeO3 via a polymerized complex technique (Figure 3). The 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 LaFeO3 [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.Figure 3
Diagram of the polymeric precursor used in the PC technique to create LaFeO3 nanoparticles [44].Fe-based Perovskite exhibits a variety of interesting properties like ferromagnetic property as in the case of BaFeO3 and increased ferromagnetism in (Ba/Ca/Sr)FeO3. 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 Fe-based Perovskites exhibit intriguing properties which are shown in Table 1.Table 1
Distinctive properties of Fe-based Perovskite.
Distinctive propertyFe-based PerovskiteFerromagnetic propertyBaFeO3,GdFeO3Electrical conductivitySrFeO3Magnetic propertyLaFeO3Catalytic propertySrFeO3, BiFeO3FerroelectricNdFeO3ElectrodeSmFeO3


## 2.7. Ferromagnetic Property

Nano-Fe-based Perovskite has superior ferromagnetic properties when compared to bulk materials. Here are a few examples. Due to its heliomagnetic order of wavelength of 62 nm, hollow BiFeO3 nanoparticles display increased ferromagnetic properties when compared to bulk material [46, 47]. Similar to the above reports, wafer-like BiFeO3 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 Pr1−yCayCo1−xFexO3 and Gd0.55Sr0.45Co1−xFexO3, 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. The 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 SmCrO3 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 BaSnO3 materials improved the material’s ferromagnetic properties. The F centre exchange interactions are responsible for the enhancing attribute [51]. Bi0.5Na0.5TiO3 materials were recently discovered to have ferromagnetism at room temperature. By adding MgFeO3 to the material, ferromagnetism was produced in the source, lowering the band gap from 3.09 eV to 2.43 eV [52]. The magnetic characteristics of Fe doped BaZrO3 were also investigated by Nisar et al. They discovered that doping Fe in the Zr site improves the material’s magnetic moment. The 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(Ti1-xFex)O3 thin films (0.15 × 0.5) and discovered that the saturation magnetization (MS) increased with the Fe content.


## 2.8. 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. The 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 et al. studied the conductivity of SrFe1-xNbxO3, where x = 0.05, 0.1, 0.2, 0.3, 0.4, and discovered that SrFe0.9Nb0.1O3 has the highest conductivity. At temperatures below 800°C, however, the conductivity remains below 1 S/cm [55]. Lan et al. studied the conductivity of SrFe0.9Nb0.1O3 and discovered that it has a conductivity of 30 S/cm in a reducing environment, which is significantly greater than previously reported values. In a reducing environment, it was also shown that partially replacing Fe with Cu in SrFe0.9Nb0.1O3 can increase conductivity even further. At 415°C and 5% H2/Ar, the Perovskite oxide SrFe0.8Cu0.1Nb0.1O3 (SFCN) has conductivities of 63 Scm1 and 60 Scm1, respectively [56].


## 2.9. Catalytic Property

The high surface activity to oxygen reduction ratio or oxygen activation that resulted from the large number of oxygen vacancies in Fe-based Perovskites was partially responsible for their high catalytic activity. Fe-based Perovskites can be used in a variety of catalytic environmental reactions. The doping of Fe in site B of Perovskites enhances the stability and catalytic activity [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 LaFeO3 film together with g-C3N4 in order to study its photocatalytic properties. In comparison to pure LaFeO3, they saw hydrogen generation of 74% at a rate of 10.8 mol/hr/cm−2 for LaFeO3/g-C3N4 [58]. Similar to this, Iervolina et al. synthesized LaFeO3 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. They found that increasing the amount of citric acid by twofold boosts the LaFeO3 surface area and the photocatalytic property of hydrogen generation [59]. Ibrahim successfully created n-type LaFeO3 Perovskite using the sol-gel process. He looked into the hydrogen-generating photocatalytic property and developed a low-cost, robust photoelectrochemical cell for solar energy conversion [60].The 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 LaFeO3 using the sol-gel method and investigated its photocatalytic ability to degrade 4-chlorophenol (4-CP) and rhodamine B (RhB). He noticed that LaFeO3 that has been calcined at 700oC 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 LaFeO3/Ag2CO3 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 LaFeO3, Vijayaraghavan developed a composite made of LaFeO3 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]. The mechanism of the photocatalytic dye degradation and water splitting by LaFeO3 is depicted in Figure 4.Figure 4
LaFeO3’s mechanism for water splitting and dye degradation [63].Microwave-prepared BiFeO3 and LaFeO3 were examined for their photoelectrochemical properties. LaFeO3 showed stronger water splitting than BiFeO3 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. They 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].


## 2.10. 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.


### 2.10.1. 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. The 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 (O2), and various Fe-based 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 LaFeO3, Sr, and Mg modified LaFeO3 nanoparticles. They found that modified LaFeO3 showed less sensitivity to CO, C2H4, and CH4 compared to unmodified LaFeO3 [68]. Recently, Fabio developed a gas sensor for detecting CO gas using Ti substituted lanthanum ferrite Perovskite (LaFe0.8Ti0.2O3) [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. Bi5Ti3FeO15 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. The gas sensing setup is depicted in Figure 5 [70]. 2% weight Pd doped LaFeO3 prepared by Xiao Feng et al. showed good response for detecting low concentration of acetone [71]. Wang et al. prepared LaFeO3 nanocrystalline powders by sol-gel method for sensing carbon dioxide gas [72]. Cao and coworkers reported the ethanol gas sensing property of chlorine doped LaFeO3 nanocrystals [73]. Ma et al. prepared mesoporous hollow PrFeO3 (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-LaFeO3 nanoparticles. They also found that the synthesized nanoparticle (Ag-LaFeO3) exhibits enhanced xylene gas sensing property [75]. Similarly, Han et al. also prepared SmFeO3 nanofibers using electrospinning method for the detection of ethylene glycol [76]. Perovskite Ag-LaFeO3 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 LaFeO3 for HCHO sensing at125°C [78]. Similarly, Queral to et al. [79] synthesized the LaFeO3 nanofibers by calcination at 600°C for the sensing of sulphur containing gases.Table 2
The use of Fe-based Perovskites as sensors for detecting various environmental pollutants.
Fe-based PerovskiteDetecting pollutantPollutant limit (ppm)Form of PerovskiteReferenceEuFe0.9Co0.1O3AcetonePowder[80]LaFeO3CO22000Powder[72]Chlorine doped LaFeO3EthanolPowder[73]PrFeO3 nanofibersAcetone500Nanofibers[74]Ag-LaFeO3Xylene10Powder[75]SmFeO3Ethylene glycol5Nanofibers[76]Ag-LaFeO3HCHONanofibers[77]LaFeO3HCHOPowder[78]LaFeO3SulphurPowder[79]Ca doped BiFeO3Hydrogen500Thin film[81]BiFeO3Carbon monoxide30Powder[82]BiFeO3SulphurLowPowder[83]SmFeO3Acetylene2–80Thin film[84]LaFeO3NOx5Thin film[85]Figure 5
Gas sensing setup of Bi5Ti3FeO15 [70].


### 2.10.2. 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).Table 3
The use of Fe-based Perovskite as adsorbent for dyes and heavy metal ions.
Fe-based PerovskiteAdsorbing pollutantContact timeRemovalReferenceCTAB-capped La0.9Sr0.1FeO3Anionic Congo red—97%[90]SrFeO3-δBisphenol A and acid orange 824 hrs83%[91]NdFeO3As(V)—126.58%[92]CuFe2O4/PMSAs(III)—63.9 mg g−1[93]LaFeO3-ACFRhodamine B—182.6 mg g−1[94]Gd0.5Sr0.5FeO3Methylene blue——[95]The adsorption of various dyes by Fe-based Perovskite was also investigated. Shima capped La0.9Sr0.1FeO3 nano-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 La0.9Sr0.1FeO3 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 LaFeO3-ACF was reported by Deng et al. [94]. They prepared activated carbon fibers (ACF) from cotton waste and decorated on LaFeO3 by sol-gel and thermal treatment. The adsorption efficiency of LaFeO3-ACF was higher compared to LaFeO3 due to the electrostatic interaction, hydrogen bonding, π-π stacking, and cation-π interactions. The adsorption of methylene blue dye from water by Gd0.5Sr0.5FeO3 Perovskite synthesized from sol-gel method was reported [95]. The adsorption of heavy metals by Fe-based Perovskite was also explored. The adsorption of As(V) by NdFeO3 synthesized by the polymeric gel precursor method was reported [92]. Removal of As(III) from the water was faster and more efficient by CuFe2O4/PMS Perovskite than by CuFe2O4 [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. LaFeO3 showed an excellent and better photocatalytic property than Fe2O3 [96]. Doping of Mn in LaFeO3 and its photocatalytic property is investigated. The doping enhances the photocatalytic property [97]. BiFeO3 was also used as a photocatalyst due to the electron-hole separation. Doping of metals in BiFeO3 and its influence on photocatalytic property was also investigated. Gd and Ca doped BiFeO3 showed enhanced photocatalytic degradation of dyes [98, 99]. Further studies have to be explored on the enhancement of photocatalytic studies of BiFeO3. Similarly, Jaffari et al. also prepared the Pd doped BiFeO3 microcomposite by hydrothermal technique. The prepared composite has coral-shaped BiFeO3 surface loaded with spherical Pd nanoparticles, which exhibits more enhanced photoactivity than pure BiFeO3. The enhanced photoactivity is due to the Pd dopant, and the composite possesses excellent recyclability with minimum leakage of Pd after six runs. They also proposed that Pd doped BiFeO3 microcomposite exhibits as potent antimicrobial agent [100].Sydorchuk et al. recently have investigated the photocatalytic properties of PrCo1−xFexO3 Perovskite powders [101]. Their studies were focused on the influence of structure and its composition on photocatalytic properties.Researchers also reported that GaFeO3 has good water splitting capacity without any cocatalyst [102]. Another report states that YFeO3 has four times enhanced photocatalytic activity than TiO2:P25 [103]. Fe-based Perovskite is extensively applied as an environmental catalyst due to its magnetic recovery. Thirumalai Rajan and his coworkers prepared floral-like LaFeO3 nanostructures by surfactant assisted hydrothermal method. The prepared floral LaFeO3 nanostructure was used for degradation of rhodamine B (RhB) and methylene blue (MB) under visible light irradiation [96].


### 2.10.3. 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. The 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].The production of electrons and holes on the surface of the catalyst causes photocatalytic degradation of dyes. The adsorbed compounds will undergo redox reaction with the produced electrons and holes on the catalyst [106]. The 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, Thirumulairajan et al. used a hydrothermal process to make LaFeO3 in three different shapes: nanocubes, nanorods, and nanospheres. The efficacy of the produced nanostructures on photocatalytic degradation of rhodamine dye is being investigated. They found that LaFeO3 nanostructures have better photocatalytic activity. Nanospheres, on the other hand, were shown to be more efficient than TiO2 [96]. Similarly, the same researchers prepared floral nanostructured LaFeO3 with a band-gap of 2.10 eV, which showed better photocatalytic efficacy in decomposing rhodamine B and methylene blue when compared to bulk LaFeO3 [107].Doping of Fe-based Perovskite at both sites gives higher photocatalytic efficiency by reducing the band-gap. The 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 H2O2, 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 TiO2, they discovered that Pb substitution increased photocatalytic efficiency. The 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]. The photocatalytic efficiency of Perovskite is improved by Fe doping. The photocatalytic degradation of RNL azo dye was investigated using Fe doped BaSnO3. Fe doping in BaSnO3 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 BiFeO3 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. The schematic representation of its mechanism and photoelectrocatalytic efficiency is given in Figure 6 [113].Figure 6
Schematic representation of photoelectrocatalytic degradation of dyes and pharmaceutical pollutants using BiFeO3 and 10% La doped BiFeO3 [113].The principal pollutant responsible for ozone depletion in the stratosphere is nitrous oxide. Infrared radiation is also absorbed by N2O, which contributes to the greenhouse effect. Thus, one of the key research areas in environmental catalysis is to control N2O emissions from industries [114]. Although there are a few strategies for reducing N2O 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 N2O reduction and decomposition reactions.By doping BaTiO3 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 BiFeO3 nanostructures and investigated their photoelectrochemical characteristics. Photo-induced water oxidation activity of single-crystalline BiFeO3 nanocubes was discovered by Joshi et al., implying that BFO could be a promising material for photocatalytic applications [36]. For photoelectrochemical water splitting, BiFeO3 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 LaFeO3 crystals have a smaller band-gap and consequently a weak O–Fe bond strength, leading to increased methane conversion activity and a large amount of detachable O [117]. Researchers have discovered LaFeO3 and La0.8Sr0.2FeO3 Perovskites with low oxygen mobility as promising partial oxidation selective catalysts, but La0.5Sr0.5Fe1–xCoxO3 (x = 0, 0.5, 1) Perovskites with high oxygen mobility are well suited to methane total combustion [118].The optical properties of Perovskite are enhanced by dopants. The capacity of Fe doping in Perovskites to improve the oxygen evolution reaction is explored here. The significance of Fe in improving OER activity and stability has remained a mystery until now.After replacing Fe3+ in the LaFeO3 Perovskite with Cu2+ and Ni2+, the catalytic activity for the N2O decomposition reaction enhanced. The catalytic activity was further boosted by increasing the reaction temperature. In the case of Ni- and Cu-included LaFeO3 samples, the increased oxygen mobility and minor increase in surface area resulted in the establishment of additional active sites for N2O 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.


## 2.10.1. 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. The 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 (O2), and various Fe-based 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 LaFeO3, Sr, and Mg modified LaFeO3 nanoparticles. They found that modified LaFeO3 showed less sensitivity to CO, C2H4, and CH4 compared to unmodified LaFeO3 [68]. Recently, Fabio developed a gas sensor for detecting CO gas using Ti substituted lanthanum ferrite Perovskite (LaFe0.8Ti0.2O3) [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. Bi5Ti3FeO15 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. The gas sensing setup is depicted in Figure 5 [70]. 2% weight Pd doped LaFeO3 prepared by Xiao Feng et al. showed good response for detecting low concentration of acetone [71]. Wang et al. prepared LaFeO3 nanocrystalline powders by sol-gel method for sensing carbon dioxide gas [72]. Cao and coworkers reported the ethanol gas sensing property of chlorine doped LaFeO3 nanocrystals [73]. Ma et al. prepared mesoporous hollow PrFeO3 (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-LaFeO3 nanoparticles. They also found that the synthesized nanoparticle (Ag-LaFeO3) exhibits enhanced xylene gas sensing property [75]. Similarly, Han et al. also prepared SmFeO3 nanofibers using electrospinning method for the detection of ethylene glycol [76]. Perovskite Ag-LaFeO3 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 LaFeO3 for HCHO sensing at125°C [78]. Similarly, Queral to et al. [79] synthesized the LaFeO3 nanofibers by calcination at 600°C for the sensing of sulphur containing gases.Table 2
The use of Fe-based Perovskites as sensors for detecting various environmental pollutants.
Fe-based PerovskiteDetecting pollutantPollutant limit (ppm)Form of PerovskiteReferenceEuFe0.9Co0.1O3AcetonePowder[80]LaFeO3CO22000Powder[72]Chlorine doped LaFeO3EthanolPowder[73]PrFeO3 nanofibersAcetone500Nanofibers[74]Ag-LaFeO3Xylene10Powder[75]SmFeO3Ethylene glycol5Nanofibers[76]Ag-LaFeO3HCHONanofibers[77]LaFeO3HCHOPowder[78]LaFeO3SulphurPowder[79]Ca doped BiFeO3Hydrogen500Thin film[81]BiFeO3Carbon monoxide30Powder[82]BiFeO3SulphurLowPowder[83]SmFeO3Acetylene2–80Thin film[84]LaFeO3NOx5Thin film[85]Figure 5
Gas sensing setup of Bi5Ti3FeO15 [70].


## 2.10.2. 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).Table 3
The use of Fe-based Perovskite as adsorbent for dyes and heavy metal ions.
Fe-based PerovskiteAdsorbing pollutantContact timeRemovalReferenceCTAB-capped La0.9Sr0.1FeO3Anionic Congo red—97%[90]SrFeO3-δBisphenol A and acid orange 824 hrs83%[91]NdFeO3As(V)—126.58%[92]CuFe2O4/PMSAs(III)—63.9 mg g−1[93]LaFeO3-ACFRhodamine B—182.6 mg g−1[94]Gd0.5Sr0.5FeO3Methylene blue——[95]The adsorption of various dyes by Fe-based Perovskite was also investigated. Shima capped La0.9Sr0.1FeO3 nano-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 La0.9Sr0.1FeO3 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 LaFeO3-ACF was reported by Deng et al. [94]. They prepared activated carbon fibers (ACF) from cotton waste and decorated on LaFeO3 by sol-gel and thermal treatment. The adsorption efficiency of LaFeO3-ACF was higher compared to LaFeO3 due to the electrostatic interaction, hydrogen bonding, π-π stacking, and cation-π interactions. The adsorption of methylene blue dye from water by Gd0.5Sr0.5FeO3 Perovskite synthesized from sol-gel method was reported [95]. The adsorption of heavy metals by Fe-based Perovskite was also explored. The adsorption of As(V) by NdFeO3 synthesized by the polymeric gel precursor method was reported [92]. Removal of As(III) from the water was faster and more efficient by CuFe2O4/PMS Perovskite than by CuFe2O4 [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. LaFeO3 showed an excellent and better photocatalytic property than Fe2O3 [96]. Doping of Mn in LaFeO3 and its photocatalytic property is investigated. The doping enhances the photocatalytic property [97]. BiFeO3 was also used as a photocatalyst due to the electron-hole separation. Doping of metals in BiFeO3 and its influence on photocatalytic property was also investigated. Gd and Ca doped BiFeO3 showed enhanced photocatalytic degradation of dyes [98, 99]. Further studies have to be explored on the enhancement of photocatalytic studies of BiFeO3. Similarly, Jaffari et al. also prepared the Pd doped BiFeO3 microcomposite by hydrothermal technique. The prepared composite has coral-shaped BiFeO3 surface loaded with spherical Pd nanoparticles, which exhibits more enhanced photoactivity than pure BiFeO3. The enhanced photoactivity is due to the Pd dopant, and the composite possesses excellent recyclability with minimum leakage of Pd after six runs. They also proposed that Pd doped BiFeO3 microcomposite exhibits as potent antimicrobial agent [100].Sydorchuk et al. recently have investigated the photocatalytic properties of PrCo1−xFexO3 Perovskite powders [101]. Their studies were focused on the influence of structure and its composition on photocatalytic properties.Researchers also reported that GaFeO3 has good water splitting capacity without any cocatalyst [102]. Another report states that YFeO3 has four times enhanced photocatalytic activity than TiO2:P25 [103]. Fe-based Perovskite is extensively applied as an environmental catalyst due to its magnetic recovery. Thirumalai Rajan and his coworkers prepared floral-like LaFeO3 nanostructures by surfactant assisted hydrothermal method. The prepared floral LaFeO3 nanostructure was used for degradation of rhodamine B (RhB) and methylene blue (MB) under visible light irradiation [96].


## 2.10.3. 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. The 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].The production of electrons and holes on the surface of the catalyst causes photocatalytic degradation of dyes. The adsorbed compounds will undergo redox reaction with the produced electrons and holes on the catalyst [106]. The 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, Thirumulairajan et al. used a hydrothermal process to make LaFeO3 in three different shapes: nanocubes, nanorods, and nanospheres. The efficacy of the produced nanostructures on photocatalytic degradation of rhodamine dye is being investigated. They found that LaFeO3 nanostructures have better photocatalytic activity. Nanospheres, on the other hand, were shown to be more efficient than TiO2 [96]. Similarly, the same researchers prepared floral nanostructured LaFeO3 with a band-gap of 2.10 eV, which showed better photocatalytic efficacy in decomposing rhodamine B and methylene blue when compared to bulk LaFeO3 [107].Doping of Fe-based Perovskite at both sites gives higher photocatalytic efficiency by reducing the band-gap. The 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 H2O2, 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 TiO2, they discovered that Pb substitution increased photocatalytic efficiency. The 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]. The photocatalytic efficiency of Perovskite is improved by Fe doping. The photocatalytic degradation of RNL azo dye was investigated using Fe doped BaSnO3. Fe doping in BaSnO3 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 BiFeO3 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. The schematic representation of its mechanism and photoelectrocatalytic efficiency is given in Figure 6 [113].Figure 6
Schematic representation of photoelectrocatalytic degradation of dyes and pharmaceutical pollutants using BiFeO3 and 10% La doped BiFeO3 [113].The principal pollutant responsible for ozone depletion in the stratosphere is nitrous oxide. Infrared radiation is also absorbed by N2O, which contributes to the greenhouse effect. Thus, one of the key research areas in environmental catalysis is to control N2O emissions from industries [114]. Although there are a few strategies for reducing N2O 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 N2O reduction and decomposition reactions.By doping BaTiO3 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 BiFeO3 nanostructures and investigated their photoelectrochemical characteristics. Photo-induced water oxidation activity of single-crystalline BiFeO3 nanocubes was discovered by Joshi et al., implying that BFO could be a promising material for photocatalytic applications [36]. For photoelectrochemical water splitting, BiFeO3 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 LaFeO3 crystals have a smaller band-gap and consequently a weak O–Fe bond strength, leading to increased methane conversion activity and a large amount of detachable O [117]. Researchers have discovered LaFeO3 and La0.8Sr0.2FeO3 Perovskites with low oxygen mobility as promising partial oxidation selective catalysts, but La0.5Sr0.5Fe1–xCoxO3 (x = 0, 0.5, 1) Perovskites with high oxygen mobility are well suited to methane total combustion [118].The optical properties of Perovskite are enhanced by dopants. The capacity of Fe doping in Perovskites to improve the oxygen evolution reaction is explored here. The significance of Fe in improving OER activity and stability has remained a mystery until now.After replacing Fe3+ in the LaFeO3 Perovskite with Cu2+ and Ni2+, the catalytic activity for the N2O decomposition reaction enhanced. The catalytic activity was further boosted by increasing the reaction temperature. In the case of Ni- and Cu-included LaFeO3 samples, the increased oxygen mobility and minor increase in surface area resulted in the establishment of additional active sites for N2O 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.


## 3. 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.The 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. The 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. There 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.The 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.



---

*Source: 6607683-2022-09-20.xml*