Facile Synthesis, Characterization, Catalytic and Photocatalytic Activity of Multiferroic BiFeO3 Perovskite Nanoparticles

We report the synthesis of multiferroic BiFeO3 perovskite nanoparticles using the microwave combustion technique. Phase evolution is investigated by XRD, which confirms that the formation of a secondary α-Bi2O3 phase with a monoclinic structure along with the existing rhombohedral (BiFeO3) structure. The average crystalline size has been found at 50 nm. The optical band gap was calculated from the Tauc’s plot it has been found 2.18 eV. The appearances of FT-IR spectra revealed bands at 550 and 444 cm−1 were correlated to the rhombohedral stretching modes of BiFeO3 nanostructure. The surface morphology showed the formation of nanosized grains with pores. The magnetization-Field (M-H) hysteresis curves revealed the appearance of ferrimagnetic behavior at room temperature. The BET surface area of BiFeO3 perovskite nanoparticles was found 44.86 m2/g. The as-fabricated BiFeO3 perovskite nanoparticles were investigated for their superior catalytic activity in two applications, which include (i) Glycerol to formic acid oxidation in the liquid phase with a high efficiency of over 98 percent, (ii) Under visible light, the photocatalytic breakdown of rhodamine B achieved maximal efficiency (almost 99 percent). Finally, we concluded that the BiFeO3 perovskite nanoparticles exhibit high performance in future multifunctional devices is demonstrated by the simultaneous enhancement of catalytic and photocatalytic activities.


Introduction
Multiferroic materials have received a lot of attention in recent years because of their various applications in spintronics, sensors, high energy density capacitors and novel magneto-electric devices [1]. They have ferroic properties like ferroelasticity, ferroelectricity, and antiferro/ferromagnetism, but they are also coupled with electric, magnetic, and structural order parameters. This material necessitates the use of vacant and/or partially filled transition metal like d-orbitals, which exhibit distinctive features [2][3][4][5][6][7][8]. The catalytic oxidation of alcohols to carbonyl molecules is one of the most prevalent forms of oxidation reactions in organic chemistry. In the production of fine molecules and intermediates, selective catalytic oxidation is crucial. From both synthetic and industrial standpoint, liquid-phase catalytic oxidation of alcohols might be a highly appealing process for the production of intermediates and fine compounds. The catalytic conversion of primary alcohols to aldehydes is an important laboratory and industrial process [9,10]. Furthermore, contemporary scientific study has focused on innovative visible light photo catalysts based on semiconductors in order to fulfill escalating environmental pollution and energy requirements through efficient solar energy usage. Photo-catalysis based on semiconductors has attracted a lot of interest because of its potential applications in solar energy consumption and environmental cleanup [11,12].
The general chemical formula for perovskites is ABO 3 or A 2 BO 4 . In ABO 3 (A cation of larger size than B) structure, where A-site trivalent cation is 12-fold coordinated, the B-site trivalent cation is 6-fold coordinated and O is an oxygen anion. In the realm of heterogeneous catalytic reactions, they are perhaps the most researched mixed-oxide system [13][14][15]. Because of their low cost, thermo-chemical stability at relatively high temperatures, and catalytic and photocatalytic activity, these perovskite oxides appear to be a potential alternative to noble metal catalysts [16]. Although these perovskite oxides are the most common and fascinating compounds, it also crystallizes in carbides, nitrides, halides, and hydrides [17]. Due to their distinctive crystal structures as well as physical and chemical characteristics, perovskite materials have attracted a lot of attention for their potential uses in solar cells, fuel cells, electro-catalysis, energy storage, catalysis, photocatalysis and so on [18]. The perovskite oxides has been studied as various catalysts such as oxidation of carbon monoxide (CO), decomposition of nitrogen oxide (NO) and NO + CO reduction and so on [19][20][21][22]. On the other hand, the perovskite oxides has been widely used for photo catalytic degradation such as organic pollutants/dyes, methyl orange, methylene blue and so on [11,12,[23][24][25]. Type 1: suprafacial, in which the catalyst surface provides a set of electronic orbitals with proper symmetry and energy for reactant and intermediate bonding; type 2: intrafacial, in which the catalyst acts as a reagent that is partially consumed and regenerated in a series of continuous redox cycles [19].
BiFeO 3 perovskite nanoparticles are characterized as combinations of two or more constituent minerals or phases and are well-known for their excellent thermal stability, high strength, chemical resistance, and increased catalytic characteristics [10,26,27]. The structure stability of A-site cation, the catalytic activity of B-site cation, and the oxygen and deficiency oxygen makes the perovskite based materials to play a significant role in the field of catalysis. Bi-based perovskite materials have shown promising visible lightresponsive photocatalyst for the organic pollutants degradation owing to reduced band gap and excellent chemical stability [28][29][30]. Despite the abundance of nanoparticles accessible, the combination of perovskite-based metal oxide is rare in the literature. Bismuth-based perovskite nanoparticles may be produced using a variety of physical and chemical methods, including sol-gel [9], solid-state reaction [31], sonochemical approach [2], and hydrothermal method [32]. Sensors, catalytic activities, photocatalytic activities, and spintronics devices utilizes BiFeO 3 perovskite-type oxides [12,31].
Ramezanalizadeh et al. used a modified sol-gel approach to make BiFeO 3 perovskite oxide. The gel was formed using 2-methoxyethanol and acetic acid, and then baked at 80 °C for 12 h before being annealed at 600 °C for 30 min in air or N 2 . The oxidation of primary and secondary alcohols was achieved using this perovskite oxide [9]. On the other hand, Li et al. have prepared BiFeO 3 perovskite by employing microwave-assisted hydrothermal using polyethylene glycol (PEG 6000, 4000, and 2000) as a precipitate agent then subjected to microwave irradiation at 190 °C for 30 min followed by vacuum drying oven at 60 °C for 12 h. These perovskite oxides have been used for the degradation of RhB [33]. However, there are few reports on the manufacture of BiFeO 3 utilizing a microwave aided combustion approach. The current work focuses on the microwave aided combustion technique for the synthesis of BiFeO 3 perovskite oxides, as well as the influence of microwave irradiation on the formation of a secondary perovskite phase (α−Bi 2 O 3 ). Microwave-assisted combustion provides several advantages over traditional heating techniques, including a faster synthesis time, lower energy usage, and the ability to fabricate materials with particular catalytic and photocatalytic characteristics. As a result, microwave-assisted methods for the production of inorganic materials are widely employed [33]. We produced BiFeO 3 perovskite nanoparticles and evaluated them using different methods such as XRD, DRS, FTIR, HR-SEM, EDX, VSM, and BET to determine structural, optical, morphological, magnetic, and textural characteristics. The as-fabricated BiFeO 3 perovskite nanoparticles are reported for the first time by our group, and its multi-functional abilities towards the oxidation of glycerol and photocatalytic degradation of RhB are described in the subsequent sections.

Synthesis of BiFeO 3 Perovskite Nanoparticles
Iron nitrate and bismuth nitrate were used as precursors and L-alanine as a fuel for the preparation of BiFeO 3 perovskite nanoparticles. To obtain a homogenous solution, the precursors and L-alanine components were dissolved in distilled water in the required quantities and stirred for 30 min at room temperature. The fuel to oxidizer ratio (F/O) was one [32] according to the propellant chemistry principle. The required amount of oxidizer-to-fuel molar ratio for the mixture was calculated using the Eq. (1) [10]. The homogenous solution was placed in a microwave oven (800 W, power) operating at 2.45 GHz for 10 min. When the precursor solution reached the threshold temperature, it was dehydrated and burnt, yielding a black fluffy powder that was calcined at 500 °C for 2 h. As a result of this, two phases are generated in the production of crystalline Bi 2 O 3 and BiFeO 3 . The prepared BiFeO 3 perovskite nanoparticles are evaluated for the catalytic oxidation of glycerol and photocatalytic degradation of RhB, respectively. The steps involved in the process of fabrication of BiFeO 3 perovskite nanoparticles is depicted schematically in Fig. 1.

Characterization
The crystalline phase of the materials was investigated using Rigaku, RXIII on a D/MAX-2500 machine with CuKα radiation (λ = 1.5406 Å) in the range between from 10° to 80°. The UV-Vis diffuse reflectance spectra (UV-DRS) of the samples were used to estimate the optical band gap using a Thermo Scientific Evolution 300 UV-Visible spectrophotometer. On a Thermo scientific Nicolet iS 10 OMNI FTIR spectrophotometer with a KBr pellet is used for sample preparation and performing Fourier transform infrared (FT-IR) studies. A high resolution field scanning electron microscope (HR-SEM, HITACHIS4800) equipped with an energy dispersive X-ray spectrometer (EDX, HORIBA EMAX) was used to examine the morphology and elemental composition of the samples. The magnetic characteristics were measured at room temperature using a vibrating sample magnetometer (VSM, PMC MicroMag 3900 model) with a maximum field of 15 kOe. After degassing at 250 °C for 6 h, the surface area of the catalyst was calculated using the Brunauer-Emmett-Teller (BET) technique  Fig. 1 Schematic representation of the preparation of BiFeO 3 perovskite nanoparticles using N 2 adsorption and desorption isotherms obtained on a Micromeritics-ASAP 2020 unit with liquid nitrogen adsorption at 77 K was using a PHS-1020 instrument.

Catalytic Activity
Glycerol oxidation was carried out in a batch reactor under air conditions. A 250 mL two-necked round bottom flask was filled with 50 mmol of glycerol as reactant and 50 mmol of hydrogen peroxide as the oxidant. Following that, 50 mg of BiFeO 3 was added as a catalyst and 50 mmol of acetonitrile was added as a solvent. The mixture was stirred at 70 °C for 6 h. After completion of the reaction, the mixture was gradually cooled to room temperature and the catalyst was separated by filtration method. The converted products were analyzed by gas chromatography (GC) with flame ionization detector (FID), using an Agilent 7890B GC system. The main by-products were detected using an Agilent Technologies Model 7890A with mass detector model 5975C and HP-5MS column in a gas chromatography mass spectrometer (GC-MS).

Photocatalytic Activity
The photocatalytic activity of BiFeO 3 perovskite nanoparticles were assessed for the inorganic dye (Rhodamine B, RhB) degradation through visible light source (Source power: 150 W Halide lamp and wavelength is λ > 420 nm).
A fixed concentration of about 0.31 g of bismuth nanoparticles was distributed in 1 L of RhB dye solutions and irradiated with visible light. To induce a Fenton-like reaction, the pH of the solution was adjusted to 2 by adding the needed amount of hydrochloric acid. Previously, the solution was placed in dark condition for overnight at room temperature to attain adsorption amid RhB dye solution and BiFeO 3 perovskite nanoparticles. In photoreaction, air was expelled for dispersing BiFeO 3 perovskite catalyst in the solution. 1 × 10 -6 L of 30% H 2 O 2 was added to the 1 L of dye solution to produce additional hydroxide radicals (OH · ) through the photo-degradation, for speedy oxidation. UV-Vis spectra were recorded for time intervals, before and after photocatalytic degradation.  [34]. It is found that crystallite size significantly affects the catalytic and photocatalytic activity due to easy transfer of charges to the surface of catalyst [29]. According to the XRD results, the lattice parameters and cell volume of the rhombohedral BiFeO 3 perovskite nanostructure were a = 5.5770, c = 13.861 and V = 384.12. In general, the relative variation of ionic radius of Bi3+ and Fe3+ atoms can easily distort the rhombohedral symmetry with simultaneous tilting of the oxygen octahedral. It is well known that oxygen octahedral rotations have paramount influence on crystal field and hence causes alter in the electronic band structures, thus affecting the behaviors of photo-generated eto h + pairs such as excitation, migration, reduction and oxidation (redox) processes in the entire photocatalytic reaction [30].

Optical Properties
The optical properties of the BiFeO 3 perovskite nanoparticles were studied using UV-Vis diffuse reflectance spectroscopy as shown in Fig. 3a. The Kubelka-Munk (K-M) function is commonly used to transform diffuse reflectance into comparable absorption coefficients, as shown in Eq. (2), and is commonly employed for powder sample analysis. where, F(R) is the K-M function and R is the reflectance. Thus, to calculate optical energy band gap as modified Tauc relation equation is αhν = A(hν−Eg) n , where α is the absorption coefficient, h is Planck's constant, A is the constant, E g is the optical energy band gap and for direct transitions n = 1/2. A graph is plotted between (F(R)hν) 2 vs hν, which upon extrapolation of the linear region to (F(R)hν) 2 = 0 gives the optical energy band gap value. The optical energy band gap of the BiFeO 3 perovskite nanostructure is 2.18 eV, which is lower than previously reported values of 2.27 eV and 2.25 eV in [34,35], perhaps due to quantum confinement processes at the nano-regime [36]. The optical band gap energy of BiFeO 3 perovskite can be seen to be narrow due to the easier recombination of e‾ and h + pair in the recombination processes, while the efficient carrier charge separation produced by photo-excitation. Hence, BiFeO 3 has small band gap and appropriate band positions to meet the demand of photocatalytic process activated by UV-Vis light energy [29].
The optical band edge potential namely conduction band (CB) and valence band (VB) edge potential of BiFeO 3 nanoparticles were determined using Eqs. (3) and (4).
The calculated CB edge potential versus NHE for bismuth ferrite is 0.302 eV. Similarly VB edge of bismuth ferrite is 2.25 eV. Figure 3b shows the Fourier transform infrared spectroscopy (FTIR) spectra of the BiFeO 3 perovskite nanoparticles. The formation of rhombohedral perovskite structure and the presence of surface functional groups were confirmed using FTIR spectrum investigation, which is plotted in the region of 4000 to 400 cm −1 as shown in

HR-SEM and EDX Analysis
The HR-SEM micrographs of the BiFeO 3 perovskite nanoparticles is shown in Fig. 4a, which reveal the presence of intragranular pores while the pore walls are fused grains with distinct grain boundaries. The average particle size calculated from the HR-SEM picture of the BiFeO 3 perovskite nanostructure is about 43 nm, which differs somewhat from the crystallite size determined by XRD measurements. Because the diffracting domain size in the nanoparticle is linked to aggregated cluster size, the former is used. The energy dispersive X-ray analysis was used to analyze the elements in this sample (EDX). The chemical composition of the BiFeO 3 perovskite nanostructure  ). In general terms, the enhanced catalytic and photocatalytic activity of BiFeO 3 was attributed to the large numbers of oxygen vacancies and unique sphere-like with intragranular pore morphology nanostructures, which led to an efficient recombination of eto h + pairs (charge transfer) in the catalytic and photocatalytic reaction. . BiFeO 3 has G-type anti-ferromagnetic ordering with a linear applied field dependency of magnetization. However these types of perovskite nanoparticles have weak ferromagnetism due to the disruption of the long-range anti-ferromagnetic order at the particle surface [40,41]. Furthermore, in nanoparticles with a high surface-to-volume ratio, there are more uncompensated spins from the particle surface, causing magnetization to develop. The size impact of the nanoparticles may also be responsible for the improved magnetic characteristics. A similar observation was previously reported [38]. The catalytic and photocatalytic activity in heterogeneous reactions of some substances is related to its magnetic properties, with distinct magnetic behaviors as ferromagnetic properties. The results suggest that the BiFeO 3 perovskite nanoparticle is an excellent magnetic catalyst, which will play an important role in recycling catalytic and photocatalytic applications [42].

N 2 Adsorption-Desorption Isotherms
The surface area has been determined from the N 2 adsorption-desorption isotherms and the specific surface area (S BET ), pore diameter (D P ) and pore volume (V P ) recorded at 77 K using the BET technique as shown in Fig. 5b. According to the International Union of Pure and Applied Chemistry (IUPAC), the particle agglomerations have a typical type II isotherm. The obtained type II isotherm was recorded at high relative pressure designating the formation of mesopores in the BiFeO 3 perovskite nanoparticles [43]. The textural property of BiFeO 3 perovskite such as the surface area and pore diameter were reported to be 44.860 m 2 g −1 and 4.319 nm respectively. The greater pore volume (V P -0.104 cc g −1 ) was caused by the higher surface area of BiFeO 3 , which is ascribed to the formation of additional gaseous products during the synthesis process. The textural features of the BiFeO 3 perovskite has acceptable porosity and surface area for catalytic and photocatalytic applications [10].

Catalytic Activities of BiFeO 3 Perovskite Nanoparticles
The multiferroic material of BiFeO 3 perovskite nanostructures have been evaluated for the catalytic oxidation of glycerol in liquid phase batch reactor under air conditions. According to the GC-MS data in Fig. 6, the catalytic conversion of glycerol and the selectivity of formic acid were determined to be 99.2 percent and 98.5 percent, respectively. BET methods revealed that the BiFeO 3 catalyst had the highest catalytic activity due to its large surface area. Figure 7a illustrates the amount of catalyst loading (10 to 70 mg) on the oxidation of glycerol using BiFeO 3 perovskite nanoparticles at 70 °C for 360 min. Gradually increasing the catalyst dosage from 10 to 50 mg, increased the proportion of glycerol conversion (61 to 99 percent) and selectivity (55 to 98 percent). Moreover 50 mg of catalyst loading causes a dramatic fall in conversion (87%) and selectivity (84%), which is comparable to the values reported in literature [12,44]. It is due to pore clogging of the catalyst surface, which leads to decrease in reactive surface area accessible to progress the reaction [36]. For the oxidation of glycerol to formic acid, a catalyst dosage of 50 mg achieves good catalytic activity.

Recycling Test
The reusability was importantly essential for the economic feasibility of reaction. The possibility of recycling performance was also investigated under the optimal reaction parameters. Following the first trail, the catalyst was collected, filtered, and washed multiple times with de-ionized water before being dried in a hot air oven at 120 °C for 120 min. After recycling for four runs under identical reaction a condition. According to the data (Fig. 7b), there is a minor decrease in the percentage of glycerol conversion and selectivity of formic acid, which is related to a slight mass loss during the catalyst recovery process [45,46].

Reaction Pathway
The proposed reaction pathway of glycerol to formic acid over a BiFeO 3 perovskite catalyst in an acetonitrile medium is shown in Fig. 8. Arcanjo et al. [47], Yang et al. [48], Lam et al. [49], Sukumar et al. [10,12], Lapenaite et.al [50] and Haider et al. [51] earlier reported that it is a two-path sequential reaction. The first pathway involves   Fig. 8 Possible reaction pathway for the conversion of glycerol using BiFeO 3 catalyst. Detected products are framed the oxidative dehydrogenation to form either glyceraldehyde or hydroxyacetone. The glyceraldehyde was further oxidized to glyceric acid. Furthermore, oxidized at the three-carbon leads to form tartronic acid. If would broke its C-C bond splitting quickly, to form glycolic acid and formic acid. Then, hydroxyacetone was oxidized to form 2-hydroxypropenal towards the pyruvaldehyde, which goes through rearrangement to form lactic acid. The second pathway, which is followed by hydroxyacetone and involves radical fragmentation, is similar to the Norrish type I process, and involves C-C bond cleavage to produce acetaldehyde and methanol precursors. Acetaldehyde is metabolized to acetic acid subsequently. Methanol, on the other hand, is oxidized to generate formaldehyde, which is further oxidized to form formic acid due to the loss of hydrogen radical.

Photocatalytic Degradation of RhB Dye Solution
Over BiFeO 3 Perovskite Nanoparticles and 558 nm tetra ethylated RhB molecule [52,53]. Control experiments were done in the dark; it was observed that after complete adsorption process there is no decrease in intensity peak. It suggests that upon addition of hydrogen peroxide (H 2 O 2 ) there is no degradation took place after 165 min. Figure 9b, indicates the photocatalytic activity of BiFeO 3 perovskite catalyst kept in dark condition, where complete degradation of RhB using BiFeO 3 perovskite catalyst with 6 mg/L of dye solution at 165 min under visible light radiation. The photocatalytic activity of BiFeO 3 perovskite nanoparticles achieves excellent photo-degradation. It is attributed to the recombination and separated electron/hole (e − /h + ) pairs and also easily available for the oxidation process, which means in terms of the charge carriers. The illumination of visible source with respect to time, leads to gently change in the color of RhB dye solution. The color changed from pink to colorless, which corresponds to the destruction of chromophoric group.

Efficiency and Recycle Performance
As shown in Fig. 9c, the effect of BiFeO 3 perovskite nanoparticles photocatalytic degradation efficiency was evaluated at an RhB concentration of 6 mg/L and a pH of 2. After 165 min, the photocatalytic degradation efficiency of BiFeO 3 was found to be 99.9%, owing to the efficient separation and suppression of electron-hole pair recombination, as well as the geometric and electronic structures of bismuth ferrite. The stability of the BiFeO 3 perovskite catalyst, as shown in Fig. 9d, even after five cycles with no decline of photocatalytic degradation efficiency. Figure 10 showed the possible photocatalytic degradation mechanism for the BiFeO 3 perovskite catalyst under visible light illumination. The e − /h + pairs are created in BiFeO 3 perovskite nanoparticles. These photo-generated h + /e − pairs facilitate the oxidation and reduction process of RhB in aqueous solution. In BiFeO 3 , the conduction band (CB) edge is at ca 0.302 eV the valence band (VB) edge is at ca 2.482 eV and against normal hydrogen electrode (NHE). In this process, the CB edge value is less than redox potential of O 2 /O ⋅− 2 (-0.33 V). This avert the electron in the CB and react to O 2 with produce the ( O ⋅− 2 ) which is also a strong oxidant. Hence, not fit for reduction process. Further, the electron in CB of catalyst surface react with H 2 O 2 to produce OH · radical which is responsible for the degradation of RhB dye. Beside, hole direct oxidation of the RhB molecules and has higher oxidation potential.

Mechanism of Dye Degradation
The redox potential of O H ⋅ /OH¯ (1.99 V), which was compared to valance band edge of BiFeO 3 is more positive. This photo-generated positive holes can oxidize water (H 2 O) that will take formation of OH¯ to form OH · radical due to large peak shift in the peak and de-ethylation of RhB molecule. The oxidant of ozone and hydrogen peroxide for RhB solution is the OH · radicals have higher oxidation potential. In this process holes, led to decolouration and de-ethylation, thereby degradation of RhB. The BiFeO 3 perovskite, the Fe 3+ cation which is present on the surface of catalyst likewise react with hydrogen peroxide, thus producing a Fenton-like process which is a most powerful tool and resulting in peroxide · OOH and OH · radical. Also, H 2 O 2 react with Fe 3+ ion surface of the catalyst could seize the electrons form OH · radicals from the BiFeO 3 system [54][55][56][57][58]. The weakness of recombination of e − /h + pairs, leads to improve photocatalytic degradation efficiency.

Conclusion
In summary, an integral analysis and discussion of the synthesis pathway and structural, optical, morphological, magnetic, textural, catalytic, and photocatalytic activities are provided in this paper. The use of L-alanine as fuel in the BiFeO 3 perovskite synthesis produces nanoparticles. The crystal structure was identified to be rhombohedral structure with R3c space group with crystallite size around 50 nm. In addition, the main byproduct formed is the α-Bi 2 O 3 phase. The optical band gap value of BiFeO 3 perovskite nanoparticles was 2.18 eV. The vibrational stretching modes of the BiFeO 3 perovskite rhombohedral structure are validated by FT-IR investigation. Morphological analyses show that the intragranular porosity is highly developed, with discrete  (12) RhB dye * + + OH ⋅ → degradation products grains on the surfaces. Energy-dispersive X-ray analysis (EDX) was done to confirm the existence of Bi 3+ , Fe 3+ , and O 2− in the samples. At room temperature, magnetic measurements show the advent of ferromagnetic activity. With aid of Brunauer-Emmett-Teller (BET) method mesopores were observed with type II isotherms, which have large surface area of 44.86 m 2 /g of the BiFeO 3 perovskite nanostructure. Subsequently, the BiFeO 3 perovskite nanoparticles were investigated for superior catalytic activity in two applications; as follows (i) the conversion of glycerol to formic acid in a selective liquid phase batch reactor at atmospheric pressure. These bismuth-based nanoparticles acted as an efficient multifunctional catalyst with high conversion and selectivity efficiency around 99.2% and 98.5%, respectively, (ii) the photocatalytic degradation of rhodamine B under visible light irradiation is found to give maximum efficiency (99.9%), when a small amount of H 2 O 2 was added during photocatalysis, indicating the samples possessed photo-Fenton like catalytic activity. The mechanism associated with the applications are described in detail and our results demonstrated high catalytic activity in diverse applications.