La 0.8 Bi 0.2 FeO 3 PEROVSKITE-TYPE: HIGH-PERFORMANCE OF PHOTOCATALYTIC DEGRADATION OF ORTHO-TOLUIDINE BLUE UNDER VISIBLE LIGHT IRRADIATION

In this study, La 1-x Bi x FeO 3 (x=0.0, 0.2, 0.4, and 0.6) perovskite nanoparticles were synthesized by a modified Pechini method. Rigorous analysis through XRD and SEM/EDX confirmed the absence of secondary phases in both pure and Bi-substituted LaFeO 3 samples, indicating the formation of a single-phase perovskite. SEM images revealed the quasi-spherical shape of the particles. The photocatalytic activity of La 1-x Bi x FeO 3 (x=0.0, 0.2, 0.4, and 0.6) was evaluated by the degradation of ortho-Toluidine Blue under visible light irradiation, indicating that La 0.8 Bi 0.2 FeO 3 exhibited excellent photocatalytic activity. The overall removal rate of o-Toluidine Blue reached 90.09% after visible light irradiation lasting for 60 min. We attribute this heightened photocatalytic activity to the grain size and optical properties of prepared sample. Consequently, the La 0.8 Bi 0.2 FeO 3 can be considered as a very promising photocatalyst in future industrial application to treat effectively wastewater of dyes.


INTRODUCTION
The rapid expansion of urbanization and industrialization, coupled with soaring global population growth, has significantly exacerbated the issue of water pollution [1].Approximately 1.2 billion people worldwide, particularly in developing nations, grapple with the alarming consequences of water contamination, which extend far beyond environmental degradation [2].Water pollutants pose a grave threat to human health, contributing to respiratory disorders, dermatitis, asthma, mutagenicity, and even cancer and other diseases [2][3][4][5][6][7].This dire scenario underscores the need for innovative and effective wastewater treatment technologies.Dye wastewater is one of the most difficult degradable industrial wastewater, due to its variety, complicated organic composition and poor biochemical degradability, even including toxic ingredients which can weaken the microbial mineralization of organic pollutants and lead to the destruction of water ecological system [8].Effective wastewater treatment plays a pivotal role in safeguarding public health and the environment.This vital process involves the removal of contaminants and pollutants from wastewater, rendering it suitable for safe discharge into the ecosystem or potential reuse.However, the complexity and scale of this challenge have prompted the exploration of innovative, sustainable solutions [9].
In recent years, photocatalytic technologies have emerged as promising strategies for advanced wastewater treatment [10][11][12].These methods harnesses the power of photocatalysts, typically semiconducting materials, to accelerate the degradation of both organic and inorganic pollutants when exposed to light irradiation.Semiconductor photocatalysis, in particular, has attracted considerable attention due to its cost-effectiveness, non-toxicity, high chemical and thermal stability, and eco-friendly nature [13,14].Traditional semiconductor photocatalysts, like TiO2 [15,16], ZnO [15,17], and SnO 2 [18], have been extensively employed in photocatalysis due to their affordability, stability, and low environmental impact.However, their limited ability to utilize solar energy, primarily in the UV range, has driven the exploration of narrow band gap semiconductors, which can absorb a broader spectrum of solar radiation, especially in the visible light region [19].
Perovskite materials, have emerged as a compelling class of narrow band gap semiconductors for photocatalytic applications [20].Their unique structural characteristics and synthesis processes significantly influence their photocatalytic efficiency, making them versatile for diverse applications, including gas sensing [21], water splitting [22], and the photocatalytic degradation of organic pollutants [23,24].Among the narrow band gap semiconductors, LaFeO3 stands out as a promising photocatalyst, with several studies emphasizing its synthesis and efficacy in the photodegradation of organic dyes under visible light irradiation [25][26][27][28].LaFeO 3 , with a narrow band gap of only 2.0 eV, exhibits exceptional potential for utilizing visible light from the solar spectrum, rendering it highly suitable for La 0.8 Bi 0.2 FeO 3 Perovskite-Type: High-Performance of Photocatalytic Degradation...
The catalytic performance of LaFeO 3 perovskite in wastewater dye removal processes has been investigated.Thirumalairajan et al. [30] studied the catalytic activity of the LaFeO 3 microsphere to degrade Rhodamine B (RhB), whereas Deng et al. [31] studied its adsorption behaviour for RhB.On another hand, Mocwana et al. [32] studied the activity of the lanthanum ferrite using the photocatalytic degradation of ortho-toluidine blue (o-TB) under visible light.To further enhance the photocatalytic performance of LaFeO 3 , modification strategies, such as doping with various elements, have been widely explored [33].In this context, Cu-doped LaFeO 3 catalysts have demonstrated remarkable efficiency in the decolorization of both cationic and anionic dyes under visible light irradiation [34].Also, experiments of radical trapping in Sr-doped porous LaFeO 3 samples reveal that •OH species are dominant intermediate oxidants involved in the oxidation of 2,4-DCP and RhB over the optimized sample [35].
The visible light-driven degradation of o-Toluidine Blue (o-TB) will be the focus of this investigation, shedding light on the potential of Bi-substituted LaFeO 3 perovskite photocatalysis in addressing these pressing challenges.Photocatalysis can be used to degrade or break down certain organic compounds, including dyes like o-TB, that may be present in water and contribute to water pollution.The effect of photocatalysis on o-TB as a water dye may result in the degradation of the dye molecules, reducing its concentration in the water.This process can be effective in treating water contaminated with certain organic dyes, which are common pollutants in industrial wastewater.In this study, the structural, morphological, optical evaluation and photocatalytic performance of a series of La 1−x Bi x FeO 3 (x=0.0,0.2, 0.4, and 0.6) would be emphatically discussed.

EXPERIMENTAL Material Preparation
The technique of modified Pechini method was employed for the synthesis of Bismuth-doped Lanthanum ferrite La 1-x Bi x FeO 3 .Specifically, precise amounts of Lanthanum nitrate (La(NO 3 ) 3 ⋅6H 2 O, Sigma-Aldrich, ≥99.0%),Bismuth nitrate (Bi(NO 3 ) 3 ⋅5H 2 O, Sigma-Aldrich, ≥98.0%), and Ferric nitrate (Fe(NO 3 ) 3 ⋅9H 2 O, Sigma-Aldrich, ≥98.0%)were utilized as initial reagents to produce La 1-x Bi x FeO 3 (x = 0, 0.2, 0.4, and 0.6).The raw materials were dissolved in 100 ml of double-distilled water to create a transparent solution, which was then evaporated at 60 °C while being continuously stirred using a magnetic stirrer.Additionally, nitric acid was incrementally introduced during the stirring process to regulate the pH.The solution was then supplemented with citric acid (Sigma-Aldrich, ≥98.0%),Ethylene Diamine Tetra Acetic acid (EDTA, Sigma-Aldrich, ≥98.0%), and ethylene glycol (Sigma-Aldrich, ≥98.0%), as well as other chemicals to create a brown precursor solution.The solution is then heated to 120 °C (10 °C/min) until it solidifies into a dry gel.The gel then turns into a puff as the temperature rises to 350 °C.The resulting puff was dried for 10 hours at 110 °C to produce a dark brown powder, and the soft powders were then calcined for 12 hours at 600 °C with a 5 °C/min rate.The light brown powder was then heated for 24 hours at 800 °C in the air using a tubular furnace (R 50/500/13 Nabertherm model).To obtain the phase, this operation is repeated twice (Fig. 1).examination of the synthesized samples, their morphology was investigated utilizing a Thermo Scientific Quattro ESEM scanning electron microscope.Simultaneously, the compositions were analyzed using energy dispersive X-ray (EDX) spectroscopy, employing a Zeiss SmartEDX detector integrated into the Zeiss Evo15 Scanning Electron Microscope (SEM).This setup is designed for observing dry and conducting samples.In addition, Fourier transform infrared (FTIR) spectra were recorded in the range of 2000-450 cm -1 using an IR Spirit -Shimadzu Fourier transform infrared (FTIR) spectrometer.The as-synthesized powders were also analyzed using a Shimadzu UV-visible spectrometer, specifically the Shimadzu UV-1900.

Photocatalytic Activity
In this study, we investigate the photocatalytic degradation of o-TB in the presence of La 1-x Bi x FeO 3 (x = 0.0, 0.2, 0.4, 0.6).The experimental work begins by adding 5 mg of the catalyst to a 5 ml o-TB solution.The prepared solution is made using distilled water at a concentration of 4x10 -5 mol/L.Following this, the solution is exposed to sunlight for varying durations (10, 20, 30, and 60 min) at room temperature and a neutral pH.On the other hand, the catalyst is separated through centrifugation, and a spectrophotometer (Shimadzu 1900 model) is used to detect absorbance values at 629 nm.Furthermore, the calculation method for degradation efficiency is presented in the following equation: Where C 0 is the concentration of o-TB solution at the reaction's beginning and C t its concentration of pollutants at time t.

RESULTS AND DISCUSSION Structural, Morphological and Elemental Analysis
In Fig. 2, the X-Ray Powder Diffraction patterns of the pure and Bi 3+ -substituted LaFeO 3 samples are displayed.The Rietveld refinement method [36] has been used for structural analysis, and using the ReX is a powder diffraction software [37].On the orthorhombic Pbnm (N° 62) space group, it is possible to refine all of the peaks of La 1-x Bi x FeO 3 (x = 0.0, 0.2, 0.4, and 0.6) samples.The calculated lattice parameters, unit cell volume, refinement factors are summarized in Table 1, and the final Rietveld refinement plot is presented in Fig. 3.The absence of a second phase in the doped samples shows that the Bi 3+ ions have completely dissolved into the host lattice by taking the place of the La 3+ .The La 1-x Bi x FeO 3 (0≤x≤0.6)unit cell volume is constant in the range of the error standard deviation (e.s.d.) with the addition of Bi.This result is due to the very closeradius of Bi 3+ ion (r(Bi 3+ )= 1.17 Å) compared to the La 3+ ion (r(La 3+ )=1.16 Å) [38], which is evident from the XRD peak's slight shift towards a higher 2θ value in Fig. 3.    Using spot measurements on powder samples, energy dispersive x-ray (EDX) spectroscopy was used to determine the quantitative composition of pure and Bi 3+ -substituted LaFeO 3 nanoparticles.Within the limit of experimental error, the EDX analysis shows consistency with the predicted stoichiometry.The EDX spectrum confirms the desired samples (Fig. 4).Also, scanning electron microscopy (SEM) was utilized to examine the surface morphology of the prepared samples.As illustrated in Figure 4, the morphology of the studied samples shows a spherical shape of grains, similar to that of sand grains.
Both the dispersion and morphology of grains remained largely unchanged despite doping samples with Bi 3+ ions.However, as the concentration of Bi substitution increased, agglomerated spherical particles began to form and grew in size.

FTIR Analysis
Figure 5 displays the FTIR spectra of pure and Bi-substituted LaFeO 3 in the range of [2000-450] cm -1 .It is important to note that the respective spectral ratios are almost similar from one to another, differing primarily in peak intensity.This difference can be attributed to variations in molar concentration composition.The strong absorption band at approximately 536 cm -1 can be attributed to the (Fe-O) bending vibration characteristics of the octahedral FeO 6 groups in La 1-x Bi x FeO 3 .Additionally, the bands between 700 and 400 cm -1 are mainly attributed to the formation of metal oxides [39].
On the other hand, the bending vibration of the La-O bonds may be responsible for the bands 717 cm -1 [23].Additionally, we observe that there are two closely bands, the first of which is at 1385 cm -1 and the second at 1485 cm -1 .They are attributed to NO 3 -and N-H respectively, and both come from reacted precursor raw materials; they are neighbors and weak [40].The La/Fe-O bending vibration modes do not change in frequency with increasing Bi content up to x=0.6, confirming that there has been no lattice deformation and, consequently, no changes in the length of the La/Fe-O bond, as supported by the XRD analysis.

Optical Study
In order to describe the optical properties of the La 1-x Bi x FeO 3 (x=0.0,0.2, 0.4 and 0.6) nanoparticles, UV-Visible spectroscopy is used.From Tauc's plot, the bandgaps of the prepared samples were taken.Tauc's equation [41] mentioned below in eqt. 2 can be used to calculate the relationship between absorption coefficient (α) and incident photon energy (hν).
( αhʋ where α is the optical absorption coefficient, hν is the photon energy, E g is the direct band gap, and A is constant.La 0.8 Bi 0.2 FeO 3 Perovskite-Type: High-Performance of Photocatalytic Degradation...

EEJP. 1 (2024)
E g for direct transitions is obtained by extrapolating the linear parts of the curves toward absorption equal to zero (Fig. 6).Pure LaFeO 3 is thought to have a direct band gap of 2.2 eV.However, E g values decrease to 1.86, 1.76, and1.54 eV, respectively, for Bi substitutions of 20, 40, and 60%.Furthermore, the value of E g can be decreased through Fe-O octahedral restructuring of molecular orbitals and nanoparticles length scale [42][43][44].This reduction could also be attributed to the direct energy transfer occurring between the excited states of the semiconductor and the 3d levels of Bi 3+ ions [45].

Evaluation of the Photocatalytic Efficiency
The photocatalytic degradation of O-toluidine blue (o-TB) was examined using La 1-x Bi x FeO 3 (x = 0.0, 0.2, 0.4, 0.6) under the influence of solar irradiation.The results show that most of the o-TB dye is removed within 60 minutes (Fig. 7).
Table 2 shows that the photocatalytic activity of La 1-x Bi x FeO 3 (x = 0.0, 0.2, 0.4 and 0.6) samples decreases with an increase in their average grain size.This can be attributed to the fact that smaller particle size with a larger surface area per unit volume provide more active sites for photocatalytic reactions, leading to a higher photocatalytic degradation rate, especially in the case of spherical nanoparticles with smaller particle sizes [46][47][48].This principle applies to the spherical shape of the grain morphology in the studied samples.EEJP. 1 (2024) Ouarda Ben Ali, et al.The highest photocatalytic activity (90.09%) was observed for La 0.8 Bi 0.2 FeO 3 , which has the smallest average grain size (76 nm).Additionally, the photocatalytic activity of La 0.6 Bi 0.4 FeO 3 (78 nm) was slightly lower (86.28%),but still higher than that of LaFeO 3 (79.50%)and La 0.4 Bi 0.6 FeO 3 , which exhibited a rate of 66.68% (Fig. 7).Therefore, these results support the notion that the average grain size and the shape of grains are an important factor in determining the photocatalytic activity of La 1-x Bi x FeO 3 .
In contrast, data obtained by Mocwana et al. [32] indicate that the optimal degradation rates for o-TB, using LaFeO 3 nanosheets, resulted in a catalytic degradation of 37% over 30 minutes.Notably, this rate remains lower than all the values obtained in our experiments for the same duration.
Furthermore, it was observed that an increase in the amount of Bi up to 20% within 60 minutes of the reaction time significantly enhances removal efficiency.This improvement can be attributed to the increased formation of hydroxyl radicals.
Conversely, a further increase in the Bi ratio decreases the removal efficiency of the o-TB dye.This can be attributed to (i) the increase in the average grain sizes, and (ii) the blocking of UV light penetration with an increasing amount of Bi [49,50].
This can be interpreted as when the catalyst load is small, the catalyst absorbs fewer photons for the photocatalytic reaction, resulting in lower photocatalytic activity.As the catalyst loading increases, the number of photon absorption centers and activity centers on the catalyst surface increases, thereby increasing the catalyst's activity.However, with further increases in catalyst loading, the number of photons tends to become saturated.This heightened catalyst loading may lead to light blockage, consequently affecting the photocatalytic efficiency [51].Thus, the narrow band gap value of La0.8Bi0.2FeO3(1.86 eV) nanoparticle suggests its potential as a promising candidate for the excellent purification of wastewater containing o-TB dye.

CONCLUSIONS
In conclusion, the perovskite-type La 1-x Bi x FeO 3 photocatalysts was successfully synthesized using the modified Pechini route, and their performance for photodegradation of o-TB dye wastewater was studied.XRD and EDX combination studies proved that all samples have an orthorhombic structure with a single-crystalline orthorhombic structure, and the average grain size samples increased by increasing of Bi-substitution to be 85 nm, 76 nm, 78 nm, and 108 nm for x=0.0, 0.2, 0.4, and 0.6 respectively.Morphological and optical properties of the perovskite-type La 1-x Bi x FeO 3 samples exhibited a well-crystalline form and excellent band gap energy respectively.A photocatalytic reaction was conducted to test the degradation ability of the synthesized samples towards o-TB dye.The reaction was monitored over a period of 60 min while using the UV-Vis to observe the degradation activity.The most photocatalytically active sample for decomposition of o-TB under visible light was La 0.8 Bi 0.2 FeO 3 .This nanoparticle acts as an excellent photocatalyst for the degradation of o-TB dye with 90.09% efficiency in 60 min under natural sunlight irradiation.This sample have the smallest average grain size and a band gap energy in the range of semiconductor materials.These results give hope for future application of this material in photocatalytic degradation of various organic pollutants present in polluted water under natural sunlight.Hence, this nanoparticle may be well exploited for the remediation of the polluted water under natural sunlight on a large scale.

Figure 1 .
Figure 1.Schematic representation of the elementary steps of La1-xBixFeO3 (x = 0.0,0.2,0.4, 0.6) perovskite preparation using the modified Pechini method Material Characterization Phase identification and unit cell parameter determination were checked using powder X-ray diffraction (XRD) at room temperature.Powder X-ray diffraction (PXRD) patterns were recorded on a Proto AXRD Benchtop diffractometer in Bragg-Brentano θ-2θ-geometry with CuK α radiation (λ=1.5418Å) operating at 30 kV and 20 mA.Regarding the

Table 1 .
Crystallographic parameters of La1-xBixFeO3 samples as obtained from Rietveld refinement