*2.1.1.2 Acid assisted sol: Gel technique*

Utilizing this process, acid is added to the solution structure. Tartaric acid (C4H6O6) is a commonly used acid synthesizing BFO nanoparticles [26–29]. By combining iron nitrate with natural solvents (2-MOE or EG), followed by tartaric acid (1: 1 molar ratio iron nitrate), raw material is synthesized using a tartaric acidassisted sol-gel method. With constant stirring at 60-80°C, until the transparent sol transforms into a brown gel, other recruited acetic acids [23, 30] could also be used to make BFO nanoparticles. The preliminary preparation is similar to the tartaric acid sol-gel method in that the first precursor is made by combining iron nitrate with an organic solvent, but this time the solvent is NH3.H2O before adding acetic acid to the precursor, it is added to adjust the pH of the solution to. The solution is then shaken and stirred at 70°C in a hot magnetic plate to form a sol. The brown gel should then be dried at around 120°C for xerogel powder. In the precursor process, acetic acid acts as an unidenttate-binding agent, and organic solvent (ethylene glycol) acts as a polarsoluble solvent. During heating, Fe ions are octahedrally mixed with condensate acetic ions after adding acetic acid and ethylene glycol. At temperatures of 70°C, this is accompanied by acetate ligands and ethylene glycol. At 120°C, +the resulting esters initiate the formation of Fe-O-Bi bonds [30].

## *2.1.1.3 The aqueous-based sol: Gel technique*

The latter method is a water-based route where no natural solvent is involved in the precursor mixing. The metal nitrate precursor system dissolves in dilute nitric

acid, followed by tartaric or acetic acid [31–34]. Using this line, the metal complex was created by adding acid to the aqueous metal nitrate precursor, and BFO nanoparticles began to form in the calculation after decomposition. Then after any of the routes as mentioned above, the dried gel powder is ground and heated over 400°C for pyrolysis to remove organic impurities and additional calcining around 500-600°C to obtain final nanoparticles [24, 35, 36]. In one study, after final immersion, the samples were repeatedly washed with distilled water, filtered, and dried at 80°C [36]. The sintering temperature is considered an appropriate strategy to monitor the size of BFO nanoparticles. Nanoparticle sizes can vary from less than 15 nm to more than 100 nm by incorporating sintering as low as 350–650°C [31, 34, 37]. In terms of lines (ii and iii), it proves that chelating acids (tartaric acid and citric acid) play an essential role in synthesizing nanoparticles, phases, and morphology. Using a precursor in water, it has been shown that the precipitation temperature of citric acid is less than 100oC than that of the preceding tartaric acid, taking temperatures into BFO nanoparticles that use as much citric acid as possible 350°C [34].

On the other hand, when citric acid was used as an irrigation agent and the calculation temperature was set at 600°C, it was also found that impurities such as Bi2O3 and Bi2Fe4O9 form BFO nanoparticles. On the other hand, the chelating agent of ethylene diamine tetraacetic acid (EDTA) in an aqueous precursor contributes to creating pure-phase BFO nanoparticles called the generation of heterometallic polynuclear complexes in solution [38, 39]. The addition of acrylamide and bisacrylamide monomers has also been beneficial in controlling the specific BFO size by providing a framework for the growth of nanoparticles and adjusting the size of the gel pores, respectively. However, the overgrowth of bisacrylamide may result from particles with no homogeneities and irregular shapes and contaminants [39]. The contaminants were obtained from BFO nanoparticles found in natural solvents (method (i) above) using citric acid as a chelating agent. This is due to a diametric of the citric complex and additional carbonaceous substances, which are pollutants usually created at high temperatures during the automatic combustion process [35, 40]. In comparison, tartaric acid creates bonds and iron ions with two groups of carboxyl and hydroxyl that contribute to a stable polynuclear complex. Esterification gel between metal complex and ethylene glycol [40]. The addition of acid chelating agents often influences the size of the nanoparticles. It is reported that the particle sizes of BFO nanoparticles found in citric acid at 350°C and tartaric acid at 450°C are as small as 4 nm and 12 nm, respectively [34]. The advantages of sol-gel synthesis techniques are easily accessible, energy saving, cost, and performance at low temperatures. The soluble temperature range in the sol-gel range is between 25°C and 200°C, and it is possible to combine a nano BFO with smaller size distribution and a controlled environment. These benefits and their flexibility in nano BFO synthesis make the sol-gel method very attractive. Other advantages of the sol-gel process include the necessary reagents that are simple compounds, producing nanoparticles, no special equipment required, dopants can be easily incorporated into the final product, there is little chance of assembling the particles, and the same grain structure. In addition, it is one of the most popular composite methods for controlling the formation of nanoparticles, microstructure, purity, and stability by adjusting various parameters such as sol concentration, vibration rate, and annealing temperature [41]. In addition, the combination of nanoparticles, films, and coating is the first commercial element of the sol-gel process [16]. The sol-gel synthesis method can synthesize various nanoparticles at a specified temperature range.
