*2.1.2.2 Hydrothermal reaction*

The solution would be placed in a hydrothermal Teflon linear autoclave at 150°C-220°C for 5–20 hours [19, 47] until the precursor was collected. The autoclave would then be naturally cooled to room temperature, the precipitate collected filtered, and cleaned with deionized water (and 10% acetic acid) to eliminate any soluble salts. Finally, the nanoparticles are obtained by drying the wet samples at 70-160°C for a few hours [19, 44]. A dissolution-crystallization process is assumed to be a synthesis of the BFO Phase by the hydrothermal method. First, Bi3+ and Fe3+ ions are converted into Fe (OH)3 and Bi (OH)3 hydroxides and later, under hydrothermal conditions, dissolved in a precursor of alkaline mineralizer (KOH, NaOH, LiOH). When the concentration of ions in the alkaline solution reaches the saturation stage, the BFO phase begins to nucleate, followed by crystal formation, and precipitates the supersaturated hydrothermal fluid [19, 48, 49]. BFO particle size and morphology rely on nucleation and crystal growth rates, which are in turn influenced by the degree of supersaturation. The subsequent strong nucleation but low growth rate and smaller particle size would be a high supersaturated precursor with high KOH content and high pH value [42, 49]. The substance's phase and morphologies are greatly influenced by the addition of water during the hydrothermal reaction Han et al. [49] and Chen et al. [19]. Analysis of the BFO hydrothermal synthesis method for the KOH concentration mineralizer, the reaction temperature, and the reaction time was performed. The precursors are held at175-225°C and 200-220°C respectively, for six hours, and produce phase-pure BFO powders using 8 M and 4 M KOH, respectively. Different raw materials and different hydrothermal equipment and conditions can contribute to the observed deviation from nominally optimal conditions (i.e., KOH concentrations). Han noticed that minor α-Bi2O3 could be generated while using a 4 M KOH solution. Scientists have noticed that metal nitrates, under some circumstances, combine with hydroxide ions to form the iron hydroxide phase.

$$\text{2Bi (NO}\_3\text{)}\_3 \star \text{6KOH} \rightarrow \text{Bi}\_2\text{O}\_3 \star \text{6 K}^\* + \text{6NO}^{3-} + \text{3H}\_2\text{O} \tag{1}$$

$$\text{Fe (NO}\_3\text{)}\_3 + \text{3KOH} \rightarrow \text{Fe (OH)}\_3 + \text{3 K}^\* + \text{3NO}\_3 \tag{2}$$

At a lower concentration of potassium hydroxide, the alpha-Bi2O3 phase can stay in the precursor, although it can be reduced into Bi3+ at a higher concentration of potassium hydroxide:

$$\text{BiO}\_3 \star \text{2Fe (OH)}\_3 \rightarrow \text{2BiFeO}\_3 \star \text{3H}\_2\text{O} \tag{3}$$

Reaction temperature has a significant effect on the reaction. At lower temperatures, alpha-Bi2O3 is produced as the key product. By raising the temperature of the solution from 150–175°C, the dissolution of alpha-Bi2O3 is significantly improved. The BFO process stays the same at the temperature of 225°C [49]. A related result was stated by Chen et al. [19]. Although increasing the pH, reaction time, and temperature will improve the solubility of alpha-Bi2O3, leading to pure BFO phases. Secondary phases such as the secondary precipitate of excess potassium hydroxide precipitation at a shift in acidic pH may occur when excessive reaction time is at high pH. As OH- ion concentration is becoming higher, particle agglomeration can be expected [19, 49]. Yang et al. figure out the second phase of Bi2Fe4O9 forms at a shallow concentration of KOH. Since Bi3+ has a greater affinity with OH than Fe3+, a low OH-concentration in the hydrothermal reaction solution would lead to more Fe3+ ions being dehydrated Bi2Fe4O9 instead of bismuth ferrite. As a consequence of an increase of potassium hydroxide concentration, the percentage of Bi3+ and Fe3+ becomes balanced, and then the BFO phase is formed [50]. When utilizing heavy alkaline NaOH instead of weak KOH for hydrothermal synthesis, both the recipes produce stoichiometric materials. At a pH = 14, a limited quantity of H2O2 can assist the reduction reaction [42], whereas a milder acidic solution will afford a crude substance with recorded nonstoichiometric Bi12Fe0.63O18.945 [42]. Thus, it relies on the techniques used to achieve synthesize the latest phases. Researchers have managed to obtain pure BFO nanoparticles at 200oC by using 7 mol/L KOH and 12 mol/L KOH; however, incorporating NaOH and LiNO3 would either interrupt Bi2Fe4O9 and Bi12(Bi0.5Fe0.5)O19.5phase development or slow it down in the final product no matter the concentration of these reagents [19]. By utilizing potassium nitrate nanoparticles, much smaller nanoparticles of the potassium nitrate content are made. This study suggests that alkali ions (K<sup>+</sup> , Na<sup>+</sup> , and Li<sup>+</sup> ) from mineralizer play a significant
