*2.1.2.1 Precursor preparation*

The most popular hydrothermal synthesis process used for the preparation of BFO nanoparticles is the mineralizer-assisted route, in which mineralizers such as KOH (NaOH) [7, 19, 42, 43] or KNO3 [19, 44] are used for the preparation of precursors. This process allows the precursor to be conveniently prepared before hydrothermal synthesis by resolving the metal nitrates in purified water and KOH solution [45]. The aqueous precursor is produced by metal nitrates immersed in dilute nitric acid more generally than others. The method is then applied to the KOH solution for precipitation of Fe3+ and Bi3+. Precipitates are purified and cleaned to extract NO3− and K+ ions by pure water. The washed precipitates are then combined with KOH or NaOH solutions and additional KNO3 under continuous agitation [19, 44, 46]. In addition to these processes, the precursor scheme for the alteration of nanoparticles can often involve other additives, such as triethanolamine (TEA) [43] or sodium lauryl sulfate (SDS). In specific, the addition of TEA is observed to create a Fe-TEA complex that inhibits Fe hydroxide precipitation and may minimize synthesis to a maximum of 130oC [43]. Without the presence of the mineralizer, hydrothermal approaches may also be modified [45]. In hydrothermal synthesis of BiFeO3, the precursor of BiFeO3 is frequently synthesized by dissolving the metal sources, Bi and Fe nitrates (Bi(NO3)3.5H2O and Fe(NO3)3.9H2O, in deionized (DI) water or citric acid. The metal and citric acid molar ratio is equal to 1:1 ratio. Then, NH3 solution is added to neutralize the unreacted citric acid and change the pH to~9 [45]. Using this updated mineralizer-free method, the dried powder is eventually calcined at 350°C for 6 hr. to extract the final nanoparticles, which is an extremely phase-pure BFO with an average particle size of 55 nm.
