Magnetic and photocatalytic properties of BiFeO$_3$ nanoparticles formed during the heat treatment of hydroxides coprecipitated in a microreactor with intense swirling flows

In this work, hydroxide deposition in a microreactor with intensively swirling flows was used to obtain BiFeO$_3$, followed by heat treatment of co-precipitated bismuth and iron hydroxides. The study of the formation of nanocrystalline bismuth orthoferrite was carried out using a set of methods: EDXMA, TEM, XRD, $^{57}$Fe Mössbauer spectroscopy, DRS, etc. The photocatalytic activity and magnetic characteristics of the material were determined. It is shown that during heat treatment of hydroxide precipitation for 1 minute at a temperature of 530$^\circ$C, BiFeO$_3$ nanocrystals with an average crystallite size of 14 $\pm$ 7 nm are formed. It was found that the resulting BiFeO$_3$ nanopowder is represented by agglomerates of individual nanoparticles. The saturation magnetization and residual magnetization values of these bismuth orthoferrite nanoparticles are 2.31 and 0.48 emu/g, respectively. According to the DRS results, band gap energy for the samples calculated at 530, 515, and 500$^\circ$C were 1.82, 1.86, and 1.91 eV, respectively, which ensures strong absorption of visible light by the samples. The sample showed higher photocatalytic activity in the X-ray amorphous state compared with nanocrystalline BiFeO$_3$ in the process of Fenton-like discoloration of methyl violet under the action of visible light with reaction rate constants of the pseudo-first order 0.0256 and 0.0072 min$^{-1}$, respectively.