Bismuth-iron-based precursor: preparation, phase composition, and two methods of thermal treatment

Bismuth ferrite (BiFeO₃) is a promising Bi-based perovskite-type material, which is multiferroic due to the coexistence of anti-ferromagnetism and ferroelectricity. During the preparation of pure BiFeO₃ nanoparticles, however, the phase structures and species of bismuth-iron-based precursor (BFOH) were still unclear, and so related precursors were prepared. X-ray diffraction, Raman, Fourier transform infrared, and X-ray absorption near-edge structure techniques were used to probe the phase structure and species of the precursors. It was found that the precursor BFOH is composed of Bi₆O₆(NO₃)₄(OH)₂·2H₂O, Bi₆O₅(NO₃)₅(OH)₃·3H₂O, Fe(OH)₃, and α-Bi₂O₃. Calcination treatment and hydrothermal synthesis were used to prepare the pure BiFeO₃ phase from the precursor BFOH. The calcination temperature was optimized as 400 °C for preparation of the pure BiFeO₃ phase. Meanwhile, hydrothermal conditions for the synthesis of the pure BiFeO₃ phase were also optimized as follows: the reaction solution was the mixture solution of Bi(NO₃)₃·5H₂O and Fe(NO₃)₃·9H₂O with cetyltrimethyl ammonium bromide (CTAB) as the surfactant and KOH as the mineralizer; the hydrothermal synthesis was performed at 180 °C for 48 h; the concentration of KOH should be at least 3 M; and the surfactant CTAB can be used to regulate the morphology of the as-prepared BiFeO₃ nanoparticles. From the point of view of the microstructure, BiFeO₃ nanoparticles prepared by calcination or hydrothermal methods have no notable differences. A formation mechanism from the precursor BFOH to the BiFeO₃ product is proposed. By providing an understanding of the precursors, this work is very helpful in the synthesis of bismuth-iron-based nanoparticles.