Easy synthesis of high-purity BiFeO₃ nanoparticles: New insights derived from the structural, optical, and magnetic characterization

Synthesis of high-purity BiFeO₃ is very important for practical applications. This task has been very challenging for the scientific community because nonstoichiometric Bi_xFe_yO_z species typically appear as byproducts in most of the synthesis routes. In the present work, we outline the synthesis of BiFeO₃ nanostructures by a combustion reaction, employing tartaric acid or glycine as promoter. When glycine is used, a porous BiFeO₃ network composed of tightly assembled and sintered nanocrystallites is obtained. The origin of high purity BiFeO₃ nanomaterial as well as the formation of other byproducts is explained on the basis of metal-ligand interactions. Structural, morphological, and optical analysis of the intermediate that preceded the formation of porous BiFeO₃ structures was accomplished. The thorough characterization of BiFeO₃ nanoparticles (NPs) included powder X-ray diffraction (XRD); scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM); thermogravimetric analysis (TGA); UV-vis electronic absorption (diffuse reflectance mode), Raman scattering, Mössbauer, and electron paramagnetic resonance (EPR) spectroscopies; and vibrating sample magnetometry (VSM). The byproducts like β-Bi₂O₃ and 5 nm Bi₂Fe₄O₉ NPs were obtained when tartaric acid was the promoter. However, no such byproducts were formed using glycine in the synthesis process. The average sizes of the crystallites for BiFeO ₃ were 26 and 23 nm, for tartaric acid and glycine promoters, respectively. Two band gap energies, 2.27 and 1.66 eV, were found for BiFeO ₃ synthesized with tartaric acid, obtained from Tauc's plots. A remarkable selective enhancement in the intensity of the BiFeO₃ A₁ mode, as a consequence of the resonance Raman effect, was observed and discussed for the first time in this work. For glycine-promoted BiFeO ₃ nanostructures, the measured magnetization (M) value at 20 000 Oe (0.64 emu g^-1) was ∼5 times lower than that obtained using tartaric acid. The difference between the M values has been associated with the different morphologies of the BiFeO₃ nanostructures.