In the present work, pure BiFeO 3 and pure Bi 2 Fe 4 O 9 single phases were successfully synthesized by tailoring hydrothermal synthesis route. The structural and morphology analyses have been achieved by X-ray diffraction and by electron microscopy. Scanning and transmission electron microscopy images revealed a strong change in the morphology from agglomerated nanoparticles with sizes less than 10 nm for BiFeO 3 to plate-like particles with larger sizes (edges of 100 to 200 nm while a thickness of * 30 nm) for Bi 2 Fe 4 O 9 . Such a small size obtained for BFO nanoparticles has never been observed before for similar synthesis method without any added reagents, to the best of our knowledge. The photocatalytic activity of the as-prepared samples for degradation of methylene blue dye under sunlight irradiation shows a good efficiency. It reaches 61% and 83% after 3 h for BiFeO 3 and Bi 2 Fe 4 O 9 nanostructures, respectively. Despite a bigger size and higher bandgap, the better efficiency of Bi 2 Fe 4 O 9 compared to BiFeO 3 is explained by a much stronger absorption especially for light with energy higher than 2.65 eV.

The multiferroic Zn‐doped BiFeO3 materials with nominal formula BiFe1−xZnxnormalO3 (0 ≤ x ≤ 0.08) are successfully synthesized using hydrothermal synthesis allowing to obtain powders with an average grain size of 8 nm. X‐ray diffraction and Raman spectroscopy analysis prove that nanopowders are pure and homogeneous and show an increase of the unit cell volume when the amount of Zn increases attesting of the substitution of Fe site. Using temperature‐dependent Raman spectroscopy, it is also found that the Néel antiferromagnetic critical temperature is weakly, if any, affected. In contrast, using UV–vis reflectance measurements, clear changes are observed in the features of the optical absorption with modifications of the crystal‐field when increasing the Zn amount. The photocatalytic activity of the pure and the most doped samples is evaluated by the degradation of the methylene blue dye under visible light irradiation. The photocatalytic activity of the Zn‐doped samples is found to be higher than that of BiFeO3. After 3 h of sunlight irradiation, the degradation efficiency increases from 61% to 83% for BiFeO3 and BiFe0.92Zn0.08O3, respectively. The crystal‐field modification after Zn substitution is considered as the principal cause of the obvious enhanced photocatalytic activity.

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