Aggregation-based growth of faceted Bi₂Fe₄O₉ micro-cuboids with a remarkable visible light photo-Fenton catalytic activity

Monodispersed and faceted Bi2Fe4O9 micro-cuboid crystals with a mullite-type structure have been successfully synthesized by a facile hydrothermal method. The single crystalline Bi2Fe4O9 micro-cuboids exhibited a length of 10-20 mu m and a square cross-section with a side length of 1-2 mu m. The preferential growth of the Bi2Fe4O9 micro-cuboid crystals has been determined to be along < 001 > with the dominant (110) and (-110) facets enclosed. These micro-cuboid crystals were probably formed by a dissolution-recrystallization mechanism, where a phase transition from irregular-shaped sillenite Bi25FeO40 agglomerates to perovskite BiFeO3 microspheres occurred and finally resulted in the micro-cuboid crystals. More interestingly, these micro-cuboids demonstrated a remarkable photo-Fenton catalytic degradation activity towards RhB under visible light irradiation (lambda > 400 nm). 90.2% RhB decomposition at 120 min can be achieved and the corresponding apparent rate constant is 0.0175 min(-1), 11.67 times and 3.65 times larger than that of Fenton-like oxidation and pure photocatalysis, respectively. The as-prepared Bi2Fe4O9 micro-cuboid crystals also showed intriguing morphology-dependent ferromagnetism at room temperature, possibly allowing the magnetic recollection and reuse of Bi2Fe4O9 samples as well as their magnetic-field-mediated photocatalytic performances.