| Summary: | Bi-Fe oxides are stable materials with potential photocatalytic activity under solar light photons. So far, however the photocatalytic potential of pure-phase nanosized mullite-Bi<sub>2</sub>Fe<sub>4</sub>O<sub>9</sub> has not been studied. Usually, synthesis of pure-phase nanosized mullite-Bi<sub>2</sub>Fe<sub>4</sub>O<sub>9</sub> is hampered by co-formation with perovskite BiFeO<sub>3</sub>. Herein we demonstrate that pure-phase mullite-Bi<sub>2</sub>Fe<sub>4</sub>O<sub>9</sub> nanoparticles prepared by Flame Spray Pyrolysis (FSP) technology are highly efficient O<sub>2</sub> production photocatalysts, achieving >1500 μmol g<sup>−1</sup>h<sup>−1</sup>. This outperforms all -so far reported- O<sub>2</sub> production Bi-Fe-O photocatalysts. We present an FSP-based process for production of a versatile Bi-Fe-O platform, that can be easily optimized to obtain 100% mullite-Bi<sub>2</sub>Fe<sub>4</sub>O<sub>9</sub> or 100% perovskite-BiFeO<sub>3</sub> or their heterojunctions. The phase-evolution of the Bi-Fe-O materials has been studied by XPS, Raman, and EPR spectroscopies. Short post-FSP annealing process impacts the photoactivity of the BiFeO<sub>3</sub> and Bi<sub>2</sub>Fe<sub>4</sub>O<sub>9</sub> in distinct ways. Fe<sup>2+</sup> centers in BiFeO<sub>3</sub> can improve dramatically its O<sub>2</sub> production efficiency, while solid-melt formation in Bi<sub>2</sub>Fe<sub>4</sub>O<sub>9</sub> is a limiting factor.
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