Highly efficient photocatalysis by BiFeO₃/α(γ)-Fe₂O₃ ferromagnetic nano p/n junctions formed by dopant-induced phase separation

A series of Bi_1−x Ca _x FeO₃ (BCFO) nanoparticles (with x = 0.0, 0.03, 0.07, 0.10, 0.15, and 0.20) have been synthesized by sol–gel reaction. X-ray diffraction patterns establish the formation of hexagonal bismuth ferrite as the prominent phase, with a small contribution of the Bi₂Fe₄O₉ phase (as reported by others as well) which diminishes rapidly with the increase in Ca concentration. Interestingly, above a calcium dopant concentration of about 10 % peaks of Fe₂O₃ (both α and γ components) are observed with a concomitant enhancement of ferromagnetism. Small contribution of the Bi₆Ca₄O₁₃ phase is also noted in these samples. This phase evolution is driven by dopant-induced strain energy and increasing oxygen vacancy concentration for local charge balance. Transmission electron microscopy (with elemental scanning) and Mössbauer spectroscopy techniques bring out the evolution of nanoparticle morphology (and elemental distribution) and phase configuration, respectively. Measurements of photocatalytic activity (and photo-Fenton activity with H₂O₂) reveal that Ca doping at the Bi site in BFO enhances the activity significantly in the concentration regime where BFO/α(γ)-Fe₂O₃ phases coexist in the form of a nanocomposite. The enhancement can thus be attributed to the carrier transfer between BFO and α(γ)-Fe₂O₃ across nano p/n junctions leading to enhanced carrier lifetime. Importantly, the magnetization of the nanocomposite (about 16 emu gm⁻¹ at x = 0.20) provides a convenient way to collect the photocatalyst with the help of an external magnet for reuse.