Highly efficient photocatalysis by BiFeO_3/α(γ)- Fe_2O_3 ferromagnetic nano p junctions formed by dopant-induced phase separation

摘要

A series of Bi_(1-x) Ca _x FeO_3 (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 Bi_2Fe_4O_9 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 Fe_2O_3 (both α and γ components) are observed with a concomitant enhancement of ferromagnetism. Small contribution of the Bi_6Ca_4O_(13) 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 M?ssbauer 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 H_2O _2) reveal that Ca doping at the Bi site in BFO enhances the activity significantly in the concentration regime where BFO/α(γ)-Fe _2O_3 phases coexist in the form of a nanocomposite. The enhancement can thus be attributed to the carrier transfer between BFO and α(γ)-Fe_2O_3 across nano p 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.