| BiFeO3is the only known single-phase compound exhibiting mutiferroism at room temperature. Both BiFeO3and Bi2Fe4O9have received considerable recent interest because of their multiferric, magnetic, catalytic and sensing properties. However, it is difficult to obtain the pure-phase bismuth ferrite because the kinetics of phase formation in the Bi2O3-Fe2O3system can easily lead to the coexistence of compounds such as BiFeO3, Bi2Fe4O9and Bi25FeO40. The intrinsic properties and applications of bismuth ferrite are still being hindered greatly by the presence of secondary phases. In this work, BiFeO3and Bi2Fe4O9nanocrystals of various morphologies have been synthesized by a facile chemical co-precipitation process. Their microstructure and growth mechanism, optical properties and energy band structure, photoelectric conversion, photocatalytic activities as well as Fenton-like ability have been investigated.Single-phase BiFeO3/Bi2Fe4O9nanocrystals with various morphologies, such as nanosheets, nanorods, nanoparticles and nanocubes, have been synthesized through a facile chemical co-precipitation process by using NaOH or NH4OH as precipitator. The Bi2Fe4O9nanorods are0.2-2μm long, their transverse sizes range from100-200nm, and its growth direction is [001] while its side facets are (110) and (110). An oriented-attachment mechanism has been suggested for the formation of Bi2Fe4O9nanorods due to the preferential adsorption of hydroxide on (110) and (110) facets of high-surface energy. The sheet-like BiFeO3nanoparticals (50-200nm) of a rhombohedrally distorted perovskite structure have large surface area of58.3m2g-1.UV-vis absorption spectra indicate that both the BiFeO3nanoparticles and nanosheets have absorptions from ultraviolet to visible light. Their bandgaps are1.93eV and1.99eV, respectively. Two bandgaps of the Bi2Fe4O9nanorods are calculated to be2.05eV and1.53eV, respectively. The bandgap values show that BiFeO3/Bi2Fe4O9nanocrystals are good at absorbing solar light, especially visible light.The photoelectric conversion properties of BiFeO3(or Bi2Fe4O9) nanocrystals have been investigated. Prompt and steady photocurrent density of41μA/cm2was observed in the BiFeO3nanoparticles under UV-vis irradiation, while35μA/cm2under visible light, which is much higher than that (～5.2μA/cm2) observed in BiFeO3nanocubes under visible light irradiation in photoelectrochemical mode. Steady photocurrent of37μA/cm2was observed in the Bi2Fe4O9nanorods under visible irradiation, while47μA/cm2under UV-vis irradiation. The photo-to-current response of Bi2Fe4O9has not been reported before. Such a high photo-to-current conversion efficiency is ascribed to the synergistic effect of energy band structure and the high reactivity of exposed (110) and (110) facets. In addition, the nanoscale size of Bi2Fe4O9nanarods facilitates the separation of the excited electrons and holes before their loss via recombination to improve the photo-to-current efficiency. Therefore, both BiFeO3and Bi2Fe4O9nanocrystals are promising to become an useful material for photoelectrode in solar cell and splitting water into hydrogen.The catalytic abilities of BiFeO3(or Bi2Fe4O9) nanocrystals for Rhodamine B (RhB) were investigated under different conditions. BiFeO3nanosheets are more efficient for the visible-light degradation of RhB than nanoparticles. BiFeO3(or Bi2Fe4O9) and H2O2form an novel Fenton-like catalyst and they exhibited a high Fenton catalytic activity. With the visible light irradiation the Fenton-like catalyst of BiFeO3nanosheets and H2O2showed the highest degradation rate93%of RhB due to the much enhanced generation of·OH radicals. Different morphology of Bi2Fe4O9has different catalytic activity. Under visible irradiation, the degradation rate of RhB was increased to54%in Bi2Fe4O9nanoparticles/H2O2system, but greatly decreased to49%in Bi2Fe4O9nanorods/H2O2system. Consequently, both the BiFeO3and Bi2Fe4O9nanocrystals are promising to be used in the treatment of organic pollutants in water.
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