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The single-crystalline Bi2Fe4O9 nanoparticles have been synthesized by hydrothermal synthesis.The shape of the samples is flake, and changes with the alkali concentration as confirmed by SEM.Furthermore, fine monocrystalline Bi2Fe4O9 nanoparticles are observed from the images of HRTEM.Bi2Fe4O9 nanocrystal, with a band gap of 1.9 eV, have a strong absorption during visible light range, and probably is the candidate of visible light photocatalyst.The degradation rate of the methly orange solution is about 67% during 2h, with Bi2Fe4O9 nanoparticles as photocatalyst.Moreover, the dielectric and ferroelectric properties are also investigated in this work.The dielectric constant and dielectric loss of Bi2Fe4O9 nanocrystal gradually decrease with increasing the frequency in the whole frequency range from 20 Hz to 1M.The dielectric constant and dielectric loss of the sample are 200 and 2.3, respectively, when the frequency is 1000 Hz.The remant polarization of Bi2Fe4O9 is 0.547 uC/cm2,which is not really saturated due to the leakage current.Thus the values only representing a partial reversal of the polarization.