Enhanced visible-light responsive photocatalytic activity of Bi25FeO40/Bi2Fe4O9 composites and mechanism investigation

Pure Bi25FeO40, Bi2Fe4O9, and different weight ratios of Bi25FeO40/Bi2Fe4O9 composite photocatalysts have been synthesized via a hydrothermal process combined with a mixing-calcination method and evaluated as visible-light responsive catalyst for the degradation of Rhodamine B (RhB). All the as-prepared samples have been characterized by a range of techniques including X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), UV-vis absorption spectra (DSR), Field Emission Scanning Electron Microscope (FE-SEM), Transmission electron microscope (TEM) and High-resolution TEM (HRTEM). The XRD, FT-IR, TEM and HRTEM results confirm that the composite only consists of Bi25FeO40 and Bi2Fe4O9. In the Bi25FeO40/Bi2Fe4O9 composites, closely contacted interfaces have been observed. Compared with the single-phase Bi25FeO40 and Bi2Fe4O9, Bi25FeO40/Bi2Fe4O9 composites exhibit enhanced visible-light responsive photocatalytic activities. The photocatalytic efficiency of optimized Bi25FeO40/Bi2Fe4O9 composite with Bi2Fe4O9 weight ratio of 30% is about 8.8 and 6.2 times higher than that of pure Bi25FeO40 and Bi2Fe4O9, respectively. On the basis of electronic energy-band structure analysis, the active species trapping experiments and the electrochemical impedance spectrum (EIS) performance, a heterojunction-type charge transfer mechanism interpreting the enhanced photocatalytic activities of the composite are proposed and discussed. In addition, the effects of different Bi25FeO40/Bi2Fe4O9 weight ratios and their geometry architecture on photocatalytic activities are also thoroughly discussed.