Mechanochemical syntheses of a series of bismuth oxyhalide composites to progressively enhance the visible-light responsive activities for the degradation of bisphenol-A

Single phase of BiOX (X = F, Cl, Br, or I) and composites of xBiOI/(1-x)BiOX (X = F, Cl, or Br) were successfully synthesized by one-step dry grinding with stable Bi oxide and halides as starting samples. Several characterization methods including X-ray diffraction, UV-Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscope-energy dispersive spectroscopy, photoluminescence and electro-chemical impedance spectroscopy were used to study the crystal phase structures and optical properties of the samples. Among the four samples of single phase BiOX, in the order from F, Cl, Br to I, the samples exhibited enhanced capacity for bisphenol-A (BPA, C15H16O2) degradation and BiOI gave the highest values around 40%, in accordance with the order in adsorption capacity of visible light. Among the composites BiOI/BiOX, BiOI/BiOCl in equal mass ratio gave the highest degradation capacity of BPA, which was nearly undetectable even under visible light irradiation. This result could be attributed to the formation of a heterojunction between BiOI and BiOCl, to efficiently separate the photogenerated carriers from the analytical results of photoluminescence and electrochemical impedance spectroscopy. Based on the correlation experiments and the calculation of the energy band structure, a possible photocatalytic mechanism is proposed.