Unexpectedly high degradation efficiency for MB on Bi3O4Br photocatalyst obtained from bismuth citrate

For the first time, pure Bi₃O₄Br with a uniform square sheet-like structure was synthesized using a hydrothermal method at 180 °C and pH of 12.5 by employing bismuth citrate as the bismuth source without any surfactant. The photocatalytic activity of Bi₃O₄Br for the degradation of methylene blue (MB), tetracycline (TC), methyl violet (MV), and rhodamine B (RhB) was evaluated under visible light illumination and the results showed that the as-prepared Bi₃O₄Br exhibited excellent photodegradation efficiency for MB with an apparent rate constant (k_a) of 0.02676 min⁻¹, superior to TC, MV and RhB with k_a = 0.00899 min⁻¹, 0.00073 min⁻¹ and 0.00409 min⁻¹, respectively. Since the direct oxidation of the holes predominates in the degradation process, the study on the photocatalytic mechanism suggested that a more positive valence band (VB) potential of Bi₃O₄Br than HOMO orbital position of MB was prerequisite and also, the smaller the difference between VB potential of a photocatalyst and HOMO energy level of a pollutant, the faster the migration of the holes and subsequent oxidation of pollutant.