In situ formed Bi/BiOBr_xI_1-x heterojunction of hierarchical microspheres for efficient visible-light photocatalytic activity

Bi nanoparticles deposited in situ in BiOBr_xI_1-x hierarchical microspheres (Bi/BiOBr_xI_1-x heterojunction) were synthesized by a facile one-step solvothermal method. The as-prepared samples were characterized via XRD, SEM, TEM, XPS, UV-vis absorption spectroscopy and N₂ adsorption-desorption. The hierarchical microspheres were composed of numerous nanosheets aggregated together compactly to form a spherical geometry. Results indicated that Bi nanoparticles were generated on the surface of BiOBr_xI_1-x microspheres via the in situ reduction of Bi³⁺ by ethylene glycol. BiOBr_xI_1-x microspheres with deposited Bi nanoparticles were employed for the degradation of RhB under visible-light irradiation and the samples exhibited exceptionally enhanced photocatalytic activity. This immense enhancement in photocatalytic activity was attributed to the contribution of Bi nanoparticles to the efficient separation of electron-hole pairs and prolongation of the lifetime of charge carriers. The behavior of Bi nanoparticles as a cocatalyst for enhancing photocatalytic activity is similar to that of noble metals in photocatalysis. The as-prepared Bi/BiOBr_0.266I_0.734 sample exhibited highest photocatalytic activity, which exceeded those of other types of visible-light photocatalysts such as N-TiO₂, Eu³⁺-BiOI, BiOBr, BiOBr_0.2I_0.8/graphene and even Ag/AgBr/BiOBr. The Bi/BiOBr_0.266I_0.734 sample displayed high photochemical stability under repeated visible-light irradiation, which is especially important for its practical application. The active species produced from Bi/BiOBr_xI_1-x under visible light were hydroxyl radicals. Bi/BiOBr_xI_1-x could generate more hydroxyl radicals due to the Bi nanoparticles, contributing to the enhance oxidation ability. This study demonstrated the high feasibility of utilizing low-cost Bi nanoparticles as a substitute for noble metals to enhance visible-light photocatalysis.