Template-free fabrication of α- and β-Bi2O3 hollow spheres and their visible light photocatalytic activity for water purification

For the first time, uniform α- and β-Bi2O3 hollow microspheres were successfully synthesized through a facile template-free solvothermal method followed by calcination of the precursor. A mixture of glycerol (Gly) and ethanol (EtOH) (volume ratio of Gly–EtOH = 1:1) was selected as the solvent, Bi(NO3)3⋅5H2O as the bismuth source. The resulting samples were characterized by thermogravimetry (TG), Fourier transform infrared spectra (FT-IR), X-ray powder diffraction (XRD), scanning and transmission electron microscopy (SEM, TEM), Brunauer–Emmett–Teller (BET) surface area and UV–Vis diffuse reflectance spectra (DRS). Based on the electron microscope observations, the formation of such hollow spheres structure was proposed as a self-assembly process followed by Ostwald ripening. The sample calcined at 270 °C exhibited β-Bi2O3 phase, whereas the sample calcined at 350 °C exhibited α phase. The band gaps of the as-prepared materials were estimated by DRS to be 3.50, 2.82, and 2.36 eV for the precursor, α- and β-Bi2O3, respectively. Upon visible light irradiation, the β-Bi2O3 exhibits much higher photocatalytic activities than the α-Bi2O3 and precursor for the degradation of rhodamine B (RhB) in aqueous solution, which could be attributed to its narrower band gap and efficient visible light absorption.