Synthesis of β-Bi2O3 nanoparticles via the oxidation of Bi nanoparticles: Size,shape and polymorph control,anisotropic thermal expansion,and visible-light photocatalytic activity

Bismuth trioxide (Bi₂O₃) is known for its simple composition but rich polymorphism that allows a number of phase-dependent physicochemical properties and technical applications. We here report our controllable synthesis of highly discrete β-Bi₂O₃ (tetragonal) nanoparticles (NPs) via the thermal oxidation of nearly-monodisperse Bi NPs. The size and shape (spherical and tear- or rod-like) of β-Bi₂O₃ NPs are tunable by the size of parent Bi NPs whereas β and α (monoclinic) polymorphs can be selectively achieved by tailoring the oxidation temperature. The phase stability of β polymorph from room temperature to 450 °C enables us to perform an in situ high-temperature XRD study on the temperature dependence of its lattice parameters, which reveals a marked thermal expansion anisotropy in β-Bi₂O₃ with a linear thermal expansion coefficient of +35.1 × 10^?6 °C^?1 in the c axis, fifteen times higher than that in the a axis. Meanwhile, the narrow band gap (2.27 eV for β vs. 2.77 eV for α) and strong visible-light absorption endow β-Bi₂O₃ NPs with a good photocatalytic activity for the visible-light Rhodamine B dye degradation. We expect that our work could be a valuable reference for the studies on the size, shape and polymorph control, thermal property, and photocatalytic application of Bi₂O₃.