Phase transformation and room temperature stabilization of various Bi2O3 nano-polymorphs: effect of oxygen-vacancy defects and reduced surface energy due to adsorbed carbon species

We report the grain growth from the nanoscale to microscale and a transformation sequence from Bi →β-Bi₂O₃→γ-Bi₂O₃→α-Bi₂O₃ with the increase of annealing temperature. The room temperature (RT) stabilization of β-Bi₂O₃ nanoparticles (NPs) was attributed to the effect of reduced surface energy due to adsorbed carbon species, and oxygen vacancy defects may have played a significant role in the RT stabilization of γ-Bi₂O₃ NPs. An enhanced red emission band was evident from all the samples attributed to oxygen-vacancy defects formed during the growth process in contrast with the observed white emission band from the air annealed Bi ingots. Based on our experimental findings, the air annealing induced oxidation of Bi NPs and transformation mechanism within various Bi₂O₃ nano-polymorphs are presented. The outcome of this study suggests that oxygen vacancy defects at the nanoscale play a significant role in both structural stabilization and phase transformation within various Bi₂O₃ nano-polymorphs, which is significant from theoretical consideration.

Structural and Enhanced Optical Properties of Stabilized γ‒Bi2O3 Nanoparticles: Effect of Oxygen Ion Vacancies

We report the synthesis of room temperature (RT) stabilized γ–Bi₂O₃ nanoparticles (NPs) at the expense of metallic Bi NPs through annealing in an ambient atmosphere. RT stability of the metastable γ–Bi₂O₃ NPs is confirmed using synchrotron radiation powder X-ray diffraction and Raman spectroscopy. γ–Bi₂O₃ NPs exhibited a strong red-band emission peaking at ~701 nm, covering 81% integrated intensity of photoluminescence spectra. Our findings suggest that the RT stabilization and enhanced red-band emission of γ‒Bi₂O₃ is mediated by excess oxygen ion vacancies generated at the octahedral O(2) sites during the annealing process.

Synthesis and radiation shielding properties of polyimide/Bi2O3 composites

Radiation shielding composites based on polyimide and Bi₂O₃ were synthesized. Surface and physical-mechanical properties of polyimide/Bi₂O₃ composites were studied. Bi₂O₃ particles were modified by polymethylphenylsiloxane for the uniform distribution of filler in composites. This paper presents data on the production of composites in two ways: hot- and cold-pressing. The hot-pressing method for the synthesis of composites is preferable compared to the cold-pressing method (the density increases by 10-12%, and the Vickers microhardness by 10-20%). The results show that the introduction of Bi₂O₃ significantly increases the thermal stability of the composites. At 680 °C, a polymer composite containing 10 wt% Bi₂O₃ retains 9.7% of its mass, and at 60 wt% Bi₂O₃, retains 58.4%. The radiation-protective characteristics of the composites with respect to gamma radiation were evaluated by experimental and theoretical methods. High radiation-protective characteristics of the composites have been established in the gamma-quanta energy range of 0.1-1 MeV.

γ-Bi2O3 - To Be or Not To Be? Comparison of the Sillenite γ-Bi2O3 and Isomorphous Sillenite-Type Bi12SiO20

The "controlled" synthesis of metastable γ-Bi₂O₃ by solution based approaches was reported several times recently, but the formation of Bi₁₂SiO₂₀ in the presence of trace amounts of silicates renders the results to be questionable. Here, the preparation of the Sillenite γ-Bi₂O₃ and the Sillenite-type Bi₁₂SiO₂₀ starting from the polynuclear bismuth oxido cluster [Bi₃₈O₄₅(O₂CC₃H₅)₂₄(DMSO)₉] is reported. γ-Bi₂O₃ crystallizes after calcination at 800 °C of the silicate-free hydrolysis product "[Bi₃₈O₄₅(OH)₂₄]" on a silver sheet. Corrosion of the substrate causes contamination with silver, which is not incorporated into the Bi-O lattice, and was removed by treatment with an aqueous KCN-solution. Bi₁₂SiO₂₀ was obtained after hydrothermal treatment of the bismuth oxido cluster in the presence of NaOH in glass vessels or Na₂SiO₃ in a Teflon-lined reactor vessel followed by calcination at 600 °C. PXRD studies, scanning electron microscopy, nitrogen adsorption measurements, IR- and Raman spectroscopy, diffuse UV-vis spectroscopy, and DSC were used for characterization. The phase transition of γ-Bi₂O₃ to give α-Bi₂O₃ occurred slowly in the temperature range of 348-510 °C ( Δ H_γ→α = 6.57 kJ·mol^-1). The silver-containing γ-Bi₂O₃ exhibits slightly increased Raman modes compared to the silver-free sample due to the SERS effect. In the diffuse UV-vis spectrum γ-Bi₂O₃ exhibits an absorption edge at λ = 485 nm ( E _g = 2.76 eV), and the contamination with silver results in an additional absorption edge at λ = 572 nm. Silver-free γ-Bi₂O₃ exhibits an absorption edge at λ = 460 nm ( E _g = 2.83 eV) and Bi₁₂SiO₂₀ at λ = 422 nm ( E _g = 3.16 eV). The photocatalytic activity of the compounds was investigated in the decomposition of aqueous rhodamine B under visible light irradiation, showing silver-containing γ-Bi₂O₃ to be slightly more effective compared to Bi₁₂SiO₂₀ and significantly more effective than the silver-free γ-Bi₂O₃.