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 (ΔH_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.