The “controlled” synthesis of metastable γ-Bi2O3by solution based approaches was reported several times recently, but the formation of Bi12SiO20in the presence of trace amounts of silicates renders the results to be questionable. Here, the preparation of the Sillenite γ-Bi2O3and the Sillenite-type Bi12SiO20starting from the polynuclear bismuth oxido cluster [Bi38O45(O2CC3H5)24(DMSO)9] is reported. γ-Bi2O3crystallizes 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. Bi12SiO20was obtained after hydrothermal treatment of the bismuth oxido cluster in the presence of NaOH in glass vessels or Na2SiO3in 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-Bi2O3to give α-Bi2O3occurred slowly in the temperature range of 348–510 °C (ΔHγ→α= 6.57 kJ·mol–1). The silver-containing γ-Bi2O3exhibits slightly increased Raman modes compared to the silver-free sample due to the SERS effect. In the diffuse UV–vis spectrum γ-Bi2O3exhibits an absorption edge at λ = 485 nm (Eg= 2.76 eV), and the contamination with silver results in an additional absorption edge at λ = 572 nm. Silver-free γ-Bi2O3exhibits an absorption edge at λ = 460 nm (Eg= 2.83 eV) and Bi12SiO20at λ = 422 nm (Eg= 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-Bi2O3to be slightly more effective compared to Bi12SiO20and significantly more effective than the silver-free γ-Bi2O3.
