γ-BiO - To Be or Not To Be? Comparison of the Sillenite γ-BiO and Isomorphous Sillenite-Type BiSiO.

The "controlled" synthesis of metastable γ-BiO by solution based approaches was reported several times recently, but the formation of BiSiO in the presence of trace amounts of silicates renders the results to be questionable. Here, the preparation of the Sillenite γ-BiO and the Sillenite-type BiSiO starting from the polynuclear bismuth oxido cluster [BiO(OCCH)(DMSO)] is reported. γ-BiO crystallizes after calcination at 800 °C of the silicate-free hydrolysis product "[BiO(OH)]" 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. BiSiO was obtained after hydrothermal treatment of the bismuth oxido cluster in the presence of NaOH in glass vessels or NaSiO 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-BiO to give α-BiO occurred slowly in the temperature range of 348-510 °C ( Δ H = 6.57 kJ·mol). The silver-containing γ-BiO exhibits slightly increased Raman modes compared to the silver-free sample due to the SERS effect. In the diffuse UV-vis spectrum γ-BiO exhibits an absorption edge at λ = 485 nm ( E = 2.76 eV), and the contamination with silver results in an additional absorption edge at λ = 572 nm. Silver-free γ-BiO exhibits an absorption edge at λ = 460 nm ( E = 2.83 eV) and BiSiO at λ = 422 nm ( E = 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-BiO to be slightly more effective compared to BiSiO and significantly more effective than the silver-free γ-BiO.