Evaluation of Synthetic Methods for Bismuth(III) Oxide Polymorphs: Formation of Binary versus Ternary Oxides

Hydrolysis of the bismuth oxido cluster [Bi 38 O 45 (O 2 CC 3 H 5 ) 24 (DMSO) 9 ] ( A ), dissolved in EtOH, with a diluted NaOH solution at ambient temperature using Teflon-lined vessels followed by heating of the amorphous product on a silver sheet gave pure γ-Bi 2 O 3 . The formation of Bi 12 SiO 20 in addition to γ-Bi 2 O 3 is observed when the hydrolysis of A is carried out in a glass vessel, which provides a low concentration of silicates as a result of glass corrosion. Hydrolysis of the cluster A under microwave-assisted heating in a Teflon-lined vessel gave β-Bi 2 O 3, whereas Bi(NO 3 ) 3 ·5H 2 O ( B ) provided α-Bi 2 O 3 and Bi 12 O 17 Cl 2 was observed from BiCl 3 ( C ). In glass vessels, alkaline induced hydrolysis of the precursors provided exclusively Bi 12 SiO 20 as final product instead of the isomorphous metastable γ-Bi 2 O 3 . This observation is in contrast to recent literature reports. Powder X-ray diffraction studies, attenuated total reflection infrared spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy were used to provide insight into the hydrolysis processes and into the concurring formation of the sillenite-type compound Bi 12 SiO 20 instead of pure bismuth(III) oxide polymorphs.