Synthesis of non-stoichiometric Bi2O4−x by oxidative precipitation

Bi2O4−x, a Bi mixed-valence phase was prepared at 95 °C, by a precipitation process, in a basic medium with a highly oxidizing K2S2O8/Na2S2O8. This phase has a low thermal stability as it decomposes below 400 °C in a multiple step process by some O2 losses prior to finally transforming into γ-Bi2O3. The as-prepared powders are 50–60 nm in size with a narrow size distribution. Optical spectra of Bi2O4−x exhibit a broad absorption band with a band gap of ∼1.4 eV as compared to 2.61 eV for Bi2O3. The composition of this non-stoichiometric phase, which crystallizes in cubic fluorite related structure with a cell parameter of 5.538(3) Å, is Bi2O3.65 ± 0.10.