Bismuth Oxide Nanoparticles Partially Substituted with Eu^III, Mn^IV, and Si^IV: Structural, Spectroscopic, and Optical...

Interest in nanostructured partially substituted bismuth oxides has been increasing over the last years. Research on new synthesis methods, properties, and possible uses for these oxides is needed. The objective of this paper is to synthesize β-Bi₂O₃, β-Bi₂O₃:Eu³⁺, β-Bi₂O₃:Mn⁴⁺, Bi₁₂Bi_0.8O_19.2, Bi₁₂Bi_0.8O_19.2/Li⁺, Bi₁₂MnO₂₀, and Bi₁₂SiO₂₀ nanoparticles and to investigate their structural, spectroscopic, and optical changes. Some of the causes that generated their properties are also discussed. These materials are important because the doping or partial substitution of bismuth oxide with these cations (Eu³⁺, Mn⁴⁺, and Si⁴⁺) modifies some properties such as optical absorption, reactivity toward CO₂, among others. X-ray diffraction (in powders), high-resolution transmission electron microscopy, Fourier transform infrared (FTIR), resonance Raman scattering, diffuse reflectance, and solid-state magic-angle-spinning ²⁹Si NMR were used for the characterization of the synthesized materials. We found that partial substitution of yellow Bi₁₂Bi_0.8O_19.2 with Mn⁴⁺ and Si⁴⁺ changed the color to green and whitish, respectively. New bands in the Raman scattering and FTIR spectra of these oxides are deeply discussed. Raman scattering spectroscopy was a valuable and reliable technique to detect the Eu³⁺ and Mn⁴⁺ cations as dopants in the bismuth oxides. The ²⁹Si chemical shift (δ) in Bi₁₂SiO₂₀ was -78.16 ppm, whereas in SiO₂, it was around -110 ppm. This considerable shift in Bi₁₂SiO₂₀ occurred because of an increased shielding of the Si nucleus in the Si(O)₄ tetrahedron. This shielding was provided by the low-electronegativity and highly polarizable Bi cations. The isovalent doping of β-Bi₂O₃ nanoparticles with Eu³⁺ enhanced their thermal stability over 400 °C. Variation in the optical absorption and reactivity toward the acidic CO₂ molecule of the partially substituted bismuth oxides was explained on the basis of the optical basicity and ionic-covalent parameter concepts. Some possible uses for the synthesized oxides are suggested.