Sol–Gel Synthesis of High-Purity Actinide Oxide ThO₂ and Its Solid Solutions with Technologically Important Tin and Zinc Ions

The applicability of epoxide-based sol–gel synthesis for actinide oxide (thoria) starting from air-stable salt, Th­(NO₃)₄, has been examined. The homogeneous gel formed from Th­(NO₃)₄ when calcined at 400 °C yielded nanostructured thoria, and with increasing tempeartures (600, 700, and 800 °C), the average crystallite size increased. Successful Rietveld refinement of the powder X-ray diffraction pattern of ThO₂ in Fm3̅m space group was carried out with a = 5.6030(35) Å. The fingerprint vibrational mode of the fluorite structure of ThO₂ was noticed as a sharp band in the Raman spectrum at 457 cm^–1. In the SEM image, a near spherical morphology of thoria was noticed. Samples showed blue emission on exciting with λ = 380 nm in the photoluminescence spectrum indicative of the presence of defects. Following this approach, 50 mol % of Sn⁴⁺ could be substituted for Th⁴⁺, retaining the fluorite structure as evidenced by the PXRD, Raman spectroscopy, electron microscopy, EDAX, and XPS measurements. Randomization of the lattice was observed for the tin-substituted samples. A significant blue shift in the absorption threshold along with a persistent blue emission in the photoluminesence spectra were evident for the tin-substituted samples. The concentration of Zn²⁺ ion in thoria was limited to 15 mol % as revealed by PXRD and XPS measurements. The Raman peak shifted to higher values for Zn²⁺-substituted samples. A change in the optical absorbance characteristics was observed for the zinc-substituted thoria. A 50 mol % Sn⁴⁺-substituted thoria degraded aqueous Rhodamine 6G dye solutions in the presence of UV–vis radiation following pseudo-first-order kinetics.