From X-ray Amorphous ThO₂ to Crystalline Nanoparticles through Long-Term Aging at Room Temperature

X-ray amorphous thorium oxide [ThO2(X-ray am)] was obtained via rapid chemical precipitation of thorium nitrate and aqueous ammonia solutions. The solubility of oxide in water (I = 0.01 M) and changes in the solid phase were simultaneously studied. The formation of well-defined crystalline ThO2 nano-particles (NPs) from X-ray amorphous samples during room-temperature aging was systematically examined via dynamic light scattering, synchrotron-based X-ray diffraction, and high-resolution transmission electron microscopy. Here, our results suggest that the particle surface becomes coarse in the near-neutral and alkaline pH regions, where the concentration of thorium in the solution does not exceed 10-7 M owing to oriented attachment. At low pH, NP growth mainly occurs via Ostwald ripening.