Hydrothermal Synthesis of CeO₂ Nanocrystals: Ostwald Ripening or Oriented Attachment?

The growth mechanism of CeO₂ nanocrystals prepared by the hydrothermal method has been studied in this article. The synthesis of CeO₂ nanocrystals follows two stages, the initial nucleation of CeO₂ nuclei and the subsequent ripening of nuclei in the hydrothermal process. The nucleation involves the precipitation of Ce³⁺ cations by OH^– ions to form Ce­(OH)₃ nanoparticles and the transition from Ce­(OH)₃ to 2–3 nm CeO₂ nuclei through an oxidation and rapid dehydration process. In the hydrothermal process, the oriented attachment of nuclei through a lattice matched surface and subsequent Ostwald ripening results in the growth of CeO₂ nanocrystals. The dominant mechanism for the ripening of nuclei in hydrothermal reactions is the oriented attachment. The addition of polyvinylpyrrolidone surfactant and adjustment of solution acidity can promote the dispersion of the nuclei and enhance the effective collision among them in the hydrothermal stage, resulting in the oriented aggregation of particles and further growth into larger CeO₂ nanocrystals (∼15 nm). Because of the low solubility of CeO₂ crystals in water, the Ostwald ripening process (dissolution/reprecipitation) only plays the second important role in the hydrothermal reactions, which converts the assembly clusters into nanocrystals with/without well-defined edges or contributes to the further growth of nuclei from 2–3 nm to 3–5 nm.