Peering into the Formation of Cerium Oxide Colloidal Particles in Solution by in-situ Small-Angle X-ray Scattering.

The formation of CeO2 colloidal particles upon heating of an aqueous solution of (NH4)2Ce(NO3)6 up to 100 °C was investigated by time-resolved, in-situ SAXS analysis using synchrotron radiation, providing absolute intensity data. In particular, the experiments were performed applying different temperatures between room temperature and 100 °C, as well as under variation of the ionic strength and concentration. Using validated SAXS evaluation tools (SASfit and McSAS software) the analyses revealed the presence of two types of particle populations possessing average dimensions of ca. 2 nm and 5 - 15 nm, the latter being agglomerates of the 2 nm particles rather than single crystallites. The analysis revealed not only the changes in the size, but also the relative volume fractions of these two CeO2 particle populations as a function of the aforementioned parameters. Surprisingly, the 5 - 15 nm agglomerate particles already form at room temperature, and increasing the temperature raises their number on the one hand by an enhanced nucleation rate of the primary particles. On the other hand, especially at high temperatures (90, 100 °C) the larger agglomerate particles precipitate, resulting in interesting trends in the fractions of the two populations as a function of time, temperature, ionic strength and precursor concentration. The experimental studies are complemented by calculating colloidal interaction energies based on classical DLVO theory. Thereby, this study provides detailed insight into the nucleation, growth and agglomeration of CeO2 nanoparticles. The primary objective of this study is to provide a better understanding of the nucleation and growth of particles by the hydrolysis of the tetravalent cerium ion in aqueous solutions.