Nanocrystalline ceria particles, with dimensions as small as 2 nm, have been prepared at room temperature by a soft chemical route using aqueous cerium nitrate and hexamethylenetetramine (HMT) as reactants. HMT decomposes slowly in aqueous solution, allowing the hydrolysis of Ce3+ ions to proceed in a homogeneous and controlled manner. Presently we report on an in situ study of homogeneous nano-ceria precipitation and growth using synchrotron-based ultra-small-angle x-ray scattering (USAXS), in combination with a new isothermal capillary flow-cell developed at NIST for applications to solution-mediated nanomaterials. Measurements were performed over a temperature range from 20 ºC to 35 ºC for post-mixing times of up to 12 hr. Previous TEM studies (Zhang, et al., J. App. Phys. 95 4319-4326, 2004) have shown that particles are single crystals with narrow size distributions. The growth process was believed to occur in two steps: an initial nucleation event followed by a slow and continuous growth at the particle surface without further nucleation. USAXS results have suggested a more complex growth mechanism, with two feature size populations that persist throughout the post-nucleation phase. One population remains constant with respect to size, while the other continues to coarsen over time. We will present USAXS data and modeling results that yield particle size distributions, mean and median sizes for each population, population surface areas and number distributions, as a function of reaction time and temperature.

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