Size-induced lattice relaxation was observed for nanoscale CeO2 single crystals with an average size from 4 to 60 nm. Results showed the finest crystallites exhibited no strain-induced line broadening, while high temperature annealing resulted in larger grain sizes and significant strains. The observed shift in the x-ray diffraction lattice parameters was assumed to be due to the formation of defects on the lattice, specifically oxygen vacancies. Modeling revealed that the oxygen vacancy concentration ([VO••]) was found to be ≈4×1020/cm3 for the 4 nm crystallites, and decreased two orders of magnitude for larger 60 nm single crystals.
Nanometer (about 4∼5 nm) CeO2 single crystals were first synthesized by room-temperature homogeneous nucleation; the size was determined by electron microscopy and specific surfaced area of the particles. Modeling revealed that the surface energy of as-synthesized nanometer single crystals was in the range of 2.8–3.7 J/m2. Crystal growth mechanisms change over the temperature regimes, from boundary diffusion over low-temperature regime (Ea=0.16 eV) to bulk diffusion (Ea=0.50 eV) over high-temperature region.
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