Oxidation state and lattice expansion of CeO 2-x nanoparticles as a function of particle size

Cerium oxide nanoparticles ${\mathrm{CeO}}_{2\ensuremath{-}x}$ (\ensuremath{\sim}3\char21{}20 nm in diameter) made by a vapor phase condensation method, have been studied by several methods of transmission electron microscopy (TEM): electron energy loss spectroscopy (EELS), high resolution imaging, and electron diffraction. The white-line ratios ${M}_{5}{/M}_{4}$ of the EELS spectra were used to determine the relative amounts of cerium ions ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Ce}}^{4+}$ as a function of particle size. The fraction of ${\mathrm{Ce}}^{3+}$ ions in the particles rapidly increased with decreasing particle size below \ensuremath{\sim}15 nm in diameter. The particles were completely reduced to ${\mathrm{CeO}}_{1.5}$ at the diameter of \ensuremath{\sim}3 nm. This reduced cerium oxide has a fluorite structure which is the same as that of bulk ${\mathrm{CeO}}_{2}.$ Also, EELS spectra taken from the edge and center of the particle indicated that for larger particles the valence reduction of cerium ions occurs mainly at the surface, forming a ${\mathrm{CeO}}_{1.5}$ layer and leaving the core as essentially ${\mathrm{CeO}}_{2}.$ A micromechanical model based on linear elasticity was used to explain the lattice expansion of the ${\mathrm{CeO}}_{2\ensuremath{-}x}$ nanoparticles. Comparing our results with previously published works indicates that the amount of ${\mathrm{CeO}}_{1.5}$ in ${\mathrm{CeO}}_{2\ensuremath{-}x}$ nanoparticles is a strong function of the particular synthesis methods used to make these particles.