Experiments reveal that the lattice constants of nanocrystallites with the number of CeO2-x formula units N more than 68 expand relative to the bulk CeO2 but are very different depending on the fabrication method. Densities of states are calculated for bulk CeO2 and CeO 1.5, and for (CeO2)13 and (CeO 1.5)13 clusters. Band gap energies of the bulk CeO 2 and CeO1.5 are estimated to be 3.20 and 0.30 eV, respectively. HOMO-LUMO gap energies of the clusters (CeO2) 13 and (CeO1.5)13 are calculated to be almost 0 and 3.05 eV, respectively. Experimental results of the ultraviolet absorption of nanocrystallites with the number of CeO2 and CeO1.5 formula units more than 68 show the blueshifts from the bulk CeO2 and CeO1.5, respectively, due to the quantum confinement effect.
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