Photoluminescence Properties of CeO 2 :Eu 3+ Nanoparticles Synthesized by a Sol-Gel Method

Nanocrystalline Eu 3+ -doped CeO 2, CeO 2, Sm 3+ -doped CeO 2, and Li +, Eu 3+ -codoped CeO 2 samples were prepared through a sol−gel process. The structure and the optical properties of the samples were characterized by X-ray diffraction, diffuse reflection spectra, and photoluminescence spectra. No luminescence was observed for nanocrystalline CeO 2 . The systematic investigation shows that the broad band in the excitation spectrum of CeO 2:Eu 3+ comes from the charge transfer (CT) transition from O 2− to Ce 4+, not from the oxygen vacancy, or from the CT of O 2− to Eu 3+ . Upon increasing the fired temperature from 600 to 800 °C, the excitation spectrum shifts to lower energy. With increasing concentrations of Eu 3+ up to 1% in CeO 2, red shifts of the excitation spectra are observed; however, when the concentration of Eu 3+ increases to 5% and 10%, blue shifts occur. The emission spectrum shows that the symmetry of the Eu 3+ site becomes lower with increasing Eu 3+ . Based on the dielectric theory of complex crystals, the environmental factor ( h e ) and the dielectric definition of average energy gap around the centers of Eu 3+ are calculated. The reasons for the shifts of the excitation spectra are discussed in detail.