Photoluminescence properties of CeO2:Eu3+ nanoparticles synthesized by a sol-gel method

Nanocrystalline Eu ³⁺-doped CeO₂, CeO₂,Sm ³⁺-doped CeO₂, and Li ⁺,Eu ³⁺- codoped CeO₂ 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 CeO₂. 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.