Surface versus volume effects in luminescent ceria nanocrystals synthesized by an oil-in-water microemulsion method

Abstract

Pure and europium (Eu³⁺) doped cerium dioxide (CeO₂) nanocrystals have been synthesized by a novel oil-in-water microemulsion reaction method under soft conditions. In-situX-ray diffraction and RAMAN spectroscopy, high-resolution transmission electron microscopy, UV/Vis diffuse-reflectance and Fourier transform infrared spectroscopy as well as time-resolved photoluminescence spectroscopy were used to characterize the nanaocrystals. The as-synthesized powders are nanocrystalline and have a narrow size distribution centered on 3 nm and high surface area of ∼250 m² g⁻¹. Only a small fraction of the europium ions substitutes for the bulk, cubic Ce⁴⁺ sites in the europium-doped ceria nanocrystals. Upon calcination up to 1000 °C, a remarkable high surface area of ∼120 m² g⁻¹ is preserved whereas an enrichment of the surface Ce⁴⁺ relative to Ce³⁺ ions and relative strong europium emission with a lifetime of ∼1.8 ms and FWHM as narrow as 10 cm⁻¹ are measured. Under excitation into the UV and visible spectral range, the europium doped ceria nanocrystals display a variable emission spanning the orange-red wavelengths. The tunable emission is explained by the heterogeneous distribution of the europium dopants within the ceria nanocrystals coupled with the progressive diffusion of the europium ions from the surface to the inner ceria sites and the selective participation of the ceria host to the emission sensitization. Effects of the bulk-doping and impregnation with europium on the ceria host structure and optical properties are also discussed.