Effect of hydrothermal ripening on the photoluminescence properties of pure and doped cerium oxide nanoparticles

We synthesized and characterized a set of undoped and doped cerium oxide nanoparticles with the following chemical formula: Ce_1-xM_xO_2-(x/2) (M: Y or Gd and x = 0 or 0.15) by different coprecipitation protocols at room temperature. Room temperature photoluminescence spectra were similar whatever the protocols and doping. After hydrothermal ripening at neutral pH, a moderate but significant particle growth was observed for doped particles whereas undoped ones remained the same. Simultaneously, the photoluminescence band at 400 nm was significantly red-shifted and practically vanished for doped particles whereas it was only slightly weaker and shifted for pure ceria. All other bands in the same region were much less affected: the bands at 363 and 378 nm became more intense, the shoulder at 420 nm remained roughly unchanged. These phenomena were observed systematically whatever the coprecipitation protocol. As a consequence, the band at 400 nm can be specifically attributed to surface defects that disappear during growth under hydrothermal conditions.