Toward a Unified Description of Luminescence-Local Structure Correlation in Ln Doped CeO2 Nanoparticles: Roles of Ln Ionic Radius, Ln Concentration, and Oxygen Vacancies

We propose a physical model for luminescence properties of trivalent lanthanide (Ln) doped into CeO2 by use of low temperature, site selective, time-gated luminescence spectroscopy seconded by X-ray diffraction, Raman, and Fourier transform infrared spectroscopy and transmission electron microscopy. The main findings can be summarized as follows: (i) Ln situated to both the left and right sides to Gd in the Ln series exhibit a two-center distribution. Both Ln centers substitute for the tetravalent Ce fluorite sites being differentiated by the local symmetry: cubic, as a result of zero vacancy in the nearest-neighbor oxygen shell (cubic Ln center), and low symmetry, likely due to one vacancy in the nearest-neighbor oxygen shell (Ln-defect associate center). (ii) A first example of Dy emission in an inversion (cubic) symmetry, characterized by relatively strong lines at 679 and 764 nm, is reported. This result is expected to challenge the way this lanthanide is currently used as a luminescence probe. (iii) The relative contribution of the Ln centers to the overall emission depends on the Ln ionic radius: Sm exists predominantly as a cubic center, while Er is found mostly as a vacancy associate. (iv) Er, La codoped CeO2 can be used as an effective model system to separate the effects of Ln concentration and subsequently induced oxygen vacancies on the efficiency of CeO2 sensitization of Ln emission. (v) Zr co-doping of CeO2 obstructs the formation of Ln-defect associates. The implications of our findings for the interpretation of data already present in the literature are also discussed.