Temperature induced conversion from surface to “bulk” sites in Eu3+-impregnated CeO2 nanocrystals

Abstract

Evolution with calcination temperature of Eu3+ sites in CeO2 nanocrystals is investigated by time-resolved photoluminescence spectroscopy. In the as-synthesized Eu3+ impregnated CeO2, most of Eu3+ ions reside on surface (S) sites. The Eu3+emission in S sites is broad and short-lived (τ = 240 μs) being dominated by the electric dipole (ED) 5D0-7F2 emission with little evidence for clustering. After calcination (between 500 and 1300 °C), Eu3+ is distributed on surface, cubic and up to three additional crystalline sites. Surface type emission could be detected until 1100 °C. In cubic sites, Eu3 substitute for the lattice Ce4+ with Oh symmetry (O sites). The emission of Eu3+ in O sites is characterized by relative long-lived (τ = 1.8–2 ms) and ultra-narrow (FWHM = 7 cm−1) magnetic dipole (MD) 5D0-7F1 emission centered at ∼591 nm. Three more crystalline sites are attributed to the oxygen vacancy charge-compensated defects: trigonal with C3v symmetry (C1 sites) and C2 and C3 sites with C2v or lower symmetry. Eu3+ in C1 sites exhibits predominant ED 5D0-7F2 emission centered at ∼610 and 632 nm with lifetime of 0.85–1 ms. The C1 sites are assigned to Eu3+-(oxygen vacancy)-associated cubic sites. The O2− to Ce4+ electronic charge-transfer band sensitizes preferentially the Eu3+ emission in O and, to a less extent, C1 sites but not the S sites. Overall, the results show that the oxygen vacancies are distributed around both Eu3+ and Ce4+ and the Eu3+-oxygen vacancy interaction mode as nearest-neighbour or next-nearest-neighbour depends on the calcination temperature.