Photoinduced electron transfer in Eu2+ and Sm3+ co-doped CaF2 nanocrystals prepared by co-precipitation

We report on the photoinduced electron transfer in Eu2+ and Sm3+ co-doped CaF2 nanocrystals prepared by a facile co-precipitation method using CaCl2, EuCl2, SmCl3 and NH4F. The doping of divalent Eu2+ was realized by employing granular zinc in the CaCl2/EuCl2 solution under nitrogen to reduce Eu3+ → Eu2+. The CaF2:xEu,0.2%Sm nanocrystals were prepared with different Eu concentrations of x = 0, 0.2, 0.5, 1.5, and 2.5%, resulting in an average crystallite size between 54 and 36 ± 1 nm. The maximum luminescence intensities of Eu2+ and Sm2+ were obtained for the x = 0.5% sample. Photoinduced electron transfer and back transfer in the CaF2:0.5%Eu,0.2%Sm nanocrystals were measured by detecting the Eu2+ and Sm2+ luminescence, upon exposure to 340 and 310 nm UV LED light. Interestingly, the photoionization of Eu2+ occurred 6 times faster for the sample co-doped with 0.2% Sm. Upon exposure to 340 nm light, a reduction and increase of the Eu2+ and the Sm2+ luminescence were observed, respectively. Subsequent exposure to 310 nm light decreased the Sm2+ luminescence, but the luminescence of Eu2+ kept on being bleached, albeit at a lower rate. Whilst the Sm valence state could be reversibly changed between the trivalent and divalent state by 340 and 310 nm light, the observation that the Eu2+ luminescence did not recover may indicate that a third centre is involved in the photoinduced electron transfer mechanisms.