Pure-phase La_2(WO_4)_3:Eu~(3+) nanocrystals and spindle-like NaLa(WO_4)_2:Yb ~(3+)/Er~(3+) nano/microcrystals: Selective synthesis, morphologies and photoluminescent properties

摘要

The selective synthesis of pure-phase La_2(WO_4) _3 nanocrystals and uniform spindle-like NaLa(WO_4) _2 nano/microcrystals with tunable size based on one reaction system has been reported for the first time. The sodium ion is crucial for the selective synthesis of La_2(WO_4)_3 and NaLa(WO_4)_2. An additional hydrothermal treatment has a great effect on the morphology of La_2(WO_4)_3, while the hydrothermal temperature and time, and the amount of glycerine, significantly influence the morphology and size of NaLa(WO_4) _2. Based on the time-dependent experiments, it is proposed that the Ostwald ripening is the formation mechanism of the spindle-like NaLa(WO _4)_2. The resultant La_2(WO_4) _3:Er~(3+) nanocrystals can emit a bright red color with a high purity under the excitation of 467 nm (blue region) and, more importantly, it can be easily dispersed in distilled water. The up-conversion emission intensity of NaLa(WO_4)_2:Yb3+/Er~(3+) is increased rapidly with the Yb3+ concentration under 980 nm laser excitation, suggesting the efficient energy transfer from Yb3+ to Er~(3+). Moreover, the emission color can be tuned from chartreuse to green by increasing the Yb~(3+) concentration. These unique properties of La_2(WO_4)_3 and NaLa(WO_4) _2 are closely related with their distinctive crystal structures, and it is anticipated that the findings in this work may give an insight into the fabrication and application of the rare earth tungstates.