Effects of Ce~(3+) Codoping and Annealing on Phase Transformation and Luminescence of Eu~(3+)-Doped YPO_4 Nanorods: D_2O Solvent Effect

摘要

Ce~(3+)- and Eu~(3+)-doped YPO_4 nanorods have been prepared at relatively low temperature (120℃). A detailed investigation of the role of Ce~(3+) concentration up to 10 atom % on the luminescence intensity of Eu~(3+) in Ce~(3+)- and Eu~(3+)-doped YPO_4 has been carried out. Phase transformation from a tetragonal to a hexagonal structure occurs with increasing Ce~(3+) concentrations, and water molecules are also associated during phase transformation. Thermal study shows that water can be retained up to 800℃ in the hexagonal structure. Interestingly, the hexagonal structure returns to the tetragonal structure on annealing above 900℃. As-prepared and 500℃ heated samples show uniform sized nanorods, whereas a 900℃ heated sample shows distorted nanorods in which pores are present. Initially, the luminescence intensity decreases sharply with increasing Ce~(3+) concentrations, even for 2 atom %. This is related to the enhanced nonradiative rate as compared to the radiative rate, since multiphonon relaxation to surrounding water molecules increases. This is not due to the possible oxidation-reduction process between Eu~(3+) and Ce~(3+) to give Eu~(2+) and Ce~(4+), as confirmed by X-ray photoelectron spectroscopy and luminescence studies. Then, a significant enhancement of luminescence intensity occurs on annealing above 900℃. This can be ascribed to the loss of water molecules during a phase transformation from the hydrated hexagonal to the dehydrated tetragonal phase. To the authors' knowledge, we for the first time performed a luminescent study with a change of solvent from H_2O to D_2O, and significant enhancement in luminescence is found.