The synthesis and mechanism exploration of europium-doped LiYF4 micro-octahedron phosphors with multilevel interiors
Multi-layered hollow LiYF4:Eu3+ micro-octahedrons, with about 400 nm of single-layer thickness and 300 nm of interlayer space, have been synthesized via a facile hydrothermal route in the presence of surfactant ethylenediamine tetraacetic acid (EDTA). The mechanisms of the morphology evolution of the LiYF4:Eu micro-octahedrons are investigated in detail. Time-dependent experiments indicate that the growth of the micro-octahedrons undergoes four different stages including the aggregation growth of the primary YF3 particle, the transformation of the substance from the orthorhombic-phase YF3 to the tetragonal-phase LiYF4 by the Kirkendall effect with the inward diffusion of quasi-steady state LiF species, adsorption and in situ crystallization, and local Ostwald ripening. The Ostwald ripening process is terminated by the organic adsorption of interlaminar leading to a hollow structure with multilevel interiors. The LiYF4:Eu micro-octahedrons are annealed under the designed temperatures, which leads to the collapse of octahedral structures indicating the role of EDTA on building the octahedron. The spectral measurements show that the calcination approach has a stronger effect on the luminescence tuning of the LiYF4:Eu micro-octahedrons due to the modification of the crystal phase, structure and size. The present study is of great importance in the preparation of rare-earth ion doped LiYF4 hollow materials as well as in applications as building blocks for functional devices.