Facile and green synthesis of (La0.95Eu0.05)2O2S red phosphors with sulfate-ion pillared layered hydroxides as a new type of precursor: controlled hydrothermal processing, phase evolution and photoluminescence

摘要

This study presents a facile and green route for the synthesis of (La_0.95Eu_0.05)₂O₂S red phosphors of controllable morphologies, with the sulfate-type layered hydroxides of Ln₂(OH)₄SO₄ 2H₂O (Ln?=?La and Eu) as a new type of precursor. The technique takes advantage of the fact that the precursor has had the exact Ln:S molar ratio of the targeted phosphor, thus saving the hazardous sulfurization reagents indispensable to traditional synthesis. Controlled hydrothermal processing at 120?°C yielded phase-pure Ln₂(OH)₄SO₄ 2H₂O crystallites in the form of either nanoplates or microprisms, which can both be converted into Ln₂O₂S phosphor via a Ln₂O₂SO₄ intermediate upon annealing in flowing H₂ at a minimum temperature of ~?700?°C. The nanoplates collapse into relatively rounded Ln₂O₂S particles while the microprisms retain well their initial morphologies at 1 200?°C, thus yielding two types of red phosphors. Photoluminescence excitation (PLE) studies found two distinct charge transfer (CT) excitation bands of O^2??→?Eu³⁺ at ~?270?nm and S^2??→?Eu³⁺ at ~?340?nm for the Ln₂O₂S phosphors, with the latter being stronger and both significantly stronger than the intrinsic intra-f transitions of Eu³⁺. The two types of phosphors share high similarities in the positions of PLE/PL (photoluminescence) bands and both show the strongest red emission at 627?nm (⁵D₀?→?⁷F₂ transition of Eu³⁺) under S^2??→?Eu³⁺ CT excitation at 340?nm. The PLE/PL intensities show clear dependence on particle morphology and calcination temperature, which were investigated in detail. Fluorescence decay analysis reveals that the 627?nm red emission has a lifetime of ~?0.5?ms for both types of the phosphors.