title: 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.
abstract: This study presents a facile and green route for the synthesis of (La0.95Eu0.05)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 Ln2O2SO4intermediate upon annealing in flowing H2at 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 O2-→ Eu3+at ∼ 270 nm and S2-→ Eu3+at ∼ 340 nm for the Ln2O2S phosphors, with the latter being stronger and both significantly stronger than the intrinsic intra-ftransitions 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→7F2transition of Eu3+) under S2-→ 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.
authors.affiliation: Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials and Metallurgy, Northeastern University, Shenyang, Liaoning, 110819, People's Republic of China.
authors.forename: Xuejiao
authors.initials: X
authors.lastname: Wang
authors.affiliation: Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials and Metallurgy, Northeastern University, Shenyang, Liaoning, 110819, People's Republic of China; Advanced Materials Processing Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan.
authors.forename: Ji-Guang
authors.initials: JG
authors.lastname: Li
authors.affiliation: Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials and Metallurgy, Northeastern University, Shenyang, Liaoning, 110819, People's Republic of China.
authors.forename: Qi
authors.initials: Q
authors.lastname: Zhu
authors.affiliation: Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials and Metallurgy, Northeastern University, Shenyang, Liaoning, 110819, People's Republic of China.
authors.forename: Xiaodong
authors.initials: X
authors.lastname: Li
authors.affiliation: Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials and Metallurgy, Northeastern University, Shenyang, Liaoning, 110819, People's Republic of China.
authors.forename: Xudong
authors.initials: X
authors.lastname: Sun
authors.affiliation: Advanced Materials Processing Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan.
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Sakka
keywords: (La; Eu)2O2S red phosphors; Hydrothermal synthesis; Photoluminescence properties; Sulfate-type layered rare-earth hydroxides (LRHs)