Photoluminescence of (La,Eu) 2 O 2 SO 4 red-emitting phosphors derived from layered hydroxide
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Qi Zhu and Xudong Sun Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, China (Received 6 February 2016; accepted 21 April 2016)
Red-emitting (La,Eu)2O2SO4 phosphors have been successfully prepared using the layered hydroxide of (La,Eu)2(OH)4SO42H2O as the precursor. The precursor compound was firstly crystallized via hydrothermal reaction (100 °C and pH 5 9.0) as well-dispersed nanoplates, followed by dehydration and dehydroxylation in the 400–1200 °C temperature range in ambient air to yield (La,Eu)2O2SO4. The phosphors show intense red emissions originated from the f–f transitions of Eu31, dominantly peaking at 617 nm, under O–Eu charge transfer excitation at 284 nm. The optimal Eu31 content was experimentally determined to be 5 at.%, agreeing well with theoretical analysis, and the concentration quenching of luminescence was suggested to be due to exchange interactions. Fluorescence decay analysis indicates that a higher calcination temperature or Eu31 content would decrease the lifetime of the 617 nm emission.
I. INTRODUCTION
Rare-earth oxysulfate (Ln2O2SO4) was widely explored as oxygen storage materials,1,2 catalysts for water–gas shift (WGS) and reverse WGS reactions,3,4 polycrystalline solid electrolytes,5 efficient host lattice for down-conversion,6,7 and up-conversion photoluminescence (PL).8 As host lattice the PL behaviors of rare-earth activators in Y2O2SO49,10 and Gd2O2SO46,7 have been intensely studied across the whole oxysulfate family; however, such study in the La2O2SO4 host is yet rather insufficient. La31 has the smallest electronegativity (;1.11)11 among all the rare-earth elements, which will probably make the charge transfer (CT) from the 2p orbital of O2 to the 4f orbital of Eu31 much easier, and thus enhance the intensities of the CT/PL bands. Moreover, comparatively, La2O3 is cheaper in price and more abundant. Therefore, it would be of scientific interest to prepare the Eu31 doped La2O2SO4 phosphor and investigate the PL properties of the phosphors. Until now, some techniques have been established to synthesize oxysulfates, including the conventional
Contributing Editor: Winston V. Schoenfeld a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.185
solid-state reaction,12 direct heating of commercial Ln2(SO4)3nH2O salts,1 and oxidation of Ln2S3 under controlled conditions.5 These methods, however, yet have difficulty in controlling the morphology of the final products, and frequently, micron-sized particles are obtained. Though the biomolecule-assisted hydrothermal synthesis8 and electrospinning9 are recently developed intending to achieve morphology control, either template, or environmentally unfriendly sulfur dioxide need to be used. In this regard, a facile and clean route was used in this work to synthesize Ln2O2SO4 with the sulfate-type layered rare-earth hydroxide (LRH) of Ln2(OH)4SO42H2O as precursor, taking
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