Sacrificial conversion of layered rare-earth hydroxide (LRH) nanosheets into (Y_1−xEu_x)PO₄ nanophosphors and investigation of photoluminescence

Abstract

Ultra-thin nanosheets (2-6 nm) of the (Y1-xEux)2(OH)5NO3·nH2O layered rare-earth hydroxide (LRH), directly crystallized without exfoliation were employed for the first time as a sacrificial precursor to synthesize (Y1-xEux)PO4 orthophosphate red phosphors via nano-conversion, and anion exchange of both the hydroxyls and NO3(-) in the LRH with the phosphate ions from diammonium hydrogen phosphate ((NH4)2HPO4) is discussed. Detailed characterization of the materials was performed by the combined techniques of XRD, FTIR, TG/DSC, FE-SEM, HR-TEM, and photoluminescence spectroscopy. The (Y1-xEux)PO4 phosphors, with crystallite sizes of up to ∼20 nm, were found to exhibit stronger (5)D0 → (7)F2 (621 nm) than (5)D0 → (7)F1 (595 nm) emissions under charge transfer excitation (236 nm) owing to the nano-size effect. The optimal Eu(3+) content was determined to be ∼10 at% (x = 0.10), and concentration quenching of luminescence was analyzed to result from exchange interactions. The effects of the Eu(3+) content and annealing temperature on the structural features and luminescence properties of the (Y1-xEux)PO4 nanophosphors were discussed in detail, including emission intensity, the asymmetry factor of luminescence, fluorescence lifetime, and CIE chromaticity coordinates.