Sacrificial Conversion Of Layered Rare-Earth Hydroxide?LRH? Nanosheets Into(Y_1-xEu_x)PO₄ Nanophosphors And Their Properties

Ultra-thin nanosheets(2-6 nm)of the(Y1-xEux)2(OH)5NO3 ·nH2O layered rare-earth hydroxide(LRH)were employed for the first time as a sacrificial precursor to synthesize(Y1-xEux)PO4 orthophosphate red phosphors via nano-conversion.Phosphate ions from both diammonium hydrogen phosphate((NH4)2HPO4)and ammonium dihydrogen phosphate(NH4H2PO4)were applied to take out the hydroxyls and NO3-in the LRH,and the process of anion exchange is discussed.Calcinations and hydrothermal reaction were carried out to yield(Y1-xEux)PO4 phosphors.Detailed characterizations were given to the materials by the combined techniques of XRD,FTIR,TG/DSC,FE-SEM,HR-TEM,and photoluminescence spectroscopies.The results can be concluded as follows:The anion-exchange process successfully conducted via employing(NN4)2HPO4 as the phosphorous source.The formation of sacrificial conversion products(YPO4 ·nH2O)can be considered as a "dissolution-reprecipitation" mechanism.Calcining the products yielded(Y1-xEux)PO4 phosphors with crystallite sizes of 10?30 nm.The phosphors were found to exhibit stronger 5D0?7F2(621 nm)than 5D0?7F1(595 nm)emissions under the excitation at 236 nm(charge transfer excitation),owing to the nano-size effect.The optimal Eu3+ content was determined to be?10 at.%(x=0.10),and concentration quenching of luminescence was resulted from exchange interactions.Altering the Eu3+ content didn't significantly change the CIE chromaticity coordinates,and the longest fluorescent lifetime was found at the Eu3+content of 5 at.%.A higher annealing temperature would make the CIE chromaticity coordinates shift to red area and prolong the lifetime of(Y0.9Eu0.1)PO4 nanophosphors.Similarly,the sacrificial conversion products(YPO4·nH2O)have been obtained via employing NH4H2PO4 as the phosphorous source.Under hydrothermal conditions,the transformation mechanism of the YPO4 ·nH2O to(Y0.95Eu0.05)PO4 was defined as the Ostwald ripening process.The crystallinity of(Y0.95Eu0.05)PO4 phosphors were improved and the grains were elongated by adjusting the molar ratio of PO43-/Y3+ from 1:1 to 1.5:1.Calcining the hydrothermal products greatly enhanced the fluorescent intensity and prolonged the lifetime.Calcining the hydrothermal products at 900? with the PO43-/Y3+molar ratio of 1.5:1 yields the phosphor with strongest fluorescent intensity.Elevating the annealing temperature didn't significantly change the fluorescence lifetime but made the CIE chromaticity coordinates shift to the red area.