Ultra-Large Unilamellar Nanosheets Efficiently Exfoliated From Y/Gd/Dy Layered Rare-Earth Hydroxides And Their Self-Assembly Into Highly Fluorescent Films

In this dissertation,ultra-large of layered rare-earth hydroxides for（LRHs）,with a series of ultra-large layered rare-earth hydroxides（LRHs）of(Y_0.975-xGd_xDy_0.025)₂（OH）₅N_O3·nH₂O have been successfully synthesized,oxide phosphors of excellent photoluminescence through proper annealing.Moreover,the hydrothermal ion exchange between the interlayer nitrate and the oleate anion,highly exfoliation of nanosheets and their self-assembly into highly fluorescent films have been syetematiclly studied.Detailed characterizations of the products were performed by the combined techniques of XRD,FT-IR,TG/DTA,FE-SEM,TEM,SAED,AFM and PLE/PL analysis.Ternary LRHs solid solutions of(Y_0.975-xGd_xDy_0.025)₂（OH）₅NO₃·nH₂O（x=0-0.975）have been successfully synthesized via hydrothermal reaction of the rare-earth nitrate/NH₄OH system in the presence of ammonium nitrate（NH₄NO₃）at 180 ℃ for 24 h.The results indicated that hexagonal shaped LRHs crystals are well crystallized with smooth surfaces.Smaller and thinner LRHs particles,and expanded lattice parameters a and b were found at a higher Gd³⁺ content,while the interlayer distance（c/2）,closely related to the hydration number n,is inversely proportional to the Gd³⁺ content.At the optimal Gd³⁺content of 2.5.at%,the fluorescence emission intensity are strongest.Increasing Gd³⁺ content leads the emission color of(Y_0.975-xGd_xDy_0.025)₂（OH）₅NO₃·nH₂O shifting from green to yellow.Calcining LRHs at 800 ℃ directly yields cubic(Y_0.975-xGd_xDy_0.025)₂O₃ phosphors,which inherited the morphologies of their precursors.Upon UV excitation at 270 nm(the ⁸S_7/2→⁶I_J transition of Gd³⁺),the oxide phosphors exhibit the characteristic yellow emission of Dy³⁺ at 576 nm(the ⁴F_9/2→⁶H_13/2 transition),suggesting the occurrence of energy transfer from Gd³⁺ to the activator,and enhanced luminescence was found for a higher Gd³⁺content.The nitrate ions,existing in the interlayer gallery of Y/Gd/Dy complex layered rare earth hydroxide,exhibit facile exchanges with oleate anions.Furthermore,the interlayer distance can thus be expanded from～0.89 nm for the pristine LRHs to～3.14 nm for the intercalated ones.A transparent colloidal solution with clear Tynall effects was obtained by dispersing the exchanged compound of(Y_0.95Gd_0.025DY_0.025)₂（OH）₅(C₁₇H₃₃COO)(C₁₇H₃₃COOH)_m·nH₂O in toluene under magnetic stirring for 24 h.Nanosheet films with a high[001]orientationwas self-assembled on a quartz substrate by a spin-coating process.Oxide films with a high[111]orientation will be constructed through calcining the LRHs films under proper annealing（800 ℃,2 h）.The strongest fluorescence emissions were found for the x=0.3 sample,and the CIE chromaticity coordinate and fluorescence lifetimes are independent on the Gd³⁺ content.