Fabrication And Luminescent Properties Of Sm³⁺,Dy³⁺ Doped NaRE?MoO₄?₂?RE=La,Gd? Phosphors With Controlled Morphologies

Owing to the excellent physical and chemical stability,non-toxicity,environmental protection and low lattice phonon energy which would increase a higher quantum efficiency,the rare-earth doped molybdate phosphor has been an important family of luminescent materials.Recently,the synthesis of molybdate luminescent materials with regular morphology,uniform size and excellent performance has become a research hotspot.The morphology,dimension,and size of materials are affected by many factors,such as synthesis method,the pH values of initial solution,reaction time,reaction temperature,template and addition and so on.Therefore,reasonably controlling various conditions is primary in the synthesis of good luminescent materials.In this paper,a series of molybdate phosphors with different morphologies have been successfully synthesized through a mild hydrothermal method without any templates and surfactant,and the formation mechanism of different morphologies has been studied.Moreover,the influences of different reaction conditions on the morphology and luminescent property of synthesized products have been investigated in detail.The contents in this paper are as follows:?1?Monodisperse ellipsoid-like Na La?MoO₄?₂:Sm³⁺ micro-particles were prepared by a hydrothermal method at 180 oC for 12 h without using any templates or organic reagents.A possible morphological formation mechanism of the ellipsoids has been presented according to time-dependent experiments.The results show that the starting pH values are responsible for the purity,phase structure and morphology.Furthermore,it is found that the luminescent intensity is influenced by the starting pH values,the hydrothermal reaction time and the concentration of Sm³⁺.When the concentration of Sm³⁺ is 3%,the emission intensity reaches a maximum.Further increasing Sm³⁺ doping concentration,the intensity of emission peaks decreases,which is attributed to the concentration quenching.The mechanism of concentration quenching of Sm³⁺ has been determined to the electric dipole-dipole interaction and cross-relxation processes ⁴G_5/2 + ⁶H_5/2 ? ⁶_H9/2 + ⁶_H9/2 between Sm³⁺ ions on the basis of the relevant theory and experiment data.Because of a strong red emission under the excitation of near-ultraviolet?NUV?,ellipsoid-like NaLa?MoO₄?₂:Sm³⁺ phosphors may have potential application in the fields of field emission displays,solid state lighting and optoelectronic devices.?2?NaGd?MoO₄?₂:Sm³⁺ microcrystals with controlled morphologies were prepared via a hydrothermal method without any additives.The realization of controlled morphologies is by changing the initial pH values.When pH values increased from 3.5 to 7.0,the morphologies of the samples evolved from truncated octahedrons,through tetragonal plates,square sandwiches without filling and spindle,to irregular slice.According to the time-dependent experiments,a possible formation mechanism of the tetragonal plate is proposed.Moreover,the luminescent properties of NaGd?MoO₄?₂:Sm³⁺ phosphors with different morphologies and Sm³⁺ concentrations are investigated.The sample with the morphology of tetragonal plates obtained under the pH value of 4.5 present the stronger red-emitting than others.?3?Well-defined Sm³⁺/Dy³⁺ ions doped NaGd?Mo O4?2 octahedral microcrystals were prepared through a hydrothermal route.The phase structure,morphology,size and luminescent property of as-synthesized products were systematically characterized by means of powder X-ray diffraction?XRD?,scanning electron microscopy?SEM?and fluorescence spectrophotometer.When pH=4.5,and reaction time is 12 h,the uniformed,perfect,monodisperse NaGd?Mo O4?2: Sm³⁺/Dy³⁺ octahedral microcrystals can be obtained.At the same time,the possible formation mechanism of octahedral has been presented by the time-dependent experiments.Furthermore,on the basis of the emission spectra of Dy³⁺,the influence of Dy³⁺ doped concentration on luminescent intensity was discussed,and the mechanism of concentration quenching of Dy³⁺ was obtained,which is the electric dipole-dipole interaction.From the emission spectra of Sm³⁺,Dy³⁺ co-doped NaGd?MoO₄?₂ phosphors,the characteristic emission peaks of Sm³⁺and Dy³⁺ can be observed at the same time.When the concentration of Sm³⁺ increased,the emission peak intensity of Dy³⁺ ions decreased gradually,and the emission peak intensity of Sm³⁺ ions increased gradually.Furthermore,according to the luminescence decay curves of Dy³⁺,Sm³⁺ co-doped phosphors,it was found that the fluorescence lifetime of Dy³⁺ ions decreased with the increase of concentration of Sm³⁺,therefore,it can be confirmed that the transmission of energy in Dy³⁺,Sm³⁺ co-doped phosphors is from Dy³⁺ to Sm³⁺.?4?The submicron spherical NaLa?MoO₄?₂:Sm³⁺ red-emitting phosphors were prepared by a microwave-assisted sol-gel method.The results show that all the samples obtained above 700 ? are pure NaLa?MoO₄?₂ phase.The submicron spherical particles of the sample calcinated at 800 ? are uniform and the size is about 700⁸00 nm.The main excitation peaks of as-synthesized NaLa?MoO₄?₂ products are located at 307 nm,364 nm,377 nm,405 nm,469 nm,and the strongest one is at 405 nm.The emission spectrum is mainly composed of three emission peaks centered at 571 nm,607 nm and 647 nm,corresponding to the ⁴G_5/2?⁶H_5/2?⁴G_5/2?⁶H_7/2?⁴G_5/2?⁶_H9/2 transitions of Sm³⁺,respectively.Among them,the strongest peak is at 647 nm,which indicates that the sample can emit red light under the excitation of the ultraviolet light,near ultraviolet light or blue light.Moreover,it is found that the intensity of NaLa1-x?MoO₄?₂:xSm³⁺ show the maximum when the calcination is 800? and Sm³⁺ doping concentration x=0.04.