Controlled Synthesis And Luminescence Properties Of NaGd?WO₄?₂:Ln³⁺?Ln=Tb,Dy,Sm,Eu?Polychromatic Phosphors

Complex tungstate AB?WO₄?₂(A=Li⁺,Na⁺,K⁺;B=Gd³⁺,La³⁺,Y³⁺)matrix luminescent materials have become a research hotspot in the field of luminescent materials due to their unique structure,good physical and chemical stability,and excellent luminescent properties.Morphology and size are important factors that determine the physical and chemical properties of materials.The synthesis of inorganic functional materials with specific morphology,size and structure is an important direction of material science research at present.In this paper,NaGd?WO₄?₂:Tb³⁺green-emitting phosphors with different morphologies such as square plate-like structures and shuttle-like tetrahedrons that can be effectively excited by n-UV light were synthesized by a facile hydrothermal method without any surfactant or templates.The novel box-like NaGd?WO₄?₂:Tb³⁺green-emitting phosphors were synthesized by adding EDTA to control the morphology.In order to reduce the reaction time,NaGd?WO₄?₂:Ln³⁺?Ln=Tb,Dy,Sm,Eu?multicolor phosphors were synthesized by combining hydrothermal method and microwave method without adding any surfactant or template for 60 min.The samples were characterized by X-ray powder diffraction?XRD?,scanning electron microscope?SEM?,energy-dispersive X-ray spectrum?EDS?,fourier transform infrared?FTIR?spectrum,photoluminescence excitation and emission spectra.The effects of different synthesis condition,the doping concentration of rare earth ion and the amount of surfactants on the phase structure,morphology and luminescent properties of the samples were investigated.The main results are as follows:?1?NaGd?WO₄?₂:Tb³⁺green-emitting phosphors with various morphologies such as shuttle-like tetrahedrons and square plate-like structures were prepared by a facile hydrothermal method without any surfactant or templates.The effects of pH value,reaction time,the doping concentration of Tb³⁺,raw material ratio and solvent ratio on the phase structure,morphology and luminescent properties of the samples were investigated.The results indicate that well-defined uniform square plate-like NaGd?WO₄?₂:Tb³⁺microcrystals can be synthesized when the raw materials ratio?n?Na₂WO₄?:n?Ln?NO₃?₃??is 3:1,the pH of the precursor solution is adjusted to 8.0,and the reaction is conducted at 180°C for 20 h.A probable formation mechanism of the square plate-like morphology has been proposed according to time-dependent experiments.The excitation spectrum of NaGd?WO₄?₂:Tb³⁺phosphor consists of charge transfer bands of W-O and Tb-O and f-f transition of Tb³⁺.The emission spectrum is composed of a series of peaks,the strongest emission peak at 545 nm is attributed to ⁵D₄?⁷F₅ transition of Tb³⁺.The luminescence intensity increases gradually with the increase of Tb³⁺doping concentration and reaches a maximum at the doping concentration of 80 mol%.The square plate-like NaGd?WO₄?₂:Tb³⁺phosphors show intensive green emission under n-UV light excitation,indicating that it can be used as green component in n-UV-excited white light emitting diodes.?2?Multi-morphologies NaGd?WO₄?₂:Tb³⁺green-emitting phosphors excited by n-UV excitation were successfully synthesized by a facile EDTA-assisted hydrothermal method.The morphology of the sample was controlled by changing the pH value of the precursor solution and the amount of EDTA.The well-defined uniform box-like NaGd?WO₄?₂:Tb³⁺phosphors can be synthesized at pH 8.0 for 20 h under 0.1 g EDTA.Taking the box-like structure as an example,the growth process of box-like NaGd?WO₄?₂ microcrystal has been investigated and a feasible mechanism is proposed by explaining the chelation roles of EDTA.The dependence of the luminescence intensity on the pH value of the precursor solution,the amount of EDTA and the reaction time were discussed in detail.Under the excitation of n-UV light?379 nm?,the sample with the morphology of box-like structure present the stronger green-emitting than others.?3?A series of NaGd?WO₄?₂:Ln³⁺?Ln=Tb,Dy,Sm,Eu?phosphors were synthesized by microwave-hydrothermal method without adding any surfactant or template.The effects of pH value,reaction temperature and reaction time on the phase structure,morphology and luminescent properties of the samples were investigated.The results indicate that the pure phase of NaGd?WO₄?₂ can be synthesized rapidly by adjusting the pH value of the precursor solution to 8.0 and reacting at 180?for 60 min,as-synthesized sample presents good crystallinity and well-defined square plate-like shape.A probable formation mechanism of the square plate-like morphology has been put forward according to time-dependent experiments.All the excitation spectra of as-synthesized NaGd?WO₄?₂:Ln³⁺phosphors are composed of two parts:the broad band between 200 nm and 300 nm,which is ascribed to the charge transfer band of W-O and Ln-O;a series of sharp lines in the range of 300-500 nm,which is due to the characteristic f-f transitions of Ln³⁺.NaGd?WO₄?₂:Ln³⁺phosphors can be excited effectively by n-UV light and exhibit bright green-,yellow-,orange-red-and red-emitting,respectively for Tb³⁺,Dy³⁺,Sm³⁺and Eu³⁺as luminescent center.