简介概要

Rapid synthesis and properties of color-tunable phosphors SrMoO₄:Eu³⁺,Tb³⁺

来源期刊：Rare Metals2017年第10期

论文作者：Ming Zhang Rui-Fang Li Xuan Li Jin-Hang Li

文章页码：828 - 832

摘要：Color-tunable phosphors Sr_0.94MoO₄ :xEu³⁺,（0.06-x）Tb³⁺ were synthesized rapidly by microwave radiation method with active carbon particle as microwave absorbent. The synthesized phosphors were investigated by X-ray powder diffraction（XRD） and fluorescence spectrophotometer. The effects of the ratio of Eu³⁺ and Tb³⁺ on the phase structure and luminescent properties of the phosphors were discussed. The results show that Eu³⁺,Tb³⁺-doped samples can be well indexed to the pure tetragonal scheelitetype SrMoO₄, indicating that Eu³⁺ and Tb³⁺ are effectively doped into the SrMoO₄ host lattices. The as-synthesized SrO 94 MoO₄:xEu³⁺,（0.06-x）Tb³⁺ phosphors have two luminescent centers(Eu³⁺ and Tb³⁺), which can show red and green emissions under ultraviolet light excitation, respectively. Doping concentration of Eu³⁺ and Tb³⁺ has great effect on the intensity of emission peaks and the chromaticity of the samples, and the full color between green and red light can be achieved by adjusting the relative concentration of Eu³⁺and Tb³⁺.

稀有金属(英文版) 2017,36(10),828-832

Rapid synthesis and properties of color-tunable phosphors SrMoO₄:Eu³⁺,Tb³⁺

Yong-Qing Zhai Rui-Fang Li Xuan Li Jin-Hang Li

College of Chemistry and Environmental Science, Hebei University

收稿日期：27 March 2014

基金：financially supported by the National Natural Science Foundation of China (No.21301046);

Rapid synthesis and properties of color-tunable phosphors SrMoO₄:Eu³⁺,Tb³⁺

Yong-Qing Zhai Rui-Fang Li Xuan Li Jin-Hang Li

College of Chemistry and Environmental Science, Hebei University

Abstract：

Color-tunable phosphors Sr_0.94MoO₄ :xEu³⁺,(0.06-x)Tb³⁺ were synthesized rapidly by microwave radiation method with active carbon particle as microwave absorbent. The synthesized phosphors were investigated by X-ray powder diffraction(XRD) and fluorescence spectrophotometer. The effects of the ratio of Eu³⁺ and Tb³⁺ on the phase structure and luminescent properties of the phosphors were discussed. The results show that Eu³⁺,Tb³⁺-doped samples can be well indexed to the pure tetragonal scheelitetype SrMoO₄, indicating that Eu³⁺ and Tb³⁺ are effectively doped into the SrMoO₄ host lattices. The as-synthesized SrO 94 MoO₄:xEu³⁺,(0.06-x)Tb³⁺ phosphors have two luminescent centers(Eu³⁺ and Tb³⁺), which can show red and green emissions under ultraviolet light excitation, respectively. Doping concentration of Eu³⁺ and Tb³⁺ has great effect on the intensity of emission peaks and the chromaticity of the samples, and the full color between green and red light can be achieved by adjusting the relative concentration of Eu³⁺and Tb³⁺.

Keyword：

Microwave radiation method; Color-tunable; SrMoO₄; Eu³⁺,Tb³⁺; Phosphors;

Author： Yong-Qing Zhai,e-mail: zhaiyongqinghbu@163.com;

Received： 27 March 2014

1 Introduction

During the past several decades,much attention has been paid to rare earth (RE) ion-doped crystals,glasses and phosphors for developing novel optoelectronic devices,such as solid-state lasers,safety indicators,displays,optical data storage [ 1, 2, 3, 4, 5] .Among the RE ions,the trivalent europium ion (Eu³⁺) is usually adopted as a red-emitting center due to its intrinsic characteristic that Eu³⁺(4f⁶) can be excited effectively by ultraviolet light or cathode ray and emit pure red light with high luminescent efficiency.Meanwhile,the tri valent terbium ion (Tb³⁺) is usually adopted as a green-emitting center due to its intrinsic characteristic that Tb³⁺(4f⁸) can be excited effectively by ultraviolet light and emit pure green light with high luminescent efficiency.Therefore,the phosphors doped with Eu³⁺or Tb³⁺attract extensive attention.However,there are few reports about phosphors co-doped with Eu³⁺and Tb³⁺.

It is well known that molybdates are important inorganic compounds and perform well in the field of catalysis,lasers,ionic conductors and phosphors.Among molybdate compounds,SrMo0₄ is a kind of representative scheelite compounds,and its central Mo metal ion is coordinated by four O^2-in tetrahedral symmetry,which makes relatively stable as a good potential host material [ 6, 7] .Many methods,such as high-temperature solid-state reaction [ 8] ,solvothermal synthesis [ 9] ,co-precipitation process [ 10] and hydrothermal approach [ 11] ,were developed to synthesize molybdate compounds.However,to the best of our knowledge,there are few reports for the preparation of SrMoO₄-host phosphors by microwave radiation method.

Since SrMoO₄:Eu³⁺can give red emission and SrMoO₄:Tb³⁺shows green emission under ultraviolet light [ 8, 9] ,it is expected to obtain the light between red and green in the SrMoO₄ host by co-doping with Eu³⁺and Tb³⁺and appropriately adjusting their concentrations.Taking this background into account,a series of colortunable phosphors Sr_0.94MoO₄:xEu³⁺,(0.06-x)Tb³⁺were synthesized by microwave radiation method in this work.The structure and luminescence properties of the obtained Sr_0.94MoO₄:xEu³⁺,(0.06-x)Tb³⁺samples were investigated in detail.

2 Experimental

2.1 Microwave synthesis of samples

Tb₄O₇(99.99%),Eu₂O₃(99.99%),MoO₃(99.99%) and SrCO₃(99.00%) were used as starting materials.Sr_0.94MoO₄:xEu³⁺,(0.06-x)Tb³⁺phosphors were synthesized by microwave radiation method successfully.Firstly,according to the stoichiometric ratio of target products,starting materials Tb₄O₇,Eu₂O₃,MoO₃ and SrCO₃ were added into a porcelain mortar and ground for30 min to ensure homogeneity and fine particle size.Then,the mixture was moved into a corundum crucible and then put into a large covered ceramic crucible.The space between the corundum and the ceramic crucible was filled with active carbon as microwave absorbent or heating medium.Subsequently,the crucibles were placed into a Galanz WG700SL2011-KG microwave oven and heated for 30 min under the middle-high power (560 W).Finally,the samples were cooled to room temperature in air and ground into powder in a porcelain mortar to obtain the goal products.

2.2 Analysis and characterization

The crystal structure and phase purity of the samples were identified by Y2000 X-ray diffractometer (XRD) using CuKαradiation (30 kV,20 mA and a scanning speed of0.06 (°)·s^-1).The photoluminescence (PL) excitation and emission spectra of the samples were recorded on an F-380fluorescence spectrophotometer using Xe lamp as light source.The color parameters of the samples were tested by PMS-50 ultraviolet-visible (UV-Vis) spectrum system.All samples were measured at room temperature.

3 Results and discussion

3.1 XRD analysis

Figure 1 shows XRD pattern of host SrMoO₄ obtained by microwave radiation method under the middle-high power(560 W) for 30 min.All the diffraction peaks of the sample can be well indexed to the tetragonal scheelite-type structure SrMoO₄ (JCPDS No.85-0809,space group:I4₁/a),which suggests that the sample is pure tetragonal phase SrMoO₄ with no other phases existing.

Under the same experimental conditions,a series of Eu³⁺,Tb³⁺-co-doped samples Sr_0.94MoO₄:xEu³⁺,(0.06-x)Tb³⁺were synthesized by microwave radiation method,and their XRD patterns are shown in Fig.2.It is clear that the diffraction peaks of samples can also agree well with tetragonal scheelite-type SrMoO₄ with I4₁/a space group.In these patterns,the peaks of Tb or Eu compounds cannot be found,which indicates that Eu³⁺and Tb³⁺enter into the host lattice and have few effects on the crystal structure of host SrMoO₄.Compared with the XRD pattern of host SrMoO₄,all diffraction peaks of samples doped with rare earth activator ions shift slightly to the higher angle side,indicating that the rare earth ions are effectively doped into the SrMoO₄ host lattice.This phenomenon can be explained by the Bragg equation,λ=2dsinθ(d is the distance between two crystal planes,θis the diffraction angle of an observed peak,andλis the X-ray wavelength).When Eu³⁺or Tb³⁺enters the host SrMoO₄,they are expected to replace Sr²⁺rather than Mo⁶⁺.The reason is that the radius of Eu³⁺(0.106 nm) or Tb³⁺(0.0923 nm) is close to that of Sr²⁺(0.118 nm),and much larger than that of Mo⁶⁺(0.041 nm) [ 12] .Because the radius of Eu³⁺or Tb³⁺is smaller than that of Sr²⁺,the substitution of Eu³⁺or Tb³⁺for Sr²⁺results in the decrease in crystal lattice constants as well as d-spacing.Hence,according to the Bragg equation,the diffraction peaks shift to higher angle side.

Fig.1 XRD patterns of as-synthesized SrMoO₄ and standard data for tetragonal SrMoO₄

Fig.2 XRD patterns of as-synthesized Sr_0.94MoO₄:x-Eu³⁺,(0.06-x)Tb³⁺and standard data for tetragonal SrMoO₄

From Figs.1 and 2,it can also be seen that the diffraction peaks of the samples are very sharp and strong,revealing that the samples with high phase purity and high crystallinity can be synthesized in only 30 min by microwave radiation method,whereas traditional solid-state reaction requires higher temperature and longer reaction time [ 8] .The reaction cycle of solvothermal synthesis is also longer,and the operating steps are more complicated [ 9] .So,microwave radiation method has the advantages of fast and uniform heating,time and energy saving,easy operation,etc.

3.2 Luminescence properties

The PL excitation and emission spectra of samples are shown in Fig.3.The excitation spectrum (Fig.3a,left) of sample SrMoO₄:Eu³⁺,obtained by monitoring at 616 nm,consists of a broad band between 200 and 350 nm with a main peak at about 282 nm and some sharp peaks between350 and 500 nm.The broad band can be assigned to charge transfer band (CTB) of Mo-O and Eu-O.The sharp peaks can be assigned to f-f transition of Eu³⁺.The dominant sharp peaks at 395 and 464 nm can be attributed to ⁷F₀^-5L₆and ⁷F₀-⁵D₂ transitions within the 4f⁶ configuration of Eu³⁺,respectively.The CTB around 282 nm demonstrates that the energy transfer occurs from groups to Eu³⁺.It can also be seen that the intensity of CTB is much stronger than that of f-f transitions of Eu³⁺,which indicates that the energy transfer from to Eu³⁺is very efficient and the excitation of Eu³⁺is realized mainly by the energy transfer from groups to Eu³⁺ [ 11] .The similar situation holds for the Tb³⁺-doped SrMoO₄(Fig.3b,left) and Eu³⁺,Tb³⁺-co-doped SrMoO₄ (Fig.3c,left) samples.

Excited by 282 nm (λ_em=282 nm),the emission spectrum (Fig.3a,right) of as-synthesized SrMoO₄:Eu³⁺is composed of a group of narrow peaks at about 592,616,656 and 701 nm,which can be ascribed to ⁵D₀-⁷F_J (J=1,2,3,4) electronic transition of Eu³⁺,respectively.According to the parity selection rule,when Eu³⁺is located at the site with an inversion symmetric center,⁵D₀-⁷F₁magnetic dipole transition is permitted,which results in orange-red emitting around 592 nm.If located at the site without an inversion symmetric center,as the opposite parity 5d configuration is mixed into 4fⁿ configuration,the parity selection rule is able to be lifted,and f-f forbidden transition is partially released;the hypersensitive ⁵D₀-⁷F₂electric dipole transition will be permitted,which results in red emitting around 616 nm.The SrMoO₄:Eu³⁺sample exhibits strong red emissions under ultraviolet excitation,and the emission spectra are dominated by ⁵D₀-⁷F₂(616 nm) transition of Eu³⁺,suggesting that Eu³⁺probably occupies non-inversion symmetric center in host lattice [ 13, 14] .

The SrMoO₄:Tb³⁺sample obtained under the same conditions exhibits strong green emission under ultraviolet excitation.The PL excitation and emission spectra of SrMoO₄:Tb³⁺are shown in Fig.3b.In the excitation spectrum monitored at 544 nm (Fig.3b,left),a broad band between 200 and 350 nm and some sharp peaks between350 and 500 nm can be observed.The broad band with a main peak at about 287 nm is ascribed to the charge transfer band of Mo-O and 4f⁸-4f⁷ 5d¹ transition of Tb³⁺.The weak peaks between 350 and 500 nm can be attributed to the f-f transitions of Tb³⁺.Excited by 287 nm,the emission spectrum (Fig.3b,right) of SrMoO₄:Tb³⁺is composed of a group of narrow peaks at about 490,544,586 and 621 nm,which can be ascribed to ⁵D₄-⁷F_J(J=6,5,4,3) electronic transition of Tb³⁺,respectively [ 15] .

Fig.3 Excitation and emission spectra of samples:a Sr_0.94MoO₄:0.06Eu³⁺;b Sr_0.94MoO₄:0.06Tb³⁺;c Sr_0.94MoO₄:0.03Eu³⁺,0.03Tb³⁺.λ_ememission wavelength,λ_ex excitation wavelength

The PL excitation and emission spectra of Sr_0.94MoO₄:0.03Eu³⁺,0.03Tb³⁺are shown in Fig.3c.In the excitation spectra monitored at 616 nm (Fig.3c,left),a broad band of the charge transfer band of Mo-O and Ln-O(Ln=Eu,Tb) with a maximum at 283 nm and some sharp peaks of f-f transitions of the rare earth activator ions(Eu³⁺,Tb³⁺) can be observed.Upon direct excitation into the at 283 nm,the emission spectrum (Fig.3c,right) is composed of a group of sharp peaks at about 490,544,592,616 and 701 nm.The first two green emission peaks can be ascribed to ⁵D₄-⁷FJ (J=6,5) electronic transition of Tb³⁺,respectively,and the others can be ascribed to ⁵D₀-⁷F_J (J=1,2,4) electronic transition of Eu³⁺,respectively.

As shown in Fig.3c,except for a strong broad band,the excitation spectrum of Sr_0.94MoO₄:0.03Eu³⁺,0.03Tb³⁺also shows a weak transition at 497 nm when monitoring with the emission of Eu³⁺at 616 nm,which is due to the ff transition of Tb³⁺(7F₆-⁵D₄).In addition,the characteristic excitation band of Eu³⁺and the ⁵D₄ characteristic emission peaks of Tb³⁺overlap to some extent,meeting the necessary condition of the resonance energy transfer.This indicates that the energy transfer can occur from Tb³⁺to Eu³⁺in the SrMoO₄ host.In other words,for Tb³⁺,Eu³⁺-co-doped sample,the emission of Eu³⁺not only results from the excitation energy of 283 nm (i.e.,the energy transfer from groups to Eu³⁺),but is also related to the energy transfer from Tb³⁺to Eu³⁺ [ 16] .But,the energy transfer from groups to Eu³⁺is dominated in the emission of Eu³⁺.

In order to investigate the influence of the concentration of Eu³⁺and Tb³⁺on the luminescent intensity,luminescent color and color temperature,a series of samples Sr_0.94MoO₄:xEu³⁺,(0.06-x)Tb³⁺were prepared.The compositions of the samples are shown in Table 1,and the emission spectra of samples are shown in Fig.4a.From Fig.4a,it can be seen that the concentration of Eu³⁺and Tb³⁺has a great effect on the intensity of emission peaks.While maintaining the total doping concentration of Eu³⁺and Tb³⁺constant (0.06),the value of x changes from 0.06 to0,the intensity of main emission peak of Eu³⁺at 616 nm decreases with the decrease in the concentration of Eu³⁺(x),and the intensity of main emission peak of Tb³⁺at 544 nm increases with the increase in the concentration of Tb³⁺(0.06-x).The data in Table 1 also confirm this point.The ratio of I_G (544 nm)/I_R(616 nm)(I_G is the green-emitting intensity at 544 nm,I_R is the red-emitting intensity at616 nm) increases gradually with the decrease in the concentration of Eu³⁺(x) and the increase in the concentration of Tb³⁺(0.06-x);meanwhile,the value of the color coordinate X decreases gradually and the value of the color coordinate Y increases gradually.The ratio ofI_G/I_R determines the color of the sample:When the luminous intensity of Eu³⁺is greater than that of Tb³⁺,the samples exhibit red or orange-red emissions under ultraviolet excitation;when the luminous intensity of Eu³⁺is less than that of Tb³⁺,the samples exhibit green or yellow-green emissions;when the luminous intensity of Eu³⁺and Tb³⁺is equivalent,the samples exhibit bright yellow emissions.Therefore,the full color between green and red light can be achieved by adjusting the relative concentration of Tb³⁺and Eu³⁺.

Figure 4b shows the corresponding CIE (Commission International de L'Eclairage,1931) chromaticity diagram of the as-prepared Sr_0.94MoO₄:xEu³⁺,(0.06-x)Tb³⁺.The symbol"×"indicates the CIE chromaticity coordinates positions.It can be seen that the chromaticity coordinates of the samples are indeed located in the region from red to green,which is consistent with expectation.Besides,the corresponding color temperatures of Sr_0.94MoO₄:xEu³⁺,(0.06-x)Tb³⁺samples are calculated as shown in Table 1.It suggests that the color temperature of the sample can be controlled by adjusting the concentrations of Eu³⁺and Tb³⁺.

下载原图

Table 1 Variations in spectral data,CIE chromaticity coordinates (X,Y),color temperature (T_c) and emission colors as a function of con-centrations of Eu³⁺and Tb³⁺in samples

Fig.4 Emission spectra of Sr_0.94MoO₄:xEu³⁺,(0.06-x)Tb³⁺a and corresponding CIE chromaticity diagram b.Numbers 1-10 corresponding to Samples 1-10 in Table 1

4 Conclusion

Color-tunable phosphors Sr_0.94MoO₄:xEu³⁺,(0.06-x)Tb³⁺were synthesized rapidly by microwave radiation method using active carbon particle as absorbent.The crystal structure and luminescent properties of the samples were investigated in detail.The kind of RE ions has few effects on the structure of the samples.As-synthesized phosphors possess the similar structure with that of SrMoO₄ with tetragonal system,belonging to I4₁/a space group.The PL color of Sr_0.94MoO₄:xEu³⁺,(0.06-x)Tb³⁺could be tuned from red to green by changing the relative doping concentrations of Eu³⁺and Tb³⁺.Because of their strong emission intensity,good CIE chromaticity,stability and low raw material cost,these phosphors have promising applications in the fields of fluorescent lamps and light-emitting diodes (LEDs).Moreover,microwave radiation method has the advantages of fast and uniform heating,time and energy saving,easy operation,so this synthesis route may be applied to the preparation of other well-defined molybdate functional materials.