Synthesis and enhanced luminescence properties of double perovskite NaLa0.95-xYxEu0.05MgWO6 phosphors
来源期刊:Rare Metals2020年第8期
论文作者:Jing Zhang Jia-Lin Shen Nai-Cen Xu
文章页码:895 - 901
摘 要:Double perovskite Na(La,Y)MgWO6:Eu3+phosphors were obtained by the sol-gel method.X-ray diffractometer(XRD) patterns,scanning electron microscopy(SEM) images and photoluminescence properties were carried out to study the effect of polyethylene glycol(PEG).Enhanced optical properties were obtained by adding 3 wt% PEG under the excitation of 395 nm.Y3+with smaller radius was selected to substitute La3+ in this host to decrease the symmetry and further enhance the luminescence properties.With Y3+concentration increasing,Na2WO4 and Y30W8O69 phases came up.The emission spectra showed different shapes,and the emission intensity was enhanced by doping Y3+.Under the excitation of 395 and 465 nm,the emission intensity reached the maximum.The Commission Internationale de L’Eclairage(CIE)coordinates of the phosphors were on the edge of the CIE diagram,illustrating that this double perovskite phosphor showed good color purity.Above luminescence properties showed that the synthesized phosphors had great potential in solid-state lighting.
稀有金属(英文版) 2020,39(08),895-901
Jing Zhang Jia-Lin Shen Nai-Cen Xu
Nanjing Center,China Geological Survey
Supervision and Testing Center of East China,Mineral Resources of the Ministry of Land and Resources
Double perovskite Na(La,Y)MgWO6:Eu3+phosphors were obtained by the sol-gel method.X-ray diffractometer(XRD) patterns,scanning electron microscopy(SEM) images and photoluminescence properties were carried out to study the effect of polyethylene glycol(PEG).Enhanced optical properties were obtained by adding 3 wt% PEG under the excitation of 395 nm.Y3+with smaller radius was selected to substitute La3+ in this host to decrease the symmetry and further enhance the luminescence properties.With Y3+concentration increasing,Na2WO4 and Y30W8O69 phases came up.The emission spectra showed different shapes,and the emission intensity was enhanced by doping Y3+.Under the excitation of 395 and 465 nm,the emission intensity reached the maximum.The Commission Internationale de L'Eclairage(CIE)coordinates of the phosphors were on the edge of the CIE diagram,illustrating that this double perovskite phosphor showed good color purity.Above luminescence properties showed that the synthesized phosphors had great potential in solid-state lighting.
作者简介:*Jia-Lin Shen,e-mail:njcgssjl@163.com;
收稿日期:23 July 2018
基金:financially supported by the Opening Project of State Key Laboratory of High-Performance Ceramics and Superfine Microstructure (No.SKL201309SIC);
Jing Zhang Jia-Lin Shen Nai-Cen Xu
Nanjing Center,China Geological Survey
Supervision and Testing Center of East China,Mineral Resources of the Ministry of Land and Resources
Abstract:
Double perovskite Na(La,Y)MgWO6:Eu3+phosphors were obtained by the sol-gel method.X-ray diffractometer(XRD) patterns,scanning electron microscopy(SEM) images and photoluminescence properties were carried out to study the effect of polyethylene glycol(PEG).Enhanced optical properties were obtained by adding 3 wt% PEG under the excitation of 395 nm.Y3+with smaller radius was selected to substitute La3+ in this host to decrease the symmetry and further enhance the luminescence properties.With Y3+ concentration increasing,Na2WO4 and Y30W8O69 phases came up.The emission spectra showed different shapes,and the emission intensity was enhanced by doping Y3+.Under the excitation of 395 and 465 nm,the emission intensity reached the maximum.The Commission Internationale de L'Eclairage(CIE)coordinates of the phosphors were on the edge of the CIE diagram,illustrating that this double perovskite phosphor showed good color purity.Above luminescence properties showed that the synthesized phosphors had great potential in solid-state lighting.
Keyword:
Double perovskite; Structure evolution; Phosphors; White light emitting diodes;
Received: 23 July 2018
1 Introduction
White light emitting diodes (W-LEDs) have been widely used as the fourth lighting source due to the excellent properties,such as environment friendliness,low power consumption,long lifetimes,high efficiency,and high brightness
Perovskite oxides ABO3 have been widely used as the host due to the micro-structures and performances
2 Experimental
Starting materials were La2O3,Eu2O3,Y2O3,NaNO3,Mg(NO3)2,(NH4)10W12O41,HNO3,NH3·H2O,citric acid(CA) and PEG.All the materials were analytically pure and directly used without purification.The amount of CA was3 mol times of the metal ions,and the weight ratio of PEG was 2 wt%of the target powders.Rare earth oxides were dissolved in HNO3 to prepare nitrate.Then stoichiometric amount of all nitrates was mixed.The CA and PEG were added into the mixed solution,and the pH value of the solution was modulated to 2.0-4.0.Then (NH4)10W12O41was added into the above solution,after which the mixed solution was heated to 80-90℃for 1 h to get gel.After then,the transparent gel was heated to 180-200℃to obtain spumous and fluffy precursor.Finally,the precursor was pre-calcined at 1100℃for 6 h.Series of NaLa0.95-xYxEu0.05MgWO6 (0≤x≤0.95) red phosphors were obtained.
The crystalline phases were determined by X-ray diffractometer (XRD,smartlabTM 9 kW,Rigaku,Japan)using Cu Kαradiation.Microstructural evolution of obtained powders was investigated by a scanning electron microscope (SEM,SU8010,Hitachi,Japan).The optical properties were measured by the fluorescent spectrophotometer (Lumina,America).
3 Results and discussion
3.1 Effect of addition of PEG
XRD patterns of the as-obtained NaLaMgWO6:Eu3+phosphors using different amounts of PEG (0 wt%,1 wt%,2 wt%,3 wt%,and 4 wt%) are shown in Fig.1.All the spectra have the same shape and can be indexed by NaLaMgWO6 (JCPDS No.88-1761).Only some small peaks at 17.1°and 43.2°can be indexed by Na2WO4 as the impurity phase.Typically,the peaks at 11.4°and 25.7°belong to the layered ordering of Na+and La3+.The peaks at 19.2°and 37.9°are observed in the patterns,indicating that MgO6 and WO6 exist in this double perovskite as a rock-salt ordering
Fig.1 XRD patterns of NaLaMgWO6:Eu3+phosphors using differ-ent amounts of PEG
SEM images of the obtained powders are shown in Fig.2.The particles exhibit irregular shape with blurry edges while no PEG was used.With PEG addition,the edges become clear and the particles become polyhedrons gradually.On the other hand,with the PEG amount increasing,the size of the particles increases firstly and reaches the maximum while 2 wt%PEG was used.Continuing adding PEG,the size of the particles reduces and the edges of the particles become clearer.Clearer edges mean better crystalline of the particles,indicating that adding PEG benefits the crystalline.With PEG addition,the ions in the solution could be modified more uniformly,which benefits the formation of the gel.During the calcination period,the uniform particles would benefit the grain growth,resulting in better crystalline.
Figure 3 shows the excitation and emission spectra of NaLaMgWO6:Eu3+and the energy level diagram of Eu3+.The excitation spectrum is composed of a strong wide band and several weak sharp peaks.The strong wide band belongs to charge transfer band (CTB) and the sharp peaks belong to characteristic.f-f transition absorption peaks of Eu3+.The CTB centered at about 320 nm is mainly attributed to W-O absorption
Fig.2 SEM images of NaLaMgWO6:Eu3+phosphors using different amounts of PEG:a 0 wt%,b 1 wt%,c 2 wt%,d 3 wt%,and e 4 wt%
Fig.3 a Excitation and emission spectra of NaLaMgWO6:Eu3+and b simplified energy level diagram of Eu3+
The emission spectra of the samples are shown in Fig.5.All the spectra show a strong electric dipole transition and a weak magnetic dipole transition,indicating that Eu3+occupies a low symmetry site in the lattice
Fig.4 Excitation (λem=615 nm) spectra of NaLaMgWO6:Eu3+phosphors using different amounts of PEG
Fig.5 Emission spectra (λex=CTB,395 nm and 465 nm) of NaLaMgWO6:Eu3+phosphors using different amounts of PEG
3.2 Structural evolution and luminescence properties of NaLa0.95-xYxEu0.05MgWO6
NaLa0.95-xYxMgWO6:Eu3+(0≤x≤0.95) phosphors are exhibited in Fig.6.For the samples prepared by sol-gel method,the Na2WO4 and Y30W8O69 phases are observed as impurity phases.When x≤0.30,double perovskite NaLaMgWO6 is the main phase and Na2WO4 is the impurity phase.Obviously,the diffraction peaks of A-site ordering and B-site ordering still exist.With Y3+concentration increasing,the Na2WO4 phase amount increases. When x is between 0.30 and 0.70,the phase is mainly composed of Na2WO4 and NaLaMgWO6.With the x value further increasing,double perovskite phase disappears and Y30W8O69 turns up as the main phase,which might result from the big radius difference between Y3+and La3+.Schematic illustration of Na(La,Y)MgWO6 is shown in Fig.7.Obviously,this double perovskite shows double ordering of A-site and B-site ions.Na+and La3+/Y3+show unique layered ordering among the double perovskite structures while Mg2+and W6+show rock-salt ordering.
Fig.6 XRD patterns of NaLa0.95-xYxEu0.05MgWO6 (0≤x≤0.95)
Figure 8a shows the excitation spectra (λem=594 nm)of Eu3+-doped Na(La,Y)MgWO6.A strong charge transfer band in the region of 250-350 nm and a series of sharp excitation ships above 350 nm could be clearly observed.With the x value increasing,the CTB shifts to the high wavelength number.The absorption intensity reaches the maximum at x=0.90,x=0.90,and x=0.70 for CTB,395 nm and 465 nm,respectively.Figure 8b shows the excitation spectra (λem=615 nm) of NaLa0.95-xYxEu0.05MgWO6 (0≤x≤0.95).Differently,the spectra show different shapes due to the phase transition.When x is between 0 and 0.30,the CTB centered at 325 nm is strong.Upon increasing the x value,CTB shifts to lower wavelength number and is centered at 280 nm,which might result from the increasing phase of Na2WO4.The absorption intensity reached the maximum at x=0.70 for395 and 465 nm.When x is bigger than 0.7,double perovskite phase disappears and the CTB weakens.
Fig.7 Schematic illustration of Na(La,Y)MgWO6
Fig.8 Excitation spectra of NaLa0.95-xYxEu0.05MgWO6 (0≤x≤0.95) withλem of a 594 nm and b 615 nm
Figure 9 shows the emission spectra (λex=CTB,395 nm and 465 nm) of NaLa0.95-xYxEu0.05MgWO6 (0≤ x≤0.95).The spectra show different shapes for different doping concentrations when the phosphors were excited by CTB.The electric dipole transition shows the dominant role when x is smaller than 0.70.Differently,the peaks split into two peaks at 612 and 618 nm while x is between 0 and0.30.The transition for x=0.50 and x=0.70 shows a single peak at 615 nm.The emission spectra show a strong peak at 615 nm while x<0.90,indicating that Eu3+occupies the La3+site in the double perovskite phase
Luminescence decay curves of the selected NaLa0.95Eu0.05MgWO6 and NaLa0.25Y0.7Eu0.05MgWO6phosphors are shown in Fig.10.Lifetimes of the phosphors were calculated based on the decay curves.Both the two curves could be fitted by the single experimental function,as shown in Eq.(1).The lifetimes were calculated to be484 and 442μs for NaLa0.95Eu0.05MgWO6 and NaLa0.25Y0.7Eu0.05MgWO6,respectively.The Commission Internationale de L'Eclairage (CIE) chromaticity coordinates were calculated based on the emission spectra,and the results are shown in Table 1 and Fig.11.The coordinates are in the range of (0.654,0.345) and (0.660,0.340)while x≤0.70.The coordinates of NaLa0.95Eu0.05MgWO6are (0.660,0.340),which is rather close to the“ideal red”(0.67,0.33).Upon increasing the x value,the coordinates come away from (0.67,0.33).In addition,the chromaticity coordinates,which are close to the border of CIE diagram,indicate that the phosphors show good color purity.
Fig.9 Emission spectra of NaLa0.95-xYxEu0.05MgWO6 (0≤x≤0.95) withλex of a CTB,b 395 nm and c 465 nm
Fig.10 Luminescence decay curves (λex=465 nm) of NaLa0.95Eu0.05MgWO6 and NaLa0.25Y0.7Eu0.05MgWO6
Table 1 CIE chromaticity coordinates of NaLa0.95-xYx-Eu0.05MgWO6 (0≤x≤0.95)
Fig.11 CIE chromaticity coordinates of NaLa0.95Eu0.05MgWO6phosphors
where I andI0 represent the emission intensity at the time of t and 0,respectively,τis the decay lifetime.
4 Conclusion
A series of NaLa0.95-xYxEu0.05MgWO6 (0≤x≤0.95)phosphors were successfully synthesized.The luminescence properties were enhanced by adding PEG,and the phosphors showed better luminescence properties while adding 3 wt%PEG under the excitation of 395 nm.The particles transferred from irregular shape to polyhedron by adding PEG.With the concentration of Y3+increasing,the Na2WO4 and Y30W8O69 phase came up and the double perovskite phase gradually disappeared.Both the excitation spectra and emission spectra showed different shapes because of the structural evolution.The coordinates of NaLa0.95Eu0.05MgWO6 were (0.660,0.340),which was rather close to the“ideal red”(0.67,0.33).The normal phosphors showed good color purity in solid-state lighting.
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