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Single-phase LiY(MoO4)2−x(WO4)x:Dy3+, Eu3+ phosphors with white luminescence for white LEDs
Materials Research Bulletin 84 (2016) 429–436
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Single-phase LiY(MoO4)2x(WO4)x:Dy3+, Eu3+ phosphors with white luminescence for white LEDs Liangjun Zhoua,b , Wenxi Wanga , Sicen Yua , Bo Nana , Yinggang Zhua , Yang Shia , Haohong Shia , Xingzhong Zhaob , Zhouguang Lua,* a b
Department of Materials Science and Engineering, South University of Science and Technology of China, Shenzhen, China School of Physics and Technology, Wuhan University, Wuhan, China
A R T I C L E I N F O
Article history: Received 27 April 2016 Received in revised form 18 August 2016 Accepted 21 August 2016 Available online 26 August 2016 Keywords: A. Inorganic compounds A. Optical materials B. Sol-gel chemistry B. Luminescence D. Phosphors
A B S T R A C T
Single-phase and Dy3+/Eu3+ co-doped novel phosphor LiY(MoO4)2x(WO4)x:Dy3+,Eu3+, emitting white luminescence under near ultraviolet (NUV) light, was prepared through sol-gel method. The preparation and characterization of this phosphor were systematically studied by X-ray diffraction and spectrofluorophotometric measurements. The molar ratio of metal ions and citric acid (Rm/c) and calcination temperature greatly influenced the phase purity of sample, revealing that the pure phase could be obtained by using Rm/c = 1:2.5 and calcination temperature = 800 C. Under NUV excitation, the as-prepared phosphor exhibited the emissions of 485, 572 and 612 nm, which intensities could be affected by the pH and the concentrations of molybdenum and tungsten ions. By doping appropriate concentrations of Dy3+ and Eu3+, white light emitting phosphor LiY(MoO4)1.2(WO4)0.8:6%Dy3+,7%Eu3+ with good responsiveness to NUV light was obtained. The Commission Internationale de L’Eclairage chromaticity coordinates were calculated to be (x = 0.334, y = 0.322), close to the D65 illuminant (x = 0.313, y = 0.329), indicating the potential application for NUV WLEDs. ã 2016 Published by Elsevier Ltd.
1. Introduction White light-emitting diodes (WLEDs) have been considered as the next generation of solid state light sources, due to their superior properties such as lower energy consumption, more efficient output and environmental friendliness [1,2]. Currently, combining blue LED chip with YAG:Ce yellow-emitting phosphor or coating ultraviolet (UV) LED with red/green/blue emission phosphors is the main approach to obtain WLEDs [3–5]. So far, amounts of investigations have been made to develop novel phosphors for WLEDs. However, some challenges, which still exist in WLEDs, are to realize high luminescent efficiency, high chromatic stability, excellent color-rending properties, and competitive price against traditional implemented fluorescent lamps [6,7]. Hence, the single-phase phosphor has become the necessary pre-requisite for the fabrication of WLEDs to solve these above challenges [6–8]. In the recent years, a great number of efforts have been made to develop single-phase white-light-emitting phosphors for near
* Corresponding author. E-mail address: [email protected] (Z. Lu). http://dx.doi.org/10.1016/j.materresbull.2016.08.028 0025-5408/ã 2016 Published by Elsevier Ltd.
ultraviolet (NUV) WLEDs [6,9,10]. It is well known that rare-earth (RE) doped materials can emit different colors of luminescence such as red, green and blue or yellow which cover the whole visible light range [1,3]. Therefore, there are some different methods to realize white light emission from a single-phase host lattice including: (i) doping a single RE ion (Eu3+, Dy3+) into single phase hosts; (ii) co-doping two or more RE ions with different emissions simultaneously, such as Ce3+/Eu2+, Tm3+/Dy3+, Tm3+/Tb3+/Eu3+; (iii) co-doping different ions to control emission through energy transfer processes (EPT); and (iv) controlling the concentration of the defect and reaction conditions of defect-related luminescent materials [6,8,9,11,12]. Compared with other host lattices, molybdates and tungstates have drawn much attention for WLEDs, due to their good chemical and physical stability, high density and low phonon threshold energy. What’s more, the (MoO4)2 and (WO4)2 groups have strong absorption in the NUV region. These groups can transfer their absorbed energy to the dopant ions, making these phosphors exhibiting strong and broad absorption in the UV region. Therefore, these materials have been considered as excellent hosts for phosphor materials [13–16]. In the past years, these above scheelite-type crystalline structures (such as LiEu(MoO4)2 [14], LiLn(MO4)2:Eu [1], Tb2(WO4)3:Eu) [13] have attracted a great of
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Scheme 1. The brief synthetic process of white phosphor powder LiY(MoO4)2x(WO4)x:Dy3+, Eu3+.
attentions. A number of papers have been reported about molybdates/tungstates phosphors activated by RE ions. However, only a few of papers have been investigated on the complex molybdate–tungstates phosphors, in which luminescence can be affected by the ratio of molybdate and tungstate ions [15]. Because that Mo6+ and W6+ ions have similar ionic radii and can be substituted for each other, introducing changes in the energy transitions and luminescence intensities of RE ions. Hence, it is of scientific importance to investigate this influence systematically. Motivated by the above-stated studies and attempts to develop phosphors excitable by NUV light for the applications of WLEDs, herein we have successfully synthesized a single-phase and white emitting phosphor LiY(MoO4)2x(WO4)x:Dy3+, Eu3+ through a simple sol-gel method, based on the consideration that Li-based tungstate/molybdate materials exhibit more promising
luminescence performance than other K and Na-based counterparts [1]. Furthermore, we have systematically studied the preparation, luminescence and color chromaticity properties of a series of phosphors LiY(MoO4)2x(WO4)x:Dy3+, Eu3+. The influence of citric acid and calcination temperature on crystal structure is studied by XRD measurement. The pH of solution and the concentrations of molybdenum and tungsten ions are adjusted to realize the highest luminescent intensities under NUV light. Moreover, the appropriate doping concentrations of Dy3+ and Eu3+ make white light emitting phosphor well responsive to NUV light. 2. Experimental section Europium oxide (Eu2O3, 99.99%), dysprosium oxide (Dy2O3, 99.99%), ammonium molybdate tetrahydrate ((NH4)6Mo7O244H2O,
Fig. 1. XRD patterns of LiY(MoO4)2 synthesized with (a) different Rm/c (1:1, 1:2, 1:2.5, 1:5); and (b) different calcination temperature (600, 700, 800, 900 and 1000 C).
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Fig. 2. PL spectra of (a) LiY(MoO4)2: 5%Eu3+ obtained at different pH (1, 3, 5, 6 and 7); and (b) LiY(MoO4)2x(WO4)x: 5%Eu3+ synthesized with different value of x (0, 0.4, 0.8, 1.0, 1.2, 1.6 and 2.0).
AR), yttrium nitrate hexahydrate (Y(NO3)36H2O, 99.9%) and lithium nitrate (LiNO3) were purchased from Alfa Aesar of Thermo Fisher Scientific Inc. Ammonium metatungstate hydrate ((NH4)10W12O41xH2O, 99.5%), ethylene glycol (C2H6O2) and citric acid (C6H8O7H2O) were provided by Aladdin Inc. Nitric acid (HNO3) was supplied from Sinopharm Chemical Reagent Co., Ltd. All reagents were used without further purification. According to the reference [17], in the typical procedure (Scheme 1), stoichiometric amounts of raw materials europium (III) oxide (Eu2O3), dysprosium (III) oxide (Dy2O3) were dissolved in nitric acid (HNO3) under vigorous stirring, then the excess HNO3 was removed after heating. A suitable volume of deionized water, stoichiometric amount of LiNO3 and RE(NO3)3xH2O were added under stirring. Stoichiometric amounts of ammonium molybdate [(NH4)6Mo7O24903;4H2O], ammonium paratungstate [(NH4)10W12O41xH2O] and citric acid were dissolved in appropriate volume of deionized water under vigorous stirring. This solution was then mixed with the above solution after adjusting the pH value of the solution. Then suitable volume of ethylene glycol was added under stirring for a while. Next, the transparent solution was continuously stirred in water bath at 80 C for 2 h to obtain a transparent light-green gel. Then the gel was further dried at 180 C in an oven for 2 h to obtain a dried gel, which was calcinated in muffle furnace for 2 h in air to obtain a white powder. All the samples were characterized by powder X-ray diffraction (XRD) using a Rigaku D/Max-2400 X-ray diffractometer with Nifiltered Cu-Ka radiation. A step size of 0.02 was used at a scanning speed of 8 /min. The photoluminescence (PL) measurements were performed on Hitachi F-7000 fluorescence spectrophotometer equipped with a 150 W Xe lamp as the excitation light source. The fluorescence decay curves were obtained on a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz), with a tunable laser as the excitation source. The CIE chromaticity coordinates were
calculated by GoCIE. All of the measurements were performed at room temperature in air. All excitation and emission spectra were measured at room temperature with the same instrumental parameters for comparison. 3. Results and discussion To our knowledge, many researches have demonstrated that lots of small organic molecules are helpful for the control of crystal structure and morphology in the synthesis of matrerials [18–20]. Among kinds of organic additives, citric acid is considered as one of the most common and important organic molecules, which have been used as the stabilizer and structure-directing agents [1,21]. In order to study the effect of citric acid on the growth of LiY(MoO4)2, different amount of citric acid have been adopted in the reaction, and the results are displayed in Fig. 1(a). These curves, which are showed with a histogram (JCPDS No.17-773) in the figure, are the XRD patterns of LiY(MoO4)2 synthesized with different molar ratios of metal ions and citric acid (Rm/c), respectively. It is clear that the structures of all samples are mostly tetragonal structures, indexed to pure phase LiY(MoO4)2 according to the stand PDF card (JCPDS No.17-773). However, it is worth noting that some diffraction peaks, which are ascribed to the existence of Li2MoO3 (JCPDS No.21-517), can be observed in all samples except the one with Rm/c = 1:2.5. No peaks of Li2MoO3 are detected, revealing a high purity of the obtained powders. In the reaction, the citric acid acts as structure-directing reagent binding to the crystal surface. It directly affects the growth of different crystal facets, resulting in the formation of the crystal structure. This result reveals that the amount of citric acid is critical for the crystal growth and purity of LiY(MoO4)2 powders [18,21–23]. Furthermore, it is known that the calcination temperature is one of the important factors, which can affect the crystal structure
Fig. 3. (a) SEM and (b) TEM images of LiY(MoO4)1.2(WO4)0.8: 6%Dy3+, 5%Eu3+. The inset is HRTEM image.
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Fig. 4. The PL excitation spectra (lem. = 572 nm) of (a) LiY(MoO4)1.2(WO4)0.8: 6%Dy3+ and (c) LiY(MoO4)1.2(WO4)0.8: 6%Dy3+, 5%Eu3+; the PL emission spectra (lex. = 393 nm) of (b) LiY(MoO4)1.2(WO4)0.8: mDy3+ (m = 1%, 3%, 6%, 8% and 10%) and (d) LiY(MoO4)1.2(WO4)0.8: 6%Dy3+, xEu3+ (x = 1%, 3%, 5%, 7% and 10%).
of the resultants [1,22]. Hence, a series of different temperatures from 600 C to 1000 C have been used in the calcination during the synthesis of LiY(MoO4)2. The XRD patterns of LiY(MoO4)2 synthesized at different calcination temperatures are displayed in Fig. 1(b), indicating that the changes of the calcination temperature in the region of 600–1000 C slightly influence the structures of the resultant phosphors. Comparing with the JCPDS No.17-773 card, it can be found that all the patterns of the prepared powders contain all the peaks of tetragonal LiY(MoO4)2 (JCPDS No.17-773). However, some other peaks, which can be ascribed to the existence of Li2MoO3 (JCPDS No.21-517), can be observed in all patterns except the one calcined at 800 C. From the above results, it can be concluded that the LiY(MoO4)2 powders can be obtained in a wide
Fig. 5. The linear plot of log (I/x) versus log x of LiY(MoO4)1.2(WO4)0.8 doped with different concentrations of Dy3+ (1%, 3%, 6%, 8% and 10%).
temperature region from 600 to 1000 C, while the one with pure phase can only be synthesized at 800 C. As considering that pH and Mo/W ratio could affect the luminescent property of the phosphor [15], the PL spectra of 5% Eu3 + doped LiY(MoO4)2 are studied. As seen from Fig. 2(a), there are two major peaks around 589 nm and 612 nm, ascribed to 5D0 ! 7 F1 and 5D0 ! 7F2 transitions of Eu3+ ions, respectively [24]. The pHdependent emission spectra of LiY(MoO4)2:5%Eu3+ under 395 nm excitation indicate that the change of pH cannot cause changes in the shape or the position of the peak, but introduces the obvious change in the luminescence intensity, which decreases with the increase of pH value. Moreover, the effect of Mo/W ratios on PL property of the phosphor is showed in Fig. 2(b). As seen from the PL spectra of LiY(MoO4)2x(WO4)x: 5%Eu3+ contained different concentrations of Mo and W ions, the shapes and positions of the two major emission peaks remain the same. However, the intensity changes as the value of x increases from 0 to 2, and reaches at the highest when the value of x is 0.8. These results demonstrate that the suitable value of pH in the reaction is proved to be 1 and LiY (MoO4)1.2(WO4)0.8 is the best choice to be the host materials. Moreover, the morphology of LiY(MoO4)1.2(WO4)0.8:6%Dy3+, 5%Eu3 + phosphor has been observed by FE-SEM and TEM. As shown in Fig. 3a, the rod-like powders are of irregular shape and size, most of which are micrometer-scale. Furthermore, the TEM and HRTEM images (Fig. 3b) suggest the high crystallinity of LiY (MoO4)1.2(WO4)0.8:6%Dy3+, 5%Eu3+ phosphor due to the clear crystal lattice stripe image. To obtain the white emitting phosphor, different doping concentrations of Dy3+ and Eu3+ in this phosphor are studied. These LiY(MoO4)1.2(WO4)0.8:6%Dy3+ phosphors are used to measure the excitation spectrum at room temperature by monitoring the emission at 572 nm. As shown in Fig. 4(a), there are four intense
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Fig. 6. Decay curves of Dy3+ luminescence in LiY(MoO4)1.2(WO4)0.8:6%Dy3+ doped with different concentrations of Eu3+ (1%, 3%, 5%, 7% and 10%) excited at 393 nm and monitored at 572 nm.
excitation peaks in near UV region around 351, 365, 390 and 451 nm, which are attributed to characteristic of intra f–f transitions of Dy3+ ion from ground state 6H15/2 to excited states 6 P7/2, 6P5/2, 4I13/2 and 4I15/2, respectively [16,25]. Hence, it is quite evident that the phosphor can be excited by near UV light, indicating the potential to be used for NUV WLEDs. Fig. 4(b) displays the PL emission spectra of LiY (MoO4)1.2(WO4)0.8 doped with different concentrations of Dy3+ (1%, 3%, 6%, 8% and 10%) under NUV excitation of 391 nm. The nature of emission spectra of all samples are similar, containing two sharp and dominating peaks at 485 nm and 572 nm, corresponding to the 4F9/2 ! 6H15/2 and 4F9/2 ! 6H13/2 transitions of Dy3+ ions [16,25], respectively. These peaks are strongly influenced by local symmetry environment of Dy3+ ion in host lattice [26]. Fig. 4(b) suggests that the luminescence intensity is observed to increase when the dopant concentration changes from x = 1% to x = 6%, while a further increase of the doped Dy3+ ions subsequently weaken the luminescence intensity. This
concentration quenching is primarily caused by the promotion of non-radiative energy transfers among Dy3+ ions, including an exchange interaction, radiation reabsorption, a multipole–multipole interaction [25]. This quenching phenomenon can be explained by Dexter theory, which can be approximated as followed [27]: logðI =xÞ ¼ c k log x where, I is correspond for the luminescence intensity, while x is the dopant concentration. c is a constant for excitation condition for the given host material. And k is the index of electric multipole, corresponding to electric dipole–dipole, electric dipole-electric quadrupole and electric quadrupole-electric quadrupole interactions, when its value is 2,2.67 and 3.33, respectively. Based on the emission spectra obtained at 391 nm, Fig. 5 illustrates the correlation between log (I/x) and log x. Although the interaction of ions in concentration quenching is more complicated, analysis of the plot indicates that those data fit well with a straight line with a
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Fig. 7. Variation of energy transfer efficiency (h, curve a) and probabilities (Pda, curve b) as a function of Eu3+ concentrations (1%, 3%, 6%, 8% and 10%).
slope of 1.999. Thus, the value of k is 1.999, which is approximate to 2. According to the Dexter theory, this result clearly indicates that the concentration quenching of Dy3+ emission is mainly caused by the electric dipole–dipole interaction [28]. On the basis of the above analysis, LiY(MoO4)1.2(WO4)0.8:6%Dy3+ phosphor is selected for further investigation of the luminescent property by doping varied concentrations of Eu3+. Fig. 4(c) displays the PL excitation spectrum of LiY(MoO4)1.2(WO4)0.8:6%Dy3+, 5%Eu3 + with emission monitored at 572 nm, which is the prominent emission of Dy3+. A broad excitation peak appears between 225 and 350 nm, due to the charge transfer band (CTB) of O2 ! metal ions (Mo6+, W6+, Eu3+ and Dy3+) [24,29]. The three peaks around 393, 415 and 465 nm are attributed to the 4f-4f transition of Eu3+ ions (7F0 ! 5L6, 7F0 ! 5D3 and 7F0 ! 5D2), indicating that the asprepared phosphor can be efficiently pumped by NUV light. Moreover, comparing with the PL excitation spectrum of LiY (MoO4)2:6%Dy3+ displayed in Fig. 4(a), other obvious changes can be observed in the excitation spectrum after doping Eu3+. All the four intense excitation peaks (351, 365, 390 and 451 nm) of Dy3+
are weakened, while both of the two sharp peaks (393 and 465 nm) of Eu3+ are enhanced, indicating that the possibility of energy transfer from Dy3+ to Eu3+. Fig. 4(d) shows the PL emission spectra at room temperature of LiY(MoO4)1.2(WO4)0.8: 6%Dy3+ doped with different concentrations of Eu3+ (1%, 3%, 5%, 7% and 10%, mol%) excited at 393 nm. The intensities of emission peaks are obviously found to change with the varied doping concentrations of Eu3+, while the shapes and positions of these peaks stay the same. It is clearly observed that there are three dominating peaks around 485, 572 and 612 nm, ascribed to the 4F9/2 ! 6H15/2 and 4F9/2 ! 6H13/2 transitions of Dy3+ ions [25,26], and the 5D0 ! 7F2 transitions of Eu3+ ions [1,29], respectively. As seen from these spectra, the Dy3+ emissions are enhanced by the introduction of Eu3+ at the beginning, but then weakened when the concentration of doped Eu3+ is higher than 0.3%. This fact can be explained as followed. At the low doping concentration of Eu3+, the increase of activator ions induces more energy transitions among Dy3+ ions, resulting in a slight enhancement of Dy3+ emissions. However, as the concentration of doped Eu3+ ions increases from 0.3% to 1%, the intensities of Dy3+ emissions correspondingly slightly decrease while those of Eu3+ are somewhat enhanced, due to the energy transfer from the Dy3+ ions to the Eu3+ ions in the LiY(MoO4)1.2(WO4)0.8 host material [30]. In order to further evaluate the energy transfer mechanism from Dy3+ to Eu3+ ions in LiY(MoO4)1.2(WO4)0.8:Dy3+, Eu3+ phosphors, the fluorescence lifetimes of Dy3+ ions (lem. = 572 nm) have been examined. The decay curves of LiY(MoO4)1.2(WO4)0.8: Dy3+, Eu3+ monitored at 572 nm are exhibited in Fig. 6a–e. All the decay curves show the multiexponential feature, which can be well fitted by a double-exponential equation as follows [31]: IðtÞ ¼ A1 expðt =t 1 Þ þ A2 expðt =t 2 Þ where t 1 and t 2 are the short and long lifetimes for the exponential components, and parometers A1 and A2 are the fitting constants, respectively. Moreover, the average lifetimes for the 4F9/2 ! 6H13/2 emission of Dy3+ can be calculated according to the equation as follows [31]: t ¼ A21 t 21 þ A22 t 22 =ðA1 t 1 þ A2 t 2 Þ
Fig. 8. The energy levels diagram of Dy3+ and Eu3+ in LiY(MoO4)1.2(WO4)0.8: Dy3+, Eu3+.
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4
Fig. 9. The CIE chromaticity coordinates of LiY(MoO4)1.2(WO4)0.8:6%Dy3+ doped with different concentration of Eu3+ (1: 0%, 2: 1%, 3: 3%, 4: 5%, 5: 7% and 6: 10%) phosphors on 1931 CIE chromaticity diagram. The insets are the digital images of corresponding samples under UV light.
The values of t are presented in Fig. 6f, showing gradually decrease with concentration of doping Eu3+, which is in good agreement with the PL intensity studies of these phosphors. The energy transfer efficiencies (h) and probabilities (Pda), which are exhibited in Fig. 7, are calculated by the following formulas [7,32]: n ¼1
Pda ¼
1
t
t t0
I15/2), respectively. They transfer to lower excited levels until arrive at 4F9/2 through multiphonon assistance, and then continuously transfer to the ground states through radiation transitions of 4 F9/2 ! 6H15/2 (485 nm) and 4F9/2 ! 6H13/2 (572 nm), respectively. Moreover, the energy transfer and emission process in Eu3+ are also presented in this diagram. Under the excitation of near ultraviolet light, electrons transfer to the excited levels (5L7, 5L6 and 5D2), then relax to lower levels 5D0, and finally transfer to the ground state (7F1, 7F2 and 7F3), respectively. Furthermore, according to the previous studies about the phosphors co-doped with Eu3+ and other RE ions (such as Sm3+, Ce3+ and Dy3+) [7,11], the energy transfer indeed exist between the other RE ions and Eu, which is consistent with the results in Fig. 3d. As illustrated in Fig. 5, when the electrons of Dy3+ relax from the higher excited levels to 4F9/2, the energy transfers from Dy3+ to Eu3+ by the resonance. Hence, the emission intensity of Dy3+ partly decreases while that of Eu3+ slightly increases. The color coordinates, a commission internationale de l’eclairage (CIE) parameter, are calculated in order to show the photometric characteristics of the as-prepared phosphors. As shown in Fig. 9, the CIE 1931 chromaticity diagram of LiY (MoO4)1.2(WO4)0.8:6%Dy3+, xEu3+ (x = 0%, 1%, 3%, 5%, 7% and 10%, mol%) phosphors are presented, locating in the near white region and changing with the increasing concentration of doped Eu3+ ions. The two digital images, obtained by the camera of mobile phone, are corresponded to the samples of No. 1 and 5 under UV light, respectively, clearly indicating the luminescence change from green to white with increasing Eu3+ concentration. The detailed photometric characteristic of these phosphors are showed in Table 1. It is clearly suggested that the powders only doped with 6% Dy3+ show emission in canary yellow region (x = 0.366, y = 0.449), which is similar as the earlier report [16,25,26]. When the concentration of doped Eu3+ ions is 7%, the CIE coordinates of this phosphor (x = 0.334, y = 0.322) are very close to the D65 illuminant (x = 0.313, y = 0.329) [25]. Hence, this phosphor can be suggested for application as WLEDs. 4. Conclusions
1
t0
It’s clearly seen that the h and Pda are linear with the concentration of acceptor (Eu3+) ions, while the increased rates gradually decrease, demonstrating that the dipole–dipole interaction is responsible for the energy transfer from Dy3+ to Eu3+ ions. When the concentration of Eu3+ reaches at 10%, the efficiency and probability increase to 53% and 1.25 103 S1, respectively, indicating that the energy transfer from Dy3+ to Eu3+ is highly effective and frequent. A summary of the emission and energy transfer process in LiY (MoO4)1.2(WO4)0.8:Dy3+, Eu3+ is showed schematically in Fig. 8. As seen from this energy level diagram, it is found that when Dy3+ ions are excited by 351, 365, 391 and 451 nm, electrons in the ground state (6H15/2) jump to the excited levels (7P7/2, 4P5/2, 4P13/2 and
In summary, we have successfully synthesized a single-phase and white light emitting novel phosphor LiY(MoO4)1.2(WO4)0.8:6% Dy3+,7%Eu3+ using citrate complexation and sol-gel method. XRD results reveal the influence of citric acid and calcination temperature on the crystal structure and the phase purity of the sample. The single-phase phosphor can be synthesized by using Rm/ c = 1:2.5 and being calcinated at 800 C. The PL emission spectra of these phosphors show three main peaks at 485, 572 and 612 nm, corresponding to the 4F9/2 ! 6H15/2 and 6H13/2 transitions of Dy3+ ions, and the 5D0 ! 7F2 transition of Eu3+ ions, respectively. What’s more, the luminescent intensities can be affected by the pH and doping concentrations of molybdenum and tungsten ions. Finally, the white light emitting LiY(MoO4)1.2(WO4)0.8 phosphor with good responsiveness to NUV light is obtained by tuning doping
Table 1 Photometric characteristic of LiY(MoO4)1.2(WO4)0.8:6%Dy3+, xEu3+ phosphors. Sample No.
x (doping concentration of Eu3+ ions, mol%)
CIE chromaticity coordinates x
y
1 2 3 4 5 6
0 1 3 5 7 10
0.366 0.374 0.311 0.314 0.334 0.362
0.449 0.421 0.395 0.360 0.322 0.312
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concentrations of Dy3+ and Eu3+ ions. The CIE coordinates of this phosphor (x = 0.334, y = 0.322) demonstrate that it can be a promising candidate for WLEDs. Acknowledgements This work was supported by the Natural Science Foundation of Shenzhen (No. JCYJ20150630145302231, JCYJ20150331101823677, JCYJ2013041144532138), the Shenzhen Peacock Plan (KQCX20140522150815065), and the Science and Technology Innovation Foundation for the Undergraduates of SUSTech (2015 19 and 2015 12). References [1] Y. Liu, Y. Wang, L. Wang, Y.-Y. Gu, S.-H. Yu, Z.-G. Lu, R. Sun, RSC Adv. 4 (2014) 4754–4762. [2] D. Chen, W. Xiang, X. Liang, J. Zhong, H. Yu, M. Ding, H. Lu, Z. Ji, J. Eur. Ceram. Soc. 35 (2015) 859–869. [3] C.-H. Huang, T.-S. Chan, W.-R. Liu, D.-Y. Wang, Y.-C. Chiu, Y.-T. Yeh, T.-M. Chen, J. Mater. Chem. 22 (2012) 20210–20216. [4] D. Chen, Y. Chen, Ceram. Int. 40 (2014) 15325–15329. [5] D. Chen, Y. Zhou, W. Xu, J. Zhong, Z. Ji, W. Xiang, J. Mater. Chem. C 4 (2016) 1704–1712. [6] X. Bai, G. Caputo, Z. Hao, V.T. Freitas, J. Zhang, R.L. Longo, O.L. Malta, R.A.S. Ferreira, N. Pinna, Nat. Commun. 5 (2014) 5702. [7] Y. Jia, R. Pang, H. Li, W. Sun, J. Fu, L. Jiang, S. Zhang, Q. Su, C. Li, R.-S. Liu, Dalton Trans. 44 (2015) 11399–11407. [8] M. Shang, C. Li, J. Lin, Chem. Soc. Rev. 43 (2014) 1372–1386. [9] A.K. Vishwakarma, K. Jha, M. Jayasimhadri, B. Sivaiah, B. Gahtori, D. Haranath, Dalton Trans. 44 (2015) 17166–17174.
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Single-phase LiY(MoO4)2x(WO4)x:Dy3+, Eu3+ phosphors with white luminescence for white LEDs Liangjun Zhoua,b , Wenxi Wanga , Sicen Yua , Bo Nana , Yinggang Zhua , Yang Shia , Haohong Shia , Xingzhong Zhaob , Zhouguang Lua,* a b
Department of Materials Science and Engineering, South University of Science and Technology of China, Shenzhen, China School of Physics and Technology, Wuhan University, Wuhan, China
A R T I C L E I N F O
Article history: Received 27 April 2016 Received in revised form 18 August 2016 Accepted 21 August 2016 Available online 26 August 2016 Keywords: A. Inorganic compounds A. Optical materials B. Sol-gel chemistry B. Luminescence D. Phosphors
A B S T R A C T
Single-phase and Dy3+/Eu3+ co-doped novel phosphor LiY(MoO4)2x(WO4)x:Dy3+,Eu3+, emitting white luminescence under near ultraviolet (NUV) light, was prepared through sol-gel method. The preparation and characterization of this phosphor were systematically studied by X-ray diffraction and spectrofluorophotometric measurements. The molar ratio of metal ions and citric acid (Rm/c) and calcination temperature greatly influenced the phase purity of sample, revealing that the pure phase could be obtained by using Rm/c = 1:2.5 and calcination temperature = 800 C. Under NUV excitation, the as-prepared phosphor exhibited the emissions of 485, 572 and 612 nm, which intensities could be affected by the pH and the concentrations of molybdenum and tungsten ions. By doping appropriate concentrations of Dy3+ and Eu3+, white light emitting phosphor LiY(MoO4)1.2(WO4)0.8:6%Dy3+,7%Eu3+ with good responsiveness to NUV light was obtained. The Commission Internationale de L’Eclairage chromaticity coordinates were calculated to be (x = 0.334, y = 0.322), close to the D65 illuminant (x = 0.313, y = 0.329), indicating the potential application for NUV WLEDs. ã 2016 Published by Elsevier Ltd.
1. Introduction White light-emitting diodes (WLEDs) have been considered as the next generation of solid state light sources, due to their superior properties such as lower energy consumption, more efficient output and environmental friendliness [1,2]. Currently, combining blue LED chip with YAG:Ce yellow-emitting phosphor or coating ultraviolet (UV) LED with red/green/blue emission phosphors is the main approach to obtain WLEDs [3–5]. So far, amounts of investigations have been made to develop novel phosphors for WLEDs. However, some challenges, which still exist in WLEDs, are to realize high luminescent efficiency, high chromatic stability, excellent color-rending properties, and competitive price against traditional implemented fluorescent lamps [6,7]. Hence, the single-phase phosphor has become the necessary pre-requisite for the fabrication of WLEDs to solve these above challenges [6–8]. In the recent years, a great number of efforts have been made to develop single-phase white-light-emitting phosphors for near
* Corresponding author. E-mail address: [email protected] (Z. Lu). http://dx.doi.org/10.1016/j.materresbull.2016.08.028 0025-5408/ã 2016 Published by Elsevier Ltd.
ultraviolet (NUV) WLEDs [6,9,10]. It is well known that rare-earth (RE) doped materials can emit different colors of luminescence such as red, green and blue or yellow which cover the whole visible light range [1,3]. Therefore, there are some different methods to realize white light emission from a single-phase host lattice including: (i) doping a single RE ion (Eu3+, Dy3+) into single phase hosts; (ii) co-doping two or more RE ions with different emissions simultaneously, such as Ce3+/Eu2+, Tm3+/Dy3+, Tm3+/Tb3+/Eu3+; (iii) co-doping different ions to control emission through energy transfer processes (EPT); and (iv) controlling the concentration of the defect and reaction conditions of defect-related luminescent materials [6,8,9,11,12]. Compared with other host lattices, molybdates and tungstates have drawn much attention for WLEDs, due to their good chemical and physical stability, high density and low phonon threshold energy. What’s more, the (MoO4)2 and (WO4)2 groups have strong absorption in the NUV region. These groups can transfer their absorbed energy to the dopant ions, making these phosphors exhibiting strong and broad absorption in the UV region. Therefore, these materials have been considered as excellent hosts for phosphor materials [13–16]. In the past years, these above scheelite-type crystalline structures (such as LiEu(MoO4)2 [14], LiLn(MO4)2:Eu [1], Tb2(WO4)3:Eu) [13] have attracted a great of
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Scheme 1. The brief synthetic process of white phosphor powder LiY(MoO4)2x(WO4)x:Dy3+, Eu3+.
attentions. A number of papers have been reported about molybdates/tungstates phosphors activated by RE ions. However, only a few of papers have been investigated on the complex molybdate–tungstates phosphors, in which luminescence can be affected by the ratio of molybdate and tungstate ions [15]. Because that Mo6+ and W6+ ions have similar ionic radii and can be substituted for each other, introducing changes in the energy transitions and luminescence intensities of RE ions. Hence, it is of scientific importance to investigate this influence systematically. Motivated by the above-stated studies and attempts to develop phosphors excitable by NUV light for the applications of WLEDs, herein we have successfully synthesized a single-phase and white emitting phosphor LiY(MoO4)2x(WO4)x:Dy3+, Eu3+ through a simple sol-gel method, based on the consideration that Li-based tungstate/molybdate materials exhibit more promising
luminescence performance than other K and Na-based counterparts [1]. Furthermore, we have systematically studied the preparation, luminescence and color chromaticity properties of a series of phosphors LiY(MoO4)2x(WO4)x:Dy3+, Eu3+. The influence of citric acid and calcination temperature on crystal structure is studied by XRD measurement. The pH of solution and the concentrations of molybdenum and tungsten ions are adjusted to realize the highest luminescent intensities under NUV light. Moreover, the appropriate doping concentrations of Dy3+ and Eu3+ make white light emitting phosphor well responsive to NUV light. 2. Experimental section Europium oxide (Eu2O3, 99.99%), dysprosium oxide (Dy2O3, 99.99%), ammonium molybdate tetrahydrate ((NH4)6Mo7O244H2O,
Fig. 1. XRD patterns of LiY(MoO4)2 synthesized with (a) different Rm/c (1:1, 1:2, 1:2.5, 1:5); and (b) different calcination temperature (600, 700, 800, 900 and 1000 C).
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Fig. 2. PL spectra of (a) LiY(MoO4)2: 5%Eu3+ obtained at different pH (1, 3, 5, 6 and 7); and (b) LiY(MoO4)2x(WO4)x: 5%Eu3+ synthesized with different value of x (0, 0.4, 0.8, 1.0, 1.2, 1.6 and 2.0).
AR), yttrium nitrate hexahydrate (Y(NO3)36H2O, 99.9%) and lithium nitrate (LiNO3) were purchased from Alfa Aesar of Thermo Fisher Scientific Inc. Ammonium metatungstate hydrate ((NH4)10W12O41xH2O, 99.5%), ethylene glycol (C2H6O2) and citric acid (C6H8O7H2O) were provided by Aladdin Inc. Nitric acid (HNO3) was supplied from Sinopharm Chemical Reagent Co., Ltd. All reagents were used without further purification. According to the reference [17], in the typical procedure (Scheme 1), stoichiometric amounts of raw materials europium (III) oxide (Eu2O3), dysprosium (III) oxide (Dy2O3) were dissolved in nitric acid (HNO3) under vigorous stirring, then the excess HNO3 was removed after heating. A suitable volume of deionized water, stoichiometric amount of LiNO3 and RE(NO3)3xH2O were added under stirring. Stoichiometric amounts of ammonium molybdate [(NH4)6Mo7O24903;4H2O], ammonium paratungstate [(NH4)10W12O41xH2O] and citric acid were dissolved in appropriate volume of deionized water under vigorous stirring. This solution was then mixed with the above solution after adjusting the pH value of the solution. Then suitable volume of ethylene glycol was added under stirring for a while. Next, the transparent solution was continuously stirred in water bath at 80 C for 2 h to obtain a transparent light-green gel. Then the gel was further dried at 180 C in an oven for 2 h to obtain a dried gel, which was calcinated in muffle furnace for 2 h in air to obtain a white powder. All the samples were characterized by powder X-ray diffraction (XRD) using a Rigaku D/Max-2400 X-ray diffractometer with Nifiltered Cu-Ka radiation. A step size of 0.02 was used at a scanning speed of 8 /min. The photoluminescence (PL) measurements were performed on Hitachi F-7000 fluorescence spectrophotometer equipped with a 150 W Xe lamp as the excitation light source. The fluorescence decay curves were obtained on a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz), with a tunable laser as the excitation source. The CIE chromaticity coordinates were
calculated by GoCIE. All of the measurements were performed at room temperature in air. All excitation and emission spectra were measured at room temperature with the same instrumental parameters for comparison. 3. Results and discussion To our knowledge, many researches have demonstrated that lots of small organic molecules are helpful for the control of crystal structure and morphology in the synthesis of matrerials [18–20]. Among kinds of organic additives, citric acid is considered as one of the most common and important organic molecules, which have been used as the stabilizer and structure-directing agents [1,21]. In order to study the effect of citric acid on the growth of LiY(MoO4)2, different amount of citric acid have been adopted in the reaction, and the results are displayed in Fig. 1(a). These curves, which are showed with a histogram (JCPDS No.17-773) in the figure, are the XRD patterns of LiY(MoO4)2 synthesized with different molar ratios of metal ions and citric acid (Rm/c), respectively. It is clear that the structures of all samples are mostly tetragonal structures, indexed to pure phase LiY(MoO4)2 according to the stand PDF card (JCPDS No.17-773). However, it is worth noting that some diffraction peaks, which are ascribed to the existence of Li2MoO3 (JCPDS No.21-517), can be observed in all samples except the one with Rm/c = 1:2.5. No peaks of Li2MoO3 are detected, revealing a high purity of the obtained powders. In the reaction, the citric acid acts as structure-directing reagent binding to the crystal surface. It directly affects the growth of different crystal facets, resulting in the formation of the crystal structure. This result reveals that the amount of citric acid is critical for the crystal growth and purity of LiY(MoO4)2 powders [18,21–23]. Furthermore, it is known that the calcination temperature is one of the important factors, which can affect the crystal structure
Fig. 3. (a) SEM and (b) TEM images of LiY(MoO4)1.2(WO4)0.8: 6%Dy3+, 5%Eu3+. The inset is HRTEM image.
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Fig. 4. The PL excitation spectra (lem. = 572 nm) of (a) LiY(MoO4)1.2(WO4)0.8: 6%Dy3+ and (c) LiY(MoO4)1.2(WO4)0.8: 6%Dy3+, 5%Eu3+; the PL emission spectra (lex. = 393 nm) of (b) LiY(MoO4)1.2(WO4)0.8: mDy3+ (m = 1%, 3%, 6%, 8% and 10%) and (d) LiY(MoO4)1.2(WO4)0.8: 6%Dy3+, xEu3+ (x = 1%, 3%, 5%, 7% and 10%).
of the resultants [1,22]. Hence, a series of different temperatures from 600 C to 1000 C have been used in the calcination during the synthesis of LiY(MoO4)2. The XRD patterns of LiY(MoO4)2 synthesized at different calcination temperatures are displayed in Fig. 1(b), indicating that the changes of the calcination temperature in the region of 600–1000 C slightly influence the structures of the resultant phosphors. Comparing with the JCPDS No.17-773 card, it can be found that all the patterns of the prepared powders contain all the peaks of tetragonal LiY(MoO4)2 (JCPDS No.17-773). However, some other peaks, which can be ascribed to the existence of Li2MoO3 (JCPDS No.21-517), can be observed in all patterns except the one calcined at 800 C. From the above results, it can be concluded that the LiY(MoO4)2 powders can be obtained in a wide
Fig. 5. The linear plot of log (I/x) versus log x of LiY(MoO4)1.2(WO4)0.8 doped with different concentrations of Dy3+ (1%, 3%, 6%, 8% and 10%).
temperature region from 600 to 1000 C, while the one with pure phase can only be synthesized at 800 C. As considering that pH and Mo/W ratio could affect the luminescent property of the phosphor [15], the PL spectra of 5% Eu3 + doped LiY(MoO4)2 are studied. As seen from Fig. 2(a), there are two major peaks around 589 nm and 612 nm, ascribed to 5D0 ! 7 F1 and 5D0 ! 7F2 transitions of Eu3+ ions, respectively [24]. The pHdependent emission spectra of LiY(MoO4)2:5%Eu3+ under 395 nm excitation indicate that the change of pH cannot cause changes in the shape or the position of the peak, but introduces the obvious change in the luminescence intensity, which decreases with the increase of pH value. Moreover, the effect of Mo/W ratios on PL property of the phosphor is showed in Fig. 2(b). As seen from the PL spectra of LiY(MoO4)2x(WO4)x: 5%Eu3+ contained different concentrations of Mo and W ions, the shapes and positions of the two major emission peaks remain the same. However, the intensity changes as the value of x increases from 0 to 2, and reaches at the highest when the value of x is 0.8. These results demonstrate that the suitable value of pH in the reaction is proved to be 1 and LiY (MoO4)1.2(WO4)0.8 is the best choice to be the host materials. Moreover, the morphology of LiY(MoO4)1.2(WO4)0.8:6%Dy3+, 5%Eu3 + phosphor has been observed by FE-SEM and TEM. As shown in Fig. 3a, the rod-like powders are of irregular shape and size, most of which are micrometer-scale. Furthermore, the TEM and HRTEM images (Fig. 3b) suggest the high crystallinity of LiY (MoO4)1.2(WO4)0.8:6%Dy3+, 5%Eu3+ phosphor due to the clear crystal lattice stripe image. To obtain the white emitting phosphor, different doping concentrations of Dy3+ and Eu3+ in this phosphor are studied. These LiY(MoO4)1.2(WO4)0.8:6%Dy3+ phosphors are used to measure the excitation spectrum at room temperature by monitoring the emission at 572 nm. As shown in Fig. 4(a), there are four intense
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Fig. 6. Decay curves of Dy3+ luminescence in LiY(MoO4)1.2(WO4)0.8:6%Dy3+ doped with different concentrations of Eu3+ (1%, 3%, 5%, 7% and 10%) excited at 393 nm and monitored at 572 nm.
excitation peaks in near UV region around 351, 365, 390 and 451 nm, which are attributed to characteristic of intra f–f transitions of Dy3+ ion from ground state 6H15/2 to excited states 6 P7/2, 6P5/2, 4I13/2 and 4I15/2, respectively [16,25]. Hence, it is quite evident that the phosphor can be excited by near UV light, indicating the potential to be used for NUV WLEDs. Fig. 4(b) displays the PL emission spectra of LiY (MoO4)1.2(WO4)0.8 doped with different concentrations of Dy3+ (1%, 3%, 6%, 8% and 10%) under NUV excitation of 391 nm. The nature of emission spectra of all samples are similar, containing two sharp and dominating peaks at 485 nm and 572 nm, corresponding to the 4F9/2 ! 6H15/2 and 4F9/2 ! 6H13/2 transitions of Dy3+ ions [16,25], respectively. These peaks are strongly influenced by local symmetry environment of Dy3+ ion in host lattice [26]. Fig. 4(b) suggests that the luminescence intensity is observed to increase when the dopant concentration changes from x = 1% to x = 6%, while a further increase of the doped Dy3+ ions subsequently weaken the luminescence intensity. This
concentration quenching is primarily caused by the promotion of non-radiative energy transfers among Dy3+ ions, including an exchange interaction, radiation reabsorption, a multipole–multipole interaction [25]. This quenching phenomenon can be explained by Dexter theory, which can be approximated as followed [27]: logðI =xÞ ¼ c k log x where, I is correspond for the luminescence intensity, while x is the dopant concentration. c is a constant for excitation condition for the given host material. And k is the index of electric multipole, corresponding to electric dipole–dipole, electric dipole-electric quadrupole and electric quadrupole-electric quadrupole interactions, when its value is 2,2.67 and 3.33, respectively. Based on the emission spectra obtained at 391 nm, Fig. 5 illustrates the correlation between log (I/x) and log x. Although the interaction of ions in concentration quenching is more complicated, analysis of the plot indicates that those data fit well with a straight line with a
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Fig. 7. Variation of energy transfer efficiency (h, curve a) and probabilities (Pda, curve b) as a function of Eu3+ concentrations (1%, 3%, 6%, 8% and 10%).
slope of 1.999. Thus, the value of k is 1.999, which is approximate to 2. According to the Dexter theory, this result clearly indicates that the concentration quenching of Dy3+ emission is mainly caused by the electric dipole–dipole interaction [28]. On the basis of the above analysis, LiY(MoO4)1.2(WO4)0.8:6%Dy3+ phosphor is selected for further investigation of the luminescent property by doping varied concentrations of Eu3+. Fig. 4(c) displays the PL excitation spectrum of LiY(MoO4)1.2(WO4)0.8:6%Dy3+, 5%Eu3 + with emission monitored at 572 nm, which is the prominent emission of Dy3+. A broad excitation peak appears between 225 and 350 nm, due to the charge transfer band (CTB) of O2 ! metal ions (Mo6+, W6+, Eu3+ and Dy3+) [24,29]. The three peaks around 393, 415 and 465 nm are attributed to the 4f-4f transition of Eu3+ ions (7F0 ! 5L6, 7F0 ! 5D3 and 7F0 ! 5D2), indicating that the asprepared phosphor can be efficiently pumped by NUV light. Moreover, comparing with the PL excitation spectrum of LiY (MoO4)2:6%Dy3+ displayed in Fig. 4(a), other obvious changes can be observed in the excitation spectrum after doping Eu3+. All the four intense excitation peaks (351, 365, 390 and 451 nm) of Dy3+
are weakened, while both of the two sharp peaks (393 and 465 nm) of Eu3+ are enhanced, indicating that the possibility of energy transfer from Dy3+ to Eu3+. Fig. 4(d) shows the PL emission spectra at room temperature of LiY(MoO4)1.2(WO4)0.8: 6%Dy3+ doped with different concentrations of Eu3+ (1%, 3%, 5%, 7% and 10%, mol%) excited at 393 nm. The intensities of emission peaks are obviously found to change with the varied doping concentrations of Eu3+, while the shapes and positions of these peaks stay the same. It is clearly observed that there are three dominating peaks around 485, 572 and 612 nm, ascribed to the 4F9/2 ! 6H15/2 and 4F9/2 ! 6H13/2 transitions of Dy3+ ions [25,26], and the 5D0 ! 7F2 transitions of Eu3+ ions [1,29], respectively. As seen from these spectra, the Dy3+ emissions are enhanced by the introduction of Eu3+ at the beginning, but then weakened when the concentration of doped Eu3+ is higher than 0.3%. This fact can be explained as followed. At the low doping concentration of Eu3+, the increase of activator ions induces more energy transitions among Dy3+ ions, resulting in a slight enhancement of Dy3+ emissions. However, as the concentration of doped Eu3+ ions increases from 0.3% to 1%, the intensities of Dy3+ emissions correspondingly slightly decrease while those of Eu3+ are somewhat enhanced, due to the energy transfer from the Dy3+ ions to the Eu3+ ions in the LiY(MoO4)1.2(WO4)0.8 host material [30]. In order to further evaluate the energy transfer mechanism from Dy3+ to Eu3+ ions in LiY(MoO4)1.2(WO4)0.8:Dy3+, Eu3+ phosphors, the fluorescence lifetimes of Dy3+ ions (lem. = 572 nm) have been examined. The decay curves of LiY(MoO4)1.2(WO4)0.8: Dy3+, Eu3+ monitored at 572 nm are exhibited in Fig. 6a–e. All the decay curves show the multiexponential feature, which can be well fitted by a double-exponential equation as follows [31]: IðtÞ ¼ A1 expðt =t 1 Þ þ A2 expðt =t 2 Þ where t 1 and t 2 are the short and long lifetimes for the exponential components, and parometers A1 and A2 are the fitting constants, respectively. Moreover, the average lifetimes for the 4F9/2 ! 6H13/2 emission of Dy3+ can be calculated according to the equation as follows [31]: t ¼ A21 t 21 þ A22 t 22 =ðA1 t 1 þ A2 t 2 Þ
Fig. 8. The energy levels diagram of Dy3+ and Eu3+ in LiY(MoO4)1.2(WO4)0.8: Dy3+, Eu3+.
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4
Fig. 9. The CIE chromaticity coordinates of LiY(MoO4)1.2(WO4)0.8:6%Dy3+ doped with different concentration of Eu3+ (1: 0%, 2: 1%, 3: 3%, 4: 5%, 5: 7% and 6: 10%) phosphors on 1931 CIE chromaticity diagram. The insets are the digital images of corresponding samples under UV light.
The values of t are presented in Fig. 6f, showing gradually decrease with concentration of doping Eu3+, which is in good agreement with the PL intensity studies of these phosphors. The energy transfer efficiencies (h) and probabilities (Pda), which are exhibited in Fig. 7, are calculated by the following formulas [7,32]: n ¼1
Pda ¼
1
t
t t0
I15/2), respectively. They transfer to lower excited levels until arrive at 4F9/2 through multiphonon assistance, and then continuously transfer to the ground states through radiation transitions of 4 F9/2 ! 6H15/2 (485 nm) and 4F9/2 ! 6H13/2 (572 nm), respectively. Moreover, the energy transfer and emission process in Eu3+ are also presented in this diagram. Under the excitation of near ultraviolet light, electrons transfer to the excited levels (5L7, 5L6 and 5D2), then relax to lower levels 5D0, and finally transfer to the ground state (7F1, 7F2 and 7F3), respectively. Furthermore, according to the previous studies about the phosphors co-doped with Eu3+ and other RE ions (such as Sm3+, Ce3+ and Dy3+) [7,11], the energy transfer indeed exist between the other RE ions and Eu, which is consistent with the results in Fig. 3d. As illustrated in Fig. 5, when the electrons of Dy3+ relax from the higher excited levels to 4F9/2, the energy transfers from Dy3+ to Eu3+ by the resonance. Hence, the emission intensity of Dy3+ partly decreases while that of Eu3+ slightly increases. The color coordinates, a commission internationale de l’eclairage (CIE) parameter, are calculated in order to show the photometric characteristics of the as-prepared phosphors. As shown in Fig. 9, the CIE 1931 chromaticity diagram of LiY (MoO4)1.2(WO4)0.8:6%Dy3+, xEu3+ (x = 0%, 1%, 3%, 5%, 7% and 10%, mol%) phosphors are presented, locating in the near white region and changing with the increasing concentration of doped Eu3+ ions. The two digital images, obtained by the camera of mobile phone, are corresponded to the samples of No. 1 and 5 under UV light, respectively, clearly indicating the luminescence change from green to white with increasing Eu3+ concentration. The detailed photometric characteristic of these phosphors are showed in Table 1. It is clearly suggested that the powders only doped with 6% Dy3+ show emission in canary yellow region (x = 0.366, y = 0.449), which is similar as the earlier report [16,25,26]. When the concentration of doped Eu3+ ions is 7%, the CIE coordinates of this phosphor (x = 0.334, y = 0.322) are very close to the D65 illuminant (x = 0.313, y = 0.329) [25]. Hence, this phosphor can be suggested for application as WLEDs. 4. Conclusions
1
t0
It’s clearly seen that the h and Pda are linear with the concentration of acceptor (Eu3+) ions, while the increased rates gradually decrease, demonstrating that the dipole–dipole interaction is responsible for the energy transfer from Dy3+ to Eu3+ ions. When the concentration of Eu3+ reaches at 10%, the efficiency and probability increase to 53% and 1.25 103 S1, respectively, indicating that the energy transfer from Dy3+ to Eu3+ is highly effective and frequent. A summary of the emission and energy transfer process in LiY (MoO4)1.2(WO4)0.8:Dy3+, Eu3+ is showed schematically in Fig. 8. As seen from this energy level diagram, it is found that when Dy3+ ions are excited by 351, 365, 391 and 451 nm, electrons in the ground state (6H15/2) jump to the excited levels (7P7/2, 4P5/2, 4P13/2 and
In summary, we have successfully synthesized a single-phase and white light emitting novel phosphor LiY(MoO4)1.2(WO4)0.8:6% Dy3+,7%Eu3+ using citrate complexation and sol-gel method. XRD results reveal the influence of citric acid and calcination temperature on the crystal structure and the phase purity of the sample. The single-phase phosphor can be synthesized by using Rm/ c = 1:2.5 and being calcinated at 800 C. The PL emission spectra of these phosphors show three main peaks at 485, 572 and 612 nm, corresponding to the 4F9/2 ! 6H15/2 and 6H13/2 transitions of Dy3+ ions, and the 5D0 ! 7F2 transition of Eu3+ ions, respectively. What’s more, the luminescent intensities can be affected by the pH and doping concentrations of molybdenum and tungsten ions. Finally, the white light emitting LiY(MoO4)1.2(WO4)0.8 phosphor with good responsiveness to NUV light is obtained by tuning doping
Table 1 Photometric characteristic of LiY(MoO4)1.2(WO4)0.8:6%Dy3+, xEu3+ phosphors. Sample No.
x (doping concentration of Eu3+ ions, mol%)
CIE chromaticity coordinates x
y
1 2 3 4 5 6
0 1 3 5 7 10
0.366 0.374 0.311 0.314 0.334 0.362
0.449 0.421 0.395 0.360 0.322 0.312
436
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concentrations of Dy3+ and Eu3+ ions. The CIE coordinates of this phosphor (x = 0.334, y = 0.322) demonstrate that it can be a promising candidate for WLEDs. Acknowledgements This work was supported by the Natural Science Foundation of Shenzhen (No. JCYJ20150630145302231, JCYJ20150331101823677, JCYJ2013041144532138), the Shenzhen Peacock Plan (KQCX20140522150815065), and the Science and Technology Innovation Foundation for the Undergraduates of SUSTech (2015 19 and 2015 12). References [1] Y. Liu, Y. Wang, L. Wang, Y.-Y. Gu, S.-H. Yu, Z.-G. Lu, R. Sun, RSC Adv. 4 (2014) 4754–4762. [2] D. Chen, W. Xiang, X. Liang, J. Zhong, H. Yu, M. Ding, H. Lu, Z. Ji, J. Eur. Ceram. Soc. 35 (2015) 859–869. [3] C.-H. Huang, T.-S. Chan, W.-R. Liu, D.-Y. Wang, Y.-C. Chiu, Y.-T. Yeh, T.-M. Chen, J. Mater. Chem. 22 (2012) 20210–20216. [4] D. Chen, Y. Chen, Ceram. Int. 40 (2014) 15325–15329. [5] D. Chen, Y. Zhou, W. Xu, J. Zhong, Z. Ji, W. Xiang, J. Mater. Chem. C 4 (2016) 1704–1712. [6] X. Bai, G. Caputo, Z. Hao, V.T. Freitas, J. Zhang, R.L. Longo, O.L. Malta, R.A.S. Ferreira, N. Pinna, Nat. Commun. 5 (2014) 5702. [7] Y. Jia, R. Pang, H. Li, W. Sun, J. Fu, L. Jiang, S. Zhang, Q. Su, C. Li, R.-S. Liu, Dalton Trans. 44 (2015) 11399–11407. [8] M. Shang, C. Li, J. Lin, Chem. Soc. Rev. 43 (2014) 1372–1386. [9] A.K. Vishwakarma, K. Jha, M. Jayasimhadri, B. Sivaiah, B. Gahtori, D. Haranath, Dalton Trans. 44 (2015) 17166–17174.
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