Luminescence enhancement of CaMoO^sub 4^:Eu^sup 3+^ phosphor by charge compensation using microwave sintering method

Abstract

CaMoO^sub 4^:Eu^sup 3+^ and CaMoO^sub 4^:Eu^sup 3+^,A^sup +^ (A = Li, Na, K) phosphors for light-emitting diode (LED) applications have been prepared by microwave sintering method (MSM), and their structure and luminescence properties are investigated. The influences of microwave reaction time and concentration of different kinds of charge compensation A^sup +^ and Eu^sup 3+^ on luminescence have also been discussed. The samples emit a red luminescence at 615 nm attributed to the ^sup 5^D^sub 0^[arrow right]^sup 7^F^sub 2^ transition of Eu^sup 3+^ under 464 nm excitation. It is observed that adding charge compensation A^sup +^ in the sample synthesis increases luminescence intensity. The optimized sample made with 32 mol% Li^sup +^ and 32 mol% Eu^sup 3+^ has an enhancement factor of 4 in photoluminescence compared to the sample made without charge compensation. The CIE (Commission Internationale de l´Eclairage) coordinates of Ca^sub 0.36^MoO^sub 4^:0.32Eu^sup 3+^,0.32Li^sup +^ are x = 0.661 and y = 0.339, which indicate that the obtained phosphor can be a promising red color candidate for white LED fabrications.

Journal of Advanced Ceramics2015, 4(4): 318325 ISSN 2226-4108 DOI: 10.1007/s40145-015-0168-7 CN 10-1154/TQ

ResearchArticle

LuminescenceenhancementofCaMoO4:Eu3+phosphorbycharge

compensationusingmicrowavesinteringmethod

Zhiping ZHOU, Yingsen YU, Xiaotang LIU*, Weihao YE, Guangqi HU, Bingfu LEI, Yun YAN

College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China

Received: May 27, 2015; Revised: August 09, 2015; Accepted: August 25, 2015 The Author(s) 2015. This article is published with open access at Springerlink.com

Abstract: CaMoO4:Eu3+ and CaMoO4:Eu3+,A+ (A = Li, Na, K) phosphors for light-emitting diode (LED) applications have been prepared by microwave sintering method (MSM), and their structure and luminescence properties are investigated. The influences of microwave reaction time and concentration of different kinds of charge compensation A+ and Eu3+ on luminescence have also been discussed. The samples emit a red luminescence at 615 nm attributed to the 5D07F2 transition of Eu3+ under 464 nm excitation. It is observed that adding charge compensation A+ in the sample synthesis increases luminescence intensity. The optimized sample made with 32 mol% Li+ and 32 mol% Eu3+ has an enhancement factor of 4 in photoluminescence compared to the sample made without charge compensation. The CIE (Commission Internationale de lEclairage) coordinates of Ca0.36MoO4:0.32Eu3+,0.32Li+ are x = 0.661 and y = 0.339, which indicate that the obtained phosphor can be a promising red color candidate for white LED fabrications.

Keywords: CaMoO4:Eu3+ phosphor; luminescence enhancement; charge compensation; microwave sintering; white light-emitting diode (LED)

1 Introduction

After the first commercial light-emitting diode (LED) based on GaN blue-emitting chip and Ce3+ activated yttrium aluminum garnet (YAG:Ce3+) yellow phosphor was fabricated in 1997 [1], white LEDs [2,3] have brought a new surge of revolution in illumination by replacing traditional incandescent or fluorescence lamps. With their advantages of high efficiency, long lifetime, and non-pollution, LEDs have been widely applied in lighting and display devices, including plasma display panel, field emission display, white light diode, etc. [47]. However, white LEDs with

GaN based blue-emitting chips and yellowred phosphors have poor color rendering index due to the deficiencies in red emission [8] in current market. In order to solve this problem, two ways are usually recommended for phosphor converted LEDs. The first one is to add red-emitting phosphor in the white LEDs based on blue-emitting chips and yellow phosphor system. The other is to adopt ultraviolet (UV) or near-UV LED chips and tricolor (red, green, and blue) phosphor system. The current commercial tricolor phosphors are mainly Y2O2S:Eu3+ for red, ZnS:Cu+,Al3+ for green, and BaMgAl10O17:Eu2+ for blue [9,10]. Nonetheless, the poor stability of Y2O2S:Eu3+ makes it less desirable due to the release of sulfide gas, lower efficiency, and shorter working lifetime under UV irradiation than the green and blue

* Corresponding author. E-mail: [email protected]

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phosphors [11]. Therefore, to exploit efficient red phosphors with high luminance, suitable chromaticity coordinates, and good stability in the near-UV region is an urgent and challenging research task.

Nowadays, some red phosphors, such as molybdates [12], tungstates [13], aluminates [14], and nitride phosphors [15], are attracting more and more attentionas they are able to overcome the drawback of poorcolor rendering index of cool white LEDs. It has been reported that Eu3+ activated CaMoO4 phosphor has higher efficient emission intensity under near-UV excitation, more satisfactory chromaticity coordinates,and better stability than sulfide phosphors [16]. However, its red emission intensity still needs to be improved. Compared to conventional research methods,such as the high temperature solid state reaction (SSR)and co-precipitation method, by which it may need several hours of calcination at 8001100 , microwave sintering is featured with faster heating up (about 30 min using a microwave oven of 800 W) andhas higher efficiency, better uniformity of sintering temperature, and an obvious effect of energy saving.

In this work, the effect of different charge compensations on the luminescence property of Eu3+

doped CaMoO4 has been investigated by the microwave sintering method (MSM). The result shows that charge compensations can significantly improve the emission intensity of Eu3+ doped CaMoO4, and the developed phosphor is expected to exhibit superior spectroscopic properties as a single-component UV-convertible phosphor in lighting devices for industrial application.

2 Experimental

2.1 Synthesis

A series of Eu3+ doped CaMoO4 phosphors were synthesized through MSM. CaCO3 (analytical reagent grade), MoO3 (99.95%), and Eu2O3 (99.99%) were used as reaction precursors. Stoichiometric amounts of CaCO3, MoO3, and Eu2O3 were mixed together and then put into a household microwave oven. The microwave was turned on with its full output (800 W) for 3560 min. For some cases, proper amounts of Li2CO3, Na2CO3, or K2CO3 were added as the charge compensators.

2.2 Characterization

The structures were analyzed on an XD-2X/M4600

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powder X-ray diffraction (XRD) with Cu K radiationwith a voltage of 36 kV and an electric current of 30 mA (Beijing Purkinje General Instrument Corporation). The XRD patterns were recorded in the range of 15 < 2 < 70. The microstructure and morphology of the phosphors were observed by Quanta 400/INCA/HKL Thermal FE Environment scanning electron microscope (SEM). The excitationand emission spectra were recorded by an RF-5301 fluorescence spectrometer equipped with a xenon lampas excitation source. The excitation slit and the emission slit were 3.0 nm and 3.0 nm, respectively. A centrifugal particle size analyzer LA-920 (HORIBA Instruments Inc.) was used to observe the distributionand size of the particles. All measurements were carried out at room temperature.

3 Resultsanddiscussion

3.1 PowderXRDandstructure

The XRD patterns of the typical Eu3+ doped CaMoO4 phosphors without and with different charge compensators are shown in Fig. 1. All of them are in agreement with the standard JADE Card PDF#29-0351 and basically maintain characteristic of scheelite structure of space group I41/a (No. 88) [17] with the cell parameters a = b = 0.5223 nm and c = 1.142 nm, implying that neither the doped Eu3+ ions nor charge compensators have apparent influence on the crystal structure.

The charge compensation mechanisms can be

Intensity (a.u.)

2 ()

Fig. 1 XRD patterns of the typical CaMoO4:Eu3+ and CaMoO4:Eu3+,A+ (A = Li, Na, K) phosphors synthesized by MSM for 30 min.

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explained as below. Two Ca2+ ions are replaced by one Eu3+ ion and one monovalent cation, 2CaCa EuCa+A

Ca, where A denotes a monovalent cation such as Li+, Na+, or K+ which is acting as a charge compensator. It can be seen that the peak shifts to lower 2 values as K+ or Na+ ions substitute Ca2+ ions, while the peak shifts to higher 2 values as Li+ ions substitute Ca2+ ions relatively [18]. These shifts should be attributed to the fact that the ionic radii of Na+

(118 pm) and K+ (138 pm) are larger than that of Ca2+ (112 pm), and the ionic radius of Li+ (92 pm) [19] is smaller than that of Ca2+. According to the Prague formula [20] 2dsin = n (d is the interplanar spacing, is the incident wavelength, is the angle between the incident light and the face of lattice), the smaller may be caused by the enlargement of interplanar spacing.

In Fig. 2(a), all diffraction peaks can be easily indexed to those of CaMoO4 scheelite structure. The phases where the peaks appear are almost the same.

However, obvious deviations can be observed in the expanded version of the XRD spectra in Fig. 2(b) when compared with the exact positions of the peaks from the standard JADE Card PDF#29-0351. The deviations reflect a slight contraction of the unit cell due to the difference in ionic size of Ca2+ (112 pm), Li+ (92 pm), and Eu3+ (107 pm) [21], which result in the distortion of scheelite structure.

The calcium molybdate crystal structure (Fig. 3) shows a high body-centered inversion symmetry [22]. This structure has eight symmetry elements, and a primitive cell includes two formula units of CaMoO4.

The Ca and Mo sites have S4 point symmetry. Each Ca site is surrounded by eight O sites in approximately octahedral symmetry. Each Mo site is surrounded by four equivalent O sites to form [MoO4]2 anions in approximately tetrahedral symmetry. Each O atom links with two Ca atoms and one Mo atom. There are two different CaO bond lengths in CaO8 and one MoO bond length in [MoO4]2. CaMoO4 has strong covalent bonds of MoO in [MoO4]2 molecular ionic units and weak coupling between [MoO4]2 anions and

Ca2+ cations [2326].

3.2 Sizedistributioncharacterization

The morphology and size of phosphors are important for their applications in the encapsulation of lighting devices since their size and shape would affect the luminescence properties, heat dissipation efficiency, and reliability when the phosphors are coated onto InCaN chips [27]. Typical SEM images of CaMoO4

samples prepared by MSM on full output (100%, 800 W) for 30 min are shown in Fig. 4(a) and Fig. 4(b) at different magnifications.

In Fig. 4(a), it can be seen that the phosphor powders are composed of slightly aggregated irregular

Intensity (a.u.)

x=0.40

x=0.36

x=0.32

x=0.28

x=0.24

x=0.20

2 ()

Intensity (a.u.)

x=0.40

x=0.36

x=0.32

x=0.28

x=0.24

x=0.20

b

c

2 ()

Fig. 2 (a) XRD patterns of the Ca12xMoO4:xEu3+,xLi+ phosphors synthesized by MSM for 30 min. (b) Enlarged

XRD patterns of the Ca12xMoO4:xEu3+,xLi+ phosphors from 26 to 32.

a

Fig. 3 Crystal structure of CaMoO4.

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(a)

(b)

Fig. 4 SEM images of CaMoO4:Eu3+ phosphors synthesized by MSM for 30 min: (a) 2000 magnification, (b) 8000 magnification.

particles which are possibly due to quick heat molding by microwave radiation. The crystalline grains of the phosphors have a mean diameter of 28 m. The enlarged SEM image in Fig. 4(b) shows that most of the particles are well dispersed octahedral shape which fits well the encapsulation requirement of white LEDs [27].

The particle size distribution shown in Fig. 5 is

narrow, suggesting an average particle diameter of 3.82 m, which is consistent with the SEM study. The particle size matches well with the phosphor size requirement for fluorescent lamp, indicating that the samples are promising for the fabrication of solid lighting devices.

3.3 Luminescenceproperties

Figure 6 displays the excitation (ex = 464 nm) and emission (em = 615 nm) spectra of Ca0.76MoO4: 0.24Eu3+ and Ca0.52MoO4:0.24Eu3+,0.24A+ (A = Li, Na, K). The excitation and emission of all the samples are similar in both shape and position. The excitation

spectra show that CaMoO4:Eu3+ has a broad charge transfer CT band (OMo) in the UV region from 230 to 350 nm. The narrow peaks from 350 to 480 nm are the transitions from Eu3+ ground state to excited state of 4f7 configuration. The absorptions at 381 and 464 nm are due to the 7F05L6 and

7F05D2 transitions of Eu3+ ions, respectively. They are stronger than other peaks and match well with the emission wavelengths of near-UV or blue LED chips [28], and are expected to improve the color rendering property of commercial white LEDs.

Under 464 nm excitation, the emission spectra show the Eu3+ characteristic emissions, such as 5D07F1 (593 nm) and 5D07F2 (615 nm). As shown in Fig. 6, the relative intensity of 5D07F2 is stronger than other peaks. The stronger red emission at 615 nm comes

em = 615 nm ex = 464 nm

Intensity (a.u.)

Fig. 6 Excitation and emission spectra of Ca0.76MoO4:

0.24Eu3+ and Ca0.52MoO4:0.24Eu3+,0.24A+ (A = Li, Na, K) synthesized by MSM for 30 min and Ca0.76MoO4:

0.24Eu3+ synthesized by SSR at 800 for 4 h.

Wavelength (nm)

Cumulative (%)

Fig. 5 Particle size distribution of Ca0.36MoO4:0.32Eu3+,

0.32Li+ phosphor.

Differential (%)

Size (m)

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from the electric dipole transition 5D07F2, while the weak orange emission located at around 593 nm is due to the magnetic dipole transition 5D07F1. According to the spin selection rule [28], if the Eu3+ ions occupy the host sites with inversion symmetry, optical transitions inside the 4f configuration are strictly forbidden as electric dipole transition, which means the electric dipole transition of 5D07F2 is forbidden. However, when the 4f configuration of Eu3+ is mixed with the opposite configurations 5d and 5g, or the symmetry is deviated from the centre of inversion, the spin selection rule is relaxed and the forbidden transition of 4f energy levels is partially relieved, which may result in the electric dipole transition of

5D07F2 and emit red light [29]. So it can be concluded that the Eu3+ ions are not located at the site with inversion symmetry for the phosphors.

The emission intensities of the phosphors synthesized by MSM with different charge compensators (Ca0.52MoO4:0.24Eu3+,0.24A+(A = Li, Na, K)) are comparable and respectively about 4.5, 3, 1.5 times stronger than that of the phosphor without charge compensation. It reveals that lithium is the best charge compensator for CaMoO4: Eu3+ because the ionic radius of Li+ (92 pm) [19] is smaller than that of Ca2+, which makes it more commodious to substitute Ca2+ ions. Besides, the red emission intensity of Ca0.76MoO4:0.24Eu3+ synthesized by MSM for 30 min is much stronger than that which is prepared by

SSR at 800 for 4 h. Consequently, an enhanced red emission is observed in charge compensated phosphor samples, and MSM gets higher efficiency comparing with SSR.

Figure 7 shows the excitation and emission spectra of Ca0.52MoO4:0.24Eu3+,0.24Li+ prepared by microwave sintering on full output (100%, 800 W) with the reaction time from 35 to 60 min. The emission intensity increases gradually as time increasing and then decreases beyond the reaction time of 55 min, as shown in the inset plot in Fig. 7. The Ca0.52MoO4:

0.24Eu3+,0.24Li+ prepared at 55 min is found to have the highest emission. Compared with traditional high temperature solid phase method using 800 for 4 h [30] or co-precipitation method which precalcines samples at 500 for 2 h and then maintains at 800 for 4 h [31], MSM has significant advantages.

For maintaining the charge balance of CaMoO4,

equal concentrations of Eu3+ and Li+ ions were used to compensate the charges of Ca2+. Figure 8 shows that

the excitation and emission intensities of the Ca12xMoO4:xEu3+,xLi+ have been strongly improvedby the increasing Eu3+ and Li+ dopant concentration. In addition, the ionic radius of Li+ (92 pm) is smaller thanthat of Ca2+, which guarantees the substitution of Ca2+ions and the entrance to lattice matrix without changing the shape and positions of spectrum peaks. However, this leads to the lattice distortion and symmetry reduction that improve the probability of the electric dipole transition 5D07F2. Thus, effective enhancement appears at red emission peak of 615 nm.

In general, the photoluminescence intensity increaseswith the concentration of the activators in its low concentration range but decreases in high concentration range due to concentration quenching [32]. As it is observed in Fig. 8, the intensities of all emissions are enhanced significantly as the Eu3+

concentration increases before it reaches 32 mol%. Thus concentration quenching occurs after this point,so this is the optimized Eu3+ concentration for maximum emission intensity for Ca12xMoO4:xEu3+, xLi+.

As shown in the inset of Fig. 8, the red emission intensity of Ca0.36MoO4:0.32Eu3+,0.32Li+ synthesizedby MSM for 30 min is no less than that which synthesized by SSR at 800 for 4 h. Furthermore, the samples synthesized by MSM are much more loose and very easy to handle, while that which synthesizedby SSR are too hard and difficult to grind. In addition,the optimized sample Ca0.36MoO4:0.32Eu3+,0.32Li+ synthesized by MSM has an enhancement factor of 4 in photoluminescence in comparison with the sample made without charge compensation compounded

Intensity(a.u.)

35 min 40 min 45 min 50 min 55 min 60 min

Intensity (a.u.)

Time (min)

em = 615 nm ex = 464 nm

Wavelength (nm)

Fig. 7 Excitation and emission spectra of Ca0.52MoO4:

0.24Eu3+,0.24Li+ synthesized by MSM at different sintering time. The inset shows the integrated emission intensity as a function of the reaction time.

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Intensity(a.u.)

Intensity (a.u.)

x

em = 615 nm ex = 464 nm

Wavelength (nm)

Fig. 8 Excitation and emission spectra of Ca12xMoO4: xEu3+,xLi+ (x = 0.200.48) synthesized by MSM for 30 min and synthesized by SSR at 800 for 4 h. The inset shows the integrated emission intensity as a function of the concentration of Eu3+ and Li+.

Fig. 9 CIE (1931) chromatic coordinates of CaMoO4: Eu3+,A+ phosphors. Numbers shown in the figure correspond to those described in Table 1.

Table 1 CIE chromatic coordinates of CaMoO4:Eu3+ phosphors excited at 464 nm

No. Composition CIE (x, y)

1

2

3

4

5

6

7

8

in the same way. All of these certify that charge compensator can lead to the luminescence enhancement and CaMoO4:Eu3+,Li+ can be heated internally and externally at the same time quickly and evenly with strong intensity, which is more time-saving as well as energy-saving than SSR. This indicates that the MSM should be considered competent for other metal molybdates or tungstates preparation.

Moreover, as the Eu3+ concentration increases, the maxima of broad charge transfer bands shifts from 292 to 310 nm (Fig. 8). As a general rule, the energy level that is influenced by the configuration of CaMoO4

changes between the ground state and the excited state or the first excited state and the ground state, which makes a difference in photon absorption or emission. Therefore, the wavelengths shift to right as shown in the photoluminescence excitation (PLE) spectra [33].

3.4 ChromaticityofCaMoO4:Eu3+,A+phosphor

The Commission Internationale de lEclairage (CIE) chromaticity coordinates of a phosphor play an important qualification sign for luminescence applications. Figure 9 and Table 1 provide a summary(in graphic and tabular forms, respectively) of the CIE chromaticity of a single-phased emission-tunable phosphor. The CIE chromaticity coordinates are calculated by CIE calculate software with the result ofx = 0.661 and y = 0.339 for the Ca0.36MoO4:0.32Eu3+, 0.32Li+ phosphor synthesized by MSM for 30 min. The obtained coordinates are very close to National Television Standard Committee (NTSC) CIE

(0.600, 0.392) (0.602, 0.389) (0.631, 0.364) (0.644, 0.353) (0.647, 0.351) (0.659, 0.340) (0.661, 0.339) (0.670, 0.330)

values for red (x = 0.670, y = 0.330). Therefore, the Ca0.36MoO4:0.32Eu3+,0.32Li+ red phosphor would be a promising candidate for future white LED fabrication.

4 Conclusions

In summary, CaMoO4:Eu3+ and CaMoO4:Eu3+,A+ (A =

Li, Na, K) phosphors have been synthesized by microwave sintering method, which exhibit red luminescence with the strongest emission and excitation peaks at 615 nm and 464 nm, respectively. The SEM images have shown that the crystalline grains of the phosphors have a mean diameter of 2 8 m, fitting well with the encapsulation requirementof white LEDs. The samples synthesized by MSM for55 min were found to have the highest emission. The introduction of charge compensators (Li+, Na+, K+) can enhance the luminescence enhancement of CaMoO4: Eu3+. The Ca0.36MoO4:0.32Eu3+,0.32Li+ phosphor, withthe average particle diameter of 3.82 m, shows the best red emission. All the results indicate that the

Ca0.76MoO4:0.24Eu3+ (SSR)

Ca0.76MoO4:0.24Eu3+ (MSM)

Ca0.52MoO4:0.24Eu3+,0.24K+ (MSM) Ca0.52MoO4:0.24Eu3+,0.24Na+ (MSM) Ca0.52MoO4:0.24Eu3+,0.24Li+ (MSM)

Ca0.36MoO4:0.32Eu3+,0.32Li+ (SSR) Ca0.36MoO4:0.32Eu3+,0.32Li+ (MSM)

NTSC standard red

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Ca0.36MoO4:0.32Eu3+,0.32Li+ phosphor would be very promising in the applications of white lighting.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No. 21271074), teamwork projects funded by Guangdong Natural Science Foundation (No. S2013030012842), and CAS-Foshan Cooperation Funding Program (No. 2012HY100685).

OpenAccess: This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.

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