Combustion synthesis and luminescence properties of Ca4Y6(SiO4)6O:Eu3+ red phosphor for white LEDs

JOURNAL OF RARE EARTHS, Vol. 31, No. 10, Oct. 2013, P. 957

Combustion synthesis and luminescence properties of Ca4Y6(SiO4)6O:Eu3+ red phosphor for white LEDs SUN Zhihua (ᄭᖫढ)1,2,3, WANG Minqiang (∾ᬣᔎ)1,2,*, SONG Xiaohui (ᅟᄱ䕝)1,2, JIANG Ziqiang (㩟㞾ᔎ)3 (1. Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education, Xi an Jiaotong University, Xi'an 710049, China; 2. International Center for Dielectric Research, Xi'an Jiaotong University, Xi'an 710049, China; 3. School of Materials Science and Engineering, Chang'an University, Xi'an 710061, China) Received 16 April 2013; revised 3 July 2013

Abstract: Eu3+ activated Ca4Y6(SiO4)6O phosphors were prepared by combustion synthesis method, and their morphologies and luminescent properties were investigated. Field scanning electron microscopy (FSEM) confirmed that the crystallite sizes of nanoparticles with narrow diameter ranging from 30 to 60 nm. The excitation spectra of Ca4Y6(SiO4)6O:Eu3+ showed that there existed two strong excitation bands at around 399 nm (7F05L6) and 469 nm (7F05D2), which were consistent with the output wavelengths of near-UV and blue LEDs, respectively. The emission spectra of Ca4Y6(SiO4)6O:Eu3+ were dominant by a red peak located at 614 nm due to the 5D07F2 transition of Eu3+. With the increase of Eu3+concentration, the luminescence intensity of the red phosphor reached maximum and then decreased. The optimum concentration for Eu3+ in Ca4Y6(SiO4)6O was 21 mol.%. Keywords: Ca4Y6(SiO4)6O:Eu3+; combustion synthesis; luminescent characteristics; white LED; rare earths

White light-emitting diodes (LEDs) are considered as the next generation light source for theirs low electric consumption, environmental friendliness, high brightness, long lifetime, good reliability, fast response, etc.[1–3]. In recent years, near-ultraviolet (NUV) InGaN-based LEDs, which range from 350 to 420 nm received more attention because NUV-LED combines a NUV chip with red, green, and blue (RGB) phosphors to generate warm white lights and can offer a highly efficient solid-state lighting[4,5]. However, the luminescent effect of red phosphor is lower than that of the green and blue phosphor at UV excitation region, and the most widely used red phosphor (such as Y2O2S:Eu3+) is also chemically unstable[6,7]. Thus, researchers are interested in the red phosphor with higher chemical stability that can be excited by NUV light. Compared with the sulfides or oxysulfides widely used for phosphor host in current tricolor phosphors for NUV LEDs, the silicate system is chemically stable in ambient environments and has been used as host materials in recent years. Ca4Y6(SiO4)6O is a ternary rare-earth-metal silicate with the oxyapatite structure (hexagonal, space group P63/m), which is very suitable for luminescent ions[8]. The most prominent structural characteristic is the two rare earth sites which are 4f (C3) with nine-coordination and 6h (Cs) with seven-coordination. Both sites are very suitable for the luminescence of rare earth ions due to their low symmetry features. It is reported that the dopant ions occupy the 4f site and 6h site simultaneously in the

silicate oxyapatites[9]. The conventional method for the synthesis of Ca4Y6(SiO4)6O is the solid-state reaction involving high temperature calcinations (1623–1873 K) of the mixture of oxides. The main drawbacks of this method are the high energy consumption (high temperature needed), long reaction time, large particle size and broad particle size distribution, agglomeration of powders, and low product purity. It is known that the surface crystallization, size, and morphology of particles affect the properties of phosphors intensely. Surface perfect and spherical shaped phosphor always has high packing density and smoother light intensity distribution[10–12]. Therefore, it is still a challenge to obtain Ca4Y6(SiO4)6O:Eu3+ phosphor with ideal luminescence intensity through a relatively simple synthesis process at low temperature. Solution combustion synthesis is a fast and effective method for producing fine and nanosized oxide powders[13–15]. Herein, we synthesized Ca4Y6(SiO4)6O:Eu3+ nanoparticles by the combustion method and the photoluminescence properties were investigated to explore the applications for LEDs.

1 Experimental Ca4Y6(SiO4)6O:Eu3+ nanoparticles were prepared by solution-combustion synthesis. The starting materials included Si(OC2H5)4 (A.R.), Ca(NO3)2·4H2O (A.R.), Y(NO3)3·6H2O (A.R.), Eu2O3 (99.99%) and CO(NH2)2 (A.R.). Y and Ca solutions were prepared by dissolving

Foundation item: Project supported by National Natural Science Foundation of China (91123019) * Corresponding author: WANG Minqiang (E-mail: [email protected]; Tel.: +86-29-82668679) DOI: 10.1016/S1002-0721(12)60385-8

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Y(NO3)3·6H2O, Ca(NO3)2·4H2O in deionized water. Eu solutions was prepared by dissolving Eu2O3 in nitric acid. Si(OC2H5)4 was dissolved in C2H6O. CO(NH2)2 was also dissolved in water. The correct stoichiometric amounts of each solution were mixed into a 100 mL beaker. The precursor solution was vigorously stirred under heating (85 ºC) until it became gelatinous, then was removed into a crucible and introduced into a muffle furnace maintained at different temperatures (550, 600, 650 ºC). Initially, the solution underwent dehydration; then, spontaneous ignition occurred followed by smoldering combustion with enormous swelling. This process produced foamy and voluminous products followed by large amounts of gases. The whole process lasted for about 5 min. Under these conditions one of the possible reaction of these processes is as follows: 2CO(NH2)2+5O22CO+4NO+4H2O (1) The phase composition and phase structure were characterized by X-ray diffraction (XRD) (Rigaku D/2400 Cu K, 40 kV, and 100 mA). The morphology of the samples was inspected using a field scanning electron microscope (FSEM, S-4800, Hitachi). The composition of the samples were inspected using an electron probe microanalyzer (EPMA, JXA-8100, Jeol) equipped with an energy dispersive X-ray spectroscope (EDS, INCA, Oxford). The photoluminescence properties of the phosphor nanoparticles were performed with fluoroSENS 9000 system at room temperature.

nanoparticles annealed at different temperatures are shown in Fig. 2. When the precursor sample is ignited at 550 ºC (Fig. 2(1)), the diffraction peaks appear, all of them can be assigned to the Ca4Y6(SiO4)6O hexagonal crystalline phase based on the data of the JCPDS file (No. 27-0093), indicating the formation of Ca4Y6(SiO4)6O nanocrystals. After annealing at 600 ºC (Fig. 2(2)) and 650 ºC (Fig. 2(3)), all the diffraction peaks increase in intensity due to grain growth of Ca4Y6(SiO4)6O crystallites. No second phase is detected at this doping level, indicating that the Eu3+ can be completely doped into the Ca4Y6(SiO4)6O host lattice by substitution for the Y3+ in the host lattice. The samples prepared by combustion synthesis method had poorer crystallinity than the samples prepared by conventional solid-state reaction. So, the luminescence performances of the samples prepared by combustion synthesis method is poorer than the samples prepared by conventional solid-state reaction for its low crystallinity. It is noticed that the diffraction peaks (2) shifts lightly to lower degree comparing with those of the standard PDF card, which indicates the expansion of lattice parameters. According to the equation of 1/d2=(h2+k2+l2)/a2, the lattice constant of calcined Ca4Y5.94(SiO4)6O:0.06Eu3+ nanoparticles is calculated to be 0.9382 nm, which is a little larger than that (a=0.9356 nm) of the blank hexagonal Ca4Y6(SiO4)6O phase. This expansion may be attributed to the Y3+ ions (r=0.096 nm, CN=7) replaced by Eu3+ ions (r=0.101 nm, CN=7) with larger ionic radius. The crystallite sizes calculated from the Scherrer’s formula is 48 nm, coincides well with the measured value (Fig. 1).

2 Results and discussion 2.1 Morphology of Ca4Y6 (SiO4)6O:Eu3+ phosphor

2.3 Excitation spectra of Ca4Y6(SiO4)6O:Eu3+ phosphor

The FSEM image and EDS of nanoparticles are shown in Fig. 1. It can be seen that the phosphor powders are irregular spherical particles, the surfaces shows a lot of voids and pores formed by the escaping gases during combustion reaction, the sizes of nanoparticles range from 30 to 60 nm. The EDS characterized the composition of the nanoparticles annealed at 650 ºC as shown in Fig. 1, ascribed to Ca4Y6(SiO4)6O:Eu3+. The presence of O, Si, Y, Ca and Eu preliminary indicates the formation of Ca4Y6(SiO4)6O:Eu3+ in the nanoparticles.

Fig. 3 exhibits the excitation spectrum of Ca4Y5.94(SiO4)6O:0.06Eu3+ monitored at 614 nm. The sharp lines are assigned to the transitions between 7F0 and 5HJ, 5D4, 5L7,6 and 5D3,2,1,0, consisting of 7F05H3 (368 nm), 7F05D4 (368 nm), 7F05L7 (388 nm), 7F0 5 L6 (399 nm), 7F05D2 (469 nm), 7F05D1 (510 nm) and 7 F05D0 (537 nm)[16–18]. Evidently, the absorptions of 7 F05L6 (399 nm) and 7F05D2 (469 nm) are stronger than other 7F0 excitation transition peaks. The excitation band centered at about 299 nm could be attributed to the

2.2 Structure of Ca4Y6(SiO4)6O:Eu3+ phosphor The XRD patterns of Ca4Y5.94(SiO4)6O:0.06Eu3+

Fig. 1 FSEM image (a) and EDS (b) of Ca4Y5.94(SiO4)6O:0.06Eu3+ nanoparticles ignited at 650 ºC

Fig. 2 XRD patterns of Ca4Y5.94(SiO4)6O:0.06Eu3+ annealed at different temperatures (1) 550 ºC; (2) 600 ºC; (3) 650 ºC

SUN Zhihua et al., Combustion synthesis and luminescence properties of Ca4Y6 (SiO4)6O:Eu3+ red phosphor …

Fig. 3 Excitation spectrum of Ca4Y5.94(SiO4)6O:0.06Eu3+ (Oem= 614 nm)

charge-transfer (CT) transition of Eu3+O2– [18–20]. The excitation intensity at 399 nm was much higher than that at 299 nm, indicating that there is a weak non-radioactive absorption and high energy conversion. Generally, in many Eu3+-doped compounds, the CT transition is always considerably more intense than f-f excitation of Eu3+. However, in this case, the host defect emission in 340–550 nm overlapped the f-f excitations of Eu3+ and thus these defect emission centers in Ca4Y6(SiO4)6O host may play important roles as sensitizers for effective energy transfer from host to f-f levels of Eu3+. The bands between 340 and 430 nm are observed, which means the Ca4Y6(SiO4)6O:Eu3+ phosphor exhibits a satisfactory red performance under UV-vis excitation. The band centred at 469 nm is observed in the excitation spectrum, which means the phosphor has sufficient absorption at the emission wavelength of blue diode. It is suitable for a red light source, which may compensate for the red deficiency of the white LED output light. This result suggested that the Ca4Y6(SiO4)6O:Eu3+ phosphor would be a promising red candidate for application in LEDs. 2.4 Emission spectra of Ca4Y6(SiO4)6O:Eu3+ phosphor Fig. 4 presents the emission spectra under excitation

Fig. 4 Emission spectrum of Ca4Y5.94(SiO4)6O:0.06Eu3+ (ex= 399 or 469 nm)

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(ex=399 or 469 nm) of the Ca4Y5.94(SiO4)6O:0.06Eu3+ nanoparticles obtained at 650 ºC. The emission spectra of the samples consist of group of lines in the =550–725 nm. Under 399 nm excitation, the spectral region corresponds to the typical emission of Eu3+: 566, 579, 591, 614, 653, 704 nm; under 469 nm excitation, the peaks at 570, 579, 590, 614, 670, 704 nm[21–23]. It is meaningful that these phosphors can strongly absorb the light from UV to visible region and transfer the excitation energy to the red radiation. Under 399 or 469 nm excitation, the most intense emission line of Eu3+ peaks at 614 nm corresponded to 5D07F2 forced electric transition of Eu3+, indicating the absence of inversion symmetry the sites occupied by the Eu3+ ions. This is in agreement with the oxyapatite structure (space group, P63/m) which provides low symmetry sites for Eu3+ ions, i.e. the 9-coordinated 4f (C3) site and/or 7-coordinated 6h (Cs) site. The emission intensity of the 5D07F2 transition is much stronger than the transition 5D07F1, the dominant emission of transition of 5D07F2 resulting in Eu3+ activated compounds Ca4Y6(SiO4)6O showing strong red emission under 399 or 469 nm excitation. The Ca4Y6 (SiO4)6O:Eu3+ phosphor is a separate red light for the white LED that can be fabricated by employing red, green and blue emitting phosphors excited by UV chip, and can compensate for the red deficiency of the output light of the white LED that was combined with a blue InGaN chip based on the yellow phosphor materials. Therefore, Ca4Y6(SiO4)6O:Eu3+ phosphor is a promising red phosphor for white LEDs. 2.5 Effect of Eu3+ concentration on red emission of Ca4Y6(SiO4)6O:Eu3+ phosphor The effect of doped-Eu3+ concentration on the emission intensity of Ca4Y6(SiO4)6O:Eu3+ was also investigated. The variations of PL intensity with different Eu3+ contents are shown in Fig. 5. The intensity of the emission transition was found to increase along with the increase in the Eu3+ concentration and then reaches a maximum value, while decreases over a critical quenching concentration. So the optimum concentration for Eu3+ in Ca4Y6 might be elucidated by the following two factors: (1) ex-

Fig. 5 Influence of Eu3+ content on emission intensity of Ca4Y6(SiO4)6O:Eu3+

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(SiO4)6O is 21 mol.%. The concentration quenching cita-

References:

tion migration due to the resonance between the activators is enhanced when the doping concentration is increased, and thus the excitation energy reaches quenching centers; (2) activators are paired or coagulated and are changed to quenching center[24]. The two characteristic emissions of Eu3+ ions are located at 614 nm (5D0 7 F2) and 590 nm (5D07F1), the emission at 614 nm is from a hypersensitive forced electric-dipole transition which depends on the symmetries of the local environment around Eu3+ ions[25], and is allowed only at the low symmetry without inversion center. The emission at 590 nm originates from a magnetic-dipole transition is insensitive to the local symmetry, and this transition is independent of the symmetry and the site occupied by Eu3+ ions in the host. Their relative intensity is determined by the crystal structure. For the hexagonal structure Ca4Y6(SiO4)6O, as state above, has two rare earth sites which are 4f (C3) with nine-coordination and 6h (Cs) with seven-coordination. The emission from 5D07F2 (614 nm) will be dominant in the hexagonal structure Ca4Y6(SiO4)6O due to the low symmetries of the local crystal fields around Eu3+ ions, which is evident in Fig. 5 (the intensity at 614 nm is stronger than that at 590 nm). The 5D07F2/5D07F1 intensity also increases with the doped-Eu3+ concentration increasing. This result can be attributed to that the local symmetry environment of Eu3+ ions decreases with the doped-Eu3+ concentration increasing. This may be related to lattice distortion because of more Eu3+ ions in the Ca4Y6(SiO4)6O obtained at higher doped-Eu3+ concentration.

3 Conclusions Eu3+ activated Ca4Y6(SiO4)6O nanoparticles phosphors were prepared by combustion method. The synthesized phosphor particles were irregular spherical particles, the sizes of nanoparticles ranged from 30 to 60 nm. The Ca4Y6(SiO4)6O:Eu3+ nanoparticles phosphor was effectively excited by NUV (399 nm) and blue light (469 nm), which matched well with the emission wavelength of NUV LED chip, and the dominated emission peak at 614 nm, was very suitable for the white LED. The concentration dependence of the emission intensity showed that the optimum doping concentration of Eu3+ was 21 mol.%. The luminescence performances could be greatly improved through post-fabrication heat treatment in the future. But heat treatment could not be carried out at high temperature, such treatment would always lead to quite a significant growth of the crystallites, the merit of nanocrystalline materials would be weaken. Acknowledgments: The authors would like to thank the support from the Special Fund for Basic Scientific Research of Central Colleges, Chang’an University.

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