A novel green luminescent material AlPO4:Tb3+

Materials Letters 65 (2011) 1853–1855

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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

A novel green luminescent material AlPO4:Tb3+ Yufeng Liu ⁎, Zhiping Yang College of Physics Science and Technology, Hebei University, Baoding 071002, China

a r t i c l e

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Article history: Received 21 February 2011 Accepted 28 March 2011 Available online 1 April 2011 Keywords: Luminescence Phosphors Tb3+

a b s t r a c t A novel green phosphor Tb3+ doped AlPO4 was synthesized by conventional solid-state reaction method. The phosphor showed prominent luminescence in green due to the 5D4–7F5 transition of Tb3+. Structural characterization of the luminescent material was carried out with X-ray powder diffraction (XRD) analysis. The XRD measurements indicated that there are no crystalline phases other than AlPO4. Luminescence properties were analyzed by measuring the excitation and photoluminescence spectra. Photoluminescence measurements indicated that the phosphor exhibited bright green emission at about 542 nm under UV excitation. It is shown that the 3 mol% of doping concentration of Tb3+ ions in AlPO4:Tb3+ phosphor is optimum. The measured chromaticity for the phosphors AlPO4:Tb3+ under UV excitation is (0.32, 0.53). © 2011 Elsevier B.V. All rights reserved.

1. Introduction Rare earth (RE) ion doped phosphates have been paid intense attention for a wide range of applications, including laser materials, optical amplifiers, optical data storage and luminescent materials because of their significant advantages, such as low cost, thermally stable and good electrochemical performance [1]. Additionally, the phosphates have their host absorption edge at rather short wavelengths which make them suitable as the host for active RE ions. Recently, extensive research has been carried out on RE-doped phosphors of LnPO4 (Ln = Y,La,Gd,Lu) [2–4]. AlPO4, as a well-known artificial material, can be used to synthesize molecular sieves of various structures because tetrahedral AlO4 and PO4 are linked alternately in three dimensions to be a regular pore structure with no defect. Zhang et al. first reported the partial substitution of Al3+ by Eu3+ in a small-pore (eight-membered ring channel) microporous aluminophosphate AlPO4-CJ2 [5]. Yan and co-workers reported the substitution of RE elements (Eu,Ce,Tb) on the large-pore aluminophosphate and mesoporous aluminophosphate [6]. These materials have potential applications in catalysis, phosphors, lasers, optical amplifiers, filters, and optical memories. However, to the best of our knowledge, few attentions have been paid to the luminescent properties of RE doped AlPO4 as a luminescent material. Tb3+-doped materials have been widely used as green emitting phosphors due to their intense 5D4–7F5 emission in the green spectral region. Previous studies have shown that Tb3+-doped phosphates, aluminates or borates exhibit relatively strong absorption in the nearUV region and intense green emission with good color purity [7]. The aim of this work is to report our investigation results on the synthesis,

photoluminescence and color chromaticity of the new green AlPO4:Tb3+ phosphors and their corresponding spectroscopic properties under UV excitation. 2. Experimental Microcrystalline powder of AlPO4:Tb3+ phosphors was prepared by high temperature solid-state reactions. Reagents Al2O3(99.9%), NH4H2PO4(99.9%) and Tb4O7(99.99%) were used for sample preparations. They were weighted on stoichiometric ratio and ground by using an agate mortar and pestle. In order to mix the raw materials homogenously, during grindings, small amount of alcohol was added. After grindings, the mixtures were preheated at 900 °C for 10 h in a muffle furnace, then reground thoroughly after being cooled down to the room temperature. Consideration of the substitution of Tb3+ for Al3+ may not be easy because of the large difference in ionic radii between them, the long firing time was necessary, so the second firing was conducted at 1200 °C for 48 h. The products were then obtained by cooling to room temperature in the furnace, ground, and pulverized for further measurements. The structure of AlPO4:Tb3+ phosphor was identified by recording the powder X-ray diffraction patterns using AXS D8 automatic diffractometer with Cu Kα1 radiation (λ = 1.5405 Å). Excitation and emissions spectra were collected in a fluorescence spectrophotometer (Hitachi F-4600) equipped with a 150 W Xenon lamp as an excitation source. The chromaticity data were taken by using the PMS-50 spectra analysis system. All the measurements were conducted at room temperature. 3. Results and discussions

⁎ Corresponding author. Tel./fax: +86 312 5079423. E-mail address: [email protected] (Y. Liu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.03.103

AlPO4 has an orthorhombic crystal structure with a space group of C2221 (No. 20). The lattice parameters were a = 7.082 Å, b = 7.098 Å,

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Y. Liu, Z. Yang / Materials Letters 65 (2011) 1853–1855

(a)

c = 6.993 Å, V = 351.52 Å3 and Z = 4. Fig. 1 represents the XRD patterns of AlPO4:0.03Tb3+, JCPDS data (No.11-0500) and the sketch map of the AlPO4 crystal structure. The comparison of measured powder XRD patterns of AlPO4:0.03Tb3+ with that of JCPDS data (No.11-0500) indicates that the as-synthesized products were well crystallized with the AlPO4 structure and no (detectable) additional phases (such as Tb4O7, TbPO4) or other crystalline impurity phase present. The concentration of Tb3+ is 3 at.% in the as-synthesized AlPO4 materials. The substitution of Al3+ by the Tb3+ ion is expected to be random in the framework. Therefore, no obvious differences in the XRD peaks positions are noted between the as-synthesized material and the JCPDS data (No.11-0500). To further rule out the existence of the TbPO4 in the final products possibility, additional experiment was performed by synthesizing the TbPO4 for spectral comparison. The fluorescent pattern of the TbPO4 (not shown) is very different from that of the AlPO4:Tb3+. This difference along with XRD results indicates that no TbPO4 exists in the final product AlPO4:Tb3+. Fig. 2a exhibits the UV excitation spectrum of the sample AlPO4:0.03Tb3+ by monitoring the emission at 542 nm. It can be seen that the excitation spectrum consists of broad bands in the range from 200 to 275 nm and a series of sharp lines (282, 300, 316, 349 and 375 nm) in the range from 275 to 400 nm. The broad bands peaking at 219 and 250 nm represent the spin-allowed and spin-forbidden 4f8–4f75d1 transitions, respectively. The sharp lines represent the spin-forbidden 4f–4f transitions [8]. Fig. 2b shows the emission spectra of AlPO4:0.03Tb3+ phosphors. The excitation wavelengths are 219 and 375 nm. The emission intensity corresponding to the 375 nm excitation is remarkably lower than that of 219 nm because of the relatively lower absorption at this wavelength. In the emission spectra of AlPO4:0.03Tb3+, two peaks at 488 and 495 nm (5D4–7F6), two peaks at 542 and 548 nm (5D4–7F5), two peaks at 583 and 594 nm (5D4–7F4), one peak at 621 nm (5D4–7F3), and one peak at 665 nm (5D4–7F2,1,0) were observed [2,8]. The intensity of the peak at 542 nm is stronger than that of the peak at 548 nm. The variation of emission intensity (5D4–7F5 transition of Tb3+ at 542 nm) of AlPO4:xTb3+ as functions of Tb3+ concentration under 375 nm and 219 nm is shown in Fig. 3a. The highest integrated emission intensity is noted at the Tb3+ concentration of x = 0.03 which is taken as the critical concentration. Lower doping concentrations and excessive doping lead to weak luminescence and concentration quenching of the Tb3+ emission, respectively. The fluorescence mechanism of Tb3+ in AlPO4:Tb3+ phosphors can be proposed as follows. The emission intensity (I) per activator ion follows the equation [9]:

where x is the activator concentration, I/x is the emission intensity (I) per activator (x). K and β are constants for a given host under the same excitation condition. According to Eq. (1), θ = 3 for the energy transfer among the nearest-neighbor ions (exchange interaction), while θ = 6, 8, and 10 for dipole–dipole (d–d), dipole–quadrupole (d–q), or quadrupole–quadrupole (q–q) interaction, respectively. Assuming that β(x) θ/3 » 1, Eq. (1) can be simplified as follows:

h i θ = 3 −1 I=x = k 1 + βðxÞ

log I=x = K ′ −θ =3 log x

ð1Þ

4000

219 nm

4f

λem=542 nm

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0 200

250

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3+

439 nm

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282 nm

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350

400

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(b) 5000

D4 -7F5

5

D4 -7F6 500

550

-7F0,1,2 4

665nm 5D

D4 - F3 621nm

5

5

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0 450

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5

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AlPO4:0.03Tb3+

488 nm 495 nm

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4000

3000

λex=219 nm

542 nm 548 nm

600

650

700

Wavelength (nm) Fig. 2. The Excitation spectrum (λ em = 542 nm) (a) and Emission spectra (λex = 219 nm and 375 nm) (b) of the AlPO4:0.03Tb3+ phosphor.



 K ′ = log K–log β :

ð2Þ

Fig. 1. The XRD patterns of AlPO4:0.03Tb3+ phosphors (a: AlPO4:0.03Tb3+ phosphors, b: Standards card JCPDS:11–0500)(Left) and the sketch map of the AlPO4 crystal structure (right).

Y. Liu, Z. Yang / Materials Letters 65 (2011) 1853–1855

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(a) λex=375 nm

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60 40 20 0 0.00

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log (I/xTb3+)

3.6 3.4 3.2 3.0 2.8 2.6 -2.4

-2.2

-2.0

-1.8

log

-1.6

-1.4

-1.2

-1.0

-0.8

(xTb3+)

Fig. 3. The emission intensity of AlPO4:Tb3+ phosphor with different Tb3+ concentration (a) and the plot of log(xTb3+) vs log(I/xTb3+) in AlPO4:xTb3+ phosphor (λex = 219 nm) (b).

From the slope of Eq. (2), the electric multipolar character (θ) can be obtained by the slope (−θ/3) of the plot log(I/x) vs log x. Since the critical concentration of Tb3+ has been determined as 3 mol%, the dependence of the emission intensity of the AlPO4:Tb3+ phosphor excited at 219 nm on the doped-Tb3+ concentration which is not less than the critical concentration (3 mol%) is determined (see Fig. 3b). It can be seen from Fig. 3b, that the dependence of log(I/x) on log x is linear and the slope is −1.95. Thus, the value of θ can be calculated as 5.85 (≈6), which means that the d–d interaction is the main mechanism for the concentration quenching in the Tb3+-doped AlPO4 phosphor. Fig. 4a shows the CIE 1931 chromaticity coordinates of AlPO4:0.03Tb3+ phosphors. The CIE chromaticity coordinates of commercial greenemitting LaPO4:Ce3+,Tb3+ phosphor are (0.34, 0.57). The measured chromaticity for the phosphors AlPO4:0.03Tb3+ is (0.32, 0.53) under excitation at 219 nm. They are in the almost same green region, indicating that the AlPO4:Tb3+ sample had good color coordinate as a green phosphor. Fig. 4b shows the excitation and emission spectra of AlPO4:Tb3+, LaPO4:Ce3+,Tb3+ and AlPO4:Ce3+,Tb3+. For comparison, all measurements were performed with the identical instrumental parameters. The LaPO4:Ce3+,Tb3+ phosphor in the florescent lamps usually contains 14–15 mol% of Tb3+ ion. Since the price of terbium compounds has become expensive recently, it is difficult to reduce the total cost in commercial mass production. Therefore, application of the new phosphate phosphor, AlPO4:Tb3+, to LEDs, lamps, CRTs and PDPs is an acceptable alternative because of inexpensive cost. However, the

Fig. 4. CIE chromaticity diagram for AlPO4:0.03Tb3+ (a) and the excitation and emission spectra of AlPO4:Tb3+, LaPO4:Ce3+,Tb3+ and AlPO4:Ce3+,Tb3+ (b).

reasons of the luminescence of AlPO4:Tb3+ enhanced by the Ce3+ ions still need further investigation. 4. Conclusions In summary, the AlPO4:Tb3+ phosphor was synthesized and the luminescent properties were studied. The results show that the excitation spectrum consists of broad bands and sharp lines in the range of 200 nm to 400 nm. The characteristic luminescence is due to 5 D4–7FJ (J = 6, 5 ,4, 3, 2, 1, 0) line emissions of the Tb3+ ions. From the luminescence study the optimum concentration of luminescence is found to be 3 mol%. The highest emission peak locates at 542 nm and the phosphor shows excellent green emission. References [1] Huang YL, Jang K, Lee HS, Cho E, Jeong J, Yi SS, et al. Phys Pro 2009;2:207–10. [2] Lai H, Bao A, Yang YM, Tao YC, Yang H, Zhang Y, et al. J Phys Chem C 2008;112: 282–6. [3] Toda A, Uematsu K, Ishigaki T, Toda K, Sato M. Mater Sci Eng B 2010;173:168–70. [4] Balakrishnaiah R, Kima DW, Yia SS, Jang K, Lee HS, Jeong JH. Mater Lett 2009;63: 2063–6. [5] Zhang L, Lu CY, Long YC. Chem Commun 2002;18:2064–5. [6] Yan WF, Zhang ZT, Xu J, Mahurin SM, Dai S. Stud Surf Sci Catal 2005;156:265–72. [7] Cai GM, Zheng F, Yi DQ, Jin ZP, Chen XL. J Lumin 2010;130:910–6. [8] Zhou LY, Choy WCH, Shi JX, Gong ML, Liang HB. Mater Chem Phys 2006;100:372–4. [9] Van Uitert LG. J Electrochem Soc 1967;114:1048–53.