LiZnPO4:Tb3+, Ce3+ green phosphors with high efficiency

JOURNAL OF RARE EARTHS, Vol. 30, No. 7, July 2012, P. 637

LiZnPO4:Tb3+,Ce3+ green phosphors with high efficiency OUYANG Chunmei (欧阳春梅), MA Shuai (马 帅), RAO Yang (饶 阳), ZHOU Xinmu (周新木), ZHOU Xuezhen (周雪珍), LI Yongxiu (李永绣) (Research Center for Rare Earths & Micro/Nano-Functional Materials, Nanchang University, Jiangxi 330031, China) Received 12 January 2012; revised 2 February 2012

Abstract: Tb3+ and Ce3+ co-activated LiZnPO4 phosphors with high luminescence efficiency were synthesized by a high temperature solid-state reaction at 1000 ºC for 3 h. The XRD patterns, photoluminescence spectra and SEM were recorded and the effects of Tb3+ and Ce3+ concentration, sintering condition on the luminescent properties of as-synthesized phosphors were investigated. The emission spectra under ultraviolet (200–300 nm) radiation showed a dominant peak at 543 nm attributed to the 5D4→7F5 transition of Tb3+, which was greatly enhanced by the co-doping of Ce3+, indicating that there occurred an efficient non-radiative energy transfer from Ce3+ to Tb3+. The optimal doping concentrations of Tb3+ and Ce3+ were determined to be 9% and 10%, respectively. Keywords: green phosphor; high luminescence efficiency; LiZnPO4:Tb3+,Ce3+; rare earths

Rare earth ion doped phosphates have been paid intense attention for a wide range of applications, including luminescent materials, laser materials, optical amplifiers and optical data storage because of their significant advantages, such as low cost, being thermally stable[1]. Therefore, extensive research has been carried out on RE-doped phosphors of LnPO4 (Ln=Y,La,Gd,Lu)[2–5]. Such as the green phosphor LaPO4:Ce, Tb (LaCeT), used in fluorescent lamps (FL) and plasma display panels (PDP)[6–8]. As efficient green phosphors, Ce3+ and Tb3+ coactivated LaPO4 powders were extensively applied in FL and PDPs due to the high-efficiency ET between Ce3+ and Tb3+ ions. This green phosphor is mainly attributed to the strong sharp emission at around 545 nm of Tb3+, which is close to the theoretical optimum wavelength for the green component of a tricolor lamp, and plays an important role in improving the luminescence efficiency of FL. The phosphates with ABPO4 formula (A and B are monoand divalent cations, respectively) are in a large family of monophosphates with the different structure types[9,10]. Hence there is an increased interest in the synthesis of new efficient luminescent materials having structures derived from the ABPO4 family[11,12]. Many ABPO4 phosphors doped with rare earth and transition metal ions have been reported[11–20], such as KSrPO4, NaCaPO4, KBaPO4, LiBaPO4, LiSrPO4, and MBaPO4 (M=Na, K), due to their excellent thermal and hydrolytic stabilities and strong luminescence efficiency that meet the requirements for efficient host materials. Recently, Mn2+ doped LiZnPO4 for potential applications as a phosphor in LEDs have been reported[11]. However, there are very few reports on rare earths doped

LiZnPO4 materials. Therefore the present investigation aims at the synthesis of green emitting phosphor LiZnPO4:Tb3+, Ce3+ (LZP:Ce,Tb) excitable by a UV radiation as well as study of their luminescence properties. The results demonstrated that LZP:Ce,Tb is a potential phosphor for FL.

1 Experimental 1.1 Preparation of LZP:Ce,Tb phosphors The samples LZP:Ce,Tb were synthesized using a conventional solid state reaction at high temperature. Mixtures of pure ZnO, Li2CO3, TbCl3, (NH4)2HPO4 and CeO2 with stoichiometric LiZn(1–x–y)PO4:Tb3+x,Ce3+y were thoroughly ground and then sintered in air at 600 ºC for 3 h followed by sintering at 1000 ºC for 3 h in a 5% H2/95% N2 gas mixture. The final products were obtained by cooling down to room temperature in the furnace and pulverized for further measurements. 1.2 Measurement procedures The phase of LZP:Ce,Tb phosphors was characterized by powder X-ray diffraction (XRD), operating at 40 kV and 40 mA and using Cu Ka radiation (λ=0.154056 nm). The photo luminescence (PL) spectra were measured by an FL-4500 spectrophotometer at room temperature.

2 Results and discussion 2.1 XRD patterns of LZP:Ce,Tb phosphors The XRD patterns of LiZn(1–x–y)PO4:Tb3+x,Ce3+y (x=0, y=0;

Foundation item: Project supported by National High Technology Research and Development Program of China, 863 Program (2010AA03A407, 2010AA03A408) and the Foundation of Training Academic and Technical Header for Main Majors of Jiangxi of China (2007GG00800) Corresponding author: Li Yongxiu (E-mail: [email protected]; Tel.: +86-791-83969240) DOI: 10.1016/S1002-0721(12)60104-5

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x=0.05, y=0.05; x=0.10, y=0.10) phosphors prepared at 1000 ºC for 3 h are shown in Fig. 1. The results indicate that all the diffraction patterns of as-synthesized phosphors can be indexed to that of a pure LiZnPO4(JCPDS 36-0306)[11]. However, with the increase of doping concentration of Ce3+ and Tb3+ ions, the diffraction peaks are weakened and widened, the peak position also shift slightly to the direction with higher 2 theta. Therefore, it is suggested that the doping of rare earth ions has no obvious influence on the type of host structure, but on the lattice parameters due to the different ionic size between rare earth ions and zinc ion, because Tb3+ and Ce3+ are expected to preferable occupy Zn2+ site.

Fig. 1 XRD patterns of LiZn(1–x–y)PO4:Tb3+x,Ce3+y (x=0, y=0; x=0.05, y=0.05; x=0.10, y=0.10) calcined at 1000 ºC for 3 h

JOURNAL OF RARE EARTHS, Vol. 30, No. 7, July 2012

2.2 Luminescence properties of LZP:Ce,Tb phosphors The PL spectra of LiZn(1–x–y)PO4:Tb3+x,Ce3+y phosphors at room temperature are shown in Figs. 2 and 3. It is clear that the phosphors with Tb3+ and Ce3+ co-doped can be efficiently excited by the UV light, yielding an intense green emission at around 543 nm. The emission spectra under the excitation at 286 nm show the characteristic optical transitions of Tb3+ ion at 488, 543, 583, 620 nm due to 5D4→7FJ (J=6,5,4,3). As the emission of Tb3+ at 542 nm was monitored, the stronger allowed f-d transitions of Ce3+ (200–300 nm) and the weaker forbidden f-f transitions of the Tb3+ ions (330–390 nm) were observed[6,7,20]. The intensity of the Tb3+ transition originating from the Ce3+-Tb3+ ET excitation is 10 times higher than that from the f-f transitions of Tb3+, implying efficient ET from Ce3+ to Tb3+ ions. Because the f-f transitions of the Tb3+ ions are electronic dipole forbidden ones, the excitation efficiency for Tb3+ itself is very low. However, the luminescent intensity can be dramatically increased through exciting Ce3+ ions in Ce3+/Tb3+-coactivated. Fig. 3 (a) shows the PL spectra of phosphors with different Tb3+ concentrations under fixed y=0.10. It is clear that the emission intensity increased with the increase of Tb3+ molar concentration up to 0.09 and then decreased because of concentration quenching. Fig. 3 (b) is the PL spectra of phosphors with different Ce3+ concentrations under fixed x= 0.09. The maximum emission intensity is observed at y=0.10.

Fig. 2 PL spectra of LiZn0.81PO4:Tb3+0.09,Ce3+0.10 calcined at 1000 ºC for 3 h and the energy level scheme for the ET and luminescence processes in LiZnPO4:Tb3+0.09,Ce3+0.10

Fig. 3 PL spectra of LiZn1–x–yPO4:Tb3+x,Ce3+y with y=0.10 and different x values (a), or with x=0.09 and different y values (b) sintered at 1000 ºC for 3 h (Ex=286 nm, Em=543 nm)

OUYANG Chunmei et al., LiZnPO4:Tb3+,Ce3+ green phosphors with high efficiency

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Therefore, the optimal doping concentrations of Tb3+ and Ce3+ are determined to be x=0.09 and y=0.10, in which Ce3+ as sensitizers and Tb3+ activators. Generally, the emission intensity per doped ion follows the equation[11]: I/x=K[1+β(x)Q⁄3]–1 (1) Where x is the terbium concentration, I the emission intensity. K and β the constants under the same excitation condition for a given host. While Q is equal to 6, 8 and 10 for dipole-dipole, dipole-quadrupole or quadrupole-quadrupole interaction, respectively. If β(x)Q⁄3 is much greater than 1, Eq. (1) can be simplified as follows[11]: lgI/x=R–Q/3lgx (2) where R can be considered as a constant, and equals to lgK–lgβ. Then, we take the data from Fig. 3 (a), and plot lgI/x vs lgx and shown in Fig. 4. A linear relation curve with slope –2.01321, and intercept 1.72077 was obtained. Using Eq. (2) the Q value is calculated to be 6.03963, very near to 6, indicating that the mechanism of concentration quenching in LiZnPO4:Tb3+,Ce3+ system follows the dipole-dipole interaction.

Fig. 5 PL spectra of phosphors with composition of LiZn0.81PO4: Tb3+0.09,Ce3+0.10 calcined at different temperatures for 3 h (a) or at 1000 ºC for different times (b)

Fig. 4 Relationship curve of lg(I/xTb) vs lg(xTb) for LiZnPO4:Tb3+, Ce3+ excited under 282 nm

2.3

Effects of calcination conditions on the luminescence properties of as-synthesized phosphors

Fig. 5 shows the PL spectra of phosphors prepared at different sintered times and temperatures. The results demonstrated that the calcinations conditions show a great effect on the luminescence intensity of as-synthesized phosphors. The optimal calcinations conditions can be determined to be at 1000 ºC for 3 h.

3 Conclusions A green-emitting phosphor LiZnPO4:Ce3+,Tb3+ suitable for UV excitation was successfully prepared by high temperature solid-state reaction at 1000 ºC for 3 h. The emission spectra showed the characteristic emission of f-f transition of Tb3+. The energy transfer from Ce3+ to Tb3+ in the LiZnPO4 system resulted a green emission under excitation by UV light. The relative intensity of green emission could be turned by adjusting the concentration of Tb3+ and Ce3+. And

the optimal doping concentrations of Tb3+ and Ce3+ in LiZn(1–x–y)PO4:Tb3+x,Ce3+y were determined to be x=0.09 and y=0.10, respectively. This phosphor promises application in tricolor based FLs.

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