Synthesis, luminescence properties and energy transfer behavior of color-tunable KAlP2O7: Tb3+, Eu3+ phosphors

Optics and Laser Technology 121 (2020) 105829

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Synthesis, luminescence properties and energy transfer behavior of colortunable KAlP2O7: Tb3+, Eu3+ phosphors

T

⁎

Mengjiao Xua,b, Yi Dingb, Wanxia Luob, Luxiang Wangb, , Suxia Lib, Yanhong Liub a

College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi, Xinjiang 830046, China Key Laboratory of Energy Materials Chemistry, Ministry of Education, Key Laboratory of Advanced Functional Materials, Autonomous Region, Institute of Applied Chemistry, Xinjiang University, Urumqi, Xinjiang 830046, China

b

H I GH L IG H T S

Tb , Eu co-doped KAlP O phosphor was firstly synthesis. • AThenovel transfer mechanism of Tb -Eu was discussed in detail. • KAlPenergy O : Tb , Eu was color tunable from green to yellow and red. • The phosphor shows excellent thermal stability. • 3+

2

7

3+

3+

2

7 3+

3+

3+

A R T I C LE I N FO

A B S T R A C T

Keywords: Phosphor LEDs Energy transfer Multicolor-emitting

A new multicolor-emitting Tb3+-Eu3+ codoped KAlP2O7 (KAPO) phosphor was synthesized by solid-state reaction under the air atmosphere. The synthesis, morphology, luminescence properties, energy transfer mechanisms and thermal stability of the synthesized phosphor were systematically investigated. The codoped phosphor exhibited the characteristic emission of Eu3+ at 612 nm (5D0-7F2) and Tb3+ at 544 nm (5D4-7F5). Under ultraviolet (UV) excitation, KAPO: Tb3+, Eu3+ shows tunable emission from green-yellow to red with increasing of Eu3+ concentration due to efficient Tb3+-Eu3+ energy transfer. Based on the decay curves, the energy transfer from Tb3+ to Eu3+ was classified to be of dipole-dipole interaction resonant type with energy transfer efficiency of ~97%. The excellent luminescent properties and remarkable thermal stabilities suggested that KAPO: Tb3+, Eu3+ is a promising UV-converting material for the fields of lighting and displays.

1. Introduction As the fourth generation of solid-state lighting, white light-emitting diodes (LEDs) have been widely used in the recent years due to their high photoelectric conversion efficiencies, long life, environmental friendliness and excellent reliability, and they have gradually been substituted for fluorescent lamps and incandescent lamps [1–3]. Blue LED chip coupled with a yellow yttrium aluminum garnet phosphor has emitted white light [4–6]. However, owing to the deficiency of the red component, the resultants exhibit a high correlated color temperature and a low color rendering index, and these constrict their applications in general illumination. Fortunately, UV-LED excited red-green-blue (RGB) phosphors can avoid these problems [7,8]. In LED chips, the phosphor is one of the most critical components in which a variety of colors is required. Rare earth (RE) doped phosphors have been widely employed in the

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lighting and imaging industry due to their abundant emission colors and high color purity based on their intra-orbital 4f transitions or 5d → 4f transitions [9–13]. Eu3+ generally exhibits intense orange or red light derived from its f-f transitions, which depend mainly on the structure of the host crystal. In addition, it can be efficiently doped into a wide variety of hosts, including borates, aluminates, silicates and phosphates [14–19]. Tb3+ is frequently used as a dopant ion in numerous hosts due to its predominant 5D4-7F5 transition, which emits green light at 544 nm. Its ultraviolet absorption and stable luminescence indicate that Tb3+ can be employed as a sensitizer [20,21]. Tb3+ and Eu3+ can serve as efficient activators in a wide range of luminescent materials because of their ideal emission colors and high luminescent efficiencies [22–27]. Here, we combine the emission characteristics of Tb3+ and Eu3+ to adjust the color of a series of luminescent materials. Phosphate based luminescent materials are of great interest due to

Corresponding author. E-mail address: [email protected] (L. Wang).

https://doi.org/10.1016/j.optlastec.2019.105829 Received 12 April 2019; Received in revised form 5 September 2019; Accepted 6 September 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.

Optics and Laser Technology 121 (2020) 105829

M. Xu, et al.

2. Experimental

atoms, respectively, and both the sites are too small to be occupied by dopant ions. The ionic radii of the dopant ions, Tb3+ (1.04 Å, CN = 8) and Eu3+ (1.14 Å, CN = 8) are larger than those of Al3+ (0.48 Å, CN = 5). K has eight coordinated O atoms, and its lattice site is suitable for substitution by Tb3+ and Eu3+ (Table 1). The Rietveld refinement of powder XRD profiles of representative KAPO: 0.08 Tb3+, 0.16Eu3+ were conducted by the TOPAS program, the cell parameters and the refinement results are presented in Fig. S1, Tables S1 and S2. As compared to un-doped KAPO, the unit cell constants of the Tb3+ and Eu3+ codoped sample slightly decrease. The refinement results shown that the dopant ions may occupy the K+ sites in the KAPO host. The morphology of the KAPO: 0.08 Tb3+, 0.16Eu3+ before and after grinding was investigated based on analysis of the SEM images shown in Figs. S2 and 1(c). The number of large particles in the sample decreased after grinding process, the ground sample consisted of irregular microparticles several micrometers in size. As shown in Fig. 1(d), EDS indicated that the KAPO: 0.08 Tb3+, 0.16Eu3+ phosphor was composed of the elements K, Al, P, O, Eu and Tb, and the molar ratios of K, Al, P, O, Eu and Tb are in good agreement with that of the KAPO phosphor.

2.1. Sample synthesis

3.2. Photoluminescence properties and mechanism

All the chemical reagents used in this study were of analytical grade and were used without further purification. K2CO3, Al2O3, NH4H2PO4 were purchased from Tianjin bodi chemical Co. Ltd., Eu2O3 and Tb4O7 were purchased from Shanghai Aladdin Biochemical Technology Co. Ltd.. A series of KAlP2O7: 0.08 Tb3+, xEu3+ (x = 0−0.20) phosphors was prepared via solid-state reaction method, where 0.08 and x are molar percentage of Tb3+ and Eu3+ relative to KAlP2O7. First, the raw materials were weighed in a specifed stoichiometric ratio and ground thoroughly with ethanol in an agate mortar to obtain a homogeneous mixture. Then, the mixture was transferred into a corundum crucible and sintered at 1073 K for 8 h in the air in a muffle furnace. The ramp rate was set to 5 K/min and the product was obtained after natural cooling to room temperature. Finally, the product was ground to a fine powder for further characterization.

The PL and PLE spectra of KAPO: 0.08 Tb3+ are shown in Fig. 2(a). Under the 378 nm excitation, four characteristic emission peaks (490, 544, 583 and 619 nm) of Tb3+ are clearly observed in the emission spectrum [38,39]. The dominant peak is located at 544 nm that is assigned to the 5D4-7F5 transition of Tb3+. By monitoring at 544 nm, the recorded excitation spectrum includes a group of sharp lines ranging from 300 to 500 nm peaking at 378 nm from the 7F6-5D3 transition of Tb3+. In Fig. 2(b), the PL and PLE spectra of KAPO: 0.16Eu3+ are displayed. The PL spectrum shows two strong characteristic red light emission sharp lines (592 nm and 612 nm resulting from the transitions of 5D0-7F1 and 5D0-7F2) with the maximum at 612 nm under the 394 nm excitation [40–42]. When using UV lamp excitation, the Eu3+ singly doped phosphor presents red light. From the observed overlap between the PL band of Tb3+ and the PLE of Eu3+, as shown in Fig. 2(a) and (b), it can be concluded that resonance type energy transfer may occur from Tb3+ to Eu3+ in the KAPO host according to Forster-Dexter theory. This can be further confirmed by the PLE and PL spectra of the codoped KAPO: 0.08 Tb3+, 0.16Eu3+ phosphor exhibited in Fig. 2(c). The PL spectrum shows the emission line of Tb3+ at 544 nm and the emission lines of Eu3+ at 592 nm and 612 nm. The PLE spectrum shows the characteristic excitation peaks of Tb3+ and Eu3+. Moreover, due to the fact that the phosphors can emit both the green emission of Tb3+ and the red emission of Eu3+, the color of the phosphors can be tuned by controlling the ratio of dopant ion concentrations. In Fig. 3(a), the PL spectra of a series of KAPO: 0.08 Tb3+, xEu3+ (x = 0.01−0.20) phosphors under the 378 nm excitation are given. As the Eu3+ doping content increases from 0.01 to 0.16, the intensity of the Tb3+ emission (544 nm) descends while that of Eu3+ (612 nm) ascends, and this reveals the energy transfer from Tb3+ to Eu3+. As the Eu3+ content continues to raise, the intensities of both the Tb3+ and Eu3+ emissions decrease. This can be attributed to concentration quenching, when activators of the same sort are close enough to transfer energy to each other in nonradiative form that results in energy loss. The intensities of Eu3+ at 612 nm and Tb3+ at 544 nm as a function of Eu3+ concentration are shown in Fig. 3(b). Thus, the optimal Eu3+ concentration is 0.16. To further validate the energy transfer process between Tb3+ and Eu3+, the lifetimes of Tb3+ in KAPO: 0.08 Tb3+, xEu3+(x = 0−0.20) are recorded, and the calculated lifetimes are given in Fig. 3 (c). The decay curves can be well fit by a single exponential function [43–45]:

their stable chemical and physical properties, which results in the stable luminescence of RE ion doped phosphors [28–32]. The KAlP2O7 (KAPO) phosphate host has a centro-symmetric monoclinic structure in space group P21/c. KAPO belongs to the AICIIIP2O7 family, where AI is a mono-valence element and CIII is a trivalent element that generates a large family of materials, such as the KYP2O7, NaFeP2O7 compounds [33,34]. Among them, the phase formation temperature has been reported to be 1073 K, which is generally lower than that of other salts [35–37]. However, to the best of our knowledge, there are only a few studies regarding the luminescent properties of KAPO phosphors. In this study, we synthesized a series of KAPO: Tb3+, Eu3+ phosphors through solid state reaction. To clarify the luminescent properties of KAPO: Tb3+, Eu3+, the photoluminescence spectra, X-ray diffraction, decay curve and fluorescent thermal stabilities of the samples were systematically investigated. The green-yellow-red emission could be tuned based on the energy transfer from Tb3+ to Eu3+ as the Eu/Tb concentration ratio is gradually increased.

2.2. Sample characterization The phase composition of the as-synthesized samples was identified by powder X-ray diffraction analysis with use of a Bruker D8 advanced diffractometer with Cu Ka radiation operating at 40 kV and 40 mA. The morphologies and sizes of the as-synthesized phosphors were studied by scanning electron microscopy (SEM, Hitachi S-4800) with attached energy dispersive spectroscopy (EDS) functionality. The photoluminescence (PL) and PL excitation (PLE) spectra of KAPO: Tb3+, Eu3+ phosphors were recorded on a Hitachi F-4500 spectrophotometer equipped with a 150 W Xe light source. The thermal stability and decay curves of samples were measured on a HORIBA JobinYvon Fluorolog-3. 3. Results and discussion 3.1. Crystal structure The XRD patterns of KAPO: 0.08 Tb3+, KAPO: 0.16Eu3+, KAPO: 0.08 Tb3+, 0.16Eu3+ and the standard diffraction peaks of the KAPO host (calculated from JCPDF 36-1459) are shown in Fig. 1(a). The XRD patterns of the singly doped and codoped KAPO phosphors can be attributed to that of the standard undoped host, indicating that the dopant ions did not change the basic structure of the host lattice. The crystal structure of the KAPO host is presented in Fig. 1(b). The cell parameters are a = 7.308 Å, b = 9.662 Å, c = 8.025 Å, β = 106.69°, V = 542.771 Å3 and Z = 4. As seen in Fig. 1(b), the unit cell of KAPO composed of four KO8 irregular polyhedrons and four AlO4 rectangular pyramids connected by [P2O7]4− groups composed of two PO4 tetrahedrons sharing one O atom. Al and P are coordinated with 5 and 4O

I = I0 exp(−t / τ )

(1)

where I and I0 are the luminescence intensities at time t and 0, respectively, and τ is the radiative decay lifetime. Based on this function, 2

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Fig. 1. (a) The XRD patterns of KAPO: 0.08 Tb3+, KAPO: 0.16Eu3+, KAPO: 0.08 Tb3+, 0.16Eu3+, and the standard diffraction peaks of KAPO; (b) three-dimensional crystal cell structure of the KAPO host and a model of KO8; (c) SEM image of the KAPO: 0.08 Tb3+, 0.16Eu3+ phosphor after grinding; (d) EDS pattern of the KAPO: 0.08 Tb3+, 0.16Eu3+ phosphor.

concentration of Eu3+ in the KAPO host. In addition, the mechanism of energy transfer from Tb3+ to Eu3+ in the KAPO host was investigated in detail. The energy transfer efficiency depends on the Tb3+-Eu3+ distance. Exchange and multipolar interactions are two main aspects responsible for the resonant energy transfer mechanism. It is well known that the critical distance between the sensitizer and activator should be less than 5 Å if the energy transfer is produced by exchange interactions. The critical distance (RC) from Tb3+ to Eu3+ can be estimated using the concentration quenching method [37]:

Table 1 The effective ionic radii for the given coordination numbers (CN) of K+, Al3+, P5+, Eu3+, and Tb3+. Element

Valence

CN

Radii (Å)

K Al P Eu Tb

+1 +3 +5 +3 +3

8 5 4 8 8

1.51 0.48 0.17 1.14 1.04

1/3

3V ⎞ RC ≈ 2 ⎛ ⎝ 4πXC N ⎠

the lifetimes of KAPO: 0.08 Tb3+, xEu3+ (x = 0, 0.01, 0.015, 0.02, 0.03, 0.04, 0.08, 0.12, 0.16 and 0.20) are 2.580, 1.679, 1.408, 1.171, 0.873, 0.679, 0.384, 0.197, 0.137 and 0.055 ms, respectively. The lifetimes of Tb3+ and Eu3+ (Fig. S3) in KAPO: 0.08 Tb3+, xEu3+ gradually go down with increasing Eu3+ content due to concentration quenching. The energy transfer efficiency between the sensitizer (Tb3+) and the activator (Eu3+) can be calculated from the decay lifetime by using Eq. (2) [45,46]:

ηT = 1 −

τ τo

⎜

⎟

(3)

where XC is the critical concentration, N is the number of cations in the unit cell, and V is the volume of the unit cell. In the KAPO host, XC is 0.24, V is 542.77 (1) Å3 and N is 4. Therefore, RC was calculated to be about 10.26 Å. This value is much more than 5 Å, indicating the crossrelaxation between the Tb3+ and Eu3+ ions mainly occurs via electric multipolar interactions. On the basis of the Dexter energy transfer formula for multipolar interaction and the Reisfeld approximation, the following relation can be obtained [43,44]:

(2)

where ηT is the energy transfer efficiency, the τ and τo are the luminescence lifetimes of the sensitizer (Tb3+) with and without the presence of the activator (Eu3+), respectively. The relationships between the luminescence lifetime of Tb3+ and energy transfer efficiency ηT and Eu3+ doping concentration (x = 0−0.20) are shown in Fig. 3(d). It is found that the ηT soars with x changing from 0.01 to 0.05, then edging up to 97% with x reaching 0.20. The result indicates that the ηT from the Tb3+ to Eu3+ ions is partial and strongly depends on the doping

⎛ η0 ∝ C n /3⎟⎞ ≅ ⎛ IS 0 ∝ C n /3⎞ ⎜ ⎠ ⎠ ⎝ IS ⎝η ⎜

⎟

(4)

where η0 and η are the luminescence quantum efficiencies of Tb in the absence and presence of Eu3+, respectively; for which the value η0/ η can be approximately replaced by the ratio of the related luminescence intensities IS0/IS; C is the total doping content; and n = 6, 8, and 10 corresponding to dipole-dipole, dipole-quadrupole, and quadrupole3+

3

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Fig. 2. The excitation and emission spectra of (a) KAPO: 0.08 Tb3+, (b) KAPO: 0.16Eu3+, and (c) KAPO: 0.08 Tb3+, 0.16Eu3+.

Fig. 3. (a) The PL spectra of a series of KAPO: 0.08 Tb3+, xEu3+(x = 0.01−0.20) phosphors under 378 nm excitation, (b) the intensities of Eu3+ at 612 nm and Tb3+ at 544 nm as a function of Eu3+ concentration; (c) the decay curves of KAPO: 0.08 Tb3+, xEu3+ (x = 0−0.20) phosphors; (d) the dependence of the luminescent lifetime of Tb3+ and the energy transfer efficiency on the doped Eu3+ concentration. 4

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Fig. 7. The PL spectra of KAPO: 0.08 Tb3+, 0.16Eu3+ over the temperature range of 273 K to 473 K under 378 nm excitation, the inset is the normalized PL emission intensities of Eu3+ at 612 nm and Tb3+ at 544 nm as a function of the temperature.

Fig. 4. The IS0/IS versus Cn/3 plots of the Tb3+, Eu3+ codoped phosphors.

quadrupole interactions, respectively. The IS0/IS versus Cn/3 plot curves are shown in Fig. 4. It can be found from the fitted results that the linear behavior of IS0/IS -C6/3 is closer to the ideal state. Therefore, the mechanism of energy transfer from Tb3+ to Eu3+ is governed by dipoledipole interactions. Based on the above analysis, the energy transfer processes between Tb3+ and Eu3+ are depicted in Fig. 5(a). In view of the Tb3+-Eu3+ transition in the KAPO host, an electron transition occurs from the Tb3+ ground state 7F6 to the excited state 5D3 under UV light excitation. A portion of the energy in the 5D3 level relaxes to the lowest 5D4 excited state that follows by the 5D4-7FJ (J = 3, 4, 5 and 6) radiation transitions of Tb3+. The other portion of the energy can be transferred from the 5 D4 level of Tb3+ to the excited levels of Eu3+ through cross relaxation. Then, the electrons can nonradiatively relax to the lowest excited level 5 D0 (Eu3+). Finally, the 5D0-7FJ radiative transition takes place, where the orange (5D0-7F1) and red (5D0-7F2) emission of Eu3+ occurs. The Commission Internationale de L’Eclairage (CIE) chromaticity diagram can simulate the color and color saturation of the phosphors upon certain excitation wavelength. The energy transfer between Tb3+ and Eu3+ offers an approach to tune the emission color, as shown in Fig. 5(b), where the CIE chromaticity diagram and digital photos of KAPO: 0.08 Tb3+, xEu3+ (x = 0−0.20) phosphors under 378 nm

Fig. 5. (a) Energy transfer diagram of KAPO: Tb3+, Eu3+; (b) CIE diagram of the KAPO: 0.08 Tb3+, xEu3+ (x = 0−0.20) phosphors under 378 nm excitation and digital photos of the phosphors under a UV lamp.

Fig. 6. (a) CIE diagram and corresponding photographs of LED devices fabricated using the KAPO: 0.08 Tb3+, xEu3+ (x = 0, 0.01 and 0.016) phosphors combined with a 380 nm LED chip; (b) PL spectra of the corresponding devices. 5

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excitation (No. 1-10) are presented. The green light emitted from KAPO: 0.08 Tb3+, xEu3+ (x = 0) gradually goes to yellow, orange and then becomes red as the Eu3+ content increase due to the nonradiation energy transfer between Tb3+ and Eu3+, which causes the emission intensity of the Tb3+ decreases, whereas the emission intensity of the Eu3+ increases, as mentioned above. Therefore, the multicolor phosphor under UV excitation has potentially significant applications in fields such as lighting and display systems. As shown in Fig. 6(a), the CIE chromaticity coordinates of LED devices fabricated using the KAPO: 0.08 Tb3+, xEu3+ (x = 0, 0.01, and 0.16) phosphors combined with a 380 nm LED chip are (0.3393, 0.4994), (0.4280, 0.4027) and (0.4398, 0.3577), respectively. The corresponding photographs show bright green, yellow and orange-red light. The PL spectra of the corresponding devices are shown in Fig. 6(b). The dominant wavelengths of the above LEDs are 544 nm, 592 and 612 nm, 592 and 612 nm, respectively.

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3.3. Thermal stability Generally, thermal stability is a critical factor for the phosphors, which seriously affects the performance of LEDs, such as the light output, lifetime, chromaticity and color rendering index. The phosphors used in LEDs must sustain a stable emission efficiency at temperatures about 150 °C over a long period. In Fig. 7, the PL spectra of KAPO: 0.08 Tb3+, 0.16Eu3+ are depicted over the temperature range of 273–473 K. As can be seen, the Eu3+ and Tb3+ emission bands remained almost the same in shape with increasing temperature, whereas the emission intensity decreases slightly due to the thermal quenching phenomenon. The inset clearly shows the intensity variations of Eu3+ emission at 612 nm and Tb3+ emission at 544 nm with increasing temperature. It was found that the emission intensity of the KAPO: 0.08 Tb3+, 0.16Eu3+ phosphor still retained about 63.5% (at 423 K) of its initial value at 298 K. 4. Conclusion A single-composition, color tunable emitting phosphor KAlP2O7 doped with Tb3+ and/or Eu3+ was synthesized through a solid-state reaction in the air atmosphere. The PL and PLE spectra show the characteristic emission and excitation lines of Tb3+ and Eu3+. Under the UV excitation, the energy transfer process between Tb3+ and Eu3+ in the KAPO host is observed. The emission color of KAPO: Tb3+, Eu3+ samples can be tuned from green to yellow to red via energy transfer by adjusting the Eu3+ doping level. The mechanism of energy transfer from Tb3+ to Eu3+ is defined to be controlled by the dipole-dipole interactions, and the energy-transfer efficiency is ~97%. Moreover, the synthesized sample exhibits good thermal stability. Thus, the KAPO: Tb3+, Eu3+ solid solution can be considered as a potential phosphor for UV LED. Acknowledgements This work was supported by the Natural Science Foundation of Xinjiang Province (No. 2017D01C037). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2019.105829. References [1] S. Ye, F. Xiao, Y.X. Pan, Y.Y. Ma, Q.Y. Zhang, Phosphors in phosphor-converted white light-emitting diodes: recent advances in materials, techniques and properties, Mater. Sci. Eng. R. 71 (2010) 1–34. [2] H.A. Hoppe, Recent developments in the field of inorganic phosphors, Angew. Chem. Int. Ed. Engl. 48 (2009) 3572–3582. [3] C. Feldmann, T. Jüstel, C.R. Ronda, P.J. Schmidt, Inorganic luminescent materials:

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