Synthesis and photoluminescence studies of novel double-perovskite phosphors, Ba2GdTaO6:Eu3+ for WLEDs

Journal Pre-proof Synthesis and photoluminescence studies of novel double-perovskite phosphors, Ba2 GdTaO6 :Eu3+ for WLEDs Jinghua Li, Xinyue Wang, Ruirui Cui, Chaoyong Deng

PII:

S0030-4026(19)31434-2

DOI:

https://doi.org/10.1016/j.ijleo.2019.163536

Reference:

IJLEO 163536

To appear in:

Optik

Received Date:

5 July 2019

Accepted Date:

3 October 2019

Please cite this article as: Li J, Wang X, Cui R, Deng C, Synthesis and photoluminescence studies of novel double-perovskite phosphors, Ba2 GdTaO6 :Eu3+ for WLEDs, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163536

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Synthesis and photoluminescence studies of novel double-perovskite phosphors, Ba2GdTaO6:Eu3+ for WLEDs

Jinghua Li†, Xinyue Wang†, Ruirui Cui†, Chaoyong Deng†*

†

Key Laboratory of Electronic Composites of Guizhou Province, College of Big Data and Information Engineering, Guizhou

University, Guiyang, 550025, China

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

Abstract

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A series of novel double-perovskite type red emitting phosphors, Ba2Gd1-xTaO6:xEu3+ were synthesized via solid-state reaction route. Powder X-ray diffraction results confirmed the cubic perovskite phase and scanning electron microscope was used to evaluate the morphology of the phosphors, respectively. Detailed studies on UV-Vis absorption showed that Eu3+ doped Ba2GdTaO6 phosphors have obvious absorption in the ultraviolet region. The excitation of UV (CTB), near UV (394 nm) and blue light (467 nm) match the output wavelength of ultraviolet and blue LED chips, and red-light emission is observed, which is mainly due to the magnetic dipole (5D0→7F1) and electric dipole transition of (5D0→7F2) of Eu3+. The results show that the energy transfer from Gd 3+ to Eu3+ enhances the luminescence. Concentration quenching was observed when Eu3+ doped at more than 10 mol%. Thermal quenching was measured and the activation energy was 0.230 eV. The prepared phosphors have potential applications in WLEDs.

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Keywords: Optical materials; Phosphors; Photoluminescence; WLEDs.

1. Introduction

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White light emitting diodes (WLEDs) are ideal for general lighting applications because they are superior to traditional light sources. Photoelectric transformation efficiency, long lifetime, low power consumption and environmental friendliness are one of its advantages.1 Effective lighting devices can be obtained by combining one or more phosphorescent materials with ultraviolet or blue LED wafers. The most commonly used white light-emitting diodes are made of blue InGaN chips and yellow blue Y3Al5O12:Ce3+ phosphors. Due to their high correlation temperature and low color rendering index due to the lack of red components, the current use of white light-emitting diodes has great limitations. Therefore, there is an urgent need for blue and ultraviolet light-emitting diodes to effectively excite red light-emitting materials with well absorption and emission properties.2-3 Europium trivalent is the most widely studied red luminescence due to the f-f transition in configurations. The Eu3+ ions in the host crystal have several emission lines, which come from the Stark level of 5 D0→7Fj transition. The local environment of Eu3+ can be detected by the relative intensity ratio and splitting degree of the energy level transition in the emission spectrum of Eu 3+ ions.4-5 Due to its good chemical and thermal stability, wide charge transfer band (CTB) in near-ultraviolet region and the ability to capture GaN-based LED radiation in a certain wavelength range, the compounds of double perovskite structure (A2BB'O6) have attracted wide attention.6-10 In addition, the B site occupied by Gd3+ is easily partially replaced by Eu3+ to maintain its ionic properties and charge neutrality. These properties are important for doping and luminescence of rare earth ions. Double-perovskite doped with Eu3+ has attracted wide attention due to its potential applications as luminescent materials, such as SrGd0.5Nb0.5O3:Eu3+,11 Sr2GaTaO6:Eu3+/Er3+,12 Ba2GdNbO6:Eu3+,13-15 and some related simulation calculations.16-18 As far as we know, there is no research report on Ba 2GdTaO6:Eu3+ phosphors so far. In the present study, double perovskite Ba2GdTaO6 is considered a good host for doping Eu3+ ions. The structure and morphology of the Ba2GdTaO6:Eu3+ phosphor is studied by X-ray diffraction (XRD)

and scanning electron microscopy (SEM) analysis. The photoluminescence (PL) properties of the luminescence spectrum and its temperature dependence are investigated. In addition, the absorption, concentration quenching and thermal quenching of Ba2GdTaO6:Eu3+ were discussed in detail. 2. Experimental Procedure 2.1 Materials synthesis

A series of Ba2Gd1-xTaO6:xEu3+ (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16) phosphors were synthesized via solid-state reaction route. The 4N high purity raw materials Ba 2CO3, Gd2O3, Ta2O5 and Eu2O3 were weighed by stoichiometry, and a small amount of alcohol was added to them for grinding by planetary ball mill. Then the mixed powders were calcined at 1250 ℃ for 6 h, and the obtained samples were grinded properly after natural cooling to room temperature. 2.2 Characterization

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X-ray powder diffractometer (Rigaku SmartLab, operated at 40 kV and 150 mA, Cu-Kα = 0.154059 nm, 2θ range = 10-80°) was used to examine the crystal structure of the samples. The morphology of the samples was studied by a scanning electron microscopy (HITACHI SU-8010, operated at 1.0 kV). Using a HITACHI U-4100 spectrophotometer and Barium sulfate as the reference, the absorbance of samples was studied in the wavelength range of 250-600 nm. The excitation and emission spectra were measured by HORIBA FluoroMax-4 fluorescence spectrophotometer with a xenon lamp (150 W) as the excitation source. The temperature-dependent PL spectra was measured between 293 K and 573 K. All measurements, excluding the temperature-dependent PL spectra, were made at room temperature.

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3. Results and Discussion 3.1 Structural studies

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The standard XRD pattern of Ba2GdTaO6 and the XRD pattern of the Ba2Gd1-xTaO6:xEu3+ (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16) are shown in Fig. 1 (a). All the diffraction peaks are well matched with the corresponding standard JCPDS card of Ba 2GdTaO6 (no. 49-1900). The result showed that Eu3+ ions are well doped into the host crystal lattice and have no obvious effect on the crystal structure of Ba2GdTaO6. With the increasing of Eu3+ concentration, the peaks move to a small angle gradually. (Fig. 1 (b)). This phenomenon just satisfies the Bragg's law 2dsinθ = nλ (where d, θ, n, and λ are the crystal interplanar spacing, diffraction angle, integer, and wavelength of the X-ray, respectively).19 When Eu3+ ions are substituted into the Gd3+ sites, the large ion radius of Eu3+ increases the interplanar crystal spacing, thereby expanding the lattice cells. Moreover, θ is inversely proportional to d. Thus, the shift toward a small angle can be explained. The average crystallites size was calculated by applying the DebyeScherrer’s equation, and the full width at half maximum (FWHM) for the strongest peak of XRD curve was used in the formula,20

D

0.941 .  cos 

(1)

Where D is the average crystallite size, λ is the wavelength of the X-ray equal to 0.154056 nm, θ is the Bragg angle and β is FWHM.

(a)

x=0.16

(b)

x=0.14

Intensity (a.u.)

x=0.12 x=0.10 x=0.08 x=0.06 x=0.04 x=0.02 JCPDS Card No.49-1900 10

20

30

40

50

60

70

2-Theta (degree)

80 29.5

30.0

30.5

Fig. 1. (a) X-ray powder diffraction patterns of Ba2Gd1-xTaO6:xEu3+ (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16) phosphors and the Ba2GdTaO6 JCPDS standard pattern. (b) XRD curves in the range of 29.5°-30.5° were magnified.

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The crystal structure of Ba2GdTaO6 is shown in Fig. 2. Ba2GdTaO6 belongs to the cubic crystal system with a space group of Fm-3m. The parameters are a = b = c = 8.478 Å and Z = 4. By sharing the same corner (O2-), TaO6 and GdO6 octahedrons alternately connect to each other. Therefore, octahedral chains are extended and arranged in each plane. Ba2+ ions exist in the middle of the gap among octahedrons. Eu3+ substitution is possible in pure octahedral and distorted octahedral symmetries. 21 Two styles of ions can be discovered in the host, which are coordinated by six oxygen ions to compose octahedrons. Considering the similarity of their radii, Eu3+ ions (CN = 6, 0.950 Å) occupy the sites of Gd3+ ions (CN = 6, 0.938 Å), instead of the sites of Ta5+ ions (CN = 6, 0.690 Å).

Fig. 2. Projection view of Ba2GdTaO6 crystal structure.

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The scanning electron microscope images of Ba2Gd1-xTaO6:xEu3+ (x = 0.06, 0.08, 0.10, 0.12) are shown in Fig. 3. The phenomenon of reunion is obvious. The grain size ranges within 3-6 µm. The distribution of BGTO:0.10Eu3+ particles is the most uniform, and the uniformity of the crystal shape enhances their density of aggregation and increases their luminescence intensity.22

Fig. 3. SEM image of BGTO:xEu3+ (x = 0.06 (a), 0.08 (b), 0.10 (c), 0.12 (d)).

3.2 Optical studies

The UV-Vis absorption spectra of Ba2Gd1-xTaO6:xEu3+ (x = 0, 0.04, 0.08, 0.10) is presented in Fig. 4 (a). The spectra exhibit a broad absorption band within 250-400 nm and 4 narrow peaks within 360-600 nm range, which indicates the suitability of the phosphors for use in LEDs to absorb UV light energy efficiently. The absorption band ranging within 250-400 nm is attributed to the Ta5+-O2- and Eu3+-O2- overlapping charge transfer band (CTB) transitions.23 Four narrow absorption lines are centered at ~363, ~394, ~467, and ~533 nm, which are from the 7F0→5D4, 7F0→5L6, 7F0→5D2 and 7F0→5D1 transitions of Eu3+, respectively. With the increase of Eu3+ concentration, the absorption band intensity increase. The energy gap (Eg) of pure and doped Ba2GdTaO6 samples can be calculated by the following equation:24

 h 

1/2

  h  Eg  .

(2)

(b) 1.8

Intensity (a.u.)

0 0.04 0.08 0.10

1.2 F0→5D4 F0→5L6 7

F0→5D2 7F0→5D1

300

400

0.6

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CTB

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7

(αhν)1/2

7

500

Wavelength (nm)

0 0.04 0.08 0.10

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(a)

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where α denotes the absorption coefficient, and h and ν represent the Planck's constant and light frequency, respectively. Fig. 4 (b) is obtained from the ultraviolet-vision absorption spectra based on Eq. (2) fitting. When (hν)1/2 = 0, the Eg value can be obtained by extrapolating the linear segment of the tails. The calculated value of the pure Ba2GdTaO6 is approximately 3.25 eV, which is close to the theoretical value of absorption edge.25 With the increase of dopant content, the band gap of Ba2Gd1-xTaO6:xEu3+ (x = 0, 0.04, 0.08, 0.10) decreases from 3.25 eV to 2.98 eV. The result shows that Eu3+ ions doping has a significant effect on the band gap of the Ba2GdTaO6 host.26

600

0.0 2.8

3.2

3.6

4.0

4.4

hν (eV)

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Fig. 4. (a) UV-Vis absorption spectra of Ba2Gd1-xTaO6:xEu3+ (0, 0.04, 0.08, 0.10). (b) The relationship between (αhν)1/2 and hν.

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The PL excitation and emission spectra of BGTO:Eu3+ phosphor at room temperature is demonstrated in Fig. 5 (a). The excitation spectra monitored at 611 nm is depicted on the left. The excitation spectra ranging from 240 nm to 550 nm is from Ta 5+-O2- and Eu3+-O2- CTB, along with the 4f-4f inner-shell transitions of Gd3+ and Eu3+ ions. The sharp excitation peaks at 363 nm, 394 nm, 467 nm and 533 nm can be attributed to 7F0→5D4, 7F0→5L6, 7F0→5D2, 7F0→5D1 transitions of Eu3+ ions in the host,27 while the typical excitation peaks at 275 nm and 313 nm are assigned to optical transition of Gd 3+ 8S7/2→6I9/2 and 8S7/2→6P3/2, respectively.28 The coexistence of both Gd 3+ excitation peaks (275 nm) and the Eu3+ CTB in the excitation spectra indicates that energy transfer from Gd 3+ to Eu3+ ions may occur.29 The emission spectra on the right excited at 467 nm exhibits Eu3+ main luminescence lines centered 594 nm and 611 nm as well as low intensity line at 651 nm. The emission intensity of electric dipole transition 5 D0→7F2 is obviously higher than that of magnetic dipole transition 5D0→7F1 of Eu3+ ions. The integral intensity ratio of the transitions from 5D0→7F2 to 5D0→7F1 is called the asymmetry ratio, which indicates the degree of distortion caused by the reversal symmetry of the local environment around Eu 3+ ions in the host. The asymmetric ratio is a good method to measure the degree of Eu 3+ site distortion. Fig. 5 (b) shows the possible scheme of energy transfer from Gd3+ to Eu3+ and the corresponding energy level scheme of Eu3+ excitation and emission.

λex = 467 nm

(b)

40

5

F0→5D1 7

7

D0→ F3 D0→7F4

D0→7F1

300

400

500

600

30

CTS 5 HJ 5 L6 5 D4

ET

IJ 6 PJ

25 20

5

15 10

D0

5

7

F6

5

5

5

0 200

Energy (103 cm-1)

D0→7F2

F0→5D2 7

F0→5L6 7 7

1x106

F0→5D4

S7/2→6I9/2 8

S7/2→6P3/2

CTB

8

Intensity (a.u.)

2x106

DJ

6

35 3x106

6

651 nm 611 nm 594 nm 467 nm 268 nm

λem = 611 nm

313 nm

4x106

275 nm

(a)

0

700

7

8

S7/2

F0

3+

3+

Gd Eu Wavelength (nm) Fig. 5. (a) PLE spectra (λem = 611 nm) and PL spectra (λex = 476 nm) of BGTO:Eu3+. (b) Energy level diagrams, the characteristic emissions of Gd3+ and Eu3+ ions as solid black line with an arrow, and possible energy transfer as solid blue line with an arrow.

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The emission intensity of Ba2GdTaO6:Eu3+ varies with different Eu3+ concentrations, as shown in Fig. 6 (a). The emission intensity of the samples increases significantly with the increase of dopant contents. When the Eu3+ concentration increases further to more than 0.10, serious concentration quenching effect can be observed. The concentration quenching mechanism is generally explained into two modes: exchange interaction and multipole-multipole interaction. In order to determine which mechanism is responsible for concentration quenching, the critical distance (Rc) could be calculated on the basis of the following formula:30 1/3

 3V  Rc  2   .  4 CN 

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(3)

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where V is the unit cell volume of the host lattice (V = 609.37 Å3), N is the number of dopant sites available in the unit cell (N = 4), and C is the critical doping concentration (C = 0.10). Therefore, the Rc is ~14.276 Å. The mechanism of exchange interaction should be considered only when Rc is less than 5 Å. When the value is significantly higher than 5 Å, the energy transfer mainly depends on multipolar interaction. According to Dexter's theory, the type of multipolar interaction mechanism between Eu3+ ions can be determined according to the following formula:31

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1

 /3 I / =K 1       .  

(4)

(b)7.6

Intensity(a.u.) 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

x

0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

Intensity (a.u.)

580

590

600

7.2

7.0

x

610

620

630

Experimental data Fitting line

7.4

lg(I/)

BGTO:xEu3+ λex = 467 nm

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(a)

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where I is the luminescence intensity for phosphors, χ is the Eu3+ ion concentration, and K and β are constants. θ represents the electric multipolar character, in which θ = 6, 8, and 10 denote dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions, respectively. The spectrum of the linear relationship between lg(I/χ) and lg(χ) is shown in Fig. 6 (b). The slope (-θ/3) of ~-1.843 can be obtained. The value of θ is close to 6, which indicates that the mechanism for concentration quenching is mainly due to the dipole-dipole interaction.

640

650

Wavelength (nm)

y = -1.843x+5.496 Slope = -1.843

6.8 -1.1

-1.0

-0.9

-0.8

lg(

Fig. 6. (a) PL spectra of Ba2GdTaO6:xEu3+ (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14) phosphors under 360 nm UV excitation. (b) The relationship between lg(χ) and lg(I/χ) according to Eq. (4).

Fluorescent thermal stability represents an essential factor to evaluate the performance of phosphors. Fig. 7 (a) shows the temperature dependence of Ba2Gd0.9TaO6:0.10Eu3+ phosphor. The temperature ranges from 293 K to 573 K, and the detection angle and mode remain unchanged during heating. Due to thermal quenching, the emission intensity decreases with the increase of temperature. In order to further understand the characteristics of thermal quenching, the activation energy ΔE evaluate via the Arrhenius equation:32

I  I 0 / 1  AeE / KBT  .

(5)

293K 323K 373K 423K 473K 523K 573K

50.2%

0.6 0.4 0.2 0.0 300

350

400

450

500

550

600

Temperature (K)

(b) 0 -1

-2

-3

y = -0.230x+4.218 ΔE = 0.230eV

-4

610

620

Wavelength (nm)

20

24

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600

Experimental data Fitting line

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λex = 467 nm

82.7% 0.8

In(I0/I-1)

1.0

Relative Intensity (a.u.)

Relative Intensity (a.u.)

(a)

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where I and I0 indicate the emission peak value at temperature T and at the initial temperature, respectively. A is a constant, and KB is the Boltzmann constant (8.629×10-5 ev-1). The relationship between ln(I0/I-1) and 1/KBT is depicted in Fig. 7 (b). At 473 K, the intensity can be maintained at 82.7%. The obtained value (0.230 eV) of the activation energy ΔE is slightly higher than that of the reported red phosphor BaGd2O4:Eu3+ (~0.185 eV).22 The reason for this result is the nonradiative transition of Eu3+ increases under high-temperature conditions.

28

32

36

-1

1/KBT(eV )

3+

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Fig. 7. (a) Temperature-dependent PL spectra for the Ba2Gd0.9TaO6:0.10Eu phosphors under excitation 467 nm over 293-573 K. (b) The linear relationship between ln(I0/I-1) and 1/kT according to Eq. (5).

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The CIE chromaticity coordinates of Ba2Gd1-xTaO6:xEu3+ (x = 0.02-0.16) are shown in Fig. 8. With the increase of Eu3+ concentration, the coordinates move to the red band, and the phosphors under ultraviolet light emit bright red light. Table 1 list the CIE data of a series of phosphors Ba 2Gd1-xTaO6:xEu3+ with different concentrations of Eu3+ (x = 0.02-0.16). These data show that the phosphors possess well optical capability and are extremely suitable for actual application for WLEDs.

Fig. 8. The CIE coordinates profile of Ba2GdTaO6:Eu3+ and phosphor photographs under day light and ultraviolet light.

4. Conclusions A series of novel double-perovskite type red emitting phosphors, Ba2Gd1-xTaO6:xEu3+ have been successfully synthesized by traditional solid-state reaction route. The host Ba2GdTaO6 with sufficient [GdO6] octahedrons is fit for doping and substituting of Eu3+ ions at Gd3+ sites. Ultraviolet-visible absorption spectra show there is a broad band in the range of 250-400 nm and several narrow peaks within 360-600 nm, which can be excited by near-UV or blue light chips with effect. The phosphors exhibit strong excitation in the near UV and blue regions and they emit intense red emissions. The temperature dependence of the PL spectra present that the luminescence intensity decreases with the increase of temperature, and the thermal quenching activation energy is 0.230 eV. These results indicate that Ba2GdTaO6:Eu3+ phosphors could meet the requirements for warm WLEDs to be developed.

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Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51762010), the High-level Innovative Talents Cultivation Program of Guizhou Province (Qian Ke He Talent [2015]4006), and the Guizhou Postgraduate Excellent Talents Program (Qian Jiao Yan He ZYRC [2014]001). References:

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Table 1. Chromaticity parameters for the Ba2Gd1-xTaO6:xEu3+ (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16).

a b c d e f g h

BGTO:0.02Eu3+ BGTO:0.04Eu3+ BGTO:0.06Eu3+ BGTO:0.08Eu3+ BGTO:0.10Eu3+ BGTO:0.12Eu3+ BGTO:0.14Eu3+ BGTO:0.16Eu3+

CIE x 0.4412 0.4996 0.5054 0.5318 0.5384 0.5312 0.5215 0.5036

re lP na ur Jo

y 0.3423 0.3520 0.3619 0.3576 0.3564 0.3551 0.3584 0.3518

ro of

BGTO:Eu3+ content (wt.%)

-p

Series