Novel Eu³⁺-activated Ba₂Y₅B₅O₁₇ red-emitting phosphors for white LEDs: high color purity, high quantum efficiency and excellent thermal stability

Abstract

Eu³⁺-activated Ba₂Y₅B₅O₁₇ (Ba₂Y_5−xEu_xB₅O₁₇; x = 0.1–1) red-emitting phosphors were synthesized by the conventional high temperature solid-state reaction method in an air atmosphere. Powder X-ray diffraction (XRD) analysis confirmed the pure phase formation of the as-synthesized phosphors. Morphological studies were performed using field emission-scanning electron microscopy (FE-SEM). The photoluminescence spectra, lifetimes, color coordinates and internal quantum efficiency (IQE) as well as the temperature-dependent emission spectra were investigated systematically. Upon 396 nm excitation, Ba₂Y_5−xEu_xB₅O₁₇ showed red emission peaking at 616 nm which was attributed to the ⁵D₀ → ⁷F₂ electric dipole transition of Eu³⁺ ions. Meanwhile, the influences of different concentrations of Eu³⁺ ions on the PL intensity were also discussed. The optimum concentration of Eu³⁺ ions in the Ba₂Y_5−xEu_xB₅O₁₇ phosphors was found to be x = 0.8. The concentration quenching mechanism was attributed to the dipole–dipole interaction and the critical distance (R_c) for energy transfer among Eu³⁺ ions was determined to be 5.64 Å. The asymmetry ratio [(⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁)] of Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors was calculated to be 3.82. The fluorescence decay lifetimes were also determined for Ba₂Y_5−xEu_xB₅O₁₇ phosphors. In addition, the CIE color coordinates of the Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors (x = 0.653, y = 0.345) were found to be very close to the National Television System Committee (NTSC) standard values (x = 0.670, y = 0.330) of red emission and also showed high color purity (∼94.3%). The corresponding internal quantum efficiency of the Ba₂Y_4.2Eu_0.8B₅O₁₇ sample was measured to be 47.2%. Furthermore, the as-synthesized phosphors exhibited good thermal stability with an activation energy of 0.282 eV. The above results revealed that the red emitting Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors could be potential candidates for application in near-UV excited white light emitting diodes.

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1. Introduction

In recent times, scientific research has been focused on new illumination sources to meet the demands of energy conservation and environmental issues.¹ Solid-state lighting (SSL) is a major revolution in the lighting industry.² Among SSL, inorganic phosphor-converted white light-emitting diodes (pc-WLEDs) are found to prevail in the lighting market owing to an extensive range of applications in versatile fields, such as flat panel displays, WLEDs, bio-markers, solid state lasers, sensors, and high energy radiation detectors.^3–10 In particular, WLEDs have attracted much attention due to their better performance such as high luminous efficiency, long operational lifetime, high rendering indices, compactness, low-cost, energy saving and environmental protection to surpass traditional incandescent and fluorescent lamps.^11,12

The most currently used WLEDs are fabricated by the combination of InGaN-based blue LED chips with the yellow emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) phosphors.¹³ However, there exist some drawbacks in its practical application, such as poor color rendering index (R_a < 80), and the high correlated color temperature (CCT ∼ 7750 K) owing to the lack of a red spectral component in the visible region.^4,14 In order to obtain high-efficiency WLEDs with appropriate CCT, high CRI and better color stability, alternative method is proposed, which combines the near-ultraviolet (n-UV) LED chips with a mixture of red, green and blue emitting phosphors.⁹ There are some commercial BaMgAl₁₀O₁₇:Eu²⁺ (blue), SrSi₂O₂N₂:Eu²⁺ and Ba₂SiO₄:Eu²⁺ (green) phosphors, but a high-efficiency, low-cost red-emitting phosphor is still needed.^2,15,16 The red-emitting phosphors are an essential component for creating high-quality white light. Currently, the lanthanide-activated sulfide and nitride (e.g., Y₂O₂S:Eu³⁺, SrGa₂S₄:Eu²⁺, CaS:Eu²⁺, Sr₂Si₅N₈:Eu²⁺ and CaAlSiN₃:Eu²⁺) red phosphors are extensively being used for WLEDs.^4,12 However, the commercially available red-emitting sulfide phosphors are chemically unstable with low luminous efficiency. Meanwhile, the nitride based phosphors have critical synthesis conditions, such as high temperature (>1800 °C) and high pressure (0.5 MPa N₂ pressure).^12,13,17,18 The search for non-toxic and highly efficient host materials has attracted researchers. Therefore, it is an urgent task to investigate new red-emitting phosphors for WLEDs with high efficiency, excellent thermal stability and high color purity, which can be excited by n-UV LEDs.

Rare earth ions activated inorganic materials have attractive optical properties because of their distinctive intra-configurational f → f transitions.⁹ Among all the rare earth ions, Eu³⁺ ion is the most often used red-emitting activator and exhibits a series of characteristic sharp emissions ascribed to the electronic transitions of ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, and 4).^19–24 Furthermore, the Eu³⁺ ion is expected to be one of the most hopeful species that offers optical devices in the red color region and numerous investigations have been executed in different host materials such as silicates²⁵ and phosphates^26,27 for the WLEDs application.

As an essential member of the family of luminescent materials, rare earth ions activated borate compounds have been widely investigated, due to their special structure, low sintering temperature, good physical and chemical stability, low cost, excellent optical damage threshold, large band gap, and strong absorption in the n-UV region.^28–30 Recently, blue-emitting Ba₂Y₅B₅O₁₇:Ce³⁺ (ref. 31) and green-emitting Ba₂Y₅B₅O₁₇:Ce³⁺,Tb³⁺ (ref. 32) phosphors have been reported. Zhang et al. investigated the Ce³⁺/Tb³⁺/Eu³⁺ tri-doped Ba₂Y₅B₅O₁₇ multi-color emitting phosphors.³³

The searching of high-performance red-emitting phosphors for WLEDs, we have systematically investigated in detail luminescence properties of Eu³⁺ activated Ba₂Y₅B₅O₁₇ red emitting phosphors. To the best of our knowledge, the elaborated spectroscopic properties of Eu³⁺ singly doped Ba₂Y₅B₅O₁₇ and its potential application in WLEDs have not yet been reported. The phosphors showed intense red emission peaking at 616 nm under a 396 nm excitation. The Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphor was found to show the highest emission intensity with internal quantum efficiency (IQE) of 47.2%. Moreover, the Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphor also exhibited good thermal stability and high color purity of red emission. The obtained results suggest that the Ba₂Y₅B₅O₁₇:Eu³⁺ phosphor has potential application for n-UV-pumped WLEDs.

2. Experimental

2.1. Materials and synthesis

Polycrystalline samples of Ba₂Y_5−xEu_xB₅O₁₇ (x = 0–1.0) phosphors were synthesized using conventional high temperature solid-state reaction method. Stoichiometric amounts of starting materials BaCO₃ (Analytical Reagent, A.R.), Y₂O₃ (99.99%), H₃BO₃ (A.R.) and Eu₂O₃ (99.99%) were weighed, and ground using an agate mortar and pestle to form homogeneous mixtures. The obtained mixtures were then transferred into an alumina crucible and pre-heated at 450 °C for 4 h in air. After that, the samples were reground and sintered in air at 1200 °C for 8 h. Finally, the as-prepared samples were gradually cooled down to room temperature in the furnace and were re-ground into homogeneous powder for further measurements.

2.2. Measurements and characterization

The crystalline phase formation of as-synthesized samples were identified by the powder X-ray diffraction (XRD) patterns using Bruker D8 advance powder diffractometer operated at 40 kV and 40 mA with CuKα radiation (λ = 1.5406 Å). The measurements were carried out in the diffraction angle (2θ) ranging from 10 to 80° with a scanning step of 0.02°. The structural parameters of Ba₂Y₅B₅O₁₇ and Ba₂Y_4.2Eu_0.8B₅O₁₇ were refined by the Rietveld refinement method using FullProf_Suite program. The morphology and elemental mapping analyzes of the phosphors were performed using a field-emission scanning electron microscope (FE-SEM; MAIA3 TESCAN). The photoluminescence (PL) and photoluminescence excitation (PLE) spectra as well as the decay curves were recorded using Edinburgh FS5 spectrofluorometer equipped with a 150 W continuous wavelength Xenon lamp and a pulsed Xenon lamp as the excitation source. The IQE of sample was measured by employing a barium sulfate coated integrating sphere attached to the Edinburgh FS5 spectrofluorometer. All the above measurements were carried out at room temperature. Furthermore, the temperature-dependent PL spectra of the phosphors within the temperature range from 303 to 483 K were obtained on the same instrument with a temperature controlling system.

3. Results and discussion

3.1. Phase identification

The XRD patterns of the Ba₂Y_5−xEu_xB₅O₁₇ (x = 0, 0.1, 0.4, 0.6, 0.8, and 1.0) phosphors are shown in Fig. 1(a). All the observed diffraction peaks of the as-obtained samples were well consistent with the Powder Diffraction File (PDF No. 00-056-0113) for Ba₂Y₅B₅O₁₇, and also matched well with the reference data.³³ No other phase or impurity were noticed in the compositions, and the result distinctly suggested that the as-prepared samples were in single phase and Eu³⁺ doping did not significantly influence the crystalline structure of the host due to the similar valence and ionic radius of Eu³⁺ and Y³⁺ [Eu³⁺ (r = 1.07 Å), Y³⁺ (r = 1.02 Å)].³³ In the meantime, a small shift in the diffraction peaks were observed after increasing Eu³⁺ content, (depicted in Fig. 1(b)) because of the ionic radius between Y³⁺ and Eu³⁺ [Y³⁺ (r = 1.02 Å), Eu³⁺ (r = 1.07 Å)]. According to previous report, the Ba₂Y₅B₅O₁₇ crystallizes as an orthorhombic structure with the space group Pbcn (no. 60).^31–33 In order to investigate the crystal structure, the structural parameters of Ba₂Y₅B₅O₁₇ and Ba₂Y_4.2Eu_0.8B₅O₁₇ were refined by the Rietveld refinement method using FullProf_Suite program. The observed, calculated and difference of XRD patterns were shown in Fig. 2(a) and (b). The structural parameters are listed in Table 1. All of the observed diffraction peaks showed a good agreement between observed and calculated patterns. The results reveals that Ba₂Y₅B₅O₁₇ crystallizes as an orthorhombic structure with space group Pbcn (no. 60), and its lattice parameters were determined to be a = 17.4486(8) Å, b = 6.64715(29) Å and c = 13.0128(6) Å and V = 1509.28(19) ų. The refinement factors finally converged to R_p = 2.54%, and R_wp = 3.67%, which was shown in Table S1 (ESI†). Similarly, the refinement results confirms that the cell parameters of Ba₂Y_4.2Eu_0.8B₅O₁₇, a = 17.45484(18) Å, b = 6.66655(9) Å, c = 13.06090(18) Å and V = 1519.813(34) ų, the refinement factors finally converged to R_p = 1.99%, and R_wp = 2.70%. The atomic coordinates were provided in the Table 2. Furthermore, the unit cell structure of Ba₂Y₅B₅O₁₇ is depicted in Fig. 3.

Fig. 1 (a) XRD patterns of as-prepared Ba₂Y_5−xEu_xB₅O₁₇ (x = 0, 0.1, 0.4, 0.6, 0.8 and 1.0) phosphors. (b) Magnified XRD patterns for Ba₂Y_5−xEu_xB₅O₁₇ (x = 0, 0.1, 0.4, 0.6, 0.8 and 1.0) phosphors.

Fig. 2 Rietveld refinement of observed, calculated and difference of the powder XRD patterns of (a) Ba₂Y₅B₅O₁₇ and (b) Ba₂Y_4.2Eu_0.8B₅O₁₇.

Table 1 Crystal structure data of Ba₂Y₅B₅O₁₇ and Ba₂Y_4.2Eu_0.8B₅O₁₇ from the Rietveld refinement

Compound	Ba2Y5B5O17	Ba2Y4.2Eu0.8B5O17
Crystal system	Orthorhombic	Orthorhombic
Space group	Pbcn (no. 60)	Pbcn (no. 60)
a (Å)	17.4486 (8)	17.45484 (18)
b (Å)	6.64715 (29)	6.66655 (9)
c (Å)	13.0128 (6)	13.06090 (18)
α = β = γ (°)	90	90
V (Å^3)	1509.28 (19)	1519.813 (34)
Z	4	4
Rp (%)	2.54	1.99
Rwp (%)	3.67	2.70

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Table 2 The atomic coordinations, occupancies and isotropic displacement parameters as determined by Rietveld refinement for Ba₂Y_4.2Eu_0.8B₅O₁₇

Atom	x	y	z	Occ.	Uiso (Å^2)	Wyck.
Ba1	0.33581	0.02396	0.18393	0.90943	−0.00532	8d
Y1	0.33581	0.02396	0.18393	0.90943	−0.00532	8d
Ba2	0.5	0.41124	0.75	0.2	0.01275	4c
Eu	0.5	0.41124	0.75	0.8	0.01275	4c
Y3	0.18165	0.04629	0.41451	1	−0.02461	8d
Y4	0.01607	0.76051	0.47887	1	0.0088	8d
B1	0.15138	0.52964	0.40624	1	−0.09	8d
O1	0.19532	0.38553	0.41003	1	−0.072	8d
O2	0.14487	0.7102	0.40012	1	0.01301	8d
O3	0.08387	0.45757	0.49893	1	0.0003	8d
B2	0.69329	0.58445	0.77545	1	−0.06048	8d
O4	0.67945	0.4438	0.75562	1	0.0293	8d
O5	0.61289	0.3519	0.8842	1	−0.00301	8d
O6	0.69615	0.58683	0.91213	1	−0.03713	8d
O7	0.94229	0.89496	0.5666	1	−0.0122	8d
O8	0.0274	0.26356	0.67037	1	−0.055	8d
O9	0.5	0.94764	0.25	1	−0.04744	4c
B3	0.0	0.63778	0.25	1	−0.09	4c

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Fig. 3 Crystal structure of Ba₂Y₅B₅O₁₇.

As reported previously, the Ba₂Y₅B₅O₁₇ host lattice contains two crystallographically independent, fully occupied Y³⁺ sites. Specifically, a distorted octahedron and a distorted capped trigonal prism around Y₃ and Y₄ site. The two Y³⁺ sites are shared with Ba²⁺, namely, Ba₁/Y₁ with a majority Ba (CN = 10) and Y₂/Ba₂ with a majority Y (CN = 7), respectively. All of the boron atoms are three-coordinated by oxygen, making slightly distorted trigonal planar units.^31–33

3.2. Morphology analysis

Fig. 4(a) and (b) depicts the FE-SEM micrographs of the Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors. The as-obtained micrographs exposed that the particles were agglomerated with irregular morphology. The estimated size of the particles could be in micrometer range which is suitable practical application for WLEDs.³⁴ Meanwhile, the elemental mapping results also suggested that the Eu were evenly distributed throughout the entire particles, as indicated in Fig. 4(c). The remaining mapping images of Ba, Y, B, and O was provided in Fig. S1 (ESI†).

Fig. 4 FE-SEM micrographs (a) and (b). The elemental mapping (c) of europium for the Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors.

3.3. Photoluminescence properties

Fig. 5 shows the PLE and PL spectra of the Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors. The excitation spectrum was monitored at 616 nm corresponding to ⁵D₀ → ⁷F₂ transition of Eu³⁺. The PLE spectrum consisted of two parts namely a broad excitation band in the wavelength range from 230 to 310 nm, and a series of sharp PLE peaks located between 315 to 550 nm. It can be clearly noted that the broad band centered at 277 nm was assigned to the O²⁻ → Eu³⁺ charge transfer band (CTB), corresponding to the electron transfer from the completely filled 2p orbital of O²⁻ ions to the empty states of 4f orbital of Eu³⁺ ions. In addition, the several sharp excitation peaks, associated with the typical intra-configurational 4f–4f forbidden transitions of Eu³⁺ ions were found with peaks positioned at 321 nm (⁷F₀ → ⁵H₆), 364 nm (⁷F₀ → ⁵D₄), 383 nm (⁷F₀ → ⁵L₇), 396 nm (⁷F₀ → ⁵L₆), 418 nm (⁷F₀ → ⁵D₃), 466 nm (⁷F₀ → ⁵D₂) and 537 nm (⁷F₀ → ⁵D₁).^9,30,35 Among all the sharp PLE peaks, the strongest excitation peak located at 396 nm matches well with the n-UV LED chip for WLEDs applications. The PL spectrum of Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphor (λ_ex = 396 nm) exhibited the well-known emission characteristics of Eu³⁺ in the spectral region from 500 nm to 750 nm. The sharp emission peaks at 580, 592, 616, 650 and 705 nm can be assigned to the transitions from the excited state ⁵D₀ to ground state ⁷F_J (J = 0, 1, 2, 3 and 4) levels of Eu³⁺ ions.^30,36 From this spectrum, it can be ascertained that the intensity of electric dipole (ED) ⁵D₀ → ⁷F₂ transition at around 616 nm was much higher than that of the magnetic dipole (MD) ⁵D₀ → ⁷F₁ transition at around 592 nm. When the Eu³⁺ ion is situated at the crystallographic site with inversion symmetry, the ⁵D₀ → ⁷F₁ (MD) transition is dominant while in a site without inversion symmetry, the ⁵D₀ → ⁷F₂ (ED) transition will be dominant.^37,38 The above result suggested that the local symmetry of Eu³⁺ sites belong to the non-centrosymmetric in Ba₂Y₅B₅O₁₇ host lattice.

Fig. 5 The PLE (λ_em = 616 nm) and PL (λ_ex = 396 nm) spectra of Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors.

3.4. Concentration quenching and energy transfer of Ba₂Y_5−xEu_xB₅O₁₇ phosphors

The PL spectra of Ba₂Y_5−xEu_xB₅O₁₇ (0.1 ≤ x ≤ 1.0) phosphors with different Eu³⁺ concentrations under 396 nm excitation were shown in Fig. 6(a). The shapes and peak positions of the PL spectra remained unchanged with different concentration of Eu³⁺ ions, except for the PL intensity (see Fig. 6(b)). The inset of Fig. 6(a) illustrates the dependence of PL intensity of various Eu³⁺ concentrations under λ_ex = 396 nm. It can be seen that the PL intensity enhanced with the amount of Eu³⁺ until it reached a maximum intensity at x = 0.8, followed by a gradual decrease with a further increase in Eu³⁺ ion content owing to the concentration quenching effect.¹³ In general, the concentration quenching phenomenon commonly takes place as a consequence of exchange interaction, radiation re-absorption or multipole–multipole interaction. In addition, the non-radiative energy transfer mechanism among Eu³⁺ ions is strongly dependent on the critical transfer distance (R_c), which was calculated by using the concentration quenching method. The R_c can be calculated according to the equation proposed by Blasse:³⁹where V represents the volume of the unit cell, x_c is the critical concentration of Eu³⁺ ions, and N is the number of host cations in the unit cell. In the case of Ba₂Y_5−xEu_xB₅O₁₇ phosphors, the values of V, x_c and N were 1509.28 ų, 0.8 and 20, respectively,^32,33 and thus the R_c was determined to be 5.64 Å. The short-distance exchange interaction becomes ineffective when the critical distance is larger than 5 Å. The radiation re-absorption is unlikely to take place as there is no overlap between PL and PLE spectra in the present case. Therefore, the concentration quenching mechanism of Eu³⁺ ions in Ba₂Y₅B₅O₁₇ system was dominated by the multipole–multipole interaction. There are three types of electric multipole interactions, including dipole–dipole (d–d), dipole–quadrupole (d–q) or quadrupole–quadrupole (q–q). Based on Dexter's theory,⁴⁰ the emission intensity (I) per activator ion can be determined by the following equation:where I/x represents the emission intensity per activator concentration, x stands for the Eu³⁺ ions concentration; k and β are constants for the given host under the same excitation conditions; θ is a function of multipolar interaction, and θ = 6, 8 and 10 corresponds to d–d, d–q or q–q interactions, respectively.⁴¹ The above mentioned equation can be rearranged further by assuming β(x) ≫ 1:⁴²where K′ = log k − log β. The dependence of log(I/x) on log(x), as shown in Fig. 7, which was found to be linear and the slope was determined to be −1.91369. Therefore, the value of θ can be calculated as ∼5.74, which approximately equals to 6, indicating that the non-radiative energy transfer mechanism among the Eu³⁺ ions can be ascribed to d–d interaction in Ba₂Y_5−xEu_xB₅O₁₇ phosphors.

Fig. 6 (a) The PL spectra of Ba₂Y_5−xEu_xB₅O₁₇ (0.1 ≤ x ≤ 1.0) phosphors under 396 nm excitation. The inset shows the corresponding PL intensity as a function of Eu³⁺ concentrations (x). (b) The magnified PL spectra for Ba₂Y_5−xEu_xB₅O₁₇ (0.1 ≤ x ≤ 1.0) phosphors.

Fig. 7 Dependence of log(I/x) on log(x) for Ba₂Y_5−xEu_xB₅O₁₇ phosphors (λ_ex = 396 nm).

Generally, the ED transition ⁵D₀ → ⁷F₂ is a hypersensitive transition as this type of transition is very sensitive to the local environment, while the MD transition ⁵D₀ → ⁷F₁ is insensitive to the local environment at Eu³⁺ site.^38,43 Meanwhile, the ratio of integrated emission intensity (asymmetry ratio (R)) of ED (⁵D₀ → ⁷F₂) to MD (⁵D₀ → ⁷F₁) transitions can be used to evaluate the site symmetry around the Eu³⁺ ions. In general, the high value of R > 1 indicates that the local symmetry around Eu³⁺ ion is non-centrosymmetric.⁴⁴ Fig. 8 shows the plot of integrated emission intensity [(⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁)] ratio values on different Eu³⁺ concentrations. The R values decreased with increasing Eu³⁺ ion concentration. The calculated R values were found to be 4.36, 4.16, 4.02, 3.95, 3.89, 3.83, 3.80, 3.82, 3.80 and 3.83 for different concentration of (0.1 ≤ x ≤ 1.0) Eu³⁺ activated phosphors, indicating that Eu³⁺ occupied the distorted cation sites with non-centrosymmetric.⁴⁴ Particularly, the calculated R value (3.82) of Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors was found to be practically better than several previous Eu³⁺-doped red phosphors, as shown in Table 3. The lack of an inversion center the Eu³⁺ ion is favorable in achieving a bright red phosphor with high color purity.³⁶

Fig. 8 The asymmetry ratio (R) values with various concentrations of Eu³⁺ in Ba₂Y_5−xEu_xB₅O₁₇ (0.1 ≤ x ≤ 1.0) phosphors.

Table 3 Asymmetry ratio (R) of (⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁) transitions of Eu³⁺ activated different red phosphors

Red phosphors	Asymmetry ratio (R) (^5D0 → ^7F2/^5D0 → ^7F1)	Ref.
Ba2Y4.2Eu0.8B5O17	3.82	This work
NaSrB5O9:7%Eu^3+	2.73	5
Ba2CaZn2Si6O17:0.9Eu^3+	2.36	6
Li6Y(BO3)3:0.5%Eu^3+	1.71	7
BaTa2O6:30%Eu^3+	1.40	8
Na3Sc2(PO4)3:0.35Eu^3+	1.37	9

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3.5. PL decay curves of Ba₂Y_5−xEu_xB₅O₁₇ phosphors

Fig. 9 illustrates the room-temperature PL decay curves of Ba₂Y_5−xEu_xB₅O₁₇ phosphors with different Eu³⁺ concentrations (0.1 ≤ x ≤ 1.0) under excitation at 396 nm and monitored at 616 nm. All the decay curves can be fitted well based on a first-order exponential decay function as given by the following equation:

I(t) = I₀ + A exp(−t/τ) (4)

where I(t) and I₀ are the luminescence intensities at time t and t = 0, respectively; A is a constant, and τ represents the characteristic decay lifetime for the exponential component.^9,13 According to eqn (4), the calculated lifetime values of the ⁵D₀ emitting level of Eu³⁺ ions in Ba₂Y_5−xEu_xB₅O₁₇ (0.1 ≤ x ≤ 1.0) phosphors were 1.347, 1.365, 1.388, 1.426, 1.461, 1.484, 1.519, 1.546, 1.580 and 1.593 ms, respectively. Obviously, the PL lifetime values slightly increased with increasing various Eu³⁺ concentrations, which can be attributed to energy migration between Eu³⁺ ions.^13,45–47

Fig. 9 PL decay curves of Ba₂Y_5−xEu_xB₅O₁₇ (0.1 ≤ x ≤ 1.0) phosphors excited at 396 nm and monitored at 616 nm.

3.6. The CIE chromaticity coordinates of Ba₂Y_4.2Eu_0.8B₅O₁₇

Fig. 10 presents the CIE chromaticity diagram of Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors under excitation at 396 nm. According to the PL spectrum, the CIE coordinates were calculated as (0.653, 0.345), which was much closer to the NTSC standard red light (0.670, 0.330) than that of the commercial red-emitting phosphor of Y₂O₂S:Eu³⁺ (0.637, 0.327) and better than other Eu³⁺ activated phosphors Na₃Sc₂(PO₄)₃:0.35Eu³⁺ (0.642, 0.353) Ca_0.9Eu_0.1TiO₃ (0.630, 0.370) CaLa_0.65Eu_0.35B₇O₁₃ (0.611, 0.388).^9,48,49 Furthermore, to better understand the red emission of the Eu³⁺-activated Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors, the color purity was calculated according to the following expression:^9,50where (x, y), (x_i, y_i) and (x_d, y_d) refers the color coordinates of emission light, the equal energy point and color coordinates of the dominant wavelength points, respectively. In the present work, (x = 0.653, y = 0.345), (x_i = 0.310, y_i = 0.316,) and (x_d = 0.675, y_d = 0.324). According to eqn (5), the color purity of Ba₂Y_4.2Eu_0.8B₅O₁₇ red phosphors was determined to be around 94.3%. We can find that the Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphor showed strong red emission intensity with good CIE chromaticity coordinates and eminent color purity. In addition, the IQE is an essential parameter for practical application. The IQE of the optimized phosphor was measured using an integrating sphere. The IQE value can be calculated according to the equation:^51,52where L_S is the luminescence emission spectrum of the sample, E_S is the spectrum of the light used for exciting the sample, and E_R is the spectrum of the excitation light without the sample in the sphere, respectively (see Fig. 11). Under the excitation of 396 nm, the IQE of Ba₂Y_4.2Eu_0.8B₅O₁₇ red-emitting phosphor was determined to be 47.2%, which was higher than the commercial Y₂O₂S:Eu³⁺ phosphors (IQE: 35%).^4,9 These results suggest that the Ba₂Y_5−xEu_xB₅O₁₇ phosphor could be a promising red phosphor for n-UV-based WLEDs.

Fig. 10 CIE color coordinates of the NTSC, commercial Y₂O₂S:Eu³⁺, Ba₂Y_4.2Eu_0.8B₅O₁₇, Na₃Sc₂(PO₄)₃:0.35Eu³⁺, Ca_0.9Eu_0.1TiO₃ and CaLa_0.65Eu_0.35B₇O₁₃ red phosphors. The inset shows the digital photographs of Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors at daylight (left) and under the 365 nm UV lamp (right).

Fig. 11 Excitation line of BaSO₄ and emission spectrum of the Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors recorded by a spectrofluorometer equipped with an integrating sphere. The inset shows the magnified emission spectrum in the wavelength range 500–750 nm.

3.7. Thermal stability of the Eu³⁺-activated Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors

The thermal stability of phosphor is one of the essential parameter to find potential application in the high-power WLEDs.¹³ Fig. 12(a) shows the PL spectra of Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors recorded at various temperatures ranging from 303–483 K under excitation at 396 nm. The spectra clearly indicated that the PL intensity decreased with increasing temperature owing to thermal quenching effect. Generally, the PL intensity of phosphors at 423 K with respect to that at room temperature (303 K) is utilized to assess the thermal stability.⁹ It is evident that the PL intensity decreased to 68.7% at 423 K in comparison with that of its initial PL intensity at room temperature (303 K), as shown in Fig. 12(b). To better understand the temperature effect on the PL intensity of the phosphors, the activation energy was calculated according to the Arrhenius equation, which can be illustrated below:⁹where I₀ and I(T) stands for the initial PL intensity and the PL intensity at different given temperature T, ΔE_a represents the activation energy for thermal quenching, c is a constant for the host, and k is the Boltzmann constant (8.629 × 10⁻⁵ eV K⁻¹). The value of ΔE_a can be estimated through the eqn (8) which is modified by the eqn (7) as follows:⁵³

Fig. 12 (a) Temperature dependent PL spectra of Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphor (λ_ex = 396 nm). (b) Normalized PL intensity of Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors as a function of different temperatures. (c) The relationship of ln(I₀/I − 1) versus 1/kT of the Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors.

The plot of ln[(I₀/I) − 1] versus 1/kT yielded a straight line, as illustrated in Fig. 12(c). The slope of the fitting line is around −0.282 and thus the activation energy ΔE_a was determined to be 0.282 eV for Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors, which was higher than the other Eu³⁺ activated Na₃Sc₂(PO₄)₃, Ca₃La(GaO)₃(BO₃)₄ and BaZrGe₃O₉:Eu³⁺ red phosphors (ΔE_a: 0.23,0.25 and 0.175 eV).^9,37,54 This high activation energy suggested that the obtained phosphors have good thermal stability and thus they are suitable for applying in WLEDs.

4. Conclusion

In summary, Eu³⁺-activated Ba₂Y_5−xEu_xB₅O₁₇ (0.1 ≤ x ≤ 1.0) red emitting phosphors have been synthesized by a high-temperature solid-state reaction. Under excitation at 396 nm, the Ba₂Y_5−xEu_xB₅O₁₇ phosphors exhibited intense red emission at 616 nm due to the electric dipole transition ⁵D₀ → ⁷F₂ of Eu³⁺. The optimum concentration of Eu³⁺ in the Ba₂Y_5−xEu_xB₅O₁₇ phosphor was determined to be x = 0.8. The corresponding concentration quenching mechanism was evidenced to be d–d interaction. Meanwhile, the Ba₂Y_4.2Eu_0.8B₅O₁₇ red phosphor also exhibited impressive thermal stability, which showed about 68.7% at 423 K of its initial emission intensity at room temperature 303 K. As a result, the optimal Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphor presented significant red emission intensity, excellent color purity with color coordinates (0.653, 0.345) and the IQE of 47.2%. These results imply that the Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors can serve as potential red emitting phosphors for n-UV pumped WLEDs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51502190, 61674074, 51402148 and 61405089), National Key Research and Development Program of China administrated by the Ministry of Science and Technology of China (No. 2016YFB0401702), Shenzhen Peacock Team Project (No. KQTD2016030111203005), Shenzhen Innovation Project (No. JCYJ20160301113356947, JCYJ20160301113537474, JCYJ20150630145302223), Distinguished Young Scholar of Natural Science Foundation of Guangdong (No. 2017B030306010), Foshan Innovation Project (No. 2014IT100072), the start-up fund from Southern University of Science and Technology, and the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology, No. 2017-skllmd-01).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra03059f

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