Highly Efficient Blue Emission and Superior Thermal Stability of

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Cite This: Chem. Mater. 2018, 30, 2389−2399

Highly Efficient Blue Emission and Superior Thermal Stability of BaAl12O19:Eu2+ Phosphors Based on Highly Symmetric Crystal Structure Yi Wei,† Ling Cao,† Lemin Lv,† Guogang Li,*,† Jiarui Hao,† Junsong Gao,† Chaochin Su,∥ Chun Che Lin,*,∥ Ho Seong Jang,§ Peipei Dang,‡ and Jun Lin*,‡ †

Engineering Research Center of Nano-Geomaterials of Ministry of Education, Faculty of Materials Science and Chemistry, China University of Geosciences, 388 Lumo Road, Wuhan, Hubei 430074, P. R. China ‡ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China ∥ Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei 106, Taiwan § Materials Architecturing Research Center, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea S Supporting Information *

ABSTRACT: Highly efficient phosphor materials with superior thermal stability are indispensable for phosphor-converted white light-emitting diodes (pc-WLEDs) solid state lighting. In order to obtain a high quality warm white light, near-ultraviolet (n-UV) chips combined with trichromatic phosphors have be extensively studied. Among them, the development of efficient blue phosphor remains a challenging task. In view of the close correlation between 5d−4f transitions of rare earth ions and the coordination environment of host lattice, many studies have been dedicated to improving the photoluminescence performances by modifying the lattice coordination environment including the lattice rigidity and symmetry. In this work, we reported highly efficient blue-emitting Eu2+-doped BaAl12O19 (BAO) phosphors with excellent thermal stability, which were prepared via the traditional high-temperature solid state reaction routes. According to the X-ray powder diffraction (XRD) Rietveld refinement analysis, BAO owned a highly symmetric layer structure with two Ba polyhedrons, marked as Ba(1)O9 and Ba(2)O10, respectively. The diffuse reflectance spectra revealed the optical band gap to be 4.07 eV. Due to the suitable optical bandgap, the Eu2+ ions could realize a highly efficient doping in the BAO matrix. The photoluminescence excitation (PLE) spectra for asprepared BAO:Eu2+ phosphors exhibited a broad absorption band in the region from 250 to 430 nm, matching well with the nUV LED chip. Under the UV radiation, it is highly luminous (internal quantum yields (IQYs) = 90%) with the peak around 443 nm. Furthermore, the color purity of BAO:Eu2+ phosphors could achieve 92%, ascribing to the narrow full width at halfmaximum (fwhm = 52 nm), which was even much better than that of commercially available BAM:Eu2+ phosphor (color purity = 91.34%, fwhm = 51.7 nm). More importantly, the as-prepared BAO:Eu2+ phosphor showed extra high thermal stability when working in the region of 298−550 K, which was a bit better than that of commercial BAM:Eu2+ phosphors. According to the distortion calculation of Ba crystallographic occupation, the superior thermal stability could be attributed to the highly symmetric crystal structure of BAO host. In view of the excellent luminescence performances of BAO:Eu2+, it is a promising blue-emitting phosphor for n-UV WLED.

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INTRODUCTION

Received: January 31, 2018 Revised: March 15, 2018 Published: March 15, 2018

Recently, phosphor-converted white light-emitting diodes (pcWLEDs) lighting has been widely integrated into our daily lives © 2018 American Chemical Society

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DOI: 10.1021/acs.chemmater.8b00464 Chem. Mater. 2018, 30, 2389−2399

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Chemistry of Materials

light with the peak around 443 nm and internal quantum yields (QYs) of about 90%. Furthermore, the color purity of BAO:xEu2+ phosphors could achieve 92.4%, ascribing to the narrow fwhm (52 nm), which is even much better than that of commercially available BAM:Eu2+ phosphor (color purity = 91.34%; fwhm = 51.7 nm). More importantly, the as-prepared BAO:xEu2+ phosphor shows extra high thermal stability when working in the region of 298−550 K, which is a bit better than that of commercial BAM:Eu2+ phosphor. According to the distortion calculation of Ba crystallographic occupation, the superior thermal stability could be attributed to the highly symmetric crystal structure and rigidity of BAO host lattice. In view of the excellent luminescence performances of BAO:xEu2+ phosphor, it is a promising blue-emitting phosphor for n-UV based WLEDs lighting sources.

due to its high energy efficiency, low energy consumption, reliability, durability, long lifetime, and eco-friendly features.1−6 A typical pc-WLEDs is commonly fabricated by the combination of a blue (460 nm) InGaN chip with a yellowish YAG phosphor (Y3Al5O12:Ce3+).7−11 However, this kind of pcWLED easily suffers from poor color-rendering indices (Ra < 75) and highly correlated color temperatures (CCT > 4500 K) due to the absence of red light components at the long wavelength region,12−15 resulting in an uncomfortable cold white light. To overcome this defect, an alternative approach to achieve high-quality warm white lighting is to match the nearultraviolet (n-UV; 380−420 nm) LED chip with trichromatic (red, blue, and green) phosphors.16 Therefore, highly efficient trichromatic phosphor materials with superior thermal stability are indispensable. Among them, Eu2+ and Ce3+ activated inorganic phosphors that have superb chemical and optical advantages are highly desired for n-UV based WLEDs solid state lighting.6,17−21 Presently, the study of high-efficiency blue emitting phosphor has drawn the attention of many researchers. The most reported one is BaMgAl10O17:Eu2+ (BAM:Eu2+), which was commonly used as commercial blueemitting phosphors for n-UV pumped WLEDs.22,23 Many strategies have been employed to improve the luminescence of BAM:Eu2+ phosphor for realizing the high-quality indoor lighting. For example, Lee et al.24 developed a “thermal-shock method” to obtain more efficient BAM:Eu2+. Yin et al.25 reported a novel method of carbon coating on BAM:Eu2+ phosphor particles through chemical vapor deposition, enhancing emission intensity and oxidation resistance at high temperature. However, those methods are complicated, timeconsuming, and costly and, thus, limit their commercial application. In view of a close correlation between 5d−4f transitions of rare earth ions and the coordination environment of host lattice, many studies have also been dedicated to improving the photoluminescence performances and designing newly highly efficient blue-emitting phosphor materials by the chemical modification of host lattice. Kim et al.26 reported a zero-thermal-quenching Na3−2xSc2(PO4)3:xEu2+ blue-emitting phosphor. Lian et al.27 obtained narrow-band blue-emitting NaBa4(AlB4O9)2X3 (X = Cl, Br):Eu2+ phosphors at low synthesized temperature. Zheng28 realized a blue emission in Sr5(PO4)3Cl:Eu2+ phosphor, which owned an excellent colorrendering index (Ra = 94.65), but those phosphors usually suffered poor absorption around the n-UV region and terrible thermal stability. Accordingly, the development of efficient blue phosphor remains a challenge and needs to be a further focus. Rare-earth activated BaAl12O19 luminescence materials have previously been reported because of a facile synthesis, low cost, excellent optical properties, and high physical−chemical stability.29−34 In this work, we realized a highly efficient blue emission in BaAl12O19 (BAO) by Eu2+ doping, accompanying an excellent thermal stability, which was prepared via the traditional high-temperature solid state route. According to the X-ray powder diffraction (XRD) Rietveld profile refinement, BAO owned a highly symmetric layer structure with two Ba polyhedrons, marked as Ba(1)O9 and Ba(2)O10, respectively. The diffuse reflectance spectra reveal the optical band gap to be 4.07 eV. Due to the suitable optical band gap, the Eu2+ ions could realize a highly efficient doping in the BAO matrix. The photoluminescence excitation (PLE) spectra for as-prepared BAO:xEu2+ phosphors exhibit a broad absorption band in the region from 250 to 430 nm, matching well with the n-UV LED chip. Under the n-UV radiation, it emits highly luminous blue

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EXPERIMENTAL SECTION

Materials Synthesis. The Ba1−xAl12O19:xEu2+ (0 ≤ x ≤ 50%) powders were made via the traditional high-temperature solid state reaction. The raw materials were BaCO3 (Sigma-Aldrich, 99.95%), Al2O3 (Sigma-Aldrich, 99.99%), Eu2O3 (Sigma-Aldrich, 99.99%), BaF2 (Aladdin, 99.9%), BaCl2·2H2O (Aladdin, 99%), and BaBr2 (Aladdin, 99%). Simultaneously, BaF2, BaCl2·2H2O, and BaBr2 were also used as fluxes. Stoichiometric amounts of the required cationic sources were weighed and ground together for 40 min with a small amount of ethanol using an agate mortar and pestle until the mixtures were almost dry. The obtained mixtures were then transferred into aluminum oxide crucibles and then treated in the horizontal tube furnace at 1400 °C for 15 h under a reduced atmosphere of H2 (10%)−N2 (90%). The annealed samples were again ground, forming the resulting phosphor materials. Characterization. The finely ground powders were used in all measurements. The crystal structure and phase purity of the asprepared samples were characterized by X-ray powder diffraction (XRD), which was performed on a D8 Focus diffractometer at as canning rate of 1° min−1 in the 2θ range from 5° to 120° with Nifiltered Cu Kα (λ = 1.540598 Å). XRD Rietveld profile refinements of the structural models and texture analysis were performed with the use of General Structure Analysis System (GSAS) software. The morphologies of the samples were inspected using a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi). The photoluminescence excitation (PLE) and emission (PL) spectra were measured by a fluorescence spectrometer (Fluoromax-4P, Horiba JobinYvon, New Jersey, U.S.A.) equipped with a 450 W xenon lamp as the excitation source, and both excitation and emission spectra were set up to be 1.0 nm with the width of the monochromator slits adjusted to 0.50 nm. The thermal stabilities of luminescence properties were measured by Fluoromax-4P spectrometer connected heating equipment (TAP-02) and using a combined setup consisting of a Xelamp, a Hamamatsu MPCD-7000 multichannel photodetector, and a computer-controlled heater. The diffuse reflectance spectra (DRS) were measured by UV−visible diffuse reflectance spectroscopy UV2550PC (Shimadzu Corporation, Japan). The photoluminescence quantum yield (QY) was measured by an absolute PL quantum yield measurement system C9920-02 (Hamamatsu photonics K.K., Japan). The photoluminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Osilloscope (1 GHz) using a tunable laser (pulse width = 4 ns; gate = 50 ns) as the excitation (Contimuum Sunlite OPO). Solid state nuclear magnetic resonance (NMR) spectra were obtained with the use of a Bruker DSX 300 MHz NMR spectrometer, equipped with a 4 mm double-resonance magic anglespinning (MAS) probe. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Mg Kα radiation source (Kratos XSAM-800 spectrometer). 2390

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Chemistry of Materials Table 1. Crystallographic Parameters Obtained from XRD Rietveld Refinements for BAO:xEu2+(0 ≤ x ≤ 20%) crystallographic parameter

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reliability factors

samples

a/b (Å)

c (Å)

V (Å3)

α/β (deg)

γ (deg)

Rp (%)

Rwp (%)

x = 0% x = 1% x = 5% x = 10% x = 20% crystal system space group

5.59488(5) 5.59485(1) 5.59464(8) 5.59413(9) 5.59256(7)

22.8755(1) 22.8737(5) 22.8740(4) 22.8725(5) 22.8617(4)

620.130(1) 620.076(2) 620.035(2) 619.883(2) 619.242(2)

90 90 90 90 90

120 120 120 120 120

7.00 7.43 6.83 8.84 7.70

5.11 5.22 4.73 6.13 5.27

hexagonal P63/mmc

620.130(11) Å3 for BAO host and a = b = 5.59485(1) Å, c = 22.8737(5) Å, V = 620.076(21) Å3 for BAO:5%Eu2+, respectively, as shown in Table 1. The atom positions, fraction factors, and thermal vibration parameters were refined by convergence and satisfied well the reflection conditions, Rwp = 7.00%, Rp = 5.11% for x = 0 and Rwp = 6.83%, Rp = 4.73% for x = 5% (Table 1). These results indicate the formation of single-phased BAO crystals. Figure 1c shows the XRD patterns of BAO:xEu2+ (1 ≤ x ≤ 25%) samples, which could be well indexed with the standard card of BaAl12O19 (ICSD No. 79359), and there are no impurity phases with the doping of Eu2+ ions (x) from 1% to 25%; the enlarged diffraction peak position of 2θ = 35−37° shifts to the larger angle direction, indicating the as-prepared samples are all pure hexagonal phase and the perfect enter into host lattice of Eu2+ ions by replacing Ba2+ ions. However, a impurity peak appeared at about 2θ = 33.5° when the value of x is more than 25%, as shown in Figure S3a, which are assigned to the EuAl12O19 phase. Therefore, the maximum doping concentration of Eu2+ ions is fixed at x = 25%. In addition, the refined cell parameters as a function of x are shown in Figure 1d. Obviously, the lattice parameters (a/b, c, and V) exhibit a monotonous, linear decreasing trend as x increases from 1% to 20%. That is because of the substitution with the smaller Eu2+ ions (r = 1.3 Å for CN = 9; r = 1.35 Å for CN = 10) where CN signifies the coordination numbers. This phenomenon further indicates the successful incorporation of Eu2+ into the Ba2+ ions (r = 1.47 Å for CN = 9, r = 1.52 Å for CN = 10) lattice by substituting Ba2+ and simultaneously confirms the formation of solid solutions at x < 25%. According to the refinement results, a schematic spatial view of the BaAl12O19:Eu2+ unit cell is shown in Figure 2. Interestingly, along the b-axis direction, the studied crystal exhibits a layered network structure. It contains five kinds of Al

RESULTS AND DISCUSSION Crystal Structure and Lattice Parameters. The phase purity and structure type of as-prepared BAO:xEu2+ (0 ≤ x ≤ 50%) powders were investigated by XRD and Rietveld profile refinements. The refined crystallographic parameters of a series of BAO:xEu2+ (1 ≤ x ≤ 25%) samples were summarized in Table 1. Three kinds of fluxes (BaF2, BaCl2·2H2O, and BaBr2) were chosen to promote the crystallinity of BAO:xEu2+ samples. In the same conditions, the as-prepared samples using BaF2 as flux had the best crystallinity (Figure S1), and the optimal concentration of BaF2 into BAO:xEu2+ was 5 wt % (5 wt % represents the weight percentage) of as-prepared powders (Figure S2). Figure 1a,b successively shows the XRD Rietveld refinement results of BAO host and the representative BAO:xEu2+ samples. The refined lattice parameters imply that the two samples all formed hexagonal phase with space group P63/mmc (194), a = b = 5.59488(5) Å, c = 22.8755(1) Å, V =

Figure 1. Representative Rietveld refinement XRD patterns of BAO:xEu2+ samples: (a) x = 0 and (b) x = 5%. The measured data, fitting data, and their difference are depicted with black crosses, red solid line, and blue solid line, respectively. The blue short vertical lines show the positions of Bragg reflections of the fitting XRD patterns. (c) Typical XRD patterns of BAO:xEu2+ (1 ≤ x ≤ 25%) samples. The standard BaAl12O19 data (ICSD #79359) is shown as a reference. The right inset is the magnified XRD patterns for the 2θ region of 35°− 37°. (d−f) Give the cell parameters (a/b, c) and volume (V), respectively, as a function of Eu2+-doped concentration (x).

Figure 2. Schematic crystal structure of BaAl12O19:Eu2+ along the baxis direction, and the coordination polyhedrons of Ba(1) site (Wyck. 2d) and Ba(2) site (Wyck. 4f) and the coordination of Al(1)−Al(5). 2391

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Figure 3. (a) The typical SEM image of Ba0.9Al12O19:10%Eu2+ sample, and it is mapping images of elements: (b) Ba L-edge, (c) Eu L-edge, (d) Al Kedge, and (e) O K-edge.

increases first and reaches the maximum value at 5 wt % of raw

sites, marked as Al(1)−Al(5). Those Al sites could be coordinated with four or six oxygen atoms, specifically, the Al(1) and Al(4) form AlO6 octahedrons, while Al(2), Al(3), and Al(5) are all AlO4 tetrahedrons. AlO4 polyhedrons connect each other with vertexes; AlO6 polyhedrons connect each other also with vertexes, and the two connections of AlO4 and AlO6 appeared alternately. These AlO4 tetrahedrons and AlO6 octahedrons construct a rigid three-dimensional network by sharing the vertexes, edges, and faces in the BAO lattice. On the basis of the above structure, there are two Ba sites: the one perches the AlO4 three-dimensional network, marked Ba(1) (Wyck. 2d), and the other one occupies the interval of AlO6 layer, marked as Ba(2) (Wyck. 4f). The sites of Ba(1) and Ba(2) are coordinated with 9 O and 10 O atoms, forming the Ba(1)O9 and Ba(2)O10 polyhedrons with 16 faces, respectively. Due to the slightly larger ion radius of Ba(1)O9 and Ba(2)O10 polyhedrons, it is suitable for accommodating Eu2+ ions, and thus, they are inferred to randomly occupy Ba(1) and Ba(2) sites. It is well-known that the luminescence intensity is closely related to the crystallinity, size, and morphology of phosphor particles. The SEM image of the representative BAO:10%Eu2+ sample is shown in Figure 3a. Clearly, the studied sample presents an irregular approximate spherical morphology with a size range of 0.5−2 μm. Moreover, a slight agglomeration of these phosphor particles could be observed. Figure 3b also gives the SEM mapping images of BAO:10%Eu2+, which demonstrates that the sample contains Ba, Al, Eu, and O elements, and they evenly distribute in the whole sample. Furthermore, the SEM-EDS analysis of BAO:10%Eu2+ demonstrates the average composition (15 points of different positions) of Al/(Ba + Eu) = 11.84:1, which is very close to the theoretical ratio of Al/Ba = 12:1. This result is consistent with the previous XRD results, which further confirms the formation of pure BaAl12O19 phase (Table S1). Photoluminescence Properties of BAO:Eu2+ Phosphors. The type and dosage of flux not only could change the crystallinity of BAO:xEu2+ phosphors but also have a significant impact on their luminescence properties. Figure S4 plots the PLE and PL spectra with changing the type and dosage of fluxes. It can be observed that the spectral profile and position of BAO:xEu2+ samples remain unchanged, while the peak intensity are obviously different. When using BaF2 as flux, the peak intensity visibly increases, compared to nonflux sample (Figure S4a). However, BaCl2·2H2O and BaBr2 do not play their role effectively. Meanwhile, the emission intensity clearly

materials as the dosage of BaF2 flux increases (Figure S4b). This consequence is identical to the XRD analysis of different doping content of BaF2. Therefore, the photoluminescence properties are measured at the series of BAO:xEu2+ (1% ≤ x ≤ 50%) samples prepared using 5 wt % BaF2 as flux.

Figure 4. Diffuse reflectance spectra of BAO:xEu2+ (0 ≤ x ≤ 25%) samples. The inset shows the relationship of [F(R∞)hv]1/2 vs photon energy hv.

Figure 4 shows the diffuse reflectance spectra of BAO:xEu2+ (0 ≤ x ≤ 25%) samples. The optical band gap of BAO could be calculated by the following equation35−40 [F(R ∞)hv]1/2 = A(hv − Eg ) F(R ∞) = (1 − R )2 /2R

(1)

where A represents the absorption constant, Eg is the optical band gap, hv represents the photon energy, F(R∞) is the absorption, and R is the reflectance (%) coefficient, respectively. The inset of Figure 4 depicts the plots of [F(R∞)hv]1/2 versus hv of BAO matrix. The calculated Eg value for BAO sample is 4.07 eV, illustrating that this host is a very appropriate matrix for rare earth luminescence. For BAO:xEu2+ samples, there are remarkable absorption in the region from 200 to 400 nm due to the 4f7 → 4f65d transitions of Eu2+ ions, which matches well with the n-UV LED chips. According to the results, it could be deduced that the Eu2+ ions could realize efficient emission in BAO host. 2392

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Figure 5. (a) The photoluminescence excitation (PLE) and emission (PL) spectra of BAO:xEu2+ (1% ≤ x ≤ 25%) samples. (b) The emission peak intensity and wavelength as a function of Eu2+ concentration. (c) The luminescence comparison of commercial BaMgAl10O17:Eu2+ (marked as BAM:Eu2+) and the as-prepared BAO:25%Eu2+. (d) XPS of BAO:25%Eu2+. The inset gives the 3d3/2 and 3d5/2 XPS peaks of the Eu element. (e) The CIE chromaticity coordinates diagram for BAM:Eu2+ and BAO:25%Eu2+. (f, g) The luminescence photos of BAM:Eu2+ and BAO:25%Eu2+ under 365 nm UV light.

Figure 5a presents the typical PLE and PL spectra of BAO:xEu2+ (1% ≤ x ≤ 25%) samples. It can be observed that the PLE spectra show a strong absorption in the range from 250 to 430 nm with the peak position at 350 nm, which is corresponding with the results of the diffuse reflectance spectra. Under the excitation wavelength of 350 nm, the BAO:xEu2+ emits bright blue light and its emission spectra consist of a narrow band from 400 to 525 nm; the maximum peak position is around 443 nm. Interestingly, the PL intensity of BAO:xEu2+ (1% ≤ x ≤ 25%) continuously increases with x, reaching a maximum at x = 25% (Figure 5a,b). There is no concentration quenching phenomenon in the pure phase range. The quantum yields of BAO:xEu2+ (1% ≤ x ≤ 25%) samples present a similar result and reach the maximum 92% at x = 25%. The possible reason is that the layered network structure and large Ba(1)O9 and Ba(2)O10 crystallographic sites could accommodate more Eu2+ ions into the lattice to efficiently emit. However, an obvious decrease in PL intensity appears at x beyond 25% (Figure S5), which is easily attributed to the existence of EuAl12O19 impurity phase (Figure S3a). Furthermore, a slight red shift can also be observed with enhancing the doping content of Eu2+ ions (Figure 5b). The following equation could help explain the red-shift phenomena41,42

Dq =

1 2 r4 Ze 6 R b5

(2)

where Dq is the crystal field splitting energy, Z is the anion charge, e is the electron charge, r is the radius of the d wave function, and Rb is the bond length. When the Ba2+ ion is substituted and occupied by a smaller Eu2+ ion, the distance between the Eu2+ and O2− ion becomes shorter. Because crystal field splitting is proportional to 1/Rb5, this shorter Eu2+−O2− distance leads to the enhancement of crystal field strength surrounding the Eu2+ ion and further results in a larger crystal field splitting of Eu2+ 5d energy levels, which makes the lowest 5d state of Eu2+ ion closer to its ground state and finally generates a red shift of the emission spectra of Eu2+ ion. In addition, Figure S6 exhibits that luminescence decay lifetimes for BAO:xEu2+ (x = 10%, 20%, 25%) gradually increase from 571 to 600 ns with the increase of x value, which also verify the red-shift emission of BAO:xEu2+. To determine the luminescence performance of the studied samples, the commercial BAM:Eu2+ blue phosphor was chosen as a comparison. Figure 5c demonstrates the PLE and PL spectra of the optimal BAO:25%Eu2+ and BAM:Eu2+ phosphors upon exciting with 350 nm wavelength UV. Unexpectedly, the 2393

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emission spectra for BAO:xEu2+ (1% ≤ x ≤ 25%) are plotted in Figure 6a. At x = 1% and 3%, the emission spectra could be

BAO:25%Eu2+ has a higher emission intensity (with the integrated intensity of about 107%) than BAM:Eu2+ at the same measurement conditions (Figure 5c). In order to clarify the highly luminous blue emission, Figure 5d shows the X-ray photoelectron spectra (XPS) of the representative BAO:25% Eu2+ sample in the binding energy range of 0−1200 eV. The binding energy signal observed around 73, 280, 532, 781, and 1134 eV could be accounted for Al 2p, C 1s, O 1s, Ba 3d, and Eu 3d, respectively.43−45 Thus, the XPS analysis further confirms the formation of BaAl12O19 phase and the presence of Eu in the lattice. Especially, the XPS peaks at 1134.6 and 1164 eV originated from the Eu 3d5/2 and Eu 3d3/2, respectively. Although there are still trace amounts of Eu3+, the main peak belongs to Eu 3d5/2, namely, meaning that the activator Eu mainly existed in the form of +2 in the BaAl12O19 matrix. This result could imply the high luminescence efficiency of the as-prepared samples. In addition, the BAO:xEu2+ powders possess a narrow fwhm (52 nm), implying a highly pure blue emission with the CIE color coordination (0.1525, 0.0499), which is even better than that of BAM:Eu2+ (0.1443, 0.0614), as shown in Figure 5c,e. Their photoluminescence photos under 365 nm UV light in Figure 5f,g also support the above situation. The CIE color coordinate, fwhm, and QYs of the as-prepared BAO:xEu2+ samples are also summarized in Table S2. In the CIE diagram, the perimeter is formed by plotting all monochromatic color coordinates. The color purity of a dominant color is the weightaverage of the emission color coordinate relative to the coordinate of the dominate wavelength and the CIE white illumination coordinate, which can be calculated by the following equation27,28 colorpurity =

(x − x i)2 + (y − yi )2 (xd − x i)2 + (yd − yi )2

× 100% (3)

where (x, y) represents the CIE color coordination of asprepared samples and commercial BAM:Eu2+ phosphor, (xi, yi) is the white light source with CIE color coordinate (0.3333, 0.3333) in the reported work, and (xd, yd) is the color coordinate corresponding to the dominant wavelength of the light source. By substituting the coordinates of (x, y), (xi, yi), and (xd, yd) in eq 3, we can obtain color purity of as-prepared BAO:xEu2+ samples and BAM:Eu2+, respectively. The corresponding calculation results are collected in Table S2. It is found that the as-prepared BAO:xEu2+ sample presents the highest color purity of 92.40%, which is better than that of the commercially available BAM:Eu2+ phosphor (the color purity is 91.35%). Generally, these observations demonstrate that the BAO:xEu2+ manifest the better luminescence efficiency, the higher color purity, and the stronger absorption in 250−430 nm than the commercially available blue-emitting BAM:Eu2+ phosphor, which is a promising blue phosphor for n-UV based WLEDs. Controlled photoluminescence tuning is highly desired for rare earth ions activated phosphor materials, which is commonly used to optimize and modify luminescence properties. Chemical modification of host materials could induce the structural and electronic alterations in the local region and thus generates anisotropic local environments for rare earth ions, which finally shift the energy levels of the emission center and tune the spectral profile.6,39,41,46−52 To expound the highly pure blue emission, the Gaussian fitting of

Figure 6. (a) Gaussian fitting of the emission spectra for BAO:xEu2+ (1% ≤ x ≤ 25%) samples. (b) The calculated average distance of Al(2)−O and Al(4)−O with Rietveld refinement. (c) 27Al NMR spectra of BAO:xEu2+ (0 ≤ x ≤ 25%). (d) The proposed mechanism of the blue-shift emission for increasing Eu concentration. (e) The preferential site-occupancy of Eu2+ into Ba(1) and Ba(2) sites when x ≥ 5%.

fitted to two single peaks at 2.82 and 2.66 eV, respectively; however, when x ≥ 5%, the emission spectra are highly symmetrical and only retain a single peak at 2.82 eV. According to the average bond length of Ba(1)−O (2.72 Å) and Ba(2)−O (2.68 Å) (Table 1), the peaks at 2.82 and 2.66 eV of Eu2+ ions should be ascribed to the occupation of Ba(1) and Ba(2) sites in the BAO lattice, respectively. In addition, Figure 6b reveals that the average Al(2)−O and Al(4)−O distances lengthened with increasing x value. Thus, a possible mechanism is proposed as follows: In the low doping concentration (x ≤ 3%), Eu2+ ions randomly occupy Ba(1) and Ba(2) sites (Figure 6d). When increasing the x, the expansion of Al(2) and Al(4) polyhedron would lead to the neighboring Ba(2) polyhedrons shrinking, and thus, the Ba(2) sites could not accommodate the Eu2+ ions (Figure 6c). Finally, in the high doping concentration (x ≥ 5%), Eu2+ ions preferentially occupy Ba(1) sites (Figure 6e), generating a narrow blue emission. To further verify the effect of Al polyhedrons on the occupation of Eu2+ ions, the 2394

DOI: 10.1021/acs.chemmater.8b00464 Chem. Mater. 2018, 30, 2389−2399

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Chemistry of Materials solid state NMR spectra of BAO:xEu2+ (0 ≤ x ≤ 25%) are measured and collected in Figure 6c. For BAO host (x = 0), there are two main peaks in the −25 to 25 ppm region with the peak position at 8.44 ppm and the 50−80 ppm region with the peak position at 73.56 ppm. The two peaks are characteristic peaks of AlO4 and AlO6, respectively. As the value of x increases, the peak position of AlO4 polyhedrons moves slightly to large chemical shift direction and that of AlO6 turns to the small chemical shift direction, meaning the change of distortion of AlO4 and AlO6 polyhedrons. It can be observed that the ratio of IAlO4/IAlO6 (peak intensity) gradually increases when enhancing x value. Moreover, the AlO6 peaks become broader compared to that of AlO4 peaks, illustrating that the distortion degree of AlO6 octahedrons is more remarkable than AlO4 tetrahedrons, which is consistent with the variation of average Al−O distances via Rietveld refinement. Thermal Quenching Properties of BAO:xEu2+ Phosphors. The thermal stability of phosphors is an important performance objective in evaluating its potential for WLED application due to its influence on the light output, service life, and color rendering index.51−56 The thermal stability can be indexed through two aspects: the quenching intensity and the spectral shape with elevating the heating temperature. The temperature-dependent PL spectra of the representative BAO:1%Eu2+ and BAM:Eu2+ phosphors recorded from 298 to 500 K with 350 nm UV light are shown in Figure 7. Figure 7a,c gives the PL spectra of BAO:1%Eu2+ and BAM:Eu2+ from 298 to 500 K, respectively. Unsurprisingly, as the temperature rises, their emission intensity all decreased, because of intensified molecular heat movement at the high working temperature commonly exacerbating the nonradiative transitions.57 Unexpectedly, the as-prepared BAO:1%Eu2+ presents a very low luminescence quenching behavior, and its PL intensity still maintains 92.1% of the initial intensity measured at 298 K when measured at 473 K (Figure 7e). This result is even a little better than that of BAM:Eu2+ (91.1%) (Figure 7e). Except for the extreme thermal stability, the color stability of BAO:1%Eu2+ was also more superior than that of BAM:Eu2+. Clearly, it could be seen from Figure 7b,d that, when temperature increased from 298 to 498 K, the emission peak and fwhm of BAO:1% Eu2+ almost remained unchanged, while the fwhm of BAM:Eu2+ phosphor gradually broadened resulting in a slight blue-shift emission. Finally, the thermal quenching behavior of BAO:xEu2+ (x = 1%, 5%, 10%, 20%) samples with Eu2+-doped concentration (x) is also investigated. As shown in Figure 7f, their thermal stability gradually declines with increasing x value. A possible reason is that the increasing substitution of Ba2+ ions by Eu2+ ions would weaken the rigidity of the BAO lattice, resulting in the degradation of luminescence intensity with temperatures. Fortunately, the as-prepared BAO:xEu2+ (x = 1%, 5%, 10%, 20%) samples also exhibit outstandingly reversible thermal stability. When the heating temperature varies from 500 to 298 K, the emission intensity nearly returns to the initial intensity (Figure S7). Generally, the as-prepared BAO:1%Eu2+ manifested excellent thermal stability even better than that of commercial BAM:Eu2+ phosphor, which further shows its promising application in WLED devices. To further explain the temperature-dependent thermal quenching phenomenon, the activation energy for the thermal quenching has been estimated using the Arrhenius equation

Figure 7. (a, b) The normal and normalized PL spectra of the representative BAO:1%Eu2+ sample at various temperatures (298−498 K) (λex = 350 nm), respectively. (c, d) The normal and normalized PL spectra of the BAM:Eu2+ phosphor at various temperatures (298−498 K) (λex = 350 nm), respectively. (e) The thermal stability comparison between BAO:1%Eu2+ and BAM:Eu2+. (f) Temperature-dependent PL intensity of BAO:1%Eu2+, and the inset is the activation energy (Ea) of BAO:xEu2+ (1% ≤ x ≤ 25%) and BAM:Eu2+.

IT =

I0 E

( )

1 + c exp − kTa

(4)

where c is a constant, k is Boltzmann’s constant with a value of 8.62 × 10−5 eV·K−1, I0 is the initial emission intensity measured at room temperature, IT represents the emission intensity measured at different temperatures, and Ea is the activation energy for the thermal quenching. Rearranging eq 5 results in ln(I0/IT − 1) = −

Ea kT

(5)

The activation energy Ea can be obtained by plotting the ln(I0/ IT−1) vs 1/kT curve. Expressly, the thermal stability of phosphor becomes better with the increase of the value of Ea. Seen from the inset of Figure 7f, the calculated Ea of BAO:xEu2+ appears to have a declining trend with the doped Eu2+ concentration increasing, which could explain the enhancive thermal quenching of the studied samples with x. It is noteworthy that the Ea values of as-prepared BAO:xEu2+ (x = 1%, 5%, 10%) are indeed slightly higher than those of 2395

DOI: 10.1021/acs.chemmater.8b00464 Chem. Mater. 2018, 30, 2389−2399

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Chemistry of Materials commercial BAM:Eu2+, which could confirm its higher thermal stability compared to BAM:Eu2+. As mentioned in the previous section, the higher the rigidity of the host lattice, the better thermal stability of phosphor materials commonly could be expected. According to the Rietveld refinement results, we have determined that the crystal structure of BAO host was highly symmetric. Therefore, we speculate that the excellent thermal stability of BAO:xEu2+ (1% ≤ x ≤ 25%) resulted from the high symmetric lattice environment of Ba sites. To prove this inference, we have calculated the distortion of Ba coordinated polyhedrons, and the calculated lattice distortion vs x for different Ba sites was illustrated in Figure 8. The equation is as follows51,52,58−60

thermal stability of blue emission of Eu2+ in BAO should be attributed to the high structure rigidity resulted from the high symmetry of Ba coordinated polyhedrons. LEDs Applications. To evaluate the device performance of the as-prepared BAO:Eu2+ phosphors, we fabricated blue LEDs and white LEDs with the representative blue BAO:20%Eu2+ phosphor combining UV (370 nm) chip and blue BAO:20% Eu2+, green Ba3Si6O12N2:Eu2+, and CaAlSiN3:Eu2+ phosphors combining UV (370 nm) chip; the electroluminescence spectra are shown in Figure 9. Under a voltage of 3.15 V and current of

n

D=

|d − d | 1 ∑ i av n i=1 dav

(6)

Figure 8. Distortion of Ba(1) and Ba(2) sites in BAO:xEu2+ (1% ≤ x ≤ 25%) with the various Eu2+-doped concentrations (x).

where D represents the lattice distortion, di is the distance from Ba to the ith coordinating O atoms, dav is the average Ba−O distance, and n is the coordination numbers. From Figure 8, the distortion index of Ba(1) site and Ba(2) site apparently increases as the doped Eu2+ concentration is raised, leading to the gradually decrease of crystal symmetry, which is consistent with the result of Figure 7f that the thermal stability declines with x. Thus, it is reasonable to infer that the thermal stability is related to the lattice symmetry, which would become much stronger with the reduction of distortion, while a high symmetry of the lattice means a better rigidity of crystal structure that is viewed as an ideal indicator for efficient photoluminescence and high thermal stability.53 According to the previous reports, Debye temperature (ΘD) is demonstrated to have positive correlation with the rigidity, and the function can be described as follows8,54,61−65 ΘD, i =

3ℏ2TNA A ikBUiso, i

Figure 9. Electroluminescence spectra of (a) blue LEDs fabricated by BAO:20%Eu2+ and 370 nm GaN chip and (b) white LEDs fabricated by BAO:20%Eu2+, Ba3Si6O12N2:Eu2+, CaAlSiN3:Eu2+, and 370 nm GaN chip. The insets of (a) and (b) are the photographs of LEDs. The working voltage and driving current are 3.15 V and 20 mA, respectively.

20 mA, the blue LEDs present high efficient luminescence with color purity of 87.8% and CIE color coordinate of (0.1578, 0.0769). In addition, the warm white with low corresponding color temperature (CCT = 3084 K), high color rendering index (Ra = 92.2), and CIE color coordinate of (0.4147, 0.3638) could be garnered. These results suggest that the as-prepared BAO:Eu2+ could be an excellent candidate for a blue-emitting phosphor material for application in near-UV LEDs.

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(7)

CONCLUSION In this work, the highly efficient blue-emitting Ba1‑xAl12O19:xEu2+ (1% ≤ x ≤ 25%) phosphors with excellent thermal stability were successfully synthesized by the traditional high-temperature solid state route. The structure refinement indicates that the as-prepared samples crystallize into a hexagonal unit cell with the space group P63/mmc (194), and two Ba sites occupy 2d and 4f Wyck. positions, forming highly symmetrical Ba(1)O9 and Ba(2)O10, respectively. The as-

It is noticeable that a smaller average atomic displacement parameter (Uiso) could result in a higher Debye temperature. In this work, the average Uiso value is calculated from the XRD Rietveld refinement for BAO:xEu2+ (x = 1%, 5%, 20%), and the corresponding results are listed in Table S3. Evidently, the average Uiso increases as the x value increases. Therefore, the rigidity gradually decreases with the increase of x value, which also proves the decreasing thermal stability. Therefore, the high 2396

DOI: 10.1021/acs.chemmater.8b00464 Chem. Mater. 2018, 30, 2389−2399

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Chemistry of Materials prepared BAO:xEu2+ phosphors exhibit an efficient absorption in the region from 250 to 430 nm, matching well with the n-UV LED chip. Under 350 nm UV excitation, they exhibit a highly efficient blue emission with the peak at around 443 nm (internal quantum yields (IQYs) = 89.6% for x = 25%). Moreover, the as-prepared samples present a high color purity (∼92%) and excellent thermal stability (>92% of room temperature intensity at 498 K), which is better than that of commercially available BAM:Eu2+ phosphor. The highly pure blue emission is ascribed to the preferential occupation of single Ba(1) sites at a high Eu2+-doping concentration. Because the expansion of Al(2) and Al(4) polyhedron leads to the neighboring Ba(2) polyhedron to shrink, the Ba(2) site could not accommodate the Eu2+ ions. Finally, the extra high thermal stability of BAO:Eu2+ is related to the highly symmetrical lattice environment of Ba sites. Generally, the as-prepared Ba1‑xAl12O19:xEu2+ (1% ≤ x ≤ 25%) phosphors exhibit high luminescence efficiency, color purity, and superior thermal stability under n-UV excitation, which can act as potential blueemitting phosphors for promising applications in n-UV based white LEDs lighting and display area.

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dependent emission spectra in BaAl12O19:Eu2+ phosphor powders.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b00464. XRD, PLE, PL, decay curves, temperature-dependent PL spectra, refined crystallographic parameters, EDS, CIE color coordinates, emission peaks and fwhm of BAO:xEu2+ (1% ≤ x ≤ 50%), fractional atomic coordinates and isotropic atomic displacement parameters (Å2) of BAO:xEu2+ (x = 1%, x = 5%, x = 20%) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ho Seong Jang: 0000-0002-2031-1303 Jun Lin: 0000-0001-9572-2134 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 51672259, 51672265, 21521092, 51750110511), Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences (Wuhan) (No. NGM2016KF002), the National College Students’ Innovative Training Program (Nos. 201710491016, 201710491115, 201710491130), the Ministry of Science and Technology of Taiwan (Contract No. MOST 104-2113-M-027-007-MY3), the Key Research Program of Frontier Sciences, CAS (Grant No. YZDY-SSW-JSC018), and projects for science and technology development plan of Jilin province (20170414003GH). The authors also thank Dr. Yixi Zhuang and Prof. Rongjun Xie from Xiamen University for their kind help in the measurement of the temperature2397

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DOI: 10.1021/acs.chemmater.8b00464 Chem. Mater. 2018, 30, 2389−2399

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DOI: 10.1021/acs.chemmater.8b00464 Chem. Mater. 2018, 30, 2389−2399