Discovery of blue-emitting Eu2+-activated sodium aluminate phosphor with high thermal stability via phase segregation

Journal Pre-proofs Discovery of Blue-Emitting Eu2+-Activated Sodium Aluminate Phosphor with High Thermal Stability via Phase Segregation Xujian Zhang, Jilin Zhang, Xiangli Wu, Liping Yu, Yongfu Liu, Xuhui Xu, Shixun Lian PII: DOI: Reference:

S1385-8947(20)30280-1 https://doi.org/10.1016/j.cej.2020.124289 CEJ 124289

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

27 December 2019 29 January 2020 30 January 2020

Please cite this article as: X. Zhang, J. Zhang, X. Wu, L. Yu, Y. Liu, X. Xu, S. Lian, Discovery of Blue-Emitting Eu2+-Activated Sodium Aluminate Phosphor with High Thermal Stability via Phase Segregation, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124289

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Discovery of Blue-Emitting Eu2+-Activated Sodium Aluminate Phosphor with High Thermal Stability via Phase Segregation Xujian Zhang,† Jilin Zhang,*,† Xiangli Wu,† Liping Yu,† Yongfu Liu,*,‡ Xuhui Xu,¶ Shixun Lian† †Key

Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of

Education of China), Key Laboratory of Sustainable Resources Processing and Advanced Materials of Hunan Province College, Hunan Normal University, Changsha 410081, China. ‡Ningbo

Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of

Sciences (CAS), Ningbo 315201, China. ¶College

of Materials Science and Engineering, Kunming University of Science and Technology,

Kunming 650093, China. Corresponding Author *[email protected] (J. Zhang). *[email protected] (Y. Liu).

ABBREVIATIONS LED, light-emitting diodes; XRD, X-ray powder diffraction; BSE, backscattered electron; CL, cathodoluminescence; CL-SEM, cathodoluminescence-combined scanning electron microscope; PLE, photoluminescence excitation; PL, emission; QE, quantum efficiency; IQE, internal QE; EQE, external QE; TEM, transmission electron microscope; EDS, energy dispersive X-ray spectroscopy; FFT, Fast Fourier Transform; CIE, International Commission on Illumination; EL, electroluminescence.

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ABSTRACT: Thermally stable phosphors are crucial for the application in phosphor-converted white light-emitting diodes. This paper reports the evolution of phase compositions in nominal Na2-xAl2B2O7:xEu (x = 0.02 – 2.0) upon replacing Na atoms by Eu atoms. Eu2+-activated blueemitting phosphor was discovered through phase segregation combined with related analyses. The structure, phase composition and related luminescent properties were investigated by X-ray powder diffraction, backscattered electron image combined with elemental mapping, cathodoluminescence-combined scanning electron microscope, and temperature-dependent emission spectra, etc. Analyses results suggest that EuBO3 phase segregates upon the gradual replacement of Na by Eu atoms, which is accompanied by phase transformation from Na2Al2B2O7:Eu2+ to NaAl11O17:Eu2+ and dramatic enhancement of blue emission. The broad emission band is peaking at around 470 nm under UV irradiation. The internal quantum efficiencies of nominal Na0.4Al2B2O7:1.6Eu and Na0.2Al2B2O7:1.8Eu are 58.6% and 60.2%, respectively, which contain both NaAl11O17 and EuBO3 phase. The emission intensity for these two samples at 150 °C remains 96% (x = 1.6) and 83% (x = 1.8) of the room-temperature values, respectively.

KEYWORDS: thermal stability; phosphor; photoluminescence; phase segregation; Eu2+.

1. Introduction Inorganic phosphors have applications in many fields, such as illumination, display, agricultural light conversion, biological and medical analyses, etc.1-3 Different applications require phosphors with different luminescent properties. When they are used in white light-emitting diodes (LED) for the general illumination, for example, phosphors should have proper excitation band matching

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the emission band of LED chip, broad emission band, high quantum efficiency, and good thermal stability, etc.1, 4 When the LED is used as the backlight of the display device, the related phosphors should have a narrow emission band to achieve a wide color gamut.5 And the phosphors should convert green light to red, or near UV to blue/red with special wavelength range if they are utilized in plant growth.6-8 Therefore, as a key component, the development of new phosphors and the adjustment of luminescence properties are critical for such applications. The luminescent properties of rare earth or transition metal ion doped phosphors depend on both host structure and activator. Therefore, it is understandable that the strategies for the exploration of phosphors and/or the adjustment of luminescence properties include but not limit to: (1) finding new host structures combined with specific activator, such as Eu2+-doped nitrides9-14 and Mn4+doped fluosilicates15-17; (2) introducing different activators into a same host18-21; (3) adjusting the coordination environment of Eu2+/Ce3+ activator by solid-solution method22-26; (4) developing phosphors that are isostructural to the present hosts, e.g. Ce3+-doped phosphors with Y3Al5O12 garnet structure27-33, Eu2+-doped UCr4C4-type phosphors34-38; (5) selective occupation of activator on different cation crystallographic sites39-44, etc. Phase segregation is a common phenomenon in synthesis of inorganic compounds or materials, which can be used to develop new compounds, materials, or to adjust the properties of materials. Anisotropic migration and phase segregation of Pt in Pt-Ni nanocrystals provide a new approach to fabricate nanocatalysts with desired compositional distribution and performance.45 Controlling surface chemistry by local elemental segregation in LiNi1-x-yMnxCoyO2 cathode material is important to successfully exploit high-energy lithium ion batteries.46 Mixed halide perovskites undergo phase segregation upon irradiation, which degrades the photovoltaic performance for solar cells or leads to red shift of emission for luminescent materials47-51, or exhibits enhanced ion

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movement in CsPbIBr2 solar cells52. Xia et al. utilized nanosegregation to tune the photoluminescence in (CaMg)x(NaSc)1-xSi2O6 solid solution.53 Xie and co-authors used a single particle diagnosis approach to discover new phosphors in multi-phase mixed phosphors powders.54-56 However, phase segregation should be generally avoided in the synthesis of inorganic phosphors, because the existence of second phase could act as quenching center or exhibit different excitation/emission bands. In the present work, phase segregation is utilized to discover Eu2+-doped phosphor. The increase of Eu content (x) from 0.02 to 1.8 in nominal composition of Na2-xAl2B2O7:xEu leads to the dramatic enhancement of blue emission intensity. Unlike the general relationship between the emission intensity and activator content, phase segregation and the transformation of host structure from Na2Al2B2O7 to NaAl11O17 are responsible for the emission enhancement, which is supported by the X-ray powder diffraction (XRD), backscattered electron (BSE) image combined with elemental mapping, and cathodoluminescence-combined scanning electron microscope (CL-SEM), etc. Although impurity phases exist, the NaAl11O17-related Eu2+-activated phosphor exhibits excellent thermal stability.

2. Experimental 2.1 Materials and Synthesis The phosphors have been prepared by a solid-state reaction at high temperature in a tube furnace. Three series of phosphors have been synthesized, namely, phosphors with nominal compositions Na2-xAl2B2O7:xEu (x = 0 – 2), Na0.2Al2ByO7:1.8Eu (y = 0 – 3), and Na0.2AlzB2O7:1.8Eu (z = 0 – 3). In a typical synthesis procedure, stoichiometic amounts of the raw materials, Al(OH)3 (A.R.), Na2CO3 (A.R.), H3BO3 (A.R.), Eu2O3 (99.99%) were thoroughly mixed for 30 min with the

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addition of alcohol in an agate mortar according to different values of varieties. The mixed raw materials were then transferred to alumina crucibles, with subsequently firing at 1000 °C for 8 h under a reducing atmosphere (N2/H2 = 95:5). After cooled down to room temperature naturally, the obtained samples were ground to powders for subsequent analyses. 2.2 Measurements and Characterization Measurement of XRD patterns was carried out by a PANalytical X’Pert Pro diffractometer (Cu Kα radiation, 40 kV, 40 mA). Photoluminescence excitation (PLE), emission (PL) spectra, and temperature-dependent PL spectra were recorded on a Hitachi F4500 spectrophotometer (Japan) equipped with a TAP-02 high-temperature controller (Orient KOJI). The absolute quantum efficiencies (QE) was measured at room temperature on an Edinburgh FLS980 fluorescence spectrometer combined with a 450 W xenon lamp and a nanosecond flash lamp. The morphology of the phosphors was observed on a TESCAN MIRA 3 LMU scanning electron microscope (SEM), which contain BSE and SEM images. An energy dispersive X-ray spectroscopy (EDS) was also combined to the microscope to analyze element composition for special points or whole in BSE image (elemental mapping). The microstructures of the samples were characterized using a transmission electron microscope (TEM, FEI Tecnai G2 F20, 200 kV). A field-emission scanning electron microscope (FE-SEM, Hitachi S-4800) that equipped with an EDS system and a cathodoluminescence (CL) system (MonoCL4, Gatan) was used to measure the morphology and related CL image. Thermoluminescence (TL) spectra were collected with a heating rate of 56 °C/min on a FJ-427A TL meter after samples exposed to the 254 and 365 nm UV light for 10 min at room temperature. The measurements for pc-LED devices were achieved on a high accurate array spectrometer (HSP6000, HOPOO). All measurements were carried out at room temperature except the TL spectra and temperature-dependent PL spectra.

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3. Results and Discussion 3.1 The Effect of Eu Concentration on the Luminescence and Phase It is well known that the emission of Eu2+-activated phosphors is generally based on the parityallowed 5d-4f transition in a strong crystal field, and the concentration quenching occurs at very low concentration of Eu2+ ions. Accordingly, the doping content of Eu2+ in phosphors is generally at around 1 mol% or even lower. Therefore, Na2-xAl2B2O7:xEu2+ phosphors were synthesized firstly with low concentration of Eu2+. However, concentration quenching was not reached at low concentration. The PLE and PL spectra of Na2-xAl2B2O7:xEu2+ (x = 0.02 – 0.50, nominal composition) are shown in Figure S1 as a representative. The phosphors exhibit a broad emission band peaking at ~470 nm. The related PLE band covers 250 – 420 nm range. The PL intensity increases with x value and no concentration quenching occurs in such doping range. It should be noted that the molar concentration is 25 mol% when x equals 0.50, which is higher than most Eu2+doped phosphors. The XRD patterns of the phosphors are shown in Figure S2. The diffraction peaks can all be indexed to Na2Al2B2O7 (PDF no. 53-1124) when x equals 0.02, and gradually weakens with increasing Eu content (x). The diffraction peaks belonging to EuAlO3 (PDF no. 300012) are observed when x = 0.05 – 0.40, and those belonging to EuBO3 (PDF no. 74-1931) appear when x = 0.15 – 0.50. Furthermore, the diffraction peaks that belong to NaAl11O17 (PDF no. 792288) appear at x = 0.15 and get stronger with increasing x. EuAlO3 and EuBO3 are both compose of Eu3+ ions, which could not promote the increase of blue emission intensity. Therefore, it is speculated that NaAl11O17 phase is responsible for the enhancement of broadband blue emission with higher Eu2+ content. A series of nominal Na2-xAl2B2O7:xEu phosphors with x ranging from 0.02 to 2.0 and ignoring charge compensation were synthesized to verify the above speculation. Figure 1 and Figure S3

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exhibit the PL and PLE spectra of nominal Na2-xAl2B2O7:xEu (x = 0.02 – 2.0), which suggest that broadband blue emission dominates. The PL spectra under excitation at 300 and 365 nm are shown in Figure 1a and b, respectively, which have similar broadband profile and variation tendency of intensity and PL peak. Upon increasing Eu content from 0.02 to 2.0, the emission intensity increases firstly, and then decreases. The emission intensity maximizes at x = 1.6 under excitation at 300 nm, while it reaches the maximum at x = 1.8 under excitation at 365 nm. The PL intensity of both broad band from Eu2+ and narrow band from Eu3+ are very weak for x equals 2.0 as shown in Figure 1 and Figure S3. Figure 1c exhibits the PLE band of the phosphors monitored at 470 nm. The PLE peak shifts to longer wavelength side with increasing Eu content. The relative excitation intensity of the shoulder at around 365 nm for x = 1.6 – 1.9 gets stronger than others. These phenomena suggest that the coordination environment for Eu2+ ions may have changed. Figure 1d plots the variation of broadband PL intensity and PLE peak versus Eu content from 0.02 to 1.9. The powder XRD patterns of the phosphors are shown in Figure 2a. Figure 2b and c exhibit the enlarged XRD patterns in 5 – 10° and 30 – 33°, respectively. It is obvious that the intensity of the diffraction peaks that belong to Na2Al2B2O7 weakens gradually upon increasing Eu content (x) and nearly disappears when x is higher than 1.4. The diffraction peaks for EuBO3 (PDF no. 74-1931) get stronger with increasing Eu content until x equals 1.8. Further increasing x leads to the formation of EuBO3 with another EuBO3 phase (PDF no. 89-7888), which dominates at x = 2.0. The diffraction intensity of the peaks that belong to NaAl11O17 (PDF no. 79-2288) increases with Eu content until x = 1.6, and then decreases. It is no doubt that both Eu2+ in Na2Al2B2O7 and NaAl11O17 phases exhibit a broadband blue emission with a peak at around 470 nm according to the evolution of PL and XRD, and the emission of latter is much stronger than the former. The obvious difference is the profile of PLE

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band, viz., it is peaking at around 300 nm for Na2Al2B2O7:Eu2+, while it is at 330 nm with a stronger shoulder at ~365 nm for NaAl11O17:Eu2+ dominated ones. The emission band of Eu2+ is influenced by its surroundings greatly. The similar emission bands suggest the similar coordination environments. The unit cell and Na polyhedrons of Na2Al2B2O7 and NaAl11O17 are shown in Figure 2d and e. There is a six-fold NaO6 phlyhedron (Na1) and a nine-fold NaO9 polyhedron (Na2) in Na2Al2B2O7 (Figure 2d). The NaO9 phlyhedron is a distorted three-capped triangular prism with the bottom of the prism rotated 60 degrees. While Na in NaAl11O17 is also coordinated with nine oxygen atoms forming a distorted three-capped triangular prism with the triangle formed by the three apexes of the capes rotated for 60° (Figure 2e). Therefore, it is expected that Eu2+ ions in both Na2Al2B2O7 and NaAl11O17 prefer to occupy the nine-fold Na sites, which results in the similar emission bands. The highly connected AlO4 and AlO6 in NaAl11O17 suggests a more rigid crystal structure than Na2Al2B2O7, which has only isolated Al2O7 with a common vertex. Therefore, it is reasonable for the higher PL intensity in NaAl11O17:Eu2+.

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Figure 1. PL spectra of nominal Na2-xAl2B2O7:xEu (x = 0.02 – 2.0) excited at (a) 300 and (b) 365 nm, inset: related normalized PL spectra and photographs under 302 or 365 nm UV lamp, (c) PLE spectra monitored at 470 nm, (d) PL intensity excited 300 and 365 nm, and PLE peak value versus Eu content (x).

Figure 2. (a) XRD patterns of nominal Na2-xAl2B2O7:xEu (x = 0.02 – 2.0), enlarged XRD peaks in (b) 5 – 10° and (c) 30 – 33° range; unit cell, Na and Al polyhedrons of (d) Na2Al2B2O7 and (e) NaAl11O17 showing at the same direction.

SEM and TEM were used to explore the phase composition of the samples. Figure 3a-e shows the BSE images of nominal compositions Na2-xAl2B2O7:xEu with x = 0.02, 0.2, 1.0, 1.6, and 1.8, respectively. The change in brightness reflects the uniformity of composition. The overall brightness of Figure 3a is relatively uniform, except for the only few small bright spots. The number of bright spots tends to increase obviously with increasing Eu concentration (x). It is known from the principle of backscatter that the heavier the element exhibit the brighter. It is speculated

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that Eu element with high atomic coefficient segregates at high Eu concentration and leads to abnormal bright spots in BSE images. The SEM image of the sample with x = 1.8 is also shown in Figure 3f as comparison. The analysis of element is further performed by energy-dispersive X-ray spectroscopy (EDS) and elemental mapping combined with BSE image. As a representative, the results of nominal Na2-xAl2B2O7:xEu (x = 1.80) are shown in Figure 4. Figure 4b and c are the EDS results of gray block and bright particle, respectively, in the BSE image shown in Figure 4a. The gray block contains mainly Na, Al, and O atoms, plus a small amount of Eu atoms. While the bright particle is mainly composed of Eu and O atoms. It should be noted that the EDS analysis cannot detect boron element. Figure 4d-g shows the elemental mapping of different elements, illustrating the distribution of elements in Figure 4a. These results combined with the XRD analysis suggest that the gray block is NaAl11O17 phase doped with Eu, while the bright particles are of EuBO3 phase for nominal Na2-xAl2B2O7:xEu (x = 1.80). The elemental analyses for other samples with x equals 0.02, 0.2, 1.0, and 1.6 are shown in Figure S4-7, exhibiting gradual phase segregation with increasing Eu content. With the phase segregation, the elemental composition of large gray particles in BSE images is gradually close to that of NaAl11O17. HR-TEM images of nominal Na2-xAl2B2O7:xEu (x = 1.60) are also shown in Figure 4. The HR-TEM images in Figure 4i and j relate to different areas in the TEM image (Figure 4h). The calculated inter-planar distances further support the co-existence of both EuBO3 and NaAl11O17 phases. The above analyses indicate that phase segregation occurs with gradually increasing Eu content. The segregation of EuBO3 phase and the reduction in the amount of raw material Na2CO3 make the molar ratio of Na/Al close to 1/11, and then force the formation of NaAl11O17 phase. The particle size of EuBO3 is around 1 m, which is smaller than that of NaAl11O17 (~10 m).

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Figure 3. BSE and SEM images of nominal Na2-xAl2B2O7:xEu (x = 0.02, 0.20, 1.00, 1.60, 1.80). (a-e) BSE, (f) SEM for x = 1.80.

Figure 4. Elemental analyses for nominal Na2-xAl2B2O7:xEu (x = 1.80). (a) BSE, (b) EDS around M site in (a), (c) EDS around N site in (a), (d-g) Elemental mapping for particles in (a), (h) TEM

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image of nominal Na2-xAl2B2O7:xEu (x = 1.60), (i, j) HRTEM images and corresponding Fast Fourier Transform (FFT) results of related areas in (h).

In order to further verify that the blue emission originates from Eu2+ in NaAl11O17 phase for nominal Na2-xAl2B2O7:xEu with higher x, the CL combined with SEM image for sample with x = 1.60 were collected and shown in Figure 5. The SEM image is shown in Figure 5a, which contains several smaller particles with a diameter of about 1 m and a bigger irregular particle with a diameter of ~10 m. The EDS analyses of these two parts are shown in Figure S8. The XRD results and elemental analyses suggest that the smaller particles belong to the EuBO3 phase and the irregular one is NaAl11O17 phase. Figure 5b is the CL image at 480 nm, and Figure 5c is the combination of Figure 5a and 5b. It is obvious that the emission at 480 nm originates from the bigger particle, not the smaller ones. This result suggests that the blue emission of samples with higher x belongs to Eu2+ in NaAl11O17 phase. Furthermore, the CL image at 612 nm as shown in Figure 5d is much darker than that at 480 nm, which is consistent with the PL intensity. The combination of SEM and CL at 612 nm is shown in Figure 5e, which indicates that the weak emission at 612 nm also originates from the NaAl11O17 phase. Figure 5f clearly exhibits that EuBO3 phase (PDF no. 74-1931) has no emission from 480 and 612 nm, which is the combination of SEM and CL at both 480 and 612 nm. This phenomenon may suggest that the high concentration of Eu3+ in EuBO3 quenches its emission, as the distance between the nearest two Eu3+ ions is only 3.84 Å according to its crystal structure.

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Figure 5. CL and related SEM image of nominal Na2-xAl2B2O7:xEu (x = 1.60). (a) SEM image, (b) CL at 480 nm, (c) overlapping of (a) in yellow and (b) in blue, (d) CL at 612 nm, (e) overlapping of (a) in cyan and (d) in red, (f) overlapping of (a), (b), and (c) in green, blue, and red, respectively. 3.2 Influence of B and Al Content on the Phase Segregation and Luminescence The above analyses on the phase segregation and luminescence of the nominal Na2-xAl2B2O7:xEu suggest that the obtained samples contain only NaAl11O17 and EuBO3 phases and exhibit better luminescent properties when x equals 1.6 and 1.8. Especially, the molar ratio of Na/Al is close to that in NaAl11O17 when x equals 1.8, and that of Eu/B is close to EuBO3, however, that of Na/Al/B is far from Na2Al2B2O7. Suitable molar ratio of raw materials forces most Eu atoms to concentrate in EuBO3, and left proper amount of Eu in NaAl11O17 phase as Eu2+. These results suggest that phase-segregation induced formation of two-phase mixture also needs proper molar ratio of raw materials, just like the formation of a pure phase.

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In order to further verify the above viewpoint, two additional series of phosphors were designed, namely, changing B content with Al content fixed, or changing Al content with B content fixed. The nominal chemical formula can be expressed as Na0.2Al2ByO7:1.8Eu and Na0.2AlzB2O7:1.8Eu. Figure 6a and b exhibit the PLE and PL spectra of nominal Na0.2Al2ByO7:1.8Eu (y = 0 – 3.0) phosphors, and Figure 6c and d exhibit those for nominal Na0.2AlzB2O7:1.8Eu (z = 0 – 3.0). The similar PLE and PL profiles are observed as those of nominal Na2-xAl2B2O7:xEu (x = 0.02 – 2.0). Upon increasing B or Al content from 0 to 3.0, the emission intensity increases firstly and then decreases, which both maximize at y or z equals 2.0, where the nominal composition Na0.2Al2B2O7:1.8Eu benefits the phase segregation for the formation of NaAl11O17 and EuBO3 (PDF no. 74-1931). The XRD patterns of Na0.2Al2ByO7:1.8Eu and Na0.2AlzB2O7:1.8Eu are shown in Figure S9 and S10, which exhibit the evolution of phase composition.

Figure 6. PLE and PL spectra of nominal (a, b) Na0.2Al2ByO7:1.8Eu and (c, d) Na0.2AlzB2O7:1.8Eu phosphors.

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3.3. Quantum Efficiency, Thermal Stability, and pc-LEDs The

internal

quantum

efficiencies

(IQE)

of

nominal

Na0.4Al2B2O7:1.6Eu

and

Na0.2Al2B2O7:1.8Eu are 58.6 and 60.2%, respectively, under excitation at 365 nm. And the calculated external QE (EQE) values are 19.8 and 24.9%, respectively. Related spectra are shown in Figure S11. Such low IQE and EQE values are attributed to the existence of impurity phase, EuBO3, which acts as luminescent quenching center. There are two obvious evidences for the quenching center. Firstly, the slight drop of the PL intensity at 467 nm is consistent with the absorption peak for the 7F0 → 5D2 transition of Eu3+. Secondly, the SEM-combined CL image for Na0.4Al2B2O7:1.6Eu indicates that the emission of Eu3+ originates from NaAl11O17 phase, not EuBO3. It is believed that Eu2+-doped NaAl11O17 phosphor has a higher QE value after the achievement of pure phase. The thermal stability of the phosphors contains two aspects, PL intensity and the PL profile (emission color). Temperature dependent PL spectra of nominal Na2-xAl2B2O7:xEu with x = 1.6 and 1.8 excited at 330 nm under temperatures ranging from 20 to 200 °C are depicted in Figure 7a and b. The PL intensity for both phosphors tends to decrease with increasing temperature. The former (x = 1.6) decreases slower than the latter (x = 1.8). Figure 7c illustrates the curves of relative emission intensity versus temperature for these two phosphors. The PL intensity at 150 °C remains 96% and 83% of that at room temperature for these two samples, respectively. The nominal Na0.4Al2B2O7:1.6Eu phosphor shows the better thermal stability than Na0.2Al2B2O7:1.8Eu when considering the PL intensity. The peak values of the PL band change little with increasing temperature, and the corresponding values of the International Commission on Illumination (CIE) coordinates for Na2-xAl2B2O7:xEu with x = 1.6 and 1.8 are shown in the inset of Figure 7c, which suggests that both samples have good thermal stability when emission color is concerned. TL

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spectra of the two samples are shown in Figure 7d. Both samples contains two TL peaks at 60 and 150 °C. The TL intensity of the two peaks for Na0.4Al2B2O7:1.6Eu are both very weak. While the TL band at 150 °C for Na0.2Al2B2O7:1.8Eu is very strong. Our previous work suggested that thermal ionization is responsible for thermal quenching at higher temperature.23 The situation may be similar to the present work. Stronger TL band at 150 °C for Na0.2Al2B2O7:1.8Eu is responsible for its heavier thermal quenching.

Figure 7. Temperature dependent PL spectra of (a) nominal Na0.4Al2B2O7:1.6Eu and (b) Na0.2Al2B2O7:1.8Eu, (c) normalized PL intensity versus temperature, (d) TL of the two samples. The excellent thermal stability suggests that the nominal Na2-xAl2B2O7:xEu phosphors are promising candidates for UV-based white pc-LEDs. Na0.2Al2B2O7:1.8Eu is used as a representative to fabricate a white pc-LED. Figure 8a shows the electroluminescence (EL) spectra of the white pc-LED under different forward bias currents, which is obtained from the combination

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of a 365 nm UV chip, Na0.2Al2B2O7:1.8Eu, commercial green phosphor (Ba,Sr)2SiO4:Eu2+, and red phosphor CaAlSiN3:Eu2+. The EL intensity increases with driving current from 50 to 350 mA. The weight ratio of the blue, green, and red phosphor is 9:1:2, and that of phosphors to silica gel is 1:11. The related properties of the pc-LED is listed in Table S1. The correlated color temperatures (CCT) are all around 3500 K, which suggests the achievement of warm white. The color rendering index (CRI) value is 91.2 for the pc-LED driven at 50 mA. Figure 8b illustrates the CIE coordinates of the white pc-LED calculated from the EL spectra, which are in warm white region. The photograph of the white pc-LED is also exhibited as an inset in Figure 8b, showing good ability of restoring color. The results suggest that Na2-xAl2B2O7:xEu phosphors could be a candidate for UV LED pumped white pc-LEDs.

Figure 8. (a) Electroluminescence spectra of the white pc-LED under different forward bias currents combined with a 365 nm UV chip and nominal Na0.2Al2B2O7:1.8Eu (NaABO:1.8Eu), and

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commercial (Ba,Sr)2SiO4:Eu2+ and CaAlSiN3:Eu2+, (b) CIE coordinates from related EL spectra of the pc-LED, inset: photograph of the pc-LED turned on.

4. Conclusions In summary, phosphors with nominal composition Na2-xAl2B2O7:xEu (x = 0 – 2) were synthesized by traditional high-temperature solid-state reaction. Broadband blue emission was observed under UV excitation, and the emission intensity increased dramatically from x = 0.02 to 1.8, which is a very high content if Eu2+ acts as activator. XRD, BSE, HR-TEM and SEM-CL analyses suggest that phase segregation occurred upon increase Eu content, and Eu concentrated in EuBO3 phase mainly, leaving a small amount of Eu2+ in NaAl11O17 phase. Furthermore, the coordination environments of Na+ in Na2Al2B2O7 and NaAl11O17 are similar to each other, which is the site for Eu2+ to occupy. Therefore, phase transformation from Na2Al2B2O7 to NaAl11O17 and phase segregation are responsible for the dramatic enhancement of broadband blue emission upon increasing Eu content. Although NaAl11O17 is isostructural with BaMgAl11O17, the well-known host for Eu2+-doped blue phosphor, there is little reports on the luminescent properties of NaAl11O17:Eu2+. Therefore, the synthesis and luminescent properties of pure-phase NaAl11O17:Eu2+ deserves further study. Phase segregation combined with related measurements is an effective method to discover new phosphor materials and other inorganic functional materials.

Supporting Information. The following files are available free of charge. PLE and PL spectra, XRD, Elemental analyses, QE measurement, Data of pc-LED. (PDF)

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ACKNOWLEDGMENT This work is financially supported by the National Natural Science Foundation of China (NSFC nos. 51402105, 21571059, 21471055), the Scientific Research Fund of Hunan Provincial Education Department (Grant No. 18B020), and the National Key Research and Development Program (2016YFB0302403).

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Highlights

► Phase segregation occurs during the high-temperature synthesis of inorganic phosphor. ► EuBO3 segregated and accompanied by transformation from Na2Al2B2O7 to NaAl11O17:Eu2+. ► More rigid structure for NaAl11O17 allows dramatical enhancement of blue emission. ► NaAl11O17:Eu2+ remains 96% of the room-temperature emission intensity at 150 °C.

Declaration of Interest Statement

The authors have no competing interests to declare.

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