Tunable Luminescence and Energy Transfer in Novel Blue-Green

6 days ago - Vijayakumar Rajagopal† , Xiaoyong Huang*† , and Yucheng Wu*‡. †College of Physics and Optoelectronics and ‡Key Laboratory of In...
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Article Cite This: ACS Omega 2019, 4, 4384−4389

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Tunable Luminescence and Energy Transfer in Novel Blue-GreenEmitting BaGd2Si3O10:Ce3+,Tb3+ Phosphors for Near-UV-Based White LEDs Vijayakumar Rajagopal,† Xiaoyong Huang,*,† and Yucheng Wu*,‡ College of Physics and Optoelectronics and ‡Key Laboratory of Interface Science and Engineering in Advanced Materials of Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, P. R. China

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ABSTRACT: A new series of Ce3+ singly doped and Ce3+ /Tb3+-co-doped BaGd2Si3O10 (BGSO) phosphors have been synthesized by a high-temperature solidstate reaction technique. The phase purity, luminescence properties, energy transfer mechanism, internal quantum efficiency, and thermal stability of as-prepared phosphors were investigated in detail. The photoluminescence spectra of BGSO:xCe3+ phosphors exhibited broad emission in the wavelength range of 350−600 nm with a peak maximum at 427 nm. Under 336 nm excitation, the tunable blue-green luminescence of BGSO:0.04Ce3+,yTb3+ phosphors was investigated as a function of Tb3+ concentration and well demonstrated by the energy transfer mechanism from Ce3+ to Tb3+ ions. The critical distance (Rc) of energy transfer was estimated using concentration quenching method, and its value was found to be 10.75 Å. The thermal stability and activation energy of thermal quenching were analyzed for composition-optimized BGSO:0.04Ce3+,0.30Tb3+ phosphors by varying the temperature from 303 to 483 K. The results obtained from the present investigation suggested that Ce3+- and Ce3+/Tb3+-activated BGSO phosphors can serve as promising materials for near-UV-based phosphor-converted white light-emitting diodes. the Tb3+ absorption in the near-UV region, Ce3+ ions can be introduced as sensitizers for Tb3+ activators, which thus can enhance the emission intensity of Tb3+ ions through Ce3+ → Tb3+ energy transfer mechanism. This could be favorable for improving the luminescence efficiency of phosphors used in LED applications.8,10 Among the host materials for phosphors, silicates are found to be suitable for the doping of RE3+ ions because of their high thermal and chemical stability, high thermal expansion, abundant resources, weather resistance, structural diversity, etc.11,12 Recently, few works have been reported on Ce3+- and Tb3+-co-doped blue-green silicate phosphors for lighting applications. For instance, Lu et al. studied the luminescence and energy transfer properties of BaMg2Al6Si9O30:Ce3+,Tb3+ phosphors for LED applications.13 The tunable luminescence of BaY2Si3O10:Ce,Tb phosphors was reported by Xia and coworkers.1 Zhang et al. achieved the color-tunable emission in K2MgSi3O8:Ce3+,Tb3+ phosphors through energy transfer mechanism.11 The excitation and emission spectra of Ce3+ singly doped and Ce3+/Tb3+-co-doped Na3LuSi2O7 phosphors were investigated by Li et al.14 Luminescence properties of multisite Ce3+ in T-phase orthosilicates and energy transfer from Ce3+ to Tb3+ were studied and accounted by Tong et al.15 Still, there is a demand for novel single-phase blue-greenemitting phosphors with high efficiency and high thermal

1. INTRODUCTION In the past few years, white-light-emitting diodes (w-LEDs) have drawn much attention because of their superior characteristics over the conventional incandescent and fluorescence lamps, such as high energy efficiency, long lifetime, high brightness, low energy consumption, and environmental-friendly nature and compactness.1−4 The most common strategy to generate white light is coating a YAG:Ce yellow phosphor on InGaN-based blue LED chips. However, this kind of w-LEDs have demerits of high correlated color temperature (CCT > 4500 K) and low color-rendering index (CRI < 80) due to lack of green and red emissions.5−7 To conquer the drawbacks of traditional w-LEDs, the combination of UV or near-UV LED chips with tricolor blue, green, and red phosphors has been proposed as an alternative approach to offer w-LEDs with high color uniformity and high CRI. Therefore, it is necessary to find novel highly efficient nearUV-excitable blue-green-emitting phosphors for the development of w-LEDs. Tb3+ ions are known to be promising activators for green-emitting phosphors due to their dominant green emission peak around 541 nm (5D4 → 7F5 transition). However, Tb3+ ions have weak absorption in the near-UV region due to strictly forbidden 4f−4f transitions. In sharp contrast, Ce3+ ions show broad absorption in the near-UV wavelength range due to spin- and parity-allowed 4f → 5d transition. Furthermore, the emission of Ce3+ ions due to 5d → 4f transition can vary from long-wavelength UV to red depending on the factors like the crystal structure, lattice symmetry, and nature of the host composition.8,9 To intensify © 2019 American Chemical Society

Received: January 21, 2019 Accepted: February 18, 2019 Published: February 28, 2019 4384

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stability for the development of phosphor-converted w-LEDs. However, no work has been reported on energy transfer from Ce3+ to Tb3+ ions in a BaGd2Si3O10 (abbreviated as BGSO) host. Hence, in this work, we reported on the energy transfer mechanism and luminescence properties of Ce3+/Tb3+-ion-coactivated BGSO blue-green-emitting phosphors for phosphorconverted w-LED applications.

2. RESULTS AND DISCUSSION 2.1. X-ray Diffraction (XRD) Analysis. The XRD patterns of BGSO:0.04Ce3+ and BGSO:0.04Ce3+,0.30Tb3+ phosphors are shown in Figure 1 as the representative cases and compared

Figure 2. (a) PLE and PL spectra of BGSO:0.04Ce3+. (b) Concentration-dependent PL spectra of BGSO:xCe3+ phosphors under 336 nm excitation. (c) Dependence of PL intensity on Ce3+ concentration. (d) Linear fit between log(I/x) and log(x) of 5d → 4f transition of Ce3+ ions.

energy transfer process will be influenced by one of the three methods, such as exchange interaction, radiation reabsorption, and electric multipole interactions. In this work, there was a small spectral overlap between excitation and emission spectra of Ce3+-doped BGSO phosphors, which indicated that the exchange and reabsorption interactions cannot account for energy transfer.18 Therefore, the multipole interaction will be responsible for the energy transfer mechanism between nearby Ce3+ ions. Based on the report of Van Uitert,19 the relation between emission intensity (I) and doping concentration of Ce3+ ions (x) can be expressed as

Figure 1. XRD patterns of BGSO:0.04Ce3+ and BGSO:0.04Ce3+,0.30Tb3+ phosphors.

with the standard data of a BGSO host. It can be clearly noticed from Figure 1 that all of the XRD patterns are in good agreement with the standard crystallography open database card no. 9016202, which confirmed the existence of singlephase Ce3+ singly doped and Ce3+/Tb3+-co-doped BGSO phosphors. According to the crystal structure reported by Li et al.,16 the as-prepared BGSO phosphors were crystallized in monoclinic structure with a space group of P21/m. It was observed that XRD patterns did not show impurity peaks corresponding to any other secondary phase, which indicated that the crystal structure of a BGSO host was not affected by the addition of Ce3+ and Tb3+ ions. 2.2. Luminescence Properties of Ce3+ Ions in BGSO Phosphors. The photoluminescence (PL) and PL excitation (PLE) spectra of BGSO:0.04Ce3+ phosphors are presented in Figure 2a. The PLE spectrum recorded by monitoring the emission at 427 nm exhibited a broad excitation band starting from 270 to 380 nm with a peak at 336 nm, which was assigned to 4f → 5d transition of Ce3+ ions. This broad excitation wavelength region (270−380 nm) is significant for the excitation absorption of near-UV LED chips.17 The PL spectrum of BGSO:0.04Ce3+ phosphors recorded under 336 nm excitation showed an asymmetric blue emission band that extended from 350 to 600 nm with a maximum at 427 nm due to 5d → 4f transition of Ce3+ ions. Figure 2b shows the concentration-dependent PL spectra of BGSO:xCe3+ phosphors measured under the excitation of 336 nm. It was observed from Figure 2c that the emission intensity was increased with the Ce3+ concentration up to 4 mol %; beyond that, it decreased to the lowest because of the concentration quenching effect. Hence, the optimum concentration of Ce3+ ions was found to be 4 mol %, and it was kept as a constant value to prepare Ce3+/Tb3+-co-doped BGSO phosphors. The nonradiative energy transfer mechanism was responsible for concentration quenching of BGSO:xCe3+ phosphors. Such an

I /x = k[1 + β(x)θ /3 ]−1

(1)

where I is the integral intensity of emission spectra, x is the Ce3+ ion concentration, and k and β are the constants for a given single host under the same pumping wavelength. θ represents the multipole interactions, and it can have the values of θ = 6, 8, and 10 for dipole−dipole (D−D), dipole− quadrupole (D−Q), and quadrupole−quadrupole (Q−Q) interactions, respectively. According to eq 1, an approximately linear relationship between log(I/x) and log(x) is plotted in Figure 2d, and the slope value obtained from the fitting result was found to be −1.847. Hence, the θ value was estimated to be 5.54, which was approximately equal to the theoretical value 6. These results revealed the fact that D−D interactions gave a major contribution to the energy transfer mechanism and concentration quenching in BGSO:xCe3+ phosphors. 2.3. Luminescence and Energy Transfer in BGSO:Ce3+,Tb3+ Phosphors. Figure 3a displays the PLE and PL spectra of BGSO:0.04Ce3+,0.30Tb3+ phosphors recorded at room temperature, in which the PLE spectra recorded by fixing the emissions of Ce3+ (427 nm) and Tb3+ (542 nm) ions are shown by the characteristic excitation band (4f → 5d) of Ce3+ ions, which indicated that the Tb3+ ions can be excited by Ce3+ ion absorption. This confirmed the existence of energy transfer from Ce3+ to Tb3+ ions in BGSO phosphors. The PL spectrum exhibited both the emission transitions of Ce3+ (5d → 4f) and Tb3+ ions (5D4 → 7F3,4,5,6) under 336 nm excitation. The 4385

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where N is the number of Z ions in the unit cell and V is the volume of the unit cell. The V and N of BGSO phosphors were acquired from the previously reported literature by Li et al.,16 and the values were found to be 442.74 Å3 and 2, respectively. xc is the critical concentration that can be referred to as the total concentration of sensitizer (Ce3+) and activator (Tb3+) ions, where the Tb3+ emission intensity is the highest.28 Here, the critical concentration was found to be xc = 0.34 [i.e., the sum of concentrations of Ce3+ (0.04) and Tb3+ (0.30) ions], and hence, the Rc value was calculated as 10.75 Å. The energy transfer will occur due to multipole interactions when the Rc is higher than 5 Å, or else, the exchange interaction will be dominant. Therefore, the energy transfer from Ce3+ to Tb3+ ions in the present BGSO phosphor was mainly due to the multipole interactions. According to the Dexter energy transfer formula of multipolar interactions and Reisfield’s approximation, the energy transfer mechanism can be further analyzed using the formula given below29,30

Figure 3. (a) PLE and PL spectra of BGSO:0.04Ce3+,0.30Tb3+. (b) PL spectra of BGSO:0.04Ce3+,yTb3+ phosphors for different Tb3+ concentrations (y = 0−0.40). (c) Dependence of Ce3+ and Tb3+ ion PL intensities on Tb3+ concentration. (d) Dependence of energy transfer efficiency on Tb3+ concentration.

Iso/Isα C n /3

(4)

where Is and Iso are the PL intensities of Ce3+ ions with and without Tb3+ ions; C is the total concentration of sensitizer (Ce3+) and activator (Tb3+) ions; and n can be 6, 8, and 10 depending on the D−D, D−Q, and Q−Q interactions, respectively. The plot between Iso/Is and Cn/3 values of asprepared phosphors is represented in Figure 4, and the linear relation was achieved for n = 6, which indicated that D−D interaction was mainly responsible for the energy transfer mechanism.

concentration-dependent PL spectra of BGSO:0.04Ce3+,yTb3+ (y = 0−0.40) phosphors were recorded by maintaining excitation at 336 nm and are presented in Figure 3b. It can be observed from Figure 3c that the PL intensity of Ce3+ ions was decreased with the increase of Tb3+ concentration. In contrast, the green emission intensity of Tb3+ ions was increased along with the Tb3+ concentration from 0.01 to 0.30 and it was decreased for a higher Tb3+ content (y = 0.40) due to the concentration quenching effect. These results revealed that tunable blue-green luminescence can be achieved from assynthesized BGSO:0.04Ce3+,yTb3+ phosphors by varying the Ce3+/Tb3+ concentration ratio. Furthermore, the efficiency (ηT) of Ce3+ → Tb3+ energy transfer in BGSO phosphors can be written as20,21 I ηT = 1 − S IS0 (2) where IS and IS0 are the emission intensities of the Ce3+ ions with and without activator (Tb3+) ions, respectively. The energy transfer efficiencies of different BGSO:0.04Ce3+,yTb3+ (y = 0.01, 0.03, 0.10, 0.20, 0.30, and 0.40) phosphors were calculated and are illustrated in Figure 3d. It was noticed that ηT values were increased gradually with the increase of Tb3+ concentration. The largest energy transfer efficiency of 78% was realized for y = 0.40, which was found to be higher than that of the similar reported silicate phosphors, such as (58.09%) BZS:0.04Ce3+,0.10Tb3+,22 (64.03%) Y4Si2O7N2:0.005Ce3+,0.4Tb3+,23 (51%) K2MgSi3O8:0.05Ce3+,0.09Tb3+,24 (16%) MgY 4 Si 3 O 1 3 :0.01Ce 3 + ,0.3Tb 3 + , 2 5 and (76%) BaMg2Al6Si9O30:0.05Ce3+,0.13Tb3+.13 All of the features of PL spectra and energy transfer efficiency proved the occurrence of energy transfer from Ce3+ to Tb3+ in a BGSO host. According to Blasse’s theory,26,27 the critical distance (Rc) between Ce3+ and Tb3+ ions can be calculated using concentration quenching method, and it can be expressed as ÄÅ É ÅÅ 3V ÑÑÑ1/3 Å ÑÑ Å R C ≈ 2ÅÅ Ñ ÅÅÇ 4πxcN ÑÑÑÖ (3)

2 Figure 4. Dependence of Iso/Is of Ce3+ ions on (a) C6/3 Ce+Tb (10 ), (b) 8/3 10/3 (103), and (c) CCe+Tb (104) for BGSO:0.04Ce3+,yTb3+ CCe+Tb phosphors.

2.4. Commission Internationale de l’É clairage (CIE) 1931 Diagram and Internal Quantum Efficiency (IQE). The dominant emission color of as-prepared BGSO:0.04Ce3+,yTb3+ phosphors can be obtained by analyzing the emission spectra using the CIE 1931 chromaticity diagram. Figure 5 represents the CIE 1931 diagram for BGSO:0.04Ce3+,yTb3+ phosphors. The (x, y) coordinates were calculated to be (0.172, 0.147), (0.175, 0.143), (0.188, 0.186), (0.217, 263), (0.250, 0.347) (0.274, 0.408), and (0.287, 0.439), which were attributed to the y = 0, 0.01, 0.03, 0.10, 0.20, 0.30, and 0.40 concentrations in BGSO:0.04Ce3+,yTb3+ phosphors, respectively. It can be noticed from Figure 5 that the CIE coordinates were initially located at the blue region for the BGSO:0.04Ce3+ sample and then moved from the greenishblue to the green region when the Tb3+ concentration increased from y = 0.01 to 0.40. The digital photographs of the BGSO:0.04Ce3+,yTb3+ phosphors are given as insets in Figure 5. These visual images revealed that emissions of as4386

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respectively. The IQE values of BGSO:0.04Ce 3+ and BGSO:0.04Ce3+,0.30Tb3+ phosphors were calculated to be 63 and 62%, respectively. It was observed that the IQE of BGSO:0.04Ce3+ phosphors was decreased when the Tb3+ ions were introduced as a co-dopant. However, the IQE (62%) of BGSO:0.04Ce3+,0.30Tb3+ phosphors was found to be higher than that of the previously reported silicate-based Na6Ca3Si6O18:0.04Ce3+,0.12Tb3+ blue-green-emitting phosphor (IQE = 43%).31 2.5. Thermal Stability. Thermal stability is one of the key issues that should be considered during the fabrication of highpower w-LEDs. Therefore, the PL spectra of optimum BGSO:0.04Ce3+,0.30Tb3+ phosphors were measured at different temperatures under 336 nm, and they are presented in Figure 7a. The integral (Ce3+ and Tb3+ ions) PL intensity was decreased gradually as the temperature increased from 303 to 483 K due to the thermal quenching effect, but no change was observed in the emission profile of Ce3+ and Tb3+ ions. Figure 7b represents the relation between integrated PL intensity and the temperature. The integral PL intensity at 423 K was found to be 44% of its initial value measured at 303 K, suggesting that Ce3+ and Tb3+ ions in as-prepared BGSO:0.04Ce3+,0.30Tb3+ phosphors exhibited high thermal stability suitable for highpower w-LEDs. The activation energy for thermal quenching can be calculated from the modified Arrhenius relation between emission intensity and temperature, and it can be expressed as32,33

Figure 5. CIE 1931 diagram for BGSO:0.04Ce3+,yTb3+ phosphors. The insets show the digital photographs of the as-prepared phosphors taken under a UV lamp.

synthesized BGSO:0.04Ce3+,yTb3+ phosphors can be tuned from blue to green by adjusting the relative Ce3+/Tb3+ concentration ratios. The internal quantum efficiencies (IQEs) of optimum concentration of Ce3+ singly and Ce3+/ Tb3+-co-doped BGSO phosphors were measured under 336 nm excitation using integrating sphere method, and the

I(T ) = I0[1 + c exp( −ΔEa /kT )]−1

where C is a constant, ΔEa is the activation energy, k is the Boltzmann constant (8.629 × 10−5 eV K−1), and T is the temperature. I0 and I are the PL intensities of BGSO:0.04Ce3+,0.30Tb3+ phosphors at room temperature and at different testing temperatures, respectively. Figure 7c shows the plot of ln[(I0/I) − 1] versus 1/kT, and its fitted line gave the slope of −0.224. Thus, the activation energy of BGSO:0.04Ce3+,0.30Tb3+ phosphors was estimated to be 0.224 eV. The high activation energy and significant thermal stability of the studied BGSO:0.04Ce3+,0.30Tb3+ phosphor suggested the fact that it can be used as a potential candidate for high-power wLEDs.

Figure 6. Excitation line of BaSO4 and the emission spectra of BGSO:0.04Ce3+ and BGSO:0.04Ce3+,0.30Tb3+ phosphors measured by integrating sphere method.

3. CONCLUSIONS Novel tunable blue-green-emitting BGSO:Ce3+,Tb3+ phosphors were successfully prepared via a solid-state reaction route. The PL spectra of BGSO:xCe3+ exhibited strong blue emission under 336 nm excitation. The CIE 1931 diagram revealed that the color-tunable blue-green luminescence can be achieved from BGSO:0.04Ce3+,yTb3+ phosphors when the Tb3+ concentration is increased from y = 0 to 0.40, which was

corresponding emission spectra are given in Figure 6. The IQE value can be estimated by the following expression21 ηQE =

∫ LS ∫ ER − ∫ ES

(6)

(5)

where LS refers to the emission from the sample and ES and ER are the excitation light spectrum with and without the sample,

Figure 7. (a) Temperature-dependent PL spectra of BGSO:0.04Ce3+,0.30Tb3+ phosphors under 336 nm excitation. (b) Variation of PL intensity with respect to temperature. (c) Linear fit for the ln(I0/I − 1) vs 1/kT plot of BGSO:0.04Ce3+,0.30Tb3+ phosphors. 4387

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mainly due to energy transfer from Ce3+ to Tb3+ ions. The optimum concentrations of Ce3+ and Tb3+ ions were estimated to be 4 and 30 mol %, respectively. The D−D interactions play a major role in the energy transfer process, and the energy transfer efficiency reached the maximum of 78% for the BGSO:0.04Ce3+,0.40Tb3+ sample. The IQE values were calculated to be about 63 and 62% for optimized BGSO:0.04Ce3+ and BGSO:0.04Ce3+,0.30Tb3+ phosphors, respectively. Thermal studies showed that the PL intensity of the BGSO:0.04Ce3+,0.30Tb3+ phosphor measured at 423 K retained 44% of its initial value at 303 K and the ΔEa value was calculated as 0.224 eV. All of these results obtained from the present investigation indicated that the BGSO:0.04Ce3+,yTb3+ phosphors can be utilized as blue-green emitters for near-UVbased phosphor-converted w-LEDs.

tunable BaY2Si3O10: Ce,Tb phosphors. Opt. Laser Technol. 2014, 56, 387−392. (2) Yang, M.; Liu, L.; Chen, F. Enhanced luminescence properties and energy transfer in novel Ce3+ and Tb3+ co-doped Na3La2(BO3)3 green-emitting phosphor. Mater. Lett. 2012, 88, 116−118. (3) Yang, Z.; Hu, Y.; Chen, L.; Wang, X.; Ju, G. Fluorescence and energy transfer in CaMgP2O7:Ce3+,Tb3+ phosphor. Mater. Sci. Eng. B 2015, 193, 27−31. (4) Huang, X. Solid-state lighting: Red phosphor converts white LEDs. Nat. Photonics 2014, 8, 748−749. (5) Huang, X.; Li, B.; Guo, H.; Chen, D. Molybdenum-dopinginduced photoluminescence enhancement in Eu3+-activated CaWO4 red-emitting phosphors for white light-emitting diodes. Dyes Pigm. 2017, 143, 86−94. (6) Huang, X.; Wang, S.; Li, B.; Sun, Q.; Guo, H. High-brightness and high-color purity red-emitting Ca3Lu(AlO)3(BO3)4:Eu3+ phosphors with internal quantum efficiency close to unity for nearultraviolet-based white-light-emitting diodes. Opt. Lett. 2018, 43, 1307−1310. (7) Zhang, M.; Liang, Y.; Xu, S.; Zhu, Y.; Wu, X.; Liu, S. Investigation of luminescence properties and the energy transfer mechanism of tunable emitting Sr3Y2(Si3O9)2:Eu2+,Tb3+ phosphors. CrystEngComm 2016, 18, 68−76. (8) Zhang, Q.; Ni, H.; Wang, L.; Xiao, F. Luminescence properties and energy transfer of GdAl3(BO3)4:Ce3+,Tb3+ phosphor. Ceram. Int. 2016, 42, 6115−6120. (9) Patel, N. P.; Srinivas, M.; Modi, D.; Vishwnath, V.; Murthy, K. Luminescence study and dosimetry approach of Ce on an α-Sr2P2O7 phosphor synthesized by a high-temperature combustion method. Luminescence 2015, 30, 472−478. (10) Zhu, G.; Wang, Y.; Ci, Z.; Liu, B.; Shi, Y.; Xin, S. Ca8Mg (SiO4)4Cl2:Ce3+,Tb3+: a potential single-phased phosphor for whitelight-emitting diodes. J. Lumin. 2012, 132, 531−536. (11) Zhang, H.; Zhang, X.; Cheng, Z.; Xu, Y.; Yang, J.; Meng, F. Tunable luminescence of K2MgSi3O8:Ce3+,Tb3+ phosphors through energy transfer. Ceram. Int. 2018, 44, 2547−2551. (12) Lin, H. C.; Yang, C. Y.; Das, S.; Lu, C. H. Photoluminescence Properties of Color-Tunable Ca3La6(SiO4)6:Ce3+,Tb3+ Phosphors. J. Am. Ceram. Soc. 2014, 97, 1866−1872. (13) Lu, Z.; Fu, A.; Xia, S.; Guan, A.; Meng, Y.; Zhou, L. Luminescence and Energy Transfer of Color-Tunable BaMg2Al6Si9O30:Ce3+,Tb3+ Phosphor. J. Electron. Mater. 2018, 47, 4929− 4935. (14) Li, Q.; Zhang, X.; Jiang, L.; Zhu, S.; Tang, H.; Zhang, W. Luminescence properties and energy transfer investigations of Ce3+ singly doped and Ce3+/Tb3+ codoped Na3LuSi2O7 phosphors. Opt. Mater. 2019, 88, 313−319. (15) Tong, X.; Zhang, X.; Wu, L.; Zhang, H.; Seo, H. J. On the photoluminescence of multi-sites Ce3+ in T-phase orthosilicate and energy transfer from Ce3+ to Tb3+. J. Alloys Compd. 2018, 748, 871− 875. (16) Li, G.; Wang, Y. Photoluminescence properties of novel BaGd2Si3O10:RE2+/3+(RE = Eu or Ce) phosphors with trichromatic emission for white LEDs. New J. Chem. 2017, 41, 9178−9183. (17) Wu, X.; Jiao, Y.; Hai, O.; Ren, Q.; Lin, F.; Li, H. Photoluminescence, energy transfer, color tunable properties of Sr3La(BO3)3:Ce,Tb phosphors. J. Alloys Compd. 2018, 730, 521−527. (18) Zhang, X.; Song, J.; Zhou, C.; Zhou, L.; Gong, M. High efficiency and broadband blue-emitting NaCaBO3:Ce3+ phosphor for NUV light-emitting diodes. J. Lumin. 2014, 149, 69−74. (19) Van Uitert, L. Characterization of energy transfer interactions between rare earth ions. J. Electrochem. Soc. 1967, 114, 1048−1053. (20) Han, B.; Zhang, J.; Lü, Y. Novel Emitting-Color Tunable Phosphors Ba3Y2(BO3)4:Ce3+,Tb3+ with Efficient Energy Transfer for Near-UV Light-Emitting Diodes. J. Am. Ceram. Soc. 2013, 96, 179− 183. (21) Li, B.; Sun, Q.; Wang, S.; Guo, H.; Huang, X. Ce3+ and Tb3+ doped Ca3Gd(AlO)3(BO3)4 phosphors: synthesis, tunable photo-

4. EXPERIMENTAL SECTION 4.1. Sample Preparation. The white powder samples with the composition of BaGd2(1−x)Si3O10:xCe3+ (abbreviated as BGSO:xCe3+; x = 0.02, 0.04, 0.06, 0.08, and 0.10) and BaGd 2 ( 0 . 9 6 − y ) Si 3 O 1 0 :0.04Ce 3 + ,yTb 3 + (abbreviated as BGSO:0.04Ce3+,yTb3+; y = 0.01, 0.03, 0.10, 0.20, 0.30, and 0.40) were successfully synthesized through solid-state reaction method. Appropriate amounts of BaCO3 (analytical reagent, AR), SiO 2 (AR), Gd 2 O 3 (99.99%), Ce(NO 3 ) 3 ·6H 2 O (99.99%), and Tb(NO3)3·6H2O (99.99%) were mixed in an agate mortar and placed in an alumina crucible. Then, the chemical mixture was sintered at 800 °C for 4 h in a COreduced atmosphere. The final products were re-ground into fine powders for further structural and optical characterizations. 4.2. Characterization. The phase purity was checked by XRD analysis using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (1.5406 Å) operating at 40 kV and 40 mA. The fluorescence properties including excitation and emission spectra were analyzed with the help of an Edinburgh FS5 spectrophotometer, where a 150 W xenon lamp was used as an excitation source. The IQE of the phosphors was determined using the same instrument equipped with an integrating sphere coated with BaSO4. The temperaturedependent luminescence spectra were recorded for composition-optimized BGSO:0.04Ce3+,0.30Tb3+ phosphors with the help of an Edinburgh FS5 spectrophotometer equipped with a temperature controller.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.H.). *E-mail: [email protected] (Y.W.). ORCID

Xiaoyong Huang: 0000-0003-4076-7874 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 (No. 51502190).

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REFERENCES

(1) Xia, Z.; Liang, Y.; Yu, D.; Zhang, M.; Huang, W.; Tong, M.; Wu, J.; Zhao, J. Photoluminescence properties and energy transfer in color 4388

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DOI: 10.1021/acsomega.9b00182 ACS Omega 2019, 4, 4384−4389