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Color-tunable emissions via energy transfer in Bi3+ and Eu3+ doped β-LaB5O9: Sol-gel synthesis and photoluminescence
Journal of Luminescence 219 (2020) 116880
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Color-tunable emissions via energy transfer in Bi3þ and Eu3þ doped β-LaB5O9: Sol-gel synthesis and photoluminescence Ruirui Yang a, Yuxuan Qi a, Yan Gao a, Pengfei Jiang a, Rihong Cong a, b, **, Tao Yang a, b, * a b
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 401331, People’s Republic of China Chongqing Key Laboratory of Theoretical and Computational Chemistry, Chongqing University, Chongqing, 401331, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: Rare earth Bismuth Borates Color-tunable emissions Energy transfer
Pentaborate β-LaB5O9 was reported to be a good phosphor host for rare earth activators, like Ce3þ, Eu3þ, Tb3þ, Sm3þ and Dy3þ. Herein this work, Bi3þ was incorporated into the pentaborate structure β-LaB5O9 for the first time using the sol-gel method, and the doping upper limit is confirmed to be 17 atom% by careful analyses on the powder X-ray diffraction data. Bi3þ exhibits an intense emission band with the maximum at 440 nm and it has a slight concentration quenching at 12 atom%. Bi3þ to Eu3þ energy transfer was confirmed in β-La0.9-yBi0.1EuyB5O9 (0.025 � y � 0.15) phosphors according to the fast decreasing of the Bi3þ emission in both intensity and lifetime along with the increase of Eu3þ dopant concentration. Benefiting from the energy transfer, color-tunable emis sions could be realized from blue to pink under UV excitation, indicated by the CIE coordinate evolution from (0.163, 0.152) to (0.318, 0.211).
1. Introduction Rare earth borates have been extensively studied for decades in various applications in phosphors, nonlinear optical, and laser crystals due to their remarkable structural characteristics [1–6]. For example, the strong covalent boron-oxygen bonds ensure the high thermal sta bility, damage threshold, and high transparency in ultra violet (UV); the remarkable structural flexibility due to the diverse interconnections between BO3 and BO4 species offers a number of noncentrosymmetric or polar structures [7–9]. Rare earth elements are important to function alize the borates while there are only a few thermodynamically stable structures in the RE2O3–B2O3 phase diagram [10], including oxyborate [11–14], orthoborate [15], and metaborate [16], if high temperature solid state reactions were applied. In order to expand the scope of metal borates, new synthetic methods were developed, including hydrother mal [17–19], boric acid flux [20,21], sol-gel [22–24], combustion [25], and high-temperature/high-pressure methods [26–30]. In order to avoid photoluminescent quenching effect, we prefer to select polyborates with B/M � 3 (M ¼ metal) as the host candidates, where the dopant activators could be well separated by large polyborate anions. In literature, Eu3þ-doped Gd[B6O9(OH)3] [22], La[B8O11(OH)5] [31], and Sm[B9O13(OH)4]⋅H2O [32] were prepared using the boric acid
flux method, and one of their common features is the absence of con centration quenching. The calcination of these hydrated rare earth polyborates at appropriate temperatures can give rise to two new anhydrous pentaborates, α- and β-REB5O9, which were proved to be excellent hosts for various lanthanide activators, like Ce3þ, Eu3þ, Tb3þ [20–22,31–34]. Such a thermal decomposition method needs the prep aration of precursors using hydrothermal method, and the exact dopant concentration is hard to be controlled accurately, therefore, sol-gel method is superior in this respect. On the other hand, Bi3þ has a similar cationic radius with that of lanthanide as well as the coordination behavior. In some cases, the Bi3þRE3þ mutual substitutions could also help the formation of meta-stable borates [35–38]. Interestingly, Bi3þ is a post-transition metal ion with the [Xe]4f145d96s2 electron configuration, which can be efficiently excited by UV irradiation due to the parity-allow 1S0→3P1 absorption, such as in the cases of Ca4YO(BO3)3:Bi3þ/Eu3þ [39], KBaY(BO3)2: Bi3þ/Eu3þ [40], and YBO3:Bi3þ/Dy3þ [41]. Moreover, efficient energy transfers could be expected when Bi3þ was combined with other lanthanide activators. As the most representative case, we were moti vated to investigate the co-doping of Bi3þ and Eu3þ in β-LaB5O9 using the sol-gel preparation. Phase purity and the Bi3þ doping upper limit was confirmed by careful analyses on powder X-ray diffraction (XRD)
* Corresponding author. College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 401331, People’s Republic of China. ** Corresponding author. College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 401331, People’s Republic of China. E-mail addresses: [email protected] (R. Cong), [email protected] (T. Yang). https://doi.org/10.1016/j.jlumin.2019.116880 Received 30 July 2019; Received in revised form 29 October 2019; Accepted 5 November 2019 Available online 6 November 2019 0022-2313/© 2019 Elsevier B.V. All rights reserved.
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Journal of Luminescence 219 (2020) 116880
Fig. 1. (a) XRD patterns for β-La1-xBixB5O9 (0 � x � 0.17) with the simulated pattern for β-LaB5O9 at the bottom; (b) unit cell volume refined from the Le Bail fitting against the doping content of Eu3þ (x).
data for both Bi3þ singly-doped and Bi3þ/Eu3þ co-doped β-LaB5O9 phosphors, and benefiting from the Bi3þ to Eu3þ energy transfer, color-tunable emissions from blue to pink were realized by simply controlling the doping concentrations of Bi3þ and Eu3þ.
Table 1 Refined cell lattice parameters for β-La1-xBixB5O9 (0 � x � 0.17) in the space group P21/c from Le Bail fittings.
2. Experimental section All phosphors investigated in this study were synthesized by sol-gel method. Stoichiometric La2O3, Bi(NO3)3⋅6H2O, Eu2O3 were first dis solved by concentrated HNO3 aqueous solution under gentle stirring and heating. Then, H3BO3 (with 50 mol% excess in order to compensate for the high temperature volatilization of boron), an appropriate amount of citric acid and deionized water were added into the solution. After slowly evaporating the water upon heating on a hot plate, a clear and homogeneous sol formed, which was further loaded into an oven at 90, 150, and 180 � C for 5 h at each temperature. Light-brown foam-like precursors could be obtained. The precursors were then ground and heated in a muffle furnace at 250 and 550 � C for 15 h, respectively. After cooling to room temperature, the resultant dark-brown powder was further heated at 680 and 830 � C for 10 h, respectively. The final product is white powder and a small amount of residual boron oxide could react with moisture to form H3BO3 in the final product, which would not affect the PL study. Phase purity was proved by powder XRD technique, which was performed on a PANalytical Empyrean diffractometer equipped with a PIXcel 1D detector (Cu Kα, 40 kV and 40 mA). Le Bail refinements were performed to obtain the cell parameters by TOPAS [42]. Photo luminescence spectra were measured with a Hitachi F4600 fluorescence spectrometer equipped with the solid accessory. The voltage of the Xe lamp was 700 V, and both of the input and output slit parameters were set to be 1.0 nm. The emission intensity was calculated according to the integral of the corresponding peaks after subtracting the background signal. Scanning electron microscopies (SEM) were performed on JSM-7800F electron microscope. Fluorescence lifetimes were recorded on an Edinburgh FLS-920 spectrophotometer equipped with the pulse xenon lamp (UF900) as the light sources.
x in β-La1xBixB5O9
a (Å)
b (Å)
c (Å)
β (o)
V (Å3)
0
6.4436 (1) 6.4432 (1) 6.4430 (1) 6.4424 (1) 6.4422 (1) 6.4411 (1) 6.4407 (1) 6.4395 (1) 6.4390 (1) 6.4380 (1)
11.6917 (2) 11.6912 (1) 11.6903 (1) 11.6890 (1) 11.6879 (1) 11.6860 (1) 11.6847 (1) 11.6813 (1) 11.6801 (1) 11.6775 (1)
8.1733 (1) 8.1730 (1) 8.1727 (1) 8.1720 (1) 8.1719 (1) 8.1713 (1) 8.1708 (1) 8.1697 (1) 8.1698 (1) 8.1683 (1)
105.1716 (8) 105.1720 (7) 105.1783 (7) 105.1812 (7) 105.1882 (7) 105.1928 (7) 105.1998 (7) 105.2162 (7) 105.2201 (7) 105.2289 (8)
594.29 (2) 594.20 (1) 594.10 (1) 593.92 (1) 593.82 (1) 593.57 (1) 593.41 (1) 592.99 (1) 592.88 (1) 592.53 (1)
0.01 0.025 0.03 0.05 0.07 0.10 0.125 0.15 0.17
structure, the doping limit should be carefully determined. Powder XRD patterns for β-La1-xBixB5O9 (0 � x � 0.17) are presented in Fig. 1a, which share the same characteristic pattern with the one calculated on the basis of the reported structure of β-LaB5O9 [21]. Le Bail fittings were performed by TOPAS and the refined cell parameters in the space group P21/c are listed in Table 1. As representative, the final convergence of Le Bail fitting on the powder XRD for β-La0.9Bi0.1B5O9 is provided in Fig. S1 in the Electronic Supplementary Information, ESI. No impurity peak is observed as indicated by the good match between the calculated and observed patterns, and the very sharp diffraction peaks indicate the high crystallinity. The cell volume (V) versus the Bi3þ concentration (x) was plotted in Fig. 1b. The linear decreasing tendency of V along with x is a strong proof of the successful cationic doping of Bi3þ. Experimentally, if the initial loading content of Bi3þ was higher than 17 atom%, the ob tained product presented a light yellow color due to the appearance of amorphous Bi2B8O15, which could be detected after additional anneal ing at temperature lower than its melting point (i.e. 670 � C). Therefore it is proved that the partial solid solutions of β-La1-xBixB5O9 (0 � x � 0.17) can be successfully prepared with the doping upper limit of 17 atom%. In addition, SEM images of two representatives with low and high Bi3þ contents are presented in Fig. 2, where the increasing content of Bi3þ apparently helps the crystallization of the particles.
3. Results and discussion 3.1. Phase identification In literature, partial solid solutions of Eu3þ singly-doped β-LaB5O9 phosphors have been prepared by either thermal decomposition or solgel methods, and the doping upper limit of Eu3þ is 15 atom%, because EuB5O9 belongs to the α-type pentaborate [24,31,33]. Here, it is the first time to introduce non-rare earth element (Bi3þ) into the pentaborate 2
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Journal of Luminescence 219 (2020) 116880
Fig. 2. SEM images of (a) β-La0.99Bi0.01B5O9 and (b) β-La0.83Bi0.17B5O9.
Fig. 3. (a) XRD patterns for β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15); (b) unit cell volume refined from the Le Bail fitting against the doping content of Eu3þ (y).
Fig. 4. (a) Excitation and (b) emission spectra for β-La1-xBixB5O9 (0.01 � x � 0.17) phosphors. The inset shows the variation of the integral intensity of Bi3þ emission band against the doping concentration of Bi3þ (x).
A series of Bi3þ and Eu3þ co-doped β-LaB5O9 samples were also prepared by sol-gel method, in order to utilize the Bi3þ to Eu3þ energy transfer. Here, the concentration of Bi3þ was fixed at 10 atom%, while the content of Eu3þ varied from 0 to 15 atom%. Fig. 3a shows the XRD patterns for β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15) and most samples share the same characteristic pattern with the simulated pattern for β-LaB5O9, except the sample with y ¼ 0.15, where several tiny impurity peaks are observed. The refined cell parameters (provided in Table S1 in ESI) from
Le Bail fitting exhibits a similar shrinkage tendency like the case for Bi3þ-singly doped samples (see Fig. 3b), because of the smaller cationic radius of Eu3þ compared to that of La3þ [43]. Although there exist some tiny impurity peaks in β-La0.75Bi0.10Eu0.15B5O9, its cell volume is smaller than that of La0.775Bi0.10Eu0.125B5O9, thus it is reasonable to conclude that the upper limit of Eu3þ in these co-doped samples is around 15 atom %. In fact, such a small amount of impurity are not supposed to affect the photoluminescent performance. In summary, two partial solid solutions 3
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Journal of Luminescence 219 (2020) 116880
Fig. 5. Excitation spectra of β-La0.9-yBi0.1EuyB5O9 (0.025 � y � 0.15) by monitoring the emission of (a) λem ¼ 440 nm and (b) λem ¼ 615 nm, respectively.
with the formulas β-La1-xBixB5O9 (0 � x � 0.17) and β-La0.9-y Bi0.1EuyB5O9 (0 � y � 0.15) were successfully prepared by sol-gel method, ensuring the following photoluminescence studies. 3.2. Photoluminescence of β-La1-xBixB5O9 (0.01 � x � 0.17) Bi3þ is a commonly used sensitizer in phosphors and sometimes it behaves as an activator as well [36,39–41]. The excitation and emission spectra for β-La1-xBixB5O9 (0.01 � x � 0.17) with λem ¼ 440 nm and λex ¼ 267 nm, respectively, are presented in Fig. 4. For each sample, a broad excitation band and a strong emission band in the region of 230–300 nm and 330–625 nm are observed. Bi3þ has a ground sate 1S0 and the corresponding excited states are 3P0, 3P1, 3P2 and 1P1. For the electron transition of Bi3þ, 1S0→1P1 is allowed for ΔS ¼ 0 according to the spin-election rule, which is usually located below 200 nm. The 1 S0→3P0 transition is strongly forbidden, because total angular mo mentum is not changed (ΔJ ¼ 0). 1S0→3P1 is usually forbidden by the spin-election rule, but it can be partially allowed by mixing singlet and triplet states (ΔJ ¼ 1) [36,44,45]. Therefore, we interpret the observed broad excitation band to be the 1S0→3P1 absorption. Moreover, the in tensity of this absorption band first increases gradually along with the increase of Bi3þ doping content, and reaches its maximum at x ¼ 0.12, and then a rapid drop was observed. The intense emission band with the maximum at 440 nm for β-La13 1 3þ xBixB5O9 (0.01 � x � 0.17) is attributed to the P1→ S0 transition of Bi by referring to the literature [36,44,45]. A similar variation tendency was observed for the emission spectra. For clarity, the inset in Fig. 4b gives the variation of the calculated integral intensities of Bi3þ emission along with the doping content (x). A clear increase is observed when x is below 0.12, after which an obvious concentration quenching effect is detected. These results indicate the introduction of Bi3þ ions into β-LaB5O9 leads to a high light absorption efficiency in the UV region. It guarantees the following study on Bi3þ and Eu3þ co-doped phosphors with the rationale of utilizing the efficient Bi3þ to Eu3þ sensitization effect.
Fig. 6. Emission spectra of β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15) under the 267 nm excitation.
decreases gradually as y increases. This is a typical indication that Bi3þ behaves as a sensitizer and transfers part of its absorbed energy to Eu3þ. When monitoring the Eu3þ 5D0→7F2 emission at λ ¼ 615 nm, the excitation spectra for β-La0.9-yBi0.1EuyB5O9 (0.025 � y � 0.15) contain both absorptions from Bi3þ and Eu3þ (see Fig. 5b). First, it is clear that the sharp peaks in the wavelength of 350–480 nm belongs to the 4f6→4f6 transitions of Eu3þ [24], which are assigned in detail as shown in Fig. 5b. While the intense absorption in the shorter wavelength needs to be clarified due to the overlapping between the Bi3þ 1S0→3P1 and O2 →Eu3þ charge transfer (CT) absorptions. For comparison, the PL spectra for β-La0.9Bi0.1B5O9 and β-La0.95Eu0.05B5O9 are presented in Fig. S2, ESI. The Bi3þ 1S0→3P1 absorption band is relatively sharp; in addition, the O2 →Eu3þ CT absorption band has a slightly asymmetric shape and a longer wavelength at the maximum value (267 nm for Bi3þ 1 S0→3P1; 271 nm for O2 →Eu3þ CT). According to the above compari son, we interpret the wide absorption band in Fig. 5b to be composed of both Bi3þ 1S0→3P1 and O2 →Eu3þ CT components. Consequently, this further proves the Bi3þ to Eu3þ energy transfer in Bi3þ and Eu3þ co-doped phosphors. A gradual increase for both the band absorptions and the Eu3þ f-f transitions when increasing the Eu3þ doping content. Fig. 6 shows the emission spectra for β-La0.9-yBi0.1EuyB5O9 (0.025 � y � 0.15) under the excitation of λex ¼ 267 nm. Obviously, the broad blue emission band at λ ¼ 350–575 nm is originated from the Bi3þ 3 P1→1S0 transition and it decreases when y increases due to the increasing efficiency of Bi3þ to Eu3þ energy transfer. The emission peaks in the range of λ ¼ 575–720 nm are typical Eu3þ 5D0→7FJ (J ¼ 0–4)
3.3. Bi3þ→Eu3þ energy transfer in β-La0.9-yBi0.1EuyB5O9 (0.025 � y � 0.15) Herein, a series of phosphors of β-La0.9-yBi0.1EuyB5O9 (0.025 � y � 0.15) were prepared with Bi3þ concentration fixed at 10 atom% and Eu3þ concentration varied from 2.5 to 15 atom%. Fig. 5a gives the excitation spectra by monitoring the Bi3þ emission at λ ¼ 440 nm. Similar to the Bi3þ singly-doped samples, only a typical Bi3þ 1 S0→3P1 absorption band is observed in the range of 230–300 nm, and no absorptions belonging to Eu3þ can be observed. This excludes any energy transfer from Eu3þ to Bi3þ. The intensity of Bi3þ absorption 4
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Journal of Luminescence 219 (2020) 116880
Fig. 7. (a) The concentration-dependent integral intensity of Bi3þ and Eu3þ emissions under the 267 nm excitation; (b) the estimated efficiency of Bi3þ to Eu3þ energy transfer in β-La0.9-yBi0.1EuyB5O9 against y.
crystal field around Eu3þ leads to a large R/O [46]. Here the calculated R/O values for the whole series of samples are ~2.5 (see Fig. S3, ESI), indicating that the surrounding environment of Eu3þ is non centrosymmetric [21]. The integral intensity of Bi3þ emission exhibits an opposite evolution trend comparing with the Eu3þ emission as shown in Fig. 7a. Although the Bi3þ content is fixed, the Bi3þ emission mono tonically declines; while the Eu3þ emission increases monotonically. This is the consequence of energy transfer, and the efficiency (ηET) can be estimated according to the equation ηET ¼ 1-Iy/I0 [47,48], in which I0 and Iy are the integral emission intensity for Bi3þ in β-La0.9Bi0.1B5O9 and β-La0.9-yBi0.1EuyB5O9 (0.025 � y � 0.15), respectively. As shown in Fig. 7b, the value of ηET increases gradually with the increase of Eu3þ concentration, and reaches 78.7% at y ¼ 0.15. Fig. 8 presents the emission spectra under the excitation of λex ¼ 392 nm, where only the characteristic emissions for Eu3þ 5D0→7FJ (J ¼ 0–4) can be observed. The integral emission intensity for Eu3þ in creases linearly along with increasing y (see the inset of Fig. 8). As we expected, there is no concentration quenching for Eu3þ, which is due to the well separation of Eu3þ by large polyborate ions. To further evaluate the Bi3þ→Eu3þ energy transfer, the fluorescence decay curves of Bi3þ for β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15) are pre sented in Fig. 9a (λex ¼ 270 nm and λem ¼ 473 nm). All lifetime are calculated by fitting the one-order exponential function, and the simu lated values of A and τ were given in Table S2. As shown in Fig. 9b, the calculated lifetime decreases monotonically as y increases, indicating the existence of energy transfer from Bi3þ to Eu3þ. And the energytransfer efficiency ηET can be estimated based on the equation [49]: ηET ¼ 1-τ/τ0, where τ and τ0 are the corresponding lifetimes of sensitizer (Bi3þ) in the presence and absence of activator (Eu3þ). A monotonous
Fig. 8. Emission spectra of β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15) under the 392 nm excitation.
transitions. The only peak at 578 nm for 5D0→7F0 emission is an indi cation that there is only one type of Eu3þ cation, which agrees with the crystal structure of β-LaB5O9. It is known that the electrical dipole transitions 5D0→7F2 (in the range of λ ¼ 605–630 nm) are sensitive to the local environment around Eu3þ, while the magnetic dipole transi tions 5D0→7F1 (in the range of λ ¼ 583–605 nm) are not. Then the in tensity ratio of red/orange (R/O) ¼ I(5D0→7F2)/I(5D0→7F1) is a sensitive probe of the site symmetry for Eu3þ. In general, a low symmetry of the
Fig. 9. (a) Photoluminescent decay curves for Bi3þ emission in β-La0.9-yBi0.1EuyB5O9 with different Eu3þ concentrations; (b) energy transfer efficiency for Bi3þ to Eu3þ in β-La0.9-yBi0.1EuyB5O9 against y. 5
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Journal of Luminescence 219 (2020) 116880
Fig. 10. Fitting of I0/Iy in β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15) on CBiþEu 6/3, C8/ BiþEu 3 , and C10/3 BiþEu, respectively.
cell volume, and yc is the critical concentration when the energy transfer efficiency is 0.5. Accordingly, Rc in β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15) is estimated to be 11.6 Å, which is much longer than 4 Å. Therefore, the Bi3þ to Eu3þ energy transfer is governed by the electric multipolar interaction. The energy transfer mechanism for multipolar interactions is usually assessed by the equation of η0/ηy ¼ Cn/3, in which η0 and ηy are the luminescence quantum efficiencies of Bi3þ in the absence and presence of Eu3þ, respectively [51]. Usually, the values of η0/ηy can be approxi mately calculated from the ratio of luminescence intensities (Iy/I0); C is the total content of Bi3þ and Eu3þ; n ¼ 6, 8, 10 correspond to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole in teractions, respectively. Fig. 10 provides the fitness of Iy/I0 against Cn/3 (n ¼ 6, 8, 10) for β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15), however, the fitting values (χ2) seem to be very close to each other. So we propose that the energy transfer mechanism from Bi3þ to Eu3þ in β-LaB5O9 host may contain various electric multipolar interactions. The CIE chromaticity coordinate for β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15) phosphors were determined on the basis of their corre sponding emission spectra under 267 nm excitation, and the data are presented in Fig. 11 and Table S3. The chromaticity coordinate can be tuned linearly from blue (0.163, 0.152) to pink (0.318, 0.211), by manipulating the concentration ratio between Bi3þ and Eu3þ. Moreover, digital photographs of β-La0.925Bi0.075B5O9 and β-La0.75Bi0.10Eu0.15B5O9 upon excitation with a λ ¼ 254 nm UV lamp are presented in Fig. 11, which show bright blue-white and pink emissions, respectively. 4. Conclusion Bi3þ was incorporated into the pentaborate structure β-LaB5O9 for the first time using the sol-gel method, and the doping upper limit is confirmed to be 17 atom% by careful analyses on the powder XRD data. Bi3þ exhibits an intense emission band with the maximum at 440 nm (blue) and it has a slight quenching at 12 atom%. By utilizing the Bi3þ to Eu3þ energy transfer in co-doped phosphors β-La0.9-yBi0.1EuyB5O9 (0.025 � y � 0.15), color-tunable emissions could be realized from blue to pink under UV excitation, indicated by the CIE coordinate evolution from (0.163, 0.152) to (0.318, 0.211). Here the preliminary results will stimulate further investigations to combine Bi3þ and other lanthanide activators in the pentaborate family. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21771027, 21671028, 21805020), Natural Sci ence Foundation of Chongqing (cstc2019jcyj-msxmX0330), Chongqing Special Postdoctoral Science Foundation (XmT2018004), and Post doctoral Research Foundation of China (2018M643402). We also acknowledge the support from the Fundamental Research Funds for the Central Universities (2019CDXYHG0013, 2019CDQYWL009).
Fig. 11. CIE chromaticity diagram for β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15) phosphors excited by 267 nm irradiation. The insets show the photographs for the representative samples β-La0.925Bi0.075B5O9 and β-La0.75Bi0.10Eu0.15B5O9 under a 254 nm UV lamp.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jlumin.2019.116880.
increase tendency was observed for ηET with increasing y (see Fig. 9b). It achieves 76.7% for β-La0.75Bi0.10Eu0.15B5O9, which is consistent with the value estimated on the basis of luminescent emission intensity (see Fig. 7). In general, the resonant-type energy transfer from a sensitizer to an activator in a phosphor may take place through an exchange interaction or electric multipolar interaction. For the former case, the critical dis tance (Rc) between the sensitizer and the activator should be shorter than 4 Å. The value of Rc can be calculated according to the concen tration quenching method by using the equation: Rc � 2(3 V/4πycN)1/3 [50], where N is the number of host cations in the unit cell; V is the unit
Electronic Supplementary Information Cell lattice parameters from Le Bail refinements and fitting param eters from the decay cures for β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15), CIE chromaticity coordinate for β-La1-xBixB5O9 and β-La0.9-yBi0.1EuyB5O9 phosphors under the excitation of 267 nm, final convergence of Le Bail fitting for β-La0.9Bi0.1B5O9, Photoluminescence spectra for β-La0.9Bi0.1B5O9 and β-La0.95Eu0.05B5O9, R/O values for β-La0.9yBi0.1EuyB5O9 (0.025 � y � 0.15) phosphors under the excitation of 267 nm. 6
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Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Color-tunable emissions via energy transfer in Bi3þ and Eu3þ doped β-LaB5O9: Sol-gel synthesis and photoluminescence Ruirui Yang a, Yuxuan Qi a, Yan Gao a, Pengfei Jiang a, Rihong Cong a, b, **, Tao Yang a, b, * a b
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 401331, People’s Republic of China Chongqing Key Laboratory of Theoretical and Computational Chemistry, Chongqing University, Chongqing, 401331, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: Rare earth Bismuth Borates Color-tunable emissions Energy transfer
Pentaborate β-LaB5O9 was reported to be a good phosphor host for rare earth activators, like Ce3þ, Eu3þ, Tb3þ, Sm3þ and Dy3þ. Herein this work, Bi3þ was incorporated into the pentaborate structure β-LaB5O9 for the first time using the sol-gel method, and the doping upper limit is confirmed to be 17 atom% by careful analyses on the powder X-ray diffraction data. Bi3þ exhibits an intense emission band with the maximum at 440 nm and it has a slight concentration quenching at 12 atom%. Bi3þ to Eu3þ energy transfer was confirmed in β-La0.9-yBi0.1EuyB5O9 (0.025 � y � 0.15) phosphors according to the fast decreasing of the Bi3þ emission in both intensity and lifetime along with the increase of Eu3þ dopant concentration. Benefiting from the energy transfer, color-tunable emis sions could be realized from blue to pink under UV excitation, indicated by the CIE coordinate evolution from (0.163, 0.152) to (0.318, 0.211).
1. Introduction Rare earth borates have been extensively studied for decades in various applications in phosphors, nonlinear optical, and laser crystals due to their remarkable structural characteristics [1–6]. For example, the strong covalent boron-oxygen bonds ensure the high thermal sta bility, damage threshold, and high transparency in ultra violet (UV); the remarkable structural flexibility due to the diverse interconnections between BO3 and BO4 species offers a number of noncentrosymmetric or polar structures [7–9]. Rare earth elements are important to function alize the borates while there are only a few thermodynamically stable structures in the RE2O3–B2O3 phase diagram [10], including oxyborate [11–14], orthoborate [15], and metaborate [16], if high temperature solid state reactions were applied. In order to expand the scope of metal borates, new synthetic methods were developed, including hydrother mal [17–19], boric acid flux [20,21], sol-gel [22–24], combustion [25], and high-temperature/high-pressure methods [26–30]. In order to avoid photoluminescent quenching effect, we prefer to select polyborates with B/M � 3 (M ¼ metal) as the host candidates, where the dopant activators could be well separated by large polyborate anions. In literature, Eu3þ-doped Gd[B6O9(OH)3] [22], La[B8O11(OH)5] [31], and Sm[B9O13(OH)4]⋅H2O [32] were prepared using the boric acid
flux method, and one of their common features is the absence of con centration quenching. The calcination of these hydrated rare earth polyborates at appropriate temperatures can give rise to two new anhydrous pentaborates, α- and β-REB5O9, which were proved to be excellent hosts for various lanthanide activators, like Ce3þ, Eu3þ, Tb3þ [20–22,31–34]. Such a thermal decomposition method needs the prep aration of precursors using hydrothermal method, and the exact dopant concentration is hard to be controlled accurately, therefore, sol-gel method is superior in this respect. On the other hand, Bi3þ has a similar cationic radius with that of lanthanide as well as the coordination behavior. In some cases, the Bi3þRE3þ mutual substitutions could also help the formation of meta-stable borates [35–38]. Interestingly, Bi3þ is a post-transition metal ion with the [Xe]4f145d96s2 electron configuration, which can be efficiently excited by UV irradiation due to the parity-allow 1S0→3P1 absorption, such as in the cases of Ca4YO(BO3)3:Bi3þ/Eu3þ [39], KBaY(BO3)2: Bi3þ/Eu3þ [40], and YBO3:Bi3þ/Dy3þ [41]. Moreover, efficient energy transfers could be expected when Bi3þ was combined with other lanthanide activators. As the most representative case, we were moti vated to investigate the co-doping of Bi3þ and Eu3þ in β-LaB5O9 using the sol-gel preparation. Phase purity and the Bi3þ doping upper limit was confirmed by careful analyses on powder X-ray diffraction (XRD)
* Corresponding author. College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 401331, People’s Republic of China. ** Corresponding author. College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 401331, People’s Republic of China. E-mail addresses: [email protected] (R. Cong), [email protected] (T. Yang). https://doi.org/10.1016/j.jlumin.2019.116880 Received 30 July 2019; Received in revised form 29 October 2019; Accepted 5 November 2019 Available online 6 November 2019 0022-2313/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) XRD patterns for β-La1-xBixB5O9 (0 � x � 0.17) with the simulated pattern for β-LaB5O9 at the bottom; (b) unit cell volume refined from the Le Bail fitting against the doping content of Eu3þ (x).
data for both Bi3þ singly-doped and Bi3þ/Eu3þ co-doped β-LaB5O9 phosphors, and benefiting from the Bi3þ to Eu3þ energy transfer, color-tunable emissions from blue to pink were realized by simply controlling the doping concentrations of Bi3þ and Eu3þ.
Table 1 Refined cell lattice parameters for β-La1-xBixB5O9 (0 � x � 0.17) in the space group P21/c from Le Bail fittings.
2. Experimental section All phosphors investigated in this study were synthesized by sol-gel method. Stoichiometric La2O3, Bi(NO3)3⋅6H2O, Eu2O3 were first dis solved by concentrated HNO3 aqueous solution under gentle stirring and heating. Then, H3BO3 (with 50 mol% excess in order to compensate for the high temperature volatilization of boron), an appropriate amount of citric acid and deionized water were added into the solution. After slowly evaporating the water upon heating on a hot plate, a clear and homogeneous sol formed, which was further loaded into an oven at 90, 150, and 180 � C for 5 h at each temperature. Light-brown foam-like precursors could be obtained. The precursors were then ground and heated in a muffle furnace at 250 and 550 � C for 15 h, respectively. After cooling to room temperature, the resultant dark-brown powder was further heated at 680 and 830 � C for 10 h, respectively. The final product is white powder and a small amount of residual boron oxide could react with moisture to form H3BO3 in the final product, which would not affect the PL study. Phase purity was proved by powder XRD technique, which was performed on a PANalytical Empyrean diffractometer equipped with a PIXcel 1D detector (Cu Kα, 40 kV and 40 mA). Le Bail refinements were performed to obtain the cell parameters by TOPAS [42]. Photo luminescence spectra were measured with a Hitachi F4600 fluorescence spectrometer equipped with the solid accessory. The voltage of the Xe lamp was 700 V, and both of the input and output slit parameters were set to be 1.0 nm. The emission intensity was calculated according to the integral of the corresponding peaks after subtracting the background signal. Scanning electron microscopies (SEM) were performed on JSM-7800F electron microscope. Fluorescence lifetimes were recorded on an Edinburgh FLS-920 spectrophotometer equipped with the pulse xenon lamp (UF900) as the light sources.
x in β-La1xBixB5O9
a (Å)
b (Å)
c (Å)
β (o)
V (Å3)
0
6.4436 (1) 6.4432 (1) 6.4430 (1) 6.4424 (1) 6.4422 (1) 6.4411 (1) 6.4407 (1) 6.4395 (1) 6.4390 (1) 6.4380 (1)
11.6917 (2) 11.6912 (1) 11.6903 (1) 11.6890 (1) 11.6879 (1) 11.6860 (1) 11.6847 (1) 11.6813 (1) 11.6801 (1) 11.6775 (1)
8.1733 (1) 8.1730 (1) 8.1727 (1) 8.1720 (1) 8.1719 (1) 8.1713 (1) 8.1708 (1) 8.1697 (1) 8.1698 (1) 8.1683 (1)
105.1716 (8) 105.1720 (7) 105.1783 (7) 105.1812 (7) 105.1882 (7) 105.1928 (7) 105.1998 (7) 105.2162 (7) 105.2201 (7) 105.2289 (8)
594.29 (2) 594.20 (1) 594.10 (1) 593.92 (1) 593.82 (1) 593.57 (1) 593.41 (1) 592.99 (1) 592.88 (1) 592.53 (1)
0.01 0.025 0.03 0.05 0.07 0.10 0.125 0.15 0.17
structure, the doping limit should be carefully determined. Powder XRD patterns for β-La1-xBixB5O9 (0 � x � 0.17) are presented in Fig. 1a, which share the same characteristic pattern with the one calculated on the basis of the reported structure of β-LaB5O9 [21]. Le Bail fittings were performed by TOPAS and the refined cell parameters in the space group P21/c are listed in Table 1. As representative, the final convergence of Le Bail fitting on the powder XRD for β-La0.9Bi0.1B5O9 is provided in Fig. S1 in the Electronic Supplementary Information, ESI. No impurity peak is observed as indicated by the good match between the calculated and observed patterns, and the very sharp diffraction peaks indicate the high crystallinity. The cell volume (V) versus the Bi3þ concentration (x) was plotted in Fig. 1b. The linear decreasing tendency of V along with x is a strong proof of the successful cationic doping of Bi3þ. Experimentally, if the initial loading content of Bi3þ was higher than 17 atom%, the ob tained product presented a light yellow color due to the appearance of amorphous Bi2B8O15, which could be detected after additional anneal ing at temperature lower than its melting point (i.e. 670 � C). Therefore it is proved that the partial solid solutions of β-La1-xBixB5O9 (0 � x � 0.17) can be successfully prepared with the doping upper limit of 17 atom%. In addition, SEM images of two representatives with low and high Bi3þ contents are presented in Fig. 2, where the increasing content of Bi3þ apparently helps the crystallization of the particles.
3. Results and discussion 3.1. Phase identification In literature, partial solid solutions of Eu3þ singly-doped β-LaB5O9 phosphors have been prepared by either thermal decomposition or solgel methods, and the doping upper limit of Eu3þ is 15 atom%, because EuB5O9 belongs to the α-type pentaborate [24,31,33]. Here, it is the first time to introduce non-rare earth element (Bi3þ) into the pentaborate 2
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Fig. 2. SEM images of (a) β-La0.99Bi0.01B5O9 and (b) β-La0.83Bi0.17B5O9.
Fig. 3. (a) XRD patterns for β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15); (b) unit cell volume refined from the Le Bail fitting against the doping content of Eu3þ (y).
Fig. 4. (a) Excitation and (b) emission spectra for β-La1-xBixB5O9 (0.01 � x � 0.17) phosphors. The inset shows the variation of the integral intensity of Bi3þ emission band against the doping concentration of Bi3þ (x).
A series of Bi3þ and Eu3þ co-doped β-LaB5O9 samples were also prepared by sol-gel method, in order to utilize the Bi3þ to Eu3þ energy transfer. Here, the concentration of Bi3þ was fixed at 10 atom%, while the content of Eu3þ varied from 0 to 15 atom%. Fig. 3a shows the XRD patterns for β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15) and most samples share the same characteristic pattern with the simulated pattern for β-LaB5O9, except the sample with y ¼ 0.15, where several tiny impurity peaks are observed. The refined cell parameters (provided in Table S1 in ESI) from
Le Bail fitting exhibits a similar shrinkage tendency like the case for Bi3þ-singly doped samples (see Fig. 3b), because of the smaller cationic radius of Eu3þ compared to that of La3þ [43]. Although there exist some tiny impurity peaks in β-La0.75Bi0.10Eu0.15B5O9, its cell volume is smaller than that of La0.775Bi0.10Eu0.125B5O9, thus it is reasonable to conclude that the upper limit of Eu3þ in these co-doped samples is around 15 atom %. In fact, such a small amount of impurity are not supposed to affect the photoluminescent performance. In summary, two partial solid solutions 3
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Fig. 5. Excitation spectra of β-La0.9-yBi0.1EuyB5O9 (0.025 � y � 0.15) by monitoring the emission of (a) λem ¼ 440 nm and (b) λem ¼ 615 nm, respectively.
with the formulas β-La1-xBixB5O9 (0 � x � 0.17) and β-La0.9-y Bi0.1EuyB5O9 (0 � y � 0.15) were successfully prepared by sol-gel method, ensuring the following photoluminescence studies. 3.2. Photoluminescence of β-La1-xBixB5O9 (0.01 � x � 0.17) Bi3þ is a commonly used sensitizer in phosphors and sometimes it behaves as an activator as well [36,39–41]. The excitation and emission spectra for β-La1-xBixB5O9 (0.01 � x � 0.17) with λem ¼ 440 nm and λex ¼ 267 nm, respectively, are presented in Fig. 4. For each sample, a broad excitation band and a strong emission band in the region of 230–300 nm and 330–625 nm are observed. Bi3þ has a ground sate 1S0 and the corresponding excited states are 3P0, 3P1, 3P2 and 1P1. For the electron transition of Bi3þ, 1S0→1P1 is allowed for ΔS ¼ 0 according to the spin-election rule, which is usually located below 200 nm. The 1 S0→3P0 transition is strongly forbidden, because total angular mo mentum is not changed (ΔJ ¼ 0). 1S0→3P1 is usually forbidden by the spin-election rule, but it can be partially allowed by mixing singlet and triplet states (ΔJ ¼ 1) [36,44,45]. Therefore, we interpret the observed broad excitation band to be the 1S0→3P1 absorption. Moreover, the in tensity of this absorption band first increases gradually along with the increase of Bi3þ doping content, and reaches its maximum at x ¼ 0.12, and then a rapid drop was observed. The intense emission band with the maximum at 440 nm for β-La13 1 3þ xBixB5O9 (0.01 � x � 0.17) is attributed to the P1→ S0 transition of Bi by referring to the literature [36,44,45]. A similar variation tendency was observed for the emission spectra. For clarity, the inset in Fig. 4b gives the variation of the calculated integral intensities of Bi3þ emission along with the doping content (x). A clear increase is observed when x is below 0.12, after which an obvious concentration quenching effect is detected. These results indicate the introduction of Bi3þ ions into β-LaB5O9 leads to a high light absorption efficiency in the UV region. It guarantees the following study on Bi3þ and Eu3þ co-doped phosphors with the rationale of utilizing the efficient Bi3þ to Eu3þ sensitization effect.
Fig. 6. Emission spectra of β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15) under the 267 nm excitation.
decreases gradually as y increases. This is a typical indication that Bi3þ behaves as a sensitizer and transfers part of its absorbed energy to Eu3þ. When monitoring the Eu3þ 5D0→7F2 emission at λ ¼ 615 nm, the excitation spectra for β-La0.9-yBi0.1EuyB5O9 (0.025 � y � 0.15) contain both absorptions from Bi3þ and Eu3þ (see Fig. 5b). First, it is clear that the sharp peaks in the wavelength of 350–480 nm belongs to the 4f6→4f6 transitions of Eu3þ [24], which are assigned in detail as shown in Fig. 5b. While the intense absorption in the shorter wavelength needs to be clarified due to the overlapping between the Bi3þ 1S0→3P1 and O2 →Eu3þ charge transfer (CT) absorptions. For comparison, the PL spectra for β-La0.9Bi0.1B5O9 and β-La0.95Eu0.05B5O9 are presented in Fig. S2, ESI. The Bi3þ 1S0→3P1 absorption band is relatively sharp; in addition, the O2 →Eu3þ CT absorption band has a slightly asymmetric shape and a longer wavelength at the maximum value (267 nm for Bi3þ 1 S0→3P1; 271 nm for O2 →Eu3þ CT). According to the above compari son, we interpret the wide absorption band in Fig. 5b to be composed of both Bi3þ 1S0→3P1 and O2 →Eu3þ CT components. Consequently, this further proves the Bi3þ to Eu3þ energy transfer in Bi3þ and Eu3þ co-doped phosphors. A gradual increase for both the band absorptions and the Eu3þ f-f transitions when increasing the Eu3þ doping content. Fig. 6 shows the emission spectra for β-La0.9-yBi0.1EuyB5O9 (0.025 � y � 0.15) under the excitation of λex ¼ 267 nm. Obviously, the broad blue emission band at λ ¼ 350–575 nm is originated from the Bi3þ 3 P1→1S0 transition and it decreases when y increases due to the increasing efficiency of Bi3þ to Eu3þ energy transfer. The emission peaks in the range of λ ¼ 575–720 nm are typical Eu3þ 5D0→7FJ (J ¼ 0–4)
3.3. Bi3þ→Eu3þ energy transfer in β-La0.9-yBi0.1EuyB5O9 (0.025 � y � 0.15) Herein, a series of phosphors of β-La0.9-yBi0.1EuyB5O9 (0.025 � y � 0.15) were prepared with Bi3þ concentration fixed at 10 atom% and Eu3þ concentration varied from 2.5 to 15 atom%. Fig. 5a gives the excitation spectra by monitoring the Bi3þ emission at λ ¼ 440 nm. Similar to the Bi3þ singly-doped samples, only a typical Bi3þ 1 S0→3P1 absorption band is observed in the range of 230–300 nm, and no absorptions belonging to Eu3þ can be observed. This excludes any energy transfer from Eu3þ to Bi3þ. The intensity of Bi3þ absorption 4
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Fig. 7. (a) The concentration-dependent integral intensity of Bi3þ and Eu3þ emissions under the 267 nm excitation; (b) the estimated efficiency of Bi3þ to Eu3þ energy transfer in β-La0.9-yBi0.1EuyB5O9 against y.
crystal field around Eu3þ leads to a large R/O [46]. Here the calculated R/O values for the whole series of samples are ~2.5 (see Fig. S3, ESI), indicating that the surrounding environment of Eu3þ is non centrosymmetric [21]. The integral intensity of Bi3þ emission exhibits an opposite evolution trend comparing with the Eu3þ emission as shown in Fig. 7a. Although the Bi3þ content is fixed, the Bi3þ emission mono tonically declines; while the Eu3þ emission increases monotonically. This is the consequence of energy transfer, and the efficiency (ηET) can be estimated according to the equation ηET ¼ 1-Iy/I0 [47,48], in which I0 and Iy are the integral emission intensity for Bi3þ in β-La0.9Bi0.1B5O9 and β-La0.9-yBi0.1EuyB5O9 (0.025 � y � 0.15), respectively. As shown in Fig. 7b, the value of ηET increases gradually with the increase of Eu3þ concentration, and reaches 78.7% at y ¼ 0.15. Fig. 8 presents the emission spectra under the excitation of λex ¼ 392 nm, where only the characteristic emissions for Eu3þ 5D0→7FJ (J ¼ 0–4) can be observed. The integral emission intensity for Eu3þ in creases linearly along with increasing y (see the inset of Fig. 8). As we expected, there is no concentration quenching for Eu3þ, which is due to the well separation of Eu3þ by large polyborate ions. To further evaluate the Bi3þ→Eu3þ energy transfer, the fluorescence decay curves of Bi3þ for β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15) are pre sented in Fig. 9a (λex ¼ 270 nm and λem ¼ 473 nm). All lifetime are calculated by fitting the one-order exponential function, and the simu lated values of A and τ were given in Table S2. As shown in Fig. 9b, the calculated lifetime decreases monotonically as y increases, indicating the existence of energy transfer from Bi3þ to Eu3þ. And the energytransfer efficiency ηET can be estimated based on the equation [49]: ηET ¼ 1-τ/τ0, where τ and τ0 are the corresponding lifetimes of sensitizer (Bi3þ) in the presence and absence of activator (Eu3þ). A monotonous
Fig. 8. Emission spectra of β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15) under the 392 nm excitation.
transitions. The only peak at 578 nm for 5D0→7F0 emission is an indi cation that there is only one type of Eu3þ cation, which agrees with the crystal structure of β-LaB5O9. It is known that the electrical dipole transitions 5D0→7F2 (in the range of λ ¼ 605–630 nm) are sensitive to the local environment around Eu3þ, while the magnetic dipole transi tions 5D0→7F1 (in the range of λ ¼ 583–605 nm) are not. Then the in tensity ratio of red/orange (R/O) ¼ I(5D0→7F2)/I(5D0→7F1) is a sensitive probe of the site symmetry for Eu3þ. In general, a low symmetry of the
Fig. 9. (a) Photoluminescent decay curves for Bi3þ emission in β-La0.9-yBi0.1EuyB5O9 with different Eu3þ concentrations; (b) energy transfer efficiency for Bi3þ to Eu3þ in β-La0.9-yBi0.1EuyB5O9 against y. 5
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Fig. 10. Fitting of I0/Iy in β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15) on CBiþEu 6/3, C8/ BiþEu 3 , and C10/3 BiþEu, respectively.
cell volume, and yc is the critical concentration when the energy transfer efficiency is 0.5. Accordingly, Rc in β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15) is estimated to be 11.6 Å, which is much longer than 4 Å. Therefore, the Bi3þ to Eu3þ energy transfer is governed by the electric multipolar interaction. The energy transfer mechanism for multipolar interactions is usually assessed by the equation of η0/ηy ¼ Cn/3, in which η0 and ηy are the luminescence quantum efficiencies of Bi3þ in the absence and presence of Eu3þ, respectively [51]. Usually, the values of η0/ηy can be approxi mately calculated from the ratio of luminescence intensities (Iy/I0); C is the total content of Bi3þ and Eu3þ; n ¼ 6, 8, 10 correspond to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole in teractions, respectively. Fig. 10 provides the fitness of Iy/I0 against Cn/3 (n ¼ 6, 8, 10) for β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15), however, the fitting values (χ2) seem to be very close to each other. So we propose that the energy transfer mechanism from Bi3þ to Eu3þ in β-LaB5O9 host may contain various electric multipolar interactions. The CIE chromaticity coordinate for β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15) phosphors were determined on the basis of their corre sponding emission spectra under 267 nm excitation, and the data are presented in Fig. 11 and Table S3. The chromaticity coordinate can be tuned linearly from blue (0.163, 0.152) to pink (0.318, 0.211), by manipulating the concentration ratio between Bi3þ and Eu3þ. Moreover, digital photographs of β-La0.925Bi0.075B5O9 and β-La0.75Bi0.10Eu0.15B5O9 upon excitation with a λ ¼ 254 nm UV lamp are presented in Fig. 11, which show bright blue-white and pink emissions, respectively. 4. Conclusion Bi3þ was incorporated into the pentaborate structure β-LaB5O9 for the first time using the sol-gel method, and the doping upper limit is confirmed to be 17 atom% by careful analyses on the powder XRD data. Bi3þ exhibits an intense emission band with the maximum at 440 nm (blue) and it has a slight quenching at 12 atom%. By utilizing the Bi3þ to Eu3þ energy transfer in co-doped phosphors β-La0.9-yBi0.1EuyB5O9 (0.025 � y � 0.15), color-tunable emissions could be realized from blue to pink under UV excitation, indicated by the CIE coordinate evolution from (0.163, 0.152) to (0.318, 0.211). Here the preliminary results will stimulate further investigations to combine Bi3þ and other lanthanide activators in the pentaborate family. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21771027, 21671028, 21805020), Natural Sci ence Foundation of Chongqing (cstc2019jcyj-msxmX0330), Chongqing Special Postdoctoral Science Foundation (XmT2018004), and Post doctoral Research Foundation of China (2018M643402). We also acknowledge the support from the Fundamental Research Funds for the Central Universities (2019CDXYHG0013, 2019CDQYWL009).
Fig. 11. CIE chromaticity diagram for β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15) phosphors excited by 267 nm irradiation. The insets show the photographs for the representative samples β-La0.925Bi0.075B5O9 and β-La0.75Bi0.10Eu0.15B5O9 under a 254 nm UV lamp.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jlumin.2019.116880.
increase tendency was observed for ηET with increasing y (see Fig. 9b). It achieves 76.7% for β-La0.75Bi0.10Eu0.15B5O9, which is consistent with the value estimated on the basis of luminescent emission intensity (see Fig. 7). In general, the resonant-type energy transfer from a sensitizer to an activator in a phosphor may take place through an exchange interaction or electric multipolar interaction. For the former case, the critical dis tance (Rc) between the sensitizer and the activator should be shorter than 4 Å. The value of Rc can be calculated according to the concen tration quenching method by using the equation: Rc � 2(3 V/4πycN)1/3 [50], where N is the number of host cations in the unit cell; V is the unit
Electronic Supplementary Information Cell lattice parameters from Le Bail refinements and fitting param eters from the decay cures for β-La0.9-yBi0.1EuyB5O9 (0 � y � 0.15), CIE chromaticity coordinate for β-La1-xBixB5O9 and β-La0.9-yBi0.1EuyB5O9 phosphors under the excitation of 267 nm, final convergence of Le Bail fitting for β-La0.9Bi0.1B5O9, Photoluminescence spectra for β-La0.9Bi0.1B5O9 and β-La0.95Eu0.05B5O9, R/O values for β-La0.9yBi0.1EuyB5O9 (0.025 � y � 0.15) phosphors under the excitation of 267 nm. 6
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