Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 682–686
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Luminescence and energy transfer of color-tunable Li6Gd(BO3)3:Ce3+, Tb3+ phosphor Peican Chen, Fuwang Mo, Siyu Xia, Guofang Wang, Anxiang Guan, Liya Zhou ⇑ School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
3+ 3+ were synthesized for the first time. The phosphors exhibited the tunable-color with adjusting the ratio of Ce3+ to Tb3+. 3+ 3+ With tuning the ratio of Ce to Tb , the color change from blue to green.
The Li6Gd(BO3)3:Ce , Tb
a r t i c l e
i n f o
Article history: Received 14 November 2014 Received in revised form 11 March 2015 Accepted 29 April 2015 Available online 11 May 2015 Keywords: Photoluminescence Phosphor Li6Gd(BO3)3
a b s t r a c t A series of novel color-tunable phosphors of Ce3+, Tb3+-codoped Li6Gd(BO3)3 was synthesized through a classic solid-state reaction. The color of these phosphors changes from blue to green by adjusting the ratio of Ce3+ to Tb3+. The photoluminescence properties of the synthesized phosphors were investigated, and several major emission bands that belong to Ce3+ and Tb3+ ions were irradiated with near ultraviolet light. Moreover, the energy transfer mechanism between Ce3+ and Tb3+ in Li6Gd(BO3)3 was explored. The photoluminescence decay curves were performed to validate the energy transfer. The analysis demonstrated that the energy transfer from Ce3+ to Tb3+ arose from dipole–dipole interaction with a critical distance of approximately 17.6 Å. Ó 2015 Elsevier B.V. All rights reserved.
Introduction Material, energy, and information have modernized the trend of state-of-the-art technology. Luminescent materials demonstrate applications in electronic products, laser, solar cell, and advanced lightings. These materials are expected to have irreplaceable functions in the foreseeable future. In general, luminescence materials can be classified into three groups: host luminophores, ‘‘host + activator’’ type, and ‘‘host + activator + sensitizer’’ type [1]. Lanthanide is usually used as a sensitizer or activator in luminescent materials because the 4f–4f transitions of Ln3+ ions confer ⇑ Corresponding author. Tel./fax: +86 771 3233718. E-mail address: [email protected] (L. Zhou). http://dx.doi.org/10.1016/j.saa.2015.04.108 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
unique characteristics, such as high color purity, large Stoke shift, and high luminescent quantum efficiency; these characteristics are highly significant in lighting and display applications [2]. Nevertheless, light-emitting diodes (LEDs) still exhibit deficiencies in because the traditional fabrication of yellow phosphor Y3Al5O12 with blue GaInN chip involves low color rendering index, lack of red component, and reabsorption [3–5]. To address these problems, single-phase color-tunable phosphors are critically needed. Various methods can be employed to yield color-tunable phosphors, such as Ce3+–Tb3+ [6–8], Eu3+–Tb3+ [9], Eu2+–Mn2+ [10], and Eu3+–Tb3+–Eu3+ [11]. Borate serves as an excellent host for luminescent materials because of its low synthesis temperature, various structures, and good chemical and physical temperature stability [12]. Numerous
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P. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 682–686
works focused on borate phosphors, such as Li6Lu(BO3)3:Tb3+, Dy3+ [13] and Sr6YSc(BO3)6:Yb3+ [14]. However, insufficient information is available on Ce3+, Tb3+-codoped Li6Gd(BO3)3. In this study, color-tunable Ce3+, Tb3+ codoped Li6Gd(BO3)3 phosphors were synthesized through a conventional solid-state reaction. The photoluminescence (PL) properties of the phosphors were discussed in detail, and the energy transfer (ET) mechanism for Li6Gd(BO3)3:Ce3+, Tb3+ was investigated. Experimental Sample preparation A series of Li6Gd(BO3)3:0.05Ce3+, xTb3+ (0.05, x, mole concentration) samples was prepared via a traditional solid-state reaction. The initial materials of Li2CO3 (A.R.), Gd2O3 (99.99%), CeO2 (99.99%), and Tb4O7 (99.99%) were weighed in accordance with the stoichiometric proportion, mixed, and then ground thoroughly. As an exception, H3BO3 (A.R.) had an excess of 5%. The samples were transferred to aluminum crucibles and then preheated at 450 °C for 2 h in air. After being ground, the powder mixtures were sintered at 700 °C under the reducing atmosphere of H2 (30%) and N2 (70%) for 5 h. Finally, white powder products were obtained. Sample characterization Power X-ray diffraction data were recorded on a Rigaku D/max-IIIA diffractometer at a scanning rate of 10 min1 in the 2h range of 10° to 70° with Cu Ka radiation. The photoluminescence excitation (PLE) and PL were studied with a Hitachi-2500 fluorescence spectrometer at 150 W excitation. Moreover, the luminescence decay curves were measured with a Horiba Jobin Yvon FL3-P-TCSPC lifetime fluorescence spectrofluorometer. All of the measurements above were conducted at room temperature.
introducing Ce3+ or Tb3+ does not affect the phase formation of Li6Gd(BO3)3. In consideration of ion radius and valence, Ce3+ or Tb3+ ions preferably incorporate Gd3+ instead of Li+ ions.
Luminescent properties Ce3+, which has a 4f1 ground state and a 5d1 excited state, can act as a sensitizer or activator. Both of the 4f–5d absorption transition and the emission are strong broad bands because the 5d electron orbital is strongly affected by the neighboring ions. Therefore, the luminescent color or wavelength of Ce3+ ions widely changes from the near ultraviolet (NUV) to the red region [17]. As shown in Fig. 2, the emission band of Li6Gd(BO3)3:0.05Ce3+ ranges from 370 nm to 500 nm centered at 420 nm at the excitation of 350 nm. The emission spectrum of Li6Gd(BO3)3:Ce3+ can be decomposed into two Gaussian profiles with peaks at 418 nm (23,946 cm1) and 443 nm (22,573 cm1) through Gaussian deconvolution. The Ce3+ emission band, which generally originates from the 5d to 2F5/2 and 2F7/2 transitions of Ce3+ with a theoretical energy difference of 2000 cm1, consists of double bands [18]. However, the energy difference between 418 and 443 nm is 1373 cm1, which is much smaller than the theoretical value of 2000 cm1. Considering the four positions of Gd3+ ions, we can deduce that Ce3+ may build into two of the four positions of Gd3+. Fig. 3(a) shows the PL and PLE spectra of Li6Gd(BO3)3:0.05Ce3+, 0.25Tb3+. At 350 nm excitation, the PL spectrum exhibits the emission bands of Ce3+ and Tb3+. In addition, the intensity of the major characteristic peak of Tb3+ at 547 nm is evidently enhanced compared with that of Fig. 3(b). This finding indicates the ET from Ce3+ to Tb3+. Furthermore, the PLE spectrum of Ce3+ exists at a
5000
Structure and phase purity Li6Gd(BO3)3 crystallizes in the monoclinic system with the space group P21/c with the following parameters: a = 7.244(3) Å, b = 16.510(5) Å, c = 6.694(3) Å, b = 105.37(5), and Z = 4 [15]. Gd3+ ions exist in four positions, whereas they possess the same coordination environment with eight oxygen ions [16]. Fig. 1 illustrates the XRD patterns of Li6Gd(BO3)3, Li6Gd(BO3)3:0.25Tb3+, and Li6Gd(BO3)3:0.05Ce3+, 0.25Tb3+. In comparison with the standard JCPDS card (No. 54-1119), diffraction peaks are consistent, and no impurity peaks are discernable. This result reflects that
3+
Intensity (a.u.)
Li6 Gd(BO)3:0.05Ce ,0.25Tb
Intensity(a.u.)
4000
Results and discussion
3000 2000 1000 0 400
450
500
3+
10
20
30
40
50
Intensity(a.u.) 60
(b)
Li6Gd(BO3)3:0.25Tb3+
700
λem=356 nm λex=547 nm λem=545 nm λex=350 nm
Li6Gd(BO3)3:0.05Ce3+
(c)
JCPDS 54-1119Li 6 Gd(BO3 )3
650
Li6Gd(BO3)3 :0.05Ce3+, 0.25Tb3+ λem=420 nm λex=350 nm
(a)
Li6Gd(BO)3
600
Fig. 2. The Gaussian peaks fitting of Ce3+ emission excited at 350 nm.
3+
Li6Gd(BO)3:0.25Tb
550
Wavelength(nm)
70
2-Theta (degree)
300
350
400
450
500
550
600
650
700
Wavelength(nm) Fig. 1. XRD patterns of Li6Gd(BO3)3, Li6Gd(BO3)3:0.25Tb3+, and Li6Gd(BO3)3:0.05Ce3+, 0.25Tb3+ phosphors. The standard data for Li6Gd(BO3)3 (JCPDS card No. 54-1119) is shown as a reference.
Fig. 3. PLE and PL spectra of Li6Gd(BO3)3:0.05Ce3+ (a), Li6Gd(BO3)3:0.25Tb3+ (b), and Li6Gd(BO3)3:0.05Ce3+, 0.25Tb3+ (c)phosphors.
P. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 682–686
8
(a) R2 = 0.97792
Iso/Is of Ce3+
6
4
2
0 0.08
8
In general, the resonant energy transfer mechanism results from two aspects, namely, exchange interaction and electric multipolar interaction [24]. The exchange interaction predominates the energy transfer when the distance between the sensitizer and the
0.24
0.32
(b) R2 = 0.9986
3+
6
4
2
0 0
20
40
60
80
100
CCe+Tb 6/3 (10 3) 8
Iso/Is of Ce
Energy transfer
0.16
CCe+Tb
(c) R2 = 0.99545
3+
maximum intensity irradiation of Tb3+ at 547 nm, which also confirms the occurrence of ET from Ce3+ to Tb3+. The information of Tb3+, which possesses the ground state of 7Fj (j = 6, . . . ,0) and the excited states 5D3 and 5D4, is difficult to be derived from the spectra because of the high degeneracy of the Tb3+ levels involved in several transitions [19]. When the Tb3+ concentration in the host is low, the 5D3–7Fj transition dominates, thus yielding the blue emission [20]. As the Tb3+ content increases, green emission is produced, which can cause cross relaxation and enhance the 5D4–7Fj transition [21,22]. Fig. 3(b) depicts the PL and PLE spectra of Li6Gd(BO3)3:0.25Tb3+. When pumped at 356 nm, the PL spectrum is observed with a prominent peak at 547 nm (5D4–7F5) and two weak peaks at 491 nm (5D4–7F6) and 585 nm (5D4–7F4). In agreement with the magnetic dipole transition selection rule DJ = ± 1, Laporte’s forbidden transition 5D4–7F5 (547 nm) shows bright green luminescence compared with other emission transitions [23]. The PLE spectrum, which represents a series of spectra bands, is ascribed to the 4f–4f transition of Tb3+. Fig. 3(c) shows the typical PL and PLE spectra of Li6Gd(BO3)3:0.05Ce3+. The PLE spectrum is composed of a broad band from 275 nm to 400 nm with peaks at 305 and 350 nm, respectively, which match well with the NUV chips. Furthermore, the PL spectrum of Ce3+-doped Li6Gd(BO3)3 overlaps the PLE spectrum of Li6Gd(BO3)3:Tb3+, which favors the resonance-type ET from Ce3+ to Tb3+. The PL and PLE spectra of Li6Gd(BO3)3:0.05Ce3+, xTb3+ with x varying from 0 to 0.25 are shown in Fig. 4. At 350 nm excitation, the intensity of Ce3+ at 420 nm gradually decreases with increasing Tb3+ content. By contrast, the intensity of the major emission band of Tb3+ at 545 nm initially increases and then decreases when x > 0.2. Fig. 4. clearly illustrates the phenomenon and implies the existence of energy transfer between Ce3+ and Tb3+.
Iso/Is of Ce
684
6
4
2
0
activator is shorter than 5 Å A [21]. On the basis of Blasse, the critical distance can be estimated using the following equation [25]:
3V 4p X c Z
80
160
240
320
400
CCe+Tb 8/3 (10 4)
1=3 ð1Þ 8
4000
Ce3+ Tb3+
Iso/Is of Ce3+
Rc ¼ 2
0
3000 2000
(d) R2 = 0.98654
6
4
1000 0
2 0.00 0.05 0.10 0.15 0.20 0.25
Tb3+or Ce3+ content (x)
0
800
1200
1600
2000
x)
CCe+Tb10/3(105) Fig. 5. Dependence of Iso/Is of Ce3+on (b), C8/3(Ce + Tb) (c), and C10/3(Ce + Tb) (d).
C(Ce + Tb)
(a)
and
C6/3(Ce + Tb)
300
400
500
600
700
Tb
3+
co
nt en
t(
0 0.01 0.03 0.05 0.09 0.15 0.20 0.25
400
Wavelength(nm) Fig. 4. PLE and PL spectra of Li6Gd(BO3)3:0.05Ce3+, xTb3+ (x = 0, 0.03, 0.05, 0.09, 0.15, 0.20, 0.25).
where Z stands for the number of cation sites in the unit cell, Rc refers to the critical distance, and V is the volume of the unit cell. Therefore, the Rc of Li6Gd(BO3)3:Ce3+, Tb3+ is determined to be 17.59 Å, excluding the possibility of exchange interaction. Thus,
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P. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 682–686
16000
Li6Gd(BO3)3:0.05Ce3+, xTb3+
Intensity (a.u.)
12000
x=0 x = 0.01 x = 0.05 x = 0.09 x = 0.15 x = 0.20 x = 0.25
8000
4000
τ *(ns) 116.6 108.9 108.3 103.0 96.7 87.0 73.6
0 0
200
400
600
800
1000
Time (ns) Fig. 7. The CIE chromaticity Li6Gd(BO3)3:0.05Ce3+, xTb3+.
coordinates
and
selected
photographs
of
Fig. 6. The schematic diagram for ET process of Ce3+ to Tb3+.
the electric multipolar interaction should be considered for the ET mechanism. In accordance with Dexter’s ET formula of multipolar interaction and Reisfeld’s approximation, the multipolar interaction can be discussed by the following relationship [26,27]:
gso / C n=3 gs
ð2Þ
where gso and gs are the luminescence quantum efficiencies of Ce3+ in the absence and presence of Tb3+, respectively, and C represents the content of sensitizer and activator. The plots of versus Cn/3 with n = 6, 8, 10 correspond to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions. The value of g is approximately calculated on the basis of luminescent intensity:
Iso / C n=3 Is
ð3Þ
Fig. 5. shows the dependence of Iso/Is on Cn/3. The linear relationships perform the best when n is 6. This finding indicates that the dipole–dipole interaction is responsible for the ET from Ce3+ to Tb3+. Fig. 6 illustrates the schematic for the ET of Ce3+ to Tb3+. ET can effectively occur between Ce3+ and Tb3+ because of the small band gap between the lowest excited state of Ce3+ and the 5D3 energy level of Tb3+. Electrons of Ce3+ transfer to the excited state under NUV. Meanwhile, parts of the excess energy shift to the excited state of Tb3+ through cross relaxation, and other parts of the energy return to the ground state of Ce3+ by emitting blue light. Simultaneously, the level of 5D4 reverts to the ground state of Tb3+, accompanying green light. To deeply understand the ET, the decay lifetimes of Li6Gd(BO3)3:0.05Ce3+, xTb3+ phosphors at 350 nm excitation and 420 nm irradiation are performed as shown in Fig. 7. The decay time curves can be fitted with the second-order exponential decay mode by using the following formula [28]:
IðtÞ ¼ A1 expðt=s1 Þ þ A2 expðt=s2 Þ
ð4Þ
where I is the luminescence intensities, t is for the time, s1 and s2 are short and long lifetimes for exponential components, respectively; and A1 and A2 are constants. Ce3+ ions occupying two
Fig. 8. Decay curves for the Li6Gd(BO3)3:0.05Ce3+, xTb3+ samples (excited at 350 nm, monitored at 420 nm).
positions of Gd3+ ions are supported by the fitting results, which are consistent with the Gaussian deconvolution. The average decay times were calculated by the following equation:
R1
s ¼ R01 0
tIðtÞdt tIðtÞdt
ð5Þ
The average decay times (s⁄) are 116.6, 108.9, 108.3, 103.0, 96.7, 87.0, and 73.6 ns for Li6Gd(BO3)3:0.05Ce3+, xTb3+ when x = 0, 0.01, 0.05, 0.09, 0.15, 0.20, 0.25, respectively. The decay time is shortened with increasing Tb3+ content because of the ET from Ce3+ to Tb3+. The ET between the donor (Ce3+) and the acceptor (Tb3+) is assigned to multipolar interaction in this system. The energy transfer probability via multipolar interaction can be described by the following equation [29]:
PðRÞ /
QA Rb sD
Z
f D ðEÞF A ðEÞ dE Ec
ð6Þ
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P. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 682–686
Table 1 CIE chromaticity coordinates (x, y) of Li6Gd(BO3)3:0.05Ce3+, xTb3+ (x = 0, 0.03, 0.05, 0.09, 0.15, 0.20, 0.25). Sample
CIE x
CIE y
Li6Gd(BO3)3:0.05Ce3+ Li6Gd(BO3)3:0.05Ce3+, Li6Gd(BO3)3:0.05Ce3+, Li6Gd(BO3)3:0.05Ce3+, Li6Gd(BO3)3:0.05Ce3+, Li6Gd(BO3)3:0.05Ce3+, Li6Gd(BO3)3:0.05Ce3+, Li6Gd(BO3)3:0.05Ce3+,
0.158 0.166 0.180 0.189 0.208 0.226 0.237 0.250
0.037 0.072 0.155 0.203 0.302 0.389 0.441 0.506
0.01Tb3+ 0.03Tb3+ 0.05Tb3+ 0.09Tb3+ 0.15Tb3+ 0.20Tb3+ 0.25Tb3+
where P is the energy transfer probability, sD is the decay time of donor emission, QA is the total absorption cross-section of the acceptor, R is the distance between the donor and the acceptor, and b and c are the parameters that depend on ET type. P is inversely proportional to decay time sD. This relation is validated by the decay curves. CIE of:0.05Ce3+, xTb3+ Fig. 8 depicts the CIE chromaticity coordinates of Li6Gd(BO3)3:0.05Ce3+, xTb3+, where x varies from 0 to 0.25, and the chromaticity coordinates are recorded in Table 1. Selected photographs of Li6Gd(BO3)3:0.05Ce3+, xTb3+ (0, 0.05, 0.15, 0.25) and Li6Gd(BO3)3:0.25Tb3+ are illustrated in the upper and bottom insets of Fig. 8, respectively. As the Tb3+ content increases, the CIE chromaticity coordinates regularly shift from blue tone to green tone. Therefore, Ce3+, Tb3+-codoped Li6Gd(BO3)3 phosphors have potential applications in NUV-based LEDs.
Conclusion A series of Ce3+, Tb3+-codoped Li6Gd(BO3)3 phosphors was prepared and investigated for the first time. The as-prepared phosphors exhibit a broad intense band ranging from 275 nm to 400 nm, matching well with NUV LED chips. The color of the phosphors gradually changes from blue to green by tuning the ratio of Ce3+ to Tb3+. The ET is demonstrated by the PL spectrum and decay time curves. The dipole–dipole interaction mechanism should mainly result from the ET of Ce3+ to Tb3+. Furthermore, the CIE is
utilized to depict the color tone alteration of the corresponding phosphors. Acknowledgements This work was supported by grants from the National Natural Science Foundation of China (No. 61264003), Innovation Project of Guangxi Graduate Education (No. YCBZ2014010). References [1] M. Shang, C. Li, J. Lin, Chem. Soc. Rev. 43 (2014) 1372–1386. [2] L. Han, L. Zhao, J. Zhang, Y. Wang, L. Guo, Y. Wang, RSC Adv. 3 (2013) (1831) 21824. [3] E. Schubert, J. Kim, Science 308 (2005) 1274–1278. [4] L. Yi, X. He, L. Zhou, F. Gong, R. Wang, J. Sun, J. Lumin. 130 (2010) 1113–1117. [5] X. Yan, W. Li, K. Sun, J. Alloy. Compd. 508 (2010) 475–479. [6] D. Geng, G. Li, M. Shang, D. Yang, Y. Zhang, Z. Cheng, J. Lin, J. Mater. Chem. 22 (2012) 14262–14271. [7] Z. Yang, Y. Hu, L. Chen, X. Wang, G. Ju, Mater. Sci. Eng., B 193 (2015) 27–31. [8] Y. Tosaka, S. Adachi, J. Lumin. 156 (2014) 157–163. [9] J. Zhou, Z. Xia, J. Mater. Chem. C 2 (2014) 6978–6984. [10] X. Zhang, Z. Wang, J. Shi, M. Gong, Mater. Res. Bull. 57 (2014) 1–5. [11] X. Zhang, L. Zhou, Q. Pang, J. Shi, M. Gong, J. Phys. Chem. C 118 (2014) 7591– 7598. [12] X. Zhang, J. Song, C. Zhou, L. Zhou, M. Gong, J. Lumin. 149 (2014) 69–74. [13] U. Fawad, O.H. Myeongjin, H. Park, S. Kim, H.J. Kim, J. Alloy. Compd. 610 (2014) 281–287. [14] F. Yuan, L. Zhang, Y. Huang, S. Suna, Z. Lin, G. Wang, Mater. Sci. Eng. B 187 (2014) 32–38. [15] J.P. Chaminade, O. Viraphong, F. Guillen, C. Fouassier, B. Czirr, IEEE Trans. Nucl. Sci. 48 (2001) 1158–1161. [16] J.F. Rivas-Silva, A. Flores-Riveros, M. Berrondo, Int. J. Quantum Chem. 94 (2003) 105–112. [17] J. Chen, W. Zhao, J. Zhong, L. Lan, J. Wang, N. Wang, Ceram. Int. 40 (2014) 15241–15248. [18] Z. Yang, P. Liu, L. Lv, Y. Zhao, Q. Yu, X. Liang, J. Alloy. Compd. 562 (2013) 176– 181. [19] D. Geng, G. Li, M. Shang, C. Peng, Y. Zhang, Z. Cheng, J. Lin, Dalton Trans. 41 (2012) 3078–3086. [20] X. Liu, L. Yan, J. Zou, J. Electrochem. Soc. 157 (2010) P1–P6. [21] G. Blasse, B.C. Grabmaier, Lumin. Mater., Springer-Verlag, Berlin and Heideberg, 1994 [ch. 4-5]. [22] K.S. Sohn, Y.Y. Choi, H.D. Park, Y.G. Choi, J. Electrochem. Soc. 147 (2000) P2375–P2379. [23] V. Naresh, S. Buddhudu, J. Lumin. 137 (2013) 15–21. [24] J. Zheng, C. Guoa, X. Ding, Z. Ren, J. Bai, Curr. Appl. Phys. 12 (2012) 643–647. [25] G. Blasse, J. Solid State Chem. 62 (1986) 207–211. [26] D.L. Dexter, J. Chem. Phys. 22 (1954) 1063–1070. [27] R. Reisfeld, E. Greenberg, R. Velapoldi, B. Barnett, J. Chem. Phys. 56 (1972) 1698–1705. [28] Z. Wang, P. Li, Q. Guo, Z. Yang, Spectrochim. Acta. A. 137 (2015) 871–876. [29] D.L. Dexter, J. Chem. Phys. 21 (1953) 836–850.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Luminescence and energy transfer of color-tunable Li6Gd(BO3)3:Ce3+, Tb3+ phosphor Peican Chen, Fuwang Mo, Siyu Xia, Guofang Wang, Anxiang Guan, Liya Zhou ⇑ School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
3+ 3+ were synthesized for the first time. The phosphors exhibited the tunable-color with adjusting the ratio of Ce3+ to Tb3+. 3+ 3+ With tuning the ratio of Ce to Tb , the color change from blue to green.
The Li6Gd(BO3)3:Ce , Tb
a r t i c l e
i n f o
Article history: Received 14 November 2014 Received in revised form 11 March 2015 Accepted 29 April 2015 Available online 11 May 2015 Keywords: Photoluminescence Phosphor Li6Gd(BO3)3
a b s t r a c t A series of novel color-tunable phosphors of Ce3+, Tb3+-codoped Li6Gd(BO3)3 was synthesized through a classic solid-state reaction. The color of these phosphors changes from blue to green by adjusting the ratio of Ce3+ to Tb3+. The photoluminescence properties of the synthesized phosphors were investigated, and several major emission bands that belong to Ce3+ and Tb3+ ions were irradiated with near ultraviolet light. Moreover, the energy transfer mechanism between Ce3+ and Tb3+ in Li6Gd(BO3)3 was explored. The photoluminescence decay curves were performed to validate the energy transfer. The analysis demonstrated that the energy transfer from Ce3+ to Tb3+ arose from dipole–dipole interaction with a critical distance of approximately 17.6 Å. Ó 2015 Elsevier B.V. All rights reserved.
Introduction Material, energy, and information have modernized the trend of state-of-the-art technology. Luminescent materials demonstrate applications in electronic products, laser, solar cell, and advanced lightings. These materials are expected to have irreplaceable functions in the foreseeable future. In general, luminescence materials can be classified into three groups: host luminophores, ‘‘host + activator’’ type, and ‘‘host + activator + sensitizer’’ type [1]. Lanthanide is usually used as a sensitizer or activator in luminescent materials because the 4f–4f transitions of Ln3+ ions confer ⇑ Corresponding author. Tel./fax: +86 771 3233718. E-mail address: [email protected] (L. Zhou). http://dx.doi.org/10.1016/j.saa.2015.04.108 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
unique characteristics, such as high color purity, large Stoke shift, and high luminescent quantum efficiency; these characteristics are highly significant in lighting and display applications [2]. Nevertheless, light-emitting diodes (LEDs) still exhibit deficiencies in because the traditional fabrication of yellow phosphor Y3Al5O12 with blue GaInN chip involves low color rendering index, lack of red component, and reabsorption [3–5]. To address these problems, single-phase color-tunable phosphors are critically needed. Various methods can be employed to yield color-tunable phosphors, such as Ce3+–Tb3+ [6–8], Eu3+–Tb3+ [9], Eu2+–Mn2+ [10], and Eu3+–Tb3+–Eu3+ [11]. Borate serves as an excellent host for luminescent materials because of its low synthesis temperature, various structures, and good chemical and physical temperature stability [12]. Numerous
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P. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 682–686
works focused on borate phosphors, such as Li6Lu(BO3)3:Tb3+, Dy3+ [13] and Sr6YSc(BO3)6:Yb3+ [14]. However, insufficient information is available on Ce3+, Tb3+-codoped Li6Gd(BO3)3. In this study, color-tunable Ce3+, Tb3+ codoped Li6Gd(BO3)3 phosphors were synthesized through a conventional solid-state reaction. The photoluminescence (PL) properties of the phosphors were discussed in detail, and the energy transfer (ET) mechanism for Li6Gd(BO3)3:Ce3+, Tb3+ was investigated. Experimental Sample preparation A series of Li6Gd(BO3)3:0.05Ce3+, xTb3+ (0.05, x, mole concentration) samples was prepared via a traditional solid-state reaction. The initial materials of Li2CO3 (A.R.), Gd2O3 (99.99%), CeO2 (99.99%), and Tb4O7 (99.99%) were weighed in accordance with the stoichiometric proportion, mixed, and then ground thoroughly. As an exception, H3BO3 (A.R.) had an excess of 5%. The samples were transferred to aluminum crucibles and then preheated at 450 °C for 2 h in air. After being ground, the powder mixtures were sintered at 700 °C under the reducing atmosphere of H2 (30%) and N2 (70%) for 5 h. Finally, white powder products were obtained. Sample characterization Power X-ray diffraction data were recorded on a Rigaku D/max-IIIA diffractometer at a scanning rate of 10 min1 in the 2h range of 10° to 70° with Cu Ka radiation. The photoluminescence excitation (PLE) and PL were studied with a Hitachi-2500 fluorescence spectrometer at 150 W excitation. Moreover, the luminescence decay curves were measured with a Horiba Jobin Yvon FL3-P-TCSPC lifetime fluorescence spectrofluorometer. All of the measurements above were conducted at room temperature.
introducing Ce3+ or Tb3+ does not affect the phase formation of Li6Gd(BO3)3. In consideration of ion radius and valence, Ce3+ or Tb3+ ions preferably incorporate Gd3+ instead of Li+ ions.
Luminescent properties Ce3+, which has a 4f1 ground state and a 5d1 excited state, can act as a sensitizer or activator. Both of the 4f–5d absorption transition and the emission are strong broad bands because the 5d electron orbital is strongly affected by the neighboring ions. Therefore, the luminescent color or wavelength of Ce3+ ions widely changes from the near ultraviolet (NUV) to the red region [17]. As shown in Fig. 2, the emission band of Li6Gd(BO3)3:0.05Ce3+ ranges from 370 nm to 500 nm centered at 420 nm at the excitation of 350 nm. The emission spectrum of Li6Gd(BO3)3:Ce3+ can be decomposed into two Gaussian profiles with peaks at 418 nm (23,946 cm1) and 443 nm (22,573 cm1) through Gaussian deconvolution. The Ce3+ emission band, which generally originates from the 5d to 2F5/2 and 2F7/2 transitions of Ce3+ with a theoretical energy difference of 2000 cm1, consists of double bands [18]. However, the energy difference between 418 and 443 nm is 1373 cm1, which is much smaller than the theoretical value of 2000 cm1. Considering the four positions of Gd3+ ions, we can deduce that Ce3+ may build into two of the four positions of Gd3+. Fig. 3(a) shows the PL and PLE spectra of Li6Gd(BO3)3:0.05Ce3+, 0.25Tb3+. At 350 nm excitation, the PL spectrum exhibits the emission bands of Ce3+ and Tb3+. In addition, the intensity of the major characteristic peak of Tb3+ at 547 nm is evidently enhanced compared with that of Fig. 3(b). This finding indicates the ET from Ce3+ to Tb3+. Furthermore, the PLE spectrum of Ce3+ exists at a
5000
Structure and phase purity Li6Gd(BO3)3 crystallizes in the monoclinic system with the space group P21/c with the following parameters: a = 7.244(3) Å, b = 16.510(5) Å, c = 6.694(3) Å, b = 105.37(5), and Z = 4 [15]. Gd3+ ions exist in four positions, whereas they possess the same coordination environment with eight oxygen ions [16]. Fig. 1 illustrates the XRD patterns of Li6Gd(BO3)3, Li6Gd(BO3)3:0.25Tb3+, and Li6Gd(BO3)3:0.05Ce3+, 0.25Tb3+. In comparison with the standard JCPDS card (No. 54-1119), diffraction peaks are consistent, and no impurity peaks are discernable. This result reflects that
3+
Intensity (a.u.)
Li6 Gd(BO)3:0.05Ce ,0.25Tb
Intensity(a.u.)
4000
Results and discussion
3000 2000 1000 0 400
450
500
3+
10
20
30
40
50
Intensity(a.u.) 60
(b)
Li6Gd(BO3)3:0.25Tb3+
700
λem=356 nm λex=547 nm λem=545 nm λex=350 nm
Li6Gd(BO3)3:0.05Ce3+
(c)
JCPDS 54-1119Li 6 Gd(BO3 )3
650
Li6Gd(BO3)3 :0.05Ce3+, 0.25Tb3+ λem=420 nm λex=350 nm
(a)
Li6Gd(BO)3
600
Fig. 2. The Gaussian peaks fitting of Ce3+ emission excited at 350 nm.
3+
Li6Gd(BO)3:0.25Tb
550
Wavelength(nm)
70
2-Theta (degree)
300
350
400
450
500
550
600
650
700
Wavelength(nm) Fig. 1. XRD patterns of Li6Gd(BO3)3, Li6Gd(BO3)3:0.25Tb3+, and Li6Gd(BO3)3:0.05Ce3+, 0.25Tb3+ phosphors. The standard data for Li6Gd(BO3)3 (JCPDS card No. 54-1119) is shown as a reference.
Fig. 3. PLE and PL spectra of Li6Gd(BO3)3:0.05Ce3+ (a), Li6Gd(BO3)3:0.25Tb3+ (b), and Li6Gd(BO3)3:0.05Ce3+, 0.25Tb3+ (c)phosphors.
P. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 682–686
8
(a) R2 = 0.97792
Iso/Is of Ce3+
6
4
2
0 0.08
8
In general, the resonant energy transfer mechanism results from two aspects, namely, exchange interaction and electric multipolar interaction [24]. The exchange interaction predominates the energy transfer when the distance between the sensitizer and the
0.24
0.32
(b) R2 = 0.9986
3+
6
4
2
0 0
20
40
60
80
100
CCe+Tb 6/3 (10 3) 8
Iso/Is of Ce
Energy transfer
0.16
CCe+Tb
(c) R2 = 0.99545
3+
maximum intensity irradiation of Tb3+ at 547 nm, which also confirms the occurrence of ET from Ce3+ to Tb3+. The information of Tb3+, which possesses the ground state of 7Fj (j = 6, . . . ,0) and the excited states 5D3 and 5D4, is difficult to be derived from the spectra because of the high degeneracy of the Tb3+ levels involved in several transitions [19]. When the Tb3+ concentration in the host is low, the 5D3–7Fj transition dominates, thus yielding the blue emission [20]. As the Tb3+ content increases, green emission is produced, which can cause cross relaxation and enhance the 5D4–7Fj transition [21,22]. Fig. 3(b) depicts the PL and PLE spectra of Li6Gd(BO3)3:0.25Tb3+. When pumped at 356 nm, the PL spectrum is observed with a prominent peak at 547 nm (5D4–7F5) and two weak peaks at 491 nm (5D4–7F6) and 585 nm (5D4–7F4). In agreement with the magnetic dipole transition selection rule DJ = ± 1, Laporte’s forbidden transition 5D4–7F5 (547 nm) shows bright green luminescence compared with other emission transitions [23]. The PLE spectrum, which represents a series of spectra bands, is ascribed to the 4f–4f transition of Tb3+. Fig. 3(c) shows the typical PL and PLE spectra of Li6Gd(BO3)3:0.05Ce3+. The PLE spectrum is composed of a broad band from 275 nm to 400 nm with peaks at 305 and 350 nm, respectively, which match well with the NUV chips. Furthermore, the PL spectrum of Ce3+-doped Li6Gd(BO3)3 overlaps the PLE spectrum of Li6Gd(BO3)3:Tb3+, which favors the resonance-type ET from Ce3+ to Tb3+. The PL and PLE spectra of Li6Gd(BO3)3:0.05Ce3+, xTb3+ with x varying from 0 to 0.25 are shown in Fig. 4. At 350 nm excitation, the intensity of Ce3+ at 420 nm gradually decreases with increasing Tb3+ content. By contrast, the intensity of the major emission band of Tb3+ at 545 nm initially increases and then decreases when x > 0.2. Fig. 4. clearly illustrates the phenomenon and implies the existence of energy transfer between Ce3+ and Tb3+.
Iso/Is of Ce
684
6
4
2
0
activator is shorter than 5 Å A [21]. On the basis of Blasse, the critical distance can be estimated using the following equation [25]:
3V 4p X c Z
80
160
240
320
400
CCe+Tb 8/3 (10 4)
1=3 ð1Þ 8
4000
Ce3+ Tb3+
Iso/Is of Ce3+
Rc ¼ 2
0
3000 2000
(d) R2 = 0.98654
6
4
1000 0
2 0.00 0.05 0.10 0.15 0.20 0.25
Tb3+or Ce3+ content (x)
0
800
1200
1600
2000
x)
CCe+Tb10/3(105) Fig. 5. Dependence of Iso/Is of Ce3+on (b), C8/3(Ce + Tb) (c), and C10/3(Ce + Tb) (d).
C(Ce + Tb)
(a)
and
C6/3(Ce + Tb)
300
400
500
600
700
Tb
3+
co
nt en
t(
0 0.01 0.03 0.05 0.09 0.15 0.20 0.25
400
Wavelength(nm) Fig. 4. PLE and PL spectra of Li6Gd(BO3)3:0.05Ce3+, xTb3+ (x = 0, 0.03, 0.05, 0.09, 0.15, 0.20, 0.25).
where Z stands for the number of cation sites in the unit cell, Rc refers to the critical distance, and V is the volume of the unit cell. Therefore, the Rc of Li6Gd(BO3)3:Ce3+, Tb3+ is determined to be 17.59 Å, excluding the possibility of exchange interaction. Thus,
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P. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 682–686
16000
Li6Gd(BO3)3:0.05Ce3+, xTb3+
Intensity (a.u.)
12000
x=0 x = 0.01 x = 0.05 x = 0.09 x = 0.15 x = 0.20 x = 0.25
8000
4000
τ *(ns) 116.6 108.9 108.3 103.0 96.7 87.0 73.6
0 0
200
400
600
800
1000
Time (ns) Fig. 7. The CIE chromaticity Li6Gd(BO3)3:0.05Ce3+, xTb3+.
coordinates
and
selected
photographs
of
Fig. 6. The schematic diagram for ET process of Ce3+ to Tb3+.
the electric multipolar interaction should be considered for the ET mechanism. In accordance with Dexter’s ET formula of multipolar interaction and Reisfeld’s approximation, the multipolar interaction can be discussed by the following relationship [26,27]:
gso / C n=3 gs
ð2Þ
where gso and gs are the luminescence quantum efficiencies of Ce3+ in the absence and presence of Tb3+, respectively, and C represents the content of sensitizer and activator. The plots of versus Cn/3 with n = 6, 8, 10 correspond to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions. The value of g is approximately calculated on the basis of luminescent intensity:
Iso / C n=3 Is
ð3Þ
Fig. 5. shows the dependence of Iso/Is on Cn/3. The linear relationships perform the best when n is 6. This finding indicates that the dipole–dipole interaction is responsible for the ET from Ce3+ to Tb3+. Fig. 6 illustrates the schematic for the ET of Ce3+ to Tb3+. ET can effectively occur between Ce3+ and Tb3+ because of the small band gap between the lowest excited state of Ce3+ and the 5D3 energy level of Tb3+. Electrons of Ce3+ transfer to the excited state under NUV. Meanwhile, parts of the excess energy shift to the excited state of Tb3+ through cross relaxation, and other parts of the energy return to the ground state of Ce3+ by emitting blue light. Simultaneously, the level of 5D4 reverts to the ground state of Tb3+, accompanying green light. To deeply understand the ET, the decay lifetimes of Li6Gd(BO3)3:0.05Ce3+, xTb3+ phosphors at 350 nm excitation and 420 nm irradiation are performed as shown in Fig. 7. The decay time curves can be fitted with the second-order exponential decay mode by using the following formula [28]:
IðtÞ ¼ A1 expðt=s1 Þ þ A2 expðt=s2 Þ
ð4Þ
where I is the luminescence intensities, t is for the time, s1 and s2 are short and long lifetimes for exponential components, respectively; and A1 and A2 are constants. Ce3+ ions occupying two
Fig. 8. Decay curves for the Li6Gd(BO3)3:0.05Ce3+, xTb3+ samples (excited at 350 nm, monitored at 420 nm).
positions of Gd3+ ions are supported by the fitting results, which are consistent with the Gaussian deconvolution. The average decay times were calculated by the following equation:
R1
s ¼ R01 0
tIðtÞdt tIðtÞdt
ð5Þ
The average decay times (s⁄) are 116.6, 108.9, 108.3, 103.0, 96.7, 87.0, and 73.6 ns for Li6Gd(BO3)3:0.05Ce3+, xTb3+ when x = 0, 0.01, 0.05, 0.09, 0.15, 0.20, 0.25, respectively. The decay time is shortened with increasing Tb3+ content because of the ET from Ce3+ to Tb3+. The ET between the donor (Ce3+) and the acceptor (Tb3+) is assigned to multipolar interaction in this system. The energy transfer probability via multipolar interaction can be described by the following equation [29]:
PðRÞ /
QA Rb sD
Z
f D ðEÞF A ðEÞ dE Ec
ð6Þ
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P. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 682–686
Table 1 CIE chromaticity coordinates (x, y) of Li6Gd(BO3)3:0.05Ce3+, xTb3+ (x = 0, 0.03, 0.05, 0.09, 0.15, 0.20, 0.25). Sample
CIE x
CIE y
Li6Gd(BO3)3:0.05Ce3+ Li6Gd(BO3)3:0.05Ce3+, Li6Gd(BO3)3:0.05Ce3+, Li6Gd(BO3)3:0.05Ce3+, Li6Gd(BO3)3:0.05Ce3+, Li6Gd(BO3)3:0.05Ce3+, Li6Gd(BO3)3:0.05Ce3+, Li6Gd(BO3)3:0.05Ce3+,
0.158 0.166 0.180 0.189 0.208 0.226 0.237 0.250
0.037 0.072 0.155 0.203 0.302 0.389 0.441 0.506
0.01Tb3+ 0.03Tb3+ 0.05Tb3+ 0.09Tb3+ 0.15Tb3+ 0.20Tb3+ 0.25Tb3+
where P is the energy transfer probability, sD is the decay time of donor emission, QA is the total absorption cross-section of the acceptor, R is the distance between the donor and the acceptor, and b and c are the parameters that depend on ET type. P is inversely proportional to decay time sD. This relation is validated by the decay curves. CIE of:0.05Ce3+, xTb3+ Fig. 8 depicts the CIE chromaticity coordinates of Li6Gd(BO3)3:0.05Ce3+, xTb3+, where x varies from 0 to 0.25, and the chromaticity coordinates are recorded in Table 1. Selected photographs of Li6Gd(BO3)3:0.05Ce3+, xTb3+ (0, 0.05, 0.15, 0.25) and Li6Gd(BO3)3:0.25Tb3+ are illustrated in the upper and bottom insets of Fig. 8, respectively. As the Tb3+ content increases, the CIE chromaticity coordinates regularly shift from blue tone to green tone. Therefore, Ce3+, Tb3+-codoped Li6Gd(BO3)3 phosphors have potential applications in NUV-based LEDs.
Conclusion A series of Ce3+, Tb3+-codoped Li6Gd(BO3)3 phosphors was prepared and investigated for the first time. The as-prepared phosphors exhibit a broad intense band ranging from 275 nm to 400 nm, matching well with NUV LED chips. The color of the phosphors gradually changes from blue to green by tuning the ratio of Ce3+ to Tb3+. The ET is demonstrated by the PL spectrum and decay time curves. The dipole–dipole interaction mechanism should mainly result from the ET of Ce3+ to Tb3+. Furthermore, the CIE is
utilized to depict the color tone alteration of the corresponding phosphors. Acknowledgements This work was supported by grants from the National Natural Science Foundation of China (No. 61264003), Innovation Project of Guangxi Graduate Education (No. YCBZ2014010). References [1] M. Shang, C. Li, J. Lin, Chem. Soc. Rev. 43 (2014) 1372–1386. [2] L. Han, L. Zhao, J. Zhang, Y. Wang, L. Guo, Y. Wang, RSC Adv. 3 (2013) (1831) 21824. [3] E. Schubert, J. Kim, Science 308 (2005) 1274–1278. [4] L. Yi, X. He, L. Zhou, F. Gong, R. Wang, J. Sun, J. Lumin. 130 (2010) 1113–1117. [5] X. Yan, W. Li, K. Sun, J. Alloy. Compd. 508 (2010) 475–479. [6] D. Geng, G. Li, M. Shang, D. Yang, Y. Zhang, Z. Cheng, J. Lin, J. Mater. Chem. 22 (2012) 14262–14271. [7] Z. Yang, Y. Hu, L. Chen, X. Wang, G. Ju, Mater. Sci. Eng., B 193 (2015) 27–31. [8] Y. Tosaka, S. Adachi, J. Lumin. 156 (2014) 157–163. [9] J. Zhou, Z. Xia, J. Mater. Chem. C 2 (2014) 6978–6984. [10] X. Zhang, Z. Wang, J. Shi, M. Gong, Mater. Res. Bull. 57 (2014) 1–5. [11] X. Zhang, L. Zhou, Q. Pang, J. Shi, M. Gong, J. Phys. Chem. C 118 (2014) 7591– 7598. [12] X. Zhang, J. Song, C. Zhou, L. Zhou, M. Gong, J. Lumin. 149 (2014) 69–74. [13] U. Fawad, O.H. Myeongjin, H. Park, S. Kim, H.J. Kim, J. Alloy. Compd. 610 (2014) 281–287. [14] F. Yuan, L. Zhang, Y. Huang, S. Suna, Z. Lin, G. Wang, Mater. Sci. Eng. B 187 (2014) 32–38. [15] J.P. Chaminade, O. Viraphong, F. Guillen, C. Fouassier, B. Czirr, IEEE Trans. Nucl. Sci. 48 (2001) 1158–1161. [16] J.F. Rivas-Silva, A. Flores-Riveros, M. Berrondo, Int. J. Quantum Chem. 94 (2003) 105–112. [17] J. Chen, W. Zhao, J. Zhong, L. Lan, J. Wang, N. Wang, Ceram. Int. 40 (2014) 15241–15248. [18] Z. Yang, P. Liu, L. Lv, Y. Zhao, Q. Yu, X. Liang, J. Alloy. Compd. 562 (2013) 176– 181. [19] D. Geng, G. Li, M. Shang, C. Peng, Y. Zhang, Z. Cheng, J. Lin, Dalton Trans. 41 (2012) 3078–3086. [20] X. Liu, L. Yan, J. Zou, J. Electrochem. Soc. 157 (2010) P1–P6. [21] G. Blasse, B.C. Grabmaier, Lumin. Mater., Springer-Verlag, Berlin and Heideberg, 1994 [ch. 4-5]. [22] K.S. Sohn, Y.Y. Choi, H.D. Park, Y.G. Choi, J. Electrochem. Soc. 147 (2000) P2375–P2379. [23] V. Naresh, S. Buddhudu, J. Lumin. 137 (2013) 15–21. [24] J. Zheng, C. Guoa, X. Ding, Z. Ren, J. Bai, Curr. Appl. Phys. 12 (2012) 643–647. [25] G. Blasse, J. Solid State Chem. 62 (1986) 207–211. [26] D.L. Dexter, J. Chem. Phys. 22 (1954) 1063–1070. [27] R. Reisfeld, E. Greenberg, R. Velapoldi, B. Barnett, J. Chem. Phys. 56 (1972) 1698–1705. [28] Z. Wang, P. Li, Q. Guo, Z. Yang, Spectrochim. Acta. A. 137 (2015) 871–876. [29] D.L. Dexter, J. Chem. Phys. 21 (1953) 836–850.
