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The formation of KF induced red-emitting phosphors K2TiF6·BaF(HF2):Mn4+ by cation exchange
Journal of Luminescence 188 (2017) 307–312
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
The formation of KF induced red-emitting phosphors K2TiF6·BaF(HF2):Mn4+ by cation exchange
MARK
⁎
Tianman Wanga, Yong Gaoa, Zhipeng Chena,c, Qiuying Huanga, Lini Wua, Yingheng Huanga,b, , ⁎⁎ ⁎ Sen Liaoa, , Huaxin Zhanga, a Guangxi Colleges and Universities Key Laboratory of Applied Chemistry Technology and Resource Development, School of Chemistry and Chemical Engineering, Guangxi University, Nanning, Guangxi 530004, China b School of Materials Science and Engineering, Guangxi University, Nanning, Guangxi 530004, China c Fangchenggang Entry-Exit Inspection and Quarantine Bureau, Fangchenggang, Guangxi, 538001, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Photoluminescence Optical materials Fluorides Red phosphor Mn4+
A series of K2TiF6 •BaF(HF2):Mn4+ phosphors with narrowband red emissions have been successfully prepared on a large scale with the cation exchange method using K2MnF6 and BaTiF6 as staring materials at ambient temperature in HF-KF solution. It is interesting that KF induces the formation of luminous phase for the samples. That is that the corresponding samples will not emit light without the presence of KF. Mechanism of KF inducing formation of luminous phase is suggested. HF-controlled effects on the fluorescent properties of the samples are discussed. Decay lifetime and the photoluminescence quantum yields of the optimal sample are 4.76 ms and 60.75 ± 0.03%, respectively. The chromaticity coordinates of the optimal sample indicate that this phosphor emits deep red light (x=0.69, y=0.31), which can be used for blue light-based white LED.
1. Introduction Mn4+ doped fluorides have been extensively studied for their high photoluminescence quantum yields and broadband absorption in the blue region, as well as narrowband emission in the red region caused by their distinct d–d transitions [1–25], having a great potential in applications to lighting, holography, laser technology, and dosimetry. There are many reports on the Mn4+-doped luminescent fluorides, including cocrystallisation [18], chemical etching [19,20], cation exchange [12,13], chemical co-precipitation [21,22], and hydrothermal method [13–15]. Currently, a series of red-emitting Mn4+ doped fluotitanates have been synthesized using above methods, such as Na2TiF6:Mn4+ [8,19], K2TiF6:Mn4+ [9–12,19], Cs2TiF6:Mn4+[13,19], BaTiF6:Mn4+[14–16], and ZnTiF6·6H2O: Mn4+ [17]. The cation exchange method is a simple and efficient way to prepare Mn4+ doped fluotitanate phosphors. Interestingly, cations of fluotitanates show monovalent and bivalent. Meanwhile, BaTiF6:Mn4+ and ZnTiF6·6H2O:Mn4+ are reported to be novel red phosphors. However, to the best of our knowledge, so far very few reports have been published on the synthesis of BaTiF6:Mn4+ and ZnTiF6·6H2O:Mn4+ phosphors by using the cation exchange method. Therefore, more
⁎
attention should focus on the synthesis of BaTiF6:Mn4+, ZnTiF6·6H2O:Mn4+ phosphors, as well as their luminescent properties. The cation exchange method can save energy during the preparation of Mn4+ doped fluotitanates since it is mild, efficient, and energy conservation [7,8]. So, in the present work, BaTiF6:Mn4+ phosphors were synthesized by the cation exchange method. Besides, the structures of the phosphors were investigated by powder X-ray diffraction (XRD) measurement. The photoluminescence (PL) properties of Mn4+ ions were investigated by excitation and emission (PLE & PL) spectra. KF controlled effects on the luminescence properties of red phosphors BaTiF6:Mn4+ were reported in this work. 2. Materials and methods All chemicals were reagent-grade pure and purchased from the Sinopharm Chemical Reagent Co. Ltd., China. XRD was performed at a scanning rate of 5°/min from 5° to 70° for 2θ at room temperature by using a Rigaku D/max 2500 V diffractometer. It was equipped with a graphite monochromator by utilizing monochromatic CuKα radiation (λ=0.154178 nm). The morphologies of the samples were examined by Hitachi S-3400 scanning electron microscopy (SEM) with an attached
Corresponding authors. Corresponding author at: Guangxi Colleges and Universities Key Laboratory of Applied Chemistry Technology and Resource Development, School of Chemistry and Chemical Engineering, Guangxi University, Nanning, Guangxi 530004, China. E-mail addresses: [email protected], [email protected] (S. Liao). ⁎⁎
http://dx.doi.org/10.1016/j.jlumin.2017.04.049 Received 22 December 2016; Received in revised form 5 April 2017; Accepted 23 April 2017 Available online 26 April 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
Journal of Luminescence 188 (2017) 307–312
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energy-dispersive X-ray spectrometer (EDS). Samples were mounted on an aluminum slice coated with Au. PLE and PL spectra were recorded at room temperature by a Shimadzu RF-53001 spectrophotometer equipped with a xenon lamp as the excitation source. The luminescence decay curves and the photoluminescence quantum yields were obtained from an Edinburgh FLS980 fluorescence spectrophotometer. The metal elemental contents of the sample were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, Perkin-Elmer Optima 3100 RL). First, the sample of K2TiF6 •BaF(HF2):xMn4+ was dissolved in 10 ml of 1:1 aqueous HCl solution which contains several drops of H2O2 (30%), then the solution was diluted to 100.00 ml with distilled water. Third, the elemental contents in the solution were determined by ICP-AES. K2MnF6 was synthesized according to the method described in the literature [26]. Specifically, 1.580 g KMnO4 and 15 g KF·2H2O were completely dissolved in 30 ml HF (40%) solution. After completely dissolving of KF·2H2O in the HF solution, H2O2 (30%) solution was added drop by drop until the solution turned to yellow. The yellow powder of K2MnF6 sample was obtained by filtering, washing with acetone and drying at 80 °C for 2 h. A typical two-step synthesis method was used to synthesize K2TiF6 •BaF(HF2):xMn4+ red phosphor. (i) Pure BaTiF6 powder was synthesized as follows: 3.947 g (20.0 mmol) BaCO3 was added into 6.555 g (20.0 mmol) H2TiF6 (50%) solution slowly with stirring. The mixture was ground together in an agate mortar for 1 h. Then the mixture was kept at 80 oC for 3 h, and dried at 120 °C for 3 h to yield BaTiF6 (sample (a)). (ii) In a typical synthesis of Mn4+-doped sample, 10 ml HF (40%) solution, 6 g KF·2H2O and 0.297 g (1.2 mmol) K2MnF6 were added into a 50 ml plastic beaker and completely dissolved with stirring. Then 5.984 g (20.0 mmol) BaTiF6 (molar ratio of K2MnF6/ BaTiF6 (Mn/Ti), x=0.06) was put into the yellow transparent solution at room temperature. After 30 min of magnetic stirring, the precipitate was collected, washed with methanol several times and dried at 80 °C for 2 h to yield sample (b). For comparison, a series of red phosphor samples were synthesized with different molar ratios of K2MnF6/ BaTiF6 (x=0.01, 0.02, 0.04, 0.06, 0.08, 0.10, and 0.12).
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3. Results and discussion
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Fig. 1 shows the XRD patterns of the as-synthesized samples. In Fig. 1a, all the diffraction peaks of the sample (a) are indexed and in agreement with the pure hexagonal BaTiF6 phase standard data (PDF#76–0269). Fig. 1b shows that the sample (b) contains mixture phases of K2TiF6 (PDF#28–0825) and BaF(HF2) (PDF#76–1301), which means that Mn4+(CN =6, r =0.53 Å) ions are at the octahedral sites of Ti4+(CN =6, r =0.61 Å). Fig. 1c shows that 2θ angles of sample (b) are slightly shifted toward shorter angles, which indicates that the lattice volume of the sample slightly expends, and there is strong distortion in the K2TiF6 lattice. Because the radius of Mn4+(0.53 Å) is smaller than that of Ti4+(0.61 Å), the doping of Mn4+ can result in the contracting of lattice volume of the samples, which was also reported by Liao et al. [9]. However, there are some differences between the results reported here and by Liao et al. [9]. The K2TiF6 reported by Liao et al. is single phase, while it is mixture phases here. The mixture phases may lead to the expanding of the lattice volume of samples, due to the lattice interaction between mixture phases. Fig. 2 shows the SEM image and the corresponding EDS spectra of sample (b) (x=0.06, wKF =6 g). The sample contains kinds of shapes of crystals, such as blockbusters, small pieces and rod-like crystals (Fig. 2a). Sample (b) is composed of K, Ba, Ti, Mn and F elements (Fig. 2b). ICP-AES of sample (b) shows that the mass percentages of Ba, Ti, K, and Mn are 30.49%, 10.62%, 18.40%, and 0.74%, respectively. So, the molar ratio of Ba, Ti, K, and Mn in sample (b) is 1.00:1.00:2.00:0.06, corresponding with the molecular formula (K2TiF6)·(BaF(HF2))·(K2MnF6) 0.06. The molar ratio of Mn/Ti is 0.06, in agreement with the raw materials ( K2MnF6/BaTiF6 =0.06). So, the
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2θ /degree Fig. 1. XRD patterns of samples: (a) sample (a), (b) sample (b), (c) An expanded XRD pattern for sample (b).
results indicate that Mn4+ and Ti4+ can replace each other in the samples. Obviously, a series of K2TiF6 •BaF(HF2): Mn4+ phosphors can be prepared on a large scale with the method of using K2MnF6 and 308
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1.2
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PL: (i) λex= 364 nm
(a) PLE:λem= 634 nm
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Fig. 2. SEM image and the corresponding EDS spectrum of sample(b): (a) SEM image, (b) EDS spectrum.
Fig. 3. Properties of sample (b): (a) PLE and PL spectra, (b) photograph for sample (b) under excitation with blue light in deionized water.
BaTiF6 as starting materials at ambient temperature in HF-KF solution. Fig. 3 shows the PLE and PL spectra of the sample (b) (x=0.06, wKF =6 g). Two strong broad excitation peaks at 364 and 463 nm are found between 300 and 550 nm, which are due to the 4A2→4T1 and 4A2→4T2 transitions of Mn4+, respectively [27,28]. The intensity of latter is 0.91 time of the former. The full-width at half-maximum of these two bands are 54 and 73 nm, respectively. Five narrow emission peaks (~612 (ν4), ~617 (ν6), ~634 (ν6), ~638 (ν4) and ~650 nm (ν3)) activated by the vibrational modes of the [MnF6]2- octahedron [29–32] are also found in Fig. 3. Among these emission peaks, those centered at 617, 634 and 650 nm are from the spin forbidden d–d transition 2Eg→4A2 of Mn4+ [27,28], indicating that the Mn4+ ions are doped in the K2TiF6 matrix. The strong broad excitation bands at 364 and 463 nm and narrow emission band at 634 nm of the sample show that it has potential application in white light-emitting diodes excited by UV light or blue light. Interestingly, compared with BaTiF6:Mn4+ sample obtained from hydrothermal synthesis method [14,15], the PLE and PL peak patterns of sample (b) are different: (i) In PLE spectra, the peak intensity of 4 A2→4T2 (463 nm) is weaker than that of 4A2→4T1(364 nm) in sample (b), while it is opposite in the another one, (ii) In PL spectra, two strong peaks centered 617 and 634 nm are split peaks in sample (b), while there are not splitting in the another one. The results indicate that the PLE and PL properties of the sample can be influenced by the distortion of crystal field [16], which is caused by distortion of the lattice. Furthermore, it is noticeable that samples prepared by this method can
emit red luminescence under the excitation with blue light in deionized water (Fig. 3b). It indicates that luminescence of Mn4+ is not quenched by water, because Mn4+ locates at the center of octahedral [MnF6]2-, and there are no interactions between Mn4+ ions and water molecules. The molar ratios of K2MnF6/BaTiF6 play a key role in the luminescent properties of samples. The luminescent properties of samples with different molar ratios of K2MnF6/BaTiF6 (x=0.01–0.12, wKF =6 g) are shown in Fig. 4. First, the luminescence intensities of the samples increase with the increase of molar ratios of K2MnF6/BaTiF6, and reach a maximum value at x=0.06. Then they reduce due to the concentration quenching. As shown in Fig. 4, the shapes of the excitation and emission spectra are not strongly affected by the Mn4+ concentrations. Compared with BaTiF6:Mn4+ sample obtained from hydrothermal synthesis method [14,15], the optimal molar ratio of Mn/Ti is different, in which the former is 0.06, while the latter is 0.08. Recently, Lee [33] reported that KF can enhance luminous efficiency of deep red emitting K2SiF6:Mn4+. So, further experiments were done with different dosage of KF. Dependent curves of PLE and PL intensity on different dosage of KF are shown in Fig. 5. It shows that the dependences of PLE and PL intensities on different dosage of KF are in curves of quintic with the mouth downward. The luminescent intensities of the samples are increased with the increase of KF dosage and
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larger than 5 g, curve (ii) is lower than curve (i). That is, intensity ratio of two PLE peaks (364 and 463 nm) or two PL peaks (excited by 364 and 463 nm) can be easily tuned by the KF dosage. The results indicate that the samples have a double application prospect, and can be used for both near UV-based white LED and blue light-based white LED. Based on the measured PL spectra, the chromaticity coordinates of the sample (b) were calculated (Fig. 5c). The calculated results show that the chromaticity coordinates from two PL spectra excited by the two excitation wavelengths (364 and 463 nm) are almost the same and given by x=0.69, y=0.31. As shown in Fig. 5c, the sample (b) emits deep red light (x=0.69, y=0.31). The decay curve of the sample (b) is shown in Fig. 5d. It is well fitted by a single-exponential function with a constant term. The lifetime is 4.76 ms, which is in agreement with BaTiF6:Mn4+ sample obtained with hydrothermal synthesis method [14,15]. The photoluminescence quantum yield of the sample (b) is 60.75 ± 0.03%, which is much higher than that of ZnTiF6·6H2O:Mn4+ [17]. Some literatures reported that BaTiF6:Mn4+ could be obtained with hydrothermal synthesis method [14,15]. But, compared with BaTiF6:Mn4+ sample obtained from hydrothermal synthesis method, the product of method used in this paper is different, which no BaTiF6:Mn4+ is formed, only (K2TiF6)·(BaF(HF2))·(K2MnF6)x is obtained. The results indicate that different synthesis method will lead to formation of different products. Based on the results of Fig. 1b and Fig. 5, mechanism for formation of luminous phase induced by the addition of KF in the cation exchange synthesis method can be suggested by the follows reactions:
2KF + BaTiF6 + HF = BaF(HF2) + K2 TiF6
(1)
x K2MnF6 + K2 TiF6 = K2 TiF6⋅x K2MnF6
(2)
From the above reactions, it can be deduced that: (i) the reasons for obtaining luminous samples in the presence of KF are as follows: first, BaTiF6 reacts with KF and HF in KF/HF solution to form BaF(HF2) and K2TiF6; second, according to the theory of similar dissolve mutually, cation exchange reaction occurs between K2MnF6 and K2TiF6 in KF/HF solution, and a part of Ti4+ sites are replaced by Mn4+ ions to form K2TiF6·xK2MnF6. So, obtained samples contain mixture crystal phases of K2TiF6:Mn4+ and BaF(HF2) (please see Fig. 1), and can emit red light under excitation (please see Fig. 3). (ii) the reasons for the formation of no luminious samples without the presence of KF are as follows: the similarity is not enough between K2MnF6 and BaTiF6, and cation exchange reaction cannot occur between K2MnF6 and BaTiF6 in HF solution without the presence of KF at room temperature and, thus, no presence of BaTiF6:Mn4+. So, the obtained sample has no fluorescence. As previously mentioned, BaTiF6:Mn4+ could be obtained from hydrothermal synthesis method [14,15]. Hence, it is obvious that cation exchange reaction can occur between K2MnF6 and BaTiF6 under hydrothermal synthesis condition, but this reaction cannot happen in ambient condition without the presence of KF. So, the luminous phase in the present sample is only K2TiF6·xK2MnF6(i.e., K2TiF6:Mn4+).
Fig. 4. PLE and PL spectra of BaTiF6: Mn4+ with different x (x=0.01–0.12, wKF =6 g): (a) PLE, (b) PL excited by 364 nm, (c) PL excited by 463 nm.
4. Conclusions In summary, a series of K2TiF6•BaF(HF2):Mn4+ samples were prepared by the cation exchange method in HF-KF solution. XRD results confirmed that the samples contain mixture phases of crystals. It is interesting that KF has induced luminescence effect for the samples. That is that the synthesized samples without the addition of KF do not emit light. Mechanism of induced luminescence effect was suggested. The addition of KF on the structural and fluorescent properties of the samples were discussed. The results indicated that the optimal dosage of KF giving the strongest PL intensity is 6 g. The chromaticity coordinates of the optimal sample indicated that this phosphor emits
reach a maximum value at wKF =6 g, and decrease when wKF is larger than 6 g, due to quenching process. That is, the intensities of two PLE peaks (364 and 463 nm) or two PL peaks (excited by 364 and 463 nm) can be greatly enhanced by the addition of KF. It is found that KF plays a key role on the luminescence properties of the phosphors synthesized using this method. Without the presence of KF, the sample does not emit light. Furthermore, enhanced effects of the two excitation peaks (364 and 463 nm) have a bit different. As shown in Fig. 5a–b, when wKF is smaller than 5 g, curve (ii) is higher than curve (i), while when wKF is
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Fig. 5. Fluorescent properties of samples (x=0.06) at different KF weight: (a) Intensities of PLE spectra, (b) Intensities of PL spectra, (c) CIE chromaticity diagram value and (d) Emission decay curve at optimal condition (x=0.06, wKF =6 g, excited at 464 nm). (2012) 18204–18213. [7] A.A. Setlur, W.J. Heward, Y. Gao, A.M. Srivastava, R.G. Chandran, M.V. Shankar, Chem. Mater. 18 (2006) 3314–3322. [8] Z.L. Wang, Y. Liu, Y.Y. Zhou, Q. Zhou, H.Y. Tan, Q.H. Zhang, J.H. Peng, RSC Adv. 5 (2015) 58136–58140. [9] J.S. Liao, L.L. Nie, L.F. Zhong, Q.J. Gu, Q. Wang, Luminescence 31 (2016) 802–807. [10] C.C. Lin, A. Meijerink, R.S. Liu, J. Phys. Chem. Lett. 7 (2016) 495–503. [11] Z.L. Wang, Y.Y. Zhou, Z.Y. Yang, Y. Liu, H. Yang, H.Y. Tan, Q.H. Zhang, Q. Zhou, Opt. Mater. 49 (2015) 235–240. [12] H.M. Zhu, C.C. Lin, W.Q. Luo, S.T. Shu, Z.G. Liu, Y.S. Liu, J.T. Kong, E. Ma, Y.G. Cao, R.S. Liu, Nat. Commun. 5 (2014) 4312. [13] Q. Zhou, Y.Y. Zhou, Y. Liu, Z.L. Wang, G. Chen, J.H. Peng, J. Yan, M.M. Wu, J. Mater. Chem. C 3 (2015) 9615–9619. [14] Y.Y. Zhou, Q. Zhou, Y. Liu, Z.L. Wang, H. Yang, Q. Wang, Mater. Res. Bull. 73 (2016) 14–20. [15] X.L. Gao, Y. Song, G.X. Liu, X.T. Dong, J.X. Wang, W.S. Yu, Cryst. Eng. Comm. 18 (2016) 5842–5851. [16] D. Sekiguchi, S. Adachi, ECS J. Solid State Sci. Technol. 3 (2014) R60–R64. [17] J.S. Zhong, D.Q. Chen, X. Wang, L.F. Chen, H. Yu, Z.G. Ji, W.D. Xiang, J. Alloy. Compd. 662 (2016) 232–239. [18] A.A. Setlur, E.V. Radkov, C.S. Henderson, J.H. Her, A.M. Srivastava, N. Karkada, et al., Chem. Mater. 22 (2010) 4076–4082. [19] Y.K. Xu, S. Adachi, J. Electrochem. Soc. 158 (2011) J58–J65. [20] L.F. Lv, Z. Chen, G.K. Liu, S.M. Huang, Y.X. Pan, J. Mater. Chem. C 3 (2015) 1935–1941. [21] H.D. Nguyen, C.C. Lin, M.H. Fang, R.S. Liu, J. Mater. Chem. C 2 (2014) 0268–10272. [22] D. Sekiguchi, S. Adachi, Opt. Mater. 42 (2015) 417–422. [23] X.Y. Jiang, Y.X. Pan, S.M. Huang, X.A. Chen, J.G. Wang, G.K. Liu, J. Mater. Chem. C 2 (2014) 2301–2306.
deep red light (x=0.69, y=0.31), which can be used for the blue lightbased white LED devices. Acknowledgements This research is supported by the National Natural Science Foundation of China (Grant No. 21561003 and No. 21661006), the Scientific Research Foundation of Guangxi University (Grant No. XDZ140116) and the Students Experimental Skills and Innovation Ability Training Fund Project of Guangxi University (No. 201610593172). References [1] E.F. Schubert, J.K. Kim, Science 308 (2005) 1274–1278. [2] H. Daicho, T. Iwasaki, K. Enomoto, Y. Sasaki, Y. Maeno, Y. Shinomiya, et al., Nat. Commun. 3 (2012) 1132. [3] C.Y. Sun, X.L. Wang, X. Zhang, C. Qin, P. Li, Z.M. Su, et al., Nat. Commun. 4 (2013) 2717. [4] W.B. Im, N. George, J. Kurzman, S. Brinkley, A. Mikhailovsky, Adv. Mater. 23 (2011) 2300–2305. [5] J.K. Sheu, S.J. Chang, C.H. Kuo, Y.K. Su, L.W. Wu, Y.C. Lin, et al., IEEE Photon. Technol. Lett. 15 (2003) 18–20. [6] K.A. Denault, N.C. George, S.R. Paden, S. Brinkley, A.A. Mikhailovsky, J. Mater. Chem. 22
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
The formation of KF induced red-emitting phosphors K2TiF6·BaF(HF2):Mn4+ by cation exchange
MARK
⁎
Tianman Wanga, Yong Gaoa, Zhipeng Chena,c, Qiuying Huanga, Lini Wua, Yingheng Huanga,b, , ⁎⁎ ⁎ Sen Liaoa, , Huaxin Zhanga, a Guangxi Colleges and Universities Key Laboratory of Applied Chemistry Technology and Resource Development, School of Chemistry and Chemical Engineering, Guangxi University, Nanning, Guangxi 530004, China b School of Materials Science and Engineering, Guangxi University, Nanning, Guangxi 530004, China c Fangchenggang Entry-Exit Inspection and Quarantine Bureau, Fangchenggang, Guangxi, 538001, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Photoluminescence Optical materials Fluorides Red phosphor Mn4+
A series of K2TiF6 •BaF(HF2):Mn4+ phosphors with narrowband red emissions have been successfully prepared on a large scale with the cation exchange method using K2MnF6 and BaTiF6 as staring materials at ambient temperature in HF-KF solution. It is interesting that KF induces the formation of luminous phase for the samples. That is that the corresponding samples will not emit light without the presence of KF. Mechanism of KF inducing formation of luminous phase is suggested. HF-controlled effects on the fluorescent properties of the samples are discussed. Decay lifetime and the photoluminescence quantum yields of the optimal sample are 4.76 ms and 60.75 ± 0.03%, respectively. The chromaticity coordinates of the optimal sample indicate that this phosphor emits deep red light (x=0.69, y=0.31), which can be used for blue light-based white LED.
1. Introduction Mn4+ doped fluorides have been extensively studied for their high photoluminescence quantum yields and broadband absorption in the blue region, as well as narrowband emission in the red region caused by their distinct d–d transitions [1–25], having a great potential in applications to lighting, holography, laser technology, and dosimetry. There are many reports on the Mn4+-doped luminescent fluorides, including cocrystallisation [18], chemical etching [19,20], cation exchange [12,13], chemical co-precipitation [21,22], and hydrothermal method [13–15]. Currently, a series of red-emitting Mn4+ doped fluotitanates have been synthesized using above methods, such as Na2TiF6:Mn4+ [8,19], K2TiF6:Mn4+ [9–12,19], Cs2TiF6:Mn4+[13,19], BaTiF6:Mn4+[14–16], and ZnTiF6·6H2O: Mn4+ [17]. The cation exchange method is a simple and efficient way to prepare Mn4+ doped fluotitanate phosphors. Interestingly, cations of fluotitanates show monovalent and bivalent. Meanwhile, BaTiF6:Mn4+ and ZnTiF6·6H2O:Mn4+ are reported to be novel red phosphors. However, to the best of our knowledge, so far very few reports have been published on the synthesis of BaTiF6:Mn4+ and ZnTiF6·6H2O:Mn4+ phosphors by using the cation exchange method. Therefore, more
⁎
attention should focus on the synthesis of BaTiF6:Mn4+, ZnTiF6·6H2O:Mn4+ phosphors, as well as their luminescent properties. The cation exchange method can save energy during the preparation of Mn4+ doped fluotitanates since it is mild, efficient, and energy conservation [7,8]. So, in the present work, BaTiF6:Mn4+ phosphors were synthesized by the cation exchange method. Besides, the structures of the phosphors were investigated by powder X-ray diffraction (XRD) measurement. The photoluminescence (PL) properties of Mn4+ ions were investigated by excitation and emission (PLE & PL) spectra. KF controlled effects on the luminescence properties of red phosphors BaTiF6:Mn4+ were reported in this work. 2. Materials and methods All chemicals were reagent-grade pure and purchased from the Sinopharm Chemical Reagent Co. Ltd., China. XRD was performed at a scanning rate of 5°/min from 5° to 70° for 2θ at room temperature by using a Rigaku D/max 2500 V diffractometer. It was equipped with a graphite monochromator by utilizing monochromatic CuKα radiation (λ=0.154178 nm). The morphologies of the samples were examined by Hitachi S-3400 scanning electron microscopy (SEM) with an attached
Corresponding authors. Corresponding author at: Guangxi Colleges and Universities Key Laboratory of Applied Chemistry Technology and Resource Development, School of Chemistry and Chemical Engineering, Guangxi University, Nanning, Guangxi 530004, China. E-mail addresses: [email protected], [email protected] (S. Liao). ⁎⁎
http://dx.doi.org/10.1016/j.jlumin.2017.04.049 Received 22 December 2016; Received in revised form 5 April 2017; Accepted 23 April 2017 Available online 26 April 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
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energy-dispersive X-ray spectrometer (EDS). Samples were mounted on an aluminum slice coated with Au. PLE and PL spectra were recorded at room temperature by a Shimadzu RF-53001 spectrophotometer equipped with a xenon lamp as the excitation source. The luminescence decay curves and the photoluminescence quantum yields were obtained from an Edinburgh FLS980 fluorescence spectrophotometer. The metal elemental contents of the sample were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, Perkin-Elmer Optima 3100 RL). First, the sample of K2TiF6 •BaF(HF2):xMn4+ was dissolved in 10 ml of 1:1 aqueous HCl solution which contains several drops of H2O2 (30%), then the solution was diluted to 100.00 ml with distilled water. Third, the elemental contents in the solution were determined by ICP-AES. K2MnF6 was synthesized according to the method described in the literature [26]. Specifically, 1.580 g KMnO4 and 15 g KF·2H2O were completely dissolved in 30 ml HF (40%) solution. After completely dissolving of KF·2H2O in the HF solution, H2O2 (30%) solution was added drop by drop until the solution turned to yellow. The yellow powder of K2MnF6 sample was obtained by filtering, washing with acetone and drying at 80 °C for 2 h. A typical two-step synthesis method was used to synthesize K2TiF6 •BaF(HF2):xMn4+ red phosphor. (i) Pure BaTiF6 powder was synthesized as follows: 3.947 g (20.0 mmol) BaCO3 was added into 6.555 g (20.0 mmol) H2TiF6 (50%) solution slowly with stirring. The mixture was ground together in an agate mortar for 1 h. Then the mixture was kept at 80 oC for 3 h, and dried at 120 °C for 3 h to yield BaTiF6 (sample (a)). (ii) In a typical synthesis of Mn4+-doped sample, 10 ml HF (40%) solution, 6 g KF·2H2O and 0.297 g (1.2 mmol) K2MnF6 were added into a 50 ml plastic beaker and completely dissolved with stirring. Then 5.984 g (20.0 mmol) BaTiF6 (molar ratio of K2MnF6/ BaTiF6 (Mn/Ti), x=0.06) was put into the yellow transparent solution at room temperature. After 30 min of magnetic stirring, the precipitate was collected, washed with methanol several times and dried at 80 °C for 2 h to yield sample (b). For comparison, a series of red phosphor samples were synthesized with different molar ratios of K2MnF6/ BaTiF6 (x=0.01, 0.02, 0.04, 0.06, 0.08, 0.10, and 0.12).
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3. Results and discussion
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Fig. 1 shows the XRD patterns of the as-synthesized samples. In Fig. 1a, all the diffraction peaks of the sample (a) are indexed and in agreement with the pure hexagonal BaTiF6 phase standard data (PDF#76–0269). Fig. 1b shows that the sample (b) contains mixture phases of K2TiF6 (PDF#28–0825) and BaF(HF2) (PDF#76–1301), which means that Mn4+(CN =6, r =0.53 Å) ions are at the octahedral sites of Ti4+(CN =6, r =0.61 Å). Fig. 1c shows that 2θ angles of sample (b) are slightly shifted toward shorter angles, which indicates that the lattice volume of the sample slightly expends, and there is strong distortion in the K2TiF6 lattice. Because the radius of Mn4+(0.53 Å) is smaller than that of Ti4+(0.61 Å), the doping of Mn4+ can result in the contracting of lattice volume of the samples, which was also reported by Liao et al. [9]. However, there are some differences between the results reported here and by Liao et al. [9]. The K2TiF6 reported by Liao et al. is single phase, while it is mixture phases here. The mixture phases may lead to the expanding of the lattice volume of samples, due to the lattice interaction between mixture phases. Fig. 2 shows the SEM image and the corresponding EDS spectra of sample (b) (x=0.06, wKF =6 g). The sample contains kinds of shapes of crystals, such as blockbusters, small pieces and rod-like crystals (Fig. 2a). Sample (b) is composed of K, Ba, Ti, Mn and F elements (Fig. 2b). ICP-AES of sample (b) shows that the mass percentages of Ba, Ti, K, and Mn are 30.49%, 10.62%, 18.40%, and 0.74%, respectively. So, the molar ratio of Ba, Ti, K, and Mn in sample (b) is 1.00:1.00:2.00:0.06, corresponding with the molecular formula (K2TiF6)·(BaF(HF2))·(K2MnF6) 0.06. The molar ratio of Mn/Ti is 0.06, in agreement with the raw materials ( K2MnF6/BaTiF6 =0.06). So, the
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results indicate that Mn4+ and Ti4+ can replace each other in the samples. Obviously, a series of K2TiF6 •BaF(HF2): Mn4+ phosphors can be prepared on a large scale with the method of using K2MnF6 and 308
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Fig. 2. SEM image and the corresponding EDS spectrum of sample(b): (a) SEM image, (b) EDS spectrum.
Fig. 3. Properties of sample (b): (a) PLE and PL spectra, (b) photograph for sample (b) under excitation with blue light in deionized water.
BaTiF6 as starting materials at ambient temperature in HF-KF solution. Fig. 3 shows the PLE and PL spectra of the sample (b) (x=0.06, wKF =6 g). Two strong broad excitation peaks at 364 and 463 nm are found between 300 and 550 nm, which are due to the 4A2→4T1 and 4A2→4T2 transitions of Mn4+, respectively [27,28]. The intensity of latter is 0.91 time of the former. The full-width at half-maximum of these two bands are 54 and 73 nm, respectively. Five narrow emission peaks (~612 (ν4), ~617 (ν6), ~634 (ν6), ~638 (ν4) and ~650 nm (ν3)) activated by the vibrational modes of the [MnF6]2- octahedron [29–32] are also found in Fig. 3. Among these emission peaks, those centered at 617, 634 and 650 nm are from the spin forbidden d–d transition 2Eg→4A2 of Mn4+ [27,28], indicating that the Mn4+ ions are doped in the K2TiF6 matrix. The strong broad excitation bands at 364 and 463 nm and narrow emission band at 634 nm of the sample show that it has potential application in white light-emitting diodes excited by UV light or blue light. Interestingly, compared with BaTiF6:Mn4+ sample obtained from hydrothermal synthesis method [14,15], the PLE and PL peak patterns of sample (b) are different: (i) In PLE spectra, the peak intensity of 4 A2→4T2 (463 nm) is weaker than that of 4A2→4T1(364 nm) in sample (b), while it is opposite in the another one, (ii) In PL spectra, two strong peaks centered 617 and 634 nm are split peaks in sample (b), while there are not splitting in the another one. The results indicate that the PLE and PL properties of the sample can be influenced by the distortion of crystal field [16], which is caused by distortion of the lattice. Furthermore, it is noticeable that samples prepared by this method can
emit red luminescence under the excitation with blue light in deionized water (Fig. 3b). It indicates that luminescence of Mn4+ is not quenched by water, because Mn4+ locates at the center of octahedral [MnF6]2-, and there are no interactions between Mn4+ ions and water molecules. The molar ratios of K2MnF6/BaTiF6 play a key role in the luminescent properties of samples. The luminescent properties of samples with different molar ratios of K2MnF6/BaTiF6 (x=0.01–0.12, wKF =6 g) are shown in Fig. 4. First, the luminescence intensities of the samples increase with the increase of molar ratios of K2MnF6/BaTiF6, and reach a maximum value at x=0.06. Then they reduce due to the concentration quenching. As shown in Fig. 4, the shapes of the excitation and emission spectra are not strongly affected by the Mn4+ concentrations. Compared with BaTiF6:Mn4+ sample obtained from hydrothermal synthesis method [14,15], the optimal molar ratio of Mn/Ti is different, in which the former is 0.06, while the latter is 0.08. Recently, Lee [33] reported that KF can enhance luminous efficiency of deep red emitting K2SiF6:Mn4+. So, further experiments were done with different dosage of KF. Dependent curves of PLE and PL intensity on different dosage of KF are shown in Fig. 5. It shows that the dependences of PLE and PL intensities on different dosage of KF are in curves of quintic with the mouth downward. The luminescent intensities of the samples are increased with the increase of KF dosage and
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larger than 5 g, curve (ii) is lower than curve (i). That is, intensity ratio of two PLE peaks (364 and 463 nm) or two PL peaks (excited by 364 and 463 nm) can be easily tuned by the KF dosage. The results indicate that the samples have a double application prospect, and can be used for both near UV-based white LED and blue light-based white LED. Based on the measured PL spectra, the chromaticity coordinates of the sample (b) were calculated (Fig. 5c). The calculated results show that the chromaticity coordinates from two PL spectra excited by the two excitation wavelengths (364 and 463 nm) are almost the same and given by x=0.69, y=0.31. As shown in Fig. 5c, the sample (b) emits deep red light (x=0.69, y=0.31). The decay curve of the sample (b) is shown in Fig. 5d. It is well fitted by a single-exponential function with a constant term. The lifetime is 4.76 ms, which is in agreement with BaTiF6:Mn4+ sample obtained with hydrothermal synthesis method [14,15]. The photoluminescence quantum yield of the sample (b) is 60.75 ± 0.03%, which is much higher than that of ZnTiF6·6H2O:Mn4+ [17]. Some literatures reported that BaTiF6:Mn4+ could be obtained with hydrothermal synthesis method [14,15]. But, compared with BaTiF6:Mn4+ sample obtained from hydrothermal synthesis method, the product of method used in this paper is different, which no BaTiF6:Mn4+ is formed, only (K2TiF6)·(BaF(HF2))·(K2MnF6)x is obtained. The results indicate that different synthesis method will lead to formation of different products. Based on the results of Fig. 1b and Fig. 5, mechanism for formation of luminous phase induced by the addition of KF in the cation exchange synthesis method can be suggested by the follows reactions:
2KF + BaTiF6 + HF = BaF(HF2) + K2 TiF6
(1)
x K2MnF6 + K2 TiF6 = K2 TiF6⋅x K2MnF6
(2)
From the above reactions, it can be deduced that: (i) the reasons for obtaining luminous samples in the presence of KF are as follows: first, BaTiF6 reacts with KF and HF in KF/HF solution to form BaF(HF2) and K2TiF6; second, according to the theory of similar dissolve mutually, cation exchange reaction occurs between K2MnF6 and K2TiF6 in KF/HF solution, and a part of Ti4+ sites are replaced by Mn4+ ions to form K2TiF6·xK2MnF6. So, obtained samples contain mixture crystal phases of K2TiF6:Mn4+ and BaF(HF2) (please see Fig. 1), and can emit red light under excitation (please see Fig. 3). (ii) the reasons for the formation of no luminious samples without the presence of KF are as follows: the similarity is not enough between K2MnF6 and BaTiF6, and cation exchange reaction cannot occur between K2MnF6 and BaTiF6 in HF solution without the presence of KF at room temperature and, thus, no presence of BaTiF6:Mn4+. So, the obtained sample has no fluorescence. As previously mentioned, BaTiF6:Mn4+ could be obtained from hydrothermal synthesis method [14,15]. Hence, it is obvious that cation exchange reaction can occur between K2MnF6 and BaTiF6 under hydrothermal synthesis condition, but this reaction cannot happen in ambient condition without the presence of KF. So, the luminous phase in the present sample is only K2TiF6·xK2MnF6(i.e., K2TiF6:Mn4+).
Fig. 4. PLE and PL spectra of BaTiF6: Mn4+ with different x (x=0.01–0.12, wKF =6 g): (a) PLE, (b) PL excited by 364 nm, (c) PL excited by 463 nm.
4. Conclusions In summary, a series of K2TiF6•BaF(HF2):Mn4+ samples were prepared by the cation exchange method in HF-KF solution. XRD results confirmed that the samples contain mixture phases of crystals. It is interesting that KF has induced luminescence effect for the samples. That is that the synthesized samples without the addition of KF do not emit light. Mechanism of induced luminescence effect was suggested. The addition of KF on the structural and fluorescent properties of the samples were discussed. The results indicated that the optimal dosage of KF giving the strongest PL intensity is 6 g. The chromaticity coordinates of the optimal sample indicated that this phosphor emits
reach a maximum value at wKF =6 g, and decrease when wKF is larger than 6 g, due to quenching process. That is, the intensities of two PLE peaks (364 and 463 nm) or two PL peaks (excited by 364 and 463 nm) can be greatly enhanced by the addition of KF. It is found that KF plays a key role on the luminescence properties of the phosphors synthesized using this method. Without the presence of KF, the sample does not emit light. Furthermore, enhanced effects of the two excitation peaks (364 and 463 nm) have a bit different. As shown in Fig. 5a–b, when wKF is smaller than 5 g, curve (ii) is higher than curve (i), while when wKF is
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Fig. 5. Fluorescent properties of samples (x=0.06) at different KF weight: (a) Intensities of PLE spectra, (b) Intensities of PL spectra, (c) CIE chromaticity diagram value and (d) Emission decay curve at optimal condition (x=0.06, wKF =6 g, excited at 464 nm). (2012) 18204–18213. [7] A.A. Setlur, W.J. Heward, Y. Gao, A.M. Srivastava, R.G. Chandran, M.V. Shankar, Chem. Mater. 18 (2006) 3314–3322. [8] Z.L. Wang, Y. Liu, Y.Y. Zhou, Q. Zhou, H.Y. Tan, Q.H. Zhang, J.H. Peng, RSC Adv. 5 (2015) 58136–58140. [9] J.S. Liao, L.L. Nie, L.F. Zhong, Q.J. Gu, Q. Wang, Luminescence 31 (2016) 802–807. [10] C.C. Lin, A. Meijerink, R.S. Liu, J. Phys. Chem. Lett. 7 (2016) 495–503. [11] Z.L. Wang, Y.Y. Zhou, Z.Y. Yang, Y. Liu, H. Yang, H.Y. Tan, Q.H. Zhang, Q. Zhou, Opt. Mater. 49 (2015) 235–240. [12] H.M. Zhu, C.C. Lin, W.Q. Luo, S.T. Shu, Z.G. Liu, Y.S. Liu, J.T. Kong, E. Ma, Y.G. Cao, R.S. Liu, Nat. Commun. 5 (2014) 4312. [13] Q. Zhou, Y.Y. Zhou, Y. Liu, Z.L. Wang, G. Chen, J.H. Peng, J. Yan, M.M. Wu, J. Mater. Chem. C 3 (2015) 9615–9619. [14] Y.Y. Zhou, Q. Zhou, Y. Liu, Z.L. Wang, H. Yang, Q. Wang, Mater. Res. Bull. 73 (2016) 14–20. [15] X.L. Gao, Y. Song, G.X. Liu, X.T. Dong, J.X. Wang, W.S. Yu, Cryst. Eng. Comm. 18 (2016) 5842–5851. [16] D. Sekiguchi, S. Adachi, ECS J. Solid State Sci. Technol. 3 (2014) R60–R64. [17] J.S. Zhong, D.Q. Chen, X. Wang, L.F. Chen, H. Yu, Z.G. Ji, W.D. Xiang, J. Alloy. Compd. 662 (2016) 232–239. [18] A.A. Setlur, E.V. Radkov, C.S. Henderson, J.H. Her, A.M. Srivastava, N. Karkada, et al., Chem. Mater. 22 (2010) 4076–4082. [19] Y.K. Xu, S. Adachi, J. Electrochem. Soc. 158 (2011) J58–J65. [20] L.F. Lv, Z. Chen, G.K. Liu, S.M. Huang, Y.X. Pan, J. Mater. Chem. C 3 (2015) 1935–1941. [21] H.D. Nguyen, C.C. Lin, M.H. Fang, R.S. Liu, J. Mater. Chem. C 2 (2014) 0268–10272. [22] D. Sekiguchi, S. Adachi, Opt. Mater. 42 (2015) 417–422. [23] X.Y. Jiang, Y.X. Pan, S.M. Huang, X.A. Chen, J.G. Wang, G.K. Liu, J. Mater. Chem. C 2 (2014) 2301–2306.
deep red light (x=0.69, y=0.31), which can be used for the blue lightbased white LED devices. Acknowledgements This research is supported by the National Natural Science Foundation of China (Grant No. 21561003 and No. 21661006), the Scientific Research Foundation of Guangxi University (Grant No. XDZ140116) and the Students Experimental Skills and Innovation Ability Training Fund Project of Guangxi University (No. 201610593172). References [1] E.F. Schubert, J.K. Kim, Science 308 (2005) 1274–1278. [2] H. Daicho, T. Iwasaki, K. Enomoto, Y. Sasaki, Y. Maeno, Y. Shinomiya, et al., Nat. Commun. 3 (2012) 1132. [3] C.Y. Sun, X.L. Wang, X. Zhang, C. Qin, P. Li, Z.M. Su, et al., Nat. Commun. 4 (2013) 2717. [4] W.B. Im, N. George, J. Kurzman, S. Brinkley, A. Mikhailovsky, Adv. Mater. 23 (2011) 2300–2305. [5] J.K. Sheu, S.J. Chang, C.H. Kuo, Y.K. Su, L.W. Wu, Y.C. Lin, et al., IEEE Photon. Technol. Lett. 15 (2003) 18–20. [6] K.A. Denault, N.C. George, S.R. Paden, S. Brinkley, A.A. Mikhailovsky, J. Mater. Chem. 22
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