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Abstract
In this paper, we present a rapid approach for synthesizing highly efficient emitting K2TiF6:Mn4+ red phosphors by ball milling. K2TiF6:Mn4+ can be obtained via cation exchange by reacting K2TiF6 and KMnO4. The synthesized time is reduced to 15 min, which is 5 times as fast as that of the simple chemical method, but the photoluminescence intensity of the obtained K2TiF6:Mn4+ increases by 34.6%. With the increase of the milling speed, the size of phosphor decreases, but the reaction of Ti4+ substituted by Mn4+ in K2TiF6 is accelerated. This is a new approach for preparing luminescent Mn4+-doped fluoride phosphors.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Red-emitting phosphors play a key role in white light emitting diodes (LEDs) for indoor illumination applications [1-4], such as CaAlSiN3:Eu2+ [5], KLa(MoO4)2:Eu3+ [6], Ca2Si5N8:Eu2+ [7]. However, the conventional solid-state reaction for preparing these phosphors needs high temperature, high pressure, and the doping of rare earth elements. Additionally, when these red nitride phosphors are mixed with the yellow phosphors, the serious re-absorption will take place, which causes lower luminous efficacy. So, more studies have focused on non-rare-earth phosphors, such as A2BF6:Mn4+ (A = Na, K, Rb, Cs, NH4+, B = Ti, Ge, Si, Sn, Zr), Ca5Zn3.92In0.08(V0.99Ta0.01O4)6 [8], CsVO3 [9], Zn3V2O8 [10], etc. Of these, Mn4+-activated A2BF6, such as K2TiF6:Mn4+, BaSiF6:Mn4+ [11], K2SiF6:Mn4+ [12,13], Na2SnF6:Mn4+, and Cs2SnF6:Mn4+ [14–18], can produce efficient red emission under blue or UV excitation, which can be used as a promising candidate to produce white LEDs with high color render index (CRI), low correlated color temperatures (CCTs), and high luminescence efficiency (LE) [19]. For example, a high-performance white LED had been fabricated, by employing K2TiF6:Mn4+ as a red phosphor, which possesses a low CCT (3556 K), a high CRI (Ra = 81) and a luminous efficacy of 116 lm/W [20]. Mn4+-activated phosphors show a good application prospect in the white LEDs field.
To date, fluoride phosphors are usually prepared by the cocrystallization [21], chemical etching [22], cation exchange [23–25] and so on. Recently, we synthesized K2TiF6:Mn4+ and Na2SiF6:Mn4+ phosphors by a modified cation exchange method and a simple chemical reaction, respectively. The typical process of the simple chemical method is as follow: firstly, an amount of KMnO4 or NaMnO4 was put in HF solution, then some K2TiF6 or Na2SiF6 particles were added, and then the H2O2 solution (10% wt) was dropped into the above mixture. The synthesized phosphors display excellent thermal stability and high performance, which can be used to produce high CRI (Ra > 85) white LEDs for indoor lighting [23,24,26]. In this paper, we propose a rapid and novel approach for synthesizing highly efficient red emitting K2TiF6:Mn4+ phosphors by ball milling.
2. Experimental
2.1 The preparation of fluoride phosphors
K2TiF6, KMnO4 and HF were purchased from Cheng Du Ke Long Chemical Reagent Co. Limited. All reagents were used as received without further purification.
The fluoride phosphors were prepared by ball milling. Typically, 3 g of K2TiF6 powders and 0.6 g of KMnO4 were added in 10 ml of HF solution (49 wt.%). Then the above mixture slurry was milled at the speed of 100 or 200 rpm for 5-20 min at room temperature. The compared samples were prepared by a simple chemical method but without H2O2 and KF. 3 g of K2TiF6 powders and 0.6 g of KMnO4 were added in 10 ml of HF solution (49 wt.%), and then the mixture was placed for 30-300 min at room temperature without stirring, heating and cooling. (CAUTION: The experimental processes have potentially dangers, such as highly corrosive, toxic, and reactive chemicals. In our experimental procedures, we had worn protective clothing, eye/face protection, safety gloves, and protective respirator. To make sure of safety, the milling balls and tank are made of polytetrafluoroethylene (PTFE). All the samples were washed by anhydrous alcohol and dried by air blowing thermostatic dry box at 80°C.
2.2 Characterization
X-ray powder diffraction (XRD) tests were taken on a diffractometer (TD-3500, Dandong, China) using monochromatized CuK (α) radiation with an accelerating voltage of 30 kV and an applied current of 20 mA. The XRD data were collected by a scanning mode in the 2θ range from 20° to 70° with a scanning step of 0.02° and a scanning rate of 2.0° min−1. Surface morphology and particle size were tested by a scanning electron microscope (SEM, Quanta 250, FEI, USA) with an accelerating voltage of 10 kV. Energy dispersive analyses of X-rays (EDAX) were performed by an energy dispersive spectrometer. Photoluminescence spectra and quantum yields were measured on a Hitachi F-7000 fluorescence spectrophotometer (F-7000, Hitachi, Japan) with a 150 W xenon lamp at room temperature, Al2O3 as a reference material was used to measure quantum efficiencies.
3. Results and discussion
Table 1 shows the synthesis parameters and Mn4+ concentration of the obtained K2TiF6:Mn4+ phosphors. According to the reaction time and revolving speed, the obtained samples were labeled as S30, S90, S150, B100-5, B100-10, B100-15, B100-20, B200-5, B200-10, and B200-20, respectively. Figure 1 shows the XRD patterns of the obtained K2TiF6:Mn4+ phosphors. It can be seen that samples S30, S90, S150, B100-5 and B100-15 display the characteristic peaks which are in accordance with K2TiF6 (JCPDS NO. 08-0488), but the diffraction peaks of sample B200-5 and sample B200-20 appear at 21.6° and 44.0° which agree with K2MnF6 phase (JCPDS NO. 34-0733). The peak position shifts to the small angle, indicating that Mn4+ is probably successful doped into the K2TiF6. It is an interesting phenomenon that the intensities of the diffraction peaks do not match with the intensities of the diffraction peaks in the reference pattern, which is similar to the literature [20]. Furthermore, additional diffraction lines were presented in the XRD pattern of B100-15 at 26° and 43°, which correspond to the standard patterns of K2MnF6 (JCPDS No. 01-089-5017). In order to confirm the component and Mn4+ concentration in K2TiF6:Mn4+, the EDAX spectrum was analyzed as shown in Fig. 2. The peaks belong to F, Ti, K, and Mn, respectively. With the increase of the speed and time of ball milling, the concentration of Mn4+ in K2TiF6 increases (Table 1).
Table 1. Synthesis parameters and Mn4+ concentration of the K2TiF6:Mn4+ phosphors
Figure 3 shows the SEM images of the synthesized K2TiF6:Mn4+ and K2TiF6 powders. In comparison with K2TiF6 particles, sample S30 has a similar size of 50–100 µm, but the sizes of samples B100-5 and B200-5 obviously decrease. Especially, sample B200-5 shows an irregular morphology with the size of less than 10 µm, due to the decrease of the lattice planes in HF corrosion process.
Mn4+-doped fluoride phosphors have attracted much interests attributed to their high luminescence properties. Figure 4 shows the images of the synthesized K2TiF6:Mn4+ and the schematic of the crystal structure of a K2TiF6 unit cell. It can be seen Mn4+ ion substitutes Ti4+ ion in the TiF62- octahedron leading to the formation of MnF62- for synthesizing K2TiF6:Mn4+. The synthesized K2TiF6:Mn4+ powders appear orange-yellow under room light and emit strong red light under UV light illumination. As shown in the Fig. 5(a), the 4A2→4T1 and 4A2→4T2 transitions lead to two wide excitation bands peaking at ~365 nm (~3.5 eV) and ~460 nm (~2.7 eV), respectively. Figure 5(b) shows under 460 nm excitation the emission spectra of phosphors consist of five narrow bands extending from 580 to 660 nm, and the strongest peak is located at 634 nm (~2 eV). Figure 6 shows the energy level diagrams of Mn4+ in light of the literatures [18,20]. Each emission line-width is not dependent on the variation of the crystal field strength, but the strong electron vibrational interaction between the electronic states of Mn4+ ions and crystal lattice vibrations [18]. The emission spectra of Mn4+-doped phosphors are controlled by the spin-forbidden 2Eg→4A2g transition [27, 28].
Fig. 4 Images of samples S90 (a), B100-5 (b) and B200-5(c) and schematic of the crystal structure of a K2TiF6 unit cell (d).
Fig. 5 Excitation spectra (λem = 634 nm) (a) and emission spectra (λex = 460 nm) (b) of the synthesized K2TiF6:Mn4+ phosphors.
Figure 7 shows the photoluminescence intensities of the synthesized K2TiF6:Mn4+ phosphors. It can be seen that relative to sample S90, the photoluminescence intensity of B100-15 prepared by the ball milling increases by 34.6%, but the reaction time reduces by 5 times. According to the XRD pattern, impurities of K2MnF6 appear in samples B200-5 and B200-20. Because of bad luminescence property of K2MnF6, samples B200-5 and B200-20 shows lower optical properties than other samples. Table 2 shows the internal quantum efficiencies (IQE) and external quantum efficiencies (EQE) of K2TiF6:Mn4+ phosphors. Sample B100-15 shows higher IQE and EQE, which indicates that the ball milling is an effective method for synthesizing K2TiF6:Mn4+ phosphors. However, B200-5 shows lower IQE and EQE than B100-15, which reveals that excessive speed of ball milling is adverse for the photoluminescence properties. There is great friction and breaking force between ball and particles in the process of ball milling, which can make the particles and solution fully contact to accelerate the reaction. In our experiments, we also prepare samples in 30 minutes by simple chemical method, but the obtained powders have no obvious luminescence. Therefore, the ball milling is a rapid, effective method for synthesizing the highly efficient fluoride phosphors.
Table 2. IQE and EQE of the obtained K2TiF6:Mn4+ phosphors.
The reference [20] reported that K2TiF6:Mn4+ phosphors with a quantum yield exceeding 90% can be synthesized by cation exchange reaction within 30 minutes. However, it will take two steps: K2MnF6 was prepared firstly and then dissolved into the HF solution. The above results show the ball milling is a highly efficient and one step method for synthesizing K2TiF6:Mn4+ phosphors. The obtained K2TiF6:Mn4+ phosphors have smaller size particles and lower quantum efficiency than these in our previous work [23,24]. However, it will be spent more than ten hours for synthesizing K2TiF6:Mn4+ phosphors in our previous work.
Some literatures pointed out that the ball milling can disperse uniformly [29] and break particles [30]. Zhao et al. [31]clarified that with the increase of milling speed, powder particle size quickly diminishes, but the specific surface area and the surface energy of the particles improves. Figure 8 shows the schematic diagram of the milling mechanisms for synthesizing K2TiF6:Mn4+ phosphors. In the reaction process, K2MnF6 (Mn4+ source materials) was produced and diffused into K2TiF6. The synthesis of K2MnF6 could be understood by the following equation:
Fig. 8 Schematic diagram of the milling mechanism for synthesizing K2TiF6:Mn4+ phosphors. (a) S10, (b) B100-5, (c) B200-5.
Regarding the XRD results, K2MnF6 phase increase with increasing the milling speed, and B100-15 shows distinctly higher photoluminescence property than B200-5 and B200-20. It can be taken into account as follows. When the milling speed increase from 100 to 200 rpm, the size of particles decrease rapidly, whereas the specific surface area and the surface energy of the particles enhance, contributing to more Mn4+ doped into the K2TiF6 (Fig. 8). But more surface and lattice defects can also degrade the photoluminescence property during the milling process, and the concentration quenching occurs owing to excessive K2MnF6 in K2TiF6 matrix. Thus, the results do not agree with the report [20] which shows the quenching concentration of Mn4+ is up to 5%.
4. Conclusions
In this paper, K2TiF6:Mn4+ phosphors were prepared via a ball milling with K2TiF6, KMnO4 and HF solution as reactants. The ball milling is a more highly efficient method than a simple chemical method for synthesizing K2TiF6:Mn4+ phosphors. On the basis of cation exchange reaction, Mn4+ replaces the Ti4+ in K2TiF6 in the reaction process. With increasing the milling speed, the powder size gradually decreases and the cation exchange reaction is accelerated, contributed to the production of more K2MnF6. However, B100-15 displays higher photoluminescence property than B200-5 and B200-20 because of concentration quenching and decrease of surface deficiency.
To the best of our knowledge, there are so far no reports on synthesizing luminescent Mn4+-doped fluoride phosphors via ball milling and this work thus presents a novel synthesis method for preparing such phosphors. Furthermore, the result shows that K2TiF6:Mn4+ can be synthesized via cation exchange (cocrystallization) by reacting the host lattice K2TiF6 and the manganese precursor KMnO4. This is a new approach, as normally cation exchange synthesis of K2TiF6:Mn4+ is done with K2Mn(IV)F6 as precursor for Mn4+ [32]. Cation exchange with KMnO4 on other fluoride lattices to synthesize Mn4+-doped fluorides has however been reported previously [33].
Funding
National Natural Science Foundation of China (51402032); Chongqing Youth Science and Technology Talent Cultivation Project (cstc2014kjrc-qnrc40006); Chongqing International Science & Technology Cooperation Program (cstc2015gjhz0003); International Science & Technology Cooperation Program of China (2014DFR50830); the open project of Key Lab of Advanced Materials of Yunnan Province (2016cx06); Basic and Frontier Research Program of Chongqing Municipality (cstc2016jcyjA0567); Chongqing Education Commission funded project (KJ1711283); Chongqing University of Arts and Sciences (M2015ME03, Z2015XC04).
References and links
1. C. C. Lin and R. S. Liu, “Advances in phosphors for light-emitting diodes,” J. Phys. Chem. Lett. 2(11), 1268–1277 (2011). [PubMed]
2. S. Pimputkar, J. S. Speck, S. P. Denbaars, and S. Nakamura, “Prospects for LED lighting,” Nat. Photonics 3, 180–182 (2009).
3. P. F. Smet and J. J. Joos, “White light-emitting diodes: Stabilizing colour and intensity,” Nat. Mater. 16(5), 500–501 (2017). [PubMed]
4. H. F. Sijbom, R. Verstraete, J. J. Joos, D. Poelman, and P. F. Smet, “K2SiF6:Mn4+ as a red phosphor for displays and warm-white leds: a review of properties and perspectives,” Opt. Mater. Express 7(9), 3332–3365 (2017).
5. X. Piao, K. Machida, T. Horikawa, H. Hanzawa, A. Y. Shimomura, and N. Kijima, “Preparation of CaAlSiN3:Eu2+ phosphors by the self-propagating high-temperature synthesis and their luminescent properties,” Chem. Mater. 19, 4592–4599 (2007).
6. L. Hou, S. Cui, Z. Fu, Z. Wu, X. Fu, and J. H. Jeong, “Facile template free synthesis of KLa(MoO4)2:Eu3+,Tb3+ microspheres and their multicolor tunable luminescence,” Dalton Trans. 43(14), 5382–5392 (2014). [PubMed]
7. P. F. Smet, K. V. Eeckhout, A. J. Bos, E. V. Kolk, and P. Dorenbos, “Temperature and wavelength dependent trap filling in M2Si5N8: Eu (M= Ca, Sr, Ba) persistent phosphors,” J. Lumin. 132(3), 682–689 (2012).
8. E. Pavitra, G. S. R. Raju, J. Y. Park, L. Wang, B. K. Moon, and J. S. Yu, “Novel rare-earth-free yellow Ca5Zn3.92In0.08(V0.99Ta0.01O4)6 phosphors for dazzling white light-emitting diodes,” Sci. Rep. 5, 10296 (2015). [PubMed]
9. T. Nakajima, M. Isobe, Y. Uzawa, and T. Tsuchiya, “Rare earth-free high color rendering white light-emitting diodes using CsVO3 with highest quantum efficiency for vanadate phosphors,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3, 10748–10754 (2015).
10. T. Nakajima, M. Isobe, T. Tsuchiya, Y. Ueda, and T. Kumagai, “A revisit of photoluminescence property for vanadate oxides AVO3(A:K, Rb and Cs) and M3V2O8(M:Mg and Zn),” J. Lumin. 129, 1598–1601 (2009).
11. X. Jiang, Y. Pan, S. Huang, X. A. Chen, J. Wang, and G. Liu, “Hydrothermal synthesis and photoluminescence properties of red phosphor BaSiF6:Mn4+ for LED applications,” J. Mater. Chem. C Mater. Opt. Electron. Devices 2, 2301–2306 (2014).
12. L. Lv, X. Jiang, S. Huang, X. A. Chen, and Y. Pan, “The formation mechanism, improved photoluminescence and LED applications of red phosphor K2SiF6:Mn4+,” J. Mater. Chem. C Mater. Opt. Electron. Devices 2, 3879–3884 (2014).
13. J. Oh, H. Kang, Y. Eo, H. Park, and Y. Do, “Synthesis of narrow-band red-emitting K2SiF6:Mn4+ phosphors for a deep red monochromatic LED and ultrahigh color quality warm-white LEDs,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3, 607–615 (2014).
14. A. Sadao and T. Takahashi, “Direct synthesis of K2SiF6: Mn4+ red phosphor from crushed quartz schist by wet chemical etching,” Electrochem. Solid St. 12(2), J20–J23 (2009).
15. Y. Arai and S. Adachi, “Optical properties of Mn4+-activated Na2SnF6 and Cs2SnF6 red phosphors,” J. Lumin. 131, 2652–2660 (2011).
16. Y. Arai, T. Takahashi, and S. Adachi, “Photoluminescent properties of K2SnF6·H2O:Mn4+ red phosphor,” Opt. Mater. 32, 1095–1101 (2010).
17. T. Nakamura, Z. Yuan, and S. Adachi, “Micronization of red-emitting K2SiF6:Mn4+ phosphor by pulsed laser irradiation in liquid,” Appl. Surf. Sci. 320, 514–518 (2014).
18. T. Takahashi and S. Adachi, “Mn4+ -activated red photoluminescence in K2SiF6 phosphor,” J. Electrochem. Soc. 155, E183–E188 (2008).
19. J. Meyer and F. Tappe, “Solid‐State Lighting: Photoluminescent materials for solid-state lighting: state of the art and future challenges,” Adv. Opt. Mater. 3, 423 (2015).
20. H. Zhu, C. C. Lin, W. Luo, S. Shu, Z. Liu, Y. Liu, J. Kong, E. Ma, Y. Cao, R. S. Liu, and X. Chen, “Highly efficient non-rare-earth red emitting phosphor for warm white light-emitting diodes,” Nat. Commun. 5, 4312 (2014). [PubMed]
21. A. A. Setlur, E. V. Radkov, C. S. Henderson, J. H. Her, A. M. Srivastava, N. Karkada, M. S. Kishore, N. P. Kumar, D. Aesram, and A. Deshpande, “Energy-efficient, high-color-rendering led lamps using oxyfluoride and fluoride phosphors,” Chem. Mater. 22, 4076–4082 (2010).
22. K. X. Yan and S. Adachi, “Properties of Mn4+-activated hexafluorotitanate phosphors,” J. Electrochem. Soc. 158, J58 (2011).
23. T. Han, T. Lang, J. Wang, M. Tu, and L. Peng, “Large micro-sized K2TiF6:Mn4+ red phosphors synthesised by a simple reduction reaction for high colour-rendering white light-emitting diodes,” Rsc. Adv. 5, 100054 (2015).
24. T. Han, J. Wang, T. Lang, M. Tu, and L. Peng, “K2MnF5·H2O as reactant for synthesizing highly efficient red emitting K2TiF6:Mn4+ phosphors by a modified cation exchange approach,” Mater. Chem. Phys. 18, 3230–3237 (2016).
25. T. Roisnel, P. Núñez, A. Tressaud, E. Molins, and J. Rodríguez-Carvajal, “Investigation of K2MnF5·H2O by neutron diffraction,” J. Solid State Chem. 150, 104–111 (2000).
26. T. C. Lang, T. Han, L. L. Peng, and M. J. Tu, “Luminescence properties of Na2SiF6:Mn4+ red phosphors for high colour-rendering white LED applications synthesized via a simple exothermic reduction reaction,” Mater. Chem. Frontiers, 1, 928–932 (2017).
27. Z. Zhong, X. Wang, J. Zhang, H. Zhong, and J. B. Han, “Optical detection of magnetic field with Mn4+: K2SiF6 phosphor from room to liquid helium temperatures,” Appl. Phys. Lett. 110(21), 212405 (2017).
28. Q. Zhou, Y. Y. Zhou, Z. L. Wang, G. Chen, J. H. Peng, J. Yan, and M. M. Wu, “A new and efficient red phosphor for solid-state lighting: Cs2TiF6:Mn4+,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3(37), 9615–9619 (2015).
29. X. Z. Wang, H. J. Gao, J. Liu, L. I. Chang-Lin, and Z. X. Liu, “Up-conversion for different substrate by ball milling method,” Faguang Xuebao 35, 312–316 (2014).
30. X. Chen, T. Lu, N. Wei, Z. Lu, W. Zhang, B. Ma, Y. Guan, W. Liu, and F. Jiang, “Effect of ball-milling granulation with PVB adhesive on the sinterability of co-precipitated Yb:YAG nanopowders,” J. Alloys Compd. 589, 448–454 (2014).
31. X. Zhao, H. Chen, M. A. Qin, X. U. Ke, “Effects of ball milling techniques on characteristics of Al2O3/Mo5Si3 composite powder by mechanical alloying,” Powder Metall. Ind. 2, 4 (2011).
32. H. D. Nguyen and R. S. Liu, “Narrow-band red-emitting Mn4+-doped hexafluoride phosphors: synthesis, optoelectronic properties, and applications in white light-emitting diodes,” J. Mater. Chem. C Mater. Opt. Electron. Devices 4, 10759–10775 (2016).
33. L. Xi, Y. Pan, S. Huang, and G. Liu, “Mn4+ doped (NH4)2TiF6 and (NH4)2SiF6 micro-crystal phosphors: synthesis through ion exchange at room temperature and their photoluminescence properties,” RSC Advances 6, 76251–76258 (2016).
