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Mn4+-activated BaSiF6 red phosphor: Hydrothermal synthesis and dependence of its luminescent properties on reaction conditions
Materials Chemistry and Physics xxx (2015) 1e6
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Mn4þ-activated BaSiF6 red phosphor: Hydrothermal synthesis and dependence of its luminescent properties on reaction conditions Qiang Zhou a, Yayun Zhou a, Fengqi Lu b, Yong Liu a, Qin Wang c, Lijun Luo a, Zhengliang Wang a, * a Key Laboratory of Comprehensive Utilization of Mineral Resources in Ethnic Regions, Joint Research Centre for International Cross-border Ethnic Regions Biomass Clean Utilization in Yunnan, School of Chemistry & Environment, Yunnan Minzu University, Kunming, 650500, China b MOE Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, College of Materials Science and Engineering, Guilin University of Technology, Guilin, 541004, PR China c College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming, Yunnan, 650500, PR China
h i g h l i g h t s The crystal structure of BaSiF6:Mn4þ has been verified using Rietveld refinement. The optimum hydrothermal reaction condition for BaSiF6:Mn4þ has been confirmed. The white LED based on YAG:CeeBaSiF6:Mn4þ mixture presents warmer white light than that only with YAG:Ce.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 August 2015 Received in revised form 6 November 2015 Accepted 12 December 2015 Available online xxx
In this work, a series of BaSiF6:Mn4þ red phosphors were synthesized through a hydrothermal route. The crystal structure and morphology were characterized by powder X-ray diffraction (XRD) with Rietveld refinement, scanning electron microscopy (SEM), and energy dispersive spectrometer (EDS) in detail. The influence of reaction conditions, including the concentration of KMnO4 and HF, reaction temperature and time, on the photoluminescence properties were investigated systematically. It can emit intense red light (~636 nm) under blue light (~458 nm) illumination. The white LED device based on YAG:CeeBaSiF6:Mn4þ mixture shows warm white light with low color temperature and high correlated color index, which reveals its potential application in WLED. © 2015 Published by Elsevier B.V.
Keywords: Optical materials Heat treatment Crystal structure Luminescence
1. Introduction Over the past decade, white light-emitting diode (WLED) fabricated with yellowish Y3Al5O12:Ce3þ (YAG:Ce) phosphor has attracted great research interest due to its low energy cost, long lifetime and environmental friendly characteristics [1e3]. The YAG:Ce phosphor can effectively absorbs blue light excited from GaN chip to emit white light [4,5]. However, the white light produced from this system lacks red light component, which causes it “cold” for indoor lighting application [6,7]. To overcome this drawback, considerable efforts have been devoted to developing red phosphors to “neutralize” the “cold” white light by mixing red
* Corresponding author. E-mail address: [email protected] (Z. Wang).
phosphor with YAG:Ce phosphor on GaN chip [8,9]. For instance, rare-earth ions Eu2þ or Ce3þ activated red-emitting phosphors have been developed and applied in WLEDs system [10e12]. However, the quantum yield of these red phosphors is desired to be improved and the preparation process of these phosphors has to suffer harsh synthesis conditions. As a typical transition-metal ion, Mn4þ was also intensively researched as an activator for aforementioned red phosphor since its distinct outer 3d3 electron distributions, which results in its typical 2E / 4A2 transition with broad excitation among blue region, sharp emission in red and near-infrared region [13e15]. Among all the Mn4þ activated phosphors, fluorosilicate with octahedral coordination host, for instance BaSiF6, exhibits intense broad excitation band (~460 nm) and sharp emissions (~630 nm) [16]. Current research on Mn4þ activated red phosphors have been
http://dx.doi.org/10.1016/j.matchemphys.2015.12.015 0254-0584/© 2015 Published by Elsevier B.V.
Please cite this article in press as: Q. Zhou, et al., Mn4þ-activated BaSiF6 red phosphor: Hydrothermal synthesis and dependence of its luminescent properties on reaction conditions, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.12.015
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focused on developing controllable fabrication methods, characterizing the resultant products and exploring their potential applications [17e20]. For example, Pan et al. reported a one-step hydrothermal route for the fabrication of single phase BaSiF6:Mn4þ red phosphor [21]. In this report, they principally demonstrated the chemical reaction mechanism for the preparation of red phosphor. But for the phosphors synthesized by the hydrothermal method, the general synthesis parameters, such as the synthesis temperature, time, and concentration of precursors play an important role on the photo-luminescent (PL) properties of the as-synthesized phosphors. Therefore, from a practical point of view, the synthesis conditions of the Mn4þ activated red phosphor with strong absorption in blue region and emission in red region should be researched and discussed comprehensively. In this work, we present a complete investigation of the synthesis parameters, including the concentration of KMnO4 and HF, synthesis temperature and time, on the photoluminescence properties (PL) of BaSiF6:Mn4þ red phosphors. The composition and morphology were studied. The PL properties of the as-prepared products have been found to be highly dependent on the concentration of HF and KMnO4, and the reaction time. The obtained pink powder phosphors emit intensely red light under blue light illumination. The WLED prepared from YAG:Ce and BaSiF6:Mn4þ mixture shows higher luminous efficiency (LE) and lower correlated color temperature (CT) than that only with YAG:Ce. 2. Experimental section 2.1. Materials and synthesis All starting materials in this work, including HF, Ba(NO3)2, (NH4)2SiF6, KMnO4 and Al(OH)3 were of analytical grade and used without any purification. The grade purity for Y2O3 and CeO2 is 99.99%. The BaSiF6:Mn4þ product was synthesized via a one-pot hydrothermal reaction from Ba(NO3)2 and (NH4)2SiF6 in HF and KMnO4 mixed solutions. In a typical synthesis, 1.314 g Ba(NO3)2, 0.836 g (NH4)2SiF6 and distilled water were mixed thoroughly under magnetic stirring and then transferred into a Teflon cup containing 50 ml HF and KMnO4 mixed solution. The Teflon-lined autoclave was tightly sealed and placed in an oven at a temperature for 8 h. After the autoclave was cooled down naturally to room temperature, the resulting pink solid powders were carefully taken out of the cup, washed with distilled water and methanol for several times, and dried at 60 C for 6 h. Yellow phosphor YAG:Ce was synthesized according to the reference [22]. The stoichiometric mixture of Y2O3, Al(OH)3 and CeO2 were ground and fired at 1300 C for 8 h in reducing atmosphere (N2: H2 ¼ 95: 5). The light-emitting diodes (LEDs) were fabricated by combing GaN chips (~460 nm) with the mixture of phosphors and epoxyresin (the mass ratio is 1:1). 2.2. Characterization The crystal structure of these samples was initially characterized using XRD with a Panalytical X'pert Pro Multi-purpose X-ray diffractometer using Cu Ka radiation (l ¼ 0.15406 nm) and a graphite monochromator operating at 40 kV and 30 mA from 15 to 70 with a scanning step of 0.02 at 4 min1. Rietveld analysis was carried out using Topas Academic software. The as-prepared products for morphologies and structures were observed by SEM (FEI Quanta 200 Thermal FE Environment scanning electron microscopy) with an attached energy-dispersive X-ray spectrometer (EDS). The valence state of a typical product was examined by a Scanning XPS Microprobe system (Phi5000Versaprobe-II, Ulvac-Phi) with an Al target working at 15 kV. PL spectra were recorded on a Cary Eclipse
FL1011M003 (Varian) spectrofluorometer, and the xenon lamp was used as excitation source. The electro-luminescence of LEDs was recorded on a high accurate array spectrometer (HSP6000). All the measurements were performed at room temperature. 3. Results and discussion 3.1. Phase identification and morphology Fig. 1 presents the XRD patterns of the as-synthesized BaSiF6 and BaSiF6:Mn4þ at 180 C for 8 h with different KMnO4 concentration. Curve a is the XRD pattern of BaSiF6 without Mn4þ, which is in accordance with the JCPDS card (No. 15-0736, a ¼ b ¼ 7.185 Å, c ¼ 7.01 Å, space group R-3m). This result shows the obtained sample is of single phase [23]. XRD patterns of the BaSiF6:Mn4þ samples are also similar with that of the BaSiF6, and no impurity peaks can be observed. This indicates that these samples are also of single phase, and the introduction of Mn4þ ion does not change the crystal structure of this BaSiF6 host. To further analyze the crystal structure of these samples, the Rietveld refinements of BaSiF6 and BaSiF6:Mn4þ were performed on Topas Academic software. The XRD patterns of the experimental, calculated and difference results are shown in Fig. 2, meanwhile the final refined structural parameters are summarized in Tables 1 and 2. According to the refinement results, Mn4þ can occupy the crystal site of Si4þ to coordinate with six F anions forming stable MnF2 6 octahedra, which resulted from the same valence state and similar ionic radius between Mn4þ (0.53 Å, CN ¼ 6) and Si4þ (0.40 Å, CN ¼ 6). The unit cell volume of BaSiF6:Mn4þ (313.82 Å3) is a little larger than that of BaSiF6 (313.77 Å3), which is due to the larger radius of the doping Mn4þ ions. The morphology of the as-prepared products was conducted on SEM and representative results are shown in Fig. 3a. It can be easily found that, the product consists of a large number of quasi-uniform micrometer rods, and their surfaces are smooth. Closely inspecting the tips of these rods, each rod has diameter about 1 mm and length around 5 mm. Fig. 3b is the corresponding EDS profile, in which the existence of Mn element can be clearly recognized. This further verifies that Mn4þ ions have occupied the lattice sites of Si4þ ions to activate BaSiF6 emitting red light under blue light illumination. Furthermore, O element cannot be found from the EDS spectrum, which implies that MnO2 is not produced in the hydrothermal reaction.
e Relatively Intensity (a.u.)
2
d c b a
JCPDS 15-0736 (BaSiF6) 20
30
40 50 o 2-Theta ( )
60
70
Fig. 1. XRD patterns of the resulting BaSiF6:Mn4þ products prepared from different KMnO4 concentration: 0 (a), 2 (b), 4 (c), 8 (d) and 12 mmol L1 (e) at 180 C for 6 h.
Please cite this article in press as: Q. Zhou, et al., Mn4þ-activated BaSiF6 red phosphor: Hydrothermal synthesis and dependence of its luminescent properties on reaction conditions, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.12.015
Q. Zhou et al. / Materials Chemistry and Physics xxx (2015) 1e6
3
Fig. 2. Experimental (crosses) and calculated (red solid line) XRD patterns and their difference (black solid line) for BaSiF6 (a) and BaSiF6:Mn4þ (b) at room temperature. The blue ticks indicate the Bragg reflection positions. The inset figures shows the projections viewed from [010] (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Table 1 Crystal data of BaSiF6 from the Rietveld refinement. atom
site
x
y
z
occupancy
Uiso (Å2)
Ba Si F
3a 3b 18h
0 0 0.2182(2)
0 0 0.1091(1)
0 1/2 0.3587(2)
1 1 1
0.73(3) 0.84(5) 0.99(4)
Space group: R3 m, a ¼ 7.1913(0) Å, c ¼ 7.0058(1) Å, a ¼ b ¼ 90 , g ¼ 120 , Z ¼ 3, cell volume: 313.77(0) Å3. Rwp ~4.71%, Rp ~3.27%, RBragg ~1.12%.
Table 2 Crystal data of BaSiF6:Mn4þ from the Rietveld refinement. atom
site
x
y
z
occupancy
Uiso (Å2)
Ba Si Mn F
3a 3b 3b 18h
0 0 0 0.2190(0)
0 0 0 0.1095(0)
0 1/2 1/2 0.3584(2)
1 0.92(0) 0.08(0) 1
0.63(2) 1.24(6) 1.24(6) 0.90(4)
Space group: R3 m, a ¼ 7.1912(0) Å, c ¼ 7.0072(0) Å, a ¼ b ¼ 90 , g ¼ 120 , Z ¼ 3, cell volume: 313.82(0) Å3. Rwp ~4.07%, Rp ~2.91%, RBragg ~1.22%.
3.2. Influence of reaction conditions on PL properties To comprehensively investigate the influence of synthesis parameters, such as the concentration of KMnO4 and HF, reaction
temperature and time, on the PL properties of BaSiF6:Mn4þ products, a series of experiments have been done. Fig. 4 exhibits the excitation and emission spectra of BaSiF6:Mn4þ red phosphor fabricated from different KMnO4 concentration (0.1, 0.5, 2, 4, 6, 8, 10, 12 mmol L1). In Fig 4a, it can be easily found that the shape of these excitation spectra is similar, which were consisted of two broadband excitation peaks locating in the UV (~356 nm) and blue (~458 nm) region respectively. According to the TanabeeSugano diagram, they are attributed to the spin-allowed transitions from ground state 4A2 to excited state 4T2 and 4T1 of Mn4þ respectively [24]. It should be noted that the stronger excitation band matches very well with the blue emission of GaN chip (~460 nm). Fig. 4b shows their corresponding emission spectra under 458 nm light excitation. There are three emission peaks located in the range from 600 to 660 nm, which strongly indicates that the emitted light is red. The first peak (~612 nm) is due to anti-Stokes vibronic side bands of the excited state 2E of Mn4þ and the latter (~636 nm and 651 nm) peaks are identified as its 2Eg / 4A2 transition [25], in which the middle emission peak locating at 636 nm is the strongest. Moreover, Fig 4b displays the influence of KMnO4 concentration on the emission property of BaSiF6:Mn4þ phosphors. It can be clearly observed that the emission intensity increases with the KMnO4 concentration until it reaches 8 mmol L1, at which the BaSiF6:Mn4þ phosphor emits the strongest red light. Its CIE
Fig. 3. SEM image (a) and EDS profile (b) of the as-prepared BaSiF6:Mn4þ product.
Please cite this article in press as: Q. Zhou, et al., Mn4þ-activated BaSiF6 red phosphor: Hydrothermal synthesis and dependence of its luminescent properties on reaction conditions, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.12.015
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a
600
12 mmol.L-1 8 mmol.L-1 4 mmol.L-1 2 mmol.L-1 0.5 mmol.L-1 0.1 mmol.L-1
500 Intensity (a.u.)
λem = 636 nm
400 300 200 100 0 300
350
400 450 500 Wavelength (nm)
550
600
Fig. 5. Emission spectrum of red phosphor BaSiF6:Mn4þ obtained at 180 C for 6 h with 8 mmol,L1 KMnO4 and 10% (a), 20% (b), 30% (c) and 40% (d) HF, the inserted figure is the relationship between HF concentration and relative emission intensity (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
500
Intensity (a.u.)
400 300
-1
12 mmol.L -1 8 mmol.L -1 4 mmol.L -1 2 mmol.L -1 0.5 mmol.L -1 0.1 mmol.L
Intensity (a.u.)
b λex = 458 nm
Content of KMnO mmol.L-1
200 100 0 575
600
625 650 Wavelength (nm)
675
700
Fig. 4. The excitation (a) and emission (b) spectra of BaSiF6:Mn4þ red phosphors obtained from 40% HF with different concentration of KMnO4 at 180 C for 6 h, insert is the relationship between KMnO4 concentration and relative emission intensity (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
(Commission Internationale de l’Eclairage, International Commission on Illumination) chromaticity coordinates are x ¼ 0.64, y ¼ 0.30, which are close to “ideal red” of the NTSC (National Television Standard Committee, x ¼ 0.67, y ¼ 0.33). Continually increase KMnO4 concentration to 12 mmol L1, the emission contrarily dropped, this is an obvious concentration quenching phenomenon in BaSiF6 crystal lattice [26]. Relationship between the KMnO4 concentration and relative emission intensity is inserted in Fig. 4b. Different HF concentration, from 10% to 40% was employed to further investigate the luminescent property of BaSiF6:Mn4þ. Fig. 5 shows a series of emission spectra of these BaSiF6:Mn4þ phosphors under 458 nm light excitation. When a low concentration, 10% is employed, only a negligible emission peak at 636 nm can be observed. However, with the addition of HF, the emission intensity at 636 nm sharply increased. When the highest concentration, 40% HF, is used, the emission intensity reaches strongest, which is about 104, 30, 2.5 times higher than that of other three samples. That is because higher HF concentration is beneficial for the formation of
stable MnF2 6 group. Otherwise, it may be hydrolyzed into MnO2, which results in the product color dark red and emission intensity decreasing. This phenomenon suggests that the emission property of this BaSiF6:Mn4þ red phosphor is highly dependent on the employed HF concentration, which plays an important role in obtaining high brightness BaSiF6:Mn4þ red phosphor. The relationship between HF concentration and relative emission intensity is inserted in Fig. 5. In the above section, we concluded the optimum concentration of KMnO4 and HF for the fabrication of BaSiF6:Mn4þ red phosphor. However, for a hydrothermal synthesis, reaction temperature and time also play an important role in obtaining high brightness red phosphor. Fig. 6 exhibits the effect of reaction temperature on the emission intensity of BaSiF6:Mn4þ with the optimum concentration of starting materials. Obviously, when the reaction temperature is higher than 100 C, the emission intensities of these red phosphors are very close. This means the synthesis temperature influenced mildly on the formation of BaSiF6:Mn4þ crystals when the synthesis temperature is higher than the boiling point of the reaction mixture. However, when a lower temperature, such as 90 C was adopted, the emission intensity diminished sharply. This may because low reaction temperature (<100 C) is not favorable for the substitution of Mn4þ for Si4þ ions. Similarly, Fig. 7 shows influence of reaction time (2 h, 6 h, 12 h and 20 h) on the emission property of BaSiF6:Mn4þ phosphor. With the increase of the reaction time from 2 h to 6 h, the emission intensity rises sharply, which means that longer reaction time leads to the stronger red light intensity. Continually extended the reaction time, the emission intensity dropped dramatically. Therefore, according to the above investigation, the optimum reaction condition is: 8 mmol L1 KMnO4, 40% HF, reacted at 180 C for 6 h. Furthermore, the influence of temperature on the PL properties of this optimum product is shown in Fig. 8. Obviously, with the temperature increasing, it can be found that the emission peak position does not shift. Up to 160 C, over 100% of the integral emission intensity can be preserved compared with that at 20 C. Actually, the BaSiF6:Mn4þ red phosphor is very stable chemically and it cannot be decomposed until 457 C (Fig. 9).
Please cite this article in press as: Q. Zhou, et al., Mn4þ-activated BaSiF6 red phosphor: Hydrothermal synthesis and dependence of its luminescent properties on reaction conditions, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.12.015
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5
350
o
20 C o 40 C o 60 C o 80 C o 100 C o 120 C o 140 C o 160 C o 180 C o 200 C
300
Intensity (a.u.)
250 200 150 100 50 0 575
600
625
650
675
Wavelength (nm) Fig. 6. Emission spectrum of BaSiF6:Mn4þ red phosphors obtained from 40% HF and 8 mmol,L1 KMnO4 for at 90 C (a), 100 C (b), 150 C (c), 180 C (d) and 200 C for 6 h, the inserted figure is the relationship between reaction temperature and relative emission intensity (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
λex = 458 nm
500
Intensity (a.u.)
400
2.0 h 6.0 h 12.0 h 24.0 h
YAG:Ce, and the YAG:CeeBaSiF6:Mn4þ mixture. The obvious red emission on Fig. 10d means that the addition of BaSiF6:Mn4þ red phosphor is favorable for obtaining a warm WLED. The performance of the WLEDs is summarized in Table 3. It should be noted that the colour rendering index (CRI) and correlated colour temperature (CCT) levels of the prepared WLED were obviously improved after the addition of BaSiF6:Mn4þ red phosphor.
Intensity (a.u.)
600
Fig. 8. Temperature-dependent thermal luminescence spectra of the BaSiF6:Mn4þ red phosphor and the relative intensity of the emission spectrum by integrating the spectral area (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Hydrothermal time / h
4. Conclusion
300
In summary, a series of BaSiF6:Mn4þ red phosphors with microrod morphology were synthesized by a hydrothermal route. The influence of synthesis parameters, including KMnO4 and HF concentration, the synthesis temperature and reaction time on its PL properties has been systematically investigated. The optimized reaction condition is: 8 mmol$L1 KMnO4 and 40% HF, reacted at
200 100 0 575
600
625 650 Wavelength (nm)
675
700 105
Fig. 7. Emission spectrum of BaSiF6:Mn4þ red phosphors obtained from 40% HF and 8 mmol,L1 KMnO4 at 180 C for 2 h (a), 6 h (b), 12 h (c) and 24 h (d). The inserted figure is the relationship between reaction temperature and relative emission intensity (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 10 displays the electro-luminescent spectra (EL) of the GaN chip, the LED based on BaSiF6:Mn4þ, YAG:Ce, YAG:CeeBaSiF6:Mn4þ mixture under 20 mA current excitation. The emission band at ~460 nm of curve a can be attributable to the emission of GaN chip. Compared with curve a, the emission peaks at 636 nm in curve b is due to the emission of red phosphor excited by the emission of GaN LED chip. Bright red light (seeing the insert of Fig. 10b) and the emission of GaN chip can be observed by naked eyes, this is beneficial for obtaining a WLED by merging it with YAG:Ce phosphor. Curve c and d are the EL spectra of the WLEDs fabricated from
95 90
Weight (%)
3.3. Application in WLEDs
100
85 80 75 70 65 60 100
200
300
400
500
600
700
o
Temperature ( C) Fig. 9. Thermogravimetrics (TG) as synthesized BaSiF6:Mn4þ under N2 atmosphere. The thermal stability the red phosphor behavior of BaSiF6:Mn4þ is investigated by thermogravimetrics analysis (TG: PerkinElmer STA 8000, at a heating rate of 10 K/min).
Please cite this article in press as: Q. Zhou, et al., Mn4þ-activated BaSiF6 red phosphor: Hydrothermal synthesis and dependence of its luminescent properties on reaction conditions, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.12.015
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Q. Zhou et al. / Materials Chemistry and Physics xxx (2015) 1e6
Science Foundation of China (21261027), the Natural Science Foundation of Yunnan Province, (2014FB147), Graduate Innovation Foundation of Yunnan Minzu University (2015YJCXY277), and Program for Innovative Research Team (in Science and Technology) in University of Yunnan Province (2011UY09) and Yunnan Provincial Innovation Team (2011HC008).
References
Fig. 10. Electro-luminescent spectra of the GaN LED chip (a), the LED based on BaSiF6:Mn4þ (b), YAG:Ce (c) and BaSiF6:Mn4þ-YAG:Ce mixture (d) under 20 mA current excitation, the inserted photograph are the images of the LEDs (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Table 3 Performance of the GaN-based WLEDs coated with: (1) YAG:Ce, (2) YAG:Ce and BaSiF6:Mn4þ at 20 mA forward current and 5 V reverse voltage. No. of LEDs samples
CCT(K)
CRI
LE (lm/W)
CIE (x, y)
1 2
6283 5903
76.0 82
45.21 51.73
(0.313, 0.366) (0.323,0.359)
180 C for 6 h. The resulting BaSiF6:Mn4þ phosphor shows intense red emission with broadband excitation in blue region. The WLED based on this BaSiF6:Mn4þ red phosphor presents warmer white light than that of the only one YAG:Ce component. Therefore, it is considered as a good candidate for improving the optical performance of indoor lighting WLEDs. Acknowledgments This work was financially supported by the National Natural
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Please cite this article in press as: Q. Zhou, et al., Mn4þ-activated BaSiF6 red phosphor: Hydrothermal synthesis and dependence of its luminescent properties on reaction conditions, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.12.015
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Mn4þ-activated BaSiF6 red phosphor: Hydrothermal synthesis and dependence of its luminescent properties on reaction conditions Qiang Zhou a, Yayun Zhou a, Fengqi Lu b, Yong Liu a, Qin Wang c, Lijun Luo a, Zhengliang Wang a, * a Key Laboratory of Comprehensive Utilization of Mineral Resources in Ethnic Regions, Joint Research Centre for International Cross-border Ethnic Regions Biomass Clean Utilization in Yunnan, School of Chemistry & Environment, Yunnan Minzu University, Kunming, 650500, China b MOE Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, College of Materials Science and Engineering, Guilin University of Technology, Guilin, 541004, PR China c College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming, Yunnan, 650500, PR China
h i g h l i g h t s The crystal structure of BaSiF6:Mn4þ has been verified using Rietveld refinement. The optimum hydrothermal reaction condition for BaSiF6:Mn4þ has been confirmed. The white LED based on YAG:CeeBaSiF6:Mn4þ mixture presents warmer white light than that only with YAG:Ce.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 August 2015 Received in revised form 6 November 2015 Accepted 12 December 2015 Available online xxx
In this work, a series of BaSiF6:Mn4þ red phosphors were synthesized through a hydrothermal route. The crystal structure and morphology were characterized by powder X-ray diffraction (XRD) with Rietveld refinement, scanning electron microscopy (SEM), and energy dispersive spectrometer (EDS) in detail. The influence of reaction conditions, including the concentration of KMnO4 and HF, reaction temperature and time, on the photoluminescence properties were investigated systematically. It can emit intense red light (~636 nm) under blue light (~458 nm) illumination. The white LED device based on YAG:CeeBaSiF6:Mn4þ mixture shows warm white light with low color temperature and high correlated color index, which reveals its potential application in WLED. © 2015 Published by Elsevier B.V.
Keywords: Optical materials Heat treatment Crystal structure Luminescence
1. Introduction Over the past decade, white light-emitting diode (WLED) fabricated with yellowish Y3Al5O12:Ce3þ (YAG:Ce) phosphor has attracted great research interest due to its low energy cost, long lifetime and environmental friendly characteristics [1e3]. The YAG:Ce phosphor can effectively absorbs blue light excited from GaN chip to emit white light [4,5]. However, the white light produced from this system lacks red light component, which causes it “cold” for indoor lighting application [6,7]. To overcome this drawback, considerable efforts have been devoted to developing red phosphors to “neutralize” the “cold” white light by mixing red
* Corresponding author. E-mail address: [email protected] (Z. Wang).
phosphor with YAG:Ce phosphor on GaN chip [8,9]. For instance, rare-earth ions Eu2þ or Ce3þ activated red-emitting phosphors have been developed and applied in WLEDs system [10e12]. However, the quantum yield of these red phosphors is desired to be improved and the preparation process of these phosphors has to suffer harsh synthesis conditions. As a typical transition-metal ion, Mn4þ was also intensively researched as an activator for aforementioned red phosphor since its distinct outer 3d3 electron distributions, which results in its typical 2E / 4A2 transition with broad excitation among blue region, sharp emission in red and near-infrared region [13e15]. Among all the Mn4þ activated phosphors, fluorosilicate with octahedral coordination host, for instance BaSiF6, exhibits intense broad excitation band (~460 nm) and sharp emissions (~630 nm) [16]. Current research on Mn4þ activated red phosphors have been
http://dx.doi.org/10.1016/j.matchemphys.2015.12.015 0254-0584/© 2015 Published by Elsevier B.V.
Please cite this article in press as: Q. Zhou, et al., Mn4þ-activated BaSiF6 red phosphor: Hydrothermal synthesis and dependence of its luminescent properties on reaction conditions, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.12.015
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focused on developing controllable fabrication methods, characterizing the resultant products and exploring their potential applications [17e20]. For example, Pan et al. reported a one-step hydrothermal route for the fabrication of single phase BaSiF6:Mn4þ red phosphor [21]. In this report, they principally demonstrated the chemical reaction mechanism for the preparation of red phosphor. But for the phosphors synthesized by the hydrothermal method, the general synthesis parameters, such as the synthesis temperature, time, and concentration of precursors play an important role on the photo-luminescent (PL) properties of the as-synthesized phosphors. Therefore, from a practical point of view, the synthesis conditions of the Mn4þ activated red phosphor with strong absorption in blue region and emission in red region should be researched and discussed comprehensively. In this work, we present a complete investigation of the synthesis parameters, including the concentration of KMnO4 and HF, synthesis temperature and time, on the photoluminescence properties (PL) of BaSiF6:Mn4þ red phosphors. The composition and morphology were studied. The PL properties of the as-prepared products have been found to be highly dependent on the concentration of HF and KMnO4, and the reaction time. The obtained pink powder phosphors emit intensely red light under blue light illumination. The WLED prepared from YAG:Ce and BaSiF6:Mn4þ mixture shows higher luminous efficiency (LE) and lower correlated color temperature (CT) than that only with YAG:Ce. 2. Experimental section 2.1. Materials and synthesis All starting materials in this work, including HF, Ba(NO3)2, (NH4)2SiF6, KMnO4 and Al(OH)3 were of analytical grade and used without any purification. The grade purity for Y2O3 and CeO2 is 99.99%. The BaSiF6:Mn4þ product was synthesized via a one-pot hydrothermal reaction from Ba(NO3)2 and (NH4)2SiF6 in HF and KMnO4 mixed solutions. In a typical synthesis, 1.314 g Ba(NO3)2, 0.836 g (NH4)2SiF6 and distilled water were mixed thoroughly under magnetic stirring and then transferred into a Teflon cup containing 50 ml HF and KMnO4 mixed solution. The Teflon-lined autoclave was tightly sealed and placed in an oven at a temperature for 8 h. After the autoclave was cooled down naturally to room temperature, the resulting pink solid powders were carefully taken out of the cup, washed with distilled water and methanol for several times, and dried at 60 C for 6 h. Yellow phosphor YAG:Ce was synthesized according to the reference [22]. The stoichiometric mixture of Y2O3, Al(OH)3 and CeO2 were ground and fired at 1300 C for 8 h in reducing atmosphere (N2: H2 ¼ 95: 5). The light-emitting diodes (LEDs) were fabricated by combing GaN chips (~460 nm) with the mixture of phosphors and epoxyresin (the mass ratio is 1:1). 2.2. Characterization The crystal structure of these samples was initially characterized using XRD with a Panalytical X'pert Pro Multi-purpose X-ray diffractometer using Cu Ka radiation (l ¼ 0.15406 nm) and a graphite monochromator operating at 40 kV and 30 mA from 15 to 70 with a scanning step of 0.02 at 4 min1. Rietveld analysis was carried out using Topas Academic software. The as-prepared products for morphologies and structures were observed by SEM (FEI Quanta 200 Thermal FE Environment scanning electron microscopy) with an attached energy-dispersive X-ray spectrometer (EDS). The valence state of a typical product was examined by a Scanning XPS Microprobe system (Phi5000Versaprobe-II, Ulvac-Phi) with an Al target working at 15 kV. PL spectra were recorded on a Cary Eclipse
FL1011M003 (Varian) spectrofluorometer, and the xenon lamp was used as excitation source. The electro-luminescence of LEDs was recorded on a high accurate array spectrometer (HSP6000). All the measurements were performed at room temperature. 3. Results and discussion 3.1. Phase identification and morphology Fig. 1 presents the XRD patterns of the as-synthesized BaSiF6 and BaSiF6:Mn4þ at 180 C for 8 h with different KMnO4 concentration. Curve a is the XRD pattern of BaSiF6 without Mn4þ, which is in accordance with the JCPDS card (No. 15-0736, a ¼ b ¼ 7.185 Å, c ¼ 7.01 Å, space group R-3m). This result shows the obtained sample is of single phase [23]. XRD patterns of the BaSiF6:Mn4þ samples are also similar with that of the BaSiF6, and no impurity peaks can be observed. This indicates that these samples are also of single phase, and the introduction of Mn4þ ion does not change the crystal structure of this BaSiF6 host. To further analyze the crystal structure of these samples, the Rietveld refinements of BaSiF6 and BaSiF6:Mn4þ were performed on Topas Academic software. The XRD patterns of the experimental, calculated and difference results are shown in Fig. 2, meanwhile the final refined structural parameters are summarized in Tables 1 and 2. According to the refinement results, Mn4þ can occupy the crystal site of Si4þ to coordinate with six F anions forming stable MnF2 6 octahedra, which resulted from the same valence state and similar ionic radius between Mn4þ (0.53 Å, CN ¼ 6) and Si4þ (0.40 Å, CN ¼ 6). The unit cell volume of BaSiF6:Mn4þ (313.82 Å3) is a little larger than that of BaSiF6 (313.77 Å3), which is due to the larger radius of the doping Mn4þ ions. The morphology of the as-prepared products was conducted on SEM and representative results are shown in Fig. 3a. It can be easily found that, the product consists of a large number of quasi-uniform micrometer rods, and their surfaces are smooth. Closely inspecting the tips of these rods, each rod has diameter about 1 mm and length around 5 mm. Fig. 3b is the corresponding EDS profile, in which the existence of Mn element can be clearly recognized. This further verifies that Mn4þ ions have occupied the lattice sites of Si4þ ions to activate BaSiF6 emitting red light under blue light illumination. Furthermore, O element cannot be found from the EDS spectrum, which implies that MnO2 is not produced in the hydrothermal reaction.
e Relatively Intensity (a.u.)
2
d c b a
JCPDS 15-0736 (BaSiF6) 20
30
40 50 o 2-Theta ( )
60
70
Fig. 1. XRD patterns of the resulting BaSiF6:Mn4þ products prepared from different KMnO4 concentration: 0 (a), 2 (b), 4 (c), 8 (d) and 12 mmol L1 (e) at 180 C for 6 h.
Please cite this article in press as: Q. Zhou, et al., Mn4þ-activated BaSiF6 red phosphor: Hydrothermal synthesis and dependence of its luminescent properties on reaction conditions, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.12.015
Q. Zhou et al. / Materials Chemistry and Physics xxx (2015) 1e6
3
Fig. 2. Experimental (crosses) and calculated (red solid line) XRD patterns and their difference (black solid line) for BaSiF6 (a) and BaSiF6:Mn4þ (b) at room temperature. The blue ticks indicate the Bragg reflection positions. The inset figures shows the projections viewed from [010] (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Table 1 Crystal data of BaSiF6 from the Rietveld refinement. atom
site
x
y
z
occupancy
Uiso (Å2)
Ba Si F
3a 3b 18h
0 0 0.2182(2)
0 0 0.1091(1)
0 1/2 0.3587(2)
1 1 1
0.73(3) 0.84(5) 0.99(4)
Space group: R3 m, a ¼ 7.1913(0) Å, c ¼ 7.0058(1) Å, a ¼ b ¼ 90 , g ¼ 120 , Z ¼ 3, cell volume: 313.77(0) Å3. Rwp ~4.71%, Rp ~3.27%, RBragg ~1.12%.
Table 2 Crystal data of BaSiF6:Mn4þ from the Rietveld refinement. atom
site
x
y
z
occupancy
Uiso (Å2)
Ba Si Mn F
3a 3b 3b 18h
0 0 0 0.2190(0)
0 0 0 0.1095(0)
0 1/2 1/2 0.3584(2)
1 0.92(0) 0.08(0) 1
0.63(2) 1.24(6) 1.24(6) 0.90(4)
Space group: R3 m, a ¼ 7.1912(0) Å, c ¼ 7.0072(0) Å, a ¼ b ¼ 90 , g ¼ 120 , Z ¼ 3, cell volume: 313.82(0) Å3. Rwp ~4.07%, Rp ~2.91%, RBragg ~1.22%.
3.2. Influence of reaction conditions on PL properties To comprehensively investigate the influence of synthesis parameters, such as the concentration of KMnO4 and HF, reaction
temperature and time, on the PL properties of BaSiF6:Mn4þ products, a series of experiments have been done. Fig. 4 exhibits the excitation and emission spectra of BaSiF6:Mn4þ red phosphor fabricated from different KMnO4 concentration (0.1, 0.5, 2, 4, 6, 8, 10, 12 mmol L1). In Fig 4a, it can be easily found that the shape of these excitation spectra is similar, which were consisted of two broadband excitation peaks locating in the UV (~356 nm) and blue (~458 nm) region respectively. According to the TanabeeSugano diagram, they are attributed to the spin-allowed transitions from ground state 4A2 to excited state 4T2 and 4T1 of Mn4þ respectively [24]. It should be noted that the stronger excitation band matches very well with the blue emission of GaN chip (~460 nm). Fig. 4b shows their corresponding emission spectra under 458 nm light excitation. There are three emission peaks located in the range from 600 to 660 nm, which strongly indicates that the emitted light is red. The first peak (~612 nm) is due to anti-Stokes vibronic side bands of the excited state 2E of Mn4þ and the latter (~636 nm and 651 nm) peaks are identified as its 2Eg / 4A2 transition [25], in which the middle emission peak locating at 636 nm is the strongest. Moreover, Fig 4b displays the influence of KMnO4 concentration on the emission property of BaSiF6:Mn4þ phosphors. It can be clearly observed that the emission intensity increases with the KMnO4 concentration until it reaches 8 mmol L1, at which the BaSiF6:Mn4þ phosphor emits the strongest red light. Its CIE
Fig. 3. SEM image (a) and EDS profile (b) of the as-prepared BaSiF6:Mn4þ product.
Please cite this article in press as: Q. Zhou, et al., Mn4þ-activated BaSiF6 red phosphor: Hydrothermal synthesis and dependence of its luminescent properties on reaction conditions, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.12.015
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a
600
12 mmol.L-1 8 mmol.L-1 4 mmol.L-1 2 mmol.L-1 0.5 mmol.L-1 0.1 mmol.L-1
500 Intensity (a.u.)
λem = 636 nm
400 300 200 100 0 300
350
400 450 500 Wavelength (nm)
550
600
Fig. 5. Emission spectrum of red phosphor BaSiF6:Mn4þ obtained at 180 C for 6 h with 8 mmol,L1 KMnO4 and 10% (a), 20% (b), 30% (c) and 40% (d) HF, the inserted figure is the relationship between HF concentration and relative emission intensity (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
500
Intensity (a.u.)
400 300
-1
12 mmol.L -1 8 mmol.L -1 4 mmol.L -1 2 mmol.L -1 0.5 mmol.L -1 0.1 mmol.L
Intensity (a.u.)
b λex = 458 nm
Content of KMnO mmol.L-1
200 100 0 575
600
625 650 Wavelength (nm)
675
700
Fig. 4. The excitation (a) and emission (b) spectra of BaSiF6:Mn4þ red phosphors obtained from 40% HF with different concentration of KMnO4 at 180 C for 6 h, insert is the relationship between KMnO4 concentration and relative emission intensity (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
(Commission Internationale de l’Eclairage, International Commission on Illumination) chromaticity coordinates are x ¼ 0.64, y ¼ 0.30, which are close to “ideal red” of the NTSC (National Television Standard Committee, x ¼ 0.67, y ¼ 0.33). Continually increase KMnO4 concentration to 12 mmol L1, the emission contrarily dropped, this is an obvious concentration quenching phenomenon in BaSiF6 crystal lattice [26]. Relationship between the KMnO4 concentration and relative emission intensity is inserted in Fig. 4b. Different HF concentration, from 10% to 40% was employed to further investigate the luminescent property of BaSiF6:Mn4þ. Fig. 5 shows a series of emission spectra of these BaSiF6:Mn4þ phosphors under 458 nm light excitation. When a low concentration, 10% is employed, only a negligible emission peak at 636 nm can be observed. However, with the addition of HF, the emission intensity at 636 nm sharply increased. When the highest concentration, 40% HF, is used, the emission intensity reaches strongest, which is about 104, 30, 2.5 times higher than that of other three samples. That is because higher HF concentration is beneficial for the formation of
stable MnF2 6 group. Otherwise, it may be hydrolyzed into MnO2, which results in the product color dark red and emission intensity decreasing. This phenomenon suggests that the emission property of this BaSiF6:Mn4þ red phosphor is highly dependent on the employed HF concentration, which plays an important role in obtaining high brightness BaSiF6:Mn4þ red phosphor. The relationship between HF concentration and relative emission intensity is inserted in Fig. 5. In the above section, we concluded the optimum concentration of KMnO4 and HF for the fabrication of BaSiF6:Mn4þ red phosphor. However, for a hydrothermal synthesis, reaction temperature and time also play an important role in obtaining high brightness red phosphor. Fig. 6 exhibits the effect of reaction temperature on the emission intensity of BaSiF6:Mn4þ with the optimum concentration of starting materials. Obviously, when the reaction temperature is higher than 100 C, the emission intensities of these red phosphors are very close. This means the synthesis temperature influenced mildly on the formation of BaSiF6:Mn4þ crystals when the synthesis temperature is higher than the boiling point of the reaction mixture. However, when a lower temperature, such as 90 C was adopted, the emission intensity diminished sharply. This may because low reaction temperature (<100 C) is not favorable for the substitution of Mn4þ for Si4þ ions. Similarly, Fig. 7 shows influence of reaction time (2 h, 6 h, 12 h and 20 h) on the emission property of BaSiF6:Mn4þ phosphor. With the increase of the reaction time from 2 h to 6 h, the emission intensity rises sharply, which means that longer reaction time leads to the stronger red light intensity. Continually extended the reaction time, the emission intensity dropped dramatically. Therefore, according to the above investigation, the optimum reaction condition is: 8 mmol L1 KMnO4, 40% HF, reacted at 180 C for 6 h. Furthermore, the influence of temperature on the PL properties of this optimum product is shown in Fig. 8. Obviously, with the temperature increasing, it can be found that the emission peak position does not shift. Up to 160 C, over 100% of the integral emission intensity can be preserved compared with that at 20 C. Actually, the BaSiF6:Mn4þ red phosphor is very stable chemically and it cannot be decomposed until 457 C (Fig. 9).
Please cite this article in press as: Q. Zhou, et al., Mn4þ-activated BaSiF6 red phosphor: Hydrothermal synthesis and dependence of its luminescent properties on reaction conditions, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.12.015
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5
350
o
20 C o 40 C o 60 C o 80 C o 100 C o 120 C o 140 C o 160 C o 180 C o 200 C
300
Intensity (a.u.)
250 200 150 100 50 0 575
600
625
650
675
Wavelength (nm) Fig. 6. Emission spectrum of BaSiF6:Mn4þ red phosphors obtained from 40% HF and 8 mmol,L1 KMnO4 for at 90 C (a), 100 C (b), 150 C (c), 180 C (d) and 200 C for 6 h, the inserted figure is the relationship between reaction temperature and relative emission intensity (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
λex = 458 nm
500
Intensity (a.u.)
400
2.0 h 6.0 h 12.0 h 24.0 h
YAG:Ce, and the YAG:CeeBaSiF6:Mn4þ mixture. The obvious red emission on Fig. 10d means that the addition of BaSiF6:Mn4þ red phosphor is favorable for obtaining a warm WLED. The performance of the WLEDs is summarized in Table 3. It should be noted that the colour rendering index (CRI) and correlated colour temperature (CCT) levels of the prepared WLED were obviously improved after the addition of BaSiF6:Mn4þ red phosphor.
Intensity (a.u.)
600
Fig. 8. Temperature-dependent thermal luminescence spectra of the BaSiF6:Mn4þ red phosphor and the relative intensity of the emission spectrum by integrating the spectral area (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Hydrothermal time / h
4. Conclusion
300
In summary, a series of BaSiF6:Mn4þ red phosphors with microrod morphology were synthesized by a hydrothermal route. The influence of synthesis parameters, including KMnO4 and HF concentration, the synthesis temperature and reaction time on its PL properties has been systematically investigated. The optimized reaction condition is: 8 mmol$L1 KMnO4 and 40% HF, reacted at
200 100 0 575
600
625 650 Wavelength (nm)
675
700 105
Fig. 7. Emission spectrum of BaSiF6:Mn4þ red phosphors obtained from 40% HF and 8 mmol,L1 KMnO4 at 180 C for 2 h (a), 6 h (b), 12 h (c) and 24 h (d). The inserted figure is the relationship between reaction temperature and relative emission intensity (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 10 displays the electro-luminescent spectra (EL) of the GaN chip, the LED based on BaSiF6:Mn4þ, YAG:Ce, YAG:CeeBaSiF6:Mn4þ mixture under 20 mA current excitation. The emission band at ~460 nm of curve a can be attributable to the emission of GaN chip. Compared with curve a, the emission peaks at 636 nm in curve b is due to the emission of red phosphor excited by the emission of GaN LED chip. Bright red light (seeing the insert of Fig. 10b) and the emission of GaN chip can be observed by naked eyes, this is beneficial for obtaining a WLED by merging it with YAG:Ce phosphor. Curve c and d are the EL spectra of the WLEDs fabricated from
95 90
Weight (%)
3.3. Application in WLEDs
100
85 80 75 70 65 60 100
200
300
400
500
600
700
o
Temperature ( C) Fig. 9. Thermogravimetrics (TG) as synthesized BaSiF6:Mn4þ under N2 atmosphere. The thermal stability the red phosphor behavior of BaSiF6:Mn4þ is investigated by thermogravimetrics analysis (TG: PerkinElmer STA 8000, at a heating rate of 10 K/min).
Please cite this article in press as: Q. Zhou, et al., Mn4þ-activated BaSiF6 red phosphor: Hydrothermal synthesis and dependence of its luminescent properties on reaction conditions, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.12.015
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Q. Zhou et al. / Materials Chemistry and Physics xxx (2015) 1e6
Science Foundation of China (21261027), the Natural Science Foundation of Yunnan Province, (2014FB147), Graduate Innovation Foundation of Yunnan Minzu University (2015YJCXY277), and Program for Innovative Research Team (in Science and Technology) in University of Yunnan Province (2011UY09) and Yunnan Provincial Innovation Team (2011HC008).
References
Fig. 10. Electro-luminescent spectra of the GaN LED chip (a), the LED based on BaSiF6:Mn4þ (b), YAG:Ce (c) and BaSiF6:Mn4þ-YAG:Ce mixture (d) under 20 mA current excitation, the inserted photograph are the images of the LEDs (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Table 3 Performance of the GaN-based WLEDs coated with: (1) YAG:Ce, (2) YAG:Ce and BaSiF6:Mn4þ at 20 mA forward current and 5 V reverse voltage. No. of LEDs samples
CCT(K)
CRI
LE (lm/W)
CIE (x, y)
1 2
6283 5903
76.0 82
45.21 51.73
(0.313, 0.366) (0.323,0.359)
180 C for 6 h. The resulting BaSiF6:Mn4þ phosphor shows intense red emission with broadband excitation in blue region. The WLED based on this BaSiF6:Mn4þ red phosphor presents warmer white light than that of the only one YAG:Ce component. Therefore, it is considered as a good candidate for improving the optical performance of indoor lighting WLEDs. Acknowledgments This work was financially supported by the National Natural
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Please cite this article in press as: Q. Zhou, et al., Mn4þ-activated BaSiF6 red phosphor: Hydrothermal synthesis and dependence of its luminescent properties on reaction conditions, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.12.015