Hydrothermal synthesis and luminescent properties of BaTiF6:Mn4+ red phosphor for LED backlighting

Hydrothermal synthesis and luminescent properties of BaTiF6:Mn4+ red phosphor for LED backlighting

Materials Research Bulletin 73 (2016) 14–20 Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.elsevier.com...

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Materials Research Bulletin 73 (2016) 14–20

Contents lists available at ScienceDirect

Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu

Hydrothermal synthesis and luminescent properties of BaTiF6:Mn4+ red phosphor for LED backlighting Yayun Zhoua , Qiang Zhoua , Yong Liua , Zhengliang Wanga,* , Hui Yanga , Qin Wangb,* a Engineering Research Center of Biopolymer Functional Materials of Yunnan, Key Laboratory of Comprehensive Utilization of Mineral Resources in Ethnic Regions, Key Laboratory of Resource Clean Conversion in Ethnic Regions, Education Department of Yunnan, School of Chemistry & Environment, Kunming, Yunnan 650500, PR China b College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming, Yunnan 650500, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 19 January 2015 Received in revised form 4 August 2015 Accepted 19 August 2015 Available online 24 August 2015

The red-emitting phosphors BaTiF6:Mn4+ (denoted as BTFM) have been synthesized from HF, H2TiF6, Ba (OH)2 and KMnO4 mixed solution using a hydrothermal route. Their structure, composition and morphology were investigated. The photo-luminescent (PL) properties of the obtained BTFM products have been investigated, which exhibit broad excitation band in the blue region and sharp emission in the red region. In order to obtain the optimum BTFM red phosphor, many important factors of hydrothermal synthesis have been studied. The single red LED fabricated by combining BTFM with GaN chip shows intense red emission with the appropriate CIE chromaticity coordinates (x = 0.660, y = 0.312), and bright red light can be observed from the red LED. Therefore, this red phosphor may finds application in blue LED backlighting. ã 2015 Published by Elsevier Ltd.

Keywords: A. Optical materials A. Fluorides B. Optical properties C. X-ray diffraction D. phosphors

1. Introduction Light emitting diodes (LEDs) have several advantages such as high efficiency, long service life, low energy consumption, and environment-friendly, which can be widely used in solid-state illumination, the backlight of liquid crystal display (LCD) and so on [1–4]. To obtain more natural illumination close to the sun light or incandescent lamps, the phosphor with a broad luminescence spectrum is favorable. However, for LED backlighting, such as the backlighting of LCD, luminescence with a narrow bandwidth is required to match with a transmittance spectrum of an optical filter [4]. Therefore, generally speaking, the adopted phosphors for LED backlighting should meet the following requirements: (1) intense broad excitation band corresponding to the emission of LED chip; (2) intense sharp emission with high color-purity. Nowadays, the manufacturing technique for blue GaN chip is very mature, which is the most commercialized blue chips for fabricating white LEDs [5]. However, the study on the red phosphors excited by blue light is not sufficient. At present, there are two kinds of red phosphors that can be efficiently excited by blue light: the first is alkaline earth metal sulfides doped with Eu2+ phosphors [6–8], and the second is alkaline earth metal silicon

* Corresponding authors. Fax: +86 871 65910017. E-mail addresses: [email protected] (Z. Wang), [email protected] (Q. Wang). http://dx.doi.org/10.1016/j.materresbull.2015.08.022 0025-5408/ ã 2015 Published by Elsevier Ltd.

nitride system [9–11]. The former one can be effectively excited by blue light (430–500 nm) to obtain a wide red emission (600– 660 nm), but its unstable chemical properties, such as hydrolysis, oxidation, and sulfide precipitates, limits its applications in LEDs [6–8]. The latter one, for instance, MAlSiN3:Eu2+, M2Si5N8:Eu2+ and MSi2O2N2:Eu2+ (M = Ca, Sr, Ba) exhibiting high QE (quantum efficiency) and outstanding temperature properties, which satisfies the demands of white LEDs [9–11]. But its preparation needs extremely harsh conditions such as high temperature and reducing atmosphere. Furthermore, all the above mentioned red phosphors exhibit a broadband red emission, this limits their applications in LEDs backlighting. Therefore, it is urgent to explore new red phosphors with narrow bandwidth red emission under blue excitation. As we known, Mn4+ ions are good luminescent centers in many phosphors [12–15]. These phosphors exhibit broad band in blue region which is due to its spin-allowed transition (4A2 ! 4T2), and red sharp emission which is ascribed to the electric-dipoleforbidden transition (2Eg ! 4A2) in octahedral site. The PL properties of Mn4+ well meet the requirement for LED backlighting. In recent years, Mn4+-activated alkaline/alkaline earth hexafluorometallate phosphors have been widely demonstrated since its excellent red emission under blue (460 nm) light excitation [16–20]. For example, Pan’s group investigated the synthesis method, optical properties and application for white LED of BTFM. As we know, though the doping amount of Mn4+ plays a crucial role

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on the PL properties of red phosphors doped with Mn4+, it is complicated to be determined, especially when the obtained phosphors synthesized from hydrothermal route. Thus, in order to avoid direct measurement of doping amount in phosphor, we adopted an indirect method to systematically investigate the influence of some general and basic synthesis parameters, such as temperature, time, KMnO4 and HF concentration on PL properties of as-synthesized phosphors. The optimum reaction condition for the preparation of this BaTiF6:Mn4+ has been determined. Moreover, the application of this red phosphor on LED devices has been studied. 2. Experimental 2.1. Synthesis The BTFM red phosphors were directly synthesized using a hydrothermal route. All the chemicals in this work, including 50% H2TiF6 aqueous solution, KMnO4, Ba(OH)2 and 40% HF aqueous solution were analytical grade without further purification prior to use. In a typical hydrothermal synthesis, 1.0 ml H2TiF6 aqueous solution, 0.065 g KMnO4, 1.577 g Ba(OH)28H2O, 50 ml HF aqueous solution were mixed thoroughly in a plastic cup for 15 min. Then the mixed solution was transferred into a Teflon cup, screwed in a stainless steel autoclave and kept at 180  C for 8 h. After hydrothermal reaction, the autoclave was taken out of the oven and cooled naturally to room temperature. Finally, the postreaction solution was carefully filtered to collect the BTFM product, then washing this product using deionized water and methanol for several times and drying in 80  C for 8 h.

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The single red LED was fabricated by combining GaN chip with BTFM. Firstly, the mixtures of BTFM and epoxy resin (mass ratio is 1:1) was coated on GaN chip and solidified. Then the device was packaged with epoxy resin and solidified at 150  C for 1 h. At last the red LED device was obtained. 2.2. Characterizations The crystal structure was initially characterized using powder X-ray diffraction (XRD) with a X-ray diffractometer using Cu Ka radiation (l = 0.15406 nm) and a graphite monochromator operating at 40 kV and 30 mA from 10 to 70 with a scanning step of 0.02 at 4 min1. The as-prepared products for morphologies and structures were observed by scanning electron microscopy (SEM, FEI Quanta 200 Thermal FE Environment scanning electron microscopy) with an attached energy-dispersive X-ray spectrometer (EDS). The XPS spectrum was recorded on an X-ray photoelectron spectroscopy (XPS, Phi5500, Ulvac-Phi, United States). Thermogravimetry (TG) and different scanning calorimeter (DSC) curves were measured on a Netzsch STA449C thermal analyzer at a heating rate 10  C/min under N2. Photoluminescence (PL) spectra were documented on a Cary Eclipse FL1011M003 (Varian) spectrofluorometer with the excitation and emission slits 2.5, and the xenon lamp was used as excitation source. The Diffuse Reflectance Ultraviolet-Visible spectrum (DRS) was collected on a Cary 5000 UV–Vis–NIR spectrophotometer, and the luminescence decay curve was obtained from an FLS920 fluorescence spectrophotometer. The performance of LEDs was recorded on a high accurate array spectrometer (HSP6000). All the measurements were performed at room temperature.

Fig. 1. XRD patterns of the red phosphors BTFM obtained from 40% HF without KMnO4 (a), 8 mmol L1 KMnO4 in 10% HF (b) and 40% (c) by hydrothermal process at 180  C for 8 h.

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Fig. 2. SEM images of as-synthesized BTFM phosphors obtained at 180  C for 8 h in 10% (a) and 40% HF (b), EDS spectrum of BTFM in 40% HF (c), XPS spectrum of BTFM in 40% HF (d).

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3. Results and discussion 3.1. Composition, morphology analysis Fig. 1 shows the XRD patterns of the hydrothermally synthesized BaTiF6 with and without the existence of KMnO4 in the starting materials, along with its standard diffraction card. Obviously, all the diffraction peaks in Fig. 1a are indexed to a pure rhombohedral BaTiF6 phase (JCPDS No. 76-0269, space group R-3m, a = b = 7.368 nm, c = 7.252 nm). With the existence of 8.0 mmol L1 KMnO4, the examined XRD patterns are identical with curve a and no impurity peaks can be detected, which implies that the obtained BTFM products are also of single pure phase. These may be due to the similar ionic radius and same valance state between Ti4+ and Mn4+ (0.605 Å, CN = 6 vs. 0.53 Å, CN = 6), which means that Mn4+ ions have successfully occupied octahedral sites of Ti4+ without change the lattice structure of BaTiF6. Furthermore,

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Fig. 1b illustrates that the BTFM product can be prepared with lower content of HF. The SEM images of the pink BTFM phosphors are shown in Fig. 2. The red phosphor synthesized from 10% HF solution shows a rod-like morphology and particle size of roughly 10 mm length and 2 mm width (Fig. 2a). While the sample prepared with 40% HF solution also has a petal-like morphology and a particle size less than 10 mm length and about 5 mm width (Fig. 2b). All of the particles gathered to one end and formed like flowers in full bloom. These may attribute to the high concentration of fluoride ion which limits the growth orientation of BTFM particles and results in its larger size in 40% HF solution. The EDS result of red phosphor BTFM was shown in Fig. 2c. The result indicates that the crystal of BTFM phosphor is composed of barium (Ba), titanium (Ti), fluorine (F) and manganese (Mn). Element Si in the spectrum is from the silicon wafer used for observation of SEM and EDS. Fig. 2d shows the XPS survey

Fig. 3. The excitation (a) and emission (b) spectra of red phosphor BTFM obtained from different concentration of HF solution with 8 mmol L1 KMnO4 at 180  C for 8 h.

Fig. 4. Excitation (a) and emission (b) spectra of BTFM red phosphors obtained from 40% HF solution with different concentration of KMnO4 at 180  C for 8 h.

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Fig. 5. Emission spectra of red phosphor BaTiF6:Mn4+ with different time (a) and different temperature (b).

spectrum for the pink powder, the elements of barium (Ba), titanium (Ti), fluorine (F), together with manganese (Mn) species, have been detected on the sample. The element composition is in accordance with the results of EDS. Other oxygen and carbon elements detected in XPS may be mainly attributed to the adsorption of CO2, CH4, etc. [21]

3.2. Optical properties and application in WLEDs Fig. 3 shows the excitation and emission spectra of BTFM in different HF solution. These samples share similar excitation spectra with broad absorption bands in the UV (350 nm) and blue (466 nm) regions, which are attributed to the 4A2 ! 4T1,4A2 ! 4T2

Fig. 6. Excitation and emission spectra of red phosphors BTFM obtained at 180  C for 8 h with 8 mmol L1 KMnO4 in 40% HF solution (a), diffuse reflection spectrum of red phosphor BTFM (b), and semi-logarithmic plot of the BTFM emission decay curve (c).

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transitions of Mn4+, respectively. Fig. 3b is the emission spectra of these phosphors. There are three sharp peaks at 613, 634, and 648 nm respectively, which are ascribed to the electric-dipoleforbidden transition 2Eg ! 4A2 of Mn4+. These sharp peaks are caused by vibrational modes of the [MnF6]2octahedral, thus the weak electron-phonon coupling for the 2Eg ! 4A2 transition can be well handled [22]. With the increasing of HF concentration, the emission intensity dramatically increases, which means that the emission property of BTFM highly dependent on the HF concentration. When the HF concentration is up to 40%, the strongest emission was obtained. This may be due to that higher HF concentration is beneficial for the formation of MnF62 octahedron [23]. Therefore, 40% is an optimum HF concentration in our following hydrothermal procedure, to obtain the highest brightness red light emitting phosphor. Another important factor to affect the luminescent properties of BTFM is the concentration of KMnO4 solution. Fig. 4 displays the according luminescent spectra of the BTFM products obtained from different KMnO4 concentration in 40% HF solution.The excitation and emission intensities increase with the addition of KMnO4 until its concentration reaches 8.0 mmol L1. The luminescent intensity decreases when KMnO4 is more than 8.0 mmol L1, which is attributable to the Mn4+ concentration quenching effect in transition metal doped phosphors. Ion-ion interaction causes cross-relaxation energy transfer and non-radiative relaxation, which makes luminescence intensity decreasing especially when activator ions are close in the lattice. Besides, the effect of hydrothermal time and temperature are also investigated in our research. Fig. 5a shows the emission spectra of red phosphor BTFM obtained from different hydrothermal time in 40% HF solution with 8 mmol L1 KMnO4 at 180  C. The luminescent intensity of BTFM increases when the reaction time extends to 8 h. However, when the reaction time exceed to 12 h, the emission spectra of BTFM decrease immediately. Meanwhile, Fig. 5b shows the emission spectra of red phosphor BTFM obtained from different hydrothermal temperature in 40% HF solution with 8 mmol L1 KMnO4 for 8 h. In this hydrothermal procedure, the temperature determines the redox reaction rates of KMnO4. The sample obtained at 180  C share the strongest emission intensity. According to the above experimental results, the optimized synthesis parameters are as following: KMnO4 concentration is 8 mmol L1, HF concentration is 40%, reaction temperature is 180  C and reaction time is 8 h. Fig. 6a is the excitation and emission spectra of red phosphors BTFM under the optimized reaction condition. The sample exhibited intense excitation band in blue (466 nm) region and intense red emission with the appropriate CIE (Commission Internationale de l’Eclairage, International Commission on Illumination) chromaticity coordinates (x = 0.695, y = 0.305). Bright red light can be observed from the red phosphor under blue light (seeing the inserted Fig. 6a). Fig. 6b is the DRS spectrum of the optimum BTFM product. Two obvious absorption bands around 350 and 460 nm can be observed, this result is consistent with that of its excitation spectrum. The decay curve of the optimized BTFM red phosphor is shown in Fig. 6c. It is well fitted into single-exponential function, and the lifetime t value is 4.5 ms. As shown in Fig. 7, TG and DSC curves of BTFM show that its thermal decomposition temperature (Td) is about 440  C. Such high decomposition temperature suggests that this phosphor has high thermal stability and it is enough to be applied in LED devices. In the above section, BTFM red phosphor displays a strong excitation band in the blue region and sharp red emission with high color purity and high decomposition temperature, so it may be a promising phosphor for GaN-LED backlighting. In order to investigate its PL properties in LED device, the single red LED was fabricated by coating the red phosphor on a blue commercial GaN

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Fig. 7. TG and DSC curves of BTFM.

Fig. 8. Electro-luminescent spectra of the red LED based on BTFM (a) and GaN chip (b) under 20 mA current excitation, the inserts are the corresponding light images. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)

chip. The emission spectra of the red LED with 20 mA current excitation were exhibited in Fig. 8. The intense red emission at 613, 634, and 648 nm are due to the emission of BTFM excited by the emission of blue LED chip. Compared with Fig 8b, no emission of LED chip can be found in Fig 8a, this result shows this red phosphor can be efficiently excited by blue LED chip. The CIE chromaticity coordinates according to the electro-luminescent spectrum of the single LED are calculated to be x = 0.660, y = 0.312, which is close to the NTSC standard CIE chromaticity coordinate values for red (x = 0.67, y = 0.33). The red LED exhibits intense red light, which can be observed by naked eyes. Hence it may finds application in blue LED backlighting. 4. Conclusions In this work, a series of BTFM red-emitting phosphors have been successfully synthesized by a hydrothermal method. In order to obtain the optimum product, the hydrothermal synthesis parameters have been investigated in details and the optimum synthesis condition has been confirmed. The optimal BTFM red

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phosphor exhibits broad excitation band in the blue region and sharp emission in red emission. The single red LED based on BTFM shows intense red emission with high color-purity, so it may be a promising red phosphor for GaN-LED backlighting. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC 21261027), the Natural Science Foundation of Yunnan Province (2014FB147), Department of Yunnan Education (22014Y253), and the Special-zone Projection from Yunnan Minzu University (No.2015TX01). References [1] H.Y. Li, H.M. Noh, B.K. Moon, B.C. Choi, J.H. Jeong, K. Jang, H.S. Lee, S.S. Yi, Wideband excited Y6(WMo)0.5O12:Eu red phosphor for white light emitting diode: structure evolution, photoluminescence properties, and energy transfer mechanisms involved, Inorg. Chem. 52 (2013) 11210–11217. [2] H.A. Höppe, Recent Developments in the Field of Inorganic Phosphors, Angew. Chem. Int. Ed. 48 (2009) 3572–3582. [3] Y.S. Tang, S.F. Hu, W.C. Ke, C.C. Lin, N.C. Bagkar, R.S. Liu, Near-ultraviolet excitable orange–yellow Sr3Al2O5Cl2:Eu2+ phosphor for potential application in light-emitting diodes, Appl. Phys. Lett. 93 (2008) 131114. [4] H. Yamamoto, White LED phosphors: the next step, Proc. SPIE 7598 (2010) 759808–759810. [5] C.C. Yang, H.Y. Tsai, K.C. Huang, Yellow-ring measurement of white LED in various lighting environments, Opt. Rev. 20 (2013) 232–235. [6] C.F. Guo, D.X. Huang, Q. Su, Methods to improve the fluorescence intensity of CaS:Eu2+ red-emitting phosphor for white LED, Mater. Sci. Eng. B 130 (2006) 189–193. [7] X.X. Duan, S.H. Huang, F.T. You, K. Kang, Hydrothermal preparation and persistence characteristics of nano-sized phosphor SrS:Eu2+, Dy3+, J. Rare Earth 27 (2009) 43–46. [8] M. Nazarov, C. Yoon, Controlled peak wavelength shift of Ca1xSrx(SySe1-y):Eu2 + phosphor for LED application, J. Solid State Chem. 179 (2006) 2529–2533.

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