Frontier of Environmental Science March 2014, Volume 3, Issue 1, PP.1-5
Photoluminescence Properties of NaLu1-xEux(WO4)2 Novel Red Phosphors Synthesized by Combustion Method Dafeng Zhanga#, Jianxiu Liub, Xipeng Puc School of Materials Science and Engineering, Liaocheng University, Liaocheng 252000, P. R. China Email: #a zhangdafeng@lcu.edu.cn, bljx3249@163.com, cxipengpu@hotmail.com
Abstract The novel red phosphors NaLu1-xEux(WO4)2 (x=0.1, 0.2, 0.4, 0.6, 0.8, 1.0) were synthesized via the combustion method. The X-ray powder diffraction patterns show that these phosphors have the same single phase with the scheelite structure. The photoluminescence spectra results indicate that the excitation spectra of the samples consist of a broad absorption band of 200-350 nm and some sharp lines around 350-500 nm which corresponds to the f-f transitions within 4f8 configuration of Eu3+. The emission spectra consist of the orange region 5D0→7F1 (594 nm) and the red region 5D0→7F2 (618 nm). Among these phosphors, NaEu(WO4)2 synthesized at 900℃ exhibits the strongest red emission under 395 nm excitation and appropriate CIE chromaticity coordinates (x=0.637, y=0.359) close to the NTSC standard value. Keywords: Phosphors; Combustion Method; NaLu1-xEux(WO4)2; Luminescence
1. INTRODUCTION Recently, White light-emitting diodes (LEDs) attract much attention due to their some advantages such as: high reliability, low energy consumption, long lifetime, and environment-friendly characteristics [1]. The new and promising white LED technique was obtained by the combination of a near UV (around 400 nm) InGaN-based LED chip and blue/green/red tricolor phosphors [1-4]. The current commercial tricolor phosphors are Y2O2S:Eu3+ for red phosphor, BaMgAl10O17:Eu2+, Mn2+ for green phosphor, and Sr5(PO4)3Cl:Eu2+ or BaMgAl10O17:Eu2+ for blue phosphor [5-7]. Compared with the blue and green phosphors, the red phosphor Y2O2S:Eu3+ has some demerits, such as lower efficiency, shorter working lifetime and instability [6-8]. Therefore, it is urgent to invent suitable red phosphors with chemical stability, high color purity, high efficiency and high absorption in the near-UV/blue spectral region, as inexpensive as possible. Double alkaline rare-earth tungstate NaLu(WO4)2 has tetragonal symmetries with a scheelite-type structure [9]. This kind of compound is known as the host materials for lasers because of their superior optical properties and chemical stability [10-12]. For instance, NaLu(WO4)2:Nd [10] and NaLu(WO4)2:Yb [11, 12] single crystal used for laser have been reported. However, NaLu(WO4)2 used as the host materials for phosphors have been studied shortly; to the best of our knowledge, the combustion synthesis of NaLu(WO4)2:Eu3+ phosphor has not been reported. In this study, NaLu1-xEux(WO4)2 phosphors were synthesized via a facile combustion method. The luminescent properties of the samples were investigated by changing doping concentration of Eu3+ ions.
2. EXPERIMENTAL PROCEDURE The NaLu1-xEux(WO4)2 (x=0.1, 0.2, 0.4, 0.6, 0.8, 1.0) samples were prepared via a facile combustion method. Lu2O3 (99.99%), Eu2O3 (99.99%), NaNO3 (AR, analytical reagent), (NH4)6W7O24·6H2O (AR) and CH4N2O (AR) were used as the starting materials without further purification and were weighted by stoichiometric ratio. Firstly, Lu(NO3)3, Eu(NO3)3 were obtained by dissolving Lu2O3 and Eu2O3 into the concentrated nitric acid, the excess HNO3 was -1http://www.ivypub.org/fes
removed by further evaporation, then solution A was obtained by adding 10 ml deionized water into the beaker with the above-obtained Lu(NO3)3, Eu(NO3)3. Secondly, solution B was prepared by dissolving NaNO3, (NH4)6W7O24·6H2O and CH4N2O into 50 ml deionized water with continuously stirring. Then solution B was added into above-mentioned solution A drop by drop under violently stirring. The mixture was continuously stirred and heated at 80℃ for about 1 h, and then the mixture gel was obtained. The sticky gel was introduced into a preheated muffle furnace with a temperature of 500℃ and maintained for 10 minutes, and then a yellowish precursor was obtained with an auto combustion process taking place. The precursor was calcined at 900℃ for 2 h, finally, the pink powder was obtained. The (XRD) patterns were recorded on a D8 Advanced diffractometer with Cu Kα-radiation (40 kV/30 mA). The emission and excitation spectra of the as-prepared products were measured by a Hitachi F-7000 spectrometer equipped with 700W Xenon lamp as excitation source. All the measurements were carried out at room temperature.
3. RESULTS and DISCUSSION FIG. 1 shows XRD patterns of the NaLu1-xEux(WO4)2 (x=0.1, 0.6, 1.0) phosphors synthesized by combustion method at 900℃. The result reveals that the diffraction peaks are well corresponding to (101), (112), (004), (200), (211), (204), (220), (116), (312), (224), (316). It is obvious that these phosphors have the same single phase with the scheelite structure which can be indexed to the standard JCPDS card of NaLu(WO4)2 (No. 27-0729) with space group of I41/a (88). No characteristic peaks of impurities are observed from these samples. Moreover, by enlarging the region from 45°to 60°(insert in the Fig.1), obviously, the peak positions of the synthesized phosphors drift to small angle with the increasing Eu3+ concentrations, because of the relative larger diameter of Eu3+(1.066 Å) than Lu3+(0.977 Å) ions [13].
FIG. 1 XRD PATTERNS of THE AS-SYNEHESIZED NaLu1-xEux(WO4)2 (X=0.1, 0.6, 1.0) PHOSPHOES.
FIG. 2 shows the excitation spectrum of the NaLu0.4Eu0.6(WO4)2 phosphor synthesized by combustion method at 900℃. The excitation spectrum by monitoring the 5D0→7F2 emission (618 nm) of Eu3+ ions represents a broad absorption band of 200-350 nm and some sharp lines around 350-500 nm which corresponds to the f-f transitions within 4f8 configuration of Eu3+, that are 7F0→5D2, 5D3, 5L6, 5G2, 5D4, respectively [14]. Among these excitation transitions, 7F0→5L6 (395 nm) is the most intense one, which is well consistent with the emission of the InGaN chip-based LED. FIG. 3 shows the emission spectra of the NaLu0.4Eu0.6(WO4)2 phosphor under 270 nm and 395 nm excitation, respectively. The emission spectrum consists of the orange region 5D0→7F1 (594 nm) and the red region 5D0→7F2 (618 nm). The high ratio of the red emission to the orange emission is advantageous to obtain a phosphor with good CIE chromaticity coordinates. The effect of the Eu3+ concentration on the emission (618 nm) intensity of the -2http://www.ivypub.org/fes
strongest 5D0→7F2 transitions is shown in the insert of Fig. 3. It is obvious that the intensity of red emission under 395 nm excitation increases and comes to the strongest at x=1.0 with the continually increasing concentrations of the Eu3+, while the intensity of red emission under 270 nm excitation increases, comes to the strongest at x=0.4, and subsequently decreases. It is easy to find that the intensities of the red emission under 270 nm are higher than that of the red emission under 395 nm in the region of 0.1-0.4; on the contrary, the intensities of the red emission under 270 nm are lower than that of the red emission under 395 nm in the region of 0.5 -1.0. Thus, x=0.4 or x=1.0 could be considered as the optimum Eu3+-doping concentration depending on the different application.
FIG. 2 EXCITATION SPECTRUM (λem=618 nm) of THE AS-SYNTHESIZED NaLu0.4Eu0.6(WO4)2 PHOSPHOR.
FIG. 3 EMISSION SPECTRA (λex=270 nm and λex=395 nm) of THE AS-SYNTHESIZED NaLu0.4Eu0.6(WO4)2 PHOSPHOR.
FIG. 4 shows the CIE diagram indicating the color coordinates from emission spectrum (under 395 nm excitation) of NaLu1-xEux(WO4)2 (x=0.4, 1.0) phosphors and the NTSC standard value. It can be clearly seen that the color coordinates of NaEu(WO4)2 phosphor is very close to the NTSC standard value. We have known that the intensity of the emission (under 394 nm excitation) of the as-synthesized sample without Lu ion existing is strongest. Therefore, the as-synthesized NaEu(WO4)2 phosphor is considered as promising red phosphor for near UV InGaN chip-based red-emitting LED light. -3http://www.ivypub.org/fes
FIG. 4 CIE DIAGRAM INDICATING THE COLOR COORDINATES FROM EMISSION SPECTRUM (UNDER 395 nm EXCITATION) of NaLu1-xEux(WO4)2 (x=0.4, 1.0) PHOSPHORS and THE NTSC STANDARD VALUE.
4. CONCLUSION The red novel phosphors NaLu1-xEux(WO4)2 with pure tetragonal scheelite structure have been synthesized via a combustion method. The as-synthesized NaEu(WO4)2 phosphor exhibits the strongest red emission under 395 nm light excitation and appropriate CIE chromaticity coordinates (x=0.637, y=0.359) close to the NTSC standard values (x=0.67, y=0.33), and it may be served as near UV InGaN chip-based red-emitting LED phosphors.
ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (No. 51002069).
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