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Synthesis and VUV luminescent properties of LaPO4:Eu3+ nanowires
Materials Chemistry and Physics 147 (2014) 831e835
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis and VUV luminescent properties of LaPO4:Eu3þ nanowires Dan Wang a, b, *, Qi Shun a, Yuhua Wang a, b, Zhiya Zhang a, b a b
Department of Materials Science, Lanzhou University, Lanzhou 730000, China Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, PR China
h i g h l i g h t s 5D0 / 7F2 integrated emission intensity of LaPO4:Eu3þ was higher than 5D0 / 7F1 transition. VUV luminescent intensities of LaPO4:Eu3þ nanowires were maintained in a certain value. Self-purification phenomenon is observed in LaPO4:Eu3þ nanowires.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 October 2013 Received in revised form 19 May 2014 Accepted 8 June 2014 Available online 25 June 2014
A series of different concentrations of LaPO4:Eu3þ phosphors with the morphology of nanowires were successfully prepared by a precipitation method. It was observed that the integrated intensity of 5 D0 / 7F2 transition of Eu3þ was higher than its 5D0 / 7F1 transition when the sintering temperature was at 600 C. The most interesting phenomenon was that the doping concentrations of LaPO4:Eu3þ nanowires were changed in a large range (5e15%), the photoluminescence intensities were maintained in a certain value. This phenomenon was not observed in the bulk LaPO4:Eu3þ phosphors. A detailed explanation to this phenomenon was given. © 2014 Elsevier B.V. All rights reserved.
Keywords: Inorganic compounds Nanostructures Precipitation Photoluminescence spectroscopy
1. Introduction Recently, much interest has focused on nanoscale phosphors due to their special luminescent properties compared to those of their bulk counterparts. It is expected that the nanosized phosphors can improve not only the luminescence quantum yield but also the resolution of display [1,2]. Moreover, in comparison with zerodimensional nanoparticles, the shape anisotropy of a onedimensional material can provide a better model to investigate the dependence of optical properties on size confinement and dimensionality. Orthophosphate LaPO4 is a potential phosphor host for it can efficiently absorb the energy in the vacuum ultraviolet (VUV) region and can be applied in the mercury-free fluorescent lamps and plasma display panels. Tb3þ doping bulk LaPO4 can efficiently transfer the VUV energy to the green light [3,4]. Eu3þ doping bulk LaPO4 had a high absorption in the VUV region too [5]. However, the strongest emission of Eu3þ doping bulk LaPO4 locates at the * Corresponding author. Department of Materials Science, Lanzhou University, Lanzhou 730000, China. Tel.: þ86 931 8912836; fax: þ86 931 8913554. E-mail address: [email protected] (D. Wang). http://dx.doi.org/10.1016/j.matchemphys.2014.06.027 0254-0584/© 2014 Elsevier B.V. All rights reserved.
orange light region under VUV excitation [6,7]. Therefore, improving the color purity of LaPO4:Eu3þ becomes a necessary work. For Eu3þ ions, the allowed transition emission locates in the orange region; the red emission belongs to forbidden transition of Eu3þ. Eu3þ ions will show high red emission only when they locate at an asymmetric center. For examples, Y(P,V)O4:Eu3þ and GdAl3(BO3)4:Eu3þ have high red emission because Eu3þ ions occupy an asymmetric center in these two phosphor [8,9]. Bulk YBO3:Eu3þ show high orange emission for Eu locates at a symmetric center [10,11], but nanosized YBO3:Eu3þ has better color purity than its bulk counterpart [12]. Therefore, the synthesis of nanosized LaPO4:Eu3þ phosphor maybe a considerable way to improve its color purity. In addition, according to Ref. [13], the decline of the sintering temperature will result in the high level of disorder of Eu3þ and the forbidden transition of Eu3þ can be released, therefore, the synthesis of LaPO4:Eu3þ with low sintering temperature maybe another possible way to improve its color purity. In this paper, we prepared a series of one-dimensional nanometer LaPO4:Eu3þ phosphors using the precipitation method and investigated their photoluminescence properties under 147 nm excitation, respectively. For a comparation, a series of bulk LaPO4:Eu3þ were prepared by a solidestate reaction.
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2. Experimental section 2.1. Preparation of LaPO4:Eu3þ products The starting materials were a stoichiometric quantities of La2O3(99.999%), Eu2O3(99.99%), (NH4)2HPO4 (98.5%), ammonia (A.R.) and HNO3 (A.R.). La(NO3)3 and Eu(NO3)3 were obtained by dissolving La2O3 and Eu2O3 in HNO3 under heating with agitation. (NH4)2HPO4 was added into the La2O3 and Eu2O3 aqueous solution under stirring. The pH value of the resultant solution was adjusted to about 4.0 using dilute HNO3 and dilute ammonia. Finally, the mixture of solutions was loaded quietly at 80 C for a certain time. After cooling to ambient temperature naturally, the precipitations were centrifugally washed with deionized water three times. Then they were dried at 80 C in the air. The final precipitations were fired at a constant heating rate of 10 C min1 from room temperature to 600 C or 1000 C in air respectively, and the temperature was maintained for 4 h to obtain the LaPO4:Eu3þ products. Finally, the temperature decreased at the cooling rate of 3 C min1. The bulk LaPO4:Eu3þ were prepared by mixing stoichiometric quantities of La2O3 (99.95%), Eu2O3 (99.99%) and (NH4)2HPO4 (98.5%, excess of 5 mol %). Excess of (NH4)2HPO4 was added to compensate the evaporation loss at higher temperature. After intimately mixing the starting materials with ethanol, the mixtures were heated at 1100 C for 6 h and cooled down to the room temperature. 2.2. Characterization All the samples were checked by X-ray diffraction (XRD) using Rigaku diffractometer with Ni-filtered CuKa radiation. The differential thermal analysis (DTA)-thermogravimetry (TG) (Du Pont Instrument 1090B) was employed to obtain the thermal decomposition behavior of the precipitations during the nitrogen annealing with a heating rate of 10 C min1. The morphology and composition of the samples were carried out using scanning electron microscopy (SEM) (Hitachi S-4800) and transmission electron microscopy (TEM) (FEI Tecnai F30, operated at 300 kV). The X-rays photoelectron spectroscopy (XPS) measurements were performed in a Kratos AXIS Ultra DLD spectrometer. The emission spectra were measured by FLS-920T fluorescence spectrophotometer with a VM504-type vacuum monochromator using a deuterium lamp as the lighting source. All the measurements were performed at room temperature. 3. Results and discussion
Fig. 1. XRD patterns of LaPO4 with the sintering temperature of 600 C, 1000 C and 1100 C.
accompanied by a heavy weight loss step at about 212 C can be found, and the peak could be due to the loss of absorbed water in the precipitate [14]. Another endothermic peak without a weightchange step around 951 C could also be found. We suppose this peaks should correspond to the phase transitions from hexagonal structure to monoclinic structure because lanthanide phosphates have several polymorphic forms; the hexagonal structure is the low-temperature phase, while the monoclinic structure is the hightemperature one, which has been studied by Rose Mooney [15]. 3.2. Morphology analysis The morphologies of the obtained LaPO4:Eu3þ did not exhibit obvious difference, when the Eu3þ concentration changes in a range of 0e20 %. Fig. 3(a) and (b) only show the SEM images of LaPO4 samples synthesized by the precipitation method with the sintering temperature of 600 C and 1000 C, respectively. Fig. 3(c) shows the SEM image of LaPO4 sample synthesized by the solid state reaction. The obtained sample has an irregular shape and the particle size in the range of 5e80 mm, which indicates the obtained sample is the bulk materials. The TEM image of the typical La0.95PO4:0.05Eu3þ with the sintering temperature of 1000 C is shown in Fig. 4. The TEM image
3.1. XRD analysis Fig. 1 shows the XRD patterns of LaPO4 with the sintering temperature of 600 C, 1000 C and 1100 C, respectively. From this figure, it is observed that the reflection planes are matched well with the reported monoclinic phase of lanthanum phosphate when the sintering temperature is at 1000 C and 1100 C, which indicates the obtained samples belong to monoclinic LaPO4 when the sintering temperature is at 1000 C and 1100 C. The reflection planes of the product which is sintered at 600 C are matched well with the reported hexagon phase of lanthanum phosphate, which implies that the obtained sample is the hexagon LaPO4 when the sintering temperature is at 600 C. No other peaks are observed in these XRD patterns, which indicate that all the products are a single phase. In order to investigate the phase transfer, DTA-TG is employed. The result of DTA-TG of the precipitated samples heat-treated in air up to 1250 C is shown in Fig. 2. A distinct endothermic peak
Fig. 2. DTA-TG of the precipitated samples heat-treated in air up to 1250 C.
D. Wang et al. / Materials Chemistry and Physics 147 (2014) 831e835
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Fig. 3. SEM images of LaPO4 samples synthesized by the precipitation method with the sintering temperature of (a) 600 C and (b)1000 C, and (c) solid state reaction.
displays nanowire morphology with the diameter about 33 nm and the length from 0.1 to 2 mm, which well indicates that the obtained samples belong to the one-dimensional nanometer materials. The growth mechanism for nanomaterials including the monoclinic LaPO4:Eu3þ nanowires considered in our work is very complicated. Nevertheless, we try to propose a possible growth mechanism of monoclinic LaPO4 nanowires. In our experiment, when the sintering temperature is lower than 600 C, LaPO4:Eu3þ possess as a hexagonal crystal structure as shown in Fig. 1. The formation of hexagonal LaPO4:Eu3þ nanowires is the result of the highly preferential growth along the c axis, which has been suggested in previous studied as the intrinsic nature in the growth of hexagonal LnPO4:Eu3þ (Ln ¼ La, Ce) [16,17]. The monoclinic LaPO4:Eu3þ nanowires might be derived from the hexagonal LaPO4:Eu3þ nanowires. That is, with the rising of the reaction
temperature, the hexagonal LaPO4:Eu3þ nanowires are first formed and then are transformed to the monoclinic LaPO4:Eu3þ nanowires when the sintering temperature is higher than 600 C. In order to confirm that whether the Eu3þ is doped into the La1xPO4:xEu3þ nanowires, the typical XPS of La1xPO4:xEu3þ are obtained as Fig. 5. The binding energy surveys from 0 to 1200 eV. The XPS profile is similar for the series of La1xPO4:xEu3þ (x ¼ 0.05, 0.10, 0.15, 0.20) nanowires, here we show the typical XPS analysis of La0.95PO4:0.05Eu3þ nanowires. The peak at 1164.5 eV can be assigned to the Eu 3d3 orbital. This confirms that the Eu3þ ions have been doped into LaPO4 successfully. The surface elemental analysis also has been adopted. The Eu concentrations on La1xPO4:xEu3þ (x ¼ 0.05, 0.10, 0.15, 0.20) nanowires surface are shown in the Table 1. The results are very surprising. The Eu concentrations on the nanowire surface are largely higher than the nominal value and show only slight change with the increasing Eu3þ content, which hints that the doping Eu3þ ions mainly locate at the nanowire
Fig. 4. TEM image of La0.95PO4:0.05Eu3þ nanowires.
Fig. 5. XPS analysis of La0.95PO4:0.05Eu3þ nanowires.
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Table 1 The Eu concentrations on La1xPO4:xEu3þ (x ¼ 0.05, 0.10, 0.15, 0.20) nanowires surface. La1xPO4:xEu3þ
x x x x
¼ ¼ ¼ ¼
5% 10% 15% 20%
Table 2 R/O Ratios and chromaticity coordinates of bulk and nanowires La0.95PO4:0.05Eu3þ samples with the sintering temperature of 600 C, 1000 C.
Eu concentration on the nanowires surface
Sample
Nominal value
NTSC Nanowire (sintering temperature 600 C) Nanowire (sintering temperature 1000 C) Bulk
5% 10% 15% 20%
Experimental value 72.8% 71.7% 75.6% 74.8%
surface. This phenomenon should be the result of the “self-purification” mechanisms of nanomaterilas. 3.3. Photoluminescence properties Fig. 6 shows the emission spectra of bulk La0.95PO4:0.05Eu3þ and nanowires La0.95PO4:0.05Eu3þ with the sintering temperature of 600 C, 1000 C, respectively. The excitation wavelengths of all the samples are 147 nm. When the Eu3þ concentration is changed from 0 to 20 %, the shapes and the wavelengths of the emission peaks are similar, and only the emission intensity is changed, thus we only show one emission spectrum for every series of samples. In these emission spectra, four emission peaks are observed with the maxima at 593 (±1) nm, 618 (±2) nm, 655 (±1) nm, 697 (±2) nm and can be assigned to the 5D0 / 7Fj(j ¼ 1,2,3,4) transitions of Eu3þ, respectively. The allowed 5D0 / 7F1 transitions are higher than the forbidden 5D0 / 7F2 transitions of Eu3þ ions when the sintering temperature is at 1000 C, this is consistent with that of bulk La0.95PO4:0.05Eu3þ which is sintered at 1100 C. But the forbidden 5 D0 / 7F2 transitions of Eu3þ ions has a higher integrated emission intensity than the allowed 5D0 / 7F1 transitions when the sintering temperature is at 600 C. Table 2 shows the R/O Ratios and chromaticity coordinates of bulk La0.95PO4:0.05Eu3þ and nanowires La0.95PO4:0.05Eu3þ samples with the sintering temperature of 600 C, 1000 C. From Table 2, it can be found that the chromaticity of La0.95PO4:0.05Eu3þ can not be improved through synthesizing the nanosized La0.95PO4:0.05Eu3þ phosphor, but can be improved when the sintering temperature declines to 600 C. One reason for the improved R/O value is that the low-temperature phase of La0.95PO4:0.05Eu3þ belongs to hexagon structure and Eu ions locate in a distortion polyhedron in this structure [18], therefore, the
Fig. 6. Emission spectra of bulk (c) and nanowires La0.95PO4:0.05Eu3þ with the sintering temperature of 600 C (a), 1000 C (b).
R/O
Chromaticity coordinates
1.379
0.67 0.633
0.33 0.367
0.951
0.610
0.386
0.942
0.616
0.383
forbidden transition (around 618 nm) of Eu3þ has higher emission intensity than the allowed transition (around 593 nm) of Eu3þ. The other reason is that the poor crystallization results in the high level of disorder of Eu3þ, so the forbidden transition is released and has high emission intensity. Fig. 7 shows the change curve of relative integrated emission intensity of La1xPO4:xEu3þ (a, sintering temperature 600 C, 0.05 x 0.20), La1yPO4:yEu3þ (b, sintering temperature 1000 C, 0.05 y 0.20) and La1zPO4:zEu3þ (c, bulk, 0.05 z 0.20) with the increase of the Eu concentration under 147 nm excitation. From this figure, we find a very interesting phenomenon that is the emission intensity of LaPO4:Eu3þ nanowires maintains a certain value when the Eu3þ doping concentrations were changed in the range of 5e15 mol %. That means that the emission intensities of LaPO4:Eu3þ nanowires under VUV excitation is not sensitive to Eu3þ doping concentrations. This phenomenon was not observed in that of bulk samples (Fig. 7(c)). The main reason for this phenomenon should be due to the penetration depth of VUV radiation and the distribution of the doping ions. According to Modi's work [19], the depth of incident radiation are all restricted to 8.0 nm (10e250 eV). Considering the penetration depth of VUV radiation can be affect by the photon energy, diffusion to the surface, dielectric permittivity of materials and incident angle, the penetration depth of VUV radiation in our test condition is not more than 10 nm [20]. It will cause that the VUV light could only excite part of the phosphor particle where it is near the surface. What's more, the activator ions concentration on the nanophosphor surface and in the interior is different. This phenomenon is called the
Fig. 7. The change curve of relative integrated emission intensity of La1xPO4:xEu3þ (a, sintering temperature 600 C, 0.05 x 0.2), La1yPO4:yEu3þ (b, sintering temperature 1000 C, 0.05 y 0.2) and La1zPO4:zEu3þ (c, bulk, 0.05 z 0.2) with the increase of the Eu concentration under 147 nm excitation.
D. Wang et al. / Materials Chemistry and Physics 147 (2014) 831e835
“self-purification” mechanisms and has been observed in many nanosystems [12,21]. It could be expressed as the impurities are hard to be doped into nanocrystals and easy to be expelled and move to the surface. That means that the doping ions preferably occupy the lattice sites that are near the nanoparticle surface. Until the surface layer reaches “saturation” concentration, then the surplus doping ions distribute from the surface to the interior. Finally, the doping ions have a high concentration in the surface of the nanophosphor particles and form a “shell” around the nanophosphor particle. This has been confirmed by the results of XPS in Table 1. Little activator exists in the interior of the nanophosphor particles. However, the penetration depth of VUV radiation is a constant, thus the emission intensities is not sensitive to Eu3þ concentrations when the depth of the “shell” is larger than the penetration depth until the concentration quenching happened. For bulk LaPO4:Eu3þ, the activator is a homogeneous distribution in the phosphor particles. The activator concentration on the phosphor surface increases along with the increase of Eu3þ concentration. Therefore, the integrated emission intensity is sensitive to the Eu concentration and increases linearly until the concentration quenching happened when Eu concentration is larger than 0.15 mol. The similar phenomenon has been observed in YBO3:Eu3þ nanoparticle [12]. Our experimental result indicates that LaPO4:Eu3þ phosphor with the one-dimensional nanostructure has the similar luminescent properties as the nanoparticle phosphors. 4. Conclusions The chromaticity of La0.95PO4:0.05Eu3þ can be improved through declining the sintering temperature to 600 C. Under 147 nm excitation, the emission intensity of LaPO4:Eu3þ nanowires phosphors maintain a certain value when Eu concentration is in the range of 0.05e0.15 mol, which due to the “self-purificationy mechanism of La1xPO4:xEu3þ with the one-dimensional nanostructure.
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Acknowledgment This work was supported by the Natural Science Foundation of Gansu Province, China (grant No. 1208RJYA022) and the National Science Foundation (51202099). References [1] R. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416e419. [2] H. Zhu, D.J. Zuo, Phys. Chem. C 113 (2009) 10402e10406. [3] W.-S. Song, H.-N. Choi, Y.-S. Kim, H. Yang, J. Mater. Chem. 20 (2010) 6929e6934. [4] D. Wang, Y. Wang, Mater. Res. Bull. 42 (2007) 2163e2169. [5] X. Wu, H. You, H. Cui, X. Zeng, G. Hong, C.-H. Kim, C.-H. Pyun, B.-Y. Yu, C.H. Park, Mater. Res. Bull. 37 (2002) 1531e1538. [6] Y. Yang, Mater. Sci. Eng. B 178 (2013) 807e810. [7] G. Li, L. Li, M. Li, Y. Song, H. Zou, L. Zou, X. Xu, S. Gan, Mater. Chem. Phys. 133 (2012) 263e268. [8] K.-S. Sohn, I.I.W. Zeon, H. Chang, S.K. Lee, H.D. Park, Chem. Mater. 14 (2002) 2140e2148. [9] Y. Wang, K. Uheda, H. Takizawa, U. Mizumoto, T. Endo, J. Electrochem. Soc. 148 (2001) G430eG433. [10] Y. Wang, T. Endo, L. He, C. Wu, J. Cryst, Growth 268 (2004) 568e574. [11] M. Ren, J. Lin, Y. Dong, L. Yang, M. Su, L. You, Chem. Mater. 11 (1999) 1576e1580. [12] Q. Dong, Y. Wang, Z. Wang, X. Yu, B. Liu, J. Phys. Chem. C 114 (2010) 9245e9250. [13] Raghvendra S. Yadav, Ranu K. Dutta, Manvendra Kumar, Avinash C. Pandey, J. Lumin. 129 (2009) 1078e1082. [14] K.C. Song, Y. Kang, Mater. Lett. 42 (2000) 283e289. [15] R. Mooney, Acta Crystallogr. 3 (1950) 337e340. [16] Y.P. Fang, A.W. Xu, R.Q. Song, H.X. Zhang, L.P. You, J.C. Yu, H.Q. Liu, J. Am. Chem. Soc. 125 (2003) 16025e16034. [17] Y. Zhang, H. Guan, J. Cryst, Growth 256 (2003) 156e161. [18] M. Yang, H. You, G. Jia, Y. Huang, Y. Song, Y. Zheng, K. Liu, L. Zhang, J. Cryst. Growth 311 (2009) 4753e4758. [19] M. Modi, G. Lodha, P. Srivastava, A. Sinha, R. Nandedkar, Phys Rev B 74 (2006), 045326/1e045326/6. [20] M. Terekhin, A. Vasil'ev, M. Kamada, E. Nakamura, S.Kubota, Phys Rev B 52 (1995) 3117e3121. [21] Steven C. Erwin, Lijun Zu, Michael I. Haftel, Alexander L. Efros, Thomas A. Kennedy, David J. Norris, Nature 436 (2005) 91e94.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis and VUV luminescent properties of LaPO4:Eu3þ nanowires Dan Wang a, b, *, Qi Shun a, Yuhua Wang a, b, Zhiya Zhang a, b a b
Department of Materials Science, Lanzhou University, Lanzhou 730000, China Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, PR China
h i g h l i g h t s 5D0 / 7F2 integrated emission intensity of LaPO4:Eu3þ was higher than 5D0 / 7F1 transition. VUV luminescent intensities of LaPO4:Eu3þ nanowires were maintained in a certain value. Self-purification phenomenon is observed in LaPO4:Eu3þ nanowires.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 October 2013 Received in revised form 19 May 2014 Accepted 8 June 2014 Available online 25 June 2014
A series of different concentrations of LaPO4:Eu3þ phosphors with the morphology of nanowires were successfully prepared by a precipitation method. It was observed that the integrated intensity of 5 D0 / 7F2 transition of Eu3þ was higher than its 5D0 / 7F1 transition when the sintering temperature was at 600 C. The most interesting phenomenon was that the doping concentrations of LaPO4:Eu3þ nanowires were changed in a large range (5e15%), the photoluminescence intensities were maintained in a certain value. This phenomenon was not observed in the bulk LaPO4:Eu3þ phosphors. A detailed explanation to this phenomenon was given. © 2014 Elsevier B.V. All rights reserved.
Keywords: Inorganic compounds Nanostructures Precipitation Photoluminescence spectroscopy
1. Introduction Recently, much interest has focused on nanoscale phosphors due to their special luminescent properties compared to those of their bulk counterparts. It is expected that the nanosized phosphors can improve not only the luminescence quantum yield but also the resolution of display [1,2]. Moreover, in comparison with zerodimensional nanoparticles, the shape anisotropy of a onedimensional material can provide a better model to investigate the dependence of optical properties on size confinement and dimensionality. Orthophosphate LaPO4 is a potential phosphor host for it can efficiently absorb the energy in the vacuum ultraviolet (VUV) region and can be applied in the mercury-free fluorescent lamps and plasma display panels. Tb3þ doping bulk LaPO4 can efficiently transfer the VUV energy to the green light [3,4]. Eu3þ doping bulk LaPO4 had a high absorption in the VUV region too [5]. However, the strongest emission of Eu3þ doping bulk LaPO4 locates at the * Corresponding author. Department of Materials Science, Lanzhou University, Lanzhou 730000, China. Tel.: þ86 931 8912836; fax: þ86 931 8913554. E-mail address: [email protected] (D. Wang). http://dx.doi.org/10.1016/j.matchemphys.2014.06.027 0254-0584/© 2014 Elsevier B.V. All rights reserved.
orange light region under VUV excitation [6,7]. Therefore, improving the color purity of LaPO4:Eu3þ becomes a necessary work. For Eu3þ ions, the allowed transition emission locates in the orange region; the red emission belongs to forbidden transition of Eu3þ. Eu3þ ions will show high red emission only when they locate at an asymmetric center. For examples, Y(P,V)O4:Eu3þ and GdAl3(BO3)4:Eu3þ have high red emission because Eu3þ ions occupy an asymmetric center in these two phosphor [8,9]. Bulk YBO3:Eu3þ show high orange emission for Eu locates at a symmetric center [10,11], but nanosized YBO3:Eu3þ has better color purity than its bulk counterpart [12]. Therefore, the synthesis of nanosized LaPO4:Eu3þ phosphor maybe a considerable way to improve its color purity. In addition, according to Ref. [13], the decline of the sintering temperature will result in the high level of disorder of Eu3þ and the forbidden transition of Eu3þ can be released, therefore, the synthesis of LaPO4:Eu3þ with low sintering temperature maybe another possible way to improve its color purity. In this paper, we prepared a series of one-dimensional nanometer LaPO4:Eu3þ phosphors using the precipitation method and investigated their photoluminescence properties under 147 nm excitation, respectively. For a comparation, a series of bulk LaPO4:Eu3þ were prepared by a solidestate reaction.
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2. Experimental section 2.1. Preparation of LaPO4:Eu3þ products The starting materials were a stoichiometric quantities of La2O3(99.999%), Eu2O3(99.99%), (NH4)2HPO4 (98.5%), ammonia (A.R.) and HNO3 (A.R.). La(NO3)3 and Eu(NO3)3 were obtained by dissolving La2O3 and Eu2O3 in HNO3 under heating with agitation. (NH4)2HPO4 was added into the La2O3 and Eu2O3 aqueous solution under stirring. The pH value of the resultant solution was adjusted to about 4.0 using dilute HNO3 and dilute ammonia. Finally, the mixture of solutions was loaded quietly at 80 C for a certain time. After cooling to ambient temperature naturally, the precipitations were centrifugally washed with deionized water three times. Then they were dried at 80 C in the air. The final precipitations were fired at a constant heating rate of 10 C min1 from room temperature to 600 C or 1000 C in air respectively, and the temperature was maintained for 4 h to obtain the LaPO4:Eu3þ products. Finally, the temperature decreased at the cooling rate of 3 C min1. The bulk LaPO4:Eu3þ were prepared by mixing stoichiometric quantities of La2O3 (99.95%), Eu2O3 (99.99%) and (NH4)2HPO4 (98.5%, excess of 5 mol %). Excess of (NH4)2HPO4 was added to compensate the evaporation loss at higher temperature. After intimately mixing the starting materials with ethanol, the mixtures were heated at 1100 C for 6 h and cooled down to the room temperature. 2.2. Characterization All the samples were checked by X-ray diffraction (XRD) using Rigaku diffractometer with Ni-filtered CuKa radiation. The differential thermal analysis (DTA)-thermogravimetry (TG) (Du Pont Instrument 1090B) was employed to obtain the thermal decomposition behavior of the precipitations during the nitrogen annealing with a heating rate of 10 C min1. The morphology and composition of the samples were carried out using scanning electron microscopy (SEM) (Hitachi S-4800) and transmission electron microscopy (TEM) (FEI Tecnai F30, operated at 300 kV). The X-rays photoelectron spectroscopy (XPS) measurements were performed in a Kratos AXIS Ultra DLD spectrometer. The emission spectra were measured by FLS-920T fluorescence spectrophotometer with a VM504-type vacuum monochromator using a deuterium lamp as the lighting source. All the measurements were performed at room temperature. 3. Results and discussion
Fig. 1. XRD patterns of LaPO4 with the sintering temperature of 600 C, 1000 C and 1100 C.
accompanied by a heavy weight loss step at about 212 C can be found, and the peak could be due to the loss of absorbed water in the precipitate [14]. Another endothermic peak without a weightchange step around 951 C could also be found. We suppose this peaks should correspond to the phase transitions from hexagonal structure to monoclinic structure because lanthanide phosphates have several polymorphic forms; the hexagonal structure is the low-temperature phase, while the monoclinic structure is the hightemperature one, which has been studied by Rose Mooney [15]. 3.2. Morphology analysis The morphologies of the obtained LaPO4:Eu3þ did not exhibit obvious difference, when the Eu3þ concentration changes in a range of 0e20 %. Fig. 3(a) and (b) only show the SEM images of LaPO4 samples synthesized by the precipitation method with the sintering temperature of 600 C and 1000 C, respectively. Fig. 3(c) shows the SEM image of LaPO4 sample synthesized by the solid state reaction. The obtained sample has an irregular shape and the particle size in the range of 5e80 mm, which indicates the obtained sample is the bulk materials. The TEM image of the typical La0.95PO4:0.05Eu3þ with the sintering temperature of 1000 C is shown in Fig. 4. The TEM image
3.1. XRD analysis Fig. 1 shows the XRD patterns of LaPO4 with the sintering temperature of 600 C, 1000 C and 1100 C, respectively. From this figure, it is observed that the reflection planes are matched well with the reported monoclinic phase of lanthanum phosphate when the sintering temperature is at 1000 C and 1100 C, which indicates the obtained samples belong to monoclinic LaPO4 when the sintering temperature is at 1000 C and 1100 C. The reflection planes of the product which is sintered at 600 C are matched well with the reported hexagon phase of lanthanum phosphate, which implies that the obtained sample is the hexagon LaPO4 when the sintering temperature is at 600 C. No other peaks are observed in these XRD patterns, which indicate that all the products are a single phase. In order to investigate the phase transfer, DTA-TG is employed. The result of DTA-TG of the precipitated samples heat-treated in air up to 1250 C is shown in Fig. 2. A distinct endothermic peak
Fig. 2. DTA-TG of the precipitated samples heat-treated in air up to 1250 C.
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Fig. 3. SEM images of LaPO4 samples synthesized by the precipitation method with the sintering temperature of (a) 600 C and (b)1000 C, and (c) solid state reaction.
displays nanowire morphology with the diameter about 33 nm and the length from 0.1 to 2 mm, which well indicates that the obtained samples belong to the one-dimensional nanometer materials. The growth mechanism for nanomaterials including the monoclinic LaPO4:Eu3þ nanowires considered in our work is very complicated. Nevertheless, we try to propose a possible growth mechanism of monoclinic LaPO4 nanowires. In our experiment, when the sintering temperature is lower than 600 C, LaPO4:Eu3þ possess as a hexagonal crystal structure as shown in Fig. 1. The formation of hexagonal LaPO4:Eu3þ nanowires is the result of the highly preferential growth along the c axis, which has been suggested in previous studied as the intrinsic nature in the growth of hexagonal LnPO4:Eu3þ (Ln ¼ La, Ce) [16,17]. The monoclinic LaPO4:Eu3þ nanowires might be derived from the hexagonal LaPO4:Eu3þ nanowires. That is, with the rising of the reaction
temperature, the hexagonal LaPO4:Eu3þ nanowires are first formed and then are transformed to the monoclinic LaPO4:Eu3þ nanowires when the sintering temperature is higher than 600 C. In order to confirm that whether the Eu3þ is doped into the La1xPO4:xEu3þ nanowires, the typical XPS of La1xPO4:xEu3þ are obtained as Fig. 5. The binding energy surveys from 0 to 1200 eV. The XPS profile is similar for the series of La1xPO4:xEu3þ (x ¼ 0.05, 0.10, 0.15, 0.20) nanowires, here we show the typical XPS analysis of La0.95PO4:0.05Eu3þ nanowires. The peak at 1164.5 eV can be assigned to the Eu 3d3 orbital. This confirms that the Eu3þ ions have been doped into LaPO4 successfully. The surface elemental analysis also has been adopted. The Eu concentrations on La1xPO4:xEu3þ (x ¼ 0.05, 0.10, 0.15, 0.20) nanowires surface are shown in the Table 1. The results are very surprising. The Eu concentrations on the nanowire surface are largely higher than the nominal value and show only slight change with the increasing Eu3þ content, which hints that the doping Eu3þ ions mainly locate at the nanowire
Fig. 4. TEM image of La0.95PO4:0.05Eu3þ nanowires.
Fig. 5. XPS analysis of La0.95PO4:0.05Eu3þ nanowires.
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D. Wang et al. / Materials Chemistry and Physics 147 (2014) 831e835
Table 1 The Eu concentrations on La1xPO4:xEu3þ (x ¼ 0.05, 0.10, 0.15, 0.20) nanowires surface. La1xPO4:xEu3þ
x x x x
¼ ¼ ¼ ¼
5% 10% 15% 20%
Table 2 R/O Ratios and chromaticity coordinates of bulk and nanowires La0.95PO4:0.05Eu3þ samples with the sintering temperature of 600 C, 1000 C.
Eu concentration on the nanowires surface
Sample
Nominal value
NTSC Nanowire (sintering temperature 600 C) Nanowire (sintering temperature 1000 C) Bulk
5% 10% 15% 20%
Experimental value 72.8% 71.7% 75.6% 74.8%
surface. This phenomenon should be the result of the “self-purification” mechanisms of nanomaterilas. 3.3. Photoluminescence properties Fig. 6 shows the emission spectra of bulk La0.95PO4:0.05Eu3þ and nanowires La0.95PO4:0.05Eu3þ with the sintering temperature of 600 C, 1000 C, respectively. The excitation wavelengths of all the samples are 147 nm. When the Eu3þ concentration is changed from 0 to 20 %, the shapes and the wavelengths of the emission peaks are similar, and only the emission intensity is changed, thus we only show one emission spectrum for every series of samples. In these emission spectra, four emission peaks are observed with the maxima at 593 (±1) nm, 618 (±2) nm, 655 (±1) nm, 697 (±2) nm and can be assigned to the 5D0 / 7Fj(j ¼ 1,2,3,4) transitions of Eu3þ, respectively. The allowed 5D0 / 7F1 transitions are higher than the forbidden 5D0 / 7F2 transitions of Eu3þ ions when the sintering temperature is at 1000 C, this is consistent with that of bulk La0.95PO4:0.05Eu3þ which is sintered at 1100 C. But the forbidden 5 D0 / 7F2 transitions of Eu3þ ions has a higher integrated emission intensity than the allowed 5D0 / 7F1 transitions when the sintering temperature is at 600 C. Table 2 shows the R/O Ratios and chromaticity coordinates of bulk La0.95PO4:0.05Eu3þ and nanowires La0.95PO4:0.05Eu3þ samples with the sintering temperature of 600 C, 1000 C. From Table 2, it can be found that the chromaticity of La0.95PO4:0.05Eu3þ can not be improved through synthesizing the nanosized La0.95PO4:0.05Eu3þ phosphor, but can be improved when the sintering temperature declines to 600 C. One reason for the improved R/O value is that the low-temperature phase of La0.95PO4:0.05Eu3þ belongs to hexagon structure and Eu ions locate in a distortion polyhedron in this structure [18], therefore, the
Fig. 6. Emission spectra of bulk (c) and nanowires La0.95PO4:0.05Eu3þ with the sintering temperature of 600 C (a), 1000 C (b).
R/O
Chromaticity coordinates
1.379
0.67 0.633
0.33 0.367
0.951
0.610
0.386
0.942
0.616
0.383
forbidden transition (around 618 nm) of Eu3þ has higher emission intensity than the allowed transition (around 593 nm) of Eu3þ. The other reason is that the poor crystallization results in the high level of disorder of Eu3þ, so the forbidden transition is released and has high emission intensity. Fig. 7 shows the change curve of relative integrated emission intensity of La1xPO4:xEu3þ (a, sintering temperature 600 C, 0.05 x 0.20), La1yPO4:yEu3þ (b, sintering temperature 1000 C, 0.05 y 0.20) and La1zPO4:zEu3þ (c, bulk, 0.05 z 0.20) with the increase of the Eu concentration under 147 nm excitation. From this figure, we find a very interesting phenomenon that is the emission intensity of LaPO4:Eu3þ nanowires maintains a certain value when the Eu3þ doping concentrations were changed in the range of 5e15 mol %. That means that the emission intensities of LaPO4:Eu3þ nanowires under VUV excitation is not sensitive to Eu3þ doping concentrations. This phenomenon was not observed in that of bulk samples (Fig. 7(c)). The main reason for this phenomenon should be due to the penetration depth of VUV radiation and the distribution of the doping ions. According to Modi's work [19], the depth of incident radiation are all restricted to 8.0 nm (10e250 eV). Considering the penetration depth of VUV radiation can be affect by the photon energy, diffusion to the surface, dielectric permittivity of materials and incident angle, the penetration depth of VUV radiation in our test condition is not more than 10 nm [20]. It will cause that the VUV light could only excite part of the phosphor particle where it is near the surface. What's more, the activator ions concentration on the nanophosphor surface and in the interior is different. This phenomenon is called the
Fig. 7. The change curve of relative integrated emission intensity of La1xPO4:xEu3þ (a, sintering temperature 600 C, 0.05 x 0.2), La1yPO4:yEu3þ (b, sintering temperature 1000 C, 0.05 y 0.2) and La1zPO4:zEu3þ (c, bulk, 0.05 z 0.2) with the increase of the Eu concentration under 147 nm excitation.
D. Wang et al. / Materials Chemistry and Physics 147 (2014) 831e835
“self-purification” mechanisms and has been observed in many nanosystems [12,21]. It could be expressed as the impurities are hard to be doped into nanocrystals and easy to be expelled and move to the surface. That means that the doping ions preferably occupy the lattice sites that are near the nanoparticle surface. Until the surface layer reaches “saturation” concentration, then the surplus doping ions distribute from the surface to the interior. Finally, the doping ions have a high concentration in the surface of the nanophosphor particles and form a “shell” around the nanophosphor particle. This has been confirmed by the results of XPS in Table 1. Little activator exists in the interior of the nanophosphor particles. However, the penetration depth of VUV radiation is a constant, thus the emission intensities is not sensitive to Eu3þ concentrations when the depth of the “shell” is larger than the penetration depth until the concentration quenching happened. For bulk LaPO4:Eu3þ, the activator is a homogeneous distribution in the phosphor particles. The activator concentration on the phosphor surface increases along with the increase of Eu3þ concentration. Therefore, the integrated emission intensity is sensitive to the Eu concentration and increases linearly until the concentration quenching happened when Eu concentration is larger than 0.15 mol. The similar phenomenon has been observed in YBO3:Eu3þ nanoparticle [12]. Our experimental result indicates that LaPO4:Eu3þ phosphor with the one-dimensional nanostructure has the similar luminescent properties as the nanoparticle phosphors. 4. Conclusions The chromaticity of La0.95PO4:0.05Eu3þ can be improved through declining the sintering temperature to 600 C. Under 147 nm excitation, the emission intensity of LaPO4:Eu3þ nanowires phosphors maintain a certain value when Eu concentration is in the range of 0.05e0.15 mol, which due to the “self-purificationy mechanism of La1xPO4:xEu3þ with the one-dimensional nanostructure.
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Acknowledgment This work was supported by the Natural Science Foundation of Gansu Province, China (grant No. 1208RJYA022) and the National Science Foundation (51202099). References [1] R. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416e419. [2] H. Zhu, D.J. Zuo, Phys. Chem. C 113 (2009) 10402e10406. [3] W.-S. Song, H.-N. Choi, Y.-S. Kim, H. Yang, J. Mater. Chem. 20 (2010) 6929e6934. [4] D. Wang, Y. Wang, Mater. Res. Bull. 42 (2007) 2163e2169. [5] X. Wu, H. You, H. Cui, X. Zeng, G. Hong, C.-H. Kim, C.-H. Pyun, B.-Y. Yu, C.H. Park, Mater. Res. Bull. 37 (2002) 1531e1538. [6] Y. Yang, Mater. Sci. Eng. B 178 (2013) 807e810. [7] G. Li, L. Li, M. Li, Y. Song, H. Zou, L. Zou, X. Xu, S. Gan, Mater. Chem. Phys. 133 (2012) 263e268. [8] K.-S. Sohn, I.I.W. Zeon, H. Chang, S.K. Lee, H.D. Park, Chem. Mater. 14 (2002) 2140e2148. [9] Y. Wang, K. Uheda, H. Takizawa, U. Mizumoto, T. Endo, J. Electrochem. Soc. 148 (2001) G430eG433. [10] Y. Wang, T. Endo, L. He, C. Wu, J. Cryst, Growth 268 (2004) 568e574. [11] M. Ren, J. Lin, Y. Dong, L. Yang, M. Su, L. You, Chem. Mater. 11 (1999) 1576e1580. [12] Q. Dong, Y. Wang, Z. Wang, X. Yu, B. Liu, J. Phys. Chem. C 114 (2010) 9245e9250. [13] Raghvendra S. Yadav, Ranu K. Dutta, Manvendra Kumar, Avinash C. Pandey, J. Lumin. 129 (2009) 1078e1082. [14] K.C. Song, Y. Kang, Mater. Lett. 42 (2000) 283e289. [15] R. Mooney, Acta Crystallogr. 3 (1950) 337e340. [16] Y.P. Fang, A.W. Xu, R.Q. Song, H.X. Zhang, L.P. You, J.C. Yu, H.Q. Liu, J. Am. Chem. Soc. 125 (2003) 16025e16034. [17] Y. Zhang, H. Guan, J. Cryst, Growth 256 (2003) 156e161. [18] M. Yang, H. You, G. Jia, Y. Huang, Y. Song, Y. Zheng, K. Liu, L. Zhang, J. Cryst. Growth 311 (2009) 4753e4758. [19] M. Modi, G. Lodha, P. Srivastava, A. Sinha, R. Nandedkar, Phys Rev B 74 (2006), 045326/1e045326/6. [20] M. Terekhin, A. Vasil'ev, M. Kamada, E. Nakamura, S.Kubota, Phys Rev B 52 (1995) 3117e3121. [21] Steven C. Erwin, Lijun Zu, Michael I. Haftel, Alexander L. Efros, Thomas A. Kennedy, David J. Norris, Nature 436 (2005) 91e94.