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Photoluminescence properties of nanocrystalline La2TeO6:Eu3+ phosphor prepared by Pechini sol–gel method
ARTICLE IN PRESS Journal of Luminescence 130 (2010) 1124–1127
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence properties of nanocrystalline La2TeO6:Eu3 + phosphor prepared by Pechini sol–gel method Jaime Llanos n, Rodrigo Castillo ´lica del Norte, Avda. Angamos 0610, Casilla 1280, Antofagasta, Chile Departamento de Quı´mica, Universidad Cato
a r t i c l e in f o
a b s t r a c t
Article history: Received 10 September 2009 Received in revised form 6 January 2010 Accepted 1 February 2010 Available online 6 February 2010
La2TeO6:Eu3 + nanophosphors were prepared by Pechini sol–gel process, using lanthanide nitrates and telluric acid as precursor. X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), thermogravimetric analysis (TG), photoluminescence spectra (PL) and fluorescence lifetime were used to characterize the resulting phosphors. The results of XRD indicate that all samples crystallized completely at 1023 K and are isostructural with the orthorhombic La2TeO6. SEM study reveals that the samples have a strong tendency to form agglomerates with an average size ranging from 50 to 80 nm. The photoluminescence intensity and chromaticity were improved for excitation at 254 and 395 nm. The optimized phosphor La1.80Eu0.10TeO6 could be considered as an efficient redemitting phosphor for solid-state lighting devices based on GaN LEDs. & 2010 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Phosphors Inorganic compounds Optical properties
1. Introduction Rare-earth-ion doped materials have attracted considerable interest because of their excellent luminescent properties and consequent applications in lasers, wide gap electroluminescent devices, scintillator detectors and phosphors [1–4]. Ions with no 4f electrons, i.e. Y3 + and La3 + , have no electronic levels that can induce luminescent process in or near the visible region. In contrast, the ions from Ce3 + to Yb3 + , which have partially filled 4f orbitals, have energy levels characteristic of each ion and show a variety of optical properties around the visible region [5–7]. Many of these ions are used as activator ions in inorganic phosphors, mostly by replacing Y3 + or La3 + in various compound crystals [8]. To prepare a phosphor is not an easy task. The shape and size of phosphor’s particles are critical in the possible application of these materials. Depending on the method of preparation, temperature, pressure, etc., the atomic arrangements in the boundary region of the nanocrystal may be disordered, shortrange ordered, or large range ordered [9]. Therefore, it is reasonable to suppose the existence of various phases on the surface of the lanthanide-doped nanophosphors. The dopant ions at different phases can experience different electronic environment, and consequently, their optical transitions can be slightly different, and the intensity pattern of spectral lines can be quite different [10]. On the other hand, a high structural defects concentration on the surface of the nanophosphor causes the
n
Corresponding author. Tel.: + 56 55 355624; fax: + 56 55 355632. E-mail address: [email protected] (J. Llanos).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.02.007
quenching of optical emission [11]. Therefore, for a lanthanidedoped nanophosphor, the control of the distribution of dopants and defects is extremely important for the development of efficient luminescent materials. All these factors can be utilized as a way to enhance the luminescent characteristics of the phosphors. In the last few years, we have been actively involved in the preparation and characterization of rare-earth powder phosphors. We have prepared and characterized the Eu3 + , Tb3 + and Eu3 + :Gd3 + -doped orthotellurates, Ln2TeO6 (Ln=La, Gd) as bulk materials [12–14]. On the other hand, it has been reported that the grain size could enhance the chromaticity of phosphors, and many methods of synthesis have been investigated for this purpose [15–17]. In this paper, the Ln2TeO6:Eu3 + phosphor was synthesized using the sol–gel process with the Pechini method in order to investigate the effect of the nanoparticles on the luminescence properties of the product [18]. The optimizations of dopant ion concentration and the luminescence decay time were also investigated.
2. Experimental 2.1. Synthesis The La2 xEuxTeO6 phosphors were prepared by the Pechini sol–gel method [18]. According to the stoichiometric formula, 4.36 10 3 mol of La2O3 (Aldrich, 99.99% pure) and Eu2O3 (Aldrich, 99.99% pure) in different ratios were dissolved in 30 ml of HNO3 (0.5 mol dm 3) under vigorous stirring. The pH of the
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La2TeO6
La1.90Eu0.10TeO6bulk
2.2. Characterization
10
20
30
40
50
60
2θ Fig. 1. XRD patterns of La2 xEuxTeO6.
100 90 80 70
% Weight
To check the phase’s purity, powder X-ray diffraction (PXD) patterns were collected with a Siemens D-5000 diffractometer fitted with a graphite monochromator, using CuKa1 radiation, l = 0.15406 nm. All measurements were carried out at room temperature. Thermogravimetric analyses were performed using a Perkin–Elmer, Pyris TGA-7 apparatus. The experiments were carried out in an atmosphere of Ar, using sample masses of approximately 40–50 mg in platinum sample pans and heating at 10 K/min from room temperature to 1473 K. The surface morphology of the nanocrystalline La2 xEuxTeO6 phosphors was inspected using scanning electron microscopy (SEM, Jeol, JSM6360 LV) and atomic force microscopy (AFM, Witec) with contact mode. The photoluminescence (PL) spectra (emission and excitation) and fluorescence lifetime were measured using the JASCO FP-6500 spectrofluorometer. All spectra were registered at room temperature. In order to compare the photoluminescence intensity, the amount of samples was the same in all experiments.
102
La1.86Eu0.14TeO6
Intensity
solution was adjusted between 1 and 2. When the oxides were completely dissolved, they were mixed with water–ethanol (v/v =1:7) solution containing citric acid (Merck, A.R.) as chelating agent for the metal ions and 4.36 10 3 mol of H6TeO6 (Aldrich, 97.5–102.5% pure). The molar ratio of telluric acid to citric acid was 1:2. Afterward, ca. 1.25 g of polyethylene glycol (PEG, M.W.= 20,000, Fluka, A.R.) was added as a cross-linking agent. Transparent sols were obtained after stirring for 2 h. The sols were dried in a 374 K water bath. When the sols were completely dry, they were annealed at 673 K in a furnace. After annealing, the resulting powders were fired to 1073 K with a heating rate of 1 K/min and kept there for 2 h. Optical inspection of the products showed homogeneous powders of white color.
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3. Results and discussion
20 10
A total of eight compositions were synthesized in the system La2 xEuxTeO6 (x =0.00, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14). The powder X-ray diffraction patterns of the compounds reveal that all the phosphors that all samples crystallized isostructurally with the orthorhombic La2TeO6-type structure in the space group P212121 [19]. The powder X-ray diffraction patterns of La2TeO6, La1.90Eu0.10TeO6 and La1.86Eu0.14TeO6 synthesized by sol–gel method, and the X-ray diffraction pattern of La1.90Eu0.10TeO6 synthesized by solid-state reaction are shown in Fig. 1. By using the Scherrer equation it is possible to estimate the crystallite size. The Scherrer equation, D=0.90lXb cos y, predicts ˚ Since crystallite thickness if crystals are smaller than 1000 A. small angular differences in angle are associated with large spatial distances (inverse space), broadening of a diffraction peak is expected to reflect some scale feature in the crystal. In this equation D is the average grain size, l is the wavelength of the radiation used in the diffraction experiments, y is the diffraction angle and b is the full-width at half-maximum (FWHM) of the observed peak [20,21]. The strongest diffraction peaks (1 2 1) were used to calculate the grain sizes of the samples. Our results show that the crystallite sizes of La2 xEuxTeO6 are in the range 80–50 nm. (DLa2 TeO6 ¼ 56 nm DLa1:86 Eu0:14 TeO6 ¼ 71 nm, and DLa1:90 Eu0:10 TeO6 ¼ 79 nm). The TG analysis of the dry sol precursor containing La and Eu, and the organic phases are shown in Fig. 2. A fast weight loss stage is observed between 100 and about 700 1C. This stage is originated by the loss of water and the burnout of the organic species in the reaction mixture; finally the La2 xEuxTeO6 is stabilized at about 725 1C.
0 400
600
800
1000
1200
Temperature (K) Fig. 2. TG curves for La2TeO6:Eu3 + precursor.
The morphology of the crystallites of La2 xEuxTeO6 was inspected using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SEM study reveals that the samples have a strong tendency to form particle aggregates, as shown in Fig. 3. It is seen from the AFM microphotograph that the average crystallite size of the nanocrystals is about 70 nm. These results are in good agreements with crystallite sizes calculated from the XRD experiments. Fig. 4 shows the excitation spectra of La1.90Eu0.10TeO6 synthesized by sol–gel method utilizing the Pechini-type process and solid-state reactions. It is observed that the CTB in the 250–300 nm ranges is shifted in the long wavelength. As a result of this red-shift, the samples prepared by Pechini-type process can absorb 250–270 nm excitation light more efficiently than the samples prepared by solid-state methods. Taking into account that the CT band is related with covalency between O2 and Eu3 + in the structure [6], the red shift could be explained by the nano-size effect. Previous work have reported that the nanocrystals prepared by sol–gel pyrolysis showed longer Eu–O bonds with decrease in the particle size [22]. The emission spectra for La2 xEuxTeO6 (x= 0.00, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14) and for La1.90Eu0.10TeO6, the latter as
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f g
λex= 254 nm
λex= 395 nm
Intensity (a. u.)
h f g h
e
e
d c
d c
b
570
600
b
a
a 630
570
600
630
Wavelength (nm) Fig. 5. Emission spectra of La2 xEuxTeO6 (x = 0.02 (b); 0.04 (c); 0.06 (d); 0.08 (e); 0.10 (f); 0.12 (g); 0.14 (h)) prepared by sol–gel method and synthesized by solid-state reaction (a). (lex =254 nm, left and lex =395 nm, right.)
sol-gel, λex= 254 nm s-s, λex= 254 nm sol-gel, λex= 395 nm
Intensity (a. u.)
s-s, λex= 395 nm
Fig. 3. SEM (a) and AFM (b) images of La1.90Eu0.10TeO6 agglomerates synthesized by Pechini sol–gel process.
7
5
F0 → D2
Sol-gel Solid-State
1
2
3
4
5
6
7
8
% Eu 7
Fig. 6. Effect of the Eu3 + concentration on the 617 nm emission peak intensity under an excitation of 254 and 395 nm.
5
Intensity (a. u.)
F0 → D1
7
5
F0 → L6
250
300
350
400
450
500
550
Wavelength (nm) Fig. 4. Excitation (lem = 617 nm) spectrum of La1.90Eu0.10TeO6 phosphor prepared by sol–gel method (solid line) and by solid-state reaction (dash line).
bulk sample are shown in Fig. 5. All the spectra show similar features, and exhibit the characteristic emission originated from the transition between the excited state 5D0 to the ground state 7 FJ (J= 0, 1, 2, 3, 4) of the 4f6 configuration of Eu3 + [23]. It is
observed that the peak for the 617 nm wavelength is stronger in the nanocrystalline samples than in the bulk samples. This enhancement in the chromaticity of La2 xEuxTeO6 phosphors is not explained by the particle size, but has to do with the degree of crystallization of the nanocrystallites. On the other hand, the CIE chromaticity coordinates are calculated to be x = 0.67 and y= 0.33 for La1.90Eu0.10TeO6 (the optimized phosphor). Compared with National Television System Committee (NTSC) standard CIE chromaticity coordinates values for red (x= 0.67 and y= 0.33), it was found that the color coordinates of the red phosphor La1.90Eu0.10TeO6 synthesized by Pechini process are in good agreement with the NSTC standard values. The color purity of these phosphors is satisfied and could be component of the tricolor phosphors. The intensity of the strong 5D0-7F2 emission lines (617 nm) on excitation at both 254 and 395 nm was found to increase with an increase in Eu3 + concentration up to 5 at%. It is shown in Fig. 6 that the sol–gel samples and the bulk samples show the same dependence of Eu3 + concentration with the same maximum
ARTICLE IN PRESS J. Llanos, R. Castillo / Journal of Luminescence 130 (2010) 1124–1127
λex = 254 nm
500
Intensity (a. u.)
1.35 1.30
1.20
La1,90 Eu
TeO6
0,10
300
monoexponential fit
200 100
0
1.15
2
4
6
8
10
Decay Time (ms)
1.10 100
1.05 1.00 0.95 0.90
80
La1,90 Eu0,10TeO6
60
monoexponential fit
40 20 0
0
2
0.85
1
2
3
4
6
Decay Time (ms)
4
5
Eu Fig. 7. Lifetimes of Eu 5 at% Eu3 + .
400
0
Intensity (a. u.)
Decay Time (ms)
1.25
3+
1127
3+
8
6
λex = 395 nm
10
7 1
2
3
4
5
6
7
concentration (% at)
as a function of its concentration (lex = 254 nm, left and lex =395 nm, right). Insets show the mono-exponential fitting of the decay curves of
intensity. However, the luminescent intensity was improved when the phosphors were prepared by sol–gel method. Fig. 7 shows the dependence of the lifetime of the rare-earth ion on its doping concentration. Representative PL decay curves, at both 254 and 395 nm, of La1.90Eu0.10TeO6 are shown in the respective inset. These curves are well fitted into a monoexponential function and can be represented by I=I0exp (t/t), where I and I0 are the luminescence intensities at time t and 0, and t represents the radiative decay time [24]. The results show that the decay mechanism of the 5D0-7F2 transition is due to only the Eu3 + ions in the structure, which are homogeneous distributed inside the host lattice without surface defects.
4. Conclusions We have prepared red-emitting Eu3 + doped powder phosphors by employing the Pechini sol–gel process using the lanthanide oxides as the main precursor. All phases were annealed at 1073 K. The phase purity has been verified by X-ray powder diffraction. The optimum concentration of Eu3 + has been identified to be 5 at% by both the bulk and the nanocrystalline samples. Results indicated that phosphors La2 xEuxTeO6 prepared by sol–gel method have high luminescent intensity and chromaticity, which is better than those of the phosphors prepared by conventional solid-state reactions. These changes are attributed to the crystallinity of the nanocrystalline particles. The red emission due to Eu3 + f-f transitions at 395 nm is also enhanced in the phosphors prepared by sol–gel method. The study reveals that the optimized phosphor (La1.90Eu0.10TeO6) could be used as red component for white lighting devices excited in the near-UV.
Acknowledgment This work 1090327).
was supported
by FONDECYT-CHILE
(Grant
References [1] M.M. Lage, R.L. Moreira, F.M. Matinaga, J.-Y. Gesland, J. Chem. Mater. 17 (2005) 4523. [2] Y.Q. Quang, A.J. Steckle, Appl. Phys. Lett. 82 (2003) 502. [3] A.J. Steckle, J.M. Zavada, MRS Bull. 24 (1999) 16. [4] X.-J. Liu, H.-L. Li, R.-J. Xie, N. Hirosaki, X. Xu, L.-P. Huang, J. Lumin. 127 (2007) 469. [5] J.L. Kropp, M.W. Windsor, J. Chem. Phys. 42 (1965) 1599. [6] B.R. Judd, Phys. Rev. 127 (1962) 750. [7] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [8] T. Kano, in: W.M. Yen (Ed.), Phosphor Handbook, Taylor and Francis Group, 2006 (Chapter 3). [9] A.S. Edelstein, R.C. Cammarata, in: Nanomaterials: Synthesis, Properties and Applications, Institute of Physics Publishing, Bristol and Philadelphia, 1997. [10] G. Blasse, B.C. Grabmaier, in: Luminescent Materials, Springer-Verlag, Berlin, 1994. [11] R. Schmechel, M. Kennedy, H. von Seggern, H. Winkler, M. Kolbe, R.A. Fischer, X. Li, A. Benker, M. Winterer, H. Hahn, J. Appl. Phys. 89 (2001) 1679. [12] J. Llanos, R. Cortes, Mater. Res. Bull. 43 (2008) 2763. [13] J. Llanos, R. Castillo, W. Alvarez, Mater. Lett. 62 (2008) 3597. [14] J. Llanos, R. Castillo, J. Lumin. 129 (2009) 465. [15] U. Rambabu, N.R. Munirathnam, T.L. Prakash, S. Buddhudu, Mater. Chem. Phys. 78 (2002) 160. [16] H.Q. Liu, L.L. Wang, W. Huang, Z.W. Peng, Mater. Lett. 61 (2007) 1968. [17] X. Cui, W. Zhuang, Z. Yu, T. Xia, X. Huang, H. Li, J. Alloys Compd. 451 (2008) 280. [18] M.P. Pechini, U.S. Patent 3,330,697, 1967. [19] S.F. Meier, T. Schleid, J. Sol. State Chem. 171 (2003) 408. [20] A.L. Patterson, Phys. Rev. 56 (1939) 978. [21] Y.W. Zhang, Y. Yang, S. Jin, S.J. Tian, G.B. Li, J.T. Jia, C.S. Liao, C.H. Yan, Chem. Mater. 13 (2001) 372. [22] Z.-G. Wei, L.-D. Sun, X.-C. Jiang, C.-S. Liao, C.-H. Yan, Y. Tao, J. Zhang, T.-D. Hu, Y.-N. Xie, Chem. Mater. 15 (2003) 3011. [23] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2. [24] D.R. Vij, in: Luminescence of Solids, Plenum Press, New York, 1998.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence properties of nanocrystalline La2TeO6:Eu3 + phosphor prepared by Pechini sol–gel method Jaime Llanos n, Rodrigo Castillo ´lica del Norte, Avda. Angamos 0610, Casilla 1280, Antofagasta, Chile Departamento de Quı´mica, Universidad Cato
a r t i c l e in f o
a b s t r a c t
Article history: Received 10 September 2009 Received in revised form 6 January 2010 Accepted 1 February 2010 Available online 6 February 2010
La2TeO6:Eu3 + nanophosphors were prepared by Pechini sol–gel process, using lanthanide nitrates and telluric acid as precursor. X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), thermogravimetric analysis (TG), photoluminescence spectra (PL) and fluorescence lifetime were used to characterize the resulting phosphors. The results of XRD indicate that all samples crystallized completely at 1023 K and are isostructural with the orthorhombic La2TeO6. SEM study reveals that the samples have a strong tendency to form agglomerates with an average size ranging from 50 to 80 nm. The photoluminescence intensity and chromaticity were improved for excitation at 254 and 395 nm. The optimized phosphor La1.80Eu0.10TeO6 could be considered as an efficient redemitting phosphor for solid-state lighting devices based on GaN LEDs. & 2010 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Phosphors Inorganic compounds Optical properties
1. Introduction Rare-earth-ion doped materials have attracted considerable interest because of their excellent luminescent properties and consequent applications in lasers, wide gap electroluminescent devices, scintillator detectors and phosphors [1–4]. Ions with no 4f electrons, i.e. Y3 + and La3 + , have no electronic levels that can induce luminescent process in or near the visible region. In contrast, the ions from Ce3 + to Yb3 + , which have partially filled 4f orbitals, have energy levels characteristic of each ion and show a variety of optical properties around the visible region [5–7]. Many of these ions are used as activator ions in inorganic phosphors, mostly by replacing Y3 + or La3 + in various compound crystals [8]. To prepare a phosphor is not an easy task. The shape and size of phosphor’s particles are critical in the possible application of these materials. Depending on the method of preparation, temperature, pressure, etc., the atomic arrangements in the boundary region of the nanocrystal may be disordered, shortrange ordered, or large range ordered [9]. Therefore, it is reasonable to suppose the existence of various phases on the surface of the lanthanide-doped nanophosphors. The dopant ions at different phases can experience different electronic environment, and consequently, their optical transitions can be slightly different, and the intensity pattern of spectral lines can be quite different [10]. On the other hand, a high structural defects concentration on the surface of the nanophosphor causes the
n
Corresponding author. Tel.: + 56 55 355624; fax: + 56 55 355632. E-mail address: [email protected] (J. Llanos).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.02.007
quenching of optical emission [11]. Therefore, for a lanthanidedoped nanophosphor, the control of the distribution of dopants and defects is extremely important for the development of efficient luminescent materials. All these factors can be utilized as a way to enhance the luminescent characteristics of the phosphors. In the last few years, we have been actively involved in the preparation and characterization of rare-earth powder phosphors. We have prepared and characterized the Eu3 + , Tb3 + and Eu3 + :Gd3 + -doped orthotellurates, Ln2TeO6 (Ln=La, Gd) as bulk materials [12–14]. On the other hand, it has been reported that the grain size could enhance the chromaticity of phosphors, and many methods of synthesis have been investigated for this purpose [15–17]. In this paper, the Ln2TeO6:Eu3 + phosphor was synthesized using the sol–gel process with the Pechini method in order to investigate the effect of the nanoparticles on the luminescence properties of the product [18]. The optimizations of dopant ion concentration and the luminescence decay time were also investigated.
2. Experimental 2.1. Synthesis The La2 xEuxTeO6 phosphors were prepared by the Pechini sol–gel method [18]. According to the stoichiometric formula, 4.36 10 3 mol of La2O3 (Aldrich, 99.99% pure) and Eu2O3 (Aldrich, 99.99% pure) in different ratios were dissolved in 30 ml of HNO3 (0.5 mol dm 3) under vigorous stirring. The pH of the
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La1.90Eu0.10TeO6bulk
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10
20
30
40
50
60
2θ Fig. 1. XRD patterns of La2 xEuxTeO6.
100 90 80 70
% Weight
To check the phase’s purity, powder X-ray diffraction (PXD) patterns were collected with a Siemens D-5000 diffractometer fitted with a graphite monochromator, using CuKa1 radiation, l = 0.15406 nm. All measurements were carried out at room temperature. Thermogravimetric analyses were performed using a Perkin–Elmer, Pyris TGA-7 apparatus. The experiments were carried out in an atmosphere of Ar, using sample masses of approximately 40–50 mg in platinum sample pans and heating at 10 K/min from room temperature to 1473 K. The surface morphology of the nanocrystalline La2 xEuxTeO6 phosphors was inspected using scanning electron microscopy (SEM, Jeol, JSM6360 LV) and atomic force microscopy (AFM, Witec) with contact mode. The photoluminescence (PL) spectra (emission and excitation) and fluorescence lifetime were measured using the JASCO FP-6500 spectrofluorometer. All spectra were registered at room temperature. In order to compare the photoluminescence intensity, the amount of samples was the same in all experiments.
102
La1.86Eu0.14TeO6
Intensity
solution was adjusted between 1 and 2. When the oxides were completely dissolved, they were mixed with water–ethanol (v/v =1:7) solution containing citric acid (Merck, A.R.) as chelating agent for the metal ions and 4.36 10 3 mol of H6TeO6 (Aldrich, 97.5–102.5% pure). The molar ratio of telluric acid to citric acid was 1:2. Afterward, ca. 1.25 g of polyethylene glycol (PEG, M.W.= 20,000, Fluka, A.R.) was added as a cross-linking agent. Transparent sols were obtained after stirring for 2 h. The sols were dried in a 374 K water bath. When the sols were completely dry, they were annealed at 673 K in a furnace. After annealing, the resulting powders were fired to 1073 K with a heating rate of 1 K/min and kept there for 2 h. Optical inspection of the products showed homogeneous powders of white color.
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3. Results and discussion
20 10
A total of eight compositions were synthesized in the system La2 xEuxTeO6 (x =0.00, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14). The powder X-ray diffraction patterns of the compounds reveal that all the phosphors that all samples crystallized isostructurally with the orthorhombic La2TeO6-type structure in the space group P212121 [19]. The powder X-ray diffraction patterns of La2TeO6, La1.90Eu0.10TeO6 and La1.86Eu0.14TeO6 synthesized by sol–gel method, and the X-ray diffraction pattern of La1.90Eu0.10TeO6 synthesized by solid-state reaction are shown in Fig. 1. By using the Scherrer equation it is possible to estimate the crystallite size. The Scherrer equation, D=0.90lXb cos y, predicts ˚ Since crystallite thickness if crystals are smaller than 1000 A. small angular differences in angle are associated with large spatial distances (inverse space), broadening of a diffraction peak is expected to reflect some scale feature in the crystal. In this equation D is the average grain size, l is the wavelength of the radiation used in the diffraction experiments, y is the diffraction angle and b is the full-width at half-maximum (FWHM) of the observed peak [20,21]. The strongest diffraction peaks (1 2 1) were used to calculate the grain sizes of the samples. Our results show that the crystallite sizes of La2 xEuxTeO6 are in the range 80–50 nm. (DLa2 TeO6 ¼ 56 nm DLa1:86 Eu0:14 TeO6 ¼ 71 nm, and DLa1:90 Eu0:10 TeO6 ¼ 79 nm). The TG analysis of the dry sol precursor containing La and Eu, and the organic phases are shown in Fig. 2. A fast weight loss stage is observed between 100 and about 700 1C. This stage is originated by the loss of water and the burnout of the organic species in the reaction mixture; finally the La2 xEuxTeO6 is stabilized at about 725 1C.
0 400
600
800
1000
1200
Temperature (K) Fig. 2. TG curves for La2TeO6:Eu3 + precursor.
The morphology of the crystallites of La2 xEuxTeO6 was inspected using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SEM study reveals that the samples have a strong tendency to form particle aggregates, as shown in Fig. 3. It is seen from the AFM microphotograph that the average crystallite size of the nanocrystals is about 70 nm. These results are in good agreements with crystallite sizes calculated from the XRD experiments. Fig. 4 shows the excitation spectra of La1.90Eu0.10TeO6 synthesized by sol–gel method utilizing the Pechini-type process and solid-state reactions. It is observed that the CTB in the 250–300 nm ranges is shifted in the long wavelength. As a result of this red-shift, the samples prepared by Pechini-type process can absorb 250–270 nm excitation light more efficiently than the samples prepared by solid-state methods. Taking into account that the CT band is related with covalency between O2 and Eu3 + in the structure [6], the red shift could be explained by the nano-size effect. Previous work have reported that the nanocrystals prepared by sol–gel pyrolysis showed longer Eu–O bonds with decrease in the particle size [22]. The emission spectra for La2 xEuxTeO6 (x= 0.00, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14) and for La1.90Eu0.10TeO6, the latter as
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f g
λex= 254 nm
λex= 395 nm
Intensity (a. u.)
h f g h
e
e
d c
d c
b
570
600
b
a
a 630
570
600
630
Wavelength (nm) Fig. 5. Emission spectra of La2 xEuxTeO6 (x = 0.02 (b); 0.04 (c); 0.06 (d); 0.08 (e); 0.10 (f); 0.12 (g); 0.14 (h)) prepared by sol–gel method and synthesized by solid-state reaction (a). (lex =254 nm, left and lex =395 nm, right.)
sol-gel, λex= 254 nm s-s, λex= 254 nm sol-gel, λex= 395 nm
Intensity (a. u.)
s-s, λex= 395 nm
Fig. 3. SEM (a) and AFM (b) images of La1.90Eu0.10TeO6 agglomerates synthesized by Pechini sol–gel process.
7
5
F0 → D2
Sol-gel Solid-State
1
2
3
4
5
6
7
8
% Eu 7
Fig. 6. Effect of the Eu3 + concentration on the 617 nm emission peak intensity under an excitation of 254 and 395 nm.
5
Intensity (a. u.)
F0 → D1
7
5
F0 → L6
250
300
350
400
450
500
550
Wavelength (nm) Fig. 4. Excitation (lem = 617 nm) spectrum of La1.90Eu0.10TeO6 phosphor prepared by sol–gel method (solid line) and by solid-state reaction (dash line).
bulk sample are shown in Fig. 5. All the spectra show similar features, and exhibit the characteristic emission originated from the transition between the excited state 5D0 to the ground state 7 FJ (J= 0, 1, 2, 3, 4) of the 4f6 configuration of Eu3 + [23]. It is
observed that the peak for the 617 nm wavelength is stronger in the nanocrystalline samples than in the bulk samples. This enhancement in the chromaticity of La2 xEuxTeO6 phosphors is not explained by the particle size, but has to do with the degree of crystallization of the nanocrystallites. On the other hand, the CIE chromaticity coordinates are calculated to be x = 0.67 and y= 0.33 for La1.90Eu0.10TeO6 (the optimized phosphor). Compared with National Television System Committee (NTSC) standard CIE chromaticity coordinates values for red (x= 0.67 and y= 0.33), it was found that the color coordinates of the red phosphor La1.90Eu0.10TeO6 synthesized by Pechini process are in good agreement with the NSTC standard values. The color purity of these phosphors is satisfied and could be component of the tricolor phosphors. The intensity of the strong 5D0-7F2 emission lines (617 nm) on excitation at both 254 and 395 nm was found to increase with an increase in Eu3 + concentration up to 5 at%. It is shown in Fig. 6 that the sol–gel samples and the bulk samples show the same dependence of Eu3 + concentration with the same maximum
ARTICLE IN PRESS J. Llanos, R. Castillo / Journal of Luminescence 130 (2010) 1124–1127
λex = 254 nm
500
Intensity (a. u.)
1.35 1.30
1.20
La1,90 Eu
TeO6
0,10
300
monoexponential fit
200 100
0
1.15
2
4
6
8
10
Decay Time (ms)
1.10 100
1.05 1.00 0.95 0.90
80
La1,90 Eu0,10TeO6
60
monoexponential fit
40 20 0
0
2
0.85
1
2
3
4
6
Decay Time (ms)
4
5
Eu Fig. 7. Lifetimes of Eu 5 at% Eu3 + .
400
0
Intensity (a. u.)
Decay Time (ms)
1.25
3+
1127
3+
8
6
λex = 395 nm
10
7 1
2
3
4
5
6
7
concentration (% at)
as a function of its concentration (lex = 254 nm, left and lex =395 nm, right). Insets show the mono-exponential fitting of the decay curves of
intensity. However, the luminescent intensity was improved when the phosphors were prepared by sol–gel method. Fig. 7 shows the dependence of the lifetime of the rare-earth ion on its doping concentration. Representative PL decay curves, at both 254 and 395 nm, of La1.90Eu0.10TeO6 are shown in the respective inset. These curves are well fitted into a monoexponential function and can be represented by I=I0exp (t/t), where I and I0 are the luminescence intensities at time t and 0, and t represents the radiative decay time [24]. The results show that the decay mechanism of the 5D0-7F2 transition is due to only the Eu3 + ions in the structure, which are homogeneous distributed inside the host lattice without surface defects.
4. Conclusions We have prepared red-emitting Eu3 + doped powder phosphors by employing the Pechini sol–gel process using the lanthanide oxides as the main precursor. All phases were annealed at 1073 K. The phase purity has been verified by X-ray powder diffraction. The optimum concentration of Eu3 + has been identified to be 5 at% by both the bulk and the nanocrystalline samples. Results indicated that phosphors La2 xEuxTeO6 prepared by sol–gel method have high luminescent intensity and chromaticity, which is better than those of the phosphors prepared by conventional solid-state reactions. These changes are attributed to the crystallinity of the nanocrystalline particles. The red emission due to Eu3 + f-f transitions at 395 nm is also enhanced in the phosphors prepared by sol–gel method. The study reveals that the optimized phosphor (La1.90Eu0.10TeO6) could be used as red component for white lighting devices excited in the near-UV.
Acknowledgment This work 1090327).
was supported
by FONDECYT-CHILE
(Grant
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