Synthesis and photoluminescent properties of YVO4:Eu3+ nano-crystal phosphor prepared by Pechini process

ARTICLE IN PRESS Journal of Luminescence 129 (2009) 1181–1185

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Synthesis and photoluminescent properties of YVO4:Eu3+ nano-crystal phosphor prepared by Pechini process Yee-Shin Chang a, Feng-Ming Huang a, Yeou-Yih Tsai b, Lay-Gaik Teoh c, a

Department of Electronic Engineering, National Formosa University, Huwei, Yunlin 632, Taiwan Department of Electronic Engineering, Kao Yuan University, Lujhu, Kaohsiung 821, Taiwan c Department of Mechanical Engineering, National Pingtung University of Science and Technology, Neipu, Pingtung 912, Taiwan b

a r t i c l e in fo

abstract

Article history: Received 20 December 2008 Received in revised form 21 May 2009 Accepted 27 May 2009 Available online 6 June 2009

A sol–gel technique emphasizing the Pechini process has been employed for the preparation of nanocrystal Eu3+-doped YVO4 phosphor. The precursor powders were heated at 800 1C for 3 h to obtain good crystallinity with better luminescence. XRD results indicate that the second phase is not presented when the Eu3+ ion concentration is increased up to 50 mol%. The absorption and photoluminescent (PL) studies indicated that the energy is absorbed first by the host and then transferred to the emitting level of the Eu3+ ions. Excitation at 318 nm in terms of Eu3+ concentrations in YVO4 powders shows that the YVO4 phosphors display bright red luminescence at about 618 nm belonging to the 5D0-7F2 electric dipole transition, and a weak band in the orange region of the 5D0-7F1 transition at 594 nm. In addition, the time-resolved 5D0-7F2 transition presents a single-exponential decay behavior, revealing the decay mechanism of the 5D0-7F2 transition is a single decay component between Eu3+ ions only. The saturation of the emission intensity excited by the CTS when the Eu3+ concentration is 10 mol%. The concentration quenching is active when the Eu3+ concentration is larger than 10 mol%, and the ˚ critical distance is about 5.75 A.

Keywords: Yttrium vanadate Europium Photoluminescence Phosphor Pechini process

& 2009 Elsevier B.V. All rights reserved.

1. Introduction It is highly desirable to develop novel low-voltage phosphors with high efficiency and chemical stability under electron beam bombardment in a high-vacuum system for the next generation of field emission displays [1–5]. Oxide phosphors have recently gained a lot of attention for applications such as screens in plasma display panels and field emission displays, and for white color light-emitting diodes, because of their higher chemical stability relative to that of sulfide phosphors. Many studies have been conducted to develop new oxide phosphors in powder form to improve luminescent performance, with regard to color purity, emission intensity, and quantum efficiency [6,7]. It has been reported that fine grain size could enhance the emission efficiency and intensity of phosphors [7–10], and many chemical methods have been investigated for this, such as sol–gel [11–13], hydrothermal [14–16], and precipitate techniques [17]. The sol–gel process with the Pechini method is an attractive route that starts from molecular precursors and forms an oxide network via inorganic polymerization reactions, offering both product and

 Corresponding author. Tel.: +886 8 7703202x7525; fax: +886 8 7740142.

E-mail address: [email protected] (L.-G. Teoh). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.05.020

processing advantages such as high purity, ultrahomogeneity, and reduction of the sintering temperature [18]. Many efforts have been made to discover host materials as well as activators with high performance for phosphor applications [19,20], and rare-earth ions have been extensively employed. Eu3+-doped compounds which emit bright red light have been widely used as phosphors in many fields [21–25], showing typical f–f transitions of europium ions. For trivalent europium ions, a given optical center in different host lattices will exhibit different optical properties due to the changes in the surroundings of the center. Yttrium vanadium oxide (YVO4) has a tetragonal type oxide structure and possesses excellent electro-optical and acousticoptical properties, making it suitable for different parts of optical devices such as solid-state laser host [26], polarizer [27,28], phosphor [29], and so on. Some previous studies [30–33] investigate that the effect of the sizes, morphologies of particles, and different calcination temperatures on the photoluminescence properties of YVO4:Eu3+ nano-crystalline phosphors. The results indicated that the emission intensities of Eu3+ ion f–f transitions increase with increasing the calcination temperatures, but the references concerning the effects of Eu3+ ion concentrations on the photoluminescence properties of YVO4:Eu3+ phosphors are deficient. In this paper, the Eu3+ ion-doped YVO4 phosphor was synthesized using the sol–gel method, and the influences of Eu3+

ARTICLE IN PRESS 1182

Y.-S. Chang et al. / Journal of Luminescence 129 (2009) 1181–1185

ion concentrations on the resulting structure and the photoluminescence properties including decay time, energy transfer behavior, and so on of YVO4:Eu3+ phosphors were also investigated.

2.1. Powder preparation The Eu3+-doped YVO4 phosphors were prepared by the sol–gel method using ammonium metavanadate (NH4VO3), yttrium acetate [Y(CH3CO2)3], and europium acetate [Eu(CH3CO2)3]. Starting materials with the purity of 99.99% were supplied from Aldrich Chemical Company, Inc, and even though they are not very sensitive to moisture, the handling of chemicals and the procedures were carried out in a dry N2 atmosphere. At first, 0.01 mole of ammonium metavanadate was dissolved with 20 ml of the ammonium hydroxide in 100 ml of deionized water, and the yttrium acetate (0.01x mole) and europium acetate (x mole, x ¼ 0.001–0.005) were separately dissolved in 100 ml of deionized water. Second, ammonium metavanadate, yttrium acetate, and europium acetate solutions were mixed in a round bottom flask. When the precursors were completely dissolved in the solution, 0.02 moles of citric acid and ethylene glycol were added. The solution was placed in an oven at 120 1C for 24 h to obtain the dried precursors, and the dried precursors were calcined at 800 1C in air for 3 h to form the YVO4:Eu3+ phosphors. The weights of obtained phosphor powders are about 4.5 g. Citric acid and ethylene glycol (propionic acid) are added to the above as a chelating agent and stabilizing agent, respectively. The amounts of citric acid and ethylene glycol were determined by the ratio of citric acid to metal cations.

50 mol%

Intensity (a.u.)

2. Experimental

800°C/3hr

30 mol%

10 mol%

5 mol% (200)

20

25

(112) (220) (301)

30

35

40

(312) (400)

45

50

(420)

55

60

1 mol%

(332)

65

70

75

80

Diffraction Angle (2θ) Fig. 1. The X-ray diffraction patterns of the YVO4 doped with different Eu3+ concentrations calcined at 800 1C for 3 h in air.

since both have the same valence. As the Eu3+ ion concentration increases further, the YVO4 phase still remains, but the intensities of the diffraction peaks decrease. This is because the grain of (YEu)VO4 decreased with the increasing Eu3+ ion concentrations losing the long-range order domains and decreasing the intensity of the diffraction peaks. 3.2. Microstructures

3. Results and discussion

Fig. 2 shows the SEM micrographs of YVO4 doped with 1, 10, and 50 mol% Eu3+ ions calcined at 800 1C in air for 3 h. It is obvious that the aggregation for calcined powder is quite acute. In lower Eu3+ ion concentration, up to 30 mol%, the particles sizes seem to increase along with the concentration, and appear to have a homogeneous and isotropic distribution. As the Eu3+ ion concentration increases further, the shape of the particles changes significantly. The particles becomes very small, and the structure of the powders seems to be fluffy when the Eu3+ ion concentration is 50 mol%. Table 1 shows the grain sizes of YVO4 doped with various Eu3+ concentrations calculated by Scherrer’s equation [35]. As can be seen, the grain sizes prepared by the Pechini process are all in the nano-scale range. The grain sizes increase to a maximum value for 30 mol% Eu3+-doped YVO4, and then decrease as the Eu3+ concentration increases. When doped with 50 mol% Eu3+ ions, the grain sizes only are about 11.6 nm, and which indicates the loss of long-range order domains and the decrease in the intensity of the diffraction peaks, as observed in the XRD analysis.

3.1. Structures

3.3. Absorption spectrum

Fig. 1 is the X-ray powder diffraction pattern of YVO4 doped with various Eu3+ concentrations calcined at 800 1C for 3 h in air. All the peaks are attributed to the YVO4 phase [JCPD no. 72-0861]. The full-width at half-maximum (FWHM)) of these peaks increase in a low-doped concentration, and best crystallinity is for YVO4 doped with 10 mol% Eu3+ ions. The trivalent europium ions ˚ [34] are introduced to substitute the trivalent yttrium (0.947 A) ˚ [34] in the (YEu)VO4 system. The variations are almost ions (0.9 A) the same for the Eu3+ and Y3+ ion radii, so they can form a solid solution easily. Additionally, for the Eu3+ ion substituting the Y3+ ion in the YVO4 lattice, there are no charge compensation issues

Fig. 3 is the optical absorption spectra for pure YVO4 and YVO4:10 mol% Eu3+ powders which were measured at room temperature. The compounds exhibited broad bands, peaking at 266 and 320 nm which are attributed to the charge transfer from the oxygen ligands to the central vanadium atom inside the VO3 4 anionic group in the host lattice [36]. After europium was added to YVO4, the compounds exhibited a charge-transfer state (CTS) between 250 and 350 nm, caused by the electron transferred from the oxygen 2p orbital to the empty 4f orbital of europium, which may be described as ligand-to-Eu3+ charge-transfer transitions (LMCT) [37]. The absorption intensity is strong, which indicates

2.2. Characterizations The powders obtained were analyzed for crystal structure with an X-ray diffractometer (XRD, Rigaku Dmax-33 X-ray diffractometer) using Cu Ka radiation to identify the possible phases formed after heat treatment. The surface morphology of powders was examined using high-resolution scanning electron microscopy (HR-SEM, S4200, Hitachi). The Hitachi U-3010 UV visible spectrophotometer was carried out to measure the optical absorption behavior of the YVO4:Eu3+ phosphors which were placed inside the closed quartz glass and measured from 200 to 700 nm at room temperature. Both excitation and luminescence spectra of these phosphors were analyzed with a Hitachi F-4500 fluorescence spectrophotometer using a 150 W xenon arc lamp as the excitation source at room temperature.

ARTICLE IN PRESS Y.-S. Chang et al. / Journal of Luminescence 129 (2009) 1181–1185

1183

Fig. 3. Absorption spectra of undoped and 10 mol% Eu-doped YVO4 powders.

Fig. 4. The excitation spectra of YVO4 doped with various concentrations of Eu3+ ion calcined at 800 1C in air for 3 h (lem ¼ 618 nm).

Fig. 2. The SEM micrographs of YVO4 doped with (a) 1, (b) 10, and (c) 50 mol% Eu3+ ions calcined at 800 1C for 3 h in air.

Table 1 The average grain sizes for YVO4 doped with various Eu3+ concentrations calculated by Scherrer’s equation. Eu3+ concentration (mol%)

Average grain sizes (nm)

1 5 10 20 30 40 50

20.5 26.5 40.5 44.5 48.8 40.5 11.6

that the host absorption is very efficient. The weak peaks in the range from 320 to 580 nm are associated with the typical f–f transitions of the Eu3+ ions that appeared at 386, 396, and 467 nm, which are attributed to the transition from the 7F0 ground state to the charge-transfer state corresponding to 7F0-5L7, 7F0-5L6 and 7 F0-5D2, respectively.

3.4. Excitation and emission spectrum For rare-earth Eu3+ ions, the electron transition caused by excitation can be classified into two types: (1) charge-transfer transition, CTS. (2) intra-4f transition. Fig. 4 shows the excitation spectra (lem ¼ 618 nm) for YVO4 doped with various concentrations of Eu3+ ion calcined at 800 1C in air for 3 h. The excitation spectrum exhibit a strong wide band centered at 318 nm, corresponding to the charge-transfer transition of the host lattice. The peak intensity of CTS increases with increasing the Eu3+ ion concentrations, and reaches a maximum value of 10 mol% Eu3+ ions and decreases with increasing Eu content indicating concentration quenching. Fig. 5 shows the emission spectra (lex ¼ 318 nm) for YVO4 doped with various concentrations of Eu3+ calcined at 800 1C in air for 3 h. According to the studies by Judd [38] and Ofelt [39], due to the absence of a center of symmetry, the 4f orbitals mix with the opposite parity orbitals resulting in the appearance of electric dipole transitions (5D0-7FJ,J ¼ 2n) [40,41]. The major emission peak at about 618 nm is due to the 5D0-7F2 electric dipole transition, in which the intensity is hypersensitive to the variation of the local structure environment. While another weak peak at 594 nm is the 5D0-7F1 magnetic dipole allowed transition, its

ARTICLE IN PRESS 1184

Y.-S. Chang et al. / Journal of Luminescence 129 (2009) 1181–1185

10000

1 mol% 5 mol% 10 mol% 30 mol% 50 mol%

λex = 318 nm

1000 Intensity (a.u.)

50 mol% Eu

D0→7F2 5

D0→7F1

1 0.1

5

D1→7F0

10 mol% Eu

λex = 318nm λem = 618nm

10

5

Intensity (a.u.)

30 mol% Eu

100

1

2

3

5

6

Time (ms) Fig. 6. The decay time curve of YVO4 doped with various Eu3+ concentrations.

5 mol% Eu

1 λex = 318 nm

4000

λem = 618 nm

0.8

560 Wavelength (nm)

600

Fig. 5. PL emission spectra of YVO4 doped with various concentrations of Eu3+ ion phosphors calcined at 800 1C in air for 3 h (lex ¼ 318 nm).

intensity hardly changes with the local structure symmetry of the Eu3+ ions [6]. In general, it is difficult to find the 5D1,2-7FJ (J ¼ 1,2,3,4) transition for Eu3+ ion-doped phosphors, because the transition transfers the energy via a relaxation process to form a cross-relaxation process [6,42,43], such as 5

D1 (Eu1)+7F0 (Eu2)-5D0 (Eu1)+7F1 (Eu2)

3.5. Decay curve and decay time In many rare-earth element activated phosphors, it is reported that there is more than one kind of site for activators to occupy, including both surface sites and lattice sites. The decay behavior of the emission could be used to identify how many emission mechanisms are employed in the luminescence process. The decay curve of 5D0-7F2 transition under an excitation of 318 nm of (Y1xEux)VO4 powders is shown in Fig. 6. The time decreases as Eu3+ concentration increases, and the curve demonstrates almost perfect single-exponential decay even though the Eu3+ ions concentration is 50 mol%. The decay curve can be represented by [44]   t ð2Þ I ¼ I0 exp 

t

where I and I0 are the luminescence intensities at time t and 0 and t the radiative decay time. The results reveal that the presence of the Eu3+ environment is unique in accordance with the crystal structure, and the decay mechanism of the 5D0-7F2 transition is a single decay component between Eu3+ ions only. The luminescence intensities of phosphor materials are always dependent on the doping concentrations. The emission intensity

0.6

3000

0.4

2000

0.2

1000

Intensity (a.u.)

520

Decay Time (ms)

1 mol% Eu

0 1

10

100

Eu3+ concentration (mol%) Fig. 7. The emission intensity and decay time of Eu3+ as a function of doping concentration under an excitation of 318 nm. The signals were detected at 618 nm.

and decay time of the 5D0-7F2 transition with different Eu3+concentrations under an excitation of 318 nm is shown in Fig. 7. In the current study, the concentration quenching effect was also observed. The emission intensity of the 5D0-7F2 transition increases with increasing Eu3+ concentration in the lower Eu3+ concentration region until the saturated photoluminescent (PL) intensity is reached, and then it diminishes. As discussed earlier, it can be attributed to the concentration quenching. The probability of an energy transfer among Eu3+ ions increases when the Eu3+ concentration increases [45], which indicates that the concentration quenching is active when x40.1. In many cases, concentration quenching is due to energy transfer from one activator to another until the energy sink in the lattice is reached. Blasse [46] suggested that the critical distance (Rc) of energy transfer can be expressed by  1=3 3V Rc ¼ 2 4pxc N

ð3Þ

where xc is the critical concentration, N the number of Y3+ ions in the YVO4 unit cell (N ¼ 4 in YVO4, Eu3+ ions are assumed to be introduced solely into Y3+ sites) [47], and V the volume of the unit cell (V ¼ 318.867  1030 m3 in this case) [47]. For the 5D0-7F2 transition, the critical concentration is estimated to be about 0.1.

ARTICLE IN PRESS Y.-S. Chang et al. / Journal of Luminescence 129 (2009) 1181–1185

Using the above equation, the Rc was determined to be about 5.75 A˚ The 5D0-7F2 transition considered is a hypersensitive-forced electric dipole transition which is sensitive to changes in the crystal field. The decreasing lifetime must be caused by the increasing nonradiative transition, because the crystallinity of the YVO4 host decreases when the Eu3+ ion concentration is larger than 10 mol%. This conclusion is agreement with the observation by XRD analysis

4. Conclusions The Eu3+-doped YVO4 with nano-crystal phosphors were prepared by the Pechini process. The X-ray diffraction profiles show that all of the peaks are attributed to the YVO4 phase when doped with the Eu3+ ions. The intensity of diffraction peaks decreases when the Eu3+ ion concentration is higher than 10 mol%. This is caused by the grain size of (Y1xEux)VO4 decreasing with increasing the Eu3+ ion concentration, which loses the long-range order domains and leads the intensity of diffraction peaks to decrease. The energy is first absorbed by the CTS of the host then transferred to the emitting level of the Eu3+ ions. The calcined powders emit bright red luminescence centered at 618 nm due to the 5D0-7F2 electric dipole transition under an excitation wavelength of 318 nm. The time-resolved 5D0-7F2 transition presents a single-exponential decay behavior revealing the decay mechanism is a single decay component between Eu3+ ions only. The concentration quenching is active when x40.1, and the ˚ critical distance is about 5.75 A.

Acknowledgements The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract no. (NSC96-2622-E-150-034-CC3). References [1] X.Q. Su, B. Yan, J. Non-Cryst. Solids 351 (2005) 3542. [2] S. Ekambaram, J. Alloys Compd. 390 (2005) L1. [3] S.D. Han, S.P. Khatkar, V.B. Taxak, G. Sharma, D. Kumar, Mater. Sci. Eng. B 129 (2006) 126.

1185

[4] K. Riwotzki, M. Haase, J. Phys. Chem. B 102 (1998) 10129. [5] B. Yan, X.Q. Su, Mater. Sci. Eng. B 119 (2005) 196. [6] S. Shionoya, W.M. Yen, Phosphor Handbook, CRC Press, Boca Raton, 1999 p. 190. [7] G. Wakefield, H.A. Keron, P.J. Dobson, J.L. Hutchison, J. Colloid Interface Sci. 215 (1999) 179. [8] H.K. Jung, D.S. Park, Y.C. Park, Mater. Res. Bull. 34 (1999) 43. [9] D.K. Williams, B. Bihari, B.M. Tissue, J.M. McHale, J. Phys. Chem. B 102 (1998) 916. [10] W.T. Hsu, W.H. Wu, C.H. Lu, Mater. Sci. Eng. B 104 (2003) 40. [11] J. Zhang, Z. Zhang, Z. Tang, Y. Lin, Z. Zheng, J. Mater. Process. Technol. 121 (2002) 265. [12] J.M. Nedelec, C. Mansuy, R. Mahiou, J. Mol. Struct. 651–653 (2003) 165. [13] J. Li, M. Kuwabara, Sci. Tech. Adv. Mater. 4 (2003) 143. [14] Y. Li, X. Duan, H. Liao, Y. Qian, Chem. Mater. 10 (1998) 17. [15] M. Hirano, J. Mater. Chem. 10 (2000) 469. [16] M. Hirano, M. Imai, M. Inagaki, J. Am. Ceram. Soc. 83 (2000) 977. [17] A.C. Tas, P.J. Majewski, F. Aldinger, J. Mater. Res. 17 (2002) 1425. [18] M. Pechini, US Patent 3 330 697, 11 July, (1967). [19] K.Y. Kim, H.K. Jung, H.D. Park, D. Kim, J. Lumin. 99 (2002) 169. [20] Q.Y. Zhang, K. Pita, W. Ye, W.X. Que, Chem. Phys. Lett. 351 (2002) 163. [21] S. Ekambaram, K.C. Patil, M. Maaza, J. Alloys Compd. 393 (2005) 81. [22] R.P. Rao, Solid State Commun. 99 (1996) 439. [23] R. Schmechel, M. Kennedy, H. von Seggern, H. Winkler, M. Kolbe, R.A. Fischer, X.M. Li, A. Benker, M. Winterer, H. Hahn, J. Appl. Phys. 89 (2001) 1679. [24] Y.C. Chen, Y.H. Chang, B.S. Tsai, Opt. Mater. 27 (2005) 1874. [25] Y.S. Chang, H.J. Lin, Y.L. Chai, Y.C. Li, J. Alloys Compd. 460 (2008) 421. [26] J.R. O’Connor, Appl. Phys. Lett. 9 (1966) 407. [27] E.A. Maunders, L.G. Deshazer, J. Opt. Soc. Am. 61 (1971) 684. [28] M. Bass, IEEE J. Quantum Electron. QE-11 (1975) 938. [29] A.K. Levine, F.C. Palilla, Appl. Phys. Lett. 5 (1964) 118. [30] Y.H. Li, G.Y. Hong, J. Solid State Chem. 178 (2005) 645. [31] H.W. Zhang, X.Y. Fu, S.Y. Niu, G.Q. Sun, Q. Xin, J. Solid State Chem. 177 (2004) 2649. [32] W.J. Park, M.K. Jung, T. Masaki, S.J. Im, D.H. Yoon, Mater. Sci. Eng. B 146 (2008) 95. [33] L.H. Tian, S.I. Mho, J. Lumin. 122–123 (2007) 99. [34] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751. [35] B.D. Cullity, Elements of X-ray Diffraction, second ed., Addison-Wesley Publishing Company, Inc., Reading, MA, 1978. [36] H. Zhang, X. Fu, S. Niu, G. Sun, Q. Xin, Solid State Commun. 132 (2004) 527. [37] M. Yun, W. Zhang, S. Xia, J.C. Krupa, J. Lumin. 68 (1996) 335. [38] B.R. Judd, Phys. Rev. 127 (1962) 750. [39] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [40] G. Blasse, A. Bril, J. Chem. Phys. 50 (7) (1969) 2974. [41] D.E. Henrie, R.L. Fellows, G.R. Choppin, Coord. Chem. Rev. 18 (1976) 199. [42] A. Kaminskii, Laser Crystal, their Physics and Properties, Springer, Berlin, 1999. [43] A.H. Kitai, Solid State Luminescence, Chapman & Hall Press, Cambridge, 1993. [44] D.R. Vij, Luminescence of Solids, Plenum Press, New York, 1998, p. 68. [45] J. Qiu, K. Miura, N. Sugimoto, K. Hirao, J. Non-Cryst. Solids 213 (314) (1997) 266. [46] G. Blasse, Philips Res. Rep. 24 (1969) 131. [47] G. Locmueller, G. Schmidt, B. Doppisch, V. Gramlich, C. Scheringer, Acta Crystllogr. B 29 (1973) 141.