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Synthesis and luminescent property of Sr2CeO4 phosphor via EDTA-complexing process
Journal of Alloys and Compounds 474 (2009) 287–291
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and luminescent property of Sr2 CeO4 phosphor via EDTA-complexing process Chunxiang Zhang, Wenjun Jiang, Xujie Yang ∗ , Qiaofeng Han, Qingli Hao, Xin Wang Key Laboratory for Soft Chemistry and Functional Materials of Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e
i n f o
Article history: Received 8 April 2008 Received in revised form 15 June 2008 Accepted 16 June 2008 Available online 25 July 2008 Keywords: Sr2 CeO4 Phosphor EDTA-complexing process Luminescence
a b s t r a c t A blue-white emitting Sr2 CeO4 phosphor with orthorhombic structure was synthesized via a simple ethylenediaminetetraacetic acid (EDTA)-complexing process using strontium nitrate and cerous nitrate as raw materials at a relatively low temperature. The crystalline phase, microstructure and morphology of the resulting particles were studied by X-ray diffraction, Raman spectra and transmission electron microscopy. The obtained Sr2 CeO4 powders consisted of uniform crotch-like grains with size of ca. 100 nm. The interplanar distance of d111 was 0.297 nm. Photoluminescence studies at room temperature showed strong luminescent behavior for the superfine particles. Two excitation bands were located at ∼280 and ∼345 nm, respectively. The intensity of the latter excitation band exceeded that of the former while the precursors were calcined at 1000 ◦ C. The emission spectrum was a broad band at 472 nm, which was suitable for the doping of rare earth ions. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The development of phosphors for the three primary colors attracts considerable interests in recent years due to their potential technological applications such as high-performance fluorescent lights and high-resolution display devices [1,2]. Red-emitting Y2 O3 :Eu3+ and green-emitting Y3 Al5 O12 :Tb3+ , Y3 Al5 O12 :Ce3+ , have found many extensive applications [3,4]. However, conventional blue-emitting rare earth or transition metal activated sulphides cannot adapt to new applications due to intrinsic problems such as their chemical instability and sensitivity to moisture. Thus, the blue luminescent materials with sufficient efficiency and chemical stability have been persistently pursued [5–7]. Since Sr2 CeO4 was found as a novel and promising blue luminescent material by combinatorial chemistry method [8], Sr2 CeO4 phosphor has been widely studied because of its great importance in the realization of a new generation of optoelectronic and displaying devices [9]. Sr2 CeO4 phosphor is conventionally synthesized by solid-state reaction [10], which requires high energy consumption to obtain pure crystalline phase and cannot always produce phosphors of an acceptable quality for practical applications. Various soft chemistry methods have been introduced to control particle morphology and to increase optical property, such as chemical co-
∗ Corresponding author. Tel.: +86 25 84315054; fax: +86 25 84315054. E-mail address: [email protected] (X. Yang). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.06.061
precipitation, spray pyrolysis, Pechini’s method, emulsion liquid membrane system [11–16]. In this study, a simple ethylenediaminetetraacetic acid (EDTA)complexing process was demonstrated for the preparation of superfine Sr2 CeO4 phosphor at relatively low temperature. Comparing with citric acid used widely as a chelating agent in the Pechini method, EDTA usually forms more stable complex with most metals, which is helpful to molecule level mixing of the metal ions in the precursor. The influence of preparation process on the structure, morphology and photoluminescence of Sr2 CeO4 phosphor was investigated. 2. Experimental 2.1. Sample preparation All chemical reagents used in this experiment were of analytical grade without further purification. An appropriate amount of ethylenediaminetetraacetic acid (EDTA) was dissolved in ethanolamine solution and formed a transparent solution, into which a stoichiometric amount of strontium nitrate (Sr(NO3 )2 ), cerous nitrate (Ce(NO3 )3 ·6H2 O) were added. The pH value of the solution was adjusted to above 5 by using ethanolamine, which was necessary to decrease acidity effect and to increase the complex ability of EDTA. Subsequently, the solution was consecutively stirred at about 90 ◦ C to form a clear and light yellow gel, which was further dried and charred at 160 ◦ C to yield a solid precursor. Finally, the precursor was calcined at different temperatures from 400 to 1000 ◦ C under static air to obtain Sr2 CeO4 phosphor. For comparison, Sr2 CeO4 phosphor was also prepared by the conventional solid-state method [8]. Stoichiometric amount of SrCO3 and CeO2 were thoroughly mixed in an agate mortar by adding ethanol. Then the mixture was calcined at 850 ◦ C for 2 h, and further treated at 1200 ◦ C for 12 h.
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Fig. 1. XRD patterns of the products. “↓” denotes Sr2 CeO4 ; “” denotes SrCO3 ; “” denotes CeO2 . 2.2. Characterization The preparation process was monitored by Fourier transform infrared spectroscopy (FTIR) with a Bruker Vector 22 spectrometer and Raman spectra with a Renishaw invia Raman microscope. The crystalline phase structures of these powders were investigated by the XRD measurement on a Bruker D8 ADVANCE Xray diffractometer (Cu K␣ = 0.15406 nm). Transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were recorded on a JEOL JEM-2100 transmission electron microscope using an accelerating voltage of 200 kV. The luminescence spectra were recorded on a FL3-TCSPC fluorescence spectrophotometer using 1 nm slit width.
3. Results and discussion 3.1. Evolution of crystalline phases The precursor powders were calcined at different calcination temperatures to investigate the evolution of crystalline phases. The corresponding XRD patterns for the resultant powders are shown in Fig. 1. The powders showed weak crystalline phases of SrCO3 and CeO2 at 400 ◦ C. With the increasing calcination temperature from 400 to 700 ◦ C, the diffraction peaks of SrCO3 and CeO2 became much sharper. When the precursor was calcined at 800 ◦ C for 1 h, Sr2 CeO4 phase with the orthorhombic symmetry appeared dominantly and a little CeO2 was still observed. As the calcination time was extended to 3 h or the temperature was raised to 900 ◦ C, a single phase of Sr2 CeO4 was completely formed which was in good agreement with the reported data (JCPDS 50-0115) [6]. Throughout the experiment, no characteristic diffraction peaks of other impurities such as Sr, SrO, Sr(NO3 )2 or SrCeO3 were detected, it can be concluded that metal EDTA complexes are first decomposed to high active SrCO3 and CeO2 , which are subsequently calcined at 800 ◦ C to produce high-quality Sr2 CeO4 .
Fig. 2. FTIR spectra of the samples calcined at different temperatures.
And some new absorption peaks at 1768, 1450, 1070, 860, 706 and 699 cm−1 are assigned to stretching characteristics of SrCO3 [18]. From 400 to 900 ◦ C, the absorption of carbonate is gradually weakened. Although the Sr2 CeO4 is the only phase above 800 ◦ C by XRD, the FTIR curves indicate that it bears amorphous carbonate phase. The carbonate ions actually disappear in the sample treated at 1000 ◦ C for 2 h. 3.3. Raman spectrometry Raman spectra are used as a complement of FTIR spectra for studying phase and structure of Sr2 CeO4 . There are some distinct differences observed from Raman spectra of Fig. 3. The Raman shift at 463 cm−1 is relative to F2g band of CeO2 with fluorite type structure [19]. The shift at 1073 cm−1 is assigned to symmetric stretching mode of SrCO3 , the shift at 703 cm−1 is attributed to antisymmetric bending vibration, the shifts at 148, 181 and 252 cm−1 are associated with librations of carbonate ion around axes [18]. With increasing temperature, the shifts at 1073 and 463 cm−1 are gradually weakened until completely disappear at 900 ◦ C. The structure of Sr2 CeO4 was found to be isostructural to that of Sr2 PbO4 [8], which was built up by edge-sharing M(IV)O6 octahedra. There are
3.2. FTIR spectrometry The IR spectra of the gel and samples subsequently calcined at different temperature are given in Fig. 2. It can be found that the asymmetric C–O stretching of the gel exhibits two peaks at 1652 and 1578 cm−1 rather than a board band of EDTA at ca. 1700 cm−1 , which indicates that the carboxylic groups of EDTA are bonded to metal ions and carboxylate-Ce(III) and -Sr(II) bondings produce C–O stretching frequency values of 1652 and 1578 cm−1 [17]. The strong absorption peak at 1384 cm−1 disappears at 400 ◦ C, which can be associated to C–O carboxylic group symmetric stretching.
Fig. 3. Raman spectra of powders calcined at different conditions.
C. Zhang et al. / Journal of Alloys and Compounds 474 (2009) 287–291
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Fig. 4. TEM and SEM images of Sr2 CeO4 calcined at 800 and 1000 ◦ C (the insert shows the corresponding FFT pattern).
two terminal O (O1) atoms and four equatorial O (O2) atoms for per octahedron [20]. The Ce–O1 bonds are about 0.1 Å shorter than the Ce–O2 bonds. Based on the group theory analysis [21], the Raman shift at 561 cm−1 may be assigned to the vibration of Ce–O1, and the shifts at 386 and 287 cm−1 may be ascribed to the stretching mode of Ce–O2. With increasing temperature, the change of the polarizability of Ce–O2 is bigger than that of Ce–O1, which can be confirmed from the corresponding Raman intensity. So the contribution of Ce–O2 bonds increases comparing with Ce–O1 bonds to induce the charge transfer, which is related to the luminescence of this material. 3.4. Morphology of samples Fig. 4(a) shows the TEM image of Sr2 CeO4 calcined at 800 ◦ C. The obtained powders are highly porous, and the particles are
linked together by crotch-like way, which is similarly observed by a PEG-2000 sol–gel route [1] and an emulsion liquid membrane system [7]. However, The average lengths and diameters of Sr2 CeO4 particles via EDTA-complexing process are smaller than those prepared by the above-mentioned methods. This suggests that the application of EDTA-complexing process can reduce the grain size and make particles more uniform. The crotch-like morphology is ascribed to slight sintering as a result of relative ductile property of the alkaline earth metals [15]. The interplanar distance measured from HRTEM image in Fig. 4(b) is 0.297 nm, corresponding to the crystal plane of (1 1 1) of the orthorhombic Sr2 CeO4 structure. The corresponding FFT pattern indicates that Sr2 CeO4 phosphor with good crystallinity is polycrystalline. No noticeable change in morphology of the particles could be detected with the increasing calcination time from 1 to 4 h in Fig. 4(c). Particle growth takes place after calcination above 800 ◦ C.
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Fig. 5. The luminescent spectra of samples calcined at (1) 800 ◦ C for 1 h, (2) 800 ◦ C for 3 h, (3) 900 ◦ C, (5) 1000 ◦ C for 2 h, (4) solid state method. (a) excitation spectra (em = 472 nm), (b) emission spectra (ex = 294 nm).
3.5. Luminescent properties of samples
4. Conclusions
Fig. 5 shows the excitation and emission spectra of Sr2 CeO4 . The excitation spectra consist of two bands observed at low and high wavelength, respectively. The intensities of both excitation bands and emission band increase with increasing temperature because of the higher crystallization. The luminescence of Sr2 CeO4 is thought to originate from a ligand-to-metal charge transfer (CT) by Danielson [8]. Two different Ce4+ –O2− bond lengths in the lattice lead to two excitation bands by different charge transfer transitions. According to the theoretical calculations of the average energy gap of the chemical bond [22], the two excitation bands at low and high wavelength, corresponding to the highest and the lowest CT energies, were assigned to the transitions O1 → Ce4+ and O2 → Ce4+ , respectively. In Fig. 5(a), the excitation bands slightly shift towards high wavelength with the increasing temperature, which is different from that reported in Refs. [1,14]. O.A. Serra [13] ascribed the change in the location of excitation peaks to the different content of oxygen in the calcination atmosphere. As it is shown in the TEM and SEM images, the particle sizes gradually increase with the increasing temperature. This will narrow the average energy gaps Eg of the chemical bonds Ce–O1 and Ce–O2 in Sr2 CeO4 [22]. From the literature, it has been known that the CT energy decreases linearly with decreasing of the average energy gap Eg [23]. Therefore, the decrease of the CT energies from the valence bands of the chemical bonds Ce–O1 and Ce–O2 to the ground state of Ce will result in the excitation bands differences of various samples. The emission spectrum is a simple broad band which center is located at 472 nm in Fig. 5(b). Based on the difference between the excitation maximum (346 nm) and the emission maximum (472 nm), the Stokes shift is 7715 cm−1 , which is within the range of CT transitions on Ce4+ ions. The single broad band emission is attributed to Ce4+ CT emission. From the luminescence spectra, we can see that the luminescence intensity of Sr2 CeO4 synthesized by our method, was higher than that of the sample prepared by the solid-state method. Similar to the excitation bands, the emission band of Sr2 CeO4 obtained at 1000 ◦ C is broadened. The broad emission band is suitable for the doping of rare earth ions in pursuing new luminescent materials.
The superfine Sr2 CeO4 phosphor was successfully synthesized by EDTA-complexing process in presence of ethanolamine at relatively low temperature and short calcination time. This process proved to be simple and convenient. The morphology of Sr2 CeO4 particles was uniform and smaller crotch-like. The excitation spectra consisted of two bands at ∼280 and ∼345 nm. The intensity of the latter excitation band exceeded that of the former when the precursors were calcined at 1000 ◦ C. The emission spectrum was a broad band at 472 nm. The luminescence intensity of phosphor at room temperature increased with the calcination temperature increasing from 800 to 1000 ◦ C. The as-synthesized Sr2 CeO4 phosphor with strong blue-white emitting could be a good candidate for advanced display devices. Acknowledgements This work was supported by the National Natural Science Fundation of China (No. 10776014), high-tech. Fundation of Jiangsu Province (BG2007047) and Natural Science Foundation of Jiangsu Province, China (No. BK2006201). References [1] S.J. Chen, X.T. Chen, Z. Yu, J.M. Hong, Z.L. Xue, X.Z. You, Solid State Commun. 130 (2004) 281. [2] T. Masui, T. Chiga, N. Imanaka, G. Adachi, Mater. Res. Bull. 38 (2003) 17. [3] J. Dhanaraj, R. Jagannathan, T.R.N. Kutty, C.H. Lu, J. Phys. Chem. B 105 (2001) 11098. [4] A. Potdevin, G. Chadeyron, D. Boyer, R. Mahiou, J. Sol–Gel Sci. Techn. 39 (2006) 275. [5] T. Jüstel, H. Nikol, Adv. Mater. 12 (2000) 7. [6] E. Danielson, M. Devenney, D.M. Giaquinta, J.H. Golden, R.C. Haushalter, E.W. McFarland, D.M. Poojary, C.M. Reaves, W.H. Weinberg, X.D. Wu, Science 279 (1998) 837. [7] T. Hirai, Y. Kawamura, J. Phys. Chem. B 108 (2004) 12763. [8] E. Danielson, M. Davenney, D.M. Giaquinta, J.H. Golden, R.C. Haushalter, E.W. McFarland, D.M. Poojary, C.M. Reaves, W.H. Weinberg, X.D. Wu, J. Mol. Struct. 470 (1998) 229. [9] N. Perea-Lopez, J.A. Gonzalez-Ortega, G.A. Hirata, Opt. Mater. 29 (2006) 43. [10] A.N. Shirsat, K.N.G. Kaimal, S.R. Bharadwaj, D. Das, Thermochim. Acta 447 (2006) 101. [11] S.K. Hong, S.H. Ju, H.Y. Koo, D.S. Jung, Y.C. Kang, Mater. Lett. 60 (2006) 334.
C. Zhang et al. / Journal of Alloys and Compounds 474 (2009) 287–291 [12] Y.B. Khollam, S.B. Deshpande, P.K. Khanna, P.A. Joy, H.S. Potdar, Mater. Lett. 58 (2004) 2521. [13] O.A. Serra, V.P. Severino, P.S. Calefi, S.A. Cicillini, J. Alloys Compd. 323/324 (2001) 667. [14] R. Ghildiyal, P. Page, K.V.R. Murthy, J. Lumin. 124 (2007) 217. [15] T. Hirai, Y. Kawamura, J. Phys. Chem. B 109 (2005) 5569. [16] D.S. Xing, J.X. Shi, M.L. Gong, Mater. Lett. 59 (2005) 948. [17] K. Nakamoto, Infrared and Raman spectra of Inorganic and Coordination Compounds, Wiley, New York, 1986.
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[18] J.M. Alia, Y. Diaz de Mera, H.G.M. Edwards, P. Gonzalez Martin, S. Lopez Andres, Spectrochim. Acta Part A 53 (1997) 2347. [19] M.A. Malecka, L. Kepinski, W. Mista, Appl. Catal. B 74 (2007) 290. [20] F. Goubin, X. Rocquefelte, M.H. Whangbo, Y. Montardi, R. Brec, S. Jobic, Chem. Mater. 16 (2004) 662. [21] R. Pisdiez, E.J. Baran, A.E. Lavat, M.C. Grasselli, J. Phys. Chem. Solids 56 (1995) 135. [22] Ling Li, Shihong Zhou, Siyuan Zhang, Chem. Phys. Lett. 453 (2008) 283. [23] Ling Li, Shihong Zhou, Siyuan Zhang, J. Phys. Chem. C 111 (2007) 3205.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and luminescent property of Sr2 CeO4 phosphor via EDTA-complexing process Chunxiang Zhang, Wenjun Jiang, Xujie Yang ∗ , Qiaofeng Han, Qingli Hao, Xin Wang Key Laboratory for Soft Chemistry and Functional Materials of Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e
i n f o
Article history: Received 8 April 2008 Received in revised form 15 June 2008 Accepted 16 June 2008 Available online 25 July 2008 Keywords: Sr2 CeO4 Phosphor EDTA-complexing process Luminescence
a b s t r a c t A blue-white emitting Sr2 CeO4 phosphor with orthorhombic structure was synthesized via a simple ethylenediaminetetraacetic acid (EDTA)-complexing process using strontium nitrate and cerous nitrate as raw materials at a relatively low temperature. The crystalline phase, microstructure and morphology of the resulting particles were studied by X-ray diffraction, Raman spectra and transmission electron microscopy. The obtained Sr2 CeO4 powders consisted of uniform crotch-like grains with size of ca. 100 nm. The interplanar distance of d111 was 0.297 nm. Photoluminescence studies at room temperature showed strong luminescent behavior for the superfine particles. Two excitation bands were located at ∼280 and ∼345 nm, respectively. The intensity of the latter excitation band exceeded that of the former while the precursors were calcined at 1000 ◦ C. The emission spectrum was a broad band at 472 nm, which was suitable for the doping of rare earth ions. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The development of phosphors for the three primary colors attracts considerable interests in recent years due to their potential technological applications such as high-performance fluorescent lights and high-resolution display devices [1,2]. Red-emitting Y2 O3 :Eu3+ and green-emitting Y3 Al5 O12 :Tb3+ , Y3 Al5 O12 :Ce3+ , have found many extensive applications [3,4]. However, conventional blue-emitting rare earth or transition metal activated sulphides cannot adapt to new applications due to intrinsic problems such as their chemical instability and sensitivity to moisture. Thus, the blue luminescent materials with sufficient efficiency and chemical stability have been persistently pursued [5–7]. Since Sr2 CeO4 was found as a novel and promising blue luminescent material by combinatorial chemistry method [8], Sr2 CeO4 phosphor has been widely studied because of its great importance in the realization of a new generation of optoelectronic and displaying devices [9]. Sr2 CeO4 phosphor is conventionally synthesized by solid-state reaction [10], which requires high energy consumption to obtain pure crystalline phase and cannot always produce phosphors of an acceptable quality for practical applications. Various soft chemistry methods have been introduced to control particle morphology and to increase optical property, such as chemical co-
∗ Corresponding author. Tel.: +86 25 84315054; fax: +86 25 84315054. E-mail address: [email protected] (X. Yang). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.06.061
precipitation, spray pyrolysis, Pechini’s method, emulsion liquid membrane system [11–16]. In this study, a simple ethylenediaminetetraacetic acid (EDTA)complexing process was demonstrated for the preparation of superfine Sr2 CeO4 phosphor at relatively low temperature. Comparing with citric acid used widely as a chelating agent in the Pechini method, EDTA usually forms more stable complex with most metals, which is helpful to molecule level mixing of the metal ions in the precursor. The influence of preparation process on the structure, morphology and photoluminescence of Sr2 CeO4 phosphor was investigated. 2. Experimental 2.1. Sample preparation All chemical reagents used in this experiment were of analytical grade without further purification. An appropriate amount of ethylenediaminetetraacetic acid (EDTA) was dissolved in ethanolamine solution and formed a transparent solution, into which a stoichiometric amount of strontium nitrate (Sr(NO3 )2 ), cerous nitrate (Ce(NO3 )3 ·6H2 O) were added. The pH value of the solution was adjusted to above 5 by using ethanolamine, which was necessary to decrease acidity effect and to increase the complex ability of EDTA. Subsequently, the solution was consecutively stirred at about 90 ◦ C to form a clear and light yellow gel, which was further dried and charred at 160 ◦ C to yield a solid precursor. Finally, the precursor was calcined at different temperatures from 400 to 1000 ◦ C under static air to obtain Sr2 CeO4 phosphor. For comparison, Sr2 CeO4 phosphor was also prepared by the conventional solid-state method [8]. Stoichiometric amount of SrCO3 and CeO2 were thoroughly mixed in an agate mortar by adding ethanol. Then the mixture was calcined at 850 ◦ C for 2 h, and further treated at 1200 ◦ C for 12 h.
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Fig. 1. XRD patterns of the products. “↓” denotes Sr2 CeO4 ; “” denotes SrCO3 ; “” denotes CeO2 . 2.2. Characterization The preparation process was monitored by Fourier transform infrared spectroscopy (FTIR) with a Bruker Vector 22 spectrometer and Raman spectra with a Renishaw invia Raman microscope. The crystalline phase structures of these powders were investigated by the XRD measurement on a Bruker D8 ADVANCE Xray diffractometer (Cu K␣ = 0.15406 nm). Transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were recorded on a JEOL JEM-2100 transmission electron microscope using an accelerating voltage of 200 kV. The luminescence spectra were recorded on a FL3-TCSPC fluorescence spectrophotometer using 1 nm slit width.
3. Results and discussion 3.1. Evolution of crystalline phases The precursor powders were calcined at different calcination temperatures to investigate the evolution of crystalline phases. The corresponding XRD patterns for the resultant powders are shown in Fig. 1. The powders showed weak crystalline phases of SrCO3 and CeO2 at 400 ◦ C. With the increasing calcination temperature from 400 to 700 ◦ C, the diffraction peaks of SrCO3 and CeO2 became much sharper. When the precursor was calcined at 800 ◦ C for 1 h, Sr2 CeO4 phase with the orthorhombic symmetry appeared dominantly and a little CeO2 was still observed. As the calcination time was extended to 3 h or the temperature was raised to 900 ◦ C, a single phase of Sr2 CeO4 was completely formed which was in good agreement with the reported data (JCPDS 50-0115) [6]. Throughout the experiment, no characteristic diffraction peaks of other impurities such as Sr, SrO, Sr(NO3 )2 or SrCeO3 were detected, it can be concluded that metal EDTA complexes are first decomposed to high active SrCO3 and CeO2 , which are subsequently calcined at 800 ◦ C to produce high-quality Sr2 CeO4 .
Fig. 2. FTIR spectra of the samples calcined at different temperatures.
And some new absorption peaks at 1768, 1450, 1070, 860, 706 and 699 cm−1 are assigned to stretching characteristics of SrCO3 [18]. From 400 to 900 ◦ C, the absorption of carbonate is gradually weakened. Although the Sr2 CeO4 is the only phase above 800 ◦ C by XRD, the FTIR curves indicate that it bears amorphous carbonate phase. The carbonate ions actually disappear in the sample treated at 1000 ◦ C for 2 h. 3.3. Raman spectrometry Raman spectra are used as a complement of FTIR spectra for studying phase and structure of Sr2 CeO4 . There are some distinct differences observed from Raman spectra of Fig. 3. The Raman shift at 463 cm−1 is relative to F2g band of CeO2 with fluorite type structure [19]. The shift at 1073 cm−1 is assigned to symmetric stretching mode of SrCO3 , the shift at 703 cm−1 is attributed to antisymmetric bending vibration, the shifts at 148, 181 and 252 cm−1 are associated with librations of carbonate ion around axes [18]. With increasing temperature, the shifts at 1073 and 463 cm−1 are gradually weakened until completely disappear at 900 ◦ C. The structure of Sr2 CeO4 was found to be isostructural to that of Sr2 PbO4 [8], which was built up by edge-sharing M(IV)O6 octahedra. There are
3.2. FTIR spectrometry The IR spectra of the gel and samples subsequently calcined at different temperature are given in Fig. 2. It can be found that the asymmetric C–O stretching of the gel exhibits two peaks at 1652 and 1578 cm−1 rather than a board band of EDTA at ca. 1700 cm−1 , which indicates that the carboxylic groups of EDTA are bonded to metal ions and carboxylate-Ce(III) and -Sr(II) bondings produce C–O stretching frequency values of 1652 and 1578 cm−1 [17]. The strong absorption peak at 1384 cm−1 disappears at 400 ◦ C, which can be associated to C–O carboxylic group symmetric stretching.
Fig. 3. Raman spectra of powders calcined at different conditions.
C. Zhang et al. / Journal of Alloys and Compounds 474 (2009) 287–291
289
Fig. 4. TEM and SEM images of Sr2 CeO4 calcined at 800 and 1000 ◦ C (the insert shows the corresponding FFT pattern).
two terminal O (O1) atoms and four equatorial O (O2) atoms for per octahedron [20]. The Ce–O1 bonds are about 0.1 Å shorter than the Ce–O2 bonds. Based on the group theory analysis [21], the Raman shift at 561 cm−1 may be assigned to the vibration of Ce–O1, and the shifts at 386 and 287 cm−1 may be ascribed to the stretching mode of Ce–O2. With increasing temperature, the change of the polarizability of Ce–O2 is bigger than that of Ce–O1, which can be confirmed from the corresponding Raman intensity. So the contribution of Ce–O2 bonds increases comparing with Ce–O1 bonds to induce the charge transfer, which is related to the luminescence of this material. 3.4. Morphology of samples Fig. 4(a) shows the TEM image of Sr2 CeO4 calcined at 800 ◦ C. The obtained powders are highly porous, and the particles are
linked together by crotch-like way, which is similarly observed by a PEG-2000 sol–gel route [1] and an emulsion liquid membrane system [7]. However, The average lengths and diameters of Sr2 CeO4 particles via EDTA-complexing process are smaller than those prepared by the above-mentioned methods. This suggests that the application of EDTA-complexing process can reduce the grain size and make particles more uniform. The crotch-like morphology is ascribed to slight sintering as a result of relative ductile property of the alkaline earth metals [15]. The interplanar distance measured from HRTEM image in Fig. 4(b) is 0.297 nm, corresponding to the crystal plane of (1 1 1) of the orthorhombic Sr2 CeO4 structure. The corresponding FFT pattern indicates that Sr2 CeO4 phosphor with good crystallinity is polycrystalline. No noticeable change in morphology of the particles could be detected with the increasing calcination time from 1 to 4 h in Fig. 4(c). Particle growth takes place after calcination above 800 ◦ C.
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Fig. 5. The luminescent spectra of samples calcined at (1) 800 ◦ C for 1 h, (2) 800 ◦ C for 3 h, (3) 900 ◦ C, (5) 1000 ◦ C for 2 h, (4) solid state method. (a) excitation spectra (em = 472 nm), (b) emission spectra (ex = 294 nm).
3.5. Luminescent properties of samples
4. Conclusions
Fig. 5 shows the excitation and emission spectra of Sr2 CeO4 . The excitation spectra consist of two bands observed at low and high wavelength, respectively. The intensities of both excitation bands and emission band increase with increasing temperature because of the higher crystallization. The luminescence of Sr2 CeO4 is thought to originate from a ligand-to-metal charge transfer (CT) by Danielson [8]. Two different Ce4+ –O2− bond lengths in the lattice lead to two excitation bands by different charge transfer transitions. According to the theoretical calculations of the average energy gap of the chemical bond [22], the two excitation bands at low and high wavelength, corresponding to the highest and the lowest CT energies, were assigned to the transitions O1 → Ce4+ and O2 → Ce4+ , respectively. In Fig. 5(a), the excitation bands slightly shift towards high wavelength with the increasing temperature, which is different from that reported in Refs. [1,14]. O.A. Serra [13] ascribed the change in the location of excitation peaks to the different content of oxygen in the calcination atmosphere. As it is shown in the TEM and SEM images, the particle sizes gradually increase with the increasing temperature. This will narrow the average energy gaps Eg of the chemical bonds Ce–O1 and Ce–O2 in Sr2 CeO4 [22]. From the literature, it has been known that the CT energy decreases linearly with decreasing of the average energy gap Eg [23]. Therefore, the decrease of the CT energies from the valence bands of the chemical bonds Ce–O1 and Ce–O2 to the ground state of Ce will result in the excitation bands differences of various samples. The emission spectrum is a simple broad band which center is located at 472 nm in Fig. 5(b). Based on the difference between the excitation maximum (346 nm) and the emission maximum (472 nm), the Stokes shift is 7715 cm−1 , which is within the range of CT transitions on Ce4+ ions. The single broad band emission is attributed to Ce4+ CT emission. From the luminescence spectra, we can see that the luminescence intensity of Sr2 CeO4 synthesized by our method, was higher than that of the sample prepared by the solid-state method. Similar to the excitation bands, the emission band of Sr2 CeO4 obtained at 1000 ◦ C is broadened. The broad emission band is suitable for the doping of rare earth ions in pursuing new luminescent materials.
The superfine Sr2 CeO4 phosphor was successfully synthesized by EDTA-complexing process in presence of ethanolamine at relatively low temperature and short calcination time. This process proved to be simple and convenient. The morphology of Sr2 CeO4 particles was uniform and smaller crotch-like. The excitation spectra consisted of two bands at ∼280 and ∼345 nm. The intensity of the latter excitation band exceeded that of the former when the precursors were calcined at 1000 ◦ C. The emission spectrum was a broad band at 472 nm. The luminescence intensity of phosphor at room temperature increased with the calcination temperature increasing from 800 to 1000 ◦ C. The as-synthesized Sr2 CeO4 phosphor with strong blue-white emitting could be a good candidate for advanced display devices. Acknowledgements This work was supported by the National Natural Science Fundation of China (No. 10776014), high-tech. Fundation of Jiangsu Province (BG2007047) and Natural Science Foundation of Jiangsu Province, China (No. BK2006201). References [1] S.J. Chen, X.T. Chen, Z. Yu, J.M. Hong, Z.L. Xue, X.Z. You, Solid State Commun. 130 (2004) 281. [2] T. Masui, T. Chiga, N. Imanaka, G. Adachi, Mater. Res. Bull. 38 (2003) 17. [3] J. Dhanaraj, R. Jagannathan, T.R.N. Kutty, C.H. Lu, J. Phys. Chem. B 105 (2001) 11098. [4] A. Potdevin, G. Chadeyron, D. Boyer, R. Mahiou, J. Sol–Gel Sci. Techn. 39 (2006) 275. [5] T. Jüstel, H. Nikol, Adv. Mater. 12 (2000) 7. [6] E. Danielson, M. Devenney, D.M. Giaquinta, J.H. Golden, R.C. Haushalter, E.W. McFarland, D.M. Poojary, C.M. Reaves, W.H. Weinberg, X.D. Wu, Science 279 (1998) 837. [7] T. Hirai, Y. Kawamura, J. Phys. Chem. B 108 (2004) 12763. [8] E. Danielson, M. Davenney, D.M. Giaquinta, J.H. Golden, R.C. Haushalter, E.W. McFarland, D.M. Poojary, C.M. Reaves, W.H. Weinberg, X.D. Wu, J. Mol. Struct. 470 (1998) 229. [9] N. Perea-Lopez, J.A. Gonzalez-Ortega, G.A. Hirata, Opt. Mater. 29 (2006) 43. [10] A.N. Shirsat, K.N.G. Kaimal, S.R. Bharadwaj, D. Das, Thermochim. Acta 447 (2006) 101. [11] S.K. Hong, S.H. Ju, H.Y. Koo, D.S. Jung, Y.C. Kang, Mater. Lett. 60 (2006) 334.
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