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A new synthetic pathway of Sr2CeO4 phosphor and its characterization
Journal of Luminescence 131 (2011) 2181–2184
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
A new synthetic pathway of Sr2CeO4 phosphor and its characterization R. Seema a,n, K. Nandakumar a,b a b
School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam 686560, Kerala, India Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam 686560, Kerala, India
a r t i c l e i n f o
abstract
Article history: Received 25 November 2010 Received in revised form 27 April 2011 Accepted 3 May 2011 Available online 8 May 2011
A blue–white emitting Sr2CeO4 phosphor was synthesized via a simple sol–gel poly vinyl alcohol (PVA)complexing process using strontium nitrate and cerium nitrate as raw materials. The samples were characterized by TG/DTA, XRD, FTIR, SEM and photoluminescence spectroscopy. The X-ray diffraction study confirms the structure of the system to be orthorhombic. The emission spectra when excited at 267 nm peaks at 470 nm. The emission band is assigned to the energy transfer between the molecular orbital of the ligand and charge transfer state of the Ce4 þ ion. The Commission International de l’Eclairage (CIE) co-ordinates for the Sr2CeO4 sample were also calculated. & 2011 Elsevier B.V. All rights reserved.
Keywords: Sr2CeO4 PVA Luminescence XRD Blue phosphor
1. Introduction Various optical devices such as plasma display panels (PDPs), field emission displays (FEDs) and light-emitting diodes (LEDs) widely utilize oxide-based phosphors due to their high thermal and chemical stability [1]. In recent years, oxide-based lanthanide phosphors have caught much attention because of their excellent optical properties [2]. Comparable red and green emitting phosphors have been widely used in FEDs and LEDs [3–5], but blue emitting phosphors suitable for practical applications remain rarely available. Sr2CeO4 was first discovered by Danielson et al. [6] in 1998 using combinatorial technique. Sr2CeO4 phosphor has been confirmed to have an orthorhombic crystal structure with onedimensional chains of edge-sharing CeO6 octahedrons linked by strontium ions [7], and it was considered a potential candidate for blue phosphors [8]. In addition, Sr2CeO4 was found to emit efficiently under ultraviolet, cathode ray and X-ray excitation [6,9]. Various chemical methods have been developed for the synthesis of pure, single-phase powders with controlled powder characteristics of Sr2CeO4 phosphor. Based on the literature survey, it is clear that the preparation method and the precursors play an important role in the formation of pure Sr2CeO4 luminescent phase. High heating temperature is usually required for the preparation of Sr2CeO4 via conventional solid state reaction
n
Corresponding author. Tel.: þ91 481 2731043. E-mail address: [email protected] (R. Seema).
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.05.004
technique [6,7,10,11]. Relatively lower heating temperature is required for the synthesis of Sr2CeO4 via PEG sol–gel process [12], citrate–gel process [13], co-precipitation method [9], ultrasonic spray pyrolysis [14] and combustion reaction [15]. Other methods like microwave-hydrothermal [16] and microwave-solvothermal methods [17] have also been adopted for the synthesis of Sr2CeO4. Although several chemical routes have been applied for the synthesis of fine Sr2CeO4 phosphor particles with good luminescence, challenges still remain in reducing process complexity, controllability and cost. Therefore, new synthesis techniques are being explored to potentially overcome some of these difficulties. Here poly vinyl alcohol –[CH2–CHOH]-n, a rather simplestructured and inexpensive polymer, has been used as polymeric carrier [18–21] for the synthesis of Sr2CeO4 phosphor. The nitrate ions act as cation sources for oxide ceramic powders, become oxidizing agents for the decomposition of PVA. The large chain molecule of PVA operates as a steric entrapment mechanism in the organic–inorganic solution. In this study the structure, morphology and photoluminescence of Sr2CeO4 phosphor are investigated.
2. Experimental The Sr2CeO4 was prepared by a simple sol–gel route using PVA as the chelating agent. The starting materials were strontium nitrate, cerium nitrate and PVA. The ratio of chelating agent to metal ions was varied from 0.5 to 4, in step of 0.5 increase to observe the optimum concentration. The sol–gel samples were prepared by taking analytical grade nitrates of strontium and
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cerium (CDH, Laboratory Reagent, purity 99.0%) and adding them in stoichiometric ratio (Sr2 þ : Ce4 þ , 2:1). The mixture of these salts was then added to the distilled water and the solution was kept on a magnetic stirrer maintained at 35 1C. PVA (CDH, Laboratory Reagent, Mw 125,000) was then added to the aqueous solution. The resultant mixture was continuously stirred for few hours with temperature maintained at 50 1C. It was further heated at 90 1C, once the mixture transformed into a brownish gel, it turned to brown fluffy powder on further heating (here after called as the precursor). The precursor powder was taken for TG/DTA analysis and the rest was kept for drying in an oven at 250 1C for 2 h. The compound obtained (white in color) was ground into powder. The obtained powder was again heated at 400 1C, 600 1C, 800 1C, 900 1C and then at 1000 1C for 2 h. Sr2CeO4 was also prepared by conventional solid state reaction method [10] taking the stoichiometric ratio of strontium carbonate and cerium oxide. The mixture was thoroughly homogenized in an agate mortar for 4 h in presence of acetone. The obtained mixture was then sintered at 1000 1C for 36 h. The TG–DTA curve was recorded using a Shimadzu DTG60 thermal analyzer at a heating rate of 10 1C in air atmosphere. The phase of the phosphor powder was checked by XRD pattern taken using a Bruker D8 ADVANCE X-Ray Diffractometer with Cu–Ka ˚ radiation. Fourier transform infrared spectroscopy (1.54056 A) (Shimadzu 8400 S FTIR) was carried out in the range of 4000– 400 cm 1 in transmission mode. The Scanning electron micrograph was taken using JEOL JSM6390, in order to characterize particle morphology. Photoluminescence measurements were performed with spectrofluorophotometer (SHIMADZU, RF-5301PC) using 150 W Xenon lamp as excitation source. The excitation and emission slit widths are 1.5 nm. All spectra were recorded at room temperature.
3. Results and discussion Fig. 1(A) shows the TG/DTA curve of the brown precursor powder (chelating agent/metal ion ¼0.5) dried in air. The two endothermic peaks around 62 and 125 1C correspond to the loss of absorbed water and decomposition of nitrates, respectively. At temperatures above 130 1C, PVA starts to degrade gradually until reaching 200 1C. There is an exothermic peak at 195 1C, which indicates the degradation of PVA. The decomposition of SrCO3 and CeO2, with the release of CO2, and formation of Sr2CeO4 occurs between 800 and 1000 1C. From 600 to 1000 1C, there is only a slight variation in the weight loss.
To investigate the evolution of crystalline phases, the precursor powders (chelating agent/metal ions ¼2) were calcined at different calcinations temperatures. Fig. 1(B) shows the corresponding XRD patterns for the resultant powders. Weak crystalline phases of SrCO3 and CeO2 were observed in powder calcined at 250 1C. There are three steps involved in the formation of Sr2CeO4; firstly, some of the nitrates may decompose into their corresponding oxides; secondly, SrO then react with carbon dioxide or carbon produced by PVA to form SrCO3; finally SrCO3 reacts with CeO2 to form Sr2CeO4 [8,22]. When the calcination temperature was increased from 400 to 800 1C, the diffraction peaks of SrCO3 and CeO2 exists. When the precursor was calcined at 900 1C for 2 h, Sr2CeO4 phase with the orthorhombic symmetry appeared dominantly. A single phased Sr2CeO4 was completely formed when calcined at 1000 1C for 2 h, which was in good agreement with the reported data (JCPDS 50-0115) [6]. Fig. 2(A) shows the IR spectra of the precursor powder (chelating agent/metal ions ¼2) calcined at different temperatures. Absorption peaks at 1770, 1450, 1070, 860, 706 and 699 cm 1 are assigned to stretching characteristics of SrCO3 [23]. From 400 to 1000 1C, the absorption of carbonate gradually weakens. XRD indicates that the precursor powder (calcined at 1000 1C) has only Sr2CeO4 phase, while the FTIR curves indicate that it bears amorphous carbonate phase. Evidently, the absorption peaks at 1400–1450 cm 1 (CH2 bending) and 850 cm 1 (CH2 rocking) characteristic to PVA are obvious in all of the spectra [15]. The absorption peak at 600–300 cm 1 is assigned to the metal oxide (Ce–O) frequency bands. Fig. 2(B) shows the SEM image of Sr2CeO4 (chelating agent/metal ion¼2) sintered at 1000 1C for 2 h. It is clear from the SEM image that the morphology of the sample is not uniform; it varies from round to irregular shape. Fig. 3(A) shows the excitation spectrum of Sr2CeO4 (chelating agent/metal ion¼2). The excitation spectrum consists of a peak at 254 nm along with a small hump around 267 nm. There are two different Ce4 þ –O2 bond lengths in the lattice, thus two excitation peaks are attributed to the two different charge transfer transitions [6]. The hump around 267 nm evident from the excitation curve may be attributed to the above-defined mechanism. As reported by Serra et al. [24] and by Ghildiyal et al. [2] the intensity of two excitations is not same. The excitation band of the sample could be attributed to the transition t1g-f, where f is the lowest excited charge transfer state of the Ce4 þ ion and t1g is the molecular orbital of the surrounding ligand in the six-fold oxygen co-ordination [6]. Fig. 3(B) shows the emission spectra of Sr2CeO4 excited at 267 nm for varying chelating agent to metal ions concentration.
Fig. 1. (A) TG–DTA curve of the precursor powder dried in air (chelating agent/metal ions¼ 0.5) and (B) XRD pattern of precursor (chelating agent/metal ions¼2) calcined at different temperatures.
R. Seema, K. Nandakumar / Journal of Luminescence 131 (2011) 2181–2184
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Fig. 2. (A) FTIR spectra of precursor (chelating agent/metal ions ¼2) calcined at different temperatures and (B) SEM image of Sr2CeO4 (chelating agent/metal ions ¼2) sintered at 1000 1C.
Fig. 3. (A) Excitation spectrum of Sr2CeO4 (chelating agent/metal ion ¼ 2) (B) emission spectra of Sr2CeO4 for varying chelating agent to metal ions concentration; (C) emission spectra of Sr2CeO4 (chelating agent/metal ion ¼2) calcined at different temperatures; (D) emission spectra of Sr2CeO4 prepared by (a) sol–gel PVA route (b) solid state route.
The spectra show broadband in the region 300–700 nm with a peak around 470 nm. The emission band is assigned to the f-t1g transitions of Ce4 þ ions. When the chelating agent to metal ions ratio is varied, the luminescence intensity also varies. Effective luminescence is observed when the chelating agent to metal ions ratio is 2. Fig. 3(C) shows the emission spectra of Sr2CeO4 (chelating agent/metal ions¼2) calcined at different temperatures. The luminescence intensity increases as the temperature is increased from 800 1C to 1000 1C, this is because the crystallization occurs at higher temperature. Pure phased Sr2CeO4 was obtained when the derived precursors were heated at 1000 1C for
2 h, which is clear from XRD. Raising the temperature results in increase of Sr2CeO4, this obviously resulted in the increase in emission intensities. Fig. 3(D) shows the emission spectra of Sr2CeO4 prepared by sol–gel PVA route and solid state reaction. It is clear from the spectra that the intensity of Sr2CeO4 prepared by sol–gel PVA route is higher than that prepared by solid state reaction method. This is the significant advantage of this synthesis technique. The CIE co-ordinates were calculated by the spectrophotometric method using the spectral energy distribution of the Sr2CeO4. The color co-ordinates are x¼0.15 and y¼0.24 for the Sr2CeO4 (chelating agent/metal ion¼2) sintered at 1000 1C.
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4. Conclusion Sr2CeO4 was successfully synthesized by a simple and convenient sol–gel PVA processing route using strontium nitrate and cerium nitrate as raw materials. The XRD study confirms the structure of the system to be orthorhombic. The excitation spectrum consisted of two bands at 254 nm and 267 nm. The emission spectrum was a broad band at 470 nm. An optimum concentration of chelating agent to metal ion was found to be two since this yield the effective luminescence. At room temperature, the luminescence intensity of Sr2CeO4 increased when the calcination temperature was increased from 800 to 1000 1C. Sr2CeO4 was completely formed when the precursor was sintered at 1000 1C; this resulted in maximum emission intensity. The sol–gel PVA route has higher luminescence intensity of Sr2CeO4 than the solid state reaction method. The strong emission makes Sr2CeO4 phosphor suitable for applications in advanced lighting and displaying devices. References [1] L. Muresan, E.J. Popovici, R. Grecu, L.B. Tudoran, J. Alloys Compd. 471 (2009) 421. [2] R. Ghildiyal, P. Page, K.V.R. Murthy, J. Lumin. 124 (2007) 217. [3] C.H. Lu, R. Jagannathan, Appl. Phys. Lett. 80 (2002) 3608.
[4] S.S. Yi, K.S. Shim, H.K. Yang, B.K. Moon, B.C. Choi, J.H. Jeong, J.H. Kim, J.S. Bae, Appl. Phys. A 87 (2007) 667. [5] L. Zhou, J. Wei, J. Wu, F. Gong, L. Yi, J. Huang, J. Alloys Compd. 476 (2009) 390. [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] 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, J. Mol. Struct. 470 (1998) 229. [8] X.M. Liu, Y. Luo, J. Lin, J. Cryst. Growth 290 (2006) 266. [9] T. Masui, T. Chiga, N. Imanaka, G. Adachi, Mater. Res. Bull. 38 (2003) 17. [10] C.H. Park, C.H. Kim, C.H. Pyun, J.H. Choy, J. Lumin. 87–89 (2000) 1062. [11] L. Van Pieterson, S. Soverna, A. Meijerink, J. Electrochem. Soc. 147 (2000) 4688. [12] S.J. Chen, X.T. Chen, Z. Yu, J.M. Hong, Z. Xue, X.Z. You, Solid State Commun. 130 (3–4) (2004) 281. [13] X. Yu, X. He, S. Yang, X. Xu, Mater. Lett. 58 (2004) 48. [14] S.K. Hong, S.H. Ju, H.Y. Koo, D.S. Jung, Y.C. Kang, Mater. Lett. 60 (2006) 334. [15] J. Junkasem, R. Rujiravanit, P. Supaphol, Nanotechnology 17 (2006) 4519. [16] Y.B. Khollam, S.B. Deshpande, P.K. Khanna, P.A. Joy, H.S. Potdar, Mater. Lett. 58 (2004) 2521. [17] Tai-Yuan Wu, Chung-Hsin Lu, Chia-Hao Hsu, J. Lumin. 130 (2010) 737. [18] S.J. Lee, W.M. Kriven, J. Am. Ceram. Soc. 81 (10) (1998) 2605. [19] S.J. Lee, W.M. Kriven, Ceram. Eng. Sci. Proc. 19 (4) (1998) 469. [20] S.J. Lee, E.A. Benson, W.M. Kriven, J. Am. Ceram.Soc. 82 (8) (1999) 2049. [21] M.H. Nguyen, S.J. Lee, W.M. Kriven, J. Mater. Res. 14 (8) (1999) 3417. [22] Y.X. Tang, H.P. Guo, Q.Z. Qin, Solid State Commun. 121 (2002) 351. [23] J.M. Alia, Y. Diaz de Mera, H.G.M. Edwards, P. Gonzalez Martin, S. Lopez Andres, Spectrochim. Acta A 53 (1997) 2347. [24] O.A. Serra, V.P. Severino, P.S. Cale., S.A. Cicillini, J. Alloys Compd. 323–324 (2001) 667.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
A new synthetic pathway of Sr2CeO4 phosphor and its characterization R. Seema a,n, K. Nandakumar a,b a b
School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam 686560, Kerala, India Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam 686560, Kerala, India
a r t i c l e i n f o
abstract
Article history: Received 25 November 2010 Received in revised form 27 April 2011 Accepted 3 May 2011 Available online 8 May 2011
A blue–white emitting Sr2CeO4 phosphor was synthesized via a simple sol–gel poly vinyl alcohol (PVA)complexing process using strontium nitrate and cerium nitrate as raw materials. The samples were characterized by TG/DTA, XRD, FTIR, SEM and photoluminescence spectroscopy. The X-ray diffraction study confirms the structure of the system to be orthorhombic. The emission spectra when excited at 267 nm peaks at 470 nm. The emission band is assigned to the energy transfer between the molecular orbital of the ligand and charge transfer state of the Ce4 þ ion. The Commission International de l’Eclairage (CIE) co-ordinates for the Sr2CeO4 sample were also calculated. & 2011 Elsevier B.V. All rights reserved.
Keywords: Sr2CeO4 PVA Luminescence XRD Blue phosphor
1. Introduction Various optical devices such as plasma display panels (PDPs), field emission displays (FEDs) and light-emitting diodes (LEDs) widely utilize oxide-based phosphors due to their high thermal and chemical stability [1]. In recent years, oxide-based lanthanide phosphors have caught much attention because of their excellent optical properties [2]. Comparable red and green emitting phosphors have been widely used in FEDs and LEDs [3–5], but blue emitting phosphors suitable for practical applications remain rarely available. Sr2CeO4 was first discovered by Danielson et al. [6] in 1998 using combinatorial technique. Sr2CeO4 phosphor has been confirmed to have an orthorhombic crystal structure with onedimensional chains of edge-sharing CeO6 octahedrons linked by strontium ions [7], and it was considered a potential candidate for blue phosphors [8]. In addition, Sr2CeO4 was found to emit efficiently under ultraviolet, cathode ray and X-ray excitation [6,9]. Various chemical methods have been developed for the synthesis of pure, single-phase powders with controlled powder characteristics of Sr2CeO4 phosphor. Based on the literature survey, it is clear that the preparation method and the precursors play an important role in the formation of pure Sr2CeO4 luminescent phase. High heating temperature is usually required for the preparation of Sr2CeO4 via conventional solid state reaction
n
Corresponding author. Tel.: þ91 481 2731043. E-mail address: [email protected] (R. Seema).
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.05.004
technique [6,7,10,11]. Relatively lower heating temperature is required for the synthesis of Sr2CeO4 via PEG sol–gel process [12], citrate–gel process [13], co-precipitation method [9], ultrasonic spray pyrolysis [14] and combustion reaction [15]. Other methods like microwave-hydrothermal [16] and microwave-solvothermal methods [17] have also been adopted for the synthesis of Sr2CeO4. Although several chemical routes have been applied for the synthesis of fine Sr2CeO4 phosphor particles with good luminescence, challenges still remain in reducing process complexity, controllability and cost. Therefore, new synthesis techniques are being explored to potentially overcome some of these difficulties. Here poly vinyl alcohol –[CH2–CHOH]-n, a rather simplestructured and inexpensive polymer, has been used as polymeric carrier [18–21] for the synthesis of Sr2CeO4 phosphor. The nitrate ions act as cation sources for oxide ceramic powders, become oxidizing agents for the decomposition of PVA. The large chain molecule of PVA operates as a steric entrapment mechanism in the organic–inorganic solution. In this study the structure, morphology and photoluminescence of Sr2CeO4 phosphor are investigated.
2. Experimental The Sr2CeO4 was prepared by a simple sol–gel route using PVA as the chelating agent. The starting materials were strontium nitrate, cerium nitrate and PVA. The ratio of chelating agent to metal ions was varied from 0.5 to 4, in step of 0.5 increase to observe the optimum concentration. The sol–gel samples were prepared by taking analytical grade nitrates of strontium and
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cerium (CDH, Laboratory Reagent, purity 99.0%) and adding them in stoichiometric ratio (Sr2 þ : Ce4 þ , 2:1). The mixture of these salts was then added to the distilled water and the solution was kept on a magnetic stirrer maintained at 35 1C. PVA (CDH, Laboratory Reagent, Mw 125,000) was then added to the aqueous solution. The resultant mixture was continuously stirred for few hours with temperature maintained at 50 1C. It was further heated at 90 1C, once the mixture transformed into a brownish gel, it turned to brown fluffy powder on further heating (here after called as the precursor). The precursor powder was taken for TG/DTA analysis and the rest was kept for drying in an oven at 250 1C for 2 h. The compound obtained (white in color) was ground into powder. The obtained powder was again heated at 400 1C, 600 1C, 800 1C, 900 1C and then at 1000 1C for 2 h. Sr2CeO4 was also prepared by conventional solid state reaction method [10] taking the stoichiometric ratio of strontium carbonate and cerium oxide. The mixture was thoroughly homogenized in an agate mortar for 4 h in presence of acetone. The obtained mixture was then sintered at 1000 1C for 36 h. The TG–DTA curve was recorded using a Shimadzu DTG60 thermal analyzer at a heating rate of 10 1C in air atmosphere. The phase of the phosphor powder was checked by XRD pattern taken using a Bruker D8 ADVANCE X-Ray Diffractometer with Cu–Ka ˚ radiation. Fourier transform infrared spectroscopy (1.54056 A) (Shimadzu 8400 S FTIR) was carried out in the range of 4000– 400 cm 1 in transmission mode. The Scanning electron micrograph was taken using JEOL JSM6390, in order to characterize particle morphology. Photoluminescence measurements were performed with spectrofluorophotometer (SHIMADZU, RF-5301PC) using 150 W Xenon lamp as excitation source. The excitation and emission slit widths are 1.5 nm. All spectra were recorded at room temperature.
3. Results and discussion Fig. 1(A) shows the TG/DTA curve of the brown precursor powder (chelating agent/metal ion ¼0.5) dried in air. The two endothermic peaks around 62 and 125 1C correspond to the loss of absorbed water and decomposition of nitrates, respectively. At temperatures above 130 1C, PVA starts to degrade gradually until reaching 200 1C. There is an exothermic peak at 195 1C, which indicates the degradation of PVA. The decomposition of SrCO3 and CeO2, with the release of CO2, and formation of Sr2CeO4 occurs between 800 and 1000 1C. From 600 to 1000 1C, there is only a slight variation in the weight loss.
To investigate the evolution of crystalline phases, the precursor powders (chelating agent/metal ions ¼2) were calcined at different calcinations temperatures. Fig. 1(B) shows the corresponding XRD patterns for the resultant powders. Weak crystalline phases of SrCO3 and CeO2 were observed in powder calcined at 250 1C. There are three steps involved in the formation of Sr2CeO4; firstly, some of the nitrates may decompose into their corresponding oxides; secondly, SrO then react with carbon dioxide or carbon produced by PVA to form SrCO3; finally SrCO3 reacts with CeO2 to form Sr2CeO4 [8,22]. When the calcination temperature was increased from 400 to 800 1C, the diffraction peaks of SrCO3 and CeO2 exists. When the precursor was calcined at 900 1C for 2 h, Sr2CeO4 phase with the orthorhombic symmetry appeared dominantly. A single phased Sr2CeO4 was completely formed when calcined at 1000 1C for 2 h, which was in good agreement with the reported data (JCPDS 50-0115) [6]. Fig. 2(A) shows the IR spectra of the precursor powder (chelating agent/metal ions ¼2) calcined at different temperatures. Absorption peaks at 1770, 1450, 1070, 860, 706 and 699 cm 1 are assigned to stretching characteristics of SrCO3 [23]. From 400 to 1000 1C, the absorption of carbonate gradually weakens. XRD indicates that the precursor powder (calcined at 1000 1C) has only Sr2CeO4 phase, while the FTIR curves indicate that it bears amorphous carbonate phase. Evidently, the absorption peaks at 1400–1450 cm 1 (CH2 bending) and 850 cm 1 (CH2 rocking) characteristic to PVA are obvious in all of the spectra [15]. The absorption peak at 600–300 cm 1 is assigned to the metal oxide (Ce–O) frequency bands. Fig. 2(B) shows the SEM image of Sr2CeO4 (chelating agent/metal ion¼2) sintered at 1000 1C for 2 h. It is clear from the SEM image that the morphology of the sample is not uniform; it varies from round to irregular shape. Fig. 3(A) shows the excitation spectrum of Sr2CeO4 (chelating agent/metal ion¼2). The excitation spectrum consists of a peak at 254 nm along with a small hump around 267 nm. There are two different Ce4 þ –O2 bond lengths in the lattice, thus two excitation peaks are attributed to the two different charge transfer transitions [6]. The hump around 267 nm evident from the excitation curve may be attributed to the above-defined mechanism. As reported by Serra et al. [24] and by Ghildiyal et al. [2] the intensity of two excitations is not same. The excitation band of the sample could be attributed to the transition t1g-f, where f is the lowest excited charge transfer state of the Ce4 þ ion and t1g is the molecular orbital of the surrounding ligand in the six-fold oxygen co-ordination [6]. Fig. 3(B) shows the emission spectra of Sr2CeO4 excited at 267 nm for varying chelating agent to metal ions concentration.
Fig. 1. (A) TG–DTA curve of the precursor powder dried in air (chelating agent/metal ions¼ 0.5) and (B) XRD pattern of precursor (chelating agent/metal ions¼2) calcined at different temperatures.
R. Seema, K. Nandakumar / Journal of Luminescence 131 (2011) 2181–2184
2183
Fig. 2. (A) FTIR spectra of precursor (chelating agent/metal ions ¼2) calcined at different temperatures and (B) SEM image of Sr2CeO4 (chelating agent/metal ions ¼2) sintered at 1000 1C.
Fig. 3. (A) Excitation spectrum of Sr2CeO4 (chelating agent/metal ion ¼ 2) (B) emission spectra of Sr2CeO4 for varying chelating agent to metal ions concentration; (C) emission spectra of Sr2CeO4 (chelating agent/metal ion ¼2) calcined at different temperatures; (D) emission spectra of Sr2CeO4 prepared by (a) sol–gel PVA route (b) solid state route.
The spectra show broadband in the region 300–700 nm with a peak around 470 nm. The emission band is assigned to the f-t1g transitions of Ce4 þ ions. When the chelating agent to metal ions ratio is varied, the luminescence intensity also varies. Effective luminescence is observed when the chelating agent to metal ions ratio is 2. Fig. 3(C) shows the emission spectra of Sr2CeO4 (chelating agent/metal ions¼2) calcined at different temperatures. The luminescence intensity increases as the temperature is increased from 800 1C to 1000 1C, this is because the crystallization occurs at higher temperature. Pure phased Sr2CeO4 was obtained when the derived precursors were heated at 1000 1C for
2 h, which is clear from XRD. Raising the temperature results in increase of Sr2CeO4, this obviously resulted in the increase in emission intensities. Fig. 3(D) shows the emission spectra of Sr2CeO4 prepared by sol–gel PVA route and solid state reaction. It is clear from the spectra that the intensity of Sr2CeO4 prepared by sol–gel PVA route is higher than that prepared by solid state reaction method. This is the significant advantage of this synthesis technique. The CIE co-ordinates were calculated by the spectrophotometric method using the spectral energy distribution of the Sr2CeO4. The color co-ordinates are x¼0.15 and y¼0.24 for the Sr2CeO4 (chelating agent/metal ion¼2) sintered at 1000 1C.
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4. Conclusion Sr2CeO4 was successfully synthesized by a simple and convenient sol–gel PVA processing route using strontium nitrate and cerium nitrate as raw materials. The XRD study confirms the structure of the system to be orthorhombic. The excitation spectrum consisted of two bands at 254 nm and 267 nm. The emission spectrum was a broad band at 470 nm. An optimum concentration of chelating agent to metal ion was found to be two since this yield the effective luminescence. At room temperature, the luminescence intensity of Sr2CeO4 increased when the calcination temperature was increased from 800 to 1000 1C. Sr2CeO4 was completely formed when the precursor was sintered at 1000 1C; this resulted in maximum emission intensity. The sol–gel PVA route has higher luminescence intensity of Sr2CeO4 than the solid state reaction method. The strong emission makes Sr2CeO4 phosphor suitable for applications in advanced lighting and displaying devices. References [1] L. Muresan, E.J. Popovici, R. Grecu, L.B. Tudoran, J. Alloys Compd. 471 (2009) 421. [2] R. Ghildiyal, P. Page, K.V.R. Murthy, J. Lumin. 124 (2007) 217. [3] C.H. Lu, R. Jagannathan, Appl. Phys. Lett. 80 (2002) 3608.
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