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Luminescence properties of red emitting phosphor NaSrBO3:Eu3+ prepared with novel combustion synthesis method
Journal of Luminescence 142 (2013) 180–183
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescence properties of red emitting phosphor NaSrBO3:Eu3+ prepared with novel combustion synthesis method Devayani Chikte (Awade) a,n, S.K. Omanwar b, S.V. Moharil c a
G.N. Khalsa College, Matunga, Mumbai 400019, India Department of Physics, S.G.B Amravati University, Amravati, India c Department of Physics, RT.M Nagpur University, Nagpur 440010, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 18 December 2012 Received in revised form 11 March 2013 Accepted 28 March 2013 Available online 16 April 2013
The red emitting phosphor NaSrBO3:Eu3+is synthesised by simple, time saving, economically modified method of solution combustion synthesis at comparatively lower temperature using urea as fuel. X-ray power diffraction (XRD) analysis confirmed the formation of the said phosphor. Photoluminescence measurements showed that the phosphor exhibited emission peak with good intensity at −614 nm, corresponding to 5D0–7F2 (614 nm) red emission andweak 5D0–7F1 (593 nm) orange emission. The excitation spectra monitored at 614 nm show broad band from 250 to 350 nm ascribed to O–Eu chargetransfer (CTB) transition and the other peaks in the range of 350–410 nm originated from f–f transitions of Eu3+ ions. The strongest band at 394 nm can be assigned to 7F0–5L6 transition of Eu3+ ions due to the typical f–f transitions within Eu3+ of 4f6 configuration. The latter lies in near ultraviolet (350–410 nm) emission of UV LED. For the excitation wavelength of 394 nm the emission intensity increases initially with the increase of Eu3+ concentration and reaches to the maximum at x ¼ 0.09.The concentration quenching observed after that is mainly due to quadrupole–quadrupole interaction. The as synthesised phosphor NaSrBO3:Eu3+by this method shows CIE colour coordinates of (0.62,0.34) with good intensity. & 2013 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Combustion synthesis Phosphor Red emission
1. Introduction: The borates possess excellent properties as host structures of phosphors due to the inherent attributes of the large band gap and covalent bond energy. Moreover they possess the advantages of low synthesising temperature and high chemical and physical stability. A variety of borate host materials doped with rare earth and other ions have been reported as phosphor materials for a variety of applications [1–5]. In order to obtain novel phosphors, researchers have tried many Sr–B based inorganic compounds as host lattices and attained various phosphors such as, Sr3B2O6:Ce [6], LiSr4(BO3)3: Dy [7],(Sr,Ca)3B2O6:Eu [8], etc. Among the strontium borates, the crystal structure of NaSrBO3 was reported by Wu et al. [9]. Until recently, Liu et al. have synthesised a high efficiency and high colour purity blue-emitting phosphor NaSrBO3:Ce [10] by the solid state reaction method. Fan Yang et al. prepared the NaSrBO3:Eu3+ phosphor by the solid state reaction method in which prolonged heating of 8 h was required at 850 1C.
n
Corresponding author. Tel.: +91 9967034316; fax: +91 712 2249875. E-mail address: [email protected] (D. Chikte (Awade)).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.03.045
We are reporting the photoluminescence properties and synthesis of NaSrBO3:Eu3+ phosphor by a simple, time saving, and economical method of modified solution combustion synthesis at comparatively lower temperature using urea as fuel. This method is easy to handle and requires no absolute control on the temperature. During the synthesis oxidiser and fuel will automatically decide the reaction temperature [11]. The same technique is now well established for borate, silicate aluminates and vanadates preparation [12–19]. As far as we know, the method is adopted for the first time for the synthesis of NaSrBO3:Eu3+phosphor. 2. Experimental The phosphors NaSr1−xBO3:xEu3+ doped with different molar concentration of Eu3+ (x ¼0.03,0.06,0.09, and 0.12) were prepared by the modified solution combustion synthesis [11] method. The synthesis is based on the exothermic reaction between the fuel (urea) and oxidiser (ammonium nitrate). For complete combustion, the oxidiser to fuel ratio should be equal to one. The constituent raw materials (AR grade) sodium nitrate, strontium nitrate, Europium nitrate, ammonium nitrate, urea and boric acid (as boron source) were weighed in stoichiometric proportion and dissolved in minimum amount of water and fired at 550 1C.
D. Chikte (Awade) et al. / Journal of Luminescence 142 (2013) 180–183
The solution boils and undergoes dehydration followed by decomposition with evolution of gases N2, CO2, etc. The mixture then froths and swells forming foam that ruptures with a flame on ignition of combustible gases and glows to incandescence. During incandescence the foam further swells to the capacity of container. The whole process completes in few minutes. Following the combustion resulting voluminous fine powder was annealed in open air at 750 1C for 2 h and quenched to room temperature. The samples were subjected to XRD analysis using an X’Pert PRO advanced automatic diffractometer with Cu Kα radiation (λ ¼1.540598 Å) operated at 40 kV and 30 mA.The XRD data was collected in a 2θ range from 101 to 801 at room temperature. The measurements of photoluminescence (PL) over the range of 450– 650 nm and photoluminescence excitation spectra (PLE) over 200– 400 nm excitation range were carried out on Hitachi-F7000 florescence spectrophotometer at room temperature. The spectral resolution of both excitation and emission spectra, width of the monochromatic slits (2.5 nm), as well the measurement conditions such as PMT detector sensitivity and scan speed were kept constant from sample to sample in measurements. The SEM image was recorded for surface morphology. The colour chromaticity coordinates were obtained according to Commission International de I’Eclairage (CIE) using Radiant Imaging colour calculator.
3. Result and discussion 3.1. XRD analysis The powder XRD pattern of the sample was analysed for the structure confirmation. Fig. 1 represents the powder XRD pattern of NaSrBO3 prepared by the solution combustion synthesis method which is in good agreement with the XRD data of NaSrBO3 reported by Wu et al. [9]. The consistent result indicated that the single phase polycrystalline NaSrBO3 powder was obtained in the study. As synthesised sample exhibit the same diffraction peaks as reported by Wu et al. corresponding to the monoclinic structure with space group of P21/c. The reported lattice parameters of NaSrBO3 are a ¼5.32446 (7) Å, b ¼9.2684(1) Å, c ¼6.06683(8) Å and β¼100.58(1)1. The cell volume (V) is 294.30(8)1 and cations in the unit cell (Z) are 4. In NaSrBO3 crystal lattice, the fundamental building units are isolated planar BO3 anionic groups, which are parallelly distributed along two different directions. The Na atoms are six-coordinated with O atoms to form octahedral, and the Sr atoms coordinated by nine O atoms are in the form of tri-capped trigonal prisms [9]. The acceptable percentage difference in ionic radii between doped and substituted ions must not exceed 30% [22]. The calculations of the radius percentage difference (Dr) between the doped ions (Eu3+) and possible substituted ions (Sr, B, Na) in
Fig. 1. X-ray diffraction pattern of as synthesised NaSrBO3.
181
NaSrBO3:Eu3+are summarised in Table 1 below. The values are based on the formula: Dr ¼
RmðCNÞ−RdðCNÞ RmðCNÞ
where CN—co-ordination number,RmðCNÞ —Radius of host cations and RdðCNÞ- Radius of dopant ion Thus it is clear that the Eu3+ ionic radius (r ¼1.12 Å, CN ¼9) is closest to that of Sr2+ (r ¼1.31 Å, CN ¼9), making it unlikely that Eu3+ ions would substitute with Na+ (r ¼1.02 A, CN ¼ 6) or B3+ (r ¼0.11, CN ¼3) in the NaSrBO3 host. Hence, it is believed that the Sr2+ sites are replaced by Eu3+ in this lattice. [20] Fig. 2 represents the SEM image showing surface morphology of as-synthesised NaSrBO3:Eu3+ phosphor. It shows the particles with an irregular shape and the particle size ranging in 5–10 μm. 3.2. Photoluminescence Fig. 3 shows the photoluminescence excitation and emission spectrum of NaSr1−xBO3:xEu (for x ¼0.09). The excitation spectrum monitored at 614 nm emission exhibits a broad band from 250 to 350 nm ascribed to O–Eu charge-transfer band (CTB) transition and the other peaks in the range of 350–400 nm originated from f–f transitions of Eu3+ ions. The strongest band at 394 nm can be assigned to 7F0–5L6 transition of Eu3+ ions. The emission spectrum exhibits five typical emission peaks in the range of 560–720 nm, which result from 5D0–7FJ (J¼ 0, 1, 2, 3 and 4) transitions of Eu3+ion, respectively. The main peak centred at 614 nm corresponds to electron dipole 5D0–7F2 transition and the peak at 592 nm ascribes to magnetic dipole 5D0–7F1 transition of Eu3+ions. Fig. 4 represents the emission spectra of NaSrBO3 doped with differentEu3+ concentrations for the excitation wavelength of 394 nm. All of the emission spectra exhibit the similar profile with different relative intensities. The emission intensity increases initially with the increase of Eu3+ concentration and reaches Table1 Ionic radii difference percentage (Dr) between matrix cations and doped ions. Ions in matrix 3+
Eu (doped) Sr2+ Na+ B3+
Ionic radius (r), Å
CN
Dr
1.12 1.31 1.02 0.11
9 9 6 3
– 14.5% −9.8% −918%
Fig. 2. SEM image of as-synthesised NaSrBO3:Eu3+.
182
D. Chikte (Awade) et al. / Journal of Luminescence 142 (2013) 180–183
Fig. 3. PLE (a) and PL (b) curve for NaSr1−xBO3:xEu3+ for x¼ 0.06.
Fig. 5. Dependence of relative intensity of emission on Eu3+ doping concentration.
Fig. 4. PL of NaSr1−xBO3:xEu3+for different ‘x’ values for excitation wavelength of 394 nm.
maximum at x ¼0.09, then gradually decreases due to the internal concentration quenching. Fig. 5 shows the dependence of the peak intensity of the emission centred at 614 nm on Eu3+doping concentration (x) in NaSr1−xBO3:xEu3+. The optimum intensity of emission is at x¼ 0.09 after that intensity drops due to internal concentration quenching. The concentration quenching mechanism is generally associated with energy transfer. Non-radiative energy transfer process from one Eu3+ ion to another Eu3+ ion can be described by three different methods:(1) exchange interaction, (2) radiation reabsorption and (3) multipolar interaction. While discussing the mechanism of energy transfer in phosphors, Blasse [21] suggested that if the activator is introduced solely on one crystallographic site (here Sr2+site), the critical energy transfer distance (Rc) is approximately equal to twice the radius of a sphere with this volume. In order to further discuss the mechanism of energy transfer between the activators in the NaSrBO3 host, the critical energy transfer distance (Rc) can be calculated by the following equation: Rc≈2
3V 4πχ c N
1=3
where χc the critical concentration, N is the number of cation sites in the unit cell, and V is the volume of the unit cell. So in this case,
Fig. 6. PL of NaSr1−xBO3:xEu3+ for different excitation wavelengths: (a) excitation wavelength 394 nm and (b) excitation wavelength 270 nm.
V¼294.30 Å, N ¼ 4 and the critical doping concentration of Eu3+ in the NaSrBO3 host is found to be 0.09. Thus, the Rc of Eu3+ in NaSrBO3:Eu3+ phosphor is determined to be 11.6 Å. Since Rc is not less than 5 Å exchange interaction is not responsible for nonradiative energy transfer process from one Eu3+ ion to another Eu3+ ion in this host. The mechanism of radiation reabsorption is the primary method only if the fluorescence spectra of the excitation and emission have obvious overlap. Thus, in view of the emission and excitation spectra of NaSrBO3:Eu3+ (Fig. 3), the radiation reabsorption is unlikely to occur. As a result, the energy transfer process of Eu3+ in NaSrBO3 phosphor would be due to multipolar interaction. Further it can be proven to be because of quadrupole–quadrupole interaction. Fig. 6 shows the variation of emission intensity for excitation wavelength of 394 nm (a) and 270 nm (b) for the same Eu3+ doping concentration (for x ¼0.06). It is obvious that the intensity of emission is greater for the excitation of 394 nm. Fig. 7 Shows the plot of CIE coordinates of the NaSrBO3:0.09Eu3+ phosphor and the commercial red phosphor Y2O2S:0.05Eu3+. The CIE coordinates of the as-synthesied sample by modified solution combustion synthesis method are (0.62,0.35) which are same as that of reported by Fan Yang et al. for the said phosphor. On the same intsity scale the intensity of the phosphor prepared by this method is
D. Chikte (Awade) et al. / Journal of Luminescence 142 (2013) 180–183
183
is calculated as 11.6 Å for the optimum doping concentration. The equadrupole–quadrupole interaction is the major mechanism for concentration quenching of fluorescence emission of Eu3+ ions in this phosphor. Eu3+ ions only occupy the Sr2+ sites and form one emission centre at 614 nm. The CIE coordinates of the assynthesied sample are (0.62,0.35) for optimum emission intensity.
Acknowledgement One of the authors (DPA) would like to thank University Grants Commission (UGC), New Delhi for availing Teacher Fellowship under Faculty Development Programme and X-ray-lab, IIT Mumbai for availing XRD facility. References
Fig. 7. CIE coordinates of the NaSrBO3:0.09Eu3+ phosphor and the commercial compound, Y2O2S:Eu3+.
better. For 394 nm excitation the emission peak is at 614 nm with FWHM of 12 nm. 4. Conclusion The red emitting phosphor NaSrBO3:Eu3+is synthesised by a simple, time saving, economically novel method of modified solution combustion synthesis at comparatively lower temperature. The XRD pattern of NaSrBO3 prepared by the combustion method is in good agreement with the XRD data of NaSrBO3 reported by Wu et al. The as synthesised phosphor shows intense red emission at 614 nm. The excitation spectrum monitored at 614 nm shows broad band centred at 250 nm along with intense line at 394 nm. The Pl spectrum at various doping concentration of Eu3+ shows maximum emission intensity at x ¼0.09 after the concentration quenching is observed. The critical transfer distance
[1] R. Chen, J.L. David, J. Electrochem. Soc. 149 (2002) S69. [2] G. Blasse, J. Chem. Phys. 46 (1967) 2583. [3] R. Velchuri, V. Kumar, V. Rama Devi, G. Prasad, Jaya Prakash D., M. Vithal, Mater.Res. Bull. 46 (8) (2011) 1219. [4] K. Kim, Y.M. Moon, S. Choi, H.K. Jung, S. Nahm, Mater. Lett. 62 (2008) 3925. [5] D.I. Shahare, B.T. Deshmukh, S.V. Moharil, S.M. Dhopte, P.L. Muthal, V. K. Kondawar, Phys. Status Solidi A 141 (1994) 329. [6] Xu Li, Chong Liu, Li Guan, Wei Wei, Zhiping Yang, Qinglin Guo, Guangsheng Fu, Mater. Lett. 87 (2012) 121–123. [7] Zhi-Wei Zhang, Xin-Yuan Sun, Lu Liu, You-shun Peng, Xi-hai Shen, Wei-Guo Zhang, Dong-Jun Wang, Ceram. Int. (2012). [8] Yanming Xue, Xuewen Xu, Long Hu, Ying Fan, Xiuqing Li, Jiajia Li, Zhaojun MoChengchun Tang, J. Lumin. 131 (9) (2011) 2016. [9] L. Wu, X.L. Chen, Y. Zhang, Y.F. Kong, J.J. Xu, Y.P. Xu, J. Solid State Chem. 179 (4) (2006) 1219. [10] W.R. Liu, C.H. Huang, C.P. Wu, Y.C. Chiu, Y.T. Yeh, T.M. Chen, J. Mater. Chem. 21 (2011) 6869. [11] K.C. Patil, Bull. Mater. 16 (6) (1993) 533. [12] R.P. Sonekar, S.K. Omanwar, S.V. Moharil, S.M. Dhopte, P.L. Muthal, V. K. Kondawar, Opt. Mater. 30 (4) (2007) 622. [13] D.S. Thakare, S.K. Omanwar, S.V. Moharil, S.M. Dhopte, P.L. Muthal, V. K. Kondawar, Opt. Mater. 29 (12) 1731 29 (12) (2007). [14] R.P. Sonekar, S.K. Omanwar, S.V. Moharil, Ind. J. Pure Appl. Phys. 47 (2009) 441. [15] P.A. Nagpure, S.K. Omanwar, J. Rare Earths 30 (9) (2012) 856. [16] P.A. Nagpure, N.S. Bajaj, R.P. Sonekar, S.K. Omanwar, Indian J. Pure Appl. Phys. 49 (12) (2011) 799. [17] D.S. Thakare, S.K. Omanwar, P.L. Muthal, S.M. Dhopte, V.K. Kondawar, S. V. Moharil, Phys. Status Solidi A 201 (3) (2004) 574. [18] V.B. Bhatkar, S.K. Omanwar, S.V. Moharil, Opt. Mater. 29 (8) (2007) 1066. [19] V.B. Bhatkar, S.K. Omanwar, S.V. Moharil, Phys. Status Solidi A 191 (1) (2002) 272. [20] R.D. Shannon, Acta Crystallogr.: Sect. A: Cryst. Phys. 32 (1976) 751. [21] G. Blasse, Phys. Lett. A 28 (1968) 444. [22] A.M. Pirees, M.R. Davolos, Chem. Mater. 13 (2001) 21.
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescence properties of red emitting phosphor NaSrBO3:Eu3+ prepared with novel combustion synthesis method Devayani Chikte (Awade) a,n, S.K. Omanwar b, S.V. Moharil c a
G.N. Khalsa College, Matunga, Mumbai 400019, India Department of Physics, S.G.B Amravati University, Amravati, India c Department of Physics, RT.M Nagpur University, Nagpur 440010, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 18 December 2012 Received in revised form 11 March 2013 Accepted 28 March 2013 Available online 16 April 2013
The red emitting phosphor NaSrBO3:Eu3+is synthesised by simple, time saving, economically modified method of solution combustion synthesis at comparatively lower temperature using urea as fuel. X-ray power diffraction (XRD) analysis confirmed the formation of the said phosphor. Photoluminescence measurements showed that the phosphor exhibited emission peak with good intensity at −614 nm, corresponding to 5D0–7F2 (614 nm) red emission andweak 5D0–7F1 (593 nm) orange emission. The excitation spectra monitored at 614 nm show broad band from 250 to 350 nm ascribed to O–Eu chargetransfer (CTB) transition and the other peaks in the range of 350–410 nm originated from f–f transitions of Eu3+ ions. The strongest band at 394 nm can be assigned to 7F0–5L6 transition of Eu3+ ions due to the typical f–f transitions within Eu3+ of 4f6 configuration. The latter lies in near ultraviolet (350–410 nm) emission of UV LED. For the excitation wavelength of 394 nm the emission intensity increases initially with the increase of Eu3+ concentration and reaches to the maximum at x ¼ 0.09.The concentration quenching observed after that is mainly due to quadrupole–quadrupole interaction. The as synthesised phosphor NaSrBO3:Eu3+by this method shows CIE colour coordinates of (0.62,0.34) with good intensity. & 2013 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Combustion synthesis Phosphor Red emission
1. Introduction: The borates possess excellent properties as host structures of phosphors due to the inherent attributes of the large band gap and covalent bond energy. Moreover they possess the advantages of low synthesising temperature and high chemical and physical stability. A variety of borate host materials doped with rare earth and other ions have been reported as phosphor materials for a variety of applications [1–5]. In order to obtain novel phosphors, researchers have tried many Sr–B based inorganic compounds as host lattices and attained various phosphors such as, Sr3B2O6:Ce [6], LiSr4(BO3)3: Dy [7],(Sr,Ca)3B2O6:Eu [8], etc. Among the strontium borates, the crystal structure of NaSrBO3 was reported by Wu et al. [9]. Until recently, Liu et al. have synthesised a high efficiency and high colour purity blue-emitting phosphor NaSrBO3:Ce [10] by the solid state reaction method. Fan Yang et al. prepared the NaSrBO3:Eu3+ phosphor by the solid state reaction method in which prolonged heating of 8 h was required at 850 1C.
n
Corresponding author. Tel.: +91 9967034316; fax: +91 712 2249875. E-mail address: [email protected] (D. Chikte (Awade)).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.03.045
We are reporting the photoluminescence properties and synthesis of NaSrBO3:Eu3+ phosphor by a simple, time saving, and economical method of modified solution combustion synthesis at comparatively lower temperature using urea as fuel. This method is easy to handle and requires no absolute control on the temperature. During the synthesis oxidiser and fuel will automatically decide the reaction temperature [11]. The same technique is now well established for borate, silicate aluminates and vanadates preparation [12–19]. As far as we know, the method is adopted for the first time for the synthesis of NaSrBO3:Eu3+phosphor. 2. Experimental The phosphors NaSr1−xBO3:xEu3+ doped with different molar concentration of Eu3+ (x ¼0.03,0.06,0.09, and 0.12) were prepared by the modified solution combustion synthesis [11] method. The synthesis is based on the exothermic reaction between the fuel (urea) and oxidiser (ammonium nitrate). For complete combustion, the oxidiser to fuel ratio should be equal to one. The constituent raw materials (AR grade) sodium nitrate, strontium nitrate, Europium nitrate, ammonium nitrate, urea and boric acid (as boron source) were weighed in stoichiometric proportion and dissolved in minimum amount of water and fired at 550 1C.
D. Chikte (Awade) et al. / Journal of Luminescence 142 (2013) 180–183
The solution boils and undergoes dehydration followed by decomposition with evolution of gases N2, CO2, etc. The mixture then froths and swells forming foam that ruptures with a flame on ignition of combustible gases and glows to incandescence. During incandescence the foam further swells to the capacity of container. The whole process completes in few minutes. Following the combustion resulting voluminous fine powder was annealed in open air at 750 1C for 2 h and quenched to room temperature. The samples were subjected to XRD analysis using an X’Pert PRO advanced automatic diffractometer with Cu Kα radiation (λ ¼1.540598 Å) operated at 40 kV and 30 mA.The XRD data was collected in a 2θ range from 101 to 801 at room temperature. The measurements of photoluminescence (PL) over the range of 450– 650 nm and photoluminescence excitation spectra (PLE) over 200– 400 nm excitation range were carried out on Hitachi-F7000 florescence spectrophotometer at room temperature. The spectral resolution of both excitation and emission spectra, width of the monochromatic slits (2.5 nm), as well the measurement conditions such as PMT detector sensitivity and scan speed were kept constant from sample to sample in measurements. The SEM image was recorded for surface morphology. The colour chromaticity coordinates were obtained according to Commission International de I’Eclairage (CIE) using Radiant Imaging colour calculator.
3. Result and discussion 3.1. XRD analysis The powder XRD pattern of the sample was analysed for the structure confirmation. Fig. 1 represents the powder XRD pattern of NaSrBO3 prepared by the solution combustion synthesis method which is in good agreement with the XRD data of NaSrBO3 reported by Wu et al. [9]. The consistent result indicated that the single phase polycrystalline NaSrBO3 powder was obtained in the study. As synthesised sample exhibit the same diffraction peaks as reported by Wu et al. corresponding to the monoclinic structure with space group of P21/c. The reported lattice parameters of NaSrBO3 are a ¼5.32446 (7) Å, b ¼9.2684(1) Å, c ¼6.06683(8) Å and β¼100.58(1)1. The cell volume (V) is 294.30(8)1 and cations in the unit cell (Z) are 4. In NaSrBO3 crystal lattice, the fundamental building units are isolated planar BO3 anionic groups, which are parallelly distributed along two different directions. The Na atoms are six-coordinated with O atoms to form octahedral, and the Sr atoms coordinated by nine O atoms are in the form of tri-capped trigonal prisms [9]. The acceptable percentage difference in ionic radii between doped and substituted ions must not exceed 30% [22]. The calculations of the radius percentage difference (Dr) between the doped ions (Eu3+) and possible substituted ions (Sr, B, Na) in
Fig. 1. X-ray diffraction pattern of as synthesised NaSrBO3.
181
NaSrBO3:Eu3+are summarised in Table 1 below. The values are based on the formula: Dr ¼
RmðCNÞ−RdðCNÞ RmðCNÞ
where CN—co-ordination number,RmðCNÞ —Radius of host cations and RdðCNÞ- Radius of dopant ion Thus it is clear that the Eu3+ ionic radius (r ¼1.12 Å, CN ¼9) is closest to that of Sr2+ (r ¼1.31 Å, CN ¼9), making it unlikely that Eu3+ ions would substitute with Na+ (r ¼1.02 A, CN ¼ 6) or B3+ (r ¼0.11, CN ¼3) in the NaSrBO3 host. Hence, it is believed that the Sr2+ sites are replaced by Eu3+ in this lattice. [20] Fig. 2 represents the SEM image showing surface morphology of as-synthesised NaSrBO3:Eu3+ phosphor. It shows the particles with an irregular shape and the particle size ranging in 5–10 μm. 3.2. Photoluminescence Fig. 3 shows the photoluminescence excitation and emission spectrum of NaSr1−xBO3:xEu (for x ¼0.09). The excitation spectrum monitored at 614 nm emission exhibits a broad band from 250 to 350 nm ascribed to O–Eu charge-transfer band (CTB) transition and the other peaks in the range of 350–400 nm originated from f–f transitions of Eu3+ ions. The strongest band at 394 nm can be assigned to 7F0–5L6 transition of Eu3+ ions. The emission spectrum exhibits five typical emission peaks in the range of 560–720 nm, which result from 5D0–7FJ (J¼ 0, 1, 2, 3 and 4) transitions of Eu3+ion, respectively. The main peak centred at 614 nm corresponds to electron dipole 5D0–7F2 transition and the peak at 592 nm ascribes to magnetic dipole 5D0–7F1 transition of Eu3+ions. Fig. 4 represents the emission spectra of NaSrBO3 doped with differentEu3+ concentrations for the excitation wavelength of 394 nm. All of the emission spectra exhibit the similar profile with different relative intensities. The emission intensity increases initially with the increase of Eu3+ concentration and reaches Table1 Ionic radii difference percentage (Dr) between matrix cations and doped ions. Ions in matrix 3+
Eu (doped) Sr2+ Na+ B3+
Ionic radius (r), Å
CN
Dr
1.12 1.31 1.02 0.11
9 9 6 3
– 14.5% −9.8% −918%
Fig. 2. SEM image of as-synthesised NaSrBO3:Eu3+.
182
D. Chikte (Awade) et al. / Journal of Luminescence 142 (2013) 180–183
Fig. 3. PLE (a) and PL (b) curve for NaSr1−xBO3:xEu3+ for x¼ 0.06.
Fig. 5. Dependence of relative intensity of emission on Eu3+ doping concentration.
Fig. 4. PL of NaSr1−xBO3:xEu3+for different ‘x’ values for excitation wavelength of 394 nm.
maximum at x ¼0.09, then gradually decreases due to the internal concentration quenching. Fig. 5 shows the dependence of the peak intensity of the emission centred at 614 nm on Eu3+doping concentration (x) in NaSr1−xBO3:xEu3+. The optimum intensity of emission is at x¼ 0.09 after that intensity drops due to internal concentration quenching. The concentration quenching mechanism is generally associated with energy transfer. Non-radiative energy transfer process from one Eu3+ ion to another Eu3+ ion can be described by three different methods:(1) exchange interaction, (2) radiation reabsorption and (3) multipolar interaction. While discussing the mechanism of energy transfer in phosphors, Blasse [21] suggested that if the activator is introduced solely on one crystallographic site (here Sr2+site), the critical energy transfer distance (Rc) is approximately equal to twice the radius of a sphere with this volume. In order to further discuss the mechanism of energy transfer between the activators in the NaSrBO3 host, the critical energy transfer distance (Rc) can be calculated by the following equation: Rc≈2
3V 4πχ c N
1=3
where χc the critical concentration, N is the number of cation sites in the unit cell, and V is the volume of the unit cell. So in this case,
Fig. 6. PL of NaSr1−xBO3:xEu3+ for different excitation wavelengths: (a) excitation wavelength 394 nm and (b) excitation wavelength 270 nm.
V¼294.30 Å, N ¼ 4 and the critical doping concentration of Eu3+ in the NaSrBO3 host is found to be 0.09. Thus, the Rc of Eu3+ in NaSrBO3:Eu3+ phosphor is determined to be 11.6 Å. Since Rc is not less than 5 Å exchange interaction is not responsible for nonradiative energy transfer process from one Eu3+ ion to another Eu3+ ion in this host. The mechanism of radiation reabsorption is the primary method only if the fluorescence spectra of the excitation and emission have obvious overlap. Thus, in view of the emission and excitation spectra of NaSrBO3:Eu3+ (Fig. 3), the radiation reabsorption is unlikely to occur. As a result, the energy transfer process of Eu3+ in NaSrBO3 phosphor would be due to multipolar interaction. Further it can be proven to be because of quadrupole–quadrupole interaction. Fig. 6 shows the variation of emission intensity for excitation wavelength of 394 nm (a) and 270 nm (b) for the same Eu3+ doping concentration (for x ¼0.06). It is obvious that the intensity of emission is greater for the excitation of 394 nm. Fig. 7 Shows the plot of CIE coordinates of the NaSrBO3:0.09Eu3+ phosphor and the commercial red phosphor Y2O2S:0.05Eu3+. The CIE coordinates of the as-synthesied sample by modified solution combustion synthesis method are (0.62,0.35) which are same as that of reported by Fan Yang et al. for the said phosphor. On the same intsity scale the intensity of the phosphor prepared by this method is
D. Chikte (Awade) et al. / Journal of Luminescence 142 (2013) 180–183
183
is calculated as 11.6 Å for the optimum doping concentration. The equadrupole–quadrupole interaction is the major mechanism for concentration quenching of fluorescence emission of Eu3+ ions in this phosphor. Eu3+ ions only occupy the Sr2+ sites and form one emission centre at 614 nm. The CIE coordinates of the assynthesied sample are (0.62,0.35) for optimum emission intensity.
Acknowledgement One of the authors (DPA) would like to thank University Grants Commission (UGC), New Delhi for availing Teacher Fellowship under Faculty Development Programme and X-ray-lab, IIT Mumbai for availing XRD facility. References
Fig. 7. CIE coordinates of the NaSrBO3:0.09Eu3+ phosphor and the commercial compound, Y2O2S:Eu3+.
better. For 394 nm excitation the emission peak is at 614 nm with FWHM of 12 nm. 4. Conclusion The red emitting phosphor NaSrBO3:Eu3+is synthesised by a simple, time saving, economically novel method of modified solution combustion synthesis at comparatively lower temperature. The XRD pattern of NaSrBO3 prepared by the combustion method is in good agreement with the XRD data of NaSrBO3 reported by Wu et al. The as synthesised phosphor shows intense red emission at 614 nm. The excitation spectrum monitored at 614 nm shows broad band centred at 250 nm along with intense line at 394 nm. The Pl spectrum at various doping concentration of Eu3+ shows maximum emission intensity at x ¼0.09 after the concentration quenching is observed. The critical transfer distance
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