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Synthesis and enhancement of luminescence intensity by co-doping of M+ (M = Li, Na, K) in Ce3+ doped strontium haloborate
Optical Materials 36 (2014) 1143–1145
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis and enhancement of luminescence intensity by co-doping of M+ (M = Li, Na, K) in Ce3+ doped strontium haloborate A.B. Gawande a,1, R.P. Sonekar b,⇑, S.K. Omanwar a a b
Department of Physics, SGB Amravati University, Amravati 444602, (M.S.), India Department of Physics, G.S. College, Khamgaon, (M.S.), India
a r t i c l e
i n f o
Article history: Received 29 December 2013 Received in revised form 17 February 2014 Accepted 18 February 2014 Available online 6 March 2014 Keywords: Strontium haloborate Charge compensation Photoluminescence
a b s t r a c t Photoluminescence properties of Ce3+ doped strontium haloborates synthesized by solution combustion technique were studied. Sr2B5O9Cl:Ce3+ produce emission band peaking at 345 nm under 307 nm excitation radiation. Enhancement of luminescence intensity was observed when M+ (Li+, Na+, K+) ions were used as co-dopant in Sr2B5O9Cl:Ce3+. Charge compensation by Na+ ion in Sr2B5O9Cl:Ce3+ show strongest luminescence intensity at 345 nm under 307 nm excitation radiation. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Materials of composition M2B5O9R:Eu2+ (M = Sr, Ba) gained special attention because of possible applications as storage phosphors for X-ray imaging [1] and thermal neutron detection [2]. Ce3+ doped alkaline earth haloborates can be of importance for practical applications as storage phosphors. The substitution of a trivalent ion for an alkaline earth ion requires the presence of a charge compensator to maintain the overall charge neutrality of the crystal. In view of its spectroscopic characteristics, the Ce3+ seems to be a suitable ion for study of the charge compensation mechanism for trivalent cations. Machida et al. [3] briefly described the crystal structure of Sr2B5O9R (R = Cl, Br) and conclude that crystal structure of Sr2B5O9R (R = Cl, Br) is almost similar to Eu2B5O9Br. The structures consist of a three-dimensional (B5O9)1 network in which three unique BO4 tetrahedra and two unique BO3 triangles are linked together. The metal ions can occupy two different C1 crystallographic sites in the host (at room temperature), and in each case the ion is surrounded by seven oxygen and two halide atoms. The actual coordination geometry of the two metal sites is that of a distorted heptagonal bipyramid, with the two halide ions at the axial positions and equatorial O atoms at the joint of the bipyramid. The two axial halides bridge to neighbouring bipyramids. The metal and halide ions are located alternately along the ⇑ Corresponding author. Tel.: +91 9422883314. E-mail addresses: [email protected] (A.B. Gawande), [email protected] (R.P. Sonekar). 1 Tel.: +91 9404689773. http://dx.doi.org/10.1016/j.optmat.2014.02.017 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
tunnels of the (B5O9)1 network, forming linear chains along the a- (b-) axis for site 1 (2). Thus, the metal ions in site 1 (2) are isolated from neighbouring metal ions by the borate units of the (B5O9)1 network in b and c directions (a and c directions) and halide ions in the a-axis (b-axis) direction. It is generally accepted that this structure and those of M2B5O9Cl (M = Ca, Sr, Ba) do not significantly differ, not withstanding their different space groups. In both the orthorhombic (Pnn2) and tetragonal (P42212) structures the metal occupies two inequivalent sites of both C1 (Pnn2) or both C2 (P42212) symmetry [4]. In this work, we discussed the synthesis of Sr2B5O9Cl:Ce3+ by using solution combustion synthesis technique, which can be potentially used in preparing the phosphors and intensity enhancement by using Li+, Na+ or K+ as charge compensator. 2. Experimental Sr2B5O9Cl:Ce3+and Sr2B5O9Cl:Ce3+, M+ (M = Li, Na or K) phosphors were prepared by solution combustion synthesis technique, discussed in detail in our previous work [5–8]. The method based on exothermic reaction in which ammonium nitrate used as oxidizer and urea is used as fuel. The stoichiometric amounts of high purity (Analytical Reagent) starting materials Sr(NO3)2, Ce(NO3)3, H3BO3, NH4Cl, CO(NH2)2, NH4NO3 have been used for preparation of phosphors. The stoichiometric amount of starting materials with little amount of double distilled water were mixed thoroughly in an alumina basin to obtain homogeneous solution. The excess water was removed by slow heating (at 70 °C) and the solution then transferred directly to the pre-heated furnace (550 ± 10 °C)
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A.B. Gawande et al. / Optical Materials 36 (2014) 1143–1145
for combustion. Following the combustion, the resulting fine powders were annealed for 3 h at temperature 800 ± 10 °C and then quenched to room temperature. The resultant powder samples were then characterised using powder XRD, and Spectrofluorometer. The structural analysis of the synthesized materials were examined using an X-ray Diffractometer (XRD: Rigaku Maniflex II, with Cu Ka irradiation (k = 1.5406 Å) with a scan speed of 2° min1. The photoluminescence excitation and emission spectra were measured at room temperature using Hitachi F-7000 Spectrofluorometer in the range 200–500 nm. 3. Results and discussion 3.1. X-ray diffraction of Sr2B5O9Cl:Ce3+ Fig. 1 shows the XRD of Ce3+ doped Sr2B5O9Cl materials which were compared with ICDD file (01-077-4284) and found in good agreement. The crystal structure can be refined to be Orthorhombic, with a = 11.3153 Å, b = 11.3838 Å, c = 6.4945 Å. The materials crystallize in the non-centrosymmetric space group Pnn2. The ionic radii of Ce3+ (0.196 nm, Coordination No. = 9) and M+ (M = Li, Na, K); Li = 0.092 nm, Na = 0.124 nm, K = 0.155 nm are close to that of Sr2+ (0.131 nm, C.N. = 9). Based on the effective ionic radii of cations with different coordination numbers, it is assumed that Ce3+ and M+ ions are preferable to replace the Sr2+ ions. Generally, when a metal cation is substituted for an ion with a different valence in the host lattice, the charge compensator, such as Li+, Na+ or K+, is employed to keep the charge balance. The charge compensation in the above mentioned structure is most likely to be described by two possible mechanisms: (i) Two Sr2+ ions are replaced by one Ce3+ ion and one alkaline cation,
2Sr2þ ! Ce3þ þ Mþ ; where M+ is an alkaline cation like Li+, Na+, and K+. (ii) Charge compensation by a strontium vacancy,
3Sr2þ ! 2Ce3þ þ V Sr :
3.2. Photoluminescence analysis Fig. 2 represents the room temperature PL and PLE spectra for samples of composition Sr2(1x)B5O9Cl. The PL emission spectrum displays a broad band peaking at 345 nm which is attributed to the transitions of Ce3+ from the lowest 5d level to the spin orbit splitting 2F5/2 and 2F7/2 ground states. The PL excitation spectrum consists of two broad bands peaking at about 272 and 307 nm, which are assigned to the 4f–5d transition of Ce3+. Furthermore, the dependence of the emission intensity on the Ce3+ ions concentration in Sr2(1x)B5O9Cl were studied in detail and found that for concentration of 0.01 mol the intensity of luminescence is maximum. In general, two mechanisms are invoked to explain concentration quenching. In the first mechanism, the excitation energy migrated to a large number of centres being finally transferred to lattice defects or impurity ions that act as energy acceptors. These kinds of energy acceptor centres are called killer or quenching traps, and acts as energy sink within the chain transfer, thus quenching the luminescence intensity. In second type of mechanism, the dissipation of excitation energy takes place via cross relaxation by means of resonant energy transfer between two identical adjacent centres (activators). Therefore it can be expressed that the concentration quenching of Ce3+ in Sr2B5O9Cl is due to migration of excitation energy to the quenching centres or to the cross-relaxation mechanisms. Generally, to substitute a trivalent activator ion such as Ce3+ into the host lattice, containing divalent metallic cation, balancing of charge is a necessary requirement of co-doped alkali metal ions [9–11]. For Sr2B5O9Cl:Ce3+, M+ (M = Li+, Na+, K+), one Ce3+ ion is expected to replace one Sr2+ ion, with the other sites occupied by a strontium vacancy (VSr) or alkali metal ions M+ (M = Li+, Na+, K+). As a result, the phosphor with efficient charge compensation exhibits intense emission. In order to find the effect of charge compensator on luminescence properties of Sr2B5O9Cl:Ce3+, M+ (M = Li+, Na+, K+), ions are co-doped in Sr2B5O9Cl:Ce3+. Fig. 3 displays the emission spectra of Sr2B5O9Cl:Ce3+, M+ (M = Li+, Na+, K+) under 307 nm radiation excitation, and excitation spectra monitored at 345 nm radiation emission. Except for the emission intensity, no other difference was observed in the sharp emission band and position under the same radiation excitation. Ionic radius of charge compensator plays an important role as it may distort,
Fig. 1. XRD pattern of Sr2B5O9Cl:Ce3+ and Li+, Na+, K+ co-doped Sr2B5O9Cl:Ce3+ phosphors.
A.B. Gawande et al. / Optical Materials 36 (2014) 1143–1145
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observed that, the luminescence intensity of Sr2B5O9Cl:Ce3+ is optimum for co-doping of Na+ in host lattice as charge compensator. This is obvious because ionic radius of Na+ (0.124 nm) is close to that of Sr2+ (0.131 nm, C.N. = 9). 4. Conclusion
Fig. 2. Emission and excitation spectra of Sr2(1x)B5O9Cl:2xCe3+ (x = 0.005, 0.007, 0.01, 0.015).
In this paper we discussed the synthesis of Ce3+ doped Sr2B5O9Cl phosphors by using solution combustion synthesis technique. The optimum concentration of Ce3+ for strongest luminescence intensity was observed for 0.01 mol concentration of Ce3+. The effect of charge compensation on luminescence intensity of Sr2B5O9Cl:Ce3+ was studied using Li+, Na+ or K+ as co-dopant. The phosphor Sr2B5O9Cl:Ce3+, Na+ shows strongest luminescence intensity. Hence, Na+ may be an optimal charge compensator for Sr2B5O9Cl:Ce3+. Acknowledgement One of the authors (ABG) wishes to thank The Chairman of FIST project, SGB Amravati University, Amravati (MS) for providing powder X-Ray Diffraction facility for this work. References
Fig. 3. Excitation (A) and emission (B) spectra of Sr1.96B5O9Cl:0.01Ce3+, (M = K+, Na+, Li+).
+ 0.01M
not greatly, the host lattice which may cause effect on luminescence intensity. The ionic radius of Li+ < Na+ < K+, are (Li+ = 0.092 nm, Na+ = 0.124 nm, K+ = 0.155 nm). From Fig. 3 it can be
[1] A. Meijerink, G. Blasse, J. Phys. D 24 (1991) 626. [2] M. Knitel, New inorganic scintillators and storage phosphors for detection of thermal neutrons, Ph.D. Thesis, Delft University of Technology, 1998. [3] K. Machida, G. Adachi, N. Yasuoka, N. Kasai, J. Shiokawa, Inorg. Chem. 19 (1980) 3807. [4] Qinghua Zeng, Nathan Kilah, Mark Riley, Hans Riesen, J. Lumin. 104 (2003) 65. [5] A.B. Gawande, R.P. Sonekar, S.K. Omanwar, Int. J. Opt. (2014), http://dx.doi.org/ 10.1155/2014/418459. [6] A.B. Gawande, R.P. Sonekar, S.K. Omanwar, Combust. Sci. Technol. (2013), http://dx.doi.org/10.1080/00102202.2013.878708. [7] A.B. Gawande, R.P. Sonekar, S.K. Omanwar, J. Lumin. 149 (2014) 200. [8] A.B. Gawande, J.T. Ingle, R.P. Sonekar, S.K. Omanwar, J. Lumin. 149 (2014) 236. [9] F. Yang, Y. Liang, M. Liu, X. Li, N. Wang, Z. Xia, Ceram. Int. 38 (2012) 6197. [10] Z. Mu, Y. Hu, L. Chen, X. Wang, J. Electrochem. Soc. 158 (2011) J287. [11] X. Yan, W. Li, K. Sun, J. Alloys Comp. 508 (2010) 475.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis and enhancement of luminescence intensity by co-doping of M+ (M = Li, Na, K) in Ce3+ doped strontium haloborate A.B. Gawande a,1, R.P. Sonekar b,⇑, S.K. Omanwar a a b
Department of Physics, SGB Amravati University, Amravati 444602, (M.S.), India Department of Physics, G.S. College, Khamgaon, (M.S.), India
a r t i c l e
i n f o
Article history: Received 29 December 2013 Received in revised form 17 February 2014 Accepted 18 February 2014 Available online 6 March 2014 Keywords: Strontium haloborate Charge compensation Photoluminescence
a b s t r a c t Photoluminescence properties of Ce3+ doped strontium haloborates synthesized by solution combustion technique were studied. Sr2B5O9Cl:Ce3+ produce emission band peaking at 345 nm under 307 nm excitation radiation. Enhancement of luminescence intensity was observed when M+ (Li+, Na+, K+) ions were used as co-dopant in Sr2B5O9Cl:Ce3+. Charge compensation by Na+ ion in Sr2B5O9Cl:Ce3+ show strongest luminescence intensity at 345 nm under 307 nm excitation radiation. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Materials of composition M2B5O9R:Eu2+ (M = Sr, Ba) gained special attention because of possible applications as storage phosphors for X-ray imaging [1] and thermal neutron detection [2]. Ce3+ doped alkaline earth haloborates can be of importance for practical applications as storage phosphors. The substitution of a trivalent ion for an alkaline earth ion requires the presence of a charge compensator to maintain the overall charge neutrality of the crystal. In view of its spectroscopic characteristics, the Ce3+ seems to be a suitable ion for study of the charge compensation mechanism for trivalent cations. Machida et al. [3] briefly described the crystal structure of Sr2B5O9R (R = Cl, Br) and conclude that crystal structure of Sr2B5O9R (R = Cl, Br) is almost similar to Eu2B5O9Br. The structures consist of a three-dimensional (B5O9)1 network in which three unique BO4 tetrahedra and two unique BO3 triangles are linked together. The metal ions can occupy two different C1 crystallographic sites in the host (at room temperature), and in each case the ion is surrounded by seven oxygen and two halide atoms. The actual coordination geometry of the two metal sites is that of a distorted heptagonal bipyramid, with the two halide ions at the axial positions and equatorial O atoms at the joint of the bipyramid. The two axial halides bridge to neighbouring bipyramids. The metal and halide ions are located alternately along the ⇑ Corresponding author. Tel.: +91 9422883314. E-mail addresses: [email protected] (A.B. Gawande), [email protected] (R.P. Sonekar). 1 Tel.: +91 9404689773. http://dx.doi.org/10.1016/j.optmat.2014.02.017 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
tunnels of the (B5O9)1 network, forming linear chains along the a- (b-) axis for site 1 (2). Thus, the metal ions in site 1 (2) are isolated from neighbouring metal ions by the borate units of the (B5O9)1 network in b and c directions (a and c directions) and halide ions in the a-axis (b-axis) direction. It is generally accepted that this structure and those of M2B5O9Cl (M = Ca, Sr, Ba) do not significantly differ, not withstanding their different space groups. In both the orthorhombic (Pnn2) and tetragonal (P42212) structures the metal occupies two inequivalent sites of both C1 (Pnn2) or both C2 (P42212) symmetry [4]. In this work, we discussed the synthesis of Sr2B5O9Cl:Ce3+ by using solution combustion synthesis technique, which can be potentially used in preparing the phosphors and intensity enhancement by using Li+, Na+ or K+ as charge compensator. 2. Experimental Sr2B5O9Cl:Ce3+and Sr2B5O9Cl:Ce3+, M+ (M = Li, Na or K) phosphors were prepared by solution combustion synthesis technique, discussed in detail in our previous work [5–8]. The method based on exothermic reaction in which ammonium nitrate used as oxidizer and urea is used as fuel. The stoichiometric amounts of high purity (Analytical Reagent) starting materials Sr(NO3)2, Ce(NO3)3, H3BO3, NH4Cl, CO(NH2)2, NH4NO3 have been used for preparation of phosphors. The stoichiometric amount of starting materials with little amount of double distilled water were mixed thoroughly in an alumina basin to obtain homogeneous solution. The excess water was removed by slow heating (at 70 °C) and the solution then transferred directly to the pre-heated furnace (550 ± 10 °C)
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A.B. Gawande et al. / Optical Materials 36 (2014) 1143–1145
for combustion. Following the combustion, the resulting fine powders were annealed for 3 h at temperature 800 ± 10 °C and then quenched to room temperature. The resultant powder samples were then characterised using powder XRD, and Spectrofluorometer. The structural analysis of the synthesized materials were examined using an X-ray Diffractometer (XRD: Rigaku Maniflex II, with Cu Ka irradiation (k = 1.5406 Å) with a scan speed of 2° min1. The photoluminescence excitation and emission spectra were measured at room temperature using Hitachi F-7000 Spectrofluorometer in the range 200–500 nm. 3. Results and discussion 3.1. X-ray diffraction of Sr2B5O9Cl:Ce3+ Fig. 1 shows the XRD of Ce3+ doped Sr2B5O9Cl materials which were compared with ICDD file (01-077-4284) and found in good agreement. The crystal structure can be refined to be Orthorhombic, with a = 11.3153 Å, b = 11.3838 Å, c = 6.4945 Å. The materials crystallize in the non-centrosymmetric space group Pnn2. The ionic radii of Ce3+ (0.196 nm, Coordination No. = 9) and M+ (M = Li, Na, K); Li = 0.092 nm, Na = 0.124 nm, K = 0.155 nm are close to that of Sr2+ (0.131 nm, C.N. = 9). Based on the effective ionic radii of cations with different coordination numbers, it is assumed that Ce3+ and M+ ions are preferable to replace the Sr2+ ions. Generally, when a metal cation is substituted for an ion with a different valence in the host lattice, the charge compensator, such as Li+, Na+ or K+, is employed to keep the charge balance. The charge compensation in the above mentioned structure is most likely to be described by two possible mechanisms: (i) Two Sr2+ ions are replaced by one Ce3+ ion and one alkaline cation,
2Sr2þ ! Ce3þ þ Mþ ; where M+ is an alkaline cation like Li+, Na+, and K+. (ii) Charge compensation by a strontium vacancy,
3Sr2þ ! 2Ce3þ þ V Sr :
3.2. Photoluminescence analysis Fig. 2 represents the room temperature PL and PLE spectra for samples of composition Sr2(1x)B5O9Cl. The PL emission spectrum displays a broad band peaking at 345 nm which is attributed to the transitions of Ce3+ from the lowest 5d level to the spin orbit splitting 2F5/2 and 2F7/2 ground states. The PL excitation spectrum consists of two broad bands peaking at about 272 and 307 nm, which are assigned to the 4f–5d transition of Ce3+. Furthermore, the dependence of the emission intensity on the Ce3+ ions concentration in Sr2(1x)B5O9Cl were studied in detail and found that for concentration of 0.01 mol the intensity of luminescence is maximum. In general, two mechanisms are invoked to explain concentration quenching. In the first mechanism, the excitation energy migrated to a large number of centres being finally transferred to lattice defects or impurity ions that act as energy acceptors. These kinds of energy acceptor centres are called killer or quenching traps, and acts as energy sink within the chain transfer, thus quenching the luminescence intensity. In second type of mechanism, the dissipation of excitation energy takes place via cross relaxation by means of resonant energy transfer between two identical adjacent centres (activators). Therefore it can be expressed that the concentration quenching of Ce3+ in Sr2B5O9Cl is due to migration of excitation energy to the quenching centres or to the cross-relaxation mechanisms. Generally, to substitute a trivalent activator ion such as Ce3+ into the host lattice, containing divalent metallic cation, balancing of charge is a necessary requirement of co-doped alkali metal ions [9–11]. For Sr2B5O9Cl:Ce3+, M+ (M = Li+, Na+, K+), one Ce3+ ion is expected to replace one Sr2+ ion, with the other sites occupied by a strontium vacancy (VSr) or alkali metal ions M+ (M = Li+, Na+, K+). As a result, the phosphor with efficient charge compensation exhibits intense emission. In order to find the effect of charge compensator on luminescence properties of Sr2B5O9Cl:Ce3+, M+ (M = Li+, Na+, K+), ions are co-doped in Sr2B5O9Cl:Ce3+. Fig. 3 displays the emission spectra of Sr2B5O9Cl:Ce3+, M+ (M = Li+, Na+, K+) under 307 nm radiation excitation, and excitation spectra monitored at 345 nm radiation emission. Except for the emission intensity, no other difference was observed in the sharp emission band and position under the same radiation excitation. Ionic radius of charge compensator plays an important role as it may distort,
Fig. 1. XRD pattern of Sr2B5O9Cl:Ce3+ and Li+, Na+, K+ co-doped Sr2B5O9Cl:Ce3+ phosphors.
A.B. Gawande et al. / Optical Materials 36 (2014) 1143–1145
1145
observed that, the luminescence intensity of Sr2B5O9Cl:Ce3+ is optimum for co-doping of Na+ in host lattice as charge compensator. This is obvious because ionic radius of Na+ (0.124 nm) is close to that of Sr2+ (0.131 nm, C.N. = 9). 4. Conclusion
Fig. 2. Emission and excitation spectra of Sr2(1x)B5O9Cl:2xCe3+ (x = 0.005, 0.007, 0.01, 0.015).
In this paper we discussed the synthesis of Ce3+ doped Sr2B5O9Cl phosphors by using solution combustion synthesis technique. The optimum concentration of Ce3+ for strongest luminescence intensity was observed for 0.01 mol concentration of Ce3+. The effect of charge compensation on luminescence intensity of Sr2B5O9Cl:Ce3+ was studied using Li+, Na+ or K+ as co-dopant. The phosphor Sr2B5O9Cl:Ce3+, Na+ shows strongest luminescence intensity. Hence, Na+ may be an optimal charge compensator for Sr2B5O9Cl:Ce3+. Acknowledgement One of the authors (ABG) wishes to thank The Chairman of FIST project, SGB Amravati University, Amravati (MS) for providing powder X-Ray Diffraction facility for this work. References
Fig. 3. Excitation (A) and emission (B) spectra of Sr1.96B5O9Cl:0.01Ce3+, (M = K+, Na+, Li+).
+ 0.01M
not greatly, the host lattice which may cause effect on luminescence intensity. The ionic radius of Li+ < Na+ < K+, are (Li+ = 0.092 nm, Na+ = 0.124 nm, K+ = 0.155 nm). From Fig. 3 it can be
[1] A. Meijerink, G. Blasse, J. Phys. D 24 (1991) 626. [2] M. Knitel, New inorganic scintillators and storage phosphors for detection of thermal neutrons, Ph.D. Thesis, Delft University of Technology, 1998. [3] K. Machida, G. Adachi, N. Yasuoka, N. Kasai, J. Shiokawa, Inorg. Chem. 19 (1980) 3807. [4] Qinghua Zeng, Nathan Kilah, Mark Riley, Hans Riesen, J. Lumin. 104 (2003) 65. [5] A.B. Gawande, R.P. Sonekar, S.K. Omanwar, Int. J. Opt. (2014), http://dx.doi.org/ 10.1155/2014/418459. [6] A.B. Gawande, R.P. Sonekar, S.K. Omanwar, Combust. Sci. Technol. (2013), http://dx.doi.org/10.1080/00102202.2013.878708. [7] A.B. Gawande, R.P. Sonekar, S.K. Omanwar, J. Lumin. 149 (2014) 200. [8] A.B. Gawande, J.T. Ingle, R.P. Sonekar, S.K. Omanwar, J. Lumin. 149 (2014) 236. [9] F. Yang, Y. Liang, M. Liu, X. Li, N. Wang, Z. Xia, Ceram. Int. 38 (2012) 6197. [10] Z. Mu, Y. Hu, L. Chen, X. Wang, J. Electrochem. Soc. 158 (2011) J287. [11] X. Yan, W. Li, K. Sun, J. Alloys Comp. 508 (2010) 475.