Research article Received: 05 July 2013,
Revised: 23 October 2013,
Accepted: 13 December 2013
Published online in Wiley Online Library: 26 February 2014
(wileyonlinelibrary.com) DOI 10.1002/bio.2633
Luminescence properties of barium – gadolinium– titanate ceramics doped with rare-earth ions (Eu3+ and Tb3+) S. Hemasundara Raju,a B. Muni Sudhakar,a B. Sudhakar Reddy,a S. J. Dhoble,c K. Thyagarajanb and C. Nageswara Rajua* ABSTRACT: Barium-gadolinium-titanate (BaGd2Ti4O12) powder ceramics doped with rare-earth ions (Eu3+ and Tb3+) were synthesized by a solid-state reaction method. From the X-ray diffraction spectrum, it was observed that Eu3+ and Tb3+: BaGd2Ti4O12 powder ceramics are crystallized in the form of an orthorhombic structure. Scanning electron microscopy image shows that the particles are agglomerated and the particle size is about 200 nm. Eu3+- and Tb3+-doped BaGd2Ti4O12 powder ceramics were examined by energy dispersive X-ray analysis, Fourier transform infrared spectroscopy, photoluminescence and thermoluminescence (TL) spectra. Emission spectra of Eu3+-doped BaGd2Ti4O12 powder ceramics showed bright red emission at 613 nm (5D0 → 7F2) with an excitation wavelength λexci = 408 nm (7F0 → 5D3) and Tb3+: BaGd2Ti4O12 ceramic powder has shown green emission at 534 nm (5D4 → 7F5) with an excitation wavelength λexci = 331 nm ((7F6 → 5D1). TL spectra show that Eu3+ and Tb3+ ions affect TL sensitivity. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: grain size; emission; excitation; FTIR; optical properties
Introduction
Luminescence 2014; 29: 861–867
Experimental Chemicals Starting chemicals, such as BaCO3, Gd2O3, TiO2, Eu2O3 and Tb2O3, were purchased from Sigma Aldrich (Vijaya Laksmi Enterprises, Kadapa-516001, A.P) with 99.9% purity and were used as received without any further purification.
* Correspondence to: C. Nageswara Raju, Department of Physics, Sri Venkateswara Degree College, Kadapa 516003, India. E-mail: [email protected] a
Department of Physics, Sri Venkateswara Degree College, Kadapa 516003, India
b
Department of Physics, J.N.T.U. College of Engineering, Pulivendula 516390, India
c
Department of Physics, R.T.M. Nagpur University, Nagpur 440 033, India
Copyright © 2014 John Wiley & Sons, Ltd.
861
One of the most important electroceramics is barium titanate. Barium titanate is a good dielectric material with a high dielectric constant and it is ferroelectric, piezoelectric and pyroelectric with good non-linear optical properties. The barium titanate compound is an electrical insulator and it has been used in a wide range of scientific and industrial applications such as capacitors, ultrasonic transducers, piezoelectric sensors, positive temperature coefficient of resistance thermistors, actuators and ferroelectric random access memories, electro-optic devices and pyroelectric infrared sensors (1,2). Barium titanate ceramic has applications in various fields such as optical limiting, switches, flat panel displays, modulated-type optical devices, second harmonic generation and ferroelectric materials (3). Titanate-based phosphors doped with rare-earth ions have attracted significant importance for potential applications in white light-emitting diodes (4). Gadolinium compounds doped with rare-earth ions are used as the red phosphors in the preparation of white light-emitting diodes and gadolinium containing host lattices are used for making green phosphors in color TV tubes, compact discs and fluorescent tubes (5). Many authors have reported on the photoluminescence (PL) analysis of titanate-based systems, such as gadolinium titanate, zirconium titanate, strontium titanates and calcium titanates, which have potential applications in optoelectronic devices (6,7). Ceramic hosts doped with rare-earth ions have a wide range of applications in various fields such as lamp phosphors, solid-state lighting in display devices and white light generation (8). Many researchers focused their attention on europium ions as luminescence centers on red light ceramics with excellent luminescent properties. Thermoluminescence (TL) is a process of emission of light from the sample, which can be observed when the sample is thermally stimulated (9–12). TL is used in various types of dosimetry such as
environmental, personal, medical and industrial (13–15). In recent years, phosphors doped with rare-earth ions play an important role as radiation detectors in the field of research. In addition, rare-earth ions may be added in to the host, which allows the opportunity to change the main features of TL as well as dose behavior (16). It is well known that the TL technique is used for radiation dosimetry. Interest in radiation dosimetry by the TL technique has resulted in numerous efforts seeking production of new, high-performance TL materials. So far, no reports have been made on the PL and TL properties of Eu3+- and Tb3+-doped barium gadolinium titanate (BaGd2Ti4O12) ceramics. In this paper, we report on the synthesis and PL and TL analysis of Eu3+ and Tb3+ ion-doped BaGd2Ti4O12 ceramics for novel applications.
S. Hemasundara Raju et al. BaGd2-xTi4O12:RE3+ (RE = Eu, Tb where x x = 0.5, 1.0, 5 and 10 mol%) ceramics were prepared by the solid-state reaction method. Starting materials such as BaCO3, Gd2O3, TiO2, Eu2O3 and Tb2O3 were taken in an appropriate stoichiometric ratio. These powders were then grounded thoroughly in an agate mortar and the mixtures were put into alumina crucibles. They were heated in an electric furnace at 1200°C for 2 h. The final samples were white powders and then used for characterization. Solid-state synthesis.
Characterization Structural characterization of these samples has been carried out from the X-ray powder diffraction (XRD) measurements on an XRD 3003 TT Seifert diffractometer (Osmania University, Hyderabad) with CuKα radiation (λ = 1.5406 Å) at 40 kV and 20 mA and the 2θ range was varied between 10° and 80°. Morphology of the ceramic powder was examined on a ZEISS-EVO-MA15 ESEM (S.V. University, Tirupati). The scanning electron microscopy (SEM) image was obtained for samples using a 35 mm camera attached to a high-resolution recording system. Elemental analysis has been carried out by energy dispersive X-ray analysis using an X-ray detector attachment to the SEM instrument. The Fourier transform infrared (FTIR) spectrum (4000–450/cm) was recorded on a Perkin Elmer spectrum2 spectrometer with KBr pellets. Both the excitation and emission spectra were obtained on a RF-5301PC SHIMADZU spectrofluorophotometer (Y.V. University, Kadapa and RTM Nagpur University, Nagpur) at room temperature using a 1.5 nm spectral slit width in the range of 200–700 nm. A xenon flash lamp with a phosphorimeter attachment was used to measure the life times of the emission transitions of these ceramics. For TL studies, samples were exposed to gamma rays from a CO60 source at room temperature at a rate of 0.995 kGy/h for 5 Gy. After the desired exposure, TL glow curves were recorded for 2 mg of sample each time at a heating rate of 2 k/s. the TL glow curves were recorded with the usual set-up consisting of a small metal plate heated directly using a temperature programmer, photomultiplier (931 B), dc amplifier and a millivolt recorder.
Results and discussion X-ray diffraction spectrum Figure 1(a,b) shows the XRD profiles of 5 mol% of Eu3+ and Tb3+: BaGd2Ti4O12 ceramics. From the XRD pattern it is observed that from diffraction peaks, which are consistent along with the standard JCPDS card no. 43–0233 and from these XRD patterns, the powder ceramics exist in orthorhombic structure with unit cell parameters a = 12.12 Å, b = 22.31 Å and c = 3.820 Å and no impurity was observed. It indicates that the doping of Eu3+ and Tb3+ ions does not influence the crystal structure of ceramics.
Scanning electron microscopy and energy dispersive X-ray analysis Figure 2(a,b), presents SEM images of Eu3+ and Tb3+:BaGd2Ti4O12 ceramics. The obtained SEM images have shown that the particles are agglomerated and the average diameter of the grain size is about 200 nm. To identify the presence of rare-earth ions in the host ceramics (Eu3+ and Tb3+:BaGd2Ti4O12 ceramics), elemental analysis has been carried out using the energy dispersive X-ray analysis technique, which is attached to the SEM system and the measured energy dispersive X-ray analysis spectrum is shown in Fig. 3(a,b), which confirms the presence of elements.
Fourier transform infrared spectroscopy studies The FTIR spectrum of Eu3+ and Tb3+:BaGd2Ti4O12 ceramics is shown in Fig. 4(a,b). The FTIR spectrum presents absorption bands in the range 2800–3800/cm with the absorption band centered at 3450/cm, which is assigned to the stretching mode of OH groups and a band at 1627/cm corresponds to the bending vibrations of H2O groups. The region 450–1100/cm shows the stretching and bending modes of titanium bonds. The band assignments are also reported by other published articles (17–19).
(a)
(b)
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Figure 1. X-ray diffraction profiles of 5 mol% of (a) Eu
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3+
and (b) Tb :BaGd2Ti4O12 ceramics.
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2014; 29: 861–867
Luminescence properties of RE3 + (RE = Eu, Tb) :BaGd2Ti4O12 ceramics
(a)
(b)
3+
Figure 2. Scanning electron microscopy images of 5 mol% of (a) Eu
(a)
3+
and (b) Tb :BaGd2Ti4O12 ceramics.
(b)
3+
Figure 3. Energy dispersive X-ray analysis spectrum of 5 mol% of (a) Eu
Photoluminescence studies of Eu3+ and Tb3+
Luminescence 2014; 29: 861–867
Figure 6, presents the emission spectra of Eu3+:BaGd2Ti4O12 ceramics when excited from 7F0 to 5D3 level. From emission spectra, the emission bands at 596 nm (5D0 → 7F1) and 613 nm (5D0 → 7F2) has been observed. The emission band assignments are also reported by others (22–24). The most intense red emission band at 613 nm (5D0 → 7F2) is the electric dipole transition with the selection rule ΔJ = 2 and is known as a hypersensitive transition because its intensity is very sensitive to the local environment of Eu3+ ions and its surrounding ligands (4). The transition 5D0 → 7F1 is the magnetic dipole transition following the selection rule ΔJ = 1 and is chosen as the reference one
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The excitation spectrum of Eu3+:BaGd2Ti4O12 powder ceramics monitored by an emission wavelength at 613 nm is shown in Fig. 5. From this spectrum, the excitation bands at 389 nm (7F0 → 5L7), 396 nm (7F0 → 5L6) and 408 nm (7F0 → 5D3) has been observed and the band assignments are also previously reported by others (20,21). Among these transitions, the transition at 7F0 → 5D3 (408 nm) is the most intense excitation band, which has been chosen for the measurement of emission spectra of Eu3+:BaGd2Ti4O12 ceramics.
3+
and (b) Tb :BaGd2Ti4O12 ceramics.
S. Hemasundara Raju et al.
(a)
(b)
Figure 4. Fourier transform infrared spectrum of 5 mol% of (a) Eu
3+
3+
and (b) Tb :BaGd2Ti4O12 ceramics.
3+
Figure 5. Excitation spectrum of 5 mol% of Eu :BaGd2Ti4O12 ceramics.
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because the crystal field does not considerably alter the intensity of this transition. If Eu3+ ions occupy an inversion symmetry site in the lattice, the orange–red emission at 596 nm and the magnetic dipole transition 5D0 → 7F1 is the dominant transition. On the other hand, if Eu3+ ions do not occupy the inversion symmetry site, the electric dipole transition 5D0 → 7F2 becomes the dominant transition. From these emission spectra, it is also observed that the emission intensities gradually increase from 0.5 to 5 mol% of Eu3+, and beyond 5 mol% they decrease as a result of concentration quenching on the luminescence. Thus, 5 mol% is the optimized concentration. It is known that a low doping ratio gives weak luminescence while an overdoping ratio yields quenching of luminescence, which is because at higher doping concentration the number of dopant ions increases and the interionic distance between them decreases, which causes an excitation energy transfer among the luminescent
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3+
Figure 6. Emission spectra of Eu :BaGd2Ti4O12 ceramics.
ions through a non-radiative process. Hence 0.5 mol% of Eu3+ is identified as the optimum dopant concentration. The excitation spectrum of (5 mol%) Tb3+:BaGd2Ti4O12 powder ceramics is shown in Fig. 7. From the measured excitation spectrum, two excitation bands at 331 nm and 399 nm are observed and are assigned to the electronic transitions (7F6 → 5D1) and (7F6 → 5D3) respectively. We have chosen the prominent excitation band at 331 nm for the measurement of emission spectrum of Tb3+:BaGd2Ti4O12 ceramics. The emission spectrum of 5 mol% of Tb3+:BaGd2Ti4O12
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2014; 29: 861–867
Luminescence properties of RE3 + (RE = Eu, Tb) :BaGd2Ti4O12 ceramics the 7FJ ground states and this occurs radiatively. Furthermore, the emission spectrum distribution depends on the concentration of Tb3+ ions. If the concentration of Tb3+ ions is low, the transitions from both the excited energy states (5D3 and 5D4) are possible and by increasing the Tb3+ ion concentrations, most of the Tb3+ ions in the 5D3 state relax non-radiatively to the 5D4 level because of multipolar interactions with Tb3+ ions in the ground state this phenomenon is known as the cross relaxation and is represented as Tb3+ (5D3) + Tb3+ (7F6) → Tb3+ (5D4) + Tb3+ (7F0) (26–28). Therefore, the emission transitions from 5D4 state are predominant at higher concentrations of Tb3+ ions. Decay curves of Eu3+ and Tb3+:BaGd2Ti4O12 ceramics can be fitted by a single exponential function as I = A exp ( t/τ) and the lifetimes 1.023 ms (Eu3+) and 0.863 ms (Tb3+) were obtained and presented in Figs. 9 and 10.
3+
Figure 7. Excitation spectrum of 5 mol% of Tb :BaGd2Ti4O12 ceramics.
ceramics has been presented in Fig. 8 and from this spectrum the emission bands at 534 nm (5D4 → 7F5) and 643 nm (5D4 → 7F2) are observed. Among these two emission bands, the band at 534 nm is the most intense and is the characteristic green emission of Tb3+: BaGd2Ti4O12 ceramics. The obtained emission transitions are due to the f–f transitions of Tb3+ within the 4f8 electronic configuration (25). It is well known that the emission transitions of Tb3+ ions are mainly from the two excited energy levels 5D3 and 5 D4 → 7FJ ground states because the large energy gap between 5 D3, 5D4 states and 7FJ ground states causes the relaxation process from these two (5D3 and 5D4) excited energy states to 3+
Figure 9. Decay curve of the emission transition of Eu :BaGd2Ti4O12 ceramics.
Luminescence 2014; 29: 861–867
3+
Figure 10. Decay curve of the emission transition of Tb :BaGd2Ti4O12 ceramics.
Copyright © 2014 John Wiley & Sons, Ltd.
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3+
Figure 8. Emission spectrum of (5 mol%) Tb :BaGd2Ti4O12 ceramics.
S. Hemasundara Raju et al. Thermoluminescence studies of Eu3+- and Tb3+-doped BaGd2Ti4O12 ceramics Generally, the TL glow curve reveals information regarding defects that result due to exposure to ionizing radiation. Luminescence intensity of previously irradiated material is recorded as a function of varying temperature and is called a glow curve. Figures 11 and 12 show the TL glow curve of BaGd2Ti4O12
ceramics doped with different concentrations of Eu3+ and Tb3+ respectively irradiated with a dose of 5 Gy by gamma rays from Co60 at a rate of 0.995 kGy/h. The glow curves of both Eu3+- and Tb3+-doped ceramics reveal the broad nature and the nature of the glow curve remains the same for different concentrations of Eu3+ and Tb3+ thus the same types of traps are due to doping of both rare-earth ions. The TL emission intensity is increased as the concentration of dopant increases, and reaches a maximum at 5 mol% for both dopants and then decreases, which may be due to non-radiative transition due to interaction between the rare-earth ions used due to increases in concentration. Thus, the single peak in the glow curve is due to one type of traps, which are shallow in nature and it is seen that the number of traps in Eu3+-doped ceramics is higher than that of Tb3+-doped ceramics.
Conclusions Eu3+- and Tb3+-doped BaGd2Ti4O12 ceramics have been synthesized by the solid-state reaction method. The XRD spectrum shows the orthorhombic phase structure of the ceramics. Morphology of the ceramic powders has been studied by using SEM images, which shows that particles are agglomerated. The TL study shows the trapping parameters of materials after gamma-ray irradiation. The PL spectra of Eu3+:BaGd2Ti4O12 ceramics have shown an intense red emission at 613 nm with an excitation wavelength λexci = 408 nm and Tb3+:BaGd2Ti4O12 ceramics have shown green emission at 534 nm (5D4 → 7F5) with λexci = 331 nm. The red and green colors emitting Eu3+ and Tb3+: BaGd2Ti4O12 ceramics are promising candidates as red-emitting ceramics for white light-emitting diodes. 3+
Figure 11. TL glow curve of Eu :BaGd2Ti4O12 ceramics. TL, thermoluminescence.
Acknowledgments This work was supported by the UGC, New Delhi in the form of Major Research Project F No. 41-919/2012 (SR) sanctioned to the author (CNR), who would like to thank, the Secretary, UGC, New Delhi, India.
References
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3+
Figure 12. TL glow curve of Tb :BaGd2Ti4O12 ceramics. TL, thermoluminescence.
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1. Kumar P, Singh S, Spah M, Juneja JK, Prakash C, Raina KK. Synthesis and dielectric properties of substituted barium titanate ceramics. J Alloys Compd 2010;489:59–63. 2. Devi S, Jha AK. Structural, dielectric and ferroelectric properties of tungsten substituted barium titanate ceramics. Asian J Chem 2009;21:S117–24. 3. Fasasi AY, Ngom BD, Kana-Kana JB, Bucher R, Maaza M, Theron C, Buttner U. Synthesis and characterization of Gd-doped BaTiO3 thin films prepared by laser ablation for optoelectronic applications. J Phys Chem Solid 2009;70:1322–9. 4. Fu J, Zhang Q, Li Y, Wang H. Highly luminescent red light phosphor 3+ CaTiO3:Eu under near-ultraviolet excitation. J Lumin 2010;130:231–5. 5. Sailaja S, Dhoble SJ, Brahme N, Sudhakar RB. Synthesis, 3+ photoluminescence and mechanoluminescence properties of Eu ions activated Ca2Gd2W3O14 phosphors. J Mater Sci 2011;46:7793–8. 6. Mohapatra M, Naik YP, Natarajan V, Seshagiri TK, Singh Z, Godbole SV. Rare earth doped lithium titanate (Li2TiO3) for potential phosphor applications. J Lumin 2010;130:2402–6. 3+ 4+ 7. Lin KM, Lin CC, Hsiao CY, Li YY. Synthesis of Gd2Ti2O7:Eu , V phosphors by sol-gel process and its luminescence properties. J Lumin 2007;127:561–7. 8. Mohapatra M, Naik YP, Natarajan V, Seshagiri TK, Godbole SV. 3+ Photoluminescence properties of Eu in lithium titanate. Physica B 2011;406:1977–82. 9. Horowitz YS. Thermoluminescence and Thermoluminescence Dosimetry. Boca Raton, FL: CRC Press, 1984.
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20. Vengala Rao B, Jang K, Sueb Lee HO, Yi S-S, Jeong J-H. Synthesis and 3+ 3+ 3+ photoluminescence characterization of RE (=Eu , Dy )-activated Ca3La(VO4) phosphors for white light-emitting diodes. J Alloy Compd 2010;496:251–5. 21. Vemasevana Raju K, Sailaja S, Nageswara Raju C, Sudhakar RB. Synthesis, the effect of rare earth ion concentration and tempera3+ ture on luminescence properties of Eu : Ba3Y2WO9 ceramics. J Lumin 2011;131:1438–42. 22. Nagpure IM, S Saha, Dhoble SJ. Photoluminescence and thermolu3+ 3+ minescence characterization of Eu - and Dy -activated Ca3(PO4)2 phosphor. J Lumin 2009;129:898–905. 23. H-Y Chen, M-H Weng, S-J Chang, R-Y Yang. Preparation of Sr2SiO4: 3+ Eu phosphors by microwave-assisted sintering and their luminescent properties. Ceram Int 2012;38:125–30. 24. Guifang J, Yihua H, L Chen, X Wang, Zhongfei M, Haoyi W, F Kang. The luminescence of bismuth and europium in Ca4YO(BO3)3. J Lumin 2012;132:717–21. 25. Yadav RS, Pandey AC. Enhanced efficiency in quantum confined 3+ YbO3:Tb nanophosphor. J Alloy Compd 2010;494(1–2):L15–19. 26. Liao J, Qiu B, Wen H, You W, Xiao Y. Synthesis and optimum luminescence of monodispersed spheres for BaWO4-based green phosphors 3+ with doping of Tb . J Lumin 2010;130:762–6. 27. Singh V, Watanabe S, Gundu Rao TK, Kwak H-Y. Luminescence and 3+ defect centres in Tb doped LaMgAl11O19 phosphors. Solid State Sci 2010;12:1981–7. 28. Sailaja S, Dhoble SJ, Brahme N, Reddy BS. Synthesis, structural, 3+ photoluminescence and mechanoluminescence properties of Tb : Ca2Gd2W3O14 novel green nanophosphors. J Mater Sci 2013;47 (5):2359–64.
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Luminescence 2014; 29: 861–867
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/luminescence
Revised: 23 October 2013,
Accepted: 13 December 2013
Published online in Wiley Online Library: 26 February 2014
(wileyonlinelibrary.com) DOI 10.1002/bio.2633
Luminescence properties of barium – gadolinium– titanate ceramics doped with rare-earth ions (Eu3+ and Tb3+) S. Hemasundara Raju,a B. Muni Sudhakar,a B. Sudhakar Reddy,a S. J. Dhoble,c K. Thyagarajanb and C. Nageswara Rajua* ABSTRACT: Barium-gadolinium-titanate (BaGd2Ti4O12) powder ceramics doped with rare-earth ions (Eu3+ and Tb3+) were synthesized by a solid-state reaction method. From the X-ray diffraction spectrum, it was observed that Eu3+ and Tb3+: BaGd2Ti4O12 powder ceramics are crystallized in the form of an orthorhombic structure. Scanning electron microscopy image shows that the particles are agglomerated and the particle size is about 200 nm. Eu3+- and Tb3+-doped BaGd2Ti4O12 powder ceramics were examined by energy dispersive X-ray analysis, Fourier transform infrared spectroscopy, photoluminescence and thermoluminescence (TL) spectra. Emission spectra of Eu3+-doped BaGd2Ti4O12 powder ceramics showed bright red emission at 613 nm (5D0 → 7F2) with an excitation wavelength λexci = 408 nm (7F0 → 5D3) and Tb3+: BaGd2Ti4O12 ceramic powder has shown green emission at 534 nm (5D4 → 7F5) with an excitation wavelength λexci = 331 nm ((7F6 → 5D1). TL spectra show that Eu3+ and Tb3+ ions affect TL sensitivity. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: grain size; emission; excitation; FTIR; optical properties
Introduction
Luminescence 2014; 29: 861–867
Experimental Chemicals Starting chemicals, such as BaCO3, Gd2O3, TiO2, Eu2O3 and Tb2O3, were purchased from Sigma Aldrich (Vijaya Laksmi Enterprises, Kadapa-516001, A.P) with 99.9% purity and were used as received without any further purification.
* Correspondence to: C. Nageswara Raju, Department of Physics, Sri Venkateswara Degree College, Kadapa 516003, India. E-mail: [email protected] a
Department of Physics, Sri Venkateswara Degree College, Kadapa 516003, India
b
Department of Physics, J.N.T.U. College of Engineering, Pulivendula 516390, India
c
Department of Physics, R.T.M. Nagpur University, Nagpur 440 033, India
Copyright © 2014 John Wiley & Sons, Ltd.
861
One of the most important electroceramics is barium titanate. Barium titanate is a good dielectric material with a high dielectric constant and it is ferroelectric, piezoelectric and pyroelectric with good non-linear optical properties. The barium titanate compound is an electrical insulator and it has been used in a wide range of scientific and industrial applications such as capacitors, ultrasonic transducers, piezoelectric sensors, positive temperature coefficient of resistance thermistors, actuators and ferroelectric random access memories, electro-optic devices and pyroelectric infrared sensors (1,2). Barium titanate ceramic has applications in various fields such as optical limiting, switches, flat panel displays, modulated-type optical devices, second harmonic generation and ferroelectric materials (3). Titanate-based phosphors doped with rare-earth ions have attracted significant importance for potential applications in white light-emitting diodes (4). Gadolinium compounds doped with rare-earth ions are used as the red phosphors in the preparation of white light-emitting diodes and gadolinium containing host lattices are used for making green phosphors in color TV tubes, compact discs and fluorescent tubes (5). Many authors have reported on the photoluminescence (PL) analysis of titanate-based systems, such as gadolinium titanate, zirconium titanate, strontium titanates and calcium titanates, which have potential applications in optoelectronic devices (6,7). Ceramic hosts doped with rare-earth ions have a wide range of applications in various fields such as lamp phosphors, solid-state lighting in display devices and white light generation (8). Many researchers focused their attention on europium ions as luminescence centers on red light ceramics with excellent luminescent properties. Thermoluminescence (TL) is a process of emission of light from the sample, which can be observed when the sample is thermally stimulated (9–12). TL is used in various types of dosimetry such as
environmental, personal, medical and industrial (13–15). In recent years, phosphors doped with rare-earth ions play an important role as radiation detectors in the field of research. In addition, rare-earth ions may be added in to the host, which allows the opportunity to change the main features of TL as well as dose behavior (16). It is well known that the TL technique is used for radiation dosimetry. Interest in radiation dosimetry by the TL technique has resulted in numerous efforts seeking production of new, high-performance TL materials. So far, no reports have been made on the PL and TL properties of Eu3+- and Tb3+-doped barium gadolinium titanate (BaGd2Ti4O12) ceramics. In this paper, we report on the synthesis and PL and TL analysis of Eu3+ and Tb3+ ion-doped BaGd2Ti4O12 ceramics for novel applications.
S. Hemasundara Raju et al. BaGd2-xTi4O12:RE3+ (RE = Eu, Tb where x x = 0.5, 1.0, 5 and 10 mol%) ceramics were prepared by the solid-state reaction method. Starting materials such as BaCO3, Gd2O3, TiO2, Eu2O3 and Tb2O3 were taken in an appropriate stoichiometric ratio. These powders were then grounded thoroughly in an agate mortar and the mixtures were put into alumina crucibles. They were heated in an electric furnace at 1200°C for 2 h. The final samples were white powders and then used for characterization. Solid-state synthesis.
Characterization Structural characterization of these samples has been carried out from the X-ray powder diffraction (XRD) measurements on an XRD 3003 TT Seifert diffractometer (Osmania University, Hyderabad) with CuKα radiation (λ = 1.5406 Å) at 40 kV and 20 mA and the 2θ range was varied between 10° and 80°. Morphology of the ceramic powder was examined on a ZEISS-EVO-MA15 ESEM (S.V. University, Tirupati). The scanning electron microscopy (SEM) image was obtained for samples using a 35 mm camera attached to a high-resolution recording system. Elemental analysis has been carried out by energy dispersive X-ray analysis using an X-ray detector attachment to the SEM instrument. The Fourier transform infrared (FTIR) spectrum (4000–450/cm) was recorded on a Perkin Elmer spectrum2 spectrometer with KBr pellets. Both the excitation and emission spectra were obtained on a RF-5301PC SHIMADZU spectrofluorophotometer (Y.V. University, Kadapa and RTM Nagpur University, Nagpur) at room temperature using a 1.5 nm spectral slit width in the range of 200–700 nm. A xenon flash lamp with a phosphorimeter attachment was used to measure the life times of the emission transitions of these ceramics. For TL studies, samples were exposed to gamma rays from a CO60 source at room temperature at a rate of 0.995 kGy/h for 5 Gy. After the desired exposure, TL glow curves were recorded for 2 mg of sample each time at a heating rate of 2 k/s. the TL glow curves were recorded with the usual set-up consisting of a small metal plate heated directly using a temperature programmer, photomultiplier (931 B), dc amplifier and a millivolt recorder.
Results and discussion X-ray diffraction spectrum Figure 1(a,b) shows the XRD profiles of 5 mol% of Eu3+ and Tb3+: BaGd2Ti4O12 ceramics. From the XRD pattern it is observed that from diffraction peaks, which are consistent along with the standard JCPDS card no. 43–0233 and from these XRD patterns, the powder ceramics exist in orthorhombic structure with unit cell parameters a = 12.12 Å, b = 22.31 Å and c = 3.820 Å and no impurity was observed. It indicates that the doping of Eu3+ and Tb3+ ions does not influence the crystal structure of ceramics.
Scanning electron microscopy and energy dispersive X-ray analysis Figure 2(a,b), presents SEM images of Eu3+ and Tb3+:BaGd2Ti4O12 ceramics. The obtained SEM images have shown that the particles are agglomerated and the average diameter of the grain size is about 200 nm. To identify the presence of rare-earth ions in the host ceramics (Eu3+ and Tb3+:BaGd2Ti4O12 ceramics), elemental analysis has been carried out using the energy dispersive X-ray analysis technique, which is attached to the SEM system and the measured energy dispersive X-ray analysis spectrum is shown in Fig. 3(a,b), which confirms the presence of elements.
Fourier transform infrared spectroscopy studies The FTIR spectrum of Eu3+ and Tb3+:BaGd2Ti4O12 ceramics is shown in Fig. 4(a,b). The FTIR spectrum presents absorption bands in the range 2800–3800/cm with the absorption band centered at 3450/cm, which is assigned to the stretching mode of OH groups and a band at 1627/cm corresponds to the bending vibrations of H2O groups. The region 450–1100/cm shows the stretching and bending modes of titanium bonds. The band assignments are also reported by other published articles (17–19).
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Figure 1. X-ray diffraction profiles of 5 mol% of (a) Eu
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Luminescence properties of RE3 + (RE = Eu, Tb) :BaGd2Ti4O12 ceramics
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Figure 2. Scanning electron microscopy images of 5 mol% of (a) Eu
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Figure 3. Energy dispersive X-ray analysis spectrum of 5 mol% of (a) Eu
Photoluminescence studies of Eu3+ and Tb3+
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Figure 6, presents the emission spectra of Eu3+:BaGd2Ti4O12 ceramics when excited from 7F0 to 5D3 level. From emission spectra, the emission bands at 596 nm (5D0 → 7F1) and 613 nm (5D0 → 7F2) has been observed. The emission band assignments are also reported by others (22–24). The most intense red emission band at 613 nm (5D0 → 7F2) is the electric dipole transition with the selection rule ΔJ = 2 and is known as a hypersensitive transition because its intensity is very sensitive to the local environment of Eu3+ ions and its surrounding ligands (4). The transition 5D0 → 7F1 is the magnetic dipole transition following the selection rule ΔJ = 1 and is chosen as the reference one
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The excitation spectrum of Eu3+:BaGd2Ti4O12 powder ceramics monitored by an emission wavelength at 613 nm is shown in Fig. 5. From this spectrum, the excitation bands at 389 nm (7F0 → 5L7), 396 nm (7F0 → 5L6) and 408 nm (7F0 → 5D3) has been observed and the band assignments are also previously reported by others (20,21). Among these transitions, the transition at 7F0 → 5D3 (408 nm) is the most intense excitation band, which has been chosen for the measurement of emission spectra of Eu3+:BaGd2Ti4O12 ceramics.
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S. Hemasundara Raju et al.
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Figure 4. Fourier transform infrared spectrum of 5 mol% of (a) Eu
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and (b) Tb :BaGd2Ti4O12 ceramics.
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Figure 5. Excitation spectrum of 5 mol% of Eu :BaGd2Ti4O12 ceramics.
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because the crystal field does not considerably alter the intensity of this transition. If Eu3+ ions occupy an inversion symmetry site in the lattice, the orange–red emission at 596 nm and the magnetic dipole transition 5D0 → 7F1 is the dominant transition. On the other hand, if Eu3+ ions do not occupy the inversion symmetry site, the electric dipole transition 5D0 → 7F2 becomes the dominant transition. From these emission spectra, it is also observed that the emission intensities gradually increase from 0.5 to 5 mol% of Eu3+, and beyond 5 mol% they decrease as a result of concentration quenching on the luminescence. Thus, 5 mol% is the optimized concentration. It is known that a low doping ratio gives weak luminescence while an overdoping ratio yields quenching of luminescence, which is because at higher doping concentration the number of dopant ions increases and the interionic distance between them decreases, which causes an excitation energy transfer among the luminescent
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Figure 6. Emission spectra of Eu :BaGd2Ti4O12 ceramics.
ions through a non-radiative process. Hence 0.5 mol% of Eu3+ is identified as the optimum dopant concentration. The excitation spectrum of (5 mol%) Tb3+:BaGd2Ti4O12 powder ceramics is shown in Fig. 7. From the measured excitation spectrum, two excitation bands at 331 nm and 399 nm are observed and are assigned to the electronic transitions (7F6 → 5D1) and (7F6 → 5D3) respectively. We have chosen the prominent excitation band at 331 nm for the measurement of emission spectrum of Tb3+:BaGd2Ti4O12 ceramics. The emission spectrum of 5 mol% of Tb3+:BaGd2Ti4O12
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Luminescence properties of RE3 + (RE = Eu, Tb) :BaGd2Ti4O12 ceramics the 7FJ ground states and this occurs radiatively. Furthermore, the emission spectrum distribution depends on the concentration of Tb3+ ions. If the concentration of Tb3+ ions is low, the transitions from both the excited energy states (5D3 and 5D4) are possible and by increasing the Tb3+ ion concentrations, most of the Tb3+ ions in the 5D3 state relax non-radiatively to the 5D4 level because of multipolar interactions with Tb3+ ions in the ground state this phenomenon is known as the cross relaxation and is represented as Tb3+ (5D3) + Tb3+ (7F6) → Tb3+ (5D4) + Tb3+ (7F0) (26–28). Therefore, the emission transitions from 5D4 state are predominant at higher concentrations of Tb3+ ions. Decay curves of Eu3+ and Tb3+:BaGd2Ti4O12 ceramics can be fitted by a single exponential function as I = A exp ( t/τ) and the lifetimes 1.023 ms (Eu3+) and 0.863 ms (Tb3+) were obtained and presented in Figs. 9 and 10.
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Figure 7. Excitation spectrum of 5 mol% of Tb :BaGd2Ti4O12 ceramics.
ceramics has been presented in Fig. 8 and from this spectrum the emission bands at 534 nm (5D4 → 7F5) and 643 nm (5D4 → 7F2) are observed. Among these two emission bands, the band at 534 nm is the most intense and is the characteristic green emission of Tb3+: BaGd2Ti4O12 ceramics. The obtained emission transitions are due to the f–f transitions of Tb3+ within the 4f8 electronic configuration (25). It is well known that the emission transitions of Tb3+ ions are mainly from the two excited energy levels 5D3 and 5 D4 → 7FJ ground states because the large energy gap between 5 D3, 5D4 states and 7FJ ground states causes the relaxation process from these two (5D3 and 5D4) excited energy states to 3+
Figure 9. Decay curve of the emission transition of Eu :BaGd2Ti4O12 ceramics.
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Figure 10. Decay curve of the emission transition of Tb :BaGd2Ti4O12 ceramics.
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Figure 8. Emission spectrum of (5 mol%) Tb :BaGd2Ti4O12 ceramics.
S. Hemasundara Raju et al. Thermoluminescence studies of Eu3+- and Tb3+-doped BaGd2Ti4O12 ceramics Generally, the TL glow curve reveals information regarding defects that result due to exposure to ionizing radiation. Luminescence intensity of previously irradiated material is recorded as a function of varying temperature and is called a glow curve. Figures 11 and 12 show the TL glow curve of BaGd2Ti4O12
ceramics doped with different concentrations of Eu3+ and Tb3+ respectively irradiated with a dose of 5 Gy by gamma rays from Co60 at a rate of 0.995 kGy/h. The glow curves of both Eu3+- and Tb3+-doped ceramics reveal the broad nature and the nature of the glow curve remains the same for different concentrations of Eu3+ and Tb3+ thus the same types of traps are due to doping of both rare-earth ions. The TL emission intensity is increased as the concentration of dopant increases, and reaches a maximum at 5 mol% for both dopants and then decreases, which may be due to non-radiative transition due to interaction between the rare-earth ions used due to increases in concentration. Thus, the single peak in the glow curve is due to one type of traps, which are shallow in nature and it is seen that the number of traps in Eu3+-doped ceramics is higher than that of Tb3+-doped ceramics.
Conclusions Eu3+- and Tb3+-doped BaGd2Ti4O12 ceramics have been synthesized by the solid-state reaction method. The XRD spectrum shows the orthorhombic phase structure of the ceramics. Morphology of the ceramic powders has been studied by using SEM images, which shows that particles are agglomerated. The TL study shows the trapping parameters of materials after gamma-ray irradiation. The PL spectra of Eu3+:BaGd2Ti4O12 ceramics have shown an intense red emission at 613 nm with an excitation wavelength λexci = 408 nm and Tb3+:BaGd2Ti4O12 ceramics have shown green emission at 534 nm (5D4 → 7F5) with λexci = 331 nm. The red and green colors emitting Eu3+ and Tb3+: BaGd2Ti4O12 ceramics are promising candidates as red-emitting ceramics for white light-emitting diodes. 3+
Figure 11. TL glow curve of Eu :BaGd2Ti4O12 ceramics. TL, thermoluminescence.
Acknowledgments This work was supported by the UGC, New Delhi in the form of Major Research Project F No. 41-919/2012 (SR) sanctioned to the author (CNR), who would like to thank, the Secretary, UGC, New Delhi, India.
References
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Figure 12. TL glow curve of Tb :BaGd2Ti4O12 ceramics. TL, thermoluminescence.
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