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Synthesis and characterization of highly luminescent green emitting BaAl2O4: Tb3+ nanophosphors
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 18 (2019) 1132–1137
www.materialstoday.com/proceedings
ICN3I-2017
Synthesis and characterization of highly luminescent green emitting BaAl2O4: Tb3+ nanophosphors Rituparna Chatterjeea*, Gopes Chandra Dasa, Kalyan Kumar Chattopadhyaya,b a
b
School of Materials Science and Nanotechnology, Jadavpur University, Kolkata-700032, India Thin film and Nanoscience Laboratory, Department of Physics, Jadavpur University, Kolkata-700032, India
Abstract In this work, highly efficient Tb3+ activated BaAl2O4 nanophosphor was synthesized by sol-gel method. Their crystal structure, morphology and microstructure have been investigated by X-ray powder diffraction (XRD), Field Emission Scanning Electronic Microscopy (FESEM) and High resolution transmission electron microscopy (HRTEM).The photoluminescence spectra (PL) revealed the influence of concentration variation of Tb3+ in the crystallinity and photoluminescence of the phosphor. The derived nanophosphors exhibit strong green emission produced by the 5D4 → 7Fj (j = 0-6) transitions of Tb3+ upon UV excitation. Obtained results show that this nanophosphor has the potential to compete in the growing field of solid-state lighting and field emission display devices. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017). Keywords: sol-gel; photoluminescence; phosphors; rare earths
* Corresponding author. Tel.: (+91) 9474048712 E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017).
R. Chatterjee et al. / Materials Today: Proceedings 18 (2019) 1132–1137
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1. Introduction Rare earth doped nanophosphors are enormously used in the field of high-performance white light LEDs (WLEDs). Several important physical properties of the phosphors facilitate their usage in multiple applications such as field emission displays (FEDs), light emitting diodes (LEDs), and plasma display panels (PDPs) [1]. Conventionally White LEDs are composed of a blue GaN chip and YAG: Ce yellow phosphor but it has a low color rendering index (CRI) and no complete range of visible light. This deficiency limits the development of LED application and it is required to establish a new strategy that produces the white light with large CRI and brightness. Recently, a commercially suggested solution is to incorporate a UV LED chip as the excitation light and combining it with red, green, and blue phosphors. To successfully produce the white LED by phosphor-converted UV LED chip, the phosphor material should function under long-wavelength UV and have larger emission efficiency. In this work, a plausible green phosphor with high emission efficiency under the excitation of UV light (250 nm) was observed. The green emission of Tb3+ considerably originates from 5D4→ 7FJ (j = 0-6) transitions [2] which are most suitable for the green component in Field Emission Displays (FEDs) because of the promising color purity, current saturation, thermal quenching and also anti-aging. Especially, BaAl2O4 is an outstanding material in this background for its exquisite dielectric, pyroelectric properties and its compatibility towards coating as well as refractory applications.[3] Most of all, the wide band gap (~4 eV) of BaAl2O4 assures narrow absorption in visible region and by this attenuates emission loss, leading it as an ultimate sensitizer in nanophosphors. Another preference of using this material as a host is its stable crystal structure. Barium aluminate belongs to the family of stuffed tridymites. In this tridymite structure the distorted AlO4 tetrahedral cluster shares the corner and connect together to form six-membered rings. Ba2+ cations are placed inside the channels of this structure.[4] These Ba2+ sites are the highly preferable sites for Tb3+ doping as the ionic radii of Tb3+ is similar to Ba2+. Hence, inspired by these favorable conditions, till now, numerous research groups have endeavored to cultivate rare-earth doped BaAl2O4 nanophosphor using different synthesis protocols. But, reports of advantageous doping of Tb3+ in BaAl2O4 matrix is remarkably inadequate. Lou et al. had followed spray pyrolysis method to develop Tb3+ doped BaAl2O4 thin films and observed their cathodoluminescence properties [5], however, the enhancement of photoluminescence emission intensity and direct optical image under UV light has not been shown by them. In the present work, we have synthesized BaAl2O4:Tb3+ nanophosphors by the sol−gel route and examined photoluminescence properties by increasing the activator concentration, which, have not been tried earlier. 2. Experimental Section 2.1. Synthesis of BaAl2O4:Tb3+ Nanophosphors Here, we report structural characterization of highly bright green-emitting BaAl2O4:Tb3+ nanophosphors prepared by simple sol-gel method under a reducing atmosphere. The tailored sol−gel route was followed to synthesize Ba(1−x)Al2O4:Tbx (x = 0.01, 0.02) nanophosphors. The precursors used for the synthesis were Ba(NO3)2 (Merck 99.9%), Al(NO3)3•9H2O (Merck, 99.9%), Tb(NO3)3 (prepared by making a solution of Tb2O3 (Sigma Aldrich 99.99%) in HNO3), citric acid (Merck 99.9%) and ethylenediamine (Merck 99.9%), all of which were of analytical reagent grade. The actual synthesis procedure is parallel to the one mentioned in our earlier report [6]. Initially, barium nitrate, aluminum nitrate and terbium nitrate were stoichiometrically weighed and dissolved in 35 mL of deionized water. After that the solution was stirred vigorously in a magnetic stirrer for 1 h. Then, the aqueous citric acid solution was incorporated drop-wise to the developed metal nitrate solution in a molar ratio such that Cit3−/(Ba2+ + Al3+) = 1 and then for another 30 min the solution was again stirred in a magnetic stirrer to make it homogeneous and transparent. Next, ethylenediamine was mixed to the transparent solution drop-wise to adjust the pH level to 6. Then, the prepared solution was put in a hot water bath and slowly evaporated, which transferred it into a viscous colloidal gel. Later, the gel was dried overnight in an oven at 120 °C and then calcined at 700 °C for 6 h to remove the organics. Lastly, the porous material was thoroughly ground by a mortar and annealed at 1000 °C for 4 h in a tube furnace under a gas flow of 10% H2 and 90% N2 to obtain ultrafine Tb3+ doped BaAl2O4 powders.
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2.2. Characterization The crystal structure of the nanophosphors is determined by XRD with a Rigaku-Ultima-III X Ray powder diffractometer using Cu Kα radiation (λ = 1.5404 Å). The morphology of the prepared sample is investigated by FESEM Hitachi S-4800 and the lattice image is obtained by HRTEM JEOL JEM 2100. The steady-state photoluminescence spectra were recorded on JASCO FP-8300 and the time-resolved photoluminescence spectra were obtained on an Edinburgh FLSP-980 luminescence spectrometer by using a microsecond flash lamp as the excitation source. 3. Results and discussion 3.1. Structural, Compositional, and Microstructural Analysis. Fig. 1(a) shows the XRD pattern of the BaAl2O4:Tb3+ nanophosphor which shows the peak positions and intensities are in good agreement with the ICDD PDF card no. 17-0306 and are indexed as (201), (202), (220), (222), (004), (402), (204), (420), (224), (422), (404), (600). The XRD pattern shows no impurity peaks which assure the formation of single phase tridymite structure with space group P6322. Generally, Tb3+ substitutes Ba2+ ion in the BaAl2O4 lattice and the ionic radius of Ba2+ and Tb3+ are 1.36 Å and 0.92 Å, respectively. The crystallite size of the prepared phosphors calculated from the Debye- Scherrer equation [7] remains within the range of 45 – 60 nm which ensures the generation of crystals in the nanoscale regime. Fig 1(b) shows FESEM images which depicts the agglomerated particles registry of the sample. Fig 1(c) is the typical HRTEM image which distinctly shows the wellresolved lattice fringes having the predicted interplanar spacing of 0.341 nm consistent with the (202) peak in the XRD pattern with hexagonal stuffed tridymite structure.
Figure 1. (a) XRD pattern of BaAl2O4:Tb3+ nanophosphors; (b) FESEM image of the BaAl2O4:2% Tb3+ nanophosphors; (c) HRTEM image depicting the interplanar spacing of 0.341 nm corresponding to the (202) plane in BaAl2O4:2% Tb3+ nanophosphors.
Fig. 2(a) is the PL emission spectra of the Tb3+ doped samples under 250 nm excitation which shows four main emission peaks located at 489, 544, 584, and 622 nm, corresponding to the four most significant transitions arising from the 4f electron configuration of Tb3+, i.e. 5D4→7F6, 5D4→7F5, 5D4→7F4 and 5D4→7F3 transitions [8]. The most intense peak at 544 nm is the characteristic green light emission which belongs to the magnetic dipole 5D4→7F5 transitions of Tb3+, and the transition rarely changes with the crystal field strength. The blue emission peak at ∼488 nm was accredited to the electric dipole 5D4→7F6 transitions of Tb3+, which is dependent on the local environment around the Tb3+ and varies with the symmetry of the crystal field [9]. For the 2% doped sample the emission intensity is much higher due to the increase in luminescence centres (defect states) in the system. Probable luminescence mechanism is at first incident light is being absorbed by the defects in BaAl2O4 matrix, then the nonradiative energy transfer starts from the defect states to Tb3+ level. The defects in the host material generate a series
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of states between the band gap of BaAl2O4, which acts as recombination centers for the excited electron–hole pairs. Thus, the recombination energy gets transferred to excite Tb3+ and drastically enhance the PL emission [10]. Fig. 2(b & c) are the corresponding Commission International del’Eclairage (CIE) chromaticity coordinates and the digital image of the samples under a UV lamp which shows brighter green emission of the BaAl2O4: 2% Tb3+ sample compared to the BaAl2O4:1% Tb3+ sample. An increase in the x coordinate as well as decrease in the y coordinate becomes clear from the chromaticity diagram of the Tb3+ doped nanophosphor. The actual evaluation of this enhancement of color purity has been calculated by employing formula [11]
Color purity=
2
2
2
2
100
where (xs,ys) are the sample point coordinates, (xi ,yi ) are the coordinates of the illuminant point, and (xd,yd) are the coordinates of the dominant wavelength. In this work, taking (xi ,yi ) = (0.333, 0.333) for the illuminant point and (xd,yd) = (0.269, 0.719) for the dominant wavelength at 544 nm [11], the color purities of the 1% Tb3+ and 2% Tb3+ doped BaAl2O4 nanophosphors were estimated to be 78.9 and 79.7%, respectively. Thus, an improvement of the colour purity with increase in Tb3+ concentration has been achieved and the colour co-ordinate reaches near the extreme green chromaticity for the National Television Standard Committee (NTSC) system with good colour saturation. Generally, elevated surface states of the nanocrystals lower the PL intensity in solid state because of the partial aggregation and nonradiative recombination [12]. Therefore, attaining high brightness in solid condition implies a pivotal benchmark for a good nanophosphor. It becomes clear from the optical images that that increasing Tb3+ concentration overcomes the problems related to the enhanced surface states and increases PL brightness.
Figure 2. (a) PL emission spectra of BaAl2O4:Tb3+ nanophosphors measured under 250 nm excitation; (b) Color co-ordinates of the sample (BaAl2O4:1% Tb3+) and corresponding digital image under UV lamp (c) Color co-ordinates of the sample (BaAl2O4:2% Tb3+) and corresponding digital image.
To corroborate the reduction in nonradiative transition with the increasing loading of Tb3+ activator, the PL decay profiles of 544 nm emission for both 1% Tb3+ and 2% Tb3+ doped BaAl2O4 nanophosphors have been carried out under 250 nm UV excitation in Fig. 3.
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Figure 3. PL decay curves of the 5D4 → 7F5 transition for BaAl2O4:1% Tb3+ and BaAl2O4:2% Tb3+ nanophosphors excited via 250 nm UV light.
The luminescence decay curves of the 5D4 → 7F5 transition for the as prepared nanophosphors were fitted well by a biexponential function, which can be demonstrated as follows [13]
exp
exp
where A1 and A2 are the weighting parameters and τ1 and τ2 are the decay components of the lifetimes of luminescence. The average lifetime for the biexponential decay can be expressed as
2 1 1
2 2 2
1 1
2 2
The calculated average lifetimes of 1% Tb3+ doped BaAl2O4 and 2% Tb3+ doped BaAl2O4 nanophosphor are 1.22 and 3.49 µs, respectively which shows that with Tb3+ concentration enhancement luminescence lifetime of the nanophosphor also increases. 4. Conclusion In summary, we have successfully demonstrated a cost-effective technique to synthesize Tb3+-doped BaAl2O4 nanophosphors. The derived nanophosphors produce bright green emission due to the 5D4 →7Fj (j = 0-6) transitions of Tb3+ when excited under UV irradiation. PL investigations of the nanophosphors depict that the incorporation of the Tb3+ activator increases the luminescence intensity and subsequent coluor purity by reducing the nonradiative transition centers in it. It is also established that the luminescence lifetime of the phosphors increases by enhancing the Tb3+ doping concentration. Finally, the obtained outcomes strongly shows that BaAl2O4:Tb3+ nanophosphors could be potential candidate for use in solid-state lighting as well as field emission display devices.
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Acknowledgements The authors sincerely acknowledge Department of Science & Technology (DST), Government of India for the financial help, for awarding an INSPIRE Fellowship during the execution of the work. They also wish to thank the University Grants Commission, for the University with Potential for Excellence (UPE-II) scheme. References [1] Y. Zhang, X. Li, K. Li, H. Lian, M. Shang, J. Lin, Crystal-Site Engineering Control for the Reduction of Eu3+ to Eu2+ in CaYAlO4:Structure Refinement and Tunable Emission Properties, ACS Appl. Mater. Interfaces, 2015, 7, pp. 2715−2725. [2] Y. Zhang, L. Lan, X. Zhang, W. Dajian, S. Zhang, Preparation and photoluminescence of Tb3+doped SrAl2O4 phosphor by composite combustion method, Journal of Rare Earths, 2008, 26, pp. 656-659. [3] Z. Zhu, F. Liu, W. Zhang, Fabricate and characterization of Ag/BaAl2O4 and its photocatalytic performance towards oxidation of gaseous toluene studied by FTIR spectroscopy, Materials Research Bulletin, 2015, 64, pp. 68–75. [4] R. Chatterjee, S. Saha, D. Sen, K. Panigrahi, U. K. Ghorai, G. C. Das, K. K. Chattopadhyay, Neutralizing the Charge Imbalance Problem in Eu3+-Activated BaAl2O4 Nanophosphors: Theoretical Insights and Experimental Validation Considering K+ Codoping, ACS Omega, 2018, 3, pp. 788-800. [5] Z. Lou, J. Hao, M. Cocivera, Luminescence studies of BaAl2O4 films doped with Tm, Tb, and Eu, J. Phys. D: Appl. Phys. 2002, 35, pp. 2841–2845. [6] S. Saha, S. Das, U. K. Ghorai, N. Mazumder, D. Ganguly, K. K. Chattopadhyay, Controlling Nonradiative Transition Centers in Eu3+ Activated CaSnO3 Nanophosphors through Na+ Co-Doping: Realization of Ultra bright Red Emission along with Higher Thermal Stability, J. Phys. Chem. C , 2015, 29, pp. 16824-16835. [7] A. Das, S. Saha, K. Panigrahi, A. Mitra, R. Chatterjee, U. K. Ghorai and B. Das, K. K. Chattopadhyay, Morphology control and photoluminescence properties of Eu3+-activated Y4Al2O9 nanophosphors for solid state lighting applications, CrystEngComm, 2018, DOI: 10.1039/C8CE00289D. [8] A. K. Parchur, A. I. Prasad, A. A. Ansari, S. B. Raia, R. S. Ningthoujam, Luminescence properties of Tb3+-doped CaMoO4 nanoparticles: annealing effect, polar medium dispersible, polymer film and core–shell formation, Dalton Trans., 2012, 41, pp. 11032- 11045. [9] S. K. Gupta, P. S. Ghosh, A. K. Yadav, N. Pathak, A. Arya, S. N. Jha, D. Bhattacharya, R. M. Kadam, Luminescence Properties of SrZrO3/Tb3+ Perovskite: Host-Dopant Energy-Transfer Dynamics and Local Structure of Tb3+, Inorganic chemistry, 2016, 55, pp. 1728-1740. [10] N. Fu, X. Wang, L. Guo, J. Zhao, X. Zhang, J. Lin, L. Gong, M. Wang, Y. Yang, Green photoluminescence in Tb3+-doped ZrO2 nanotube arrays, J Mater Sci: Mater Electron, 2017, DOI 10.1007/s10854-017-6407-7. [11] D. R. Taikar, S. Tamboli, S. J. Dhoble, Synthesis and photoluminescence properties of Li2SO4: RE (RE= Eu3+, Tb3+, Gd3+, Ce3+) phoshors, Optik, 2017, 139, pp. 111- 122. [12] Otto. T, Müller. M, Mundra, Lesnyak. V, Demir. H. V, Gaponik. N, Eychmüller. A, Colloidal Nanocrystals Embedded in Macrocrystals: Robustness, Photostability, and Color Purity, Nano Letters, 2012, 12, pp. 5348−5354. [13] K. Panigrahi, S. Saha, S. Sain, R. Chatterjee, A. Das, U. K. Ghorai, N. S. Das, K. K. Chattopadhyay, White light emitting MgAl2O4:Dy3+,Eu3+ nanophosphor for multifunctional applications, Dalton Transactions, 2018, 47, pp. 12228-12242.
ScienceDirect Materials Today: Proceedings 18 (2019) 1132–1137
www.materialstoday.com/proceedings
ICN3I-2017
Synthesis and characterization of highly luminescent green emitting BaAl2O4: Tb3+ nanophosphors Rituparna Chatterjeea*, Gopes Chandra Dasa, Kalyan Kumar Chattopadhyaya,b a
b
School of Materials Science and Nanotechnology, Jadavpur University, Kolkata-700032, India Thin film and Nanoscience Laboratory, Department of Physics, Jadavpur University, Kolkata-700032, India
Abstract In this work, highly efficient Tb3+ activated BaAl2O4 nanophosphor was synthesized by sol-gel method. Their crystal structure, morphology and microstructure have been investigated by X-ray powder diffraction (XRD), Field Emission Scanning Electronic Microscopy (FESEM) and High resolution transmission electron microscopy (HRTEM).The photoluminescence spectra (PL) revealed the influence of concentration variation of Tb3+ in the crystallinity and photoluminescence of the phosphor. The derived nanophosphors exhibit strong green emission produced by the 5D4 → 7Fj (j = 0-6) transitions of Tb3+ upon UV excitation. Obtained results show that this nanophosphor has the potential to compete in the growing field of solid-state lighting and field emission display devices. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017). Keywords: sol-gel; photoluminescence; phosphors; rare earths
* Corresponding author. Tel.: (+91) 9474048712 E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017).
R. Chatterjee et al. / Materials Today: Proceedings 18 (2019) 1132–1137
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1. Introduction Rare earth doped nanophosphors are enormously used in the field of high-performance white light LEDs (WLEDs). Several important physical properties of the phosphors facilitate their usage in multiple applications such as field emission displays (FEDs), light emitting diodes (LEDs), and plasma display panels (PDPs) [1]. Conventionally White LEDs are composed of a blue GaN chip and YAG: Ce yellow phosphor but it has a low color rendering index (CRI) and no complete range of visible light. This deficiency limits the development of LED application and it is required to establish a new strategy that produces the white light with large CRI and brightness. Recently, a commercially suggested solution is to incorporate a UV LED chip as the excitation light and combining it with red, green, and blue phosphors. To successfully produce the white LED by phosphor-converted UV LED chip, the phosphor material should function under long-wavelength UV and have larger emission efficiency. In this work, a plausible green phosphor with high emission efficiency under the excitation of UV light (250 nm) was observed. The green emission of Tb3+ considerably originates from 5D4→ 7FJ (j = 0-6) transitions [2] which are most suitable for the green component in Field Emission Displays (FEDs) because of the promising color purity, current saturation, thermal quenching and also anti-aging. Especially, BaAl2O4 is an outstanding material in this background for its exquisite dielectric, pyroelectric properties and its compatibility towards coating as well as refractory applications.[3] Most of all, the wide band gap (~4 eV) of BaAl2O4 assures narrow absorption in visible region and by this attenuates emission loss, leading it as an ultimate sensitizer in nanophosphors. Another preference of using this material as a host is its stable crystal structure. Barium aluminate belongs to the family of stuffed tridymites. In this tridymite structure the distorted AlO4 tetrahedral cluster shares the corner and connect together to form six-membered rings. Ba2+ cations are placed inside the channels of this structure.[4] These Ba2+ sites are the highly preferable sites for Tb3+ doping as the ionic radii of Tb3+ is similar to Ba2+. Hence, inspired by these favorable conditions, till now, numerous research groups have endeavored to cultivate rare-earth doped BaAl2O4 nanophosphor using different synthesis protocols. But, reports of advantageous doping of Tb3+ in BaAl2O4 matrix is remarkably inadequate. Lou et al. had followed spray pyrolysis method to develop Tb3+ doped BaAl2O4 thin films and observed their cathodoluminescence properties [5], however, the enhancement of photoluminescence emission intensity and direct optical image under UV light has not been shown by them. In the present work, we have synthesized BaAl2O4:Tb3+ nanophosphors by the sol−gel route and examined photoluminescence properties by increasing the activator concentration, which, have not been tried earlier. 2. Experimental Section 2.1. Synthesis of BaAl2O4:Tb3+ Nanophosphors Here, we report structural characterization of highly bright green-emitting BaAl2O4:Tb3+ nanophosphors prepared by simple sol-gel method under a reducing atmosphere. The tailored sol−gel route was followed to synthesize Ba(1−x)Al2O4:Tbx (x = 0.01, 0.02) nanophosphors. The precursors used for the synthesis were Ba(NO3)2 (Merck 99.9%), Al(NO3)3•9H2O (Merck, 99.9%), Tb(NO3)3 (prepared by making a solution of Tb2O3 (Sigma Aldrich 99.99%) in HNO3), citric acid (Merck 99.9%) and ethylenediamine (Merck 99.9%), all of which were of analytical reagent grade. The actual synthesis procedure is parallel to the one mentioned in our earlier report [6]. Initially, barium nitrate, aluminum nitrate and terbium nitrate were stoichiometrically weighed and dissolved in 35 mL of deionized water. After that the solution was stirred vigorously in a magnetic stirrer for 1 h. Then, the aqueous citric acid solution was incorporated drop-wise to the developed metal nitrate solution in a molar ratio such that Cit3−/(Ba2+ + Al3+) = 1 and then for another 30 min the solution was again stirred in a magnetic stirrer to make it homogeneous and transparent. Next, ethylenediamine was mixed to the transparent solution drop-wise to adjust the pH level to 6. Then, the prepared solution was put in a hot water bath and slowly evaporated, which transferred it into a viscous colloidal gel. Later, the gel was dried overnight in an oven at 120 °C and then calcined at 700 °C for 6 h to remove the organics. Lastly, the porous material was thoroughly ground by a mortar and annealed at 1000 °C for 4 h in a tube furnace under a gas flow of 10% H2 and 90% N2 to obtain ultrafine Tb3+ doped BaAl2O4 powders.
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2.2. Characterization The crystal structure of the nanophosphors is determined by XRD with a Rigaku-Ultima-III X Ray powder diffractometer using Cu Kα radiation (λ = 1.5404 Å). The morphology of the prepared sample is investigated by FESEM Hitachi S-4800 and the lattice image is obtained by HRTEM JEOL JEM 2100. The steady-state photoluminescence spectra were recorded on JASCO FP-8300 and the time-resolved photoluminescence spectra were obtained on an Edinburgh FLSP-980 luminescence spectrometer by using a microsecond flash lamp as the excitation source. 3. Results and discussion 3.1. Structural, Compositional, and Microstructural Analysis. Fig. 1(a) shows the XRD pattern of the BaAl2O4:Tb3+ nanophosphor which shows the peak positions and intensities are in good agreement with the ICDD PDF card no. 17-0306 and are indexed as (201), (202), (220), (222), (004), (402), (204), (420), (224), (422), (404), (600). The XRD pattern shows no impurity peaks which assure the formation of single phase tridymite structure with space group P6322. Generally, Tb3+ substitutes Ba2+ ion in the BaAl2O4 lattice and the ionic radius of Ba2+ and Tb3+ are 1.36 Å and 0.92 Å, respectively. The crystallite size of the prepared phosphors calculated from the Debye- Scherrer equation [7] remains within the range of 45 – 60 nm which ensures the generation of crystals in the nanoscale regime. Fig 1(b) shows FESEM images which depicts the agglomerated particles registry of the sample. Fig 1(c) is the typical HRTEM image which distinctly shows the wellresolved lattice fringes having the predicted interplanar spacing of 0.341 nm consistent with the (202) peak in the XRD pattern with hexagonal stuffed tridymite structure.
Figure 1. (a) XRD pattern of BaAl2O4:Tb3+ nanophosphors; (b) FESEM image of the BaAl2O4:2% Tb3+ nanophosphors; (c) HRTEM image depicting the interplanar spacing of 0.341 nm corresponding to the (202) plane in BaAl2O4:2% Tb3+ nanophosphors.
Fig. 2(a) is the PL emission spectra of the Tb3+ doped samples under 250 nm excitation which shows four main emission peaks located at 489, 544, 584, and 622 nm, corresponding to the four most significant transitions arising from the 4f electron configuration of Tb3+, i.e. 5D4→7F6, 5D4→7F5, 5D4→7F4 and 5D4→7F3 transitions [8]. The most intense peak at 544 nm is the characteristic green light emission which belongs to the magnetic dipole 5D4→7F5 transitions of Tb3+, and the transition rarely changes with the crystal field strength. The blue emission peak at ∼488 nm was accredited to the electric dipole 5D4→7F6 transitions of Tb3+, which is dependent on the local environment around the Tb3+ and varies with the symmetry of the crystal field [9]. For the 2% doped sample the emission intensity is much higher due to the increase in luminescence centres (defect states) in the system. Probable luminescence mechanism is at first incident light is being absorbed by the defects in BaAl2O4 matrix, then the nonradiative energy transfer starts from the defect states to Tb3+ level. The defects in the host material generate a series
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1135
of states between the band gap of BaAl2O4, which acts as recombination centers for the excited electron–hole pairs. Thus, the recombination energy gets transferred to excite Tb3+ and drastically enhance the PL emission [10]. Fig. 2(b & c) are the corresponding Commission International del’Eclairage (CIE) chromaticity coordinates and the digital image of the samples under a UV lamp which shows brighter green emission of the BaAl2O4: 2% Tb3+ sample compared to the BaAl2O4:1% Tb3+ sample. An increase in the x coordinate as well as decrease in the y coordinate becomes clear from the chromaticity diagram of the Tb3+ doped nanophosphor. The actual evaluation of this enhancement of color purity has been calculated by employing formula [11]
Color purity=
2
2
2
2
100
where (xs,ys) are the sample point coordinates, (xi ,yi ) are the coordinates of the illuminant point, and (xd,yd) are the coordinates of the dominant wavelength. In this work, taking (xi ,yi ) = (0.333, 0.333) for the illuminant point and (xd,yd) = (0.269, 0.719) for the dominant wavelength at 544 nm [11], the color purities of the 1% Tb3+ and 2% Tb3+ doped BaAl2O4 nanophosphors were estimated to be 78.9 and 79.7%, respectively. Thus, an improvement of the colour purity with increase in Tb3+ concentration has been achieved and the colour co-ordinate reaches near the extreme green chromaticity for the National Television Standard Committee (NTSC) system with good colour saturation. Generally, elevated surface states of the nanocrystals lower the PL intensity in solid state because of the partial aggregation and nonradiative recombination [12]. Therefore, attaining high brightness in solid condition implies a pivotal benchmark for a good nanophosphor. It becomes clear from the optical images that that increasing Tb3+ concentration overcomes the problems related to the enhanced surface states and increases PL brightness.
Figure 2. (a) PL emission spectra of BaAl2O4:Tb3+ nanophosphors measured under 250 nm excitation; (b) Color co-ordinates of the sample (BaAl2O4:1% Tb3+) and corresponding digital image under UV lamp (c) Color co-ordinates of the sample (BaAl2O4:2% Tb3+) and corresponding digital image.
To corroborate the reduction in nonradiative transition with the increasing loading of Tb3+ activator, the PL decay profiles of 544 nm emission for both 1% Tb3+ and 2% Tb3+ doped BaAl2O4 nanophosphors have been carried out under 250 nm UV excitation in Fig. 3.
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Figure 3. PL decay curves of the 5D4 → 7F5 transition for BaAl2O4:1% Tb3+ and BaAl2O4:2% Tb3+ nanophosphors excited via 250 nm UV light.
The luminescence decay curves of the 5D4 → 7F5 transition for the as prepared nanophosphors were fitted well by a biexponential function, which can be demonstrated as follows [13]
exp
exp
where A1 and A2 are the weighting parameters and τ1 and τ2 are the decay components of the lifetimes of luminescence. The average lifetime for the biexponential decay can be expressed as
2 1 1
2 2 2
1 1
2 2
The calculated average lifetimes of 1% Tb3+ doped BaAl2O4 and 2% Tb3+ doped BaAl2O4 nanophosphor are 1.22 and 3.49 µs, respectively which shows that with Tb3+ concentration enhancement luminescence lifetime of the nanophosphor also increases. 4. Conclusion In summary, we have successfully demonstrated a cost-effective technique to synthesize Tb3+-doped BaAl2O4 nanophosphors. The derived nanophosphors produce bright green emission due to the 5D4 →7Fj (j = 0-6) transitions of Tb3+ when excited under UV irradiation. PL investigations of the nanophosphors depict that the incorporation of the Tb3+ activator increases the luminescence intensity and subsequent coluor purity by reducing the nonradiative transition centers in it. It is also established that the luminescence lifetime of the phosphors increases by enhancing the Tb3+ doping concentration. Finally, the obtained outcomes strongly shows that BaAl2O4:Tb3+ nanophosphors could be potential candidate for use in solid-state lighting as well as field emission display devices.
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Acknowledgements The authors sincerely acknowledge Department of Science & Technology (DST), Government of India for the financial help, for awarding an INSPIRE Fellowship during the execution of the work. They also wish to thank the University Grants Commission, for the University with Potential for Excellence (UPE-II) scheme. References [1] Y. Zhang, X. Li, K. Li, H. Lian, M. Shang, J. Lin, Crystal-Site Engineering Control for the Reduction of Eu3+ to Eu2+ in CaYAlO4:Structure Refinement and Tunable Emission Properties, ACS Appl. Mater. Interfaces, 2015, 7, pp. 2715−2725. [2] Y. Zhang, L. Lan, X. Zhang, W. Dajian, S. Zhang, Preparation and photoluminescence of Tb3+doped SrAl2O4 phosphor by composite combustion method, Journal of Rare Earths, 2008, 26, pp. 656-659. [3] Z. Zhu, F. Liu, W. Zhang, Fabricate and characterization of Ag/BaAl2O4 and its photocatalytic performance towards oxidation of gaseous toluene studied by FTIR spectroscopy, Materials Research Bulletin, 2015, 64, pp. 68–75. [4] R. Chatterjee, S. Saha, D. Sen, K. Panigrahi, U. K. Ghorai, G. C. Das, K. K. Chattopadhyay, Neutralizing the Charge Imbalance Problem in Eu3+-Activated BaAl2O4 Nanophosphors: Theoretical Insights and Experimental Validation Considering K+ Codoping, ACS Omega, 2018, 3, pp. 788-800. [5] Z. Lou, J. Hao, M. Cocivera, Luminescence studies of BaAl2O4 films doped with Tm, Tb, and Eu, J. Phys. D: Appl. Phys. 2002, 35, pp. 2841–2845. [6] S. Saha, S. Das, U. K. Ghorai, N. Mazumder, D. Ganguly, K. K. Chattopadhyay, Controlling Nonradiative Transition Centers in Eu3+ Activated CaSnO3 Nanophosphors through Na+ Co-Doping: Realization of Ultra bright Red Emission along with Higher Thermal Stability, J. Phys. Chem. C , 2015, 29, pp. 16824-16835. [7] A. Das, S. Saha, K. Panigrahi, A. Mitra, R. Chatterjee, U. K. Ghorai and B. Das, K. K. Chattopadhyay, Morphology control and photoluminescence properties of Eu3+-activated Y4Al2O9 nanophosphors for solid state lighting applications, CrystEngComm, 2018, DOI: 10.1039/C8CE00289D. [8] A. K. Parchur, A. I. Prasad, A. A. Ansari, S. B. Raia, R. S. Ningthoujam, Luminescence properties of Tb3+-doped CaMoO4 nanoparticles: annealing effect, polar medium dispersible, polymer film and core–shell formation, Dalton Trans., 2012, 41, pp. 11032- 11045. [9] S. K. Gupta, P. S. Ghosh, A. K. Yadav, N. Pathak, A. Arya, S. N. Jha, D. Bhattacharya, R. M. Kadam, Luminescence Properties of SrZrO3/Tb3+ Perovskite: Host-Dopant Energy-Transfer Dynamics and Local Structure of Tb3+, Inorganic chemistry, 2016, 55, pp. 1728-1740. [10] N. Fu, X. Wang, L. Guo, J. Zhao, X. Zhang, J. Lin, L. Gong, M. Wang, Y. Yang, Green photoluminescence in Tb3+-doped ZrO2 nanotube arrays, J Mater Sci: Mater Electron, 2017, DOI 10.1007/s10854-017-6407-7. [11] D. R. Taikar, S. Tamboli, S. J. Dhoble, Synthesis and photoluminescence properties of Li2SO4: RE (RE= Eu3+, Tb3+, Gd3+, Ce3+) phoshors, Optik, 2017, 139, pp. 111- 122. [12] Otto. T, Müller. M, Mundra, Lesnyak. V, Demir. H. V, Gaponik. N, Eychmüller. A, Colloidal Nanocrystals Embedded in Macrocrystals: Robustness, Photostability, and Color Purity, Nano Letters, 2012, 12, pp. 5348−5354. [13] K. Panigrahi, S. Saha, S. Sain, R. Chatterjee, A. Das, U. K. Ghorai, N. S. Das, K. K. Chattopadhyay, White light emitting MgAl2O4:Dy3+,Eu3+ nanophosphor for multifunctional applications, Dalton Transactions, 2018, 47, pp. 12228-12242.