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Eu, Li codoped zirconia nanoparticles with enhanced optical properties
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 20 (2020) 241–244
www.materialstoday.com/proceedings
ANM 2018
Eu, Li codoped zirconia nanoparticles with enhanced optical properties Dimaral Aben, Yerkezhan Amangeldinova, Timur Sh. Atabaev* Department of Chemistry, School of Science and Technology, Nazarbayev University, Astana 010000, Kazakhstan
Abstract This study reports on the synthesis of Eu and Li-codoped zirconia ZrO2 nanoparticles (NPs) using a facile urea precipitation method. The morphology, structural and optical properties of prepared Eu, Li-codoped ZrO2 NPs were analyzed by transmission electron microscope (TEM), X-ray diffraction (XRD), and photoluminescence (PL), respectively. As the main goal of this study, we investigated the effects of Li-codoping on the optical properties of Eu-doped ZrO2 NPs. Strong eye-visible red emission due to 5D0→7F1 and 5D0→7F2 transitions was detected under continuous 250 nm excitation. Thanks to excellent optical properties, prepared Eu, Li-codoped ZrO2 NPs can be used for solid-state lighting, security printing, etc. © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of 11th International Conference on Advanced Nano Materials. Keywords: Zirconia, nanoparticles, optical properties, codoping
1. Introduction Rare-earth doped optical materials play an important role in science and technology. In particular, these optical materials can be used for solid state lighting [1], photocatalysis [2], solar cells enhancement [3], nanomedicine [4], sensors [5], etc. Among many known host materials for rare-earths doping, the zirconia one of the most popular due to low cost, excellent chemical stability, and low phonon energy (~ 470 cm-1) [6]. Therefore, various rare-earths have been doped into a zirconia to investigate their optical properties [6-8].
* Corresponding author. E-mail address: [email protected] and [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of 11th International Conference on Advanced Nano Materials.
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On the other hand, optical materials with enhanced optical properties are highly desirable for technological applications such as solid state lighting, lasers, sensors, etc. The optical properties of rare-earth doped materials can be enhanced by the codoping with some other metals. For example, trace amounts of lithium Li ions were added to enhance the optical properties of Er/Yb:Y2O3 [9]. In another report, the optical properties of Tb:CaYBO4 were enhanced by codoping with Al ions [10]. One can easily observe that Li or Al codoping strategy become attractive for optical properties enhancement. To the best of our knowledge, the optical properties enhancement of Eu and Li codoped ZrO2 NPs were not reported yet. Thus, the main aim of this study is to investigate the effects of concentration-dependent Li-codoping on the optical properties of Eu-doped ZrO2 NPs. 2. Experimental Analytical grade zirconyl nitrate hydrate (99.5 %), europium nitrate hydrate (99.99 %), lithium nitrate (99.0 %) and urea (99.0 – 100.5 %) were purchased from Sigma-Aldrich and used as received. Nearly spherical ZrO2:Eu codoped with different Li+ content were prepared according to a reported protocol [11]. In brief, zirconia/europium mixing ratio was kept 99/1 mol% (0.001 mol total per sample). Later on, the proper amount of lithium nitrate (0-5 mol%) was added to each sample. Urea (1 g) was added to each sample and each of these mixtures was dissolved in a 40 ml of deionized water. Each solution was heated in a sealed glass bottle at 90 oC for 2 h. Collected precipitates were washed with deionized water, dried and calcined in air at 800 oC for 1 h to produce the final Eu and Li codoped ZrO2 NPs. The structure of the prepared powders was examined by XRD (Bruker D8 Discover) using Cu-Kα radiation (λ = 0.15405 nm) at a 2θ scan range of 20–60o. The morphology of the particles was characterized by transmission electron microscope TEM (JEOL JEM-2100F). The fluorescence spectrophotometer (Hitachi F-7000) was used to measure the optical properties of the samples. All measurements were performed at room temperature. 3. Results and discussion TEM study was used to analyze the morphology of prepared samples. Figure 1 shows that TEM image of the ZrO2:Eu NPs codoped with 2 mol% of Li+. According to the TEM image analysis, the sample consists of quasispherical nanoparticles, 22-35 nm in size. We found that codoping with different Li-content did not alter the morphology of the samples, thus, all prepared samples have the similar morphology and size distribution (not shown here).
Fig. 1. TEM image of ZrO2:Eu NPs codoped with 2 mol% of Li+ (scale bar = 20 nm).
The XRD analysis was further used to identify the phase and structure of prepared samples. Figure 2 shows that all diffraction peaks belong to the tetragonal zirconia phase (JCPDS no. 81-1544) [11]. All samples have good crystallinity because the detected XRD peaks have a high intensity. On the other hand, no impurity diffraction peaks related to Li-ions were detected, suggesting that Li-ions incorporated into the zirconia matrix. It should be also noted that Li-codoping cannot be verified with conventional energy-dispersive X-ray spectroscopy due to the
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detection limit of the equipment. Thus, the Li-codoping into a zirconia matrix was tested with the help of fluorescence spectrophotometer.
Fig. 2. XRD patterns of ZrO2:Eu NPs codoped with different Li-ions concentrations.
The optical properties of ZrO2:Eu NPs codoped with different Li-ions concentrations were studied further with the fluorescence spectrophotometer. All samples were analyzed with identically, thus, their excitation and emission spectra could be reasonably compared. Figure 3 shows that excitation spectra consist of a broad peak centered at 247 nm which was assigned to the charge transfer from the 2p orbital of O2- to the 4f orbital of Eu3+ [6]. The emission spectra represented by two prominent peaks centered at 592 nm (5D0→7F1) and 607 nm (5D0→7F2) [6, 7]. In both cases, the intensity of the peaks increased with increasing the Li-codoping concentration up to 2 mol%, and then decreases. Thus, one can conclude that Li-codoping can enhance the optical properties of ZrO2:Eu NPs. It is known that Li-ions can distort the local symmetry around Eu3+ [9], which in turn increase the transitions probability within the Eu3+ and enhance the optical signal. But, at the same time, the enhancement of optical properties depends on the Li-codoping concentration. For example, when the Li-codoping concentration was further increased (3 and 5 mol%), the intensity of the emission spectra decreased. This can be attributed to an increase of local symmetry around Eu3+ [9]. In addition, increased Li-concentration can also form deeper levels in the forbidden bandgap, which act as nonradiative recombination centers and decrease the fluorescence intensity. Therefore, 2 mol% Li-codoping can be considered as optimal codoping concentration for ZrO2:Eu NPs produced by this method.
Fig. 3. PL excitation and emission spectra of ZrO2:Eu NPs codoped with different Li-ions concentrations.
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4. Conclusion In summary, Eu, Li-codoped zirconia NPs with enhanced optical properties were prepared using a facile urea precipitation method. The effects of Li-codoping concentration on the optical properties of ZrO2:Eu NPs were studied using the fluorescence spectrophotometer. The optical properties enhancement due to Li-codoping was attributed to the local crystal field modification around Eu3+. According to experimental data, the optimal Licodoping concentration was found to be 2 mol% for samples produced by this method. Therefore, Eu, Li-codoped zirconia NPs with enhanced optical properties have a great potential for solid-state lighting and optoelectronic devices. Acknowledgements Dimaral Aben and Yerkezhan Amangeldinova would like to acknowledge the financial support received from Nazarbayev University. References [1] D.C. Rich, A review of solid-state lighting and its impact on the marketing and production of decorative coatings, J. Coat. Technol. Res. 13 (2016) 1-9. [2] H. Chu, X. Liu, J. Liu, W. Lei, J. Li, T. Wu, P. Li, H. Li, L. Pan, Down-conversion phosphors as noble-metal-free co-catalyst in ZnO for efficient visible light photocatalysis, Appl. Surf. Sci. 391 (2017) 468-475. [3]G. Shao, C. Lou, D. Xiao, Enhancing the efficiency of solar cells by down shifting YAG:Ce3+ phosphors, J. Lumin. 157 (2015) 344-348. [4] Y. Wu, X. Xu, Q. Li, R. Yang, H. Ding, Q. Xiao, Synthesis of bifunctional Gd2O3:Eu3+ nanocrystals and their applications in biomedical imaging, J. Rare Earth. 33 (2015) 529-534. [5] H. Li, Y. Zhang, L. Shao, Z. Htwe, P. Yuan, Ratiometric temperature sensing based on non-thermal coupling levels in BaZrO3:Yb3+/Er3+ ceramics, Opt. Mater. Express 7 (2017) 3003-3010. [6] T.S. Atabaev, N.H. Hong, Enhanced optical properties of ZrO2:Eu3+ powders codoped with gadolinium ions, J. Sol-Gel Sci. Technol. 82 (2017) 15-19. [7] X. Chen, L. Li, Y. Su, G. Li, Phase evolution and photoluminescence of Eu3+-doped ZrO2, J. Nanosci. Nanotechnol. 10 (2010) 1800-1807. [8] M. Wang, J. Zhao, R. Xu, N. Fu, X. Wang, Preparation and photoluminescence properties of Tm3+-doped ZrO2 nanotube arrays, J. Alloys Compd. 574 (2016) 238-244. [9] G. Chen, H. Liu, H. Liang, G. Somesfalean, Z. Zhang, Upconversion emission enhancement in Yb3+/Er3+-codoped Y2O3 nanocrystals by tridoping with Li+ ions, J. Phys. Chem. C 112 (2008) 12030-12036. [10] K. Park, S.W. Nam, Enhanced photoluminescence properties of CaYBO4:Tb3+,Al3+ by doping Al3+ for plasma display panel applications, Met. Mater. Int. 16 (2010) 963-966. [11] T.S. Atabaev, Paramagnetic Gd-doped zirconia nanoparticles for potential T1-weighted MRI imaging, Nano Life 7 (2017) 1750007.
ScienceDirect Materials Today: Proceedings 20 (2020) 241–244
www.materialstoday.com/proceedings
ANM 2018
Eu, Li codoped zirconia nanoparticles with enhanced optical properties Dimaral Aben, Yerkezhan Amangeldinova, Timur Sh. Atabaev* Department of Chemistry, School of Science and Technology, Nazarbayev University, Astana 010000, Kazakhstan
Abstract This study reports on the synthesis of Eu and Li-codoped zirconia ZrO2 nanoparticles (NPs) using a facile urea precipitation method. The morphology, structural and optical properties of prepared Eu, Li-codoped ZrO2 NPs were analyzed by transmission electron microscope (TEM), X-ray diffraction (XRD), and photoluminescence (PL), respectively. As the main goal of this study, we investigated the effects of Li-codoping on the optical properties of Eu-doped ZrO2 NPs. Strong eye-visible red emission due to 5D0→7F1 and 5D0→7F2 transitions was detected under continuous 250 nm excitation. Thanks to excellent optical properties, prepared Eu, Li-codoped ZrO2 NPs can be used for solid-state lighting, security printing, etc. © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of 11th International Conference on Advanced Nano Materials. Keywords: Zirconia, nanoparticles, optical properties, codoping
1. Introduction Rare-earth doped optical materials play an important role in science and technology. In particular, these optical materials can be used for solid state lighting [1], photocatalysis [2], solar cells enhancement [3], nanomedicine [4], sensors [5], etc. Among many known host materials for rare-earths doping, the zirconia one of the most popular due to low cost, excellent chemical stability, and low phonon energy (~ 470 cm-1) [6]. Therefore, various rare-earths have been doped into a zirconia to investigate their optical properties [6-8].
* Corresponding author. E-mail address: [email protected] and [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of 11th International Conference on Advanced Nano Materials.
242
D. Aben et al. / Materials Today: Proceedings 20 (2020) 241–244
On the other hand, optical materials with enhanced optical properties are highly desirable for technological applications such as solid state lighting, lasers, sensors, etc. The optical properties of rare-earth doped materials can be enhanced by the codoping with some other metals. For example, trace amounts of lithium Li ions were added to enhance the optical properties of Er/Yb:Y2O3 [9]. In another report, the optical properties of Tb:CaYBO4 were enhanced by codoping with Al ions [10]. One can easily observe that Li or Al codoping strategy become attractive for optical properties enhancement. To the best of our knowledge, the optical properties enhancement of Eu and Li codoped ZrO2 NPs were not reported yet. Thus, the main aim of this study is to investigate the effects of concentration-dependent Li-codoping on the optical properties of Eu-doped ZrO2 NPs. 2. Experimental Analytical grade zirconyl nitrate hydrate (99.5 %), europium nitrate hydrate (99.99 %), lithium nitrate (99.0 %) and urea (99.0 – 100.5 %) were purchased from Sigma-Aldrich and used as received. Nearly spherical ZrO2:Eu codoped with different Li+ content were prepared according to a reported protocol [11]. In brief, zirconia/europium mixing ratio was kept 99/1 mol% (0.001 mol total per sample). Later on, the proper amount of lithium nitrate (0-5 mol%) was added to each sample. Urea (1 g) was added to each sample and each of these mixtures was dissolved in a 40 ml of deionized water. Each solution was heated in a sealed glass bottle at 90 oC for 2 h. Collected precipitates were washed with deionized water, dried and calcined in air at 800 oC for 1 h to produce the final Eu and Li codoped ZrO2 NPs. The structure of the prepared powders was examined by XRD (Bruker D8 Discover) using Cu-Kα radiation (λ = 0.15405 nm) at a 2θ scan range of 20–60o. The morphology of the particles was characterized by transmission electron microscope TEM (JEOL JEM-2100F). The fluorescence spectrophotometer (Hitachi F-7000) was used to measure the optical properties of the samples. All measurements were performed at room temperature. 3. Results and discussion TEM study was used to analyze the morphology of prepared samples. Figure 1 shows that TEM image of the ZrO2:Eu NPs codoped with 2 mol% of Li+. According to the TEM image analysis, the sample consists of quasispherical nanoparticles, 22-35 nm in size. We found that codoping with different Li-content did not alter the morphology of the samples, thus, all prepared samples have the similar morphology and size distribution (not shown here).
Fig. 1. TEM image of ZrO2:Eu NPs codoped with 2 mol% of Li+ (scale bar = 20 nm).
The XRD analysis was further used to identify the phase and structure of prepared samples. Figure 2 shows that all diffraction peaks belong to the tetragonal zirconia phase (JCPDS no. 81-1544) [11]. All samples have good crystallinity because the detected XRD peaks have a high intensity. On the other hand, no impurity diffraction peaks related to Li-ions were detected, suggesting that Li-ions incorporated into the zirconia matrix. It should be also noted that Li-codoping cannot be verified with conventional energy-dispersive X-ray spectroscopy due to the
D. Aben et al. / Materials Today: Proceedings 20 (2020) 241–244
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detection limit of the equipment. Thus, the Li-codoping into a zirconia matrix was tested with the help of fluorescence spectrophotometer.
Fig. 2. XRD patterns of ZrO2:Eu NPs codoped with different Li-ions concentrations.
The optical properties of ZrO2:Eu NPs codoped with different Li-ions concentrations were studied further with the fluorescence spectrophotometer. All samples were analyzed with identically, thus, their excitation and emission spectra could be reasonably compared. Figure 3 shows that excitation spectra consist of a broad peak centered at 247 nm which was assigned to the charge transfer from the 2p orbital of O2- to the 4f orbital of Eu3+ [6]. The emission spectra represented by two prominent peaks centered at 592 nm (5D0→7F1) and 607 nm (5D0→7F2) [6, 7]. In both cases, the intensity of the peaks increased with increasing the Li-codoping concentration up to 2 mol%, and then decreases. Thus, one can conclude that Li-codoping can enhance the optical properties of ZrO2:Eu NPs. It is known that Li-ions can distort the local symmetry around Eu3+ [9], which in turn increase the transitions probability within the Eu3+ and enhance the optical signal. But, at the same time, the enhancement of optical properties depends on the Li-codoping concentration. For example, when the Li-codoping concentration was further increased (3 and 5 mol%), the intensity of the emission spectra decreased. This can be attributed to an increase of local symmetry around Eu3+ [9]. In addition, increased Li-concentration can also form deeper levels in the forbidden bandgap, which act as nonradiative recombination centers and decrease the fluorescence intensity. Therefore, 2 mol% Li-codoping can be considered as optimal codoping concentration for ZrO2:Eu NPs produced by this method.
Fig. 3. PL excitation and emission spectra of ZrO2:Eu NPs codoped with different Li-ions concentrations.
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4. Conclusion In summary, Eu, Li-codoped zirconia NPs with enhanced optical properties were prepared using a facile urea precipitation method. The effects of Li-codoping concentration on the optical properties of ZrO2:Eu NPs were studied using the fluorescence spectrophotometer. The optical properties enhancement due to Li-codoping was attributed to the local crystal field modification around Eu3+. According to experimental data, the optimal Licodoping concentration was found to be 2 mol% for samples produced by this method. Therefore, Eu, Li-codoped zirconia NPs with enhanced optical properties have a great potential for solid-state lighting and optoelectronic devices. Acknowledgements Dimaral Aben and Yerkezhan Amangeldinova would like to acknowledge the financial support received from Nazarbayev University. References [1] D.C. Rich, A review of solid-state lighting and its impact on the marketing and production of decorative coatings, J. Coat. Technol. Res. 13 (2016) 1-9. [2] H. Chu, X. Liu, J. Liu, W. Lei, J. Li, T. Wu, P. Li, H. Li, L. Pan, Down-conversion phosphors as noble-metal-free co-catalyst in ZnO for efficient visible light photocatalysis, Appl. Surf. Sci. 391 (2017) 468-475. [3]G. Shao, C. Lou, D. Xiao, Enhancing the efficiency of solar cells by down shifting YAG:Ce3+ phosphors, J. Lumin. 157 (2015) 344-348. [4] Y. Wu, X. Xu, Q. Li, R. Yang, H. Ding, Q. Xiao, Synthesis of bifunctional Gd2O3:Eu3+ nanocrystals and their applications in biomedical imaging, J. Rare Earth. 33 (2015) 529-534. [5] H. Li, Y. Zhang, L. Shao, Z. Htwe, P. Yuan, Ratiometric temperature sensing based on non-thermal coupling levels in BaZrO3:Yb3+/Er3+ ceramics, Opt. Mater. Express 7 (2017) 3003-3010. [6] T.S. Atabaev, N.H. Hong, Enhanced optical properties of ZrO2:Eu3+ powders codoped with gadolinium ions, J. Sol-Gel Sci. Technol. 82 (2017) 15-19. [7] X. Chen, L. Li, Y. Su, G. Li, Phase evolution and photoluminescence of Eu3+-doped ZrO2, J. Nanosci. Nanotechnol. 10 (2010) 1800-1807. [8] M. Wang, J. Zhao, R. Xu, N. Fu, X. Wang, Preparation and photoluminescence properties of Tm3+-doped ZrO2 nanotube arrays, J. Alloys Compd. 574 (2016) 238-244. [9] G. Chen, H. Liu, H. Liang, G. Somesfalean, Z. Zhang, Upconversion emission enhancement in Yb3+/Er3+-codoped Y2O3 nanocrystals by tridoping with Li+ ions, J. Phys. Chem. C 112 (2008) 12030-12036. [10] K. Park, S.W. Nam, Enhanced photoluminescence properties of CaYBO4:Tb3+,Al3+ by doping Al3+ for plasma display panel applications, Met. Mater. Int. 16 (2010) 963-966. [11] T.S. Atabaev, Paramagnetic Gd-doped zirconia nanoparticles for potential T1-weighted MRI imaging, Nano Life 7 (2017) 1750007.