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Pure red upconversion photoluminescence and paramagnetic properties of Gd2O3:Yb3+, Er3+ nanotubes prepared via a facile hydrothermal process
Materials Letters 73 (2012) 147–149
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Pure red upconversion photoluminescence and paramagnetic properties of Gd2O3: Yb 3+, Er 3+ nanotubes prepared via a facile hydrothermal process Congbing Tan ⁎, Bin Ma, Jie Zhang, Yong Zuo, Wei Zhu, Yunxin Liu, Wenbin Li, Yutao Zhang School of Physics, Hunan University of Science and Technology, Xiangtan, Hunan, 411201, China
a r t i c l e
i n f o
Article history: Received 12 August 2011 Accepted 3 January 2012 Available online 13 January 2012 Keywords: Gd2O3:20% Yb3+, 2% Er3+ nanotubes Red emission Luminescence Magnetic materials
a b s t r a c t The cubic Gd2O3:20% Yb3+, 2% Er3+ nanotubes have been synthesized via a facile hydrothermal method and subsequently calcination. Field emission scanning electron microscopy images show that as-obtained Gd2O3 entirely consists of uniform nanotubes with outermost diameters of ~ 250 nm and lengths of ~ 2 μm. The corresponding Gd2O3:20% Yb 3+, 2% Er3+ nanotubes exhibit completely paramagnetism and the magnetic mass susceptibility of them is about 0.28 × 10− 4 emu g− 1 Oe− 1 at 300 K. Pure red light ranged from 645 nm to 685 nm in the nanotubes was observed and assigned to the 4F9/2 → 4I15/2 transition of Er3+ under excitation of 980 nm undergoing an upconversion and the emission intensity was found to decrease in applied magnetic field. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The bifunctional magnetic-optical materials have received growing attention for their potential use in a variety of applications, such as drug targeting or carrier [1], magnetic resonance imaging (MRI) [2,3], and distant optical detection of magnetic field [4], due to their ability to be detected at different models, optically and magnetically. Among these materials, complex containing Gd 3+ ions, especially gadolinium oxide (Gd2O3), can be considered the most perspective ones for the intrinsic magnetic and efficient upconversion properties to the Gd 3+ ions [5-9]. Although the synthesis and optical properties of the doped Gd2O3 phosphor have been extensively investigated [1014], there is rare report focused on the magnetic properties and its effect on the emission, especially the pure red upconversion emission. In this work, we reported on the fabrication and paramagnetism properties of Yb 3+/Er 3+-codoped Gd2O3 nanotubes, which emit upconverted pure red light under a 980 nm excitation.
into a 40 mL Teflon-lined autoclave and held at 180 °C for 20 h in an oven and then naturally cooled down to room temperature. The resulting white precipitates were collected by centrifugation and washed several times to remove residue and finally dried at 60 °C for 12 h in air. Subsequently, the dried powder was calcinated at 700 °C for 2 h in air to obtain the Gd2O3:Yb 3+, Er 3+ nanotubes. X-ray powder diffraction (XRD) pattern was measured on a Bruker/ AXS D8-ADVANCE X-ray diffractometer. Field-emission scanning electron microscopy (FE-SEM) analyses were performed using an FEI SFEG-XL30 apparatus. Upconversion fluorescence spectra were recorded upon excitation with 980 nm laser diode (LD) using a Jobin-Yvin U1000 spectrometer equipped with a Photocool PC104CE photomultiplier tube. Magnetic field versus magnetization (M-H) measurement was carried out with applied fields of up to 60 KOe using a SQUID NMPS-7 magnetometer (Quantum Designs, San Diego, CA). All the tests were performed at the ambient temperature. 3. Results and discussion
2. Experimental section The analytical grade reagents were used as the raw materials without further purification. Rare earth (Re) nitrate aqueous solutions were obtained by dissolving Re2O3 (Re = Gd, Yb, Er: 99.99%) in dilute nitric acid. In a typical synthesis, ammonia solution (25 wt %) was dripped to 20 mL solution containing 0.6 mmol Re (molar ratio, Gd 3+:Yb 3+:Er 3+ = 78:20:2) nitrate until pH ~ 10 under vigorously stirring. After another agitation for 10 min, the mixture was poured ⁎ Corresponding author. Tel.: + 86 731 58290433. E-mail address: [email protected] (C. Tan). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.01.043
The XRD pattern in Fig. 1 shows that the peak positions and relative intensities from 15° to 65° can be well indexed in accord with cubic Gd2O3 reported in JCPDS 11-0604. No additional peaks are observed, suggesting the high purity of the final products. It is also worthwhile to note that the diffraction peaks of the Gd2O3:Yb 3+, Er 3+ nanotubes are sharp and strong indicating that the sample has high crystallinity. This is crucial for the photoluminescence efficiency of materials, because higher crystallinity, in general, means fewer defect traps and higher emission efficiency [15]. The FE-SEM images are shown in Fig. 2a and indicate that the products have tubular morphology. Combining with the local highmagnification image (Inset in Fig. 2a), one can observe that these
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C. Tan et al. / Materials Letters 73 (2012) 147–149
Fig. 1. XRD pattern of the as-obtained Gd2O3:Yb3+, Er3+ nanotubes. The standard data of cubic Gd2O3 (JCPDS 11-0604) is a reference.
Fig. 3. Magnetization-magnetic field (M-H) plots at 300 K.
nanotubes have outermost diameters of about 250 nm and lengths of 2 μm. In addition, the ends of these nanotubes are open and the wall thickness is approximately 50 nm and the surfaces of them are very smooth. The investigation of the growth mechanism is in progress. In general, hollow structural tube can save the expensive raw materials as well as improve photoluminescence efficiency due to the stronger interaction between the under coordinated atoms near the surface [16]. Fig. 2b shows the EDS pattern for the Gd2O3:Yb 3+, Er 3+ nanotubes, which exhibits the presence of Gd, O, Yb, and Er, indicating that the Yb 3+ and Er 3+ ions have been effectively incorporated into the host lattice of the cubic Gd2O3, which is in accord with the above XRD results. Fig. 3 shows the magnetization as a function of applied magnetic field of the Gd2O3:20% Yb 3+, 2% Er 3+ nanotubes. As the strength of the applied magnetic field increasing, especially up to 6T, the linear correlation between the magnetization and the applied magnetic field in the Gd2O3:Yb 3+, Er 3+ nanotubes was observed, which is demonstrated completely paramagnetism [2]. The paramagnetic properties of the Gd2O3:Yb 3+, Er3+ nanotubes can be attributed to the Gd3+ ions, which have seven unpaired 4f electrons effectively shielded by the outer closed shell electrons 5s25p6 from the environment. The formation of Gd2O3 does not affect these 4f electrons due to the large mutual separation between different Gd3+ ions. Thus the magnetic moments associated with each Gd 3+ ion are all localized and noninteracting, giving rise to paramagnetism [5,9]. From Fig. 3, the magnetic mass susceptibility of the as-prepared Gd2O3:Yb 3+, Er3+ nanotubes are found to be 0.28 × 10− 4 emu g− 1 Oe− 1 at 300 K, which is lower than that of Gd2O3 nanoparticles reported in the literature [9]. The authors expect that hollow tubular structure should be responsible for the relatively low magnetic mass susceptibility. The upconversion emission spectra for the Gd2O3:20% Yb 3+, 2% 3+ Er nanotubes were measured under a 980 nm excitation with the power of 120 mW as shown in Fig. 4. In the range of UV to visible light spectrum, only red emission band, ranging from 645 nm to
685 nm, was observed. According to the energy level structure of Er3+ ions [17,18], this emission band is attributed to the 4F9/2 → 4I15/2 transition of Er 3+. The most likely cause of the entirely red emission is a striking increase in the efficiency of the 4F7/2 + 4I15/2 → 4F9/2 + 4F9/2 cross-relaxation mechanism, which is responsible for preferential population of the 4F9/2 level [19]. It is worth noting that from the inset in Fig. 4 the emission intensity has a slight decline as the applied magnetic field of 2.0 KOe, and get back the initial value once removal of the external field. According to Singh's report [9], magnetization under applied magnetic field reduces the transition probability of the emitting level that results in a decrease in emission intensity. However, after sudden removal of the magnetic field, the intensity of the emission should have the initial value due to only instantaneous magnetization of the paramagnetic Gd2O3 host.
a
4. Conclusion In summary, we have prepared the cubic Gd2O3:20% Yb 3+, 2% Er 3+ nanotubes with about 250 nm in outer diameters and 2 μm in length by a hydrothermal method and subsequently calcination. The as-obtained nanotubes exhibit paramagnetism and have the magnetic mass susceptibility of 0.28 × 10 − 4 emu g -1 Oe − 1 at 300 K. In particular, these nanotubes emit pure red light, ranged from 645 nm to 685 nm and attributed to the 4F9/2 → 4I15/2 transition of Er 3+, following excitation of 980 nm. The emission intensity of these Gd2O3 nanotubes was found to decrease under the loading of magnetic field which implies the efficient tuning of the external magnetic field on their optical properties. Acknowledgements The authors are grateful for the support from the National Natural Science Foundation of China (Grant No. 50807011).
b 200 nm
1µm Fig. 2. FE-SEM image (a) and EDS spectrum (b) of the Gd2O3:Yb3+, Er3+ nanotubes. Inset is the local high-magnification image.
C. Tan et al. / Materials Letters 73 (2012) 147–149
149
References
Fig. 4. Upconversion spectra of the Gd2O3:Yb3+, Er3+ nanotubes following excitation with 980 nm. Inset shows effect of applied magnetic field on emission intensity.
[1] Gai S, Yang P, Li C, Wang W, Dai Y, Niu N, et al. Adv Funct Mater 2010;20:1166–72. [2] Huang C-C, Liu T-Y, Su C-H, Lo Y-W, Chen J-H, Yeh C-S. Chem Mater 2008;20: 3840–8. [3] Park YI, Kim JH, Lee KT, Jeon K-S, Na HB, Yu JH, et al. Adv Mater 2009;21:4467–71. [4] Tikhomirov VK, Chibotaru LF, Saurel D, Gredin P, Mortier M, Moshchalkov VV. Nano Lett 2009;9:721–4. [5] Wong H-T, Chan HLW, Hao JH. Appl Phys Lett 2009;95:022512. [6] Selinsky RS, Han JH, Morales Pérez EA, Guzei IA, Jin S. J Am Chem Soc 2010;132: 15997–6005. [7] Ryu J, Park H-Y, Kim K, Kim H, Yoo JH, Kang M, et al. J Phys Chem C 2010;114: 21077–82. [8] Su J, Yang XF, Wang L, Wang CQ, Ji YQ. Mater Lett 2011;65:2852–4. [9] Singh SK, Kumar K, Srivastava MK, Rai DK, Rai SB. Opt Lett 2010;35:1575–7. [10] Xiao H, Li P, Jia F, Zhang L. J Phys Chem C 2009;113:21034–41. [11] Chang C, Kimura F, Kimura T, Wada H. Mater Lett 2005;59:1037–41. [12] Gao S, Lu H, Nie Y, Chen H, Xu D, Dai Q, et al. Mater Lett 2007;61:4003–5. [13] Jia G, You H, Liu K, Zheng Y, Guo N, Zhang H. Langmuir 2009;26:5122–8. [14] Chang WH, Chang P, Lai TY, Lee YJ, Kwo J, Hsu CH, et al. Cryst Growth Des 2010;10:5117–22. [15] Jia G, You H, Song Y, Jia J, Zheng Y, Zhang L, et al. Inorg Chem 2009;48:10193–201. [16] Zhang F, Shi Y, Sun X, Zhao D, Stucky GD. Chem Mater 2009;21:5237–43. [17] Dieke GH. Spectra and energy levels of rare earth ions in crystals. New York: Interscience; 1968. p. 294–308. [18] Wang F, Liu X. Chem Soc Rev 2009;38:976–89. [19] Vetrone F, Boyer J-C, Capobianco JA, Speghini A, Bettinelli M. J Appl Phys 2004;96: 661–7.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Pure red upconversion photoluminescence and paramagnetic properties of Gd2O3: Yb 3+, Er 3+ nanotubes prepared via a facile hydrothermal process Congbing Tan ⁎, Bin Ma, Jie Zhang, Yong Zuo, Wei Zhu, Yunxin Liu, Wenbin Li, Yutao Zhang School of Physics, Hunan University of Science and Technology, Xiangtan, Hunan, 411201, China
a r t i c l e
i n f o
Article history: Received 12 August 2011 Accepted 3 January 2012 Available online 13 January 2012 Keywords: Gd2O3:20% Yb3+, 2% Er3+ nanotubes Red emission Luminescence Magnetic materials
a b s t r a c t The cubic Gd2O3:20% Yb3+, 2% Er3+ nanotubes have been synthesized via a facile hydrothermal method and subsequently calcination. Field emission scanning electron microscopy images show that as-obtained Gd2O3 entirely consists of uniform nanotubes with outermost diameters of ~ 250 nm and lengths of ~ 2 μm. The corresponding Gd2O3:20% Yb 3+, 2% Er3+ nanotubes exhibit completely paramagnetism and the magnetic mass susceptibility of them is about 0.28 × 10− 4 emu g− 1 Oe− 1 at 300 K. Pure red light ranged from 645 nm to 685 nm in the nanotubes was observed and assigned to the 4F9/2 → 4I15/2 transition of Er3+ under excitation of 980 nm undergoing an upconversion and the emission intensity was found to decrease in applied magnetic field. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The bifunctional magnetic-optical materials have received growing attention for their potential use in a variety of applications, such as drug targeting or carrier [1], magnetic resonance imaging (MRI) [2,3], and distant optical detection of magnetic field [4], due to their ability to be detected at different models, optically and magnetically. Among these materials, complex containing Gd 3+ ions, especially gadolinium oxide (Gd2O3), can be considered the most perspective ones for the intrinsic magnetic and efficient upconversion properties to the Gd 3+ ions [5-9]. Although the synthesis and optical properties of the doped Gd2O3 phosphor have been extensively investigated [1014], there is rare report focused on the magnetic properties and its effect on the emission, especially the pure red upconversion emission. In this work, we reported on the fabrication and paramagnetism properties of Yb 3+/Er 3+-codoped Gd2O3 nanotubes, which emit upconverted pure red light under a 980 nm excitation.
into a 40 mL Teflon-lined autoclave and held at 180 °C for 20 h in an oven and then naturally cooled down to room temperature. The resulting white precipitates were collected by centrifugation and washed several times to remove residue and finally dried at 60 °C for 12 h in air. Subsequently, the dried powder was calcinated at 700 °C for 2 h in air to obtain the Gd2O3:Yb 3+, Er 3+ nanotubes. X-ray powder diffraction (XRD) pattern was measured on a Bruker/ AXS D8-ADVANCE X-ray diffractometer. Field-emission scanning electron microscopy (FE-SEM) analyses were performed using an FEI SFEG-XL30 apparatus. Upconversion fluorescence spectra were recorded upon excitation with 980 nm laser diode (LD) using a Jobin-Yvin U1000 spectrometer equipped with a Photocool PC104CE photomultiplier tube. Magnetic field versus magnetization (M-H) measurement was carried out with applied fields of up to 60 KOe using a SQUID NMPS-7 magnetometer (Quantum Designs, San Diego, CA). All the tests were performed at the ambient temperature. 3. Results and discussion
2. Experimental section The analytical grade reagents were used as the raw materials without further purification. Rare earth (Re) nitrate aqueous solutions were obtained by dissolving Re2O3 (Re = Gd, Yb, Er: 99.99%) in dilute nitric acid. In a typical synthesis, ammonia solution (25 wt %) was dripped to 20 mL solution containing 0.6 mmol Re (molar ratio, Gd 3+:Yb 3+:Er 3+ = 78:20:2) nitrate until pH ~ 10 under vigorously stirring. After another agitation for 10 min, the mixture was poured ⁎ Corresponding author. Tel.: + 86 731 58290433. E-mail address: [email protected] (C. Tan). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.01.043
The XRD pattern in Fig. 1 shows that the peak positions and relative intensities from 15° to 65° can be well indexed in accord with cubic Gd2O3 reported in JCPDS 11-0604. No additional peaks are observed, suggesting the high purity of the final products. It is also worthwhile to note that the diffraction peaks of the Gd2O3:Yb 3+, Er 3+ nanotubes are sharp and strong indicating that the sample has high crystallinity. This is crucial for the photoluminescence efficiency of materials, because higher crystallinity, in general, means fewer defect traps and higher emission efficiency [15]. The FE-SEM images are shown in Fig. 2a and indicate that the products have tubular morphology. Combining with the local highmagnification image (Inset in Fig. 2a), one can observe that these
148
C. Tan et al. / Materials Letters 73 (2012) 147–149
Fig. 1. XRD pattern of the as-obtained Gd2O3:Yb3+, Er3+ nanotubes. The standard data of cubic Gd2O3 (JCPDS 11-0604) is a reference.
Fig. 3. Magnetization-magnetic field (M-H) plots at 300 K.
nanotubes have outermost diameters of about 250 nm and lengths of 2 μm. In addition, the ends of these nanotubes are open and the wall thickness is approximately 50 nm and the surfaces of them are very smooth. The investigation of the growth mechanism is in progress. In general, hollow structural tube can save the expensive raw materials as well as improve photoluminescence efficiency due to the stronger interaction between the under coordinated atoms near the surface [16]. Fig. 2b shows the EDS pattern for the Gd2O3:Yb 3+, Er 3+ nanotubes, which exhibits the presence of Gd, O, Yb, and Er, indicating that the Yb 3+ and Er 3+ ions have been effectively incorporated into the host lattice of the cubic Gd2O3, which is in accord with the above XRD results. Fig. 3 shows the magnetization as a function of applied magnetic field of the Gd2O3:20% Yb 3+, 2% Er 3+ nanotubes. As the strength of the applied magnetic field increasing, especially up to 6T, the linear correlation between the magnetization and the applied magnetic field in the Gd2O3:Yb 3+, Er 3+ nanotubes was observed, which is demonstrated completely paramagnetism [2]. The paramagnetic properties of the Gd2O3:Yb 3+, Er3+ nanotubes can be attributed to the Gd3+ ions, which have seven unpaired 4f electrons effectively shielded by the outer closed shell electrons 5s25p6 from the environment. The formation of Gd2O3 does not affect these 4f electrons due to the large mutual separation between different Gd3+ ions. Thus the magnetic moments associated with each Gd 3+ ion are all localized and noninteracting, giving rise to paramagnetism [5,9]. From Fig. 3, the magnetic mass susceptibility of the as-prepared Gd2O3:Yb 3+, Er3+ nanotubes are found to be 0.28 × 10− 4 emu g− 1 Oe− 1 at 300 K, which is lower than that of Gd2O3 nanoparticles reported in the literature [9]. The authors expect that hollow tubular structure should be responsible for the relatively low magnetic mass susceptibility. The upconversion emission spectra for the Gd2O3:20% Yb 3+, 2% 3+ Er nanotubes were measured under a 980 nm excitation with the power of 120 mW as shown in Fig. 4. In the range of UV to visible light spectrum, only red emission band, ranging from 645 nm to
685 nm, was observed. According to the energy level structure of Er3+ ions [17,18], this emission band is attributed to the 4F9/2 → 4I15/2 transition of Er 3+. The most likely cause of the entirely red emission is a striking increase in the efficiency of the 4F7/2 + 4I15/2 → 4F9/2 + 4F9/2 cross-relaxation mechanism, which is responsible for preferential population of the 4F9/2 level [19]. It is worth noting that from the inset in Fig. 4 the emission intensity has a slight decline as the applied magnetic field of 2.0 KOe, and get back the initial value once removal of the external field. According to Singh's report [9], magnetization under applied magnetic field reduces the transition probability of the emitting level that results in a decrease in emission intensity. However, after sudden removal of the magnetic field, the intensity of the emission should have the initial value due to only instantaneous magnetization of the paramagnetic Gd2O3 host.
a
4. Conclusion In summary, we have prepared the cubic Gd2O3:20% Yb 3+, 2% Er 3+ nanotubes with about 250 nm in outer diameters and 2 μm in length by a hydrothermal method and subsequently calcination. The as-obtained nanotubes exhibit paramagnetism and have the magnetic mass susceptibility of 0.28 × 10 − 4 emu g -1 Oe − 1 at 300 K. In particular, these nanotubes emit pure red light, ranged from 645 nm to 685 nm and attributed to the 4F9/2 → 4I15/2 transition of Er 3+, following excitation of 980 nm. The emission intensity of these Gd2O3 nanotubes was found to decrease under the loading of magnetic field which implies the efficient tuning of the external magnetic field on their optical properties. Acknowledgements The authors are grateful for the support from the National Natural Science Foundation of China (Grant No. 50807011).
b 200 nm
1µm Fig. 2. FE-SEM image (a) and EDS spectrum (b) of the Gd2O3:Yb3+, Er3+ nanotubes. Inset is the local high-magnification image.
C. Tan et al. / Materials Letters 73 (2012) 147–149
149
References
Fig. 4. Upconversion spectra of the Gd2O3:Yb3+, Er3+ nanotubes following excitation with 980 nm. Inset shows effect of applied magnetic field on emission intensity.
[1] Gai S, Yang P, Li C, Wang W, Dai Y, Niu N, et al. Adv Funct Mater 2010;20:1166–72. [2] Huang C-C, Liu T-Y, Su C-H, Lo Y-W, Chen J-H, Yeh C-S. Chem Mater 2008;20: 3840–8. [3] Park YI, Kim JH, Lee KT, Jeon K-S, Na HB, Yu JH, et al. Adv Mater 2009;21:4467–71. [4] Tikhomirov VK, Chibotaru LF, Saurel D, Gredin P, Mortier M, Moshchalkov VV. Nano Lett 2009;9:721–4. [5] Wong H-T, Chan HLW, Hao JH. Appl Phys Lett 2009;95:022512. [6] Selinsky RS, Han JH, Morales Pérez EA, Guzei IA, Jin S. J Am Chem Soc 2010;132: 15997–6005. [7] Ryu J, Park H-Y, Kim K, Kim H, Yoo JH, Kang M, et al. J Phys Chem C 2010;114: 21077–82. [8] Su J, Yang XF, Wang L, Wang CQ, Ji YQ. Mater Lett 2011;65:2852–4. [9] Singh SK, Kumar K, Srivastava MK, Rai DK, Rai SB. Opt Lett 2010;35:1575–7. [10] Xiao H, Li P, Jia F, Zhang L. J Phys Chem C 2009;113:21034–41. [11] Chang C, Kimura F, Kimura T, Wada H. Mater Lett 2005;59:1037–41. [12] Gao S, Lu H, Nie Y, Chen H, Xu D, Dai Q, et al. Mater Lett 2007;61:4003–5. [13] Jia G, You H, Liu K, Zheng Y, Guo N, Zhang H. Langmuir 2009;26:5122–8. [14] Chang WH, Chang P, Lai TY, Lee YJ, Kwo J, Hsu CH, et al. Cryst Growth Des 2010;10:5117–22. [15] Jia G, You H, Song Y, Jia J, Zheng Y, Zhang L, et al. Inorg Chem 2009;48:10193–201. [16] Zhang F, Shi Y, Sun X, Zhao D, Stucky GD. Chem Mater 2009;21:5237–43. [17] Dieke GH. Spectra and energy levels of rare earth ions in crystals. New York: Interscience; 1968. p. 294–308. [18] Wang F, Liu X. Chem Soc Rev 2009;38:976–89. [19] Vetrone F, Boyer J-C, Capobianco JA, Speghini A, Bettinelli M. J Appl Phys 2004;96: 661–7.