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Er3+ nanocrystals by adjusting the reaction time
Journal of Alloys and Compounds 699 (2017) 1e6
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The colour tuning of upconversion emission from green to red in NaScF4:Yb3þ/Er3þ nanocrystals by adjusting the reaction time Ligang Zhang a, b, c, Suling Zhao a, b, *, Zhiqin Liang a, b, Junjie Zhang a, b, Wei Zhu a, b, Pu Liu c, Hongkai Sun c a b c
Key Laboratory of Luminescence and Optical Information, Beijing Jiaotong University, Ministry of Education, Beijing 100044, China Institute of Optoelectronics Technology, Beijing Jiaotong University, Beijing 100044, China School of Science, Hebei University of Architecture, Hebei, Zhangjiakou 075000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 August 2016 Received in revised form 16 November 2016 Accepted 16 December 2016 Available online 18 December 2016
A series of water-soluble NaScF4:Yb3þ/Er3þ upconversion nanoparticles (UCNPs) have been synthesized by a facile hydrothermal route. The samples display a high degree of uniformity and crystallization. The upconversion (UC) emissions from green to red in NaScF4:Yb3þ/Er3þ nanoparticles are successfully obtained by adjusting the reaction time from 12 h to 2 h. The UC emission peaks at 541 nm, and 652 nm of all the samples excited by 980 nm correspond to the transitions of 4S3/2/2H11/2 / 4I15/2 and 4F9/2 / 4I15/2 of Er3þions, respectively. The green UC emission at 541 nm decreases while the red emission at 652 nm increases gradually when the reaction time changes from 12 h to 2 h. This work provides a novel and facile way of tuning colour from green to red in NaScF4:Yb3þ/Er3þ UCNPs. These water-soluble nanoball structures NaScF4:Yb3þ/Er3þ UCNPs promise to be novel biological probes and possess other applications in many fields. © 2016 Elsevier B.V. All rights reserved.
Keywords: NaScF4 Upconversion Reaction time
1. Introduction In the past two decades, rare-earth ion-doped materials have received considerable attention due to their distinctive luminescence properties [1,2]. In particular, Ln3þ doped UCNPs, in which two or more low energy photons are sequentially absorbed and then emit a higher energy photon [1], possess many virtues, such as sharp emission band widths, low toxicity, long luminescence lifetime and high photostability, and have a wide range of potential applications in colour displays, biomarkers, drug carriers, photodynamic therapy, and photovoltaic devices [3e7]. Compared to green and blue light, red light used as a biomedical detection light has the greatest sensitivity because biotissues absorb high energy photons like blue light and emit green light. In addition, the loss of the red light signal due to scattering is reduced, owing to its relatively longer wavelength. Therefore, the red light UC emissions of Ln3þ doped UCNPs have great potential applications in the biomedical field. However, it is very difficult to obtain a pure red UC
* Corresponding author. Key Laboratory of Luminescence and Optical Information, Beijing Jiaotong University, Ministry of Education, Beijing 100044, China. E-mail address: [email protected] (S. Zhao). http://dx.doi.org/10.1016/j.jallcom.2016.12.202 0925-8388/© 2016 Elsevier B.V. All rights reserved.
spectrum because of the abundant radiative and nonradiative transition channels in the rare earth ions' 4F-4F electron state. Hitherto, it is still a necessary and important challenge to obtain UCNPs with intense red emissions. Dangli Gao et al. synthesized a series of b-NaYF4:Yb3þ/Er3þ microrods and tuned the output light colour from green to red by increasing Yb3þ ion concentrations [8]. Zhanjun Gu and coworkers synthesized NaYF4:Yb/Er nanoparticles with pure dark red emissions by Mn2þ doping [9]. Sunil Kumar Singh et al. synthesized a series of Gd:UCNPs@AuNPs and obtained a uniquely red UC emission [10]. Ho3þ is another intriguing ion for upconversion luminescence, and there are many researchers who reported the red emission of Ho3þ in host material doping with Ce3þ, such as NaYF4, NaluF4 [11e13], LiYF [14], and LiLuF4 [15]. Except for those methods used by doping different ions to produce red UC luminescence, reaction parameters such as reaction time and the reaction temperature can sometimes have a strong influence on the results of these experiments. Up to now, tuning of upconversion emission from green to red in NaScF4:Yb3þ/Er3þ nanocrystals by adjusting the reaction parameters have been rarely reports. Therefore, it is necessary to find a facile and simple way to get intense red UC emission for various applications. Scandium, as a well-known fluorophilic rare-earth element, has received little attention [16,17]. In a chemical reaction, the
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L. Zhang et al. / Journal of Alloys and Compounds 699 (2017) 1e6
2.3. Characterization The crystallization phase of the samples was performed by x-ray power diffraction (XRD) on a D/max 2200v X-ray powder diffractometer equipped with Cu Ka radiation (l ¼ 1.540 Å). Scanning electron microscope (SEM) measurements were performed using a Hitachi S-8010 SEM equipped with an energy dispersive x-ray (EDX) spectrum. Transmission electron microscopy (TEM) images were recorded on a HEOL-3000F TEM operating at an acceleration voltage of 200 kV. The upconversion luminescence spectra were recorded on a ZOLIX fluorescence spectrometer system, while the excitation source used was an external 980 nm semiconductor laser. The fluorescence images and photographs were obtained using a Canon 1D X MARK II camera. All of the measurements were performed at room temperature.
3. Results and discussion Fig. 1. XRD results (t ¼ 2,2.5,3,4,12 h).
of
NaScF4:Yb3þ/Er3þ
synthesized
at
different
time
outermost electrons are easily lost to form a stable Sc3þ ion. Its distinct atomic electron configuration and smaller ion radius may lead to different optical properties from other rare earth elements. Recently, Huang and co-workers synthesized NaxScF3þx NCs by a solvent-thermal process and tuned the phase and composition of the NaxScF3þxNCs nanocrystals by using the ratio of oleic acid (OA) to octadecene (ODE) [18]. They also found that the phase and out colour of NaxScF3þxNCs can be controlled by NaF [19]. Zhang and coworkers synthesized NaScF4:Yb3þ/Er3þ UCNPs via a facile hydrothermal route and realized red colour emission [20]. In this paper, we synthesized NaScF4:Yb3þ/Er3þ UCNPs by a hydrothermal route, whose UC emission from green to red was tuned gradually by adjusting the reaction time. Moreover, the luminescence mechanism and the influence of reaction time were investigated in detail. 2. Experiment 2.1. Chemicals YbCl3$6H2O, ErCl3$6H2O and ScCl3$xH2O were purchased from Jining Tianyi New Materials CO. Ltd. NaCl, NH4F, Ethylene glycol (EG) and Branched polyethyle-nimine (PEI) were purchased from Sinopharm chemical reagent Co. Ltd. All chemicals were used directly without further purification. 2.2. Material synthesis Series NaScF4:Yb3þ/Er3þ UCNPs were prepared by a facile hydrothermal route [21]. Herein, 2 mmol of ScCl3$xH2O, YbCl3$6H2O and ErCl3$6H2O with the molar ratio of 78:20:2 were dissolved in 30 ml EG solution and stirred, then 0.3 g PEI were added and stirred for 30 min. Then, 4 mmol NaCl and 4 mmol NH4F were added and stirred in continuously until all the solids were dissolved and form a uniform solution. Then, the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 200 C for 2 h, 2.5 h, 3 h, 4 h and 12 h, respectively. When the autoclave cooled down to the room temperature naturally, the products were collected and further purified using ethanol and deionized water several times, and finally dried at 60 C for 24 h.
The XRD results of NaScF4:Yb3þ/Er3þ nanocrystals synthesized with different reaction time from 2 h to 12 h are shown in Fig. 1. All the sharp and strong diffraction peaks could be consistent with the hexagonal NaScF4 phase's standard XRD pattern (JCPDS Card No. 20-1152). When the reaction time was reduced, there was a little monoclinic ScF3 phase (JCPDS Card No. 46-1243) formed according to the result of XRD. Fig. 2 shows TEM and SEM images, and the EDS pattern of NaScF4:Yb3þ/Er3þ UNCPs synthesized in 2 h, 3 h and 12 h, respectively. The nanoparticles are coated by small particles, whose average diameter are about 90 nm. This nanoparticles were synthesized in 2 h, as shown in Fig. 2(a) and (b). When the reaction time is extended to 3 h, the nanoparticle surface still has some defects, and is not smooth as seen in Fig. 2(c). The nanoparticles became nanoballs with smooth surfaces when the reaction time is increased to 12 h, and the size of the nanoballs reaches 50 nm, as shown in Fig. 2 (e). SEM images of NaScF4:Yb3þ/Er3þ UCNPs with the reaction times of 2 h and 12 h are shown in Fig. 2(g) and (h), respectively. The surface of these particles is very clearly shown in SEM images. When the reaction time is longer, the surface became smoother. In the EDS spectrum, shown in Fig. 2 (i), the elemental components of Yb, Er, F, Sc and Na peaks are clearly presented. The ratio of the relative content of these elements is not in agreement with the molecular formula of NaScF4, which means that other byproducts were also synthesized. Even then, the emission properties of the products are not influenced. Fig. 3(a) shows the UC luminescence spectra of NaScF4:Yb3þ/ 3þ Er UCNPs synthesized with a different reaction time under 980 nm excitation. There are three typical UC emission peaks at 520 nm (green), 541 nm (green) and 652 nm (red), which derives from the transitions of 2H11/2 / 4I15/2, 4S3/2 / 4I15/2, 4F9/2 / 4I15/2, respectively of Er3þions in Fig. 5. Strikingly, the red emission around 652 nm gradually becomes much stronger than the green emission around 541 nm when the reaction time decreases from 12 to 2 h, which are clearly observed with the naked eye. The out colour shift from green to red is related to the morphology of particles shown in the XRD spectra and TEM imaging. Increasing the reaction time, the particle size does not change significantly, while its surface becomes gradually smoother and with an increase in crystallinity, resulting in fewer defects in the particles. The reaction time is thus increased and the obtained particles' crystallinity is improved, which results in fewer defects in these particles. On the contrary, the particles synthesized in less reaction time possess more defects, which also increase the nonradiative
L. Zhang et al. / Journal of Alloys and Compounds 699 (2017) 1e6
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Fig. 2. TEM images and size distributions of NaScF4:Yb3þ/Er3þ nanocrystals with reaction time of 2 h(a)-(b), 3 h(c)-(d), 12 h(e)-(f) respectively; (g) and (h) SEM image of NaScF4:Yb3þ/Er3þ nanocrystals with the reaction time of 2 h and 12 h, respectively; (i) EDX image of NaScF4:Yb3þ/Er3þ nanocrystals synthesized in 12 h.
relaxation probability. These defects also diminish the population in the 2H11/2 and 4S3/2 levels, but also enhance the population in the 4 F9/2 energy level of Er3þ. As a result, the red emission is much stronger than the green emission. As shown in Fig. 3(b), fluorescent emission was tuned from green to red by adjusting the reaction time are clearly observed with the naked eye. Also, the corresponding CIE chromaticity coordinate (x, y) shifts from (0.2826, 0.6912) to (0.5103, 0.4710) as the reaction time decreases from 12 h to 2 h as shown in Table 1. In order to further understand the UC mechanism used in preparing NaScF4:Yb3þ/Er3þ UCNPs in 2 h and 12 h, the relationship of
the dependence of the UC emission intensity and the pumping power was measured and plotted in Fig. 4. For the UC process, the number of photons required to populate the upper excited state can be expressed by the relationship IfPn, where I is the UC emission intensity, P is the pumping power, and n is the number of pump photons required [22]. The slopes n of the green and red emissions (1.91 and 1.70, respectively), for the NaScF4:Yb3þ/Er3þ nanocrystals synthesized for 2 h are shown in Fig. 4(a). Likewise, the slopes for the NaScF4:Yb3þ/Er3þ nanocrystals with the reaction time of 12 h are shown in Fig. 4(b), with the values n of the green and red emissions being 1.56 and 1.42, respectively. This means that the
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L. Zhang et al. / Journal of Alloys and Compounds 699 (2017) 1e6
Fig. 3. (a) The UC luminescence spectra of NaScF4:Yb3þ/Er3þ UCNPs with synthesized reaction time 12-2 h under 980 nm excitation. (b) The calculated CIE chromaticity coordinate (x, y) of NaScF4:Yb3þ/Er3þ nanocrystals with different reaction time.
Table 1 The calculated CIE chromaticity coordinates (x, y) of NaScF4:Yb3þ/Er3þ UCNPs with different reaction time.
4 I13/2, then the 4I13/2 level of Er3þ ions absorb a photon from excited Yb3þ ions, and are thereby excited to the 4F9/2 level [23].
CIE chromaticity coordinate The reaction time (hour)
x
y
12 4 3 2.5 2
0.2826 0.4009 0.4541 0.4669 0.5103
0.6912 0.5756 0.5230 0.5107 0.4710
green and red emissions for both of the products are two photons processes. From the schematic diagram of the UC process in Fig. 5, we can easily deduce the UC mechanism: two successive energy transfers from Yb3þ ions excite Er3þ ions to the 4F7/2 level, then the Er3þ ions relax to the 2H11/2 and 4S3/2 levels, due to the non-radiative process, followed by the green emission. The red emission of the 4F9/2 level can also be populated, either from the upper 2H11/2 and 4S3/2level by a non-radiative process, or by non-radiative relaxation from 4I11/2 to
4. Conclusions In summary, a series NaScF4:Yb3þ/Er3þ nanoparticles have been synthesized by a simple and facile hydrothermal route. The out colour from green to red in NaScF4:Yb3þ/Er3þ nanoparticles is successfully obtained by adjusting the reaction time from 12 h to 2 h. The apparent upconversion emission peaks at 541 nm and 652 nm of all samples correspond to the transitions of 4S3/2/2H11/ 4 4 4 3þ ions. The green UC lumines2 / I15/2 and F9/2 / I15/2 of Er cence at 541 nm decreases, while red UC luminescence increases gradually when the reaction time is changed from 12 h to 2 h, which is due to the crystallization of particles with different reaction times. When the reaction time is shorter, defects increase. This results in high nonradiative probability, and the red upconversion luminescence is enhanced. This work provides a novel and facile way to tune colour from green to red in NaScF4:Yb3þ/Er3þ UCNPs and to expand the method of tuning UC emission.
Fig. 4. Pump power dependence of the green and red emissions of (a) NaScF4:Yb3þ/Er3þ nanocrystals with reaction time of 2 h, (b) NaScF4:Yb3þ/Er3þ nanocrystals with reaction time of 12 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
L. Zhang et al. / Journal of Alloys and Compounds 699 (2017) 1e6
5
Fig. 5. A schematic diagram of the energy levels in Yb3þ,Er3þ of UC process under 980-nm excitation.
Declaration of interest The authors declare no conflict of interest. Author contributions Zhang Ligang conducted the experiments and wrote the initial draft of the manuscript. Suling Zhao contributed to the characterization of the materials and revised the manuscript. All of authors contributed to the discussions.
[7]
[8]
[9]
[10]
Acknowledgments We express thanks to the National High Technology Research and Development Program of China (863 Program) under Grant No. 2013AA032205; the National Natural Science Foundation of China under Grant No. 51272022; the Research Fund for the Doctoral Program of Higher Education Grant No. 20120009130005; and the Fundamental Research Funds for the Central Universities with Grant No.2012JBZ001.
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microspheres: NIR-light-activated photopolymerization and drug delivery, Chem. Commun. 49 (2013) 1527e1529. Lele Li, Peiwen Wu, Kevin Hwang, Yi Lu, An exceptionally simple strategy for DNA-functionalized upconversion nanoparticles as biocompatible agents for nanoassembly, DNA delivery, and imaging, J. Am. Chem. Soc. 135 (2013) 2411e2414. Dangli Gao, Xiangyu Zhang, Hairong Zheng, Wei Gao, Enjie He, Yb3þ/Er3þ codoped b-NaYF4 microrods: synthesis and tuning of multicolor upconversion, J. Alloy Compd. 554 (2013) 395e399. Gan Tian, Zhanjun Gu, Liangjun Zhou, Wenyan Yin, Xiaoxiao Liu, Liang Yan, Shan jin, Wenlu Ren, Gengmei Xing, Shoujian Li, Yuliang Zhao, Mn2þ dopantcontrolled synthesis of NaYF4:Yb/Er upconversion nanoparticles for in vivo imaging and drug deliver, Adv. Mater. 24 (2012) 1226e1231. Monika Rai, Sunil Kumar Singh, Akhilesh Kumar Singh, Ritika Prasad, Biplob Koch, Kavita Mishra, Shyam Bahadur Rai, Enhanced red upconversion emission, magneto-luminescent behavior, and bio-imaging application of NaSc0.75Er0.02Yb0.18Gd0.05F4@AuNPs nanoparticles, ACS Appl. Mater. Inter. 7 (2015) 15339e15350. Daqin Chen, Zhongyi Wan, Yang Zhou, Ping Huang, Zhenguo Ji, Ce3þ dopantsinduced spectral conversion from green to red in the Yb/Ho: NaLuF4 selfcrystallized nano-glass-ceramics, J. Alloy Compd. 654 (2016) 151e156. Wei Gao, Jun Dong, Jihong Liu, Xuewen Yan, Enhancement of red upconversion emission of cubic phase NaLuF4: Yb3þ/Ho3þ/Ce3þ nanocrystals, Mater. Res. Bull. 80 (2016) 256e262. Wei Zhu, Suling Zhao, Zhiqin Liang, Yongxin Yang, Junjie Zhang, Zheng Xu, The color tuning and mechanism of upconversion emission from green to red in NaLuF4:Yb3þ/Ho3þ nanocrystals by codoping with Ce3þ, J. Alloy Compd. 659 (2016) 146e151. Wei Gao, Hairong Zheng, Enjie He, Ying Lu, Fangqi Gao, Luminescence investigation of Yb3þ/Er3þ codoped single LiYF4 microparticle, J. Lumin. 152 (2014) 44e48. Wei Gao, Jun Dong, Jihong Liu, Xuewen Yan, Effective tuning of the ratio of red to green emission of Ho3þ ions in single LiLuF4 microparticle via codoping Ce3þ ions, J. Alloy Compd. 679 (2016) 1e8. S. Hao, L. Sun, G. Chen, H. Qiu, C. Xu, T.N. Soitah, Y. Sun, C. Yang, Synthesis of Monoclinic Na3ScF6:1 mol % Er3þ/2 mol %Yb3þMicrocrystals by a facile hydrothermal approach, J. Alloy Compd. 522 (2012) 74e77. Yu Ai, Datao Tu, Wei Zheng, Yongsheng Liu, Jintao Kong, Ping Hu, Zhuo Chen, Mingdong Huang, Xueyuan Chen, Lanthanide-doped NaScF4 nanoprobes: crystal structure,optical spectroscopy and biodetection, Nanoscale 5 (2013) 6430e6438. Xue Teng, Yihan Zhu, Wei Wei, Shuchao Wang, Jingfeng Huang, Rafik Naccache, Wenbo Hu, Alfred Iing Yoong ok, Yu Han, Qichun Zhang, Quli Fan, Wei Huang, John A. Capobianco, Ling Huang, Lanthanide-doped NaxScF3þx nanocrystals: crystal structure evolution and multicolor tuning, J. Am. Chem. Soc. 134 (2012) 8340e8343. Wen-Bo Pei, Bo Chen, Lili Wang, Jiansheng Wu, Xue Teng, Raymond Lau,
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Ling Huang, Wei Huang, NaF-mediated controlled-synthesis of multicolor NaxScF3þx:Yb/Er upconversion nanocrystals, Nanoscale 7 (2015) 4048e4054. [20] Min Pang, Xuesong Zhai, Jing Feng, Shuyan Song, Ruiping Deng, Zhuo Wang, Shuang Yao, Xin Gea, Hongjie Zhang, One-step synthesis of water-soluble hexagonal NaScF4:Yb/Er nanocrystals with intense red emission, Dalton Trans. 43 (2014) 10202e10207. [21] Jingjing Cao, Li Yuan, Shanshan Hu, Jianfeng Tang, Xianju Zhou, Jun Yang, Tuning the phase, morphology and size of monodisperse ScF3 and NaScF4
crystals through lanthanide doping, CrystEngComm 18 (2016) 5940e5951. [22] H. Lin, G. Meredith, S.B. Jiang, X. Peng, T. Luo, N. Peyghambarian, E.Y.B. Pun, Optical transitions and visible upconversion in Er3þ doped niobic tellurite glass, J. Appl. Phys. 93 (2003) 186e191. [23] Liang Zhi-Qin, Su-Ling Zhao, Yue Cui, Li-Jiao Tian, Jun-Jie Zhang, Zheng Xu, Phase transformation and morphology tuning of b-NaYF4:Yb3þ,Er3þ nanocrystals through Kþ ions codoping, Chin. Phys. B 24 (2015).
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The colour tuning of upconversion emission from green to red in NaScF4:Yb3þ/Er3þ nanocrystals by adjusting the reaction time Ligang Zhang a, b, c, Suling Zhao a, b, *, Zhiqin Liang a, b, Junjie Zhang a, b, Wei Zhu a, b, Pu Liu c, Hongkai Sun c a b c
Key Laboratory of Luminescence and Optical Information, Beijing Jiaotong University, Ministry of Education, Beijing 100044, China Institute of Optoelectronics Technology, Beijing Jiaotong University, Beijing 100044, China School of Science, Hebei University of Architecture, Hebei, Zhangjiakou 075000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 August 2016 Received in revised form 16 November 2016 Accepted 16 December 2016 Available online 18 December 2016
A series of water-soluble NaScF4:Yb3þ/Er3þ upconversion nanoparticles (UCNPs) have been synthesized by a facile hydrothermal route. The samples display a high degree of uniformity and crystallization. The upconversion (UC) emissions from green to red in NaScF4:Yb3þ/Er3þ nanoparticles are successfully obtained by adjusting the reaction time from 12 h to 2 h. The UC emission peaks at 541 nm, and 652 nm of all the samples excited by 980 nm correspond to the transitions of 4S3/2/2H11/2 / 4I15/2 and 4F9/2 / 4I15/2 of Er3þions, respectively. The green UC emission at 541 nm decreases while the red emission at 652 nm increases gradually when the reaction time changes from 12 h to 2 h. This work provides a novel and facile way of tuning colour from green to red in NaScF4:Yb3þ/Er3þ UCNPs. These water-soluble nanoball structures NaScF4:Yb3þ/Er3þ UCNPs promise to be novel biological probes and possess other applications in many fields. © 2016 Elsevier B.V. All rights reserved.
Keywords: NaScF4 Upconversion Reaction time
1. Introduction In the past two decades, rare-earth ion-doped materials have received considerable attention due to their distinctive luminescence properties [1,2]. In particular, Ln3þ doped UCNPs, in which two or more low energy photons are sequentially absorbed and then emit a higher energy photon [1], possess many virtues, such as sharp emission band widths, low toxicity, long luminescence lifetime and high photostability, and have a wide range of potential applications in colour displays, biomarkers, drug carriers, photodynamic therapy, and photovoltaic devices [3e7]. Compared to green and blue light, red light used as a biomedical detection light has the greatest sensitivity because biotissues absorb high energy photons like blue light and emit green light. In addition, the loss of the red light signal due to scattering is reduced, owing to its relatively longer wavelength. Therefore, the red light UC emissions of Ln3þ doped UCNPs have great potential applications in the biomedical field. However, it is very difficult to obtain a pure red UC
* Corresponding author. Key Laboratory of Luminescence and Optical Information, Beijing Jiaotong University, Ministry of Education, Beijing 100044, China. E-mail address: [email protected] (S. Zhao). http://dx.doi.org/10.1016/j.jallcom.2016.12.202 0925-8388/© 2016 Elsevier B.V. All rights reserved.
spectrum because of the abundant radiative and nonradiative transition channels in the rare earth ions' 4F-4F electron state. Hitherto, it is still a necessary and important challenge to obtain UCNPs with intense red emissions. Dangli Gao et al. synthesized a series of b-NaYF4:Yb3þ/Er3þ microrods and tuned the output light colour from green to red by increasing Yb3þ ion concentrations [8]. Zhanjun Gu and coworkers synthesized NaYF4:Yb/Er nanoparticles with pure dark red emissions by Mn2þ doping [9]. Sunil Kumar Singh et al. synthesized a series of Gd:UCNPs@AuNPs and obtained a uniquely red UC emission [10]. Ho3þ is another intriguing ion for upconversion luminescence, and there are many researchers who reported the red emission of Ho3þ in host material doping with Ce3þ, such as NaYF4, NaluF4 [11e13], LiYF [14], and LiLuF4 [15]. Except for those methods used by doping different ions to produce red UC luminescence, reaction parameters such as reaction time and the reaction temperature can sometimes have a strong influence on the results of these experiments. Up to now, tuning of upconversion emission from green to red in NaScF4:Yb3þ/Er3þ nanocrystals by adjusting the reaction parameters have been rarely reports. Therefore, it is necessary to find a facile and simple way to get intense red UC emission for various applications. Scandium, as a well-known fluorophilic rare-earth element, has received little attention [16,17]. In a chemical reaction, the
2
L. Zhang et al. / Journal of Alloys and Compounds 699 (2017) 1e6
2.3. Characterization The crystallization phase of the samples was performed by x-ray power diffraction (XRD) on a D/max 2200v X-ray powder diffractometer equipped with Cu Ka radiation (l ¼ 1.540 Å). Scanning electron microscope (SEM) measurements were performed using a Hitachi S-8010 SEM equipped with an energy dispersive x-ray (EDX) spectrum. Transmission electron microscopy (TEM) images were recorded on a HEOL-3000F TEM operating at an acceleration voltage of 200 kV. The upconversion luminescence spectra were recorded on a ZOLIX fluorescence spectrometer system, while the excitation source used was an external 980 nm semiconductor laser. The fluorescence images and photographs were obtained using a Canon 1D X MARK II camera. All of the measurements were performed at room temperature.
3. Results and discussion Fig. 1. XRD results (t ¼ 2,2.5,3,4,12 h).
of
NaScF4:Yb3þ/Er3þ
synthesized
at
different
time
outermost electrons are easily lost to form a stable Sc3þ ion. Its distinct atomic electron configuration and smaller ion radius may lead to different optical properties from other rare earth elements. Recently, Huang and co-workers synthesized NaxScF3þx NCs by a solvent-thermal process and tuned the phase and composition of the NaxScF3þxNCs nanocrystals by using the ratio of oleic acid (OA) to octadecene (ODE) [18]. They also found that the phase and out colour of NaxScF3þxNCs can be controlled by NaF [19]. Zhang and coworkers synthesized NaScF4:Yb3þ/Er3þ UCNPs via a facile hydrothermal route and realized red colour emission [20]. In this paper, we synthesized NaScF4:Yb3þ/Er3þ UCNPs by a hydrothermal route, whose UC emission from green to red was tuned gradually by adjusting the reaction time. Moreover, the luminescence mechanism and the influence of reaction time were investigated in detail. 2. Experiment 2.1. Chemicals YbCl3$6H2O, ErCl3$6H2O and ScCl3$xH2O were purchased from Jining Tianyi New Materials CO. Ltd. NaCl, NH4F, Ethylene glycol (EG) and Branched polyethyle-nimine (PEI) were purchased from Sinopharm chemical reagent Co. Ltd. All chemicals were used directly without further purification. 2.2. Material synthesis Series NaScF4:Yb3þ/Er3þ UCNPs were prepared by a facile hydrothermal route [21]. Herein, 2 mmol of ScCl3$xH2O, YbCl3$6H2O and ErCl3$6H2O with the molar ratio of 78:20:2 were dissolved in 30 ml EG solution and stirred, then 0.3 g PEI were added and stirred for 30 min. Then, 4 mmol NaCl and 4 mmol NH4F were added and stirred in continuously until all the solids were dissolved and form a uniform solution. Then, the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 200 C for 2 h, 2.5 h, 3 h, 4 h and 12 h, respectively. When the autoclave cooled down to the room temperature naturally, the products were collected and further purified using ethanol and deionized water several times, and finally dried at 60 C for 24 h.
The XRD results of NaScF4:Yb3þ/Er3þ nanocrystals synthesized with different reaction time from 2 h to 12 h are shown in Fig. 1. All the sharp and strong diffraction peaks could be consistent with the hexagonal NaScF4 phase's standard XRD pattern (JCPDS Card No. 20-1152). When the reaction time was reduced, there was a little monoclinic ScF3 phase (JCPDS Card No. 46-1243) formed according to the result of XRD. Fig. 2 shows TEM and SEM images, and the EDS pattern of NaScF4:Yb3þ/Er3þ UNCPs synthesized in 2 h, 3 h and 12 h, respectively. The nanoparticles are coated by small particles, whose average diameter are about 90 nm. This nanoparticles were synthesized in 2 h, as shown in Fig. 2(a) and (b). When the reaction time is extended to 3 h, the nanoparticle surface still has some defects, and is not smooth as seen in Fig. 2(c). The nanoparticles became nanoballs with smooth surfaces when the reaction time is increased to 12 h, and the size of the nanoballs reaches 50 nm, as shown in Fig. 2 (e). SEM images of NaScF4:Yb3þ/Er3þ UCNPs with the reaction times of 2 h and 12 h are shown in Fig. 2(g) and (h), respectively. The surface of these particles is very clearly shown in SEM images. When the reaction time is longer, the surface became smoother. In the EDS spectrum, shown in Fig. 2 (i), the elemental components of Yb, Er, F, Sc and Na peaks are clearly presented. The ratio of the relative content of these elements is not in agreement with the molecular formula of NaScF4, which means that other byproducts were also synthesized. Even then, the emission properties of the products are not influenced. Fig. 3(a) shows the UC luminescence spectra of NaScF4:Yb3þ/ 3þ Er UCNPs synthesized with a different reaction time under 980 nm excitation. There are three typical UC emission peaks at 520 nm (green), 541 nm (green) and 652 nm (red), which derives from the transitions of 2H11/2 / 4I15/2, 4S3/2 / 4I15/2, 4F9/2 / 4I15/2, respectively of Er3þions in Fig. 5. Strikingly, the red emission around 652 nm gradually becomes much stronger than the green emission around 541 nm when the reaction time decreases from 12 to 2 h, which are clearly observed with the naked eye. The out colour shift from green to red is related to the morphology of particles shown in the XRD spectra and TEM imaging. Increasing the reaction time, the particle size does not change significantly, while its surface becomes gradually smoother and with an increase in crystallinity, resulting in fewer defects in the particles. The reaction time is thus increased and the obtained particles' crystallinity is improved, which results in fewer defects in these particles. On the contrary, the particles synthesized in less reaction time possess more defects, which also increase the nonradiative
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Fig. 2. TEM images and size distributions of NaScF4:Yb3þ/Er3þ nanocrystals with reaction time of 2 h(a)-(b), 3 h(c)-(d), 12 h(e)-(f) respectively; (g) and (h) SEM image of NaScF4:Yb3þ/Er3þ nanocrystals with the reaction time of 2 h and 12 h, respectively; (i) EDX image of NaScF4:Yb3þ/Er3þ nanocrystals synthesized in 12 h.
relaxation probability. These defects also diminish the population in the 2H11/2 and 4S3/2 levels, but also enhance the population in the 4 F9/2 energy level of Er3þ. As a result, the red emission is much stronger than the green emission. As shown in Fig. 3(b), fluorescent emission was tuned from green to red by adjusting the reaction time are clearly observed with the naked eye. Also, the corresponding CIE chromaticity coordinate (x, y) shifts from (0.2826, 0.6912) to (0.5103, 0.4710) as the reaction time decreases from 12 h to 2 h as shown in Table 1. In order to further understand the UC mechanism used in preparing NaScF4:Yb3þ/Er3þ UCNPs in 2 h and 12 h, the relationship of
the dependence of the UC emission intensity and the pumping power was measured and plotted in Fig. 4. For the UC process, the number of photons required to populate the upper excited state can be expressed by the relationship IfPn, where I is the UC emission intensity, P is the pumping power, and n is the number of pump photons required [22]. The slopes n of the green and red emissions (1.91 and 1.70, respectively), for the NaScF4:Yb3þ/Er3þ nanocrystals synthesized for 2 h are shown in Fig. 4(a). Likewise, the slopes for the NaScF4:Yb3þ/Er3þ nanocrystals with the reaction time of 12 h are shown in Fig. 4(b), with the values n of the green and red emissions being 1.56 and 1.42, respectively. This means that the
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Fig. 3. (a) The UC luminescence spectra of NaScF4:Yb3þ/Er3þ UCNPs with synthesized reaction time 12-2 h under 980 nm excitation. (b) The calculated CIE chromaticity coordinate (x, y) of NaScF4:Yb3þ/Er3þ nanocrystals with different reaction time.
Table 1 The calculated CIE chromaticity coordinates (x, y) of NaScF4:Yb3þ/Er3þ UCNPs with different reaction time.
4 I13/2, then the 4I13/2 level of Er3þ ions absorb a photon from excited Yb3þ ions, and are thereby excited to the 4F9/2 level [23].
CIE chromaticity coordinate The reaction time (hour)
x
y
12 4 3 2.5 2
0.2826 0.4009 0.4541 0.4669 0.5103
0.6912 0.5756 0.5230 0.5107 0.4710
green and red emissions for both of the products are two photons processes. From the schematic diagram of the UC process in Fig. 5, we can easily deduce the UC mechanism: two successive energy transfers from Yb3þ ions excite Er3þ ions to the 4F7/2 level, then the Er3þ ions relax to the 2H11/2 and 4S3/2 levels, due to the non-radiative process, followed by the green emission. The red emission of the 4F9/2 level can also be populated, either from the upper 2H11/2 and 4S3/2level by a non-radiative process, or by non-radiative relaxation from 4I11/2 to
4. Conclusions In summary, a series NaScF4:Yb3þ/Er3þ nanoparticles have been synthesized by a simple and facile hydrothermal route. The out colour from green to red in NaScF4:Yb3þ/Er3þ nanoparticles is successfully obtained by adjusting the reaction time from 12 h to 2 h. The apparent upconversion emission peaks at 541 nm and 652 nm of all samples correspond to the transitions of 4S3/2/2H11/ 4 4 4 3þ ions. The green UC lumines2 / I15/2 and F9/2 / I15/2 of Er cence at 541 nm decreases, while red UC luminescence increases gradually when the reaction time is changed from 12 h to 2 h, which is due to the crystallization of particles with different reaction times. When the reaction time is shorter, defects increase. This results in high nonradiative probability, and the red upconversion luminescence is enhanced. This work provides a novel and facile way to tune colour from green to red in NaScF4:Yb3þ/Er3þ UCNPs and to expand the method of tuning UC emission.
Fig. 4. Pump power dependence of the green and red emissions of (a) NaScF4:Yb3þ/Er3þ nanocrystals with reaction time of 2 h, (b) NaScF4:Yb3þ/Er3þ nanocrystals with reaction time of 12 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. A schematic diagram of the energy levels in Yb3þ,Er3þ of UC process under 980-nm excitation.
Declaration of interest The authors declare no conflict of interest. Author contributions Zhang Ligang conducted the experiments and wrote the initial draft of the manuscript. Suling Zhao contributed to the characterization of the materials and revised the manuscript. All of authors contributed to the discussions.
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Acknowledgments We express thanks to the National High Technology Research and Development Program of China (863 Program) under Grant No. 2013AA032205; the National Natural Science Foundation of China under Grant No. 51272022; the Research Fund for the Doctoral Program of Higher Education Grant No. 20120009130005; and the Fundamental Research Funds for the Central Universities with Grant No.2012JBZ001.
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