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NaYF4:Yb3+/Er3+ nanocrystals were prepared using a hydrothermal method. The products were characterized by XRD, SEM, TEM, HRTEM, and photoluminescence (FL). The XRD results indicated that the sample’s phase transformed from cubic to hexagonal when EDTA was added at low reaction temperatures. The SEM results revealed that the products were transformed from granular to rod-shaped with an increase in temperature. TEM images revealed the diameter of sample prepared at 120°C is about 10-50nm. HRTEM showed that crystals prepared at 120°C grew along {111} surfaces. The PL results indicated that samples prepared at 180°C without EDTA have the strongest luminescence property.
© 2016 Optical Society of America
1. Introduction
Lanthanide-doped fluoride-based upconverting nanoparticles (UCNPs) have garnered significant interest in recent years, as they can be prepared and are expected to be integrated into applications ranging from display development to in vitro and in vivo imaging [1–3]. Upconversion materials based on anti-Stokes fluorescence have been developed quickly. Compared to organic dyes and quantum dots, UCNPs have many potential advantages, such as weak background fluorescence, deep detection range, high emission intensity, low toxicity, good stability, great application prospects, and cause less photo damage to living organisms [4–8]. Developing new synthetic technologies and related surface modification approaches for these luminescent nanomaterials is important for fundamental research and practical applications.
Among upconversion materials, hexagonal β-phase sodium yttrium fluoride (β-NaYF4) co-doped with Yb3+-Er3+ or Yb3+-Tm3+ ion couples, which have low phonon energies (<400 cm−1) and a high refractive index (1.430–1.470 m2 W−1), are one of the most efficient near-infrared-to-visible materials [9,10]. The green luminescence of NaYF4:Yb/Er appears from the 2H11/2/4S3/2 levels via a two-photon process with sequential energy transfer wherein Er is excited first from 4I15/2 to 4I11/2 and, subsequently, from 4I11/2 to 4F7/2, which relaxes nonradiatively to the thermalized emitting-state manifold of 2H11/2/4S3/2 [11].
Preparation of high-quality β-phase NaYF4 upconversion materials under simple experimental conditions has received worldwide attention, and synthesizing good quality rare earth ion co-doped NaYF4 UCNPs is of significance. Wei [12] synthesized NaYF4:Yb/Ho UCNPs using a simple, inexpensive precipitation method. However, the sizes of the samples prepared using this method were not uniform and the luminescent property is weak. Du [13] and Wei [14] and other researchers have prepared UCNPs using a thermal decomposition method, but this process is complex and harsh. A hydrothermal method is a convenient way to prepare NaYF4 microcrystals, and sample size can be controlled [15,16]. Many researchers have used different ligands to determine the importance of various factors during preparation of UCNPs [17]. Samples with different morphologies can be obtained by adding different compounds, changing the solvents, and adjusting reaction temperature. However, few studies have reported on the effects of EDTA on phase evolution of β-NaYF4 crystals, and the corresponding mechanism is unclear.
In this study, polyacrylic acid (PAA) was used as the main sample solvent, and the entire reaction time was 24 h. We initially adjusted temperature from 120 to 180°C and determined that temperature could be used to control size, phase, and morphology of NaYF4:Yb3+/Er3+ samples. We were interested in determining the function of EDTA, which is known to affect particle morphology. Many researchers use EDTA to control sample morphology and size [18,19]. However, we determined that EDTA promoted phase transformation of samples from cubic to hexagonal, which may provide a new method to produce samples with strong luminescence property at a low temperature. The morphologies and phases of the products are summarized in Table 1. Table 1 shows that temperature and EDTA have significant effects on the morphologies and phases of the final products.
Table 1. Summary of experimental conditions and corresponding sample morphologies and phases.
2. Experimental
2.1 Chemicals
All chemical reagents were analytical grade and were used without further purification. The water used was distilled or deionized, and absolute ethanol, Ln2O3 (>99.9%), including yttrium oxide (Y2O3), ytterbium oxide (Yb2O3), and erbium oxide (Er2O3) were purchased from Shanghai Yuelong Non-Ferrous Metals Ltd. (Shanghai, China). Stearic acid (A.R., 99%), PAA (A.R. 98%), and trichloromethane (99%) were purchased from Xiya Chemical Industry Co., Ltd. (Shandong, China). NaF (A.R., 98%) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). EDTA (A.R., 99%) was purchased from Ruijinte Chemical Co., Ltd. (Tianjin, China), and NaOH (A.R., 96%) was purchased from Tieta Chemical Co., Ltd. (Yantai, China).
2.2 Preparation
First, 3.9 mM Y2O3, 1.0 mM Yb2O3, and 0.1 mM Er2O3 were dissolved in concentrated nitric acid, heated, and stirred constantly until the liquid evaporated. Absolute ethanol (80 mL) was added to form a solution and stirred for 10 min. The solution was poured into a three-armed flask (500 mL), 30 mM stearic acid was added, and heated in a water bath with continuous stirring. Temperature was adjusted to 50°C until all components had dissolved. Then, temperature was increased to 78°C with vigorous stirring. Another 20 mL of absolute ethanol was added with 30 mM NaOH, and the solution refluxed for about 1 h. Precursor was got by pumping filtration and drying at 60°C for 12h.
Then, 10 mL H2O, 15 mL absolute ethanol, and 5 mL PAA were added to a beaker, containing 1 mM rare earth stearates and 5 mM NaF (and 0.1g EDTA if needed). Ultrasound was used to form a solution, which was transferred to a 50 mL Teflon bottle and heated for 24 h. The supernatant was removed. The samples were collected after centrifugation, washed several times with trichloromethane, absolute ethanol, and deionized water, and dried under air at 60°C for 12 h.
2.3 Characterization
Sample structure was investigated by X-ray diffraction (XRD) using Cu Kα radiation (λ = 0.15418). The 2θ angle ranged from 10° to 80°. Size, morphology, and dispersivity of the samples were determined by scanning electron microscopy (SEM) (JSM-6700F; JEOL, Tokyo, Japan). Transmission electron microscopy and high resolution-transmission electron microscopy (HR-TEM) were performed on a JEOL JEM-2010 instrument at an accelerating voltage of 200 kV. The upconversion spectra of the samples were obtained with a fluorescence spectrometer (F-4600; Hitachi, Tokyo, Japan), and the excitation source was a 980 nm laser diode. All samples were examined under the same conditions to compare their luminescent intensities. All measurements were taken at room temperature.
3. Results and discussion
3.1 Effect of reaction temperature
The XRD patterns of the samples prepared under different reaction temperatures are shown in Fig. 1, which clearly demonstrates the cubic and hexagonal structures of NaYF4:Yb3+/Er3+. Some researchers have controlled crystallization of the nanocrystals and transformed them from the cubic to the hexagonal phase by increasing reaction time or adjusting concentrations of the precursors [20,21]. The XRD patterns show that the phase of NaYF4 was almost all cubic when temperature was 120°C, and that the hexagonal phase only represented a small percentage of NaYF4, Fig. 1(a). The hexagonal to cubic phase ratio increased significantly when temperature was increased to 150°C and most of the sample was in the hexagonal phase, Fig. 1(b), compared with the literature values (JCPDS nos. 77-2042 and 28-1192). Crystals in all samples were transformed from cubic to hexagonal at a reaction temperature of 180°C. The diffraction peaks of the samples prepared at 180°C, Fig. 1(c), were indexed as a pure hexagonal phase, which coincides well with the literature value (JCPDS no. 28-1192).
Fig. 1 X-ray diffraction (XRD) patterns of NaYF4:Yb3+/Er3+ prepared at temperatures of (a) 120°C, (b) 150°C, and (c) 180°C. The standard cards of α-NaYF4 (PDF file no. 77-2042) and β-NaYF4 (PDF file no. 28-1192) are provided as references.
Figure 2 shows the morphologies of samples prepared under different temperatures. The samples obtained at 120°C were granular (diameter, 10–50 nm; Fig. 2(a) and 2(d)) with good dispersivity and homogeneity. The HR-TEM images, Fig. 2(e) and 2(f), revealed lattice fringes of 0.32 nm, which agree well with the value of 0.316 nm calculated from XRD data (JCPDS card no. 77-2042) and corresponded to the (111) lattice planes of cubic NaYF4, which clearly shows that the crystals grew along the {111} surfaces. The samples changed to rod-shaped when the reaction temperature was increased 150°C. However, some irregular particles remained, Fig. 2(b). Sample morphology changed completely from granular to prisms when reaction temperature was increased further to 180°C, Fig. 2(c). Notably, the top of the prism was not planar but looked more like a crown. This may have occurred because the adjacent monomers were consumed, which decreased their concentrations near the prism. Thus, the monomers in other locations would move toward the prism and would meet first at the top corner of the prism. Thus, the top corner would grow faster than other locations, and the prism would form into a crown shape [22].
Fig. 2 Scanning electron microscopic (SEM) images of NaYF4 at different reaction temperatures of (a) 120°C, (b) 150°C, and (c) 180°C. Transmission electron microscopic image (TEM) of NaYF4 prepared at (d) 120°C and a high resolution-transmission electron microscopic (HR-TEM) image of NaYF4 prepared at (e) (f) 120°C.
3.2 Effect of EDTA
Notably, EDTA promoted phase transformation from cubic to hexagonal, Fig. 3, which has rarely been reported.
Fig. 3 X-ray diffraction (XRD) patterns of NaYF4:Yb3+/Er3+ (a) samples without EDTA at 120°C, (b) with EDTA at 120°C, (c) without EDTA at 150°C, and (d) with EDTA at 150°C. The standard cards for α-NaYF4 (PDF file no. 772042) and β-NaYF4 (PDF file no. 281192) are provided as references.
The XRD pattern clearly demonstrates that almost all samples were in the cubic phase when the reaction was performed without EDTA at 120°C, Fig. 3(a). The phase transformed from cubic to hexagonal when 0.1 g EDTA was added to the reaction system, as shown by the XRD pattern, Fig. 3(b). This phase transformation was also observed when temperature was increased 150°C, Fig. 3(c) and 3(d), and may have occurred due to the solid-liquid reaction mechanism. Re3+ is released freely from an alcohol/water/PAA system when the precursor is heated. The Re3+ reacts with Na+ and F− at the interface between alcohol/water and PAA. However, Re3+ would complex with EDTA after EDTA was added and would require more time to escape from complexing, which would allow the Re3+ to be heated longer than before and it would react with Na+ and F−. The energy of Re3+, Na+, and F− would subsequently increase, tend to transform to the hexagonal phase, and develop a microrod morphology.
Adding EDTA to the hydrothermal reaction produced more regularly shaped hexagonal microrods, Fig. 4. Some researchers have controlled the size and morphology of nanocrystals using EDTA as a chelating agent [23,24]. Our results show that the crown morphology transformed to a smooth prism with a length of about 1.5 µm after adding EDTA. The morphology was transformed from granular to rod shaped at 120°C when EDTA was added, Fig. 4(a).
Fig. 4 Scanning electron microscopic (SEM) images of NaYF4:Yb3+/Er3+ with EDTA at different reaction temperatures of (a) 120°C, (b) 150°C, and (c) 180°C.
Adding EDTA at reaction temperatures of 150 and 180°C produced perfect, smooth-surfaced hexagonal prisms from the microrods. EDTA reduces particle size due to its chelating action [25] and prevents tiny particles from aggregating, as seen during the crown-forming process. EDTA stops the monomers from reacting at locations near the hexagonal prisms. Otherwise, the samples formed standard smooth-surfaced hexagonal prisms. The progression of this reaction is shown in Fig. 5.
3.3 Upconversion luminescence
Figure 6(a)-6(c) show that intensity strengthened when reaction temperature was increased. The properties of luminescent materials can be changed by controlling their phase, shape, and particle size. NaYF4:Yb3+/Er3+ samples in the hexagonal phase had a much stronger luminescent property than that of cubic phase NaYF4:Yb3+/Er3+. Thus, samples with a greater proportion of the hexagonal phase had much higher fluorescence intensities when the reaction temperature was increased.
Fig. 6 NaYF4:Yb3+/Er3+ upconversion photoluminescence spectra at different reaction temperatures of (a) 120°C, (b) 150°C, and (c)180°C.
EDTA plays an important role in phase transformation and the change in morphology, as shown by our results. Fluorescent intensity was stronger after adding EDTA than that of samples prepared at 120°C before EDTA was added, Fig. 7(a) and 7(b), because the phase of as-obtained samples changed from cubic to hexagonal.
Fig. 7 Upconversion photoluminescence spectra of samples prepared (a) without EDTA at 120°C, (b) with EDTA at 120°C, (c) without EDTA at 150°C, (d) with EDTA at 150°C, (e) without EDTA at 180°C, and (f) with EDTA at 180°C.
However, although products prepared with EDTA at 150°C changed completely to the hexagonal phase, their luminescent properties decreased, Fig. 7(c) and 7(d). The as-obtained samples without EDTA and reacted at 180°C had stronger luminescent intensity than those prepared with EDTA, Fig. 7(e) and 7(f), because the as-obtained samples may have absorbed organic molecules and ions on their surfaces, which decreases upconversion emission intensity because of faster nonradiative decay [26].
Samples prepared with EDTA at 120, 150 and 180°C, Fig. 7(b), 7(d) and 7(f), were transformed to the hexagonal phase but emitted different luminescent intensities. Products synthesized under a lower temperature were irregularly shaped with rough surfaces, representing the many defects in the host lattice. Samples prepared with EDTA at 120°C were rods not hexagonal prisms, Fig. 3(a), which may be related to low crystallinity and the large number of crystal defects. The energy for excitation is transferred easily to the quenching trap because the excited center of luminescence is close to the trap. High reaction temperature favors the formation of perfect crystals [27].
4. Conclusion
NaYF4:Yb3+/Er3+ was synthesized using a hydrothermal method. Samples with good dispersivity and a uniform size were prepared in an alcohol/water/PAA system. Higher temperatures promoted a phase transition and controlled the morphology of NaYF4:Yb3+/Er3+. Adding 0.1 g EDTA also promoted a phase transition and controlled the morphology, as shown by the XRD patterns, the SEM, TEM, and HRTEM images, and the luminescence spectra. The luminescence spectra also revealed that a perfect hexagonal phase, higher reaction temperature, and fewer defects were key factors associated with a stronger luminescent property.
Acknowledgments
This research was supported by the National Natural Science Foundation of China (Grants: 51372127 and 51072086)
References and links
1. F. Wang and X. Liu, “Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals,” Chem. Soc. Rev. 38(4), 976–989 (2009). [CrossRef] [PubMed]
2. R. Scheps, “Upconversion laser processes,” Prog. Quantum Electron. 20(4), 271–358 (1996). [CrossRef]
3. E. Downing, L. Hesselink, J. Ralston, and R. Macfarlance, “A three-color, solid-state, three-dimensional display,” Science 273(5279), 1185–1189 (1996). [CrossRef]
4. B. Liu, C. Li, D. Yang, Z. Hou, M. Ping’an, Z. Cheng, H. Lian, S. Huang, and J. Lin, “Upconversion-luminescent core/mesoporous-silica-shell-structured β-NaYF4:Yb3+,Er3+@SiO2@mSiO2 composite nanospheres: fabrication and drug-storage/release properties,” Eur. J. Inorg. Chem. 42(11), 1906–1913 (2014). [CrossRef]
5. D. K. Chatterjee, A. J. Rufaihah, and Y. Zhang, “Upconversion fluorescence imaging of cells and small animals using lanthanide doped nanocrystals,” Biomaterials 29(7), 937–943 (2008). [CrossRef] [PubMed]
6. Q. Ju, D. Tu, Y. Liu, R. Li, H. Zhu, J. Chen, Z. Chen, M. Huang, and X. Chen, “Amine-functionalized lanthanide-doped KGdF4 nanocrystals as potential optical/magnetic multimodal bioprobes,” J. Am. Chem. Soc. 134(2), 1323–1330 (2012). [CrossRef] [PubMed]
7. D. Tu, L. Liu, Q. Ju, Y. Liu, H. Zhu, R. Li, and X. Chen, “Time-resolved FRET biosensor based on amine-functionalized lanthanide-doped NaYF4 nanocrystals,” Angew. Chem. Int. Ed. Engl. 50(28), 6306–6310 (2011). [CrossRef] [PubMed]
8. Z. Li, W. Park, G. Zorzetto, J. S. Lemaire, and C. J. Summers, “Synthesis protocols for δ-doped NaYF4:Yb,Er,” Chem. Mater. 26(5), 1770–1778 (2014).
9. D. K. Ma, S. M. Huang, Y. Y. Yu, Y. F. Xu, and Y. Q. Dong, “Rare-earth-ion-doped hexagonal-phase NaYF4 nanowires:controlled synthesis and luminescent properties,” PhysChemComm 113(19), 8136–8142 (2009).
10. D. Li, Q. Shao, Y. Dong, and J. Jiang, “Anomalous temperature-dependent upconversion luminescence of small-sized NaYF4:Yb3+,Er3+ nanoparticles,” PhysChemComm 118(39), 22807–22813 (2014).
11. B. R. Anderson, S. J. Smith, P. S. May, and M. T. Berry, “Revisiting the NIR-to-visible upconversion mechansim in β-NaYF4:Yb3+Er3+,” Phys. Chem. Lett 5(5), 36–42 (2013).
12. Y. Wei, F. Q. Lu, X. R. Zhang, and D. P. Chen, “Synthesis and characterization of efficient near-infrared upconversion Yb and Tm codoped NaYF4 nanocrystal reporter,” Alloys Compd. 427(1–2), 333–340 (2007). [CrossRef]
13. Y. P. Du, Y. W. Zhang, Z. G. Yan, L. D. Sun, S. Gao, and C. H. Yan, “Single-crystalline and near-monodispersed NaMF3 (M = Mn, Co, Ni, Mg) and LiMAlF6 (M = Ca, Sr) nanocrystals from cothermolysis of multiple trifluoroacetates in solution,” Chem. Asian J. 2(8), 965–974 (2007). [CrossRef] [PubMed]
14. Y. Wei, F. Q. Lu, X. R. Zhang, and D. P. Chen, “Synthesis of oil-dispersible hexagonal-phase and hexagonal-shape NaYF4:Yb,Er nanoplates,” Chem. Mater. 18(24), 5733–5737 (2006). [CrossRef]
15. J. Zhao, Y. Sun, X. Kong, L. Tian, Y. Wang, L. Tu, J. Zhao, and H. Zhang, “Controlled synthesis, formation mechanism, and great enhancement of red upconversion luminescence of NaYF4:Yb3+, Er3+ nanocrystals/submicroplates at low doping level,” J. Phys. Chem. B 112(49), 15666–15672 (2008). [CrossRef] [PubMed]
16. F. Zhang, Y. Wan, T. Yu, F. Q. Zhang, Y. F. Shi, S. H. Xie, Y. G. Li, B. Tu, and D. Y. Zhao, “Uniform nanostructured arrays of sodium rare-earth fluorides for highly efficient multicolor upconversion luminescence,” Angew. Chem. 119(42), 8122–8125 (2007). [CrossRef]
17. Suli Wu, Ye Liu, Jie Chang, and S. Zhang*, “Ligand dynamic effect on phase and morphology control of hexagonal NaYF4,” RSC. 16(21), 4472–4477 (2014).
18. C. Li, Z. Quan, J. Yang, P. Yang, and J. Lin, “Highly uniform and monodisperse β-NaYF4:Ln3+ (Ln = Eu, Tb, Yb/Er, and Yb/Tm) hexagonal microprism crystals: hydrothermal synthesis and luminescent properties,” Inorg. Chem. 46(16), 6329–6337 (2007). [CrossRef] [PubMed]
19. Z. J. Wang, F. Tao, L. Z. Yao, W. L. Cai, and X. G. Li, “Selected synthesis of cubic and hexagonal NaYF4 crystal via a complex-assisted hydrothermal route,” J. Cryst. Growth 290(1), 296–300 (2006). [CrossRef]
20. C. Liu, H. Wang, X. Zhang, and D. Chen, “Morphology-and phase-controlled synthesis of mono disperse lanthanide-doped NaGdF4 nanocrystals with multicolor photoluminescence,” J. Mater. Chem. 19(4), 489–496 (2009). [CrossRef]
21. F. Wang, Y. Han, C. S. Lim, Y. Lu, J. Wang, J. Xu, H. Chen, C. Zhang, M. Hong, and X. Liu, “Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping,” Nature 463(7284), 1061–1065 (2010). [CrossRef] [PubMed]
22. X. Liang, X. Wang, J. Zhuang, Q. Peng, and Y. Li, “Synthesis of NaYF4 nanocrystals with predictable phaes and shape,” Adv. Funct. Mater. 17, 2757–2765 (2007). [CrossRef]
23. Y. Sun, Y. Chen, L. Tian, Y. Yu, X. Kong, J. Zhao, and H. Zhang, “Controlled synthesis and morphology dependent upconversion luminescence of NaYF4:Yb,Er nanocrystals,” Nanotech. 18(27), 447 (2007). [CrossRef]
24. K. S. Yang, Y. Li, C. Y. Yu, L. P. Lu, C. H. Ye, and X. Y. Zhang, “Upconversion luminescence properties of Ho3+,Tm3+,Yb3+ co-doped nanocrystal NaYF4 synthesized by hydrothermal method,” Rare Earth 24(6), 757–760 (2006). [CrossRef]
25. J. H. Zeng, J. Su, Z. H. Li, R.-X. Yan, and Y.-D. Li, “Synthesis and upconversion luminescence of hexagonal-phase NaYF4:Yb3+,Er3+ phosphors of controlled size and morphology,” Adv. Mater. 17(17), 2119–2123 (2005). [CrossRef]
26. J. Zhuang, L. Liang, H. H. Sung, X. Yang, M. Wu, I. D. Williams, S. Feng, and Q. Su, “Controlled hydrothermal growth and up-conversion emission of NaLnF4 (Ln = Y, Dy-Yb),” Inorg. Chem. 46(13), 5404–5410 (2007). [CrossRef] [PubMed]
27. C. Li, J. Yang, Z. Quan, P. Yang, D. Y. Kong, and J. Lin, “Different microstructures of β-NaYF4 fabricated by hydrothermal process:effects of pH values and fluride sources,” Chem. Mater. 19(20), 4933–4942 (2007). [CrossRef]
