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Ionic Liquid-Based Route to Spherical NaYF4 Nanoclusters with the Assistance of Microwave Radiation and Their Multicolor Upconversion Luminescence Cheng Chen,†,‡ Ling-Dong Sun,† Zhen-Xing Li,† Le-Le Li,† Jun Zhang,*,‡ Ya-Wen Zhang,† and Chun-Hua Yan*,† † Beijing National Laboratory for Molecular Sciences, State Key Lab of Rare Earth Materials Chemistry and Applications, PKU-HKU Joint Lab in Rare Earth Materials and Bioinorganic Chemistry, Peking University, Beijing 100871, P. R. China, and ‡College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, P. R. China
Received December 2, 2009. Revised Manuscript Received January 3, 2010 An ionic liquid (IL) (1-butyl-3-methylimidazolium tetrafluoroborate)-based route was introduced into the synthesis of novel spherical NaYF4 nanoclusters with the assistance of a microwave-accelerated reaction system. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected area electron diffraction (SAED), energy-dispersive X-ray spectroscopy (EDS) and upconversion (UC) luminescence spectroscopy were used to characterize the obtained products. Interestingly, these spherical NaYF4 nanoclusters with diameters ranging from 200 to 430 nm are formed by the self-assembly of small nanoparticles. The diameters of the nanoclusters could be easily tuned just by changing the amounts of the precursors. By conducting the control experiments with different ILs or precursors, it is proven that the ILs have played key roles, such as the solvents for the reaction, the absorbents of microwave irradiation, and the major fluorine sources for the formation of the NaYF4 nanocrystals. The UC luminescence properties of the Ln3þ codoped NaYF4 were measured, and the results indicate that the nanoclusters obtained in BmimBF4 exhibit excellent UC properties. Since this IL-based and microwave-accelerated procedure is efficient and environmentally benign, we believe that this method may have some potential applications in the synthesis of other nanomaterials.
1. Introduction Ionic liquids (ILs) have recently attracted much attention since they have a variety of potential applications in organic synthesis, electrochemistry, catalysis, chemical separation, and so on.1 ILs can be used as green solvents to replace conventional organic solvents in many chemical processes because of their unique properties, such as their negligible vapor pressure, good thermal and chemical stability, extremely high ionic conductivity, wide electrochemical windows, and so on.2,3 In recent years, ILs have emerged as one of the most promising categories of medium for the fabrication of nanomaterials with various morphologies, since ILs possess tunable properties so that they can easily interact with various surfaces and chemical reaction environments.4-6 Smarsly et al. designed a low-temperature route to synthesize rutile nanorods just by the stabilization of an amorphous phase, which then converted to rutile by a simple extraction of the stabilizer (IL).7 Chen et al. demonstrated a novel route for the preparation of IL-stabilized ZnO nanocrystals with remarkably high *Corresponding author. Fax: þ86-10-6275-4179; e-mail:
[email protected] (C.-H.Y.). Fax: þ86-471-499-2278; e-mail:
[email protected] (J.Z.).
(1) Parvulescu, V. I.; Hardacre, C. Chem. Rev. 2007, 107, 2615. (2) Antonietti, M.; Kuang, D. B.; Smarsly, B.; Zhou, Y. Angew. Chem., Int. Ed. 2004, 43, 4988. (3) Gao, H. X.; Li, J. C.; Han, B. X.; Chen, W. N.; Zhang, J. L.; Zhang, R.; Yan, D. D. Phys. Chem. Chem. Phys. 2004, 6, 2914. (4) Lodge, T. P. Science 2008, 321, 50. (5) Kim, T. Y.; Kim, W. J.; Hong, S. H.; Kim, J. E.; Suh, K. S. Angew. Chem., Int. Ed. 2009, 48, 3806. (6) Zhou, Y.; Antonietti, M. Adv. Mater. 2003, 15, 1452. (7) Kaper, H.; Endres, F.; Djerdj, I.; Antonietti, M.; Smarsly, B. M.; Maier, J.; Hu, Y. S. Small 2007, 3, 1753. (8) Liu, D. P.; Li, G. D.; Su, Y.; Chen, J. S. Angew. Chem., Int. Ed. 2006, 45, 7370.
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photoluminescence quantum yields.8 Moreover, because of their high polarity, ILs are also considered to be excellent microwave absorbents.9 Microwave dielectric heating has aroused many interests in recent years because there are many advantages compared to conventional heating for chemical reactions, such as higher heating rate, uniform heating without thermal gradients, selective heating properties, and higher yields in shorter reaction time.10,11 Because of these special properties, microwave irradiation is becoming an important pathway for the preparation of nanomaterials. It not only benefits the spontaneous nucleation of inorganic materials,12-14 but also enables reactions to be performed at higher temperatures, which results from the superheating phenomenon of the solvents caused by the microwave dielectric heating.15,16 The exploration of the combination between ILs and microwave radiation has just begun, which would facilitate the fabrication of novel nanomaterials with designed structures and functions.17-19 (9) Ding, K. L.; Miao, Z. J.; Liu, Z. M.; Zhang, Z. F.; Han, B. X.; An, G. M.; Miao, S. D.; Xie, Y. J. Am. Chem. Soc. 2007, 129, 6362. (10) Gabriel, C.; Gabriel, S.; Grant, E. H.; Halstead, B. S. J.; Mingos, D. M. P. Chem. Soc. Rev. 1998, 27, 213. (11) Hu, X. L.; Yu, J. C. Adv. Funct. Mater. 2008, 18, 880. (12) Gerbec, J. A.; Magana, D.; Washington, A.; Strouse, G. F. J. Am. Chem. Soc. 2005, 127, 15791. (13) Kim, S. H.; Lee, S. Y.; Yi, G. R.; Pine, D. J.; Yang, S. M. J. Am. Chem. Soc. 2006, 128, 10897. (14) Hu, X. L.; Gong, J. M.; Zhang, L. Z.; Yu, J. C. Adv. Mater. 2008, 20, 4845. (15) Baghurst, D. R.; Mingos, D. M. P. J. Chem. Soc. Chem. Commun. 1992, 9, 674. (16) Saillard, R.; Poux, M.; Berlan, J. Tetrahedron 1995, 51, 4033. (17) Yang, L. X.; Zhu, Y. J.; Wang, W. W.; Tong, H.; Ruan, M. L. J. Phys. Chem. B 2006, 110, 6609. (18) Buhler, G.; Feldmann, C. Angew. Chem., Int. Ed. 2006, 45, 4864. (19) Lovingood, D. D.; Strouse, G. F. Nano. Lett. 2008, 8, 3394.
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Recent decades have witnessed an explosion in research devoted to preparing upconversion (UC) nanocrystals because of their potential applications in solid-state lasers, optical storage, flat-panel displays, optical fiber-based telecommunications, low-intensity IR imaging, and so on.20,21 As an important category of rare earth fluoride compounds, AREF4 (A = alkali; RE = rare earth) is regarded as an excellent host matrix for UC phosphors.22,23 Great efforts have been devoted to the synthesis of shape-controllable NaYF4 nanocrystals via different routes and the study of their optical properties.24-26 Zhao et al. reported the synthesis of NaYF4 nanotubes via an in situ ion exchange procedure from the corresponding hydroxides.27 van Veggel’s group developed an efficient route to prepare a UC nanoparticle-polymer composite.28 A core/shell structure of NaYF4/silica was prepared by Zhang’s group, and multicolor UC fluorescence properties of the nanoparticles were measured.29,30 Recently, IL was also applied to synthesize hexagonal-phase NaYF4 UC nanophosphors through an ionothermal method.31 However, the unique properties of ILs, such as their high polarity and the resulting excellent microwave absorbing ability, have not been reflected and emphasized. Additionally, the effects of different ILs on the formation process of the final nanostructures have not been investigated. In this work, we have introduced a microwave-accelerated reaction system into the synthesis of novel spherical NaYF4 nanoclusters in 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4), with which the reactions could be completed in a significantly short time and highly crystallized products could be obtained. Interestingly, these NaYF4 nanoclusters are formed by the self-assembly of small nanoparticles and exhibit excellent UC luminescent property. By conducting control experiments, it is proven that ILs play key roles in the formation of the final nanostructure, since they act not only as the solvents and microwave absorbents in the synthetic process, but also as the major fluorine sources for the preparation of NaYF4 nanocrystals.
2. Experimental Section 2.1. Materials. Trifluoroacetic acid (99%, Acros), Na(CF3COO) (>97%, Acros), Na(CH3COO) (99.0%, Beijing Chemical Plant), 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4, 99%, Alpha), 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF6, 98%, Alpha), and 1-butyl-3-methylimidazolium bromide (BmimBr, 99%, Alpha) were used as received. RE(CF3COO)3 (RE = Y, Yb, Er, Tm) and Y(CH3COO)3 were prepared from the corresponding lanthanide oxides following the literature method.32 2.2. Synthesis of NaYF4 with Trifluoroacetate Salts in BmimBF4. For a typical synthesis, 0.5 mmol Na(CF3COO) (20) Zhang, F.; Wan, Y.; Yu, T.; Zhang, F. Q.; Shi, Y. F.; Xie, S. H.; Li, Y. G.; Xu, L.; Tu, B.; Zhao, D. Y. Angew. Chem., Int. Ed. 2007, 46, 7976. (21) Sivakumar, S.; van Veggel, F. C. J. M.; May, P. S. J. Am. Chem. Soc. 2007, 129, 620. (22) Boyer, J. C.; Cuccia, L. A.; Capobianco, J. A. Nano. Lett. 2007, 7, 847. (23) Li, C. X.; Yang, J.; Yang, P. P.; Zhang, X. M.; Lian, H. Z.; Lin, J. Cryst. Growth Des. 2008, 8, 923. (24) Mai, H. X.; Zhang, Y. W.; Si, R.; Yan, Z. G.; Sun, L. D.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2007, 129, 6362. (25) Wang, L. Y.; Li, Y. D. Chem. Mater. 2007, 19, 727. (26) Wang, H. Q.; Nann, T. ACS Nano 2009, 3, 3804. (27) Zhang, F.; Zhao, D. Y. ACS Nano 2009, 3, 159. (28) Boyer, J. C.; Johnson, N. J. J.; van Veggel, F. C. J. M. Chem. Mater. 2009, 21, 2010. (29) Li, Z. Q.; Zhang, Y.; Jiang, S. Adv. Mater. 2008, 20, 4765. (30) Qian, H. S.; Guo, H. C.; Ho, P. C. L.; Mahendran, R.; Zhang, Y. Small 2009, 5, 2285. (31) Liu, X. M.; Zhao, J. W.; Sun, Y. J.; Song, K.; Yu, Y.; Du, C.; Kong, X. G.; Zhang, H. Chem. Commun. 2009, 43, 6628. (32) Roberts, J. E. J. Am. Chem. Soc. 1961, 83, 1087.
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and 0.5 mmol Y(CF3COO)3 were taken as the precursors and added into 5 mL IL (BmimBF4) followed by vigorously magnetic stirring at room temperature for 2 h to obtain a transparent solution, which was then heated to 200 °C by microwave irradiation and maintained for 5 min. The temperature ramping process was accomplished by two steps (20 °C/min from room temperature to 100 °C and then 10 °C/min from 100 to 200 °C) in a CEM microwave-accelerated system at 400 W. After the microwave reaction was completed, the temperature of the system was reduced to room temperature. Then after washing and centrifugation several times, white precipitates were collected and dried at 65 °C.
2.3. Synthesis of NaYF4 with Acetate Salts in BmimBF4. The synthetic procedure was the same as that used to synthesize cubic NaYF4 in BmimBF4, except that 0.5 mmol Na(CH3COO) and 0.5 mmol Y(CH3COO)3 were taken as the precursors instead of the trifluoroacetate salts. 2.4. Reaction of Trifluoroacetate Salts in BmimBr. The synthetic procedure was the same as that used to synthesize cubic NaYF4 by the reaction of Na(CF3COO) and Y(CF3COO)3, except that BmimBr was used as the solvent instead of BmimBF4.
2.5. Synthesis of NaYF4 with Trifluoroacetate Salts in BmimPF6. The synthetic procedure was the same as that used to synthesize cubic NaYF4 by the reaction of Na(CF3COO) and Y(CF3COO)3, except that BmimPF6 was used as the solvent instead of BmimBF4.
2.6. Synthesis of NaYF4:Yb,Er and NaYF4:Yb,Tm with Trifluoroacetate Salts in BmimBF4. The synthetic procedure was the same as that used to synthesize cubic NaYF4 in BmimBF4, except that 0.5 mmol Na(CF3COO) and stoichiometric amounts of Y(CF3COO)3, Yb(CF3COO)3, Er(CF3COO)3/Tm(CF3COO)3 were taken as the precursors.
2.7. Synthesis of NaYF4:Yb,Er and NaYF4:Yb,Tm with Trifluoroacetate Salts in BmimPF6. The synthetic procedure was the same as that used to synthesize cubic NaYF4 in BmimPF6, except that 0.5 mmol Na(CF3COO) and stoichiometric amounts of Y(CF3COO)3, Yb(CF3COO)3, Er(CF3COO)3/Tm(CF3COO)3 were taken as the precursors. 2.8. Characterization. Powder X-ray diffraction (XRD) patterns of the dried powders were recorded on a Rigaku D/ MAX-2000 diffractometer (Japan) with Cu KR radiation (λ = 1.5406 A˚). Scanning electron microscopy (SEM) observations were carried out with DB-235 focused ion beam (FIB) system operated at an acceleration voltage of 15 kV. Transmission electronic microscopy (TEM), and selected area electron diffraction (SAED) were performed with a JEOL-2100 transmission electron microscope (Japan) operated at 200 kV. High-resolution TEM (HRTEM) characterization and energy-dispersive X-ray spectroscopy (EDS) were taken on a JEOL-2100F transmission electron microscope (Japan) equipped with an EDS detector. The UC luminescence spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer equipped with an external tunable 2 W 980 nm laser diode (Pmax = 500 mW at 1000 mA).
3. Results and Discussion The combination of ILs and microwave dielectric heating could provide us with a facile and green route to fabricate nanomaterials. The XRD pattern of the product synthesized with trifluoroacetate salts (Na(CF3COO) and Y(CF3COO)3) in BmimBF4 is shown in Figure 1a. All the peaks can be well indexed as a cubic phase of NaYF4 (JCPDS card No. 39-0724). The well-resolved four peaks between 20° and 60° in 2θ value could be assigned to (111), (200), (220), and (311) planes of cubic NaYF4 nanocrystals. The SEM image (Figure 2a) indicates that NaYF4 nanospheres with diameters ranging from 200 to 430 nm could be obtained. The TEM image (Figure 2b) shows that these spherical nanoclusters have rough surfaces, and they are formed by the self-assembly of small nanoparticles. The inset of Figure 2b is the SAED pattern Langmuir 2010, 26(11), 8797–8803
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Article Scheme 1. Schematic Representation of the Formation of NaYF4 Nanocrystals Obtained in Different ILs
Figure 1. (a) XRD pattern of NaYF4 synthesized via the reaction of trifluoroacetate precursors in BmimBF4. (b) XRD pattern of NaYF4 synthesized via the reaction of trifluoroacetate precursors in BmimPF6. (c) XRD pattern of NaYF4 synthesized via the reaction of acetate precursors in BmimBF4. (d) XRD pattern of the product synthesized via the reaction of trifluoroacetate precursors in BmimBr.
Table 1. Average Diameters of the NaYF4 Nanoclusters Obtained via the Reaction of Different Amounts of Trifluoroacetate Precursors in BmimBF4 sampleNa(CF3COO)/mmolY(CF3COO)3/mmol average diameters/nm 1 2 3
Figure 2. (a) SEM image of the NaYF4 nanoclusters obtained in BmimBF4. (b) TEM image of two NaYF4 nanoclusters obtained in BmimBF4 and the ED pattern (inset). (c) HRTEM image taken on the edge of a NaYF4 nanocluster.
taken on a single sphere. The discontinuous rings again reveal that the cluster is formed by the aggregation of many small nanoparticles. Observation from the HRTEM image (Figure 2c) reveals that the size of the small nanoparticles in these nanoclusters is about 10 to 15 nm. Moreover, the nanoparticles are single crystals with an interplanar spacing of 0.31 nm corresponding to the (111) facets of cubic NaYF4, which confirms that the nanoparticles are highly crystallized. Scheme 1 illustrates the formation process of the spherical NaYF4 nanoclusters. Since the ILs with high polarity have strong microwave absorbing ability, the whole reaction system could reach a high temperature rapidly under microwave irradiation, which results in spontaneous nucleation.9 Followed by the rapid growth of these nuclei, large quantities of NaYF4 nanocrystals are formed within an extremely short time. Therefore, the size of the as-synthesized nanoparticles is very small. Because of their high surface energy, the freshly formed small nanoparticles tend to aggregate rapidly into spherical nanoclusters.14 Furthermore, the size of the spherical NaYF4 nanoclusters could be tuned by changing the amounts of the trifluoroacetate precursors. As shown in Table 1, samples 1-3 were prepared by Langmuir 2010, 26(11), 8797–8803
0.5 0.25 0.1
0.5 0.25 0.1
302 163 79
changing the amounts of the trifluoroacetate salts from 0.5 to 0.1 mmol. It could be observed from the TEM images in Figure 3a,c,e that the size of the NaYF4 nanoclusters decreases with the decrease of the precursors’ amounts. By measuring 200 nanoclusters for each sample, we could get the size distributions of the nanoclusters (Figure 3b,d,f), and the average sizes of these three samples are 302, 163, and 79 nm, respectively. This size tunability could be resulted from the differences in crystal nuclei caused by varying additions of the precursors.33 Because higher concentration of trifluoroacetate salts could accelerate the decomposition of Na(CF3COO) and Y(CF3COO)3, more nuclei would form in the solution, which leads to the formation of larger NaYF4 nanoclusters. The factors governing the formation of the spherical NaYF4 nanoclusters were studied, and it was found that the IL had played key roles in the self-assembly process of the final nanostructure. As is known, the existence of sodium, yttrium, and fluorine sources is indispensable for the formation of NaYF4 nanocrystals. In this method, both the precursors (trifluoroacetate salts) and the solvent (BmimBF4) might serve as the supplier of the fluoride ions. In order to investigate where the fluoride ions came from and the role that IL played on the synthetic procedure of NaYF4, control experiments were carried out by changing the inorganic precursors or the ILs while keeping other synthetic parameters constant. Figure 1c shows the XRD pattern of the sample obtained by the reaction of acetate salts Na(CH3COO) and Y(CH3COO)3 in BmimBF4. It can be clearly observed that cubic NaYF4 could also be obtained without the existence of CF3COO-. This indicates that the IL provided the fluorine source for the formation of NaYF4 by the decomposition of BmimBF4, which compares well with the reported results that BF4- ions are prone to thermal decomposition to produce F- at a certain temperature.34,35 To further confirm it, another control experiment was carried out just by changing the molecular composition of the ILs. The XRD pattern (Figure 1d) shows that NaYF4 could (33) Ge, J. P.; Hu, Y. X.; Biasini, M.; Beyermann, W. P.; Yin, Y. D. Angew. Chem., Int. Ed. 2007, 46, 4342. (34) Fox, D.; Gilman, J.; Long, H. D.; Trulove, P. J. Chem. Thermodyn. 2005, 37, 900. (35) Koval’chuk, E.; Reshetnyak, O.; Kozlovs’ka, Z.; Bza’ejowski, J.; Gladyshevs’kyj, R.; Obushak, M. Thermochim. Acta 2006, 444, 1.
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Figure 3. TEM images and corresponding size distributions of samples 1 (a,b), 2 (c,d), and 3 (e,f).
not be synthesized under the same condition by the use of trifluoroacetate salts (Na(CF3COO) and Y(CF3COO)3) and IL bearing bromide (BmimBr), and no signal of fluorine was detected by EDS for the final product (Figure 4a,b), which also pointed out that CF3COO- did not decompose during the microwave heating process and did not provide the indispensable F- for the preparation of NaYF4. The results of the above experiments illuminated that BmimBF4 not only served as the solvent and microwave absorbent in the whole synthetic process, but also acted as the building block and the major fluorine source for the formation of NaYF4 nanoclusters. Based on the above analysis, it is reasonable to deduce that NaYF4 nanocrystals may also be synthesized in other ILs bearing fluorine. So we applied the same synthetic procedure to prepare NaYF4 nanocrystals in BmimPF6. As we expected, the XRD pattern (Figure 1b) shows that cubic NaYF4 could be achieved, but a peak of NaF was observed at 38.4° in 2θ value. This probably could be explained as follows: The thermal degradation of BmimPF6 is relatively easier than that of BmimBF4,36 because the bond energy of P-F is weaker compared with that of B-F.19 Therefore, high concentration of fluoride ions could be presented in the system of BmimPF6, which might lead to the formation of NaF. The TEM image in Figure 5a illustrates that spherical to ellipsoidal nanoparticles could be obtained in BmimPF6. The interplanar spacing shown in the HRTEM image (Figure 5b) is about 0.31 nm corresponding to the (111) facets of cubic NaYF4, which confirms that the nanoparticles are single crystals with high crystallinity. By comparison of Figure 2b and Figure 5a, we can (36) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Green Chem. 2001, 3, 156.
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easily see that the morphology of the NaYF4 nanocrystals obtained in BmimPF6 is different from that of the nanoclusters obtained in BmimBF4. This may be caused by the different viscosities of these two ILs. As is reported by Afonso´s group, the viscosity of BmimPF6 is higher than that of BmimBF4.37 Thus the assembly and aggregation of the small nanoparticles might be prevented to some extent in BmimPF6, thus affecting the final morphology. In order to elucidate the effect of microwave dielectric heating on the preparation of the NaYF4 nanoclusters, the comparison between the samples synthesized through the ionothermal method and the microwave method was investigated. The XRD pattern (Figure S1 in the Supporting Information) indicates that the product of the ionothermal treatment is cubic NaYF4. TEM images (Figure S2 in the Supporting Information) show that the aggregates of NaYF4 nanoparticles could also be obtained. However, the shape of these aggregates is not as regular as the spherical nanoclusters obtained through the microwave method. At the same time, some rectangular structures could also be observed in the same sample. The existence of different structures in the product may have resulted from the thermal gradients in the ionothermal reaction system. The vessel for the ionothermal reaction serves as an intermediary in the whole process, through which the energy could transfer from the oven to the solvent and then to the reactants. This could result in thermal gradients throughout the bulk solution, which lead to the formation of nonuniform reaction conditions.12 However, the effect of thermal gradients could be eliminated in the microwave-accelerated system, which leads to the uniform morphology of the NaYF4 nanocrystals. The UC emission can be realized by doping cubic NaYF4 with lanthanide ions. The morphologies of the UC nanocrystals are not affected by doping with Ln3þ (Figure 6). Figure 7a,b shows the fluorescence spectra for a 1 wt % colloidal solution of NaYF4:20%Yb3þ,2%Er3þ and NaYF4:20%Yb3þ,0.2%Tm3þ in these two different ILs (BmimBF4 and BmimPF6) under the excitation of a 980 nm laser diode. The spectra of NaYF4:Yb3þ, Er3þ nanocrystals exhibit two emission bands, which could be attributed to 2H11/2f4I15/2, 4S3/2f4I15/2, and 4F9/2f4I15/2 transitions of Er3þ. The emission bands of NaYF4:Yb3þ,Tm3þ nanocrystals at 450-500 nm and 630-670 nm could be assigned to the 1 G4f3H6 and 1G4f3F4 transition of Tm3þ.38 Interestingly, the UC emission intensity of nanoclusters obtained in BmimBF4 was enhanced nearly eight times compared with that of the nanoparticles in BmimPF6. It is well-known that the UC intensity may be influenced by the surface state of nanoparticles. As the ratio of the surface defects decreases, the nonradiative decay is reduced, which would cause the increase of the emission intensity.39 In our work, the formation of the NaYF4 nanoclusters might result in the surface reduction of the primary nanoparticles, which is caused by the hard connection among the nanoparticles formed during the self-assembling process. Therefore, the nonradiative centers existing on the surface of the nanocrystals will be eliminated partially, which finally enhances the intensity of the NaYF4 nanoclusters.40,41 Figure 7c shows the strong UC luminescence photographs for the 1 wt % colloidal solution of the NaYF4:20%Yb3þ,2%Er3þ and NaYF4:20%Yb3þ,0.2%Tm3þ (37) Branco, L. C.; Rosa, J. N.; Ramos, J. J. M.; Afonso, C. A. M. Chem.;Eur. J. 2002, 8, 3671. (38) Wang, F.; Liu, X. G. J. Am. Chem. Soc. 2008, 130, 5642. (39) Vetrone, F.; Naccache, R.; Mahalingam, V.; Morgan, C. G.; Capobianco, J. A. Adv. Funct. Mater. 2009, 19, 2924. (40) Bovero, E.; van Veggel, F. C. J. M. J. Phys. Chem. C 2007, 111, 4529. (41) Mai, H. X.; Zhang, Y. W.; Sun, L. D.; Yan, C. H. J. Phys. Chem. C 2007, 111, 13721.
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Figure 4. (a) TEM image and (b) corresponding EDS spectrum of the product obtained in BmimBr.
Figure 5. (a) TEM image, (b) HRTEM image, and (c) size distribution of NaYF4 nanoparticles obtained in BmimPF6.
nanoclusters in BmimBF4, implying that these nanoclusters are excellent UC hosts. The emission intensity of UC nanomaterials could be easily tuned just by changing the doping amounts of the lanthanide ions (Figure 8). For the NaYF4:Yb3þ,Er3þ nanoclusters obtained in BmimBF4, the UC emission intensity obviously decreases when the concentration of Er3þ increases from 0.2% to 2% with the Yb3þ concentration fixed at 20%. For the NaYF4: Yb3þ,Tm3þ nanoclusters obtained in BmimBF4, the UC emission intensity also decreases with the increase of the Tm3þ concentration from 0.2% to 2%, while keeping the concentration of Yb3þ at 20%. According to these results, it could be concluded that higher concentrations of Er3þ or Tm3þ may cause concentration quenching of the spherical UC nanoclusters.41 Furthermore, we investigated how the size distribution affected the UC properties of the spherical NaYF4:Yb3þ,Er3þ/Tm3þ nanoclusters. As shown in Figure 9, the UC emission intensity of NaYF4:20%Yb3þ,2%Er3þ and NaYF4:20%Yb3þ,2%Tm3þ decreased when the average size of the nanoclusters was reduced Langmuir 2010, 26(11), 8797–8803
Figure 6. TEM images of (a) R-NaYF4:20%Yb3þ,2%Er3þ and
(b) R-NaYF4:20%Yb3þ,0.2%Tm3þ nanoclusters obtained in BmimBF4. TEM images of (c) R-NaYF4:20%Yb3þ,2%Er3þ and (d) R-NaYF4:20%Yb3þ,0.2%Tm3þ nanoparticles obtained in BmimPF6.
from 302 to 79 nm. Because of the higher surface area of the smaller nanoclusters,14 the nonradiative centers existing on the surface of the nanoparticles increased with the decrease of the size of the nanoclusters. Therefore, the UC emission intensity of the large nanoclusters would be enhanced compared with that of the small nanoclusters. The influence of different thermal treatment methods (microwave irradiation and ionothermal) on the UC properties of the NaYF 4 :20%Yb 3þ,2%Er 3þ and NaYF 4 :20% Yb 3þ,2%Tm 3þ nanocrystals was also studied. The results (Figure S3 in the Supporting Information) illuminated that the emission intensity of the UC nanocrystals synthesized via the microwave dielectric heating was slightly increased compared with that of the nanocrystals synthesized via the ionothermal method. The reason is that DOI: 10.1021/la904545a
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Figure 7. (a) UC luminescence spectra of R-NaYF4:20%Yb3þ,2%Er3þ nanoclusters in BmimBF4 and R-NaYF4:20%Yb3þ,2%Er3þ
nanoparticles in BmimPF6. (b) UC luminescence spectra of R-NaYF4:20%Yb3þ,0.2%Tm3þ nanoclusters in BmimBF4 and R-NaYF4: 20%Yb3þ,0.2%Tm3þ nanoparticles in BmimPF6. (c) UC luminescence photographs for 1 wt % colloidal solution of R-NaYF4:20% Yb3þ,2%Er3þ (left) and R-NaYF4:20%Yb3þ,0.2%Tm3þ (right) nanoclusters in BmimBF4.
Figure 8. (a) UC luminescence spectra of R-NaYF4:Yb3þ,Er3þ nanoclusters with different doping concentrations of Er3þ. (b) UC luminescence spectra of R-NaYF4:Yb3þ,Tm3þ nanoclusters with different doping concentrations of Tm3þ.
Figure 9. (a) UC luminescence spectra of R-NaYF4:20%Yb3þ,2%Er3þ nanoclusters with different average diameters. (b) UC luminescence spectra of R-NaYF4:20%Yb3þ,2%Tm3þ nanoclusters with different average diameters.
relative higher crystallinity and uniformity of the nanoclusters could be achieved through the microwave irradiation, which resulted in the increase of the UC emission intensity. 26,29
4. Conclusion In summary, we have developed a rapid microwave-assisted process to synthesize cubic NaYF4 in fluorine-contained ILs. It shows that small nanoparticles could form spontaneously in BmimBF4 due to the microwave irradiation, and then spherical cubic NaYF4 nanoclusters could be obtained by the self-assembly of these primary nanoparticles. From the control experiments with different precursors or ILs, it can be concluded that ILs play key roles, such as the solvent for the reaction, the absorbent of microwave irradiation, and the source of fluoride ions for the formation of NaYF4 nanocrystals. By the investigation of 8802 DOI: 10.1021/la904545a
different thermal treatment methods, it is also found that higher crystallinity and uniformity of the nanocrystals could be achieved in the microwave-accelerated system. The various experimental results of the UC properties indicate that the NaYF4:Yb3þ,Er3þ and NaYF4: Yb3þ,Tm3þ nanoclusters synthesized in BmimBF4 exhibit excellent luminescent properties. Therefore, the rare earth fluoride nanoclusters are expected to be applied in solid-state laser, three-dimensional flat-panel displays, light emitting diodes, and some other optics devices. Since this IL-based and microwave-accelerated procedure is efficient and environmentally benign, it may have some potential applications in the synthesis of other nanomaterials. Acknowledgment. Grants-in-aid from NSFC (20821091, 20961005, and 20971005) and MOST of China (2006CB601104) are gratefully acknowledged. Langmuir 2010, 26(11), 8797–8803
Chen et al.
Supporting Information Available: Synthesis of NaYF4: Yb,Er and NaYF4:Yb,Tm with trifluoroacetate salts in BmimBF4 via the ionothermal method. XRD pattern and TEM images of NaYF4 nanocrystals synthesized via the ionothermal method. UC luminescence spectra of
Langmuir 2010, 26(11), 8797–8803
Article
R-NaYF4:20%Yb3þ,2%Er3þ and R-NaYF4:20%Yb3þ, 2%Tm3þ nanocrystals synthesized via the microwave irradiation and the ionothermal method. This material is available free of charge via the Internet at http://pubs. acs.org.
DOI: 10.1021/la904545a
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