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Controlled Construction of Monodisperse La2(MoO4)3:Yb,Tm Microarchitectures with Upconversion Luminescent Property Zhenxing Chen,†,‡ Wenbo Bu,*,† Na Zhang,†,‡ and Jianlin Shi*,† State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, People’s Republic of China, and Graduate School of the Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China ReceiVed: NoVember 27, 2007
Monodisperse La2(MoO4)3:Yb,Tm microarchitectures with uniform waxberry-like morphology have been successfully constructed in large scale by a facile surfactant-assisted hydrothermal route, in which sodium lauryl sulfate (SLS) was used as the structure directing agent. It was found that the pH value was a crucial factor controlling the phase composition and purity, which was unaffected by surfactant SLS. The growth process of these waxberry-shaped microarchitectures has been examined in detail, and it was proved that a special dissolution-recrystallization transformation mechanism as well as a preferential adsorption of SLS process was responsible for the morphology evolution of the La2(MoO4)3:Yb,Tm microarchitectures. A mechanism for the formation of the waxberry-shape La2(MoO4)3:Yb,Tm microarchitectures was put forward. The specially shaped architectures showed blue up-conversion emission properties.
Introduction During the past decade, low-dimensional nanoscale materials, including nanotubes,1,2 nanorods,3 nanospheres,4 nanosprings,5 nanowires,6,7 and nanobelts,8,9 have attracted considerable attention due to their interesting physical properties and potential applications in nanodevices. Recently, keen efforts have been put into using these low-dimensional nanoscale materials as building blocks to assemble micro-, meso-, and nanostructures that have highly specific morphology, novel properties, and great application potential in many fields such as photochemistry, superconductors, optoelectronics, solar cells, and catalysis.10-14 Several methods have been found quite effective, e.g., mesoscopic structured arrays and hierarchical patterns could be fabricated using the colloidal-crystal-assisted imprint (CCAIP) approach;15 3D continuous macroscopic metal or semiconductor nanowire networks were formatted by a templated electrodeposition technique;16 Si-SiO2 hierarchical heterostructures were prepared through a Sn-catalyzed vapor-liquid-solid process,17 and R-Fe2O3 “micro-pines” were synthesized by a hydrothermal method,18 etc. Among these various methods, the hydrothermal method with its great facility and flexibility can produce many kinds of materials with low cost and large-scale production capability. Especially by adding different surfactants, which have efficient self-assembly properties in aqueous solution, novelstructure materials with controlled morphology can be obtained via a facile hydrothermal process. For example, Zhengrong R. Tian and co-workers created multiscale hierarchical structures through self-similar assembly of polyhedral mesophase crystals using hexadecyltrimethylammonium chloride (CTAC) as a surfactant;19 Deren Yang’s group fabricated flowerlike ZnO nanostructures by a cetyltrimethylammonium bromide-assisted * To whom correspondence should be addressed. Phone: 86-2152412712. Fax: 86-21-52413122. E-mail:
[email protected] (J.S.);
[email protected] (W.B.). † Shanghai Institute of Ceramics, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.
hydrothermal process.20 Up to now, hydrothermal synthesis theory and surfactant action theory are still not perfect, and there is much room for the construction of various nanostructures and architecture; therefore it remains a challenge to select proper surfactants for the large-scale controlled synthesis of specific material with unique architectures and properties. Rare-earth molybdate compounds have been widely used in various fields, such as high-quality phosphors, up-conversion materials, catalysts, and otherwise.21 On the basis of their unique optical, catalytic, and magnetic properties, these compounds have attracted intensive interest till now.22,23 The composition and structure of rare-earth compounds, especially the complexation state and the crystal field of the matrix, in which rareearth ions are coordinated, all have strong impacts on these available properties. For instance, up-conversion phosphors (UCP), such as ytterbium- and erbium-co-doped lanthanum molybdate (La2(MoO4)3:Yb,Er) nanocrystals with an average diameter of 50 nm, were prepared by Depu Chen’s group, and the fluorescent intensity was much stronger than that of bulk materials because of the surface effect generated by the decreased size.24 To the best of our knowledge, previous work about rare-earth molybdate compounds has been focused mainly on the synthesis of the bulk crystals,25 nanorods, and nanoparticles.24 Very recently, we have successfully constructed uniform three-dimensional La2(MoO4)3 nanostructures with pompon shape via a facile hydrothermal process without using any surfactants.26 Nevertheless, to fabricate novel micro- and nanoarchitectures of lanthanum molybdate by suitably applying surfactants as directing agents still remains interesting and challenging. In this paper, we report a sodium lauryl sulfate (SLS)-assisted hydrothermal approach for the preparation of monodisperse upconversion luminescent La2(MoO4)3:Yb,Tm micro-architectures with uniform waxberry-like morphology. The formation mechanism and the blue up-conversion emission property of the architectures are preliminarily investigated.
10.1021/jp711213r CCC: $40.75 © 2008 American Chemical Society Published on Web 02/23/2008
Controlled Construction of Monodisperse La2(MoO4)3
J. Phys. Chem. C, Vol. 112, No. 11, 2008 4379 6700F microscope. Transmission electron microscopy (TEM) images and the selected area electron diffraction (SAED) patterns were recorded on a JEOL200CX microscope with an accelerating voltage of 200 kV. Near-IR absorption spectra were recorded on a Shimadzu UV-3101 spectrophotometer equipped with an integrating sphere, using BaSO4 as reference. Upconversion fluorescent spectra were obtained with a Fluorolog-3 fluorescence spectrophotometer using 970 nm LD as the excitation source. Results and Discussion
Figure 1. XRD patterns of La2(MoO4)3:Yb,Tm products synthesized at different pH values in the presence of SLS by aging at 180 °C for 12 h.
Experimental Section All reagents were analytical grade and used without further purification. In a typical synthesis, solution A was prepared by dissolving 0.371 g of La2O3 (1.14 mmol, 99.99%), 0.124 g of Yb2O3 (0.315 mmol, 99.99%), and 0.018 g of Tm2O3 (0.045 mmol, 99.99%) in diluted nitric acid, according to the molar ratio of La3+:Yb3+:Tm3+ ) 76:21:3. The solution was heated to drive away the unreacted nitric acid, and the residue was redissolved in 10 mL of deionized water and stirred for 1 h at room temperature. 0.865 g (3 mmol) of SLS and 0.79 g (0.64 mmol) of (NH4)6Mo7O24‚4H2O were dissolved in 20 and 30 mL of deionized water, respectively, named solution B and solution C. Solution B was then added slowly into solution A under constant magnetic stirring, and white precipitate yielded in solution A immediately. After stirring for about 15 min, solution C was added dropwise into the precipitate under vigorous stirring, and the precipitate became a white homogeneous suspension. Then the pH value of the solution was adjusted to 8-9 using NaOH solution (5 M) under stirring, and the resulting precursor suspension was further stirred for half an hour. After that, the suspension was poured into an 80-mL capacity Teflon-lined stainless steel autoclave and heated subsequently to 180 °C for 12 h. After cooling down to roomtemperature naturally, the product could be directly collected at the bottom of the vessel. The product was filtered, washed several times with deionized water and absolute ethanol, and dried in oven at 100 °C for 5 h. X-ray powder diffraction (XRD) patterns were obtained on a Rigaku D/Max-II X-ray diffractometer with graphite-monochromatized Cu KR radiation. Field emission scanning electron microscopy (FE-SEM) images were taken from a JEOL JSM-
The phase composition and purity of the La2(MoO4)3:Yb,Tm product after hydrothermal treatment at different pH values were examined by XRD technique. As shown in Figure 1, the pure and well-crystallized La2(MoO4)3:Yb,Tm product has been obtained within a very narrow pH range around 8 at 180 °C for 12 h. All the peaks of the product at pH ) 8 can be perfectly indexed as the tetragonal La2(MoO4)3 [space group: I41/a], which is consistent with the reported values (JCPDS No. 450407). As our previous work has shown that without using any surfactants the pure and well crystallized tetragonal La2(MoO4)3 can also be obtained within a very narrow pH range of 8-9,26 it seems that surfactant SLS has no influence on the phase composition and purity, which strongly depend on the pH value of aqueous solution. Parts a and b of Figure 2 show the low-magnification FESEM images of the La2(MoO4)3:Yb,Tm microarchitectures formed after 12 h of the hydrothermal treatment at pH ) 8 and 180 °C. From these images, it turns out that the La2(MoO4)3: Yb,Tm micro-architectures are monodisperse with an average diameter of about 2.5 µm and that these high-yield microarchitectures have a uniform waxberry-like morphology. Figure 2c reveals the high-magnification FE-SEM image of a single waxberry-like architecture. It is clearly demonstrated that the waxberry-like microarchitecture is highly porous and is composed of several tens of well aligned nanostrips with a lot of small surface protuberances, which are rough and coarse. The protuberances are tips of many assembled nanostrips, which adhibit with each other and extend outward from the center of the microstructure, as also can be identified by TEM observation of the unique La2(MoO4)3:Yb,Tm micro-architectures in Figure 3. Figure 3a shows the TEM image of the La2(MoO4)3:Yb,Tm samples after long-time ultrasonic treatment. As we can see, these microstructures are strong enough and could not be destroyed by a long ultrasonic treatment, though only a small amount of La2(MoO4)3:Yb,Tm nanoflake conglomerations can be found, as being better revealed in Figure 3b. It looks like these nanoflakes have parallelogram-shaped morphology and smooth surfaces, which is similar with what we have obtained
Figure 2. FE-SEM images of monodisperse La2(MoO4)3:Yb,Tm microarchitecture withwaxberry-like morphology obtained at pH ) 8 in the presence of SLS by aging at 180 °C for 12 h. (a) Low-magnification image; (b) and (c) higher-magnification images.
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Figure 3. TEM images of the same La2(MoO4)3:Yb,Tm products after prolonged ultrasonic dispersion obtained at pH ) 8 in the presence of SLS by aging at 180 °C for 12 h. (a) TEM image of La2(MoO4)3:Yb,Tm micro-architectures; (b) the corresponding enlarged image of the area marked with a white rectangle in panel (a). The insert SAED pattern in (b) was recorded from one of these nanoflakes.
Figure 4. Time-dependent XRD patterns of the La2(MoO4)3:Yb,Tm products obtained by aging at pH ) 8 and 180 °C for different time: (a) 1 h, (b) 3 h, (c) 6 h, and (d) 12 h.
by constructing La2(MoO4)3 microstructures without surfactants.26 The corresponding SAED pattern of these nanoflakes as shown at the bottom right corner of Figure 3b displays their characteristic of a tetragonal La2(MoO4)3, which is consistent with the XRD result. Furthermore, the SAED patterns taken both from different areas on a single nanoflake and from different nanoflakes were found to be identical counting experimental accuracy, which indicates that these nanoflakes are single crystalline, and have the same crystallization habit. To understand the formation mechanism of these unique waxberry-like microarchitectures, the growth process was carefully examined by using XRD and FE-SEM. The XRD pattern in Figure 4a indicates that most precursors remain noncrystalline after the reaction for 1 h. After 3 h of reaction, the XRD pattern in Figure 4b shows that the tetragonal crystalline phase appears, corresponding to the narrowing of (112) peak at 2θ ) 28.1°, though most of the amorphous phase is still prevailing. When the hydrothermal reaction lasted for 12 h, a well-crystallized tetragonal phase (Figure 4d) finally formed. Figure 5 shows FE-SEM images of the same samples obtained for different reaction time with other synthetic conditions remaining unchanged. As shown in Figure 5a, in 1 h of reaction time, the products were composed of large irregularly shaped
and twisted pieces that were almost amorphous according to the XRD pattern in Figure 4a. When the reaction time was prolonged to 3 h, several aggregates with obviously different morphology came forth (see Figure 5b), and on the basis of the corresponding XRD pattern in Figure 4b they were in the tetragonal phase. From Figure 5c, we can see that these aggregates were formed by many rugged nanostrips that were assembled radiating and adhering with each other. As this process continued, more nanostrips were regularly assembled and more aggregates appeared after 6 h (As shown in parts d and e of Figure 5). It should be noted that we could observe these nanostrips had coarse-grain-like edges on the long sides (Figure 5c) and the nanostrips extended outward from the center of the aggregates showing that they were assembled regularly (Figure 5e). When the reaction time was extended to 12 h, the products turned into uniform waxberry-like morphology, and most of the amorphous precursor transformed into wellcrystallized, monodisperse microarchitecture (Figure 5f). As can be seen in Figure 5e, more nanostrips are needed to generate and assemble into the microarchitecture to make it grow up, which means this is not a one-off nucleation-growth process but a continual nucleation-growth process. Therefore even when aging at 180 °C for 12 h, there still have some nanoflakes (Figure 3b) that are the initial morphology of nanostrips but cannot grow up into nanostrips because all of the amorphous twisted chips have been consumed. It seems that these nanoflakes cannot be found in FE-SEM images because their size (about 50 nm) is too small to be seen. We have reported the growth process of La2(MoO4)3 with the absence of surfactants, but other synthetic conditions remain similar.26 In that process, nanoflakes also formed but with much larger size and higher aspect ratio larger than 5; nanostrips did not show up, and the final product had pompomlike morphology. All these differences could be related to the absence or presence of the surfactant SLS. Herein we propose a possible mechanism for the formation of waxberry-like microarchitecture under the presence of SLS as follows. When the amorphous precursor suspension with SLS is heated to a certain temperature, some precursor begins to dissolve. At the same time the hydrophilic group of SLS, namely, the sulfonic group, can capture Ln3+ (Ln ) La, Yb, Tm) to form complexing agent. This metastable complexing agent and MoO42- react with each other under
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Figure 5. Representative FE-SEM images of the La2(MoO4)3:Yb,Tm products obtained by aging at pH ) 8 and 180 °C for different time intervals: (a) 1 h, (b) and (c) 3 h, (d) and (e) 6 h.
hydrothermal conditions, producing special La2(MoO4)3:Yb,Tm crystal nuclei coated with SLS. Accompanying the continuing appearance and growth of these nuclei, the amorphous precursor precipitates are consumed slowly but continuously. Because of the different growth rates of different crystal facets of tetragonal La2(MoO4)3:Yb,Tm crystallites, parallelogram-like nanoflakes yield by following an Ostwald ripening process and the crystallization habit. Though the adsorption quantity of SLS on different crystal facets should not be the same, thus SLS has different effect on slowing down the growth of different facets; however, in the present case, it seems that SLS can adsorb onto all the crystal facets to produce much smaller nanoflakes than those obtained without using any surfactants.26
In addition, the preferential adsorption of SLS onto the different facets gives a low aspect ratio smaller than three of the nanoflakes, as compared to five in our previous report (Figure 3b). These SLS-coated nanoflakes keep on growing and evolve into specific nanostrips that have coarse-grain-like edges. These rough edges can be explained by the adsorption of SLS cumbering the Ostwald ripening process to make them smooth. As we have expatiated,26 such nanostrips are charged, and the charge on the flat surface should be different from that on the edge flanks of short sides due to the different atomic arrangement, thus such opposite surface charges in peculiar distribution lead to some kind of edge-to-flat-surface conjunction through the self-assembly by the electrostatic interaction, and the
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Figure 8. Up-conversion luminescence spectrum and assignments of La2(MoO4)3:Yb,Tm microarchitectures after excitation at 10 309 cm-1 (970 nm).
Figure 6. Schematic illustration of the growth process of the unique La2(MoO4)3:Yb,Tm microarchitecture in the presence of SLS. The nanostrips in procedures 5 and 6 have the same morphology with the nanostrips in procedure 4, which is not shown in the figure.
Figure 9. Energy level and up-conversion scheme for the La2(MoO4)3: Yb,Tm microarchitectures. Full, dotted, and dash-dotted arrows indicate radiative, nonradiative energy transfer, and multiphonon relaxation processes, respectively.
Figure 7. Near-IR absorption spectrum and assignments of La2(MoO4)3:Yb,Tm microarchitectures.
waxberry-shaped La2(MoO4)3:Yb,Tm microarchitectures are finally constructed. Figure 6 shows the schematic illustration. As we can see, SLS plays the key role in the morphology control of uniform waxberry-like architecture. Its preferential adsorption onto different crystal facets makes those nanoflakes evolve into specific nanostrips that have a special surface charge distribution. Furthermore, such surface charge distribution induces these nanostrips self-assembled into the waxberry-like microarchitectures. Figure 7 shows the near-IR absorption spectrum of the La2(MoO4)3:Yb,Tm microarchitectures. We can see the strong 2F7/2 f 2F5/2 absorption band of Yb3+ centered at about 10 380 cm-1 (964 nm), which is in agreement with Stephan Heer and his
fellow’s work,27 and two weak absorption bands centered at about 12 619 cm-1 (792 nm) and 14 597 cm-1 (685 nm) due to the transitions of 3H6 f 3H4 and 3H6 cfar f 3F3 of Tm3+, respectively.28 The whole absorption range is about 9 51015 365 cm-1 (1052-651 nm), which can be used to estimate the excitation range. As Yb3+ ions have a broad and very strong absorption at ∼970 nm, the up-conversion fluorescent spectrum of the La2(MoO4)3:Yb,Tm microarchitectures was obtained by using a 970-nm diode laser for excitation, as depicted in Figure 8. From this figure we can see the dominant blue emission band around 21 191 cm-1 (472 nm) assigned to the transition of 1G4 f 3H6 of the Tm3+ ion and a very weak emission band around 15 445 cm-1 (647 nm), which is caused by the 1G4 to 3F4 transition of the Tm3+ ion.29 In comparison with the literature,30 the emission band induced by the transition of 1D2f3F4 of the Tm3+ ion does not show up in Figure 8. We presume that is because in the matrix of La2(MoO4)3 the fluorescence life time of the 1G4 energy level is not long enough to absorb another photon to transit into the 1D2 energy level. Figure 9 gives a
Controlled Construction of Monodisperse La2(MoO4)3 schematic energy level diagram showing the up-conversion mechanism of the La2(MoO4)3:Yb,Tm microarchitectures. Further investigation on ion doping and up-conversion fluorescent property of these unique waxberry-shaped La2(MoO4)3 microarchitectures is under way, as well as the possible potential application. Conclusion Monodisperse up-conversion luminescent La2(MoO4)3:Yb,Tm microarchitectures with uniform waxberry-like morphology have been prepared in large scale by an SLS-assisted hydrothermal process. These microarchitectures are pure and wellcrystallized tetragonal phases obtained by conventional hydrothermal treatment in a very narrow pH range around 8. It has been found that the preferential adsorption of SLS is crucial to construct such unique morphology. Such La2(MoO4)3:Yb,Tm microarchitectures have shown good blue up-conversion emission property. Acknowledgment. The authors would like to acknowledge support from the National Natural Science Foundation of China Research (Grant No. 50672115, 50502037), the National Project for Fundamental Research (Grant No.2002CB613300), and the Shanghai Rising-Star Program (Grant No. 07QA14061). References and Notes (1) Li, Y. D.; Li, X. L.; He, R. R.; Zhu, J.; Deng, Z. X. J. Am. Chem. Soc. 2002, 124, 1411. (2) Liu, H.; Hu, C.; Wang, Z. L. Nano Lett. 2006, 6, 1535. (3) Lou, X. W.; Zeng, H. C. Chem. Mater. 2002, 14, 4880. (4) Ma, Y.; Qi, L.; Ma, J.; Cheng, H. Langmuir 2003, 19, 4040. (5) Liu, H.; Cui, H.; Wang, J.; Gao, L.; Han, F.; Boughton, R. I.; Jiang, M. J. Phys. Chem. B 2004, 108, 13254. (6) Qi, L.; Ma, J.; Cheng, H.; Zhao, Z. J. Phys. Chem. B 1997, 101, 3460. (7) Wang, X.; Li, Y. J. Am. Chem. Soc. 2002, 124, 2880. (8) Liu, H.; Cui, H.; Han, F.; Li, X.; Wang, J.; Boughton, R. I. Cryst. Growth Des. 2005, 5, 1711.
J. Phys. Chem. C, Vol. 112, No. 11, 2008 4383 (9) Bai, J.; Qin, Y.; Jiang, C.; Qi, L. Chem. Mater. 2007, 19, 3367. (10) Wang, L. Z.; Omomo, Y.; Sakai, N.; Fukuda, K.; Nakai, I.; Ebina, Y.; Takada, K.; Watanabe, M.; Sasaki, T. Chem. Mater. 2003, 15, 2873. (11) Zhang, G. Q.; Lu, X. L.; Zhang, T.; Qu, J. F.; Wang, W.; Li, X. G.; Yu, S. H. Nanotechnology 2006, 17, 4252. (12) May, S. J.; Zheng, J. G.; Wessels, B. W.; Lauhon, L. J. AdV. Mater. 2005, 17, 598. (13) Wang, L. Z.; Ebina, Y.; Takada, K.; Sasaki, T. J. Phys. Chem. B 2004, 108, 4283. (14) Kale, B. B.; Baeg, J. O.; Lee, S. M.; Chang, H.; Moon, S. J.; Lee, C. W. AdV. Funct. Mater. 2006, 16, 1349. (15) Chen, X.; Sun, Z. Q.; Zheng, L. L.; Chen, Z. M.; Wang, Y. F.; Fu, N.; Zhang, K.; Yan, X.; Liu, H.; Jiang, L.; Yang, B. AdV. Mater. 2004, 16, 1632. (16) Wang, D. H.; Luo, H. M.; Kou, R.; Gil, M. P.; Xiao, S. G.; Golub, V. O.; Yang, Z. Z.; Brinker, C. J.; Lu, Y. F. Angew. Chem., Int. Ed. 2004, 43, 6169. (17) Hu, J. Q.; Bando, Y.; Zhan, J. H.; Yuan, X. L.; Sekiguchi, T.; Golberg, D. AdV. Mater. 2005, 17, 971. (18) Cao, M.; Liu, T.; Gao, S.; Sun, G.; Wu, X.; Hu, C.; Wang, Z. Angew. Chem., Int. Ed. 2005, 44, 4197. (19) Tian, Z. R.; Liu, J.; Voigt, J. A.; Mckenzie, B.; Xu, H. F. Angew. Chem., Int. Ed. 2003, 42, 414. (20) Zhang, H.; Yang, D.; Ji, Y.; Ma, X.; Xu, J.; Que, D. J. Phys. Chem. B 2004, 108, 3955. (21) Naruke, H.; Yamase, T. Inorg. Chem. 2002, 41, 6514. (22) Smet, F. D.; Ruiz, R.; Delmon, B.; Devillers, M. J. Phys. Chem. B 2001, 105, 12355. (23) Mandrik, A. V.; Troshin, A. E.; Kisel, V. E.; Yasukevich, A. S.; Klavsut, G. N.; Kuleshov, N. V.; Pavlyuk, A. A. Appl. Phys. B 2005, 81, 1119. (24) Yi, G. S.; Sun, B. Q.; Yang, F. Z.; Chen, D. P.; Zhou, Y. X.; Cheng, J. Chem. Mater. 2002, 14, 2910. (25) Huang, X. Y.; Hu, Z. S.; Lin, Z. B.; Wang, G. F. J. Cryst. Growth 2005, 276, 177. (26) Bu, W.; Xu, Y.; Zhang, N.; Chen, H.; Hua, Z.; Shi, J. Langmuir 2007, 23, 9002-9007. (27) Heer, S.; Lehmann, O.; Haase, M.; Gu¨del, H. U. Angew. Chem., Int. Ed. 2003, 42, 3180. (28) Gu¨ell, F.; Mateos, X.; Gavalda`, J.; Sole´, R.; Aguilo´, M.; Dı´az, F.; Galan, M.; Massons, J. Opt. Mater. 2004, 25, 73. (29) Heer, S.; Ko¨mpe, K.; Gu¨del, H. U.; Haase, M. AdV. Mater. 2004, 16, 2103. (30) Wang, F.; Chatterjee, D. K.; Li, Z. Q.; Zhang, Y.; Fan, X. P.; Wang, M. Q. Nanotechnology 2006, 17, 5789.
