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Langmuir 2007, 23, 9002-9007
Controlled Construction of Uniform Pompon-Shaped Microarchitectures Self-Assembled from Single-Crystalline Lanthanum Molybdate Nanoflakes Wenbo Bu, Yunpeng Xu, Na Zhang, Hangrong Chen, Zile Hua, and Jianlin Shi* State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China ReceiVed February 12, 2007. In Final Form: April 20, 2007 Uniform three-dimensional La2(MoO4)3 nanostructures with a pompon shape have been successfully constructed by a simple surfactant-free hydrothermal approach via self-assembly from single-crystalline nanoflakes. The formation of the uniform pompon-shaped La2(MoO4)3 microarchitectures is closely related to the presence of a proper amount of ammonium ions, and it is proposed that the pompon-shaped microarchitecture forms through an electrostatic attraction/repulsion effect between the oppositely charged flat surface and the edge of nanoflakes. Without the introduction of ammonium ions, no pompon-shaped microarchitectures can be formed, and while under the presence of excess ammonium ions, the nanoflakes on the micropompons become amorphous, twisted, and rugged. The novel microarchitectures of the product can be successfully modified from spherical to columelliform by using a mixed solvent of water/ethanol. This simple and efficient method may provide a practical reference to the controlled synthesis of other microarchitectures.
Introduction In recent years, large-scale self-assembly of micro-, meso-, and nanostructured building components has attracted great interest in the fields of materials synthesis and device fabrication.1-3 The synthesis of inorganic materials of complex morphology and texture, controlled crystallography, and microor nanoscale architectures is an important goal in various research and application fields such as adsorption, catalysis, optoelectronics, and drug or micromolecule transport systems.4 Several types of nanoarchitectures have been directly synthesized with different methods. For example, ZnO, AlN, and β-SiC “combs” were synthesized through a chemical vapor transportation method,5-9 CuO “dandelions” and R-Fe2O3 “micro-pines” were prepared by a hydrothermal method,10,11 fishbonelike or penniform BaWO4 was obtained from a microemulsion,12,13 polymer “hierarchical nanotrees” were cultured by electron irradiation,14 and so forth. It is highly desirable that the materials can be produced with low-cost and large-scale production capability; therefore, many efforts have been devoted to the exploration of * To whom correspondence should be addressed. Telephone: 86-2152412712. Fax: 86-21-52413122. E-mail:
[email protected]. (1) Bowden, N.; Terfort, A.; Carbeck, J.; Whitesides, G. M. Science 1997, 276, 233. (2) Cui, Y.; Lieber, C. M. Science 2001, 291, 851. (3) Wu, H.; Thalladi, V. R.; Whitesides, S.; Whitesides, G. M. J. Am. Chem. Soc. 2002, 124, 14495. (4) Mann, S.; Ozin, G. A. Nature 1996, 382, 313. (5) Yan, H. Q.; He, R. R.; Johnson, J.; Law, M.; Saykally, R. J.; Yang, P. D. J. Am. Chem. Soc. 2003, 125, 4728. (6) Liu, B.; Zeng, C. J. Am. Chem. Soc. 2004, 126, 8124. (7) Shen, G.; Bando, Y.; Chen, D.; Liu, B.; Zhi, C.; Golberg, D. J. Phys. Chem. B 2006, 110, 3973. (8) Yin, L.; Bando, Y.; Zhu, Y.; Li, M.; Li, Y.; Golberg, D. Cryst. Growth Des. 2007, 7, 35. (9) Shen, G.; Bando, Y.; Golberg, D. J. Phys. Chem. B 2006, 110, 3973. (10) Arabatzis, I. M.; Falaras, P. Nano Lett. 2003, 3, 249. (11) Cao, M.; Liu, T.; Gao, S.; Sun, G.; Wu, X.; Hu, C.; Wang, Z. Angew. Chem., Int. Ed. 2005, 44, 4197. (12) Zhang, X.; Xie, Y.; Xu, F.; Tian, X. B. J. Colloid Interface Sci. 2004, 274, 118. (13) Shi, H.; Wang, X.; Zhao, N.; Qi, L.; Ma, J. J. Phys. Chem. B 2006, 110, 748. (14) Cho, S. O.; Lee, E. J.; Lee, H. M.; Kim, J. G.; Kim, Y. J. AdV. Mater. 2006, 18, 60.
low temperature solution routes for the synthesis of inorganic materials. Among various aqueous synthetic methods, hydrothermal/solvothermal methods have shown great facility and flexibility. The morphology and crystal structure of the final products can be tuned by varying reaction conditions, such as reactant source, stoichiometry, pH value, reaction temperature and time, and ambient circumstances, during synthesis. Various types of surfactants have been widely used in most solution routes in the synthesis of well structured materials with controlled morphology thanks to their efficient self-assembly properties.15-17 However, the use of surfactants may introduce heterogeneous impurities and increase production costs, which is undesirable for the further development of research and applications.18 Therefore, it remains a significant challenge to develop facile and effective methods for the large-scale synthesis of novel hierarchical architectures assembled from independent and discrete nanobuilding blocks with controlled assembly properties. Molybdate and tungstate materials comprise a large class of inorganic compounds that exhibit interesting physical properties and thus have technological applications in the fields of catalysis and quantum electronics.19 Intensive research efforts have been focused on rare-earth molybdate compounds due to their unique optical, catalytic, and magnetic properties.20,21 These compounds have been widely used in various fields, such as high-quality phosphors, up-conversion materials, catalysts, and so forth.22 Most of these important properties originate from electron transitions within the 4f shell, and they are affected greatly by the composition and structures of rare-earth compounds, espe(15) Liu, Z. P.; Li, S.; Yang, Y.; Peng, S.; Hu, Z. K.; Qian, Y. T. AdV. Mater. 2003, 15, 1946. (16) Zhang, H.; Yang, D.; Ji, Y.; Ma, X.; Xu, J.; Que, D. J. Phys. Chem. B 2004, 108, 3955. (17) Zhao, N.; Qi, L. AdV. Mater. 2006, 18, 359. (18) Wu, C. Z.; Xie, Y.; Wang, D.; Yang, J.; Li, T. W. J. Phys. Chem. B 2003, 107, 13583. (19) Ano, T.; Ogata, N.; Miyaro, Y. J. Catal. 1996, 161, 78. (20) Smet, F. D.; Ruiz, R.; Delmon, B.; Devillers, M. J. Phys. Chem. B 2001, 105, 12355. (21) Pashchenko, V. A.; Jansen, A. G. M.; Kobets, M. I.; Khats’ko, E. N.; Wyder, P. Phys. ReV. B 2000, 62, 1197. (22) Naruke, H.; Yamase, T. Inorg. Chem. 2002, 41, 6514.
10.1021/la700404n CCC: $37.00 © 2007 American Chemical Society Published on Web 07/18/2007
Pompon-Shaped La2(MoO4)3 Microarchitectures
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Table 1. Experimental Conditions and Results of the Products sample/ condition
reaction system
solvent
pH adjustors
results
A
La(NO3)3 (0.12 M) + (NH4)6Mo7O24 (0.02557 M)
water
NaOH
spherical micropompons assembled with flat and well-crystallized nanoflakes
B
La(NO3)3 (0.12 M) + Na2MoO4 (0.18 M)
water
NaOH
flat and well-crystallized nanoflakes, no assembly property
C
La(NO3)3 (0.12 M) + (NH4)6Mo7O24 (0.02557 M)
water
ammonia
spherical micropompons assembled with twisted and amorphous nanoflakes with rough surfaces
D
La(NO3)3 (0.12 M) + (NH4)6Mo7O24 (0.02557 M)
water/ethanol (1:1)
NaOH
microcolums assembled with flat and well-crystallized nanoflakes
cially by the complexation state and the crystal field of the matrix in which rare-earth ions are coordinated. The R2(MO4)3 (R ) Al, Sc, and rare-earth ions; M ) W and Mo) family has promising trivalent cation conducting properties that attracted great research attention.23 Previous work has been focused on the synthesis of the bulk crystals,24 nanorods, and nanoparticles.25 Very recently, we have demonstrated the successful synthesis of threedimensional flowerlike europium-doped lanthanum molybdate microarchitectures via a facile hydrothermal process in the presence of the surfactant bis(2-ethylhexyl)-sulfosuccinate(AOT).26 However, the controlled construction of threedimensional lanthanum molybdate microarchitectures from independent and discrete nanobuilding blocks such as nanorods and nanoflakes via chemical self-assembly routes in the absence of surfactant still remains a challenge. Herein, the present research focuses on the surfactant-free hydrothermal recrystallization approach to construct uniform pompon-shaped La2(MoO4)3 microarchitectures assembled with nanoflakes. The influences of several variables (e.g., pH adjuster and ambient circumstances) on the final product’s crystallinity and architecture are addressed, and a self-assembly mechanism of such a microarchitecture is proposed.
Results and Discussion The pH value was found to be an important factor influencing the phase composition, purity, and morphology of the final product in the hydrothermal procedure.27-29 As shown in Figure 1, pure and well crystallized tetragonal La2(MoO4)3 can only be obtained within a very narrow pH range of 8-9 under condition A in Table 1, and hence the pH value of the reaction system was maintained at 9. Figure 2 shows the XRD patterns of samples A-D, all synthesized at pH ) 9. When using NaOH to adjust the pH value (samples A, B, and D), all the peaks of the samples can be perfectly indexed as tetragonal La2(MoO4)3 (space group: I41/a), which is consistent with the literature values (JCPDS no. 45-0407), instead of the monoclinic phase reported previously.30 The calculated cell parameters based on XRD data are a ) 5.343 and c ) 11.78, which are in good agreement with the bulk material (JCPDS no. 45-0407). When using ammonia as the pH adjuster (sample C), only amorphous La2(MoO4)3 can be obtained. This is believed to result from the effect of the ammonium ions, as will be discussed later. Sample A was prepared in a pure water system. As shown in Figure 3, the sample is composed of nanoflakes that selfassembled into micrometer-scale spheres named micropompons.
Experimental Section All chemicals were analytically pure and used without further purification. For the synthesis of La2(MoO4)3 pompon-shaped microarchitectures, 25 mL of La(NO3)3‚6H2O aqueous solution (0.12 M) was added slowly into 25 mL of (NH4)6Mo7O24‚4H2O aqueous solution (0.02557 M) dropwise under strong magnetic stirring. The amorphous white precipitate formed immediately. The different pH values of the solution were then adjusted using concentrated aqueous ammonia or NaOH under stirring. After being vigorously stirred for 30 min, the resulting precursor suspension (∼50 mL) was transferred into an 80 mL capacity Teflon-lined stainless steel autoclave, which was subsequently heated to 180 K and maintained at that temperature for 12 h. The autoclave was then allowed to cool to room temperature. The product was filtered, washed several times with deionized water and absolute ethanol, and dried in vacuum at 130 K for 6 h. Four samples (named A-D) were obtained. The different experimental conditions are summarized in Table 1. X-ray powder diffraction (XRD) patterns were obtained on a Rigaku D/Max-X-ray diffractometer with graphite-monochromatized Cu KR radiation. Field emission scanning electron microscopy (FESEM) images were taken on a JEOL JSM-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. (23) Imanaka, N.; Ueda, T.; Okazaki, Y.; Tamura, S.; Adachi, G. Chem. Mater. 2000, 12, 1910. (24) Huang, X. Y.; Hu, Z. S.; Lin, Z. B.; Wang, G. F. J. Cryst. Growth 2005, 276, 177. (25) Yi, G. S.; Sun, B. Q.; Yang, F. Z.; Chen, D. P.; Zhou, Y. X.; Cheng, J. Chem. Mater. 2002, 14, 2910. (26) Zhang, N.; Bu, W.; Xu, Y.; Jiang, D.; Shi, L. J. Phys. Chem. C 2007, 111, 5014.
Figure 1. XRD patterns of La2(MoO4)3 samples synthesized at different pH values under condition A in Table 1.
Figure 2. XRD patterns of La2(MoO4)3 samples obtained under different hydrothermal conditions. A-D are the sample names as listed in Table 1.
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Figure 3. Images of sample A: (a and b) SEM images of La2(MoO4)3 micropompons; (c) TEM images of one La2(MoO4)3 micropompon; and (d) TEM image of La2(MoO4)3 nanoflakes after prolonged ultrasonic dispersion and its SAED pattern. Scale bar: (a) 100 nm, (b) 1 µm, (c) 0.5 µm, and (d) 200 nm.
Figure 4. Images of sample B: (a) TEM image of La2(MoO4)3 nanoflakes and (b) SEM image of the nanoflakes. Scale bar: (a) 200 nm and (b) 100 nm.
Figure 3a shows a uniform micropompon of a diameter of ∼3.5 µm. The micropompons are highly porous, and most of the nanoflakes are linked together by an edge-to-flat-surface conjunction. No obvious flat-to-flat or edge-to-edge attachments can be identified. Therefore, such an architecture is a result of some kind of self-assembly, rather than random aggregation between nanoflakes. From the several erectly assembled nanoflakes in the radial direction on the surface of the micropompon, the thickness of the nanoflakes can be judged to be 30-40 nm. Figure 3b shows several micropompons that are tangentially attached to each other. The size and morphology of the micropompons are uniform, and no single nanoflake can be found, showing that almost all of the nanoflakes have been self(27) Yu, S. H.; Liu, B.; Mo, M. S.; Huang, J. H.; Liu, X. M.; Qian, Y. T. AdV. Funct. Mater. 2003, 13, 639. (28) Li, Q. S.; Feng, C. H.; Jiao, Q. Z.; Guo, L.; Liu, C. M.; Xu, H. B. Phys. Status Solidi 2004, 201, 3055. (29) Fang, Y. P.; Xu, A. W.; Qin, A. M.; Yu, R. J. Cryst. Growth Des. 2005, 5, 1221. (30) Xu, Y. P.; Jiang, D. Y.; Bu, W. B.; Shi, J. L. Chem. Lett. 2005, 34, 978.
assembled into micropompons. Figure 3c is the representative TEM image of a single micropompon. A TEM image (Figure 3d) from the same sample after prolonged ultrasonic dispersion shows that the micropompon is composed of striplike regularly shaped nanoflakes with smooth surfaces, and these nanoflakes are 20-200 nm in width. Its corresponding SAED pattern at the top right corner of Figure 3d reveals that the ED pattern of these nanoflakes is characteristic of tetragonal La2(MoO4)3, in accordance with the XRD result. Moreover, the SAED patterns taken both from different areas on a single nanoflake and from different nanoflakes were found to be identical within experimental accuracy, indicating that the La2(MoO4)3 nanoflakes are single-crystalline and the different nanoflakes have an identical crystallization habit. In addition to pH adjustment, the starting materials also play an important role in determining the shape and crystallinity of the product. As shown in Figure 4, when using Na2MoO4 as the Mo source instead of (NH4)6Mo7O24 (sample B), nanoflakes
Pompon-Shaped La2(MoO4)3 Microarchitectures
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Figure 5. Representative SEM images of sample C: (a) surface of a single micropompon composed of twisted La2(MoO4)3 nanoflakes and (b) several micropompons of 3-4 µm in diameter. Scale bar: (a) 100 nm and (b) 1 µm.
cannot assemble with each other to form spherical micropompons. The crystallinity of the final product remains unchanged. When ammonia is used to raise the pH value of the reactant solution (sample C), the resultant morphology is shown in Figure 5. Irregularly shaped and twisted nanoflakes, rather than well crystallized and flat La2(MoO4)3 nanoflakes, have been obtained due to the presence of the excessive amount of ammonium ions, as seen in Figure 5a. The rugged nanoflakes are twisted into a foamlike morphology. The thickness of the flake is still 30-40 nm, but the surface has become rough and rugged. Figure 5b shows the porously assembled micropompon sphere of twisted La2(MoO4)3 nanoflakes with a diameter of 4-5 µm. Such morphological differences from sample A in both the nanoflakes and micropompon spheres should be caused by the existence of the ammonium ions. The corresponding XRD pattern in Figure 2c shows that the twisted La2(MoO4)3 nanoflakes are almost noncrystalline. In the current synthesis route, the obtained La2(MoO4)3 microstructures with varied morphologies synthesized from the precipitated amorphous precursor might be formed through a dissolution-recrystallization growth process as widely used in the hydrothermal process.31 To investigate the growth process of pompon-shaped microarchitectures self-assembled from singlecrystalline La2(MoO4)3 nanoflakes, we conducted a series of parallel experiments for different reaction times with the other synthetic conditions remaining unchanged. The XRD patterns and FE-SEM images of the samples synthesized under condition A (see Table 1) but with different reaction times are shown in Figures 6 and 7. Three obvious evolution stages could be clearly observed as shown in Figure 7. At the early stages, an examination of the intermediate products collected after 1 h of hydrothermal treatment showed that there existed a large number of spherelike nanoparticles (Figure 7a), while the intermediates collected after 3 h were two-dimentional nanoflakes (Figure 7b). It was clear that the nanoparticles grew and/or orientedly attached with each other to form two-dimentional flakes. Further increasing the reaction time to 6 h resulted in the formation of a mixture of nanoflakes and pompon-shaped microarchitectures (Figure 7c). After 12 h of reaction, uniform pompon-shaped microarchitectures self-assembled from single-crystalline La2(MoO4)3 nanoflakes were finally synthesized, as shown in Figure 7d. Based on the above experimental results, the formation process of the pompon-shaped microarchitectures self-assembled from single-crystalline La2(MoO4)3 nanoflakes can be illustrated as follows. When the simple precipitation reaction took place, amorphous precipitates of lanthanum molybdate formed. When the precipitate was transferred into the autoclave, the tiny nuclei (31) Dawson, W. J. Am. Ceram. Soc. Bull. 1988, 67, 1673.
Figure 6. XRD patterns of La2(MoO4)3 synthesized under condition A in Table 1 but with different reaction times.
formed in advance under hydrothermal conditions. The formation mechanism for the La2(MoO4)3 nanoflakes can be simply depicted as an Ostwald ripening process: tiny crystalline nuclei of La2(MoO4)3 in a supersaturated medium formed in advance which was followed by crystal growth, or oriented attachment, at the cost of the amorphous precipitates and/or the small crystals. Because the growth rates of different facets are different, La2(MoO4)3 nanoflakes are formed. The nanoflakes could then grow into varied microstructures at the final stage.32 Under certain conditions, that is, at pH ) 9 and in the presence of a certain amount of ammonium ions introduced from the raw material (sample A), the surfaces of the nanoflakes would most probably be charged. The charges on the flat surface and on the edge flanks should be different due to the different atomic arrangements, as schematically shown in Figure 8. Therefore, in this case, it is referred that such opposite surface charges should lead to a kind of edge-to-flat-surface linkage through self-assembly by electrostatic attraction/repulsion. Finally, highly porous pompon-shaped microarchitectures self-assembled from singlecrystalline La2(MoO4)3 nanoflakes formed in homogeneous solutions due to such an electrostatic interaction, as can be identified in Figure 3a. A schematic illustration is presented in Figure 8. From the above analyses, it can be seen that ammonium ions contribute very much to the assembly of the architectures, as no such architecture could form under the absence of ammonium ions (sample B), and in this case La2(MoO4)3 nuclei can only grow freely into flat flakes. However, the presence of an excess amount of ammonium ions (sample C, by using ammonia as the (32) Zhang, H.; Yang, D. R.; Li, D. S.; Ma, X. Y.; Li, S. Z.; Que, D. L. Cryst. Growth Des. 2005, 5, 547.
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Figure 7. FE-SEM images of La2(MoO4)3 synthesized under condition A in Table 1 but with different reaction times: (a) 1 h, (b) 3 h, (c) 6 h, and (d) 12 h.
Figure 8. Schematic representation of the assembly mechanism of pompon-shaped microarchitectures self-assembled from singlecrystalline La2(MoO4)3 nanoflakes.
pH adjuster) would lead to the twisted morphology of amorphous nanoflakes with rugged surfaces, though spherical architectures still remain. During the recrystallization process in hydrothermal treatment, the presence of an excess amount of ammonium ions may interact with the surface -OH groups on the particle surface due to its strong affinity to the hydroxyl groups and the possible complexing effect with MoO42-, which interferes with the crystallization process. Therefore, the periodic atomic arrangement was prevented, and the product became amorphous after hydrothermal treatment. The spherical architectures still remain in the excess of ammonium ions due to the electrostatic attraction/ repulsion effect mentioned above. Also due to such a strong interaction, the nanoflakes become twisted and the surfaces become rugged. Such an interaction between the ammonium ions and -OH groups has been formerly reported by Arabatzis et al.10 in the synthesis of TiO2 foam with twisted flakes, whose morphology is similar to that in Figure 5a. In their case, TiO2 nanocrystals were twisted by a strong interaction between the hydrophilic amine headgroups on hexadecylamine (HDA) and the surface hydroxyl groups present on the TiO2 surface.
To modify the assembling properties of the nanoflakes, a mixed solvent of C2H5OH/H2O (1:1) was used, and the result is shown in Figure 9. In this case, nanoflake-assembled microcolumns formed. The columns are 1-2 µm in diameter and 4-6 µm in length. Figure 9b shows the surface of a column. The nanoflakes are nearly parallel with each other, and the thickness of the nanoflakes is ∼30-40 nm. Figure 9a and c also shows that the nanoflakes are mostly inclined to the axes of the column. Figure 9d shows one parallelogram nanoflake from the same sample after prolonged ultrasonic dispersion. Ethanol is a common and widely used solvent. The introduction of ethanol into the hydrothermal procedure can efficiently decrease the temperature for YAG synthesis.33,34 As ethanol has no chelating property, no hydrogen bonding was possible.35 However, the introduction of ethanol could greatly change the solution polarity, and therefore, the solubility and recrystallization process of La2(MoO4)3 in hydrothermal treatment would be changed. Similarly, the state of the surface charges on the nanoflakes also changed, which affected the assembly properties of the nanoflakes, and the morphology of the microarchitectures varied from spherical to columelliform. The study of the detailed mechanism is still in progress. Lanthanum molybdate has been shown to be a useful host for other lanthanide ions, which generate phosphors emitting in the UV-vis region and show a potential application for the microarchitectures to serve as efficient UV-vis phosphors in luminescent nanodevices.26 Considering the similarity of their crystal structures and lattice constants, rare-earth-doped lanthanum molybdate pompon-shaped microarchitectures could be prepared by the same hydrothermal growth process as the undoped products, and doping alters neither the crystal structure nor the morphology of the host materials. Further investigations of the (33) Inoue, M.; Otsu, H.; Kominami, H.; Inui, T. J. Am. Ceram. Soc. 1991, 74, 1452. (34) Zhang, X. D.; Liu, H.; He, W.; Wang, J. Y.; Li, X.; Boughton, R. I. J. Alloys Compd. 2004, 372, 300. (35) Gorai, S.; Ganguli, D.; Chaudhuri, S. Cryst. Growth Des. 2005, 5, 875.
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Figure 9. Images of sample D: (a and b) SEM images of La2(MoO4)3 columns; (c) TEM image of La2(MoO4)3 columns; and (d) single La2(MoO4)3 nanoflake found after prolonged ultrasonic dispersion. Scale bar: (a) 100 nm, (b) 100 nm, (c) 1 µm, and (d) 100 nm.
rare-earth-doped lanthanum molybdate pompon-shaped microarchitectures with high luminescence efficiency are in progress.
Conclusion In summary, we have demonstrated a simple and mild surfactant-free hydrothermal approach for the synthesis of La2(MoO4)3 microarchitectures (micropompons and microcolumns) composed of nanoflakes with 3D microarchitecture. It has been proved that the formation of the micropompon is closely related to the presence of a proper amount of ammonium ions probably through an electrostatic attraction/repulsion effect between the oppositely charged flat surface and edge of the nanoflakes. However, when excess ammonium ions were introduced, the nanoflakes become amorphous and twisted, but the spherical
assembly property remained. The assembly property of the product was successfully modified from spherical to columelliform by introducing ethanol to the system. It is believed that there are still great spaces in the conventional hydrothermal approach to modify the architecture morphology of the products through this facile and low-cost process without the addition of surfactants or chelating reagents. Acknowledgment. The authors would like to acknowledge support from the National Natural Science Foundation of China Research (Grant Nos. 50672115 and 50502037) and the National Project for Fundamental Research (Grant No. 2002CB613300). LA700404N
