Hydrothermal Synthesis of Bi2WO6 Uniform Hierarchical

Jun 19, 2007 - Xin-Yao Yu , Ren-Xia Xu , Chao Gao , Tao Luo , Yong Jia , Jin-Huai Liu , and ...... Yumin Liu , Zijing Li , Hua Lv , Haibo Tang , Xinya...
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Hydrothermal Synthesis of Bi2WO6 Uniform Hierarchical Microspheres Yuanyuan Li,*,† Jinping Liu,† Xintang Huang,*,†,‡ and Guangyun Li† Department of Physics, Central China Normal UniVersity, Wuhan 430079, P. R. China and Key Laboratory of Ferroelectric and Piezoelectric Materials and DeVices of Hubei ProVince, Hubei UniVersity, Wuhan 430062, P. R. China

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 7 1350-1355

ReceiVed April 8, 2007; ReVised Manuscript ReceiVed May 10, 2007

ABSTRACT: Bi2WO6 uniform hierarchical microspheres were grown on a large scale at 180 °C by a simple hydrothermal method with the help of the surfactant poly(vinyl pyrrolidone) (PVP). X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) were used to characterize the product. The result indicated that three-dimensional (3D) Bi2WO6 microspheres were constructed layer-by-layer from a large number of two-dimensional (2D) sheets, which were composed of numerous interconnected square nanoplates with a mean side length of 65 nm. Pore-size distribution analysis showed that both mesopores and macropores existed in the 3D microstructures. The formation mechanism was discussed on the basis of the results of time-dependent experiments. It was demonstrated that PVP played a key role in the formation of such hierarchical microspheres. By adjusting the amount of PVP, Bi2WO6 with different morphologies can be attained accordingly. UV-vis spectroscopy was further employed to estimate the band gap energy of the hierarchical structures. Our work may shed some light on the design of other well-defined complex nanostructures, and the as-grown architectures may have potential applications. 1. Introduction It is believed that there is a close relationship between the morphology and the properties of inorganic materials. For example, isotropic or anisotropic behavior and region-dependent surface reactivity are strongly related to the size, shape, and dimensionality of materials.1 As a result, there has been considerable effort in synthesizing inorganic materials with controlled shapes of architectures, which are desirable for many applications in optics, electronics, biology, medicine, and energy/ chemical conversions. In particular, the alignment of nanostructure building blocks (nanoparticles, nanorods, and nanoribbons/ nanoplates) into three-dimensional (3D) ordered superstructures by bottom-up approaches has been an exciting field in recent years.2 To date, a wide variety of inorganic materials, including metal, metal oxide, sulfide, hydrate, and other minerals, have been successfully prepared with hierarchical shapes.3-7 A solution-phase chemical method was one of the most promising routes in these reports, due to its low cost and potential advantage for large-scale production.4,6,7 Exploration of reasonable synthetic methods for controlled construction of complex 3D architectures of other functional materials via a chemical self-assembly route is still an intensive and hot research topic. For the self-assembly of nanostructures, organic capping reagents usually play critical roles in reducing the activity of nanobuilding blocks to promote or tune the ordered selfassembly.6b,7b Relative to the isotropic self-assembly of spherical or near-spherical nanoparticles, the self-assembly of anisotropic nanostructures, such as nanoplates, nanosheets, nanorods, and nanotubes, requires more effort. Metal tungstates, as one kind of multicomponent metal oxide compound, have received great interest over the past few years because of their potential applications in photoluminescence, microwave, optical fibers, catalysts, magnetic devices, and humidity sensors.8 Bismuth tungstate (Bi2WO6), which has a * To whom correspondence should be addressed: Fax +86-02767861185;e-mail: [email protected](X.H.);[email protected] (Y.L.). † Central China Normal University. ‡ Hubei University.

layered structure with the perovskite-like slab of WO6 and [Bi2O2]2+ layers and possesses excellent intrinsic chemical properties, is one of the simplest members of the Aurivillius oxide family.9 In addition, Bi2WO6 has many interesting and important physical properties such as pyroelectricity, ferroelectricity/ piezoelectricity, oxide anion conducting, and a nonlinear dielectric susceptibility.9,10 It can also be used as an excellent photocatalyst and a solar energy transfer material. Kudo11 and Zou12 have reported that Bi2WO6 had photocatalytic activities for O2 evolution/water splitting and could degrade the organic compound (CHCl3 and CH3CHO) under visible light irradiation. However, all these works focused on the high-temperature solidstate metathesis reaction for the preparation of Bi2WO6 materials, the shortcomings of which were the high-energy consumption and the generation of particles with large sizes. Very recently, a soft chemical method was demonstrated to be efficient in synthesizing nanosized Bi2WO6.8b,13,14 Simple nanostructures of Bi2WO6 including low-dimensional nanoparticles,13 nanorods,8b and nanoplates14 were attained accordingly. Since photocatalysts with special 3D morphologies and high efficiency of electron-hole separation usually possess high photocatalytic activities and can be more easily separated and recycled, the synthesis of hierarchical architectures with pores is essentially necessary from the practical point of view. Nevertheless, the morphological control of Bi2WO6 nanostructures, especially of hierarchical structures, is relatively unexplored due to the lack of synthetic capability. Herein, we report for the first time the hydrothermal synthesis of Bi2WO6 uniform hierarchical microspheres at 180 °C in the presence of poly(vinyl pyrrolidone) (PVP). The as-grown spheres consist of a number of stacked two-dimensional (2D) sheets, revealing a layer-by-layer growth style. In addition, each sheet is assembled by numerous square nanoplates via the “oriented attachment” mechanism. It is found that capping ligand PVP in the reaction mixture can prevent the randomly aggregation of nanoplates and promote the delicate assembly of these plates into 3D microspheres. Different amounts of PVP can result in different morphologies of Bi2WO6. The band gap energy of the microspheres is determined to be 2.825 eV by optical absorption. Pore-size distribution analysis results indicate

10.1021/cg070343+ CCC: $37.00 © 2007 American Chemical Society Published on Web 06/19/2007

Bi2WO6 Hierarchical Microspheres

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Figure 1. (a) Low-magnification and (b) high-magnification SEM images of the as-prepared hierarchical microspheres. (c) SEM image of an individual microsphere. (d) Enlarged image of the surface of the sphere.

that there are both mesopores (2-50 nm) and macropores (>50 nm) in the 3D spherical microstructures. 2. Experimental Section All chemicals are of analytical grade used without further treatments. In a typical procedure, 0.329 g (0.001 mol) of Na2WO4‚2H2O and 0.971 g (0.002 mol) of Bi(NO3)3‚5H2O were put into two beakers. Then, 40 mL of distilled water was added respectively to the two beakers under magnetic stirring at room temperature. After that, 0.15 g of PVP K30 was added to the Na2WO4 aqueous solution to form a homogeneous mixture, and the mixture was subsequently dropped to the Bi(NO3)3 aqueous solution slowly. Plenty of precipitation appeared after the addition. The resulting precursor suspension was transferred into a Teflon-lined stainless steel autoclave (V ) 100 mL). The autoclave was sealed and maintained at 180 °C for 12 h under self-generated pressure and then allowed to cool to room temperature naturally. The product was filtered off, washed several times with absolute alcohol and distilled water, and finally dried in a vacuum at 60 °C for 5 h. Because of the absorption of blue light, the color of the powder appeared to be light yellow. Control experiments were carried out by varying the amount of PVP while keeping other experimental conditions unchanged. The morphology and composition of the as-prepared product were characterized by field-emission scanning electron microscopy (FESEM, JEOL, JSM-6700F) operated at an acceleration voltage of 5.0 kV and equipped with an energy-dispersive X-ray spectroscopy (EDS). The crystalline structure of the product was analyzed by an X-ray diffractometer (XRD, Y-2000) with CuKR radiation (λ ) 1.5418 Å) at a scan rate of 0.04° s-1. Transmission electron microscopy (TEM) and highresolution ransmission electron microscopy (HRTEM) observations were carried out on a JEOL JEM-2010 instrument in bright field and selected area electron diffraction (SAED) modes and on a HRTEM JEM-2010FEF instrument (operated at 200 kV). Room temperature UV-vis absorption spectrum was recorded on a UV-2550 spectrophotometer in the wavelength range of 200-900 nm. The nitrogen adsorption-desorption isotherm and Barrett-Joyner-Halenda (BJH) methods were analyzed on a Micromeritics ASAP 2010 analyzer (accelerated surface area and porosimetry system).

3. Results and Discussion 3.1. Synthesis of Bi2WO6 Hierarchical Microspheres. Figure 1a is a typical low-magnification scanning electron microscopy (SEM) image of the as-grown product, from which a number of uniform spheres with an average diameter of 4 µm are clearly observed. No other morphologies can be detected, indicating a high yield of these 3D microspheres. Higher

Figure 2. (a) XRD and (b) EDS results of the as-prepared hierarchical microspheres.

magnification SEM images shown in Figure 1b-d demonstrate the detailed structural information of the sample. As can be seen in Figure 1b, the observed spheres are constructed by many 2D sheets, which are densely packed and form a multilayered structure. Careful examination shows that some microspheres actually contain a concave in the middle section (marked by an arrowhead in Figure 1c), which is related to the formation process and will be discussed in the later section. In fact, many concaves can be observed from other microspheres in Figure 1a (see arrowheads), although they are sometimes filled by multilayers. In Figure 1d, we can further find that each layer/ sheet is made up of numerous square nanoplates with an average side length of ∼65 nm. The tiny plates are attached side by side into integrated sheets, which is similar to the process of forming a CuO elliptical nanosheet from small nanoribbons.4e On the basis of the above results, the as-prepared microspheres can be generally classified as hierarchical structures. It is worth mentioning that these hierarchical microspheres are sufficiently stable that they cannot be destroyed into dispersed nanoplates even after long periods of ultrasonication. XRD pattern of as-prepared product is shown in Figure 2a. All of the diffraction peaks can be clearly indexed as an orthorhombic phase of Bi2WO6 and match very well with the reported data (JCPDS card No. 73-1126, a ) 5.457 Å, b ) 5.436 Å, c ) 16.42 Å). No impurity peaks are observed, indicating that our product is pure. In the inset of Figure 2a, a standard XRD pattern of Bi2WO6 is given. The intensity ratio of the (200) or (020) peak to the (113) peak in our result is 0.64, obviously larger than the standard value which is only 0.185. This important result indicates that the crystal has special anisotropic growth along the (200)/(020) plane, in good agreement with the platelike morphology of Bi2WO6. Similar phenomena can be obtained in our other experiments. Consistent with the XRD result, the EDS result (Figure 2b) also demon-

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Figure 3. (a) TEM image and SAED result of an individual Bi2WO6 sphere. (b) TEM image and its corresponding SAED of a sheet/layer. (c) Enlarged TEM image of a single nanoplate, showing a 90° angle between adjacent sides. (d) HRTEM image of the nanoplate, indicating its single-crystal nature.

strates only the elements of Bi, W, and O are contained in the sample and the atomic ratio of Bi to W to O is equal to 2:1:6. Further investigation was carried out by TEM to reveal the organization of such self-assembled complex structures. Figure 3a presents an individual microsphere with a zigzag circle, in accordance with the SEM images (such as Figure 1d). The light

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color in the center indicates the Bi2WO6 microsphere should be loose in the middle part. Combined with the observed concaves in SEM, it is reasonable to conclude that the central section of the hierarchical microspheres is thinner than the edges of such crystals. Select area electron diffraction (SAED) results of the whole microsphere are further illustrated in the inset of Figure 3a, revealing its polycrystalline structure rather than welldefined single crystal. However, the pattern is not a ring; the arc-like spots have obvious symmetry, which directly demonstrates that the spheres should consist of single-crystal building blocks.4f,6b Figure 3b is the enlarged TEM image of the area marked by a red rectangle (in Figure 3a). Obviously, square nanoplates can be seen, which assemble into one sheet/layer with their sides. The SAED of this part for the [001] zone is shown in the inset. The pattern exhibits a regular and clear square diffraction spot array and can be indexed to single crystalline Bi2WO6. The 2D exposed surface and four edges of Bi2WO6 plates also can be determined to be (001), (220), (22h0), (2h20), and (2h2h0) planes, respectively. High-resolution TEM (HRTEM) image shown in Figure 3c was recorded on the corner of a square plate. Some round particles of 10 nm on the surface are observed. They appeared just after a few seconds of irradiation of a high-energy electron beam, indicating the asprepared nanoplates are unstable in that case. From Figure 3d, the lattice interplanar spacing is measured to be 0.193 nm, corresponding to the (220) plane of orthorhombic Bi2WO6. This result also confirms the single-crystal structure of nanoplates. To reveal the growth process of Bi2WO6 hierarchical microspheres, time-dependent experiments were carefully conducted. The products were collected at different stages from the reaction mixture, and then their morphologies were investigated by SEM. The images of the two typical products attained after 2 and 7 h of reaction are displayed in Figure 4a and b, respectively. As shown in Figure 4a, square nanoplates are attained at first, which

Figure 4. SEM images of Bi2WO6 products attained after (a) 2 h and (b) 7 h of reaction. (c-e) Different morphologies observed in Figure 4b. The rectangle shown in panel a indicates the nanosheets assembled by tiny square nanoplates.

Bi2WO6 Hierarchical Microspheres

Figure 5. Schematic illustration of the growth process of Bi2WO6 hierarchical microspheres.

have similar shape and size to those observed in Figure 1d. In addition, it can be detected that some plates tend to organize into a sheet (marked by a rectangle). After an additional 5 h of reaction, hierarchical architectures of Bi2WO6 are formed, as illustrated in Figure 4b. Close examination demonstrates that there are mainly three kinds of hierarchical structures, the SEM images of which are shown in Figure 4c-e. The first kind is a flat nanodisk with a multilayered structure. Each layer is made up of tiny square nanoplates (Figure 4c); the second kind is a distorted and curved disk that can be considered as a flat disk with some tilted sheets grown on it (see arrowheads in Figure 4d). The tilted sheets prefer to assemble at the edges of the flat disks and interconnect with each other around the edges, resulting in concaves in the middle part; the third type of the architecture is multilayered quasi-microsphere with larger size (Figure 4e), which is very similar to that shown in Figure 1. Concaves that are smaller than those in Figure 4d can be seen on the top of these microspheres. A further increase in the reaction time leads to the formation of more 3D microspheres. For time close to 12 h, high-yield and uniform hierarchical microstructures could be produced (Figure 1). 3.2. Growth Mechanism. On the basis of the SEM and TEM observations, along with crystal structure and chemical composition analyses, it can be concluded that the formation of such intricate microspheres is achieved via a hierarchical assembly process. The main evolving steps are schematically illustrated in Figure 5. Bi2WO6 tiny nanoplates should be formed at the early stage, followed by the oriented attachment of these building blocks into 2D nanosheets, and finally the arrangement of the sheets into 3D hierarchical microspheres would take place. In our study, the formation of primary Bi2WO6 nanoplates was a typical Ostwald ripening process. At first, the generation of tiny crystalline nuclei in a supersaturated solution occurred and then followed by crystal growth. The larger particles grew at the cost of the smaller ones because there was different solubilities between relatively larger and smaller particles according to the Gibbs-Thomson law.15 The further crystal growth for the formation of 2D nanostructures was strongly related to the intrinsic crystal structure of Bi2WO6. It has been reported that orthorhombic Bi2WO6 is constructed by a cornershared WO6 octahedral layer and [Bi2O2]2+ atom layers sandwiched between WO6 octahedral layers.9a The layer is parallel to the (001) facets. On the basis of this structure, the chains of octahedral-W equally exist along the a- and b-axes, which indicates that the (200) and (020) faces have a much

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higher chemical potential in comparison with other facets.8b As discussed previously,14a the above feature resulted in faster growth rates of the (200) and (020) faces, and thus the crystal growth was preferentially along the layer. After 2 h of reaction, Bi2WO6 square nanoplates were attained. In the subsequent assembly process, PVP played an important role in the formation of integrated nanosheets and multilayered structures. It is generally believed that the building blocks for oriented aggregation/attachment are usually nanoparticles with surfaces stabilized by organic coating, and weakly protected nanoparticles often undergo entropy-driven random aggregation.6a,b In that regard, selective adsorption and subsequent controlled removal of organic additives at interfaces play important roles in rotating adjacent nanoparticles so that they share an identical 3D crystallographic orientation.6b In the past years, PVP has been applied as an important surfactant for the synthesis of nanomaterials, and various nanostructures were successfully fabricated with the assistance of PVP.16 In the present case, it was believed that the selective adsorption of PVP on various crystallographic planes of Bi2WO6 nanoplates was of great importance at the initial growth stage. As evidenced by TEM and SAED results, the assembled nanosheets/layers were constructed with tiny nanoplates, with their top and lateral surfaces enclosed by {001} and {220} planes, respectively. Thus, PVP should have stronger interactions with {001} planes. In contrast, it might be adsorbed relatively more sparsely on {220} planes, which enabled it to be removed from these crystal planes more easily, resulting in preferential assembly along the layer (a × b layer plane) rather than along the c-axis ([001] direction). That is, the initially formed plates assembled in an edge-to-edge way with the gradual enlargement of the 2D surface areas. With further rotation of adjacent nanoplates to share the same 2D crystallographic orientation and subsequent coalescence between these building blocks, well-shaped nanosheets/layers could be formed. For the formation of 3D microspheres, a layer-by-layer growth style should be considered (Figure 5). As observed in SEM, the in-situ formed nanosheets tended to stack along the [001] direction at first, forming flat multilayered disks. This process could further reduce the surface energy of nanosheets, and the stacking in this fashion was facilitated by the special 2D geometrical shape. With a longer reaction time, however, instead of piling up to form a column, the disks preferred to wrinkle up and form the notched template for further development. This specific growth fashion should be due to lattice tension or surface interaction in the edge areas of the sheets.17 The following growth was confined within the concave portion. More new tilted sheets would be added into the notched template, and repeating the above process could result in thickening of the edges of structures and a stagnating growth in the middle section. Simultaneously, the shape of Bi2WO6 evolved from 2D flat disks to semiconvex superstructures, and finally to 3D hierarchical microspheres with a smaller concave. Thus, it is reasonable that the thickness of the edge of spheres is always larger than that of the middle section. The above stacking process is analogous to that observed previously for some biomaterials.18 In these cases, the interaction between inorganic materials and polymers was also the main driving force for self-assembly. We further emphasize the importance of PVP in the formation of well-defined hierarchical structures. Our control experiments have shown that the amount of PVP is also crucial in the synthesis. By adjusting the amount of PVP, Bi2WO6 with different morphologies can be obtained accordingly, and the corresponding SEM images are shown in Figure 6a-f. When

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Figure 7. XRD results of the Bi2WO6 products prepared by using different amounts of PVP.

Figure 6. SEM images of Bi2WO6 powders obtained by using (a, b) 0.05 g of PVP. (c, d) 0.10 g of PVP. (e) 0.00 g of PVP. (f) 0.20 g of PVP.

there was no PVP used, the primary nanoplates suffered from entropy-driven random aggregation; thus, ill-defined nanostructures and their assemblies were formed (Figure 6e). When the amount of PVP was 0.05 g, 2D disklike shapes and 3D complex structures could be detected, although the dominant product remained dispersed and aggregated plates (Figure 6a). Figure 6b illustrates a typical SEM image of a quasi-sphere of Bi2WO6 attained in this case. It can be seen that this structure is loose with an irregular shape and it seems to be underdeveloped. It cannot undergo strong ultrasonication, which is very different from the hierarchical microspheres shown in Figure 1. Further increase in the amount of PVP (0.10 g) led to the generation of relatively regular 2D and 3D superstructures (Figure 6c,d), and nearly no individual nanoplates can be observed. These results directly confirmed that PVP could promote the delicate assembly. However, excessive PVP was not beneficial for the formation of well-defined microstructures (Figure 6f, 0.20 g of PVP). We proposed that PVP might be strongly adsorbed even on some {220} planes of nanoplates when plenty of PVP was used, which reduced the assembly potential necessary for the growth of well-constructed hierarchical microspheres. The XRD results of the products obtained by using various amounts of PVP are summarized in Figure 7, from which a pure orthorhombic phase of Bi2WO6 can be identified. 3.3. Optical Absorption Property and Pore Distribution. Diffuse reflectance spectroscopy is a useful tool for characterizing the electronic states in optical materials. Figure 8 shows the UV-visible diffuse reflectance spectrum of as-prepared Bi2WO6 powders. It can be seen that the Bi2WO6 microsphere has a steep absorption edge in the visible range, indicating that the absorption relevant to the band gap is due to the intrinsic transition of the nanomaterials rather than the transition from impurity levels. According to the equation REphoton ) A(Ephoton - Eg)1/2 (where R, Ephoton, and Eg are the absorption coefficient, the discrete photon energy, and the band gap energy, respectively; A is a constant),19 a classical Tauc approach is further

Figure 8. UV-vis diffuse reflectance spectrum of as-prepared Bi2WO6 powders. The inset is the (REphoton)2 ∼ Ephoton curve.

Figure 9. Typical N2 gas adsorption-desorption isotherm of Bi2WO6 microspheres. The inset is the corresponding pore-size distribution.

employed to estimate the Eg of Bi2WO6 microspheres. The plot of (REphoton)2 ∼ Ephoton based on the direct transition is shown in the inset of Figure 8. The extrapolated value (the straight line to the X-axis) of Ephoton at R ) 0 gives an absorption edge energy corresponding to Eg ) 2.825 eV. This value is larger than that of powder produced at high temperature.12 The increase in the band gap of the as-prepared Bi2WO6 microarchitectures is indicative of quantum confinement effects arising from the nanosized building blocks. The nitrogen adsorption-desorption isotherms and porosity of the hierarchical microspheres were further investigated. As can be seen in Figure 9, the nitrogen adsorption and desorption isotherms are characteristic of a type IV isotherm with a type

Bi2WO6 Hierarchical Microspheres

H3 hysteresis loop,20 indicating the presence of mesopores in the size range of 2-50 nm. In addition, the observed hysteresis loop shifts to a higher relative pressure on approaching P/P0 ≈ 1, which suggests that macropores (size > 50 nm) are also present.20 This is confirmed by the corresponding pore-size distribution that shows two peaks for mesopores along with macropores up to 120 nm in size (inset of Figure 9). As can been observed, the sizes of mesopores centralize on two areas: the peak values are 4 and 20 nm, respectively, in the desorption curve, corresponding to the interspace between neighboring Bi2WO6 nanoplates (within a sheet/layer) and the mesopores between edges of two closely packed sheets/layers (Figure 1d). The mesopores and macropores will endow the as-prepared hierarchical microspheres with novel application potentials. 4. Conclusions In summary, hierarchical Bi2WO6 uniform microspheres composed of multilayered sheets were successfully synthesized by means of a hydrothermal method by using PVP as the surfactant. The sheets was further found to be constructed by numerous tiny square nanoplates in an edge-to-edge way. The plate growth was limited to the [001] direction and predominately occurred along the layer of orthorhombic Bi2WO6 (a × b layer plane). PVP played an important role in the formation of such well-constructed microstructures. Their high quality should be more valuable for further theoretical and practical explorations of their shape- and size-dependent properties, and their intriguing self-assembly capability enables them to serve as novel nanobuilding blocks for new nanodevice applications. Furthermore, this method also presents a way for the controlled synthesis of multicomponent metal oxides. Acknowledgment. The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 50202007). References (1) (a) Wang, Z. L.; Song, J. H. Science 2006, 312, 242. (b) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. AdV. Mater. 2003, 15, 353. (c) Huang, J. X.; Tao, A. R.; Connor, S.; He, R. R.; Yang, P. D. Nano Lett. 2006, 6, 524. (2) Yang, H. G.; Zeng, H. C. Angew. Chem. Int. Ed. 2004, 43, 5930. (3) (a) Shen, G. Z.; Bando, Y.; Golberg, D. Cryst. Growth Des. 2007, 7, 35. (b) Shen, G. Z.; Bando, Y.; Lee, C. J. J. Phys. Chem. B. 2005, 109, 10578. (c) Teng, X. W.; Yang, H. Nano. Lett. 2005, 5, 885. (d) Fang, X. S.; Ye, C. H.; Zhang, L. D.; Zhang, J. X.; Zhao, J. W.; Yan, P. Small 2005, 1, 422. (e) Zhao, Q.; Xie, Y.; Zhang, Z.; Bai, X. Cryst. Growth Des. 2007, 7, 153. (f) Wu, Z.; Pan, C.; Yao, Z.; Zhao, Q.; Xie, Y. Cryst. Growth Des. 2006, 6, 1717. (4) (a) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 16744. (b) Leng, N.; Gao, L. Z.; Ping, F.; Zhang, J. Y.; Fu, X. Q.; Liu, Y. G.; Yan, X. Y.; Wang, T. H. Small 2006, 2, 621. (c) Jiang, C.; Zhang, W.; Liu, Y.; Qian, Y. Cryst. Growth Des. 2006, 6, 2603. (d) Chen, X. Y.; Wang, X.; Wang, Z. H.; Yang, X. G.; Qian, Y. T. Cryst. Growth Des. 2005, 5, 347. (e) Liu, J. P.; Huang, X. T.; Li, Y. Y.; Sulieman, K. M.; He, X.; Sun, F. L. Cryst. Growth Des. 2006, 6, 1690. (f) Liu, J. P.; Huang, X. T.; Sulieman, K. M.; Sun, F. L.; He, X. J. Phys. Chem. B. 2006, 110, 10612.

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