Chelating Ligand-Mediated Crystal Growth of Cerium Orthovanadate

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Chelating Ligand-Mediated Crystal Growth of Cerium Orthovanadate Feng

Luo,†

Chun-Jiang

Jia,†

Wei

Song,†

Li-Ping

You,‡

and Chun-Hua

Yan*,†

State Key Laboratory of Rare Earth Materials Chemistry and Applications & PKU-HKU Joint Laboratory on Rare Earth Materials and Bioinorganic Chemistry, Peking University, Beijing 100871, China, and Electron Microscopy Laboratory, Peking University, Beijing 100871, China Received February 9, 2004;

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 1 137-142

Revised Manuscript Received October 18, 2004

ABSTRACT: An ethylenediaminetetraacetic acid (EDTA)-mediated hydrothermal route to cerium orthovanadate (CeVO4) microcrystals has been developed. The EDTA/Ce3+ ratio, solution pH, and temperature of the reaction systems are found to play important roles in determining the morphologies and growth process of the CeVO4 products. During the process of synthesis, EDTA plays important roles such as chelating ligand and capping reagent. These products have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Moreover, the results of X-ray photoelectron spectra (XPS) and magnetic measurements reveal the existence of Ce4+ induced by surface oxidation of air. The existence of Ce4+ ions may be helpful for oxidative dehydrogenation of propane of CeVO4. On the basis of the experimental results, we suppose this method may have wide applications in exploring the crystal growth process and may provide guidance for the morphology controllable synthesis. Introduction Controllable synthesis of well-defined nano- or microcrystals with uniform size and morphology such as wires, belts, tubes, cubes, and rods have sparked a worldwide interest due to their unique electronic, optical, and magnetic properties associated with the reduced dimensionality, and their potential applications in nanotechnology.1-5 As a consequence, various nanostructures such as elemental semiconductors, II-VI and III-V semiconductors, metal oxides, metals, and inorganic salts on large scale have been fabricated through a variety of methods including templating direction, catalytic growth, electrochemistry, chemical vapor deposition, and solution-based solvothermal or hydrothermal treatment.6-13 In these fabrications, the growth habit of crystals always plays an important role in determining the final morphology; meanwhile, sometimes a special “capping reagent” is also used for tailoring the crystal growth dynamically. Although it is a convenient choice for morphology control to follow the crystal growth habit, the morphology is restricted by the nature of crystals, and always deviates from our research target. Moreover, the use of “capping reagent”, on the other hand, affords the possibility of breaking this limitation and resulting in various morphologies and thus may have wider applications.14-16 With respect to crystallization, an interesting phase in the ternary system of Ce-V-O is cerium orthovanadate (CeVO4), which has a tetragonal zircon-type structure belonging to the space group I1/amd. This tetragonal zircon-type structure stabilizes Ce3+ cations even in oxidizing conditions.17 Besides as a high-activity * Chun-Hua Yan, College of Chemistry, Peking University, Beijing 100871, China. Fax & Tel: 86-10-62754179. E-mail: [email protected]. † State Key Laboratory of Rare Earth Materials Chemistry and Applications & PKU-HKU Joint Laboratory on Rare Earth Materials and Bioinorganic Chemistry, Peking University. ‡ Electron Microscopy Laboratory, Peking University.

catalyst in oxidative dehydrogenation of propane at lowtemperature,18 CeVO4 represents a new class of optically inactive material, with favorable properties for use as counter electrodes.19 Moreover, CeVO4-based phases are of great interest for high-temperature electrochemical applications because of the significant ionic and electronic conductivity.20 The conventional methods for the preparation of rare earth orthovanadates include solidstate reaction,21 coprecipitation,22 and the hydrothermal method,23 but no efficient control over their size and morphology has been achieved yet. Therefore, to extend the application of CeVO4 and to deepen the comprehension of its crystal growth behaviors, it is necessary to have a suitable choice of “capping reagent” possessing both stability and simplicity for the preparation of CeVO4 microcrystals with distinguished shapes and especially good uniformity. Herein, our choice of ethylenediaminetetraacetic acid (EDTA) is proven to have such merits.24 Experimental Section All reagents (analytical grade reagents) were purchased from Beijing Chemicals Co. Ltd. and used as received without further purification. Ce(NO3)3‚6H2O was used as the cerium source and Na3VO4 as the vanadium source. In a typical procedure for the preparation of CeVO4, 0.8685 g (0.02 mol) of Ce(NO3)3‚6H2O was dissolved in distilled water in a 100.0 mL flask, and then appropriate EDTA solution dissolved by 1:1 (v/v) ammonia was added, forming a chelated cerium complex. Subsequently, 0.8000 g (0.02 mol) of Na3VO4 in 10 mL of water was dripped into the flask with vigorous stirring. This flask was then filled with water up to 80% of the total volume and the solution pH was adjusted from 1 to 14 with 1:1 (v/v) ammonia or dilute nitric acid solution. Finally, the mixture was transferred into a Teflon-lined stainless steel autoclave of 100 mL capacity. After the autoclave was sealed tightly, it was placed in a temperature-controlled electric oven, maintained at the temperature from 140 to 240 °C. As the autoclave cooled to room temperature, the precipitated powders were separated by centrifugation, washed with deionized

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Figure 1. X-ray diffraction patterns of CeVO4 samples (a) at different reaction temperatures, (b) at different pH. water and ethanol for several times, and then dried in a vacuum oven at about 80 °C for 12 h. The size and morphology of CeVO4 samples were characterized by scanning electron microscopy (SEM, Amary FE-1910 and JEOL JSM-6700F), operated at an acceleration voltage of 3.0 kV, and transmission electron microscopy (TEM, 200CX, JEOL, Japan) employing a microscope working at 160 kV. High-resolution TEM (H-9000, Hitachi, Japan) coupled with energy-dispersive X-ray analysis (EDAX) was performed at a resolution of 159 eV at 300 kV. The samples for SEM were prepared by dispersing the products in ethanol with 30 min ultrasonicating, and then dropping a few drops of the resulted suspension onto a silicon wafer substrate and allowing them to dry naturally. Samples for TEM and HRTEM are obtained by dispersing a small drop of the suspension with a much lower concentration onto a copper grid precoated with amorphous carbon. The X-ray diffraction (XRD) pattern is recorded on a powder sample with a Rigaku D/max-2000 diffractometer employing Cu KR radiation (λ ) 1.5418 Å) at a scanning rate of 0.02 degrees per second ranging from 10 to 80°. The purity and composition of the sample were determined by X-ray photoelectron spectra (XPS), which were carried out in an ionpumped chamber (evacuated to 2 × 10-9 Torr) of an Escalab5 (UK) spectrometer, employing Mg-KR radiation (binding energy (BE) ) 1253.6 eV). Magnetization measurements were performed on a MagLab System 2000 (Oxford, UK) in the temperature range of 300-5 K.

Results and Discussion Morphological Control. All the obtained cerium orthovanadate are of a tetragonal zircon-type structure (space group I1/amd). In the XRD patterns of CeVO4 with typical morphologies displayed in Figure 1a, all the diffraction peaks agree well with those of tetragonal phase CeVO4 (JCPDS No. 12-0757), although the diffraction peaks are broadened owing to the small particle size. At a reaction temperature of 140 °C, the crystallinity of as-prepared samples is considerably low and the uniformity is not perfect. However, a longer time at a higher temperature favors the formation of the thermodynamically stable, well-crystallized and uniform samples. Since the beginning decomposition temperature of EDTA is about 220 °C, when the reaction temperature reaches 240 °C, the obtained CeVO4 samples are inhomogeneous because of the decomposition of EDTA ligand. Meanwhile, the indexed powder XRD in Figure 1b indicates that CeO2 is precipitated at pH values up to 14. In contrast to reactions in alkaline

Table 1. Summarized Reaction Conditions of CeVO4 and Corresponding Morphologies EDTA/Ce pH temp (°C) 1.0 1.0 1.0 1.0 1.0 1.5 1.5 1.5 1.5 1.5 1.5 0 0.25 0.5 3.0 3.0

10 10 10 10 10 1 3 6 8 10 12 10 10 10 10 10

140 180 200 220 240 180 180 180 180 180 180 180 180 180 180 220

morphology nanorod, 5 nm × 2 µm nanorod, 5 nm × 2 µm (Figure 2d) nanorod, 10 nm × 1 µm nanorod, 100 nm × 0.5 µm flowerlike, inhomogeneous woolen sphere, 0.5 µm (Figure 2b) hollow sphere, 1 µm (Figure 2c) dumbbell-like, 1.5 µm nanorod, 200 nm × 1 µm nanorod, 200 nm × 3 µm nanorod, 300 nm × 2 µm nanoparticles, 20 nm (Figure 2a) nanorod, 50 nm × 400 nm nanorod, 100 nm × 1 µm nanorod, 250 nm × 1 µm (Figure 2e) cube, 400 nm (Figure 2f)

media, there is no extra phase detected in an acidic medium except for phase-pure CeVO4. Moreover, the relative intensity of (200) over (112) obtained for the as-prepared nanocrystals in Figure 1a is different from that of bulk material, indicating the existence of somewhat preferential orientation in the CeVO4 nanocrystals. From Figure 1b, we can observe that the relative intensity of (200) over (112) is rapidly decreased with the pH value decreasing, indicating that the preferential orientation is strongly dependent on the pH of aqueous solution. The summarized morphologies and reaction conditions of CeVO4 are illustrated in Table 1 and the typical SEM and TEM images of tetragonal CeVO4 nanocrystals obtained at different reaction conditions are displayed in Figure 2, respectively. Controlled experiments are carried out to screen the optimal conditions for the preparation of pure phase monocrystalline CeVO4 materials. The crystallinity, phase purity, and morphological uniformity of the products is found to be highly correlative with the heat treatment temperature, the starting acidity in the mother liquors, and the addition of EDTA medium.24 Because the chelation and capping capabilities of EDTA are effectively suppressed by raising the reaction temperature, it is very clear that the aspect ratio of obtained samples decreases with the increase of reaction temperature when fixing the EDTA/ Ce ratio and solution pH values, as shown in Table 1. The pH value also plays an important role. When the

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Figure 3. HRTEM images of CeVO4 samples obtained at pH ) 10, 180 °C, 24 h with EDTA/Ce ratio of 3.0.

Figure 2. SEM and TEM images of CeVO4 samples with typical shapes obtained at (a) pH ) 10, 180 °C, 24 h, EDTA/ Ce ) 0; (b) pH ) 1, 180 °C, 24 h, EDTA/Ce ) 1.5; (c) pH ) 3, 180 °C, 24 h, EDTA/Ce ) 1.5; (d) pH ) 10, 180 °C, 24 h, EDTA/ Ce ) 1.0; (e) pH ) 10, 180 °C, 24 h, EDTA/Ce ) 3.0; and (f) pH ) 10, 220 °C, 24 h, EDTA/Ce ) 3.0.

pH is lower than 4, the obtained samples are hollow woolen spheres or spheres about 0.5-1 µm in diameter, as shown in Figure 2b,c. Meanwhile, when the pH is higher than 8, the obtained rodlike samples are about 1 µm in length. Between the two pH values, the dumbbell-shaped structure appears. These phenomena can be easily understood by taking the following considerations. Under a pH below 4, VO43- begins to transform into poly-orthovanadate anion,25 forming a hollow framework in solution. Since Ce3+ exists near the surrounding of anion framework, we suppose the nucleation happens at the interface. When the framework is broken due to the reaction temperature increase, poly-orthovanadate anion can easily turn into VO43ions and diffuse forth from the center, forming a hollow sphere structure. When pH is fixed at 10, VO43- anion is the main form of orthovanadate, so no hollow spheres are observed. Moreover, under the low pH, the aspect ratio of the CeVO4 is also reduced because of the weakened chelation and capping abilities of EDTA. Besides the heat treatment temperature and the starting acidity in the mother liquors, both the concentration of the Ce3+ ions and the molar ratio of EDTA to Ce3+ are found to play important roles in determining the shape of the product. When the molar ratio of EDTA to Ce3+ rises up to 0.1, only nanorods are formed; meanwhile, without the addition of EDTA while keeping other conditions, mainly small particles are observed, as shown in Figure 2a. While increasing the molar ratio of EDTA to Ce3+, the aspect ratio of the nanocrystals gradually increases and then decreases as indicated in Table 1, reaching a maximum at the molar ratio of EDTA to Ce3+ equal to 1.0 due to the appropriate chelation and capping abilities. By varying the reaction temperature and the molar ratio of EDTA to Ce3+, the

aspect ratio of the CeVO4 is controllable as shown in Figure 2d-f. Growth Process. Usually, the tetragonal CeVO4 colloidal precipitates are aggregates of highly isotropic nanoparticles with sizes around 20 nm when treated at 180 °C. However, when mediated by EDTA, CeVO4 samples with a high aspect ratio can be obtained, if the pH value is well controlled in a narrow range of 8-12. These initial colloidal precipitates can be mediated by the adsorbed ligand on the crystal surface and serve as seeds for the growth of highly anisotropic nanostructures in the solution-solid process via the dissolution and crystallization mechanism.26,27 As mentioned before, our choice of EDTA is mainly based on its appropriately chelating and capping effects, which influence the growth rate of different facets distinguishingly. Once the EDTA is dissociated from the Ce3+ ions, it will bind to the specific surface of the precipitated cerium orthovanadate. With the increase of temperature, the influence of EDTA is greatly weakened due to the decrease of surface adsorption, consistent with the results of Figure 2e,f. To further understand the fine structure of the nanorods, HRTEM studies of CeVO4 samples obtained at pH ) 10, 24 h with different values of EDTA/ Ce ratio are employed. The energy-dispersive X-ray (EDX) analysis confirms that there is no EDTA remaining in the product. The clear lattice fringes in the HRTEM images in Figure 3 confirm the high crystallinity of the CeVO4 nanorods. The spacing of all the samples between two adjacent horizontal and vertical lattice planes is 3.7 ( 0. 1 Å and 3.2 ( 0. 1 Å, close to the d200 (3.677 Å) and d002 (3.244 Å), respectively. Combined with the results of ED analysis (inset of Figure 3), they can be owed to (200) and (002) planes of CeVO4, respectively.28 The perpendicularity of the (002) plane to the nanorods axis indicates a c-axis preferential growth direction. Normally, the (002) reflection shown in Figure 3 is forbidden in the zircon type structure (space group I1/amd) as shown in X-ray diffraction patterns (see Figure 1). However, we believe that the appearance of the (002) reflection indexed in ED patterns originates from the effect of double diffraction. This anomaly arises because the electron scattering is much heavier than that assumed in the kinematical theory. The extra spots that can result from the double diffraction process are obtained by translating the

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Figure 4. XPS analysis of Ce 3d (A), V 2p and O 1s (B) of as-prepared CeVO4 samples. (a) pH ) 3, Ce4+, (b) pH ) 10, Ce4+, (c) pH ) 3, EDTA/Ce ) 1.5, (d) pH ) 8, EDTA/Ce ) 1.5, (e) pH ) 10, EDTA/Ce ) 1.5, and (f) pH ) 10, Ce3+.

primary diffraction pattern, without rotation, so that its origin coincides successively with all the strong spots of the primary patterns. It is clear that the indices of the double diffraction spots are given by the sum of the indices of the two component primary diffraction spots. Thus, all spots of the form h1 + h2, k1 + k2, l1 + l2 are possible double diffraction spots, where h1k1l1 and h2k2l2 are any two allowed primary diffraction spots.28 For tetragonal CeVO4, it consists of VO4 tetrahedra sharing corners and edges with CeO8 dodecahedra. The chain of CeO8, interrupted by distorted VO4 units, extends along the c-crystal axis. From the plain view of {020} for CeVO4, we can find that only oxygen atom appears on the {020} plane. Differently, on the {002} plane, cerium atoms, vanadium atoms, as well as oxygen atoms are alternately arranged. Once the EDTA is dissociated from the Ce3+ ions, the Ce3+ ion has a stronger binding to oxygen atom located on the {200} or {020} planes of CeVO4, rather than binding to those anions located on the {002} plane. Since the Ce3+ ion forms a stable coordination complex with EDTA in aqueous solutions, then the surface adsorption of EDTA on the {020} plane has a higher coverage than on the {002} plane, leading to the crystal growth direction along [001].29 In other words, EDTA has the capability of balancing the growth rate of different facets, leading to the seeming restriction of crystal growth. When the EDTA/Ce ratio is lower than 1.0, it preferentially binds to the faces vertical to the c-crystal axis, resulting in a high growth aspect ratio of CeVO4. While the EDTA/ Ce ratio reaches 3.0, those faces vertical to the c-crystal axis are fully covered and other faces parallel to the c-crystal axis are also allowed to be adsorbed, resulting in a lower aspect ratio, according to Figure 2d,e. Apparently, the kinetically controlled shape evolution discussed here cannot be well explained by the diffusioncontrolled growth process proposed by Peng et al.,30 which shows that elevating the temperature and extending the heating time may increase the aspect ratio of nanocrystals. It is clear that the strong ligand, EDTA, is not only required to form a stable complex with Ce3+, but also acting as a capping reagent binding to the surface of crystal, which directly affects the facet growth and crystallinity of the nanocrystals.29 It plays double roles as chelating ligand and capping reagent. Therefore, we can easily control the aspect ratio of CeVO4 nanocrystals by adjusting the addition of EDTA and the reaction temperature. To study the effects of the medi-

ated reagent, other ligands such as citric acid and acetic acid, lacking of chelation capability, are also used to mediate the crystal growth of CeVO4 in this hydrothermal reaction when fixing other conditions. However, the obtained samples are only spherical nanocrystals with further crystallization. Meanwhile, DTPA, whose chelation capability is larger than EDTA, is also used for comparison. It favors the lower anisotropic growth and leads to the formation of tetragonal nanocakes due to the lack of good selectivity in the capping process. XPS and Magnetic Measurements. To examine the component and purity of CeVO4 nanorod, the asprepared samples are determined by X-ray photoelectron spectra (XPS). Since the assignment of Ce4+ 3d photoelectron peaks is ambiguous due to the complex nature of the spectra, which arises not only from the multiple oxidation states but also from the mixing of Ce 4f levels and O 2p states during the primary photoemission process. This hybridization leads to the splitting of the peaks into doublets.31 The complex spectrum of Ce 3d can be decomposed to eight components with the assignment as defined in Figure 4A. The bands labeled v represent the Ce 3d5/2 ionization, and the bands labeled u represent the Ce 3d3/2 ionization. The band (v0, v) at (880.4-881.1 eV) is the Ce 3d5/2 ionization and the band (u0, u) at (898.9-899.3 eV) is the Ce 3d3/2 ionization for Ce3+ and Ce4+. The bands labeled v′ (884.1-884.6 eV), v′′ (887.6-888.1 eV), and v′′′ (896.4-897.1 eV) are satellites arising from the Ce 3d5/2 ionization, while the bands u′ (902.9-903.4 eV), u′′ (906.0-906.3 eV), u′′′ (915.1-915.3 eV) are satellites arising from the Ce 3d3/2 ionization.32,33 Since the bands labeled u′ and v′ represent the 3d104f 1 initial electronic state corresponding to Ce3+, while the peaks labeled u′′′ and v′′′ represent the 3d104f 0 state of Ce4+ ions, the ratio of Ce3+/(Ce4+ + Ce3+) can be quasi-quantitatively determined by the ratio of (Iu′ + Iv′)/(Iu′′′ + Iv′′′ + Iu′ + Iv′).34 For both samples (a) and (b), the Ce3+/(Ce4+ + Ce3+) are 20 and 25%, respectively, indicating the main valence of cerium in these samples is +4, due to the +4 valances of the starting cerium salts. However, strikingly, the spectra of (c)-(f) appear much more intense in the BE of 884.3 and 903.4 eV (an additional doublet characteristic peaks of Ce2O3) compared to the samples of (a) and (b), revealing that (c)-(f) contain much more content of Ce3+ than Ce4+. With increasing pH value from (c) to (e), the Ce3+/(Ce4+ + Ce3+) range from 99 to 98%, indicating a decrease in the surface content of Ce3+. As

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Figure 5. (A) Temperature dependence of susceptibility (χM) and χMT for (a) 180 °C, pH ) 10, EDTA/Ce ) 3.0; (b) 180 °C, pH ) 10, EDTA/Ce ) 1.0; (c) 180 °C, pH ) 10, Ce3+; and (d) 180 °C, pH ) 10, Ce4+; (B) Temperature dependence of inverse of susceptibility and the curves of M-H measured at 5 K are displayed in the inset for (a) 180 °C, pH ) 10, EDTA/Ce ) 3.0; (b) 180 °C, pH ) 10, EDTA/Ce ) 1.0; (c) 180 °C, pH ) 10, Ce3+; and (d) 180 °C, pH ) 10, Ce4+.

for sample (f) without addition of EDTA, the Ce3+/(Ce4+ + Ce3+) is 95%, which indicates that the presence of EDTA can prevent the oxidation of Ce3+ ions at the surface. This is also confirmed by the solution redox analysis experiments that the formation of Ce4+ is slower in the presence of EDTA. From Figure 4B, it is very clear that the BE of V 2p3/2 and O 1s of Ce4+ varies toward the low value, compared with that of Ce3+. This BE decrease of O 1s originates from the valence increase of Ce, which leads to the suppression of electron transfer from O 2p to Ce 4f, and hence the enhancement of BE of O 1s. The magnetic susceptibility measurement is performed in the temperature range of 5-300 K under 1000 Oe for samples prepared from different starting cerium salts and EDTA/Ce ratios. For the samples of (a), (b), and (c), the χMT decreases with decreasing temperature and almost vanishes when temperature reaches zero as shown in Figure 5A. This means that these samples behave as a paramagnetic state due to the existence of three-valence cerium. Meanwhile, for the sample (d), the χMT decreases with increasing temperature and almost remains negative, which is the characteristic of the diamagnetic property of Ce4+. Therefore, we can find that with the decrease of EDTA (from a to c), the content of Ce4+ is increased because of the oxidation of Ce3+ during the hydrothermal process, which is consistent with the results of XPS. Moreover, temperature dependence of the reciprocal of susceptibility (1/χm) and the curves of M-H measured at 5 K obtained from Figure 5B also identify the coexistence of Ce3+and Ce4+ in these Ce-V-O oxides. Since the hopping between Ce3+ and Ce4+ ions is the possible mechanism of electron hole transport of CeVO4,20 and the deoxidization of V5+ is the key factor for oxidative dehydrogenation of propane,18 this variation of Ce3+/Ce4+ ratio may affect the oxidative dehydrogenation of propane process as well as potential applications in ionic and electronic conductivity for CeVO4. Our results of improved selectivity of oxidative dehydrogenation of propane for CeVO4 are obtained and will be published elsewhere. Conclusion In this paper, we have developed an EDTA-mediated hydrothermal route for the crystal growth of CeVO4

microcrystals. The EDTA/Ce3+ ratio, solution pH, and temperature of the reaction systems are found to play important roles in determining the morphologies and aspect ratios of the CeVO4 products. Both as capping reagent and chelating ligand, EDTA is not only found to be responsible for the especially good uniformity and high crystallinity of the products, but also plays an important role in restricting the natural growing habit of CeVO4 due to the possible selective interaction between EDTA and certain crystal facets, thus having a great impact over its final morphology. Our recent experiments also showed that monocrystalline zircontype LaVO4 and luminescent active LaVO4:Eu nanorods can also be prepared via the same synthetic approach.35 Due to the simplicity of this system and the efficient control over the morphology it has achieved, we suppose this method may have wide applications in exploring the crystal growth process and provide guidance for the morphology controllable synthesis. Acknowledgment. The authors would like to acknowledge the supports of MOST of China (G19980613), NSFC (Nos. 50272006, 20221101, and 20423005), the Training Project for Doctoral Student of MOE, and Founder Foundation of PKU. References (1) Hu, J. T.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (2) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. Adv. Mater. 2003, 15, 353. (3) Wu, Y. Y.; Yan, H. Q.; Huang, M.; Messer, B.; Song, J. H.; Yang, P. D. Chem. Eur. J. 2002, 8, 1260. (4) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446. (5) Klein, D. I.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; McEuen, P. L. Nature 1997, 389, 699. (6) Zhu, H. W.; Xu, C. L.; Wu, D. H.; Wei, B. Q.; Vajtai, R.; Ajayan, P. M. Science 2002, 296, 884. (7) Feldman, Y.; Wasserman, E.; Srolovitz, D. J.; Tenne, R. Science 2001, 267, 222. (8) Li, Y. D.; Li, X. L.; Deng, Z. X.; Zhou, B. C.; Fan, S. S.; Wang, J. W.; Sun, X. M. Angew. Chem., Int. Ed. 2002, 41, 333. (9) Duan, X. F.; Hang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241. (10) Zhang, J.; Sun, L. D.; Su, H. L.; Liao, C. S.; Yin, J. L.; Yan, C. H. Chem. Mater. 2002, 14 4172. (11) Zhang, D. F.; Sun, L. D.; Yin, J. L.; Yan, C. H. Adv. Mater. 2003, 15, 1022.

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