Synthesis and Characterization of Novel Three-Dimensional Metallic

Publication Date (Web): March 5, 2008. Copyright © 2008 American Chemical ... 15 µm were successfully prepared by a simple hydrothermal reduction ro...
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CRYSTAL GROWTH & DESIGN

Synthesis and Characterization of Novel Three-Dimensional Metallic Co Dendritic Superstructures by a Simple Hydrothermal Reduction Route

2008 VOL. 8, NO. 4 1113–1118

Lu-Ping Zhu,†,‡ Hong-Mei Xiao,† Wei-Dong Zhang,†,‡ Yang Yang,†,‡ and Shao-Yun Fu*,† Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China, and Graduate School of Chinese Academy of Sciences, Beijing 100039, China ReceiVed October 19, 2007; ReVised Manuscript ReceiVed January 7, 2008

ABSTRACT: Novel three-dimensional (3D) Co dendritic superstructures with an average diameter of ca. 15 µm were successfully prepared by a simple hydrothermal reduction route. The as-obtained products were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and vibrating sample magnetometry. It was shown that the 3D Co superstructures were comprised of dozens of well-aligned metallic Co dendrites with hierarchical assemblies radiating from the center. On the dendritic hierarchical structures, several leaves with different lengths and widths are connected to the main branch. The length of the main branch is several micrometers, and that of each leaf is about 0.5–1.5 µm with a width of 100-600 nm. A rational formation mechanism was proposed on the basis of the contrasting experiment. Compared with bulk cobalt, the 3D Co superstructures exhibited a decreased saturation magnetization (Ms) but an enhanced coercivity (Hc) due to their peculiar morphology.

Introduction. Large-scale self-assembly of micro-/nanostructured building components with highly specific morphology and novel properties are of great interest to chemists and material scientists not only for their role in better understanding the concept of self-assembly with artificial building blocks but also for their great potential for technological applications.1 Remarkable progress has been made in the self-assembly of highly organized building blocks of metals,2 semiconductors,3 copolymers,4 organic–inorganic hybrid materials,5 and biomaterials6 based on different driving mechanisms. However, it is still a big challenge to develop simple and reliable synthetic methods for hierarchically self-assembled architectures with novel and interesting morphologies, which would lead to novel properties of micro-/nanomaterials. Metallic cobalt has been extensively studied as an important magnetic material not only due to its multiple crystal structures (hexagonal-close-packed (hcp), face-centered-cubic (fcc), ε) but also because of its structuredependent magnetic and electronic properties.7 Various micro-/ nanostructures with different morphologies of metallic Co, such as nanorods, micro/nanowires, nanofibers, nanobelts, nanotubes, nanorings, nanodisc/nanoplatelets, two- and three-dimensional (2D and 3D) superlattices, and chain-like structures, have been successfully synthesized via different methods, including thermal decomposition organometallic precursors,8 electrodeposition technology,9 templated-mediated synthesis,10 electrospinning technique,11 magnetic-field-induced process,12 and hydro-/solvothermal methods.13 Recently, using polystyrene spheres as a template via the infiltration of precursors, Srivastava et al.14 prepared bowl-like Co nanocrystals. Qian and co-workers15 synthesized flower-like Co nanostructures by reduction of a Co 2-hydroxy-4-(1-methylheptyl) benzophenone oxime (N530) complex. Furthermore, Ohta et al. assembled Co nanoplatelets into microspheres through a surfactantassisted hydrothermal procedure.16 On the other hand, a dendrite is a kind of material that has a main stem from which many side branches grow out. Recently, dendritic structured materials have received much attention in view of their importance in physics studies and their potential applications.17 Qiu and co-workers17a synthesized CuNi dendritic materials by the electrochemical method and applied them in glucose sensor. Tao et al.17b synthesized dendritic silver hierarchical structures by carrying out the silver mirror reaction on the surface of a porous * Corresponding author. E-mail: [email protected]. † Technical Institute of Physics and Chemistry. ‡ Graduate School of Chinese Academy of Sciences.

anodic aluminum oxide (AAO) template. Fang et al.18a,b prepared dentritic silver nanostructures by a replacement reaction and 3D monocrystalline gold dendritic nanostructures through a fast electroless metal deposition route. Yan et al.18c synthesized dendritic superstructural and fractal BaCrO4 crystals via facile vegetal bitemplates. Moreover, Zhu and co-workers19 synthesized dendritic cobalt nanocrystals in ethanol via a reduction synthetic route. Liu et al.20 prepared snowflake-like cobalt microcrystals based on a precipitate slow-release controlled process. However, to our best knowledge, highly ordered 3D metallic cobalt dendritic superstructures, which have a hierarchical structure with primary, secondary, and higher-order branches, have not been reported yet. Herein we describe a simple hydrothermal reduction route to synthesize highly ordered 3D metallic cobalt dendritic superstructures with high yield. The 3D Co superstructures can be expected to bring about new opportunities in vast research and application fields, such as sensors, micro-/nanodevices, etc. The chemical reduction can be formulated as follows: 2[Co(C6H5O7)2]4-+N2H4+4OH- f 2Co V +N2 v +4H2O+4C6H5O72- (1) The proposed mechanism for the formation and the magnetic behaviors of the 3D magnetic Co superstructures is then studied.

Experimental Section Synthesis. All the reagents used were purchased from Beijing Chemical Reagent Ltd. and used without further purification. In a typical procedure, an aqueous solution of 100 mL was first prepared by dissolving CoCl2 · 6H2O (50 mM), sodium citrate (C6H5O7Na3 · 2H2O, 0.75 M), and NaOH (5 M) in distilled water. After N2H4 · H2O (10 mL, 40%(v/v)) was added, the solution was vigorously stirred and then transferred into a Teflon cup in a stainless steel-lined autoclave. The autoclave was maintained at 110 °C for 60 min and then was allowed to cool down to room temperature. A gray fluffy solid product was deposited on the bottom of the Teflon cup. The final product was centrifuged and rinsed with distilled water and ethanol for several times, and then was dried in a vacuum oven at 60 °C for 4 h. Characterization. The phase purity of the products was examined by X-ray powder diffraction (XRD) using a Rigaku D/max 2500 diffractometer with Cu KR radiation (λ ) 1.5406 Å). Scanning electron microscopy (SEM) images were obtained using a Hitachi

10.1021/cg701036k CCC: $40.75  2008 American Chemical Society Published on Web 03/05/2008

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Figure 1. XRD patterns of the prepared samples for the reaction time of 60 and 180 min.

S-4300 microscope (Japan). Transmission electron microscopy (TEM) images were taken on a Hitachi-600 transmission electron microscope. Fourier transform infrared (FT-IR) spectrum was recorded for the as-synthesized product with a Varian 3100 FT-IR spectrometer using a KBr wafer. Magnetic measurements for the samples in the powder form were carried out at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7307, USA) with a maximum magnetic field of 10 kOe. Thermogravimetric analysis (TGA) was carried out on a NETZSCH STA-409 PC thermal analyzer with a heating rate of 10 °C · min-1 in flowing oxygen atmosphere.

Results and Discussion. The crystalline nature of the phase and its purity were determined by XRD. Representative XRD patterns of the as-obtained product are shown in Figure 1 in the 2θ range of 30–80°. The characteristic peaks of the as-prepared products arise at 2θ ) 41.68°, 44.24°, 47.44°, and 75.89°, which matches the reflection planes of (100), (002), (101), and (110). All the diffraction peaks can be well indexed to hexagonal-phase cobalt,

Communications which is consistent with the standard values reported in JCPDS 05-0727. No characteristic peaks due to the impurities of cobalt oxides or hydroxides are detected, indicating that the as-prepared products obtained by the present synthetic route consist of a pure hexagonal-close-packed (hcp) phase. The strong and sharp diffraction peaks display that the as-obtained samples are well crystallized. Notably, when the reaction time is prolonged to 180 min, all the peaks belonging to the Co hexagonal phase are markedly sharpened with high intensities, suggesting that the longer reaction time favors the crystallization of Co phase. In addition, the relative intensity of the peaks corresponding to the (002)/(100) and (002)/(101) planes is significantly higher than the standard values, which indicates that the preferred growth orientation of the product was in the [001] direction. Typical SEM images of the as-prepared product with different magnifications are presented in Figure 2. Figure 2a is the overall morphology of the sample, which indicates the obtained product consists of a large number of novel 3D dendritic superstructures with an average diameter of ca. 15 µm. X-ray energy dispersive spectroscopy (EDS) microanalysis of the 3D magnetic Co dendritic superstructures as shown in the inset of Figure 2a displays that the as-prepared sample is essentially pure cobalt. Only a very small amount of oxygen is detected, which results possibly from slight oxidation of the surface. Considering that the experiment was carried out in air, it is not difficult to understand this. A magnified image of an individual crystallite indicates that the 3D superstructures comprise dozens of dendrites with hierarchical assemblies radiating from the center (see Figure 2b). On the dendritic hierarchical structures, several leaves (first branches, see the red arrow of Figure 2c) with different lengths and widths are connected to the main branch. The length of the main branch is several micrometers, and that of each leaf (second branches, see the dashed arrow) is about 0.5–1.5 µm with a width ranging from 100 to 600 nm. Interestingly, close inspection of the dendritic hierarchical structures under higher magnification clearly shows that each leaf connected to the main branch also acts as a secondary main branch to be connected by

Figure 2. SEM images of the as-prepared products: (a) a low magnification SEM image, the inset shows EDS spectrum of the as-prepared sample; (b) a high magnification SEM image of an individual 3D Co dendritic superstructure; (c) a high magnification SEM image of an individual dendrite composing of one main branch, second branches and leaves; and (d) a high magnification SEM image of the top view of an individual dendrite.

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Figure 3. (a) TEM image of metallic Co 3D superstructure. The inset shows the selected-area electron diffraction (SAED) pattern for the boxed area, and (b) SAED pattern of the circled area in (a).

Figure 4. SEM images of the samples prepared at 110 °C for different reaction times of (a) 10 min, (b) 30 min, and (c) 60 min, respectively.

smaller leaves (tertiary branches, see the blue arrow), as shown in Figure 2c. A top-view image of an individual Co dendrite is shown in Figure 2d, exhibiting a naturally symmetrical character. Furthermore, Figure 2d also reveals that the Co dendrites are composed of a great number of nanoparticles. Figure 3 shows the transmission electron microscopy (TEM) images of the as-prepared cobalt samples. The 3D metallic cobalt superstructures with dendrites can be seen in Figure 3a. The inset of Figure 3a is the corresponding selected-area electron diffraction (SAED) pattern of the boxed area. The SAED pattern illustrates an imperfect single crystal nature of the cobalt dendrite, where some separated and elongated diffraction spots in the SAED could be explained as the deformation of several branches in the metallic cobalt 3D superstructures. A perfect single-crystal character of the SAED pattern is shown in Figure 3b, which corresponds to the circled area in Figure 3a. The [001] direction, corresponding to the (002) plane of hcp-Co as shown in Figure 3b, is parallel to the growth direction of the main branch as displayed by the arrow in Figure 3a, indicating that the growths of the main branch and leaves are along [001] directions, which agrees well with the XRD result. In order to reveal the formation process of the 3D metallic cobalt dendritic superstructures in more detail, experiments were conducted at 110 °C for various reaction times of 10, 30, 60, and 180 min. The representative SEM images of the products prepared at certain reaction time intervals are shown in Figure 4. Underdeveloped dendritic superstructures are formed after treatment for 10 min (Figure 4a). When the reaction time is prolonged to 30 min, the underdeveloped 3D superstructures as well as dendrites with hierarchical assemblies appear as a result of oriented attachment and self-assembly, as exhibited in Figure 4b, indicating that selfassembly is still underway. After the reaction time is further prolonged to 60 min, the sample is composed entirely of the wellassembled 3D dendritic superstructures, as shown in Figure 4c. From this point, the size and morphology of the product remain the same even at longer reaction time such as 180 min. However, the prolonged reaction time favors the crystallization of Co phase, as shown in Figure 1. Moreover, no obvious crack or destruction

Figure 5. SEM images of the samples prepared without sodium citrate (a) and 0.05 M sodium citrate solution (b).

Figure 6. Schematic illustration of the formation process of the 3D cobalt dendritic superstructures.

of the product was observed even under vigorous stirring, indicating a high stability of the product. It is well-known that usage of templates can induce the formation of hierarchical and complex micro-/nanostructures. In this work, metallic Co 3D dendritic superstructures are synthesized without using any template; thus, it is supposed that citrate has played a crucial role in the formation of the self-assembled metallic 3D Co dendritic superstructures, which is confirmed by the experimental result in which citrate is not used. Only Co particles or flakes (Figure 5a) can be obtained in the hydrothermal process without using

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Figure 7. SEM images of the samples prepared in (a) 20% (v/v), (b) 10% (v/v), (c) 5% (v/v) N2H4 · H2O solution, and (d) 1.25 M NaOH solution.

citrate. At a low concentration of citrate ions (0.05 M), the product is a flower-like aggregation of particles and flakes, as shown in Figure 5b. Moreover, the snowflake-like metallic Co microcrystals are obtained at an appropriate concentration of NaOH in the absence of citrate (see the Supporting Information, Figure S1) similar to the previous report.20 As we know, citrate is an important biological ligand for metal ions. It has been widely employed as a reductant and capping agent in the synthesis of elemental Ag, Au, and Ag-Au alloy nanoparticles.21 It also serves as shape controller and stabilizer in the synthesis of Ni(OH)2, doughnut-shaped ZnO microparticles, CuI crystals, CoFe2O4 ocahedra, and one-dimensional (1D), 2D, and 3D CuO nanostructures.22 On the basis of the literature,21,22 and the investigations described above, we believe that citrate plays two major roles in our system. On the one hand, citrate ions can coordinate with cobalt ions to form [Co(C6H5O7)2]4- complexes in the excess of citrate solution,23,24 which decreases the free Co2+ concentration in solution and results in the slow generation of Co nanoparticles. Reaction velocity can be adjusted through the complexation slow-release method, which can regulate the kinetics of nucleation and growth of the products and further efficiently control the morphology and structure of the final products. As described in the diffusion limited aggregation (DLA) model,25 the interaction between the stochastic diffusive force and directive force may lead to formation of kinetically roughened branch structures. On the other hand, citrate can also serve as a shape modifier and controller, which may bind to certain crystal faces of the cobalt particles through its COO- and -OH functions. The FT-IR spectrum of the sample (see the Supporting Information, Figure S2) exhibits strong characteristic absorptions of the carboxylate group of the citrate ligands in the asymmetric and symmetric vibration regions. Specifically, νas-COO- vibrations appear at 1588 cm-1 and νs-COOvibrations are observed at 1436 (shoulder) and 1392 cm-1. The asymmetric νas-COO- and symmetric νs-COO- stretching modes in the samples are shifted to lower frequencies compared to those of free citrate, denoting some interaction between metal and citrate ligand.13b The interaction between metal and citrate ligand could force the nanoparticles to be assembled. Such a role of citrate has been reported in the literature.22 Thus, the formation of the 3D dendritic Co superstructures in hydrothermal condition would result from the nucleation and continuous assembly with orientedattachment mechanism26 assisted by citrate. On the basis of the above experimental observations, it becomes possible to interpret the formation process, as shown in Figure 6.

First, at an early reaction stage the primary Co nanocrystals are formed through reduction and conventional nucleation process and these Co nanocrystals form the final Co dendrites as shown in Figure 2d. Second, with rotation of adjacent nanoparticles to share the same crystallographic orientation and subsequent coalescence, the loose dendritic aggregates are formed through the orientedattachment and self-assembly as shown in Figure 4a, which are similar to the report on formation of silver dendrites by Fang et al.18a After the self-assembly process, the loose dendritic structures become compact gradually as shown in Figure 4b through Ostwald ripening (i.e., coarsening),3f the interface between two aggregates nearly disappears, and the aggregates share the same single crystallographic orientation. As a result, the individual dendritic structures could organize so well that they exhibited the feature of single crystals. This provides strong evidence that oriented attachment mechanism is a major path for the formation of the Co dendritic structures. Finally, the dendritic structures are further assembled in the presence of citrate to obtain the 3D metallic Co dendritic superstructures as shown in Figure 4c. In addition, we also tested the effects of other synthetic parameters on the formation of Co superstructures and found that the concentration of N2H4 · H2O is also an important factor in determining the shape of the final product when keeping other parameters unchanged. Figure 7 are SEM images of the products obtained at 110 °C for 1 h with 20%, 10%, and 5% (v/v) N2H4 · H2O solution. As the concentration of aqueous hydrazine in the synthetic system decreases, it can be clearly seen that the shape of the final product is transformed from microspheres assemblied with nanoplatelets to scattered nanoflakes. Furthermore, we found that a low concentration of NaOH (such as 1.25 M) results in aggregated particles and few 3D dendritic Co superstructures (shown in Figure 7d). The magnetic hysteresis measurement of the as-synthesized 3D metallic Co superstructures is carried out at room temperature in the applied magnetic field sweeping from –10 to 10 kOe. Figure 8 shows that the 3D metallic Co superstructures reveal ferromagnetic behavior with the saturation magnetization (Ms) and coercivity (Hc) values of 104.2 emu/g and 226.5 Oe, respectively. The saturation magnetization value of the sample is found to be lower than that of the bulk (168 emu/g).27 Surface oxidation at grain boundaries has been shown to cause a decrease in saturation magnetization.27,28 This might be the reason for the decrease of Ms. In order to confirm this speculation, the product was characterized using thermogravimetric analysis (TGA). As shown in Figure S3, Supporting

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Figure 8. Magnetic hysteresis loop for the prepared sample at room temperature.

Information, the as-synthesized cobalt product began to be oxidized at around 200 °C, receiving a weight gain. This indicates that the as-synthesized product was slightly oxidized, which is basically consistent with the EDS result. Moreover, Figure S3, Supporting Information shows that at around 700 °C the product was fully oxidized and the final weight gain was around 36.7%, which was slightly lower than the theoretical weight gain (40.7%) for the perfect conversion of pure Co to Co2O3. On the other hand, compared to the coercivity value of bulk Co (a few tens of oersteds),29 the 3D Co dendritic superstructures exhibit an enhanced value. As ultrafine ferromagnetic particles of anisotropic shape often exhibit enhanced coercivity relative to the corresponding bulk material, the shape anisotropy of the 3D Co superstructures may be responsible for the increased Hc. However, this value is much lower than that of the 2D and 3D superlattice of Co nanorods (740, 3200, and 7200 Oe)8b or Co nanobelts (410.6 Oe)13a with smaller sizes. It is well-known that the physical and chemical properties of magnetic materials strongly depend on the size, size distribution, defect structures and dimensions. Herein, because of the radial orientation of the dendrite, it is difficult for all of them to be aligned simultaneously along the direction of the external magnetic field compared with the nanorods or nanobelts. Therefore, a relatively low Hc value is then observed.

Conclusions. In summary, 3D magnetic metallic Co dendritic superstructures with an average diameter of ca. 15 µm have been successfully prepared by a simple hydrothermal reduction process. The 3D Co superstructures are comprised of dozens of well-aligned metallic Co dendrites with hierarchical assemblies radiating from the center. On the dendritic hierarchical structures, a number of leaves with different lengths and widths are connected to the main branch. The length of the main branch is several micrometers, and that of each leaf is about 0.5–1.5 µm with a width ranging from 100 to 600 nm. A significantly enhanced magnetic coercivity has been observed for the magnetic Co 3D dendritic superstructures compared to the bulk metal. The present hydrothermal route can be easily controlled and hence can be extended to preparation of other metal or alloy superstructures.

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Acknowledgment. This work is supported by the Beijing Municipal Natural Science Foundaiton (No. 2082023) and the National Natural Science Foundation of China (Grant Nos.: 10672161 and 50573090). We also thank Prof. Xiang-Min Meng for helpful discussions.

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Supporting Information Available: SEM image, FTIR spectrum, and TCG curve. This material is available free of charge via the Internet at http://pubs.acs.org.

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References (1) (a) Noble, P. F.; Cayre, O. J.; Alargova, R. G.; Velev, O. D.; Paunov, V. N. J. Am. Chem. Soc. 2004, 126, 8092. (b) Dinsmore, A. D.; Hsu,

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M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006. (c) Yan, P.; Ye, C. H.; Fang, X.; Zhao, J. W.; Wang, Z. Y.; Zhang, L. D. Chem. Lett. 2005, 34, 384. (d) Fang, X. S.; Ye, C. H.; Zhang, L. D.; Xie, T. AdV. Mater. 2005, 17, 1661. (e) Fang, X. S.; Ye, C. H.; Xie, T.; Wang, Z. Y.; Zhao, J. W.; Zhang, L. D. Appl. Phys. Lett. 2006, 88, 013101. (f) Fang, X. S.; Zhang, L. D. J. Mater. Sci. Technol. 2006, 22, 1. Kaltenpoth, G.; Himmelhaus, M.; Slansky, L.; Caruso, F.; Grunze, M. AdV. Mater. 2003, 15, 1113. (a) Yuan, J.; Laubernds, K.; Zhang, Q.; Suib, S. L. J. Am. Chem. Soc. 2003, 125, 4966. (b) Yada, M.; Taniguchi, C.; Torikai, T.; Watari, T.; Furuta, S.; Katsuki, H. AdV. Mater. 2004, 16, 1448. (c) Fan, H. Y.; Yang, K.; Boye, D. M.; Sigmon, T.; Malloy, K. J.; Xu, H.; López, G. P.; Brinker, C. J. Science 2004, 304, 567. (d) Gao, P.; Wang, Z. J. Am. Chem. Soc. 2003, 125, 11299. (e) Hu, J.; Ren, L.; Guo, Y.; Liang, H.; Cao, A.; Wan, L.; Bai, C. Angew. Chem., Int. Ed. 2005, 44, 1269. (f) Zhu, L.-P.; Xiao, H.-M.; Fu, S.-Y. Cryst. Growth Des. 2007, 7, 177. (a) Jenekhe, S. A.; Chen, X. L. Science 1998, 279, 1903. (b) Ikkala, O.; Brinke, G. T. Science 2002, 295, 2407. Du, J.; Chen, Y. Angew. Chem., Int. Ed. 2004, 43, 5084. Shenton, W.; Pum, D.; Sleytr, U. B.; Mann, S. Nature 1997, 389, 585. Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (a) Dumestre, F.; Chaudret, B.; Amiens, C.; Fromen, M.; Casanove, J.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2002, 41, 4286. (b) Dumestre, F.; Chaudret, B.; Amiens, C.; Fromen, M.; Respaud, M.; Fejes, P.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2003, 42, 5213. (c) Vivekchand, S. R. C.; Gundiah, G.; Govindaraj, A.; Rao, C. N. R. AdV. Mater. 2004, 16, 1842. (d) Ma, W. W.; Yang, Y.; Chong, C. T.; Eggeman, A.; Piramanayagam, S. N.; Zhou, T. J.; Song, T.; Wang, J. P. J. Appl. Phys. 2004, 95, 6801. (a) Cao, H.; Wang, L.; Qiu, Y.; Wu, Q.; Wang, G.; Zhang, L.; Liu, X. ChemPhysChem 2006, 7, 1500. (b) Rivas, J.; Bantu, A. K. M.; Zaragoza, G.; Blanco, M. C.; López-Quintela, M. A. J. Magn. Magn. Mater. 2002, 249, 220. (c) Henry, Y.; Iovan, A.; George, J. M.; Piraux, L. Phys. ReV. B 2002, 66, 184430. Knez, M.; Bittner, A. M.; Boes, F.; Wege, C.; Jeske, H.; Mai, E.; Kern, K. Nano Lett. 2003, 3, 1079. Wu, H.; Zhang, R.; Liu, X.; Lin, D.; Pan, W. Chem. Mater. 2007, 19, 3506. (a) Niu, H. L.; Chen, Q. W.; Zhu, H. F.; Lin, Y. S.; Zhang, X. J. Mater. Chem. 2003, 13, 1803. (b) Athanassiou, E. K.; Grossmann, P.; Grass, R. N.; Stark, W. J. Nanotechnology 2007, 18, 165606. (a) Xie, Q.; Dai, Z.; Huang, W. W.; Liang, J. B.; Jiang, C. L.; Qian, Y. T. Nanotechnology 2005, 16, 2958. (b) Xie, Q.; Qian, Y. T.; Zhang, S.; Fu, S.; Yu, W. Eur. J. Inorg. Chem. 2006, 2454. (c) Guo, L.; Liang, F.; Wen, X.; Yang, S.; He, L.; Zheng, W.; Chen, C.; Zhong, Q. AdV. Funct. Mater. 2007, 17, 425. Srivastava, A. K.; Madhavi, S.; White, T. J.; Ramanujan, R. V. J. Mater. Chem. 2005, 15, 4424. Zhu, Y.; Yang, Q.; Zheng, H.; Yu, W.; Qian, Y. T. Mater. Chem. Phys. 2005, 91, 293. Hou, Y.; Kondoh, H.; Ohta, T. Chem. Mater. 2005, 17, 3994. (a) Qiu, R.; Zhang, X. L.; Qiao, R.; Li, Y.; Kim, Y. I.; Kang, Y. S. Chem. Mater. 2007, 19, 4174. (b) Tao, F.; Wang, Z.; Chen, D.; Yao, L.; Cai, W.; Li, X. Nanotechnology 2007, 18, 295602. (a) Fang, J.; You, H.; Kong, P.; Yi, Y.; Song, X.; Ding, B. Cryst. Growth Des. 2007, 7, 864. (b) Fang, J.; Ma, X.; Cai, H.; Song, X.; Ding, B. Nanotechnology 2006, 17, 5841. (c) Yan, Y.; Wu, Q. S.; Li, L.; Ding, Y.-P. Cryst. Growth Des. 2006, 6, 769. Zhu, Y.; Zheng, H.; Yang, Q.; Pan, A.; Yang, Z.; Qian, Y. J. Cryst. Growth 2004, 260, 427. Liu, X.; Yi, R.; Wang, Y.; Qiu, G.; Zhang, N.; Li, X. J. Phys. Chem. C 2007, 111, 163. (a) Pillai, Z. S.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 945. (b) Brown, K. R.; Walter, D. G.; Natan, M. Chem. Mater. 2000, 12, 306. (c) Mallin, M. P.; Murphy, C. Nano Lett. 2002, 2, 1235. (d) Zhou, G. J.; Lu, M. K.; Yang, Z. S.; Zhang, H. P.; Zhou, Y. Y.; Wang, S. M.; Wang, S. F.; Zhang, A. Y. J. Cryst. Growth 2006, 289, 255. (a) Meyer, M.; Bée, A.; Talbot, D.; Cabuil, V.; Boyer, J. M.; Répetti, B.; Garrigos, R. J. Colloid Interface Sci. 2004, 277, 309. (b) Liang, J. B.; Liu, J. W.; Xie, Q.; Bai, S.; Yu, W. C.; Qian, Y. T. J. Phys. Chem. B 2005, 109, 9463. (c) Zhang, L. P.; Guo, F.; Liu, X. Z. Mater. Res. Bull. 2006, 41, 905. (d) Liu, X. M.; Fu, S. Y.; Zhu, L. P. J. Solid State Chem. 2007, 180, 461. (e) Xiao, H. M.; Fu, S. Y.; Zhu, L. P.; Li, Y. Q.; Yang, G. Eur. J. Inorg. Chem. 2007, 1966. Bresson, C.; Bion, L.; Moulin, C.; Lamouroux, C. Radiochim. Acta

1118 Crystal Growth & Design, Vol. 8, No. 4, 2008 2005, 93, 672. (24) Matzapetakis, M.; Dakanali, M.; Raptopoulou, C. P.; Tangoulis, V.; Terzis, A.; Moon, N.; Giapintzakis, J.; Salifoglou, A. J. Biol. Inorg. Chem. 2000, 5, 469. (25) (a) Well, A. F. Structure Inorganic Chemistry; Oxford University Press: Oxford, 1975788. (b) Halsey, T. C.; Duplantier, B.; Honda, K. Phys. ReV. Lett. 1997, 78, 1719. (26) (a) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188. (b) Zhu, L. P.; Xiao, H. M.; Liu, X. M.; Fu, S. Y. J.

Communications Mater. Chem. 2006, 16, 1794. (27) Dumpich, G.; Krome, T. P.; Hausmans, B. J. Magn. Magn. Mater. 2002, 248, 241. (28) Hausmanns, B.; Krome, T. P.; Dumpich, G. J. Appl. Phys. 2003, 93, 8095. (29) Cao, H. Q.; Xu, Z.; Sang, H.; Sheng, D.; Tie, C. Y. AdV. Mater. 2001, 13, 121.

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