Self-Assembled Growth of Hollow Spheres with Octahedron-like Co

May 21, 2008 - ... Growth of Hollow Spheres with Octahedron-like Co Nanocrystals via One-Pot Solution Fabrication. Xi Wang ... Phone: +86-10-82627058 ...
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J. Phys. Chem. C 2008, 112, 8773–8778

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Self-Assembled Growth of Hollow Spheres with Octahedron-like Co Nanocrystals via One-Pot Solution Fabrication Xi Wang,†,‡ Fangli Yuan,*,† Peng Hu,†,‡ Lingjie Yu,†,‡ and Liuyang Bai†,‡ State Key Laboratory of Multi-phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed: September 19, 2007; ReVised Manuscript ReceiVed: March 3, 2008

In this work, we report a simple one-pot approach to prepare hollow Co microspheres via self-assembly of octahedron-like nanocrystals. Uniform hollow Co spherical assemblies, prepared via a facile solvothermal reduction approach in the presence of NaCl, were composed of ordered nanooctahedra with a thickness of about 80–120 nm. The formation mechanism (NaCl-assisted Ostwald ripening) has been investigated with TEM/HRTEM/SEM/XRD methods. Surface morphologies of Co hollow spheres have been finely modulated from rough to smooth surface through altering reaction conditions (such as time, temperature, and amount of NaCl). The interior-cavity size of the Co hollow sphere has been well tunable by controlling reaction time or temperature. These materials exhibit good ferromagnetic characteristics, showing their potential applications for catalysts and other related devices. The influence of surface morphology, the interior cavity size, and surface modification upon the magnetic properties of hollow Co spheres have been systematically studied. The possible reasons for diverse magnetic properties of products obtained under different conditions are also discussed. 1. Introduction In recent years, much attention has been focused on the fabrication of nano- and microscale hollow spheres because of their potential applications in catalysts, artificial cells, coatings, and especially in delivery-vehicle systems for inks and dyes.1 Above all, metal nanoparticles with hollow structures exhibit a range of interesting properties that are superior to their solid counterparts, such as the controlled release of drugs or cosmetics, their low density, high specific surface, and large surface permeability without much sacrifice of mechanical/thermal stability. 2 For instance, hollow metal nanostructures are often prepared by templating with use of existing entities such as silica beads,3 polymer beads,4 micelles,5 or by seed-mediated methods.6 In addition, nanosized Ni hollow particles were also prepared successfully via hydrothermal treatment of an alkaline solution of Ni(DS)2 and NaH2PO2 at 100 °C.7 Furthermore, the nanoscale Kirkendall effect has been applied to form hollow Co nanocrystals.8 In most cases, the magnetic nanocrystals are superparamagnetic at room temperature because of their small dimensions and, thus, are not usable for many applications, such as magnetic recording. One way to increase the magnetic anisotropy of the particles is to modify their shape.9 Another possible way to improve the magnetic anisotropy is to assemble nanocrystals into multidimensional morphologies, especially hollow structures.10 These ordered nanospheres are expected to have potential applications in catalysts, drug carriers, and magnetic recording due to their dimensions and high surface areas.11 The ability to synthesize uniform hollow nanospheres with diameters ranging from nano- to microscale size is desirable. To the best of our * Corresponding author. Phone: +86-10-82627058. Fax: +86-1062561822. E-mail: [email protected]. † Institute of Process Engineering. ‡ Graduate University of the Chinese Academy of Sciences.

knowledge, self-assemblies of octahedron-like nanocrystals into metal hollow microspheres have not yet been reported. As an important magnetic metal material, nanosized Co crystals have extensive applications in heterogeneous catalysis, sensors, and ultra-high-density magnetic recording.12 Nanostructured Co crystals, including nanorods, nanodiscs, and 2D superlattices nanoaggregates, have been prepared to modify or promote their intrinsic properties.13 Here, we demonstrate a new method for the synthesis of Co hollow microspheres consisting of octahedron-like nanocrystals through a one-pot solution route. 2. Experimental Section 2.1. Chemicals and Reagents. All chemicals (analytical grade) were purchased from Beijing Chemicals Corp., China, and used as received without further purification. 2.2. Particle Synthesis. A typical synthesis was carried out as follows: 1 g of C4H6CoO4 · 4H2O and 0.2–1 g of NaCl were loaded into a 100 mL Teflon-lined stainless steel autoclave, which was then filled with 75 mL of absolute ethanol. The autoclave was sealed and maintained at 140–180 °C for 12–48 h, respectively, and then was allowed to cool to room temperature. The final product was centrifugated then rinsed with distilled water and ethanol several times to remove any chloride that remained in the final products. 2.3. Characterization. The size and morphologies of the asobtained samples were characterized by a field emission scanning electron microscope (FESEM, JSM-6700F, JEOL, Japan), a transmission electron microscope (TEM, H-700, Hitachi, Japan), and a high-resolution transmission electron microscope (HRTEM, JSM-2010, JEOL, Japan) operating at 200 kV. The as-prepared products were characterized by FTIR (Perkin-Elmer Spectrum GX). Phases were identified in an X-ray diffractometer (XRD, X-Pert, PANalytic, The Netherlands) with

10.1021/jp0775404 CCC: $40.75  2008 American Chemical Society Published on Web 05/21/2008

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Figure 1. XRD patterns of as-synthesized products.

Figure 2. (A) TEM image of Co hollow spheres obtained at 160 °C and 48 h. (B) Corresponding SEM image. (C) TEM image of Co particles obtained under 60 min ultrasonication treatment in an ultrasonic water bath. It also suggests that the spheres are assembled by octahedron-like nanoparticles. (D) HRTEM image of Co octahedronlike nanoparticles shown in part C. Scale bar: 500 nm.

Co KR radiation (40 kV, 30 mA). The magnetic properties of the sample were measured with a VSM (Lake Shore 7410 VSM). 3. Results and Discussion 3.1. Structure. The crystal structure of the product was confirmed by X-ray diffraction. As shown in Figure 1, the recorded diffraction peaks are well-assigned to the structure of Co with hexagonal phase, indicating the formation of hcp Co metals (space group P63/mmc; a ) 2.5031 Å, c ) 4.0605 Å; JCPDS No. 5-727). Broadening of the peaks exhibited the nanocrystalline nature of the sample. No impurities such as cobalt oxide or precursor compounds were detected, indicating the formation of pure cobalt with hexagonal structure. In particular, the crystallinity of the products is indeed increased gradually with the reaction time prolonged (e.g., from 12 to 48 h; Figure 1), which indicates that Ostwald ripening (crystallites grow at the expense of the smaller ones, discussion detailed later)14 is an underlying mechanism operative in this hollowing process. 3.2. Morphologies. Figure 2A shows a typical TEM image of Co products, where there is a strong contrast difference in all of the spheres with a bright center surrounded by a much darker edge, confirming their hollow architecture. The average outer diameter of the hollow spheres is about 1 µm, showing a relatively narrow size distribution. Note that the yield of the

Wang et al. assembled spheres is as high as 98%. The enlarged image of an individual sphere is presented in Figure 2B and the detailed morphology of particles on the surface of a single sphere can be found in Figure S1 (Supporting Information), which all indicates that the spheres are assembled by octahedron-like nanoparticles with the edge length of about 80–120 nm. The TEM image in Figure 2C shows that the cracked Co sphere obtained under 60 min ultrasonication treatment in an ultrasonic water bath is comprised of many nanoparticles with uniform tetragonal projected shape, consistent with the octahedron-like morphology observed by SEM. It is clear that the shell is composed of octahedron-like nanocrystals with diameters in the range from 80 to 120 nm, which is in agreement with the highmagnification SEM observation. The HRTEM image (Figure 2D) of the selected area in Figure 2C shows a crystalline character with a lattice spacing of 0.22 nm, which can be indexed to the (100) plane of hexagonal close-packed (hcp) Co.15 The single-crystalline structure of the single octahedron-like Co nanocrystal is also mirrored in the fast Fourier transform (FFT) pattern shown in the inset of Figure 1D. 3.3. Influence of Reaction Time on the Interior-Cavity Size of the Co Hollow Sphere. Panels A, B, and C of Figure 3 show high-magnification TEM images of samples obtained at different reaction time. From these images, obvious changes occur in the shell thickness or interior-cavity sizes of these hollow microspheres. As shown in Figure 3A-C, these spheres clearly exhibit hollow structure and the shell thickness and cavity can be easily identified. Compared with Figure 3B, Figure 3C clearly reveals a decrease of Co shell thickness and an increase in interior cavity size. This is derived from continuous mass transportation from the core into the shell due to the symmetric ripening. On the basis of the above results, it is believed that Ostwald ripening should be the main formation mechanism for these novel hierarchical architectures. It is well-known that the Ostwald ripening process involves the growth of larger crystals from those of smaller size which have a higher solubility than the larger ones.16 The original driving force for this ripening could be attributed to the existence of intrinsic density variations inside the starting solid aggregates. The formation and evolution of nanospheres seem to be as follows: at first, driven by the minimization of the total energy of the system, the small primary octahedron-like Co nanocrystals aggregated together to form 3D solid nanospheres, which exhibit various packing densities along the radial direction. The outer crystallites packed loosely would serve as starting growth sites for the subsequent recrystallization. As the mass was transported, the void space between the core and the shell was generated mainly through the Ostwald ripening. The schematic images in Figure 3 illustrate the formation process of hollow Co nanospheres. As indicated, the solid Co spheres are composed of numerous smaller crystallites. Compared to those in the outer surfaces, the crystallites located in the inner cores have high surface energies, because they can also be visualized as a smaller sphere having a higher curvature (i.e., higher surface energies and thus easy to dissolve). When the reaction time is short, the Co spheres have a solid core (entirely dark; Figure 3A). A hollowing effect is observed for those with a longer reaction time. Reported in parts B and C of Figure 3, the inner nanospace (the lighter part) of the spheres is further increased in comparison to the shell (darker spherical circle) when the reaction time is longer (24 h vs 48 h). It should be mentioned that a longer reaction time will give a thinner shell due to a gradually ripening. Furthermore, a higher starting

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Figure 3. Evolution (TEM images) of Co spheres synthesized at 160 °C with different reaction times: (A) 12, (B) 24, and (C) 48 h. Scale bar: 500 nm. Schematic illustration (down views) of the ripening process of hollow structures.

Figure 4. The detailed surface morphology of Co hollow spheres obtained at 160 °C with different reaction time: (A) 12, (B) 24, and (C) 60 h.

concentration of C4H6CoO4 · 4H2O will give a thicker shell due to a higher growth rate. The size evolution of octahedral building blocks in the spherical shell can also prove the Ostwald Ripening mechanism. Figure 4 shows the detailed surface morphology of a single Co hollow sphere harvested at different reaction time. From these images, one can see that all the spheres are composed of numerous octahedral nanocrystals, note that the size of octahedral building blocks changed from 50 to 60, 80 to 100, to 100 to 120 nm as the reaction time increased from 12, to 48, to 60 h. 3.4. Influence of Reaction Temperature on the Surface Morphology and Interior-Cavity Size of the Co Hollow Sphere. It is demonstrated that the surface feature of Co microspheres can be actively controlled by adjusting the reaction temperature. When the reaction temperature is at a lower temperature (140 °C), only microspheres with a rough surface are observed (Figure 5A-C). From the enlarged image of an individual sphere in Figure 5B, one can see that the sphere is assembled by the octahedron-like nanoparticles with an edge length of 100–150 nm. Increasing reaction temperature up to 160 °C, the octahedron-like nanoparticles with size of about 80–120 nm assembled microspheres begin to transform into the smooth microspheres (Figure 5D-F), where the smooth microspheres and octahedron-assembled microspheres coexist. Elevating the reaction temperature to 180 °C causes the more compact and smooth surface structure of microspheres (Figure 5G-I), in comparison with the microspheres obtained at 140 and 160 °C, and the size of octahedron-like nanoparticles on the assembled Co spheres is about 60–80 nm. It is known that more nuclei can always be formed at a higher reaction

temperature duo to a higher crystal growth rate than at a lower temperature, so the nanocrystals with relatively smaller size can be obtained at the higher reaction temperature. As a result, more octahedral building blocks with relatively smaller size can aggregate into a ∼1 µm microsphere with a smooth surface at higher temperature; on the contrary, relatively big and a smaller number of octahedral nanocrystals are comprised of ∼1 µm microsphere with a rough surface. It can be found that the change of the surface condition of microstructures may be attributed to a dissolution-recrystallization process followed by Ostwald ripening16 under solvothermal conditions. In our synthetic system, it is evident that the formation of Co hollow architectures with different surface morphology at various temperatures is another example of the Ostwald ripening, completely different from the templatedirected growth for the formation of simple hollow nanostructure. The appearance of different surface morphology at various temperatures suggests that ripening on the outer shell happened as well. The smooth surface on the shell at higher temperature was formed via fast dissolution and recrystallization due to the much higher ripening rate on the outer shell on the crystallographic planes with high surface energy (anisotropic). It is obvious that the cores could be excavated at the same time, resulting in hollow nanospheres. Once the cores in the center of the microspheres were consumed completely, the final hollow spheres were created. On the other hand, at lower temperature (140 °C) for 48 h, the rough surface (Figure 5B) should be formed due to the relatively lower ripening rate, while the much thicker shell of these hollow spheres was formed then via selfassembly of nanocrystals and the hollowing process already discussed above shown in Figure 3C.

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Wang et al.

Figure 5. Low-magnification, high-magnification FESEM, and TEM images of the Co hollow spheres obtained at different temperatures (48 h): (A-C) 140, (D-F) 160, and (G-I) 180 °C. Scale bar: 500 nm.

It is also noticed that the reaction temperature plays an important role in interior-cavity size control of the Co spheres due to the different ripening rate. Note that the outer surfaces of these hollow spheres gradually changed from rough to smooth as temperature elevated, which partially testified that the different ripening rate existed in different reaction temperatures. From TEM images shown in parts C, F, and I of Figure 5, one can see that the obtained spheres clearly exhibit hollow structure and the shell thickness and cavity can be easily identified. Compared with parts F and I of Figure 5, part C clearly reveals an increase of Co shell thickness and a decrease in interiorcavity size. The formation of samples with different shell thickness may be attributed to the fact that at higher temperatures under solvothermal conditions for the same reaction time, the ripening process proceeded at a rather higher rate, and in this hollowing (ripening) process Co aggregates were inclined to form a thin shell, while at lower temperatures the ripening process proceeded at a lower rate and formed a thick shell. As this ripening occurs slowly, the octahedron-like Co nanoparticles were dissolved slowly in both the interior and exterior of the spheres, and eventually evolve into a thick and rough shell. The change on the shell of the outer surface may be attributed to the effective reconstruction of Co to form a smooth and highly crystalline structure through the Ostwald ripening process to lower surface energy.17 3.5. Influence of NaCl on the Surface Morphology of the Co Hollow Sphere. Our conditional experiments show that the addition of a different amount of NaCl can also affect the surface morphology of Co hollow spheres. Figure 6 shows SEM images of Co hollow spheres synthesized with different amounts of NaCl. The particle surface experiences a corresponding alteration from rough to relatively smooth when the amount of NaCl changes from 0.25 to 1 g. It can be seen that the surface of the Co hollow spheres synthesized with 0.25 g of NaCl is not smooth, as shown in Figure 6A; however, it is smooth for the

Figure 6. SEM images of Co hollow spheres synthesized with different amounts of NaCl at 160 °C and 48 h: (A) 0.25 g of NaCl and (B) 1 g of NaCl. Scale bar: 1µm.

Co hollow spheres synthesized with 1 g of NaCl, as shown in Figure 6B. On the basis of these phenomena, it is reasonable to conclude that the octahedron-like particles on the shell might experience the dissolution process due to NaCl-induced ripening. It is reported that the inorganic soluble salts can act as complexing agents in solution to affect the deposition rate of crystals, and thus, it is reasonable that the deposition rate of Co could be adjusted in a certain range of the NaCl concentration because of the strong coordination of the most electronegative Cl- anions to Co2+ cations in our reaction system.18,19 In addition, some studies demonstrate that inorganic soluble salts can change the ionic strength, ζ potential of chemical species, chemical potential, and viscosity of the solution in the reaction system, which would provide a suitable chemical environment for the growth of the desired crystals.20 It is also found that NaCl may provide a suitable chemical environment to direct the specific nucleation and growth of hollow microspheres in the reaction system.21 In our reaction system, the structure of the Co hollow spheres can be tuned by the amount of NaCl, and the formation mechanism of the hollow Co microspheres might also be controlled by a NaCl-induced Ostwald ripening process. The core–shell structure of the partial microspheres is

Self-Assembled Growth of Hollow Spheres

Figure 7. Magnetic hysteresis curve of the products obtained at different reaction temperature: 140, 160, and 180 °C, respectively.

formed in this period, which could be demonstrated by TEM observation of the morphology of a few intermediates of core–shell Co structures obtained at the reaction period of 36 h, as shown in Figure S2 (see Supporting Information). It welltestified the Ostwald ripening mechanism. However, further investigation is needed to better understand the exact effect of NaCl and formation mechanism of the hollow Co microspheres obtained only in the presence of NaCl. 3.6. Magnetic Properties. The magnetic characterization of samples was performed at room temperature. At this temperature all the particles exhibited ferromagnetic characteristics. It is interesting to observe that the values of both HC and MR decreased while the value of MS increased with a increase in the reaction temperature. In comparison, for the samples obtained at the same temperature but different reaction time, the striking feature is that the values of both HC and MR increased, while the value of Ms did not obviously change, as the reaction time increased. 3.6.1. The Saturation Magnetization (MS) of the Co Hollow Spheres. The saturation magnetization (MS) is, in all cases, lower than the reported value for bulk Co (163.1 emu g-1)22 but is close to the values measured in Co of very small particle size.13 The reduction of the saturation magnetization for magnetic nanoparticles compared to their bulk counterparts has been studied extensively but still remains an open issue. Many possible mechanisms have been proposed, for example, the existence of impurities,23 surface antiferromagnetic oxidation,24 surface spin disorder,22–25 crystallinity, etc. It is interesting to note that the reduction of MS has been observed with the jinglebell-shaped hollow Co spheres, 550–750 nm,26 and Co shell with a diameter of 550 nm and a thickness of 40 nm.27 In addition, a dramatic reduction of MS has been observed with the Ni hollow sphere of about 50 nm in size.28 On the other hand, the saturation magnetization has shown an increase with the increasing reaction temperature (Figure 7). Such an increase has usually been attributed to a decreasing proportion of the pinned surface magnetic moments in overall magnetization as the crystallinity increases in size. The nanocrystals with different surface shapes have demonstrated the distinctly different coercivities that imply different surface pinning. The reduction in the MS, especially significant for the samples heated at 180 °C, is associated with spin canting at both the disordered surface layer and the whole volume of the particles. The last contribution originated from the different degree of cationic vacancy disorder or crystallinity of samples.29 Supporting this interpretation MS values increased as the temperature did (i.e., increase in crystallinity) and for the samples with different reaction time

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Figure 8. Magnetic hysteresis curve of the products obtained at different reaction time: 12, 24, and 48 h (140 °C).

but similar crystallite size (samples heated at 140 °C), Ms values were similar (Figure 8). 3.6.2. The CoerciWity (HC) and Remanence (MR) of the Co Hollow Spheres. Hysteresis loops of samples were registered at room temperature. As shown in Figure 8, the particles exhibited ferromagnetic characteristics including coercivity (HC) and remanence (MR). For the samples obtained at 140 °C but different reaction time, the striking feature is that the values of both HC and MR increased, as the reaction time was prolonged. In this case, HC and MR of these particles are interior-cavity size dependent. For solid Co spheres, the value of the coercivity (HC) is determined as 72.8 Oe. With interior-cavity size increasing, HC values are greatly enhanced and reach as large as 142.8 and 179.9 Oe, respectively. The values of MR are also applied to the above rule. In addition to interior-cavity size, reaction temperature during the synthesis also plays an important role in determining HC and MR. It was observed that when the mixture was reacted at 140 °C, the coercivity is determined as HC ≈ 179.9 Oe with a remanence, MR, ∼10.8 emu/g. On the other hand, by raising the reaction temperature to 160 and 180 °C, the HC values were decreased to 114.7 and 73.7 Oe, respectively, while MR values are as low as 7.4 and 4.3 emu/g. That is, the values of both HC and MR decreased as the temperature increased. Néel’s early calculations as well as several recent theoretical studies have suggested that coercivity decreases with decreasing surface anisotropy.30–32 The magnetic anisotropy in nanoparticles includes the anisotropy in the core and the surface anisotropy. The surface anisotropy mainly has the effect on coercivity of nanoparticles. Certainly, the extent of the surface anisotropy effect is usually strong on magnetic coercivity. The surface usually exhibits some degree of spin disorder and pinning. Compared to the rough surface, the smooth surface makes the symmetry of the surface metal atom more closely resemble that of the core, and therefore reduces the spin disorder and pinning. In contrast, due to lacking a solid core in hollow spheres, the symmetry of the surface metal atom less resembles that of the core with the interior-cavity size increasing, and therefore induces the spin disorder and pinning. These changes are certain to affect the surface anisotropy and consequently the coercivity of nanoparticles. Compared to the Co hollow spheres with the rough surface, Co crystals with the smooth surface enable the surface metal atom to possess a more symmetric coordination and smaller degree of spin disorder and pinning. Therefore, the surface anisotropy should be much smaller in Co crystals with a smooth surface than the one with a rough surface. In comparison, Co hollow spheres with the large interior-cavity

8778 J. Phys. Chem. C, Vol. 112, No. 24, 2008 size have a much higher coercivity than that having the small one. Clearly, the surface anisotropy determines the coercivity, which is proportional to the interior-cavity size but opposite that of the reaction temperature (Figure 7). 3.6.3. Influence of Surface Modification on the Magnetic Properties of the Co Hollow Sphere. As we found in the cobalt acetate and ethanol system under solvothermal conditions, some organic materials including residual the RCOOH and ROH groups can modify the as-synthesized particles.33 It is reasonable that the surface of the as-synthesized Co hollow spheres has been modified with some organic materials including residual RCOOH and ROH groups. The encapsulation of residual organic materials over the particle surface may have an enhancement effect on the surface spin disorder.34 A core–shell model was proposed to study the magnetism associated with the surface layer.35 The surface random potential would result in a spindisordered state without magnetic ordering, leading to the reduction of ferromagnetism. This effect is expected to be more pronounced with a hollow mesosphere structure consisting of a Co shell and a vacuum core. Surface magnetism is, therefore, expected to be manifest. The observed increase or no change in the saturation magnetization in all of our particulate samples can also be considered in terms of reducing surface anisotropy. Surface anisotropy is often pictorially viewed in terms of the degree of spin disorder and/or spin pinning at the surface. It appears that coating the surface with the ligands “frees” the surface spins, and hence, they are more easily able to align with the overall magnetization direction of the particle. As a result of such increasing alignment, the magnetization of nanoparticles is observed to increase after surface modification. 4. Conclusions Hollow Co microspheres composed of octahedron-like Co nanocrystals were synthesized by a one-step solution approach. Surface morphology of the Co hollow sphere has been modulated from a rough, to slightly rough, to smooth surface through adjusting the reaction conditions. In addition, the interior-cavity size of the Co hollow sphere was testified to be related to the reaction time and temperature. Thus, through controlling the reaction parameters, the interior-cavity size of the Co hollow sphere can be adjusted. The formation of the hollow microspheres might be a NaCl-induced Ostwald ripening process. The magnetic properties of the hollow Co spheres were systematically studied. Increase or no change of MS might be due to different surface morphology, surface spin disorder, and modification. The surface anisotropy determines HC and MR, which is proportional to the interior-cavity size but opposite that of the reaction temperature. The hollow Co microspheres prepared with this method may find potential applications in the fabrication of magnetic devices and as material encapsulators or carriers. This method is very simple, mild, environmentally safe, and economical, and it may be general to synthesize other metal microspheres with a hollow interior. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 50574083).

Wang et al. Supporting Information Available: Detailed morphology of synthesized products: Figure S1 reveals detailed morphology of particles on the surface of obtained Co hollow spheres shown in Figure 2B of the manuscript and Figure S2 shows the highmagnification TEM image of a few intermediates of core–shell Co structures obtained at the reaction period of 36 h. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Zhang, D.; Qi, L.; Ma, J.; Cheng, H. AdV. Mater. 2002, 14, 1499. (2) Sun, Y.; Xia, Y. Nano Lett. 2003, 3, 1569. (3) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (4) Yoshikawa, H.; Hayashida, K.; Kozuka, Y.; Horiguchi, A.; Awaga, K.; Bandow, S.; Iijima, S. Appl. Phys. Lett. 2004, 85, 5287. (5) Bao, J.; Liang, Y.; Xu, Z.; Si, L. AdV. Mater. 2003, 15, 1832. (6) Yang, J.; Lee, J. Y.; Too, H. P.; Valiyaveettil, S. J. Phys. Chem. B 2006, 110, 125. (7) Li, Q.; Liu, H. J.; Han, M.; Zhu, J. M.; Liang, Y. Y.; Xu, Z.; Song, Y. AdV. Mater. 2005, 17, 1995. (8) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Sicence 2004, 304, 711. (9) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (10) Dumestre, F.; Chaudret, B.; Amiens, C.; Respaud, M.; Fejes, P.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2003, 42, 5213. (11) Love, J. C.; Urbach, A. R.; Prentiss, M. G.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 12696. (12) Kim, H.; Achermann, M.; Balet, L. P.; Hollingsworth, J. A.; Klimov, V. I. J. Am. Chem. Soc. 2005, 127, 544. (13) Legrand, J.; Ngo, A. T.; Petit, C.; Pileni, M. P. AdV. Mater. 2001, 13, 58. (14) Ostwald, W. Z. Phys. Chem. 1900, 34, 495. (15) Hou, Y. L.; Kondoh, H.; Ohta, T. Chem. Mater. 2005, 17, 3994. (16) Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 3–492. (17) Teo, J. J.; Chang, Yu.; Zeng, H. C. Langmuir 2006, 22, 7369. (18) Yamabi, S.; Imai, H. J. Mater. Chem. 2002, 12, 3773. (19) Xu, F.; Du, G. H.; Halasa, M.; Su, B. L. Chem. Phys. Lett. 2006, 426, 129. (20) Filankembo, A.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 5865. (21) Wang, W. S.; Zhen, L.; Xu, C. Y.; Zhang, B. Y.; Shao, W. Z. J. Phys. Chem. B 2006, 110, 23154–23158. (22) Hou, Y.; Kondoh, H.; Ohta, T. Chem. Mater. 2005, 17, 3994. (23) Zhang, L.; Manthiram, A. Phys. ReV. B 1996, 54, 3462. (24) Lin, D.; Nunes, A. C.; Majkrzak, C. F.; Berkowitz, A. E. J. Magn. Magn. Mater. 1995, 145, 343. (25) Biasi, E. D.; Zysler, R. D.; Ramos, C. A. Phys. ReV. B 2005, 71, 104–408. (26) Liang, F.; Guo, L.; Zhong, Q.; Wen, X.; Yang, S.; Zheng, W.; Chen, C.; Zhang, N.; Chu, W. Appl. Phys. Lett. 2006, 89, 103105. (27) Yoshikawa, H.; Hayashida, K.; Kozuka, Y.; Horiguchi, A.; Awaga, K. Appl. Phys. Lett. 2004, 85, 5287. (28) Liu, Q.; Liu, H. J.; Han, M.; Zhu, J. M.; Liang, Y.; Xu, Z.; Song, Y. AdV. Mater. 2005, 17, 1995. (29) Morales, M. P.; Veintemillas-Verdaguer, S.; Montero, M. I.; Serna, C. J.; Roig, A.; Clasas, L.; Martinez, B.; Sandiumenge, F. Chem. Mater. 1999, 11, 3058. (30) Ne′el, L. J. Phys. Radium 1954, 15, 225. (31) Vestal, C. R.; Zhang, Z. J. J. Am. Chem. Soc. 2003, 125, 9828. (32) Kodoma, R. H.; Berkowitz, A. E. Phys. ReV. B 1999, 59, 6321. (33) Ye, Y.; Yuan, F.; Li, S. Mater. Lett. 2006, 60, 3175. (34) Guo, L.; Liang, F.; Wen, X.; Yang, S.; He, L.; Zheng, W.; Chen, C.; Zhong, Q. AdV. Funct. Mater. 2007, 17, 425. (35) Kodama, R. H.; Berkowitz, A. E.; McNiff, E. J.; Foner, S. Phys. ReV. Lett. 1996, 77, 394.

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