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Morphology-Controllable Synthesis and Characterization of Hierarchical 3D Co1-xMnxO Nanostructures Hai-Tao Zhang and Xian-Hui Chen* Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ReceiVed: February 20, 2006; In Final Form: March 27, 2006
Multirod and hierarchically spherical 3D Co1-xMnxO (2/3 e x e 1) nanostructures have been successfully synthesized by the decomposition of acetylacetonate precursors. Their morphologies could be controlled through tuning the heat rating which affects the nucleation. The rods grew along [110] directions to reduce the appearance of high-energy crystallographic {110} planes. The hierarchically spherical superstructures were formed by a three-dimensional oriented-attachment mechanism. Magnetic measurement indicates that the MnO nanomaterials with hierarchically spherical superstructures show an antiferromagnetic transition temperature at 121 K, similar to that of bulk, and a ferromagnetic ordering exists at low temperature. Such anomalous magnetic properties arise plausibly from their microstructure characteristics.
Introduction A great deal of effort has been devoted to the synthesis of nanostructures with well-controlled shapes and sizes.1,2 This is mainly due to the ability of nanomaterials to exhibit novel electronic, magnetic, optical, chemical, and mechanical properties compared to those of corresponding bulk materials. These novel properties make them highly attractive for many technological applications.2,3 Obviously, understanding the fundamental physical properties of nanostructures is principal to the rational technological applications of nanomaterials. Therefore, coupling synthetic studies for rapid variation and optimization of desired properties is an efficient mechanism for the discovery of potentially new electronic, catalysis, thermal, optical, and magnetic properties.4 The properties of materials are highly size and morphology dependent when their dimensionality exits in the nanometer scale.2,5 To form complex architectures with hierarchy across an extended scale, especially in the nanometer and micrometer scales, is a real challenge in the design of integrated materials with advanced functions.6 Fortunately, morphosynthesis offers researchers the opportunities to design higher-order architectures at the macroscopic scale with embedded structures at the microscopic scale.7 Morphosynthesis of inorganic solid involves the chemically based strategies to control the size, shape, and organization of materials over multiscales beyond the unit cell.7,8 In addition to the single crystals with complex morphologies that can be formed through the organization of nanoparticles, superstructures interspaced by organic additives can be formed through the oriented organization of nanoparticles. Furthermore, the fusion of the nanoparticles leads to single-crystalline structures with organic additives as defects.6 Among different inorganic nanoparticles, transition metal oxide nanostructures are promising for their potential technological applications in magnetic data storage, promising audio speakers, biosensors, powder compacts, magnetic targeted drug delivery, contrasting agents in magnetic resonance imaging, and * To whom correspondence should be addressed. E-mail: chenxh@ ustc.edu.cn. Phone: +86-551-3601654. Fax: +86-551-3601654.
alternatives to radioactivity.9 Manganese oxide (MnO) is an antiferromagnetic (AFM) oxide with a Ne´el temperature of 122 K even though it was predicted to be ferromagnetic by theory.10 Manganese oxides have important technological applications such as catalysts and electrode materials.10,11 Synthetically chemical methods have been proved to be very effective for synthesizing transition metal oxide nanostructures with monodispersed size and shape. Up to date, zero- and one-dimensional nanostructures of manganese oxides with uniform size and shape have been synthesized successfully in the past few years.12 These nanostructures exhibit interestingly novel magnetic properties that are very helpful in understanding new science on a “small” scale. Herein, we report the synthesis of three-dimensional (3D) MnO nanostructures: multirod 3D nanostructures and hierarchically spherical 3D superstructure nanostructures by controlling the kinetics of the thermal decomposition of organometal precursor complexes. They are the first hierarchical 3D nanostructure of manganese oxide. Experimental Section Chemicals. Manganese(III) acetylacetonate (Mn(acac)3, Alfa), cobalt(II) acetylacetonate (Co(acac)2, Alfa), dibenzyl ether (Alfa, 99%), oleylamine (technical purity), and oleic acid (99%) were used as purchased without further purification. Anhydrous ethanol and dichloromethane were purchased from Shanghai Chem. Corp. All reactions were conducted in a three-neck flask equipped with a reflux condenser and a Teflon-coated magnetic stirring bar under flowing N2 gas. Synthesis of 3D MnO Nanostructures. In a typical synthesis process of multirod 3D MnO nanostructures, 2 mmol of Mn(acac)3, 20 mL of dibenzyl ether, 2 mL of oleic acid, and 2 mL of oleylamine were mixed and stirred under a flow of nitrogen. The solution was heated to 200 °C at a rate of 10 °C/min under stirring, and then, the solution was kept at 200 °C for 60 min. Following this, the solution was carefully heated, at a rate of 1-2 °C, up to slight refluxing for another 60 min. Then, a black colloidal solution was formed. The colloidal nanostructures were
10.1021/jp061088r CCC: $33.50 © 2006 American Chemical Society Published on Web 04/27/2006
Hierarchical 3D Co1-xMnxO Nanostructures
J. Phys. Chem. B, Vol. 110, No. 19, 2006 9443
Figure 1. (a) XRD patterns of multirod (top) and spherical (bottom) 3D MnO nanostructures, separately. (b) XPS spectra of multirod (hollow circles) and spherical (solid triangles) 3D MnO nanostructures, respectively. The inset of part b shows core-level XPS spectra of the C 1s.
separated upon the addition of 50 mL of alcohol, centrifuged, and washed using a mixture of alcohol and dichloromethane solvent. In the end, the products were dried in an oven at room temperature overnight. On the other hand, hierarchically spherical 3D MnO nanostructures were synthesized following the above process. The only difference is that the reaction medium was heated to refluxing at a higher heating rate, 5-15 °C/min, and after it had been kept at 200 °C for 60 min. Under identical conditions, reaction of Co(acac)2 (0.67 mmol) with Mn(acac)3 (1.33 mmol) would lead to 3D Co1/3Mn2/3O nanostructures by thermal decomposition of precursors. Nanostructure Characterization. Samples for transmission electron microscopy (TEM) analysis were prepared by drying a dispersion of the particles on amorphous carbon-coated copper grids. The morphologies of the products were characterized using a Hitachi H-800 transmission electron microscope (200 kV) and a JSM-6700F field emission scanning electron microscope (FE-SEM). The microstructures of the nanostructures were characterized using high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) on a JEOL 2100 transmission electron microscope (200 kV). X-ray powder diffraction patterns of the products were characterized by a Rigaku D/max-A X-ray diffractometer (XRD) with graphite monochromatized Cu KR1 radiation in the 2θ range 20-100° with a step of 0.02° at room temperature. X-ray photoelectron spectroscopy (XPS) was performed on a VGESCALAB MKII X-ray photoelectron spectrometer, using nonmonochromatized Mg KR X-rays as the excitation source. Mn and Co elemental analyses of the as-synthesized nanoparticle powders were performed by an Atomscan Advantage inductively coupled plasma atomic emission spectrometer (ICP-AES). Fourier transform infrared (FTIR) spectroscopy was carried out on a Bruker Vector-22 FTIR spectrometer. Magnetic studies were carried out with a superconducting quantum interference device (SQUID) magnetometer (Quantum design, MPMS-XL). The magnetic susceptibility was measured under both field cooling (FC) and zero field cooling (ZFC) processes under an applied field of 100 Oe. Hysteresis loops were measured at different temperatures after FC with a field of 1000 Oe. Results and Discussion The XRD studies (Figure 1a) of multirod and hierarchical 3D MnO nanostructures reveal a cubic rock salt crystal structure
with high crystallinity (Fm3m, JCPDS no. 72-1533). The crystalline size of multirod nanostructures calculated by the Scherrer equation is 11.5 nm, in good agreement with the diameter observed by TEM studies. XPS was employed to detect the valance state of manganese in the surface of the nanostructures, and the core-level XPS spectra in the Mn 2p region are shown in Figure 1b. To exclude the charging effects, the spectra were standardized by C 1s core-level spectra, as shown in the inset of Figure 1b. The spectra of hierarchically spherical 3D MnO nanostructures show the 2p3/2 peak at 641.2 eV with a weak satellite peak at 647.5 eV. These results are consistent with that of bulk MnO, indicating that the manganese atoms at the surface have not been oxidized to higher oxidation states.13 The binding energy of the Mn 2P3/2 core level for multirod nanostructures is 642.0 eV, being between these of bulk Mn2O3 and MnO2.13a,14 The two kinds of nanostructures were synthesized under almost the same synthetic conditions except for the heating rate and strength of refluxing. It is obvious that the only difference in the resulting samples is morphologies through the analyses of XRD patterns and HRTEM images. Compared to the Mn 2p core level of hierarchically spherical 3D MnO nanostructures, the Mn element in the surface of multirod nanostructures has higher oxidation states than that of hierarchically spherical nanostructures. The FTIR absorption spectra (not shown here) of the samples indicate that they are coated with substantial surfactants, oleylamine and oleic acid, which might play some role in the hierarchical growth pattern. The multirod 3D nanostructures were synthesized on a large scale and in a highly uniform way. Figure 2 shows typical TEM images of multirod 3D MnO and Co1/3Mn2/3O nanostructures. The low-magnification TEM image of MnO in Figure 2a shows that the product consists almost entirely of such multirod nanostructures, and no isolated nanorods could be observed in our TEM studies. These results indicate that the nanostructures, which have an average size of 100 nm, are highly yielded and exceedingly monodispersed. The SAED pattern shown in Figure 2b of the multirod 3D nanostructures consists of cubic MnO (JCPDS card no. 72-1533) with strong ring patterns from the (111), (200), and (220) planes. Figure 2c shows an enlarged magnification TEM image of an isolated multirod 3D MnO nanostructure whose rods have an average diameter of 10 nm and lengths of 50 nm. The microstructures of the multirod nanostructures were characterized in detail; one representative HRTEM image of one rod is shown in Figure 2d. The clear
9444 J. Phys. Chem. B, Vol. 110, No. 19, 2006
Figure 2. (a) Low-magnification TEM image of multirod 3D MnO nanostructures with high yield and good uniformity. (b) SAED pattern of multirod 3D MnO nanostructures. (c) Enlarged magnification TEM image of an individual multirod nanostructure. (d) HRTEM image recorded in a rod of one multirod 3D MnO nanostructure. (e) Lowmagnification TEM image of multirod 3D Co1/3Mn2/3O nanostructures with high yield and good uniformity. (f) HRTEM image recorded in a 3D Co1/3Mn2/3O nanostructure. The inset of part f shows a magnified image of the region where the two rods connect with each other.
lattice image indicates the high crystallinity and single-crystalline nature of the rod. A lattice spacing of 0.26 nm for the (111) planes and a lattice spacing of 0.22 nm for the (200) planes, which are perpendicular to the rod, could be readily resolved. These HRTEM analyses indicate that the rod grows along the [110] direction. The solid solution Co1/3Mn2/3O multirod nanostructures could also be formed through this process. Figure 1e shows a TEM image of the Co1/3Mn2/3O multirod nanostructures. The nanostructures were found to be homogeneous in size and morphology, too. The multirod nanostructures have a uniform size of 100 nm. In addition, every nanostructure has several dozens of rods. All of the rods were single crystalline and have a diameter of 11 nm and lengths up to 60 nm, as evidenced by high-resolution research. More interesting, the nanorods attached with one another and fused at the bottom tips. Figure 2f shows a typical HRTEM image of two nanorods that fused at tips. The lattice spacing at the fused position of 0.25 nm for the (111) planes is very clear. ICP analysis reveals that the Co/Mn atom ratio of the Co1/3Mn2/3O multirod nanostructures is 1:2, which is in good agreement with that of precursors. Furthermore, study reveals that such nanostructures will not be formed with increasing Co concentration.
Zhang and Chen
Figure 3. (a) High-magnification FE-SEM image of spherical 3D MnO nanostructures. (b and c) TEM image and the corresponding SAED pattern, respectively, of an isolated spherical 3D MnO nanostructure. (d) HRTEM image of a smaller particle which belongs to a spherical 3D MnO nanostructure.
The AFM MnO crystallizes in the high-symmetry rock salt (face-centered cubic, fcc) structure, whose surface density of atoms in the corresponding planes should follow n(111) > n(200) > n(110). As to the fcc structured metal, the surface energy for the three lowest index planes follows γ{111} < γ{200} < γ{110}.15 Therefore, the {110} planes are the most thermodynamically unstable and the {111} planes are the most thermodynamically stable. Hence, growth in the [110] direction is highly favored. This agrees well with the high-resolution analyses of MnO nanorods. The result indicates that a slow heating rate and weak refluxing favor the anisotropic growth of nanorods along [110] directions. Few nuclei would form and nucleate slowly under weak heating. Therefore, the high shape anisotropic nanorods were probably formed through a diffusion-controlled reaction under slight refluxing. Growth of nanorods would reduce highenergy crystallographic planes and the curvature imposed on the surface-anchored surfactants; such favorable changes in the inorganic lattice and organic bending energies can occur by anisotropic growth. Interestingly, the morphology of the product changed from a multirod 3D nanostructure to a hierarchically spherical 3D superstructure when the heating rate was enhanced to 5-15 °C/ min and the solution was allowed to reflux vigorously. It should be pointed out that an overheated phenomenon appeared under such conditions. Figure 3a shows the FE-SEM image of hierarchically spherical 3D particles with a relatively uniform diameter of 180 nm. The FE-SEM image at high magnification shows that the larger spheres were composed of relatively
Hierarchical 3D Co1-xMnxO Nanostructures
Figure 4. TEM image of MnO samples formed through refluxing for 3 min. The arrows indicate the fused section between attached nanoparticles.
smaller nanoparticles. A TEM image of one isolated nanoparticle further confirms that the spheres were composed of nanoparticles with an average diameter of 25 nm (Figure 3b). It should be pointed out that aggregation of the nanoparticles is oriented. The ED pattern (as shown in Figure 3c) of the particle reveals that the smaller nanoparticles are oriented-attached and the larger one has the characteristics of a single crystal. More particles were characterized and found to be directionally attached. This indicates that the nanoparticles are hierarchical structures and can be classed as mesocrystals.6 More interestingly, high-resolution studies show that the relatively smaller nanoparticles are composed of much smaller primary nanoparticles with an average diameter of 5 nm. Figure 3d shows a HRTEM image of one nanoparticle of a hierarchical nanostructure. The high-resolution analyses reveal that the nanoparticles are composed of 5 nm nanoparticles. The clear lattice image indicates that the smaller primary nanoparticles in a relatively larger nanoparticle were coalesced in high order and fused with the attached nanoparticles. Furthermore, the clear lattice at the contact between relatively larger nanoparticles with an average diameter of 25 nm indicates that they coalesced together in high order and fused, too. The results indicate that the hierarchically spherical nanostructures were formed through the threedimensionally oriented aggregation of nanoparticles with an average diameter of 25 nm, which were formed through the self-organization of smaller nanoparticles that have an average diameter of 5 nm.6 The nanoparticles self-organized threedimensionally and fused to form textured superstructures in order to reduce the surface energy. The formation process of the spherical hierarchical 3D nanostructures was studied systemically. To obtain the products, the reactions which were conducted under a higher heating rate were quenched promptly by adding alcohol in different reaction times. The HRTEM characterization shows that relatively monodispersed nanocrystals with a diameter of 5 nm were formed when the reaction was quenched in 3 min. The HRTEM image in Figure 4 shows that the relatively monodispersed nanocrystals self-organized together and fused on the joint (as indicated by the arrows). With prolonged reaction time, the nonoparticles coerced further. Bigger superstructures with an average diameter of 40 nm (not shown here) and 180 nm (Figure 3a) were formed when the reaction time was extended to 6 and 60 min, respectively. Together with the HRTEM analyses shown in Figure 3d, it is reasonable to conclude that the spherical hierarchical 3D nanostructures were formed through the three-
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Figure 5. Schemes of hierarchical 3D MnO spherical nanostructures formed through a two-stage oriented-attachment mechanism.
dimensionally oriented attachment of nanoparticles. As discussed above, the high-symmetry cubic structure MnO should have 18 high-energy crystallographic planes: 6 {100} planes and 12 {110} planes. It should be pointed out that a higher heating rate would result in brief overheated reaction conditions. Compared to the conditions of a slow heating rate, a great nucleus formed instantaneously and nucleated rapidly. In addition, effective collision of nanoparticles was improved by vigorous refluxing. Therefore, smaller nanoparticles would have much more chances to attach with one another through crystallographic alignment along the high-energy crystallographic planes in three dimensions. Once they aggregate along the same crystallographic planes, they would fuse to eliminate high-energy crystallographic planes in order to depress the surface energy of nanostructures. Then, the hierarchical superstructures were formed through the three-dimensional oriented attachment of nanoparticles. On the other hand, no products were formed in the reaction which was allowed to slightly reflux for 3 min under a slow heating rate. The results indicate that nucleation was slow under the weak refluxing conditions. The hierarchical superstructures might be formed through a two-stage self-alignment of nanoparticle building blocks, as shown in the hypothetic scheme shown in Figure 5. Ordinarily, building blocks with high shape anisotropy, such as nanorods and nanodisks, will spontaneously align to produce crystals with analogous morphologies.6,16 In other words, the superstructures always have similar morphologies to those of building blocks and, therefore, the morphologies of superstructures could be controlled through tuning the morphologies of building blocks. Remarkably, ovoid and spindle-shaped superstructures of copper oxide could be formed through the vectorial aggregation of spherical primary nanoparticles.17 It is significant that current hierarchical superstructures were formed through two-step organization of spherical primary nanoparticles. The formation process might be similar to that of magic nuclearity giant clusters of metal nanoparticles formed by mesoscale self-assembly.16b The primary nanoparticles with a diameter of 5 nm have roles as the metal atoms. First, the primary nanoparticles with a diameter of 5 nm self-assembled homogeneously to form bigger spherical superstructures with a diameter of 25 nm, as shown in Figure 5b-d. In this process, the superstructures were formed by the three-dimensional oriented-attachment mechanism. To reduce system energy, the nanoparticles self-organized and fused with one another along high-energy surfaces under crystallographic fusion elimination of the high-energy faces.18 Second, the spherical superstructures with a diameter of 25 nm selfassembled further. In the second step, the spherical nanoparticles
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Figure 6. (a) Temperature dependence of magnetic susceptibility for hierarchically spherical 3D MnO nanostructures under the FC and ZFC processes. (b) Hysteresis loops of hierarchically spherical 3D MnO nanostructures conducted at 4, 100, and 200 K after field cooling with an applied field of 1000 Oe. The inset of part b shows an enlarged area of the center of the M-H loops.
formed in the first step acted as building blocks, which are similar to the primary nanoparticles with a diameter of 5 nm in the first step, to form hierarchical superstructures with an average diameter of 180 nm, as shown in Figure 5e and f. Similarly, the nanoparticles self-organized and fused together with their high-energy surfaces under crystallographic fusion elimination of the high-energy faces in order to reduce the total energy. To our knowledge, it is the first hierarchical superstructures formed through a two-step self-assembly. It is very general that the magnetic properties of magnetic nanomaterials are greatly sensitive to the size, morphology, and composition, even to the formation conditions. Therefore, contrary and/or anomalous magnetic properties are usually reported for the same magnetic nanomaterials, including MnO nanomaterials.12 Figure 6a shows the temperature-dependent susceptibility measured under ZFC and FC processes of hierarchically spherical superstructures. The magnetization initially increases slowly with decreasing temperature from 300 to 50 K and then increases vigorously from 50 to 4 K in the FC process, indicating ferromagnetic ordering shows up in the low-temperature range. Similar ferromagnetic behavior has been found in nanoscale NiO and CoO.19 Remarkably, the magnetization initially increases starting from 4 K with increasing temperature in the ZFC process. Then, the magnetization reaches one maximum point at 18 K, which is defined as the blocking temperature, TB. Eventually, the magnetization decreases with
Zhang and Chen increasing temperature above TB and reaches one minimum point around 106 K. After that, it increases gradually with increasing temperature and reaches another maximum point at 121 K, which is the AFM transition temperature, the Ne´el temperature, TN. The AFM transition temperature agrees well with that of the bulk, 122 K. In the end, the magnetization decreases gradually with increasing temperature from 121 to 300 K. The hierarchically spherical 3D MnO nanostructures exhibit a similar AFM temperature to that of bulk and a ferromagnetic transition at low temperature. Such anomalous magnetic behavior plausibly arises from the complex superstructures of nanostructures. As analyzed by HRTEM, the superstructures were composed of three-dimensionally attached and fused nanoparticles. Therefore, the superstructures are endowed with the characteristics of smaller nanoparticles with an average diameter of 5 nm and of bigger nanoparticles with an average diameter of 180 nm. First, the superstructures could act as single crystals with defects interspaced by organic additives. It might be possible that the long-range magnetic correlations in the superstructures are similar to those in bulk MnO. Consequently, the superstructures have an AFM transition temperature as that of the corresponding bulk MnO because the superstructures have a characteristic scale of 180 nm. Secondarily, the superstructures exhibit many more surfaces in the interspaced defects. As a result, the superstructures possess a relatively large surface-to-volume ratio and a great many broken bonds exist on the surface of interspaced defects. The broken bonds will induce a great many surface spins that might order differently from the spins in the core of the smaller nanoparticles of superstructures. Therefore, the lowtemperature ferromagnetism arose plausibly from the uncompensated surface spins. Figure 6b shows the hysteresis loop measured at 4, 100, and 200 K for the spherical superstructures in FC with 1000 Oe from 250 K. The loops measured at 100 and 200 K are linear. Interestingly, the coercivity and exchange bias of the M-H loop measured at 4 K are 3870 and 550 Oe, respectively. Such a large coercivity and shifted hysteresis loop reveal that a strong ferromagnetic interaction is exhibited in the surface of interspaces owning to the uncompensated surface spins. Therefore, there is a strong ferromagnetic and AFM exchange coupling between the ferromagnetic shell on the surface of interspaced defects and the AFM core of fused nanoparticles of superstructures.20 Conclusion In summary, we have successfully fabricated uniform multirod and hierarchically spherical 3D Co1-xMnxO (2/3 e x e 1) nanostructures using a novel one-pot procedure. Our results indicate that the morphology is controllable through controlling the heating rate. Obviously, the refluxing will inevitably stir the reaction solution, and the intensity of refluxing could affect nucleation and aggregation.21 A slow heating rate and weak refluxing resulted in slower nucleation and higher monomer concentration; then, multirod 3D nanostructures might be formed through a diffusion-controlled reaction. Remarkably, a higher heating rate and vigorous refluxing would lead to hierarchically spherical 3D nanostructures formed by the three-dimensionally oriented-attachment mechanism. Strong ferromagnetic interaction existed in the low-temperature range, and weak antiferromagnetism which was similar to the bulk was exhibited in the hierarchically spherical 3D MnO nanostructures. The anomalous magnetic properties plausibly arose from the novel microstructure characteristics of the superstructures. This facile one-pot approach to hierarchical structures with novel morphologies
Hierarchical 3D Co1-xMnxO Nanostructures could be potentially extended to other transition metal oxides. The obtained 3D MnO nanostructures may find potential applications in catalysts, electrode materials, and building blocks of nanoelectronic devices. Acknowledgment. This work was supported by the National Science Foundation of China, by the Ministry of Science and Technology of China (973 Project No. 2006CB601001), and by the Knowledge Innovation Project of Chinese Academy of Sciences. References and Notes (1) (a) Schmid, G. Nanoparticles: From Theory to Application; WileyVCH: 2004. (b) Sugimoto, T. Monodispersed Particles; Elsevier: Amsterdam, The Netherlands, 2001. (c) Heath, J. R. Acc. Chem. Res. 1996, 29, 388. (d) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (e) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 54. (f) Zhang, J. Z.; Wang, Z. L.; Jiu, J.; Chen, S.; Liu, G. Y. Self-Assembled Nanostructures; Kluwer: New York, 2003. (2) (a) Alivisatos, A. P. Science 1996, 271, 933 (b) Duan, X. F.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241 (c) 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. (d) Wang, Z. W.; Daemen, L. L.; Zhao, Y. S.; Zha, C. S.; Downs, R. T.; Wang, X. D.; Wang, Z. L.; Hemlry, R. J. Nat Mater. 2005, 4, 922. (3) (a) Ajayan, P. M.; Schadler, L. S.; Braun, P. V. Nanocomposite Science and Technology; Wiley-VCH Verlag GmbH Co.: 2003. (b) Klabunde, K. J. Nanoscale Materials in Chemistry; Wiley-Interscience: New York, 2001. (4) (a) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2, 382. (b) Law, M.; Goldberger, J.; Yang, P. D. Annu. ReV. Mater. Res. 2004, 34, 83. (c) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L. W.; Alivisatos, A. P. Nature 2004, 430, 190. (5) (a) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (b) Peng, X.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343. (c) Murray, C. B.; Sun, S. H.; Doyle, H.; Betley, T. MRS Bull. 2001, 26, 985. (d) Sun, S. H.; Fullerton, E. E.; Weller, D.; Murray, C. B. IEEE Trans. Magn. 2001, 37, 1239. (e) Murray, C. B.; Sun, S. H.; Gaschler, W.; Doyle, H.; Betley, T. A.; Kagan, C. R. IBM J. Res. DeV. 2001, 45, 47. (f) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (6) (a) Co¨lfen, H.; Antonietti, M.; Angew. Chem., Int. Ed. 2005, 44, 5576. (b) Co¨lfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350. (7) Maan, S.; Davis, S. A.; Hall, S. R.; Li, M.; Rhodes, K. H.; Shenton, W.; Vaucher, S.; Zhang, B. J. J. Chem. Soc., Dalton Trans. 2000, 3753.
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