Morphology-Controllable Synthesis and Characterization of Hierarchical 3D Co1-xMn xONanostructures

Hai-Tao Zhang and Xian-Hui Chen*Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, UniVersity ofScience 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 successfullysynthesized by the decomposition of acetylacetonate precursors. Their morphologies could be controlled throughtuning the heat rating which affects the nucleation. The rods grew along [110] directions to reduce theappearance of high-energy crystallographic{110} planes. The hierarchically spherical superstructures wereformed by a three-dimensional oriented-attachment mechanism. Magnetic measurement indicates that theMnO nanomaterials with hierarchically spherical superstructures show an antiferromagnetic transitiontemperature at 121 K, similar to that of bulk, and a ferromagnetic ordering exists at low temperature. Suchanomalous magnetic properties arise plausibly from their microstructure characteristics.

Introduction

A great deal of effort has been devoted to the synthesis ofnanostructures with well-controlled shapes and sizes.1,2 This ismainly due to the ability of nanomaterials to exhibit novelelectronic, magnetic, optical, chemical, and mechanical proper-ties compared to those of corresponding bulk materials. Thesenovel properties make them highly attractive for many techno-logical applications.2,3 Obviously, understanding the fundamentalphysical properties of nanostructures is principal to the rationaltechnological applications of nanomaterials. Therefore, couplingsynthetic studies for rapid variation and optimization of desiredproperties is an efficient mechanism for the discovery ofpotentially new electronic, catalysis, thermal, optical, andmagnetic properties.4

The properties of materials are highly size and morphologydependent when their dimensionality exits in the nanometerscale.2,5 To form complex architectures with hierarchy acrossan extended scale, especially in the nanometer and micrometerscales, is a real challenge in the design of integrated materialswith advanced functions.6 Fortunately, morphosynthesis offersresearchers the opportunities to design higher-order architecturesat the macroscopic scale with embedded structures at themicroscopic scale.7 Morphosynthesis of inorganic solid involvesthe chemically based strategies to control the size, shape, andorganization of materials over multiscales beyond the unit cell.7,8

In addition to the single crystals with complex morphologiesthat can be formed through the organization of nanoparticles,superstructures interspaced by organic additives can be formedthrough the oriented organization of nanoparticles. Furthermore,the fusion of the nanoparticles leads to single-crystallinestructures with organic additives as defects.6

Among different inorganic nanoparticles, transition metaloxide nanostructures are promising for their potential techno-logical applications in magnetic data storage, promising audiospeakers, biosensors, powder compacts, magnetic targeted drugdelivery, contrasting agents in magnetic resonance imaging, and

alternatives to radioactivity.9 Manganese oxide (MnO) is anantiferromagnetic (AFM) oxide with a Ne´el temperature of 122K even though it was predicted to be ferromagnetic by theory.10

Manganese oxides have important technological applicationssuch as catalysts and electrode materials.10,11 Syntheticallychemical methods have been proved to be very effective forsynthesizing transition metal oxide nanostructures with mono-dispersed size and shape. Up to date, zero- and one-dimensionalnanostructures of manganese oxides with uniform size and shapehave been synthesized successfully in the past few years.12 Thesenanostructures exhibit interestingly novel magnetic propertiesthat 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 hierarchi-cally spherical 3D superstructure nanostructures by controllingthe kinetics of the thermal decomposition of organometalprecursor complexes. They are the first hierarchical 3D nano-structure 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%) wereused as purchased without further purification. Anhydrousethanol and dichloromethane were purchased from ShanghaiChem. Corp. All reactions were conducted in a three-neck flaskequipped with a reflux condenser and a Teflon-coated magneticstirring bar under flowing N2 gas.

Synthesis of 3D MnO Nanostructures.In a typical synthesisprocess of multirod 3D MnO nanostructures, 2 mmol of Mn-(acac)3, 20 mL of dibenzyl ether, 2 mL of oleic acid, and 2 mLof oleylamine were mixed and stirred under a flow of nitrogen.The solution was heated to 200°C at a rate of 10°C/min understirring, and then, the solution was kept at 200°C for 60 min.Following this, the solution was carefully heated, at a rate of1-2 °C, up to slight refluxing for another 60 min. Then, a blackcolloidal solution was formed. The colloidal nanostructures were

* To whom correspondence should be addressed. E-mail: chenxh@ustc.edu.cn. Phone:+86-551-3601654. Fax:+86-551-3601654.

9442 J. Phys. Chem. B2006,110,9442-9447

10.1021/jp061088r CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 04/27/2006

separated upon the addition of 50 mL of alcohol, centrifuged,and washed using a mixture of alcohol and dichloromethanesolvent. In the end, the products were dried in an oven at roomtemperature overnight.

On the other hand, hierarchically spherical 3D MnO nano-structures were synthesized following the above process. Theonly difference is that the reaction medium was heated torefluxing at a higher heating rate, 5-15 °C/min, and after ithad 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 thermaldecomposition of precursors.

Nanostructure Characterization. Samples for transmissionelectron microscopy (TEM) analysis were prepared by dryinga dispersion of the particles on amorphous carbon-coated coppergrids. The morphologies of the products were characterizedusing a Hitachi H-800 transmission electron microscope (200kV) and a JSM-6700F field emission scanning electron micro-scope (FE-SEM). The microstructures of the nanostructures werecharacterized using high-resolution transmission electron mi-croscopy (HRTEM) and selected-area electron diffraction(SAED) on a JEOL 2100 transmission electron microscope (200kV). X-ray powder diffraction patterns of the products werecharacterized by a Rigaku D/max-A X-ray diffractometer (XRD)with graphite monochromatized Cu KR1 radiation in the 2θrange 20-100° with a step of 0.02° at room temperature. X-rayphotoelectron spectroscopy (XPS) was performed on a VGES-CALAB MKII X-ray photoelectron spectrometer, using non-monochromatized Mg KR X-rays as the excitation source. Mnand Co elemental analyses of the as-synthesized nanoparticlepowders were performed by an Atomscan Advantage inductivelycoupled plasma atomic emission spectrometer (ICP-AES).Fourier transform infrared (FTIR) spectroscopy was carried outon a Bruker Vector-22 FTIR spectrometer. Magnetic studieswere carried out with a superconducting quantum interferencedevice (SQUID) magnetometer (Quantum design, MPMS-XL).The magnetic susceptibility was measured under both fieldcooling (FC) and zero field cooling (ZFC) processes under anapplied field of 100 Oe. Hysteresis loops were measured atdifferent temperatures after FC with a field of 1000 Oe.

Results and Discussion

The XRD studies (Figure 1a) of multirod and hierarchical3D MnO nanostructures reveal a cubic rock salt crystal structure

with high crystallinity (Fm3m, JCPDS no. 72-1533). Thecrystalline size of multirod nanostructures calculated by theScherrer equation is 11.5 nm, in good agreement with thediameter observed by TEM studies. XPS was employed to detectthe valance state of manganese in the surface of the nanostruc-tures, and the core-level XPS spectra in the Mn 2p region areshown in Figure 1b. To exclude the charging effects, the spectrawere standardized by C 1s core-level spectra, as shown in theinset of Figure 1b. The spectra of hierarchically spherical 3DMnO nanostructures show the 2p3/2 peak at 641.2 eV with aweak satellite peak at 647.5 eV. These results are consistentwith that of bulk MnO, indicating that the manganese atoms atthe surface have not been oxidized to higher oxidation states.13

The binding energy of the Mn 2P3/2 core level for multirodnanostructures is 642.0 eV, being between these of bulk Mn2O3

and MnO2.13a,14The two kinds of nanostructures were synthe-sized under almost the same synthetic conditions except for theheating rate and strength of refluxing. It is obvious that the onlydifference in the resulting samples is morphologies through theanalyses of XRD patterns and HRTEM images. Compared tothe Mn 2p core level of hierarchically spherical 3D MnOnanostructures, the Mn element in the surface of multirodnanostructures has higher oxidation states than that of hierarchi-cally spherical nanostructures. The FTIR absorption spectra (notshown here) of the samples indicate that they are coated withsubstantial surfactants, oleylamine and oleic acid, which mightplay some role in the hierarchical growth pattern.

The multirod 3D nanostructures were synthesized on a largescale and in a highly uniform way. Figure 2 shows typical TEMimages of multirod 3D MnO and Co1/3Mn2/3O nanostructures.The low-magnification TEM image of MnO in Figure 2a showsthat the product consists almost entirely of such multirodnanostructures, and no isolated nanorods could be observed inour TEM studies. These results indicate that the nanostructures,which have an average size of 100 nm, are highly yielded andexceedingly monodispersed. The SAED pattern shown in Figure2b 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 enlargedmagnification TEM image of an isolated multirod 3D MnOnanostructure whose rods have an average diameter of 10 nmand lengths of 50 nm. The microstructures of the multirodnanostructures were characterized in detail; one representativeHRTEM image of one rod is shown in Figure 2d. The clear

Figure 1. (a) XRD patterns of multirod (top) and spherical (bottom) 3D MnO nanostructures, separately. (b) XPS spectra of multirod (hollowcircles) and spherical (solid triangles) 3D MnO nanostructures, respectively. The inset of part b shows core-level XPS spectra of the C 1s.

Hierarchical 3D Co1-xMnxO Nanostructures J. Phys. Chem. B, Vol. 110, No. 19, 20069443

lattice image indicates the high crystallinity and single-crystal-line 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 nano-structures could also be formed through this process. Figure 1eshows a TEM image of the Co1/3Mn2/3O multirod nanostructures.The nanostructures were found to be homogeneous in size andmorphology, too. The multirod nanostructures have a uniformsize of 100 nm. In addition, every nanostructure has severaldozens of rods. All of the rods were single crystalline and havea diameter of 11 nm and lengths up to 60 nm, as evidenced byhigh-resolution research. More interesting, the nanorods attachedwith one another and fused at the bottom tips. Figure 2f showsa 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 atomratio of the Co1/3Mn2/3O multirod nanostructures is 1:2, whichis in good agreement with that of precursors. Furthermore, studyreveals that such nanostructures will not be formed withincreasing Co concentration.

The AFM MnO crystallizes in the high-symmetry rock salt(face-centered cubic, fcc) structure, whose surface density ofatoms in the corresponding planes should follown(111) > n(200)

> n(110). As to the fcc structured metal, the surface energy forthe three lowest index planes followsγ{111} < γ{200} < γ{110}.15

Therefore, the{110} planes are the most thermodynamicallyunstable and the{111} planes are the most thermodynamicallystable. Hence, growth in the [110] direction is highly favored.This agrees well with the high-resolution analyses of MnOnanorods. The result indicates that a slow heating rate and weakrefluxing favor the anisotropic growth of nanorods along [110]directions. Few nuclei would form and nucleate slowly underweak heating. Therefore, the high shape anisotropic nanorodswere probably formed through a diffusion-controlled reactionunder slight refluxing. Growth of nanorods would reduce high-energy crystallographic planes and the curvature imposed onthe surface-anchored surfactants; such favorable changes in theinorganic lattice and organic bending energies can occur byanisotropic growth.

Interestingly, the morphology of the product changed from amultirod 3D nanostructure to a hierarchically spherical 3Dsuperstructure when the heating rate was enhanced to 5-15 °C/min and the solution was allowed to reflux vigorously. It shouldbe pointed out that an overheated phenomenon appeared undersuch conditions. Figure 3a shows the FE-SEM image ofhierarchically spherical 3D particles with a relatively uniformdiameter of 180 nm. The FE-SEM image at high magnificationshows that the larger spheres were composed of relatively

Figure 2. (a) Low-magnification TEM image of multirod 3D MnOnanostructures with high yield and good uniformity. (b) SAED patternof multirod 3D MnO nanostructures. (c) Enlarged magnification TEMimage of an individual multirod nanostructure. (d) HRTEM imagerecorded in a rod of one multirod 3D MnO nanostructure. (e) Low-magnification TEM image of multirod 3D Co1/3Mn2/3O nanostructureswith high yield and good uniformity. (f) HRTEM image recorded in a3D Co1/3Mn2/3O nanostructure. The inset of part f shows a magnifiedimage of the region where the two rods connect with each other.

Figure 3. (a) High-magnification FE-SEM image of spherical 3D MnOnanostructures. (b and c) TEM image and the corresponding SAEDpattern, respectively, of an isolated spherical 3D MnO nanostructure.(d) HRTEM image of a smaller particle which belongs to a spherical3D MnO nanostructure.

9444 J. Phys. Chem. B, Vol. 110, No. 19, 2006 Zhang and Chen

smaller nanoparticles. A TEM image of one isolated nanoparticlefurther confirms that the spheres were composed of nanoparticleswith an average diameter of 25 nm (Figure 3b). It should bepointed out that aggregation of the nanoparticles is oriented.The ED pattern (as shown in Figure 3c) of the particle revealsthat the smaller nanoparticles are oriented-attached and thelarger one has the characteristics of a single crystal. Moreparticles were characterized and found to be directionallyattached. This indicates that the nanoparticles are hierarchicalstructures and can be classed as mesocrystals.6 More interest-ingly, high-resolution studies show that the relatively smallernanoparticles are composed of much smaller primary nanopar-ticles with an average diameter of 5 nm. Figure 3d shows aHRTEM image of one nanoparticle of a hierarchical nanostruc-ture. The high-resolution analyses reveal that the nanoparticlesare composed of 5 nm nanoparticles. The clear lattice imageindicates that the smaller primary nanoparticles in a relativelylarger nanoparticle were coalesced in high order and fused withthe attached nanoparticles. Furthermore, the clear lattice at thecontact between relatively larger nanoparticles with an averagediameter of 25 nm indicates that they coalesced together in highorder and fused, too. The results indicate that the hierarchicallyspherical nanostructures were formed through the three-dimensionally oriented aggregation of nanoparticles with anaverage diameter of 25 nm, which were formed through theself-organization of smaller nanoparticles that have an averagediameter of 5 nm.6 The nanoparticles self-organized three-dimensionally and fused to form textured superstructures inorder to reduce the surface energy.

The formation process of the spherical hierarchical 3Dnanostructures was studied systemically. To obtain the products,the reactions which were conducted under a higher heating ratewere quenched promptly by adding alcohol in different reactiontimes. The HRTEM characterization shows that relativelymonodispersed nanocrystals with a diameter of 5 nm wereformed when the reaction was quenched in 3 min. The HRTEMimage in Figure 4 shows that the relatively monodispersednanocrystals self-organized together and fused on the joint (asindicated by the arrows). With prolonged reaction time, thenonoparticles coerced further. Bigger superstructures with anaverage diameter of 40 nm (not shown here) and 180 nm (Figure3a) were formed when the reaction time was extended to 6 and60 min, respectively. Together with the HRTEM analyses shownin Figure 3d, it is reasonable to conclude that the sphericalhierarchical 3D nanostructures were formed through the three-

dimensionally oriented attachment of nanoparticles. As discussedabove, the high-symmetry cubic structure MnO should have 18high-energy crystallographic planes: 6{100} planes and 12{110} planes. It should be pointed out that a higher heatingrate would result in brief overheated reaction conditions.Compared to the conditions of a slow heating rate, a greatnucleus formed instantaneously and nucleated rapidly. Inaddition, effective collision of nanoparticles was improved byvigorous refluxing. Therefore, smaller nanoparticles would havemuch more chances to attach with one another throughcrystallographic alignment along the high-energy crystallo-graphic planes in three dimensions. Once they aggregate alongthe same crystallographic planes, they would fuse to eliminatehigh-energy crystallographic planes in order to depress thesurface energy of nanostructures. Then, the hierarchical super-structures were formed through the three-dimensional orientedattachment of nanoparticles. On the other hand, no productswere formed in the reaction which was allowed to slightly refluxfor 3 min under a slow heating rate. The results indicate thatnucleation was slow under the weak refluxing conditions.

The hierarchical superstructures might be formed through atwo-stage self-alignment of nanoparticle building blocks, asshown in the hypothetic scheme shown in Figure 5. Ordinarily,building blocks with high shape anisotropy, such as nanorodsand nanodisks, will spontaneously align to produce crystals withanalogous morphologies.6,16 In other words, the superstructuresalways have similar morphologies to those of building blocksand, therefore, the morphologies of superstructures could becontrolled through tuning the morphologies of building blocks.Remarkably, ovoid and spindle-shaped superstructures of copperoxide could be formed through the vectorial aggregation ofspherical primary nanoparticles.17 It is significant that currenthierarchical superstructures were formed through two-steporganization of spherical primary nanoparticles. The formationprocess might be similar to that of magic nuclearity giant clustersof metal nanoparticles formed by mesoscale self-assembly.16b

The primary nanoparticles with a diameter of 5 nm have rolesas the metal atoms. First, the primary nanoparticles with adiameter of 5 nm self-assembled homogeneously to form biggerspherical superstructures with a diameter of 25 nm, as shownin Figure 5b-d. In this process, the superstructures were formedby the three-dimensional oriented-attachment mechanism. Toreduce system energy, the nanoparticles self-organized and fusedwith one another along high-energy surfaces under crystal-lographic fusion elimination of the high-energy faces.18 Second,the spherical superstructures with a diameter of 25 nm self-assembled further. In the second step, the spherical nanoparticles

Figure 4. TEM image of MnO samples formed through refluxing for3 min. The arrows indicate the fused section between attachednanoparticles.

Figure 5. Schemes of hierarchical 3D MnO spherical nanostructuresformed through a two-stage oriented-attachment mechanism.

Hierarchical 3D Co1-xMnxO Nanostructures J. Phys. Chem. B, Vol. 110, No. 19, 20069445

formed in the first step acted as building blocks, which aresimilar to the primary nanoparticles with a diameter of 5 nm inthe first step, to form hierarchical superstructures with anaverage diameter of 180 nm, as shown in Figure 5e and f.Similarly, the nanoparticles self-organized and fused togetherwith their high-energy surfaces under crystallographic fusionelimination of the high-energy faces in order to reduce the totalenergy. To our knowledge, it is the first hierarchical super-structures formed through a two-step self-assembly.

It is very general that the magnetic properties of magneticnanomaterials are greatly sensitive to the size, morphology, andcomposition, even to the formation conditions. Therefore,contrary and/or anomalous magnetic properties are usuallyreported for the same magnetic nanomaterials, including MnOnanomaterials.12 Figure 6a shows the temperature-dependentsusceptibility measured under ZFC and FC processes ofhierarchically spherical superstructures. The magnetizationinitially increases slowly with decreasing temperature from 300to 50 K and then increases vigorously from 50 to 4 K in theFC process, indicating ferromagnetic ordering shows up in thelow-temperature range. Similar ferromagnetic behavior has beenfound in nanoscale NiO and CoO.19 Remarkably, the magne-tization initially increases starting from 4 K with increasingtemperature in the ZFC process. Then, the magnetization reachesone maximum point at 18 K, which is defined as the blockingtemperature,TB. Eventually, the magnetization decreases with

increasing temperature aboveTB and reaches one minimum pointaround 106 K. After that, it increases gradually with increasingtemperature 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 ofthe bulk, 122 K. In the end, the magnetization decreasesgradually with increasing temperature from 121 to 300 K. Thehierarchically spherical 3D MnO nanostructures exhibit a similarAFM temperature to that of bulk and a ferromagnetic transitionat low temperature. Such anomalous magnetic behavior plausiblyarises from the complex superstructures of nanostructures. Asanalyzed by HRTEM, the superstructures were composed ofthree-dimensionally attached and fused nanoparticles. Therefore,the superstructures are endowed with the characteristics ofsmaller nanoparticles with an average diameter of 5 nm and ofbigger nanoparticles with an average diameter of 180 nm. First,the superstructures could act as single crystals with defectsinterspaced by organic additives. It might be possible that thelong-range magnetic correlations in the superstructures aresimilar to those in bulk MnO. Consequently, the superstructureshave an AFM transition temperature as that of the correspondingbulk MnO because the superstructures have a characteristic scaleof 180 nm. Secondarily, the superstructures exhibit many moresurfaces in the interspaced defects. As a result, the superstruc-tures possess a relatively large surface-to-volume ratio and agreat many broken bonds exist on the surface of interspaceddefects. The broken bonds will induce a great many surfacespins that might order differently from the spins in the core ofthe smaller nanoparticles of superstructures. Therefore, the low-temperature ferromagnetism arose plausibly from the uncom-pensated surface spins.

Figure 6b shows the hysteresis loop measured at 4, 100, and200 K for the spherical superstructures in FC with 1000 Oefrom 250 K. The loops measured at 100 and 200 K are linear.Interestingly, the coercivity and exchange bias of the M-H loopmeasured at 4 K are 3870 and 550 Oe, respectively. Such alarge coercivity and shifted hysteresis loop reveal that a strongferromagnetic interaction is exhibited in the surface of inter-spaces owning to the uncompensated surface spins. Therefore,there is a strong ferromagnetic and AFM exchange couplingbetween the ferromagnetic shell on the surface of interspaceddefects and the AFM core of fused nanoparticles of superstruc-tures.20

Conclusion

In summary, we have successfully fabricated uniform mul-tirod and hierarchically spherical 3D Co1-xMnxO (2/3 e x e 1)nanostructures using a novel one-pot procedure. Our resultsindicate that the morphology is controllable through controllingthe heating rate. Obviously, the refluxing will inevitably stirthe reaction solution, and the intensity of refluxing could affectnucleation and aggregation.21 A slow heating rate and weakrefluxing resulted in slower nucleation and higher monomerconcentration; then, multirod 3D nanostructures might be formedthrough a diffusion-controlled reaction. Remarkably, a higherheating rate and vigorous refluxing would lead to hierarchicallyspherical 3D nanostructures formed by the three-dimensionallyoriented-attachment mechanism. Strong ferromagnetic interac-tion existed in the low-temperature range, and weak antiferro-magnetism which was similar to the bulk was exhibited in thehierarchically spherical 3D MnO nanostructures. The anomalousmagnetic properties plausibly arose from the novel microstruc-ture characteristics of the superstructures. This facile one-potapproach to hierarchical structures with novel morphologies

Figure 6. (a) Temperature dependence of magnetic susceptibility forhierarchically spherical 3D MnO nanostructures under the FC and ZFCprocesses. (b) Hysteresis loops of hierarchically spherical 3D MnOnanostructures conducted at 4, 100, and 200 K after field cooling withan applied field of 1000 Oe. The inset of part b shows an enlargedarea of the center of the M-H loops.

9446 J. Phys. Chem. B, Vol. 110, No. 19, 2006 Zhang and Chen

could be potentially extended to other transition metal oxides.The obtained 3D MnO nanostructures may find potentialapplications in catalysts, electrode materials, and building blocksof nanoelectronic devices.

Acknowledgment. This work was supported by the NationalScience Foundation of China, by the Ministry of Science andTechnology of China (973 Project No. 2006CB601001), andby the Knowledge Innovation Project of Chinese Academy ofSciences.

References and Notes

(1) (a) Schmid, G.Nanoparticles: From Theory to Application; Wiley-VCH: 2004. (b) Sugimoto, T.Monodispersed Particles; Elsevier: Am-sterdam, The Netherlands, 2001. (c) Heath, J. R.Acc. Chem. Res.1996,29, 388. (d) Pan, Z. W.; Dai, Z. R.; Wang, Z. L.Science2001, 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.Science1996, 271, 933 (b) Duan, X. F.; Huang,Y.; Agarwal, R.; Lieber, C. M.Nature2003, 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.NanocompositeScience and Technology; Wiley-VCH Verlag GmbH Co.: 2003. (b)Klabunde, K. J.Nanoscale Materials in Chemistry; Wiley-Interscience: NewYork, 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) Colfen, 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.

(8) Dujardin, E.; Mann, S.AdV. Mater. 2002, 14, 775.(9) (a) Willard, M. A.; Kurihara, L. K.; Carpenter, E. E.; Calvin S.;

Harris, V. G.Int. Mater. ReV. 2004, 49, 125. (b) Gates, B. C.Chem. ReV.1995, 95, 511. (c) Lewis, L. N.Chem. ReV. 1993, 93, 2693. (d) Beecroft,L. L.; Ober, C. K.Chem. Mater. 1997, 9, 1302. (e) Morup, S.Studies ofSuperparamagnetism in Samples of Ultrafine Particles, Nanomagnetism;Hernando, A., Ed.; Kluwer Academic Publishers: Boston, MA, 1993; pp93-99. (f) Aharoni, A. Introduction to the Theory of Ferromagnetism;Oxford University Press: New York, 1996; pp 133-152. (g) Babes, L.;Denizot, B.; Tanguy, G.; Le Jeune, J. J.; Jallet, P.J. Colloid Interface Sci.1999, 212, 474. (h) Hafeli, U., Schutt, W., Teller, J., Zborowski, M., Eds.Scientific and Clinical Applications of Magnetic Carriers; Plenum Press:New York, 1997. (i) Safarik, I.; Safarikova, M.Monatsh. Chem.2002, 133,737.

(10) Nayak, S. K.; Jena, P.J. Am. Chem. Soc. 1999, 121, 644.(11) (a) Kim, S. H.; Kim, S. J.; Oh, S. M.Chem. Mater.1999, 11, 557.

(b) Li, J.; Wang, Y. J.; Zou, B. S.; Wu, X. C.; Lin, J. G.; Guo, L.; Li, Q.S. Appl. Phys. Lett.1997, 70, 3047. (c) Nardi, J. C.J. Electrochem. Soc.1985, 132, 1787.

(12) (a) Seo, W. S.; Jo, H. H.; Lee, K.; Kim, B.; Oh, S. J.; Park, J. T.Angew. Chem., Int. Ed.2004, 43, 1115. (b) Zitoun, D.; Pinna, N.; Frolet,N.; Belin, C.J. Am. Chem. Soc.2005, 127, 15034. (c) Lee, G. H.; Huh, S.H.; Jeong, J. W.; Choi, B. J.; Kim, S. H.; Ri, H. C.J. Am. Chem. Soc.2002, 124, 12094. (d) Park, J.; Kang, E.; Bae, C. J.; Park, J. G.; Noh, H.J.; Kim, J. Y.; Park, J. H.; Park, H. M.; Hyeon, T.;J. Phys. Chem. B 2004,108, 13594. (e) Na, C. W.; Han, D. S.; Kim, D. S.; Park, J.; Jeon Y. T.;Lee, G. Jung, M. H.;Appl. Phys. Lett.2005, 87, 142504. (f) Jana, N. R.;Chen, Y. F.; Peng,Chem. Mater.2004, 16, 3931.

(13) (a) Wagner, C. D.; Moulder, J. F.; Davis, L. E.; Riggs, W. M.Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer CorporationPhysical Electronics Division. (b) Oku, M.; Hirokawa, K.; Ikeda, S.J.Electron Spectrosc. Relat. Phenom. 1975, 7, 465. (c) Lenglet, M.;D’Huysser, A.; Kasperek, J.; Boulle, J. P.; Durr, J.Mater. Res. Bull.1985,20, 745.

(14) Briggs, D.; Seah, M. P.Practical Surface Analysis, 2nd ed.; JohnWiley & Sons: 1993; Vol. 1.

(15) Wang, Z. L.J. Phys. Chem. B2000, 104, 1153.(16) (a) Zhang, H. T.; Wu, G.; Chen, X. H.Langmuir2005, 21, 4281.

(b) Thomas, P. J.; Kulkarni, G. U.; Rao, C. N. R.J. Phys. Chem. B2001,105, 2515.

(17) Lee, S. H.; Her, Y. S.; Matijevic, E.J. Colloid Interface Sci.1997,186, 193.

(18) Penn, R. L.; Banfield, J. F.Geochim. Cosmochim. Acta1999, 63,1549.

(19) (a) Kodama, R. H.; Makhlouf, S. A.; Berkowitz, A. E.Phys. ReV.Lett.1997, 79, 1393. (b) Zhang, Chen, H. T.; X. H.Nanotechnology2005,16, 2288.

(20) Nogues, J.; Schuller, I. K.J. Magn. Magn. Mater.1999, 192, 203.(21) Li, D.; Kaner, R. B.J. Am. Chem. Soc.2006, 128, 968.

Hierarchical 3D Co1-xMnxO Nanostructures J. Phys. Chem. B, Vol. 110, No. 19, 20069447