Controllable Synthesis and Enhanced Electrochemical Properties of

J. Phys. Chem. B 2006, 110, 24305-24310

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Controllable Synthesis and Enhanced Electrochemical Properties of Multifunctional AucoreCo3O4shell Nanocubes Jianqiang Hu, Zhenhai Wen, Qiang Wang, Xin Yao, Qian Zhang, Jianhua Zhou, and Jinghong Li* Department of Chemistry, Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed: May 25, 2006; In Final Form: October 2, 2006

Multifunctional AucoreCo3O4shell nanocubes were synthesized through the introduction of chloroauric acid (HAuCl4) into a typical hydrothermal system after a solvothermal process was completed to form metastable Co3O4 hollow nanospheres in the presence of sodium dodecyl benzenesulfonate (SDBS), which served as the surfactant. The strategy suggested that HAuCl4 played a vital role in the shape transformation and core/shell structure formation, and the sizes of the nanocubes can be tunable through control of the acid concentration. The core/shell structure of the nanocubes was demonstrated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and element analysis (EA) measurements. Moreover, Li ion battery measurement indicated that trace Au intercalation altered not only the size and shape of the Co3O4 nanoparticles but also greatly increased their electrochemical properties. These multifunctional nanocubes will be not only helpful to study physical chemistry properties of magnetic nanocrystals but also are expected to find use in many fields such as biomolecular detection and analysis, sensor, electrochemistry, and Li ion batteries.

1. Introduction Multifunctional nanometer-scale building blocks constructed with two or more components have attracted increasing interest because they usually exhibit unique electrical, optical, and magnetic, etc., properties in comparison to nanostructures with a single component.1-4 In effect, one doped component potentially amplifies the properties of the other component and even generates new properties. For example, multifunctional hybrid nanoparticles consisting of Au, Fe3O4, and PbS or PbSe combine magnetic, plasmonic, and semiconducting properties, and the properties of each component within the hybrids can be effectively adjusted by the conjugating components aided by the coherent interfaces between them.1 These complex nanomaterials displaying novel or amplified properties commonly possess wider application in biomolecular detection and analysis,5-8 lithium ion batteries,9 and electrochemistry.10 Many methodologies are utilized to fabricate multifunctional nanoparticles, which can be generally classified into two types. One method functionalizes nanoparticles with polymers,11 dye,12 alkanethiols (e.g., self-assembled monolayers),13 antibodies/ antigens,14 biotins/avidins,15 DNA,16 proteins,17 and peptides,18 etc. Another straightforward and effective method for synthesizing multifunctional nanoparticles involves direct chemical synthesis of nanoparticles mainly with alloy19 and core/shell20 structures. Co3O4 nanoparticles as anode materials for Li ion batteries have extensively been studied and used because of their high discharge-charge capacity and good cycle life.21 Currently, many research groups have prepared Co3O4 nanoparticles with different sizes and shapes22-24 and found that their electrochemical properties intimately depend on the size and shape.25 Nevertheless, it is still difficult to commercialize single* Corresponding author. E-mail: [email protected]; tel and fax: +86-10-62795290.

component Co3O4 nanoparticles as the anode material of Li ion batteries due to its high irreversible capacity. Therefore, it is desirable to expect that multifunctional core/shell Co3O4 nanoparticles obtained through doping other acceptable elements possess better electrochemical properties for Li ion batteries. However, to the best of our knowledge, the synthesis of multifunctional Co3O4 nanoparticles with the core-shell structure has still been rarely reported to date. In the present work, multifunctional AucoreCo3O4shell nanocubes were synthesized through using a combined solvothermal and hydrothermal method. In this process, metastable Co3O4 hollow nanospheres were first formed by refluxing cobalt nitrate (Co(NO3)2) and absolute ethanol (C2H5OH) in the presence of sodium dodecyl benzenesulfonate (SDBS), which served as the surfactant, and then chloroauric acid (HAuCl4) was added to induce the formation of cube-shaped AucoreCo3O4shell nanocrystals under a hydrothermal atmosphere. It was found that the acid played a central role in the shape transformation and core/ shell structure formation of the nanocubes, whose sizes can be tunable through controlling the acid concentration or reaction time. For verifying multifunctional properties of the AucoreCo3O4shell nanocubes, we studied the influence of Au intercalation on electrochemical properties of Co3O4 nanomaterials served as an anode material for Li ion batteries. 2. Experimental Section 2.1. Reagents and Materials. Chloroauric acid (99%) was purchased from Aldrich. The following analytical reagent-grade reagents were purchased from Tianjin VAS Chemical Reagent Co.: Co(NO3)2‚6H2O, sodium dodecyl benzenesulfonate, concentrated hydrochloric acid, concentrated nitric acid, and absolute ethanol. All above reagents were used without further purification. Milli-Q water (>18 MΩ cm-1) was used to prepare all aqueous solutions. All glassware used was washed with aqua regia and rinsed with >18 MΩ cm-1 water prior to use.

10.1021/jp063216h CCC: $33.50 © 2006 American Chemical Society Published on Web 11/14/2006

24306 J. Phys. Chem. B, Vol. 110, No. 48, 2006 2.2. Preparation of Hollow Co3O4 Nanospheres and AucoreCo3O4shell Nanocubes. In a typical procedure for the preparation of hollow Co3O4 nanospheres and AucoreCo3O4shell nanocubes, two round-bottom flasks labeled A and B containing 3.0 mmol of Co(NO3)2‚6H2O were prepared, respectively. Then, 27.5 mL of saturated SDBS solutions in absolute ethanol was introduced to A and B containers, respectively, and the two mixtures were stirred and refluxed at 78-79 °C (the boiling point of absolute ethanol) for 0.5 h. Next, the above solvothermal products were put into two 30 mL Teflon-linear autoclaves correspondingly marked with A and B. Finally, 1 mL of milli-Q water was introduced to A autoclave while 1 mL of 1 mM HAuCl4 in aqueous solution was added to B autoclave. The final products were obtained by heating the two autoclaves at 180 °C for 6.5 h, repeatedly washing with absolute ethanol and milli-Q water, and separating centrifugally while the two samples were cooled to room temperature. 2.3. Instruments. TEM measurements were performed with a Hitachi Model H-800 microscope operated at 100 kV. Scanning electron microscopy (SEM) was carried out with a field-emission microscope (Leo, 1530) operated at an accelerating voltage of 20 kV. X-ray photoelectron spectroscopy (XPS) spectra were obtained using a PHI-5300 ESCA spectrophotometer. Elemental analysis (EA) was performed by inductively coupled plasma optical emission spectrometry (ICP-OES) with an IRIS Intepid II XSP instrument operated at 22 °C and relative humidity of 52%. UV-vis spectra were acquired using a Shimadzu UV-21005 spectrophotometer using two 1 cm quartz cells. The XRD pattern was recorded on a powder sample using a Bruker D8 Advance X-ray diffractometer with CuKR radiation (λ ) 1.5418 Å) in a range from 10° to 80°. Electrochemical measurements were performed using the following procedure: (i) Electrode and electrolyte solution preparation. A lithium pellet served as both the counter and the reference electrode, the working electrode was fabricated by compressing the mixture of 80 wt % active materials, 10 wt % acetylene black, and 10 wt % polytetrafluoroethylene (PTFE) onto an aluminum foil, and the electrolyte solution was prepared by dissolving 1 M LiPF6 in a solution of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) with the volume ratio of EC:DMC:DEC ) 1:1:1. (ii) The cell assembly was operated in an Ar-filled Labconco glovebox. (iii) Dischargecharge curves were recorded between 3.0 and 0.1 V at a current density of 0.1 mA/cm2 using a Roofer Battery Tester (Shenzhen, China). 3. Results and Discussions 3.1. TEM and SEM Characterization of Hollow Co3O4 Nanospheres and AucoreCo3O4shell Nanocubes. Figure 1 shows typical TEM images of hollow Co3O4 nanospheres and cubeshaped AucoreCo3O4shell nanoparticles prepared using the combined solvothermal and hydrothermal synthetic route. Co3O4 nanospheres with average inside and external diameters of ca. 60 and 80 nm were observed (Figure 1A). In the experiment adding HAuCl4, cube-shaped AucoreCo3O4shell nanoparticles with ca. 126 nm as an average length of side were obtained (Figure 1B). Their corresponding SEM images are shown in Figure 2, from which the cube shape can be clearly identified, too. These images indicated that chloroauric acid played a critical role in the shape transformation and core/shell structure formation of the nanocubes, and multifunctional AucoreCo3O4shell nanoparticles could be obtained only through adding a rather low HAuCl4 concentration. In the top left corner of Figure 1B, a tiny quantity of hollow-shaped nanospheres not being converted into cubic

Hu et al.

Figure 1. TEM images of (A) hollow Co3O4 nanospheres and (B) cube-shaped AucoreCo3O4shell nanoparticles prepared both using the combined solvothermal and hydrothermal synthetic route. Scale bar: 50 nm.

nanoparticles could be clearly seen due to the shortage of HAuCl4 during the hydrothermal reaction, further demonstrating the importance of HAuCl4 in the shape transformation and core/ shell structure formation. In addition, the dimensions of the AucoreCo3O4shell nanocubes were also found to strongly depend on the concentrations of HAuCl4. Figures 3A and 3B give representative TEM images of AucoreCo3O4shell nanocubes prepared using 1 mL of 2 mM and 4 mM HAuCl4, respectively. In the experiment using 1 mL of 2 mM HAuCl4, AucoreCo3O4shell nanocubes had the average length of side of ca. 160 nm (Figure 3A). When the concentration of HAuCl4 added (up to 1 mL 4 mM) to the reaction solution was increased, AucoreCo3O4shell nanocubes with the average length of side of ca. 200 nm were obtained (Figure 3B). Also, different reaction times also led to the formation of different size AucoreCo3O4shell nanocubes. Figures 3C and 3D show typical TEM images of different size AucoreCo3O4shell nanocubes prepared using 1 mL of 1 mM HAuCl4 and the reaction times of 3.5 and 4.5 h, respectively, wherein their corresponding sizes were 75 and 100 nm, respectively. 3.2. Characterization of AucoreCo3O4shell Core/Shell Structure. Figure 4 shows X-ray diffraction (XRD) patterns of the SDBS-protected elliptic Au (see Supporting Information), hollow Co3O4, and cubic AucoreCo3O4shell nanoparticles obtained by using the present method, respectively. The data (curve A) shows diffraction peaks at 2θ ) 38.2°, 44.4°, 64.6°, and 77.5°, which could be indexed to (111), (200), (220), and (311) planes of pure gold in a face-center-cubic (fcc) phase (JCPDS: 4-0784, space group: Fm3m), respectively. Curve B gives the XRD pattern taken from the hollow Co3O4 nanospheres prepared without HAuCl4. All the diffraction peaks at 2θ ) 18.9°, 31.2°, 36.7°, 39.2°, 44.7°, 59.1°, 65.0°, 73.8°, and 78.1° could be indexed to (111), (220), (311), (222), (400), (511), (440), (620), and (622) planes of a pure cubic phase of Co3O4 spinel (JCPDS: 80-1542, space group: Fd3m). Clearly, the XRD peaks for AucoreCo3O4shell nanocubes (curve C) synthesized using 1 mL of 1 mM HAuCl4 were similar to those for these Co3O4 nanospheres (curve B); thus, all XRD peaks from the Au component could be not observed. Moreover, none of isolated gold nanoparticles were discovered in any of the TEM and SEM images of the AucoreCo3O4shell nanocubes. Thus, three probabilities are inferred: the removal of isolated Au nanoparticles by centrifugation, the extremely low content of Au in the AucoreCo3O4shell nanocubes, and the Au nanoparticles coated with

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Figure 2. SEM images of (A) hollow Co3O4 nanospheres and (B) cube-shaped AucoreCo3O4shell nanoparticles prepared both using the combined solvothermal and hydrothermal synthetic route. Scale bar: (A) 200 nm and (B) 300 nm.

Figure 4. XRD patterns of the SDBS-protected (A) elliptic Au, (B) hollow Co3O4, and (C, D) cubic AucoreCo3O4shell nanoparticles obtained through using the present method. Wherein, (C, D) cubic AucoreCo3O4shell nanoparticles were prepared using 1 mL of 1 mM and 4 mM HAuCl4, respectively. The crystalline planes of various diffraction peaks of Au and Co3O4 are shown in the figure, wherein * and 9 represent diffraction peaks of Au and Co3O4, respectively.

Figure 3. TEM images of different size AucoreCo3O4shell nanocubes prepared using different HAuCl4 concentrations and reaction times. (A) 160 nm (1 mL 2 mM HAuCl4 and 6.5 h). (B) 200 nm (1 mL 4 mM HAuCl4 and 6.5 h). (C) 75 nm (1 mL 1 mM HAuCl4 and 3.5 h). (D) 100 nm (1 mL 1 mM HAuCl4 and 4.5 h). Scale bar: (A, B, D) 50 nm and (C) 100 nm.

Co3O4. To verify the three probabilities, element analysis (EA) measurement was performed. EA results indicated that trace Au composition in the AucoreCo3O4shell nanocubes could be detected by the ICP-OES (see Supporting Information), indicating that the Au/Co3O4 nanocomposites had potential core-shell structure. To increase the Au component weight ratio of AucoreCo3O4shell nanocubes, we synthesized AucoreCo3O4shell nanocubes with an average length of side of ca. 200 nm by using 1 mL of 4 mM HAuCl4 (Figure 3B). Besides diffraction peaks of a pure cubic phase of Co3O4 spinel similar to those of the hollow nanospheres, the fcc diffraction peaks of Au, especially the two peaks at 2θ ) 38.2° and 77.5°, could also be clearly observed in the XRD pattern taken from the AucoreCo3O4shell nanocubes prepared using 1 mL of 4 mM HAuCl4 (curve D), which indicated the undoubted existence of Au composition in the AucoreCo3O4shell nanocubes.

To further confirm the core/shell structure of the AucoreCo3O4shell nanocubes, X-ray photoelectron spectroscopy (XPS) studies were performed. It is well-known that XPS is an effective analytical technique for evaluating the composition and the chemical state of materials.26 For nanomaterials with core/shell structures, XPS could not only report detailed information on the near-surface elemental compositions but also provide strong evidence for the formation of core/shell structures.27-29 In the Au/Co3O4 nanocubes produced using 1 mL of 4 mM HAuCl4, their powder XPS analysis showed significant Co 2p and O 1s peaks, respectively, corresponding to the binding energy of the Co metal (Figure 5A) and O nonmetal (Figure 5B). The two strongest peaks at 781.0 and 797.0 eV, corresponding, respectively, to Co 2p3/2 and Co 2p1/2, are characteristic of the Co3O4 phase.23 Furthermore, two satellites located at about 6.0 eV above the primary binding energy peaks were detected, further confirming the Co3O4 phase.30 The O 1s peak at approximately 530.0 eV refers to the Co3O4 while its shoulder peak at about 532.0 eV is attributed to the presence of hydroxyl species or adsorbed water on the surface, which are ubiquitous in airexposed samples.30,31 However, XPS spectra (Figure 5C) corresponding to the Au metal could not be detected directly (Figure 5C-a), revealing that the surface composition of asprepared cube-shaped Au/Co3O4 nanoparticles was dominated by Co and O atoms. Many studies27-29 demonstrated that if particles possessed a real core and shell structure, the core would be screened by the shell and became gradually more dominant when the sample was etched. Therefore, along with increasing etching depth, the intensity ratio of the core/shell spectra would

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Figure 5. XPS spectra of the (A) Co, (B) O, and (C) Au lines obtained, respectively, in powder AucoreCo3O4shell nanocubes synthesized by adding 1 mL of 4 mM HAuCl4.

gradually increase. When we etched the as-prepared nanocubes for 1 min according to Borchert’s method,29 two weak characteristics of the XPS signals of Au metal were discerned (Figure 5C-b). The quantitative XPS analysis data of the nanocube sample indicated that the Au/Co atomic ratio increased from 0 to 0.006 during the above etching procedure. These results confirmed that core-shell Au-Co3O4 nanocubes were formed in the present study. 3.3. Possible AucoreCo3O4shell Core/Shell Structure Formation and Growth Mechanism. To explain the primary core/ shell structure formation and growth mechanism of the AucoreCo3O4shell nanocubes prepared in the present study, we put forward a proposed schematic illustration shown in Figure 6. Our results clearly demonstrated that the introduction of HAuCl4 played a critical role in the formation of the core and shell structure of these AucoreCo3O4shell nanocubes produced thus. Its implication was straightforward that crystallographic planes of gold crystals (e.g., cluster) that were grown from gold monomers generated from HAuCl4 controlled the growth pattern and speed of association processes.32

Hu et al. As has been known, saturated SDBS in absolute ethanol will form micelles with sphere-like shape.33 After the mixture was stirred and refluxed for 30 min, hydrophilic Co(NO3)2‚6H2O was gradually dissolved and the reaction mixture turned uniformly turbid and deep blue from two phases (colorless SDBS ethanol solution and red Co(NO3)2‚6H2O solid), indicating the formation and growth of Co3O4 monomers generated from Co(NO3)2. To fall their surface free energy, the hydrophilic Co3O4 monomers potentially grew along the hydrophilic group of the sphere-like micelles according to Ostwald ripening.34 In the assistance of SDBS, active Co3O4 monomers gradually grew into clusters and simultaneously favored aggregation among clusters. Finally, metastable Co3O4 nanospheres consisting of clusters were formed with hollow structure. HAuCl4 was added to the as-prepared reaction mixture and entered into the hydrophilic end of the sphere-like micelles (i.e., inside of the hollow nanospheres). Meanwhile, high-activity Au monomers were instantaneously generated through reducing HAuCl4 with ethanol under the hydrothermal atmosphere. Our XRD measurements demonstrated that the Au core of the AucoreCo3O4shell nanocubes had the same crystalline structure as that of the elliptical Au nanoparticles, e.g., (100), (110), and (111) crystallographic planes. The (100), (110), and (111) crystallographic planes of the Au crystals are well-known to have different chemical reactivities and surface free energies (γ (110) > γ (100) > γ (111)).35 As elucidated by Wang’s and Xia’s groups,35,36 the final shape of an fcc nanocrystal growth mainly depends on the ratio (R) between the growth rates along and directions. In the present study, SDBS in the hydrothermal atmosphere was supersaturated due to the solvent (C2H5OH and H2O) evaporation and thus R decreased to 0.58. Therefore, the supersaturated SDBS greatly restricted the direction growth rate of the fcc Au cores and quickened their directional growth rate. As well the metastable Co3O4 nanospheres gradually dissolved into the solution and grew again onto the different crystallographic planes of the gold cores under the high-activity gold-core catalysis. As a result, the growth rate of Co3O4 monomers generated from dissolved metastable Co3O4 nanospheres was quicker along the direction of the gold cores than that along the direction and thus facilitated the formation of the cube-shaped nanoparticles. Finally, the nanocubes with the Au core and Co3O4 shell structure were obtained. 3.4. Lithium Ion Discharge-Charge Property Investigation. Co3O4 has received considerable attention because it can be used as an electrode material in lithium ion batteries.21-23,25 On the basis of previous research,23,25,37 the formation and decomposition of Li2O have been considered as the chargedischarge mechanism of Co3O4 for anodes. Nevertheless, the single sloping discharge or charge performance, which resulted from the irreversible formation or decomposition of Li2O after the first discharge, prohibited its use in practice for a lithium ion battery. To widen the application of Co3O4 nanomaterials in Li ion batteries, it is desirable to investigate Co3O4 nanomaterials with different parameters (e.g., size, shape, and doped element) and thus find the optimum Co3O4 electrode material. Now, Chen and his co-workers have demonstrated that differently shaped Co3O4 nanomaterials that served as anode materials for rechargeable lithium batteries possessed different electrochemical properties and found that the electrochemical properties of Co3O4 nanotubes were much better than those of Co3O4 nanospheres or nanorods.25 In the present study, we studied the influence of element intercalation on electrochemical performance of Co3O4 electrode

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Figure 6. Schematic illustration of the core/shell structure formation and growth of multifunctional AucoreCo3O4shell nanocubes prepared using the combined solvothermal and hydrothermal method.

Figure 7. Charge-discharge profiles of the anode electrodes made respectively from (A) cubic Co3O4 and (B) cubic Au-doped Co3O4 nanoparticles between 0.1 and 3.0 V at the current density of 0.1 mA/ cm2.

material. The electrochemical performance toward lithium of cubic Au-doped Co3O4 nanoparticles has been characterized and contrasted against that of pure cubic Co3O4 nanoparticles by galvanostatic measurements between 0.1 and 3.0 V at a constant current density of 0.1 mA/cm2. The cube-shaped Co3O4 nanoparticles were synthesized as described in the procedure with

little modification (see Supporting Information). Figure 7A shows voltage-capacity profiles obtained using the as-prepared cubic Co3O4 nanoparticles. The first discharge process was characterized by a well-defined potential plateau corresponding to a specific capacity of about 650 mAh/g, but in the following discharge, the voltage-capacity profile evolved from a welldeveloped plateau toward an abrupt sloping voltage curve, resulting in a dramatic fading of capacity. Such a phenomenon had already been observed for bulk or film Co3O4 and was associated with the occurrence of electrochemical Li-driven irreversible structural changes and some kinetics limitations of the Co3O4 electrode.25,37 As for cubic Au-doped Co3O4 nanoparticles prepared using 1 mL of 1 mM HAuCl4, its voltagecapacity profiles are shown in Figure 7B. It is worth noting that its second discharge followed the voltage plateau characteristic observed in the first discharge process (about 850 mAh/ g), with a well-pronounced plateau instead of a sharp drop of potential for the pure Co3O4 electrode, suggesting better plateau capacity retention for the former. The better electrochemical properties of the Au-doped Co3O4 that serves as an anode material of Li ion batteries can be explained by regarding the catalytic effect of Au as tremendously decreasing the binding energy of Li2O and further facilitating Li2O reversible formation or decomposition during the charge or discharge process. It is especially worth noting that the AucoreCo3O4shell nanocubes only containing trace Au component (about 0.0265% (wt %)) were prepared only consuming 10-6 mol (i.e., 0.34 µg) HAuCl4 in the present study although HAuCl4 was a relatively expensive compound. Therefore, the multifunctional AucoreCo3O4shell nanocubes may be due to its well-pronounced plateau capacity retention and low-cost, potentially exploited into a general and effective anode material for Li ion batteries. This method is also expected to be extendable to the synthesis of other AucoreMshell (M represents metal oxide) with well-characterized electrochemical properties. 4. Conclusions In summary, multifunctional AucoreCo3O4shell nanocubes were successfully synthesized by using a combined solvothermal and

24310 J. Phys. Chem. B, Vol. 110, No. 48, 2006 hydrothermal method in the introduction of HAuCl4. The sizes of these nanocubes were tunable by controlling the acid concentration or reaction time. In the synthesis, HAuCl4 played a vital role in the cubic shape and core/shell structure formation, whereas without the participation of HAuCl4, the main product was hollow Co3O4 nanospheres. The shape transformation (from hollow to cube) and core/shell structure of the AucoreCo3O4shell nanocubes were confirmed by TEM, SEM, XRD, EA, and XPS measurements. In addition, we also put forward a primary experiment model to explain the shape transformation and core/ shell structure formation, which is perhaps helpful to understand the growth mechanism of other core/shell nanocrystals. Meanwhile, the electrochemical properties of the AucoreCo3O4shell nanocubes served as an anode material of Li ion batteries were investigated. The results indicated that the electrochemical performance of the AucoreCo3O4shell electrode was, due to the trace Au intercalation, greatly enhanced as compared with that of the pure Co3O4 electrode. Therefore, the AucoreCo3O4shell nanocubes may be potentially exploited into a general and effective anode material for Li ion batteries. These multifunctional nanocubes are also expected to scale-up preparation and find use in many fields such as catalysis, sensor, electrochemistry, magnetism, and biomolecular detection and analysis. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20435010, No. 20125513, No. 20575032) and PCSIRT (No. IRT0404). We also acknowledged the financial support from Postdoctoral Science Foundation of China (023205035). Supporting Information Available: Synthetic procedure of the preparation of elliptical Au nanoparticles and its TEM image, element analysis of the AucoreCo3O4shell nanocubes synthesized, respectively, using 1 mL of 1, 4, and 20 mM HAuCl4 in the present study, whole XPS spectra of the cubic AucoreCo3O4shell nanoparticles prepared using 1 mL of 4 mM HAuCl4 in the present study before and after etching, and synthetic procedure of the preparation of cubic Co3O4 nanoparticles and its TEM image. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Shi, W. L.; Zeng, H.; Sahoo, Y.; Ohulchanskyy, T. Y.; Ding, Y.; Wang, Z. L.; Swihart, M.; Prasad, P. N. Nano Lett. 2006, 6, 875. (2) Ederth, J.; Johnsson, P.; Niklasson, G. A.; Hoel, A.; Hultåker, A.; Heszler, P.; Granqvist, C. G.; van Doom, A. R.; Jongerius, M. J.; Burgard, D. Phys. ReV. B 2003, 68, 155410. (3) Luo, J.; Xing, Y. J.; Zhu, J.; Yu, D. P.; Zhao, Y. G.; Zhang, L.; Fang, H.; Huang, Z. P.; Xu, J. AdV. Funct. Mater. 2006, 16, 1081. (4) Yoon, T. J.; Kim, J. S.; Kim, B. G.; Yu, K. N.; Cho, M. H.; Lee, J. K. Angew. Chem., Int. Ed. 2005, 44, 1068. (5) Azzazy, H. M. E.; Mansour, M. M. H.; Kazmierczak, S. C. Clin. Chem. 2006, 52, 1238. (6) Xie, H. Y.; Zuo, C.; Liu, Y.; Zhang, Z. L.; Pang, D. W.; Li, X. L.; Gong, J. P.; Dickinson, C.; Zhou, W. Z. Small 2005, 1, 506. (7) Wang, H.; Wu, Y. P.; Lassiter, B.; Nehl, C. L.; Hafner, J. H.; Nordlander, P.; Halas, N. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 10856.

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