Microscale Mn2O3 Hollow Structures: Sphere, Cube, Ellipsoid

Sep 16, 2009 - Calcining these MnO2 shells at 500 °C, polycrystalline .... results of (a) MnCO3 spheres, (b) MnCO3 cubes, (c) MnCO3 ellipsoids, and (...
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J. Phys. Chem. C 2009, 113, 17755–17760

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Microscale Mn2O3 Hollow Structures: Sphere, Cube, Ellipsoid, Dumbbell, and Their Phenol Adsorption Properties Jie Cao, Yongchun Zhu, Keyan Bao, Liang Shi, Shuzhen Liu, and Yitai Qian* Hefei National Laboratory for Physical Science at Microscale and Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ReceiVed: June 11, 2009; ReVised Manuscript ReceiVed: August 24, 2009

Various Mn2O3 hollow structures, such as spheres, cubes, ellipsoids, and dumbbells have been synthesized through the following process: The surfaces of the prepared MnCO3 microspheres, microcubes, and microellipsoids were oxidized by KMnO4 to form a core/shell structure. Similarly, the surface of a dumbbelllike MnCO3 intermediate can also be oxidized by KMnO4. As the MnCO3 or MnCO3 intermediate cores were dissolved by acid, the MnO2 shells were formed. Calcining these MnO2 shells at 500 °C, polycrystalline Mn2O3 hollow structures were obtained. The morphologies of these hollow structures were similar to their precursors. The as-prepared hollow Mn2O3 materials were used as adsorbents in water treatment, and the hollow Mn2O3 spheres, cubes, ellipsoids, and dumbbells could respectively remove about 77%, 83%, 81%, and 78% of phenol. 1. Introduction Hollow micro- and nanostructures have high specific surface areas, low densities, and better permeation, and they are potential materials for waste removal, catalysis, photonic crystals, bimolecular-release systems, and so on.1-6 In this regard, remarkable progress has been made for the fabrication of hollow materials with varying sizes and shapes. Among them, soft templates formed from amphiphilic molecules through self-assembly and hard templates, such as polymers and inorganic nanospheres, have been demonstrated to be most effective for the preparation of hollow-structured materials.7-14 Considering that the properties of hollow structures can be tuned by tailoring the external morphologies, it is thus desirable to explore hollow structures with various well-defined, novel geometrical architectures.15 Mn2O3 has been extensively investigated as an inexpensive, environmentally-friendly catalyst to remove carbon monoxide and nitrogen oxide from waste gases.16 In addition, it can be an ideal candidate for the preparation of soft magnetic materials, and for the electrode materials of rechargeable lithium batteries.17 A wide variety of morphological forms of Mn2O3, ranging from one- to three-dimensional structures, such as rods, wires, cubes, octahedra, and hollow spheres, have been synthesized through various methods.14,18-21 However, the morphologycontrolled synthesis of Mn2O3 hollow nanostructures/microstructures, especially those with novel and attractive morphologies, is difficult and remains a great challenge. Herein, we describe the synthesis of various Mn2O3 hollow structures, such as spheres, cubes, ellipsoids, and dumbbells. The method developed here was based on the calcination of the MnO2 shells, the morphologies of which can be easily controlled by varying the morphologies of the premade MnCO3 and MnCO3 intermediate. The distinguished property of removing the pollutant phenol, which is found in disinfectants and antiseptics and is widely used as the intermediate for dyes in organic synthesis processes, was observed for hollow Mn2O3 materials. * To whom correspondence should be addressed. Tel: 86-0551-360-6647. Fax: 86-551-360-7402. E-mail: [email protected].

2. Experimental Section 2.1. Materials. All of the reagents (analytical grade purity) were purchased from Shanghai Chemical Reagent Co. (China) and used as received without further purification. KMnO4 and HCl were dissolved in distilled water to make stock solutions with concentrations of 0.032 and 2.4 M, respectively. 2.2. Synthesis. The synthesis of MnCO3 microspheres and microcubes were carried out on the basis of the previous work of Li’s group with amelioration.3 In brief, 0.169 g of MnSO4 · H2O and 0.84 g of NaHCO3 were separately dissolved in 70 mL of distilled water. A 7-mL portion of of ethanol was then added to the MnSO4 solution under stirring, and the NaHCO3 solution was directly put into the mixture solution mentioned above. The mixture was then maintained at room temperature for 3 h. The as-obtained MnCO3 microspheres were washed with distilled water and absolute alcohol several times. MnCO3 microcubes were prepared by adding 1.321 g of (NH4)2SO4 into the initial mixture and maintaining them for 7 h in an oven at 50 °C. By changing the experimental parameters, the sizes of the prepared MnCO3 micorspheres and microcubes decreased by 50%, which may help in the observation of the final hollow structures by TEM. MnCO3 ellipsoids were prepared in a procedure described by Tang’s group, with minor modification.20 Briefly, 0.474 g of KMnO4, 0.595 g of C16H1206 · H2O, and 0.15 g of C4H6O6 were added to a given amount of distilled water, and the mixture was dispersed to form a homogeneous solution by constant vigorous stirring. The resulting mixture was placed in a Teflonlined stainless-steel autoclave (50 mL capacity) that was then sealed and maintained at 150 °C for 10 h. After cooling to room temperature, the final product was washed with distilled water and absolute alcohol several times. Finally, the product was dried under vacuum at 60 °C. Preparation of MnCO3 dumbbell intermediate: 0.5 g of MnCl2 · 4H2O and 0.6 g of C6H5Na3O7 · 2H2O were added to 40 mL of distilled water under stirring. A 3-mL portion of ammonia was then added to the solution at room temperature, which was continually stirred for several minutes. The above solution was transferred into a Teflon-lined stainless steel autoclave, which

10.1021/jp905482z CCC: $40.75  2009 American Chemical Society Published on Web 09/16/2009

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was sealed and heated at 160 °C for 2 h. The product was collected, washed, and dried under vacuum. Mn2O3 preparation: The as-prepared various morphologies of MnCO3 (0.1 g) and MnCO3 dumbbell intermediate (0.038 g) were respectively dispersed in 20 mL of distilled water, and then KMnO4 solution (5 mL, 0.032 M) was added under stirring. After 1-2 min, HCl solution (5 mL, 2.4 M) was added into the above solutions, and the mixtures were maintained with stirring for 1 min. To obtain Mn2O3 hollow structures, all of the acidtreated products were washed with distilled water and absolute alcohol several times, respectively. Next, the products were calcined in an electronic furnace at 500 °C for 24 h in air. 2.3. Characterization. The products were characterized by X-ray diffraction (XRD) by using a Japanese Rigaku D/maxγA rotating anode X-ray diffractometer equipped with monochromatic high-intensity Cu KR radiation (λ ) 1.54178 Å). The scanning electron microscopy (SEM) images were taken using a JEOL-JSM-6700F field-emitting (FE) scanning electron microscope. The transmission electron microscopy (TEM) images were captured on a Hitachi model H-800 instrument, and the JEOL-2010 transmission electron microscopy was performed at an accelerating voltage of 200 kV. The X-ray photoelectron spectra (XPS) were collected on an ESCALab MKII X-ray photoelectron spectrometer using nonmonochromatizedMgKRX-rayastheexcitationsource.TheBrunauer-Emmett-Teller (BET) surface areas were measured on a Micromeritics ASAP2020 nitrogen adsorption apparatus. 2.4. Water Treatment Experiment. In the experiments, phenol was used as the organic contaminant in the wastewater. As an example of potential applications, the as-prepared hollow Mn2O3 with four different morphologies were used as adsorbents. The phenol solution containing 100 mg L-1 of phenol was prepared. Then, four kinds of the prepared samples (30 mg) were respectively added to 20 mL of the above solution under stirring. After a given time, the solid and liquid were separated, and a TU-1901 UV-vis spectrometer was used to measure the phenol concentration remaining in the solutions. The same experiment is performed on commercial Fe2O3 material. 3. Results and Discussion X-ray diffraction (XRD) was carried out to determine the structures and composition of the prepared samples, and the results of (a) MnCO3 spheres, (b) MnCO3 cubes, (c) MnCO3 ellipsoids, and (d) MnCO3 dumbbell intermediate are given in Figure S1 of the Supporting Information. All of the diffraction peaks correspond to the pure rhombohedral phase of MnCO3 (JCPDS No. 83-1763) for patterns a, b, and c, whereas for pattern d, apart from the peaks of the rhombohedral MnCO3, some peaks that originate from the MnCO3 intermediate can also be found. The XRD patterns shown in Figure 1 correspond to the samples obtained from MnCO3 and MnCO3 intermediate via potassium permanganate oxidation, acid treatment, and calcination. All of the diffraction peaks of a, b, c, and d can be assigned as cubic Mn2O3 (JCPDS No. 89-4836), and the corresponding average sizes of crystallites calculated using Scherrer’s equation are 48, 51, 33, and 38 nm, respectively. Morphologies and sizes of the as-prepared samples of MnCO3 and MnCO3 intermediate were examined using SEM. As shown in Figure 2a,b, the microspheres exhibit a diameter of about 2 µm, and the average edge length of a microcube is approximately 2.5 µm. As for sample c, the microcrystals reveal an ellipsoid shape with a mean length of 1.7 µm and a width in the range of 0.7-1 µm (Figure 2c). Figure 2d shows the SEM

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Figure 1. XRD patterns a-d Mn2O3 hollow spheres, hollow cubes, hollow ellipsoids, and hollow dumbbells, respectively.

Figure 2. SEM images for different shaped MnCO3 structures: (a) spheres, (b) cubes, (c) ellipsoids, and (d) MnCO3 dumbbell intermediate.

image of the dumbbell-like MnCO3 intermediate. There exists a large number of dumbbells with a length of several micrometers. Various Mn2O3 hollow structures, such as spheres, cubes, ellipsoids, and dumbbells, were synthesized from MnCO3 and MnCO3 intermediate with similar morphologies, as described in detail in the Experimental Section. Figure 3a shows the SEM image of Mn2O3 hollow spheres. The shape and size of the Mn2O3 architecture are almost identical to the precursor of MnCO3. As shown in Figure 3b, the as-synthesized product exhibits a hollow spherical morphology and about 2 µm in diameter, in accordance with the TEM results. The homogeneous hollow spheres in the architecture are rough and well-compacted by tiny particles. The TEM image of such prepared spheres (Figure 3c) shows that the final product with hollow structure is in high yields. Through the magnified TEM image (Figure 3d), it can be seen that the shell of the spherical cage is about 100 nm in thickness. Figure 4 shows both the TEM and SEM images of the hollow Mn2O3 microcubes. The cubic cages have an edge-length ranging from 2 to 3 µm (Figure 4a) and exhibit rough surfaces on their sides (shown in Figure 4b). The interior space of the hollow cubic Mn2O3 is clearly revealed on the SEM image for some broken Mn2O3 cubes (Figure 4b). Figure 4c,d gives the typical TEM images of the cubic Mn2O3 hollow structure. Each cube-like structure has a uniform shell composed of a dense

Microscale Mn2O3 Hollow Structures

Figure 3. (a) SEM image of Mn2O3 spherical structure with hollow interiors. (b) SEM image of the sample with higher magnification. (c) TEM image of Mn2O3 spherical structures with hollow interiors. (d) TEM image taken from the same sample with slightly higher magnification.

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Figure 5. (a) SEM image of Mn2O3 ellipsoids. (b) Enlarged view of a region in (a). (c) Representative TEM images of the as-synthesized Mn2O3 ellipsoids in general view. (d) TEM image of some isolated hollow ellipsoids.

Figure 4. SEM images of Mn2O3 cubic structure with hollow interior (a, b). TEM images of Mn2O3 hollow cubes (c, d).

Figure 6. Images of Mn2O3 hollow dumbbells: (a) low- and (b) highmagnification SEM images, and (c) low- and (d) high-magnification TEM images.

aggregation of tiny Mn2O3 particles, and the shell is about 100-200 nm in thickness (estimated from Figure 4c,d). By carefully adjusting the experimental conditions, such as the proper precursor, the reactant concentration, and the reaction time, etc., various Mn2O3 hollow structures with other welldefined shapes were explored. The Mn2O3 hollow ellipsoid discussed below is a good example further demonstrating the interesting concept. The representative SEM pattern of the hollow ellipsoid shown in Figure 5a clearly indicates that there exist a large number of microellipsoids with a mean length of 1.7 µm and a mean width of 1 µm. As shown by the high magnification SEM image in Figure 5b, the incomplete ellipsoids reveal that the architecture of the ellipsoid is empty in the interior and the surface of them is composed of small nanoparticles. Figure 5c,d shows the corresponding TEM images of the sample, which indicate that the boundary of the shell of the hollow ellipsoids is quite defined, and the thickness of the shell is about 50 nm. Mn2O3 hollow structures were synthesized from MnCO3 with similar morphologies, which inspired us to extend the precursor to MnCO3 intermediate. To validate the feasibility of this strategy, MnCO3 dumbbell intermediate was used as precursor

to prepare Mn2O3 hollow dumbbells. The morphology of asprepared hollow dumbbells was studied by SEM. Figure 6a shows that the as-prepared sample is mainly composed of dumbbells with lengths of several micrometers. Some broken dumbbells that result from the ultrasonic treatment can also be found. Figure 6b is the higher-magnification SEM image of the dumbbell-like product. It is clearly seen that the dumbbell has a hollow interior. The structure of the sample was further characterized by TEM. As shown in Figure 6, parts c and d, the dumbbell-like morphology and obvious hollow nature of the Mn2O3 microstucture can be observed. The dumbbell with hollow structure may be formed by the self-assembly of tiny nanoparticles. The pathway leading to the Mn2O3 hollow structures is shown in Scheme 1. In brief, the MnO2 shell with MnCO3 or MnCO3 intermediate core was first prepared at room temperature using a freshly prepared solution of dilute KMnO4 to oxidize the surface of the premade MnCO3 or MnCO3 intermediate. After removal of the MnCO3 or MnCO3 intermediate core with HCl, less crystallized MnO2 hollow structure was obtained. To further determine the process of formed hollow MnO2 shell, ellipsoidlike MnO2 was selected as an example. Figure 7a shows XPS data for Mn 2p region of the as-synthesized hollow ellipsoids.

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SCHEME 1: Schematic Illustration of the Synthesis of Mn2O3 Hollow Structuresa

a

(a) MnCO3 and MnCO3 intermediate; (b) the MnCO3 or MnCO3 intermediate core with the shell of MnO2 structures; (c) MnO2 hollow shells; (d) Mn2O3 hollow structures. (i): reaction with KMnO4; (ii): removal of MnCO3 or MnCO3 intermediate core; and (iii): thermal treatment at 500 °C for 24 h.

There are two strong peaks at about 642 and 654 eV, which are attributed to Mn 2p3/2 and 2p1/2, respectively. The values are in agreement with those reported for MnO2.3,22 Figure 7b shows the corresponding XRD pattern of the MnO2 ellipsoid shell. The broad reflections indicate its poor crystallinity, in accordance with the previous report.23 Figure 7c gives the typical SEM image of the MnO2 ellipsoid. The hollow nature of the MnO2 ellipsoid is clearly revealed on the SEM image for broken MnO2 ellipsoid shell. More structural information is provided by the TEM image in Figure 7d, confirming that the MnO2 ellipsoids indeed have a vivid hollow structure. Calcining these MnO2 shells at 500 °C, polycrystalline Mn2O3 hollow structures were obtained. All of the above reveals that the poor crystallinity of the MnO2 hollow shells were really obtained in the intermediate process, which is crucial to the formation of morphology-preserved hollow Mn2O3 structures. To investigate the influence of experimental parameters, a series of comparison experiments were carried out. With the other conditions kept constant, the thicknesses of the hollow MnO2 ellipsoid will increase by prolonging the reaction time of KMnO4 and MnCO3 ellipsoid (Figure 8a-c) or by enhancing the amount of reacted KMnO4 (Figure 8d-f). Therefore, the thickness of the final hollow Mn2O3 products can be tuned by adjusting the reaction parameters. We also investigated the effect of calcination temperature on the crystallization and morphology of final product. The related XRD patterns (see Supporting InformationFigure S2) confirm that the temperature of MnO2 transformed to crystlline Mn2O3 is 500 °C and the products remain in the same phase until 700 °C. The TEM images of Mn2O3 obtained at different temperature are shown in Figure 8g-i. As the calcination temperature was elevated, the mean size of the nanoparticles that construct the hollow Mn2O3 ellipsoids increased from 30 nm to 67 nm. It is known that transition metal oxides could be capable of removing organic waste from water by adsorption and subsequent catalytic combustion at room temperature.3,24 Herein, the prepared hollow Mn2O3 microstructures were used as adsorbents to investigate their application of water treatment. The Brunauer-Emmett-Teller (BET) surface areas of hollow Mn2O3 spheres, cubes, ellipsoids, and dumbbells were found

Figure 7. (a) XPS date for Mn 2p region of MnO2 hollow ellipsoids and (b) XRD pattern of MnO2 hollow ellipsoids. (c) SEM image of MnO2 hollow ellipsoids. (d) TEM image of MnO2 hollow ellipsoids.

to be 15, 20, 24, and 19 m2 g-1, respectively. The hollow structure with high surface area is beneficial for adsorption and various catalytic reactions. Phenol, a common intermediate of dyes in organic synthesis processes, was chosen as the organic contaminant in the wastewater. The high level of toxicity of phenols has been of substantial concern with regard to it being harmful to the health of humans. Hence, removal of phenol from water at relatively low temperatures is very important for ameliorating the quality of environmental water. UV-vis absorption spectroscopy was applied to record the adsorption behavior of the phenol solution before and after adsorption by the prepared Mn2O3 hollow materials (Figure 9a-e). The characteristic absorption of phenol at 270 nm was chosen for monitoring the adsorption process. It was found that all four as-prepared Mn2O3 exhibited excellent adsorption capability for phenol. For example, 30 mg of different hollow

Microscale Mn2O3 Hollow Structures

J. Phys. Chem. C, Vol. 113, No. 41, 2009 17759 than the commercial Fe2O3. The better performances could be attributed to the high surface area and stable hollow structure of the prepared Mn2O3 products, which can provide more electrostatic attraction sites on the surface. Furthermore, the materials containing phenol (such as hollow Mn2O3 ellipsoids) could be recovered by catalytic combustion at 300 °C in air for 3 h and reused. The recovered hollow Mn2O3 ellipsoids displayed almost the same adsorption performance as newly prepared Mn2O3 ellipsoids, and they both exhibited better performance than that of preformed MnO2 ellipsoids (inset of Figure 9). From the above discussion, the Mn2O3 hollow materials would be highly promising candidates to be applied in water treatment for environmental protection. 4. Conclusions

Figure 8. TEM images of the MnO2 hollow ellipsoid obtained under MnCO3 reacted with KMnO4 for different times while other experiment conditions remain the same: (a) 2 min; (b) 30 min; and (c) 60 min. TEM images of different thickness of the MnO2 hollow ellipsoid by varying the concentration of M (KMnO4): (d) 0.038; (e) 0.076; and (f) 0.114. TEM images of the Mn2O3 hollow ellipsoid obtained at different temperature: (g) 500 °C; (h) 600 °C; and (i) 700 °C.

In summary, hollow Mn2O3 with four different morphologies have been selectively prepared. The method developed here was based on the calcination of the MnO2 shells, the morphologies of which can be controlled easily by varying the morphologies of the premade MnCO3 and MnCO3 intermediate. The asprepared Mn2O3 hollow materials exhibited excellent adsorption performance for phenol in wastewater and are expected to be useful in many other applications. Such control over the morphology suggests great potential for shape-controlled syntheses of other manganese oxide hollow structures such as Mn3O4, which can be prepared directly using Mn2O3 as the precursor. Acknowledgment. This work was supported by National Natural Science Foundation of China (No. 20431020) and the 973 Project of China (No. 2005CB623601). Supporting Information Available: Figure S1, showing the XRD patterns of MnCO3 and MnCO3 intermediate, and Figure S2, showing the XRD patterns of products prepared by calcining the hollow MnO2 ellipsoid at different temperatures. This information is available free of charge via the Internet at http:// pubs.acs.org. References and Notes

Figure 9. UV-vis absorption spectra of phenol. Curve a corresponds to the spectrum of phenol without adsorption by Mn2O3; curves b-f respectively correspond to the spectra of phenol after treated with hollow Mn2O3 spheres, cubes, ellipsoids, dumbbells, and commercial Fe2O3. Inset shows the spectra of phenol after adsorption by as-prepared Mn2O3 ellipsoids (1), recovered Mn2O3 ellipsoids (2), and preformed MnO2 ellipsoids (3).

Mn2O3 products were respectively added into 20 mL of wastewater containing 100 mg L-1 of phenol, and all of the mixtures were maintained for 300 min under stirring; the asprepared hollow Mn2O3 spheres, cubes, ellipsoids, and dumbbells could respectively remove about 77%, 83%, 81%, and 78% of phenol without any other additives, as shown by UV-vis absorption spectra (Figure 9b-e). The mechanism for the adsorption was thought to be the electrostatic attraction between the surface of Mn2O3 hollow products and phenol molecules.24 Many researchers have used commercial Fe2O3 as a reference to evaluate the effectiveness of the adsorbents.3,24 In this work, the UV-vis result for the phenol solution treated with commercial Fe2O3 powder is shown in Figure 9f. It was found that the as-prepared Mn2O3 products exhibited better removal ability

(1) Chen, D.; Ye, J. H. AdV. Funct. Mater. 2008, 18, 1922. (2) Caruso, F. Chem.sEur. J. 2000, 6, 413. (3) Fei, J. B.; Cui, Y.; Yan, X. H.; Qi, W.; Yang, Y.; Wang, K. W.; He, Q.; Li, J. B. AdV. Mater. 2008, 20, 452. (4) Wang, J. X.; Ding, H. M.; Tao, X.; Chen, J. F. Nanotechnology 2007, 18, 245705. (5) Xu, X. L.; Asher, S. A. J. Am. Chem. Soc. 2004, 126, 7940. (6) Wang, L. Y.; Bao, J.; Wang, L.; Zhang, F.; Li, Y. D. Chem.sEur. J. 2006, 12, 6341. (7) Garcia, R. M.; Song, Y. J.; Dorin, R. M.; Wang, H. R.; Li, P.; Qiu, Y.; Swol, F. V.; Shelnutt, J. A. Chem. Commun. 2008, 2535. (8) Kim, S. J.; Ah, C. S.; Jang, D. J. AdV. Mater. 2007, 19, 1064. (9) Zhang, X. J.; Li, D. Angew. Chem., Int. Ed. 2006, 45, 5971. (10) Zhang, Z. M.; Sui, J.; Zhang, L. J.; Wan, M. X.; Wei, Y.; Yu, L. M. AdV. Mater. 2005, 17, 2854. (11) Miao, J. J.; Ren, T.; Dong, L.; Zhu, J. J.; Chen, H. Y. Small 2005, 1, 802. (12) Wang, N.; Gao, Y.; Gong, J.; Ma, X. Y.; Zhang, X. L.; Guo, Y. H.; Qu, L. Y. Eur. J. Inorg. Chem. 2008, 3827. (13) Zhao, Q. R.; Xie, Y.; Dong, T.; Zhang, Z. G. J. Phys. Chem. C 2007, 111, 11598. (14) Wang, L. Z.; Ebina, Y.; Takada, K.; Sasaki, T. Chem. Commun. 2004, 1074. (15) Ye, L. N.; Wu, C. Z.; Guo, W.; Xie, Y. Chem. Commun. 2006, 4738. (16) Imamura, S.; Shono, M.; Okamoto, N.; Hamada, A.; Ishida, S. Appl. Catal., A 1996, 142, 279. (17) Jiao, F.; Bao, J. L.; Hill, A. H.; Bruce, P. G. Angew. Chem., Int. Ed. 2008, 47, 1. (18) Chen, Y. C.; Zhang, Y. G.; Yao, Q. Z.; Zhou, G. T.; Fu, S. Q.; Fan, H. J. Solid State Chem. 2007, 180, 1218.

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(19) Yuan, Z. Y.; Zhang, Z. L.; Du, G. H.; Ren, T. Z.; Su, B. L. Chem. Phys. Lett. 2003, 378, 349. (20) Lei, S. J.; Tang, K. B.; Fang, Z.; Liu, Q. C.; Zheng, H. G. Mater. Lett. 2006, 60, 53. (21) Li, W. N.; Zhang, L. C.; Sithambaram, S.; Yuan, J. K.; Shen, X. F.; Aindow, M.; Suib, S. L. J. Phys. Chem. C 2007, 111, 14694. (22) Li, B. X.; Rong, G. X.; Xie, Y.; Huang, L. F.; Feng, C. Q. Inorg. Chem. 2006, 45, 6404.

Cao et al. (23) (a) Huang, X. K.; Lv, D. P.; Yue, H. J.; Attia, A.; Yang, Y. Nanotechnology 2008, 19, 225606. (b) Reddy, R. N.; Reddy, R. G. J. Power Sources 2004, 132, 315. (24) (a) Yu, C. C.; Dong, X. P.; Guo, L. M.; Li, J. T.; Qin, F.; Zhang, L. X.; Shi, J. L.; Yan, D. S. J. Phys. Chem. C 2008, 112, 13378. (b) Zhong, L. S.; Hu, J. S.; Liang, H. P.; Cao, A. M.; Song, W. G.; Wan, L. J. AdV. Mater. 2006, 18, 2426.

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