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J. Phys. Chem. C 2007, 111, 11598-11603
Oxidation-Crystallization Process of Colloids: An Effective Approach for the Morphology Controllable Synthesis of SnO2 Hollow Spheres and Rod Bundles Qingrui Zhao,† Yi Xie,*,† Ting Dong,‡ and Zhigao Zhang† Department of Nanomaterials and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, and Department of Chemical Physics, BCL Lab, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ReceiVed: April 12, 2007; In Final Form: June 1, 2007
In this article, we have demonstrated that the oxidation-crystallization method of amorphous colloids in the microreactor systems generated by soft templates is convenient and feasible in the large-scale production of hollow and one-dimensional (1D) structures. Through the air oxidation and crystallization of colloidal spheres in the different systems, SnO2 hollow spheres and rod bundles, respectively, were controllably obtained. The detailed growth processes of SnO2 hollow spheres and rod bundles were investigated. When SnO2 hollow spheres and rod bundles were used as an anode material for lithium-ion batteries, they exhibited relatively high electrochemical capacities. Thus, it is rational to expect that this simple method can be extended to systems of other inorganic materials with desired morphologies.
Introduction It is well-known that the particle size and morphology of materials have a great influence on their properties.1 Recently, there has been an increasing interest in the morphology control of inorganic substances due to their fundamental and technological importance.2,3 At present, the developed strategies for the preparation of nanomaterials include chemical vapor deposition (CVD),4 the irradiation method,5 electrochemical deposition,6 and the solvothermal method.7 For most of these strategies, however, high temperature or special apparatuses are required. Thus, designing a simple approach to synthesize inorganic material under mild condition is quite important from the viewpoint of fundamental issues and application. Previously, few works directly used the crystallization method as an effective tool to control the morphologies of inorganic materials.8 Our group has successfully applied this method to fabricate orthorhombic antimony sulfide (Sb2S3) dendrites,9 orthorhombic bismuth sulfide (Bi2S3) nanorods, and trigonal selenium (t-Se) nanowires.10 In this article, we developed the crystallization process to the oxidation-crystallization process and further combined the soft template strategy to synthesize inorganic functional nanomaterials with controllable morphology. The advantages are obvious because of its simplicity, convenience, low temperature, and capability of producing large quantities of samples. In this article, we concentrated on tin oxide (SnO2) to illustrate the feasibility of the oxidation-crystallization process combined the soft templates strategy in the production of hollow spheres and rod bundles. SnO2 is an n-type semiconductor with a large band gap (Eg ) 3.6 eV) and is well-known for its applications in gas sensors and dye-based solar cells.11,12 Recently, attention has been focused on its possible application in the use of SnO2 as an anode material for lithium-ion batteries. For these * To whom correspondence should be addressed. Phone and Fax: 86551-3603987. E-mail:
[email protected]. † Department of Nanomaterials and Nanochemistry. ‡ Department of Chemical Physics.
Figure 1. XRD pattern of the as-prepared (a) amorphous colloids, (b) SnO2 hollow spheres, and (c) SnO2 rod bundles.
applications, the performance is strongly influenced by the size and morphology.13 With specific morphologies, SnO2 materials such as hollow spheres and rods have attracted intensive research interests because of their size- and dimensionality-dependent properties and potential applications in the fields of lithiumion batteries.14 Usually, Sn(II) is easily oxidized to Sn(IV) in the atmosphere,15 and most of the amorphous colloids have a strong transformation tendency from the thermodynamically metastable amorphous state to the stable crystalline state under appropriate conditions. Inspired by this idea, we have successfully fabricated SnO2 hollow spheres and rod bundles on a large scale, through oxidation and crystallization in air at room temperature, respectively. The successful synthesis of SnO2 hollow spheres and rod bundles in the present article verifies the feasibility of this strategy. Moreover, the electrochemical Li+ intercalation capacities of the as-obtained SnO2 materials were investigated by using the as-prepared samples as the anode materials in lithium-ion batteries. The result indicated that SnO2 hollow
10.1021/jp072858h CCC: $37.00 © 2007 American Chemical Society Published on Web 07/17/2007
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Figure 2. XPS spectra of SnO2 hollow spheres: (a) Sn region; (b) O region. The XPS spectrum of SnO2 rod bundles is similar to that of SnO2 hollow spheres.
Figure 3. Electron microscopy images of the SnO2 nanostructures: (a) FESEM image; (b) TEM image; (c) high-magnification TEM image; (d) the corresponding ED pattern. The FESEM and TEM image showed the “opened” structure of a hollow sphere.
Figure 4. Electron microscopy images of the SnO2 rod bundles: (a) FESEM image; (c) high-magnification FESEM image; (b) TEM image; (d) high-magnification TEM image. The upper inset is the corresponding ED pattern.
spheres exhibited a better electrochemical property, suggesting that the morphology of the materials has exerted a noticeable influence on the electrochemical performance. Experimental Section Chemicals. Stannous sulfate (SnSO4) (90%+), sodium dodecyl benzenesulfonate (SDBS), sodium dodecyl sulfate (SDS), cetyltrimethyl ammonium bromide (CTAB), and poly(vinyl pyrrolidone) (PVP, polymerization degree 360) were of analytical grade and were used as received. Synthesis of SnO2 Hollow Spheres. SnSO4 (2 mmol) and SDBS (2 mmol) were dissolved in 20 mL of distilled water and stirred for 8-10 h in a 50 mL beaker at room temperature. After the reaction was completed, the white products were collected from solution, rinsed several times with distilled water and absolute ethanol, and then dried under vacuum at 40 °C for 4 h. Synthesis of SnO2 Rods. SDBS (2 mmol), PVP (0.12 g), and 20 mL of distilled water were mixed in a 50 mL beaker, stirred for several minutes, and then SnSO4 (2 mmol) was added
Figure 5. TEM images of the micelles formed in the reaction system: (a) SDBS micelle; (b) SDBS-PVP micelle.
in the solution. The mixed solution was stirred for 8-10 h at room temperature. After the reaction was completed, the white products were collected from solution, rinsed with distilled water and absolute ethanol, and then dried under vacuum at 40 °C for 4 h. Characterization. The samples were characterized by X-ray powder diffraction (XRD) with a Japan Rigaku D/max rA X-ray diffractometer equipped with graphite-monochromatized highintensity Cu KR radiation (λ ) 1.54178 Å), recorded with the 2θ ranging from 20° to 70°. The transmission electron microscopy (TEM) images and electron diffraction (ED) patterns were performed with a Hitachi model H-800 instrument with a
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Figure 6. IR spectra of (a) pure SDBS and (b) the intermediates collected at the reaction stage of 10 min prepared in the presence of 2 mmol of SDBS, 2 mmol of SnSO4, and 20 mL of H2O.
SCHEME 1: Schematic Illustration of the Formation Process of (a) SnO2 Hollow Spheres and (b) Rod Bundles
tungsten filament, using an accelerating voltage of 200 kV. The field emission scanning electron microscopy (FESEM) images were taken on an FEI Sirion-200 SEM. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB MKII with Mg KR (hν ) 1253.6 eV) as the excitation source. The binding energies obtained in the XPS spectral analysis were corrected for specimen charging by referencing the C1s to 284.6 eV. Electrochemical Measurements. The electrochemical properties were measured using a Maccor series 2000 battery tester. The working electrode consisted of 80 wt % of the active material (tin oxide), 10 wt % of a conductivity agent (carbon black, Super-P), and 10 wt % of binder (polyvinylidene difluoride, Aldrich). Lithium was used as the reference electrode. The electrolyte was 1 M LiPF6 in a 50:50 w/w mixture of ethylene carbonate and diethyl carbonate. The cells were charged and discharged at a constant current of ca. 0.2 C, and the fixed voltage limits were between 2.75 and 0.01 V. Results and Discussions In the initial reaction stage (1 h) of synthesizing hollow spheres, SnO2 colloids formed primarily. The typical XRD
Zhao et al. pattern of the SnO2 colloids is shown in Figure 1a. The weak peaks exhibit in the pattern, indicating its amorphous characteristics. The XRD pattern of the initial reaction product for synthesizing SnO2 rod bundles is similar to that of SnO2 hollow spheres. The XRD patterns after stirring for 10 h are shown in Figure 1, parts b and c. All the sharp and strong diffraction peaks could be readily indexed to the tetragonal phase of SnO2 (JCPDS card no. 41-1445), with lattice constants of a ) 4.738 Å and c ) 3.187 Å, which were found to match well with the standard XRD pattern. No characteristic peaks are observed for the impurities such as SnO. To further characterize the product, XPS was carried out to investigate the surface compositions and chemical states of the as-prepared products obtained in different systems. Figure 2 is a typical spectrum of SnO2 hollow spheres, which resemble that of SnO2 rod bundles. The binding energies obtained in the XPS analysis were corrected for specimen charging by referencing the C1s to 284.6 eV. The sample appeared as a spin-orbit doublet at ∼486.8 eV (3d5/2) and ∼495.2 eV (3d3/2) (Figure 2a), which was in agreement with the reported value in the literature.16 The O1s binding energy of 531.5 eV (Figure 2b) indicated that the oxygen atoms existed as O2- species in the compounds.13 Consequently, on the basis of the results of XRD and XPS measurements, the as-synthesized products could be determined as SnO2. In this approach, it has been found that the morphology and size of the SnO2 materials are sensitive to the microreactor systems generated by the surfactants. The as-prepared products are examined by FESEM and TEM, and their typical images are displayed in Figures 3 and 4. SnO2 hollow spheres can be formed in the microreactor systems with the presence of surfactant SDBS. From the SEM image shown in Figure 3a, uniform nanospheres with high yields of 95% have been observed clearly. The diameter of the spheres with rough exteriors is about 200 to ∼250 nm. Some broken spheres with apparent cavities can be obviously observed, further demonstrating the hollow nature of the products. The panoramic morphology (Figure 3b) shows the bright contrast (dark/bright) between the boundary and the center of the spheres, confirming their hollow nature. The thickness of the shell is ca. 5 to ∼10 nm, which is comparable to the SEM results. The structural details are investigated in a high-magnification TEM image (Figure 3c), revealing that the shells of hollow SnO2 nanospheres were thin. The corresponding SAED pattern (Figure 3d) performed on the shell indicates the polycrystallinity of hollow SnO2 spheres. Interestingly, the morphology of the product changed from hollow spheres to rodlike bundles as the used surfactant was changed from SDBS to PVP-SDBS complexes. It could be observed from Figure 4a that the yield of the rods is over 90%. A high-magnification SEM image (Figure 4b) displays that the smooth rods are connected by a common core with width of about 200 nm and length up to several micrometers. Figure 4c shows the representative TEM images of the product in the PVP-SDBS system, which indicates a bundle of rods splitting from the same root. The upper right inset in Figure 4d shows a selected area electron diffraction (SAED) pattern from an individual rod with a [11h0] zone axis, which indicates that the as-prepared SnO2 rods are singlecrystalline. Their preferential growth direction for the rods is the [001] direction. And this orientation is maintained for all rods, which is learned from SAED investigation of more individual rods.
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Figure 7. TEM image of as-obtained SnO2 materials when the surfactant used was (a) CTAB, (b) PVP, and (c) SDS and PVP.
It is known that Sn(II) could be easily oxidized to Sn(IV) in the atmosphere.15 The involved chemical reaction that we employed for the synthesis of SnO2 materials can be formulated as
2SnSO4 + O2 + 2H2O f 2SnO2 + 2H2SO4 Only black precipitate can be obtained if the hydrolysis of SnSO4 was carried out under N2. XRD analysis confirmed that the black precipitate was SnO, when SnSO4 (2 mmol) and SDBS (2 mmol) were dissolved in 20 mL of distilled water and stirred for 8-10 h under N2, as shown in Figure S1 (see the Supporting Information). On the basis of the above observation, the formation of SnO2 hollow spheres can be explained by a surfactant-assisted oxidation-crystallization mechanism. The surfactant-assisted synthesis has been proved to be effective and appealing because surfactants can act as the soft templates as well as structuredirecting agents for the assembly and subsequent mineralization of inorganic precursors at the surfactant-solution interface. It is widely known that SDBS has the tendency to form micelles, which could act as the soft templates. TEM images shown in Figure 5a for the diluted solutions of the SDBS system indicate the well-defined micelles. As SDBS is an anionic surfactant, Sn2+ can adsorb on the surface of the micelles, which provide domains for the subsequent oxidation and crystallization in the solutions. Further evidence for the adsorption of Sn2+ on the SDBS micelle is obtained by infrared (IR) spectroscopy measurements. The IR spectroscopy of pure SDBS and the intermediates collected at the reaction stage (10 min) of synthesizing hollow spheres are shown in Figure 6. The characteristic spectroscopy of SO4- group in SDBS in Figure 6a appears as a doublet peak at 580-620 cm-1. However, in Figure 6b there is a triplet peak at 580-620 cm-1, implying that Sn2+ may interact with the sulfate headgroup of the surfactant SDBS.17 The room-temperature synthesis avoids the rapid reaction and guarantees that SnO2 nanoparticles grow on the surface before their assembly. With the reaction proceeding, the formed SnO2 nanoparticles aggregate on the surfaces of the micelles gradually. Then, the soft template can be successfully eliminated by the washing process without destroying the spherical morphologies. Finally, SnO2 hollow spheres form. When the amount of SDBS was changed to 1 mmol and other experimental conditions were kept unchanged, the product was mainly solid particles with few hollow particles as shown in Figure S2a (see the Supporting Information). When the amount of SDBS was 4 mmol and other experimental conditions were kept unchanged, the product was hollow structures with irregular morphology as shown in Supporting Information Figure S2b. Therefore, the concentrations of the surfactant in the system play a significant role, which should be carefully controlled.
However, by adding PVP to the reaction system, the morphology of the SnO2 sample is changed to rod bundles, which can be explained by a polymer (PVP)-surfactant (SDBS)-assisted oxidation-crystallization mechanism. PVP, as a polymer, has been proved to play an important role in the synthesis of one-dimensional (1D) structure, as a protective agent against the particles agglomeration and face-inhibited function surfactant favoring the 1D growth. For example, Se tubes and rods were reported recently in the presence of PVP.18 It was known that the polymer PVP could interact strongly with anionic surfactants in an aqueous system. Meanwhile, previous investigation of the poly(ethylene oxide) (PEO)/SDS system showed that the anionic surfactant interacted strongly with the polymer molecules, forming micellar aggregates attached to the polymer chains.17 Thus, in the present work, we employ another polymer-surfactant system, i.e., the PVP-SDBS system, to fabricate 1D nanostructure. Similar to the PEO-SDS complex system, the mixture of PVP and SDBS could also form complex micelles, as PVP has both hydrophilic and hydrophobic groups.19 While in a water solution of PVP, the existing two kinds of dynamic structures are as follows:
Due to the unoccupied electron pairs of the nitrogen atoms, the electron cloud moves to the oxygen atom, which results in the oxygen atom showing negative charge. After the formation of the hydroxyl group, the whole molecule shows a positive charge. Thus, there exists an interaction between PVP and the SDBS micelle due to the electric attraction between the anionic SDBS micelle and the positively charged PVP, resulting in the adherence of SDBS micelles to PVP.20 Therefore, the surfactant SDBS associating with the polymer PVP chains form micellar aggregates as shown in Figure 5b, which can act as a template for the precipitation and crystallization of SnO2. Meanwhile, SDBS can also act as stabilizers for the crystals, preventing their precipitation and aggregation. And the SDBS-stabilized particles adsorbed on the PVP chains combine through a polymer-mediated bridging flocculation mechanism to produce rodlike aggregates.21 The complex of PVP and SDBS brings particles together, similar to those described in the computational work of Stoll and Buffle.21a With the reaction proceeding, the formed SnO2 rods are well crystallized. And the formation
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Figure 8. Electrochemical performance of (a) SnO2 hollow spheres electrode, (b) SnO2 rod bundles electrode, (c) SnO2 hollow spheres after annealing electrode, and (d) SnO2 rod bundles after annealing electrode. (e) Cycling performance of SnO2 hollow spheres after annealing in the current work.
mechanism of SnO2 hollow spheres and rods can be illustrated in Scheme 1. To investigate the effect of reaction conditions such as surfactants on the formation of SnO2 materials, a series of comparative experiments were carried out through similar processes. When the surfactant used was SDS (2 mmol) and other experimental conditions were kept unchanged, irregular SnO2 particles were obtained. When the surfactant was varied to CTAB (2 mmol), SnO2 nanoparticles were observed (Figure
7a). When only PVP (0.12 g) was present in the solution, SnO2 nanochains were observed (Figure 7b), which could be attributed to the effect of PVP. Possibly PVP acted as some face-inhibited function surfactant to help the growth of the chain.22 If the surfactant was modified to the complexes of SDS (2 mmol) and PVP (0.12 g), SnO2 short chains (Figure 7c) were obtained. All results indicate that the surfactant used is the prerequisite for the formation of SnO2 materials.
Oxidation-Crystallization Process of Colloids It is well-known that SnO2 is a potential anode material for Li-ion batteries with a theoretical specific capacity of 790 mA‚ h/g, which is much higher than that of the currently used graphite (372 mA‚h/g).23 Thus, the obtained SnO2 samples were fabricated as Li-ion battery anodes, and their electrochemical properties were experimentally measured with a cutoff voltage window of 0.01 to 2.75 V. The measured capacities of SnO2 hollow sphere and rod electrodes (Figure 8, parts a and b) were 698 and 470 mA‚h/g, respectively. And the capacity of SnO2 hollow spheres electrode was higher than that of a solid SnO2 nanoparticles electrode (650 mA‚h/g) prepared by microwaveassisted synthesis.24 It is known that the crystallinity of the materials play a critical role in determining the performance of the materials as Li-ion battery anodes.25 Inspired by this idea, we calcined the samples at 400 °C in air atmosphere for 2 h to enhance the crystallinity. And the morphologies of the samples as shown in Figure S3 (see the Supporting Information) were kept unchanged. Figure 8, parts c and d, shows the voltagecapacity curves of the SnO2 hollow spheres and rods after annealing as anode materials, indicating the obvious increase of the initial specific capacity of SnO2 samples after annealing. Notably, for SnO2 hollow spheres after annealing, a high capacity of 1440 mA‚h/g was obtained in the first discharge process, which was greater than that of solid SnO2 nanoparticles, while the specific capacity of rods also increased compared to that of SnO2 rods before annealing, which was 1093 mA‚h/g. Both the values obtained after annealing were more than the theoretical capacity of SnO2. It was evident that the electrochemical property was susceptible to the crystallinity. Moreover, compared to that of previously reported SnO2 materials,26 SnO2 hollow spheres after annealing manifested large improvements in electrochemical performance. It could be realized by combining a higher degree of crystallinity with SnO2 hollow spheres. Figure 8e shows the cycling performance of as-prepared SnO2 hollow nanospheres after annealing. After 30 cycles, the capacity was still comparable to the theoretical capacity of SnO2, which manifested its good structural stability during the chargedischarge process. In a word, the prepared SnO2 hollow sphere electrode possessed superior lithium storage capacity when used as an anode material in lithium-ion batteries. Conclusions In summary, we have used the successful morphology control of SnO2 as examples to illustrate the process and mechanism of oxidation-crystallization in the microreactor systems. The growth mechanism is identified through the investigations on the intermediate and a series of comparative experiments. SnO2 hollow spheres can be formed by an SDBS-assisted process, whereas the formation of SnO2 rod bundles can be explained by a polymer (PVP)-surfactant (SDBS)-assisted mechanism. Therefore, the microreactor systems generated by the surfactants and polymers play an important role in the controllable synthesis, which can be used to control the size and shape of inorganic substances. We believe that oxidation-crystallization in the microreactor systems will be applied in the production of other inorganic materials with desired morphologies.
J. Phys. Chem. C, Vol. 111, No. 31, 2007 11603 Acknowledgment. This work was supported by the National Natural Science Foundation of China (20621061) and the State Key Project of Fundamental Research of Nanomaterials and Nanostructures (2005CB623601). Supporting Information Available: XRD pattern of the product and TEM images of SnO2 products and samples. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Kolkamov, A.; Zhang, Y.; Cheng, G.; Moskovites, M. AdV. Mater. 2003, 15, 997. (b) Ramgir, N. S.; Mulla, I. S.; Vijayamohanan, K. P. J. Phys. Chem. B 2004, 108, 14815. (c) Law, M.; Kind, H.; Messer, B.; Kim, F.; Yang, P. Angew. Chem., Int. Ed. 2002, 41, 2405. (d) Maiti, A.; Rodriguez, J. A.; Law, M.; Kung, P.; Mckinney, J. R.; Yang, P. Nano Lett. 2003, 3, 1025. (2) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (3) Lu, J.; Xie, Y.; Xu, F.; Zhu, L. Y. J. Mater. Chem. 2002, 12, 2755. (4) Liu, Y.; Liu, M. AdV. Funct. Mater. 2005, 15, 57. (5) Zhang, X.; Xie, Y.; Xu, F.; Xu, D.; Liu, X. H. Inorg. Chem. Commun. 2004, 7, 417. (6) Macak, J.; Tsuchiya, H.; Schmuki, P. Angew. Chem., Int. Ed. 2005, 44, 2100. (7) Zhang, D. F.; Sun, L. D.; Yin, J. L.; Yan, C. H. AdV. Mater. 2003, 15, 1022. (8) Lu, K. AdV. Mater. 1999, 11, 1127. (9) Cao, X. B.; Xie, Y.; Li, L. Y. J. Solid State Chem. 2004, 177, 202. (10) Cao, X. B.; Xie, Y.; Li, L. Y. J. Colloid Interface Sci. 2004, 273, 175. (11) (a). Chen, Y. J.; Nie, L.; Xue, X. Y.; Wang, Y. G.; Wang, T. H. Appl. Phys. Lett. 2006, 88, 83105. (b) Huang, H.; Tan, O. K.; Lee, Y. C.; Tran, T. D.; Tse, M. S.; Yao, X. Appl. Phys. Lett. 2005, 87, 163123. (12) (a) Bergeron, B. V.; Marton, A.; Oskam, G.; Meyer, G. J. J. Phys. Chem. B 2005, 109, 937. (b) Park, N. G.; Kang, M. G.; Kim, K. M.; Ryu, K. S.; Chang, S. H.; Kim, D. K.; Van de Lagemaat, J.; Benkstein, K. D.; Frank, A. J. Langmuir 2004, 20, 4246. (13) (a) Cao, Q. H.; Gao, Y. Q.; Chen, X. Y.; Mu, L.; Yu, W. C.; Qian, Y. T. Chem. Lett. 2006, 35, 178. (b) Wang, Y.; Su, F. B.; Lee, J. Y.; Zhao, X. S. Chem. Mater. 2006, 18, 1347. (c) Zhao, Q. R.; Gao, Y.; Bai, X.; Wu, C. Z.; Xie, Y. Eur. J. Inorg. Chem. 2006, 1643. (14) (a) Martinez, C. J.; Hockey, B.; Montgomery, C. B.; Semancik, S. Langmuir 2005, 21, 7937. (b) Cheng, B.; Russell, J. M.; Shi, W. S.; Zhang, L.; Samulski, E. T. J. Am. Chem. Soc. 2004, 126, 5972. (15) Uchiyama, H.; Ohgi, H.; Imai, H. Cryst. Growth Des. 2006, 6, 109. (16) Zhao, Q. R.; Zhang, Z. G.; Dong, T.; Xie, Y. J. Phys. Chem. B 2006, 110, 15152. (17) Leontidis, E.; Kyprianidou-Leodidou, T.; Caseri, W.; Robyr, P.; Krumeich, F.; Kyriacou, K. C. J. Phys. Chem. B 2001, 105, 4133. (18) Zhang, B.; Dai, W.; Ye, X. C.; Zuo, F.; Xie, Y. Angew. Chem., Int. Ed. 2006, 45, 2571. (19) Zhai, L. M.; Lu, X. H.; Chen, W. J.; Hu, C. B.; Zheng, L. Colloids Surf., A 2004, 236, 1. (20) Bakshi, M. S.; Kaur, R.; Kaur, I.; Mahajan, R. H.; Sehgal, P.; Doe, H. Colloid Polym. Sci. 2003, 281, 716. (21) (a) Stoll, S.; Buffle, J. J. Colloid Interface Sci. 1996, 180, 548. (b) Leontidis, E.; Kyprianidou-Leodidou, T.; Caseri, W.; Kyriacou, K. C. Langmuir 1999, 15, 3381. (c) Cao, X. B.; Yu, F.; Li, L. Y.; Yao, Z. Y.; Xie, Y. J. Cryst. Growth 2003, 254, 164. (22) Wiley, B.; Sun, Y. G.; Mayers, B.; Xia, Y. N. Chem. Eur. J. 2005, 11, 454. (23) Wang, Y.; Lee, J. Y. J. Phys. Chem. B 2004, 108, 17832. (24) Wang, Y.; Lee, J. Y. J. Power Sources 2005, 144, 220. (25) Besenhard, J. O. In Progress in Intercalation Research; Mu¨llerWarmuth, W., Scho¨llhorn, R., Eds.; Kluwer: Dordrecht, The Netherlands, 1994; p 457. (26) Lou, X. W.; Wang, Y.; Yuan, C. L.; Lee, J. Y.; Archer, L. A. AdV. Mater. 2006, 18, 2325.
