Enhanced Li Storage Capacity in 3 nm Diameter SnO2 Nanocrystals

Enhanced Li Storage Capacity in 3 nm Diameter SnO2 Nanocrystals Firmly Anchored on Multiwalled Carbon Nanotubes. Yun-Ho Jin, Kyung-Mi Min, ...
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Enhanced Li Storage Capacity in 3 nm Diameter SnO2 Nanocrystals Firmly Anchored on Multiwalled Carbon Nanotubes Yun-Ho Jin, Kyung-Mi Min, Seung-Deok Seo, Hyun-Woo Shim, and Dong-Wan Kim* Department of Materials Science and Engineering, Ajou University, Suwon 443-749, Korea

bS Supporting Information ABSTRACT: A simple synthesis route is demonstrated for the preparation of hybrid nanocomposite electrodes with a combination of SnO2 nanoparticles and conducting multiwalled carbon nanotubes (MWCNTs) for Li ion battery applications. The MWCNTs were initially treated using strong acid solutions to generate functional groups with negative charges on their surfaces. For the formation of the nanocomposites, the next process was driven by the mutual electrostatic interactions between the functionalized MWCNTs and the Sn4+ ion species and was followed by spontaneous oxidation during a hydrothermal reaction at 150 °C. The SnO2 nanoparticles with diameters of less than 3 nm were uniformly loaded onto the MWCNT surfaces, providing an extremely large Brunauer Emmett Teller (BET) surface area of ≈240 m2 g 1. Furthermore, we demonstrate that the incorporation of multiwalled carbon nanotubes gives rise to an enhanced reversible capacity estimated to be 420 mA h g 1 after 100 cycles, which is at least three times greater than that of pure SnO2 nanoparticles.

’ INTRODUCTION Recently, Li ion batteries (LIBs) have become widely used in portable electronic products, electric/hybrid vehicles, and stationary applications because of their desirable features such as high energy, high power, and long-term cyclability. The electrode materials employed play important roles in LIB performance. For the anode, the current carbon-based materials have the inherent capacity limitation of 372 mA h g 1 based on the Li-insertion mechanism. Some noncarbonaceous materials, Li-alloying elemental materials (silicon, germanium, tin, etc.), have been shown to provide the possibility of increasing the capacity by reversibly transferring 4.4 Li ions per formula unit.1 3 However, a huge volume expansion/contraction (≈300%) associated with the alloying/dealloying process occurs, which can generate severe internal strain, in turn leading to cracking and electrical isolation that limits the cycle life of the electrode.4 6 In this respect, tin oxide (SnO2), with a theoretical capacity of 782 mA h g 1, has been proposed as a promising alternative for improving the cycling performance of Li-alloying elemental anodes.7,8 During the first discharge process, the SnO2 undergoes an electrochemical reaction whereby the Sn metal cation is reduced and embedded into an inactive Li2O matrix (SnO2 + 4Li+ + 4e f Sn + 2Li2O) and subsequently alloys with Li (Sn + xLi+ + xe T LixSn). For the continued exploitation of SnO2 electrodes in practical applications, more attention has been paid to designing nanostructured SnO2 materials with different sizes and morphologies to improve the energy-storage performance.9 13 Kim et al. reported a systematic correlation between the particle size of SnO2 nanoparticles and the cycling stability and suggested a critical size (≈3 nm) to improve the cyclability.14 However, nanoparticles tend to attract one another, and the resulting increased r 2011 American Chemical Society

interparticle contact resistance limits the electronic conduction paths to the current collector.15 Mechanical mixing of the active nanostructured materials with carbon black, which is commonly used as a conducting additive, is not a proper solution for reducing the interparticle chargetransfer resistance. To bypass the drawbacks suffered through the facile aggregation of nanoparticles, researchers have previously devoted a large amount of effort to the development of carbon nanotube (CNT)/SnO2 nanocomposites using various methods including chemical vapor deposition (CVD), atomiclayer deposition (ALD), and different chemical-solution processes.16 22 CVD and ALD processes benefitted the deposited SnO2 with tunable morphologies from nanoparticles to nanofilms on CNTs. However, they are not scalable and require the use of hazardous gas precursors (SnCl4, SnH4). The most common chemical process is to use structure-directing agents such as cetyltrimethylammonium bromide or long-chain polymers such as poly(vinylpyrrolidone) and other polyelectrolytes for the uniform overlaying of CNTs by SnO2. More recently, SnO2 carbon core shell nanochains have been demonstrated using a simple hydrothermal route and subsequent carbonization at high temperature, providing a high reversible capacity and excellent cycling performance.23,24 In this work, we report a simple hydrothermal reaction for the uniform assembly of optimized SnO2 nanoparticles (≈3 nm in diameter) without appreciable particle aggregation to the multiwalled CNTs (MWCNTs), the surfaces of which are functionalized Received: August 20, 2011 Revised: October 3, 2011 Published: October 04, 2011 22062

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The Journal of Physical Chemistry C using an acid treatment. We show that the selective growth of SnO2 nanoparticles on MWCNTs allows a large surface area to be obtained and enables wiring up to a current collector through electrically conducting MWCNT networks. Furthermore, we demonstrate the superior electrochemical properties of MWCNT/SnO2 nanocomposite electrodes compared to their pure SnO2 counterparts.

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Scheme 1. Schematic Illustration of the Functionalization of MWCNTs and Hydrothermal Synthesis of MWCNT/SnO2 Nanocomposites

’ EXPERIMENTAL SECTION Preparation of Pure SnO2 and MWCNT/SnO2 Nanocomposite Powders. Similarly to the hydrothermal process intro-

duced by Zhu et al., SnCl4 3 5H2O (6.73 g, 98.0%, Sigma Aldrich, USA) and N2H4 3 H2O (3.84 g, 98.0%, Sigma Aldrich) were mixed with DI water (500 mL), and the resulting solution was then transferred into an autoclave for the hydrothermal preparation of pure SnO2.25 The autoclave was maintained at 150 °C for 24 h. After the hydrothermal treatment, the suspension was centrifuged, and the precipitate was washed and dried. The typical synthetic process for the preparation of MWCNT/SnO2 nanocomposite powders can be described as follows. First, MWCNTs (2 g, Hanwha Nanotech Co., Ltd., Korea) were refluxed with a solution of H2SO4/HNO3 (3:1 ratio) for 30 min, then filtered, and washed with distilled water until the pH of the solution obtained was neutral. The MWCNTs were then dried overnight at 80 °C under vacuum. Subsequently, MWCNTs (0.3 g) were dispersed and sonicated for 3 h in DI water (500 mL) using a high-power ultrasonicator. After the complete dispersion of the MWCNTs, the same steps were carried out as for the SnO2 nanopowders. Structural and Electrochemical Characterization. The crystal structures and morphologies of each powder were investigated using X-ray powder diffraction (XRD; model D/MAX-2500 V/ PC, Rigaku, Tokyo, Japan), a Fourier transform infrared (FTIR) spectrometer (Spectrum 2000, Perkin-Elmer), and high-resolution transmission electron microscopy (HRTEM; model JEM3000F, JEOL, Tokyo, Japan). In addition, the specific surface areas were examined using the Brunauer Emmett Teller (BET; Belsorp-mini, BEL Japan Inc., Osaka, Japan) method with a nitrogen adsorption/desorption process. The contents of adsorbed water and MWCNTs in the nanocomposites were determined using thermogravimetric analysis (TGA; model DTG-60, Shimadzu, Japan). The electrochemical performance of each powder was evaluated by assembling Swagelok-type half cells, using Li metal foil as the negative electrode. The positive electrodes were cast on Cu foil by mixing the prepared powders (2.0 3.0 mg) with Super P carbon black (MMM Carbon, Brussels, Belgium) and Kynar 2801 binder (PVdF-HFP) in a mass ratio of 70:15:15 in 1-methyl-2pyrrolidinone (NMP; Sigma-Aldrich, St. Louis, MO, USA). A separator film of Celgard 2400 and liquid electrolyte (ethylene carbonate and dimethyl carbonate (1:1 by volume) with 1.0 M LiPF6, Techno Semichem Co., Ltd., Seongnam, South Korea) were also used. The assembled cells were cycled galvanostatically between 3.0 and 0.01 V using an automatic battery cycler (WBCS 3000, WonaTech, Seoul, South Korea). The capacity of only SnO2 in the MWCNT/SnO2 was calculated after eliminating the contribution from MWCNTs (and carbon black) based on exact mass ratio by TGA. All cyclic voltammetry measurements were carried out at a scan rate of 0.1 mV s 1.

’ RESULTS AND DISCUSSION According to previous reports, acid treatment can generate functional groups on the surfaces and ends of MWCNTs.26 The

Figure 1. XRD patterns of the functionalized MWCNTs, pure SnO2 nanoparticles, and MWCNT/SnO2 nanocomposites.

surface functionalization and MWCNT/SnO2 nanocomposite preparation processes are shown in Scheme 1. First, the MWCNTs were functionalized by heating at reflux in sulfuric/ nitric acid, which led to a large concentration of carboxylate ions ( COO ) on the MWCNT surfaces.27 In the second step, the MWCNTs with negative charge were surrounded by Sn4+ ions through electrostatic attraction. Afterward, the Sn4+ ions were mineralized and subsequently oxidized during the hydrothermal process in the presence of N2H4 3 H2O.25 The phase and purity of the as-prepared pure SnO2 and MWCNT/SnO2 nanocomposite powders were determined from their X-ray diffraction (XRD) patterns, which are shown in Figure 1. For comparison, the XRD pattern of functionalized MWCNTs is also included in Figure 1. All the diffraction peaks of each sample can be perfectly indexed to the tetragonal SnO2 structure (JCPDS no. 41-1445). The rather broad peaks indicate the small crystallite size of the SnO2. The presence of MWCNTs in the MWCNT/SnO2 nanocomposites could not be successfully confirmed using the characteristic peak of carbon at 26° because of overlapping with the broad (110) peak of SnO2. Therefore, 22063

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Figure 2. TGA of pure SnO2 nanoparticles and MWCNT/SnO2 nanocomposites. Figure 4. (a, b) Typical TEM and HRTEM images of MWCNT/SnO2 nanocomposites, respectively. Inset in (a) shows the corresponding SAED pattern. (c, d) Typical low- and high-magnification TEM images of nf-MWCNT/SnO2.

Figure 3. (a, b) Typical TEM images of pure SnO2 nanoparticles showing considerable aggregations of particles. (c, d) HRTEM image and SAED patterns of pure SnO2 nanoparticles, respectively.

thermogravimetric analysis (TGA) was performed for the quantitative determination of the amounts of MWCNTs present in the MWCNT/SnO2 nanocomposite powders; the results are shown in Figure 2. The final weight loss of the pure SnO2 powders was ≈8.3%, possibly due to the adsorbed water and organic molecules.28,29 Notably, it can be observed that the additional weight loss took place mainly between 350 and 550 °C for the MWCNT/ SnO2 nanocomposite powders. Thus, the incorporated amount of MWCNTs in the MWCNT/SnO2 nanocomposite powders could easily be determined to be ≈10% by weight. The particle size and crystal structure of the pure SnO2 were characterized by TEM and HRTEM (Figure 3). The pure SnO2 powders were composed of homogeneous ultrafine nanocrystallites

with an average diameter ≈3 nm, but the primary particles overlapped considerably and formed large aggregates, as shown in the typical TEM images of Figures 3a and b. Figure 3c shows the HRTEM image, which reveals that each nanocrystal is indexed as a phase-pure tetragonal SnO2 with high crystallinity and single-crystalline nature. The representative lattice fringes with d-spacings of 0.354 and 0.264 nm correspond to the (110) and (101) planes of SnO2, respectively (JCPDS no. 41-1445). In addition, the corresponding selective-area electron diffraction (SAED) pattern taken from the pure SnO2 powders was successfully indexed to SnO2 (Figure 3d). Figures 4a and b show TEM and HRTEM images of the MWCNT/SnO2 nanocomposite powders, and they clearly show that all the MWCNTs were uniformly covered with SnO2 nanoparticles. The SAED pattern (lower right inset of Figure 4a) also indicates typical rings for the crystal planes of SnO2, as seen in Figure 3d. Indeed, single particles of SnO2 of ≈3 nm in diameter were distributed without any appreciable agglomeration, and lattice fringes of the nanoparticle are clearly observed in the HRTEM image (Figure 4b), with the adjacent fringe spacing of 0.335 0.338 nm corresponding to the (110) plane of SnO2. To exploit the effect of the surface functionalization of the MWCNTs, we performed a control experiment in which MWCNT/SnO2 nanocomposite powders were prepared under the same synthetic conditions but using as-received MWCNTs without acid treatment (hereafter designated as nf (nonfunctional)-MWCNT/ SnO2). In these nf-MWCNT/SnO2 powders, it was observed that only a small proportion of the SnO2 particles were attached to the MWCNTs, while most of them aggregated free of the MWCNTs, as shown in Figures 4c and d. This observation confirmed the important role played by the acid treatment in the anchoring of the SnO2 particles. The FTIR spectra confirm that MWCNTs were successfully modified by acid, based on the features at wave numbers 1610 1550 cm 1 ( COO asymmetric stretching) and 1720 cm 1 ( COOH) (Figure S1, Supporting Information).26,27 Therefore, the preferential nucleation on the surface 22064

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Figure 5. Cyclic voltammetry of pure SnO2 nanoparticles and MWCNT/SnO2 nanocomposites in the first ten cycles.

of MWCNTs and the resulting reliable formation of MWCNT/SnO2 nanocomposites are therefore made possible through the use of MWCNTs negatively charged by functionalization with COO groups, which are then surrounded by Sn4+ ions, as mentioned above in Scheme 1. Ultrafine homogeneous nanoparticles can provide extremely large surface areas. The Brunauer Emmett Teller (BET) surface areas of pure SnO2 particles and functionalized MWCNTs were estimated to be 245 and 210 m2 g 1, respectively. More importantly, the MWCNT/SnO2 nanocomposite particles also had a large BET surface area of 240 m2 g 1. Nanocomposite electrode materials with large surface areas have led to multiple advances in the performance of Li ion batteries by providing shorter path lengths for both electron and Li ion transport, a higher electrode/electrolyte contact area, better accommodation of the strain of the Li ion insertion/extraction, and much easier conductive pathways via the MWCNTs.15,30 32 Cyclic voltammetry (CV) curves were recorded for pure SnO2 and MWCNT/SnO2 composite electrodes, as shown in Figure 5. For both samples, two major pairs of redox peaks (0.85/1.25 V and 0.2/0.6 V) were observed in the CV profiles, which is in accordance with the pairs of peaks reported for SnO2 nanostructures.33,34 During the initial ten scans of the pure SnO2 electrode, the characteristic redox peaks were significantly weakened, as marked by the arrows in Figure 5. However, there was a relatively small decrease in the redox peaks of the MWCNT/SnO2 electrode until the tenth cycle, revealing its enhanced cyclability. Therefore, it is believed that the incorporation of MWCNTs improved the Li electroactivity of the SnO2 nanoparticles because of their beneficial effect on the conductivity, efficient electron paths, and aggregation control of active nanoparticles. The galvanostatic, voltage-specific capacity profiles of the pure SnO2 and MWCNT/SnO2 composite electrodes in the voltage range 0.0 3.0 V at a current density of 0.2 C (=156 mA g 1) over 100 cycles are presented in Figure 6a. The first specific discharge capacity and Coulombic efficiency of pure SnO2 nanoparticles reached ≈1655 mA h g 1 and 47%, respectively. The high first discharge capacity and resultant low Coulombic efficiency could be attributed to the irreversible decomposition of SnO2 into metallic, active Sn nanodomains surrounded by an amorphous,

Figure 6. (a) Galvanostatic discharge/charge voltage profiles of pure SnO2 nanoparticles and MWCNT/SnO2 nanocomposites at a rate of 0.2 C. (b) Comparison of specific capacities versus cycle number for pure SnO2 nanoparticles and MWCNT/SnO2 nanocomposites at a rate of 0.2 C. Coulombic efficiency of MWCNT/SnO2 nanocomposites is also indicated.

Figure 7. Typical TEM image of SnO2 nanoparticles in MWCNT/ SnO2 nanocomposites in the fully discharged state after 37 cycles, showing the formation of the Li alloy Li4.4Sn.

inactive Li2O matrix and the irreversible formation of a solid electrolyte interface (SEI).6,8 Subsequently, the reversible capacities faded drastically from ≈830 mA h g 1 (2nd cycle) to 126 mA h g 1 (100th cycle). For the MWCNT/SnO2 nanocomposite electrodes, a slightly lower first discharge capacity of 1546 mA h g 1 and better first Coulombic efficiency of 56% were observed compared to those of the pure SnO2 electrodes (Figure 6a). More importantly, the MWCNT/SnO2 nanocomposites exhibited a much improved capacity retention, delivering a high reversible capacity of 420 mA h g 1 even after 100 cycles, while maintaining an excellent Coulombic efficiency of over 98% (Figure 6b). This capacity value was better or comparable to any of the various MWCNT/SnO2 nanocomposites,18,19,22 even though 22065

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The Journal of Physical Chemistry C SnO2 carbon core shell nanochains, which were more recently reported, exhibited a superior capacity as high as 650 mA h g 1 after 100 cycles at a current density of 300 mA g 1.23,24 This capacity value was still higher than the theoretical specific capacity of a conventional graphite electrode. This remarkable difference in the cycling stability is a clear demonstration of the beneficial effects of the incorporation of MWCNTs on the cycling characteristics of Sn-based systems. This electrochemical performance was better than or comparable to any of the various types of SnO2 and MWCNT/SnO2 nanostructured electrodes reported previously.33,35 37 SnO2 has poor intrinsic electronic conductivity and high interparticle contact resistance, as the facile aggregation of tiny nanoparticles results in the poor realization of Li electroactivity for all nanoparticles. On the other hand, the incorporation of MWCNTs and the better dispersion of SnO2 nanoparticles in MWCNT/SnO2 nanocomposites give rise to a sufficient degree of electronic connection between the current collector and host material (SnO2) as well as the efficient reaction of each SnO2 nanoparticle with Li.15 We found that the SnO2 in the MWCNT/ SnO2 nanocomposites was used in the Li-alloying reaction, as shown in the typical HRTEM image taken of the sample in the discharged state (Figure 7). The HRTEM image which was taken at the fully discharged state shows the formation of the Lialloying phase with an interplanar spacing of about 0.279 nm, corresponding to the (640) lattice spacing of the Li4.4Sn. In addition, compared with the XRD pattern of the MWCNT/ SnO2 deposited onto Cu foil current collector before cycling, only LixSn peaks were observed in the MWCNT/SnO2 nanocomposite without any trace of SnO2 at the tenth discharged state, which indicates the Li-alloying reaction of almost all the SnO2 in this nanocomposite (Figure S2, Supporting Information). Furthermore, the homogeneous dispersion of SnO2 nanoparticles on the surface of MWCNTs could effectively accommodate the large volume change of the SnO2 particles during the charging and discharging processes, thereby leading to a significant improvement in the electrochemical performance.

’ CONCLUSIONS In summary, we have presented the successful formation of a nanocomposite electrode, fabricated by using a simple hydrothermal process to attach synthesized SnO2 nanoparticles to MWCNTs with surfaces functionalized by acid treatment. The carboxylate anions on the MWCNT surfaces served as the linkers for the electrostatic interaction between the MWCNTs and the SnO2 nanoparticles with diameters of ≈3 nm. Pure SnO2 nanoparticles showed a poor electrochemical performance because of their high aggregation and the resulting interparticle resistance. However, SnO2 nanoparticles were uniformly anchored on the surfaces of the MWCNTs without any appreciable aggregation in the MWCNT/SnO2 nanocomposites, allowing the favorable utilization of the high surface area by the nanoparticles and in the Li electrochemical reactions. Therefore, the MWCNT/SnO2 nanocomposites could deliver reversible high capacities of ≈420 mA h g 1 even after 100 cycles, which are much higher than those of graphite-based anodes. ’ ASSOCIATED CONTENT

bS

Supporting Information. FTIR spectra of pristine and acid-functionalized MWCNTs and ex situ XRD patterns of the

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MWCNT/SnO2 deposited onto Cu foil before cycling and after the tenth cycling. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: +82 31 219 2468. Fax: +82 31 219 1956.

’ ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2010-0029027, 2010-0029617, and 2011-0019119). ’ REFERENCES (1) Chan, C. K.; Peng, H.; Liu, G.; Mcilwarth, K.; Zhang, X. F.; Huggins., R. A.; Cui., Y. Nat. Nanotechnol. 2008, 3, 31. (2) Ko, Y. D.; Kang, J. G.; Park, J. G.; Park, K. S.; Jin, Y. H.; Kim, D. W. Nanoscale 2011, 3, 3371. (3) Lee, K. T.; Jung, Y. S.; Oh, S. M. J. Am. Chem. Soc. 2003, 125, 5652. (4) Boukamp, B. A.; Lesh, G. C.; Huggins, R. A. J. Electrochem. Soc. 1981, 128, 725. (5) Idota, Y.; Kubota, T.; Matsufuji, A.; Maekawa, Y.; Miyasaka, T. Science 1997, 276, 1395. (6) Courtney, I. A.; Dahn, J. R. J. Electrochem. Soc. 1997, 144, 2045. (7) Ko, Y. D.; Kang, J. G.; Park, J. G.; Lee, S. J.; Kim, D. W. Nanotechnology 2009, 20, 455701. (8) Sivashanmugam, A.; Kumar, T. P.; Renganathan, N. G.; Gopukumar, S.; Wohlfahrt-Mehrens, M.; Garche, J. J. Power Sources 2005, 144, 197. (9) Wang, Z.; Luan, D.; Boey, F. Y. C.; Lou, X. W. J. Am. Chem. Soc. 2011, 133, 4738. (10) Li, J.; Zhao, Y.; Wang, N.; Guan, L. Chem. Commun. 2011, 47, 5238. (11) Wang, J.; Du, N.; Zahng, H.; Yu, J.; Yang, D. J. Phys. Chem. C 2011, 115, 11302. (12) Wang, H.; Liang, Q.; Wang, W.; An, Y.; Li, J.; Guo, L. Cryst. Growth Des. 2011, 11, 2942. (13) Lei, D.; Zhang, M.; Hao, Q.; Chen, L.; Li, Q.; Zhang, E.; Wang, T. Mater. Lett. 2011, 65, 1154. (14) Kim., C.; Noh, M.; Choi, M.; Cho, J.; Park, B. Chem. Mater. 2005, 17, 3297. (15) Lee, D. H.; Kim, D. W.; Park, J. G. Cryst. Growth Des. 2008, 8, 4506. (16) Meng, X.; Zhong, Y.; Sun, Y.; Banis, M. N.; Li, R; Sun, X. Carbon 2011, 49, 1133. (17) Kuang, Q.; Li, S. F.; Xie, Z. X.; Lin, S. C.; Zhang, X. H.; Xie, S. Y. Carbon 2006, 44, 1166. (18) Du, N.; Zhang, H.; Chen, B.; Ma, X.; Huang, X.; Tu, J. Mater. Res. Bull. 2009, 44, 211. (19) Wen, Z; Wang, Q; Zhang, Q; Li, J. Adv. Funct. Mater. 2007, 17, 2772. (20) Zhang, H. X.; Feng, C.; Zhai, Y. C.; Jiang, K. L.; Li, Q. Q.; Fan, S. S. Adv. Mater. 2009, 21, 2299. (21) Myung, Y.; Jang, D. M.; Cho, Y. J.; Kim, H. S.; Park, J.; Kim, J. U. J. Phys. Chem. C 2009, 113, 1251. (22) Chen, G.; Wang, Z.; Xia, D. Chem. Mater. 2008, 20, 6951. (23) Zhang, B.; Yu, X.; Ge, C.; Dong, X.; Fang, Y.; Li, Z.; Wang, H. Chem. Commun. 2010, 46, 9188. (24) Yu, X.; Yang, S.; Zhang, B.; Shao, D.; Dong, X.; Fang, Y.; Li, Z.; Wang, H. J. Mater. Chem. 2011, 21, 12295. (25) Zhu, H.; Yang, D.; Yu, G.; Zhang, H.; Yao, K. Nanotechnology 2006, 17, 2386. (26) Yuen, S. M.; Ma, C. C. M.; Lin, Y. Y.; Kuan, H. C. Compos. Sci. Technol. 2007, 67, 2546. 22066

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