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SnO2 Nanoparticles with Controlled Carbon Nanocoating as High-Capacity Anode Materials for Lithium-Ion Batteries Jun Song Chen,† Yan Ling Cheah,‡ Yuan Ting Chen,† N. Jayaprakash,† Srinivasan Madhavi,‡ Yan Hui Yang,† and Xiong Wen Lou*,† School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, 70 Nanyang DriVe, Singapore 637457, Singapore, and School of Materials Science and Engineering, Nanyang Technological UniVersity, 50 Nanyang AVenue, Singapore 639798, Singapore ReceiVed: August 26, 2009; ReVised Manuscript ReceiVed: September 25, 2009
We demonstrate a facile route for the scalable synthesis of SnO2 nanoparticles with controlled carbon nanocoating for use as high-capacity anode materials for next-generation lithium-ion batteries. SnO2 nanoparticles with size in the range of 6 -10 nm are produced via a simple hydrothermal method with high yield, which are then encapsulated by a carbon layer through a modified method. The weight fraction of carbon present in the final product can be readily tuned by varying the concentration of glucose used during the hydrothermal coating process. A systematic study has been carried out to examine the effect of carbon content upon lithium-ion battery performance. It is found that the optimized SnO2@carbon nanoparticles manifest excellent lithium storage properties. As an example, SnO2@carbon with 8 wt % carbon can deliver a capacity as high as 631 mA h g-1 even after 100 charge/discharge cycles at a current drain of 400 mA g-1. Introduction Lithium-ion batteries (LIBs) have been the power sources for various electronic devices because of their unmatched energy density per unit volume or per unit mass.1,2 SnO2-based materials have attracted a lot of attention due to their theoretical reversible capacity of 790 mA h g-1, which is more than twice that of the currently used graphite (370 mA h g-1). In a LIB where SnO2based material serves as the working electrode, the electrochemical process is governed by two principal reactions: SnO2 + 4Li+ + 4e- f Sn + 2Li2O (1); Sn + xLi+ + xe- T LixSn (0 e x e 4.4) (2). The first reaction involving the reduction of SnO2 to Sn is widely believed to be irreversible, although recent findings by us and others suggested partial reversibility.3-5 Together with this reduction process, formation of a solid electrolyte interface (SEI) at low voltage contributes to the huge initial capacity loss during the first few charge-discharge cycles. The second reaction, on the other hand, is reversible. By forming alloyed LixSn during insertion, and dealloyed Sn during extraction, lithium ions shuttle between the two electrodes repeatedly. This process plays a dominant role in determining the lithium capacity; yet, it also brings a major drawback to the SnO2-based anodes. When dealloyed Sn switches to the fully lithiated state of Li4.4Sn, it induces a volume change of more than 200%,6 generating a large internal stress. This will cause the disintegration of the electrode material and subsequently the loss of electrical contact,7,8 eventually leading to quick capacity fading. To date, many efforts have been devoted to resolve the above so-called “pulverization” problem.9-11 As discussed in a recent review,12 exploitation of materials with unique nanostructure, e.g., hollow spheres,13-17 core-shell structures,18 nanorods,16 nanoneedles,19 and nanowires,20,21 can effectively alleviate the problem and improve the cyclability to a large extent. However, * To whom correspondence should be addressed. E-mail: xwlou@ ntu.edu.sg. † School of Chemical and Biomedical Engineering. ‡ School of Materials Science and Engineering.
practical use of such nanostructures might not be feasible if one considers the scalability of the synthesis process. Another strategy of nanocoating the active material with carbon is proved to be very practical with promising LIB performance.3,7,8,22-26 Carbon itself has been intensively researched as the anode material.27,28 Carbon nanocoating is believed to play two different roles upon cycling. It acts as a buffer layer that cushions the stress induced by volume expansion/contraction. Its elastic nature renders it very effective in accommodating the volume strain,25 leading to a better capacity retention. At the same time, it also increases the electronic conductivity of the nanocomposite electrode. Despite these good features, this method is a compromise between cycle life and capacity, since carbon is barely active3 and has very limited lithium storage capacity. It is thus desirable to carry out a systematic study on the effect of carbon coating upon LIB performance.25 Herein, we report a simple two-step process for making SnO2 nanoparticles (NP) with controlled carbon nanocoating and their comparative lithium storage properties. Pure SnO2 NPs with narrow size distribution can be synthesized with a nearly perfect yield through a simple hydrothermal treatment. SnO2 NPs are then deposited with a very thin layer of glucose-derived, carbonrich polysaccharide (GCP). After carbonization at a moderate temperature under an inert atmosphere, carbon-coated SnO2 NPs are obtained. We show that the carbon content in the final product can be readily tuned by changing the glucose concentration. We also carry out a systematic investigation on the influence of carbon coating upon the cyclability and capacity. By carefully adjusting the carbon content and the electrochemical parameters applied for charge-discharge, a good balance could be established between the cycle life and lithium storage capacity. Results and Discussion The crystal structure of the as-prepared SnO2@carbon nanocomposites is determined by X-ray diffraction (XRD), as shown
10.1021/jp908244m CCC: $40.75 2009 American Chemical Society Published on Web 10/16/2009
SnO2 Nanoparticles with Controlled Carbon Nanocoating
Figure 1. X-ray diffraction patterns of the as-prepared samples: pure SnO2 nanoparticle (I); SnO2@C nanocomposite using 0.05 M (II), 0.2 M (III), and 0.5 M (IV) glucose.
in Figure 1. All the identified peaks can be assigned to tetragonal SnO2 (JCPDS card no. 41-1445, SG: P42/mnm, ao ) 4.738 Å, co ) 3.187 Å).3 It can be observed that the intensities of the peaks are relatively low, suggesting the small size of the SnO2 NPs. The average crystallite sizes of these four samples are listed in Table 1, which are calculated using Scherrer’s formula based on the (110) peak. These values indicate that the carbonization process helps the crystallization of SnO2 NPs. Additionally, the XRD analysis confirms that using a carbonization temperature below 600 °C will not trigger the carbothermal reduction of SnO2 to Sn.3,5,7,23 Figure 2 displays the transmission electron microscopy (TEM) images of the as-prepared SnO2@carbon NPs. As shown in Figure 2A, the pure SnO2 powder contains NPs with size ranging from 5 to 10 nm. A high resolution TEM (HRTEM) image of a few nanoparticles with visible lattice fringes is shown in the inset. Figure 2B displays the morphology of the SnO2@carbon NPs (Sample 2, see Table 1) prepared with 0.05 M glucose. Clearly, the nanoparticles are no longer well dispersed; instead, they appear like little clusters as the carbon coating joins nearby particles together. From the HRTEM image (Figure 1B, inset), a thin layer of carbon (less than 1 nm) can be observed on the edges of the particles (indicated by black arrows).7 It is suggested that this thin carbon sheath can act as a barrier to prevent the aggregation of SnO2 NPs during Li+ insertion/ extraction,22 thus mitigate the pulverization problem and improve the cyclic retention. When the glucose concentration is increased to 0.2 M, even larger clusters of nanoparticles are formed (Figure 2C), and the carbon content is also increased accordingly, which can be seen as a thicker (ca. 1-2 nm) layer around the SnO2 nanoparticles (Figure 2C, inset; indicated by black arrows). By using an even higher glucose concentration of 0.5 M, the carbon coating of the as-prepared SnO2@carbon composite becomes much more pronounced (Figure 2D), and numerous SnO2 NPs are embedded inside an interconnected carbon matrix. From the HRTEM image (Figure 2D, inset), the lattice fringes of the nanoparticles are still visible even if encapsulated by carbon layers as thick as 10 to 20 nm. Thermogravimetric analysis (TGA) is performed to quantitatively determine the amount of carbon present in the final products, with the result shown in Figure 3. It can be observed that the weight loss mainly takes place between 200 and 500 °C. In this temperature range, there is no carbothermal reduction of SnO2 to Sn as confirmed by the above XRD analysis. Thus, the carbon contents of the as-prepared SnO2@carbon nanocomposites using 0.05 M, 0.2 M, and 0.5 M glucose can be easily
J. Phys. Chem. C, Vol. 113, No. 47, 2009 20505 determined to be about 8%, 22%, and 65% by weight, respectively. This result is in good agreement with the TEM analysis. To understand the lithium storage properties of the asprepared samples, a series of electrochemical measurements were performed. Figure 4 shows some typical data obtained for SnO2-8 wt % C (Sample 2). From the cyclic voltammograms (CV, Figure 4A), the behavior is generally consistent with previous reports.3,23,29 Two characteristic pairs (cathodic, anodic) of current peaks are observed at the potential (V) of (0.05, 0.60 V), and (0.40, 1.30 V) in the first cycle. The first pair is attributed to the alloying (cathodic sweep) and dealloying (anodic sweep) given by reaction 2 during charge-discharge, and it is much more pronounced than the second pair, revealing its dominant contribution to lithium storage capacity. This reaction is shown to be highly reversible, as there is no substantial change of the positions of both peaks in subsequent cycles. Interestingly, the current of the anodic peak at 0.60 V increases significantly in the second cycle and stays at a constant level for five cycles, indicating that there might be an activating process in the electrode material and that the reaction is reversible in nature. The second pair of peaks appearing at (0.40, 1.30 V) is presumably related to reaction 1, which is believed to be irreversible, as the cathodic peak shifts to a higher voltage (ca. 1 V) in the second cycle. However, no noticeable change of current or voltage can be observed for the anodic peak at 1.30 V, suggesting the partial reversibility of this reaction.3 Figure 4B depicts the cyclic performance of Sample 2 with a voltage window of 0.01- 2 V at different current rates of 400 and 1000 mA g-1. Similar behavior is observed over 100 charge-discharge cycles at both rates. The capacity fades rapidly in the course of the first few cycles and then stabilizes at ca. 700 mA h g-1 for about 20 cycles, followed by a gradual decay thereafter. After 100 cycles, a capacity as high as 492 mA h g-1 at the rate of 400 mA g-1, versus 408 mA h g-1 at 1000 mA g-1, can still be retained. It is commonly known that the voltage window applied for charge-discharge can greatly affect the cyclic performance; we thus investigated the properties using a wider voltage window of 0.01-2.5 V. Figure 4C shows the comparison of the cyclic performance for Sample 2 at 400 mA g-1 between the two voltage windows. Interestingly, the as-prepared SnO2@carbon nanocomposite delivers a significantly higher capacity when the wider voltage window is applied, without significant effect on the cyclability, as can be seen from the nearly parallel orientation of the two curves. As a result, a much higher reversible capacity of 631 mA h g-1 can be obtained after 100 charge-discharge cycles. With a closer look at the anodic sweep (Figure 4A), it can be observed that there is a small peak appearing at around 1.85 to 2.2 V, which is not present in the CV of pure SnO2 NPs (see Supporting Information, Figure S1). We believe that this accounts for the significant increase in the lithium storage capability when applying a voltage window of 0.01-2.5 V, where additional amount of lithium can be repeatedly extracted above 2 V. Figure 4D displays the cyclic performance of Sample 2 and pure SnO2 NPs (Sample 1) with the voltage window of 0.01 V - 2.5 V at different current rates. The pure SnO2 NPs have a very high initial capacity of ca. 1800 mA h g-1 because of the absence of less active carbon and relatively large surface area (ca. 120 m2/g; see Supporting Information, Figure S2). The capacity then fades dramatically to a value below the theoretical capacity of graphite in less than 40 cycles. In comparison, the as-prepared SnO2@carbon nanocomposite shows greatly im-
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TABLE 1: Key Parameters and Electrochemical Results Obtained for the As-Prepared SnO2@Carbon Nanocomposites capacity (mA h g-1) after 100 cycles 0.01- 2 V
0.01- 2.5 V
sample number
glucose concentration (M)
carbon content (wt %)
crystallite size (nm)
400 mA/g
1000 mA/g
400 mA/g
1000 mA/g
1 2 3 4
0 0.05 0.2 0.5
0 8 22 65
1.05 1.73 2.76 1.94
197 492 514 -
408 414 234
59 631 440 -
190 353 396 -
proved capacity retention upon extended cycling, which evidently demonstrates the beneficial effect of the ultrathin carbonaceous layer coated on the SnO2 NPs. Some important electrochemical results are summarized in Table 1 for easy comparison. Similar electrochemical tests were conducted for the other two samples with higher carbon contents. Figure 5A shows the
Figure 2. Transmission electron microscopy (TEM) images of the asprepared SnO2 nanoparticles (A) and SnO2@carbon nanoparticles with different glucose concentrations: 0.05 M (B), 0.2 M (C), and 0.5 M (D). The insets are the high resolution TEM images of the corresponding sample.
CV of SnO2-22 wt % C (Sample 3). It generally exhibits the same behavior as that of Sample 2, with the two pairs of current peaks appearing at specific voltages. Nonetheless, there are some apparent dissimilarities. It can be clearly observed that the current of the cathodic peak at about 0.1 V decreases significantly in the second cycle, indicating irreversible processes have taken place during the first sweep. On the other hand, there is no noticeable change in the current of the anodic peak at about 0.6 V, showing its high reversibility and also the absence of the activating process observed from Sample 2 (Figure 4A). Figure 5B shows the first-cycle discharge voltage profiles of all the four samples at a current rate of 400 mA g-1. The patterns are generally consistent with that of SnO2-based materials. For Sample 1, there exist two broadly defined plateaus. The first plateau appearing at 1.0 - 0.75 V can be ascribed to the reduction of SnO2 into Sn; while the more pronounced second plateau shown at 0.5 V could be attributed to the process of LixSn alloy formation. It is interesting to observe that the first plateau becomes less prominent with increasing carbon content and even completely disappears for Sample 4 with 65 wt % of carbon, but Sample 4 still delivers a discharge capacity of 1080 mA h g-1. This can be understood if one considers that the firstcycle discharge capacity of bare carbon can be as high as 879 mA h g-1,3 and that of pure SnO2 NPs is ca. 1776 mA h g-1. Figure 5C shows the cyclic performance of Sample 3 with a voltage window of 0.01-2 V at different current rates. While noting the parallel behavior for both curves I and II, the initial capacity of 1379 mA h g-1 at a slower rate of 400 mA g-1 is slightly higher than 1229 mA h g-1 at 1000 mA g-1. Reversible capacities of 514 mA h g-1 and 414 mA h g-1 can be retained after 100 charge-discharge cycles at 400 mA g-1 and 1000 mA g-1, respectively. It is worth mentioning that unlike the Sample 2, using a wider voltage window of 0.01-2.5 V does not lead to a higher capacity for Sample 3 (see Table 1). This can be easily understood from the CV (Figure 5A) where no current peak is observed above 2.0 V, suggesting that no electrochemical reaction will further contribute to the lithium storage capacity. The performance of SnO2-65 wt % C (Sample 4) at 1000 mA g-1 with the same voltage window is also shown here. It conveys a much lower initial capacity of 813 mA h g-1, mainly because of the much higher weight percentage of inactive carbon present. Owning to this fact, Sample 4 can only deliver a capacity below the theoretical value of graphite throughout the measurement, and a reversible capacity of only 234 mA h g-1 after 100 cycles. Surprisingly, even with much thicker protective carbon layers (see TEM image in Figure 2D), it fails to demonstrate an improved cyclic retention compared to Samples 2 and 3. Summary
Figure 3. Thermogravimetric analysis of the SnO2@carbon nanocomposites prepared with different glucose concentrations: 0.05 M (I), 0.2 M (II), and 0.5 M (III). Measurement was performed from room temperature to 600 °C at a ramping rate of 10 °C/min in air.
In summary, carbon-coated SnO2 NPs can be prepared via a simple two-step hydrothermal method followed by carbonization. The carbon content in the as-formed nanocomposites can be readily tuned by varying the glucose concentration used
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J. Phys. Chem. C, Vol. 113, No. 47, 2009 20507
Figure 4. Electrochemical results of SnO2-8 wt % C (Sample 2). (A) Representative cyclic voltammograms taken at a scan rate of 0.2 mV s-1 from 5 mV to 2.5 V. (B) The cyclic performance (0.01- 2 V) performed at a current rate of 400 mA g-1 (I), and 1000 mA g-1 (II). (C) The comparison of the cyclic performance between the two voltage windows taken at a current rate of 400 mA g-1. (D) The cyclic performance (0.01-2.5 V) taken at a current rate of 400 mA g-1 (I) and 1000 mA g-1 (II), and the performance of pure SnO2 NPs at 1000 mA g-1 in the same voltage window for comparison (III).
Figure 5. (A) Representative cyclic voltammograms of SnO2-22 wt % C (Sample 3) taken at a scan rate of 0.2 mV s-1 from 5 mV to 2.5 V. (B) First-cycle discharge voltage profiles for the as-prepared SnO2@C nanocomposites with different carbon contents: 0 wt % (I), 8 wt % (II), 22 wt % (III), and 65 wt % (IV). (C) Cyclic performance of Sample 3 with a voltage window of 0.01-2 V at a current rate of 400 mA g-1 (I) and 1000 mA g-1 (II). The performance of SnO2-65 wt % C (Sample 4) at 1000 mA g-1 (III) with the same voltage window is included for comparison.
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during the synthesis. With a systematic study on the effect of carbon coating upon the lithium storage performance, it is found that SnO2@C NPs with only 8 wt % carbon, which exists as an ultrathin carbon layer on the surface of SnO2 NPs, is able to deliver a capacity as high as 631 mA h g-1 after 100 charge-discharge cycles. When the carbon content is increased to 22 wt %, the as-prepared SnO2@C NPs exhibit improved capacity retention upon extended cycling. With a further increased carbon content of 65 wt %, the nanocomposite is unable to demonstrate a satisfactory performance in terms of both capacity as well as cyclic retention. Besides SnO2, this versatile carbon coating technique with tunable carbon content can be easily extended to other electrode materials suitable for next-generation lithium-ion batteries. Experimental Section Materials Preparation. Well dispersed SnO2 nanoparticles with high yield are prepared through a simple hydrothermal method. Typically, a certain amount of tin(IV) chloride pentahydrate (SnCl4 · 5H2O, Sigma-Aldrich, 98%) was dissolved in 20 mL of ultrapure water to achieve a concentration of 0.5 M. This solution is then transferred into a Teflon-lined stainless steel autoclave (60 mL in volume) and kept in an air-flow electric oven at 180 °C for 2 h. Afterward, the autoclave was taken out to cool down naturally. The white precipitate was then harvested by centrifugation and washed thoroughly with ultrapure water before drying at 60 °C overnight. A 0.2 g amount of the as-prepared white powder was added into 40 mL of glucose solution with concentration in the range of 0.05-0.5 M. After being fully dispersed, the reaction solution was put into the electric oven at 180 °C for 4 h. The dark-brown precipitate was collected and dried at 60 °C overnight. For carbonization, the as-prepared dark-brown powder was kept in a tube furnace at 500 °C for 4 h under N2 at a ramping rate of 5 °C/min. Materials Characterization. The products were characterized by X-ray powder diffraction (Bruker, D8 - Advance X-ray Diffractometer, Cu KR, λ ) 1.5406 Å). Morphology and structure of the samples were revealed with a transmission electron microscope (JEOL, JEM-2100F, 200 kV). The carbon content was determined by thermogravimetric analysis (TGA; Shimadzu, DRG-60). The surface area of SnO2 nanoparticles was measured using BET (Quantachrome Instruments, Autosorb AS-6B). Electrochemical Analysis. The electrochemical measurements were performed using two-electrode Swagelok-type cells with lithium serving as both the counter and reference electrodes under ambient temperature. The working electrode was composed of 70 wt % of active material (e.g., SnO2 nanoparticles with or without carbon coating), 20 wt % of conductivity agent (carbon black, Super-P-Li), and 10 wt % of binder (polyvinylidene difluoride, PVDF, Aldrich). The electrolyte used was 1 M LiPF6 in a 50:50 (w/w) mixture of ethylene carbonate and diethyl carbonate. Cell assembly was carried out in an argonfilled glovebox with both moisture and oxygen contents below
Chen et al. 1 ppm. Cyclic voltammetry (CV, 5 mV to 2.5 V, 0.2 mV s-1) was performed using an electrochemical workstation (CHI 660C). Galvanostatic charge/discharge was performed using a battery tester (NEWAER). Acknowledgment. We are grateful to the Nanyang Technological University for financial support through a start-up grant (SUG). Supporting Information Available: Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nature 2000, 407, 496. (2) Tarascon, J. M.; Armand, M. Nature 2001, 414, 359. (3) Lou, X. W.; Chen, J. S.; Chen, P.; Archer, L. A. Chem. Mater. 2009, 21, 2868. (4) Demir-Cakan, R.; Hu, Y. S.; Antonietti, M.; Maier, J.; Titirici, M. M. Chem. Mater. 2008, 20, 1227. (5) Sun, X. M.; Liu, J. F.; Li, Y. D. Chem. Mater. 2006, 18, 3486. (6) Larcher, D.; Beattie, S.; Morcrette, M.; Edstroem, K.; Jumas, J. C.; Tarascon, J. M. J. Mater. Chem. 2007, 17, 3759. (7) Lou, X. W.; Deng, D.; Lee, J. Y.; Archer, L. A. Chem. Mater. 2008, 20, 6562. (8) Wang, Y.; Wu, M.; Jiao, Z.; Lee, J. Y. Chem. Mater. 2009, 21, 3210. (9) Beaulieu, L. Y.; Eberman, K. W.; Turner, R. L.; Krause, L. J.; Dahn, J. R. Electrochem. Solid-State Lett. 2001, 4, A137. (10) Fransson, L.; Nordstrom, E.; Edstrom, K.; Haggstrom, L.; Vaughey, J. T.; Thackeray, M. M. J. Electrochem. Soc. 2002, 149, A736. (11) Veeraraghavan, B.; Durairajan, A.; Haran, B.; Popov, B.; Guidotti, R. J. Electrochem. Soc. 2002, 149, A675. (12) Lou, X. W.; Archer, L. A.; Yang, Z. C. AdV. Mater. 2008, 20, 3987. (13) Han, S. J.; Jang, B. C.; Kim, T.; Oh, S. M.; Hyeon, T. AdV. Funct. Mater. 2005, 15, 1845. (14) Lou, X. W.; Wang, Y.; Yuan, C. L.; Lee, J. Y.; Archer, L. A. AdV. Mater. 2006, 18, 2325. (15) Yang, H. X.; Qian, J. F.; Chen, Z. X.; Ai, X. P.; Cao, Y. L. J. Phys. Chem. B 2007, 111, 14067. (16) Zhao, Q. R.; Xie, Y.; Dong, T.; Zhang, Z. G. J. Phys. Chem. B 2007, 111, 11598. (17) Liu, H. M.; Wang, Y. G.; Wang, K. X.; Hosono, E.; Zhou, H. S. J. Mater. Chem. 2009, 19, 2835. (18) Deng, D.; Lee, J. Y. Chem. Mater. 2008, 20, 1841. (19) Lou, X. W.; Deng, D.; Lee, J. Y.; Archer, L. A. J. Mater. Chem. 2008, 18, 4397. (20) Meduri, P.; Pendyala, C.; Kumar, V.; Sumanasekera, G. U.; Sunkara, M. K. Nano Lett. 2009, 9, 612. (21) Kim, H.; Cho, J. J. Mater. Chem. 2008, 18, 771. (22) Lee, K. T.; Jung, Y. S.; Oh, S. M. J. Am. Chem. Soc. 2003, 125, 5652. (23) Lou, X. W.; Li, C. M.; Archer, L. A. AdV. Mater. 2009, 21, 2536. (24) Wang, Y.; Su, F. B.; Lee, J. Y.; Zhao, X. S. Chem. Mater. 2006, 18, 1347. (25) Zhang, W. M.; Hu, J. S.; Guo, Y. G.; Zheng, S. F.; Zhong, L. S.; Song, W. G.; Wan, L. J. AdV. Mater. 2008, 20, 1160. (26) Cui, G. L.; Hu, Y. S.; Zhi, L. J.; Wu, D. Q.; Lieberwirth, I.; Maier, J.; Mullen, K. Small 2007, 3, 2066. (27) Yoshio, M.; Wang, H. Y.; Fukuda, K.; Umeno, T.; Abe, T.; Ogumi, Z. J. Mater. Chem. 2004, 14, 1754. (28) Kaskhedikar, N. A.; Maier, J. AdV. Mater. 2009, 21, 2664. (29) Park, M. S.; Kang, Y. M.; Wang, G. X.; Dou, S. X.; Liu, H. K. AdV. Funct. Mater. 2008, 18, 455.
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