Nanostructures: Inorganic Salt-Controlled Synthesis and Their

16824

J. Phys. Chem. C 2008, 112, 16824–16829

r-Fe2O3 Nanostructures: Inorganic Salt-Controlled Synthesis and Their Electrochemical Performance toward Lithium Storage Xing-Long Wu,† Yu-Guo Guo,* Li-Jun Wan,* and Chang-Wen Hu‡ Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China, and Beijing National Laboratory for Molecular Sciences (BNLMS), Beijing 100190, China ReceiVed: July 1, 2008; ReVised Manuscript ReceiVed: August 26, 2008

By applying the concept of an inorganic structure-directing agent, uniform R-Fe2O3 nanospheres of about 300 nm in diameter and well-defined nanorhombohedra of about 50-80 nm in size have been successfully synthesized using the simple inorganic sodium salt of NaAc and NaCl as the only structure-directing agent in the hydrothermal system, respectively. In comparison, only micrometer sphere-like aggregates composed of irregular nanoparticles of about 80-120 nm were obtained without the presence of any inorganic salt additives. All three nanostructures are investigated with XRD, SEM, TEM, and electrochemical tests toward lithium storage. It is found that the particle size and shape has a remarkable effect on the lithium insertion/ extraction behavior. Among the three R-Fe2O3 nanostructures, nanospheres show a very high specific capacity of >600 mA h g-1 in the initial 10 cycles and >414 mA h g-1 after 60 cycles as well as good cycling performance, exhibiting great potential as anode materials in lithium-ion batteries. It benefits from the proper submicrometer size with the right surface area and the spherical shape. 1. Introduction Hematite (R-Fe2O3) has attracted great technological interest due to its attractive features, such as low cost, good stability, nontoxicity, and environmentally friendly properties. It is very useful in ordinary1 and photocatalysis,2 gas sensors,3 magnetic recording, field emission,4 and pigments as well as electrochemical devices. Recently, use of nanostructured R-Fe2O3 as anode materials for lithium-ion batteries (LIBs)5-10 has attracted more and more interest, which is largely promoted by the successful synthesis of diverse R-Fe2O3 nanostructures, including nanoparticles,11 nanocubes,12 nanorods,13 and nanotubes.6 For example, Thackeray et al.14,15 first explored the structural changes of R-Fe2O3 during lithium insertion and predicted the potential application for LIBs. Chen et al.6 synthesized aligned R-Fe2O3 nanotubes by a templating technique, which exhibit enhanced electrochemical activity toward lithium insertion. Larcher et al.16,17 disclosed the dissimilarity of the lithium insertion process between micrometer- and nanometer-sized R-Fe2O3. So far numerous groups5-10,16-19 have demonstrated that the particle size and morphology of R-Fe2O3 nanostructures has a remarkable effect on their electrochemical performance toward lithium storage. However, it still remains a challenge to develop a simple approach to synthesize controllable R-Fe2O3 nanostructures. Solving this challenge will facilitate our in-depth understanding of the influence of particle size and shape on the electrochemical properties. It is well known that functionalized organic molecules, such as surfactants and polymer electrolytes, are effective structuredirecting agents for achieving morphology and size control in the synthesis procedures of various nanostructures.20-25 Re* To whom correspondence should be addressed. Phone/Fax: (86)1062557908. E-mail: [email protected]. † Also at the Graduate School of CAS, Beijing 100064, China. ‡ Department of Chemistry, Institute for Chemical Physics and State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China.

cently, some groups24,26-30 have found that the added or residual inorganic species in the organic molecule-based synthesis systems also play a vital role similar to the organic molecules on controlling the morphology and size of as-synthesized nanostructures. Therefore, the small inorganic ions can be considered as part of the structure-directing agent or even the only one.10,31 They have shown their advantages as structuredirecting additives over the organic ones in terms of environmental benignity and low cost. However, there are few reports on the investigation of the roles of inorganic species in controlling the size and shape of nanostructures so far.10,28-31 By applying the concept of an inorganic structure-directing agent, herein we report the controllable synthesis of R-Fe2O3 nanostructures. Well-defined R-Fe2O3 nanorhombohedra and uniform nanospheres have been successfully synthesized using the simple inorganic sodium salt of sodium acetate (NaAc) and sodium chloride (NaCl) as the only structure-directing agent, respectively. Without use of any inorganic salt additive, irregular R-Fe2O3 aggregates were obtained. Since neither noxious raw materials nor environmentally unfriendly organic solvents are used in the whole synthesis processes, the inorganic salt-based method is totally “green”. In this paper, we also report on the electrochemical performance of the three R-Fe2O3 nanostructures, which show different electrochemical behaviors toward lithium storage. It is found that the particle size and shape has a remarkable effect on the lithium insertion/extraction behavior. Among the three R-Fe2O3 nanostructures, nanospheres show a very high specific capacity of >600 mA h g-1 in the initial 10 cycles and >414 mA h g-1 after 60 cycles as well as good cycling performance, exhibiting great potential as anode materials in lithium-ion batteries. 2. Experimental Section 2.1. Preparation of Hematite Nanostructures. R-Fe2O3 nanostructures were prepared by hydrothermally forced hydrolyzation of low-cost FeCl3 in a medium of distilled water with

10.1021/jp8058307 CCC: $40.75  2008 American Chemical Society Published on Web 10/04/2008

R-Fe2O3 Nanostructures or without inorganic additives, which was quite straightforward and excellently repeatable. In a typical procedure, 1 mmol of FeCl3 was dissolved into 50 mL of distilled water to form a transparent solution. To control its size and morphology, 3 mmol of NaCl or NaAc was added to the above solution, respectively. After stirring for 30 min, the obtained solution was transferred and sealed into a 75 mL Teflon-lined autoclave, heated at 180 °C for 24 h, and then cooled to room temperature naturally. The as-prepared precipitate was collected by centrifuging, repeatedly washed several times with distilled water and ethanol, and finally dried in vacuum for further characterization and electrochemical tests. 2.2. Characterization. X-ray powder diffraction (XRD) patterns were collected on a Shimadzu-6000 X-ray diffractometer with Cu KR radiation (λ ) 1.5406 Å). The accelerating voltage was set at 40 kV with 30 mA flux at a scanning rate of 5°/min in the 2θ range of 10-80°. A Hitachi model H-800 transmission electron microscope (TEM, operating at 200 kV) and Hitachi S-4800 field-emission scanning electron microscope (SEM, operating at 10 kV) were used to investigate the morphology and size of the as-obtained products. For zetapotential measurements, 0.5 mg of Fe2O3 was ultrasonically dispersed in 10 mL of distilled water and tested on a zetapotential analyzer (ZetaPALS/90plus, Brookhaven). A ST-2000 specific surface area detecting instrument (Beifen Instrument Co., Ltd.) was used to measure the specific surface areas of Fe2O3 samples. 2.3. Electrochemical Characterization. Electrochemical measurements were performed using two-electrode Swageloktype cells assembled in an argon-filled glovebox. For preparing working electrodes, a mixture of R-Fe2O3 nanostructures, acetylene black, and poly(vinyl difluoride) (PVDF) at a weight ratio of 70:20:10 was pasted on a Cu foil. A glass fiber (GF/D) from Whatman was used as a separator. Lithium foil was used as the counter electrode. The electrolyte consisted of a solution of 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1:1:1, in wt %) obtained from Ferro Corp. A galvanostatic cycling test of the assembled cells was carried out on an Arbin BT2000 system in the voltage range of 0.01-3.0 V (vs Li+/Li) at a discharge/charge rate of C/10 (100.7 mA g-1). 3. Results and Discussion 3.1. Characterization. The composition and purity of the as-obtained samples were determined by XRD. Figure 1 shows XRD patterns of the samples synthesized with or without inorganic salts as additives in which all diffraction peaks are in good agreement with rhomb-centered hexagonal (rch) R-Fe2O3 [JCPDS Card No. 03-0664, space group R3jc(167), a0 ) 5.035 Å, c0 ) 13.74 Å]. No other peaks are observed, indicating the high purity of the as-prepared R-Fe2O3. The size and morphology of the as-prepared products were characterized by SEM and TEM. As revealed in the SEM image shown in Figure 2a, without adding any inorganic salt to the reaction system as additive irregular R-Fe2O3 aggregates were obtained. The particles were mainly composed of irregular nanoparticles of about 80-120 nm in diameter, which finally congregate into micrometer sphere-like aggregates (Figure 2b). When NaAc was used as the structure-directing agent, welldispersed R-Fe2O3 particles were produced (Figure 2c and 2d). These particles have the morphology of a rhombohedron with a size of about 50-80 nm. With NaCl as the structure-directing agent, well-dispersed and uniform R-Fe2O3 spheres of about 300 nm in diameter were obtained (Figure 2e and 2f). In the

J. Phys. Chem. C, Vol. 112, No. 43, 2008 16825

Figure 1. XRD patterns of Fe2O3 nanoaggregates, nanorhombohedra, and nanospheres.

following, nanoaggregates, nanorhombohedra, and nanospheres refer to the three types of R-Fe2O3 nanostructures. 3.2. Effects of Inorganic Salts. It has been demonstrated that in addition to organic surfactants and polymers,20-24 inorganic species can also be used as capping agents or structuredirecting agents for size- and morphology-controlled synthesis of nanoparticles.10,31 Owing to their relatively small sizes, it is expected that inorganic ions might have a more pronounced influence on the nucleation and growth process of nanocrystallites than organic surfactants and polymers. On the basis of the above structure and morphology characterizations of the three R-Fe2O3 nanostructures, it is clear that the inorganic salts have the following effects. First, the added inorganic salts (NaCl and NaAc) are able to change the pH (from 2.02 to 2.09 and 3.58, respectively) and ionic strength (from 6.105 to 7.86 and 8.565, respectively) of the solution32 (i.e., the reaction medium), which may increase the viscosity of the reaction system, so as to decrease the mobility of the reactive ions and crystal nuclei formed and, eventually, influence the size and morphology of the obtained nanostructures. Second, inorganic anions added can coordinate with existing reactant cations via different coordination modes and strengths. For example, one Fe3+ could chelate three CH3COO- (Ac-) but simply coordinate with six Cl(chloride) to form two types of octahedral molecular structures with different stability, which may change the dynamics of the nuclei and growth rate of crystals, resulting in anisotropic growth and oriented attachment. This kind of function is also considered as selective adsorption of added inorganic ions. The selective adsorption on the surface of formed crystal nuclei and primary nanoparticles can alter its surface termination and atomic arrangement. These changes may affect the anisotropic growth process and result in a different size and morphology of asformed products. In this context, formation of R-Fe2O3 nanorhombohedra should be ascribed to the different adsorption ability of Ac- on different planes of R-Fe2O3. Third, the Ac- or Cladsorbates stemming from the added NaAc or NaCl can charge the surfaces of as-obtained colloid nanoparticles negatively, which has been confirmed by zeta-potential measurements. Due to the same negative charge, interactional repulsion exists among particles. As a result, well-dispersed R-Fe2O3 nanoparticles (nanorhombohedra and nanospheres) can be obtained with addition of NaAc or NaCl, which is consistent with SEM and TEM observations. Finally, the adsorbed inorganic ions can also

16826 J. Phys. Chem. C, Vol. 112, No. 43, 2008

Wu et al.

Figure 2. Typical SEM and TEM images of the three R-Fe2O3 nanostructures: (a, b) nanoaggregates, (c, d) nanorhombohedra, and (e, f) nanospheres.

Figure 3. First discharge/charge curves of the three R-Fe2O3 nanostructures. (Inset) Enlarged view of the curves in the capacity range of 0-50 mA h g-1. (b) Differential capacity plot of R-Fe2O3 nanospheres.

be considered as capping agents which have a pronounced influence on the uniformity of as-grown particles. Usually the stronger the inorganic ions adsorb the better uniformity the resulted particles are. In view of the monodispersed R-Fe2O3 nanospheres, Cl- is expected to have a stronger adsorption than Ac- on the surface of R-Fe2O3 particles. 3.3. Electrochemical Performance toward Lithium Storage. To investigate the particle size and shape effects on the electrochemical properties of the three R-Fe2O3 nanostructures, we carried out a preliminary investigation into their electrochemical performance with respect to Li insertion/extraction. Figure 3 shows the first discharge/charge voltage profiles for

the R-Fe2O3 nanoaggregates, nanorhombohedra, and nanospheres at a rate of C/10 (100.7 mA g-1) in the voltage window of 0.01-3.0 V (vs Li+/Li). It was found that the shape of the three curves is very similar, implying that the morphology and size does not change the lithium storage nature of R-Fe2O3. The first discharge curves can be divided into four regions, marked as I, II, III, and IV. For regions I and II there are two plateaus at about 1.62 and 1.08 V, respectively, which can be clearly seen in the curves for R-Fe2O3 nanoaggregates and nanorhombohedra, while there is only a sloped curve for nanospheres. However, two short plateaus at 1.62 and 1.34 V do also exist as shown from the enlarged view of the profile below 50 mA

R-Fe2O3 Nanostructures

J. Phys. Chem. C, Vol. 112, No. 43, 2008 16827

h g-1 (inset of Figure 3a) and two sharp peaks in the differential capacity plot (Figure 3b). For region III a long plateau appears at about 0.85 V in all three curves. The three plateau capacities (I + II + III) for all three R-Fe2O3 nanostructures are almost the same (ca. 1090 mA h g-1, i.e., 6.5 Li per R-Fe2O3), which is very close to the theoretical capacity of 1007 mA h g-1 (6 Li per R-Fe2O3), indicating the following three lithiation steps I

II

III

R-Fe2O3 98 R-LixFe2O3 98 cubic Li2Fe2O3 y\z Fe + Li2O At the early stage of lithium insertion (plateau I) a small amount of lithium can be inserted into the crystal structure of R-Fe2O3 before the structural transformation of the close-packed anionic array from hexagonal to cubic stacking occurs. As reported in the literature, the inserted amount of lithium (the value of x in LixFe2O3) is related to the particle size. For instance, up to 1 mol of Li can be inserted into nanometersized R-Fe2O3 particles with a diameter of about 20 nm, while only 0.03 mol of Li can be inserted into submicrometer-sized R-Fe2O3 particles with a diameter of about 0.5 µm.16,17 In the present investigation, only a small degree of Li insertion (0.05 mol) into R-Fe2O3 nanospheres (ca. 300 nm in diameter) was observed. In contrast, in the case of both R-Fe2O3 nanoaggregates containing nanoparticles of ca. 80-120 nm in diameter and nanorhombohedra (ca. 50-80 nm in size) a substantially higher amount of 0.7 mol of Li can be inserted in the first insertion step. The results are very consistent with the reported results.16,17 On the following stage of lithium insertion (plateau II) a similar size effect of lithium storage as plateau I was found for the three R-Fe2O3 nanostructures.6 For region III a long plateau appears at approximately 0.85 V, corresponding to the reversible reaction between cubic Li2Fe2O3 and Fe in step III. Almost the same length of plateau III for the three samples indicates that step III of lithium insertion is not sensitive to the particle size.4,5,16-19 On further lithium insertion the sloped region IV appears in the three samples, which is the biggest different part in the discharge curves for the three R-Fe2O3 nanostructures. On the basis of a complete reduction of Fe3+ to Fe0, a maximum capacity uptake of 1007 mA h g-1 (e.g., 6 Li per R-Fe2O3) is expected for R-Fe2O3. However, as revealed in Figure 3, the initial discharge capacity of R-Fe2O3 nanoaggregates, nanorhombohedra, and nanospheres is 1700, 1550, and 1398 mA h g-1, corresponding to 10.1, 9.3, and 8.3 Li per R-Fe2O3, respectively, all of which are larger than the theoretical capacity (1007 mA h g-1). The phenomenon that the first discharge capacity considerably exceeds the theoretical capacity has been widely reported for transition metal oxides.19,33,34 Usually the exceeded capacity is ascribed to the electrolyte being reduced at low voltage (generally below 0.8V vs Li+/Li)16 to form a solid electrolyte interphase (SEI) layer and possibly interfacial lithium storage.35,36 The results indicate that the difference of the lithium storage ability among the three R-Fe2O3 nanostructures should be ascribed to the different capacity in region IV (0.01-0.8 V), which is related to the size and surface area of the samples. It is worth noting that the nanospheres exhibit the best cycling performance among the three R-Fe2O3 nanostructures (Figure 4). After 60 cycles, the reversible capacity for nanospheres is still as high as 414 mA h g-1 while that for nanoaggregates and nanorhombohedra is only 164 and 95 mA h g-1, respectively. It is much higher than the theoretical specific capacity of currently used graphite (LiC6, 372 mA h g-1) and makes the R-Fe2O3 nanosphere a promising anode material for lithium-ion batteries.

Figure 4. Cycling performance of R-Fe2O3 nanoaggregates, nanorhombohedra, and nanospheres.

So far it is known that the crystalline texture and surface atomic arrangement of synthesized nanostructures are the primary factors for the size- and morphology-dependent electrochemical properties. Though nanometer-sized particles have multifold merits, for example, preferable accommodation to the strain of Li+ insertion/extraction in the processes of discharge and charge to maintain the integrity of electrode materials,36,37 the high surface areas raise the risk of secondary reactions, including formation and (or) decomposition of SEI layers. The secondary reactions may cause a large irreversible capacity loss (i.e., low Coulombic efficiency) and consumption of active materials and result in a poor cycle life. As shown in Figure 5d, all three R-Fe2O3 nanostructures exhibit the same low Coulombic efficiency of about 68% for the first cycle. However, after the first cycle, the Coulombic efficiency is above 95% for the nanospheres and thereafter stabilizes at about 98-100%. In contrast, both the nanoaggregates and the nanorhombohedra show a quite different behavior in which the Coulombic efficiency slowly increases and reaches 95% until the 12th cycle. The above results may be ascribed to the larger specific areas of nanoaggregates (ca. 22 m2 g-1) and nanorhombohedra (ca. 36 m2 g-1) than that of nanospheres (ca. 6 m2 g-1). Another reason may be related to the different morphology of the three R-Fe2O3 nanostructures, which holds different crystalline surfaces exposed to electrolyte. It has been reported that the SEI layer formed during Li uptake may decompose completely catalyzed by transition metal upon Li extraction,35,38 which leads to irreversible capacity and capacity fade. Our results indicate that the morphology of the electrode materials has a pronounced influence on the stability of the SEI layers formed on the surfaces. The uniform spherical particle is the best system of choice among the three R-Fe2O3 nanostructures in terms of the stability of the SEI layers formed on the surfaces. Furthermore, the nearly monodispersed submicrometer-sized R-Fe2O3 nanospheres also enable a high packing density of R-Fe2O3, which guarantees high absolute capacities of lithium-ion batteries. Figure 5a-c shows the discharge/charge voltage profiles for the three R-Fe2O3 nanostructures. For the nanoaggregates and nanorhombohedra it can be observed that plateau III of the first discharge curve gradually disappears in the following discharge curves and finally changes to a sloped region at the fifth discharge curve and thereafter. However, in the case of nanospheres plateau III at approximately 0.85 V is still present in the 20th discharge curve (Figure 5c). The results indicate that the crystalline structure of R-Fe2O3 had changed irreversibly

16828 J. Phys. Chem. C, Vol. 112, No. 43, 2008

Wu et al.

Figure 5. Galvanostatic discharge/charge voltage profiles for R-Fe2O3 nanoaggregates (a), nanorhombohedra (b), and nanospheres (c) cycled at a rate of C/10. (d) Corresponding Coulombic efficiency profiles of the three R-Fe2O3 nanostructures.

for nanoaggregates and nanorhombohedra. Usually irreversible formation of a ‘nanocomposite’ of crystalline grains of metals and amorphous matrix of Li2O is considered as the reason for the sloped behavior at the discharge process of metal oxides.38 However, why the nanospheres maintain the plateau behavior is not fully understood so far and needs further study. 4. Conclusions Using inorganic salts (NaAc and NaCl) as the only structuredirecting agent we developed a simple “green” hydrothermal method for large-scale synthesis of three R-Fe2O3 nanostructures with well-defined shapes: nanoaggregates, nanorhombohedra, and nanospheres. Electrochemical measurements show that the size and morphology of R-Fe2O3 particles have remarkable effects on their electrochemical performance toward lithium storage. It is found that both the nanoaggregates and the nanorhombohedra have very poor cycling performance due to the high surface reactions relating to the high surface area of nanometer-sized particles and low stability of the SEI layers formed on the surfaces, which may be affected by the morphology of R-Fe2O3. However, the nanospheres exhibit good electrochemical performance in terms of cycling behavior and capacity, which can be attributed to the following two factors. First, the submicrometer size (ca. 300 nm) of R-Fe2O3 particles provides the right specific surface area (ca. 6 m2 g-1). Second, the spherical shape leads to a stable SEI layer formed during lithium uptake. The nearly monodispersed submicrometer-sized R-Fe2O3 nanospheres can also guarantee a high packing density. Our results make the R-Fe2O3 nanospheres a promising anode

material for high-performance lithium-ion batteries and provide further examples of the strategy of improving electrode performance based on structure optimization. The cost-effective hydrothermal method could also be extended to other metal oxides. Acknowledgment. This work was supported by the National Natural Science Foundation of China (grant nos. 50730005, 20701038, and 20673121), the National Key Project on Basic Research (grant no. 2009CB930400), and the Chinese Academy of Sciences. C.W.H. thanks the National Natural Science Foundation of China (grant nos. 20671011 and 20731002). References and Notes (1) Zheng, Y.; Cheng, Y.; Wang, Y.; Bao, F.; Zhou, L.; Wei, X.; Zhang, Y.; Zheng, Q. J. Phys. Chem. B 2006, 110, 3093–3097. (2) Cesar, I.; Kay, A.; Gonzalez-Martinez, J. A.; Gratzel, M. J. Am. Chem. Soc. 2006, 128, 4582–4583. (3) Gou, X. L.; Wang, G. X.; Park, J.; Liu, H.; Yang, J. Nanotechnology 2008, 19, 125606–125606. (4) Yu, T.; Zhu, Y.; Xu, X.; Yeong, K.-S.; Shen, Z.; Chen, P.; Lim, C.-T.; Thong, J. T.-L.; Sow, C.-H. Small 2006, 2, 80–84. (5) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nature 2000, 407, 496–499. (6) Chen, J.; Xu, L.; Li, W.; Gou, X. AdV. Mater. 2005, 17, 582–586. (7) Feng, J.; Jianli, B.; Peter, G. B. Electrochem. Solid-State Lett. 2007, 10, A264-A266. (8) Hirokazu, K.; Kenji, T.; Fuminori, M.; Akitoshi, H.; Kiyoharu, T.; Masahiro, T. J. Electrochem. Soc. 2007, 154, A725-A729. (9) Reddy, M. V.; Yu, T.; Sow, C. H.; Shen, Z. X.; Lim, C. T.; Rao, G. V. S.; Chowdari, B. V. R. AdV. Funct. Mater. 2007, 17, 2792–2799. (10) Wu, C.; Yin, P.; Zhu, X.; OuYang, C.; Xie, Y. J. Phys. Chem. B 2006, 110, 17806–17812.

R-Fe2O3 Nanostructures (11) Jin, J.; Ohkoshi, S.; Hashimoto, K. AdV. Mater. 2004, 16, 48–51. (12) Pu, Z.; Cao, M.; Yang, J.; Huang, K.; Hu, C. Nanotechnology 2006, 17, 799–804. (13) Woo, K.; Lee, H. J.; Ahn, J. P.; Park, Y. S. AdV. Mater. 2003, 15, 1761–1764. (14) Thackeray, M. M.; David, W. I. F.; Goodenough, J. B. Mater. Res. Bull. 1982, 17, 785–793. (15) Thackeray, M. M.; David, W. I. F.; Bruce, P. G.; Goodenough, J. B. Mater. Res. Bull. 1983, 18, 461–472. (16) Larcher, D.; Masquelier, C.; Bonnin, D.; Chabre, Y.; Masson, V.; Leriche, J. B.; Tarascon, J. M. J. Electrochem. Soc. 2003, 150, A133A139. (17) Larcher, D.; Bonnin, D.; Cortes, R.; Rivals, I.; Personnaz, L.; Tarascon, J. M. J. Electrochem. Soc. 2003, 150, A1643-A1650. (18) Zeng, S.; Tang, K.; Li, T. J. Colloid Interface Sci. 2007, 312, 513–521. (19) Morimoto, H.; Tobishima, S.-i.; Iizuka, Y. J. Power Sources 2005, 146, 315–318. (20) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664–670. (21) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Nature 2005, 437, 121–124. (22) Cao, M.; Hu, C.; Peng, G.; Qi, Y.; Wang, E. J. Am. Chem. Soc. 2003, 125, 4982–4983. (23) Cao, M.; Wu, X.; He, X.; Hu, C. Langmuir 2005, 21, 6093–6096. (24) Pileni, M.-P. Nat. Mater. 2003, 2, 145–150. (25) Subramanian, V.; Burke, W. W.; Zhu, H.; Wei, B. J. Phys. Chem. C 2008, 112, 4550–4556.

J. Phys. Chem. C, Vol. 112, No. 43, 2008 16829 (26) Guo, Y.-G.; Lee, J.-S.; Hu, Y.-S.; Maier, J. J. Electrochem. Soc. 2007, 154, K51-K60. (27) Liu, Y.; Chu, Y.; Zhuo, Y.; Li, M.; Li, L.; Dong, L. Cryst. Growth Des. 2007, 7, 467–470. (28) Houtepen, A. J.; Koole, R.; Vanmaekelbergh, D.; Meeldijk, J.; Hickey, S. G. J. Am. Chem. Soc. 2006, 128, 6792–6793. (29) Herricks, T.; Chen, J.; Xia, Y. Nano Lett. 2004, 4, 2367–2371. (30) Wiley, B.; Herricks, T.; Sun, Y.; Xia, Y. Nano Lett. 2004, 4, 1733– 1739. (31) Siegfried, M. J.; Choi, K. S. J. Am. Chem. Soc. 2006, 128, 10356– 10357. (32) Filankembo, A.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 5865– 5868. (33) Hang, B. T.; Doi, T.; Okada, S.; Yamaki, J.-i. J. Power Sources 2007, 174, 493–500. (34) Liu, S.; Zhang, L.; Zhou, J.; Xiang, J.; Sun, J.; Guan, J. Chem. Mater. 2008, (35) Balaya, P.; Li, H.; Kienle, L.; Maier, J. AdV. Funct. Mater. 2003, 13, 621–625. (36) Maier, J. Nat. Mater. 2005, 4, 805–815. (37) Hu, Y.-S.; Kienle, L.; Guo, Y. G.; Maier, J. AdV. Mater. 2006, 18, 1421–1426. (38) Jamnik, J.; Maier, J. Phys. Chem. Chem. Phys. 2003, 5, 5215–5220.

JP8058307