Synthesis of Hexagonal-Shaped SnO2 Nanocrystals and SnO2@C

Jan 18, 2008 - Nanosized SnO2 has been used as promising gas sensors1 and electrode ... steel 3/4' ' union part that was plugged from both sides by st...
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J. Phys. Chem. C 2008, 112, 1825-1830

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Synthesis of Hexagonal-Shaped SnO2 Nanocrystals and SnO2@C Nanocomposites for Electrochemical Redox Supercapacitors Ramakrishnan Kalai Selvan, Ilana Perelshtein, Nina Perkas, and Aharon Gedanken* Department of Chemistry and Kanbar Laboratory for Nanomaterials, Center for AdVanced Materials and Nanotechnology, Bar-Ilan UniVersity, Ramat-Gan, 52900, Israel ReceiVed: August 31, 2007; In Final Form: NoVember 6, 2007

To realize a suitable supercapacitor nanomaterial, the recently developed technique of reaction under autogenic pressure at elevated temperature has been employed by us to synthesize SnO2 hexagonal nanocrystals and SnO2@C nanocomposites. The synthesis at different temperatures (viz. 500, 600, and 700 °C) yields three different composites. Characterization of these composites by various methods confirms the structural (XRD, Raman, FT-IR) and nanoparticulate (TEM, HRTEM) nature of the synthesized materials. TEM studies including HRTEM reveal that all the synthesized SnO2 and SnO2@C nanomaterials are highly crystalline with hexagonal shape. Cyclic voltammetric studies carried out to examine the capacitance of SnO2@C in 1 M H2SO4 show that the nanocomposite prepared at 700 °C has a maximum specific capacitance of 37.8 F/g at a scan rate of 5 mV/s.

1. Introduction In recent years, oxide-based nanostructured materials have attracted special attention because of their tailor-made properties in a wide range of applications. Among the oxide nanomaterials, SnO2 has gained prominent interest because of its potential applications in various fields. Nanosized SnO2 has been used as promising gas sensors1 and electrode for lithium ion batteries,2 in dye-sensitized solar cells,3 electrochromic windows,4 transparent conducting electrodes,5 transistors,6 catalyst supports,7 catalysts,8 supercapacitors,9 and so forth. It is well known that the physical and chemical properties of the materials depend on the size, shape, and composition of the particular material. Therefore, researchers are actively engaged in developing tin oxide nanostructures with different sizes and shapes, including nanospheres,10 hollow microspheres,11 nanofibers,12 nanoribbons,13 nanomesh,14 mesopores,15 springs,16 rings,16 spirals,16 and nanorods17 for a wide range of applications. The abovementioned nanostructures have been prepared by hydrothermal methods,10,11 electrospinning,12 rapid oxidation of elemental tin,13 solution phase methods,14 evaporation-induced selfassembly,15 solid vapor process,16 and oriented aggregation17 methods. In a few studies, the properties of SnO2 have been modified by adding a suitable dopant or forming a composite for specific necessities. Doping of SnO2 with Eu3+, Ni2+, Sb3+, Au2+, and Fe3+ ions resulted in enhancing photoluminescence,18 dilute magnetic semiconductors,19 electrochromic properties,20 CO adsorption,21 and gas sensing,22 respectively. A great deal of attention has been paid to the development of energy devices for portable electronic applications. Therefore, the search for a suitable alternate material for supercapacitors to meet the requirements of high power density devices is an active area of research in recent times. Thus far, RuO2 is considered a good electrode material for supercapacitors because of its high specific capacity of 720 F/g in aqueous acid electrolytes.23 However, its disadvantages are its high cost and * Corresponding author. Phone: +972-3-5318315. Fax: +972-37384053. E-mail: [email protected].

toxic behavior. Therefore, the capacitance behavior of different oxide materials such as MnO2,24 CoO,25 NiO,26 Bi2O3,27 Fe3O4,28 SnO2,29 SnO,30 and others was studied using various electrochemical techniques. Carbon-based materials have usually been added with the active materials for improving the electrical, electrochemical, and electrocatalytic properties, especially in energy devices. This inhomogeneous mechanical dispersion reduces the performance of the active materials. For this reason, we have tried to employ the recently developed reactions under autogenic pressure at elevated temperatures (RAPET) technique to prepare MO2@C nanocomposites, as well as to make particles of different shapes for a wide range of applications. Using this novel method, we have recently reported the synthesis of octahedral MnO nanocrystals and MnO@C core-shell composites,31 Fe3O4 pyramids and carbon sheets,32 WS2 breeds and wormlike nanostructures,33 and TiO2@C core-shell composites.34 In continuation, here we have synthesized hexagonal-shaped SnO2 nanocrystals and SnO2@C nanocomposites by adopting the RAPET technique using tin(II) acetate and urea as the starting precursors, and to the best of our knowledge this is the first report on hexagonal-shaped SnO2. The products were examined for their application as supercapacitor material. 2. Experimental Method The SnO2 hexagonal nanocrystals and SnO2@C nanocomposites were prepared in a Swagelok union. Sn(II) acetate (Aldrich) and urea (Aldrich) were used without any further purification. Tin(II) acetate (1.775 g) and urea (0.31 g) were introduced into a 2-mL closed vessel cell under atmospheric conditions. The details of the experimental setup are given elsewhere.32 The cell was assembled from a Swagelok stainless steel 3/4′′ union part that was plugged from both sides by standard caps. The closed cell was heated at different temperatures (500, 600, and 700 °C) for 3 h at a heating rate of 10 °C/min in a horizontal tube furnace, keeping it inside the iron pipe. The reaction took place at the autogenic pressure of the precursor. The cell was gradually cooled to room temperature

10.1021/jp076995q CCC: $40.75 © 2008 American Chemical Society Published on Web 01/18/2008

1826 J. Phys. Chem. C, Vol. 112, No. 6, 2008 and opened with the release of a little pressure. The products obtained at temperatures of 500, 600, and 700 °C were called SC-500, SC-600, and SC-700, respectively. The X-ray diffraction measurements were carried out with a Bruker AXS D* advance powder X-ray diffractometer with a Cu KR (λ ) 1.5418 Å) radiation source. The diffuse reflectance UV-vis (DRUV) spectra were recorded on a Perkin-Elmer UV-visible spectrophotometer. An Olympus BX41 (Jobin Yvon Horiba) Raman spectrometer was employed using the 514.5-nm line of an Ar laser as the excitation source to analyze the nature of the carbon present in the products. The particles’ morphology was studied by transmission electron microscopy on a JEOL-JEM 100 SX microscope, working at an 80 kV accelerating voltage, and a JEOL-2010 HRTEM instrument, using an accelerating voltage of 200 kV. Samples for TEM and HRTEM measurements were prepared by ultrasonically dispersing the products into absolute ethanol, then placing a drop of this suspension onto a copper grid coated with an amorphous carbon film, and then drying in air. The elemental analysis of the sample was carried out by an Eager C, H, N analyzer. The Brunauer-Emmett-Teller (BET) surface area measurements were performed by a Micromeritics (Gemini 2375) surface area analyzer. For the electrochemical measurements, a conventional three-electrode glass cell was used. The measurements were carried out with a potentiostat/ galvanostat model 273 A (Princeton Applied Research). A 0.076 cm2 area with glassy carbon (GC) was used as the working electrode. The detail of the electrode preparation has already been reported from this lab.35 In brief, 10 mg of SnO2@C nanocomposites was dispersed in 0.5 mL of water for 20 min in an ultrasonicator. The dispersed material (10 µL) was placed on a GC and dried in an oven at 90 °C for 2 min. Five microliters of 5% Nafion was dropped on a GC and dried at room temperature. Pt wire and Ag/AgCl (3 M NaCl) electrodes served as counter and reference electrodes, respectively. The capacitance studies were carried out in 1 M H2SO4 electrolyte. 3. Results and Discussion Figure 1 shows the XRD patterns of SnO2@C nanocomposites synthesized by RAPET at different temperatures of 500, 600, and 700 °C. The patterns show sharp and well-defined peaks, indicating the crystallinity of the synthesized materials. The observed diffraction patterns at 2θ ) 26.7, 33.9, 38.0, 39.05, 51.8, 54.8, 57.85, 61.9, 64.8, 65.9, 71.2, and 78.72° correspond to the planes of (110), (101), (200), (111), (211), (220), (002), (310), (112), (301), (202), and (321), respectively, which enumerate the tetragonal structure of SnO2. The observed 2θ values are consistent with the standard JCPDS values (JCPDS No. 01-088-0287). It was found that the intensity of the diffraction peaks increases with the increase in the reaction temperature, revealing the enhancement of the crystallization of the materials. That no peaks corresponding to SnO or elemental Sn are detected indicates the phase purity of the prepared materials. The average particle size has been calculated using the Debye-Scherrer formula using the three different prominent planes of (110), (101), and (211) with mean values of 15.6, 17.0, and 19.3 nm for SC-500, SC-600, and SC-700, respectively. This shows that the operating temperature has a slight influence on the primary particle size of the SnO2 hexagonal nanocrystals and SnO2@C nanocomposites. When the reaction was conducted at 800 and 900 °C, metallic tin was obtained. The formation of metallic Sn is a result of a reduction of the Sn+4 by the carbon layer. That is why in the article only the SC-500, SC-600, and SC-700 are presented. The calculated elemental percentages of carbon are 6.088, 6.597, and 6.579% for SC-500, SC-600, and SC-700, respec-

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Figure 1. XRD patterns of SC-500 (a), SC-600 (b), and SC-700 (c) nanocomposites.

Figure 2. FT-IR spectra of SC-500 (a), SC-600 (b), and SC-700 (c) nanocomposites.

tively. Some trace amount of nitrogen is observed (i.e., 3.825% for SC-500, 3.903% for SC-600, and 1.397% for SC-700), which originated from the precursor, urea. The amount of hydrogen decreases with the increasing reaction temperature: 0.906, 0.572, and 0.237% for SC-500, SC-600, and SC-700 nanocomposites. The measured BET specific surface areas of the SC-500, SC600, and SC-700 nanocomposites are 20.09, 25.26, and 39.66 m2/g, respectively. The FT-IR spectrum is an important tool for characterizing materials. Figure 2 shows the FT-IR spectra of SnO2@C nanocomposites of the three samples. It is well known that the low wavenumber region of 400-800 cm-1 depicts the lattice vibration of SnO2.36 The main peaks, observed at 644, 623, and 614 cm-1, are typical for the Sn-O-Sn antisymmetric and symmetric vibrations for the three nanocomposites, respectively.37 The observed low intensity peak around ∼1620 cm-1 is due to the small amount of adsorbed water. Similarly, the peak at 3400-3500 cm-1 corresponds to the OH stretching mode, which disappears for the SC-700 nanocomposite prepared at high temperature. The UV-vis spectra is useful to understand the structural variation of the materials via the calculated band gap values.

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Figure 3. UV-vis spectra of SC-500 (a), SC-600 (b), and SC-700 (c) nanocomposites.

The observed UV-vis absorption spectra of SnO2@C nanocomposites are shown in Figure 3. It can be seen that a strong absorption peak around 253, 256, and 258 nm and their corresponding band gap values of 4.9, 4.84, and 4.8 eV were observed for SC-500, SC-600, and SC-700 nanocomposites, respectively. The band gap values are calculated using the relation of Eg ) 1240/λ, where λ is the wavelength of the absorption peak in nanometers. It is well known that nanocrystalline tin oxide has an absorption peak position of around 306 nm (4.05 eV) for bulk nanoparticles.38 However, in the present case, the band gap values are shifted to higher energies (i.e., a blue shift). This band gap shift suggests that the size and shape of the nanoparticles influence the electronic properties of the materials. Similar observations have been reported for boron nitride-functionalized SnO2 nanotubes, where the band gap value was reported as 4.8 eV.39 The edge of the absorption band of SnO2@C and the hexagonal SnO2 nanocrystals are measured at 350 nm (3.54 eV), which is a higher value than the reported values of 298 nm for SnO2 nanocrystals40 and 315 nm for SnO2 nanofibers.12 This further confirms that particle morphology also influences the band gap values.41,42 Figure 4 shows the Raman spectra of SnO2@C nanocomposites. The spectrum has been measured in the wavenumber range of 200-1800 cm-1. The peaks observed in the low wavenumber region from 200 to 1000 cm-1 are assigned to nanocrystalline SnO2. According to group theory,43 the SnO2 structure has a 15 lattice vibrational mode, such as A1g + A2g + A2u + B1g + B2g + 2Bu + Eg + 3Eu, where A1g, B1g, B2g, and Eg are Raman active, and A2u and Eu belong to the transverse optical (TO) vibration) and longitudinal optical (LO) vibration) modes, respectively. The A2g and B1g vibrational modes are attributed to the linear O-Sn-O base and the incipient instability of the lattice. The observed strong peaks at 632 cm-1 for all the samples SnO2@C are assigned to the Raman active modes of A1g. In addition to the above intense peaks (632 cm-1), SC-700 nanocomposite contains two less intense peaks at 475.5 and 776.3 cm-1. The peak at 475.5 cm-1 is attributed to the Eg vibration mode of SnO2.44 The other at 776.3 cm-1 corresponds to the B2g vibration mode of SnO2.44 These Raman bands are in agreement with previous Raman studies and confirm the high crystalline nature of the SnO2 nanocrystals. The existence of carbon in the SnO2@C nanocomposites has been indicated by the Raman spectrum at a higher wavenumber region of 10001800 cm-1. Characteristic carbon peaks are observed at 1356

Figure 4. Raman spectra for SC-500 (a), SC-600 (b), and SC-700 (c).

Figure 5. Photoluminescence spectra of SC-500 (a) and SC-700 (b) nanocomposites.

and 1590 cm-1 for SC-500, 1343 and 1603 cm-1 for SC-600, and 1337 and 1603 cm-1 for SC-700 nanocomposites. This reveals the presence of carbon in the SnO2@C nanocomposites in the form of a disordered structure. It is well known that the band around 1350 cm-1, the D band, is assigned to disordered carbon and the G band at 1590 cm-1 represents the graphitic carbon. The peak at 1590 cm-1 corresponds to an E2g mode of graphite, which is due to the sp2-bonded carbon atoms in a twodimensional hexagonal graphitic layer.35 Similarly, the D band around 1350 cm-1 corroborates the occurrence of defects in the hexagonal graphitic layers. The intensity ratios (ID/IG) of graphitic D and G bands are 1.39, 1.30, and 1.29 for SC-500, SC-600, and SC-700, respectively. It shows that the materials contain disordered amorphous carbon with a high content of lattice defects.45 The intensity ratio (ID/IG) decreases with the increase in the reaction temperature, showing the enhancement of the structural order of disordered carbon.46

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Figure 7. HRTEM image (a) and EDAX patterns (b) of an SC-500 nanocomposite.

Figure 6. TEM images of SC-500 (a), SC-600 (b), and SC-700 (c) nanocomposites. Inset shows the corresponding SAED patterns.

Figure 5 shows the PL spectra of SnO2@C nanocomposites measured at room temperature and excited at 325 nm. The fluorescence spectra show two strong emission peaks at 372 and 386 nm and five weak emission peaks at 421.9, 432, 459.8, 484, and 528 nm. It is known that the emission wavelength of the oxide material depends mainly on the particle’s shape, size, and excitation wavelength. For example, an emission peak at 400 nm (λex ) 300 nm) is observed for 2.8-nm-sized SnO2,47 392 and 439 nm (λex ) 280 nm) for SnO2 nanoribbons,48 399 and 470 nm for SnO2/Sn nanocables,49 and 525 nm for SnO2 nanofibers.12 Using the same excitation wavelength of 325 nm,

we have observed different emission peaks. This is accounted for by arguing that the particle size, particle shape, and defects strongly influence the emission spectrum. The origin of the PL emission is mainly based on the electron transfer made by lattice defects and oxygen vacancies. In addition to the effect of size and shape, the surface effect also plays an important role. When the size of the particles approaches nanosize, the large number of atoms residing on the surface of the particles forms a damaged lattice site with a higher energy state. Hence, the surface will play an important role in quenching the luminescence. The low-resolution TEM images of SC-500, SC-600, and SC700 presented in Figure 6a-c indicate the synthesized particles are nanocrystalline and the average particle size is about 20 nm, irrespective of the reaction temperature. Figure 6a shows the large number of hexagonal crystals together with the octahedral/tetrahedral crystals, with a size range from 15 to 20 nm. The size of the particles increased marginally when the temperature was increased from 500 to 600 °C. The TEM images also reveal that the product has a core shell structure in which the SnO2 particles are the core and the carbon is the shell. The product is therefore marked as a SnO2@C nanocomposite. The size of the composite particles also increases with the increase in the temperature. Interestingly, no particle agglomeration is observed, from which it is inferred that the carbon shell is a hindering agent, preventing the agglomeration of the products, confirming the confinement effect of the particle’s size.50 This effect is an important phenomenon for providing the superior electrochemical activity of the materials during intercalation and deintercalation. The size of the nanocrystals further increases with the increase in the temperature from 600

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Figure 8. Cyclic voltammograms of SC-500 (a), SC-600 (b), and SC-700 (c) nanocomposites at different scan rates. The variation between capacitance and scan rate (d) for all the nanocomposites.

to 700 °C. The average particle size of the nanocrystals is around 25 nm. The SAED (inset Figure 6) image presents the polycrystalline behavior of the materials due to the spot and ring patterns. Figure 7a shows the representative HRTEM image of an SC-500 nanocomposite. It shows that the synthesized particles are perfectly hexagonal in shape and contain a welldefined lattice fringe that indicates the high crystallinity of the materials. The measured interplanar spacing “d” ) 3.33 Å corresponds to the lattice plane of (110). These values are in agreement with the reported value of d110 ) 3.35 Å.51 The EDAX (Figure 7b) measurements carried out on an HRTEM instrument also confirm the presence of Sn, O, and C. Some amount of Cu, which came from the grid, has also been identified. The capacitance behavior of SnO2@C nanocomposites has been studied in a 1 M H2SO4 electrolyte with a potential window of 0-1 V vs Ag/AgCl. Figure 8a-c shows the cyclic voltammetric (CV) responses of the SC-500, SC-600, and SC-700 electrodes with scan rates of 5, 10, 20, 30, 40, 50, and 100 mV/s. The specific capacitance was obtained from the CV curve according to the following equation: Csp ) i/sm, where i is the average cathodic current, s is the potential sweep rate, and m is the mass of each electrode. It can be seen that the shapes of the CV curves are more or less rectangular within the measured potential window, and no redox peaks are observed, except for the SnO2@C-700 nanocomposite. Similarly, the current under curve was slowly increased by increasing the scan rate, which indicates that the voltammetric currents are directly proportional to the scan rate.52 This also reflects the ideal capacitance behavior of the synthesized nanocomposites. The measured specific capacitances were 7.02, 18.8, and 37.8 F/g for SC-500, SC-600, and SC-700, respectively, at the lowest scan rate of 5

mV/s. Similarly, a capacitance of 23.85 F/g was obtained even at a higher scan rate of 100 mV/s for the SC-700 nanocomposite. This high capacitance value, compared with that of other composites, may be due to the increase in crystallinity of the product as a result of the higher reaction temperature of 700 °C. The high crystallinity has been confirmed by the XRD and Raman spectrum. This leads to an increase in the mobility of the charge carriers, which increases the space charge capacitance.30 The large specific surface area of the SC-700 composite compared with that of the other composites may be the cause for getting the best capacitance among the studied systems. Similarly, the observed specific capacitance values are higher than the reported values of SnO2 synthesized by other wet chemical techniques. Hu et al. obtained a specific capacitance of 5.3, 1.8, 0.8, and 0.6 F/g for SnO2‚xH2O heat treated at 150, 200, and 250 °C, respectively, at 25 mV/s in a 0.1 M H2SO4 electrolyte.53 Wu reported specific capacitance value of 5 and 10-16 F/g for untreated and treated SnO2 at 4 mV/s in a 1 M KOH electrolyte. The materials were synthesized by a precipitation technique.54 On the other hand, a maximum specific capacitance value of 285 F/g was reported for electrochemically deposited SnO2 at a scan rate of 10 mV/s in a 0.1 M Na2SO4 electrolyte.29 The high capacitance of this above electrode is due to the higher concentration of the hydroxyl group for the Na2SO4 solution, whose pH is around 7, while our experiments were conducted at a pH ≈ 1. In addition, the electrodeposited SnO2 has a microporous structure, leading to the interaction of the protons from the hydroxide group with the constitutional water and the electrolyte. This wide range of capacitance variation may be due to the different synthetic method, specific resistance, specific surface area, electrode preparation, thickness/

1830 J. Phys. Chem. C, Vol. 112, No. 6, 2008 loading of active materials, hydration number, carbon matrix, the electrolyte and its concentration, and so forth.55 Figure 8d shows the effect of specific capacitance with the scan rate of the three different nanocomposites measured in the potential range of 0-1 mV. It can be observed that the capacitance values decrease with the increase in scan rate. This is the normal behavior of electrochemical systems. Generally, two different mechanisms have been proposed for the charge storage mechanisms of the oxide materials.56 One is the intercalation/deintercalation of protons or alkaline metal cations, which leads to the full utilization of the electrode material. This may be the reason for obtaining a higher specific capacitance at a lower scan rate. The second explanation relates to the surface adsorption process for a higher scan rate. This is based on the diffusion effect of the proton within the electrode materials. Hence, it is believed that part of the surface of electrode materials contributed at a high charging/discharging rate, which decreases the specific capacitance. 4. Conclusions By employing the RAPET technique, three samples of hexagonal-shaped SnO2 nanocrystals and SnO2@C naocomposites have been synthesized. XRD, FT-IR, and Raman spectral studies of the as-prepared samples confirm the formation of SnO2@C. The observed photoluminescence of the nanocomposites is attributed to electron transfer by lattice defects and oxygen vacancies. Whereas TEM studies reveal the hexagonal shape of SnO2@C naocomposites, the high order crystallinity of these materials has been confirmed by HRTEM images. Electrochemical studies show that the material synthesized at 700 °C is a suitable candidate for supercapacitor application as it has a specific capacitance of 37.8 F/g. Acknowledgment. This research was financed by a grant from the Horowitz Foundation for Renewable Energy. References and Notes (1) Chiu, H. C.; Yeh, C. S. J. Phys. Chem. C 2007, 111, 7256. (2) Zhu, J.; Lu, Z.; Aruna, S. T.; Aurbach, D.; Gedanken, A. Chem. Mater. 2000, 12, 2557. (3) Srivastava, D. N.; Chappel, S.; Palchik, O.; Zaban, A.; Gedanken, A. Langmuir 2002, 18, 4160. (4) Cummins, D.; Boschloo, G.; Regan, M.; Corr, D.; Rao, S. N.; Fitzmaurice, D. J. Phys. Chem. B 2000, 104, 11449. (5) He, Y. S.; Cambell, J. C.; Murphy, R. C.; Arendt, M. F.; Swinner, J. S. J. Mater. Res. 1993, 8, 3131. (6) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J.; Liaber, C. M. Nature 2001, 409, 66. (7) Dazhi, W.; Shulin, W.; Jun, C.; Suyuan, Z.; Fangqing, L. Phys. ReV. B 1994, 49, 282. (8) Zheng, Y.; Kalmakor, A.; Lilach, Y.; Maskovits, M. J. Phys. Chem. B 2005, 109, 1928. (9) Prasad, K. R.; Miura, N. Electrochem. Commun. 2004, 6, 849. (10) Miao, Z.; Wu, Y.; Zhang, X.; Liu, Z.; Han, B.; Ding, K.; Au, G. J. Mater. Chem. 2007, 17, 1791. (11) Du, F.; Guo, Z.; Li, G. Mater. Lett. 2005, 59, 2563. (12) Dharmaraj, D.; Kim, C. H.; Kim, K. W.; Suh, E. K. Spectrochim. Acta 2006, 64, 136. (13) Hu, J. Q.; Ma, X. L.; Shang, N. G.; Xie, Z. Y.; Wong, N. B.; Leem, C. S.; Lee, S. T. J. Phys. Chem. B 2002, 106, 3823. (14) Uchiyama, H.; Imai, H. Cryst. Growth Des. 2007, 7, 841. (15) Velasquez, C.; Ojeda, M. L.; Camero, A.; Esparza, J. M.; Rojas, F. Nanotechnology 2006, 17, 3347. (16) Yang, R.; Wang, Z. L. J. Am. Chem. Soc. 2006, 128, 1466. (17) Sun, J. Q.; Wang, J. S.; Wu, X. C.; Zhang, G. S.; Wei, J. Y.; Zhang, S. Q.; Li, H.; Chen, D. R. Cryst. Growth Des. 2006, 6, 1585.

Selvan et al. (18) Moon, T.; Hwang, S. T.; Jung, D. R.; Son, D.; Kim, C.; Kim, J.; Kang, M.; Park, B. J. Phys. Chem. C 2007, 111, 4164. (19) Archer, P. I.; Radovanovic, P. V.; Heald, S. M.; Gamelin, D. R. J. Am. Chem. Soc. 2005, 127, 14479. (20) Felde, U. Z.; Haase, M.; Weller, H. J. Phys. Chem. B 2000, 104, 9388. (21) Mazzone, A. M.; Morandi, V. Physica E 2007, 37, 287. (22) Rani, S.; Roy, S. C.; Bhatnagar, M. C. Sens. Actuators, B 2007, 122, 204. (23) (a) Zheng, J. P.; Jow, T. R. J. Electrochem. Soc. 1995, 142, L6. (b) Kim, H. H.; Kim, K. B. Electrochem. Solid State Lett. 2001, 4, A62. (24) Prasad, K. R.; Miura, N. Electrochem. Commun. 2004, 6, 1004. (25) Kim, H. J.; Park, S. G. Electrochemistry 2001, 69, 848. (26) (a) Yuan, C. Z.; Zhang, X. G.; Wu, Q.; Gao, B. Solid State Ionics 2006, 177, 1237. (b) Wu, M. G.; Gao, J. H.; Zhang, S.; Chen, A. J. Porous Mater. 2006, 13, 407. (c) Wang, Y. G.; Xia, Y. Y. Electrochim. Acta 2006, 51, 3223. (27) Gujar, T. P.; Shinde, V. R.; Lokhande, C. D.; Han, S. H. J. Power Sources 2006, 161, 1479. (28) (a) Cottineau, T.; Toupin, M.; Brousse, T.; Belanger, D. Appl. Phys. A 2006, 82, 599. (b) Wang, S. Y.; Ho, K. C.; Kuo, S. L.; Wu, N. L. J. Electrochem. Soc. 2006, 153, A75. (29) Prasad, K. R.; Miura, N. Electrochem. Commun. 2004, 6, 849. (30) Rao, M. M.; Jeyalakshmi, M.; Reddy, B. R.; Madhavendra, S. S.; Kantam, M. L. Chem. Lett. 2005, 34, 712. (31) Shanmugam, S.; Gedangen, A. J. Phys. Chem. B 2006, 110, 24486. (32) Pol, S. V.; Pol, V. G.; Gedanken, A.; Felner, I.; Sung, M. G.; Asai, S. Inorg. Chem. 2007, 46, 4951. (33) Pol, V. G.; Pol, S. V.; Perkas, N.; Gedanken, A. J. Phys. Chem. C 2007, 111, 134. (34) Shanmugam, S.; Gabashvili, A.; Jacob, D. S.; Yu, J. S.; Gedanken, A. Chem. Mater. 2006, 18, 2275. (35) Shanmugam, S.; Gedanken, A. Electrochem. Commun. 2006, 8, 1099. (36) Miao, Z.; Wu, Y.; Zhang, X.; Liu, Z.; Han, B.; Ding, K.; An, G. J. Mater. Chem. 2007, 17, 1791. (37) Fujihara, S.; Maeda, T.; Ohgi, H.; Hosono, E.; Imai, H.; Kim, S.H. Langmuir 2004, 20, 6476. (38) Lee, J. H.; Ribeiro, C.; Giraldi, T. R.; Longo, E.; Leite, E. R.; Varela, J. A. Appl. Phys. Lett. 2004, 84, 1745. (39) Zhi, C.; Bando, Y.; Tang, C.; Golberg, D. J. Phys. Chem. B 2006, 110, 8548. (40) Gu, F.; Wang, S. F.; Song, C. F.; Lu, M. K.; Qi, Y. X.; Zhou, G. J.; Xu, D.; Yuan, D. R. Chem. Phys. Lett. 2003, 372, 451. (41) Singh, A. K.; Kumar, V.; Note, R.; Kawazoe, Y. Nano Lett. 2006, 6, 920. (42) Tang, Q.; Zhou, W.; Zhang, W.; Ou, S.; Jiang, K.; Yu, W.; Qian, Y. Cryst. Growth Des. 2005, 5, 147. (43) Yu, K. N.; Xiong, Y.; Liu, Y.; Xiong, C. Phys. ReV. B 1997, 55, 2666. (44) (a) Duan, J.; Gong, J.; Huang, H.; Zhao, X.; Cheng, G.; Yu, Z. Z.; Yang, S. Nanotechnology 2007, 18, 055607. (b) Bouaine, A.; Brihi, N.; Schmerber, G.; Bouillet, C. U.; Colis, S.; Dinia, A. J. Phys. Chem. C 2007, 111, 2926. (45) Pol, V. G.; Pol, S. V.; Moreno, J. M. C.; Gedanken, A. Carbon 2006, 44, 3285. (46) Cao, Y. G.; Chen, X. L.; Lan, Y. C.; Li, J. Y.; Xu, Y. P.; Xu, T. Appl. Phys. A 2000, 71, 465. (47) Gu, F.; Wang, S. F.; Lu, M. K.; Zhou, G. J.; Xu, D.; Yuan, D. R. J. Phys. Chem. B 2004, 108, 8119. (48) Hu, J. Q.; Ma, X. L.; Shang, N. G.; Xie, Z. Y.; Wong, N. B.; Lee, C. S.; Lee, S. T. J. Phys. Chem. B 2002, 106, 3823. (49) Wang, B.; Yang, Y. H.; Yang, G. W. Nanotechnology 2006, 17, 4682. (50) Wu, M. S.; Chiang, P. C. J.; Lee, J. T.; Lin, J. C. J. Phys. Chem. B 2005, 109, 23279. (51) Dong, Q.; Su, H.; Zhang, D.; Cao, W.; Wang, N. Langmuir 2007, 23, 8108. (52) Gujar, T. P.; Shinde, V. R.; Lokhande, C. D.; Han, S. H. J. Power Sources 2006, 161, 1479. (53) Hu, C. C.; Wang, C. C.; Chang, K. H. Electrochim. Acta 2007, 52, 2691. (54) Wu, N. L. Mater. Chem. Phys. 2002, 75, 6. (55) Lee, B. L.; Sivakkumar, S. R.; Ko, J. M.; Kim, J. H.; Jo, S. M.; Kim, D. Y. J. Power Sources 2007, 168, 546. (56) Kuratani, K.; Tatsumi, K.; Kuriyama, N. Cryst. Growth Des. 2007, 7, 1375.