Supercapacitance of Solid Carbon Nanofibers Made from Ethanol

Feb 19, 2008 - and Electronic Engineering, School of Mechanical and Aerospace ... UniVersity, Singapore, 639798, Singapore, Department of Physics and ...
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J. Phys. Chem. C 2008, 112, 3612-3618

Supercapacitance of Solid Carbon Nanofibers Made from Ethanol Flames Qiaoliang Bao,†,‡ Shujuan Bao,† Chang Ming Li,*,† Xiang Qi,†,‡ Chunxu Pan,*,‡ Jianfeng Zang,§ Zhisong Lu,† Yibin Li,| Ding Yuan Tang,§ Sam Zhang,| and Keryn Lian⊥ School of Chemical and Biomedical Engineering, Center for AdVanced Bionanosystems, School of Electrical and Electronic Engineering, School of Mechanical and Aerospace Engineering, Nanyang Technological UniVersity, Singapore, 639798, Singapore, Department of Physics and Key Laboratory of Acoustic and Photonic Materials and DeVices of Ministry of Education, Wuhan UniVersity, Wuhan, 430072, China, and Department of Materials Science and Engineering, UniVersity of Toronto, Toronto, Ontario, Canada ReceiVed: October 29, 2007; In Final Form: January 8, 2008

Solid carbon nanofibers (CNFs) made from ethanol flames were used to prepare supercapacitors. Their microstructure, crystallinity, porosity, chemical properties, and electrochemical activity were compared with the multiwalled carbon nanotubes (MWCNTs) synthesized by chemical vapor deposition. The produced CNFs have a unique microstructure with a solid core and porous surface. The specific surface area of CNFs was comparable to that of MWCNTs because of their larger amount of micropores on the surface. The synthesis environment also resulted in abundant functional groups absorbed on the surface of the CNFs. Electrochemical characterization shows that CNFs have much larger capacitance than that of MWCNTs. The capacitance of CNFs consists of both double-layer capacitance contributed by micropores and pseudo-capacitance produced from redox reactions of the absorbed oxygen functional groups. In comparison to the reported MWCNTsbased supercapacitors, the CNF demonstrates more promising potential in energy storage applications because of its larger electrochemical capacitance.

1. Introduction Electrochemical capacitors are attractive energy storage devices because of their high-energy density, high power density, and long cycle life.1 According to the energy storage mechanism, electrochemical capacitors can be classified2 as electrochemical double-layer capacitors based on the separation of electronic and ionic charges at the electrode-electrolyte interface of high specific area and faradic pseudo-capacitors based on fast electrochemical reactions taking place on the electrode. Carbon material due to different allotropes (graphite, diamond, fullerences, nanotubes/nanofibers) and abundant forms (powders, fibers, foams, fabrics, and composites) with high specific surface area is a promising material for electrochemical capacitors.3 During the past few years carbon nanotubes (CNTs) have been formed onto electrodes for supercapacitors4-8 because of their high conductivity, chemical stability, low mass density, and high specific surface area and can be theoretically up to the order of 1000 m2 g-1.9 However, CNTs have shown lower energy densities than porous carbons. It is generally believed that the central canal of multiwalled carbon nanotubes (MWCNTs) could be utilized to expand the surface area so as to support an electrical double layer. Although many methods are developed to increase the specific surface area of CNTs, that is, acid * Corresponding authors. E-mail: (C.M.L.) [email protected]; (C.X.P.) [email protected]. † School of Chemical and Biomedical Engineering, Nanyang Technological University. ‡ Wuhan University. § School of Electrical and Electronic Engineering, Nanyang Technological University. | School of Mechanical and Aerospace Engineering, Nanyang Technological University. ⊥ University of Toronto.

treatments10,11 for disintegration, cutting nanotubes, opening the central canal and modification with functional groups, heat treatment12 and KOH activation,13 etc., it is still difficult to activate the materials with high graphitization degree or develop their microporosity even by the chemical activation.14 In this work, a novel solid carbon nanofiber (CNF) synthesized from ethanol flame,15,16 which processes a distinctive microstructure is used to make an electrochemical capacitor without necessitation of extensive electrode pretreatment for surface activation. The microstructure, crystallinity, porosity, chemical properties, and electrochemical activity of CNFs are thoroughly studied to compare with MWCNTs synthesized by chemical vapor deposition (CVD). The mechanism of the high capacitance of CNF is proposed, and its supercapacitor application is explored. 2. Experimental Section In the experiment, a nanocrystalline Fe layer was electrodeposited on the sampling surface of the substrate by a pulse plating process,17 which was the catalysts for synthesizing solid CNFs in ethanol flames.15,16,18 The as-produced CNFs were then collected and purified by HCl to remove the metal catalysts. The CNFs were very stable in ambient conditions after purification, and no significant further oxidation or “aging” was observed after measurements and storage for more than 1 year. The MWCNTs used in this experiments were commercially purchased, which were produced by CVD and purified by refluxing in nitric acid. The morphology and microstructure of the CNFs and MWCNTs were characterized by using scanning electron microscope (SIRION SEM, FEI, The Netherlands), transmission electron microscope (TEM, JEOL JEM 2010, Japan), and highresolution transmission electron microscope (HRTEM, JEOL

10.1021/jp710420k CCC: $40.75 © 2008 American Chemical Society Published on Web 02/19/2008

Solid Carbon Nanofibers Made from Ethanol Flames

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Figure 1. (a) SEM, (b) TEM and (c,d) high magnification TEM of CNFs from flame.

JEM 2010FEF, Japan). Raman spectra were measured with a confocal Raman microscope system (WITec CRM 200, Germany) excited by a 488 nm Ar-ion laser. The functional groups absorbed on the surface of MWCNTs and CNFs were examined by using a Fourier transform infrared spectroscopy (Nicolet MAGNA-IR 560, FTIR, U.S.). The chemical properties of the functional groups were further determined using X-ray photoelectron spectroscopy (XPS).19 XPS shallow surface measurements were performed by the Kratos AXIS Ultra spectrometer with a monochromatized Al KR X-ray source (1486.71 eV) operated at a reduced power of 150 W (15 kV and 10 mA). The base pressure in the analysis chamber was 2.66 × 10-7 Pa. The core-level spectra were obtained at a photoelectron takeoff angle of 90° measured with respect to the sample surface and were recorded in 0.1 eV step with the pass energy of 40 eV. The binding energy scale of the XPS spectrum was calibrated with the C1s peak (neutral C-C peak at 285.0 eV). Nitrogen adsorption isotherms were measured with an automated gas sorption system (AUTOSORB-1, Quantachrome Instruments) at liquid nitrogen temperature. The specific surface area was calculated using Brunauer-Emmett-Teller (BET) method. Pore size and distribution were calculated by the nonlinear density function theory (NLDFT) method and Barrett-Joyner-Halenda (BJH) method. Electrochemical characterization were performed using a CHI 660B electrochemical workstation (CH Instruments, Austin, TX) in a three-electrode arrangement, including a glassy carbon working-electrode, a platinum counterelectrode, and a 3 M KCl/ Ag/AgCl reference electrode. All potentials were quoted versus the 3 M KCl/Ag/AgCl reference electrode. The electrode preparation was similar to that in ref 20. CNFs and MWCNTs with the same mass (5 mg) were dispersed in deionized water (15 mL) followed by high-shear mixing and sonication to get a uniform suspension. Glassy carbon electrodes were then modified by a 5 µL drop of CNFs or MWCNTs suspension and dried in air. Next, uniform films containing a network of CNFs or MWCNTs were formed due to self-adhesion. Then, the cyclic

Figure 2. Typical microstructure of CNFs from flame. (a) TEM image of a helical CNF. (b-d) HRTEM images of the dotted area in in panel a.

voltammograms (CV) and galvanostatic charge/discharge tests were carried out in 1 M H2SO4. 3. Results and Discussion Generally, the CNT is a filament with a center canal, while the CNF has no central channel or is just a solid-cored fiber, regardless of the graphitic wall structures.15,16 Both they are considered to be promising one-dimensional nanomaterials for energy storage applications. However, their inherent microstructure has a profound effect on observed electrochemical behavior such as capacitance and electron-transfer rates.21 Therefore, it is of importance to distinguish accurately the particular type and structural features of the carbon nanomate-

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Figure 3. (a) Raman and (b) FTIR spectra of CNFs from flame and MWCNTs by CVD.

Figure 4. XPS spectra of MWCNTs by CVD and CNFs from flame. (a) Wide scan spectra. (b) Curve fitting of the C1s spectrum of MWCNTs. (c) Curve fitting of the C1s spectrum of CNFs.

rials. In our previous work, we controlled the particular type of carbon nanomaterials by using Ni- or Fe-based substrates.15,16 High yield of solid CNFs has been achieved from ethanol flames on the nanocrystalline Fe-based substrates. The detail micropore structure and surface functional groups are not thoroughly studied although its surface morphology is reported.16,22 Figure 1a shows SEM morphology of as-grown CNFs on the Fe-based substrate. Figure 1b shows typical TEM morphology of purified CNFs. It is found that the product is a compound of nonhelical and helical CNFs, which are both totally solid. The high magnification TEM images in Figure 1c,d further demonstrate the solid structure that has coarse surface with discernible grooves. The HRTEM characterization in Figure 2 (also in Figure S2, Supporting Information) reveals that the CNFs consist

of disordered graphite layers inside and there is a layer of porous carbon with plenty of micropores in the outer wall, which is apparently different from that of MWCNTs. The porous carbon layer has a similar morphology as reported in refs 23 and 24, which has been demonstrated to have anomalous capacitance contribution. The outside porous carbon structure also indicates that CNFs have a lot of edge sites on the surface, which are supposed to be chemically active sites.25 Figure 3a shows the Raman spectra of CNFs of MWCNTs. The two main modes D at 1347/1357 cm-1 and G at 1566/ 1596 cm-1 are observed. Generally, G peak corresponds to the tangential stretching (E2g) mode of highly oriented pyrolytic graphite, which indicates the presence of crystalline graphitic carbon in the carbon nanomaterials, while D peak represents

Solid Carbon Nanofibers Made from Ethanol Flames TABLE 1: Relative Concentrations of Functional Components Obtained from Curve Fitting the C1s XPS Spectra of CNFs and MWCNTs functional groups C-C (285.0 eV) C-OH/C-O-C (286.6 eV) >CdO/C-Cl (287.7 eV) COOH (289.5 eV)

MWCNTs by CVD CNFs from flame 68.0% 5.8% 7.4% 6.3%

56.8% 22.6% 10.6% 4.6%

the disorder-induced feature due to the finite particle size effect or lattice distortion.26,27 The ratio of the intensity of D peak (ID) to G peak (IG) is also used to measure the amount of disorder in the carbon nanostructure. It is found that the intensity of disordered D-band and the ID/IG ratio of CNFs (ID/IG ) 1.15) are much larger than those of MWCNTs (ID/IG ) 0.75), which further reveals that CNFs are highly disordered graphite structure with increased number of edge plane sites on which presumably more functional groups present. This can significantly increase the capacitance. The FTIR spectra in Figure 3b show the existence of functional groups, such as carbonyl (>CdO), carboxyl (-COOH), and alkyl (-C-H). Clearly, for CNFs several strong peaks in the FTIR spectrum confirm the stretching vibration of >CdO (1750 cm-1), -C-O (1190 cm-1), and -C-H (2950 cm-1, 2870 cm-1).28-30 The bending deformation of O-H at 1380 cm-1 in carboxylic acids is also assigned. However, these peaks are relatively weak for MWCNTs even though the sample has been purified by nitric acid.31 XPS analysis was applied to determine the chemical constituents of the functional groups, as shown in Figure 4. Wide

J. Phys. Chem. C, Vol. 112, No. 10, 2008 3615 scan spectra of MWCNTs and CNFs in Figure 4a show distinct carbon and oxygen peaks, representing the major constituents of the sample surfaces. The intensity of C1s peaks has been normalized to the same scale and serves as reference.32 As can be seen from the spectra, the intensity of the O1s peak of CNFs is much higher than that of MWCNTs, indicating higher oxygenrelated contents. It is associated with the abundant functional groups absorbed on the surface of CNFs. Figure 4b,c, shows the curve fittings of high-resolution C1s spectra of MWCNTs and CNFs. The C1s spectra have been resolved into five individual component peaks representing C-C (285.0 eV), C-OH/C-O-C (286.6 eV), >CdO/C-Cl (287.7 eV), -COOH (289.5 eV), and shakeup satellite peaks (291.7 eV) due to π-π* transitions in aromatic rings.32-40 Table 1 lists the quantitative peak analysis of C1s peak region by calculating the integrated peak area. These data further support the conclusion that CNFs have much higher concentrations of oxygen-related functional groups. The curve fittings of high-resolution O1s spectra of MWCNTs and CNFs (Figure S3, Supporting Information) further quantitatively demonstrate the presence of these functional groups: >CdO (530.5 eV), and C-OH/C-O-C (531.9 eV). The results discussed above can be explained by the different structure characteristics of CNFs and MWCNTs (Supporting Information, Figure S4). There are plenty of edge-plane sites that are chemically active centers on the sidewall surface of CNFs, while only tube ends and defect sites of MWCNTs are chemically active.25,41-44 As discussed before,30,45 the O2 molecule can be dissociatively adsorbed at the vacancy sites of

Figure 5. (a) Nitrogen adsorption/desorption isotherms of CNFs from flame. (b) Nitrogen adsorption/desorption isotherms of MWCNTs by CVD. (c) Histogram of pore diameter distribution by NLDFT method. (d) Pore size distributions calculated from desorption branch by BJH method.

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Figure 6. Electrochemical characterization of CNFs from flame and MWCNTs by CVD. (a) CV curves for MWCNTs with various scan rates. (b) CV curves for CNFs with various scan rates. (c) CV curves for CNFs and MWCNTs at scan rate of 200 mV s-1. (d) Chronopotentiograms of CNFs and MWCNTs.

graphite exothermally without an energy barrier and a selfsustaining chain reaction with oxygen can take place. In addition to the synthesis environment in the open air, the functional groups are apt to form and absorb on the sidewall of CNFs. Therefore, it is ascertained that not only the unique synthesis environment but also the novel structure of the CNFs is responsible for the abundant functional groups absorbed at those reactive sites. Figure 5a,b shows nitrogen sorption isotherms of the CNFs and MWCNTs. The hysteresis loops suggest typical features of the micropores, that is, the absorbed volume increases at low pressure and saturates up to 0.8 P/P0.11 The total pore volume of the CNFs with diameter less than 347.55 nm at P/P0 ) 0.99445 is 0.30 cm3 g-1. The total pore volume of MWCNTs with diameter less than 357.24 nm at P/P0 ) 0.99460 is 1.79 cm3 g-1. Although the total volume of the CNFs is much smaller than that of MWCNTs, the multipoint BET surface area of the CNFs is equivalent to that of MWCNTs, which are 149.40 and 148.40 m2 g-1, respectively. The average pore diameter of the CNFs is about 8.05 nm, which is 40 nm smaller than that of MWCNTs (48.12 nm). The result suggests that the solid CNFs exhibit a larger amount of small size pores than MWCNTs, which contribute to the comparative high surface area. In addition, the increase of the specific surface area is also contributed from the number of edge plane sites, which is shown by comparing their Raman Spectra. Figure 5c,d illustrates the pore diameter distribution of CNFs and MWCNTs by using the NLDFT and BJH methods. The histogram of the pore diameter distribution calculated by NLDFT method clearly shows the micropores with the diameter

TABLE 2: Physical Characteristics and Electrochemical Properties of CNFs and MWCNTs MWCNTs CNFs by CVD from flame multipoint BET surface (m2 g-1) pore volume (cm3 g-1) average pore diameter (nm) BJH method desorption pore diameter (nm) NLDFT pore volume (ccg-1) NLDFT pore width (nm) specific capacitance (Cg, Fg-1)

148.40 1.79 48.12 1.02 0.76 28.75 32.40

149.40 0.30 8.05 0.91 0.17 1.18 73.20

between 1.3 and 1.6 nm in CNFs. In contrast, the porosity of MWCNTs shows indistinctive diameter distribution. The pore size distributions calculated from desorption branch by BJH method show that CNFs exhibit a complex pore system that consists of not only the particular pores around 3.7 nm but also a large amount of micropores with the diameter smaller than 2 nm. In comparison to the regular CNTs synthesized using CVD or arc discharge, the CNFs exhibit distinct features, that is, totally disordered graphite layers in the sidewall, much smaller pores including a large amount of micropores, and an abundance of chemical groups absorbed at those edge sites on the outer wall. These unique properties should be mainly introduced from the synthesizing process:30 (1) synthesizing in atmosphere where oxygen and nitrogen are abundant; (2) unstable supply of carbon sources; (3) thermal nonuniformity in flame without protection from airflow. Figure 6a,b shows two sets of CV curves measured at scan rates of 10, 50, 100, and 200 mV s-1 for MWCNTs (Figure

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6a) and CNFs (Figure 6b), respectively, both presenting a pair of electrochemical redox peaks around 0.3/0.4 V, which is a signature of pseudocapacitance.7,46 The linear relationship of the plots of cathodic peak currents versus the scan rates (Supporting Information, Figure S5) indicates that surface redox reactions occur, further demonstrating its pseudocapacitance nature. As the experimental results have shown above, there are an abundance of oxygen functional groups bound to the surface of the CNFs and MWCNTs, and the redox peaks are possibly due to the following reported surface redox reactions: 3,5

>C-OH S >C ) O + H++ e+

-

-COOH S -COO + H + e -

> C ) O + e S >C-O

-

(1) (2) (3)

Figure 6c compares CVs of the CNFs and MWCNTs at scan rate of 200 mV s-1. Obviously, the CV curve of the CNFs covers a much larger area than that of MWCNTs, indicating that CNFs have significantly larger specific capacitance. Figure 6d shows the galvanostatic charge-discharge behavior of the CNFs and MWCNTs electrodes with an applied current of 20 µA in a potential range between 0 and +1.0 V in a 1.0 M H2SO4 solution. Both curves are very symmetric to their corresponding discharge counterparts, which indicates that both CNFs and MWCNTs possess excellent capacitive characteristics or a strongly reversible oxidation reaction.5,8,47,48 Therefore, the capacitance are further determined from the dc discharge with a 1 V potential window of capacitor device. The specific capacitance (Cg) could be estimated by the following equation: 47

Cg ) [(i dt/dV)]/m ) i/[(dV/dt)m]

(4)

where i is the applied current and m is the mass of each electrode material. The calculated specific capacitance (Cg) of CNFs and MWCNTs is 73.20 and 32.40 Fg-1, respectively. The capacitance of CNFs is more than two times larger than that of MWCNTs. The capacitance value of CNFs made from the ethanol flame without excessive surface activation and/or processing prior to electrochemical experiments is comparable with that (20∼65 Fg-1) of CNFs after chemical activation reported in literatures.14,49 The detailed physical characteristics and electrochemical properties of these two kinds of electrode materials are listed in Table 2. In our work, the solid CNFs exhibit much higher electrochemical capacitance than the regular MWCNTs. One of the reasons is that the much higher surface concentrations of oxygen functional groups on CNFs can significantly increase the Faradic current for pseudocapacitance.50 It has been reported that the oxygen functional groups influence the capacitor performance, suchaswettability,chemicalreactivityandelectricalproperties3-5,50,51 and the CO-type oxygen groups provide a positive contribution to the capacitance.50,52 In addition, although a higher internal surface area of a porous electrode could give a higher electrochemical rate per unit apparent surface area of electrode, the internal area cannot, in general, be completely utilized at high discharge rates because of internal pore effect on mass transfer and ohmic polarization in electrolyte.53 The specific double layer capacitance is contributed by the ionic double layer at the electrode/electrolyte interface and the accessibility of the active layer is explicitly related to the diffusion of solvated ions and the pore size distribution during charge-discharge process. The smaller pores, especially micropores, are more effective

for solvated ions in aqueous electrolytes to access to electrode/ electrolyte interface electrochemically.3,4 Even though the MWCNTs have the central hollow core that is also accessible for the double layer charging beside the pores formed by entanglement,3,5,54 the mesopores with the diameter in the range of 2-50 nm have lower efficiency while being accessed by solvated ions of aqueous electrolytes.55 By contrast, CNFs without an inside canal have a large amount of micropores less than 2 nm on the surface that are more effective for solvated ions to access, and their smaller pores allow closer approach of the ion center to the electrode surface, which can also lead to enhanced double layer capacitance.24 Thus, although both CNF and MWCNT have identical specific surface area, the superior pore structure and pore distributions of CNF over MWCNT can also produce much larger electrochemical capacitance. 4. Summary In conclusion, the solid CNFs synthesized from ethanol flames exhibit much larger capacitance than regular MWCNTs from CVD, because of its porous surface with abundant micropores and plenty of functional groups at the edge sites of the outer wall. A large amount of micropores less than 2 nm also contribute to the larger capacitance of CNFs. The results demonstrate that the CNF is more promising as a supercapacitor than the MWCNT to be used in energy storage applications without necessitation for extensive electrode pretreatment of surface activation. Acknowledgment. This work was jointly supported by the Foundation for the Author of National Excellent Doctoral Dissertation of People’s Republic of China (FANEDD No. 200233) as well as the Center for Advanced Bionanosystems of Nanyang Technological University, Singapore. SupportingInformationAvailable: TEMimageofMWCNTs, HRTEM image of nonhelical CNF, curve fittings of highresolution O1s XPS spectra of MWCNTs and CNFs, schematic model of MWCNT and CNF with absorbed functional groups, plots of cathodic peak current versus the scan rates. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Conway, B. E. J. Electrochem. Soc. 1991, 138, 1539. (2) Sarangapani, S.; Tilak, B. V.; Chen, C. P. J. Electrochem. Soc. 1996, 143, 3791. (3) Frackowiak, E.; Beguin, F. Carbon 2001, 39, 937. (4) Niu, C. M.; Sichel, E. K.; Hoch, R.; Moy, D.; Tennent, H. Appl. Phys. Lett. 1997, 70, 1480. (5) Frackowiak, E.; Metenier, K.; Bertagna, V.; Beguin, F. Appl. Phys. Lett. 2000, 77, 2421. (6) Chen, J. H.; Li, W. Z.; Wang, D. Z.; Yang, S. X.; Wen, J. G.; Ren, Z. F. Carbon 2002, 40, 1193. (7) Chen, Q. L.; Xue, K. H.; Shen, W.; Tao, F. F.; Yin, S. Y.; Xu, W. Electrochim. Acta 2004, 49, 4157. (8) Du, C. S.; Yeh, J.; Pan, N. Nanotechnology 2005, 16, 350. (9) Peigney, A.; Laurent, C.; Flahaut, E.; Bacsa, R. R.; Rousset, A. Carbon 2001, 39, 507. (10) Li, C. S.; Wang, D. Z.; Wu, J. J.; Lu, W. Z.; Liang, J. J. Inorg. Mater. 2003, 18, 1010. (11) Kim, H. J.; Jeon, K. K.; An, K. H.; Kim, C.; Heo, J. G.; Lim, S. C.; Bae, D. J.; Lee, Y. H. AdV. Mater. 2003, 15, 1757. (12) An, K. H.; Kim, W. S.; Park, Y. S.; Choi, Y. C.; Lee, S. M.; Chung, D. C.; Bae, D. J.; Lim, S. C.; Lee, Y. H. AdV. Mater. 2001, 13, 497. (13) Raymundo-Pinero, E.; Cazorla-Amoros, D.; Linares-Solano, A.; Delpeux, S.; Frackowiak, E.; Szostak, K.; Beguin, F. Carbon 2002, 40, 1614. (14) Yoon, S. H.; Lim, S.; Song, Y.; Ota, Y.; Qiao, W. M.; Tanaka, A.; Mochida, I. Carbon 2004, 42, 1723. (15) Pan, C. X.; Liu, Y. L.; Cao, F.; Wang, J. B.; Ren, Y. Y. Micron 2004, 35, 461.

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