Carbon Nanotube Nanocomposite Synthesized in

Pacific Northwest National Laboratory, 902 Battelle BouleVard, P.O. Box 999, Richland, ... fuel cells through increased utilization of platinum.8d It ...
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J. Phys. Chem. B 2005, 109, 14410-14415

Platinum/Carbon Nanotube Nanocomposite Synthesized in Supercritical Fluid as Electrocatalysts for Low-Temperature Fuel Cells Yuehe Lin* and Xiaoli Cui† Pacific Northwest National Laboratory, 902 Battelle BouleVard, P.O. Box 999, Richland, Washington 99352

Clive Yen and Chien M. Wai* Department of Chemistry, UniVersity of Idaho, Moscow, Idaho 83844 ReceiVed: March 21, 2005; In Final Form: May 9, 2005

Carbon nanotube (CNT)-supported Pt nanoparticle catalysts have been synthesized in supercritical carbon dioxide (scCO2) using platinum(II) acetylacetonate as metal precursor. The structure of the catalysts has been characterized with transmission electron micrograph (TEM) and X-ray photoelectron spectroscopy (XPS). TEM images show that the platinum particles’ size is in the range of 5-10 nm. XPS analysis indicates the presence of zero-valence platinum. The Pt-CNT exhibited high catalytic activity both for methanol oxidation and oxygen reduction reaction. The higher catalytic activity has been attributed to the large surface area of carbon nanotubes and the decrease in the overpotential for methanol oxidation and oxygen reduction reaction. Cyclic voltammetric measurements at different scan rates showed that the oxygen reduction reaction at the Pt-CNT electrode is a diffusion-controlled process. Analysis of the electrode kinetics using Tafel plot suggests that Pt-CNT from scCO2 provides a strong electrocatalytic activity for oxygen reduction reaction. For the methanol oxidation reaction, a high ratio of forward anodic peak current to reverse anodic peak current was observed at room temperature, which implies good oxidation of methanol to carbon dioxide on the Pt-CNT electrode. This work demonstrates that Pt-CNT nanocomposites synthesized in supercritical carbon dioxide are effective electrocatalysts for low-temperature fuel cells.

Introduction Direct methanol fuel cells (DMFCs) are considered to be one of the most promising options for addressing future energy needs.1 DMFCs provide a clean and mobile power source with high-energy conversion efficiency and low pollutant emissions. The basic principle employed in DMFCs involves methanol oxidation and oxygen reduction over precious metal catalysts, such as platinum and platinum-ruthenium alloy dispersed over a carbon support. There are some obstacles inhibiting the applications of DMFCs; one of the main problems is the low catalytic activity of electrodes both for oxygen reduction reaction (ORR) and for methanol oxidation reaction. Both the carbon support and the dispersion of catalysts are two main concerns to enhance the catalytic activity. Carbon black (Vulcan XC-72, a registered trade name from CABOT) has been the most widely used support for preparing fuel cell catalyst because of its good compromise between electronic conductivity and surface area.2 However, to increase or improve activity, a new carbon support with a high surface area may provide better utilization of the electrocatalysts. Other carbon types such as carbon tubule membranes,3 ordered porous carbon,4 graphite nanofibers,5 films of C60 clusters,6a hard carbon spherules,7 and carbon nanotubes (CNT) have been used as supports8 for DMFCs. In particular, CNT have attracted special attention because of their excellent catalytic and electronic properties and extensive applications in many areas, for example, * Authors to whom correspondence should be addressed. Tel: 509-3760529; fax: 509-376-5106; e-mail: [email protected] (Y.L.). Tel: 208885-6787; fax: (208) 885-6173; e-mail: [email protected] (C.M.W.). † Permanent address: Department of Materials Science, Fudan Univesity, Shanghai, 200433, China. E-mail: [email protected].

in chemical sensors,9 for hydrogen storage,10 and as a new type of support for heterogeneous catalysts.11 Previous studies have shown that multiwalled carbon nanotubes exhibit better performance than carbon black for the electrooxidation of methanol.8a Metal particles supported on CNT appear to be less susceptible to carbon monoxide poisoning than traditional catalyst systems. The enhancement of activity in ORR was also observed on CNT electrodes12 and after modification with Pt13 and Pd nanoparticles.14 Yan et al. used CNT as a platinum support for proton exchange membrane fuel cells as a way to reduce the cost of fuel cells through increased utilization of platinum.8d It is also reported that CNT with 12 wt % Pt deposition can give 10% higher voltage than carbon black with 29% Pt deposition in polymer electrolyte fuel cells.8c Thus, CNT show great potential for use in designing electrodes for miniature fuel cells. It is well-known that the particle shape and size, as well as dispersion, of Pt-based catalysts are key factors that determine their ORR activity and cell performance for DMFCs. Several approaches, including impregnation and chemical reduction2a,2c,8b,8e,15 and electrodeposition,6,8d,8g,8h have been developed to load Pt or other metal catalyst particles on the surface of supports. Platinum nanocatalysts supported on Vulcan XC-72 carbon have been synthesized through the reduction of chloroplatinic acid with formic acid, using surfactant tetraoctylammonium bromide as the stabilizer in the solvent tetrahydrofuran.2c Through chemical reduction in H2 at 580 °C, Che et al. were able to fill the very narrow size Pt-Ru alloy particles in CNT membranes.3 Liu et al.8e loaded Pt nanoparticles on the CNT by an electroless plating method through two-step sensitization and activation processes. However, conventional preparation techniques based on wet impregnation and chemical reduction

10.1021/jp0514675 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/12/2005

Platinum/Carbon Nanotube Nanocomposite of the metal precursors often do not provide adequate control of particle shape and size. In addition, these procedures can be time-consuming and labor-intensive. Consequently, continuing efforts are underway to develop alternative synthesis methods to generate colloids and clusters on the nanoscale with greater uniformity. Electrodeposition of platinum particles has the advantage of high-purity deposits and a simple deposition procedure.6,8d,8g,8h One of the problems, however, is the likely concurrent reduction of H+ during the electrodeposition process. The loading mass of the metallic catalyst is not easy to estimate according to the deposition charge. The current efficiency for Pt deposition is not equal to 100%.6e The synthesis of nanoscale particles with good dispersion over the carbon as electrocatalytic materials still remains a challenge. In recent years, the use of supercritical fluids (SCFs) for the synthesis and processing of nanomaterials has proven to be a rapid, direct, and clean approach to develop nanomaterials and nanocomposites.11c,16 In a recent paper, platinum/carbon aerogel nanocomposites were synthesized using a supercritical deposition method.17 SCFs are ideal solvents to synthesize and process many types of nanomaterials, including nanoparticales, nanocrystals, nanotubes, nanowires, and nanocomposites. The application of supercritical fluid technology can result in products (and processes) that are cleaner, less expensive, and of higher quality than those that are produced using conventional technologies and solvents. Through hydrogen reduction of metalβ-diketone complexes in supercritical carbon dioxide (scCO2), CNT can be decorated by metal nanoparticles such as Pd16g,16j and Rh16f with uniformity to achieve nanocomposites.11c,16h In this paper, platinum nanoparticles were decorated on CNT surfaces in scCO2 and were characterized by transmission electron micrograph (TEM) and X-ray photoelectron spectroscopy (XPS). The Pt-CNT powder was loaded on the glassy carbon (GC) electrode through a casting process, and the electrocatalytic activity for methanol oxidation and oxygen reduction was investigated at room temperature using cyclic voltammetry (CV) and chronoamperometry (CA). Experimental Section Reagents. Multiwalled carbon nanotubes (>95% purity, diameter 20∼50 nm, length 1∼5 µm) were purchased from NanoLab, Inc. (Newton, MA). Nafion-perfluorinated ionexchange resin (5 wt % solution) was purchased from Aldrich, and H2SO4 was purchased from Fischer Chemicals. Ultrapure water (∼18.3 MΩ‚cm) was used to prepare the solutions. Platinum precursor, platinum(II) acetylacetonate, Pt(acac)2, 97%, purchased from Aldrich, was used as received. High-purity hydrogen, carbon dioxide, nitrogen, and oxygen gas were used in all experiments. A pure oxygen flow was introduced into the solution to supply oxygen for the oxygen reduction experiments and was passed over the top of the solution. All measurements were conducted at room temperature. Decorating Platinum Nanoparticles on Carbon Nanotubes. The supercritical fluid reaction system for the deposition of Pt nanoparticle on CNT was described in a previous report.16f The Pt nanoparticles were synthesized using the following procedures. The CNT (20 mg) and the metal precursor Pt(acac)2 (50 mg) with a small amount of methanol as a modifier were loaded into a high-pressure reaction cell located in an oven heated to 200 °C. CO2 gas was introduced into the reaction cell and pressurized to 80 bar to produce a supercritical fluid. H2 gas at 10 bar was initially in the H2 + CO2 mixer cell and was then added to the CO2 gas of 120 bar. After 1 h, when the precursor was completely dissolved in the scCO2, the H2 + CO2 gas was introduced into the reaction cell by pressurizing it to 160 bar.

J. Phys. Chem. B, Vol. 109, No. 30, 2005 14411 The reduction of the Pt2+ to Pt0 was fast, occurring within only ∼15 min. After the reaction cell was depressurized, Pt-CNT powder was recovered, washed with methanol, and sonicated five times. Electrode Preparation and Modification. A 0.5 wt % Nafion solution was prepared by diluting the 5 wt % Nafion solution with water. Catalyst powder was dispersed ultrasonically in the 0.5 wt % Nafion solution to obtain a homogeneous black suspension solution with 1 mg/mL Pt-CNT, and a 5-µL aliquot of this solution was pipetted onto the surface of a 3-mmdiameter glassy carbon (BAS, West Lafayette, IN) electrode. Before the surface modification, the GC electrode was polished with 0.3-µm and 0.05-µm alumina slurries, washed with water and acetone, and then subjected to ultrasonic agitation for 1 min in ultrapure water and dried under an air stream. The coating was dried at room temperature in the air for 1 h. The modified electrode surface was then washed carefully with ultrapure water before measurement. Apparatus. The TEM images of the decorated CNT were taken using a JEOL JEM 2010 microscope equipped with an Oxford ISIS system. The operating voltage on the microscope was 200 keV. All images were digitally recorded with a slow-scan CCD camera (image size 1024 × 1024 pixels), and image processing was carried out using a Digital Micrograph (Gatan). To obtain TEM images, the as-synthesized platinum-modified CNT powder was dispersed in ethanol solution under ultrasonic agitation for 1 min and then was deposited on a copper-carbon grid. For the XPS analysis, a Kratos AXIS 165 multitechnique electron spectrometer was used to confirm the presence of zerovalence platinum. The loading of Pt is measured by energydispersive X-ray spectroscopy. The instrumentation is LEO SUPRA 35VP (FESEN). Cyclic voltammetric and chronoamperometry experiments were performed with a CHI 660 electrochemical workstation (CH Instruments Inc, Austin, TX). All experiments were conducted in a conventional three-electrode system at room temperature. The working electrode was GC coated with PtCNT composite films. A Ag/AgCl (saturated by KCl solution) reference electrode was used for all electrochemical measurements, and all the potentials were reported versus this reference electrode. A platinum wire was used as a counter electrode. To obtain reproducible and reliable results, a fresh methanol solution was used in every measurement. Results and Discussion Characterization of Carbon Nanotubes Decorated with Platinum Nanoparticles. Figure 1 shows the typical TEM image of the CNT decorated with platinum nanoparticles synthesized in scCO2. Nanoparticles of platinum with a size of 5∼10 nm can be observed clearly on the surface of CNT with relative uniformity. The loading platinum on the surface of CNT was 25%, which was estimated by energy-dispersive X-ray spectroscopy. The chemical composition of the platinum nanoparticles deposited onto functionalized CNT was also analyzed by XPS. Figure 2 displays a typical XPS spectrum of CNT coated with platinum nanoparticles. The binding energy for a metallic Pt 4f7/2 peak is 71.2 eV, according to the literature.18 Our sample has a Pt 4f7/2 peak of 71.1 eV, which is very close to that value. We also find a small shoulder peak near 72-73 eV, which may correspond to the platinum oxides. The spectra shown in Figure 2 indicate that most of the platinum particles in our catalyst are zero-valence. Characterization of Platinum from Cyclic Voltammetry. Cyclic voltammogramms for Pt-CNT electrodes at different

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Figure 1. A typical TEM image of CNT decorated by platinum nanoparticles synthesized in supercritical carbon dioxide.

Figure 3. Cyclic voltammogram in 0.5 M H2SO4 solution saturated by N2 for Pt-CNT electrode at 20 mV/s. (A) short potential range between -0.2 and 0.6 V and (B) long potential range between -0.22 and 1.3 V.

Figure 2. XPS spectra of CNT decorated with platinum nanoparticles.

scan ranges in 0.5 M H2SO4 solution saturated by nitrogen are presented in Figure 3. Fine structures of hydrogen absorption/ desorption peaks clearly appeared (Figure 3A). A reduction peak centered at 0.45 V can be observed during the negative-going potential sweep (Figure 3B). This reduction peak can be attributed to the reduction of platinum oxide. This feature of the curve is consistent with those of the cyclic voltammogramm curves for Pt electrodes.19 Thus, it may also be concluded that the platinum nanoparticles have a very clean active surface. This is further evidence of the presence of platinum on the electrode surface. Electrocatalytic Activity of Oxygen Reduction Reaction. The ORR is especially important for realizing highly efficient fuel cells, batteries, and many other electrode applications. Platinum and platinum alloy particles on a variety of carbon supports are the most widely used and efficient catalysts for the fuel cell cathode.13,20 For the oxygen reduction experiments with the Pt-CNT electrode, a solution of 0.1 M H2SO4 was purged with ultrapure oxygen for ∼15 min. The solution became completely saturated with oxygen. The electrode was scanned over a potential range from 0.7 V to 0 V, involving five cycles at different scan rates

Figure 4. Typical cyclic voltammograms of Pt-CNT electrode for oxygen reduction reaction in 0.1 M H2SO4 saturated with oxygen at 20 mV/s. A larger reduction current can be observed in the first cycle. The cyclic voltammograms at other scan rates have a similar phenomenon.

to ensure reproducibility. Figure 4 shows typical cyclic voltammograms for the ORR at Pt-CNT electrodes in a solution of 0.1 M H2SO4 saturated by oxygen. It is interesting that the first cycle always involves a larger reduction current at all the scan

Platinum/Carbon Nanotube Nanocomposite

Figure 5. Cyclic voltammograms at a fresh Pt-CNT electrode for oxygen reduction reaction at various scan rates, 0.01, 0.02, 0.04, 0.06, 0.08, and 0.10 V/s, from inside to outside. The electrolyte was 0.1 M H2SO4 saturated with oxygen. The scan involved five cycles at each scan rate, and the fifth cycle (last cycle) is shown here. The dependence of peak current on scan rates is shown in the inset.

rates and becomes stable from the second cycle. Other electrodes such as Pd-CNT on glassy carbon surface also experience this phenomenon, which may be related to the absorption of oxygen on the electrode surface and to the reduction of platinum oxide which is formed when the working electrode is polarized to the starting potential. More experimental studies are necessary to give suitable explanations for this phenomenon. The cathodic current flowing during the reduction of oxygen should also contain the cathodic current of the reduction of platinum oxide. It is impossible to separate these two contributions accurately. However, the cathodic limiting currents of oxygen reduction under present conditions were much larger than the current of platinum oxide reduction, which has been confirmed from the cathodic current in nitrogen-saturated solution. Thus, it is reasonable to conclude that the major contribution of the cathodic current in oxygen-saturated solution resulted from oxygen reduction. Figure 5 shows the cyclic voltammograms of the Pt-CNT electrode at different scan rates. The scan was performed with successive five cycles to obtain the stable response and the last cycle (fifth) is shown in Figure 5. The peak current increases linearly with the square root of the scan rates as shown in the Figure 5 inset. This fact indicates that the ORR process on PtCNT is controlled by the diffusion of oxygen to the electrode surface. Similar results were observed on a glassy carbon electrode modified by Pd-CNT nanocomposite14 and a hybrid thin film containing platinum nanoparticles and [tetrakis(Nmethylpyridyl)porphyrinato]cobalt modified CNT on a glassy carbon electrode surface fabricated by Dong and colleagues.13 A Tafel plot was recorded from 1.0 V to 0 V in 0.1 M H2SO4 solution saturated by oxygen. A plot of log i versus potential for a fresh Pt-CNT electrode is shown in Figure 6. The exchange current density can be obtained by extrapolating the linear region to zero overpotential. We find the Tafel slope for Pt/CNT to be -21 mV/decade in the potential range from 0.63 to 0.76 V. The resulting Tafel slope and exchange current density are listed in Table 1. A Tafel slope of -38 mV/decade at OTE/SWCNT/Pt was reported by Kamat et al.8h The exchange current density on Pt-CNT electrode is about 1 order larger than that of commercial Pt/C even though the loading of catalysts is much less in our system.12a The difference between GC/Pt-CNT and OTE/SWCNT/Pt may be caused by the

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Figure 6. Tafel polarization curve for Pt-CNT electrode in 0.1 M H2SO4 solution saturated by oxygen at 1 mV/s.

different loading of platinum, different carbon nanotubes, or different methods of preparing catalysts and electrodes. Electrocatalytic Activity for Methanol Oxidation Reaction. Cyclic voltammetry is a valuable and convenient tool for studying methanol oxidation catalysts. The electrocatalytic activity for methanol oxidation of Pt-CNT prepared in scCO2 was characterized by cyclic voltammetry in an electrolyte of 1 M H2SO4 and 2 M CH3OH at 50 mV/s, and the resulting voltammograms are shown in Figure 7. The current from methanol oxidation becomes apparent as the potential rises above 0.35 V. In the forward scan, methanol oxidation produced a prominent symmetric anodic peak around 0.70 V. In the reverse scan, an anodic peak appeared at around 0.53 V. This anodic peak in the reverse scan could be attributed to the removal of the incompletely oxidized carbonaceous species formed in the forward scan.2a The reaction mechanism of electrooxidation methanol on the surface of platinum is a complex one which involves many carbonaceous species as intermediates. There are several different versions of the reaction mechanism. However, it is generally agreed that the most abundant surface intermediate is chemisorbed carbon monoxide.20c This feature of the cyclic voltammetric curve is in agreement with the reports for Pt/C catalysts.2a,2c The ratio of the forward anodic peak current (If) to the reverse anodic peak current (Ib) can be used to describe the catalyst tolerance to carbonaceous species accumulation.2a,2c A high If/Ib value implies good oxidation of methanol to CO2. In our experiments, the ratio was estimated to be 1.4 for the Pt-CNT electrode from the first cycle and 1.6 and 1.3 for the second and third cycles. Such a high value indicates that most of the intermediate carbonaceous species were oxidized to CO2 in the forward scan. For comparison, the ratio 0.87 was reported with a nanosized Pt on XC-72 synthesized by a microwave-assisted polyol process.2a These experimental results highlighted the high activity for methanol oxidation of Pt-CNT prepared from scCO2. It also implies that the major deficiency of all Pt catalysts, that is, the accumulation of intermediate carbonaceous species on the catalysts’ surface leading to “catalyst poisoning”, can be partly overcome using this novel Pt-CNT in designing fuel cell electrodes. The high activity may be a result of the high surface of CNT and the nanostructure of platinum particles. The oxidative current increases with repetitive scans in the initial period and reaches a plateau after three cycles (Figure 7). In contrast, the peak current in the reverse scan increases with the

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TABLE 1: Electrochemical Parameters for Oxygen Reduction Reaction at Pt-CNT Electrodes electrodes

slope/mV

exchange current/A/cm2

refs

-21

8.9e-7 1.09e-7 6.3e-4

this work 12a 8h

mg/cm2

Pt/CNT (25% Pt/CNT, 0.07 ) Pt/C/Nafion (commercial, 20% Pt/C, 0.4 mg/cm2) OTE/SWCNT/Pt

cycle, which reflects the accumulation of intermediate carbonaceous species on the catalysts’ surface. The effect of potential scan limit on the reverse anodic peak current is shown in Figure 8. This feature is consistent with that reported for Pt/C.2a As shown in Figure 8, the reverse anodic peak current decreases with increasing the anodic limit in the forward scan. This behavior also indicates that the reverse anodic peak current is primarily associated with residual carbon species on the surface of the Pt-CNT electrode. It is reasonable that the If/Ib ratio increases with the anodic limit. Chronoamperometric curves were measured at different potentials 0.3 (a), 0.6 (b), and 0.8 V (c) at the Pt-CNT electrode for 10 min, as shown in Figure 9. For this experiment, the potential was stepped from the open-circuit potential (∼0.3 V) to 0.8 V. After 2 s, the potential was stepped to 0.3 V for 2 s, then stepped to the desired potential, and the current-time curve

-38

was recorded. As shown, the largest current was observed at 0.6 V, and the current decay with time was observed. This result is in agreement with its behavior in the cyclic voltammogram. DMFCs Based on Electrocatalysts from ScCO2. The use of scCO2 can thus provide an environmentally sound alternative to other conventional solvents. ScCO2 has been used in many areas, including material cleaning, natural product extraction, chemical reactions, sample preparation, and environmental remediation. It is also possible to prepare catalysts in SCFs applied to the actual miniaturized fuel cells. The results above show that Pt-CNT synthesized in scCO2 have a high activity both for methanol oxidation and oxygen reduction. The experimental conditions differ significantly from those used in an actual fuel cell. The experiments in this study were carried out at room temperature, while commercial DMFCs are operated at 40-100 °C. The catalyst loadings on the electrode surface do not match those typically used for DMFCs, which resulted in the smaller current density. However, the high If/Ib value for methanol oxidation indicated the advantages of the catalysts prepared in scCO2 and the use of carbon nanotubes. Furthermore, many metal precursors can be used as starting materials for the nanoparticle production in scCO2. A better electrocatalytic performance of bimetallic alloy Pt-Ru on the surface of CNT synthesized in scCO2 is expected for the methanol oxidation reaction. More detailed studies are underway in this laboratory, and the results will be reported in due course.

Figure 7. Cyclic voltammograms of room-temperature methanol oxidation on Pt-CNT electrode cycled between potentials 0 V to 1.0 V vs Ag/AgCl at 50 mV/s in 1 M H2SO4, 2 M CH3OH. Results for cycles 1-5 (from downside to upside) correspond to successive scans showing the stabilization of the current peak.

Figure 9. Current-time curves at 0.3, 0.6, and 0.8 V for the PtCNT electrode in 1 M H2SO4, 2 M CH3OH.

Conclusions

Figure 8. Cyclic voltammograms of room-temperature methanol oxidation on Pt-CNT electrode at 50 mV/s in 1 M H2SO4, 2 M CH3OH for different forward potential scan limits.

Using a supercritical fluid technique, we have successfully deposited platinum nanoparticles on carbon nanotube surfaces. This approach provides a new way to develop catalysts with nanostructure and uniformity. Platinum deposited on carbon nanotubes in supercritical fluids has been shown to possess a higher catalytic activity, both for methanol oxidation and for oxygen reduction reaction. The higher catalytic activity has been attributed to the larger surface area of carbon nanotubes and the decrease in the overpotential for methanol oxidation and oxygen reduction reaction. The results presented in this paper demonstrated the use of scCO2 to be an efficient way to prepare

Platinum/Carbon Nanotube Nanocomposite electrocatalysts and the feasibility of using carbon nanotube based electrocatalysts for the development of low-temperature fuel cells. Acknowledgment. This work is supported by a laboratory directed research and development program at Pacific Northwest National Laboratory (PNNL) and a grant from the Electricity Innovation Institute and the Electric Power Research Institute (E2-P261/C8273). The research described in this paper was performed partially at the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the U.S. Department of Energy’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the U.S. Department of Energy under Contract DE-AC05-76RL01830. The authors would like to thank Ms. Sue Gano (PNNL) for editing the manuscript. References and Notes (1) (a) Arico, A. S.; Srinivasan, S.; Antonucci, V. Fuel Cells 2001, 1, 133. (b) Chan, K. Y.; Ding, J.; Ren, J. W.; Cheng, S. A.; Tsang, K. Y. J. Mater. Chem. 2004, 14, 505. (2) (a) Liu, Z.; Ling, X. Y.; Su, X.; Lee, J. Y. J. Phys. Chem. B 2004, 108, 8234. (b) Lu, Q.; Yang, B.; Zhuang, L.; Lu, J. J. Phys. Chem. B 2005, 109, 1715. (c) Prabhuram, J.; Wang, X.; Hui, C. L.; Hsing, I.-M. J. Phys. Chem. B 2003, 107, 11057. (d) Moore, J. T.; Corn, J. D.; Chu, D.; Jiang, R.; Boxall, D. L.; Kenik, E. A.; Lukehart, C. M. Chem. Mater. 2003, 15, 3320. (e) Lizcano-Valbuena, W. H.; Azevedo, D. C.; Gonzalez, E. R. Electrochim. Acta 2004, 49, 1289. (3) Che, G. L.; Lakshmi, B. B.; Fisher, E. R.; Martin, C. R. Nature 1998, 393, 346. (4) Chai, G. S.; Yoon, S. B.; Yu, J.-S.; Choi, J.-H.; Sung, Y.-E. J. Phys. Chem. B 2004, 108, 7074. (5) (a) Bessel, C. A.; Laubernds, K.; Rodriguez, N. M.; Baker, R. T. K. J. Phys. Chem. B 2001, 105, 1115. (b) Steigerwalt, E. S.; Deluga, G. A.; Cliffel, D. E.; Lukehart, C. M. J. Phys. Chem. B 2001, 105, 8097. (c) Steigerwalt, E. S.; Deluga, G. A.; Lukehart, C. M. J. Phys. Chem. B 2002, 106, 760. (6) (a) Vinodgopal, K.; Haria, M.; Meisel, D.; Kamat, P. Nano Lett. 2004, 4, 415. (b) Wei, Z. D.; Chan, S. H. J. Electroanal. Chem. 2004, 569, 23. (c) Antoine, O.; Durand, R. Electrochem. Solid-State Lett. 2001, 4, A55. (d) Chen, A.; La Russa, D. J.; Miller, B. Langmuir 2004, 20, 9695.(e) He, Z.; Chen, J.; Liu, D.; Tang, H.; Deng, W.; Kuang, Y. Mater. Chem. Phys. 2004, 85, 396. (7) Yang, R.; Qiu, X.; Zhang, H.; Li, J.; Zhu, W.; Wang, Z.; Huang, X.; Chen, L. Carbon 2005, 43, 11. (8) (a) Li, W.; Liang, C.; Qiu, J.; Zhou, W.; Han, H.; Wei, Z.; Sun, G.; Xin, Q. Carbon 2002, 40, 791. (b) Li, W.; Liang, C.; Zhou, W.; Qiu, J.; Zhou, Z. H.; Sun, G.; Xin, Q. J. Phys. Chem. B 2003, 107, 6292. (c) Matsumoto, T.; Komatsu, T.; Arai, K.; Yamazaki, T.; Kijima, M.; Shimizu, H.; Takasawa, Y.; Nakamura, J. Chem. Commun. 2004, 840. (d) Wang, C.;

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