Synthesis and Application of Pt Nanocrystals with Controlled

Sep 23, 2009 - As an example for possible applications, it is demonstrated that the electrocatalytic activity of the prepared sample for the oxygen re...
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J. Phys. Chem. C 2009, 113, 18115–18120

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Synthesis and Application of Pt Nanocrystals with Controlled Crystallographic Planes Bao Yu Xia,† Jian Nong Wang,*,‡ and Xiao Xia Wang‡ Shanghai Key Laboratory for Laser Processing and Materials Modification, School of Materials Science and Engineering, Shanghai Jiao Tong UniVersity, 800 Dong Chuan Road, Shanghai 200240, People’s Republic of China and Shanghai Key Laboratory for Metallic Functional Materials, Key laboratory for AdVanced CiVil Engineering Materials (Ministry of Education), School of Materials Science and Engineering, Tongji UniVersity, 1239 Siping Road, Shanghai 200092, People’s Republic of China ReceiVed: July 16, 2009; ReVised Manuscript ReceiVed: August 25, 2009

Pt nanocrystals play a key role in many catalytic reactions. However, direct synthesis of such crystals with crystallographic plane control and thus enhanced catalytic activity is still a great challenge. In this work, we report a facile route for synthesizing Pt nanocrystals supported on a graphitic carbon and mediating the growth by addition of a small amount of Fe ions. It is shown that with changing Fe addition, the density ratio of (100) to (111) planes can be changed over a wide range. As an example for possible applications, it is demonstrated that the electrocatalytic activity of the prepared sample for the oxygen reduction reaction is mainly determined by this density ratio, and appears to be independent of the Pt crystal size over the observed range of 2-5 nm. That is, the exposure of populous (100) planes results in enhanced activity. The present synthesis with crystallographic plane control is achieved without the addition of a capping agent or surfactant, which is beneficial for practical applications. It is suggested that the present approach may be applied for preparing any catalysts wherein controlling the crystallographic planes is critical. 1. Introduction Platinum plays an important role in a wide variety of applications. For example, it serves as an excellent catalyst for partial oxidation, hydrogenation, and dehydrogenation of many industrial processes.1 In the technology of fuel cells, platinum acts as the most effective electrocatalyst for the oxygen reduction reaction (ORR) and fuel (including hydrogen, methanol, ethanol, and formic acid) oxidation reaction.2 As the demand for Pt and its price grows, we must find ways to reduce the amount of Pt used in a specific application by increasing its catalytic activity in order to lower the overall cost. It is now known that the catalytic and electrocatalytic activities depend not only on the size of the Pt crystals3 but also on their crystallographic plane. Studies have shown that by altering the crystallographic plane of a bulk single crystal, one can manipulate the catalytic properties of a Pt-based catalyst.4 For instance, the hexagonal {111} surface of Pt was found to be 5× more active than that of the square {100} surface for the dehydrocyclization of n-heptane, whereas the {100} surface was shown to be more active than the {111} surface for the isomerization of isobutene.5 Studies on the structural effect of Pt single crystal electrodes on formic acid oxidation revealed that formic acid oxidation to CO2 proceeded favorably on (111) planes with significantly less CO poisoning compared to that of other planes, such as (100) and (110).6 It was also found that compared to (111) and (110) planes, (100) showed higher activity, whereas (111) showed the least activity toward the C-C bond cleavage involved during ethanol oxidation.7 In the case of ORR, in the nonadsorbing electrolyte HClO4, the activity of ORR decreased in the order of (110) > (111) > (100), due to the decreased interaction strength between O2 and the * To whom correspondence should be addressed. Tel: 86-21-6598 2867. Fax: 86-21-6598 5385. E-mail: [email protected]. † Shanghai Jiao Tong University. ‡ Tongji University.

different surface structures and increased adsorption of the OH species on these surfaces.8 In comparison, for the adsorbing H2SO4 electrolyte, the activity of ORR decreased in the order of (111) < (100) < (110), as determined by the adsorption and inhibiting effect of bisulfate anion.8 The shape of Pt particles was found in recent years to affect the catalytic activity.9 Thus, great effort has been made to synthesize shape-controlled Pt particles by varying the nucleation and growth rates of Pt clusters.10 This is because through shape-controlled synthesis, it is possible to maneuver the shape of a nanocrystal to expose only a specific set of crystallographic planes. For example, a tetrahedron is bounded by {111} facets and a cube is covered by {100} facets, whereas a cuboctahedron is enclosed by a mix of {100} and {111} facets. For an fcc single crystal, the surface energies associated with the low-index crystallographic planes are in the order of (111) < (100) < (110).11 One can alter the surface energies and thus growth rates by using various capping agents in a shape-controlled synthesis. In general, Pt nanocrystals of different shapes, such as tubes,12 cubes,13 nanowires/rods,14 prisms,15 and urchin16 were recently synthesized in the presence of a capping agent or surfactant17 via reduction of a Pt precursor, decomposition of an organometallic complex, or a combination of these two routes, such as hydrogenated decomposition of Pt(acac)2.9a,18 One of the major limitations of these approaches is the utilization of strong capping molecules or surfactants, which in most cases hinder or even prevent catalysis by blocking some of their active sites and by inducing steric effects.9a Efficiently removing the surface ligands while keeping the morphology of the nanoparticles remains a big challenge. Furthermore, the catalytic efficiency is not improved in terms of mass activity since the Pt particles have large sizes (in most cases, >7 nm) and/or high aspect ratios as compared with a conventional spherical particle.13b,19 Thus, new approaches for synthesizing

10.1021/jp906734d CCC: $40.75  2009 American Chemical Society Published on Web 09/23/2009

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Pt nanoparticles with controlled shapes and improved catalytic activity are still needed. In this work, a new method is proposed for controlling the crystallographic planes of Pt nanocrystals supported on a graphitic carbon. This is achieved by the addition of a trace amount of Fe ions in the reaction solution, in which no capping agent/surfactant is included. The prepared catalyst shows apparent improvement in electrocatalytic activity over the traditional one. 2. Experimental Section 2.1. Materials. Dihydrogen hexachlorplatinate (H2PtCl6•6H2O, 99.9%) was purchased from the Shanghai Chemical Reagent Research Institute. Silver nitrate (AgNO3, 99.8%) and ethylene glycol (99.0%, EG) were obtained from the Shanghai Lingfeng Chemical Reagent Co., Ltd. Ferric chlcride (99.0%, FeCl3) was purchased from the Sinopharm Chemical Reagent Co., Ltd. Other solvents were of analytical grade and used without further purification. 2.2. Preparation of Pt/Carbon Catalysts. The typical procedure for preparing the Pt catalyst started with a carbon nanaocage (CNC) powder (30 mg) which was ultrasonicated in EG to form a dispersing solution. Then, a predetermined amount of Pt (30 mg) precursor H2PtCl6•6H2O dissolved in EG was slowly added. In order to introduce iron ions, a given amount of ferric chloride (1M, FeCl3) was added into the mixed solution of 120 mL. The volume of the added FeCl3 ranged from 0.1 to 1 mL. The mixed solution was heated at the desired temperature of around 140 °C for 3 h under continuous magnetic stirring. After having been cooled to room temperature, the catalyst was then filtered and washed with excess deionized water until chloride ions were not detected by AgNO3 solution. The Pt loading on the CNC support was controlled to be 45 wt.%. Such catalysts prepared with and without iron additions are denoted as Pt/CNC-Fe and Pt/CNC, respectively. 2.3. Characterization. X-ray diffraction (XRD) experiments were performed to study the crystallization of the prepared catalysts. The XRD patterns were recorded using a Bruker diffractometer with Cu KR radiation (Bruker D8 Advanced, 40 kV and 40 mA). The microstructure and morphology of the catalysts were studied by transmission electron microscopy (TEM) and high resolution TEM (HRTEM) (JEOL-2010F, 200 kV). Selected area electron diffraction (SAED) was performed to study the crystallinity and structure of the Pt nanoparticles. Energy dispersive X-ray spectroscopy (EDX) was carried out to analyze the chemical compositions of the selected area. The weight ratio of Pt to CNC for each catalyst was controlled by stoichiometric calculation and confirmed by EDX measurements. 2.4. Measurement of Electrochemical Activity. The electrochemical activities of the two catalysts (Pt/CNC, Pt/CNCFe) were characterized by the cyclic voltammetry (CV) technique. The experiments were performed in a three-electrode cell using an EG&G potentiostat (Model 366A) at ambient temperature. The working electrode was Pt/CNC and 5% Nafion mixed in ethanol and coated on a glassy carbon cylinder with a diameter of 3 mm. The amount of catalyst loading on the electrode was controlled to be 0.4 mg cm-2. A saturated calomel electrode (SCE) and a large-area Pt plate were used as the reference electrode and counter electrode, respectively, and 0.5 M H2SO4 or 1 M CH3OH + 0.5 M H2SO4 was used as the electrolyte. The CV profiles were recorded at a scan rate of 100 mV s-1 from the potential of -0.241 to 1 V vs SCE.

Figure 1. XRD patterns of Pt/CNC catalysts prepared with different FeCl3 additions.

TABLE 1: Results from XRD and Electrochemical Measurement for Samples with Different Additions of FeCl3 added volume of FeCl3 (mL)

0

0.1

0.2

0.5

1

intensity ratio of (100)/(111) 19.6% 37.2% 40.4% 38.3% 29.1% Pt particle size (nm) 1.8 3.0 3.6 4.0 4.6 electroactive surface area 56.3 65.5 72.5 62.3 60.9 (m2 g-1)

3. Results The synthesis process and characteristics of the support material of CNCs have been reported in our previous publications.20 The CNCs have a hollow interior and graphitic shell. Their sizes are generally in the range of 20 to 50 nm. See Figure S1 in Supporting Information (SM). The constituent crystalline phases and structures of the Pt nanoparticles deposited on CNCs were examined by means of XRD analysis, and the results are shown in Figure 1. The peak at 2θ ) 26° is associated to the (002) diffraction of graphitic CNCs, and the peaks at around 39.7°, 46.3°, and 67.4° can be assigned to (111), (200), and (220) planes of Pt, respectively. At the same time, the intensities of Pt peaks become stronger with the increase of iron addition, especially for the diffraction of (200) planes. The intensity ratio of (100)/(111) for each sample can be obtained from the corresponding XRD pattern. Such data are included in Table 1. The intensity ratio appears to increase with increasing Fe addition up to 0.2 mL and then decrease with more additions. It should be noted that the intensity of a peak actually means the integrated value obtained from the area covered by the diffraction. However, the height of a peak also gives a rough indication for the strength of the diffraction. This can be seen in Figure 1. Here, we have considered the intensity ratio rather than the intensity value so that normalized numbers are more effective.9a,21 The average crystallite sizes of Pt supported on CNCs were calculated based on the Pt(220) peak from Scherrer’s equation:

d ) 0.9λ/Bcosθ,

(1)

where d is the average particle size (nm), λ the wavelength of X-ray radiation, and B the width (in radius) of the diffraction peak at the half height. The results presented in Table 1 suggest that the Pt particle size increases with iron addition. The samples with and without 0.2 mL addition of FeCl3 were selected for TEM studies. Typical images of these two samples are shown in Figure 2(a,b). As can be seen, Pt particles are highly dispersed on CNC surfaces. By counting hundreds of Pt nanoparticles on representative TEM photographs, the size distributions are shown in Figure 2(c,d) for the two samples

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Figure 2. Typical TEM images of Pt/CNC (a) and Pt/CNC-Fe (b) and Pt size distribution histograms measured for Pt/CNC (c) and Pt/CNC-Fe (d).

examined. It is evident that the average sizes for Pt/CNC and Pt/CNC-Fe samples are 2.3 and 4.3 nm, respectively, and these sizes are in close agreement with those estimated by eq 1. The crystallinity and chemical composition of Pt nanoparticles in the Pt/CNC-Fe sample were further examined, and the results are shown in Figure S2 (of the Supporting Information) and Figure 3. The SAED pattern illustrates four rings which could be indexed to the diffractions of {111}, {200}, {220}, and {311} planes (Figure S2 of the Supporting Information). EDX analysis was conducted in many areas. A typical result is shown in Figure 3(a,b). Except for Pt and C, no peak of Fe was found, suggesting that no detectable iron was left in the sample. Figure 3(c,d) illustrates the morphologies of some Pt crystals outlined by white dashes. Many Pt crystals are found to be of faceted type and show rectangular or cubic morphologies with some exhibiting hexagonal morphologies. The lattice spacing between (100) planes is about 0.195 nm, which is in agreement with that for bulk Pt as shown in Figure 3c.The atomic images of the Pt crystals marked 1 and 2 in Figure 3d are shown in Figure 3, parts e and f, respectively. Detailed inspection shows that the cuboctahedrons may be enclosed by both {100} and {111} facets. The potential application of the as-prepared Pt/CNC and Pt/ CNC-Fe catalysts is examined for the case of their electrochemical performance in fuel cells. Figure 4a shows the stabilized CVs obtained for the catalysts prepared with different iron additions at a sweep rate of 100 mV s-1 in 0.5 M H2SO4. The CVs, which contain well-defined hydrogen adsorption and desorption peaks, show that the Pt/CNC-Fe catalysts have better catalytic performance than the Pt/CNC catalyst. Figure 4b illustrates the magnified profiles of hydrogen desorption peaks for all samples. In the potential range from -0.241 to 0.10 V, there are three hydrogen desorption peaks at about -0.1, -0.04, and 0.01 V, corresponding to the catalytic effects of (111), (110),

and (100) planes of Pt, respectively.22 The Pt/CNC-Fe catalysts with different Fe additions show higher current peaks at all of these three voltages. The increase in current density at 0.01 V is especially obvious. CVs are also used to estimate the electroactive surface area, which could truly reflect the catalytic activity of different Pt catalysts. The electroactive surface area R (m2g-1) for a catalyst can be estimated from the following equation:23

R ) Q/(mβ),

(2)

where Q is the charge for hydrogen desorption (mC cm-2), m the quantity of Pt used () 0.4 mg cm-2 in the present study), and β the charge required to oxidize a monolayer of H2 on bright Pt () 0.21 mC cm-2). The Q value for each catalyst can be calculated from its cyclic voltamogram. In doing this, the contribution of the charge from the electric double layer to the overall amount of charge exchanged during the electro-adsorption/desorption of H2 on Pt sites was deducted. Such calculated values of effective R for different catalysts are listed in Table 1. As can be seen, the R value increases with the addition of iron into the reaction solution. However, when the addition of FeCl3 is more than 0.2 mL, the catalytic activity decreases, indicating an unfavorable effect of excess iron ions. The Pt/ CNC-Fe (0.2 mL) catalyst demonstrates the strongest hydrogen desorption/adsorption peak and thus the largest R value (72.5 m2g-1). This value is apparently larger than that for the catalyst without the addition of Fe (56.3 m2g-1). The advantage of the Pt/CNC-Fe catalyst over Pt/CNC persists even after a long period of testing (Figure S3of the Supporting Information). Electrocatalytic activities of the catalysts of Pt/CNC and Pt/ CNC-Fe (0.2 mL) were also measured in a CH3OH + H2SO4 solution between -0.241 and 1 V with a sweep rate of 100

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Figure 3. (a) TEM image of Pt/CNC-Fe (0.2 mL) and (b) EDS of the circled region in (a). (c),(d) HRTEM images of some regions of the sample. (e),(f) Enlarged HRTEM images of the Pt crystals 1 and 2 in (d), respectively. Crystal 3 in (d) also has exposed crystallographic planes of (111) and (100).

mV s-1 (Figure 4c). It is illustrated that the Pt/CNC-Fe catalyst also has an excellent catalytic activity on methanol electrooxidation. It is noteworthy that the methanol oxidation curves are the ones after 2000 cycles. In other words, the stability and durability of the catalytic performance of Pt/CNC-Fe is also better than Pt/CNC in methanol. 4. Discussion The first important observation of the present work is that with the addition of Fe ions in the synthesis process, the Pt nanocrystals contain more (100) planes on their surfaces. The direct evidence for this observation comes from the CV curves (Figure 4b). The curves show the increase of the current density at 0.01 V, which results from the fact that more (100) planes are exposed on the surfaces of Pt nanocrystals.13c,24 This is because the voltammetry data concern surface sensitive reactions and are thus a direct measure of the area of the exposed planes. The enrichment of (100) planes on Pt surfaces concluded from electrochemistry is supported by the XRD patterns exhibiting the apparent increase of the diffraction intensity of (100) planes (Figure 1). The intensity ratio does not give direct information

about the surface of the nanoparticle or the exposed planes. However, there has been experimental observation that when the Pt crystal shape was controlled to be cubic with most exposed planes being (100), the corresponding XRD curve showed dominant (100) diffraction with very weak diffractions from other planes.13d,e TEM imaging further verifies the wide presence of Pt crystals with (100) exposed planes (Figure 3). The second important observation is that the electrocatalytic activity of the prepared catalyst is mainly determined by the density ratio of (100) and (111) planes. As can be seen from Table 1, with the addition of Fe and alternation of the density ratio, the electroactive surface area changes accordingly. Such change is observed regardless of the change of the particle size of Pt in the range of 2 to 5 nm. This is the first systematic demonstration that the catalytic activity of Pt crystal may be independent of size, at least in the present size range of 2 to 5 nm (Table 1). Previous control of the Pt crystal shape was mostly aimed at having cubic crystals with most exposed planes being (100).13d,e The difference in electrocatalytic activity between different catalysts may be attributed to the structure sensitivity of (bi)

Synthesis and Application of Pt Nanocrystals

Figure 4. (a) Cyclic voltamograms of different catalysts in 0.5 M H2SO4 solution, (b) the enlarged part of H desorption in (a), (c) cyclic voltamograms of Pt/CNC and Pt/CNC-Fe catalysts in 0.5 M H2SO4 + 1 M CH3OH solution. The potential sweep rate was 100 mV s-1.

sulfate anion adsorption rate on (100) and (111) facets and its inhibiting effect.25 Sulfate ions tend to bind to the Pt atoms on (111) facets in solution and impede the ORR. The (110) and (100) facets have more active sites than (111) in H2SO4. Thus, the catalyst with a high density ratio of (100)/(111) shows a high electrocatalytic activity. Nevertheless, it should be noted that the advantage of such catalyst may be lost for a catalytic reaction taking place favorably on (111) facets. This is the reason why the crystallographic planes should be controlled. The third important observation is that the present synthesis with the crystallographic planes controlled is achieved without addition of a capping agent or a surfactant. EDX shows that iron cannot be detected on the surface of the Pt nanocrystals (Figure 3b), indicating that the trace amount of iron added is only involved in the formation of nuclei. Capping agents used in the preparation procedure have deleterious effects on the catalytic application, as they induce passivation of catalytic surfaces.9c There have been reports that surfactants would interact strongly with Pt nanoparticles, blocking the contact between the catalyst and reactant.26 The blocking would decrease the active sites on the catalytic surface and result in a low catalytic activity. Furthermore, the as-prepared Pt particles with a capping agent/surfactant usually need to be deposited on a carbon support.27 However, the presence of the capping agent is not conductive to the anchoring of Pt deposited on the support.26b This is a disadvantage for catalytic activity, stability, and durability during the continued operation. The Pt catalysts

J. Phys. Chem. C, Vol. 113, No. 42, 2009 18119 become less useful unless further treatments are applied to remove the surfactants, which make the synthesis more complex.28 So far, the best way to remove the surface surfactants is thermal annealing.29 The annealing, however, would lead to aggregation of Pt nanoparticles and destroy the nanostructure of the catalyst.30 Therefore, the present catalyst, with the crystallographic planes controlled without inclusion of a capping agent or surfactant, has advantages over previous ones in terms of practical application. In general, Pt crystal structures expose the most stable planes such as {100}, {110}, or {111} in any particular synthesis condition. However, the exposed plane could be selected by modulating the growth rates of different crystal planes.31 Since Pt nanocrystals have different electronic structures and atomic arrangements for various planes, Fe ions may absorb onto these surfaces differently. The preferred adsorption of Fe ions onto (111) surfaces over (100) surfaces should result in different growth rates. The solute atoms of Pt would more likely attach to those less protected (100) surfaces, leading to an anisotropic growth. It has been reported that Pt4+ is reduced by EG in two steps: 4+ Pt f Pt2+ f Pt0.32 This process is too fast to induce anisotropic growth, although the presence of oxygen could slow the reduction rate. However, when Fe ions are added into the reaction mixture, they could reduce the supersaturation of Pt atoms by the following reaction: 2Fe3+ + Pt0 f 2Fe2+ + Pt2+. That is, Fe3+ species could oxidize both Pt atoms and nuclei back to Pt2+ species and thus significantly slow down the overall reduction rate. As a result, anisotropic growth could take place at a slow reduction rate. However, when excessive Fe ions are added, they could cover all of the Pt planes, such as (100), (110), and (111). In this case, anisotropic growth becomes less important or even destructive, leading to fewer (100) planes exposed on Pt surfaces. 4. Conclusions IThe crystallographic planes of Pt nanocrystals supported on CNCs can be controlled with the addition of a small amount of Fe ions. The electrocatalytic activity of the prepared catalyst for ORR is mainly determined by the density ratio of (100) and (111) planes, and appears to be independent of the Pt crystal size. The exposure of popular (100) planes is favorable for ORR. The present synthesis with crystallographic plane control is achieved without the addition of a capping agent or surfactant, which is beneficial for practical applications. Acknowledgment. J.N.W. is thankful for research funding from The National Natural Science Foundation of China (Project No. 50871067) and The Ministry of Science and Technology of China (National 863 Project of 2007AA05Z128). Supporting Information Available: HRTEM image of CNC supports, SEAD pattern of Pt/CNC-Fe catalysts, and electroactive surface area (EAS) of a catalyst calculated for the testing up to 2000 cycles. This material is available free of charge via the Internet at http://pubs.acs.org or from the authors. References and Notes (1) (a) Lipkowski, J. Ross, P. N., Electrocatalysis; Wiley-VCH: New York, 1998; (b) Ertl, I. G. Handbook of Heterogeneous Catalysis; WileyVCH: Weinheim, 2008. (2) (a) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal. B 2005, 56, 9. (b) Xu, C. W.; Wang, H.; Shen, P. K.; Jiang, S. P. AdV. Mater. 2007, 19, 4256. (c) Jensen, B. W.; Jensen, O. W.; Forsyth, M.; MacFarlane, D. R. Science 2008, 321, 671.

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(3) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (4) Somorjai, G. A. Chemistry in Two Dimensions: Surfaces; Cornell University Press: Ithaca, NY, 1981. (5) Somorjai, G. A.; Zaera, F. J. Phys. Chem. 1982, 86, 3070. (6) (a) Adzic, R. R.; Tripkovic, A. V.; O’Grady, W. E. Nature 1982, 296, 137. (b) Tripkovic, A. V.; Popovic, K. D.; Lovic, J. D. J. Serb. Chem. Soc. 2003, 68, 849. (7) Xia, X. H.; Liess, H.-D.; Iwasita, T. J. Electroanal. Chem. 1997, 437, 233. (8) Markovic, N. M.; Adzic, R. R.; Cahan, B. D.; Yeager, E. B. J. Electroanal. Chem. 1994, 377, 249. (9) (a) Subhramannia, M.; Pillai, V. K. J. Mater. Chem. 2008, 18, 5858. (b) Tian, N.; Zhou, Z. Y.; Sun, S. G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732. (c) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343. (d) Feldheim, D. L. Sicence 2007, 316, 699. (10) Njoki, P. N.; Luo, J.; Zhong, L. Langmuir 2005, 21, 1623. (b) Gu, Y. J.; Wong, W. T. Langmuir 2006, 22, 11447. (c) Wu, G.; Li, D. Y.; Dai, C. S.; Wang, D. L.; Li, N. Langmuir 2008, 24, 3566. (d) Miyazaki, A.; Balint, I.; Nakano, Y. J. Nanopart. Res. 2003, 5, 69. (e) Ferreria, P. J.; Shao-Horn, Y. Electrochem. Solid State Lett. 2007, 10, b60. (11) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (12) Kijima, T.; Yoshimura, T.; Uota, M.; Ikeda, T.; Fujikawa, D.; Mouri, S.; Uoyama, S. Angew. Chem., Int. Ed. 2004, 43, 228. (13) (a) Petroski, J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 3316. (b) Ren, J. T.; Tilley, R. D. J. Am. Chem. Soc. 2007, 129, 3287. (c) Yamada, M.; Kon, S.; Miyake, M. Chem. Lett. 2005, 34, 1050. (d) Wang, C.; Daimon, H.; Lee, Y.; Kim, J.; Sun, S. J. Am. Chem. Soc. 2007, 129, 6974. (e) Wang, C.; Daimon, H.; Onodera, T.; Koda, T.; Sun, S. Angew. Chem., Int. Ed. 2008, 47, 3588. (14) (a) Teng, X. W.; Yang, H. Nano Lett. 2005, 5, 885. (b) Zhong, X.; Feng, Y.; Lieberwirth, I.; Knoll, W. Chem. Mater. 2006, 18, 2468. (c) Fenske, D.; Borchert, H.; Kehres, J.; Kro¨ger, R.; Parisi, J.; Kolny-Olesiak, J. Langmuir 2008, 24, 9011. (d) Chen, J; Xiong, Y; Yin, Y.; Xia, Y. Small 2006, 2, 1340. (e) Liu, Z.; Sakamoto, Y.; Ohsuna, T.; Hiraga, K.; Terasaki, O.; Ko, C. H.; Shin, H. J.; Ryoo, R. Angew. Chem., Int. Ed. 2000, 39, 3107. (f) Shen, Z.; Yamada, M.; Miyake, M. Chem. Commun. 2007, 3, 245. (15) Lim, B.; Wang, J.; Camargo, P. H. C.; Jiang, M.; Kim, M. J.; Xia, Y. N. Nano Lett. 2008, 8, 2535. (16) Wang, L.; Guo, S. J.; Zhai, J. F.; Dong, S. J. J. Phys. Chem. C 2008, 112, 13372. (17) (a) Chen, W. F.; Huang, H. Y.; Lien, C. H.; Kuo, P. L. J. Phys. Chem. B 2006, 110, 9822. (b) Susut, C.; Nguyen, T. D.; Chapman, G. B.; Tong, Y. Y. J. Cluster Sci. 2007, 18, 773. (c) Park, J. J.; Soon, G. K.; Jang, Y. J.; Hyeon, T. Angew. Chem., Int. Ed. 2007, 46, 4630. (18) (a) Chen, J.; Lim, B.; Lee, E. P.; Xia, Y. Nano Today 2009, 4, 81. (b) Peng, Z.; Yang, H., Nano Today 2008,doi:10.1016/j.nantod.2008.10.010.

Xia et al. (19) (a) Ren, J.; Tilley, R. D. Small 2007, 3, 1508. (b) Wu, C. W.; Mosher, B. P.; Zeng, T. F. Chem. Mater. 2006, 18, 2925. (c) Yu, Y. T.; Wang, J.; Zhang, J. H.; Yang, H. J.; Xu, B. Q.; Sun, J. C. J. Phys. Chem. C 2007, 111, 18563. (20) (a) Subhramannia, M.; Ramaiyan, K.; Pillai, V. K. Langmuir 2008, 24, 3576. (b) Cui, H.; Ye, J.; Zhang, W.; Wang, J.; Sheu, F. J. Electroanal. Chem. 2005, 557, 295. (21) (a) Wang, J. N.; Zhang, L.; Niu, J. J.; Yu, F.; Sheng, Z. M.; Zhao, Y. Z.; Chang, H.; Pak, C. Chem. Mater. 2007, 19, 453. (b) Xia, B. Y.; Wang, J. N.; Wang, X. X.; Niu, J. J.; Sheng, Z. M.; Hu, M. R.; Yu, Q. C. AdV. Funct. Mater. 2008, 18, 1790. (22) Solla-Gullo´n, J.; Rodrı´guez, P.; Herrero, E.; Aldaz, A.; Feliu, J. M. Phys. Chem. Chem. Phys. 2008, 10, 1359. (23) (a) Lee, S. J.; Mukerjee, S.; McBreen, J.; Rho, Y. W.; Kho, Y. T.; Lee, T. H. Electrochim. Acta 1998, 43, 3693. (b) Perez, J.; Gonzalez, E. R.; Ticianelli, E. A. Electrochim. Acta 1998, 44, 1329. (c) Antolini, E.; Giorgi, L.; Pozio, A.; Passalacqua, E. J. Power Source 1999, 77, 136. (d) Lee, E. P.; Peng, Z.; Cate, D. M.; Yang, H.; Campbell, C. T.; Xia, Y. J. Am. Chem. Soc. 2007, 129, 10634. (24) (a) Markovic, N. M.; Gasteiger, H. A.; Ross, P. N. J. Phys. Chem. 1995, 99, 3411. (b) Lee, H.; Habas, S. E.; Somorjai, G. A.; Yang, P. J. Am. Chem. Soc. 2008, 130, 5406. (25) Solla-Gullo´n, J.; Vidal-Iglesias, F. J.; Herrero, E.; Feliu, J. M.; Aldaz, A. Electrochem. Commun. 2006, 8, 189. (26) (a) Lee, H.; Habas, S. E.; Kweskin, S.; Butcher, D.; Somorjai, G. A.; Yang, P. Angew. Chem., Int. Ed. 2006, 45, 7824. (b) Lim, D. H.; Lee, W. D.; Choi, D. H.; Park, D. R.; Lee, H. I. J. Power Source 2008, 185, 159. (27) Hui, C. L.; Li, X. G.; Hsing, I. M. Electrochim. Acta 2005, 51, 711. (28) (a) Rioux, R.; Song, H.; Grass, M.; Habas, S.; Niesz, K.; Hoefelmeyer, J.; Yang, P.; Somorjai, G. Top. Catal. 2006, 39, 167. (b) Liu, Z.; Gan, L. M.; Hong, L.; Chen, W.; Lee, J. Y. J. Power Source 2005, 139, 73. (29) Oh, H. S.; Oh, J. G.; Hong, Y. G.; Kim, H. Electrochim. Acta 2007, 52, 7278. (30) (a) Yang, D. Q.; Sun, S.; Dodelet, J. P.; Sacher, E. J. Phys. Chem. C 2008, 112, 11717. (b) Kim, S. W.; Park, J.; Jang, Y.; Chung, Y.; Hwang, S.; Hyeon, T.; Kim, Y. W. Nano Lett. 2003, 3, 1289. (c) Prabhuram, J.; Wang, X.; Hui, C. L.; Hsing, I. M. J. Phys. Chem. B 2003, 107, 11057. (d) Bo¨nnemann, H.; Britz, P.; Vogel, W. Langmuir 1998, 14, 6654. (31) Lee, E. P.; Chen, J.; Yin, Y.; Campbell, C. T.; Xia, Y. AdV. Mater. 2006, 18, 3271. (32) (a) Chen, J.; Herricks, T.; Xia, Y. Angew. Chem., Int. Ed. 2005, 44, 2589. (b) Chen, J.; Herricks, T.; Geissler, M.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 10854.

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