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Controlled Synthesis of Pd-Pt Alloy


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Controlled Synthesis of Pd?Pt Alloy Hollow Nanostructures with Enhanced Catalytic Activities for Oxygen Reduction
Jong Wook Hong,? Shin Wook Kang,? Bu-Seo Choi,? Dongh

eun Kim,? Sang Bok Lee,?,§ and Sang Woo Han?,*
? §

Department of Chemistry and KI for the NanoCentury and ?Graduate School of Nanoscience and Technology (WCU), KAIST, Daejeon 305-701, Korea, and Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States

mong various alternative energy sources, proton exchange membrane fuel cells (PEMFCs) have attracted considerable attention due to their high e?ciency, low operation temperature, and environmentally benign products.1?3 However, for the commercialization of PEMFCs, the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode must be overcome.4?6 To address this issue, a great deal of e?orts have been devoted to improving the ORR activity with Pt-based catalysts because Pt is the most e?ective catalytic material toward ORR in PEMFCs.7,8 A number of previous works showed that the electrocatalytic activities of Pt could be improved by tailoring the shape of Pt nanostructures.9?12 For example, Pt nanowires and nanocubes exhibited higher ORR performances than spherical Pt nanoparticles.9,10 Noticeably, Adzic and co-workers reported that Pt hollow nanocrystals (NCs) are the most suitable catalyst to enhance sustainable ORR activity, with this structure increasing both durability and Pt mass activity for oxygen reduction in acid fuel cells.11 On the other hand, the use of Ptbased bimetallic NCs (Pt-M NCs, where M = Pd, Co, Ni, Fe, Au, Cu, etc.) in place of pure Pt as electrocatalysts is also an e?ective strategy to increase the speci?c activity and mass activity of Pt catalysts.13?22 The introduction of other metal compositions not only decreases the amount of Pt required in electrocatalysis but also modi?es the crystallographic and electronic structures of Pt, which can optimize the binding energy between Pt and oxygen.23 In particular, Pd is known for its capacity to enhance the ORR activity and durability of Pt.24?26 On the basis of this background,
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ABSTRACT Pd?Pt alloy nanocrystals (NCs)

with hollow structures such as nanocages with porous walls and dendritic hollow structures and Pd@Pt core?shell dendritic NCs could be selectively synthesized by a galvanic replacement method with uniform Pd octahedral and cubic NCs as sacri?cial templates. Fine control over the degree of galvanic replacement of Pd with Pt allowed the production of Pd?Pt NCs with distinctly di?erent morphologies. The synthesized hollow NCs exhibited considerably enhanced oxygen reduction activities compared to those of Pd@Pt core?shell NCs and a commercial Pt/C catalyst, and their electrocatalytic activities were highly dependent on their morphologies. The Pd?Pt nanocages prepared from octahedral Pd NC templates exhibited the largest improvement in catalytic performance. We expect that the present work will provide a promising strategy for the development of e?cient oxygen reduction electrocatalysts and can also be extended to the preparation of other hybrid or hetero-nanostructures with desirable morphologies and functions.
KEYWORDS: electrocatalysis . hollow nanocrystals . oxygen reduction . palladium . platinum

Pd?Pt bimetallic NCs with hollow structures are considered to be a highly promising candidate for e?cient ORR electrocatalysts. However, the controlled synthesis of Pd?Pt hollow NCs with a range of morphologies and their electrocatalytic properties toward ORR have been rarely studied to date. The galvanic replacement reaction has been widely applied to synthesize NCs with hollow features.27 To exploit galvanic replacement in the formation of a hollow nanostructure, the reduction potential of a sacri?cial metallic template should be su?ciently lower than that of the metal composing the ?nal hollow structure. In this regard, Ag NCs have frequently been used as sacri?cial templates to produce Pd and/ or Pt hollow nanostructures owing to the
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* Address correspondence to sangwoohan@kaist.ac.kr. Received for review December 2, 2011 and accepted February 23, 2012. Published online 10.1021/nn2046828
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Scheme 1. Schematic illustration for the synthetic parameters to produce various types of Pd?Pt bimetallic NCs from Pd NC templates.

much lower reduction potential of Ag relative to those of Pd and Pt.28?31 In the present work, we used Pd NCs with well-de?ned morphologies, such as octahedral and cubic structures, as sacri?cial templates to generate Pd?Pt NCs with hollow interiors. Interestingly, maneuvering the degree of galvanic replacement of Pd with Pt yielded hollow Pd?Pt alloy NCs with distinctly di?erent morphologies, that is, nanocages with porous walls and dendritic hollow structures of which the hollow interiors were closely related to the original Pd NC templates. The synthesized hollow NCs exhibited higher ORR activities than those of Pd@Pt core?shell NCs without a hollow feature, and their electrocatalytic activities were highly dependent on their morphologies. RESULTS AND DISCUSSION To generate Pd?Pt hollow NCs, uniform octahedral and cubic Pd NCs were ?rst prepared in aqueous solutions using cetyltrimethylammonium chloride (CTAC) and ascorbic acid (AA) as a surfactant and reductant, respectively (see Methods section). The average edge lengths of octahedral and cubic Pd NCs were adjusted to approximately the same value of 25 nm (Figure S1 in the Supporting Information (SI)). These Pd NCs were then used as seeds for the formation of Pd?Pt bimetallic hollow NCs. We successfully synthesized four di?erent types of Pd?Pt bimetallic NCs with hollow structures: octahedral nanocages (ONCs), cubic nanocages (CNCs), octahedral dendritic hollow (ODH) NCs, and cubic dendritic hollow (CDH) NCs. For comparison, we also prepared two types of dendritic Pd@Pt core? shell NCs without a hollow interior: octahedral dendritic (OD) NCs and cubic dendritic (CD) NCs. Since the catalytic functions of NCs are in?uenced by the surfactant molecules, all of the Pd?Pt bimetallic NCs were synthesized in the presence of the same surfactant,
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CTAC, for precise comparison of inherent ORR properties between di?erent NCs. Scheme 1 summarizes the general synthetic routes for the preparation of Pd?Pt bimetallic NCs with di?erent morphologies. The ONCs and CNCs with porous walls and hollow interiors were prepared via the galvanic replacement reaction with a Pt precursor, K2PtCl4, for 24 h using octahedral and cubic Pd NC templates, respectively, in the absence of AA. On the other hand, the galvanic replacement reaction for 16 h in the presence of AA (?nal concentration of 2.5 mM) as a reductant produced Pd?Pt NCs with hollow interiors and numerous dendritic branches. The shapes of hollow interiors of these four types of NCs followed those of their corresponding templates. When a higher amount of AA (10 mM) was used, dendritic Pd@Pt core?shell NCs with no hollow interiors were formed within 3 h. The as-prepared Pd?Pt bimetallic NCs were characterized by transmission electron microscopy (TEM) (Figure 1) and scanning electron microscopy (SEM) measurements (Figure S2 in SI). The TEM and SEM images clearly show the formation of uniform Pd?Pt NCs with well-de?ned morphologies. As shown in Figure 1a?d, the ONCs, CNCs, ODH NCs, and CDH NCs have hollow interiors in common. The ONCs and CNCs have porous walls with an average thickness of ?1.5 nm (Figure 1a,b), whereas ODH and CDH NCs have numerous dendritic branches on their surfaces (Figure 1c,d). In comparison with ODH and CDH NCs, OD and CD NCs have similar dendritic branches, but their central parts, unlike ODH and CDH NCs, are composed of Pd seeds instead of empty space (Figure 1e,f). The average lengths of the dendritic branches of the ODH, CDH, OD, and CD NCs are 8.5, 8.2, 7.9, and 8.1 nm, respectively. The hollow interiors or Pd cores of the prepared NCs have the same shapes as and similar sizes (?25 nm) to the Pd NC templates.
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Figure 1. TEM images of (a) ONCs, (b) CNCs, (c) ODH NCs, (d) CDH NCs, (e) OD NCs, and (f) CD NCs. High-magni?cation TEM images are shown in each inset. Scale bars in the insets indicate 10 nm.

The high-angle annular dark-?eld scanning TEM (HAADFSTEM) images of the Pd?Pt bimetallic NCs further con?rmed their structural characteristics (Figure 2), and corresponding energy-dispersive X-ray spectroscopy (EDS) mapping (Figure 2) and cross-sectional compositional line pro?les (Figure 3) clearly revealed their compositional structures. Figure 2a,b and Figure 3a,b show that the thin porous walls of the ONCs and CNCs comprise a Pd?Pt alloy. For the ODH and CDH NCs, Pd?Pt alloy features were indenti?ed at their dendritic branches as well as at their walls (Figure 2c,d and Figure 3c,d), whereas the dendritic shells and cores of the OD and CD NCs were formed only by Pt and Pd, respectively (Figure 2e,f and Figure 3e,f). Inductively coupled plasma atomic emission spectroscopy
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(ICP-AES) exhibited that the actual composition ratios of Pt in the synthesized NCs were 62.9, 61.5, 69.3, 68.7, 52.6, and 49.6% for the ONCs, CNCs, ODH NCs, CDH NCs, OD NCs, and CD NCs, respectively. The surface compositions of Pt in the ONCs, CNCs, ODH NCs, and CDH NCs were maesured to be 80.8, 80.3, 84.8, and 81.7%, respectively, by Auger electron spectroscopy, also indicating the alloy nature of their surfaces with similar compostions. We further characterized the Pd?Pt bimetallic NCs by high-resolution TEM (HRTEM) to obtain more detailed information on the growth manner and surface features of the NCs, as shown in Figure 4. The corresponding fast Fourier transform (FFT) patterns are also shown in the insets of HRTEM images. The d spacing for
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Figure 2. (Left panels) HAADF-STEM images and (right panels) corresponding HAADF-STEM-EDS mapping images of a (a) ONC, (b) CNC, (c) ODH NC, (d) CDH NC, (e) OD NC, and (f) CD NC. The yellow and green colors in HAADF-STEM-EDS mapping images indicate Pd and Pt, respectively. The scale bars indicate 10 nm. Figure 3. HAADF-STEM images and cross-sectional compositional line pro?les of a (a) ONC, (b) CNC, (c) ODH NC, (d) CDH NC, (e) OD NC, and (f) CD NC. For the hollow NCs including ONC, CNC, ODH NCs, and CDH NCs, Pt traces match well with those of the Pd traces, identifying the Pd?Pt alloy structures. The compositional line pro?les for OD and CD NCs clearly con?rm the Pd@Pt core?shell structures. HONG ET AL. VOL. XXX


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Figure 4. (Left panels) HRTEM images of a (a) ONC, (b) CNC, (c) ODH NC, (d) CDH NC, (e) OD NC, and (f) CD NC. The scale bars indicate 10 nm. Corresponding FFT patterns are shown in each inset. (Right panels) High-magni?cation images of the square regions in the HRTEM images shown in left panels. The scale bars indicate 2 nm.

Figure 5. TEM images of CDH NCs collected at di?erent reaction times: (a) 3, (b) 6, (c) 9, (d) 12, and (e) 16 h.

adjacent lattice fringes of the wall of the ONCs is 2.24 ?, which corresponds to that of the (111) planes of facecentered cubic (fcc) Pd, Pt, or Pd?Pt alloy (Figure 4a).32
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ODH and OD NCs also showed identical d spacing values (Figure 4c,e). These results indicate that the surfaces of all of the ONCs, ODH NCs, and OD NCs prepared using
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{111}-faceted octahedral Pd NC templates are bound by {111} facets. On the contrary, d spacings of 2.00 ? between adjacent lattice fringes of CNCs, CDH NCs, and CD NCs reveal the growth of the {100}-faceted surfaces of Pd?Pt NCs from the cubic Pd NC templates enclosed by {100} facets (Figure 4b,d,f).33 These ?ndings de?nitely demonstrate that the crystallographic as well as structural characteristics of the Pd NC templates have a profound in?uence on the growth habit of the Pd?Pt NCs. The close correlation in crystallographic properties between the Pd?Pt NCs and Pd templates can be due to the very small lattice mismatch between Pd and Pt (0.77%).34,35 The X-ray di?raction (XRD) patterns of the Pd?Pt NCs also show the distinct di?raction peaks from the re?ections of the fcc structure of the metal (Figure S3 in SI), identifying the crystalline nature of the prepared NCs. For comparison, the XRD patterns of pristine Pd NC templates are also shown in Figure S4 in SI. In the present work, Pd?Pt bimetallic NCs with di?erent morphologies were obtained by adjusting the degree of galvanic replacement. When Pd NCs are used as sacri?cial templates, reduction of the Pt precursors is less e?cient and thus slow compared to the case where Ag NCs are employed as templates; the di?erence in the reduction potential between PtCl42? and Ag? is 0.5 V, while that between PtCl42? and PdCl42? is 0.2 V.36 Therefore, the reductant can considerably a?ect the degree of galvanic replacement, such that the ?nal morphology of the bimetallic NCs can be tuned by altering the amount of reductant. Addition of a su?cient amount of reductant into the reaction mixture, which can reduce all the Pt precursors (PtCl42?), will prevent the galvanic replacement of Pd with Pt.37,38 In relation to this, when 10 mM of AA was used as a reductant, Pt branches grown on the surface of Pd NCs with no observable galvanic replacement reaction, thus forming dendritic Pd@Pt core? shell NCs without a hollow structure (OD and CD NCs). When 2.5 mM of AA, which can reduce some portion of PtCl42?, was employed, dendritic hollow NCs (ODH and CDH NCs) were generated through both Pt deposition and galvanic replacement (vide infra). In the absence of AA, PtCl42? could be reduced solely by the galvanic replacement reaction, and Pd?Pt alloy nanocages (ONCs and CNCs) were thereby produced.39?41 Very recently, Xia et al. reported the synthesis of Pd?Pt alloy cubic nanocages using Pd nanocubes as templates and their enhanced performance for CO oxidation.42 Contrary to our case, they found that bromide ion-induced galvanic replacement complemented with a coreduction process through the introduction of citric acid as a reductant was inevitable for the formation of the nanocages. Pd?Pt NCs with a concave structure instead of nanocages were generated without citric acid. This was attributed to the bromide ion-induced galvanic replacement selectively from the {100} facets of Pd nanocubes. As such, Pd atoms were dissolved mainly
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Figure 6. (a) H2O2 yield plots and (b) ORR polarization curves for the Pd?Pt bimetallic NCs and Pt/C obtained using a RRDE in O2-saturated 0.1 M HClO4 at scan rate of 10 mV s?1 and a rotation rate of 1600 rpm. The ring potential was held at 1.2 V vs RHE. The current densities were normalized to geometric surface area of the GC electrode (0.1256 cm2). (c) Mass and area-speci?c activities at 0.85 V vs RHE for the various catalysts.

from the {100}-faceted nanocube faces and Pt atoms preferentially deposited on the {111}-faceted corners of Pd nanocubes in the absence of an additional reductant, eventually Pd?Pt concave nanocubes without a hollow feature were evolved. Furthermore, controlling the morphology of Pd?Pt nanocages by altering the shape of Pd NC templates was very di?cult due to the high selectivity of bromide ion-induced galvanic replacement for the {100} facets of Pd. For instance, Pd?Pt nanocages could not be produced by using truncated octahedral Pd NCs, which were mainly enclosed by {111} facets, as sacri?cial templates. These facts thus imply that our method is very simple and versatile for the synthesis of Pd?Pt hollow nanostructures with tailored morphologies. To decipher the formation mechanism of hollow structures, the shape evolution of the CDH NCs, as a representative example, was monitored by taking TEM images of the products sampled at di?erent reaction times (Figure 5). Initially, Pt branches formed on the Pd NC seeds within 3 h through the deposition of Pt atoms, which were reduced by AA. After 3 h, small hollow interiors started to appear in the Pd seed regions. As the
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reaction proceeded, the hollow interior of the NCs gradually increased in size. Eventually, Pd?Pt nanostructures with dendritic branches and hollow interiors were produced. On the basis of these experimental results, we posited that the reduction of PtCl42? by AA produced Pt branches on the Pd NC surfaces at the beginning stage. After exhaustion of AA, simultaneous dissolution of Pd atoms and reduction of remaining PtCl42? were initiated by the galvanic replacement reaction, producing hollow interior in the Pd seed parts. Concomitantly, some amounts of dissolved Pd (PdCl42?) were reduced onto the Pt branches. From this process, hollow Pd?Pt NCs with alloyed surfaces could be generated. To further con?rm the proposed mechanism for the formation of Pd?Pt hollow structures, we tested the possibility of the preparation of CDH NCs by taking CD NCs and subjecting them to galvanic replacement in a decoupled step without AA. In fact, injection of the presynthesized CD NCs into a solution containing CTAC and K2PtCl4 successfully produced CDH NCs (Figure S5 in SI). Taken together, these ?ndings show that manipulation of the degree of galvanic replacement is the key to the formation of various hollow structures. The morphology-dependent electrocatalytic activities of various Pd?Pt bimetallic NCs toward ORR were investigated, and the results were compared with those of a commercial Pt/C catalyst (40 wt %, average Pt particle size = 3 nm). Cyclic voltammetry curves of Pd?Pt NCs obtained in 0.1 M HClO4 exhibited distinct peaks associated with hydrogen adsorption and desorption, which were not signi?cantly di?erent from those of Pt/C (Figure S6 in SI). The electrochemically active surface areas (ECSA) for each NC were calculated by measuring the Coulombic charge for the desorption of the monolayer of hydrogen (Figure S6 in SI). The speci?c ECSA values for the ONCs, CNCs, ODH NCs, CDH NCs, OD NCs, CD NCs, and Pt/C were estimated to be 40.3, 42.3, 41.5, 39.8, 22.2, 25.9, and 57.7 m2 g?1, respectively (Figure S7 in SI). Although the prepared Pd?Pt hollow NCs showed lower speci?c ECSA values than that of Pt/C due to their larger NC sizes, the hollow structures endowed ONCs, CNCs, ODH NCs, and CDH NCs with higher speci?c ECSA compared to OD and CD NCs. Panels a and b of Figure 6 display, respectively, H2O2 yield plots and ORR polarization curves for the prepared Pd?Pt bimetallic NCs and Pt/C obtained using a rotating ring disk electrode (RRDE), with a glassy carbon (GC) disk and a Pt ring, in O2-saturated 0.1 M HClO4. The ORR polarization curves obtained from disk currents were normalized to the geometric surface area of the GC electrode (0.1256 cm2). The very low yields of H2O2 for all the catalysts (<0.15%), which were calculated from the ring current (see Methods section), imply high selectivity for four-electron reduction of oxygen.43,44 The ORR polarization curves show that hollow structures generally have enhanced ORR activities as compared to Pd@Pt core?shell NCs. The ORR activities of the various
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Pd?Pt bimetallic NCs, estimated from the half-wave potential (E1/2), follow the order ONCs > ODH NCs > CDH NCs > OD NCs > Pt/C > CNCs > CD NCs. To gain more insight into the di?erent ORR activities, the kinetic current (jk) associated with the intrinsic catalytic activity was obtained using a Koutecky?Levich plot for the ORR with various catalysts (see Methods section). At 0.85 V versus reversible hydrogen electrode (RHE), the ONCs exhibited a mass activity of 764.7 mA mgPd?Pt?1, the highest value among the various catalysts and 1.9 and 3.8 times higher than those of the ODH NCs (393.9 mA mgPd?Pt?1) and Pt/C (200.5 mA mgPd?Pt?1), respectively (Figure 6c). The mass activities of the other catalysts are 324.4, 286.4, 178.1, and 128.5 mA mgPd?Pt?1 for the CDH NCs, OD NCs, CNCs, and CD NCs, respectively. The Pt mass activities (jk, mA mgPt?1) of the various NCs are also shown in Figure S8 in SI. Moreover, the area-speci?c activities of Pd?Pt NCs, obtained by normalization of jk against ECSA, show a similar trend with mass activity (Figure 6c). Together with the most positive E1/2 value (0.905 V vs RHE), these results signify that the ONCs drastically outperformed other Pd?Pt NCs and Pt/C. The accelerated durability tests further demonstrated that the ONCs have excellent electrochemical stability (Figure S9 in SI). Through the electrocatalytic experiments, we found that the Pd?Pt NCs prepared from Pd octahedral NC templates exhibited higher ORR performances than the NCs from Pd cubic NC templates for each structure (ONCs vs CNCs, ODH NCs vs CDH NCs, and OD NCs vs CD NCs): ONCs, ODH NCs, and OD NCs had more positive E1/2 values and showed, respectively, 4.3, 1.2, and 2.2 times higher mass activities than their counterparts prepared from Pd cubic NCs. Furthermore, the speci?c activities of Pd?Pt NCs produced by Pd octahedral NCs were higher than those of the NCs from Pd cubic NCs, irrespective of their structures (Figure 6c). These ?ndings unambiguously reveal that the type of surface facet of NCs, which can be controlled by the morphology of sacri?cial templates, plays a decisive role in determining the ORR activities of Pd?Pt NCs. Previous studies on the ORR in a HClO4 solution have shown that {111} planes are more active than {100} planes.45?47 For example, Zhang et al. have shown that mass and speci?c activity of Pt3Ni octahedral NCs enclosed by {111} planes are, respectively, 2.8 and 4.0 times higher than those of Pt3Ni cubic NCs enclosed by {100} planes.46 Accordingly, the improved ORR activities of the ONCs, ODH NCs, and OD NCs compared to Pd?Pt NCs from the cubic Pd NC templates can be attributed to their {111} facet-dominant surfaces. On the other hand, of the NCs bound by the same surface facet, NCs with hollow structures exhibited enhanced mass activities toward ORR compared to the Pd@Pt core?shell NCs due to their larger speci?c ECSA: mass activities of the ONCs and ODH NCs were 2.67 and 1.38 times higher than that of the OD NCs, and CNCs and
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CDH NCs showed 1.39 and 2.52 times higher mass activities than the CD NCs. The prominent ORR activity of the ONCs compared to ODH NCs can be ascribed to the relatively smooth surface feature of the ONCs compared to that of the ODH NCs. The amount of undercoordinated Pt atoms should be larger on the surface of ODH NCs than those on the ONCs due to their dendritic morphology. It is well-known that these active surface sites have higher oxygen binding energies, thus decrease the ORR activities of NCs.11,48 However, unlike the case of the ONCs and ODH NCs, the CDH NCs showed better ORR activity than that of the CNCs. This can be understood by the observation that the {111} facets also developed to some extent in the branch tip regions of the CDH NCs, due to a release of strain imposed by the {100} facets at the interface (Figure S10 in SI).49 A similar phenomenon was also observed in our previous work on Au@Pt heteronanostructures.38 This ?nding further demonstrates that the type of surface facet is the most important parameter to determine the ORR activity of Pd?Pt NCs. In this regard, it is also noticeable that the OD NCs, which are enclosed by {111} facets, have higher mass and speci?c activities than those of the {100}-faceted

CNCs, despite their lower speci?c ECSA compared to that of CNCs. CONCLUSION Pd?Pt bimetallic alloy NCs with hollow structures and dendritic Pd@Pt core?shell NCs could be selectively synthesized by a galvanic replacement method with Pd NCs with di?erent morphologies as sacri?cial templates. Fine control over the degree of galvanic replacement of Pd with Pt allowed the production of Pd?Pt NCs with distinctly di?erent morphologies. The hollow NCs exhibited considerably enhanced oxygen reduction activities compared to those of Pd@Pt core? shell NCs, and their electrocatalytic activities were highly dependent on their morphologies. We found that the type of surface facet plays a crucial role in determining the ORR activities of Pd?Pt NCs. The Pd?Pt nanocages prepared from octahedral Pd NC templates exhibited the largest improvement in ORR performance. We expect that the present work will provide a promising strategy for the development of e?cient ORR electrocatalysts and can also be extended to the preparation of other hybrid or hetero-nanostructures with desirable morphologies and functions.

METHODS
Chemicals and Materials. K2PdCl4 (Aldrich, 98%), K2PdBr4 (Aldrich, 98%), K2PtCl4 (Aldrich, 99.9%), L-ascorbic acid (Dae Jung Chemicals & Metals Co., 99.5%), cetyltrimethylammonium chloride (CTAC, Aldrich, solution in water, 25 wt %), HClO4 (Aldrich, 60%), and Pt/C (Johnson Matthey, 40 wt %, average Pt particle size = 3 nm) were all used as received. Other chemicals, unless specified, were reagent grade, and high purified water with a resistivity of greater than 18.0 MΩ 3 cm was used in the preparation of aqueous solutions. Preparation of Pd Octahedral and Cubic NCs. In a typical synthesis of Pd octahedral NCs, a 2 mL aqueous solution of K2PdCl4 (5 mM) and a 0.05 mL aqueous solution of L-ascorbic acid (50 mM) were injected to an aqueous solution of CTAC (10 mM, 7.95 mL) with gentle shaking, and then the resultant solution was heated at 90 °C for about 3 h in a conventional forced-convection drying oven. In a typical synthesis of Pd cubic NCs, an aqueous solution of CTAC (50 mM, 24 mL) was heated to 100 °C, and then 1 mL of L-ascorbic acid (300 mM) and 10 mL of K2PdBr4 (5 mM) were consequently added into the solution with vigorous stirring. The reaction was continued for 5 h at 100 °C with vigorous stirring. Preparation of Pd?Pt Bimetallic NCs. In a typical synthesis of ONCs, 2 mL of Pd octahedral NC seed solution (1 mMPd) and 1 mL of K2PtCl4 (5 mM) were added to a 7 mL aqueous solution of CTAC (20 mM). The mixture was then heated to 100 °C and maintained at this temperature for 24 h with vigorous stirring. In a typical synthesis of ODH NCs, 2 mL of Pd octahedral NC seed solution (1 mMPd), 1 mL of K2PtCl4 (5 mM), and 0.5 mL of L-ascorbic acid (50 mM) were added to a 6.5 mL aqueous solution of CTAC (20 mM). The mixture was then heated to 100 °C and maintained at this temperature for 16 h with vigorous stirring. In a typical synthesis of OD NCs, 2 mL of Pd octahedral NC seed solution (1 mMPd), 1 mL of K2PtCl4 (5 mM), and 1 mL of L-ascorbic acid (100 mM) were added to a 6 mL aqueous solution of CTAC (20 mM). The mixture was then heated to 100 °C and maintained at this temperature for 3 h with vigorous stirring. For the synthesis of CNCs, CDH NCs, and CD NCs, Pd

cubic NC seed solution (1 mMPd) instead of Pd octahedral NC seed solution was used under identical experimental conditions. The prepared Pd?Pt NCs were cleaned by repeated centrifugation of hydrosols at 9000 rpm for 8 min followed by redispersion of the NCs in deionized water to remove excess reagents. Preparation of the Catalysts. The Ketjen Black carbon was used as a support for preparing the catalysts. First, the Ketjen Black carbon was dispersed in a high purified water. Then, a predetermined amount of the NCs was added to this dispersion. The resultant dispersion was sonicated for 2 h and then stirred for 24 h. The resulting dispersion was dried in a vacuum drying oven at 60 °C. The amounts of ONCs, CNCs, ODH NCs, CDH NCs, OD NCs, and CD NCs loaded on the Ketjen Black carbon determined by ICP-AES were 37.3, 42.8, 30.6, 45.4, 38.0, and 28.8 wt %, respectively. Characterization of NCs. SEM images of the samples were taken using a field-emission scanning electron microscope (FESEM, Phillips Model XL30 S FEG). TEM, HRTEM, and HAADF-STEM characterizations were performed using a FEI Tecnai G2 F30 Super-Twin transmission electron microscope operated at 300 kV after placing a drop of hydrosol on carbon-coated Cu grids (200 mesh). The effective electron probe size and dwell time used in HAADF-STEM-EDS mapping experiments were 1.5 nm and 200 ms per pixel, respectively. The compositions of Pd?Pt bimetallic NCs were determined by ICP-AES (CIROS VISION). The surface compositions of Pd?Pt NCs were determined by Auger electron spectroscopy (Perkin-Elmer, SAM 4300). XRD patterns were obtained using a RIGAKU D/MAX-2500 diffractometer with Cu KR (0.1542 nm) radiation. Electrochemical measurements were carried out in a three-electrode cell using a CH Instrument model 760D bipotentiostat. Pt wire and Ag/AgCl (in 3 M NaCl) were used as the counter and reference electrodes, respectively. All electrochemical data were obtained at room temperature, and all the potentials are reported with respect to the RHE. The electrolyte solutions were purged with high-purity N2 gas before use for about 1 h. To prepare the working electrode,

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10 μL of the catalyst dispersion (0.3 mgmetal/mL based on ICPAES) was dropped onto a GC disk (diameter = 4 mm) of a RRDE with a Pt ring and dried at room temperature. The electrode was then washed several times with ethanol and water to remove stabilizing agents, and a thin layer of Nafion was placed on the catalyst-loaded GC electrode by dropping 15 μL of Nafion solution (0.05%). The dried working electrode was cleaned again by sequentially washing with ethanol and water and then electrochemically cleaned by 30 potential cycles between 0.1 and 1.3 V versus RHE at a scan rate of 50 mV s?1 to remove residual organic materials on the surface of the NCs. RRDE experiments were carried out using a rotator (ALS, RRDE-3A) in 0.1 M HClO4 aqueous solution purged with O2 prior to use for 30 min. The ring potential was held at 1.2 V versus RHE.43 The specific kinetic current densities (jk) associated with the intrinsic activity of the catalysts can be obtained by Koutecky?Levich eq (eq 1):50 1=j ? 1=jk ? 1=jd ? 1=jk ? 1=Bω1=2 (1)

where jk is the kinetic current density, jd is the diffusion-limited current density, ω is the angular frequency of rotation. The B parameter is defined as eq 2: B ? 0:62nFCo Do 2=3 υ ?1=6 (2)

where n is the overall number of electrons, F is the Faraday constant (96485 C mol?1), Co is the concentration of molecular oxygen in the electrolyte (1.26 ? 10?6 mol cm?3), Do is the diffusion coefficient of the molecular O2 in 0.1 M HClO4 solution (1.93 ? 10?5 cm2 s?1), and υ is the viscosity of the electrolyte (1.009 ? 10?2 cm2 s?1).10 The yield of H2O2 was calculated by the following equation (eq 3):43 yield of H2 O2 (%) ? 200 ? (Ir =N)=((Id ? Ir )=N) (3)

where Ir is ring current, Id is disk current, and N is current collection efficiency of the Pt ring. N was experimentally determined to be 0.35 from the reduction of K3Fe[CN]6. Con?ict of Interest: The authors declare no competing ?nancial interest. Acknowledgment. This work was supported by Basic Science Research Programs (2010-0029149), EPB Center (2008-0062042), Future-based Technology Development Program (Nano Fields) (2009-0082640), and WCU Program (R31-2008-000-10071-0) through the National Research Foundation (NRF) funded by the Korean government (MEST). Supporting Information Available: Experimental details and additional ?gures. This material is available free of charge via the Internet at http://pubs.acs.org.

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