Liquid-Phase Templateless Synthesis of Pt-on-Pd0.85Bi0.15

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Liquid-Phase Templateless Synthesis of Pt-on-Pd0.85Bi0.15 Nanowires and PtPdBi Porous Nanoparticles with Superior Electrocatalytic Activity Hanbin Liao and Yanglong Hou* Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: This article reports the synthesis of Pt-onPd0.85Bi0.15 nanowires (NWs) and PtPdBi porous nanoparticles (PNPs) by a facile, one-pot, wet-chemical, and templateless method in the presence of oleylamine (OAm) and NH4Br. The relationship between the morphology and composition in the PtPdBi trimetallic system was systematically studied. Interestingly, it is verified that adding only 5% Bi will produce Pd NWs, which offers a novel approach to synthesize Pd NWs in the oil phase without any template. On the basis of the fact of synthesizing Pd0.85Bi0.15 NWs, Pt-on-Pd0.85Bi0.15 NWs with hetero-nanostructures were successfully synthesized by a onestep method. Furthermore, the number of Pt nanobranches for Pt-on-Pd0.85Bi0.15 NWs could be easily controlled via simply changing the synthetic parameters, which could tune the catalytic properties. PtPdBi PNPs were obtained by the acid pickling of PtPdBi2 intermetallic compounds. Most importantly, a catalytic study indicates that the as-obtained Pt-on-Pd0.85Bi0.15 NWs and PtPdBi PNPs exhibited much higher electrocatalytic activity and durability for the oxygen reduction reaction (ORR) than the commercial Pt/C catalyst. We expect that this work will provide a promising strategy for the development of efficient ORR electrocatalysts and can also be extended to the preparation of other nanowires or hetero-nanostructures with desirable functions. KEYWORDS: PtPdBi, nanowire, porous nanoparticle, fuel cell, oxygen reduction reaction as CdSe/Pt, CdS/Pt, and CdSe/Pd.12−14 However, up to now, the progress of developing a facile approach to synthesize Pt/Pd nanostructures with heterostructures and 1-D noble metal structures has only limited success and is still a grand challenge.15 So far, the synthetic strategies of Pd or Pt 1-D nanostructures can be summarized as follows: (1) Template-directed approach, which is achieved by using Te or Ag NWs as template.10 The synthetic procedure is relatively complicated and cumbersome, which might hinder its further applications. (2) Vapor phase deposition, which is a facile method to synthesize large amount of products, but the size and morphology of products are often difficult to control. (3) Wet-chemical method, of which the most important advantage is easy control of the shape and size of products.16 However, few works reported hydrophobic Pd or Pt 1-D nanostructures by using wet-chemical route because of as the intrinsic symmetry of Pt or Pd crystal structures.11,17 Therefore, to explore a facile and reliable strategy for the preparation of Pd or Pt NWs is great of significance for expanding their potential applications.

1. INTRODUCTION Platinum group elements, especially Pt and Pd, are usually employed as major catalysts in one of new energy, polymer electrolyte membrane fuel cells (PEMFCs), and many other catalytic applications due to their unique electronic and chemical properties.1−3 In particular, Pt or Pd bimetallic heteronanostructures, one-dimensional (1-D) nanostructures, and their porous nanostructures have been emerging as novel structures, showing excellent performances compared with their 0-D monometallic nanostructures.4−7 It can be generally accepted that the catalytic performances of hetero-nanostructures can be effectively tuned by manipulating their compositions or structural features, as the synergistic effects between different components are mostly able to enhance physical and chemical properties.8,9 In addition, 1-D nanostructures, such as nanowires (NWs), nanorods (NRs), and nanotubes (NTs), possess anisotropy structures, which are more conducive to the reaction kinetics on the catalyst surfaces and less vulnerable to dissolution and aggregation than nanoparticles.10,11 Therefore, it is of great significance to combine the respective advantages of 1-D nanostructures and hetero-structures to achieve catalytic materials with more excellent performances. With the unremitting efforts of many groups, these hetero-nanostructures have been prepared in a wide range of 1-D quantum dots (QDs), such © 2013 American Chemical Society

Received: November 17, 2012 Revised: January 13, 2013 Published: January 16, 2013 457

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Figure 1. (a, b) TEM images of Pt-on-Pd0.85Bi0.15 NWs with low resolutions. The arrows are Pt NFs at the end of Pt-on-Pd0.85Bi0.15 NWs. TEM (c, d) and HRTEM (e, f) images of the obtained Pt-on-Pd0.85Bi0.15 NWs after acid pickling. Inset: TEM image of Pt NFs grown at the end of Pt-on-Pd0.85Bi0.15 NWs.

(3.92 Å for Pt and 3.89 Å for Pd). The low lattice mismatch facilitates the epitaxial growth of the components. Second, the Pt on Pd surface shows significantly higher catalytic activity than pure Pt toward ORR.9,21 Apart from Pt-on-Pd0.85Bi0.15 NWs, PtPdBi PNPs is also a promising structure, which is obtained by a traditional corrosion method. Compared with their solid counterparts, porous metallic nanostructures not only hold the advantages of low density, saving of material, and reduction of costs but also usually exhibit unique catalytic activities due to high surface areas and interconnected structures.22−24

On the other hand, in addition to Pt/Pd bimetallic materials, the controllable synthesis of Pt/Pd-based trimetallic nanostructures has attracted much attention.18 For example, Sun’s group has synthesized a set of FePtM (M = Pd, Au) trimetallic nanostructures with controllable morphology and composition, which are a new class of catalysts for fuel cell catalytic applications.19,20 The addition of another metal could change the atomic scale structures (Pt−Pt, Pd−Pd, or Pt−Pd bond distance and coordination number) and electronic structures of PtPd alloys, and these microcosmic changes will bring two significant influences to PtPd alloys, that is, variant morphologies and improved catalytic performance. Therefore, Pt/Pd-based trimetallic nanostructure is being considered as promising materials with enhanced catalytic performance. Herein, we report a one-step, facile, wet-chemical, templateless strategy to synthesize controllable Pt/Pd-based nanostructures including Pt-on-Pd0.85Bi0.15 NWs and PtPdBi trimetallic porous nanoparticles (PNPs) and then systematically studied their electrochemical performances as catalysts for oxygen reduction reaction (ORR) in PEMFCs. Significantly, we find that the amount of Bi in the reaction is the critical factor to control the morphology of the products. The surfaces of Pt-on-Pd0.85Bi0.15 NWs were covered with Pt NPs, which exhibit heterostructure and high surface area. It is worth noting that such heterogeneous bimetallic nanocrystals are expected to integrate several different functionalities in one structure, which is difficult to accomplish in a single-component material. In a typical Pt-on-Pd0.85Bi0.15 NWs structure, Pd0.85Bi0.15 NWs play three key roles in ORR: (1) to provide channels for electron transfer; (2) to fix surface Pt particles and to prevent them from aggregation; and (3) to enhance the catalytic activity of Pt through synergistic effect. The reasons for choosing Pd as a metallic support for Pt nanoparticles are based on the following two major factors: First, both metals have a face-centered cubic (fcc) phase with similar lattice spacing

2. EXPERIMENTAL SECTION 2.1. Materials. Platinum(II) 2,4-pentanedionate [Pt(acac)2, Pt 48.0%] as Pt source was purchased from Strem Chemicals, Inc. (America), palladium(II) 2,4-pentanedionate [Pd(acac)2, Pd 34.7%] as Pd source was purchased from Alfa Aesar (America), and bismuth(III) neodecanoate [Bi(NE)3] as Bi source was purchased from SigmaAldrich Co. LLC. (America). Oleylamine (OAm) and NH4Br were obtained from Sinopharm Chemical Reagent Co. Ltd. (China). 2.2. Synthesis of Pt-on-Pd0.85Bi0.15 NWs and PtPdBi PNPs. A total of 0.1000 g of Pt(acac)2, 0.0775 g of Pd(acac)2, 0.0356 g of Bi(NE)3, 1.0 g of NH4Br, and 40 mL of OAm were loaded into a fourneck flask. Under vacuum conditions, the mixture was heated to 90 °C with vigorous stirring and kept there for 30 min, and then the solution turned to primrose yellow. After removing the air out of the system, the solution was heated to 200 °C at 2.5 °C/min under nitrogen stream and kept there for 5 min. Then the product was cooled to room temperature and collected by centrifugation. The black product was dispersed in 10 mL of ethanol and 20 mL of glacial acetic acid solution and then refluxed for 6 h at 80 °C. The PtPd NFs were prepared by the same process but without mixing Bi(NE)3 into the precursors. Similar to this method, the synthesis of PtPdBi PNPs just used 0.3556 g of Bi(NE)3. It must be noted that if we want to get PtPdBi2 NPs, we should avoid using acetic acid to wash the product. 2.3. Characterizations. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and electron diffraction patterns were 458

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Figure 2. (a) TEM image of Pt-on-Pd0.85Bi0.15 NWs. The mapping images of (b) Bi, (c) Pd, and (d) Pt. (e) The merged mapping image of Bi, Pd, and Pt. carried out on FEI Tecnai F20 and T20 microscopes at 200 kV. Elemental mapping analysis was performed on a JEOL JEM-2100 Field Emission TEM operating in the STEM mode. X-ray diffraction (XRD) measurements of products were carried out on a Philips X-Pert Pro diffractometer with Cu Kα (λ = 1.5405 Å) radiation source (40 kV, 40 mA), using a step-scanning mode (4°/min) with a narrow receiving slit (0.2). X-ray photoelectron spectroscopy (XPS) measurements were done on an Axis Ultra imaging photo-electron spectrometer (Kratos Analytical Ltd.) using a monochromatized Al Kα anode, and the C 1s peak at 284.8 eV was taken as an internal standard. 2.4. Electrocatalytic Experiment. All electrochemical measurements were performed at room temperature using a CH Instruments 760C electrochemical workstation in three electrodes configuration. The working electrode was a glassy carbon rotating disk electrode (RDE) (the geometric area is 0.196 cm2), the reference electrode was a KCl-saturated Ag/AgCl electrode, and the counter electrode was Pt foil. Before each experiment, the RDE was mechanically polished with 0.3 and 0.05 μm alumina powder successively until a mirror-finish surface was obtained. The polished electrode was then rinsed with ultrapure water and sonicated in ethanol and water. As-prepared Pt-on-Pd0.85Bi0.15 NWs, PtPdBi PNPs were loaded on Vulcan XC-72 carbon (20% product loading) and then suspended in 5 mL of ethanol to create approximately 1 mg/mL suspensions. The suspensions were sonicated for 30 min. A total of 20 μL of the catalyst suspension was dropped onto the RDE surface and dried in air. Then 5 μL of 0.1% nafion solution was also pipetted onto the RDE surface to avoid the catalyst dropping. Before electrochemical measurements, the RDE electrode supported catalysts was treated by cyclic voltammetry (CV) measurement in 0.5 M HClO4 solutions with a sweep speed of 200 mV/s between −0.20 and 1.00 V. CV measurements of these catalysts were conducted in deaerated 0.5 M HClO4 solutions (−0.20 V ∼ 1.00 V) with a sweep speed of 50 mV/s in order to establish ECSAs. The activity of the products toward oxygen reduction were measured by obtaining polarization curves in an O2saturated 0.5 M HClO4 electrolyte at a rate of 2000 rpm and at a scan rate of 10 mV/s. Durability tests were performed by cycling the potential between 0.1 and 0.9 V (vs Ag/AgCl) in O2-saturated 0.5 M HClO4 at a scan rate of 50 mV/s.

(TEM) images of the Pt-on-Pd0.85Bi0.15 NWs under different magnifications, which show high aspect ratios: the diameter is 8.3 ± 1.1 nm and the length is 387 ± 105 nm (Supporting Information Figure S2). Figure 1c,d obviously indicate that small Pt nanobranches grew on the bodies of Pd NWs, and lighter Pd NWs were surrounded by many darker Pt nanobranches in this heterostructure. For the Pt branches the diameter is about 5 nm. It is interesting to note that, at the tips of some Pt-on-Pd0.85Bi0.15 NWs, Pt nanoflowers (NFs) are formed, which contributes to the facilitating nucleation of Pt NPs at the active sites of the Pd0.85Bi0.15 NWs (Figure 1b,d, inset). In order to further reveal the detailed structure of Pt-on-Pd0.85Bi0.15 NWs, the highresolution TEM (HRTEM) images of typical Pt-on-Pd0.85Bi0.15 NWs are shown in Figure 1e. The measured interplanar spacing for the lattice fringes is 2.25 Å, which corresponds to the (111) lattice plane of fcc-Pt or Pd. The continuous fringe patterns between Pt nanobranches and Pd0.85Bi0.15 NWs indicate the Pt nanobranches had grown from Pd0.85Bi0.15 NWs, not only a simple mixing of them. More interestingly, the HRTEM image shows that there is about a 2 nm thick layer of amorphous carbon on the Pt-on-Pd0.85Bi0.15 NWs (Figure 1f). The amorphous carbon shells come mainly from the cracking of OAm at high temperature, and the Pd and Pt may provide catalytic functions. This mechanism is similar to that of Fe5C2 nanoparticles reported by our group.25 The uniformly coated amorphous carbon layer on the Pt-on-Pd0.85Bi0.15 NWs surfaces can offer a large effective electronic conductive area, which means that each active site on the Pt-on-Pd0.85Bi0.15 NWs surfaces is able to speed up the electron transfer. Therefore, the amorphous carbon layer can improve the conductivity of the products. Many works have proved that the carbon nanotubes or other organized carbonsupported Pt catalysts are more stable than amorphous carbon supported due to the resistant of electrochemical oxidation.26,27 However, the amorphous carbon shell can effectively prevent the Pt NPs from dropping and agglomerating, which can also greatly improve the stability of Pt-on-Pd0.85Bi0.15 NWs from another aspect. The elemental mapping images of Pt-on-Pd0.85Bi0.15 NWs are showed in Figure 2, which clearly reveals the compositional structure. The pattern of Pt (Figure 2d) is broader than that of Bi

3. RESULTS AND DISCUSSION Pt-on-Pd0.85Bi0.15 NWs and PtPdBi PNPs were synthesized by the same procedure except for the amount of Bi precursors, which was described in the Experimental Section. Figure 1 presents a representative set of transmission electron microscope 459

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or Pd (Figure 2b,c). It can clearly illustrate the heterostructures of Pt-on-Pd0.85Bi0.15 NWs. In this work, Bi is a key factor of forming Pt-on-Pd0.85Bi0.15 NWs. When none of the third elemental Bi was added, the products would form PtPd NFs (Supporting Information Figure S1). The formation mechanism is similar with Yan’s previous work, which is ascribed to the alkaline environment and Br ions.28 It is well-known that one of the mechanisms for the anisotropic growth of nanostructures involves twin or stacking faults.29 A similar mechanism was observed in this work. At the early stage of reaction, that is, the nucleation process, Bi can induce the Pd to produce multiple-twinned seeds, which can be observed in the HRTEM image of Pt-on-Pd0.85Bi0.15 in Figure S2 (Supporting Information) and then facilitate the formation of NWs. If the amount of Bi is increased to twice that of Pt or Pd, it will form about 70 nm PtPdBi PNPs or a PtPdBi2 intermetallic compound (Figure 3). It reveals that the morphologies and

Figure 4. XRD patterns of (a) Pt-on-Pd0.85Bi0.15 NWs before and after acid pickling, (b) PtPdBi2 NPs before and after acid pickling (PtPdBi PNPs). The red lines are the XRD peaks of Pt (JCPDF Card File 040802), green lines are the XRD peaks of Pd (JCPDF Card File 46-1043), and blue lines are the XRD peaks of Bi (JCPDF Card File 44-1246).

not change before and after acid pickling. All detectable peaks in both patterns can be assigned to the fcc-Pt or Pd structure by their peak positions and intensities. Before acid pickling, PtPdBi2 PNPs shows ordered intermetallic structures, which are analogous to the PtBi ordered intermetallic compound, with half of the Pt atoms replaced by Pd atoms.30−32 No evidence is found to indicate the existence of bismuth oxides and hydroxides or other impurities in the products. However, after acid pickling, a portion of Bi, which can be dissolved in acid, is washed away to form fcc-PtPdBi trimetallic alloy PNPs. The lattice constant is calculated to be 3.98 Å, which is larger than that of Pt (3.92 Å) and Pd (3.89 Å). The angle shifts of the disordered fcc phase are attributed to the addition of larger Bi atoms. The broadening of the reflection peaks assigned to PtPdBi PNPs distinctly indicated its nanoscale crystalline size (∼5 nm estimated by Scherrer’s formula) as compared with the sample before acid pickling (∼70 nm). This can also be proved by inductively coupled plasma− atomic emission spectroscopy (ICP-AES): Either before acid pickling or after acid pickling, the composition of Pt-onPd0.85Bi0.15 NWs did not change, and both of them were Pt48Pd44Bi8. The small amount of Bi cannot make the XRD peaks shift to smaller 2θ degree, apparently. However, after acid pickling, the composition of PtPdBi2 NPs changes from Pt26Pd25Bi49 (Pt:Pd:Bi = ∼1:1:2) to Pt33Pd35Bi32 (Pt:Pd:Bi = ∼1:1:1). The chemical oxidation states of Pt, Pd, and Bi in Pt-onPd0.85Bi0.15 NWs and PtPdBi PNPs were examined by the X-ray

Figure 3. (a, b) TEM and HRTEM images of the PtPdBi PNPs (PtPdBi2 NPs were processed by acid pickling). (c, d) TEM and HRTEM images of the PtPdBi2 NPs before acid pickling.

components of the products are apparently changed with increasing the quantity of the third element Bi. Figure 3a clearly shows the porous structures of PtPdBi PNPs. It is easily understood that the porous structure is formed by acid pickling, which is demonstrated by the TEM images after and before acid pickling (Figure 3a,c). Before acid pickling, the measured interplanar spacing for the lattice fringes is 3.07 Å, which corresponds to the (101) lattice plane of hexagonal close-packed (hcp) PtPdBi2 intermetallic compound (Figure 3d). And after acid pickling, the measured interplanar spacing for the lattice fringes is 2.30 Å, which corresponds to the (111) lattice plane of fcc-PtPdBi alloy (Figure 3b). And in the whole process of acid pickling, the sizes of PtPdBi trimetallic NPs remain unchanged. Pt and Pd are impossible to dissolve away by acid pickling, but part of Bi was dissolved, thus forming a porous nanostructure. The chemical structures of Pt-on-Pd0.85Bi0.15 NWs and PtPdBi PNPs were also investigated by X-ray diffraction (XRD) analysis. Figure 4a indicates that the phase of Pt-on-Pd0.85Bi0.15 NWs did 460

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photoelectron spectroscopy (XPS) analysis. Table S1 (Supporting Information) gives the surface element composition ratio of various products. The C content is as high as over 50%, and this combined with HRTEM image and implies the existence of amorphous carbon on the NP surface. The XPS spectra of all the products in the Bi 4f region are shown in Figure 5.

Table 1. Surface Element Composition Obtained by XPS Pt-on-Pd0.85Bi0.15 NWs Pt-on-Pd0.85Bi0.15 NWS after acid cleaning PtPdBi2 NPs PtPdBi2 NPs after acid cleaning (PtPdBi PNPs)

Pt

Pd

Bi

83.7% 82.7% 6.3% 25.0%

14.8% 15.6% 18.5% 45.5%

1.5% 1.7% 75.2% 29.5%

products. It is worth noting that the amount of Pt (83.7%) in the Pt-on-Pd0.85Bi0.15 NWs is much higher than that of Pd (14.8%), which further demonstrates that Pt NPs are grown on the surfaces of Pd0.85Bi0.15 NWs. The construction processes of the Pt-on-Pd0.85Bi0.15 NWs and PtPdBi PNPs are shown in Scheme 1a. The decomposition Scheme 1. (a) Reaction Mechanism for the Formation of Pton-Pd0.85Bi0.15 NWs and PtPdBi PNPs and (b) Schematic Illustration of Shape Evolution Mediated by Bi, Pt, and Pd Elements

Figure 5. XPS spectra: The Bi 4f spectra of (a) Pt-on-Pd0.85Bi0.15 NWs before and after acid pickling; (b) PtPdBi2 NPs before and after acid pickling (PtPdBi PNPs).

Corresponding to the XRD data, XPS data show similar trends before and after acid pickling. The Bi 4f spectrum can be deconvoluted into two pairs of peaks at 157.5 eV (4f7/2) and 159.1 eV (4f7/2) and at 162.9 (4f5/2) and 164.4 (4f5/2). The binding energy (BE) values at 157.5 and 162.9 eV were assigned to pure metallic Bi(0). The 4f7/2 peak at 159.1 eV and 4f5/2 peak at 164.4 eV might be contributed to the Bi(III) oxidation state on the surface of products. There are closely similar oxidation states of Bi between the Pt-on-Pd0.85Bi0.15 NWs before and after acid pickling, changed from Bi(0):Bi(III) = 78.12:21.88 to Bi(0):Bi(III) = 80.67:19.33. However, during the acid pickling process from PtPdBi2 NPs to PtPdBi PNPs, the oxidation state of Bi observably changed from Bi(0):Bi(III) = 24.3:75.7 to Bi(0):Bi(III) = 67.4:32.6. Thus it is evident that acid pickling removed the high oxidation state of Bi on the surfaces. The spectra of Pt and Pd in Pt-on-Pd0.85Bi0.15 NWs, PtPdBi2 NPs, and PtPdBi PNPs, either before or after acid pickling, are shown in Figure S5 (Supporting Information). Their oxidation states are mainly zero-valent metal, and the few higher binding energies can be attributed to Pt(II) or Pd(II) in the form of PtO, Pt(OH)2, PdO, and Pd(OH)2, which can also be found in other works.33,34 Table 1 gives the metal composition ratio of the surfaces of various

temperatures (DT) of Bi(EC)3, Pd(acac)2, and Pt(acac)2 are about 170, 175, and 200 °C, respectively, in the presence of Br ions. So with the increase of heating temperature, the precursors of Bi and Pd decompose first to form Pd multiple-twinned NWs at about 175 °C. Subsequently, when the temperature rises to 200 °C, the precursor of Pt starts to decompose and grow on the surfaces of Pd NWs. However, if the amount of Bi to Pt or Pd was increased two times, the reaction process will change, and a large amount of elemental Bi, which has strong reducibility, can 461

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Figure 6. TEM images of (a) Pd NFs (proportion of precursor: Pt:Pd:Bi = 0:1:0), (b) Pd NWs (proportion of precursor: Pt:Pd:Bi = 0:1:0.1), and (c) PdBi NWs (proportion of precursor: Pt:Pd:Bi = 0:1:1).

Figure 7. TEM images of Pt-on-Pd0.85Bi0.15 NWs using (a) 50%, (b) 100%, and (c) 200% Pt precursor, respectively.

Figure 8. (a) CVs obtained from commercial Pt/C (green), Pt-on-Pd0.85Bi0.15 NWs (red), and PtPdBi PNPs (blue) in 0.5 M HClO4 solution at a scan rate of 50 mV/s normalized to the geometrical area of an RDE. (b) ORR polarization curves for commercial Pt/C (green), Pt-on-Pd0.85Bi0.15 NWs (red), and PtPd PNPs (blue) in O2-saturated 0.5 M HClO4 solution normalized to the geometrical area of an RDE. The potential scan rate was 10 mV/s, and the electrode rotation speed was 2000 rpm. (c) A potential vs specific activity plot (E vs Jk) for these three catalysts. Specific activity (d) and mass activity (e) for these three catalysts, in which the kinetic current densities Jk are normalized in reference to the ECSA and loading amount of metal at 0.50, 0.55, 0.60, 0.65, and 0.70 V, respectively.

462

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Table 2. Surface Areas and ORR Activities at 0.60 V (vs Ag/AgCl) for Commercial Pt/C, Pt-on-Pd0.85Bi0.15 NWs and PtPdBi PNPs catalyst commercial Pt/C Pt-on-Pd0.85Bi0.15 NWs PtPdBi PNPs

catalyst loading (μg/cm2Geo)

specific ECSA (m2/gmetal)

half-wave potential (V)

onset potential (V)

specific activity at 0.60 V (mA/cm2metal)

mass activity at 0.60 V (mA/μgPt+Pd)

20.4 26.1

76.5 78.7

0.54 0.60

0.67 0.73

0.23 1.48

0.17 1.16

25.6

59.8

0.57

0.71

1.12

0.67

Figure 9. Stability test of (a) commercial Pt/C, (b) Pt-on-Pd0.85Bi0.15 NWs, and (c) PtPd PNPs were operated before (solid line) and after (dotted line) 2000 cycles. Insets: Comparison of mass activity for the respective catalysts before and after durability test at 0.55, 0.60, and 0.65 V. The potential scan rate was 10 mV/s, and the electrode rotation speed was 2000 rpm.

The D range is special, which will form PtBi nanoplatelets in the limited range (Supporting Information Figure S4a). Figure 8a shows the cyclic voltammetry (CV) curves for the commercial Pt/C, Pt-on-Pd0.85Bi0.15 NWs, and PtPdBi PNPs catalysts recorded at room temperature in N2-purged 0.5 M HClO4 solutions. The oxide reduction peaks (around 0.6 V) of the Pt-on-Pd0.85Bi0.15 NWs and PtPdBi PNPs are significantly shifted to higher potentials, compared with the commercial Pt/ C, suggesting that Pt-on-Pd0.85Bi0.15 NWs and PtPdBi PNPs give improved ORR performance.4 The electrochemically active surface areas (ECSAs) of these three catalysts can be estimated by measuring the total charge of hydrogen adsorption (Q) between −0.14 and 0.206 V (vs Ag/AgCl) after deduction of the double-layer region on the CV curves by adopting an assumption of 210 μC/cm2 corresponding to adsorption of a hydrogen monolayer.35 Then, the specific ECSAs (the ECSAs per unit weight of metal) were calculated based on the following relation (eq 1):

simultaneously reduce Pd(acac)2 and Pt(acac)2 to Pd and Pt at about 170 °C. This process is so rapid that it will form PtPdBi2 ordered intermetallic compound NPs instead of NWs. In order to investigate the role of Bi in the formation of Pd NWs, the products obtained at different ratios of Pd to Bi were synthesized (Figure 6). Only adding small amounts (17 mg) of Bi precursor, it can form high yield Pd NWs (Figure 6b), which declares the significance of Bi in forming Pd NWs. To further elucidate the synthetic progress, the products of Pt-on-Pd0.85Bi0.15 NWs synthesized under different amounts of Pt precursors were observed by TEM (Figure 7). Keeping the amount of Pd and Bi precursors as constant, we found that the quantity of Pt nanobranches grown on the surface of NWs was increased with the increase of the amount of Pt precursor, but the thickness of substrate Pd NWs did not change significantly. Combined with previous elemental mapping and XPS data, it can be pretty clear that the structure of Pt-on-Pd0.85Bi0.15 NWs is exactly as described above. On the other hand, the results definitely illustrate that the density of Pt nanobranches can be easily regulated and controlled via simply changing the molar ratios of Pd to Pt precursor. By the systematic experiments, we can conclude the relationship between the morphology and composition in this trimetallic system as shown in Scheme 1b. In the A range, it will tend to form NWs. In the B range, because of the lack of Bi products it will form Pt, Pd, or PtPd NFs. In the C range, due to the excessive Bi, the reaction is very rapid to form NPs, and then if washed with acid, the product will form PNPs.

specific ECSA =

Q ECSA = m m × 210 μC/cm 2

(1)

where m is the loading amount of metal. The specific ECSAs of the commercial Pt/C, Pt-on-Pd0.85Bi0.15 NWs and PtPdBi PNPs are 76.5, 78.7, and 59.8 m2/gmetal, respectively. The PtPdBi PNPs exhibited a small specific ECSA mainly because of extensive agglomeration. 463

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has excellent durability toward ORR. By comparison, the PtPdBi PNPs had durability only slightly better than the commercial Pt/ C. The small drop in the catalytic activities of commercial Pt/C and PtPdBi PNPs may be due to two reasons: (1) loss of Pt or PtPdBi NPs, including separation from the carbon support and dissolution of Pt and Pd; (2) Pt or PtPdBi NPs aggregation driven by surface-energy minimization or Ostwald ripening.36 However, Pd core NWs and amorphous carbon layers would effectively prevent Pt NPs from agglomeration in Pt-onPd0.85Bi0.15 NWs, as Pt NPs were fixed on the surfaces of Pd NWs. After the stability test, there are no noticeable morphology changes for Pt-on-Pd0.85Bi0.15 NWs, as revealed by TEM imaging (Supporting Information Figure S6), and the Pt branches did not agglomerate or separate from Pd NWs, suggesting that Pt-onPd0.85Bi0.15 NWs is an efficient and stable catalyst for ORR.

Figure 8b displays the ORR polarization curves for commercial Pt/C, Pt-on-Pd0.85Bi0.15 NWs, and PtPdBi PNPs in O2-saturated 0.5 M HClO4 solutions obtained at 2000 rpm. The metal loadings were 20.4, 26.1, and 25.6 μg/cm2Geo for the commercial Pt/C, Pt-on-Pd0.85Bi0.15 NWs, and PtPdBi PNPs respectively. The half-wave potentials of Pt-on-Pd0.85Bi0.15 NWs (0.596 V) and PtPdBi PNPs (0.567 V) are higher than that of commercial Pt/C catalyst (0.545 V), indicating that Pt-on-Pd0.85Bi0.15 NWs and PtPdBi PNPs have better electrocatalytic activity toward ORR than Pt/C. Moreover, Pt-on-Pd0.85Bi0.15 NWs have a more positive onset potential (0.73 V) than Pt/C (0.67 V) and PtPdBi PNPs (0.71 V). The intrinsic ORR activity of these three catalysts can be compared by normalizing the kinetic current densities (Jk), which can be calculated from the ORR polarization curve by using the mass-transport correction and normalized to the ESCA or loading amount of metal. According to the Levich-Koutecky equation (eq 2): 1 1 1 1 1 = + = + J Jk Jd Jk Bω1/2

4. CONCLUSION In summary, a facile, wet-chemical, templateless synthetic procedure of Pt-on-Pd0.85Bi0.15 NWs and PtPdBi PNPs was developed. We have systematically investigated the relationship between the morphology and composition in the Pt/Pd/Bi trimetallic system. It is interesting that adding only 5% Bi will produce Pd NWs, which is a novel method to synthesize Pd NWs in the oil phase without a template. In addition, a catalytic study demonstrated high electrochemical activity and durability of the as-obtained Pt-on-Pd0.85Bi0.15 NWs and PtPdBi PNPs for the oxygen reduction reaction. Pt-on-Pd0.85Bi0.15 NWs have unique hetero- and 1-D nanostructures, which showed much enhanced catalytic activity and durability for ORR in 0.5 M HClO4 solution. It is expected that these unique 1-D Pt-on-Pd0.85Bi0.15 NWs are scientifically interesting and can serve as advanced electrochemical sensors.

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where J is the experimentally measured current density, Jk is the kinetic current density, Jd is the diffusion-limiting current density, B is a constant at certain conditions, and ω is the velocity of the disk. Jk is independent of ω, so in the 1/Jlimit − 1/ω1/2 plot, the slope is 1/B, where Jlimit is the limit measured current density below 0.2 V (vs Ag/AgCl) at each ω. The B factor can be applied to obtain Jd in the ORR. Then the kinetic current densities were calculated based on this equation (eq 3): Jk =

Jd × J Jd − J

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Figure 8c depicts Jk normalized to the ECSAs in the potential range of 0.5−0.7 V. It is evident that both the specific (1.48 mA/ cm2metal) and mass (1.16 mA/μgPt+Pd) activity of the Pt-onPd0.85Bi0.15 NWs are higher than those of commercial Pt/C (0.23 mA/cm2metal and 0.17 mA/μgPt+Pd, respectively) at 0.6 V (vs Ag/ AgCl), which are 6.4-fold and 6.8-fold greater than the corresponding value of commercial Pt/C (Figure 8d,e). The enhanced catalytic activity can be contributed to the bifunctional mechanism and the electronic effects: (1) Pd providing electrons to Pt and (2) Pt-on-Pd0.85Bi0.15 1-D NWs structures, which is conductive to the transmission of electrons. These results prove that Pt-on-Pd0.85Bi0.15 NWs maintain significantly enhanced activity compared with both commercial Pt/C and PtPdBi PNPs. The specific (1.12 mA/cm2metal) and mass (0.67 mA/μgPt+Pd) activity of the PtPdBi PNPs are obviously higher than those of commercial Pt/C at 0.6 V (vs Ag/AgCl) despite being lower than those of Pt-on-Pd0.85Bi0.15 NWs (Figure 8d,e). Table 2 summarizes the specific performances of those three catalysts. Durability tests were performed by cycling the potential between 0.1 and 0.9 V (vs Ag/AgCl) in O2-saturated 0.5 M HClO4 at a scan rate of 50 mV/s. Figure 9 shows the ORR catalytic activities of commercial Pt/C, Pt-on-Pd0.85Bi0.15 NWs, and PtPdBi PNPs before and after 2000 potential sweeps. Commercial Pt/C and PtPdBi PNPs show an obvious shift to negative potentials after the stability test. As a comparison, the curve of Pt-on-Pd0.85Bi0.15 NWs almost remains unchanged under the same reaction conditions. The insets in Figure 9 show the comparison of mass activity for the respective samples before and after the durability test at 0.55, 0.60, and 0.65 V (vs Ag/AgCl). Those results can intuitively demonstrate that the Pt-on-Pd0.85Bi0.15 NWs catalyst

ASSOCIATED CONTENT

* Supporting Information S

Additional characterization data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the National Natural Science Foundation of China (51125001, 51172005, 90922033), the National Basic Research Program of China (2010CB934601), the Natural Science Foundation of Beijing (2122022), and the New Century Excellent Talents Program (NCET-09-0177) and the Doctoral Program (20120001110078) in the Ministry of Education of China, the Yok Ying Tung Foundation (122043), and the New Star Program of Beijing Municipal Science & Technology Committee (2008B02).

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