A Facile Method for Synthesizing Dendritic Core–Shell Structured

Oct 11, 2016 - ... toward the oxygen reduction reaction compared to that of commercial Pt/C. The unique dendritic core–shell structures, Pt3Pd alloy...
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A Facile Method for Synthesizing Dendritic Core−Shell Structured Ternary Metallic Aerogels and Their Enhanced Electrochemical Performances Qiurong Shi,† Chengzhou Zhu,*,† Yijing Li,‡ Haibing Xia,‡ Mark H. Engelhard,§ Shaofang Fu,† Dan Du,† and Yuehe Lin*,† †

School of Mechanical and Material Engineering, Washington State University, Pullman, Washington 99164, United States Institute of Crystal Materials, Shandong University, Jinan 250100, P. R. China § Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ‡

S Supporting Information *

ABSTRACT: Currently, three-dimensional self-supported metallic structures are attractive for their unique properties of high porosity, low density, excellent conductivity, etc., that promote their wide application in fuel cells. Here, for the first time, we report a facile synthesis of Au@Pt3Pd ternary metallic aerogels with a unique dendritic core−shell structure via a one-pot self-assembly gelation strategy. This strategy is simple and saves time without any concentration or destabilizer steps. The as-prepared Au@Pt3Pd ternary metallic aerogels demonstrated enhanced electrochemical performance toward the oxygen reduction reaction compared to that of commercial Pt/C. The unique dendritic core−shell structures, Pt3Pd alloyed shells, and cross-linked network structures are beneficial for the electrochemical oxygen reduction reaction via the electronic effect, geometric effect, and synergistic effect. This strategy of fabrication of metallic hydrogels and aerogels as well as their exceptional properties holds great promise in a variety of applications.

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INTRODUCTION Self-assembly is one of the bottom-up approaches for obtaining well-defined complex architectures. Nanoparticles (NPs) with rationally designed sizes, shapes, morphologies, and structures usually act as building blocks for construction of template-free two-dimensional (2D) or three-dimensional (3D) superstructures.1−4 Metallic hydrogels (MHs) and metallic aerogels (MAs), a very promising class of 3D unsupported materials, have attracted intense interest because of their unique physical and chemical properties as determined by the pioneering work of Eychmüller and co-workers in 2009.5,6 From the macroscopic perspective, a MH is a solid, jellylike material, while from the microscopic perspective, it is a cross-linked system that is mainly a result of self-assembly of metal NPs. MHs and MAs integrate the merits of traditional aerogels of high hierarchical porosity, large active surface area, and ultralow density with metals of excellent electrical and thermal conductivity and catalytic activity.6 Therefore, MAs could be potentially used in a variety of applications such as electrocatalysis, optoelectronics, sensors, etc.7−9 For their application as electrode materials in proton exchange membrane fuel cells (PEMFCs),10−14 the construction of porous self-supported Pt-based bi- or multi-MAs offers an effective way of improving the electrochemical catalytic activities and durability toward oxygen reduction reaction (ORR) along with reducing the total cost.1,15−25 This unique 3D structure mainly has two advantages. (1) The self-supported network structure could eliminate the corrosion of the carbon © 2016 American Chemical Society

support or the detachment of electrocatalyst NPs from the carbon support to enhance their durability. (2) The macroporous 3D structure offers a large active surface area and allows the accessibility of the fuel molecules to the active sites to promote their mass transfer for catalytic activities.2,10,17,26 For another, alloyed Pt surface structures are beneficial in both lowering the expensive Pt content and enhancing the comprehensive catalytic performance via geometric and electronic effects. Density functional theory has confirmed that both of these two effects could enhance the electrocatalytic performance by decreasing the absorption strength of the oxygen-containing intermediates caused by a downshift of the d-band center.27−30 Thus, a number of Pt-, Pd-, Au-, and Agbased bimetallic or ternary MHs and MAs5,7−9,31−34 were developed in recent years in an effort to boost catalytic performances. Currently, the synthesis of MHs employs a one-step in situ spontaneous gelation process 5,9 or a two-step gelation process.6,7,32 The two-step gelation process requires the concentration and gelation steps induced by adding destabilizers or increasing temperature. In many cases, the gelation would take several weeks or months at low temperatures for the self-assembly of generated NPs into 3D structures. For the onestep spontaneous gelation method, a hydrogel is formed Received: August 23, 2016 Revised: October 10, 2016 Published: October 11, 2016 7928

DOI: 10.1021/acs.chemmater.6b03549 Chem. Mater. 2016, 28, 7928−7934

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Chemistry of Materials directly from the in situ reduction of metal ions into the interconnected structures. Generally, most of the alloyed MHs obtained via these two strategies exhibited nanowire-like morphologies. In our case, for the first time, we reported a facile synthesis of Au@Pt3Pd ternary metallic hydrogels (TMHs) through a combination of the two strategies mentioned above that takes only 4 h at 35 °C. More importantly, the as-synthesized Au@ Pt3Pd TMHs are assembled from building blocks with dendritic and core−shell structures. We first added HAuCl4 and citrate for generation of Au NPs at room temperature followed by the addition of K2PtCl4, Na2PdCl4, and ascorbic acid (AA) at 35 °C for in situ growth. What is important is that the gelation of hydrogel takes only ∼4 h, which is much faster than the previous reported synthesis of Pt-based MHs at low temperatures.5,9 In addition, the distinct advantage lies in the fact that it avoided the time-consuming filter concentration step, which could be attributed to the assistance of salt generated during the gelation process. As expected, the dendritic core−shell structured Au@Pt3Pd TMHs exhibited excellent catalytic activity and stability toward ORR; on the other hand, the mass activity reached 0.812 A/mgPt+Pd and the half-potential (E1/2) negatively shifted by only 11.8 mV after an accelerated durability test (ADT). Therefore, this gelation method of fabrication of TMHs or TMAs is significant in the mass production of efficient electrode materials for fuel cells because of the superior catalytic performances as well as its convenience and time-saving features in the synthesis process.

Figure S2 displays interconnected structures with observable macro-sized pores. The surface areas of Au@PtPd TMAs, Au@ Pt2Pd TMAs, and Au@Pt3Pd TMAs are 22.2, 24.7, and 29.2 m2/g, respectively, based on the Brunauer−Emmett−Teller (BET) model. The N2 physisorption isotherms shown in Figure S3A−C show the typical features of MAs with interlinked macroporous structures. The pore size distribution plots and cumulative volumes shown in panels D and E of Figure S3 were calculated according to the Barrett−Joyner−Halenda (BJH) theory.6 Combined with the SEM image, the pore size ranges from micropores to mesopores and macropores. The cumulative volume plots indicate that the most of the volumes were derived from macro-sized pores. It is noted that the hybrid pore system plays a key role in enhancing diffusion rates because 10−50 nm pores could act as open channels for the diffusion of molecules.35 The building blocks of dendritic core− shell structured Au@Pt3Pd NPs are shown in the transmission electron microscopy (TEM) image in Figure 1D. The calculated average diameter of these NPs is ∼24.50 ± 3 nm. The high-resolution transmission electron microscopy (HRTEM) image in Figure 1E shows the lattice spacings marked 0.235 and 0.226 nm are indexed to the Au(111) facet and Pt(111) facet, respectively. The X-ray energy dispersive spectroscopy (EDS) result shown in Figure 2A shows the Pt/Pd atomic ratio is ∼3, which

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RESULTS AND DISCUSSION Characterization of Dendritic Core−Shell Structured Au@Pt3Pd TMHs. The optical photos of as-prepared Au@ Pt3Pd TMHs and TMAs shown in panels A and B of Figure 1, respectively, exhibited macro 3D structures. Besides, the video in the Supporting Information also proves the unique solid and jellylike features of hydrogels. The scanning electron microscopy (SEM) image of the hydrogels in Figure 1C and

Figure 2. (A) EDS result, (B) XRD pattern, and XPS of the (C) Pt 4f and (D) Pd 3d and Pt 4d peaks of Au@Pt3Pd TMAs.

is in agreement with the feeding ratio of Pt and Pd precursors in the synthetic process, indicating a complete reduction of metal precursors. The crystal structure of as-fabricated Au@ Pt3Pd TMAs was further characterized by X-ray diffraction (XRD) in Figure 2B. The diffraction peaks were indexed with (111), (200), (211), and (311) planes of the typical facecentered cubic (fcc) Pt (JADE PDF No. 65-2868) and Pd (JADE PDF No. 46-1043) lattice, which is in agreement with the HRTEM image.26,36,37 The surface composition of Au@ Pt3Pd TMAs was analyzed by X-ray photoelectron spectroscopy (XPS) in panels C and D of Figure 2. The observed peaks that appear at 71.0 and 74.2 eV in Figure 2C are assigned to Pt

Figure 1. Optical photos of Au@Pt3Pd (A) TMHs and (B) TMAs. (C) Scanning electron microscopy, (D) transmission electron microscopy, and (E) high-resolution transmission electron microscopy images of TMAs. 7929

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Chemistry of Materials 4f7/2 and Pt 4f5/2, respectively. They could be further deconvoluted into four individual peaks, i.e., 71.0 and 74.6 eV and 72.4 and 76.6 eV, corresponding to the metallic state (Pt0) and oxide state (Pt2+) of Pt, respectively. Similarly, the peaks that appear at 340.0 and 335.0 eV in Figure 2D are assigned to Pd 3d3/2 and Pd 3d5/2, respectively. Pd 3d could be also deconvoluted into four individual peaks, i.e., 335.0 and 340.2 eV and 336.7 and 341.7 eV, corresponding to the metallic state (Pd0) and oxide state (Pd2+) of Pd, respectively. Therefore, the shell of Au@Pt3Pd TMAs is mainly composed of Pt0 and Pd0. Besides, there is no typical peak of Au detected via XPS in Au@Pt3Pd TMAs for a shell thickness of >8 nm. Therefore, we can conclude that Au mainly formed the inner core and the PtPd alloy formed the shell. The high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) image in Figure 3A

Scheme 1. Illustration of the Synthetic Process of the Au@ Pt3Pd TMAs

surface energy under TEM conditions. Au(0) atoms could evolve into a spherical morphology (Figure S4B) after addition of Pt(II) and Pd(II) at room temperature.38−41 This fast nucleation was promoted by the addition of Pt(II) to the citrate/HAuCl4 mixture solutions because Pt(II) atoms are oxidized into Pt(IV) while Au(III) atoms are reduced to Au(0) based on their different reduction potentials.40 Therefore, Pt(II) could favor the formation of quasi-spherical Au NPs possibly because of fast nucleation.40,42 In this step, the incubation time (approximately 5−10 min) is important for the generation of quasi-spherical Au NPs before the addition of AA. In the second step, AA was added to reduce the Pt and Pd precursors to form the dendritic shells via selecting Au NPs as the core after incubation shown in panels C and D of Figure S4. In this step, the compositions and feeding molar ratios are important for the generation of TMHs. The formation of a bimetallic Au/Pt hydrogel from gold and platinum sol has not yet been reported.5 Only certain compositions like Pd, Ag, or Cu with an appropriate ratio range could favor the formation of Au@PtM (M = Pd, Ag, Cu, etc.) TMHs.9 As proven in our work, when we adjusted the atomic ratio of Pt and Pd from Au@Pt, Au@Pt4Pd, Au@Pt3Pd, Au@Pt2Pd, and Au@PtPd to Au@Pd, all the hydrogels could form except Au@Pt and Au@ Pt4Pd hydrogels while keeping the amount of Pt(II) constant (shown in Figure S5). It can be seen from TEM images in panels A and B of Figure S6 that the dendritic structure became less clear with a growing amount of Pd. The Au/Pt/Pd atomic ratios for Au@PtPd TMAs and Au@Pt2Pd TMAs in the EDS results (Figure S6C) were all in agreement with the feeding ratios of their precursors. In addition, the typical Pd and Pt crystal lattice patterns were confirmed via XRD patterns (Figure S6D). Therefore, Pd plays an important role as a linking metal in the aqueous solution for generating a “metastable state”, leading to the gelation of a Au@Pt3Pd hydrogel.9,43 Also, the addition of Cu could also favor the formation of a AuPtCu hydrogel with a Pt/Cu atomic ratio of 1. Hence, Cu could also act as a linking metal with appropriate Pt/Cu ratios to form AuPtCu TMHs shown in Figure S9. Although the underlying mechanism is not clearly understood, it could be assumed that Pd or Cu plays an important role in favoring the formation of the TMHs. Another reasonable explanation for the rapid gelation process is probably to the contribution of salts (e.g., KCl and NaCl) generated during the reaction, which is critical for increasing the ionic strength of the solution and weakening the electrostatic repulsion (ER) between the NPs. In addition, they may accelerate the transformation of the isotropic ER between NPs and the anisotropic ER, resulting in the fast anisotropic assembly of NPs stabilized by a weak capping agent

Figure 3. (A) HAADF-STEM image, (B) cross-sectional compositional line profiles, and (C−F) HAADF-STEM−EDS mapping images of the as-obtained Au@Pt3Pd TMAs.

shows the shells are dendritic structures. In addition, crosssectional compositional line profiles in Figure 3B also verified the element distribution of the Au core and PtPd alloy shell. Because the lattice parameters of Pt and Pd are quite similar, it is difficult to distinguish them via HRTEM or XRD. Therefore, we employed the HAADF-STEM−energy dispersive spectroscopy (HAADF-STEM−EDS) mapping images to further confirm the detailed composition distribution of as-prepared Au@Pt3Pd TMAs. The elemental distributions in HAADFSTEM−EDS mapping images (Figure 3D−F) demonstrate that aerogels were composed of Au, Pt, and Pd. Element Au mainly forms the inner core, while Pt and Pd cooperatively form the shells with small branches. From Figure 3, we can assume that the as-obtained Au@Pt3Pd TMAs were composed of dendritic core−shell structured NPs with Au as the core and the PtPd alloy as the shell. Mechanism of the Formation of Dendritic Core−Shell Structured Au@Pt3Pd TMHs. The synthesis of core−shell structured Au@Pt3Pd TMHs employs a self-assembly gelation strategy that includes the formation of a Au core and the in situ growth of the dendritic Pt3Pd alloy shells, as illustrated by Scheme 1. In the first step, it has already been determined that HAuCl4 could be quickly reduced to Au nanoclusters (NCs) by citrate via autocatalysis; that is, Au(III) is first reduced to Au(I) by citrate, and then Au(I) decomposes into Au(0) and Au(III).38,39 Here, the evolution of the Au nanowire in Figure S4A was a result of fusion of the Au NCs due to their high 7930

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Chemistry of Materials (i.e., citrate).6,7 Therefore, the salt generated during the reaction could promote the anisotropic self-assembly process and accelerate the formation of hydrogels. Electrochemical Performance of Dendritic Core−Shell Structured Au@Pt3Pd TMAs. It is well-known that multimetallic aerogels with core−shell structures, alloyed shells, and high porosity could improve the electrocatalytic performances toward ORR because of their enlarged active surface area, the electronic and geometric effect among different compositions of Au, Pt, and Pd,17,44−46 the easy accessibility to the reactants, etc. The calculated electrochemically active surface areas (ECSA)47 (see the Supporting Information) of the Au@PtPd, Au@Pt2Pd, and Au@Pt3Pd TMAs and commercial Pt/C from the hydrogen adsorption peak in cyclic voltammetry (CV) curves (Figure 4A) were 30.5 m2/gPt+Pd, 33.3 m2/gPt+Pd, 58.7

electrocatalytic behaviors of Pt-based electrode catalysts toward ORR. Figure 4C displays the linear sweep voltammetry (LSV) curves for ORR employing the four electrocatalysts on a rotating disk electrode (RDE) in an O2-saturated 0.1 M HClO4 solution at a rotating rate of 1600 rpm and a scan rate of 20 mV/s. The E1/2 of Au@Pt3Pd TMAs (0.897 V) shows positive shifts of 30.7, 60, and 70 mV of commercial Pt/C, Au@PtPd TMAs, and Au@Pt2Pd TMAs, respectively. The much more positive E1/2 of Au@Pt3Pd TMAs indicates that they were more active than the other three electrocatalysts toward ORR. Kinetic current (jk), which was calculated according to the Levich− Koutecky equation (see the Supporting Information), is usually used to evaluate mass activity.48 Figure 4D displays the calculated mass activity and specific activity of Au@Pt3Pd TMAs (0.812 A/mgPt+Pd and 24.3 mA/cm2, respectively) that were much higher than those of commercial Pt/C (0.412 A/ mgPt and 10.5 mA/cm2, respectively), Au@PtPd TMAs (0.085 A/mgPt+Pd and 3.3 mA/cm2, respectively), and Au@Pt2Pd TMAs (0.090 A/mgPt+Pd and 2.9 mA/cm2respectively) at 0.85 V versus the RHE. Also, the mass acitivity of the as-obtained PAu@Pt3Pd NPs calculated from the LSV in Figure S7C was ∼0.172 A/mgPt+Pd, which was much smaller than that of Au@ Pt3Pd TMAs, indicating the high porosity of the Au@Pt3Pd TMAs facilitated the mass transfer rate through interconnected channels and offered more active sites for catalytic reactions. In addition, it is reported that the observed improvement in the catalytic efficiency in the Pt3M alloys is mainly attributed to the modified electronic structure on the Pt surface.49,50 The electronic structure of the Pt3M surface was modified to reduce the Pt d-band state for reducing the chemisorption energy toward the intermediate.27,48,51 In addition, the geometric effect caused by the lattice strain could lower the bonding strength of intermediate oxygenated adsorbates on the Pt surface.29 Therefore, the dendritic alloyed shells and the high porosity facilitate the electron and mass transfer rate for improving ORR catalytic activity. The potential of ADT was determined between 0.6 and 1.1 V in an O2-saturated 0.1 M HClO4 solution with a scan rate of 100 mV/s. Panels A and D of Figure 5 show the CV curves of Au@Pt3Pd TMAs and commercial Pt/C before and after 5000 cycles. The ECSA degradation values of the Au@Pt3Pd TMAs and commercial Pt/C were ∼21 and ∼23.7%, respectively. The LSV of the Au@Pt3Pd TMAs before and after ADT in Figure 5C exhibited an ∼11.8 mV negative potential shift, while that of commercial Pt/C shifted ∼20.8 mV (Figure 5D). The enhanced stability of Au@Pt3Pd TMAs was also confirmed via the TEM image of the sample after ADT shown in Figure S8A, revealing that the dendritic core−shell structures of Au@ Pt3Pd NPs were well-maintained, while the Pt NPs of commercial Pt/C after ADT suffered from serious aggregation shown in Figure S8B. The superior stability of the Au@Pt3Pd TMAs was probably a result of the core−shell supportless structures. For one thing, it has been verified that Au was usually selected as the inner core for improving the durability of the Pt-based electrocatalysts by increasing the Pt oxidation potential45,51 or by accelerating the removal of the intermediate from the Pt surface through electron transfer.52,53 For another, the stiffness of the self-supported structure could eliminate the issues concerning the corrosion of the carbon support, aggregation, or dissolution of the building blocks for enhancing the long-term operation durability.2,6,17,54

Figure 4. CV curves of Au@PtPd, Au@Pt2Pd, and Au@Pt3Pd TMAs and commercial Pt/C with a sweep rate of 50 mV/s in (A) N2sauturated and (B) O2-sauturated 0.1 M HClO4 aqueous solutions. (C) LSV curves of Au@PtPd, Au@Pt2Pd, and Au@Pt3Pd TMAs and commercial Pt/C at a rotation rate of 1600 rpm and a scan rate of 20 mV/s. (D) Mass activity and specific activity of Au@PtPd, Au@Pt2Pd, and Au@Pt3Pd TMAs and commercial Pt/C.

m2/gPt+Pd, and 66.7 m2/gPt, respectively. Au@Pt3Pd TMAs demonstrated the largest ECSA value among the three TMAs. In addition, we have also synthesized precipitated Au@Pt3Pd NPs (P-Au@Pt3Pd NPs) with the same method except with the inclusion of stirring for 4 h, followed by precipitation overnight. From the TEM image in Figure S7A, the as-synthesized P-Au@ Pt3Pd NPs also display the core−shell dendritic structures. The calculated ECSA of P-Au@Pt3Pd NPs from CV in Figure S7B is ∼37.4 m2/gPt+Pd, which is smaller than that of Au@Pt3Pd TMAs, indicating that the ECSA could be effectively enhanced via employing the aerogel structures. The enhanced ECSA was mainly attributed to the macro-sized pores, which could reduce the mass transfer barrier and favor the accessibility of molecular fuel to active sites to promote the catalytic activity.6 The typical ORR peak between 0.7 and 1.0 V in Figure 4B is obvious in the CV curves of an O2-sautured acid solution, indicating the 7931

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for ∼11 min at room temperature until the color changed from yellow to green and black. Then, 5 mL of 0.01 M K2PtCl4 and 0.333 mL of 0.05 M Na2PdCl4 were added to the as-obtained nanowire-like Au NPs, followed by addition of 0.5 mL of 0.25 M AA, and then the glassware was moved into the water bath at 35 °C without stirring. After ∼4 h, Au@Pt3Pd TMHs were generated at the bottom of the glassware. After being washed three times with water, Au@Pt3Pd TMAs could be obtained via a supercritical fluid CO2 drying technique. Au@PtPd and Au@Pt2Pd TMAs were all obtained via the same method with the exception of the different feeding ratio of the Pt and Pd precursors. Au@PtCu TMAs were synthesized by changing the precursors and the feeding ratio. Precipitate Au@Pt3Pd NPs were also synthesized via the same method but were kept stirring during reactions for 4 h, followed by precipitation overnight. Preparation and Electrochemical Measurements of the Electrode. The homogeneous Pt/C catalyst ink (2 mg/mL) was prepared via dispersing commercial Pt/C powders into 2 mL of 2propanol, 0.05 mL of Nafion (5%), and 8 mL of deionized water via ultrasonic agitation. The electrochemical measurements were taken on a standard threeelectrode electrochemical workstation (CHI 630E) at room temperature. A Pt wire was used as the counter electrode, and a Hg/HgCl2 electrode filled with a saturated potassium chloride aqueous solution was the reference electrode. The working electrode was prepared by loading 5 μL of Au@Pt3Pd TMHs (1 mgPt/mL) on a rotating disk electrode (RDE) (5 mm in diameter, 0.19625 cm2 in geometric area) and dried at 60 °C, and the calculated loading was 25.5 μg/cm. The electrolyte was 120 mL of a 0.1 M HClO4 solution for the ORR test. CV and LSV measurements on RDE were taken on an electrode rotator (Princeton Applied Research). CV measurements with a scan rate of 50 mV/s for ORR tests were performed after nitrogen or oxygen had been purged into a 0.1 M HClO4 solution for 30 min. LSV measurements were taken in an O2-saturated electrolyte. The scan rate was 10 mV/s. Characterization. Transmission electron microscopy (TEM) images were recorded with a Philips CM200 UT instrument [Field Emission Instruments (FEI)]. An FEI sirion field emission scanning electron microscope (FESEM) was used for imaging and energy dispersive X-ray analysis (EDX). The tube was operated at an accelerating voltage of 40 kV and a current of 15 mA. X-ray diffraction (XRD) characterization was performed with a Rigaku Miniflex 600 instrument. X-ray photoelectron spectroscopy (XPS) measurements were taken with a Physical Electronics Quantera Scanning X-ray Microprobe. This system uses a focused monochromatic Al Kα X-ray (1486.7 eV) source for excitation and a spherical section analyzer. The instrument has a 32-element multichannel detection system. An 85 W X-ray beam focused to a 100 μm diameter was rastered over a 1.3 mm × 0.1 mm rectangle on the sample. The X-ray beam is incident normal to the sample, and the photoelectron detector is 45° off normal. Highenergy resolution spectra were recorded using a pass energy of 69.0 eV with a step size of 0.125 eV. The binding energy scale is calibrated using the Cu 2P3/2 feature at 932.62 ± 0.05 eV and the Au 4f feature at 83.96 ± 0.05 eV using monatomic Ar+ ion sputter-cleaned high-purity Au and Cu foils. The sample experienced variable degrees of charging. Low-energy electrons at 1 eV and 20 μA and 7 eV Ar+ ions were used to minimize this charging.

Figure 5. (A and C) CV curves of Au@Pt3Pd TMAs and commercial Pt/C on RDE before and after 5000 cycles in a N2-sauturated 0.1 M HClO4 solution with a scan rate of 50 mV/s. (B and D) ORR LSV curves of Au@Pt3Pd TMAs and commercial Pt/C before and after 5000 cycles at a rotation rate of 1600 rpm and a scan rate of 20 mV/s.

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CONCLUSION In summary, for the first time, we have successfully synthesized Au@Pt3Pd TMHs and TMAs with dendritic and core−shell structures through a facile and green method under mild conditions. The as-obtained Au@Pt3Pd TMHs exhibited superior electrochemical performances compared to that of commercial Pt/C. The outstanding catalytic activities were a result of its unique structures. (I) The 3D cross-linked structure provided a high porosity for mass transportation. (II) Its dendritic surfaces offer a large active surface area. (III) The alloyed shells and core−shell structures endow the structures with the geometric and electronic effect that favors the desorption of oxygen-containing intermediates from the Pt active sites. (IV) The self-support 3D structure could eliminate the corrosion and detachment problems that occurred in the carbon support to some degree. Taken together, because of the exceptional and adjustable properties, these unique core−shell TMHs and TMAs could be potentially used in heterogeneous catalytic reactions, biosensing devices, surface-enhanced Raman spectroscopy, transparent conductive substrates, etc.

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EXPERIMENTAL SECTION

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Chemical and Materials. Ascorbic acid (AA), potassium tetrachloroplatinate(II) (K 2 PtCl 4 ), gold(III) chloride hydrate (HAuCl4·2H2O), sodium tetrachloropalladate(II) (Na2PdCl4), and sodium citrate dihydrate were all purchased from Sigma-Aldrich Co. Ltd. All glasswares and stirring bars were cleaned with aqua regia [3:1 (v/v) HCl (37%)/HNO3 (65%) solutions] and then rinsed thoroughly with water before use. (Caution: aqua regia solutions are dangerous and should be used with extreme care; never store these solutions in closed containers.) The water in all experiments was prepared in a three-stage Millipore Milli-Q plus 185 purification system and had a resistivity of >18.2 MΩ cm. Synthesis of TMHs, TMAs, and P-Au@Pt3Pd NPs. In a typical synthesis of Au@Pt3Pd TMHs, 1.5 mL of citrate (1 wt %) was added to 1 mL of a 0.0125 M HAuCl4 aqueous solution and mixture stirred

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03549. ORR evaluation descriptions, BET plots, growth procedures and mechanisms of TMHs, TEM image and ORR performance of P-AuPt3Pd NPs, and TEM image of AuPtCu TMHs (PDF) Jellylike features of TMHs (AVI) 7932

DOI: 10.1021/acs.chemmater.6b03549 Chem. Mater. 2016, 28, 7928−7934

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Chemistry of Materials

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by start-up funds from Washington State University. We thank the Franceschi Microscopy & Image Center at Washington State University for TEM and SEM measurement. Q.S. thanks the China Scholarship Council for the financial support. XPS measurements were performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory.

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