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Nov 27, 2017 - and Sang Woo Han*,†. †. Center for Nanotectonics, Department of Chemistry and KI for the NanoCentury, KAIST, Daejeon 34141, Korea. ...
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Dendritic Ternary Alloy Nanocrystals for Enhanced Electrocatalytic Oxidation Reactions Young Wook Lee, Mintaek Im, Jong Wook Hong, and Sang Woo Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14763 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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Dendritic Ternary Alloy Nanocrystals for Enhanced Electrocatalytic Oxidation Reactions Young Wook Lee,†,§ Mintaek Im,†,§ Jong Wook Hong,*,‡ and Sang Woo Han*,† † ‡

Center for Nanotectonics, Department of Chemistry and KI for the NanoCentury, KAIST, Daejeon 34141, Korea Department of Chemistry, University of Ulsan, Ulsan 44610, Korea

ABSTRACT: Engineering the morphology and composition of multi-metallic nanocrystals composed of noble and 3d transition metals has been of great interest due to its high potential to the development of high-performance catalytic materials for energy and sustainability. In the present work, we developed a facile aqueous approach for the formation of homogeneous ternary alloy nanocrystals with a dendritic shape, Pt-Pd-Cu nanodendrites, of which synthesis is hard to be achieved due to synthetic difficulties. Proper choice of stabilizer and fine control over the amount of stabilizer and reductant allowed the successful formation of Pt-PdCu nanodendrites with controlled sizes and compositions. The prepared ternary alloy nanodendrites exhibited considerably improved electrocatalytic performance toward methanol and ethanol oxidation reactions compared to their binary alloy counterparts and commercial Pt and Pd catalysts, as well as to previously reported Pt- and Pd-based nanocatalysts due to synergism between their morphological and compositional characteristics. We anticipate that the present approach will be helpful to develop efficient electrocatalysis systems for practical applications. KEYWORDS: nanodendrites, alloy nanocrystals, Pt-Pd-Cu, electrocatalysis, oxidation reactions

INTRODUCTION Noble metal nanocrystals (NCs) have attracted tremendous interest in a wide range of research fields including heterogeneous catalysis, bio/chemical sensing, plasmonics, cancer therapy, and electrocatalysis due to their distinct catalytic and optical characteristics.1-4 In particular, noble metal NCs with precisely designed shapes and sizes have shown fascinating electrocatalytic performance in fuel cell applications.5-10 Among myriad shapes of noble metal NCs, porous dendritic structures, so-called nanodendrites, have recently received enormous attention owing to their large surface areas, high mass transport, and high density of undercoordinated surface atoms at their dendritic branches, which led to the enhancement in catalytic properties for various electrocatalytic reactions.11-15 Together with the control of NC shapes, tuning the composition of NCs is also an effective strategy to the improvement of catalytic function of NCs. As the incorporation of secondary and tertiary metals into mono- and bimetallic NCs, respectively, can not only drive the change in the binding strength of reactive species on NC surfaces but also facilitate the removal of poisoning intermediates, such as CO, adsorbed on the active sites of NCs through bifunctional, ligand, and/or strain effects,16,17 bi- and tri-metallic NCs with controlled compositional structures have exhibited enhanced catalytic performance.18-49 In particular, Pt- and/or Pd-based trimetallic NCs with judiciously tuned compositions showed pronounced catalytic properties toward fuel (methanol, ethanol, and formic acid) oxidation reactions compared to mono- and bi-metallic NCs.50-55 Furthermore, Pt- and/or Pdbased trimetallic NCs containing 3d transition metals, such as

Fe, Co, Ni, and Cu, have exhibited promising electrocatalytic function due to the modified electronic structures of constituent elements as a result of charge transfer between them and enhanced tolerance to poisoning species.30,56-62 Based on these previous findings, it can be expected that the development of a facile synthetic method for the realization of ternary alloy nanodendrites consisting of Pt, Pd, and 3d transition metal is a desirable approach for the preparation of effective catalysts toward fuel oxidation reactions. However, the synthesis of such ternary alloy nanodendrites with controlled morphologies and compositions is a formidable task due to difficulties in the manipulation of NC growth in the presence of multiple metal precursors with different reduction habits. Herein, we report a facile aqueous synthetic route toward the formation of Pt-Pd-Cu ternary alloy nanodendrites with good size and shape homogeneity. Maneuvering the NC growth through the proper choice of stabilizing agent and the control of the concentration of stabilizing and reducing agents is critical to the generation of the well-defined dendritic shape and the Pt-Pd-Cu alloy compositional structure. In the preparation of Pt-Pd-Cu ternary alloy nanodendrites, cetyltrimethylammonium chloride (CTAC) and ascorbic acid (AA) were employed as a stabilizer and a reductant, respectively. Interestingly, the adjustment of the amounts of CTAC and Cu precursor in the synthesis gave fine control over the size and relative Cu composition of Pt-Pd-Cu nanodendrites. Notably, the prepared Pt-Pd-Cu nanodendrites with controlled size and composition exhibited excellent electrocatalytic function toward both methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR) under

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various electrolyte conditions compared to their binary alloy counterparts, such as Pt-Pd, Pt-Cu, and Pd-Cu nanodendrites, and commercial Pt and Pd catalysts. Furthermore, the MOR and EOR activities of the Pt-Pd-Cu nanodendrites surpass those of previously reported Pt- and Pd-based nanocatalysts. This can be attributed to their unique dendritic morphology and ternary alloy structure.

EXPERIMENTAL SECTION Chemicals and materials. K2PtCl4 (Aldrich, 99.9%), K2PdCl4 (Aldrich, 98%), CuCl2 (Aldrich, 99.999%), CTAC (Aldrich, solution in water, 25 wt%), AA (Dae Jung Chemicals & Metals Co., 99.5%), poly(vinyl pyrrolidone) (PVP, Aldrich, MW = 55,000), cetyltrimethylammonium bromide (CTAB, Aldrich, 95%), sodium dodecyl sulfate (SDS, Fluka, 99%), tetraoctylammonium bromide (TOAB, Aldrich, 98%), Pt black (Aldrich, 99.9%), and Pd black (Aldrich, 99.9%) were used as received. Other chemicals were reagent grade and Milli-Q water with a resistivity of greater than 18.0 MΩ·cm was used in the preparation of aqueous solutions. Synthesis of Pt-Pd-Cu nanodendrites. In a typical synthesis of Pt-Pd-Cu nanodendrites, aqueous solutions of K2PtCl4 (0.5 mL, 5 mM), K2PdCl4 (0.5 mL, 5 mM), CuCl2 (0.5 mL, 5 mM), and AA (0.5 mL, 100 mM) were injected into an aqueous solution of CTAC (5 mL, 10 mM) via a syringe within few seconds with gentle stirring. The whole system was sealed, and heated at 95 °C for 40 min in a conventional forced-convection drying oven. For the preparation of Pt-PdCu0.25 and Pt-Pd-Cu2 nanodendrites, CuCl2 (0.5 mL, 2.5 mM) + CTAC (5 mL, 1 mM) and CuCl2 (0.5 mL, 10 mM) + CTAC (5 mL, 100 mM) combinations were used, respectively. Synthesis of Pt-Pd, Pt-Cu, and Pd-Cu nanodendrites. In the synthesis of Pt-Pd nanodendrites, aqueous solutions of K2PtCl4 (0.5 mL, 5 mM) and K2PdCl4 (0.5 mL, 5 mM) were injected into an aqueous solution of CTAC (5 mL, 50 mM) via a syringe within few seconds. For the preparation of Pt-Cu nanodendrites, aqueous solutions of K2PtCl4 (0.5 mL, 5 mM), CuCl2 (0.5 mL, 5 mM), and AA (0.5 mL, 100 mM) were injected into an aqueous solution of CTAC (5 mL, 50 mM), and aqueous solutions of K2PdCl4 (0.5 mL, 5 mM), CuCl2 (5 mL, 5 mM), and AA (0.5 mL, 100 mM) were injected into an aqueous solution of CTAC (5 mL, 50 mM) for the synthesis of Pd-Cu nanodendrites. Then the resultant reaction mixtures were sealed, and heated at 95 °C for 2 h in the drying oven. Characterization. Transmission electron microscopy (TEM) images were obtained with a JEOL JEM-2010 transmission electron microscope operating at 200 kV. Highresolution TEM (HRTEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) characterizations were performed with a FEI Tecnai G2 F30 Super-Twin transmission electron microscope operating at 300 kV after placing a drop of hydrosol on carbon-coated Ni grids (200 mesh). The compositions of products were determined by inductivelycoupled plasma-mass spectrometry (ICP-MS, Agilent 7700S). X-ray diffraction (XRD) patterns were obtained with a Bruker AXS D8 DISCOVER diffractometer using Cu Kα (0.1542 nm) radiation. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo VG Scientific Sigma Probe spectrometer with Al Kα X-ray (1486.6 eV) as

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the light source. XPS data were calibrated using the C 1s peak at 284.5 eV. Electrochemical measurements. Electrochemical measurements were carried out in a three-electrode cell using a CH Instruments Model 600C and 760D potentiostats. Pt wire and Ag/AgCl (in saturated NaCl) were used as the counter and reference electrodes, respectively. All cyclic voltammograms (CVs) were obtained at room temperature. Electrolyte solutions were purged with high-purity N2 gas before use for 40 min. The drop-casting films of catalysts on glassy carbon electrodes (GCEs, diameter = 3 mm) served as working electrodes. Prior to conducting CV measurements, 4 µL of an aqueous catalyst solution (0.25 mg mL-1) was dropped onto the GCE to yield a metal loading of 1 µg. After the solution was dried, a 4 µL Nafion solution (0.05 wt%) was dropped, followed by drying in a dry-keeper. The dried GCE was cleaned by washing with acetone, water, and ethanol and then electrochemically cleaned by 50 potential cycles at a scan rate of 50 mV s-1 between -0.2 and 1.0 V vs. Ag/AgCl in an acidic electrolyte solution (HClO4 or H2SO4) or -0.8 and 0.3 V vs. Ag/AgCl in an alkaline electrolyte solution (KOH) to remove residual stabilizing agents on the catalyst surfaces. For COstripping experiments, the surface of the catalysts on GCE was saturated with CO by bubbling CO gas in 0.1 M KOH while holding the working electrode at -0.35 V vs. Ag/AgCl for 20 min and then the remaining CO was purged by high-purity N2 gas for 20 min.

RESULTS AND DISCUSSION Figure 1a shows a representative TEM image of the product, demonstrating that majority of the prepared NCs were multibranched nanodendrites with an average size of 20.9 ± 3.3 nm. The average thickness of branches in the nanodendrites was 3.5 ± 0.5 nm. Fast Fourier transform (FFT) pattern obtained from a nanodendrite along the [011] zone axis indicates the highly crystalline nature of the prepared NCs (inset of Figure 1a). The HRTEM image of a nanodendrite shows the dspacing of 2.19 Å between adjacent lattice fringes, which matches well with that of the (111) planes of face-centered cubic (fcc) Pt-Pd-Cu alloy (Figure 1b).60 Notably, numerous under-coordinated surface atoms at the branches of the nanodendrite were clearly identified in the HRTEM image (denoted by an arrow in Figure 1b). This inherent structural feature of the Pt-Pd-Cu nanodendrites would lead to the improvement in electrocatalytic performance.63,64 To investigate the detailed structural characteristics of the nanodendrites, HAADF-STEM image of a nanodendrite was obtained (Figure 1c). The strong contrast was explicitly observed at multiple spots in the nanodendrite, indicating the high porosity of the prepared nanodendrites. The compositional structure of the nanodendrites was investigated by elemental mapping (Figure 1c) and cross-sectional compositional line profile analysis (Figure 1d) with HAADFSTEM-energy-dispersive X-ray spectroscopy (HAADFSTEM-EDS), which unequivocally demonstrated the formation of homogeneous Pt-Pd-Cu alloy structure. The Pt:Pd:Cu atomic ratio of the nanodendrites estimated by ICPMS was 34:37:29. The XRD pattern of the nanodendrites shows that all diffraction peaks appeared between those of pure fcc Pt/Pd and Cu references, further corroborating their ternary alloy nature (Figure 1e). The position of the (111) diffraction peak of the nanodendrites was 41.20, which is in

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good agreement with that calculated from Vegard’s law65 by using the ICP-MS-determined Pt:Pd:Cu atomic ratio, i.e., 41.10. The average crystallite size of the nanodendrites, calculated using the Scherrer equation,66 was 3.8 nm, which is very close to the average thickness of branches in the nanodendrites. In addition, XPS measurements for the Pt 4f, Pd 3d, and Cu 2p core levels of the Pt-Pd-Cu nanodendrites revealed that the Pt, Pd, and Cu in the nanodendrites were mostly in metallic state [Figure S1 in the Supporting Information (SI)].

Figure 1. (a) TEM and (b) HRTEM images of Pt-Pd-Cu nanodendrites. Inset in a shows a FFT pattern obtained from a nanodendrite along the [011] zone axis. (c) HAADF-STEM image and corresponding EDS elemental mapping images of a nanodendrite. Scale bar indicates 10 nm. (d) HAADF-STEM image and corresponding EDS cross-sectional compositional line profiles of a nanodendrite. Scale bar indicates 50 nm. (e) XRD pattern of Pt-Pd-Cu nanodendrites. The positions of Pt, Pd, and Cu references were taken from the JCPDS database.

The shape evolution of Pt-Pd-Cu nanodendrites during the reaction was monitored by measuring the TEM images of products collected at different reaction times (Figure 2a-f). Initially, nanodendrites with immature branches rapidly formed within 5 min (Figure 2a). As the reaction proceeded, the branches of nanodendrites gradually developed until 15 min (Figure 2b,c). Beyond 15 min, no significant morphological change was observed (Figure 2d-f). To investigate the change in the composition of nanodendrites during the course of the reaction, the Pt:Pd:Cu atomic ratios of the products obtained at different reaction times were also measured by ICP-MS (Figure 2g). The Pt:Pd:Cu atomic ratio of the product obtained at 5 min was 38:57:5. The lower Cu content in the initially formed nanodendrites compared to that in the final product can be attributed to much lower reduction potential of Cu2+ (0.3419 V vs. standard hydrogen electrode

(SHE)) than those of PtCl42- (0.755 V vs. SHE) and PdCl42(0.591 V vs. SHE).67 Notably, despite the lower reduction potential of PdCl42- than that of PtCl42-, the relative composition of Pd in nanodendrites was much higher than that of Pt at the initial stage. This can be ascribed to the stronger binding affinity of PtCl42 to the stabilizer, CTA+, than PdCl42-, which could retard its reduction kinetics.51 As the reaction further proceeded, the relative compositions of Cu and Pd in nanodendrites gradually increased and decreased, respectively, until 15 min and then plateaued. This might be due to the catalytic function of growing nanodendrites for the reduction of remaining metal precursors in the reaction solution, which could accelerate the reduction kinetics of Cu and Pt precursors.68

Figure 2. TEM images of Pt-Pd-Cu nanodendrites collected at different reaction times: (a) 5, (b) 10, (c) 15, (d) 20, (e) 30, and (f) 40 min. (g) Changes in the composition of each metal element during the reaction estimated by ICP- MS.

Given that stabilizing and reducing agents employed in the synthesis of NCs have profound influences on the nucleation and growth of NCs, the proper choice of the type and amount of stabilizer and reductant is critical to the formation of NCs with a desired morphology. To explore the effect of stabilizer on the shape of resultant NCs, we conducted reactions with other stabilizers, such as PVP, CTAB, SDS, and TOAB, instead of CTAC under experimental conditions identical to those employed in the synthesis of the Pt-Pd-Cu nanodendrites. TEM images of products obtained in the presence of the other stabilizers showed the formation of NCs with irregular shapes and wide size distributions (Figure S2 in the SI), demonstrating the pivotal role of CTAC in the formation of the Pt-Pd-Cu nanodendrites. Furthermore, the size of Pt-Pd-Cu nanodendrites was sensitive to the amount of CTAC stabilizer used in the synthesis. As abovementioned, the standard synthesis with 10 mM of CTAC solution yielded the Pt-Pd-Cu

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nanodendrites with an average size of 20.9 ± 3.3 nm. As shown in Figure 3, the average size of nanodendrites increased monotonically as the amount of CTAC increased: Pt-Pd-Cu nanodendrites with average sizes of 28.7 ± 3.1, 34.5 ± 2.3, and 43.0 ± 4.5 nm were generated when 30, 50, and 100 mM of CTAC solutions were used in the reaction, respectively. This can be attributed to the augmented interaction between CTAC and metal precursors with the increase in the amount of CTAC, which can lead to the decrease in the number of initially formed seeds during the formation of nanodendrites, resulting in the generation of larger nanodendrites.69 The ICP-MSdetermined Pt:Pd:Cu atomic ratios of the nanodendrites produced using different amounts of CTAC were almost same despite of their different sizes (Table S1 in the SI). On the contrary, when the amount of CTAC was smaller than that used in the standard synthesis, nanodendrites with immature branches were produced together with irregularly shaped small nanoparticles (Figure S3 in the SI). On the other hand, the influence of the amount of reductant, AA, on the morphological and compositional structures of nanodendrites was also examined (Figure S4 in the SI). In the absence of AA, dendritic nanostructures with an average size of 53.2 ± 10.2 nm were formed (Figure S4a in the SI). Notably, the HRTEM image of the product showed the d-spacing of 2.23 Å between adjacent lattice fringes, which is close to that of the (111) planes of fcc Pt-Pd alloy (Figure S4b in the SI).70,71 Crosssectional compositional line profile analysis with HAADFSTEM-EDS further revealed their Pt-Pd alloy structure (Figure S4c in the SI). These results indicate that Cu could hardly be reduced in the absence of AA under our experimental conditions. In the presence of AA, of which the final concentration ranged from 3.0 to 13.3 mM (standard protocol = 7.1 mM), nanodendrites with analogous shape and size to the typical Pt-Pd-Cu nanodendrites were obtained irrespective of the amount of AA (Figure S4d,g,j in the SI). The d-spacing values for adjacent lattice fringes (Figure S4e,h,k in the SI) and EDS cross-sectional compositional line profiles (Figure S4f,i,l in the SI) of the nanodendrites prepared with different amounts of AA indicated that they had the PtPd-Cu alloy compositional structure. Taken together, these results of control experiments confirm the importance of the employment of a proper stabilizing agent and the control of the concentration of stabilizing and reducing agents in the synthesis of Pt-Pd-Cu ternary alloy nanodendrites with welldefined structural and compositional characteristics.

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Figure 3. TEM images of nanodendrites prepared with (a) 30, (b) 50, and (c) 100 mM of CTAC solutions. (d) Size distributions of nanodendrites prepared with different amounts of CTAC.

To prove our expectation that ternary alloy nanodendrites consisting of Pt, Pd, and 3d transition metal can be effective catalysts toward fuel electrooxidation reactions, the electrocatalytic properties of the Pt-Pd-Cu nanodendrites toward both MOR and EOR were investigated. To find an optimum size of Pt-Pd-Cu nanodendrites for electrocatalysis, the size-dependent electrocatalytic activity of the Pt-Pd-Cu nanodendrites was explored first with the nanodendrites prepared using different amounts of CTAC. Based on the Coulombic charges for oxygen desorption in the CVs of the nanodendrites in 0.1 M KOH (Figure S5a in the SI), the electrochemically active surface areas (ECSAs) of the nanodendrites with average sizes of 20.9, 28.7, 34.5, and 43.0 nm, which were prepared using 10, 30, 50, and 100 mM of CTAC solutions, respectively, were calculated to be 39.3, 37.1, 34.6, and 31.4 m2 g-1, respectively (Figure S5b in the SI).22 The Pt-Pd-Cu nanodendrites produced with 10 mM of CTAC solution (the standard protocol) showed the highest ECSA due to their smallest size among the different nanodendrites. Furthermore, they exhibited the highest mass and specific activities toward both MOR and EOR among the different nanodendrites (Figure S5c-f in the SI). The mass and specific activities of a catalyst were estimated by the normalization of peak current values during the forward scans in the CVs of MOR and EOR with the catalyst to the mass and ECSA of the catalyst, respectively. Based on these results, we chose the PtPd-Cu nanodendrites obtained from the standard synthesis for further electrochemical studies. To examine the morphological and compositional advantages of the Pt-Pd-Cu nanodendrites in electrocatalysis, their electrocatalytic properties were benchmarked against binary alloy counterparts, such as Pt-Pd, Pt-Cu, and Pd-Cu alloy nanodendrites, and commercial Pt and Pd black catalysts. Pt-Pd, Pt-Cu, and Pd-Cu nanodendrites, which were prepared using binary metal precursors, had dendritic morphologies similar to the Pt-Pd-Cu nanodendrites with average sizes of 23.1 ± 3.5, 21.6 ± 2.9, and 21.9 ± 3.7 nm, respectively, and had homogeneous alloy compositional structures (Figure S6 in the SI). The ICP-MS-determined Pt:Pd, Pt:Cu, and Pd:Cu atomic ratios of the prepared Pt-Pd, Pt-Cu, and Pd-Cu

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nanodendrites were 43:57, 63:37, and 61:39, respectively. In addition, the Pt-Pd, Pt-Cu, and Pd-Cu nanodendrites exhibited ECSAs of 39.6, 35.4, and 38.5 m2 g-1, respectively, which are very close to that of the Pt-Pd-Cu nanodendrites (Figure S7a,b in the SI). The ECSA values of commercial Pt and Pd black catalysts were 24.2 and 23.5 m2 g-1, respectively (Figure S7a,b in the SI). The larger ECSAs of the prepared ternary and binary alloy nanodendrites compared to the commercial catalysts can be attributed to their dendritic morphologies. XPS measurements for the Pt 4f, Pd 3d, and Cu 2p core levels of the binary alloy nanodendrites and Pt and Pd black catalysts showed that the Pt, Pd, and Cu in the catalysts were mostly in metallic state (Figure S1 in the SI). Figure 4a shows the CVs of MOR obtained with the different catalysts in 0.1 M KOH containing 0.5 M methanol. Current values were normalized to the mass of the catalysts loaded on the GCE. Notably, the PtPd-Cu nanodendrites showed the most pronounced mass activity for the MOR among the different catalysts: the mass activities of the Pt-Pd-Cu nanodendrites, Pt-Pd nanodendrites, Pt-Cu nanodendrites, Pd-Cu nanodendrites, Pt black, and Pd black were 1447, 1066, 809, 626, 568, and 419 mA mg-1, respectively (Figure 4a,b). The similar trend across the different Pt-containing catalysts was also observed in the specific activity (Figure 4b and Figure S7c in the SI). Furthermore, the Pt-Pd-Cu nanodendrites also exhibited promoted electrocatalytic activity for the EOR compared to their binary alloy counterparts and commercial monometallic catalysts (Figure 4c,d and Figure S7d in the SI).

large ECSAs11,14 and the high density of under-coordinated surface atoms at their branches,63,64 and synergism between constituent elements, i.e., ligand and bifunctional effects.9,16,17 Notably, despite the structural similarity between the alloy nanodendrites, the Pt-Pd-Cu ternary alloy nanodendrites exhibited superior MOR and EOR activities over their binary alloy counterparts. This can be attributed to the synergistic integration of the promotional effect of Cu on the catalytic function of both Pt and Pd, which is evidenced by the improved MOR and EOR activities of the Pt-Cu and Pd-Cu nanodendrites, respectively, and mutual promotion effect between Pt and Pd, which is corroborated by the enhanced electrocatalytic activity of the Pt-Pd nanodendrites toward the both MOR and EOR compared to their monometallic counterparts. These promotion effects may be due to the abovementioned ligand effect, which is associated with the modification of the electronic structures of Pt and Pd upon the alloy formation. To examine the surface electronic structure modification of the nanodendrites, XPS measurements on the valence band region were performed and the positions of the d-band center of the different catalysts relative to the Fermi level were calculated based on the XPS spectra (Figure S8 in the SI). Indeed, the XPS measurements of the alloy nanodendrites distinctly demonstrated the modification of the electronic structures with the alloy formation. The d-band center positions of the Pt-Pd-Cu, Pt-Pd, Pt-Cu, and Pd-Cu nanodendrites were estimated to be -4.16, -3.92, -3.96, and 3.85 eV, respectively, and those of the Pt and Pd black

Figure 4. CVs obtained with various catalysts in (a) 0.1 M KOH + 0.5 M methanol and (c) 0.1 M KOH + 0.5 M ethanol at a scan rate of 50 mV s-1. Catalytic activities of various catalysts for (b) MOR and (d) EOR.

The significant enhancement in the catalytic activity of the alloy nanodendrites compared to monometallic catalysts can be attributed to their morphological characteristics, such as

catalysts were -3.22 and -2.56 eV, respectively. Notably, the Pt-Pd-Cu nanodendrites exhibited the lowest d-band center position among the different catalysts and the binary alloy

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nanodendrites showed lower d-band center positions compared to the Pt and Pd black catalysts. Given the fact that catalysts having low-lying d-band centers weakly bind poisoning intermediates, such as CO, produced during electrocatalytic reactions, which can lead to the promoted electrocatalytic activity,72-74 the enhanced catalytic activity of the alloy nanodendrites compared to the Pt and Pd black catalysts could be attributed to the decrease in the binding energy for poisoning intermediates on their surfaces as well as to their structural characteristics. The most pronounced catalytic activity of the Pt-Pd-Cu nanodendrites among the different nanodendrites can thus be ascribed to the highest tolerance for poisoning reaction intermediates. This was further corroborated by CO stripping experiments. Apparently, the PtPd-Cu nanodendrites exhibited a current peak for CO oxidation at the most negative potential among the nanodendrites in CO stripping voltammograms (Figure S9 in the SI), indicating their higher CO-removal capability than the binary alloy nanodendrites.17,23 To further explore the promotional effect of Cu on the electrocatalytic function of the Pt-Pd-Cu nanodendrites, Pt-PdCu alloy nanodendrites with ICP-MS-determined Pt:Pd:Cu atomic ratios of 41:48:11 (Pt-Pd-Cu0.25 nanodendrites) and 24:28:48 (Pt-Pd-Cu2 nanodendrites) were also prepared by controlling the amount of CTAC and CuCl2 during the synthesis (see Experimental Section for synthesis details) and their electrocatalytic properties were compared with those of the standard Pt-Pd-Cu nanodendrites. The prepared Pt-PdCu0.25 and Pt-Pd-Cu2 nanodendrites had dendritic morphologies similar to the standard Pt-Pd-Cu nanodendrites with homogeneous Pt-Pd-Cu ternary alloy structures (Figure S10 in the SI). The ECSAs of the Pt-Pd-Cu0.25 and Pt-Pd-Cu2 nanodendrites were 42.0 and 36.3 m2 g-1, respectively, which are similar to that of the standard Pt-Pd-Cu nanodendrites (Figure S11 in the SI). Interestingly, the electrocatalytic activity of Pt-Pd-Cu nanodendrites highly depended on their compositions (Figure S12 in the SI). The standard Pt-Pd-Cu nanodendrites showed the highest MOR and EOR activities among the different Pt-Pd-Cu nanodendrites, demonstrating that the relative composition of Cu has a profound influence on the catalytic function of Pt-Pd-Cu nanodendrites. To elucidate the plausible origin of the higher electrocatalytic activity of the standard Pt-Pd-Cu nanodendrites compared to the Pt-Pd-Cu0.25 and Pt-Pd-Cu2 nanodendrites, the CO removal capacities of the different Pt-Pd-Cu nanodendrites were investigated by CO stripping experiments. Evidently, the standard Pt-Pd-Cu nanodendrites showed enhanced resistance against CO poisoning compared to the Pt-Pd-Cu0.25 and Pt-PdCu2 nanodendrites (Figure S13 in the SI). Taken together, the controlled composition of nanodendrites can endow them with optimized electronic structures and high CO tolerance, thus leading to the promotion of their electrocatalytic performance.

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Figure 5. Comparison of electrocatalytic activities of various catalysts for (a) MOR in acidic media and (b) MOR and EOR in alkaline media. Corresponding references are denoted in parentheses. The electrocatalytic activities and electrolyte conditions of various catalysts are summarized in Table S2-4 in the SI.

To further demonstrate that the present strategy is promising for the development of high-performance electrocatalysts toward fuel electrooxidation reactions, the MOR and EOR activities of the Pt-Pd-Cu nanodendrites were extensively compared with those of recently reported Pt- and Pd-based nanocatalysts with various structures and compositions.12,17,2251,53,54,61,62 For comparative purpose, electrocatalysis experiments under other electrolyte conditions, which were employed in some of the previous studies, such as 0.1 M HClO4 + 0.5 M methanol, 0.5 M H2SO4 + 0.5 M methanol, 1 M KOH + 1 M methanol, and 1 M KOH + 1 M ethanol, were also performed (Figure S14 in the SI). The MOR and EOR activities of various catalysts including the Pt-Pd-Cu nanodendrites are summarized in Figure 5 and Table S2-4 in the SI. The electrocatalytic activities of the Pt-Pd-Cu nanodendrites shown in Figure 5 were obtained under the above electrolyte conditions, and normalized to the Pt and (Pt + Pd) mass of the Pt-Pd-Cu nanodendrites for MOR in acidic media (Figure 5a) and MOR and EOR in alkaline media (Figure 5b), respectively, to compare them with those of the previous catalysts. Apparently, the Pt-Pd-Cu nanodendrites exhibited superb catalytic activities over the previously reported catalysts, which can be attributed to their unique dendritic morphology and Pt-Pd-Cu ternary alloy compositional structure.

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ACS Applied Materials & Interfaces In summary, we developed an efficient aqueous synthetic method for the preparation of homogeneous Pt-Pd-Cu ternary alloy nanodendrites with a high density of under-coordinated surface atoms at their branches. The morphological and compositional structures of nanodendrites were highly dependent on the concentration of stabilizing and reducing agents. The prepared Pt-Pd-Cu nanodendrites showed excellent electrocatalytic performance toward both methanol and ethanol oxidation reactions in comparison to their binary alloy counterparts and commercial catalysts, as well as to previously reported Pt- and Pd-based nanocatalysts, which can be attributed to their unique morphological characteristics and Pt-Pd-Cu ternary alloy compositional structure. We expect that the present approach will find uses in the development of advanced electrocatalysts for fuel cell applications and it can be extended to designing multi-component nanomaterials with unprecedented morphologies and functions.

ASSOCIATED CONTENT Supporting Information. Additional experimental data (Figures S1-15 and Table S1) and electrocatalytic activities and electrolyte conditions of various catalysts (Tables S2-4). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

Figure 6. CVs obtained before and after 200 MOR (blue) and EOR (red) cycles for (a) Pt-Pd-Cu nanodendrites, (b) Pt-Pd nanodendrites, (c) Pt black, and (d) Pd black catalysts in 0.1 M KOH at a scan rate of 50 mV s-1. Electrocatalytic stabilities of different catalysts in (e) MOR and (f) EOR.

The stability of electrocatalysts is one of the most critical issues in their applications. In this regard, the electrocatalytic stabilities of the different catalysts were evaluated by repeated MOR and EOR cyclings (Figure 6a-d). The Pt-Pd nanodendrites were chosen as representative binary alloy nanodendrites for the stability test. After 200 MOR cycles, the Pt-Pd-Cu nanodendrites, Pt-Pd nanodendrites, Pt black, and Pd black catalysts showed losses of 12, 8, 35, and 25% in mass activity, respectively (Figure 6e). Meanwhile, 16, 13, 31, and 34% activity losses were observed for the Pt-Pd-Cu nanodendrites, Pt-Pd nanodendrites, Pt black, and Pd black catalysts, respectively, after 200 EOR cycles (Figure 6f). These results demonstrate the higher electrocatalytic stability of the nanodendrites relative to the commercial Pt and Pd catalysts. TEM images of the nanodendrites after the stability tests further revealed that their morphologies were almost preserved, whereas considerable agglomeration was observed for Pt and Pd catalysts after the tests (Figure S15 in the SI). The enhanced stability of the nanodendrites during the oxidation reactions can thus be attributed to their strong tolerance toward undesirable aggregation that can lead to the decrease in the number of active surface sites. After the stability test, the Cu content of the Pt-Pd-Cu nanodendrites somewhat decreased due to the oxidation of Cu during the repeated electrocatalytic cycles. For instance, the ICP-MSdetermined Pt:Pd:Cu atomic ratio of the Pt-Pd-Cu nanodendrites after 200 MOR cycles was 38:42:20.

CONCLUSIONS

[email protected] (SWH); [email protected] (JWH)

Author Contributions §

These authors contributed equally to this work.

ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program (2015R1A3A2033469) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT.

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