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Composition Controllable Synthesis of PtCu Nanodendrites with Efficient Electrocatalytic Activity for Methanol Oxidation Induced by High Index Surface and Electronic Interaction Linfang Lu, Shutang Chen, Sravan Thota, Xudong Wang, Yongchen Wang, Shihui Zou, Jie Fan, and Jing Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05629 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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The Journal of Physical Chemistry

Composition Controllable Synthesis of PtCu Nanodendrites with Efficient Electrocatalytic Activity for Methanol Oxidation Induced by High Index Surface and Electronic Interaction Linfang Lu,†,‡ Shutang Chen,‡ Sravan Thota,‡ Xudong Wang,‡ Yongchen Wang,‡ Shihui Zou,† Jie Fan*,† and Jing Zhao*,‡# †Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, Hangzhou 310027, China ‡Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269, United States #

Institute of Materials Science, University of Connecticut, Storrs, CT, 06269-3136, USA

ACS Paragon Plus Environment

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ABSTRACT

Metal nanodendritic structures have attracted a lot of attention because of their high activity towards catalytic reactions. Herein, we present a facile method for the one-pot synthesis of highly branched PtCu alloy nanodendrites. The composition of the PtCu nanodendrites can be easily tuned by changing the molar ratio of the precursors. The PtCu nanodendrites exhibit efficient catalytic activity towards the methanol oxidation reaction (MOR). Particularly, the Pt1Cu1 nanodendrites exert 4.6 times increase in the specific activity and 3.8 times increase in the mass activity compared to the commercial Pt/C catalyst. The mechanism of the enhancement was comprehensively studied. The enhanced catalytic activities can be ascribed to the high index surface of the branched structure and the electronic effect between the alloy metals. Specifically, the addition of Cu downshifts the binding energy of Pt, increasing the CO-tolerance ability of PtCu nanodendrites, and hence improves their MOR activities. Moreover, the PtCu nanodendrites display better stability and durability for MOR compared to Pt/C. The approach can be adapted to synthesize desired Pt-based nanodendrites for various catalytic reactions.


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1. INTRODUCTION Direct methanol fuel cells (DMFCs) are highly promising power sources for electrical vehicles and portable electronic devices because of their high energy density, convenient fuel storage, and low environmental pollution.1-5 Catalysts are the critical components in DMFCs due to the sluggish kinetics of the methanol oxidation reaction (MOR) at the anode. To date, Pt is generally considered as the most efficient catalyst for MOR.6-7 However, the high cost of the precious platinum and loss of catalytic activity over long term usage still need to be addressed before large-scale commercialization of DMFCs can be realized.8-9 What’s more, the intermediates, CO molecules, generated during the electro-oxidation of methanol tend to strongly adsorb onto monometallic Pt catalysts, thus reducing their catalytic activity and stability.10 Alloying Pt with less expensive 3d-transition metals (M) such as Ni, Co, Cu, Fe to form PtM alloy is an effective way to improve the CO-tolerance ability of Pt and increase its electrocatalytic activity, accompanied by reducing the consumption of Pt.11-13 Although great progress has been made in this area, the activity, stability and durability of nano-sized PtM alloy for MOR still need to be improved.14 Morphology of the catalysts is a crucial factor that affects their catalytic activity and stability. Nanoparticles with various morphologies such as cubes,15-16 cages,17-18 dendrites,19-20 wires,21-22 octahedras,23 frames24 and mesoporous structures25-26 were synthesized to promote the activity and stability of the nanoparticles for different catalytic reactions. Among the morphologies mentioned above, nanodendritic structures have attracted great interest since they exhibit outstanding properties towards catalytic reactions.27-28 Nevertheless, the reasons why the dendritic structure shows better performance needs to be studied and confirmed. Several methods have been developed to synthesize monometallic or alloy nanodendrites, such as seed mediated


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growth,29-30 co-chemical reduction,31-33 and electrochemical depositon.34 For example, Lim et al. used Pd nanoparticles as seeds to synthesize bimetallic Pd-Pt nanodendrites, which show good activity towards oxygen reduction reaction (ORR).35 Fu and coworkers used a co-reduction approach to prepare Pt-Pd alloy nanodendrites, which show enhanced ORR activity and remarkable methanol-tolerant ability in acid media.36 Feng et al. prepared surface clean Au nanodendrites through a low-potential electrochemical approach.37 However, the above methods require relatively costly materials, multiple synthetic steps, and rigorous fabricating conditions. What’s more, the composition of the PtM nanodendrites is uneasy to be precisely controlled. From the above discussion, it is highly desirable to prepare composition controlled PtM nanodendrites with a high catalytic activity and stability, as well as a good CO-tolerance ability towards methanol oxidation reaction. Furthermore, the reasons for the enhcancd activity need to be studied and confirmed. Herein, we report a facile synthesis to fabricate branched PtCu alloy nanodendrites. The compositions of Pt and Cu can be readily tuned by adjusting the molar ratio of the precursors. Specifically, Pt1Cu1 nanodendrites exhibit 4.6 times specific activity and 3.8 times mass activity enhancement towards methanol oxidation, as well as better stability and durability compared to the commercial Pt/C. The mechanism of the enhancement was carefully studied. The enhanced catalytic activity of PtCu nanodendrites can be ascribed to the high index surface of nanodendritic structure and the modified electronic structure of Pt because of electron donation from Cu to Pt. The synthetic approach can be used to prepare PtFe nanodendrites or other PtM nanodendrites. 2. Experimental Section 2.1 Chemicals. Copper 2,4-pentanedionate hydrate (Cu(acac)2, 97%), iron 2,4-pentanedionate hydrate (Fe(acac)3, 97%), chloroplatinic acid hexahydrate (H2PtCl6 6H2O, 99.9%), oleylamine


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(OLA, 70%), and hexadecylamine (HDA, 90%) were purchased from Sigma Aldrich. Commercial Pt/C (20 wt% of Pt, Pt nanoparticles: 3.0 nm) catalyst, methanol (CH3OH, 99.9999%), sulfuric acid (H2SO4, 99.99%), and Nafion (5%) were purchased from Alfa Aesar. All chemicals were used as received. 2.2 Synthesis of PtCu alloy nanodendrites. Pt1Cu1 alloy nanodendrites were synthesized using the following procedure. Briefly, HDA (17 mmol, 4.6 g) and H2PtCl6·6H2O (0.05 mmol, 25.9 mg) were added to a 50 mL three-neck round bottom flask. Nitrogen was used in advance to protect the reaction system. Then the temperature was raised under magnetic stirring with a heating rate of 20 oC min-1. When the temperature was raised to 150 oC, Cu(acac)2 (13.1 mg, 0.05 mmol) in 1.0 mL of OLA (dissolved by mild heating in advance) was injected into the flask. The temperature was raised to 200 oC and the reaction was allowed to proceed for 45 min at 200 o

C. The collected samples were cooled to 100 oC and toluene was added, followed by

centrifugation for 2 min (5000 rpm). The synthesis of Pt3Cu1 and Pt1Cu3 nanodendrites also followed the same protocol except that the amount of Cu(acac)2 was changed to 4.4 mg and 39.3 mg, respectively. 2.3 Synthesis of PtFe alloy nanodendrites. The above method was slightly modified to synthesize PtFe alloy nanodendrates expect that the reaction temperature was at 240 oC. To synthesize Pt3Fe1, Pt1Fe1 and Pt1Fe3 nanodendrites, the amount of Fe(acac)3 used was 5.9 mg, 17.7 mg and 53.0 mg, respectively. All the other reaction conditions remained the same. 2.4 Characterizations. Transmission electron microscopy (TEM) images were recorded on a JEOL 2010 microscope with an accelerating voltage of 200 kV. High-resolution TEM (HRTEM) images, high angle annular dark field scanning transmission electron microscopy (HAADFSTEM) images, and energy-dispersive X-ray (EDS) mapping were obtained using an FEI-Talos


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microscope at an accelerating voltage of 200 kV. X-ray diffraction (XRD) patterns were acquired using a Rigaku Ultima IV power X-ray Diffractometer with Cu Ka radiation operated at a tube voltage of 40 kV and current of 40 mA. X-ray photoelectron spectroscopy (XPS) measurements were performed in a VG Scientific ESCALAB Mark II spectrometer equipped with two ultrahigh vacuum chambers. All binding energies were referenced to the C 1s peak at 284.6 eV of the surface adventitious carbon to correct the shift caused by charging effect. Inductively coupled plasma mass spectrometry (ICP-MS) analysis was performed on the ELAN DRC-e equipment. 2.5 Electrochemical measurements. All the electrochemical measurements were carried out on an electrochemical workstation (CH Instruments, Inc., Model CHI 627E) at room temperature (~25 oC), using a three-electrode electrochemical system. The cell consisted of a glassy carbon working electrode (5 mm diameter, 0.196 cm2), a carbon rod counter electrode, and a saturated calomel electrode (SCE). The SCE was isolated by a double reference electrode to ensure the stability of SCE during the entire measurement. The potential measured against a SCE was converted to the potential versus the reversible hydrogen electrode (RHE) according to EvsRHE = EvsSCE + 0.2438. To prepare catalyst-modified working electrodes, a desired amount of catalyst was dispersed in 2 mL of ethanol and then 50 µL of nafion was added. The mixture was sonicated to form a 1 mgPt mL-1 catalyst ink. ~3 µL of this ink was dropped onto the surface of the GC electrode (the actual Pt loadings on electrode were controlled at 15 µgPt cm-2 by ICP-MS), and then dried under the ambient condition. The electrochemical active surface areas (ECSAs) were calculated by integrating the coulombic charge for hydrogen desorption in the cyclic voltammetries (CVs) recorded in N2 saturated 0.5 M H2SO4 aqueous solution.38 The methanol oxidation reaction was conducted in a


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N2-saturated 0.5 M H2SO4 + 0.5 M CH3OH solution. CV profiles were recorded at a sweep rate of 50 mV s-1 or 10 mV s-1, and in a potential window between 0.03 and 1.24 V vs. RHE. Chronoamperommetry (CA) curves were recorded in 0.5 M H2SO4 + 0.5 M CH3OH aqueous solution at 0.85 V vs. RHE for 1,000 s. The accelerated durability test was performed by continuous cycling between 0.6 V and 1.0 V vs. RHE in 0.5 M H2SO4 solution. 3. RESULTS AND DISCUSSION Using the synthetic method described above, Pt-Cu nanodendrites of varying compositions were synthesized. In the following discussion, PtxCuy is used to describe the nanodendrites, where x : y is the molar ratio of the Pt and Cu precursors. We also found that the molar ratio of Pt and Cu in the PtCu nanodendrites was similar to the molar ratio of Pt and Cu precursors (discussed below).


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Figure 1. Morphology and structure characterizations of Pt1Cu1 nanodendrites. (a) TEM image, (b) XRD pattern, (c) HAADF-STEM images and (d) HRTEM image of as-synthesized Pt1Cu1 nanodendrites.

The morphology, structure and composition analyses for the Pt1Cu1 nanodendrites are shown in Figure 1. The transmission electron microscopy (TEM) image (Figure 1a) shows that the asprepared Pt1Cu1 nanodendrites are uniformly dispersed with branched morphology and an average size of ~100 nm. The crystal phase of Pt1Cu1 nanodendrites was determined by X-ray diffraction (XRD) (Figure 1b). The peaks appeared at 41.2o and 47.9o can be indexed to the (111) and (200) facets of Pt1Cu1 alloy (PDF #48-1549). The high-resolution TEM (HRTEM) image in Figure 1d shows that the calculated lattice spacings are 0.219 nm and 0.19 nm, corresponding to the (111) and (200) facets of Pt1Cu1 alloy. The XRD and HRTEM data show that the atomic ratio of Pt and Cu in the Pt1Cu1 nanodendrites is 1:1, consistent with the molar ratio of Pt and Cu precursors used in the synthesis. The average crystallite size of Pt1Cu1 nanodendrites was estimated to be ~10 nm by calculating the width of Pt1Cu1 (111) peak from XRD. This value is much smaller than that evaluated from TEM (~100 nm), which indicates the nanodendrites are composed of many small nanocrystals. The structure and element distribution were further evaluated by high-angle annular dark field-scanning transmission electron microscopy (HAADFSTEM) and energy-dispersive X-ray (EDS) elemental mapping. As shown in Figure 1c, the Pt and Cu elements were uniformly distributed across the whole particle, confirming its alloy structure. The atom ratio of Pt and Cu calculated from EDS mapping is 48 to 52, in agreement with that determined by XRD and HRTEM. In addition, from the dark field TEM image in Figure 1c, the dendritic particles are consisted of many interconnected poles while the surfaces


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of the poles are highly accessible. This special structure can potentially increase the accessibility of molecules to the active sites on the nanoparticle surface, and hence promote the catalytic activity of the nanoparticles.

Figure 2. Representative TEM and HRTEM images of the reaction time-dependent intermediates of Pt1Cu1 nanodendrites when the reaction time is (a, d) 1 min, (b, e) 5 min, (c, f) 30 min. Distribution of (g) the size of the nanopaticles and (h) the width of the branches of Pt1Cu1 nanodendrites acquired at three reaction stages mentioned above. Although the size of NPs increased over time, the width of branches was nearly unchanged.


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To understand how the PtCu nanodendrites were formed, reaction intermediates were collected at desired reaction times by adding toluene to quench the reaction, and then followed by centrifuging at 5000 rpm. The morphology of the reaction intermediates were studied by TEM and HRTEM (Figure 2). As shown in Figure 2a,d, some branched nanoparticles (including tripod, tetrapod and pentapod) were formed at the initial stage (1 min of reaction). As the reaction proceeded for 5 min, branches began to develop (Figure 2b,e). When the reaction time was increased to 30 min, nanoparticles with a highly branched structure were obtained (Figure 2c,f). The morphology and size of the PtCu nanodendrites were almost unchanged when further prolonging the reaction time to 1 h (data not shown). The average size of the nanoparticles is 15.9 ± 2.5 nm for the 1 min sample, 49.8 ± 5.5 nm for the 10 min sample and 89.4 ± 9.1 nm for the 30 min sample (Figure 2g). Although the particle size was increased during the reaction, the average width of the branches remained nearly unchanged (5.4 ± 0.9 nm for 1 min, 5.3 ± 0.5 nm for 5 min and 6.6 ± 0.5 for 30 min samples) (Figure 2h). The small branches help to maintain a relatively high surface area of the PtCu nanodendrites, which promotes their catalytic performance (discussed below). The elemental distribution of the intermediates was evaluated by EDS mapping, as displayed in Figure S1. Both Pt and Cu were uniformly distributed in the particles during the entire growth process. Moreover, we measured the lattice spacing of the nanodendrites obtained at different reaction stages, as shown in the bottom right corner of Figure 2d-f (enlarged in the solid box). The lattice spacing of the Pt1Cu1 nanodendrites obtained at 1 min, 5 min and 30 min were 0.221 nm, 0.218 nm and 0.217 nm, respectively. Based on the lattice spacing of the alloy nanoparticles, the corresponding compositions of Pt and Cu were


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calculated to be Pt59Cu41, Pt51Cu49 and Pt47Cu53 according to the Vegard's law,39-41 showing that compositions of the intermediates did not change significantly at different reaction stages.

Figure 3. XRD patterns of different PtCu nanodendrites. The solid lines at 39.7o and 46.2o are corresponding to the standard (111) and (200) facets of Pt. The Pt and Cu compositions of the PtCu nanodendrites can be easily tuned by changing the ratio of precursors.

A key advantage of the synthesis is that the composition of PtCu nanodendrites can be easily tuned by adjusting the amount of Cu(acac)2 added to the reaction while keeping the concentration of H2PtCl6 the same. Figure S2a-c show the TEM images of PtCu nanodendrites obtained when the feeding molar ratio of Pt and Cu were adjusted to 3:1, 1:1, 1:3. From the TEM images, all of the three samples have dendritic structures and an average size of ~100 nm (Figure S2a-c). The XRD patterns of the different PtCu nanodendrites in Figure 3 show that the (111) facet of PtCu nanodendrites gradually shifted to a higher diffraction angle when more Cu


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was added. As mentioned above, the XRD peaks of the Pt1Cu1 sample appearing at 41.2o and 47.9o fit well with the (111) and (200) facets of PtCu alloy (PDF #48-1549). Moreover, for the Pt1Cu3 sample, the diffraction peaks at 42.2o and 49.3o are corresponding to the (111) and (200) facets of PtCu3 alloy (PDF #35-1358). For the Pt3Cu1 sample, although there is no standard Pt3Cu XRD PDF card, the calculated molar ratio of Pt and Cu was close to 3:1 according to the Vegard's law. Thus, the composition of PtCu nanodendrites was successfully tuned by adjusting the ratio of metal precursors. The surface composition of the synthesized PtCu nanodendrites was also measured by X-ray photoelectron spectroscopy (XPS). As shown in Table S1, the atomic percentage of Cu in Pt/C, Pt3Cu1 nanodendrites, Pt1Cu1 nanodendrites and Pt1Cu3 nanodendrites are 0%, 31.3%, 51.9%, 60.79%, respectively. This result means the surface compositions of Pt and Cu are basically consistent with the overall composition. In brief, the TEM, XRD, and XPS measurements confirmed that the compositions of PtCu nanodendrites can be simply tuned by adjusting the molar ratio of Pt and Cu precursors. Meanwhile, the dendritic structure can be maintained for the PtCu particles with varying compositions.

Figure 4. (a) Pt 4f XPS spectra of commercial Pt/C, Pt1Cu1 nanodendrites (NDs)/C and Pt1Cu3 NDs/C. (b) Cu 2p spectra of Pt3Cu1 NDs/C, Pt1Cu1 NDs/C and Pt1Cu3 NDs/C. The downshifting


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of the binding energy of Pt after adding Cu indicates the strong electronic interaction between Pt and Cu on the surface.

The electronic interaction between Pt and Cu was examined by XPS (Figure 4). As shown in Figure 4a, each Pt 4f peak can be fitted to two doublets. The peaks at 71.4 and 72.4 eV for the commercial Pt/C correspond to metallic Pt0 4f7/2 and oxidized Pt2+ 4f7/2, respectively.42-43 After the addition of Cu, the binding energy of Pt0 4f7/2 was shifted to 71.1 eV and that of Pt2+ 4f7/2 was shifted to 71.7 eV for Pt1Cu1 nanodendrites. With more Cu added, the binding energy of Pt0 4f7/2 further decreased to 71.0 eV and that of Pt2+ 4f7/2 decreased to 71.6 eV for Pt1Cu3 nanodendrites. The change in the binding energy is resulted from the strong electronic interaction between Pt and Cu after alloying. The downshifting binding energy of Pt indicates that Cu donates electrons to Pt, leading to a substantial increase in the electron density around the Pt sites. Theoretically, this increase would result in the weaker chemisorption of CO (improve the CO-tolerance ability) thus promote methanol oxidation.44-45 On the other hand, as shown in Figure 4b, Cu 2p3/2 can also be fitted with two peaks. The peak at lower binding energy is ascribed to metallic Cu 2p3/2 and the peak at higher energy is ascribed to oxidized Cu 2p3/2.46-47 Generally, monometallic Cu would be oxidized to Cu2+; however, the absence of shake-up satellite peak for Cu2+ suggests that metallic Cu occupies the most of the surface composition, which can be ascribed to the stabilizing effect from Pt by forming the alloy structure.48


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Figure 5. Morphology characterization of Pt1Fe1 nanodendrites. (a) TEM image and (b) HAADF-STEM image and EDS mapping of as synthesized Pt1Fe1 nanodendrites. The results show that the synthetic protocol can be used to synthesize PtFe nanodendrites.

The synthetic method developed here can also be applied to synthesize other Pt-based alloy nanodendrites. To demonstrate the generality of the synthetic method, PtFe nanodendrites were synthesized via the same protocol except that the reaction temperature was higher than that for PtCu (see details in the experimental section). TEM image in Figure 5a shows the as prepared Pt1Fe1 nanoparticles have dendritic structure and the average size is ~100 nm. The elements of Pt and Fe were homogeneously dispersed across the particle, as evidenced by EDS mapping (Figure 5b). The compositions of Pt and Fe can also be easily tuned by changing the feeding molar ratio of precursors. Figure S3a-c show the TEM images of PtFe nanodendrites when the feeding molar ratio of Pt and Fe was 3:1, 1:1 and 1:3, respectively. The XRD patterns of different PtFe nanodendrites in Figure S3d shows that, with more Fe added, the (111) facet of


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PtFe nanodendrites gradually shifted to a higher diffraction angle, which means more Fe was alloyed with Pt.

Figure 6. Electrochemical properties of the as-prepared PtCu nanodendrites and commercial Pt/C catalysts. (a) Cyclic voltammogram curves recorded in N2 saturated 0.5 M H2SO4 aqueous solution. (b) Specific activity and (c) mass activity of MOR recorded in 0.5 M H2SO4 + 0.5 M CH3OH aqueous solution. (d) The comparison of the specific activity and mass activity of


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different samples. (e) Chronoamperometry curves of different catalysts in 0.5 M H2SO4 + 0.5 M CH3OH aqueous solution at 0.65 V. (f) Time dependent relative current curves of different catalysts. Previous works have shown that PtCu alloy exhibited a good performance for methanol oxidation.49-52 Herein, the electrocatalytic properties of PtCu nanodendrites with different compositions were further studied and compared to that of the commercial Pt/C. Figure 6a shows the cyclic voltammograms (CVs) of Pt3Cu1 nanodendrites, Pt1Cu1 nanodendrites, Pt1Cu3 nanodendrites and Pt/C in 0.5 M H2SO4 aqueous solution. The electrochemical surface area (ECSA) was evaluated by integrating the coulombic charge for hydrogen desorption, assuming an adsorbed hydrogen monolayer on metal surface is 210 µC/cm2.35, 53 It is noted that the Cu atoms on the surface of PtCu nanodendrites were dissolved during the pretreatment in acid solution to form a thin Pt shell, which has already been discussed in previous report.54-55 As shown in Figure S4, the anode peak at around 0.7 V of first scan of Pt1Cu1 nanodendrites corresponds to the oxidation peak of Cu. The peak was gradually decreased with more scans and then disappeared, indicating that the Pt shell was formed. On the other hand, the peaks of hydrogen adsorption and desorption between 0.03 V to 0.4 V became stable, further demonstrating the formation of Pt shell. The ECSAs of PtCu nanodendrites were then estimated based on the Pt shell. The calculated ECSAs were 32.7 m2 g-1 for Pt3Cu1 nanodendrites, 46.5 m2 g-1 for Pt1Cu1 nanodendrites, 45.4 m2 g-1 for Pt1Cu3 nanodendrites, and 56.9 m2 g-1 for Pt/C. The branched structure of the PtCu nanodendrites provides a relatively high surface area although their overall particle size (~100 nm) is much larger than that of Pt/C (3~4 nm). Figure 6b and Figure 6c show the CV profiles of methanol oxidation of different catalysts. The current in Figure 6b was normalized to the actual surface area of Pt and the current in Figure 6c was


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normalized to the mass of Pt. Although the ECSAs of PtCu nanodendrites were smaller than that of Pt/C, all of the PtCu nanodendrites displayed better specific activity and mass activity than Pt/C. The increased activities of PtCu nanodendrites were also confirmed by the CVs at a lower scan rate (10 mV s-1) (Figure S5). Figure 6d summarized the specific activity and mass activity of MOR for Pt3Cu1 nanodendrites, Pt1Cu1 nanodendrites, Pt1Cu3 nanodendrites, and Pt/C. For the specific activity, the MOR current densities of Pt3Cu1 nanodendrites (2.54 mA cm-2), Pt1Cu1 nanodendrites (3.01 mA cm-2) and Pt1Cu3 nanodendrites (2.71 mA cm-2) are 3.9, 4.6 and 4.2 times as high as that of the commercial Pt/C (0.65 mA cm-2). The mass activities of Pt3Cu1 nanodendrites, Pt1Cu1 nanodendrites, and Pt1Cu3 nanodendrites are 0.83 A mg-1, 1.4 A mg-1, and 1.23 A mg-1, which are 2.2, 3.8 and 3.3 times better than that of Pt/C (0.37 A mg-1), respectively. The above results clearly show the PtCu nanodendrites exhibit much better catalytic activities than Pt/C towards methanol oxidation. Specifically, Pt1Cu1 nanodendrites exhibit a specific activity of 3.01 mA cm-2 and a mass activity of 1.4 A mg-1, which show the outstanding activities compared to those of other catalysts in recent papers (Table S2). The stability of PtCu nanodendrites was further evaluated. Figure 6e shows the chronoamperometry (CA) curves of MOR by holding the potential at 0.65 V. The Pt1Cu1 nanodendrites have the best mass activity after 1,000 s, showing the best stability. In addition, the relative current-time curves were plotted by normalizing the initial current to 100%. As shown in Figure 6f, Pt3Cu1 nanodendrites, Pt1Cu1 nanodendrites, Pt1Cu3 nanodendrites and Pt/C maintained 33.1%, 39.7%, 41.3% and 24.6% of their initial activity, showing that the Pt1Cu1 nanodendrites displayed the best stability among all the catalysts.


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Figure 7. (a) LSV curves of Pt/C, Pt nanodendrites and Pt1Cu1 nanodendrites for MOR at a scan rate of 10 mV s-1. The current was normalized to the actual surface area of Pt. (b) The relationship between peak potential of CO stripping and the binding energy of Pt0 4f7/2 of different samples. (c) Comparison of specific activity of Pt/C, Pt nanodendrites (NDs) and Pt1Cu1 nanodendirtes (NDs) for MOR at 0.65 V. (d) Comparison of CO stripping peak potential of Pt/C, Pt nanodendrites (NDs) and Pt1Cu1 nanodendirtes (NDs) for MOR. The CO-tolerance ability of PtCu nanodendrites was enhanced by the addition of Cu.


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Figure 8. (a) HRTEM image of Pt nanodendrites. (b) Magnified HRTEM image taken from (a) maked by the rectangle. (c) HRTEM image of Pt1Cu1 nanodendrites. (d) Magnified HRTEM image taken from (c) marked by the rectangle.


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Figure 9. CO stripping curves of Pt/C, Pt3Cu1 nanodendrites, Pt1Cu1 nanodendrites and Pt1Cu3 nanodendrites. The negative shift of peak potential indicates the enhanced CO tolerance ability of PtCu nanodendrites.

The enhanced activities of the PtCu nanodendrites for MOR can be explained by the following two reasons. First, according to previous reports, high index surfaces of the special dendritic structure can boost the catalytic activity because of the low coordinated atoms on the surface.56-58 To examine if our nanodendrites have high index surfaces, and also to decouple the morphology effect of the dendritic structure from the electronic effect caused by alloying Pt with Cu, we synthesized monometallic Pt nanodendrites and compared their catalytic properties with that of the commercial Pt/C and the PtCu nanodendrites. The Pt nanodendrites were synthesized by the same method describe in the experimental method section expect that only Pt precursor was added to the growth solution. As shown in Figure S6, we were able to obtain uniform Pt


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nanodendrites with branched structure. Monometallic Pt nanodendrites displayed much better MOR specific activity than Pt/C (see Figure 7a,c). This means the special surface of the dendritic structure plays an important role in improving the MOR activity. The fast Fourier transform (FFT) pattern transferred from the HRTEM image of Pt nanodendrites in Figure S7 shows the existence of high-index facet, which can promote the electrocatalytic activity.24, 27-28 To further study the detailed surface conditions of Pt and PtCu nanodendrites, the HRTEM images with higher magnification were obtained. Figure 8 shows the HRTEM images and enlarged HRTEM images of Pt nanodendrites (8a,b) and Pt1Cu1 nanodendrites (8c,d). It is distinct that the surfaces of these nanodendrites have low-coordinated steps or high-index facets, which are beneficial to promote the catalytic activity. Thus, the higher specific activity of Pt nanodendrites and PtCu nanodendrites compared to Pt/C can be partly related to the exposure of high-index facets, as the small Pt nanoparticles in Pt/C catalyst usually take the shape of a truncated octahedron and are thus enclosed by a mix of low-index and facets.27, 35 Second, the regulated electronic structure of Pt by alloying with Cu also promotes the MOR. The methanol oxidation reaction can be divided into two routes: direct oxidation and indirect oxidation.59-60 The direct oxidation oxidizes methanol to CO2 directly. In contrast, the indirect oxidation usually makes methanol firstly oxidized to CO or other carbonaceous intermediates and consequently cause poisoning of the Pt/C catalysts.59-60 The addition of Cu downshifts the binding energy of Pt from the XPS data in Figure 4. Theoretically this will weaken the adsorption of oxygen species (such as CO), and hence promote the direct oxidation of methanol.44-45 The CO stripping experiment was used to evaluate the tolerance of catalysts towards CO poisoning. As shown in Figure 9, it can be seen that the peak potentials of CO stripping curves of PtCu nanodendrites were all negatively shifted compared to Pt/C, which indicates the CO molecules are easier to be


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removed on PtCu nanodendrites, further demonstating the enhanced CO tolerance ability of PtCu nanodendrites. More importantly, as shown in Figure 7b, the peak potential of CO stripping curve was nearly linear with the binding energy of Pt0 4f7/2, which experimentally proved that the addition of Cu to Pt catalysts improves their CO-tolerance ability and thus enhances their MOR activity. Furthermore, the CO-tolerance ability of Pt1-xCux alloy can be regulated by adjusting the composition of the alloy. For comparison, the value of peak potential of CO stripping curve for Pt nanodendrites was almost same as the value for Pt/C (Figure S8), which means the COtolerance ability of Pt nanodendrites and Pt/C are the same. When Pt nanodendrites alloyed with Cu, the MOR activity further increased (Figure 7a,c), because of the increased CO-tolerance ability (Figure 7d).

Figure 10. Electrochemical durability of Pt1Cu1 nanodendrites. CVs of Pt/C (a) and Pt1Cu1/C (b) for consecutive cycles conducted in 0.5 M H2SO4 + 0.5 M CH3OH. TEM images of Pt1Cu1/C nanodendrites (c) before and (d) after the accelerated durability tests.


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We also performed the accelerated durability tests by cycling the potentials in 0.5 M H2SO4 solution. The Pt1Cu1 nanodendrites catalyst was selected because of its best MOR activity. As shown in Figure 10a, for the Pt/C electrocatalyst, CV measurements of MOR showed an activity loss of 13.9% after 1000 cycles, and 30.2% after 2000 cycles. In contrast, Pt1Cu1 nanodendrites showed a loss of only 5.2% after 1000 cycles and 14.4% after 2000 cycles (Figure 10b), suggesting that the Pt1Cu1 nanodendrites have better durability than the Pt/C catalyst. TEM images in Figure 10c,d confirmed that the dendritic structure and the size of Pt1Cu1 nanodendrites were preserved after the accelerated durability tests. It is worth noting that more branches emerged on the surface of the Pt1Cu1 nanodendrites compared to the fresh ones (Figure 10c,d), which will provide more accessible active sites for the catalytic reactions. The morphology of the Pt1Cu1 nanodendrites was maintained after durability test because the branches are connected with each other, stabilizing the dendritic structure. 4. CONCLUSION In conclusion, PtCu alloy nanodendrites with controllable compositions were synthesized. The one-pot synthetic approach can be used to synthesize other Pt based nanodendrites such as PtFe. Compared to the commercial Pt/C electrocatalysts, Pt1Cu1 nanodendrites exhibit much enhanced catalytic activities for methanol oxidation. The mechanism of the enhancement is ascribed to the special surface of nanodendrites and the change in the electronic structure of Pt due to alloying with different amount of Cu with Pt. The downshifting of the binding energy of Pt improves the CO-tolerance ability of the PtCu nanodendrites, contributing to the boost of the MOR activity. The systematic study can be used to rationally understand the increased catalytic performance by dendritic structures. ASSOCIATED CONTENT


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Supporting Information. Supporting information include TEM and EDS mapping images, XPS data, electrochemical measurements and the comparison of the activity of different MOR catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *(J. Z.) Email: [email protected] *(J. F.) Email: [email protected] Funding Sources American Chemical Society Petroleum Research Fund (PRF# 54004-DN15) National Natural Science Foundation of China (21373181 and 91545113) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was finally supported by American Chemical Society Petroleum Research Fund (PRF# 54004-DN15) and National Natural Science Foundation of China (21373181 and 91545113). L.L. appreciates a scholarship from Zhejiang University. The TEM studies were performed using the facilities in the UConn/FEI Center for Advanced Microscopy and Materials Analysis (CAMMA).


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