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Shape-Control of Pt-Ru Nanocrystals: Tuning Surface Structure for Enhanced Electrocatalytic Methanol Oxidation Liang Huang, Xueping Zhang, Qingqing Wang, Yujie Han, Youxing Fang, and Shaojun Dong J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12353 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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Journal of the American Chemical Society

Shape-Control of Pt-Ru Nanocrystals: Tuning Surface Structure for Enhanced Electrocatalytic Methanol Oxidation Liang Huang,†,§ Xueping Zhang,†,‡ Qingqing Wang,† Yujie Han,† Youxing Fang,† and Shaojun Dong*†,‡ †

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun, Jilin 130022, PR China § University of Science and Technology of China, Hefei, Anhui 230026 (P. R. China) ‡ University of Chinese Academy of Sciences, Beijing, 100049, PR China ABSTRACT: Despite that both electrochemical experiments and density functional theory calculations have testified the superior electrocatalytic activity and CO-poisoning tolerance of platinum-ruthenium (PtRu) alloy nanoparticles toward the methanol oxidation reaction (MOR), the facet-dependent electrocatalytic properties of PtRu nanoparticles are scarcely revealed because it is extremely difficult to synthesize well-defined facets enclosed PtRu nanocrystals. Herein, we for the first report a general synthesis of ultrathin PtRu nanocrystals with tuneable morphologies (nanowires, nanorods and nanocubes) through a one-step solvothermal approach, and systematically investigate the structure-directing effects of different surfactants and the formation mechanism by control experiments and time-dependent studies. In addition, we utilize these {100} and {111} facets enclosed PtRu nanocrystals as model catalysts to evaluate the electrocatalytic characteristics of MOR on different facets. Remarkably, the {111}-terminated PtRu nanowires exhibit much higher stability and electrocatalytic mass activity toward MOR, which are 2.28- and 4.32-times higher than {100}-terminated PtRu nanocubes and commercial Pt/C, respectively, indicating that PtRu {111} facets possess superior methanol oxidation activity and CO-poisoning resistance relative to {100} facets. Our present work provides a series of well-defined PtRu nanocrystals with tuneable facets, which would be ideal model electrocatalysts for fundamental research of fuel cell electrocatalysis.

INTRODUCTION Fuel cells, such as proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), are promising renewable energy devices in converting chemical energy into electric power with no fossil fuels and environmental sensitivity.1-5 As the methanol oxidation reaction (MOR) is a basic anode reaction of DMFCs, developing highly efficient electrocatalysts for MOR is the key to fabricating high energy/power density fuel cells. Platinum (Pt) nanomaterials have received intensive research interest due to their superior catalytic nature in MOR.6-10 And increasing efforts are being devoted to promoting the catalytic activity by precisely controlled synthesis of Pt nanostructures with tailored shapes and compositions.11-17 For example, bimetallic Pt-M alloy (M = Pd, Ru, Fe, Co, Ni, Cu, Sn and Pb) with various morphologies (nanopolyhedra, nanowires, nanosheets and nanodendrites).1826 However, the CO poisoning on catalysts surface is still an inevitable challenge, where the CO originates from dissociation of methanol molecules during the electrocatalytic reaction.27-29 Both density functional theory and model experiments have revealed the possible CO poisoning route: the Pt atom site catalyzes methanol molecule oxidation to form strongly adsorbed COads, which can only be further oxidized to CO2 at large overpotentials. Hence Pt-COads is considered as a poisoning intermediate, and hinders the further methanol oxidation on Pt catalyst surface.30-34 According to the poison resistance research of Pt-based catalysts in MOR, PtRu bimetallic nanomaterials are recognized as the best performing CO-

poisoning tolerant electrocatalysts to date, and be explained by the Watanabe-Motoo bifunctional mechanism. The addition of an oxophilic metal Ru to Pt can provide adsorbed hydroxyl groups (OHads) at lower potential than that on pristine Pt, which served as the oxidant to remove the poisoning species (COads) by oxidizing it to CO2.35-39 Meanwhile, various reported PtRu catalysts exhibit outstanding MOR electrocatalytic activity, but the high reduction potential and lattice mismatch of Ru make it a great challenge to synthesize shape-controlled PtRu nanocrystals rather than irregular nanoparticles.40-47 In this regard, it is significant to systematically research the correlations of well-defined Pt-Ru nanocrystals with the MOR catalytic properties. Herein, we report the systematic synthesis of ultrathin PtRu nanocrystals with tuneable morphologies (nanowires, nanorods, nanocubes and nanoparticles) through a one-step wetchemical approach, and investigate their electrocatalytic activities in the oxidation of methanol. The PtRu nanocrystals are typically synthesized by one-pot hydrothermal method in oleylamine (OAm)-based system, Pt(acac)2 and Ru(acac)3 as the metal precursors, W(CO)6 as reducing agent and additional surfactants as structure-directing agent. Noteworthily, the morphologies of the nanocrystals are facilely controlled by different kinds/amount of surfactants (Scheme 1). For instance, appropriate amount of DDAC can lead to ultrathin PtRu nanowires (PtRu NWs), CTAB can lead to PtRu nanocubes (PtRu NCs), and increasing amount of HDBAC can shorten the length of PtRu nanorods (denoted as PtRu NRs1, PtRu NRs2, PtRu NRs3, corresponding to the length of 60 nm,

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45 nm and 30 nm NRs, respectively). Furthermore, the asobtained uniform PtRu nanocrystals exhibit excellent electrocatalytic activity and stability toward MOR. And the PtRu NWs show the highest mass activity of 0.82 A mgPt -1 and specific activity of 1.16 mA cm-2, which are superior to other shape of PtRu catalysts and commercial Pt/C catalyst.

Scheme 1. Schematic illustration of the synthetic route to PtRu nanocrystals with tunable morphologies and facets.

EXPERIMENTAL SECTION Chemicals. Platinum(II) acetylacetonate (Pt(acac)2, 97%), Ruthenium(III) acetylacetonate (Ru(acac)3, 97%), Tungsten hexacarbonyl (W(CO)6, 97%), hexadecyldimethylbenzyl ammonium chloride (HDBAC, 99%) and cetyltrimethylammonium bromide (CTAB, >99%) were purchased from SigmaAldrich. Dimethyldioctadecylammonium chloride (DDAC, 98%) and Oleylamine (OAm, >90%) were purchased from Energy Chemical. Commercial Pt/C catalyst (20 wt.% loading of Pt on carbon black), commercial PtRu/C catalysts (20 wt. % loading of Pt and 10 wt. % loading of Ru on carbon black) and Nafion (5%) were purchased from Alfa Aesar. All the chemicals in the experiment were used without further purification. Synthesis of Pt-Ru nanowires (PtRu NWs), Pt-Ru nanorods (PtRu NRs), and Pt-Ru nanocubes (PtRu NCs). In a typical synthesis of PtRu NWs, Pt(acac)2 (25.0 mg), Ru(acac)3 (5.0 mg) and DDAC (90.0 mg) were dissolved in 5.0 mL of OAm, and followed by sonication and vigorous stirring for 1 h. The resulting homogeneous solution was transferred into a 10 ml Teflon-lined high-pressure vessel, and then W(CO)6 (30 mg) was injected into the vessel with stirring for another 30 min under a nitrogen atmosphere. The sealed vessel was heated up to 185 oC from room temperature in an electric oven within 45 min, and maintained at this temperature for 5 h. After the solution was cooled down to room temperature, the product was precipitated by ethanol and washed five times with an ethanol-hexane mixture, and finally dispersed in 10 mL hexane. The synthesis of PtRu NRs was similar to that of PtRu NWs, except that DDAC was replaced by different amount of HDBAC, 60 mg for PtRu NRs1, 120 mg for PtRu NRs2 and 180 mg for PtRu NRs3. The PtRu NCs were synthesized by replacing DDAC with 90 mg of CTAB, while keeping the other reaction parameters the same. Characterization. The morphological and structural characterizations of these PtRu nanocrystals were conducted on a Hitachi H-600 Analytical transmission electron microscope (TEM) with an accelerating voltage of 100 kV. The HRTEM and HAADF-STEM

images were obtained with a JEM-2010 operating at 200 kV equipped with an energy dispersive spectrometer (EDS). The X-ray diffraction (XRD) patterns were collected on D8 ADVANCE (Bruker AXS, Germany) diffractometer with Cu Kα radiation. The X-ray photoelectron spectroscopy (XPS) were investigated using an ESCALAB-MKII spectrometer (VG Co., United Kingdom) with Al Kα X-ray radiation as the X-ray source for excitation. The element mole ratio of nanocrystals was measured by the inductively coupled plasma atomic emission spectroscopy (ICP-AES), which was conducted on a Thermo Scientific iCAP6300 (Thermo Fisher Scientific, US). For post-durability characterization of the catalysts, the products were collected by ultrasonication of the modified glassy-carbon electrode in ethanol, and then washed several times with ethanol. Electrochemical Measurements. A three-electrode system was used to conduct the electrochemical measurements. Ag/AgCl (saturated KCl) electrode was served as reference electrode, Pt wire was served as counter electrode, and a working electrode. The working electrode was a glassy-carbon electrode (GCE) (diameter: 3 mm, area: 0.07065 cm2). Typically, 2.0 mg of the PtRu nanocrystals were dispersed in 5 mL hexane and 8.0 mg of Vulcan XC-72 carbon was dispersed in 15 mL ethanol and sonication for 2 h. Then these carbon supported catalysts were collected by centrifugation and redispersed in 10 mL of acetic acid, followed by heating at 70 °C for 12 h to remove the residual OAm. Afterward, the catalysts were washed, dried and re-dispersed in a mixture of deionized water, isopropanol and Nafion (V/V/V = 4/1/0.05). Then 5.0 µL of the mixture (the concentration of the catalysts was 1.0 mg mL-1) was cast on GCE and dried under ambient condition. The electrochemical active surface areas (ECSA) were determined by integrating the hydrogen adsorption charge on the cyclic voltammetry (CV) at room temperature in 0.1 M HClO4 solution. The MOR measurements were conducted in 0.1 M HClO4 + 0.5 M CH3OH aqueous solution at a sweep rate of 50 mV s-1. The accelerated durability tests (ADT) were performed at room temperature in 0.1 M HClO4 + 0.5 M CH3OH solution by sweeping cyclic potential cycles between -0.2 V and 1.0 V (versus Ag/AgCl electrode) at a scan rate of 50 mV s-1 for 1000 cycles. The ECSA was estimated by measuring the charge associated with Hupd adsorption (QH) between -0.2 V and 0.1 V, ECSA = QH / (qH×m) and assuming 210 µC cm-2 for the adsorbed monolayer of hydrogen on Pt surface (qH). The Hupd adsorption charge (QH) could be determined by QH = 0.5 × Q, where Q was the charge in the Hupd adsorption/desorption area obtained after double-layer correction.

RESULTS AND DISCUSSION


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Figure 1. Morphological and structural characterizations of PtRu nanocrystals. Representative TEM images of (a,d) PtRu NWs, (b,e) PtRu NRs1 and (c,f) PtRu NCs. HRTEM images of an individual (g) PtRu NW and (h) PtRu NC, and the corresponding FFT pattern (i) of (h). The morphology and structure of the as-synthesized PtRu nanocrystals are characterized by transmission electron microscopy (TEM). Generally, the products are all uniform and monodispersed with approaching 100% of synthetic yield. As shown in Figure 1a, the ultrathin PtRu NWs have highly uniform diameter of 1.8 nm and average length of 130 nm (Figure S1 and 1d). The high-resolution TEM (HRTEM) images (Figure 1g and S2) reveal that the PtRu NWs have smooth surface along the whole length with ultrathin feature, and the coherent lattice fringes with lattice spacings of 0.23 nm, implying that the PtRu NWs are enclosed by {111} facets. When DDAC is replaced by HDBAC with keeping the other synthesis conditions unchanged, the PtRu NRs (Figure 1b and 1e) with an average diameter of 2.0 nm and length of 65 nm are obtained (Figure S3). Moreover, the length of the nanorods can be effectively adjusted to 45 nm (Figure S4) and 30 nm (Figure S5) by simply increasing the amount of surfactants. The HRTEM images of PtRu NRs show coherent lattice fringes, consistent with the {111} facets of PtRu nanocrystals (Figure S5). In addition, monodispersed PtRu NCs (Figure 1c, 1f and S6) with an average size of 6.2 nm are simultaneously synthesized with the assistance of CTAB. And the lattice spacings are measured to be 0.19 nm (Figure 1h), corresponding to the {100} facets (Figure 1i) of PtRu nanocrystals. Therefore, the controllable synthesis of well-defined facets enclosed PtRu nanocrystals turns out to be meaningful model electrocatalysts. The element distribution of the obtained PtRu nanocrystals are analyzed by HAADF-STEM energy dispersive X-ray (EDX) elemental mapping (Figure 2a and S7). Both Pt and Ru elements are uniformly distributed throughout these nanowires and nanoro-

Figure 2. (a) HADDF-STEM image and the corresponding elemental mapping images of the PtRu NWs. (b) XRD pattern, and XPS patterns for (c) Pt 4f region and (d) Ru 3p region of the PtRu nanocrystals. ds, which confirmed the homogeneous alloyed nanostructure. The energy-dispersive spectroscopy (EDS) (Figure S8) of PtRu nanocrystals reveal that the composed of PtRu NWs with the Pt/Ru atomic ratio at 9.85/1.0, which is in accordance with the inductively coupled plasma atomic emission spectroscopy (ICP-AES) results.



Figure 3. Electrocatalytic performance of the PtRu nanocrystals and commercial Pt/C catalyst. CVs of (a) PtRu NRs and (b) PtRu NWs, PtRu NRs1, PtRu NCs, commercial PtRu/C and commercial Pt/C for MOR in 0.1 M HClO4 + 0.5 M CH3OH solution at a sweep rate of 50 mV s-1. (c) Histograms of the mass and specific activities of different catalysts. (d) Schematic depiction of the synergistic effect between Pt and Ru for the COads oxidation by OHads. The additional W(CO)6 will decompose into W0 and CO, which are served as powerful reductants for reducing Pt2+ and Ru3+ species, and there is no residual W element detected in the products. The powder X-ray diffraction (XRD) patterns (Figure 2b) of these nanoparticles display the typical facecentered cubic (fcc) structure, which are associated with the alloyed PtRu nanomaterials. The X-ray photoelectron spectroscopy (XPS) patterns show that both Pt (Figure 2c) and Ru (Figure 2d) are mainly in the zerovalent state on the surface of the nanocrystals. Notably, the Pt 4f binding energy in PtRu nanocrystals emerges a negative shift from 71.1 to 70.8 eV compared to the Pt/C, indicating the charge transfer from Ru to Pt due to the higher electronegativity of Pt.21,22,48 To understand the formation mechanism of the alloyed PtRu nanocrystals, the morphologies of the intermediate nanoparticles obtained at different reaction times are investigated by TEM (Figure S9a). The ultra-small PtRu nanoparticles are produced at the initial reaction stage (Figure S9b). When the reaction time is prolonged to 1 h, the nanoparticles grow into nanorods (Figure S9c) with a ultrathin diameter of 1.5 nm. And the length of nanocrystals will increase to 60 nm during the another 1 h of reaction time (Figure S9d). After the reaction been proceeded for 3 h, the nanorods are excavated into nanowires (Figure S9e), and the length and diameter have no obvious variation. In addition, a set of control experiments is conducted by altering the reaction parameters to investigate the structure-directing effects of surfactants on the specific crystal facets growth of PtRu nanocrystals. Appropriate kinds of surfactants introduced in the synthesis are proved to be the key for anisotropic growth of PtRu nanocrystals.13,20,49 On the one hand, high aspect ratio nanowires will generate with sufficient DDAC in the synthetic system, while by-products of nanorods and nanoparticles will form with insufficient/excess surfactants (Figure S10 and S12a). On the other hand, when DDAC is replaced with HDBAC, the samples become lower aspect ratio nanorods, and the length is controllable by altering the amount

of HDBAC. These could be ascribed to the different carbon number of the alkyl chain and micelle concentration of surfactants. The double alkyl chains in OAm might promote longer and stable micelle wrapping the nanocrystals and prolong their anisotropic growth into nanowire structure. For nanorods, the micelle concentration and morphology will change with increasing the amount of surfactants, higher concentration of surfactants will shorten the anisotropic length of individual micelle as well as the wrapped nanorods. Furthermore, attributed to the stronger coordination ability of Br- than Cl-, the anion (Br-) of CTAB boosts the selectivity growth of {100} facets, which leads to the form of nanocubes structure.18 Besides, the lack of powerful reductant of W(CO)6 directly decelerates the reduction rate of Pt2+ and Ru3+, and results in the growth of irregular nanowires and nanoparticles (Figure S11). The amount of Ru3+ and reaction temperature are also found to influence the morphology of products. The absence of Ru(acac)3 is adverse to the form of longer 1D nanostructure (Figure S12b), and the higher or lower reaction temperature will change nucleation and growth rates of the PtRu nanocrystals (Figure S13). The PtRu catalysts usually exhibit high activity toward MOR, while the systematic study about the structuredependent electrocatalytic properties of PtRu nanocrystals is scarce. Thus we use MOR as a model reaction to evaluate the catalytic activity of these tunably well-defined PtRu nanocrystals by manipulating exposed {111}/{100} facets on the surface. We benchmark their electrocatalytic performance against commercial PtRu/C (Figure S14) and commercial Pt/C catalyst. The typical cyclic voltammetry curves (CVs) of these PtRu catalysts and commercial Pt/C in 0.1 M HClO4 solution at a sweep rate of 50 mV s-1 are shown in Fig. S15. The electrochemical active surface areas (ECSAs) are calculated to be 72.1 m2 g-1 for PtRu NWs, 65.3 m2 g-1 for PtRu NRs1, 67.8 m2 g-1 for PtRu NRs2, 63.4 m2 g-1 for PtRu NRs3, 58.7 m2 g-1 for PtRu NCs, 54.5 m2 g-1 for commercial PtRu/C and 62.5 m2 g-1


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Journal of the American Chemical Society for commercial Pt/C. Figure 3a and 3b show MOR catalytic activity catalyzed by these catalysts, which are measured in 0.1 M HClO4 aqueous solution with 0.5 M CH3OH at a sweep rate of 50 mV s-1. As displayed in Figure 3a, the PtRu NRs1 show a higher methanol oxidation current density compared to the other PtRu NRs with shorter length. Then we evaluate the MOR activity of the PtRu NWs, PtRu NRs1, PtRu NCs and commercial Pt/C in Figure 3b. PtRu NWs exhibit lower onset potential and greater If/Ib ratio (where If is the forward current density, and Ib is the backward current density) than other catalysts, especially than commercial Pt/C.23 The negative shift of the onset potential toward MOR (Figure S16) on PtRu NWs indicates that the methanol oxidation would be inclined to occur at a lower potential, and the higher If/Ib ratio implies that the methanol would be effectively oxidized during the forward potential scan and producing less poisoning species, thereby possessing higher tolerance toward CO-poisoning.30,42 As shown in Figure 4c, the specific and mass activity are calculated by normalizing the MOR current densities with the corresponding ECSA and the loading amount of Pt, respectively. The PtRu NWs exhibit the highest mass activity of 0.82 A mgPt -1 at 0.70 V, which is 1.21-, 2.28-, 3.04- and 4.32- times greater than that of the PtRu NRs1 (0.68 A mgPt-1), PtRu NCs (0.36 A mgPt-1), commercial PtRu/C (0.27 A mgPt-1) and commercial Pt/C (0.19 A mgPt-1), respectively. Meanwhile, the PtRu NWs exhibit the highest specific activity of 1.16 mA cm2 , which is 2.43- and 3.87- times higher than that of the commercial PtRu/C (0.48 mA cm-2) and Pt/C (0.30 mA cm-2), respectively. The mechanism analysis of PtRu bimetallic synergistic effects catalyzing methanol molecules oxidation is shown in Figure 3d. It is widely acknowledged that the electrooxidation of methanol on Pt-based catalyst surface will go through a dual-path mechanism, direct pathway (reactive intermediates) and indirect pathway (poisoning intermediates).50,51 Among them, the indirect pathway is the ratedetermining step and the poisoning intermediates are determined mainly as adsorbed COads species, which are derived from the dissociative adsorption of methanol molecules.38,39 The adsorbed COads would be scarcely stripped out until the oxygen-containing species (OHads) generate on Pt surfaces under high electrode potential. Fortunately, the additional Ru atoms can provide adsorbed OHads at a lower potential, which are served as the oxidant to effectively oxidize the COads. Besides, previous density functional theory calculations indicate that the Pt-COads and Ru-OHads species are unable to interact to form a transition state (Pt-CO•••OH-Ru) unless the distance between the Ru and Pt atoms reach (or below) the critical distance of 4.0 Å, otherwise the OHads and COads tend to stay away from each other.41 Thus, the alloying structure of PtRu catalyst has more advantages than core-shell and heterostructure for the close connection between Pt and Ru atoms. Additionally, the adsorption energies of COads and OHads on {111} facets enclosed PtRu NWs are closer to the optimal value than it on {100} facets enclosed PtRu NCs, hence the PtRu NWs show much enhanced MOR activity than PtRu NCs.40,41 And ascribed to the longer length and thinner diameter, the PtRu NWs expose more {111} facets active sites and exhibit higher electrocatalytic activity in comparison to the PtRu NRs. The electrochemical durability of the PtRu NWs is evaluated by an accelerated durability test (ADT) in 0.1 M HClO4 + 0.5 M CH3OH aqueous solution at room temperature. Figure 4a and S17 show the CVs of MOR on PtRu NWs and commercial Pt/C at different scan rates (ranging from 10-100 mV

s-1), respectively. It is obvious that the current density increases as well as the peak poten-

Figure 4. Electrocatalytic durability of the PtRuNWs, PtRu NRs1 and commercial Pt/C. (a) CVs of MOR on PtRu NWs at different scan rates and the inset is the corresponding plot of forward peak current (jm) versus the square root of the scan rate (v1/2). (b) CVs of these catalysts before and after 800 potential cycles. (c) Current-time curves of these catalysts recorded at 0.7 V. (d) HRTEM image and (the inset) TEM image of the PtRu NWs/C after 800 cycles. The scale bars in (d) and the inset are 10 nm and 30 nm, respectively. tial shifts positively with the increase of scan rate. And the linear relationship is found between the square root of the scan rate (ν1/2) and the forward peak current density (jm), which demonstrate that the MOR follows the diffusion-controlled process, and the higher slope value of the PtRu NWs relative to that of the commercial Pt/C indicate the great improvement of methanol electrooxidation kinetics on PtRu NWs.52-54 As shown in Figures 4b and S18, the current density on PtRu NWs, PtRu NRs1 and commercial Pt/C is slowly decreasing with the CV cycles increase. After 800 CV cycling, the PtRu NWs reserve 63.67% of the initial catalytic activity, much better than commercial PtRu/C (51.57%) and commercial Pt/C (43.65%). These results are consistent with the current-time (it) curves in Figure 4c. The current density remaining on PtRu NWs is much higher than that of the commercial Pt/C even throughout the 4000 s test, which further reveals the high electrochemical durability of the as-prepared PtRu NWs. The morphological variations of these catalysts after the durability tests are characterized by TEM. Figure 4d and S19 show that there is negligible change of the PtRu NWs after long-term cycles, whereas obvious aggregation occurs on the commercial Pt/C under the same conditions (Figure S20). Therefore, it can be concluded that the special alloy composition and the ultrathin one-dimensional nanostructure effectively prevent the PtRu NWs from aggregation and promote the electrochemical stability as well.

CONCLUSIONS To summarize, we report a general synthesis of ultrathin PtRu nanocrystals with tuneable morphologies (nanowires, nanorods and nanocubes) through a one-step solvothermal approach, and systematically investigate the structure-directing effects of



different surfactants and the formation mechanism by control experiments and time-dependent studies. What's more, we for the first time evaluate the facet-dependent MOR electrocatalytic activities of PtRu catalysts by manipulating the exposed {111}/{100} facets on the surface of uniform nanocrystals. Surprisingly, the {111}-terminated PtRu NWs exhibit much higher stability and electrocatalytic mass activity toward MOR, which are 2.28- and 4.32-times higher than {100}terminated PtRu NCs and commercial Pt/C, respectively, indicating the superior methanol oxidation activity and COpoisoning resistance of PtRu {111} facets relative to {100} facets. We believe that these as-prepared PtRu nanocrystals with tuneable facets would be ideal model electrocatalysts for fundamental research of fuel cell electrocatalysis.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Additional TEM images, EDS, XRD results and electrocatalytic curves.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21375123 and 21675151) and the Ministry of Science and Technology of China (Nos. 2013YQ170585 and 2016YFA0203203).

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