Ultrathin PtâAg Alloy Nanotubes with Regular Nanopores for

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Ultrathin Pt-Ag Alloy Nanotubes with Regular Nanopores for Enhanced Electrocatalytic Activity Hongpo Liu, Kai Liu, Ping Zhong, Jing Qi, Jihong Bian, Qikui Fan, Kui Ren, Haoquan Zheng, Lu Han, Yadong Yin, and Chuanbo Gao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03085 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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Chemistry of Materials

Ultrathin Pt-Ag Alloy Nanotubes with Regular Nanopores for Enhanced Electrocatalytic Activity Hongpo Liu,† Kai Liu,† Ping Zhong,† Jing Qi,‡ Jihong Bian,† Qikui Fan,† Kui Ren,¶ Haoquan Zheng,‡ Lu Han,§ Yadong Yin,# and Chuanbo Gao†* †Frontier Institute of Science and Technology and State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710054, China. ‡School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an, Shaanxi 710119, China. ¶State Key Laboratory of Catalytic Materials and Reaction Engineering, Research Institute of Petroleum Processing, Sinopec, Beijing 100083, China. §School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. #Department of Chemistry, University of California, Riverside, California 92521, United States.

ABSTRACT: While creating open nanostructures represents a popular strategy toward improved utilization efficiency of Ptbased catalysts for electrochemical reactions, the exposed facets should be precisely controlled for further enhancement in the catalytic activity. Here, we report a novel strategy to create regularly shaped nanopores in ultrathin nanotubes of bimetallic noble metals. By templating against Ag nanowires and then applying a thermal ripening process, we have successfully produced ultrathin (with a wall thickness of ~ 1 nm) Pt-Ag alloy nanotubes containing high-density well-defined rectangular nanopores and a collapsed double-layer structure. The resulting porous nanotubes expose {100} facets at the basal sides and {110} facets with step sites at the edges of the rectangular nanopores. The particular surface structure and the bimetallic composition enable suppressed CO poisoning of the catalysts and consequently enhanced electrocatalytic activity in the methanol oxidation reaction (MOR). The typical specific and mass activities are 6.63 mA cm–2 and 2.08 mA μg–1 Pt, respectively, in an acidic medium, superior to the commercial Pt/C and many previously reported catalysts. We believe this work opens new opportunities in the design of noble metal open nanostructures for enhanced performance in a broad range of catalytic applications.

nanostructures demonstrate extraordinary catalytic activities, they expose random crystal facets in an uncontrollable Platinum (Pt) has attracted tremendous attention in promanner.12 It is highly desirable that these open nanostructon-exchange membrane (PEM) fuel cell applications due to tures could be obtained with an ultrathin size, alloy compotheir excellent performance as a catalyst in both oxidation sition, and more importantly, regular arrangements of the of the fuels (such as methanol in a direct methanol fuel cell) Pt atoms at the nanopores for further enhancement in the at the anode1-5 and reduction of oxygen at the cathode.6-8 electrocatalytic activity, which, to the best of our knowledge, Due to the low abundance of Pt in the earth’s crust and thus remains a significant challenge. the high price, there has been urgent stress to enhance the In this work, we report a novel strategy to create open utilization efficiency of Pt, which is critically important to nanostructures with regular shapes in the ultrathin noble realize the commercial viability of the PEM fuel cells at an metal nanotubes. Specifically, we show that Pt-Ag alloy industrial scale. Great efforts have been made toward this nanotubes with a collapsed double-layer structure, an uldirection by synthesizing Pt nanocrystals with an ultrasmall trasmall wall thickness of ~ 1 nm, a length of tens of mithickness for increasing proportion of Pt atoms exposed on crometers, and an exposing facet of {100} could be synthethe surface,9-14 bimetallic compositions for introducing sized by templating against Ag nanowires, and high-density strong synergistic effects,15-18 and controllable facets for rectangular nanopores that expose {110} facets and abunachieving the appropriate binding strength of the reactants dant step sites could be produced in these ultrathin nanoon the catalyst surface.19-22 In addition, the creation of open tubes by a robust post-synthetic thermal ripening process. nanostructures such as nanoframes12, 14 and porous nanoThe newly-formed {110} facets and step sites at the edge of crystals23-25 represents another attractive strategy in the the rectangular nanopores, in addition to the {100} facets at design of Pt-based catalysts with enhanced exposure of surthe basal sides, the Pt-Ag alloying, and the ultrasmall thickface active sites, which promises significantly boosted cataness, proved to be effective in enhancing the electrocatalytic lytic activity. Conventionally, these open nanostructures activity of the nanotubes in the methanol oxidation reaction could be synthesized by involving soft-templating,26-27 self(MOR). The porous ultrathin Pt-Ag nanotubes demonassembly28-29 or a dealloying process.12, 30-31 Although such strated a specific activity of 6.63 mA cm–2 and a mass activity of 2.08 mA μg–1 Pt in an acidic medium, which are ~ 7.1 ACS Paragon Plus Environment

INTRODUCTION

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and 4.1 times those of the commercial Pt/C catalyst, respectively, in a typical demonstration.

RESULTS AND DISCUSSION

Figure 1. Epitaxial growth of Pt on the Ag nanowires using CH3CN as a coordination ligand. (a) TEM image of the Ag@Pt core/shell nanowires. Inset: A model for the nanowire; the blue and yellow colors represent Ag and Pt, respectively. (b) HRTEM image of a sliced Ag@Pt nanowire obtained by cutting with a microtome, showing the structure of the cross-section. Inset: an enlarged HRTEM image. (c) EDS elemental mapping of the sliced cross-section of the Ag@Pt core/shell nanowire. (d) EDS elemental mapping of a Ag@Pt core/shell nanowire lying flat on the TEM copper grid. The red and green colors represent Ag and Pt, respectively.

Ultrathin Pt-Ag alloy nanotubes were first prepared by epitaxial growth of Pt on Ag nanowires and subsequent chemical etching of the Ag nanowires (Figure 1). One challenge is that the galvanic replacement reaction between the Ag nanowires and the Pt salt may be involved as a side reaction, which leads to a hollow structure with remarkable polycrystallinity and an unfavorable rough surface, instead of a controlled layer-by-layer epitaxial growth.32-38 One strategy that may solve this problem is to introduce a strong reducing agent to compete with and thereby suppress the galvanic replacement between the noble metal salt and the less-stable metal nanocrystals from the perspective of the reaction kinetics.39-45 In parallel, our group developed a strategy based on thermodynamic consideration. By coordinating to appropriate ligands, the reduction potential of the noble metal salt could be significantly reduced, which leads to effective suppression of the galvanic replacement.46-48 Both strategies may be applicable to the controlled epitaxial growth of Pt on Ag nanocrystals.42-46 In this work, we chose acetonitrile (CH3CN) and sodium nitrite (NaNO2) as a ligand to complex with the Pt salt (H2PtCl6). As a result, an epitaxial growth of Pt on the Ag nanowires was achieved without involving the galvanic replacement, which can be confirmed by the transmission electron microscopy (TEM) imaging,

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showing nanowires of a smooth surface without forming a hollow structure (Figure 1a, Figure S1). To further inspect the nanostructure, the Ag@Pt core/shell nanowires were embedded into an epoxy and cut into slices along the fivefold-twinned cross-section (thickness: ~ 30 nm) by a microtome for electron microscopy analysis (Figure 1b, c). The high-resolution TEM (HRTEM) image of the thin slice shows lattice fringes extending from the center of the cross-section (Ag core) to its edges (Pt shell) without differences in the fringe spacing, suggesting that the lattice of Pt follows that of the Ag core due to the epitaxy in the crystal growth (Figure 1b, X-ray diffraction see Figure S2). The elemental mapping of the thin slice clearly reveals a Ag core enclosed by a Pt shell by the energy-dispersive X-ray spectroscopy (EDS) (Figure 1c). The thickness of the Pt shell can be therefore estimated to be ~ 1.3 nm. Additionally, the core/shell nanostructure can be also evidenced by the EDS elemental mapping of an individual nanowire lying flat on the TEM grid (Figure 1d). All the evidence confirms the successful epitaxial growth of a uniform thin layer of Pt on the Ag nanowires without involving the galvanic replacement and validates the strategy of the synthesis. It is worth noting that during the epitaxial growth of Pt on the Ag nanowires, atomic inter-diffusion of Ag and Pt across their interfaces readily occurs,9 which accounts for the alloying of Pt with Ag in the epitaxial ultrathin shells. Although our idea to deposit an ultrathin layer of a noble metal on Ag nanowires is similar to some previous efforts,33-38 the latter usually involve galvanic replacement, which leads to a less control over the shell thickness, composition, and the surface crystal structure. By our method, we can achieve high controllability of the synthesis.

Figure 2. Collapsed ultrathin Pt-Ag nanotubes obtained by etching the Ag core from the Ag@Pt core/shell nanowires. (a) A low-magnification TEM image. Inset: a simplified model for the collapsed Pt-Ag nanotubes. (b) TEM image of the collapsed Pt-Ag nanotubes with a curled edge, showing the ultrasmall thickness (~ 1.3 nm) and a double-layer structure. (c) TEM image of the collapsed Pt-Ag nanotubes. The black lines indicate the thin areas of the nanotubes with a smaller thickness of Pt

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grown at the 5-fold twin boundaries of the Ag nanowires. (d) HRTEM image. Inset: Corresponding Fourier diffractogram.

The Ag cores could be conveniently etched due to the presence of pinholes in the ultrathin Pt-Ag alloy nanoshells.9 The remaining Pt-Ag nanostructure collapses due to the ultrasmall thickness, giving rise to nanotubes with a collapsed double-layer structure as confirmed by the TEM (Figure 2a–b, Figure S3). The nanotubes are tens of micrometers in length (Figure 2a), and ~ 1.3 nm in thickness as measured from their curled edges (Figure 2b). The thickness of the nanotubes can also be confirmed by the atomic force microscopy (AFM) measurement (Figure S4). A smooth surface has been obtained, suggesting that the Pt growth on the Ag nanowires was in the Frank-van der Merwe mode, rather than the Volmer-Weber mode that leads to Pt islands.49 A close investigation of the ultrathin Pt-

Ag nanotubes reveals bright lines that are parallel to the edges of the nanotubes (Figure 2c). This image contrast can be ascribed to a smaller thickness of Pt at the five twin boundaries of the Ag nanowires than the lateral {100} facets, which indicates a crystal growth behavior that is different from those observed in the growth of Pt on Pd nanocrystals.50 These ultrathin Pt-Ag nanotubes expose {100} facets exclusively, as determined by the HRTEM image and the Fourier diffractogram (Figure 2d). This facet structure is consistent with the lateral facets of the Ag nanowires, confirming the determinant role of the template in the rational engineering of the exposing facet of the ultrathin Pt nanostructures. The lattice fringes of the {200} planes show a spacing of 1.98 Å (Figure 2d), which is slightly larger than that of pure Pt (1.96 Å), confirming the Pt-Ag alloying in the nanotubes.

Figure 3. Collapsed porous ultrathin Pt-Ag nanotubes induced by a thermal ripening process. (a) A model for the nanotubes, showing the porous thin walls with a collapsed double-layer structure. (b) SEM image. (c, d) Low-magnification TEM images. (e) HRTEM image of a collapsed nanotube showing the {110} facet at the edge of the rectangular nanopores. (f) Corresponding Fourier diffractogram. (g) A scheme illustrating the orientation of the nanopores with respect to the Pt-Ag lattice. (h) HRTEM image of a Pt-Ag nanotube at an edge of a rectangular nanopore (enclosed area by the rectangle in e). Circles indicate the arrangement of the surface atoms, showing the presence of step sites on the {110} facets (indicated by the arrows).

The creation of the regular nanopores within the ultrathin Pt-Ag nanotubes was achieved by a convenient thermal ripening process (a model, see Figure 3a). Typically, the ultrathin Pt-Ag nanotubes (thickness: ~ 1.1 nm, Ag: 47.5 %, Figure S5–S7) were dispersed in ethylene glycol (EG). The dispersion was then heated at 150 °C for 9 h to produce the ultrathin Pt-Ag nanotubes with regular rectangular nanopores. Both the scanning electron microscopy (SEM) and TEM images confirm that after the thermal treatment, abundant nanopores were formed in the collapsed nanotubes, while the morphology was virtually unchanged (Figure 3b– d). In particular, the nanopores exhibit a rectangular shape, showing a typical edge length of ~ 8–10 nm (Figure 3d). The

electron diffraction pattern further confirms that the porous ultrathin Pt-Ag nanotubes expose the {100} facet, which is in good agreement with those before the thermal treatment (Figure 3e, f). The X-ray diffraction (XRD) pattern of the porous ultrathin Pt-Ag nanotubes deposited on a glass substrate showed typical reflections of a face-centered cubic (fcc) lattice, with the (200) reflection particularly strong, confirming the preferential exposure of the {100} facet (Figure S8). For comparison, the XRD pattern of the ultrathin Pt-Ag nanotubes showed very weak reflections before the thermal treatment. The increase in the peak intensities suggests that the periodicity of the Pt lattice was significantly improved during the thermal treatment, which



can be attributed to an increased thickness of the Pt-Ag nanotubes (from ~ 1.1 to 1.3 nm during the thermal treatment, Figure S9). During the thermal process, a negligible change (from 47.5 % to 46.7 %) in the mole fraction of Ag can be detected by the inductively coupled plasma mass spectrometry (ICP-MS), and the resulting porous nanotubes are uniform alloys of elemental Pt and Ag, as evidenced by the X-ray photoelectron spectroscopy (XPS) and EDS elemental mappings (Figure S10–S12). Therefore, it is reasonable that under the thermal condition, Pt and Ag atoms migrate to allow an effective ripening process, which formed thicker nanotubes compensated by the formation of nanopores. We found that the Ag alloying in the ultrathin PtAg nanotubes favors the formation of the rectangular nanopores, which may arise from the reduced stability of Pt in the nanotubes and thus the effective migration of the metal atoms under the thermal conditions (Figure S13). In addition, the solvent plays an important role in the thermal ripening process. Besides EG, rectangular nanopores could

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also be created when the ultrathin Pt-Ag nanotubes were thermally treated in N, N-dimethylformamide (DMF) (Figure S14). Both EG and DMF are capable of interacting with metal ions, and by this means, these solvents may accelerate the atom migration of the metals to form the nanopores. The dissociation and deposition of Pt and Ag atoms were in a thermodynamic equilibrium, which accounts for the regular shapes of the nanopores with well-faceted edges. Interestingly, the HRTEM image reveals that the edges of the newlyformed rectangular nanopores were parallel to the {110} plane of the nanotubes (Figure 3e, g). A closer investigation into the structure indicates that abundant step sites are present at the edges of the rectangular nanopores (Figure 3h). It is worth noting that although nanoporous structures can be obtained by many other methods, the regular nanopores with controlled exposing facets are created for the first time in the noble metal nanocrystals.

Figure 4. (a–c) Electrocatalytic activities of the porous ultrathin Pt-Ag nanotubes (Ag: 46.7 %), the ultrathin Pt-Ag nanotubes before the thermal treatment (Ag: 47.5 %) and the commercial Pt/C catalyst in the MOR. (d–f) Electrocatalytic activities of the porous ultrathin Pt-Ag nanotubes with different molar fractions of Ag: 46.7 %, 15.8 %, and 4.6 %. (a, d) CV curves of the catalysts in N2saturated HClO4 (0.1 M) + CH3OH (1 M) at a scan rate of 50 mV s–1. (b, e) Comparison of the specific and mass activities of the catalysts. (c, f) CO stripping voltammetry of the catalysts.

These ultrathin Pt-Ag nanotubes with regular rectangular nanopores and a collapsed double-layer structure represent a novel family of electrocatalysts that promise high catalytic activities. To demonstrate it, we investigated the catalytic activity of the porous ultrathin Pt-Ag nanotubes (wall thickness: ~ 1.3 nm; Ag: 46.7 %) in the methanol oxidation reaction (MOR), in comparison with the nanotubes before the thermal treatment (wall thickness: ~ 1.1 nm; Ag: 47.5 %) and the commercial Pt/C catalyst (Pt 20 %) (Figure 4a–c). The electrocatalytic performances of the catalysts in the MOR were examined by cyclic voltammetry (CV) in N2-saturated CH3OH(1 M)/HClO4(0.1 M) at a scan rate of 50 mV s– 1. The specific (Figure 4a) and mass activities (Figure S15) were calculated by normalizing the current density to the electrochemically active surface area (ECSA, Figure S16)

and the mass of Pt, respectively. It is clear that the both the specific and the mass activity increase in the order of commercial Pt/C < ultrathin Pt-Ag nanotubes without rectangular nanopores < porous ultrathin Pt-Ag nanotubes (Figure 4b). The specific activity of the porous ultrathin Pt-Ag nanotubes was 6.63 mA cm–2 in terms of the current density at the anodic peak, which was 2.3 times that of the nanotubes without rectangular nanopores (2.88 mA cm–2) and 7.1 times that of the commercial Pt/C catalyst (0.93 mA cm–2). The mass activity of the porous ultrathin Pt-Ag nanotubes was 2.08 mA μg–1Pt, which was ~ 2.5 times that of the nanotubes without rectangular nanopores (0.84 mA μg–1 Pt) and ~ 4.1 times that of the commercial Pt/C catalyst (0.51 mA μg–1 Pt). Therefore, of the three materials, the porous ultrathin Pt-Ag nanotubes represent the champion catalyst


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for the electrocatalytic MOR. It is also worth noting that the porous ultrathin Pt-Ag nanotubes outperformed most Ptbased catalysts reported in the literature in catalyzing the MOR (a comparison, see Table S1). From these results, it is inferred that the ultrathin Pt-Ag nanotubes without rectangular nanopores already showed significantly enhanced catalytic activity in the MOR compared with the commercial Pt/C catalyst, which can be attributed to the ultrasmall thickness, the selective exposure of the favorable {100} facet,19-20 and the synergistic ligand effect arising from the Pt-Ag alloying (discussed later).51 The porous ultrathin Pt-Ag nanotubes showed even enhanced catalytic activity, compared with both the nanotubes without the rectangular nanopores and the Ag@Pt core/shell nanowires (Figure S17), which well elucidated the significant role of the rectangular nanopores in enhancing the catalytic activity. It is recognized that the step sites on the newly-formed {110} facets (Figure 3h) favor the direct oxidation pathway of methanol (a pathway without involving COads as an intermediate).20 In addition, the step sites also allow convenient oxidation of COads at the Pt surface, which is the rate-limiting step in an indirect methanol oxidation pathway (a pathway involving COads as an intermediate).52-54 As confirmed by the CO stripping, the CO molecules adsorbed on the porous ultrathin Pt-Ag nanotubes could be oxidized at a much lower potential than those adsorbed on the nanotubes without rectangular nanopores (Figure 4c). Therefore, besides the ultrasmall thickness, the exposing {100} facets (Figure S18, S19) and the Pt-Ag alloying, the newly-formed {110} facets with step sites particularly contributed to the significantly boosted electrocatalytic activity of the porous ultrathin Pt-Ag nanotubes in the MOR. The synergistic effect of Ag was further examined on the electrocatalytic activity of the porous ultrathin Pt-Ag alloy nanotubes (Figure 4d–f). To tune the mole fraction of Ag, the porous ultrathin Pt-Ag nanotubes with a high fraction of Ag (46.7 %) were subjected to a dealloying process with concentrated HNO3 at a high temperature, which gave rise to continuously decreased fractions of Ag (15.8 % and 4.6 %) without causing a significant change in the structure (Figure S20). It is found that the electrocatalytic activity of the porous Pt-Ag nanotubes increased monotonically with the fraction of Ag (Figure 4d, e). With an increasing fraction of Ag in the nanotubes, the oxidation of CO occurs at a much lower potential, as inferred from the CO stripping results (Figure 4f). Therefore, the enhanced electrocatalytic activity by the Pt-Ag alloying could be ascribed to the facile oxidation of COads in an indirect oxidation pathway of methanol. This enhancement may arise from a charge transfer from Ag to Pt in the alloy nanotubes (ligand effect, Figure S10), causing a weakening of the CO adsorption and thus its convenient oxidation.55-56

Figure 5. (a, b) Adsorption energy of CO and methanol, respectively, on Pt {100} and Pt-Ag alloy surfaces, calculated by DFTD.

To better understand the roles of the surface structure and the composition of the porous ultrathin Pt-Ag nanotubes, we further performed density functional theory plus dispersion (DFT-D) calculations to investigate the adsorption energies of CO and methanol molecules on Pt and Pt-Ag alloy surfaces with different exposing facets (Figure 5).57 The modeling of the alloy surface follows Xia’s method in a previous report.58 We consider two layers of a Pt-Ag alloy. The Pt:Ag ratios in the first and second layers are Ag4/Pt4, Pt1Ag3/Pt3Ag1, Pt2Ag2/Pt2Ag2, Pt3Ag1/Pt1Ag3, and Pt4/Ag4, respectively, each maintaining an overall 1:1 Pt:Ag ratio. Our calculation results indicate that except for Pt3Ag1/Pt1Ag3{100}, the Pt-Ag alloy surfaces bind CO much more weakly than a monometallic Pt surface (Figure 5a), which is in line with the CO stripping results (Figure 4f). It is also clear that the adsorption energies of CO on the Pt-Ag {110} surfaces are generally lower than those of CO on the Pt-Ag {100} surfaces (Figure 5a). Therefore, both the Pt-Ag alloying and the Pt-Ag {110} facets at the edges of the rectangular nanopores are confirmed to contribute to the easy oxidization of the COads intermediates in the MOR, leading to significantly enhanced activities. The DFT-D calculation also hints that the {110} facets of a Pt-Ag alloy bind more strongly with methanol molecules, compared with the {100} facets, which represents another effect of the exposing facet that contributes to the enhanced MOR activity (Figure 5b). In addition, the porous ultrathin Pt-Ag nanotubes show superior electrocatalytic durability as demonstrated by the chronoamperometric measurement (Figure S21). The excellent durability can be attributed to the improved poisoning tolerance of the catalyst against the COads intermediates (Figure 4c, f), thanks to the presence of step sites and the PtAg alloying. The structural integrity of the nanotubes during the catalysis also contributed to the excellent catalytic durability (Figure S20).



CONCLUSION In summary, we demonstrate the synthesis of ultrathin Pt-Ag alloy nanotubes with regular rectangular nanopores and a collapsed double-layer structure in a high density by a templated synthesis of Pt against Ag nanowires and a subsequent thermal ripening process. The porous nanotubes expose {100} facets at the basal sides and {110} facets with step sites at the edges of the rectangular nanopores. Benefitting from the ultrasmall thickness, the exposure of the {100}/{110} facets and step sites, and the bimetallic synergistic effect, these porous Pt-Ag collapsed nanotubes exhibit excellent catalytic activity in MOR due to the suppressed CO poisoning of the catalysts, with the typical specific and mass activities being ~ 7.1 and 4.1 times those of the commercial Pt/C catalyst, respectively. We believe our work provides a robust strategy for the synthesis of noble metal open nanostructures with regular shapes of nanopores, which promises enhanced performance in a broad range of electrocatalytic applications.

EXPERIMENTAL SECTION Materials. Chloroplatinic acid hexahydrate (H2PtCl6·6H2O), silver nitrate (AgNO3), iron(III) chloride (FeCl3), sodium nitrite (NaNO2), L-ascorbic acid (AA), sodium hydroxide (NaOH), polyvinylpyrrolidone (PVP) (Mw 55,000), perchloric acid (HClO4, 70 %), acetonitrile (CH3CN), methanol (CH3OH), and ethylene glycol (EG) were purchased from Sigma-Aldrich. All chemicals were used as purchased without further purification. Synthesis of Ag nanowires. Ag nanowires were synthesized according to a previous report with modification.59 Typically, 200 mg of PVP, 250 mg of AgNO3 and 3.5 g of a FeCl3 solution (0.6 mM in EG) were added into 25 mL of EG in order. The mixture was shaken quickly and kept undisturbed at 130 °C for 5 h. Ag nanowires were then collected by centrifugation at 4000 rpm, washed twice with H2O, and redispersed 25 mL of H2O (Ag: ~ 0.059 M). Synthesis of Ag@Pt core/shell nanowires. In a typical synthesis of Ag@Pt core/shell nanowires using CH3CN as a coordination ligand, 13 mL of CH3CN, 8 mL of PVP (5 wt%), 800 µL of AA (0.5 M), 800 µL of NaOH (1 M) and 200 µL of H2PtCl6 (0.1M) were added into 25 mL of a sol of the Ag nanowires (Ag: ~ 0.059 M). The reaction system was transferred to a high-pressure tube, filled with H2 of atmospheric pressure, and stirred at 140 °C for 12 h. Finally, Ag@Pt core/shell nanowires were collected by centrifugation at 4000 rpm, washed twice with H2O, and redispersed in 40 mL of H2O. The thickness of the Pt layer was ~ 1.3 nm. When a nitrite was used as a ligand, a growth solution of Pt was first prepared by incorporating 200 µL of H2PtCl6 (0.1 M) and 800 µL of NaNO2 (0.2 M) in 3 mL of H2O, which was left undisturbed overnight before use. In a typical epitaxial growth, 10.1 mL of H2O, 2 mL of PVP (5 wt%), 400 µL of AA (0.5 M), 400 µL of NaOH (1 M), 100 µL of NaNO2 (0.2 M) and 2 mL of the growth solution of Pt were added into 5 mL of a sol of the Ag nanowires (Ag: ~ 0.059 M). The reaction system was transferred to a high-pressure tube, filled with N2 of atmospheric pressure, and stirred at 30 °C for 12 h. Finally, Ag@Pt core-shell nanowires were collected by

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centrifugation at 4000 rpm, washed twice with H2O, and redispersed in 8 mL of H2O. The wall thickness of the Pt layer was ~ 1.1 nm. Synthesis of ultrathin Pt-Ag nanotubes. Ultrathin Pt-Ag nanotubes were obtained by etching away the Ag templates from the Ag@Pt core/shell nanowires. In a standard procedure, 8 mL of Fe(NO3)3 (0.4 M) was added into 8 mL of the sol of the Ag@Pt core/shell nanowires under stirring. After 2 h, the porous ultrathin Pt-Ag nanotubes were collected by centrifugation at 6000 rpm, washed with H2O for 3 times, and redispersed in 10 mL of EG. Thermal ripening of the Pt-Ag nanotubes. The ultrathin Pt-Ag nanotubes (wall thickness ~ 1.1 nm) in 10 mL of EG were stirred at 150 °C for 9 h. The products were collected by centrifugation at 6000 rpm and washed with H2O for 3 times. Electrochemical measurements. Electrochemical measurements were carried out using a three-electrode system at 25 °C, with a rotating disk electrode (RDE, 0.196 cm2) connected to an Autolab PGSTAT302N electrochemical workstation. A Pt foil (1 × 1 cm2) and a Ag/AgCl (3 M) electrode were used as the counter and reference electrodes, respectively. All potentials were converted into values with reference to a reversible hydrogen electrode (RHE). The (porous) ultrathin Pt-Ag nanotubes were supported on carbon black (Ketjen Black EC-300J) with a metal loading of ~ 20 % (precise loading determined by ICP-MS). Commercial Pt/C (E-Tek, 20 wt% Pt) was used as a reference catalyst. The catalyst was dispersed in a mixture of isopropanol and 5 % Nafion (volume ratio, 1:0.004) under ultrasonication for 1 h, producing a homogeneous ink with a Pt concentration of 0.1 mg mL−1. Then, 10 μL of the ink (Pt, 1 µg) was dropped and dried onto a pre-cleaned glassy carbon RDE. The CV curves were recorded in N2-saturated HClO4 (0.1 M) in the potential range of 0.05−1.2 V at a sweep rate of 50 mV s–1. The ECSAs were calculated based on the charges associated with the adsorption of a monolayer hydrogen on the Pt surface in the region of 0.05−0.4 V after double-layer correction with a reference value of 210 C cm−2. The electrocatalytic activities of the catalysts in MOR were evaluated by CV and chronoamperometry (CA) techniques. The MOR experiments were performed in N2-saturated CH3OH (1 M)/HClO4 (0.1 M) at a scan rate of 50 mV s–1. The CA curves were recorded at 0.8 V for 3000 s to investigate the stability of the electrocatalysts. To obtain the CO stripping voltammograms, the working electrode was held at 0.1 V for 30 min in CO-saturated 0.1 M HClO4, followed by N2 purging for 30 min to remove the excess CO. The CO stripping profiles were collected with the potential scanned from 0.05 to 1.2 V at a rate of 50 mV s–1. DFT-D calculations. All the DFT-D calculations were performed with the program package DMol3 in Materials Studio (version 8.0, Accelrys Inc).60-61 The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerh (PBE) formulation of the exchange-correlation functional was employed.62 The valence electron wave functions were expanded into a set of atomic orbitals composed of the double numerical plus polarization (DNP) basis set, and the cutoff radius is 4 Å.63 We use 2×2×1 Monkhorst-Pack k-point meshes to sample the Brillouin zones. The models of Pt and the Pt-Ag alloy (Pt:Ag = 1:1) surface were constructed by


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following Xia’s method.58 The surfaces were modeled as four-layer slabs, with the uppermost two layers relaxed together with the adsorbate, whereas the remaining two substrate layers were fixed in their bulk positions. Specifically, the composition (denoted as the Pt:Ag ratio) of the uppermost two layers are Ag4/Pt4, Pt1Ag3/Pt3Ag1, Pt2Ag2/Pt2Ag2, Pt3Ag1/Pt1Ag3, and Pt4/Ag4, respectively, each maintaining an overall 1:1 Pt:Ag ratio on a {100} or {110} surface. The vacuum thickness between the slabs is taken as 20 Å. The convergence tolerance of energy, maximum force, and maximum displacement for geometry optimization are 2×10−5 Ha, 0.004 Ha/Å, and 0.005 Å, respectively. The adsorption energies, Eads, are calculated by Eads = Eadsorbate+surface – (Eadsorbate + Esurface). Here, Eadsorbate+surface, Eadsorbate, and Esurface are the total energy of the surface covered with adsorbates, the energy of the adsorbate, and the energy of a clean surface, respectively. A negative value of Eads means a release of energy or a stable adsorption mode of the adsorbate on the surface. Characterizations. HRTEM and HAADF-STEM were acquired using FEI Tecnai F20 FEG-TEM and JEOL JEM-F200 microscopes operated at 200 kV. Low-magnification TEM was performed on a Hitachi HT-7700 microscope equipped with a LaB6 filament, operated at 100 kV. Atomic force microscopy (AFM) analysis was conducted on an Asylum Research Cypher S microscope. X-ray diffraction (XRD) patterns were recorded on a Rigaku SmartLab Powder X-ray diffractometer equipped with Cu Kα radiation and D/teX Ultra detector, scanning from 30 to 90° (2θ) at a rate of 5 °/min. ICP-MS was performed on a NexION 350D.

ASSOCIATED CONTENT Supporting Information. Additional electron microscopy and AFM images, EDS elemental mappings, XRD, XPS, and electrocatalytic results. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected].

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant numbers: 21671156 (C.G.), 21301138 (C.G.), 21601118 (H.Z.), and 21571128 (L.H.)). C.G. acknowledges the support from the Fundamental Research Funds for the Central Universities, the World-Class Universities (Disciplines) and the Characteristic Development Guidance Funds for the Central Universities, and the Tang Scholar Program from Cyrus Tang Foundation. Y.Y. acknowledges the support from the UC-KIMS Center of Innovative Materials for Energy and Environment (UC-KIMS CIME). Acknowledgment is also made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research (55904-ND10). The authors thank Jiao Li and Guoqing Zhou at Instrument Analysis Center of Xi’an Jiaotong University for assistance with HRTEM and ICP-MS.

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