Concave Platinum–Copper Octopod Nanoframes Bounded with

Sep 23, 2016 - Multimetallic nanoframes with three-dimensional (3D) catalytic surfaces represent an emerging class of efficient nanocatalysts. However...
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Concave Platinum−Copper Octopod Nanoframes Bounded with Multiple HighIndex Facets for Efficient Electrooxidation Catalysis Shuiping Luo†,‡ and Pei Kang Shen*,†,‡ †

School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China Collaborative Innovation Center of Sustainable Energy Materials, Guangxi University, Nanning 530004, P. R. China

‡

S Supporting Information *

ABSTRACT: Multimetallic nanoframes with three-dimensional (3D) catalytic surfaces represent an emerging class of efficient nanocatalysts. However, it still remains a challenge in engineering nanoframes via simple and economical methods. Herein, we report a facile one-pot synthetic strategy to synthesize Pt−Cu nanoframes bounded with multiple high-index facets as highly active electrooxidation catalysts. Two distinct octopod nanoframes, namely, concave PtCu2 octopod nanoframes (PtCu2 CONFs) and ultrathin PtCu octopod nanoframes (PtCu UONFs) were successfully synthesized by simply changing the feeding Pt and Cu precursors. Interestingly, the PtCu2 CONFs are constructed by eight symmetric feet with sharp tips, which are enclosed by high-index facets of n (111)−(111), such as {553}, {331}, and {221}. Benefiting from their 3D accessible surfaces and multiple high-index facets, the self-supported PtCu2 CONFs catalysts exhibit excellent electrocatalytic performance and superior CO-tolerant ability. For methanol oxidation reaction, the PtCu2 CONFs catalysts exhibit more than 7-fold increase in activities, 205 mV lower in the onset potential compared with commercial Pt/C. More importantly, when facing harsh electrochemical reaction conditions, the PtCu2 CONFs are well-preserved in the catalytic activities, architectural features, and stepped surfaces. The PtCu UONFs with 12 ultrathin edges, however, suffer from breakdown. The present work provides guidelines for the rational design and synthesis of nanoframe catalysts with both high activity and stability. KEYWORDS: nanoframes, high-index facets, platinum, methanol oxidation reaction, formic acid oxidation reaction

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Improving the stability of Pt-based nanocatalysts is another key challenge because the traditional Pt-based electrocatalysts normally suffers from (1) carbon corrosion, which leads to the inactivation of nanoparticles that are detached from the support; (2) sintering of the nanoparticles, which leads to the decrease in atomic utilization; and (3) dissolution of transition metals, which leads to the morphology evolution and causes damage to the Nafion membrane.13−15 These problems would result in rapid degradation of catalysts under realistic reaction conditions. Therefore, it still remains urgent in the rational design and synthesis of new nanocatalysts with both high activity and stability. Recently, polyhedral Pt−Ni nanoframes have been developed and represent a promising electrocatalysts

latinum (Pt) is the most efficient monometallic catalyst in many significant applications, such as fuel cells, catalytic converters and metal-air batteries.1 However, it remains great challenges in optimizing the catalytic performance and reducing the costs of catalysts to enable its broad deployment in real world applications. Catalytic activity can be significantly improved by engineering the structure (size, morphology, composition and surface) of Pt at atomic scale. Intense research efforts have been focused on: (1) Pt-M alloys (M = Ni, Co, Cu, etc.), which can reduce the precious metallic content and tailor the electronic properties of Pt;2−6 (2) highindex facets, which exhibit higher activity than low-index planes;7−10 and (3) nanoframe structures, which offer 3D catalytic surfaces and high utilization of precious metals.1,11,12 However, synthesizing Pt-based alloy nanoframes with highindex facets remains an important challenge. © 2016 American Chemical Society

Received: July 6, 2016 Accepted: September 23, 2016 Published: September 23, 2016 11946

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Figure 1. (a) TEM image and (b) HAADF-STEM image of the PtCu2 CONFs. The inset in (a) shows the TEM image of individual CONFs. Scale bar, 10 nm. (c) Aberration corrected TEM image of a typical foot on the CONFs. The inset in (c) shows the corresponding fast Fourier transition (FFT) pattern, and the white arrows in (c) show the high-index facets of n (111)−(111). (d) HAADF-STEM image and EDX mapping images of individual PtCu2 CONFs.

for fuel cells.1,11 However, those nanoframe catalysts are usually fabricated by a multistep synthesis method, making cost-saving mass production a significant challenge toward real world applications. Thus, engineering self-supported and highperformance nanoframe electrocatalysts via one-pot method is of great interest.16 Herein, we report the facile one-pot synthesis of selfsupported concave PtCu2 octopod nanoframes (CONFs) bounded by high-index facets as highly active and stable electrooxidation catalysts. The well-defined CONFs architectures meet several critical design criterions for efficient nanocatalysts: (1) the highly open nanostructures offer 3D catalytic surfaces, high surface to volume ratio, and high atomic utilization; (2) the stepped surfaces composed of multiple highindex facets serve as highly active sites, which exhibit enhanced catalytic activities for electrooxidation reactions; (3) selfsupported CONFs catalysts eliminate the carbon corrosion problems and facilitate the reactions by enhancing mass transport and electron conduct; and (4) the CONFs are exclusively constructed by eight symmetric feet, which exhibit excellent physical and electrochemical stability. Benefiting from the aforementioned advantages, such free-standing PtCu2 CONFs catalysts exhibit extraordinary activity and stability in both methanol oxidation reaction and formic acid oxidation reaction.

directly prepared by a facile stirring-assisted solvothermal method. Platinum acetylacetonate ([Pt(acac)2]) and copper acetylacetonate ([Cu(acac)2]) were simultaneously dissolved and reduced by oleylamine (OAM), in the presence of CTAB, which serves as surfactant and structure-directing agent. In a typical synthesis, Pt(acac)2, Cu(acac)2, CTAB, and OAM were mixed together by stirring under ambient atmosphere (see the experimental section for details). The autoclave was sealed in a stainless steel and solvothermally treated for 24 h in a preheated oil bath (170 °C) with moderate magnetic stirring. After cooling down to room temperature naturally, the products were precipitated using ethanol via centrifugation, washed with ethanol and further dispersed in hexane. The transmission electron microscope (TEM) and highangle annular dark-field scanning TEM (HAADF-STEM) images in Figure 1 show that the product are uniform nanocrystals with shape yield approaching 100% (Figure S1). The well-defined nanocrystals has an even size distribution with average edge length of 20.7 nm. Interestingly, the open nanostructure appears as a concave nanoframe, which possesses eight symmetric feet with average length of 9.8 nm (Figure S2).2,8,10,17−19 Figure 1c shows the atomic-resolution TEM image of an individual foot oriented along the [110] zone axis, where abundant step and terrace atoms are clearly observed on the surfaces. As shown in Figure S3, the detailed analysis demonstrate that the stepped surfaces are enclosed by continuously distributed high-index facets with (111) terraces and (110) steps, such as {331}, {221}, and {553}, which usually serves as highly active sites.20−22 These high-index facets

RESULTS AND DISCUSSION Synthesis and Characterization of Concave PtCu2 Octopod Nanoframes. The intriguing PtCu2 CONFs were 11947

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Figure 2. (a−c) TEM images of ultrathin PtCu octopod nanoframes at low, middle, and high magnifications, respectively. The inset in (b) shows the structure model of UONFs. (d) HAADF-STEM image and EDX mapping images of individual PtCu UONFs.

a narrow size distribution of ∼20 nm. Each individual nanocrystal is constructed by eight symmetrical feet with average length of 8.6 nm and 12 ultrathin edges (2 nm thick) with average length of 14.1 nm. As shown in Figure S6, the octopods also possess abundant high-index facets of n (111)− (111), such as {331}, {221}, and {553}, on the border surface. Besides the obvious difference in their overall structure features (size, feet, and edges), these two distinct nanoframes also show much difference in compositions. The high-resolution elemental mapping images of UONFs show that Pt and Cu elements are both present in the whole nanoframe and that the Pt element has relatively higher content on the surfaces (Figure 2d). As shown in Figure S7, the XRD patterns of PtCu2 CONFs and PtCu UONFs exhibit similar face-centered-cubic (FCC) and single-phase alloy structure. The diffraction peaks of CONFs shift to higher 2θ values (0.6 degree) compared with the UONFs, indicating a higher Cu content in the CONFs alloy. As shown in Figure S8, no impure peaks are observed in the EDS spectrum. Detailed EDX analysis, taken from five different regions on the TEM grid, show that the average atomic percentage of Pt in the CONFs is 33.1%, whereas the UONFs have an average Pt atomic percentage of 47.8%. As shown in the EDX line profiles (Figure S9), the PtCu2 CONFs show a similar changing trend in the intensity of Pt and Cu elements, indicating the uniformly distributed elements throughout the feet of CONFs. However, the PtCu UONFs has a relatively higher Pt content on the surface and higher Cu content in the core, indicating the presence of Pt-rich surfaces on the feet of UONFs. Growth Mechanism. To understand the formation mechanism of CONFs, a set of control experiments regarding

composed of (111) terraces and (110) steps are usually represented by n (111)−(111) or equivalently (n − 1) (111)(110), as (110) surface can be further divided into one unit of (111) terrace and (111)̅ step.20,22 As illustrated by the atomic models of high-index facets (Figure S3), n = 2, 3, 4, and 5 in the notation are corresponding to the surfaces of (110), (331), (221), and (553), respectively. The HAADF-STEM micrograph shown in the Figure 1d clearly demonstrates that the highly open CONFs are exclusively constructed by interconnected feet with sharp tips. Specially, the PtCu2 CONFs do not contain ultrathin edges, which normally appear in the polyhedral nanoframes. The high-resolution elemental mapping images of an individual CONFs show that the Pt and Cu elements are overlapped throughout the octopods. The brightness in the HAADF-STEM images represents the intensity associated with the number of atoms in the column (sample thickness) and the number of electrons per atom (Z). The higher contrast on the corners of CONFs is ascribed to the relatively higher Pt content and sample thickness on the corners compared with other sites, as ZPt (78) is significantly higher than ZCu (29) and the thickness of corners is relatively greater than other sites. The XPS results clearly demonstrate that the electronic coupling exists between Pt and Cu atoms and that Pt and Cu elements are mainly in metallic state on the surfaces (Figure S4). Synthesis and Characterization of Ultrathin PtCu Octopod Nanoframes. By using H2PtCl6·6H2O + Cu(NO3)2·3H2O as precursors instead of Pt(acac)2 + Cu(acac)2, well-dispersed ultrathin PtCu octopod nanoframes (PtCu UONFs) were obtained in high shape yield approaching 100% (Figure 2, Figure S5).23,24 The well-defined UONFs have 11948

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Figure 3. Representative TEM images of the intermediates obtained at different reaction time during the synthesis of concave PtCu2 octopod nanoframes: (a) 0.5 h, (b) 1 h, (c) 12 h, and (d) 18 h, respectively. (e) Schematic of the major steps involved in the formation of PtCu2 CONFs.

the synthesis reagents were performed at first. In the present synthesis system, OAM alone would facilitates the formation of 3D Pt nanorod-assemblies growing along ⟨111⟩ directions (Figure S10a). No product was detected in absence of Pt precursors and CTAB, indicating that it is difficult for OAM to reduce the Cu species due to its mild reducing ability.25 Generally, Pt species are more easily reduced than Cu species in the same conditions because the standard redox potential for Cu2+/Cu (0.34 V vs RHE) are more negative than PtCl62−/Pt (0.74 V vs RHE). The Pt nuclei can induce the underpotential reduction and deposition of Cu species, resulting in the formation of Pt−Cu alloy.26 CTAB in our synthesis recipe appears to be the key for the preparation of concave PtCu2 octopod nanoframes. Solid polyhedral nanoparticles were obtained in absence of CTAB (Figure S10b), and these solid nanocrystals were etched into hollow nanoparticles with ultrathin shell after acid-treatment (1.0 M HCl for 0.5 h), indicating that the recipe in absence of CTAB are able to produce polyhedral Pt−Cu nanocrystals with Cu-rich core (Figure S10c).6 The results also demonstrate that CTAB functions as the structure-directing agent to induce the formation of well-defined PtCu2 CONFs. When 150 mg CTAB was added in the synthesis recipes, concave octopods were obtained. Further increasing the amount of CTAB to 250 mg, the morphology became concave octopod nanocages with side facets (Figure S11a,b). The results suggest that the concave octopod nanostructures are preferably formed in the presence of CTAB, and a high concentration of CTAB is the key to produce perfect CONFs.27 In the CTAB molecules, Br¯ is the functional group in maneuvering CONFs, by selectively binding on the {100} facets and serving as oxidation etchant (Br¯/O2).8 CTA+ is vital for the dispersion of NCs and formation of stepped surfaces because its long alkyl chain is comparable in size to the atomic spacing on the high-index facets.28,29 The conclusion was experimentally confirmed by the results that typical CONFs were prepared as well when CTAB was replaced by the same concentrations of KBr and TBAB (Figure S11c,d). To gain insight into the morphological evolution processes of PtCu2 CONFs, the intermediates produced after different reaction period were analyzed by TEM and ICP-AES (Figure 3a−d). After 0.5 h of reaction, the truncated cubes were obtained with edge length of 5.2 nm, and the Pt yield was 28.9%. When the reaction time was increased to 1 h, concave

cubes with average size of 7.8 nm and Pt yield of 42.7% were obtained. Specially, the interior and corners of concave cubes were partly etched. The results imply that the Pt(acac)2 species are quickly reduced and that the corrosion are present at the early stage. By further increasing the reaction time to 12, 18, and 24 h, the size of concave cubes and the yield of Pt increased continuously until the Pt(acac)2 precursors were completely depleted. Meanwhile, the side facets were gradually disappeared, and finally the octopod nanoframes could be obviously observed. The results indicate that the overgrowth from concave cubes to concave octopods is relatively slow, and that the octopod nanoframes evolved from solid octopods by continuously etching. Figure 3e illustrates the schematic of typical formation processes of PtCu2 CONFs. At the nucleation stage, nuclei under the corrosive conditions would preferentially take a thermodynamic stable single-crystalline cuboctahedron bounded by a mix of (111) and (100) facets, which is a typical wulff polyhedron with lowest surface energy.30,31 At the growth stage, considering that the {100} facets are capped by Br¯ and the Pt(acac)2 species (1.18 V vs RHE) are fast reduced, the rate for deposition on corners would be faster than the rates for diffusion along ⟨110⟩ directions and etch along ⟨100⟩ directions (Vdepo > Vdiff ≈ Vetch). As a result, the nuclei would selectively and quickly grow into concave cubes.8,28,32 Then, the slowly deposition of newly generated atoms on the corners, and overgrowth along the ⟨111⟩ directions are preferred, as a relatively low concentration of Pt species are remained in the solution. Oxidative etching by Br¯/O2 is vital for synthesizing 3D open CONFs. Solid nanoparticles were obtained, when the oxidative etching was efficiently eliminated by avoiding halide ions or removing oxygen (heating the reaction mixture before sealing the autoclave) (Figure S12).33 Being able to control the fabrication of two distinct octopod nanoframes by simply changing the feeding precursors is the most striking feature reported herein.24 During the synthesis of PtCu UONFs (Figure S13a−e), no product were detected within 0.5 h. The result indicates the slow reduction rate of H2PtCl6·6H2O species, which has a relatively low redox potential of 0.74 V vs RHE. In these conditions that the {100} facets are capped by Br¯ and the H2PtCl6 species are slowly reduced, the rates for deposition on corners, diffusion along ⟨110⟩ directions, and etch along ⟨100⟩ directions would be comparable (Vdepo ≈ Vdiff ≈ Vetch). As a result, cubic 11949

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Figure 4. Cyclic voltammetry (CV) curves of different electrocatalysts for methanol oxidation reaction (a) recorded at scanning rate of 50 mV s−1 in 0.5 M H2SO4 + 1 M CH3OH solution, and formic acid oxidation reaction (b) recorded at scanning rate of 50 mV s−1 in 0.5 M H2SO4 + 0.25 M HCOOH solution, respectively. (c,d) Mass and specific activities of PtCu2 CONFs, PtCu UONFs, Pt black, and Pt/C electrocatalysts for methanol oxidation reaction.

comparable with that of Pt/C (46.1 m2/g) and Pt black (41.4 m2/g). The ECSA of PtCu UONFs (52.1 m2/g) is higher than that of PtCu2 CONFs, due to the smaller size and more accessible interior of nanoframes in the former. The onset potentials for CO-stripping CVs on PtCu2 CONFs and PtCu UONFs are both negatively shifted by more than 70 mV compared to pure Pt catalysts (Pt black and commercial Pt/C), indicating the enhanced CO antipoisoning ability of the Pt−Cu alloy nanoframes (Figure S16). As shown in Figure 4a, the peak current density of PtCu2 CONFs for methanol oxidation reaction is 155.9 mA cm−2, which is 1.1, 3.4, and 7.2 times higher than that of PtCu UONFs (137.5 mA cm−2), Pt black (46.3 mA cm−2), and Pt/C (21.8 mA cm−2), respectively. The PtCu2 CONFs exhibit maximum mass activity of 3355.6 mA mg−1 and specific activity of 7.5 mA cm−2, which are 7.0 and 7.2 times higher than that of commercial Pt/C (Figure 4). More importantly, the onset potential for methanol oxidation reaction on PtCu2 CONFs (213 mV vs SCE) is 205 mV lower than that of commercial Pt/C (418 mV vs SCE) (Figure S17). The higher current density at lower potentials indicates a greatly enhanced catalytic activity on the PtCu2 CONFs.13 As shown in Figure 4b, the peaks for direct oxidation of formic acid to CO2 at about 0.4 V in the forward scan correspond to current densities of 0.47, 0.50, 20.1, and 27.3 mA cm−2 for Pt/ C, Pt black, PtCu UONFs, and PtCu2 CONFs, respectively. The result indicates significantly enhanced formic acid oxidation catalytic activity on the PtCu2 CONFs.37 The enhanced electrooxidation catalytic activity of PtCu2 CONFs should be attributed to the synergistic effect resulting from the composition (alloy with Cu), surface structures (high-index facets), and shape effects (concave nanoframes).38 Pt atoms on the surface are surrounded by more Cu atoms on the subsurface in the PtCu2 CONFs compared with that in the PtCu UONFs. The presence of more subsurface Cu atoms can further weaken the binding of surface Pt atoms to intermediates and expedite the electrooxidation reactions.39

nanoframes were obtained after 6 h. By further increasing the reaction time, cubic nanoframes with edge length of 14.0 nm and Pt yield of 54.8% were obtained. Specially, small branches with length ranging from 2 nm up to 7.2 nm were observed on the corners. After 18 h of reaction, the average length of feet increased to 7.9 nm and the Pt yield increased to 86.2%. The results indicate the fast anisotropic-growth of feet on the corners, when a relatively high concentration of Pt species are remained in the reaction solution (Figure S13f). Therefore, the precursors with different ligand and oxidation state would affect the reduction kinetics, resulting in the difference of structure features.23 In addition, the replacement of Pt(acac)2 with H2PtCl6· 6H2O would introduce considerable Cl¯ ions, which can adsorb on specific metal surfaces such as {110}, and act as a wellknown oxidative etchant when combined with oxygen (Cl¯/ O2).34,35 In the present synthesis system, dendritic nanocrystals were produced when H2PtCl6·6H2O, Cu(NO3)2·3H2O, and OAM were used in the recipe, as shown in Figure S14a. The result indicates that the Cl¯/O2 pair as oxidative etchant is introduced after changing precursors, as the Cl¯/O2 pair could oxidize the metallic atoms back to ions while Cl¯ ions could help coordinate and stabilize the newly generated metallic ions; thus, the competition of reduction and oxidation could result in dendritic nanostructures.34 As shown in Figure S14b, rhombic dodecahedral nanoframes were produced when H2PtCl6·6H2O, Cu(NO3)2·3H2O, CTAC (100 mM), and OAM were used in the recipe. The result indicates that the introduction of Cl¯ ions would facilitate the etching process to produce highly open nanoframes, and that a high concentration of Cl¯ ions are required to determine the shape of nanoframes through selectively capping on {110} surfaces.35 Electrooxidation Catalytic Performance. The freestanding PtCu2 CONFs with high-index facets are expected to exhibit enhanced activity for electrooxidation reactions.5,7,22,36 The PtCu2 CONFs exhibit high electrochemical active surface area (ECSA) of 44.5 m2/g (Figure S15), which is 11950

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nitrate trihydrate [Cu(NO3)2·3H2O, 99%], copper acetylacetonate (Cu(acac) 2, 97%), oleylamine (OAM, 80−90%), cetyltrimethylammonium bromide (CTAB, 99%), and tetrabutylammonium bromide (TBAB, 99%) were purchased from Aladdin. All chemicals were used as received without further purification. Solvents such as hexane, ethanol, methanol, and formic acid were analytical grade and used as received without further purification. The water used in all experiments was ultrapure (18.2 MΩ). Synthesis of Concave PtCu2 Octopod Nanoframes. In a typical synthesis of concave PtCu2 octopod nanoframes, 20.7 mg Pt(acac)2 and 69.2 mg Cu(acac)2 are added into the 25 mL Teflon-lined autoclave containing 10 mL of OAM and 350 mg of CTAB. The mixed solution is stirred for 10 min in the autoclave under ambient atmosphere, and then sealed in a stainless steel. The device is transferred into an oil bath with the temperature holding at 170 °C and solvothermally treated for 24 h with moderate magnetic stirring. After cooling down to room temperature naturally, the products were precipitated using ethanol via centrifugation (3500 rpm for 5 min), washed with ethanol, and further dispersed in hexane. Synthesis of Ultrathin PtCu Octopod Nanoframes. In a typical synthesis of ultrathin PtCu octopod nanoframes, 26.7 mg H2PtCl6·6H2O and 64.2 mg Cu (NO3)2·3H2O are added into the 25 mL Teflon-lined autoclave containing 10 mL of OAM and 350 mg of CTAB. The mixed solution is stirred for 10 min in the autoclave under ambient atmosphere, and then sealed in a stainless steel. The device is transferred into an oil bath with the temperature holding at 170 °C, and solvothermally treated for 24 h with moderate magnetic stirring. After cooling down to room temperature naturally, the products were precipitated using ethanol via centrifugation (3500 rpm for 5 min), washed with ethanol, and further dispersed in hexane. Materials Characterizations. The X-ray diffraction (XRD) measurements were collected on a D/Max-IIIA using Cu−Kα radiation (Rigaku Co., Japan). TEM studies were performed on a Tecnai G2 F30 transmission electron microscope operating at 300 kV. High resolution-TEM (HRTEM), high angle annular dark field scanning transmission electron microscope (HAADFSTEM) and EDS mapping were carried out on Titan G2 60300 equipped with image corrector and highly sensitive SuperX energy dispersive X-ray (EDX) detector system. The samples for TEM characterizations were prepared by dropping hexane dispersion of nanocrystals onto 300-mesh Mo grids and immediately evaporating the solvent. X-ray photoelectron spectroscopy (XPS) was conducted with two separate systems equipped with monochromatic Al K sources to analyze the chemical composition of the samples (ESCALab250, U.S.A.). The loadings of catalysts were determined by the inductively coupled plasma atomic emission spectroscopy (TJA RADIAL IRIS 1000 ICP-AES, U.S.A.). Electrochemical Study. The electrochemical measurements were conducted in a three-compartment electrochemical cell with a Pine rotational disk electrode (RDE) setup connected with a biopotentiostat (AFCBP1E, Pine Instrument Co., U.S.A.), Glassy carbon (GC) covered with catalyst acts as a working electrode, Pt flag as a counter electrode and saturated calomel electrode (SCE) as a reference electrode. GC disk electrode (5 mm in diameter) served as the substrate for the support, and was polished using aqueous alumina suspension prior to use. To prepare the working electrodes for catalytic studies, the black products were collected after reaction and

Stability of Two Distinct Octopod Nanoframes. The durability tests were carried out by sweeping the potential from −0.2 to 1.0 V vs SCE at a scan rate of 100 mV s−1 for 2000 cycles. After such potential cycles, the ECSAs of the PtCu2 CONFs, PtCu UONFs, Pt black, and Pt/C decreased by 8.5%, 20.5%, 45.2%, 80.0%, respectively. The well-retained ECSA of PtCu2 CONFs demonstrated its high stability in acid. The peak current densities for methanol oxidation reaction are 112.3, 74.2, 20.1, and 5.5 mA cm−2 on PtCu2 CONFs, PtCu UONFs, Pt black, and commercial Pt/C, which are 72.0%, 53.9%, 43.4%, and 25.2% of their initial values, respectively. The peak current density of PtCu2 CONFs after durability test is more than 20 times higher than that of commercial Pt/C (Figure S18). After durability tests, the PtCu2 CONFs are well preserved in both performance and structure (overall morphology and stepped surfaces) (Figure 5a). However, some ultrathin edges

Figure 5. HAADF-STEM images of PtCu2 CONFs (a) and PtCu UONFs (b) obtained after 2000 potential cycles. The insets show the structure models of nanoframes after durability tests. Scale bar, 20 nm.

in the PtCu UONFs are broken, leading to the collapse of nanoframes and loss of activities (Figures 5b and S19). The instability of ultrathin edges in the PtCu UONFs may be ascribed to the fact that Cu atoms at the narrow regions cannot be fully covered by Pt surfaces, and thus suffer from continuous dissolution in acid (0.5 M H2SO4).40 The results demonstrate that the feet are more stable than the ultrathin edges in the octopod nanoframe architectures.

CONCLUSIONS In summary, we have successfully prepared two distinct platinum−copper octopod nanoframes bounded with multiple high-index facets, via a facile yet robust synthetic strategy. CTAB was demonstrated to be the structure-directing agent to induce the formation of well-defined CONFs with high-index facets. Because of the 3D accessible surfaces enclosed by abundant high-index facets, the self-supported PtCu2 CONFs catalyst exhibited greatly enhanced activity and durability than Pt black and commercial Pt/C for the electrooxidation reactions. Our results also demonstrated that the feet are more stable than ultrathin edges in the octopod nanoframe architectures under harsh reaction conditions. We expect the present research would inspire the rational design and synthesis of nanoframe electrocatalysts with highly active and stable components for real world applications. MATERIALS AND METHODS Chemicals. Chloroplatinic acid hexahydrate (H2PtCl6· 6H2O), platinum(II) acetylacetonate (Pt(acac)2, 97%), copper 11951

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washed with sufficient ethanol by centrifugation for three times (3500 rpm for 5 min). The catalyst dispersion was prepared by mixing nanocrystals with ethanol, followed by untrasonication for 15 min. An aliquot of catalyst suspension was pipetted using a micropipette onto the clean GC surface, leading to a metal loading in the range of 50 μgPt cm−2 for commercial Pt/C, platinum black, PtCu2 CONFs, and PtCu UONFs. The Pt loadings of catalysts on the electrodes were determined by the ICP-AES. After evaporation of ethanol in air, 10 μL of 0.05 wt % Nafion suspension (DuPont, U.S.A.) was transferred onto the electrode surface to attach the catalyst particles to the GC RDE. Then the electrodes were electrochemical activated by sweeping from −0.2 to 1.0 V versus SCE in N2-saturated 0.5 M H2SO4 solution at a scan rate of 100 mV s−1. After the electrochemical activation (100 cycles), overlapped cyclic voltammetry (CV) curves were obtained at scanning rate of 50 mV s−1 in N2-saturated 0.5 M H2SO4 solution. The electrochemical surface areas were calculated from the H2 desorption peak of the CV curve. The methanol oxidation reaction and formic acid oxidation reaction experiments were carried out by sweeping the potential from −0.2 to 1.0 V versus SCE at a scan rate of 50 mV s−1 in 0.5 M H2SO4 + 1 M CH3OH and 0.5 M H2SO4 + 0.25 M HCOOH solution, respectively. For CO-stripping experiments, the working electrode was held at −0.14 V versus SCE for 15 min under a flow of CO. The remaining CO in the electrolyte was thoroughly purged with N2 for 15 min, and then the CV curves were obtained at a scan rate of 50 mV s−1. The durability tests were carried out by sweeping the potential from −0.2 to 1.0 V at a scan rate of 100 mV s−1 for 2000 cycles. All electrochemical experiments were carried out at 30 °C.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b04458. Experimental details, TEM images, XRD patterns, XPS analysis, and EDX spectrum of concave PtCu2 octopod nanoframes, HRTEM, HAADF-STEM, and EDS mapping images of ultrathin PtCu octopod nanoframes, TEM analysis of ultrathin PtCu octopod nanoframes obtained at different reaction time points, TEM analysis of products prepared using different surfactants, and cyclic voltammetry (CV) curves of catalysts for electrooxidation reactions. (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Major International (Regional) Joint Research Project (51210002), the National Basic Research Program of China (2015CB932304), and the Natural Science Foundation of Guangdong province (2015A030312007). REFERENCES (1) Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H. L.; Snyder, J. D.; Li, D.; Herron, J. A.; Mavrikakis, M.; et al. Highly 11952

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DOI: 10.1021/acsnano.6b04458 ACS Nano 2017, 11, 11946−11953