High-Index Facets Bounded Platinum–Lead Concave Nanocubes with

However, the high surface energy, thermodynamic instability, and lattice mismatch make it a significant challenge to synthesize well-defined bimetalli...
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High-Index Facets Bounded Platinum-Lead Concave Nanocubes with Enhanced Electrocatalytic Properties Liang Huang, Xueping Zhang, Yujie Han, Qingqing Wang, Youxing Fang, and Shaojun Dong Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 02 May 2017 Downloaded from http://pubs.acs.org on May 3, 2017

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Chemistry of Materials

High-Index Facets Bounded Platinum-Lead Concave Nanocubes with Enhanced Electrocatalytic Properties Liang Huang,†,§ Xueping Zhang,†,‡ Yujie Han,† Qingqing Wang,† 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: High-index facets bounded platinum-based alloy nanocrystals usually exhibit enhanced electrocatalytic activity. However, the high surface energy, thermodynamic instability and lattice mismatch make it a significant challenge to synthesize well-defined bimetallic nanocrystals with high-index facets. Here we developed a one-step wet-chemical synthesis of uniform PtPb concave nanocubes (CNCs) enclosed by {520} high-index facets. The as-prepared PtPb CNCs exhibited high synthetic yield and highly concave structure with an average size of 14 nm. Moreover, the synergistic effects and exposed {520} facets endowed the PtPb CNCs with excellent stability and electrocatalytic mass activity toward the methanol oxidation reaction (MOR), which was 2.16 and 4.62 times higher than PtPb nanocubes (NCs) and commercial Pt/C catalysts, respectively. This work provides a new way for rational design and practical application of efficient catalysts.

INTRODUCTION Platinum (Pt) nanostructure electrocatalysts for renewable energy conversion devices, such as polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), are becoming irreplaceable.1-6 However, the high cost, low reserves and poor stability predominantly hindered the large-scale commercialization.7-8 Hence, the primary goal for the design of Pt nanomaterials is to maximize the atomutilization efficiency and electrocatalytic activities.9-12 A major trend is alloying Pt with non-noble metals, which could tune the electronic properties of Pt effectively by the second metal and exhibit enhanced performance for electrocatalysis.13-15 On the other hand, the catalytic active sites are generally located on the surfaces of the heterogeneous catalysts, thus the nanocrystals equipped with highly active surfaces, such as welldefined planes and high-index facets, would significantly promote the mass and specific activities of Pt.16-19 Based on the above viewpoint, Pt-based bimetallic nanoparticles with high-index facets will be an ideal model for the electrocatalysts. High-index facets are considered as a kind of open structures, which are denoted by a set of Miller indices {hkl} with at least one index being greater than unity.20-21 These facets possess a high density of low-coordinated atoms in the sites of steps, edges, and kinks, which could serve as more active catalytical sites than low-index facets.22 To date, there are only a few reports about Pt-based nanocrystals exposed with highindex facets, for example, convex nanopolyhedra,17,23 concave nanopolyhedra16,18,24 and 1D nanowires.25-26 The synthetic methods mainly include electrochemical methods17,22 and wetchemical methods24-27 (site-specific dissolution and directionally controlled overgrowth). However, it is a great challenge to preserve high-index facets during the growth of nanocrystals

due to the high surface energy and thermodynamic instability. Therefore, developing simple synthetic methods for Pt-based bimetallic nanocrystals exposed with high-index facets is crucial for electrocatalytical research. Herein, we report the synthesis of PtPb concave nanocubes (CNCs) enclosed with high density {520} high-index facets through a one-step wet-chemical approach and served as robust electrocatalysts for methanol oxidation reaction (MOR). During the methanol molecules oxidation process, the strongly adsorbed carbon monoxide (CO) intermediates on Pt atom sites led to the electrocatalyst poisoning.28-29 While both experiments and density functional theory calculations indicate that Pt-Pb bimetallic alloy have superior electrocatalytic properties and stability toward MOR for its synergetic effects and COpoisoning tolerance.30-31 Integrated the synergistic effects of Pt-Pb bimetal with high-index facets, the PtPb CNCs have the mass activity of 0.97 A mgPt-1 and specific activity of 2.09 mA cm-2 for MOR, which are 4.6 and 5.2 times higher than those of commercial Pt/C catalyst, respectively. Besides, the PtPb CNCs show higher catalytic activity and stability than other PtPb bimetallic nanocrystals with various morphologies. We synthesized PtPb CNCs by one-step hydrothermal method using platinum(II) acetylacetonate (Pt(acac)2) and lead(II) iodide (PbI2) as the metal precursors, oleylamine (OAm) and oleic acid (OA) mixture as solvents and surfactants, cetyl trimethylammonium bromide (CTAB) as the structure-directing agent and glucose as reducing agent. In a typical synthesis, Pt(acac)2, PbI2, CTAB, glucose, OAm and OA were added into a vial, and then ultrasonicated for 30 min. The resulting homogeneous solution was heated at 180℃ for 5 h in an oil bath with stirring, before it was cooled to room temperature (see the experimental part for details).

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EXPERIMENTAL SECTION Chemicals. Platinum(II) acetylacetonate (Pt(acac)2, 97%), lead(II) iodide (PbI2, 99.999%), lead(II) bromide (PbI2, 99.999%), glucose (>99.5%) and cetyl trimethylammonium bromide (CTAB, >99%) were purchased from Sigma Aldrich. oleylamine (OAm, >90%) and oleic acid (OA, 98%) were purchased from Energy Chemical. Commercial Pt/C catalysts (20 wt. % loading of Pt on carbon black) and Nafion (5%) were purchased from Alfa Aesar. All the chemicals in the experiment were used without further purification. Synthesis of PtPb CNCs. In a typical synthesis of the PtPb CNCs, 12 mg of Pt(acac)2, 12 mg of PbI2, 12 mg of glucose and 36 mg CTAB were dissolved in 4 mL of OAm and 2 mL of OA. Then the mixture was ultrasonicated for around 30 min to form a homogeneous solution. The solution was heated at 180℃ for 5 h in an oil bath with stirring before it was cooled to room temperature. The products were collected by centrifugation and washed several times with an ethanol-hexane mixture. Characterization. The structure and composition of the nanocrystals were investigated using a Hitachi H-8100 EM 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). X-ray diffraction (XRD) patterns were collected on D8 ADVANCE (Bruker AXS, Germany) diffractometer using Cu Kα radiation. Xray photoelectron spectroscopy (XPS) measurements were conducted on an ESCALAB-MKII spectrometer (VG Co., United Kingdom) with Al Kα X-ray radiation as the Xray source for excitation. The element mole ratio of nanocrystals was measured by the inductively coupled plasma mass spectrometry (ICP-MS), which was obtained by a Thermo Scientific iCAP6300 (Thermo Fisher Scientific, US). For postdurability characterization of the catalysts, the products were collected by ultrasonication of the modified glassy-carbon electrode in ethanol, and then centrifugation and washed three times with ethanol. Electrochemical Measurement. A three-electrode cell 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). The catalysts were dried and redispersed in a mixture of deionized water, isopropanol and Nafion (V/V/V = 4/1/0.05). Then 3 µL of the mixture (the concentration of the catalysts was 1.0 mg mL-1) was cast on the working electrode and dried under ambient condition. The electrochemical active surface areas (ECSA) measurements were determined by integrating the hydrogen adsorption charge on the cyclic voltammetry (CV) at room temperature in 0.1 M HClO4 solution. 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 were performed at room temperature in 0.1 M HClO4 + 0.5 M CH3OH solution by applying cyclic potential sweeps between -0.2 V and 1.0 V versus Ag/AgCl (saturated KCl) electrode at a sweep rate of 50 mV/s 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/cm2 for the adsorption of a monolayer of hydrogen on a Pt surface (qH). The Hupd adsorption charge (QH) can be determined using QH = 0.5 × Q, where Q is the

charge in the Hupd adsorption/desorption area obtained after double-layer correction.37-38

RESULTS AND DISCUSSION

Figure 1. (a), (c) and (d) HAADF-STEM images and (b) TEM image of the as-prepared PtPb CNCs. e), g) HRTEM images and (f) corresponding FFT pattern of an individual PtPb CNC. (h) Atomic model of PtPb (520) plane with high density of stepped surface atoms.

The morphology of the as-prepared PtPb CNCs were first characterized by transmission electron microscopy (TEM). Typical TEM (Figure 1b and S1) and high-angle annular darkfield scanning TEM (HAADF-STEM) images (Figure 1a,1c and 1d) show that the product consisted of uniform cubic shape nanocrystals with highly concave structure and the synthetic yield approaching 100%. The concave nanocubes are highly monodisperse with an average size of 14 nm (Figure S2a). Figure 1e and 1g show the high-resolution TEM (HRTEM) images of an individual PtPb CNC. The coherent lattice fringes indicate the nanoparticles have good crystallinity and the majority of displayed lattice fringes with an interplanar spacing of 0.20 nm, corresponding to the (200) plane of the fast Fourier transform (FFT) pattern, which indicate that the PtPb CNCs are single crystalline.31 The corresponding FFT pattern revealed that the HRTEM image of an individual fourarmed nanocrystal was projected along the [001] zone axis.

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Chemistry of Materials Meanwhile, the angles between the facets of the projected concave nanocube and the {100} facets of an ideal cube were determined to be 21.8°, 21.4°, 22.1° and 21.8°, which are in good agreement with theoretical value of angle α=21.8° of {520} facets.23,32 Furthermore, as shown in Figure 1g, the (310) and (210) planes made of subfacets of (100) and (010) planes can be observed in the (520) surface oriented in the [001] direction, consistent well with the atomic model (Figure 1h and S2). Thus, the PtPb CNCs are mainly bounded by {520} high-index facets.

Figure 2. (a) HADDF-STEM image corresponding elemental mapping images of PtPb CNCs. (b) XRD pattern of the PtPb nanocrystals collected at different reaction times. (c) XPS patterns for the Pb 4f region of PtPb CNCs and (d) Pt 4f region of different catalyst. The element distribution of the obtained PtPb CNCs was analyzed by HAADF-STEM energy dispersive X-ray spectroscopy (EDX). As seen in elemental mapping images (Figure 2a), both Pt and Pb elements were uniformly distributed throughout these nanoparticles, confirming the homogeneous alloyed nanostructure. The overall Pt/Pb atomic ratio was 3.2/1.0, as measured by inductively coupled plasma mass spectrometry (ICP-MS), which was consistent with the TEMEDX result (Figure S3a). The X-ray diffraction (XRD) (Figure 2b) and selected area electron diffraction (SAED) (Figure S3b) were conducted to determine the phase of the PtPb CNCs, the positions of diffraction peaks were corresponding to those of the hexagonal close packed (hcp) PbPt (JCPDS no. 06-0374) and face-centered cubic (fcc) PbPt (JCPDS no. 06-0574), indicating the presence of both PbPt phase and PbPtx phase in the PtPb CNCs.31,33 The X-ray photoelectron spectroscopy (XPS) patterns showed that both Pt and Pb were mainly in the zerovalent state on the surface of the nanocrystals. Remarkably, divalent Pb was gradually reduced with the increase of reaction time (Figure 2c and S5), while the Pt 4f binding energy in PtPb nanocrystals emerged a negative shift from 70.81 eV to 70.36 eV compared to the Pt/C (Figure 2d). Which indicate that the obvious electronic structure change and charge transfer from Pb to Pt for the higher electronegativity of Pt compared to Pb.30-31

To elucidate the formation mechanism of the PtPb CNCs, the intermediate nanocrystals collected at different reaction times were investigated by TEM and XRD. As shown in Figure 3a and 3b, spherical nanoparticles with a size of 6 nm formed in the initial reaction stage. The corresponding XRD pattern showed that the phase of the nanoparticles was mainly fcc PtPb, indicated that the Pt was reduced and nucleated at first, consistent with the FFT pattern in Figure 3b. When the reaction time prolonged to 2 h, the nanoparticles grew into nanocubes with a edge length of 10 nm (Figure 3c, 3d and S4) and possessed of fcc and hcp structure. The formation of cubic structure could be attributed to selectivity of crystal growth direction with the effects of shape-directing agents (CTAB, OAm and I-). After the reaction been proceeded for 3 h, the nanocubes were excavated into concave nanocubes, and the average size increased to 14 nm. Thus, the growth process indicates that the concave nanocubes formed by directional growth over the vertexes of the cubes.27,34 In addition, a set of control experiments were conducted by altering the reaction parameters to investigate their effects on the growth of the PtPb CNCs. As shown in Figure S6, the size distribution of the PtPb nanocrystals closely depended on the proportions of the OAm and OA, only proper ratio of the two solvents can obtain uniform PtPb CNCs. And the size of the nanoparticles gradually increased and became nonuniform with the decrease of the OAm/OA ratio. Meanwhile, the quantity of reducing agent would affect the reduction reaction rate of the precursors. Excess glucose led to form small particles, while deficient glucose caused cube-like nanocrystals (Figure S7). Furthermore, the surfactant and reaction temperature directly affect the yield of the PtPb CNCs, moderate CTAB (Figure S8) and 180℃ (Figure S9) contribute to form homogeneous nanocrystals. Combined Figure S10 with Figure S11, we can see that the concentration of PbI2 is a crucial parameter for the synthesis of cubic and concave structure. As shown in Figure S11, replaced PbI2 with PbBr2 in the synthesis just resulted in the form of nanocubes rather than concave nanocubes, which indicated that I- could promote the nanoparticles directional growth into concave structure.

Figure 3. (a-b), (c-d) and (e-f) TEM images and HRTEM images of the PtPb nanocrystals collected at 1 h, 2h and 3h of the

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reaction times, respectively. (g) Schematic illustration of the growth mechanism of PtPb CNCs. Corresponding FFT patterns inset in (b) and (d). The electrocatalytic properties toward MOR were investigated to evaluate the performance of the PtPb CNCs. The intermediate PtPb NCs and commercial Pt/C catalysts were selected as references. Figure S12a shows the cyclic voltammetry (CV) curves of the PtPb CNCs, PtPb NCs and commercial Pt/C catalysts in N2-purged 0.1 M HClO4 solution at a sweep rate of 50 mV s-1. And the electrochemically active surface areas (ECSA) were calculated to be 46.6 m2 g-1 for PtPb CNCs, 34.6 m2 g-1 for PtPb NCs and 52.5 m2 g-1 for commercial Pt/C catalysts. The PtPb CNCs showed greater ECSA than PtPb NCs because of concave nanostructure. Figure 4a shows MOR catalyzed by these catalysts, which were measured in 0.1 M HClO4 aqueous solution with 0.5 M CH3OH at a sweep rate of 50 mV s-1. The MOR current density on PtPb CNCs was much higher than that on commercial Pt/C catalysts, and PtPb CNCs exhibited greater If/Ib ratio (where If is the forward current density and Ib is the backward current density), which implied that the methanol would be more effectively oxidized on PtPb CNCs during the forward potential scan and generating less poisoning species, thereby exhibited higher tolerance toward CO-poisoning compared to commercial Pt/C catalysts.26,31,35 Furthermore, the negative shift of the onset potential toward MOR (Figure S12b) on PtPb CNCs indicated that the methanol electrooxidation would be inclined to start at a lower potential than PtPb NCs and commercial Pt/C catalysts. As shown in Figure 4b,

Figure 4. Electrocatalytic performance of the PtPb CNCs, PtPb NCs and commercial Pt/C catalysts. a) CV and b) histogram of mass and specific activities of different catalysts for MOR in 0.1 M HClO4 + 0.5 M CH3OH solution. c) The changes on mass activities of these catalysts during 1000 potential cycles. d) Current-time curves recorded at 0.7 V, the inset is the magnified image of the region indicated by the square.

the mass and specific activities were calculated by normalizing the MOR current densities with the loading amount of Pt and the ECSA, respectively. The PtPb CNCs exhibited the highest

mass activity of 0.97 A mgPt-1 at 0.68 V, which was 2.16 and 4.62 times greater than that of the PtPb NCs (0.45 A mgPt-1) and commercial Pt/C catalysts (0.21 A mgPt-1), respectively. Meanwhile, the PtPb CNCs exhibited the specific activity of 2.09 mA cm-2, which was 5.16 times higher than that of the Pt/C catalysts (0.40 mA cm-2), and been greatly promoted compared to various reported electrocatalysts (Table S1). Figure S13 shows the CV curves of MOR on these catalysts at different scan rates, ranging from 10 mV s-1 to 100 mV s-1. Obviously, the current density increased as well as the peak current potential shifted positively with the increase of scan rate. Moreover, the linear relationship between the square root of the scan rate (v1/2) and the forward peak current density ( jm) was found on these catalysts, which indicated that MOR follows the diffusion-controlled process and the higher slope value of PtPb CNCs relative to that of the commercial Pt/C catalysts implied the improvement of methanol electrooxidation kinetics on PtPb CNCs.35 The electrochemical durability of the PtPb CNCs was evaluated by long-term stability test in 0.1 M HClO4+ 0.5 M CH3OH aqueous solution. As shown in Figure S14 and Figure 4c, the current densities on these catalysts were gradually decreased with the CV cycles increase and the current density on commercial Pt/C catalysts obviously reduced faster. Even after 1000 potential cycles, the PtPb CNCs reserved 68.7% of the initial mass activity, much better than PtPb NCs (52.6%) and commercial Pt/C catalysts (39.5%). This result was consistent with the current-time curves in Figure 4d. Throughout the 6000 s test, the current density remaining on PtPb CNCs was still higher than that of the PtPb NCs and commercial Pt/C catalysts, which further confirmed the high durability of the PtPb CNCs. The structure and composition of the catalysts after the durability tests were characterized by TEM and XPS. Figure S15 shows that there was negligible change of the morphology and composition in PtPb CNCs after long-term cycles. However, under the same conditions, the commercial Pt/C catalysts obvious aggregation occured and uniform small Pt nanoparticles were aggregated into large irregular particles after the durability test (Figure S16). Hence these structure change of the catalysts could also illuminate the decrease of electrocatalytic activities. The excellent electrocatalytic activity and stability of the PtPb CNCs on MOR should be attributed to the combination of unique structure and alloy composition. As illuminated by the equations of methanol molecules oxidation (Figure S17), it is widely accepted that the electrooxidation of methanol on Pt surfaces is via a dual-path mechanism, reactive intermediates (direct pathway) and poisoning intermediates (indirect pathway). On the one hand, the C-H and O-H cleavage in initial steps of methanol decomposition is more favorable at high-index facets Pt step sites. On the other hand, the poisoning intermediates are determined mainly as adsorbed CO (COad) species, which are derived from the dissociative adsorption of methanol and can be scarcely stripped out until electrode potential is above 0.6 V (RHE), where oxygen-containing species are generated on Pt surfaces. High-index {520} planes can catalyze both the direct and the indirect pathways, and the catalytic activity of the investigated Pt planes is in the order of Pt(111) < Pt(100) < Pt(110) < Pt(520). Therefore, high-index planes exhibit much higher activity than that of the basal planes.29,36,39 Moreover, the change of Pt electronic structure through charge transfer from Pb will weaken the adsorption energy (Eads) of adsorbed CO intermediate on Pt atoms, and alloyed Pb facilitates the ad-

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Chemistry of Materials sorption of oxygen-like species to oxidize the adsorbed CO.30,35 Thus the CO-poisoning tolerance of PtPb CNCs been greatly promoted. Meanwhile, the concave structure provides higher ECSA as well as abundant low-coordinated atoms on steps, and decreases the contact area of each nanoparticle, which can act as highly catalytic sites and effectively prevent PtPb CNCs from aggregation. CONCLUSIONS In summary, uniform PtPb CNCs enclosed with high density {520} high-index facets were successfully synthesized via a one-step wet-chemical approach. The formation mechanism and the effects of small molecules on concave structure were investigated by time-dependent studies and control experiments. Because of the synergistic effects and the exposed {520} facets, the as-prepared PtPb CNCs exhibited high stability and electrocatalytic mass activity toward MOR, which was 2.16 and 4.62 times higher than PtPb NCs and commercial Pt/C catalysts, respectively. We believe that the present work will open up possibilities for the practical application of the catalysts and provide new insights into rational design of efficient catalysts. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Additional TEM images, XPS, 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 2016YFA0203201). REFERENCES (1) Chen, A.; Holt-Hindle, P., Platinum-based nanostructured materials: synthesis, properties, and applications. Chem. Rev. 2010, 110, 3767-3804. (2) Wu, B.; Zheng, N., Surface and interface control of noble metal nanocrystals for catalytic and electrocatalytic applications. Nano Today 2013, 8, 168-197. (3) Wang, D.; Li, Y., Bimetallic nanocrystals: liquid-phase synthesis and catalytic applications. Adv. Mater. 2011, 23, 1044-1060. (4) Wang, L.; Nemoto, Y.; Yamauchi, Y., Direct Synthesis of Spatially-Controlled Pt-on-Pd Bimetallic Nanodendrites with Superior Electrocatalytic Activity. J. Am. Chem. Soc. 2011, 133, 9674-9677. (5) Malgras, V.; Ataee-Esfahani, H.; Wang, H.; Jiang, B.; Li, C.; Wu, K. C.; Kim, J. H.; Yamauchi, Y., Nanoarchitectures for Mesoporous Metals. Adv. Mater. 2016, 28, 993-1010. (6) Wang, L.; Yamauchi, Y., Metallic nanocages: synthesis of bimetallic Pt-Pd hollow nanoparticles with dendritic shells

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