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Facile Synthesis of Sea-Urchin-Like Pt and Pt/Au Nanodendrites and Their Enhanced Electrocatalytic Properties Ruigang Xie,† Shuanglong Lu,‡ Yaoyao Deng,‡ Sujuan Mei,‡ Xueqin Cao,‡ Lingli Zhou,§ Cuiling Lan,*,† and Hongwei Gu*,‡

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/13/19. For personal use only.

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Guangxi Colleges and Universities Key Laboratory of Regional Ecological Environment Analysis and Pollution Control of West Guangxi & College of Chemistry and Environment Engineering, Baise University, Baise 533000, China ‡ College of Chemistry, Chemical Engineering and Materials Science & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, China § Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, Guilin University of Technology, Guilin 541004, China S Supporting Information *

can be synthesized using templates or nanoreactors.19−25 Moreover, many of the reported dendritic structures comprise small nanoparticle aggregates. Therefore, facile and efficient chemical methods for synthesizing 3D nanodendrites are highly desired. In this Communication, we describe a facile method to synthesize sea-urchin-like Pt nanodendrites formed by thin single-crystalline nanowires with well-ordered perpendicular channels. Compared with the work reported by Lacroix et al., our work used o-phthalic acid as the capping agent, leading to higher branched density and thinner nanowires, which is favored for catalytic applications. The formation mechanism was investigated, and the superior electrocatalytic performance of the Pt nanodendrites for methanol oxidation reactions (MORs) was demonstrated. Additionally, these Pt nanodendrites can be used as templates for the formation of heterogeneous Pt/Au nanodendrites, further improving their catalytic reactivity and durability. As shown in Scheme 1, Pt nanodendrites were prepared according to a one-pot reduction procedure: reducing Pt(acac)2

ABSTRACT: This Communication demonstrates a novel and facial approach to achieving monodispersed seaurchin-like Pt nanodendrites under a 1 bar hydrogen environment at 165 °C. These Pt nanodendrites can be further used as seeds for the formation of Pt/Au nanodendrites. Both Pt and Pt/Au nanodendrites exhibit the desired eletrocatalytic activities for the methanol oxidation reaction.

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oble metals, in particular Pt, are excellent catalysts in many important fields, such as in fuel cells and fine chemical synthesis, because of their unique ability to facilitate both oxidation and reduction reactions.1−4 Given the high demand and price for Pt, it is necessary to reduce the amount of Pt used in specific applications by increasing its catalytic efficiency and selectivity.5,6 To this end, morphological control of nanoparticles has been extensively pursued because it is generally accepted that the size and shape of Pt nanomaterials determine their surface area and exposed crystalline facets, and consequently their catalytic reactivity and selectivity.7,8 Earlier studies focused mainly on zero-dimensional Pt nanocrystals, one-dimensional Pt nanowires and nanotubes, which possess high surface energy and easily changeable morphology as a result of Ostwald ripening and grain growth under the harsh conditions during the fuel cell operation.9 Moreover, corrosion of the carbon support and subsequent catalyst detachment, dissolution, and aggregation can further deteriorate the catalyst durability.10 More recently, various strategies for the solution-phase synthesis of three-dimensional (3D) metal nanodendrites have received considerable attention.11−14 The dendritic structures possess high surface area with open pores and rich edge/corner atoms, making them highly favorable for low-cost catalytic applications. The formation of dendritic nanostructures is usually kinetically controlled, which is typically achieved by means of changing the surface binding ligands,11,12,15,16 pH,13 and temperature17 or by adding oxidizing reagents to limit the monomer concentration.18 This typically requires a long reaction time, from many hours to days. Alternatively, they © XXXX American Chemical Society

Scheme 1. Pathway of Pt Nanodendrite Formation

in oleylamine, with the addition of o-phthalic acid, under a H2 atmosphere. Transmission electron microscopy (TEM) images show that the as-synthesized Pt nanostructures are uniform 3D assemblies (≈120 nm in size) of thin Pt nanowires (Figure 1A), which appear like orthogonally placed nanowire meshes. The diameter of each nanowire is about 3.6 nm (Figure 1B). The selected-area electron diffraction (SAED) pattern (Figure 1A, inset) further demonstrates the single-crystalline nature of these Received: November 29, 2018

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DOI: 10.1021/acs.inorgchem.8b03321 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

the reaction rates were varied by changing the Pt precursor concentrations in the reaction mixture, it was observed that welldefined Pt nanodendrites could only be obtained in a narrow concentration range, with all other experimental conditions held constant. When the amount of Pt(acac)2 was changed from 50 mg to either 25 or 75 mg, multiarmed structures were observed (Figure 3A,B).

Figure 1. (A and B) TEM images of the Pt nanodendrites and the corresponding SAED pattern. (C) TEM images of a single Pt nanodendrite at different tilt angles. The scale bar represents 20 nm.

Pt nanodendrites, indicating that the dendritic 3D nanostructure formations were based on grain growth from the initial nuclei rather than aggregation of the smaller nanoparticles. The X-ray diffraction (XRD) patterns (Figure S1) match well with the database values of face-centered-cubic (fcc) Pt. In addition, the high-resolution TEM (HRTEM) image of Pt nanodendrites is shown in Figure S4, and the distance of a set of lattice fringes is 0.22 nm, which matched with that of Pt(111). For another set of lattice fringes, the interfringe distance is measured to be 0.196 nm, which matched with the (200) planes. The 3D morphologies are further characterized by measuring the TEM images of the single Pt nanostructure at different tilt angles (Figure 1C). To better understand the growth mechanism of these Pt nanodendrites, TEM images were taken of sample aliquots withdrawn at different reaction stages (Figure 2). Branched Pt

Figure 3. TEM images of Pt nanostructures with (A) 25 mg of Pt(acac)2 and (B) 75 mg of Pt(acac)2.

In the synthetic method presented here, H2 and o-phthalic acid were critical for anisotropic Pt nanostructure growth. When the experiment was conducted under a N2 instead of a H2 atmosphere, under otherwise identical reaction conditions, no anisotropic Pt nanostructures were obtained (Figure S2). We conducted reactions with different amounts of o-phthalic acid, and TEM images revealed that the dendritic morphologies of the nanostructures were kept, while the densities were changed (Figure 4). The benzene moiety, which adsorbed on the Pt(111) surface, is important for Pt nanodendrite formation (Figure S3).26

Figure 4. TEM images of Pt nanodendrites formed with different amounts of o-phthalic acid: (A) 0.25 mmol; (B) 0.5 mmol; (C) 1.0 mmol; (D) 1.5 mmol.

Furthermore, when o-phthalic acid was replaced by benzoic acid or m-phthalic acid (Figure S5), very similar branched nanodendritic structures with different porosities were achieved, which was affected by the size of the capping agent. When ophthalic acid was replaced by 1,2-cyclohexanedicarboxylic acid or hexahydrophthalic acid, only randomly assembled straight Pt nanowires with small branches at the tips were observed (Figure S6), which may be due to a possible electronic interaction between the capping group and Pt surface. The Pt nanodendrites can be further used as building blocks for the formation of Pt/Au nanodendrites.27 As illustrated in Figure 5A, a Au precursor reduced by oleylamine preferentially nucleated on the tip of the Pt branches to form well-defined Pt/ Au hybrid nanostructures. The average size of the particle was around 130 nm, which was about 10 nm larger than the asprepared Pt nanodendrites. The Pt/Au nanodendrites were further characterized by HRTEM (Figures 5B and S7). The high-angle annular dark-field scanning transmission electron

Figure 2. TEM images of the Pt nanodendrites obtained after different reaction times: (A) 1 min; (B) 3 min; (C) 5 min; (D) 10 min; (E) 20 min; (F) 30 min; (G) 60 min; (H) 120 min.

nanocrystals were observed as early as 1 min (Figure 2A,B); these Pt multipods continued to grow in length and branch out in different directions. After 5 min, the Pt nanodendrites took their initial form, with a cubic outer contour (Figure 2C). At longer reaction time, the density of Pt nanowires increased in each Pt nanodendrite (Figure 2D). No significant changes in the morphology were observed after 20 min (Figure 2E−H). As a highly symmetric fcc crystal, the anisotropic growth of Pt nanostructures is usually kinetically controlled and typically requires the use of templates or shape-directing ligands. When B

DOI: 10.1021/acs.inorgchem.8b03321 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 5. (A) TEM and (B) HRTEM images of the Pt/Au nanodendrites. (C) HAADF-TEM image and elemental mapping of Pt and Au elements with overlaid images. (D) XRD pattern of the Pt/ Au nanodendrites showing the existence of Pt and Au rather than a Pt/ Au bimetallic alloy. (E) TEM images of a single Pt/Au nanodendrite at different tilt angles. The scale bar represents 20 nm.

Figure 6. (A) CV curves of the Pt/Au and Pt nanodendrites and Pt nanoparticles measured in N2-saturated 0.5 M H2SO4. (B) Linearsweep voltammetry, (C) CV, and (D) chronoamperometric curves of the MOR catalyzed by the Pt/Au and Pt nanodendrites and Pt nanoparticles in N2-saturated 0.5 M H2SO4 and 0.5 M CH3OH aqueous solution. The chronoamperometric curves were recorded at 0.5 V. The scanning rate in all cases was 50 mV s−1.

microscopy (HAADF-STEM) and elemental mapping images for the Pt/Au nanodendrites (Figure 5C) indicate that Au was distributed on the outside of the Pt nanodendrites, which was further demonstrated by the line profiles of the Pt/Au nanodendrites (Figure S8B). The Pt/Au atomic ratio determined by energy-dispersive X-ray analysis was 66:34 (Figure S8A), which is in good agreement with the result from inductively coupled plasma atomic emission spectrometry analysis (Pt:Au = 63:37). The XRD pattern matches well with the database values of both Pt and Au (Figure 5D), and tiltingangle TEM measurements reveal the 3D structures of the Pt/Au nanodendrites (Figure 5E). The electrocatalytic performance of the Pt and Pt/Au nanodendrites toward MOR was investigated using electrochemical cyclic voltammetry (CV) and chronoamperometric curves. For comparison, Pt nanoparticles were prepared following the method reported by the Sun group.28 The CV curves of the Pt nanoparticles, Pt nanodendrites, and Pt/Au nanodendrites were measured in 0.5 M H2SO4 from −0.25 to 1.0 V at a sweep rate of 50 mV s−1 (Figure 6A). According to the CV curves, the electrochemical active surface areas (ECSAs) of the Pt nanoparticles, Pt nanodendrites, and Pt/Au nanodendrites were calculated to be around 18.03, 63.79, and 46.41 m2 g−1 respectively. As a reference, the ECSA of the Pt nanodendrites is also higher than those of the existing Pt catalysts, such as porous Pt nanoballs (23 m2 g−1),29 porous Pt nanoassemblies (40.8 m2 g−1),30 and Pt dendritic nanoparticles (56 m2 g−1).31 The high ECSA of the Pt nanodendrites is ascribed to their thin singlecrystalline nanowires with well-ordered perpendicular channels, which can provide enough active sites. All of the obtained currents were normalized to the mass of loaded Pt. As seen in Figure 6B, Pt/Au nanodendrites showed the highest current density (185 mA mg−1). This value is 1.23 times higher than that of the Pt nanodendrites (150 mA mg−1) and also around 5.3 times higher than that of the Pt nanoparticles (35 mA mg−1). The current density ratio of the anodic peak in the reverse (Ib) and forward (If) directions in the CV for methanol electrooxidation has been widely used to characterize the resistance of catalysts to intermediate carbonaceous species, especially CO.32−34 Pt and Pt/Au nanodendrites have greatly improved

electrocatalytic activity compared with the Pt nanoparticles (Figure 6C). The forward peak is due to the oxidation of methanol, and the reverse peak is due to the CO or CO-like intermediates, which are created by the incomplete oxidation of methanol and make the active site of Pt poison. As illustrated in Figure 6C, the If/Ib value of the Pt/Au nanodendrites (1.18) is higher than that of the Pt nanodendrites (0.74) and Pt nanoparticles (1.09). As a comparison, the If/Ib value of the Pt/Au nanodendrites (1.18) is greater than those of dendritic Pt nanoparticles (0.96) and mesoporous Pd@Pt nanoparticles (1.08).35 The above results imply that the Pt/Au nanodendrites have better tolerance toward CO poisoning. Furthermore, the long-term stability and tolerance toward CO intermediates were evaluated from chemical analysis measurements performed in 0.5 M H2SO4 and 0.5 M CH3OH at 0.5 V potential. The chemical analysis results (Figure 6D) showed that the nanodendrites maintained a higher methanol oxidation current density (normalized to the mass of Pt) than the Pt nanoparticles over the entire time range studied, demonstrating their excellent stability and tolerance toward CO poisoning in the MOR. Besides, the nanodendrites can retain their original structure after electrochemical tests, which also indicate their good stability (Figure S9). In summary, we have successfully prepared 3D Pt and Pt/Au nanodendrites, which were structured by thin single-crystalline nanowires with well-ordered perpendicular channels, via a facile wet-chemical route under a H2 atmosphere. The use of ophthalic acid was demonstrated to be most essential for the formation of well-defined nanodendritic structures. The Pt and Pt/Au nanodendrites exhibit higher electrocatalytic activity than Pt nanoparticles and were highly beneficial as anodic electrocatalysts for MOR. C

DOI: 10.1021/acs.inorgchem.8b03321 Inorg. Chem. XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03321.

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Synthetic procedure and additional characterization data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hongwei Gu: 0000-0001-9962-4662 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work received financial support from the Science and Technology Program of Suzhou (Grant SYG201732), the Project of Scientific and Technologic Infrastructure of Suzhou (Grant SZS201708), the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Improvement Project of Basic Ability for Young and Middle-aged Teachers in Guangxi Colleges and Universities (Grant KY2016YB418), the Guangxi Colleges and Universities Key Subject of Material Physics and Chemistry (Grant KS16YB01), and a start-up fund from Baise University.

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DOI: 10.1021/acs.inorgchem.8b03321 Inorg. Chem. XXXX, XXX, XXX−XXX