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Ultrathin Pt-Based Alloy Nanowire Networks: Synthesized by CTAB Assistant Two-Phase Water-Chloroform Micelles Shengchun Yang,†,‡ Feng Hong,† Liqun Wang,† Shengwu Guo,‡ Xiaoping Song,† Bingjun Ding,‡ and Zhimao Yang*,† MOE Key Laboratory for Non-equilibrium Synthesis and Modulation of Condensed Matter, School of Science, Xi’an Jiaotong UniVersity, and State Key Laboratory for Mechanical BehaVior of Materials, Xi’an Jiaotong UniVersity, Shaan Xi 710049, People’s Republic of China ReceiVed: September 11, 2009; ReVised Manuscript ReceiVed: October 21, 2009
Ultrathin Pt-based PtM (M ) Pd, Ru, Au, Fe) alloy nanowire networks could be facilely synthesized using a soft template formed by cetyltrimethylammonium bromide in a two-phase water-chloroform system. The as-synthesized Pt-based alloyed nanowires show the fcc crystal phase. They have an average diameter of ∼2.3 nm and form porous nanonetworks, which can be potentially used for a number of catalytic applications. It was found that PtRu alloy nanonetworks showed more efficient catalytic activity for the hydrogenation of azo bonds in methyl orange than that of PtPd, PtAu, and pure Pt. 1. Introduction The fantastic shape dependent physical and chemical properties of nanostructures have attracted a great deal of attention and interest from worldwide scientists for their potential applications in the field of catalysts,1-7 magnetic storage,8-10 biosensors,11 disease detection,12,13 and optics.14-16 It has been shown that catalytic reactions on a platinum surface can be much enhanced by incorporating other metal atoms to form an alloy with the proper composition and crystal phase.17,18 Pt3Ni was an example for such cases, which exhibited a much higher activity than platinum in oxygen reduction reaction (ORR).19 Our recent work shows that, by controlling the atomic ratios or choosing the proper crystal phase of platinum-lead alloys, multifold increases in electrocatalytic performance can be achieved.20 PtPd alloy was verified to be much more efficient in the hydrogen oxidation reaction (HOR) than that of pure Pt.21 Recent studies indicated that the catalytic properties of platinum could be potentially improved by combining both the advantages of shape effect and alloy. For example, platinumbased alloy nanoparticles with designed morphologies have been the choice of catalysts in the reactions of electrochemistry.19,20,22,23 In such cases, PtPd alloy nanotubes were suggested to potentially possess high surface area, high utilization, high activity, and high durability compared with the Pt/C, Pt nanotubes, and Pt black.5 PtRu alloy nanodendrites with defined 3D shapes and composition were found to be catalytically more active in the direct methanol oxidation reaction (MOR) than commercially used Pt/C catalyst.7 Therefore, synthesizing platinum-based alloy nanomaterials with special morphologies has been the focused subject. Up to date, the particle-templating approach to prepare alloy nanostructures with well-controlled morphologies and compositions via galvanic replacement reaction by selecting the sacrificial particles with defined shapes as a template has been developed to be one of the most important strategies for the preparation of alloy nanostructures since it was reported by Xia * To whom correspondence should be addressed. E-mail: zmyang@ mail.xjtu.edu.cn. Fax: +86-29-82665995. † MOE Key Laboratory for Non-equilibrium Synthesis and Modulation of Condensed Matter. ‡ State Key Laboratory for Mechanical Behavior of Materials.
et al.24 Besides the galvanic method, coreducing or decomposing the different metal precursors in organic solution in the presence of capping agents, such as oleic acid and oleylamine, can be used to synthesize the Pt-based alloy nanostructures, including FePt cubes25 and ultrathin rods/wires,8,9 PtPb nanorods,20,26 and CuPt nanorods.27 However, to investigate a facile and common method to synthesize the Pt-based nanowires with ultrathin diameters was still a remaining difficulty because of the synthetic challenges combined with inducing anisotropic growth as well as controlling multielement composition. In the current case, we report a facile and common method for the preparation of PtM (M ) Pd, Ru, Au, Fe) alloy nanowire networks with diameters of ∼2.3 nm by using a soft template, in which both the composition and morphology can be well controlled. This method can be readily extended to synthesis of other alloy nanomaterials with 1D structure. 2. Experimental Section Synthesis. All chemicals were used as received without further purification. Deionized water was used throughout. Pt-M alloy nanowire networks were synthesized with a soft surfactant template formed by cetyltrimethylammonium bromide (CTAB) in a two-phase water-chloroform system as reported by Song et al., in which the formation of swollen wormlike micelle networks within the chloroform droplets with the CTAB molecules was considered as the perfect microplace for the reduction reaction of platonic ions and template for metal growth.28 In a typical experiment for the synthesis of PtRu nanowire networks, a mixture of H2PtCl6 (2.5 mL, 20 mM) and RuCl3 (2.5 mL, 20 mM) aqueous solution was mixed with 5 mL of chloroform containing 40 mM cetyltrimethylammonium bromide (CTAB) and then 40 mL of pure water was added. After stirring for 30 min, 0.2 g of sodium borohydride dissolved in 5 mL of pure water was added into the flask while stirring at a speed of 1000 rpm for 20 min. The mixture immediately turned into a gray dark color, indicating the formation of nanocrystals. The as-synthesized nanowires could be separated out by precipitation. In a typical procedure, 2 mL of the mixture was centrifuged at 6000 rpm for 2 min. After centrifuging, the
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Figure 1. TEM (a) and HRTEM (b) images of PtRu nanonetworks synthesized with a Pt and Ru atomic ratio of 1:1; (c) EDX spectra for PtRu nanonetworks. The Cu signal came from the copper grid on which the sample is supported.
supernatant was decanted and the precipitate was dispersed in 2 mL of ethanol followed by centrifuging for 2 min. Characterization. The X-ray diffraction (XRD) patterns were recorded on a Shimadzu X-ray diffractometer (XRD-7000) with a Cu KR X-ray source (λ ) 1.5405 Å). TEM specimens were prepared by drop-casting nanowire dispersions onto carbon coated TEM grids. A JEOL JEM-3010 instrument was used for recording the TEM and HRTEM images. The typical accelerating voltage of the electron beam was set at 300 kV. EDX analysis was obtained with an Oxford INCA detector installed on the JEOL JEM-3010 instrument. The magnetic properties wererecordedbyaLakeShore-7307vibratingsamplemagnetometer. Catalytic Properties Study. The resultant products were dispersed in ethanol at a concentration of 1 mg of nanowires/ mL of solvent. A 20 µL portion of alloy nanowires suspension was injected into a solution of methanol orange (50 mL, 1 mM) bobbled with hydrogen under severe stirring. The reaction was stopped at various time periods by taking out about 1.5 mL of the sample and centrifuging immediately to remove the catalyst. The solution was collected and diluted to a certain concentration for the recording of UV-vis spectra on the UV-vis spectrophotometer with quartz cuvette. 3. Results and Discussion Figure 1 shows the transmission electron microscopy (TEM) images of the nanowire networks of PtRu. The lower magnification image (Figure 1a) indicates that the nanowires have an average diameter of 2.3 nm quite similar to the pure platinum nanowires reported by Song and co-workers.28 The lattice spacing of the alloy nanowires is measured as 2.20 Å, slightly less than that from the (111) diffraction of the fcc phase of platinum, 2.26 Å (Figure 1b). Misoriented grain can be remarkably observed between two nanocrystals, as highlighted by an arrow in the inset of Figure 1b, indicating formation of polycrystalline interconnected nanowires. The energy dispersive
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Figure 2. TEM (a) and HRTEM (b) images of PtAu nanonetworks synthesized with a Pt and Au atomic ratio of 1:1; (c) EDX spectra for PtAu nanonetworks. The Cu signal came from the copper grid on which the sample is supported.
X-ray (EDX) analysis indicates that both platinum and ruthenium could be clearly detected from the nanonetworks. The Pt/ Ru atomic ratio is close to 1:1, as summarized in the inset of Figure 1c. Bimetallic PtAu alloy nanowire networks also can be synthesized using similar chemistry. In this case, a solution of a mixture of H2PtCl6 (2.5 mL, 20 mM) and HAuCl4 (2.5 mL, 20 mM) was used as the precursor. After addition of NaBH4, the mixture of micelle solution started to turn a gray-reddish color, indicating the reduction of Au3+ and Pt4+ to zero-valence colloids. TEM analysis taken from this sample reveals the formation of PtAu nanowires with an average diameter of 2.2 nm, similar to that of PtRu nanowires (Figure 2a), which can be attributed to the fact that the size of the nanowires is strictly limited by the wormlike micelle networks. High-resolution TEM (HRTEM) images (Figure 2b) show that the atomic lattice fringes along an individual continuous wire are misoriented and have an average d-spacing of 2.3 Å which can be signed to the (111) plane of the fcc phase PtAu alloy, as verified by the XRD pattern from the sample of PtAu alloy nanowires (it will be discussed later). The atomic ratio of Pt and Au analyzed by EDX approaches 1:1, revealing that Pt4+ and Au3+ ions can be coreduced by NaBH4. Herein, the above synthetic method can be facilely extended to other systems for the preparation of platinum-based alloy nanowires with ultrathin diameter, such as PtPd and PtFe nanowires. The TEM images in Figure 3a show the netlike PtPd nanonetworks with an average diameter of 2.5 nm. The HRTEM image (Figure 3b) shows PtPd nanowires with a lattice spacing of 2.3 Å. The EDX result indicates the precursors of both Pt and Pd can be coreduced to form alloy nanowires with an atomic ratio of about 1:1. Figure 3c shows the TEM image of PtFe nanowires with an average diameter of 2.3 nm. The corresponding HRTEM image and EDX results are shown in Figure 3d. The lattice spacing of the PtFe nanowires is 2.2 Å nm with an atomic ratio of about 1:1. It is well-known that the standard
Ultrathin Pt-Based Alloy Nanowire Networks
J. Phys. Chem. C, Vol. 114, No. 1, 2010 205 calculated to be 3.9114, 3.8766, and 3.8765 Å, respectively. The decrease of the lattice parameters and display of the diffraction characteristics of the Pt (fcc) structure can be attributed to the formation of alloys, suggesting the alloy formation based on the substitution of the Pt lattice sites partly replaced by other atoms.30 It can also be found that the addition of such metals leads to the (220) diffraction peak shift to higher 2θ degree, while the addition of Au to Pt induces the movement to lower position. The calculated value of lattice parameters of PtAu is 4.004 based on the 2θ values of the (220) peak, which is higher than that of the bulk platinum but smaller than pure gold (4.079). The peak 2θ values of the PtAu diffraction patterns fall exactly between those of pure Pt and pure Au metals, indicating the formation of the alloy and the substitution of platinum atoms by Au atoms.31 To study the catalytic activity of the alloy nanowire networks, we chose the reaction of hydrogenation of the azo bond in the organic dye molecules, such as methyl orange, which has been used to test the catalytic activity of the Pt-nanorod-coated TiO2 nanofibers.32 The hydrogenation reaction can be illustrated by the following chemical reaction equation (eq 1):
Figure 3. TEM and HRTEM images of PtPd (a, b) and PtFe (c, d) nanonetworks. The atomic ratios for PtPd and PtFe are 1:1. The inset of part c shows the room-temperature magnetic hysteresis loop of the as-synthesized FePt nanowires, showing a superparamagnetic property. EDX results for PtPd and PtFe nanonetworks are summarized and listed in the insets of parts b and d, respectively.
Figure 4. XRD patterns of platinum-based nanowire networks.
reduction potential of Pt4+ in [PtCl6]2- is 0.691 V ([PtCl6]2- + 4e f Pt) and Fe2+ is -0.447 V (Fe2+ + 2e f Fe) in aqueous solutions. Therefore, it is quite difficult to synthesize the PtFe alloy nanostructures in such conditions. Herein, the excessive and stronger reducing agent NaBH4 is expected to reduce Pt and Fe ions simultaneously to atoms before the separate nucleation of previous reduced Pt atoms occurring, and ultimately lead to the formation of alloy nanowires. The inset of part c shows the magnetic hysteresis loop of the as-synthesized FePt nanowires, showing the chemically disordered phases have small magnetocrystalline anisotropy, which leads to their superparamagnetic behavior at room temperature.29 The different diffractograms of the above-mentioned Pt-based alloy nanowires are shown in Figure 4. The diffraction peaks close to 39, 46, 68, and 81° can be assigned to Pt(111), (200), (220), and (311) diffractions, respectively, which represents the typical character of a crystalline Pt fcc phase (JCPDS databaseInternational Centre for Diffraction Data, 1999, PCPDFWIN version 2.02). Obviously, no other distinct diffraction peaks in all spectra than those of the peaks as mentioned above are observed, indicating that all Pt-based nanowires have a prevailing Pt (fcc) crystal structure. The lattice parameter of platinum is estimated to be 3.9241 Å on the basis of PCPDFWIN version 2.02, while the PtRu, PtFe, and PtPd lattice parameters are
As shown in the equation, the hydrogenation results in the break of π-conjugation via azo cleavages, inducing the solution color transformation from orange to clear, which allows one to quantitatively measure and monitor the catalytic performance of the Pt-based alloy nanowire networks by taking UV-vis spectra from a methyl orange solution with different reaction times. Herein, the evolutions of UV-vis absorbance in the range 400-500 nm were recorded. The hydrogenation reaction was performed by dispersing 20 µg of as-synthesized nanowires into 50 mL of methyl orange aqueous solution with a concentration of 1 mM. Then, hydrogen was bubbled into the mixture with a vigorous stirring. The reaction was stopped at various time periods by taking out about 1.5 mL samples and centrifuging immediately to remove the catalyst. The solution is collected for the recording of UV-vis spectra on the UV-vis spectrophotometer with quartz cuvette. Herein, PtRu, PtPd, and PtAu alloys and Pt pure nanowires (see the Supporting Information for a TEM image, Figure S1) are selected as the catalysts. Figure 5 shows the UV-vis spectra taken from solutions of methyl orange with increasing hydrogenation time catalyzed by the above different kinds of catalysts. The top curves for both figures are taken from the untreated methyl orange aqueous solution (by diluting the original solution 1 mM to 50 µM), showing an absorption band from 400 to 500 nm and the λMAX locating at 462 nm. As one can see from these figures, the hydrogenation reaction leads to an obvious decrease of the absorbance, indicating the azo group in the methyl molecule is catalytically hydrogenated to form an amine group, which induces the fading of the dye gradually. The absorbance reduction at 462 nm for the dye solution catalyzed by PtRu was found to be much faster than that of others. For PtPd and PtAu nanowires, even after hydrogenation reaction for 32 min, they still keep a relatively high absorbance, indicating a certain amount of dye still remained. The variation of methyl orange concentration with the hydrogenation reaction is calculated and summarized as shown
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Figure 5. Azo bond cleavage of methyl red via hydrogenation using the alloy and Pt nanowire networks of PtRu (a), PtPd (b), PtAu (c), and Pt (d) monitored through the changes of UV-vis spectra peak intensity from 1 mM methyl red solution for 32 min. In order to match the scope of equipment in the measurements of the UV-vis spectra, the solutions are diluted to a certain concentration.
Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 50901056 and 50834003) and National High-Tech Research and Development Program of China (863 Program, No. 2009AA03Z320) Supporting Information Available: TEM image of Pt nanowire networks. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 6. Variation of methyl orange concentration with the hydrogenation reaction calculated and summarized from Figure 5.
in Figure 6. The activity of the PtRu alloy nanowires is found to be much higher than that of other alloys and Pt nanowires, which can be seen from the hydrogenation kinetics, where the solution color fades within 30 min, indicating that almost all of the methyl orange molecules were hydrogenated. For the cases of pure Pt, PtPd, and PtAu nanowires, the remaining concentration of dye was about 3.0, 28.7, and 30.5%, repectively. 4. Conclusions In summary, we demonstrated a facile synthetic method for the preparation of ultrathin Pt-based PtRu, PtPd, PtAu, and PtFe alloy nanowire networks with an average diameter of 2.3 nm through the soft template formed by CTAB in the chloroform/ water system. The changes of lattice parameter and the shifts of (220) 2θ values for the products verified the formation of alloys. The produced PtRu alloy nanowire networks proved to show much more catalytic activities for the hydrogenation of methyl orange than that of pure Pt, PtPd, and PtAu nanowires. The synthetic method described in the current work can be extended to the preparation of other kindsof alloy nanowires with ultrathin diameters, which are expected to have potential applications in catalysis and electrocatalysis, such as MOR or HOR.
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