Pt Hybrid Nanowire with Nanoporous Surface: A Catalytically

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J. Phys. Chem. C 2010, 114, 4797–4802

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Novel Te/Pt Hybrid Nanowire with Nanoporous Surface: A Catalytically Active Nanoelectrocatalyst Shaojun Guo, Shaojun Dong, and Erkang Wang* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, China ReceiVed: October 7, 2009; ReVised Manuscript ReceiVed: January 25, 2010

We have developed a facile procedure to synthesize semiconductor/metal Te/Pt hybrid nanowire (NW) with nanoporous surface through the Te/Pt contact without prior Te NWs functionalization, which represents a new type of semiconductor/metal heterostructure. Specifically, the growth of Pt nanoparticles (NPs) on the surface of Te NWs could be performed at room temperature, without the need for templates and surfactants. The resulting Te/Pt hybrid NWs were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-visible spectra, energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and electrochemical techniques. It is found that ultrahigh density small Pt NPs of about 3 nm directly adhere to Te NWs and further form a morphology with nanoporous structures, which exhibit an enlarged electroactive surface area relative to that of commercial platinum black (CPB) catalyst. Most importantly, the as-prepared Te/Pt hybrid NWs exhibit high electrocatalytic activity toward the methanol oxidation reaction (MOR), indicating that they have potential for fuel-cell applications. Introduction One-dimensional (1D) nanomaterials have been of great interest recently for a number of possible applications in electronics, optics, optoelectronics, and nanosensors as well as in catalysis.1 In particular, 1D nanostructured platinum has received more attention, primarily due to its superb performance as a high-efficiency electrocatalyst for fuel-cell technology.2 To date, various chemical protocols have been developed for preparing 1D platinum nanostructures such as nanowires (NWs), nanotubes, etc.2–4 For instance, Xia and co-workers3a–d have demonstrated the synthesis of single crystalline platinum NWs in large quantities by a polyol process. Platinum nanotubes have been facilely prepared via a galvanic replacement reaction of silver NWs with Pt precursor or by the reduction of Pt salts confined to lyotropic mixed liquid crystals (LCs) of two different-sized surfactants.2,4 Despite these, the development of a mild, template-free, surfactant-free route for the production of 1D Pt nanoparticle (NP) assembling architecture with rough surface remains a grand challenge to date. On the other hand, Pt nanostructures with size- or shapetailoring catalytic properties have shown enormous potential for many types of industrial reactions such as fuel-cell reactions, CO oxidation in a catalytic converter, nitric acid production, petroleum cracking, etc.5–11 Although several excellent examples of novel Pt nanostructure,s such as tetrahexahedral Pt, multiarmed Pt and highly faceted multioctahedral Pt nanocrystals, reveal that the maximization of high-index surfaces and abundant corner and edge sites should be the criteria for selection of an excellent nanoelectrocatalyst,6,7,12 there is still a grand challenge to decrease the size of Pt NPs with high-index facets down to a size comparable to commercial nanocatalysts * Corresponding author: fax +86-431-85689711; e-mail ekwang@ ciac.jl.cn.

at present (a proper size, i.e., 3-4 nm). Therefore, one has to control the size, making it smaller, which gives the catalyst a higher specific surface area on a mass basis.13 Furthermore, in order to further maximize the activity of Pt and minimize the use of previous Pt, it is very necessary to load small Pt NPs with high activity on the surface of supporting nanomaterials with high surface area, which not only maximizes the availability of nanosized electrocatalyst surface area for electron transfer but also provides better mass transport of reactants to the electrocatalyst. One-dimensional nanomaterials, particularly carbon nanotubes (CNTs), supporting Pt nanostructures have been shown to be promising candidates as high-efficiency nanoelectrocatalysts because of their advanced advantages such as their providing higher electrochemically active surface area (ECSA) and stability than individual NPs. Prominent examples include 3D flowerlike Pt NPs electrodeposited onto CNTs by using a three-step protocol,14a Pt NWs assembled on the surface of CNTs via a wet-chemical route,14b and Pt NPs supported on graphitic carbon nanofibers (GCNFs) with “herringbone” atomic structure.14c It is interestingly found that when hybrid nanomaterials containing Pt nanostructures were used as catalysts, the performance of electrocatalytic reactions could be greatly enhanced relative to that recorded for an unsupported catalyst. Despite these nice demonstrations, there still exist some issues for the 1D hybrid nanoelectrocatalysts reported. (a) The density of Pt NPs on the surface of CNTs was low, which is not beneficial for supplying high ECSA. (b) One-dimensional supporting nanomaterials such as CNTs are hydrophobic and usually needed to conduct the complex modification procedures before use. Generally, harsh oxidizing acids are usually used to produce carboxylic acid sites on the surface of CNTs; or polymer (or surfactant) is employed to functionalize CNTs in order to make them be soluble in water. Therefore, it is very necessary to search a

10.1021/jp909623x  2010 American Chemical Society Published on Web 02/25/2010

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new 1D supporting nanomaterial for loading ultrahigh density small Pt NPs as a high-efficiency nanoelectrocatalyst. In this paper, we describe an efficient approach for the synthesis of high-density Pt NP assembling architecture supported on the surface of Te NWs, through Te/Pt contact without prior Te NWs functionalization. Our work represents a new type of semiconductor/metal heterostructure with several important benefits.15 First, the procedure for the growth of Pt NPs can be performed at room temperature, using commercially available reagents, without the need for templates and surfactants. Second, no complex CNT functionalization process is required. Third, ultrahigh density small “bare” Pt NPs of about 3 nm are grown directly onto the surfaces of the Te NWs without the use of any linkers. Fourth, small Pt NPs located on the surface of Te NWs form an electronic network and thus lead to some nanoporous structures on the surface of Te NWs, which is in favor of providing high ECSA. Fifth, the as-prepared Te/Pt hybrid NWs with nanoporous surface exhibit higher electrocatalytic activity toward the methanol oxidation reaction (MOR) than the commercial platinum black (CPB) catalyst. Experimental Section Materials. Poly(N-vinyl-2-pyrrolidone) (PVP · K30, molecular weight 30 000-40 000), H2PtCl6 · 6H2O, H2SO4, ammonium hydroxide (NH4OH), acetone, hydrazine (50%), and ethanol were purchased from the Shanghai Chemical Factory (Shanghai, China) and used as received without further purification. Na2TeO3 and Nafion (perfluorinated ion-exchange resin, 5 wt % solution in a mixture of lower aliphatic alcohols and water) were obtained from Aldrich. CPB was purchased from Johnson Matthey Co. Water used throughout all experiments was purified with the Millipore system. Apparatus. A XL30 ESEM scanning electron microscope was used to determine the morphology and composition of products. Transmission electron microscopy (TEM) measurements were made on a Hitachi H-8100 EM with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurement was performed on an Escalab-MKII spectrometer (VG Co.) with Al KR X-ray radiation as the X-ray source for excitation. Cyclic votalmmogram (CV) experiments were performed with a CHI 832 electrochemical analyzer (CH Instruments, Chenhua Co., Shanghai, China). The composition of Te/Pt hybrid NWs was determined by inductively coupled plasma-mass spectroscopy (ICP-MS, X Series 2, Thermo Scientific). A conventional three-electrode cell was used, including an Ag/AgCl (saturated KCl) electrode as reference electrode, a platinum wire as counterelectrode, and modified GC as working electrode. Synthesis of Te Nanowires. Te NWs of small size (∼9 nm) were obtained according to reports.16 The synthesis of larger Te NWs was carried out through the following strategy. First, 0.6 g of PVP was dissolved with 27 mL of double-distilled water under vigorous magnetic stirring to form a homogeneous solution at room temperature. Then, 0.1107 g of sodium tellurite (Na2TeO3, 0.5 mmol) was added into the previous solution and dissolved, followed by the addition of hydrazine hydrate (1.7 mL, 50% w/w) and 2 mL of an ammonia solution (25% w/w). Then, the obtained solution was transferred into the container of Teflon-lined stainless steel autoclave, sealed, and maintained at 180 °C for 4 h. Finally, the product was centrifuged and washed several times with double-distilled water and absolute ethanol.

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Figure 1. (A, B) SEM and (C) TEM images of larger Te NWs at different magnifications. (D) TEM image of smaller Te NWs.

Synthesis of Te/Pt Hybrid Nanowires with Nanoporous Surface. Three milligrams of Te NWs was dissolved with 18 mL of double-distilled water, followed by the addition of 2 mL of 1% H2PtCl6 and 1 mL of HCOOH. The mixture was stored at room temperature for about 2 days until the Pt precursor was reduced completely. Then the solution was centrifuged and washed several times with double-distilled water and dissolved in water. Electrocatalytic Experiment. Prior to the surface coating, the GC electrode was polished carefully with 1.0, 0.3, and 0.05 µm alumina powder, respectively, and rinsed with deionized water, followed by sonication in acetone and double-distilled water successively. Then the electrode was allowed to dry under nitrogen. Catalyst dispersions were prepared by ultrasonically dispersing 50 µL of Te/Pt hybrid NWs (1.5 mg/mL Pt) or 100 µL of CPB catalyst (1.5 mg/mL Pt) in 0.50 mL of an aqueous solution containing 0.1 mL of ethanol and 20 µL of a 0.5 wt % Nafion solution. Twenty microliters of these inks was dispersed onto 5 mm glassy-carbon disk electrodes and dried at room temperature. Methanol electrocatalytic oxidation measurements were carried out in a solution of 0.1 M HClO4 containing 1 M methanol at the scan rate of 50 mV/s. Results and Discussion Te/Pt hybrid NWs with nanoporous surface were synthesized as follows. First, a large-scale and reproducible hydrothermal route was employed for preparing well-defined Te NWs according to the references with a slight modification.16 Then the as-prepared Te NWs were used to load ultrahigh density small Pt NPs for producing novel semiconductor/metal Te/Pt hybrid NWs via a simple and surfactantless route at room temperature. The morphologies of the resulting products were investigated by SEM and TEM. Parts A and B of Figure 1 show the typical SEM images of the as-prepared Te NWs coated on the silicon substrate at different magnifications. As shown in Figure 1A, the substrate is covered with a great deal of Te NWs with high purity. From the magnified image (Figure 1B), it is observed that these Te NWs have a diameter of about 75-150 nm. TEM was further used to characterize the morphology of the sample. Figure 1C provides the TEM overview image of Te NWs. The magnified images (Figure S1, Supporting Information) indicate that Te NWs have a smooth surface. Figure 2A shows a typical SEM image of the as-prepared Te/Pt hybrid NWs (adding 2 mL of 1% H2PtCl6). It is observed

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Figure 3. EDX spectrum of the Te/Pt hybrid NWs with nanoporous surface.

Figure 2. (A) SEM image of semiconductor/metal Te/Pt hybrid NWs with nanoporous surface. (B-D) TEM images of semiconductor/metal Te/Pt hybrid NWs with nanoporous surface at different magnifications. Reaction conditions: 3 mg of Te NWs, 2 mL of 1% H2PtCl6, 1 mL of HCOOH, and 18 mL of water. (E) Size distribution of Pt NPs supported on Te NWs.

that the as-obtained hybrid NWs have a rougher surface than pristine Te NWs (Figure 1B), indicating Pt NPs were facilely adsorbed on the surface of Te NWs. In order to reveal the detailed structure of Te/Pt hybrid NWs, the corresponding TEM images are shown in Figure 2B-D. Novel NWs containing ultrahigh density Pt NPs on their surface were observed (Figure 2C,D and Figure S2, Supporting Information). Most importantly, these small Pt NPs form a NP network, and thus lead to many nanoporous structures on the surface of hybrid NWs, which will probably be in favor of high electrocatalytic activity (discussion later). The size distribution of Pt NPs on the surface of Te NWs is shown in Figure 2E. It is observed that most of these Pt NPs are about 3 nm. The formation of Te/Pt hybrid NWs was further characterized by UV-visible spectra, energy-dispersive X-ray spectroscopy (EDX), and XPS. The UV-visible spectrum of the upper solution (Figure S3, Supporting Information) shows that the peak, which can be ascribed to ligand-to-metal charge-transfer transition of the PtCl62- ions, completely disappears after the reaction, indicating that the PtCl62- ions have been reduced to form Pt NPs.17 The formation of such Pt NPs can be attributed to direct redox between HCOOH and PtCl62-, because there are no other reducing reagents in the solution. Thus, the Pt loading of Te/Pt hybrid NWs (3 mg of Te and 7.5 mg of Pt) was calculated to be about 71.4%, which supports the conclusion that ultrahigh density Pt NPs have been assembled on the surface of Te NWs. The EDX

spectrum (Figure 3) shows the peaks corresponding to Te and Pt elements, confirming the existence of Pt NPs on the surface of the Te NWs. XPS patterns (Figure 4) of the resulting Te/Pt hybrid NWs show significant Pt 4f signals, corresponding to the binding energy of Pt (Figure 4A), and significant Te 3d signals, corresponding to the binding energy of Te NWs (Figure 4B), which further supports the conclusion that Pt NPs have been effectively assembled on the surface of Te NWs. Note that all the binding energies (BEs) were corrected for the C 1s signal at 284.6 eV as an internal standard. In order to identify the different chemical states of Pt, the spectra were deconvoluted into three components, as labeled by Pt(0), Pt(II), and Pt(IV), attributed to Pt metal, PtO, and PtO2, respectively.18 Thus, the relative amount of Pt(0) was calculated to be about 67.65% from the relative intensities of these three peaks, which is higher than that of CNT/poly(diallyldimethylammonium chloride) (PDDA)/Pt hybrids (60.2%) reported by Jiang and co-workers18b,c This result shows that Te/Pt hybrid NWs has a lower oxophilicity than CNT/PDDA/Pt hybrids, which has been proven to facilitate the methanol oxidation reaction.18a It should be noted that the downshift of the d-band center of Pt NPs can weaken the chemisorption with oxygen-containing species, such as OHad and COad, and therefore benefit the methanol oxidation reactions in fuel cells.18 Tsukuda and co-workers18a studied the effect of poly(N-vinyl-2-pyrrolidone) (PVP) as a stabilizer of Pt NPs on their electronic structures. They found that the BE of Pt NPs could downshift when they were protected by PVP. For our present system, the linker between Te NWs and Pt NPs is PVP, which may also functionalize as electron donors to Pt because of its electron-rich properties. As shown in Figure 4A, the Pt BEs for Te/Pt hybrid NWs (70.94 and 74.25 eV for Pt 4f 7/2 and Pt 4f 5/2) show a negative shift relative to CNTs/PDDA/Pt hybrids (PDDA has electron-poor characteristics) reported by Jiang and coworkers18b (71.9 and 75.1 eV for Pt 4f 7/2 and Pt 4f 5/2, respectively). This further indicates that Te/Pt hybrid NWs have good potential for enhancing the electrocatalytic activity toward methanol oxidation. In addition, the as-obtained Te/ Pt hybrid NWs have high stability (Pt NPs will not fall off when the Te/Pt NWs are sonicated in solution), which could be ascribed to the strong coordination interaction between poly(N-vinyl-2-pyrrolidone) (PVP, existing on the surface of Te NWs) and Pt NPs.3 The concentration of H2PtCl6 was found to have significant influence on the morphologies of the resulting products. When 4 mL of 1% H2PtCl6 was added into the solution containing

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Figure 4. XPS patterns of the Te/Pt hybrid NWs with nanoporous surface. (A) Deconvolution of Pt 4f; (B) Te.

Figure 5. (A, B) SEM images of necklace-like Te@Pt nanocables at different magnifications. (C) TEM image of necklacelike Te@Pt nanocables. (D) TEM image of Te/Pt hybrid NWs with low-density Pt NPs loading. (E, F) TEM images of smaller Te/Pt hybrid NWs at different magnifications.

Te NWs, it is found that necklacelike Te@Pt nanocables were generated (Figure 5A-C). If the concentration of H2PtCl6 was reduced (by addition of 1 mL of 1% H2PtCl6), only a few larger Pt NPs supported on the surface of Te NWs were formed (Figure 5D). These results indicate that a proper concentration of H2PtCl6 will be favorable for obtaining welldefined Te/Pt hybrid NWs with nanoporous surface. In addition, when smaller Te NWs (∼9 nm, Figure 1D) were employed as a supporting material, the diameter of Te/Pt hybrid NWs could be easily controlled to a smaller size (∼45 nm) (Figure 5E,F). Te/Pt hybrid NWs and CPB catalyst were coated on the surface of electrode, and their electrocatalytic activity toward methanol oxidation was studied. Figure 6A shows CVs of CPB (trace a) and Te/Pt hybrid NWs (trace b) in N2-purged 0.1 M HClO4 solution. Through measurement of the charge collected in the hydrogen adsorption/desorption region after double-layer correction and assumption of a value of 210

µC/cm2 for adsorption of a hydrogen monolayer, the specific ECSA (i.e., ECSA per unit weight of metal) of the Te/Pt hybrid NWs (54.0 m2/gPt) was found to be 285% that of the CPB catalyst (18.9 m2/gPt), indicating that the as-prepared Te/Pt hybrid NWs can probably be used as a good electrocatalyst. Figure 6B,C shows the CVs (B, specific current; C, mass current) of CPB catalyst (trace a) and Te/Pt hybrid NWs (trace b) modified GC electrodes in 0.1 M HClO4 solution containing 1 M CH3OH at a scan rate of 50 mV/s. It is observed that the onset potential of Te/Pt hybrid NWs for MOR was slightly higher than that of CPB catalyst, indicating that the existence of Te NWs did not greatly influence the conductivity of Te/Pt hybrid NWs (probably caused by the fact that suprahigh density Pt NPs were supported on the surface of Te NWs and further formed the NP electric network). The higher CH3OH oxidation peak current densities observed from CVs suggest that Te/Pt hybrid NWs catalyst exhibited better electrooxidation activity of CH3OH than the CPB catalyst. It should be noted that the mass activity of Te/Pt hybrid NWs (Figure 6C) is also higher than those of some state-of-the-art nanomaterials such as carbon nanofiber or CNT supported Pt NPs,14c carbon nanotube/ionic liquid/Pt NPs hybrid,19a CNT/Pt composite catalyst,19b etc. Furthermore, as indicated by dashed line in Figure 6D, the corresponding potential on Te/Pt hybrid NWs is lower than that on CPB at a given oxidation current density, which further indicates that Te/Pt hybrid NWs exhibit enhanced catalytic activity for MOR. Figure 6E shows the chronoamperometric curves for the above two electrodes at a fixed potential of 400 mV. It is observed that Te/Pt hybrid NWs exhibited higher mass current density than the CPB catalyst during the entire testing time. Thus, all the above data reveal that the as-prepared Te/Pt hybrid NWs show high electrocatalytic activity toward MOR. It is worthwhile to say that higher activity in electrochemical performance observed here can probably be attributed to three major factors: (i) The size of Pt NPs is small (Pt NPs with small and proper size show higher electroactivity than larger ones), which is comparable with commercial nanocatalysts. (ii) Small Pt NPs located on the surface of Te NWs form NPs electronic network and thus lead to some nanoporous structures on the surface of Te NWs. (iii) The downshift of the BE of Pt NPs and high amount of Pt(0) revealed by XPS may also be responsible for the high electrocatalytic activity of Te/Pt hybrid NWs toward MOR. As shown in Figure 6B, an oxidation peak is observed in the reverse scan, which is primarily associated with removal of the residual carbon species formed in the forward scan. Thus, the ratio of the forward oxidation current peak to the

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Figure 6. (A) CVs of CPB catalyst (trace a) and Te/Pt hybrid NWs (trace b) modified GC electrodes in a N2-saturated 0.1 M HClO4 solution at a scan rate of 50 mV/s. (B, C) CVs and (D) linear sweep voltammetry of CPB catalyst (trace a) and Te/Pt hybrid NWs (trace b) modified GC electrode in 0.1 M HClO4 solution containing methanol (1 M). Scan rate: 50 mV/s. Current density (panel B) was normalized in reference to the active surface of Pt. (E) Current density-time curve of CPB catalyst (trace a) and Te/Pt hybrid NWs (trace b) in 0.1 M HClO4 solution containing 1 M CH3OH at 0.4 V (vs Ag/AgCl).

reverse current peak, If/Ib, is an important index of the catalyst tolerance to the poisoning species, PtdCdO.20 A higher ratio indicates better tolerance of catalyst for the poisoning species. Figure 6B shows that the present catalyst tolerance to the poisoning species is not gratifying. Recently, several reports have demonstrated that Pt@Ru (or Rh) core/shell or Pt/Ru (or Rh) alloy NPs not only exhibited higher electrocatalytic activity than individual Pt NPs but also provided the particular advantage of effectively removing the poisoning species on the catalyst surface through a bifunctional mechanism.21 Thus, the activity and tolerance of hybrid NWs can probably be further improved by forming some new nanostructures such as suprahigh density Pt@Ru core/shell NPs or Pt/Ru alloy NPs supported Te NWs.21 Conclusions We have developed a facile procedure to synthesize semiconductor/metal Te/Pt hybrid NWs with nanoporous surface through the Te/Pt contact without prior Te NW functionalization. The ultrahigh density small Pt NPsof about 3 nm directly adhere to Te NWs and further form a morphology with many nanoporous structures, which exhibit an enlarged electroactive surface area relative to that of CPB catalyst. Most importantly, the as-prepared Te/Pt hybrid NWs with nanoporous surface exhibit high electrocatalytic activity toward MOR, indicating that they have potential for fuel-cell applications. The new hybrid nanostructure is scientifically interesting and probably has some potential in sensors and nanoelectronics and other electrochemical applications. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20735003 and 20820102037) and the 973 Project (Nos. 2009CB930100 and 2010CB933600).

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