Platinum Hybrid Nanoparticles Supported on Multiwalled Carbon

Jan 29, 2008 - Isabel Schick , Steffen Lorenz , Dominik Gehrig , Anna-Maria Schilmann , Heiko Bauer , Martin Panthöfer , Karl Fischer , Dennis Strand...
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J. Phys. Chem. C 2008, 112, 2389-2393

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Gold/Platinum Hybrid Nanoparticles Supported on Multiwalled Carbon Nanotube/Silica Coaxial Nanocables: Preparation and Application as Electrocatalysts for Oxygen Reduction Shaojun Guo, Shaojun Dong,* and Erkang Wang State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China, and Graduate School of the Chinese Academy of Sciences, Beijing, 100039, People’s Republic of China ReceiVed: September 10, 2007; In Final Form: NoVember 23, 2007

A simple approach combining sonication and sol-gel chemistry was employed to synthesize silica coated carbon nanotube (CNTs) coaxial nanocables. It was found that a homogeneous silica layer can be coated on the surface of the CNTs. This method is simple, rapid, and reproducible. Furthermore, gold nanoparticle supported coaxial nanocables were facilely obtained using amino-functionalized silica as the interlinker. Furthermore, to reduce the cost of Pt in fuel cells, designing a Pt shell on the surface of a noble metal such as gold or silver is necessary. High-density gold/platinum hybrid nanoparticles were located on the surface of 1-D coaxial nanocables with high surface-to-volume ratios. It was found that this hybrid nanomaterial exhibits a high electrocatalytic activity for enhancing oxygen reduction (low overpotential associated with the oxygen reduction reaction and almost four-electron electroreduction of dioxygen to water).

Introduction The unique physical, chemical, electronic, and mechanical properties of carbon nanotubes (CNTs) have led researchers to explore their use in composite materials for a variety of applications such as ultrahigh-strength engineering fibers,1 nanocatalysts,2 electrochemical sensors,3 etc. However, a major barrier for studying the properties and potential applications of CNTs is their poor solubility and processibility. As such, for scientists to functionalize CNTs to force them to dissolve in different solvents is still a central topic. Noncovalent functionalization of CNTs is particularly attractive because it not only provides the possibility of attaching a chemical species without affecting the electronic structure of CNTs but also endows some functional performance for biomolecule immobilization. One of the approaches that has been widely used to prepare individual CNTs is the noncovalent wrapping of CNTs by polymers,4 polynuclear aromatic compounds,5 surfactants,6 and biomolecules.7 Sol-gel chemistry8-13 also provides an advanced approach to functionalize CNTs due to its simplicity and ease of processibility. For example, Liu et al.8 reported a new method of coating single-walled carbon nanotubes (SWNTs) with a thin layer of SiO2 using 3-aminopropyltriethoxysilane as the coupling layer. Our previous work13 demonstrated a three-step process for synthesizing CNT/silica coaxial nanocables through the functionalization of CNTs using the polymer first. However, a precursor layer of silica should be introduced in the subsequent step, which makes the preparation process complex. In this paper, a simple method combining sonication and a sol-gel process is reported to synthesize silica homogeneously coated on the surface of CNTs to form well-defined coaxial nanocables. On the other hand, inorganic nanoparticles have continued to draw unwavering interest due to their particular optical, electronic, and magnetic properties and potential applications in catalysis, sensors, biomedicine,14 and surface enhanced Raman spectroscopy.15 Thus, functionalizing CNTs with inor* Corresponding author. E-mail: [email protected]; fax: +86-43185689711.

ganic nanoparticles that combines the properties of two functional nanomaterials to achieve a wider range of applications will play an important role in the development of nanoscience and nanotechnology. In the past few years, considerable effort has been made to functionalize CNTs with various nanomaterials such as metallic nanoparticles,11,16,17 quantum dots,18 and magnetic nanoparticles.19 Most current methods of binding metal or semiconductor nanocrystals to CNTs often make use of functional molecules as glue, such as small organic bridging molecules,20 polyelectrolytes,16,17,21-23 and DNA,24 to improve the adhesion between the nanocrystals and the CNTs. However, few publications have reported the synthesis of metallic nanoparticle modified CNTs using homogeneous silica as a spacer. In this study, we explored a facile, efficient, and economical route to obtain gold/platinum hybrid nanoparticles with a high density supported on 1-D CNT/silica coaxial nanocables (GPCSCN). This hybrid nanostructure obtained greatly increases the efficient surface-to-volume ratios of Pt. The rapid development of fuel cells has inspired us to investigate the electrocatalytic properties for dioxygen reduction of this novel hybrid nanomaterial. The hybrid nanomaterial mentioned here exhibits a high electrocatalytic activity for dioxygen reduction (low overpotential associated with the oxygen reduction and almost four-electron electroreduction of dioxygen to water). Experimental Procedures Chemicals. MWNTs (Shenzhen Nanotech Port Co., Ltd.) with a diameter of 20-40 nm were obtained through the CVD method and purified with 3 M HNO3 at 120° for 24 h. Poly(N-vinyl-2-pyrrolidone) (PVP‚K30), sodium citrate, tetraethoxysilane (TEOS), ascorbic acid (AA), HAuCl4‚4H2O, H2PtCl6‚ 6H2O, ammonium hydroxide (NH4‚OH), HNO3, and ethanol were purchased from the Shanghai Chemical Factory and used as received without further purification. 3-Aminopropyltrimethoxysilane (APTMS) and NaBH4 were obtained from Acros. Water used throughout all experiments was purified with the Millipore system.

10.1021/jp0772629 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/29/2008

2390 J. Phys. Chem. C, Vol. 112, No. 7, 2008 Instrumentation. A XL30 ESEM scanning electron microscope was used to determine the morphology of the hybrid nanomaterial. TEM measurements were made on a Hitachi H-8100 EM with an accelerating voltage of 200 kV. The sample for TEM characterization was prepared by placing a drop of prepared solution on a carbon-coated copper grid and drying at room temperature. The XPS measurement was performed on an ESCALAB-MKII spectrometer (VG Co.) with Al KR X-ray radiation as the X-ray source for excitation. The sample for XPS characterization was dropped on an Al substrate. Optical spectra were acquired using a Cary 500 UV-vis-NIR spectrometer (Varian). Electrochemical experiments were performed with a CHI 830 electrochemical workstation in a conventional threeelectrode electrochemical cell using twisted platinum wire as the auxiliary electrode and Ag/AgCl as the reference electrode. The gold nanoplate electrode (2 mm) loaded with different nanomaterials was employed as the working electrode. An EG&G PARC Model 366 bi-potentiostat was used for rotating ring-disk electrode (RRDE) experiments. A rotating glassy carbon (GC, 5 mm) disk-platinum ring electrode was used as a working electrode. The collection efficiency (N) of the ring electrode obtained by reducing ferricyanide at the disk electrode was 0.139. CNT Noncovalent Functionalization. The MWNTs were wrapped with polymer (PVP). The PVP modified CNTs can be easily dispersed in polar solvents such as water, DMF, and ethanol. In a typical experiment, 50 mg of CNTs was dispersed in a 0.5 wt % PVP (30 000-40 000) solution (250 mL) and sonicated for 2 h. Excess polymer was removed through a 450 nm polycarbonate filtration membrane. Then, a stable, homogeneous CNT ethanol suspension was obtained. Synthesis and Functionalization of CNT/Silica Coaxial Nanocables. A total of 2 mL of ammonia (28 wt % in water) was added to 30 mL of a CNT-PVP ethanol solution (3 mg of CNTs). Immediately after this, 0.1 mL of TEOS solution was added and stirred for 2 min. Then, the mixture was sonicated for 6 h and kept overnight at room temperature. The resulting product was then functionalized with APTMS. Briefly, 40 µL of APTMS was added to the previous solution, followed by the addition of 40 µL of ammonia. The resulting solution was stirred for 8 h. Finally, the solution was centrifuged and washed with ethanol and water 3 times, respectively. Preparation of Gold Nanoparticle Supported Coaxial Nanocables. Colloidal gold nanoparticles were prepared according to the reported literature.25 Gold nanoparticle supported coaxial nanocables were synthesized by the following approach. A total of 80 mL of the gold colloid was added to APTMSfunctionalized coaxial nanocables (30 mL). After being stirred for 2 h, the resulting solution was centrifuged and washed with water 3 times. The purified gold nanoparticle supported coaxial nanocables were redispersed in water (80 mL) until use. Preparation of GP-CSCN. The nanocomposites (80 mL) were heated to a boil. Then, 2 mL of 1% H2PtCl6 and 3 mL of 1% citrate were added to the previous solution, followed by the addition of 0.2 M AA (1 mL). Herein, AA acted as a reductant for the reduction of H2PtCl6. After being heated for 30 min, GP-CSCN was obtained. The resulting solution was centrifuged 4 times and redispersed in water (30 mL). Electrocatalytic Experiments. The electrode was loaded with hybrid nanomaterial (3 µL). Electrocatalytic dioxygen reduction measurements were carried out in a 0.5 M H2SO4 solution at a scan rate of 50 mV/s. For RRDE voltammetry experiments, 10 µL of the GP-CSCN solution was dropped on the GC electrode (5 mm) and allowed to dry at room temperature. Then, 5 µL of

Guo et al. SCHEME 1: Procedure To Design Gold/Platinum Hybrid Nanoparticle Supported Coaxial Nanocables

Nafion (0.2%) was placed on the surface of the GP-CSCN modified GC electrode. Results and Discussion The whole preparation strategy is shown in Scheme 1. First, a simple method combining the sonication and sol-gel process was employed to synthesize silica homogeneously coated on the surface of CNTs to form well-defined coaxial nanocables. Herein, sonication provided a good homogeneous environment (preventing CNT sedimentation) for silica growth. Second, these coaxial nanocables were functionalized with NH2. Third, aminofunctionalized coaxial nanocables were mixed with gold nanoparticles to obtain gold nanoparticle supported coaxial nanocables. Fourth, GP-CSCN was obtained via heating the assembly system and H2PtCl6 solution in the presence of a reductant (AA). As is known, nanostructured silica has gained great interest because it can offer excellent surface chemistry properties and provides good biocompatibility for biosensors and bioassays. Herein, a simple sonication approach to functionalize CNTs with a homogeneous silica layer is reported. The structure and morphology of the coaxial nanocables were characterized using TEM. Figure 1A shows the typical TEM image of the asprepared coaxial nanocables. A great number of 1-D hollow nanofibers with diameters of 60-80 nm are observed. The highmagnification image (Figure 1B) shows an individual coaxial nanocable. A contrasted difference in the coaxial nanocable with a dark center surrounded by a lighter edge is observed, confirming the core/shell nanostructure. In fact, the thickness of silica on the surface of CNTs can be easily controlled via changing the weight ratio of CNTs to TEOS or using an asprepared coaxial nanocable as a seed (data not shown). When the coaxial nanocables are modified by gold nanoparticles, a great number of gold nanoparticles distribute almost uniformly on the walls of the coaxial nanocables, although a few aggregates consisting of several nanoparticles are observed (Figure 2), as were previously demonstrated for the selfassembly of nanoparticles on a planar substrate.26 In fact, the adsorption of nanoparticles was also supported by UV-vis spectra. Figure 3a,b shows the UV-vis absorption spectra of gold nanoparticles before (a) and after (b) attachment onto the surface of coaxial nanocables. For gold nanoparticles,

Au/Pt Nanoparticle Supported Nanotubes/Nanocables

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Figure 4. Typical SEM images (A and B) of the as-prepared gold/ platinum hybrid nanoparticle supported coaxial nanocables at different magnifications. Figure 1. Typical TEM images of coaxial nanocables at different magnifications (A and B).

Figure 5. Typical TEM images (A and B) of the as-prepared gold/ platinum hybrid nanoparticle supported coaxial nanocables at different magnifications. Figure 2. Typical TEM images (A and B) of the as-prepared gold nanoparticle supported coaxial nanocables at different magnifications.

Figure 3. UV-vis spectra of gold nanoparticles (a), the as-prepared gold nanoparticle supported coaxial nanocables (b), and coaxial nanocables (c).

the plasmon band appears at 515 nm, as expected. For gold nanoparticle supported coaxial nanocables, a much wider visible absorption peak appears at 524 nm, albeit at a decreased intensity. The red-shift of the surface-plasmon band can be attributed to the interparticle interactions adsorbed on the coaxial nanocables, as was previously demonstrated by Giersig et al.27 As a comparison, the UV-vis absorption spectrum of coaxial nanocables is shown in Figure 3c. No absorption peak is observed. The previous result indicates that the gold nanoparticles are indeed bound to the surface of coaxial nanocables. To obtain GP-CSCN, the coaxial nanocables decorated with gold nanoparticles were dispersed in H2PtCl6 and then heated at 100 °C for 30 min in the presence of AA. Figure 4 shows the typical scanning electron microscopy (SEM) image of GPCSCN. It is found that high-density nanoparticles are supported on the surface of coaxial nanocables. To reveal the detailed structure of GP-CSCN, a typical TEM image of GP-CSCN is shown in Figure 5. Nanoparticles with a larger size than the preformed gold nanoparticles were found, revealing that a gold/ platinum hybrid nanostructure was produced. To confirm the existence of Au, Pt, silica, and CNTs in the resulting hybrid

nanomaterials, an XPS experiment was employed for the surface analysis of the sample. XPS patterns of the resulting GP-CSCN show a significant Au4f signal corresponding to the binding energy of metallic Au (Figure 6A), a Pt4f signal characteristic of metallic Pt (Figure 6B), Si2p and O1s signals characteristic of silica (Figure 6C,D), a N1s signal characteristic of NH2 (Figure 6E), and a C1s signal characteristic of CNTs (Figure 6F). Thus, we can further affirm that gold/platinum hybrid nanoparticles supported on coaxial nanocables can be obtained via the previous method. As is known, the large overpotential associated with oxygen reduction reactions (ORR) is one of the major challenges that calls for the development of a high-performance cathode catalyst. Previously, we obtained and characterized the Au/Pt hybrid nanoparticles supported on coaxial nanocables. The hybrid nanostructure is expected to have a good and highly efficient electrocatalytic activity because high-density small gold/platinum hybrid nanospheres are located on the surface of 1-D coaxial nanocables. Therefore, the electrocatalytic activity of GP-CSCN has primarily been investigated for dioxygen reduction. Figure 7c,d shows the typical cyclic voltammograms (CVs) of dioxygen reduction at the GP-CSCN modified gold electrode in a 0.5 M H2SO4 solution in the presence of air (Figure 7c) and saturated dioxygen (Figure 7d). In the presence of air, a remarkable catalytic reduction current occurs at 0.44 V (Figure 7c) at a scan rate of 50 mV/s. A higher catalytic current for dioxygen reduction is observed at 0.38 V in the presence of saturated dioxygen (Figure 7d), while no catalytic reduction current can be observed at the bare gold (Figure 7a) and gold nanoparticle supported coaxial nanocable (Figure 7b) modified electrode in the potential range employed. The previous results indicate that the as-prepared hybrid nanostructures have an excellent electrocatalytic activity for dioxygen reduction (low overpotential associated with the oxygen reduction reaction). The four-electron electroreduction of dioxygen to water, which is the cathode reaction of most fuel cells, is greatly valued by scientists around the world in view of its important application in fuel cells. Herein, the ORR was also probed via RRDE experiments to demonstrate the ORR process of the GPCSCN film modified electrode. It was found that the as-prepared

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Figure 6. XPS spectra of the as-prepared gold/platinum hybrid nanoparticle supported coaxial nanocables: (A) Au4f, (B) Pt4f, (C) Si2p, (D) O1s, (E) N1s, and (F) C1s.

Figure 7. CVs of O2 reduction at the as-prepared GP-CSCN modified gold electrode (c and d); as-prepared gold nanoparticle supported coaxial nanocables modified gold electrode (b); and bare gold electrode (a) in air-saturated (a-c) and O2-saturated (d) 0.5 M H2SO4 solutions. The scan rate was 50 mV/s.

GP-CSCN modified GC electrode reduced O2 predominantly by four electrons to H2O, as confirmed by the RRDE technique. Figure 8 shows the voltammetric curves for dioxygen reduction,

Figure 8. Current-potential curves for the reduction of air-saturated O2 at a rotating platinum ring-GC disk electrode with GP-CSCN adsorbed on the disk electrode. The potential of the ring electrode was maintained at 1.0 V. Rotation rate was 200 rpm. Scan rate was 50 mV/s. The supporting electrolyte was 0.5 M H2SO4.

recorded at the RRDE with GP-CSCN immobilized on the GC disk electrode. In this experiment, the disk potential was scanned from +0.95 to 0.1 V while the ring potential was kept at +1.0 V to detect any H2O2 evolved at the disk. A large disk current

Au/Pt Nanoparticle Supported Nanotubes/Nanocables was obtained, whereas almost no ring current was observed, suggesting that the as-prepared nanostructured electrocatalysts reduce O2 predominantly by four electrons to H2O. From the ratio of the ring-disk current, the electron-transfer number (n) is calculated to about 4 (3.90) according to the equation n ) 4 - 2(IR/IDN).28 Conclusion In conclusion, we developed a simple and reproducible strategy for the synthesis of CNT/silica coaxial nanocables. It was found that a homogeneous silica layer was coated on the surface of CNTs. Furthermore, gold nanoparticle supported coaxial nanocables were facilely obtained using amino-functionalized silica as interlinkers. The versatility of this method can be extended to join other functional materials, such as biomolecules, quantum dots, magnetic nanoparticles, and nonconducting nanoparticles of different sizes and shapes, which will result in a range of nanohybrids with different properties. Moreover, to reduce the cost of Pt in fuel cells, designing a Pt shell on the surface of a noble metal such as gold and silver is useful. High-density gold/platinum hybrid nanoparticles were located on the surface of 1-D coaxial nanocables with high surface-to-volume ratios. It was found that this hybrid nanomaterial exhibited a high electrocatalytic activity for enhancing oxygen reduction, which is expected to be useful for applications in fuel cells. Acknowledgment. This work was supported by the National Science Foundation of China (20575064, 20675076, and 20427003). References and Notes (1) Treacy, M.; Ebbesen, T.; Gibson. J. Nature (London, U.K.) 1996, 381, 678. (2) Cao, Lin.; Scheiba, F.; Roth, C.; Schweiger, F.; Cremers, C.; Stimming, U.; Fuess, H.; Chen, L.; Zhu, W.; Qiu, X. Angew. Chem., Int. Ed. 2006, 45, 5315.

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