A General Route to Prepare One- and Three-Dimensional Carbon

Nov 6, 2006 - A General Route to Prepare One- and Three-Dimensional Carbon ... Chinese Academy of Sciences, Beijing, 100039, People's Republic of ...
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Langmuir 2007, 23, 6352-6357

A General Route to Prepare One- and Three-Dimensional Carbon Nanotube/Metal Nanoparticle Composite Nanostructures Xiaoge Hu, Tie Wang, Liang Wang, Shaojun Guo, and Shaojun Dong* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, People’s Republic of China, and Graduate School of the Chinese Academy of Sciences, Beijing, 100039, People’s Republic of China ReceiVed NoVember 6, 2006. In Final Form: February 26, 2007 Adsorption of polyethyleneimine (PEI)-metal ion complexes onto the surfaces of carbon nanotubes (CNTs) and subsequent reduction of the metal ion leads to the fabrication of one-dimensional CNT/metal nanoparticle (CNT/M NP) heterogeneous nanostructures. Alternating adsorption of PEI-metal ion complexes and CNTs on substrates results in the formation of multilayered CNT films. After exposing the films to NaBH4, three-dimensional CNT composite films embedded with metal nanoparticles (NPs) are obtained. UV-visible spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy are used to characterize the film assembly. The resulting (CNT/M NP)n films inherit the properties from both the metal NPs and CNTs that exhibit unique performance in surfaceenhanced Raman scattering (SERS) and electrocatalytic activities to the reduction of O2; as a result, they are more attractive compared to (CNT/polyelectrolyte)n and (NP/polyelectrolyte)n films because of their multifunctionality.

Introduction Considerable attention has been drawn to carbon nanotubes (CNTs) over the past decade because of their unique electronic, mechanical, and chemical properties, as well as potential applications in material science, sensor technology, biomedical applications, and nanoscale electronic devices.1,2 In pursuit of these potential applications, many attempts have been carried out to functionalize CNTs in various ways.3 However, the possible applications are still suppressed by progress in functionalization of CNTs and limitations in assembly methods. For this reason, new strategies for modification and assembly of CNTs are needed. In the past few years, considerable effort has been made to functionalize CNTs with various nanomaterials.4 The organization of metal or semiconductor nanoparticles (NPs) on CNTs produces novel nanocomposites that combine the properties of two functional nanoscale materials to achieve a wider range of * Corresponding author. Tel: +86-431-5262101. Fax: +86-431-5689711. E-mail: [email protected]. (1) (a) Terrones, M. Annu. ReV. Mater. Res. 2003, 33, 419. (b) Ouyang, M.; Huang, J.-L.; Lieber, C. M. Acc. Chem. Res. 2002, 35, 1018. (c) Avouris, P. Acc. Chem. Res. 2002, 35, 1026. (d) Sun, Y.-P.; Fu, K.; Lin, Y.; Huang, W. Acc. Chem. Res. 2002, 35, 1096. (e) Kam, N. W. S.; Dai, H. J. Am. Chem. Soc. 2005, 127, 6021. (f) Hou, P.-x.; Yang, Q.-h.; Bai, S.; Xu, S.-t.; Liu, M.; Cheng, H.-m. J. Phys. Chem. B 2002, 106, 963. (g) Su, L.; Gao, F.; Mao, L. Anal. Chem. 2006, 78, 2651. (2) (a) Franklin, N. R.; Li, Y.; Chen, R. J.; Javey, A.; Dai, H. Appl. Phys. Lett. 2001, 79, 4571. (b) Bachtold, A.; Hadley, P.; Nakanishi, T.; Dekker, C. Science 2001, 294, 1317. (c) Rueckes, T.; Kim, K.; Joselevich, E.; Tseng, G. Y.; Cheung, C.; Lieber, C. M. Science 2000, 289, 94. (d) Sheeney-Haj-Ichia, L.; Basnar, B.; Willner, I. Angew. Chem., Int. Ed. 2005, 44, 78. (3) (a) Satake, A.; Miyajima, Y.; Kobuke, Y. Chem. Mater. 2005, 17, 716. (b) Liu, Y.; Wu, D.; Zhang, W.; Jiang, X.; He, C.; Chung, T. S.; Goh, S. H.; Leong. K. W. Angew. Chem., Int. Ed. 2005, 44, 4782. (c) Nguyen, C. V.; Delzeit, L.; Cassell, A. M.; Li, J.; Han, J.; Meyyappan, M. Nano Lett. 2002, 2, 1079. (d) Jiang, K.; Schadler, L. S.; Siegel, R. W.; Zhang, X.; Zhang, H.; Terrones, M. J. Mater. Chem. 2004, 14, 37. (e) Chattopadhyay, D.; Galeska, I.; Papadimitrakopoulos, F. J. Am. Chem. Soc. 2003, 125, 3370. (f) Artyukhin, A. B.; Bakajin, O.; Stroeve, P.; Noy, A. Langmuir 2004, 20, 1442. (g) Sun, J.; Gao, L.; Iwasa, M. Chem. Commun. 2004, 7, 832. (4) (a) Xing, Y. J. Phys. Chem. B 2004, 108, 19255. (b) Hu, X.; Wang, T.; Qu, X.; Dong, S. J. Phys. Chem. B 2006, 110, 853. (c) Correa-Duarte, M. A.; Sobal, N.; Liz-Marza’n, L. M.; Giersig, M. AdV. Mater. 2004, 16, 2179. (c) Cao, J.; Sun, J.-Z.; Hong, J.; Li, H.-Y.; Chen, H.-Z.; Wang, M. AdV. Mater. 2004, 16, 84. (e) Correa-Duarte, M. A.; Perez-Juste, J.; Sanchez-Iglesias, A.; Giersig, M.; Liz-Marzan L. M. Angew. Chem., Int. Ed. 2005, 44, 4375. (f) Fu, Q.; Lu, C.; Liu, J. Nano Lett. 2002, 2, 329.

applications.4,5 Most current methods of binding metal or semiconductor nanocrystals to CNTs often make use of functional molecules such as small organic bridging molecules,6 polyelectroyetes,7 and DNA8 to improve the adhesion between the nanocrystals and CNTs. On the other hand, to investigate the optical, optoelectronic, and electrical properties on a macroscopic scale, a homogeneous CNT thin film is required.9 Therefore, facile and effective methods for controlled thin-film fabrication are still needed. There have been a number of reports on the preparation of CNT thin films, including solution casting,10 electrophoretic11 and Langmuir-Blodgett deposition,12 dipcoating,13 adsorption on modified surfaces,14 stretching polymer films loaded with CNTs,15 and self-assembly,16 but the layerby-layer (LBL) assembly technique is perhaps the most versatile and common method today to fabricate robust and uniform thin (5) (a) Qu, L.; Dai, L. J. Am. Chem. Soc. 2005, 127, 10806. (b) Qu, L.; Dai, L.; Osawa, E. J. Am. Chem. Soc. 2006, 128, 5523. (c) Robel, I.; Bunker, B. A.; Kamat, P. V. AdV. Mater. 2005, 17, 2458. (d) Correa-Duarte, M. A.; Grzelczak, M.; Salgueirino-Maceira, V.; Giersig, M.; Liz-Marzan, L. M.; Farle, M.; Sierazdki, K.; Diaz, R. J. Phys. Chem. B 2005, 109, 19060. (6) (a) Shi, J. H.; Qin, Y. J.; Wu, W.; Li, X. L.; Guo, Z.-X.; Zhu, D. B. Carbon 2004, 42, 455. (b) Ravindran, S.; Chaudhary, S.; Colburn, B.; Ozkan, M.; Ozkan, C. S. Nano Lett. 2003, 3, 447. (7) (a) Jiang, K.; Eitan, A.; Schadler, L. S.; Ajayan, P. M.; Siegel, R. W.; Grobert, N.; Mayne, M.; Reyes-Reyes, M.; Terrones, H.; Terrones, M. Nano Lett. 2003, 3, 275. (b) Kim, B.; Sigmund, W. M. Langmuir 2004, 20, 8239. (8) Moghaddam, M. J.; Taylor, S.; Gao, M.; Huang, S.; Dai, L.; McCall, M. J. Nano Lett. 2004, 4, 89. (9) Correa-Duarte, M. A.; Kosiorek, A.; Kandulski, W.; Giersig, M.; LizMarzan, L. M. Chem. Mater. 2005, 17, 3268. (10) Meitl, M. A.; Zhou, Y.; Gaur, A.; Jeon, S.; Usrey, M. L.; Strano, M. S.; Rogers, J. A. Nano Lett. 2004, 4, 1643. (11) Gao, B.; Yue, G. Z.; Qiu, Q.; Cheng, Y.; Shimoda, H.; Fleming, L.; Zhou, O. Adv. Mater. 2001, 13, 1770. (12) (a) Guo, Y.; Minami, N.; Kazaoui, S.; Peng, J.; Yoshida, M.; Miyashita, T. Physica B 2002, 323, 235. (b) Guo, Y.; Wu, J.; Zhang, Y. Chem. Phys. Lett. 2002, 362, 314. (13) Spotnitz, M. E.; Ryan, D.; Stone, H. A. J. Mater. Chem. 2004, 14, 1299. (14) Zhu, J.; Yudasaka, M.; Zhang, M.; Kasuya, D.; Iijima, S. Nano Lett. 2003, 3, 1239. (15) Ichida, M.; Mizuno, S.; Kataura, H.; Achiba, Y.; Nakamura, A. AIP Conf. Proc. 2001, 590, 121. (16) (a) Shimoda, H.; Oh, S. J.; Geng, H. Z.; Walker, R. J.; Zhang, X. B.; Mcneil, L. E.; Zhou, O. Adv. Mater. 2002, 14, 899. (b) Wu, B.; Zhang, J.; Wei, Z.; Cai, S.; Liu, Z. J. Phys. Chem. B 2001, 105, 5075. (c) Liu, Z.; Shen, Z.; Zhu, T.; Hou, S.; Ying, L. Langmuir 2000, 16, 3569.

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Scheme 1. Formation of Metal Nanoparticles in 1D CNT/M NP Composite Nanstructures (A) and in 3D (CNT/M NP)n Films (B)

film of CNTs.17 The LBL assembly has several advantages as follows: it allows precise control over the film thickness and its properties, allows generation of complex film architectures with several different components, makes the uniform film coverage, reduces the phase segregation, and makes composites highly homogeneous.18 Interactions used in the LBL assembly of CNT films include van der Waals,19 covalent bonding,20 and hydrogen bonding interactions,21 but electrostatic self-assembly22 is the most versatile and facile method. As demonstrated previously, the acidic oxidation procedure used for purification of CNTs, mainly to remove metal catalyst and amorphous carbon, usually creates anionic groups (such as carboxylic groups) at the open tube ends but also at the defect sites generated along the tube surface.7a,23 The as-treated CNTs carry negative charges and can interact electrostatically with polycations, e.g., with poly(diallyldimethylammonium chloride) (PDDA) and poly(ethyleneimine) (PEI).7b,9,17b,22,24-26 So far, there are many reports about the fabrication of CNT thin films by sequential deposition of oppositely charged CNTs and polyelectrolytes (PEs) on substrates through electrostatic interacton.9,17,22,25,27,28 In these (17) (a) Mamedov, A. A.; Kotov, N. A.; Prato, M.; Guldi, D. M.; Wicksted, J. P.; Hirsch, A. Nat. Mater. 2002, 1, 190. (b) Olek, M.; Ostrander, J.; Jurga, S.; Mohwald, H.; Kotov, N.; Kempa, K.; Giersig, M. Nano Lett. 2004, 4, 1889. (c) Shim, B. S.; Kotov, N. A. Langmuir 2005, 21, 9381. (18) (a) Cho, J.; Jang, H.; Yeom, B.; Kim, H.; Kim, R.; Kim, S.; Char, K.; Caruso, F. Langmuir 2006, 22, 1356. (b) Lojou, E.; Bianco, P. Langmuir 2004, 20, 748. (19) Sato, M.; Sano, M. Langmuir 2005, 21, 11490. (20) Qin, S. H.; Qin, D. Q.; Ford, W. T.; Zhang, Y. J.; Kotov, N. A. Chem. Mater. 2005, 17, 2131. (21) Qin, S. H.; Qin, D. Q.; Ford, W. T.; Herrera, J. E.; Resasco, D. E. Macromolecules 2004, 37, 9963. (22) (a) Rouse, J. H.; Lillehei, P. T. Nano Lett. 2003, 3, 59. (b) Zhang, M. N.; Yan, Y. M.; Gong, K. P.; Mao, L. Q.; Guo, Z. X.; Chen, Y. Langmuir 2004, 20, 8781. (23) Yu, R.; Chen, L.; Liu, Q.; Lin, J.; Tan, K.-L.; Ng, S. C.; Chan, H. S. O.; Xu, G.-Q.; Hor, T. S. A. Chem. Mater. 1998, 10, 718. (24) He, P.; Bayachou, M. Langmuir 2005, 21, 6086. (25) Kovtyukhova, N. I.; Mallouk, T. E. J. Phys. Chem. B 2005, 109, 2540. (26) Kim, B.; Park, H.; Sigmund, W. M. Langmuir 2003, 19, 2525. (27) Kim, B.; Sigmund, W. M. Langmuir 2003, 19, 4848.

CNT/PE multilayered films, some novel properties inherited from the NPs or the active biomolecules29 are lacking. The introduction of metal NPs to the CNT films can generate new nanostructures with excellent functions in the fields of optics, electronics, and electrocatalysis, which are very attractive for practical applications. It has been reported that PEI can form PEI-metal ion complexes (PEI-M) with several metal compound such as AgNO3,30,31 Pt(NH3)4Cl2,30 and K2PdCl4.32 The PEI-M complexes hold positive charges and can be used as polycations in the LBL assembly of CNT multilayers. In addition, the PEI-M can easily adsorb on the surface of CNTs via electrostatic interaction. In this paper, we report a general method for the synthesis of one-dimensional (1D) CNT/metal nanoparticle (CNT/M NP) heterogeneous nanostructures and the corresponding three-dimensional (3D) layered nanocomposites (CNT/M NP)n through adsorption of a PEI-M on the surface of CNTs and layer-by-layer adsorption of the cationic PEI-M and anionic CNTs on a solid surface, respectively, and finally reduction of the metal ions. Scheme 1 illustrates the formation of 1D and 3D composite nanostructures. This method has the virtues of precise control over the CNT films, and the NPs are dispersed throughout the film becasuse the metal ions are well-distributed along the polymer chains. Moreover, the process overcomes the need to synthesize uniform and isolated NPs in solution. Experimental Section Reagents and Materials. Polyethyleneimine (PEI, Mn ) 423), 4-aminothiophenol (4-ATP), and sodium borohydride (NaBH4, 99%) were purchased from Aldrich. Multiwalled CNTs (MWNTs) were (28) Paloniemi, H.; Lukkarinen, M.; Aaritalo, T.; Areva, S.; Leiro, J.; Heinonen, M.; Haapakka, K.; Lukkari, J. Langmuir 2006, 22, 74. (29) Yan, Y.; Zhang, M.; Gong, K.; Su, L.; Guo, Z.; Mao, L. Chem. Mater. 2005, 17, 3457. (30) Dai, J.; Bruening, M. L. Nano Lett. 2002, 2, 497. (31) Zhou, Y.; Ma, R.; Ebina, Y.; Takada, K.; Sasaki, T. Chem. Mater. 2006, 18, 1235. (32) Kidambi, S.; Bruening, M. L. Chem. Mater. 2005, 17, 301.

6354 Langmuir, Vol. 23, No. 11, 2007 obtained from Nanotech Port Ltd. Co. (95% purity, Shenzhen, China). The MWNTs were treated by refluxing in 3 M HNO3 for 48 h, then were filtered with a 2.5 µm minipore size membrane with the aid of a pump and thoroughly washed with water to obtain a neutral state; finally, they were dried under vacuum at 60 °C overnight to obtain purified MWNTs of about 20-45 nm diameter.33 Finally, the MWNTs were dispersed in water, obtaining a stable dispersion of oxidized MWNTs with carboxylic groups on the walls, providing a negative surface charge. AgNO3, K2PtCl4, and PdCl2 were of analytical reagent grade and used as received. Ultrapure water purified with Milli-Q plus system (Millipore Co.) was exclusively used in all aqueous solutions and rinsing procedures. Its resistivity was 18 MΩ·cm. Preparation of PEI-M Complexes (M ) Ag(I), Pt(II), and Pd(II)). 10 mL of 4 mM AgNO3 solution was slowly added into 10 mL of 2 mg/mL PEI solution at pH ) 9.0-9.1 under stirring. The mixture solution was then stirred for 1 h, driving completion of the coordination reaction. The resulting complex aqueous solutions have an 11:1 concentration ratio of PEI (repeating unit) to metal ion. In the UV-vis experiment, the molar ratio of PEI and AgNO3 was adjusted to 6:1. As for the preparation of PEI-Pd(II) and PEI-Pt(II), the same procedure was used except that K2PtCl4 or K2PdCl4 was used. Synthesis of 1D CNT/M NP Composites in Solution. The CNT aqueous solution (0.03 mg/mL) was prepared by dispersing purified CNTs (0.18 mg) in 6 mL water with the help of ultrasonic treatment. To each 2 mL aliquot of this CNT solution, 0.5 mL of the abovementioned PEI-M complex solutions were added, followed by a combination of vigorous stirring and sonication. After they were thoroughly mixed, 0.3 mL of 0.1 M NaBH4 solution was added quickly. Several minutes later, the mixed solution was centrifuged and washed for two cycles, and the resulting products were dispersed in 1 mL water. Preparation of 3D (CNT/M NP)n Composite Films by Layerby-Layer Deposition. The PEI-M complexes and CNTs were LBL assembled onto an indium tin oxide (ITO)-coated glass plate alternately. The treated plates were first immersed in a 2 wt % aqueous solution of positively charged PEI containing 0.2 M NaCl for 2 h to form a positively charged layer onto the surface. The PEI-treated plates were then immersed in 0.1 mg/mL CNT aqueous solution for 30 min. After being rinsed with water and dried with N2, the CNT/PEI-treated ITO plates were immersed into PEI-M complex solutions for 30 min. LBL stepwise assembly of the PEIM/CNT bilayer onto the PEI-treated ITO plates was conducted by alternately repeating the last two procedures. During each assembly interval, the plates were rinsed with water to remove the excess materials and dried with N2. Finally, the resulting multilayered thin films were immersed in freshly prepared 0.1 M aqueous NaBH4 solution, during which time the Ag(I), Pt(II), or Pd(II) component was reduced to Ag, Pt, and Pd metal, respectively, forming metal NPs between the CNT layers. The resulting multilayered nanocompsites were referred to as (CNT/M NP)n. After that, the films were rinsed with deionized water for 1 min and finally dried in a N2 stream. Characterization. UV-vis absorption spectra were recorded on a Cary 500 UV-vis-NIR spectrometer (Varian, U.S.A.). X-ray photoelectron spectroscopy (XPS) measurements were performed on a thermo ESCALAB 250 photoelectron spectrometer with an Al KR X-ray radiation as the X-ray source for excitation. Scanning electron microscopy (SEM) observations were carried out on a XL30 ESEM FEG field emission scanning electron microscopy (SEM, FEI Company with 20 kV operating voltage) equipped with energydispersive X-ray (EDX). Transmission electron microscopy (TEM) observations were carried out on a Hitachi H-8100 EM with accelerating voltage of 200 kV. SERS spectra were recorded with a Renishaw 2000 equipped by an Ar+ ion laser giving the excitation (33) (a) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y.-S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253. (b) Qu, J.; Shen, Y.; Qu, X.; Dong, S. Chem. Commun. 2004, 34.

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Figure 1. TEM images of purified CNTs (A) and 1D CNT/Ag NP heterogeneous nanostructures (B). (C) The selected area electron diffraction (SAED) of the as-prepared CNT/Ag NP nanocomposites. (D,E) TEM images of 1D CNT/Pt NP and CNT/Pd NP nanocomposites, respectively. The inset of E is the higher-magnification image of E. line of 514.5 nm and an air-cooling charge-coupled device (CCD) as the detector (Renishaw Co., U.K.). Electrochemical experiments were carried out on an Autolab PGSTAT30 potentiostat (Utrecht, The Netherlands) in a conventional one-compartment electrochemical cell. The cell was housed in a homemade Faraday cage to reduce stray electrical noise. All measurements were done using standard three-electrode systems. A Ag/AgCl electrode was used as the reference electrode, a Pt foil was used as the counter electrode, and the modified ITO slides were used as the working electrode.

Results and Discussion In order to explore the feasibility of the PEI-M complexes on the assembly of the CNTs into multilayers, we first carry out the assembly of PEI-M on the surface of CNTs in solution by direct mixing. The PEI-M complexes are positively charged complex ions, which can be adsorbed on the surfaces of negatively charged CNTs. After reduction, the 1D CNT/M NP heterogeneous nanostructures are received. Figure 1A shows the TEM image of purified CNTs. The surfaces of the CNTs are clean, indicating that the initial catalyst nanoparticles have been removed. Figure 1B shows the TEM image of 1D CNT/Ag NP nanocomposites. Ag NPs with average diameter of 4.7 ( 1.1 nm are distributed on the CNT surfaces. The selected area electron diffraction (SAED) pattern (Figure 1C) shows that the Ag NPs are crystallized in a face-centered cubic (fcc) structure. The Ag NPs selectively adsorbed onto the CNT surfaces, and the majority of assembled NPs display a spatially isolated feature. This strategy provides a general route to obtain other sorts of CNT/M NP composites. Figure 1D,E shows the TEM images of 1D CNT/Pt NP and CNT/Pd NP nanocomposites prepared by the same method, respectively. Pt and Pd NPs with sizes of 2.6 ( 0.5 nm and 2.8 ( 0.4 nm were also located on the surface of CNTs.

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Figure 2. SEM images of (CNTs/PEI)6 film (A) and (CNT/Ag NP)6 composite film (B) assembled onto PEI-treated ITO plate. Scale bar, 200 nm.

Figure 4. UV-vis spectra of (CNT/PEI-Ag(I))n multilayers assembled onto quartz slide before (solid line) and after (doted line) reduction with NaBH4. The small discontinuity at 350 nm is due to the change of grating and detector. The insert plot shows that the absorbance at 263 nm increases linearly with the number of bilayers.

Figure 3. SEM images of 3D (CNT/Ag NP)n composite films assembled onto PEI-treated ITO plate. (A) n ) 1, (B) n ) 3, (C) n ) 6. Scale bar, 1 µm. (D) shows the EDX spectrum of (CNT/Ag NP)3 films on PEI-treated ITO plate.

In many cases, the material is needed in the form of a thin uniform film on the substrate of choice, requiring facile methods for the controlled thin-film fabrication. By using the alternative deposition of the PEI-M complex and CNTs on the substrate, the metal NPs are easily introduced into the CNT multilayered film, endowing the film with novel properties compared to the previous (CNT/PE)n films. The formation of Ag NPs in the composite film is confirmed by the SEM images (Figure 2). Figure 2A shows the SEM image of the (CNT/PEI)6 film without incorporation of metal NPs. It can be seen that the CNT surface is smooth. Figure 2B shows a representative image for the Ag NP-incorporated CNT film in which the surface of the CNTs is rough. The obvious difference in the morphology shows the presence of the metal NPs in the composite film. Figure 3 displays typical SEM images of 3D (CNT/Ag NP)n composite films through LBL assembly on an ITO plate. These SEM images reveal an obvious increase in nanotube coverage with increasing number of LBL assembly treatments, indicative of stepwise assembly of the CNTs and PEI-M into a multilayered film. Most of the surface-confined CNTs are in the form of small bundles and single tubes with 20-45 nm diameter and 0.5-1.5 µm length. It is found that these films comprised many randomly oriented, uniformly distributed CNTs. The stepwise growth of the film is also confirmed by the increasing amount of Ag NPs in the films with different deposition layers after the films were subject to reduction by NaBH4. It can be observed that the surfaces of CNTs are covered by Ag NPs, which are distributed throughout the CNT films. The presence of Ag metal in the CNT films is proven by EDX analysis (as shown in Figure 3D). The progress of multilayer formation was followed by UVvisible spectroscopy after the deposition of each bilayer on a quartz slide. Figure 4 shows UV-visible absorption spectra of

the self-assembled multilayers upon stepwise deposition of CNTs and PEI-Ag(I). Clearly, there is an absorption peak at 263 nm in each bilayer. The strong absorbance was characteristic of the absorption of the assembled CNTs as described previously,22a indicating the presence of CNTs. With the number of CNT/ PEI-Ag(I) bilayers increasing, the absorption band intensity at 263 nm increases gradually, demonstrating that the multilayered film composed of CNTs and PEI-Ag(I) has been fabricated (solid line in Figure 4). As seen from the inset of Figure 4, the linear increase in absorbance indicates that almost the same amounts of CNTs are assembled in each deposition bilayer, confirming that the film grows uniformly. The dashed line in Figure 4 is the absorbance of (CNT/PEI-Ag(I))8 film after being reduced by NaBH4. There is an additional absorbance at 390 nm, which is characteristic of the absorption of Ag NPs, indicating the formation of Ag NPs in the film. The absorbance at 390 nm is somewhat broad because the molar ratio of PEI to AgNO3 was adjusted to 6:1 to increase the content of Ag in the film; aggregates may occur in the (CNT/PEI-Ag)8 film. In order to confirm the existence of both Ag NPs and CNTs in the resulting (CNT/Ag NP)n hybrid multilayered films, XPS experiments were employed for the surface analysis of the sample (Figure 5). Figure 5a is the XPS spectrum of the (CNT/Ag NP)6 film in a wide scan. Figure 5b shows the high-resolution XPS spectrum of Ag in the multilayered film. The peaks at 367.7 ( 0.2 eV and 373.5 ( 0.2 eV are assigned to Ag 3d5/2 and Ag 3d3/2, respectively, indicating the existence of Ag NPs in the film. The C 1s peak is also detected at a binding energy of 284.6 eV, which is assigned to the C component of CNTs (Figure 5c). The resolved XPS peak of the binding energy assigned to the N 1s component of PEI is detected at 400.1 ( 0.2 eV as shown in Figure 5d. The incorporation of metal NPs in the multilayered film of CNTs brings some novel properties such as optics, catalysis, and so on, which are absent in the simple CNT/PE multilayer. In addition, Ag NPs show good SERS enhancement to the probe molecules, and the enhancement is related to the aggregation degree of the NPs. It has been demonstrated that intentional aggregation of gold and silver NPs significantly increases the observed SERS enhancement.34 After adjusting the Ag content (34) (a) Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. A 2003, 107, 3372. (b) Faulds, K.; Littleford, R. E.; Graham, D.; Dent, G.; Smith, W. E. Anal. Chem. 2004, 76, 592.

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Figure 5. XPS data of the (CNT/Ag NP)6 multilayered film: (a) wide scan spectrum, (b) Ag 3d, (c) C 1s, and (d) N 1s.

in the PEI-Ag(I) complex, some bigger and highly aggregated Ag NP-supported CNT films can be fabricated, which can be used as substrates for increased SERS enhancement. 4-ATP is selected as the probe molecule. Figure 6A compares the SERS spectra acquired from the (CNT/Ag NP)6 films (PEI-Ag(I) with molar ratio of 3:1 was used) and that from the (CNT/PEI)6 films. In the (CNT/Ag NP)6 films, Ag NPs with sizes of 30-50 nm are highly aggregated along the surface of CNTs and distributed all over the surface (Figure 6C), but are absent in the (CNT/PEI)6 films (Figure 6B). Obviously, an enhanced SERS signal of 4-ATP is observed in the (CNT/Ag NP)6 films (Figure 6A, curve b), while in the (CNT/PEI)6 films, only two peaks at 1586 cm-1 (G band), and 1356 cm-1 (D band) are observed, which are ascribed to the characteristic Raman peaks of CNTs (curve a). In Figure 6A (curve b), the SERS spectrum of 4-ATP on the (CNT/Ag NP)6 films is similar to that reported on the Ag surface,35 in which the intensities of four b2 modes at 1580, 1436, 1390, and 1144 cm-1 and of one a1 mode at 1080 cm-1 increase. The corresponding intensity of the b2 mode increases, which is associated with the charge transfer of the metal to the adsorbed molecules. As reported, the obvious enhancement of b2 modes at the visible light excitation was interpreted in terms of the metal-to-molecule charge transfer (CT) theory,35 while the electromagnetic effect (EM) cannot be neglected because the a1 mode (1080 cm-1) was also enhanced. The electromagnetic effect may be derived from the localized surface plasmon resonance of the Ag NPs encapsulated in the CNTs film. The introduction of Ag NPs into the CNT films gives the CNT films a new property, namely, the application in SERS that is unattainable in the pure CNTs films. (35) (a) Zheng, J. W.; Zhou, Y. G.; Li, X. W.; Ji, Y.; Lu, T. H.; Gu, R. A. Langmuir 2003, 19, 632. (b) Osawa, M.; Matsuda, N.; Yoshii, K.; Uchida, I. J. Phys. Chem. 1994, 98, 12702. (36) (a) Garcia-Martinez, J. C.; Lezutekong, R.; Crooks, R. M. J. Am. Chem. Soc. 2005, 127, 5097. (b) Wilson, O. M.; Knecht, M. R.; Garcia-Martinez, J. C.; Crooks, R. M. J. Am. Chem. Soc. 2006, 128, 4510. (c) Oh, S.-K.; Niu, Y.; Crooks, R. M. Langmuir 2005, 21, 10209. (d) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181.

Figure 6. (A) The SERS spectra of 4-ATP on the (CNT/PEI)6 multilayered film (a) and on the (CNT/Ag NP)6 multilayered film (b). TEM images of (CNT/PEI)6 multilayered film (B) and (CNT/ Ag NP)6 multilayered film (C) used for SERS.

Metal NPs are especially attractive for catalysis because of their high surface area-to-mass ratio.36 Pt and Pd NPs are good catalysts for the O2 reduction reaction.37 The CNTs/Pt NP and

1D and 3D CNT/Metal Nanoparticle Composites

Langmuir, Vol. 23, No. 11, 2007 6357

Figure 7. (A) CVs obtained at (CNT/PEI)6 film modified ITO electrode (dotted line) in air-saturated 0.1 M KCl solution and (CNT/Pd NP)6 film modified ITO electrode in the N2-saturated (dashed line) and air-saturated (solid line) 0.1 M KCl solutions. Scan rate: 100 mV/s. (B) Cyclic voltammograms at (CNT/PEI)6 film (dotted line), (CNT/Pt NP)6 film electrode in the O2-saturated (solid line) and N2-saturated (dashed line) 0.5 M H2SO4. Scan rate: 100 mV/s.

CNTs/Pd NP films have electrocatalytic properties for the reduction of O2. Figure 7A shows the cyclic voltammograms (CVs) obtained for the reduction of O2 with (CNT/Pd NP)6 and (CNT/PEI)6 multilayered film modified ITO electrode in a 0.1 M KCl solution. The doted line is the CV of the (CNT/PEI)6 films in air-saturated 0.1 M KCl solution. A low catalytic current is observed, which arises from the catalysis of CNTs to O2. The dashed line corresponds to CV of the (CNT/Pd NP)6 films in a N2-saturated 0.1 M KCl solution. In the presence of O2, a remarkable catalytic reduction peak current occurs at -0.52 V, which indicates that the electrocatalytic activity of the films is greatly enhanced upon the encapsulation of Pd NPs in the CNT films. Figure 7B shows the cyclic voltammogram (CV) of (CNT/ Pt NP)6 and (CNT/PEI)6 film electrodes in a 0.5 M H2SO4 solution. The doted line corresponds to CV of the (CNT/PEI)6 films in an O2-saturated 0.5 M H2SO4 solution. Little reduction current appears from +0.8 to 0 V. The dashed line corresponds to CV of the (CNT/Pt NP)6 films in a N2-saturated 0.5 M H2SO4 solution. In the presence of O2, a remarkable catalytic reduction peak current occurs at 0.29 V, which indicates that the electrocatalytic activity of the films is greatly enhanced upon the encapsulation of Pt NPs in the CNTs film. The combination of Pd or Pt NPs (37) (a) Ye, H.; Crooks, R. M. J. Am. Chem. Soc. 2005, 127, 4930. (b) Shen, Y.; Bi, L.; Liu, B.; Dong, S. New J. Chem. 2003, 27, 938. (c) Liu, J.; Cheng, L.; Song, Y.; Liu, B.; Dong, S. Langmuir 2001, 17, 6747.

with CNT films has the advantage of good electrocatalytic properties over CNT/PEI films and electrical conductivity over polyelectrolyte PEI/PAA films.

Conclusions In summary, the ability of PEI to form complex ions with Ag(I), Pt(II), and Pd(II) provides a versatile tool for the formation of CNT/M NP heterogeneous nanostructures and NP-containing CNT mutilayered films. Adsorption of the PEI-M complexes on the surface of CNTs and subsequent reduction of the metal ions readily facilitated the fabrication of variable CNT/M NP complexes. Alternating adsorption of PEI-M complexes and CNTs on the substrate results in the formation of multilayered CNT films. Reduction of the metal ions by exposure to NaBH4 then yields composite films containing metal NPs. The incorporation of metal NPs into the CNT films forms a new functional nanostructure that is superior to the (CNT/PE)n film in the applications of catalysis and SERS. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20575064 and 20427003). LA063246B