- Email: [email protected]
Structural and electrochemical properties of gold-deposited carbon nanotube composites
Current Applied Physics 10 (2010) S201–S205
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Structural and electrochemical properties of gold-deposited carbon nanotube composites Jungsoo Kim a, Mohammad Mahbub Rabbani a, Dongmin Kim b, Moonhor Ree b, Jeong Hyun Yeum c, Chang Hyun Ko d, Yangdo Kim e, Jong-Seong Bae f,*, Weontae Oh a,* a
Department of NanoEngineering, Electronic Ceramics Center, Dong-Eui University, Busan 614-714, Republic of Korea Department of Chemistry, National Research Lab for Polymer Synthesis and Physics, Pohang Accelerator Laboratory, Center for Integrated Molecular Systems, Polymer Research Institute, and BK School of Molecular Science, POSTECH, Pohang 790-784, Republic of Korea c Department of Natural Fiber Science, Kyungpook National University, Daegu 702-701, Republic of Korea d Greenhous Gas Research Center, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea e School of Material Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea f Busan Center, Korea Basic Science Institute, Busan 609-735, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 21 December 2008 Accepted 31 July 2009 Available online 20 November 2009 Keywords: Carbon nanotubes Gold nanoparticles Composite Electrostatic interaction Energy band gap
a b s t r a c t Gold-decorated multi-walled carbon nanotubes were prepared by electrostatic interaction in solution states and they have been studied in detail with UV/Vis spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). Gold nanoparticles and their composites showed the similar surface plasmon absorptions. XPS results were in good agreement with the composite morphologies of TEM images. HOMO/LUMO energies and the energy band gaps of MWNTs in their composites were analyzed by CV measurements. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) are a steady candidate in active research fields for the advanced materials because of their outstanding mechanical, chemical and electrical properties [1]. However, the intrinsic characteristics of high agglomeration caused by strong attractive van der Waals interaction among CNTs is a crucial obstacle for many industrial and academic applications [2–5]. To overcome this problem, the CNTs bundled in their native state should be treated either with strong acids or some surfactants to improve their solubility and processibility [6,7]. Through these treatments, the sidewalls of CNTs are chemically or physically modified by ionic polar functional groups. In order to improve the feasibility of CNTs, their sidewalls could further be treated by chemical and/or physical methods [6–8]. Nanoparticles of metal, metal oxide, and semiconductor are also of great interest due to their potential applications in microelectronics, optoelectronics, catalysis, and information storage [9–
* Corresponding authors. Tel.: +82 51 510 3944; fax: +82 51 517 2497 (J.-S. Bae), tel.: +82 51 890 1721; fax: +82 51 890 1714 (W. Oh). E-mail addresses: [email protected] (J.-S. Bae), [email protected] (W. Oh). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.07.034
11]. The unique characteristics of nanoparticles are considerably dependent on their sizes and shapes. A versatile building block for the construction of modern nanodevices can be provided by combining these representative nanostructured materials. For instance, the composites of CNTs and some nanoparticles expand their applications from CNT-assisted catalytic systems to complex conductor networks in microelectronics [1,11]. In particular, nanoparticle-deposited CNTs are important due to their potential applications as the electrode system of solar cell devices [12]. The introduction of nanoparticles to the sidewalls of CNTs is mainly stimulated by their characteristic interfacial interactions, leading to the resultant nano-sized composites [13,14]. In order to prepare the CNT composites in which nanoparticles are well distributed, the soluble CNTs should be prepared prior to the composite preparation. In this work we have studied the structural and electrochemical properties of some gold-decorated multi-walled carbon nanotubes, which were prepared by the electrostatic attractive interaction of gold nanoparticles and CNTs. Gold nanoparticles were stabilized by poly(vinylpyrrolidone) (PVP), sodium dodecyl sulfate (SDS) and poly(sodium 4-styrene sulfonate) (PSS) in water, and multi-walled carbon nanotubes (MWNTs) were modified by poly(diallyldimethylammonium chloride) (PDDA).
S202
J. Kim et al. / Current Applied Physics 10 (2010) S201–S205
2. Experimental
2.3. Characterizations
2.1. Materials
Transmission electron microscopic (TEM) images were obtained in Tecnai F30S-Twin (FEI) microscope. TEM samples were prepared on carbon grids of 400 mesh by dip-coating in dilute solutions (1.0 wt.% solid content). UV/Vis spectra were collected using an S-3100 Scinco spectrophotometer in single-beam mode and in the range of 300–900 nm with water as blank. X-ray photoelectron spectroscopy (XPS) measurements were carried out by VG Scientific Escalab 250 at KBSI (Busan, Korea). Survey and high-resolution spectra were obtained at resolutions of 1 and 0.05 eV, respectively. All of the binding energies were determined with the C1s core level peak at 284.6 eV as a reference. Cyclic voltammetry (CV) was carried out in 0.5 M H2SO4 by using an electrochemical workstation (IM6ex impedance analyzer) with a platinum gauze counter electrode and a Ag/AgCl (3.8 M KCl) reference electrode, and the composite thin films were deposited on the Au-bottomed electrode on a Si wafer. A scan rate of 100 mV/s was used for the measurement. The composite thin films for CV measurement were prepared as follows; a thin film of G–MWNT composite was deposited on a macroporous alumina disc supporter (200 nm pore size) with penetrating G–MWNT solution under suction filtration. This composite thin film was floated on 3 M aqueous NaOH solution to dissolve and remove the alumina supporter. The composite thin film floating on the solution was diluted and washed several times with fresh distilled water. Thereafter, the composite thin film was transferred to a gold-deposited Si wafer.
Multi-walled carbon nanotubes (MWNTs, CMP310F) were supplied from Hanwha Nanotech Co. Ltd. Their diameters and lengths were 3–5 nm and 10–20 lm, respectively. Because of the thinner characteristics of the MWNTs used in this work, the pristine MWNTs were supplied as bundles of tens of individual nanotubes. Hydrogen tetrachloroaurate trihydrate (HAuCl43H2O), sodium borohydride (NaBH4), poly(diallyldimethylammonium chloride) (PDDA, M w 100; 000), poly(sodium 4-styrene sulfonate) (PSS, M w 70; 000), sodium dodecyl sulfate (SDS), and polyvinylpyrrolidone (PVP, M w 10; 000) were purchased from Sigma–Aldrich, and nitric acid (HNO3) from Junsei. All the chemicals were used as received without further purification. Water was distilled using a water purification system (Daihan Labtech Co. Ltd., LWD-108S).
2.2. Preparation of gold–decorated multi-walled carbon nanotubes (G–MWNTs) Pristine MWNTs were treated with a strong nitric acid aqueous solution to carboxylate on the sidewalls of MWNTs, which is a well-known method for the carboxylation of CNTs [15]. Further purification of CNTs was conducted simultaneously during this acid treatment. MWNTs (300 mg) were added to 60 mL nitric acid aqueous solution, in which the concentration of nitric acid was adjusted to 30% in water. The MWNT-dispersed solution was sonicated at ambient condition for 3 h, and further reacted on reflux condition at 100 °C for 5 h. The carboxylated MWNTs (c-MWNTs) were diluted and washed several times with water. These procedures were repeated until the c-MWNT dispersion in water was neutralized to pH value of 6–7. c-MWNTs were further treated to prepare c-MWNTs wrapped with PDDA (p-MWNTs) [16]; 16 mg of c-MWNTs were added to 20 mL aqueous solution of 8% PDDA and sonicated for 1 h to make a homogeneous dispersion in the aqueous solution. Thereafter, the unreacted excess PDDA molecules were removed by repeating centrifugation and washing several times with water. The resultant products were carboxylated MWNTs wrapped with PDDA (pMWNTs) and they were dispersed in water before use. Gold nanoparticles (GNPs) stabilized by surfactants were synthesized by the method described in a literature [17]; 55.8 mg PVP was dissolved in 150 mL water. 70.8 mg (0.18 mmol) HAuCl43H2O was added to the above aqueous PVP solution with vigorous stirring. After 10 min of reaction, NaBH4 aqueous solution (0.712 mmol, 10 mL) was added to the reaction mixture with stirring. Then the color of the final mixture solution immediately changed from yellow to purple due to the reduction of gold ions. The reaction was stopped after further reaction for 1 h with stirring. Unreacted chemicals or byproducts were washed out with water after the completion of reaction. The same procedures mentioned above were separately used to prepare the other gold nanoparticles stabilized by SDS and PSS. In order to prepare the composites of GNPs stabilized by PVP (PVP–GNPs) and p-MWNTs, 10 mL PVP–GNPs (0.44 mg/mL) solution was mixed with 10 mL p-MWNTs (0.80 mg/mL), and the mixture solution was stirred for 2 h at ambient temperature. Centrifugation and washing with water were repeated three times to purify the final composite of PVP–GNPs and p-MWNTs (G(PVP)– MWNTs). The same protocol was separately conducted to prepare the G–MWNT composites with PSS–GNPs and SDS–GNPs. PSS– and SDS–GNPs denote gold nanoparticles stabilized by PSS and SDS, respectively.
3. Results and discussion Fig. 1 shows the absorption spectra of gold nanoparticles stabilized by various surfactants, and G–MWNT composites. All of the GNPs showed the characteristic surface plasmon bands at 515 nm. The absorption bands were not changed in G–MWNT composites except for absorption weakening. This result indicates that there was no change in structural characteristics of GNPs in their composites. A little broadening of the band shapes might be caused by the weak electrostatic interfacial interaction between GNPs and MWNTs [18]. It was found from UV/Vis absorption results that all of the gold nanoparticles were well synthesized in solution state and their structural characteristics were similar to each other. The nanostructures and the morphologies of GNPs and their composites with p-MWNTs were deliberately investigated by TEM images as shown in Fig. 2. It was found from these images that their sizes were diversely distributed from 5 to 10 nm, and inhomogeneous. This explanation was different from the discussion of UV/Vis results above. This might be caused by the difference of the agglomeration tendency of the gold nanoparticles during the drying process of composites. In particular, SDS–GNPs of 20 nm sizes were identified in the image. However, the structure shapes of all the GNPs were almost similar to each other regardless of the surfactants. These large particles were caused by the interparticular agglomeration of SDS–GNPs, indicating that SDS is relatively less effective than the other surfactants to stabilize gold nanoparticles. The morphologies of G–MWNT composites were compared in Fig. 2d–f. The contact surfaces of p-MWNTs with nanoparticles were considerably restricted due to the characteristics of very thin walls (<5 nm diameters) and high curvatures. Therefore, the morphologies of the composites might be changed by a small variation of GNP–MWNT interaction [19–23]. The diameters of p-MWNTs were estimated to be 10 nm in the images. The thickening of their diameters was caused by the wrapping of the surfactants on the sidewalls of MWNTs. In the G(SDS)–MWNT composite, some GNPs were aggregated to be 100 nm scale due
J. Kim et al. / Current Applied Physics 10 (2010) S201–S205
S203
(a) (d)
Intensity (a.u.)
(b) (e) (c) (f)
Wavelength (nm)
Wavelength (nm)
Fig. 1. UV/Vis absorption spectra of gold nanoparticles and their composites with p-MWNTs; (a) PVP–GNPs, (b) PSS–GNPs, (c) SDS–GNPs, (d) G(PVP)–MWNTs, (e) G(PSS)– MWNTs, and (f) G(SDS)–MWNTs. All of the spectra were collected in water solutions. PVP–, PSS–, and SDS–GNPs denote the gold nanoparticles stabilized by PVP, PSS, and SDS, respectively. G(PVP)–, G(PSS), and G(SDS)–MWNTs represent the p-MWNT composites with PVP–GNPs, PSS–GNPs, and SDS–GNPs, respectively.
Fig. 2. TEM images of gold nanoparticles and their composites with p-MWNTs; (a) SDS–GNPs, (b) PSS–GNPs, (c) PVP–GNPs, (d) G(SDS)–MWNTs, (e) G(PSS)–MWNTs, and (f) G(PVP)–MWNTs.
to the high agglomeration tendency of SDS–GNPs and the GNPs were not evenly distributed on the sidewalls of p-MWNTs. GNPs in G(PSS)–MWNTs were poorly distributed in p-MWNTs and some large agglomerations were also found in the composite. GNPs were well distributed only in the G(PVP)–MWNTs. Unlike SDS and PSS, PVP stabilized GNPs and enhanced the interaction between GNPs and MWNTs simultaneously. These morphological changes were obviously caused by the variation of surfactants stabilizing GNPs. According to our previous study, surfactants in the composite of nanoparticles and MWNTs give an effect on its morphological change [24]. The similar explanation could be given to describe the TEM results in this work; the differences of ionic and non-ionic polar functional groups, and the interfacial charging balances between counter-polar groups can cause the morphological changes.
Further discussion will be given in the following CV and XPS studies. Cyclic voltammetry is an effective method for probing the electrochemical behaviors of a G–MWNT composite system and analyzing the mechanism of oxidation–reduction. Fig. 3 shows the CV curves of p-MWNTs and G–MWNT composite thin films in a 0.5 M H2SO4 solution at a scan rate of 100 mV/s. The characteristic oxidation (1.5 V) and reduction (1.0 V) peaks of gold were identified in CV curves through a complete cyclic scan, and their exact positions were slightly shifted with change of GNPs. Accordingly with this change, oxidation–reduction peaks of p-MWNTs in the composites were enhanced. The enlarged A and B of Fig. 3 represent the oxidation–reduction positions of p-MWNTs in the composites, respectively. These chemically-treated p-MWNTs showed
S204
J. Kim et al. / Current Applied Physics 10 (2010) S201–S205
Fig. 3. Cyclic voltammograms of the G–MWNT composite films. The measurements were conducted in a 0.5 M H2SO4 solution at a scan rate of 100 mV/s. The p-MWNTs used in this work were carboxylated and then wrapped with PDDA.
a pair of the weak oxidation–reduction peaks in the region 0.4– 0.5 V. This might be caused by electron transfer between carbon and oxygen on the surface of c-MWNTs [25]. As the p-MWNTs formed a composite with GNPs, the oxidation–reduction peaks of p-MWNTs shifted towards the higher voltages. In addition, their peak positions of oxidation–reduction were changed with variation of the surfactants stabilizing GNPs. For instance, the reduction peaks were shifted towards the higher position in the order of pMWNTs < G(SDS)–MWNTs < G(PSS)–MWNTs < G(PVP)–MWNTs. Although the order of oxidation peak positions was unclear in the case of G(PSS)–MWNTs and G(PVP)–MWNTs, the shift trend of oxidation position was, as a whole, similar to the reduction. HOMO/ LUMO energy levels could be analyzed from the CV results by using ferrocene standard [26–29]. In opposite to the order of peak positions on oxidation–reduction, HOMO and LUMO energy levels of pMWNTs in G–MWNT composites were changed in the order of pMWNTs > G(SDS)–MWNTs > G(PSS)–MWNTs > G(PVP)–MWNTs. Their energy band gaps could also be estimated with these HOMO/ LUMO energies. The energy band gaps slightly decreased in the order of p-MWNTs > G(SDS)–MWNTs > G(PSS)–MWNTs > G(PVP)– MWNTs. Conclusively, these CV results show that the interfacial interaction between gold nanoparticles and MWNTs can induce the re-distribution of the surface electrons in G–MWNT compos-
ites, and correspondingly change the HOMO/LUMO energy levels and the energy band gaps. The comparison of the binding energies of gold atoms in G– MWNT composites is another evidence for the interfacial electronic interaction between elements of the composite as shown in XPS spectra of Fig. 4. Au 4f peaks of G–MWNT composites were compared with the Au 4f peaks of the corresponding GNPs. Au 4f peaks in G(PVP)–MWNT composite were shifted towards the higher binding energies than the corresponding PVP–GNP, while those in G(PSS)–MWNTs and G(SDS)–MWNTs were shifted towards the lower binding energies than their corresponding PSS–GNP and SDS–GNP, respectively. These XPS results suggested that the surface electrons of gold nanoparticles in the composites were re-distributed and changed their binding energies due to the interfacial interaction between GNPs and p-MWNTs. Our previous work has reported that Au 4f peaks shifted towards the lower binding energies when the positively-charged GNPs formed a homogeneous composite with the negatively-charged MWNTs [24]. The lower shift of binding energy is caused by strengthening of the electrical contact [14,30]. However, the G–MWNT composites in this work consisted of the negatively-charged GNPs and the positivelycharged MWNTs. In this case, the lower shift of binding energy implies that the electrical contact is weakened in the G–MWNT
(b)
(c)
Intensity (CPS)
(a)
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
Fig. 4. Au 4f peaks of XPS spectra collected from G–MWNT composites; (a) G(SDS)–MWNTs, (b) G(PSS)–MWNTs, and (c) G(PVP)–MWNTs. Red and blue lines represent GNP peaks measured in the G–MWNT composites and the corresponding GNPs, respectively. The higher and the lower peaks of two sharp binding energies represent Au 4f5/2 and Au 4f7/2, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
J. Kim et al. / Current Applied Physics 10 (2010) S201–S205
composites because the charged characteristics of MWNTs and GNPs are reversed to our previous case [24]. Actually the binding energies of Au 4f peaks in G(SDS)–MWNTs and G(PSS)–MWNTs were shifted towards the lower energies than those of the corresponding SDS–GNPs and PSS–GNPs, respectively. On the other hand, the binding energies of Au 4f peaks in G(PVP)–MWNTs were shifted towards the higher energies. These XPS results indicate that gold nanoparticles can be poorly distributed in G(SDS)–MWNT and G(PSS)–MWNT composites, but gold nanoparticles can be homogeneously distributed in G(PVP)–MWNT composite. This estimation obtained from XPS results agreed with TEM images as described above. 4. Conclusions Gold–decorated MWNT composites were prepared with GNPs stabilized by various surfactants and MWNTs treated with PDDA, and their structural and electrochemical properties were studied in detail. Gold nanoparticles were stabilized by SDS, PSS, and PVP and all of the nanoparticles were negatively charged in aqueous solution. On the other hand, c-MWNTs wrapped with PDDA (pMWNTs) were positively charged on their sidewalls. UV/Vis spectra collected in solution states represent that the characteristic absorption bands ware related to the surface plasmons of gold nanoparticles, and were similar to each other regardless of the species of surfactants. TEM images demonstrated that gold nanoparticles were well distributed in p-MWNTs only in G(PVP)–MWNT composite. GNPs were poorly distributed in the other G–MWNT composites. These TEM analyses for the composite morphologies well agreed with XPS results. It was found through CV studies on the composites that the oxidation–reduction behaviors of MWNTs were dependent on their composite conditions. In addition, HOMO/LUMO energies and the energy band gaps of MWNTs in their composites were analyzed by CV measurements. Acknowledgement This work was supported by KBSI Grant (T29608) to J.S. Bae. References [1] E. Unger, G.S. Duesberg, M. Liebau, A.P. Graham, R. Seidel, F. Kreupl, W. Hoenlein, Decoration of multi-walled carbon nanotubes with noble- and transition-metal clusters and formation of CNT–CNT Networks, Appl. Phys. A: Mater. Sci. Process. 77 (2003) 735–738. [2] W. Zhao, C. Song, P.E. Pehrsson, Water-soluble and optically pH-sensitive single-walled carbon nanotubes from surface modification, J. Am. Chem. Soc. 124 (2002) 12418–12419. [3] A. Star, J.F. Stoddart, Dispersion and solubilization of single-walled carbon nanotubes with a hyperbranched polymer, Macromolecules 35 (2002) 7516– 7520. [4] A. Thess, R. Lee, P. Nikolaev, H.J. Dai, P. Petit, J. Robert, C.H. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fischer, R.E. Smalley, Crystalline ropes of metallic carbon nanotubes, Science 273 (1996) 483–487. [5] K.D. Ausman, R. Piner, O. Lourie, R.S. Ruoff, M. Korobov, Organic solvent dispersions of single-walled carbon nanotubes: toward solutions of pristine nanotubes, J. Phys. Chem. B 104 (2000) 8911–8915.
S205
[6] A. Hirsch, Functionalization of single-walled carbon nanotubes, Angew. Chem. Int. Ed. 41 (2002) 1853–1859. [7] C.A. Dyke, J.M. Tour, Overcoming the insolubility of carbon nanotubes through high degrees of sidewall functionalization, Chem. Eur. J. 10 (2004) 812–817. [8] D. Tasis, N. Tagmatarchis, V. Georgakilas, M. Prato, Soluble carbon nanotubes, Chem. Eur. J. 9 (2003) 4000–4008. [9] D.L. Feldheim, C.A. Foss, Metal Nanoparticles: Synthesis, Characterization and Applications, Dekker, New York, 2000. [10] K.R. Gopidas, J.K. Whitesell, M.A. Fox, Synthesis, characterization, and catalytic applications of a palladium-nanoparticle-cored dendrimer, Nano Lett. 3 (2003) 1757–1760. [11] V. Georgakilas, D. Gournis, V. Tzitzios, L. Pasquato, D.M. Guldi, M. Prato, Decorating carbon nanotubes with metal or semiconductor nanoparticles, J. Mater. Chem. 17 (2007) 2679–2694. [12] G.G. Wildgoose, C.E. Banks, R.G. Compton, Metal nanoparticles and related materials supported on carbon nanotubes: methods and applications, Small 2 (2006) 182–193. [13] D.-Q. Yang, E. Sacher, Interaction of evaporated nickel nanoparticles with highly oriented pyrolytic graphite: back-bonding to surface defects, as studied by X-ray photoelectron spectroscopy, J. Phys. Chem. B 109 (2005) 19329– 19334. [14] C. Bittencourt, A. Felten, B. Douhard, J.-F. Colomer, G.V. Tendeloo, W. Drube, J. Ghijisen, J.-J. Pireaux, Metallic nanoparticles on plasma treated carbon nanotubes: nano2hybrids, Surf. Sci. 601 (2007) 2800–2804. [15] G.-W. Lee, J. Kim, J. Yoon, J.-S. Bae, B.C. Shin, I.S. Kim, W. Oh, M. Ree, Structural characterization of carboxylated multi-walled carbon nanotubes, Thin Solid Films 516 (2008) 5781–5784. [16] D. Wen, Y. Liu, G.Y., S. Donga, Electrochemistry of glucose oxidase immobilized on the carbon nanotube wrapped by polyelectrolyte, Electrochim. Acta 52 (2007) 5312–5317. [17] E. Hao, T. Lian, Buildup of polymer/Au nanoparticle multilayer thin films based on hydrogen bonding, Chem. Mater. 12 (2000) 3392–3396. [18] S. Link, M.A. El-Sayed, Size and temperature dependence of the plasmon absorption of colloidal gold nanoparticles, J. Phys. Chem. B 103 (1999) 4212– 4217. [19] Y. Liu, J. Tang, X. Chen, W. Chen, G.K.H. Pang, J.H. Xin, A wet-chemical route for the decoration of CNTs with silver nanoparticles, Carbon 44 (2006) 381– 392. [20] Y. Wang, X. Xu, Z. Tian, Y. Zong, H. Cheng, C. Lin, Selective heterogeneous nucleation and growth of size-controlled metal nanoparticles on carbon nanotubes in solution, Chem. Eur. J. 12 (2006) 2542–2549. [21] M.S. Raghuveer, S. Agrawal, N. Bishop, G. Ramanath, Microwave-assisted single-step functionalization and in situ derivatization of carbon nanotubes with gold nanoparticles, Chem. Mater. 18 (2006) 1390–1393. [22] B. Yoon, C.M. Wai, Microemulsion-templated synthesis of carbon nanotubesupported Pd and Rh nanoparticles for catalytic applications, J. Am. Chem. Soc. 127 (2005) 17174–17175. [23] R. Yu, L. Chen, Q. Liu, J. Lin, K.-L. Tan, S.C. Ng, H.S.O. Chan, G.-Q. Xu, T.S.A. Hor, Platinum deposition on carbon nanotubes via chemical modification, Chem. Mater. 10 (1998) 718–722. [24] M.M. Rabbani, C.H. Ko, J.-S. Bae, J.H. Yeum, I.S. Kim, W. Oh, Comparison of some gold/carbon nanotube composites prepared by control of electrostatic interaction, Colloids Surf. A: Physicochem. Eng. Aspects 336 (2009) 183–186. [25] N. Alexeyeva, K. Tammeveski, Electroreduction of oxygen on gold nanoparticle/PDDA–MWCNT nanocomposites in acid solution, Anal. Chim. Acta 618 (2008) 140–146. [26] G. Gritzner, J. Kuta, Recommendations on reporting electrode potentials in nonaqueous solvents, J. Pure Appl. Chem. 56 (1984) 461–466. [27] J.L. Bredas, R. Silbey, D.S. Boudreaux, P.R. Chance, Chain-length dependence of electronic and electrochemical properties of conjugated systems: polyacetylene, polyphenylene, polythiophene, and polypyrrole, J. Am. Chem. Soc. 105 (1983) 6555–6559. [28] J.H. Kim, H. Lee, Efficient poly(p-phenylenevinylene) derivative with 1,2diphenyl-20 -cyanoethene for single layer light-emitting diodes, Synth. Met. 139 (2003) 471–478. [29] J.H. Kim, J.H. Park, H. Lee, Highly efficient novel poly(p-phenylenevinylene) derivative with 1,3,4-oxadiazole pendant on a vinylene unit, Chem. Mater. 15 (2003) 3414–3416. [30] C. Bitterncourt, A. Felten, B. Douhard, J. Ghijsen, R.L. Johnson, W. Drube, J.-J. Pireaux, Photoemission studies of gold clusters thermally evaporated on multiwall carbon nanotubes, Chem. Phys. 328 (2006) 385–391.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Structural and electrochemical properties of gold-deposited carbon nanotube composites Jungsoo Kim a, Mohammad Mahbub Rabbani a, Dongmin Kim b, Moonhor Ree b, Jeong Hyun Yeum c, Chang Hyun Ko d, Yangdo Kim e, Jong-Seong Bae f,*, Weontae Oh a,* a
Department of NanoEngineering, Electronic Ceramics Center, Dong-Eui University, Busan 614-714, Republic of Korea Department of Chemistry, National Research Lab for Polymer Synthesis and Physics, Pohang Accelerator Laboratory, Center for Integrated Molecular Systems, Polymer Research Institute, and BK School of Molecular Science, POSTECH, Pohang 790-784, Republic of Korea c Department of Natural Fiber Science, Kyungpook National University, Daegu 702-701, Republic of Korea d Greenhous Gas Research Center, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea e School of Material Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea f Busan Center, Korea Basic Science Institute, Busan 609-735, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 21 December 2008 Accepted 31 July 2009 Available online 20 November 2009 Keywords: Carbon nanotubes Gold nanoparticles Composite Electrostatic interaction Energy band gap
a b s t r a c t Gold-decorated multi-walled carbon nanotubes were prepared by electrostatic interaction in solution states and they have been studied in detail with UV/Vis spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). Gold nanoparticles and their composites showed the similar surface plasmon absorptions. XPS results were in good agreement with the composite morphologies of TEM images. HOMO/LUMO energies and the energy band gaps of MWNTs in their composites were analyzed by CV measurements. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) are a steady candidate in active research fields for the advanced materials because of their outstanding mechanical, chemical and electrical properties [1]. However, the intrinsic characteristics of high agglomeration caused by strong attractive van der Waals interaction among CNTs is a crucial obstacle for many industrial and academic applications [2–5]. To overcome this problem, the CNTs bundled in their native state should be treated either with strong acids or some surfactants to improve their solubility and processibility [6,7]. Through these treatments, the sidewalls of CNTs are chemically or physically modified by ionic polar functional groups. In order to improve the feasibility of CNTs, their sidewalls could further be treated by chemical and/or physical methods [6–8]. Nanoparticles of metal, metal oxide, and semiconductor are also of great interest due to their potential applications in microelectronics, optoelectronics, catalysis, and information storage [9–
* Corresponding authors. Tel.: +82 51 510 3944; fax: +82 51 517 2497 (J.-S. Bae), tel.: +82 51 890 1721; fax: +82 51 890 1714 (W. Oh). E-mail addresses: [email protected] (J.-S. Bae), [email protected] (W. Oh). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.07.034
11]. The unique characteristics of nanoparticles are considerably dependent on their sizes and shapes. A versatile building block for the construction of modern nanodevices can be provided by combining these representative nanostructured materials. For instance, the composites of CNTs and some nanoparticles expand their applications from CNT-assisted catalytic systems to complex conductor networks in microelectronics [1,11]. In particular, nanoparticle-deposited CNTs are important due to their potential applications as the electrode system of solar cell devices [12]. The introduction of nanoparticles to the sidewalls of CNTs is mainly stimulated by their characteristic interfacial interactions, leading to the resultant nano-sized composites [13,14]. In order to prepare the CNT composites in which nanoparticles are well distributed, the soluble CNTs should be prepared prior to the composite preparation. In this work we have studied the structural and electrochemical properties of some gold-decorated multi-walled carbon nanotubes, which were prepared by the electrostatic attractive interaction of gold nanoparticles and CNTs. Gold nanoparticles were stabilized by poly(vinylpyrrolidone) (PVP), sodium dodecyl sulfate (SDS) and poly(sodium 4-styrene sulfonate) (PSS) in water, and multi-walled carbon nanotubes (MWNTs) were modified by poly(diallyldimethylammonium chloride) (PDDA).
S202
J. Kim et al. / Current Applied Physics 10 (2010) S201–S205
2. Experimental
2.3. Characterizations
2.1. Materials
Transmission electron microscopic (TEM) images were obtained in Tecnai F30S-Twin (FEI) microscope. TEM samples were prepared on carbon grids of 400 mesh by dip-coating in dilute solutions (1.0 wt.% solid content). UV/Vis spectra were collected using an S-3100 Scinco spectrophotometer in single-beam mode and in the range of 300–900 nm with water as blank. X-ray photoelectron spectroscopy (XPS) measurements were carried out by VG Scientific Escalab 250 at KBSI (Busan, Korea). Survey and high-resolution spectra were obtained at resolutions of 1 and 0.05 eV, respectively. All of the binding energies were determined with the C1s core level peak at 284.6 eV as a reference. Cyclic voltammetry (CV) was carried out in 0.5 M H2SO4 by using an electrochemical workstation (IM6ex impedance analyzer) with a platinum gauze counter electrode and a Ag/AgCl (3.8 M KCl) reference electrode, and the composite thin films were deposited on the Au-bottomed electrode on a Si wafer. A scan rate of 100 mV/s was used for the measurement. The composite thin films for CV measurement were prepared as follows; a thin film of G–MWNT composite was deposited on a macroporous alumina disc supporter (200 nm pore size) with penetrating G–MWNT solution under suction filtration. This composite thin film was floated on 3 M aqueous NaOH solution to dissolve and remove the alumina supporter. The composite thin film floating on the solution was diluted and washed several times with fresh distilled water. Thereafter, the composite thin film was transferred to a gold-deposited Si wafer.
Multi-walled carbon nanotubes (MWNTs, CMP310F) were supplied from Hanwha Nanotech Co. Ltd. Their diameters and lengths were 3–5 nm and 10–20 lm, respectively. Because of the thinner characteristics of the MWNTs used in this work, the pristine MWNTs were supplied as bundles of tens of individual nanotubes. Hydrogen tetrachloroaurate trihydrate (HAuCl43H2O), sodium borohydride (NaBH4), poly(diallyldimethylammonium chloride) (PDDA, M w 100; 000), poly(sodium 4-styrene sulfonate) (PSS, M w 70; 000), sodium dodecyl sulfate (SDS), and polyvinylpyrrolidone (PVP, M w 10; 000) were purchased from Sigma–Aldrich, and nitric acid (HNO3) from Junsei. All the chemicals were used as received without further purification. Water was distilled using a water purification system (Daihan Labtech Co. Ltd., LWD-108S).
2.2. Preparation of gold–decorated multi-walled carbon nanotubes (G–MWNTs) Pristine MWNTs were treated with a strong nitric acid aqueous solution to carboxylate on the sidewalls of MWNTs, which is a well-known method for the carboxylation of CNTs [15]. Further purification of CNTs was conducted simultaneously during this acid treatment. MWNTs (300 mg) were added to 60 mL nitric acid aqueous solution, in which the concentration of nitric acid was adjusted to 30% in water. The MWNT-dispersed solution was sonicated at ambient condition for 3 h, and further reacted on reflux condition at 100 °C for 5 h. The carboxylated MWNTs (c-MWNTs) were diluted and washed several times with water. These procedures were repeated until the c-MWNT dispersion in water was neutralized to pH value of 6–7. c-MWNTs were further treated to prepare c-MWNTs wrapped with PDDA (p-MWNTs) [16]; 16 mg of c-MWNTs were added to 20 mL aqueous solution of 8% PDDA and sonicated for 1 h to make a homogeneous dispersion in the aqueous solution. Thereafter, the unreacted excess PDDA molecules were removed by repeating centrifugation and washing several times with water. The resultant products were carboxylated MWNTs wrapped with PDDA (pMWNTs) and they were dispersed in water before use. Gold nanoparticles (GNPs) stabilized by surfactants were synthesized by the method described in a literature [17]; 55.8 mg PVP was dissolved in 150 mL water. 70.8 mg (0.18 mmol) HAuCl43H2O was added to the above aqueous PVP solution with vigorous stirring. After 10 min of reaction, NaBH4 aqueous solution (0.712 mmol, 10 mL) was added to the reaction mixture with stirring. Then the color of the final mixture solution immediately changed from yellow to purple due to the reduction of gold ions. The reaction was stopped after further reaction for 1 h with stirring. Unreacted chemicals or byproducts were washed out with water after the completion of reaction. The same procedures mentioned above were separately used to prepare the other gold nanoparticles stabilized by SDS and PSS. In order to prepare the composites of GNPs stabilized by PVP (PVP–GNPs) and p-MWNTs, 10 mL PVP–GNPs (0.44 mg/mL) solution was mixed with 10 mL p-MWNTs (0.80 mg/mL), and the mixture solution was stirred for 2 h at ambient temperature. Centrifugation and washing with water were repeated three times to purify the final composite of PVP–GNPs and p-MWNTs (G(PVP)– MWNTs). The same protocol was separately conducted to prepare the G–MWNT composites with PSS–GNPs and SDS–GNPs. PSS– and SDS–GNPs denote gold nanoparticles stabilized by PSS and SDS, respectively.
3. Results and discussion Fig. 1 shows the absorption spectra of gold nanoparticles stabilized by various surfactants, and G–MWNT composites. All of the GNPs showed the characteristic surface plasmon bands at 515 nm. The absorption bands were not changed in G–MWNT composites except for absorption weakening. This result indicates that there was no change in structural characteristics of GNPs in their composites. A little broadening of the band shapes might be caused by the weak electrostatic interfacial interaction between GNPs and MWNTs [18]. It was found from UV/Vis absorption results that all of the gold nanoparticles were well synthesized in solution state and their structural characteristics were similar to each other. The nanostructures and the morphologies of GNPs and their composites with p-MWNTs were deliberately investigated by TEM images as shown in Fig. 2. It was found from these images that their sizes were diversely distributed from 5 to 10 nm, and inhomogeneous. This explanation was different from the discussion of UV/Vis results above. This might be caused by the difference of the agglomeration tendency of the gold nanoparticles during the drying process of composites. In particular, SDS–GNPs of 20 nm sizes were identified in the image. However, the structure shapes of all the GNPs were almost similar to each other regardless of the surfactants. These large particles were caused by the interparticular agglomeration of SDS–GNPs, indicating that SDS is relatively less effective than the other surfactants to stabilize gold nanoparticles. The morphologies of G–MWNT composites were compared in Fig. 2d–f. The contact surfaces of p-MWNTs with nanoparticles were considerably restricted due to the characteristics of very thin walls (<5 nm diameters) and high curvatures. Therefore, the morphologies of the composites might be changed by a small variation of GNP–MWNT interaction [19–23]. The diameters of p-MWNTs were estimated to be 10 nm in the images. The thickening of their diameters was caused by the wrapping of the surfactants on the sidewalls of MWNTs. In the G(SDS)–MWNT composite, some GNPs were aggregated to be 100 nm scale due
J. Kim et al. / Current Applied Physics 10 (2010) S201–S205
S203
(a) (d)
Intensity (a.u.)
(b) (e) (c) (f)
Wavelength (nm)
Wavelength (nm)
Fig. 1. UV/Vis absorption spectra of gold nanoparticles and their composites with p-MWNTs; (a) PVP–GNPs, (b) PSS–GNPs, (c) SDS–GNPs, (d) G(PVP)–MWNTs, (e) G(PSS)– MWNTs, and (f) G(SDS)–MWNTs. All of the spectra were collected in water solutions. PVP–, PSS–, and SDS–GNPs denote the gold nanoparticles stabilized by PVP, PSS, and SDS, respectively. G(PVP)–, G(PSS), and G(SDS)–MWNTs represent the p-MWNT composites with PVP–GNPs, PSS–GNPs, and SDS–GNPs, respectively.
Fig. 2. TEM images of gold nanoparticles and their composites with p-MWNTs; (a) SDS–GNPs, (b) PSS–GNPs, (c) PVP–GNPs, (d) G(SDS)–MWNTs, (e) G(PSS)–MWNTs, and (f) G(PVP)–MWNTs.
to the high agglomeration tendency of SDS–GNPs and the GNPs were not evenly distributed on the sidewalls of p-MWNTs. GNPs in G(PSS)–MWNTs were poorly distributed in p-MWNTs and some large agglomerations were also found in the composite. GNPs were well distributed only in the G(PVP)–MWNTs. Unlike SDS and PSS, PVP stabilized GNPs and enhanced the interaction between GNPs and MWNTs simultaneously. These morphological changes were obviously caused by the variation of surfactants stabilizing GNPs. According to our previous study, surfactants in the composite of nanoparticles and MWNTs give an effect on its morphological change [24]. The similar explanation could be given to describe the TEM results in this work; the differences of ionic and non-ionic polar functional groups, and the interfacial charging balances between counter-polar groups can cause the morphological changes.
Further discussion will be given in the following CV and XPS studies. Cyclic voltammetry is an effective method for probing the electrochemical behaviors of a G–MWNT composite system and analyzing the mechanism of oxidation–reduction. Fig. 3 shows the CV curves of p-MWNTs and G–MWNT composite thin films in a 0.5 M H2SO4 solution at a scan rate of 100 mV/s. The characteristic oxidation (1.5 V) and reduction (1.0 V) peaks of gold were identified in CV curves through a complete cyclic scan, and their exact positions were slightly shifted with change of GNPs. Accordingly with this change, oxidation–reduction peaks of p-MWNTs in the composites were enhanced. The enlarged A and B of Fig. 3 represent the oxidation–reduction positions of p-MWNTs in the composites, respectively. These chemically-treated p-MWNTs showed
S204
J. Kim et al. / Current Applied Physics 10 (2010) S201–S205
Fig. 3. Cyclic voltammograms of the G–MWNT composite films. The measurements were conducted in a 0.5 M H2SO4 solution at a scan rate of 100 mV/s. The p-MWNTs used in this work were carboxylated and then wrapped with PDDA.
a pair of the weak oxidation–reduction peaks in the region 0.4– 0.5 V. This might be caused by electron transfer between carbon and oxygen on the surface of c-MWNTs [25]. As the p-MWNTs formed a composite with GNPs, the oxidation–reduction peaks of p-MWNTs shifted towards the higher voltages. In addition, their peak positions of oxidation–reduction were changed with variation of the surfactants stabilizing GNPs. For instance, the reduction peaks were shifted towards the higher position in the order of pMWNTs < G(SDS)–MWNTs < G(PSS)–MWNTs < G(PVP)–MWNTs. Although the order of oxidation peak positions was unclear in the case of G(PSS)–MWNTs and G(PVP)–MWNTs, the shift trend of oxidation position was, as a whole, similar to the reduction. HOMO/ LUMO energy levels could be analyzed from the CV results by using ferrocene standard [26–29]. In opposite to the order of peak positions on oxidation–reduction, HOMO and LUMO energy levels of pMWNTs in G–MWNT composites were changed in the order of pMWNTs > G(SDS)–MWNTs > G(PSS)–MWNTs > G(PVP)–MWNTs. Their energy band gaps could also be estimated with these HOMO/ LUMO energies. The energy band gaps slightly decreased in the order of p-MWNTs > G(SDS)–MWNTs > G(PSS)–MWNTs > G(PVP)– MWNTs. Conclusively, these CV results show that the interfacial interaction between gold nanoparticles and MWNTs can induce the re-distribution of the surface electrons in G–MWNT compos-
ites, and correspondingly change the HOMO/LUMO energy levels and the energy band gaps. The comparison of the binding energies of gold atoms in G– MWNT composites is another evidence for the interfacial electronic interaction between elements of the composite as shown in XPS spectra of Fig. 4. Au 4f peaks of G–MWNT composites were compared with the Au 4f peaks of the corresponding GNPs. Au 4f peaks in G(PVP)–MWNT composite were shifted towards the higher binding energies than the corresponding PVP–GNP, while those in G(PSS)–MWNTs and G(SDS)–MWNTs were shifted towards the lower binding energies than their corresponding PSS–GNP and SDS–GNP, respectively. These XPS results suggested that the surface electrons of gold nanoparticles in the composites were re-distributed and changed their binding energies due to the interfacial interaction between GNPs and p-MWNTs. Our previous work has reported that Au 4f peaks shifted towards the lower binding energies when the positively-charged GNPs formed a homogeneous composite with the negatively-charged MWNTs [24]. The lower shift of binding energy is caused by strengthening of the electrical contact [14,30]. However, the G–MWNT composites in this work consisted of the negatively-charged GNPs and the positivelycharged MWNTs. In this case, the lower shift of binding energy implies that the electrical contact is weakened in the G–MWNT
(b)
(c)
Intensity (CPS)
(a)
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
Fig. 4. Au 4f peaks of XPS spectra collected from G–MWNT composites; (a) G(SDS)–MWNTs, (b) G(PSS)–MWNTs, and (c) G(PVP)–MWNTs. Red and blue lines represent GNP peaks measured in the G–MWNT composites and the corresponding GNPs, respectively. The higher and the lower peaks of two sharp binding energies represent Au 4f5/2 and Au 4f7/2, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
J. Kim et al. / Current Applied Physics 10 (2010) S201–S205
composites because the charged characteristics of MWNTs and GNPs are reversed to our previous case [24]. Actually the binding energies of Au 4f peaks in G(SDS)–MWNTs and G(PSS)–MWNTs were shifted towards the lower energies than those of the corresponding SDS–GNPs and PSS–GNPs, respectively. On the other hand, the binding energies of Au 4f peaks in G(PVP)–MWNTs were shifted towards the higher energies. These XPS results indicate that gold nanoparticles can be poorly distributed in G(SDS)–MWNT and G(PSS)–MWNT composites, but gold nanoparticles can be homogeneously distributed in G(PVP)–MWNT composite. This estimation obtained from XPS results agreed with TEM images as described above. 4. Conclusions Gold–decorated MWNT composites were prepared with GNPs stabilized by various surfactants and MWNTs treated with PDDA, and their structural and electrochemical properties were studied in detail. Gold nanoparticles were stabilized by SDS, PSS, and PVP and all of the nanoparticles were negatively charged in aqueous solution. On the other hand, c-MWNTs wrapped with PDDA (pMWNTs) were positively charged on their sidewalls. UV/Vis spectra collected in solution states represent that the characteristic absorption bands ware related to the surface plasmons of gold nanoparticles, and were similar to each other regardless of the species of surfactants. TEM images demonstrated that gold nanoparticles were well distributed in p-MWNTs only in G(PVP)–MWNT composite. GNPs were poorly distributed in the other G–MWNT composites. These TEM analyses for the composite morphologies well agreed with XPS results. It was found through CV studies on the composites that the oxidation–reduction behaviors of MWNTs were dependent on their composite conditions. In addition, HOMO/LUMO energies and the energy band gaps of MWNTs in their composites were analyzed by CV measurements. Acknowledgement This work was supported by KBSI Grant (T29608) to J.S. Bae. References [1] E. Unger, G.S. Duesberg, M. Liebau, A.P. Graham, R. Seidel, F. Kreupl, W. Hoenlein, Decoration of multi-walled carbon nanotubes with noble- and transition-metal clusters and formation of CNT–CNT Networks, Appl. Phys. A: Mater. Sci. Process. 77 (2003) 735–738. [2] W. Zhao, C. Song, P.E. Pehrsson, Water-soluble and optically pH-sensitive single-walled carbon nanotubes from surface modification, J. Am. Chem. Soc. 124 (2002) 12418–12419. [3] A. Star, J.F. Stoddart, Dispersion and solubilization of single-walled carbon nanotubes with a hyperbranched polymer, Macromolecules 35 (2002) 7516– 7520. [4] A. Thess, R. Lee, P. Nikolaev, H.J. Dai, P. Petit, J. Robert, C.H. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fischer, R.E. Smalley, Crystalline ropes of metallic carbon nanotubes, Science 273 (1996) 483–487. [5] K.D. Ausman, R. Piner, O. Lourie, R.S. Ruoff, M. Korobov, Organic solvent dispersions of single-walled carbon nanotubes: toward solutions of pristine nanotubes, J. Phys. Chem. B 104 (2000) 8911–8915.
S205
[6] A. Hirsch, Functionalization of single-walled carbon nanotubes, Angew. Chem. Int. Ed. 41 (2002) 1853–1859. [7] C.A. Dyke, J.M. Tour, Overcoming the insolubility of carbon nanotubes through high degrees of sidewall functionalization, Chem. Eur. J. 10 (2004) 812–817. [8] D. Tasis, N. Tagmatarchis, V. Georgakilas, M. Prato, Soluble carbon nanotubes, Chem. Eur. J. 9 (2003) 4000–4008. [9] D.L. Feldheim, C.A. Foss, Metal Nanoparticles: Synthesis, Characterization and Applications, Dekker, New York, 2000. [10] K.R. Gopidas, J.K. Whitesell, M.A. Fox, Synthesis, characterization, and catalytic applications of a palladium-nanoparticle-cored dendrimer, Nano Lett. 3 (2003) 1757–1760. [11] V. Georgakilas, D. Gournis, V. Tzitzios, L. Pasquato, D.M. Guldi, M. Prato, Decorating carbon nanotubes with metal or semiconductor nanoparticles, J. Mater. Chem. 17 (2007) 2679–2694. [12] G.G. Wildgoose, C.E. Banks, R.G. Compton, Metal nanoparticles and related materials supported on carbon nanotubes: methods and applications, Small 2 (2006) 182–193. [13] D.-Q. Yang, E. Sacher, Interaction of evaporated nickel nanoparticles with highly oriented pyrolytic graphite: back-bonding to surface defects, as studied by X-ray photoelectron spectroscopy, J. Phys. Chem. B 109 (2005) 19329– 19334. [14] C. Bittencourt, A. Felten, B. Douhard, J.-F. Colomer, G.V. Tendeloo, W. Drube, J. Ghijisen, J.-J. Pireaux, Metallic nanoparticles on plasma treated carbon nanotubes: nano2hybrids, Surf. Sci. 601 (2007) 2800–2804. [15] G.-W. Lee, J. Kim, J. Yoon, J.-S. Bae, B.C. Shin, I.S. Kim, W. Oh, M. Ree, Structural characterization of carboxylated multi-walled carbon nanotubes, Thin Solid Films 516 (2008) 5781–5784. [16] D. Wen, Y. Liu, G.Y., S. Donga, Electrochemistry of glucose oxidase immobilized on the carbon nanotube wrapped by polyelectrolyte, Electrochim. Acta 52 (2007) 5312–5317. [17] E. Hao, T. Lian, Buildup of polymer/Au nanoparticle multilayer thin films based on hydrogen bonding, Chem. Mater. 12 (2000) 3392–3396. [18] S. Link, M.A. El-Sayed, Size and temperature dependence of the plasmon absorption of colloidal gold nanoparticles, J. Phys. Chem. B 103 (1999) 4212– 4217. [19] Y. Liu, J. Tang, X. Chen, W. Chen, G.K.H. Pang, J.H. Xin, A wet-chemical route for the decoration of CNTs with silver nanoparticles, Carbon 44 (2006) 381– 392. [20] Y. Wang, X. Xu, Z. Tian, Y. Zong, H. Cheng, C. Lin, Selective heterogeneous nucleation and growth of size-controlled metal nanoparticles on carbon nanotubes in solution, Chem. Eur. J. 12 (2006) 2542–2549. [21] M.S. Raghuveer, S. Agrawal, N. Bishop, G. Ramanath, Microwave-assisted single-step functionalization and in situ derivatization of carbon nanotubes with gold nanoparticles, Chem. Mater. 18 (2006) 1390–1393. [22] B. Yoon, C.M. Wai, Microemulsion-templated synthesis of carbon nanotubesupported Pd and Rh nanoparticles for catalytic applications, J. Am. Chem. Soc. 127 (2005) 17174–17175. [23] R. Yu, L. Chen, Q. Liu, J. Lin, K.-L. Tan, S.C. Ng, H.S.O. Chan, G.-Q. Xu, T.S.A. Hor, Platinum deposition on carbon nanotubes via chemical modification, Chem. Mater. 10 (1998) 718–722. [24] M.M. Rabbani, C.H. Ko, J.-S. Bae, J.H. Yeum, I.S. Kim, W. Oh, Comparison of some gold/carbon nanotube composites prepared by control of electrostatic interaction, Colloids Surf. A: Physicochem. Eng. Aspects 336 (2009) 183–186. [25] N. Alexeyeva, K. Tammeveski, Electroreduction of oxygen on gold nanoparticle/PDDA–MWCNT nanocomposites in acid solution, Anal. Chim. Acta 618 (2008) 140–146. [26] G. Gritzner, J. Kuta, Recommendations on reporting electrode potentials in nonaqueous solvents, J. Pure Appl. Chem. 56 (1984) 461–466. [27] J.L. Bredas, R. Silbey, D.S. Boudreaux, P.R. Chance, Chain-length dependence of electronic and electrochemical properties of conjugated systems: polyacetylene, polyphenylene, polythiophene, and polypyrrole, J. Am. Chem. Soc. 105 (1983) 6555–6559. [28] J.H. Kim, H. Lee, Efficient poly(p-phenylenevinylene) derivative with 1,2diphenyl-20 -cyanoethene for single layer light-emitting diodes, Synth. Met. 139 (2003) 471–478. [29] J.H. Kim, J.H. Park, H. Lee, Highly efficient novel poly(p-phenylenevinylene) derivative with 1,3,4-oxadiazole pendant on a vinylene unit, Chem. Mater. 15 (2003) 3414–3416. [30] C. Bitterncourt, A. Felten, B. Douhard, J. Ghijsen, R.L. Johnson, W. Drube, J.-J. Pireaux, Photoemission studies of gold clusters thermally evaporated on multiwall carbon nanotubes, Chem. Phys. 328 (2006) 385–391.