Copyright © 2007 American Scientific Publishers All rights reserved Printed in the United States of America
Journal of Nanoscience and Nanotechnology Vol. 7, 3386–3393, 2007
Synthesis and Characterization of Processable Multi-Walled Carbon Nanotubes—Sulfonated Polydiphenylamine Graft Copolymers Kwang-Pill Lee1 2 , Anantha Iyengar Gopalan1 2 3 ∗ , Kyu Soo Kim1 2 , and Padmanabhan Santhosh1 2 RESEARCH ARTICLE
1
Department of Chemistry Graduate School, Kyungpook National University, Daegu 702-701, South Korea 2 Nano Practical Application Center, Daegu, South Korea 3 Department of Industrial Chemistry, Alagappa University, Karaikudi 630003, Tamil Nadu, India
Delivered by Ingenta to:of multi-walled carbon nanotubes Water soluble and processable nanocomposites composed Max-Planck-Institut (MWNTs) and poly(diphenylamine sulfonic acid) (PDPASA) are synthesized and characterized. Two IP : 134.105.184.136 types of methodologies are adopted. MWNTs are covalently functionalized with 2,5-diaminobenzene Mon, Oct 2007with 10:53:12 sulfonic acid (DABSA) and further in situ08 polymerized diphenylamine-4-sulfonic acid (DPASA). This results in the formation of nanocomposites, MWNT(DABSA)-g-PDPASA, in which PDPASA is presented as the graft chains onto MWNTs. In another approach, DPASA is in situ polymerized in presence of unfunctionalized MWNTs, results in a nanocomposite in which MWNTs are present as entrapped mass in PDPASA matrix. Both nanocomposites are found to be water soluble and can form free standing films. The conductivity of MWNT(DABSA)-g-PDPASA and MWNT/PDPASA is found to be 1.25 mS · cm−1 and 0.65 mS · cm−1 , respectively, which is higher than that of pristine PDPASA (025 × 10−5 S · cm−1 ). The nanocomposites are characterized for their structure, morphology, optical and thermal properties. Keywords: Multi-Walled Carbon Nanotubes, Poly(diphenylamine sulfonic acid), Soluble, Processable, Nanocomposites.
1. INTRODUCTION Carbon nanotubes (CNTs) have demonstrated a wealth of exceptional electrical, mechanical, and thermal properties that have made them useful for potential applications ranging from nano-electronics to biomedical devices.1 Recently, variety of conducting polymers has been tried for making CNT/polymer nanocomposites toward various target applications in order to obtain a new material that would possess synergic properties of CNT and polymer.2–4 The formation of CNTs/conducting polymer composites has proved to be a promising approach to the effective incorporation of CNTs into devices. Nanocomposites such as CNT/polypyrrole,5 CNT/polyaniline,6 7 CNT/polydiphenylamine,8 CNT/polythiophene,9 and CNT/poly(phenylene vinylene)10 have been prepared through chemical and electrochemical methods. However, CNT itself is insoluble in organic solvent or water, which makes it difficult to form thin films and ∗
Author to whom correspondence should be addressed.
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limits its applications. Considerable effort has been made on the preparation of soluble CNTs both in fundamental and applied research areas.11 12 Metallothionein proteins were trapped inside and placed onto the outer surfaces of open-ended MWNTs.13 Porphyrin was found to adsorb on SWNTs presumably via Van-der-Waals attraction between the nanotubes and porphyrin.14 Polyaniline covered MWNTs via acid–base reaction between emeraldine and MWNTs were also realized.15 16 However, issues such as dispersion/orientation, interfacial bonding, and nanotube deformation within matrix have still not been investigated in detail. In particular, the solubility and processability of the nanotubes, which are the prime factors for device applications, have not been achieved. It is well established that sulfonated polyaniline derivatives are self-doped and are found to be soluble in water and other common organic solvents.17 18 Kim et al. synthesized water-soluble poly(4-anilino-1-butane sulfonic acid) by an electrochemical method.17 A water-soluble selfdoped conducting poly(aniline-co-N -sulphopropyl aniline) was reported.18 Further, sulfonated polyaniline derivatives 1533-4880/2007/7/3386/008
doi:10.1166/jnn.2007.822
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Synthesis and Characterization of Processable Multi-Walled Carbon Nanotubes
are known for their unique electrochemical and optical properties.19 20 In the present investigation, we have prepared water soluble and processable multi-walled carbon nanotubes (MWNTs)/poly(diphenylamine sulfonic acid) (PDPASA) nanocomposites. MWNTs were covalently functionalized with 2,5-diaminobenzene sulfonic acid (DABSA) and further polymerized with diphenylamine-4-sulfonic acid (DPASA). Also, DPASA is in situ polymerized in presence of unfunctionalized MWNTs. Both nanocomposites were found to be soluble in water. The nanocomposites were characterized for their structure, morphology, optical and thermal properties.
as MWNT(DABSA)-g-PDPASA) was filtered through a sintered glass crucible and washed. The composites were then dried under dynamic vacuum at room temperature. In another approach, in situ polymerization was carried out in the solution containing unfunctionalized MWNTs and DPASA using APS as oxidizing agent. The green precipitate, MWNT/PDPASA was formed, then filtered and dried. 2.4. Characterization
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Fourier transform infrared (FT-IR) spectra of the nanocomposites were recorded using a Bruker IFS 66v FT-IR spectrophotometer in the region 400 cm−1 to 4000 cm−1 using KBr pellets. The morphology of the nanocompos2. EXPERIMENTAL DETAILS ites was examined by field emission scanning electron 2.1. Materials microscope (FESEM)—Hitachi S-4300 and field emission transmission electron microscope (FETEM)—JOEL JEM2,5-diaminobenzene sulphonic acid and diphenyalmine2000EX to: with a field emission gun operated at 200 kV. Delivered 4-sulphonic acid sodium salt were from Aldrich, used by Ingenta Elemental compositions of the samples (% of carbon, Max-Planck-Institut as received. MWNTs (10 ∼ 50 nm in diameter, CNT nitrogen, hydrogen, and sulfur) were determined by Fision : 134.105.184.136 Co., Ltd., Incheon, Korea) were rinsed withIP doubleEA H10 elemental analyzer equipped with flash combusMon, 08 broOct 2007 10:53:12 distilled water and dried. Cetyltrimethyl ammonium tion furnace. The room-temperature conductivity of the mide (CTAB), N -methylpyrrolidinone (NMP), ammonium free standing composites films was determined using the persulphate (APS) of analytical grade from Aldrich were conventional two-point probe method. UV-Visible spectra used as received. were recorded using Varian UV-Visible spectrophotometer. Thermo gravimetric analysis was performed using a TA 2.2. Functionalization of MWNTs 2950 Hi-Res TGA at a heating rate of 10 C/min under Following procedure was adopted for the preparation of nitrogen atmosphere. diaminobenzene sulphonic acid functionalized MWNTs (MWNT-DABSA).21 50 mg of MWNT was refluxed in 4 M 3. RESULTS AND DISCUSSION HNO3 for 24 h and filtered through a polycarbonate membrane (0.2 m pore size). The residue, MWNT-COOH (carMWNT/PDPASA nanocomposites were prepared by boxylated MWNTs) was washed with deionized water and two methodologies. In one of the approach, MWNTs dried under vacuum at 60 C for 12 h. 50 mg of MWNTwere functionalized with diaminobenzene sulphonic acid COOH was refluxed in 100 ml of thionyl chloride at 65 C (MWNT-DBASA) and followed by in situ polymerization for 24 h to get MWNT-COCl. MWNT-COCl was filtered, of the solution containing MWNT-DBASA and monomer, washed with THF and dried under vacuum at room temdiphenyalmine-4-sulphonic acid (DPASA). In another perature. To prepare the functionalized MWNTs, MWNTapproach, polymerization of DPASA was carried out COCl was refluxed with 2,5-diaminobenzene sulphonic in presence of unfunctionalized MWNTs. Both methods acid using THF at 60 C for 24 h. The functionalized yield soluble and processable conducting nanocomposiMWNTs (MWNT-DABSA) were separated by filtration tes. However, the first approach resulted nanocomposites, and dried under vacuum. in which poly(diphenylamine sulphonic acid), (PDPASA) is presented as a grafted chain onto MWNT(DBASA). 2.3. Preparation of Nanocomposites PDPASA wrapped or coated MWNT was obtained in another approach. There are few similarities and difTwo types of methodologies were adopted for the prepaferences between the two types of MWNT/PDPASA ration of nanocomposites of MWNTs and PDPASA. The nanocomposites. typical polymerization procedure is outlined. To a soluMechanisms that describe the formation of two types of tion of DPASA (40 mM) in 0.05 M CTAB, 20 mg of nanocomposites prepared through in situ polymerization of MWNT-DABSA was added and sonicated for 1 h. The DPASA in presence of MWNT(DBASA) and unfunctionmixture was cooled to 4 C using a freezing mixture. alized MWNTs are presented in Scheme 1. In the former A pre-cooled (4 C) solution of ammonium persulphate case, it is anticipated that the amine groups present in (0.1 M) was added drop wise to the mixture with stirring MWNT(DBASA) and DPASA could be simultaneously and the mixture was stirred for 1 h. The resulting green precipitate, PDPASA grafted MWNT-DABSA (designated oxidized by APS. As a consequence, a cross-reaction
Synthesis and Characterization of Processable Multi-Walled Carbon Nanotubes
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(A) Preparation of MWNT(DABSA)-g-PDPASA COCl
COOH
ClOC HNO3
HOOC
SOCl2
COOH
COCl
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NH2
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HNOC
Di phenylamine-4-sulphonic acid Chemi cal Polymerization
SO3H
CONH H2N SO3H
NH2
Diaminobenzene sulphonic acid 60 ˚C, 24 h
Delivered HNOC by Ingenta to:SO3H Max-Planck-Institut IP : 134.105.184.136 H2N SO3H08 Oct 2007 10:53:12 Mon,
g-PDP ASA
CONH NH HNOC
N
n
SO3H n N
NH SO3H
CONH
SO3H NH
HNOC
SO3H n
N
N n
SO3H NH SO3H
SO3H
MWNT(DABSA)-g-PDPASA SO3H
(B) Preparation of MWNT/PDPASA
PDPASA
Diphenylamine-4-sulphonic acid Chemical Polymerization
MWNT
MWNT/PDPASA
Scheme 1. Mechanism of formation of MWNT/PDPASA composite by (A) graft copolymerization and (B) in situ polymerization.
could occur between the amine cation radicals formed from MWNT-DBASA and DPASA. This would result in the grafting of PDPASA onto MWNT-DABSA (Scheme 1(A)). However, in the later case of composite formation, grafting 3388
is not feasible (Scheme 1(B)), when polymerization of DPASA was carried out in presence of unfunctionalized MWNTs. Hence, a composite of MWNTs and PDPASA is expected. J. Nanosci. Nanotechnol. 7, 3386–3393, 2007
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(i)
IP : 134.105.184.136 Mon, 08 Oct 2007 10:53:12 FT-IR spectroscopy was used to characterize the chemi-
cal composition of the nanocomposites. FT-IR spectra of MWNT(DABSA)-g-PDPASA, MWNT/PDPASA and PDPASA are compared. FT-IR spectrum of PDPASA (Fig. 2(c)) shows sharp peaks at 1587 and 1488 cm−1 corresponding to oxidized form of PDPASA consisting of diphenoquinodimine (quinoid) and diphenyl benzidine (benzenoid) structures.23 The presence of band around 1150 cm−1 is attributed to the presence of diphenoquinone type units.24 The absorption band around 3390 cm−1
c
Transmittance (%)
(ii)
b
a
4000
Fig. 1. Typical photographs of (i) MWNT(DABSA)-g-PDPASA in water (a) before and after (b) neutralization with ammonia, (ii) MWNT (DABSA)-g-PDPASA film coated on the glass plate; doped (a) and neutral (b) form.
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3000
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Fig. 2. FT-IR spectra of (a) MWNT(DABSA)-g-PDPASA, (b) MWNT/ PDPASA, and (c) PDPASA.
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Table I. Elemental composition of the nanocomposites. It is interesting to note that both approaches of preparation of nanocomposites, MWNT(DABSA)-g-PDPASA and Components MWNT(DABSA)-g-PDPASA MWNT/PDPASA MWNT/PDPASA, yield soluble and processable conductCarbon 61.39 60.83 ing nanocomposites. Soluble and stable (green coloured) Nitrogen 6.49 6.25 dispersion of the nanocomposites are resulted. On adding Hydrogen 3.36 2.82 few drops of aqueous ammonia to a green solution in Sulfur 4.65 6.03 water, changes the solution colour from green to blue. Typical snapshots of solutions of MWNT(DABSA)-gThe elemental analysis (EA) results of MWNT PDPASA dissolved in water before and after addition of (DABSA)-g-PDPASA and MWNT/PDPASA are presented ammonia are presented in Figure 1(i). Also, it is noted in Table I. A 85% of grafting percentage of PDPASA was that blue solution was also stable. There was no settling achieved. A 17% (w/w) of SWNT in the SWNT-poly(mof agglomerates in the solution. 22 The grafting aminobenzene sulfonic acid) was reported. Further, the neutralized composites are soluble in efficiency was calculated as 42.2% from the EA results N -methylpyrrolidinone (NMP). Free standing films of (Table I). MWNT(DABSA)-g-PDPASA are obtained by casting A comparative account of the structure, morpholNMP solutions of MWNT(DABSA)-g-PDPASA onto ogy, optical and thermal properties of the nanocomposi glass, plate followed by drying at 50 C for 2 h tes (MWNT(DABSA)-g-PDPASA, MWNT/PDPASA) are (Fig. 1(ii)). The free-standing blue film of neutralized presented. MWNT(DABSA)-g-PDPASA could be transformed into a by Ingenta to: Delivered green film on exposure to HCl vapors (Fig. 1(ii)). Max-Planck-Institut 3.1. FT-IR spectroscopy
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Delivered by Ingenta to: Max-Planck-Institut30 IP : 134.105.184.136 Mon, 08 Oct 2007 10:53:12 20 10
0 40
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Diameter (nm) Fig. 3.
FESEM images and diameter distribution curves of (a) MWNT(DABSA)-g-PDPASA and (b) MWNT/PDPASA.
correspond to -NH stretching mode of secondary amine. The band around 1306 cm−1 is assigned to stretching vibration of C–N groups with partial double bond characteristics. Further, PDPASA shows peaks at 1010 and 1088 cm−1 , corresponds to symmetric and asymmetric stretches of sulfonic acid groups in PDPASA,25 respectively. FT-IR spectra of MWNT(DABSA)-g-PDPASA and MWNT/PDPASA (Figs. 2(a, b)) show prominent peaks at 3395, 1300, 1480, 1595, and 1010 cm−1 , which are characteristics of doped PDPASA. Further, the band corresponding to the stretching of B–NH+ Q (1150 cm−1 ) showed variation in position and intensity between MWNT(DABSA)-g-PDPASA and MWNT/PDPASA. Typically, the shift in position is more pronounced in MWNT(DABSA)-g-PDPASA (1172 cm−1 ) than in MWNT/PDPASA (1160 cm−1 ). Hence, we anticipate that B–NH+ Q groups in PDPASA might have interactions with MWNTs. This can happen by grafting of PDPASA chains onto MWNTs through reactions with -NH groups present in DABSA. 3.2. Morphology Morphology of the nanocomposites was determined through field emission scanning electron microscopy (FESEM) 3390
and field emission transmission electron microscopy (FETEM) measurements. Figure 3 shows the FESEM images of MWNT(DABSA)-g-PDPASA and MWNT/ PDPASA and the diameter distribution of the nanocomposites are presented. Strikingly, MWNT(DABSA)-gPDPASA shows a narrow distribution of diameters with average diameter around 55 nm. On the other hand, MWNT/PDPASA has different ranges of distribution of diameters. Particularly, the average diameter is higher (∼70 nm) than noticed for MWNT(DABSA)-g-PDPASA. Hence, it is concluded that a compact coating of PDPASA is present in MWNT(DABSA)-g-PDPASA than in MWNT/PDPASA. Also, the coating of PDPASA is uniform in MWNT(DABSA)-g-PDPASA (Fig. 3(a)). However, in the case of MWNT/PDPASA (Fig. 3(b)), an uneven or roughed coating of PDASA over MWNTs is observed. FETEM images of the nanocomposites also informed that PDPASA was formed over MWNTs as a layer in both cases. In the case of MWNT/PDPASA (Fig. 4(b)), the composite had a microstructure in which MWNT was presented as entrapped mass in PDPASA matrix. Formation of a smooth layer of PDPASA on the surface of MWNTDABSA was noticed in the case of MWNT(DABSA)-gPDPASA (Fig. 4(a)). Compared with MWNT/PDPASA, J. Nanosci. Nanotechnol. 7, 3386–3393, 2007
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Synthesis and Characterization of Processable Multi-Walled Carbon Nanotubes
(a)
Delivered by Ingenta to: patterns of (a) MWNT(DABSA)-g-PDPASA, Fig. 5. XRD (b) MWNT/PDPASA, (c) PDPASA, and (d) MWNT. Max-Planck-Institut IP : 134.105.184.136 Mon, 08 Oct 2007 3.4.10:53:12 XRD Analysis
Fig. 4. FETEM images of (a) MWNT(DABSA)-g-PDPASA (scale bar: 10 nm) and (b) MWNT/PDPASA (scale bar: 20 nm).
the surface of MWNT(DABSA)-g-PDPASA was smooth, as revealed from FESEM (Fig. 3) and FETEM images (Fig. 4). It is clear from the FETEM images that both nanocomposites have a layer of PDPASA on the surface of MWNTs. However, PDPASA layer in MWNT/ PDPASA is more dense than noticed for MWNT(DABSA)-g-PDPASA (note the different scales in Figs. 4((i) and (ii)). 3.3. Conductivity Measurements Conductivity measurements of the free standing films of MWNT(DABSA)-g-PDPASA and MWNT/PDPASA was made at 25 C. The conductivity of MWNT(DABSA)-gPDPASA and MWNT/PDPASA was found to be 1.25 mS · cm−1 and 0.65 mS · cm−1 , respectively. The observed conductivity for the nanocomposites is higher than that of PDPASA (025 × 10−5 S · cm−1 ). J. Nanosci. Nanotechnol. 7, 3386–3393, 2007
The structural characteristics of the MWNT(DABSA)g-PDPASA, MWNT/PDPASA was analyzed by X-ray powder diffraction measurements and the XRD profiles are shown in Figure 5. MWNTs exhibit the typical peaks at 25.2, 44.8, 55.6, and 74 corresponding to the graphite (002), (100), (101), (004), and (110) reflections, respectively26 (Fig. 5(d)). PDPSA shows a strong peak at 21.8 , a characteristic peak of its doped form (Fig. 5(c)). XRD profiles of MWNT(DABSA)-g-PDPASA and MWNT/PDPASA are similar to one observed for PDPASA, suggesting that the MWNTs were entirely encapsulated by PDPASA. Also, MWNT(DABSA)-gPDPASA and MWNT/PDPASA exhibits peak around 25.2 , which is a characteristic of (002) graphite reflection. Both, MWNT(DABSA)-g-PDPASA and MWNT/ PDPASA display typical peaks of the emeraldine salt form of PDPASA with suppressed intensities of the peaks of MWNTs. 3.5. UV-Visible Spectroscopy Figure 6 shows the UV-Visible spectra of MWNT (DABSA)-g-PDPASA, MWNT/PDPASA and PDPASA in NMP solution. UV-Visible spectrum of PDPASA shows the characteristic bands around 330 and 650 nm (Fig. 6(c)). The band observed at 330 nm is due to the inter-band transition (– ∗ ), whereas the peak at 550 nm is assigned to the n- ∗ transition from the non-bonding nitrogen lone pair to the conduction band ( ∗ 27−29 UV-Visible spectra of MWNT(DABSA)-g-PDPASA, MWNT/PDPASA in NMP solution are also presented (Figs. 6(a) and (b), respectively). A more pronounced shift of the n- ∗ transition 3391
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(b)
Synthesis and Characterization of Processable Multi-Walled Carbon Nanotubes Table II.
Lee et al. Summary of the thermal behavior of the nanocomposites.
c 1.2 b
Samples a
MWNT MWNT(DABSA)-g-PDPASA
c
Abs.
0.8 500
550
600
MWNT/PDPASA
650
PDPASA 0.4 a
Temperature range ( C)
Weight loss (%)
600–800 100–160 320–380 380–800 100–160 320–380 380–800 25–100 100–160 270–380 380–800
32.5 04.2 22.5 70.0 05.6 20.5 72.6 08.5 10.2 03.1 65.2
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b 0 300
400
500
600
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700
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of more amount of PDPASA in MWNT/PDPASA is consistent with the observation made through FETEM.
4. CONCLUSIONS Delivered by Ingenta to:
Fig. 6. UV-Visible spectra of (a) MWNT(DABSA)-g-PDPASA, Max-Planck-Institut (b) MWNT/PDPASA, and (c) PDPASA. In summary,
we have prepared water soluble and pro-
IP : 134.105.184.136 cessable nanocomposites composed of multi-walled carMon, Oct 2007 10:53:12 in the spectrum of MWNT(DABSA)-g-PDPASA 08 is witbon nanotubes (MWNTs) and poly(diphenylamine sulfonic nessed than for MWNT/PDPASA. This infers that the MWNTs are not only embedded in a PDPASA molecule, rather MWNTs act as a dopant for PDPASA, which resulted in the bathochromic shift of n- ∗ band, from 550– 605 nm. A similar kind of dopant effect was reported in MWNT-(OSO3 H)/PANI composites.30 Also, a new band was observed around 330 nm for both composites and that may be attributed to – ∗ staking interactions between graphitic MWNTs and phenyl rings in PDPASA. 3.6. Thermal analysis Thermal transitions of MWNT(DABSA)-g-PDPASA, MWNT/PDPASA, MWNT, and PDPASA, were recorded by the thermogravimetric analysis under N2 atmosphere. Further, TGA was used to estimate the amount of PDPASA present in the nanocomposites. The amount of grafting to MWNTs in the composite was determined from the weight loss of the composite at 400 C. Thermogram of MWNTs displays steady without evident weight loss below 600 C.31 On the contrary, MWNT(DABSA)g-PDPASA and MWNT/PDPASA, show three major weight loss (Table II). The first two weight losses observed at 160 C and >320 C corresponding to the removal of dopants and degradation of backbone units, respectively.32 33 As for PDPASA, the first two loss stages in the temperature range 100–160 C is presumably due to the elimination of water and the loss of dopant. The third weight loss upto 270 C are attributed mainly to the release of solvent. Above 380 C, decomposition of PDPASA was noticed. It is interesting to note that more amount of PDPASA is present in MWNT/PDPASA (58%) than in MWNT(DABSA)-g-PDPASA (43%). The presence 3392
acid) (PDPASA) with two different morphologies. The nanocomposites are found to be soluble in water and other common organic solvents. Further, it is demonstrated that the nanocomposites can be formed as free standing films, and the films can be easily transformed from conducting (green) to non-conducting (blue) form. These nanocomposites are expected to find wide range of applications in electrochromic devices, sensors, etc. Acknowledgments: The work was supported by Korean Research Foundation Grant (KRF-2004-00500009) and Brain Pool Program. The authors acknowledge the Korea Basic Science Institute (Daegu) and Kyungpook National University Center for Scientific Instrument.
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Delivered by Ingenta to: Received: 19 February 2006. Accepted: 12 January 2007. Max-Planck-Institut IP : 134.105.184.136 Mon, 08 Oct 2007 10:53:12
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