Materials Science and Engineering B 151 (2008) 210–219

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Materials Science and Engineering B

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Synthesis and characterization of water-soluble and conducting sulfonatedpolyaniline/para-phenylenediamine-functionalized multi-walled carbonnanotubes nano-composite

Jun Xua,b,c, Pei Yaoa,∗, Xuan Lia,b, Fei Hed

a Centre for Analysis, Tianjin University, Tianjin City 300072, PR Chinab School of Material Science and Engineering, Tianjin University, Tianjin City 300072, PR Chinac College of Science, Civil Aviation University of China, Tianjin City 300300, PR Chinad School of Chemical Engineering and Technology, Tianjin University, Tianjin City 300072, PR China

a r t i c l e i n f o

Article history:Received 22 February 2008Received in revised form 2 July 2008Accepted 6 July 2008

Keywords:PolyanilineMulti-walled carbon nanotubes

a b s t r a c t

Water-soluble and conducting sulfonated polyaniline (SPAN)/phenylamine groups contained MWNTs(p-MWNTs) nano-composite were synthesized by in situ oxidation polymerization followed by sulfona-tion and hydrolysis. TEM, Raman spectroscopy, FTIR, XPS, TGA and standard four-probe methods wereemployed to characterize morphology, chemical structure and performance of the nano-composite. Theresults show that phenylamine groups are grafted on the surface of p-MWNTs via amide bond and oxidizedphenylamine groups initiate polyaniline polymerized on the surface of p-MWNTs. SPAN chains covalentlyattached to p-MWNTs render p-MWNTs compatibility with SPAN matrix and lead to SPAN/p-MWNTs

Nano-compositesWater-solubleConductingC

nano-composite highly soluble and stable in water. Improved thermal stability illuminate existence ofa new phase in the nano-composite where there is chemical interaction between p-MWNTs and SPANcoatings. Owing to incorporation of p-MWNTs conductivity of the nano-composite at room temperature

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. Introduction

Carbon nanotubes (CNTs) including multi-walled and single-alled carbon nanotubes (MWNTs and SWNTs, respectively) with

xceptional structural, mechanical and electronic properties haveeceived much interest in fabricating advanced functional mate-ials [1–4]. However, insolubility in common solvents and poorispersibility of pristine CNTs block their further development inevice applications. Recently much attention is paid to the for-ation of CNTs/conducting polymer composites considered as a

romising approach to exerting synergetic effects and showingotential for many electronic device applications such as organic

ight-emitting diodes, photovoltaic cells, energy storage devicesnd sensors [5–8].

Polyaniline (PANI) is a promising conducting polymer and

eceived special attention owing to its good processibility, envi-onmental stability and reversible control of electrical propertiesy both charge-transfer doping and protonation [9]. Among PANIerivatives, sulfonated polyaniline is the most successful candi-

∗ Corresponding author. Tel.: +86 22 27405694; fax: +86 22 27405694.E-mail address: pyaotjueducn@163.com (P. Yao).

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921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2008.07.003

s of magnitude over neat SPAN.© 2008 Elsevier B.V. All rights reserved.

ate for practical applications in electronic devices due to its waterolubility, electroactive properties, thermal stability, optical prop-rties, unique self-doping and external doping mechanism [10–18].owever, strong electron-withdrawing of sulfonic acid functionalroups make conductivity of SPAN much lower than that of PANI19].

Generally, combination of a conducting polymer with CNTsased on chemical interaction or grafting of polymer chains ontohe surface of CNTs by covalent bonding [19] and formation ofonducting polymer in the presence of CNTs (in situ polymer-zation) are two important approaches to prepare CNT/polymeromposites [20]. Recent study shows that the latter method (in situolymerization) is a good approach for synthesizing homogeneousolymer/MWNTs nano-composites and covalent functionalizationf MWNTs helps dispersion of nanotubes in the reaction systems21,22]. Philip et al. [22] introduced the monomers of polyani-ine on the surface of MWNTs by covalently functionalized the

WNTs with p-PDA and prepared homogeneous core–shell nano-

omposite of PANI/phenylamine groups contained MWNT by then situ polymerization. For comparison, they conducted a blankxperiment in which polymerization of PANI in the presence ofWNTs–COOH. The results showed that phenylamine functional-

zation helped to disperse MWNTs homogeneously in the reaction

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edium and a tubular composite with an ordered polyanilinehell of uniform thickness was obtained. The phenylamine groupsere no longer impurities in the composite and were converted

o polyaniline during the composite formation. Furthermore, theovalent functionalization of MWNTs with polyaniline can ensurehe compatibility of carbon nanotubes in the polyaniline matrixhich can avoid potential microscopic phase separation in theano-composite. On the other hand, in order to promote water sol-bility of MWNTs and exert synergetic effects of MWNTs and SPAN,hang et al. [23] prepared a water-soluble nano-composite of PANInd pristine MWNT by in situ polymerization of aniline followedy sulfonation with chlorosulfonic acid in an inert solvent and byydrolysis in water. The results showed that quinoid structure ofPAN preferentially interacts with the nanotubes by strong �–�nteraction and SPAN/MWNTs composite is highly dispersible inater.

In this paper, we report a new water-soluble and conductingPAN/MWNTs nano-composite, in which we introduced pheny-

amine groups (–C6H4–NH2) the monomers of polyaniline onhe surface of MWNTs (designated as p-MWNTs) by using anffective chemical route, oxidized phenylamine groups on surfacef p-MWNTs via amide bond (O C–NH) initiate polymerizationnd externally HCl-doped sulfonated PANI (SPAN) was chemi-

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Fig. 1. Reaction routes for p-MWNTs and

ineering B 151 (2008) 210–219 211

ally grafted onto the surface of p-MWNTs by in situ oxidationolymerization followed by sulfonation and hydrolysis for therst time to the best of our knowledge. After sulfonation andydrolysis, the covalent bonding between SPAN chains and p-WNTs renders p-MWNTs compatibility with SPAN matrix. As a

esult, SPAN/p-MWNTs nano-composite was highly soluble andtable in water. Also, the thermal stability and electrical con-uctivity of SPAN/MWNTs nano-composite were investigated inarticular. Although chemical modification affects the perfor-ance of p-MWNTs, thermal stability and electronic conductivity

t room temperature of SPAN/p-MWNTs nano-composites areighly increased by incorporation of p-MWNTs and covalent bond-

ng between SPAN chains and carbon nanotubes. The chemicaleactions for p-MWNTs and SPAN/p-MWNTs nano-composite arellustrated in Fig. 1.

. Experimental

.1. Fabrication of p-PDA-functionalized MWNTs

MWNTs diameter of 20–30 nm were produced by CCVD methodn which CH4 was converted into MWNTs in the presence of La2NiO4atalysts and the as-grown MWNTs were purified by using H2O2

SPAN/p-MWNTs nano-composite.

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olution and HCl solution in ultrasonic as described in previoustudy [24,25]. Purified MWNTs were ultrasonically treated with a:1 mixture of concentrated H2SO4:HNO3 at 50–60 ◦C for 12 h andhen dispersed in excess SOCl2 and N,N-dimethyl formamide (DMF)nd refluxed at 80 ◦C for 24 h converting the carboxylic acid groupsnto acylchloride groups. At last, SOCl2 was removed by distillationnd excess p-PDA was added in the mixture stirring at 120 ◦C for2 h in nitrogen atmosphere. The resulting p-MWNTs were filtered,ashed and dried under vacuum.

.2. Synthesis of SPAN/p-MWNTs nano-composite

PANI/p-MWNTs nano-composite was synthesized by in situolymerization. After p-MWNTs was ultrasonicated in 1.0 M HClolutions for 0.5 h, aniline (mass ratio of p-MWNTs:aniline is:10) was added and ultrasonicated for more than 0.5 h. Ammo-ium peroxydisulfate (APS) dissolved in 1.0 M HCl solution wasripped slowly into the suspension (molar ratio of aniline:APS

s 1:1). In order to investigate the formation mechanism of theano-composite, the polymerization was continued for 2 and 4 h,espectively. The whole polymerization process was carried out at

–5 ◦C with constant mechanical stirring and ultrasonication. Theesulting black-green suspension was filtered, washed and driednder vacuum.

SPAN/p-MWNTs nano-composite was prepared according torevious method [18] and modified somewhat to prevent the

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ig. 2. Stability of SPAN in water for 2 months (a), SPAN/p-MWNTs nano-composite inANI-EB in NMP for 2 months (d) and PANI-EB/p-MWNTs composite in NMP for 2 months

ineering B 151 (2008) 210–219

ano-composite from aggregation during sulfonation and hydrol-sis. Dried PANI/p-MWNTs nano-composite polymerized for 4 h1 g) was ultrasonicated in 200 ml of 1,2-dichloroethane (DCE) andtirred at 80 ◦C for 1 h. Chlorosulfonic acid (2 g) diluted with 20 mlf DCE was dripped into the suspension for 60 min, and then theeaction mixture was held for 4 h with vigorous stirring. Chloro-ulfonated polyaniline was separated by filtration and immersedn 100 ml of isopropyl alcohol (IPA) containing 10% water with stir-ing and ultrasonication for 4 h at 60 ◦C to promote its hydrolysis.he product was filtered, washed by IPA and dried under vacuumt 60 ◦C for 12 h.

For comparative study, neat SPAN was prepared under the sameondition without p-MWNTs.

.3. Characterization of p-MWNTs and SPAN/p-MWNTsano-composite

Morphology of the samples was performed by using a FEI TECNAI2-F20 Field Emission Transmission Electron Microscope (TEM)perated at 200 kV after purified MWNTs, p-MWNTs, PANI/p-WNTs and SPAN/p-MWNTs nano-composites were dispersed

n ethanol, or SPAN/p-MWNTs nano-composite was dissolved inater and dripped on carbon copper grids. Integrity and disor-er of p-MWNTs induced by chemical modification, the influencesf polymerization of PANI and the treatments of sulfonationnd hydrolysis on the structural properties of MWNTs were

water for 2 months (b) and PANI/p-MWNTs nano-composite in water for 1 h (c);(e).

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haracterized by using Raman spectroscopy recorded under aenishaw inVia Raman Microscope using an Argon ion laser oper-ting at 514.5 nm. Fourier transform infrared (FTIR) spectroscopypectra of p-MWNTs, PANI/p-MWNTs composite polymerized forh, SPAN/p-MWNTs nano-composite and neat SPAN in KBr were

ecorded on a Nicolet Avata 330 at room temperature. X-ray pho-oelectron spectroscopy (XPS) measurements of the samples werearried out with a ESCA PHI-1600 PE XPS spectrometer with a MgK�) X-ray source. Low-resolution survey scans were performedt 187.85 eV with a step of 0.8 eV, high-resolution survey scansere done at pass energy of 29.35 eV with a step of 0.25 eV. All

ore-level spectra were referenced to the C 1s neutral carboneak at 284.6 eV and were deconvoluted into Gaussian compo-ent peaks. The curve fitting was done by PHI Multipack 8.0oftware. Thermal stabilities of purified MWNTs, p-MWNTs, SPAN,ANI/p-MWNTs nano-composites with different reaction times andPAN/p-MWNTs nano-composite were operated using TA SDT-600 TG/DTA thermogravimetric analysis (TGA) system at a heating

ate of 10 ◦C/min in N2. Electrical conductivities of the samples wereeasured by standard four-probe methods using a programmable

DY-5 voltage/current detector (Guangzhou Semi-conductor Insti-ute) at room temperature, all powder materials were pressed intoisk pellets with 12.7 mm in diameter and about 0.5 mm in thick-ess.

. Results and discussion

.1. Solubility and stability

Solubility of SPAN/p-MWNTs nano-composite in water wasetermined as follows: excess dried SPAN/p-MWNTs powder wasdded to 20 ml of deionized water with ultrasonication and stirringor 10 h at room temperature. The resulting solution was restedor more than 24 h. The supernatant liquid was carefully trans-erred into another vessel with accurate volume. Water in thePAN/p-MWNTs-saturated solution was removed by vacuum dry-ng and the recovered MWNT/SPAN composite in the vessel was

eighted. According to the method, solubility of SPAN/p-MWNTsano-composite in water is measured to be 43.56 mg/ml. Assum-

ng that no p-MWNTs lost in the chemical process, filtering andashing cycles, and accounting for 7.52% of free polymer being

ost during the filtering and washing cycles which is obtainedrom the yield of neat SPAN prepared under the same conditionithout p-MWNTs, the solubility of p-MWNTs can be estimated

o be 4.25 mg/ml. In order to investigate the solubility of PANI/p-WNTs composite in organic solvents, the polymerization yield

ANI and PANI/p-MWNTs composite with reaction time of 4 here de-doped by stirring for 2 h with 3 wt.% ammonium hydrox-

de (NH4OH) to transform emeraldine salt (ES) into emeraldinease (EB) and followed by wash and vacuum drying as previousescribed [26]. According to the method mentioned above, solubil-

ty of PANI-EB/p-MWNTs composite in N-methyl-2-pyrrolidinoneNMP) is measured to be 37.19 mg/ml.

Stability of neat SPAN, SPAN/p-MWNTs composite and PANI/p-WNTs composite polymerized for 4 h in water, PANI-EB and

ANI-EB/p-MWNTs composite in NMP are shown in Fig. 2. SPANFig. 2(a)) is soluble in water and results in a stable green and trans-arent solution. After PANI/p-MWNTs nano-composite is dispersed

n water, fall-out of small agglomerates appears in the suspen-

ion and then most of agglomerates settle down at the bottom forh (Fig. 2(c)). SPAN/p-MWNTs composite is completely soluble inater and forms a stable black-green solution as can be seen in

ig. 2(b). Like PANI-EB in NMP (Fig. 2(d)) which shows a transpar-nt and blue solution, PANI-EB/p-MWNTs composite (Fig. 2(e)) is

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ineering B 151 (2008) 210–219 213

ompletely dispersed in NMP and shows a stable black-blue solu-ion. Both of the resulting solutions of SPAN/p-MWNTs compositen water and PANI-EB/p-MWNTs composite in NMP are stable ando floating particles or fall-out of small agglomerates appears inhe solutions at least for 2 months.

.2. Morphology

Morphology and nano-structures of purified MWNTs, p-WNTs, PANI/p-MWNTs and SPAN/p-MWNTs nano-composites

re observed by TEM and shown in Fig. 3. Fig. 3(a1) and (a2)hows the entangled purified MWNTs with integrate and smoothidewalls, there are not much defects and amorphous layers pre-ented on the surface. As can be seen in Fig. 3(b1) and (b2),idewalls of p-MWNTs are rough and there are detectable amor-hous prominences on outer graphite sheet, which indicates thatodifications introduce functional groups such as phenylamine

roups on the surface of p-MWNTs. In the images of PANI/p-WNTs nano-composite polymerized for 4 h (Fig. 3(c1) and (c2)),

here are no dissociative PANI granules and as self-assemblyemplates, p-MWNTs are encapsulated by PANI coatings form-ng a core–shell nano-structure with diameter in the range of00–120 nm. To investigate the formation mechanism of PANI/p-WNTs nano-composite and influence of the treatments of

ulfonation and hydrolysis on SPAN/p-MWNTs nano-composite,orphology of PANI/p-MWNTs nano-composite polymerized forh and SPAN/p-MWNTs nano-composite samples dispersed inthanol were performed by TEM and shown in Fig. 3(d1) and (d2)nd Fig. 3(e1) and (e2). Fig. 3(d1) and (d2) show that when in situolymerization within 2 h, p-MWNTs are discontinuously coated,here are prominences like knaps lying along the sidewalls of p-

WNTs and heaps locating at the ends of p-MWNTs. Images ofPAN/p-MWNTs nano-composite in ethanol (Fig. 3(e1) and (e2))how that diameters of the core (p-MWNTs)–shell (SPAN) nano-tructure decrease to 90–110 nm and the taper-shape outer layersisappear. Combined with the result of XPS quantitative analysis ofPAN/p-MWNTs nano-composite that molar ratio of –SO3H groupso nitrogen atoms is closed to 1:1, the phenomena maybe attributeo the PANI outer layers partially destructed during sulfonationnd hydrolysis. Images of SPAN/p-MWNTs nano-composite dis-olved in water (Fig. 3(f1) and (f2)) show that SPAN coatings areissolved in water, carbon nanotubes do not aggregate and arembedded within the dissolved SPAN films. Furthermore, there iso distinct interface between SPAN and nanotubes, and both of theomponents are combined together in the film-like structures. Thenteresting morphology naturally account for the high stability ofPAN/p-MWNTs nano-composite in water, which can be explaineds follows.

After purified MWNTs are covalently functionalized by p-PDA,henylamine groups are grafted at the defective sites of sidewallsnd ends of p-MWNTs via amide bond. Two kinds of cation rad-cals initiate PANI polymerized on the surface of p-MWNTs afterddition of APS. One is oxidized phenylamine groups grafted onhe surface of p-MWNTs, the other is oxidized aniline cation radi-als after aniline absorbed on the surface of p-MWNTs from HClolution and reacted with APS. Despite both of them initiate initu polymerization within 2 h, the former dominates the forma-ion of the inner layer coatings including PANI prominences alonghe sidewalls of p-MWNTs and PANI heaps at tube ends wherehere are more phenylamine groups grafted on the surface of p-

WNTs, which is consistent with the results of previous study22]. As reaction time increasing, more aniline monomers join inhe in situ polymerization. Some are entrapped into the polymerhains initiated by oxidized phenylamine groups, others incorpo-ate the polymer chains initiated by oxidized aniline cation radicals

214 J. Xu et al. / Materials Science and Engineering B 151 (2008) 210–219

Fig. 3. TEM images of purified MWNTs (a1 and a2), p-MWNTs (b1 and b2), PANI/p-MWNTs nano-composite polymerized for 4 h (c1 and c2), PANI/p-MWNTs nano-compositepolymerized for 2 h (d1 and d2), SPAN/p-MWNTs nano-composite (e1 and e2) dispersed in ethanol and SPAN/p-MWNTs nano-composite (f1 and f2) dissolved in water.

J. Xu et al. / Materials Science and Engineering B 151 (2008) 210–219 215

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Factbbalized reactions the integrity and order of the curved graphemesheets on the surface of p-MWNTs has not been destroyed too much.The Raman spectrum of PANI/p-MWNTs nano-composite (Fig. 4c)shows the typical bands of coated PANI on MWNTs including C–Hbending of the quinoid ring at 1169 cm−1, C–H bending of the

Fig. 3.

on-covalently linked with p-MWNTs and assembly mainly on theuter layers of PANI coatings owing to low activation energy and therinciples of heterogeneous catalysis [22]. Correspondingly, afterulfonation and hydrolysis, there are two kinds of SPAN chainsssembly on the surface of p-MWNTs. One is covalently attachedo p-MWNTs via amide bond, the other is non-covalently linkedith p-MWNTs [23] and mainly distributed in the outer layers of

he SPAN coatings. When SPAN/p-MWNTs nano-composite is dis-ersed in water, most of the latter are completely dissolved in waternd form the irregular SPAN films, while the SPAN chains covalentlyttached to p-MWNTs via amide bond (the former) are entanglednd linked between the dissolved macromolecules and carbon nan-tubes forming the indistinct interface in Fig. 3(f1) and (f2). As aesult, p-MWNTs are compatible with SPAN matrix and are stabi-ized in water via strengthen of chemical bonding between CNTsnd SPAN as well as the solubility of SPAN.

.3. Raman and FTIR analysis

Raman spectra of purified MWNTs, p-MWNTs, PANI/p-MWNTsano-composite polymerized for 4 h and SPAN/p-MWNTs nano-omposite are shown in Fig. 4. There are strong bands in the spectraf purified MWNTs and p-MWNTs samples including peaks at345 cm−1 (D mode) indicating the amorphous carbon and disorderriginating from defects in the curved grapheme sheets and tubends, peaks at 1569 cm−1 (G mode) revealing the order and integrity

f MWNTs from the Raman-allowed phonon high-frequency mode,nd shoulders around 1600 cm−1 assigned to the D′-line (disorderine) [27,28]. The IG/ID ratio of purified MWNTs equal to 1.38 indi-ating that there are amount of defects and amorphous carbon inhe nanotube which favors the chemical modification of MWNTs.

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or p-MWNTs sample, the intensities of D- and D′-line enhancend the peak intensity ratio IG/ID is 1.28, which reveal that chemi-al treatment and further p-PDA-functionalized reactions decreasehe degree of order and increase the number of amorphous car-on on the surface of p-MWNTs. At the same time, IG/ID ratio noteing decreased excessively means after a long time of function-

ig. 4. Raman spectra of purified MWNTs (a), p-MWNTs (b), PANI/p-MWNTs nano-omposite polymerized for 4 h (c) and SPAN/p-MWNTs nano-composite (d).

216 J. Xu et al. / Materials Science and Eng

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ig. 5. FTIR spectra of p-MWNTs (a), PANI/p-MWNTs nano-composite polymerizedor 4 h (b), SPAN/p-MWNTs nano-composite (c) and SPAN (d).

enzenoid ring at 1260 cm−1, C–N•+ stretching at 1339 cm−1, C Ntretching vibration at 1484 cm−1 and C–C stretching of the benzeneing at 1600 cm−1, which reveals the presence of the doped PANI-S structure and formation of core (p-MWNTs)–shell (HCl-dopedANI) nano-composite [23,27]. Similar Raman spectrum is alsobserved for SPAN/p-MWNTs nano-composite (Fig. 4d) revealinghat the further chemical treatments of sulfonation and hydrolysiso not change the core–shell tubular structures of the compos-

te, which is consistent with the result of morphology mentionedbove. Also, it is necessary to point out that compared with that ofANI/p-MWNTs composite, the relative intensity of C–H bending ofhe quinoid ring at 1169 cm−1, C–H bending of the benzenoid ringt 1260 cm−1 and C N stretching vibration at 1484 cm−1 decrease,hich indicates the presence of sulfonated rings in the SPAN/p-WNTs composite materials.FTIR spectra of p-MWNTs, PANI/p-MWNTs nano-composite

olymerized for 4 h, SPAN/p-MWNTs nano-composite and SPANre shown in Fig. 5. In the spectrum of p-MWNTs (Fig. 5(a)),eaks at 1656 and 1616 cm−1 originate from amide I band C Otretching and amide II band N–H bending, absorptions at 3417nd 838 cm−1 correspond to amine N–H stretching and asym-etry 1,4-disubstituted benzenoid ring, respectively [22]. These

ndicate that after purified MWNTs treated with a mixture of con-entrated acids, refluxing with SOCl2 and then reacted with p-PDA,henylamine groups are introduced on the surface of p-MWNTs viamide bond. In the spectrum of PANI/p-MWNTs nano-compositeFig. 5(b)), peaks at 1573, 1492 and 1297 cm−1 corresponding to

C stretching of quinoid rings, C C stretching of benzenoid ringsnd C–N stretching mode indicate that PANI assembly on the sur-ace of p-MWNTs is in its ES form [29]. The strong characteristicbsorption at 1133 cm−1 considered as “electron-like band” [30]s shifted to lower frequency compared with that of pure PANI-S owing to the higher protonated state and intensive interactionetween MWNTs and PANI chains [29,31]. Also, there is a weakbsorption at 1652 cm−1 originating from the carbonyl stretch ofmide bond, which verifies that PANI chains are grafted on theurface of p-MWNTs via covalent bonding instead of physical wrap-ing and is consistent with the results of previous study [19].

n the spectrum of SPAN/p-MWNTs nano-composite (Fig. 5(c)),

xcept for a weak absorbance at 1650 cm−1 corresponding to thearbonyl stretch of amide bond, there are peaks at 1170 and060 cm−1 assigned to asymmetric and symmetric O S O stretch-ng vibrations, peaks at 702 and 593 cm−1 assigned to S–O and

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–S stretching vibrations, peak at 850 cm−1 from out-of-planeending of 1,2,4-trisubstituted aromatic rings and a broad peakround 2500–3700 cm−1 for externally doped SPAN [18], whichre consistent with the presence of SPAN (Fig. 5(d)). These resultshow that in the nano-composite, PANI coatings have been sul-onated after sulfonation in DCE and hydrolysis in water, and somef SPAN chains are covalently bonded to p-MWNTs via amideond.

.4. XPS analysis

XPS spectra of purified MWNTs and p-MWNTs are shown inig. 6. Wide scan spectra identify elements presented on the surfacef carbon nanotubes. In spectra of purified MWNTs and p-MWNTsFig. 6(a) and (b)), C 1s and O 1s signals appear at 284 and 533 eV.

1s signal in the spectrum of purified MWNTs arises from intro-uction of groups such as COOH, OH and C O during purificationnd oxygen absorbed on the surface of the MWNTs [32]. Atomiconcentrations of O 1s and N 1s increase from 5.5% and 0% forurified MWNTs to 11.7% and 7.9% for p-MWNTs with a low peakf s 2p at the same time. These indicate that functional groupsonsisting of oxygen and nitrogen atoms are introduced on theurface of p-MWNTs after MWNTs functionalized with p-PDA asso-iated with newly created species containing S atoms [33]. Fig. 6(c)hows that C 1s peak for sp2-hybridized carbon in p-MWNTs ishifted lower to 284.19 eV suggesting acceptor behaviors such asCO–NH– groups attached to carbon nanotube [34,35]. Peaks ofp3-hybridized carbon (285.4 eV) and C–C bond (284.6 eV) com-ine into a mild peak at 285.29 eV, C–N bond at 285.8 eV and C–Oond at 286.6 eV merge into another peak at 286.42 eV. Further-ore, the peak appearing at 287.78 eV is assigned to C 1s of amide

ond and C O band, peak at 290.47 eV is assigned to �–�* transi-ion of phenylene plane [36,37]. N 1s spectrum of p-MWNTs showshree peaks in Fig. 6(d). Peak at 399.15 eV with area of 47.93% ariserom N 1s of amide nitrogen, which is shifted to lower bondingnergy than 400 eV attributed to hydrogen bond effect [38]. Peakt 400.57 eV with area of 46.47% is arise from amine nitrogen, andweak peak at 402.2 may be a byproduct when p-PDA modifica-

ion. These data indicate phenylamine groups with concentrationf 3.7% are covalently grafted on the surface of p-MWNTs via amideond.

XPS spectra with surface elements and quantitative analysis forANI/p-MWNTs nano-composite polymerized for 4 h and SPAN/p-WNTs nano-composite are shown in Fig. 7. In both wide scan

pectra of the samples (Fig. 7(a) and (b)), N 1s and Cl 2p signalsppearing at 399 and 197 eV reveal that p-MWNTs are wrappednderneath PANI and SPAN coatings in the PANI/p-MWNTs andPAN/p-MWNTs composite materials, respectively, and that SPANoatings in the SPAN/p-MWNTs composite are doped by Cl− ions.oreover, atomic concentrations of O 1s and S 2p increase from

.7% and 0% for PANI/p-MWNTs composite to 26.1% and 5.6% forPAN/p-MWNTs composite, in which increment of oxygen atoms isbout three times as much as the concentration of sulfur atoms.he quantitative analysis data indicate that –SO3H groups haveeen attached to the backbones of SPAN chains with molar ratiof –SO3H groups to nitrogen atoms closed to 1:1, and that the high/N ratio lead to SPAN/p-MWNTs nano-composite highly solublen water [11]. Deconvolution of N 1s spectrum of SPAN/p-MWNTsano-composite is shown in Fig. 7(c) in which N 1s can be deconvo-

–NH–), 401.48 eV ( N+–) and 402.58 eV (–N+–) with area frac-ions of 7.79%, 40.86%, 38.11% and 13.24%, respectively. The resultslluminate that most of the imine nitrogens have been convertedo positively charged species, and SPAN in the SPAN/p-MWNTs

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ano-composite are externally doped by Cl− ions and in its ESorm.

.5. TGA

Thermal stabilities of purified MWNTs, p-MWNTs, PANI/p-WNTs nano-composites with different reaction times of 2 andh, SPAN/p-MWNTs nano-composite and SPAN are shown in Fig. 8.he first weight loss period of all the samples below 200 ◦C isainly attributed to release of water molecules and lower molec-

lar weight oligomers along surface of samples [41]. PurifiedWNTs sample (Fig. 8(a)) is stable and hardly decomposes below

00 ◦C. p-MWNTs sample (Fig. 8(b)) displays a weight loss about.4% in the interval of 200–500 ◦C owing to the decomposition ofrganic groups such as phenylamine groups grafted on the sur-ace of p-MWNTs, after that it decomposes slowly in the regionf 500–800 ◦C. SPAN sample (Fig. 8(f)) shows an obvious weightoss about 42.1% in the interval of 200–330 ◦C from its de-doping

rocess and decomposition of sulfonic acid groups, then its back-one structure and leftovers are burning in the range of 330–800 ◦C42,43]. TGA curves of PANI/p-MWNTs nano-composite polymer-zed for 2 and 4 h (Fig. 8(c) and (d)) comprise early terminatedANI de-doping processes in 200–300 ◦C, long platforms with mild

obtMS

Fig. 6. Wide scan XPS spectra of purified MWNT (a) and p-MWNT (b), de

ineering B 151 (2008) 210–219 217

ecomposition speed in the range of 300–500 ◦C and burningf PANI and leftovers in the range of 500–800 ◦C. For SPAN/p-WNTs nano-composite (Fig. 8(e)), its TGA curve is almost identicalith the combination of the thermal behaviors of neat SPAN- andCl-doped PANI/p-MWNTs composite polymerized for 4 h, includ-

ng de-doping process and decomposition of sulfonic acid groupsn the range of 200–330 ◦C, a platform with mild decomposi-ion speed in the range of 330–480 ◦C, and burning of polymerackbones and carbon nanotubes in the range of 500–800 ◦C.he thermal behaviors of HCl-doped PANI/p-MWNTs and SPAN/p-WNTs nano-composites in the range of 300–500 ◦C illuminate

he presence of MWNTs in the polymer matrix and existence of aew phase in these nano-structures [44] where there exits chem-

cal interaction between p-MWNTs and PANI or SPAN coatings.n the PANI or SPAN coatings, some polymer chains are cova-ently grafted on the surface of p-MWNTs. Weight loss behaviorsf the nano-composites after de-doping process and decomposi-ion of sulfonation groups are controlled by both the degradation

f polymer chains and the decomposition of covalent bondingsetween PANI or SPAN and p-MWNTs in the new phase. As a result,he thermal stability of HCl-doped PANI/p-MWNTs and SPAN/p-

WNTs nano-composites are improved compared with that of neatPAN.

convolutions of C 1s for p-MWNTs (c) and N 1s for p-MWNTs (d).

218 J. Xu et al. / Materials Science and Engineering B 151 (2008) 210–219

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Acknowledgment

ig. 7. Wide scan XPS spectra of PANI/p-MWNTs nano-composite polymerized forh (a), SPAN/p-MWNTs nano-composite (b) and deconvolution of N 1s spectrum ofPAN/p-MWNTs nano-composite (c).

.6. Conductivity

Electrical conductivities measured by four-probe method atoom temperature for purified MWNTs, p-MWNTs, SPAN andPAN/p-MWNTs nano-composite are 116.3, 71.69, 2.87 × 10−4 and

aj

ig. 8. TGA curves of purified MWNTs (a), p-MWNTs (b), PANI/p-MWNTs nano-omposite polymerized for 2 h (c), PANI/p-MWNTs nano-composite polymerizedor 4 h (d), SPAN/p-MWNTs nano-composite (e) and SPAN (f).

.65 × 10−2 S/cm, respectively. The lower conductivity of p-MWNTshan that of purified MWNTs due to chemical modification intro-ucing defects and functional groups on the curved graphite sheetsnd the ends of nanotube which affect the expedite charge trans-ort on p-MWNTs. For SPAN sample, high S/N ratio (closed to 1:1)

ead to its high solubility in water at the price of sacrificing elec-ronic conductivity for strong electron-withdrawing of sulfonic acidroups. However, p-MWNTs still have considerable charge trans-ort ability and serve as ideal conducting matrix when incorporated

n SPAN/p-MWNTs nano-composite. Furthermore, phenylamine–C6H4–NH2) groups on the surface of p-MWNTs join in the initu polymerization, become a part of polyaniline chains and act asridges to facilitate charge transportation between p-MWNTs andPAN. As a result, the room temperature conductivity of SPAN/p-WNTs nano-composite is dramatically increased by about two

rders of magnitude compared to that of neat SPAN.

. Conclusions

Attachment of phenylamine groups on the surface of p-MWNTselp p-MWNTs disperse well in reaction system, phenylamineroups join in polymerization and act as chemical bridges between-MWNTs and SPAN chains, which is significant for the forma-ion of stable and homogeneous water-soluble nano-compositend exerting synergetic effects of SPAN and p-MWNTs in theomposite for electronic device applications. TEM images, Ramanpectroscopy, FTIR and XPS quantitative analysis reveal that pheny-amine groups with concentration of 3.7% are covalently graftedn the surface of p-MWNTs via amide bond, high ratio of S/N inhe SPAN/p-MWNTs nano-composite as well as water-soluble SPANhains covalently attached to p-MWNTs result in high solubilityf SPAN/p-MWNTs nano-composite and further p-MWNTs stableispersed in water. Although chemical modification affects theerformance of p-MWNTs, thermal stability and electronic conduc-ivity at room temperature of SPAN/p-MWNTs nano-compositesre highly increased by incorporation of p-MWNTs and covalentonding between SPAN chains and carbon nanotubes.

We gratefully acknowledge the financial support by the Sciencend Technology Development Project (No. 05YFJZJC00200) of Tian-in Natural Science Foundation.

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