Thermoelectric Enhancement in Polyaniline Compositeswith Polypyrrole-Functionalized Multiwall Carbon Nanotubes

JIE LIU1,2 and HUI-QUN YU1

1.—East China University of Science and Technology, Shanghai 200237, People’s Republic ofChina. 2.—e-mail: liujieuste@126.com

This work suggests a facile method to improve the thermoelectric properties ofpolyaniline (PANi) composites. Carbon multiwall nanotubes (MWNTs) werenoncovalently functionalized with polypyrrole (PPy-MWNTs) based on in situpolymerization, and these PPy-MWNTs were used to synthesize PPy-MWNT/PANi composites. The surface-functionalized PPy nanolayer on the MWNTswas found to yield a homogeneous dispersion of MWNTs and strong interfacialadhesion. The resulting composites demonstrated a remarkable enhancementin both electrical conductivity and Seebeck coefficient, and exhibited a highpower factor of 3.1 lW/m K2 compared with the values of 0.006 lW/m K2 forPANi and 0.1 lW/m K2 for MWNT/PANi composite at 28.6 wt.% MWNT load-ing. The obtained results indicate that this method is useful for synthesizingconductive polymer composites with improved thermoelectric performance.

Key words: Carbon nanotubes, conductive polymer, thermoelectric material,power factor

INTRODUCTION

Thermoelectric materials have attracted muchattention for decades due to their application insolid-state heat engines, which generate electricalpower from a temperature difference or convertelectrical power into cooling.1 The performance ofthermoelectric materials is quantified by a figure ofmerit, given by ZT = S2rT/k, where S, r, T, and kare the Seebeck coefficient, electrical conductivity,absolute temperature, and thermal conductivity,respectively.2 The power factor in the dimensionlessfigure of merit (P = S2r) comprises competing fac-tors and is important because it is directly related tothe usable power attainable from the thermoelec-tric. A high ZT is achieved by creating a materialwith a high power factor and low thermal conduc-tivity. The power factor and thermal conductivityare determined by the details of the electronicstructure and scattering of charge carriers, andthus are not independently controllable.3

In past decades, significant improvements in ZThave been achieved in nanostructured inorganicmaterials such as Bi2Te3

4 and BiSbTe5 alloys.Nanostructures of these inorganic materials such asnanodispersions and nanoscale heterogeneitieshave been found to be effective in reducing thethermal conductivity to a greater degree than thepower factor, resulting in an enhanced ZT for suchbulk thermoelectric materials.6 However, inorganicthermoelectric materials have distinct disadvan-tages including high cost, poor processability,shortage of raw materials, and toxicity, whichimpede widespread use of thermoelectric generatorsfor energy harvesting on a large scale.7

Organic materials, especially conductive polymers(c-polymers) such as polyacetylene (PC), polypyrrole(PPy), polyaniline (PANi), and polythiophene (PTh),show promise for use as alternatives to inorganicmaterials due to their increasing electrical conduc-tivity, low thermal conductivity, environmentalstability, nontoxicity, flexibility, rich resources, andfacile processing into versatile forms.7,8 However,the poor power factors of single c-polymers excludethem as feasible candidate thermoelectric materials.Recently, it was reported that, when single-wall(Received September 29, 2013; accepted December 17, 2013)

Journal of ELECTRONIC MATERIALS

DOI: 10.1007/s11664-013-2958-4� 2014 TMS

nanotubes (SWNTs) or multiwall nanotubes (MWNTs)of carbon are incorporated into c-polymers, thethermoelectric performance of the composites couldbe remarkably enhanced compared with both oftheir components.8–10 This is mainly due to theintrinsic high electrical conductivity as well as theextremely stable one-dimensional (1D) nanostruc-ture of CNTs, which are highly advantageous toimprove the power factor of the polymer matrix byenhancing the carrier mobility.11 Therefore, thecombined advantages of CNTs and c-polymer makeCNT/c-polymer composites competitive materials foruse in thermoelectric applications.

In practical development of CNT/c-polymer com-posites, agglomeration of CNTs due to the stronginternanotube van der Waals interactions is a majorobstacle to utilization of CNTs in polymer compos-ites.12 In solving this CNT dispersion problem, cova-lent functionalization of CNTs allows high-qualitydispersions, and improves compatibility with polymermatrixes.13 However, covalent functionalization ofCNTs can cause severe disruption of the p-system,resulting in extensive degradation of the electronictransport properties of the nanotubes, which in turnleads to reduced thermoelectric performance.14 Acompromise can be achieved through noncovalentattachment via a third phase, such as surfactants,micromolecules, or conjugated polymers. Such non-covalent bonding is realized by p–p-type interactionswith the CNTs and the third phase and hence does notunduly perturb the electronic properties of the CNTs.Nevertheless, most dispersants and micromoleculesare electrical insulators that hinder electron transportacross the CNT–matrix interface. Conjugated poly-mers are quite efficient dispersants because of theirlong-chain structure that allows them to wrap aroundthe CNTs by disrupting the van der Waals interac-tions between their walls.15 Recently, it has beenreported that conjugated PPy can effectively stabilizeand disperse CNTs, thereby enhancing the electricalconductivity by preventing settling and aggregation ofCNTs.16 PPy emerged as an important conductivepolymer and has been used in a wide range of appli-cations, including electrodes for capacitors, photodi-odes, solar cells, and chemical sensing.17–19

Here, we suggest a facile method for obtainingMWNTs noncovalently functionalized with PPy(PPy-MWNTs) based on in situ polymerization andtheir use to synthesize PPy-MWNT/PANi compos-ites exhibiting enhanced thermoelectric perfor-mance. PANi was chosen as a matrix materialbecause of its good electrical transport propertiesamong all the polymers.20 The structure and mor-phology of these composites were studied, and theirthermoelectric properties are discussed as a func-tion of MWNT loading.

EXPERIMENTAL PROCEDURES

MWNTs with average diameter of 30 nm andlength of 10 lm were purchased from Shenzhen

Nanotech Port Co. Ltd. (China). Pyrrole (99.8%;Capchem) was distilled prior to use and stored at�10�C in nitrogen atmosphere. All other reagentswere of analytical reagent grade and used withoutfurther purification.

MWNTs were functionalized with PPy by in situpolymerization.21 The as-received MWNTs werepreviously treated with lithium dodecyl sulfate(LDS). Approximately 0.2 g MWNTs was dispersedin a mixture of ethanol solution (100 mL) and2.0 mL LDS under ultrasonication for 1 h. Themixture was washed several times with deionizedwater using a centrifuge. The resulting dispersedsolution was filtered onto 1-lm-pore-size polytetra-fluoroethylene (PTFE) membrane and dried in avacuum oven at 50�C for 24 h. In the polymerizationprocedure, the pretreated MWNTs were dispersed indeionized water and then ultrasonicated for 30 min.Pyrrole monomer (0.02 mol) was then added, andthe solution was stirred for 1 h. Polymerization wascarried out by dropwise addition of 0.02 mol H2O2

and 0.45 g ammonium peroxydisulfate to the solu-tion for 24 h under stirring. Finally, the obtainedproducts were washed several times with distilledwater, filtered, and dried in vacuum for 24 h. NeatPPy was prepared under conditions similar to thoseused for synthesis of PPy-MWNTs but with theomission of the MWNTs.

To fabricate PPy-MWNT/PANi composites, vari-ous weight fractions of commercial PANi powderand PPy-MWNTs were mixed in 50 mL ethanol andfurther ultrasonicated for 1 h to form a homoge-neous dispersion. The mixed powders were dried inan oven at 60�C for 24 h, then molded into com-posite pellets using a hot-press machine at 90�Cunder 15 MPa for 20 min in argon atmosphere.Finally, the MWNT content (0 wt.%, 3.7 wt.%,8.2 wt.%, 15.1 wt.%, 28.6 wt.%) in the compositeswas determined based on the initial dry weight ofMWNTs and the final dry weight of the composites.For comparison, r-MWNT/PANi composites withraw MWNTs at the same filler contents were pre-pared using the same processing.

Scanning electron microscopy (SEM) observationswere conducted using a Leo Supra 50VP micro-scope. Fourier-transform infrared (FT-IR) spectrawere recorded on a Nicolet 6700 Fourier-transforminfrared instrument from KBr disks. x-Ray photo-electron spectroscopy (XPS) measurements wereperformed using an x-ray photoelectron spectrome-ter (AXIS ULTRA) with monochromated Al Ka

radiation. Raman spectra were recorded on a Jobin–Yvon spectrometer (Labram 1B) equipped with amicroscope. Electrical conductivity was measuredon compressed pellets using a resistance meter(Advantest) by a four-probe method.22 A homemadedevice was used to measure the Seebeck coefficient.The measurements were carried out by establishinga temperature gradient DT between the ends andmeasuring the resulting potential difference DV fordifferent values of DT. The Seebeck coefficient was

Liu and Yu

determined as the ratio between the electricalpotential, DV, and the temperature difference, DT,given as23

S ¼ DV

DT: (1)

The thermal diffusivity of the samples at roomtemperature was measured using an LFA 447Nanoflash (NETZSCH, Capitola, CA, USA). Thethermal conductivity, k, of the samples was calcu-lated according to24,25

k ¼ aqCp; (2)

where a is the thermal diffusivity (m2/s), Cp is thespecific heat (J/kg K), and q is the bulk density (kg/m3) of the material. Cp was measured on a differ-ential scanning calorimetry (DSC) system, and theq of specimen was measured by the Archimedesmethod.

RESULTS AND DISCUSSION

The morphology of raw MWNTs (r-MWNTs) andPPy-MWNTs was characterized by SEM, as shownin Fig. 1. It is apparent from Fig. 1a that ther-MWNTs have a smooth surface and their bundlesrandomly intertwine together to form a MWNTnetwork (Fig. 1a). The MWNT diameters were inthe range of about 20 nm to 40 nm. In contrast,after functionalization with PPy, it is seen fromFig. 1b that the whole surfaces of the MWNTs wereuniformly covered with a PPy layer. After func-tionalization, the originally smooth MWNTs nowexhibited a rather rough surface; however, they stillretained the original diameter of the MWNTs,indicating the formation of a thin coating on thenanoscale. The average thickness of the PPy nano-layer coated on the MWNTs was determined to be�8 nm by image analysis.

Figure 2 shows the FT-IR spectra of the r-MWNTs,PPy, and PPy-MWNTs. No obvious absorption peaksare observed for the r-MWNTs, indicating very fewfunctional groups on their surface. In the FT-IR

spectrum for neat PPy, the absorption peaks at1549 cm�1 and 1465 cm�1 are associated with C–Cand C–N stretching vibrations in the pyrrole ring,respectively. The peak at 1198 cm�1 is related to C–Nstretching vibration, and the peaks at 1043 cm�1 and1312 cm�1 are assigned to C–H in-plane vibration.These observations are in good agreement with pre-vious reports.26,27 The characteristic peaks of PPy arealso observed in the FT-IR spectrum of PPy-MWNTs.In addition, it is found that the characteristic PPypeaks in PPy-MWNTs slightly shift to lower wave-number compared with those of neat PPy. The shift ofthese peaks is attributed to the p–p interactionbetween the carbon rings of MWNTs and pyrrolemonomer of PPy.28 These results suggest that theMWNTs were successfully functionalized by PPy vianoncovalent p–p interaction. Successful functionali-zation of MWNTs by PPy is also proved by XPS.Figure 3 shows the high-resolution N 1s XPS spec-trum of the PPy-MWNTs. The N 1s XPS spectrum canbe deconvoluted into three distinct curves with peakslocated at 399.5 eV, 401.3 eV, and 402.6 eV. Thecharacteristic binding energy peak at 399.5 eV cor-responds to the benzenoid amine (–NH–). The peaksat 401.3 eV and 402.6 eV are related to the proton-ated benzenoid amine (–NH+–) and protonatedquinonoid imine (NH+–), respectively.29,30 Thesecharacteristic binding energy peaks (399.5 eV,401.3 eV, and 402.6 eV) assigned to PPy indicate thepresence of functionalized PPy on the MWNTs, con-sistent with the FT-IR results.

Raman spectra were used to determine theinteractions in the PPy-MWNT/PANi composites, asshown in Fig. 4. In the spectrum for the r-MWNTs,two strong characteristic peaks at 1356 cm�1 and1578 cm�1 can be attributed to the graphite struc-ture D band and G band, respectively. The D band isattributed to the reduction in size of the in-plane Csp2 atoms, while the G band represents first-orderscattering of the E2g vibrational mode.31 In thespectrum for the PPy-MWNTs, both the D and Gbands slightly shift to 1353 cm�1 and 1578 cm�1,revealing the p–p interaction between the PPy andthe MWNTs.32 Additionally, two small peaks at

Fig. 1. SEM images of (a) r-MWNTs and (b) PPy-MWNTs.

Thermoelectric Enhancement in Polyaniline Composites with Polypyrrole-FunctionalizedMultiwall Carbon Nanotubes

930 cm�1 and 1040 cm�1 corresponding to thebipolaron and polaron structure of PPy33 appear inthe spectrum for the PPy-MWNTs. Thus, theRaman spectrum of the PPy-MWNTs shows curvesrelated to both the PPy and MWNT components.These are in good agreement with the FT-IR andXPS results.

On incorporation of the PPy-MWNTs into the PANimatrix, the spectrum of the PPy-MWNT/PANi com-posites exhibits the characteristic bands of each com-ponent, in which the typical PANi peaks due to C–Hbending of the quinoid/benzenoid ring (1162 cm�1),C–N stretching (1217 cm�1), C=N stretching of thequinoid ring (1485 cm�1), and C–C stretching of thebenzenoid rings (1589 cm�1)34 are also observed.Nevertheless, compared with neat PANi, the intensityof the Raman peaks of the composites at around1485 cm�1 assigned to C–H bending of the quinoidring decreases after the introduction of the PPy-MWNTs into the PANi matrix. The decrease of the

intensity of the Raman peak at 1483 cm�1 is attrib-uted to site-selective interactions between the PANiquinoid rings and aromatic PPy rings of PPy-MWNTs,which induce the chemical transformation to moreordered PANi chains.11,35 Thus, the introduction ofPPy-MWNTs helps to align the PANi chains towardsan ordered molecular arrangement, favoring chargetransport. Previously, it has been reported that in situpolymerization of aniline in the presence of MWNTsleads to effective site-selective charge-transfer inter-actions between the MWNTs and quinoid ring ofPANi.35 In this study, we demonstrate that such typeof site-selective charge-transfer interaction could alsooccur between the quinoid rings of PANi and PPy-MWNTs. In addition, it is apparent that the peaks at1589 cm�1, 1485 cm�1, 1217 cm�1, and 1160 cm�1

downshift to lower wavenumber after the addition ofPPy-MWNTs, This indicates a strong interaction be-tween the PPy-MWNTs and PANi. These interactionsinclude electrostatic forces, hydrogen bonding, and p–p interaction between the PPy-MWNTs andPANi.36,37

SEM images of fracture surfaces of r-MWNT/PANiand PPy-MWNT/PANi composites at 28.6 wt.% fillerloading are shown in Fig. 5. As seen, large MWNTclusters and long MWNT pull-out (poor interfacialbonding) can be clearly observed for the r-MWNT/PANi composites, in turn leading to the defectivestructure of the composites. In contrast, it is clear thatthe MWNTs are well dispersed in the PPy-MWNT/PANi composites. This is attributed to the role of thesurface-functionalized PPy which avoids tube–tubecontacts and formation of agglomerations. Also, mostMWNTs are embedded within the PANi matrix,implying that the interaction between the MWNTsand the PANi matrix is strong. Therefore, theattachment of the PPy nanolayer on the nanotubesurfaces can lead to homogeneous dispersion of theMWNTs in the PANi matrix and strong PANi–MWNT

2500 2000 1500 1000 500

PPy-MWNTs

PPy

r-MWNTs 1549

1043

1198

1312

Wavenumber (cm-1)

Tra

nsm

itta

nce

(%

)

1465

Fig. 2. FT-IR spectra of r-MWNTs, PPy, and PPy-MWNTs.

NH+-

-NH+-

-NH-

Inte

nsi

ty (

a.u

.)

Binding evergy (eV)410 408 406 404 402 400 398 396 394 392

Fig. 3. N 1s XPS spectrum of PPy-MWNTs.

1800 1600 1400 1200 1000

9301040

1573

1353

1578

15891217

PPy-MCNTs

r-MCNTs

PANi

PPy-MCNT/PANi

Inte

nsi

ty (

a.u

.)

Raman shift (cm-1)

11601485

1356

Fig. 4. Raman spectra of r-MWNTs, PPy-MWNTs, PANi, and PPy-MWNT/PANi composites.

Liu and Yu

adhesion, which is expected to improve the thermo-electric properties of the composites.

The thermoelectric properties of the compositeswere measured at room temperature, as shown inFig. 6. Figure 6a shows the electrical conductivity ofthe PPy-MWNT/PANi and r-MWNT/PANi compos-ites as a function of MWNT loading. As seen, theelectrical conductivity of the PPy-MWNT/PANicomposites increased dramatically with increasingMWNT loading, reaching a maximum value of3034 S/m at 28.4 wt.% MWNT loading. This value iscomparable to that of previous CNT/PANi compos-ites.9,11 This large conductivity enhancement isattributed to the formation of a highly electricallyconductive network, as realized by the high con-ductivity quality and large aspect ratio of theMWNTs and their homogeneous dispersion throughthe noncovalent functionalization by PPy. In addi-tion, the strong filler–matrix interactions promotetransfer of charge carriers across the interface of thecomposites, thus largely increasing the electricalconductivity.38 However, the electrical conductivityof the r-MWNT/PANi composite is 912 S/cm, whichis more than three times lower than that of the PPy-MWNT/PANi composite with the same MWNTloading of 28.6 wt.%. The underperformance of theelectrical conductivity in the r-MWNT/PANi com-posites is mainly related to the presence of MWNTclusters, which impede the formation of an electricalnetwork and thus dramatically reduce the electricalconductivity enhancement due to the MWNTs.

Figure 6b shows the Seebeck coefficient of PPy-MWNT/PANi and r-MWNT/PANi composites withdifferent MWNT contents. The Seebeck coefficientof the PPy-MWNT/PANi composites increases from4.7 lV/K to 31.2 lV/K (564% enhancement) withincrease of the MWNT loading from 0 wt.% to28.4 wt.%. The increased Seebeck coefficient is dueto the energy filtering effect, which allows prefer-ential scattering of low-energy charge carriersover high-energy charge carriers at the interface.3 Itis noteworthy that the introduction of the PPynanolayer significantly increases the number ofnanointerfaces, since the original, single type of

MWNT–PANi nanointerface is split into two typesof MWNT–PPy and PPy–PANi nanointerface due tothe presence of the PPy nanolayer on the surface ofthe MWNTs. Such a larger number of nanointer-faces can enhance the energy filtering effect, whichprovides a remarkable increase in the Seebeckcoefficient. Although increased nanointerfaces alsocause a decrease in the electrical conductivity due toa possible interfacial electron scattering effect, thegood conductive nature of the PPy nanolayer cancompensate well for this electron scattering. Inaddition, interchain and intrachain hopping haveimportant effects on charge-carrier transport inpolymers.39 The carrier mobility is strongly depen-dent on the conformation and arrangement of thepolymer chains. Highly oriented polymer chains canreduce the barriers to interchain and intrachainhopping and allow carriers to move easily.10 Previ-ous Raman analysis suggested a preferably orientedstructure of the PANi matrix due to the introductionof PPy-MWNTs (Fig. 4). Therefore, the ordering ofthe polymer chains in the hybrids enhances thecarrier mobility and results in a further improvedSeebeck coefficient. Similar to the electrical con-ductivity behavior, the r-MWNT/PANi compositesexhibit a greatly reduced Seebeck coefficient com-pared with the PPy-MWNT/PANi composites, whichis related to the MWNT clusters. The reason is thatthe MWNT clusters, on the one hand, cause moreconjugated defects when assembling with the PANimatrix. On the other hand, the MWNT clustershinder the growth of crystallization in the PANichains and lead to more disordered PANi. Both theconjugated defects and the disordered PANi chainsgreatly decrease the carrier mobility, resulting in amuch lower Seebeck coefficient.7 In addition, theabsence of a PPy nanolayer between the MWNTsand PANi matrix would effectively weaken theenergy filtering effect owing to the reduced numberof nanointerfaces, leading to a further reduction inthe Seebeck coefficient of the r-MWNT/PANi com-posites.

The power factor (S2r) was calculated based onthe values of the electric conductivity and Seebeck

Fig. 5. Fracture surfaces of (a) r-MWNT/PANi and (b) PPy-MWNT/PANi composites with 28.6 wt.% MWNTs.

Thermoelectric Enhancement in Polyaniline Composites with Polypyrrole-FunctionalizedMultiwall Carbon Nanotubes

coefficient and is shown in Fig. 6c. Compared withthe values of 0.006 lW/m K2 for the PANi matrixand 0.1 lW/m K2 for the r-MWNT/PANi composites,a high power factor of 3.1 lW/m K2 at room tem-perature was attained for the PPy-MWNT/PANicomposite at 28.4 wt.% MWNT loading, being morethan two and one order of magnitude larger thanthat of neat PANi and r-MWNT/PANi, respectively.This implies that the power factor of conductingpolymers could be significantly enhanced by incor-poration of PPy-MWNTs.

Regarding the thermal conductivity, it is clear fromFig. 6d that only a slight increase in the thermalconductivity is observed with the addition of thePPy-MWNTs into the PANi matrix: the thermal con-ductivity remains at a very low level of 0.34 W/m K,corresponding to a 51% increment even at the highestMWNT concentration of 28.6 wt.%. Note that anenhancement of 125% in the thermal conductivity hasbeen observed for SWNT/epoxy composites on addi-tion of only 1 wt.% SWNTs.40 These results indicatethat incorporation of PPy-MWNTs enables effectivecharge-carrier transport between the PANi matrixand MWNTs but does not provide good thermaltransport in the composites. The reduced thermalenhancement is thought to be primarily due to a large

thermal resistance induced by the PPy nanolayer thatformed on the MWNT surfaces. Since PPy is thermallyinsulating, the PPy nanolayer on the MWNTs acts as athermal barrier and limits the MWNT–PANi thermaltransport. In addition, previous theoretical andexperimental investigations have indicated thatnanostructures, including nanoinclusions and nano-interfaces in composites, can effectively scatter pho-nons and reduce the thermal conductivity.6 Asmentioned above, the PPy-MWNT/PANi nanostruc-tures form a great amount of nanointerfaces, whichmay act as effective scattering centers for phonons,thus decreasing the thermal conductivity. Conse-quently, the composites maintain almost the samethermal conductivity as neat PANi. Interestingly, thelow thermal conductivity is also observed for ther-MWNT/PANi composites, with both the r-MWNT/PANi and PPy-MWNT/PANi composites exhibitingnearly the same thermal conductivity behavior withMWNT loading. Owing to the highly resistive thermaljunctions, MWNT clusters have thermal conductivityof �1 W/m K, three orders of magnitude lower thanthat of individual nanotubes.41 MWNT clusters canthus be treated as low-conductive inclusions in thePANi matrix, which leads to a large diminishment inthe thermal conductivity enhancement due to the

0

1000

2000

3000

4000

5000

MWNT/PANi

PPy-MWNT/PANi

Ele

ctri

cal c

on

du

ctiv

ity,

S/m

MWNT loading, wt.%

(a)

0

10

20

30

40

50

MWNT/PANi

PPy-MWNT/PANi

(b)

MWNT loading, wt.%

See

bec

k co

effi

cien

t,

V/K

10-3

10-2

10-1

10

µ

0

101

MWNT/PANi

PPy-MWNT/PANi

(c)

Po

wer

fac

tor,

W

/mK

2

MWNT loading, wt.%

0 5 10 15 20 25 30 0 5 10 15 20 25 30

0 5 10 15 20 25 30 0 5 10 15 20 25 300.1

0.2

0.3

0.4

0.5(d)

MWNT loading, wt.%

Th

erm

al c

on

du

ctiv

ity,

W/m

KPPy-MWNT/PANi

MWNT/PANi

µ

Fig. 6. Thermoelectric properties of r-MWNT/PANi and PPy-MWNT/PANi composites with various MWNT loadings: (a) electrical conductivity,(b) Seebeck coefficient, (c) power factor, and (d) thermal conductivity.

Liu and Yu

MWNTs. The formation of MWNT clusters alsodecreases the total contact area between the MWNTsand the PANi matrix,42 further reducing the effectivethermal conductivity of the r-MWNT/PANi compos-ites.

An enhanced power factor and reduced thermalconductivity are both beneficial to the thermoelectricperformance. Therefore, in all respects, the methodreported herein has great potential to enhance thethermoelectric performance of PANi composites.Nevertheless, the maximum power factor obtained inthe PPy-MWNT/PANi composites is still much lowerthan that of conventional thermoelectric materialssuch as Bi2Te3,43 suggesting that there is still muchroom for further enhancement in the thermoelectricperformance of the present PPy-MWNT/PANi com-posites. In view of this, work is ongoing to improve thepower factor by improving the material processing, byfurther increasing the filler loading, by better tailoringthe nanostructure of the PPy layer, and by selectingother conductive polymers such as PTh with betterthermoelectric properties for use as a coating layer.

CONCLUSIONS

We demonstrated an effective method for improvingthe thermoelectric performance of the conductivepolymer PANi by mixing with PPy-functionalizedMWNTs. SEM, FT-IR, XPS, and Raman studies indi-cated that MWNTs were successfully functionalizedby a PPy nanolayer through noncovalent p–p inter-action. Raman analysis also confirmed the formationof ordered PANi chains in the composites by incorpo-ration of PPy-MWNTs. The surface-functionalizedPPy nanolayer on the MWNTs yielded a homogeneousdispersion of MWNTs and strong PANi matrix–MWNT adhesion, while MWNT clusters were clearlypresent in the r-MWNT/PANi composites. Comparedwith neat PANi and MWNT/PANi composites, PPy-MWNT/PANi composites showed significantenhancement in both electrical conductivity and See-beck coefficient. As a result, a maximum power factorof 3.1 lW/m K2 at room temperature was attained,being more than two and one order of magnitude lar-ger than that of neat PANi and MWNT/PANi,respectively. At the same time, both r-MWNT/PANiand PPy-MWNT/PANi composites retained the lowthermal conductivity of PANi even with high MWNTloading. The present approach may potentially beextended to other material systems and provide afacileandgeneral strategy for synthesizing conductivepolymer composites with enhanced thermoelectricperformance.

REFERENCES

1. M.S. Dresselhaus, G. Chen, M.Y. Tang, R. Yang, H. Lee, D.Wang, Z. Ren, J.P. Fleurial, and P. Gogna, Adv. Mater. 19,1043 (2007).

2. J.R. Sootsman, D.Y. Chung, and M.G. Kanatzidis, Angew.Chem. Int. Ed. 48, 8616 (2009).

3. D. Medlin and G. Snyder, Curr. Opin. Colloid Interface Sci.14, 226 (2009).

4. X. Tang, W. Xie, H. Li, W. Zhao, Q. Zhang, and M. Niino,Appl. Phys. Lett. 90, 012102 (2007).

5. W. Xie, X. Tang, Y. Yan, Q. Zhang, and T.M. Tritt, Appl.Phys. Lett. 94, 102111 (2009).

6. C.J. Vineis, A. Shakouri, A. Majumdar, and M.G. Kanatzidis,Adv. Mater. 22, 3970 (2010).

7. O. Bubnova and X. Crispin, Energy Environ. Sci. 5, 9345(2012).

8. D. Kim, Y. Kim, K. Choi, J.C. Grunlan, and C. Yu, ACSNano 4, 513 (2009).

9. C. Yu, Y.S. Kim, D. Kim, and J.C. Grunlan, Nano Lett. 8,4428 (2008).

10. C. Meng, C. Liu, and S. Fan, Adv. Mater. 22, 535 (2010).11. Q. Yao, L. Chen, W. Zhang, S. Liufu, and X. Chen, ACS

Nano 4, 2445 (2010).12. X.L. Xie, Y.W. Mai, and X.P. Zhou, Mater. Sci. Eng. R 49, 89

(2005).13. A. Hirsch, Angew. Chem. Int. Ed. 41, 1853 (2002).14. J. Zhao, J.P. Lu, J. Han, and C.K. Yang, Appl. Phys. Lett. 82,

3746 (2003).15. P. Petrov, F. Stassin, C. Pagnoulle, and R. Jerome, Chem.

Commun. 2904 (2003).16. J. Wang, Y. Xu, X. Chen, and X. Sun, Compos. Sci. Technol.

67, 2981 (2007).17. A. Ramanavicius, A. Ramanavicien _e, and A. Malinauskas,

Electrochim. Acta 51, 6025 (2006).18. L.Z. Fan and J. Maier, Electrochem. Commun. 8, 937 (2006).19. J. Wu, Q. Li, L. Fan, Z. Lan, P. Li, J. Lin, and S. Hao,

J. Power Sources 181, 172 (2008).20. R. Chan Yu King, F. Roussel, J.-F. Brun, and C. Gors,

Synth. Met. 162, 1348 (2012).21. T.M. Wu, H.L. Chang, and Y.W. Lin, Compos. Sci. Technol.

69, 639 (2009).22. K. Chu, C. Jia, W. Li, and P. Wang, Phys. Status Solidi A

210, 594 (2013).23. Y. Klein, S. Hebert, A. Maignan, S. Kolesnik, T. Maxwell,

and B. Dabrowski, Phys. Rev. B 73, 052412 (2006).24. K. Chu, W. Li, and H. Dong, Appl. Phys. A 111, 221 (2013).25. K. Chu, C. Jia, and W. Li, Appl. Phys. A 110, 269 (2013).26. G.I. Mathys and V.T. Truong, Synth. Met. 89, 103 (1997).27. N.G. Sahoo, Y.C. Jung, H.H. So, and J.W. Cho, Synth. Met.

157, 374 (2007).28. J. Jin, T. Iyoda, C. Cao, Y. Song, L. Jiang, T.J. Li, and D.B.

Zhu, Angew. Chem. Int. Ed. 113, 2193 (2001).29. E. Benseddik, M. Makhlouki, J. Bernede, S. Lefrant, and A.

Pron, Synth. Met. 72, 237 (1995).30. K. Cheah, M. Forsyth, and V.T. Truong, Synth. Met. 101, 19

(1999).31. W. Maser, A. Benito, M. Callejas, T. Seeger, M. Martınez, J.

Schreiber, J. Muszynski, O. Chauvet, Z. Osvath, and A.Koos, Mater. Sci. Eng. C 23, 87 (2003).

32. S. Tsuzuki, K. Honda, T. Uchimaru, M. Mikami, and K.Tanabe, J. Am. Chem. Soc. 124, 104 (2002).

33. Y.C. Liu and B.J. Hwang, Synth. Met. 113, 203 (2000).34. M.C. Bernard and A. Hugot-Le Goff, Synth. Met. 85, 1145

(1997).35. M. Cochet, W.K. Maser, A.M. Benito, M.A. Callejas, M.T.

Martınez, J.-M. Benoit, J. Schreiber, and O. Chauvet, Chem.Commun. 1450 (2001).

36. M. Trojanowicz, Trac-Trend. Anal. Chem. 25, 480 (2006).37. J. Kim, Pure Appl. Chem. 74, 2031 (2002).38. L. Hu, D. Hecht, and G. Gruner, Nano Lett. 4, 2513 (2004).39. H. Sirringhaus, P. Brown, R. Friend, M. Nielsen, K.

Bechgaard, B. Langeveld-Voss, A. Spiering, R.A. Janssen, E.Meijer, and P. Herwig, Nature 401, 685 (1999).

40. M. Biercuk, M.C. Llaguno, M. Radosavljevic, J. Hyun, A.T.Johnson, and J.E. Fischer, Appl. Phys. Lett. 80, 2767 (2002).

41. K. Chu, Q. Wu, C. Jia, X. Liang, J. Nie, W. Tian, G. Gai, andH. Guo, Compos. Sci. Technol. 70, 298 (2010).

42. K. Chu, C. Jia, L. Jiang, and W. Li, Mater. Des. 45, 407(2013).

43. D. Vashaee and A. Shakouri, Phys. Rev. Lett. 92, 106103(2004).

Thermoelectric Enhancement in Polyaniline Composites with Polypyrrole-FunctionalizedMultiwall Carbon Nanotubes