Preparation and characterization of polymer -copper composites by electrical explosion of wire

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Copyright © 2013 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 13, 7728–7733, 2013

Synthesis and Characterization of ConductingPolyaniline-Copper Composites

Aijie Liu1, Luong Huu Bac2, Ji-Soon Kim1, Byoung-Kee Kim1, and Jin-Chun Kim1�∗1School of Materials Science and Engineering, University of Ulsan, Daehak-ro 102, Nam-gu,

Ulsan, 680-749, South Korea2School of Engineering Physics, Hanoi University of Science and Technology Hanoi, Vietnam

Conducting polymer composites have many interesting physical properties and important applica-tion potentials. Suitable combinations of metal nanoparticles with conductive polymers can result incomposite materials having unique physical and chemical properties that can have wide applicationpotential in diverse areas. In this work, copper nanoparticles were fabricated by electrical explo-sion of wire (EEW) in solution of polyacrylic acid (PAA) and ethanol. Conductive polyaniline-copper(PANI-Cu) composites have been synthesized by in-situ polymerization of aniline in the fabricatedcopper suspension. Optical absorption in the UV-visible region of these suspensions was measuredin the range of 200–900 nm. Morphology and structure of the composites were characterized byscanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier-transform infrared spec-tra (FTIR). Pure copper nanoparticles were uniformly dispersed into the polymer matrix. Thermalstability of the composites was characterized by thermogravimetric analysis (TGA). Electrical con-ductivity measurements indicated that the conductivity of the composites was higher than that ofpure polyaniline and increased with increasing content of copper.

Keywords: Polyaniline, Copper Nanoparticles, Composites, Electrical Explosion of Wire (EEW),Electrical Conductivity.

1. INTRODUCTION

Metal-polymer composites have attracted much atten-tion recently due to their interesting properties and havebeen widely used in pratical applications.1�2 In additionto the advantageous properties of metals and polymers,these composites exhibit many new characteristics that thesingle-phase materials do not have. Among these materi-als, nanocomposites formed by metal nanoparticles (NPs)dispersed in electrically conducting polymers, such aspolyaniline (PANI) or polypyrrole (PPy), have receivedmuch attention in the past few years. These compositenanomaterials are expected to display several synergisticproperties between the polymer and the metal nanopar-ticles, making them potential candidates for applicationin several fields such as catalysis,3 sensors,4 memorydevices,5 colored particulate emulsifier or colored filmsetc.6 PANI has three kinds of structures:(1) fully oxidized polymer chains (pernigraniline base)containing imines groups only,(2) fully reduced PANI chains (leucoemeraldine base)containing amine groups only and

∗Author to whom correspondence should be addressed.

(3) intermediate oxidized state (emeraldine base) contain-ing equal number of amine groups and imine groups.7

In these structures, only emeraldine base can be easilytransformed to conducting state by dopping with suitabledopants, forming salt forms. The emeraldine salt is themost conductive form of PANI. Due to the great chromaticvariation resulting from the different oxidation states of thePANI by doping and dedoping, the PANI has commonlyused as active component in various fields.Cu nanoparticles have been applied in many fields

because it has not only size effects but also high conduc-tivity. Conductivity of Cu is in the same order of mageni-tude as noble metals like Au and Ag. Cu is much cheaperthan the noble metals; it can reduce the cost of productsin practical applications. On the other hand, Cu nanopar-ticles can act as catalyst during polymerization to increaseproduction yield and change in the nanoparticles contentmay result in different thermal, mechanical, and electricalproperties of the prepared nanocomposites.8

Several processing methods are available for fabricat-ing metal-polymer composites. They mainly include phys-ical mixing,9 in-situ polymerization,10 sol–gel technique,11

melt blending12 processes. Although inherently different

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Liu et al. Synthesis and Characterization of Conducting Polyaniline-Copper Composites

processing methods have been attempted, they all addressimportant issues that affect composite properties, such asexfoliation of Cu nanoparticles’ bundles and ropes, homo-geneous dispersion in the matrix, alignment, and interfacialbonding.In the present study, copper nanocolloid was fabricated

by electrical explosion of wire (EEW) in PAA ethanolsolution, in which PAA act as a dispersant and protector.EEW is a simple method to fabricate metallic nanopar-ticles and metallic colloids.13–15 In this method, metallicnanoparticles were formed from the vapor state and con-densed into the solid in the liquid medium. PANI powderused for fabricating composite was commercially obtainedfrom Alfa Aesar in previous work,16 and it was in-situsynthesized by a rapid chemical polymerization in thecopper nanocolloid to form PANI-Cu nanocomposite inthis work. The synthesized composites was characterizedby field emission scanning electron microscopy (FESEM),Fourier transform infrared spectroscopy (FTIR), and X-raydiffraction (XRD), UV-vis absorbance spectroscopy. Ther-mal stability and degradation behaviors of the compositewere studied by thermal gravimetric analysis (TGA), andconductivity of the nanocomposite were carried out bystandard four-probe method.

2. EXPERIMENTAL DETAILS

2.1. Materials

All chemicals were purchased from Sigma Aldrich andused as received: aniline, ammonium peroxydisulfate(APS), poly(acrylic acid) (PAA), hydrochloric acid (HCl).Thin copper wire with diameter of 0.1 mm was obtainedfrom Nilaco Corp., Japan. All materials were of analyticalgrade. All aqueous solutions were prepared using distilledwater.

2.2. Synthesis

PAA-stabilized copper nanocolloid was prepared by EEW.PAA was disolved in 400 mL ethanol to get 0.01 M. Thissolution was used for the working medium of EEW to fab-ricate Cu nanoparticles. In brief, a Cu wire with a length of25 mm was installed between two electrodes and immersedinto the PAA solution. A high-density current was passedthrough the wire via discharging of a 30 �F capacitor. Thewire was melted, evaporated and then plasma formed dueto a high rate of the energy injection. The Cu vapor cooleddown and condensed into nanoparticles which dispersed inthe PAA solution. The Cu content was estimated from thenumber of explosion shots and the dimension of Cu wiresection.The PANI-Cu nanocomposites were obtained in the fol-

lowing manner: 292 �L of aniline (3.2 mmol) was mixedin 10 mL of 1 M HCl aqueous solution, and then thissolution was added with 20 mL of Cu colloid whichwas obtained as described earlier. The mixed solution was

maintained under vigorous stirring during 30 min. Ammo-nium peroxydisulfate (10 mL, 0.8 mmol) in 10 mL of 1MHCl aqueous solution was rapidly poured into the aniline-Cu NPs solution. The mixture was maintained under stir-ring for 12 hrs at room temperature. The precipitatedPANI-Cu nanocomposite was collected by centrifuge andrepetitively washed with distilled water, ethanol, and ace-tone until the upper liquid became colorless. The productswere dried under vacuum at 60 �C for 24 hrs. Pure PANIwithout copper was synthesized in a similar manner. In thiswork, pure PANI and three composite samples with differ-ent Cu concentrations (2, 4 and 6 wt.%) were fabricated,and they were lebelled as S0, S1, S2, S3, respectively.

2.3. Characterization

PANI-Cu composite particles were obtained after dryingin vacuum. Morphology of composites was evaluated byFE-SEM. Nanopowders were placed on an aluminium stuband sputter-coated with gold in order to avoid sample-charging problems. X-ray diffraction characterization wasperformed with Cu-K� in 2� range from 20� to 80�. TheX-ray beam was nickel-filtered Cu-K� radiation from asealed tube operated at 40 kV and 30 mA.FTIR spectra were recorded in the region of 500–

4000 cm−1. Solid specimens of PANI-Cu composites wereprepared by grinding powdery samples with KBr pow-der and then pressing the mixtures into tablets. The opti-cal absorption of the samples was recorded by UV-visiblespectrophotometer using n-methyl-2-pyrrolidinone (NMP)as a solvent.For measurement of the electrical conductivity, the dried

nanopowders were pressed into a pellet under a pressureof 60 MPa. The pellets were around 0.6 mm in thicknessand 10 mm in diameter. The electrical conductivity of thesamples was measured by a standard four-probe techniqueat room temperature.

3. RESULTS AND DISCUSSION

XRD patterns of pure PANI and PANI-Cu composites withdifferent Cu contents were presented in Figure 1. In XRDpattern of S0 sample, a broad peak at 2� = 25�1� is relatedto the diffraction of amorphous PANI in its emeraldine saltform. In XRD pattern of S1, S2 and S3 samples, beside thepeak of the PANI other peaks at 2� values of 43.3�, 50.4�

and 74.0� match copper in the face-centered cubic crystalstructure and correspond to the (hkl) planes (111), (200),(311), respectively (JCPDF). It can be observed that theintensity of these peaks increased with the increasing con-centration of the Cu nanoparticles. No cupric oxide and/orcuprous oxide peaks can be found in XRD patterns. It indi-cates that Cu nanoparticles were not oxidized during thepolymerization.Figure 2 shows the FTIR spectra of the bulk PANI

and PANI-Cu composites in 500 to 4000 cm−1 region.

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Fig. 1. XRD patterns of (S0� PANI and PANI-Cu composites with dif-ferent Cu content: (S1� 2, (S2� 4 and (S3� 6 wt.%.

The spectrum of S0 shows major characteristic vibrationalbands of PANI at 1575, 1496, 1390, 1311, 1157, and816 cm−1. The peak at 1575 cm−1 is assigned to C Cvibration in N Q N (Q denoting the quinoid moiety),while the C C stretching of N B N (B denoting thebenzene moiety) form is observed at 1496 cm−1.17 TheC N stretching bands of amine appear at 1391 cm−1 and1240 cm−1, which indicate C N stretching in QBQ andBBB, respectively. The peak at 1148 cm−1 is due to thequinoid unit of polyaniline (–N Q N–), and the peakat 816 cm−1 is from the C C and C H stretching of thebenzoid unit of polyaniline. The PANI-Cu nanocompositeshave similar peaks as PANI, however they shift slightlyto higher wavelength. The peaks at 1575, 1496, 1390 and1311 cm−1 are shifted to 1581, 1504, 1396 and 1334 cm−1,respectively. In addition three more peaks appeared in thespectrum of PANI-Cu composites. The peaks at 2978 cm−1

and 2900 cm−1 which indicate the symmetric and asym-metric aliphatic C H stretching vibrations18�19 are very

Fig. 2. FTIR spectra of (S0� pure PANI and PANI-Cu composite (S2�.

noticeable on the spectra of nanocomposite samples, theoccurrence of these bands on the nanocomposite spectrais a signal that the dodecanethiol molecules remain on thecopper NPs surface after the polumerization step.20 Thepeak at 1072 cm−1 is attributed to the in-plane bending ofC H bands of aromatic ring. The shifting and the addi-tion of peaks in the spectrum of the PANI-Cu compositesindicate the interaction of Cu with different sites of PANIduring polymerization.The structure formula of PANI can be described by the

schematic:

where y is the proportion of reduced unit and 1-y is theproportion of oxidized unit, 0 ≤ �1−y�≤ 1. When y = 0,polyaniline is in the form of fully oxidized pernigraniline(PAN); y= 0�5, polyaniline is in the form of half oxidized;y = 1, polyaniline is in fully reduced form. The intensityratio of the peaks at ∼1600/∼1500 cm−1 is a quantitativemeasure of the oxidation state of the polymer.21 From thespectrum we can see that the synthesized PANI is in thehalf-oxidized state.Figure 3 shows FESEM images of pure PANI and PANI-

Cu composites with different Cu content. It is observedthat the morphology of the pure PANI is uniformly in shortrod with diameter of 50 nm and length ∼400 nm. Thenanorods tend to agglomerate into interconnected nanorodnetworks. The polymerization of aniline was stronglyaffected by the presence of the Cu nanoparticles; the diam-eter and length of the PANI nanorod decreased drammati-cally. The average diameter of the rods reduced to ∼30 nmand the length was ∼120 nm, much smaller than purePANI. The structural change of the composites may becaused by surfactant PAA. This material can act as tem-plate during polyaniline polymerizaiton.22�23 Morphologyand size of polymer rod showed no change with changingof Cu contents. It can be seen in Figure 3 that microstruc-ture of the PANI-Cu composites with different Cu contentsall stayed all the same. The dispersion of the Cu nanopar-ticles in the polymer matrix is shown in Figure 4. It showsthat the Cu nanoparticles are homogeneously dispersed inthe polymer matrix.The PAA surfactant acts not only as a stabiliser but also

as a protector for Cu nanoparticles. When Cu nanoparticleswere fabricated in PAA solution, a thin PAA layer mayform on their surface.24 This PAA layer may protect theCu nanoparticles from oxidation by APS during polymer-ization. because Cu is an active element, it is easy tobe oxidized. And also, nanosized particles can be easilyoxidized due to high surface energy. During the polymer-ization, with presence of a strong oxidizing agent (APS),both copper and aniline may be oxidized. Guo showedthat copper is easier to be oxidized than aniline by APS.25

However, as seen in XRD results, the oxide phases of the

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Fig. 3. FESEM images of (S0� PANI and PANI-Cu composites with different Cu content: (S1� 2, (S2� 4 and (S3� 6 wt.%.

Fig. 4. Mapping image of PANI-Cu composite (S2�.

Cu were not detected; the Cu nanoparticles was not oxi-dized. The PAA polymer coating on the Cu nanoparticlesprotected Cu from the oxidation by APS. Otherwise, PAAfilm coating on the Cu nanoparticles surface will absorbaniline and guide aniline growing on the surface of the Cunanoparticle to get the PANI coating layer on Cu surface.UV-vis absorption spectra of pure PANI (S0� and PANI-

Cu composites (S1, S2, S3� are demonstrated in Figure 5.Two main absorption broad bands at 320 nm and 640 nmwere observed in the pure PANI. The first band is

attributed to –∗ transition in the benzenoid rings ofPANI and the second band is due to –∗ transition in thequinoid rings of PANI, respectively. The absorption banddue to –∗ transition in the quinoid rings has shifted to620 nm in PANI-Cu composites. The –∗ transition inthe benzenoid rings has also shifted to lower wavelength.Higher Cu content in composite causes more shift to thelower wavelength. The surface plasma absorption of theCu nanoparticles is about 560 nm and has not been foundin the optical absorption of all PANI-Cu composites. The




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Fig. 5. Optical absorption spectra of (S0� PANI and PANI-Cu compos-ites with different Cu content: (S1� 2, (S2� 4 and (S3� 6 wt.%.

reason is because the Cu content is small and PANI hasstrong absorption at 640 nm which overlaps surface plas-mon absorption of the Cu nanoparticles.Figure 6 shows the thermogravimetric curves for pure

PANI and PANI-Cu composite containing the 6 wt.%Cu (S2�. The TG curve of the pure PANI shows threestep weight loss processes. The first-step with about 10%weight loss at temperatures up to ∼250 �C can beattributed to the release of water molecule and the dopantions (HCl) or oligomer from the polymer matrix. The sec-ond and the third weight loss occur at temperature higherthan 300 �C is due to degradation of the polymer chain.The trend of the TG curve of the composite is similar tothat of the pure PANI. The starting temperature of thirdweight loss is 365 �C for pure PANI and this temper-ature is significantly shifted to a higher temperature byabout 82 �C for PANI-Cu composite. This result suggeststhat the composite is more thermally stable than the purePANI. This enhanced thermal stability of the composites

Fig. 6. Thermogravimetric curves of PANI and PANI-Cu composite.

can be ascribed to the interaction between the PANI andCu nanoparticles.The electrical conductivity of pure PANI and PANI-Cu

with different Cu contents was measured by a standardfour-probe technique at room temperature. The conductiv-ity of S0, S1, S2, S3 were 3.0, 6.4, 7.9 and 8.8 S cm−1,respectively. It is evident that the electrical conductivity ofthe PANI-Cu composites is higher than that of pure PANI.It increased with increasing content of the Cu in compos-ites. The reason for the improved electrical conductivitycan be explained via electronic tunnelling.26 Free electronsof Cu metal can be moved through Cu nanoparticles. Asmore Cu nanoparticles are added to the composites, theconductivity increased appreciably which can be due tothe increased enhancement in electron mobility owing toincreased tunneling probability.27

4. CONCLUSIONS

In summary, we have prepared copper nanocolloid byEEW and PANI-Cu composites with different copper con-tents by rapid chemical oxidative method. FESEM imagesshowed that nanostructure of the PANI was formed in shortnanorod with diameter of 30 nm and length of 120 nm.Copper nanoparticles were hormogeneously dispersed inthe polymer matrix. FTIR data showed that the incorpo-ration of the Cu nanoparticles resulted in some shifts inplasmon vibrations of the PANI. The synthesized PANIwas in the form of half-oxidized state. TG analysis indi-cated enhancement of thermal stability of the composite.Electrical measurement showed increase in conductivity ofthe composites. Conductivity of the PANI-Cu compositeincreased with increasing content of Cu.

Acknowledgments: This work was supported bythe 2012 Research Fund of University of Ulsan.

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Received: 1 May 2012. Accepted: 21 January 2013.


Preparation and characterization of polymer -copper composites by electrical explosion of wire

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