Electropolymerization of Polypyrrole/Carbon Nanotube Nanocomposite Films over anElectrically Nonconductive Membrane

Pejman Hojati-Talemi and George P. Simon*Department of Materials Engineering, Monash UniVersity, Clayton, Victoria 3800, Australia

ReceiVed: February 12, 2010; ReVised Manuscript ReceiVed: July 15, 2010

A novel method for the electropolymerization of polypyrrole/carbon nanotube composites on an electricallyinsulating, porous membrane is presented. In order to study the mechanisms relating to this process, differentsamples using carbon nanotubes with varying concentrations of functional groups were prepared and themorphology, electrical conductivity, thermal stability, and X-ray photoelectron spectroscopy (XPS) spectraof the resulting samples measured and used to confirm the explanation for the formation of these compositestructures.

1. Introduction

The relatively high conductivity of electrically conductivepolymers and their interesting physical and chemical properties,has meant that these materials have been intensively studied inrecent years.1 To promote high levels of conductivity, nano-composites combining conductive carbon nanotubes (CNTs) andan intrinsically conducting polymer (ICP) such as polyanilineand polypyrrole (PPy) have been reported by many researchgroups. Conductive nanomaterials such as carbon nanotubes witha high surface area allow electrochemical deposition or polym-erization of significant amounts of conductive polymer in theform of a thin layer of polymer on the surface of the nanotubes,leading to a porous, conductive, and electrochemically activestructure with a high surface area.2,3 Such structures have manypotential applications, such as electrodes for rechargeablebatteries,4-6 biosensors,7,8 field emission devices,9,10 and assupercapacitors.11 For some applications, the space between thetwo separated electrodes is filled with an electrolyte, a dielectric,or just a vacuum. The ability to deposit conductive, compositematerials on the surface of a nonconductive membrane whichcan also act as a spacer would greatly facilitate the process ofmaking such devices. Electropolymerization of ICPs has beenproven to be a good method for the synthesis and deposition ofsuch conductive polymers, allowing the production of polymercoatings with high levels of conductivity and chemical stability.12,13

This method is, however, largely limited to conductive sub-strates. Although some specially synthesized or modified ICPscan be dissolved in solvents,14,15 most of them are intractableand insoluble in common organic solvents in their original form;therefore, preparation of the desired composite structures is atime-consuming process and requires a series of different stepsand components.11,16,17 Because of this technological barrier,some research groups have tried to use electrodeless methodsfor the deposition of conductive polymers on differentsubstrates.18-20 While there has been some success in formingthin films of conductive polymers over surfaces using suchtechniques, the slow rates of polymerization and unavoidablepolymerization in the solution make these methods inefficient.

In this article, we describe a new method for fast and directelectropolymerization of PPy/CNT composites on an electrically

insulating nano- or microporous substrate such as polycarbonateor anodized aluminum oxide (AAO) membranes. Interestingly,the presence of CNTs as solid and bulky counterions plays akey role in limiting the polymer deposition to the surface ofthe membrane, keeping it away from the working electrode, aswell as contributing to the intrinsic conductivity of the hybridsystem.

2. Experimental Section

2.1. Preparation. Pyrrole (0.2 g) (Sigma-Aldrich, reagentgrade) was passed through a column of neutralized alumina forpurification and was dissolved in 20 mL of deionized water.The carbon nanotube component was added by dispersing 15mg of carbon nanotubes (Nanocyl 3150, Nanocyl S.A, Belgium)in the solution using a sonic bath for 10 min. In order to studythe effect of the number of functional groups on carbonnanotubes, the concentration of groups was varied (increasedor decreased compared to the as-received tubes) using twodifferent treatments. An acid treatment of the as-receivednanotubes was used to increase the concentration of functionalgroups and involved refluxing the as-received nanotubes in a3:1 mixture of concentrated sulfuric and nitric acids for 1 h at100 °C, collecting them over a polycarbonate membrane, andrepeatedly washing them with deionized water. These acid-treated nanotubes were then analyzed and used in the next stepsof the process,21 the yield of this step being ca.25%. The reducednanotubes (defunctionalized) were obtained by heating as-received nanotubes in a tube furnace at 1000 °C in an argonatmosphere, with a heating rate of 2 °C/min; the yield of thisprocess was ∼90%.

For the electropolymerization process, the AAO membrane(Anodisc 25, pore size 100 nm, Whatman, USA) was adheredto the surface of a copper electrode and sealed so that the onlycontact between the copper electrode and solution was throughthe membrane pores. The electropolymerization process wasperformed in a standard three electrode cell, a constant potentialof +0.8 V versus SCE was applied using a potentiostat setup(Faraday M1 Obbligato Objectives, Canada) for 3 min. The filmthus prepared was washed with deionized water and characterized.

2.2. Characterization. To estimate the number of functionalgroups on the nanotubes, a conventional micro-Raman gratingspectrograph Renishaw RM2000 using a laser beam at 782 nmwavelength was used. A JEOL 6300F FEG SEM and a Hitachi

* To whom correspondence should be addressed Phone: +61-3-990 54936. Fax: +61-3-99054934. E-mail: george.simon@eng.monash.edu.au.

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10.1021/jp101371u 2010 American Chemical SocietyPublished on Web 07/30/2010

H7500 TEM were used for morphological studies. Thermo-gravimetric analysis (TGA) of the samples was performed onthe AAO membranes coated with nanocomposite film, using aSII6300 TGA instrument in air, with a heating rate of 10 °C/min. The conductivity measurement was performed by using aJandel four-Probe system. XPS analysis was carried out on aKratos AXIS Ultra XPS (Kratos Analytical Ltd., UK) equippedwith a monochromated X-ray source (Al Kalpha, hν ) 1486.6eV). Radiation was provided at a source power of 150 W anda takeoff angle of 30° relative to the sample surface. The analysisareas were nominally 700 × 300 µm2. Peak deconvolutions wereperformed using Gaussian components after Shirley backgroundsubtraction.

3. Results and Discussion

Figure 1 shows the SEM and TEM images of the compositelayer which forms over the membrane, the SEM imagesdeliberately taken in an area close to some uncoated membraneto allow comparison. The electron microscopy images confirmthe formation of the polypyrrole/nanotube composite over thenonconducting, nanoporous membranes. The following mech-anism is suggested to explain this morphology, and the resultsof further characterization techniques are then used to supportthis hypothesis.

In this process, the functional groups on the surface of CNTs(mainly carboxylic acid groups, which become negativelycharged due to hydrolysis) can act as strong, conductive dopantanions in the polymerization of polypyrrole.2 By the applicationof a voltage across the electrodes of the electrochemical cell,the negatively charged nanotubes migrate toward the anodewhere, because of their large size, they cannot pass through thepores of the membrane to the underlying electrode and thusbecome deposited on the surface of the membrane. In contrast,the small pyrrole monomer molecules are able to pass throughthe holes in the membrane, reach the electrode, and becomingoxidized. In conventional electropolymerization using standardcounterions, the PPy monomer would then polymerize andprecipitate on the surface of the electrode in a columnar fashion.

Indeed, nanofibrous structures due to the confining effects ofthe membrane channels and PPy nanotubes produced in thisway have been previously reported.22,23 However, in the processwe describe here and in the absence of the conventionalcounterions, the only anions available for doping the positivelycharged PPy are the anionic groups on the surface of nanotubeswhich lie on top of the nanoporous, nonconductive membrane.This causes the polymerized or oligomerized pyrrole to leavethe positive electrode and migrate back to the surface of themembrane and form a layer of PPy around the nanotubes onthe surface of membrane, resulting in the formation of a PPy/CNT composite layer. As the process proceeds, the nanocom-posite layer becomes impenetrable, but since it is itselfconductive, it can also act as an electrode for further polym-erization of the composite material. As has been reportedpreviously,24 in low anion concentration electro-polymerizationconditions, the pyrrole oxidation rate is directly proportional tothe anion concentration, and since the anionic moieties on thesurface of the CNTs are the only accessible counterions, theratio of PPy to CNTs remains constant and is clearly dependenton the concentration of functional groups on the surface ofCNTs.

To confirm the proposed mechanism, three samples with thesame concentration of nanotubes but a different concentrationof functional groups on the nanotube surface were prepared andanalyzed, in order to see the influence of functional groupcontent of the nanotubes on the resultant composite. Asdescribed above, these nanotube samples include the as-received,defunctionalized (by heat treatment in 1000 °C in argonatmosphere), and acid-treated CNTs.

The Raman spectra of the nanotubes before and after thementioned treatments were obtained to better understand nano-tube structure. Comparing the intensity of the G peak at around1580 cm-1 (related to the vibration of sp2-bonded carbon atoms)and the D band at around 1320 cm-1 (the dispersive, defect-induced vibrations) of the Raman spectra of CNTs is a well-accepted indication of functional groups on nanotubes22 (thehigher the ID/IG ratio, the greater the degree of functionalization).The Raman study of our three nanotube systems showed thatthe ID/IG ratio for the as-received nanotubes is 1.2, whiledecreasing the functionality by heat treatment in argon causedthe value to decrease to 0.8. In comparison, acid treatment ofthe as-received sample resulted in a higher ID/IG ratio of 1.8,which confirms that the acid-treated nanotubes have morefunctional groups, usually in the form of carboxylic acidunits.25,26 The TEM images of nanotubes before and afteracid treatment (Figure 2) show that due to the short time ofacid treatment, there is no significant damage to the structureof the carbon nanotubes. XPS was also used to analyze thechemical properties of carbon nanotubes after each treatment.

Figure 1. (a) SEM images of NNP and (b) ANP composite filmsformed over a membrane, and (c) TEM images of NNP and (d) ANPcomposites.

Figure 2. TEM images of carbon nanotubes before (a) and after (b)acid treatment.

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The analysis of the as-received nanotube shows that this sampleconsists of 84.3 atomic percent of carbon atoms and 15.7%oxygen. The oxygen atoms are mainly in the form of carboxylicacid (22.9%) and carbonyl (42.4%) and hydroxyl (34.7%)functional groups. The acid treatment has increased the oxygencontent to 26.8%. The oxygen atoms in the functional groupsformed by acid treatment are in the form of carboxylic acid(34.9%) and carbonyl (28.3%) and hydroxyl (36.8%) groups.No remaining sulfur was observed in this sample, whichindicates that washing of the nanotubes after treatment wassuccessful in removing all of the acids from carbon nanotubes.The thermal treatment in inert atmosphere decreased the oxygencontent to 4.7%, which is in the form of carboxylic acid (4.9%)and carbonyl (51.3%) and hydroxyl (43.8%) functional groups.

A chrono-amperometery graph of the electropolymerizationprocess using these different nanotubes is shown in Figure 3.A considerable difference in current between the differentsamples can be observed, where samples with nanotubes witha higher concentration of functional groups lead to a highercurrent. The formation of polypyrrole increases in the order ofthe lowest value for defunctionalized nanotubes (DNP), thenthe as-received nanotubes (NNP), and finally the acid treatednanotubes (ANP) sample, which has the highest concentrationof functional groups and thus displays the highest levels ofcurrent.

A closer look at Figure 1a and b also shows that the sampleusing acid treated nanotubes (ANP) has the nanotubes embeddedwell in a higher polypyrrole content, while more nanotubes arevisible in the sample using the as-received nanotubes (NNP).Comparing TEM images of the composites after sonication inacetone to loosen the composite structure, it can be seen (Figure1c and d) that the as-received nanotubes are not completelycoated with PPy and that in some locations (presumably defectsites of nanotubes) the PPy is coagulated. In the case of acid-treated nanotubes where there are many more bonding sites (and

thus a higher content of PPy in the composite), it was notpossible to disentangle the composite. Further sonication onlyresulted in breaking the composite as a whole into smallersubparts, and an examination of these fractured sectionsconfirmed the stronger bonding and entanglement in this sample.

To further investigate and understand the resultant compositestructures described above, thermo-gravimetric analysis (TGA)was undertaken (Figure 4) on the samples on the basis of thedifferent nanotubes. The TGA of AAO membrane was alsomeasured and used as baseline data. The DNP sample onlyshows a small weight loss at around 270 °C, which indicatesthat only a small amount of polypyrrole is formed in this sample,as there is no weight loss observed for the nanotubes. It can beassumed that this material is in the form of a low molecularweight polypyrrole without any dopant or is doped with somecontaminating ions from other sources. The other two weightloss patterns for NNP and ANP show that these two samplescontain two different species, one that degrades at around 270°C (polypyrrole) and the second related to carbon nanotubeswhich oxidize at 489 °C (ANP) and 502 °C (NNP). This slightdifference in degradation temperature is probably due to a higherlevel of defects in acid treated CNTs, and its greater sensitivityto oxidation. From this graph, and by using the weight losspattern of the AAO membrane as the baseline, the weightpercentage of each component in the tested composite can beestimated. These results show that the weight percentages ofCNTs in the NNP and ANP samples are 70% and 23.3%,respectively, which confirm the suggested mechanism andelectron microscopy observations.

DC conductivity of the two samples was also measured, withANP showing a lower conductivity of 2.05 S cm-1, while NNPrecorded a greater conductivity of 39.65 S cm-1, due to thehigher concentration of nanotubes in this sample.

An XPS study of composite films prepared using ANP andNNP also demonstrated that there is a direct relationship betweenthe concentration of oxygen (and consequently oxygenated

TABLE 1: Quantitative Analysis of the XPS Spectra of Composite Samples

C% N%

sample O-C*dO (289.4) C*-N (285.9) O% (530) -Nd (398.7) -NH- (400.0) -NH · +- (401.1) )NH+- (402.4)Cu %(932)

NNP 81.89 14.23 2.72 0.893.2 7.00 56.2 41.4 2.4 0

ANP 72.98 19.78 4.74 2.395.69 15.64 16.6 60.0 14.6 8.8

Figure 3. Chrono-amperometery graph of the electro-polymerizationprocess.

Figure 4. Thermogravimetric analysis graphs of the AAO membraneas the baseline and prepared samples.

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functional groups) and nitrogen (as an indication of polypyrrole)in the samples (Table 1) and that the higher oxygen content inthe ANP sample results in the formation of more polypyrrolein this sample. After decovolution of the C 1s peak to itscomponents (Figure 5a) (sp2 (284.38 eV), C-N (285.9 eV),C-O (286.76 eV), CdO (288.10 eV), and OdC-O (289.42eV)), the same trend between the atomic concentration of acidgroups and nitrogen bonded carbon atoms can be observed. TheN1s peak (Figure 5b) can also be quantitatively differentiatedinto three different nitrogen types, the amine-like -NH- (400eV), the imine-like dN- (398.7 eV), the positively chargedpolaron -NH.+- (401.1 eV), and the bipolarondNH+- (402.4eV).27 The presence of a polaron (-NH.+-) and bipolaron(dNH+-) in samples and the higher N+/N ratio (23.4%) in ANPsuggest that the PPy in these samples is partly doped bycarboxylic acid groups on the surface of MWCNTs and alsothat a much better doped PPy is formed with ANP, incomparison to NNP (2.4%).

A possible issue that we had to take into account is thepossibility of some sulfate ions remaining in the nanotubesamples after acid treatment, which could act as a counterion,resulting in formation of more PPy in ANP. For this reason,the XPS spectra of samples were used to allow the detection ofany sulfur in the samples. The absence of a peak at 169.0 eV(S2p) indicates that none of the samples contains any sulfurcontamination.

As described above, after the composite layer forms on thesurface of the AAO membrane, it will act as the electrode forfurther electropolymerization, which means that two parallelelectrochemical cells will form in the system. The first cell isthe main cell consisting of the main counterion as the cathode(where hydrogen is being reduced) and the PPy/CNT compositelayer over the membrane as the anode (where pyrrole is beingoxidized). The other cell will be between a growing compositeelectrode and the copper electrode. In this case, the copperelectrode acts as the anode (the copper is oxidized), and thecomposite film behaves as the cathode (the copper beingreduced). Therefore, we can expect that as electropolymerizationproceeds, some copper deposition may occur in the compositelayer. This small amount of copper should be detected as a peaklocated in 932 eV in the XPS spectra of samples. Indeed, theNNP and ANP samples contain 0.89 and 2.39 atomic percent

of copper, respectively, which means that parallel electrochemi-cal reaction in two cells has shown a direct relationship betweenthe polymerization of pyrrole and the deposition of copper,further confirming the proposed theory and mechanism.

4. Conclusions

A new method for electro-polymerization of PPy-basednanocomposites over an electrically insulating, porous substratehas been developed. The underlying efficacy of this process isbased on the use of a solid, bulky particle (functionalizednanotubes) as the counterion for the polymerization of anintrinsically conducting polymer, a counterion system whichcannot penetrate a porous membrane for steric reasons. Mor-phological, thermal, electrical, and spectroscopic analyses ofthe prepared samples confirm the proposed mechanism, and thedegree of functionalization of the nanotube provides a routefor controlling the relative concentration of PPy and nanotubesin the final nanocomposite hybrid.

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Figure 5. (a) Details of the C 1s XPS spectra of NNP (top) and ANP (bottom). (b) Details of the N 1s XPS spectra of NNP (top) and ANP(bottom).

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