Carbon Nanotube

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Electropolymerization of Polypyrrole/Carbon Nanotube Nanocomposite Films over an Electrically 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 electrically insulating, porous membrane is presented. In order to study the mechanisms relating to this process, different samples using carbon nanotubes with varying concentrations of functional groups were prepared and the morphology, electrical conductivity, thermal stability, and X-ray photoelectron spectroscopy (XPS) spectra of the resulting samples measured and used to confirm the explanation for the formation of these composite structures. 1. Introduction The relatively high conductivity of electrically conductive polymers and their interesting physical and chemical properties, has meant that these materials have been intensively studied in recent years.1 To promote high levels of conductivity, nanocomposites combining conductive carbon nanotubes (CNTs) and an intrinsically conducting polymer (ICP) such as polyaniline and polypyrrole (PPy) have been reported by many research groups. Conductive nanomaterials such as carbon nanotubes with a high surface area allow electrochemical deposition or polymerization of significant amounts of conductive polymer in the form of a thin layer of polymer on the surface of the nanotubes, leading to a porous, conductive, and electrochemically active structure with a high surface area.2,3 Such structures have many potential applications, such as electrodes for rechargeable batteries,4-6 biosensors,7,8 field emission devices,9,10 and as supercapacitors.11 For some applications, the space between the two separated electrodes is filled with an electrolyte, a dielectric, or just a vacuum. The ability to deposit conductive, composite materials on the surface of a nonconductive membrane which can also act as a spacer would greatly facilitate the process of making such devices. Electropolymerization of ICPs has been proven to be a good method for the synthesis and deposition of such conductive polymers, allowing the production of polymer coatings with high levels of conductivity and chemical stability.12,13 This method is, however, largely limited to conductive substrates. Although some specially synthesized or modified ICPs can be dissolved in solvents,14,15 most of them are intractable and insoluble in common organic solvents in their original form; therefore, preparation of the desired composite structures is a time-consuming process and requires a series of different steps and components.11,16,17 Because of this technological barrier, some research groups have tried to use electrodeless methods for the deposition of conductive polymers on different substrates.18-20 While there has been some success in forming thin films of conductive polymers over surfaces using such techniques, the slow rates of polymerization and unavoidable polymerization in the solution make these methods inefficient. In this article, we describe a new method for fast and direct electropolymerization of PPy/CNT composites on an electrically * To whom correspondence should be addressed Phone: +61-3-990 5 4936. Fax: +61-3-99054934. E-mail: [email protected].

insulating nano- or microporous substrate such as polycarbonate or anodized aluminum oxide (AAO) membranes. Interestingly, the presence of CNTs as solid and bulky counterions plays a key role in limiting the polymer deposition to the surface of the membrane, keeping it away from the working electrode, as well as contributing to the intrinsic conductivity of the hybrid system. 2. Experimental Section 2.1. Preparation. Pyrrole (0.2 g) (Sigma-Aldrich, reagent grade) was passed through a column of neutralized alumina for purification and was dissolved in 20 mL of deionized water. The carbon nanotube component was added by dispersing 15 mg of carbon nanotubes (Nanocyl 3150, Nanocyl S.A, Belgium) in the solution using a sonic bath for 10 min. In order to study the effect of the number of functional groups on carbon nanotubes, the concentration of groups was varied (increased or decreased compared to the as-received tubes) using two different treatments. An acid treatment of the as-received nanotubes was used to increase the concentration of functional groups and involved refluxing the as-received nanotubes in a 3:1 mixture of concentrated sulfuric and nitric acids for 1 h at 100 °C, collecting them over a polycarbonate membrane, and repeatedly washing them with deionized water. These acidtreated nanotubes were then analyzed and used in the next steps of the process,21 the yield of this step being ca.25%. The reduced nanotubes (defunctionalized) were obtained by heating asreceived nanotubes in a tube furnace at 1000 °C in an argon atmosphere, with a heating rate of 2 °C/min; the yield of this process was ∼90%. For the electropolymerization process, the AAO membrane (Anodisc 25, pore size 100 nm, Whatman, USA) was adhered to the surface of a copper electrode and sealed so that the only contact between the copper electrode and solution was through the membrane pores. The electropolymerization process was performed in a standard three electrode cell, a constant potential of +0.8 V versus SCE was applied using a potentiostat setup (Faraday M1 Obbligato Objectives, Canada) for 3 min. The film thus prepared was washed with deionized water and characterized. 2.2. Characterization. To estimate the number of functional groups on the nanotubes, a conventional micro-Raman grating spectrograph Renishaw RM2000 using a laser beam at 782 nm wavelength was used. A JEOL 6300F FEG SEM and a Hitachi

10.1021/jp101371u  2010 American Chemical Society Published on Web 07/30/2010

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

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

H7500 TEM were used for morphological studies. Thermogravimetric analysis (TGA) of the samples was performed on the AAO membranes coated with nanocomposite film, using a SII6300 TGA instrument in air, with a heating rate of 10 °C/ min. The conductivity measurement was performed by using a Jandel four-Probe system. XPS analysis was carried out on a Kratos AXIS Ultra XPS (Kratos Analytical Ltd., UK) equipped with a monochromated X-ray source (Al Kalpha, hν ) 1486.6 eV). Radiation was provided at a source power of 150 W and a takeoff angle of 30° relative to the sample surface. The analysis areas were nominally 700 × 300 µm2. Peak deconvolutions were performed using Gaussian components after Shirley background subtraction. 3. Results and Discussion Figure 1 shows the SEM and TEM images of the composite layer which forms over the membrane, the SEM images deliberately taken in an area close to some uncoated membrane to allow comparison. The electron microscopy images confirm the formation of the polypyrrole/nanotube composite over the nonconducting, nanoporous membranes. The following mechanism is suggested to explain this morphology, and the results of further characterization techniques are then used to support this hypothesis. In this process, the functional groups on the surface of CNTs (mainly carboxylic acid groups, which become negatively charged due to hydrolysis) can act as strong, conductive dopant anions in the polymerization of polypyrrole.2 By the application of a voltage across the electrodes of the electrochemical cell, the negatively charged nanotubes migrate toward the anode where, because of their large size, they cannot pass through the pores of the membrane to the underlying electrode and thus become deposited on the surface of the membrane. In contrast, the small pyrrole monomer molecules are able to pass through the holes in the membrane, reach the electrode, and becoming oxidized. In conventional electropolymerization using standard counterions, the PPy monomer would then polymerize and precipitate on the surface of the electrode in a columnar fashion.

Indeed, nanofibrous structures due to the confining effects of the membrane channels and PPy nanotubes produced in this way have been previously reported.22,23 However, in the process we describe here and in the absence of the conventional counterions, the only anions available for doping the positively charged PPy are the anionic groups on the surface of nanotubes which lie on top of the nanoporous, nonconductive membrane. This causes the polymerized or oligomerized pyrrole to leave the positive electrode and migrate back to the surface of the membrane and form a layer of PPy around the nanotubes on the surface of membrane, resulting in the formation of a PPy/ CNT composite layer. As the process proceeds, the nanocomposite layer becomes impenetrable, but since it is itself conductive, it can also act as an electrode for further polymerization of the composite material. As has been reported previously,24 in low anion concentration electro-polymerization conditions, the pyrrole oxidation rate is directly proportional to the anion concentration, and since the anionic moieties on the surface of the CNTs are the only accessible counterions, the ratio of PPy to CNTs remains constant and is clearly dependent on the concentration of functional groups on the surface of CNTs. To confirm the proposed mechanism, three samples with the same concentration of nanotubes but a different concentration of functional groups on the nanotube surface were prepared and analyzed, in order to see the influence of functional group content of the nanotubes on the resultant composite. As described above, these nanotube samples include the as-received, defunctionalized (by heat treatment in 1000 °C in argon atmosphere), and acid-treated CNTs. The Raman spectra of the nanotubes before and after the mentioned treatments were obtained to better understand nanotube structure. Comparing the intensity of the G peak at around 1580 cm-1 (related to the vibration of sp2-bonded carbon atoms) and the D band at around 1320 cm-1 (the dispersive, defectinduced vibrations) of the Raman spectra of CNTs is a wellaccepted indication of functional groups on nanotubes22 (the higher the ID/IG ratio, the greater the degree of functionalization). The Raman study of our three nanotube systems showed that the ID/IG ratio for the as-received nanotubes is 1.2, while decreasing the functionality by heat treatment in argon caused the value to decrease to 0.8. In comparison, acid treatment of the as-received sample resulted in a higher ID/IG ratio of 1.8, which confirms that the acid-treated nanotubes have more functional groups, usually in the form of carboxylic acid units.25,26 The TEM images of nanotubes before and after acid treatment (Figure 2) show that due to the short time of acid treatment, there is no significant damage to the structure of the carbon nanotubes. XPS was also used to analyze the chemical properties of carbon nanotubes after each treatment.

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Hojati-Talemi and Simon

Figure 3. Chrono-amperometery graph of the electro-polymerization process.

The analysis of the as-received nanotube shows that this sample consists of 84.3 atomic percent of carbon atoms and 15.7% oxygen. The oxygen atoms are mainly in the form of carboxylic acid (22.9%) and carbonyl (42.4%) and hydroxyl (34.7%) functional groups. The acid treatment has increased the oxygen content to 26.8%. The oxygen atoms in the functional groups formed 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, which indicates that washing of the nanotubes after treatment was successful in removing all of the acids from carbon nanotubes. The thermal treatment in inert atmosphere decreased the oxygen content 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 electropolymerization process using these different nanotubes is shown in Figure 3. A considerable difference in current between the different samples can be observed, where samples with nanotubes with a higher concentration of functional groups lead to a higher current. The formation of polypyrrole increases in the order of the lowest value for defunctionalized nanotubes (DNP), then the as-received nanotubes (NNP), and finally the acid treated nanotubes (ANP) sample, which has the highest concentration of functional groups and thus displays the highest levels of current. A closer look at Figure 1a and b also shows that the sample using acid treated nanotubes (ANP) has the nanotubes embedded well in a higher polypyrrole content, while more nanotubes are visible in the sample using the as-received nanotubes (NNP). Comparing TEM images of the composites after sonication in acetone to loosen the composite structure, it can be seen (Figure 1c and d) that the as-received nanotubes are not completely coated with PPy and that in some locations (presumably defect sites of nanotubes) the PPy is coagulated. In the case of acidtreated nanotubes where there are many more bonding sites (and

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

thus a higher content of PPy in the composite), it was not possible to disentangle the composite. Further sonication only resulted in breaking the composite as a whole into smaller subparts, and an examination of these fractured sections confirmed the stronger bonding and entanglement in this sample. To further investigate and understand the resultant composite structures described above, thermo-gravimetric analysis (TGA) was undertaken (Figure 4) on the samples on the basis of the different nanotubes. The TGA of AAO membrane was also measured and used as baseline data. The DNP sample only shows a small weight loss at around 270 °C, which indicates that only a small amount of polypyrrole is formed in this sample, as there is no weight loss observed for the nanotubes. It can be assumed that this material is in the form of a low molecular weight polypyrrole without any dopant or is doped with some contaminating ions from other sources. The other two weight loss patterns for NNP and ANP show that these two samples contain two different species, one that degrades at around 270 °C (polypyrrole) and the second related to carbon nanotubes which oxidize at 489 °C (ANP) and 502 °C (NNP). This slight difference in degradation temperature is probably due to a higher level of defects in acid treated CNTs, and its greater sensitivity to oxidation. From this graph, and by using the weight loss pattern of the AAO membrane as the baseline, the weight percentage of each component in the tested composite can be estimated. These results show that the weight percentages of CNTs in the NNP and ANP samples are 70% and 23.3%, respectively, which confirm the suggested mechanism and electron microscopy observations. DC conductivity of the two samples was also measured, with ANP showing a lower conductivity of 2.05 S cm-1, while NNP recorded a greater conductivity of 39.65 S cm-1, due to the higher concentration of nanotubes in this sample. An XPS study of composite films prepared using ANP and NNP also demonstrated that there is a direct relationship between the concentration of oxygen (and consequently oxygenated

TABLE 1: Quantitative Analysis of the XPS Spectra of Composite Samples C% sample

O-C*dO (289.4)

NNP

N% C*-N (285.9)

81.89 3.2

ANP

-Nd (398.7)

-NH- (400.0)

56.2

41.4

16.6

60.0

14.23 7.00

72.98 5.69

O% (530)

)NH+- (402.4)

2.72

19.78 15.64

-NH · +- (401.1)

Cu % (932) 0.89

2.4

0

4.74

2.39 14.6

8.8

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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).

functional groups) and nitrogen (as an indication of polypyrrole) in the samples (Table 1) and that the higher oxygen content in the ANP sample results in the formation of more polypyrrole in this sample. After decovolution of the C 1s peak to its components (Figure 5a) (sp2 (284.38 eV), C-N (285.9 eV), C-O (286.76 eV), CdO (288.10 eV), and OdC-O (289.42 eV)), the same trend between the atomic concentration of acid groups and nitrogen bonded carbon atoms can be observed. The N1s peak (Figure 5b) can also be quantitatively differentiated into three different nitrogen types, the amine-like -NH- (400 eV), the imine-like dN- (398.7 eV), the positively charged polaron -NH.+- (401.1 eV), and the bipolaron dNH+- (402.4 eV).27 The presence of a polaron (-NH.+-) and bipolaron (dNH+-) in samples and the higher N+/N ratio (23.4%) in ANP suggest that the PPy in these samples is partly doped by carboxylic acid groups on the surface of MWCNTs and also that a much better doped PPy is formed with ANP, in comparison to NNP (2.4%). A possible issue that we had to take into account is the possibility of some sulfate ions remaining in the nanotube samples 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 of any sulfur in the samples. The absence of a peak at 169.0 eV (S2p) indicates that none of the samples contains any sulfur contamination. As described above, after the composite layer forms on the surface of the AAO membrane, it will act as the electrode for further electropolymerization, which means that two parallel electrochemical cells will form in the system. The first cell is the main cell consisting of the main counterion as the cathode (where hydrogen is being reduced) and the PPy/CNT composite layer over the membrane as the anode (where pyrrole is being oxidized). The other cell will be between a growing composite electrode and the copper electrode. In this case, the copper electrode acts as the anode (the copper is oxidized), and the composite film behaves as the cathode (the copper being reduced). Therefore, we can expect that as electropolymerization proceeds, some copper deposition may occur in the composite layer. This small amount of copper should be detected as a peak located in 932 eV in the XPS spectra of samples. Indeed, the NNP and ANP samples contain 0.89 and 2.39 atomic percent

of copper, respectively, which means that parallel electrochemical reaction in two cells has shown a direct relationship between the 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-based nanocomposites over an electrically insulating, porous substrate has been developed. The underlying efficacy of this process is based on the use of a solid, bulky particle (functionalized nanotubes) as the counterion for the polymerization of an intrinsically conducting polymer, a counterion system which cannot penetrate a porous membrane for steric reasons. Morphological, thermal, electrical, and spectroscopic analyses of the prepared samples confirm the proposed mechanism, and the degree of functionalization of the nanotube provides a route for controlling the relative concentration of PPy and nanotubes in the final nanocomposite hybrid. References and Notes (1) MacDiarmid, A. G. Angew. Chem., Int Ed. 2001, 40, 2581. (2) Chen, G. Z.; Shaffer, M. S. P.; Coleby, D.; Dixon, G.; Zhou, W.; Fray, D. J.; Windle, A. H. AdV. Mater. 2000, 12, 522. (3) Chen, J. H.; Huang, Z. P.; Wang, D. Z.; Yang, S. X.; Li, W. Z.; Wen, J. G.; Ren, Z. F. Synth. Met. 2001, 125, 289. (4) Wang, J.; Too, C. O.; Zhou, D.; Wallace, G. G. J. Power Sources 2005, 140, 162. (5) Chen, J.; Liu, Y.; Minett, A. I.; Lynam, C.; Wang, J.; Wallace., G. G. Chem. Mater. 2007, 19, 3595. (6) Sivakkumar, S. R.; Oh, J.-S.; Kim, D.-W. J. Power Sources 2006, 163, 573. (7) Luo, X.; Killard, A. J.; Morrin, A.; Smyth, M. R. Anal. Chim. Acta 2006, 575, 39. (8) Wang, J.; Musameh, M. Anal. Chim. Acta 2005, 539, 209. (9) Rakhi, R. B.; Sethupathi, K.; Ramaprabhu, S. Appl. Surf. Sci. 2008, 254, 6770. (10) Jin, Y. W.; Jung, J. E.; Park, Y. J.; Choi, J. H.; Jung, D. S.; Lee, H. W.; Park, S. H.; Lee, N. S.; Kim, J. M.; Ko, T. Y.; Lee, S. J.; Hwang, S. Y.; You, J. H.; Yoo, J.-B.; Park, C.-Y. J. Appl. Phys. 2002, 92, 1065. (11) Sivakkumar, S. R.; Kim, W. J.; Choi, J.-A.; MacFarlane, D. R.; Forsyth, M.; Kim, D.-W. J. Power Sources 2007, 171, 1062. (12) Tantavichet, N.; Pritzker, M. D.; Burns, C. M. J. Appl. Electrochem. 2001, 31, 281. (13) Huang, J.-E.; Li, X.-H.; Xu, J.-C.; Li, H.-L. Carbon 2003, 41, 2731. ¨ .; Tanyeli, C.; Akhmedov, I. M.; Toppare, (14) Arslan, A.; Tu¨rkarslan, O L. Mater. Chem. Phys. 2007, 104, 410.

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(15) Yigitsoy, B.; Varis, S.; Tanyeli, C.; Akhmedov, I. M.; Toppare, L. Thin Solid Films 2007, 515, 3898. (16) Sivakkumar, S. R.; Kim, D.-W. J. Electrochem. Soc. 2007, 154, A134. (17) Sivakkumar, S. R.; MacFarlane, D. R.; Forsyth, M.; Kim, D.-W. J. Electrochem. Soc. 2007, 154, A834. (18) Attout, A.; Yunus, S.; Bertrand, P. Surf. Interface Anal. 2008, 40, 657. (19) Liao, C.; Gu, M. Thin Solid Films 2002, 408, 37. (20) Ayad, M. M. J. Mater. Sci. Lett. 2003, 22, 1577. (21) Hojati-Talemi, P.; Cervini, R.; Simon, G. J. Nanopart. Res. 2010, 12, 393.

Hojati-Talemi and Simon (22) Xu, Q.; Meng, G.; Han, F.; Zhao, X.; Kong, M.; Zhu, X. Mater. Lett. 2009, 63, 1431. (23) Xiao, R.; Cho, S. I.; Liu, R.; Lee, S. B. J. Am. Chem. Soc. 2007, 129, 4483. (24) Li, Y. J. Electroanal. Chem. 1997, 433, 181. (25) Rosca, I. D.; Watari, F.; Uo, M.; Akasaka, T. Carbon 2005, 43, 3124. (26) Shieh, Y.-T.; Liu, G.-L.; Wu, H.-H.; Lee, C.-C. Carbon 2007, 45, 1880. (27) Martin, P.; rcaron; etislav, S.; Xinsheng, P.; Dmitri, G.; Jie, T.; Izumi, I. Chem.sEur. J. 2007, 13, 7644.

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