Enhanced Electrochemical Growth and Redox Characteristics of Poly

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J. Phys. Chem. C 2007, 111, 11320-11328

Enhanced Electrochemical Growth and Redox Characteristics of Poly(o-phenylenediamine) on a Carbon Nanotube Modified Glassy Carbon Electrode and Its Application in the Electrocatalytic Reduction of Oxygen Pandi Gajendran and Ramiah Saraswathi* Department of Materials Science, School of Chemistry, Madurai Kamaraj UniVersity, Madurai 625021, India

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ReceiVed: March 7, 2007; In Final Form: May 2, 2007

The electrochemical growth current of poly(o-phenylenediamine) as well as its redox current were enhanced several-fold when deposited on a carbon nanotube modified glassy carbon electrode compared to the same at a glassy carbon surface. The current enhancements during the growth of the polymer were not due to any surface area effects but are attributed to a facilitated nucleation involving acidic sites of the carbon nanotube. The poly(o-phenylenediamine)-carbon nanotube composite electrode showed a polymer redox-mediated electrocatalytic effect for oxygen reduction reaction with a 5-fold enhancement in current and a favorable potential shift of 130 mV compared to the values obtained at a poly(o-phenylenediamine) electrode.

1. Introduction Recent studies on the electrochemistry of a number of active compounds at carbon nanotube electrodes have proved beyond doubt their excellent electrocatalytic properties.1 Particularly, the advancements accomplished toward the functionalization of carbon nanotubes resulting in their enhanced solubility in aqueous solutions have helped in the preparation of stable carbon nanotube electrodes.2 Glassy carbon has been invariably the preferred substrate for casting carbon nanotube electrodes. Such carbon nanotube modified glassy carbon electrodes have been extensively used and rapidly exploited in the development of electrochemical biosensors and in other electroanalytical applications.3-5 Composites of carbon nanotubes with several conjugated polymers like polyaniline, polypyrrole, and poly(p-phenylenevinylene) have been prepared using a variety of chemical and electrochemical polymerization techniques and have been shown to exhibit superior performance especially in hybrid supercapacitors, electrochemical sensors, and photovoltaic devices.6-8 One of the first reports on the use of carbon nanotube electrodes for electrochemical deposition of conducting polymers demonstrated the superior electrodic performance of carbon nanotube over other conventional electrodes.9 It was shown that a polyaniline film deposited on multiwall carbon nanotube was oxidized more readily and produced a higher current density during the anodic oxidation compared to the results obtained on a platinum electrode. The efficient polymerization of aniline was attributed to a large surface area and unique surface topology of carbon nanotubes. There have been some studies on the electrochemical film formation of conducting polymers such as polyaniline and polypyrrole from aqueous solutions containing dispersed carbon nanotubes, and the carbon nanotube was shown to act as a dopant for these polymers.10,11 However, this method of preparation of conducting polymer-carbon nanotube composite may have drawbacks owing to agglomerations and precipitation of carbon nanotubes in strong electrolyte solutions and also may not be suitable for the preparation of * Corresponding author. Tel.: 91-452-2458247. Fax: 91-452-2459181. E-mail: [email protected].

some conducting polymers in nonaqueous media. Further, the amount of carbon nanotubes that is transferred from the bulk electrolyte solution to electrode surface during electropolymerization may be extremely negligible resulting in the absence of any noticeable improvements in the electrochemical performance of the film deposited. In another approach, the carbon nanotubes were covalently functionalized with the monomer before electropolymerization,12-15 and the resulting composite electrodes were used as new electrocatalysts for oxygen reduction and methanol and hydrogen peroxide oxidations.16-18 But the covalent functionalization of carbon nanotubes with monomers required tedious procedures. In this report, we present a simple and direct procedure for the electrochemical preparation of poly(o-phenylenediamine)carbon nanotube composite. An aqueous dispersion of multiwall carbon nanotube was cast on a glassy carbon electrode (GCE) surface and used for electrochemical deposition of the polymer film. There have been some attempts in the literature19-21 to explain the interaction between polyaniline and carbon nanotube. But in these studies, the polyaniline-carbon nanotube composites were mainly prepared by chemical polymerization procedures. On the basis of the results obtained from absorption, IR, Raman, and X-ray photoelectron spectral analysis and also from morphological data, different authors have attributed the interaction between polyaniline and carbon nanotube to a number of effects including adsorption, charge-transfer complex formation, covalent bonding, π-π interactions, wrapping, and so on. However, there appears to be no common consensus on the specific interaction yet. The present work is perhaps the first of its kind where a simple cyclic voltammetric growth of poly(o-phenylenediamine) (PoPD) provides direct evidence for the presence of a specific interaction between the polymer and carbon nanotube. We expect that the present results will help to understand and explain the exact nature of interaction with a different perspective. In addition to the preparation and characterization of PoPD-carbon nanotube composite, herein its usefulness in the electrocatalytic reduction of oxygen in an acid medium is demonstrated.

10.1021/jp071848d CCC: $37.00 © 2007 American Chemical Society Published on Web 07/10/2007

Poly(o-phenylenediamine)-Carbon Nanotube Electrode

Figure 1. Cyclic voltammograms of K3[Fe(CN)6] (5 × 10-3 M) in 0.1 M KCl at 10 mV s-1: (a) GCE and (b) CNT/GCE.

2. Experimental Section 2.1. Materials. Multiwall carbon nanotubes (purity 95%, diameter 10-20 nm, length 0.5-200 µm) were purchased from Aldrich. o-Phenylenediamine (s.d fine) was used after recrystallization from hot water.22 AR grade sodium sulfate (SRL) and sulfuric acid (s.d fine) were used as received. Doubledistilled water was used to prepare all electrolyte solutions. 2.2. Pretreatment of Multiwall Carbon Nanotube and Its Characterization. The commercial sample of carbon nanotube is not dispersible in water even by prolonged sonication. A good dispersion of carbon nanotubes is essential to cast a film on the GCE surface, and the following procedure was adopted to accomplish the same. The commercial powder sample of the multiwall carbon nanotubes was subjected to a chemical oxidation treatment with a mixture of concentrated nitric and sulfuric acids in a ratio of 1:3, respectively.2,23 About 50 mg of the sample was added to 24 cm3 of the acid mixture in a roundbottomed flask, and the mixture was refluxed for 5 h. After cooling, the mixture was washed with a copious amount of distilled water on a Millipore polycarbonate membrane filter (0.2 µ GTTP) until the washings showed no acidity. The acidtreated sample of carbon nanotubes can be easily dispersed in aqueous solution due to the presence of hydrophilic functional groups such as -COOH, -OH, etc. The FT-IR spectrum of the acid-treated sample agreed well with that reported in the literature.24 2.3. Surface Area and Stability of Carbon Nanotube Electrodes. GCE (0.07 cm2) procured from CH instruments, U.S.A., was used in the experiments after polishing with fine alumina powder. About 1 mg of the acid-treated multiwall carbon nanotube was dispersed in 1 mL of water, and 5 µL of the dispersion was cast on the polished GCE surface and dried in air at room temperature. The 5 µL volume of the dispersion was quite sufficient to cover the entire surface of the GCE with a uniform film. The true surface areas of GCE and carbon nanotube modified glassy carbon electrode (CNT/GCE) were determined from the data obtained on the cyclic voltammetric reduction of 5 × 10-3 M K3[Fe(CN)6] in 0.1 M KCl (Figure 1), assuming a diffusion coefficient value of 6.3 × 10-6 cm2 s-1 for K3[Fe(CN)6].25 The calculated surface area values were 0.069 and 0.072 cm2 for GCE and CNT/GCE, respectively. The true surface areas for both electrodes were very close to the

J. Phys. Chem. C, Vol. 111, No. 30, 2007 11321 geometric area of the GCE (0.07 cm2). The stability of the CNT/ GCE electrode was ascertained from its repeated cycling in the above experiment which gave stable current values. This result indicates that the carbon nanotubes were strongly adhered and confined to the GCE surface and did not get redispersed in aqueous electrolyte solutions. 2.4. Characterization. Electrochemical experiments were performed on a EG and G PAR potentiostat/galvanostat (model 263A). The rotating disc electrode (RDE) experiments were carried out using an assembly model RDE-001 supplied by Conserv Enterprises, Mumbai. The cyclic voltammetric growth and redox characteristics of the polymer were obtained from deaerated electrolyte solutions. The absorption spectra were recorded using JASCO (V550) spectrophotometer. For this purpose, the polymer was coated on an optically transparent indium tin oxide coated glass electrode (ITO), and the spectrum was recorded. The solution spectra of the acid-treated carbon nanotube and the electrodeposited polymer-carbon nanotube composite were obtained by dissolving the samples in water under mild sonication for about 2-3 min. The FT-IR spectra of the powder samples were recorded using Shimadzu (8400S) spectrometer. The surface morphology was obtained using a scanning electron microscope (LEO 1455 U.K.). The dry electrodeposited polymer and its carbon nanotube composite from the GCE surface were pressed onto a scotch tape and then sputter-coated with gold for imaging. Powder samples of the commercial and acid-treated carbon nanotubes were also inspected for their morphology. 3. Results and Discussion 3.1. Electrochemical Growth of Poly(o-phenylenediamine) (PoPD) on CNT/GCE. The electrodeposition of PoPD was carried out from a nitrogen-purged aqueous electrolyte solution containing 5 × 10-2 M monomer and 0.2 M Na2SO4 at pH 1 by cycling the potential at 50 mV s-1 between -250 and 1050 mV versus Ag/AgCl. A thin film made with 10 deposition cycles was used in all experiments. A comparison of the cyclic voltammetric growth profile of PoPD at GCE and CNT/GCE (Figure 2) shows that the monomer is more easily oxidized at the latter electrode (Ep ) 670 mV on the GCE and 540 mV at CNT/GCE). The Ip value for monomer oxidation is 1.6 times higher at the CNT/GCE than at the GCE. A plot of the oxidation peak current of the monomer (at Ep ) 670 mV on the GCE and 540 mV at CNT/GCE) against the cycle number at the two electrodes shows that the current drops drastically at GCE, whereas it decreases gradually at CNT/GCE (Supporting Information I). At both electrodes, the polymer peaks began to appear during first cathodic sweep itself at potential less than 0 mV (Figure 2). During initial stages of growth of polymer at the GCE, two redox couples (Ep1C ) 80 mV, Ep1A ) 10 mV; Ep2C ) -160 mV, Ep2A ) -100 mV) were observed. With increasing number of cycles, the more cathodic redox couple (P2) decreased in current becoming barely observable, while the other redox couple (P1) showed a concomitant increase in current during cycling, and eventually the growth of polymer was defined by only one redox couple (P1). The anodic and cathodic peak potentials of P1 showed a cathodic shift so that eventually after about 12 cycles, the redox couple corresponding to the polymer (EpC ) 0 mV, EpA ) -15 mV) appeared at a potential intermediate between the two redox couples (P1 and P2) (figure not shown). At the CNT/GCE electrode, the two redox couples (Ep1C ) 50 mV, Ep1A ) 40 mV; Ep2C ) -170 mV, Ep2A ) -100 mV)

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Figure 3. Cyclic voltammograms of PoPD at 50 mV s-1 in 0.2 M Na2SO4 at pH 1: (a) PoPD/GCE and (b) PoPD/CNT/GCE.

Figure 2. Cyclic voltammetric growth of PoPD at 50 mV s-1 from a solution containing 5 × 10-2 M monomer and 0.2 M Na2SO4 at pH 1. The numbers in the figure denote the cycle number: (a) GCE and (b) CNT/GCE. The inset shows the first growth cycle.

were observed together only for first two cycles, and in this case the more cathodic redox couple, P2, grew while P1 vanished after second cycle. In this case, the potentials of the redox couple P2 shifted anodically to appear as a single redox couple (EpC ) -150 mV, EpA ) -58 mV) with increasing cycle number. We have reported earlier mechanistic details for growth of PoPD on platinum and carbon electrodes.26 A ladder structure containing phenazine units for the polymer was suggested. Recently, Tu et al.27 studied cyclic voltammetric growth of PoPD in aqueous electrolytes at various pH values using scanning electrochemical microscopy in combination with quartz crystal impedance analysis. The polymer porosity and stability were shown to depend on potential sweep ranges used for its growth. In the present study, monomer oxidation peak current progressively diminished with cycling indicating that the monomer was rapidly converted to its cation radical and thereafter its oxidation was limited by diffusion of monomer from bulk solution28 o-Phenylenediamine can be oxidized to form a dication diradical with a loss of two electrons. Unlike polyaniline, polymerization of o-phenylenediamine is not autocatalytic,26 and dimerization of dication diradical involves a cyclization reaction to form a 2,3-diamino-9,10-dihydrophenazine which needs to undergo further oxidation for effective polymerization. The two redox couples observed at the GCE during initial stages of polymer growth (Figure 2) can be attributed to two one-electron transfers in the oxidation of 2,3diamino-9,10-dihydrophenazine moieties which eventually be-

comes a one-step two-electron transfer process. Since carbon nanotube is a good electron acceptor, it is possible that the oxidation of 2,3-diamino-9,10-dihydrophenazine is more facile and rapid resulting in a one-step two-electron process even during the initial stages of growth of polymer. Considering the nearly equal surface area of both electrodes, the several-fold increase in current for monomer oxidation and also polymer growth with a simultaneous shift in the monomer/polymer peak potentials at the CNT/GCE electrode indicate a definite interaction between the monomer/polymer and carbon nanotube during the growth of polymer itself. This is discussed in detail in section 3.3. 3.2. Characterization of the Composite. Figure 3 shows redox characteristics of PoPD deposited on GCE and CNT/GCE. Like the facile oxidation of monomer resulting in a rapid growth of polymer at the CNT/GCE, the polymer redox process was also found to be more facile at CNT/GCE. At 50 mV s-1, the oxidation potential of the polymer was favorably shifted by about 90 mV at the CNT/GCE. The total charge passed during polymerization for the first 10 cycles at GCE and CNT/GCE was found to be 13.5 and 77.5 mC, respectively. The redox peaks of the polymer appear in the first reverse cycle itself, and therefore the above total charge would include the charge due to the redox process of the polymer. Therefore, a correction is necessary to evaluate the actual charge passed for polymerization. This is accomplished by subtracting the charge passed in the potential range between -250 and +200 mV during 10 cycles of growth from the total charge. After this correction, the actual charge involved in the polymerization has been estimated to be 10.75 and 44.7 mC at GCE and CNT/GCE, respectively. This means that the amount of polymer deposited was 4.15 times higher on the CNT/GCE than at the GCE itself. Taking this into consideration, the redox charge can also be expected to be higher by 4.15 times at the CNT/GCE compared to that at the GCE. The polymer redox charge obtained in the

Poly(o-phenylenediamine)-Carbon Nanotube Electrode

J. Phys. Chem. C, Vol. 111, No. 30, 2007 11323

Figure 4. SEM images of (a) commercial carbon nanotube sample, (b) carbon nanotube sample after acid oxidative treatment, (c) PoPD deposited on carbon nanotube, (d) one magnified bundle in (c), and (e) PoPD deposited on GCE.

background electrolyte solution (Figure 3) at GCE was 0.18 mC, whereas that at CNT/GCE was 2.33 mC, and therefore the ratio of the two redox charges is 12.9 instead of the expected value of 4.15. This unique enhancement in the redox charge of the polymer deposited on the CNT/GCE is yet another indication of a strong interaction between the polymer and carbon nanotubes. The Ip was proportional to the scan rate passing through the origin at both the CNT/GCE and GCE, and this feature is characteristic of a surface-active film.29 Further evidence for the interaction between the carbon nanotube and

polymer can be obtained from the morphological inspection of the polymer-coated carbon nanotube electrodes. Figure 4 shows the morphology of multiwall carbon nanotube powder samples, obtained by a scanning electron microscope, before and after functionalization along with that of the polymer deposited on GCE and CNT/GCE. The commercial sample shows the existence of carbon nanotubes in several bundles with an orderly arrangement (Figure 4a). The acid oxidative treatment does not seem to break the bundles into individual carbon nanotubes although some breaking and shortening of bundles

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Figure 5. Absorption spectrum of (a) aqueous dispersion of acid-treated carbon nanotube, (b) aqueous dispersion of PoPD deposited on carbon nanotube, and (c) PoPD film deposited on an optically transparent ITO substrate.

into smaller ones are obvious. The individual strands of carbon nanotubes cannot be visually observed in the oxidized samples. It appears that the entire surface of each bundle is modified by the acid treatment. The average diameter of the acid-treated carbon nanotube bundle is about 7 µm (Figure 4b). The sample obtained on CNT/GCE shows the preferential deposition of polymer on the carbon nanotube bundles indicating a strong interaction between the polymer and carbon nanotubes. The diameter of the polymer-coated carbon nanotube bundle is 14 µm, which is nearly twice that of the acid-treated carbon nanotubes indicating clearly that the carbon nanotube is entirely encapsulated or wrapped by the polymer to form a tubular composite (Figure 4, parts c and d). The sample of PoPD obtained electrochemically on a GCE shows spherical particles with an average size of about 700 nm (Figure 4e). The acid-treated carbon nanotube sample can be readily dispersed in water to give a black-colored solution. Whereas the PoPD deposited on GCE cannot be dissolved or dispersed in aqueous solution, the sample deposited on the carbon nanotube electrode can be readily dispersed in water with mild sonication. This observation again indicates a definite interaction between the polymer and carbon nanotubes. In order to ascertain the nature of the interaction, UV-vis and FT-IR spectral measurements were made. Figure 5 shows the absorption spectra of water-dispersed solutions of carbon nanotube and PoPD-coated carbon nanotube. The spectrum of PoPD film coated on an optically transparent ITO substrate is also given. The spectrum of the carbon nanotube dispersion in water showed a peak with λmax at 236 nm which is analogous to that reported by many authors in the literature.30,31 Although the peak itself had not been assigned to any specific electronic transition in carbon nanotubes, the continuous increase in the absorption from the NIR region to the UV region had been ascribed to the overlap between the electrons of several double bonds and carbonyl groups present in the carbon nanotubes.31 In our study, we found the peak at 236 nm varied its intensity in proportion to the concentration of the carbon nanotube in accordance with Beer’s law. The spectrum of PoPD film coated on an optically transparent ITO substrate showed two maxima at 295 and 475 nm. The peak at 295 nm can be assigned to π-π* transition and that at 475 nm can be assigned to a chargetransfer exciton-like transition related to the quinoid unit which is a measure of oxidation state of the polymer.28 The PoPDcoated carbon nanotube showed two peaks at 267 and 444 nm. There is a 30 nm blue shift in the λmax values of PoPD-CNT composite in comparison to the values obtained for PoPD. Such

Gajendran and Saraswathi a remarkable blue shift has also been reported for polyanilinecarbon nanotube composite15 and also for poly(p-phenylenevinylene)-carbon nanotube composite.21 Conjugated polymer structures are known to be strongly coupled to carbon nanotubes through π-π stacking interactions.32 In the case of the polyaniline-carbon nanotube composite, the number of quinoid units was reported to be increased reducing the effective π-conjugation.15,33,34 The blue shift was attributed to an increase in band gap due to this structural change in the polymer.15 The IR spectra obtained for the acid-treated carbon nanotube, the polymer sample coated on GCE, and CNT/GCE electrodes are given in the Supporting Information II. The peaks at 3414 and 1739 cm-1 correspond to the presence of functional groups -OH and -COOH, respectively, in the acid-treated carbon nanotubes.24 The IR spectrum of PoPD sample coated on the GCE is similar to that reported by us earlier for the sample deposited on platinum electrodes.26 PoPD is a ladder polymer, and the characteristic peaks at 3458, 1636, 1213, and 878 cm-1 can be assigned to N-H stretching, quinoid stretching, C-N stretching, and the presence of the phenazine ring, respectively.35 The bands at 1057 and 588 cm-1 are assigned to the sulfate dopant. The spectrum of the PoPD sample obtained on the CNT/ GCE electrode is similar to that of PoPD coated on GCE except that the peak corresponding to C-N stretching is observed at 1224 cm-1 instead of at 1213 cm-1 and the peak corresponding to the phenazine ring is observed at 873 cm-1 instead of at 878 cm-1. No new peak is observed. The shifts in the wavenumbers in the IR spectrum, although small, definitely indicate some subtle changes in the structure of the polymer formed on the surface of carbon nanotubes. However, the absence of any new peak suggests that there is a least possibility of any new covalent bonding between the carbon nanotube and polymer. 3.3. Nature of Interaction between PoPD and Carbon Nanotube. The acid-treated carbon nanotubes are known to introduce functional groups like -COOH, -CdO, -OH, and quinone moieties at the defect sites on the nanotube ends and sidewalls36,37 Besides, the acid treatment of carbon nanotubes may increase their porosity and surface area. The electrochemistry of some redox molecules like ferrocene monocarboxylic acid and hexaamineruthenium(III) at the acid-treated carbon nanotube paste electrode was not altered much, whereas for other molecules including ferricyanide, catechol, and dopamine, the reversibility and current increased significantly.38,39 The exact reason for this behavior of the carbon nanotube electrode is not ascertained so far, although the involvement of oxygencontaining functional groups in the activation mechanism has been suggested in analogy to the electrochemically pretreated GCEs.40 There have been several recent reports in the literature on the preparation and characterization of conjugated polymerscarbon nanotube composites. Except for a few attempts involving electrochemical preparation of the composites,10,11,41 most of the methods involved the use of chemical polymerization in the presence of an aqueous dispersion of carbon nanotubes. Of the several conjugated polymers, extensive work has been reported on polyaniline-, polypyrrole-, and poly(p-phenylenevinylene)-based carbon nanotube composites.6-8 Both single-wall and multiwall carbon nanotubes have been used with and without acid oxidative treatment. A few earlier reports revealed that there was an increase in the rate of polymerization of conjugated monomeric compounds in the presence of carbon nanotubes.42,43 Especially, it has been shown that the presence of 1 wt % carbon nanotubes decreased aniline polymerization time to one-fourth.41 The prepared

Poly(o-phenylenediamine)-Carbon Nanotube Electrode composites are also known to be easily soluble in aqueous solutions.44,45 Another interesting common observation found in the literature is the preferential formation of the polymers on the carbon nanotubes, either by chemical or electrochemical procedures.21,46-48 The electron microscopic characterization has revealed a clear wrapping of the carbon nanotubes by the conjugated polymers irrespective of their different structural characteristics. Such polymer-wrapped carbon nanotube composites are also shown to be very much soluble in water paving a new way for the preparation of aqueous dispersions of conjugated polymers for better technological applications.21 The polymer wrapping of carbon nanotubes resulting in good solubility was also suggested to be useful in quantifying the purity of arc-discharge carbon nanotube powder itself, separating it from the unwanted amorphous graphitic particulate matter which was not dispersed in solution.49 The number of reports on the preparation, characterization, and applications of polyaniline-carbon nanotube composite is steadily increasing since the first report by Downs et al.9 On the basis of the experimental observations and the various characterization data, several explanations have been provided in the literature for the enhanced properties of the composite.9 The interaction between polyaniline and carbon nanotubes has been explained on the basis of adsorption effects,12,19 chargetransfer complex formation,19,50 the carbon nanotube acting as a p-dopant in polyaniline,33 extended π-π interaction between the carbon nanotube and polyaniline,20,34 polyaniline wrapping of the carbon nanotube,21,46 and covalent binding between the carbon nanotube and aniline.51 In the present study, we have observed a significant difference in the growth of PoPD on bare GCE and CNT/GCE. The difference is not only in the several-fold increase in the polymer growth current but also in the manner by which the monomer oxidation current decreased with increasing growth cycles. While there was a rapid decrease in the monomer oxidation peak current on GCE which is usually observed for the growth of any conducting polymer on any substrate like Pt, Au, or ITO, a very gradual decrease in current was observed at CNT/GCE. Also, the redox charge of PoPD is nearly 12.9 times higher at CNT/GCE than at GCE, and this is for the same surface area of the two electrodes. In addition to this interesting observation, the spectral and morphological characterization of the PoPD sample coated on the CNT/GCE electrode definitely indicated a specific interaction between the monomer/polymer and carbon nanotube. The observations of easy solubility of PoPD coated on CNT/GCE, the positional shifts in the UV and IR spectra, and the unique wrapping of the carbon nanotubes by PoPD are somewhat similar to the observations reported in the literature for other conjugated polymers-carbon nanotube composites, prepared by different methods. We have also carried out some experiments on the electropolymerization of aniline, N-methylaniline, N,N-dimethylaniline, and diphenylamine on carbon nanotube modified glassy carbon electrodes52 and in all these cases there was considerable enhancement in both growth and redox currents. The extent of increase in current depended on the nature of the substituent present in the aniline ring and other experimental conditions. However, such a current enhancement is inferred to be unique to the anilinic polymers as our experiment on electropolymerization of pyrrole52 on the carbon nanotube modified glassy carbon electrode did not give any enhancement in electrochemical properties. Therefore, the present results cannot be just explained on the basis of π-π interactions between the carbon nanotube and PoPD. Although we do not preclude the prevailing

J. Phys. Chem. C, Vol. 111, No. 30, 2007 11325 of such interactions, they may not be just sufficient to cause such an enormous increase in the electrochemical current. The π-π interactions may be the reason for the observed CNT core-polymer shell morphology or the CNT being wrapped by the polymer as revealed in the SEM characterization. The absence of a new peak in the absorption and IR spectra eliminates the possibilities of any covalent bond formation between the -COOH groups present on CNT and the anilinic radicals and also the carbon nanotube acting as dopant in the polymer. The absence of any covalent bond formation has also been excluded in the study of the oxidative coupling of 4-aminobenzylamine to single-wall carbon nanotubes.53 Although the adsorption of aniline on carbon nanotube is wellknown,54 it has been reported recently that the adsorptive capacity of acid-treated multiwall carbon nanotube for aniline decreased markedly due to the presence of oxygenous groups on the carbon nanotube’s surface.55 The oxidation of o-phenylenediamine at the CNT/GCE electrode is very facile as indicated by the favorable shift in the peak potential, and this may be due to the presence of electron-rich amino groups interacting with the electronwithdrawing groups in the acid-treated carbon nanotubes. The carboxylic sites at the carbon nanotubes are the most likely sites for the interaction of the monomer. The gradual decrease in the monomer oxidation current at CNT/GCE compared to the rapid decrease at GCE indicates a difference in the nucleation process at the two electrodes. The monomer cation radicals can preferentially attach themselves to all the acidic sites at the carbon nanotubes rather than being available for polymerizing and precipitating at the interface. As a result, the number of nucleation sites for the polymerization reaction can be increased enormously, making it more efficient. In other words, since more nucleation sites are available, the polymer aggregates and precipitates at these sites preferentially. This kind of interaction between the acidic sites in the carbon nanotube and the amino group is possible for other anilinic polymers including polyaniline, poly(N-methylaniline), poly(N,N-dimethylaniline), and polydiphenylamine but is not likely to occur with pyrrole cation radical. Therefore, no current enhancement can be expected for the growth of polypyrrole, and this was what experimentally observed in the present study.52 3.4. Electrocatalytic Reduction of Oxygen at PoPD/CNT/ GCE. The electrochemical reduction of oxygen at carbon-based modified electrodes has been recently reviewed.56 It is welldocumented in the literature that the oxygen reduction reaction on a carbon electrode proceeds predominantly as a two-electron process producing hydrogen peroxide as the product. The literature on oxygen reduction reaction at a plain GCE is very scanty. However, the reduction of oxygen on quinone-modified GCE has been extensively investigated. A variety of quinones have been employed for the modification of GCE. Adsorption, covalent binding, and electrochemical reduction of appropriate diazonium salts are some of the methods employed for electrode surface modification by quinones.57 There have been several reports demonstrating the electrocatalytic activity of both plain and modified carbon nanotube electrodes. The improved charge transfer for oxygen reduction at a carbon nanotube electrode was first demonstrated by Britto et al.58 A multilayered film of multiwall carbon nanotubes assembled on a GCE by a layer-by-layer method was used for oxygen reduction in alkaline medium.59 A Nafion solution of functionalized palladium-modified carbon nanotube cast on a GCE was found to show a two-step two-electron process for oxygen reduction in acid medium.60 In addition to the above

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Figure 7. Chronoamperometric response for oxygen reduction at -200 mV during alternative purging of N2 and O2 in 0.2 M Na2SO4 at pH 1: (a) PoPD/GCE, (b) CNT/GCE, and (c) PoPD/CNT/GCE. Figure 6. Cyclic voltammograms for the reduction of dissolved oxygen in 0.2 M Na2SO4 at pH 1 at 10 mV s-1: (a) GCE, (b) PoPD/GCE, (c) CNT/GCE, (d) PoPD/CNT/GCE, and (e) GCE in N2 atmosphere.

reports, there have been some recent systematic efforts to develop platinum or gold nanoparticles incorporated carbon nanotubes as electrocatalysts for the oxygen reduction and methanol oxidation, the two important fuel cell reactions.61-65 To our knowledge, there has been only one very recent report on the use of a conducting polymer modified carbon nanotube electrode for oxygen reduction.16 We have recently reported the reduction of oxygen at platinum nanoparticles incorporated PoPD electrode and showed that a 70-fold increase in oxygen reduction current could be obtained for a platinum loading of about 170 µg/cm2 in the PoPD electrode compared to bulk platinum electrode.66 There has been tremendous interest in the investigation of oxygen reduction reaction at PoPD electrode. Ohsaka et al.67 reported that the oxygen reduction reaction can be catalyzed through the redox process of PoPD in acid medium when the polymer was coated on a GCE. Li et al.68 reported that the current density for the reduction of oxygen at a platinum nanoparticles incorporated PoPD-coated GCE was 4 times higher than that obtained with other modified electrodes such as porphyrin-based electrodes in neutral buffer solution. In this study, the electrolyte used for oxygen reduction experiments was 0.2 M Na2SO4 solution adjusted to pH 1 by the addition of concentrated sulfuric acid. Pure oxygen gas was purged into the electrolyte for about 1 h. Trial experiments have shown that this time duration was sufficient to get a well-defined peak for the reduction of oxygen. Any further increase in oxygen purging time did not increase the peak current. Pure N2 was used for the deaeration of electrolyte solution. The electrochemical reduction of oxygen has been carried out at GCE, CNT/GCE, PoPD/GCE, and PoPD/CNT/GCEmodified electrodes at a sweep rate of 10 mV s-1 (Figure 6). At GCE, the cyclic voltammogram shows an ill-defined reduction peak at potentials greater than -600 mV. At CNT/GCE, the cyclic voltammogram shows the oxygen reduction peak at -170 mV. The oxygen reduction potential for the CNT/GCE electrode is shifted by about 465 mV with a nearly 2.5 times increase in current compared to GCE. At PoPD/GCE, the reduction of oxygen is observed at -230 mV, whereas at PoPD/ CNT/GCE the cathodic peak potential is -95 mV. The cathodic peak current is higher by about 5 times at PoPD/CNT/GCE than

that at PoPD/GCE. A comparison of the cyclic voltammetric data at the four electrodes indicates that the most favorable response for oxygen reduction in terms of both potential and current is observed at the PoPD/CNT/GCE electrode. The above results are further corroborated with chronoamperometry experiments. Figure 7 shows the current-time curves obtained at -200 mV at CNT/GCE, PoPD/GCE, and PoPD/ CNT/GCE electrodes by purging nitrogen and oxygen alternatively into the electrolyte solution. The initial purging of nitrogen in the electrolyte results in a very low background current of about 0.5 µA. After about 5 min, the nitrogen purging was stopped and a steady flow of oxygen gas was let in which resulted in a rapid increase in current within a few seconds attaining a constant value. At this stage pure nitrogen gas was purged into the electrolyte again, and oxygen gas flow was stopped. The nitrogen purging immediately decreased the oxygen reduction current until it became equal to the original background current. Repetition of the process of alternative purging of nitrogen and oxygen showed highly reproducible values of steady-state current indicating the good stability of the electrode. The magnitude of the steady-state current is found to be dependent on the nature of the electrode and also on the amount of the oxygen purged into the electrolyte. This result demonstrates the good selectivity and also oxygen sensitivity of the PoPD/CNT/GCE electrode under the conditions employed in the experiment. The potential step chronoamperometry experiment (0 to -200 mV) at PoPD/CNT/GCE under a continuous purging of oxygen into the electrolyte was carried out to ascertain the stability of the electrodes (Supporting Information III). In the case of PoPD/GCE, the current dropped initially but then eventually reached a steady state. Both CNT/ GCE and PoPD/CNT/GCE electrodes exhibited good stability over a period of 3 h of the experiment. In order to calculate the number of electrons (n) involved in the reduction of oxygen, RDE experiments have been carried out, and the data are used to make Koutecky-Levich plots (Supporting Information IV). The RDE experiment with a bare GCE did not give well-defined current-voltage curves. Assuming66 a diffusion coefficient of oxygen as 2.1 × 10-5 cm2 s-1 and the dissolved oxygen concentration of 1.03 × 10-3 mol dm-3, the “n” values calculated from the Koutecky-Levich plots are 1.3, 1.5, and 1.7 for PoPD/GCE, CNT/GCE, PoPD/ CNT/GCE, respectively.

Poly(o-phenylenediamine)-Carbon Nanotube Electrode

J. Phys. Chem. C, Vol. 111, No. 30, 2007 11327

SCHEME 1: Oxygen Reduction on PoPD/CNT/GCE

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The interesting aspect of oxygen reduction at a PoPD/GCE is the polymer redox reaction mediated electrocatalysis which is not the case with some of the other conjugated polymers like polyaniline and polypyrrole.67 In the present study also, the oxygen reduction at the PoPD/CNT/GCE is observed to be occurring through the redox reactions of the polymer. The RDE experimental results have shown that two electrons are involved in the charge-transfer process, and this means that hydrogen peroxide is the product formed as shown in Scheme 1. 4. Conclusions The study has demonstrated the presence of a specific interaction of carbon nanotubes with PoPD resulting in a very good enhancement of the electrochemical properties of the polymer. Our preliminary experiments with other conjugated polymers indicate that this kind of enhancement is unique to anilinic polymers. The results obtained on oxygen reduction show a favorable potential shift of about 130 mV and a 4-5fold increase in the current at the PoPD/CNT/GCE electrode compared to the values obtained at the CNT/GCE and PoPD/ GCE. The present results may be significant in that the tremendous enhancement in the redox properties of the polymer-carbon nanotube composites will increase the usefulness of such materials in electrochemical sensors, especially in the context of PoPD being considered to be an ideal polymeric matrix for a number of biosensor applications.69-73 The sensitivity of these biosensors can be expected to be enormously improved if the conventional PoPD matrix is replaced by the PoPD-CNT matrix. Further, the readily soluble nature of the PoPD-CNT composite in aqueous solutions may be helpful in realizing several applications of such anilinic polymers including the construction of electronic devices and as coatings for corrosion protection. Acknowledgment. The authors are grateful to the Department of Science and Technology, Government of India, for financial support of this work (SR/S5/NM-12/2003). The authors wish to record their gratitude to Dr. C. Srinivasan, Emeritus Professor in their department, for several interesting discussions while preparing this manuscript. Supporting Information Available: Plots of the monomer oxidation peak current against cycle number, FT-IR spectra, electrode stability data for oxygen reduction, RDE and KouteckyLevich plots. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wildgoose, G. G.; Banks, C. E.; Leventis, H. C.; Compton, R. G. Microchim. Acta 2006, 152, 187.

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