Journal of Materials Chemistry PAPER. Sulfonated porphyrin doped polyaniline nanotubes and nanofibers: synthesis and characterization

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1 Journal of Materials Chemistry Dynamic Article Links C < Cite this: J. Mater. Chem., 2012, 22, Sulfonated porphyrin doped polyaniline nanotubes and nanofibers: synthesis and characterization Mohd. Khalid,* Jose J. S. Acu~na, Milton A. Tumelero, Jeison A. Fischer, Vinicius C. Zoldan and Andre A. Pasa* Received 22nd February 2012, Accepted 3rd April 2012 DOI: /c2jm31116j PAPER The nanotubes and nanofibers of polyaniline (PANI) were successfully synthesized by a template-free and interfacial polymerization method respectively in the presence of tetrakis(4-sulfonatophenyl) porphyrin (TSPP) as a dopant. The tubular and fibrous morphologies of the TSPP PANI were confirmed by TEM images. The morphology of TSPP PANI can be changed from nanotube to cauliflower by simply changing the volume ratio of the dopant to the aniline monomer as confirmed by SEM images. It was observed that the formation of nanotubes (by the template-free method) was independent of the dopant, but the dopant was associated with interfacial polymerization for the formation of nanofibers. It might be the electrostatic and hydrophobic interactions between the water dispersible porphyrin complex [TSPP (NH 4 ) 2 S 2 O 8 ] and the dimer cation-radical surfactant which are responsible for the formation of nanofibers. The resulting materials were also characterized by conductivity, solubility, FTIR and UV-vis spectroscopy, X-ray diffraction and cyclic voltammetry. Introduction The investigation of conducting polymers with nanostructures is of great importance from both scientific and technological points of view due to their unique properties and promising potential applications in nanodevices. 1 4 To exploit nanostructural effects in these devices, researchers have developed several methods for fabricating conducting polymers as thin films, nanospheres, nanofibers, nanotubes, and core shell structures. 5,6 Particularly conducting polymer nanofibers and nanotubes have received growing interest in recent years. Although various approaches such as hard-template methods, soft-template methods, electrospinning technology, and so on are widely employed to synthesize conducting polymer nanostructures, each of the currently used methods possesses disadvantages. Polyaniline nanotubes or nanofibers with diameters <100 nm can be made by template-guided polymerization within channels of zeolites 7 or nanoporous membranes Since all these methods are dependent on either a template or a specific complex chemical reagent, these processes are rather tedious in order to remove the template to recover the polyaniline nanostructures. Therefore developing syntheses that do not rely on templates, structural directing molecules, or specific dopants are important, especially for scaling up to produce large quantities of nanostructured materials. Recently, Laboratorio de Filmes Finos e Superfıcies, Departamento de Fisica, Universidade Federal de Santa Catarina, Florianopolis, Brazil. mkansarister@gmail.com; aapasa@gmail.com polypyrrole and polyaniline micro- and nanotubes have been prepared directly from the aqueous medium without using any template However, if polyaniline nanostructures with diameters <100 nm are desired, then very complex dopants with bulky side groups are needed. 16,17 Kaner has synthesized nanofibers with diameters near 50 nm, using camphor sulfonic acid as dopant without using any template, which are among the smallest reported for polyaniline nanofibers by the templatefree method. 18 Huang and Kaner developed rapid mixing and interfacial polymerization methods to make polyaniline nanofibers chemically. 19,20 Liu and co-workers devised a three-step electrochemical route to polyaniline nanowires. 21 However, the means of controlling their morphology without the use of templates are so far rather limited. Herein, we found that the template-free method for the fabrication of polyaniline nanotubes does not depend on any specific template or dopant but the morphology of the polyaniline could be varied from nanotube to cauliflower shape just by changing the concentration of sulfonated porphyrin as dopant. However, polyaniline nanofibers were obtained via the interfacial polymerization method when sulfonated porphyrin acts as an in situ dopant. In the absence of sulfonated porphyrin, granules of PANI were obtained as in traditional polymerization of PANI. The formation mechanism and electrical properties of PANI nanostructures were also investigated. In addition, the morphological characterization, molecular structure and crystalline characterization of the PANI nanostructures were discussed J. Mater. Chem., 2012, 22, This journal is ª The Royal Society of Chemistry 2012

2 Experimental detail Synthesis Sulfonated porphyrin doped PANI nanotubes were synthesized by a template-free method. A typical polymerization procedure for the PANI nanotubes is as follows: aniline monomer (0.1 ml) and 0.1 ml of TSPP were dissolved in deionized water (20 ml) under supersonic stirring for 10 min. Then precooled aqueous solution of ammonium peroxydisulfate (APS, 0.54 g in 40 ml deionized water) was rapidly mixed into the above reaction mixture. The mixture was left overnight in an ice bath without stirring. The product was filtered and washed with deionized water, methanol, and ether, and then dried in vacuum for 24 h to obtain a green-black PANI powder. In order to optimize the synthetic conditions, the influence of the TSPP/aniline ratio on the formation of the nanotubes was investigated. The volume of the TSPP ( M) as dopant was varied from 0.1 to 0.4 ml with the same volume (0.1 ml) of aniline in 20 ml of deionized water to study their effects on the formation of the PANI nanostructure. In all reactions, the molar concentrations (0.06 M) were kept same for aniline and APS to optimize the synthetic conditions. PANI nanotubes were also obtained when the synthesis was same as those for synthesizing nanotubes of TSPP doped PANI except for using sulfonated porphyrin as dopant. In a typical interfacial polymerization reaction, we took 0.1 ml of aniline and dissolved it in 10 ml of methylene chloride as organic phase. This reaction was performed in a 20 ml glass vial. For the second solution, 0.1 ml of TSPP ( M) was dissolved in 10 ml of APS (0.06 M). These two precooled reaction mixtures were added directly and kept in a refrigerator overnight without stirring. We obtained a dark-green precipitate and the product was filtered and washed with deionized water, methanol, and ether, and then dried in vacuum for 24 h. Finally we obtained a green-black PANI powder. To optimize the synthesis for the formation of nanofibers, we varied the amount of TSPP from 0.1 to 0.4 ml with the same amount of aniline (0.1 ml in 10 ml of methylene chloride) and APS (10 ml of 0.06 M). a diameter of 13 mm and a thickness of 0.4 mm using a vacuum press at 8 MPa for 5 min. Cyclic voltammetry The electrochemical response of the material was determined using an Autolab PGSTAT30 electrochemical workstation. Each sample was first dispersed in CHCl 3 and the dispersion was dropped onto a gold plate ( cm 2 ) working electrode which was allowed to dry at room temperature. The cell was filled with 0.1 M H 2 SO 4 and purged with N 2 for approximately 15 min. Following this, N 2 was allowed to flow over the solution to prevent O 2 from re-entering the cell for the remaining experiment. Cyclic voltammograms were recorded in the potential range from 0.5 V to 1.0 V at scan rates of 60 mv s 1, using saturated calomel as reference electrode and a Pt foil as counter electrode. Results and discussion Morphology Typical FESEM and TEM images obtained demonstrate the fibrous, tubular and cauliflower morphology of PANI, which correspond to the case of the presence or absence of the TSPP as the dopant. The FESEM images (Fig. 1A) show that the PANI synthesized by interfacial polymerization in the presence of TSPP is composed of nanofibers. The average wall thickness of these fibers is 50 nm, and a single nanofiber was imaged by TEM as shown in Fig. 1B. In the absence of TSPP as dopant, the polyaniline was obtained in a granular form like in the traditional chemical polymerization of polyaniline (Fig. 1C). 22 Characterizations The morphologies of the products were investigated with a JEOL JSM-6701F field-emission scanning electron microscope (FESEM) and a JEOL JEM-1011 transmission electron microscope (TEM). The samples for SEM were mounted on aluminium studs without sputter-coating of gold. Samples for TEM measurements were dispersed in ethanol and coated on copper microgrids with a carbon support film. Infrared spectra in the range cm 1 on sample pellets made with KBr were measured by means of an infrared spectrophotometer (Perkin- Elmer Tensor 100). UV-visible spectra of the products were recorded from 190 to 1000 nm using a Perkin Elmer 750 spectrophotometer. X-ray scattering of the samples was carried out on an X-ray diffraction instrument (with CuK N radiation). Electrical characterization was performed by a four probe method with linear contact geometry in compressed pallets. The thermal conductivity behavior of the material was measured at temperatures ranging from 100 to 300 K, on base pressure of 10 5 Torr. The nanotube/nanofiber samples were pelletized to Fig. 1 (A) SEM images of polyaniline nanofibers made by interfacial polymerization in a water methylene chloride system. (B) TEM image of a single nanofiber, volume ratio of TSPP/aniline ¼ 4: 1, reaction conditions: 16 hours, 5 C and (C) PANI granules. This journal is ª The Royal Society of Chemistry 2012 J. Mater. Chem., 2012, 22,

3 reactions and controlling the shape and size of the polyaniline nanotubes. The surfaces of the nanotubes are not smooth, showing the Iceland type morphology. Mechanism Fig. 2 TEM images of PANI nanotubes made by a template-free method: (A) TEM image of a PANI nanotube with TSPP doping [TSPP/ aniline ¼ 1 : 1] and (B) TEM image of a PANI nanotube without TSPP doping, inset: the inner diameter near about 20 nm. Fig. 3 SEM images of PANI nanotubes made by a template-free method: (A) TSPP doped PANI nanotubes [TSPP/aniline ¼ 1 : 1], (A 0 ) single PANI nanotube, and (B and B 0 ) cauliflower shape of PANI at different magnifications [TSPP/aniline ¼ 4 : 1]. The tubular morphology of polyaniline synthesized by the template-free polymerization method has been confirmed by TEM images (Fig. 2). The average outer and inner diameters of the TSPP doped PANI nanotube were about and 5 10 nm, respectively. In the absence of TSPP, the nanotubes were also obtained with almost the same outer diameters of about nm besides bigger inner diameters of nm. The inner diameter of the resulting nanotubes was then affected by the TSPP dopant used in the polymerization. The morphology of PANI tubes was also depended on the TSPP/aniline ratio. Typical influence of the TSPP/aniline ratio on the morphology of TSPP PANI is shown in Fig. 3. When the volume ratio of TSPP/ aniline was 4 : 1 the resulting TSPP PANI was composed of a cauliflower shape structure (Fig. 3B). While when the volume ratio was 1 : 1 of TSPP/aniline, the nanotubes were observed. In general hydrogen bonding plays an important role in the selfassembly of the different nanostructures. It is reasonable to expect that hydrogen bonds are responsible for the aggregation behavior of the cauliflower shape structure of the PANI. In fact, many papers are dealing with hydrogen bond in polyaniline. 23,38 This may be potentially a great advantage for scaling up the Wan et al. have demonstrated that PANI nanotubes can be obtained by the self-assembly method with organic acids, such as 2-pyrrolidone-5-carboxylic acid and b-naphthalene sulfonic acid, as dopants. 24,25 They proposed that the micelles of anilium salt act as a tubular supramolecular template for the formation of PANI nanotubes. In our system, the polyaniline nanotubes dominate morphology when unprotonated aniline was dissolved in deionized water in the absence of organic acids. Polyaniline nanotubes were shown to form spontaneously during the chemical oxidative polymerization of aniline. The polyaniline nanotube morphology does not require any template or surfactant, and appears to be intrinsic to polyaniline nanotubes synthesized in water. Our results support the proposed model of Epstein et al. for the formation of self-assembled polyaniline nanotubes. 26 In this model, aniline is oxidized by (NH 4 ) 2 S 2 O 8 to form reactive aniline cation-radicals. Two initially formed aniline cation-radicals combined into a dimer which is further oxidized by (NH 4 ) 2 S 2 O 8 to form a dimer cation-radical. Such a dimer cation could act as a surfactant to template the formation of nanostructures. 27,28 This indicates that the formation mechanism of polyaniline nanotubes is independent of the dopant, but rather associated with the oxidative chemical polymerization process conditions. The dimer cation-radical surfactant could be aggregated in different sizes and types of micelles on introducing counterions. 29 The concentration and interaction between surfactants and counterions have an effect on the shape of the micelles including tubular, fibrillar, spherical, cylinder and interconnected network. For example, in our case, the volume ratio 1 : 1 of TSPP ( M)/aniline (0.06 M) produced nanotubes, while when the ratio was 4 : 1, the resulting TSPP PANI nanotubes almost disappeared and formed a cauliflower like structure as shown in Fig. 3. On the basis of the model discussed above, it is reasonable to expect that the liberated counterion SO 3 from water dispersed sulfonated porphyrin interacting with the cation-radical of aniline might be a driving force for the self-assembling cauliflower type shape. We can say that an appropriate amount of counterions is necessary to assist the cation-radical of aniline to remarkably alter to the other shape. This result indicates that changing the ratio of dopant to monomer might be one way to improve the size controllable ability of the self-assembly method to synthesize conducting polymer tubes. In the interfacial polymerization, it is important to note that the fibrillar morphology was obtained in the presence of TSPP. Interestingly, in the absence of TSPP absolute granular morphology of PANI was observed like traditionally synthesized polyaniline during the subsequent polymerization. This suggests that there is a remarkable relation between APS and the water dispersible TSPP complex. This may be attributed to the nanofibers structure driven by electrostatic and hydrophobic interactions between porphyrin and the dimer cation-radical surfactant to template the formation of nanofibers. Based on common knowledge, TSPP with a planar hydrophobic section J. Mater. Chem., 2012, 22, This journal is ª The Royal Society of Chemistry 2012

4 and hydrophilic edge group ( SO 3 ) is water soluble and has anionic characteristics in aqueous solution. Unlike some large organic dopant anions such as naphthalenesulfonic acid, TSPP has no surfactant characteristics because of the absence of critical micelle concentration. Evidently, TSPP in the complex [TSPP (NH 4 ) 2 S 2 O 8 ] plays a role of not only a nanofibers directing template but also serve as a dopant. Electrical conductivity The conductivity at room temperature of the TSPP doped PANI nanotubes and nanofibers was almost the same, about 10 3 S m 1. This conductivity value was lower than the conductivity of PANI without TSPP of 10 S m 1. This is because H 2 SO 4 produced during the polymerization of aniline with APS and SO 4 anion is certainly incorporated in the PANI main chain as counter-ion serving as dopant to form an emeraldine salt (i.e. conducting state). This is in good agreement with the ph value of the reaction mixture consisting of aniline and APS in de-ionized water when the polymerization time was measured. It was observed that the color of the solution undergoes a change of yellow to blue to green and the ph value of the reaction solution rapidly decreases down to 2.4 with the polymerization time. X-ray analysis (EDX) was used to check the incorporation of dopant into the polyaniline nanostructures. The samples were mainly composed of C, N, O and S. The presence of S in all samples indicates the doped state of the polyaniline products. In TSPP doped PANI nanotubes/nanofibers sample, a lower amount of S (1.28%) was detected than undoped (in the absence of TSPP) PANI nanoparticles (2.20%), indicating that the lower doping level means lower conductivity of TSPP doped PANI nanotubes/nanofibers. The origin of the lower conductivity may be attributed to the reason that the presence of the side substitute bulky group of TSPP led to the separation enhancement of polymer main-chains and reduced inter-chain diffusion of charge carriers, subsequently reducing the conductivity of nanotube and nanofibers. Moreover, the interaction between the SO 3 group of porphyrin and the aniline cationic radical nitrogen atoms is likely to force the polymer main chain out of planarity and lower the overlap of orbitals along the conjugated chains by twisting the aromatic rings relative to one another. 30 Consequently, the charge carrier s mobility along the polymer chain is reduced. Furthermore, the conductivity of the cauliflower like structure was also lower than the conductivity of the nanotube structure. This difference may be due to the highly random aggregation of nanotubes in the cauliflower like structure. Fig. 4A shows I V curves obtained for the different temperatures tested. From these curves we see that the sample has an ohmic behavior and calculated electrical conductivity, which is displayed in Fig. 4B. The conductivity has a thermal activated behavior typical of non-degenerated semiconductors and insulators. In polymers, this thermal activated behavior is usually assigned to hopping conduction, given by the equation: g s ¼ s 0 e T0 T where s is the electrical conductivity, T is the temperature in Kelvin, s 0 and T 0 are characteristic constants of the system, and the coefficient g allows discrimination between the various Fig. 4 Temperature dependence of electrical conductivity for TSPP doped PANI nanotubes (A) I V curves at different temperatures, (B) ohmic behavior and (C) logarithm of conductivity as a function of square root of reciprocal temperature, reaction condition: TSPP/aniline ¼ 1: 1. hopping models. In Fig. 4C, the logarithm of conductivity is plotted as a function of square root of reciprocal temperature, the linearity of the data at high temperatures indicates that the samples follow a 1D variable range hopping, characteristic for g ¼ 1/2. This result is in good agreement with the ones obtained by Reedijk et al. 31 for low doped conjugated polymers. Polyaniline without TSPP doping was easily dispersed in deionized water with the assistance of ultrasonics. The resulting suspension was stable from up to several minutes to days. The good dispersability of PANI without TSPP doping indicates that the protonated polyaniline in the emeraldine salt form is hydrophilic. This result indicates that H 2 SO 4 is produced during polymerization with APS and served as dopant to form an emeraldine salt. However, TSPP doped PANI nanotubes/nanofibers were not dispersed in deionized water, because the amine groups of the polyaniline chains are consumed by bulky sulfonated porphyrin. This indicates that the polyaniline nanotubes/nanofibers strongly interact with the sulfonated group of porphyrin. The possible combining mode of TSPP and polyaniline is presented in Scheme 1; this proposed structure was supported by FTIR characterization. Structural characterization The molecular structure of the PANI nanostructures was characterized by FTIR and UV-vis spectroscopies, as well as X-ray spectroscopy. FTIR spectra of polyaniline nanotubes and nanofibers are almost identical and are in good agreement with previous reported results. 32 For example, nanotubes/nanofibers show the characteristic peaks of PANI at 1600 and 1497 cm 1 (quinoid and benzenoid ring, respectively), 1301 cm 1 (C N This journal is ª The Royal Society of Chemistry 2012 J. Mater. Chem., 2012, 22,

5 Scheme 1 Proposed possible structure of TSPP doped polyaniline. stretching), 1172 cm 1 (C]N stretching), and 823 cm 1 (1,4- substituted phenyl ring stretching) (see Fig. 5), which are identical to the emeraldine salt form of PANI. 33 The peaks at 1115 and 1080 cm 1 can be assigned to the asymmetric and symmetric O]S]O stretching vibrations, respectively, indicating the existence of SO 3 groups. 34,35 The S O stretching peak is at 750 cm 1, while the peak at 630 cm 1 represents the C S stretching vibration. The absorption peaks at 1080, 750, and 630 cm 1 in the FTIR spectra of the TSPP doped PANI nanotubes/nanofibers are consistent with the presence of the SO 3 group attached to the aromatic rings. 36 In the spectrum of the porphyrin sample the region from around 3400 to 2800 cm 1 is more pronounced indicating more stretching vibration modes from free N H. Also, the C H region (the C H stretching bands from the aromatic rings are clearly visible at 3061, 2969, and 2842 cm 1 ) has a higher intensity in the spectrum of the porphyrin sample compared with the same region in the TSPP doped polyaniline sample spectrum. In the TSPP doped PANI samples, the lack of vibrational modes of C H bonds and surprisingly no bands of significant intensity at 3400 cm 1 corresponding to the free N H stretching vibration of the amine ( NH) group confirm the electrostatic interaction and favor hydrogen bonding S O/H N, respectively, 37,38 as can be seen in Scheme 1. By comparison, the spectrum of the sulfonated porphyrin doped PANI nanotubes/nanofibers illustrates the obvious presence of PANI characteristic vibrations, suggesting that the PANI is in a doped state. Sulfonated porphyrin doped PANI is also supported by the UV-vis absorption spectra of the nanotube/nanofiber dispersed in N-methyl-2-pyrrolidone, as shown in Fig. 6. The as prepared PANI sample (without TSPP doping) has sharp intense peaks at 290 and 330 nm and a broad peak at 640 nm. The first and second peaks are related to the molecule conjugation and the third absorption peak with high conductivity (10 S m 1 ) originates in the charged cationic species. 39,40 In the case of TSPP doped polyaniline nanotubes and nanofibers, the first two peaks exhibit higher absorption intensity, which correspond to the strong interaction between sulfonic group of porphyrin and imine nitrogen of polyaniline. 41 The third peak of nanotubes and nanofibers is blue shifted to 570 and 600 nm, respectively. The reasons might be the lower conductivity (10 3 Sm 1 ) of the PANI nanotubes and nanofibers compared to that of PANI nanotubes/grains without TSPP doping. Moreover, the presence of these three peaks indicates that the PANI nanotubes and nanofibers are in a doped state. 42 X-Ray scattering patterns of PANI nanotubes, nanofibers and grains are presented in Fig. 7. For nanotubes and nanofibers the two peaks centered at 2q z 20 and 2q z 25 are observed, which indicates that these nanotubes and nanofibers are partially crystalline. However, there is little difference in the crystallinity between the nanotubes and nanofibers. For nanotubes, a sharp Fig. 5 FTIR spectra of PANI nanotubes, nanofibers and sulfonated porphyrin; reaction conditions: for PANI nanotube TSPP/aniline ¼ 1: 1, nanofibers TSPP/aniline ¼ 4 : 1, reaction time 12 h. Fig. 6 UV-visible absorption spectra of PANI nanostructures dissolved in N-methyl-2-pyrrolidone (A) TSPP doped PANI nanotubes; TSPP/ aniline ¼ 1 : 1, (B) PANI grains; without TSPP doping, and (C) PANI nanofibers TSPP/aniline ¼ 4: J. Mater. Chem., 2012, 22, This journal is ª The Royal Society of Chemistry 2012

6 presence of sulfonated porphyrin or to the degradation of PANI (soluble species such as benzoquinone and hydroquinone), 46 while the PANI grains voltammogram is characterized by a single redox pair AA 0, with broad anodic and cathodic waves. From the cyclic voltammograms, we can observe that the voltammograms are different for PANI nanotubes/nanofibers and grains. This fact was attributed to the electrostatic interaction of the dopant with the chemically flexible NH group of the polymer. Conclusions Fig. 7 XRD scattering pattern of PANI nanotubes; TSPP/aniline ¼ 1 : 1, PANI nanofibers TSPP/aniline ¼ 4 : 1, and PANI grains; without TSPP doping. peak at about 2q z 6.49 is observed, which arises from aniline and dopant interaction during the polymerization; this has been discussed elsewhere. 43 This weak peak is also observed in nanofibers and particles. This is consistent with the mechanism of the formation of PANI nanotubes in the above discussions. Cyclic voltammetry The electrochemical characteristics of the synthetic PANI nanotubes, nanofibers and grains were studied by voltammetry as shown in Fig. 8. The cyclic voltammogram of the TSPP doped PANI nanotubes and nanofibers in 0.1 M H 2 SO 4 solution displays three pairs of redox peaks (under the scanning rate of 60 mv s 1 ). The first redox pair AA 0 corresponds to the reversible transition between leucoemeraldine (reduced form of polyaniline) and emeraldine (half-oxidized form of the polymer), 44 and the redox pair CC 0 is ascribed to reversible transition between emeraldine and pernigraniline (fully oxidized form of the polymer). 45 The middle pair BB 0 has been attributed either to the Polyaniline nanotubes and nanofibers have been synthesized by a template-free and interfacial polymerization process from solutions containing tetrakis(4-sulfonatophenyl)porphyrin dopant. It was found that the morphology of the obtained PANI nanotubes was changed from a one-dimensional nanotube to a three-dimensional cauliflower shape by simply changing the ratio of TSPP to aniline monomer. In the interfacial polymerization, the fibrillar morphology was obtained in the presence of TSPP while in the absence of TSPP absolute granular morphology of PANI was obtained. This may be accounted for by the electrostatic and hydrophobic interactions between the water dispersible porphyrin complex [TSPP (NH 4 ) 2 S 2 O 8 ] and aniline dimer cation-radicals which could act as effective surfactants to shape the PANI nanofibers morphology. The structural characterizations by using FTIR and UV-vis spectroscopy, and XRD, EDX, solubility, conductivity, as well as cyclic voltammetric measurements provided the evidence that the PANI nanotubes and nanofibers were in a doped state identical to the emeraldine salt form. Acknowledgements One of the authors (Mohd. Khalid) would like to place on record his special thanks to the CNPq, Govt. of Brazil for providing a post-doctoral fellowship to carry out part of the research work in the physics department of UFSC, Florianopolis. The authors also thank Prof. A. Neves (LBC/UFSC, Brazil) for providing FTIR and hydraulic pressure machine facilities and the Brazilian agencies CAPES, FINEP, CNPQ and FAPESC. References Fig. 8 Cyclic voltammetric response of polyaniline nanotubes, nanofibers and grains in 0.1 M H 2 SO 4 solution at a scan rate of 60 mv s 1. The nanostructures of PANI were confined on the surface of a gold electrode. 1 D. Normile, Science, 1999, 286, J. Kong, N. R. Franklin, C. W. Zhou, M. G. Chapline, S. Peng, K. J. Cho and H. J. Dai, Science, 2000, 287, C. R. Martin, Science, 1994, 266, J. Huang and M. X. Wan, J. Polym. Sci., Part A: Polym. Chem., 1999, 37, J. Jang, Adv. Polym. Sci., 2006, 199, G. F. Li, C. Martinez and S. Semancik, J. Am. Chem. Soc., 2005, 127, C. G. Wu and T. Bein, Science, 1994, 264, C. R. Martin, Acc. Chem. Res., 1995, 28, C. W. Wang, Z. Wang, M. K. Li and H. L. Li, Chem. Phys. Lett., 2001, 341, Z. Wang, M. A. Chen and H. L. Li, Mater. Sci. Eng., A, 2002, 328, N. Gospodinova, P. Mokreva and L. Terlemezyan, Polymer, 1993, 34, J. Huang and M. Wan, J. Polym. Sci., Part A: Polym. Chem., 1999, 37, Y. Shen and M. Wan, J. Polym. Sci., Part A: Polym. Chem., 1999, 37, This journal is ª The Royal Society of Chemistry 2012 J. Mater. Chem., 2012, 22,

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