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Optical and Electrochemical Properties of Poly(o-toluidine) Multiwalled Carbon Nanotubes Composite Langmuir-Schaefer Films Valter Bavastrello,*,† Sandro Carrara,† Manoj Kumar Ram,‡ and Claudio Nicolini†,§ Department of Biophysical M&O Science and Technologies, University of Genoa, Corso Europa 30, 16132 Genoa, Italy, Fractal Systems Inc., 14200 Carlson Circle, Tampa, Florida 33626, and Fondazione EL.B.A, Via delle Testuggini 181, 00143, Rome, Italy Received July 28, 2003. In Final Form: November 5, 2003 Conducting poly(o-toluidine) (POT) with multiwalled carbon nanotubes (MWNTs) nanocomposite (POTMWNTs) was synthesized by oxidative polymerization. Chloroform solutions of the material were used for the optical characterizations by means of UV-visible spectroscopy and for the fabrication of LangmuirSchaefer (LS) films. LS films were fabricated at the air-liquid interface by using 0.1 M HCl aqueous solution as the subphase to study the electrochemical properties of the nanocomposite by means of cyclic voltammetry and photoelectrochemical techniques. The optical characterizations gave proof that the presence of MWNTs inside the polymeric matrix produced no change in the (π-π*) transition of POT structure, indicating that the polymeric chains were simply wrapped around and not doped by MWNTs. The electrochemical investigations highlighted significant changes in the redox properties of POT-MWNTs LS films with respect to pure POT. The cyclic voltammetric study also revealed high electrochemical stability, confirmed by the estimation of the diffusion coefficient and the photoelectrochemical response of the nanocomposite LS films. This characteristic turned out to be more evident than that obtained in our earlier studied poly(o-anisidine)-MWNTs (POAS-MWNTs) system.
1. Introduction Carbon nanotubes (CNTs) show a great promise for engineering applications due to their high strength and stiffness, as well as unique physical and electrical properties.1 CNTs find application in scanning probes,2,3 electron field emission sources,4 actuators,5 nanoelectronic devices,6 medical devices,7 hydrogen storage,8 and nanocomposites materials.9,10 The intractability of CNTs, both singlewalled (SWNTs) and multiwalled nanotubes (MWNTs) however poses an obstacle to the further development in CNTs science and significantly limits the scope of their practical applications.11 It has been envisioned that nanocomposite formation and functionalization would highlight the physical properties of CNTs. The issues that * To whom correspondence may be addressed. Phone: +39-010-3538145. Fax: +39-010-3538541. E-mail:
[email protected]. † University of Genoa. ‡ Fractal Systems Inc. § Fondazione EL.B.A. (1) Dresselhaus, M. S.; Dresselhaus, G.; Eklund P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, CA, 1996. (2) Dai, H.; Hafner, J. H.; Rinzler, A. G.; Colbert, D. T.; Smalley, R. E. Nature 1996, 384, 147. (3) Wong, S.; Joselevich, E.; Woolley, A.; Cheung, C.; Lieber, C. Nature 1998, 394, 52. (4) de Heer, W. A.; Chatelain, A.; Ugarte, D. Science 1995, 270, 1179. (5) Baughman, R. H.; Cui, C.; Zakhidov, A. A.; Iqbal, Z.; Barisci, J. N.; Spinks, G. M.; Wallace, G. G.; Mazzoldi, A.; De Rossi, D.; Rinzler, A. G.; Jaschinski, O.; Roth, S.; Kertesz M. Science 1999, 284, 1340. (6) Tans, S.; Verschueren, A.; Dekker, C. Nature 1998, 393, 49. (7) Cao, X. N.; Lin, L.; Zhou, Y. Y.; Shi, G. Y.; Zhang, W.; Yamamoto, K.; Jin,L. T. Talanta 2003, 60, 1063. (8) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (9) Wagner, H. D.; Lourie, O.; Feldman, Y.; Tenne, R. Appl. Phys. Lett. 1998, 72, 188. (10) Dagani, R. Chem. Eng. News 1929, 7, 25. (11) Ajayan, P. M.; Charlier, J. C.; Rinzler, A. G. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 14199.
are important to nanocomposite performance are CNTs dispersion/orientation, interfacial bonding, and CNTs deformation within a matrix or CNTs-wrapped polymers.12 No suitable solvents for CNTs have been found, so they are simply dispersed in a solvent containing the soluble polymer to form a nanocomposite material. PolymerCNTs nanocomposites have recently been studied using various materials by taking into account epoxy resins, thermoplastic polymers, and conducting polymers as the organic matrix.13-23 Furthermore, in the field of conducting polymers, novel electrochemical and chemical methods for the synthesis of polypyrrole-CNTs, polythiopheneCNTs, polyphenylenevinylene-CNTs, and polyanilineCNTs nanocomposites have been studied.24-30 (12) Bahr, J. L.; Mickelson, E. T.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. Chem. Commun. 2001, 2, 193. (13) Bower, C.; Rosen, R., Jin, L.; Han, J.; Zhou, O. Appl. Phys. Lett. 1999, 74, 3317. (14) Lourie, O.; Wagner, H. D. Appl. Phys. Lett. 1998, 73, 3527. (15) Wagner, H. D.; Lourie, O.; Feldman, Y.; Tenne, R. Appl. Phys. Lett. 1998, 72, 188. (16) Schadler, L. S.; Giannaris, S. C.; Ajayan, P. M. Appl. Phys. Lett. 1998, 73, 3842. (17) Qian, D.; Dickey, E. C.; Andrews, R.; Rantell, T. Appl. Phys. Lett. 2000, 76, 2868. (18) Grimes, C. A.; Mungle, C.; Kouzoudis, D.; Fang, S.; Eklund, P. C. Chem. Phys. Lett. 2000, 319, 460. (19) Gong, X.; Liu, J.; Baskaran, S.; Voise, R. D.; Young, J. S. Chem. Mater. 2000, 12, 1049. (20) Stephan, C.; Nguyen, T. P.; Lamy de la Chapelle, M., Lefrant, S.; Journet, C.; Bernier, P. Synth. Met. 2000, 108, 139. (21) Sandle, J.; Shaffer, M. S. P.; Prasse, T.; Bauhofer, W.; Schulte, K.; Windle, A. H. Polymer 1999, 40, 5967. (22) Woo, H. S.; Czerw, R.; Webster, S.; Carroll, D. L.; Park, J. W.; Lee, J. H. Synth. Met. 2001, 116, 369. (23) Jin, Z.; Sun, X.; Xu, G.; Hong, S.; Wei, J. G. Chem. Phys. Lett. 2000, 318, 505. (24) Tang, B. Z.; Xu, H. Macromolecules 1999, 32, 2569. (25) Gao, M.; Huang, S.; Dai, L.; Wallace, G.; Gao, R.; Wang, Z. Angew. Chem., Int. Ed. 2000, 39, 3664. (26) Mussa, I.; Baxendale, M.; Amaratunga, G. A.; Eccleston, W. Synth. Met. 1999, 102, 1250.
10.1021/la035372a CCC: $27.50 © 2004 American Chemical Society Published on Web 01/08/2004
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Figure 1. Representation of POT in emeraldine base (A) and emeraldine salt (B) states.
Polyaniline is an important conjugated conducting polymer because of its good environmental stability.31-35 It can be doped to its conducting form, without changing the number of π electrons, through protonation by exposure to an appropriate protic acid in aqueous solution.36 To obtain additional insight into polyaniline, a methyl group (-CH3) was used to block the ortho position of the aromatic ring of aniline.37-39 It may be remarked that methyl-substituted polyaniline called poly(o-toluidine) (POT) (Figure 1) was found to have additional advantage with respect to polyaniline due to its faster switching time between the oxidized and the reduced states.40 It can be ascertained at this juncture that the thickness and orientation of polymeric molecules play a role on the electroactivity of POT films. This is a processable polyaniline system. The aim of this work was to study the changed properties of POT conducting polymer due to the presence of MWNTs inside the polymeric matrix. However the results pointed out even more differences with respect to our earlier investigations on the nanocomposite synthesized with poly(o-anisidine) (POAS) conducting polymer.29 2. Experimental Techniques 2.1. Synthesis of Nanocomposite. Monomers of o-toluidine, ammonium persulfate [(NH4)2S2O8] as oxidizing agents, and other reagents were obtained from Sigma, while MWNTs were purchased from MER Corporation, Tucson, AZ. The MWNTs employed in the synthesis had a diameter ranging between 2 and 15 nm and length between 1 and 10 µm, with 5-20 graphitic layers. To have comparable systems, the same supply of MWNTs utilized in our previous work based on POAS-MWNTs nanocomposite was used.29 For the synthesis of POT-MWNTs material, 100 mg of accurately weighed MWNTs were dispersed in 100 mL of 1 M HCl and ultrasonicated for 1 h so that the MWNTs were homogeneously dispersed in the solution. The oxidizing agent was added in the MWNTs dispersion, and the monomer was (27) Coleman, J. N.; Curran, S.; Dalton, A. B.; Davey A. P.; McCarthy, B.; Blau, W.; Barklie, R. C. Synth. Met. 1999, 102, 1174. (28) Jin, L.; Bower, C.; Zhou, O. Appl. Phys. Lett. 1998, 73, 1197. (29) Bavastrello, V.; Ram, M. K.; Nicolini, C. Langmuir 2002, 18, 1535. (30) 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. (31) Ram, M. K.; Sundaresan, N. S.; Malhotra, B. D. J. Phys. Chem. 1993, 97, 11580. (32) Ram, M. K.; Mehrotra, R.; Pandey, S. S.; Malhotra, B. D. J. Phys. Conden. Mater. 1994, 6, 8913. (33) Ram, M. K.; Sundaresan, N. S.; Malhotra, B. D. J. Mater. Sci. Lett. 1994, 13, 1490. (34) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1989, 32, 263. (35) Syed, A. A.; Dinesan, M. K.; Genies, E. M. Bull. Electrochem. 1988, 4, 183. (36) Huang, W. S.; Humphery, B. D.; MacDiarmid, A. G. J. Chem. Soc., Faraday Trans. 1986, I 82, 2385. (37) Kumar, D.; Dhawan, S. K.; Ram, M. K.; Sharma, R. C.; Malhotra, B. D.; Chandra, S. Indian J. Chem., Sect. A: Inorg., Bio-inorg., Phys., Theor. Anal. Chem. 1997, 36, 290. (38) Kumar, D.; Sharma, R. C.; Ram, M. K.; Dhawan, S. K.; Malhotra, B. D.; Chandra, S. S Indian J. Chem., Sect. A: Inorg., Bio-inorg., Phys., Theor. Anal. Chem. 1997, 36, 14. (39) Ram, M. K.; Joshi, M.; Mehrotra, R.; Dhawan, S. K.; Chandra, S. Thin Solid Films 1997, 304, 65. (40) Ram, M. K.; Adami, M.; Sartore, M.; Salerno, M.; Paddeu, S.; Nicolini, C. Synth. Met. 1999, 100, 249.
Bavastrello et al. then added slowly, maintaining the temperature at around 4 °C by means of an ice bath. The reaction was continued for 12 h. A dark green precipitate recovered from the reaction vessel was filtered and washed by using 1 M HCl to remove the oxidant and oligomers, respectively. The precipitate was subsequently washed in deionized water several times and later in methanol and diethyl ether in order to eliminate the polymers of low molecular weight and oligomers (violet in color). POT-MWNTs material was heated to 90-100 °C. The powder thus obtained, the emeraldine salt (ES) form of the nanocomposite, was washed in distilled water and acetone several times and then dried for 6 h at a temperature of 100 °C.40-42 Later, the ES was treated with 1 M ammonium hydroxide for 24 h to obtain the emeraldine base (EB) form of POT-MWNTs nanocomposite. Such material was washed with excess water and acetone and dried at 100 °C. The EB form of POT-MWNTs nanocomposite was soluble in chloroform. 2.2. Formation of Nanocomposite Langmuir-Schaefer (LS) Films. Different concentrations of POT-MWNTs spreading solutions in chloroform were prepared, and the correspondent isotherms were recorded similar to our earlier investigation on POT conducting polymer.39 The solutions concentration was kept at a minimum level, showing no immediate change in the collapse pressure. So, a stock solution was prepared by dissolving 1 mg of the nanocomposite in 2 mL of chloroform for immediate use. The solution was sonicated for 30 min to reach the maximum solubility in the solvent. The resulting solution was filtered with a solvent-resistant filter. Volumes of 100-150 µL of each solution were spread onto 0.1 M HCl as the subphase. In fact, our previous results showed high collapse pressure at the air-liquid interface by using such a subphase, and it was thus utilized for the fabrication of LS films. The Langmuir monolayers were formed in a Langmuir-Blodgett trough, 240 mm × 100 mm in size and 300 mL in volume (MDT corp., Russia), having a compression speed of 1.67 mm/s (100 cm2/min). Different compression speeds were also utilized for the recording of pressure-area isotherms. Different numbers of monolayers were transferred onto glass, glass indium-tin oxide (ITO) plates, and interdigitated electrode substrates (containing chromium electrodes, which were previously cleaned with ethanol and chloroform) by the LS technique. The stability of Langmuir monolayers was checked at various surface pressures using a feedback (gain) of 5 mN/m for the deposition of LS films. 2.3. Optical Measurement. The UV-vis spectra of the nanocomposite material were recorded by using a UV-vis spectrophotometer Jasco model V530, with software. 2.4. Electrical and Electrochemical Characterizations. To perform the electrical characterizations, a Keithley model 6517 electrometer was employed. Current-voltage (I-V) characteristics were obtained by a potential step of 0.05 V. Each strip of the interdigitated electrodes was spaced 50 µm and 40 nm in thickness. The electrochemical measurements were made by a potentiostat/galvanostat (EG & G PARC model 163), through M270 supplied software. A standard three-electrode configuration was used, where POT-MWNTs LS films onto a glass ITO plate acted as a working electrode, platinum as a counter electrode, and Ag/AgCl as a reference electrode. Cyclic voltammograms of the nanocomposite LS films in 0.1 M HCl were measured. The electrical conductivity of the 1 M HCl treated nanocomposite 40 layer LS films showed a conductivity of 8 × 10-2 S/cm. 2.5. Photocurrent Study. The photoelectrochemical study was carried out in a cell containing the electrode setup previously described. A 150 W visible lamp served as the light source, with the entire wavelength range passing through the ITO/POTMWNTs side of the electrochemical cell. The electrochemical photocurrent was observed by applying potential from a model 263A potentiostat at fixed bias, and the photocurrent was measured.
3. Results and Discussion 3.1. Pressure-Area (π-A) Isotherms. The stability of Langmuir monolayers for POT-MWNTs was found to (41) Ram, M. K.; Carrara, S.; Paddeu, S.; Nicolini, C. Thin Solid Films 1997, 302, 89. (42) Paddeu, S.; Ram, M. K.; Nicolini, C. J. Phys. Chem. B 1997, 101, 4759.
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Figure 2. Pressure-area isotherms of POT-MWNTs nanocomposite obtained by spreading the nanocomposite chloroform solution onto 0.1 M HCl as a subphase (barrier speed ) 1.67 mm/s).
be associated to a high collapse pressure in the condensed phase. The π-A isotherms showed an increment in the pressure of the condensed phase, with the possibility to obtain the yielding (breaking) points at a surface pressure greater than 30 mN/m for POT-MWNTs molecules. Wrinkles on the aqueous subphase could be observed when the pressure was at about 30 mN/m. The -CH3 groups could have caused different arrangements of POT-MWNTs molecules at the air-liquid interface, though the films were formed at pH 1 and possibly behaved a bit differently than parent POT molecules.40 When the monolayers at the air-liquid interface were kept for 2 h, the surface pressure remained constant at 22 mN/m. Figure 2 shows the isotherm as a function of the area per molecules, calculated by considering the repeat unit (r.u.) illustrated in Figure 1. The comparison of this curve with that obtained in the case of pure POT Langmuir monolayer (Figure 2b, curve 2 of ref 40) is proof that the composite is present at the air-liquid interface. In fact, the molecular area at the condensed phase was estimated to be 42 Å2/r.u. for the nanocomposite, while the molecular area was found to be 25 Å2/r.u. in the case of pure POT.40 The same phenomenon was also confirmed by spreading onto deionized water. In fact we obtained a value of 53 Å2/r.u. for the nanocomposite, while a value of 45 Å2/r.u. was found for pure POT.40 3.2. UV-visible Absorption of POT-MWNTs Nanocomposite. In Figure 3a is illustrated the optical absorption spectrum of POT-MWNTs solution in chloroform. It shows the characteristic absorption bands at 310 and 570 nm for the EB form of POT. The band seen at 310 nm is due to (π-π*) interband transition, whereas the observed peak at 570 nm is assigned to (n-π*) transition from the nonbonding nitrogen lone pair to the conduction band (π*). There is no shift in the (π-π*) transition of the POTMWNTs nanocomposite with respect to POT pure polymer, while there is a shift in the (n-π*) transition. By taking into account that the excitation of π electrons requires smaller energy, the band gap was calculated for the (π-π*) transition.43 In a semiconductor, the band gap is the energy gap between the valence band and the conduction band. In the case of a conducting polymer, the band gap is defined as the minimum photon energy required to excite an electron from the highest occupied molecular orbital to the lowest unoccupied molecular orbital. In both cases, the band gap is diminished by the insertion of doping (43) Phukan, T.; Kanjilal, D.; Goswami, T. D.; Das, H. L. Radiat. Meas. 2003, 36, 611.
Figure 3. (a) UV-vis spectrum of POT-MWNTs nanocomposite in CHCl3. (b) UV-vis spectra of POT-MWNTs nanocomposite (]) and POT pure polymer (4) in CHCl3 plotted as (Rhν)2 vs hν. The continuous line corresponds to the best fitting of the equation (Rhν/B)2 ) hν - Eg, and the band gap (Eg) is obtained by the intercept on the abscissa
agents inside the material. The band gap was thus calculated by means of the Tauc equation44
R)B
(hν - Eg)n hν
(1)
where R is the absorption coefficient, B is a fitting parameter, h is the Planck constant, ν is the photon frequency, Eg is the band gap, and n takes into account different possible electronic transitions responsible for the light absorption. In the case of polyaniline and its derivatives, n ) 1/2.45 Figure 3b reports the UV-vis spectra plotted as (Rhν)2 vs hν, and the energy gap was obtained by the intercept on the abscissa of the best fitting of eq 1. The band gap of POT-MWNTs was estimated to be 3.4 eV, and this value was found to match the band gap calculated for POT pure polymer, indicating that the polymeric chains are simply wrapped around and not doped by MWNTs. 3.3. Electrochemical Study. The cyclic voltammetry mesaurements carried out on polyaniline and its derivatives are usually characterized by the presence of three redox couples, corresponding to the leucoemeraldine/ emeraldine, intermediate emeraldine, and emeraldine/ (44) Pal, M.; Hirota, K.; Sakata, H. Phys. Status Solidi A 2003, 196, 396. (45) Huang, L. M.; Wen, T. C.; Gopalan A.; Ren, F. Materials Sci. Eng., B 2003, 104, 88.
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Figure 4. Cyclic voltammetry of POT-MWNTs nanocomposite in 0.1 M HCl, sweep rate 5 mV/s. The graphic also shows the Gaussian decomposition related to the not well resolved oxidation peaks.
pernigraniline transitions.46 In Figure 4 is illustrated the cyclic voltammetry of POT-MWNTs nanocomposite LS films obtained in 0.1 M HCl at a sweep rate of 5 mV/s, where it can be seen the presence of more peaks than common polyaniline systems. It is however possible to observe the presence of three not well resolved oxidation peaks (between 50 and 500 mV) and three evident reduction peaks (10, 190, and 350 mV) corresponding to the leucoemeraldine/emeraldine, intermediate emeraldine, and emeraldine/pernigraniline transitions. The extra oxidation/reduction peaks are probably due to the interactions between POT molecules and MWNTs embedded in the polymeric matrix. By the support of the Gaussian decomposition, as shown in Figure 4, the oxidation peaks were assigned values of 74, 238, and 403 mV. It can be also observed that the oxidation/reduction peaks are shifted toward lower bias with respect to the correspondent transitions of POT LS films.40 This behavior is coherent with the results obtained for the nanocomposite materials based on POAS conducting polymer,29 where the reduction peaks were revealed at lower potentials with respect to POAS pure polymer.40 It can be therefore deduced that the presence of MWNTs inside the polymeric matrix results in an increasing of the electronic density states which facilitates the protonation of the amine group and the formation of the polaron/bipolaron states. Despite the presence of MWNTs increasing the oxidation capability of the polymer, the resulting system is electrochemically slower with respect to POT pure polymer (Figure 3 of ref 39). In fact, the cyclic voltammograms of POT-MWNTs nanocomposite carried out at increasing scan rate show a fusion of the signals, as shown in Figure 5, evidenced by the presence of only two redox peaks at 394 and 262 mV. It is also possible to observe that the peak current sweeps linearly as a function of the applied scan rate, as shown in Figure 6, confirming that the currents correspond to redox processes involving surface-confined electroactive species. The fusion of the redox peaks in POT-MWNTs arises due to the decreased mobility of the polymer chains. In fact, taking into account that POT conducting polymer is wrapped around MWNTs, the cause of the decreased mobility is generated by the sterical hindrance of the -CH3 groups along the graphitic layers of MWNTs. This suggests that the -CH3 groups affect the mechanisms involved in the formation of the previously mentioned species during the redox processes. To have an immediate vision of the results so far discussed, the experimental data are (46) Tawde, S.; Mukesh, D.; Yakhmi, J. V. Synth. Met. 2002, 125, 401.
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Figure 5. Cyclic voltammetry of POT-MWNTs nanocomposite in 0.1 M HCl as a function of scan rate.
Figure 6. Anodic and cathodic current variations in function of the sweep rate. Table 1. Electrochemical Parameter of Thin Films Based on POT, POAS, POT-MWNTs, and POAS-MWNTs type of film
oxidation potential (mV)
reduction potential (mV)
refs
POAS POAS-MWNTs POT POT-MWNTs
707, 506, 282 632, 430, 200 662, 532, 304 403, 238, 74a
680, 410, 144 572, 364, 48 627, 499, 165 354, 190, 10a
40 29 40 present work
a Data acquired at 5 mV/s due to the slow electrochemical activity of POT-MWNTs while other data were obtained at 50 mV/s.
summarized in Table 1. The analyses of the cyclic voltammograms also outline that the ratio of the anodic to cathodic currents, being less than unity, implies that the electrochemical phenomena in POT-MWNTs LS films may be governed by diffusion-controlled processes. Hence the diffusion coefficient was calculated for POT-MWNTs LS films by means of the Randles-Sevcik equation47
Ip ) (2.687 × 105)n3/2AD01/2Cν1/2
(2)
where n is the number of electrons transferred in the reaction, A is the electrode area, C is the concentration of diffusing species in the bulk of the electrolyte, and ν is the sweep potential rate. By using values of n ) 1 and C ) 0.1 M, the diffusion coefficient D0 was found to be 0.5 × 10-8 cm2 s-1. This value is less than one-third of the equivalent diffusion coefficient obtained in the case of POAS-MWNTs nanocomposite.29 This result is coherent with the previous considerations related to the slower electrochemical system based on POT-MWNTs. Figure 7 shows cyclic voltammograms as a function of higher potential range scanned at 50 mV/s in a threeelectrode cell system. An applied potential between -1.0 and 1.0 V generated the appearance of an oxidation peak at around -500 mV, besides the conventional peaks of (47) Hesse, K.; Schlettwein, D. J. Electroanal. Chem. 1999, 476, 148.
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Figure 8. Photocurrent response of POT-MWNTs nanocomposite LS films at zero bias in 0.1 M HCl.
interior of POT-MWNTs film. Under the illumination of light to the film, a net positive space charge is expected to form, which impairs the current flow. By application of higher light energy (hν > Eg), the effect is maintained slightly more. The phenomenon (the influence of electrochemical photocurrent in dark) may be due to the dark oxidation of the reduced forms of POT, which are accumulated on the electrode surface as a result of reduction processes of the adsorbed ions by photoelectron activity of this conjugated polymer. The following sequence of photoelectrical sensitivity for the substituted polyaniline system is given as CN . H > OCH3 > CH3.48 So POTMWNTs nanocomposite shows less photoconductive activity than POAS-MWNTs LS films.29 Moreover, the photocurrent was observed to decrease upon the applied potential. This supports the idea that an understanding of the transient behavior of photoelectrochemical cells solely depends on the consideration of ion motion. This final result gives further support to the fact that POTMWNTs nanocomposite is a slower electrochemical system with respect to POAS-MWNTs material. 4. Conclusion
Figure 7. Cyclic voltammetry of POT-MWNTs nanocomposite in 0.1 M HCl sweep rate 50 mV/s as a function of applied potential: (a) -1.0 to 1.0 V; (b) -1.0 to 1.2 V; (c) -1.0 to 2.0 V.
POT system (Figure 7a). A wider potentials range between -1.0 and 1.2 V showed the appearance of oxidation peaks at roughly -584, -498, and 440 mV as well as reduction peaks at around 154 and -772 mV (Figure 7b). New signals were revealed in a potential range of -1.0 to 2.0 V corresponding to oxidation peaks at roughly -734, -662, -510, and 874 mV, as well as reduction peaks at around 896, -184, and -772 mV (Figure 7c). It can be thus deduced that POT conducting polymer is duly overoxidized, giving proof of electrochemical interactions between POT conducting polymer and MWNTs. These results also show that POT-MWNTs nanocomposite is a stable system even in the case of applied wide bias ranges. 3.4. Photocurrent through Electrochemical Study. Figure 8 shows the photocurrent for POT-MWNTs LS films at zero bias in 0.1 M HCl. The uniform photocurrent was observed in each switch-on and switchoff condition for the LS film. The slow photocurrent transits are associated with the consumption of oxygen in the
A processable POT-MWNTs nanocomposite material was synthesized by oxidative polymerization. The presence of MWNTs in POT polymeric matrix produced no change in the (π-π*) transition of POT structure, indicating that the polymeric chains were simply wrapped around and not doped by MWNTs. Different results were obtained in our previous work on POAS-MWNTs nanocomposite. In fact, the presence of MWNTs inside the polymeric matrix seemed to have major interactions, producing a sort of doping effect on the conducting polymer. Further investigations on POT-MWNTs LS films showed different electrochemical properties with respect to POT pure polymer and to POAS-MWNTs nanocomposite LS films. The experimental data highlighted that POTMWNTs LS films were less electroactive than POT LS films, but they were found to be electrochemically more stable. A minor diffusion coefficient and a lower photoelectrochemical response of POT-MWNTs LS films with respect to POAS-MWNTs LS films gave further proof of a diminished electrochemical activity. The analysis of the experimental data demonstrated that the insertion of MWNTs inside a polymeric matrix can generate some interactions between the materials, and these interactions can vary by changing the conducting polymer employed in the synthesis. LA035372A (48) Handbook of Conducting Polymers; Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker, Inc.: New York, 1998; p 399.
