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Preparation and Characterization of Polypyrrole-Coated Nanosized Novel Ceramics Gyoujin Cho,*,†,‡ Bing M. Fung,‡ Daniel T. Glatzhofer,‡ Jae-Suk Lee,§ and Yong-Gun Shul| Department of Chemical Engineering, Sunchon National University, Sunchon, Cheon Nam 540-742, Korea, Department of Chemistry and Biochemistry, The University of Oklahoma, Norman, Oklahoma 73019, Department of Chemical Engineering, Yonsei University, Seoul 1202-749, Korea, and Department of Materials Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), Kwangju 500-712, Korea Received August 29, 2000. In Final Form: November 7, 2000 Surfactant bilayers adsorbed on TS-1 zeolite were used as templates to produce colloidal nanocomposites with a polypyrrole (Ppy) shell. The adsorbed surfactant was cetylpyridinium chloride, and it plays a critical role for attaining both the colloidal stability of the nanocomposites and an enhanced conductivity of the Ppy sheath on the TS-1 core. The observed contact conductivity of the nanocomposites was 5 S/cm for a sample with 8 wt % of Ppy incorporation while bulk Ppy powder had a contact conductivity of 0.03 S/cm.
Introduction One of the current interests in research on nanostructured materials is the preparation of nanocomposite conducting colloidal particles consisting of inorganic core particles homogeneously covered with ultrathin films of conducting polymers. The optical and electrical properties of these materials differ from their individual nanoparticles or macroscopic equivalents.1-3 Therefore they can be effectively applied in the fields of sensors, optics, and electronics. Techniques for the preparation of nanocomposites containing conducting polymers have been recently reviewed.4 Among a number of conducting polymers, polypyrrole (Ppy) has been extensively investigated by numerous research groups because it offers reasonably high conductivity and has fairly good environmental stability. Many investigations on Ppy with nanoscopic dimensions, for example Ppy nanotubules and Ppy nanocomposites, have been published.5 If Ppy nanocomposite systems can be prepared in stable colloidal form, the poor processibility of Ppy can be overcome, at least partly, so that its applications can be extended to wide areas. Armes and co-workers have carried out pioneer studies on stable colloidal Ppy nanocomposites using a technique for encapsulating colloidal metal oxide nanoparticles with * Corresponding author. E-mail:
[email protected]. Fax: 405-325-6111. † Sunchon National University. ‡ The University of Oklahoma. § Yonsei University. | Kwangju Institute of Science and Technology. (1) Wegner, G. Angew. Chem., Int. Ed. Engl. 1981, 20, 361. Meyer, W. H. Adv. Mater. 1993, 5, 254. Millellar, E.; Musio, F.; Alba, M. B. Thin Solid Films 1996, 284, 908. Cao, J.; Heeger, A. J.; Lee, J. K.; Kim, C. Y. Synth. Met. 1996, 82, 221. (2) Godovski, D. Y. Adv. Polym. Sci. 1995, 119, 79. (3) Sayre, C. N.; Collard, D. M. Langmuir 1997, 13, 714. Marinakos, S. M.; Brousseau, L. C.; Jones, A.; Feldheim, D. L. Chem. Mater. 1998, 10, 1214. Phanabalan, A.; Mello, S. V.; Oliveria, O. N., Jr. Macromolecules 1998, 31, 1827. (4) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608. (5) Martin, C. R. Science 1994, 266, 1961. Martin, C. H. Acc. Chem. Res. 1995, 28, 61. Goren, M.; Qi, Z.; Lennox, R. B. Chem. Mater. 1995, 7, 171. Wong, H. P.; Dave, B. C.; Leroux, F.; Harreld, J.; Dunn, B.; Nazar, L. F. J. Mater. Chem. 1998, 8, 1019.
Ppy or polyaniline.6-10 They were able to produce stable colloids of Ppy-metal oxide nanocomposites by slowing down the rate as well as the degree of polymerization and enhancing polymerization on the substrate rather than in bulk.6 The morphology of the resulting nanocomposites showed that nanosized metal oxide particles were adhered together by raspberry-shaped domains of Ppy.10 Matijevic and co-workers have reported a core-shell type of nanocomposite using a host of inorganic oxides as core materials that can polymerize pyrrole without using extraneous oxidizing agents.11,12 Recently, Ppy-coated gold nanoparticles were reported by using polystyrene-block-poly(2vinylpyridine) as a template.13 The diblock copolymer acts as a core on which the composite was formed and stabilization could thereby take place. On the other hand, the Ppy from all of above Ppy nanocomposites was normal Ppy without any degree of molecular orientation. Therefore, it is of interest to investigate whether the formation of stable colloidal nanocomposites with ordered Ppy would improve their properties. Many studies have been carried out to attain a high molecular orientation of Ppy using templates such as the nanopores of zeolites14 and membranes,5,15 Langmuir-Blodgett films,16 and liquid crystal(6) Armes, S. P.; Gottesfeld, S.; Beery, J. G.; Garzon, F.; Agnew, S. F. Polymer 1991, 32, 2325. (7) Maeda, S.; Armes, S. P. Chem. Mater. 1995, 7, 171. (8) Flitton, R.; Johal, J.; Maeda, S.; Armes, S. P. J. Colloid Interface Sci. 1995, 173, 135. (9) Gill, M.; Mykytiuk, J.; Armes, S. P.; Edwards, J. L.; Yeats, T.; Moreland, P.; Mollett, C. J. Chem. Soc., Chem. Commun. 1992, 108. McCarthy, G. P.; Armes, S. P.; Greaves, S. J.; Watts, J. F. Langmuir 1997, 13, 3686. (10) Maeda, S.; Armes, S. P. J. Colloid Interface Sci. 1993, 159, 257. Maeda, S.; Armes, S. P. J. Mater. Chem. 1994, 4, 935. (11) Huang, C. L.; Partch, R. E.; Matijevic, E. J. Colloid Interface Sci. 1995, 170, 275. (12) Huang, C. L.; Matijevic, E. J. Mater. Res. 1995, 10, 1327. (13) Selvan, S. T. Chem. Commun. 1998, 351. Selvan, S. T.; Spatz, J. P.; Klok, H. A.; Moller, M. Adv. Mater. 1998, 10, 132. (14) Bein, T.; P. Enzel. Angew. Chem., Int. Ed. Engl. 1989, 28, 1692. Enzel, P.; Bein, T. Chem. Commun. 1989, 1326. Larsen, G.; Haller, G. L.; Marquez, M. J. Phys. Chem. 1992, 96, 4145. Miller, G. J.; Lewis, A. M.; Browmake, G. A.; Cooney, R. P. J. Mater. Chem. 1993, 3, 867. (15) Menon, V. P.; Lei, J.; Martin, C. R. Chem. Mater. 1996, 8, 2382. (16) Skotheim, T. A.; Yang, X. Q.; Chen, J.; Hale, P. D.; Inagaki, T.; Samelson, L.; Tripathy, S.; Hong, K.; Rubner, M. F. Synth. Met. 1989, 28, 229.
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Polypyrrole-Coated Nanosized Novel Ceramics Scheme 1. Descriptive Illustration for Preparation of Core-Shell Nanocomposites
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fields of rechargeable batteries and capacitors because of synergistic effects from the unique properties of both Ppy and zeolites.19,20 The objective of this paper is to demonstrate that surfactant SAAs can be used as a template to prepare colloidal core-shell nanocomposites and that the technique offers unique advantages over other techniques in attaining microscopic homogeneity and enhanced molecular ordering of the Ppy thin films in the nanocomposites. Experimental Section
line media.17 From the results it can be seen that a confined environment (30 nm) should be deposited on the surface of nanoparticles to avoid overgrowth to form amorphous Ppy and precipitation by gravity. Furthermore, Ppy should not aggregate the resulting nanocomposites. To meet the above conditions simultaneously, Ppy should start to grow on the surface up to a limited thickness. Using a template can control the thickness of Ppy on the surface if pyrrole only polymerizes inside the template. Self-assembled monolayers and adsorbed amphiphilic molecular layers on the surface have been used as the template for Ppy growth. Such templates can afford ordered molecular arrays on the surfaces and be used as 2-D reaction media to produce ultrathin polymer films. Adsorbed surfactants bilayers have comparable properties and are also used as templates for practical and convenient purposes.18 In this process, amphiphilic molecules are first adsorbed from aqueous solutions as self-assembled arrays (SAAs) onto a solid surface. Depending on factors governing the adsorption behavior (e.g. surface charge for ionic surfactants), the SAAs can be monolayers or bilayers. In the second step, pyrrole is allowed to partition from water into the SAAs, and the nature of the surface and interior properties of the SAAs control the rate of this process. Finally, polymerization on the solid surface is initiated by adding chemical oxidants. After polymerization, free Ppy is generally washed away with water, leaving thin Ppy coatings on substrates. This process may be applicable to various kinds of nanoparticles to yield colloidal nanocomposites. In this paper, a simple method (Scheme 1), which is an extension of the above technique, for the preparation of a stable colloid of a zeolite, titanium silicate-1 (TS-1), -Ppy core-shell nanocomposite with microscopic structural homogeneity will be reported. One reason for using a zeolite as a core particle is for potential application in the (17) Torres, W.; Fox, M. A. Chem. Mater. 1992, 4, 583. (18) Funkhouser, G. P.; Arevalo, M. P.; Glatzhofer, D. T.; O’Rear, E. A. Langmuir 1995, 11, 1443. Huang, Z.; Wang, P.; MacDiarmid, A. G.; Xia, Y.; Whitesides, G. Langmuir 1997, 13, 6480. Cho, G. Bull. Chem. Soc. Jpn. 1997, 70, 2309. Cho, G.; Glatzhofer, D. T.; Fung, B. M.; Yuan, W. L.; O’Rear, E. A. Langmuir 2000, 16, 4424.
Materials. The zeolite system used in our work was a silicontitanium zeolite called TS-1, which can be prepared as nanoparticles with an average size of 100 nm and surface area of 550 m2/g by following reported procedures.21,22 The TS-1 particles were redispersed (0.02 wt % of TS-1) in deionized water (1 L), and the pH was adjusted to a value of pH ) 8 using a 1 N stock solution of NaOH. Cetylpyridinium chloride (CPC) was purchased from Aldrich and used as received. Pyrrole (98%, Aldrich) was purified by passing it through a short column of basic alumina, activity grade I (Sigma). Ferric chloride (97%, Aldrich) was used as an oxidant without further purification. Double distilled and deionized water was used for all the systems. Adsorption Isotherms and Pyrrole Adsolubilization. When the TS-1 zeolite particles were redispersed in water at pH ) 8, no precipitation was observed due to the electrostatic repulsive forces between the nanoparticles. Since the zero point charge of TS-1 is pH ) 6.7,22 its surface is negatively charged at pH ) 8. Therefore, CPC was used to form the template. To determine the formation of SAAs on TS-1, CPC adsorption studies were first carried out. For the adsorption studies, various amounts of CPC were added to 5 mL of a TS-1 stock solution. The resulting mixtures were allowed to equilibrate at 25 °C for 1 day. After the equilibration period, the TS-1 nanoparticles were removed by centrifugation at 3000 rpm. Adsorption isotherms were constructed by measuring the concentration of CPC in the supernatant after 5 h centrifugation using UV-visible spectroscopy (Hewlett-Packard 4852A). The amount of CPC adsorbed from solution onto the surface of the TS-1 nanoparticles was calculated using eq 1,
G ) (Ci - Cf)V/m
(1)
where G is the number of moles of CPC adsorbed per gram of solid substrate at adsorption equilibrium, Ci is the initial molar concentration of CPC in solution before adsorption, Cf is the molar concentration of CPC in solution after adsorption, V is the volume of solution, and m is the weight of solid substrate. The solubilization of pyrrole into adsorbed CPC was studied in the following way. From the adsorption studies, the initial amount of CPC to form the SAAs was calculated from the isotherm and was found to be 24 mM. We used a slightly lower initial concentration of surfactant (16 mM) to reduce the formation of free micelles. In the TS-1 colloidal solution, 16 mM of CPC was added and the solution was equilibrated for 24 h at room temperature. After 24 h of equilibration, various amounts of pyrrole were added to the solution which was further equilibrated for another 24 h. The same experiment was carried out without using CPC to observe the adsorption of pyrrole on the TS-1 nanoparticles. The final equilibrium concentrations of pyrrole after removing the TS-1 nanoparticles were determined by GC (Hewlett-Packard with a flame ionization detector) using an aqueous amine analysis column (Supelco; 60/80 Carbopack B, 4% carbowax 20M, 0.8% KOH) at 160 °C. Polypyrrole Formation on the Surface of Nanoparticles. Polymerization of pyrrole was slowly initiated by adding an (19) McCann, G. F.; Millar, G. J.; Bowmaker, G. A.; Cooney, R. J. Chem. Soc., Faraday Trans. 1995, 91, 4321. (20) Uehara, H.; Miyake, M.; Matsuda, M.; Sato, M. J. Mater. Chem. 1998, 8, 2133. (21) Cho, G.; Lee, J. S.; Glatzhofer, D. T.; Fung, B. M.; Yuan, W. L.; O’Rear, E. A. Adv. Mater. 1999, 11, 497. (22) Zhang, G.; Sterte, J.; Schoeman, B. J. Chem. Mater. 1997, 9, 210.
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Figure 1. TEM image for TS-1 nanoparticles. equimolar amount of ferric trichloride, which is dissolved in 0.5 mL of water, to the loaded pyrrole. To demonstrate the role of SAAs in the formation of Ppy ultrathin film on TS-1, TS-1-Ppy nanocomposites were prepared under the same conditions except without CPC presents. Particle sizes and morphologies of the resulting TS-1-Ppy nanocomposites were studied using transmission electron microscopy (TEM; Philips EM 400T) before and after removing free Ppy by repeating the process of centrifugation and redispersion. Diffuse reflectance infrared spectra of the powders of the nanocomposites were recorded on a Shimazu 7100 FTIR spectrometer with a diffuse reflection attachment. After casting of the solution of the dispersed free Ppy and redispersed nanocomposites on mica plate, the morphologies of Ppy coated on TS-1 nanoparticles were studied using AFM (Parks Science Instruments) using the contact mode.18 After removal of free Ppy by repeating the process of centrifugation and redispersion, the Ppy-coated TS-1 nanoparticles were dried under vacuum at room temperature, and the contact conductivities were measured using a two-probe technique by following a reported procedure.18
Results and Discussion The dispersed TS-1 nanoparticles, hereafter called TS1, are stable for a couple of months in water at pH ) 8 because high surface negative charges cause them to repel each other.23 The TEM image of dispersed TS-1 indicates that the particles have irregular but rarely spherical shape with diameters of about 100 nm (Figure 1). The aggregation of the TS-1 is originated from drying the solution on the copper grid for taking TEM images. The formation of SAAs of CPC on the dispersed TS-1 was studied at 25 °C and pH ) 8. The resulting adsorption isotherm (Figure 2) is typical for the adsorption of an ionic surfactant on a charged surface. The concentration of adsorbed CPC at plateau region was 0.23 mmol/g. Considering that the surface area of adsorbed CPC at the silica/water interface24 is 1.66 nm2, this is about the right amount to form bilayered structures of SAAs on the nanoparticles. By using eq 1, the initial concentration of CPC to form bilayered SAAs on the TS-1 was recalculated to be 23 mM. At this concentration, even after much of the CPC molecules are adsorbed on TS-1, the remaining CPC in solution is still above the critical micellar concentration (CMC) of CPC, which is 1.06 mM.25 On the basis of our previous results, the presence of micelles in the bulk solution is not good for the formation of Ppy thin films in SAAs because a large amount of pyrrole is solubilized in the micelles instead of partitioning into SAAs. Therefore, (23) Vold, R. D.; Vold, M. In Colloid and Interface Chemistry; AddisonWesley Publishing Co., Inc.; Reading, MA, 1983; pp 223-336. (24) Favoriti, P.; Treiner, C. Langmuir 1998, 14, 7493. (25) Moulik, S. P.; Haque, Md. E.; Das, A. R. J. Phys. Chem. 1996, 100, 701.
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Figure 2. Adsorption isotherm of CPC on TS-1 nanoparticles at 25 °C.
Figure 3. Adsorption of pyrrole onto SAAs on TS-1 (filled squares) and bare TS-1 (open squares).
to avoid an appreciable amount of pyrrole going into the bulk solution, the initial CPC concentration was chosen to be 16 mM so that the equilibrium concentration is slightly lower than its CMC. The results of the adsorption studies of pyrrole both in the SAAs and on the bare nanoparticles are shown at Figure 3. The saturated adsorption amount of pyrrole on bare TS-1 is about 40 mmol/g. This indicates that pyrrole can be adsorbed strongly in micropores of TS-1. In the case of adsorption on zeolite micropores of which the pore width is less than ca. 2 nm, the interactions between adsorbate molecules and surface of micropores are enhanced by overlapping of the potential field of pore walls thereby promoting strong adsorption.26 On the other hand, in the presence of the SAAs, the adsorbed amount of pyrrole is only about 5 mmol/g at the saturation point, because the pores are mostly covered by the SAAs. In fact, at the SAAs system, pyrrole would rather be solubilized by SAAs on TS-1 than direct adsorption onto the surface. This is actually advantageous for forming ultrathin polypyrrole films on the surface of TS-1 while maintaining the stability of resulting nanocomposites, a process requiring Ppy to grow on the surface but not be overgrown to avoid macroprecipitations. Furthermore, this process not only regulates the thickness of deposited Ppy films via controlling the solubilized amount of pyrrole on the surface but also offers well-ordered, quasi-two-dimensional environments comparable to the LB and chemisorbed SAA.27 On the basis of the adsorption data, we used initial (26) Stoeckli, F. Helv. Chim. Acta 1974, 57, 237. Everett, D. H.; Powl, J. C. J. Chem. Soc., Faraday Trans. 1 1976, 72, 619. (27) Soderlind, E. Langmuir 1994, 10, 1122.
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Table 1. Summary of Ppy Contents and Conductivitya m pyrrole (mM)
CPC (mM)
wt %b
wt %c
precipitation
conductivity (S/cm)
17.3 14.4 8.6 4.3 17.3 14.4 8.6 4.3
0 0 0 0 16 16 16 16
88 60 45 20 88 67 51 23
nad nad nad nad nad 10 8 8
yes yes yes yes yes no no no
3.82 0.000 64 0.000 072 0.000 002 4 5.09 3.82 5.09 4.3
a For each sample, the volume of the initial pyrrole solution was 10 mL and the amount of TS-1 was 0.002 g; CPC ) cetylpyridinium chloride. b wt % of Ppy and CPC in the composites before the centrifugation to remove free Ppy. c wt % of Ppy and CPC in the composites after the centrifugation to remove free Ppy. d Free Ppy could not be removed due to an aggregation.
pyrrole loading concentrations from 4.3 to 100 mM for polymerization on the surface. The polymerization process was initiated by adding an equimolar amount of ferric chloride into the dispersion. Samples with SAAs and having pyrrole concentrations of 20, 50, and 100 mM quickly turned black, and macroprecipitations were observed. Since the solubilized amount of pyrrole on the surface is very low in comparison with that in the aqueous phase in these systems, the polymerization of pyrrole would start quickly in solution, and the resulting Ppy will rapidly aggregate causing the dispersed nanoparticles to be precipitated out. Therefore, we did not further investigate systems with high pyrrole concentrations and focused on initial pyrrole concentrations ranging from 4.3 to 17.3 mM. The results are summarized in Table 1. From the data in Table 1, it is clear that SAAs can stabilize the nanocomposites in an aqueous medium at low pyrrole loading because they act as a template and render a confined environment to control Ppy growth. However, without CPC, all the nanocomposites were settled out from the solution even under low loading of pyrrole because a relatively large amount of pyrrole was adsorbed on the surface and then coagulated by overgrown Ppy. Further comparison of conductivities of the precipitated nanocomposites without using the SAAs showed an abrupt change in conductivity due to the percolation of heterogeneously mixed insulators (TS-1) and conductors (Ppy), while the nanocomposites prepared with SAAs did not show any percolation threshold.18 In this case, the lack of abrupt percolation behavior indirectly supports the assumption that Ppy thin films were completely coated on TS-1. Furthermore, for 14.4 to 4.3 mM of initial loading, the Ppy content in the nanocomposites was about same (less than 10 wt %). From those results, we can speculate that the growth of Ppy in the SAAs is limited by the confined environment of SAAs regardless of the amount of pyrrole loading. Although, there are a number of unknown factors effecting the colloidal stability of the nanocomposites, we can assume that TS-1 attains its colloidal stability by ionic repulsion. Therefore, TS-1 completely coated with Ppy will mask the surface charge and lead to the loss of colloidal stability. However, the results in Table 1 indicate that the TS-1 (core)/Ppy (shell) nanocomposites formed in the presence of CPC attain colloidal stability. Therefore, in those systems, we postulate that the SAAs of CPC on TS-1 only act as a template for the growth of Ppy films but also render steric repulsion on coated Ppy via adsorption of free CPC in the solution.28-30 This could be proved indirectly by observing that the nanocomposites (28) Gao, Z.; Zi, M.; Chen, B. J. Electroanal. Chem. 1994, 373, 14.
Figure 4. TEM images for the nanocomposites of 8.6 mM (a) and 4.3 mM (b) pyrrole loading before removing free Ppy.
lost their stability after repeating centrifugation and redispersion processing (desorption of CPC), but they regained the stability by adding CPC to the redispersed solution. In fact, the preparation of stable dispersions of Ppy with surfactants alone has been reported.29 Transmission electron microscopy (TEM) studies (Figure 4) of the bulk colloidal nanocomposites indicate that the Ppy-coated TS-1 particles seem to interconnect with a large amount of free Ppy. This is because free Ppy and Ppy-coated TS-1 were aggregated during drying on a copper grid. As the initial loading of pyrrole is reduced from 8.6 mM to 4.3 mM, the free Ppy morphology turns from raspberry-like to plain sheetlike. The dramatic transition of free Ppy morphology may stem from the different pyrrole concentrations in the aqueous phase. As shown at Figure 3, the solubilized pyrrole in SAAs and supernatant solution is smaller than the 4.3 mM concentration of the pyrrole system. Therefore, Ppy would be grown solely onto the surface because more pyrrole will be transferred from the solution as the polymerization is proceeding.31 If the free Ppy was predominantly from growth in the solution with CPC, the resulting morphology of Ppy was raspberry-like. On the other hand, if the Ppy was predominantly from growth in SAAs on the surface, the plain sheetlike morphology was observed. After a trial to remove free Ppy using centrifugation, the Ppy morphology and thickness on the surface was rechecked using (29) Warren, L. F.; Anderson, D. P. J. Electrochem. Soc. 1987, 134, 101. John, R.; John, M. J.; Wallace, G. G.; Zhao, H. In Electrochemistry in Colloids and Dispersions; Mackay, R. A., Texter, J., Eds.; VCH: New York, 1992; Chapter 17. (30) DeArmitt, C.; Armes, S. P. Langmuir 1993, 9, 652. (31) Lorell, P. A., ElAasser, M. S., Eds. Emulsion Polymerization and Emulsion Polymers; Wiley: Chichester, U.K., 1997; Chapter 2.
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Figure 6. AFM topological image of the nanocomposites from 4.3 mM pyrrole loading.
Figure 7. Surface diffuse reflectance IR for the nanocomposites: 8.6 mM of pyrrole system with SAAs (a); 8.6 mM of pyrrole system without using SAAs (b); free Ppy removed by the centrifugation from 8.6 mM of pyrrole system with SAAs (c).
Figure 5. TEM images for the nanocomposites from 8.6 mM (a), 4.3 mM (b), and 4.3 mM (c) pyrrole loading after removing free Ppy.
TEM. Figure 5 shows TEM images of the nanocomposites after the process of centrifugation to remove the free Ppy and redispersion of the Ppy-coated TS-1 particles. The results indicate that free Ppy was not grafted to the nanocomposites and that the morphologies of Ppy on the surface are the same in the two cases. The thickness of the Ppy film on the particles of the nanocomposites is 10-30 nm (Figure 5). The nanoscopic morphology of Ppy on the surface of TS-1 (4.3 mM of pyrrole system) was further investigated using AFM (Figure 6). The results indicates that there is no presence of bare TS-1 surface, and the morphology of Ppy is quite similar to that observed for thin films of Ppy. The sizes of each of the granules at the AFM image were 110-130 nm, which are consistent with the results from TEM studies. It has been reported that if there is growth of Ppy on the surface with thickness on the order of nanometers, the Ppy can be a layer of a highly ordered polymer chain.32 Since Ppy is grown in confined SAAs on the surface, Ppy
films on TS-1 may have an enhanced supermolecular order. To investigate the enhanced molecular order of Ppy, we adopted the method of analysis of Zerbi33,34 and Martin.15 According to their analysis, the extent of delocalization is inversely proportional to the intensity ratio between the bands at 1560 and 1480 cm-1, because the intensity of the antisymmetric ring streching mode at 1560 cm-1 will decrease relative to the intensity of the symmetric mode at 1480 cm-1 as the conjugation length increases.35 By using diffuse reflection IR, we investigated Ppy films on both nanocomposites without and with SAAs. From the spectrum, the strong Si-O stretching bands at 1250-1000 cm-1 from the TS-1 were not seen for both nanocomposites with and without SAAs (Figure 7). For those samples, it was shown that the ratios of I1560/I1480 were respectively 8.7 for the 8.6 mM pyrrole system with SAAs and 11.7 for the 8.6 mM pyrrole system without SAAs. On the other hand, the free Ppy removed by centrifugation from the 8.6 mM of pyrrole system with SAAs showed no indication of a band at 1480 cm-1 (Figure 7c). If we consider the ratios and thickness of Ppy nanofabrils obtained from Martin’s group, our ratio of 8.7 and thickness of 20-30 nm (Figure 5c) well agreed with their values.15 In fact, as we expected from the ratio, the attained contact conductivity was 5.1 S/cm for the nano(32) Cai, Z.; Martin, C. R. J. Am. Chem. Soc. 1989, 111, 4138. Cai, Z.; Lei, J.; Liang, W.; Menon, V.; Martin, C. R. Chem. Mater. 1991, 3, 960. (33) Tian, B.; Zerbi, G. J. Chem. Phys. 1990, 92, 3886. (34) Tian, B.; Zerbi, G. J. Chem. Phys. 1990, 92, 3892. (35) Lei, J. T.; Cai, Z.; Martin, C. R. Synth. Met. 1992, 46, 53.
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composites prepared in the presence of SAAs while the value for the bulk Ppy powder was 0.03 S/cm. In this sample, the content of Ppy in the nanocomposite was 8 wt % (Table 1). On the basis of our previous results, the Ppy prepared using SAAs as a template usually showed higher conductivity than the bulk Ppy.18 Furthermore, thin films of the nanocomposites can be simply fabricated by slowly evaporating the water in the colloidal solution of the nanocomposites at 70 °C; they had a conductivity of about 0.3 S/cm at 25 °C measured by the four-probe method. The temperature dependence of the conductivity of the films was studied between 30 and -70 °C under nitogen atmosphere,18 and no abrupt conductivity change was observed in this temperature range. The results strongly indicate that the film was well connected, and there was no ionic contribution to the observed conductivity. Conclusions The present study shows that formation of ultrathin Ppy films (10-30 nm) on nanosized particles (100 nm) can be accomplished on a preparative scale using SAAs of CPC on TS-1 zeolite nanoparticles as a template. In this technique, CPC plays a critical role for attaining the colloidal stability of the resulting nanocomposites and providing a nanoscopically confined environment for the growth of ordered Ppy films. Surface polymerization of pyrrole at the interface yields Ppy-coated TS-1 nanopar-
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ticles that were characterized by TEM, AFM, contact conductivity, diffuse reflectance IR, and calcination. Even with a fairly low amount of Ppy in the nanocomposites (8%), a high contact conductivity (5 S/cm) was obtained. The high contact conductivity was originated from the enhanced molecular order of polymer chains that were grown in the nanoscopically confined environment. TEM studies show that the morphologies of free Ppy and Ppy grown in the SAAs are quite different from TEM studies. Studies of surface diffuse reflectance IR show that the Ppy thin films have a longer extended conjugation. Therefore, we propose that the Ppy prepared using adsorbed SAAs of CPC as a template may have both microscopically and macroscopically different structures than Ppy prepared under normal condition. The use of SAAs as template to form ordered Ppy films may be useful for constructing a wide variety nanostructured materials. Indeed, we can prepare 10-nm-thick of Ppy on 25-nm-diameter gold particles using this method. This result will be published separately. Acknowledgment. This paper was funded by the Research Foundation of the Engineering College, Sunchon National University (to G.C.), and the industrial sponsors of the Institute of Applied Surfactant Research at the University of Oklahoma (to B.M.F.). LA0012485
