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polyphosphate organic–inorganic nanocomposites
Journal of Non-Crystalline Solids 351 (2005) 3704–3708 www.elsevier.com/locate/jnoncrysol
Polypyrrole/polyphosphate organic–inorganic nanocomposites Eryza G. Castro a, Aldo J.G. Zarbin a, Andre´ Galembeck
b,*
a
b
Departamento de Quı´mica, Universidade Federal do Parana´, CP 19081, CEP 81531-990 Curitiba, PR, Brazil Departamento de Quı´mica Fundamental, Universidade Federal de Pernambuco, CEP 50670-901 Recife, PE, Brazil Received 17 March 2005; received in revised form 26 September 2005
Abstract Polypyrrole/polyphosphate organic–inorganic nanocomposites were synthesized by a single-step synthesis in which the inorganic host network develops initially from a sol–gel transition and provide a restricted environment for pyrrole polymerization. The organic polymer forms, entrapped within the polyphosphate gel. The samples were characterized by UV–visible–NIR, infrared, Raman and electron paramagnetic resonance spectroscopies and thermogravimetric analysis. Self-standing thick films and powdered samples were obtained by adjusting the pyrrole and oxidant total amounts and their stoichiometric ratio. Self-standing samples in which the organic polymer is dispersed in nanosized domains within the inorganic matrix were prepared with up to a 10.8% polypyrrole load in its oxidized conducting form. 2005 Elsevier B.V. All rights reserved. PACS: 81.07.Pr; 81.20.Fw; 82.35.Cd; 82.33.Ln
1. Introduction Organic–inorganic hybrid materials present unusual and unprecedented properties, which can be tailored in order to optimize their performance for specific applications. When the organic fraction of these materials is composed by intrinsically conductive polymers, like polyaniline (PAni) and polypyrrole (PPy) [1–3], one can foresee a wide range of applications such as electronic, electrochromic and photoelectrochemical devices [4,5], cation exchange materials and membranes [6,7], ion conductors [8], photochromic devices [9], corrosion protection [10] and electrodes [11]. The choice of the synthetic pathway strongly affects the hybrid material composition, microstructure and morphology and, consequently, the resulting properties [12]. Hence, the composites can be designed to get the most desirable properties of the individual constituents and, also, synergistic and/or complementary behavior arising from interac-
*
Corresponding author. Tel.: +55 81 2126 7474; fax: +55 81 2126 8442. E-mail address: [email protected] (A. Galembeck).
0022-3093/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.09.024
tions between the conducting polymer and the inorganic matrix [13]. Even better ability to tune the properties may be reached if at least one phase can be prepared at nanoscopic level. Several routes to synthesize hybrid materials with conducting polymers were recently presented in the literature. The sol–gel process is a very versatile and useful approach [14–20]. Template synthesis, that is, reactions carried out within nanometric void spaces of inorganic host materials (pores, cavities, tunnels, layers, etc.) also provides interesting alternatives [21]. Our research group has reported nanocomposites of polypyrrole and polyaniline with several inorganic materials like porous glasses [22–24], inorganic oxides [25,26], three-dimensional framework materials [27], layered materials [28,29], metal nanoparticles [30] and inorganic gels [31,32]. The hybrid materials described in this work are based in aluminum polyphosphate gels (APP) which are supramolecular ionic swollen networks that result from the association of polyphosphate polyanions and Al3+ ions in aqueous solutions [33,34]. The system cohesion results from electrostatic interactions and hydrogen bonding
E.G. Castro et al. / Journal of Non-Crystalline Solids 351 (2005) 3704–3708
rather the covalent bonds observed in alkoxide-based sol–gel materials. APP gels are transparent to visible light, permeable to water and hydrophilic vapors and may be synthesized at or near room temperature within 24 h. They can be processed into flexible freestanding films and little monolithic bulks, thus providing very attractive host structures to new hybrid materials [31,32,35]. We reported recently a hybrid material formed from APP gels and the conducting polymer polyaniline [35]. In this work we show that a similar approach is also successful to obtain a similar material with polypyrrole. The synthesis and characterization of these APP/PPy materials is presented and compared with the model proposed for polyaniline-based hybrids. 2. Experimental Pyrrole was vacuum distilled before use. Aluminum nitrate, sodium polyphosphate and ammonium persulfate, (NH4)2S2O8, were used as received. APP gels were prepared starting from sodium polyphosphate (NaPP) and aluminum nitrate aqueous solutions, as described earlier in the literature [34]. In order to synthesize polypyrrole/aluminum polyphosphate (APP/PPy) composites, the pyrrole (monomer) and ammonium persulphate (initiator) were dissolved in the NaPP and Al(NO3)3 solutions, respectively. The NaPP–pyrrole and Al(NO3)3–(NH4)2S2O8 solutions were added under strong stirring. Gelation proceeds within 1 min and the initially white viscous samples changes to light blue, blue and black within 10 min (sample APP/ PPy-1). After 30 min the samples were centrifuged, the supernatant liquid was discarded and the resulting composites were washed with distilled water and stored in closed
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vials overnight. Self-standing thick films were prepared by pressing the samples between glass plates. Several experiments were carried out in order to optimize the synthesis. Our discussion will be focused in three samples with the same pyrrole-to-oxidant molar ratio, in which the pyrrole and (NH4)2S2O8 amounts were 10.0 lL and 0.016 mg (sample APP/PPy-1); 5 lL and 0.0058 mg (APP/PPy-2); 40 lL and 0.456 mg (APP/PPy-3). UV–Vis–NIR absorption spectra were collected directly from APP gels self-standing films in the 190–1000 nm range, using air as reference. Infrared spectra were obtained in the 4000–400 cm 1 region, from KBr pellets. Raman spectra were measured in a spectrophotometer coupled to an optical microscope that focused the incident radiation in a 1.0 lm spot, using a 632.8 nm laser as the excitation source and 4.0 mW power. Spectra were obtained from 28 scans, over the 2000–100 cm 1 region. Thermogravimetric analysis (TGA) was performed at an 8 C min 1 heating rate, under static air. The samples were pre-submitted to dynamic vacuum (10 2 Torr) for 8 h before the analysis. EPR spectra were obtained in the solid state at room temperature in quartz tubes. The spectrometer was used operating at a frequency of 9.5 GHz (Xband), with a 100 kHz modulation frequency, 10.145 G modulation amplitude and 2.0 mW microwave power. 3. Results An aluminum polyphosphate gel sample is presented in Fig. 1(a). A polyphosphate/polyaniline (APP/PAni) sample described in an earlier work is also shown (Fig. 1(b)) [35]. Nanocomposites with polypyrrole, APP/PPy-1 and APP/ PPy-2 are presented in Fig. 1(c) and (d), respectively. Twenty four hours after the synthesis, the gel has plasticity
Fig. 1. Freestanding films: (a) APP; (b) APP/PPy-1; (c) APP/PPy-2; (d) APP/PAni.
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enough to be pressed into the flexible, freestanding thick films shown in Fig. 1. Black powdered samples (APP/ PPy-3) resulted when the pyrrole amount was increased four times in comparison to APP/PPy-1. The very first evidence of pyrrole polymerization within the polyphosphate gel network is the color changes that proceed during the synthesis, described in the preceding section. The oxidized conducting form of polypyrrole is black. All samples are amorphous to X-rays. Absorption spectrum from APP/PPy-1 could not be acquired because its absorbance was excessively high. The APP/PPy-2 absorption spectrum, which was synthesized with the half the pyrrole load added to APP/PPy-1, is presented in Fig. 2. The bands centered at 460 and 750 nm arise from polaronic transitions in highly oxidized polypyrrole (Fig. 2(b)) [36,37]. Aluminum polyphopsphate gels are transparent to visible radiation. Polypyrrole formation was confirmed by Raman and FT-IR spectroscopies. Raman spectra from APP gel and APP/PPy samples are shown in Fig. 3. The aluminum polyphosphate gel has a poor scattering power in comparison to APP/PPy samples. Hence, APP/PPy spectra are dominated by polypyrrole vibrations, which were ascribed as follows: 1609 cm 1 (m symmetric C@C), 1500 cm 1 (m symmetric C–N, very weak), 1362 cm 1 (m asymmetric C–N), 1244 cm 1 (d C–H, in plane), 1099 and 1045 cm 1 (d C– H), 981 cm 1 (ring deformation associated to the bipolaron) and 932 cm 1 (m C–N due to the radical cation, i.e., the polaron). The identification of polaronic and bipolaronic bands confirms that polypyrrole is formed in its oxidized conducting form [38,39]. Aluminum polyphosphate gel bands appear at 940, 1120 and 1294 cm 1 (middle-chain groups), 1060 cm 1 from end-chain phosphate groups and 1330 cm 1 from P@O stretching from (Fig. 3(a)). Bending modes for phosphate chains appear as broad bands centered at 380 and 550 cm 1 (not shown) [40].
Fig. 2. UV–Vis–NIR absorption spectra collected directly from the films presented in Fig. 1: (a) APP; (b) APP/PPy-2.
Fig. 3. Raman spectra: (a) APP; (b) APP/PPy-1; (c) APP/PPy-2.
FT-IR spectra of the samples are presented in Fig. 4. The APP gel and APP/PPy-2 spectra are almost identical. No signal from the organic polymer was detected in the APP/PPy-2 spectrum due to the low polypyrrole amount. The absorptions showed in Fig. 4(a) and (b) can be divided in three groups: (i) phosphate bands at 761 cm 1 (m symmetric P–O–P), 930 cm 1 (shoulder from P–(OH)), 980 cm 1 (m symmetric PO3 ), 1167 cm 1 (m asymmetric POH), a shoulder at 1262 cm 1 (m asymmetric O–P–O) and 2420 cm 1 (m P–OH, not shown); (ii) water absorptions at 1642 cm 1 and 3640–3000 cm 1 (not shown) (H–O–H bending and O–H stretching, respectively) and; (iii) a sharp peak from nitrate ions (counter ion from aluminum salt) stretching at 1383 cm 1 [41]. Some bands ascribed to polypyrrole can be already observed in the APP/PPy-1 spectrum (Fig. 4(c)). These absorptions are more intense in the spectrum of the sample with the highest polypyrrole load (APP/PPy-3). All are related to the oxidized polypyrrole, as described on the following: 3440 cm 1 (m N–H, not shown), 1566 cm 1
E.G. Castro et al. / Journal of Non-Crystalline Solids 351 (2005) 3704–3708
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hybrid material was estimated as 7.9 · 1015, 4.7 · 1012 and 2.7 · 1017 for the samples APP/PPy-1, APP/PPy-2 and APP/PPy-3, respectively. 4. Discussion
Fig. 4. FT-IR spectra: (a) APP; (b) APP/PPy-2; (c) APP/PPy-1; (d) APP/ PPy-3.
(m C–C), 1279 cm 1 (d N–H in plane) 1099 cm 1 (d C–H in plane) [42]. Thermogravimetric curves of APP/PPy samples (not shown) present a weight-loss event in the 110–540 C range, which is not observed to APP gel samples and must arise to the degradation of the skeletal polypyrrole chain structure. The polypyrrole amount in APP/PPy samples (43.0% wt) were 10.8%, 5.2% and 43% for APP/PPy-1, APP/PPy-2 and APP/PPy-3, respectively. The EPR spectra (not shown) of the APP/PPy hybrid samples showed a single peak typical from free radical species, which appear as a consequence of the presence of polarons in these samples [43,44]. The g value observed for all spectra (g = 2.0027) is very close to the related for pure polypyrrole (g = 2.0026) [36,37]. The intensity of this signal is directly related with the amount of polymer present in the sample, and the number of spins per gram of
In a previous work we observed an upper limit for polyaniline loading within the polyphosphate gel network that allows to obtain monolithic transparent samples. Beyond this limit, the conducting polymer provides a barrier to gel contraction and powdered samples are obtained [35]. A similar behavior was observed in this case, although, homogeneous monolithic polyphosphate gel samples, which can be processed into flexible self-standing films, can attain up to a 10.8% (w/w) polypyrrole amount. This is nearly 4.5 times greater than the result reported for AAP/PAni samples (2.4%). These samples also seem to fit to our model in which the organic conducting polymer chains might be formed in small domains homogeneously dispersed within the gel matrix [35]. These domains are too small to scatter visible light. An excessive polypyrrole addition tends to break the Al-polyphosphate coercive interactions, which are essential to generate the gel network. This observation explains the fact that there is an upper limit of conducting polymer load that allows synthesizing transparent and selfstanding films. Above this limit powdered samples result. One must keep in mind that polypyrrole is water insoluble and the aluminum polyphosphate gel is a water-rich environment with nearly 30% (w/w) water amount [45]. It is important to notice that, when oxidized polypyrrole is formed, there are cationic and dicationic segments (polarons and bipolarons) in the polymer chains. Hence, negative charged species are needed to act as counter-anions. This role is played by the polyphosphate chains and, in a lesser extent, by the nitrate ions from the aluminum salt, which are also entrapped within the gel network when gelation proceeds. 5. Conclusion A single-step route to novel organic/inorganic nanocomposites in which the conducting polymer polypyrrole is formed entrapped within the aluminum polyphosphate gel network was described in this paper. Transparent, flexible and freestanding films were synthesized with up to a 10.8% polypyrrole load. The characteristics of the nanocomposites reported here make them suitable for optical sensors and electrochromic applications. Acknowledgments Authors are grateful for the Brazilian agency CNPq, CT-ENERG/CNPq, Rede de Materiais Nanoestruturados (MCT) and Rede de Nanotecnologia Molecular e de Interfaces (MCT) for the financial support and Professor
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Antoˆnio S Mangrich (DQ-UFPR) for the EPR measurements and clarifying discussions. References [1] T.A. Kerr, H. Wu, L.F. Nazar, Chem. Mater. 8 (1996) 2005. [2] M. Lira-Cantu´, P. Go´mez-Romero, Chem. Mater. 10 (1998) 698. [3] D.J. Maia, M.-A. De Paoli, O.L. Alves, A.J.G. Zarbin, S. Neves, Quim. Nova 23 (2000) 204. [4] E.A.R. Duek, M.-A. De Paoli, M. Mastragostino, Adv. Mater. 4 (1992) 287. [5] H.P. Oliveira, C.F.O. Graeff, C.A. Brunello, E.M. Guerra, J. NonCryst. Solids 273 (2000) 193. [6] A.A. Khan, M.M. Alam, Anal. Chim. Acta 504 (2004) 253. [7] A.A. Khan, M.M. Alam, F. Mohammad, Elctrochim. Acta 48 (2003) 2463. [8] Q.Y. Wu, X.F. Xie, Mater. Chem. Phys. 77 (2003) 621. [9] P. Gomez-Romero, M. Chojak, K. Cuentas-Gallegos, J.A. Asensio, P.J. Kulesza, N. Casan-Pastor, M. Lira-Cantu, Electrochem. Commun. 5 (2003) 149. [10] T.P. Chou, C. Chandrasekaran, S.J. Limmer, S. Seraji, Y. Wu, M.J. Forbess, C. Nguyen, G.Z. Cao, J. Non-Cryst. Solids 290 (2001) 153. [11] G. Torres-Gomez, E.M. Tejada-Rosales, P. Gomez-Romero, Chem. Mater. 13 (2001) 3693. [12] T. Asefa, C. Yoshina-Ishii, M.J. MacLachlan, G.A. Ozin, J. Mater. Chem. 10 (2000) 1751. [13] P. Gomez-Romero, Adv. Mater. 13 (2001) 163. [14] Q.G. Wang, N.J. Liu, X.H. Wang, J. Li, X.J. Zhao, F.S. Wang, Macromolecules 36 (2003) 5760. [15] W.M. de Azevedo, D.J. Brondani, J. Non-Cryst. Solids 296 (2001) 224. [16] S. Roux, P. Audebert, J. Pagetti, M. Roche, J. Sol–Gel Sci. Technol. 26 (2003) 435. [17] Y.J. Wang, X.H. Wang, J. Li, Z.S. Mo, X.J. Zhao, X.B. Jing, F.S. Wang, Adv. Mater. 13 (2001) 1582. [18] F. Leroux, J.P. Besse, Chem. Mater. 13 (2001) 3507. [19] S. Roux, P. Audebert, J. Pagetti, M. Roche, New J. Chem. 24 (2000) 885. [20] R.O.R. Costa, W.L. Vasconcelos, J. Non-Cryst. Solids 304 (2002) 84. [21] V. Mehrotra, J.L. Keddie, J.M. Miller, E.P. Gianellis, J. Non-Cryst. Solids 136 (1991) 97.
[22] A.J.G. Zarbin, M.-A. De Paoli, O.L. Alves, Synth. Met. 99 (1999) 227. [23] D.J. Maia, A.J.G. Zarbin, O.L. Alves, M.-A. De Paoli, Adv. Mater. 7 (1995) 792. [24] P.T. Sotomayor, I.M. Raimundo, A.J.G. Zarbin, J.J.R. Rohwedder, G.O. Neto, O.L. Alves, Sensor. Actuact. B – Chem. 74 (2001) 157. [25] D.C. Schnitzler, M. Meruvia, I.A. Hummelguen, A.J.G. Zarbin, Chem. Mater. 15 (2003) 4658. [26] D.C. Schnitzler, A.J.G. Zarbin, J. Braz. Chem. Soc. 15 (2004) 378. [27] F.A. Beleze, A.J.G. Zarbin, J. Braz. Chem. Soc. 12 (2001) 542. [28] A.J.G. Zarbin, D.J. Maia, M.-A. De Paoli, O.L. Alves, Synth. Met. 102 (1999) 1277. [29] A.B. Gonc¸alves, A.S. Mangrich, A.J.G. Zarbin, Synth. Met. 114 (2000) 119. [30] M.M. Oliveira, D. Zanchet, D. Ugarte, A.J.G. Zarbin, Prog. Colloid Polym. Sci. 128 (2004) 126. [31] M.S. Amorim, A. Galembeck, H. Bazin, G. Mathis, C.M. Donega´, G.F. de Sa´, S. Alves Jr., Mol. Cryst. Liq. Cryst. 374 (2002) 267. [32] A. Galembeck, S.B.C. Silva, J.A.P. Silva, J. Del Nero, Opt. Mater. 24 (2004) 637. [33] M. Draoui, P. Vast, G. Palavit, Rev. Chem. Min. 22 (1985) 256. [34] E.C.O. Lima, F. Galembeck, J. Colloid Interf. Sci. 166 (1994) 309. [35] E.G. Castro, A.J.G. Zarbin, H.P. Oliveira, A. Galembeck, Synth. Met. 146 (2004) 57. [36] K. Yakushi, L.J. Lauchlan, T.C. Clarke, G.B. Street, J. Chem. Phys. 79 (1983) 4774. [37] D.Y. Kim, J.Y. Lee, D.K. Moon, C.Y. Kim, Synth. Met. 69 (1995) 471. [38] Y.-C. Liu, B.-J. Hwang, W.-J. Jian, R. Santhanam, Thin Solid Films 374 (2000) 85. [39] S. Ghosh, A.B. Bowmaker, C.J.M. Seakins, Synth. Met. 95 (1998) 63. [40] A. Mogus-Milankovic, A. Gajovic, A. Santic, D.E. Day, J. NonCryst. Solids 289 (2001) 204. [41] L. Montagne, G. Palavit, M. Draoui, J. Non-Cryst. Solids 155 (1993) 115. [42] J. Wang, K.G. Neoh, E.T. Kang, Thin Solid Films 446 (2004) 205. [43] A. Bartl, L. Dunsch, H. Naarmann, D. Schmeißer, W. Go¨pel, Synth. Met. 61 (1993) 167. [44] G. Larsen, G.L. Haller, M.J. Marquez, Phys. Chem. 96 (1992) 4145. [45] A. Galembeck, J. Braz. Chem. Soc. 14 (2003) 930.
Polypyrrole/polyphosphate organic–inorganic nanocomposites Eryza G. Castro a, Aldo J.G. Zarbin a, Andre´ Galembeck
b,*
a
b
Departamento de Quı´mica, Universidade Federal do Parana´, CP 19081, CEP 81531-990 Curitiba, PR, Brazil Departamento de Quı´mica Fundamental, Universidade Federal de Pernambuco, CEP 50670-901 Recife, PE, Brazil Received 17 March 2005; received in revised form 26 September 2005
Abstract Polypyrrole/polyphosphate organic–inorganic nanocomposites were synthesized by a single-step synthesis in which the inorganic host network develops initially from a sol–gel transition and provide a restricted environment for pyrrole polymerization. The organic polymer forms, entrapped within the polyphosphate gel. The samples were characterized by UV–visible–NIR, infrared, Raman and electron paramagnetic resonance spectroscopies and thermogravimetric analysis. Self-standing thick films and powdered samples were obtained by adjusting the pyrrole and oxidant total amounts and their stoichiometric ratio. Self-standing samples in which the organic polymer is dispersed in nanosized domains within the inorganic matrix were prepared with up to a 10.8% polypyrrole load in its oxidized conducting form. 2005 Elsevier B.V. All rights reserved. PACS: 81.07.Pr; 81.20.Fw; 82.35.Cd; 82.33.Ln
1. Introduction Organic–inorganic hybrid materials present unusual and unprecedented properties, which can be tailored in order to optimize their performance for specific applications. When the organic fraction of these materials is composed by intrinsically conductive polymers, like polyaniline (PAni) and polypyrrole (PPy) [1–3], one can foresee a wide range of applications such as electronic, electrochromic and photoelectrochemical devices [4,5], cation exchange materials and membranes [6,7], ion conductors [8], photochromic devices [9], corrosion protection [10] and electrodes [11]. The choice of the synthetic pathway strongly affects the hybrid material composition, microstructure and morphology and, consequently, the resulting properties [12]. Hence, the composites can be designed to get the most desirable properties of the individual constituents and, also, synergistic and/or complementary behavior arising from interac-
*
Corresponding author. Tel.: +55 81 2126 7474; fax: +55 81 2126 8442. E-mail address: [email protected] (A. Galembeck).
0022-3093/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.09.024
tions between the conducting polymer and the inorganic matrix [13]. Even better ability to tune the properties may be reached if at least one phase can be prepared at nanoscopic level. Several routes to synthesize hybrid materials with conducting polymers were recently presented in the literature. The sol–gel process is a very versatile and useful approach [14–20]. Template synthesis, that is, reactions carried out within nanometric void spaces of inorganic host materials (pores, cavities, tunnels, layers, etc.) also provides interesting alternatives [21]. Our research group has reported nanocomposites of polypyrrole and polyaniline with several inorganic materials like porous glasses [22–24], inorganic oxides [25,26], three-dimensional framework materials [27], layered materials [28,29], metal nanoparticles [30] and inorganic gels [31,32]. The hybrid materials described in this work are based in aluminum polyphosphate gels (APP) which are supramolecular ionic swollen networks that result from the association of polyphosphate polyanions and Al3+ ions in aqueous solutions [33,34]. The system cohesion results from electrostatic interactions and hydrogen bonding
E.G. Castro et al. / Journal of Non-Crystalline Solids 351 (2005) 3704–3708
rather the covalent bonds observed in alkoxide-based sol–gel materials. APP gels are transparent to visible light, permeable to water and hydrophilic vapors and may be synthesized at or near room temperature within 24 h. They can be processed into flexible freestanding films and little monolithic bulks, thus providing very attractive host structures to new hybrid materials [31,32,35]. We reported recently a hybrid material formed from APP gels and the conducting polymer polyaniline [35]. In this work we show that a similar approach is also successful to obtain a similar material with polypyrrole. The synthesis and characterization of these APP/PPy materials is presented and compared with the model proposed for polyaniline-based hybrids. 2. Experimental Pyrrole was vacuum distilled before use. Aluminum nitrate, sodium polyphosphate and ammonium persulfate, (NH4)2S2O8, were used as received. APP gels were prepared starting from sodium polyphosphate (NaPP) and aluminum nitrate aqueous solutions, as described earlier in the literature [34]. In order to synthesize polypyrrole/aluminum polyphosphate (APP/PPy) composites, the pyrrole (monomer) and ammonium persulphate (initiator) were dissolved in the NaPP and Al(NO3)3 solutions, respectively. The NaPP–pyrrole and Al(NO3)3–(NH4)2S2O8 solutions were added under strong stirring. Gelation proceeds within 1 min and the initially white viscous samples changes to light blue, blue and black within 10 min (sample APP/ PPy-1). After 30 min the samples were centrifuged, the supernatant liquid was discarded and the resulting composites were washed with distilled water and stored in closed
3705
vials overnight. Self-standing thick films were prepared by pressing the samples between glass plates. Several experiments were carried out in order to optimize the synthesis. Our discussion will be focused in three samples with the same pyrrole-to-oxidant molar ratio, in which the pyrrole and (NH4)2S2O8 amounts were 10.0 lL and 0.016 mg (sample APP/PPy-1); 5 lL and 0.0058 mg (APP/PPy-2); 40 lL and 0.456 mg (APP/PPy-3). UV–Vis–NIR absorption spectra were collected directly from APP gels self-standing films in the 190–1000 nm range, using air as reference. Infrared spectra were obtained in the 4000–400 cm 1 region, from KBr pellets. Raman spectra were measured in a spectrophotometer coupled to an optical microscope that focused the incident radiation in a 1.0 lm spot, using a 632.8 nm laser as the excitation source and 4.0 mW power. Spectra were obtained from 28 scans, over the 2000–100 cm 1 region. Thermogravimetric analysis (TGA) was performed at an 8 C min 1 heating rate, under static air. The samples were pre-submitted to dynamic vacuum (10 2 Torr) for 8 h before the analysis. EPR spectra were obtained in the solid state at room temperature in quartz tubes. The spectrometer was used operating at a frequency of 9.5 GHz (Xband), with a 100 kHz modulation frequency, 10.145 G modulation amplitude and 2.0 mW microwave power. 3. Results An aluminum polyphosphate gel sample is presented in Fig. 1(a). A polyphosphate/polyaniline (APP/PAni) sample described in an earlier work is also shown (Fig. 1(b)) [35]. Nanocomposites with polypyrrole, APP/PPy-1 and APP/ PPy-2 are presented in Fig. 1(c) and (d), respectively. Twenty four hours after the synthesis, the gel has plasticity
Fig. 1. Freestanding films: (a) APP; (b) APP/PPy-1; (c) APP/PPy-2; (d) APP/PAni.
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enough to be pressed into the flexible, freestanding thick films shown in Fig. 1. Black powdered samples (APP/ PPy-3) resulted when the pyrrole amount was increased four times in comparison to APP/PPy-1. The very first evidence of pyrrole polymerization within the polyphosphate gel network is the color changes that proceed during the synthesis, described in the preceding section. The oxidized conducting form of polypyrrole is black. All samples are amorphous to X-rays. Absorption spectrum from APP/PPy-1 could not be acquired because its absorbance was excessively high. The APP/PPy-2 absorption spectrum, which was synthesized with the half the pyrrole load added to APP/PPy-1, is presented in Fig. 2. The bands centered at 460 and 750 nm arise from polaronic transitions in highly oxidized polypyrrole (Fig. 2(b)) [36,37]. Aluminum polyphopsphate gels are transparent to visible radiation. Polypyrrole formation was confirmed by Raman and FT-IR spectroscopies. Raman spectra from APP gel and APP/PPy samples are shown in Fig. 3. The aluminum polyphosphate gel has a poor scattering power in comparison to APP/PPy samples. Hence, APP/PPy spectra are dominated by polypyrrole vibrations, which were ascribed as follows: 1609 cm 1 (m symmetric C@C), 1500 cm 1 (m symmetric C–N, very weak), 1362 cm 1 (m asymmetric C–N), 1244 cm 1 (d C–H, in plane), 1099 and 1045 cm 1 (d C– H), 981 cm 1 (ring deformation associated to the bipolaron) and 932 cm 1 (m C–N due to the radical cation, i.e., the polaron). The identification of polaronic and bipolaronic bands confirms that polypyrrole is formed in its oxidized conducting form [38,39]. Aluminum polyphosphate gel bands appear at 940, 1120 and 1294 cm 1 (middle-chain groups), 1060 cm 1 from end-chain phosphate groups and 1330 cm 1 from P@O stretching from (Fig. 3(a)). Bending modes for phosphate chains appear as broad bands centered at 380 and 550 cm 1 (not shown) [40].
Fig. 2. UV–Vis–NIR absorption spectra collected directly from the films presented in Fig. 1: (a) APP; (b) APP/PPy-2.
Fig. 3. Raman spectra: (a) APP; (b) APP/PPy-1; (c) APP/PPy-2.
FT-IR spectra of the samples are presented in Fig. 4. The APP gel and APP/PPy-2 spectra are almost identical. No signal from the organic polymer was detected in the APP/PPy-2 spectrum due to the low polypyrrole amount. The absorptions showed in Fig. 4(a) and (b) can be divided in three groups: (i) phosphate bands at 761 cm 1 (m symmetric P–O–P), 930 cm 1 (shoulder from P–(OH)), 980 cm 1 (m symmetric PO3 ), 1167 cm 1 (m asymmetric POH), a shoulder at 1262 cm 1 (m asymmetric O–P–O) and 2420 cm 1 (m P–OH, not shown); (ii) water absorptions at 1642 cm 1 and 3640–3000 cm 1 (not shown) (H–O–H bending and O–H stretching, respectively) and; (iii) a sharp peak from nitrate ions (counter ion from aluminum salt) stretching at 1383 cm 1 [41]. Some bands ascribed to polypyrrole can be already observed in the APP/PPy-1 spectrum (Fig. 4(c)). These absorptions are more intense in the spectrum of the sample with the highest polypyrrole load (APP/PPy-3). All are related to the oxidized polypyrrole, as described on the following: 3440 cm 1 (m N–H, not shown), 1566 cm 1
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hybrid material was estimated as 7.9 · 1015, 4.7 · 1012 and 2.7 · 1017 for the samples APP/PPy-1, APP/PPy-2 and APP/PPy-3, respectively. 4. Discussion
Fig. 4. FT-IR spectra: (a) APP; (b) APP/PPy-2; (c) APP/PPy-1; (d) APP/ PPy-3.
(m C–C), 1279 cm 1 (d N–H in plane) 1099 cm 1 (d C–H in plane) [42]. Thermogravimetric curves of APP/PPy samples (not shown) present a weight-loss event in the 110–540 C range, which is not observed to APP gel samples and must arise to the degradation of the skeletal polypyrrole chain structure. The polypyrrole amount in APP/PPy samples (43.0% wt) were 10.8%, 5.2% and 43% for APP/PPy-1, APP/PPy-2 and APP/PPy-3, respectively. The EPR spectra (not shown) of the APP/PPy hybrid samples showed a single peak typical from free radical species, which appear as a consequence of the presence of polarons in these samples [43,44]. The g value observed for all spectra (g = 2.0027) is very close to the related for pure polypyrrole (g = 2.0026) [36,37]. The intensity of this signal is directly related with the amount of polymer present in the sample, and the number of spins per gram of
In a previous work we observed an upper limit for polyaniline loading within the polyphosphate gel network that allows to obtain monolithic transparent samples. Beyond this limit, the conducting polymer provides a barrier to gel contraction and powdered samples are obtained [35]. A similar behavior was observed in this case, although, homogeneous monolithic polyphosphate gel samples, which can be processed into flexible self-standing films, can attain up to a 10.8% (w/w) polypyrrole amount. This is nearly 4.5 times greater than the result reported for AAP/PAni samples (2.4%). These samples also seem to fit to our model in which the organic conducting polymer chains might be formed in small domains homogeneously dispersed within the gel matrix [35]. These domains are too small to scatter visible light. An excessive polypyrrole addition tends to break the Al-polyphosphate coercive interactions, which are essential to generate the gel network. This observation explains the fact that there is an upper limit of conducting polymer load that allows synthesizing transparent and selfstanding films. Above this limit powdered samples result. One must keep in mind that polypyrrole is water insoluble and the aluminum polyphosphate gel is a water-rich environment with nearly 30% (w/w) water amount [45]. It is important to notice that, when oxidized polypyrrole is formed, there are cationic and dicationic segments (polarons and bipolarons) in the polymer chains. Hence, negative charged species are needed to act as counter-anions. This role is played by the polyphosphate chains and, in a lesser extent, by the nitrate ions from the aluminum salt, which are also entrapped within the gel network when gelation proceeds. 5. Conclusion A single-step route to novel organic/inorganic nanocomposites in which the conducting polymer polypyrrole is formed entrapped within the aluminum polyphosphate gel network was described in this paper. Transparent, flexible and freestanding films were synthesized with up to a 10.8% polypyrrole load. The characteristics of the nanocomposites reported here make them suitable for optical sensors and electrochromic applications. Acknowledgments Authors are grateful for the Brazilian agency CNPq, CT-ENERG/CNPq, Rede de Materiais Nanoestruturados (MCT) and Rede de Nanotecnologia Molecular e de Interfaces (MCT) for the financial support and Professor
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Antoˆnio S Mangrich (DQ-UFPR) for the EPR measurements and clarifying discussions. References [1] T.A. Kerr, H. Wu, L.F. Nazar, Chem. Mater. 8 (1996) 2005. [2] M. Lira-Cantu´, P. Go´mez-Romero, Chem. Mater. 10 (1998) 698. [3] D.J. Maia, M.-A. De Paoli, O.L. Alves, A.J.G. Zarbin, S. Neves, Quim. Nova 23 (2000) 204. [4] E.A.R. Duek, M.-A. De Paoli, M. Mastragostino, Adv. Mater. 4 (1992) 287. [5] H.P. Oliveira, C.F.O. Graeff, C.A. Brunello, E.M. Guerra, J. NonCryst. Solids 273 (2000) 193. [6] A.A. Khan, M.M. Alam, Anal. Chim. Acta 504 (2004) 253. [7] A.A. Khan, M.M. Alam, F. Mohammad, Elctrochim. Acta 48 (2003) 2463. [8] Q.Y. Wu, X.F. Xie, Mater. Chem. Phys. 77 (2003) 621. [9] P. Gomez-Romero, M. Chojak, K. Cuentas-Gallegos, J.A. Asensio, P.J. Kulesza, N. Casan-Pastor, M. Lira-Cantu, Electrochem. Commun. 5 (2003) 149. [10] T.P. Chou, C. Chandrasekaran, S.J. Limmer, S. Seraji, Y. Wu, M.J. Forbess, C. Nguyen, G.Z. Cao, J. Non-Cryst. Solids 290 (2001) 153. [11] G. Torres-Gomez, E.M. Tejada-Rosales, P. Gomez-Romero, Chem. Mater. 13 (2001) 3693. [12] T. Asefa, C. Yoshina-Ishii, M.J. MacLachlan, G.A. Ozin, J. Mater. Chem. 10 (2000) 1751. [13] P. Gomez-Romero, Adv. Mater. 13 (2001) 163. [14] Q.G. Wang, N.J. Liu, X.H. Wang, J. Li, X.J. Zhao, F.S. Wang, Macromolecules 36 (2003) 5760. [15] W.M. de Azevedo, D.J. Brondani, J. Non-Cryst. Solids 296 (2001) 224. [16] S. Roux, P. Audebert, J. Pagetti, M. Roche, J. Sol–Gel Sci. Technol. 26 (2003) 435. [17] Y.J. Wang, X.H. Wang, J. Li, Z.S. Mo, X.J. Zhao, X.B. Jing, F.S. Wang, Adv. Mater. 13 (2001) 1582. [18] F. Leroux, J.P. Besse, Chem. Mater. 13 (2001) 3507. [19] S. Roux, P. Audebert, J. Pagetti, M. Roche, New J. Chem. 24 (2000) 885. [20] R.O.R. Costa, W.L. Vasconcelos, J. Non-Cryst. Solids 304 (2002) 84. [21] V. Mehrotra, J.L. Keddie, J.M. Miller, E.P. Gianellis, J. Non-Cryst. Solids 136 (1991) 97.
[22] A.J.G. Zarbin, M.-A. De Paoli, O.L. Alves, Synth. Met. 99 (1999) 227. [23] D.J. Maia, A.J.G. Zarbin, O.L. Alves, M.-A. De Paoli, Adv. Mater. 7 (1995) 792. [24] P.T. Sotomayor, I.M. Raimundo, A.J.G. Zarbin, J.J.R. Rohwedder, G.O. Neto, O.L. Alves, Sensor. Actuact. B – Chem. 74 (2001) 157. [25] D.C. Schnitzler, M. Meruvia, I.A. Hummelguen, A.J.G. Zarbin, Chem. Mater. 15 (2003) 4658. [26] D.C. Schnitzler, A.J.G. Zarbin, J. Braz. Chem. Soc. 15 (2004) 378. [27] F.A. Beleze, A.J.G. Zarbin, J. Braz. Chem. Soc. 12 (2001) 542. [28] A.J.G. Zarbin, D.J. Maia, M.-A. De Paoli, O.L. Alves, Synth. Met. 102 (1999) 1277. [29] A.B. Gonc¸alves, A.S. Mangrich, A.J.G. Zarbin, Synth. Met. 114 (2000) 119. [30] M.M. Oliveira, D. Zanchet, D. Ugarte, A.J.G. Zarbin, Prog. Colloid Polym. Sci. 128 (2004) 126. [31] M.S. Amorim, A. Galembeck, H. Bazin, G. Mathis, C.M. Donega´, G.F. de Sa´, S. Alves Jr., Mol. Cryst. Liq. Cryst. 374 (2002) 267. [32] A. Galembeck, S.B.C. Silva, J.A.P. Silva, J. Del Nero, Opt. Mater. 24 (2004) 637. [33] M. Draoui, P. Vast, G. Palavit, Rev. Chem. Min. 22 (1985) 256. [34] E.C.O. Lima, F. Galembeck, J. Colloid Interf. Sci. 166 (1994) 309. [35] E.G. Castro, A.J.G. Zarbin, H.P. Oliveira, A. Galembeck, Synth. Met. 146 (2004) 57. [36] K. Yakushi, L.J. Lauchlan, T.C. Clarke, G.B. Street, J. Chem. Phys. 79 (1983) 4774. [37] D.Y. Kim, J.Y. Lee, D.K. Moon, C.Y. Kim, Synth. Met. 69 (1995) 471. [38] Y.-C. Liu, B.-J. Hwang, W.-J. Jian, R. Santhanam, Thin Solid Films 374 (2000) 85. [39] S. Ghosh, A.B. Bowmaker, C.J.M. Seakins, Synth. Met. 95 (1998) 63. [40] A. Mogus-Milankovic, A. Gajovic, A. Santic, D.E. Day, J. NonCryst. Solids 289 (2001) 204. [41] L. Montagne, G. Palavit, M. Draoui, J. Non-Cryst. Solids 155 (1993) 115. [42] J. Wang, K.G. Neoh, E.T. Kang, Thin Solid Films 446 (2004) 205. [43] A. Bartl, L. Dunsch, H. Naarmann, D. Schmeißer, W. Go¨pel, Synth. Met. 61 (1993) 167. [44] G. Larsen, G.L. Haller, M.J. Marquez, Phys. Chem. 96 (1992) 4145. [45] A. Galembeck, J. Braz. Chem. Soc. 14 (2003) 930.