Synthesis and characterization of polypyrrole–TiO2 nanocomposites in supercritical CO2

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Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 300–303

Synthesis and characterization of polypyrrole–TiO2 nanocomposites in supercritical CO2 Haldorai Yuvaraj a , Eun Ju Park a , Yeong-Soon Gal b , Kwon Taek Lim a,∗ a

b

Division of Image and Information Engineering, Pukyong National University, Busan 608-739, Republic of Korea Polymer Chemistry Laboratory, College of Engineering, Kyung Il University, Gyeongsang buk-do 712-701, Republic of Korea Received 15 November 2006; accepted 27 April 2007 Available online 2 June 2007

Abstract The semiconducting polymer and inorganic metal oxide nanocomposites composed of polypyrrole/titanium dioxide (PPy/TiO2 ) were synthesized by in situ chemical oxidative polymerization of pyrrole in the presence of TiO2 particles in supercritical carbon dioxide (scCO2 ). TiO2 nanoparticles with average particle size of 5 nm were surface modified by 3-(trimethoxysilyl)propyl methacrylate (␥-MPS) in order to disperse effectively in CO2 before the polymerization. Transmission electron microscope (TEM) image of the nanocomposite revealed well-dispersed TiO2 particles in PPy matrix. The nanocomposites were also confirmed by X-ray diffraction (XRD), thermogravimetric analysis (TGA) and FT-IR spectroscopy. © 2007 Elsevier B.V. All rights reserved. Keywords: Supercritical carbon dioxide; Semiconducting polymer; Nanocomposites; Polypyrrole; Titanium dioxide

1. Introduction Conducting polymers such as polypyrrole, polythiophene, and polyaniline are of interest in a number of applications. Among them, polypyrrole (PPy) is one of the most extensively studied polymer because of its ease of synthesis as well as its relatively high air-stability and conductivity [1,2]. Conducting polymer/inorganic oxide nanocomposites have recently attracted great attention owing to their unique microstructure, outstanding physiochemical and electro-optical properties, and wide range of potential uses as a battery cathode and also in constructing nanoscopic assemblies in sensors and microelectronics [3]. Armes et al., showed that colloidal nanocomposites were formed when pyrrole or aniline was oxidatively polymerized in the presence of ultra fine silica or tin(IV) oxide [4–6]. Since then several research groups have prepared nanocomposites using silica sols or various kinds of inorganic metal oxides [3,7–11]. A wide variety of methodologies have been employed to synthesize conducting polymer/inorganic oxide nanocomposites [3]. However, processing of the polymers generally employs large quantity of organic solvents that are noxious and harmful

∗

Corresponding author. Tel.: +82 51 620 1692; fax: +82 51 625 2229. E-mail address: [email protected] (K.T. Lim).

0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.114

to the environment. Thus, the processing with an environmentally benign supercritical fluid such as carbon dioxide attracts a great interest as an alternative to the conventional processing. In recent years, scCO2 technology has been widely applied in material science because of its unique characteristics such as low viscosity, high diffusivity and near zero surface tension. In addition, it is non-toxic, non-flammable, chemically inert and having a moderate critical temperature (31.1 ◦ C) and critical pressure (7.38 MPa). Moreover, carbon dioxide offers high mass transport rates and allows in situ removal of unreacted monomer and other impurities [12]. Since supercritical CO2 has a strong solvent power for dissolving some organic compounds and a swelling property for most organic polymers [12], it has been successfully utilized in the synthesis of polymer/polymer composites [13,14]. Though few studies have been reported based on polymer/polymer composites, reports on the synthesis of inorganic compound/polymer composites in scCO2 are scarce in the literature. Particularly there has been no report in the literature so far based on the synthesis of conducting polymer/inorganic oxide nanocomposites in scCO2 . In this work, PPy/TiO2 nanocomposites were synthesized by the oxidative polymerization of pyrrole in the presence of TiO2 nanoparticles dispersed in scCO2 using ferric chloride as the oxidant. The resulting composites are characterized by various techniques including FT-IR, XRD, TGA and TEM.

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2. Experimental 2.1. Materials Pyrrole (Aldrich) was purified using a column of activated basic alumina. Titanium dioxide nanoparticles with average particle size 5 nm (anatase, Aldrich), ferric chloride (Aldrich), 3-(trimethoxysilyl)propyl methacrylate (␥-MPS, Aldrich), methanol (Aldrich), and research grade CO2 (Daeyoung Co., 99.99%) were used as received. Toluene (Aldrich) was distilled over CaH2 prior to use. 2.2. Grafting of γ-MPS onto TiO2 particles The grafting reaction was carried out according to the procedure given in the literature [15]. After dispersing 10 g of TiO2 nanoparticles in 200 mL of toluene, an excess amount of ␥-MPS was added and the resulting solution was stirred for 24 h under argon atmosphere. Modified TiO2 was isolated by centrifugation and washed repeatedly with toluene. Finally, it was dried at 50 ◦ C under vacuum for 24 h. 2.3. Synthesis of polypyrrole–TiO2 colloidal nanocomposites In a typical experiment, 0.4 g of TiO2 was dispersed into 1 g of pyrrole to form a suspension, and then the suspension was moved

Fig. 1. FT-IR spectra of (a) pristine TiO2 and (b) surface modified TiO2 .

Fig. 2. TEM pictures of PPy/TiO2 nanocomposites synthesized using (a) pristine TiO2 and (c) surface modified TiO2 . Pictures (b) and (d) are different magnifications of (a) and (c).

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into a 40 mL high-pressure reactor vessel. A required amount of ferric chloride with small amount of methanol was added into the vessel. Then it was sealed and CO2 was introduced at 40 ◦ C and 2000 psi. The in situ polymerization was carried out with the stirring speed of 600 rpm for 2 h, and then the vessel was cooled to room temperature. The resulting precipitate was collected and repeatedly washed with methanol and distilled water to remove the oxidant. Finally, the sample was dried at 50 ◦ C for 24 h in a vacuum oven. 2.4. Characterization The XRD patterns of the composite were collected on a powder X-ray diffractometer (Philips, X’Pert-MPD) with Cu K␣ radiation. The TEM images were obtained on a transmission electron microscope (JEOL, JEM-2010) operated with an accelerating voltage of 200 kV and equipped with an energy-dispersive X-ray spectrometer (EDX). FT-IR characterizations of pristine TiO2 , functionalized TiO2 , and PPy/TiO2 nanocomposites were performed using a BOMEM Hartman & Braun spectrometer. Thermal stability of polypyrrole and its nanocomposite with TiO2 were investigated by thermal gravimetric analyzer (Perkin Elmer, TGA-7) under a nitrogen flow (35 mL/min). The heating rate was 10 ◦ C/min. 3. Results and discussion In order to obtain PPy/TiO2 colloidal nanocomposite in scCO2 , it is necessary to disperse the TiO2 nanoparticles prior to the polymerization. The as-received TiO2 particles were not dispersible in CO2 but could be dispersed after modification with the silane coupling agent. Yue et al. have demonstrated a successful dispersion of silica nanoparticles in scCO2 with ␥-MPS modification, which is due to the favorable interaction between ␥-MPS molecules and CO2 [16]. The ␥-MPS modified TiO2 was also found to be dispersed well in scCO2 with stirring. The surface modified TiO2 was characterized by FTIR (see Fig. 1). The spectrum showed characteristic absorption bands: CH3 (2950–2920 (not shown) and 1430 cm−1 ), C O (∼1720 cm−1 ), C C (∼1635 cm−1 ), C C (∼1500–1400 cm−1 ) and Si O (∼1300–1250 cm−1 ; ␣,␤-unsaturated ester bands (1300, 1200 cm−1 ) asymmetrical stretching, 1000–1100 cm−1 ; symmetrical stretching), which indicate the availability of silane group on the surface of the filler [17]. For the purpose of comparison, PPy/TiO2 nanocomposites were synthesized in CO2 by using both pristine TiO2 and the surface modified TiO2 at 40 ◦ C and 2000 psi. The apparent physical nature of PPy changed remarkably after composite formation. Transmission electron micrographs of PPy/TiO2 nanocomposites synthesized in scCO2 using pristine TiO2 and surface modified TiO2 are shown in Fig. 2. The nanocomposite synthesized using pristine TiO2 (without surface modified) showed severe aggregation of TiO2 particles (Fig. 2a), whereas the composite synthesized using ␥-MPS modified TiO2 showed an image of well-dispersed TiO2 in the polymer matrix (Fig. 2c). In Fig. 2, the lighter portion shows the polymer and the dark portion indicates the nanoparticles. The size of the TiO2 nanoparticles used in the synthesis was

Fig. 3. TGA curves obtained from (a) PPy and (b) PPy/TiO2 nanocomposite.

in good agreement with the TEM images shown in Fig. 2. Nature of the association between particles and the conducting polymer showed that all TiO2 nanoparticles were encapsulated by PPy. There were no separate TiO2 particles observed. The chemical composition of PPy/TiO2 nanocomposite synthesized using surface modified TiO2 was examined by EDX analysis. The result indicates that the existence of elements Ti (from TiO2 ), C, O, and N (from pyrrole). TGA curve of PPy and its nanocomposite are shown in Fig. 3. All the samples follow the similar decomposition curves. As expected earlier, PPy and the composite show a gradual decomposition trend [8,18]. The residual percentages of PPy and

Fig. 4. XRD curves of (a) PPy/TiO2 nanocomposite (b) pristine TiO2 nanoparticles and (c) PPy.

H. Yuvaraj et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 300–303

PPy/TiO2 composites after TGA analysis in nitrogen are 58.9 and 66.5%, respectively. The residual amount of PPy decomposition in nitrogen possibly due to the carbonization is accordance with the previous result [8]. TGA results also suggest that the PPy/TiO2 nanocomposite has slightly higher thermal stability than neat PPy. Fig. 4 shows XRD curves of pristine TiO2 , PPy, and PPy/TiO2 nanocomposite synthesized using surface modified TiO2 . The broad peak in the region of 2θ = 20–30◦ in XRD curve of PPy shows that PPy prepared in the absence of TiO2 nanoparticles is amorphous. The main peaks of PPy/TiO2 nanocomposite are similar to those of TiO2 particles. The broad weak diffraction peak of PPy still exists, but its intensity decreases. It implies that the composite sample has a more ordered arrangement than the bare polymer owing to the TiO2 . These results show that the introduction of TiO2 do not affect the crystalline behavior of PPy. FT-IR spectroscopy studies yielded useful qualitative information on the PPy/TiO2 nanocomposite. The spectrum of pristine TiO2 (Fig. 1a) had one major band at 471 cm−1 and a week feature at 1633 cm−1 in the 400–2000 cm−1 range. The spectrum of PPy bulk powder prepared with the FeCl3 oxidant confirmed the formation of PPy (Fig. 5a) with the usual characteristic bands at ∼1559, ∼1480, ∼1305, ∼1214, ∼1038 and

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∼929 cm−1 . The bands at ∼1559 and ∼1480 cm−1 are assign to C C and C C stretching modes of PPy [8]. As expected, the spectrum of PPy/TiO2 nanocomposites (Fig. 5b) clearly exhibited adsorption bands attributed to both PPy (∼1570, ∼1410, ∼1300, ∼1194, ∼1110 and ∼910 cm−1 ) and pristine TiO2 (∼510 and ∼1626 cm−1 ). In FT-IR spectra of PPy and PPy/TiO2 nanocomposites, similar bands are observed from 400 to 2000 cm−1 , indicating that the main components of each specimen have the same chemical structures. However, the incorporation of TiO2 leads to the obvious shift of some FT-IR bands of PPy. 4. Conclusion A simple and efficient method is developed to fabricate semiconducting polymer/inorganic metal oxide composites in supercritical CO2 . PPy/TiO2 nanocomposites are successfully synthesized by in situ chemical oxidative polymerization of pyrrole in the presence of TiO2 nanoparticles dispersed in supercritical CO2 . The typical nanocomposite synthesized using ␥-MPS modified TiO2 in CO2 showed an image of welldispersed TiO2 in the polymer matrix by TEM analysis. TGA, XRD, and FT-IR data also confirmed that TiO2 nanoparticles are encapsulated by polypyrrole. Acknowledgements This subject is supported by Ministry of Environment as “The Eco-technopia 21 project” and the second stage of BK21 program. References

Fig. 5. FT-IR spectra of (a) PPy and (b) PPy/TiO2 nanocomposite.

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