Journal of Nanoparticle Research

, Volume 11, Issue 6, pp 1441–1452

Fabrication and characterization of iron oxide nanoparticles filled polypyrrole nanocomposites

Authors

    • Multifunctional Composites Lab (MCL), Mechanical & Aerospace Engineering and Materials Science & Engineering DepartmentUniversity of California
    • Chemical Engineering DepartmentLamar University
  • Koo Shin
    • Department of Applied ChemistrySejong University
  • Amar B. Karki
    • Department of Physics and AstronomyLouisiana State University
  • David P. Young
    • Department of Physics and AstronomyLouisiana State University
  • Richard B. Kaner
    • Department of Chemistry and BiochemistryUniversity of California
  • H. Thomas Hahn
    • Multifunctional Composites Lab (MCL), Mechanical & Aerospace Engineering and Materials Science & Engineering DepartmentUniversity of California
Research Paper

DOI: 10.1007/s11051-008-9531-8

Cite this article as:
Guo, Z., Shin, K., Karki, A.B. et al. J Nanopart Res (2009) 11: 1441. doi:10.1007/s11051-008-9531-8

Abstract

The effect of iron oxide nanoparticle addition on the physicochemical properties of the polypyrrole (PPy) was investigated. In the presence of iron oxide nanoparticles, PPy was observed in the form of discrete nanoparticles, not the usual network structure. PPy showed crystalline structure in the nanocomposites and pure PPy formed without iron oxide nanoparticles. PPy exhibited amorphous structure and nanoparticles were completely etched away in the nanocomposites formed with mechanical stirring over a 7-h reaction. The thermal stability of the PPy in the nanocomposites was enhanced under the thermo-gravimetric analysis (TGA). The electrical conductivity of the nanocomposites increased greatly upon the initial addition (20 wt%) of iron oxide nanoparticles. However, a higher nanoparticle loading (50 wt%) decreased the conductivity as a result of the dominance of the insulating iron oxide nanoparticles. Standard four-probe measurements indicated a three-dimensional variable-range-hopping conductivity mechanism. The magnetic properties of the fabricated nanocomposites were dependent on the particle loading. Ultrasonic stirring was observed to have a favorable effect on the protection of iron oxide nanoparticles from dissolution in acid. A tight polymer structure surrounds the magnetic nanoparticles, as compared to a complete loss of the magnetic iron oxide nanoparticles during conventional mechanical stirring for the micron-sized iron oxide particles filled PPy composite fabrication.

Keywords

Polymer nanocompositesConductivityStirring methodsMagnetic propertyThermal stabilityCorrosion-resistanceNanomanufacturing

Introduction

Polypyrrole (PPy), a conducting conjugated polymer, has attracted much interest due to its low cost, easy synthesis, good stability, and environmentally benign performance (Yeh et al. 2003). The conductivity of a conductive polymer is strongly dependent on the doping agents (dopant) with electron donor or acceptor abilities. The doping process can even transform an intrinsically conjugated polymer insulator to a near-metallic conductor (Lee et al. 2006). Conductive PPy has been reported to serve as polymeric rechargeable batteries for energy-storage purposes (Song and Palmore 2006), electrode materials used in the electrochemical supercapacitors (Ingram et al. 2004; Noh et al. 2003), metal corrosion protection coating materials (Ferreira et al. 1990; Zaid et al. 1994), matrix for structural composite materials (Han et al. 2005), electromagnetic interference (EMI) shielding, electrochemomechanical devices (Asavapiriyanont et al. 1984), and sensors for pH (Lakard et al. 2007), gas, and humidity (Tandon et al. 2006; An et al. 2004) testing. In addition, granular polypyrrole nanocomposites have been reported as a candidate for photovoltaic (solar cell) materials (Kwon et al. 2004).

Polymer nanocomposites with nanoparticles (NPs) have attracted much interest due to their homogeneity, easy processability, and tunable physical (mechanical, magnetic, electrical, thermoelectric, and electronic) properties (Castro et al. 2000; Wang et al. 2000; Gangopadhyay et al. 2000; Corbierre et al. 2001; Li et al. 2002; Wetzel et al. 2003; Mack et al. 2005; Chen et al. 2005; Vivekchand et al. 2005; Mammeri et al. 2005; Guo et al. 2006; Lee et al. 2008). High particle loading is required for certain industrial applications, such as electromagnetic wave absorbers (Brosseau and Talbot 2005; Guo et al. 2007a), photovoltaic cells (solar cells) (Beek et al. 2004), photo detectors, and smart structures (Gall et al. 2004; Mohr et al. 2006; Guo et al. 2007b). Magnetic nanoparticles, due to their unique magnetic and electronic properties, are used in various applications such as biomedical drug delivery, specific site targeting, magnetic data storage and sensors (Toal and Trogler 2006; Podlaha et al. 2006; Lei and Bi. 2007; Bi et al. 2008). Successfully incorporating magnetic nanoparticles into conductive polymer matrices will definitely widen their applicability in the fields of electronics, biomedical drug delivery, and optics. However, one of the challenges so far is the ability to integrate a high fraction of nanoparticles into the polymer matrix in a strong acidic environment. The acid, which is normally required for the PPy synthesis, will etch away the nanoparticles in aqueous solutions. A balance between the need for polymerization in an acidic solution and the prevention of dissolution of reactive iron oxide nanoparticles will be a determining factor for high-quality nanocomposite fabrication.

Polypyrrole nanocomposites with iron oxide and other nanoparticles have been prepared by several methods. For example, an in situ chemical oxidative polymerization approach with either an ultrasonication approach (Yen et al. 2008) or mechanical stirring approach (Li et al. 2006) was reported. The nanocomposites showed particle-loading magnetic properties and electric conductivity. In addition, the supercritical fluid was also reported to be used as a media in the in situ chemical oxidative polymerization for the fabrication of the conductive polymer magnetic nanocomposites with a consideration of green chemistry (Yuvaraj et al. 2008). The stirring method (ultrasonication or mechanical stirring) is believed to have a significant effect on the formed nanocomposites and the subsequent physicochemical properties. However, there are few papers reported in the literature.

In this paper, the effect of iron oxide nanoparticle addition on the morphology of PPy, thermal stability, magnetic properties, and electrical conductivity of the resulting Fe2O3/PPy nanocomposites was reported. The effect of the stirring method, i.e., ultrasonic and mechanical stirring on the composite fabrication was also reported. The electric conductivity was investigated by a standard four-probe method and found to be strongly dependent on the particle loadings. The iron oxide nanoparticles were observed to be stable even after exposure to a strong acid with a pH value of 1.0 for more than 3 weeks.

Experimental details

Materials

The pyrrole monomer (Aldrich) was distilled under reduced pressure. γ-Fe2O3 nanoparticles were obtained from Nanophase Technologies Corporation with a reported average size of 23 nm and a specific surface area of 45 m2/g. Ammonium persulfate (APS) and p-toluenesulfonic acid (CH3C6H4SO3H, p-TSA) were all purchased from Aldrich and used as received without further treatment.

Polypyrrole and nanocomposite preparation

A dispersion of γ-Fe2O3 nanoparticles was made by adding a desired amount of γ-Fe2O3 in 20 mL deionized water under sonication. The p-TSA (6.0 mmol) and pyrrole (7.3 mmol) were added into the above nanoparticle suspended solution under constant sonication and continuously stirred for 10 min. APS (3.6 mmol) was rapidly mixed into the above solution at room temperature, and the resulting solution was kept under sonication for 1 h. In addition, the effect of reaction time was investigated by sonication for 7 h as used previously in a study of the micron-sized iron oxide particles (Li et al. 2006). Both mechanical and ultrasonic stirring were explored, and the resulting nanocomposite properties were characterized accordingly. All the products were washed thoroughly with deionized water (to remove any unreacted APS and p-TSA) and methanol (to remove any oligomers), respectively. The precipitated powder was dried at 50 °C for further analysis. As a control experimental for comparison purposes, pure PPy was also synthesized following the same procedure as described before but without iron oxide nanoparticles.

Characterization

A Fourier transform infrared (FT–IR) spectrometer (Jasco, FT–IR 420) in transmission mode under dry nitrogen flow (10 cubic centimeters per minute, ccpm) was used to test the physicochemical interactions between PPy and Fe2O3 nanoparticles. The dried PPy powder was mixed with powder KBr, ground, and compressed into a pellet. Its spectrum was recorded as a reference to be compared with that of the Fe2O3/PPy nanocomposites.

The thermal degradation of the nanocomposites with different particle loadings was studied with a thermo-gravimetric analysis (TGA, Perkin Elmer). TGA was conducted on pure PPy and Fe2O3/PPy nanocomposites from 25 °C to 600 °C with an argon flow rate of 50 ccpm and a heating rate of 10 °C/min.

The dispersion quality of the nanoparticles within the PPy matrix, and the nanostructures of the polymer and nanocomposites were investigated using a scanning electron microscope (SEM, JEOL field emission scanning electron microscope, JSM-6700F). The SEM specimens were prepared by spreading a thin layer of powder onto a double-side carbon tape. The microstructure and crystallinity were investigated with a transmission electron microscope (TEM, JEOL, 100CX) with an accelerating voltage of 100 keV. The samples were prepared by dispersing the powder in anhydrous ethanol, dropping some suspended solution onto a carbon-coated copper grid and drying naturally under ambient conditions.

The magnetic properties of the nanocomposite were investigated in a 9-Tesla physical properties measurement system (PPMS) by Quantum Design. The electrical conductivities were measured using a standard four-probe method. The samples were prepared by the cold-press method. The applied pressure was 10,000 psi and the pressing duration time was 10 min.

Results and discussion

Figure 1 shows the SEM microstructures of the synthesized pure PPy and Fe2O3/PPy nanocomposites fabricated by the conventional method, as used for iron oxide micron-sized particles filled PPy composite fabrication (Li et al. 2006). The conventional method is based on mechanical stirring over a long period of time. In contrast to the network structure of pure PPy as shown in Fig. 1a, discrete spherical nanoparticles with uniform size distribution are observed in the nanocomposite counterparts fabricated by the conventional method as shown in Fig. 1b. However, no attraction was observed when a permanent magnet was placed nearby, and further quantitative magnetic characterization did not show any sign of magnetization in the nanocomposite. The disappearance of the magnetic iron oxide nanoparticles is believed to be due to the slow dissolution over time caused by the acidic solution used in the pyrrole polymerization. This observation also suggests the formation of a porous PPy shell rather than a solid tight one that can protect the magnetic nanoparticles from dissolution.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9531-8/MediaObjects/11051_2008_9531_Fig1_HTML.jpg
Fig. 1

SEM micrographs of a pure PPy formed without iron oxide nanoparticles, b pure PPy formed with iron oxide nanoparticles completely dissolved (mechanical stirring), c Fe2O3/PPy nanocomposites with 20 wt% iron oxide nanoparticles (sonication), and d Fe2O3/PPy nanocomposites with 50 wt% iron oxide nanoparticles (sonication)

For the in situ formation of the conductive magnetic nanocomposite, a short reaction time (1 h) was used to balance the PPy formation and the iron oxide nanoparticle dissolution. Ultrasonic stirring was used rather than mechanical stirring to minimize the contamination and achieve better particle dispersion. The red particles turned black after the polymerization, indicating the formation of PPy. Unlike the network structure as observed with pure PPy, the SEM micrographs as shown in Fig. 1c, d of the nanocomposites with different initial particle loadings show discrete nanoparticles without any obvious adhesion between them. In stark contrast to the obvious loss of magnetic nanoparticles when mechanical stirring was used for the 7-h polymerization, the dried nanocomposite powder in Fig. 1c, d did get attracted toward a permanent magnet, indicating the presence of the magnetic nanoparticles.

Figure 2 shows the FT–IR spectra of the pure PPy and Fe2O3/PPy nanocomposites. Characteristic peaks of PPy are observed in all the samples, indicating the formation of PPy. The peaks at 1547 and 1464 cm−1 can be assigned to C = C and C–N stretching vibrations, respectively. The peaks at 1,172 and 901 cm−1 are due to the C–H in-plane bending and ring deformation, respectively. Similar patterns were also observed in PPy-Fe[OH] microcomposites (Li et al. 2006). The obvious spectral differences between pure PPy and the composites indicate that PPy exhibits a different PPy chain structure and there are physicochemical interactions between the nanoparticles and PPy. The presence of iron oxide nanoparticles in the 20 and 50 wt% composites is strongly supported by new peaks at 485 cm−1 and 588 cm−1 as shown in Fig. 2c, d, which are characteristic peaks of Fe2O3 (e.g., polyhedral Fe3+–O2- stretching vibrations of iron oxide) (Ram 1995; Sepulveda-guzman et al. 2007; Guo et al. 2007c). This observation indicates that a magnetic nanocomposite can be synthesized with PPy if polymerization can be achieved in a short period of time. The prolonged polymerization is characterized by the disappearance of the characteristic IR peaks of iron oxide as shown in Fig. 2b. The almost similar spectra between the PPy and PPy fabricated by the long time reaction also indicate the loss of iron oxide nanoparticles.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9531-8/MediaObjects/11051_2008_9531_Fig2_HTML.gif
Fig. 2

FT–IR spectra of a pure PPy without nanoparticles, b PPy with iron oxide nanoparticles that have been attacked by acid, c a Fe2O3/PPy composite with a 20 wt% particle loading, d composite with a 50 wt% loading, and e as-received iron oxide nanoparticles

The thermal stability of pure PPy and Fe2O3/PPy nanocomposites was investigated by TGA measurements. Figure 3a shows the thermogravimetric profiles of nanocomposites with different particle loadings (0, 20, and 50 wt%). The black nanocomposites were observed to turn red upon test completion, characteristic of iron oxide rather than black carbon, indicating the complete loss of PPy. The weight loss at temperatures lower than 120 °C is due to loss of moisture, while the major loss at temperatures higher than 240 °C is due to the decomposition of PPy. The differences in the residue reflect the different amounts of iron oxide nanoparticles present. In addition, pure PPy formed with the aid of iron oxide nanoparticles shows higher stability than that formed without them. The thermal stability increases slightly with increasing nanoparticle loading, which is believed to be due to both the lower mobility of the polymer chains when the polymer chains are bound onto the nanoparticles and stronger chemical interaction (Chen et al. 2003). Similar to the TGA observation, a higher decomposition temperature (308.0 °C) as shown in Fig. 3b was observed in the PPy formed with the aid of nanoparticles than that (298.4 °C) of the pure PPy formed without them. Whereas only one peak was observed in the pure PPy samples, two exothermic peaks are observed in the DTA curves of the nanocomposites. These are due to the decomposition of PPy at 307 °C and the possible phase transition of iron oxide at 480 °C, as reported in the Fe2O3/PPy nanocomposites fabricated by the simultaneous gelation and polymerization (sol-gel) method (Suri et al. 2001; Suri et al. 2003), respectively. As compared with no obvious phase transition in the pure iron oxide nanoparticles, the observed phase transition is likely due to an intermediate product of PPy (Suri et al. 2001; Suri et al. 2003).
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9531-8/MediaObjects/11051_2008_9531_Fig3_HTML.gif
Fig. 3

a TGA weight changes and b corresponding DTA curves of pure PPy, and Fe2O3/PPy nanocomposites with different particle loadings

Figure 4 shows the magnetic hysteresis loops of the nanocomposites with an initial particle loading of 20 and 50 wt%, respectively. There is no hysteresis observed in the sample formed from the 7-h reaction due to the dissolution of magnetic nanoparticles in the acidic solution. The saturation magnetization (Ms) is about 29.4 emu/g and 45.1 emu/g based on the total weight of the nanocomposites with an initial loading of 20 and 50 wt%, respectively. The saturation magnetization of iron oxide was reported to be 74 emu/g and independent of the surface chemistry of the nanoparticles. Thus, the real particle loading of the nanoparticles in the nanocomposites with an initial particle loading of 20 and 50 wt% was calculated to be 27 and 68 wt%, respectively. The calculated particle loading based on the magnetic data is much higher than the initial particle loading considering the partial particle loss from the dissolution during the nanocomposite fabrication. This indicates that the yield of the PPy is lower. In other words, the pyrrole monomers are partially polymerized into PPy, which leads to higher particle loading. In addition, the polymer yield was observed to be lower at the higher initial particle loading than that at the lower initial particle loading.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9531-8/MediaObjects/11051_2008_9531_Fig4_HTML.gif
Fig. 4

Magnetic hysteresis loops of nanocomposites at different loadings

The electrical conductivity was measured by a standard four-probe method. Figure 5a shows the temperature-dependent resistivity of pure PPy and nanocomposites with different particle loadings. The PPy synthesized with the presence of the nanoparticles over a long-term reaction was observed to have large resistance beyond the ability of the utilized equipment due to the poor contact between small particles (Dey et al. 2005) and the observed amorphous structure. At lower temperature the resistance is too large to be measured by the utilized equipment. The much lower resistivity in the pure PPy prepared by the conventional method is due to the network structure formation as shown in the SEM image of Fig. 1a, which favors electron transport. The resistivity change is not due to the doping extent based on the fact that all the samples were washed and doped with the same solution (Huang et al. 2005). The resistance was observed to decrease dramatically in the Fe2O3/PPy nanocomposites. The nanocomposite with an initial particle loading of 50 wt% was observed to have a higher resistivity, which is due to the known insulating properties of iron oxide (Ortiz et al. 1988; Mei et al. 1987) and consistent with the silicon carbide/polypyrrole nanocomposites (Omastova et al. 2005), silica/polypyrrole nanocomposites (Dutta and De 2006), and titanium dioxide/polyaniline nanocomposites (Su and Kuramoto 2000).
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9531-8/MediaObjects/11051_2008_9531_Fig5_HTML.gif
Fig. 5

Temperature-dependent a resistivity and b conductivity of pure PPy and nanocomposites with different loadings

Figure 5b shows the temperature-dependent conductivity (σ). The conductivity of the pure PPy and nanocomposites with two different particle loadings can be linearly correlated to temperature with the quasi-three-dimensional variable range hopping (quasi-3D-VRH) model. The variable range hopping theory analysis indicates that three-dimensional hopping dominates in these samples, and the correlation between conductivity and temperature can be expressed in Eq. 1,
$$ \sigma \left( T \right) = \sigma_{0} \exp \left[ { - \left( {\frac{{T_{0} }}{T}} \right)^{1/4} } \right] $$
(1)
where T0 is the characteristic Mott temperature and the pre-exponential factor σ0 is the conductivity at infinite temperature. The σ0 was calculated to be 3000.1 S cm−1, 978.4 S cm−1, and 298.8 S cm−1 for pure PPy and nanocomposites with an initial particle loading of 20 and 50 wt%, respectively. The T0 was calculated to be 1204137, 138346, and 212010 K for pure PPy and the nanocomposites with an initial particle loading of 20 and 50 wt%, respectively. The parameter T0 is inversely proportional to the localization length of the charge carriers. A larger T0 implies a stronger localization of the charge carriers, which results in an increase of the resistance at low temperatures, whereas a small T0 implies a weak localization (Gangopadhyay et al. 2000). The observed T0 variation with the component is consistent with our observed resistivity changes in the synthesized PPy and Fe2O3/PPy systems.
It was reported that the electrical conductivity strongly depends on the ordered states of the conductive polymer (Abthagir and Saraswathi 2005). In other words, the structural and electronic properties of PPy can be easily modified due to a soft lattice, which can be reflected in the crystalline structure of the nanomaterials. Iron oxide nanoparticles are thought to serve as a template for the subsequent PPy matrix formation. Ultrasonic energy has been reported to have strong capability to alter the electronic structure of a polymer or even produce various nanoparticles. Here, the crystalline structural change with the nanocomposite formation was investigated using selected area electron diffraction (SAED) and dark field TEM microstructures. Figure 6a, b show the bright field TEM microstructure of pure PPy and the corresponding selected area electron diffraction (SAED). Similar to the SEM observation, linked nanoparticles were observed in the bright field TEM images as shown in Fig. 6a. The clear ring pattern as shown in Fig. 6b indicates a crystalline structure for the formed PPy. The lattice distances and crystalline planes (from the inner to the outward) of the pure PPy calculated from the electron diffraction patterns are 0.117 nm (100), 0.0666 nm (111), 0.0577 nm (200), 0.0434 nm (220), and 0.0386 nm (300), respectively. However, no ring pattern is observed in PPy formed in the presence of the iron oxide nanoparticles over a long time reaction, indicating an amorphous structure. The observed discrete spherical structure as shown in Fig. 7c is consistent with the SEM observation, which leads to poor contact and high resistivity.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9531-8/MediaObjects/11051_2008_9531_Fig6_HTML.jpg
Fig. 6

a TEM bright field microstructure of pure PPy formed without iron oxide nanoparticles, b selected area electron diffraction (SAED) pattern of pure PPy in a, and c TEM bright field microstructure of PPy formed with iron oxide nanoparticles completely dissolved. d TEM bright field microstructure of the Fe2O3/PPy nanocomposites with an initial loading of 20 wt%, e SAED pattern of pure PPy in d, f dark field microstructure of the corresponding sample in d, g TEM bright field microstructure of the Fe2O3/PPy nanocomposites with an initial loading of 50 wt%, h SAED pattern of pure PPy in g, and i dark field microstructure of the corresponding sample in g

https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9531-8/MediaObjects/11051_2008_9531_Fig7_HTML.gif
Fig. 7

a Magnetic hysteresis loops of pure iron oxide nanoparticles and the nanocomposite, b temperature-dependent resistivity/conductivity of the nanocomposite, c TEM bright field microstructure, d selected area electron diffraction (SAED) pattern, and e dark field microstructure of the Fe2O3/PPy nanocomposite (nanocomposite has an initial loading of 50 wt% and is ultrasonically stirred for 7 h)

Figure 6d–i shows the bright field microstructure, selected area electron diffraction, and dark field micrograph of the prepared nanocomposites with an initial particle loading of 20 and 50 wt%, respectively. Discrete nanoparticles are observed in the nanocomposites and consistent with the SEM observations. The image contrast arises from different molecular weights of PPy and iron oxide. The dark and gray regions correspond to iron oxide and PPy, respectively. The lattice distance of the related SAED of the nanocomposite with an initial particle loading of 20 wt% as shown in Fig. 6e is indexed to 0.167 nm (2 1 1, Fe2O3), 0.137 nm (2 0 8, Fe2O3), 0.114 nm (100. PPy), 0.086 nm (2 3 8, Fe2O3), and 0.0804 nm (2 4 4, Fe2O3). The SAED pattern of the nanocomposite with an initial particle loading of 50 wt% is indexed to 0. 167 nm (2 1 1, Fe2O3), 0.137 nm (2 0 8, Fe2O3), 0.111 nm (100, PPy), 0.0863 nm (2 3 8, Fe2O3), 0.0804 nm (2 4 4, Fe2O3), 0.0648 nm (111, PPy), 0.0569 nm (200, PPy), and 0.0376 nm (300, PPy). The dark field micrographs of the nanocomposites corresponding to the bright field images as shown in Fig. 6f, i also indicate a crystalline structure. The high crystallinity observed in the nanocomposites is most likely responsible for the increased conductivity. In addition, the lattice distance of PPy was observed to be smaller in the nanocomposite with a higher particle loading, indicating that the nanoparticles favor a compact PPy structure with a lower resistivity. However, the high resistivity in the higher particle loaded nanocomposite is due to the insulating iron oxide nanoparticles, which dominate the electron transport at higher loadings.

The effect of the stirring method, i.e., mechanical versus ultrasonic stirring was investigated by polymerizing for 7 h as used in the mechanical stirring. The final product exhibits a strong attraction to a permanent magnet, indicating the presence of iron oxide nanoparticles. Figure 7a shows the hysteresis loop of the as-received iron oxide nanoparticles and the nanocomposite (an initial particle loading of 50 wt%) synthesized with ultrasonic stirring over 7 h, respectively. The weight percentage of iron oxide nanoparticles in the nanocomposite was estimated to be 20.2% based on the saturation magnetization of the nanocomposite and the as-received nanoparticles. This weight percentage is much lower than the initial particle loading and the composite sample synthesized with a 1-h ultrasonic stirring. This indicates that more particles are lost due to the dissolution over the long-time reaction between the nanoparticles and the protons. The coercivity was observed to be much larger in the nanocomposite (65 Oe) than in the as-received samples (18 Oe), due to the dispersion of the single-domain size nanoparticles. Figure 7b shows the temperature-dependent resistivity and conductivity (σ) of the nanocomposite with an initial particle loading of 50 wt% and ultrasonic stirring for 7 h. In comparison to the high resistivity of the nanoproduct (pure PPy, complete loss of the iron oxide nanoparticles), the lower resistivity and the presence of magnetic hysteresis indicate a significant effect of the stirring methods on the composite preparation, and the ultrasonic stirring favors the protection of nanoparticles from dissolution. The linear relation between ln(σ) and T^(−1/4) indicates a quasi-three-dimensional variable range hopping (quasi-3D-VRH) mechanism.

Figure 7c–e shows the bright field microstructures, corresponding selected area electron diffraction, and dark field microstructures of the nanocomposites with ultrasonic stirring, respectively. The contrast as shown in Fig. 7c represents iron oxide nanoparticles (black region) and polymer PPy (gray region). The nanoparticles are observed to be enclosed and protected from complete dissolution by the polymer matrix. The ring patterns with clear spots shown in Fig. 7d indicate the highly crystalline structure of the nanocomposites, which can be indexed to iron oxide nanoparticles and the formed PPy. The dark field microstructures of the nanocomposites also indicate a crystalline structure. The crystalline structural parameter of pure PPy and electric resistivity/conductivity of the nanocomposites are summarized in Table 1. The crystalline structural parameter is observed to be larger in the nanocomposites with a 7-h ultrasonic stirring and an initial particle loading of 50 wt%, which is due to the majority of the PPy matrix, as observed in the TEM (Fig. 7c) and also lower final iron oxide particle loading. With a similar crystalline structural parameter, the presence of iron oxide nanoparticles makes the nanomaterials less conductive. With an amorphous structure, PPy formes, with the presence of iron oxide nanoparticles and long-time mechanical stirring, insulating materials.
Table 1

Physical properties of pure PPy and Fe2O3/PPy nanocomposites

Material name

Conductivity at 290 K (S cm−1)

Conductivity at 10 K (S cm−1)

Final particle loading (wt%)d

Pure PPya

0.9

1.2 × 10−4

0

Nanocomposite 20 wt%b (1 h ultrasonic stirring)

8.3

105.4 × 10−4

27.0

Nanocomposite 50 wt% (1 h ultrasonic stirring)

1.4

9.0 × 10−4

68.0

Nanocomposite 50 wt% (7 h ultrasonic stirring)

0.6

1.0 × 10−5

20.2

Nanocomposite 50 wt%c (7 h mechanical stirring)

<1.2 × 10−10

<1.2 × 10−10

0

aPure PPy formed without iron oxide nanoparticles

bThe weight percentage was the initial particle loading before polymerization and calculated based on the total mass of monomers and the nanoparticles

cThis is pure PPy and the nanoparticles were completely dissolved due to the reaction between the nanoparticles and the protons

dParticle loading was estimated from the saturation magnetization

The stability of the Fe2O3/PPy nanocomposites in harsh environments was tested by dispersing the nanocomposites in an aqueous hydrogen chloride solution with a pH value of 1.0 for more than 3 weeks. The nanocomposites were observed to be re-dispersible in water as shown in Fig. 8a and exhibited a strong tendency to be attracted onto the permanent magnet as shown in Fig. 8b. This indicates that the iron oxide nanoparticles in the nanocomposites have been effectively protected by the formed PPy during the ultrasonic stirring polymer formation, in stark contrast with the complete loss of the nanoparticles in the mechanical stirring composite sample. In addition, the nanocomposites were attracted to the magnet very quickly, and the black solution turned to transparent within minutes. The stabilized iron oxide nanoparticles within the conductive polymer matrix in acidic environments allow these nanocomposites to be used in biomedical areas.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9531-8/MediaObjects/11051_2008_9531_Fig8_HTML.jpg
Fig. 8

a Nanocomposites redispersed in the aqueous solution, and b the attraction of the nanocomposite onto a permanent magnet in a hydrogen chloride solution

Summary

The effect of iron oxide nanoparticles on the chemical polymerization of pyrroles in an acidic solution was investigated and found to significantly influence the morphology (size and shape) and other physicochemical properties of the PPy. Pure discrete PPy nanoparticles with a much higher resistivity are formed over a long reaction time in the presence of iron oxide nanoparticles. Similar to pure PPy formed with iron oxide nanoparticles and different from the network structure of the pure PPy formed without iron oxide nanoparticles, discrete nanoparticles are observed in all the nanocomposites with an initial particle loading of 20 and 50 wt%. The subsequent nanocomposites are observed to have an improved thermal stability with a higher decomposition temperature. FT–IR, TGA/DTA, and TEM/SAED analyses indicate a strong interaction between the iron oxide nanoparticles and the polymer matrix. PPy was observed to have a lower yield in the nanocomposite with a higher initial particle loading. The saturation magnetization in the nanocomposite with high particle loading was larger, and the conduction behavior follows a three-dimensional variable range hopping mechanism. The presence of the iron oxide nanoparticles in the nanocomposites is observed to produce a more condensed structured PPy. The decreased conductivity in the high particle loading is due to the insulating behavior of iron oxide. Compared to mechanical stirring, ultrasonic stirring plays a critical role in the iron oxide-PPy nanocomposite formation and provides protection of iron oxide nanoparticles from dissolution by protons arising from a tight PPy matrix formed surrounding the iron oxide nanoparticles.

Acknowledgments

The present paper is based on work supported by QuantumSphere Research Grant (QuantumSphere Inc.), UC-discovery Grant ELE06-10268, and the Air Force Office of Scientific Research Grant F9550-05-1-0138. DPY kindly acknowledges support from the National Science Foundation under Grant No. DMR 04-49022.

Copyright information

© Springer Science+Business Media B.V. 2008