Fabrication and characterization of iron oxide nanoparticles filled polypyrrole nanocomposites
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- Guo, Z., Shin, K., Karki, A.B. et al. J Nanopart Res (2009) 11: 1441. doi:10.1007/s11051-008-9531-8
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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.
KeywordsPolymer nanocompositesConductivityStirring methodsMagnetic propertyThermal stabilityCorrosion-resistanceNanomanufacturing
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.
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.
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
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 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.
Physical properties of pure PPy and Fe2O3/PPy nanocomposites
Conductivity at 290 K (S cm−1)
Conductivity at 10 K (S cm−1)
Final particle loading (wt%)d
1.2 × 10−4
Nanocomposite 20 wt%b (1 h ultrasonic stirring)
105.4 × 10−4
Nanocomposite 50 wt% (1 h ultrasonic stirring)
9.0 × 10−4
Nanocomposite 50 wt% (7 h ultrasonic stirring)
1.0 × 10−5
Nanocomposite 50 wt%c (7 h mechanical stirring)
<1.2 × 10−10
<1.2 × 10−10
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.
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.