Synthesis of electrically conductive and superparamagnetic monodispersed iron oxide-conjugated polymer composite nanoparticles by in situ chemical oxidative polymerization
Graphical abstract
Conducting and superparamagnetic monodispersed Fe3O4–poly(3, 4-ethylenedioxythiophene) core–shell composite nanoparticles with the size about 15 nm, were synthesized via an in situ chemical oxidative polymerization in the micellar solution of lignosulfonic acid (LSA).
Introduction
Conducting polymers (CPs) such as polyaniline (PANI), polypyrrole (PPy) and polythiophene (PTh) are important due to their relatively easy processability, thermal and environmental stabilities and tunable electrical conductivities [1], [2]. CPs offer potential applications in the domain of composite materials, separation membranes, tissue engineering, actuator, robot manipulators, supercapacitor, molecular motors, electronic and electro optic devices [3], [4]. Recent advances in the field of polymer science have increased research interest in nanostructured CPs [5], [6]. A current challenge in this field of nanotechnology is the ability to synthesize small enough monodispersed CP nanostructures, and their composites with metal nanoparticles that have multifunctional properties.
Encapsulation of metal nanoparticles into CPs results in hybrid organic–inorganic nanocomposites that exhibit enhanced thermal stability, electrochemical, catalytic, magnetic, mechanical, optical, dielectrical and electro-rhelogical properties [7], [8], [9], [10]. Also, the metal nanoparticles in conducting polymeric matrixes allow for the development of materials in various applications in the field of electrocatalsis, sensors, microelectronics, and magnetism. The unique properties of such polymer-coated metal composite materials are strongly dependent on their size and shape.
Various CPs-metal/metal oxide composites have been synthesized with different strategies and studied their properties and applications [11], [12], [13], [14], [15], [16], [17]. For an example, Parvatikar et al. [11] have synthesized WO3–PANI composites that have significantly higher electrical conductivity than that of pure PANI, and they used the composite as a humidity sensor. Sadek et al. [12] have studied In2O3–PANI composite nanofibers for sensing H2, NO2 and CO gases. Xu et al. [13] reported the preparation of a Pt–PANI composite and its utility in detecting glucose. De et al. [14] reported that a TiO2–PANI nanocomposite, with its high dielectric constant, can be used in integrated electronic circuits such as capacitors and gate oxides. V2O5–PANI composite offers considerable promise for the utilization as a cathode material for lithium secondary batteries [15]. Poly(3,4-ethylenedioxythiophene) Stochmal et al. [16] synthesized a Rh–PANI composite by reduction of RhCl3 with NaBH4 in the presence of PANI, and this composite was utilized as a catalyst for the conversion of isopropyl alcohol. Zhang et al. [17] employed radiation polymerization process to synthesize a composite of PANI derivative with SiO2 nanoparticles, which acts as glucose sensor. Therefore, synthesis of novel nanocomposites with metal nanoparticles and CPs would be of importance, and such novel materials can have interesting characteristics and features for nano-technological applications.
Among metal oxide nanoparticles, magnetic ferrite (Fe3O4) particles are attracting tremendous interest largely due to their unique properties and potential applications in magnetic storage media, contrast agents for magnetic resonance imaging (MRI), separation of biomolecules, ferro-fluid technology, heterogeneous catalysis, environmental and food analyzes, immunoassays and magnetic targeted site-specific drug delivery, for example [18], [19], [20]. Also, magnetic materials are the most important substance for the application of microwave absorbers due to their low microwave loss, low magnetic anisotropy and low magnetostriction. All of these applications require that the magnetic nanoparticles be chemically stable, have small particles with a narrow size distribution, and disperse well in an aqueous solution.
Among various CPs, polythiophene derivatives, poly(3,4-ethylenedioxythiophene) (PEDOT) is the most successful because of its excellent electrochemical activity, high electrical conductivity, moderate band gap, low redox potential, excellent environmental stability, simple acid/base doping/dedoping chemistry and high optical transparency in thin oxidized films [21], [22]. However, synthesis of uniform size and diameter of nanostructured PEDOT in various morphologies such as nanorods, nanowires is difficult without the use of external templates, in contrast to PANI [23]. Recently, composites consisting of different inorganic metals/CNTs and polythiophene (PTh), or its derivatives, have been prepared through different strategies such as irradiation and electrochemical polymerization, and evaluated [24], [25], [26].
Among various metal–polythiophene oxide nanocomposites, Fe3O4–PEDOT nanocomposites have received great attention because of their both unique electrical and magnetic properties as well as extensive applications in various fields. Shin et al. [27], [28] reported the synthesis of Fe3O4–PEDOT nanocomposite by one pot reaction of EDOT monomer with Fe3O4 in the presence of strong acid (HCl), and used this material for adsorption of heavy metal ions and for photocatalytic degradation of organic dyes. This method has some disadvantages: (i) the strong acid can disintegrate the nanostructured magnetic clusters, (ii) the magnetic nanoparticles dissolves in HCl to form water soluble Fe3+/Fe2+ ions, leading to reduce magnetite content [29] and (iii) obtained composite with impurities that do not favor conductive network formation result in material with low conductivity. Thus the development of stable conducting and magnetic nanocomposites remains a challenge and demands careful design.
Here, we report the facile synthesis of well dispersed core–shell type more stable composites of Fe3O4 nanoparticles and PEDOT by a simple, inexpensive and environmentally friendly in situ polymerization of EDOT with magnetic nanoparticles in the micellar solution of lignosulfonic acid (LSA). The LSA that is adsorbed onto the surface of the Fe3O4 nanoparticles acts as soft template. Nanocomposites were characterized for morphological, crystalline, structural, conducting and magnetic properties by HRTEM, XRD, FT-IR, four-probe conductivity and SQUID, respectively. Formation mechanism of the core–shell morphology of the nanocomposites and effect of magnetic nanoparticles on electrical conductivity and magnetic properties of the nanocomposites are discussed. We expect that the combination of electrically conductive PEDOT and magnetic Fe3O4 can form a composite with electro-magnetic properties, and such materials are expected to find applications in microwave absorbers, shielding materials, data storage, coil core, removal of industrial toxic waste, etc.
Section snippets
Materials
3,4-Ethylenedioxythiophene (EDOT), ammonium peroxydisulfate (APS) and lignosulfonic acid (LSA) were purchased from Aldrich, USA. Other reagents, iron (III) chloride hexahydrate (FeCl3 · 6H2O), iron (II) chloride tetrahydrate (FeCl2 · 4H2O), hydrochloric acid (HCl) and methanol were used without further purification. All the reagents were AR, and milli-Q purified water was used for all the experiments.
Synthesis of Fe3O4 nanoparticles
Fe3O4 nanoparticles were synthesized using a co-precipitation method according to reported
Morphology of the Fe3O4–PEDOT core–shell nanocomposites
Using HRTEM, we compared the morphology and size of the Fe3O4 nanoparticles and Fe3O4–PEDOT nanocomposites prepared with 20 wt.% of Fe3O4 content. The Fe3O4 nanoparticles (Fig. 1a) synthesized by a co-precipitation process are spherical in shape, with very small size nanometer diameter, and there were few aggregates due to their large surface area and the magnetic dipole–dipole interactions between the particles. A tremendous change in the morphology of the composite (Fig. 2a) was observed after
Summary
Uniform core–shell Fe3O4−PEDOT composite nanoparticles were successfully synthesized by in situ chemical oxidative polymerization of EDOT with Fe3O4, directed by LSA as a both structure-directing agent and a dopant. The composites are well defined in size and shape, superparamagnetic with magnetization values of ∼13 emu/g; and they exhibit good electrical conductivity (∼0.2 S/cm). Upon the dispersion of magnetic particles into a polymer matrix, they reduced their aggregation and the particles
Acknowledgment
This work was supported by the 2008 Research Fund of University of Ulsan.
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