Synthesis and microwave absorption properties of electromagnetic functionalized Fe3O4–polyaniline hollow sphere nanocomposites produced by electrostatic self-assembly
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Highly regulated Fe3O4–polyelectrolyte-modified polyaniline (Fe3O4–PE@PANI) hollow sphere nanocomposites were successfully synthesized using an electrostatic self-assembly approach. The morphology and structure of the Fe3O4–PE@PANI nanocomposites were characterized using field-emission scanning electron microscopy, transmission electron microscopy, Fourier-transform infrared spectroscopy, X-ray powder diffraction, thermogravimetric analysis, and X-ray photoelectron spectroscopy. The results showed that the as-prepared nanocomposites had well-defined sizes and shapes, and the average size is about 500 nm. The assembly process was investigated. Magnetization measurements showed that the saturation magnetization of the nanocomposites was 38.6 emu g−1. It was also found that the Fe3O4–PE@PANI nanocomposites exhibited excellent reflection loss abilities and wide response bandwidths compared with those of PANI hollow spheres in the range 0.5–15 GHz. The Fe3O4–PE@PANI nanocomposites are, therefore, promising for microwave absorption applications.
KeywordsFe3O4 nanoparticles Polyaniline hollow sphere Nanocomposite Polyelectrolyte Microwave absorption
Combinations of conducting polymers and inorganic magnetic nanoparticles have recently attracted significant interest because the resultant materials exhibit both conductive and magnetic properties, and take advantage of the properties of both conducting polymers and inorganic nanoparticles. Electromagnetic functionalized conducting polymer nanocomposites have great potential for applications in microwave-absorbing materials, electrochemical displays, nonlinear optics, and electromagnetic shielding (Shen et al. 2010; Zhou et al. 2011; Kang et al. 1998; Kawaguchi 2000; Gomez-Romero 2001; Zhang and Wan 2003; Marchessault et al. 1992; Fang et al. 2011). Interest in the design and controlled fabrication of materials with specific conducting and magnetic properties, therefore, continues to grow. Polyaniline (PANI), which is an excellent conducting polymer, has been known for more than a century and studied in many fields because of its excellent environmental stability and ease of doping (MacDiarmid 2001). Particularly, PANI-based nanocomposite, the most important material for the twenty-first century, has received special attention owing to their potential wide applications arising from the unique nanofiller-introduced thermal stability, electrochemical, mechanical, magnetic, and dielectric properties (Zhang et al. 2013). For example, grapheme/PANI (Wei et al. 2012a, b ), BaTiO3/PANI (Zhang et al. 2013; Zhu et al. 2013), and multi-walled carbon nanotube/PANI (Wei et al. 2013; Gu et al. 2013) for supercapacitors, stealth materials, and environmental remediations have been recently explored and investigated (Wei et al. 2012a, b).
As magnetic nanoparticles, magnetite (Fe3O4) nanoparticles are mostly investigated among the many magnetic materials owing to their interesting magnetic properties and are easy to synthesize, and have a wide range of potential applications in various fields, such as magnetic recording media, photonic crystals, microwave-absorbing materials, and biomedical applications (Wei et al. 2012a, b; Umare et al. 2010; Zhang et al. 2011; Kim et al. 2008). Recently, various Fe3O4–PANI micro/nanostructures have been the focus of research because their properties are different from those of the corresponding bulk forms. Shape-controlled synthesis of Fe3O4–PANI nanocomposites with desired morphologies is, therefore, a hot research topic. Conducting polymer hollow spheres have potential applications in reactors, catalysts, sensors, carriers, combinatorial analytics, and photochemistry (Meier 2000; Shchukin and Sukhorukov 2004; Peyratout and Dahne 2004), and are also promising as ideal microwave resonators because of their special structures, low densities, and light weights. The development of microwave absorbers has been an important technology for eliminating electromagnetic wave pollution. Recently, the demand for various microwave absorbers for commercial and military applications has increased. The preparation of Fe3O4–PANI hollow sphere nanocomposites for use as microwave absorbers is, therefore, of interest.
In the past few decades, various techniques have been developed for the fabrication of PANI and magnetic nanocomposites, mainly by in situ synthesis of a conjugated polymer via oxidative (Deng et al. 2003) and electrochemical oxidation polymerizations (Bidan et al. 1994). However, most electromagnetic functionalized nanocomposites prepared by these processes typically produce an uncontrolled structure and unpredictable material properties. It is, therefore, important to find methods of fabricating Fe3O4–PANI nanocomposites with desired shapes and high Fe3O4 contents. Electrostatic self-assembly has been shown to be a promising approach to produce large-scale periodic structures with the desired properties. This method permits the fabrication of thin-film assemblies on solid supports by spontaneous sequential adsorption of oppositely charged species from dilute aqueous solutions onto charged substrates (Caruso et al. 1998; Liu and Choi 2012).
In the present work, we describe a facile, general, eco-friendly, and effective approach to the fabrication of Fe3O4–PANI hollow sphere nanocomposites based on polyelectrolyte-modified PANI hollow spheres (PE@PANI). We then explore the potential application of the as-prepared Fe3O4–polyelectrolyte-modified PANI hollow sphere (Fe3O4–PE@PANI) nanocomposites as microwave absorbers. The morphology, structure, magnetic properties, and formation mechanism of the Fe3O4–PE@PANI nanocomposites were investigated in detail. To the best of our knowledge, this is the first report of Fe3O4–PE@PANI hollow sphere nanocomposites prepared by electrostatic self-assembly.
Analytical grade aniline (99 %), H2O2 (30 %), FeCl3·6H2O, FeCl2·4H2O, ammonia (28 %), and ethanol (99.5 %) were purchased from Wako Pure Chemical Industries, Ltd., Osaka, Japan. Analytical grade poly(sodium 4-styrenesulfonate) (PSS, average M w ~ 70,000) and poly(allylamine hydrochloride) (PAH, average M w ~ 56,000) were purchased from Sigma-Aldrich, St Louis, MO, USA. Deionized water was used in all the experiments. All chemicals were used without further purification.
Synthesis of Fe3O4 nanoparticles
The Fe3O4 magnetic nanoparticles were prepared using a modified co-precipitation method. The reaction was carried out in a 250-mL three-necked round-bottomed flask equipped with a stirrer. FeCl2·4H2O (0.994 g, 5 mmoL) in 10 mL of deionized water and FeCl3·6H2O (2.73 g, 10 mmoL) in 10 mL of deionized water were mixed by vigorous stirring, and the mixed solution was kept in a water bath at 80 °C. Preheated ammonia solution (1.5 M, 20 mL) was added rapidly to the solution, followed by dropwise addition of aqueous ammonia, with stirring, until the pH reached 10–12; stirring was then continued for 1 h. The Fe3O4 nanoparticles formed were collected by magnetic field separation, washed three times with deionized water, and dried under vacuum at 50 °C for 24 h.
Synthesis of Fe3O4–PE@PANI hollow sphere nanocomposites
PANI hollow spheres were synthesized according to the method reported in the literature (Zhang et al. 2009a, b ): 1 mmoL (0.1 mL) of aniline monomer was added to 40 mL of aqueous H3PO4 solution (0.4 M) at room temperature, with vigorous stirring, over several minutes, to form a uniform solution. Then 0.12 mL of H2O2 and 0.02 mL of FeCl3 (0.1 M) aqueous solution, in turn, were mixed with this solution. After fully mixing by stirring, the mixture was transferred to a Teflon-lined stainless-steel autoclave. The autoclave was sealed and maintained at 140 °C for a specific time. After immediately cooling to room temperature, the precipitate was filtered and washed several times with deionized water and ethanol, and dried under vacuum at 60 °C for 12 h.
The as-prepared PANI hollow spheres were immersed in 0.5 M NaCl aqueous solution (pH = 4) with ultrasonication for 20 min. PAH was then added to the mixture to give a final concentration of 1 mg mL−1. The PAH adsorption time was 20 min under ultrasonication. Excess PAH was removed by three centrifugation/washing/redispersion cycles. Negatively changed PSS was then deposited on the coated PANI hollow spheres using the same conditions and procedures. Finally, the polyelectrolyte-coated PANI hollow spheres (PE@PANI) were formed.
The Fe3O4–polyelectrolyte-modified PANI hollow sphere (Fe3O4–PE@PANI) nanocomposites were formed by adding 1 mg mL−1 of an Fe3O4 uniformly magnetic fluid to PE@PANI dispersed in deionized water under the adsorption conditions (pH = 4), allowing 20 min for Fe3O4 adsorption, and removing excess Fe3O4 by three centrifugation/washing/redispersion cycles. The precipitate was dried under vacuum at 60 °C for 24 h.
The zeta (ζ)-potentials of the different samples were measured using a Zetaplus analyzer (Nano ZS, Malvern Instruments Ltd., Malvern, UK) by an electrophoretic light-scattering method. The morphologies and microstructures of the products were characterized using field-emission scanning electron microscopy (FE-SEM; JEOL S-5000, JEOL Ltd., Tokyo, Japan) and transmission electron microscopy (TEM; JEOL JEM-2010) with an accelerating voltage of 200 kV. The crystal structures of the prepared powders were analyzed by X-ray diffraction (XRD; RINT 2550H diffractometer) using Cu Kα radiation. Fourier-transform infrared (FT-IR) spectra were obtained with a Shimadzu IR Prestige-21 spectrometer (Shimadzu, Kyoto, Japan) using KBr pellets. Thermogravimetric analysis (TGA; TG8120, Rigaku Denki, Tokyo, Japan) was carried out under an air atmosphere from room temperature to 600 °C at a heating rate of 5 °C min−1. X-ray photoelectron spectroscopy (XPS; Kratos AXIS Ultra DLD) was used to analyze the chemical compositions and chemical states of the samples. Magnetic measurements were carried out at room temperature using a vibrating sample magnetometer (TM-VSM5250, Japan) with a maximum magnetic field of 10 kOe.
Results and discussion
Design of synthesis strategy
Morphology and structure of Fe3O4–PE@PANI nanocomposites
Magnetic properties of Fe3O4–PE@PANI nanocomposites
Electromagnetic wave absorption properties of Fe3O4–PE@PANI nanocomposites
A facile approach to the synthesis of Fe3O4–PE@PANI nanocomposites was presented. A polyelectrolyte film was successfully coated onto PANI hollow sphere surfaces. Based on the strong electrostatic attractions between opposite charges and other interactions, paramagnetic Fe3O4 nanoparticles were assembled on the convex surfaces of PE@PANI, affording paramagnetic nanocomposites. This approach has the advantages of generality, reproducibility, controllability, tailorability, high loading capability, and stability, and is promising for producing a wide range of functional nanocomposites. The Fe3O4–PE@PANI nanocomposites exhibited excellent magnetic properties and produced magnetic resonance and loss in the nanocomposites. The reflection loss, calculated using the absorbing-wall theory, showed that the Fe3O4–PE@PANI nanocomposites exhibited better reflection loss abilities and wider response bandwidths than those of PANI hollow spheres in the range 0.5–15 GHz. In summary, the synthesized electromagnetically functionalized Fe3O4–PE@PANI nanocomposites are promising for applications in microwave-absorbing materials.
This work was partly supported by Grants for Excellent Graduate Schools, MEXT, Japan.
- Liu YD, Choi HJ (2012) Carbon nanotube-coated silicated soft magnetic carbonyl iron microspheres and their magnetorheology. J Appl Phys 111:07B502-1-07B502-3. doi: 10.1063/1.3670603
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