In situ preparation of monodispersed Ag/polyaniline/Fe3O4 nanoparticles via heterogeneous nucleation
© Bian et al.; licensee Springer. 2013
Received: 27 May 2013
Accepted: 26 June 2013
Published: 3 July 2013
Acrylic acid and styrene were polymerized onto monodispersed Fe3O4 nanoparticles using a grafting copolymerization method. Aniline molecules were then bonded onto the Fe3O4 nanoparticles by electrostatic self-assembly and further polymerized to obtain uniform polyaniline/Fe3O4 (PANI/Fe3O4) nanoparticles (approximately 35 nm). Finally, monodispersed Ag/PANI/Fe3O4 nanoparticles were prepared by an in situ reduction reaction between emeraldine PANI and silver nitrate. Fourier transform infrared and UV-visible spectrometers and a transmission electron microscope were used to characterize both the chemical structure and the morphology of the resulting nanoparticles.
Researches regarding polymer-metal and polymer-inorganic multicomponent hybrid composites such as polyaniline/silver (PANI/Ag), poly(ethylene oxide)/aurum (PEO/Au), PANI/Fe3O4, etc. have attracted much attention during the last two decades due to their large potential applications in the fields of electromagnetic interference (EMI) shielding [1–3], energy storage devices [4–6], catalysis [7–9], and sensors [10–14]. These hybrid composites show better chemical and physical properties than bulk materials.
Among various polymers, PANI as a versatile conducting polymer is usually selected to compound with noble metals or inorganic particles owing to its easy preparation, anticorrosion, and the low cost of raw material. Recently, Kamchi et al.  have elaborated serials of camphor-doped PANI/FeNi nanoparticle-based EMI shielding composites. The maximum conductivity value of 104 S m-1 and the shielding effectiveness (SE) of 90 dB of the prepared multilayer composites have been detected over the frequency band of 8 to 18 GHz. Leyva et al.  have successfully synthesized kinds of hybrid organic/inorganic PANI-Ag powders via in situ chemical oxidation of PANI-emeraldine base by the capped Ag+ on the PANI-emeraldine base surface. The electrical direct current conductivity of the resulting PANI-Ag reaches 3.5 × 103 S m-1 at room temperature, showing a good conductivity. Moreover, Shukla et al.  have also prepared homogeneous PANI-Ag core-shell nanorods synthesized via a mild photolysis-initiated ultraviolet radiation. The core-shell nanorods display a strong blueshift in the UV-visible (UV–vis) absorption spectrum and have instant application as a highly sensitive hydrazine and hydrogen peroxide sensor. However, the EMI shielding properties have not been studied. In addition, relevant PANI-based nanowires, nanorods, and core-shell nanoparticle EMI composites have been successfully prepared elsewhere [17–20]. Actually, many researches have been done to improve both the EMI SE and the cost performance by enhancing the conductivity and lowering the magnetic loss. Unfortunately, most of the developed hybrid EMI shielding materials are binary composites comprised of polymer/metal, polymer/inorganic, or metal/inorganic. These materials still suffer disadvantages of low EMI SE, limited shielding frequency range, high density, and high cost. Furthermore, from the angle of the crystal growth dynamics, most of the developed binary composites are simple blends or epitaxial blends. In the in situ preparation process of the second layer of the binary hybrid nanoparticles or nanocomposites, an obvious contradiction between the formation of more homogeneous nucleations and the heterogeneous nucleation and epitaxial growth of the second layer should be firstly solved. Usually, the formation of more homogeneous nucleations implies the formation of more separated nanoparticles, i.e., low efficiency to obtain the monodispersed binary nanoparticles. A few papers report the synthesis method to prepare monodispersed binary nanoparticles such as Ag or Au/Fe3O4 nanoparticles [21–25]. Supermagnetic and conductive properties of the performed monodispersed nanoparticles have also been particularly studied. However, these methods are only facilitated to prepare metal/inorganic binary nanoparticles.
To the best of our knowledge, almost no papers regarding synthesis and EMI shielding application of monodispersed multilayer nanoparticles have been reported. Consequently, studies about the synthesis and the application evaluation of PANI multicomponent nanocomposites such as PANI/metal/inorganic, metal/PANI/inorganic, or metal/inorganic/PANI ternary nanocomposites, especially the monodispersed nanocomposites, are necessary since noble metals, e.g., Au and Ag, usually own high electronic conductivity and PANI possesses both a low density and a considerable conductivity. To achieve this aim, the following two points should be considered prior to the preparation: (1) Mild reaction conditions are necessary to obtain the monodispersed nanoparticles. (2) A suitable solvent (polar or non-polar) is also an important factor to both the nucleation and the growth of the nanoparticles. Herein, we have prepared a monodispersed Ag/PANI/Fe3O4 ternary nanoparticle via a typical grafting copolymerization, an electrostatic self-assembly, and an in situ reduction of Ag+ on the surface of the PANI-emeraldine base polymeric chains.
Fourier transform infrared (FTIR, Nicolet 560, Nicolet Instruments, Inc., Madison, WI, USA) and UV–vis (Shimadzu UV-2100, Shimadzu Corporation, Kyoto, Japan) spectrometers have been used to monitor the preparation process of the nanoparticles. The morphology of the prepared PANI/Fe3O4 binary nanoparticles and Ag/PANI/Fe3O4 ternary nanoparticles has also been extensively evaluated using a JEOL JEM-2100 electron microscope (JEOL Ltd., Akishima-shi, Japan) operating at an accelerating voltage of 200 kV.
Results and discussion
Figure 4c,d shows the morphology of the Ag/PANI/Fe3O4 nanoparticles at different TEM views. In the case of Figure 4c, many gray, even dark, pre-spheral particles with a size range of 30 to 50 nm are detected. The color of the nanoparticles is apparently darker than that of PANI/Fe3O4 nanoparticles, demonstrating the possible formation of Ag/PANI/Fe3O4 nanoparticles. The TEM morphology of the Ag/PANI/Fe3O4 nanoparticles at another view (different district) can be also used to confirm this assumption even if the background of the TEM graph is coarse (see Figure 4d) because the color of the observed nanoparticles is almost dark, originating from the existence of heavy metal Ag. Figure 4d also reveals that the obtained Ag/PANI/Fe3O4 nanoparticles are still monodisperse and that the distance between two particles further increases in comparison with the PANI/Fe3O4 nanoparticles. Furthermore, a high-resolution TEM (HR-TEM) technique is also performed, and the HR-TEM images are shown on the right top inset of Figure 4c,d. As can be seen from the HR-TEM images, obvious lattices originating from Ag are observed. In the lattice structures, the d-space of the (111) lattice is about 0.24 nm, which is the characteristic of Ag [22–24]. In addition, the HR-TEM images show that there are transitional layers between the lattice fringes of Ag and the PANI/Fe3O4 nanoparticles. Ag+ in the silver nitrate solution can be in situ reduced to gradually form Ag particles and coat onto PANI layers by interacting with the N atoms of emeraldine PANI polymer chains, while separated Ag particles cannot be observed at all due to the lack of an effective reducer in the solution, i.e., the homogeneous nucleation of Ag particles is thoroughly restrained. This is the reason why the monodispersed Ag/PANI/Fe3O4 nanoparticles can be obtained by the mild reduction reaction.
In summary, monodispersed Ag/PANI/Fe3O4 ternary nanoparticles with an average size of approximately 50 nm can be successfully obtained by incorporating grafting copolymerization, electrostatic self-assembly, and mild reduction reaction method between the N atoms of PANI chains and the silver cations of silver nitrate solution. The control of heterogeneous nucleation and corresponding epitaxial growth of both PANI and Ag is crucial to prepare monodispersed Ag/PANI/Fe3O4 nanoparticles. The obtained monodispersed Ag/PANI/Fe3O4 nanoparticles have large potential applications in the fields of EMI shielding materials, biology, catalysis, etc.
This research is supported by the National Natural Science Foundation of China under grant no. 21204076/B040307.
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