One-pot synthesis of polyaniline/Fe 3 O 4 nanocomposite in ionic liquid: electrical conductivity and magnetic studies

. One-pot synthesis of polyaniline/Fe 3 O 4 nanocomposite in 1-methyl-3-alkylcarboxylic acid imidazolium chloride ([CMMIm]Cl) ionic liquid (IL) was introduced for the first time in this work. Transmission electron microscopy (TEM), X-ray diffraction (XRD), four probes method and vibrating sample magnetometer (VSM) were used to explore the influence of IL on the structure, conductivity and magnetic properties of polyaniline/Fe 3 O 4 composite. Compared with Fe 3 O 4 particles prepared in water, the results show that Fe3O4 particles prepare in imidazolium-based ionic liquid were more regular in shape and dispersed uniformly. So the Fe 3 O 4 nanoparticles prepared in IL can easier serve as cores to form the polyaniline/Fe 3 O 4 nanocomposite via in situ chemical oxidative polymerization of aniline molecule. The saturation magnetization of polyaniline/Fe 3 O 4 nanocomposite prepared in ionic liquid shows about 2 times higher than polyaniline/Fe 3 O 4 composite prepared in water. And the conductivities of PANI/Fe 3 O 4 composite prepared in IL decreased and the saturated magnetization increased with the increasing amount of Fe 3 O 4 .


Introduction
Owing to the unique electronic and magnetic properties of electromagnetic functionalized conducting polymers, they have recently received more and more attention for kinds of applications, such as microwave absorption [1], electromagnetic interference [2], catalytic materials [3],and sensor [4].Among the known conducting polymers, polyaniline (PANI) has been extensively studied due to its unique proton doping ability, low cost, thermal and chemical stability [5,6]. Combining magnetic nanoparticles (Fe 3 O 4 ), which contain ferromagnetic and supermagnetic properties, the polyaniline/Fe 3 O 4 (PANI/Fe 3 O 4 ) nanocomposite has been studied in various fields [7,8].
Usually, the traditional methods of synthesizing electromagnetic composite contain two steps. First, magnetic nanoparticles are prepared in aqueous solution and dried for further use. Second, electromagnetic nanocomposite is prepared via in situ polymerization of aniline with magnetic nanoparticles [9,10]. Recently, ionic liquid (IL) has introduced to prepare electromagnetic materials due to its excellent thermal stability, good electrical conductivity [11,12]. However, the magnetic nanoparticles, which dried at first step are hard to disperse in IL again and easy to agglomerate [13].
In this work, we introduced one-pot reaction method to synthesize PANI/Fe 3 O 4 nanocomposite directly. Firstly, Fe 3 O 4 nanoparticles were synthesized in IL, and then, after adding aniline in IL, the polymerization carried out without any other treatment process. This method prevents the agglomeration of Fe 3 O 4 nanoparticles effectively and enhances the saturation magnetization of PANI/Fe 3 O 4 nanocomposite. The influence of IL on the structure, electrical conductivity and magnetic properties were discussed in detail. This novel method was found to be a straightforward, fast way to obtain conducting PANI coated magnetic Fe3O4 electromagnetic composite.

Experimental
Aniline monomer was purified by distilled under reduced pressure prior to use. The other chemicals and reagents were used as received from commercial sources without further purification.

Preparation of ionic liquid
1-methyl-3-alkylcarboxylic acid imidazolium chloride ([CMMIm]Cl) ionic liquid was prepared according to the previous report [14]. 0.17 mol chloroacetic acid solid was added to 0.15 mol N-Methylimidazole liquid with stirring until the mixed solution became uniformly. Then the solution was heated at 70 ℃ for 3-5 h. Finally, the sample was dried in the oven at 40℃ for 12 h to yield the white solid of [CMMIm]Cl.

Preparation of PANI/Fe3O4
The PANI/Fe 3 O 4 nanocomposite was synthesized by one-pot reaction. Firstly, Fe 3 O 4 nanoparticles were prepared by co-precipitation method with IL as synthetic environment. FeCl 3 •6H 2 O and the according amounts of FeCl 2 •4H 2 O (Table 1) were dissolved in distilled water with vigorous stirring under nitrogen protection. 5.0 g of IL was added to the mixture above. The molar ratio of FeCl 3 •6H 2 O to FeCl 2 •4H 2 O was 3:2. Then the solution was heated to 90℃ and the pH value of the reaction mixture was kept in the range of 10-11 with addition of NaOH. As soon as the reaction started, black precipitate appeared immediately. The solution was kept under stirring for 1 h. Then the reaction mixtures were cooled to room temperature under ultrasonic treatment. Secondly, The PANI/Fe 3 O 4 nanocomposites were prepared by in situ chemical oxidative polymerization of aniline with Fe 3 O 4 . After adding 10 mmol of aniline, 10 mmol sulfamic acid (SA) and 30 ml HCl (1 M) to the above mixture under vigorous stirring, 10 ml 7.5 mmol ammonium persulphate (APS), which dissolved in 1M HCl solution, was added dropwise over 30 min (Table 1). Then, the reaction mixture was stirred for 2 h. The produced blackish green precipitate was filtered, washed with distilled water till colorless filtrate was obtained and then dried at 50℃ for 24 h under vacuum.
The PANI/Fe 3 O 4 composite with ionic liquid was denoted as PF-I. For comparison, we also prepared PANI/Fe 3 O 4 sample using the same condition like sample 3 just without IL (PF) ( Table 1)

Sample characterization
The powder XRD patterns were recorded on an X-ray diffractometer (Panalytical X'Pert PRO) equipped with CuKα as radiation source. Transmission electron microscope (TEM) analysis was carried out on a Tecnai G2 F30 microscope, working at 100 KV. A vibrating sample magnetometer (VSM, model Lake Shore, new 7304 series) was used to investigate the magnetic properties of the samples at room temperature. Four probes method was used to measure the conductivity of the obtained sample by a four-probe resistivity/square resistance tester (Kund Technology Co. Ltd., Guangzhou, China), the test samples were prepared in pellet form (diameter: 13mm, thickness: 0.6mm).

Results and Discussion
The morphology of Fe 3 O 4 and PANI/Fe 3 O 4 composite which prepared in water and IL were investigated (Fig.  1). It is obviously to see that, the size distribution of Fe 3 O 4 prepared in water (Fig. 1a) is about 4-17 nm with aggregation. On the other hand, for PF (Fig 1b), Fe 3 O 4 prepared in water is hard to be dispersed in polymeric matrix evenly. However, by introducing IL in the synthesis process, large-scale Fe 3 O 4 nanoparticles with a relatively uniform size of 5-11 nm were obtained (Fig.  1c). And the Fe 3 O 4 nanoparticles are nearly spherical. Compared with PF, the Fe 3 O 4 nanoparticles prepared in IL (Fig. 1d) are dispersed in polymeric matrix evenly.
And PANI is well coated or grown on the surface of   511) and (440) reflections. The crystallite size for the most intense peak (311) at 2θ=35.40° was calculated from the XRD data using the Scherer formula and found to be 11 nm. The crystallite size is consistent with the result obtained from TEM. The broad peak at 2θ=20-30° is ascribed to the polymer chains of PANI, suggesting that PANI is almost amorphous [15]. It is obviously to see that Fe 3 O 4 retained its cubic structure on dispersion in PANI matrix during in situ polymerization reaction. On the other hand, it also conformed the formation of PANI/Fe 3 O 4 composite.  Fig. 3a shows the room temperature conductivity of PF-I series. The conductivity of pure PANI prepared in IL is 3.3 S/cm. As we know, with the adding of Fe 3 O 4 particles, the conductivity of PANI/Fe 3 O 4 composite decreases dramatically [9].The decreased electrical conductivity is mainly due to the presence of semiconducting Fe 3 O 4 nanoparticles that lowers conductivity of PANI/Fe 3 O 4 composite and partially baffle the formation of conductivity path .To figure out the influence of IL on materials electromagnetic property, we also prepared PF sample using the same condition like sample 3 just without IL (Table 1). Compared with PF-I, the conductivity of PF changed randomly from 2.0S/cm to 0.1S/cm at different points. It's perhaps due to two reasons. First, the existence of IL reduced the intercontact resistance which caused by PANI conducting nanofibers enwinding around the Fe 3 O 4 magnet.Second,Fe 3 O 4 nanoparticles which were synthesized in IL directly are much easier to be incorporated into PANI matrix during one-pot in situ chemical oxidative polymerization of aniline than traditional methods. The new methods results the uniform dispersal of Fe 3 O 4 nanoparticles in PANI, which is consistent with the result obtained from TEM. Fig. 3b shows the magnetic properties of PF-I, PF and Fe 3 O 4 prepared in IL. It can be seen that the saturation magnetization (Ms) of PF-I are less than that of pure Fe 3 O 4 (54.08 emu/g). The Ms values of PF-I decreased with the decreasing of Fe 3 O 4 nanoparticles. It due to the fact that conducting PANI is not magnetic, and the magnetic properties of nanocomposite are strongly dependent on the contents of Fe 3 O 4 nanoparticles. However, the Ms values of PF-I (sample 3, 18.11 emu/g) is much higher than that of PF (8.04 emu/g, Fig. 3b, inset) which prepared at the same condition just without IL. This can be attributed to the fact that Fe3O4 nanoparticles were easier aggregation without IL. In conclusion, electromagnetic Fe 3 O 4 /PANI nanocomposite was successfully synthesized via the in situ polymerization using IL as synthetic environment by one-pot method. For PF-I, structure analysis showed that the ferrite nanoparticles were incorporated in the polyaniline matrix evenly to form a core-shell structure. The introduction of IL in the composite synthesis process not only helped to form uniform structure but also helped to enhance electromagnetic property of composite. It perhaps provide a new strategy to fabricate functionalized nanocomposite easily