Fabrication of electromagnetic Fe₃O₄@polyaniline nanofibers with high aspect ratio

Abstract

Electromagnetic Fe₃O₄@polyaniline (PANI) nanofibers with high aspect ratio were realized by combining magnetic-field-induced (MFI) self-assembly and in situ surface polymerization of aniline. The key to fabrication is to induce chaining of the amino-Fe₃O₄ microspheres during the PANI coating process, which allows additional deposited PANI to warp entire chains into nanofibers. Hydrogen bonds are thought to be the driving force that makes aniline form a PANI coating shell instead of irregular sheets. Compared to the Fe₃O₄@PANI microspheres, higher magnetization saturation value and conductivity value were achieved in the Fe₃O₄@PANI nanofibers, which hold high promise for potential applications in microwave absorbents, electromagnetic shielding coatings, and other usage associated with conventional electromagnetic techniques.

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Introduction

Electromagnetic materials demonstrate simultaneous conductive and magnetic characteristics, which are extremely useful in batteries, electrochemical displays, sensors, microwave absorbing applications, and electromagnetic shielding.^1–3 Among the large family of conductive polymers, polyaniline (PANI) is a remarkable polymer which is relatively easy to process and has simple, reversible acid/base doping/dedoping chemistry.^4–7 Regarding magnetic materials, Fe₃O₄ microspheres show unusual phenomena such as controllable sizes,^8,9 fast magnetic response,^10–12 and desirable surface properties.^13,14 Recently, to realize the full potential of practical applications, composites of conductive polymers PANI containing magnetic Fe₃O₄ microspheres have attracted much attention, because the resultant composites may concurrently possess good magnetic and electrical properties. Until now, many efforts have been made to prepare electromagnetic Fe₃O₄–PANI composites. For example, superparamagnetic Fe₃O₄@PANI core@shell microspheres with well-defined blackberry-like morphology were synthesized via a simple in situ surface polymerization method, and these unique core@shell spherical materials could find applications in catalyst supports or biomedical areas.¹⁵ Fe₃O₄ nanocrystal-embedded PANI hybrids with well-defined cluster-like morphology were prepared through macromolecule-induced self-assembly, which showed flow ability at room temperature in the absence of any solvent and offered great potential in applications such as microwave absorbents and electromagnetic shielding coatings.¹⁶ Composites consisting of Fe₃O₄ microspheres and PANI have been successfully prepared through a two-step oxidative polymerization of aniline monomers in the presence of Fe₃O₄ microspheres. It could be found that embedding Fe₃O₄ microspheres in the polymer matrixes would not only modulate the complex permittivity, but also produce magnetic resonance and loss.¹⁷ However, one can notice that the morphologies of these existing samples either stacked disorderly or isolated in the form of core@shell structures.^2,18,19 The anisotropic Fe₃O₄@PANI composites have not been reported yet, and its physical properties, such as the magnetic and conductive properties, have not been sufficiently explored.

In general, morphology is the crucial factor in controlling the practical properties and applications of macroscopic materials.²⁰ For example, charge transport can be efficiently carried out by the aligned/oriented arrays of one dimensional PANI nanostructures such as wires, tubes, and fibers, because these PANI nanostructures presented the smallest dimensional structures.²¹ Therefore, the Fe₃O₄@PANI nanofibers are expected to yield better performances in these already established areas and even create new properties. To date, two basic strategies have been proposed to obtain one dimensional (1D) magnetic nanostructures, including the use of nanostructured templates^22–25 and a controlled assembly process.^26–30 Template-assisted method is a controlled approach for synthesis of 1D nanostructures. But the demanding templates and the tedious post-treatment to remove the templates not only result in a complex preparation process, but also may destroy the nanostructures. For the latter, MFI assembly represents a versatile method to fabricate magnetic chainlike structures.^31,32 However, owing to the weak or negligible anisotropic dipolar interaction between the magnetic building blocks, these ordered structures can hardly be maintained after removal of the external field. In this regard, inorganic materials such as silica^33,34 and titanium dioxide³⁵ or polymers^36–38 have been utilized to stabilize 1D magnetic chains, but the electromagnetic 1D nanostructures have not been successfully prepared. Therefore, the development of a facile preparation method for electromagnetic 1D Fe₃O₄@PANI nanofibers is attractive and emergent.

Herein, we presented a facile approach to obtain electromagnetic Fe₃O₄@PANI nanofibers with high aspect ratio, that is, in situ surface polymerization of aniline under MFI self-assembly. Compared to the Fe₃O₄@PANI microspheres, Fe₃O₄@PANI nanofibers demonstrate higher magnetization saturation value and conductivity value, which offer great potential in applications, such as microwave absorbents and electromagnetic shielding coatings.

Experimental section

Materials

Aniline (Hongyan Chemical Reagent Co., Ltd.) was purified by distilling at a reduced pressure and stored in a refrigerator. Ferric chloride hexahydrate (FeCl₃·6H₂O; Hongyan Chemical Reagent Co., Ltd.), ethylene glycol (EG; Jinshan Chemical Reagent Co., Ltd.), sodium acetate (NaAc; Jinhua Chemical Reagent Co., Ltd.), polyethylene glycol 4000 (PEG4000; Kemiou Chemical Reagent Co., Ltd.), absolute ethanol (Fuyu Chemical Reagent Co., Ltd.), 3-aminopropyltriethoxysilane (APTES; Chenguang Chemical Reagent Co., Ltd.), ammonium persulfate (APS; Hongyan Chemical Reagent Co., Ltd.), ammonium hydroxide (NH₃·H₂O; Sanpu Chemical Reagent Co., Ltd.), and hydrochloric acid (HCl; Beijing Chemical Reagent Co., Ltd.) were analytical grade reagents and used without further purification. Deionized water was used through all the synthetic process.

Synthesis of Fe₃O₄ microspheres

Fe₃O₄ microspheres were prepared through a solvothermal reaction. Briefly, FeCl₃·6H₂O (2.7 g) was dissolved in EG (80 ml) to form a clear solution, followed by the addition of NaAc (7.2 g) and PEG4000 (2.0 g). The mixture was stirred vigorously for 30 min and then sealed in a Teflon-lined stainless-steel autoclave (100 ml). The autoclave was heated to and maintained at 200 °C for 8 h, then cooled to room temperature. The black Fe₃O₄ microspheres were washed several times with ethanol and deionized water, then dried at 60 °C for 12 h.

Modification of Fe₃O₄ microspheres

The amino-Fe₃O₄ microspheres were prepared through the hydrolysis of APTES. Typically, Fe₃O₄ (100 mg) and APTES (4.0 ml) were dispersed in a mixed solution of ethanol (40 ml) and deionized water (40 ml). NH₃·H₂O (1.0 ml) as catalyst was added into the solution. Then, the solution was refluxed for 24 h at 70 °C. The amino-Fe₃O₄ microspheres were obtained.

Synthesis of electromagnetic Fe₃O₄@PANI nanofibers

Electromagnetic Fe₃O₄@PANI nanofibers were prepared via in situ surface polymerization under MFI self-assembly. In a representative procedure, the amino-Fe₃O₄ microspheres (10 mg) and purified aniline (0.10 ml) were dissolved in a 90 ml hydrochloric acid solution (0.010 mol l⁻¹) to form a homogeneous solution by using ultrasonic (KQ-300DE, Kun Shan Ultrasonic Instruments Co., Ltd; frequency: 40 kHz; out power: 50 W) for half an hour. The solution was poured into a flask and a plane magnet (0.50 T) was placed to the side of the vessel with a distance of about 4 cm. 10 ml hydrochloric acid solution (0.010 mol l⁻¹) containing APS (54.7 mg) was quickly injected into the above solution to initiate the polymerization. The polymerization was performed under static condition at 20 °C for 10 h. The resulting sediment was washed with ethanol and deionized water several times and then obtained through magnetic separation.

Characterization

Field emission scanning electron microscopy (FE-SEM) images were generated with a JEOL JSM-6700F microscope. Samples dispersed in ethanol were deposited onto silicon wafers and sputtered with platinum by a JFC-1600 auto fine coater at a current of 20 mA for 300 s before observation. Transmission electron microscopy (TEM) images were taken on a JEOL JEM-3010 microscope with Oxford 794-CCD camera at an accelerating voltage of 200 kV. Samples suspended in ethanol were dropped onto copper grids coated with carbon support film. The magnetite content of samples was determined through HENVEN thermogravimetric analysis (TGA) in the temperature range from 20 °C to 800 °C with a heating rate of 10 °C min⁻¹. Magnetic property of samples was acquired by using a Quantum Design VersaLab vibrating sample magnetometer (VSM). The conductivity value of the samples, which were pressed into pellets in advance, was received by a RTS-9 four probes measurement.

Results and discussion

Synthesis of electromagnetic Fe₃O₄@PANI nanofibers

The schematic illustration of fabrication process of Fe₃O₄@PANI nanofibers is displayed in Fig. 1. Fe₃O₄ microspheres were firstly hydrolytically treated by APTES to immobilize the amino groups onto their surface. These prepared amino-Fe₃O₄ microspheres were subsequently coated with a layer of aniline, which were then assembled into chains due to the effect of the magnetic field. The formed chains were further coated with an additional PANI coating shell and stabilized as the polymerization of aniline proceeded. Upon polymerization and condensation, PANI deposited on the surface of the amino-Fe₃O₄ microspheres, thus gradually improving their hydrophobicity in the low concentration hydrochloric acid solution.^39–41 Continued deposition of PANI covered the entire surface of each chain, producing Fe₃O₄@PANI nanofibers with further increased mechanical stability. The resultant Fe₃O₄@PANI nanofibers were found on the bottom of the reactor due to gravity and robust hydrophobicity. The chainlike morphology and core@shell structures of the final Fe₃O₄@PANI nanofibers can be readily demonstrated by employing SEM and TEM (in Fig. 2).

Fig. 1 Schematic illustration of fabrication process of electromagnetic Fe₃O₄@PANI nanofibers.

Fig. 2 (a) SEM image of Fe₃O₄ microspheres (inset image: TEM image of Fe₃O₄ microspheres); (b) and (c) SEM and (d) TEM images of Fe₃O₄@PANI nanofibers (inset image in (d): TEM image of Fe₃O₄@PANI nanofibers at low magnification).

The SEM image (Fig. 2(a)) demonstrates that Fe₃O₄ microspheres synthesized using solvothermal method⁴² are monodisperse, with an average diameter of 500 nm, which is in accordance with that in TEM image (inset image in Fig. 2(a)). Fig. 2(b) and (c) show typical SEM images of Fe₃O₄@PANI nanofibers at different magnifications. These images display that most of Fe₃O₄@PANI nanofibers have chainlike morphology rather than isolated Fe₃O₄@PANI microspheres, and their length spans from tens to hundreds of microns. The core@shell structure of Fe₃O₄@PANI nanofibers was further investigated by TEM. As illustrated in Fig. 2(d), the periodically arranged black Fe₃O₄ microspheres form chains by head-to-tail interactions in a continuous rough gray PANI shell. The thickness of gray PANI coating shell is estimated to be 50–130 nm. Note that Fe₃O₄ microspheres are linear (inset image in Fig. 2(d)), suggesting they were pre-organized to form chainlike nanostructures under MFI self-assembly before in situ surface polymerization of aniline.^33,34,37 The pre-organized chains of Fe₃O₄ microspheres act as templates and the PANI coating shell is prone to encircle the chains, resulting in the formation of Fe₃O₄@PANI nanofibers.

Effects of MFI self-assembly

To illuminate the importance of MFI self-assembly, we carried out a comparative experiment in the absence of the external magnetic field. In Fig. 3(a) and (b), no Fe₃O₄@PANI nanofibers and only unordered Fe₃O₄@PANI microspheres are observed. It should be aware of that the pre-organization of the amino-Fe₃O₄ microspheres via MFI self-assembly is crucial to the formation of Fe₃O₄@PANI nanofibers before coating with PANI. The core@shell structures can be better observed in the inset TEM image of Fig. 3(b). Comparing with the Fe₃O₄@PANI nanofibers (Fig. 2(d)), they have a thicker and rougher PANI coating shell with a thickness of 50–260 nm. It is easily understood that the weight of a single Fe₃O₄@PANI microsphere is lighter than that of a Fe₃O₄@PANI nanofiber. Thus, when MFI self-assembly is absent, a single Fe₃O₄ microsphere is able to get more PANI on its surface due to it is relatively hard to precipitate from the solution. During the formation of thicker PANI coating shell, to minimize the interfacial energy between the solution and Fe₃O₄@PANI microspheres, a lot of additional nucleation centers of PANI take place on the as-synthesized PANI coating shell,^43–47 which is followed by the rapid precipitation of PANI in a disordered manner that yields the rougher PANI coating shell. From Fig. 3, it is clear to see that MFI self-assembly is a vital procedure for preparing Fe₃O₄@PANI nanofibers.

Fig. 3 (a) and (b) SEM images of Fe₃O₄@PANI microspheres obtained without the external magnetic field at different magnifications (inset image in (b): TEM image of Fe₃O₄@PANI microspheres).

Effects of hydrogen bonds

During the preparation of Fe₃O₄@PANI nanofibers, two difficulties must be overcome. The first is how to induce chaining of the amino-Fe₃O₄ microspheres during PANI coating process, which have been realized by MFI self-assembly. The second is that how to improve contact between Fe₃O₄ microspheres and aniline. To overcome the intrinsic hydrophilicity of the Fe₃O₄ microspheres and to favor the growth of PANI on their surface, a coupling agent is needed.¹⁵ Therefore, the periphery of Fe₃O₄ microspheres was modified with amino groups to improve their affinity to PANI. This procedure is thought to be important for the formation of Fe₃O₄@PANI nanofibers, which can be verified from the Fig. 4(a) and (b). The SEM image (Fig. 4(a)) exhibit the sample synthesized without using APTES to modify Fe₃O₄ microspheres in the absence of the external magnetic field, in which white Fe₃O₄ microspheres and gray PANI sheets are seen and no Fe₃O₄@PANI nanofibers are found. For the sample in Fig. 4(b), even when the external magnetic field is used, the resultant Fe₃O₄@PANI nanofibers cannot be got, and only some short chains of Fe₃O₄ microspheres (the marked circles in Fig. 4(b)) are found deriving from the weak dipolar interaction between the Fe₃O₄ microspheres under the external magnetic field. From the morphology in Fig. 4(a) and (b), we know, when the Fe₃O₄ microspheres don't have amino groups on their periphery, the Fe₃O₄@PANI nanofibers and even the Fe₃O₄@PANI microspheres cannot be obtained.

Fig. 4 (a) SEM image of samples obtained with no using APTES to modify Fe₃O₄ microspheres in the absence of the external magnetic field; (b) SEM image of samples obtained as with (a) but in the presence of the external magnetic field; (c) schematic illustration of interaction between the amino-Fe₃O₄ microspheres and the amino groups of aniline and PANI.

According to the distinct morphology in Fig. 4(a) and (b), it is obvious that the amino groups on the periphery of Fe₃O₄ microspheres play a decisive role in realizing in situ surface polymerization of aniline. On the basis of the importance of amino groups, it is reasonable to expect that the hydrogen bonds,^48–51 which are formed spontaneously between the amino-Fe₃O₄ and aniline, should be the driving force for the formation of PANI coating shell and that is why the PANI coating shell cannot be prepared once Fe₃O₄ microspheres are not modified with APTES. The interaction between the amino-Fe₃O₄ microspheres and the amino groups of aniline or PANI is schematically illustrated in Fig. 4(c). As shown in Fig. 4(c), these amino-Fe₃O₄ microspheres can adsorb a layer of aniline onto their surface through hydrogen bonds. After the APS solution is added, a few aniline radical cations generate and oxidize the aniline to form PANI. Since the oxidative polymerization of aniline mainly takes place at the surface of the amino-Fe₃O₄ microspheres due to hydrogen bonds, in situ surface polymerization of aniline happens. As the polymerization proceeds, PANI coating shells are formed, leading to the final core@shell structures. Therefore, hydrogen bonds between the amino-Fe₃O₄ microspheres and aniline are regarded to be responsible for the formation of PANI coating shell.

Magnetic properties of electromagnetic Fe₃O₄@PANI nanofibers

The weight loss of Fe₃O₄ microspheres, Fe₃O₄@PANI nanofibers, and Fe₃O₄@PANI microspheres in TGA data (Fig. 5(a)) is 3.7, 13.0, and 15.0%, respectively. For Fe₃O₄ microspheres, weight loss at 20–300 °C is contributed from residual water and surfactant PEG4000 which are against particle agglomeration during the formation of Fe₃O₄ microspheres. A sharp weight loss of Fe₃O₄@PANI nanofibers and Fe₃O₄@PANI microspheres at 200–500 °C is mainly ascribed to the thermal degradation of PANI coating shell, consistent with the results reported by others.^15,16,19 In Fig. 5(b), zero coercivity in VSM data indicates that all three samples have superparamagnetic property. The magnetization saturation value of Fe₃O₄ microspheres, Fe₃O₄@PANI nanofibers, and Fe₃O₄@PANI microspheres is estimated to 78.01, 33.18, and 24.33 emu g⁻¹, respectively. The gradually reduced magnetization saturation values are rationalized on two reasons: one is that some part of Fe₃O₄ microspheres are oxidized when they experienced the APTES modification at high temperature (70 °C); the other is that the magnetic content of Fe₃O₄@PANI nanofibers and Fe₃O₄@PANI microspheres is lower than that of the Fe₃O₄ microspheres. Particular attention should be paid to the fact that the Fe₃O₄ content of Fe₃O₄@PANI nanofibers is almost the same as the Fe₃O₄@PANI microspheres (TGA data in Fig. 5(a)). Whereas the magnetization saturation value of Fe₃O₄@PANI nanofibers increases 36.37% relative to Fe₃O₄@PANI microspheres. The increased magnetization saturation value is probably attributed to the ordered dipole moments in the 1D chainlike morphology. Thus, we think Fe₃O₄@PANI nanofibers are a promising candidate for maintaining the magnetization saturation value.

Fig. 5 (a) TGA and (b) VSM (at room temperature) of Fe₃O₄ microspheres, Fe₃O₄@PANI nanofibers, and Fe₃O₄@PANI microspheres. Optical microscope images of Fe₃O₄@PANI nanofibers dispersed in water encapsulated between two glass slides (c) in the absence of the external magnetic field, (d) in the presence of the external magnetic field (0.50 T), scale bar is 60 μm.

The superparamagnetic property and the high magnetization saturation value of Fe₃O₄@PANI nanofibers offer easy magnetic manipulation. The high magnetic sensitivity and effortless manipulation of Fe₃O₄@PANI nanofibers in water solution were proved through optical microscopy (Fig. 5(c) and (d)). In Fig. 5(c), in the absence of the external magnetic field, the Fe₃O₄@PANI nanofibers disperse well in water, showing various chain lengths due to their random orientations. When the external magnetic field is applied, nearly all of the Fe₃O₄@PANI nanofibers are aligned along the magnetic force line of the external magnetic field (Fig. 5(d)). The new nanofibers appear significantly longer than the individual nanofibers, implying additional chaining of the Fe₃O₄@PANI nanofibers owing to the induced magnetic inter-nanofiber attraction.⁵²

Electrical properties of electromagnetic Fe₃O₄@PANI nanofibers

The conductivity value of Fe₃O₄@PANI microspheres and Fe₃O₄@PANI nanofibers was also investigated to elucidate the influence of the ordered 1D nanostructures on electrical property. The conductivity value of Fe₃O₄@PANI nanofibers and microspheres is 4.42 × 10⁻⁵ and 2.13 × 10⁻⁶ S cm⁻¹, respectively. The conductivity value of the sample is very low, as expected, since washing the samples with deionized water removes the unreacted monomers and oligomers, and it undopes the PANI since the PH of deionized water is around 7.⁵³ From TGA data (Fig. 5(a)), the PANI content of Fe₃O₄@PANI nanofibers is slightly less than that of Fe₃O₄@PANI microspheres. Remarkably, the conductivity value of Fe₃O₄@PANI nanofibers has one order of magnitude higher than that of Fe₃O₄@PANI microspheres. This dramatically increased conductivity value is deemed to result from the effect of 1D chainlike morphology. The charge transport of Fe₃O₄@PANI microspheres and Fe₃O₄@PANI nanofibers is illustrated in Fig. 6. As indicated by arrows, to make charge transport come true, there must be accessible paths to transfer charge carriers. For Fe₃O₄@PANI microspheres, so many interspaces are among them, which cut paths off and lead to impossible electrons transferring. But for Fe₃O₄@PANI nanofibers, the situation is totally different. Some accessible paths can be easily formed via the accumulation of the nanofibers with high aspect ratio, which are greatly beneficial for charge transport. Hence, the Fe₃O₄@PANI nanofibers with high aspect ratio have a significant effect on the enhancement of conductivity value.

Fig. 6 Schematic illustration of the charge transport of Fe₃O₄@PANI microspheres and Fe₃O₄@PANI nanofibers.

Conclusions

In summary, the electromagnetic Fe₃O₄@PANI nanofibers with high aspect ratio were successfully prepared via a simple and practical approach. During the fabrication of Fe₃O₄@PANI nanofibers, the amino-Fe₃O₄ microspheres were induced to form chainlike arrays with the aid of MFI self-assembly, and PANI coating shell, which connected and fixed the chainlike arrays, could be realized via in situ surface polymerization. Hydrogen bonds are believed to be the driving force that makes aniline form coating shell rather than bulk sheets. Saturated magnetization value and conductivity value of Fe₃O₄@PANI nanofibers are 33.18 emu g⁻¹ and 4.42 × 10⁻⁵ S cm⁻¹, while those of Fe₃O₄@PANI microspheres are 24.33 emu g⁻¹ and 2.13 × 10⁻⁶ S cm⁻¹, respectively. Compared to the Fe₃O₄@PANI microspheres, these prepared Fe₃O₄@PANI nanofibers show much higher magnetization saturation value and conductivity value, which allows them to serve as ideal candidates for microwave absorbents, electromagnetic shielding coatings, and other usage associated with conventional electromagnetic techniques.

Acknowledgements

We gratefully appreciate the support of the National Natural Science Foundation of China (Grant 51173146), the National High Technology Research and Development Program of China (863 Program) (Grant 2012AA02A404), and the National Key Basic Research Program of China (Grant 2010CB635111).

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Footnote

† Electronic supplementary information (ESI) available: SEM and TEM images of Fe₃O₄@PANI nanofibers; FTIR spectra and XRD patterns of Fe₃O₄ microspheres, Fe₃O₄@PANI nanofibers, and Fe₃O₄@PANI microspheres; XPS spectra of Fe₃O₄@PANI nanofibers and Fe₃O₄@PANI microspheres. See DOI: 10.1039/c4ra14723e

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