Single Flexible Nanofiber to Simultaneously Realize Electricity-Magnetism Bifunctionality

Electrospinning is an outstanding technique to process viscous solutions or melts into continuous fibers with diameters ranging from micrometer to submicron or nanometer. This technique attracts extensive academic investigations,1-3 and is also applied in many areas such as filtration,4 optical and chemical sensors,5 biological scaffolds6 and electrode materials.7 PANI is one of the most important conducting polymers because of its high electrical conductivity, good redox reversibility, processibility, and environmental stability8,9 as well as its a variety of potential applications. 9,10 In recent years, one-dimensional (1D) structures of PANI including nanowires, rods and tubes have been studied with the expectation that such materials will combine the advantages of both low-dimensional systems and organic conductors.11 Sub-micron fibers of pure PANI doped with sulfuric acid or hydrochloric acid were prepared by electrospinning PANI with suitable molecular weight dissolved in hot sulfuric acid,12 but it remains a great challenge to apply electrospinning to PANI as limited by its molecular weight and solubility. To overcome this problem, most of the researchers electrospun PANI through blending it with other spinnable polymers.13,14 Magnetic Fe3O4 nanocrystals have been extensively studied owing to their unique and tunable magnetic properties. Their magnetic features have found widespread uses in applications as diverse as environmental remediation, magnetic recording, biomacromolecule separation, catalyst separation, drug/gene delivery and release, and magnetic resonance imaging.15-18 In the past few years, multifunctional composite materials have attracted inevasible attention of scientists.19-21 Some preparations of Fe3O4@RE complex (RE=rare earth) core-shell structure NPs have been reported.22-24 Recently, Ma, et al. have prepared Fe3O4/RE complex magnetic-fluorescent bifunctional composite nanofibers and nanobelts via electrospinning process.25-29 Wang et al., have prepared PANI particles/rare earth complex/PVP luminescent-electrical bifunctional composite nanofibers via electrospinning process.30 It is found from the above discussion that multifunctional composite materials have attracted extensive academic researches and become one of the popular subjects of study in materials science. To the best of our knowledge, there have been rare reports on the preparation and performance of electricity-magnetism bifunctional nanofibers in the literature. In the present study, we successfully assembled electricity-magnetism bifunction into single PANI/ Fe3O4/PVP flexible nanofiber via a facile electrospinning technique using PANI as a conductive polymer and Fe3O4 NPs as a magnetic material. The structure, morphology and performances of the bifunctional flexible nanofibers were systematically studied, and some new results were obtained. Single Flexible Nanofiber to Simultaneously Realize Electricity-Magnetism Bifunctionality


Introduction
Electrospinning is an outstanding technique to process viscous solutions or melts into continuous fibers with diameters ranging from micrometer to submicron or nanometer.This technique attracts extensive academic investigations, [1][2][3] and is also applied in many areas such as filtration, 4 optical and chemical sensors, 5 biological scaffolds 6 and electrode materials. 7ANI is one of the most important conducting polymers because of its high electrical conductivity, good redox reversibility, processibility, and environmental stability 8,9 as well as its a variety of potential applications. 9,10In recent years, one-dimensional (1D) structures of PANI including nanowires, rods and tubes have been studied with the expectation that such materials will combine the advantages of both low-dimensional systems and organic conductors. 11ub-micron fibers of pure PANI doped with sulfuric acid or hydrochloric acid were prepared by electrospinning PANI with suitable molecular weight dissolved in hot sulfuric acid, 12 but it remains a great challenge to apply electrospinning to PANI as limited by its molecular weight and solubility.To overcome this problem, most of the researchers electrospun PANI through blending it with other spinnable polymers. 13,14agnetic Fe 3 O 4 nanocrystals have been extensively studied owing to their unique and tunable magnetic properties.6][27][28][29] Wang et al., have prepared PANI particles/rare earth complex/PVP luminescent-electrical bifunctional composite nanofibers via electrospinning process. 30It is found from the above discussion that multifunctional composite materials have attracted extensive academic researches and become one of the popular subjects of study in materials science.To the best of our knowledge, there have been rare reports on the preparation and performance of electricity-magnetism bifunctional nanofibers in the literature.
In the present study, we successfully assembled electricity-magnetism bifunction into single PANI/ Fe 3 O 4 /PVP flexible nanofiber via a facile electrospinning technique using PANI as a conductive polymer and Fe 3 O 4 NPs as a magnetic material.The structure, morphology and performances of the bifunctional flexible nanofibers were systematically studied, and some new results were obtained.

Preparation of Fe 3 O 4 NPs
Fe 3 O 4 NPs were obtained via a facile coprecipitation synthetic method, and PEG was used as a protective agent to prevent the particles from aggregation.One typical synthetic procedure was as follows: 5.4060 g of FeCl 3 •6H 2 O, 2.7800 g of FeSO 4 •7H 2 O, 4.04 g of NH 4 NO 3 and 1.9 g of PEG were added into 100 mL of deionized water to form uniform solution under vigorous mechanical stirring (400 r•min -1 ) at 50 °C.To prevent the oxidation of Fe 2+ , the reactive mixture was kept under argon atmosphere.After the mixture had been bubbled with argon for 30 min, 0.1 mol•L -1 of NH 3 •H 2 O was dropwise added into the mixture to adjust the pH value above 11.Then the system was continuously bubbled with argon for 20 min at 50 °C, and black precipitates were formed.The precipitates were collected from the solution by magnetic separation, washed with deionized water for three times, and then dried in an electric vacuum oven at 60 °C for 12 h with the vacuum degree of 0.08 MPa.

Fabrication of electricity-magnetism bifunctional composite nanofibers via electrospinning
In the preparation of spinning solution, the masses of ANI and Fe 3 O 4 NPs were respectively proportional to the mass of PVP, and the molar ratio among ANI, CSA and APS were 2:1:2.ANI was dissolved in 3.100 g of DMF with magnetic stirring at room temperature, and then 0.9 g of PVP and CSA were slowly added into the above solution.The mixture was then cooled down to 0 °C in an ice-bath.The mixture was denoted as solution I. A mixed solution of ammonium persulfate (APS) and 2.000 g of DMF at 0 °C was prepared as the solution II.The solution II was added dropwise into the solution I under mechanical stirring (200 r•min -1 ).The final mixture was allowed to react at 0 °C for 24 h and then Fe 3 O 4 NPs were added into the mixture under mechanical stirring at room temperature, thus the spinning solution was prepared.The dosages of these materials were summarized in Table 1.
During the electrospinning process, a traditional electrospinning apparatus was used to prepare the PANI/Fe 3 O 4 /PVP composite nanofibers.Spinning solution was loaded into a plastic syringe with a plastic spinneret, and the inner diameter of the spinneret was 1 mm.Carbon rod used as anode was inserted into spinning solution.A flat iron net was used as cathode and collector and put about 11 cm away from the tip of the spinneret.A positive direct current (DC) voltage of 13 kV was applied between the spinneret and the collector to generate stable, continuous PVP-based composite nanofibers under the ambient temperature of 20 °C-25 °C, and the relative humidity was 45%-50%.

Characterization Methods
The as-prepared Fe 3 O 4 NPs and PANI/Fe 3 O 4 /PVP composite nanofibers were identified by an X-ray powder diffractometer (XRD, Bruker, D8 FOCUS) with Cu Kɑ radiation.The morphology and internal structure of PANI/Fe 3 O 4 /PVP composite nanofibers were observed by a field emission scanning electron microscope (FESEM, XL-30) and a transmission electron microscope (TEM, JEM-2010), respectively.The elementary compositions of the PANI/Fe 3 O 4 /PVP nanofibers were determined by an energy dispersive X-ray spectrometer (EDX, GENESIE 2000).The electrical conductivity was detected by a Hall effect measurement system (ECOPIA HMS-3000).Then, the magnetic performances of Fe 3 O 4 NPs and PANI/Fe 3 O 4 /PVP composite nanofibers were measured by a vibrating sample magnetometer (VSM, MPMS SQUID XL).All measurements were performed at room temperature in air atmosphere, and the relative humidity was 40%-45%.In order to characterize the size and morphology of the

Magnetic Property
The magnetic properties of Fe   2, implying that the magnetic properties of the composite nanofibers can be tuned via addition of various amounts of Fe 3 O 4 NPs.

Electrical Conductivity Analysis
The average electrical conductivity values of the samples are summarized in Table 3.The electrical conductivity is greatly increased from 1.357×10 -6 to 7.175×10 -3 S•cm -1 at the beginning when the PANI is incorporated into composite nanofibers, meaning that the electrical conductivity of the composite nanofibers can be tunable by adjusting amount of PANI.With further increase in PANI content, the electrical conductivity of composite nanofibers is only slightly increased.The increase in electrical conductivity of PANI/Fe 3 O 4 /PVP composite nanofibers with increasing in PANI content is due to forming a better continuous net structure for PANI polymer.Furthermore, it is found from Table 3 that the electrical conductivity of the composite nanofibers is slightly increased with increasing in the mass ratios of Fe 3 O 4 to PVP when fixing the other parameters, as shown in samples S3, S6, S7 and S8.

Formation Mechanism for PANI/Fe 3 O 4 /PVP Composite Nanofibers
We propose the formation mechanism for PANI/Fe 3 O 4 /PVP composite nanofibers, as shown in Fig. 5. Firstly, ANI and PVP are dissolved in DMF to form solution with certain viscosity.Secondly, ammonium persulfate (APS) is added into above solution and allowed to react at 0 °C for 24 h.

XRD patterns of Fe 3 O 4
NPs and composite nanofibers (S3) are shown in Fig. 1.The XRD patterns in Fig. 1a reveal that the as-prepared Fe 3 O 4 NPs are conformed to the cubic structure of Fe 3 O 4 (PDF 74-0748).No characteristic diffraction peaks of any other impurities are detected.XRD analysis result of the PANI/Fe 3 O 4 /PVP composite nanofibers demonstrates that the composite nanofibers contain Fe 3 O 4 NPs, as indicated in Fig. 1b.The morphology of the as-prepared Fe 3 O 4 NPs is observed by means of TEM, as presented in Fig. 2a.The size distribution of the spherical Fe 3 O 4 NPs is almost uniform, and the diameter of the Fe 3 O 4 NPs is 9-13 nm.
Fig.2bshows FESEM image of PANI/Fe 3 O 4 /PVP composite nanofibers, indicating that the as-prepared nanofibers are relatively smooth.The Image-Pro Plus 6.0 software is used to measure the diameters of the nanofibers, and the results are analyzed with statistics.The diameter for composite nanofibers is 400±20 nm under the confidence level of 95%, as indicated in Fig.3.The TEM image of PANI/Fe 3 O 4 /PVP composite nanofibers is presented in Fig.2c.It is obviously seen from the TEM image that Fe 3 O 4 NPs are well dispersed in the composite nanofibers.A slightly agglomeration phenomenon which can be observed via FESEM and TEM is caused by the large surface energy of the nano-sized Fe 3 O 4 particles.31The EDX analysis, as shown in Fig.2d, reveals that the composite nanofibers consist of C, N, O, S, Fe and Au elements.The Au peak in the spectrum comes from gold conductive film plated on the surface of the sample for SEM observation.No other elements are found in the sample, implying that the PANI/Fe 3 O 4 /PVP composite nanofibers are highly pure.

3 O 4
NPs and PANI/Fe 3 O 4 /PVP composite nanofibers are examined by a vibrating sample magnetometer.No remanence is detected for the as-prepared Fe 3 O 4 NPs.The zero coercivity and the reversible hysteresis

Finally, the prepared Fe 3 O 4
NPs are added into the mixture under magnetic stirring at room temperature, thus the spinning solution is prepared.The spinning solution is loaded into a plastic syringe, and then PANI/Fe 3 O 4 /PVP composite nanofibers are fabricated by electrospinning.Fe 3 O 4 NPs are well dispersed in the composite nanofibers.PANI also forms a continuous network in the nanofibers, leading to high electrical conductivity of the electricity-magnetism bifunction composite nanofibers.

Table 1 .
Compositions of the spinning solutions presented PANI/Fe 3 O 4 /PVP composite nanofibers, FESEM observation is conducted.