Structure and Ferromagnetism in Carbon Nanofibers from PAN/PVP


 We report on the room-temperature ferromagnetism in carbon nanofibers. Carbon nanofibers were fabricated using sequential electrospinning of polyacrylonitrile (PAN) and polyvinylpyrrolidone (PVP). The morphologies, crystal structures, chemical bonding states and magnetic properties were characterized for three different polyacrylonitrile (PAN) to polyvinylpyrrolidone (PVP) weight ratios (10:0, 7:3 and 6:4) of PAN/PVP. The as-spun PAN/PVP were carbonized in three steps; stabilization, carbonization and activation at 800 ºC to obtain carbon nanofibers. The morphology and structure of the carbon nanofibers (CNFs) were completely characterized by field emission scanning electron microscopy (FE-SEM), x-ray diffraction (XRD) and Raman spectroscopy. The elemental composition and the chemical bonding of CNFs were analyzed by x-ray photoelectron spectroscopy (XPS), the magnetic properties of CNFs were measured by vibrating sample magnetometer (VSM) at room-temperature. XRD patterns showed the phase of amorphous carbon structure. The average diameter sizes of the carbon nanofibers ranged from 340 to 484 nm. Raman analysis was used to determine the carbon qualities in the samples by the numbers of sp3/sp2 hybridized atoms. Chemical analysis with XPS indicated that there were no magnetic contaminants in the samples. The PAN/PVP weight ratio of 6:4 showed ferromagnetic carbon nanofibers with the highest specific magnetization as ~144.2 memu/g at 300 K. These results inspire us to further research the potential of carbon materials, as a completely new class of magnetic devices. This will aid the development of new technologies in the near future.


Introduction
The origin of magnetism in conventional ferromagnetic materials that are composed metal elements is due to the presence of cations with partially lled d or f shells, such as iron, nickel and cobalt [1]. Ferromagnetic material can be used in many applications; including memory devices such as magnetic tapes, magnetic hard drives, and magnetic random access memory, information (magnetic recording), magnetic shielding and magnetic recording permanent magnets electromagnets core of the transformer magnetic tapes and memory store [2]. Carbon-based materials are included in the list of ferromagnetic elements that contains only s and p electrons in contrast to traditional ferromagnets [3]. Magnetism of carbon-based material has been attracting great interest due to high temperature ferromagnetic behavior and application. For application, if it is technologically feasible to produce a bulk carbon magnetic material in gram amounts, it would be expected that they would be applicable to a wide eld of modern technology for medical devices; adaptable, lightweight, exible information storage systems; have low production cost, molding capability, environmental friendliness and biocompatibility. Biotechnology applications may include biosensors, targeted drug delivery, arti cial muscles, magnetic buckles, magnetic markers and magnetic resonance imaging [4][5][6][7].
Conventional, carbon-based structure were initially believed to be diamagnetic or, in special cases, paramagnetic [8]. Ferromagnetism in carbon exhibiting spontaneous magnetization at room-temperature was formerly reported as early as 1987 [9]. There have been a number of publications appearing in which reproducible observations of the room-temperature magnetic ordering in different carbon allotropes, such as graphite [5,[10][11][12], graphene [13], proton-irradiated highly oriented pyrolytic graphite (HOPG) [5,14,15], carbon nanotubes [6, 16, 17], polymerized fullerenes [18], carbon nanofoam [19] and amorphous carbon [20] were reported. In addition, Ma et al. [21] suggested the observation of room temperature ferromagnetism in a polymer which includes Te on tape subjected to simple mechanical stretching, heating or cutting. Several reports have been published describing magnetism observed in carbon systems. The magnetic moments produced in carbon materials are suspected to be due to the existence of defects [5,22]. For example, hydrogen passivated zigzag and armchair edges [23]; carbon dangling bonds or vacancy-interstitial complexes [3] [21], adatom defects [24,25] and hydrogen adsorption defects [26] were predicted to induce local magnetic moments and magnetic order in graphite or graphene [15].
The ferromagnetism in graphitic materials has been attributed to lattice imperfections. The induction of ferromagnetism was demonstrated in proton-irradiated graphite spots [14]. The pyrolytic graphite contained a high defect concentration showing increased magnetic signals [27]. Makarova and Palacio reported that a major essential for magnetism in an all-carbon structure is the presence and stability of carbon radicals [26]. The phenomenon of radicals can introduce an unpaired spin and can bring down the potent susceptibility of pairing of all valence electrons in covalent bonds. However, the mechanism of ferromagnetism in pure carbon remains unclear [23]. Since Ovchinnikov and Spector [28] prepared ferromagnetic carbon material from polyacrylonitrile, many scientists have attempted the di cult task of reproducing ferromagnetic carbon materials. However, mostly PAN only prepared, no works have yet been reported about ferromagnetic carbon material from PAN/PVP blends, furthermore, PVP polymer is cost effective. In considering the future potential of such materials it is also necessary to take into account the lower costs of material.
In this work, the metal-free carbon materials were prepared from the mass ratios of polyacrylonitrile (PAN) and polyvinyl pyrrolidone (PVP) by electrospinning method. The all as-spuns were calcined in three steps; stabilization, carbonization and activation using a quartz tube furnace. The structural and magnetic properties of the CNFs were investigated. The CNFs are pure phase and reveal ferromagnetic behavior at room temperature. Typical characterization techniques including X-ray diffraction (XRD), Raman spectroscopy (RS), Fourier transform infrared spectroscopy (FT-IR), eld emission scanning electron microscopy (FE-SEM), and vibrating sample magnetometer (VSM) were employed. Furthermore and importantly, X-ray photoelectron Spectroscopy (XPS) was performed and contributed an important role in con rming and understanding in terms of bonding con guration of the carbon system and the contamination causing a ferromagnetic behavior in the CNFs.

Experimental Procedure
In this study, polyacrylonitrile (M w =150000, Aldrich) and polyvinyl pyrrolidone (M w =1300000, Aldrich) N,N-dimethylformamide (DMF, 99.8%) were used as the starting chemicals. We rst prepared three different mass ratios of PAN and PVP at 10:0, 7:3, and 6:4 in 40 ml of DMF under vigorous stirring for 5 h and with ultrasonic-assisted vibration (500W) for 30 min until a homogeneous solution was obtained. The mixed total mass of PAN and PVP in DMF was xed to 10 wt% [29]. The PAN or PAN/PVP electrospun ber mats were prepared using a drum electrospinning process which is shown in Fig. 1. The voltage used for electrospinning was maintained between 15.6-16.0 kV. The distance between the syringe needle tip and the aluminum foil collector was set at 15 cm. The solution was fed at a ow rate of 0.5 ml/h. The collection of electrospun bers was set to 6 h. All as-spun bers were stored at room temperature. The carbon nano bers obtained after pyrolysis of polymer bers were placed inside a tube furnace an processes in three steps, stabilized at 280°C for 2 h in an air atmosphere at the heating rate of 2 ºC/min, and then carbonized at 500 ºC for 2 h in an argon atmosphere at the heating rate of 5 ºC/min and activation at 800°C for 2 h in an argon atmosphere at the heating rate of 5 ºC/min. Finally, the carbonized nano bers were activated by CO 2 ambience at 800 ºC for 30 mins directly after carbonization.

Results And Discussion
The morphology of nano bers was analyzed by FE-SEM. and PAN/PVP (6: 4) obtained after carbonization at 800°C; their diameters were 511 ± 81, 440 ± 63, and 340 ± 60 nm, respectively. This average diameter was measured just on the brous morphology, it was not in interconnection areas. The reduction in average diameter of the nano bres after calcination could be attributed to the loss of PVP and PAN from the nano bers [29][30][31]. The average diameter of samples has been found to decrease with increasing the PVP content because PVP completely degrades during the carbonization process. However, the size of bers could be increased with the PVP content because of ber-ber interconnection. Niu et al. [30] reported that ber diameter increased with the PAN/PVP ratio at 13:87 and 8:92. There was almost no ber -ber interconnection at a low PVP content. In this case the interconnection ber was formed only in small areas, which were mostly invisible, explaining why the ber diameter decreased with the PVP content. α λ Figure 3 shows the XRD pattern of all samples used for investigation of their crystal structures. For all samples, diffraction peaks corresponding to the (002) and (101) peaks of graphitic carbon were observed at approximately 2θ = 25° and 44° [20]. The broad diffraction peaks suggest the disorder and amorphous characteristics of the materials [32]. These results imply that the crystal structure of PAN and PAN/PVP nano bers is not changed by increasing PVP content; only the intensity is changed, which in the sample PAN/PVP (6:4) is the lowest intensity and is in agreement with the previous report by Lv et al. [33]. This may result from the incorporation of carbon nanoparticles from PVP into the sample. The decrease in terms of diffraction peaks led to a decrease of degree of crystallinity of PAN/PVP nano ber [34], but it is far from the perfect graphitized structure [35].
The surface elemental composition and their binding energies of samples were investigated by X-ray photoelectron spectroscopy (XPS). The main peak contained two distinct peaks, indicating the existence of carbon and oxygen elements as shown in Fig. 4.  Fig. 6a, b, and c) [7]. The comparison of the proportion of C = O and C-O in all samples is shown in Fig. 6d and e. FT-IR spectra was characterized in the samples in order to understand the relevant functional groups after the heat treatment activity. For comparison, pure PAN and PAN/PVP show the absorption peak Fig. 7. The FT-IR spectra show that the PVP had no effect on the functional groups in the samples, which were similar in the appearance; however, a small shift in PAN/PVP (6:4) towards the higher wavenumber observed and may be due to incorporation of carbon nanoparticles in the template. The broad absorption band at ~ 1160 cm − 1 represents the stretching mode of C-O from the carboxyl group (-COOH) [40]. The peak at 1563 cm − 1 represents the stretching mode of the C = C group from an aromatic ring [41,42]. The band at ~ 1800 cm − 1 may be assigned to the C = O bond stretching vibration mode [43][44][45]. The peak at 1892 cm − 1 is assigned to the aromatic combination band [46]. The peak around 2082 cm − 1 indicates the C C bending [47].
Raman analysis was used to determine the carbon quality of PAN and PAN/PVP (6:4) samples by using a Gaussian tting of the observed band positions. In addition, Raman spectroscopy is a commonly used technique for identifying secondary phase in doped/composites materials. The rst-order zone boundary phonon mode component with D and G band, D band has sp 3 carbon bonded (C-C), associated with the disorder that was induced by defects (vacancies) in the carbon nano bers. The G band results from the stretching of the C = C bonds in the hexagonal ring of sp 2 bonded carbon in graphitic structures [32,48]. Figure 8a and b show the D bands associated with PAN and PAN/PVP (6:4) are located at 1361.2 and 1359.9 cm − 1 respectively. Th G band showed peaks at 1580.5 and 1581.8 cm − 1 that corresponded to an ≡ ideal graphitic lattice vibration mode with E 2g symmetry [49]. The intensity ratio of D band to G band (R = I D /I G ) re ects the degree of the defects in the carbon nano bers. This work showed an I D /I G ratio of 1.78 for PAN obtained from the tting (see Fig. 8a). The I D /I G value of carbon nano bers increased to 1.89 with decreasing ratio of PAN/PVP(6:4) (see Fig. 8b). The comparison of spectra for PAN and PAN/PVP96:4) is shown in Fig. 8c. An increase in the I D /I G ratio indicates an increase in the number of defects on nano bers, which indicates the decrease in the average diameter size from the FE-SEM result [43]. The  Table 1). The coercivity is the demagnetizing eld needed to reduce the remnant magnetization to zero [51]. This study shows that the coercivity values of samples increased with  Fig. 6d and e). The oxygen coverage was small in PAN/ PVP(7:3) so that it was possible for magnetic ordering to occur because the magnetic domain could develop in graphite oxide [52], while increasing of PAN/PVP (6:4) leading to an increase in a speci c magnetization of content that shows the highest magnetic moment of our sample. This may be due to increasing amorphization that correlated with the increasing ratio of I D /I G from Raman analysis.
The I D /I G is monotonically increased with the PVP content, which indicates an increased density of vacancy defects [5]. The results show that the amount of vacancy in samples tends to increase with PVP content. The vacancy in samples is predicted to have local magnetic moments [53], the local moments could couple each other and produce a long-range order, and could give rise to ferromagnetic behavior in samples, which explained the increase in the magnetization in the PAN/PVP(6:4) sample. Furthermore, the carbon atoms neighboring to defects in samples will adjust their bonds to accommodate the defects and produce dangling sp 2 bonds, which generates a magnetic moment. Thus, the origin of the ≡ ferromagnetism was suggested to be attributed to the mixture of carbon atoms with sp 2 and sp 3 bonds according to the above XPS and Raman analysis, and this creates vacancies in the presence of unpaired electron spins in sp 3 carbon bonds that contributed the ferromagnetism [10,54], In addition, the FE-SEM shows the smallest average diameter of nano bers were possible magnetically ordered due to the large amount of graphitized carbons edge sites magnetism [20,55]. The observed ferromagnetism could be based on the rich carbon defects of carbon dangling bonds and strong ferromagnetic coupling between them [3]. This suggest that the increasing disorder in the graphite structure in samples clearly played an important role in improving the ferromagnetic properties of graphite, in agreement with the results of the ratio of I D /I G [51]. This observation is consistent with previous research results of Saito et al [11] reported that the carbon material prepared by pyrolysis of PAN at 1273 K for 1 h that was attracted by the permanent magnet and which is a ferromagnetic carbon material. The authors suggested that the ferromagnetic behavior in carbon exhibited a saturation magnetization of 1.22 emu/g at 300 K.

Conclusion
In this study, we report the existence of room temperature ferromagnetism in carbon nano bers. The carbon nano bers were prepared by an electrospinning technique and carbonization of PAN and PVP at high temp, which is an easy and low-cost process. The as-spuns were carbonized at 800 ºC to obtain a carbon nano bers by three steps of stabilization, carbonization and activation. The PAN/PVP weight ratio of 6:4 was shown to have ferromagnetic carbon nano bers with the highest speci c magnetization of ~ 144.2 memu/g at 300 K. Chemical analysis with XPS indicated that there was no magnetic impurity in the samples. The ferromagnetic behavior reported here is considered to be associated with the mixed fraction of sp 2 and sp 3 carbon atom in carbon nano bers; furthermore the nanostructure could also play an important role in the net magnetic moment obtained. These results inspire us to further research the potential of carbon materials as a completely new class and generation of magnetic devices with, for instance, low mass density or biocompatibiliy. This will aid the development of new technologies in the near future.
Declarations Figure 1 Schematic diagram setup for the electrospinning of carbon nano bers.