Synthesis and magnetic properties of Fe3O4/helical carbon nanofiber nanocomposites from the catalytic pyrolysis of ferrocene

High purity Fe3O4/helical carbon nanofiber composites were obtained on a large scale by the catalytic pyrolysis of ferrocene in the presence of tin powder at 500°C over 12 h. The sizes of Fe3O4 nanoparticles are 35–65 nm in size, and the diameters of the helical carbon nanofibers range from 40–70 nm. The shapes and compositions of the nanocomposites are simply controlled by adjusting the reaction temperatures. On the basis of the obtained experimental results the formation of the helical Fe3O4/carbon nanofiber composites was investigated and discussed. The magnetic hysteresis loop of the products shows ferromagnetic behavior with saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (Hc) values of ca. 29.8 emu/g, 9.6 emu/g and 306.6 Oe, respectively.

Carbon nanotubes were first observed in 1991 [1] and this discovery stimulated intense interest in the synthesis and physical properties of carbon nanotubes because of their potential applications. Apart from cylindrical carbon nanotubes, differently shaped carbon nanomaterials have also been observed. Much attention has been paid to the study of coiled carbon nanofibers and carbon nanotubes because of their novel coiled morphologies, which gives interesting physical properties and diverse applications [2][3][4][5][6][7][8][9]. They can be used as electromagnetic wave absorbers and filters, microdevices, micro-sensors, hydrogen storage materials, electrode materials, catalyst supports, elastic materials, strengthening fibers in composite materials, mechanical components and field emission materials [10][11][12][13][14][15]. Their elastic and mechanical properties are among their most important properties. Luo et al. [16] reported the synthesis of helically coiled carbon nanotubes by reducing the amount of ethyl ether and using Zn as a catalyst at 700°C. Yang et al. [17] reported that three-dimensional spring-like carbon nanocoils were obtained in high purity and high yield by the catalytic pyrolysis of acetylene at 750-790°C using an Fe-based catalyst. Bajpai et al. [18] demonstrated that large scale aligned helical carbon nanotube arrays perpendicular to the substrate surface can be prepared by the co-pyrolysis of Fe(CO) 5 and pyridine onto pristine quartz glass plates in a tube furnace at 900-1100°C under a mixted flow of Ar and H 2 . Carbon microcoils, a type of novel carbon material with a 3D double-helix/spiral structure similar to DNA were obtained by the Ni-catalyzed pyrolysis of acetylene at 750°C [19]. In general, they were prepared by the high-temperature (>600°C) catalytic decomposition of an organic vapour of acetylene or pyridine with transition-metal catalysts such as Ni and Fe. In this study, we describe the novel synthesis of Fe 3 O 4 /helical carbon nanofiber nanocomposites by the pyrolysis of ferrocene in the presence of tin at 500°C.

Experimental
All the reagents are commercially available and were used without further purification. In a typical experiment, ferrocene (0.50 g) and Sn powder (0.50 g) were added to an autoclave with 20 mL capacity. The autoclave was tightly sealed and heated in an electric stove. The stove temperature was raised to 500°C at a heating ramp rate of 10 °C min 1 and maintained at 500°C for 12 h upon which it was allowed to cool to room temperature naturally. We found that the final products in the autoclave included many dark precipitates and some residual gases. The dark products on the wall were collected and washed with absolute ethanol and distilled water. The final products were dried in a vacuum at 50°C over 6 h.
The X-ray diffraction (XRD) patterns were obtained on a Rigaku (Japan) D/max-A X-ray diffractometer with Cu K radiation ( = 1.54178 Å). Raman spectra were obtained at ambient temperature on a Spex 1403 Raman spectrometer (Ar ion laser, 514.5 nm). FESEM (JEOL JSM-6700F) was used to observe the morphologies of products. The TEM images and HRTEM image were obtained on a JEOL-2010 transmission electron microscope. The magnetic properties (M-H curve) were measured at room temperature on an MPMS XL magnetometer made by Quantum Design Corporation.

Results and discussion
Figure 1(a) shows the powder XRD pattern of the products and these were prepared at 500°C over 12 h, and washed several times using distilled water and ethanol. According to the diffraction peaks in the figure, the main phases in the products can be indexed as face-centered cubic (fcc) Fe 3 O 4 (JCPDS Card no. 85-1436) and hexagonal graphite (JCPDFS Card no. 48-1487). The representative Raman spectrum ( Figure 1(b)) of the products shows a typical carbon nanofiber feature. In detail, two strong peaks are present at 1594 and 1335 cm 1 , which correspond to typical Raman peaks of graphitized carbon nanostructures. The peak at 1594 cm 1 is attributed to the Raman-active E2g in-plane vibration mode and is related to the vibration of sp 2 -bonded carbon atoms in a two-dimensional hexagonal lattice. The peak at 1335 cm 1 is associated with the vibrations of carbon atoms with dangling bonds at the plane terminations of disordered graphite. The intensity of the D-band peak is strong, which originates from the in-plane defects of the products preventing the layer from extending.
The morphology of the products was investigated using FESEM and TEM observation, and these indicated that the products are composed of helical structures and the yield was more than 95%. Figure 2 Figure 2(c) shows a TEM image of the products, which agrees with the above FESEM images. A notable result is that the average diameters of the carbon nanofibers and the Fe 3 O 4 nanoparticles are all about 35-65 nm, which indicates that the size of the Fe 3 O 4 nanoparticles determines the diameter of the carbon nanofibers. In Figure 2(d), the lattice plane spacing calculated from the HRTEM image is ca. 0.295 nm, which corresponds to the hkl (220) of a typical fcc Fe 3 O 4 structure.
To understand the possible formation processes of Fe 3 O 4 / helical carbon nanofiber composites, a series of relevant experiments were carried out by altering the experimental parameters. It is obvious that the reaction temperature plays a critical role in the formation of Fe 3 O 4 /helical carbon nanofiber composites. At a reaction temperature of 500°C the products are Fe 3 O 4 /helical carbon nanofiber composites. Figure 3 shows TEM images and XRD patterns of the products obtained at 600 and 700°C. Figure 3(a) shows that the main products are flake-like structures for a reaction temperature of 600°C. Figure 3(b) indicates that the products consist of two parts: an outer shell and an inner core while  the shell thickness of the large flakes is about 6 nm. XRD (Figure 3(e)) confirms that the products are carbon and Fe 3 O 4 , which agrees with the TEM images. By increasing the reaction temperature to 700°C, carbon coated nanoparticles were obtained, as shown in Figure 3(c). Figure 3(d) indicates that the size of nanoparticles is about 48 nm and the thickness of shells is about 4 nm. TEM images and XRD (Figure 3(f)) indicate that the products are carbon coated Fe nanoparticles.
In this system, we found that the reaction temperature plays a crucial role in the controlled synthesis of the different composites. Although the detailed mechanism is not fully understood at present it is likely that Fe atoms are released by the decomposition of ferrocene (decomposition temperature: 400°C [20]) and Fe nanoparticles form with an increase in temperature. At 500°C Fe nanoparticles react with O 2 in a sealed reaction system to form Fe 3 O 4 nanoparticles but the catalytic decomposition of C 5 H 5 to carbon is slow because of the lower temperature, and this results in that the formed carbon having adequate time to grow vertically in the (002) crystal plane direction, leading to the formation of Fe 3 O 4 /helical carbon nanofiber composites. However, a question arises about why helical carbon nanofibers are obtained. To create carbon helical structures, transition metal tin compounds have been used as catalysts [21]. At a reaction temperature of 600°C, the lower amount of O 2 causes the nucleation rate of the Fe 3 O 4 particles to slow but the catalytic decomposition of C 5 H 5 to carbon is fast because of the high temperature. Compared with fast nucleation and aggregation growth the slower aggregation growth of Fe 3 O 4 allows for an adequate rotation of the crystals to find a low energy configuration interface and to form perfectly oriented aggregates. Similar to the gold or silver triangular fcc crystals [22,23] the (111) plane of Fe 3 O 4 may also possess the lowest surface energy. Therefore, flake-like Fe 3 O 4 might be formed with the (111) plane as a basal plane. At the same time, some carbon feed stock would cover the flake-like Fe 3 O 4 particles to form flake-like Fe 3 O 4 @C composites [24]. At a reaction temperature of 700°C the as-formed iron nanoparticles are quickly wrapped by a small number of carbon atoms (forming a thin carbon lamella) to reduce their surface energy and form Fe nanoparticles encapsulated within thin carbon capsules. These thin carbon lamella restrict the growth of the formed Fe nanoparticles. As carbon atoms are released in these experiments the continued addition (or diffusion) of carbon atoms finally leads to the formation of carbon coated Fe nanoparticles, as shown in Figure 3(c).
Magnetization measurements were carried out to examine the magnetic properties at room temperature with an applied field from 5000 to 5000 Oe, as shown in Figure 4. The magnetic hysteresis loop of the products at 500°C shows ferromagnetic behavior with saturation magnetization (M s ), remanent magnetization (M r ), and coercivity (H c ) values of ca. 29.8 emu/g, 9.6 emu/g and 306.6 Oe, respectively. From these results, the saturation magnetization value of the products is lower than that of bulk Fe 3 O 4 (85-100 eum/g). The decrease in the M s found in this work might be caused by two factors. On the one hand, the wide existence of carbon nanofibers is likely. The results of the burring experiments indicate that the calculated Fe 3 O 4 to Fe 3 O 4 /helical carbon nanofibers mass ratio of the products is about 0.384. On the other hand, the presence of a detrimental surface/ crystal-structure is also related to the demagnetization effects. However, the coercivity value is higher than that of sub-microspheres (124.7 Oe) and Fe 3 O 4 nanopartiicles (274 Oe) [25,26]. These research results indicate that size and morphology may be influencing factors because the coercivity is sensitive to many structural parameters such as the internal stress, the orientation, defects and shapes.

Conclusions
In summary, Fe 3 O 4 /helical carbon nanofiber composites were synthesized at 500°C for 12 h in a sealed reaction system. The yield of Fe 3 O 4 /helical carbon nanofiber composites was more than 95%. By controlling the reaction temperature, flake-like Fe 3 O 4 nanocomposites and carbon coated Fe nanoparticles were obtained at 600 and 700°C over 12 h. The growth mechanism of the Fe 3 O 4 /helical carbon nanofibers composites is discussed. The Fe 3 O 4 /helical carbon nanofibers composites show that the saturation magnetization (29.8 emu/g) and coercivity (306.6 Oe) are different from those of bulk Fe 3 O 4 , sub-microspheres and Fe 3 O 4 nanoparticles, which can be attributed to the different carbon content, size and morphology of the products.