Superparamagnetic Behaviour and Surface Analysis of Fe3O4/PPY/CNT Nanocomposites

The superparamagnetic property of nanomaterials such as Fe3O4 has been considered to be promising for various applications. In this paper, Fe3O4/PPY/CNT nanocomposites were synthesized with utilizing natural iron sand by a coprecipitation method. The as-precipitated Fe3O4 NPs were combined with carbon nanotubes (CNTs) using conductive polypyrrole (PPY) as linking agents. The Fe3O4/PPY/CNT nanocomposites were systematically characterized by FE-SEM, EDS, XRD, BET, and FTIR. Furthermore, the effects of CNTs on magnetic and thermal properties of nanocomposites were investigated by VSM and thermal gravimetric analysis (TGA), respectively. The composites exhibited significant decrease of coercivity value with the content of CNTs increasing. The VSM result confirmed that Fe3O4/PPY/CNT nanocomposites were superparamagnetic. It was found that by increasing the amounts of CNTs, the magnetization of Fe3O4/PPY/CNT nanocomposites gradually decreased. The addition of CNTs is intended to improve the mesoporous property as proved by BET analysis which has the potential application as a nanocatalyst.


Introduction
Iron oxide is one of the most abundant metal oxides on the earth. There are four kinds of iron oxides such as magnetite, hematite, maghemite, and wüstite [1]. Among them, magnetite (Fe 3 O 4 ) is the most studied due to its stability and wide range applications. The nanosized Fe 3 O 4 has been used in several fields such as sensor, medical devise, drug delivery, data storage, and catalysts [2][3][4][5][6]. Superparamagnetism is an important property that is owned by Fe 3 O 4 nanoparticles. Superparamagnetic Fe 3 O 4 nanoparticles with a stable domain have anisotropy crystals that allow the reorientation and exchange of energy to easily produce hyperthermia magnetism. Superparamagnetic has the huge potential for various applications including magnetic resonance imaging (MRI), magnetic hyperthermia (MH), drug delivery, bioseparation, and biosensing [7][8][9].
One of the disadvantages of superparamagnetic Fe 3 O 4 is being susceptible to agglomerate. A key strategy to overcome this issue is by adding polymers (e.g., PPY) during the preparation process. The usage of polymer not only can decrease particle size but also can improve the surface area and adsorption capability. Several studies have been developed to synthesize binary and ternary materials with Fe 3 O 4 base for different purposes. For example, Phadtare and coworkers designed Fe 3 O 4 /polymer composites for microwaveabsorption applications [10]. Zhu et al. synthesized core shell Fe 3 O 4 /PANI for chromium reduction [11].
Carbon nanotubes (CNTs) have high modulus, high tensile strength, low density, and good electronic conductivity [12]. CNTs have attracted a great attention of scientists due to their unique structural, electrical, and mechanical properties. To improve the function of the ingredients, CNTs are supported by other materials to expand its applications. In an energy storage field, large numbers of ultrafine Fe 3 O 4 nanoparticles covered on the surfaces of CNTs enhanced the electrochemical activity and mechanical property of the electrodes during the repeatedly charged and discharged processes and furthermore, the as-prepared CNT@Fe 3 O 4 also exhibited mesoporous properties [13]. The hybrid structures are expected to improve electrochemical performance due to their unique structures, relative specific surface area, and high-porosity properties. The highly flexible and conductive CNT backbone provides a three-dimensional structure to facilitate electron transfer and provides a large contact area for higher Li + diffusion between electrodes and electrolytes [14].
In this contribution, we synthesize Fe 3 O 4 /PPY/CNTs with a low-cost and environmentally friendly coprecipitation method. Fe 3 O 4 are coated with CNTs to enhance the proper-ties of nanocomposites which are polymerized by polypyrrole (PPY). Furthermore, the effect of different amounts of CNT loading on the magnetic and surface properties is systematically investigated.

Experimental Sections
2.1. Materials. Natural iron sands were taken from Buaya River in Deli Serdang, North Sumatera. PPY was kindly received from Sigma-Aldrich. Hydrochloric acid (Merck 37%), ammonium solution (Merck 32%), and polyethylene glycol (PEG 6000) were purchased from Merck. Carbon nanotubes (CNTs) with a diameter of 25 nm were provided by Hanwha Chemical, Korea. Deionized (D.I.) water was used throughout the experiment for washing and neutralizing the pH.

Results and Discussion
The morphology of Fe 3 O 4 /PPY/CNT nanocomposites with different amounts of CNTs was observed by a fieldemission scanning electron microscope (FE-SEM). As shown in Figure 1(a), Fe 3 O 4 has a spherical shape with a particle size of 20-50 nm which is also similar to our previous work [15]. FE-SEM equipped with an energy dispersive spectroscopy (EDS) analysis was further carried out to study the atomic composition of prepared samples. Figure 2 shows the EDS with elemental mapping of Fe 3 O 4 /PPY/CNT nanocomposite to show the presence of carbon, iron, and oxygen as indicated in Figures 2(b)-2(d). The contrast of elemental mapping showed that carbon was the major phase in the composites as compared to other elements, which was originated from CNT and PPY.
The XRD results of Fe 3 O 4 /PPY/CNT nanocomposite are shown in Figure 3. XRD patterns of as-prepared samples indicated that Fe 3 O 4 phase had the cubic spinel crystal structure based on JCPDS card No. 19-0629. The main peaks of Fe 3 O 4 located at 30.09, 35.20, 37.03, 43.05, 53.39, 56.94, and 62.51°for two theta correspond to crystal planes of (220), (311), (222), (400), (422), (511), and (440), respectively. The cubic structures of Fe 3 O 4 were still clearly observed as the CNT amounts were increased. The crystallite size of the as-prepared samples was further estimated using the Scherrer equation [16]: where d is the crystallite size, k is a Scherrer constant, λ is the wavelength of X-ray, and B is the full width at half maximum (FWHM The magnetic properties of Fe 3 O 4 /PPY/CNT nanocomposite were studied using vibrating-sample magnetometer. Figure 4 exhibits the magnetization of the as-prepared samples with different contents of CNTs. As there was no   where the magnetization of magnetite decreased from 30 to 9 emu/gr after the incorporation of silica [17]. Furthermore, due to the small coercivity value, Fe 3 O 4 /PPY/CNTs can be classified as a soft magnet material group. Figure 5 displays the FTIR spectra of Fe 3 O 4 /PPY/CNT nanocomposites with different contents of CNTs. The peak at 570.2 cm -1 revealed Fe-O stretching. The presence of PPY was confirmed at 1072.42 cm -1 due to the C-O-C stretching [18]. The other peaks at 1627.9 and 3402.43 cm -1 were related to O-H bending and stretching bonds due to adsorbed water. Moreover, the results of FTIR also confirmed that PPY successfully interacted with nanocomposites as the correlation peak was shown at 1072.42 cm -1 . Figure 6 indicates the thermogravimetry analysis of Fe 3 O 4 /PPY/CNT nanocomposites, which are examined from room temperature to 800°C in nitrogen atmosphere. The decrease of spectra for all samples at the temperatures below 100°C was related to the adsorbed water evaporation. The data indicated that more water was adsorbed on nanocomposites as the amount of CNTs increases. The sample weight began to decrease from a temperature range of 50°C for all samples. NC2C and NC3C samples experienced the same mass shrinkage which was around 30%. The samples of NC4C and NC5C indicated a mass shrinkage about 35% and 40%, respectively. The mass shrinkage is caused by the thermal decomposition of adsorbed organic substances and CNTs [19].
To investigate the surface area, pore size, and pore volume, BET analysis was conducted with absorption and desorption process. Figure 7 represents the BET nitrogen adsorption/desorption isotherm curve of the as-prepared NC4C nanocomposite. The isotherm curve closely matches to a typical type IV isotherm graph confirming the mesoporous property of the nanocomposite [20]. The BET analysis is conducted mainly to provide surface areas, pore sizes, and pore volumes of the as-prepared nanocomposites. The results of BET analysis are provided in Table 1. The NC4C sample had the largest surface area compared to the other samples. It can be concluded that the NC4C sample had a very small particle size compared to the others. Based on the BET analysis, it may be concluded that sample NC4C has a good adsorption property as compared to other samples.

Conclusions
Fe 3 O 4 /PPY/CNT nanocomposite has been successfully synthesized by the coprecipitation method. The XRD result indicated a cubic structure of the as-prepared Fe 3 O 4 nanoparticles. The SEM analysis revealed that Fe 3 O 4 nanoparticles were deposited on CNTs with a diameter of 100 nm. FTIR of all samples showed that iron oxide and PPY were confirmed at 570.2 cm -1 and 1072.42 cm -1 , respectively. The BET isotherm curve closely matched to a typical type IV isotherm graph that confirms the mesoporous property of the as-prepared nanocomposites with high surface area of about 130 m 2 /g. The VSM results confirmed that the as-prepared nanocomposite was superparamagnetic and had soft magnetic property. Based on the characterization results, the as-prepared Fe 3 O 4 /PPY/CNT nanocomposites are promising for many applications such as battery, as supercapacitor, and also as a substrate with a magnetic property to support catalyst development.

Data Availability
The data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.