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Influence of TiO2 addition on the magnetic properties of carbon-based iron oxide nanocomposites synthesized using submerged arc-discharge

The influence of TiO2 addition on the magnetic properties of Fe3O4 and carbon nanocomposites was studied. The nanocomposites were synthesized using a one-step submerged arc-discharge method in a liquid medium of 50% ethanol solution using two carbon electrodes. The magnetic properties of the resulting materials were characterized using a Vibrating Sample Magnetometer (VSM). The changes in the crystalline structures were studied using X-ray Diffraction (XRD). We confirmed that TiO2 addition caused no changes in magnetite phase Fe3O4. Several intense peaks in the XRD diffractogram at 2θ: 35.5, 43.1 and 57° corresponded to diffracted Fe3O4 magnetite panes of (311), (400) and (511), respectively. The hysteresis pattern of the magnetic properties analysed by VSM showed that the addition of TiO2 did not affect their superparamagnetic character. However, the TiO2/Fe3O4/C nanocomposites had higher saturated magnetization (Ms) than the Fe3O4/C nanocomposites without TiO2 addition.


Introduction
Magnetic nanocomposites are materials of interest for study and development because they have a variety of potential applications in various fields-for example, for magnetic storage, bioimaging, biosensor and drug delivery applications [1][2][3][4][5][6]. This is because magnetic nanocomposite materials have different chemical and physical properties [7]. One example of a magnetic material is Fe3O4 magnetite, which could be used in the environmental field for the degradation of wastewater, especially dye wastewater, when combined with a photocatalyst material (TiO2) [8].
Fe3O4 is one of the iron oxides other than maghemite (γ-Fe2O3) and hematite (α-Fe2O3); it is a more powerful magnetic material than other iron oxides [9]. Magnetite (Fe3O4) has an inverse cubic spinel structure [10]. The crystal structure of Fe3O4 consists of a unit cell which contains oxygen ions in closepacked (face-centred cubic) FCC structure and iron ions that form the tetrahedral and octahedral structures, represented by [Fe 3+ ]A-[Fe 2+ Fe 3+ ]BO4, in which the tetrahedral position is occupied by Fe 3+ ions and the octahedral position is occupied by eight Fe 2+ and eight Fe 3+ ions, as shown in figure 1 [11,12].
Carbon-based magnetic nanocomposites have also received attention due to their beneficial properties. Carbon is a non-metallic element with interesting properties such a high surface area and a porous structure [13]. Carbon forms multiple allotropes, which come in crystalline carbon and noncrystalline forms. Examples of crystalline carbon include graphite, diamond and fullerene, while noncrystalline carbon includes amorphous and activated carbon [14,15]. Incorporating carbon material with 2 magnetic iron oxide makes the magnetic nanocomposite more stable because it can prevent further metal and/or metal oxide oxidation. Moreover, the carbon layer is suitable for surface functionalization with the other molecules due to its greater surface area, making it more easily modifiable [16]. Meanwhile, titanium dioxide (TiO2) is the most effective semiconductor photocatalyst because it has a relatively large energy gap of 3.2 eV, which is suitable for photocatalysts, and it is non-toxic, cheap and abundant in nature [17][18][19]. TiO2 has three phases of crystal structure-anatase, rutile and brookite [17,20]. However, only the anatase and rutile phases are stable enough, whilst the brookite crystalline phase may change at elevated temperatures [20,21].
Taking into account the physical and chemical properties of TiO2, Fe3O4 and carbon materials, their combination is interesting to examine. There are several method reported for producing their combination such as sol-gel [8,22,23], co-precipitation [24], hydrothermal [25]. Saraswati et al. [26], Saraswati et al. [27] and Arikawati and Saraswati [28] conducted coupling of TiO2 with carbon using arc-discharge in a liquid medium [26][27][28][29]. This method is simple, low-cost and efficient in producing combined nanomaterials.
In this study, the addition of TiO2 to an Fe3O4/carbon nanocomposite was performed using the arcdischarge method. The changes in the physical and chemical properties of the resultant material are interesting to study, as they affect its role in a particular application. Therefore, this article will discuss the crystallinity and magnetic properties of Fe3O4/C nanocomposites with and without the addition of TiO2 nanoparticles, synthesized using the arc-discharge method in ethanol-based liquid media.

Synthesis of Nanocomposite TiO2/Fe3O4/C.
A graphite electrode was filled with a mixture of TiO2 anatase, Fe3O4 powder, carbon powder and fructose as a binder in a weight ratio of 1:1:3:2 for nanocomposite TiO2/Fe3O4/C production, while for the preparation of the Fe3O4/C nanocomposite, a graphite electrode was filled with a mixture of Fe3O4 powder and carbon powder with fructose as binder in a weight ratio of 1:3:2. The electrode was heated at a temperature of 180°C for 6 hours. The liquid medium consisted of 300 ml of 50% ethanol solution. Two graphite electrodes, an unfilled carbon electrode and filled carbon electrode, were set in the liquid medium at a very close distance. The synthesis process was conducted by passing a voltage of 20 V and current of 10 A between them. The resulting nanocomposites were collected from the surface and bottom of the liquid medium. The collected TiO2/Fe3O4/C nanocomposite was characterized using X-Ray Diffraction (XRD) Bruker DS to determine the crystallinity and crystal structure. A Vibration Sample Magnetometer (VSM) was used to determine the magnetic properties of the materials at room temperature.

Results and Discussion
After the synthesis process, the products were characterised to confirm their properties. The crystallinity of the synthesized TiO2/Fe3O4/C nanocomposite was analysed using XRD, as shown in figure 2. The resulting diffractogram shows the peaks of the starting materials of the Fe3O4 nanomaterials that appear at 2θ: 35  The surface structure morphology of nanocomposites fabricated using arc-discharge was characterised by scanning electron microscopy (SEM) and EDX elemental analysis, as shown in figure  3 and table 1, respectively. The morphology shown by the SEM analysis of the Fe3O4/C and Fe3O4/TiO2/C nanocomposites at a magnification of 15,000X was spherical and regular. As shown in the SEM images in figure 2, Fe3O4/C materials aggregated in a spherical and regular structure at a nanometre scale, where Fe3O4/C clusters had a smaller size than Fe3O4/TiO2/C, forming microsphere aggregates. It can be concluded the that the structural and morphological properties of the products of Fe3O4/C nanocomposites did not change regardless of the addition of TiO2. Based on the results of the EDX analysis shown in table 1, the C, O and Fe elements in the Fe3O4/C nanocomposite had weight percentages of 66.49%, 27.77% and 5.74%, respectively, while the percentages of C, O, Fe and Ti elements in the Fe3O4/TiO2/C nanocomposite were 64.74%, 29.32%, 4.43%, and 1.50%. In the Fe3O4/C nanocomposite, about 2.033% of O atoms bonded with iron to form Fe3O4; the remaining wt% of oxygen bonded to other atoms to form oxygen-containing functional groups attached to C elements, such as hydroxyl and carboxyl groups. In the Fe3O4/TiO2/C nanocomposite, O bonded with Ti and Fe to form TiO2 and Fe3O4, and to other atoms to form oxygencontaining functional groups and oxygen-containing compounds such as titanium oxide (TixOy). The magnetic properties were then further characterised using VSM. The saturated magnetization (Ms) and coercive magnetic field (Hc) are shown in table 2. Compared to TiO2, Fe3O4 has greater magnetic properties. Fe3O4 has a magnetic moment of 28 μB [31], while TiO2 and C have zero magnetic moment [32][33][34]. Therefore, the magnetic moment depends on the presence and the crystalline structure of Fe3O4.  34]. However, the carbon atoms interacted more with TiO2 due to the wider cavity size of TiO2 compared to that of Fe3O4, as shown in figure 4. Therefore, the magnetization in TiO2/Fe3O4/C was higher because there was less carbon interference in Fe3O4 crystals than in Fe3O4/C. In addition, the Hc of both materials was close to zero, indicating that the superparamagnetic property of the material products was maintained [35].The preferable interaction induced a carbothermal reduction reaction between TiO2 and C, as evidenced by the appearance of a new TiC material peak after the addition of TiO2, which agrees with the XRD spectra shown in figure 1.

Conclusion
The synthesis of Fe3O4/C and TiO2/Fe3O4/C nanocomposites was performed using the arc-discharge method in 50% ethanol liquid media. According to the VSM results, the saturated magnetization of the TiO2/Fe3O4/C nanocomposite is greater than that of the Fe3O4/C nanocomposite because the carbon atoms more easily interfered with the TiO2 crystalline structure than that of Fe3O4. However, the superparamagnetic properties of both materials were maintained.