One-step synthesis of TiO₂/Fe₃O₄/C hydrophilic magnetic nanocomposites using the magnetized submerged arc discharge method

Abstract

This study investigates the production of TiO₂/Fe₃O₄/C hydrophilic magnetic nanocomposites produced via a one-step novel method using the submerged arc discharge method applied in liquid media of ethanol and ethanol–ammonia under the influence of external magnetic field. The magnetized submerged arc discharge forms a longer-lasting plasma arc, thereby affecting the growth of spherical and tubular carbon nanocomposites. More reactive species enrolled in the carbon growth in the magnetized submerged arc discharge, resulting in the ordered crystalline structure. The nanocomposites synthesized in ethanol–ammonia show hydrophilic characteristics, indicated by their dispersion improvement in the water and the presence of the vibrations of hydroxyl, amine, and –CN functional groups in Fourier transfer infrared (FTIR) spectra. The preparation technique described in this study using magnetized submerged arc discharge is supposed to be a simple and less costly preparation method that can potentially be further applied to produce numerous other nanocomposites.

GRAPHICAL ABSTRACT

Keywords:

• Magnetic nanocomposite
• carbon
• TiO₂
• Fe₃O₄
• submerged arc discharge
• ethanol
• ammonia
• magnetic field

Introduction

Composites can be formed by combining two or more inorganic materials or organic materials [1]. Nanocomposites are hybrid materials created on a nanometer scale produced by mixing two or more materials, wherein each component material might exhibit different physical and chemical characteristics. After being combined, these component materials form a new substance via van der Waals interactions, hydrogen bonding, weak electrostatic interactions, and covalent bonding [2], influenced by their individual components’ structure, composition, interfacial interactions, and properties. The nanocomposites possibly exhibit different physical and chemical characteristics; the constituent materials maintain their respective identities and characteristics, but the resulting nanocomposites exhibit new characteristics different from those of their constituent components [3,4]. Magnetic nanocomposites, which exhibit multifunctional properties, have been demonstrating rapid developments, making them suitable for imaging, drug delivery systems [5], biological separations [6], detection of cancer cells [7,8], and pathogens [9], and magnetic hyperthermia treatment for cancer [10]. In obtaining magnetic nanocomposites that exhibit superior properties potentially applied in the biomedical field, combining nanomaterial with magnetic components in the synthesis process is necessary.

Titanium dioxide (TiO₂) is an inorganic material that exhibits attractive characteristics, such as ubiquity, low cost, non-toxicity, and good biocompatibility [11], and the American Food and Drug Administration has approved its use. Thus, TiO₂ is widely used in food and medicinal products [12,13]. TiO₂ has potential use as an antibacterial agent in dental and orthopedic implants and postoperative drug release implants [11,14], that can enhance the osteogenic activity of implants [15,16], and coating material to filter UV light composited with the other metal oxides [3,4]. Moreover, in biomedicine application, TiO₂ can be applied as a nanocarrier (carrier material) for drug delivery [14,17] and gene delivery [18,19]; however, TiO₂ does not display magnetic strength, making it challenging to target a treatment location.

On the other hand, iron oxide of Fe₃O₄ is an inorganic material that exhibits high magnetization; therefore, it can target an intended location with minimal side effects in its bioapplication. This property renders magnetic nanoparticles attractive for application in several biomedical fields, such as drug delivery [20], enhancing and targeting gene delivery [21], hyperthermia, tumor imaging [22], theranostics [23], and tissue engineering. However, Fe₃O₄ requires a support material to prevent oxidation and to facilitate further functionalization [5,21,24]. One of the support materials that could address this problem is carbon, which generally exhibits good biocompatibility and permeability, and its surface could be easily modified [25].

Various methods can be used to form carbon nanocomposites, including sol-gel [26], hydrothermal [27], self-assembly process [28], chemical coprecipitation [29], microemulsion [30], chemical vapor deposition (CVD) [31], thermal deposition [32], and arc discharge [33,34]. Compared with the other methods, arc discharge offers the advantages of being simple, easy to develop, economical, and able to produce various products [35–37]. The arc discharge method created using a liquid medium as a substitute for the vacuum system is known as submerged arc discharge; this substitution renders the method simpler, cheaper, and able to produce high-quality products [33,34].

In addition, submerged arc discharge is commonly used to produce nanoparticles, such as carbon nanoparticles [38], fullerenes [39], carbon onions [34,40], CNTs [41], magnetic metals [42], alloys [43,44], metal carbides [45], and non-magnetic nanoparticles [46]. The approach involving variations in liquid medium has received considerable attention because such an approach allows for nanocomposites to be synthesized and modified in a single step.

Our previous study carried out nanomaterial synthesis using the submerged arc discharge method in an ethanol/acetic acid medium, resulting in nanomaterial modified by the oxygen-containing functional groups that can increase photodegradation activity [47]. The study was developed to prepare nanomaterial using the submerged arc discharge method in ethanol with urea added, producing nanomaterials that increased their hydrophilicity [48]. The submerged arc discharge method was further modified using an ethanol and ethylenediamine mix medium, producing aminated nanomaterial [49,50]. However, ethylenediamine is more reactive and flammable compared to ammonia.

Keidar et al. [51] show that the arc discharge method in a vacuum system with the addition of an external magnetic field can influence the growth of CNTs. However, using a vacuum system in this synthesis process results in high costs and a longer time for preparing the sophisticated chamber and instrument without vacuum leaks. The arc discharge method submerged in a liquid medium needs less simple equipment. By contrast, the idea of ​​using an external magnetic field in the submerged arc discharge method has not yet been fully developed. Varying the chemical composition in the liquid medium allows nanocomposite preparation and modification in a single step.

Therefore, this study aims to investigate the effect of the external magnetic field on the particle structure of TiO₂/Fe₃O₄/C nanocomposites produced via the submerged arc discharge method. In this study, ethanol and ethanol–ammonia were used as liquid media. Ethanol donates carbon atoms during a synthesis process. Ammonia solution may be played role in surface modification of nanocomposites. Interestingly, the application of an external magnetic field affects particle growth during a synthesis process.

Experimental details

Materials and tools

The materials used in this study were graphite electrodes (Qingdao Tenry Cardon Co., Ltd.; carbon: 99%; density: 1.95 g/mL; electrical resistance: 7–10 Ω; dimensionless electrodes (ϕ = 10 mm; t = 100 mm); dimensions of the perforated electrode (ϕ_outer = 10 mm; ϕ_inner = 7 mm; t = 100 mm)), titanium(IV) oxide anatase powder (Sigma-Aldrich; 99.7% trace metal base; density: 3.9 g/mL at 25 °C; molar mass: 79.87 g/mol), graphite powder (Merck; molar mass: 12.01 g/mol), fructose, iron(II, III) oxide powder (Sigma-Aldrich; 97% trace metal base; density: 4.8–5.1 g/mL at 25 °C; molar mass: 231.53 g/mol), ethanol 70% (technical grade), toluene (Mallinckrodt Chemicals; molar mass: 92.14 g/mol), ammonia 25% (Merck; boiling point: 32 °C; density: 0.90 g/mL), and distilled water. The tools used in this research were as follows: assembled ball milling tools (50 Hz, 220 V), a series of submerged arc discharge tools, DC power supply (Krisbow; 160 A; 220 V; 1PH), 1200X OTF furnace with quartz tube (ϕ = 75 mm; t = 1 mm), neodymium magnet (10 mm × 10 mm × 5 mm; 11,000–14,000 Gauss).

Preparation and characterization of TiO₂/Fe₃O₄/C nanocomposites

The nanocomposites were synthesized using the planetary ball milling method, followed by the submerged arc discharge method. In the ball milling method, a mixture of TiO₂, Fe₃O₄, and graphite was prepared at a 1:1:3 ratio (w/w/w). Next, the material was milled for five hours with a constant rotating frequency of 20 Hz (±1200 rpm) [52]. The milled material was sieved using a 200-mesh sieve and then was used to synthesize modified graphite electrodes consisting of the mixture of TiO₂, Fe₃O₄, and graphite via the submerged arc discharge method.

In the submerged arc discharge method, two electrodes were used: an anode and a cathode. The graphite cathode was tapered, whereas the anode had a flat surface. The anode was made by punching holes in the graphite electrodes (ϕ_hole = ⁓7 mm) and then filling them with a paste of the milling products, that is, the mixture of TiO₂, Fe₃O₄, and graphite powders with fructose as a binder at a weight ratio of 5:2. The paste was prepared by adding a sufficient amount of ethanol to the mixed powders to form a paste with the help of sonication for 10 min. The paste was then filled into the perforated electrode, which was then heated in the OTF furnace at a temperature of 250 °C for six hours.

Both the anode and cathode were cleaned with ethanol before use. The electrodes were mounted on electrode holder, positioned relatively close to each other (approximately 1 mm), and then immersed in the desired liquid medium. The external magnetic field was applied 1 cm above the liquid medium in a beaker glass (500 mL in size), which served as the submerged arc discharge chamber. Two various liquid media were used, consisting of (1) a mixture of 70% ethanol and 10% ammonia at volume ratio of 1:1 (v/v) with a total volume of 300 mL and (2) a 300 mL 70% ethanol. The electrode holders were connected to a power supply that was set to 26.4 V and 40 A. When the power supply was turned on, sparks appeared fairly quickly (in seconds [38]) around the anode and the cathode submerged in a liquid medium. The gas produced from the arc discharge process was evacuated by a vacuum pump connected to an inlet and outlet gas hose. The cathode was brought closer to the anode within a certain distance (±1 mm) to maintain the arc. The arc emission was analyzed using optical spectroscopy (Ocean Insight Maya 2000 Pro Series High Sensitivity Spectrometer). After the arc discharge occurred, the color of the liquid medium changed from clear to black, showing that carbon-based nanocomposites had begun to form. This arc discharge process was also compared without an external magnetic field.

The resulting nanocomposite was separated from the liquid media through centrifugation and drying in a vacuum chamber. The nanocomposite synthesized in the ammonia-containing liquid medium was washed with distilled water to remove the excess ammonia. TiO₂/Fe₃O₄/C nanocomposites were further washed with toluene and ethanol to remove amorphous C and prevent particle agglomeration. The remaining solvent was re-evaporated using a vacuum chamber. After the purification stage, the nanocomposite powders were further characterized using X-ray diffraction (XRD Bruker D8 Advance; Cu 1.54 nm; 40 V and Miniflex XRD Rigaku; Cu 1.54 nm), Fourier Transform Infrared (FTIR), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX Brand Hitachi SU 3500), transmission electron microscopy (TEM; HT7700), vibrating sample magnetometry analysis (VSM Oxford type 1.2 H), and Raman spectroscopy (Raman iHR320 Horiba).

In this study, four types of nanocomposites were produced, namely, TiO₂/Fe₃O₄/C-E, TiO₂/Fe₃O₄/C-EA, TiO₂/Fe₃O₄/C-E (M), and TiO₂/Fe₃O₄/C-EA (M). These TiO₂/Fe₃O₄/C nanocomposites correspond to the purified nanocomposites produced via the submerged arc discharge method using 70% ethanol without magnetic field, 70% ethanol–10% ammonia without magnetic field, 70% ethanol with magnetic field, and 70% ethanol–10% ammonia with magnetic field, respectively.

Results and discussion

Figure 1 shows the mechanism of the formation of TiO₂/Fe₃O₄/C nanocomposites and the representative arc profiles obtained in the submerged arc discharge synthesis process performed in different liquid media without (left) and with a magnetic (right) field. The plasma arc formed under a magnetic field was larger and more stable than without a magnetic field. Moreover, the experiments showed that applying a magnetic field above the liquid medium (horizontal plane) could stabilize the formed plasma. As a result, the plasma rendered to last longer and showed increased ion density, especially the ion density of carbon plasma species. C atoms evaporate more easily and have a much higher condensation temperature than other atoms (e.g. Ti, Fe, and O); thus, C condenses into its solid phase more quickly. Greater C density might increase carbon graphitization (more sp² bonds being formed), resulting in a more significant number of graphitic layers. In addition, a longer plasma arc generation possibly causes more elongated or tubular carbon nanostructure to form [34,53].

Figure 1. The schematic illustration of TiO₂/Fe₃O₄/C nanocomposites synthesis via the submerged arc discharge method in a liquid medium without (left) and with (right) a magnetic field.

When a high current flows from the cathode to the anode, an electron jump occurs, marked by the appearance of plasma arcs between the cathode and the anode, known as a hot plasma zone. The plasma arc occurs quickly (in seconds) [38]. The submerged arc discharge with a magnetic field (Figure 1, images on the right side) produces a more stable plasma arc with a longer processing time than submerged arc discharge without a magnetic field. This phenomenon affects the structures of the formed carbon nanomaterials. A rapid increased temperature creates gas bubbles surround this plasma zone formed by the interaction between the electrodes and the liquid medium. These gas bubbles facilitate the cooling of the region around the sparks. In this hot plasma zone, the hollow rod filled with TiO₂, Fe₃O₄, and carbon graphite evaporated and was further ionized. Due to water turbulence during arc discharging, the resulting ions spread at a region in the liquid medium having a lower temperature (cold zone); as a result, they quickly condense into a solid phase, forming TiO₂/Fe₃O₄/C nanocomposites [34,54–57]. The two condensation zones produce particles with different shapes. Zone I created elongated (tubular) nanostructures, whereas zone II formed arbitrary spherical shapes resulting from isotropic growth [34,58].

The different liquid media in the synthesis process produced various reactive species contributing to the plasma arc identified through OES analysis. The emission spectrum obtained when 70% ethanol was used as a liquid medium is shown in Figure 2(a). The dominant peak at 656.9 nm is the Balmer H_α series emitted by the H atoms released from the breakdown of dielectric molecules. Two other 516.0 and 468.3 nm emission peaks emerged from the C₂ swan bands. Another peak is also observable at 777.4 nm, representing the excited atomic oxygen plasma emission (O1). This result indicates that the discharge nearly completely dissociated dielectric organic molecules.

Figure 2. The OES spectra of plasma arc revealed in the submerged arc discharge method using (a) ethanol and (b) ethanol–ammonia as liquid media; the inset images are the dispersion (in water) of nanocomposites A–D for TiO₂/Fe₃O₄/C-E, TiO₂/Fe₃O₄/C-EA, TiO₂/Fe₃O₄/C-E (M), and TiO₂/Fe₃O₄/C-EA (M), respectively; (c,d) the FTIR spectra of nanocomposites A–D.

The emission spectrum obtained when ethanol–ammonia was used as a liquid medium is shown in Figure 2(b). The dominant peak at 656.1 nm is the H_α Balmer series emitted by hydrogen atoms; this peak is significantly higher than that obtained in the OES of plasma arc using ethanol-only liquid medium, suggesting the presence of more H atoms that possibly came from the ethanol and ammonia molecules. The other peaks were as follows: the peak of H₂ emission at 612.0 nm, three peaks from the C₂ swan bands with emission at 547.1, 485.2, and 467.5 nm, two peaks from N₂ emission spectra at 438.2 and 385.0 nm, and the resonance peak of O1 with emission spectra at 777.0 nm. The appearance of the emission spectra from N₂ indicates the ammonia decomposition in the liquid medium used. The overall emission spectrum results align with the standards of the Atomic Spectroscopy Databases of the National Institute of Standards and Technology. Based on the standard databases, the emission spectra of H_α, H₂, C₂ (swan bands), N₂, and O₁ were located at 656.3 nm, 612 nm, 465–590 nm, 300–450 nm, and 777 nm, respectively. Both emission spectra in Figure 2(a,b) show that the dominant observed peaks were atomic peaks, while no ionic peaks were visible [57,59–63].

During the synthesis process, the plasma arc generally formed quickly (in seconds) at high temperatures, causing the interaction of TiO₂, Fe₃O₄, and C in the liquid media. Furthermore, the addition of ammonia induced the nitrogen N element addition, as confirmed by the EDX analysis. In the synthesis process, a high-temperature plasma arc generated the interaction of TiO₂, Fe₃O₄, and C with the liquid medium, resulting in the carbon nanocomposites of these materials. The gases formed in the process are highly plausible of H₂, O₂, N₂, and CO [34,56,64], which agree with the presented emission spectra, as results of the proposed reaction before carbon nanomaterial condensation, as shown in Equations (1–8). (1) $${\text{During arc plasma: 2C}}_2{\mathrm{H}}_5\text{OH}\to {\text{4C + 6H}}_2\uparrow {\text{+\hspace{0.17em}O}}_2\uparrow$$(1) (2) $${\text{NH}}_4\text{OH}\to {\text{NH}}_3{\text{\hspace{0.17em}+\hspace{0.17em}H}}_2\mathrm{O}$$(2) (3) $${\text{NH}}_3\to {\text{H}}_2\uparrow {\text{+ N}}_2\uparrow$$(3) (4) $${\text{C + H}}_2\text{O}\mathrm{\hspace{0.17em}\to \hspace{0.17em}}\text{CO}\uparrow {\text{\hspace{0.17em}+ H}}_2\uparrow$$(4) (5) $${\text{C + C}}_2{\mathrm{H}}_5{\text{OH + 2NH}}_4\text{OH}\to {\text{8H}}_2\uparrow \text{+\hspace{0.17em}3CO}\uparrow {\text{+\hspace{0.17em}N}}_2\uparrow$$(5) (6) $${\mathrm{C}}_2{\mathrm{H}}_5{\text{OH + NH}}_4\text{OH}\to {\text{C}}_2{\mathrm{H}}_5{\text{NH}}_2{\text{\hspace{0.17em}+\hspace{0.17em}H}}_2\mathrm{O}$$(6) (7) $$\text{C}\to \to \to {\text{C}}_{\text{nanomaterial}}$$(7) (8) $${\mathrm{C}}_2{\mathrm{H}}_5{\text{NH}}_2\to \to \to {\text{C}}_{\text{nanomaterial}}{\text{+ H}}_2\uparrow {\text{+ N}}_2\uparrow$$(8)

As shown in the inset of Figure 2(a,b), the nanocomposites produced in the submerged arc discharge using ammonia-added liquid medium have better dispersion (in water) than those made in liquid medium without ammonia, indicating that the ammonia gas vaporized during the arc discharge can convert to amine-based plasma species and further attach to carbon surface as amine groups, resulting in a hydrophilic nanocomposite surface. The success of the surface modification of the nanocomposites using amine groups can also be confirmed by analyzing the FTIR spectra, as shown in Figure 2(c,d).

The FTIR findings suggest that TiO₂/Fe₃O₄/C-EA and TiO₂/Fe₃O₄/C-EA (M) have –OH, –NH, and –CN groups, leading them to interact better with water. The dispersity of TiO₂/Fe₃O₄/C-E and TiO₂/Fe₃O₄/C-E (M) (produced without ammonia) significantly improved after sonication, indicating that the amount of hydrophilic group on the particle surface was possibly less than in the nanocomposites produced in the ammonia-containing liquid media that is in good agreement with FTIR data showing that only the –OH vibration peak was observed in TiO₂/Fe₃O₄/C-E and TiO₂/Fe₃O₄/C-E (M).

The FTIR spectra, as presented in Figure 2(c,d), show the similar existence of several functional groups, such as O–H, N–H, =C–H, –C–H, –C=C–, –C=O–, –C–OH, –C–N, and MO (M = metal). The four synthesized nanocomposites had similar FTIR spectra. For instance, the peaks at ∼2850 and ∼2920 cm⁻¹ correspond to the symmetric vibration –C–H (possibly from CH₃; sp³) and to the asymmetric vibration = C–H (and the asymmetric vibration CH₂; sp²). Furthermore, the vibrational peaks at ∼1640 cm⁻¹ were assigned as an overlap between C=C aromatic and C=O vibrations. In addition, the absorption values of ∼1410, 500–700, and ∼3400 cm⁻¹ are the bending vibrations of –C–OH, the metal oxide, and the –OH vibration, respectively. This –OH vibration at ∼3400 cm⁻¹ possibly overlapped with the –NH vibrational peak [54,65,66]. However, as observed in Figure 2(d), FTIR spectra of TiO₂/Fe₃O₄/C-EA (spectra B) and TiO₂/Fe₃O₄/C-EA (M) (spectra D) showed slightly different patterns at ∼1000 cm⁻¹, which correspond to the –CN (N heterocyclic ring modes) vibrational peak. This peak was seen only in FTIR spectra of TiO₂/Fe₃O₄/C-EA and TiO₂/Fe₃O₄/C-EA (M), indicating the successful surface modification by the amine groups that highly possibly attach to carbon atoms covalently.

The use of different liquid media with and without magnets also influenced the particle yields. As shown in Figure 3(a), adding ammonia and applying a magnetic field increased the yield, possibly due to the availability of more energetic atomic species that eventually condensed as composite carbon particles.

Figure 3. (a) The yield and (b) magnetism hysteresis loop (VSM graph) of the TiO₂/Fe₃O₄/C nanocomposites resulting in submerged arc discharge under different conditions and the interaction of the nanocomposites with the magnets (inset).

Using different liquid media and an external magnetic field also affected the physical and chemical characteristics of the resulting nanocomposites. Figure 3(b) shows the hysteresis curves analyzed through the VSM instrument. The inset shows the photographs of TiO₂/Fe₃O₄/C nanocomposite interactions with the magnet bar, confirming that TiO₂/Fe₃O₄/C nanocomposites have magnetic properties because they were attracted to the magnet. The detailed values of the magnetization and coercive field are listed in Table 1.

Table 1. Magnetic properties, crystallite size, and I_D/I_G values of TiO₂/Fe₃O₄/C nanocomposites produced via the submerged arc discharge method.

Nanocomposites	Ms^a (emu/g)	Hc^a (T)	Mr^a (emu/g)	Crystallite sizes^b (nm)	ID/IG^c
TiO2/Fe3O4/C-E (M)	20.90	0.02	2.63	31.74	0.07
TiO2/Fe3O4/C-E	19.21	0.03	2.46	10.60	0.16
TiO2/Fe3O4/C-EA (M)	17.18	0.03	1.92	35.64	0.04
TiO2/Fe3O4/C-EA	15.79	0.03	1.73	20.83	0.16

^a The data were taken from VSM graphs.

^b The values were calculated using Scherrer equation for C (002) peak.

^c The data were taken from Raman spectra.

In terms of magnetization characteristics, TiO₂/Fe₃O₄/C-E (M) and TiO₂/Fe₃O₄/C-E had higher magnetization saturation (M_s) values at 20.90 and 19.21 emu/g, respectively, than TiO₂/Fe₃O₄/C-EA (M) and TiO₂/Fe₃O₄/C-EA, the M_s values of which were 17.18 and 15.79 emu/g, respectively. These results indicate that the ammonia in the liquid medium affected the magnetic properties of the TiO₂/Fe₃O₄/C nanocomposites. Ammonia acts in iron oxide leaching [67], possibly changing the structure of Fe₃O₄ and turning it into less-magnetic iron oxide (e.g. α-Fe₂O₃). In addition, the external magnetic field influenced the magnetic properties of the nanocomposites, as evidenced by the higher magnetization of the TiO₂/Fe₃O₄/C nanocomposites produced under a magnetic field compared with their counterpart. TiO₂/Fe₃O₄/C-E (M) had the highest M_s value at 20.90 emu/g. The TiO₂/Fe₃O₄/C nanocomposites shown in Figure 3(b) exhibit nearly superparamagnetic properties, as their coercive magnetic field (H_c) and remanent magnetization (M_r) values ​​were close to the coordinates (0,0). This hysteresis pattern is similar to the superparamagnetic hysteresis pattern of other superparamagnetic nanoparticles [68,69].

Figure 4(a,b) shows the diffractogram of the starting materials, the milled TiO₂/Fe₃O₄/C, and the synthesized TiO₂/Fe₃O₄/C nanocomposites. The starting materials of TiO₂, Fe₃O₄, and C show a good match with the references in the XRD database, PDF No. 21-1272 (TiO₂), No. 89-0691 (Fe₃O₄), and No. 41-1487 (graphite), respectively. As shown in Figure 4(a), the milled TiO₂/Fe₃O₄/C has a crystalline structure showing the diffraction peaks of TiO₂ at 2θ 25.32° (101), 48.06° (200), 53.58° (105), 55.36° (211), 62.78° (204); of carbon at 2θ 26.64° (002), 54.68° (004); and of Fe₃O₄ at 2θ 30.20° (220), 35.56° (311), 43.20°(400), 57.12°(511), and 62.70° (440); which are similar to their initial diffraction pattern. These features indicate that milling results only in a mixture rather than new crystalline forms.

Figure 4. The XRD patterns of (a) the starting materials consisting of A→D for graphite, TiO₂, Fe₃O₄, and milled of mixture TiO₂, Fe₃O₄, and graphite, respectively; (b) the resulting nanocomposites consisting of E→H for TiO₂/Fe₃O₄/C-E, TiO₂/Fe₃O₄/C-EA, TiO₂/Fe₃O₄/C-E (M), and TiO₂/Fe₃O₄/C-EA (M), respectively; and (c) Raman spectra of the resulting nanocomposites E→H.

Figure 4(b) shows that TiO₂/Fe₃O₄/C-E, TiO₂/Fe₃O₄/C-EA, TiO₂/Fe₃O₄/C-E (M), and TiO₂/Fe₃O₄/C-EA (M) have similar diffraction patterns. These patterns not only show the intense diffraction peaks of the starting materials, namely, the peaks at 2θ ⁓25.30° for TiO₂ (101), 2θ ⁓26.5° and ⁓54.5° for C (002) and C (004), and 2θ ⁓35.5° for Fe₃O₄ (311), but also several other new peaks. Interestingly, the diffraction peaks of TiO₂ (101) and Fe₃O₄ (311) remained lower than the new peaks after the arc discharging process, indicating that both materials almost completely reacted in this process. For instance, the new peaks which correspond to titanium carbide TiC (111), (200), (220), and (311) emerged at 2θ ⁓36.0°, ⁓41.8°, ⁓60.5°, and ⁓72.5°, respectively, in line with the JCPDS standard No. 32-1383 (Khamrabaevite, TiC). In addition, Fe₃C peaks are observed at 2θ ⁓43.2° (102), ⁓44.2° (220), ⁓45.0° (031), and ⁓54.5°, (230) which match with the JCPDS standard No. 35-0772 (Cohenite, Fe₃C). The Fe peak is also observable at 2θ ⁓44.7° (110) following the JCPDS standard No. 06-0696 (Iron, Fe). The metal carbides and iron metals, such as TiC, Fe₃C, and Fe, resulting in arc plasma, were formed in the proposed reaction shown in Equations (9–13). (9) $${\text{In arc plasma: TiO}}_2\text{+ C}\to {\text{TiC + O}}_2\uparrow$$(9) (10) $${\text{Fe}}_3{\mathrm{O}}_4\text{\hspace{0.17em}+\hspace{0.17em}C}\to {\text{Fe}}_3{\text{C + 2O}}_2\uparrow$$(10) (11) $${\text{Fe}}_3{\mathrm{O}}_4\text{\hspace{0.17em}+\hspace{0.17em}4C}\to \text{3Fe + 4CO}\uparrow$$(11) (12) $${\text{2Fe}}_3{\text{C + O}}_2\to \text{6Fe + 2CO}\uparrow$$(12) (13) $${\text{Fe}}_3{\text{C\hspace{0.17em}+\hspace{0.17em}O}}_2\to {\text{3Fe + CO}}_2\uparrow$$(13)

As presented in Table 1, the crystallite size value calculated by the Scherrer equation [70–72] shows that the crystallite sizes of carbon graphite assigned to C (002) at 2θ ⁓26.5° for TiO₂/Fe₃O₄/C-E (M) and TiO₂/Fe₃O₄/C-EA (M) are much larger than those of the carbon graphite in TiO₂/Fe₃O₄/C-E and TiO₂/Fe₃O₄/C-EA. The larger crystallite size of the nanoparticle produced in the submerged arc discharge with a magnetic field added indicates that the magnetic field induced the formation of more graphitic layers encapsulating the metal oxide, resulting in larger particles, as confirmed by the TEM analysis.

Carbon graphitization in the composite materials was further identified by Raman spectroscopy, as shown in Figure 4(c). Graphitic structure regularity was predicted from the peak values of the D, G, and 2D bands on the Raman shift, namely, ∼1350, ∼1580, and ∼2700 cm⁻¹, respectively [73,74]. All synthesized TiO₂/Fe₃O₄/C nanocomposites have a G band with higher intensity than the D band, indicating that the sp² bond was more dominant than the sp³ bond. Therefore, the TiO₂/Fe₃O₄/C nanocomposites produced have less amorphous carbon content and ordered graphitized structures with low defects, especially for the nanocomposites produced with magnetic field. The magnetic field guides the active species in the plasma arc to have confined plasma, and therefore, carbon species plasma can interact with each other optimally, resulting in the graphitic structure with dominant atom carbon in sp² hybridization. The presence of a magnetic field reveals the downshift of the G band in Raman spectra, indicating the elongation of the tubular structure [75].

Furthermore, the peak shifting of the D and G bands, as shown in the emphasized insets, is highly possible due to the doping of Fe and Ti to carbon, resulting in metal carbide, as earlier discussed, as well as the other elements coming from the molecules in liquid medium (i.e. ammonia, ethanol). These elements also might influence the carbon structure, giving the band shifting in Raman spectra [74,76].

Table 1 also shows the I_D/I_G ratio of the TiO₂/Fe₃O₄/C nanocomposites produced in submerged arc discharge. An I_D/I_G value is generally used to estimate carbon structure defects. The higher the I_D/I_G value, the higher the defect structure; as a result, carbon graphitization of nanocomposites possibly decreases. In agreement with the earlier discussion, the external magnetic field affected the I_D/I_G parameters. This is evidenced by the lower I_D/I_G values of the TiO₂/Fe₃O₄/C nanocomposites produced under a magnetic field (i.e. TiO₂/Fe₃O₄/C-E (M) and TiO₂/Fe₃O₄/C-EA (M)) than those of the TiO₂/Fe₃O₄/C nanocomposites produced without a magnetic field (i.e. TiO₂/Fe₃O₄/C-E and TiO₂/Fe₃O₄/C-EA). Therefore, the significant change in the I_D/I_G ratios translates into a reduction in structural irregularity and an increase in the graphitization structure.

In addition to the D and G bands, the peaks of the 2D band are also shown. The intensity of the 2D band determines the number of graphene layers [74]. The presence of the 2D band indicates a structural modification in the graphitic multilayers, which could represent the multilayers in carbon nanotubes, carbon nanostructured materials, or the other structural forms of carbon allotropes. In addition to the D, G, and 2D bands, which display graphitization regularity, other relatively weak peaks were also observable at a Raman shift of 400–1000 cm⁻¹, corresponding to the presence of metal carbide compounds in the resulting nanocomposite [77–80], as also detected in the XRD patterns.

Moreover, the EDX analysis was employed to determine the elemental composition in TiO₂/Fe₃O₄/C nanocomposites. The use of ethanol and ethanol–ammonia as liquid media produced variations in the composition of the constituent elements, as shown in Table 2. The use of ethanol contributes more C elements. This is evidenced by the higher percentages of C element in TiO₂/Fe₃O₄/C-E and TiO₂/Fe₃O₄/C-E (M) than in the nanocomposite produced using ethanol–ammonia. The N element was found in TiO₂/Fe₃O₄/C produced using ethanol–ammonia (TiO₂/Fe₃O₄/C-EA) but absent in TiO₂/Fe₃O₄/C produced using liquid ethanol medium (TiO₂/Fe₃O₄/C-E). The detected N element most likely came from the amine groups attached to the carbon nanomaterial surface. The absence of N element in TiO₂/Fe₃O₄/C-E and TiO₂/Fe₃O₄/C-E (M) indicated they do not have an amine group, as shown in the FTIR spectra and dispersion test results. Regarding magnetic properties, the percentage of Fe in both weight and atomic percent was directly proportional to the VSM results. The higher the percentage of Fe, the higher the magnetization.

Table 2. The constituent elements in TiO₂/Fe₃O₄/C nanocomposites produced using the submerged arc discharge method.

Nanocomposites	Atomic Percent (At%)
	C	O	Ti	Fe	N
TiO2/Fe3O4/C-E	66.9	26.4	2.1	4.6	–
TiO2/Fe3O4/C-EA	52.5	22.5	0.6	1.0	23.4
TiO2/Fe3O4/C-E (M)	64.6	26.6	2.9	5.9	–
TiO2/Fe3O4/C-EA (M)	48.2	27.7	1.7	3.3	19.1

The nanocomposites obtained are having spherical and tubular in shape, as shown in Figure 5, in which the diameter of the spherical and tubular materials produced under a magnetic field was wider than that of their counterpart. The applied magnetic field dramatically altered the geometry and density of the plasma arc. A magnetic field produces a plasma arc that is more stable and longer lasting; also, it can change the direction of a plasma arc, and the distribution of ion and electron density can be significantly improved. Therefore, the changes in the plasma arc and the nucleation process induce nanocomposite growth with varying diameters [81,82].

Figure 5. The TEM images and the diameter histogram of nanocomposites in spherical (a–d) and tubular (e–h) structures of TiO₂/Fe₃O₄/C-E, TiO₂/Fe₃O₄/C-EA, TiO₂/Fe₃O4/C-E (M), and TiO₂/Fe₃O₄/C-EA (M), respectively.

The dominant diameters of the spherical and tubular particles are ∼20 nm and ∼15 nm for TiO₂/Fe₃O₄/C-E and ∼15 nm and ∼10 nm for TiO₂/Fe₃O₄/C-EA; both of these nanocomposites were produced without a magnetic field, as shown in Figure 5(a,e) and Figure 5(b,f), respectively. As for the nanocomposites produced under a magnetic field, the dominant diameters of the spherical and tubular particles are ∼30 nm and ∼25 nm for TiO₂/Fe₃O₄/C-E (M); ∼25 nm and ∼20 nm for TiO₂/Fe₃O₄/C-EA (M), as shown in Figure 5(c,g) and Figure 5(d,h).

Apart from the spherical and tubular particles, solid spherical and core–shell spherical particles also formed, wherein the metal with a darker area is the core and the carbon with a lighter area is the shell. These results suggest that the products of the submerged arc discharge synthesis were formed in two zones (see Figure 1): zone 1 formed the elongated structures, whereas zone II formed the arbitrary spherical structures. A magnetic field is known to affect the generation of plasma arcs, thereby affecting the growth of particles.

The particles produced under a magnetic field had a larger diameter than their counterpart. This is because the nanocomposites synthesized under a magnetic field would consist of more graphitic layers because the magnetic field can maintain more reactive species as the arc discharge stays longer [34,56,58]. This suggests that the nanocomposites produced under a magnetic field have lower defects than nanocomposites produced without a magnetic field, which is consistent with the results of the Raman analysis.

Conclusion

Using different liquid media with and without a magnetic field, TiO₂/Fe₃O₄/C nanocomposites with tubular and spherical forms were formed through the submerged arc discharge method. The arc plasma in liquid medium with ammonia added shows the emission peaks of N atoms and N₂ in its OES spectra, suggesting the decomposition of ammonia vapor. Moreover, this process produces nanocomposites with additional N atom in addition to the other detectable elements of C, O, and Fe atoms. The addition of ammonia to the liquid medium resulted in the nanocomposite surface modification, which is highly possible by the amine group attached on carbon atoms in graphitic layers of nanocomposites, strengthen by the FTIR analysis confirming the presence of the hydroxyl, amine, and –CN vibration peaks. In addition, the dispersion test the collected nanocomposites produced in ethanol-ammonia medium demonstrated good dispersibility in water, indicating their hydrophilic characteristics. Moreover, the resulting plasma arc with magnetic field is more stable and longer lasting, resulting in nanocomposites in spherical and tubular forms with more graphitic layers, less defected structure, and improved magnetic property. The magnetized plasma arc allows reactive species to stay longer, causing further growth in the carbon nanocomposites resulting a larger diameter size. The different properties and characteristics displayed in all TiO₂/Fe₃O₄/C nanocomposites have potential applications in various fields of nanoengineering. The results of controlling and optimizing the method presented in this study provide essential information as a reference in developing a much more efficient technique for synthesizing carbon-based nanomaterials with unique properties that might be applicable in various nanoengineering, such as the electronic engineering devices, bioelectronic sensor, biomedicine applications, and the other application which required carbon nanomaterials with hydrophilic surface characteristics.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors received the financial support from the Ministry of Research and Technology/National Research and Innovation Agency of the Republic of Indonesia, under grant research with project number 221.1/UN27.22/HK.07.00/2021, 673.1/UN27.22/PT.01.03/2022 and 380.1/UN27.22/PT.01.03/2023.

Notes on contributors

Teguh Endah Saraswati

Teguh Endah Saraswati is an associate professor at the Department of Chemistry, Faculty of Mathematics and Natural Sciences, Sebelas Maret University. She received a M.Sc. in Physical Chemistry from Nagoya University and a Ph.D. in Nanovision Technology from Shizuoka University. Her research interest is plasma science for developing carbon nanomaterials. Her contributions to this article are designing the research project and experiments and supervising the experiment's conduct. She analyzed and validated the data. She also wrote, reviewed, and revised the article manuscript.

Annisa Dinan Ghaisani

Annisa Dinan Ghaisani is a graduated master’s student at the Department of Chemistry, Faculty of Mathematics and Natural Sciences, Sebelas Maret University. She conceived the project and experimental works, assembled the experimental setup, synthesized the materials, and analyzed the material characterization data. She wrote and revised the article manuscript.

Kusumandari Kusumandari

Kusumandari Kusumandari is an associate professor at the Department of Physics, Faculty of Mathematics and Natural Sciences, Sebelas Maret University. She earned her doctoral degree in engineering from Nagoya University. Her research projects focused on physics and nanotechnology. In this article, she validated, supervised, and analyzed the data. She reviewed and revised the article manuscript.