Microstructures and Dielectric Permittivity Properties of Fe3O4/Cdots Nanocomposites Synthesized by Green Route Utilizing Moringa Oleifera Extract and Watermelon Peel

Dielectric materials are beneficial for storing electrical energy due to their insulating and polarization properties in response to external electric fields. Magnetite has shown promise as a dielectric material among other materials due to its good magnetic properties, low toxicity, and biocompatibility. However, the weakness of Fe3O4, which has low stability and easy agglomeration, requires a modification on its surface by using Carbon dots (Cdots). This research investigates the dielectric properties of Fe3O4/Cdots obtained through the green synthesis method. Fe3O4 nanoparticles were synthesized using the co-precipitation method with Moringa oleifera leaf extract as a reducing and stabilizing agent. In contrast, Cdots were synthesized using the hydrothermal method with watermelon peel waste as a carbon source. The Fe3O4 composite nanoparticles were characterized using X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray (SEM-EDX), ultraviolet-visible spectroscopy (UV-Vis), and impedance spectroscopy. The XRD spectra revealed the existence of cubic inverse spinel and a reduction in crystal size as the concentration of Cdots increased, measuring 7.8 and 7.1 nm, respectively. SEM-EDX revealed that the sample is composed of Fe, O, and C elements and has a spherical shape with Cdots distributed on the surface of Fe3O4. The UV-Vis spectrum showed the absorption peak of Cdots at 282 nm. The Fe3O4 absorption peak is identical to the Fe3O4/Cdots absorption peak at 193 nm. The increase in band gap energy from 2.96 to 3.33 eV is related to the increase in Cdots concentration. In the 10-900 kHz frequency range, dielectric property tests demonstrated peak dielectric permittivity values (real and imaginary). A substantial decrease was observed between 10 kHz and 200 kHz, followed by a relatively stable pattern up to 900 kHz. The loss tangent value obtained has a tanδ value <0.5, which means that the addition of Cdots affects reducing the energy loss stored in Fe3O4.


Introduction
Dielectric material is an insulator material that can be polarized when influenced by an external magnetic field [1].This polarization ability is usually used for energy storage in capacitors with storage capacity based on the magnitude of the dielectric permittivity of the material [2].Various kinds of dielectric capacitors are also often used in machinery and operate in harsh environments, such as in cars, airplanes, geothermal power plants, and spacecraft.Dielectric materials also determine the progress in various fields of human life, one of which is the field of microelectronics, such as frequency modulation of smart grids, electronic products, and high-speed trains [3].
Magnetic materials such as Fe3O4 [4,5], CoFe2O4 [6], NdFeO3 [7], and GO-ZnO [8] are used for the development of dielectric applications.Among the other materials, Fe3O4 nanoparticles were chosen to be analyzed for their dielectric properties due to their abundant availability in nature as well as their superparamagnetic, biodegradable, biocompatible, and low toxicity [9].The Curie temperature of Fe3O4 is high, and its spin polarization is quite intense at room temperature.In this case, it is very supportive of its usefulness as a dielectric material when influenced by external magnetic fields [9,10].However, the shortcomings of Fe3O4 nanomaterials in the form of easy oxidation and agglomeration require surface modification.For this surface modification, Carbon dots [4], Mn [11], and folate-chitosan [12] can used.Among these substances, carbon dots (Cdots) exhibit outstanding characteristics, including strong water solubility, chemical durability, absence of toxicity, straightforward customization, compatibility with biological systems, and resilience against photodegradation [13,14].Cdots are produced through topdown and bottom-up approaches.Top-down techniques involve the decomposition of substances like graphite and activated carbon into Cdots using methods such as laser ablation, arc discharge, and electrochemical processes.On the other hand, bottom-up methods create Cdots from precursor molecules of various sizes, including carbohydrates, organic acids, and natural products, using solvothermal, hydrothermal, chemical, and microwave synthesis methods.[15].The hydrothermal technique provides a simple and environmentally friendly means of producing Cdots that is economical and ensures uniform particle sizes [16].So, this Fe3O4 modification can be done by adding Cdots [17,18].The addition of Cdots does not change the magnetic properties of Fe3O4, instead increasing the surface area and dispersibility of Fe3O4 nanoparticles [4].
Various conventional methods are often used to prepare nanoparticles, such as sol-gel, coprecipitation, hydrothermal, and sonochemical methods.However, these methods usually use precursor chemicals that are harmful to the environment [4].That way, the green synthesis method can be an alternative because it uses materials that are more environmentally friendly, namely utilizing plant or microbial extracts that are easily available so that they are also cheaper than chemicals in conventional methods.Another advantage of green synthesis in making nanoparticles is non-toxicity and more energy efficiency.Moringa oleifera (MO) is a plant often used in green synthesis, including the manufacture of Fe3O4.MO, which contains phenolic substances and flavonoids, can be a metal ion reducer and stabilizer.Our recent study successfully synthesized using the green synthesis method utilizing MO extract [19,20] Additionally, we have reported the potential of Fe3O4 as a dielectric material [21].However, previous studies have not reported Fe3O4/Cdots as dielectric materials.Further research is needed to explore the potential of green-synthesized nanocomposites.Therefore, this study focused on the successful synthesis of Fe3O4/Cdots nanocomposites using the green synthesis method, which provides promising opportunities for dielectric applications.The optical and microstructure of the greensynthesized Fe3O4/Cdots nanocomposites were also studied to obtain a comprehensive study.

a. Materials
The precursor chemicals used in this study were Ferric chloride hexahydrate (FeCl3-6H2O), Ferrous sulfate heptahydrate (FeSO4-7H2O), ammonia solution (NH4OH) 25%, ethanol (CH3CH2OH) 96%, and distilled water (DI water).All chemicals were purchased from Merck (Germany) and Sigma Aldrich (USA) with analytical quality without further purification.Then, MO leaf extract and watermelon peel waste were produced by PT Timurasa and the local market (Indonesia), respectively.

b. Preparation of MO
The 5 grams of MO powder was dissolved in 60 mL of distilled water and stirred for one hour at 60⁰ C.Then, cooling and filtration using Whatman paper.The filtration results (MO extract solution) were stored in a refrigerator covered with wrap and aluminium foil for use in the following process.

c. Green Synthesis of Fe3O4
The FeSO4 and FeCl3 powders were mixed with distilled water 7.5 mL each and stirred for 15 minutes, 600 rpm.Both were mixed and stirred again for 15 minutes, temperature 60⁰ C, 600 rpm. 10 mL MO solution was added to the mixture and stirred again for 30 minutes, temperature 60⁰ C. A total of 30 mL of 10% NH4OH solution was titrated into the mixture while stirring for 90 minutes.Cooling was done to room temperature.Next, washing was done on the solution using distilled water with the help of an outside magnet.The resulting precipitate was dried at 100⁰ C for 2 hours.

d. Green Synthesis of Cdots
The preparation of Cdots was carried out by hydrothermal process using a mixture of watermelon peel juice, 96% ethanol, and distilled water (1:1:1) stirred for 30 minutes, 500 rpm.The mixture was put into an autoclave and put in a 180⁰ C furnace for 3 hours.The results were filtered with the Whatman paper.

e. Fabrication of Fe3O4/Cdots
The Fe3O4 powder was prepared and mixed with Cdots 30 mL.The mixture was sonicated for 30 minutes.Next, it was allowed to stand for 24 hours at room temperature.Then, washing was done with distilled water and the help of an outside magnet.Fe3O4/Cdots precipitates/samples were dried at 90⁰ C for 4 hours.
f. Characterization Characterization of Fe3O4/Cdots samples was carried out using X-ray diffraction (XRD) Shimadzu XD-3H with Cu-kα radiation (λ = 1.5406Å) to determine its crystal structure and phase.Scanning electron microscopy-energy dispersive X-ray (SEM-EDX) JSM-6510LA to determine the morphology of Fe3O4/Cdots nanocomposite.Furthermore, characterization using ultraviolet-visible spectroscopy (UV-Vis) Shimadzu UV-1900 to determine its optical properties.Characterization using Impedance Spectroscopy to determine the dielectric properties of Fe3O4/Cdots.

g. Dielectric Study
Dielectric testing was carried out by placing the compaction results of Fe3O4/Cdots on a capacitor plate that was connected to an impedance spectroscopy circuit.The frequency range was set from 10-900 kHz with the results in the form of dielectric permittivity (real and imaginary), loss tangent, and impedance, as well as the cole-cole graph displayed on the connected computer.

Result and Discussion
Based on the results of XRD spectra, the structure of Fe3O4/Cdots nanoparticles is inverse cubic spinel.Data analysis carried out based on XRD testing also shows a reduction in crystal size along with the addition of Cdots concentration, measuring 7.8 and 7.1 nm, respectively.In figure 2, the X-ray diffraction (XRD) pattern of Fe3O4 and Fe3O4/Cdots 30 mL nanocomposites was analyzed using the Rietveld refinement method implemented in the MAUD program.The XRD analysis revealed distinct diffraction peaks for Fe3O4, located at approximately 30.1°, 35.5°, 43.3°, 53.5°, 57.1°, and 62.8°, corresponding to the diffraction planes indexed as (220), (311), (400), ( 422), (511), and (440), respectively.The diffraction peaks corresponds to the ICDD standard data No. 01-075-0033 which shows a face-centered cubic (FCC) inverse spinel magnetite (Fe3O4) crystal structure.Also, the peak which identified at angle 2θ = 32.1⁰,corresponding to the diffraction plane indexed as (330) indicates the presence of Fe2O3 in Fe3O4 and Fe3O4/Cdots samples.This occurs due to the oxidation process during the modification and drying of the nanoparticles, with the chemical reaction 4Fe3O4 + O2 → 6Fe2O3.
Then, the Fe3O4/Cdots sample has a small peak at diffraction angle 2θ = 23.1⁰.Based on ICDD standards, the diffraction peak shows the characteristics of graphite (002) which identifies the presence of Cdots formed in samples with amorphous properties with peaks tending to be wide at 23⁰.The Cdots diffraction peak formed is quite low, meaning that the composition of Cdots in the sample tends to be low [4].
The widening and reduction in the intensity of the dominant peak (311) in the Fe3O4 diffraction pattern in the nanocomposite samples are in direct correlation with the rising concentration of Cdots.This indicates the synthesis of Fe3O4/Cdots was successfully carried out.Based on the XRD spectra data, the main peak (311) was analyzed, and the crystallite size was obtained at 7.8 nm and a decrease in size after the addition of Cdots to 7.1 nm.Thus, there is a widening of the peak with the addition of Cdots.Crystal size is calculated using the Debye-Scherrer Eq. ( 1) where λ is the wavelength of the incident X-ray (Cu Kα radiation, λ = 1.5418Å), ߚ is the full width at half maximum (FWHM) of the diffraction peak and θ is the Bragg diffraction peak (in radians) [22].
Based on the lattice parameter results obtained in table 1, the lattice parameter values for Fe3O4 nanoparticles and Fe3O4/Cdots nanocomposites are 8.16 and 8.23 Å, respectively.These results show that the lattice parameter values obtained from experiments are close to the ICDD reference of 8.38 Å [4].In this case, to determine the lattice parameters, equations ( 2) and ( 3) can be used.

݀ = ݊ߣ ‫ߠ݊݅ݏ2‬
(2) where ܽ is the lattice parameter, ݀ is the distance between planes, ߠ is the diffraction angle, ݊ is the diffraction order, and ℎ݈݇ is the miller index.
XRD pattern was analyzed using Rietveld Refinement method using the MAUD program also provides data results in the form of the percentage value of the phase composition of the sample (table 1).Based on the table, the detection of Fe3O4 phase composition indicates that Fe3O4 was successfully formed using MO extract.Then, the presence of Fe3O4 and carbon phases shows that Fe3O4/Cdots was successfully synthesized.The increase in carbon phase shows the addition of Cdots concentration is increasing.A high concentration of Cdots can increase the solubility or dispersion ability in nanocomposites.This is due to the ability of functional groups such as hydroxyl to easily bind to the surface of Cdots.Furthermore, to determine the chemical composition, Fe3O4/Cdots were characterized using SEM-EDX.The results of the EDX spectrum characterization of Fe3O4 (figure 3(a)) showed a mass percentage of Fe 70.87% and O 29.13%, while the EDX spectrum of Fe3O4/Cdots (figure 3(c)) showed a mass percentage of Fe 59.69%, O 30%, and C 10.31%.These show that Fe3O4/Cdots were successfully formed from Fe, O, and C elements, with no other impurities present in the structure.Based on the results in figure 3(b,d), it appears that Fe3O4 is in the form of chunks, and when Cdots are added, they appear to be distributed on its surface.The UV-Vis spectrum shows the absorption peak of Cdots at 282 nm.In addition, the Fe3O4 absorption peak is identical to the Fe3O4/Cdots absorption peak at 193 nm.Based on the UV-visible characterization results, Fe3O4 shows the highest absorption peak at around 193 nm, accompanied by a broader shoulder peak extending from 365 to 400 nm.Although the presence of Cdots in the absorption spectra of Fe3O4/Cdots nanocomposites may not be immediately apparent, a slight peak in the range of 212 to 223 nm confirms the ongoing presence of carbon.This specific peak is not exclusive to Fe3O4/Cdots; it is also found in Fe3O4 due to the use of MO, which contains flavonoid carbon chains.Although the peak is subtle, it is essential to highlight that increasing the concentration of Cdots leads  (4) The band gap energy of Fe3O4 and Fe3O4/Cdots 30 mL can be observed by plotting (ߙℎ‫)ݒ‬ 2 versus ℎ‫ݒ‬ and extrapolating the linear part of curve to the x-axis as shown in Figure 5.The band gap energy of samples can be obtained in table 2, respectively.The increase in band gap energy from 2.96 to 3.30 eV corresponds to an increase in the concentration of Cdots.The increase in band gap energy indicates that the crystallite size of the sample is getting smaller and the excitation energy also increases.Dielectric characterization for Fe3O4/Cdots yielded several constants: dielectric permittivity (real and imaginary), loss tangent, and impedance as a function of frequency as shown in figure 6.A substantial decrease in real and imaginary permittivity was observed between 10 kHz and 200 kHz, followed by a relatively stable pattern up to 900 kHz.The results of the dielectric constant, both real and imaginary, show an increasing trend with the addition of Cdots concentration (table 3 and table 4).At a frequency of 10 kHz, the highest absolute permittivity is by Fe3O4/Cdots with 30 mL variation, which is 285.2.This largest constant shows a better energy storage ability than the 20 mL and 25 mL Cdots variations.At the same time, the resulting loss tangent is tanδ <0.5, which indicates that the addition of Cdots affects the decrease in energy loss stored in Fe3O4 samples.The resulting impedance measurement decreased with the addition of frequency.

Conclusion
The Fe3O4/Cdots nanocomposite was successfully synthesized using the green method by combining Fe3O4 and Cdots using MO leaf extract and watermelon rind waste, respectively, as the dielectric material.The crystallite size of Fe3O4/Cdots of 7.1 nm with inverse cubic spinel structure was successfully analyzed using XRD.The composition of the elements contained was successfully proven by SEM-EDX, namely Fe, O, and C.Then, there is also an increase in band gap with the addition of Cdots as a result of the UV-Visible test, which is also in accordance with the results of dielectric analysis, namely an increase in the dielectric constant, which states that the addition of Cdots is getting better for dielectric materials.These results confirm that Fe3O4/Cdots indicate the potential utility of dielectric materials.The comprehensive data obtained from this research holds immense value and will serve as an inspiration for researchers to delve deeper into the advancement of efficient magnetic nanomaterials for dielectric materials.

Figure 1 .
Figure 1.Illustration of synthesis of nanocomposites.(a) The extraction of MO, (b) Green synthesis of Fe3O4 with the co-precipitation method, (c) Green synthesis of Cdots with the hydrothermal method, and (d) Fabrication of Fe3O4/Cdots.

Table 3 .
Real Permittivity of Fe3O4/Cdots nanoparticles with various concentrations at the current frequency.

Table 4 .
Imaginer Permittivity of Fe3O4/Cdots nanoparticles with various concentrations at the current frequency.