Synthesis and Properties of Magnetic-Luminescent Fe 3 O 4 @ZnO/ C Nanocomposites

A Fe 3 O 4 @ZnO/C nanocomposite with a core-shell structure was synthesized using the co-precipitation method. To prevent the aggregation of the Fe 3 O 4 magnetic particles, polyethylene glycol (PEG) was added. Te X-ray difractometer (XRD) results confrmed the formation of Fe 3 O 4 and ZnO phases, with Fe 3 O 4 having a cubic crystal system and ZnO having a hexagonal crystal system. Carbon in Fe 3 O 4 @ZnO/C had no efect on the crystal structure of Fe 3 O 4 @ZnO. Images from transmission electron microscopy (TEM) and scanning electron microscopy (SEM) revealed that the nanocomposite formed a core-shell structure. Te Fourier transform infrared (FTIR) spectra verifed the presence of bonds among ZnO, Fe 3 O 4 , and carbon. Te appearance of the stretching vibration of the C ≡ C bond on the Fe 3 O 4 @ZnO/C sample revealed the nanocomposites’ carbon coupling. Photo-luminescence (PL) spectroscopy was used to characterize the optical properties of the nanocomposites. Based on the results of the PL, the sample absorption of visible light was in the wavelength range of 400–700 nm. Te photoluminescence of Fe 3 O 4 @ZnO difered from that of the Fe 3 O 4 @ZnO/C, especially in the deep-level emission (DLE) band. Tere was a phenomenon of broadening and shift of the band at a shorter wavelength, namely, in the blue wavelength region. Magnetic properties were characterized by vibrating-sample magnetometry (VSM). Based on the VSM results, the sample coupled with carbon exhibited a decrease in magnetic saturation. Te presence of carbon changed photon energy into thermal energy. So, this material, apart from being a bioimaging material, can also be developed as a photothermal therapy material.


Introduction
Fe 3 O 4 nanoparticles are considered a potential candidate for application as magnetic bioimaging materials, by making Fe 3 O 4 nanoparticles covered by biocompatible materials [1][2][3].Magnetic nanoparticles made of Fe 3 O 4 have a high surface-to-volume ratio and a high surface energy.As a result, the magnetic nanoparticles tend to agglomerate to reduce the surface energy.In addition, these nanoparticles are highly reactive and easily oxidized which decrease their magnetic properties and dispersibility.Various strategies are further being developed by researchers to solve these problems.Tese strategies include doping Fe 3 O 4 and surface coating of magnetic nanoparticles with organic molecules (such as surfactants, polymers, and biomolecules) or nonorganic materials (such as SiO 2 and Au).Te coating material for magnetic nanoparticles must be compatible and maintain the stability of the magnetic nanoparticles [4][5][6].Several coating materials for Fe 3 O 4 have been proposed, including polymers (such as dextran, albumin, polyethylene glycol, polyvinylpyrrolidone [7], folic acid [8,9], chitosan [10], and silica [11]).
Combining magnetic and luminescent materials, such as lanthanide [12,13], carbon/graphene [14,15], and semiconductor [16,17], produces materials with unique properties for diferent applications.Several researchers combined Fe 3 O 4 @ZnO nanocomposites with other materials to be utilized for antibacterial application [16], photodegradation of organic pollutants [18], and targeted drug delivery [19].However, only a few studies have developed Fe 3 O 4 @ZnO nanocomposites as bioimaging materials.Te main advantage of such nanocomposites for biological applications is nontoxicity and biocompatibility.Te surface modifcation must not signifcantly change the biocompatibility, photoluminescence, and magnetic properties.One of the materials that can be employed as a combiner for Fe 3 O 4 @ZnO is carbon.Carbon can also transfer the generated heat by the electron recombination process on the ZnO surface.Terefore, this material has multiple functions in biological applications, such as bioimaging and photothermal therapy of cancerous cells [20,21].
In this study, a class of multifunctional nanocomposites is presented that combines superparamagnetic Fe 3 O 4 , ZnO, and surface modifcation of Fe 3 O 4 @ZnO with carbon.Fe 3 O 4 @ZnO/C is the core-shell structure of Fe 3 O 4 -based nanocomposite materials.Te co-precipitation method was used to create a Fe 3 O 4 @ZnO nanocomposite.To prevent Fe 3 O 4 agglomeration, polyethylene glycol (PEG) was used as a coating material.Te carbon shell acted as a protective shell making it stable, free from external environmental infuence, and biocompatible.XRD (Bruker D8 Advance) was utilized to determine the phase and crystal structure.Te qualitative phase analysis of the XRD characterization's difraction pattern will be compared to established crystallographic databases, including the International Center for Difraction Data (ICDD).To investigate optical properties, photoluminescence (PL, Horiba MicO Photoluminescence Microspectrometer) was used.SEM (SU3500) and TEM (FEI Tecnai G2 20 S-Twin) were used to examine the morphology and particle size of the samples.Te chemical bonds formed were determined using FTIR (FTIR, Nicolet iS50 FTIR).In addition VSM (VSM250) was used to examine the magnetic properties of materials, which were then represented as a hysteresis curve.

Synthesis of Fe 3 O 4 @
ZnO/C Nanocomposites.In 25 mL of distilled water, 2 g of PEG was dissolved.Following that, 1 g of glucose was added, stirred for 30 minutes, heated in an oven at 300 °C for 1 hour, and then dissolved in 10 mL of distilled water.Fe 3 O 4 @ZnO (0.1 g) and 5 mL of carbon solution were mixed, stirred, and then heated in the furnace at 250 °C.Te resulting powder was named Fe 3 O 4 @ZnO/C nanocomposite.

Results and Discussion
XRD analyzed the crystal structure, phase, and purity of the nanomaterials (Figure 1).Measurement results for Fe Figure 2 shows the results of FTIR measurements in the wavenumber range of 400-4000 cm −1 .Several absorption peaks in the sample Fe 3 O 4 @ZnO were observed, namely, the wavenumber 3352.As shown in Figure 3(a), TEM characterization was performed to determine the core-shell structure of the sample represented by Fe 3 O 4 @ZnO.Te morphology of Fe 3 O 4 nanoparticles depicts a spherical shape with an approximate size of 15 nm.Tese clusters resemble a chain-like structure due to magnetic dipole interactions between nearby Fe 3 O 4 particles.Based on Figure 3(a), it can be seen clearly that the black Fe 3 O 4 is coated by the gray ZnO, which confrms the core-shell structure of the sample.Tese results were also in good agreement with the literature [17,26].Figure 3(b) shows a TEM image of the Fe 3 O 4 @ZnO/C nanocomposite.Based on the analysis of the difraction pattern, it was found that Fe 3 O 4 and ZnO, had a particle size of less than 20 nm, while carbon was in the form of nanorods composed of Fe 3 O 4 @ZnO particles.
Te SEM images of the Fe 3 O 4 @ZnO sample demonstrate aggregated spherical particles with sizes ranging from 50 to 100 nm (Figure 4), which can be distinguished from the white color assigned to ZnO nanoparticles covering the Fe 3 O 4 particles.In the preparation steps, the dispersion of Fe 3 O 4 in Zn 2+ can lead to the adsorption of Zn 2+ ions on the Fe 3 O 4 surface.Te growth of this bound ZnO nanoparticle can be caused by Zn 2+ ions from nearby Fe 3 O 4 surfaces and the freely available Zn 2+ ions.Tis can also result in the attachment of a specifc portion of every ZnO particle to the Fe 3 O 4 particles.
PL spectroscopy and VSM were used to investigate the room-temperature luminescence and magnetic properties, respectively.Te PL spectrum of Fe 3 O 4 @ZnO (Figure 5) shows a UV emission peak at 381.86 nm for Fe 3 O 4 @ZnO and 382.49nm for Fe 3 O 4 @ZnO/C, as well as a broad visible emission peak ranging from 400 to 800 nm.Te visible photoluminescence emission centers in the Fe 3 O 4 @ZnO sample are determined by ZnO vacancies and surface defects.Although the PL mechanism of the visible ZnO band is unknown, a photoluminescence mechanism can be proposed.According to Figure 5, emissions in the 400 nm to 800 nm range are commonly referred to as "deep-level emissions" (DLEs).Te DLE is caused by levels allowed inside the ZnO band gap.Transitions with energy in the visible range of the spectrum are produced by the allowed levels.Te band broadness presumably resulted from a superposition of many diferent deep levels (yellow peak, green peak, and blue peak) emitting simultaneously [27,28].Te presence of a strong and broad emission peak in the visible region indicates that ZnO has a higher concentration of defects.Te most common surface defects reported in ZnO are oxygen vacancies, and the intensity of the green emission depends on the concentration of the oxygen vacancies.Te diference between the DLE of Fe 3 O 4 @ZnO and Fe 3 O 4 @ ZnO/C is visible.Tere is a broadening of the DLE band in Fe 3 O 4 @ZnO/C, and there is a shift in the band to a shorter wavelength, namely, in the blue wavelength region caused by the presence of carbon atoms.Carbon has two signifcant peaks at 450 nm and 510 nm attributing to the sp 2 domain's π-π * transition and the n-π * transition of the surface functional group, respectively.Interstitial oxygen cause yellow emission in ZnO which can be reduced by the presence of C atoms resulting in a shift in the blue region emission.Te green and yellow peaks were observed in the Fe 3 O 4 @ZnO sample.Besides, the blue shift was observed in the Fe 3 O 4 @ZnO/C sample.Free exciton (FE) emissions dominated the UV emission band which is generally ascribed to the band-to-band transition.
Furthermore, a slight decrease in the intensity of photoluminescence of the Fe 3 O 4 @ZnO/C sample was identifed compared to that of the sample Fe 3 O 4 @ZnO.A reduction in the intensity of the Fe 3 O 4 @ZnO/C photoluminescence is closely related to the recombination of the electron-hole pairs.It can be concluded that the weaker the PL intensity, the slower the recombination of photogenerated electronhole pairs.
Te results of measuring the magnetic properties of Fe 3 O 4 , Fe 3 O 4 @ZnO, and Fe 3 O 4 @ZnO/C nanocomposites are shown in Figure 6.Te coercivity values (H c ) of Fe 3 O 4 , Fe 3 O 4 @ZnO, and Fe 3 O 4 @ZnO/C were calculated as 0.0031 T, 0.0037 T, and 0.0038 T, respectively, while the saturation magnetization (M s ) values were diferent, as shown in Table 1.
Based on Table 1 and Figure 6, the M s value of Fe 3 O 4 nanoparticles is 68.10 emu/g.It decreased to 66.53 emu/g for the Fe 3 O 4 @ZnO sample.Te decrease in M s value of the Fe 3 O 4 @ZnO nanocomposite was due to the addition of nonmagnetic PEG and ZnO and also the presence of oxygen-containing groups in the matrix of Fe 3 O 4  nanoparticles which could reduce the amount of magnetic moment in the sample.Te addition of carbon to the sample Fe 3 O 4 @ZnO caused a decrease in M s to 44.65 emu/g.Tis was followed by a slight increase in the value of the coercivity feld due to the presence of ZnO and C around Fe 3 O 4 .Te three samples had high magnetic saturation values and low Hc, close to zero.So, these three materials are classifed as superparamagnetic.Te magnetic saturation value decreases in direct proportion to particle size.Te smaller the particle size, the lower the crystallinity.Terefore, reduced crystallinity decreases magnetic saturation [29].TEM investigation indicates that the particle size of the Fe 3 O 4 @ZnO/C nanocomposite was smaller than that of the Fe 3 O 4 @ZnO nanocomposite.Smaller particles caused Fe 3 O 4 @ZnO/C nanocomposite to have a lower magnetic saturation.However, the decrease in magnetic saturation in the Fe 3 O 4 @ ZnO/C sample was still within the proper range for biomedical applications.

Conclusion
Te Fe 3 O 4 @ZnO nanocomposite has a cubic and a hexagonal wurtzite structure for Fe 3 O 4 and ZnO, respectively.Te addition of carbon increases the absorption of Fe 3 O 4 @ZnO UV emission.It also broadens and shifts the visible emission to shorter wavelengths.Based on the VSM results, it can be  concluded that there is a decrease in magnetic saturation of the Fe 3 O 4 @ZnO/C sample which is associated with a reduction of particle size based on TEM results.Te presence of carbon also causes a change in photon energy into thermal energy.Te addition of carbon to the Fe 3 O 4 @ZnO nanocomposite increases its biocompatibility as well.However, this does not signifcantly afect the photoluminescent and magnetic properties of the Fe 3 O 4 @ZnO/C nanocomposite.Terefore, these materials have the potential to be further developed as biological application materials, especially as bioimaging and photothermal therapy materials.

Table 1 :
Magnetic properties of Fe 3 O 4 and nanocomposites.