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

Astuti, undefined; Arief, Syukri; Muldarisnur, undefined; Zulhadjri, undefined; Usna, R. A.

doi:https://doi.org/10.1155/2023/2381623

Journal of Nanotechnology

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Abstract Introduction Materials and Methods Results and Discussion Conclusion Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 2381623 | https://doi.org/10.1155/2023/2381623

Synthesis and Properties of Magnetic-Luminescent Fe₃O₄@ZnO/C Nanocomposites

Astuti,¹Syukri Arief,² Muldarisnur,¹ Zulhadjri,²and R. A. Usna¹

Academic Editor: Mozhgan Afshari

Received05 Oct 2022

Revised18 Mar 2023

Accepted29 Mar 2023

Published08 Apr 2023

Abstract

A Fe₃O₄@ZnO/C nanocomposite with a core-shell structure was synthesized using the co-precipitation method. To prevent the aggregation of the Fe₃O₄ magnetic particles, polyethylene glycol (PEG) was added. The X-ray diffractometer (XRD) results confirmed the formation of Fe₃O₄ and ZnO phases, with Fe₃O₄ having a cubic crystal system and ZnO having a hexagonal crystal system. Carbon in Fe₃O₄@ZnO/C had no effect on the crystal structure of Fe₃O₄@ZnO. Images from transmission electron microscopy (TEM) and scanning electron microscopy (SEM) revealed that the nanocomposite formed a core-shell structure. The Fourier transform infrared (FTIR) spectra verified the presence of bonds among ZnO, Fe₃O₄, and carbon. The appearance of the stretching vibration of the C≡C bond on the Fe₃O₄@ZnO/C sample revealed the nanocomposites’ carbon coupling. Photoluminescence (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. The photoluminescence of Fe₃O₄@ZnO differed from that of the Fe₃O₄@ZnO/C, especially in the deep-level emission (DLE) band. There 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. The 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.

1. Introduction

Fe₃O₄ nanoparticles are considered a potential candidate for application as magnetic bioimaging materials, by making Fe₃O₄ nanoparticles covered by biocompatible materials [1–3]. Magnetic nanoparticles made of Fe₃O₄ 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. These strategies include doping Fe₃O₄ and surface coating of magnetic nanoparticles with organic molecules (such as surfactants, polymers, and biomolecules) or nonorganic materials (such as SiO₂ and Au). The coating material for magnetic nanoparticles must be compatible and maintain the stability of the magnetic nanoparticles [4–6]. Several coating materials for Fe₃O₄ 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 different applications. Several researchers combined Fe₃O₄@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 Fe₃O₄@ZnO nanocomposites as bioimaging materials. The main advantage of such nanocomposites for biological applications is nontoxicity and biocompatibility. The surface modification must not significantly change the biocompatibility, photoluminescence, and magnetic properties. One of the materials that can be employed as a combiner for Fe₃O₄@ZnO is carbon. Carbon can also transfer the generated heat by the electron recombination process on the ZnO surface. Therefore, 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 Fe₃O₄, ZnO, and surface modification of Fe₃O₄@ZnO with carbon. Fe₃O₄@ZnO/C is the core-shell structure of Fe₃O₄-based nanocomposite materials. The co-precipitation method was used to create a Fe₃O₄@ZnO nanocomposite. To prevent Fe₃O₄ agglomeration, polyethylene glycol (PEG) was used as a coating material. The carbon shell acted as a protective shell making it stable, free from external environmental influence, and biocompatible. XRD (Bruker D8 Advance) was utilized to determine the phase and crystal structure. The qualitative phase analysis of the XRD characterization’s diffraction pattern will be compared to established crystallographic databases, including the International Center for Diffraction 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. The 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.

2. Materials and Methods

2.1. Synthesis of Fe₃O₄@ZnO Nanocomposites

The synthesis of nanocomposite Fe₃O₄@ZnO/C begins with the synthesis of Fe₃O4 nanoparticle combined with ZnO by the co-precipitation method [12, 22, 23]. A modified co-precipitation method was used to create Fe₃O₄@ZnO nanoparticles as done by Astuti et al. [24].

2.2. Synthesis of Fe₃O₄@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. Fe₃O₄@ZnO (0.1 g) and 5 mL of carbon solution were mixed, stirred, and then heated in the furnace at 250°C. The resulting powder was named Fe₃O₄@ZnO/C nanocomposite.

3. Results and Discussion

XRD analyzed the crystal structure, phase, and purity of the nanomaterials (Figure 1). Measurement results for Fe₃O₄ match the ICDD code 01-071-6339. Fe₃O₄ lattice parameters were a = b = c = 8.3153 Å, according the cubic system of Fe₃O₄ nanoparticles. The crystalline peaks of Fe₃O₄ observed at 2θ values of 30.4593°, 35.7909°, 43.3784°, 53.9629°, 57.3681°, 63.2529°, and 90.5352° can be assigned to the planes with Miller indices of (220), (311), (400), (422), (511), (440), and (731).

Fe₃O₄ and ZnO were present in the Fe₃O₄@ZnO sample, and they had simple cubic and hexagonal wurtzite crystalline structures, respectively. The Fe₃O₄ phase had a cubic crystal structure with lattice parameters of a = b = c = 8.3761 Å, according to the ICDD code 01-076-7171. ZnO crystal structure has lattice parameters of a = 3.2525 Å, b = 3.2525 Å, and c = 5.2111 Å and is depicted in ICDD 01-075-7917. The crystalline ZnO peaks were observed at the 2θ values of 31.7748°, 34.4273°, 36.2561°, 47.5718°, 67.9733°, 69.1152°, and 95.3214° which can be, respectively, assigned to the planes with Miler indices of (100), (002), (101), (102), (112), (201), and (211). The formation of the core-shell structure of Fe₃O₄@ZnO can be seen from the emergence of peaks (311) in Fe₃O₄ and (101) in ZnO, while the sample Fe₃O₄@ZnO/C showed similar peaks and crystal structure to Fe₃O₄@ZnO. This result shows that the presence of carbon has no effect on the crystal structure of Fe₃O₄@ZnO. The FTIR results confirm XRD measurements, indicating the presence of carbon in the Fe₃O₄@ZnO/C nanocomposite.

Figure 2 shows the results of FTIR measurements in the wavenumber range of 400–4000 cm⁻¹. Several absorption peaks in the sample Fe₃O₄@ZnO were observed, namely, the wavenumber 3352.34 cm⁻¹ related to the vibration of the -OH (hydroxyl) group. The absorption peaks illustrated the formation of PEG in the C-O and C-C vibrations of the sample, namely, at 1247.00 cm⁻¹, 1087.87 cm⁻¹, and 940.31 cm⁻¹. There were vibrations of tetrahedral Fe-O and octahedral Fe-O bonds at wavenumbers of 545.86 cm⁻¹ and 484.14 cm⁻¹, respectively, and Zn-O bonds at 424.35 cm⁻¹ [25]. Apart from the same absorption peaks, Fe-O, Zn-O, C-H, and C-O, no differences were found in the Fe₃O₄@ZnO/C sample. However, no absorption peak was found in the Fe₃O₄@ZnO sample, namely, the C≡C bond. Wavenumbers of 2086.05 cm⁻¹ and 2860.48 cm⁻¹ indicated C≡C bonds and C-H bonds formed in the sample, respectively. The absorption peaks at wavenumbers of 1713.78 cm⁻¹, 1599.98 cm⁻¹, and 1348.27 cm⁻¹ corresponded to bond stretching vibrations (C=O). The appearance of the stretching vibration of the C≡C bond on the Fe₃O₄@ZnO/C sample resulted from carbon demonstrating the sample coupling with carbon. Besides, a more substantial shift and enhancement of absorption peaks of Fe₃O₄@ZnO/C were detected compared to Fe₃O₄@ZnO which can be possibly due to the bond between the C and O atoms in ZnO or Fe₃O₄. This result shows that the synthesis of Fe₃O₄@ZnO/C using the co-precipitation method was successful.

As shown in Figure 3(a), TEM characterization was performed to determine the core-shell structure of the sample represented by Fe₃O₄@ZnO. The morphology of Fe₃O₄ nanoparticles depicts a spherical shape with an approximate size of 15 nm. These clusters resemble a chain-like structure due to magnetic dipole interactions between nearby Fe₃O₄ particles. Based on Figure 3(a), it can be seen clearly that the black Fe₃O₄ is coated by the gray ZnO, which confirms the core-shell structure of the sample. These results were also in good agreement with the literature [17, 26]. Figure 3(b) shows a TEM image of the Fe₃O₄@ZnO/C nanocomposite. Based on the analysis of the diffraction pattern, it was found that Fe₃O₄ and ZnO, had a particle size of less than 20 nm, while carbon was in the form of nanorods composed of Fe₃O₄@ZnO particles.

(a)

(b)

The SEM images of the Fe₃O₄@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 Fe₃O₄ particles. In the preparation steps, the dispersion of Fe₃O₄ in Zn²⁺ can lead to the adsorption of Zn²⁺ ions on the Fe₃O₄ surface. The growth of this bound ZnO nanoparticle can be caused by Zn²⁺ ions from nearby Fe₃O₄ surfaces and the freely available Zn²⁺ ions. This can also result in the attachment of a specific portion of every ZnO particle to the Fe₃O₄ particles.

PL spectroscopy and VSM were used to investigate the room-temperature luminescence and magnetic properties, respectively. The PL spectrum of Fe₃O₄@ZnO (Figure 5) shows a UV emission peak at 381.86 nm for Fe₃O₄@ZnO and 382.49 nm for Fe₃O₄@ZnO/C, as well as a broad visible emission peak ranging from 400 to 800 nm. The visible photoluminescence emission centers in the Fe₃O₄@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). The 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. The band broadness presumably resulted from a superposition of many different deep levels (yellow peak, green peak, and blue peak) emitting simultaneously [27, 28]. The presence of a strong and broad emission peak in the visible region indicates that ZnO has a higher concentration of defects. The 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. The difference between the DLE of Fe₃O₄@ZnO and Fe₃O₄@ZnO/C is visible. There is a broadening of the DLE band in Fe₃O₄@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 significant peaks at 450 nm and 510 nm attributing to the sp² 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. The green and yellow peaks were observed in the Fe₃O₄@ZnO sample. Besides, the blue shift was observed in the Fe₃O₄@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 Fe₃O₄@ZnO/C sample was identified compared to that of the sample Fe₃O₄@ZnO. A reduction in the intensity of the Fe₃O₄@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 electron-hole pairs.

The results of measuring the magnetic properties of Fe₃O₄, Fe₃O₄@ZnO, and Fe₃O₄@ZnO/C nanocomposites are shown in Figure 6. The coercivity values (H_c) of Fe₃O₄, Fe₃O₄@ZnO, and Fe₃O₄@ZnO/C were calculated as 0.0031 T, 0.0037 T, and 0.0038 T, respectively, while the saturation magnetization (M_s) values were different, as shown in Table 1.

Based on Table 1 and Figure 6, the M_s value of Fe₃O₄ nanoparticles is 68.10 emu/g. It decreased to 66.53 emu/g for the Fe₃O₄@ZnO sample. The decrease in M_s value of the Fe₃O₄@ZnO nanocomposite was due to the addition of nonmagnetic PEG and ZnO and also the presence of oxygen-containing groups in the matrix of Fe₃O₄ nanoparticles which could reduce the amount of magnetic moment in the sample. The addition of carbon to the sample Fe₃O₄@ZnO caused a decrease in M_s to 44.65 emu/g. This was followed by a slight increase in the value of the coercivity field due to the presence of ZnO and C around Fe₃O₄. The three samples had high magnetic saturation values and low Hc, close to zero. So, these three materials are classified as superparamagnetic. The magnetic saturation value decreases in direct proportion to particle size. The smaller the particle size, the lower the crystallinity. Therefore, reduced crystallinity decreases magnetic saturation [29]. TEM investigation indicates that the particle size of the Fe₃O₄@ZnO/C nanocomposite was smaller than that of the Fe₃O₄@ZnO nanocomposite. Smaller particles caused Fe₃O₄@ZnO/C nanocomposite to have a lower magnetic saturation. However, the decrease in magnetic saturation in the Fe₃O₄@ZnO/C sample was still within the proper range for biomedical applications.

4. Conclusion

The Fe₃O₄@ZnO nanocomposite has a cubic and a hexagonal wurtzite structure for Fe₃O₄ and ZnO, respectively. The addition of carbon increases the absorption of Fe₃O₄@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 Fe₃O₄@ZnO/C sample which is associated with a reduction of particle size based on TEM results. The presence of carbon also causes a change in photon energy into thermal energy. The addition of carbon to the Fe₃O₄@ZnO nanocomposite increases its biocompatibility as well. However, this does not significantly affect the photoluminescent and magnetic properties of the Fe₃O₄@ZnO/C nanocomposite. Therefore, these materials have the potential to be further developed as biological application materials, especially as bioimaging and photothermal therapy materials.

Data Availability

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

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research was financially supported by the RKAT FMIPA, Andalas University, with research contract number: 30/UN.16.03.D/PP/FMIPA/2022.

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