Magnetite-Silver Core–Shell Nanoparticles: Synthesis, Characterizes, and Optical Properties

Fe3O4 and Fe3O4/Ag core–shell nanocomposite powders were synthesized via the co-precipitation method. The structure, microstructure, magnetic, and optical properties of Fe3O4/Ag and Fe3O4 were studied. XRD patterns and UV–Vis spectra showed that nanostructure Fe3O4 and Fe3O4/Ag particles were successfully synthesized. AFM and MFM mode micrographs of Fe3O4 and Fe3O4/Ag powders confirm the formation of Fe3O4 particles in the nano-scale range. Both Fe3O4 and Fe3O4/Ag composite powders represented the superparamagnetic behavior due to the formation of nanosized Fe3O4 particles. Furthermore, in-situ synthesis of Ag on the surface of magnetite nanoparticles increased the particle size, resulting in a decrease in saturation magnetization. Moreover, based on the Maxwell-Garnet effective medium theory, a theoretical model was developed to determine the optical properties of suspended core–shell nanoparticles. A very good agreement was found between the theoretical and experimental results. In addition, the local electric field in the particles, evaluated using the numerical finite element method, showed that the electric field in the magnetite core might be amplified up to 20 times at the symmetrical SPR wavelength mode, depending on the silver shell thickness.


Introduction
Noble metal nanostructures have attracted much attention thanks to their optical properties linked to the surface plasmon resonance (SPR) phenomenon [1]. This phenomenon is related to the coherent oscillation of the free electrons of the metal and results in an exaltation of the local electric field at a wavelength known as SPR wavelength. The SPR wavelength depends on the different parameters, especially on the shape and size of nanoparticles [2]. So these materials were proposed for various applications such as photonic devices [3][4][5], catalysts [6,7], and biomedicines [8]. On the other hand, many researchers have been interested in magnetite nanoparticles due to their special properties and environmental compatibility [9][10][11][12][13][14]. However, their optical applications are limited because of their intrinsic optical properties. Perhaps, suitable nanostructures, including noble metal and magnetite, at the same time have been proposed by many groups to increase the desired properties [15][16][17]. In particular, the core-shell nanoparticles were studied for their optical and magneto-optical properties [18][19][20]. Thus, Fe 3 O 4 /Ag nanoparticles are employed in various application fields, including catalysts [21][22][23], sensors [24], antibacterial applications [25], cancer detection [26], and magnetic application [27][28][29].
The aim of the present study was to synthesize Fe 3 O 4 / Ag composite nanopowders. First, Fe 3 O 4 particles were prepared. Then, Ag particles were in situ-synthesized in the presence of magnetite nanopowders via the precipitation method in the presence of a NaHB 4 reducer agent. The structure, microstructure, magnetic, and optical properties of Fe 3 O 4 /Ag and Fe 3 O 4 were then studied. Moreover, a theoretical model based on the effective medium theory was proposed to determine the optical properties of embedded core-shell nanoparticles. This model predicts the symmetrical and antisymmetrical SPR modes linked to the shell silver very well. In addition, the numerical approach, based on the finite element method (FEM) in the frequency domain, was applied to evaluate the local electrical field at the SPR wavelengths.

Synthesis of Fe3O4 Nanoparticles
Magnetite powders were synthesized based on the work of Tahmasebi and Yamini [38]. Synthesis of Fe 3 O 4 particles were carried out in a 500 ml three-necked round glass flask under N 2 atmosphere together with reflux condition. First, 8.48 g iron(III) chloride hexahydrate and 2.25 g iron(II) chloride tetrahydrate were dissolved in 400 ml of deionized water under magnetic stirring at 80 °C. Then 20 ml of diluted ammonia (25 vol.%, Merck) was added drop-wise to the solution for 5 min. until a black precipitate was obtained. The gained product was washed with deionized water several times. Finally, the powders were dried in an electric oven at 70 °C for 2 h.

Synthesis of Fe3O4/Ag Nanocomposite Particles
First, 1 g of synthesized magnetite was added to 20 ml of AgNO 3 (0.885 molar) solution and agitated (1000 rpm) for 30 min. Then 50 ml of NaBH 4 (3.172 molars) was added drop-wise to the solution for 30 min until a dark olive green precipitate was obtained. Then, the synthesized powders were washed with deionized water several times and dried in an electric oven at 70 °C for 2 h.
In some cases, Ag particles were synthesized solely by removing nano magnetite particles from the reactor flask.
Structural characterization of the samples was performed using the X-ray diffraction (XRD, Bruker Advance D8) technique. The mean magnetite crystallite diameter (d Scherrer ) was determined from the half-height width (β) of the (311) diffraction peak of magnetite using the Scherrer equation (d Scherrer = 0.9λ/β cos θ). One T80 UV-Vis spectrophotometer (PG Instruments Ltd) was applied to measure UV-visible absorption spectra. Scanning probe microscope with atomic force and magnetic force modes (AFM/ MFM, Ara Research Co. Iran) were used for topographic and fly phase of magnetic characterizations. The magnetic behavior of powders was characterized using the vibration sample magnetometer (VSM, Meghnatis Daghigh Kavir Co., Kashan, Iran).  Fig. 2b). Moreover, in-situ synthesis of Ag on the surface of magnetite nanoparticles increased particle dimensions (Fig. 2c) and so resulted in a decrease in magnetic behavior (Fig. 2d). These magnetic behaviors were studied by VSM mesurements.

Results and Discussion
The magnetization of Fe 3 O 4 and Fe 3 O 4 /Ag powders as a function of the applied magnetic field at room temperature is shown in Fig. 3 Fig. 4. Fe 3 O 4 absorbance decreases gradually with the wavelength, which confirms the formation of Fe 3 O 4 in nano-scale dimensions [40]. As illustrated, the Ag absorbance spectrum (red curve) presents a maximum value at 400 nm of wavelength. This peak is associated with the surface plasmon resonance effect of Ag nanoparticles [41]. Also, the Fe 3 O 4 /Ag spectrum (blue curve) presents two peaks at 435 and 780 nm of wavelengths, attributed to asymmetric and symmetric surface plasmon resonance, respectively [42,43].

Model
The optical properties of suspended core-shell magnetitesilver nanoparticles were studied using the Maxwell-Garnet effective medium theory. This approximation is valid for a negligible scattering nanocomposite material, including the low volume fraction of nanoparticles embedded in a host medium as the following: where p, d , and ̃ p are nanoparticle volume fraction, dielectric function of host medium, and the core-shell magnetitesilver nanoparticle, respectively. This earlier may be given using the effective medium theory for sphere multilayer [44]: where ̃ c and ̃ sh are the dielectric function of the magnetite core and that of the silver shell, and = (R c ∕R sh ) 3 represents the volume ratio of the magnetite core to the particle volume. The absorbance then may be given as. The model was applied to evaluate the optical properties of the core-shell magnetite-silver nanoparticle suspended in the water. The magnetite particle radius was considered 10 nm for the different silver shell thicknesses of 2, 5, and 10 nm. Based on the model, the absorbance spectra of different core-shell magnetite-silver nanoparticles are depicted in Fig. 5. The absorbance of the magnetite and silver nanoparticles suspended in the water is also shown in Fig. 5. The magnetite nanoparticle absorbance is magnified by a factor of 50. As illustrated, the absorbance of suspended magnetite NPs regularly decreases by wavelength, while the suspended silver NPs show a maximum value at about 400 nm of wavelength. This maximum is related to the surface plasmon resonance phenomena [2]. As can be observed, the absorbance of the core-shell magnetite-silver NPs presents two maxima. These maxima are linked to the combination of the dark and bright modes of SPR [39, 40, [45]. Indeed, the first pic of SPR, located at the shorter wavelength, is due to the antisymmetrical combination of the dark and bright mods. In contrast, the second SPR pic, located at the longer wavelength, is related to the symmetrical combination of the dark and light mods. A very good agreement may be seen between the experimental results (Fig. 4) and the theoretical ones (Fig. 5). Figure 6 represents the symmetrical and antisymmetrical combination of dark and bright modes schematically. The finite element method was also used for the study of the absorption cross-section as well as the electrical field in the nanoparticle according to reference 2.  The local field in the particle for symmetrical and antisymmetrical SPR modes is depicted in Fig. 7 for different shell thicknesses. The local field in the core for the symmetrical mode is much higher than that of the antisymmetrical mode for each particle. This behavior can also explain the higher amplitude of the symmetrical mode compared to the antisymmetrical one. By increasing the shell thickness, the first maximum, located at the low wavelength, presents a slightly red-shift displacement, whereas the second one, located at the longer wavelength, reveals an important blueshift. Indeed, for a very thin shell thickness, there is a high electric coupling between the dark and bright modes of SPR. By increasing the silver shell thickness, the distance between the inner and outer of the silver shell surfaces increases. Therefore, the electric coupling becomes weaker. As the magnetite core size is considered the same for different shell thicknesses, the first SPR mode is slightly affected by the shell thickness. In addition, the SPR amplitude increases by increasing the shell thickness.

Conclusion
The present work aimed to synthesize sented the superparamagnetic behavior due to the formation of nano-sized Fe 3 O 4 particles. In-situ synthesis of Ag on the surface of magnetite nanoparticles increased particle sizes, resulting in a decrease in magnetic behavior. 4. The theoretical model predicts well two modes of symmetry and asymmetry SPRs. By increasing the silver shell thickness, the symmetrical mode presents a blueshift, and its amplitude increases. 5. Electric local field, calculated by FEM, showed amplification of the electric local field in the particle at the symmetric SPR wavelength mode depending on the silver shell thickness.