One-pot synthesis of monodisperse CoFe2O4@Ag core-shell nanoparticles and their characterization

In recent years, monodispersed magnetic nanoparticles with a core/shell structure are expected for their wide applications including magnetic fluid, recoverable catalysts, and biological analysis. However, their synthesis method needs numerous processes such as solvent substitution, exchange of protective agents, and centrifugation. A simple and rapid method for the synthesis of monodispersed core-shell nanoparticles makes it possible to accelerate their further applications. This paper describes a simple and rapid one-pot synthesis of core (CoFe2O4)-shell (Ag) nanoparticles with high monodispersity. The synthesized nanoparticles showed plasmonic light absorption owing to the Ag shell. Moreover, the magnetic property of the nanoparticles had a soft magnetic behavior at room temperature and a hard magnetic behavior at 5 K. In addition, the nanoparticles showed high monodispersity with a low polydispersity index (PDI) value of 0.083 in hexane. Electronic supplementary material The online version of this article (10.1186/s11671-018-2544-z) contains supplementary material, which is available to authorized users.

Among the magnetic nanoparticles, a spinel ferrite nanoparticle has frequently been employed as a magnetic core because of its excellent magnetic and electrical properties [12]. Particularly, cobalt ferrite (CoFe 2 O 4 ) nanoparticles have a large maximum coercive field (H c ), even with a small size as well as a remarkable chemical stability and a mechanical hardness [13][14][15][16][17]. Although many different chemical methods have been developed to fabricate CoFe2O4 nanoparticles, the thermal decomposition method has recently been employed one of the most promising procedures to obtain highly, structurally, and morphologically controlled nanoparticles with a high crystallinity [13,17,18].
Magnetic nanoparticles with a core/shell structure have attracted a great deal of attention due to their multifunctionality including optical, electronic, and magnetic properties [6,8,10,19]. In particular, the Au shell-coated magnetic nanoparticles have widely been studied in order to provide not only the surface plasmon properties but also a reactive surface for strong binding to organic compounds containing thiol groups [3,20]. Typically, an approach of combined two-step thermal decomposition process can continuously synthesize from cores to shells, resulting in the formation of Au-coated magnetic nanoparticles with a high monodispersity [20]. On the other hand, Ag shell-coated magnetic nanoparticles have not been synthesized by this approach in spite of excellent plasmonic properties, a higher extinction coefficient, a sharper extinction band, a higher light scattering-toextinction effect, and strong local electromagnetic fields of Ag shells.
In this study, we succeeded in synthesizing Ag shellcoated CoFe 2 O 4 nanoparticles by a simple and rapid one-pot method involving two thermal decomposition processes. It was confirmed that our synthesized nanoparticles formed a precise core-shell structure, as compared with those synthesized in a previous paper [21,22]. In addition, we demonstrated that the CoFe 2 O 4 @Ag showed the localized surface plasmon resonance (LSPR) originated from the Ag shells. In the investigation of the magnetic property, this core-shell nanoparticle revealed soft magnetic behavior with H c of 70 Oe at 300 k and hard magnetic behavior with 11 k Oe at 5 K.

Synthesis of CoFe2O4@Ag
The CoFe2O4@Ag were synthesized by the two-step high thermal decomposition method (Scheme 1). Fe(acac) 3 (0.353 g, 1 mmol), Co(acac) 2 (0.129 g, 0. 5 mmol), and OA (3.39 g, 12 mmol) were dissolved in 30 mL of diphenyl ether, which was pre-treated by heating at 180°C for 30 min. A mixture was heated at 180°C for 16 h under vigorous stirring. The solution color gradually turned from dark red to fine black. After cooling at room temperature, a mixture of OA (1.48 g, 5.2 mmol), OAm (8.13 g, 30.4 mmol), and silver acetate (0.61 g, 3.6 mmol) dissolved in 100 mL of diphenyl ether was added to the mixture, followed by heating at 180°C for 1.5 h. The color of the mixture further turned to metallic dark purple during heating. After cooling, 400 mL of methanol as a poor solvent was added to the mixture solution, followed by centrifugation (5000 rpm, 5 min) and the redispersion in 60 mL of hexane. Although the nanoparticles dispersed in the solution might be able to be magnetically separated, it takes time to recover. The centrifugation process was repeated several times to remove the unreacted precursors. Finally, by centrifuging the colloidal hexane solution (14,000 rpm, 20 min), the resulting precipitates were removed. The net weight of nanoparticles by this method is about 60 mg as 1 mg/mL of the colloidal hexane solution. The CoFe 2 O 4 nanoparticles as a reference were prepared by performing only step 1 in Scheme 1.

Characterization and Calculation
The morphology of nanoparticles was observed using field-emission transmission electron microscopy (TEM) (Hitachi, Ltd., FE 2000). The crystal structures were measured with X-ray diffraction (XRD) (PANalytical, X'Pert PRO MPD) in the range of 2θ = 20°to 80°by using the CuK α-ray. Element composition of nanoparticles was analyzed by X-ray photoelectron spectroscopy (XPS) (KARATOS ESCA 3400). Etching operation was performed with Ar ion gun. The magnetization measurements were performed by a superconducting quantum interference device (SQUID) (Cryogenic, S700X-R). The optical properties were measured on a UV-visible spectrophotometer (Jasco, V-670). Dynamic light scattering (DLS) (Malvern, zetasizer-nano-zs) was measured with 633-nm laser line. For the optical properties of our synthesized core-shell nanoparticles, the experimental data are supported by Mie scattering calculations which were carried out by Bohren and Huffman's solution [23] using the MATLAB code written by Mätzler [24]. Dielectric functions for the Ag were taken from Reference [25]. Figure 1 shows the TEM images of CoFe 2 O 4 nanoparticles and CoFe 2 O 4 @Ag core-shell nanoparticles. As shown in the insets of Fig. 1, the size distributions of both nanoparticles are narrow. The average sizes (mean ± S.D.) of them are 3.5 ± 0.76 and 5.5 ± 0.77 nm, respectively. From these results, the thickness of Ag shell was estimated to be ca. 1 nm. Aggregation of CoFe 2 O 4 particles occurred but not for CoFe 2 O 4 @Ag nanoparticles. This is possibly due to a  [17]. On the other hand, the diffraction peaks of CoFe 2 O 4 @Ag at 2θ = 38. 42°, 44.50°, 64.91°, 77.75°, and 81.83°correspond to those of the standard face-centered cubic (fcc) phase of Ag [10]. The intensity of the diffraction peaks of CoFe 2 O 4 are relatively weak, and its main peak overlaps with Ag; therefore, all emerge into those of Ag. The crystallite size was calculated from the full width at half maximum (FWHM) of the highest intensity diffraction peak, which is based on the Debye-Scherrer equation,

Results and Discussion
where t is the crystallite size, l is the wavelength of Cu-Ka radiation, b is the FWHM, and y is the diffraction angle of the strongest peak. The crystal sizes evaluated from the diffraction patterns were 7.1 and 3.6 nm for CoFe 2 O 4 nanoparticles and CoFe 2 O 4 @Ag nanoparticles, respectively. The crystal size of CoFe 2 O 4 nanoparticles was observed to be larger than the size of TEM because of the residue of CoFe 2 O 4 nanoparticles out of size distribution, which could not be removed by centrifugation in hexane. On the other hand, the crystal size from XRD showed an agreement in CoFe2O4@Ag nanoparticles considering that the crystal size of Ag shell has to be smaller than the size of TEM. The size of the colloid after the silver coating reaction enables to select by centrifugation due to its heavyweight in hexane.
To evaluate the internal composition of the obtained nanoparticles with a core-shell structure, the nanoparticle surfaces were etched using Ar ion gun in the chamber [27]. According to the previous studies, when the  particles had a precise core-shell structure, the peak intensity of the element contained in the core should be increased as the etching progresses. As shown in Fig. 3a-d, to determine the surface composition of CoFe 2 O 4 @Ag nanoparticles, we measured the XPS spectra before the Ar ion etching. In the initial surfaces, the peek C (1 s) were easily observed in nanoparticles due to the presence of the protective agent on the surface of the nanoparticles (Fig. 3a). The spectrum of C (1 s) was decomposed, and a peak derived from C-O-C was observed, which is derived from oleic acid modified on the surface. While the peaks of Ag(3d) were observed, those of Fe(2p) and Co(2p) could not be observed, indicating that the core was completely covered with the Ag shells ( Fig.  3b-d). On the other hand, the peaks of Fe(2p) and Co(2p) were observed in the nanoparticles after the etching operation with argon ion (Fig. 3f, g). The peaks of Fe(2p) and Co(2p) are decomposed and can be assigned to Fe 2+ , Fe 3+ , Co 2+ , and Co 3+ ,  respectively. The formation of both types of charge carriers results from the loss of oxygen during the high-temperature reaction process [28,29]. For the charge compensation, a part of Fe3+ is converted to Fe2+, and a part of Co2+ is converted to Co3+. Furthermore, each of the Ag(3d) peak after the etching can be decomposed into two peaks (Fig. 3h), due to the difference in electronic state at between the nanoparticle surfaces and the inside of the shells. These results indicate that the precise core-shell structure is formed.
The magnetic hysteresis loops of films made up of the CoFe 2 O 4 and the CoFe 2 O 4 @Ag nanoparticles were measured at 300 and 5 K, as shown in Fig. 4. These hysteresis loops were normalized as the magnetic susceptibility per unit cobalt weight. Due to the analysis of the crystallographic phase using XRD (Fig. 2), the crystalline densities of CoFe 2 O 4 and CoFe 2 O 4 @Ag nanoparticles were estimated to be 5.3 and 10.5 g/cm 3 , respectively. Also, the volumes of CoFe 2 O 4 and CoFe 2 O 4 @Ag nanoparticles were calculated using the results from the TEM observation (Fig. 1). CoFe 2 O 4 nanoparticles showed a superparamagnetic behavior at room temperature (Fig. 4a). As mentioned by López-Ortega et al. [17], the CoFe 2 O 4 nanoparticles with the size below 20 nm showed the superparamagnetic behavior at room temperature. The magnetic properties of each sample at the two temperatures are summarized in Table 1. Magnetic saturation (M s ) of the CoFe 2 O 4 nanoparticles was 11 (emu/g CoFe 2 O 4 ), which is lower than the previous results [17,30,31]. This is possibly owing to the smaller particle size obtained in this study. On the other hand, the M s of the CoFe 2 O 4 @Ag was even smaller with a value of 3.3 (emu/g, CoFe 2 O 4 ). As mentioned in the previous literature for Fe 3 O 4 @Ag nanoparticles [8][9][10][32][33][34], the M s of CoFe 2 O 4 @Ag decreases possibly due to the diamagnetic contribution of the Ag shell. Moreover, CoFe 2 O 4 @Ag showed 77 Oe, which is high H c value at 300 k. The H c of the CoFe 2 O 4 @Ag is also different from that of CoFe 2 O 4 under the low temperature (Fig. 4b). Both of the nanoparticles exhibited ferromagnetism at 5 K despite their relatively small sizes. On the basis of the data near zero magnetization, the value of H c increases for CoFe 2 O 4 @Ag nanoparticles (7 k Oe for CoFe 2 O 4 and 11 k Oe for CoFe 2 O 4 @Ag). This interesting behavior has also been observed in other core-shell nanoparticles such as Fe@Ag [10] and Fe 3 O 4 @Au nanoparticles [5]. Taking these facts into account, the increase of the H c of the CoFe 2 O 4 @Ag nanoparticles can be derived from a lesseffective coupling of magnetic dipole moment [5,20].
Next, optical properties of the CoFe 2 O 4 nanoparticles were investigated by UV-visible spectral measurements. Ag nanoparticles are known to show significant light extinction in the visible region due to the excitation of localized surface plasmon resonance (LSPR) by the coupling of the irradiated light with the coherent oscillation of surface electrons within the Ag nanoparticles. Although the CoFe 2 O 4 nanoparticles showed no LSPR extinction band in the visible region (Fig. 5), the colloidal solution of our core-shell type CoFe 2 O 4 @Ag nanoparticles showed a sharp extinction peak at 416 nm. This can be attributed to the plasmon absorption (dipole mode) of the Ag shell, which is theoretically supported by the Mie theory (see Additional file 1). This interesting behavior has been observed for Fe@Ag nanoparticles [10] and Co@Ag nanoparticles [7]. In addition, the spectroscopic properties of the CoFe 2 O 4 @Ag nanoparticles were not changed for 1 month, indicating the superior stability of the nanoparticles under air.
The colloidal stability of the CoFe 2 O 4 and the CoFe 2 O 4 @Ag nanoparticles was evaluated by measuring the size distributions of the nanoparticles in hexane using DLS (Fig. 6). The average sizes of the CoFe 2 O 4 and CoFe 2 O 4 @Ag nanoparticles were measured to be 19.67 and 9.27 nm, respectively. The sizes of these   Table 2. The main difference in sizes measured by these two techniques is due to the presence of an adsorption layer consisting of the OA and OAm on the surface of the particles [35]. Organic compounds such as OA and OAm did not appear in TEM images due to the electron permeability (Fig. 1). Given that the chain lengths of the OA and the OAm are roughly 2 nm [36,37], the size of CoFe 2 O 4 @Ag estimated by the TEM is slightly (ca. 4 nm) larger than that by the DLS. On the other hand, it is reasonable that the size of CoFe 2 O 4 by the DLS is far larger than that estimated from this assumption. These results suggest that CoFe2O4 nanoparticles are agglomerated in hexane. This factor includes not only the size effect of the particles described above but also the low affinity between the CoFe 2 O 4 surfaces and the protective agents. The tendency of agglomeration of the CoFe 2 O 4 may not only due to the size effect of the particles described above but also due to the low affinity between the CoFe 2 O 4 surfaces and the protective agents. Precipitation of CoFe 2 O 4 nanoparticles was observed much more frequently than CoFe 2 O 4 @Ag nanoparticles in the process of redispersion by increasing the number of methanol washing. The high monodispersity of CoFe 2 O 4 @Ag is strongly supported by the low polydispersity index (PDI) obtained by the DLS measurements [38]. These results indicate that the coating with Ag adds not only an optical function but also the stability in solution to the CoFe 2 O 4 nanoparticles.

Conclusions
The CoFe 2 O 4 @Ag nanoparticles synthesized by a simple and rapid one-pot process were found to be formed on having a uniform core-shell structure with a narrow size distribution from TEM images (Fig. 6). Also, these nanoparticles showed a multifunctionality consisting of the plasmonic light extinction property and a superparamagnetic behavior at room temperature. Furthermore, the core-shell nanoparticles showed higher H c than CoFe 2 O 4 nanoparticles at 5 K and 300 k. In addition, these nanoparticles maintained high monodispersity in an organic solvent. The uniform nanoparticles synthesized by the simple process have a great potential in various fields owing to the multifunctionality as well as the stability.