Preparation of Multilayered Core–Shell Fe3O4-SnO2-C Nanoparticles via Polymeric/Silane–Amino Functionalization

Multilayered core–shell Fe3O4-SnO2-C nanoparticles were prepared via surface treatment and carbonization at atmospheric pressure. Fe3O4-SnO2 nanoparticles were prepared by the carboxylation of the pivotal particles (Fe3O4) with an anionic surfactant to immobilize SnO2 nanoparticles. A method was proposed to externally surround hydrophilic carbon with amine-forming materials, polyethyleneimine (PEI), and (3-Aminopropyl) triethoxysilane (APTES). The synthesis strategy was based on the electrostatic bonding of the introduced amine group with the hydroxyl group on the carbon precursor and the carbonization of the coating layer by the catalytic reaction of sulfuric acid.

Among the various materials that are used for forming composites with Fe 3 O 4 , tin dioxide (SnO 2 ), an n-type semiconductor with a wide bandgap (Eg = 3.6 eV at 300 K), has been extensively investigated [16]. Owing to its unique characteristics, SnO 2 can stabilize the electronic, thermal, and chemical properties of Fe 3 O 4 through the proximity effect and equilibration of potentials [17][18][19], and various efforts have been made to realize the electrochemical applications of the composites of Fe 3 O 4 and SnO 2 . However, Fe 3 O 4 -SnO 2 composite materials show poor electrical conductivity, which limits their applications. Therefore, it is necessary to combine these composites with high-conductivity materials, such as precious metals and carbon materials, to enhance their conductivity [20,21].
In particular, numerous studies have been carried out on the preparation of composites of Fe 3 O 4 -SnO 2 particles with carbon-based materials to improve their conductivity and performance for application in various fields. Wang et al. [22] prepared a composite obtained by combining core-shell Fe 3 O 4 @SnO 2 with reduced graphene oxide (rGO), a carbon-based oxide, for application in EMI shielding. Iron oxide with high magnetic permeability and low complex permittivity improved the dielectric properties of the composite, thus improving its EM wave absorption performance. Furthermore, the combination of rGO (with high electrical conductivity) with Fe 3 O 4 @SnO 2 resulted in electric dipolar polarization and interfacial polarization. Hence, the composite showed an enhanced electromagnetic wave absorption effect in the high-frequency region. Chen et al. [23] fabricated Fe 3 O 4 /SnO 2 /C composite particles for application as an anode for lithium-ion batteries. The complexation of Fe 3 O 4 and SnO 2 improved the reversibility of the LiO 2 conversion reaction, thereby increasing the specific capacity and charge/discharge efficiency of the anode. In addition, the surface modification of the composite with carbon improved the conductivity of the active material and its affinity with the electrolyte. Thus, the Fe 3 O 4 /SnO 2 /C composite showed a more stable rate performance than the Fe 3 O 4 /SnO 2 particles during cycling. Therefore, the preparation of Fe 3 O 4 -SnO 2 -C composites has become a topic of great scientific interest.
The fabrication of SnO 2 -decorated Fe 3 O 4 composites through facile surface modification has been reported [24]. Fe 3 O 4 nanoparticles were carboxylated using polyacrylic acid (PAA) or tri-sodium citrate dihydrate (tSCD), and the surface modification method using tSCD was more effective than that using PAA. The results demonstrated the feasibility of the preparation of spherical nanoparticles surrounded by ultrafine SnO 2 nanoparticles. Motivated by these results, in this study, we prepared Fe 3 O 4 -SnO 2 -C composite particles with a three-layered core-shell structure through the dehydration of glucose under atmospheric pressure ( Figure 1). Fe 3 O 4 -SnO 2 nanoparticles were amino-functionalized using a polymeric precursor, PEI, and a silane precursor, APTES. Subsequently, the effects of the functionalizing amine groups on the structure of the multilayered particles were investigated. In addition, the mechanism underlying the formation of the core-shell structure by the reaction between the polymerized layer formed by combining glucose, a monosaccharide-based carbon precursor, and the aminated nanoparticles and sulfuric acid was investigated.
Nanomaterials 2021, 11, 2 of 15 polarization and interfacial polarization. Hence, the composite showed an enhanced electromagnetic wave absorption effect in the high-frequency region. Chen et al. [23] fabricated Fe3O4/SnO2/C composite particles for application as an anode for lithium-ion batteries. The complexation of Fe3O4 and SnO2 improved the reversibility of the LiO2 conversion reaction, thereby increasing the specific capacity and charge/discharge efficiency of the anode. In addition, the surface modification of the composite with carbon improved the conductivity of the active material and its affinity with the electrolyte. Thus, the Fe3O4/SnO2/C composite showed a more stable rate performance than the Fe3O4/SnO2 particles during cycling. Therefore, the preparation of Fe3O4-SnO2-C composites has become a topic of great scientific interest. The fabrication of SnO2-decorated Fe3O4 composites through facile surface modification has been reported [24]. Fe3O4 nanoparticles were carboxylated using polyacrylic acid (PAA) or tri-sodium citrate dihydrate (tSCD), and the surface modification method using tSCD was more effective than that using PAA. The results demonstrated the feasibility of the preparation of spherical nanoparticles surrounded by ultrafine SnO2 nanoparticles. Motivated by these results, in this study, we prepared Fe3O4-SnO2-C composite particles with a three-layered core-shell structure through the dehydration of glucose under atmospheric pressure ( Figure 1). Fe3O4-SnO2 nanoparticles were amino-functionalized using a polymeric precursor, PEI, and a silane precursor, APTES. Subsequently, the effects of the functionalizing amine groups on the structure of the multilayered particles were investigated. In addition, the mechanism underlying the formation of the core-shell structure by the reaction between the polymerized layer formed by combining glucose, a monosaccharide-based carbon precursor, and the aminated nanoparticles and sulfuric acid was investigated.

Preparation of Core-Shell Structured Fe 3 O 4 -SnO 2 Nanoparticles
Fe 3 O 4 -SnO 2 nanoparticles were prepared as described in our previous report [24]. To prepare the Fe 3 O 4 nanoparticles, 45 g of FeCl 3 ·6H 2 O was added to distilled water under stirring at room temperature, and 0.5 M NaOAc was dissolved in 1000 mL of EG. These two solutions were then transferred to a three-necked round-bottom flask, and the resulting solution was heated to boiling with mechanical stirring and maintained at this temperature for 18 h. After the completion of the reaction, the solution was naturally cooled to room temperature, and the reactants were separated from the solution using a magnet. Subsequently, the reactants were rinsed with distilled water and ethanol several times to eliminate the organic and inorganic by-products.
The prepared Fe 3 O 4 nanoparticles (1 g) were dispersed in 100 mL of distilled water. The resulting suspension was then transferred to a three-neck round-bottom flask, followed by the addition of 0.5 M tSCD. The mixed solution was mechanically stirred at 300 rpm for 24 h at room temperature. After the completion of the reaction, the surface-modified particles were separated from the solution and subsequently rinsed several times with distilled water.
The surface-modified Fe 3 O 4 nanoparticles (0.5 g) were ultrasonically redispersed in 300 mL of distilled water for 30 min. Subsequently, 1 g of Na 2 SnO 3 ·3H 2 O was added to the suspension under stirring at 300 rpm for 24 h at 80 • C. The reaction-terminated suspension was naturally cooled to room temperature, and the particles were separated using a magnet. The resultant particles were washed several times with distilled water.

Amino Functionalization of the Core-Shell Fe 3 O 4 -SnO 2 Nanoparticles
The surface amino-functionalization process was carried out using two different methods. In the case of the inorganic-precursor-based amino functionalization, the Fe 3 O 4 -SnO 2 nanoparticles (0.5 g) were ultrasonically dispersed for 15 min in a 100 mL solution of distilled water and ethanol (1:1 vol%). The dispersed Fe 3 O 4 -SnO 2 suspension was then transferred to a three-neck round-bottom flask, followed by the injection of 0.09 M of APTES. The suspension was mechanically stirred at 350 rpm and 70 • C for 48 h. After the reaction, the solution was naturally cooled to room temperature and the reactants were separated from the solution using a magnet. The resultant particles were washed several times with ethanol and distilled water.
In the case of the polymeric-precursor-based amino functionalization, the Fe 3 O 4 -SnO 2 nanoparticles (0.5 g) were ultrasonically redispersed for 20 min in 200 mL of distilled water with 5 wt% of PEI. Subsequently, the dispersed suspension was mechanically stirred at 300 rpm in a round-bottom flask and heat-treated at 80 • C for 18 h. The solution was cooled to room temperature, and the resulting particles were washed with distilled water several times to eliminate the by-products.

Carbonization of the Core-Shell Fe 3 O 4 -SnO 2 Nanoparticles
The amino-functionalized Fe 3 O 4 -SnO 2 nanoparticles (0.5 g) were redispersed in 400 mL of EG for 30 min via ultrasonication. The mixture solution was transferred to a three-neck round-bottom flask equipped with a reflux condenser, followed by the addition of 30 g of D-(+)-glucose. Subsequently, the solution was mechanically stirred at 300 rpm for 20 h at 180 • C under reflux conditions. Subsequently, the heated solution was slowly cooled to room temperature and immediately reheated to 60 • C. Then, 2 mL of 0.1 M H 2 SO 4 was added to the solution, and the resulting solution was heated to 180 • C and maintained for 24 h. After the reaction, the solution was naturally cooled to room temperature, and the particles were separated from the solution using a magnet. The synthesized particles were washed several times with ethanol and distilled water.

Characterization
The surface characteristics of the surface-modified particles were investigated using Fourier transform infrared (FTIR) spectroscopy (Nicolet 5700, Thermo Electron, Waltham, Nanomaterials 2021, 11, 2877 4 of 14 MA, USA). The surface charges and dispersion properties of the particles were evaluated by carrying out zeta potential and particle size distribution analyses (Zetasizer Nano ZS, Malvern, UK). The crystal structures of the synthesized nanoparticles were analyzed using X-ray diffraction (XRD, UltimaIV, Rigaku, Japan) with Cu Kα radiation (λ = 1.5418 Å). The morphologies of the nanoparticles were investigated using high-resolution transmission electron microscopy (HRTEM, Tecnai G2 F30 S-Twin, FEI, Hillsboro, OR, USA). The degree of graphitization of the carbon layer on the particle surface was investigated using Raman spectroscopy (NRS-3100, Jasco, Easton, PN, USA), with an excitation wavelength of 532 nm. Moreover, the magnetic properties of the particles were evaluated using a vibrating sample magnetometer (Lake Shore 7400, Cryotronics Inc., Westerville, OH, USA) at the applied field of −10-10 kOe at room temperature.

Results and Discussion
To identify the surface functional groups of the as-prepared nanoparticles, their FTIR spectra were analyzed, as shown in Figure 2. The Fe 3 O 4 nanoparticles exhibited a peak at 582 cm −1 , corresponding to the stretching of the Fe-O bond in the tetrahedral sites [8]. In contrast, the SnO 2 nanoparticles synthesized under the same experimental conditions exhibited peaks corresponding to the stretching of the Sn-OH, O-Sn-O, and Sn-O bonds at 550, 618, and 941 cm −1 , respectively [25]. The peaks at 1639 and 3415 cm −1 can be ascribed to the -OH stretching vibration of the particle surface of the hydroxyl group, which was generated by the absorption of H 2 O from the ambient atmosphere [25]. The tSCD-treated Fe 3 O 4 nanoparticles exhibited -CH 2 and -CH 3 vibration peaks at 2857 and 2920 cm −1 , respectively. In addition, peaks corresponding to the vibration of the C-O and COO-Fe bonds were observed at 1393 and 1619 cm −1 , respectively [26]. This confirms the presence of carboxyl groups on the particle surface. temperature, and the particles were separated from the solution using a magnet. The synthesized particles were washed several times with ethanol and distilled water.

Characterization
The surface characteristics of the surface-modified particles were investigated using Fourier transform infrared (FTIR) spectroscopy (Nicolet 5700, Thermo Electron, Waltham, MA, USA). The surface charges and dispersion properties of the particles were evaluated by carrying out zeta potential and particle size distribution analyses (Zetasizer Nano ZS, Malvern, UK). The crystal structures of the synthesized nanoparticles were analyzed using X-ray diffraction (XRD, UltimaIV, Rigaku, Japan) with Cu Kα radiation (λ = 1.5418 Å). The morphologies of the nanoparticles were investigated using high-resolution transmission electron microscopy (HRTEM, Tecnai G2 F30 S-Twin, FEI, Hillsboro, OR, USA). The degree of graphitization of the carbon layer on the particle surface was investigated using Raman spectroscopy (NRS-3100, Jasco, Easton, PN, USA), with an excitation wavelength of 532 nm. Moreover, the magnetic properties of the particles were evaluated using a vibrating sample magnetometer (Lake Shore 7400, Cryotronics Inc., Westerville, OH, USA) at the applied field of −10-10 kOe at room temperature.

Results and Discussion
To identify the surface functional groups of the as-prepared nanoparticles, their FTIR spectra were analyzed, as shown in Figure 2. The Fe3O4 nanoparticles exhibited a peak at 582 cm −1 , corresponding to the stretching of the Fe-O bond in the tetrahedral sites [8]. In contrast, the SnO2 nanoparticles synthesized under the same experimental conditions exhibited peaks corresponding to the stretching of the Sn-OH, O-Sn-O, and Sn-O bonds at 550, 618, and 941 cm −1 , respectively [25]. The peaks at 1639 and 3415 cm −1 can be ascribed to the -OH stretching vibration of the particle surface of the hydroxyl group, which was generated by the absorption of H2O from the ambient atmosphere [25]. The tSCD-treated Fe3O4 nanoparticles exhibited -CH2 and -CH3 vibration peaks at 2857 and 2920 cm −1 , respectively. In addition, peaks corresponding to the vibration of the C-O and COO-Fe bonds were observed at 1393 and 1619 cm −1 , respectively [26]. This confirms the presence of carboxyl groups on the particle surface. an absorption peak was observed at approximately 3300 cm −1 , corresponding to the symmetric and asymmetric stretching modes of NH/NH 2 [27,28]. The peak at 1048 cm −1 can be ascribed to the Si-O-Si bond, which originated from the silane group of APTES [27]. On the other hand, the PEI-treated Fe 3 O 4 -SnO 2 nanoparticles exhibited -CH 2 and -CH 3 vibration peaks at 2843 and 2919 cm −1 , respectively [29]. In addition, symmetric and asymmetric NH/NH 2 stretching modes were observed at 1639 and approximately 3300 cm −1 , respectively [30]. Therefore, it can be stated that various organic functional groups were present on the surface of the PEI-and APTES-modified Fe 3 O 4 -SnO 2 nanoparticles with different structures.
The zeta potentials of the nanoparticles were measured to examine the changes in their surface properties after their surface modification under distilled water conditions ( Table 1). In the table, each sample displacement is the result of 10 measurements under the same conditions. The zeta potential value of the Fe 3 O 4 nanoparticles was −5.4 mV, which indicates that these particles showed a small amount of surface charge. The Fe 3 O 4 -SnO 2 nanoparticles showed a zeta potential of −42.3 mV. In contrast, the zeta potentials of both the APTES-and PEI-treated particles were positive (31.8 and 32.9, respectively) because of the formation of amine groups on their surfaces. Although these modified nanoparticles showed similar zeta potential values, the average value and standard deviation of the PEI-treated particles were slightly larger. Therefore, the initial glucose conjugation amount could be set identically by minimizing the effect of electrostatic attraction that might have been caused by the surface charge on each particle during the carbon coating process.   [33]. The diffraction peaks corresponding to SnO 2 were broad because of its crystalline size of less than 5 nm [34]. This was further confirmed by calculating the average crystallite sizes of the prepared nanoparticles using the Scherrer's formula, D = Kλ/βcosθ, where D is the average crystallite size, K is the shape factor (K = approximately 0.94 for spherical crystallites), λ is the X-ray wavelength (λ = 1.5418 Å for Cu K α radiation), β is the full width at half maximum of the high-intensity diffraction peak (in radians), and θ is the Bragg's angle (in radians). The average crystallite sizes of Fe 3 O 4 and SnO 2 were calculated to be 18.09 and 4.36 nm, respectively. Meanwhile, the XRD patterns of the APTES and PEI-treated Fe 3 O 4 -SnO 2 nanoparticles were the same as that of the Fe 3 O 4 -SnO 2 nanoparticles. This indicates that the Fe 3 O 4 -SnO 2 nanoparticles maintained their crystallinity even after the amino-functionalization treatment.  [33]. The diffraction peaks corresponding to SnO2 were broad because of its crystalline size of less than 5 nm [34]. This was further confirmed by calculating the average crystallite sizes of the prepared nanoparticles using the Scherrer's formula, D = Kλ/βcosθ, where D is the average crystallite size, K is the shape factor (K = approximately 0.94 for spherical crystallites), λ is the Xray wavelength (λ = 1.5418 Å for Cu Kα radiation), β is the full width at half maximum of the high-intensity diffraction peak (in radians), and θ is the Bragg's angle (in radians). The average crystallite sizes of Fe3O4 and SnO2 were calculated to be 18.09 and 4.36 nm, respectively. Meanwhile, the XRD patterns of the APTES and PEI-treated Fe3O4-SnO2 nanoparticles were the same as that of the Fe3O4-SnO2 nanoparticles. This indicates that the Fe3O4-SnO2 nanoparticles maintained their crystallinity even after the amino-functionalization treatment.   Figure 5 shows the TEM and HR-TEM images showing the morphologies and microstructures of the synthesized particles before carbon coating. As shown in the figure, all the synthesized particles were spherical, and the Fe3O4 particles, which acted as the core, had a diameter of approximately 300 nm (Figure 5a). The magnified TEM ( Figure 5(b-1)) and HRTEM ( Figure 5(b-2)) images revealed that SnO2 particles with a diameter of ap-  Figure 5 shows the TEM and HR-TEM images showing the morphologies and microstructures of the synthesized particles before carbon coating. As shown in the figure, all the synthesized particles were spherical, and the Fe 3 O 4 particles, which acted as the core, had a diameter of approximately 300 nm (Figure 5a). The magnified TEM ( Figure 5(b-1)) and HRTEM ( Figure 5(b-2)) images revealed that SnO 2 particles with a diameter of approximately 4.5 nm were formed on the surface of the Fe 3 O 4 particles to a thickness of approximately 20 nm. This is almost consistent with the average crystallite sizes of the Fe 3 O 4 and SnO 2 nanoparticles, as calculated from the XRD data ( Figure 4) according to Scherrer's formula. In addition, lattice patterns with the interplanar spacings of 0.268, 0.334, and 0.233 nm corresponding to the (101), (110), and (200) planes, respectively, were observed on the particle surface. In Figure 5(c-1), the outer layer of the PEI-treated Fe 3 O 4 -SnO 2 nanoparticles can be clearly distinguished from that of the untreated nanoparticles. This outer layer was confirmed to be grafted onto the particle surface with a thickness of approximately 6.5 nm because of the polymerization of the polymeric precursor. In contrast, no significant difference was observed in the images of the Fe 3 O 4 -SnO 2 nanoparticles before and after the APTES treatment ( Figure 5(d-1)).  (110) and (200) planes of SnO 2 , respectively. This is consistent with the XRD results and confirms the successful amino functionalization of the Fe 3 O 4 -SnO 2 nanoparticles through the polymerization of PEI or the silane bonding of APTES. However, the surface microstructures of the amino-functionalized nanoparticles prepared using these functionalization methods were significantly different. Figure 6 shows the carbon coating procedure of the amino-functionalized Fe 3 O 4 -SnO 2 nanoparticles using glucose. EG, which was used as the solvent, played an important role in forming a stable dispersion of the reaction medium and functionalized magnetic nanoparticles. First, owing to its negative charge, the hydroxyl group of glucose bonded strongly to the amine group (with a positive charge) on the surface of the Fe 3 O 4 -SnO 2 particles by electrostatic attraction. Glucose transformed into oligosaccharide chains by the condensation reaction and surrounded the particle surface. In addition, the rotational energy generated by stirring facilitated the deposition of a uniform layer of the oligosaccharide chains on the surface of the particles. Then, the added sulfuric acid dehydrated the oligosaccharide chains bonded to the surface, and a hydrophilic carbon layer with a hydroxyl group at the end was finally formed.
The TEM images of the amino-functionalized nanoparticles subjected to the carbon coating process are shown in Figure 7. In the case of the PEI-treated nanoparticles, only naked nanoparticles with the polymer layer removed were observed after the carbon coating process (Figure 7a). The polymer layer disappeared because of dissolution by high temperature or sulfuric acid during the reaction. Since PEI, as a binding polymeric material, has a melting point of approximately 75 • C, it is sensitive to heat and acidic environments [35]. However, it seems that the SnO 2 particles formed on the surface of the Fe 3 O 4 particles by electrostatic attraction were not removed by sulfuric acid (Figure 7b). To analyze the structure of the nanoparticles in detail, their selected area electron diffraction (SAED) patterns were recorded, which confirmed the presence of the (101), (110), and (200) planes of SnO 2 in the composite nanoparticles (Figure 7c) [36].  strongly to the amine group (with a positive charge) on the surface of the Fe3O4-SnO2 particles by electrostatic attraction. Glucose transformed into oligosaccharide chains by the condensation reaction and surrounded the particle surface. In addition, the rotational energy generated by stirring facilitated the deposition of a uniform layer of the oligosaccharide chains on the surface of the particles. Then, the added sulfuric acid dehydrated the oligosaccharide chains bonded to the surface, and a hydrophilic carbon layer with a hydroxyl group at the end was finally formed. Figure 6. Schematic for the carbon-coating mechanism of amino-functionalized Fe3O4-SnO2 nanoparticles through glucose and sulfuric acid.
The TEM images of the amino-functionalized nanoparticles subjected to the carbon coating process are shown in Figure 7. In the case of the PEI-treated nanoparticles, only naked nanoparticles with the polymer layer removed were observed after the carbon coating process (Figure 7a). The polymer layer disappeared because of dissolution by high temperature or sulfuric acid during the reaction. Since PEI, as a binding polymeric material, has a melting point of approximately 75 °C, it is sensitive to heat and acidic environments [35]. However, it seems that the SnO2 particles formed on the surface of the Fe3O4 particles by electrostatic attraction were not removed by sulfuric acid (Figure 7b). To analyze the structure of the nanoparticles in detail, their selected area electron diffraction (SAED) patterns were recorded, which confirmed the presence of the (101), (110), and (200) planes of SnO2 in the composite nanoparticles (Figure 7c) [36].  In contrast, the particles treated with APTES formed a new coating layer on the surface with a thickness of approximately 17 nm (Figure 7d,e). In addition, the lattice pattern corresponding to the (110) plane was observed with an interplanar spacing of 0.334 nm in the HRTEM image. Ring patterns corresponding to the (333) and (311) planes of Fe3O4 and the (101) plane of SnO2 were observed, as shown in Figure 7f [29]. This can be attributed to the carbonization of the nanoparticles after their electrostatic bonding with glucose because the amino-functional group formed at the end of the particles by the silane bond of APTES was resistant to high temperatures and acidic environments. Therefore, it can be stated that the formation of the carbon layer was directly related to the thermal/chemical stability of the amine retention layer, and the surface treatment using APTES was more effective in carbon layer formation than that using the polymeric precursor, PEI. Finally, multilayered core-shell Fe3O4-SnO2-C nanoparticles were synthesized by the amino functionalization of the Fe3O4-SnO2 particles through APTES. Figure 8a shows the XRD pattern of the Fe3O4-SnO2-C composite nanoparticles. The nanoparticles showed peaks of Fe3O4 and SnO2 corresponding to the (110), (101), (200), In contrast, the particles treated with APTES formed a new coating layer on the surface with a thickness of approximately 17 nm (Figure 7d,e). In addition, the lattice pattern corresponding to the (110) plane was observed with an interplanar spacing of 0.334 nm in the HRTEM image. Ring patterns corresponding to the (333) and (311) planes of Fe 3 O 4 and the (101) plane of SnO 2 were observed, as shown in Figure 7f [29]. This can be attributed to the carbonization of the nanoparticles after their electrostatic bonding with glucose because the amino-functional group formed at the end of the particles by the silane bond of APTES was resistant to high temperatures and acidic environments. Therefore, it can be stated that the formation of the carbon layer was directly related to the thermal/chemical stability of the amine retention layer, and the surface treatment using APTES was more effective in carbon layer formation than that using the polymeric precursor, PEI. Finally, multilayered core-shell Fe 3 O 4 -SnO 2 -C nanoparticles were synthesized by the amino functionalization of the Fe 3 O 4 -SnO 2 particles through APTES. Figure 8a shows  (440) planes. In addition, the lowintensity peak at 2θ = 22.1 • corresponds to the (002) plane of carbon, which originated from the carbon layer formed by the carbonization of glucose [37]. As shown in Figure 8b, the Raman spectrum of the Fe3O4-SnO2-C composite nanoparticles confirmed the formation of carbon on their surface. Two strong characteristic carbon peaks were observed over the wavenumber range of 1000-2800 cm −1 . In general, the D band, which appears only when disordered or finite-sized graphite crystals are present, is not identified in well-crystallized graphite [38]. The D band at 1371 cm −1 indicates the presence of sp 3 defects in the carbon network, whereas the G band at 1584 cm −1 corresponds to the E2g stretching vibration of graphite with sp 2 electronic configuration.
The intensity ratio of the D-and G-bands (ID/IG) provides useful information about the crystallinity of carbon material. The closer the ID/IG value to zero, the higher is the degree of structural alignment of the formed carbon material [39]. The ID/IG ratio of the Fe3O4-SnO2-C composite nanoparticles was approximately 0.5706, as determined using Lorentz fitting. This indicates that the formed carbon layer had a fairly ordered structure. However, this carbon layer can be considered amorphous carbon because of its high ID value. Thus, the layer formed on the surface of the Fe3O4-SnO2 nanoparticles was confirmed to be amorphous graphitized carbon, which was formed when the sulfuric acid used in the coating process promoted the dehydration reaction of the oligosaccharide obtained from glucose. Thus, the TEM and Raman analysis results indicate that the Fe3O4-SnO2-C nanoparticles were synthesized by the formation of a structurally aligned carbon layer in the reflux process under atmospheric pressure. Figure 9a shows the FTIR spectra of the Fe3O4-SnO2 and Fe3O4-SnO2-C nanocomposites. Unlike Fe3O4-SnO2, the Fe3O4-SnO2-C nanocomposites exhibited a -OH stretching vibration peak over a broad spectral region of 3630-3200 cm −1 . The peaks at 2917 and 2871 cm −1 correspond to the vibration of the -CH2 and -CH3 groups, respectively. The strong peak at 1636 cm −1 can be ascribed to the C-C stretching vibration. In addition, the characteristic peaks at approximately 1387-1301 and 1042-1083 cm −1 correspond to the stretching vibration of the C-O and C-OH bonds, respectively, which were probably formed because of the hydroxyl group formed on the carbon layer [40]. As a result, the Fe3O4-SnO2-C nanoparticles exhibited high hydrophilicity because of the large number of hydroxyl groups present in the carbon coating layer. As shown in Figure 8b, the Raman spectrum of the Fe 3 O 4 -SnO 2 -C composite nanoparticles confirmed the formation of carbon on their surface. Two strong characteristic carbon peaks were observed over the wavenumber range of 1000-2800 cm −1 . In general, the D band, which appears only when disordered or finite-sized graphite crystals are present, is not identified in well-crystallized graphite [38]. The D band at 1371 cm −1 indicates the presence of sp 3 defects in the carbon network, whereas the G band at 1584 cm −1 corresponds to the E 2g stretching vibration of graphite with sp 2 electronic configuration.
The intensity ratio of the D-and G-bands (I D /I G ) provides useful information about the crystallinity of carbon material. The closer the I D /I G value to zero, the higher is the degree of structural alignment of the formed carbon material [39]. The I D /I G ratio of the Fe 3 O 4 -SnO 2 -C composite nanoparticles was approximately 0.5706, as determined using Lorentz fitting. This indicates that the formed carbon layer had a fairly ordered structure. However, this carbon layer can be considered amorphous carbon because of its high I D value. Thus, the layer formed on the surface of the Fe 3 O 4 -SnO 2 nanoparticles was confirmed to be amorphous graphitized carbon, which was formed when the sulfuric acid used in the coating process promoted the dehydration reaction of the oligosaccharide obtained from glucose. Thus, the TEM and Raman analysis results indicate that the Fe 3 O 4 -SnO 2 -C nanoparticles were synthesized by the formation of a structurally aligned carbon layer in the reflux process under atmospheric pressure. Figure 9a shows the FTIR spectra of the Fe 3 O 4 -SnO 2 and Fe 3 O 4 -SnO 2 -C nanocomposites. Unlike Fe 3 O 4 -SnO 2 , the Fe 3 O 4 -SnO 2 -C nanocomposites exhibited a -OH stretching vibration peak over a broad spectral region of 3630-3200 cm −1 . The peaks at 2917 and 2871 cm −1 correspond to the vibration of the -CH 2 and -CH 3 groups, respectively. The strong peak at 1636 cm −1 can be ascribed to the C-C stretching vibration. In addition, the characteristic peaks at approximately 1387-1301 and 1042-1083 cm −1 correspond to the stretching vibration of the C-O and C-OH bonds, respectively, which were probably formed because of the hydroxyl group formed on the carbon layer [40]. As a result, the  Figure 9b shows the size distribution curves of the Fe3O4-SnO2-C nanoparticles. As can be observed, the estimated mean size of the particles was 669 nm, and their size distribution was in the range of 225-1700 nm. As compared with the Fe3O4-SnO2 nanoparticles (mean size of 476 nm with a zeta potential of −42.3 mV), the Fe3O4-SnO2-C nanoparticles (with a higher zeta potential of −51 mV) showed aggregation. This can be attributed to the carbonization of the Fe3O4-SnO2 nanoparticles in a bonded state between the oligosaccharide coating layers during the carbon coating process. Nevertheless, the Fe3O4-SnO2-C nanoparticles showed high dispersibility in aqueous solvents because of the hydroxyl groups present on the surface of the carbon layer.
As can be observed from the lower right inset of Figure 10, the coercivity (Hc) and residual magnetization (Mr) of the Fe3O4, Fe3O4-SnO2, and Fe3O4-SnO2-C nanoparticles were almost zero. This suggests that all the prepared nanoparticles were superparamagnetic. As the prepared Fe3O4 particles gradually evolved into the Fe3O4-SnO2-C particles, their saturation magnetization (MS) value tended to decrease. The measured MS values of the Fe3O4, Fe3O4-SnO2, and Fe3O4-SnO2-C nanoparticles were 121, 95, and 81 emu/g, respectively ( Figure 10). This decrease in the MS of the Fe3O4-SnO2-C nanoparticles can be attributed to the increase in the weight of the single particles because of the formation of non-magnetic SnO2 and carbon in the Fe3O4 nanoparticles. As can be seen from the optical image inserted, the Fe3O4-SnO2-C nanoparticles reacted strongly to the external magnetic fields despite their relatively lower magnetization than that of the Fe3O4-SnO2 nanoparticles.  Figure 9b shows the size distribution curves of the Fe 3 O 4 -SnO 2 -C nanoparticles. As can be observed, the estimated mean size of the particles was 669 nm, and their size distribution was in the range of 225-1700 nm. As compared with the Fe 3 O 4 -SnO 2 nanoparticles (mean size of 476 nm with a zeta potential of −42.3 mV), the Fe 3 O 4 -SnO 2 -C nanoparticles (with a higher zeta potential of −51 mV) showed aggregation. This can be attributed to the carbonization of the Fe 3 O 4 -SnO 2 nanoparticles in a bonded state between the oligosaccharide coating layers during the carbon coating process. Nevertheless, the Fe 3 O 4 -SnO 2 -C nanoparticles showed high dispersibility in aqueous solvents because of the hydroxyl groups present on the surface of the carbon layer.
As can be observed from the lower right inset of Figure

Conclusions
Multilayered core-shell-structured Fe3O4-SnO2-C nanoparticles were fabricated via surface treatment and carbonization at atmospheric pressure. The carboxylation of Fe3O4 nanoparticles using tSCD fixed SnO2 nanoparticles electrostatically on them, with a mean size of 4.5 nm while maintaining a layer thickness of 20 nm. Thereafter, the surface amino functionalization of the Fe3O4-SnO2 nanoparticles using PEI and APTES was carried out at the hydroxyl group at the end of the glucose molecule by electrostatic attraction. Unfortunately, when PEI was used, the delicate polymer layer was destroyed by high temperatures and sulfuric acid during the carbon coating process, and the final product was Fe3O4-SnO2. Therefore, the polymer-based amino functionalization was not suitable for the carbonization process. When APTES was used, the amine group formed on the surface of the nanoparticles by silane bonding facilitated the formation of a carbon layer even under severe coating conditions. The carbonized layer formed by the dehydration of glucose with sulfuric acid was hydrophilic amorphous graphitized carbon. The Fe3O4-SnO2-C nanoparticles showed an MS of approximately 81 emu/g and showed excellent separation performance, which is typical of superparamagnetic nanoparticles.

Data Availability Statement:
All data used to support the findings of this study are included in this article.

Conflicts of Interest:
The authors declare no conflict of interest.

Conclusions
Multilayered core-shell-structured Fe 3 O 4 -SnO 2 -C nanoparticles were fabricated via surface treatment and carbonization at atmospheric pressure. The carboxylation of Fe 3 O 4 nanoparticles using tSCD fixed SnO 2 nanoparticles electrostatically on them, with a mean size of 4.5 nm while maintaining a layer thickness of 20 nm. Thereafter, the surface amino functionalization of the Fe 3 O 4 -SnO 2 nanoparticles using PEI and APTES was carried out at the hydroxyl group at the end of the glucose molecule by electrostatic attraction. Unfortunately, when PEI was used, the delicate polymer layer was destroyed by high temperatures and sulfuric acid during the carbon coating process, and the final product was Fe 3 O 4 -SnO 2 . Therefore, the polymer-based amino functionalization was not suitable for the carbonization process. When APTES was used, the amine group formed on the surface of the nanoparticles by silane bonding facilitated the formation of a carbon layer even under severe coating conditions. The carbonized layer formed by the dehydration of glucose with sulfuric acid was hydrophilic amorphous graphitized carbon. The Fe 3 O 4 -SnO 2 -C nanoparticles showed an M S of approximately 81 emu/g and showed excellent separation performance, which is typical of superparamagnetic nanoparticles.

Data Availability Statement:
All data used to support the findings of this study are included in this article.