Co7Fe3 and Co7Fe3@SiO2 Nanospheres with Tunable Diameters for

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Co7Fe3 and Co7Fe3@SiO2 Nanospheres with Tunable Diameters for High-Performance Electromagnetic Wave Absorption Na Chen,†,‡ Jian-Tang Jiang,*,† Cheng-Yan Xu,†,‡ Yong Yuan,§ Yuan-Xun Gong,∥ and Liang Zhen*,†,‡ †

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China MOE Key Laboratory of Micro-System and Micro-Structures Manufacturing, Harbin Institute of Technology, Harbin 150080, China § Precision Machinery Research Institute, Shanghai Space Flight Academy, Shanghai 201600, China ∥ Aerospace Research Institute of Special Materials and Processing Technology, Beijing 100074, China ‡

S Supporting Information *

ABSTRACT: Ferromagnetic metal/alloy nanoparticles have attracted extensive interest for electromagnetic wave-absorbing applications. However, ferromagnetic nanoparticles are prone to oxidization and producing eddy currents, leading to the deterioration of electromagnetic properties. In this work, a simple and scalable liquid-phase reduction method was employed to synthesize uniform Co7Fe3 nanospheres with diameters ranging from 350 to 650 nm for high-performance microwave absorption application. Co7Fe3@SiO2 core−shell nanospheres with SiO2 shell thicknesses of 30 nm were then fabricated via a modified Stöber method. When tested as microwave absorbers, bare Co7Fe3 nanospheres with a diameter of 350 nm have a maximum reflection loss (RL) of 78.4 dB and an effective absorption with RL > 10 dB from 10 to 16.7 GHz at a small thickness of 1.59 mm. Co7Fe3@SiO2 nanospheres showed a significantly enhanced microwave absorption capability for an effective absorption bandwidth and a shift toward a lower frequency, which is ascribed to the protection of the SiO2 shell from direct contact among Co7Fe3 nanospheres, as well as improved crystallinity and decreased defects upon annealing. This work illustrates a simple and effective method to fabricate Co7Fe3 and Co7Fe3@SiO2 nanospheres as promising microwave absorbers, and the design concept can also be extended to other ferromagnetic alloy particles. KEYWORDS: Co7Fe3 nanospheres, liquid-phase reduction, SiO2 coating, microwave absorption, effective absorption bandwidth process.17,32,33 The eddy current effect as well as dielectric loss is related to conductivity22,23,34 and particle size.35 Ferromagnetic resonance is highly sensitive to particle morphology36 and particle−particle distance.37,38 The microstructure design of ferromagnetic particles can influence their conductivity, dispersion, and ferromagnetic properties; thus, it would be beneficial to adjust their EMA properties. Inspired by these findings, numerous strategies have been developed to modify and tailor the morphology and structure of ferromagnetic particles. Previous studies have suggested that the EM properties of ferromagnetic particles could be improved via annealing.39 However, the morphologies are partially destructed at high temperature,40 which can be an obstacle for developing EMA fillers. Coating of ferromagnetic particles with a dielectric shell is an efficient strategy to tailor their EMA performance.41−44 In addition, the presence of a dielectric shell can also adjust the spacing between ferromagnetic particles.34,45 Kuang et al. fabricated porous Co/C nanomaterials with improved electromagnetic wave absorption properties due to

1. INTRODUCTION Many efforts have been made to develop electromagnetic waveabsorbing (EMA) materials to meet the increasing requirements for absorbing electromagnetic waves.1−4 These efforts have cultivated a great scope wherein various materials have been explored as EMA fillers.5−12 Among these materials, ferromagnetic metal/alloy particles possess unique properties, such as a high saturation magnetization (Ms), a high Curie temperature (Tc), and composition controllability,13−16 which endow them with a superior potential to present high and tailorable electromagnetic properties in the gigahertz (GHz) range.17−21 However, these intrinsic superiorities have not been fully revealed because of various obstacles, including the eddy current effect, low filling fraction, or decay.22−24 For instance, eddy current in a single ferromagnetic particle or local aggregations may induce an extremely high permittivity but decreased permeability, leading to deteriorated electromagnetic matching.25−28 On the other hand, the filling ratio in EMA coatings is usually limited to a low level, which restricts improvement of the coatings’ permeability.29−31 Additionally, ferromagnetic nanoparticles are prone to oxidization when exposed to a high temperature or corrosive environment, resulting in a degenerated EMA performance during the serving © 2017 American Chemical Society

Received: March 19, 2017 Accepted: June 1, 2017 Published: June 1, 2017 21933

DOI: 10.1021/acsami.7b03907 ACS Appl. Mater. Interfaces 2017, 9, 21933−21941

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ACS Applied Materials & Interfaces

Figure 1. Characterization of the structure, morphology, and compositions of Co7Fe3 nanospheres. SEM images of Co7Fe3 spheres with different diameters: (a) CF350 and (b) CF650. SEM image (c) and EDS mappings (d, e) of a single Co7Fe3 sphere (CF350). (f, g) TEM images, (h) HRTEM image, and (i) SAED pattern of CF350. similar to that in a previous report.40 In a typical synthesis procedure, 15 mmol of CoCl2·6H2O and 5 mmol of FeCl2·4H2O were dissolved in 100 mL of EG and the mixture was magnetically stirred for about 20 min. At the same time, 1 g of PVP was dissolved in another 100 mL of EG, and the PVP-EG solution was slowly added to the above mixture. For comparison, a process without the addition of PVP was also carried out. Subsequently, the resultant solution was stirred for several minutes at 85 °C in a water bath before adding 0.375 mol of NaOH (1.875 mol/L). After 20 min, 20 mL of N2H4·H2O was introduced. After the reaction was allowed to proceed for 1 h, the products were collected by magnetic separation, followed by washing with deionized water and ethanol. Co7Fe3 nanospheres were finally obtained after drying at 60 °C. For size control, different dosages of N2H4·H2O (20 and 40 mL) were introduced to obtain spheres with diameters of 350 and 650 nm, designated as CF350 and CF650, respectively. 2.3. Synthesis of Core−Shell Co7Fe3@SiO2 Composite Particles. Co7Fe3@SiO2 composite particles were fabricated using a modified Stöber process.51 The as-prepared Co7Fe3 nanospheres were dispersed in a mixture of ethanol (160 mL) and deionized water (40 mL) by ultrasonic agitation for 30 min. Then, the mixture was mixed with 4 mL of aqueous ammonia solution and mechanically stirred for 5 min at room temperature. Afterward, 0.2 mL of TEOS was added and the reaction proceeded for 4 h. The obtained products were separated by centrifugation, washed with ethanol several times, and finally dried at 60 °C overnight. Co7Fe3@SiO2 with a 30 nm thick SiO2 shell is denoted as CFS350. The obtained Co7Fe3@SiO2 spheres were annealed in a H2 atmosphere at 500 °C for 120 min. 2.4. Characterization. The phase compositions of the products were characterized by X-ray diffraction (XRD, Rigaku D/max-rB, Cu Kα). The morphology was observed under a scanning electron microscope (SEM, FEI Quanta 200F) and transmission electron microscope (TEM, JEOL JEM-2100). The compositions of the asprepared samples were probed by X-ray photoelectron spectroscopy (XPS, recorded on a Thermo Fisher Scientific VG Kα Probe spectrometer) using Al Kα radiation as the excitation source. 2.5. Magnetic and Electromagnetic Property Measurements. The magnetic properties of the samples were measured using a vibrating sample magnetometer (VSM, Lakeshore 7300). The electromagnetic properties of the specimens were analyzed using a network analyzer (VNA, Agilent N5230A) at a 2−18 GHz band. Co7Fe3 or Co7Fe3@SiO2 nanospheres (20 vol %) were dispersed in paraffin homogeneously, and then, the mixer was pressed into a mold to fabricate specimens for VNA measurement. The fabricated VNA specimens are coaxial rings with an outer diameter of 7.0 mm, inner diameter of 3.04 mm, and thickness of 3 mm.

the synergetic effects between the porous structure and multiple components.11 Kolhatkar et al. coated FeCo nanocubes with a thin layer of SiO2, in which the cubic shape of the FeCo particles was well retained.19 In our previous study, the introduced SiO2 shell was found to isolate the cross-particle diffusion and prevent the aggregation of Co particles, as well as enhance the anti-oxidation capability.46 Ni et al. found that SiO2 coating enabled Fe particles to maintain better dispersity by reducing their magnetic coupling effect.47,48 Previous studies have proved that improved EMA efficiency or oxidation resistance was obtained via dielectric coating.45,49,50 However, systematic investigation of the effects of sizes and coating of ferromagnetic particles on the electromagnetic properties is still limited. Thus, deep insight into the electromagnetic properties of ferromagnetic particle composites is restricted. The current study aimed at fabricating composite particles based on ferromagnetic metal/alloy particles and exploring the feasibility of tailoring their electromagnetic properties. Co7Fe3 nanospheres were selected as electromagnetic dissipation components, and a SiO2 shell was introduced to avoid the particle−particle contact of Co7Fe3 nanospheres. Co7Fe3 nanospheres were synthesized through a liquid-phase-reduction method that possesses the feasibility to control the reaction kinetically and then give the spheres of desired size. The size-control process of Co7 Fe 3 was investigated, and its size-dependent microwave absorption performance was examined. In addition, we also investigated the effects of annealing on the microstructures of Co7Fe3@SiO2 nanospheres and further explored their EMA properties. The composites with different sizes exhibited strong absorption and wide absorption bandwidths, which met the current requirement for high-efficiency microwave-absorbing materials.

2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals were of analytical grade and used as received without further purification. Cobalt chloride hexahydrate (CoCl2·6H2O), iron chloride tetrahydrate (FeCl2·4H2O), sodium hydroxide (NaOH), hydrazine hydrate (N2H4·H2O, 85%), ethylene glycol (EG), tetraethyl orthosilicate (TEOS), ammonia (28 wt %), polyvinyl pyrrolidone (PVP, Mw = 55 000), and absolute ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. 2.2. Synthesis of Co7Fe3 Nanospheres. Co7Fe3 nanospheres were fabricated by a PVP-assisted liquid-phase reduction method 21934


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Figure 2. (a) XPS survey spectrum of as-prepared Co7Fe3 nanospheres. High-resolution XPS spectra of Co 2p (b) and Fe 2p (c).

3. RESULTS AND DISCUSSION The phase compositions of the as-prepared nanospheres were characterized by XRD (Figure S1). The diffraction peaks centered at around 45.1, 65.7, and 83.2° correspond to the (110), (200), and (211) planes of cubic Co7Fe3 (space group Im3̅m), with cell parameters a = b = c = 2.8403 Å (JCPDS no. 50-0795). No characteristic peaks corresponding to impurities were found, suggesting a high purity of the as-obtained Co7Fe3 nanospheres. The SEM image in Figure 1a reveals the spherelike shape of Co7Fe3 prepared via the liquid-phase-reduction process. Uniform nanospheres with an average diameter of 350 nm were achieved when 20 mL of N2H4·H2O was added. The energy-dispersive X-ray spectroscopy (EDS) characterization of the CF350 nanosphere in Figure S2 indicates that the stoichiometry ratio of Co/Fe is close to 7:3, suggesting compositions of Co7Fe3 for the obtained product. Meanwhile, the distribution of both Co and Fe within a single sphere (Figure 1c) is rather homogeneous, as shown in Figure 1d,e. The Co7Fe3 nanospheres were further characterized by TEM. The TEM images in Figure 1f,g illustrate an average diameter of about 350 nm and a narrow size distribution of the as-prepared Co7Fe3 nanospheres. The surface of the nanosphere is smooth. The high-resolution TEM (HRTEM) image and selected area electron diffraction (SAED) pattern obtained from a single sphere are shown in Figure 1h,i, respectively. The well-resolved lattice fringes with distances of 0.201 nm were ascribed to the (101) and (110) facets of cubic Co7Fe3. Moreover, the intersection angle between the (101) and (110) planes is 60°, which is in line with the theoretical value. The SAED pattern exhibited well-defined sharp diffraction spots corresponding to the (110), (101), and (211) planes of cubic Co7Fe3, which is in accordance with the XRD results. The HRTEM and SAED results clearly indicate high crystallinity of the as-prepared Co7Fe3 nanospheres with a single crystalline structure. It was found that the presence of PVP is a crucial factor to obtain uniform spheres. As shown in Figure S3, the products prepared in the absence of PVP are nonuniform spheres. During the reaction, Co7Fe3 particles were prevented from aggregating into large ones due to the presence of PVP macromolecules, which could be chemically absorbed onto the surfaces of Co7Fe3 particles. A similar phenomenon of surfactants adjusting and controlling the morphology has also been observed in previous reports.52−54 Moreover, we find that a suitable amount of NaOH is another factor for the formation of spheres of uniform size. A low concentration of NaOH leads to no or incomplete reaction to form Co7Fe3 particles, whereas a high concentration of NaOH results in the formation of

irregular Co7Fe3 particles (Figure S4). This is because strong alkaline conditions are necessary to prepare metallic particles via the liquid-phase-reduction process with N2H4·H2O as the reducing agent. More importantly, alkaline conditions affect the thermodynamics of nucleation and growth.55,56 Furthermore, the diameter of Co7Fe3 spheres can be adjusted by changing the amount of N2H4·H2O. Upon doubling the amount of N2H4·H2O, Co7Fe3 spheres with an average diameter of 650 nm were obtained, as shown in Figure 1b. Further increase in the amount of N2H4·H2O resulted in much larger Co7Fe3 spheres with a diameter of about 2 μm (Figure S5). Despite having different sizes, Co7Fe3 spheres present both uniform size and a smooth surface. This is because the size of the particles depends on the initial nucleation and subsequent growth processes of the primary crystals, which can be adjusted by controlling the reaction rate.57 The kinetics of nucleation and growth of reduced metal atoms can be well regulated and efficiently controlled by varying the amount of N2H4·H2O. For a high concentration of N2H4·H2O, a higher reaction rate, abundance of nuclei formed, and concentration in a small area contribute to the coalescence of primary particles to grow into spheres. Therefore, a higher nucleation rate means more raw material to form spheres in the nucleation process, which, in turn, results in a larger size. However, at a low concentration of N2H4·H2O, the reaction rate was lower, leading to lower nucleation and, subsequently, smaller size.58 From the SEM and TEM observations, it is clear that Co7Fe3 nanospheres with uniform sizes can be successfully prepared through a facile liquid-phase reduction method, and the diameter can be adjusted by changing the concentration of N2H4·H2O. The chemical compositions of Co7Fe3 nanospheres were examined by XPS. Figure 2a displays the XPS survey spectrum of CF350 spheres. The sharp peaks at about 778, 707, 531, and 284 eV correspond to Co 2p, Fe 2p, O 1s, and C 1s, respectively, indicating a high purity of the prepared sample. To further investigate the valence states of elements, highresolution XPS spectra were recorded. As shown in Figure 2b, besides the Co 2p3/2 peak (at 778.5 eV) and Co 2p1/2 peak (at 794.3 eV) corresponding to metallic cobalt, there are two peaks (802.1 and 783.7 eV) located at the high-binding-energy side of the main peaks. The appearance of these two peaks indicates the presence of Co2+ ions,59 suggesting the surface oxidation of Co7Fe3 spheres. The Fe 2p spectrum in Figure 2c presents two dominant peaks corresponding to Fe 2p3/2 (at 707.3 eV) and Fe 2p1/2 (at 719.2 eV), representing metallic Fe. The weak peak at 711.4 eV is attributed to oxidized Fe species,60 demonstrating a slight oxidation of Fe. Moreover, no 21935


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Figure 3. SEM (a) and TEM (c) images of Co7Fe3@SiO2 spheres (CFS350); (b) SEM image of an individual sphere showing the full coating of SiO2; (d) TEM image of an individual sphere with a SiO2 thickness of 30 nm.

Figure 4. (a) Room-temperature hysteresis loops of Co7Fe3 and Co7Fe3@SiO2 nanospheres; (b) magnetic properties of Co7Fe3 and Co7Fe3@SiO2 nanospheres.

can be maintained upon annealing at 500 °C. No characteristic peaks attributed to SiO2 can be observed in the XRD patterns (Figure S9), suggesting that the SiO2 coating shell is amorphous. Compared to those of as-prepared Co7Fe3@SiO2 nanospheres, the XRD peaks of the annealed sample are much stronger and sharper, indicating an improvement in crystallinity and elimination of defects. These results demonstrate that the SiO2 shell was successfully fabricated onto the surface of Co7Fe3 spheres by the Stöber process. The presence of shell enables Co7Fe3 cores to well isolate from each other and protects Co7Fe3 from possible oxidation at a high temperature. Co7Fe3@SiO2 nanospheres with a unique structure possess the potential for high-temperature applications. The magnetic properties of Co7Fe3 and Co7Fe3@SiO2 composite spheres were measured using VSM. The hysteresis loops in Figure 4a indicate the ferromagnetic behavior of all samples. The saturation magnetization (Ms) and coercivity (Hc) of the spheres with difference diameters are displayed in Figure 4b. It can be seen that the Ms of CF350 and CF650 are

XRD peaks of oxides were detected, further confirming surface oxidation.61 To improve the chemical stability and optimize microwave absorption ability, Co7Fe3 spheres were coated by SiO2 layer. Figure 3 shows SEM and TEM images of Co7Fe3@SiO2 spheres. The SEM images in Figure 3a,b indicate that the introduction of the SiO2 shell does not change the sphere configuration of Co3Fe7 but the surface becomes rougher. The SiO2 shell was uniformly coated onto the surface of the Co7Fe3 spheres in the process. The XPS survey spectrum of Co7Fe3@ SiO2 in Figure S6 shows that SiO2 was successfully coated onto the surface of Co7Fe3 nanospheres. The elemental mappings of Co, Fe, O, and Si in Figure S7 reveal homogeneous element distribution within the core−shell structures. The TEM images in Figure 3c,d clearly demonstrate the core−shell structure of Co7Fe3@SiO2 nanospheres, with a shell thickness of about 30 nm. The SiO2 shell is dense and fully covers the metallic core, which is beneficial for chemical stability. SEM analysis in Figure S8 indicates that the morphology of Co7Fe3@SiO2 nanospheres 21936


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Figure 5. Complex permittivity (a) and permeability (b) of CF350- and CF650/paraffin composites with different filling ratios.

Figure 6. Complex permittivity (a) and permeability (b) of CFS350- and CFS350-annealed/paraffin composites.

almost the same despite the differences in size. However, Hc decreases from 72.5 to 64.5 Oe when the particle size increases. The Ms of the samples was found to be lower than that of bulk materials. As a SiO2 shell of 30 nm thickness was introduced, the Ms decreased to 155.0 emu/g. The decrease in Ms was ascribed to the introduction of nonmagnetic SiO2. Ms increased to 183.9 emu/g after annealing at 500 °C due to the elimination of crystal defect and improvement in crystallinity. The significantly increased Ms contributes to an increase in the permeability and enhancement of magnetic loss. The complex permittivity and permeability were measured on VNA, and the results are illustrated in Figure 5. The real part of permittivity (ε′) of all specimens decreases slightly with increasing frequency in 2−18 GHz band, whereas ε″ first increased with increasing frequency to reach a peak of 2.1 at around 12 GHz and then decreased gradually, as shown in Figure 5a. The variations in ε′ and ε″ suggest a mild dielectric relaxation in the band. Compared to that of CF350, the ε′ of CF650 is lower; for instance, ε′ of the specimen containing CF350 remains at around 9.6 in the 2−12 GHz range, whereas the specimen containing CF650 presents a ε′ of around 7.7. Both ε′ and ε″ for CF350 increased all over the 2−18 GHz band with an increase in the filling ratio from 20 to 50 vol %. The dielectric relaxation is also enhanced greatly. The relaxation is dominated by the particle’s interface areas and conductivity.62−64 An increased filling ratio could enhance the conductivity of the filler. Consequently, the dielectric relaxation was immensely improved. An increase in the permittivity as well as enhanced relaxation is attributed to an increase in the surface charge polarization.

As shown in Figure 5b, the permeability (μ′) of specimens containing CF350 and CF650 decreases with an increase in frequency, whereas a broad peak at the curve of μ″ indicates a mild ferromagnetic resonance. Compared to that of the CF350based specimen, the μ′ of CF650-based specimen is apparently lower in the 2−4 GHz range. For example, μ′ of the specimen containing CF350 is about 2.5 at 2 GHz, whereas that of the specimen containing CF650 is 2.3. Specifically, μ″ of the CF650-based specimen is lower in the whole frequency range. When increasing the filling ratio to 50 vol %, μ′ as well as the μ″ increases significantly all over the 2−18 GHz band. An evident and broad peak (covering the 6−13 GHz range) is observed in the curve of imaginary part, revealing the enhancement of the natural resonance. The influence of eddy current is quite weak in specimens containing Co3Fe7 spheres as fillers, even at a high filling ratio of 50 vol %, as demonstrated in Figure 5b. Spheres prepared via solution chemistry method are believed to possess a low conductivity due to the imperfections developed during their preparation,65 which then suppress the intensity of eddy current. The fine size of the spheres is also helpful for reducing the effect of eddy current, as noted previously.66 The inherently high resistivity of Co3Fe7 nanospheres is of technical significance for developing EMA materials with a higher filling ratio. The electromagnetic properties of Co7Fe3@SiO2 spheres with a SiO2 shell thickness of 30 nm were also investigated. As shown in Figure 6a, the ε′ of the Co7Fe3@SiO2-based specimen decreases apparently (from 9.3 to 7.3) compared to that of the Co7Fe3-based specimen, and the dielectric relaxation weakens. The surface resistivity of Co7Fe3@SiO2 spheres is improved as 21937


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Figure 7. EMA performance of Co7Fe3/paraffin composites: (a) CF350, 20 vol %; (b) CF650, 20 vol %; (c) CF350, 50 vol %.

enhanced EM loss, which can be favorable for obtaining a high EMA efficiency. To evaluate the microwave absorption properties, the reflection loss (RL) of coatings containing different fillers was calculated according to transmit-line theory on the basis of the measured EM parameters. The plots of RL versus frequency at different thicknesses are presented in Figure 7. The coating containing Co7Fe3 nanospheres as fillers exhibits excellent EMA performance. As shown in Figure 7a, an RLmax of 78.4 dB together with an effective absorbing bandwidth (EAB) (RL higher than 10 dB, EAB10) of 6.7 GHz was simultaneously obtained in a thin coating (1.59 mm) using CF350 as fillers. Moreover, an EAB10 of 5.2 GHz (7.4−12.6 GHz) is observed at X band (8−12 GHz) in a thin coating (2 mm). The RLmax of the coating containing CF650 as fillers shifts to a higher frequency compared to that in the case of CF350. For instance, an EAB10 width of up to 6.5 GHz is observed at 9.0−15.5 GHz when the thickness is 2.0 mm, as shown in Figure 7b. The RLmax shifts to a lower frequency as the filling ratio of CF350 increases from 20 to 50 vol %. The RLmax is 64.0 dB and EAB10 is 4.3 GHz (4.3−8.6 GHz) at 5.8 GHz, with a thickness of 1.93 mm. Specifically, the coating with a small thickness of 1.5 mm presents an EAB5 (RL higher than 5 dB) width up to 14 GHz in 4.0−18.0 GHz (Figure 7c), suggesting a high EMA efficiency at very wide band range. These coatings are excellent candidates for EMA application that require small thickness, high efficiency, and wide absorbing band, as the EAB5 covers the C, X, and Ku bands. The excellent EMA performance of these highly filled coatings is related to the high and descending EM properties. The application of a high filling ratio is quite helpful to reveal

Co7Fe3 is separated completely by the SiO2 layers, which then weakened the electric dipole polarization that occurred in lathy and conductive aggregations,67 leading to a decrease in the permittivity of the Co7Fe3@SiO2 core−shell spheres. The permittivity of the specimens containing Co7Fe3@SiO2 nanospheres as fillers increased obviously after annealing at 500 °C for 2 h. As shown in Figure 6a, ε′ increases over the whole frequency range. For instance, ε′ of the specimen containing CFS350 is about 7.3 at 2 GHz, whereas that of the specimen containing annealed product is 8.8. Meanwhile, the dielectric relaxation enhanced apparently. The dielectric relaxation mainly depends on the particle’s conductivity and interface area. The conductivity of Co7Fe3@SiO2 spheres increases as defects are eliminated during annealing, thus contributing to the enhanced relaxation. A similar variation in microstructures as well as the contribution to dielectric loss was previously observed in Co coatings.68 Figure 6b shows the permeability of a specimen containing Co7Fe3@SiO2 nanospheres as fillers. It can be observed that μ′ decreases slightly compared to that of the Co7Fe3 spheres. The μ′ was observed to increase slightly, from 2.08 to 2.35, after the filler was annealed at 500 °C. The improvement in permeability is related to the microstructure evolution that occurs during annealing. The saturation magnetization of Co7Fe3@SiO2 spheres increased from 155.0 to 183.9 emu/g after annealing (Figure 4b), which then contributed to an improved permeability. These results imply that upon SiO2 coating and following annealing, the electromagnetic matching is improved at the cost of a slight decrease in permeability. The microstructure evolution, including the elimination of imperfections and improvement in crystallinity, contributes to the 21938


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Figure 8. EMA performance of Co7Fe3@SiO2/paraffin composites: (a) CFS350; (b) CFS350 annealed at 500 °C.

the superior penitential of ferromagnetic particles and thus endues high permittivity and permeability to this series of coatings, which, in turn, contributes greatly to the high EMA efficiency and small thickness.69 On the other hand, the intense dielectric relaxation and natural resonances in the targeting band lead to a slowly descending index, which is crucial for obtaining broad band absorbing. Figure 8 shows the RL of coatings containing Co7Fe3@SiO2 nanospheres as fillers. As shown in Figure 8a, an RLmax of 68.5 dB and EAB10 of 7.0 GHz (11.0−18.0 GHz) were obtained with a thickness of 1.8 mm. It is notable that EAB10 can reach up to 7.8 GHz (9.6−17.4 GHz) when the thickness is 2 mm, which is over that of the X and Ku band and is actually much wider (2.6 GHz) than that of the coating containing CF350 as a filler with the same thickness. In our previous work, the effects of the SiO2 shell thicknesses on the EM parameters of Co@ SiO2 composites have been investigated.46 It was found that with a proper SiO2 shell thickness, excellent reflection characteristics can be obtained. In the current study, we investigated the effects of SiO2 thickness (Figure S10) on the microwave-absorbing performance. As shown in Figure S11, RLmax is 61.3 dB and EAB10 is 4.3 GHz (13.7−18.0 GHz), with a SiO2 thickness of 10 nm. When the SiO2 thickness is up to 50 nm, an RLmax of 54.5 dB and EAB10 of 3.8 GHz (14.2−18.0 GHz) are achieved. It is notable that these two samples show slightly decreasing microwave absorption capability in both the maximum RL and absorption bandwidth compared to that of CFS350. After annealing at 500 °C, the microwave absorption capability can be effectively maintained and the absorption band shifts to a low frequency. As shown in Figure 8b, an RLmax of 66.2 dB and an EAB of 6.5 GHz (8.5−15.0 GHz) were obtained with a thickness of 1.93 mm. The excellent EMA performance was caused by the complementarity between magnetic loss contributed by Co7Fe3 cores and dielectric loss from SiO2 shells. Table S1 shows the typical CoFe-based composites and their corresponding EMA performances in the recent literature. According to the comparison, the composite nanospheres in our study are more competitive than other CoFe-based agents for microwave absorption application in terms of strong absorption and a wide absorption frequency range. On the basis of the above results, we can conclude that the composites can be used as promising microwave-absorbing agents in a wide frequency range and high-temperature application (see thermogravimetric analysis data in Figure

S12 and the associated discussion in the Supporting Information).

4. CONCLUSIONS In summary, we have described a simple liquid-phase reduction process to synthesize size-controllable Co7Fe3 nanospheres via adjusting the reaction parameters. The Co7Fe3 absorbers exhibited excellent microwave absorption properties in terms of a high maximum RL (78.4 dB) as well as a broad absorption bandwidth (6.7 GHz). Upon SiO2 coating, a significantly enhanced microwave absorption ability, with an RLmax of up to 68.5 dB and an absorption bandwidth of up to 7.0 GHz (11.0− 18.0 GHz), was obtained. The morphology and outstanding microwave absorption capability of Co7Fe3@SiO2 nanospheres could be well maintained after annealing. Moreover, the SiO2 shells play an important role in protecting Co7Fe3 nanospheres from oxidation. Our results reveal that the Co7Fe3 nanospheres achieved in this study with the features of small thickness, strong RL, and wide absorption frequency range are attractive candidates for new types of EMA materials. It can be believed that this study opens up new discernment and afflatus to novel EMA agents.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03907. XRD, EDS, and thermogravimetric analysis of asprepared Co7Fe3 and Co7Fe3@SiO2 nanospheres; SEM images of Co7Fe3 particles obtained with different experimental parameters; XPS spectra of Co7Fe3@SiO2 spheres; SEM images of Co7Fe3@SiO2 spheres annealed at 500 °C; TEM images and EMA performance of Co7Fe3@SiO2 spheres with different SiO2 thicknesses; comparison of electromagnetic properties with those in previous reports (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.-T.J.). *E-mail: [email protected] (L.Z.). ORCID

Cheng-Yan Xu: 0000-0002-7835-6635 Liang Zhen: 0000-0001-6159-8972 21939


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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Natural Science Foundation of China (grant no. 51201048), Ph.D. Programs Foundation of Ministry of Education of China (grant no. 20112302120021), and the SAST Foundation.

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Co7Fe3 and Co7Fe3@SiO2 Nanospheres with Tunable Diameters for

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