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Facile, Large-Scale and Expeditious Synthesis of Hollow Co and Co@Fe Nanostructures: Application for Electromagnetic Wave Absorption Peipei Yang, Xiuchen Zhao, Ying Liu, and Yue Gu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11284 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Facile, Large-scale and Expeditious Synthesis of Hollow Co and Co@Fe Nanostructures: Application for Electromagnetic Wave Absorption Peipei Yang,† Xiuchen Zhao,*† Ying Liu,† Yue Gub‡ †

School of Materials Science and Engineering, Beijing Institute of Technology, Beijing, 100081, China

‡

Materials Science and Engineering Program, nanoEngineering Department, 9500 Gilman Drive La Jolla, CA 92093, United States

ABSTRACT. Hollow metal materials have drawn a lot of attention owing to their excellent performance for wide potential applications. Here, we have successfully synthesized hollow Co nanostructure with controllable structures and compositions, including hollow Co nanospheres, hollow Co nanochains and hollow Co@Fe nanospheres. Uniform Fe nanospheres and nanochains are first synthesized, and then Fe@Co nanospheres are achieved by electroless plating cobalt on iron surfaces. Hollow Co nanostructures are obtained easily by galvanic cell reaction between Co shells and Fe cores in hydrochloric acid at room temperature. Furthermore, hollow Co@Fe nanospheres form after plating iron on the as-synthesized hollow Co nanospheres, which acted as templates. Electromagnetic (EM) wave absorption properties of hollow Co nanostructures are investigated. Hollow Co nanochains, when 1

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blended with 40 wt.% in paraffin-based filler, exhibit better EM wave absorption (-42.5 dB) than the hollow Co nanospheres (-30.4 dB) with same ratio of filler. On the contrary, as the filler content is 60 wt.%, the reflection loss of hollow Co nanochains degrades to -14.0 dB, which is much worse compared to hollow Co nanospheres (-41.7 dB). Moreover, hollow Co@Fe nanospheres (with 60 wt.% filler) show excellent EM wave absorption properties with minimum RL of -47.3 dB and effective bandwidth of 4.8 GHz compared with hollow Co nanospheres. The method of electroless plating followed by galvanic cell reaction to synthesize hollow nanostructures is simple, robust, and widely applicable for some metals or composites with various potential.

1. Introduction In recent years, hollow materials are of widespread interest owing to their application potential in lithium ion batteries, adsorption, wastewater treatment, biosensor and catalysis.1-8 Many nano-sized hollow materials, including metals, polymers, and oxides, have been successfully synthesized.9-13 Hollow metal materials are superior to their solid structure counterparts, due to their high specific surface area, low density, optical absorption spectra and surface plasmon resonance.14-17 Kong and co-workers demonstrated porous hollow Pd spheres, which is able to greatly increase the specific surface area and improve electrochemical properties as an anode material of direct ethanol fuel cells.14 Hollow magnetic metals, such as Fe, Co, Ni and Fe-Co alloys, could be investigated as electromagnetic (EM) wave absorption materials. It is proved 2


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that, other than granularity, phase structure, or shell thickness, the morphology is equally important in enhancing EM wave absorption property of hollow magnetic metallic materials.18-20 A variety of chemical methods, including self-assembly and template method, have been employed to successfully synthesize hollow metal materials.21-25 Guo and his co-workers synthesized hollow Co nanochains by self-assembly method, and the reaction conditions controlled strictly in the process of preparation.21 Hollow metal materials are also able to be synthesized based on sacrificial templates, those examples including silver sphere, silica sphere, polymer beads and PSA sphere. In most of the cases, templates would be removed by various methods according to the different natures of materials, which include acid/alkali dissolution, high temperature calcination, and galvanic replacement reaction.26-29 However, there are some disadvantages by using templates methods. First, when we prepare hollow metal materials of various morphologies, it is necessary to use corresponding template of different materials synthesized by various approaches. However, the template materials always differ from each other, thus synthesis processes would be much more complex. Besides, high temperature and extremely long time are often required while removing the templates.30-32 Most importantly, non-metallic materials are common used as templates, such like SiO2, carbon and polystyrene.33-36 The benefit of using these templates is that they would not damage the metal coating or shell. Even so, there is not any feasible method to coat metal on those non-metallic cores, owing to the intrinsic chemical or physical property 3



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difference. Therefore, surface modification on templates are necessary before coating metal shell, which would complicate the synthesis process. In general, the best alternative template should have various morphologies, and also has the capability to be removed easily and quickly. In this paper, a facile, large-scale, and expeditious synthesis method of hollow Co nanostructures by electroless plating on Fe templates is demonstrated. The morphologies of hollow Co nanostructures could be manipulated simply by controlling ammonia amount during the synthesis process. Notably, the Co shell is plated on the Fe core directly, hence no surface modification is required to acquire the hollow nanostructures. Then the Fe cores dissolve in hydrochloric acid at room temperature based on the galvanic cell principle. Further, the mechanisms of forming Fe template and hollow Co nanostructure are discussed. Finally, the EM wave absorption properties of hollow Co nanospheres, hollow Co nanochains and hollow Co@Fe nanospheres are investigated.

2. Experimental Methods 2.1. Raw materials. Ferrous sulfate heptahydrate (FeSO4·7H2O), sodium borohydride (NaBH4), cobalt sulfate heptahydrate (CoSO4·7H2O), potassium sodium tartrate tetrahydrate (KNaC4H12O10·4H2O), ammonium hydroxide (NH3·H2O, 25%, AR), hydrazine hydrate (N2H4·H2O, 80%, AR), hydrochloric acid (HCl, 36%-38%, AR) and anhydrous ethanol were of analytical purity without further purification. The deionized water had a resistivity higher than 18.25 MΩ·cm. 4


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2.2. Preparation of Fe nanospheres and nanochains. Fe nanospheres and nanochains were prepared with the chemical reduction method. First, 0.5 mol/L FeSO4·7H2O was heated up to 40 oC in a water bath. Then, an appropriate amount of NaBH4 solution (2.0 mol/L) and 140 mL NH3·H2O were mixed, and were added into FeSO4 solution slowly with mechanical stirring. In a similar way, Fe chains were obtained by adding only NaBH4 solution into FeSO4 solution. It is worth noting that the solution was mixed by mechanical stirring instead of magnetic stirring, because magnetic stirring could induce out-of-order assembly of the magnetism. After 15 min, the products were collected and rinsed with deionized water. 2.3. Preparation of Fe@Co core-shell nanospheres and nanochains. Fe@Co core-shell nanoparticles were synthesized by electroless plating. CoSO4·7H2O and KNaC4H12O10·4H2O of the same mass were dissolved into 500 mL deionized water and 7 mol/L of NaOH solution was introduced into the mixture to adjust PH equal to 13.5. Then, the obtained Fe nanoparticles were also added into the mixture with mechanical stirring, sonication and heating (up to 50 oC) in a sonicator. Subsequently, 8 mL N2H4·H2O was diluted to 50 mL and added into the mixture with Fe nanoparticles. The reaction was kept for 20 min and then rinsed with deionized water. 2.4. Preparation of hollow Co nanospheres and nanochains. In order to remove Fe core, the prepared Fe@Co core-shell nanoparticles were immersed into 1 vt.% HCl aqueous solution assisted by mechanical and ultrasonic stirring at room temperature. Eventually, hollow Co nanostructures were obtained based on the principle of galvanic cell reaction. The products were collected and rinsed sequentially with 5



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deionized water and anhydrous ethanol, and then were dried in a vacuum chamber. 2.5. Preparation of hollow Co@Fe nanospheres. Hollow Co nanoparticles were used as template to synthesize hollow Co@Fe nanostructures. Same masses of FeSO4·7H2O and KNaC4H12O10·4H2O were dissolved in deionized water first, and mixed with the as-synthesized hollow Co nanospheres while heating up to 40 oC. Subsequently, 200 mL NaBH4 solution and 300 mL N2H4·H2O were added into the mixture solution at a rate of 5 mL/min. The reaction was kept for 30 min and then the products were obtained after rinsed with deionized water. 2.6. Characterization. The crystal structure of Fe template and hollow Co@Fe nanospheres were analyzed by X-ray diffraction (XRD; Rigaku Ultima-III X-ray diffractmeter) using Cu Kα radiation, and the crystal structure of hollow Co nanospheres and nanochains were analyzed by XRD (Bruker D8 Advance diffractometer) using Mo Kα radiation. The morphologies of the samples were observed by transmission electron microscopy (TEM; JEOL JEM-2100) at an acceleration voltage of 20 kV and scanning electron microscopy (SEM; Quanta 200F) equipped with energy-dispersive spectrometry (EDS). Surface states were confirmed by X-ray photoelectron spectroscopy (XPS; PHI QUANTERA-II), Fourier transform infrared spectroscopy (FT-IR; IRPRESTIGE) and Thermo gravimetric Analysis (TG; HITACHI STA7300). In addition, Brunauer-Emmett-Teller method was utilized to calculate the specific surface areas of hollow Co nanospheres and nanochains by a N2 adsorption apparatus (BET; JW-BK112). The average pore size was derived from desorption curve. The magnetic properties were characterized by vibration sample 6


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magnetometer (VSM; LakeShore 730T) and superconducting quantum interference device (SQUID; Quantum Design MPMS-3). The EM wave absorption properties of hollow Co nanospheres, nanochains and hollow Co@Fe nanospheres were studied through a vector network analyzer (VNA; Agilent E5071C) in the frequency range of 2-18 GHz. The specimens applied for the EM measurements were prepared by homogeneously blending the samples with paraffin wax (the weight ratio of the hollow Co particles was 40 wt.% and 60 wt.%), and then the mixture was pressed into a ring shape (outer diameter of 7 mm and inner diameter of 3.04 mm). The relative complex permittivity (εr=ε′−jε″) and permeability (µr=µ′−jµ″) were obtained by the experimental scattering parameters S11 and S21 through the standard Nicolson-Ross theoretical calculations. 37

3. Results and Discussion 3.1. Morphology and Microstructure of hollow Co nanospheres and nanochains The morphological features of Fe nanospheres and nanochains are shown in Figure 1a and 1b. The particles are spherical within a region of 150-200 nm when adding ammonia. Fe nanochains are obtained without ammonia and the average diameter is approximately 100 nm. Typical selected area electron diffraction (SAED) patterns are also shown in insets of Figure 1a and 1b. The SAED pattern of Fe nanospheres corresponds to amorphous structure, however, SAED pattern of Fe nanochains can be well indexed to a BCC phase that the inner rings are assigned to (110), (100) and (200) planes. The HRTEM images of Fe nanospheres and nanochains are also demonstrated 7



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that Fe nanospheres is amorphous structure and Fe nanochains is BCC structure, as shown in Figure S1. Then the Co shell is plated on Fe core, the morphologies and structure are shown in Figure S2 and S3. Then an etching process with 1 vt.% HCl solution is applied to remove Fe core of Fe@Co core-shell nanospheres and nanochains, resulting in the production of hollow Co nanospheres and nanochains. The morphology of hollow Co nanospheres and nanochains is verified by TEM to confirm the nature of samples, as shown in Figure 1c and 1d. On the basis of TEM examination, a sharp contrast between the dark edges and the pale center can be seen. The average diameter and shell thickness of hollow Co nanospheres is about 280 nm and 32 nm, respectively. Figure 1d reveals the hollow Co nanochains with an average diameter of 113 nm and shell thickness of 28 nm. The corresponding SAED patterns (Figure 1e and 1f) indicate that the hollow Co nanospheres and nanochains are polycrystalline structure. The interlunar spacing, obtained from the SAED patterns, are 2.16, 2.04, 1.91, 1.25 and 1.06 Å which can be indexed to the (100), (002), (101), (110), and (112) planes of the HCP-Co, and 2.04, 1.25 and 1.06 Å also corresponding to the (111), (220), and (311) planes of FCC-Co. The high-resolution TEM (HRTEM) image of hollow Co nanospheres is shown in Figure 1g. The lattice fringe of hollow Co nanospheres corresponds to 2.032 Å, which could be attributed to the (002) planes of HCP-Co or (111) planes of FCC-Co, and lattice fringe of 1.910 Å can be ascribed to (101) planes of HCP-Co. Figure 1h displays the HRTEM image of hollow Co nanochains, in which the lattice fringe is assigned to 2.035 Å of (002) plane for HCP-Co or (111) planes for FCC-Co. 8


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Figure 1. (a, b) TEM images of Fe nanospheres and nanochains, (c, d) TEM images, (e, f) SAED patterns and (g, h) HRTEM images of hollow Co nanospheres and nanochains.

Scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS) 9



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are used to further confirm the surface morphology of hollow Co nanospheres and nanochains, as shown in Figure 2. The well dispersed hollow Co nanospheres and nanochains are observed in Figure 2a and 2c. Fe and Co elements can be detected in hollow Co nanospheres and nanochains as shown in Figure 2b and 2d. The atom content ratio of Fe and Co is 1.47:98.53 and 1.78:98.22, which confirms that Fe cores were almost entirely removed, and hollow Co nanospheres and nanochains were almost only composed with cobalt. In addition, it is worthwhile noting that some voids are observed on the surface of hollow Co nanospheres and nanochains, as shown in insets of Figure 2b and 2d.

Figure 2. SEM images and EDS of (a-b) hollow Co nanospheres and (c-d) hollow Co nanochains.

Figure 3 shows the XRD patterns of as-prepared samples using Mo Kα radiation. It reveals that the XRD patterns of hollow Co nanospheres and nanochains are the same. The XRD peaks can be indexed to the (111), (220), (311), and (222) planes of 10


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FCC-Co, and the (100), (002), (101), (110), (103), and (112) planes of HCP-Co, which is in complete agreement with the results of SAED and HRTEM data. It should be pointed out that Fe diffraction peaks would not present in the both samples in accordance with the XRD results. From the above results, we can confirm that the as-synthesized hollow Co nanospheres and nanochains are mixture of HCP and FCC Co structures. HCP-Co is usually synthesized at low temperature and it has be proved that the amount of HCP-cobalt is related to the [C4H4O6]2-/Co2+ ratio in our previous work.38 It is widely known that FCC-Co is a high temperature phase which is metastable at room temperature. However, it has been reported that FCC-Co can be synthesized successfully at room temperature in nanocrystals. As a metastable phase, FCC-Co occurs as a resultant because of the existence of stacking faults, which would be generated during the rapid deposition process of Co atomic controlled by the reaction conditions.39

Figure 3. XRD patterns of as-prepared hollow Co nanospheres and nanochains.

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3.2. The controlling of Fe template and formation mechanism of hollow Co nanospheres and nanochains Fe nanospheres and nanochains served as template are synthesized to fabricate hollow Co nanospheres and nanochains. Figure 4a shows the XRD patterns of Fe nanospheres and nanochains and theoretical XRD simulation pattern of Fe (JCPDS No. 06-0696). The structure of Fe nanospheres becomes amorphous after adding ammonia in the reaction solution. However, Fe nanochains are crystal structure, and also the peaks can be assigned to BCC structure of (110), (200), and (211) planes. The XRD results correspond to the SAED patterns of Fe templates as shown in insets of Figure 1a and 1b. The magnetic properties of Fe templates are measured at room temperature, as shown in Figure 4b. It is not difficult to find that Fe nanochains have a high remanence (Mr) of 33 emu/g, while Mr of Fe nanospheres is 17 emu/g. The remanence difference between Fe nanochains and nanospheres is due to their different crystal structures. It is well known that the crystalline structure provides a geometric basis for a reasonable orientation arrangement of the magnetic moment, and the low Mr can be found for the smallest structural correlation lengths like amorphous of no grain boundaries and dislocation.40 Hence, the Mr value of Fe nanospheres (amorphous) is lower than that of Fe nanochains (crystalline), which leads to the difference morphologies of Fe templates. The Fe particles with crystalline structure is easy to connect together to form chains due to its higher Mr and strong magnetic interaction between the particles. On the contrary, the Fe particles with a low Mr and weak magnetic interaction, due to its amorphous structure, result in the spherical Fe 12


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template formation.

Figure 4. (a) XRD patterns and (b) hysteresis loop of Fe nanospheres and nanochains.

The formation mechanism of hollow Co nanospheres and nanochains is very interesting, and demonstrated with the schematic diagram shown in Figure 5. The galvanic cell reaction between Fe core and Co shell happens in HCl solution due to the difference of electrochemical potentials between Fe2+/Fe (−0.44 V) and Co2+/Co(−0.28 V). However, the galvanic cell reaction does not occur immediately when the Fe@Co particles are immersed in a diluted HCl solution (1 vt.%). The Co shells of Fe@Co particles start to have pitting corrosion under the existence of H+, usually originating from some defects in the Co shells of Fe@Co particles, such as stacking faults, vacancies and crystal boundary, which are formed during electroless plating process. Once Fe cores contact HCl solution, it starts to be corroded, and the corrosion of Co shells ceases immediately based on galvanic cell reaction. Thus, the small voids induced by pitting corrosion can be observed clearly on the surface of hollow Co nanospheres and nanochains as shown in insets of Figure 2b and 2d. Most importantly, Fe core and Co shell need to contact during the entire corrosion process. 13



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Fe core as anode is dissolved (Fe−2e-→Fe2+) and Co shell as cathode is prevented from being etching off. Electrons flow from anode (Fe) to cathode (Co). The H+ gathers on the cathode of Co (with 2H++2e-→H2↑ as cathodic reaction). The reaction is very intensive, and bubbles can be seen during the reaction. When Fe is almost completely corroded, the hollow Co structures are formed, which can be observed in Figure 5.

Figure 5. Schematic depiction of the synthesis procedure of hollow Co structure.

3.3. Properties of hollow Co nanospheres and nanochains X-ray photoelectron spectroscopy (XPS) is characterized to ascertain the surface oxidation state of hollow Co nanostructure. The wide scan XPS spectrum of hollow Co nanospheres (Figure 6a) indicates the existence of C, O and Co in the composites. 14


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The introduction of C is possible due to the decomposed residuals of organic compounds. The core level binding energy for the Co 3p primary peak at 61.0 eV is clearly observed. The binding energy peaks at 101.9 eV and 781.9 eV for Co 3s and Co 2p further support the presence of cobalt and cobalt oxides. The distinct two states of Co (2p3/2) and Co (2p1/2) observed at 781.4 and 797.3 eV are due to spin-orbit splitting in the inset of Figure 6a.41 The XPS result of hollow Co nanochains is consistent with hollow Co nanospheres as shown in Figure 6b. From the above analysis, we can deduce that the presence of Co and cobalt oxides on the surface of hollow Co nanostructure. The FT-IR spectra of hollow Co nanospheres and nanochains are shown in Figure 6c. The absorption of hollow Co nanospheres at approximately 2922 cm-1 reflects the stretching vibration of the C-H groups. The absorption at 1631 cm-1 is due to the C=O stretching vibration. The absorption at 1396 cm-1 and 613 cm−1 is due to the C-H bending vibration and C-O stretching vibration of anhydrous ethanol.42 The introduction of these groups is possible due to the chemical adsorption of alcohol. In order to apply the materials in harsh conditions (such as high temperature), the thermal stability should be considered. The thermal stability of hollow Co nanospheres and nanochains is characterized by TG in Figure 6d. The first weight loss from ambient temperature to 250 oC is attributed to the removal of physically adsorbed water and alcohol. Then both hollow Co nanospheres and nanochains exhibit an obvious weight gain owing to the oxidation at the increasing temperature. From XPS, FT-IR and TG results, it can be concluded that some organic derivant or cobalt oxides exist on the surface of hollow Co nanospheres and 15



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nanochains, but the contents of them are too low to be detected by XRD. Hollow Co nanospheres and nanochains also show porous structures, as revealed in Figure 2. Furthermore, N2 adsorption-desorption measurements (Figure 6e and 6f) demonstrate that hollow Co nanospheres are porous in nature. Accordingly, the curves display a single peak with a maximum at 3.6 nm for hollow Co nanospheres and 4.4 nm for hollow Co nanochains (insets of Figure 6e and 6f). It also shows that BET surface area of hollow Co nanospheres is 13.847 m2/g, and the obtained hollow Co nanochains have a BET surface area of 35.255 m2/g, respectively. It demonstrates that hollow Co nanochains exhibit a higher specific surface area than that of hollow Co nanospheres.

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Figure 6. Hollow Co nanospheres and nanochains: (a) and (b) XPS spectra, (c) and (d) FT-IR and TG spectra, (e) and (f) nitrogen adsorption-desorption isotherms and pore size distribution curves.

The magnetic properties of hollow Co nanospheres and nanochains are measured at room temperature. As displayed in Figure 7a, both samples show typical hysteresis loops. Hollow Co nanospheres show a saturation magnetization (Ms) of 91 emu/g, remanent magnetization (Mr) of 19.1 emu/g, and coercivity (Hc) of 300 Oe, and those of hollow Co nanochains are 100.5 emu/g, 22.6 emu/g and 395.5 Oe, respectively. It is known that the difference in Ms values maybe attribute to the specific surface 17



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area.43 Due to a larger specific surface area of hollow Co nanochains, Ms values of hollow Co nanochains is higher than that of hollow Co nanospheres. In addition, Hc is also a vital parameter for assessing the magnetic properties. From this point of view, Hc depends on shape anisotropy,44-45 thus hollow Co nanochains possess an increased Hc value compared with hollow Co nanospheres. It is known to all that magnetic properties are sensitive to temperature. The zero-field cooling (ZFC) magnetization curves were recorded to illustrate the magnetism of hollow Co nanospheres and nanochains (Figure 7b). The blocking temperature (TB) represents the temperature required for materials to overcome energy barrier via thermal activation.46-48 As it can be observed, when the measuring temperature decreases from 300 to 5 K, the ZFC curves begin to drop at a certain TB, suggesting a magnetic anisotropy energy barrier distribution existed in the hollow Co materials. In addition, compared with hollow Co nanospheres with TB of 20-30 K, hollow Co nanochains have an increased TB of 12-25 K a, indicating that hollow Co nanochains need more thermal energy to overcome energy barrier, which could be attributed to the dipolar interactions among the surface.

Figure 7. (a) Hysteresis loops and (b) zero-field-cooled curves for the hollow Co nanospheres and 18


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hollow Co nanochains at 0 Oe.

3.4. Morphology, microstructure and magnetic properties of hollow Co@Fe nanospheres The morphology features of hollow Co@Fe nanospheres are displayed in Figure 8a. The average diameter of hollow Co@Fe nanospheres is about 330 nm, which is also demonstrated in TEM images in Figure S4. Fe and Co elements can be detected by EDS in hollow Co@Fe nanospheres as shown in Figure 8b, in which the atom content ratio of Fe and Co is 48.8:51.2. Figure 8c shows the XRD pattern of hollow Co@Fe nanospheres by using Cu Kα radiation. The peak of amorphous structure corresponds to the outermost layer of Fe which is completely agreeable with the SAED pattern. The XRD peaks of the samples can be indexed to the BCC and HCP structure. As displayed in Figure 8d, the sample shows a typical hysteresis loop in magnetic behavior with a saturation magnetization (Ms) of 200.7 emu/g, remanent magnetization (Mr) of 6.5 emu/g, and coercivity (Hc) values of 76 Oe. The zero-field cooling (ZFC) magnetization curves of hollow Co@Fe spheres are recorded (Figure 8e). As can be observed, the ZFC curves begin to drop at a certain blocking temperature (TB) of 150-200 K, which is higher than that of hollow Co nanospheres, suggesting an increasing magnetic anisotropy energy barrier distribution existed in the hollow Co@Fe spheres.

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Figure 8. (a) SEM, (b) EDS, (c) XRD, (d) Hysteresis loop and (e) zero-field-cooled curves for the hollow Co@Fe spheres.

3.5. EM wave absorption properties To reveal the EM wave absorption of the samples, the reflection loss (RL) of absorbers can be calculated from the measured relative complex permittivity and permeability values according to the transmission line theory by the following equations:49-50

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RL( dB ) = −20 log

Z in − Z 0 Z in + Z 0

µr Zin 2πfd = tanh( j Z0 c εr

µ rε r )

(1)

(2)

where Zin is the input impedance, Z0 is the characteristic impedance of free space, µr and εr are relative complex permeability and permittivity, f is the frequency, d is the thickness and c is the speed of light. The RL for different thickness is calculated according to Eqs. (1) and (2) to analyze the change of EM wave absorption properties with thickness. The RL values of paraffin-based samples are calculated as shown in Figure 9 and Table 1. For hollow Co nanospheres of 40 wt.%, it can be clearly seen that a minimum RL of -30.4 dB at 14.8 GHz is exhibited corresponding to a thickness of 6.0 mm and the effective bandwidth of RL values below -10 dB (90% of EM wave absorption) is 2.7 GHz (13.5-16.2 GHz) at this thickness (Figure 9a). While the RLmin of hollow Co nanochains of 40 wt.% is -42.5 dB at 16 GHz, and effective bandwidth is 1.86 GHz (15.04-16.9 GHz) for the thickness of 4.9 mm (Figure 9b). However, when the filler content is 60 wt.%, for hollow Co nanospheres, the RLmin is -41.7 dB with the thickness of 1.5 mm and the effective bandwidth is 3.2 GHz (8.8-12 GHz), which is better than those of hollow Co nanochains (-14 dB, 1.5 mm, 2.9 GHz), as shown in Figure 9c and 9d. For hollow Co@Fe nanospheres, there is only one sharp and strong peak at 12.8 GHz with the RLmin of -47.3 dB when the thickness is 1.5 mm, and effective bandwidth is 4.8 GHz (10.7-15.5 GHz) (Figure 9e). Compared with the EM wave absorption properties of hollow Co nanospheres of 60 wt.%, hollow Co@Fe 21



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nanospheres show a better EM wave absorption properties with a lower RLmin and wider effective bandwidth. The EM wave absorption of hollow Co@Fe nanospheres are much better than that of Fe@Co nanospheres, and we also supply the EM wave absorption contrast of these two samples in supporting information (Figure S5, S6 and Table S1).

22


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Figure 9. Reflection loss in the frequency range of 2-18 GHz: (a) hollow Co nanospheres of 40 wt.%, (b) hollow Co nanochains of 40 wt.%, (c) hollow Co nanospheres of 60 wt.%, (d) hollow Co nanochains of 60 wt.% and (e) hollow Co@Fe nanospheres of 60 wt.%. 23



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Table 1. The comparison of EM wave absorption of five samples Sample

RLmin(dB)

f(GHz)

d(mm)

Bandwidth(GHz)

Hollow Co nanospheres-40 wt.% Hollow Co nanochains-40 wt.% Hollow Co nanospheres-60 wt.% Hollow Co nanochains-60 wt.% hollow Co@Fe nanospheres-60 wt.%

-30.4 -42.5 -41.7 -14.0 -47.3

14.8 16.0 10.3 15.0 12.8

6.0 4.9 1.5 1.5 1.5

2.7(13.5-16.2) 1.9(15.0-16.9) 3.2(8.80-12.0) 2.9(13.5-16.4) 4.8(10.7-15.5)

It is well known that the anisotropy energy (Ha) plays an important role in increasing the EM wave absorption. It is worth noting that Ha is related to Hc value based on the following equations:51 K1=µ0MsHc/2 and Ha=4|K1|/3µ0Ms , where µ0 stands for the universal value of permeability in free space, K1 is the anisotropic coefficient. The Hc of hollow Co nanochains is higher than that of hollow Co nanospheres. Thus the EM wave absorption properties of hollow Co nanochains are much better than that of hollow Co nanospheres when the filler content is 40 wt.%. At the same time, due to high specific surface area of hollow Co nanochains, the improvement of space polarization and dipole polarization enhance the permittivity of hollow Co nanochains when the filler content is 40 wt.%, which is beneficial to enhance EM wave absorption properties.52-53 However, when the filler content is 60 wt.%, some continuous networks are formed in the hollow Co nanochains-paraffin composites decreased the energy loss, which is harmful to the EM wave absorption. The hollow Co@Fe spheres of 60 wt.% show an excellent EM wave absorption properties including the minimum RL and effective bandwidth, compared with hollow Co nanospheres, which is attributed to the special shell structures.54 This structure has the following advantages: first, the samples have an interfacial polarization between Fe and Co; second, the magnetic properties are improved due to the composition of Fe 24


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and Co; finally, owing to the Fe coating layer on hollow Co nanospheres, the rest of the incident EM wave will undergo a multi reflection at the surface and can be further absorbed. To investigate the EM wave absorption mechanism of the above samples, the EM parameters of paraffin-based samples are investigated in the frequency range of 2-18 GHz, as shown in Figure 10. Notably, the real part of the complex permittivity (ε′) and imaginary part (ε″) of hollow Co nanospheres are lower than those of hollow Co nanochains when the filler concentration is 40 wt.%. The ε′ values of hollow Co nanospheres and nanochains of 40 wt.% increase gradually from 5.75 to 5.9 and 8.2 to 8.7 and a fluctuation is observed at the frequency of 12.8 GHz and 10.88 GHz, respectively. The ε″ values vary from 0.05 to 0.22 and 0.2 to 0.64 and a fluctuation appears at 13.2 GHz and 11.4 GHz which lags behind the vibration peaks of ε′. On the contrary, the ε′ and ε″ of hollow Co nanospheres are higher than that of hollow Co nanochains when the filler content is 60 wt.%, as shown in Figure 10a and 10b. A remarkable peak of hollow Co nanospheres with the filler content of 60 wt.% appears on the curve of ε′ and ε″. The µ′ and µ″ of the hollow Co nanospheres and nanochains are shown in Figure 10c and 10d. It can be seen that the µ′ and µ″ values of hollow Co nanospheres is higher than that of hollow Co nanochains when the filler content is 40 wt.%. The tendency of µ′ is decreased and that of µ″ firstly increases and then decrease with the increasing frequency. However, the µ′ and µ″ of hollow Co nanospheres are less than that of hollow Co nanochains when the filler content is 60 wt.%. A remarkable peak of hollow Co nanospheres are observed on the curve of µ′ 25



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and µ″ compared with hollow Co nanochains of 60 wt.%. Figure 10 also shows the ε', ε'', µ' and µ'' for hollow Co@Fe spheres. For hollow Co@Fe spheres, the ε' value negligibly increases from 12.3 to 12.7 in the 2-18 GHz range, and the ε'' value exhibits strong peaks in the 7-16 GHz range, indicating a resonance behavior. Moreover, for hollow Co@Fe spheres, the µ' value obviously decreases with increasing frequency from 1.6 to 1.0 in the 2-18 GHz range, and the µ'' value first increases and then decreases. Especially, hollow Co@Fe spheres also exhibits a peak at 9.2 GHz. Generally, the ε′ and ε″ value are associated with dissipation of the EM energy and polarization existing in the material under the alternating electromagnetic fields, which can be evaluated by the Cole-Cole semicircle (ε″ vs ε′) and each semicircle is correlated with one Debye relaxation process.55-58 In Figure 11a to 11d, it shows the Cole-Cole semicircles for hollow Co nanospheres and nanochains of 40 wt.% and 60 wt.% in the frequency range of 2-18 GHz. Three and four Cole-Cole semicircles are observed in the hollow Co nanospheres and nanochains of 40 wt.% respectively. The amount of semicircles of hollow Co nanospheres with 60 wt.% is higher than that of hollow Co nanochains. It also demonstrates that multiple dielectric relaxation processes take place in the hollow Co@Fe spheres of 60 wt.% (Figure 11e). On the one hand, the powders dispersed in paraffin can provide space polarization occurs at the interface between the samples and paraffin. On the other hand, such phenomenon attributes to dipole polarization at high frequencies. Notably, due to high specific surface area of hollow Co nanochains compared to simple hollow spherical particles, 26


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the contact area of hollow Co nanochains and paraffin is increased. Such a process may increase the attenuation electromagnetic ability. It also can be deduced that when the content of hollow Co nanochains increases to 60 wt.%, some continuous networks are formed in the composites owing to high aspect ratio of hollow Co nanochains. This phenomenon would enhance the electric conductivity of the samples, resulting the decrease of the permittivity, which means the energy induced by dissipative current will be decrease. In addition, its special structure also plays an important role in improving EM wave absorption for hollow Co@Fe spheres. The magnetic hysteresis loss, domain wall resonance, eddy current effect and natural resonance is the main cause for magnetic loss of the magnetic materials.54,

59-60

The magnetic

hysteresis and domain wall resonance could be neglected, because these two magnetic loss occur in the weak electromagnetic field or low frequency (

Application for Electromagnetic Wave Absor - ACS Publications

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