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Heterostructured Nanorings of Fe-Fe3O4@C Hybrid with Enhanced Microwave Absorption Performance Xian Jian, Xiangyun Xiao, Longjiang Deng, Wei Tian, Xin Wang, Nasir Mahmood, and Shixue Dou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18324 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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ACS Applied Materials & Interfaces

Heterostructured Nanorings of Fe-Fe3O4@C Hybrid with Enhanced Microwave Absorption Performance Xian Jiana,b,*, Xiangyun Xiaoa, Longjiang Dengb, Wei Tiana, Xin Wangb, Nasir Mahmoodc,d,* and ShiXue Doud a

School of Materials and Energy, University of Electronic Science and Technology of China,

Chengdu, 611731, China. b

National Engineering Research Center of Electromagnetic Radiation Control Materials,

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, China c

School of Engineering, RMIT University, 124 La Trobe Street, 3001 Melbourne,

Victoria, Australia d

Institute for Superconducting and Electronic Materials, Australian Institute for Innovative

Materials, University of Wollongong, North Wollongong, 2500, Australia

KEYWORDS: hybrid nanorings; catalytic chemical vapour deposition; iron oxide; carbon overcoat; microwave absorption ABSTRACT: Microwave absorption is a critical challenge with progression in electronics, where fine structural designing of absorbent materials plays an effective role in optimizing their microwave absorption properties. Here, we have developed Fe3O4@C (FC) and FeFe3O4@C (FFC) hybrid nanorings via hydrothermal method coupled with chemical catalytic vapour deposition (CCVD) technique. FC and FFC hybrid nanorings have fine carbon coating 1 ACS Paragon Plus Environment

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while their size can easily be tuneable in a certain range from 80~130 nm and 90~140 nm, respectively. The optimized FC and FFC hybrid nanorings bear minimum reflection loss (RL) values of −39.1 dB at 15.9 GHz and −32.9 dB at 17.1 GHz, respectively, while FFC shows effective absorption bandwidth (RL values < −10 dB) ranged from 5.2 to 18 GHz. Such an enhanced microwave absoprtion performance of hybrid nanorings is mainly due to the sutiable impedance characteristic, multilevel interfaces and polarization feature in nanorings. This work provides an approach to design hybrid materials having complex structure to enchance the microwave absorption properties. 1. INTRODUCTION Continues advancement in society and globalization demands high-tech communication devices, which release excessive radiations and cause electromagnetic (EM) wave pollution, having adverse effects on human health.1-2 To resolve the problem of EM pollution especially by the microwaves in GHz band, extensive efforts were devoted to design better microwave absorbing materials (MAMs) possesing the features of light weight, thinner, effective in broad frequency and strong absorption properties, etc.3 According to the energy dissipation inside the MAMs, the loss mechanisms are categorically divided into resistance loss, dielectric loss and magnetic loss.1 However, the MAMs with single loss factor cannot result in an excellent absorption performance due to the lack of well impedance matching.4 Thus, it is urgent to develop excellent MAMs having strong dielectric and magnetic loss simultaneously through mutual cooperation of the electromagnetic parameters (complex permittivity and permeability). Till now, extensive research has been carried out to improve the performance of MAMs via component optimization and/or structural designing.5-6 In terms of component optimization, enhancement in dielectric or magentic properties of many MAMs are the point of focus.7 Carbon-based materials having tuneable conductivity, low density, and excellent mechanical 2 ACS Paragon Plus Environment

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strentgh are typical dielectric materials and widely applied for microwave absoprtion, however, less effective in higher bandwidth.8-10 For example, pure helical carbon nanofibers don’t have effective absorption bandwidth less than −10 dB.11 On the other hand, the magnetic materials including Fe, Co, Ni and their oxides are also potential candidates for large magnetic loss.5, 12 However, their poor resistance to corrison and high density are the main hurdals in their rapid developemnt. Thus, the development of single-component absorbers restrict with poor perforamnce as they cannot produce reasonable dielectric and sufficiently high magnetic losses simultaneously. Therefore, constructing a composite or hybrid structure of several materials to achieve suitable dielectric loss and high magnetic loss was found a possible solution and remains a hot topic of MAMs.13-15 Both the homogeneous and heterogeneous structures are developed likewise Zhou et al. reported 3D SA-Ni(Fe3O4/CNTs)-X composite having maximum RL value of -32 dB at 10.8 GHz.16 However, tuning the structure with better design is favourable for high dielectric as well as magnetic loss and can come up with fascinating results, as it can facilitate the attenuation of incident electromagnetic waves.17 For example, Tian et al. developed yolk-shell Fe3O4@C microspheres possessing high performance due to unique structure that enhance the reflection and scattering characteristic.18 Recently, Cao and co-workers have developed many kinds of novel materials such as ultrathin graphene8 and composites including rGO/SiO2,19, NiO/SiC,7 NiFe2O4-rGO,20 graphene/Fe3O4,21 etc., to achieve high microwave absorption performance with low density even at high temperatures. Therefore, the designing of carbon based composites to improve their microwave absorption ability becomes the topic of interest. In addition, the nanoring struture presents the enhanced performance due to effective polarization and coupling.4, 22 Keeping in mind these impressive advantages, construction of optimal hybrid structures by tuning both components and structure will be a promising and effective method to improve the performance. In this work, we have developed Fe3O4@C (FC) and Fe-Fe3O4@C (FFC) hybrid nanorings 3 ACS Paragon Plus Environment


through hydrotheral and chemical catalytic vapour deposition (CCVD) technology, shown in Figure 1. The resulting hybrid nanorings come up with unique core-shell structure and wellmatched permittivity and impedance. The optimized FC and FFC hybrid nanorings show minimum reflection loss (RL) values of −39.1 dB and −32.9 dB at 15.9 and 17.1 GHz, respectively. Further, FFC shows effective absorption bandwidth (RL < −10 dB) from 5.2 to 18 GHz. We believe that such a hybrid designing of materials having good control over impedance matching and dielectric-magnetic multiple loss mechanism will assist the researchers to resolve the issue of EM pollution. 2. EXPERIMENTAL SECTION Synthesis of α-Fe2O3 nanorings: The α-Fe2O3 nanorings were obtaind from a hydrothermal method based on a previous report.23 In a typical processes, 64.88 mg of FeCl3, 0.42 mg of NaH2PO4 and 1.56 mg of Na2SO4 were weighted and dissolved in 20 mL of deionized water and kept striring for 30 min. The as-designed solution was put into a hydrothermal reactor and heated up to 220 °C for 40 h. The as-obtained powders possess reddish brown colour after washing by deionized water and ethanol alternately for 3 times and were dried in oven at 80 °C. Synthesis of FC hybrid nanorings: The FC was prepared by thermal annealing acetylene with the α-Fe2O3 nanorings. Initially, α-Fe2O3 nanorings were transferred to the middle part of horizontal furnace in silica boat. Then reaction conditions were achieved by attaining the reaction temperture of 400°C under protective gas of Ar at 5 °C/min. Afterwards, acetylene (C2H2) was introduced at the flow of 15 mL/min at 400 °C and reaction was continued for 30 mints. At the end, Ar was introduced to stop the reaction and the furacne was cooled down to room temperature. Synthesis of FFC hybrid nanorings: The FFC was prepared by hydrogen reduction of FC nanorings. Typically, the dried FC were reduced at 450 ℃ for 1 h under the mixed gas of H2 4 ACS Paragon Plus Environment

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and Ar at the flow of 30 mL/min and 60 mL/min, respectively. The as-prepared powders were collected at the end of the reaction. Characterization: The as-obtained products of α-Fe2O3 nanorings, FC and FFC were studied by field-emission scanning electron microscopy (FESEM) on Fei Inspect-F, high-resolution transmission electron microscopy (HRTEM) on Fei-F200, and powder x-ray diffraction (XRD) on an XRD-7000X, and Raman spectra were obtained on Renishaw, In Via with the help of 532 nm laser. The M-H loops were recored in range of 0-5000 Oe/0-10000 Oe on vibrating sample magnetometer of BHV-525 from Japan. The Brunauer–Emmett–Teller (BET) surface area measurements were done using JW-BK123F from China. The relative complex permeability and permittivity of the sample-wax composites were determined through vector network analyzer system of Agilent N5234A. 3. RESULTS AND DISCUSSION Figure 1 is showing the preparation process schematically, where initially α-Fe2O3 nanorings with average particle size of ~100 nm are synthesized by hydrothermal method at 220 ºC for 40 h using a mixed solution of FeCl3, NaH2PO4 and Na2SO4. The phosphate and sulfate ions support the generation of ring-like structure. The formation mechanism of αFe2O3 nanorings is based on the dissolution process where it undergoes an shape-evolution from capsule to nanorods and sequentially to nanorings. Where adsorption of phosphate ions on α-Fe2O3 surface significantly affected the dissolution process. In detail, the sulfate ions act as ligand and promote the dissolution of α-Fe2O3 due to the coordination effect leading to the gerneration of hollow structure.24 In a subsequent step, a uniform carbon coating on α-Fe2O3 nanorings were carried out in an acetylene based CCVD process at 400 ºC. Intrestingly, αFe2O3 nanorings were oxidized into Fe3O4 nanorings during the carbon coating and results in FC structure. Finally, the FC nanorings were reduced under hydrogen at 450 ºC for 60 min to

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obtain FFC nanorings. However, it is found that the outside layers have a partially graphitic nature of carbon overcoat.

Figure 1. Schematic presentation of synthesis process of α-Fe2O3, FC and FFC hybrid nanorings. The phase structure of pure α-Fe2O3 nanorings and FC and FFC hybrid nanorings was determined by XRD as shown in Figure 2. The peaks at 33.1º, 35.6º, 40.8º, 49.4º, 53.9º and 62.3º are assigned to (104), (110), (113), (024), (116), (214) planes of α-Fe2O3, respectively, confirming its high purity. Furthermore, after carbon coating on the surface of α-Fe2O3, a new phase of iron oxide appears as peaks at 35.3º, 43.0º, 56.8º and 62.4º are well-matched with the standard pattern of Fe3O4 (ICDD, PDF, file no. 01-075-0033). The complete diminishing of αFe2O3 peaks illustrate that the α-Fe2O3 is converted into Fe3O4 under C2H2/Ar=15:60 atmosphere at T=400 ºC for 30 min. It is also observed that the diffraction peaks of Fe (44.6º) and Fe3O4 (35.4º) appear in the hybrid, when it was reduced at 450 ºC under a mixed gas of hydrogen/argon with ratio of H2/Ar=30:60. The presence of pure Fe peak in the XRD of Fe3O4 nanorings confirm the existance of heterogeneous structure having both Fe and Fe3O4 in hybrid nanorings, named as FFC. 6 ACS Paragon Plus Environment

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Figure 2. XRD patterns of pure α-Fe2O3 nanorings, FC hybrid nanorings and FFC hybrid nanorings. The scanning electron microscopic (SEM) images of as-obtained α-Fe2O3 delineate its ring like morphlogy with clear inner hole of 20 to 40 nm and uniform round wall as shown in Figure 3a, b.The Figure 3c shows the corresponding particles size distribution of α-Fe2O3, having avrage size of 90 nm. However, the inner diameter of hollow structure becomes smaller after the reaction between C2H2 and α-Fe2O3 particles to consturct the carbon overcoat (Figure 3d, e). Furthermore, the FC formed because of the acetylene reduction effect during the CCVD process, resulting in the formation of Fe3O4 from α-Fe2O3, while the average particle size increases to 110 nm, shown in Figure 3f. Interestingly, in the formation process of FFC from FC by the reduction under H2 at 450 ºC results in a little irregularity in nanoring morphology, presented in Figure 3g, h. While the particle size further increases to ~120 nm (Figure 3i), which suggests a phase change and recrystallization happened during H2 reduction process to form FFC nanorings as suggested by the XRD.



Figure 3. SEM images of (a)-(b) pure α-Fe2O3 nanorings (d)-(e) FFC hybrid nanorings. (g)(h) FFC hybrid nanorings. Size distribution of (c) pure α-Fe2O3 nanorings, (f) FC hybrid nanorings and (i) FFC hybrid nanorings. The transmission electron microscope (TEM) image of pure α-Fe2O3 clearly shows its ring like morphology (Figure 4a). High magnification TEM observation and fast Fourier transform (FFT) pattren for pure α-Fe2O3 nanorings further illustrate its ring like morphology and high crystallinity (Figure 4b).The high resoltion TEM (HRTEM) image shows the lattice spacing of 2.54 and 2.71 Å corresponds to the (110) and (104）planes of pure α-Fe2O3 nanorings (Figure 4c). Furthermore after carbon coating, TEM studies shows that FC nanorings well-maintained their morphology as inner hole and uniform exterior boundary of 8 ACS Paragon Plus Environment

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nanorings can be clearly observed in Figure 4d. Figure 4e further shows that there is a uniform carbon overcoat of ~3 nm that homogenously covered the well-crystalline Fe3O4 nanostructures as can be seen from the clear lattice fringes and bright diffraction spots of FFT (present in the inset of Figure 4e). The interplanar spacing of 2.53 Å is atrributed to the Fe3O4 (311) facet, which clearly demostrates the sucessful conversion of α-Fe2O3 into Fe3O4 nanorings (Figure 4f). It is believed that this carbon overcoat will prevent the oxidation and aggregation of magenitic material.25

Figure 4. HRTEM imagines of (a-c) pure α-Fe2O3 nanorings and (d-f) FC hybrid nanorings (Note: Figure f is the enlarge image of pointed area from Figure e, having same scale bar). To further enhance the magnetic features of core of hybrid nanorings and heterogenity in the structure as well as to improve the crystalization of carbon overcoat, reduction of hybrid was carried out at high temperature. The TEM studies were done to observe the morphology and crystal structure afte reduction. From Figure 5a-b, the hybrid keeps the ring structure well, however, a little discontinuity of encapsulated matter is observed might be due to reduction and recrystalization afterwards. Figure 5c shows the high magnification TEM 9 ACS Paragon Plus Environment


image to highlight the internal structure and outer cover that looks amorphous but continues with ~4.72 nm thikness both on interior and exterior sides of ring. The lattice spacings of 2.03 Å and 2.53 Å confirmedfrom the HRTEM iamge of FFC nanorings corresponds to (110) and (311) planes of Fe and Fe3O4, respectively, shown in Figure 5d. The exitance of nanocrystals of both Fe and Fe3O4 approves our hypothesis to creat heterogenity in the hybrid nanorings for tuning the microwave absorption properties of FC and FFC nanorings. Figure 5e-g presents the FFT of three selected zones (shown in Figure 5c) of FFC, the interior and exterior FFT patterns show the amorphous nature of carbon. But the FFT of core containing Fe/Fe3O4 shows clear diffraction spots correspond to both Fe (mentioned with doted triangle) and Fe3O4 (mentioned with doted circle), bit different from that of FC nanorings due to the formation of Fe nanocrystals in FFC nanorings. Raman analysis was applied to confirm the nature of carbon overcoat. Figure S1 illustrates the Raman spectra of pure α-Fe2O3, FC and FFC hybrid nanorings, where α-Fe2O3 nanorings have intense peaks at 235, 308, 427 and 1319 cm-1, while FC nanorings show thepeaks at 665, 1339 and 1591 cm-1.26 The peak at 665 cm-1 is assigned to Fe3O4, and 1339 and 1591 cm-1 coresponds to D band and G band of carbon overcoat, respectively.24, 27 Similarly, FFC have a distinct intense peaks at 217, 283 and 392 cm-1 coresponds to interior hybrid part (Fe/Fe3O4) of nanorings, while peaks at1348 and 1589 cm-1suggest the existence of carbon. Magnetic property was also investigated to clarify the relationship of composition and magnetic behavior of hybrids. Figure S2 illustrates the hysteresis loops of pure α-Fe2O3, FC and FFC hybrid nanorings. The M−H curves of both FC and FFC samples display S-type shape, but pure α-Fe2O3 sample presents a straight line due to its poor magnetism (Figure S2). The FC and FFC saturation magnetization (Ms) exhibits an increment from 70.35 to 96.72 emu·g−1 and coercively (Hc) approximately from 20.89 to 22.75 Oe. The M-H curves indicate that FC and FFC have enhanced magnetic properties than pure α-Fe2O3, which contribute positively towards their better microwave absorption performance. Furthermore, surface area 10 ACS Paragon Plus Environment

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measurements were also done to see its effect on the microwave absorption property. Figure S3 shows N2 adsorption-desorption isotherms of α-Fe2O3, FC and FFC while insets show their pore size distribution. According to Figure S3a, α-Fe2O3 nanorings bear surface area of 7.69 m2/g and pore diameter of 3.1 nm in average. While, FC and FFC hybrid nanorings have surface areas of 25.01 m2/g and 100.27 m2/g with average pore diameters of 4.1 and 4.0 nm, respectively (Figure S3b&c). From BET analysis, it is found that carbon coating and reduction bring higher surface areas for FC and FFC ultimately yield better microwave absorption performance.4

Figure 5. (a-c) TEM (d-f) the FFT patternscorespoding to the three zones in Figure 5c (mentioned as internal, middle and external) of FFC hybrid nanorings. (g-i) HRTEM images 11 ACS Paragon Plus Environment


of FFC hybrid nanorings (Note: Figure g & i are the enlarge images of pointed areas from Figure h, having same scale bars). Generally, to improve the microwave absorption performance, a reasonable balance among the complex permittivity (εr = ε′– jε′′) and permeability (µr = µ′- jµ′′) is developed,28 where ε′ and µ′ determin the ability of storing electric and magnetic energy, respectively, while ε″ and µ″ represent their corresponding loss capabilities.29-30 Single-type loss in material leads to inferior electromagnetic impedance matching, therefore, hybrid materials are point of focus here. Furthermore, εr and µr are determined by main factors in terms of various polarizations and magnetic performance, etc. which strongly based on the physical and chemcial properties of the used materials.4 The recorded curves for ε′, ε′′, µ′ and µ′′ are shown in Figure 6 and both the dielectric loss tangent (tanδε= ε″/ε′) and permeability loss tangent (tanδµ = µ″/µ′) were calculated. Figure 6a&b show that ε′ and ε″ for α-Fe2O3 dispersing into paraffin, the ratio of sample is 30 wt% (F30), FC30 and FFC30 keep constant value in the frequency range of 0.5−18 GHz. The ε″ values are nearly zero confirming week dielectric loss of F30, FC30 and FFC30. When the ratio of FC and FFC increased up to 50%, both ε′ and ε″values increased due to larger space charge polarization. The large amount of nanorings results in enhanced the surface area at particles and matrix interfaces, which brings enhanced interfacial polarization. The increased concnetration of nanorings also enhanced the dipole moment, which has positive effect on complex polarization. There are two relaxation peaks appear at 5.2 and 13.8 GHz for FFC50 and 6.1 and 14.4 GHz for FFC60, while the dielectric loss factor for both FFC50 and 60 exhibit similar fluctuation in 0.5−18 GHz.31 The largest dielectric loss for FFC50 and FFC60 are at 11.2 and 9.8 GHz, implying that the hybrid possesses better dielectric loss ability. Similiarly, the dielectric loss curve of FC50 is smooth at 0.5-10 GHz and has a great improvement at 16-18 GHz showing dielectric loss in this region. Considering the special dielectric feature is rarely found in normal materials, we assign this result to the hybrid 12 ACS Paragon Plus Environment

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nanorings having core-shell microstructure. This kind of behavior is due to the defective carbon shell resutling in the formation of polarization centered at carbon vacancy in case of microwave irradiation.32 Besides, the core/shell microstructure also play a critical role to enhance the interfacial polarization.33 According to the polarization models, carbon shell having approperiate thickness will have lots of charges accumulating at the position of interface that will reverse the enough charges, a conventional dielectric behavior under the applied EM field, indicating the carbon shell enhances the dielectric loss properties of these hybrid nanorings.34 The µ′ and µ′′ values for F30 and FFC30 remain almost constant in 0.5−18 GHz. The µ′ and µ′′ curve of FC30 and FC50 have resonance peaks at low-frequence, which indicates there is a natural resonance.27 The values of tanδε and tanδµ are usually used to estimate the loss ability of microwave, and the relatively higher values of tanδε and tanδµ imply higher dielectric and magnetic losses, respectively.1, 35 In case of too large ε′ and ε″ of absorber, it will lead to enhancement of the reflection of EM wave, which lowers the absorption of EM wave. Furthermore, the enhanced microwave absorption performance is mainly due to the good impedance matching.36 It is necessary to optimize the electromagnetic parameters to design materials with as high absorbing energy as possible by the incident wave. The bigger values of µ′ and µ″ is better to match sufficiently with large values of ε′ and ε″ to address the impedance matching problem in practical applications. Based on the transmission line theory,37 when single layer material is used as absorbing medium, the input impedance of the EM wave incident at the interface is Zin as given below:38

= ℎ

√ .

(1)

Where thers concrete notations are difined as the impedance of air (Z0), complex permeability (µr) and permittivity (εr), frequency of microwaves (f), thickness of absorber (d), and the velocity of light (c). It is well-known that when Zin/Z0 value is 1, implying that no EM wave 13 ACS Paragon Plus Environment


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reflection happens on the MAMs surface, which means well-matched impedance was achieved for the MAMs. Figure S4 shows that the Zin/Z0 value curves vary with respect to frequency at thickness of 3 mm. The absorber will dissipate the EM wave incident and this ability can be evaluated by attenuation constant α.31 The value of α can be figured out by the equation as below:39

=

√

× − + " − ′′

+ − ′′′

(2)

The α values of samples are presented in Figure S5. The α values of FFC60 are the highest among those of others in the range of 10-14 GHz, which suggests its stronger attenuation ability for EM wave. Furthermore, the reflection loss (RL) is figured out based on the transmission line theory:40

$% = 20lg|

+,- ./0 +,- 1/0

|

(3)

The calculated RL curves for the pure Fe2O3 nanoring and FC and FFC in the frequency range 0.5-18 GHz in case of 3 mm absorber thickness were presented in Figure S6. The minimum RLvalues for F30, FC30, FC50, FFC30, FFC50, and FFC60 are -1.4 dB, -9.7 dB, 11.9, -3.3 dB, -28.1 dB and -18.4 dB, respectively. It is found that the FC and FFC possess improved performance in terms of minimum RLvalue. Besides, the mass ratio of active material and paraffin has great effects on the microwave abortion behavior and FFC50 presents best performance. The effective bandwidths ( RL< −10 dB) of FC50, FFC50 and FFC60 are 1.2 GHz (10.7-11.9 GHZ),4.1 GHZ (9-13.1 GHZ) and 3.4 GHZ (7.5-10.9 GHZ), respectively. So FFC50 has the widest bandwidth of effective absorption as well as minimum RL value.


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Figure 6. Complex (a, b) permittivity and (d, e) permeability, and (c, f) tangent losses of pure α-Fe2O3 nanorings, FC hybrid nanorings and FFC hybrid nanorings. Furthermore, the three-dimensional (3D) simulated curves for EM wave loss RL values of the hybrids are presented in Figure 7. From RL curves, F30, FC30 and FFC30 do not present effective microwave absorption performance (RL < −10 dB) shown in Figure 7a, 7b and 7d, respectively. While FC50, FFC50 and FFC60 have relatively better microwave absorption properties. In detail, the minimum RL of the FC50 is -39.1 dB in case of 2 mm thickness at 15.9 GHz. The absorption bandwidth (RL < −10 dB) is from 4.5 to 8.1 GHz and from 10.7 to 18 GHz in the absorber thickness range of 2-5 mm presented in Figure 7c. Besides, for FFC50 sample, the minimum RL is -32.9 dB in case of 2 mm thickness at 17.1 GHz. The absorption bandwidth is from 5.2 to 18 GHz in range of 2-5 mm (Figure 7e). The minimum RL of FFC60 is -18.4 dB in case of 3 mm at 8.9 GHz. The absorption bandwidth is from 4.4 to 18 GHz for in range of 2-5 mm(Figure 7f). Thus, the FC and FFC hybrid nanorings have improved microwave absorption property which is attributed to their unique structure and composition.



Figure 7. Simulated three dimentional (3D) RL patterns of (a) F30, (b) FC30, (c) FC50, (d) FFC30, (e) FFC50 and (f) FFC60. The microwave absorption mechanism can be mainly ascribed to two factors i.e. magnetic and dielectric loss.41 Magnetic loss of absorber was mainly from many impacts including hysteresis, resonance and eddy current effect, etc.42 Here, the magnetic hysteresis is negligible in case of weak magnetic field. While the loss from domain wall resonance often exists in relatively low frequency of below GHz, it is also negligible at 0.5-18 GHz.20 So in present case, the magnetic loss mainly originates from other two factors: natural ferromagnetic 16 ACS Paragon Plus Environment

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resonance and eddy current effect. The hollow geometry and carbon shells of FFC can reduce eddy current loss (P). In case of the alternating magnetic field, the P of nanorings is determined by the following equation:4 23 = 4 5 67 89 − 8:9 ;

(4)

Where symbols are defined as the frequency (f), magnetic induction (Bm), conductivity (σ), the outer diameter (r1) and inner diameter (r2) of the nanoring. From the above equation, it is proved that in the present case nanoring struture inhibits the increment in eddy current. Furthermore, another equation is applied to judge the eddy current loss:43

= 24 ;. 5/3

(5)

where the definitions for the composite are the permeability of vacuum (µ0), thickness (d), and electrical conductivity (σ). Then the outer carbon shell promotes the increment of eddy current loss due to the higher conductivity, which is positive for the enhancement of magnetic loss. According to the criterion of skin-effect, the plots of C0 (µ″(µ′)−2f−1) remain stable as a constant when changing frequency ( f), in case of magnetic loss only originating from eddy current loss.44, 45 From the curves of µ″(µ′)−2f−1 vs. f shown in Figure S7, the values of F30, FFC30 and FFC50 are constant at 6-18 GHz; and FC30, FC50 and FFC60 are constant in the range of 10 to 18 GHz, confirming that magnetic loss in the range of 10-18 GHz is mainly from the loss of eddy current. In addition to these ranges, the values of C0 beyond these ranges are unstable, which neglects the other absorption mechanisms. The above analysis suggested that natural resonance exists and play a role in the ability of absorber. The natural resonance can be eveluated by the following equations:4, 27

5 = 8>? /24 >? = 4|K: |/3> BC

(6) (7)

where the definitions are the frequency of natural resonance (fr), gyro magnetic ratio (r), anisotropic energy (Ha), anisotropic coefficient (|K1|), and saturation magnetization (Ms). 17 ACS Paragon Plus Environment


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During the process of EM wave attenuation for FFC, another contribution is considered in forms of dielectric loss including interfacial polarization and relaxation. While the dipole relaxation polarizations are positve to improve microwave absorption performance. From Debye theory, the complex permittivity (εr) is defined as below:28, 29

= + D E = F +

G . H

(8)

:1IJ0

Here, the definitions are the relaxation time (τ0), static dielectric constant (εs) and dielectric constant at infinite frequency (ε∞). Then the ε′andε″ are deduced as another forms as below: .

G H = F + :1IJ K

E =

(9)

0

IJ0 G . H

(10)

:1IJ0 K

From eqs 9 and 10, it can be further deduced that

−

G 1 H

+

G . H

=

(11)

From the eq 11, the curves of ε″ versus ε′ is shape of a semicircle, named as Cole-Cole semicircle. Every semicircle corresponds to a Debye dipolar relaxation process. Figure S8 presents the ε′-ε″ plots for the samples of F30, FC30, FC50, FFC30, FFC50 and FFC60 in 0.518 GHz. Among them, the plots in Figure S8a and S8d are totally disordered indicating that the F30, FC30 and FFC30 samples don’t have any dielectric relaxation process. Importantly, several semicircles are found for FC50, FFC50 and FFC60, indicating there are several continuous Debye relaxation processes. The FFC and FC have much more variable interfaces than Fe2O3 nanorings including Fe-Fe3O4, Fe-C, C-Fe3O4 interfaces (as shown in Figure 8), and interfacing with wax. The multi-level rich interfaces in the hybrids favor the interfacial polarization and the Maxwell-Wagner effect. It is confirmed that the nanoring hybrids with multiple interfacial polarization among Fe, Fe3O4 and hollow carbon nanoring lead to the enhancement of dielectric relaxation processes.


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Figure 8. The microwave absoption mechanisms and the related polarization and coupling illustration for the FFC hybrid nanorings (Note: Fe shows as particle to clearly describe the grain boundary although it exist as crystallite in the nanoring). The FC and FFC hybrids have some advantages for improving the microwave absorption performance. First, introducing nanoring structure is beneficial to electricity-magnetism transformation, abundant and positive modes of polarization and coupling. In the changing electromagnetic field, there are effective polarized dipoles due to the existence of excited electrons on the surface of nanorings, assigning to four kinds of modes of the surface plasmon resonance, shown in Figure 8. Second, benefiting the multiple interfacial polarization, the hybrid nanorings have an enhancement of microwave absorption. Third, the multilevel structure of nanoring hybrid contains the dielectric matter (carbon) and magnetic components (Fe, Fe3O4), which is flexible to control and adjust the optimal component for well-matched impedance. CONCLUSIONS In summary, we have developed hydrothermal method coupled with CCVD to designed FC and FFC hybrid nanorings for enahnced microwave absorption performance compared to bulk 19 ACS Paragon Plus Environment


iron oxide. As-developed hybrid nanorings possess unique core-shell structure, where Fe/Fe3O4 magentic components are completely encapsulated in the amorphous carbon having defects to create a unique interface and well-matched impedance.The resulted FFC hybrid nanorings have minimum reflection loss value of -32.9 dB at 17.1 GHz, while the absorption bandwidth (RL < −10 dB) is from 5.2 to 18 GHz. The effective microwave absorption bandwidth for FFC hybrid nanorings is larger than pure α-Fe2O3 and FC hybrid nanorings due to the unique composition and structure. Besides, the FC hybrid also bear minimun RL value of −39.1 dB at 15.9 GHz, but absorption bandwidth becomes smaller than FFC. The engineering at nanoscale level presented here to develop unique hybrid structure will provide a new way to design materials having better capabilities for microwave absorption and various other applications. ASSOCIATED CONTENT Supporting Information. Raman images, hysteresis loops of FC, FFC and α-Fe2O3, Impedance matching of F30, FC30, FC50, FFC30, FFC50 and FFC60 in case of 3 mm in 0.518 GHz, nitrogen adsorption-desorption isotherms and BJH pore size distribution curves of αFe2O3, FC and FFC. Attenuation constant of F30, FC30, FC50, FFC30, FFC50 and FFC60 in case of 3 mm in 0.5-18 GHz, RL curves for F30, FC30, FC50, FFC30, FFC50 and FFC60 in case of 3 mm in the frequency range of 0.5-18 GHz, Plots of µ″(µ′)−2f−1 vs frequency for F30, FC30, FC50, FFC30, FFC50, FFC60 and the plot of Cole-Cole semicircles for (a) F30 and (b) FC30 (c) FC50 (d) FFC30 (e) FFC50 (f) FFC60. AUTHOR INFORMATION Corresponding Authors Dr. Jian Xian ([email protected]) Dr. Nasir Mahmood ([email protected]) 20 ACS Paragon Plus Environment

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Author Contributions Dr. Xian Jian and Xiangyun Xiao designed all the experiments and carried out the testing of all properties, and draft the manuscript. Prof. Longjiang Deng, Wei Tian and Dr. Xin Wang assist with analysis of microwave absorption performance. Dr. Nasir Mahmood and Prof. Shi Xue Dou provided guidance on designing the experiments, analyzing the results. Acknowledgements Many thanks for the funding supported by the Fundamental Research Funds for the Chinese Central Universities (Grant No. ZYGX2016J139), Science and Technology Support Program of Sichuan Province (2016RZ0054), China Postdoctoral Science Foundation (2015M582539), and the National Hi-Tech Research and Development Program (863 Program) of China (No. 2015AA034202). The authors would also like to acknowledge the technical support provided by the RMIT University. References (1) You, W.; Bi, H.; She, W.; Zhang, Y.; Che, R. Dipolar-Distribution Cavity γ-Fe2O3@C@αMnO2 Nanospindle with Broadened Microwave Absorption Bandwidth by Chemically Etching. Small 2017, 13 (5), 1602779. (2) Zhang, Y.; Huang, Y.; Zhang, T.; Chang, H.; Xiao, P.; Chen, H.; Huang, Z.; Chen, Y. Broadband and Tunable High-Performance Microwave Absorption of an Ultralight and Highly Compressible Graphene Foam. Adv. Mater. 2015, 27 (12), 2049-2053. (3) Liu, J.; Che, R.; Chen, H.; Zhang, F.; Xia, F.; Wu, Q.; Wang, M. Microwave Absorption Enhancement of Multifunctional Composite Microspheres with Spinel Fe3O4 Cores and Anatase TiO2 Shells. Small 2012, 8 (8), 1214-1221. (4) Wu, T.; Liu, Y.; Zeng, X.; Cui, T.; Zhao, Y.; Li, Y.; Tong, G. Facile Hydrothermal Synthesis of Fe3O4/C Core–shell Nanorings for Efficient Low-frequency Microwave Absorption. ACS Appl. Materi. Interfaces 2016, 8 (11), 7370-7380. 21 ACS Paragon Plus Environment


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