Low-Cost Carbothermal Reduction Preparation of Monodisperse Fe

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Energy, Environmental, and Catalysis Applications

Low-Cost Carbothermal Reduction Preparation of Monodisperse Fe3O4/C Core-Shell Nanosheets for Improved Microwave Absorption Yun Liu, Yiwei Fu, Lin Liu, Wei Li, Jianguo Guan, and Guoxiu Tong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02770 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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ACS Applied Materials & Interfaces

Low-Cost Carbothermal Reduction Preparation of Monodisperse Fe3O4/C Core-Shell Nanosheets for Improved Microwave Absorption Yun Liu,†,‡ Yiwei Fu,† Lin Liu,‡ Wei Li,† Jianguo Guan,†Guoxiu Tong* ‡ †

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,

International School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China ‡

College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, China

* Corresponding Author. E-mail: [email protected] Tel.: +86-579-82282269; Fax: +86-57982282269.

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ABSTRACT: This paper demonstrates a facile and low-cost carbothermal reduction preparation of monodisperse Fe3O4/C core-shell nanosheets (NSs) for greatly improved microwave absorption. In this protocol, the redox reaction between sheet-like hematite (α-Fe2O3) precursors and acetone under inert atmosphere and elevated temperature generates Fe3O4/C core-shell NSs with the morphology inheriting from α-Fe2O3. Thus, Fe3O4/C core-shell NSs of different sizes (a) and Fe3O4/C core-shell nanopolyhedrons are obtained by using different precursors. Benefited from the high crystallinity of Fe3O4 core and thin carbon layer, the resultant NSs exhibit high specific saturation magnetization larger than 82.51 emu·g−1. Simultaneously, the coercivity enhances with the increase of a, suggesting a strong shape anisotropy effect. Furthermore, due to the anisotropy structure and the complementary behavior between Fe3O4 and C, the as-obtained Fe3O4/C core-shell NSs exhibit strong natural magnetic resonance at a high frequency, enhanced interfacial polarization and improved impedance matching, ensuring the enhancement of the microwave absorption. The 250 nm NSs-paraffin composites exhibit reflection loss (RL) lower than −20 dB (corresponding to 99% absorption) in a large frequency (f) range of 2.08−16.40 GHz with a minimum RL of –43.95 dB at f = 3.92 GHz when the thickness is tuned from 7.0 mm to 1.4 mm, indicating that the Fe3O4/C core-shell NSs are a good candidate to manufacture highperformance microwave absorbers. Moreover, the as-developed carbothermal reduction method could be applied for the fabrication of other composites based on ferrites and carbon.

KEYWORDS: Fe3O4/C composites; core-shell structures; nanosheets; carbonthermal reduction; magnetic properties; microwave absorption


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1. INTRODUCTION

Massive electromagnetic (EM) waves generated from widely used electronic and communication devices in military and civil fields cause growing EM interference and pollution problems and threat the health of human beings. Researchers have devoted much efforts to develop effective EM shielding or absorbing materials to suppress the aforementioned problems.1−4 Compared with EM shielding materials, microwave absorbing materials (MAMs) could absorb EM waves efficiently by converting them into thermal energy via various attenuation mechanisms, avoiding secondary contaminations.5 The absorption of MAMs are closely associated with their complex permittivity ( εr = ε ′ − jε ′′ ), complex permeability ( µr = µ′ − jµ′ ), and the impedance matching, which are greatly influenced by the composition, interface, shape and dimension.6 The so far developed MAMs generally include resistive loss materials, dielectric loss materials and magnetic loss materials.7,8 Among them, magnetic loss materials, including ferrites9 and magnetic metals, such as Fe,7,10−12 Co,13 Ni,8 FeCo14 exhibit strong EM wave absorptions with thin coating layers due to the coexistence of magnetic loss and dielectric loss. The transition metal or metal alloy magnetic materials exhibit good EM wave absorption performances, but they generally suffer from the drawbacks of high density and easy oxidation.15−17 In contrast, ferrite materials show a much better chemical stability. Magnetite (Fe3O4), as the simplest ferrite, has further advantages of low-cost, easy preparation and nontoxicity. Up to date, various morphologies of Fe3O4 nanomaterials, such as nanorings,18 nanodiscs,19 nanodendrites20 and nanoflowers21 have been reported regarding EM wave absorption performances. The sheet-



shaped particles are regarded to promise excellent microwave absorption for the following reasons. First, they may achieve high permeability for the large planar-anisotropy and high anisotropy field, adjusting the demagnetizing factor.22 Second, the sheet-shaped structure endows them with high magnetic resonant frequency due to the high magnetocrystalline anisotropy field and shape anisotropy field.23 Additionally, sheet-shaped particles would exhibit increased permittivity for the enhancement of interface polarization caused by the large specific surface. Several approaches have been investigated for the preparation of Fe3O4 nanosheets, including oxidization from pure Fe substrates,24 and wet reduction from anisotropic γ-Fe2O3 or α-Fe2O3 precursors.19, 25, 26 However, single component Fe3O4 materials show low permittivity and weak absorption at a thin thickness, which restrict their further applications.2 The combination of Fe3O4 with dielectric materials, such as ZnO, TiO2, MnO2, CuO, and carbon can improve impedance matching and dielectric loss while almost maintaining the permeability of Fe3O4, which is an efficient approach to develop thin-thickness, light-weight, strong-absorption and broad-band MAMs. 3,4,27, 28 Carbon materials distinguish themselves from the aforementioned dielectric materials for their features of high permittivity, low density, and good chemical and thermal stability.29, 30 During the past decades, various Fe3O4/C composites MAMs have been fabricated, 2,15, 31 which, however, usually contain high carbon content, and thus weakens the magnetic permeability. Fabricating Fe3O4/C composites with high magnetic content and disclosing their morphology and composition dependent EM properties will promote the development of Fe3O4/C MAMs for practical application. In this paper, monodisperse Fe3O4/C core-shell NSs have been prepared by a facile and lowcost carbothermal reduction method, in which α-Fe2O3 NSs were used as precursors and acetone functioned as reductant and carbon source. The effect of calcination temperature (Tc) on the 4 Environment ACS Paragon Plus

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morphology heritage and phase transition process has been explored. The developed approach is capable for the preparation of Fe3O4/C core-shell NSs with different sizes (a) and Fe3O4/C coreshell nanopolyhedrons (NPs). The obtained NSs exhibit high specific saturation magnetization and the covercivity increases with the increase of particle size, exhibiting a strong shape anisotropy effect. Paraffin composites containing 250 nm NSs show strong absorption (more than 99%) within 2.08−16.40 GHz when the thickness (d) changes from 7.0 mm to 1.4 mm, indicating that the obtained NSs are a good candidate to manufacture high performance microwave absorbers. Moreover, the facile and low-cost preparation method shown here may have a great potential for the fabrication of other composites based on ferrites and carbon.

2. EXPERIMENTAL SECTION 2.1 Chemical reagents. Ferric chloride hexahydrate (FeCl3⋅6H2O), absolute ethanol (C2H5OH), anhydrous sodium acetate (NaAc), and acetone (CH3COCH3) were of analyticalgrade supplied by Shanghai Sinopharm Chemical Reagent Co., Ltd. 2.2 Preparation of Fe2O3 NSs and NPs. Uniform hexagonal α-Fe2O3 NSs with tunable sizes and morphologies (Figure S1) were obtained by a modified mixed solvothermal method in a H2O−C2H5OH system.32 The detailed experiment process was described in the supporting information. 2.3 Preparation of Fe3O4/C core-shell NSs and NPs. The magnetic NSs and NPs were prepared via calcining α-Fe2O3 NSs and acetone at 400 °C for 2 h under the protection of Ar or N2. The black powders with strong magnetic feature were obtained after cooling to room temperature. Control experiments were conducted under different calcination temperatures (350 °C and 500 °C) with other conditions kept unchanged to investigate the influence of Tc. 5 Environment ACS Paragon Plus


2.4 Characterizations. The micromorphologies and microstructures of the samples were observed by field-emission scanning electron microscope (FE-SEM, Hitachi S-4800, 5 kV) and high resolution-transmission electron microscope (HR-TEM, JEM-2100F, 200 kV). The element components of the products were determined by energy dispersive X-ray spectrometer (EDX, Horiba, EX-250). The phases of the obtained NSs and NPs were characterized by X-ray diffraction (XRD, D/MAX-IIIA, CuKα radiation, λ = 0.15406 nm, 10 °/min).Thermogravimetric (TG) curves were recorded by a thermal analyzer (Netzsch, STA 449C) from 25 ºC to 700 ºC under air atmosphere with a heating rate of 10 ºC⋅min-1. Raman spectra were obtained by a Raman spectrometer (Renishaw RM10000) to analyze the crystallization of carbon. X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi) was applied to record the XPS spectra of the typical sample. 2.5 Measurement of magnetic and EM properties. The magnetic hysteresis loops of the final products were recorded by a vibrating sample magnetometer (VSM, LakeShore 7404) at room temperature. The EM parameters of the sample-paraffin composites in 2−18 GHz were measured by a vector network analyzer (Keysight N5230A, USA). The preparation of cylindrical toroidal samples and the calculation of reflection loss (RL) are same as those reported in our previous work.33

3. RESULTS AND DISCUSSION 3.1. Synthesis and characterizations of Fe3O4/C core-shell NSs. We developed a simple carbothermal reduction approach for the preparation of Fe3O4/C core-shell nanosheets (NSs). In


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this method, monodisperse α-Fe2O3 NSs were obtained by a solvothermal method32 and then they reacted with acetone at elevated temperature under inert atmosphere. In the reaction system, acetone functioned as both reducing agent and carbon source. Consequently, black powders were obtained as the products.

Figure 1. (a) SEM image, (b) TEM image, (c) HR-TEM image, (d) XRD pattern, (e) TG curve, (f) Raman spectrum of the typical Fe3O4/C core-shell NSs. The insert in (c) is the corresponding fast Fourier transform (FFT) pattern. Figure 1 shows the detailed characterization of a typical product. SEM image reveals that the products are uniform NSs of high quality and large scale (Figure 1a). The diameter and thickness of the NSs are about 250 nm and 10 nm, respectively, corresponding to an aspect ratio of 25. Compared to their precursors (Figure S1a), the morphology and size of the obtained magnetic NSs remain unchanged. Figure 1b further confirms the good dispersibility and thin thickness of the NSs. In addition, Figures 1c and S2 reveal that the NSs have a core-shell heterostructure with



a thin shell layer of about 2–3 nm. The long-range order lattice fringes and the corresponding strong diffraction spots (Figure 1c) confirm that the Fe3O4 core features high crystallinity. Furthermore, the observed lattice fringes (0.25 nm) fit well to the (311) plane of Fe3O4. In the EDX spectrum (Figure S3), the elements of C, O, Fe, Al, Pt are detected in the sample, among which, Al comes from the Al foil used for spreading the sample and Pt comes from the sprayed Pt film. The characterizations demonstrate that the final products are composed by Fe3O4 and C. The phase and average crystal size (D) of the typical sample were further confirmed by XRD (Figure 1d). Using the software of Jade 5, D and internal strain (ε) were calculated based on the Hall-Williamson equation34 and the D and ε values were 22.5 nm and 0.42, respectively. The diffraction peaks in Figure 1d match well with those in the cubic inverse spinel structure of Fe3O4 (magnetite) [JCPDS 65–3107]. Sharp and strong peaks further demonstrate high crystallinity of the product. However, no obvious diffraction peak could be indexed to C, indicating low content or low crystallinity of C in the final products.6 TG characterization was used to determine the exact amount of C. As shown in Figure 1e, the TG curve of the sample shows a three-step weight loss and a one-step weight increase. The first mass loss step before 200 ºC is related to the remove of adsorbed water. The following mass increase in the temperature range of 200–300 ºC is caused by the oxidation of Fe3O4 to Fe2O3 in air.29 The main weight loss occurring between 300 ºC and 400 ºC is due to the combustion of carbon shell. The last weight loss between 600–700 ºC is attributed to the further decomposition of carbon residue.35 During the heating process in air, Fe3O4 was oxidized to Fe2O3 and carbon was completely burnt to carbon dioxide, thus the remaining product was only Fe2O3. Based on the analysis above, the exact amount of carbon can be calculated by


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wt % carbon = 1 − wt % H 2O − wt % R emains

M Fe3O4 1.5 M Fe2O3

,6 where wt % H O and wt % Rrmains are the mass 2

percentage of water and the remaining product, respectively. The calculated mass percentage of carbon in Fe3O4/C core-shell NSs is 5.4%. As shown in the Raman spectrum (Figure 1f), there is a clear characteristic scattering peak at around 675 cm−1, corresponding to the A1g vibration mode of Fe3O4.36 In addition, the peaks around 1380 and 1595 cm−1 are the characteristic peaks of the D and G bands from the carbon shell. The intensity ratio of D and G peaks (ID/IG) is calculated to be 1.15. The relative high value of ID/IG implies a low crystallinity of the carbon shell,37, 38 which consists with the XRD result. XPS analysis is conducted to investigate the surface chemical composition of the typical Fe3O4/C core-shell NSs. All the binding energies were standardized using C 1s at 284.8 eV as the reference. Obvious peaks at about 285, 531, 711, and 724 eV can be observed in the survey spectrum (Figure 2a), which guarantes the existence of C, O, and Fe,39 consistent with the EDX data. Figure 2b shows the fitted spectra of Fe 2p, in which binding energies centred at about 711.5 and 725.2 eV are characteristic peaks of Fe3+, while the peaks centred at about 710.4 and 723.4 eV are corresponding to Fe2+.40 The relative atomic ratio of Fe3+/Fe2+ (γ) can be calculated by the equation

ni I = i × nj Ij

E kj E ki

, where ni and nj are the number of surface atoms, Ii and Ij are the

peak areas, and Ekj and Eki are the corresponding photoelectron kinetic energies. Based on the peak areas of Fe3+ and Fe2+ shown in Figure 2b, γ is calculated to be 1.99, which is very close to the theoretical value of 2 in Fe3O4, confirming the exact composition of Fe3O4. In addition, the presence of Fe3O4 is further confirmed by the O 1s peak at 530.5 eV, corresponding to the lattice oxygen in Fe3O4 phase (Figure 2c). 41 Meanwhile, the peak located at 532.1 eV in Figure 2c indicates the presence of oxygen-containing groups.42 The carbon-containing function groups are



further confirmed by analyzing the C 1s spectra (Figure 2d). The asymmetric and broad peak of C 1s is fitted into three peaks, which correspond to C–C (284.8 eV), C–OH or C–O–C groups (286.0 eV), and carboxyl group (288.4 eV),43 respectively. These data demonstrate that the components of the NSs are Fe3O4 and C, consistent with the TEM and EDX results.

Figure 2. XPS spectra of typical Fe3O4/C core-shell NSs: (a) Survey spectrum and (b-d) deconvolution of (b) Fe 2p peak, (c) O 1s, and (d) C 1s peak. 3.2 Controlled preparation principles for the resultant Fe3O4/C core-shell products. In this carbothermal reduction preparation method, the calcining temperature (Tc) has a vital effect on the morphology and phase of the resultant products. Figure 3 shows that the α-Fe2O3


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precursors exhibit uniform hexagonal sheet-like morphology with the diameter of about 150 nm. When Tc = 350−400 ºC, the sheet-like configuration were well maintained, as shown in Figure 3b–c. However, when Tc = 500 ºC, only molten NSs with irregular morphology (Figure 3d) were obtained. Figure 3e indicates that α-Fe2O3 and Fe3O4 coexist in the products obtained at Tc=350 ºC, while for the products obtained at Tc=400−500 ºC, their diffraction peaks are well indexed to Fe3O4 [JCPDS 65–3107] without any other impure peaks. The above results indicate that simply controlling Tc can obtain monodisperse Fe3O4/C core-shell NSs. During the carbothermal reduction process, acetone acted as carbon source and reducing agent. At elevated reaction temperature, acetone was carbonized into amorphous carbon,33 and the α-Fe2O3 precursors were reduced to Fe3O4, resulting in the resultant products. As it is known, α-Fe2O3 has a rhombohedrally centered hexagonal structure while Fe3O4 has a cubic inverse spinel structure. Their corresponding three-dimensional crystal structures are shown in Figure 3f, g. The conversion from α-Fe2O3 to Fe3O4 involves the change of hexagonal close-packed oxide ion array to cubic close-packed array. Despite the changed packing style, the original morphology could be maintained due to the mediation function of oxygen vacancies in α-Fe2O3. The phenomenon of morphology heritage and phase conversion from α-Fe2O3 to Fe3O4 is consistent with the reported works.20, 44 The effect of Tc on the morphology and component of the samples can be illustrated as follows. At Tc=350 °C, the carbonization rate is relative low and most of acetone is carried out by the Ar flow, leading to the incomplete reduction of α-Fe2O3 NSs to Fe3O4. At Tc= 400 °C, the carbonization kinetics of acetone makes the complete conversion of α-Fe2O3 NSs to Fe3O4/C core-shell NSs. When Tc is further increased to 500 °C, the reaction kinetics is significantly



enhanced and the NSs tend to melt into irregular particles to minimize their surface energy.45,46 As a result, only irregular products are produced.

Figure 3. (a-d) SEM images and (e) XRD patterns of (a) the α-Fe2O3 precursors and (b-d) the corresponding samples obtained at different Tc. The scale bars are all 500 nm. (f-g) Threedimensional crystal structures of (f) α-Fe2O3 and (g) Fe3O4, respectively.


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Figure 4. (a-b) SEM images, (c) TEM image and (d) HR-TEM image of the Fe3O4/C core-shell NSs and NPs: (a) 100 nm NSs, (b-e) 60 nm NPs. The insert in (e) is the relevant fast Fourier transform (FFT) pattern. The as-developed carbonthermal reduction method is flexible and capable for the preparation of Fe3O4/C nanoparticles with different sizes and morphologies by adjusting precursors. Figures S4 and 4 show the EDX spectra, XRD patterns, SEM and TEM images of the products obtained by using 100 nm α-Fe2O3 nanosheets and 60 nm nanopolyhedrons (Figure S1c–d) as precursors. Seen from Figure 4a–c, the morphology and diameter of the magnetic samples remain unchanged compared to their precursors, which is consistent with the typical sample. EDX spectra and XRD patterns (Figure S4) demonstrate that both the two samples are composed of Fe3O4 and C. The TEM images (Figure 4d–e) confirm the high crystallinity nature and core-shell structure for NPs. Figure S5 shows the Raman spectrum and TG curve of the NPs. The calculated ID/IG is 0.74,



indicating the low graphitization degree. The carbon content of 6.28% is slightly higher than that in the typical NSs. The increased carbon content is related to the small size and large surface area of the NPs. To make a visualized comparison, the related physical parameters of the obtained NSs and NPs were summarized in Table S1. 3.3 Static magnetic properties of the resultant Fe3O4/C core-shell products. Figure 5 shows that all the products display a typical S-type magnetic hysteresis loops, indicating their ferromagnetic behavior. The magnetization is saturated under an external magnetic field of 5 000 Oe. The specific saturation magnetization (Ms) fluctuates within 82.08−84.47 emu·g−1 (Figure 5 and Table S1). The Ms value is close to 92 emu·g−1 of bulk Fe3O4 and much larger than those of Fe3O4 nanospheres (30.9−77.71 emu·g−1),34,47,48 Fe3O4 nanorings (60.75 emu·g−1),49 and Fe3O4 flowers (65.8 emu·g−1).50 This is attributed to the large crystal sizes (Table S1), high crystallinity and low carbon content of the as-obtained samples.

Figure 5. Magnetic hysteresis loops of the Fe3O4/C core-shell NSs and NPs. The inset at the lower right is the expanded hysteresis curves near the origin.


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Figure 5 and Table S1 indicate that when a increases from 60 nm to 250 nm, the coercivity (Hc) of the obtained samples increases monotonously. At a = 250 nm, Hc reaches up to 283.98 Oe, which is greatly larger than the corresponding values of octahedral Fe3O4 particles (161.67 Oe),51 Fe3O4 nanospheres (55.65 Oe),34 and Fe3O4 nanorings (20.69 Oe).18 Hc is usually related to anisotropy. As shown in Table S1, The shape and stress anisotropies of the NSs increase with increasing a, leading to the increased Hc. In addition, crystal size (D) is also an important factor that determines Hc. The as-obtained NSs have a crystal size of 22.5−26.9 nm. This value is close to the critical size of Fe3O4 with a single magnetic domain (around 25 nm), enabling the enhancement of Hc. The large Hc of the 250 nm NSs is attributed to their large shape anisotropy and appropriate particle/crystal size. Results indicate that the as-obtained Fe3O4/C core-shell NSs exhibit high Ms with Hc adjusted by the aspect ratio. 3.4 EM parameters and microwave absorption of the resultant Fe3O4/C core-shell products. Figure 6 reveals the EM parameters of the paraffin-sample composites in the frequency range of 2–18 GHz. Seen from Figure 6a, the NSs with a = 250 nm exhibit the highest real permittivity (ε′), ranging from 7.94 to 8.46. When a decreases to 150 nm, ε′ fluctuates between 8.06 and 8.25, showing little changes compared to that of 250 nm. When a further decreases to 100 nm, ε′ rapidly declines to 5.77–6.34. However, ε′ exhibits an increase for 60 nm NPs. As to the imaginary permittivity (ε″) (Figure 6b), the NSs with a = 250 nm and 150 nm show little difference, while those with a = 100 nm manifest the lowest ε″ of 0.12–1.11. Compared to the NSs, the 60 nm NPs exhibit enhanced ε″ (0.93–2.47). This relates to their increased interfacial polarization caused by the increased specific surface area. In addition, obvious relaxation peaks can be observed in 12–18 GHz for both NSs and NPs, corresponding to dipole polarization from the interfaces.52 This suggests that for the as-obtained Fe3O4/C core-



shell products, the ε′ and ε″ show strong dependences on the particle anisotropy (diameter/thickness). They affect the permittivity by influencing the orientation polarization and interfacial polarization, respectively. The similarity in ε′ and ε″ of the 250 nm and 150 nm NSs is due to the co-effect of shape anisotropy and interfacial polarization while the rapid decrease of permittivity in 100 nm NSs is due to the great decrease of aspect ratio from 25 to 6.0, which significantly deduces the orientation polarization. The highest ε″ of 60 nm NPs is related to the increased interfacial polarization induced by the decreased particle size. According to the free electron theory,53 ε ′′ ∝σ / 2π f ε0 , where σ is the conductivity,the highest ε″ of the 60 nm NPs also benefits from its relative high carbon content, which causes the increase of σ and ε″.

Figure 6. (a) Real permittivity (ε′), (b) imaginary permittivity (ε″), (c) real permeability (µ′), and (d) imaginary permeability (µ′′) of the paraffin composites containing 50 wt.% Fe3O4/C coreshell NSs and NPs.


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In addition to the influence of anisotropy and particle size, the core-shell heterostructures of Fe3O4/C NSs and NPs also contribute greatly to the permittivity. As the permittivity characteristics of Fe3O4 and carbon are very different, many free charges exist at their interface. When an altering electromagnetic field is applied, the free charges would transfer between Fe3O4 and carbon to counter the change of field, leading to strong interfacial polarization relaxation and thus the dissipation of EM waves into thermal energy.54 Based on the Debye theory, the relationship between ε′ and ε″ follows the equation: (ε ′ − ε s + ε ∞ ) 2 + (ε ′′)2 = ( ε s − ε ∞ ) 2 , where εs is the 2

2

static dielectric constant and ε∞ is the relative dielectric constant at the high frequency limit.55 The plot of ε′ versus ε″ is defined as Cole−Cole semicircle. Figure S6 shows the Cole−Cole curves of the as-obtained samples. Two Cole−Cole semicircles with different radius could be found in all the four samples, indicating the dual relaxation processes, which mainly originate from interfacial polarization between C and Fe3O4, as well as between Fe3O4/C composites and paraffin matrix. In comparison, only one Cole−Cole semicircle could be observed in the pure RGO or Fe3O4 nanoparticles.56 Notably, the curves shift to lower value as a decreases from 250 nm to 100 nm and then shift to higher value in a = 60 nm. The changing trends and the different semicircle radiuses are attributed to the different Debye dielectric relaxation process in NSs and NPs with different aspect ratio and specific surface area, which strongly affect the interfacial polarization. These results indicate that multiple dielectric relaxations can generate in the monodisperse Fe3O4/C core-shell NSs and NPs and modulation over the morphology, size and composition can expediently adjust the permittivity. Benefited from the core-shell anisotropy structure, complementary effect between Fe3O4 and C, as well as the low content of C, the obtained Fe3O4/C NSs and NPs possess larger permittivity than pure Fe3O4,29 and remain the high permeability.



As shown in Figure 6c,d, the Fe3O4/C core-shell NSs show distinguished complex permeability from the Fe3O4/C core-shell NPs. The formers exhibit obvious magnetic resonance peaks within 2−12 GHz while the magnetic resonance frequency (fr) of the latter appears below 2 GHz. The 150 nm NSs show fr at 4.4 GHz, which is higher than that of Fe3O4 microspheres (3.5 GHz),47 Fe3O4 hollow spherical chains (3.6 GHz),57 Fe3O4 nanorings (3.85 GHz),49 and bulk Fe3O4 (approximately 1.5 GHz).58 The relationship between fr and anisotropy can be expressed by f r = (γ 0 2π ) × H eff and H eff = 4 K1 (3µ0 M S ) ,18 where γ0 is the gyromagnetic ratio, and K1 is the anisotropy coefficient. Therefore, the large fr for the 150 nm NSs is attributed to the relative high aspect ratio, which guarantees the large anisotropy coefficient. The slightly lower fr of the 250 nm NSs than that of the 150 nm NSs is attributed to the decreasing surface anisotropic energy caused by the increased particle size.59 The fact that fr is below 2 GHz for the Fe3O4/C core-shell NPs is consistent with the theoretical fr of single crystal Fe3O4 (1.5 GHz), which is calculated based on K1 = −1.2 × 104 J/m3 and Ms = 4.6 × 105 A/m.60 High fr could suppress the rapid decline of real permeability (µ′) in low frequency range and guarantee high imaginary permeability (µ′′) at a high frequency range, leading to high permeability. Consequently, µ′ at 2−8 GHz and µ′′ at 4.5−18 GHz of the Fe3O4/C core-shell NSs are both larger than those of the Fe3O4/C core-shell NPs. The size independence of the complex permeability of the obtained NSs could be illustrated by the co-effect of particle size and shape anisotropy on the permeability. Figure S7 indicates that there form horizontal lines within 12−18 GHz for the NSs and 8−18 GHz for the NPs, in accordance with the skin-effect criterion that µ′′(µ′)−2 f −1 = 2πµ0σ d 2 / 3 remains unchanged when f changes.18 This suggests that the high-frequency permeability is also caused by the suppression of eddy current loss. The sheet-shape structure facilitates the enhancement of the permeability at 8-12 GHz for the as-obtained Fe3O4/C core-shell NSs.


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Figure 7. Frequency dependence of calculated reflection loss curves (a, c, e, g) and 2D color maps (b, d, f, h) for paraffin composites containing Fe3O4/C core-shell NSs and NPs with a volume fraction of 21 vol.%. (a, b) 250 nm NSs; (c, d) 150 nm NSs; (e, f) 100 nm NSs and (g, h) 60 nm NPs. 19 Environment ACS Paragon Plus


Figure 7 shows the calculated reflection loss (RL) of the paraffin-sample composites. For the Fe3O4/C core-shell NSs with a = 250 nm, the minimum RL value of −43.93 dB is observed at 3.92 GHz when d = 4.3 mm. The efficient absorption (RL ≤ –20 dB) covers a broad frequency range of 2.08−16.40 GHz when d changes between 7.0−1.4 mm. Those with d = 1.4−2.0 mm exhibit a bandwidth (RL ≤ –10 dB) beyond 3.0 GHz (Figure 7b). Seen from Figure 7c–f, the minimum RL values for 150 nm NSs and 100 nm NSs are −36.94 dB at 17.28 GHz with d = 1.5 mm and −38.40 dB at 17.12 GHz with d = 4.9 mm, respectively. As a decreases from 150 nm to 100 nm, the absorbing band (RL ≤ –20 dB) sharply decreases from 13.47 GHz (4.53−18.0 GHz) to 4.88 GHz (3.44−7.6 GHz and 16.8−17.52 GHz). The sample thicknesses for efficient absorption (RL ≤ –20 dB) are at the ranges of 1.38−4.2 mm and 3.3−5.8 mm (Figure 7c,e) for 150 nm NSs and 100 nm NSs, respectively, indicating thinner sample thickness for 150 nm Fe3O4/C NSs. As shown in Figures 7g and 7h, the 60 nm Fe3O4/C core-shell NPs also exhibit strong absorption. The minimum RL value is −41.90 dB at 12.8 GHz and the efficient absorption band with RL ≤ –20 dB is 8.56 GHz (5.60−14.16 GHz) corresponding to the thickness of 4.0−1.85 mm (Figure 7g). A wide absorption bandwidth of 4.85 GHz (RL ≤ –10 dB) has been achieved in 60 nm NPs with d = 2.0 mm, demonstrating broadband, thin thickness absorption. The results from Figure 7 further reveal that the microwave absorption performance is closely related to the particle size and morphology. Compared with the reported Fe3O4/C based nanomaterials, such as Fe3O4/C nanospindles,2 Fe3O4/C nanorings,6 Fe3O4@C microspheres,29 and Fe3O4@N-doped carbon nanochains,61 the 250 nm Fe3O4/C core-shell NSs obtained in this work exhibit strong absorption (more than 99%) in a broad frequency range (2.08−16.40 GHz) by tuning d in 7.0−1.4 mm. (Table 1) The excellent microwave absorption performance of the obtained NSs are benefited from their core-shell anisotropy structure, strong magnetic properties


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and the complementary behavior between Fe3O4 and C. The detailed microwave absorption mechanisms are illustrated by analyzing the attenuation and impedance matching properties based on the EM parameters. Table 1. Comparison of microwave absorption performance between the reported Fe3O4/C composites and the obtained Fe3O4/C core-shell NSs.

Absorber Fe3O4/C core-shell nanospindles Fe3O4/polyaniline hybrid materials Fe3O4/C core-shell nanorings Fe3O4@C microspheres 2D Fe3O4/C composites Fe3O4/Fe@C nanorings Fe3O4@N-doped carbon nanochains RGO–Fe3O4 composites Fe3O4@polypyrrole composites 250 nm Fe3O4/C core-shell NSs

Maximum RL (dB)

Matching frequency (GHz)

dw (mm) (RL < – 20dB)

Frequency range (GHz) (RL < –20 dB)

Absorption band (GHz) (RL < –20 dB)

Ref.

−37.0

3.5

5.0−3.0

3.3−7.8

4.5

2

−37.4

15.4

4.0−1.75

6.8−18

11.2

5

−55.68

3.44

9.0−2.7

2.11−10.99 16.5−17.26

9.55

6

−40.0

15.8

5.0−1.5

4.4−16.4

12.0

29

−39.3

15.5

4.5−2.5

7.0−16.8

9.8

30

−28.18

4.94

7.0−3.0 1.5

3.20−9.50 16.24−16.64

6.7

33

-63.09

11.91

6.0−2.8

4.8−14.4

9.6

61

−44.6

6.6

2.5−1.5

9.5−17.3

7.8

62

−41.9

13.3

5.0−2.5

6.0−14.7

8.7

63

−43.95

3.92

7.0−1.4

2.08−16.40

14.32

This work



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Figure 8. Illustrations of EM wave absorption mechanisms in Fe3O4/C core-shell NSs. It is generally accepted that a good microwave absorber should simultaneously features the characteristics of strong microwave attenuation ability and good impedance matching. EM wave attenuation ability of an absorber can be evaluated by the attenuation constant (α), which can be expressed as: α = 2π f c

( µ ′′ε ′′ − µ ′ε ′) + ( µ ′ε ′′ + µ ′′ε ′) 2 + ( µ ′′ε ′′ − µ ′ε ′) 2

,64 where f is the frequency

and c is the velocity of light. Figure S8a shows the curves of attenuation constant versus frequency for the obtained NSs and NPs. It can be seen that the composites have large α values in the measured frequency range, confirming their excellent dissipation properties. The enhanced dissipation properties are mainly attributed to the following factors: First, NSs with core-shell structure can generate multiple dielectric relaxations and enhance the permittivity loss due to the


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co-effect of shape anisotropy and interfacial polarization. Second, NSs with large Hha/Hea ratio can enhance the high-frequency permeability because of the suppressed eddy current in high frequency. Third, NSs with large shape anisotropy and strong magnetic property exhibit broad and strong natural magnetic resonance, leading to strong magnetic loss. Finally, sheet-like structures can enhance the absorption by multiple scattering. The microwave absorption mechanisms of the NSs are summarized in Figure 8. Absorbers with good impedance matching will reduce the reflection of microwave at the interface between the free space and absorbers and thus guarantee the successful incidence of microwave to the absorber. The impedance matching (Z) could be expressed as Z = Z1 / Z0 , where Z1 = Z0 µr / ε r

65,66

. According to the formula, Z is closely related to εr and µr, which are strongly

affected by the particle size and composition. As shown in Figure S8b, 100 nm Fe3O4/C coreshell NSs with the lowest permittivity exhibit the highest Z values, indicating the best impedance matching. Relative high Z values (Figure S8b) indicate that all the obtained samples exhibit enhanced impedance matching. The good impedance matching is mainly thanks to the combination of strong magnetic Fe3O4 core with dielectric carbon shell.

4. CONCLUSIONS In summary, monodisperse Fe3O4/C core-shell NSs with significantly improved microwave absorption performance were synthesized by a facile and low-cost carbothermal reduction method using α-Fe2O3 NSs as precursors. The morphology heritage and phase conversion process were determined by the redox reaction between α-Fe2O3 and acetone, which is strongly influenced by the calcining temperature. The morphology heritage conversion occurs optimally at 400 °C. The developed carbonthermal reduction method is capable for the preparation of



Fe3O4/C core-shell particles with different sizes and morphologies by using different α-Fe2O3 precursors. The obtained Fe3O4/C core-shell NSs exhibit high Hc (198.35–283.98 Oe) and Ms (82.51–84.47 emu·g−1), as well as a large magnetic resonance frequency and high permeability due to the high shape anisotropy, appropriate crystal sizes, high crystallinity and low carbon content. Consequently, the composites containing 250 nm NSs reach a minimum RL of –43.95 dB at 3.92 GHz and exhibit efficient absorption (RL ≤ –20 dB) in a broad frequency range (2.08−16.40 GHz) when the thickness is tuned from 7.0 nm to 1.4 mm. The results here indicate that the obtained magnetic NSs are promising candidates for microwave absorption applications. Furthermore, the simple, low-cost, and high yield method developed in this work also can be utilized for the design of other ferrite and carbon composites with different morphologies, sizes, and compositions.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM images of the precursors with different sizes and morphologies. HR-TEM images and EDX spectrum of the typical Fe3O4/C NSs. EDX spectra and XRD patterns of the 100 nm NSs and 60 nm NPs. Raman spectrum and TG curve of the 60 nm NPs. Data of crystal size, internal strain, Ms and Hc of the obtained NSs and NPs with different sizes. Cole-Cole curves, curves of eddy current loss versus frequency, attenuation constant versus frequency, and impendance matching versus frequency for paraffin composites containing 50 wt.% samples.

AUTHOR INFORMATION


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Corresponding Author * E-mail: [email protected] (G.T.). ORCID Jianguo Guan: 0000-0002-2223-4524 Guoxiu Tong: 0000-0002-8772-1251 Author Contributions Y.L. and G.T. conceived the idea. Y.L., Y.F., and L.L. participated in the fabrication and characterization of samples. Y.L. and W.L. engaged in the analysis of results. Y.L. and G.T. assisted in the interpretation discussion and manuscript preparation. Y.L. and G.T. wrote the manuscript. J.G. revised the manuscript. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (51672252, 51577138 and 51521001), Public Utility Items of Zhejiang Province (2015C31022), Natural Science Foundation of Zhejiang Province (LY14B010001), National Innovation and Entrepreneurship Training Program of Undergraduates (201610345010), and the Fundamental Research Funds for the Central Universities (2017-YB-003).

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87x63mm (300 x 300 DPI)

ACS Paragon Plus Environment

Low-Cost Carbothermal Reduction Preparation of Monodisperse Fe

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