Porous rGO Composites

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Controllable Synthesis of #-Fe2O3 Nanotube/ Porous rGO Composites and Their Enhanced Microwave Absorption Properties Shanshan Wang, Qingze Jiao, Xiufeng Liu, Yingchun Xu, Quan Shi, Song Yue, Yun Zhao, Hongbo Liu, Caihong Feng, and Daxin Shi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06729 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 14, 2019

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Controllable Synthesis of γ-Fe2O3 Nanotube/ Porous rGO Composites and Their Enhanced Microwave Absorption Properties Shanshan Wang†, Qingze Jiao†‡, Xiufeng Liu†, Yingchun Xu†, Quan Shi†, Song Yue†, Yun Zhao*†, Hongbo Liu‡, Caihong Feng†, and Daxin Shi* † † School of Chemistry and Chemical Engineering, Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian District, Beijing 100081, China. ‡ School of Materials and the Environment, Beijing Institute of Technology, Zhuhai, Jinfeng Road No.6, Xiangzhou District, Zhuhai 519085, China. Corresponding Authors *Yun Zhao. E-mail: [email protected] , *Daxin Shi. E-mail: [email protected] Abstract: γ-Fe2O3 nanotube/porous reduced graphene oxide (rGO) composites were prepared using a controllable method. Uniform γ-Fe2O3 nanotubes with a diameter of about 85 nm and a length of about 230 nm are well distributed between porous rGO sheets. Compared with the γ-Fe2O3 nanorod/porous rGO and γ-Fe2O3 nanotube/non-porous rGO, the γ-Fe2O3 nanotube/porous rGO composites with both unique hollow and porous structure show an advantage for the attenuation of microwaves. The appropriate dielectric

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loss and magnetic loss result in a good impedance matching. The minimum reflection loss of γ-Fe2O3 nanotube/porous rGO composite reaches -34.20 dB with a thickness of 2.0 mm and the absorption bandwidth is 4.59 GHz. These results reveal that the synthesis of hollow and porous composites is a promising way for getting light weight microwave materials with high performances. Keywords: porous composite, nanotube, γ-Fe2O3, reduced graphene oxide, microwave absorbing performance

Introduction Recently, the electromagnetic radiation pollution has been a serious problem in both military and civil fields. In order to solve this expanded problem, extensive efforts have been devoted toward exploiting absorbing materials, which can convert microwave energy into heat or dissipate through interference.1,2 In particular, the novel microwave absorbing materials with the strengths of light weight, thin thickness, wideband and strong absorption are in urgent needed for the demands of information safety.3,4 It is well known that the microstructure of the microwave absorbing material is important for the microwave absorption performance.5 Over the past few years, numerous magnetic loss materials (ferrites,6-8 magnetic metals9,10 ), dielectric loss materials (SiC,11,12 ZnO,13,14 carbon materials15,16 ) and their composites17-19 as microwave absorption materials have been developed. Magnetic materials have been investigated widely for their special applications.20-22 Among these researches, the magnetic materials with hollow or porous structures have attracted


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attention due to the enhanced microwave absorption performance and the lower density. Ni et al. synthesized hollow CoFe2O4 microspheres, and the minimum reflection loss was 15.23 dB with the thickness of 5.5 mm at a loading of 60 wt%.23 Li et al. reported that highquality MnxFe3-xO4 hollow/porous spherical chains exhibited a broader absorption band in a mass fraction of 60 wt%.24 He et al. obtained Fe3O4 nanotubes by a two-step route and their minimum reflection loss was -50.94 dB in the mass filling ratio of 70 wt%.25 These single magnetic materials with hollow structure can reduce the density of the absorbers but a large mass filling ratio is still needed for the effective absorption. Carbon materials are investigated widely due to their excellent properties and practicability.26-28 In particular, reduced graphene oxide (rGO) with many defects and high specific surface is considered as a promising candidate for the advantages of low density and high dielectric loss.29 Quan et al. reported that the minimum reflection loss of nitrogendoped graphene achieved -11.3 dB with the mass filling ratio of 30 wt%.30 Kuang et al. discussed the relationship between (rGO) and chemical reduction degrees. Its minimum reflection loss was -37.2 dB at 30 wt% loading.31 Zhang et al. constructed graphene foam and the minimum reflection loss reached -34 dB.32 It is clear that porous graphene shows better microwave absorbing properties than non-porous rGO. According to microwave absorbing principle, the well-balance of dielectric loss and magnetic loss is significant for the microwave absorption performance.33 Therefore, the absorbers with both dielectric loss and magnetic loss have been the main focuses. Wang et al. synthesized a 2D hybrid material: PSS-Fe3O4-rGO, the microwave absorbing property



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was -61.4 dB at the loading of 50 wt%.34 Ma et al. prepared a follower-like Co3O4@rGO/SiO2 composite and the optimal reflection loss reached -52.6 dB.35 Zong et al. synthesized rGO/cobalt ferrite composites through a one-pot hydrothermal process. The minimum reflection loss was -47.9 dB with the loading of 50 wt%.36 In a previous work, we prepared novel flower-like CoFe2O4@graphene through a spray drying route, and the minimum reflection loss was -42 dB with the loading of 45 wt%.37 Based on the abovementioned reports, it can be inferred that the composites of hollow magnetic materials and porous graphene might exhibit excellent wave-absorbing performances due to the dielectric loss, magnetic loss and the unique hollow/porous structures. Therefore, we design a composite with hollow ferrite and porous rGO and hope it will be a practical and efficient microwave absorbing material. In detail, γ-Fe2O3 nanotube/porous rGO composites were prepared using an easy chemical reduction reaction in combination with a hydrothermal method. Their microwave absorbing performances in 1-18 GHz were investigated. In order to understand the influence

of

pore

structure

for

microwave

absorbing

performances,

γ-Fe2O3

nanorod/porous rGO and γ-Fe2O3 nanotube/non-porous rGO composites were also synthesized and compared. The results show that the γ-Fe2O3 nanotube/porous rGO with both hollow and porous structure show the features of low density, small mass filling ratio and the stronger microwave absorbing performance. It is believed that the γ-Fe2O3 nanotube/porous rGO is a potential candidate with lightweight and strong absorption.

Experimental


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Materials. Graphite powder with 325 mesh was received from Beijing Creative Biological Engineering Materials Co. Ltd. Ferric chloride hexahydrate (FeCl3•6H2O), Diammonium phosphate ((NH4)2HPO4) and L-Ascorbic acid (L-AA) were bought from Beijing Chemicals. All reagents were analytical grade and used without further decontamination. Synthesis of α-Fe2O3 nanotubes and α-Fe2O3 nanorods. The α-Fe2O3 nanotubes were prepared by a simple hydrothermal process. 1.89 mL FeCl3•6H2O (0.5 M) aqueous solution and 1.70 mL (NH4)2HPO4 (0.02 M) aqueous solution were added into 43.68 mL of distilled water. The solution was then transferred into a 60 mL Teflon-lined autoclave and heated at 220 ℃ for 48h and cooled to room temperature. Then, the brick red products were washed and dried under vacuum at 80 ℃ for 12h to obtain α-Fe2O3 nanotubes. For synthesis of α-Fe2O3 nanorods, the same process was used and the volume of FeCl3•6H2O (0.5 M) solution, (NH4)2HPO4 (0.02 M) solution and distilled water were 1.89 mL, 0.81 mL and 44.57 mL, respectively. Preparation of γ-Fe2O3/porous rGO composites. For the preparation of γ-Fe2O3/rGO composites, GO was firstly obtained by a modified Hummers method, and 30 mL of GO (1 mg/mL) suspension was sonicated for 30 min.38 Then, 120 mg of α-Fe2O3 nanotubes were added into the GO suspension and sonicated for another 20 min to obtain a homogeneous dispersion. Subsequently, 120 mg of L-AA was added to the suspension and dissolved by sonication for 20 min. Finally, the suspension was sealed in a 50 mL isotope bottle and heated at 90 ℃ for 2 h without stirring. After they were collected, washed with distilled water and freeze-dried at -50 ℃ for 48 h, the products were annealed in a tube furnace



under Ar and H2 at 360 ℃ for 10 min. Then, γ-Fe2O3 nanotube/porous rGO composites were obtained. The synthetic process is shown in Figure 1. γ-Fe2O3 nanorod/porous rGO composites were prepared by the same procedure using α-Fe2O3 nanorods instead of αFe2O3 nanotubes. γ-Fe2O3 nanotube/non-porous rGO composites were obtained with mechanical stirring in a flask and dried in a vacuum oven at 60 ℃ for 12 h.

Figure 1. Schematic illustration for the fabrication of γ-Fe2O3 nanotube/porous rGO. Characterization. The morphology for the composites was analyzed by a scanning electron microscopy (SEM, Hitachi S-4800), a transmission electron microscopy (TEM, Hitachi HT7700) and a high-resolution transmission electron microscopy (HRTEM, JEM-2010). The structure and composition were characterized by X-ray diffractometer (XRD, Ultima IV, Cu Kα), Raman Microscope (Renishaw, λ=632.8 nm) and X-ray photoelectron spectroscopy (XPS, PHI5300X). The magnetic properties were recorded by vibrating


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sample magnetometer (VSM, Lakeshore7407). The electrical conductivity was tested by a Solartron 1287 electrochemical workstation (Advanced Measurement Technology Inc. USA) with the DC measurement model.

The electromagnetic parameters were

investigated using a vector network analyzer (HP 8722ES). The samples were obtained by mixing the products (17 wt%) with paraffin wax and pressed into a ring with ϕout of 7.0 mm and ϕin of 3.0 mm. Results and discussion The morphologies are shown by the SEM and HRTEM images. As shown in Figure S1a and b, the pure GO exhibits a typical layered and wrinkle sheet. In Figure S1c-f, uniform αFe2O3 nanotubes are about 230 nm in length and 85 nm in diameter and α-Fe2O3 nanorods are about 240 nm in length and 95 nm in diameter. It can be seen from Figure 2a-d that the γ-Fe2O3 with two kinds of morphologies (nanotubes and nanorods) are uniformly loaded between the sheets of rGO. Moreover, the rGO sheets overlap randomly to form porous reticulum-like frameworks for two composites. While in Figure 2e and f, the composites show non-porous morphology, although the rGO sheets are also obviously decorated by γFe2O3 nanotubes. The further insight into the morphology and detailed structure of γ-Fe2O3 and rGO sheets can be confirmed by TEM and HRTEM images in Figure 3.



Figure 2. SEM images for the composites of γ-Fe2O3 nanotube/porous rGO (a, b), γ-Fe2O3 nanorod/porous rGO (c, d), and γ-Fe2O3 nanotube/non-porous rGO (e, f). The microstructure of the composites is shown in Figure 3. In Figure 3a, the γ-Fe2O3 tubes exhibit a hollow structure and their wall thickness are about 10-25 nm. As shown in Figure 3b, the γ-Fe2O3 present distinct rod shape with some holes, and they are about 240 nm in length and 95 nm in diameter. The typical transparent tulle-like rGO sheets can also be obviously seen in Figure 3a and b. While owing to the large size of pores, the porous structure of the porous composites cannot be observed in TEM images. Hence, the TEM image (Figure 3c) of γ-Fe2O3 nanotube/non-porous rGO composites is similar to that of γFe2O3 nanotubes/porous rGO. Figure 3d reveals the lattice fringes and the selected area electron diffraction (SAED) of the γ-Fe2O3 nanotubes/porous rGO composites. The lattice spacing of rGO is 0.35 nm, which is lower than that of GO (0.76 nm), implying the most oxygen-containing functional groups were removed. The lattice spacing of 0.48 nm corresponds to the (111) crystal phase of the γ-Fe2O3. The clear lattice indicates a good


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crystallinity of γ-Fe2O3. The clear and regular lattice diffraction spots further confirm the perfect crystallinity of γ-Fe2O3 for the γ-Fe2O3 nanotube/porous rGO.

Figure 3. TEM images for the composites of γ-Fe2O3 nanotube/porous rGO (a), γ-Fe2O3 nanorod/porous rGO (b), γ-Fe2O3 nanotube/non-porous rGO (c), HRTEM image and SAED pattern of γ-Fe2O3 nanotube/porous rGO (d SAED pattern of γ-Fe2O3 nanotube/porous rGO inset in d). The XRD patterns of α-Fe2O3 and γ-Fe2O3/rGO composites are shown in Figure 4. In Figure 4a and 4b, the diffraction peaks of α-Fe2O3 nanotubes and nanorods consistent with hematite (JCPDS: 33-0664). When α-Fe2O3 nanotubes and nanorods were partially reduced by H2 during an annealing process, the maghemites were obtained. As shown in Figure S2 and Figure 4c-e, the peaks located at 2θ of 18.38, 30.24, 35.63, 37.24, 43.28, 53.73, 57.27, 62.92°correspond to the (111), (220), (311), (222), (400), (422), (511), (440) crystal planes of



γ-Fe2O3 (JCPDS: 39-1346). The narrow sharp peaks indicate the high crystallinity and high purity of both the hematite and maghemite Fe2O3 nanoparticles. The result is consistent with above TEM analyses. Furthermore, no peak is observed for rGO, which suggests that the γ-Fe2O3 effectively prevents the agglomeration of rGO sheets.

Figure 4. XRD patterns of α-Fe2O3 nanotubes (a), α-Fe2O3 nanorods (b), γ-Fe2O3 nanotube/porous rGO (c), γ-Fe2O3 nanorod/porous rGO (d) and γ-Fe2O3 nanotube/nonporous rGO (e). Raman spectra are given to analyze the detailed structure of rGO for the composites. As shown in Figure 5, the D and G bands can be observed at 1350 and 1590 cm-1, which reveal the disordered carbon and the stretching motion of sp2 pairs, respectively.39,40 The relative intensity ratio of D band to G band (ID/IG) is an indicator to assess the defect degree of carbon materials, and a higher ID/IG ratio suggests a lower degree of graphitization in carbon.41,42 Compared to 0.90 of GO, the higher ID/IG of 1.17 for γ-Fe2O3 nanotube/porous rGO composites implies that rGO of the composites contains more defects after reduction. The reduction process removes the oxygen-containing functional groups while introducing


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some defects.43,44 In Figure S3, two peaks of 210 and 272 cm-1 attributed to γ-Fe2O3 can also be observed for the γ-Fe2O3 nanotube/porous rGO composite. This also proves the existence of γ-Fe2O3 in the composites.

Figure 5. Raman spectra of GO and γ-Fe2O3 nanotube/porous rGO. The elemental components of GO and γ-Fe2O3 nanotube/porous rGO are presented in Figure 6. Figure 6a confirms that γ-Fe2O3 nanotube/porous rGO is made of C, O and Fe elements. It can be clearly seen from the spectrum that the intensity of O1s decreases for the composites, which suggests that an effective reduction of GO. As shown in Figure 6b and c, the C1s XPS spectra of GO and γ-Fe2O3 nanotube/porous rGO can be deconvoluted into three different peaks. The binding energies at 284.5、286.5 and 288.3 eV are typically belonged to C–C/C=C, C–O and C=O, respectively.45 Furthermore, the peak of C-O and



C=O for C1s XPS spectrum of γ-Fe2O3 nanotube/porous rGO composites decrease rapidly, suggesting the elimination of most oxygen-containing functional groups of GO after the process of reduction, which is consistent with the analysis for Figure 6a. As shown in Figure S4 and Figure 6d, both γ-Fe2O3 nanotubes and the composite of γ-Fe2O3 nanotube/porous rGO display two peaks at 711.12 and 724.55 eV, which can be attributed to Fe 2p3/2 and Fe 2p1/2 of γ-Fe2O3 (710.99 and 724.69 eV). 20,46 Moreover, small shakeup satellite peak at ~718.98 eV is observed, which are fingerprints of the electronic structure of γ-Fe2O3 or αFe2O3. While in case of magnetite, no shakeup satellite peak can be observed in the XPS spectrum and the peaks of Fe 2p3/2 and Fe 2p1/2 should be at lower binding energy (710.29 and 724.09 eV) due to the coexistence of Fe3+ and Fe2+.47,48 Based on the XPS spectrum and XRD pattern, we can attribute our materials to γ-Fe2O3.

Figure 6. XPS spectra of the survey scan of GO and γ-Fe2O3 nanotube/porous rGO (a), C1s of GO (b), C1s of γ-Fe2O3 nanotube/porous rGO (c) and Fe2p region of γ-Fe2O3 nanotube/porous rGO (d).


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As shown in Figure 7(A), the saturation magnetizations (Ms) of γ-Fe2O3 nanotube/nonporous rGO, γ-Fe2O3 nanotube/porous rGO and γ-Fe2O3 nanorod/porous rGO composites are 51.81, 52.55 and 64.83 emu g-1, respectively. These maybe attributed to the shape anisotropy and surface area of the samples.49-51 Therefore, the magnetic properties of materials can be controlled through changing the microstructures of the samples. The Ms of γ-Fe2O3 nanotube/non-porous rGO is very close to γ-Fe2O3 nanotube/porous rGO. This is because the magnetic properties of the samples mainly affected by the γ-Fe2O3 nanotubes, when the composition of composites are the same, the morphology of rGO does not affect the magnetic properties of the materials. As shown in Figure 7(B), the electrical conductivity of γ-Fe2O3 nanotube/non-porous rGO composite is 14.96 S/m. The electrical conductivities of the composites with porous rGO are much higher than that of the composite with non-porous rGO because the porous structure effectively avoids the self-agglomeration of rGO and improves the formation of conductive network. The electrical conductivity of γ-Fe2O3 nanotube/porous rGO (71.88 S/m) is slightly higher than that of γ-Fe2O3 nanorod/porous rGO (66.04 S/m). The electrical conductivity is an important factor of the resistance loss, which is beneficial to improve the microwave absorbing properties.



Figure 7. Magnetic hysteresis loops (A) and electrical conductivity (B) for γ-Fe2O3 nanotube/non-porous rGO (a), γ-Fe2O3 nanorod/porous rGO (b) and γ-Fe2O3 nanotube/ porous rGO (c). The electromagnetic parameters of the samples with the mass filling ratios of 17 wt% were measured for the research of microwave absorption performances. The relative complex permittivity (𝜀𝑟=𝜀′-𝑗𝜀″) and relative complex permeability (𝜇𝑟=𝜇′-𝑗𝜇″) are important for the microwave absorption properties. The real parts of permittivity (𝜀′) and permeability (𝜇′) relate to the storage capabilities of electric and magnetic energy, while the imaginary parts of permittivity (𝜀″) and permeability (𝜇″) stand for the dissipation or loss of electric and magnetic energy.52,53 The dielectric loss tangent (tanδe= 𝜀″/𝜀′) and the magnetic loss tangent (tanδm= 𝜇″/𝜇′) are calculated and shown in Figure 8c and f. In Figure 8a, the 𝜀′ values of γ-Fe2O3 nanotube/porous rGO are much higher than those of γ-Fe2O3 nanorod/porous rGO and γ-Fe2O3 nanotube/non-porous rGO composites, which is almost a constant with slight fluctuations between 8.88 and 9.51. The 𝜀′ values of γ-Fe2O3 nanorod/porous rGO are keeping at about 6.45, and the 𝜀′ values of γ-Fe2O3 nanotube/non-porous rGO are the lowest, which only fluctuate around 4.25. In Figure 8b, the patterns of the 𝜀″ values are similar to those of 𝜀′ values. The highest 𝜀″ of γ-Fe2O3 nanotube/porous rGO reaches 3.11, and the 𝜀″values fluctuate between 2.51 and 3.11. While the 𝜀″ values of γ-Fe2O3 nanorod/porous rGO are close to 2.19, slightly less than those of γ-Fe2O3 nanotube/porous rGO. The 𝜀″values of γ-Fe2O3 nanotube/non-porous rGO vacillate between 0 and 0.46. These differences can be explained by the Debye theory in equation (1).


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ε𝑠 ― ε∞

𝜎

𝜀″ = 1 + 𝜔2𝜏2𝜔𝜏 + 𝜔ε0

(1)

Where ε𝑠 is the static permittivity, ε∞ is the relative dielectric permittivity at the highfrequency limit, 𝜔 is the angular frequency, 𝜏 is polarization relaxation time and 𝜎 is the electrical conductivity. Based on the theory, the electrical conductivity is an important factor for the 𝜀″. As mentioned above, the electrical conductivity of the composites with porous rGO are much higher than that of γ-Fe2O3 nanotube/non-porous rGO, thus the 𝜀″ of γ-Fe2O3 nanotube/porous rGO and γ-Fe2O3 nanorod/porous rGO are increased.54,55 In Figure 8d and e, all the samples have similar 𝜇′ and 𝜇″ values, they all fluctuate around 1 and 0.05 respectively, the 𝜇′ and 𝜇″ values of γ-Fe2O3 nanotube/porous rGO and γ-Fe2O3 nanotube/non-porous rGO are little larger than those of γ-Fe2O3 nanorod/porous rGO in a certain frequency range. In Figure 8, it can be noted that the wave shapes of tanδe and tanδm curves are similar to those of 𝜀″and 𝜇″, respectively. It demonstrates that the loss tangent is mainly influenced by the imaginary part. As shown in Figure 8c, the dielectric loss tangents of γ-Fe2O3 nanorod/porous rGO slightly exceed γ-Fe2O3 nanotube/porous rGO due to the relative higher 𝜀″ and relative lower 𝜀′. Nevertheless, in Figure 8f, the composites with γ-Fe2O3 nanotubes have a slight advantage in high-frequency. As shown in Figure 8a-c, not only does the porous rGO influence the relative complex permittivity and dielectric loss, but also the microstructure of γ-Fe2O3 acts as a pivotal part in regulating the dielectric constant. In Figure 8d-f, the magnetic permeability and magnetic loss tangent are mainly affected by the microstructure of γ-Fe2O3 and have nothing to do with the structure of rGO.



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Figure 8. Frequency dependences of the permittivity real part (a), permittivity imaginary part (b), dielectric loss tangent (c), permeability real part (d), permeability imaginary part (e) and magnetic loss tangent (f) for different γ-Fe2O3/rGO composites. The reflection loss (RL) can be summarized using the electromagnetic parameters by the following equations.56 𝑧𝑖𝑛 ― 𝑧0

(2)

RL(dB) = 20log | 𝑧𝑖𝑛 +𝑧0 | 𝑧𝑖𝑛 = 𝑧0

𝜇𝑟

(𝑗2𝜋𝑓𝑑 𝑐

𝜀𝑟 tan ℎ

)

𝜇𝑟𝜀𝑟

(3)

Where 𝑧0 is the impedance of the free space, 𝑧𝑖𝑛 is the normalized input impedance, 𝜀𝑟 and 𝜇𝑟 are the relative complex permittivity and permeability. 𝑓 is the microwave frequency, d is the thickness of the absorber, and c is the velocity of light. The 3D reflection loss curves of the composites at different thicknesses are shown in Figure 9. It’s obvious that the microstructure of composites is an important factor for microwave


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absorbing performances. In Figure 9a, the minimum reflection loss of γ-Fe2O3 nanorod/porous rGO reaches -20.37 dB at 15.62 GHz with a thickness of 2.0 mm and the absorption bandwidth (RL

Porous rGO Composites

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