Enhanced Microwave Absorption Property of Fe Nanoparticles

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Enhanced Microwave Absorption Property of Fe Nanoparticles Encapsulated within Reduced Graphene Oxide (RGO) with Different Thickness Youjiang Li, Miao Yu, Pingan Yang, and Jie Fu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01732 • Publication Date (Web): 17 Jul 2017 Downloaded from http://pubs.acs.org on July 18, 2017

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Industrial & Engineering Chemistry Research

Enhanced Microwave Absorption Property of Fe Nanoparticles Encapsulated within Reduced Graphene Oxide (RGO) with Different Thickness Youjiang Li 1,

Miao Yu 1,* Pingan Yang 1,

Jie Fu 2, 1

1. Key Lab for Optoelectronic Technology and Systems, Ministry of Education, College of Optoelectronic Engineering, Chongqing University, Chongqing 400044, China 2. Anhui Weiwei Rubber Parts (Group) Co., Ltd, Tongcheng 231400, People's Republic of China

Corresponding Author: *E-mail: [email protected](M. Yu); Fax: +86 23 65111016

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ACS Paragon Plus Environment


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Abstract: Reduced graphene oxide (RGO)/Fe nanocomposites were synthesized by a facile one-step reduction reaction, using reduced graphene oxide and FeSO4·7H2O as raw materials. These RGO/Fe nanocomposites combine Fe nanoparticles with graphene in various thickness. The results show that the residual defects and groups in chemically RGO brought about defect polarization relaxation and groups' electronic dipole relaxation, which both make RGO/Fe nanocomposites have better electromagnetic absorbing performance than pure Fe nanoparticles. Most importantly, this paper discoveries that the graphene with different thickness has a great influence on the microwave absorbing properties of the composites. There was a serious agglomeration in RGO/Fe nanocomposites synthesized by thin graphene, while the graphene a certain thickness could support the nanoparticles on its surface thereby reducing the effect of agglomeration on the composite material and giving full play to dielectric loss properties of the graphene.

KEYWORDS: graphene thickness; RGO/Fe; nanocomposites; microwave absorption

For Table of Contents Only

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1. INTRODUCTION With the widespread use of electronic equipment in recent decades, there is more and more electromagnetic radiation generated and released, and electromagnetic radiation pollution has become a serious problem. Electromagnetic radiation not only affect the normal operation of electronic equipment, but also have significant impact on human health.1 On the contrary, electromagnetic wave has great application in military affairs, for example, it can be applied in radar detection.2 In order to suppress radar detection, radar stealth has attracted tremendous attention. Therefore, how to reduce the adverse effects of electromagnetic waves has been becoming the serious issue for many years. Absorbing material can solve these issues effectively, for which can reduce the electromagnetic waves reflection by transform the electromagnetic wave into heat or other energy. Ideal absorbing material need to meet the characteristics of strong absorption capability, broad absorption frequency, light weight and thin matching thickness.3-6 In the conventional absorbing materials, soft magnetic metal materials high saturation magnetization, high magnetic permeability and large dielectric constant.7,8 These advantages are conductive to absorbing microwaves. However, high density of magnetic metal is the challenge for their applications

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Fortunately, carbon-based materials possess the advantages of low

density and high complex permittivity.4,12-17 Among them, graphene has the characteristics of small density, large specific surface area and higher dielectric constant, which are favorable for the polarization relaxation of external electrons and resulting in the electromagnetic loss in the electromagnetic field.5,18,19 However, the 3



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low magnetic loss of graphene is an adverse factor for the microwave absorption properties of it. In order to overcome their shortcomings to obtain ideal absorbing material, the combination of soft magnetic particles and carbon based materials has attracted more and more researchers' attention.20,21 The residual defects and groups in chemically reduced graphene oxide introduce defect polarization relaxation and groups’ electronic dipole relaxation, shows enhanced microwave absorption compared with graphite and carbon nanotubes, and can be expected to display better absorption than high quality graphene.22 Singh et al. 23

synthesized layered and porous structure graphene, and RGO/nitrile butadiene

rubber composites showed a very high dielectric loss tangent under low concentration of 10 %. He, et al.24 reported a facile solvothermal route to synthesize laminated magnetic graphene RGO/Fe3O4 composites that integrate the respective advantages of Fe3O4 nanoparticles and graphene, making full use of the contributions to electromagnetic wave absorbing materials that derive from magnetic and dielectric materials. This could be defined as an electromagnetic complementary effect. Das et al.25 prepared graphene/CuFe10Al2O19 composites in acid modified graphene, finally they got a broad absorption band with a maximum reflection loss of -60.6 dB at 2.5 mm thickness. This excellent microwave absorption of the prepared nanocomposites was due to good impedance matching characteristics, good complementarity of dielectric loss and magnetic loss. In the composites of magnetic materials and graphene, the influence of magnetic materials on the electromagnetic wave absorption of composites had be taken into consideration to some extent. Zhao et al.26 prepared 4


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NiFe2O4

nanoparticle-graphene

composites

and

NiFe2O4

nanorod-graphene

composites respectively, and studied the effects of different shapes of the same kind of magnetic materials on the electromagnetic wave absorption composite materials in the field of electromagnetic wave absorption. It suggested that the different shapes of magnetic materials had a great influence on the electromagnetic wave absorption composite materials. In the study of graphene microwave absorbing composites, the researchers have studied the influence of different magnetic materials, morphologies, synthesis methods and multicomponent and so on. Most of them are concerned at the preparation method and the final microwave absorption properties. However, published papers neglected the fact that the thickness of graphene is small relative to the size of the magnetic nanoparticles, and there will be a serious agglomeration after drying. In this context, Zhang et al.27 found that multi-walled carbon nanotubes have bigger loss tangent than single-walled carbon nanotubes, indicating that the thickness of carbon nanotubes will also affects the absorptive properties. The thickness of the graphene is small relative to the size of the magnetic material, and the influence of the graphene thickness on the wave absorption should be considered. A certain thickness of graphene in the composites may exhibit a better absorbing performance. In the published reseaches, they little focus on the influence of graphene thickness on microwave absorbing materials. In this paper, we reported a method of preparation of composite materials by a facile one-step reaction, and studied the influence of two different thickness graphene on the microwave absorbing properties of the composites. 5



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2. EXPERIMENTAL SECTION 2.1. Preparation of graphene Two kinds of graphene oxide with different thickness were prepared from natural graphite (purchased from Shanghai Huayuan Chemical Co. Ltd., China) with the improved Hummer method.28,29 Reduced them with zinc powder to obtain two kinds of graphene with different thickness. The thinner one was named RGO-1 and the other named RGO-2. The two kinds of graphene were respectively used for the next step to synthesize composites.

2.2. Synthesis of RGO/Fe nanocomposites The RGO/Fe nanocomposites were synthetised by a simple in situ method. In a typical procedure, 5.56 g FeSO4·7H2O was dissolved in 400 mL deionized water to form a homogeneous solution. RGO-1 containing 60 mg graphene was scattered into above FeSO4 solution, and stirred well for 10 min homogeneously to obtain mixed solution A. 9.07 g NaBH4 was dissolved in 200 mL deionized water, followed by 0.8 g NaOH dissolved in 40 mL deionized water, and the two solutions were mixed and stirred for 10 min uniformly to obtain solution B. The solution A and solution B slowly trickled into the beaker, making ⁄ = 4: 1. After that, the beaker was allowed to stand 6 h to react sufficiently. The resulting product was vacuum-filtered and dried to obtain RGO/Fe-1 nanocomposites. Similarly, RGO/Fe-2 nanocomposites was obtained by repeating the above process with adding RGO-2 containing 60 mg graphene into FeSO4 solution.

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2.3. Preparation of Fe nanoparticles As a contrast, Fe nanoparticles were prepared under the same conditions. 5.56 g FeSO4·7H2O, 9.07 g NaBH4 and 0.8 g NaOH were dissolved in 400 mL, 200 mL and 40 mL deionized water respectively. At first, Mixed the NaBH4 solution and NaOH solution. Then FeSO4 solution and the mixed solution slowly and evenly added into the beaker. After that, the beaker was allowed to stand 6 h to react sufficiently. The resulting product was vacuum-filtered and dried in vacuo to obtain Fe nanoparticles.

2.4. Characterization The structural characterization of nanocomposites was carried out by X-ray powder diffraction (XRD) performed on a PANalytical X' Pert Powder by using CuKα1 (λ = 0.154 nm) radiation. The morphology and elemental composition of the nanocomposites were respectively performed by field-emission scanning electron microscopy (FESEM, JEOL JSM-7800F Japan) and energy dispersive spectroscopy (EDS).

The

synthesized

graphene

layers

were

characterized

by

AFM

(Asylum Research MFP-3D-BIO USA). The electromagnetic parameters of the samples were measured in a vector network analyzer (Agilent N5234A) in the range of 2-18 GHz. The measured samples were prepared by uniformly mixing 40 wt% of the sample with a paraffin matrix.

3. RESULTS AND DISCUSSION Natural flake graphite is oxidized by strong oxidizing agents with the modified Hummer’s method. Because layer and layer were modified by the intercalated -OH, -COOH and other functional groups when graphite was oxidized, the interlayer 7



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distance became large. Graphene oxide (GO) solution with a larger interlayer distance was sonicated, resulting in GO with fewer layers. GO is reduced to graphene via zinc powder, and lay the obtained graphene sheets under ultrasonic to obtain a graphene suspension, whose thickness is thinner and more uniform. Graphene suspension and containing Fe2+ ions solution were homogeneously mixed. After in situ growth, Fe2+ ions reduction and directly deposited onto the graphene surface without the need for other ways to anchored Fe nanoparticles and graphene. In addition to simplicity and ease of operation, the high stability of synthesized nanocomposites by in situ growth method suggest strong interaction between the graphene and Fe nanoparticles.

3.1. XRD Analysis The XRD analysis was conducted to determine the phase composition of Fe nanoparticles and RGO/Fe nanocomposites. As shown in Figure 1, Fe nanoparticles have a wide angle diffraction peak at 45°. Peaks are not obvious corresponding to Fe oxides observed in the XRD pattern. RGO/Fe nanocomposites have a weak and broad diffraction peak at 23°, which is a typical amorphous carbon structure pattern, and indicates that GO has been a good reduction by zinc powder, and the stacking of graphene is disordered. The obtained composites do not have the crystal structure, which indicates that Fe nanoparticles are present in the form of amorphous or microcrystalline.

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Figure 1. XRD patterns of Fe and RGO/Fe.

3.2. Morphological Study and Energy-Dispersive Spectroscopy To investigate the morphology of as-prepared composites, field-emission scanning electron microscopy (FESEM) reveals a planar morphology of test samples. Figures 2 (a) and 2 (b) respectively show the surface topographies of RGO-1 and RGO-2. It is clearly that graphene shows a morphology of silk-like lamina with some wrinkles and foldings on the surface and edge. Figure 2 (c) shows the morphology of Fe nanoparticles. Fe nanoparticles are well spaced and dispersed without serious agglomeration. Figures 2 (d) and 2 (e) respectively show the microstructures of RGO/Fe-1 and RGO/Fe-2, Fe nanocomposites wrapped in graphene sheets can be seen clearly. It indicates that the usage of in situ growth method to synthetise nanocomposites is successful. A large number of uniform Fe nanoparticles are uniformly dispersed on the graphene surface without agglomeration. The graphene 9



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after decoration with the Fe nanoparticles is not easy to aggregate, as a result of the surface tension of the RGO/Fe nanocomposites is reduced. As indicated in Figures 2 (f) and 2 (g), the EDS of the nanocomposites mainly consist of C, Fe and O, and confirms that nanocomposites are composites of graphene and Fe nanoparticles. A small amount of oxygen may be derived from the oxygen-containing functional groups.

Figure 2. FESEM images of (a) RGO-1, (b) RGO-2, (c) Fe, (d) RGO/Fe-1 and (e) RGO/Fe-2; (f) and (g) energy dispersive spectroscopy (EDS) graph of RGO/Fe-1and RGO/Fe-2.

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3.3. Thickness Measurements AFM was used to measure the thickness of the prepared graphene. The height profiles along the red solid lines in the upper insets of Figures 3 (a) and 3 (b) show that the average thicknesses of RGO-1 and RGO-2 are ∼8.5 and ∼19 nm, respectively. It is shown that the thickness of the prepared graphene is obviously different, and the thickness of RGO-2 is about twice that that of RGO-1.

Figure 3. AFM topographic images of (a) RGO-1 and (b) RGO-2 and the lower insets show AFM height profiles.

3.4. Electric and Magnetic Properties The electromagnetic wave absorbing properties of materials should be estimated from their dielectric and magnetic properties. Fe nanoparticles and RGO/Fe nanocomposites respectively pulverized into powder to mix with the paraffin in a mass fraction of 40 %. The Fe nanoparticles and composite materials were uniformly 11



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mixed with paraffin and then poured into a standard mold. The samples were made into a circular ring with an inner diameter of 3.04 mm, an outer diameter of 7 mm and a thickness of 3 mm for electromagnetic parameter test. The electromagnetic parameters of the samples were measured using Agilent N5234A vector network analyzer. Due to the loss of the electromagnetic wave of the graphene without the magnetic material is very small,22 the electromagnetic parameters of the pure graphene which prepared in this work are not study. The permittivity ε and the permeability µ are two basic parameters to evaluate microwave absorption in the electromagnetic field. It is usually expressed in the form of complex numbers εr (εr = ε' - jε") and µr (µr = µ' - jµ"). The real part of the complex permittivity represents the storage capacity of the electromagnetic wave energy of absorbing material. The imaginary part represents the energy loss capacity of material. The real and imaginary parts of the complex permeability are directly proportional to the energy density and magnetic loss power stored in the magnetic medium. It can be seen from Figure 4 (a), the real part of permittivity of Fe and RGO/Fe-1 are relatively smooth, it mainly between 3.5 to 5.5, and the real part of the permittivity of RGO/Fe-2 is much larger than Fe and RGO/Fe-1. The real part of permittivity of RGO/Fe-2 below 14 GHz is greater than 13, while there is a significant decline in the frequency band above 14 GHz, and it fluctuates continuously at 10. Likewise, as shown in Figure 4 (b), the imaginary part of the permittivity of RGO/Fe-2 is much larger than that of Fe and RGO/Fe-1. In addition, the imaginary part of the permittivity of RGO/Fe-1 is larger than that of Fe in the other frequency ranges except 12


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for the frequency range of 4.8-5.4 GHz and 13.9-15.9 GHz. Therefore, the dielectric loss tangent of RGO/Fe-1 is greater than that of Fe in all test ranges except for 4.9-5.5 GHz and 13.9-16 GHz. As shown in Figure 5 (a), except that the dielectric loss tangent of Fe appeared a peak at 15.6 GHz, the dielectric loss tangent of RGO/Fe-1 and Fe has a fluctuation from 0 to 0.1. The dielectric loss tangent of RGO/Fe-2 is larger than that of Fe and RGO/Fe-1 in the testing whole frequency band, and appears the trend of increasing first and then decreasing. Figure 4 (c) shows the real part of permeability of nanomaterials in the 2.0-6.6 GHz fluctuate relatively powerful and stable at more than 6.6 GHz. In the lower frequency range, the real part of the permeability of Fe and RGO/Fe-2 increases with the frequency increasing, but the real part of the permeability of RGO/Fe-2 is smaller than Fe, which is mainly ascribed to the additional nonmagnetic graphene in the RGO/Fe-2. As the real part of the permeability is relatively stable in the higher frequency band, the fluctuation trend of the magnetic loss tangent in the frequency range about 4 GHz or more is very similar to that of the imaginary part of the permeability, as shown in Figures 4 (d) and 5 (b). Fe and RGO/Fe-2 in the short frequency range above 2 GHz is a rising trend and the value is relatively small, resulting in their magnetic loss tangent in low-frequency range is relatively large. In general, the dielectric loss tangent of RGO/Fe-2 is higher than that of Fe and RGO/Fe-1, and the magnetic loss tangent of RGO/Fe-2 is lower than that of Fe and RGO/Fe-1. RGO/Fe-1 dielectric loss tangent of the other band outside of the 4.9-5.5 GHz and 13.9-16.0 GHz are higher than Fe, and the magnetic loss tangent showing an alternate trend. 13



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Figure 4. Frequency dependence on (a) real and (b) imaginary parts of the complex permittivity of Fe nanoparticles and RGO/Fe nanocomposites, (c) real and (d) imaginary parts of the complex permeability.

Figure 5. The corresponding (a) dielectric and (b) magnetic loss tangents of Fe nanoparticles and RGO/Fe nanocomposites.

3.5. Microwave Absorbing Properties As was shown above, the introduction of RGO is beneficial to improve the 14


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dielectric loss tangent of nanomaterials. In order to evaluate and compare the microwave absorbing properties of nanomaterials, the reflection loss (RL) values of Fe and RGO/Fe are estimated by the complex permittivity and complex permeability varying with different frequencies and thicknesses according to the transmit line theory by the following equations: 30-32

Z = Z tanh j

√

RL = 20 log( |*Z + Z ,⁄*Z + Z ,|

(1) (2)

Where Z0 is the impedance of free space, Zin is the input impedance of the composites, εr is the complex permittivity, µr is the complex permeability, f is the microwave frequency, c is the velocity of light, and d is the thickness of the composites. The relative complex permeability and permittivity were tested by a network analyzer with a frequency in the 2–18 GHz range.

Figure 6. Calculated reflection loss of Fe nanoparticles and RGO/Fe nanocomposites.

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The

calculated

reflection

losses

for

Fe

nanoparticles

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and

RGO/Fe

nanocomposites synthesized by different thickness graphene are shown in Figure 6. Fe nanoparticles have a major absorption peak near 15.6 GHz. There is a slight fluctuation and a weak microwave absorption capacity in other frequency bands. The microwave absorbing properties of RGO/Fe-1 nanocomposites were significantly higher than those of Fe nanoparticles. RGO/Fe-1 nanocomposites had a maximum absorption peak near 6.5 GHz, and the maximum reflection loss can reach -20.5 dB. Microwave absorption is very weak in the frequency band below 5 GHz, it only has a certain degree of microwave absorption capacity above 8 GHz, but the reflection loss is not more than -10 dB. The reflection loss curve of RGO/Fe-2 is significantly different from that of Fe and RGO/Fe-1. The maximum absorption peak of RGO/Fe-2 is 5.6 GHz, and the maximum reflection loss is -36.5 dB. The reflection loss of RGO/Fe-2 is lower than -10 dB in the frequency range of 5.0-6.4 GHz, and its’ bandwidth reaches 1.4 GHz. For comparison, the microwave absorption properties of some graphene-based absorbing materials are summarized in Table 1.

Table 1. Microwave absorption properties of some graphene-based absorbing materials. Optimal RL

fm GHz

Frequency range

dm (mm)

value (dB)

(optimal RL)

(GHz) (RL

Enhanced Microwave Absorption Property of Fe Nanoparticles

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