Room temperature fabrication of an RGO–Fe₃O₄ composite hydrogel and its excellent wave absorption properties

Abstract

As a result of their lightweight properties and high dielectric loss, graphene and their composites have attracted great attention for potential applications in wave absorption. Herein, we report room temperature conditions for the synthesis of a 3D composite hydrogel composed of reduced graphene oxide nanosheets and Fe₃O₄ nanoparticles (RGO–Fe₃O₄). The experimental results show that the composite has an interconnected 3D porous network with micrometer-sized pores, and that the Fe₃O₄ nanoparticles with a small size of about 5–10 nm are uniformly dispersed onto the thin graphene nanosheets. The as-prepared RGO–Fe₃O₄ composite hydrogel shows excellent microwave absorbability compared with previously reported nanocomposites based on graphene and Fe₃O₄. The obtained composite with a coating layer thickness of only 2.5 mm exhibits a maximum absorption of −47.9 dB at 10.1 GHz. In particular, the product with a coating layer thickness of only 2.0 mm shows a bandwidth of 5.3 GHz (from frequency of 11.3–16.6 GHz) corresponding to reflection loss at −10 dB (90% absorption). Additionally, the fabrication method is simple, low cost and easily done on a large scale. This further confirms that nanoscale Fe₃O₄ particles on graphene networks give the composite hydrogel the ability to realize practical applications for wave absorption.

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Introduction

Recently, severe electromagnetic (EM) radiation has been generated by the increasing use of wireless communication tools, local area networks, personal digital assistants, and so on. The rapid development of EM wave devices produces a highly harmful living environment for human beings and problems with regards to their military applications.^1,2 Thus, great effort has been made to achieve the preparation of high performance EM wave absorption materials. Besides strong absorption abilities, a wide absorption range, lightweight properties, good thermal stability and antioxidation capability are other important requirements for ideal EM wave absorption materials.^3,4 For this reason, graphene is a promising candidate because of its low density and high complex permittivity values. Nevertheless, pure graphene has very weak EM wave absorption properties.^5,6 The main problems are firstly, that graphene alone is non-magnetic, and so its contribution to microwave energy absorption involves dielectric loss but no magnetic loss. Secondly, with the poor balance between dielectric permittivity and magnetic permeability, graphene has bad impedance match characteristics relating to microwave absorption.²

In order to solve the above problems, many researchers have synthesized magnetic Fe₃O₄ nanoparticles coupled with graphene to form graphene based magnetic composites. Shi et al.⁷ synthesized a composite of reduced graphene oxide (r-GO) coated with Fe₃O₄, through a facile method involving the decomposition of Fe(OH)₃ in an argon atmosphere and the reduction of graphene oxide in a hydrogen and argon mixed atmosphere. The composite demonstrated a reflectivity loss (RL) below −10 dB of 3.7 GHz, and a maximum absorption of −22.2 dB. Bi et al.³ prepared a novel kind of bowl-like hollow Fe₃O₄–r-GO composite, exhibiting a maximum absorption of −24 dB. He et al.² reported a facile solvothermal route to prepare laminated graphene oxide–Fe₃O₄ composites. The bandwidth of the as-prepared composite corresponding to the RL below −10 dB was 2 GHz and the maximum absorption was −26.4 dB with a coating layer thickness of 4.0 mm. Li et al.⁸ demonstrated a simple polyol method to fabricate a composite containing spinel Fe₃O₄ and graphene, for which the smallest RL reached −30 dB. Wang et al.⁹ prepared graphene–Fe₃O₄ nanohybrids by first depositing β-FeOOH crystals and then converting them to Fe₃O₄ on the surface of the graphene sheets; the maximum absorption was up to −40.36 dB with a thickness of 5.0 mm. Huang et al.¹⁰ synthesized RGO coated with Fe₃O₄ through a facile one-pot simplified coprecipitation method, and the composite showed a maximum absorption of −44.6 dB with a thickness of 3.9 mm. However, the above researchers were confined to studying the microwave absorption properties of composites of two-dimensional (2D) graphene and Fe₃O₄. Macroscopic three-dimensional (3D) graphene structures such as foams, gels, and other networks have recently attracted extensive attention because of the desire to realize their practical applications.^11–17 3D graphene has a porous structure and better mechanical stability, so it is a promising candidate for the construction of lightweight materials. Nevertheless, there have been few reports on studying the microwave absorption properties of 3D graphene-based composites. We¹⁸ have fabricated a macroscopic rGO–α-Fe₂O₃ composite hydrogel with 3D interconnected networks by a hydrothermal process. The bandwidth of the as-prepared composite corresponding to an RL below −10 dB was 7.12 GHz, and the maximum absorption was −33.5 dB. Qu et al.¹⁹ also prepared 3D graphene–Fe₃O₄ composites by a hydrothermal method. However, the maximum absorption of the composite was −27.0 dB, which was lower than some of the 2D graphene–Fe₃O₄ composites.

In this paper, an RGO–Fe₃O₄ composite hydrogel with 3D interconnected networks is reported. Compared with Qu's work, the first difference is that we fabricate the composite at room temperature, so our method is simple, low cost and easily done on a large scale. Secondly, we use Fe₃O₄ sol, not Fe³⁺, as a precursor, because the nanoscale Fe₃O₄ nanoparticles can be homogeneously dispersed in the graphene oxide (GO) aqueous suspension and can be easily embedded into the graphene network.¹⁵ Thirdly, the diameters of the Fe₃O₄ nanoparticles in the composite are about 5–10 nm, which is much smaller than used in Qu's work. Most important of all, the maximum absorption of our RGO–Fe₃O₄ composite hydrogel is −47.9 dB, which is superior to all nanocomposites based on graphene and Fe₃O₄ reported before. Moreover, the amount of the composite hydrogel added into the paraffin matrix is only 8 wt%, which establishes a lightweight system for wave absorption. Thus, the as-prepared RGO–Fe₃O₄ composite hydrogel is very promising as a lightweight and high-performance EM wave absorbing material.

Experimental section

Materials

Graphite powder (325 mesh) was purchased from Qingdao Huatai Lubricant Sealing S&T Co. Ltd. Concentrated sulfuric acid (H₂SO₄), hydrochloric acid (HCl), potassium permanganate (KMnO₄), sodium nitrate (NaNO₃), hydrogen peroxide (H₂O₂) and sodium hydroxide (NaOH) were obtained from Chemical Shanghai Reagent Co. Ferric trichloride (FeCl₃·6H₂O), ferrous chloride (FeCl₂·4H₂O) and ascorbic acid (C₆H₈O₆) were purchased from Aladdin Chemical Reagent Co. All the reagents used for the experiments were of analytical grade and used directly without further purification. De-ionized (DI) water was used in all aqueous solution preparation and washing.

Preparation

Graphite oxide was synthesized from natural graphite powder using a modified Hummers' method.^20,21 To prepare suspensions of GO, the graphite oxide was bath sonicated (using a KQ 800KDV, Kunshan, China) in water for 1 h to give a brown colloidal solution, and then centrifuged (using a TG16-WS, Changsha, China) at 4000 rpm for 20 min to remove any unexfoliated materials.

The cationic Fe₃O₄ nanoparticles were prepared by the method used in ref. 15 and 22. In detail, 5.2 g of FeCl₃·6H₂O (32.0 mmol) and 3.18 g of FeCl₂·4H₂O (16.0 mmol) were dissolved in 25 mL of ultrapure water with 0.86 mL of HCl, and then the mixture was added dropwise into 250 mL of 1.5 M NaOH aqueous solution under vigorous stirring. A black precipitation was obtained by centrifugation at 6000 rpm (3 min), and was washed with ultrapure water 3 times. Then, 250 mL of 0.01 M HCl solution was added to neutralize the anionic charges on the nanoparticles. The cationic Fe₃O₄ nanoparticles were collected by centrifugation at 10 000 rpm (10 min) and then freeze dried for future use. The positively charged Fe₃O₄ nanoparticles could easily interact with the negatively charged GO. Therefore, they could be homogeneously dispersed into the GO aqueous suspension.

The RGO–Fe₃O₄ composite hydrogels were synthesized at room temperature. In detail, different amounts of Fe₃O₄ nanoparticles were dispersed into 4 mg mL⁻¹ GO solution with vigorous stirring. Then we added 20 mg mL⁻¹ ascorbic acid as a reductant to the above mixture. The mixture was sealed in a 50 mL beaker using plastic films and kept at room temperature (about 20 °C) for one week. The as-prepared hydrogels were taken out with a tweezer and freeze dried under a vacuum. Three composites with weight ratios of GO to Fe₃O₄ nanoparticles of 4 : 5 (sample 1), 2 : 5 (sample 2) and 4 : 15 (sample 3) were prepared. The corresponding as-synthesized products were named as product 1, product 2 and product 3, respectively.

Characterization

The as-synthesized GO, pure graphene oxide (GH), pure Fe₃O₄ nanoparticles and RGO–Fe₃O₄ composite hydrogels were characterized by X-ray diffraction (XRD) using a DX-2700 X-ray diffractometer equipped with a Cu Kα sealed tube (λ = 1.5406 Å). The samples were scanned in a range between 8° and 80° with a step size of 0.02°. Scanning electron microscopy (SEM) images were performed on a Hitachi SU1510 scanning electron microscope with energy-dispersive X-ray (EDX) analysis (INCA x-ACT). Raman spectroscopy was performed on an inVia-Reflex Raman Microscope equipped with a 532 nm laser in a range of 100–2000 cm⁻¹. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB-MKII spectrometer (VG Co., UK) with Al Kr X-ray radiation as the X-ray source for excitation. Transmission electron microscopy (TEM) images on a Cu grid were obtained using a JEM 2100 microscope and an accelerating voltage of 100 kV. The thermogravimetric (TG) analysis of the composite hydrogels was performed on a Q2000 thermogravimetric analyzer at a heating rate of 10 °C min⁻¹ in air.

Electromagnetic measurements

The electromagnetic parameters of the samples (pure GH, RGO–Fe₃O₄ composite hydrogels) were measured in a VNA, AV3629D vector network analyzer in a range of 1–18 GHz after a full two-port calibration (SHORT-OPEN-LOAD-THRU). The measured samples were prepared by uniformly mixing 8 wt% of the sample with a paraffin matrix. The mixture was then pressed into toroidal shaped samples with an outer diameter of 7.00 mm and inner diameter of 3.04 mm.

Results and discussions

Fig. 1a shows the XRD patterns of GO, pure GH, pure Fe₃O₄ nanoparticles, and the resulting composite hydrogels of RGO–Fe₃O₄ from product 1 to product 3. It can be seen that the feature diffraction peak of GO appears at 10.51 (002) representing the AB stacking order with a layer-to-layer distance (d-spacing) of 0.849 nm.²³ Meanwhile, the pure GH shows a very broad diffraction peak at 2θ of ca. 25.0°, which means that the GO has been transformed to reduced GO.²⁴ The peak positions of the RGO–Fe₃O₄ composite hydrogels (product 1 to product 3) are matched well with the Fe₃O₄ nanoparticles; eight peaks at 2θ = 18.5°, 30.2°, 35.5°, 43.3°, 53.6°, 57.2°, 62.9° and 74.6° are assigned to reflections from the (111), (220), (311), (400), (422), (511), (440) and (533) crystal planes (JCPDS no. 01-076-5337), respectively. Notably, a diffraction hump around 20°–30° appears in the composite hydrogels, indicating a relatively low diffraction intensity for the reduced GH in the RGO–Fe₃O₄ composite hydrogel. Fig. 1b shows the Raman spectra of the GO and RGO–Fe₃O₄ composite hydrogel (product 2). The Raman peaks of the original GO at 1594 cm⁻¹ (G band) is due to the presence of isolated double bonds, and the D band at 1351 cm⁻¹ indicates the reduction in size of the in-plane sp² domains due to extensive oxidation. The Raman spectrum of the RGO–Fe₃O₄ composite hydrogel also exhibits the presence of D and G bands. The increase in the D/G intensity ratio indicates a decrease in the size of the in-plane sp² domains and a partially ordered crystal structure of the reduced GO.^25,26 Notably, the characteristic Raman peak of the Fe₃O₄ nanoparticles appears at around 675 cm⁻¹ as shown inset in Fig. 1b.¹⁵

Fig. 1 (a) XRD patterns of GO (1), pure GH (2), pure Fe₃O₄ nanoparticles (3) and the RGO–Fe₃O₄ composite hydrogels (4–6 corresponding to product 1–product 3); (b) Raman spectra of GO and the RGO–Fe₃O₄ composite hydrogel (product 2).

The elemental components of the RGO–Fe₃O₄ composite hydrogel were identified by XPS. Fig. 2a shows the general XPS survey for the RGO–Fe₃O₄ composite hydrogel. It reveals that the composite hydrogel is completely composed of three elements, Fe, O and C. No other elemental signals are detected in the general XPS spectrum. The strong C1s peak displayed arises from the graphene in the sample. In Fig. 2c, two peaks at 711.2 and 724.8 eV can be observed, which are the characteristic peaks of Fe 2p3/2 and Fe 2p1/2 for Fe₃O₄.²⁷ Fig. 2d shows the C1s region of the pure GO. Here four different peaks centered at 284.8, 286.8, 287.6, and 289.4 eV are observed, corresponding to C–C in aromatic rings, C–O, C=O, and C–C=O groups, respectively. After the composite hydrogel of RGO and Fe₃O₄ is formed, the peak intensity of C–O decreases dramatically (Fig. 2b) indicating that most of the GO is reduced. The XPS surveys of product 1 and product 3 are similar to product 2.

Fig. 2 (a) XPS survey, (b) C1s region, (c) Fe2p region of the RGO–Fe₃O₄ composite hydrogel (product 2) and (d) C1s region of GO.

The SEM image of the RGO–Fe₃O₄ composite hydrogel is shown in Fig. 3a. From it, we can see that the composite hydrogel has an interconnected three-dimensional porous network with micrometer-sized pores. We couldn't find any Fe₃O₄ nanoparticles on the graphene surface, because the diameters of the Fe₃O₄ nanoparticles are very small (5–10 nm). In order to discover the composition of the composite hydrogel again, we performed EDX analysis (Fig. 3b). The result suggested that only three elements (C, O and Fe) are present in the hydrogel. Fig. 3c shows the TEM images of Fe₃O₄ nanoparticles on the surface of the graphene wall inside the composite hydrogel. We found that the nanoparticles are uniformly dispersed onto the thin graphene nanosheets, and no apparent aggregates appear. Fig. 3d and e show the magnified TEM images of the composite hydrogel. Compared with the TEM image of the pure Fe₃O₄ nanoparticles (Fig. 3f), the morphologies of the nanoparticles in the composite hydrogel are very similar to pure Fe₃O₄ nanoparticles. The morphology and composition of product 1 and product 3 are similar to product 2.

Fig. 3 (a) SEM image, (b) EDX analysis and (c–e) TEM images of the RGO–Fe₃O₄ composite hydrogel (product 2) and (f) TEM image of pure Fe₃O₄ nanoparticles.

We used TG analysis to clarify the weight ratio of Fe₃O₄ nanoparticles in the three as-prepared composite hydrogels (product 1 to product 3); the results are shown in Fig. 4. The weight loss process can be divided into two processes. The slight weight loss below 150 °C is ascribed to the loss of absorbed water from the product. Then, a significant weight loss occurs between 150 °C and 500 °C, indicating the removal of non-reduced oxygen-containing functional groups and the pyrolysis of graphene. We also draw a conclusion that the mass loading of Fe₃O₄ nanoparticles in the composite hydrogels is about 16.5 wt%, 27.2 wt% and 39.8 wt% in product 1, product 2 and product 3, respectively.

Fig. 4 TG analyses of the RGO–Fe₃O₄ composite hydrogels (product 1 to product 3) measured from 50 to 700 °C at a heating rate of 10 °C min⁻¹ in air.

We investigated the electromagnetic parameters (complex permittivity and permeability) of the RGO–Fe₃O₄ composite hydrogels to reveal their microwave absorbing properties, as shown in Fig. 5a–d. Fig. 5a and b show the real part (ε′) and imaginary part (ε′′) of the complex permittivity of pure GH and its composites in the frequency range of 1–18 GHz. The ε′ and ε′′ values of pure GH and the RGO–Fe₃O₄ composite hydrogels decrease with increasing frequency at 1–18 GHz, which may be related to a resonance behavior that has been reported before.^28–30 The composites have much lower ε′ and ε′′ than pure GH. With the increasing content of Fe₃O₄ nanoparticles, lower ε′ and ε′′ are achieved because Fe₃O₄ is a kind of semiconductor material, which leads to the conductivity of the RGO–Fe₃O₄ composite hydrogels decreasing. According to the free electron theory, low conductivity would result in low permittivity.^5,30,31 We calculated the dielectric tangent loss (tan δ_E = ε′′/ε′) based on the permittivity of the samples measured above, as shown in Fig. 5e. We found that the samples with higher (RGO)/(Fe₃O₄) ratios showed higher dielectric tangent losses. Does this mean that the samples possess higher wave absorption abilities when the (RGO)/(Fe₃O₄) ratios are higher? Apparently, the answer is no. Besides the dielectric loss, impending match and magnetic loss are other factors which influence the wave absorption. Therefore, we demonstrate the real part of permeability (μ′), imaginary part of permeability (μ′′) and the calculated magnetic tangent loss (tan δ_M = μ′′/μ′) of pure GH and its composites in the frequency range of 1–18 GHz, as shown in Fig. 5c, d and f, respectively. It is seen that the μ′ and μ′′ are increased by the introduction of Fe₃O₄ nanoparticles. For the magnetic tangent loss, that of the pure GH is lower than for the composite hydrogels, indicating the magnetic loss in the composites. However, for the pure GH, the tan δ_E is much higher than tan δ_M. This can easily cause a bad impedance match, which is harmful to wave absorption.

Fig. 5 Frequency dependence of (a) real and (b) imaginary parts of the complex permittivity, (c) real and (d) imaginary parts of the complex permeability, the corresponding (e) dielectric and (f) magnetic loss tangents of pure GH and the RGO–Fe₃O₄ composite hydrogels (product 1 to product 3).

To reveal the microwave absorption properties of the composites, the reflection loss (RL) values were calculated according to the transmission line theory as follows:

Z_in = Z₀(μ_r/ε_r)^1/2tanh[j(2πfd/c)(μ_rε_r)^1/2] (1)

RL = 20 log|(Z_in − Z₀)/(Z_in + Z₀)| (2)

where Z_in is the input impedance of the absorber, and Z₀ is the intrinsic impedance of free space. μ_r and ε_r are the relative complex permeability and permittivity of the absorber medium, f is the frequency of electromagnetic wave, d is the coating thickness, and c is the velocity of light. The calculated results are shown in Fig. 6. For pure GH, the RL values are more than −8 dB with a thickness of 2–5 mm over 1–18 GHz. This demonstrates that pure GH has a very weak ability to absorb EM waves. The bad impedance match is an important reason for this phenomenon. However, the EM absorption properties of RGO–Fe₃O₄ composite hydrogels are significantly enhanced. It can be clearly seen that when the Fe₃O₄ nanoparticles’ weight ratio is 27.2 wt% with a coating layer thickness of only 2.5 mm they exhibit a maximum absorption of −47.9 dB at 10.1 GHz. The bandwidth of RL values below −10 dB (90% of EM wave absorption) is 5.3 GHz in the range of 11.3–16.6 GHz with a weight ratio for the Fe₃O₄ nanoparticles of 16.5 wt%. The enhanced wave absorption properties of the RGO–Fe₃O₄ composite hydrogels are contributed to the compensatory properties of graphene and Fe₃O₄ nanoparticles. But when the weight ratio of Fe₃O₄ is too high, the wave absorption properties are decreased, as shown in Fig. 6d. Maybe permittivity is reduced too much when more Fe₃O₄ nanoparticles introduced, which caused too small dielectric loss.

Fig. 6 Reflection loss curves for the (a) pure GH, (b–d) the RGO–Fe₃O₄ composite hydrogel with Fe₃O₄ nanoparticles weight ratio is 16.5 wt%, 27.2 wt% and 39.8 wt% with different thicknesses in the frequency range of 1–18 GHz, respectively.

Conclusions

In summary, a 3D RGO–Fe₃O₄ composite hydrogel with prominently enhanced microwave absorption properties has been successfully synthesized at room temperature conditions. The formation of Fe₃O₄ nanoparticles in the 3D framework of GH results in the decrease in permittivity and the increase in permeability of the composite hydrogels. Therefore not only a larger wave absorption value, but also a wider absorption band in the frequency range of 1–18 GHz has been obtained. The reason for this originates from the dielectric loss, magnetic loss and impedance match characteristics of microwave absorption. Additionally, the main body of the composite hydrogels is reduced graphene oxide with a porous network and has the advantage of low density. Therefore, the as-prepared 3D RGO–Fe₃O₄ composite hydrogels are a great potential EM wave absorbing material with lightweight properties and a high efficiency for practical applications.

Acknowledgements

This work is supported by The National Nature Science Foundation of China (91022032, 21171001 and 21173001), the Science Foundation for Excellent Youth Scholars of Higher Education of Anhui Province (2012SQRL011) and the Nature Science Foundation of Anhui Province (1308085QB43).

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