Using organic solvent absorption as a self-assembly method to synthesize three-dimensional (3D) reduced graphene oxide (RGO)/poly(3,4-ethylenedioxythiophene) (PEDOT) architecture and its electromagnetic absorption properties

Abstract

A novel self-assembling 3D-RGO/PEDOT architecture has been synthesized through organic solvent absorption and gentle heating. It gives a new way to obtain RGO-based 3D materials, and the results also indicate that this 3D-RGO/PEDOT architecture has a promising application in the area of electromagnetic absorption.

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Electromagnetic pollution poses a great challenge to modern society. Electromagnetic interference (EMI) has harmful effects on electronic devices as well as on organisms; therefore, shielding materials are being developed to protect electronic devices and human beings from this hazard. A good shielding material should prevent both incoming and outgoing EMI;¹ therefore, electromagnetic absorption is more important than electromagnetic reflection during the development of this shielding material. Conventional electromagnetic absorption materials are based on metal oxides, especially the ferrites. However, they only have good electromagnetic absorption performance with large content ratio. Being lightweight is one of the most important technical requirements for the development of effective and practical electromagnetic absorption applications, especially in the area of stealth aircraft manufacturing, aerospace engineering, and in the production of fast-growing next-generation flexible electronics. An ideal electromagnetic absorption material should exhibit low density, reduced thickness, strong absorption and it should possess a broad bandwidth.² In recent years, several electromagnetic absorbers have been devised based on dielectric materials such as CuS,^3,4 Bi₂S₃,⁵ intercalated graphite,⁶ α-MnO₂,^7,8 and ZnO nanorods.⁹ The efficient nano-structure and low content ratio result into an optimal electromagnetic absorption performance (less than 20 wt%).

Reduced graphene oxide (RGO) is a kind of two dimensional (2D) carbon material that can be synthesized from graphene oxide (GO) using chemical,¹⁰ thermal,¹¹ optical,^12,13 and hydrothermal¹⁴ methods. Because of the intense interest in graphene all over the world in the last decade (2004–2014), RGO has also succeeded in attracting researchers' attention because its carbon skeleton structure is highly similar to that of pure graphene. Although RGO does not exhibit the excellent electronic,¹⁵ thermal,¹⁶ mechanical¹⁷ and optical¹⁸ properties like graphene, it is easy to synthesize, gives high yield, and can be modified easily. The dielectric loss and low density of RGO enables it to be used as an electromagnetic absorption material. Using a suitable preparation process, pure RGO can display good electromagnetic absorption performance.¹⁹ In order to further enhance its absorption properties, macro- or nano-particles, such as Fe₂O₃,^20,21 Fe₃O₄,^22,23 hematite,²⁴ Co₃O₄,²⁵ MnFe₂O₄,²⁶ polyaniline,^27,28 and carbon nanotubes, were added or crystallized onto the RGO surfaces.²⁹ Furthermore, the core–shell structure, for example, Fe₃O₄@ZnO,³⁰ SiO₂@Fe₃O₄,³¹ and Fe₃O₄@Fe,³² can also improve the absorption property. Moreover, recently, polyvinylidene fluoride (PVDF) has been widely used as a matrix in electromagnetic absorbers. This polymer possesses a high operating temperature and dielectric strength. The high permittivity of PVDF helps to achieve a uniform distribution of the local electric field in the filled materials, and simultaneously results into a lower dispersion of the complex permittivity in the required frequency range.³ According to a recent study, PVDF-based composites exhibit better electromagnetic absorption performance compared to the paraffin matrix.^4,5,9,19

Poly(3,4-ethylenedioxythiophene) (PEDOT), a conducting polymer, can be used as an electromagnetic absorber because of its high electrical conductivity.³³ Crystalline 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) can be easily synthesized through a facile halogenation approach (N-bromosuccinimide, NBS)^34,35 and gentle heating (50–70 °C) to afford a highly conductive solid-state polymerization (SSP) of bromine-doped poly(3,4-ethylenedioxythiophene).³⁴ An excellent electromagnetic absorption performance was observed when mixed with paraffin with the weight ratio of 1 : 1.

In this study, the unique sorbent property of hydrothermal 3D RGO for organic solvents was used as a self-assembly process to build the 3D-RGO/PEDOT architecture. Compared with former studies, the new architecture makes complete use of 3D-RGO's pores. Note that a DBEDOT solution in chloroform (CHCl₃) was easily absorbed into 3D RGO, and after gentle heating, DBEDOT was polymerized in these pores rather than on the carbon skeleton surface (Fig. 1). Fig. 2a and b shows that an 3D-RGO with 2 mm of length can put on a dandelion, it imply that this 3D architecture is light enough, and it contributes to the pores that were formed during the hydrothermal process. As revealed by field emission scanning electron microscopy (FE-SEM), the pore sizes of the 3D architecture are in the range of submicrometer to several micrometers (Fig. 2c and d). The hydrophobicity determined from the contact angle measurements performed on the 3D-RGO and on the air interface revealed a contact angle of 113.5° (Fig. 2e). However, organic solvents such as CHCl₃ were quickly absorbed. Because of the physical adsorption process, when the 3D-RGO was added into the solution, the solvent and solute would both get absorbed into the 3D-RGO architecture. When the architecture was heated up to 70 °C, which was little higher than the boiling point of CHCl₃ (61.3 °C), the liquid gets evaporated and recollected elsewhere, and the DBEDOT monomer in the form of an SSP would be obtained in the 3D-RGO pores. Fig. 2f and g show the evidences of this phenomenon. The specific surface area decreased from 276 m² g⁻¹ of 3D-RGO to 29 m² g⁻¹ of 3D-RGO/PEDOT, and this result indicates that the pores had been crammed by the SSP PEDOT. Because of the hydrophobic property of PEDOT, the self-assembling 3D-RGO/PEDOT expresses a higher contact angle (123.4°) than that of the 3D-RGO under the same testing condition (Fig. 2h).

Fig. 1 Synthesis strategy for 3D RGO/PEDOT.

Fig. 2 (a and b) Optical image of 3D-RGO. (c and d) FE-SEM of 3D-RGO. (e) Contact angle of 3D-RGO (113.5°). (f and g) FE-SEM of 3D-RGO/PEDOT. (h) Contact angle of 3D-RGO/PEDOT (123.4°).

The electromagnetic absorption properties were tested by uniformly mixing 10 wt% of 3D-RGO/PEDOT with a paraffin matrix under coaxial wire analysis. The thickness of the sample is an important parameter, which is related to the intensity and the position, as well as the frequency range of the electromagnetic energy absorption. As shown in Fig. 3, with growth in the sample thickness, the absorbing peaks shift to lower frequencies. There are apparent absorbing ranges deeper than −10 dB (which means it can yield a 90% of microwave attenuation) under different thicknesses. When the sample thickness reaches 2 mm, it not only shows the maximum absorption value (−35.5 dB), but also shows the widest bandwidth with a reflection loss (RL) deeper than −10 dB, which is nearly 5 GHz (from 11.5 to 16.5 GHz). These results show that the self-assembling 3D-RGO/PEDOT possesses good electromagnetic absorption capabilities in both low- and high-frequency bands under different thicknesses with a low content ratio of 3D-RGO/PEDOT in composite.

Fig. 3 Reflection loss curves for samples of 3D-RGO/PEDOT with different thicknesses (2.0 to 4.0 mm) in the frequency range of 2–18 GHz.

Conclusions

In summary, we have presented a simple method to synthesize a 3D RGO/PEDOT using organic solvent absorption as a self-assembly method. It gives a new synthesis strategy to obtain 3D RGO composites with low cost and high yield. The 3D RGO/PEDOT architecture shows excellent electromagnetic absorption properties in low contents and thickness situation. It can have promising applications in military camouflage and the protection of electronic devices. In addition, this architecture also warrants other novel applications, e.g., it can serve as an electrode in dye-sensitized solar cells (DSSCs) or supercapacitors.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51403236, 51021001) and the Opening Project of State Key Laboratory of Disaster Prevention & Mitigation of Explosion & Impact (DPMEIKF201310).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra08717h

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