Elsevier

Applied Surface Science

Volume 537, 30 January 2021, 147840
Applied Surface Science

Defect reconstructions in graphene for excellent broadband absorption properties with enhanced bandwidth

https://doi.org/10.1016/j.apsusc.2020.147840Get rights and content

Highlights

  • Surface defects and defect reconstructions on Graphene for broadband absorptions.

  • Graphene-polyurethane nanocomposite coating for scalable applications of broadband-RAM.

  • Increasing defect-reconstructions, absorption bandwidth increases with high absorption.

  • Free space impedance matching signifies high absorption bandwidth in GHz range.

Abstract

A rapid and controlled reduction method of graphene oxide (GO) along with their performances as broadband frequency absorbers is demonstrated in this report. The microwave-controlled rapid GO reduction technique was adopted to control the exfoliation, structural defects, and the localized electron concentrations while keeping the carbon network in graphene intact. The reduction technique is facile and scalable to produce gram-scale graphene with a controlled manner for preparing a sprayable, lightweight coating (coating thickness ~100–120 μm) throughout a large-area (~225 mm diagonal distances) substrate. The as-prepared coating was explored as active broadband frequency absorbing material in the 4–15 GHz frequency region. We obtained extraordinary absorption properties with an absorptivity average ~96% along with the successive broadening of the bandwidth of ~8.5 GHz throughout the absorption band. The retrieved real and imaginary parts of the surface impedance signify proper impedance matching with the free space; thereby providing high absorption at the aforementioned frequency band. The presence of residual oxygen-containing functional groups results in the differential ability to accumulate electrons, which generate the electric dipole under alternating electromagnetic field while electron hysteresis in these dipoles results in additional surface polarization, which effectively increases the microwave absorption potential of rGO compared to pristine GO.

Introduction

Significant advances in the use of electronic materials in communication devices experiencing electromagnetic interference, which has led researchers to broaden the studies of microwave absorption devices. Recently, a continuous search of superior radar absorbing materials (RAMs) and electromagnetic shielding devices have been performed to fulfill the increased demand of stealth technology and EMI protection [1], [2], [3]. A wide range of microwave absorbing materials are reported to date, however, ideal microwave absorbing materials should have wider absorption bandwidth with good absorption properties, ultrathin structures, and lightweight, for their use in aircraft and space technology [2], [4]. With decreasing thickness, the absorption properties of any thin-coating decrease and it is quite challenging to achieve all these properties in a single material or structure [5], [6], [7]. Materials with high bandwidth are often compromised in terms of thickness and weight since the number of layers of RAM, or the specific gravity of RAM has to be increased and unsuitable for a wide variety of practical applications. Traditional ferrites and ferrites based nanocomposites have been effective in achieving wide absorption properties, but a high density and poor corrosion resistance limit their applicability [5], [8]. On the contrary, a few metasurface-based absorbers have been proposed; however, either they are limited by narrow bandwidth or loaded by lumped elements, thereby making it difficult to realize in practice [9], [10]. Conventional absorbers such as dielectric absorbers show low dielectric permeability and high losses due to relaxation dispersion [11]. Hence, a variety of carbon nanomaterials, like carbon black, carbon fiber, graphene, and carbon nanotubes have been introduced as performance-enhanced lightweight microwave absorbers [12], [13], [14].

Graphene and graphene-based composites have been used for electromagnetic applications owing to its remarkable properties, including low density, very high specific surface area, exceptional carrier mobility, large aspect ratios, excellent thermal conductivity, and mechanical properties [15], [16], [17], [18], [19], [20]. Recently, the incorporation of graphene in the metasurface design finds application as a polarization converter, absorber, etc. [21], [22]. Reduced graphene oxide (rGO) is a form of graphene, which resembles graphene containing residual oxygen and other functional heteroatoms along with structural defects [23]. In specific, the reduction of graphene oxide partially recover the pure hexagonal 2D carbon framework due to the presence of residual functional moieties and defects such as missing carbon atoms and wrinkles [24]. As rGO reduces its electrical conductivity to the permittivity of rGO also decreases as per the free-electron theory, which improves rGO’s microwave absorption capabilities compared to pristine graphene. Due to the weak magnetism of graphitic materials, dielectric loss is the main mechanism of microwave absorption corresponding to the Debye relaxation [25]. Moreover, the partly re-constructed structure of rGO shows smaller Cole-Cole semicircles during the dielectric relaxation process owing to insufficient induced charges, which is due to the incomplete lattice structure or rGO [26]. Similarly, two additional relaxation processes in rGO became prominent due to the defects present in the lattice structure and the presence of additional functional groups [27], [28]. These induced defects in graphene act as polarization centers, which generate polarization relaxation under alternating electromagnetic field and enhances the microwave loss in the structure. The presence of residual oxygen-containing chemical bonds results in the differential ability to catch electrons, which gives rise to electric dipole polarization under alternating electromagnetic field where electron hysteresis in these dipoles results in additional polarization, which effectively increase the microwave absorption efficiency of rGO [28], [29]. In addition, because of microwave irradiation, the energy transfer occurred in rGO from its neighboring states to the Fermi level, which successively results in the improved absorbance of electromagnetic waves due to the elevated spin states at Fermi level originated due to the lattice defects [30].

In this work, we report the controlled reduction of GO using microwave reduction techniques where the extent of reduction of graphene was presented as a function of time. Various graphene-based thin coating was prepared to explore the broadband absorption properties of the coating on a metal substrate covering a broad microwave spectrum including C, X, and significant region of Ku bands. Also, we have demonstrated the broadband absorption properties of the graphene as a function of its degree of transformation from GO to rGO. This work will have potential many-fold applications in the stealth technology, radar absorbing material, electromagnetic shielding device, and radiation absorption for sensor technologies.

Section snippets

Synthesis of graphene oxide and rGO

Graphene oxide was synthesized by the improved method reported elsewhere [31]. The as-synthesized GO was taken in a glass crucible and heated in a microwave at 750 W for 5, 10, and 20 min to produce partially reduced graphene oxide. The samples were denoted as GO0, GO5, GO10, and GO20 where GO0 is pristine graphene oxide sample and GO5, GO10, and GO20 represents partially reduced GO for 5, 10, and 20 min, respectively.

X-Ray diffraction analysis

Structural and phase analysis of the graphene and reduced graphene oxide was

Results and discussions

Fig. 3a shows the comparative X-ray diffraction (XRD) data of the pristine GO and microwave treated GO with their characteristic’s peaks at the respective positions. The pristine GO shows the characteristics (0 0 2) peak at its corresponding 2θ position of 10.12° (Blue curve, Fig. 3a while the GO5, GO10 and GO20 samples (i.e., the various microwave treated GO samples with MW treatment time of 5 min, 10 min, and 20 min, respectively) show their (0 0 2) peak positions at respective 2θ positions of

Conclusions

In conclusion, we have developed a rapid and controlled microwave technique of GO reduction and demonstrated its efficacy as broadband frequency absorber materials in the 4–15 GHz frequency region. The as-produced rGO exhibits mono to few layers with wrinkles and localized surface defects, which causes the enhancement in surface polarization domains of GO to increase the microwave absorption and the broadening of the absorbing band. The XRD results show that with increasing MW treatment time,

CRediT authorship contribution statement

Rajarshi Bhattacharyya: Data curation, Writing - original draft, Formal analysis. Vivek Kumar Singh: Data curation, Formal analysis, Writing - original draft. Somak Bhattacharyya: Writing - review & editing, Methodology, Data curation, Formal analysis. Pralay Maiti: Writing - review & editing. Santanu Das: Conceptualization, Methodology, Investigation, Supervision, Writing - review & editing, Formal analysis, Funding acquisition, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Dr. Das acknowledge the STAR project from Ministry of Human Resource and Development (MHRD), Govt. of India (Grant No: STARS/APR2019/NS/428) for providing financial support for this work. Dr. Das also acknowledges ECR project from SERB (ECR/2016/001112) for a partial support for this work. Authors are highly thankful to the in-charge, Department of Physics, Banaras Hindu University for extending their Raman Spectroscopy measurement facilities for this work. SB gratefully acknowledge SERB

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