Controllable fabrication of elastomeric and porous graphene films with superior foldable behavior and excellent electromagnetic interference shielding performance
Graphical abstract
Introduction
Currently, high-performance electromagnetic interference (EMI) shielding materials are urgently needed to avoid the increasingly serious radiation pollution created by the explosive use of electronic devices, which has detrimental impacts on human health and sensitive electronic equipment and systems [1]. In particular, the explosive growth of portable devices and smart wearable electronics has led to more stringent requirements for lighter, thinner, more flexible and even foldable EMI shielding materials with higher shielding efficiency [2,3]. However, most of the present materials cannot simultaneously integrate these intriguing characteristics [4]. Macroscopic layered graphene films (GFs) tightly and orderly assembled by two-dimensional nanometer-scale graphene sheets are highly promising for excellent EMI shielding performance by taking full advantage of graphene’s superior electrical property, and they are considered a candidate to replace heavy and easily-corroded metal-based shielding materials [[5], [6], [7]]. As pristine graphene can only be dispersed in a few choice solvents at extremely low concentrations (<1 mg ml−1) [8], graphene oxide (GO) has been used as a reliable chemical functionalized graphene with wide utilization for constructing GFs due to its good solubility in water. However, the electrical conductivity of GO is seriously reduced because of its disrupted sp2 structures. To improve the electrical conductivity of constructed GO film and in turn achieve higher EMI shielding effectiveness (SE) of the resulting GFs, ultrahigh-temperature annealing (over 2000 °C) for graphitization is commonly used to repair the crystal defects of GO nanosheets (e.g., an 8.4 μm-thick graphene film obtained 20 dB after 2000 °C annealing [5], a 13 μm-thick film obtained 48.9 dB after 2800 °C annealing [9], and 8.8 μm-thick GFs achieved 37 dB after 3000 °C annealing [10]). Nevertheless, such harsh conditions not only tremendously increase the processing cost, time and energy consumption and make mass production infeasible but also inevitably reduce the mechanical properties of GFs [[11], [12], [13]].
Elastic GFs, especially those that allow reversible large-strain deformation, are highly desirable for applications in foldable and wearable devices [14]. Note that the introduction of structural hierarchy and porous architecture can significantly improve material utilization with further enlarged elastic strain range and ductility [15]. Coincidentally, the foaming strategy is undoubtedly an effective way to decrease the weight or density of materials, and the resultant foam structure in GFs is beneficial to improve its shielding efficiency and microwave absorption property [3,16,17]. Therefore, the foaming of compact layered GFs into porous graphene films (PGFs) is an efficient strategy to combine excellent elastic and lightweight properties with remarkable EMI shielding into one material. Recently, graphene aerogel film was developed by 3000 °C self-expansion of reduced graphene film and exhibited a high EMI SE of over 60 dB [4]. However, the pores generated in structure were in a nonuniform distribution and uncontrollable, and the flexibility of these films was not satisfactory for the diverse purposes of foldable and wearable devices. Furthermore, such extremely high temperatures are inevitably accompanied by flexibility deterioration, and this unpleasant ultrahigh temperature treatment is extremely energy-consuming, which limits the industrial use of the films. To the best of our knowledge, few papers have focused on the fabrication of elastic PGFs and have considered their EMI shielding performance. Thus far, although many great improvements in graphene-based shielding materials have been achieved, the daunting challenge of the facile and controllable fabrication of fully flexible or foldable, ultrathin, lightweight, and porous graphene films with outstanding EMI shielding performance remains unresolved [18].
Herein, we demonstrate the controllable fabrication of thin, lightweight, elastic and porous graphene films by assembling GO nanosheets into films followed by a simple leavening-accompanied reduction with a spatial confinement strategy. We used a confined space to constrain the expanding films to a desired and uniform thickness. In contrast to conventional unconstrained foamed GFs (brittle), our uniformly expanded PGFs with engineered porosity exhibit excellent elastomeric behavior and strong 1000-cycle repeated folding stability. Such PGFs present unchanged flexibility even after high-temperature annealing (1000 °C) and ultralow liquid nitrogen temperature (−196 °C) treatment. More importantly, a greatly enhanced EMI-shielding performance is achieved for the porous GFs compared to their unfoamed compact film counterpart reduced by HI because of the favorable porous network. Also, the EMI shielding behavior and its enhancement mechanism of the foam structure are elucidated.
Section snippets
Results and discussion
The methodology and mechanism of controllably fabricating PGFs is schematically illustrated in Fig. 1a. The nacre-like GO layered film is prepared by evaporating an aqueous suspension of GO at room temperature, and then it is confined between two parallel glass plates with a gap formed by placing spacers of defined thickness between the plates. A small amount of hydrazine as the reducing agent is precoated on the surface of the bottom glass plate. The two plates are then clamped together to
Conclusions
Herein, a graphene film with engineered porosity is prepared by a simple leavening-accompanied reduction treatment of its precursor GO films. This treatment is found to introduce an abundance of homogeneous microvoids between the graphene layers. In our method, by confining the GO film between two glass plates, the film experiences a highly controlled expansion in thickness. The resultant porous graphene films can be folded and even crumpled but spring back to their original shape without
Experimental section
The experimental details and characterizations are provided in the Supporting Information.
Notes
The authors declare no competing financial.
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.
Acknowledgments
This work is supported by the China-Japan Research Cooperative Program (2016YFE0118000), the Industry Leading Key Projects of Fujian Province (2019H0056), the National Natural Science Foundation of China (21806163) and the “Strategic Priority Research Program (A)" of the Chinese Academy of Sciences (XDA23030301).
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These authors contributed equally to this work.