Regular ArticleTailoring conductive network nanostructures of ZIF-derived cobalt-decorated N-doped graphene/carbon nanotubes for microwave absorption applications
Graphical abstract
Introduction
Irradiated electromagnetic pollution problems have aroused widespread attention accompanied by the advancement and evolution of electronic apparatuses. Radioactive electromagnetic energy plagues humanity by obstructing precise operation in various fields ranging from industrial manufacturing to aerospace equipment, and may threaten the health of the human body [1], [2], [3]. It is generally insufficient to focus on the absorption intensity and effective absorption frequency bandwidth but neglect the application environment. Thin, lightweight, corrosion-resistant, easily-assembled, and upscaled production are also important considerations in developing next-generation microwave absorption devices.
Microwave absorption materials can be classified as dielectric loss materials and magnetic loss materials according to their working mechanism [4], [5]. Amounts of researchers have chosen carbon-based materials [6], [7], [8], [9], such as graphene and carbon nanotubes (CNTs), rather than ceramics [10], [11] or conductive polymers [12], [13], [14], [15], for their superior advantages including high electrical conductivity, low density, corrosion resistance, high carrier concentration, and adjustable nanostructures [16], [17], [18]. Magnetic loss behaviours can be mainly obtained by ferromagnetic particles (Fe, Co, Ni) and their alloy compounds [19], [20], [21]. However, the aggregation of magnetic particles at the nanoscale often exerts limited microwave absorption performance at low frequencies with a narrow absorption bandwidth, attributed to the Snoke’s limit [22] around bulk magnetic materials. Furthermore, unilateral of composition cannot simultaneously provide sufficient attenuation ability and effective matched impedance in an electromagnetic field. The combination of carbon-based materials and magnetic materials, which have synergetic effects, has gained much attention. Furthermore, the introduction of N atoms in the carbonaceous framework can direct specific dipole movement, which helps adjust the electrical conductivity of N-doped graphene (NG). Apart from the chemical compositions involved, the design of the nanostructure also has an indispensable impact on influencing the material microwave absorption performance, but is rarely emphasized. Notably, carbon-based conductive networks have been suggested as competitive candidates because the numerous efficient electric transfer pathways within nanocomposites can accumulate space charges, progressively increasing the conductivity loss of the materials. In addition, the increasing interfacial areas in the heterostructure have been proven to strengthen interfacial polarization behaviours. The multi-reflection effect can be facilitated in hollow frameworks at the same time, which are auxiliary methods to transfer and dissipate electromagnetic energy.
Metal-Organic Frameworks (MOFs) are materials composed of organic ligands and metal ions [23]. Their attractive properties of high specific surface area, porosity and active sites have great potential in the fields of energy storage and transformation [24], [25], [26], [27]. The popular posttreatment of MOFs [28], namely high-temperature calcination under an inert atmosphere, can transform them into metal/carbon core/shell structures while maintained their porous morphology, enabling upgrading performance in potential applications. Our group [29] synthesized a Co/C core/shell structure by pyrolyzing relative Co-MOF-74 under a N2 atmosphere to evaluate its microwave absorption performance. The reflection loss reached −62.12 dB with an effective absorption bandwidth (EAB) of 4.6 GHz at a thickness of 2.4 mm. To enhance the microwave absorption performance towards MOF-based materials, researchers [30], [31], [32], [33], [34] often modify MOFs on graphene surfaces through two-step processes, including graphene prefabrication and growing MOFs on the surfaces. However, it is difficult to govern uniform MOF nucleation on graphene nanosheets (GNs) due to the irregular distribution of the functional groups. In addition, it requires complicated GN prefabrication. It is a challenging but highly pursued mission to synthesize nanocomposites with controllable structures and superior functions.
To fulfil the purpose of simultaneously tailoring the microwave absorption performance and controlling the nanostructure, along with integrating the above-mentioned multiple advantages, we chose Co-ZIF-67, melamine and g-C3N4 in this work. Hollow voids were created insides the Co-ZIF-67 nanoparticles when introducing tannic acid to create hollow core by surface functionalization-assisted etching strategy. The NG/CNT conductive networks were constructed in-situ with the decoration of nanoscale cobalt under specific conditions. NG nanoshells grew and covered the surfaces of residual cobalt nanoparticles from ZIF-based structures, while the CNTs were interconnected and limited within the ZIF frameworks. Distinct morphologies have direct relationships with microwave absorption performance, which can be precisely adjusted by tailoring the carbonization temperature and composition proportions. Excellent microwave absorption features of thin, lightweight, wide frequency bandwidth, and strong absorption intensity, could be obtained by a facile synthesis process and met strict requirements for next-generation microwave absorption devices.
Section snippets
Materials
Sodium Nitrate Hexahydrate (Co(NO3)2·6H2O), 2-methylimidazole (2-mIM), methanol, absolute ethanol (99.7%), melamine, graphitic carbon nitride (g-C3N4), and tannic acid (TA) were supplied by Sigma-Aldrich Co., Ltd. All chemicals were used without any further purification.
Fabrication and pretreatment of Co-ZIF-67
Co-ZIF-67 was prepared according to procedures reported previously [35]. Specifically, Co(NO3)2·6H2O (6.87 mmol) and 2- -mIM (24.36 mmol) were separately dissolved in methanol (50 mL) under magnetic stirring for 20 min at room
Results and discussion
The Co-ZIF-67 synthesis was facile and scalable since mixing methanolic solutions of Co(NO3)2·6H2O and 2-mIM supported the Co-ZIF-67 nucleation at room temperature. Tannic acid (TA) acts as both protecting agent and etching agent during the corrosion process of Co-ZIF-67 [36], which was beneficial to exposing more active sites on interfaces. It also avoided aggregation of nanosized cobalt when compared with the traditional Co-ZIF-67 carbonization process. The carbonization process under a
Conclusions
In conclusion, Co-ZIF-67 was adopted as a self-template to synthesize N-doped graphene/carbon nanotube interlinked conductive networks modified with cobalt nanoparticles through a facile carbonization process. Both the morphology and the microwave absorption performance could be precisely controlled by either tailoring the content of the precursor or the carbonization temperature. Appropriate ratios and calcination temperatures not only guaranteed the construction of conductive networks but
CRediT authorship contribution statement
Kaifeng Wang: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization. Shunzhe Zhang: Investigation, Writing - review & editing, Visualization. Wenshuang Chu: Formal analysis, Writing - review & editing, Visualization. Hua Li: Formal analysis, Investigation, Resources, Funding acquisition. Yujie Chen: Resources, Funding acquisition. Biqiong Chen: Project administration. Bingbing Chen: Methodology. Hezhou Liu: Project
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
Hua Li is thankful for the support from the National Natural Science Foundation of China (No. U1733130 and 81772432); Basic Research Field of Shanghai Science and Technology Innovation Program (No.16JC1401500); Science and Technology Innovation Special Zone Program (No.18-163-13-ZT-008-003-06); CALT Foundation; Cross Research Fund of Biomedical Engineering of Shanghai Jiao Tong University (YG2016MS70, YG2017MS11); and Joint Foundation from the Ministry of Education of China (6141A02022264).
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These authors contribute equally to the article.