General Fabrication of 3D Hierarchically Structured Bamboo-like Nitrogen-Doped Carbon Nanotube Arrays on 1D Nitrogen-Doped Carbon Skeletons for Highly Efficient Electromagnetic Wave Energy Attenuation.

Hierarchically three-dimensional (3D) micro-nanostructures have promising applications in multifarious fields. Herein, we report a general strategy, that is, in situ catalysis process, for fabrication of nitrogen-doped carbon nanotube (NCNT) arrays on one-dimensional (1D) nitrogen-doped carbon (NC) skeletons. The NCNT arrays branch out from the 1D NC surfaces, resulting in the formation of hierarchically 3D micro-nanostructures. The strategy is involved in the pyrolysis of M-precursor (M = Fe, Co, and Ni) nanowires with the assistance of dicyandiamide. During the synthesis process, the metal components in the precursors serve as catalysts for growing NCNTs, while dicyandiamide provides carbon and nitrogen sources. With the ongoing reaction, the NCNTs were catalytically grown and branched out from 1D NC skeletons. Through the strategy, three kinds of hierarchically 3D structures with encapsulated Fe/FeC, Co, and Ni nanoparticles, respectively, were fabricated successfully. As functional materials for attenuating electromagnetic wave energy, these hierarchically 3D structures exhibit satisfactory performances even at a low matching thickness, exceeding most of the carbon-based materials. Typically, the minimal reflection losses of the 3D structures can reach -10.0 dB even as the matching thickness is in the range of 1.4-2.0 mm. Experimental results demonstrate that the excellent attenuation properties toward electromagnetic wave energy are relative to high conduction loss at a low frequency and high dielectric relaxations at a high frequency as well as better impedance matching with the input impedance of the free space. Our method presented here opens a general way for the development of hierarchically 3D carbon-based micro-nanostructures for their practical applications.