No abstract

Rational manipulation of hollow structure with uniform heterojunctions is evolving as an effective approach to meet the lightweight and high‐performance microwave absorption for metal‐organic frameworks (MOFs) derived absorbers. Herein, a new and controlled synergistic protecting‐etching strategy is proposed to construct shelled ZIF‐67 rhombic dodecahedral cages using tannic acid under theoretical guidance, then hollow Co@N‐doped carbon nanocages with uniform heterojunctions and hierarchical micro‐meso‐macropores are obtained via a pyrolysis process, which addresses the shortcomings of using sacrificing templates or corrosive agents. The outer Co@N‐doped carbon shell, composed of highly dispersive core‐shell heterojunctions, possesses micro‐mesopores while the inner hollow macroporous cavity endows the absorbers with lightweight characteristics. Accordingly, the maximum reflection loss is −60.6 dB at 2.4 mm and the absorption bandwidth reaches 5.1 GHz at 1.9 mm with 10 wt% filler loading, exhibiting superior specific reflection loss compared with the vast majority of previous MOFs derived absorbers. Furthermore, this synergistic protecting‐etching strategy provides inspiration for precisely creating a hollow void inside other MOFs crystals and broadens the desirable candidates for lightweight and high‐efficient microwave absorbers.

The reduced graphene oxide (RGO)/Fe 3 O 4 composites with an obviously enhanced microwave absorption property had been successfully fabricated by rational one-pot simplified co-precipitation route, which avoided the usage of inert gas and any additional chemical agents (such as surfactants and stabilizer).Given these advantages, the strategy described in this study can be developed as a simple and large-scale yield route to RGO/Fe 3 O 4 composites. The morphology, structure, thermal stability, magnetic and microwave electromagnetic properties of as-prepared composites were characterized by XRD, XPS, TEM, FT-IR, Raman, TG and VSM. Those composites exhibit excellent microwave absorption properties, which are attributed to effective complementarities between the dielectric loss and the magnetic loss. The microwave absorption mechanism of the RGO/Fe 3 O 4 composites was studied in detail. For RGO/Fe 3 O 4 -3 composite, the maximum R L reaches -44.6 dB at 6.6 GHz with a thickness of 3.9 mm, and the bandwidth of RL less than -10 dB can reach up to 4.3 GHz (from 12.2 to 16.5 GHz) with a thickness of 2.0 mm. Moreover, the microwave absorption property can be tuned easily by varying the (RGO)/(Fe 3 O 4 ) ratio and layer thickness of the samples. It is believed that such composites will find their wide applications in microwave absorbing area.-2 -expanded EM interference problems. 3-5 They can absorb microwaves effectively and convert EM energy into thermal energy or make microwaves dissipated by interference. The microwave absorbing materials are now requested to have strong absorption characteristics, wide absorption frequency, lightweight and antioxidation. 4 There are a number of microwave absorbing materials such as ferrites, carbonyl iron, conducting polymers and carbon-based materials etc. 6-9 However, the traditional microwave absorbing materials can not meet all of the requirements such as strong, wide, lightweight and thin at the same time.Hence, extensive studies have been made to develop novel microwave absorping materials with high absorption and wide absorption frequency.Graphene, a novel carbon nanomaterial consisting of one-atom-thick, hexagonally arranged carbon atoms, has attracted extensive attention owing to its superior electronic, thermal and mechanical properties as well as its chemical stability. 10,11 Due to its special surface properties and layered structure, graphene becomes potential nanoscale building blocks for new hybrid materials. 12 Recent research shows that inorganic nanoparticles, such as Au, Ag, Fe 3 O 4 and Co 3 O 4 , etc., could be attached to graphene or graphene oxide (GO) to form hybrid materials, which have potential applications in surface enhanced Raman scattering (SERS), lithium ion batteries, hydrogen storage, microwave absorption and magnetic resonance imaging (MRI) contrast agent, etc.. [13][14][15][16] As a class of promising microwave absorping materials, carbon-based materials exhibited several exceptional properties including lightweight, wide absorption frequency, high...

Hybrid nanocomposites with enhanced microwave absorption properties have been designed by growing CuS nanoflakes on magnetically decorated graphene, and the effect of special nanostructures on microwave absorption properties has been investigated. The structure of the nanocomposites was characterized by Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), N2 adsorption-desorption, and vibrating sample magnetometer (VSM). The influence of cetyltrimethylammonium bromide (CTAB) on the morphology of CuS nanoflakes was also investigated. A possible formation process of the nanocomposites and the mechanism of microwave absorption were explained in detail. As an absorber, the nanocomposites with a filler loading of 20 wt % exhibited enhanced microwave absorption properties due to the special nanostructures, extra void space, and synergistic effect. The maximum reflection loss can reach -54.5 dB at 11.4 GHz, and the absorption bandwidths exceeding -10 dB are 4.5 GHz with a thickness of 2.5 mm, which can be adjusted by the thickness. The results indicate that the hybrid nanocomposites with enhanced microwave absorption properties and lightweight have a promising future in decreasing electromagnetic wave irradiation.

Hierarchical structures of graphene@Fe3O4@SiO2@NiO nanosheets were prepared by combining the versatile sol-gel process with a hydrothermal reaction. Graphene@Fe3O4 composites were first synthesized by the reduction reaction between FeCl3 and diethylene glycol (DEG) in the presence of GO. Then, graphene@Fe3O4 was coated with SiO2 to obtain graphene@Fe3O4@SiO2. Finally, NiO nanosheets were grown perpendicularly on the surface of graphene@Fe3O4@SiO2 and graphene@Fe3O4@SiO2@NiO nanosheet hierarchical structures were formed. Moreover, the microwave absorption properties of both graphene@Fe3O4 and graphene@Fe3O4@SiO2@NiO nanosheets were investigated between 2 and 18 GHz microwave frequency bands. The electromagnetic data demonstrate that graphene@Fe3O4@SiO2@NiO nanosheet hierarchical structures exhibit significantly enhanced microwave absorption properties compared with graphene@Fe3O4, which probably originate from the unique hierarchical structure with a large surface area and high porosity.