Achieving super-broad effective absorption bandwidth with low filler loading for graphene aerogels/raspberry-like CoFe2O4 clusters by N doping

A Review of Nitrogen-Doped Graphene Aerogel in Electromagnetic Wave Absorption

Graphene aerogels (GAs) possess a remarkable capability to absorb electromagnetic waves (EMWs) due to their favorable dielectric characteristics and unique porous structure. Nevertheless, the introduction of nitrogen atoms into graphene aerogels can result in improved impedance matching. In recent years, nitrogen-doped graphene aerogels (NGAs) have emerged as promising materials, particularly when combined with magnetic metals, magnetic oxides, carbon nanotubes, and polymers, forming innovative composite systems with excellent multi-functional and broadband absorption properties. This paper provides a comprehensive summary of the synthesis methods and the EMW absorption mechanism of NGAs, along with an overview of the absorption properties of nitrogen-doped graphene-based aerogels. Furthermore, this study sheds light on the potential challenges that NGAs may encounter. By highlighting the substantial contribution of NGAs in the field of EMW absorption, this study aims to facilitate the innovative development of NGAs toward achieving broadband absorption, lightweight characteristics, and multifunctionality.

Synthesis of hollow CuFe2O4 microspheres decorated nitrogen-doped graphene hybrid composites for broadband and efficient electromagnetic absorption

The development of graphene-based electromagnetic wave (EMW) absorbers with broad bandwidth, strong absorption and low filling ratio remains a big challenge. In this work, hollow copper ferrite microspheres decorated nitrogen-doped reduced graphene oxide (NRGO/hollow CuFe₂O₄) hybrid composites were prepared by a two-step route of solvothermal reaction and hydrothermal synthesis. Results of microscopic morphology analysis showed that the NRGO/hollow CuFe₂O₄ hybrid composites had a special entanglement structure between hollow CuFe₂O₄ microspheres and wrinkled NRGO. Moreover, the EMW absorption properties of as-prepared hybrid composites could be regulated by changing the additive amounts of hollow CuFe₂O₄. It was worth noting that when the additive amount of hollow CuFe₂O₄ was 15.0 mg, the attained hybrid composites showed the optimal EMW absorption performance. The minimum reflection loss reached up to -34.18 dB at a thin matching thickness of 1.98 mm and a low filling ratio of 20.0 wt%, and the corresponding effective absorption bandwidth was as large as 5.92 GHz, covering almost the whole Ku band. Furthermore, when the matching thickness was increased to 3.02 mm, the EMW absorption capacity was significantly enhanced, and the optimal reflection loss value of -58.45 dB was achieved. In addition, the possible EMW absorption mechanisms were proposed. Therefore, the structural design and composition regulation strategy presented in this work would provide a great reference value for the preparation of broadband and efficient graphene-based EMW absorbing materials.

Controlled formation of multiple core-shell structures in metal-organic frame materials for efficient microwave absorption

The design of metal-organic frameworks (MOF) derived composites with multiple loss mechanisms and multi-scale micro/nano structures is an important research direction of microwave absorbing materials. Herein, multi-scale bayberry-like Ni-MOF@N-doped carbon composites (Ni-MOF@NC) are obtained by a MOF assisted strategy. By utilizing the special structure of MOF and regulating its composition, the effective improvement of Ni-MOF@NC's microwave absorption performance has been achieved. The nanostructure on the surface of core-shell Ni-MOF@NC can be regulated and N doping on carbon skeleton by adjusting the annealing temperature. The optimal reflection loss of Ni-MOF@NC is -69.6 dB at 3 mm, and the widest effective absorption bandwidth is 6.8 GHz. This excellent performance can be attributed to the strong interface polarization caused by multiple core-shell structures, the defect and dipole polarization caused by N doping, and the magnetic loss caused by Ni. Meanwhile, the coupling of magnetic and dielectric properties enhances the impedance matching of Ni-MOF@NC. The work proposes a particular idea of designing and synthesizing an applicable microwave absorption material that possesses excellent microwave absorption performance and promising application potential.

Bioinspired multiple-degrees-of-freedom responsive metasurface by high-entropy-alloy ribbons with hierarchical nanostructures for electromagnetic wave absorption

The compound eyes of the dragonfly, Pantala flavescens Fabricius, are covered by micro-scaled ocelli capable of sensing polarized light, an attractive property for radar stealth and counterreconnaissance. In this work, we fabricated biomimetic electromagnetic wave absorption materials (EAMs) by analyzing the covert information identifications of biological systems and focusing on the design of metastructures and microstructures. Several bionic metasurfaces with anisotropic double-V meta atoms made up of (FeCoNiSi_8.9Al_8.9)C_0.2 high-entropy-alloy (HEA) ribbons for multiple-degrees-of-freedom recognition and broadband absorption are presented. The covert phase, amplitude, and angular momentum of electromagnetic waves were controlled and recognized as information by manipulating the rotation angle θ of meta atoms. A vortex wave with a topological charge of 1 was generated to recognize linearly polarization and left- and right-handed circular polarization. In addition, the polarization conversion enhanced absorption. The hierarchical nanostructures of HEA ribbons give rise to suitable electromagnetic loss and a superior impedance match. Finally, inspired by the structure of compound eyes, the designed multilayer metamaterials realized effective absorption (reflection loss (RL) ≤  - 10 dB) within the 4.5-18 GHz regime under 2.8 mm thickness. These materials provide evidence for a new way for integrated EAMs and metamaterials.

High-efficiency adsorption removal of CR and MG dyes using AlOOH fibers embedded with porous CoFe2O4 nanoparticles

Owing to the toxicity and difficulty in degradation, how to the effective separation for the residual dyes in the aqueous solution is still an issue with great challenge in the area of environmental protection. Now, to high-efficiency removal of organic dyes from the aqueous solution, we design a unique AlOOH/CoFe₂O₄ adsorbent with porous CoFe₂O₄ nanoparticles embedded on the AlOOH fibers using a simple hydrothermal technique and calcination process. The structural properties and surface characteristics of the AlOOH/CoFe₂O₄ composites are detailedly analyzed by XRD, FTIR, XPS, TEM and SEM. Here, the high S_BET and specific porous structure are beneficial to improve the adsorption performance of AlOOH/CoFe₂O₄ adsorbents. Especially, when the molar ratio of AlOOH to CoFe₂O₄ in the AlOOH/CoFe₂O₄ fibers is 1:1, an optimal performance on adsorbing anionic Congo red (CR) and cationic methyl green (MG) dyes can be obtained at pH = 6.29, where the corresponding maximum adsorption capacities reach up to 565.0 and 423.7 mg g^-1, respectively. Factors leading to the change in the ability of adsorbing CR and MG dyes are systematically discussed, including contact time, temperature, initial concentrations, and pH values of the solutions. Meanwhile, the uptake of CR and MG dyes can best conform to Langmuir isotherm model and pseudo-second-order adsorption kinetics. The thermodynamic analysis verifies that the dye adsorption process is spontaneous and endothermic. Moreover, from the point view of practical application, the good reusability further makes the as-synthesized magnetic AlOOH/CoFe₂O₄ composite be a perfect adsorbent with efficiently removing both anionic and cationic dyes from aqueous solutions.

Fabrication of nitrogen-doped reduced graphene oxide/hollow copper ferrite composite aerogels as lightweight, thin and high-efficiency electromagnetic wave absorbers in the X band

The development of lightweight, thin and high-efficiency electromagnetic (EM) wave absorbers remains a huge challenge in the field of EM absorption. Graphene aerogels with three-dimensional (3D) network structure and low bulk density have been considered as potential EM absorbing materials. In this work, nitrogen-doped reduced graphene oxide/hollow copper ferrite (NRGO/hollow CuFe₂O₄) composite aerogels were fabricated by the three-step method of solvothermal reaction, hydrothermal self-assembly and calcination treatment. The as-prepared composite aerogels had a unique 3D hierarchical porous network structure. Furthermore, results demonstrated that the EM absorption performance of attained composite aerogels could be improved by adjusting the calcination temperature. Notably, the obtained composite aerogel calcined at 400.0 ℃ exhibited the best EM absorption performance. When the loading ratio was as low as 15.0 wt%, the minimum reflection loss reached up to -54.5 dB with a matching thickness of 2.0 mm, and the maximum effective absorption bandwidth of 5.0 GHz could be achieved under an extremely thin thickness of 1.6 mm. Additionally, the probable EM attenuation mechanisms of attained composite aerogels were proposed. The results of this work could be helpful for developing graphene-based 3D composites as lightweight, thin and high-efficiency EM wave absorbers.

Synthesis of hierarchical porous nitrogen-doped reduced graphene oxide/zinc ferrite composite foams as ultrathin and broadband microwave absorbers

Magnetic graphene foams with three-dimensional (3D) porous structure, low bulk density and multiple electromagnetic loss mechanisms have been widely recognized as the potential candidates for lightweight and high-efficiency microwave attenuation. Herein, zinc ferrite hollow microspheres decorated nitrogen-doped reduced graphene oxide (NRGO/ZnFe₂O₄) composite foams were prepared via a solvothermal and hydrothermal two-step method. Results demonstrated that the attained magnetic composite foams possessed the ultralow bulk density (12.9-13.5 mg·cm^-3) and 3D hierarchical porous netlike structure constructed through stacking of lamellar NRGO. Moreover, the microwave dissipation performance of binary composite foams could be notably improved through annealing treatment and further elaborately regulating the annealing temperature. Remarkably, the attained composite foam with the annealing temperature of 300.0 °C presented the integrated excellent microwave attenuation capacity, i.e. the strongest reflection loss reached -40.2 dB (larger than 99.99% absorption) and broadest bandwidth achieved 5.4 GHz (from 12.4 GHz to 17.8 GHz, covering 90.0% of Ku-band) under an ultrathin thickness of only 1.48 mm. Furthermore, the probable microwave dissipation mechanisms were illuminated, which derived from the optimized impedance matching, strengthened dipole polarization, interfacial polarization and multiple reflection, notable conduction loss, natural resonance and eddy current loss. Results of this work would pave the way for developing graphene-based 3D lightweight and high-efficiency microwave absorption composites.

3D porous coral-like Co1.29Ni1.71O4 microspheres embedded into reduced graphene oxide aerogels with lightweight and broadband microwave absorption

In this work, three-dimensional (3D) porous coral-like Co_1.29Ni_1.71O₄ microspheres were successfully combined with reduced graphene oxide (rGO) to form Co_1.29Ni_1.71O₄/rGO aerogels as an efficient microwave absorber by a facile calcination and hydrothermal method. The elemental composition, microstructure, and morphology of the as-synthesized composites were characterized, and the electromagnetic wave absorption performance were analyzed in the frequency range of 2.0-18.0 GHz. The results show that adjusting the mass ratio of Co_1.29Ni_1.71O₄ microspheres and rGO in the composites can effectively tune the electromagnetic parameters, which in turn improves their microwave absorption performance. Here, the minimum reflection loss (RL_min) of the Co_1.29Ni_1.71O₄/rGO aerogels is -51.76 dB with an effective absorption bandwidth (RL < -10 dB) of 7.04 GHz (10.96-18 GHZ) at the thickness of 2.66 mm and a low filling ratio of 15 wt%. It can be demonstrated that the superior microwave absorption performance is attributed to the synergistic effect of impedance matching and dielectric loss, the unique 3D porous structure as well as the abundant interface of the composites. In brief, this study provides a new strategy for the design of magnetic/dielectric high-performance microwave absorbing materials.

Synthesis of three-dimensional porous netlike nitrogen-doped reduced graphene oxide/cerium oxide composite aerogels towards high-efficiency microwave absorption

Three-dimensional (3D) graphene aerogels with porous structure and lightweight feature have been regarded as promising candidates for microwave attenuation. Herein, nitrogen-doped reduced graphene oxide/cerium oxide (NRGO/CeO₂) composite aerogels were fabricated via a hydrothermal route. The obtained composite aerogels possessed low bulk density and unique 3D porous netlike structure constructed by the stacking of lamellar NRGO. Moreover, it was found that the microwave dissipation performance of NRGO aerogel could be notably improved through complexing with CeO₂ nanoparticles and carefully regulating the contents of CeO₂ in the composite aerogels. Remarkably, the attained NRGO/CeO₂ composite aerogel with the content of CeO₂ of 44.11 wt% presented the comprehensively excellent microwave attenuation capacity, i.e. the optimal reflection loss reached -50.0 dB (larger than 99.999% absorption) at a thickness of 4.0 mm and wide bandwidth achieved 5.7 GHz (from 12.3 GHz to 18.0 GHz, covering 95.0% of Ku-band) under an ultrathin thickness of only 1.9 mm. Furthermore, the probable microwave dissipation mechanisms of as-synthesized composite aerogels were clarified, which included the optimized impedance matching, strengthened interfacial polarization and dipole polarization relaxation, notable oxygen vacancy effect and enhanced conduction loss. This work could shed light on developing graphene-based 3D broadband microwave absorption composites.

3D core-shell Fe3O4@SiO2@MoS2 composites with enhanced microwave absorption performance

In this work, a 3D ternary core-shell Fe₃O₄@SiO₂@MoS₂ composite is synthesized by a hydrothermal technique and a modified Stöber method, where magnetic Fe₃O₄@SiO₂ microsphere with the core of raspberry-like Fe₃O₄ nanoparticles is completely coated by the flower-like MoS₂. Herein, the electromagnetic parameters of the composites are effectively tuned by the combination of magnetic Fe₃O₄ with dielectric SiO₂ and MoS₂. The obtained ternary composites exhibit remarkable enhancement of microwave absorption. The measurement results indicate that the minimum reflection loss (RL) of Fe₃O₄@SiO₂@MoS₂ composites reaches -62.98 dB at 1.83 mm with the effective absorption bandwidth (RL < -10 dB) of 5.76 GHz (from 11.28 to 17.04 GHz) at 1.92 mm, much higher than those of pure Fe₃O₄ particles and Fe₃O₄@SiO₂ microsphere. It is believed that the improved performances come from the specific structural design and the plentiful interfacial construction. Further, the synergistic effect of the dielectric and magnetic loss as well as the promoted impedance matching also help to enhance the microwave absorption of the composites. The microwave absorption behavior of the composites conforms to the quarter-wavelength cancellation theory. Our study offers an effective and promising strategy in the structural design and interfacial construction of the novel magnetic/dielectric composites with high-efficiency microwave absorption.

MoS2-Decorated/Integrated Carbon Fiber: Phase Engineering Well-Regulated Microwave Absorber

Phase engineering is an important strategy to modulate the electronic structure of molybdenum disulfide (MoS₂). MoS₂-based composites are usually used for the electromagnetic wave (EMW) absorber, but the effect of different phases on the EMW absorbing performance, such as 1T and 2H phase, is still not studied. In this work, micro-1T/2H MoS₂ is achieved via a facile one-step hydrothermal route, in which the 1T phase is induced by the intercalation of guest molecules and ions. The EMW absorption mechanism of single MoS₂ is revealed by presenting a comparative study between 1T/2H MoS₂ and 2H MoS₂. As a result, 1T/2H MoS₂ with the matrix loading of 15% exhibits excellent microwave absorption property than 2H MoS₂. Furthermore, taking the advantage of 1T/2H MoS₂, a flexible EMW absorbers that ultrathin 1T/2H MoS₂ grown on the carbon fiber also performs outstanding performance only with the matrix loading of 5%. This work offers necessary reference to improve microwave absorption performance by phase engineering and design a new type of flexible electromagnetic wave absorption material to apply for the portable microwave absorption electronic devices.