Recently, biomass-derived three-dimensional (3D) porous carbon materials have been gaining more interest as promising microwave absorbers due to their low cost, vast availability, and sustainability. Here, a novel 3D interconnected porous magnetic carbon foams are in-situ synthesized via a combination of sol-gel and carbonization process with wheat straw as the carbon source and FeCl₃·6H₂O as the magnetic regulating agent. During the process of foams formation, the lignocelluloses from the steam-exploded wheat straw are converted into interconnected carbon sheet networks with hierarchical porous structures, and the precursor FeCl₃·6H₂O is converted into magnetic nanoparticles uniformly embedded in the porous carbon foams. The generated magnetic nanoparticles are benefit to enhance the interface polarization and magnetic loss ability to improve the efficient complementarities between the dielectric and magnetic loss, thus increasing the impedance matching. The obtained sample treated at 600 ℃ displays the best microwave absorption (MA) performance. It presents a minimal reflection loss (RL) of −43.6 dB at 7.1 GHz and the effective bandwidth (RL < −10 dB) is 3.3 GHz with the thickness of 4.7 mm. The 3D porous structure, multi-interfaces and the synergy of dielectric loss and magnetic loss make great contribution to the outstanding MA performance.

Helical carbon nanotubes (HCNTs) are highly desirable due to their unique geometrical elegance and inherent physical properties; however, high-efficiency synthesis of high-purity HCNTs with high yield and full elucidation of their growth mechanism remains challenging. Traditional methods to achieve the high-yield growth of HCNTs mainly focus on controlling the size of catalytic particles. Herein, we found that addition of trace water greatly benefits large-scale synthesis of HCNTs. Uniform HCNTs with ~ 100% purity can be obtained, and the yield of HCNTs can reach ~ 8, 078% in a run of 6 h, much higher than that obtained without trace water and any of the reported yields. Experiments and theoretical simulations are performed to reveal that the trace water can react with the dangling bond on carbon, thus inhibiting the generation of amorphous species. Furthermore, the trace water can enhance the anisotropy of the catalyst surface. This results in different segregation rates of carbon atoms coming out of different crystal planes and further periodic mismatch of the graphite layers, thus leading to the formation of HCNTs. Therefore, this new and efficient method is promising for practical, large-scale production of HCNTs.

Despite recent progress in the synthesis and application of graphene-based aerogels, some challenges such as scalable and cost-effective production, and miniaturization still remain, which hinder the practical application of these materials. Here we report a large-scale electrospinning method to generate graphene-based aerogel microspheres (AMs), which show broadband, tunable and high-performance microwave absorption. Graphene/Fe₃O₄ AMs with a large number of openings with hierarchical connecting radial microchannels can be obtained via electrospinning-freeze drying followed by calcination. Importantly, for a given Fe₃O₄: graphene mass ratio, altering the shape of aerogel monoliths or powders into aerogel microspheres leads to unique electromagnetic wave properties. As expected, the reflection loss of graphene/Fe₃O₄ AMs-1:1 with only 5 wt.% absorber loading reaches?51.5 dB at 9.2 GHz with a thickness of 4.0 mm and a broad absorption bandwidth (R_L < -10 dB) of 6.5 GHz. Furthermore, switching to coaxial electrospinning enables the fabrication of SiO₂ coatings to construct graphene/Fe₃O₄@SiO₂ core?shell AMs. The coatings influence the electromagnetic wave absorption of graphene/Fe₃O₄ AMs significantly. In view of these advantages, we believe that this processing technique may be extended to fabricate a wide range of unique graphene-based architectures for functional design and applications.