Hierarchical structures of graphene@Fe₃O₄@SiO₂@NiO nanosheets were prepared by combining the versatile sol–gel process with a hydrothermal reaction.

Reduced graphene oxide (RGO)–Fe₃O₄ composites with obviously enhanced microwave absorption properties were successfully fabricated by a rational one-pot simplified co-precipitation route, which avoided the usage of an inert gas and any additional chemical agents (such as surfactants and stabilizers). Given these advantages, the strategy described in this study can be developed as a simple and large-scale route to yield RGO–Fe₃O₄ composites. The morphology, structure, thermal stability, magnetic and microwave electromagnetic properties of the as-prepared composites were characterized by XRD, XPS, TEM, FT-IR, Raman, TG and VSM. These 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–Fe₃O₄ composites was studied in detail. For the RGO–Fe₃O₄-3 composite, the maximum R_L reaches −44.6 dB at 6.6 GHz with a thickness of 3.9 mm, and the bandwidth of R_L 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 properties can be tuned easily by varying the (RGO)/(Fe₃O₄) ratio and layer thickness of the samples. It is believed that such composites will find wide applications in the microwave absorbing area.

A two-step strategy combining in situ polymerization and a hydrothermal process has been developed for coupling polyaniline (PANI) with porous TiO₂ anchored on magnetic graphene. The microstructure and morphology of magnetic graphene@PANI@porous TiO₂ were characterized by FETEM, FESEM, XRD, XPS and VSM in detail. The results indicated that magnetic graphene@PANI was completely covered by porous TiO₂ with random orientations and the saturation magnetization value of the composite was 19.2 emu g⁻¹. PANI was used to decrease the absorber thickness, while the porous TiO₂ with a large surface area was designed to enhance the interaction between the electromagnetic (EM) wave and the absorber through multiple reflections, thus enhancing EM wave absorption properties. As an EM wave absorber, the maximum reflection loss of the composite was up to −45.4 dB due to the better normalized characteristic impedance (close to 1) at a thickness of only 1.5 mm and the absorption bandwidths exceeding −10 dB were 11.5 GHz when the thickness ranged from 1 to 3.5 mm. The excellent EM wave absorption performance was ascribed to the combined contribution from the enhanced dielectric relaxation processes, the unique porous nanostructures, the quarter-wave length matching model and the well-matched normalized characteristic impedance. Consequently, it is believed that the composite could be used as an excellent EM wave absorption material and the two-step strategy offered an effective way to design a high-performance EM wave absorber with a relatively thin thickness.