Ferromagnetic Co20Ni80 nanoparticles encapsulated inside reduced graphene oxide layers with superior microwave absorption performanceElectronic supplementary information (ESI) available. See DOI: 10.1039/c8tc05800h

An excellent microwave absorber featuring lightweight, broadband, low filling content, strong absorption intensity and tunable electromagnetic properties is urgently needed for innovative electromagnetic interference shielding. Herein, a series of Co 20 Ni 80 nanoparticles with different sizes encapsulated inside reduced graphene oxide layers (rGO/Co 20 Ni 80 ) were synthesized via a facile hydrothermal method followed by a continuous freeze-drying process. The size of the Co 20 Ni 80 nanoparticles played a crucial role in the microwave absorption properties of the rGO/Co 20 Ni 80 nanocomposites in terms of both the maximum reflection loss value and the absorption bandwidth. In particular, for rGO/Co 20 Ni 80 with Co 20 Ni 80 nanoparticles of 700 nm diameter, a maximum reflection loss value of −54.4 dB at 7.36 GHz was achieved with a low loading content of only 16%. Moreover, compared with previously reported absorption bandwidth values, a wider absorption bandwidth was observed for the rGO/Co 20 Ni 80 nanocomposites, which exceeded 6.02 GHz and covered the entire Ku band. The superior microwave absorption performance can be attributed to the better impedance matching and synergistic effects between the ferromagnetic Co 20 Ni 80 nanoparticles and the dielectric rGO layers, as confirmed by electron holography. This work may provide a new perspective for the development of lightweight, broadband, low-loading-content and high-performance microwave absorbers. The size regulation of rGO/Co 20 Ni 80 nanocomposites improves microwave loss properties in terms of the reflection loss value and absorption bandwidth.