Enhanced electromagnetic wave absorption induced by void spaces in hollow nanoparticles.

We developed a facile method for the growth of hollow structured NiCo2 O4 nanoparticles on a graphene sheet (NiCo2 O4 -h/G). The hollow NiCo2 O4 nanoparticles have an average diameter of approximately 10.0 nm and a shell thickness of merely 2.5 nm. The NiCo2 O4 -h/G hybrid exhibited excellent electromagnetic wave absorption with minimal reflection loss below -20 dB at absorber thickness ranging from 2.0 to 5.0 mm, outperforming the solid NiCo2 O4 nanoparticles on the graphene sheet. Remarkably, even for a thickness as small as 1.5 mm, the efficient absorption bandwidth and the minimal reflection loss of the hybrid can reach 2.6 GHz and -20.3 dB, respectively. Experimental results and theoretical calculations indicate that the void space in the hollow NiCo2 O4 nanoparticles plays a crucial role in the excellent electromagnetic wave absorption property, which greatly increases the dielectric loss and impedance matching characteristics. Our results demonstrate that growing the hollow nanoparticles on a graphene sheet is an efficient way to produce high-performance electromagnetic wave absorbers.