Yolk–shell ternary composites composed of a Ni sphere core and a SnO₂(Ni₃Sn₂) shell were successfully prepared by a facile two-step method. The size, morphology, microstructure, and phase purity of the resulting composites were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy (TEM), high-resolution TEM, selected-area electron diffraction, and powder X-ray diffraction. The core sizes, interstitial void volumes, and constituents of the yolk–shell structures varied by varying the reaction time. A mechanism based on the time-dependent experiments was proposed for the formation of the yolk–shell structures. The yolk–shell structures were formed by a synergistic combination of an etching reaction, a galvanic replacement reaction, and the Kirkendall effect. The yolk–shell ternary SnO₂ (Ni₃Sn₂)@Ni composites synthesized at a reaction time of 15 h showed excellent microwave absorption properties. The reflection loss was found to be as low as–43 dB at 6.1 GHz. The enhanced microwave absorption properties may be attributed to the good impedance match, multiple reflections, the scattering owing to the voids between the core and the shell, and the effective complementarities between the dielectric loss and the magnetic loss. Thus, the yolk–shell ternary composites are expected to be promising candidates for microwave absorption applications, lithium ion batteries, and photocatalysis.

One of the major hurdles of Ni-based microwave absorbing materials is the preparation of two-dimensional (2D) Ni flakes that can improve magnetic anisotropy to tune complex permeability. In this study, we successfully synthesized porous 2D sponge-like Ni/derivative heterostructures composed of Ni, NiO and Ni(OH)₂ through a controllable hydrogen reduction method. Thanks to the larger grain size of the Ni/derivative heterostructure prepared at 600 °C (Ni-600) under hydrogen flow, good magnetic properties and high magnetic loss could be obtained, which is beneficial for the enhancement of microwave absorption properties. For the Ni-600 samples, the minimal reflection loss (RL) is −37.3 dB at 7.1 GHz and the effective bandwidth (RL < −10 dB, 90% microwave dissipation) could be tuned in the range of 4.5–18.0 GHz with the thickness of 1.5–4.5 mm. High attenuation ability, including dielectric loss and magnetic loss, and good impedance matching are the requirements for excellent microwave absorption properties. In addition, the porous 2D heterostructure flake structure also significantly contributes to microwave absorption. Multiple reflections and scattering caused by the porous flakes, interfacial polarizations in the heterostructures, tunable impedance matching in the porous structure, strong natural resonance induced by the 2D flakes and plentiful micro-capacitors in the separate flakes account for the enhanced microwave absorption performance. This study demonstrates a fresh exploration of designing novel electromagnetic wave absorbing materials.

The Ni/ZnO composites show two wide attractive absorption bands, which could broaden the effective absorption bandwidth.