Facile Synthesis and Hierarchical Assembly of Flowerlike NiO Structures with Enhanced Dielectric and Microwave Absorption Properties

In this work, two novel flowerlike NiO hierarchical structures, rose-flower (S1) and silk-flower (S2), were synthesized by using a facial hydrothermal method, coupled with subsequent postannealing process. Structures, morphologies, and magnetic and electromagnetic properties of two NiO structures have been systematically investigated. SEM and TEM results suggested that S1 had a hierarchical rose-flower architecture with diameters in the range of 4-7 μm, whereas S2 exhibited a porous silk-flower architecture with diameters of 0.7-1.0 μm. Electromagnetic performances indicated that the NiO hierarchical structures played a crucial role in determining their dielectric behavior and impedance matching characteristic, which further influenced the microwave attenuation property of absorbers based on them. Due to its hierarchical and porous architectures, S2 had higher microwave absorption performances than S1. The maximum R_L value for sample S2 can reach -65.1 dB at 13.9 GHz, while an efficient bandwidth of 3 GHz was obtained. In addition, the mechanism of the improved microwave absorption were discussed in detail. It is expected that our NiO hierarchical structures synthesized in this work could be used as a reference to design novel microwave absorption materials.