Tailoring the shape and size of Fe₃O₄ nanocrystals by oxidation–precipitation processes for microwave absorption enhancement

Abstract

Microwave absorbing materials refined into nanoscale are known to have fascinating electromagnetic properties, whereas investigations on their shape and size dependence to microwave absorption performance are insufficient. In this study, single-crystal Fe₃O₄ with various shapes of nanoblocks, nanowires and nanospheres were prepared by surfactant-assisted oxidation–precipitation processes in oxygen-free environment without autoclaves. In microstructural characterizations, a broad size distribution of 30–200 nm for Fe₃O₄ nanoblocks and a uniform size around 50 nm for Fe₃O₄ nanospheres were obtained respectively, while Fe₃O₄ nanowires exhibited non-uniform length-to-diameter ratios. In electromagnetic analysis, Fe₃O₄ nanoblocks have higher saturation magnetization and increased coercivity in contrast to Fe₃O₄ nanowires and nanospheres. Both permeability and permittivity of Fe₃O₄ nanospheres are limited, while an increased attenuation constant is obtained at higher frequency due to the effective electromagnetic matching. As the absorbent thickness reaches 3.5 mm, the reflection loss of Fe₃O₄ nanowires achieves the minimum value of − 29.7 dB around 7.3 GHz, while the effective absorbing bandwidth (reflection loss ≤ − 10 dB) of Fe₃O₄ nanoblocks covers 5.6 GHz, demonstrating their potential applications in electromagnetic devices.