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

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 30-200nm for Fe3O4 nanoblocks and a uniform size around 50nm 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 3.5mm, the reflection loss of Fe3O4 nanowires achieves the minimum value of -29.7dB around 7.3GHz, while the effective absorbing bandwidth (reflection loss -10dB) of Fe3O4 nanoblocks covers 5.6GHz, demonstrating their potential applications in electromagnetic devices.