Ethylenediamine-assisted hydrothermal synthesis of NiCo2O4 absorber with controlled morphology and excellent absorbing performance.

In recent years, developing excellent electromagnetic wave (EMW) absorbers with small thickness and large bandwidth is an effective strategy to deal with the seriously EMW interferes in military and civil field. The morphology of absorbers has profound effects on their EMW absorption performance. Herein, ethylenediamine (EDA) was applied to EMW absorbing field as morphology control agent for the first time (as far as we know) for the synthesis of nickel cobaltate (NiCo2O4) absorber, and the effect of EDA content on the morphology and EMW absorbing performance was also investigated elaborately. As a bidentate ligand and structure-directing reagent, EDA controlled the morphology of NiCo2O4 by reducing the rate of nucleation and crystal growth through the complexation with metal ions, and adjusting the growth rate of different facets through the selective binding of amino to certain surfaces. The study found that the morphology of NiCo2O4 changed from three-dimensional urchin-like structures to two-dimensional nanosheets with the increase of EDA content, and meanwhile the dielectric loss property decreased, which led to decline in EMW attenuation properties. The urchin-like NiCo2O4 absorber at a molar ratio of 0.5 (EDA: metal ions) exhibited optimum absorption properties with a small thickness of 1.70 mm and large effective absorption bandwidth (EAB) of 5.81 GHz (12.19-18 GHz). The excellent EMW absorbing properties mainly originate from remarkable dipole polarization induced by oxygen vacancies and lattice defects, interface polarization stemming from the interfaces of NiCo2O4 fibers, and multiple reflections and scattering in its unique urchin-like structures. This work provides a simple method for absorber with controlled morphologies, and also expands the family members of Co-based ferrite with outstanding absorbing performance.