Sodium citrate assisted hydrothermal synthesis of nickel cobaltate absorbers with tunable morphology and complex dielectric parameters toward efficient electromagnetic wave absorption

NiCo₂O₄ was synthesized by sodium citrate assisted hydrothermal method. Four NiCo₂O₄ samples were fabricated by changing the ratio between sodium citrate and metal ions (0.5, 1, 1.5 and 2). Morphology evolution from 3-dimensional flower-like structure to 2-dimensional nanosheet structure is observed as the ratio between sodium citrate and metal ions increased from 0.5 to 2. In addition to the morphology, the complex permittivity of NiCo₂O₄ can also be simply adjusted by regulating the addition amount of sodium citrate. The decent complex permittivity was obtained when the ratios between sodium citrate and metal ions were 0.5, 1 and 1.5. The optimal absorber (sodium citrate: metal ions = 1.5: 1) displays high absorption capacity of −47.9 dB and wide effective absorption bandwidth (EABs) of 4.28 GHz (from 13.72 GHz to 18 GHz) at ultrathin thickness of 1.39 mm. This NiCo₂O₄ absorber, possessing high absorption capacity as well as wide adsorption bandwidth, is the thinnest pristine NiCo₂O₄ absorbers currently reported. The high EM wave attenuation performance is ascribed to the well matched impedance originated from the rational design of complex permittivity values, and other dissipation mechanism, including dipole polarization, interfacial polarization and conduction loss, etc. This work may provide us a novel strategy for regulating the effective absorption band of other Co-based spinel structure EM wave absorption materials.