Co²⁺/Co³⁺ ratio dependence of electromagnetic wave absorption in hierarchical NiCo₂O₄-CoNiO₂ hybrids

Amorphous hierarchical NiCo₂O₄-CoNiO₂ hybrids have been successfully fabricated via a facile one-pot hydrothermal route, followed by morphological conversion into urchin-like structured NiCo₂O₄-CoNiO₂ nanorods and irregular-shaped hierarchical NiCo₂O₄-CoNiO₂ polyhedral nanocrystals through air-annealing treatment at 450 °C and 650 °C, respectively. The phase structure, morphology and chemical composition have been characterized in detail. Calcined hierarchical NiCo₂O₄-CoNiO₂ hybrids show improved microwave absorption properties, which are ascribed to the synergistic effect of dielectric CoNiO₂ and NiCo₂O₄ phases. In particular, the calcined hierarchical NiCo₂O₄-CoNiO₂ hybrids at 450 °C exhibit significant enhancement in complex permittivity with respect to others due to their remarkable dipole polarization and interfacial polarization. The maximum reflection loss (RL) of the calcined hierarchical NiCo₂O₄-CoNiO₂ hybrids at 450 °C reaches -42.13 dB at 11.84 GHz with a matching thickness of 1.55 mm, and a relatively broad absorption bandwidth (RL ≤ -10 dB) in the 13.12-17.04 GHz range. Very interestingly, the electromagnetic (EM) wave absorption performance of the hierarchical NiCo₂O₄-CoNiO₂ hybrids shows dependence on the Co²⁺/Co³⁺ ratio. The calcined NiCo₂O₄-CoNiO₂ hybrids at 450 °C of the most defect concentration possess the best EM wave absorption ability among all the samples. The results suggest that appropriate interactions between the building blocks in hybrids can guide us to design and fabricate highly efficient EM wave absorption materials.