Co super(2+)/Co super(3+) ratio dependence of electromagnetic wave absorption in hierarchical NiCo sub(2)O sub(4)-CoNiO sub(2) hybrids

Amorphous hierarchical NiCo sub(2)O sub(4)-CoNiO sub(2) hybrids have been successfully fabricated viaa facile one-pot hydrothermal route, followed by morphological conversion into urchin-like structured NiCo sub(2)O sub(4)-CoNiO sub(2) nanorods and irregular-shaped hierarchical NiCo sub(2)O sub(4)-CoNiO sub(2) polyhedral nanocrystals through air-annealing treatment at 450 degree C and 650 degree C, respectively. The phase structure, morphology and chemical composition have been characterized in detail. Calcined hierarchical NiCo sub(2)O sub(4)-CoNiO sub(2) hybrids show improved microwave absorption properties, which are ascribed to the synergistic effect of dielectric CoNiO sub(2) and NiCo sub(2)O sub(4) phases. In particular, the calcined hierarchical NiCo sub(2)O sub(4)-CoNiO sub(2) hybrids at 450 degree 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 NiCo sub(2)O sub(4)-CoNiO sub(2) hybrids at 450 degree C reaches -42.13 dB at 11.84 GHz with a matching thickness of 1.55 mm, and a relatively broad absorption bandwidth (RL less than or equal to -10 dB) in the 13.12-17.04 GHz range. Very interestingly, the electromagnetic (EM) wave absorption performance of the hierarchical NiCo sub(2)O sub(4)-CoNiO sub(2) hybrids shows dependence on the Co super(2+)/Co super(3+) ratio. The calcined NiCo sub(2)O sub(4)-CoNiO sub(2) hybrids at 450 degree 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.