Morphology-controlled synthesis and excellent microwave absorption performance of ZnCoO nanostructures via a self-assembly process of flake units.

Pure dielectric microwave absorbers with strong attenuation capability and wide-band response become a challenge for efficient electromagnetic wave energy absorption. Herein, a series of ZnCo2O4 hierarchical structures with superior absorption performance have been achieved by tuning their surface architectures from ball-, hydrangea- to cabbage-, and pineapple-like morphologies. A facile one-step synthesis strategy using a self-assembly process with ZnCo2O4 crystalline flakes as structural units was proposed. The deionized water solution and urea addition were found to critically determine the formation of our unique cabbage-like ZnCo2O4 self-assembled morphology. The wide band and distinct absorption was dominantly contributed from dielectric ZnCo2O4 flakes, which could be furthermore adjusted by the above-mentioned morphologies. Due to its abundant void volume stacked by flakes, the cabbage-like ZnCo2O4 demonstrated the best absorption performance where the RLmax reached -36.33 dB at 9.5 GHz with an efficient bandwidth of 5.11 GHz (RL < -10 dB, 11.17-16.28 GHz). Adjusting the simulating thickness from 1 to 5 mm, the bandwidths range from 5.8 to 18 GHz. This unique structure has the polarization, conduction loss and strong dissipation capability resulting from the high density of accumulated charges trapped by the flake gap, confirmed by the analysis of electromagnetic parameters and electronic holography. It is expected that the self-assembled ZnCo2O4 microsphere might shed new light on the design of novel microwave absorption materials.