Construction of Co@C nanocapsules by one-step carbon reduction of single-crystal Co3O4 nanoparticles: Ultra-wideband microwave absorber verified via coaxial and arch methods

• Single-crystal Co 3 O 4 NPs of 20 nm were successfully prepared by nitrate pyrolysis. • Uniform core–shell Co@C nanocapsules synthesized via one-step carbon reduction. • Co@C NCs exhibit an ultra-wide EAB of 15.2 GHz (≤-10 dB) and 7.1 GHz (≤−20 dB). • Polarization and magnetic coupling behaviors are analyzed by using HFSS simulation. • Plate coating sample with ultra-wide EAB was also verified via the arch method. It is highly demanding and challenging to construct the nano-scale microwave absorber with wide-frequency responding feature. Herein, a series of Co@C nanocapsules (NCs) are fabricated via one-step carbon reduction of Co 3 O 4 nanoparticles (NPs) of only 20 nm obtained by the nitrate pyrolysis method. The electromagnetic parameters of the samples can be effectively regulated by flexibly adjusting the carbon shell thickness. Surprisingly, all samples exhibit ultra-wide microwave absorption (MA) performance investigated by the coaxial method. For the Co@C NCs with a carbon shell of 25 nm, especially, the effective absorption bandwidth (EAB) for reflection loss (RL) below −10 dB reaches a record high of 15.2 GHz (2.8–18 GHz), which completely covers the whole C-, X-, and Ku-bands. More excitingly, the absorption bandwidth for RL ≤ −20 dB is up to 7.1 GHz at only 2.0 mm thickness. Such outstanding MA properties are attributed to nano-size effect, synergistic effects of strong dielectric/magnetic loss, and superior impedance matching characteristics. Notably, the polarization and magnetic coupling behaviors are clarified with the aid of electric field and magnetic field simulations using High-Frequency Structure Simulator (HFSS). The plate coating sample is further prepared and measured by the arch method, which also displays an ultrawide MA bandwidth. This work provides a new design strategy toward the facile synthesis of ultra-broadband microwave absorbers.