MOF-derived Co/CoO particles prepared by low temperature reduction for microwave absorption

Ameliorating the absorption capability and thickness of microwave-absorbing materials is still a formidable challenge. Herein, novel nanoporous Co particles are synthesized successfully via the thermal decomposition of Co-based zeolitic imidazolate framework (ZIF-67) and subsequent hydrogen reduction process at relatively low temperatures. Through controlling passivation process, the nanoporous Co/CoO particles are generated as a result of the surface oxidation of Co particles. The crystalline size of Co/CoO particles ranges from 15.1 to 27.7 nm. The Co/CoO sample reduced at 200 ℃ exhibits the highest microwave absorption intensity with a minimum reflection loss (RL) value of −87.2 dB at a matching thickness of merely 1.0 mm, whose effective absorption bandwidth (EAB) for RL ≤ −10 dB is 6.2 GHz. The absorption bandwidth of Co/CoO increases with the increasing reduction temperature, but the matching thickness gets larger. Co/CoO reduced at 300 ℃ possesses extremely broad EAB of 8.2 GHz at a thickness of 1.5 mm. Notably, such superior microwave absorbing properties are attributed to the facts that nanoporous structure can adjust the effective permittivity of the material, and CoO improves the impedance match by promoting dipolar polarization and interfacial polarization processes. The nanoporous Co/CoO particles with strong absorption ability and ultrathin thickness demonstrate a potential application in the electromagnetic wave absorbing field.