Bimetallic MOFs-derived yolk-shell structure ZnCo/NC@TiO₂ and its microwave absorbing properties

Bimetallic MOFs with core–shell morphology are prepared by feeding step by step. Sandwich type ZIF-67@ZIF-8@TiO₂ is prepared by coating TiO₂ shell layer on the surface of the obtained ZIF-67@ZIF-8 via hydrolysis technology. After pyrolysis of ZIF-67@ZIF-8@TiO₂, ZnCo/NC@TiO₂ composite wave absorber with yolk-shell structure is fabricated. The evaporation of zinc, volatilization of organic components and shrinkage of carbon layer at high temperature result in a larger cavity between the core and shell. Large specific surface area, pore volume and cavity structure are favorable for multiple interface reflection. Existence of multiple heterogeneous interfaces, unevenly distribution of space charges accumulated between the interfaces, abundant crystal defects and doped atoms lead to the continuous occurrence of interface/dipole polarization. These make the absorber exhibits excellent microwave absorption performance. The study shows that under the optimum carbonization temperature (700 ℃) and filler content (37%), minimum reflection loss (RL_min) of ZnCo/NC@TiO₂ reaches −46.21 dB@9.5 GHz with a matching thickness of 2.8 mm. Meanwhile, by adjusting the thickness of absorber, the effective bandwidth can cover 4.3–18 GHz. Maximum effective absorption bandwidth is up to 4.5 GHz. This work will provide a new strategy for the effective construction of yolk-shell microwave absorbing materials derived from bimetallic MOFs as well as performance optimization of existing microwave absorbing materials.