Three-dimensional hierarchical metal-organic framework-based flexible films for efficient microwave absorption

Hierarchical structures have garnered significant attention in the development of microwave-absorbing materials due to their tunable microstructures and synergistic multi-loss mechanisms. This work demonstrates a facile strategy to fabricate ZIF-67-derived flexible microwave-absorbing films on cotton cloth by systematically regulating melamine content. This approach enabled the fabrication of a three-dimensional, multi-scale, hierarchical composite consisting of (1) a layer of porous carbon particles (hundreds of nanometers in diameter) grown on the surfaces of cotton fibers (10 μm in diameter), (2) nitrogen-doped carbon nanotubes (10–20 nm in diameter) grown on the surfaces of the porous carbon particles, and (3) Co nanoparticles encapsulated by the carbon nanotubes. The multi-scale hierarchical structure was confirmed by comprehensive morphological, microstructural, and structural characterizations. The data also show that changes in melamine concentration result in variations in the length and loading of the carbon nanotubes as well as the size of the porous carbon particles. These variations, in turn, give rise to changes in the static magnetic and microwave electromagnetic properties of the composites, which were observed through vibrating sample magnetometer and vector network analyzer measurements. The analysis based on microwave permittivity and permeability data shows that the sample prepared with a melamine mass of 1.5 g exhibited the highest microwave absorption performance—an effective absorption bandwidth of 6.3 GHz (11.7–18.0 GHz) at a thickness of 2.06 mm and a filler loading of 15 wt%. This exceptional performance originates from improved impedance matching and enhanced conductive and polarization losses enabled by the unique multi-scale hierarchical structure. This study provides novel insights and experimental foundations for designing flexible, highly efficient microwave absorption materials.