Hollow Zn_1-xCo_2-yNi_xFe_yO₄ spinel via multi-metal ion doping: ultralight and broadband microwave absorber

Hollow architectures offer significant advantages in achieving simultaneous weight reduction and efficient electromagnetic (EM) wave absorption; however, their practical application is often constrained by inherent structural limitations. In this study, Zn_1-xCo_2-yNi_xFe_yO₄ composites were synthesized through an integrated self-sacrificing templating and ion-doping approach. Specifically, mixed zeolitic imidazolate frameworks (ZIFs) were utilized as sacrificial templates to fabricate hollow dodecahedral nanocages. Subsequent ion doping was facilitated by the chelating effect of tannic acid (TA), followed by oxidative annealing in a tube furnace. Interestingly, the introduction of hetero-metal ions disturbed the original spinel lattice structure, leading to the extensive precipitation of a secondary ZnO phase. This spontaneous phase separation generated a high density of heterogeneous interfaces, which significantly enhanced interfacial polarization and thereby improved overall EM wave attenuation performance. These structural and compositional features enable the material to exhibit excellent microwave absorption capabilities even at low filler loadings. The hollow architecture not only reduces the intrinsic density of spinel ferrites but also extends the effective absorption bandwidth by optimizing impedance matching characteristics. As a result, a minimum reflection loss of -57.6 dB and an effective absorption bandwidth of 10.27 GHz were achieved with a filler content as low as 30 wt.%. This work presents a new strategy for the rational design of high-performance electromagnetic absorbers through the synergistic optimization of structural architecture and compositional modulation.