Coaxial multi-interface hollow Ni-Al₂O₃-ZnO nanowires tailored by atomic layer deposition for selective- frequency absorptions

In this work, atomic layer deposition (ALD) was employed to fabricate coaxial multi-interface hollow Ni-Al₂O₃-ZnO nanowires. The morphology, microstructure, and ZnO shell thickness dependent electromagnetic and microwave absorbing properties of these Ni-Al₂O₃-ZnO nanowires were characterized. Excellent microwave absorbing properties with a minimum reflection loss (RL) of approximately –50 dB at 9.44 GHz were found for the Ni-Al₂O₃-100ZnO nanowires, which was 10 times of Ni-Al₂O₃ nanowires. The microwave absorption frequency could be effectively varied by simply adjusting the number of ZnO deposition cycles. The absorption peaks of Ni-Al₂O₃-100ZnO and Ni-Al₂O₃-150ZnO nanowires shifted of 5.5 and 6.8 GHz towards lower frequencies, respectively, occupying one third of the investigated frequency band. The enhanced microwave absorption arose from multiple loss mechanisms caused by the unique coaxial multi-interface structure, such as multi-interfacial polarization relaxation, natural and exchange resonances, as well as multiple internal reflections and scattering. These results demonstrate that the ALD method can be used to realize tailored nanoscale structures, making it a highly promising method for obtaining high- efficiency microwave absorbers, and opening a potentially novel route for frequency adjustment and microwave imaging fields.