Wire-in-tube ZnO@carbon by molecular layer deposition: Accurately tunable electromagnetic parameters and remarkable microwave absorption

Yolk–shell and one-dimensional structures are promising microwave absorption structures due to the microwave multi-reflection and scattering sites of yolk–shell structure, and the conductive network that is easily formed by one-dimensional structure. The wire-in-tube structure may be a more effective microwave absorption structure because it combines the advantages of one-dimensional and yolk–shell structures. However, conventional methods are difficult to realize such a structure because of the weak controllability of the structure, and it is impossible to tune the absorption band or optimize the absorption performance by precisely tailoring the structure. Molecular layer deposition (MLD), with excellent structure controllability, is an effective strategy to overcome this problem. In this work, a novel wire-in-tube ZnO@carbon nanostructure was realized by polyimide MLD–calcination strategy. The ZnO cores and carbon shells form voids between them by a redox process during calcination, conducive to microwave multi-refection and scattering. More importantaly, by tuning the number of deposition cycles, the carbon shell thickness can be adjusted at the atomic scale so as to modulate the absorption bands effectively. Maximum absorption of −50.05 dB and a bandwidth of 5.68 GHz are simultaneously achieved at a matching thickness of 2.0 mm. The polyimide MLD–calcination strategy not only provides a novel wire-in-tube structure with remarkable microwave absorption performance, but diversifies carbonaceous material fabrication methods, extending the applications to supercapacitors, sensing, catalysis, and the biomedical fields.