Rational design of ZnO/ZnO nanocrystal-modified rGO foam composites with wide-frequency microwave absorption properties

In this study, ZnO/ZnO nanocrystal-modified reduced graphene oxide (rGO) foam composites having various morphologies and mass ratios were synthesized using the hydrothermal and freeze-drying processes. The free Zn²⁺ present in the composites controlled their morphology by regulating the pH value, and the Zn²⁺ ions adsorbed onto the rGO sheets to form ZnO nanocrystals. The rGO sheets were dispersed by growing ZnO and ZnO nanocrystals on them, effectively preventing their agglomeration. The effects of the ZnO morphology and ZnO:rGO mass ratio on the complex permittivity and microwave absorption properties of the composites were systematically studied. The results revealed that the composite with small hexagonal cylindrical ZnO and the ZnO:rGO mass ratio of 8:1 (pH = 8.5) showed considerable microwave absorption, and the bandwidths of the reflection loss (≤−10 dB) were 8.7–11.1 and 11.5–12.4 GHz when the thickness was 3.4 mm. The microwave absorption mechanism of the composites was investigated systematically. The results indicated the potential of the composites as an absorber material with high performance, broad bandwidth, and light weight.