Transformation between nanosheets and nanowires structure in MnO2 upon providing Co2+ ions and applications for microwave absorption

The phase transition process of the MnO2 phase with increases in Co2+ doping can be described by the Ouroboros symbol. The undoped sample is pure delta-MnO2 with nanosheets structures. Then a small amount of Co2+ ions changes the final products and generates alpha-MnO2 nanofibres in the delta-MnO2 matrix. The products become pure alpha-MnO2 with an appropriate amount of Co added. However, when the Co amount continues to increase, the amount of alpha-MnO2 decreases in the products and turns back to form pure delta-MnO2 in the end. Analysing the electromagnetic absorption performance, the relationship between the properties and the proportion of delta-MnO2/alpha-MnO2 in the powders adjusted by Co2+ doping has been explored, and the composites of delta-/alpha-MnO2 show better absorption ability than the single-phase samples. As a result, the optimal reflection loss (RL) is -54.8 dB, and the effective absorption bandwidth can cover the Ku band at a thickness of 2.2 mm and the X band at a thickness of 3.1 mm with 50 wt.% filler loading ratios. This research might shed new light on the improvement of novel microwave absorption materials.