Synthesis of pomegranate-like Mo2C@C nanospheres for highly efficient microwave absorption

Transition metal carbides/carbon composites are appearing as attractive microwave absorption materials (MAMs) due to their positive synergistic effects as well as good chemical stability. In this study, we trigger the polymerization of pyrrole monomer in the presence of phosphomolybdic acid (PMo12) at room temperature, and then convert the homogeneous PMo12/polypyrrole (PMo12/PPy) nanospheres into Mo2C@C nanospheres under high-temperature inert atmosphere. The carbonization of PPy conjugated chains also induces the in situ formation of Mo2C nanoparticles, resulting in a unique pomegranate-like microstructure. The pyrolysis temperature plays important roles in determining the size and crystalline phase of Mo2C nanoparticles, chemical composition, and the relative graphitization degree of carbon framework. As a dual dielectric system, conductivity loss is primarily responsible for the consumption of electromagnetic energy in Mo2C@C nanospheres, and dipole orientation polarization and interfacial polarization provide auxiliary contribution in this process. It is found that high pyrolysis temperature favors powerful dielectric loss capability. However, the desirable microwave absorption is established in the composite from moderate pyrolysis temperature (700 degrees C), where the strongest reflection loss reaches -48.0 dB at 12.5 GHz and the integrated qualified bandwidth covers the frequency range of 3.5-18.0 GHz. This is because moderate pyrolysis temperature endows Mo2C@C nanospheres with proper attenuation capability and matched characteristic impedance. In view of its excellent microwave absorption performance and simple preparation method, such Mo2C@C nanospheres with pomegranate-like heterostructure will be a kind of promising candidate for lightweight and broadband MAMs in the future.