Peculiar porous α-Fe₂O₃, γ-Fe₂O₃ and Fe₃O₄ nanospheres: Facile synthesis and electromagnetic properties

We reported a facile approach to prepare peculiar porous α-Fe₂O₃, γ-Fe₂O₃ and Fe₃O₄ nanospheres by combining a facile hydrothermal route with a calcination process in Ar or H₂ atmosphere. The synthesized monodisperse porous α-Fe₂O₃ nanospheres with uniform average diameters of ~60nm in fact contained randomly distributed pores. A close view further revealed that there are two types of pores, one is large mesopores (ca. 15-20nm) in the center, and the other is small mesopores (ca. <10nm) in the outside. After calcining in Ar or H₂, the obtained α-Fe₂O₃, γ-Fe₂O₃ and Fe₃O₄ nanospheres preserved the similar morphology and particle size as the uncalcined α-Fe₂O₃ nanospheres, indicating the as-prepared α-Fe₂O₃ nanospheres are stable under Ar and H₂-annealing heat treatment. Comparing with all the paraffin composites, it was found that the porous α-Fe₂O₃ nanosphere/paraffin composites exhibit a higher permittivity level. A minimum reflection loss (RL) of -25dB was observed at ~13GHz for the porous α-Fe₂O₃ nanosphere/paraffin composites with a thickness of 3.5mm, and the effective absorption frequency (RL<-10dB) ranged from 9.9 to 15.1GHz. The composites exhibited better absorption properties than the magnetic porous γ-Fe₂O₃ and Fe₃O₄ nanosphere/paraffin composites.