Synthesis and microwave absorption of uniform hematite nanoparticles and their core-shell mesoporous silica nanocomposites

Abstract

Single-crystal α-iron oxide (denoted as FO) particles with uniform sub-micrometer size and polyhedron-like shape have been successfully fabricated by using polyvinylpyrrolidone (PVP) capping agent-mediated hydrolysis of iron nitrate under mild hydrothermal conditions (200 °C). The hematite products were characterized via combined techniques including scanning electronic microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The single-crystal hematite particles have relatively uniform sizes of 180–360 nm and octahedron-shaped structures with comparatively smooth surfaces. Furthermore, the as-made hematite particles can be used as cores to prepare core-shell mesoporous silica composites. The intermediate nonporous silica layer was coated first via a sol-gel process, and then the mesoporous silica structure was coated as the outer shell layer by a surfactant-assembly method, resulting in uniform core-shell mesoporous silica FO@nSiO₂@mSiO₂ composites. TEM images show that the FO@nSiO₂@mSiO₂ composites possess distinct two-layer coating core-shell structures with ordered hexagonal mesostructure in the outer silica shell layer. N₂ sorption measurements show that the uniform accessible mesochannel size for the FO@nSiO₂@mSiO₂ nanocomposites is ∼2.10 nm, the surface area is as high as ∼445 m²/g, and the pore volume is as large as ∼0.29 cm³/g. Furthermore, the reflection loss (dB) spectra measured in the frequency range 2–18 GHz showed that the FO@nSiO₂@ mSiO₂ composites have improved electromagnetic interference (EMI) shielding effectiveness (SE) compared to that of pure hematite materials. This is mainly attributed to the better impedance match and multiple-interfacial polarization among the FO@nSiO₂@mSiO₂ nanocomposites.