Gram-scale synthesis, thermal stability, magnetic properties, and microwave absorption application of extremely small Co–C core–shell nanoparticles

Co–C core–shell nanoparticles have been synthesized in large quantity (in grams) by metal-organic chemical vapor deposition with analytical cobalt (III) acetylacetonate as precursor. Extremely small nanoparticles with an average core diameter of 3 nm and a shell thickness of 1–2 nm, and relatively large nanoparticles with an average core diameter of 23 nm and a shell thickness of 5–20 nm were obtained, depending on the deposition regions. The 3 nm Co nanocores are thermally stable up to 200 °C in air atmosphere, and do not exhibit visible structural and morphological changes after exposure to air at room temperature for 180 d. The extremely small core–shell nanoparticles exhibit typical superparamagnetic behaviors with a small coercivity of 5 Oe, while the relative large nanoparticles are a typical ferromagnetic material with a high coercivity of Oe. In the microwave absorption tests, a low reflection loss (RL) of −80.3 dB and large effective bandwidth (frequency range for dB) of 10.1 GHz are obtained in the nanoparticle-paraffin composites with appropriate layer thicknesses and particle contents. This suggests that the as-synthesized Co–C core–shell nanoparticles have a high potential as the microwave-absorbing materials.