Low-Cost Carbothermal Reduction Preparation of Monodisperse Fe<sub>3</sub>O<sub>4</sub>/C Core–Shell Nanosheets for Improved Microwave Absorption

This paper demonstrates a facile and low-cost carbothermal reduction preparation of monodisperse Fe<sub>3</sub>O<sub>4</sub>/C core–shell nanosheets (NSs) for greatly improved microwave absorption. In this protocol, the redox reaction between sheet-like hematite (α-Fe<sub>2</sub>O<sub>3</sub>) precursors and acetone under inert atmosphere and elevated temperature generates Fe<sub>3</sub>O<sub>4</sub>/C core–shell NSs with the morphology inheriting from α-Fe<sub>2</sub>O<sub>3</sub>. Thus, Fe<sub>3</sub>O<sub>4</sub>/C core–shell NSs of different sizes (<i>a</i>) and Fe<sub>3</sub>O<sub>4</sub>/C core–shell nanopolyhedrons are obtained by using different precursors. Benefited from the high crystallinity of the Fe<sub>3</sub>O<sub>4</sub> core and the thin carbon layer, the resultant NSs exhibit high specific saturation magnetization larger than 82.51 emu·g<sup>–1</sup>. Simultaneously, the coercivity enhances with the increase of <i>a</i>, suggesting a strong shape anisotropy effect. Furthermore, because of the anisotropy structure and the complementary behavior between Fe<sub>3</sub>O<sub>4</sub> and C, the as-obtained Fe<sub>3</sub>O<sub>4</sub>/C core–shell NSs exhibit strong natural magnetic resonance at a high frequency, enhanced interfacial polarization, and improved impedance matching, ensuring the enhancement of the microwave absorption. The 250 nm NSs–paraffin composites exhibit reflection loss (RL) lower than −20 dB (corresponding to 99% absorption) in a large frequency (<i>f</i>) range of 2.08–16.40 GHz with a minimum RL of −43.95 dB at <i>f</i> = 3.92 GHz when the thickness is tuned from 7.0 to 1.4 mm, indicating that the Fe<sub>3</sub>O<sub>4</sub>/C core–shell NSs are a good candidate to manufacture high-performance microwave absorbers. Moreover, the as-developed carbothermal reduction method could be applied for the fabrication of other composites based on ferrites and carbon.