Using elliptical iron glycolate nanosheets as precursors, elliptical Fe3O4/C core-shell nanorings (NRs) [25 ± 10 nm in wall thickness, 150 ± 40 nm in length, and 1.6 ± 0.3 in long/short axis ratio] are synthesized via a one-pot hydrothermal route. The surface-poly(vinylpyrrolidone) (PVP)-protected-glucose reduction/carbonization/Ostwald ripening mechanism is responsible for Fe3O4/C NR formation. Increasing the glucose/precursor molar ratio can enhance carbon contents, causing a linear decrease in saturation magnetization (Ms) and coercivity (Hc). The Fe3O4/C NRs reveal enhanced low-frequency microwave absorption because of improvements to their permittivity and impedance matching. A maximum RL value of -55.68 dB at 3.44 GHz is achieved by Fe3O4/C NRs with 11.95 wt % C content at a volume fraction of 17 vol %. Reflection loss (RL) values (≤-20 dB) are observed at 2.11-10.99 and 16.5-17.26 GHz. Our research provides insights into the microwave absorption mechanism of elliptical Fe3O4/C core-shell NRs. Findings indicate that ring-like and core-shell nanostructures are promising structures for devising new and effective microwave absorbers.

This paper demonstrates a facile and low-cost carbothermal reduction preparation of monodisperse FeO/C core-shell nanosheets (NSs) for greatly improved microwave absorption. In this protocol, the redox reaction between sheet-like hematite (α-FeO) precursors and acetone under inert atmosphere and elevated temperature generates FeO/C core-shell NSs with the morphology inheriting from α-FeO. Thus, FeO/C core-shell NSs of different sizes ( a) and FeO/C core-shell nanopolyhedrons are obtained by using different precursors. Benefited from the high crystallinity of the FeO core and the thin carbon layer, the resultant NSs exhibit high specific saturation magnetization larger than 82.51 emu·g. Simultaneously, the coercivity enhances with the increase of a, suggesting a strong shape anisotropy effect. Furthermore, because of the anisotropy structure and the complementary behavior between FeO and C, the as-obtained FeO/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 ( f) range of 2.08-16.40 GHz with a minimum RL of -43.95 dB at f = 3.92 GHz when the thickness is tuned from 7.0 to 1.4 mm, indicating that the FeO/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.

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Elliptical Fe3O4 nanorings (NRs) with continuously tunable axes that range from 40 nm to 145 nm in length were prepared through a precursor-directed synthetic route to determine the electromagnetic responses generated at 2–18 GHz. The tunability of the dielectric properties of Fe3O4 NRs depends on the long axis rather than on the specific surface area, internal stress, and grain size. Elliptical Fe3O4 NRs exhibit the excellent microwave absorbing properties due to the unique ring-like configuration, which significantly enhances permittivity, multiple scattering, oscillation resonance absorption, microantenna radiation, and interference. These findings indicate that ring-like nanostructures are promising for devising effective microwave absorbers.

The morphological modulation and phase conversion of a-and b-Ni(OH) 2 complex architectures with varying subunits from nanopetals, nanocolumns, nanocones, and nanoflakes were investigated using a facile coordination homogeneous precipitation method in the Ni(NO 3 ) 2 + urea system. Slow growth and nucleation rates due to relatively low reaction temperatures and molar ratios of CO(NH 2 ) 2 to Ni(NO 3 ) 2 induced the formation of uniform flower-like a-Ni(OH) 2 architectures. Such flower-like architectures originated from subordinate nanopetals that grow perpendicular to the primordial nanopetal surface and are driven by minimum surface free energy effects. At relatively high reaction temperatures, flower-like a-Ni(OH) 2 can transform into b-Ni(OH) 2 microspheres assembled from nanocolumns, nanocones, and even nanoflakes by varying the reaction time. These processes could be related to the synergetic effect of the anisotropic growth and continuous increase in mass transportation along the [001] direction. Flower-like a-Ni(OH) 2 exhibited better electrochemical activity for glucose oxidation compared with b-Ni(OH) 2 microspheres consisting of nanocones because of its special flower-like morphology with high specific surface areas, well-ordered pores, and layered structures intercalated by water and anions. The approach in this study can be used to fabricate other metal hydroxide nanostructures. Flower-like Ni(OH) 2 nanoarchitectures have potential applications in rechargeable batteries, photonic catalysis, and non-enzymatic sensors for glucose.