Well-defined porous Fe3O4 flower-like nanostructures have been synthesized by decomposition of the iron alkoxide precursors that are prepared by heating up the solution of FeCl3·6H2O, urea, and surfactant in ethylene glycol. Time-dependent SEM studies indicate that the structure evolution of iron alkoxide precursors contains a fast nucleation of primary nanoparticles followed by a subsequent growth. By varying the amount of surfactant in the solution, the morphology and microstructure of the iron alkoxide precursors can be controlled. After calcination, the flower-like nanostructures of the precursors are maintained in the final products Fe3O4, with each petal of the flower being transformed from a dense structure with a smooth surface into a highly porous structure consisting of interconnected nanoparticles due to the removal of organic species in the iron alkoxide by pyrolysis. Compared to traditional ferrites and ferromagnetic alloys, the complex permittivity of the flower-like porous Fe3O4 samples is modified, and the permeability presents natural magnetic resonance at about 3.0 GHz, which is higher than that of usual Fe3O4 nanoparticles and symbolizes a break-through of the Snoek’s limit. A maximum reflection loss of the flower-like porous Fe3O4 can reach −28.31 dB at 13.2 GHz with a thickness of 2 mm due to an improved impedance matching that is associated with complex permittivity, complex permeability, and the structure of the material. We believe the prepared porous Fe3O4 nanostructures can be good candidates for electromagnetic absorbing materials.

Integrating nitrogen species into sp 2-hybridized carbon materials has proved an efficient means to improve their electrochemical performance. Nevertheless, an inevitable mixture of nitrogen species in carbon materials, due to the uncontrolled conversion among different nitrogen configurations involved in synthesizing nitrogen-doped carbon materials, largely retards the precise identification of electrochemically active nitrogen configurations for specific reactions. Here, we report the preparation of single pyrrolic N-doped carbon materials (SPNCMs) with a tunable nitrogen content from 0 to 4.22 at.% based on a strategy of low-temperature dehalogenation-induced and subsequent alkaline-activated pyrolysis of 3-halogenated phenol-3-aminophenol-formaldehyde (X-APF) co-condensed resins. Additionally, considering that the pseudocapacitance of SPNCMs is positively dependent on the pyrrolic nitrogen content, it could be inferred that pyrrolic nitrogen species are highly active pseudocapacitive sites for nitrogen-doped carbon materials. This work gives an ideal model for understanding the contribution of pyrrolic nitrogen species in N-doped carbon materials.

The excellent microwave absorption performance of the rose-like porous Fe@C is due to the enhancement of matched impedance and collective multiple loss mechanism.

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Heterogeneous Fe 3 O 4 and Fe composites are highly desirable for microwave absorption application because of their complementary electromagnetic (EM) properties. With three-dimensional (3D) Fe 2 O 3 as a sacrificing template, we realize the construction of Fe 3 O 4 /Fe composites with tunable chemical composition, and more importantly, these composites inherit the unique 3D microstructure from their precursor. The change in chemical composition produces significant impacts on the EM functions of these composites. On the one hand, dielectric loss can be improved greatly through positive interfacial polarization and reach the peak when the mass contents of Fe 3 O 4 and Fe are 72.1 and 27.9 wt %, respectively. On the other hand, high Fe content slightly pulls down magnetic loss in the low-frequency range but favors strong magnetic loss in the high-frequency range because of the breakthrough of Snoek's limitation. The attenuation constant reveals that dielectric loss dominates overall consumption of incident EM waves. As a result, the optimized composite, F-350 (the reduction of Fe 2 O 3 is conducted at 350 °C), shows the best microwave absorption performance, whose strongest reflection loss is −56.0 dB at 17.5 GHz and the effective bandwidth can cover the frequency range of 12.0−15.5 GHz with the thickness of 1.5 mm. Furthermore, an ultrawide effective bandwidth of 15.3 GHz can be achieved with the integrated thickness of 1.0−5.0 mm. Such a performance is superior to those of many reported Fe 3 O 4 /Fe composites, and a comparative analysis manifests that good microwave absorption of F-350 is also benefited from its unique 3D architecture.