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The understanding of the relationships between the coordination configuration of single-atoms (SAs) and their properties remains a great challenge. In this manuscript, a facile method is developed to construct Fe-SAs on onion-like nitrogen-doped nanocarbons supported by graphene (ONCG). In contrast to the symmetrical planar Fe–N4 moieties, the Fe-SAs coordinated with N atoms are located above the plane of the curved graphene and exhibited antenna-like structures. The ONCG with non-planar Fe–N4 moieties possesses greatly increased dielectric loss property compared to their counterparts with symmetrical planar Fe-SAs. In-depth theoretical calculations reveal that the unique geometric structure of the non-planar Fe-SAs improves both conduction and polarization losses significantly, which is attributed to the increased dielectric property. The increased dielectric property endows Fe-SAs@ONCG with an excellent electromagnetic wave absorption at a low filler ratio of 10 wt. %. Our results describe an efficient way for the development of non-planar SAs for dielectric applications. [ABSTRACT FROM AUTHOR]

Highlights: A general method was developed to fabricate a series of honeycomb-like N-doped nanocarbons (3D M–NxC) doped with metal single atoms (Mn, Fe, Co, Cu, or Ni) with a high yield. The intrinsic dielectric properties of 3D M–NxC were identified for the first time at the atomic-level, revealing that the introduction of metal single atoms greatly increases both conductive loss and polarization loss. 3D Mn–NC exhibited high-performance electromagnetic wave absorption at a low filler loading of 10 wt% outperforming most reported absorbers. Atomically dispersed metals on N-doped carbon supports (M–NxCs) have great potential applications in various fields. However, a precise understanding of the definitive relationship between the configuration of metal single atoms and the dielectric loss properties of M–NxCs at the atomic-level is still lacking. Herein, we report a general approach to synthesize a series of three-dimensional (3D) honeycomb-like M–NxC (M = Mn, Fe, Co, Cu, or Ni) containing metal single atoms. Experimental results indicate that 3D M–NxCs exhibit a greatly enhanced dielectric loss compared with that of the NC matrix. Theoretical calculations demonstrate that the density of states of the d orbitals near the Fermi level is significantly increased and additional electrical dipoles are induced due to the destruction of the symmetry of the local microstructure, which enhances conductive loss and dipolar polarization loss of 3D M–NxCs, respectively. Consequently, these 3D M–NxCs exhibit excellent electromagnetic wave absorption properties, outperforming the most commonly reported absorbers. This study systematically explains the mechanism of dielectric loss at the atomic level for the first time and is of significance to the rational design of high-efficiency electromagnetic wave absorbing materials containing metal single atoms. [ABSTRACT FROM AUTHOR]

Highlights: A general method was developed to fabricate a series of honeycomb-like N-doped nanocarbons (3D M–NxC) doped with metal single atoms (Mn, Fe, Co, Cu, or Ni) with a high yield. The intrinsic dielectric properties of 3D M–NxC were identified for the first time at the atomic-level, revealing that the introduction of metal single atoms greatly increases both conductive loss and polarization loss. 3D Mn–NC exhibited high-performance electromagnetic wave absorption at a low filler loading of 10 wt% outperforming most reported absorbers. Atomically dispersed metals on N-doped carbon supports (M–NxCs) have great potential applications in various fields. However, a precise understanding of the definitive relationship between the configuration of metal single atoms and the dielectric loss properties of M–NxCs at the atomic-level is still lacking. Herein, we report a general approach to synthesize a series of three-dimensional (3D) honeycomb-like M–NxC (M = Mn, Fe, Co, Cu, or Ni) containing metal single atoms. Experimental results indicate that 3D M–NxCs exhibit a greatly enhanced dielectric loss compared with that of the NC matrix. Theoretical calculations demonstrate that the density of states of the d orbitals near the Fermi level is significantly increased and additional electrical dipoles are induced due to the destruction of the symmetry of the local microstructure, which enhances conductive loss and dipolar polarization loss of 3D M–NxCs, respectively. Consequently, these 3D M–NxCs exhibit excellent electromagnetic wave absorption properties, outperforming the most commonly reported absorbers. This study systematically explains the mechanism of dielectric loss at the atomic level for the first time and is of significance to the rational design of high-efficiency electromagnetic wave absorbing materials containing metal single atoms. [ABSTRACT FROM AUTHOR]

Metal-organic frames derived magnetic nanostructures have potential applications in various fields. However, the magnetic particles in the derivatives are frequently agglomerated, limiting their physicochemical properties. Herein, we cross CoZn-MOF particles through ultra-long MnO 2 nanowires (MnO 2 @CoZn-MOF) as precursor for high-performance functional materials. After the carbonization of the MnO 2 @CoZn-MOF, MnO 2 nanowires were transformed into MnO nanowires coated with N -doped carbon layer (MnO@CN), while the CoZn-MOFs were converted into N -doped carbon polyhedron particles encapsulated with Mn 3 Co 7 alloy nanoparticles (Mn 3 Co 7 @CN), leading to the formation of hierarchically pearl necklace-like nanostructure (CoMn@CN). The magnetic Mn 3 Co 7 @CN nanoparticles are separated uniformly along one-dimensional MnO@CN nanowires, and thus the self-agglomeration of magnetic nanoparticles is avoided efficiently. As a result, the CoMn@CN exhibits excellent electromagnetic wave absorption property with an effective absorption bandwidth of 5.24 GHz as the matching thickness is only 2.0 mm. Experimental results and theory calculations indicate that the improved conductive loss and dielectric relaxation loss caused by abundant heterointerfaces and the electronic interaction between Co and Mn atoms are responsible for the excellent electromagnetic wave absorption performance. Our strategy provides an efficient way for the synthesis of high-performance MOFs-derived nanostructures. [Display omitted] [ABSTRACT FROM AUTHOR]

Combining hollow materials with magnetic particles to construct hierarchical structures favors enhancement of microwave absorption properties. However, the fabrication of the well-defined materials possessing both hollow and hierarchical features still faces a huge challenge. Herein, a facile two-step method is developed to synthesize hierarchically nitrogen-doped carbon hollow microspheres assembled with loose and porous magnetic carbon sheets. Especially, many magnetic Fe/Fe 3 C NPs covered with N -doped graphitic layers are embedded in the mesoporous N -doped carbon sheets layers. Benefited from the special structure, the optimal nitrogen-doped carbon hollow microspheres exhibit an excellent microwave absorption property with an EAB of ∼4.00 GHz at a thickness of 2.5 mm and the minimal reflection loss of −39.87 dB at a frequency of 7.76 GHz, while the filler ratio is only 10 wt% in the paraffin wax. Electromagnetic parameters analysis reveals that unique hierarchical structure and rich heterogeneous interfaces result in the enhancement of dielectric and magnetic losses as well as optimized the impedance matching. The computer simulation technology (CST) simulation further verifies the practical applicability of the optimal nitrogen-doped carbon hollow microspheres. This work provides an effective way for the efficient synthesis of hierarchical and hollow materials for microwave absorption. [Display omitted] • The hierarchical and hollow microspheres based on magnetic particles were prepared. • The microspheres were assembled with loose and porous magnetic carbon sheets. • EAB of the optimal sample is about 4.0 GHz with a loading of only 10 wt%. • The enhanced property is due to the hierarchical and hollow features of the sample. [ABSTRACT FROM AUTHOR]

Hierarchical and hollow heteronanostructures had attached wide attention in electromagnetic wave (EMW) absorption fields due to their adjustable impedance matching and loss property. However, fabrication of well-defined heteronanomaterials with hierarchical and hollow features is challenging. Here, we develop a simple method to prepare N-doped carbon hollow spheres supported by N-doped carbon nanotubes (Ni@NCNT/NCHSs). Due to the hierarchical and hollow features, the Ni@NCNT/NCHSs exhibit high-efficiency EMW absorption performance. With a thickness only 1.6 mm, the optimized heteronanostructure has an RL min of −64.75 dB and an EAB of 4.17 GHz. The experimental results demonstrate that the hierarchical and hollow heteronanostructures have significantly enhanced dielectric loss and conductive loss of the Ni@NCNT/NCHSs as well as the impedance matching characteristics, which are responsible for their enhanced EMW absorption performances. This work presents an effective method to synthesize hierarchical and hollow heteronanostructures for high-performance EMW absorption. [Display omitted] • N-doped CNTs were prepared and supported by N-doped hollow carbon sphere for electromagnetic wave absorption. • The hierarchical and heteronanostructures are beneficial to enhance the interfacial loss. • The hollow carbon sphere reduces the material density and improves the impedance matching characteristics. • The NCNT/NCHS shows good absorption performance with a filling load of only 15 wt%. [ABSTRACT FROM AUTHOR]

Three-dimensional magnetic metal nanoparticle-encapsulated nitrogen-doped carbon nanotube arrays (3D M@NCNTS) are fabricated through a facile method. The unique 3D architectures have abundant defects, various N dopants, numerous interfaces, and void spaces. Benefitting from the advantages above, our designed 3D M@NCNTSs exhibit excellent electromagnetic wave absorption properties including strong attenuation abilities, thin thickness of absorber film, and low filler mass loading, superior to most of reported magnetic metal-based absorbers. Typically, the minimal reflection loss of the optimized 3D architecture is – 49.82 dB at a frequency of 7.92 GHz, and efficient absorption bandwidth is 4 GHz as the thickness of the absorber film is merely 2 mm. Furthermore, we also find that the dielectric losses of the 3D architectures can be tuned by the metal composition. Thus, the controllable electromagnetic wave absorption properties can be achieved. Our strategy here may be extended to synthesize other types of 3D architectures as high-performance electromagnetic wave absorbers. Image 1 • CoNi NP-encapsulated N-doped CNT arrays are fabricated through a facile method. • 3D architectures have defects, N dopants, numerous interfaces, and void spaces. • 3D CoNi@NCNTSs exhibit excellent electromagnetic wave absorption properties. • Dielectric losses of 3D architectures can be tuned by metal composition. [ABSTRACT FROM AUTHOR]

Micro-nanospheres assembled with helically coiled nitrogen-doped carbon nanotube (HNCNT) are successfully fabricated using CoZn double hydroxide nanosheets as substrates and catalytic precursors and dicyandiamide as carbon and nitrogen sources. The micro-nanospheres have diameters of 3–5 μm, composed of helically coiled NCNTs with diameters of 40–100 nm. The metallic Co NPs covered with graphene shell were encapsulated in HNCNTs. Experimental results demonstrate that both Co and Zn are important to the formation of the specially coiled NCNTs. The optimized micro-nanostructures exhibit excellent microwave absorption property with a minimal reflection loss of −44.2 dB at the thickness of absorber film (d) of only 1.7 mm, and efficient absorption bandwidth (reflection loss less than −10 dB) of 4.8 GHz at d of 2.0 mm. Furthermore, even at a thin thickness (1.5–2.0 mm) the minimal reflection losses of the optimized micro-nanostructures can reach −10 dB. The enhanced microwave absorption property can be explained by increased dielectric loss caused by defects and N dopants, better impedance matching characteristic, and the unique helically coiled structure. Image 1 • Micro-nanospheres assembled with helically coiled carbon nanotube are designed. • Both cobalt and zinc are important to produce the coiled carbon nanotubes. • Nitrogen-doping, single atom sites and defects are formed in carbon nanotubes. • The micro-nanospheres have excellent microwave absorption property. [ABSTRACT FROM AUTHOR]