Although transition metal carbides/carbonitrides (MXenes) exhibit immense potential for electromagnetic wave (EMW) absorption, their absorbing ability is hindered by facile stacking and high ...permittivity. Layer stacking and geometric structures are expected to significantly affect the conductivity and permittivity of MXenes. However, it is still a formidable task to simultaneously regulate layer stacking and microstructure of MXenes to realize high‐performance EMW absorption. Herein, a simple and viable strategy using electrostatic adsorption is developed to integrate 2D Ti3C2Tx MXene nanosheets into 3D hollow bowl‐like structures with tunable layer stacking thickness. Density functional theory calculations indicate an increase in the density of states of the d orbital from the Ti atom near the Fermi level and the generation of additional electrical dipoles in the MXene nanosheets constituting the bowl walls upon reducing the layer stacking thickness. The hollow MXene bowls exhibit a minimum reflection loss (RLmin) of −53.8 dB at 1.8 mm. The specific absorbing performance, defined as RLmin (dB)/thickness (mm)/filler loading (wt%), exceeds 598 dB mm−1, far surpassing that of the most current MXene and bowl‐like materials reported in the literature. This work can guide future exploration on designing high‐performance MXenes with “lightweight” and “thinness” characteristics for superior EMW absorption.
A simple and viable strategy using electrostatic adsorption is developed to integrate 2D Ti3C2Tx MXene nanosheets into 3D hollow bowl‐like structures with tunable layer stacking thickness. The layer stacking and geometric structures of the MXenes are simultaneously regulated to realize high‐performance electromagnetic wave absorption due to improved Ti vacancies, lattice distortions, degree of aggregation/stacking, and hollow structure.
Surface chemistry and interlayer engineering determines the electrical properties of 2D MXene. However, it remains challenging to regulate the surface and interfacial chemistry of MXene ...simultaneously. Herein, simultaneous modulation of Ti3C2Tx MXene surface termination and layer spacing by alkali treatment are achieved. The electrical and electromagnetic properties of Ti3C2Tx are investigated in detail with respect to KOH and ammonia concentration dependence. A high concentration of KOH caused the Ti3C2Tx layer spacing to expand to 13.7 Å and the surface O/F ratio to increase to 33.84. Because of its weaker ionization effect, ammonia provides finer tuning compared to the drastic intercalation of KOH with a thorough sweeping of the F‐containing groups. Ti3C2Tx is enriched with conductive ‐OH termination after ammonia treatment, which achieves an effective balance with the increased interlayer resistance. Therefore, NH3H2O‐Ti3C2Tx achieves broad‐band impedance matching and exhibits an efficient microwave loss of −49.1 dB at a low thickness of 1.7 mm, with an effective frequency bandwidth of 3.9 GHz. The results herein optimize the electrical properties of Ti3C2Tx using surface and interfacial chemistry to achieve broad microwave absorption, providing a framework for enhancing the electromagnetic wave loss of intrinsic MXene.
Surface chemistry and interlayer engineering determines the electrical properties of 2D MXene. The electrical and electromagnetic properties of Ti3C2Tx are investigated in detail with respect to KOH and ammonia concentrations, and the electrical properties of Ti3C2Tx are optimized using surface and interfacial chemistry to provide a framework for enhancing the electromagnetic wave loss of intrinsic MXene.
High‐efficiency electromagnetic (EM) wave (EMW)‐absorbing materials have attracted extensive scientific and technical interest. Although identifying the dominant EM loss mechanism in dielectric‐loss ...materials is indispensable, it is challenging due to a complex synergism between dipole/interfacial polarization and conduction loss. Modulation of defects and microstructures can be a possible approach to determine the dominant EM loss mechanism and realize high‐efficiency absorption. Herein, 2D reduced graphene oxide (rGO) flakes are integrated into a 3D hollow bowl‐like structure, which increases defect sites (i.e., oxygen vacancy and lattice defect) and reduces the stacked thickness of rGO. Despite their lower stacked thicknesses, the hollow rGO bowls with more defects exhibit lower conductivities but higher permittivities. Accompanied by the transformation from 2D flakes to 3D hollow bowls, the dominant EM loss mechanism of rGO transforms from conduction loss to defect‐induced polarization. Furthermore, the defect engineering and structural design endow rGO with well‐matched impedance and strong EMW‐absorbing capacity. A minimum reflection loss of −41.6 dB (1.3 mm) and an effective absorption bandwidth of 4.8 GHz (1.5 mm) is achieved at a filler loading of 5 wt%. This study will provide meaningful insights into the development of materials with superior EMW‐absorbing performances via defect engineering and structural design.
Accompanied by the transformation from 2D flakes to 3D hollow bowls, the dominant electromagnetic loss mechanism of reduced graphene oxide transforms from conduction loss to defect‐induced polarization. Furthermore, the defect‐induced polarization gradually intensifies with the increase in the defect content, ultimately forming cumulative dominance in the multiple loss mechanisms, namely, the Matthew effect.
Abstract
Although transition metal carbides/carbonitrides (MXenes) exhibit immense potential for electromagnetic wave (EMW) absorption, their absorbing ability is hindered by facile stacking and high ...permittivity. Layer stacking and geometric structures are expected to significantly affect the conductivity and permittivity of MXenes. However, it is still a formidable task to simultaneously regulate layer stacking and microstructure of MXenes to realize high‐performance EMW absorption. Herein, a simple and viable strategy using electrostatic adsorption is developed to integrate 2D Ti
3
C
2
T
x
MXene nanosheets into 3D hollow bowl‐like structures with tunable layer stacking thickness. Density functional theory calculations indicate an increase in the density of states of the d orbital from the Ti atom near the Fermi level and the generation of additional electrical dipoles in the MXene nanosheets constituting the bowl walls upon reducing the layer stacking thickness. The hollow MXene bowls exhibit a minimum reflection loss (RL
min
) of −53.8 dB at 1.8 mm. The specific absorbing performance, defined as RL
min
(dB)/thickness (mm)/filler loading (wt%), exceeds 598 dB mm
−1
, far surpassing that of the most current MXene and bowl‐like materials reported in the literature. This work can guide future exploration on designing high‐performance MXenes with “lightweight” and “thinness” characteristics for superior EMW absorption.
Dual-phase high entropy oxide (HEO) ceramics (FeCoNiZn)xV2Oy were prepared using a simple sintering process. Vanadium oxide was selected due to the characteristics of reduction ability and multiphase ...transformation under different conditions. The crystal structure, microstructure, elemental chemical state, magnetic property and microwave absorption (MA) properties of HEO ceramics were studied. The results show that the two-phase structure is spinel phase and vanadate phase, and there are oxygen vacancies in the samples. Magnetic properties can be promoted with high content of spinel phase. Lots of oxygen vacancies were found in the sample sintered at 900 °C, which exhibited good MA ability. The minimum reflection loss (RL) of −36.5 dB was obtained at 10.72 GHz with a thickness of 2.2 mm, while the effective bandwidth reached 2.77 GHz. Large number of oxygen vacancies can speed up the migration of electrons or ions, thereby improving the dielectric loss capability.
•Comparison between bowl-like and honeycomb composites is reported for the first time.•Bowl-like structure raises polarization and conduction losses, multiplies reflections.•Honeycomb structure is ...conducive to optimizing impedance matching.•This work provides platform for effect of structural design on EMW absorption.
Materials that can efficiently absorb electromagnetic waves (EMWs) are required to deal with electromagnetic pollution. Structure design appears to be an efficient way to improve the EMW-absorption performance of such materials, particularly when adjustment of the constitution or mixing ratio is limited. In this study, bowl-like and honeycomb titanium dioxide/carbon nanotube (TiO2/CNT) composites with different CNT contents were fabricated using the methods of hierarchical and mixing vacuum-assisted filtration, respectively. Compared to the honeycomb structure, the bowl-like structure simultaneously facilitated greater interfacial polarization and conduction loss in favor of dielectric polarization, and augmented multiple reflections. The high porosity of the honeycomb structure was conducive to optimizing the impedance matching characteristics. The bowl-like TiO2/CNT composite exhibited a minimum reflection loss (RLmin) of −38.6 dB (1.5 mm) with a wide effective absorption band (EAB; <−10 dB) of 4.2 GHz, while the honeycomb TiO2/CNT composite showed an RLmin of −34.8 dB (2.1 mm) with an EAB of 4.3 GHz. The required mixing ratio in the matrix was only 15 wt%, outperforming that of the most closely related composites. Thus, both the bowl-like and honeycomb TiO2/CNT composites are ideal candidates for light-weight and highly efficient EMW-absorbing materials.
Sodium-ion batteries (SIBs) have received much attention for scalable electrical energy storage because of the abundance and wide availability of sodium resources. However, it is still unclear ...whether carbon anodes can realize large-scale commercial application in SIBs as in lithium-ion batteries. Recently, great attention has been devoted to hard carbon which has been treated as a promising choice. Herein, we observe that the turbostratic lattice of kelp-derived hard carbon (KHC) is repeatedly expandable and shrinkable upon cycling, where the interlayer distance varies between 3.9 and 4.3 Aa. Such interlayer spacing dilation is highly reversible, giving rise to high rate capability (a stable capacity of 96 mA h g-1 at 1000 mA g-1) and excellent cycling performance (205 mA h g-1 after 300 cycles at 200 mA g-1). Furthermore, kelp-derived hard carbon exhibits a good specific capacity at potentials higher than 0.05 V, which make it an essentially dendrite-free anode for SIBs.
