Broadband and Lightweight Microwave Absorber Constructed by in Situ Growth of Hierarchical CoFe2O4/Reduced Graphene Oxide Porous Nanocomposites

Heterogeneous interfaces in 3D interconnected networks of flower-like 1T/2H Molybdenum disulfide nanosheets and carbon-fibers boosts superior EM wave absorption

With the wide application of electromagnetic waves in national defense, communication, navigation and home appliances, the electromagnetic pollution problem is becoming more and more prominent. Therefore, high-performance, and low-density composite wave-absorbing materials have attracted much attention. In this paper, three-dimensional (3D) network structures of flower-like 1T/2H Molybdenum disulfide nanosheets anchored to carbon fibers (1T/2H MoS2/CNFs) were prepared by electrostatic spinning technique and calcination process. The morphology and electromagnetic wave absorption properties were tuned by changing the content of flower-like MoS2. The optimized 1T/2H MoS2/CNFs composite exhibits superior electromagnetic wave absorption with minimum reflection (RLmin) of -42.26 dB and effective absorption bandwidth (EAB) of 6.48 GHz at 2.5 mm. Multi-facts contribute to the super performance. First, the uniquely designed nanosheet and 3D interconnected networks leads to multiple reflection and scattering of electromagnetic waves, which promotes the attenuation of electromagnetic waves. Second, the propriate content of CNFs and MoS2 with different phase regulates its impedance matching characteristic. Third, Numerous heterogeneous interfaces existed between CNFs and MoS2, 1T and 2H MoS2 phase results in interface polarization. Besides, the 1T/2H MoS2 rich in defects induces defect polarization, improving the dielectric loss. Furthermore, the electromagnetic wave absorption performance was proved via radar reflectance cross section simulation. This work illustrates 1T/2H MoS2/CNFs is a promising material for electromagnetic absorption with wide bandwidth, strong absorption, low density, and high thermal stability.

In Situ Loading of Ni3ZnC0.7 Nanoparticles with Carbon Nanotubes to 3D Melamine Sponge Derived Hollow Carbon Skeleton toward Superior Microwave Absorption and Thermal Resistance

The simple and low-cost construction of a 3D network structure is an ideal way to prepare high-performance electromagnetic wave (EMW) absorption materials. Herein, a series of carbon skeleton/carbon nanotubes/Ni3ZnC0.7 composites (CS/CNTs/Ni3ZnC0.7) are successfully prepared by in situ growth of Ni3ZnC0.7 and CNTs on 3D melamine sponge carbon. With the increase of precursor, Ni3ZnC0.7 nanoparticles nucleate and catalyze the generation of CNTs on the surface of the carbon skeleton. The minimum reflection loss (RL) value of the S60min composite (loading time of 60 min) reaches -86.6 dB at 1.6 mm and effective absorption bandwidth (EAB, RL≤-10 dB) is up to 9.3 GHz (8.7-18 GHz). The 3D network sponge carbon with layered micro/nanostructure and hollow skeleton promotes multiple reflection and absorption mechanisms of incident EMW. The N-doping and defects can be equivalent to an electric dipole, providing dipole polarization to increase dielectric relaxation. The uniform Ni3ZnC0.7 nanoparticles and CNTs play a key role in dissipating electromagnetic energy, blocking heat transfer, and enhancing the mechanical properties of the skeleton. Fortunately, the composite displays a quite low thermal conductivity of 0.09075 W m·K-1 and good flexibility, which can provide insulation and quickly recover to its original state after being stressed.

Radio-Absorbing Magnetic Polymer Composites Based on Spinel Ferrites: A Review

Ferrite-containing polymer composites are of great interest for the development of radar-absorbing and -shielding materials (RAMs and RSMs). The main objective of RAM and RSM development is to achieve a combination of efficient electromagnetic wave (EMW) absorption methods with advantageous technological and mechanical properties as well as acceptable weight and dimensions in the final product. This work deals with composite RAMs and RSMs containing spinel-structured ferrites. These materials are chosen since they can act as efficient RAMs in the form of ceramic plates and as fillers for radar-absorbing polymer composites (RAC) for electromagnetic radiation (EMR). Combining ferrites with conducting fillers can broaden the working frequency range of composite RAMs due to the activation of various absorption mechanisms. Ferrite-containing composites are the most efficient materials that can be used as the working media of RAMs and RSMs due to a combination of excellent dielectric and magnetic properties of ferrites. This work contains a brief review of the main theoretical standpoints on EMR interaction with materials, a comparison between the radar absorption properties of ferrites and ferrite-polymer composites and analysis of some phenomenological aspects of the radar absorption mechanisms in those composites.

Effect of carbon allotropes and thickness variation on the EMI shielding properties of PANI/NFO@CNTs and PANI/NFO@RGO ternary composite systems

The innovative design of thin, multiphase flexible composite systems with good mechanical properties, low density and improved EMI shielding properties at low filler content has become a key area of research. In this work, we report the low temperature synthesis of three-dimensional ternary composites (PANI/NFO@CNTs and PANI/NFO@RGO) by oxidative chemical polymerization of aniline in the presence of two different binary composites, viz. NFO@CNTs and NFO@RGO. Enhanced impedance matching is achieved by varying the ratio of the carbon allotropes (CNTs and RGO) to the ferrite component. The synthesis of NFO, PANI/NFO@CNTs and PANI/NFO@RGO is validated by XRD and FTIR spectroscopy. Field emission scanning electron microscopy (FE-SEM) confirmed the synthesis of core-shell structures of PANI/NFO@CNTs and PANI/NFO@RGO, where the binary composites (NFO@CNTs and NFO@RGO) serve as a core onto which a tubular PANI layer was coated. Shielding effectiveness of 22.36 dB (99.41% attenuation) is exhibited by the ternary composite PANI/NFO@CNTs (8 : 1), while for PANI/NFO@RGO (20 : 1) a total shielding effectiveness of 31 dB equivalent to 99.92% attenuation was observed at a thickness of 2 mm. The ternary composite PANI/NFO@RGO (20 : 1) 4 mm showed a maximum SET of 43 dB corresponding to 99.996% attenuation of incident EM waves. The enhanced EMI shielding properties of the synthesized ternary composite systems are accredited to good impedance matching, effective dielectric and magnetic loss mechanisms and good conductivity, which facilitate multiple reflections and scattering of incident radiation.

High-Temperature, Lightweight Ceramics with Nano-Sized Ferrites for EMI Shielding: Synthesis, Characterisation, and Potential Applications

The present study focuses on the synthesis and characterisation of a lightweight ceramic material with electromagnetic interference (EMI) shielding properties, achieved using mullite containing micrometre-sized hollow spheres (cenospheres) and CoFe2O4 nanoparticles. This research explores compositions with varying CoFe2O4 contents ranging from 0 up to 20 wt.%. Conventional sintering in an air atmosphere is carried out at a temperature between 1100 and 1300 °C. The addition of ferrite nanoparticles was found to enhance the process of sintering cenospheres, resulting in improved material density and mechanical properties. Furthermore, this study reveals a direct correlation between the concentration of ferrite nanoparticles and the electromagnetic properties of the material. By increasing the concentration of ferrite nanoparticles, the electromagnetic shielding effect of the material (saturation magnetisation (Ms) and remanent magnetisation (Mr)) was observed to strengthen. These findings provide valuable insights into designing and developing lightweight ceramic materials with enhanced electromagnetic shielding capabilities. The synthesized ceramic material holds promise for various applications that require effective electromagnetic shielding, such as in the electronics, telecommunications, and aerospace industries.

Microstructure optimization strategy of ZnIn2S4/rGO composites toward enhanced and tunable electromagnetic wave absorption properties

Although microstructure optimization is an effective strategy to improve and regulate electromagnetic wave (EMW) absorption properties, preparing microwave absorbents with enhanced EMW absorbing performance and tuned absorption band by a simple method remains challenging. Herein, ZnIn2S4/reduced graphene oxide (rGO) composites with flower-like and cloud-like morphologies were fabricated by a convenient hydrothermal method. The ZnIn2S4/rGO composites with different morphologies realize efficient EMW absorption and tunable absorption bands, covering a wide frequency range. The flower-like structure has an optimal reflection loss (RL) of up to -49.2 dB with a maximum effective absorption bandwidth (EAB) of 5.7 GHz, and its main absorption peaks are concentrated in the C and Ku bands. The minimal RL of the cloud-like structure can reach -36.3 dB, and the absorption peak shifts to the junction of X and Ku bands. The distinguished EMW absorption capacity originates from the uniquely optimized microstructure, complementary effect of ZnIn2S4 and rGO in dielectric constant, and synergy of various loss mechanisms, such as interfacial polarization, dipole polarization, conductive loss, and multiple reflections. This study proposes a guide for the structural optimization of an ideal EMW absorber to achieve efficient and tunable EMW absorption performance.

Combination strategy of large interlayer spacing and active basal planes for regulating the microwave absorption performance of MoS2/MWCNT composites at thin absorber level

Recently, molybdenum disulfide (MoS₂)/carbon has become a promising candidate for efficient microwave absorption. However, it is still challenging to simultaneously optimize the synergy of impedance matching and loss capability at the level of a thin absorber. Here, a new adjustment strategy is proposed by changing the concentration of precursor l-cysteine for MoS₂/multi-walled carbon nanotubes (MWCNT) composites to unlock the basal plane of MoS₂ and expand the interlayer spacing from 0.62 nm to 0.99 nm, leading to improved packing of MoS₂ nanosheets and more active sites. Therefore, the tailored MoS₂ nanosheets exhibit abundant sulfur-vacancies, lattice-oxygen, more metallic 1T-phase, and higher surface area. Such sulfur-vacancies and lattice-oxygen promote the electronic asymmetric distribution at the solid-air interface of MoS₂ crystals and induce stronger microwave attenuation through interface/dipole polarization, which is further verified by first-principles calculations. In addition, the expansion of the interlayer spacing induces more MoS₂ to deposit on the MWCNT surface and increases the roughness, improving the impedance matching and multiple scattering. Overall, the advantage of this adjustment method is that while optimizing impedance matching at the thin absorber level, composite still maintains a high attenuation capacity, which means enhancing the attenuation performance of MoS₂ itself offsets the weakening of the composite's attenuation ability caused by the decrease in the relative content of MWCNT components. Most importantly, adjusting impedance matching and attenuation ability can be easily implemented by separate control of l-cysteine content. As a result, the MoS₂/MWCNT composites achieve a minimum reflection loss value of -49.38 dB and an effective absorption bandwidth of 4.64 GHz at a thickness of only 1.7 mm. This work provides a new vision for the fabrication of thin MoS₂-carbon absorbers.

Synthesis of a one-dimensional carbon nanotube-decorated three-dimensional crucifix carbon architecture embedded with Co7Fe3/Co5.47N nanoparticles for high-performance microwave absorption

Low-dimensional cell-decorated three-dimensional (3D) hierarchical structures are considered excellent candidates for achieving remarkable microwave absorption. In the present work, a one-dimensional (1D) carbon nanotube (CNT)-decorated 3D crucifix carbon framework embedded with Co₇Fe₃/Co_5.47N nanoparticles (NPs) was fabricated by the in-situ pyrolysis of a trimetallic metal-organic framework (MOF) precursor (ZIF-ZnFeCo). Co₇Fe₃/Co_5.47N NPs were uniformly dispersed on the carbon matrix. The 1D CNT nanostructure was well regulated on the 3D crucifix surface by changing the pyrolysis temperature. The synergistic effect of 1D CNT and the 3D crucifix carbon framework increased the conductive loss, and Co₇Fe₃/Co_5.47N NPs induced interfacial polarization and magnetic loss; thus, the composite manifested superior microwave absorption performance. The optimum absorption intensity was -54.0 dB, and the effective absorption frequency bandwidth reached 5.4 GHz at a thickness of 1.65 mm. The findings of this work could provide significant guidance for the fabrication of MOF-derived hybrids for high-performance microwave absorption applications.

In-situ preparation of CoFe2O4 nanoparticles on eggshell membrane-activated carbon for microwave absorption

This study explores the potential of using cobalt ferrite (CF) nanoparticles grown in situ on eggshell membranes (ESM) to mitigate the increasing problem of electromagnetic interference (EMI). A simple carbonization process was adopted to synthesize CF nanoparticles on ESM. The study further examines the composites' surface morphology and chemical composition and evaluates their microwave absorption performance (MAP) at X-band frequency. Results showed that the composite of CF and ESM - CESM@CF, exhibited a strong RL peak value of -39.03 mm with an optimal thickness of 1.5 mm. The combination of CF and ESM demonstrates excellent impedance matching and EM wave attenuation. The presence of numerous interfaces, conduction loss from the morphology, interfacial polarisation, and dual influence from both CF and ESM contribute to the high MAP of the composite. CESM@CF composite is projected as an excellent biomass-based nano-composite for EM wave absorption applications.

Synthesis and characterization of Ag2O, CoFe2O4, GO, and their ternary composite for antibacterial activity

Currently, nanomaterials with exceptional antibacterial activity have become an emerging domain in research. The optimization of nanomaterials against infection causing agents is the next step in dealing with the present-day problem of antibiotics. In this research work, Ag₂O, CoFe₂O₄, and Ag₂O/CoFe₂O₄/rGO are prepared by chemical methods. Ag₂O was prepared by co-precipitation method, while solvothermal technique was utilized for the synthesis of CoFe₂O₄. The ternary nanocomposite was synthesized by a simple in situ reduction using a two-step approach. The structural and morphological properties were studied by UV-Vis spectroscopy, X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (SEM), dynamic light scattering (DLS), and Fourier-transform infrared spectroscopy (FTIR). From the X-ray diffraction analysis, the crystallite size is found to be 14 nm, 5 nm, and 6 nm for Ag₂O, CoFe₂O₄, and Ag₂O/CoFe₂O₄/rGO respectively. The synthesized nanomaterials were investigated for antibacterial activities against gram-positive strain Staphylococcus aureus (S. aureus) and gram-negative strain Escherichia coli (E. coli) using Agar well diffusion method. Ag₂O and CoFe₂O₄ showed zones of inhibition (ZOI) of 13 mm and 11 mm against gram positive bacteria while 12 mm against gram negative bacteria respectively, while ternary nanocomposite showed 14 mm and 13 mm of ZOI. The antibacterial activity of nanomaterials showed a gradual increment with an increase in the concentration of the materials. Ag₂O, CoFe₂O₄, and Ag₂O/CoFe₂O₄/rGO showed minimum inhibitory concentration (MIC) values of 4.5, 6.5, and 4.5 μg/mL for S. aureus and 6.5, 7.2, and 4.8 μg/mL for E. coli respectively. Minimum bactericidal concentrations were found to be same as the MIC values. Additionally, a time-kill curve analysis was performed and for ternary nanocomposite; the killing response was most effective as the complete killing was achieved at 3 h of incubation at 3-MIC (9.75 μg/mL). These results demonstrate that all the nanomaterials, as a kind of antibacterial material, have a great potential for a wide range of biomedical applications.

Morphology-Evolved Succulent-like FeCo Microarchitectures with Magnetic Configuration Regulation for Enhanced Microwave Absorption

The regulation of magnetic configuration through diverse morphologies to achieve a rapid magnetic response has attracted considerable academic favor on account of the unique application prospects in various fields. Herein, porous FeCo alloys with morphology evolved from spheres to succulent-like microstructures are successfully constructed via a facile hydrothermal reaction-hydrogen reduction synthetic strategy. A multiple balance/competition mechanism is proposed, including the coexistence of the dissolution-precipitation balance of hydroxides and the dissociation-stability balance of coordination compounds, the Fe³⁺-Co²⁺ competition, and the precipitation-coordination reaction contest. As the morphology evolves to a succulent-like assembly, the multidomain features with a stable combination of vortex states and the violent motion of magnetic vectors contribute to the improvement of magnetic storage capacity and loss capability, which are evidenced by the off-axis electron holography and micromagnetic simulation. Consequently, the succulent-like FeCo exhibits enhanced permeability and microwave absorption performance. The effective absorption bandwidth reaches 5.68 GHz, and the maximum reflection loss is elevated to -53.81 dB. This work sheds considerable insight into the microstructure regulation with an application in microwave absorption and offers guidance in research for the topological magnetic configuration and dynamic response mechanism of magnetic alloys.

Architecture Design and Interface Engineering of Self-assembly VS4/rGO Heterostructures for Ultrathin Absorbent

The employment of microwave absorbents is highly desirable to address the increasing threats of electromagnetic pollution. Importantly, developing ultrathin absorbent is acknowledged as a linchpin in the design of lightweight and flexible electronic devices, but there are remaining unprecedented challenges. Herein, the self-assembly VS4/rGO heterostructure is constructed to be engineered as ultrathin microwave absorbent through the strategies of architecture design and interface engineering. The microarchitecture and heterointerface of VS4/rGO heterostructure can be regulated by the generation of VS4 nanorods anchored on rGO, which can effectively modulate the impedance matching and attenuation constant. The maximum reflection loss of 2VS4/rGO40 heterostructure can reach - 43.5 dB at 14 GHz with the impedance matching and attenuation constant approaching 0.98 and 187, respectively. The effective absorption bandwidth of 4.8 GHz can be achieved with an ultrathin thickness of 1.4 mm. The far-reaching comprehension of the heterointerface on microwave absorption performance is explicitly unveiled by experimental results and theoretical calculations. Microarchitecture and heterointerface synergistically inspire multi-dimensional advantages to enhance dipole polarization, interfacial polarization, and multiple reflections and scatterings of microwaves. Overall, the strategies of architecture design and interface engineering pave the way for achieving ultrathin and enhanced microwave absorption materials.

Lightweight and broadband 2D MoS2 nanosheets/3D carbon nanofibers hybrid aerogel for high-efficiency microwave absorption

Three-dimensional (3D) porous molybdenum disulfide nanosheets/carbon nanofibers (MoS₂/CNF) hybrid aerogels were synthesized by using solvothermal method and following carbonization, where two-dimensional (2D) MoS₂ nanosheets were homogenously in-situ grown on the interconnected CNF skeleton derived from bacterial cellulose, forming a hierarchical porous structure. This unique heterogeneous structure of the MoS₂/CNF hybrid aerogels were conducive to electromagnetic loss, including conduction, polarization, multi-scatterings, and reflections, thus resulting in a balanced impedance matching and microwave attenuation capacity. It was found that the resulted MoS₂/CNF hybrid aerogels demonstrate excellent microwave absorbing performance when the only 5.0 wt% fillers were loaded in paraffin. Particularly, MoS₂/CNF-2-900 hybrid aerogel displayed an effective absorption bandwidth of 5.68 GHz and minimum reflection loss (RL_min) value of -36.19 dB at a thickness of 2.0 mm. As the thickness increases to 4.4 mm, the RL_min value of MoS₂/CNF-2-900 hybrid aerogel reaches -48.53 dB. Electromagnetic loss mechanism analysis indicates that such improved microwave attenuation is attributed to proper component, multiple heterogenous interface and hierarchical porous structures. All the results in this work pave the avenue for the development of ultralight microwave absorber with high absorption capacity as well as broad effective absorption bandwidth.

Fabrication of Electrospun Ni0.5Zn0.5Fe2O4 Nanofibers Using Polyvinyl Pyrrolidone Precursors and Electromagnetic Wave Absorption Performance Improvement

Ni0.5Zn0.5Fe2O4 nanofibers with an average diameter of 133.56 ± 12.73 nm were fabricated by electrospinning and calcination. According to our thermogravimetric-differential thermal analysis and X-ray diffraction results, the calcination temperature was 650 °C. The microstructure, crystal structure, and chemical composition of the nanofibers were observed using field-emission scanning electron, X-ray diffraction, and energy-dispersive X-ray spectroscopy. Commercial particle samples and samples containing 10 wt% and 20 wt% nanofibers were fabricated, and the electromagnetic properties were analyzed with a vector network analyzer and a 7.00 mm coaxial waveguide. Regardless of the nanofiber content, Ni0.5Zn0.5Fe2O4 was dominantly affected by the magnetic loss mechanism. Calculation of the return loss based on the transmission line theory confirmed that the electromagnetic wave return loss was improved up to -59.66 dB at 2.75 GHz as the nanofiber content increased. The absorber of mixed compositions with Ni0.5Zn0.5Fe2O4 nanofibers showed better microwave absorption performance. It will be able to enhance the performance of commercial electromagnetic wave absorbers of various types such as paints and panels.

Implantation of WSe2 nanosheets into multi-walled carbon nanotubes for enhanced microwave absorption

Microwave absorption materials can protect humanity from harmful electromagnetic radiation, but it is still a challenge to absorb electromagnetic radiation with different bands simultaneously. Herein, an effective strategy for obtaining WSe₂@CNTs nanohybrids is reported. The conductive network and polarization of WSe₂@CNTs nanohybrids can be tailored by confinedly implanting WSe₂ nanosheets on multi-walled carbon nanotubes. The electromagnetic properties and microwave absorption performance of the nanohybrids are effectively adjusted via changing the hybrid ratio of WSe₂ and CNTs. Multi-band microwave absorption is achieved with up to three bands. The reflection loss (RL) of the sample can reach -60.1 dB, and the bandwidth can reach 4.24 GHz (RL ≤ -10 dB). The excellent microwave absorption performance is attributed to the conductance and multiple relaxations, as well as the synergistic effect of the two. This result confirms that WSe₂@CNTs nanohybrids are potential candidates for high-efficiency microwave absorbers and provide a valuable pathway for designing high-performance microwave absorption materials in the future.

High-Performance, Lightweight, and Flexible Thermoplastic Polyurethane Nanocomposites with Zn2+-Substituted CoFe2O4 Nanoparticles and Reduced Graphene Oxide as Shielding Materials against Electromagnetic Pollution

The development of flexible, lightweight, and thin high-performance electromagnetic interference shielding materials is urgently needed for the protection of humans, the environment, and electronic devices against electromagnetic radiation. To achieve this, the spinel ferrite nanoparticles CoFe2O4 (CZ1), Co0.67Zn0.33Fe2O4 (CZ2), and Co0.33Zn0.67Fe2O4 (CZ3) were prepared by the sonochemical synthesis method. Further, these prepared spinel ferrite nanoparticles and reduced graphene oxide (rGO) were embedded in a thermoplastic polyurethane (TPU) matrix. The maximum electromagnetic interference (EMI) total shielding effectiveness (SET) values in the frequency range 8.2-12.4 GHz of these nanocomposites with a thickness of only 0.8 mm were 48.3, 61.8, and 67.8 dB for CZ1-rGO-TPU, CZ2-rGO-TPU, and CZ3-rGO-TPU, respectively. The high-performance electromagnetic interference shielding characteristics of the CZ3-rGO-TPU nanocomposite stem from dipole and interfacial polarization, conduction loss, multiple scattering, eddy current effect, natural resonance, high attenuation constant, and impedance matching. The optimized CZ3-rGO-TPU nanocomposite can be a potential candidate as a lightweight, flexible, thin, and high-performance electromagnetic interference shielding material.

Improvement of microwave absorption properties of polyester coatings using NiFe2O4, X-doped g-C3N4 (X = S, P, and O), and MTiO3 (M = Fe, Mg, and Zn) nanofillers

In recent decades, to reduce electromagnetic pollution, scientists focus on finding new microwave absorbers with effective performance, thin thickness, and broad bandwidth. In this work, the nanoparticles of NiFe₂O₄, X-doped g-C₃N₄ (M = S, P, and O), and MTiO₃ (M = Fe, Mg, and Zn) were successfully synthesized using co-precipitation, specific heat program, and semi-wet sol–gel methods, respectively. The synthesized nanoparticles were utilized as absorption agents and polyester resin as the matrix. Morphology, particle size, crystal structure, and chemical composition of the prepared nanocomposites were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffractometer (XRD), and energy dispersive X-Ray analysis (EDX), respectively. The microwave absorption performance of the coatings was also investigated by a vector network analyzer (VNA). Moreover, the effect of different parameters on the performance of absorbent coatings was studied by the Taguchi method and optimized to achieve an optimal absorbent. The results showed that the optimal nanocomposite has the reflectance loss (RL) less than − 30 dB (equal to absorption > 99%) at a high-frequency range (8–12 GHz) and 1 mm thickness. Furthermore, the addition of such novel nanoparticles to absorbents resulted in high values of attenuation constant (more than 200 dB/m) at the X-band. Therefore, the polyester coating filled with ZnTiO₃, O-doped g-C₃N₄, and NiFe₂O₄ nanofillers can be considered a high-efficiency and low-density absorber.

Flexible and Waterproof 2D/1D/0D Construction of MXene-Based Nanocomposites for Electromagnetic Wave Absorption, EMI Shielding, and Photothermal Conversion

ABSTRACT

High-performance electromagnetic wave absorption and electromagnetic interference (EMI) shielding materials with multifunctional characters have attracted extensive scientific and technological interest, but they remain a huge challenge. Here, we reported an electrostatic assembly approach for fabricating 2D/1D/0D construction of Ti₃C₂T_x/carbon nanotubes/Co nanoparticles (Ti₃C₂T_x/CNTs/Co) nanocomposites with an excellent electromagnetic wave absorption, EMI shielding efficiency, flexibility, hydrophobicity, and photothermal conversion performance. As expected, a strong reflection loss of -85.8 dB and an ultrathin thickness of 1.4 mm were achieved. Meanwhile, the high EMI shielding efficiency reached 110.1 dB. The excellent electromagnetic wave absorption and shielding performances were originated from the charge carriers, electric/magnetic dipole polarization, interfacial polarization, natural resonance, and multiple internal reflections. Moreover, a thin layer of polydimethylsiloxane rendered the hydrophilic hierarchical Ti₃C₂T_x/CNTs/Co hydrophobic, which can prevent the degradation/oxidation of the MXene in high humidity condition. Interestingly, the Ti₃C₂T_x/CNTs/Co film exhibited a remarkable photothermal conversion performance with high thermal cycle stability and tenability. Thus, the multifunctional Ti₃C₂T_x/CNTs/Co nanocomposites possessing a unique blend of outstanding electromagnetic wave absorption and EMI shielding, light-driven heating performance, and flexible water-resistant features were highly promising for the next-generation intelligent electromagnetic attenuation system. [Image: see text]

HIGHLIGHTS

The 2D/1D/0D Ti₃C₂T_x/carbon nanotubes/Co nanocomposite is successfully synthesized via an electrostatic assembly. Nanocomposites exhibit an excellent electromagnetic wave absorption and a remarkable electromagnetic interference shielding efficiency. The flexible, waterproof, and photothermal conversion performances are achieved.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40820-021-00673-9.

Rationally designed structure of mesoporous carbon hollow microspheres to acquire excellent microwave absorption performance

In this study, we used a novel and facile hard-template etching method to manufacture mesoporous carbon hollow microspheres (MCHMs). We prove that the dielectric ability and microwave absorption of MCHMs can be adjusted by structural characteristics. When the average particle size of MCHMs is 452 nm, the paraffin composite material mixed with 10 wt% MCHMs can achieve a maximum reflection loss value of -51 dB with a thickness of 4.0 mm at 7.59 GHz. When the average particle size of MCHMs is 425 nm, the effective absorption bandwidth of the paraffin composite material mixed with 10 wt% MCHMs can achieve a broad bandwidth of 7.14 GHz with a thickness of 2.5 mm. Compared with other microwave absorbers, MCHMs possess high microwave absorption capacity and broad microwave absorption bandwidth with as low as a 10 wt% filler ratio. This excellent microwave absorption performance is due to the internal cavity and the mesoporous shell of MCHMs. By rationally designing the structure of MCHMs, excellent microwave absorption performance can be acquired. Meanwhile, this design concept based on a rational design of spherical structure can be extended to other spherical absorbers.

Construction of a Cu-Sn Heterojunction Interface Derived from a Schottky Junction in Cu@Sn/rGO Composites as a Highly Efficient Dielectric Microwave Absorber

Developing high-performance dielectric absorbers, low filler loading, and a broad absorption band remains a great challenge for wireless data communication systems, household appliances, local area network, and so on. Herein, we report a facile green method to design and fabricate a copper-coated tin/reduced graphene oxide (Cu@Sn/rGO) composites with a heterojunction obtained by modifying a Schottky junction. The unique heterojunction can enable an appropriate balance between impedance and strong loss capacity. Meanwhile, it can not only promote the carrier migration but also obtain the rich interfaces. Consequently, a Cu@Sn/rGO composite with a heterojunction exhibits superior absorption intensity, far surpassing that of other absorbing materials reported. With a weight content of only 5 wt %, the maximum absorptivity reaches -49.19 dB at 6.08 GHz, and an effective absorption bandwidth (RL < -10 dB) of 13.94 GHz is achieved when the absorber's thickness ranges from 1.7 to 5.5 mm. This study provides new insights into the design and synthesis of a novel microwave absorption material with lightweight, smaller filler loading, and strong reflection loss.

Magnetoelectric effect in flexible nanocomposite films based on size-matching

Flexible magnetoelectric (ME) nanocomposites with a strong coupling between ferromagnetism and ferroelectricity are of significant importance from the point of view of next-generation flexible electronic devices. However, a high loading of magnetic nanomaterials is needed to achieve preferable ME response due to the size mismatch of the magnetostrictive phase and piezoelectric phase. In this work, ultra-small CoFe2O4 nanoparticles were prepared to match the size of the polar crystal in poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), and 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) is introduced to enhance the interplay between P(VDF-TrFE) and CoFe2O4. The above multiple effects promote a good connection between the magnetostrictive phase and the piezoelectric phase. Therefore, an effective transference of stress from CoFe2O4 to P(VDF-TrFE) can be achieved. The as-prepared P(VDF-TrFE)/CoFe2O4@POTS exhibits a high ME coupling coefficient of 34 mV cm-1 Oe-1 when the content of CoFe2O4@POTS is 20 wt%. The low loading of fillers ensures the flexibility of ME nanocomposite films.

MOF Induces 2D GO to Assemble into 3D Accordion-Like Composites for Tunable and Optimized Microwave Absorption Performance

Three-dimensional (3D) materials assembled by 2D layered lamella can provide abundant interfaces which are greatly advantageous for high-performance microwave absorbers. Herein, accordion-like CeO_{2- x} /reduced graphene oxide (CeO_{2- x} /RGO) hybrid materials can be successfully synthesized by a solvothermal and hydrothermal method, which are composed of laminated RGO sheets and confined CeO_{2- x} nanoparticles (NPs). The multilayer structure is attributed to the process of Ce-MOF dissolving into NPs, then the NPs combining with graphene oxide (GO) to induce the 2D GO assembled into 3D accordion-like composites. The 3D accordion-like CeO_{2- x} /RGO simultaneously utilizes the insulated CeO_{2- x} and highly conductive RGO to assemble into the laminated structure with moderate electromagnetic parameters. The 3D-laminated lightweight CeO_{2- x} /RGO composite exhibits excellent attenuation ability of an ultrabroad bandwidth (5.84 GHz) and a maximum reflection loss (-50.6 dB) which can be ascribed from the glorious impedance matching, synergistic effect between RGO sheets and the embedded CeO_{2- x} NPs. An off-axis electron holography is carried out to visualize the spatial electrical potential and charge distribution around the CeO_{2- x} /RGO heterojunction, which clarifies the dipole polarization and interfacial polarization. This work enlightens a simple strategy to fabricate an excellent 3D laminated RGO-based microwave absorber.

Design of MOF-derived hierarchical Co@C@RGO composite with controllable heterogeneous interfaces as a high-efficiency microwave absorbent

Carbon-based composites have triggered tremendous attention in the development of high-efficiency microwave absorbers, due to their compatibility, light weight, and high microwave absorption. However, fabricating carbon-based absorbers with a strong absorption ability in a broad frequency range is challenging. Hence, a facile strategy was used to produce Co@C derived from a zeolitic imidazolate framework (ZIF)@ graphene. The Co@C@RGO composite was obtained by annealing the ZIF67/GO nanocomposite precursor at 650 °C in a nitrogen atmosphere. Due to the magnetic loss induced by the Co particles, the dielectric loss generated by the carbon skeletons and graphene, and the interfacial polarization between the components, the hierarchical composite exhibits superior electromagnetic (EM) wave absorption properties. The optimal reflection loss (R_L) of the Co@C@ RGO composite can be up to -67.5 dB at 2.6 mm, and the effective bandwidth (≥-10 dB) is 5.4 GHz (10-15.4 GHz) with a thickness of 2 mm at 20 wt% loading. The dipolar polarization caused by graphene, as well as enhanced impedance matching, synergistic effect and interfacial effect among the components, increase the microwave absorption performance of the composite. This work may open a new path to use the Co@C@RGO composite with its high-efficiency EM wave properties as an absorber.

Direct Synthesis of Magnetic CoFe2O4 Nanoparticles as Recyclable Photo-Fenton Catalysts for Removing Organic Dyes

Herein, CoFe₂O₄ nanoparticles were directly synthesized through a solution combustion method using ferric nitrate, cobalt nitrate, and glycine as raw materials. The effects of glycine on the phase composition and magnetic properties of the CoFe₂O₄ products were investigated. When the fuel/ferric nitrate ratio was 0.8, the obtained product was pure CoFe₂O₄ with an average particle size of 25 nm. Furthermore, the saturation magnetization is 77.3 emu/g, which is about 95.7% that of CoFe₂O₄ bulk materials at room temperature and good for recycling. The photo-Fenton catalytic properties of CoFe₂O₄ were investigated for assessing its efficacy in removing dyes. It could degrade the 20 ppm MB in 75 min. To improve the photo-Fenton catalytic performance, NH₄HCO₃ and glucose were employed as additives. Due to the pores formed by NH₄HCO₃ and glucose, the G-CoFe₂O₄ and N-CoFe₂O₄ could degrade the 20 ppm MB in 40 and 25 min, respectively. The results indicated that these additives can effectively improve the catalytic activity of CoFe₂O₄. The modified CoFe₂O₄ is a promising alternative recyclable photo-Fenton catalyst for removing organic dyes.

Efficacy and Molecular Effects of a Reduced Graphene Oxide/Fe3O4 Nanocomposite in Photothermal Therapy Against Cancer

PURPOSE

Expanded research on the biomedical applications of graphene has shown promising results, although interactions between cells and graphene are still unclear. The current study aims to dissect the cellular and molecular effects of graphene nanocomposite in photothermal therapy against cancer, and to evaluate its efficacy.

METHODS

In this study, a reduced graphene oxide and iron oxide (rGO-Fe3O4) nanocomposite was obtained by chemical synthesis. The nanocomposite was fully characterized by Raman spectroscopy, TEM, VSM and thermal profiling. Cell-nanocomposite interaction was evaluated by confocal microscopy and viability assays on cancer cell line HeLa. The efficacy of the thermal therapy and changes in gene expression of Bcl-2 and Hsp70 was assessed.

RESULTS

The resulting rGO-Fe3O4 nanocomposite exhibited superparamagnetic properties and the capacity to increase the surrounding temperature by 18-20°C with respect to the initial temperature. The studies of cell-nanocomposite interaction showed that rGO-Fe3O4 attaches to cell membrane but there is a range of concentration at which the nanomaterial preserves cell viability. Photothermal therapy reduced cell viability to 32.6% and 23.7% with 50 and 100 µg/mL of nanomaterial, respectively. The effect of treatment on the molecular mechanism of cell death demonstrated an overexpression of anti-apoptotic proteins Hsp70 and Bcl-2 as an initial response to the therapy and depending on the aggressiveness of the treatment.

CONCLUSION

The results of this study contribute to understanding the interactions between cell and graphene and support its application in photothermal therapy against cancer due to its promising results.

Ultrathin MoS2 Nanosheets Encapsulated in Hollow Carbon Spheres: A Case of a Dielectric Absorber with Optimized Impedance for Efficient Microwave Absorption

A dielectric loss-type electromagnetic wave (EMW) absorber, especially over a broad frequency range, is important yet challenging. As the most typical dielectric attenuation absorber, carbon-based nanostructures were highly pursued and studied. However, their poor impedance-matching issues still exist. Here, to further optimize dielectric properties and enhance reflection loss, ultrathin MoS₂ nanosheets encapsulated in hollow carbon spheres (MoS₂@HCS) were prepared via a facile template method. The diameter and shell thickness of the as-prepared HCSs were ∼250 and ∼20 nm. The encapsulated MoS₂ nanosheets presented high dispersity and crystallinity. Compared to a pure HCS or MoS₂ absorber, MoS₂@HCS exhibited an optimized impedance characteristic, which can be attributed to the synergistic effects between HCSs (ensuring rapid electron transmission and compensating the low conductivity of MoS₂) and MoS₂ nanosheets (exposing sufficient numbers of active sites for polarizations and multi-reflection). Consequently, the MoS₂@HCS was endowed with -65 dB EMW attenuation ability under 2 mm and the effective attenuation bandwidth under -20 dB was ∼3.3 GHz over the K-band under 1.2 mm and ∼3.4 GHz over the Ka-band under merely 0.7 mm. These results suggested that the MoS₂@HCS is a promising dielectric absorber for practical applications. Meanwhile, this work introduces a facile and versatile strategy, which could in principle be extended to other transition metal sulfide@HCS for designing novel EMW absorbers.

Ultralight reduced graphene oxide aerogels prepared by cation-assisted strategy for excellent electromagnetic wave absorption

In this work, to maximize the unique attributes of reduced graphene oxide (RGO) for excellent microwave absorption, the ultralight RGO aerogels with improved dispersion and interface polarization performance were fabricated via a facile cation-assisted hydrothermal treatment process. The prepared RGO/paraffin composite exhibits excellent microwave absorption (MA) performance in a wideband frequency range of 8.0 ∼ 18.0 GHz with an ultralow absorbent content of 0.5 wt.%. Such performance is comparable with most previously reported results on RGO-based composites but required much higher absorbent content. The mechanisms for the enhancement of polarization relaxation loss and conductive loss were investigated in detail. This study provides a promising and facile method for preparing RGO-based excellent microwave absorption materials with ultra-low filler content, which is significant for designing efficient MA absorbers.

Microwave Absorption of Crystalline Fe/MnO@C Nanocapsules Embedded in Amorphous Carbon

Crystalline Fe/MnO@C core-shell nanocapsules inlaid in porous amorphous carbon matrix (FMCA) was synthesized successfully with a novel confinement strategy. The heterogeneous Fe/MnO nanocrystals are with approximate single-domain size which gives rise to natural resonance in 2-18 GHz. The addition of MnO2 confines degree of graphitization catalyzed by iron and contributes to the formation of amorphous carbon. The heterogeneous materials composed of crystalline-amorphous structures disperse evenly and its density is significantly reduced on account of porous properties. Meanwhile, adjustable dielectric loss is achieved by interrupting Fe core aggregation and stacking graphene conductive network. The dielectric loss synergistically with magnetic loss endows the FMCA enhanced absorption. The optimal reflection loss (RL) is up to - 45 dB, and the effective bandwidth (RL < - 10 dB) is 5.0 GHz with 2.0 mm thickness. The proposed confinement strategy not only lays the foundation for designing high-performance microwave absorber, but also offers a general duty synthesis method for heterogeneous crystalline-amorphous composites with tunable composition in other fields.

Facile Design of Three-Dimensional Nitrogen-Doped Reduced Graphene Oxide/Multi-Walled Carbon Nanotube Composite Foams as Lightweight and Highly Efficient Microwave Absorbers

Graphene foams with three-dimensional (3D) network structure, high porosity, and ultralow density have been regarded as lightweight microwave absorption materials. Herein, nitrogen-doped reduced graphene oxide/multi-walled carbon nanotube composite foams were prepared through a two-step strategy of hydrothermal self-assembly and subsequent high-temperature calcination. Morphology analysis indicated that the 3D networks were composed of overlapped flaky reduced graphene oxide. In addition, the influences of nitrogen doping, calcination temperature, and filler ratios on microwave absorption of composite foams were explored. Results manifested that the microwave absorption of composite foams was remarkably improved with the calcination temperature increased. Dramatically, it was noteworthy that the composite foam obtained under 600 °C calcination (bulk density of ∼10.8 mg/cm³) with an 8 wt % mass filler ratio presented the strongest microwave absorption of -69.6 dB at 12.5 GHz and broadest absorption bandwidth achieved 4.3 GHz (13.2-17.5 GHz) at an extremely low matching thickness equal to 1.5 mm. Moreover, the microwave absorption performance could be conveniently adjusted through modifying the thicknesses, filler ratios, and calcination temperature. The excellent microwave absorption performance of as-prepared composite foams was greatly derived from a well-constructed 3D network structure, significant nitrogen doping, enhanced polarization relaxation, and improved conduction loss. This work proposed a new strategy for fabricating graphene-based composites with a 3D network structure as high-efficiency microwave absorbers.

NiFe2O4 Nanoparticles Synthesized by Dextrin from Corn-Mediated Sol-Gel Combustion Method and Its Polypropylene Nanocomposites Engineered with Reduced Graphene Oxide for the Reduction of Electromagnetic Pollution

In this work, nickel ferrite (NiFe2O4) nanoparticles were synthesized by dextrin from corn-mediated sol-gel combustion method and were annealed at 600, 800, and 1000 °C. The structural and physical characteristics of prepared nanoparticles were studied in detail. The average crystallite size was 20.6, 34.5, and 68.6 nm for NiFe2O4 nanoparticles annealed at 600 °C (NFD@600), 800 °C (NFD@800), and 1000 °C (NFD@1000), respectively. The electromagnetic interference shielding performance of prepared nanocomposites of NiFe2O4 nanoparticles (NFD@600 or NFD@800 or NFD@1000) in polypropylene (PP) matrix engineered with reduced graphene oxide (rGO) have been investigated; the results indicated that the prepared nanocomposites consisted of smaller-sized nickel ferrite nanoparticles exhibited excellent electromagnetic interference (EMI) shielding characteristics. The total EMI shielding effectiveness (SET) for the prepared nanocomposites have been noticed to be 45.56, 36.43, and 35.71 dB for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP nanocomposites, respectively, at the thickness of 2 mm in microwave X-band range (8.2-12.4 GHz). The evaluated values of specific EMI shielding effectiveness (SSE) were 38.81, 32.79, and 31.73 dB·cm3/g, and the absolute EMI shielding effectiveness (SSE/t) values were 388.1, 327.9, and 317.3 dB·cm2/g for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP, respectively. The prepared lightweight and flexible sheets can be considered useful nanocomposites against electromagnetic radiation pollution.

Broadband microwave absorber constructed by reduced graphene oxide/La0.7Sr0.3MnO3 composites

High-performance microwave absorbing materials require optimized impedance matching and high attenuation ability. Here we meet the challenge by incorporating electric loss with magnetic loss materials to prepare carbon-based/magnetic hybrids. The reduced graphene oxide (rGO)/La_0.7Sr_0.3MnO₃ (LSMO) composites were prepared by dispersing the LSMO powders into 4.25, 6.25, 8.16, and 10 wt% of the graphene oxide aqueous solution, then the rGO/LSMO composites were formed by hydrothermal method. The pure rGO, LSMO, and rGO/LSMO composites were studied using X-ray diffraction and SEM. Microwave absorption properties were investigated by using coin method. Simulation studies show that 6.25 wt% of rGO/LSMO in a wax matrix exhibits the strongest reflection loss of −47.9 dB @ 10.7 GHz at a thickness of 2.5 mm. Moreover, the effective absorption bandwidth with the reflection loss below −10 dB is up to 14.5 GHz, ranged from 3.5 to 18 GHz for the composites with a thickness of 1.5–5.5 mm, due to a synergism between dielectric loss of rGO and magnetic loss of magnetic LSMO, which is an interesting exploration in the applications of rGO and LSMO. This method can be extended to design and fabricate hybrid absorbers with effective microwave absorption.

High-performance microwave absorbing materials require optimized impedance matching and high attenuation ability. To meet these requirements, rGO/LSMO composites were prepared thereby incorporating electric loss with magnetic loss materials.

Ultralight Three-Dimensional Hierarchical Cobalt Nanocrystals/N-Doped CNTs/Carbon Sponge Composites with a Hollow Skeleton toward Superior Microwave Absorption

It is extremely desirable but remains greatly challenging to obtain high-performance microwave absorption (MA) materials with thin thickness, lightweight, wide frequency bandwidth, and strong absorption by facile and low-cost preparing methods. In this work, by utilizing an inexpensively commercial melamine-formaldehyde sponge (MFS) as a template for growth of a Co-based metal-organic framework (ZIF-67) and subsequently carbonizing this ZIF-67-decorated MFS in a nitrogen atmosphere, an ultralight (8 mg cm^-3), three-dimensional hybrid carbon sponge composite with a hierarchical micro/nanostructure and hollow skeleton is successfully prepared to acquire excellent MA performances for the first time. The as-obtained composite consisted of interconnected carbon microtubes as a skeleton, intertwined N-doped carbon nanotubes (CNTs) grew on the outer surface of the carbon microtubes, and metallic Co nanocrystals encapsulated at the tips of the CNTs. Benefiting from the unique architecture and hierarchical composite which contribute to a good conductive network, moderate magnetic loss, strong matched impedance, and multiple polarization, the composite (Co/CNTs/CS) exhibited a minimum reflection loss (RL) of -51.2 dB and an effective absorption bandwidth (EAB, RL < -10 dB) of 4.1 GHz with a matching thickness of 2.2 mm at a filler loading of as low as 10 wt % in paraffin wax. Even with the thickness of 1.6 mm or at the filler loading of 5 wt %, the composites can also gain the low minimum RL value of -30.9 or -17.9 dB, respectively. In addition, the largest EAB was 5.4 GHz at the thickness of 2.0 mm, and the tunable EAB can be achieved in the range of 3.6-18 GHz, covering 90% of the measured frequency range via adjusting the absorber thickness between 1 and 5.5 mm. The results offer new insights for designing advanced microwave absorbers with lightweight, thin thickness, strong RL, and wide absorption frequency range.

High-Efficiency Electromagnetic Wave Absorption of Cobalt-Decorated NH2-UIO-66-Derived Porous ZrO2/C

Broadband absorbers derived from metal-organic frameworks are highly desirable in the electromagnetic (EM) wave absorption field. Herein, a strategy for cobalt-decorated porous ZrO₂/C hybrid octahedrons by pyrolysis of Co(NO₃)₂-impregnated NH₂-UIO-66 was developed. The hybridization of Co nanoparticles with ZrO₂/C results in remarkable EM wave absorption performance with a minimum reflection loss (RL) of -57.2 dB at 15.8 GHz, corresponding to a matching thickness of 3.3 mm. The maximum effective absorption bandwidth (RL ≤ -10 dB) reaches 11.9 GHz (6.1-18 GHz), covering 74.4% of the whole measured bandwidth. The textural properties of nanocomposites have been thoroughly characterized by powder X-ray diffraction, electron microscopy, X-ray photoelectron spectroscopy, and nitrogen adsorption-desorption isotherms. The corresponding results show that the face-centered cubic-phased ∼50 nm Co nanoparticles are evenly distributed on the surface of porous ZrO₂/C hybrid octahedrons. The excellent performance of Co/ZrO₂/C can be ascribed to the strong interface polarization and the suitable impedance matching, originating from the synergistic effect among the components.

Rational Construction of Hierarchically Porous Fe-Co/N-Doped Carbon/rGO Composites for Broadband Microwave Absorption

Developing lightweight and broadband microwave absorbers for dealing with serious electromagnetic radiation pollution is a great challenge. Here, a novel Fe-Co/N-doped carbon/reduced graphene oxide (Fe-Co/NC/rGO) composite with hierarchically porous structure was designed and synthetized by in situ growth of Fe-doped Co-based metal organic frameworks (Co-MOF) on the sheets of porous cocoon-like rGO followed by calcination. The Fe-Co/NC composites are homogeneously distributed on the sheets of porous rGO. The Fe-Co/NC/rGO composite with multiple components (Fe/Co/NC/rGO) causes magnetic loss, dielectric loss, resistance loss, interfacial polarization, and good impedance matching. The hierarchically porous structure of the Fe-Co/NC/rGO enhances the multiple reflections and scattering of microwaves. Compared with the Co/NC and Fe-Co/NC, the hierarchically porous Fe-Co/NC/rGO composite exhibits much better microwave absorption performances due to the rational composition and porous structural design. Its minimum reflection loss (RLmin) reaches - 43.26 dB at 11.28 GHz with a thickness of 2.5 mm, and the effective absorption frequency (RL ≤ - 10 dB) is up to 9.12 GHz (8.88-18 GHz) with the same thickness of 2.5 mm. Moreover, the widest effective bandwidth of 9.29 GHz occurs at a thickness of 2.63 mm. This work provides a lightweight and broadband microwave absorbing material while offering a new idea to design excellent microwave absorbers with multicomponent and hierarchically porous structures.

Graphene oxide-ferrite hybrid framework as enhanced broadband absorption in gigahertz frequencies

The present investigation is focused on the in-situ synthesis of Graphene oxide (GO)-ferrite nanoparticle hybrid framework by gel-combustion method followed by fabrication of homogeneous, structurally stable thin (~100–120 μm) hybrid-polyurethane coating on a metallic aluminum substrate and its application on the properties of broadband absorption over the microwave frequency region. Microstructure studies of hybrid materials illustrated that small sized ferrite nanoparticles (~17 nm) are grafted on and through the graphene layers, which forms a homogeneous coating thereby. The hybrid-nanocomposite coating demonstrated superior broadband absorption properties with absorptivity higher than 90% throughout a bandwidth of ~6 GHz, and moreover, it was found that with increased loading of GO in the nanocomposite, the bandwidth range of absorption frequency increases with enhanced absorptivity. The real part and imaginary part of the surface impedance values of the coating was obtained as 377 Ω and 0 Ω, respectively, which imply that the free-space impedance of the hybrid-nanocomposite coating is matching correctly. The nanocomposite coating showed ultra-high absorptivity over the frequency band of 8–12 GHz, which has numerous practical applications as radar absorbing materials (RAM), stealth technology, electromagnetic shielding, and radiated electromagnetic interference (EMI) management in onboard spacecraft and many more.

Fabrication of NiO/NiCo2O4 Mixtures as Excellent Microwave Absorbers

The NiO/NiCo₂O₄ mixtures with unique yolk-shell structure were synthesized by a simple hydrothermal route and subsequent thermal treatment. The elemental distribution, composition, and microstructure of the samples were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and scanning electron microscope (SEM), respectively. The microwave absorption property was investigated by using vector network analysis (VNA). The results indicated that the excellent electromagnetic wave absorption property of the NiO/NiCo₂O₄ mixtures was achieved due to the unique yolk-shell structure. In detail, the maximum reflection loss (RL) value of the sample reached up to - 37.0 dB at 12.2 GHz and the absorption bandwidth with RL below - 10 dB was 4.0 GHz with a 2.0-mm-thick absorber. In addition, the NiO/NiCo₂O₄ mixtures prepared at high temperature, exhibited excellent thermal stability. Possible mechanisms were investigated for improving the microwave absorption properties of the samples.

Cactus-Inspired Bimetallic Metal-Organic Framework-Derived 1D-2D Hierarchical Co/N-Decorated Carbon Architecture toward Enhanced Electromagnetic Wave Absorbing Performance

Metal-organic framework (MOF)-derived magnetic metal/carbon nanocomposites have shown tremendous potential for lightweight electromagnetic wave (EMW) absorption. However, it is a challenge but highly significant to design and construct mixed-dimensional hierarchical architectures with synergistically integrated characteristics from individual MOFs for advancing the EMW absorption performance. Inspired by the structure of cactus, a novel hierarchical one-dimensional (1D)-two-dimensional (2D) mixed-dimensional Co/N-decorated carbon architecture comprising carbon nanotubes grafted on carbon flakes (abbreviated as CoNC/CNTs) has been fabricated by the pyrolysis of bimetallic CoZn-ZIF-L. The CoNC/CNTs integrate the advantages of 1D nanotubes for the extra polarization of EMW and 2D nanoflakes with an interconnected porous structure for multiple reflection losses of EMW and optimization of impedance matching. The resultant CoNC/CNTs demonstrate excellent EMW absorbing performance. For the optimal EMW absorbing material of CoNC/CNT-3/1, minimum reflection loss reaches -44.6 dB at 5.20 GHz with a low filler loading of 15 wt %. Moreover, the largest effective bandwidth range achieves 4.5 GHz with a thickness of 1.5 mm and a filled ratio of 20 wt %. These findings indicate that such a mixed 1D-2D hierarchical architecture synergistically enhances EMW absorbing performance. This work sheds light on the rational design of a mixed-dimensional carbon architecture derived from MOFs for desirable functionalities.

Interface Modulating CNTs@PANi Hybrids by Controlled Unzipping of the Walls of CNTs To Achieve Tunable High-Performance Microwave Absorption

Making full use of the interface modulation-induced interface polarization is an effective strategy to achieve excellent microwave absorption (MA). In this study, we develop an interfacial modulation strategy for achieving this goal in the commonly reported dielectric carbon nanotubes@polyaniline (CNTs@PANi) hybrid microwave absorber by optimizing the CNT nanocore structure. The heterogeneous interfaces from PANi and CNTs can be well regulated by longitudinal unzipping of the walls of CNTs to form 1D CNT- and 3D CNT-bridged graphene nanoribbons and 2D graphene nanoribbons. By controlling the oxidation peeling degree of CNTs, their interface area and defects are enhanced, thus producing more polarization centers to generate interfacial polarization and polarization relaxation, and also introducing more PANi loadings. Furthermore, more interface contact area can be produced between CNTs and PANi. This could induce a strong dielectric resonant and further improve the impedance matching, leading to significant enhancement of MA performance. With filler loading of only 10 wt % and a thinner coating thickness of 2.4 mm, the optimized CNTs@PANi exhibits excellent MA performance with the minimum reflection loss (RL_min) value of -45.7 dB at 12.0 GHz and the effective bandwidth is from 10.2 to 14.8 GHz. Meanwhile, the broadest effective bandwidth reaches 5.6 GHz, covering the range of 12.4-18.0 GHz with a thin thickness of 2.0 mm and its RL_min reaches -29.0 dB at 14.6 GHz. It is believed that the proposed interfacial modulation strategy can provide new opportunities for designing efficient MA absorbers.

Novel nanocomposites of cobalt ferrite covalently-grafted on graphene by amide bond as superior electromagnetic wave absorber

Unique covalently bonded cobalt ferrite (CoFe₂O₄)/graphene nanocomposites are successfully fabricated via an amino-ester-amide reaction process. The morphology, component, functional groups and electromagnetic properties are detected by Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared Spectra (FTIR), Vibrating Sample Magnetometer (VSM) and Vector Network Analyzer (VNA). Compared to non-covalently bonded nanocomposites, the covalently bonded CoFe₂O₄/graphene nanocomposites have outstanding electromagnetic wave absorption properties. We found that the maximum reflection loss value reached at -55.2 dB and the absorption bandwidth with reflection loss below -10 dB was about 5.4 GHz at 1.7 mm of thickness. The efficiency is attributed to the introduction of amide bonds in the nanocomposites. As a stable carrier channel, amide bonds can promote the migration rate of electrons and binding degree between CoFe₂O₄ and graphene nanosheets, which provide a crucial impact on electromagnetic parameters and polarization modes of materials, thus improving the absorption capacity of electromagnetic waves. It can be inferred that the nanocomposites have a broad application prospect in the field of electronic instruments, aerospace, military radars and national defense security fields.

Green Synthesis of Porous Cocoon-like rGO for Enhanced Microwave-Absorbing Performances

A novel porous cocoon-like reduced graphene oxide (rGO) with high porosity and low density was fabricated by a simple and green reduction reaction using ascorbic acid as the reductant in combination with a freeze-drying process without annealing. The bulk density of porous cocoon-like rGO is only 28.49 mg/cm³, and the porosity reaches 94.57%. The reaction times have an important influence on the formation of porous cocoon-like rGO and the reduction degree of rGO. The porous cocoon-like rGO exhibits an excellent microwave-absorbing property with a low mass filling ratio of 7.0 wt %; its minimum reflection loss (RL) is -29.05 dB at 15.96 GHz with a sample thickness of 2.0 mm and the effective absorption bandwidth (RL < -10 dB) is 5.27 GHz. The microwave-absorbing property of porous cocoon-like rGO is much better than that of GO and other porous rGO. The in-depth analyses of the reduction degree, porosity, and microwave-absorbing performance illustrate that the microwave-absorbing performance of rGO is significantly related to the reduction degree and porosity. In addition, the synthetic route for porous cocoon-like rGO is simple, has low energy consumption, and is environmentally friendly. Our work demonstrates that the porous cocoon-like rGO is a promising lightweight microwave absorber with high performance.

Solid and macroporous Fe3C/N-C nanofibers with enhanced electromagnetic wave absorbability

A series of solid and macroporous N-doped carbon nanofibers composed of Fe₃C nanoparticles (named as solid Fe₃C/N-C NFs, solid Fe₃C/N-C NFs-1, solid Fe₃C/N-C NFs-2, macroporous Fe₃C/N-C NFs, macroporous Fe₃C/N-C NFs-1 and macroporous Fe₃C/N-C NFs-2, respectively) were prepared through carbonization of as-electrospun nanofiber precursors. The results show that the magnetic Fe₃C nanoparticles (NPs) dispersed homogeneously on the N-doped carbon fibers; as-prepared six materials exhibit excellent microwave absorption with a lower filler content in comparison with other magnetic carbon hybrid nanocomposites in related literatures. Particularly, the solid Fe₃C/N-C NFs have an optimal reflection loss value (RL) of −33.4 dB at 7.6 GHz. For solid Fe₃C/N-C NFs-2, the effective absorption bandwidth (EAB) at RL value below −10 dB can be up to 6.2 GHz at 2 mm. The macroporous Fe₃C/N-C NFs have a broadband absorption area of 4.8 GHz at 3 mm. The EAB can be obtained in the 3.6–18.0 GHz frequency for the thickness of absorber layer between 2 and 6 mm. These Fe₃C–based nanocomposites can be promising as lightweight, effective and low-metal content microwave absorption materials in 1–18 GHz.

Enhanced electromagnetic wave absorption induced by void spaces in hollow nanoparticles

We developed a facile method for the growth of hollow structured NiCo₂O₄ nanoparticles on a graphene sheet (NiCo₂O₄-h/G). The hollow NiCo₂O₄ nanoparticles have an average diameter of approximately 10.0 nm and a shell thickness of merely 2.5 nm. The NiCo₂O₄-h/G hybrid exhibited excellent electromagnetic wave absorption with minimal reflection loss below -20 dB at absorber thickness ranging from 2.0 to 5.0 mm, outperforming the solid NiCo₂O₄ nanoparticles on the graphene sheet. Remarkably, even for a thickness as small as 1.5 mm, the efficient absorption bandwidth and the minimal reflection loss of the hybrid can reach 2.6 GHz and -20.3 dB, respectively. Experimental results and theoretical calculations indicate that the void space in the hollow NiCo₂O₄ nanoparticles plays a crucial role in the excellent electromagnetic wave absorption property, which greatly increases the dielectric loss and impedance matching characteristics. Our results demonstrate that growing the hollow nanoparticles on a graphene sheet is an efficient way to produce high-performance electromagnetic wave absorbers.