Synthesis of Hierarchical ZnFe2O4@SiO2@RGO Core-Shell Microspheres for Enhanced Electromagnetic Wave Absorption

Investigation of Electromagnetic Wave Absorption Properties of Ni-Co and MWCNT Nanocomposites

BACKGROUND

In recent years, severe electromagnetic interference among electronic devices has been caused by the unprecedented growth of communication systems. Therefore, microwave absorbing materials are required to relieve these problems by absorbing the unwanted microwave. In the design of microwave absorbers, magnetic nanomaterials have to be used as fine particles dispersed in an insulating matrix. Besides the intrinsic properties of these materials, the structure and morphology are also crucial to the microwave absorption performance of the composite. In this study, Ni-Co- MWCNT composites were synthesized, and the changes in electric permittivity, magnetic permeability, and reflectance loss of the samples were evaluated at frequencies of 2 to 18 GHz.

METHODS

Nickel-Cobalt-Multi Wall Carbon Nanotubes (MWCNT) composites were successfully synthesized by the co-precipitation chemical method. The structural, morphological, and magnetic properties of the samples were characterized and investigated by X-ray diffractometer (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Vibrating Sample Magnetometer (VSM), and Vector Network Analyzer (VNA).

RESULTS

The results revealed that the Ni-Co-MWCNT composite has the highest electromagnetic wave absorption rate with a reflectance loss of -70.22 dB at a frequency of 10.12 GHz with a thickness of 1.8 mm. The adequate absorption bandwidth (RL <-10 dB) was 6.9 GHz at the high-frequency region, exhibiting excellent microwave absorbing properties as a good microwave absorber patent.

CONCLUSION

Based on this study, it can be argued that the Ni-Co-MWCNT composite can be a good candidate for making light absorbers of radar waves at frequencies 2- 18 GHz.

In-modified Sn-MOFs with high catalytic performance in formate electrosynthesis from aqueous carbon dioxide

Electrochemical reduction of carbon dioxide (CO2ER) has become an effective solution to relieve the energy crisis and tackle climate change. In this study, a series of tin-based organic frameworks modified by In (Sn-MOF/Inx) were successfully synthesized via a simple hydrothermal method and explored for high formate-selective CO2ER. The pure Sn-MOF exhibits maximum formate selectivity with a faradaic efficiency (FEformate) of approximately 85.0% and a current density of 15 mA cm-2 at -1.16 VRHE, while the In (6%)-modified Sn-MOF (Sn-MOF/In6) delivers a much higher maximum FEformate (around 97.5%) and a current density of 16 mA cm-2 at -0.96 VRHE. Remarkably, the Sn-MOF/In6 exhibits a significantly larger specific surface area (183.3 m2 g-1) compared to the Sn-MOF (65.2 m2 g-1). These findings indicate that introducing In, an alien element with a slightly different outer orbital electron number from that of Sn, can significantly boost the selectivity and activity for CO2ER to formate. This study presents an efficient way to modify MOF catalysts through a well-designed introducing process.

A Highly Efficient Electromagnetic Wave Absorption System with Graphene Embedded in Hybrid Perovskite

To cope with the explosive increase in electromagnetic radiation intensity caused by the widespread use of electronic information equipment, high-performance electromagnetic wave (EMW)-absorbing materials that can adapt to various frequency bands of EMW are also facing great demand. In this paper, CH3NH3PbI3/graphene (MG) high-performance EMW-absorbing materials were innovatively synthesized by taking organic-inorganic hybrid perovskite (OIHP) with high equilibrium holes, electron mobility, and accessible synthesis as the main body, graphene as the intergranular component, and adjusting the component ratio. When the component ratio was 16:1, the thickness of the absorber was 1.87 mm, and MG's effective EMW absorption width reached 6.04 GHz (11.96-18.00 GHz), achieving complete coverage of the Ku frequency band. As the main body of the composite, CH3NH3PbI3 played the role of the polarization density center, and the defects and vacancies in the crystal significantly increased the polarization loss intensity; graphene, as a typical two-dimensional material distributed in the crystal gap, built an efficient electron transfer channel, which significantly improved the electrical conductivity loss strength. This work effectively broadened the EMW absorption frequency band of OIHP and promoted the research process of new EMW-absorbing materials based on OIPH.

Emerging Two-Dimensional Materials for Electromagnetic Interference Shielding Application

Graphene is the first two-dimensional material that becomes the center material in various research areas of material science, chemistry, condensed matter, and engineering due to its advantageous properties, including larger specific area, lower density, outstanding electrical conductivity, and ease of processability. These properties attracted the attention of material researchers that resulted in a large number of publications on EMI shielding in a short time and play a central role in addressing the problems and challenges faced in this modern era of electronics by electromagnetic interference. After the popularity of graphene, the community of material researchers investigated other two-dimensional materials like MXenes, hexagonal boron nitride, black phosphorous, transition metal dichalcogenides, and layered double hydroxides, to additionally enhance the EMI shielding response of materials. The present article conscientiously reviews the current progress in EMI shielding materials in reference to two-dimensional materials and addresses the future challenges and research directions to achieve the goals.

Core-Shell Structured Silica-Coated Iron Nanowires Composites for Enhanced Electromagnetic Wave Absorption Properties

In this study, we successfully prepared core-shell heterostructured nanocomposites (Fe NWs@SiO₂), with ferromagnetic nanowires (Fe NWs) as the core and silica (SiO₂) as the shell. The composites exhibited enhanced electromagnetic wave absorption and oxidation resistance and were synthesized using a simple liquid-phase hydrolysis reaction. We tested and analyzed the microwave absorption properties of Fe NWs@SiO₂ composites with varied filling rates (mass fractions of 10 wt%, 30 wt%, and 50 wt% after mixing with paraffin). The results showed that the sample filled with 50 wt% had the best comprehensive performance. At the matching thickness of 7.25 mm, the minimum reflection loss (RL_min) could reach -54.88 dB at 13.52 GHz and the effective absorption bandwidth (EAB, RL < -10 dB) could reach 2.88 GHz in the range of 8.96-17.12 GHz. Enhanced microwave absorption performance of the core-shell structured Fe NWs@SiO₂ composites could be attributed to the magnetic loss of the composite, the core-shell heterogeneous interface polarization effect, and the small-scale effect induced by the one-dimensional structure. Theoretically, this research provided Fe NWs@SiO₂ composites with highly absorbent and antioxidant core-shell structures for future practical applications.

Construction of a binary S-scheme S-g-C3N4/Co-ZF heterojunction with enhanced spatial charge separation for sunlight-driven photocatalytic performance

A step-scheme (S-scheme) photocatalyst made of sulfurized graphitic carbon nitride/cobalt doped zinc ferrite (S-g-C3N4/Co-ZF) was constructed using a hydrothermal process because the building of S-scheme systems might increase the lifespan of highly reactive charge carriers. Utilizing cutting-edge methods, the hybrid photocatalyst was evaluated by employing TEM, XPS, XRD, BET, FTIR, transient photo-response, UV-vis, EIS and ESR signals. In order to create a variety of binary nanocomposites (NCs), nanoparticles (NPs) of 6% cobalt doped zinc ferrite (Co-ZF) were mixed with S-g-C3N4 at various concentrations, ranging from 10 to 80 wt%. For photocatalytic dye removal, a particular binary NC constructed between S-g-C3N4 and Co-ZF produces a huge amount of catalytic active sites. The findings showed that loading of S-g-C3N4 on 6% Co-ZF NPs serves as a good heterointerface for e-/h+ separation and transportation through the S-scheme S-g-C3N4/Co-ZF heterojunction. By boosting the hybrid system's BET surface area for the photocatalytic process, the addition of 6% Co-ZF improves the system's ability to absorb more sunlight and boosts its photocatalytic activity. The highest photo-removal effectiveness (98%), which is around 2.45 times higher than that of its competitors, was achieved by the hybrid photocatalyst system with an ideal loading of 48% Co-ZF. Furthermore, the trapping studies showed that the primary species involved in the MB aqueous photo-degradation were ˙OH- and h+.

Multi-nanocavities and multi-defects synergetic enhancement for the electromagnetic absorption of the rGO-NG film

Dielectric loss is an important way to eliminate electromagnetic pollution. In order to achieve high dielectric loss, a graphene film reduced graphene oxide-N doped graphene (rGO-NG) was constructed from graphene oxide-Ni@polydopamine (GO-Ni@PDA) via thein situsynthesis of hollow graphene spheres between graphene sheets. Thisin situwas achieved by means of electrostatic self-assembly and metal-catalyzed crystallization. Owing to the synergetic effect of multi-nanocavities and multi-defects, the prepared rGO-NG film shows an average shielding effectiveness (SE) of 50.0 dB in the range of 8.2-12.4 GHz with a thickness of 12.2μm, and the SE reflection is only 7.3 dB on average. It also exhibits an average dielectric loss tangent (tanδ) of 23.1, which is 26 and 105 times higher than those of rGO and rGO-Ni, respectively. This work provides a simple but effective route to develop high performance graphene-based materials for application as an electromagnetic interference shielding film in today's electronic devices.

A General Way to Fabricate Chain-like Ferrite with Ultralow Conductive Percolation Threshold and Wideband Absorbing Ability

The magnetic nanochain-like material has been regards as one of the most promising electromagnetic (EM) absorbing material but remains a challenging. Herein, magnetic chain-like ferrite (included Fe3O4, CoFe2O4 and NiFe2O4) are successfully produced through a general solvothermal method, using PVP as the structural-liking agent. Experimental results confirm the ultimate sample possess a 3-dimensional chain-like structure which are constructed by numerous ferrite's nanoparticles with ~60 nm in diameter. Their electromagnetic parameters can be also manipulated by such a chain structure, especially the dielectric loss, where a sharply increases can be observed on within a lower filling ratio. It greatly benefits to the EM absorbing property. In this article, the electromagnetic absorption layer made with a lower content of ferrite possess the excellent electromagnetic absorption ability, where the optimized effective absorption band was nearly 6.4 GHz under a thickness of 1.8 mm. Moreover, the filling ratio is only 30 wt%. Our method for designing of chain-like magnetic material can be helpful for producing wideband electromagnetic absorption in a low filling ratio.

NiMnO3 Anchored on Reduced Graphene Oxide Nanosheets: A New High-Performance Microwave Absorbing Material

With the increasing influence of electromagnetic radiation on precision instruments and organisms, there is an urgent need for research on lightweight and high-strength electromagnetic wave absorbing materials. This study has probed into a new composite absorbing material based on reduced graphene oxide (rGO)-NiMnO₃, where the like-core-shell NiMnO₃ is anchored on the rGO nanosheets to significantly improve the electromagnetic wave dissipation ability of the composite material using the inter-component dipole polarization and interface polarization. At the same time, NiMnO₃ can effectively adjust the impedance matching ratio of rGO so that electromagnetic waves can effectively enter the absorbing material. At a thickness of 3.73 mm, the maximum absorption strength of rGO-NiMnO₃ reaches −61.4 dB at 6.6 GHz; at a thickness of 2.5 mm, the adequate absorption bandwidth is 10.04–18.00 GHz, achieving a full coverage for the Ku band. As a new option for preparing lightweight and broadband electromagnetic wave absorbing materials, rGO-NiMnO₃ is an ideal material for electromagnetic wave protection.

Dimensional Design and Core-Shell Engineering of Nanomaterials for Electromagnetic Wave Absorption

Electromagnetic (EM) wave absorption materials possess exceptionally high EM energy loss efficiency. With vigorous developments in nanotechnology, such materials have exhibited numerous advanced EM functions, including radiation prevention and antiradar stealth. To achieve improved EM performance and multifunctionality, the elaborate control of microstructures has become an attractive research direction. By designing them as core-shell structures with different dimensions, the combined effects, such as interfacial polarization, conduction networks, magnetic coupling, and magnetic-dielectric synergy, can significantly enhance the EM wave absorption performance. Herein, the advances in low-dimensional core-shell EM wave absorption materials are outlined and a selection of the most remarkable examples is discussed. The derived key information regarding dimensional design, structural engineering, performance, and structure-function relationship are comprehensively summarized. Moreover, the investigation of the cutting-edge mechanisms is given particular attention. Additional applications, such as oxidation resistance and self-cleaning functions, are also introduced. Finally, insight into what may be expected from this rapidly expanding field and future challenges are presented.

Construction of Ni-Zn bimetal sulfides Heterostructured-hybrids for High-performance electromagnetic wave absorption

Utilizing the synergistic effect of multiple components in heterostructured composites has been regarded as a promising strategy for achieving high-performance electromagnetic wave absorption. Nonetheless, rationally collocate the components of absorbers in order to legitimately achieve synergy remains an intractable problem. By adjusting the NiS and ZnS composition ratios in the ZnS/NiS/C composites, the optimal impedance matching and dissipation capability can be obtained. The formation of a ZnS/NiS heterostructure is found to significantly enhance polarization relaxation, and the relative ratios of ZnS and NiS have a significant effect on the electromagnetic properties. The optimal performance was obtained on Z1N2, with a minimum reflection loss of -51.45 dB at 4.72 GHz and -56.69 dB at 11.12 GHz, respectively, and an effective absorption bandwidth of up to 3.68 GHz at 1.16 mm. The potential of heterogeneous bimetal sulfides as high-performance absorbers is demonstrated in this study.

High-performance microwave absorption of MOF-derived Co3O4@N-doped carbon anchored on carbon foam

Absorbing materials can convert electromagnetic wave (EMW) energy into heat and other energy and dissipate it. Carbon materials can attenuate EMW by generating large conduction losses due to their high conductivity. The introduction of low dielectric materials can improve impedance matching caused by high conductivity. However, the density of materials compounded with carbon materials is too large, which affects the overall density of composite materials. Therefore, this problem is solved by matching melamine foam with ZIF-67. As an ultra-light material, the melamine foam-based carbon material can significantly reduce the density of composite materials, and its developed three-dimensional structure can cause multiple scattering of EMW. The large specific surface area and evenly distributed metal oxides obtained after annealing of ZIF-67 can provide ultra-low-density carbon materials and abundant interfacial polarization to further attenuate EMW. So far, the methods of self-growing materials on the surface of melamine foam have not been reported. We prepared a 500 nm Co₃O₄ nanosheet/carbon foam (CF) composite material coated on the surface by a two-step method. The sample had a maximum reflection loss of -46.58 dB at 10.72 GHz, and an effective absorption bandwidth (EAB) of 5.4 GHz. This research provides a new idea for the growth of porous materials on the surface of melamine foam-based carbon materials.

Carbon-Based MOF Derivatives: Emerging Efficient Electromagnetic Wave Absorption Agents

To tackle the aggravating electromagnetic wave (EMW) pollution issues, high-efficiency EMW absorption materials are urgently explored. Metal-organic framework (MOF) derivatives have been intensively investigated for EMW absorption due to the distinctive components and structures, which is expected to satisfy diverse application requirements. The extensive developments on MOF derivatives demonstrate its significantly important role in this research area. Particularly, MOF derivatives deliver huge performance superiorities in light weight, broad bandwidth, and robust loss capacity, which are attributed to the outstanding impedance matching, multiple attenuation mechanisms, and destructive interference effect. Herein, we summarized the relevant theories and evaluation methods, and categorized the state-of-the-art research progresses on MOF derivatives in EMW absorption field. In spite of lots of challenges to face, MOF derivatives have illuminated infinite potentials for further development as EMW absorption materials.

Three-dimensional graphene encapsulated Ag-ZnFe2O4 flower-like nanocomposites with enhanced photocatalytic degradation of enrofloxacin

Three-dimensional (3D) Ag–ZnFe₂O₄-reduced graphene oxide (rGO) was successfully synthesized using a hydrothermal and photo-reduction method, and the morphological differences of the materials were observed. Their photocatalytic activity was evaluated by photocatalytic degradation of enrofloxacin (ENR) under visible-light irradiation. The results indicated that Ag–ZnFe₂O₄–rGO exhibited superior photocatalytic properties and good stability. In this research, the enhancement of photocatalytic performance is mainly attributed to the electron channelization ability of rGO, which traps the photoexcited electrons of ZnFe₂O₄ on its π framework, and reduces the electron–hole recombination rate. Moreover, the high surface area of 3D pompon mum flower-like ZnFe₂O₄ provides more reactive sites. In addition, free radical capture and ESR experiments as well as pathway analysis of degradation also confirmed that superoxide radicals (˙O₂⁻) and photo-generated holes from Ag–ZnFe₂O₄–rGO were the main active species in the degradation progress of ENR.

Three-dimensional (3D) Ag–ZnFe₂O₄-reduced graphene oxide (rGO) was successfully synthesized using a hydrothermal and photo-reduction method, and the morphological differences of the materials were observed.

Constructing and optimizing hollow ZnxFe3-xO4@polyaniline composites as high-performance microwave absorbers

In this study, a series of hollow Zn_xFe_3-xO₄@polyaniline composites (ZFO@PANI) were synthesized by a facile solvothermal process and followed by in-situ chemical oxidation polymerization method, and then evaluated as microwave absorption (MA) absorbers. The effect of ZFO content on the electrical conductivity, electromagnetic parameters and MA performance of the ZFO@PANI composites was also elaborately investigated. As anticipated, the optimized composites of S2 exhibits the minimum reflection loss (RL_min) of -59.44 dB at 11.04 GHz with a matching thickness of 2.31 mm, and the broadest effective absorption bandwidth (EAB, RL < -10 dB, >90% absorption) of up to 4.65 GHz (13.35-18.0 GHz) at 1.72 mm. Noticeably, by adjusting the thickness from 1.5 to 5.0 mm, it can be observed that its RL_min values are all much lower than -10 dB and the qualified EAB can cover the entire C, X and Ku bands. The enhanced MA performance of S2 is mainly due to the efficient synergistic effect between dielectric loss (PANI) and magnetic loss (ZFO nanosphere), and thus achieving the relative balance of impedance matching (appropriate ZFO content) and attenuation capability. Therefore, it has great prospect to be explored as attractive candidate in practical application.

MXene/wood-derived hierarchical cellulose scaffold composite with superior electromagnetic shielding

Electromagnetic-interference (EMI) shielding materials that are green, lightweight, and with high mechanical properties need to be urgently developed to address increasingly severe radiation pollution. However, limited EMI shielding materials are successfully used in practical applications, due to the intensive energy consumption or the absence of sufficient strength. Herein, an environmentally friendly and effective method was proved to fabricate wood-based composites with high mechanical robustness and EMI shielding performance by a MXene/cellulose scaffold assembly strategy. The lignocellulose composites with a millimeter-thick mimic the "mortar-brick" layered structure, resulting in excellent mechanical properties that can achieve the compressive strength of 288 MPa and EMI shielding effectiveness of 39.3 dB. This "top-down" method provides an alternative for the efficient production of robust and sustainable EMI shielding materials that can be used in the fields of structural materials for next-generation communications and electronic devices.

Three-Dimensional Bi2Fe4O9 Nanocubes Loaded on Reduced Graphene Oxide for Enhanced Electromagnetic Absorbing Properties

Bi₂Fe₄O₉(BFO) nanocubes were prepared in proportion using a simple and easy hydrothermal method, and were then assembled on reduced graphene oxide (rGO) multilayered sheets. The excellent microwave absorption properties of Bi₂Fe₄O₉/rGO nanohybrids were achieved by properly adjusting the impedance matching and getting a high attenuation capability contributed from different ratios of the BFO and rGO. A minimum reflection loss value of −61.5 dB at 12.8 GHz was obtained with a Bi₂Fe₄O₉/rGO ratio of 2:1, and the broadest bandwidth below −10 dB was up to 5.0 GHz (from 10.8 to 15.8 GHz) with a thickness of 2.4 mm. Additionally, the elementary mechanism of wave absorption performance is also investigated.

Multicolor tunable luminescence and energy transfer of core-shell structured SiO2@Gd2O3 microspheres co-activated with Dy3+/Eu3+ under single UV excitation

Optimizing structure and varying doped ions are two main strategies to obtain excellent luminescence performance. Spherical morphology is considered to be the most ideal phosphor structure due to the least surface defects. Herein, a series of spherical and monodispersed Dy³⁺/Eu³⁺ co-activated SiO₂@Gd₂O₃ core-shell phosphors with multicolor tunable luminescence were successfully prepared via a facile urea assisted precipitation method. Related chemical reactions and the possible growth mechanism of Gd₂O₃:Ln³⁺ directional deposition on the surface of SiO₂ microspheres were put forward. Upon 273 nm UV radiation excitation, SiO₂@Gd₂O₃:Dy³⁺ and SiO₂@Gd₂O₃:Eu³⁺ samples exhibited characteristic yellow (⁴F_9/2-⁶H_13/2) and blue (⁴F_9/2-⁶H_15/2) emissions of Dy³⁺ and red (⁵D₀-⁷F₂) emission of Eu³⁺, respectively. Meanwhile, multicolor emissions (warm white, yellow and orange) could be easily obtained by modulating the relative content of Dy³⁺ and Eu³⁺ in SiO₂@Gd₂O₃:Ln³⁺ samples under a single excitation wavelength. Moreover, it was confirmed that Dy³⁺ could transfer energy to Eu³⁺ in the form of quadrupole-quadrupole interaction and further improved the luminescence intensity of Eu³⁺ by comparing experimental data with theoretical calculations. These results imply that tunable luminescence Dy³⁺,Eu³⁺ co-doped SiO₂@Gd₂O₃ microspheres have great potential applications in multicolor displays and biolabeling fields.

MOF-derived yolk–shell Ni/C architectures assembled with Ni@C core–shell nanoparticles for lightweight microwave absorbents

The yolk–shell Ni/C microspheres assembled by Ni@C core–shell nanoparticles with excellent microwave absorption performance can be simply fabricated by decomposition of a Ni-based metal–organic framework (Ni-MOF).

A New Broadband and Strong Absorption Performance FeCO3/RGO Microwave Absorption Nanocomposites

A novel composite of FeCO₃ nanoparticles, which are wrapped with reduced graphene oxide (RGO), is fabricated using a facile one-spot solvothermal method. The composite consists of a substrate of RGO and FeCO₃ nanoparticles that are embedded in the RGO layers. The experimental results for the FeCO₃/RGO composite reveal a minimum refection loss (-44.5 dB) at 11.9 GHz when the thickness reaches 2.4 mm. The effective bandwidth is 7.9 GHz between 10.1 and 18 GHz when the refection loss was below -10 dB. Compared to GO and RGO, this type of composite shows better microwave absorption thanks to improved impedance matching. Overall, this thin and lightweight FeCO₃/RGO composite is a promising candidate for absorbers that require both strong and broad absorption.

NiS2@rGO Nanosheet Wrapped with PPy Aerogel: A Sandwich-Like Structured Composite for Excellent Microwave Absorption

To reduce electromagnetic pollution as well as increase the accuracy of high-precision electronic equipment, more attention has been paid to new electromagnetic wave (EMW) absorbing materials, which have the advantages of strong absorption, wide absorption bands, and a narrow thickness. In this study, a novel ternary type of the NiS2@rGO/polypyrrole (PPy) sandwich-like structured composites was synthesized via a facile two-step method, in which the hydrothermal method was used to prepare NiS2@rGO binary composites and then the in situ polymerization method was used to synthesize the PPy, which acted as the outer layer of the sandwich-like structure. The morphologies and electromagnetic absorption performance of the NiS2@rGO/PPy were measured and investigated. A sample with 6 wt% NiS2@rGO/PPy loading paraffin-composite obtained an outstanding reflection loss (RL) of -58.7 dB at 16.44 GHz under a thickness of 2.03 mm. Simultaneously, the effective electromagnetic wave absorption bandwidth for RL < -10 dB, which covered 7.04 to 18.00 GHz (10.96 GHz), was achieved by changing the thickness of the absorber from 2.0 to 3.5 mm. The results not only suggest that the NiS2@rGO/PPy composite has excellent performance in the field of EMW absorption but also prove that the novel sandwich-like structure can contribute to appropriate impedance matching through multiple relaxation and interfacial polarization processes.

Photocatalytic Simultaneous Removal of Nitrite and Ammonia via a Zinc Ferrite/Activated Carbon Hybrid Catalyst under UV-Visible Irradiation

Nitrite and ammonia often coexist in waters. Thus, it is very significant to develop a photocatalytic process for the simultaneous removal of nitrite and ammonia. Herein, zinc ferrite/activated carbon (ZnFe₂O₄/AC) was synthesized and characterized by X-ray diffraction spectroscopy, transmission electron microscopy, Raman spectroscopy, and ultraviolet-visible diffuse reflectance spectroscopy. The valence band level of ZnFe₂O₄ was measured by X-ray photoelectron spectroscopy-valence band spectroscopy, and first-principles calculation was performed to confirm the band structure of ZnFe₂O₄. The as-synthesized ZnFe₂O₄/AC species functioned as a photocatalyst to simultaneously remove nitrite and ammonia under anaerobic conditions upon UV-visible light irradiation at the first stage. The results indicated that an average removal ratio of 92.7% with ±0.2% error for nitrite degradation for three runs was achieved in 50.0 mg/L nitrite + 100.0 mg/L ammonia solution with pH 9.5 under anaerobic conditions for 3 h at this stage; simultaneously, the removal ratio of 64.0% with ±0.2% error for ammonia was also achieved. At the second stage, oxygen gas was bubbled in the reactor to photocatalytically eliminate residual ammonia under aerobic conditions upon continuous irradiation. The results demonstrated that the removal ratios for nitrite, ammonia, and total nitrogen reached to 92.0, 90.0, and 90.2% at 12th hour, respectively, and the product released during photocatalysis is N₂ gas, detected by gas chromatography, fulfilling the simultaneous removal of nitrite and ammonia. The reaction mechanism was exploited.

Toward the Application of High Frequency Electromagnetic Wave Absorption by Carbon Nanostructures

With the booming development of electronic information technology, the problems caused by electromagnetic (EMs) waves have gradually become serious, and EM wave absorption materials are playing an essential role in daily life. Carbon nanostructures stand out for their unique structures and properties compared with the other absorption materials. Graphene, carbon nanotubes, and other special carbon nanostructures have become especially significant as EM wave absorption materials in the high-frequency range. Moreover, various nanocomposites based on carbon nanostructures and other lossy materials can be modified as high-performance absorption materials. Here, the EM wave absorption theories of carbon nanostructures are introduced and recent advances of carbon nanostructures for high-frequency EM wave absorption are summarized. Meanwhile, the shortcomings, challenges, and prospects of carbon nanostructures for high-frequency EM wave absorption are presented. Carbon nanostructures are typical EM wave absorption materials being lightweight and having broadband properties. Carbon nanostructures and related nanocomposites represent the developing orientation of high-performance EM wave absorption materials.

Crystalline-Amorphous Permalloy@Iron Oxide Core-Shell Nanoparticles Decorated on Graphene as High-Efficiency, Lightweight, and Hydrophobic Microwave Absorbents

The exploration of high-efficiency microwave absorption materials with lightweight and hydrophobic features is highly expected to reduce or eliminate the electromagnetic pollution. Graphene-based nanocomposites are universally acknowledged as promising candidates for absorbing microwaves due to their remarkable dielectric properties and lightweight characteristic. However, the hydrophilicity of graphene may reduce their stability and restrict the applications in moist environment. Herein, a well-designed heterostructure composed of crystalline permalloy core and amorphous iron oxide shell was uniformly adhered on oleylamine-modified graphene nanosheets by a one-pot thermal decomposition method. Compared with the recognized hydrophilic graphene-based hybrid materials, the permalloy@iron oxide/graphene nanocomposites show excellent hydrophobic and water-resistant features with a water contact angle of 136.5°. Besides, the nanocomposites show high-efficiency microwave absorption performance, benefiting from the tunneling effect, polarization, interface interaction, impedance matching condition, and synergistic effect between core-shell permalloy@iron oxide nanoparticles and graphene nanosheets. A broad effective absorption bandwidth with reflection loss (RL) value exceeding -10 dB can be obtained from 4.25 to 18 GHz, covering about 86% measured frequency range when the absorber thickness is 2.0-5.0 mm. Also, the microwave absorption performance of nanocomposites can be tuned by changing the amount of graphene. More importantly, a greatly improved microwave absorption effectiveness of -71.1 dB can be achieved for the nanocomposites in comparison with the bare permalloy@iron oxide nanoparticles (-5.6 dB) and oleylamine-modified GO nanosheets (-3.56 dB). The lightweight and hydrophobic permalloy@iron oxide/graphene nanocomposites with high-efficiency microwave absorption performance are highly promising to improve the environmental adaptability of electric devices, especially in the wet environment.

One-step hydrothermal synthesis of Ni–Fe–P/graphene nanosheet composites with excellent electromagnetic wave absorption properties

Ni–Fe–P nanoparticles/graphene nanosheet (Ni–Fe–P/GNs) composites were successfully synthesized by a simple one-step hydrothermal method. Specifically, Ni²⁺ and Fe²⁺ were reduced by using milder sodium hypophosphite as a reducing agent in aqueous solution. SEM and TEM images show that a large number of Ni–Fe–P nanoscale microspheres are uniformly deposited on graphene nanosheets (GNs). At the thickness of 3.9 mm, the minimum reflection loss (RL) of Ni–Fe–P/GNs reaches −50.5 dB at 5.3 GHz. In addition, Ni–Fe–P/GNs exhibit a maximum absorption bandwidth of 5.0 GHz (13.0–18.0 GHz) at the thickness of 1.6 mm. The significant electromagnetic absorption characteristics of the Ni–Fe–P/GN composites can be attributed to the addition of magnetic particles to tune the dielectric properties of graphene to achieve good impedance matching. Therefore, Ni–Fe–P/GN is expected to be an attractive candidate for an electromagnetic wave absorber.

Ni–Fe–P nanoparticle/graphene nanosheet composites synthesized by a one-step hydrothermal method have excellent performance in the field of electromagnetic wave absorption, with a minimum reflection loss of −50.5 dB and a maximum effective absorption bandwidth of 5 GHz.

Correlation between Structural Changes and Electrical Transport Properties of Spinel ZnFe2O4 Nanoparticles under High Pressure

The structural phase transition of synthetic ZnFe₂O₄ nanoparticles (ZFO NPs) is investigated as a function of pressure up to 40.6 GPa at room temperature for the first time, and its associated intriguing electrical transport properties are resolved from in situ impedance spectra and magnetoresistivity measurements. Significant anomalies are observed in the properties of the grain boundary resistance ( R_gb), the relaxation frequency ( f_max), and the relative permittivity (ε_r) in the ZFO NPs under the pressures around 17.5-21.5 GPa. These anomalies are believed to be correlated with a cubic-to-orthorhombic phase transition of ZnFe₂O₄ at the pressures between 21.9 and 25.7 GPa, which is found to be partially reversible. The pressure-tuned dielectric properties are measured for the cubic and the orthorhombic phases of ZFO, respectively. Remarkably, R_gb decreases up to 6 orders of magnitude as a function of pressure in the cubic phase. The dielectric polarization is obviously strengthened with increased f_max and decreased ε_r with pressure in the orthorhombic phase. Furthermore, it is confirmed that the external pressure effectively improves the electrochemical stability of the sample based on the cycled measurements of the impedance spectra at various pressures. The changes in the complex permittivity (ε', ε″) and the dielectric loss tangent (tan δ) with frequency reveal the irreversible increase in the dielectric loss accompanied by phase transition. The MR measurements indicate that ZFO NPs are superparamagnetic under high pressure of up to 40 GPa. The transmission electron microscopy images reflect the decrease in the grain boundary number and some local amorphization of grains after compression, which provides good explanations for the changes in the electrical transport properties as a function of pressure. Herein, the structural and electrical properties of ZnFe₂O₄ NPs generated are preserved by quenching the high-pressure phase to ambient conditions, thus providing great choices of ferrites materials for a variety of applications.

Rational Construction of Uniform CoNi-Based Core-Shell Microspheres with Tunable Electromagnetic Wave Absorption Properties

Core-shell particles with integration of ferromagnetic core and dielectric shell are attracting extensive attention for promising microwave absorption applications. In this work, CoNi microspheres with conical bulges were synthesized by a simple and scalable liquid-phase reduction method. Subsequent coating of dielectric materials was conducted to acquire core-shell structured CoNi@TiO₂ composite particles, in which the thickness of TiO₂ is about 40 nm. The coating of TiO₂ enables the absorption band of CoNi to effectively shift from K_u to S band, and endows CoNi@TiO₂ microspheres with outstanding electromagnetic wave absorption performance along with a maximum reflection loss of 76.6 dB at 3.3 GHz, much better than that of bare CoNi microspheres (54.4 dB at 17.8 GHz). The enhanced EMA performance is attributed to the unique core-shell structures, which can induce dipole polarization and interfacial polarization, and tune the dielectric properties to achieve good impedance matching. Impressively, TiO₂ coating endows the composites with better microwave absorption capability than CoNi@SiO₂ microspheres. Compared with SiO₂, TiO₂ dielectric shells could protect CoNi microspheres from merger and agglomeration during annealed. These results indicate that CoNi@TiO₂ core-shell microspheres can serve as high-performance absorbers for electromagnetic wave absorbing application.

Nickel Nanoparticle Encapsulated in Few-Layer Nitrogen-Doped Graphene Supported by Nitrogen-Doped Graphite Sheets as a High-Performance Electromagnetic Wave Absorbing Material

Herein we develop a facile strategy for fabricating nickel particle encapsulated in few-layer nitrogen-doped graphene supported by graphite carbon sheets as a high-performance electromagnetic wave (EMW) absorbing material. The obtained material exhibits sheetlike morphology with a lateral length ranging from a hundred nanometers to 2 μm and a thickness of about 23 nm. Nickel nanoparticles with a diameter of approximately 20 nm were encapsulated in about six layers of nitrogen-doped graphene. As applied for electromagnetic absorbing material, the heteronanostructures exhibit excellent electromagnetic wave absorption property, comparable to most EMW absorbing materials previously reported. Typically, the effective absorption bandwidth (the frequency region falls within the reflection loss below -10 dB) is up to 8.5 GHz at the thicknesses of 3.0 mm for the heteronanostructures with the optimized Ni content. Furthermore, two processes, carbonization at a high temperature and subsequent treatment in hot acid solution, were involved in the preparation of the heteronanostructures, and thus, mass production was achieved easily, facilitating their practical applications.

Porous Graphene Microflowers for High-Performance Microwave Absorption

Graphene has shown great potential in microwave absorption (MA) owing to its high surface area, low density, tunable electrical conductivity and good chemical stability. To fully realize graphene's MA ability, the microstructure of graphene should be carefully addressed. Here we prepared graphene microflowers (Gmfs) with highly porous structure for high-performance MA filler material. The efficient absorption bandwidth (reflection loss ≤ -10 dB) reaches 5.59 GHz and the minimum reflection loss is up to -42.9 dB, showing significant increment compared with stacked graphene. Such performance is higher than most graphene-based materials in the literature. Besides, the low filling content (10 wt%) and low density (40-50 mg cm^-3) are beneficial for the practical applications. Without compounding with magnetic materials or conductive polymers, Gmfs show outstanding MA performance with the aid of rational microstructure design. Furthermore, Gmfs exhibit advantages in facile processibility and large-scale production compared with other porous graphene materials including aerogels and foams.

Magnetically Aligned Co-C/MWCNTs Composite Derived from MWCNT-Interconnected Zeolitic Imidazolate Frameworks for a Lightweight and Highly Efficient Electromagnetic Wave Absorber

Developing lightweight and highly efficient electromagnetic wave (EMW) absorbing materials is crucial but challenging for anti-electromagnetic irradiation and interference. Herein, we used multiwalled carbon nanotubes (MWCNTs) as templates for growth of Co-based zeolitic imidazolate frameworks (ZIFs) and obtained a Co-C/MWCNTs composite by postpyrolysis. The MWCNTs interconnected the ZIF-derived Co-C porous particles, constructing a conductive network for electron hopping and migration. Moreover, the Co-C/MWCNTs composite was aligned in paraffin matrix under an external magnetic field, which led to a stretch of the MWCNTs along the magnetic field direction. Due to the anisotropic permittivity of MWCNTs, the magnetic alignment considerably increased the dielectric loss of the Co-C/MWCNTs composite. Benefiting from the conductive network, the orientation-enhanced dielectric loss, and the synergistic effect between magnetic and dielectric components, the magnetically aligned Co-C/MWCNTs composite exhibited extremely strong EMW absorption, with a minimum reflection loss (RL) of -48.9 dB at a filler loading as low as 15 wt %. The specific RL value (RL/filler loading) of the composite was superior to that of the previous MOF-derived composite absorbers. It is expected that the proposed strategy can be extended to the fabrication of other lightweight and high-performance EMW-absorbing materials.

Constructing hierarchical porous nanospheres for versatile microwave response approaches: the effect of architectural design

Hierarchical porous nanospheres via functionalized structural design were obtained to achieve a promising microwave absorption performance.