Simply and effectively achieving the tunability of the composition and chemical state of each component remains a challenge for modifying the electromagnetic performance of metal–organic-framework-derived (MOF-derived) composites. In this work, quaternary ZnO/Fe/Fe3C/carbon composites have been successfully synthesized by thermal decomposition of FeIII-MOF-5. The composition and chemical state of each component can be effectively controlled by changing the heating temperature. In detail, with increasing temperature, the Fe element would be transformed from Fe3+ to Fe3C and Fe, which also leads to the graphitization and weight loss of carbon. The effects on electromagnetic properties are also investigated, and the ZFC-700 sample possesses optimized reflection-loss (RL) performance with an RL value of −30.4 dB and a broad effective frequency bandwidth of 4.96 GHz at a thin thickness of only 1.5 mm. Conduction loss, interfacial polarization, ferromagnetic resonance, and interference cancelation should be responsible for ideal electromagnetic absorption. The porous quaternary composites not only convert incident electromagnetic energy to heat rather than reflect it back which is in favor of solving electromagnetic pollution, but also reduce the consumption of the metal source and poisonous raw materials for traditional microwave-absorbing materials.Keywords: Composition design; Metallic carbides; Microwave absorption; MOF-derived composites; Tunable electromagnetic properties;

The magnet/dielectric composites with tunable structure and composition have drawn much attention because of their particular merits in magnetoelectric properties compared with the sole dielectric or magnetic composites. In addition, porous materials at the nanoscale can satisfy the growing requirements in many industries. Therefore, constructing porous metal alloy/carbon nanocomposites is to be an admirable option. Unfortunately, traditional synthesis methods involve multistep routes and complicated insert-and-remove templates approaches. Here we report a facile process to synthesize CoxNiy/C composites via a spontaneous cross-linking reaction and subsequent calcination process, during which multiple processes, including reducing polyvalent metal ions, forming alloy, and encapsulating alloy nanoparticles into porous carbon matrix, are achieved almost simultaneously. By adjusting the feed ratio of Co2+ to Ni2+ ions, controllable composition of CoxNiy/C composites can be gained. It should be noted that the CoxNiy/C composites are demonstrated to be excellent microwave absorbers from every aspect of assessment criteria including reflection loss, effective bandwidth, thickness, and weight of absorber. Our study opens up a promising technique for the synthesis of alloy/carbon composites with porous nanostructures with target functionalities.Keywords: alginate; alloy; CoxNiy/C; electromagnetic wave absorption; porous carbon;

Owing to the fast development of wireless information technologies at the high-frequency range, the electromagnetic interference problem has been of increasing significance and attracting global attention. One key solution for this problem is to develop materials that are able to attenuate the unwanted electromagnetic waves. The desired properties of these materials include low reflection loss value, wide attenuation band, light weight, and low cost. This review gives a brief introduction to graphene-based composites and their electromagnetic absorption properties. The ultimate goal of these graphene absorbers is to achieve a broader effective absorption frequency bandwidth (fE) at a thin coating thickness (d). Representative and popular composite designs, synthesis methods, and electromagnetic energy attenuation mechanisms are summarized in detail. The two key factors, impedance matching behavior and attenuation ability, that determine the electromagnetic behavior of graphene-based materials are given particular attention in this article.

In this work, a one-pot strategy was proposed to synthesize carbon-coated Fe3O4 and carbon-coated Fe3C via the pyrolysis of colloidal Fe3O4 nanocrystals capped with oleic acid (OA) at different calcination temperatures. After exploring the microwave absorption performance of these composites, we found that carbon-coated Fe3C obtained at 700 °C possesses higher reflection loss (RL) and broader effective bandwidth (RL ≤ −10 dB) at low thickness. So the further investigation of the microwave absorption performance for carbon-capped Fe3C composites mixed with different mass percentages of paraffin was also carried out. The results demonstrated that the multi-dielectric relaxation process and better impedance matching, especially at low thickness and high frequency, contributed to the preferable microwave absorption greatly.

Porous carbon materials have long been regarded as promising candidate for high-performance lightweight microwave absorption materials owing to their strong attenuation abilities, tunable dielectric properties and low density. Nevertheless, previous work mainly focused on binary composites (usually carbon and magnetic fillers), which show limited loss mechanisms. The effect (except temperature) on the interfacial polarization and electromagnetic properties has rarely been investigated. Thus, a series of bimetallic zeolitic imidazolate frameworks (BMZIFs) with designed compositions and highly porous structures were selected to be converted to porous carbon-wrapped semiconductors (ZnO, Co3ZnC) and magnetic metal (Co) composites. Strong dielectric loss capabilities could be provided by graphitic carbon and enhanced interfacial polarization induced by multiple components and unique microstructures. By changing the molar ratio of Zn/Co under a fixed carbonization temperature, the interface of this multicomponent system could be adjusted, which influenced the electromagnetic properties. When evaluated as microwave absorption material, a reflection loss of −32.4 dB could be achieved with a broad effective frequency bandwidth of 5.24 GHz at only 1.9 mm. This work may provide an effective method to modify the physical and chemical properties of porous carbon materials with a desired complex structure and excellent microwave absorption performance.

Owing to their immense potential in functionalized applications, tremendous interest has been devoted to the design and synthesis of nanostructures. The introduction of sufficient amount of microwaves into the absorbers on the premise that the dissipation capacity is strong enough remains a key challenge. Pursuing a general methodology to overcome the incompatibility is of great importance. There is widespread interest in designing the materials with specific architectures. Herein, the common absorber candidates were chosen to feature the hierarchical porous Fe3O4@C@Fe3O4 nanospheres. Due to the reduced skin effect (induced by low-conductivity Fe3O4 outer layer), multiple interfacial polarizations and scattering (due to the ternary hierarchical structures and nanoporous inner core) as well as the improved magnetic dissipation ability (because of multiple magnetic components), the material design enabled a promising microwave absorption performance. This study not only illustrates the primary mechanisms for the improved microwave absorption performance but also underscores the potential in designing the particular architectures as a strategy for achieving the compatibility characteristics.

Design of an interface to arouse interface polarization is an efficient route to attenuate high-frequency electromagnetic waves. The attenuation intensity is highly related to the contact area. To achieve stronger interface polarization, growing metal oxide granular film on graphene with a larger surface area seems to be an efficient strategy due to the high charge carrier concentration of graphene. This study is devoted to fabricating the filmlike composite by a facile thermal decomposition method and investigating the relationship among contact area, polarization intensity, and the type of metal oxide. Because of the high-frequency polarization effect, the composites presented excellent electromagnetic wave attenuation ability. It is shown that the optimal effective frequency bandwidth of graphene/metal oxide was close to 7.0 GHz at a thin coating layer of 2.0 mm. The corresponding reflection loss value was nearly −22.1 dB. Considering the attenuation mechanism, interface polarization may play a key role in the microwave-absorbing ability.Keywords: contact area; effective absorption frequency; electromagnetic interference; interface polarization; metal oxide granular film;

One-dimensional (1D) microwave absorbers have been verified to have a predominant morphology due to their significant anisotropy, large surface area, and great dielectric attenuation compared to other microstructures. Consequently, in this research, novel 1D mesoporous MoO2/C heteronanowires have been designed through an in situ facile synthesis process. As well as their attractive morphology, building multiple interfaces for polarization between MoO2 and carbon, inducing dipole polarization of MoO2 and constructing conductive networks among nanowires endow the composites with outstanding dielectric dissipated properties, allowing excellent microwave absorption (MA) performance. Therefore, at the appropriate filling condition of 25 wt%, the MoO2/C nanowire-paraffin achieved a minimum reflection loss of −47.6 dB at 11.1 GHz and a bandwidth of 3.8 GHz in the range of 9.9–13.7 GHz with a thickness of 2 mm, thus it has the potential to be a lightweight candidate of microwave absorbing material.

To satisfy the diverse requirements of low reflection and high absorption of microwave attenuation, the construction of multiple heterojunction structure is imperative. On the one hand, the impedance mismatching could be ameliorated via the addition of new component; on the other hand, the multiple interface polarizations derived from the architecture of heterojunction make for the dissipation of microwave. In this work, the ternary TiO2/RGO/Fe2O3 composites exhibit tremendous superiority compared with single TiO2 or RGO no matter the absorption coefficient or effective bandwidth. The maximum absorption value of the TiO2/RGO/Fe2O3 composites is −44.05 dB at 14.48 GHz with a low thickness of 2.0 mm. In addition, the effective bandwidth (RL < −10 dB) reaches 5.6 GHz from 11.96 to 17.56 GHz. The superior electromagnetic wave absorbing performance of the TiO2/RGO/Fe2O3 composites derived from the appropriate impedance matching as well as the multiple polarization effect. The results adequately demonstrate the accessibility of the prepared TiO2/RGO/Fe2O3 composites as a preeminent absorber.The obtained ternary heterojunction structures of TiO2/RGO/Fe2O3 composites exhibit enhanced microwave absorption ability compared to the single TiO2 or RGO due to the ameliorated impedance mismatching characteristics and the multiple interface polarizations derived from the architecture of heterojunctions.Download high-res image (137KB)Download full-size image

•The titanium based MOFs were firstly used to prepare nanoporous TiO2/C composites.•The dielectric loss and microwave absorption properties can be regulated by adjusting the calcination temperature.•An optimal reflection loss and wide frequency bandwidth is achieved with an ultrathin matching thickness.•The synthetic process of TiO2/C composites is simple.Nanoporous carbon materials derived from metal organic frameworks (MOFs) have been regarded as an important members in the microwave absorption field. Nevertheless, most attentions only focus on Co-based, Ni-based MOFs and the impedance matching. In this study, a novel nanoporous carbon material (TiO2/C) has been directly synthesized by annealing titanium based MOFs (MIL-125 (Ti), MIL stands for Material from Institute Lavoisier), which possess low toxicity, redox activity and good stability. The obtained TiO2/C composites show outstanding electromagnetic wave absorbing properties. In detail, the minimum reflection loss of −49.6 dB and broad effective bandwidth of 4.6 GHz (13.4–18 GHz) can be reached with an absorbent thickness of 1.6 mm. Our study not only opens a new avenue for artificially designed diverse nanoporous carbon but also introduce a new candidate to directly synthesize absorber by a simple method.Download high-res image (215KB)Download full-size image

Design of dielectric/magnetic heterostructure and multiple interfaces is a challenge for the microwave absorption. Thus, in this study, a novel C/Fe3C nanocomposites have been fabricated by annealing the precursors obtained by the facile chemical blowing of polyvinyl pyrrolidone (PVP) and Fe(NO3)3·9H2O. By changing the content of Fe(NO3)3·9H2O, the honeycomb-like structure with scads of pores and electromagnetic parameters could be successfully tailored. When the addition of Fe(NO3)3·9H2O is ranging from 1 to 2 g, honeycomb-structured nanocomposites possess high performance microwave absorption when mixed with 90 wt% paraffin. The minimal reflection loss is −37.4 dB at 13.6 GHz and effective bandwidth can reach to 5.6 GHz when the thickness is 2.0 mm, indicating its great potential in microwave absorbing field. Its outstanding microwave performance is tightly related to the porous structure and substantial interface such as carbon/air and carbon/Fe3C, which are in favor of the impedance matching and interfacial polarization. Thus, our study may provide a good reference for the facile synthesis of light-weight carbon-based nanocomposites with effective interfacial polarization.Porous C/Fe3C heterojunctions with effective interfacial polarization have been fabricated by a facile strategy. By changing the addition of Fe(NO3)3·9H2O, its morphology and EM absorption could be regulated.Download high-res image (89KB)Download full-size image

In this work, the 3-D honeycomb-like FeCo/C nanocomposites were synthesized through the carbon thermal reduction under an inert atmosphere. The enhanced microwave absorption properties of the composites were mainly attributed to the unique three dimensional structure of the FeCo/C nanocomposites, abundant interfaces and junctions, and the appropriate impedance matching. The Cole-Cole semicircles proved the sufficient dielectric relaxation process. The sample calcinated at 600 °C for 4 h showed the best microwave absorption properties. A maximum reflection loss of −54.6 dB was achieved at 10.8 GHz with a thickness of 2.3 mm and the frequency bandwidth was as large as 5.3 GHz. The results showed that the as-prepared FeCo/C nanocomposite could be a potential candidate for microwave absorption.The FeCo/C core-shell nanocomposites have been synthesized by the carbon thermal reduction method and show an optimal microwave absorption performances with adjustable 3-D morphology and appropriate impedance matching.Download high-res image (229KB)Download full-size image

Permittivity plays crucial but reverse roles in impedance matching and attenuation loss in terms of electromagnetic wave absorption. In addition to unilateral superior performance, an ideal absorber needs to take into consideration both impedance matching and energy conservation. In order to acquire absorbing materials with moderate impedance matching and attenuation ability simultaneously, we have fabricated MoS2/rGO composites via a facile and effective hydrothermal approach. The dielectric constant of the obtained composite can be regulated by varying the molar ratio of the precursors and an optimal balance between impedance matching and energy conservation is eventually obtained upon addition of 6 mL GO. The maximum reflection loss is −67.1 dB at 14.8 GHz and the effective electromagnetic wave absorption bandwidth for RL < −10 dB covers from 12.08 to 18.00 GHz (5.92 GHz) with a small thickness of 1.95 mm. Moreover, the relationship between the matching thickness and the highest reflection loss value has also been discussed in detail. The results not only suggest that the MoS2/rGO composites developed through a simple procedure here can act as an ideal absorber with strong absorption, a broad frequency bandwidth and small thickness, but also offer a good reference for the design of microwave absorbers, including the compatibility of impedance matching and attenuation loss ability as well as matching thickness.

The preparation of nanocomposites of reduced graphene oxide with loaded TiO2 nanoparticles (TRGO) by a facile one-step hydrothermal treatment is reported. We have successfully increased the contact area of TiO2 and RGO to enhance polarization point, which is in favor of strengthening interfacial polarization. The interfaregioncial polarization has been regarded as an important role on the attenuation of high-frequency electromagnetic waves. Therefore, a good absorber is prepared by inserting the polarization point on the graphene aerogel, which shows excellent electromagnetic wave absorbing properties. In detail, the minimum reflection loss value at 2.1 mm is up to −27.2 dB for the TRGO-1.5 composite and the frequency bandwidth of 5.2 GHz can be obtained. Thus, it demonstrates that the adjustment of interface polarization would play a key role in the microwave-absorbing ability.Taking full advantage of the interface is an efficient strategy to achieve good electromagnetic absorption performance. Therefore a simple strategy to improve the electromagnetic absorption performance is achieved by inserting the polarization point on the graphene aerogel. The obtained TRGO samples show the excellent absorbing property.Download high-res image (79KB)Download full-size image

In this study, two-dimensional MoS2 nanosheets synthesized by a hydrothermal method were firstly investigated for microwave absorbing performance. The obtained MoS2 nanosheets are highly desirable as an electromagnetic wave (EM) absorber because of its larger interfacial polarization and high dielectric loss. Our results show that the real and imaginary parts of permittivity of MoS2 prepared at 180 °C are higher than those of other samples. A broad bandwidth absorption at a thin thickness can be obtained between 2 and 18 GHz. The microwave reflection loss (RL) of MoS2 nanosheets prepared at 180 °C reaches as high as −47.8 dB at 12.8 GHz due to its high electrical conductivity and the polarization effect. It can also be found that MoS2 exhibits an effective electromagnetic wave absorption bandwidth of 5.2 GHz (<−10 dB) at the thicknesses of 1.9 and 2.0 mm. The results showed that the MoS2 nanosheets can be a candidate for microwave absorption with a broad effective absorption bandwidth at thin thicknesses.

This study is focused on the spinel structure of metal oxides and sulfides, from which the ternary (NiCo2O4/NiCo2S4) and quaternary (Fe0.5Ni0.5Co2O4/Fe0.5Ni0.5Co2S4) samples with hollow sphere structures were prepared. Among these samples, Fe0.5Ni0.5Co2S4 was highly effective in its ability to attenuate electromagnetic waves, wherein a broader absorption bandwidth of 6.2 GHz could be achieved with a thinner coating layer of 1.3 mm. The cation distribution rule for Fe, Co and Ni ions in the spinel structure is given according to hybrid orbital theory to support the excellent electromagnetic absorption properties. Relying on the distribution of Fe, Co and Ni cations, the probable electron transmission and coupling between Fe3+/Co2+ and Fe3+/Fe3+ adjacent cation ion pairs could occur at the octahedral site (B site), which reflects the enhanced dielectric loss.

To overcome the shortcomings (poor impedance mismatching and weak electromagnetic wave attenuation) of the Co nanoparticles embedded into nanoporous carbon (Co@NPC) derived from the thermal decomposition of zeolitic imidazolate framework-67 (ZIF-67), two coated titanium oxide (TiO2) routes are designed to prepare core–shell Co@NPC@TiO2 and multi-interfaced yolk–shell C–ZIF-67@TiO2 (obtained from the thermal decomposition of ZIF-67@TiO2) structures. The permittivity and permeability of C–ZIF-67@TiO2 significantly depend on the thickness of the TiO2 shell in ZIF-67@TiO2, and the thickness of the TiO2 shell in the as-obtained samples can be easily controlled via changing the addition content of tetrabutyl titanate in the hydrolyzation process. The as-prepared samples have remarkable absorbing characteristics in wide frequency bands from 2–18 GHz with thicknesses of 1.0–5.0 mm. 50 wt% of the C–ZIF-67@TiO2-2 (the addition amount of tetrabutyl titanate is 2 mL) nanocomposite filled within paraffin shows a maximum reflection loss (RL) of −51.7 dB at an absorbing thickness of 1.65 mm, meanwhile, for the Co@NPC@TiO2-1.2 (the addition amount of tetrabutyl titanate is 1.2 mL) nanocomposite, a maximum RL can be achieved of −31.7 dB at 1.5 mm. This study provides a good reference for the future preparation of other carbon-based lightweight microwave absorbing materials derived from metal organic frameworks.

An improved hydrogenation strategy for controllable synthesis of oxygen-deficient anatase TiO2 (H-TiO2) is performed via adjusting the particle size of starting rectangular anatase TiO2 nanosheets from 90 to 30 nm. The morphology and structure characterizations obviously demonstrate that the starting materials of TiO2 nanosheets are transformed into nanoparticles with distinct size reduction; meanwhile, the concentration of oxygen vacancy is gradually increased with the decreasing particle size of starting TiO2. As a result, the Li-storage performance of H-TiO2 is not only much better than that of the pure TiO2 but also elevated stage by stage with the decreasing particle size of starting TiO2; especially the H-TiO2 with highest concentration of oxygen vacancy from smallest TiO2 nanosheets shows the best Li-storage performance with a stable discharge capacity 266 mAh g–1 after 100 cycles at 1 C. Such excellent performance should be attributed to the joint action from oxygen vacancy and size effect, which promises significant enhancement of high electronic conductivity without weakening Li+ diffusion via hydrogenation strategy.Keywords: anatase TiO2 nanosheets; anode materials; controllable oxygen vacancy; hydrogenation process; lithium-ion battery; size−reactivity correlation

Among all polarizations, the interface polarization effect is the most effective, especially at high frequency. The design of various ferrite/iron interfaces can significantly enhance the materials’ dielectric loss ability at high frequency. This paper presents a simple method to generate ferrite/iron interfaces to enhance the microwave attenuation at high frequency. The ferrites were coated onto carbonyl iron and could be varied to ZnFe2O4, CoFe2O4, Fe3O4, and NiFe2O4. Due to the ferrite/iron interface inducing a stronger dielectric loss effect, all of these materials achieved broad effective frequency width at a coating layer as thin as 1.5 mm. In particular, an effective frequency width of 6.2 GHz could be gained from the Fe@NiFe2O4 composite.Keywords: carbonyl iron; coating; dielectric loss; ferrite; microwave absorption; polarization

Quasi-noble-metal graphene quantum dots (GQDs) deposited stannic oxide (SnO2) with oxygen vacancies (VOs) were prepared by simply sintering SnO2 and citric acid (CA) together. The redox process between SnO2 and GQDs shows the formation of oxygen vacancy states below the conduction band of stannic oxide. The produced VOs obviously extend the optical absorption region of SnO2 to the visible-light region. Meanwhile, GQDs can effectively improve the charge-separation efficiency via a quasi function like noble metal and promote the visible-light response to some degree. In addition, the samples calcinated at 450 °C reveals the best performance because of its relatively high concentrations of VOs. What is more, the possible degradation mechanism has been inferred as extended visible-light response as well as raised charge-separation efficiency has also been put forward. Our work may offer a simple strategy to combine the defect modulation and noble metal deposition simultaneously for efficient photocatalysis.The nanocomposite photocatalysts of quasi-noble-metal graphene quantum dots (GQDs) deposited SnO2 with oxygen vacancies (VOs) were prepared through a simple two-step reactions process. The obtained nanocomposites exhibited much enhanced photodegradation activity for the extended visible-light response as well as raised charge-separation efficiency.

•Ti3+ doped TiO2 crystals were synthesized via a facile hydrothermal reaction.•Ti3+ doped TiO2 crystals showed higher photocatalytic activity.•HF was responsible for the formation of anatase phase and Ti3+.In the present study, Ti3+ doped TiO2 single anatase crystals were synthesized via a facile hydrothermal reaction between Ti powder, HCl and HF. The Ti3+ doped TiO2 single crystals possess truncated bipyramidal nanostructure with particle size ranging from ca. 100 nm to 500 nm and demonstrate better visible light absorption as compared to TiO2–HCl and pure TiO2 samples. Photo-decomposition of methylene blue (MB) experiment shows that Ti3+ doped TiO2 single crystals showed higher decomposition rate than TiO2–HCl and undoped TiO2 samples benefited from the Ti3+ doping, which improved the light absorption ability and concentration of photo-generated electron-hole pairs.Ti3+ doped TiO2 single anatase crystals with truncated bipyramidal nanostructure and high photocatalytic activity has been successfully prepared via hydrothermal reaction of Ti nanopowder, HCl and HF. Ti3+ doping extends the light absorption from the UV into the visible range and results in enhanced photocatalytic degradation of MB. The adding of HF was found to be responsible for the formation of anatase phase, truncated bipyramidal nanostructure and Ti3+ doping.

Hierarchical hollow carbon@Fe@Fe3O4 nanospheres were synthesized by a simple template method and another pyrolysis process. Interestingly, the thickness of hollow carbon spheres is tunable by a simple hydrothermal approach. The as-prepared carbon@Fe@Fe3O4 shows excellent microwave absorption properties. In detail, the maximum effective frequency is up to 5.2 GHz with an optimal reflection loss value of −40 dB while the coating thickness is just 1.5 mm. Meanwhile, such absorption properties can be maintained via controlling the thickness of the hollow carbon. For instance, in another coating layer of 2 mm, the effective frequency is still more than 5 GHz as the carbon thickness declines to 12 nm. As novel electromagnetic absorbers, the composites also present the lower density feature due to the hollow carbon sphere frame. The excellent electromagnetic absorption mechanism may be attributed to the obvious interface polarization, and strong magnetic loss ability resulting from the Fe and Fe3O4 shell. Besides, owing to the dielectric feature of carbon, the hollow carbon core is beneficial for the attenuation ability.

A novel FeCo nanoparticle embedded nanoporous carbon composite (Fe–Co/NPC) was synthesized via in situ carbonization of dehydro-ascorbic acid (DHAA) coated Fe3O4 nanoparticles encapsulated in a metal–organic framework (zeolitic imidazolate framework-67, ZIF-67). The molar ratio of Fe/Co significantly depends on the encapsulated content of Fe3O4 in ZIF-67. The composites filled with 50 wt% of the Fe–Co/NPC-2.0 samples in paraffin show a maximum reflection loss (RL) of −21.7 dB at a thickness of 1.2 mm; in addition, a broad absorption bandwidth for RL < −10 dB which covers from 12.2 to 18 GHz can be obtained, and its minimum reflection loss and bandwidth (RL values exceeding −10 dB) are far greater than those of commercial carbonyl iron powder under a very low thickness (1–1.5 mm). This study not only provides a good reference for future preparation of carbon-based lightweight microwave absorbing materials but also broadens the application of such kinds of metal–organic frameworks.

Impedance matching and the attenuation constant, α, are two key parameters in determining electromagnetic absorption properties. Although materials with single magnetic or dielectric loss properties have a high α value, they nonetheless suffer from poor impedance matching. The design of magnetic and dielectric composites might possibly be an effective method of solving this problem, but unfortunately the introduction of magnetic material may give a poor value of α. In order to obtain absorptive materials with high impedance matching and a high value of α, we have designed a novel ternary composite of MnO2@Fe–graphene. A 30 nm wide rod-like strip of MnO2 was first obtained by a simple liquid process. Liquid decomposition of Fe(CO)5 was then carried out to deposit iron on the surface of the rod-like structure, and the MnO2@Fe was finally loaded on graphene by a liquid deposition technique. The resulting ternary composite exhibited attractive electromagnetic absorption properties, in which the optimal reflection loss of up to −17.5 dB obtained with a thin coating thickness of 1.5 mm was able to satisfy the requirements of lightness of weight and a high degree of absorption. The effective bandwidth frequency of MnO2@Fe–GNS is broader than that of pure MnO2 or MnO2@Fe, possibly due to its moderate impedance matching and attenuation ability. The possible attenuation mechanism will also be discussed.

In this paper, we designed a novel core–shell composite for microwave absorption application in which the α-Fe2O3 and the porous CoFe2O4 nanospheres served as the core and shell, respectively. Interestingly, during the solvothermal process, the solvent ratio (V) of PEG-200 to distilled water played a key role in the morphology of α-Fe2O3 for which irregular flake, coin-like, and thinner coin-like forms of α-Fe2O3 can be produced with the ratios of 1:7, 1:3, and 1:1, respectively. The porous 70 nm diameter CoFe2O4 nanospheres were generated as the shell of α-Fe2O3. It should be noted that the CoFe2O4 coating layer did not damage the original shape of α-Fe2O3. As compared with the uncoated α-Fe2O3, the Fe2O3@CoFe2O4 composites exhibited improved microwave absorption performance over the tested frequency range (2–18 GHz). In particular, the optimal reflection loss value of the flake-like composite can reach −60 dB at 16.5 GHz with a thin coating thickness of 2 mm. Furthermore, the frequency bandwidth corresponding to the RLmin value below −10 dB was up to 5 GHz (13–18 GHz). The enhanced microwave absorption properties of these composites may originate from the strong electron polarization effect (i.e., the electron polarization between Fe and Co) and the electromagnetic wave scattering on this special porous core–shell structure. In addition, the synergy effect between α-Fe2O3 and CoFe2O4 also favored balancing the electromagnetic parameters. Our results provided a promising approach for preparing an absorbent with good absorption intensity and a broad frequency that was lightweight.Keywords: core−shell structure; lightweight absorber; microwave absorption properties; α-Fe2O3; α-Fe2O3@CoFe2O4

The porous three-dimensional (3-D) flower structures assembled by numerous ultrathin flakes were favor for strengthen electromagnetic absorption capability. However, it still remains a big challenge to fabricate such kind of materials. In this study, an easy and flexible two-step method consisting of hydrothermal and subsequent annealing process have been developed to synthesize the porous 3-D flower-like Co/CoO. Interestingly, we found that the suitable heat treatment temperature played a vital role on the flower-like structure, composition, and electromagnetic absorption properties. In detail, only in the composite treated with 400 °C can we gain the porous 3-D flower structure. If the annealing temperature were heated to 300 °C, the Co element was unable to generate. Moreover, when the annealing temperature increased from 400 to 500 °C, these flower-like structures were unable to be kept because the enlarged porous diameter would wreck the flower frame. Moreover, these 3-D porous flower-like structures presented outstanding electromagnetic absorption properties. For example, such special structure enabled an optimal reflection loss value of −50 dB with the frequency bandwidth ranged from 13.8 to 18 GHz. The excellent microwave absorption performance may attribute to the high impedance matching behavior and novel dielectric loss ability. Additionally, it can be supposed that this micrometer-size flower structure was more beneficial to scatter the incident electromagnetic wave. Meanwhile, the rough surface of the ultrathin flake is apt to increase the electromagnetic scattering among the leaves of the flower due to their large spacing and porous features.Keywords: Co/CoO; electromagnetic absorption properties; impedance matching; lightweight; porous 3-D flower structure;

Blue oxygen-deficient nanoparticles of anatase TiO2 (H-TiO2) are synthesized using a modified hydrogenation process. Scanning electron microscope and transmission electron microscope images clearly demonstrate the evident change of the TiO2 morphology, from 60 nm rectangular nanosheets to much smaller round or oval nanoparticles of ∼17 nm, after this hydrogenation treatment. Importantly, electron paramagnetic resonance and positronium annihilation lifetime spectroscopy confirm that plentiful oxygen vacancies accompanied by Ti3+ are created in the hydrogenated samples with a controllable concentration by altering hydrogenation temperature. Experiments and theory calculations demonstrate that the well-balanced Li+/e– transportation from a synergetic effect between Ti3+/oxygen vacancy and reduced size promises the optimal H-TiO2 sample a high specific capacity, as well as greatly enhanced cycling stability and rate performance in comparison with the other TiO2.Keywords: anatase TiO2; hydrogenation treatment; Li-storage performance; oxygen vacancy dependence; synergetic effect

Hollow hierarchical microspheres of Bi/BiOBr (SBB) with oxygen vacancies were prepared using a one step solvothermal method. It was found that the stannous chloride dihydrate played key roles in the formation of Bi, defects and the stacking mode of hierarchical construction units. Positron annihilation lifetime spectroscopy (PALS) was used to demonstrate the oxygen vacancies in Bi/BiOBr samples. The density of states (DOS) of the valence band of BiOBr can be modulated by the introduction of oxygen vacancies according to the valence band XPS and Density Functional Theory (DFT) calculations. Analyses of photoluminescence and BET demonstrated that SBB hollow hierarchical microspheres with higher specific surface area have a lower recombination rate of photo-generated electrons and holes. The photocatalytic and adsorptive performances showed that the samples exhibited stronger adsorption capacity toward rhodamine B (RhB) and highly efficient photocatalytic activity in the degradation of RhB, which were attributed to the higher adsorption ability and synergistic effect of oxygen vacancies and construction of the heterojunction structure (Bi/BiOBr).

In this paper a novel electromagnetic absorbent, porous coin-like iron with a diameter of ∼10 μm and a thickness of 2 μm, was fabricated using a hydrogen gas reduction process. This special porous coin-like structure was attributed to a decrease in density and exceeded the Snoek limitation. It was observed that these coin-like iron structures exhibit excellent microwave absorption properties. An optimal reflection loss value of −53.2 dB was obtained at 16 GHz, moreover, the effective frequency bandwidth could be up to 6.3 GHz (11.7–18 GHz) at a thickness of 1.4 mm. The microwave absorption mechanism may have originated from the following factors: firstly, these coin-like irons were favorable for obtaining a lower real part of permittivity value and thus gained the improvement of impedance matching behavior, as compared with other reported irons. Secondly, the coin-like morphology exhibited a strong magnetic loss ability. Further analysis revealed that the magnetic loss mechanism may rely mainly on the resonance. In addition, the porous feature of the coin-like iron offered a rough surface on the large size of the coin-like structure, which was beneficial for electromagnetic wave scattering and further enhanced their microwave absorption properties.

Magnetic/dielectric core–shell structures have been regarded as ideal high-performance electromagnetic absorption materials due to their novel multiple-loss mechanism. However, the poor impedance matching property of a dielectric shell may lead to the high reflection of electromagnetic waves from the interface of the shell. Thus, we ingeniously use the magnetic material as the shell while the dielectric material is used as the core. Such a change not only decreases the electromagnetic wave reflection, but also causes a strong interface polarization. Subsequently, the wave-transparent material SiO2 was further coated on the surface of the magnetic shell which not only protected it from oxidation but also increased the impedance matching performance. Based on the above design, in this study, we fabricated a CNT@Fe@SiO2 ternary core–shell structure composite using a simple two-step approach consisting of pyrolysis and decomposition processes. As compared with pure CNTs and CNT@Fe materials, the obtained CNT@Fe@SiO2 composite shows obviously enhanced electromagnetic absorption properties. In particular at a thin thickness of 1.5 mm, the optimal reflection loss value is as high as −14.2 dB which is better than most of the reported CNT based absorbers. The improved electromagnetic absorption properties can be attributed to the perfect impedance matching behavior and the multiple interface polarization effect.

An easy, template-free one-pot method was carried out for mass preparation of microscale octahedral Fe3O4. Thermal decomposition and a flux-mediated method were combined by directly introducing ferric acetate into the molten flux, which is called an in situ molten salt method. The morphology, magnetic properties, electromagnetic parameters, and microwave absorption behaviors were characterized. With the right octahedral temperature (800 °C), the iron oxide has a strong electrostatic interaction with salt ions leading to a perfect octahedron morphology. The reflection loss of the sample annealed at 800 °C (named F-800) can reach −23.67 dB at a frequency of 15.24 GHz with only a 1.4 mm coating thickness. The strongest microwave absorption ability may be ascribed to the unique morphology, best impedance matching and large attenuation value.

In the present investigation, monochromatic 355-nm pulsed laser radiations generated by third harmonic of Nd:YAG laser (1,060 nm) were applied as an excitation light source for conversion of CO2 into value-added hydrocarbon (methanol) with high selectivity over ZnS particles through photo-catalytic process. The microstructure and optical band absorption of as-prepared ZnS nanoparticles were characterized by means of XRD, UV–Vis absorption spectrum, TEM, SAED, and EDX spectrum. The effect of irradiation time as well as the pulsed laser energy on methanol yield and photoefficiency were investigated and discussed in detail. Furthermore, a possible mechanism for such conversion process is also discussed based on the band edge of ZnS semiconductor and the unique monochromatic characteristic of the adopted 355-nm pulsed laser.

FeCo/ZnO composites were successfully synthesized by a simple one-step hydrothermal process. It is clearly seen that the pencil-like ZnO particles with 3–4 μm in length and 200–300 nm in width are found to grow along the surface of the hexagonal-cone FeCo particles, which form a discontinuous conductive network. Enhanced microwave absorption properties can be obtained for the FeCo/ZnO composites as compared to those of pure FeCo alloy, which is mainly attributed to the better resonance according to an isotropic antenna mechanism. Further investigation confirms that the hydrothermal temperature (T) and time (t) play a key role on the real part of the permittivity values of the FeCo/ZnO composites and indirectly affect the microwave absorbing properties. It is surprising to find out that the composite prepared at 150 °C for 12 hours, exhibited the optimal reflection loss of −31 dB with a 5.5 GHz effective frequency bandwidth.

•Fe3O4/GNS nanoparticles have the significantly enhanced dielectric properties when compared with pure Fe3O4.•The absorbing bandwidth of Fe3O4/GNS composites exhibits more wider than pure Fe3O4.•The peak value of the reflection loss of Fe3O4/GNS composites has a tendency to be more lower with a thinner thickness.Fe3O4/graphene nanosheet (Fe3O4/GNS) composites were prepared from acetylacetone iron (III) and graphene oxide (GO) with ethylene glycol (EG) as the solvent and reducing agent. The SEM and TEM images show that the Fe3O4 nanoparticles with relatively uniform size are well-distributed on the surface of graphene nanosheet. The structure guiding agents have a great influence on the size and surface property of Fe3O4 particles by control the nucleation and growth of Fe3O4. The effect of GNS in the composites was also discussed by regulation the mass ratio of acetylacetone iron (III) and GO. The optimal Fe3O4/GNS composites have significantly enhanced dielectric properties when compared with pure Fe3O4. Especially, it can be found that the calculated minimum RL for Fe3O4/GNS composites with a thickness of 1.5 mm is −8.75 dB at 8.11 GHz, the microwave absorption values less than −5 dB is in the ranges of 7.78–10.36 GHz, moreover, it can be modulated by tuning the mass ratio of Fe3O4 and GO. Therefore, the composites with the thin, light-weighted and broadband absorbing properties probably have a huge potential for electromagnetic wave absorption applications.Graphical abstractThe calculated minimum RL for Fe3O4/GNS composites with a thickness of 1.5 mm is −8.75 dB at 8.11 GHz, the microwave absorption values less than −5 dB is in the ranges of 7.78–10.36 GHz, moreover, it can be modulated by tuning the mass ratio of Fe3O4 and GO.

•FexOy@SiO2 nanoparticles were kept well-defined in size and shape as Fe3O4@SiO2.•FexOy@SiO2 exhibit enhanced magnetic properties (Ms = 76.032 emu/g).•FexOy@SiO2 exhibit enhanced microwave absorption properties (RL = −23 dB).Fe3O4@SiO2 core–shell nanostructures with different core diameter and shell thicknesses were prepared by modified stöber method and then converted into FexOy@SiO2 by H2. The structure, morphology and electromagnetic properties of FexOy@SiO2 core–shell nanostructures were investigated. Experimental results demonstrate that the as-prepared FexOy@SiO2 nanoparticles with deeply reduction not only exhibit significantly enhanced magnetic properties with maximum saturation magnetization values of 76.032 emu/g vs. 50.310 emu/g of Fe3O4@SiO2 core–shell spheres, but also show enhanced microwave absorption properties with the maximum reflection loss value of −23 dB at 9.2 GHz under a thickness of 2.5 mm, which give forceful evidences that these FexOy@SiO2 core–shell nanomaterials are attractive materials for microwave absorption applications.Graphical abstractThe FexOy@SiO2 nanoparticles were kept well-defined in size and shape as the same with the Fe3O4@SiO2 core–shell nanoparticles completely. The results also show that FexOy@SiO2 core–shell nanoparticles have significantly enhanced microwave absorption properties with the maximum reflection loss value of −23 dB at 9.2 GHz with a thickness of 2.5 mm.

Hollow structured magnetite spheres were fabricated by a simple solvothermal process, with the assistance of various ammonium salts, where ethylene glycol was used as the solvent and reducing agent. The results of X-ray diffraction, scanning electron microscopy and transmission electron microscopy show that the as-synthesized products are pure single-phase Fe3O4 with good crystalline state, and all the samples are hollow structures except for the S-d sample, which was obtained by the assistance of NH4HCO3. The X-ray photoelectron spectrometry manifests that the Fe3+ ions on the surface of the hollow spheres exist in the form of Fe3O4, and Mössbauer measurements reveal that the hollow spheres are similar to the stoichiometric Fe3O4. The possible formation mechanism of hollow magnetite spheres with various sizes was discussed in detail. Meanwhile magnetic properties were determined by using a vibrating sample magnetometer at room temperature. The magnetic properties investigation showed that the Fe3O4 spheres were ferromagnetic with small hysteresis loops. The values of saturation magnetization are 82.989, 78.049 and 87.417 emu g−1 for the S-a, S-b and S-c samples, which decreases with increasing particle size. This phenomenon may be caused by the content of Fe3+ on A- and B-sites of the spinel ferrite. Based on experimental results, the relationship between their microcomponents also has been studied.

This study reported, for the first time systematically, photodegradation of Rhodamine B (RhB) in aqueous solution over BiOCl and BiOBr semiconductors. Under visible light irradiation (λ > 400 nm, λ > 420 nm and λ = 550 ± 15 nm), RhB adsorbed on the surface of BiOCl and BiOBr was photosensitized and decomposed effectively over unexcited BiOCl and BiOBr. The degradation of Methyl Orange (MO) and Methylene Blue (MB) over BiOCl and BiOBr was investigated as well, and the results were compared with RhB photodegradation. It was found that MB molecules having the lowest LUMO could not be degraded by this process. Utilizing the quantum chemical calculation (Gaussian 03 program), the relationship between frontier orbital energy of selected dye molecules and photodegradation rate was established for the first time in this study.Graphical abstractHighlights► Photosensitized degradation of Rodamine B was investigated on BiOX (X = Cl, Br). ► Methylene Blue could not be degraded on BiOX under visible light irradiation. ► The relationship between LOMO of dye molecules and degradation rate was established.

Magnetic Co–Cu alloy nanowires with low Cu content were prepared by SC electrodeposition in pores of anodic aluminum oxide templates. The as-deposited Co–Cu nanowires, with a diameter of 15 nm, show distinctive magnetic anisotropy as an applied magnetic field parallel to the axis of nanowires. With increase in the molar ratio of Co and Cu, the coercivity along nanowire axis increases and reaches a maximum value of 1977.5 Oe at the Co/Cu molar ratio of 60:1, but the maximum value of coercivity increases to 1743.6 Oe with the decrease of frequency to 2 Hz.Highlights► The coercivity increased with Co:Cu ratio in electrolyte. ► The coercivity decreased with the frequency of power and reached an average. ► With the increase of voltage, the Ms and Hc increased to some extent.

Molten salts have been reported to be excellent media for preparation of crystals with special morphology. Via the molten salt synthesis (MSS) route, well defined octahedral CoFe2O4 crystals with the side length of about 0.5–1.5 μm were obtained in a series of molten salt systems, using superfine CoFe2O4 particles pre-synthesized through the hydrothermal process as the precursor. It was found that the calcining temperature, the reaction time, as well as the salt species and the content could significantly affect the crystallization habit and morphology of CoFe2O4 crystals. The magnetic test revealed that the synthesis conditions also had a remarkable influence on the magnetic parameters of the products.

Arrays of cobalt nanorods consisting of a number of nanowires have been fabricated by the electrochemical method using an anodic-aluminum oxide (AAO)/mesoporous-silica (SBA-15) composite. Microscopic studies clearly display that each nanorod (with a diameter of ∼200 nm) of the array was consisting of a number of cobalt nanowires which exhibit an average diameter of 3 nm. The observed hysteresis loops measured at room temperature indicate that the magnetic shape anisotropy of cobalt mesostructures, i.e. the parallel and perpendicular squarenesses of 0.5 and 0.1, respectively have been estimated. The maximum value of the coercivity measured perpendicular to the sample axis shows a value of 330 Oe and it was found that the coercivity decreases by increasing the temperature which is possibly caused by thermal disturbance inside the arrays.Highlights► Mesostructural Co nanorods synthesized using AAO/SBA-15 composite membrane. ► Co nanorods are composed of a number of 3 nm cobalt nanowires. ► The Co mesostructures obtained possess obvious magnetic shape anisotropy. ► The maximum value of the perpendicular coercivity is 330 Oe.

Ultrafine nanowires of Fe–Co with a diameter around 15 nm have been fabricated by electrodeposition method using anodic porous alumina as a template. The alloy nanowires were in the form of arrays and consisting of polycrystalline structures. They showed obvious shape anisotropy parallel to the axis of nanowires and the perpendicular coercivity (Hc⊥) was found to be 2576.8 Oe which is higher than any coercivity value reported in the literature. The effects of critical factors such as heat treatment and temperature of annealing on the structure and magnetic properties of the ultrafine nanowire arrays were studied and discussed.Highlights► Ultrafine 15 nm Fe–Co nanowires have been synthesized using AAO template. ► The ultrafine nanowires obtained possess obvious magnetic shape anisotropy. ► The perpendicular coercivity of the as-prepared nanowires is 2576.8 Oe. ► The maximum value of the squareness is 0.92.

Cobalt and iron thin films were successfully electrodeposited on the KIT-6/ITO substrate. The films consisted of mesoporous structure (the average pore size of which was estimated at ~5 nm) and large specific surface area (431 m2/g). The nanofilms were further characterized by X-ray diffractometer, scanning electron microscope, and vibrating sample magnetometer. The results indicated that the Fe magnetic films tend to grow preferentially along Fe (002) crystal direction while the Co film grew along both (101) and (220) crystal directions. The as-obtained magnetic metal films exhibited ferromagnetic behavior and both the values of coercivities parallel to the films were examined below 50 Oe.

Based on the nanocasting strategy, highly crystalline mesoporous CoFe2O4 were synthesized for the first time through the pyrolysis of hybrid metal nitrates within the pores of two kinds of mesoporous silica, SBA-15 and KIT-6, respectively. The Brunauer–Emmet–Teller (BET) surface areas of the samples could be as high as 155.7 m2 g−1 (templated by SBA-15) and 129.4 m2 g−1 (templated by KIT-6). Characterization results revealed that the mesoporous structures of the templates can be negatively replicated by the crystals of CoFe2O4, and the probable phase formation mechanism was also proposed initially. Magnetic test showed that both samples had rather low coercivity (106.5 and 212.1 Oe, respectively) due to the presence of a prominent proportion of superparamagnetic particles. It has also been found that the impregnation steps had a great influence on the stucture and properties of the products.

Cu2O nanowires were successfully fabricated by a simple liquid reaction in the presence of amphiphilic triblock copolymer (P123). The concentration of copolymer and aging time drastically changed the morphology of Cu2O samples. XPS results show that tiny amount of CuO as well as some absorbed O2 molecules existed on the surface of the Cu2O nanowires. Thermal analysis shows that the triblock copolymer owns a higher decomposition temperature (∼645 °C) than that of pervious research results and this may be the result of strong interaction between P123 and Cu2O. The band gap of Cu2O nanowires was estimated about 2.23 eV and shifting of the band gap with decreasing of the particle size could also be found. The photoluminance property study reveals that the intrinsic emission peak of Cu2O nanowires can be observed at 546 nm under the excitation wavelength of 400 nm (Xe lamp was used as the excitation light source).

Ni–Pb/Al2O3 nanostructures having composition of Ni 84 at.%, Pb 16 at.% were successfully fabricated by AC electrochemically assembled within the nanopores of ordered porous alumina films prepared by a two-step anodization. SEM images revealed that the pore arrays are regularly arranged throughout the alumina film and parallel each other. TEM analyses showed that the Ni–Pb nanoarrays are polycrystalline with dimension uniformity around 20 nm in diameter and lengths up to several μm. X-Ray diffraction results revealed that the Ni–Pb nanoarrays do not form metastable alloy phase. Hysteresis loops determined by VSM indicated that the Ni–Pb/Al2O3 nanostructures obtained possess obvious magnetic anisotropy, and the perpendicular coercivity is lower than that of Ni nanowires before and after annealing.

•Fe3O4/Fe flaky composites were prepared by a simple one-step ball-milling process.•The magnetization of Fe3O4/Fe composites has been improved compared with raw Fe3O4.•The optimal RL value of Fe3O4/Fe composites can reach −25.9 dB at 16.1 GHz under 1.25 mm.In this study, an extremely simple one-step ball milling has been applied to fabricate the flaky Fe3O4/Fe composites. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), and vector network analysis (VNA). It is acknowledged that the ball-milling process has huge influence on the morphology and electromagnetic absorption capability. Compared with the raw Fe3O4, the saturation magnetization of Fe3O4/Fe composites improved greatly. When the thickness is just 1.25 mm, the reflection loss values below −10 dB can be achieved from 13.8 to 18 GHz, with a maximum reflection loss of −25.9 at 16.1 GHz. Obviously, the Fe3O4/Fe composites can be used as magnetic microwave absorbers due to its simple synthesis for the mass production standards. Moreover, this study also gives new enlightenment to design other magnetism/magnetism composites that can be applied to Ku-band such as satellite communications.

•The FeCo alloys were prepared by a simple liquid-thermal process.•The atomic ratio of FeCo has significant influence on the morphology and microwave absorption.•The effective frequency width can reach 6.8 GHz covering the whole Ku-band with thickness of 1.55 mm for Fe7Co3.The different atomic ratios of FeCo alloys with various morphologies have been prepared through a simple liquid-thermal reduction method. It is found that the atomic ratio of ferrum and cobalt has great influence on the formation of FeCo alloys. Via adjusting Fe/Co atomic ratio (3:7, 5:5, 7:3), the surface of alloy becomes smooth gradually from distinct cone structure, and the saturation of alloys have been improved with the increase of Fe proportion. The impedance matching is also related tightly with atomic ratio and the best condition can be achieved by Fe7Co3 with lower dielectric constant and higher permeability. It is delighted to find that the optimal reflection loss (RL) value of Fe7Co3 can reach −53.6 dB at 14.3 GHz with a thin thickness of 1.55 mm and the RL values less than −10 dB can be gained from 11.2 to 18 GHz, which covering the whole Ku-band. The excellent performance derived from strong magnetic loss and well dielectric loss guarantees it as an ideal high-frequency microwave absorbent.

Porous carbon materials have long been regarded as promising candidate for high-performance lightweight microwave absorption materials owing to their strong attenuation abilities, tunable dielectric properties and low density. Nevertheless, previous work mainly focused on binary composites (usually carbon and magnetic fillers), which show limited loss mechanisms. The effect (except temperature) on the interfacial polarization and electromagnetic properties has rarely been investigated. Thus, a series of bimetallic zeolitic imidazolate frameworks (BMZIFs) with designed compositions and highly porous structures were selected to be converted to porous carbon-wrapped semiconductors (ZnO, Co3ZnC) and magnetic metal (Co) composites. Strong dielectric loss capabilities could be provided by graphitic carbon and enhanced interfacial polarization induced by multiple components and unique microstructures. By changing the molar ratio of Zn/Co under a fixed carbonization temperature, the interface of this multicomponent system could be adjusted, which influenced the electromagnetic properties. When evaluated as microwave absorption material, a reflection loss of −32.4 dB could be achieved with a broad effective frequency bandwidth of 5.24 GHz at only 1.9 mm. This work may provide an effective method to modify the physical and chemical properties of porous carbon materials with a desired complex structure and excellent microwave absorption performance.

Nanoporous carbon materials derived from metal organic frameworks (MOFs) have attracted considerable attention due to their low density for microwave absorption. Nevertheless, their poor impedance matching has reduced the absorber performance. The design and fabrication of complex nanocarbon materials with outstanding impedance matching is still a challenge. Here, we prepared a core–shell structured ZIF-8@ZIF-67 crystal through a new seed-mediated growth method. After the thermal treatment of ZIF-8@ZIF-67 crystals, we obtained selectively nanoporous carbon materials consisting of ZnO/NPC as the cores and highly graphitic Co/NPC as the shells. The shell thicknesses of ZIF-67 can be tuned simply by varying the feeding molar ratios of Co2+/Zn2+. The composites exhibited excellent impedance matching and strong absorption. The composite ZnO/NPC@Co/NPC-0.5 samples filling with 50 wt% of paraffin show a maximum reflection loss (RL) of −28.8 dB at a thickness of 1.9 mm. In addition, a broad absorption bandwidth for RL <−10 dB which covers from 13.8–18 GHz can be obtained. Our study not only bridges diverse carbon-based materials with infinite metal–organic frameworks but also opens a new avenue for artificially designed nano-architectures with target functionalities.

In this work, a one-pot strategy was proposed to synthesize carbon-coated Fe3O4 and carbon-coated Fe3C via the pyrolysis of colloidal Fe3O4 nanocrystals capped with oleic acid (OA) at different calcination temperatures. After exploring the microwave absorption performance of these composites, we found that carbon-coated Fe3C obtained at 700 °C possesses higher reflection loss (RL) and broader effective bandwidth (RL ≤ −10 dB) at low thickness. So the further investigation of the microwave absorption performance for carbon-capped Fe3C composites mixed with different mass percentages of paraffin was also carried out. The results demonstrated that the multi-dielectric relaxation process and better impedance matching, especially at low thickness and high frequency, contributed to the preferable microwave absorption greatly.

Owing to the fast development of wireless information technologies at the high-frequency range, the electromagnetic interference problem has been of increasing significance and attracting global attention. One key solution for this problem is to develop materials that are able to attenuate the unwanted electromagnetic waves. The desired properties of these materials include low reflection loss value, wide attenuation band, light weight, and low cost. This review gives a brief introduction to graphene-based composites and their electromagnetic absorption properties. The ultimate goal of these graphene absorbers is to achieve a broader effective absorption frequency bandwidth (fE) at a thin coating thickness (d). Representative and popular composite designs, synthesis methods, and electromagnetic energy attenuation mechanisms are summarized in detail. The two key factors, impedance matching behavior and attenuation ability, that determine the electromagnetic behavior of graphene-based materials are given particular attention in this article.

Hollow hierarchical microspheres of Bi/BiOBr (SBB) with oxygen vacancies were prepared using a one step solvothermal method. It was found that the stannous chloride dihydrate played key roles in the formation of Bi, defects and the stacking mode of hierarchical construction units. Positron annihilation lifetime spectroscopy (PALS) was used to demonstrate the oxygen vacancies in Bi/BiOBr samples. The density of states (DOS) of the valence band of BiOBr can be modulated by the introduction of oxygen vacancies according to the valence band XPS and Density Functional Theory (DFT) calculations. Analyses of photoluminescence and BET demonstrated that SBB hollow hierarchical microspheres with higher specific surface area have a lower recombination rate of photo-generated electrons and holes. The photocatalytic and adsorptive performances showed that the samples exhibited stronger adsorption capacity toward rhodamine B (RhB) and highly efficient photocatalytic activity in the degradation of RhB, which were attributed to the higher adsorption ability and synergistic effect of oxygen vacancies and construction of the heterojunction structure (Bi/BiOBr).

Based on the nanocasting strategy, highly crystalline mesoporous CoFe2O4 were synthesized for the first time through the pyrolysis of hybrid metal nitrates within the pores of two kinds of mesoporous silica, SBA-15 and KIT-6, respectively. The Brunauer–Emmet–Teller (BET) surface areas of the samples could be as high as 155.7 m2 g−1 (templated by SBA-15) and 129.4 m2 g−1 (templated by KIT-6). Characterization results revealed that the mesoporous structures of the templates can be negatively replicated by the crystals of CoFe2O4, and the probable phase formation mechanism was also proposed initially. Magnetic test showed that both samples had rather low coercivity (106.5 and 212.1 Oe, respectively) due to the presence of a prominent proportion of superparamagnetic particles. It has also been found that the impregnation steps had a great influence on the stucture and properties of the products.

Impedance matching and the attenuation constant, α, are two key parameters in determining electromagnetic absorption properties. Although materials with single magnetic or dielectric loss properties have a high α value, they nonetheless suffer from poor impedance matching. The design of magnetic and dielectric composites might possibly be an effective method of solving this problem, but unfortunately the introduction of magnetic material may give a poor value of α. In order to obtain absorptive materials with high impedance matching and a high value of α, we have designed a novel ternary composite of MnO2@Fe–graphene. A 30 nm wide rod-like strip of MnO2 was first obtained by a simple liquid process. Liquid decomposition of Fe(CO)5 was then carried out to deposit iron on the surface of the rod-like structure, and the MnO2@Fe was finally loaded on graphene by a liquid deposition technique. The resulting ternary composite exhibited attractive electromagnetic absorption properties, in which the optimal reflection loss of up to −17.5 dB obtained with a thin coating thickness of 1.5 mm was able to satisfy the requirements of lightness of weight and a high degree of absorption. The effective bandwidth frequency of MnO2@Fe–GNS is broader than that of pure MnO2 or MnO2@Fe, possibly due to its moderate impedance matching and attenuation ability. The possible attenuation mechanism will also be discussed.

Hierarchical hollow carbon@Fe@Fe3O4 nanospheres were synthesized by a simple template method and another pyrolysis process. Interestingly, the thickness of hollow carbon spheres is tunable by a simple hydrothermal approach. The as-prepared carbon@Fe@Fe3O4 shows excellent microwave absorption properties. In detail, the maximum effective frequency is up to 5.2 GHz with an optimal reflection loss value of −40 dB while the coating thickness is just 1.5 mm. Meanwhile, such absorption properties can be maintained via controlling the thickness of the hollow carbon. For instance, in another coating layer of 2 mm, the effective frequency is still more than 5 GHz as the carbon thickness declines to 12 nm. As novel electromagnetic absorbers, the composites also present the lower density feature due to the hollow carbon sphere frame. The excellent electromagnetic absorption mechanism may be attributed to the obvious interface polarization, and strong magnetic loss ability resulting from the Fe and Fe3O4 shell. Besides, owing to the dielectric feature of carbon, the hollow carbon core is beneficial for the attenuation ability.

Nanostructured carbon materials with hollow structures derived from metal organic frameworks (MOFs) have attracted considerable attention due to their low density for microwave absorption. However, their poor impedance matching worsens the absorption properties. The rational design and fabrication of complex hollow nanocarbon materials with excellent impedance matching still remains a challenge. Herein, we report a simple strategy to fabricate porous CuO/carbon composites by nitrate impregnation into a MOF template (thermal decomposition of zeolitic imidazolate frameworks, ZIF-67). When used as microwave absorbing materials, these hollow CuO/carbon composite polyhedra exhibited excellent impedance matching, light weight and strong absorption. An optimal reflection loss (RL) of −57.5 dB is achieved at 14.9 GHz with a matching thickness of 1.55 mm and RL values less than −10 dB can be gained in the range of 13–17.7 GHz. The best absorbing performance of the composites mainly originates from the high loss of the porous carbon obtained by the carbonization of ZIF-67, and the improvement of the impedance matching with the embedding of CuO. This work may provide a general way for fabricating porous metal oxides/carbon composites for lightweight microwave absorbing materials.

This study is focused on the spinel structure of metal oxides and sulfides, from which the ternary (NiCo2O4/NiCo2S4) and quaternary (Fe0.5Ni0.5Co2O4/Fe0.5Ni0.5Co2S4) samples with hollow sphere structures were prepared. Among these samples, Fe0.5Ni0.5Co2S4 was highly effective in its ability to attenuate electromagnetic waves, wherein a broader absorption bandwidth of 6.2 GHz could be achieved with a thinner coating layer of 1.3 mm. The cation distribution rule for Fe, Co and Ni ions in the spinel structure is given according to hybrid orbital theory to support the excellent electromagnetic absorption properties. Relying on the distribution of Fe, Co and Ni cations, the probable electron transmission and coupling between Fe3+/Co2+ and Fe3+/Fe3+ adjacent cation ion pairs could occur at the octahedral site (B site), which reflects the enhanced dielectric loss.

To overcome the shortcomings (poor impedance mismatching and weak electromagnetic wave attenuation) of the Co nanoparticles embedded into nanoporous carbon (Co@NPC) derived from the thermal decomposition of zeolitic imidazolate framework-67 (ZIF-67), two coated titanium oxide (TiO2) routes are designed to prepare core–shell Co@NPC@TiO2 and multi-interfaced yolk–shell C–ZIF-67@TiO2 (obtained from the thermal decomposition of ZIF-67@TiO2) structures. The permittivity and permeability of C–ZIF-67@TiO2 significantly depend on the thickness of the TiO2 shell in ZIF-67@TiO2, and the thickness of the TiO2 shell in the as-obtained samples can be easily controlled via changing the addition content of tetrabutyl titanate in the hydrolyzation process. The as-prepared samples have remarkable absorbing characteristics in wide frequency bands from 2–18 GHz with thicknesses of 1.0–5.0 mm. 50 wt% of the C–ZIF-67@TiO2-2 (the addition amount of tetrabutyl titanate is 2 mL) nanocomposite filled within paraffin shows a maximum reflection loss (RL) of −51.7 dB at an absorbing thickness of 1.65 mm, meanwhile, for the Co@NPC@TiO2-1.2 (the addition amount of tetrabutyl titanate is 1.2 mL) nanocomposite, a maximum RL can be achieved of −31.7 dB at 1.5 mm. This study provides a good reference for the future preparation of other carbon-based lightweight microwave absorbing materials derived from metal organic frameworks.

In this study, two-dimensional MoS2 nanosheets synthesized by a hydrothermal method were firstly investigated for microwave absorbing performance. The obtained MoS2 nanosheets are highly desirable as an electromagnetic wave (EM) absorber because of its larger interfacial polarization and high dielectric loss. Our results show that the real and imaginary parts of permittivity of MoS2 prepared at 180 °C are higher than those of other samples. A broad bandwidth absorption at a thin thickness can be obtained between 2 and 18 GHz. The microwave reflection loss (RL) of MoS2 nanosheets prepared at 180 °C reaches as high as −47.8 dB at 12.8 GHz due to its high electrical conductivity and the polarization effect. It can also be found that MoS2 exhibits an effective electromagnetic wave absorption bandwidth of 5.2 GHz (<−10 dB) at the thicknesses of 1.9 and 2.0 mm. The results showed that the MoS2 nanosheets can be a candidate for microwave absorption with a broad effective absorption bandwidth at thin thicknesses.