Rational design on the microstructure of electromagnetic composites offers immense potential for overcoming the challenges related to the microwave absorption performance. In this study, uniform core-shell Co@C microspheres are innovatively fabricated through an in situ transformation from Co3O4@phenolic resin precursor. Carbon shells restrain the agglomeration of Co particles during high-temperature treatment, which accounts for the survival of uniform core-shell microstructure. Electromagnetic analysis reveals that Co@C microspheres show quite different electromagnetic functions as compared with pure Co derived from Co3O4 microspheres. On one hand, carbon shells can effectively regulate the complex permittivity of Co@C microspheres by reducing conductivity loss and introducing polarization relaxations; on the other hand, carbon shells suppress the skin effect from cross-linked Co networks and make the isolated Co cores produce stronger magnetic loss. The microwave absorption properties are evaluated in the frequency range of 2.0–18.0 GHz, and as expected, core-shell Co@C microspheres exhibits excellent reflection loss characteristics, where strong reflection loss (−68.7 dB at 10.6 GHz), ultra-wide response bandwidth (2.7–18.0 GHz over −10 dB), and thin matched thickness (1.65 mm) can be achieved. A delta-function method and attenuation constant validate that the matched characteristic impedance and improved loss ability in Co@C microspheres account for the significant enhancement.

Nitrogen-doped porous carbon (NPC) is synthesized through a direct pyrolysis of zeolitic imidazolate framework (ZIF)-8 under N2 flow followed by acid washing process. It is found that NPC-800 pyrolyzed at 800 °C can inherit the perfect rhombic dodecahedron morphology of ZIF-8 crystals and achieve the considerable nitrogen-doping content of 15.20%. When NPC-800 is applied as the heterogeneous catalyst in peroxymonosulfate (PMS) activation for the degradation of Rhodamine B (RhB) and phenol, NPC-800 will exhibit its better performance than some conventional transition metal-based oxides and common carbon materials. The active sites can be primarily ascribed to nitrogen modification and sp2-hybridized carbon frameworks. Besides, the influence of several parameters such as the dosage of catalyst, the concentration of oxidant, and reaction temperature is conducted systematically. More importantly, NPC-800 can maintain its considerable degradation in the presence of some anions and natural organic matters, even under some actual water background conditions. Although NPC-800 displays mild deactivation in repeated experiments, its catalytic performance can be easily recovered through heat treatment at 350 °C in air. Radical quenching tests reveal that both sulfate and hydroxyl radicals are responsible for the removal of organic pollutants. This research may provide a new way for the application of novel metal-free carbocatalysts in terms of PMS activation.

Composites of carbon and magnetic metal are always the most attractive candidates for high-performance microwave absorbing materials (MAMs) due to their synergetic loss mechanisms and tunable electromagnetic properties. Herein, FeCo alloy nanoparticles have been innovatively coupled with ordered mesoporous carbon through impregnation and in situ hydrogenthermal reduction. Incorporation of FeCo alloy nanoparticles into mesoporous carbon provides a significant enhancement in microwave absorption, and both strong reflection loss (−73.8 dB at 10.6 GHz) and wide response bandwidth (4.0–18.0 GHz over −10.0 dB) can be simultaneously achieved in the optimized composite. This improved microwave absorption property is much better than that of many mesoporous carbon-based composites ever reported. Electromagnetic analysis reveals that FeCo alloy nanoparticles display dual electromagnetic functions. On the one hand, they produce obvious magnetic loss ability in the composites, and on the other hand, they moderately weaken the dielectric loss ability as compared to pristine mesoporous carbon. A delta function calculation indicates that the narrowed gap between complex permittivity and complex permeability is beneficial to creating the matched characteristic impedance, which affords the prerequisite for the formation of desirable microwave absorption performance. This study not only identifies FeCo alloy/mesoporous carbon as promising MAMs and reveals the origin of the microwave absorption enhancement, but also paves a new way for designing magnetic carbon-based MAMs in the future.

Metal cobalt is one of the most promising candidates for high-performance microwave absorbers due to its compatible dielectric loss and magnetic loss abilities. Rational design on the microstructure of metal cobalt became a popular way to upgrade its microwave absorption performance in the past decade, while much less attention has been paid to the electromagnetic functions derived from its different crystal structures. Herein, we report the microwave absorption of porous cobalt assemblies with varied composition of close-packed hexagonal (hcp) and face-centered cubic (fcc) phases. Electromagnetic analysis reveals that the change of phase composition can significantly impact the complex permittivity and complex permeability of metal cobalt, where hcp-cobalt favors high complex permittivity and fcc-cobalt produces high complex permeability. The optimum phase composition in these porous cobalt assemblies will promise well-matched characteristic impedance and good performance in strong reflection loss (−41.0 dB at 9.4 GHz) and wide response bandwidth (4.0–17.4 GHz over −10.0 dB). The enhanced microwave absorption is superior to many cobalt absorbers ever reported. It is believed that these results will provide a new pathway to the design and preparation of highly effective metal cobalt and cobalt-based composites as novel microwave absorbers in the future.

•SnO2-encapsulated α-Fe2O3 nanocubes have been rationally designed.•SnO2 shells play an important role in stabilizing the microstructure of Fe2O3.•SnO2 shells can promote the phase variation of Fe2O3 from γ/β-phase to α-phase.•α-Fe2O3@SnO2 nanocubes exhibit excellent photo-Fenton performance.In situ transformation of metal-organic frameworks (MOFs) is becoming a fascinating strategy to construct porous metal oxides with excellent performance in many fields. In this work, Prussian blue (PB) nanocubes are employed as the precursor of porous Fe2O3 to fabricate SnO2-encapsulated α-Fe2O3 (Fe2O3@SnO2) nanocubes by pre-coating Sn(OH)Cl on the surface of PB nanocubes. It is very interesting to find that SnO2 shells can not only preserve the microstructure of Fe2O3 nanocubes from high-temperature treatment, but also facilitate the phase variation from metastable γ/β-phase to stable α-phase. The thickness of SnO2 shells can be controlled by manipulating the amount of stannous chloride. When Fe2O3@SnO2 nanocubes are applied as heterogeneous photo-Fenton catalysts, they will exhibit much better catalytic efficiency for the degradation of Rhodamine B (RhB) than PB-derived Fe2O3 and commercial α-Fe2O3. The characterization results reveal that Fe2O3@SnO2 nanocubes have similar catalytic mechanism to conventional α-Fe2O3, and stable microstructure and preferable crystalline phase are primarily responsible for this significant enhancement. Some influential factors, including H2O2 concentration, catalyst dosage, pH value, and reaction temperature are investigated and analyzed in details. Moreover, Fe2O3@SnO2 nanocubes can maintain their catalytic efficiency during the repeated batch experiments. We believe Fe2O3@SnO2 nanocubes can be a new kind of high-performance green heterogeneous catalyst for the degradation of organic pollutants, and this study may provide a new idea to upgrade the performance of some conventional catalysts by rational design in the future.Download high-res image (130KB)Download full-size image

Sulfate radical-based advanced oxidation processes (SR-AOPs) are receiving more and more attention for the removal of recalcitrant organic pollutants. In this study, we employ CoMoO4 as a novel heterogeneous catalyst for peroxymonosulfate (PMS) activation to release powerful sulfate radicals for the first time. The CoMoO4, prepared through a hydrothermal route and high-temperature calcination, displays a hierarchical microstructure assembled from ultrathin nanosheets and a large surface area (61.9 m2 g−1). Methylene blue (MB) is selected as a model organic pollutant, and it is found that the CoMoO4/PMS system can realize 100% degradation of MB in 40 min and maintain its removal efficiency during three recycling experiments. Such a catalytic performance of CoMoO4 is indeed superior to those of conventional Co3O4 and CoFe2O4. The effects of some potential influential factors, including reaction temperature, dosages of PMS and CoMoO4 and the initial pH value are systematically evaluated. More importantly, the CoMoO4/PMS system not only shows its universality in the degradation of other organic dyes (e.g. orange II and rhodamine B), but also exhibits considerable degradation efficiency under some actual water background conditions. The quenching experiments confirm that sulfate radicals are the main active species for the degradation of dyes, and XPS spectra reveal that Co sites on the surface of CoMoO4 are the primary active sites for the generation of sulfate radicals.

Rational design on the microstructure of microwave-absorbing materials is paving the way for upgrading their performances in electromagnetic pollution prevention. In this study, a Fe3O4/C composite with unique yolk–shell microstructure (YS-Fe3O4@C) is successfully fabricated by a silica-assisted route. It is found that carbon shells in this composite can make up the shortages of Fe3O4 microspheres in dielectric loss ability, while they may more or less attenuate the intrinsically magnetic loss of Fe3O4 microspheres. The microwave absorption properties of YS-Fe3O4@C are evaluated in the frequency range of 2.0–18.0 GHz in terms of the measured complex permittivity and complex permeability. The results demonstrate that YS-Fe3O4@C can exhibit much better performance than bare Fe3O4 microspheres and individual carbon materials, as well as core–shell Fe3O4/C composite (CS-Fe3O4@C), where strong reflection loss and wide response bandwidth can be achieved simultaneously. With an absorber thickness of 2.0 mm, the maximum reflection loss is −73.1 dB at 14.6 GHz and a bandwidth over −10.0 dB is in the range of 12.3–18.0 GHz. It can be proved that the unique yolk–shell microstructure is helpful to reinforce the dielectric loss ability and create an optimized matching of characteristic impedance in the composite.

Carbon materials, as a typical dielectric loss medium, are always the most attractive candidates for microwave absorption due to their characteristic advantages; however, much less attention has been paid to upgrading their performance by rational design on the microstructure. According to the transmission behavior and loss mechanism of electromagnetic waves, uniform yolk-shell C@C microspheres are innovatively fabricated through a “coating-coating-etching” route as a novel microwave absorber. The unique microstructure endows yolk-shell C@C microspheres with improved BET surface and pore volume as compared to solid carbon microspheres. The microwave absorption properties are evaluated in the frequency range of 2–18 GHz, and as expected, yolk-shell C@C microspheres exhibit excellent reflection loss characteristics, where strong reflection loss (−39.4 dB at 16.2 GHz) and ultra-wide response bandwidth (4.5–18.0 GHz over −20 dB) can be achieved. Such good performance is indeed superior to most carbon absorbers ever reported. Electromagnetic parameters reveal that the yolk-shell structure is favorable for the matching of characteristic impedance, and more importantly, desirable dielectric loss ability can be achieved at matched characteristic impedance. It is believed that the multiple reflections between cores and shells are responsible for the improved dielectric loss.

•High-purity Co/C composites are prepared by optimizing pyrolysis of ZIF-67.•The as-prepared Co/C composites show strong microwave absorption.•Dual loss mechanism and matched impedance account for their microwave absorption.•Ordered structure and pure Co phase contribute to upgrading their performances.•The effective microwave absorption frequency can be tuned by the polyhedron size.In situ pyrolysis of metal-organic frameworks (MOFs) is becoming a popular technique to construct uniform carbon-based composites with excellent performance in many research fields. In this study, Co/C composites derived from a zeolitic imidazolate framework, ZIF-67, are selected as novel microwave absorbers. The obtained Co/C composites with uniform polyhedron microstructure are actually composed of amorphous carbon frameworks and highly dispersed core-shell Co@graphite nanoparticles. The pyrolysis conditions are carefully optimized, and the effects of pyrolysis temperature on carbon content, graphitization degree, magnetic property, and porous structure are also investigated. It is very interesting that these Co/C composites present different dielectric loss ability and similar magnetic loss ability, resulting in their distinguishable reflection loss characteristics. Among these candidates, the Co/C composite pyrolyzed at 800 °C (Co/C-800) shows the best microwave absorption due to its dual loss mechanisms and well matched characteristic impedance. The control experiments indicate that both high-purity Co phase and ordered microstructure are indeed helpful to improving their performances. Moreover, the effective microwave absorption frequency can be further manipulated by the polyhedron size of Co/C composites, which may provide an exciting clue for the design and fabrication of lightweight and highly effective microwave absorbers in the future.Download high-res image (335KB)Download full-size image

Nickel ferrites (NiFe2O4) were prepared through thermal decomposition of homogeneous nickel oxalate and ferrous oxalate, and the product displayed typical spinel structure, small nanoparticle size (ca. 12 nm), high BET surface (53.5 m2 g−1), and good magnetic response (19.3 emu g−1). The as-prepared NiFe2O4 was applied in heterogeneous catalysis to generate powerful radicals from peroxymonosulfate (PMS) for the removal of recalcitrant pollutant. Herein, benzoic acid (BA) was employed as a stable model organic pollutant, and it was found that NiFe2O4/PMS system could realize 82.5% degradation in 60 min and maintain its catalytic efficiency during four recycling experiments. Such a catalytic performance of NiFe2O4 was indeed superior to those from Fe2O3 (23.5%), Fe3O4 (48.0%), NiO (57.6%), and MnFe2O4 (63.8%). Although NiFe2O4 performed a slightly inferior BA degradation to CoFe2O4 (86.2%), its nickel leaching (0.265 mg L−1) was much less than cobalt leaching (0.384 mg L−1) from CoFe2O4. In addition, some potential influential factors, including the dosages of PMS and NiFe2O4, the initial pH value, ion strength, and the concentrations of bicarbonate, natural organic matter, halide, were systemically investigated. More importantly, NiFe2O4/PMS were also effective for BA removal under some actual water background conditions, especially in surface water and the finished water from drinking water treatment plant: the degradation efficiencies of BA were still close up to 60%. Sulfate and hydroxyl radicals were confirmed to be the main reactive species in the NiFe2O4/PMS system. XPS spectra revealed that Ni sites on the surface of NiFe2O4 were the primary active sites, and Raman spectra suggested that inner-sphere complexation between PMS and Ni sites derived peroxo species on the surface, which were further responsible for the efficient generation of radicals.

Gamma irradiation induced synthesis of metal or metal oxides has received increasing attention due to its mild reaction conditions. Here, we demonstrate the synthesis of aligned CoxNi1−x (x = 0.25, 0.33, 0.5, 0.67, 0.75) alloy nanobundles via a gamma irradiation induced simultaneous reduction of Co2+ and Ni2+ ions with the assistance of an external magnetic field. The formation of alloy structures was confirmed by element mapping and X-ray absorption studies. Natural ferromagnetic resonance and dielectric loss mainly contribute to the electromagnetic wave absorption of the CoxNi1−x alloy materials, and the composition also has great influence. Co1Ni2 (x = 0.33) has the strongest absorption of −10.5 dB at 13.6 GHz at a thickness of 2 mm, and the electromagnetic absorption properties can be tuned by the thickness of the CoxNi1−x alloys. We believe synthesis of metal alloys through the gamma irradiation induced reduction technique will be appealing to other areas.

Here we demonstrate the surface plasmon (SP) induced nitration of aromatic rings by an in situ surface enhanced Raman spectroscopy (SERS) technique. The size feature of the as-prepared Au, Ag and Ag@PDA@Au hierarchical structures allows monitoring the entire reaction process on a single hierarchical structure. With benzenethiol (BT) and HNO3 as reactants, SP induced aromatic nitration can be successfully realized without the assistance of a conventional acid catalyst, H2SO4. Experimental and theoretical studies confirm that the nitration reaction leads to para-nitrothiophenol (p-NTP). While control experiments show that SP here functions as a local heating source and the presence of metal is also necessary for this nitration reaction. This SP induced aromatic nitration reaction also displays SERS substrate-dependent reaction kinetics, which proceeds more rapidly on the Au surface. Higher laser power can generate a stronger photothermal effect, and thus an accelerated reaction rate for this reaction. We believe this finding may broaden the research areas in the SP assisted or induced catalytic reactions.

Composites of magnetic metal nanoparticles and carbon materials are highly desirable for high-performance microwave absorbers due to their compatible dielectric loss and magnetic loss abilities. In this article, novel nanocomposites, Fe/C nanocubes, have been successfully prepared through an in situ route from a metal–organic framework, Prussian blue, by controlled high-temperature pyrolysis. The resultant nanocubes are actually composed of a cubic framework of amorphous carbon and uniformly dispersed core–shell Fe@graphitic carbon nanoparticles. Within the studied pyrolysis temperature range (600–700 °C), the porous structure, iron content, magnetic properties, and graphitization degree of the Fe/C nanocubes can be well modulated. Particularly, the improved carbon graphitization degree, both in amorphous frameworks and graphitic shells, results in enhanced complex permittivity and dielectric loss properties. The homogeneous chemical composition and microstructure stimulate the formation of multiple dielectric resonances by regularizing various polarizations. The synergistic effect of dielectric loss, magnetic loss, matched impedance, and dielectric resonances accounts for the improved microwave absorption properties of the Fe/C nanocubes. The absorption bands of the optimum one obtained at 650 °C are superior to most composites ever reported. By considering the good chemical homogeneity and microwave absorption, we believe that the as-fabricated Fe/C nanocubes will be promising candidates as highly effective microwave absorbers.

Highly uniform core–shell composites, polypyrrole@polyaniline (PPy@PANI), have been successfully constructed by directing the polymerization of aniline on the surface of PPy microspheres. The thickness of PANI shells, from 30 to 120 nm, can be well controlled by modulating the weight ratio of aniline and PPy microspheres. PPy microspheres with abundant carbonyl groups have very strong affinity to the conjugated chains of PANI, which is responsible for the spontaneous formation of uniform core–shell microstructures. However, the strong affinity between PPy microspheres and PANI shells does not promote the diffusion or reassembly of two kinds of conjugated chains. Coating PPy microspheres with PANI shells increases the complex permittivity and creates the mechanism of interfacial polarization, where the latter plays an important role in increasing the dielectric loss of PPy@PANI composites. With a proper thickness of PANI shells, the moderate dielectric loss will produce well matched characteristic impedance, so that the microwave absorption properties of these composites can be greatly enhanced. Although PPy@PANI composites herein consume the incident electromagnetic wave by absolute dielectric loss, their performances are still superior or comparable to most PANI-based composites ever reported, indicating that they can be taken as a new kind of promising lightweight microwave absorbers. More importantly, microwave absorption of PPy@PANI composites can be simply modulated not only by the thickness of the absorbers, but also the shell thickness to satisfy the applications in different frequency bands.Keywords: core−shell; interfacial polarization; microwave absorption; polypyrrole@polyaniline; shell thickness

Core–shell composites, Fe3O4@C, with 500 nm Fe3O4 microspheres as cores have been successfully prepared through in situ polymerization of phenolic resin on the Fe3O4 surface and subsequent high-temperature carbonization. The thickness of carbon shell, from 20 to 70 nm, can be well controlled by modulating the weight ratio of resorcinol and Fe3O4 microspheres. Carbothermic reduction has not been triggered at present conditions, thus the crystalline phase and magnetic property of Fe3O4 micropsheres can be well preserved during the carbonization process. Although carbon shells display amorphous nature, Raman spectra reveal that the presence of Fe3O4 micropsheres can promote their graphitization degree to a certain extent. Coating Fe3O4 microspheres with carbon shells will not only increase the complex permittivity but also improve characteristic impedance, leading to multiple relaxation processes in these composites, thus the microwave absorption properties of these composites are greatly enhanced. Very interestingly, a critical thickness of carbon shells leads to an unusual dielectric behavior of the core–shell structure, which endows these composites with strong reflection loss, especially in the high frequency range. By considering good chemical homogeneity and microwave absorption, we believe the as-fabricated Fe3O4@C composites can be promising candidates as highly effective microwave absorbers.Keywords: carbon; composites; core−shell; Fe3O4; microwave absorption

A facile synthesis of homogeneous Ag nanostructures on modified polypyrrole (PPy) films through direct chemical deposition has been demonstrated. The as-prepared Ag nanostructures are highly sensitive and reproducible in the SERS detection of the target analyte, methylene blue (MB).

Composites of magnetic metal nanoparticles and carbon materials are highly desirable for high-performance microwave absorbers due to their compatible dielectric loss and magnetic loss abilities. In this article, novel nanocomposites, Fe/C nanocubes, have been successfully prepared through an in situ route from a metal–organic framework, Prussian blue, by controlled high-temperature pyrolysis. The resultant nanocubes are actually composed of a cubic framework of amorphous carbon and uniformly dispersed core–shell Fe@graphitic carbon nanoparticles. Within the studied pyrolysis temperature range (600–700 °C), the porous structure, iron content, magnetic properties, and graphitization degree of the Fe/C nanocubes can be well modulated. Particularly, the improved carbon graphitization degree, both in amorphous frameworks and graphitic shells, results in enhanced complex permittivity and dielectric loss properties. The homogeneous chemical composition and microstructure stimulate the formation of multiple dielectric resonances by regularizing various polarizations. The synergistic effect of dielectric loss, magnetic loss, matched impedance, and dielectric resonances accounts for the improved microwave absorption properties of the Fe/C nanocubes. The absorption bands of the optimum one obtained at 650 °C are superior to most composites ever reported. By considering the good chemical homogeneity and microwave absorption, we believe that the as-fabricated Fe/C nanocubes will be promising candidates as highly effective microwave absorbers.

Here we demonstrate the surface plasmon (SP) induced nitration of aromatic rings by an in situ surface enhanced Raman spectroscopy (SERS) technique. The size feature of the as-prepared Au, Ag and Ag@PDA@Au hierarchical structures allows monitoring the entire reaction process on a single hierarchical structure. With benzenethiol (BT) and HNO3 as reactants, SP induced aromatic nitration can be successfully realized without the assistance of a conventional acid catalyst, H2SO4. Experimental and theoretical studies confirm that the nitration reaction leads to para-nitrothiophenol (p-NTP). While control experiments show that SP here functions as a local heating source and the presence of metal is also necessary for this nitration reaction. This SP induced aromatic nitration reaction also displays SERS substrate-dependent reaction kinetics, which proceeds more rapidly on the Au surface. Higher laser power can generate a stronger photothermal effect, and thus an accelerated reaction rate for this reaction. We believe this finding may broaden the research areas in the SP assisted or induced catalytic reactions.