•A new approach to break Snoek's limit and enhance electromagnetic parameters.•Modulation over the composition and size in EG/Fe3O4 nanoring composites.•Investigating composition and size -dependent electromagnetic characteristics.•Revealing the enhancement mechanism of electromagnetic properties.Fe3O4 nanoring (NR) composites anchored on the nanolayers of expanded graphite (EG) were prepared through solvothermal–surface modification–sintering approach to break Snoek's limit. The samples were characterized by XRD, EDX, XPS, FESEM, TEM, STEM, and Raman spectroscopy analyses. Fe3O4 NR content, NR size, and EG size were well adjusted by changing precursor mass and size and ball milling EG for various times. The composites exhibited controllable saturation magnetization and remarkably enhanced permittivity and permeability. A proper Fe3O4 content (6.38 wt.%–22.12 wt.%), large EG size, and large NR size favor the enhancement of permeability and permittivity due to the synergistic effect of easy-plane anisotropy, high magnetization (Ms), microcurrent induced plasmonic resonance, and microcurrent induced electromagnetic coupling. Specifically, the composites exhibit light weight, strong absorption, and broad-absorbing band compared with those of pure EG and Fe3O4 NRs. The optimal microwave absorption property was found in composites containing 29.82 wt.% Fe3O4 with a minimum RL value of −45.8 dB at 8.55 GHz, frequency range (RL ≤ −20 dB, 99% absorption) of 12.97 GHz, and mass fraction of 10 wt.%. This work provides a significant guide for designing and synthesizing microwave absorbers with high Snoek's limit, broad bandwidth, and light weight.Download high-res image (304KB)Download full-size image

•An easy one-step low-temperature chemical etching route for ZnO NR/rGO composites.•Modulation over the ZnO morphology and content in ZnO NR/rGO composites.•Investigating shape and content-dependent optical and photocatalytic properties.•Revealing the enhancement mechanism of optical and photocatalytic properties.ZnO with various morphologies and contents was used to decorate reduced graphene oxide (rGO) sheets via an easy one-step low-temperature chemical etching route to improve photocatalytic properties. The ZnO shape and content in ZnO/rGO composites were adjusted by changing aging time, heating mode, and rGO mass added. Shape and content-dependent optical and photocatalytic properties are observed in ZnO/rGO composites. A moderate amount of ZnO nanorings (NRs) decorated with rGO can significantly improve the light absorption and photo-luminescence emission because of plasmonic resonant absorption and plasmonic nanoantenna radiation, respectively. ZnO NR/rGO composites with a moderate ZnO content of 29.54 wt.% exhibit the optimum photocatalytic activity with a 0.025 min−1 apparent rate constant, which is significantly higher than those of pure rGO (0.0085 min−1) and ZnO NRs (0.018 min−1). The improved performance is ascribed to the synergistic effect of enhanced adsorption capacity, plasmonic light absorption, plasmonic nanoantenna radiation, and the prolonged lifetime of photogenerated electron-hole pairs. Our findings not only offer insights into the plasmon enhanced optical and photocatalytic properties of ZnO NR/rGO composites but also suggest the possibility of fabricating ZnO NR/rGO photocatalyst with enhanced performance.Download high-res image (252KB)Download full-size imageZnO/reduced graphene oxide sheets composites made from a facile low-temperature chemical etching route exhibit interesting morphology and composition dependent photocatalytic properties.

To break Snoek's limit and obtain high permeability, expanded graphite/Fe3O4 nanoring composites have been synthesized via a solvothermal-surface modification-sintering approach. A series of characterizations have confirmed the formation of the composites. Studies of the influence of compound mode, Fe3O4 shape, and filling mass fraction on the EM parameters reveal that the recombination of Fe3O4 NRs and EG can distinctly enhance permeability and permittivity. The ε′ and ε′′ values of the composites are 1.5–70.0 and 4.0–858.0 times as many as EG′ and 22.2–26.0 and 214.0–611.0 times as many as Fe3O4 NRs', respectively. Their μ′ and μ′′ values are around 2.8–3.0 and 2.2–100 times the Fe3O4 NRs', respectively. This significant enhancement is caused by the synergistic effect of the planar anisotropy, plasmon resonance, electromagnetic coupling, and interfacial polarization. The EG/Fe3O4 NR composites with a mass fraction of 10 wt% achieved the maximum RL value of −24.8 dB at 6.8 GHz and the corresponding frequency range (RL ≤ −20 dB, 99% absorption) is 8.0 GHz. Our findings confirm that the above composites are not only excellent microwave absorbers of broad bandwidth but are also light weight and can break Snoek's limit.

High-quality MnxFe3−xO4 (0 ≤ x ≤ 1.09) hollow/porous spherical chains (H/PSCs) were prepared via a facile one-pot solvothermal approach. These chains were formed through a magnetic field induced-Oswald ripening mechanism. The external magnetic field induced nanocrystals to aggregate into microspheres along the [111] direction, and these microspheres further assembled into 1D H/PSCs. Characterization confirmed that an increase in the Mn2+/Fe3+ molar ratio decreased the crystal size, diameter, and aspect ratio as well as increased internal strain, lattice constant, Mn doping amount, and specific surface area. Consequently, saturation magnetization and coercivity decreased because of the crystal size and Mn2+ substitution. Mn2+ substitution induced a dual-frequency absorption at 2–18 GHz, in which the absorption band at λ/4 decreased and that at 3λ/4 increased with increasing x. Compared with Fe3O4 solid spheres and hollow spheres, Mn0.746Fe2.254O4 H/PSCs exhibited a broader absorption band (RL ≤ −20 dB) of 9.86 GHz (2.05–7.91 and 14–18 GHz, respectively). The enhanced absorption performance may be related to hollow and porous structures, oriented arrangement of nanocrystals, and Mn2+ substitution.

Heterostructured nanorings (NRs) with Fe3O4 and/or Fe cores and carbon shells (Fe3O4@C and Fe3O4/Fe@C) were synthesized by a facile and controllable two-step process. The NRs were formed through a synchronous reduction/carbonization/diffusion growth mechanism. Their composition, crystal size, and phase structure could be controlled by selecting the sintering temperature of iron glycolate nanosheets in the presence of acetone. Fe3O4/Fe@C NRs formed at 600 °C to 650 °C exhibit higher specific saturation magnetization (Ms) than Fe3O4@C NRs obtained at 300 °C to 500 °C because of increased Fe content and crystal size; the former also shows higher coercivity (Hc) because of large crystal size and surface pinning function. In addition, Fe3O4/Fe@C NRs show lower density and broader absorption bandwidth than Fe3O4@C NRs and Fe3O4 NRs. Fe3O4/Fe@C NRs formed at 600 °C with a mass fraction of 40 wt% exhibit an absorption bandwidth (RL ≤ −20 dB) of 6.7 GHz and a minimum RL value of −28.18 dB at 4.94 GHz. The enhanced absorption properties are ascribed to the heterostructured and ring-shape configuration, which generates multiple dielectric relaxations, enhanced electromagnetic parameters, and plasmon resonance absorption.

Using elliptical iron glycolate nanosheets as precursors, elliptical Fe3O4/C core–shell nanorings (NRs) [25 ± 10 nm in wall thickness, 150 ± 40 nm in length, and 1.6 ± 0.3 in long/short axis ratio] are synthesized via a one-pot hydrothermal route. The surface-poly(vinylpyrrolidone) (PVP)-protected-glucose reduction/carbonization/Ostwald ripening mechanism is responsible for Fe3O4/C NR formation. Increasing the glucose/precursor molar ratio can enhance carbon contents, causing a linear decrease in saturation magnetization (Ms) and coercivity (Hc). The Fe3O4/C NRs reveal enhanced low-frequency microwave absorption because of improvements to their permittivity and impedance matching. A maximum RL value of −55.68 dB at 3.44 GHz is achieved by Fe3O4/C NRs with 11.95 wt % C content at a volume fraction of 17 vol %. Reflection loss (RL) values (≤−20 dB) are observed at 2.11–10.99 and 16.5–17.26 GHz. Our research provides insights into the microwave absorption mechanism of elliptical Fe3O4/C core–shell NRs. Findings indicate that ring-like and core–shell nanostructures are promising structures for devising new and effective microwave absorbers.Keywords: Fe3O4/C; hydrothermal method; magnetic property; microwave absorption; nanoring

•An easy H2O-governed solvothermal synthesis of uniform Fe3O4 microspheres.•Revealing variation regularities of sphere diameter, crystal size and surface chemistry.•Investigating the size-dependence of static magnetic and electromagnetic properties.We demonstrate an easy solvothermal method for the synthesis of monodisperse polycrystalline Fe3O4 microspheres ranging in size from 82 nm to 1116 nm. Results show that controlling H2O volume fraction (γ) modulates the crystal size (D) and sphere diameter (d) of Fe3O4 microspheres and also changes their surface state. D and d values of Fe3O4 microspheres change with γ in a Gauss model and a Boltzmann mode, respectively. Adding nonreductive H2O favors the increased O content. In these cases, the saturation magnetization (Ms) and coercivity (Hc) values follow various Gauss models of variation with γ because of the weakened interaction with decreased d, the enhanced surface spin disorder with D, the increased O content, and the critical size for multidomain–monodomain behavior. The cooperation of size (D and d  ) with surface state induces ε″ε″and tanδE to gradually decrease with γ  , and μ″μ″and tanδM are significantly enhanced at γ = 9.2% (d = 728 nm). This work clearly shows the potential to tune magnetic and microwave properties of polycrystalline magnetic nanomaterials by controlling crystal/particle size and surface state.

An easy mixed solvent solvothermal/hydrothermal method was developed for the one-step synthesis of monodisperse, single-crystal Fe3O4 and α-Fe2O3 nanomaterials. The morphologies can be varied from spherical, to octahedral, to rice like, and even to fusiform; the size can be continuously tuned to a range within 30–290 nm. The morphology-, dimension-, and phase-controlled growth of FexOy nanocrystals can be achieved by tuning kinetic factors, such as the H2O volume fraction (γ), Fe3+ concentration, reaction temperature, and the ratio of alkali/Fe3+. The threshold value γ (about 25%) for the H2O-steered size and phase evolutions was theoretically and experimentally inferred. The size- and phase-dependent saturation magnetization (Ms) and coercivity (Hc) were systematically investigated. High Ms was observed in single-crystal Fe3O4 nanomaterials because of high crystallinity; significantly enhanced Hc was exhibited by the as-obtained Fe3O4–α-Fe2O3 hybrid nanocrystals because of the unique unidirectional anisotropy and exchange bias. This study provided insights into the size and phase evolution mechanisms of nanocrystals in the EG–H2O system and served as efficient guidance for the tunable synthesis of monodisperse nanomaterials in a mixed solvent system. The uniform single-crystalline Fe3O4 and α-Fe2O3 nanomaterials can provide better platforms for studying their size- and phase-dependent optical, electric, and magnetic performances.

This paper describes an original and facile polyol-mediated solvothermal synthesis of elliptical iron glycolate nanosheets (IGNSs) combined with precursor thermal conversion into γ-Fe2O3 and α-Fe2O3/γ-Fe2O3 porous nanosheets (PNSs), α-Fe2O3 nanochains (NCs), and elliptical Fe3O4 nanorings (NRs). The IGNSs were produced via the oxidation–reduction and co-precipitation reactions in the presence of iron(III) salts, ethylene glycol, polyethylene glycol, and ethylenediamine. Control over Fe3+ concentration, temperature, and time can considerably modulate the size and phase of the products. The IGNSs can be transformed to γ-Fe2O3 and α-Fe2O3/γ-Fe2O3 PNSs, α-Fe2O3 NCs, and elliptical Fe3O4 NRs by heat treatment under various annealing temperatures and ambiences. The PNSs and NCs exhibited high soft magnetic properties and coercivity, respectively. Visible-light photocatalytic activity toward RhB in the presence of H2O2 by PNSs and NCs was phase-, SBET, size-, porosity-, and local structure-dependent, following the order: α-Fe2O3 NCs > α-Fe2O3/γ-Fe2O3 PNSs > γ-Fe2O3 PNSs > IGNSs. In particular, α-Fe2O3/γ-Fe2O3 PNSs possessed significantly enhanced photocatalytic activity with good recyclability and could be conveniently separated by an applied magnetic field because of high magnetization. We believe that the as-prepared α-Fe2O3/γ-Fe2O3 PNSs have potential practical use in waste water treatment and microwave absorption.

•The selective, mass preparation of porous ZnO/Ni/ZnxNiyFe3−x−yO4 hybrid micro-hexahedra.•Modulation over composition, grain size, and SBET modulation.•Revealing relationships between structure and properties.•Insight into the absorption mechanism of hybrid materials.A green versatile glucose-engineered precipitation/sintering process was developed for the selective, large-scale growth of porous ZnO/Ni/ZnxNiyFe3−x−yO4 micro-polyhedra. Modulation over composition, grain size, and specific surface area can be expediently achieved by changing the Fe3+/Ni2+/Zn2+ molar ratio (γ) and sintering temperature (Ts). Relationships between structure and properties were investigated. High Ts and the appropriate addition of Zn2+ improved the microwave electromagnetic properties of the materials. The ZnO/Ni/ZnxNiyFe3−x−yO4 hybrid micro-hexahedra obtained at 700 °C and γ = 2:0.5:0.5 showed a minimum reflection loss (RL) of −36.52 dB at 10.0 GHz with absorbing frequency (RL ≤ −20 dB) from 4.59 GHz to 18.0 GHz, corresponding to 1.7 mm–5.5 mm coating thickness. The enhanced absorption performances were attributed to the additional multiphase interface, multiresonance, and good matching and absorbing properties of the hybrid materials. The obtained magnetic hybrid materials exhibited promising applications in magnetic devices, catalysis, solar energy conversion, and electromagnetic wave-absorbing materials.

•A gas-flow-induced in situ two-step CVD method for centipede-shaped Fe/Fe3C/MWCNT heterostructures.•Modulation over composition, structure, and morphology.•Revealing relationships between structure and properties.•Insight into the absorption mechanism of hybrid materials.Centipede-shaped iron/cementite/multiwalled carbon nanotube (Fe/Fe3C/MWCNT) heterostructures were synthesized via a novel and easy gas-flow-induced in situ two-step chemical vapour deposition (CVD) method for the first time. The formation mechanism was studied via X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The cooperation between the carrier gas flow and the spontaneous magnetization caused the assembly of Fe nanocrystals from the pyrolysis of Fe(CO)5 into Fe nanofibers, which subsequently functioned as substrates and catalyst precursors for Fe3C-catalyzed MWCNT growth. The aspect ratio and the MWCNT content of the products were easily controlled by changing the feeding ratio of Fe(CO)5 volume to polyethylene glycol 20 000 (PEG 20 000) mass. The electromagnetic property study revealed that increasing the aspect ratio and length can improve the material's dielectric properties. Simulation studies show that 40 wt.% of Fe/Fe3C/MWCNT heterostructures in a wax matrix exhibits high values of reflection loss (<− 20 dB) over a wide frequency range 3.3 to 7.5 GHz with the minimum reflection loss value of − 32.3 dB at 4.2 GHz.Easy gas-flow-induced in situ two-step CVD synthesis has been developed for tunable preparation of centipede-shaped Fe/Fe3C/MWCNT heterostructures.

Rambutan-like heterostructures consisting of Ni microspheres coated with oriented multiwall carbon nanotubes (MWCNTs) were synthesized by the one-pot thermal decomposition of a mixture of organic matter and Ni precursors. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy were used to reveal the formation mechanism. The growth of MWCNTs capped by Ni nanoparticles on the surface of the Ni nanoparticle-built microspheres followed a tip-growth mode. The composition and morphology of the rambutan-like heterostructures were easily controlled by changing the reaction time, mass ratio δ of polyethylene glycol (PEG) 20000 to NiO, as well as type of C source and Ni precursor. Increasing the δ favored not only the increased C mass fraction but also the morphological conversion from Ni/C film core–shell structures to rambutan-like Ni/MWCNT heterostructures. Such changes caused the decreased saturation magnetization and enhanced permittivity properties with δ. Owing to intensive eddy current loss and multiresonance behaviors, rambutan-like Ni/MWCNT heterostructures with long MWCNTs exhibited significantly improved complex permeability and magnetic loss. Ni/MWCNT heterostructures coated by short MWCNTs showed an optimal microwave absorption property with a minimum RL value of −37.9 dB occurring at 12.8 GHz. This work provides effective guidelines for devising and synthesizing highly efficient microwave-absorbing materials.

•A novel gas bubble-assisted self-assembly strategy for Co3O4 nanobowl arrays.•Revealing the morphological evolution mechanism.•Analysis of electrochemical properties of polymorphic Co3O4 nanostructures.•Enhanced electrochemical activity was exhibited by nanobowl and nanotube arrays.A novel and versatile gas bubble-assisted self-assembly technique was developed for the first-time preparation of Co3O4 nanobowl arrays by the rapid thermal decomposition of Co(NO3)2⋅6H2O on a flat substrate. The morphological modulation from novel nanobowl arrays, to nanotube arrays, to nanorods, and even to microspheres can be realized by only tuning decomposition temperature from 150 °C to 700 °C. The in situ generated (O2, H2O, NO2) bubbles guided the growth of Co3O4 nuclei, resulting in the final morphology of Co3O4 nanostructures. The Co3O4 nanostructures were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption–desorption isotherms. Analysis of electrochemical properties revealed that Co3O4 nanobowl and nanotube arrays obtained at low temperatures displayed significant enhancement of electrochemical activity because of low crystallization, small grain size, high specific surface area, and hierarchically porous structure. This simple process was applicable to large-scale production and may be extended to other materials. The porous/hollow structure and high specific surface area of the as-obtained Co3O4 nanobowl and nanotube arrays can enable their potential use in catalysis, chemical sensing, luminescence, energy storage, controlled release, and cellular applications.Graphical abstract

Complex ZnO architectures with tunable morphologies and structures were obtained by modulating only the base type and molar ratio of base to Zn2+ (α) using an easy one-pot hydrothermal approach without any template or organic additive. Characterizations by X-ray diffraction, Fourier-transform infrared spectrometry, scanning electron microscopy, transmission electron microscopy, and surface area analysis were performed. The effect of the base type and base/Zn2+ molar ratio on the morphology and corresponding mechanism were determined. The correlations between the microstructure and properties were established. The antibacterial effect of the ZnO samples was probably due to a combination of variable factors. Better antibacterial activity is derived from more effective antibacterial surfaces, which are mainly associated with the specific surface area and Zn-polar plane. Thus, flower-like architectures with larger specific surface areas and more highly exposed (0001) Zn-polar surfaces outwards are promising structures for ZnO antibacterial agents. This work provides a guide for devising and synthesizing highly efficient antibacterial materials.

In this study, sponge-like ZnO/ZnFe2O4 hybrid micro-hexahedra with diverse textures and compositions were fabricated by the thermal decomposition of hexahedral zinc/iron oxalate precursors, starting from a glucose-engineered co-precipitation process. The resulting ZnO/ZnFe2O4 micro-hexahedra were systematically characterized by X-ray powder diffraction, Fourier-transform infrared spectroscopy, scanning electronic microscopy, transmission electron microscopy (TEM), high-resolution TEM, and surface area analysis. Moreover, modulation in crystal size, composition, and textural properties of spongy ZnO/ZnFe2O4 micro-hexahedra was easily achieved by varying the Zn2+/Fe3+ feeding ratio and the annealing temperature. The antibacterial property of the products was analyzed by testing ATP (adenosine triphosphate) and inhibition zones. Results showed that oxidative stress was the governing mechanism for the antibacterial activity of ZnO/ZnFe2O4 hybrid materials. Moreover, we found that the higher reactive oxygen species yields and the resulting antibacterial activity were exhibited by the ZnO/ZnFe2O4 micro-hexahedra formed at lower sintering temperatures rather than the pure ZnO and Fe2O3. The enhanced antibacterial properties were likely caused by the spongy ZnO/ZnFe2O4 heterostructures, improving the probability of photoinduced charge separation and broadening the visible-light absorption.

The current study describes a facile one-step ethanediamine (en)-assisted hydrothermal approach for the selective synthesis of ZnO architectures with morphologies that evolve from nanocones, to twinned nanoroses, to dispersed microneedles, and even to complex, flower-shaped architectures. Kinetic factors, such as time, temperature, en-to-Zn(NO3)2 molar ratio (δ), [Zn2+], and Zn sources can be easily utilized to control the oriented attachment growth of [Zn(OH)4]2− on the (0001) polar surface, thereby regulating the morphology and growth direction of the ZnO architectures. Time lengthening as well as increases in temperature, δ, and [Zn2+] can promote the morphological evolution from needle-like to flower-shaped and can change the structurally oriented growth from along the c-axis to along the a-axis. The flower-shaped ZnO–wax composites exhibit enhanced permittivity and microwave-absorbing properties as mass fraction increases. However, this distinct morphology is prone to high dielectric loss. Thus, the flower-shaped ZnO showed stronger microwave absorption performances than the needle-like ZnO, with a minimum reflection loss (RL) of −21.85 dB at 8.4 GHz, corresponding to a matching thickness of 3.0 mm. In particular, interesting nesting microwave absorption peaks can be observed in the reflection loss plots of the flower-shaped ZnO. The current work provides insights into the absorption mechanism of flower-shaped ZnO absorption materials.

Flower-like Co superstructures composed of leaf-like flakes were synthesized via a facile hydrothermal approach independent of surfactants or complex precursors. The evolution of the morphology and crystal phase was closely related to the variation of the electrode potentials, in which NaOH and hydrazine hydrate played crucial roles. The microwave electromagnetic and absorbing properties of the flower-like Co/wax composites varied strongly with the mass ratios (λ) of Co powder to wax. At the low λ of Co powder to wax, flower-like Co superstructures functioned as the random distributed patches in wax matrix and, therefore composites exhibited frequency selective surface (FSS) behaviors. Owing to high conductance and eddy current losses, however, composites with high λ showed excellent microwave absorption performances, with a minimum reflection loss (RL) of −40.25 dB observed at 6.08 GHz, corresponding to a matching thickness of 2.5 mm. In particular, the absorption bandwidth (RL ≤ −20 dB) was 13.28 GHz. The current work provides insights into the absorption mechanism of flower-like complex absorption materials.

For the first time, detailed investigation into the formation of porous iron particles (PIPs) via a facile corrosion technique is reported. The complex permittivity, permeability, and electromagnetic (EM) wave absorption properties of the products were characterized at 2–18 GHz. Compared with compact iron particles, PIPs had higher complex permittivity and permeability, which were strongly dependent on their microstructure. Wax composites containing 20 vol% PIPs showed a strong absorption peak of −42.2 dB at 13.2 GHz, a low area density of 0.387 g cm−2, and a bandwidth (≤−20 dB) of about 10.0 GHz. The enhanced multipolarization, multiple dispersion, and interference of EM waves in pores led to excellent EM wave absorption properties. The PIPs may be candidates for EM wave absorption materials with thinness, lightness, width, and strength.Highlights► An original and facile corrosion technique for preparation of porous iron particles. ► Microstructure control of porous iron particles. ► Microstructure dependence of static magnetic and microwave electromagnetic characteristics of porous iron particles.

Urchin-like α-Fe2O3 and Fe3O4 nanostructures were prepared from the precursor urchin-like α-FeOOH under reducing atmosphere. The dependence of reduction temperature on their morphology, microstructure, and microwave electromagnetic and absorbing characteristics were systematically studied. It is found that the reduction temperature plays an important role in the microstructure and electromagnetic characteristics of the resulting products. In present study, the urchin-like α-Fe2O3 with dual absorption peaks can be formed at the relatively low temperature (e.g. 300 °C). Urchin-like Fe3O4 can be obtained just at 350–400 °C, which presents excellent microwave absorption property, with the minimum reflection loss of −29.96 dB and below −20 dB in 3.76–8.15 GHz corresponding to 3–4 mm thickness. The excellent microwave-absorption properties are a consequence of a proper electromagnetic matching and enhanced absorbing abilities resulting from the urchin-like shape and inverse spinel-type crystal structure.Graphical abstractUrchin-like α-Fe2O3 and Fe3O4 with excellent microwave absorbing characteristics were selectively synthesized by a facile glucose-guided hydrolyzing-reducing approach at different reducing temperatures.Research highlights▶ The morphology and microstructure evolution of urchin-like FeOOH with reducing temperature. ▶ The analysis of the microwave electromagnetic characteristics of urchin-like α-Fe2O3 and Fe3O4. ▶ The investigation of the absorption mechanism of urchin-like α-Fe2O3 and Fe3O4.

The electromagnetic (EM) characteristics of the carbon nanotubes/carbonyl iron powders (CNTs/CIPs) complex absorbers synthesized by mixing CNTs with CIPs were studied at 2–18 GHz, for the aim of the absorbing coating with thinness, lightness, width, and strength. Compared with CIPs, the CNTs/CIPs composites had higher electrical conductivity, permittivity, and dielectric loss, which gradually increased with the increasing CNTs content (WCNTs). Among them, with WCNTs = 2.2 %, a reflection loss (RL) exceeding −20 dB was obtained in the frequency range of 6.4–14.8 GHz for a coating thickness of 1.2–2.5 mm. Particularly, a minimum RL of −33.3 dB was found at 11.2 GHz corresponding to a matching thickness of 1.5 mm. The excellent EM-wave absorption properties are a consequence of a proper EM matching and enhanced absorption abilities resulting from the addition of a small quantity of CNTs with high electrical conductivity, permittivity, and dielectric loss. Thus, CNTs/CIPs complex absorbers may be promising candidates for EM-wave-absorption materials with strong-absorption, thin-thickness, light-weight, and low-cost.Research highlightsThis study suggests that with the permeability unchanged, the absorbing coating with thinness, lightness, width, and strength can be achieved by raising the dielectric constant and dielectric loss within a certain range. Therefore, the carbon nanotubes/carbonyl iron powders (CNTs/CIPs) complex absorbers were synthesized by mixing a small quantity of the CNTs of lightweight, high permittivity, and high conductivity with the CIPs of high magnetic loss and large density. The investigation of the electromagnetic characteristics showed that conductivity, permittivity, and dielectric loss corresponding to the CNTs/CIPs complex absorbers regularly augmented with the increase of the CNTs mass fraction (WCNTs). Thereinto, the wax-composites containing 35 vol% CNTs/CIPs with WCNTs = 2.2 % exhibited excellent electromagnetic-wave absorption properties, with a minimum RL value of −33.3 dB at 11.4 GHz on a coating with a matching thickness of 1.5 mm and <−20 dB (99 % attenuation of the incidence wave) with thickness of 1.2–2.5 mm in the 6.4–14.8 GHz. The enhanced electromagnetic-wave absorption properties are a consequence of a proper EM matching and high absorption abilities originating from the addition of a small quantity of CNTs with high permittivity and dielectric loss. Compared to the reported literatures, obviously, the method reported here is more concise and efficient and the absorbing bandwidth (RL ≤ −20 dB) gained in the 6.4–14.8 GHz are broader. Thus CNTs/CIPs complex absorbers may be promising candidates for electromagnetic-wave absorption materials with strong-absorption, thin-thickness, light-weight, and low-cost.

Polymorphous Fe/FexOy core–shell and urchin-like composites were synthesized via a facile oxidation process at relatively low temperatures (100–300 °C) in the absence of surfactants or an external magnetic field. The oxidation temperature plays a key role in determining the morphology, crystal size, and composition of the resulting products. The static magnetic and electromagnetic (EM) properties of Fe/FexOy composites are influenced by their morphology, crystal size, and composition. In this study, excellent soft magnetic properties and enhanced permeability were obtained from core–shell Fe/FexOy composites with low FexOy shell contents and low surface anisotropy. In contrast, high coercivity and dielectric performance were exhibited by urchin-like Fe/FexOy composites with high shape and surface anisotropy. This work provides insights into the absorption mechanism of urchin-like complex absorption materials.Highlights► The FexOy nanoshell morphology and composition evolution of Fe/FexOy core–shell and urchin-like composites with oxidation temperature. ► The establishment of the relationship between microstructure of FexOy nanoshells and properties of Fe/FexOy core–shell and urchin-like composites (e.g. static magnetic, and microwave electromagnetic and absorbing properties). ► The analysis of the absorption mechanism of Fe/FexOy composites.

This paper describes an original and facile method for preparing goethite (α-FeOOH) hierarchical nanostructures (HNSs), as well as chemically converting into hematite (α-Fe2O3) under the preservation of α-FeOOH complex morphology. This method was based on a forced hydrolysis–oxidation of iron (II) salts in the presence of d-(+)-glucose as structure-directing agents, where control over the concentration and feeding way of d-(+)-glucose can considerably modulate the morphologies of the α-FeOOH HNSs from 1D unique architecture composed of arrayed nanoplates to 3D sea urchin-like superstructures. Moreover, using the sea urchin-like α-FeOOH nanostructures as precursors, the α-Fe2O3 HNSs with the same morphology but tailored crystallization and texture characteristics are robustly achieved only by adjusting annealing temperature. The formation mechanism of the α-FeOOH HNSs is proposed to be the synergistic effects of polar interaction and magnetic interaction between the building blocks of the nanoparticles. Due to the high BET specific surface area and favorable crystallization, the α-Fe2O3 HNSs obtained at 300 °C show excellent photocatalytic efficiency. Taking advantages of environmental benign biocompatibility, chemical stability and potential for mass generation, the α-Fe2O3 HNSs described in this work are believed to have a wide range of applications including catalysis, gas sensing.Graphical abstractGoethite hierarchical nanostructures synthesized in the presence of glucose are easily converted into hematite with the same morphology but excellent photocatalytic properties only by adjusting annealing temperature.Research highlights► An original and facile method based on a forced hydrolysis–oxidation of iron (II) salts in the presence of d-(+)-glucose. ► The microstructure evolution of goethite (α-FeOOH) and hematite (α-Fe2O3) hierarchical nanostructures (HNSs). ► The investigation of the formation mechanism of the HNSs. ► The analysis of the photocatalytic performance for the α-Fe2O3 HNSs.

•A metal-ion-steered solvothermal method for synthesizing NixFe3-xO4 nanospheres.•Proposing an in situ-reduction, coordination-precipitation transformation mechanism.•Investigating size- and composition-dependent static magnetic properties.•Investigating size- and composition-dependent microwave absorbing properties.An easy metal-ion-steered solvothermal method was developed for the one-step synthesis of monodisperse, uniform NixFe3-xO4 polycrystalline nanospheres with tunable sphere diameter (40–400 nm) and composition (0 ≤ x ≤ 0.245) via changing just Ni2+/Fe3+ molar ratio (γ). With g increased from 0:1 to 2:1, sphere diameter gradually decreased and crystal size exhibited an inversed U-shaped change tendency, followed by increased Ni/Fe atom ratio from 0% to 0.0888%. An in situ-reduction, coordination-precipitation transformation mechanism was proposed to interpret the metal-ion-steered growth. Size- and composition-dependent static magnetic and microwave absorbing properties were systematically investigated. Saturation magnetization declines with g in a Boltzmann model due to the changes of crystal size, sphere diameter, and Ni content. The coercivity reaches a maximum at γ = 0.75:1 because of the critical size of Fe3O4 single domain (25 nm). Studies on microwave absorption reveal that 150–400 nm Fe3O4 nanospheres mainly obey the quarter-wavelength cancellation model with the single-band absorption; 40–135 nm NixFe3-xO4 nanospheres (0 ≤ x ≤ 0.245) obey the one and three quarter-wavelength cancellation model with the multi-band absorption. 150 nm Fe3O4 nanospheres exhibit the optimal EM wave-absorbing property with an absorbing band of 8.94 GHz and the maximum RL of −50.11 dB.

Complex ZnO architectures with tunable morphologies and structures were obtained by modulating only the base type and molar ratio of base to Zn2+ (α) using an easy one-pot hydrothermal approach without any template or organic additive. Characterizations by X-ray diffraction, Fourier-transform infrared spectrometry, scanning electron microscopy, transmission electron microscopy, and surface area analysis were performed. The effect of the base type and base/Zn2+ molar ratio on the morphology and corresponding mechanism were determined. The correlations between the microstructure and properties were established. The antibacterial effect of the ZnO samples was probably due to a combination of variable factors. Better antibacterial activity is derived from more effective antibacterial surfaces, which are mainly associated with the specific surface area and Zn-polar plane. Thus, flower-like architectures with larger specific surface areas and more highly exposed (0001) Zn-polar surfaces outwards are promising structures for ZnO antibacterial agents. This work provides a guide for devising and synthesizing highly efficient antibacterial materials.

High-quality MnxFe3−xO4 (0 ≤ x ≤ 1.09) hollow/porous spherical chains (H/PSCs) were prepared via a facile one-pot solvothermal approach. These chains were formed through a magnetic field induced-Oswald ripening mechanism. The external magnetic field induced nanocrystals to aggregate into microspheres along the [111] direction, and these microspheres further assembled into 1D H/PSCs. Characterization confirmed that an increase in the Mn2+/Fe3+ molar ratio decreased the crystal size, diameter, and aspect ratio as well as increased internal strain, lattice constant, Mn doping amount, and specific surface area. Consequently, saturation magnetization and coercivity decreased because of the crystal size and Mn2+ substitution. Mn2+ substitution induced a dual-frequency absorption at 2–18 GHz, in which the absorption band at λ/4 decreased and that at 3λ/4 increased with increasing x. Compared with Fe3O4 solid spheres and hollow spheres, Mn0.746Fe2.254O4 H/PSCs exhibited a broader absorption band (RL ≤ −20 dB) of 9.86 GHz (2.05–7.91 and 14–18 GHz, respectively). The enhanced absorption performance may be related to hollow and porous structures, oriented arrangement of nanocrystals, and Mn2+ substitution.

An easy mixed solvent solvothermal/hydrothermal method was developed for the one-step synthesis of monodisperse, single-crystal Fe3O4 and α-Fe2O3 nanomaterials. The morphologies can be varied from spherical, to octahedral, to rice like, and even to fusiform; the size can be continuously tuned to a range within 30–290 nm. The morphology-, dimension-, and phase-controlled growth of FexOy nanocrystals can be achieved by tuning kinetic factors, such as the H2O volume fraction (γ), Fe3+ concentration, reaction temperature, and the ratio of alkali/Fe3+. The threshold value γ (about 25%) for the H2O-steered size and phase evolutions was theoretically and experimentally inferred. The size- and phase-dependent saturation magnetization (Ms) and coercivity (Hc) were systematically investigated. High Ms was observed in single-crystal Fe3O4 nanomaterials because of high crystallinity; significantly enhanced Hc was exhibited by the as-obtained Fe3O4–α-Fe2O3 hybrid nanocrystals because of the unique unidirectional anisotropy and exchange bias. This study provided insights into the size and phase evolution mechanisms of nanocrystals in the EG–H2O system and served as efficient guidance for the tunable synthesis of monodisperse nanomaterials in a mixed solvent system. The uniform single-crystalline Fe3O4 and α-Fe2O3 nanomaterials can provide better platforms for studying their size- and phase-dependent optical, electric, and magnetic performances.

In this study, sponge-like ZnO/ZnFe2O4 hybrid micro-hexahedra with diverse textures and compositions were fabricated by the thermal decomposition of hexahedral zinc/iron oxalate precursors, starting from a glucose-engineered co-precipitation process. The resulting ZnO/ZnFe2O4 micro-hexahedra were systematically characterized by X-ray powder diffraction, Fourier-transform infrared spectroscopy, scanning electronic microscopy, transmission electron microscopy (TEM), high-resolution TEM, and surface area analysis. Moreover, modulation in crystal size, composition, and textural properties of spongy ZnO/ZnFe2O4 micro-hexahedra was easily achieved by varying the Zn2+/Fe3+ feeding ratio and the annealing temperature. The antibacterial property of the products was analyzed by testing ATP (adenosine triphosphate) and inhibition zones. Results showed that oxidative stress was the governing mechanism for the antibacterial activity of ZnO/ZnFe2O4 hybrid materials. Moreover, we found that the higher reactive oxygen species yields and the resulting antibacterial activity were exhibited by the ZnO/ZnFe2O4 micro-hexahedra formed at lower sintering temperatures rather than the pure ZnO and Fe2O3. The enhanced antibacterial properties were likely caused by the spongy ZnO/ZnFe2O4 heterostructures, improving the probability of photoinduced charge separation and broadening the visible-light absorption.

Rambutan-like heterostructures consisting of Ni microspheres coated with oriented multiwall carbon nanotubes (MWCNTs) were synthesized by the one-pot thermal decomposition of a mixture of organic matter and Ni precursors. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy were used to reveal the formation mechanism. The growth of MWCNTs capped by Ni nanoparticles on the surface of the Ni nanoparticle-built microspheres followed a tip-growth mode. The composition and morphology of the rambutan-like heterostructures were easily controlled by changing the reaction time, mass ratio δ of polyethylene glycol (PEG) 20000 to NiO, as well as type of C source and Ni precursor. Increasing the δ favored not only the increased C mass fraction but also the morphological conversion from Ni/C film core–shell structures to rambutan-like Ni/MWCNT heterostructures. Such changes caused the decreased saturation magnetization and enhanced permittivity properties with δ. Owing to intensive eddy current loss and multiresonance behaviors, rambutan-like Ni/MWCNT heterostructures with long MWCNTs exhibited significantly improved complex permeability and magnetic loss. Ni/MWCNT heterostructures coated by short MWCNTs showed an optimal microwave absorption property with a minimum RL value of −37.9 dB occurring at 12.8 GHz. This work provides effective guidelines for devising and synthesizing highly efficient microwave-absorbing materials.

Heterostructured nanorings (NRs) with Fe3O4 and/or Fe cores and carbon shells (Fe3O4@C and Fe3O4/Fe@C) were synthesized by a facile and controllable two-step process. The NRs were formed through a synchronous reduction/carbonization/diffusion growth mechanism. Their composition, crystal size, and phase structure could be controlled by selecting the sintering temperature of iron glycolate nanosheets in the presence of acetone. Fe3O4/Fe@C NRs formed at 600 °C to 650 °C exhibit higher specific saturation magnetization (Ms) than Fe3O4@C NRs obtained at 300 °C to 500 °C because of increased Fe content and crystal size; the former also shows higher coercivity (Hc) because of large crystal size and surface pinning function. In addition, Fe3O4/Fe@C NRs show lower density and broader absorption bandwidth than Fe3O4@C NRs and Fe3O4 NRs. Fe3O4/Fe@C NRs formed at 600 °C with a mass fraction of 40 wt% exhibit an absorption bandwidth (RL ≤ −20 dB) of 6.7 GHz and a minimum RL value of −28.18 dB at 4.94 GHz. The enhanced absorption properties are ascribed to the heterostructured and ring-shape configuration, which generates multiple dielectric relaxations, enhanced electromagnetic parameters, and plasmon resonance absorption.