Guoxiu Tong

Facile Hydrothermal Synthesis of Fe3O4/C Core–Shell Nanorings for Efficient Low-Frequency Microwave 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.

One-pot low temperature solution synthesis, magnetic and microwave electromagnetic properties of single-crystal iron submicron cubes

Large quantities of single-crystal iron submicron cubes with controllable dimensions and surface structures were synthesized by a facile low-temperature solution reduction approach under normal atmosphere. The influence of kinetic parameters such as NaOH concentration, reaction temperature, reaction time, species of solvents and reducing agents, etc. on the morphology, size and crystal structure of the as-synthesized products were investigated in detail. The resultant products experience multistep phase transformations and morphology evolutions from Fe(OH)2 nanosheets, to weak crystalline Fe3O4 ultrafine particles and to high crystalline Fe cubes. It is believed that both the control over the quasi-equilibrium crystal nucleation and growth rate by adjusting kinetic parameters, and the selective interaction between ethylenediamine and various crystallographic planes of bcc iron play crucial roles in the formation of the slightly truncated cubes with {100} planes at the sides and small {111} planes on the corners. The as-synthesized iron cubes exhibit a strong and wide resonance behavior for the imaginary permittivity and imaginary permeability over the frequency range of 2–14 GHz as a result of the submicron size and anisotropy morphology. The paraffin-based composites containing 26 vol% iron submicron cubes show good electromagnetic wave absorbing characteristics and the reflection loss values less than −20 dB are obtained in the frequency range of 6.5–18.0 GHz when the thickness of the composites is 1.0–2.5 mm, suggesting that the iron submicron cubes synthesized here are promising as a strong-absorption, thin-thickness, light-weight and low-cost microwave absorber.

Flower-like Co superstructures: Morphology and phase evolution mechanism and novel microwave electromagnetic characteristics

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.

Rambutan-like Ni/MWCNT heterostructures: Easy synthesis, formation mechanism, and controlled static magnetic and microwave electromagnetic characteristics

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) 20 000 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.

Polymorphous ZnO complex architectures: selective synthesis, mechanism, surface area and Zn-polar plane-codetermining antibacterial activity

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.

Morphology evolution, magnetic and microwave absorption properties of nano/submicrometre iron particles obtained at different reduced temperatures

Nano/submicrometre iron particles were prepared by a hydrogen reduction method in a fluidized bed furnace using ?-FeOOH nanorods as precursors. The effect of the reducing temperature (T) on the microstructure, static magnetic properties, microwave electromagnetic parameters and microwave absorption properties of the resultant iron particles was investigated. When T increases from 450 to 650??C, the as-obtained iron particles show an obvious morphology evolution from anisotropic nanorods to isotropic submicrometre polyhedra. As a result, the saturation magnetization, the complex permittivity and the real permeability all increase, while the coercivity and the imaginary permeability decrease due to the reducing surface effect and shape anisotropy. Nanocomposites containing 30?wt% iron nanorods obtained at 450??C show a minimal reflection loss (RL) as low as ?36.8?dB at 14.1?GHz and an absorption band with RL under ?10?dB from 11.6 to 17.0?GHz when the thickness is 1.5?mm, suggesting that they are promising as a strong absorption, thin and lightweight microwave absorber.

Tunable dielectric properties and excellent microwave absorbing properties of elliptical Fe3O4 nanorings

Elliptical Fe3O4 nanorings (NRs) with continuously tunable axes that range from 40 nm to 145 nm in length were prepared through a precursor-directed synthetic route to determine the electromagnetic responses generated at 2–18 GHz. The tunability of the dielectric properties of Fe3O4 NRs depends on the long axis rather than on the specific surface area, internal stress, and grain size. Elliptical Fe3O4 NRs exhibit the excellent microwave absorbing properties due to the unique ring-like configuration, which significantly enhances permittivity, multiple scattering, oscillation resonance absorption, microantenna radiation, and interference. These findings indicate that ring-like nanostructures are promising for devising effective microwave absorbers.

Excellent microwave-absorbing properties of elliptical Fe₃O₄ nanorings made by a rapid microwave-assisted hydrothermal approach.

High-quality elliptical polycrystalline Fe3O4 nanorings (NRs) with continuously tunable size have been synthesized in large amounts via a rapid microwave-assisted hydrothermal approach. The surface-protected glucose reducing/etching/Ostwald ripening mechanism is responsible for the formation of NRs. Ring size can be modulated by selecting iron glycolate nanosheets with various sizes as precursors. The size-dependent magnetic behavior of the NRs was observed. Our research gives insights into the understanding of the microwave absorption mechanism of elliptical Fe3O4 NRs. Owing to their large specific surface area, shape anisotropy, and closed ring-like configuration, elliptical polycrystalline Fe3O4 NRs exhibited significantly enhanced microwave absorption performance compared with Fe3O4 circular NRs, nanosheets, microspheres, nanospindles, and nanotubes. An optimal reflection loss value of -41.59 dB is achieved at 5.84 GHz and R(L) values (≤-20 dB) are observed at 3.2-10.4 GHz. Some new mechanisms including multiple scattering, oscillation resonance absorption, microantenna radiation, and interference are also crucial to the enhanced absorption properties of NRs. These findings indicate that ring-like nanostructures are a promising structure for devising new and effective microwave absorbers.

Controllable synthesis of elliptical Fe3O4@C and Fe3O4/Fe@C nanorings for plasmon resonance-enhanced microwave absorption

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.

Selective preparation and enhanced microwave electromagnetic characteristics of polymorphous ZnO architectures made from a facile one-step ethanediamine-assisted hydrothermal approach

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.