Zhengwang Liu

Synthesis and Microwave Absorption Properties of Yolk–Shell Microspheres with Magnetic Iron Oxide Cores and Hierarchical Copper Silicate Shells

Yolk-shell microspheres with magnetic Fe3O4 cores and hierarchical copper silicate shells have been successfully synthesized by combining the versatile sol-gel process and hydrothermal reaction. Various yolk-shell microspheres with different core size and shell thickness can be readily synthesized by varying the experimental conditions. Compared to pure Fe3O4, the as-synthesized yolk-shell microspheres exhibit significantly enhanced microwave absorption properties in terms of both the maximum reflection loss value and the absorption bandwidth. The maximum reflection loss value of these yolk-shell microspheres can reach -23.5 dB at 7 GHz with a thickness of 2 mm, and the absorption bandwidths with reflection loss lower than -10 dB are up to 10.4 GHz. Owing to the large specific surface area, high porosity, and synergistic effect of both the magnetic Fe3O4 cores and hierarchical copper silicate shells, these unique yolk-shell microspheres may have the potential as high-efficient absorbers for microwave absorption applications.

Hierarchical Fe3O4@TiO2 Yolk–Shell Microspheres with Enhanced Microwave‐Absorption Properties

A facile and efficient strategy for the synthesis of hierarchical yolk-shell microspheres with magnetic Fe3O4 cores and dielectric TiO2 shells has been developed. Various Fe3O4@TiO2 yolk-shell microspheres with different core sizes, interstitial void volumes, and shell thicknesses have been successfully synthesized by controlling the synthetic parameters. Moreover, the microwave absorption properties of these yolk-shell microspheres, such as the complex permittivity and permeability, were investigated. The electromagnetic data demonstrate that the as-synthesized Fe3O4@TiO2 yolk-shell microspheres exhibit significantly enhanced microwave absorption properties compared with pure Fe3O4 and our previously reported Fe3O4@TiO2 core-shell microspheres, which may result from the unique yolk-shell structure with a large surface area and high porosity, as well as synergistic effects between the functional Fe3O4 cores and TiO2 shells.

Morphology-dominant microwave absorption enhancement and electron tomography characterization of CoO self-assembly 3D nano-flowers

To meet the demands of strengthening microwave absorption capability, self-assembly nanoparticles (NP) with flexible morphology and abundant interfaces are important and their synthesis remains great challenge. In this paper, cobalt monoxide (CoO) nanostructures with octahedral and 3-dimensional (3D) nano-flower morphologies were controllably synthesized by decomposition of cobalt acetylacetonate at 280 °C, chelated with dual-surfactants of oleylamine and oleic acid with various volume ratios (10 : 1–7 : 4). The basic structural units of both octahedrons and 3D nano-flowers are octahedral CoO NP, which was demonstrated by advanced 3D transmission electron microscopy tomography. Dependency of the tunable microwave absorption properties on the 3D geometric morphologies of CoO NP was well established. The absorption bandwidth with a reflection loss (RL) of less than −10 dB is larger than 6 GHz for both octahedrons and nano-flowers. Compared to the spherical CoO NP, the octahedral nano-flowers have highly enhanced microwave absorption capability. Moreover, the maximum RL peak of the nano-flower CoO NP reached as high as −37 dB at 10.5 GHz, compared to that of −17 dB at 12 GHz for the octahedral CoO NP and −6.3 dB at 7.5 GHz for the spherical CoO NP. These results indicate a remarkable dependency of the dielectric polarization absorption and magnetic coupling absorption on the geometric morphology of CoO nano-architecture. It can be supposed from our findings that various morphologies of self-assembling CoO NP might become an effective path to achieve high-performance microwave absorption for electromagnetic shielding and stealth camouflage applications.

Predominant growth orientation of Li1.2(Mn0.4Co0.4)O2 cathode materials produced by the NaOH compound molten salt method and their enhanced electrochemical performance

Li-rich layered cathode materials Li1.2(Mn0.4Co0.4)O2 with excellent crystal structure and enhanced electrochemical performance were synthesized by a facile compound molten salt method with different molar ratios of mineralizer NaOH to transition metals R (0, 2.5, 5 and 10). The effects of the molar ratio of NaOH to R on the morphology, selective orientation growth, and electrochemical properties of the as-prepared material were investigated through X-ray diffraction (XRD), transmission electron microscopy (TEM), cyclic voltammetry (CV), and galvanostatic charge–discharge tests. With the introduction of NaOH into the molten salt reaction system, the crystals grew along the [0001] crystal axis predominantly and the particle morphology changed from hexagonal tablets to columns, which resulted in enhanced electrochemical performance by facilitating Li ion migration. The initial capacity increased from 220 mA h g−1 (R = 0) to 258 mA h g−1 (R = 5), and the capacity retention improved from 70.0% (R = 0) to 89.9% (R = 5) at a current density of 0.1 C after 50 cycles. Furthermore, by using high resolution TEM (HRTEM) and electron energy loss spectroscopy (EELS), the crystal local structure variation and Mn ion valence reduction (Mn4+ to Mn3+) were investigated, which are relevant to the capacity loss after charge–discharge cycling. Our work demonstrated that the prepared particle crystal structure was improved in NaOH flux, and the additional formation of a spinel-like structure was remarkably suppressed during cycling, which contributed to the improved electrochemical properties.

High-temperature annealing of an iron microplate with excellent microwave absorption performance and its direct micromagnetic analysis by electron holography and Lorentz microscopy

To meet the demand of electromagnetic interference shielding, cheap and easily available microwave absorbers are urgently required. Recently, most of the related research has been focussed on a number of complicated absorbers comprising multi-components because of their better electromagnetic match. However, it is still a great challenge to develop an absorber that simultaneously possesses the advantages of easy fabrication, low-cost, ultra-wide bandwidth, and strong absorption. Hence, development of a simple and convenient absorber with efficient performance is attracting significant attention because of the urgent requirement of this type of absorbers. Herein, a series of single-component iron-based absorbers with different morphologies and grain sizes was successfully prepared. Strong absorption intensity (∼−43.4 dB) was found in plate-like samples, which could even match those of some multi-component absorbers. Electron holography and Lorentz microscopy analysis were used for the further comprehension of the relationships among the microstructure, electromagnetic property, and microwave absorption performance. The primary grain size of the present iron microplate was found fundamentally important for microwave absorption performance. This cheap and available absorber is believed to be an optimal choice for single-component absorbers and useful in the research of absorption mechanism.

Doping of Ni and Zn Elements in MnCO3: High‐Power Anode Material for Lithium–Ion Batteries

Herein, Ni and Zn elements are doped simultaneously in MnCO3 and microspheric Mnx Niy Znz CO3 is successfully obtained. Atomic mapping images reveal that the Ni and Zn elements have been successfully doped in MnCO3 and thus the prepared sample is not a mixture of MnCO3 , NiCO3 , and ZnCO3 . It is the first time that the atomic mapping images of ternary transition metal carbonates have been demonstrated so far. The scanning transmission electron microscopy - annular bright field (STEM-ABF) image successfully confirms the formation of oxygen vacancies in Mnx Niy Znz CO3 , which is beneficial to improve the electrical conductivity. The evolution of the microstructure from crystal to amorphization during cycling process confirmed by the fast Fourier transform patterns effectively lowers the overpotential of the conversion reaction and accelerates the conversion between Mn2+ and much higher valence of Mn element, contributing to the superior capacity of Mnx Niy Znz CO3 electrode. As anode material for lithium-ion batteries, the prepared Mnx Niy Znz CO3 exhibits excellent long-term cycling stability and outstanding rate performance, delivering the superior reversible discharge capacities of 1066 mA h g-1 at 500 mA g-1 after 500 cycles and 760 mA h g-1 at 1 A g-1 after 1000 cycles. It is the first time that Mnx Niy Znz CO3 has been synthesized and used as anode for lithium-ion batteries so far.

Flexible Graphene-Wrapped Carbon Nanotube/Graphene@MnO2 3D Multilevel Porous Film for High-Performance Lithium-Ion Batteries

The ingenious design of a freestanding flexible electrode brings the possibility for power sources in emerging wearable electronic devices. Here, reduced graphene oxide (rGO) wraps carbon nanotubes (CNTs) and rGO tightly surrounded by MnO2 nanosheets, forming a 3D multilevel porous conductive structure via vacuum freeze-drying. The sandwich-like architecture possesses multiple functions as a flexible anode for lithium-ion batteries. Micrometer-sized pores among the continuously waved rGO layers could extraordinarily improve ion diffusion. Nano-sized pores among the MnO2 nanosheets and CNT/rGO@MnO2 particles could provide vast accessible active sites and alleviate volume change. The tight connection between MnO2 and carbon skeleton could facilitate electron transportation and enhance structural stability. Due to the special structure, the rGO-wrapped CNT/rGO@MnO2 porous film as an anode shows a high capacity, excellent rate performance, and superior cycling stability (1344.2 mAh g-1 over 630 cycles at 2 A g-1 , 608.5 mAh g-1 over 1000 cycles at 7.5 A g-1 ). Furthermore, the evolutions of microstructure and chemical valence occurring inside the electrode after cycling are investigated to illuminate the structural superiority for energy storage. The excellent electrochemical performance of this freestanding flexible electrode makes it an attractive candidate for practical application in flexible energy storage.

Janus-like Fe3O4/PDA vesicles with broadening microwave absorption bandwidth

Microstructural design of the magnetic composites with ultra-wide microwave absorption bandwidth has been attracting extensive attention in the fields of both electromagnetic shielding and radar stealth, but it still remains a great challenge. Herein, asymmetric Janus nanocomposites of Fe3O4/PDA vesicles were successfully prepared, and they exhibited strong absorption intensity (−50.0 dB) and ultra-wide bandwidth (6.4 to 18 GHz) covering 73% of the frequency range of 2–18 GHz with a film thickness of only 2.5 mm. The excellent absorption properties could be due to the asymmetric polarization and magnetic coupling effects, which allow most incident microwaves propagating from the nonmagnetic PDA shell into the magnetic core to be exhausted (−50.0 dB). Electron holography analysis confirmed the intense magnetic coupling of the high-density flux lines within the adjacent Fe3O4-derived core@shell network. Our findings suggest that the anisotropic Janus absorber effectively possesses a broad bandwidth, which might be extended to other asymmetric nanomaterials for electromagnetic interference applications.

Aspect ratio tuned red-shift of photoluminescence emission of PbSe nanorods investigated by electron holography

The physical properties of nanometer scale semiconductors are known to be sensitively influenced by their aspect ratios, but the intrinsic mechanisms still remain unclear. Shape-controlled anisotropic PbSe nanorods were obtained by means of the addition of MnCl2, and the aspect ratio of the nanorods can be continuously tuned from 1 to 10 by simply modulating the amount of chloride ions. It was demonstrated that an optimized concentration of Cl- anions is about 0.04mmol, which controls the competition between thermodynamics and kinetics mechanisms. The emission peaks of the infrared absorbance and photoluminescence spectra were significantly tuned from 1664nm to 1840nm and from 1459nm to 1938nm only by the aspect ratios, respectively. A strong electric dipole phenomenon localized onside the surface of PbSe nanorods terminated by Pb2+ charge was found by using high-spatial-resolution off-axis electron holography, which was furthermore evidenced by the quantitative analysis of the mean inner potential and the surfaces charge. The charge intensity depended on the aspect ratio of PbSe nanorods. The results provide clear evidence that the energy gap interval reduces as a result of the increasing of conduction charge amounts. A novel strategy to facilely shift the peak position of absorbance and photoluminescence emission was therefore proposed.