Showing papers by "Renchao Che published in 2013"

Hierarchical Fe3O4@TiO2 yolk-shell microspheres with enhanced microwave-absorption properties.

Abstract: 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.

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

Abstract: 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 abso...

Double-Shelled Yolk–Shell Microspheres with Fe3O4 Cores and SnO2 Double Shells as High-Performance Microwave Absorbers

Abstract: Double-shelled yolk–shell microspheres with Fe3O4 cores and SnO2 double shells have been successfully synthesized by combining the versatile sol–gel process and hydrothermal shell-by-shell deposition method. The as-synthesized double-shelled Fe3O4@SnO2 yolk–shell microspheres have uniform size, unique morphology, well-defined shells, favorable magnetization, large specific surface area, and high porosity and exhibit significantly enhanced microwave absorption properties in terms of both the maximum reflection loss value and the absorption bandwidth. The excellent microwave absorption properties of these microspheres may be attributed to the unique double-shelled yolk–shell structure and synergistic effect between the magnetic Fe3O4 cores and dielectric SnO2 shells.

Ultrathin BaTiO3 nanowires with high aspect ratio: a simple one-step hydrothermal synthesis and their strong microwave absorption.

Abstract: In this paper, we report the facile synthesis of ultrathin barium titanate (BaTiO3) nanowires with gram-level yield via a simple one-step hydrothermal treatment. Our BaTiO3 nanowires have unique features: single crystalline, uniform size distribution and ultra high aspect ratio. The synergistic effects including both Ostwald ripening and cation exchange reaction are responsible for the growth of the ultrathin BaTiO3 nanowires. The microwave absorption capability of the ultrathin BaTiO3 nanowires is improved compared to that of BaTiO3 nanotorus,1 with a maximum reflection loss as high as −24.6 dB at 9.04 GHz and an absorption bandwidth of 2.4 GHz (<−10 dB). Our method has some novel advantages: simple, facile, low cost and high synthesis yield, which might be developed to prepare other ferroelectric nanostructures. The strong microwave absorption property of the ultrathin BaTiO3 nanowires indicates that these nanowires could be used as promising materials for microwave-absorption and stealth camouflage tec...

Binary Li4Ti5O12-Li2Ti3O7 Nanocomposite as an Anode Material for Li-Ion Batteries

Abstract: Li4Ti5O12 typically shows a flat charge/discharge curve, which usually leads to difficulty in the voltage-based state of charge (SOC) estimation In this study, a facile quench-assisted solid-state method is used to prepare a highly crystalline binary Li4Ti5O12-Li2Ti3O7 nanocomposite While Li4Ti5O12 exhibits a sudden voltage rise/drop near the end of its charge/discharge curve, this binary nanocomposite has a tunable sloped voltage profile The nanocomposite exhibits a unique lamellar morphology consisting of interconnected nanograins of ≈20 nm size with a hierarchical nanoporous structure, contributing to an enhanced rate capability with a capacity of 128 mA h g−1 at a high C-rate of 10 C, and excellent cycling stability

Uniform wurtzite MnSe nanocrystals with surface-dependent magnetic behavior

Abstract: Manganese selenide (MnSe) possesses unique magnetic properties as an important magnetic semiconductor, but the synthesis and properties of MnSe nanocrystals are less developed compared to other semiconductor nanocrystals because of the inability to obtain high-quality MnSe, especially in the metastable wurtzite structure. Here, we have successfully fabricated wurtzite MnSe nanocrystals via a colloidal approach which affords uniform crystal sizes and tailored shapes. The selective binding strength of the amine surfactant is the determining factor in shape-control and shape-evolution. Bullet-shapes could be transformed into shuttle-shapes if part of the oleylamine in the reaction solution was replaced by trioctylamine, and tetrapod-shaped nanocrystals could be formed in trioctylamine systems. The three-dimensional (3D) structure of the bullet-shaped nanorods has been demonstrated by the advanced transmission electron microscope (TEM) 3D-tomography technology. High-resolution TEM (HRTEM) and electron energy-loss spectroscopy (EELS) show that planar-defect structures such as stacking faults and twinning along the [001] direction arise during the growth of bullet-shapes. On the basis of careful HRTEM observations, we propose a “quadra-twin core” growth mechanism for the formation of wurtzite MnSe nanotetrapods. Furthermore, the wurtzite MnSe nanocrystals show lowtemperature surface spin-glass behavior due to their noncompensated surface spins and the blocking temperatures increase from 8.4 K to 18.5 K with increasing surface area/volume ratio of the nanocrystals. Our results provide a systematic study of wurtzite MnSe nanocrystals. Open image in new window

Direct evidence of antisite defects in LiFe0.5Mn0.5PO4via atomic-level HAADF-EELS

Abstract: Using comprehensive transmission electron microscopy (TEM) techniques, the associations between the Mn dopant content, microstructure and improved rate performance of LiFe(1−x)MnxPO4 (0 ≤ x ≤ 0.5) were well established. Via the synergistic mechanism including both templating and chelating effects contributed by cetyltrimethyl ammonium bromide (CTAB) and citric acid, a series of LiFe(1−x)MnxPO4 (0 ≤ x ≤ 0.5) olivine crystals with adjustable Mn doping content were synthesized. No impurity phase was detected. Accidentally, a novel type of roughness phenomenon at the particle boundaries of LiFe(1−x)MnxPO4 particles was observed, which depended on citric acid chelation. At the atomic level, the Mn ions were confirmed to be homogeneously substituted at the iron sites, which were furthermore examined by the combined analysis of electron energy loss spectroscopy (EELS), high angle annular dark-field (HAADF) imaging, magnetic susceptibility measurements and X-ray diffraction (XRD). Li/Fe antisite defects were found in the doped LiFe(1−x)MnxPO4 rather than in pure LiFePO4 by HAADF-EELS acquired from a single-atom column at high spatial resolution. The rate performance of LiFe0.9Mn0.1PO4 and LiFe0.8Mn0.2PO4 was improved compared to that of LiFePO4. Our findings might provide new insights into the understanding of Li-ion battery cathode materials with Mn dopant from a microstructural point of view.

Microwave absorption enhancement, magnetic coupling and ab initio electronic structure of monodispersed (Mn1−xCox)3O4 nanoparticles

Abstract: Monodispersed manganese oxide (Mn1−xCox)3O4 (0 ≤ x ≤ 0.5) nanoparticles, less than 10 nm size, are respectively synthesized via a facile thermolysis method at a rather low temperature, ranging from 90 to 100 °C, without any inertia gas for protection. The influences of the Co dopant content on the critical reaction temperature required for the nanoparticle formation, electronic band structures, magnetic properties, and the microwave absorption capability of (Mn1−xCox)3O4 are comprehensively investigated by means of both experimental and theoretical approaches including powder X-ray diffraction (XRD), electron energy loss spectroscopy (EELS), super conductivity quantum interference device (SQUID) examination, and first-principle simulations. Co is successfully doped into the Mn atomic sites of the (Mn1−xCox)3O4 lattice, which is further confirmed by EELS data acquired from one individual nanoparticle. Therefore, continuous solid solutions of well-crystallized (Mn1−xCox)3O4 products are achieved without any impurity phase or phase separation. With increases in the Co dopant concentration x from 0 to 0.5, the lattice parameters change systemically, where the overall saturation magnetization at 30 K increases due to the more intense coupling of the 3d electrons between Mn and Co, as revealed by simulations. The microwave absorption properties of the (Mn1−xCox)3O4 nanoparticles are examined between 2 and 18 GHz. The maximum absorption peak −11.0 dB of the x = 0 sample is enhanced to −11.5 dB for x = 0.2, −12.7 dB for x = 0.25, −15.6 dB for x = 0.33, and −24.0 dB for x = 0.5 respectively, suggesting the Co doping effects. Our results might provide novel insights into the understanding of the influences of metallic ion doping on the electromagnetic properties of metallic oxide nanomaterials.