Guangwu Wen

Co-enhanced SiO2-BN ceramics for high-temperature dielectric applications

Abstract Two typical high-temperature dielectric materials, fused silica and BN, have been used to form a composite with an attempt to overcome their own drawbacks. In the resultant BN–SiO 2 composites, BN platelike grains were preferentially orientated by hot pressing and homogeneously distributed in the fused silica matrix. An evident co-operative enhancement has been achieved by the combination of the constituents. The sinterability and the thermal shock resistance of the BN materials were increased and the ablation surface temperature was decreased by the involvement of the fused silica. On the other hand, the strength, fracture toughness, and flame ablation resistance of the fused silica were increased due to the addition of BN. Furthermore, an amorphous Si–B–O–N structure was identified in the surface layer of the ablated composites, to which attention should be further paid in the development of new elevated temperature dielectric materials.

NiCo2O4 nanosheet supported hierarchical core–shell arrays for high-performance supercapacitors

Two types of homogeneous NiCo2O4 nanosheet@NiCo2O4 nanorod and heterogeneous NiCo2O4 nanosheet@NiO nanoflake hierarchical core–shell arrays are synthesized via facile solution methods in combination with a simple thermal treatment. In both cases, the NiCo2O4 nanosheets serve as the core backbone for anchoring the shell materials. The two as-prepared hierarchical nanoarrays are evaluated as supercapacitor electrodes and demonstrate excellent electrochemical performance with high specific capacitance (1925 and 2210 F g−1 for NiCo2O4@NiCo2O4 and NiCo2O4@NiO at 0.5 A g−1, respectively), good rate capability, and superior cycling stability. The superior capacitive performance is mainly due to the unique hierarchical core–shell architecture with faster ion/electron transfer, improved reactivity, and enhanced structural stability. Our work can allow for the fabrication of various NiCo2O4 nanosheet supported hierarchical nanostructures for applications in energy storage, catalysis, and sensing.

Novel SiOC nanocomposites for high-yield preparation of ultra-large-scale SiC nanowires.

Novel SiOC nanocomposites were successfully synthesized from commercial silica sol and sucrose via a simply designed route. The formation of SiOC nanocomposites was studied using thermogravimetry and differential scanning calorimetry. The synthesized nanocomposites were characterized by Fourier transform infrared spectroscopy, x-ray fluorescence spectrometer, x-ray photoelectron spectroscopy, x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results indicate that the synthesized composites are amorphous in nature and homogeneous with the microstructure of close packed SiO(2) and carbon at nanoscale. The SiOC nanocomposites exhibit very high reactivity and can be annealed to produce SiC nanocrystals at 1200 degrees C which is about 300 degrees C lower than the value obtained by thermodynamic calculation. Ultra-large-scale beta-SiC nanowires with high quality were prepared by directly annealing the synthesized SiOC nanocomposites at 1500 degrees C under Ar atmosphere, where the yield of SiC nanowires was up to 59%. The SiC nanowires grow along the [111] direction with highly uniform diameters of about 100 nm. Experimental results indicate that the close contact between SiO(2) and carbon at nanoscale plays a vital role in the high yield of SiC nanowires. The present work provides an efficient strategy for the large scale production of high-quality SiC nanowires.

A novel one-step strategy toward ZnMn2O4/N-doped graphene nanosheets with robust chemical interaction for superior lithium storage

Ingenious hybrid electrode design, especially realized with a facile strategy, is appealing yet challenging for electrochemical energy storage devices. Here, we report the synthesis of a novel ZnMn2O4/N-doped graphene (ZMO/NG) nanohybrid with sandwiched structure via a facile one-step approach, in which ultrafine ZMO nanoparticles with diameters of 10-12 nm are well dispersed on both surfaces of N-doped graphene (NG) nanosheets. Note that one-step synthetic strategies are rarely reported for ZMO-based nanostructures. Systematical control experiments reveal that the formation of well-dispersed ZMO nanoparticles is not solely ascribed to the restriction effect of the functional groups on graphene oxide (GO), but also to the presence of ammonia. Benefitting from the synergistic effects and robust chemical interaction between ZMO nanoparticles and N-doped graphene nanosheets, the ZMO/NG hybrids deliver a reversible capacity up to 747 mAh g(-1) after 200 cycles at a current density of 500 mA g(-1). Even at a high current density of 3200 mA g(-1), an unrivaled capacity of 500 mAh g(-1) can still be retained, corroborating the good rate capability.

Hierarchically constructed NiCo2S4@Ni(1-x)Co x (OH)2 core/shell nanoarrays and their application in energy storage.

We report a new type of core-shell heterostructure consisting of a rod-like NiCo2S4 (NCS) core and an urchin-like Ni(1-x)Co x (OH)2 (NCOH) shell via a simple hydrothermal route coupled with a facile electrodeposition. NCS nanorod arrays (NRAs) can not only act as excellent electrochemically active materials by themselves, but they can also serve as hierarchical porous scaffolds capable of fast electron conduction and ion diffusion for loading a large amount of additional active materials. Moreover, it is observed that the urchin-like NCOH nanosheets coating could bind the inner NCS nanorods together and thereby reinforce the whole structure mechanically. Meanwhile, more effective pathways for electrons are available in the NCS@NCOH hybrids than an individual NCS nanorod. Benefiting from both structural and compositional features, the NCS@NCOH electrode exhibits greatly improved electrochemical performance with high capacity (3.54 C cm(-2) at 1 mA cm(-2)) and excellent cycling stability (78% capacity retention after 4000 cycles). Moreover, a battery-type device is also fabricated by using NCS@NCOH as a positive electrode and activated carbon (AC) as a negative electrode, displaying high capacity (2.51 C cm(-2) at 2 mA cm(-2)) and good durability (88.8% capacity retention after 4000 cycles) as well.

Ultra-long Sialon nanobelts: large-scale synthesis via a pressure enhanced CVD process and photoluminescence characteristics

Large quantities of core–shell structured Sialon nanobelts containing amorphous carbon in the shell were synthesized by pressure enhanced CVD at 1450 °C without a catalyst. The products are high-quality nano single-crystalline with a uniform morphology. The yield and aspect ratio are systematically controlled by changing the growth temperature and chamber pressure. The nanostructures were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption–desorption isotherms and high-resolution transmission electron microscopy (HRTEM). A stepwise reaction model and luminescence characteristics were proposed based on the experimental results.

Metal–semiconductor Zn/ZnO core–shell nanocables: facile and large-scale fabrication, growth mechanism, oxidation behavior, and microwave absorption performance

A new and facile synthetic route has been developed for the fabrication of metal–semiconductor Zn/ZnO core–shell nanocables on a large scale. Zn/ZnO nanocables were grown by heating a ball-milled mixture of boron and ZnO powders at 1300 °C under ammonia atmosphere. The structure and chemical composition of the as-prepared products were characterized by a variety of techniques including powder X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. The nanocables were approximately 30–200 nm in diameter and tens of microns in length. The core was a Zn single crystal and the shell was an epitaxially grown ZnO layer of 3–10 nm thickness. It was found that the Zn/ZnO nanocables transformed into mace-like nanostructures or ZnO nanotubes when oxidized at 300 °C in air. The formation mechanism of the Zn/ZnO nanocables as well as the oxidized products has been clarified based on the experimental observations. The Zn/ZnO nanocable–paraffin composites showed good microwave absorption properties, and the reflection loss could reach −23 dB at 13.22 GHz. The mechanism for the enhanced absorption performance is discussed.

General synthesis of graphene-supported bicomponent metal monoxides as alternative high-performance Li-ion anodes to binary spinel oxides

Engineering two transition metals into an integrated spinel oxide anode provides great opportunity towards high-performance lithium-ion batteries (LIBs). Spinels with high-valence transition metal oxides (TMOs) however tend to exhibit low initial coulombic efficiency (ICE) due to the irreversible Li2O generated during the first discharge process. Herein, we report a simple and general strategy to synthesize elaborate graphene framework (GF) supported low-valence bicomponent transition metal monoxide anodes (e.g., ZnO–MnO microcubes, ZnO–CoO polyhedra, NiO–CoO nanowires, and (FeO)0.333(MnO)0.667 microspheres, etc.), which can efficiently address the low ICE issue. As a proof of concept demonstration, we show that the ZnO–MnO/GF is indeed more advantageous as an LIB anode over the spinel ZnMn2O4/GF counterpart as well as many other ZnMn2O4-based anodes. Benefiting from the enhanced reversibility of Li+ uptake/extraction and graphene hybridization, the ZnO–MnO/GF electrode exhibits significantly improved ICEs at various current densities, superior rate capability (286 mA h g−1 even at a high current density of 6 A g−1; ∼2.9 min charging/discharging), and extended cycling life (1123 mA h g−1 after 300 cycles) with respect to ZnMn2O4/GF. Such improvements have also been observed for the ZnO–CoO/GF electrode and other analogues. This versatile electrode design could advance our understanding and control of complex TMO-based anodes to gain high ICE and capacity.

Enhanced microwave absorption properties of ferroferric oxide/graphene composites with a controllable microstructure

Fe3O4/graphene composites were synthesized as an advanced electromagnetic wave absorption material by a solvothermal method in a system of ethylene glycol. The Fe3O4 nanoparticles were homogeneously anchored on the graphene sheets and the structures of the nanoparticles could be experimentally controlled from ring-like spheres, flower-like spheres to solid spheres by changing the concentration of the oxide graphene. Microwave absorption tests demonstrated that the structures of the nanoparticles had a positive influence on the microwave absorption properties. Especially, for the Fe3O4/graphene composite with a flower-like structure, the minimum reflection loss value (RL) could reach −53.2 dB and the bandwidth of RL less than 10 dB (90% absorption) ranged from 8.1 to 16 GHz at a thickness of 2.5 mm, which is among the best-reported performances of Fe3O4/graphene materials, showing a huge potential to be used as a candidate for microwave absorbing materials.

Large-Scale Fabrication of Boron Nitride Nanotubes via a Facile Chemical Vapor Reaction Route and Their Cathodoluminescence Properties

Cylinder- and bamboo-shaped boron nitride nanotubes (BNNTs) have been synthesized in large scale via a facile chemical vapor reaction route using ammonia borane as a precursor. The structure and chemical composition of the as-synthesized BNNTs are extensively characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and selected-area electron diffraction. The cylinder-shaped BNNTs have an average diameter of about 100 nm and length of hundreds of microns, while the bamboo-shaped BNNTs are 100–500 nm in diameter with length up to tens of microns. The formation mechanism of the BNNTs has been explored on the basis of our experimental observations and a growth model has been proposed accordingly. Ultraviolet–visible and cathodoluminescence spectroscopic analyses are performed on the BNNTs. Strong ultraviolet emissions are detected on both morphologies of BNNTs. The band gap of the BNNTs are around 5.82 eV and nearly unaffected by tube morphology. There exist two intermediate bands in the band gap of BNNTs, which could be distinguishably assigned to structural defects and chemical impurities.