Youxiang Zhang

Current rectification in a single GaN nanowire with a well-defined p-n junction

This letter discusses Mg incorporation in GaN nanowires with diameters ∼35 nm, fabricated by vapor–liquid–solid synthesis in p-type nanowires. Turning on the Mg doping halfway through the synthesis produced nanowires with p–n junctions that showed excellent rectification properties down to 2.6 K. The nanowires are shown to possess good-quality, crystalline, hexagonal GaN inner cores surrounded by an amorphous GaN outer layer. Most wires grow such that the crystalline c axis is normal to the long axis of the nanowire. The temperature dependence of the current–voltage characteristics is consistent with electron tunneling through a voltage-dependent barrier.

Three-Dimensional Macroporous Graphene–Li2FeSiO4 Composite as Cathode Material for Lithium-Ion Batteries with Superior Electrochemical Performances

Three-dimensional macroporous graphene-based Li2FeSiO4 composites (3D-G/Li2FeSiO4/C) were synthesized and tested as the cathode materials for lithium-ion batteries. To demonstrate the superiority of this structure, the composites performances were compared with the performances of two-dimensional graphene nanosheets-based Li2FeSiO4 composites (2D-G/Li2FeSiO4/C) and Li2FeSiO4 composites without graphene (Li2FeSiO4/C). Due to the existence of electronic conductive graphene, both 3D-G/Li2FeSiO4/C and 2D-G/Li2FeSiO4/C showed much improved electrochemical performances than the Li2FeSiO4/C composite. When compared with the 2D-G/Li2FeSiO4/C composite, 3D-G/Li2FeSiO4/C exhibited even better performances, with the discharge capacities reaching 313, 255, 215, 180, 150, and 108 mAh g(-1) at the charge-discharge rates of 0.1 C, 1 C, 2 C, 5 C, 10 C and 20 C (1 C = 166 mA g(-1)), respectively. The 3D-G/Li2FeSiO4/C composite also showed excellent cyclability, with capacity retention exceeding 90% after cycling for 100 times at the charge-discharge rate of 1 C. The superior electrochemical properties of the 3D-G/Li2FeSiO4/C composite are attributed to its unique structure. Compared with 2D graphene nanosheets, which tend to assemble into macroscopic paper-like structures, 3D macroporous graphene can not only provide higher accessible surface area for the Li2FeSiO4 nanoparticles in the composite but also allow the electrolyte ions to diffuse inside and through the 3D network of the cathode material. Specially, the fabrication method described in this study is general and thus should be readily applicable to the other energy storage and conversion applications in which efficient ionic and electronic transport is critical.

Ag size-dependent visible-light-responsive photoactivity of Ag–TiO2 nanostructure based on surface plasmon resonance

Anatase TiO2 nanocrystals with regular polyhedron morphology and co-exposed {001} and {101} facets are prepared, and then ultrafine Ag nanoparticles (SAg NPs) with different sizes are loaded on those TiO2 facets (denoted as SAg–TiO2) through a novel in situ photoreduction approach. For comparison, Ag nanoparticles (PAg NPs) are also loaded on TiO2 facets (denoted as PAg–TiO2) by a traditional photodeposition method. The experimental results show that PAg NPs with a size of ∼20 nm only appeared on the {101} facets of TiO2, while SAg NPs with a much smaller size (1–6 nm) are highly dispersed on both {001} and {101} facets of TiO2. The size of Ag NPs plays an important role in surface plasmon resonance (SPR) effect and the photogenerated carrier separation process, and those Ag NPs loaded on TiO2 facets show size-dependent photoactivity for the RhB degradation. SAg–TiO2 with ultrafine SAg NPs on both {001} and {101} facets of TiO2 results in more substantial visible-light-responsive photoactivity and stability than PAg–TiO2 with larger Ag NPs on {101} facets of TiO2. Moreover, the RhB photodegradation reaction rate constant of SAg–TiO2 with Ag particle size of ∼1 nm is 10 and 21.3 times higher than that of PAg–TiO2 and the pristine TiO2, respectively.

Catalytic and catalyst-free synthesis of CdSe nanostructures with single-source molecular precursor and related device application.

Air-stable single-source molecular precursor is applied for controlled size and morphology synthesis of one-dimensional and quasi-one-dimensional CdSe nanostructures. Two different growth approaches are compared to control the growth of nanostructures. When combined with well-defined Au colloidal catalysts, the use of single-source molecular precursor allows diameter control synthesis of monodispersed CdSe nanowires from 10-30 nm via a vapor-liquid-solid mechanism. In addition, a variety of CdSe nanostructures with different morphologies can be achieved and tuned without assistance of metallic catalysts by carefully manipulating dynamic thermal decomposition process of single-source molecular precursor. The new level of synthetic control afforded by our present work opens up new opportunities for using as-synthesized CdSe nanostructures as model systems for fundamental studies as well as building blocks for larger scale functional device assembly. Importantly, we demonstrate that a single CdSe tripod can be natively configured as a nanoscale phototransistor in which photocurrent created between two tripod arms can be efficiently modulated by applying a gate voltage through the third arm.

Two-electron migration orthosilicate cathode materials for Na-ion batteries

For the first time, sodium iron orthosilicates were obtained through electrochemical Li–Na ion-exchange and tested as cathode materials for Na-ion batteries. Discharge capacities as high as 330 mA h g−1 could be reached in the Na cells at the rate of 10 mA g−1 and at room temperature, it showed that two Na+ ions per molecule could intercalate into these orthosilicates.

Spinel-layered integrate structured nanorods with both high capacity and superior high-rate capability as cathode material for lithium-ion batteries

Spinel phase LiMn2O4 was successfully embedded into monoclinic phase layeredstructured Li2MnO3 nanorods, and these spinel-layered integrate structured nanorods showed both high capacities and superior high-rate capabilities as cathode material for lithium-ion batteries (LIBs). Pristine Li2MnO3 nanorods were synthesized by a simple rheological phase method using α-MnO2 nanowires as precursors. The spinel-layered integrate structured nanorods were fabricated by a facile partial reduction reaction using stearic acid as the reductant. Both structural characterizations and electrochemical properties of the integrate structured nanorods verified that LiMn2O4 nanodomains were embedded inside the pristine Li2MnO3 nanorods. When used as cathode materials for LIBs, the spinel-layered integrate structured Li2MnO3 nanorods (SL-Li2MnO3) showed much better performances than the pristine layered-structured Li2MnO3 nanorods (L-Li2MnO3). When charge–discharged at 20 mA·g−1 in a voltage window of 2.0–4.8 V, the SL-Li2MnO3 showed discharge capacities of 272.3 and 228.4 mAh·g−1 in the first and the 60th cycles, respectively, with capacity retention of 83.8%. The SL-Li2MnO3 also showed superior high-rate performances. When cycled at rates of 1 C, 2 C, 5 C, and 10 C (1 C = 200 mA·g−1) for hundreds of cycles, the discharge capacities of the SL-Li2MnO3 reached 218.9, 200.5, 147.1, and 123.9 mAh·g−1, respectively. The superior performances of the SL-Li2MnO3 are ascribed to the spinel-layered integrated structures. With large capacities and superior high-rate performances, these spinel-layered integrate structured materials are good candidates for cathodes of next-generation high-power LIBs.

Oxygen Vacancies and Stacking Faults Introduced by Low-Temperature Reduction Improve the Electrochemical Properties of Li2MnO3 Nanobelts as Lithium-Ion Battery Cathodes

Among the Li-rich layered oxides Li2MnO3 has significant theoretical capacity as a cathode material for Li-ion batteries. Pristine Li2MnO3 generally has to be electrochemically activated in the first charge-discharge cycle which causes very low Coulombic efficiency and thus deteriorates its electrochemical properties. In this work, we show that low-temperature reduction can produce a large amount of structural defects such as oxygen vacancies, stacking faults, and orthorhombic LiMnO2 in Li2MnO3. The Rietveld refinement analysis shows that, after a reduction reaction with stearic acid at 340 °C for 8 h, pristine Li2MnO3 changes into a Li2MnO3-LiMnO2 (0.71/0.29) composite, and the monoclinic Li2MnO3 changes from Li2.04Mn0.96O3 in the pristine Li2MnO3 (P-Li2MnO3) to Li2.1Mn0.9O2.79 in the reduced Li2MnO3 (R-Li2MnO3), indicating the production of a large amount of oxygen vacancies in the R-Li2MnO3. High-resolution transmission electron microscope images show that a high density of stacking faults is also introduced by the low-temperature reduction. When measured as a cathode material for Li-ion batteries, R-Li2MnO3 shows much better electrochemical properties than P-Li2MnO3. For example, when charged-discharged galvanostatically at 20 mA·g-1 in a voltage window of 2.0-4.8 V, R-Li2MnO3 has Coulombic efficiency of 77.1% in the first charge-discharge cycle, with discharge capacities of 213.8 and 200.5 mA·h·g-1 in the 20th and 30th cycles, respectively. In contrast, under the same charge-discharge conditions, P-Li2MnO3 has Coulombic efficiency of 33.6% in the first charge-discharge cycle, with small discharge capacities of 80.5 and 69.8 mA·h·g-1 in the 20th and 30th cycles, respectively. These materials characterizations, and electrochemical measurements show that low-temperature reduction is one of the effective ways to enhance the performances of Li2MnO3 as a cathode material for Li-ion batteries.

Effects of V2O5 nanowires on the performances of Li2MnSiO4 as a cathode material for lithium-ion batteries

The effects of vanadium pentoxide on the electrochemical properties of Li2MnSiO4 as a cathode material for lithium-ion batteries were tested by synthesizing a V2O5 nanowire-modified in situ carbon coated Li2MnSiO4 composite (LMS/C/V2O5) and comparing its performances with that of a Li2MnSiO4 composite without V2O5. In LMS/C/V2O5, the V2O5 nanowires, with diameters of around 10–20 nm and lengths up to tens of micrometers, entangled together and formed a 3D conductive network; the Li2MnSiO4 nanoparticles, with sizes around 30 nm, distributed uniformly in the network frame and tended to adhere to the V2O5 nanowires. In this structure, the LMS/C/V2O5 composite showed a superior performance as a cathode of lithium-ion batteries even with very low carbon content (3.4 wt%). Ex situ X-ray diffraction patterns, electrochemical impedance spectroscopies of the electrodes and the concentration of Mn ions in the electrolyte during the charge–discharge processes explained the effects of the V2O5 nanowires as an additive in the Li2MnSiO4 cathode material. The benefits of the nanowires include maintaining the crystal structure of Li2MnSiO4 during the charge–discharge cyclings, reducing the charge-transfer resistances at the solid–electrolyte interfaces, increasing the lithium ions diffusion coefficient in the cathode and alleviating the dissolution of manganese into the electrolyte of the batteries.

Interfacial effects of MnOx-loaded TiO2 with exposed {001} facets and its catalytic activity for the photoreduction of CO2

Herein, anatase TiO2 with different percentages of exposed {001} and {101} facets was prepared, and subsequently, Ag and MnOx were co-loaded on these TiO2 facets through a classical photodeposition method; the loaded TiO2 samples were labeled as Ag–TiO2–MnOx (N%), where N% denotes the percentage of the exposed {001} facets. The as-prepared samples were analyzed via scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and UV-vis diffuse reflectance spectroscopy (DRS), and the photocatalytic reduction of CO2 was carried out. The results showed that Ag–TiO2–MnOx (40%) exhibited the highest photocatalytic activity among the other dual-loaded samples and pure TiO2. The XPS spectra demonstrated that Ti–O–Mn and Ti–O–H bonds were formed at the interfaces of the cocatalyst-TiO2. The PL and VB spectra revealed that the chemical bond (Ti–O–Mn) at the Ag–TiO2–MnOx (40%) interface greatly increased the hole mobility rate and the photoinduced carrier separation efficiency that resulted in higher catalytic activity for the photoreduction of CO2.

A Study about the Synthesis Characterization and Electrochemical Properties of γ-MnOOH Nanowires

In this paper, manganite (γ-MnOOH) nanowires have been synthesized, using KMnO4 and CTAB as raw materials, by a hydrothermal method at 180°C for 12h. The samples were characterized by X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), scanning electron microscope (SEM),and thermogravimetric analysis (TG) for the information of crystal form, size, d-spacings, morphology and the weight-loss course. The results showed the manganite nanowires diameter range from 15 nm to 150 nm,length range from 5μm to 20 μm. Moreover, the properties of manganite (γ-MnOOH) nanowires as anode materials for Li-ion batteries had been studied. The first-discharge capacity is 1660 mAh g-1 and corresponds to a consumption of about 5.5 moles of Li per mole of MnOOH, which made this material maybe one of the candidates for the negative materials.