Morphology-control of VO2 (B) nanostructures in hydrothermal synthesis and their field emission properties

VO₂ (B) nanostructures were synthesized via a facile hydrothermal process using V₂O₅ as source material and oxalic acid as reductant. Three nanostructures of nanorods, nanocarambolas and nanobundles were found existing in the products, and a continuous changing of morphology was found in the synthesis process, during which the proportion of these three types of nanostructures can be adjusted by altering the concentrations of oxalic acid. The microstructures were evaluated using X-ray diffraction and scanning and transmission electron microscopies, respectively. FE properties measurement of these three types of nanostructures showed that the nanobundles have the best field emission performance with a turn-on field of ∼1.4 V/μm and a threshold field of ∼5.38 V/μm. These characteristics make VO₂ (B) nanostructures a competitive cathode material in field emission devices.     

Self-assembled 3D La(OH)3 and La2O3 nanostructures: Fast microwave synthesis and characterization

Self-assembled three-dimensional (3D) urchin-like and flower-like La(OH)₃ nanostructures were successfully prepared for the first time via a facile and fast microwave-assisted solution-phase chemical method in 15 min. The obtained products were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The SEM results reveal that the urchin-like and flower-like La(OH)₃ nanostructures are ca. 3 μm and 6 μm in diameter, respectively. The urchin-like La(OH)₃ nanostructures are constructed by nanorods with diameters of about 300 nm and lengths of about 500 nm. The flower-like La(OH)₃ nanostructures are built from nanopetals about 100 nm thick. The effects of reaction time, microwave power, amount of tetraethyl ammonium bromide (TEAB), and surfactants on the preparation were systematically investigated. The possible formation mechanism of the 3D La(OH)₃ nanostructures was preliminarily discussed. Urchin-like and flower-like La₂O₃ nanostructures were obtained after calcining the La(OH)₃ nanostructures at 800 °C for 4 h. Urchin-like and flower-like La₂O₃:Eu³⁺ nanostructures were also prepared and their photoluminescence (PL) properties were investigated.     

Synthesis of urchin-like Co3O4 hierarchical micro/nanostructures and their photocatalytic activity

Urchin-like Co₃O₄ hierarchical micro/nanostructures have been successfully synthesized by calcining urchin-like precursor CoCO₃, which are prepared by a facile hydrothermal route. The particle size of the urchin-like Co₃O₄ could be easily controlled by altering the calcination temperature. The morphology and structure of the as-prepared urchin-like products were characterized by XRD, FESEM and TEM. Photocatalytic measurement demonstrates that these urchin-like Co₃O₄ micro/nanostructures show good photocatalytic effect and their degradation efficiency is strongly dependent on their particle size. Furthermore, a plausible reaction mechanism is also proposed to illustrate the photocatalytic processes of Co₃O₄.     

Novel urchin-like Pd nanostructures prepared by a simple replacement reaction and their catalytic properties

Novel urchin-like Pd nanostructures were firstly synthesized using a simple replacement reaction between Al and PdCl₂ aqueous solution at room temperature without the assistance of any surfactant or ligand. Their phase structure, morphology, specific surface area, and catalytic property were characterized by XRD, TEM, EDS, BET and DSC/TG. The results show numerous one-dimensional Pd nanorods radiate from the center of the aluminum templates to form an urchin-like shape with a diameter of ∼0.5-1.0 μm. The as-prepared urchin-like Pd nanostructures can serve as a promising additive to accelerate the thermal decomposition of ammonium perchlorate (AP), a key oxidizer in composite solid propellants.     

Hierarchical hollow urchin-like structured MnO2 microsphere/carbon nanofiber composites as anode materials for Li-ion batteries

In this work, novel hollow urchin-like MnO₂ microspheres (u-MnO₂), consisting of a hollow core with nanotubes, are synthesized by a simple hydrothermal process. The morphology of the MnO₂ structures could be tuned from round particles to a hierarchical hollow urchin structure by controlling the hydrothermal reaction time, with no need for surfactant or templates. The nanostructures of the obtained u-MnO₂ are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The X-ray diffraction (XRD) pattern of the u-MnO₂ reveals a tetragonal structure of α-MnO₂. The carbon nanofibers (CNFs) are uniformly deposited on u-MnO₂ to improve the electrical conductivity and to utilize the hierarchical architecture of u-MnO₂. As the anode electrode of Li-ion batteries, the u-MnO₂/CNFs nanocomposites exhibit discharge capacity of 988 mAh·g⁻¹ after 100 cycles with a good rate capability. The superior electrochemical performances of the u-MnO₂/CNFs nanocomposites can be attributed to the hierarchical urchin-like structures and the superior electrical conductivity of the nanocomposites, which can facilitate fast electron and ion transport and accommodate a large volume change during charge/discharge.     

A facile and environmentally friendly chemical route for the synthesis of metastable VO2 nanobelts

Metastable VO₂ nanobelts, designated as VO₂ (B), were successfully fabricated by a facile hydrothermal route in the presence of V₂O₅ and glucose. The samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), transmission electron microscopy (TEM), selected area electronic diffraction (SAED), high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) techniques. The main synthesis parameters such as temperature, reaction time and molar ratio of the starting materials have been also discussed. The results showed that pure B phase VO₂ nanobelts with high crystallinity can be prepared easily at 180 °C in 24 h at the molar ratio of V₂O₅:glucose=1:1. Typically, the belt-like products were 0.6-1.2 μm long, 80-150 nm wide and 20-30 nm thick. It is noted that the whole process is free of any harmful reducing reagents and surfactants, and valuable gluconic acid can be formed as the main by-product. From an economic and environmental point of view, the present approach is particularly fit for the synthesis of VO₂ (B) nanobelts on a large scale.     

Structural and electrical properties of VO2/ZnO nanostructures

We examined the structural and electrical properties of uniformly-oriented VO₂/ZnO nanostructures. VO₂ was deposited on ZnO templates by using a direct current-sputtering deposition. Scanning electron microscope and transmission electron microscope measurements indicated that b-oriented VO₂ were uniformly crystallized on ZnO templates with different lengths. VO₂/ZnO formed nanorods on ZnO nanorods with length longer than 250 nm. X-ray absorption fine structure at the V K edge of VO₂/ZnO showed M₁ and R phases of VO₂ at 30 and 100 °C, respectively, suggesting structural-phase transition occurring between the two temperatures. Temperature-dependent resistance measurements of VO₂/ZnO nanostructures revealed metal-to-insulator transition at 65 °C and 55 °C during a heating and a cooling, respectively, regardless of ZnO length. Asymmetry behavior of resistance curves from VO₂/ZnO nanostructure during a heating and a cooling was attributed from a strong bond of VO₂ and ZnO.     

Fabrication of urchin-like NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript> microspheres assembled by using SDS as soft template for anode materials of Lithium-ion batteries

In this paper, the urchin-like NiCo₂O₄ microspheres assembled by using sodium dodecyl sulfate (SDS) as soft template are successfully fabricated by a facile procedure including microemulsion-solvothermal reaction and subsequent calcination at 400 °C for 4 h. The structure and morphology of synthesized NiCo₂O₄ particles are investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It has been clearly revealed that the prepared three-dimensional urchin-like NiCo₂O₄ microspheres are constituted by one-dimension nanowires. As it is applied to anode for lithium-ion batteries (LIBs), the initial coulombic efficiency is up to 75.7%, and the specific reversible capacity retains up to 1034.2 mAh/g even after 40 cycles at a current density of 100 mA/g. Furthermore, as the current density gradually increases to 800 mA/g, it still delivers the reversible capacity of 895.4 mAh/g. The high reversible specific capacity, perfect cyclability, and rate performance are attributed to the unique urchin-like NiCo₂O₄ microspheres, which can alleviate the volume expansion and shorten the diffusion path of ions and electrons during lithiation/delithiation process. The self-standing urchin-like NiCo₂O₄ microspheres may be a very promising candidate in place of the commercial graphite-based anode materials for high-performance LIBs.     

Synthesis of sea urchin-like LiMn2O4 hollow macrospheres via in situ conversion for rechargeable lithium-ion batteries

Among several materials (transition metal oxide) under development for use as a cathode in lithium-ion batteries, cubic spinel LiMn₂O₄ is one of the most promising cathode materials. In this study, the sea urchin-like LiMn₂O₄ hollow macrospheres were synthesized by using sea urchin-like α-MnO₂ precursors through solid-state in situ self-sacrificing conversion route. The as-prepared LiMn₂O₄ was assembled by many single-crystalline “thorns” of ca.10–20 nm in diameter and ca. 400–500 nm in length. Galvanostatic battery testing showed that sea urchin-like LiMn₂O₄ had an initial discharge capacity of 126.8 mAh/g at the rate of 0.2 C in the potential range between 3.0 and 4.5 V. More than 96.67 % of the initial discharge capacity was maintained for over 50 cycles. The improved electrochemical properties were attributed to the reduced particle size and enhanced electrical contacts by the materials. This particular sea urchin-like structured composite conceptually provides a new strategy for designing electrodes in energy storage applications.     

Formation mechanisms and crystallographic characteristics of metastable VO2(A) nanofiber hydrothermally synthesised in V2O5–H2C2O4–H2O system

There are many similarities between VO₂(B) and VO₂(A) from crystallographic view. However, missing of VO₂(A) during the preparation of VO₂ polymorph confused many researchers. Here, the preparation of VO₂(A) was studied systemically via a hydrothermal method in V₂O₅–H₂C₂O₄–H₂O system. As a metastable phase, it can be transferred from VO₂(B) by assembling process. Usually, poly-crystal VO₂(A) nano-fibers are formed by this process. On contrast, owing to the small energy gap between meta-stable VO₂(A) and stable VO₂(R), single crystal VO₂(A) with regular shape can also be obtained by exfoliating some parts of VO₂(R) during non-equilibrium cooling process. VO₂(A) has higher phase transition temperature than stable VO₂(R). The hysteresis in this phase transition can be observed by DSC measurement and the phase transition temperature of VO₂(A) can be tuned down by tungsten doping.     

Impact of growth temperature on the crystal habits,forms and structures of VO<Subscript>2</Subscript> nanocrystals

We investigated the impact of the process temperature on the habits, forms and crystal structure of VO₂ nanocrystals grown by a vapor-transport method on (0001) quartz substrates. Four distinct growth regimes were discerned: orthorhombic nanowires, sheets, hemispheres, and nanowires with a monoclinic structure. The nanostructures were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). I/V characterization of individual nanowires was enabled by Ti/Au contact formation via electron beam lithography and lift-off techniques. The expected metal–insulator transition (MIT) was found in monoclinic VO₂ nanowires.     

VOxH2O (x= 1——-5)团簇的结构及稳定性研究

用密度泛函理论(DFT),在B3LYP/DZP水平上对H₂O分子与VO_x形成的团簇VO_xH₂O (x= 1---5)进行结构优化、能量和频率的计算,研究了团簇的稳定结构、稳定性和频率特性.结果表明VO_xH₂O (x= 1---5) 团簇的基态构型的电子态均为²A, 对称性均属C₁对称点群,其中x= 1, 4, 5时基态构型中水分子已被解离.水分子倾向于吸附在团簇VO_x上, 形成VO_xH₂O (x= 1---5)团簇. VO_xH₂O (x= 1---5)团簇中, VO_xH₂O (x= 1,4,5) 的化学活性小于VO_xH₂O (x= 2, 3)的化学活性.此外, H₂O体系与VO_x之间的结合强弱顺序为 VO₄H₂O > VO₅H₂O > VOH₂O > VO₃H₂O > VO₂H₂O. VOH₂O中离解出H原子的能量为2.88 eV和从VO₅H₂O中离解出OH基团的能量为2.38 eV, 均在可见光能量范围内,这两个化学过程有可能在可见光催化条件下进行.可以通过团簇的红外和拉曼谱特征, 初步判断水分子在VO_xH₂O团簇中是否离解.     

Thermally tunable optical constants of vanadium dioxide thin films measured by spectroscopic ellipsometry

Smart materials with reversible tunable optical constants from visible to near-infrared wavelengths could enable excellent control over the resonant response in metamaterials, tunable plasmonic nanostructures, optical memory based on phase transition and thermally tunable optical devices. Vanadium dioxide (VO₂) is a promising candidate that exhibits a dramatic change in its complex refraction index or complex dielectric function arising from a structural phase transition from semiconductor to metal at a critical temperature of 70 °C. We demonstrated the thermal controllable reversible tunability of optical constants of VO₂ thin films. The optical/dielectric constants showed an abrupt thermal hysteresis which confirms clearly the electronic structural changes. Temperature dependence of dielectric constants as well as optical conductivity of sputtered VO₂ thin films was also reported and compared to previous theoretical and experimental reports.     

二氧化钒纳米棒的相变行为

研究了VO₂(M)纳米棒的金属绝缘体相变(MIT)行为．在VO₂(M)纳米棒的DSC分析曲线上发现了两个MIT,分别位于低温和高温区．低温MIT总是伴随出现VO₂(B)纳米棒,而独立的高温MIT出现在纯VO₂(M)纳米棒．分析和讨论了这两个MIT的机制．     

Urchin-like α-MnO<Subscript>2</Subscript> formed by nanoneedles for high-performance lithium batteries

MnO₂ nanoneedles (NNs) were synthesized by sol-gel assisted by a redox reaction between ascorbic acid and KMnO₄. X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), Raman, far-infrared spectroscopy, and magnetic measurements confirm the tunnel structure of the tetragonal α-MnO₂ phase. The MnO₂ NNs prepared by sol-gel at moderate temperature (T ≈ 350 °C) aggregate with an urchin-like morphology observed by scanning electron (SEM) and high-resolution transmission electron (TEM) microscopy. Electrochemical investigations show an outstanding initial specific capacity ca. 230 mAh g^?1 and 45 % capacity retention at 100th cycle was obtained for these MnO₂ nanoneedles.     

Structure and electrical properties of SrO-borovanadate (V2O5)0.5(SrO)0.5−y(B2O3)y glassses

SrO-borovanadate glasses with nominal composition (V₂O₅)_0.5(SrO)_0.5−y(B₂O₃)_y, 0.0≤y≤0.4 were prepared by a normal quench technique and investigated by direct current (DC) electrical conductivity, inductively coupled plasma (ICP) spectroscopy, infrared (IR) spectroscopy and X-ray powder diffraction (XRD) studies in an attempt to understand the nature of mechanism governing the DC electrical conductivity and the effect of addition of B₂O₃ on the structure and electrical properties of these glasses. XRD patterns confirm the amorphous nature of the present glasses and actual compositions of the glasses were determined by ICP spectroscopy. The temperature dependence of DC electrical conductivity of these glasses has been studied in terms of different hopping models. The IR results agree with previous investigations on similar glasses and it has been concluded that similar to SrO-vanadate glasses, metavandate chain-like structures of SrV₂O₆ and individual VO₄ units also occur in SrO-borovanadate glasses. The SrV₂O₆ and VO_n polyhedra predominate in the low B₂O₃-containing SrO-borovanadate glasses as B substitutes into the V sites of the various VO_n polyhedra and only when the concentration of B₂O₃ exceeds the SrO content do BO_n structures appear. This qualitative picture of three distinct structural groupings for Sr-vanadate and Sr-borovanadate glasses is consistent with the proposed glass structure on previous IR and extended X-ray absorption fine structure (EXAFS) studies on these types of glasses. The conductivity results were analyzed with reference to theoretical models existing in the literature and the analysis shows that the conductivity data are consistent with Mott's nearest neighbor hopping model. Analysis of the conductivity data shows that they are consistent with Mott's nearest neighbor hopping model. However, both Mott VRH and Greaves models are suitable to explain the data. Schnakenberg's generalized polaron hopping model is also consistent with temperature dependence of activation energy. However, various model parameters such as density of states, hopping energy, etc. obtained from the best fits were not found to be in accordance with the prediction of the Mott model.     

Magnetic characterization of vanadium oxide/polyaniline nanotubes

We present a magnetic study of vanadium oxide nanotubes (VO_x-NTs) with polyaniline (PAni). Transmission electron microscopy (TEM) shows the tubular shape and the multi-wall structure of the nanotubes. The static magnetic susceptibility measured at different magnetic fields shows a Curie behavior, while the magnetization versus magnetic field presents a non-linear dependence at low temperatures. Both experiments can be explained by the presence of paramagnetic ions with S = 1/2. Using Electron Paramagnetic Resonance technique, we identified as V⁴⁺(3d¹) the paramagnetic ions present in the nanostructures. All the experimental results can be explained by a fraction between 14% and 18% of V⁴⁺ with respect to the total V atoms in the system.     

Optical limiting in pulsed laser deposited VO2 nanostructures

Being a Mott type oxide, at a temperature of ~ 68 °C and ambient pressure, stoichiometric VO₂ undergoes a first order metal-insulator transition, which is accompanied by a reversible abrupt change in the band gap opening. From an optical point of view, this metal-insulator transition manifests itself by a significant and reversible variation of the refractive index under either a thermal stimuli or by photo-induction. This contribution reports on the ultrafast optical limiting in the IR regime of pulse laser deposited VO₂ nanostructures.     

The influence of PEO on the synthesis and electrochemical properties of VO2 and V3O7·nH2O nanobelts as a cathode for lithium battery

In this paper, we report the hydrothermal synthesis of VO₂, poly(ethylene oxide) (PEO)/VO₂,V₃O₇·nH₂O and PEO/V₃O₇·nH₂O nanobelts by using 1,2-propylene carbonate (1,2-PC (C₄H₆O₃)) and poly(ethylene glycol) (PEG) as templates, respectively. Structure and morphology of the samples were investigated by XRD, FTIR, SEM, and TEM. The vanadium oxide (VO₂) nanobeltcomposite show the initial specific capacity 152?mA?h?g^?1, whereas PEO/VO₂ shows 182?mA?h?g^?1. The pure V₃O₇·nH₂O nanobelts shows the initial specific capacity 192?mA?h?g^?1, while PEO/V₃O₇·nH₂O nanobelts show 297?mA?h?g^?1. It was found that PEO/VO₂ and PEO/V₃O₇·nH₂O nanocomposites show better cyclic performance and high discharge stability compared to pure vanadium oxide nanomaterials. The role of the polymeric PEO component of the hybrid material seems to be the stabilization and improvement of the specific capacity due to probable homogeneous distribution between the nanobelts. The TEM images indicate that PEO works as a surfactant to decrease the dimensions of nanobelts.