Formation of V2O3 nanocrystals by thermal reduction of V2O5 thin films

We report the formation of homogeneous and stable V₂O₃ nanocrystals, directly from V₂O₅ thin films, at 600 °C, as observed by using in situ electron microscopy experiments. Thermally-induced reduction of V₂O₅ thin films in vacuum is remarkably different when compared to reduction of V₂O₅ single crystals and results in the formation of nanophase V₂O₃. Thermally grown V₂O₃ nanocrystals exhibit hexagon or square shape and are stable at higher temperature as well as room temperature. The formation of stable nanocrystals through the reduction process in a non-chemical environment (vacuum) could provide a basis for understanding the complex processes of vanadium oxide phase transitions and for controlling the chemical processes to produce oxide nanocrystals.     

X-ray emission spectra and electron structure of neutron irradiated V3Si

The results of the X-ray VKβ₅ emission spectra of V₃Si irradiated with fast neutrons are reported. A comparison with the spectrum for unirradiated V₃Si as well as with cluster calculations suggested that the changes observed in the electron structure of irradiated V₃Si result from antisite defects and inhomogeneous atomic distribution.     

Thermal and low energy electron bombardment induced oxygen loss of V2O5 single crystals: Transition into V6O13

Thermal treatment in UHV of clean V₂O₅ single crystals results in homogeneous oxygen loss, involving a rate-limiting surface reaction. Depending upon the pretreatment, aircleaved samples transform topotactically into V₆O₁₃, or into what we call a phase Q of probable composition V₄O₉ or V₆O_13.5. Low energy electron bombardment of clean UHV-cleaved V₂O₅(010) surfaces produces the transition V₂O₅ → V₆O₁₃ at room temperature. This effect is attributed to electron beam stimulated reactions. The influence on the transition of carbon-containing impurities is discussed. The nucleation of V₆O₁₃ on V₂O₅ is explained by a model based on a surface reaction, the rate of which is enhanced by the interaction with contaminating molecules and low energy electron bombardment. The presence of shear planes at the boundary between V₂O₅ and the V₆O₁₃ nuclei locally enhances the oxygen loss rate and allows the V₆O₁₃ nuclei to grow into the bulk.The enhanced mobility of the oxygen at these boundaries is thought to influence favorably the oxidation-regeneration rate of the V₂O₅-catalyst.     

Characterization and electrical properties of V2O5-CuO-P2O5 glasses

Characterization and electrical properties of vanadium-copper-phosphate glasses of compositions xV₂O₅-(40−x)CuO-60P₂O₅ have been reported. X-ray diffraction (XRD) confirms the amorphous nature of these glasses. It was observed that, the density (d) decreases gradually while the molar volume (V_m) increases with the increase of the vanadium oxide content in such glasses. This may be due to the effect of the polarizing power strength, PPS, which is a measure of ratio of the cation valance to its diameter. The dc conductivity increases while the activation energy decreases with the increase of the V₂O₅ content. The dc conductivity in the present glasses is electronic and depends strongly upon the average distance, R, between the vanadium ions. Analysis of the electrical properties has been made in the light of small polaron hopping model. The parameters obtained from the fits of the experimental data to this model are reasonable and consistent with glass composition. The conduction is attributed to non-adiabatic hopping of small polaron.     

基于旋涂法和电子束蒸发法制备的V2O5/Ag/V2O5叠层透明导电薄膜

利用溶胶-凝胶技术与电子束蒸镀相结合的方法在常温下制备了叠层V₂O₅/Ag/V₂O₅（VAV）透明导电薄膜,研究了各层薄膜厚度对叠层结构光电特性的影响。用原子力显微镜、紫外-可见光分光光度计、四探针电阻仪及开尔文探针对样品的表面形貌、光电性能及功函数等性质进行了表征。实验结果表明,该薄膜具有良好的光学和电学性质,可见光（380~780 nm）平均透过率达75%,迁移率为16.89 cm²/（V·s）,载流子浓度为－1.043×10²² cm^-3,方块电阻值为15.1 Ω/□,功函数为5.17 eV。该制备方法降低了V₂O₅薄膜的工艺制备难度,为该材料在太阳能电池中的应用创造了良好的前期基础。     

基于旋涂法和电子束蒸发法制备的V_2O_5/Ag/V_2O_5叠层透明导电薄膜

利用溶胶-凝胶技术与电子束蒸镀相结合的方法在常温下制备了叠层V2O5/Ag/V2O5(VAV)透明导电薄膜,研究了各层薄膜厚度对叠层结构光电特性的影响。用原子力显微镜、紫外-可见光分光光度计、四探针电阻仪及开尔文探针对样品的表面形貌、光电性能及功函数等性质进行了表征。实验结果表明,该薄膜具有良好的光学和电学性质,可见光(380~780 nm)平均透过率达75%,迁移率为16.89 cm2/(V·s),载流子浓度为-1.043×1022cm-3,方块电阻值为15.1Ω/□,功函数为5.17 eV。该制备方法降低了V2O5薄膜的工艺制备难度,为该材料在太阳能电池中的应用创造了良好的前期基础。     

Angular dependence of auger electron emission from Cu (111) and (100) surfaces

Experimental angular emission profiles of the M_2,3M_4,5M_4,562 eV Cu Auger electrons from clean copper (100) and (111) surfaces in several azimuths are presented. A simple single scattering theory to account for elastic scattering of the Auger electrons by other ion cores in the solid is presented, and calculations have been performed to assess the importance of this process in contributing to the observed angular dependence. These calculations produce angular structure having a similar magnitude and temperature dependence to that observed experimentally, and some featural similarity in peak positions or shapes. It appears that elastic scattering is an important source of angular dependence, and that studies of adsorbed species on surfaces should provide a very sensitive method of surface structure determination.     

Electronic properties of single-walled V2O5 nanotubes

Atomistic models of quasi-one-dimensional vanadium pentoxide nanostructures—single-walled nanotubes formed by rolling (010) layers of V₂O₅ are constructed and their electronic properties and bond indices are studied using the tight-binding band method. We show that all zigzag (n,0)- and armchair (n,n)-like nanotubes are uniformly semiconducting, and the band gap trends to vanish as the tube diameters decrease. The V-O covalent bonds were found to be the strongest interactions in V₂O₅ tubes, whereas V-V bonds proved to be much weaker.     

Preparation effects on the spectroscopic properties of V2O5-GeO2 glass

The spectroscopic studies of V₂O₅-GeO₂ (45:55) glasses fused and equilibrated at various temperatures (T) in air were measured. Their spectroscopic properties are shown to be sensitive to microstructure. This observation is consistent with previously reported results that the structure of glasses depend upon the temperature at which the glass was fused and equilibrated.     

Substrate effects on V2O3 thin films

We apply density functional theory and the augmented spherical wave method to analyze the electronic structure of V₂O₃ in the vicinity of an interface to Al₂O₃. The interface is modeled by a heterostructure setup of alternating vanadate and aluminate slabs. We focus on the possible modifications of the V₂O₃ electronic states in this geometry, induced by the presence of the aluminate layers. In particular, we find that the tendency of the V 3d states to localize is enhanced and may even cause a metal-insulator transition.     

Dielectric dispersion in PbO-Sb2O3-As2O3:V2O5 glasses

Dielectric properties, viz. dielectric constant ε′, loss tan δ and a.c conductivity σ_ac (over a wide range of frequency and temperature) and dielectric breakdown strength of PbO-Sb₂O₃-As₂O₃ glasses doped with V₂O₅ (ranging from 0 to 0.5 mol%) are studied. Analysis of these results, based on optical absorption and ESR spectra, indicates that the insulating strength of the glasses is comparatively high when the concentration of V₂O₅ is about 0.3 mol% in the glass matrix.     

碳热还原法制备V2O3-C双层包覆磷酸铁锂及其电化学性能

通过V₂O₅的碳热还原反应制备了具有优异倍率性能和循环稳定性的V₂O₃-C双层包覆的磷酸铁锂正极材料. 粉末X射线衍射、元素分析、高分辨投射电镜和拉曼光谱研究表明V2O3相与碳层共包覆于磷酸铁锂颗粒表面. 在V₂O₅的碳热还原反应后,碳含量明显降低,但石墨化程度未发生明显改变. 电化学测试结果表明少量V₂O₃显著改善了磷酸铁锂正极材料的倍率性能和高温循环性能,包含1%氧化钒的复合正极材料在0.2 C放电容量为167 mAh/g,5 C时放电容量为129 mAh/g,并且循环稳定性优异;在55 oC和1 C时放电容量为151 mAh/g,循环100次后无明显容量衰减.     

Molecular model for interstitial impurities in V2O5

A simple molecular model, including the four nearest vanadium and oxygen neighbours of an interstitial impurity in V₂O₅, is presented. The main result of the present model calculations are the symmetry of the ground state, and an order of magnitude for the defect level energies with respect to the bottom of the conduction band. These two results allow for an interpretation of the infrared spectrum of V₂O₅ doped with Cu or alkali-ions.     

A simple statistical model for V2O5 catalysis

In this work we propose a model to describe the selective oxidation of hydrocarbons at the surface of the V₂O₅ catalyst. The main ingredients of the model are the concentration of vanadium active sites, the surface and bulk diffusion rates of oxygen vacancies and the probability rate of a hydrocarbon reaction. The reactions take place at the free V₂O₅ (0 1 0) surface, and the diffusion of vacancies occur along the [0 1 0] (bulk) and [0 0 1] (surface) directions. The coupling between V₂O₅ and a given metal oxide support determines the concentration of the active vanadium sites, where the reactions can occur. Only the oxygen atoms, which are coordinated to three vanadium sites, take part of the oxidation process. In our calculations we employed two different approaches, single site and pair approximations, and some Monte Carlo simulations. We have found the dependence of the critical concentration of vacancies on the diffusion rates, probability of reaction, and fraction of active vanadium sites, for the catalyst to operate in an active steady state.     

The Raman spectra and structure of pure vitreous P2O5

We report the polarized Raman spectra of pure anhydrous vitreous P₂O₅ and conclude that this glass is a 3-connected network in which each P atom is surrounded almost tetrahedrally by three bridging O atoms and one double-bonded O terminator. Vibrational calculations on an anchored fragment are used to determine atomic motions in the higher frequency modes; these calculations confirm that the dominant Raman modes involve in-phase compression of neighboring PO bonds.     

Photoelectron spectroscopic study of Li oxides on Li over-deposited V2O5 thin film surfaces

The Li oxides species formed on Li over-deposited V₂O₅ thin film surfaces have been studied by using X-ray and UV induced photoelectron spectroscopy (XPS and UPS). The photoelectron spectroscopic data show that the Li over-deposited V₂O₅ system itself is not stable. Further chemical decomposition reactions are taken place even under UHV conditions and lead to form Li₂O and Li₂O₂ compounds on the surface. The formation of Li₂O₂ causes to arise an emission line at about 11.3 eV in the valence band spectra.     

XAS and electrochemical characterization of lithium intercalated V2O5 xerogels

The use of sol-gel processes in the preparation of cathode materials is of growing interest because of their ease and flexibility. The electrochemical properties, e.g. the rate of lithium intercalation, appear to depend on the morphology of the thin-film vanadium oxide xerogels that can be changed by modifying the preparation. In this context, in order to extend the study to bulk materials, xerogel powder samples with surface areas in the range 2–5 m²/g have been prepared from pure vanadium pentoxide hydrogels, or in the form of composites, from carbon powder added to hydrogels. The electrochemical properties have been correlated with the morphological and structural changes induced by the presence of carbon using X-ray and XAS spectroscopy.     

Observation of the V2O5(001) surface using ambient atomic force microscopy

Constant force images of the V₂O₅(001) surface were recorded in ambient conditions with atomic force microscopy. All images exhibit the 11.5 Å × 3.5 Å. periodicity expected for a bulk terminated surface. However, images reveal differences from the ideal structure. The experimental results are interpreted in terms of preferential adsorption sites for water molecules. Because these sites are thought to influence the catalytic properties of the surface, their characterization is an important step towards understanding how the atomic-scale structure of a surface influences its properties.     

Microstructural, dielectric and spectroscopic properties of Li2O-Nb2O5-ZrO2-SiO2 glass system crystallized with V2O5

Li₂O-Nb₂O₅-ZrO₂-SiO₂ glasses mixed with different concentrations of V₂O₅ were crystallized. The samples were characterized by XRD, SEM and DTA techniques. The SEM pictures indicated that the samples contain well defined and randomly distributed crystal grains. The X-ray diffraction studies have revealed the presence of several crystalline phases in these samples. Optical absorption, ESR and photoluminescence spectral studies on these samples have indicated that a considerable proportion of vanadium ions do exist in V⁴⁺ state in addition to V⁵⁺ state and the redox ratio seems to be increasing with increase in the concentration of crystallizing agent V₂O₅. The infrared spectral studies have pointed out the existence of conventional SiO₄, ZrO₄, NbO₆, VO structural units in the glass ceramic network. The study of dielectric properties suggested a decrease in the insulating character of the glass ceramics with increase in the crystallizing agent. A.C. conductivity in the high temperature region seems to be connected mainly with the polarons involved in the process of transfer from V⁴⁺↔V⁵⁺ ions.