Structural characterization of the V2O5/TiO2 system obtained by the sol–gel method

The influence of the vanadium load and calcination temperature on the structural characteristics of the V₂O₅/TiO₂ system was studied by X-ray diffraction and X-ray absorption spectroscopy (XAS) techniques. Samples of the V₂O₅/TiO₂ system were prepared by the sol–gel method under acid conditions and calcined at different temperatures. The rutile phase was found to predominate in pure TiO₂ calcined at 450 °C as a result of the reduction of phase transition temperature promoted by the sol–gel method under acid conditions. The anatase phase became predominant at 450 °C as the amount of vanadium increased from 6 to 9 wt%. A structural change in the TiO₂ phase from predominantly anatase to totally rutile with increased calcination temperature was observed in 6 wt% samples. An analysis of the vanadium X-ray Absorption Near Edge Structure (XANES) spectra showed that the oxidation state of vanadium atoms in the samples containing 6 and 9 wt% of vanadium and calcined at 450 °C was predominantly V⁴⁺. However, the presence of V⁵⁺ atoms cannot be ruled out. A qualitative analysis of extended X-ray absorption fine structure (EXAFS) spectra of the samples containing 6 and 9 wt% of vanadium calcined at 450 °C showed that the local structure around vanadium atoms is comparable to that of VO₂ crystalline phase, in which vanadium atoms are fourfold coordinated in a distorted structure. For the sample after calcination at 600 °C, the EXAFS and XANES results showed that a significant portion of vanadium atoms were incorporated in the rutile lattice with a V_xTi_(1−x)O₂ solid solution formation. The conditions of sample preparation used here to prepare V₂O₅/TiO₂ samples associated with different amounts of vanadium and calcination temperatures proved to be useful to modifying the structure of the V₂O₅/TiO₂ system.     

Conductivity and spectroscopic studies of Li<Subscript>2</Subscript>O-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

Lithium vanadium-borate glasses with the composition of 0.3Li₂O–(0.7-x)B₂O₃–xV₂O₅ (x?=?0.3, 0.325, 0.35, 0.375, 0.4, 0.425, 0.45, and 0.475) were prepared by melt-quenching method. According to differential scanning calorimetry data, vanadium oxide acts as both glass former and glass modifier, since the thermal stability of glasses decreases with an increase in V₂O₅ concentration. Fourier transform infrared spectroscopy data show that the vibrations of [VO₄] structural units occur at V₂O₅ concentration of 45 mol%. It is established that the concentration of V⁴⁺ ions increases exponentially with the growth of vanadium oxide concentration. Direct and alternative current measurements are carried out to estimate the contribution both electronic and ionic conductivities to the value of total conductivity. It is shown that the electronic conductivity is predominant in the total one. The glass having the composition of 0.3Li₂O-0.275B₂O₃-0.475V₂O₅ shows the highest electrical conductivity that has the value of 7.4?×?10^?5 S cm^?1 at room temperature.     

DO<Subscript>3</Subscript>-type cubic structure of nonstoichiometric vanadium monoxide

An X-ray diffraction study indicates that nonstoichiometric vanadium monoxide VO _y ≡ V _x O _z (y = z/x) has a cubic structure of the DO₃ type (space group Fm $ \bar 3 $ \bar 3 m), where vanadium atoms are not only at the 4(a) sites of the metal fcc sublattice, but also at the tetrahedral 8(c) sites. This circumstance fundamentally distinguishes monoxide VO _y from strongly nonstoichiometric MX _y compounds with the B1 structure and the same space group, where atoms M and X and structural vacancies ▪ and ▭ of the metal and nonmetal sublattices, respectively, are distributed over the 4(a) and 4(b) sites. The dependence of the filling factor q of the tetrahedral interstices by vanadium atoms on the relative content y of oxygen in VO _y has been obtained. It has been shown that the composition of cubic vanadium monoxide should be represented as VO _y ≡ V _x O _z ≡ V _{x − 2q} V_2q ^(t)▪_{1 − x + 2q} O _z ▭_{1 − z} , taking into account the structure.     

High‐performance a‐Si1–xOx:H/c‐Si heterojunction solar cells realized by the a‐Si:H/a‐Si1–xOx:H stack buffer layer

We used amorphous silicon oxide (a‐Si_1–xO_x:H) and microcrystalline silicon oxide (µc‐Si_1–xO_x:H) as buffer layer and p‐type emitter layer, respectively, in n‐type silicon hetero‐junction (SHJ) solar cells. We proposed to insert a thin (2 nm) intrinsic amorphous silicon (a‐Si:H) thin film between the thin (2.5 nm) a‐Si_1–xO_x:H buffer layer and the p‐layer to form a stack buffer layer of a‐Si:H/a‐Si_1–xO_x:H. As a result, a high open‐circuit voltage (V_OC) and a high fill factor (FF) were obtained at the same time. Finally, a high efficiency of 19.0% (J_SC = 33.46 mA/cm², V_OC = 738 mV, FF = 77.0%) was achieved on a 100 μm thick polished wafer using the stack buffer layer.

     

Low‐temperature synthesis and Raman scattering of Mn‐doped ZnO nanopowders

Nanocrystalline Mn‐doped zinc oxides Zn_1−xMn_xO (x = 0–0.10) were synthesized by the sol–gel technique at low temperature. The calcination temperature of the as‐prepared powder was found at 350 °C using differential thermal analysis. A thermogravimetric analysis showed that there is a mass loss in the as‐prepared powder till 350 °C and an almost constant mass till 800 °C. The X‐ray diffraction patterns of investigated nanopowders calcined at 350 °C correspond to the hexagonal ZnO structure without any foreign impurities. The average grain size of the nanocrystal that was observed around ∼25–40 nm from transmission electron microscopy matched well with the crystallite size calculated from the line shape of X‐ray diffraction. The chemical bonding structure in Zn_1−xMn_xO nanopowders was examined using X‐ray photoelectron spectroscopy techniques, which indicate substitution of Mn²⁺ ions into Zn²⁺ sites in ZnO lattice. Micro Raman spectroscopy confirmed the insertion of Mn ions in the ZnO host matrix, and similar wurtzite structure of Zn_1−xMn_xO (x < 10%) nanocrystals. Temperature‐dependent Raman spectra of the nanocrystals displayed suppression of luminescence and enhancement in full width at half maximum in pure ZnO nanocrystals with increase in temperature, which suggests an enhancement in particle size at elevated temperature. Copyright © 2009 John Wiley & Sons, Ltd.     

Ordered monoclinic vanadium suboxide V<Subscript>14</Subscript>O<Subscript>6</Subscript>

The monoclinic (space group C2/m) superstructure of the suboxide V₁₄O₆, which is formed as a result of the atomic and vacancy ordering of the tetragonal solid solution of oxygen in vanadium, is investigated using X-ray diffraction and symmetry analysis. The monoclinic suboxide V₁₄O₆ is observed in the vanadium oxide samples VO_0.57, VO_0.81, and VO_0.86 synthesized at 1770 K and the samples VO _y (0.87 ≤ y ≤ 0.98) additionally annealed at 1470 K after the synthesis. It is established that the channel of the disorder-order phase transition associated with the formation of the monoclinic suboxide V₁₄O₆ includes six superstructure vectors belonging to three non-Lifshitz stars of one type {k ₁}. The distribution function of the oxygen atoms in the monoclinic superstructure of the suboxide V₁₄O₆ is calculated. It is demonstrated that the displacements of vanadium atoms distort the body-centered tetragonal metal sublattice, thus preparing the formation of the facecentered cubic sublattice and the transition from the suboxide V₁₄O₆ to the cubic vanadium monoxide with the B1 structure.     

Vanadium oxide thin films deposited on indium tin oxide glass by radio—frequency magnetron sputtering

Highly oriented VO₂(B), VO₂(B) + V₆O₁₃ films were grown on indium tin oxide glass by radio-frequency magnetron sputtering. Single phase V₆O₁₃ films were obtained from VO₂(B) +V₆O₁₃ films by annealing at 480℃ in vacuum. The vanadium oxide films were characterized by x-ray diffraction and x-ray photoelectron spectra (XPS). It was found that the formation of vanadium oxide films was affected by substrate temperature and annealing time, because high substrate temperature and annealing were favourable to further oxidation. Therefore, the formation of high valance vanadium oxide films was realized. The V₆O₁₃ crystalline sizes become smaller with the increase of annealing time. XPS analysis revealed that the energy position for all the samples was almost constant, but the broadening of the V_2p3/2 line of the annealed sample was due to the smaller crystal size of V₆O₁₃.     

Vanadium-Al2O3 nanostructured thin films prepared by pulsed laser deposition: Optical switching

The formation and optical response of VO_x nanoparticles embedded in amorphous aluminium oxide (Al₂O₃) thin films by pulsed laser deposition is studied. The thin films have been grown by alternate laser ablation of V and Al₂O₃ targets, which has resulted in a multilayer structure with embedded nanoparticles. The V content has been varied by changing the number of pulses on the V target. It is found that VO_x nanoparticles with dimensions around 5 nm have been formed. The structural analysis shows that the vanadium nanoparticles are oxidized, although probably there is not a unique oxide phase for each sample. The films show a different optical response depending on their vanadium content. Optical switching as a function of temperature has been observed for the two films with the highest vanadium content, at transition temperatures of about −20 °C and 315 °C thus suggesting the presence of nanoparticles with compositions V₄O₇ and V₂O₅, respectively.     

Oxygen vacancy induced ferromagnetism in rutile TiO2–x

First‐principles LDA + U calculations have been performed to study the effects of oxygen vacancies (V_O) on the electronic structure and magnetism in undoped rutile TiO_2–x . Instead of treated as an adjustive parameter, the value of U was determined by constrained‐density‐functional calculations. The calculated electronic structure reveals that the valence electrons released by V_O would occupy mainly the neighboring Ti:3d orbital which then becomes spin‐polarized due to intra‐atomic exchange interaction, thereby giving rise to the half‐metallic ferromagnetism. The magnetization induced by V_O in rutile TiO_2–x is almost proportional to the V_O concentration (x) for x > 0.0625, and becomes 0 for x ≤ 0.0417. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Wet‐Etching Induced Abnormal Phase Transition in Highly Strained VO2/TiO2 (001) Epitaxial Film

The metal–insulator transition (MIT) behavior in vanadium dioxide (VO₂) epitaxial film is known to be dramatically affected by interfacial stress due to lattice mismatching. For the VO₂/TiO₂ (001) system, there exists a considerable strain in ultra‐thin VO₂ thin film, which shows a lower T_c value close to room temperature. As the VO₂ epitaxial film grows thicker layer‐by‐layer along the “bottom‐up” route, the strain will be gradually relaxed and T_c will increase as well, until the MIT behavior becomes the same as that of bulk material with a T_c of about 68 °C. Whereas, in this study, we find that the VO₂/TiO₂ (001) film thinned by “top‐down” wet‐etching shows an abnormal variation in MIT, which accompanies the potential relaxation of film strain with thinning. It is observed that even when the strained VO₂ film is etched up to several nanometers, the MIT persists, and T_c will increase up to that of bulk material, showing the trend to a stress‐free ultra‐thin VO₂ film. The current findings demonstrate a facial chemical‐etching way to change interfacial strain and modulate the phase transition behavior of ultrathinVO₂ films, which can also be applied to other strained oxide films.     

Preparation of (As x Co3−x O4)(x=0–0.024) nanoparticles by rheological phase reaction method

Nanoparticles of a series of arsenic–cobalt mixed valency spinel oxides of theoretical formula As _x Co_3?x O₄, (x=0, 0.005, 0.01, 0.015, 0.024) have been successfully prepared by the rheological phase reaction and the pyrolysis method. The products were characterized by X-ray powder diffraction, scanning electron microscope, thermogravimetric analysis and simultaneous differential thermal analysis. Calcination of the precursor at 500 ^°C resulted in the formation of arsenic-doped cobalt oxide nanoparticles of 48 nm in crystal size. The effect of the calcination temperature on the crystal size of arsenic-doped Co₃O₄ was discussed.     

Study on electrical conductivity and phase transition in singly doped BIPBVOX (Bi2V1−xPbxO5.5−x/2) solid electrolyte

Samples of bismuth lead vanadium oxide (BIPBVOX) (Bi₂V_1–xPb_xO_5.5–x/2) singly substituted system in the composition range 0.05 ≤ x ≤ 0.20 were prepared by sol–gel synthesis route. Structural investigations were carried out by using a combination of differential thermal analysis (DTA) and powder X-ray diffraction (PXRD) technique. Energy dispersive X-ray spectroscopy analysis (EDXA) of doped samples was carried out to predict the sample purity and doping concentration. Transitions, α?β, β?γ and γ′?γ were detected by XRD, DTA and variation in the Arrhenius plots of conductivity. The ionic conductivity was measured by AC impedance spectroscopy. The solid solutions with composition x ≤ 0.07 undergo α?β phase transition, at 329 °C and β?γ phase transition at 419 °C. The highly conducting γ′-phase was effectively stabilized at room temperature for compositions with x ≥ 0.17 whose thermal stability increases with Pb content. At 300 °C, the highest value of conductivity 6.234 × 10^?5 S cm^?1 was obtained for composition x = 0.15 and at 600 °C the highest value of conductivity 0.65 S cm^?1 is observed for x = 0.17. AC impedance plots reveal that the conductivity is mainly due to the grain contribution to oxide ion conductivity.     

XPS study of vanadium surface oxidation by oxygen ion bombardment

Oxidation of vanadium metal surfaces at room temperature by low-energy oxygen ion beams is investigated by X-ray photoelectron spectroscopy (XPS). It is observed that ion-beam irradiation of clean V results in formation of thin oxide layer containing vanadium in oxidation states corresponding to VO, V₂O₃, VO₂ and V₂O₅ oxides. The composition of the products of ion-beam oxidation depends markedly on oxygen ion fluence. The results of angle-resolved XPS measurements are consistent with a structure of oxide film with the outermost part enriched in V₂O₅ and VO₂ oxides and with V₂O₃ and VO oxides located in the inner region of the oxide layer.     

Hydrothermal route to VO<Subscript>2</Subscript> (B) nanorods: controlled synthesis and characterization

One-dimensional vanadium dioxides have attracted intensive attention owing to their distinctive structure and novel applications in catalysis, high energy lithium-ion batteries, chemical sensors/actuators and electrochemical devices etc. In this paper, large-scale VO₂ (B) nanorods have been successfully synthesized via a versatile and environment friendly hydrothermal strategy using V₂O₅ as vanadium source and carbohydrates/alcohols as reductant. The obtained samples are characterized by XRD, FT-IR, TEM, and XPS techniques to investigate the effects of chemical parameters such as reductants, temperature, and time of synthesis on the structure and morphology of products. Results show that pure B phase VO₂ with homogeneous nanorod-like morphology can be prepared easily at 180 °C for 3 days with glycerol as reluctant. Typically, the nanorod-like products are 0.5–1 μm long and 50 nm width. Furthermore, it is also confirmed that the products are consisted of VO₂, corresponding to the B phase. More importantly, this novel approach is efficient, free of any harmful solvents and surfactants. Therefore, this efficient, green, and cost-saving route will have great potential in the large-scale fabrication of 1D VO₂ (B) nanorods from the economic and environmental point of view.     

Composition dependent self‐regenerative property of perovskite‐type oxides

The self‐regenerative property of LaCo_{1–x –y}Pd_x Zn_y O_3±δ and LaFe_{1–x –y}Pd_x Zn_y O_3±δ solid solutions with monometallic Pd or bimetallic Pd/Zn substituents for Co or Fe is studied under a redox cycle by high angular annular dark‐field scanning transmission electron microscopy (STEM‐HAADF) and energy dispersive X‐ray spectroscopy (EDX) and X‐ray diffraction (XRD). These results reveal that the composition of perovskites determines the self‐regenerative property that occurs largely in LaCo_{1–x –y}Pd_x Zn_y O_3±δ but is limited greatly in LaFe_{1–x –y}Pd_x Zn_y O_3±δ. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

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.     

Activation energy and conductivity of glasses in the P2O5–V2O5–Li2O system

The conductivity of glasses in the 50\textP_\text2 \textO_\text5 - x\textV_\text2 \textO_\text5 - ( 50 - x )\textLi_\text2 \textO50{\text{P}}_{\text{2}} {\text{O}}_{\text{5}} - x{\text{V}}_{\text{2}} {\text{O}}_{\text{5}} - \left( {50 - x} \right){\text{Li}}_{\text{2}} {\text{O}} system was studied as a function of temperature and composition. For all compositions, the conductivity variation as a function of temperature followed an Arrhenius type relationship. Isothermal variation of conductivity as a function of composition showed a minimum for a molar ratio x near 20. Probable mechanisms for decrease of conductivity with decrease of vanadium oxide concentration were explained. The minimum in room temperature was attributed to increase of V⁴⁺/V⁵⁺ with decrease of vanadium oxide in specific concentrations of vanadium oxide. Activation energy increased with decrease of V₂O₅ content. This behavior was attributed to increase of average spacing between vanadium ions.     

Highly stable amorphous indium‐zinc‐oxide thin‐film transistors with back‐channel wet‐etch process

The stabilities of amorphous indium‐zinc‐oxide (IZO) thin film transistors (TFTs) with back‐channel‐etch (BCE) structure are investigated. A molybdenum (Mo) source/drain electrode was deposited on an IZO layer and patterned by hydrogen peroxide (H₂O₂)‐based etchants. Then, after etching the Mo layer, SF₆ plasma with direct plasma mode was employed and optimized to improve the bias stress stability. Scanning electron microscopy and X‐ray photoelectron spectroscopic analysis revealed that the etching residues were removed efficiently by the plasma treatment. The modified BCE‐ TFTs showed only threshold voltage shifts of 0.25 V and –0.20 V under positive/negative bias thermal stress (P/NBTS, V_GS = ±30 V, V_DS = 0 V and T = 60 °C) after 12 hours, respectively. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The study of deNOx catalyst in low temperature using nano-sized supports

The present study pertains to a vanadium/titania-based catalyst for removing nitrogen oxides at a relatively low temperature window. More specially, the present study relates to a vanadium/titania-based catalyst containing VO_x (x < 2.5) and having excellent ability to remove nitrogen oxides at a wide temperature window, particularly at a relatively low temperature window and a process for removing nitrogen oxides using the same. In this study, various TiO₂ supports have been tested to determine the role of support. Raw TiO₂ were examined a variety of physical properties. Also comparing with commercial V₂O₅/TiO₂ catalyst, the activity of various VO_x (x < 2.5)/TiO₂ in this study have quite different values.To find the source of lattice oxygen in vanadium oxides, the effect of calcination conditions on the removal efficiency of nitrogen oxides was examined. When nitrogen instead of air was introduced as a balance gas in calcination step, the activity of catalysts in this study was not changed. That may indicate the source of lattice oxygen in vanadium oxides as that of TiO₂. The results of X-ray photoelectron spectroscope (XPS) revealed that after vanadium oxides loaded the support, TiO₂ was reduced to Ti₂O₃, etc. In the test of calcination temperature of a variety of vanadium/titania-based catalysts, it has been found that TiO₂ supports affects the optimal calcination temperature, indicating that the difference of crystal structure, defect and binding energy in TiO₂ may make inherent VO_x (x < 2.5)/V₂O₅ molar ratios, respectively. Its ratio seems to be an index of activity.