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.     

An XPS study on the surface reduction of V2O5(0 0 1) induced by Ar ion bombardment

The effect of the irradiation with Al Kα X-rays during an XPS measurement upon the surface vanadium oxidation state of a fresh in vacuum cleaved V₂O₅(0 0 1) crystal was examined. Afterwards, the surface reduction of the V₂O₅(0 0 1) surface under Ar⁺ bombardment was studied. The degree of reduction of the vanadium oxide was determined by means of a combined analysis of the O1s and V2p photoelectron lines. Asymmetric line shapes were needed to fit the V³⁺2p photolines, due to the metallic character of V₂O₃ at ambient temperature. Under Ar⁺ bombardment, the V₂O₅(0 0 1) crystal surface reduces rather fast towards the V₂O₃ stoichiometry, after which a much slower reduction of the vanadium oxide occurs.     

Dispersion of V2O5 supported on a TiO2 by x-ray photoelectron spectroscopy surface

The dispersion of V₂O₅ in V₂O₅/TiO₂ catalysis has been studied by x-ray photoelectron spectroscopy, transmission electron microscopy and a thermal gravimetric analysis for samples with various V₂O₅ content. The coverage, thickness and dispersion of vanadium oxide were calculated by a quantitative x-ray photoelectron spectroscopy (XPS) analysis. The experimental results show that V₂O₅ is well dispersed on a TiO₂ surface at low VEOs content. The coverage increases rapidly with the V₂O₅ content up to 10 wt%. Beyond that, the coverage increases very slowly, and considerable parts of the TiO₂ surface remain bare even at a V₂O₅ content as high as 40 wt%. The vanadium oxide is probably present on the TiO₂ surface as isolated patches or islands. This investigation shows that x-ray photoelectron spectroscopy is also a powerful tool for obtaining information about the dispersion of supported species in catalytic research.     

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.     

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.     

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.     

Spin glass phase in the iron vanadium oxide bronze θ − Fe0.35V2O5

Low temperature magnetic susceptibility measurements performed on Fe_0.35V₂O₅ and Al_0.35V₂O₅ indicate for the first time for vanadium oxide bronzes that the magnetically ordered spin glass phase occurs in the iron bronze below 8 K.     

Chemical stability and surface stoichiometry of vanadium oxide phases studied by reactive molecular dynamics simulations

Compositional stability of various vanadium oxides and oxide growth on vanadium surfaces have been studied using reactive molecular dynamics simulation methods. Vanadium dioxide (VO₂), sesquioxide (V₂O₃), pentoxide (V₂O₅), and hexavanadium tridecaoxide (V₆O₁₃) are studied in bulk crystalline and thin film structures, investigating charge distribution and pair distribution functions of particle interactions. The stability is estimated to be pentoxide, hexavanadium tridecaoxide, sesquioxide, and dioxide respectively in decreasing order in thin film structures. We then analyze oxide growth kinetics on vanadium (100) and (110) surfaces. The oxidation rate, stoichiometry, charge distribution, and the effect of surface orientation on kinetic phenomena are noted. In the early stages of surface oxidation of our simulation configurations, sesquioxide is found to be the dominant component. The modeling and simulation results are compared with experiments where available.     

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.     

From crystalline V2O5 to nanostructured vanadium oxides using aromatic amines as templates

Nanoneedles, nanorods of B-VO₂, and vanadium oxide nanotubes with high crystallinity were synthesized via a one-step hydrothermal treatment using crystalline V₂O₅ as a precursor and aromatic amines (C₆H₅-(CH₂)_n-NH₂ with n=0, 1, 3) as structure-directing templates. Samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), thermal analysis, nitrogen adsorption/desorption isotherms and infrared spectroscopy. Nanoneedles, 0.5-5 μm in length and about 50 nm in average diameter and VO₂(B) nanorods about 20-100 nm wide and up to 2.5 μm long, have been obtained. The inner and the outer diameters of the vanadium oxide nanotubes vary, respectively, between 15-25 and 70-100 nm with a length up to 4 μm.     

Vanadium oxide thin films produced by magnetron sputtering from a V2O5 target at room temperature

Vanadium oxide thin films were grown by RF magnetron sputtering from a V₂O₅ target at room temperature, an alternative route of production of vanadium oxide thin films for infrared detector applications. The films were deposited on glass substrates, in an argon–oxygen atmosphere with an oxygen partial pressure from nominal 0% to 20% of the total pressure.X-ray diffraction (XRD) and X-ray photon spectroscopy (XPS) analyses showed that the films were a mixture of several vanadium oxides (V₂O₅, VO₂, V₅O₉ and V₂O₃), which resulted in different colors, from yellow to black, depending on composition. The electrical resistivity varied from 1 mΩ cm to more than 500 Ω cm and the thermal coefficient of resistance (TCR), varied from −0.02 to −2.51% K⁻¹.Computational thermodynamics was used to simulate the phase diagram of the vanadium–oxygen system. Even if plasma processes are far from equilibrium, this diagram provides the range of oxygen pressures that lead to the growth of different vanadium oxide phases. These conditions were used in the present work.     

Synthesis, characterization and applications of nanostructural/nanodimensional metal oxides

Molybdenum oxide nanorods (MOx-NR) and vanadium oxide nanotubes (VOx-NT) have been prepared using MoO₃ and V₂O₅ powders as precursors and hexadecylamine as surfactant via hydrothermal route. Porous nanocrystalline MgO powder has been prepared by a simple and instantaneous solution combustion process using corresponding magnesium nitrate as oxidizer and glycine as fuel. The compounds are characterized by XRD, TG-DTA, SEM, TEM, surface area and porosity measurements. Because of the porous nature having large surface area (107 m²/g) with nanodimension (12-23 nm), MgO powder has been successfully employed as defluoridizing agent for the removal of fluoride (75%) in ground water     

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.     

The effect of thermal annealing and laser irradiation on the microstructure of vanadium oxide nanotubes

The microstructure of vanadium oxide nanotubes (VONTs) have been characterized using FTIR spectroscopy and Raman spectroscopy. The temperature effects on the VONTs were studies by changing the laser irradiation power and thermal annealing temperature in air. Raman spectroscopy studies showed that the VONTs could be decomposed even at low laser power irradiation. Also, together with scanning electron microscopy, it was found that thermal annealing in air could lead to the collapse of the tubular structure and convert the nanotubes into V₂O₅ nanoparticle. It was found that the thermal stability of VONTs was relatively low and the tubular morphology was destroyed at temperatures higher than 300 °C. The spectroscopic analyses showed that the Raman signature of the VONTs could be established for probing tubular structure.     

A synchrotron XPS study of the vanadia–titania system as a model for monolayer oxide catalysts

The deposition of vanadium metal onto stoichiometric TiO₂(110) has been studied with photoelectron spectroscopy from low to high coverages of vanadium. A synchrotron source was employed in XPS experiments for the study of submonolayer coverages of vanadium in order to determine the oxidation state of the vanadia species formed at submonolayer coverages. The exposure of the titania surface to vanadium metal results in charge-transfer from vanadium to titania at the interface. At low doses of the metal vigorous interaction between the metal and titania surface yields reduction of the Ti⁴⁺ species to Ti³⁺ at the interface, as evidenced by both changes in the lineshape of the Ti 2p XPS spectra and Ti LIII-edge spectra. Concurrent with this surface reduction vanadium metal is oxidized. At higher vanadium doses the vanadium 2p binding energy indicates the formation of metallic vanadium. When metallic vanadium deposition is followed by exposure of the surface to oxygen, only one vanadium species remains on the surface, the binding energy of which corresponds to that of the oxide present initially at low doses of vanadium metal. By comparison of the V 2p binding energies to those of bulk oxides, it appears that the oxidation state of the vanadium in the oxide species is +3, suggesting the formation of V₂O₃ on the surface. Vanadium LIII-edge data also suggest that V₂O₅ is not formed by the oxidation of predosed vanadium metal.     

Influence of surface modification by vanadium oxide and carbon on the electrochemical performance of LiFePO4/C

Surface modification with metal oxides is an efficient method to improve the performance of LiFePO₄. Carbon and V₂O₃ co-coated LiFePO₄ is synthesized by carbothermal reduction method combined with star-balling technique, and vanadium oxide is produced in situ. The structure and pattern of LiFePO₄/C modified with different amounts of vanadium oxide (0–5 mol%) were studied by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and micro-Raman spectroscopy. The electrochemical performance of material electrodes was analyzed by constant current charge–discharge and electrochemical impedance spectra (EIS). Electrochemical test results show that sample B (1.0 mol%) exhibits the best electrochemical performance, whose discharge capacity is up to 160.1, 127.2, and 88.4 mAh?g^?1 at 1, 5, and 10 °C, respectively. It indicates that V₂O₃ modification efficiently improves specific capacity and rate capability. The EIS experiment demonstrates that catalytic activity and reversibility of the cathode electrode are obviously increased by the surface modification of vanadium oxide.     

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.     

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.     

On the validity of the electron transfer model in photon emission from ion bombarded vanadium surfaces

The spectral structure of the radiation (250–500 nm) emitted during sputtering of clean and oxygen-covered polycrystalline vanadium and V₂O₅ by 5 keV Kr⁺ ions is presented. The optical spectra obtained by bombarding the vanadium target consist of series of sharp lines, which are attributed to neutral and ionic excited V. The same lines are observed in the spectra of V₂O₅ and vanadium when oxygen is present. The absolute intensities of VI and VII lines are measured under similar conditions for all spectra. The difference in photon yield from the clean and oxide vanadium targets is discussed in terms of the electron-transfer processes between the excited sputtered and electronic levels of the two types of surfaces. We have examined the existing models of ionisation, excitation, neutralisation and de-excitation of atomic particles in the vicinity of solid surfaces. Continuum radiation was also observed and interpreted as a result of the emission of excited molecules of the metal-oxide.     

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.