Amorphous V2O5 Research Articles

Electrosynthesis of Macroporous Polyaniline−V2O5 Nanocomposites and Their Unusual Magnetic Properties

This paper reports a novel two-step one-pot all-electrochemical method for the preparation of interpenetrating conducting-polymer (polyaniline, PANI) semiconducting oxide (V2O5) nanocomposites. In the method, a spongy interconnected PANI network is first deposited on a titanium metal substrate. The electrodeposited PANI network has pores on the order of a few micrometers and is used as a template for the V2O5 component which is also deposited electrochemically. The dimensionality of the amorphous V2O5 that forms can be controlled through control of the current density during the deposition, and this in turn reduces the porosity. As the current density increases and more V2O5 is deposited, Raman and X-ray photoelectron spectroscopy (XPS) indicate that the conductivity of the PANI decreases. Regardless of the current density used in the range 1−5 mA/cm2, the 51V solid-state NMR spectrum of the V2O5 component shows a major resonance at about −8500 ppm which is ascribed to a Knight shift due to interaction of the PANI conduction electrons with the 51V nuclear spin. The magnitude of this 51V Knight shift is unprecedented exceeding by a significant margin any of those previously reported for vanadium oxide compounds.

Spin–spin exchange in vanadium-containing catalysts studied by in situ-EPR: a sensitive monitor for disorder-related activity

In unsupported V-containing catalysts, V4+ sites are frequently present in amorphous oxidic clusters and/or crystalline paramagnetic bulk phases in which they are coupled by effective spin–spin exchange interactions. This work presents two EPR procedures for evaluating these interactions and relating their strength to the degree of disorder and catalytic performance, namely (1) evaluating permanent structural disorder by calculating exchange energies and integrals and (2) monitoring transient electronic disorder caused by catalyst–reactant interaction in working catalysts using the ratio of the fourth and the square of the second moment of the EPR signal. Selected application examples comprise VPO catalysts such as (i) (VO)2P2O7, in which a certain degree of structural disorder revealed to be essential for high catalytic performance in the selective oxidation of n-butane to maleic anhydride, (ii) (NH4)2(VO)3(P2O7)2 in which transient perturbation of spin–spin exchange sensitively reflects the strength of reactant adsorption during ammoxidation of toluene and (iii) (NH4)2VOP2O7 in which an amorphous VO2+-containing phase could be identified as active component besides inactive crystalline phases. Based on operando-EPR measurements, reasons for activity differences in MoVTeNb oxide catalysts could be suggested.

V2O5 Nanorods on TiO2 Nanofibers: A New Class of Hierarchical Nanostructures Enabled by Electrospinning and Calcination

Electrospinning provides a simple approach to fabricating nanofibers and assemblies with controllable hierarchical structures. In this communication, we demonstrate that electrospinning can be combined with calcination to further maneuver the morphology and phase structure of nanofibers. More specifically, single-crystal V2O5 nanorods could be grown on rutile nanofibers by carefully calcining composite nanofibers consisting of amorphous V2O5, amorphous TiO2, and poly(vinylpyrrolidone). The size of the resulting V2O5 nanorods could be conveniently controlled by varying the composition of the nanofibers and/or the calcination temperature. In addition to the nanorod-on-nanofiber hierarchical structure, we believe this approach can also be extended to fabricate other more complex architectures.

Sol−Gel Synthesis of Vanadium Oxide within a Block Copolymer Matrix

Films consisting of a vanadium pentoxide (i.e., V2O5) phase formed within a rubbery block copolymer were developed for their potential use as nanocomposite cathodes in lithium rechargeable batteries. Nanocomposite films were prepared by sol−gel synthesis from vanadyl triisopropoxide precursor in poly(oligooxyethylene methacrylate)-block-poly(butyl methacrylate), POEM-b-PBMA. The in situ growth of amorphous V2O5 was confirmed by wide-angle X-ray scattering (WAXS), X-ray photoelectron spectroscopy (XPS), and small-angle X-ray scattering (SAXS). Scanning transmission electron microscopy (STEM) and differential scanning calorimetry (DSC) demonstrated the selective incorporation of vanadium oxide within the ion-conducting POEM domains, while the oxide morphology was revealed by TEM to be a filamentous network. Cyclic voltammetry and impedance spectroscopy confirmed the preservation of the redox properties of the vanadium oxide and the ion-conductive properties of the polymer in hybrid films. Co-assembled nanocomposite films incorporating up to 34 wt % (13 vol %) vanadium oxide were flexible and semi-transparent.

Incorporation of Vanadium Oxide in Silica Nanofiber Mats via Electrospinning and Sol‐Gel Synthesis

Submicron scale vanadia/silica hybrid nanofiber mats have been produced by electrospinning silica sol‐gel precursor containing vanadium oxytriisopropoxide (VOTIP), followed by calcinations at high temperature. The properties of the resulting inorganic hybrid nanofiber mats are compared to those of electrospun pure silica nanofibers. SEM images show fibers are submicron in diameter and their morphology is maintained after calcination. Physisorption experiments reveal that silica nanofiber mats have a high specific surface area of 63 m2/g. FT‐IR spectra exhibit Si—O vibrations and indicate the presence of V2O5 in the fibers. XPS studies reveal that the ratio of Si to O is close to 0.5 on the surface of fibers and the amount of vanadium on the surface of fibers increases with calcination. XRD diffraction patterns show that silica nanofibers are amorphous and orthorhombic V2O5 crystals have formed after calcination. EFTEM images demonstrate the growth of crystals on the surface of fibers containing vanadium after calcination. SEM images of fibers with high‐vanadium content (50 mol% V: Si) show that vanadia crystals are mostly aligned along the fiber axis. XPS shows an increase in vanadium contents at the surface, and XRD patterns exhibit an increase in the degree of crystallinity. A coaxial electrospinning scheme has successfully been employed to selectively place V2O5 in the skin layer.

Spontaneous and Self-Assembled Line Formations on Silicon Substrates with Vanadium Pentoxide Sol−Gels

A simple one-pot method has been developed to deposit discreet nanometer line formations on silicon substrates without any surface pretreatment starting with vanadium pentoxide sol-gels. These vanadium suspensions were made by hydrolyzing amorphous V2O5 in water. The properties of the vanadium clusters were determined through X-ray diffraction, elemental analyses, pH calculations, and concentration calculations. Morphology of the lines was examined with optical microscopy, atomic force microcoscopy, and scanning electron microscopy. Infrared spectroscopy was used to inspect the organic components. The vanadium sol-gel used formed discreet and regular lines with high reproducibility and on the same order of magnitude as other patterning techniques. Previous research with a low solubility, 8 g/L, metal oxide for line, ring, or helix formation has not been found in the literature; this work could lead to novel applications of metal oxides such as porous catalysts, battery materials, and resistive electronic materials.

Effect of silver co-sputtering on amorphous V2O5 thin-films for microbatteries

Abstract The effect of silver co-sputtering on the microstructural and electrical properties of amorphous V2O5 films is investigated. The films are grown by dc reactive sputtering and are applicable to thin-film microbattery cathodes. Analysis with glancing-angle X-ray diffraction (GXRD), scanning electron microscopy (SEM), X-ray photon spectroscopy (XPS), and Fourier transformation infrared (FT-IR) spectroscopy confirm that the growing rate, silver content and microsturucture of the V2O5 films are dominantly affected by the r.f. power supplied on the silver metal targets. The discharge capacity and cycle ability of microbatteries increases with increasing silver content in the thin cathode films. In addition, it is found that silver co-sputtered V2O5 cathode films, if only obtained at not more than 30 W, exhibit better discharge capacity, cycle ability and lithium-ion diffusivity than an undoped V2O5 cathode film. This is due to increases both in the electronic conductivity with doping elements of high electronic conductivity and in the distance between two adjacent V2O5 layers by co-sputtered silver atoms.

Anodic oxidation of vanadium and properties of vanadium oxide films

Thin films of amorphous vanadium oxide have been prepared by electrochemicalanodic oxidation. The phase composition of anodic films on vanadium hasbeen shown to depend on the oxidation conditions (electrolyte composition,oxidation current, and time), and the stoichiometry can be controlled fromV O2 toV2O5. Physical properties of the oxide films, including the metal–insulator transition in amorphousV O2, are studied. In addition, it is shown that non-equilibrium electrochemical oxidation leadsto the formation of metastable vanadium oxides with extremely high sensitivity to laser() and electron-beam () irradiation. Such films are of considerable technical interest, particularly because ofpotential applications as an efficient resist material for both photonanolithography andelectron-beam nanolithography.

Lithium intercalation on amorphous V2O5 thin film, obtained by r.f. deposition, using in situ sample transfer for XPS analysis

AbstractSpectroscopic and electrochemical properties of amorphous V2O5 films, prepared by r.f. sputter deposition, were compared before and after electrochemical lithium intercalation. The films gave a cyclic voltammogram typical of vanadium pentoxide with two/three steps of reversible electrochromism (yellow–green–blue). The expected effect of lithiation on chemical state and surface composition was monitored using XPS. A careful curve‐fitting procedure of the detailed spectra was adopted and the in‐depth profile analysis of the intercalated films clearly showed their surface reorganization as a function of time and ultrahigh vacuum conditions. The first objective of this work was to establish the influence of lithium intercalation while minimising degradation effects due to its high reactivity with the atmosphere, taking advantage of the in situ sample transfer facility. Copyright © 2003 John Wiley & Sons, Ltd.

Phase change induced degradation of amorphous vanadium oxide cathode thin film during charge-discharge

Amorphous vanadium oxide (V2O5) is a very good candidate as material for cathode thin film since it has a relatively high capacity. In addition, the room temperature deposition process is valuable in V2O5 thin film fabrication. Due to these advantages, much effort to grow amorphous V2O5 thin film has been made. In this research, we successfully grew amorphous V2O5 thin film using room temperature sputtering. Based on a Li/LiPON/V2O5 full cell structure, charge-discharge performances were measured according to cycling number. Even though the full cell structure showed an average capacity of 15 μAh/cm2 over more than 500 cycles, a capacity fade was shown after a few cycles. Many reports revealed that the phase change of V2O5 from amorphous to crystalline made this kind of capacity fade. In order to investigate this phenomenon, high-resolution transmission electron microscopy (HRTEM) was employed. The as-deposited V2O5 thin film consisted of a homogeneous amorphous without any grain-boundary and/or polycrystalline island. However, the microcrystalline V2O5 phase was randomly distributed in the amorphous V2O5 thin film matrix after 450 cycles by cross sectional TEM (XTEM). That is, some amorphous phase in the V2O5 thin film matrix changed to the crystalline phase. This crystalline phase strongly prevented the extraction of Li ions during the charge process, which induced the irreversible diffusion of Li ions from cathode to anode. From this result, a high efficiency thin film battery based on amorphous V2O5 can be fabricated by preventing amorphous-crystal phase transformation during cycling.

Growth and properties of completely amorphous vanadium oxide cathode thin film with Pt co-sputtering method

The effect of Pt co-sputtering on the characteristics of amorphous V2O5 films, grown by de reactive sputtering, was investigated using glancing angle x-ray diffraction (GXRD), transmission electron microscopy (TEM), transmission electron diffraction (TED) and charge-discharge measurement. It was found that the Pt co-sputtering processes influence the growth mechanism as well as the characteristics of V2O5 films. In addition, it was found that the Pt co-sputtered V2O5 cathode film exhibits cyclibility better than the undoped V2O5 cathode film, due to the absence of short range order, which is generally shown in undoped V2O5 cathode films. Possible explanations for the cycling behavior of the Pt doped V2O5 cathode film and Pt co-sputtering effect on the electrochemical properties of thin film batteries are suggested.

Effect of crystallization of amorphous vanadium dioxide films on the parameters of a semiconductor-metal phase transition

The hysteresis loops of reflectivity and capacitance of amorphous and polycrystalline vanadium dioxide films in the range of a semiconductor-metal phase transition were studied. The morphology of these films was studied by an atomic-force microscope. It is established that the small number (2–3) of temperature cycles suppress the phase transition due to possible diffusion of oxygen from VO2 clusters into adjacent clusters consisting of the lowest oxides of the Magnelli series. It is shown that the annealing of an amorphous film of vanadium dioxide in oxygen leads to an additional oxidation of low oxides and the formation of VO2, the formation of a polycrystalline film, and the recovery of the phase transition. The above results and the data obtained by atomic-force microscopy are indicative of the high optical quality of the polycrystalline vanadium dioxide films produced by the annealing of amorphous VO2 films. The results also show that these films can be used in interferometers and optical limiters.

Microstructure study of amorphous vanadium oxide thin films using raman spectroscopy

Microstructural changes of amorphous V2O5 films with lithium intercalation are studied using Raman-scattering measurements. The Raman spectra of as-deposited films show two broad peaks around at 520 and 650 cm−1, due to the stretching modes of the V3–O and V2–O bonds, respectively, and a relatively sharp peak at 1027 cm−1 due to the V5+=O stretching mode of terminal oxygen atoms. In addition, there is a peak at 932 cm−1 that we attribute to the V4+=O bonds. Comparison of the Raman spectra of V2O5 films with different oxygen deficiencies confirms this assignment. This Raman peak due to the stretching mode of the V4+=O bonds develops and shifts toward lower frequencies with increasing lithium concentration. Comparison to results from gasochromic hydrogen insertion indicates that the 932 cm−1 Raman peak is not a result of vibrations which involve Li or H atoms. We propose that the V4+=O bonds are created by two different mechanisms: a direct conversion from V5+=O bonds and the breaking of the single oxygen bonds involving V4+ ions.

UV-visible and IR spectroelectrochemical properties of V2O5 crystalline films charged/discharged in extended potential range

Electrochemical properties of amorphous and crystalline V2O5 films, dip-coated from V-oxoisopropoxide sols and thermally treated at various temperatures (100, 150, 200 and 300°C), have been studied in extended potential range, i.e. from 1.4 to −1.6 V vs. Ag/AgCl in 1M LiClO4/propylen carbonate (PC) electrolyte. The formation of various lithiated (α-, δ-, e- and γ-LixV2O5) phases was correlated with the values of insertion coefficient x obtained from cyclic voltammograms (CV) of crystalline V2O5 films (300°C). Reversible charging was observed when films were cycled up to −1.0 V vs. Ag/AgCl, while the extension of the potential to −1.3 V vs. Ag/AgCl change the CV of films irreversibly. Charging of crystalline V2O5 films was followed by the help of in-situ UV-visible spectroscopy, that revealed the intensity variations of the polaron absorption above 600 nm and the presence of the absorbing V3+ species between 550 and 650 nm. Ex-situ IR spectra of the crystalline films charged/discharged at −1.6V/1.4V vs. Ag/AgCl confirmed the amorphisation of the films' structure.

Characterization of Surface Vanadia Forms on V/Ti–Oxide Catalyst via Temperature-Programmed Reduction in Hydrogen and Spectroscopic Methods

Surface vanadia species formed on vanadia/titania catalysts consisting of 0.2–2.6 monolayers (ML) of VOx have been characterized by FT–Raman spectroscopy under controlled atmosphere, temperature-programmed reduction in hydrogen (TPR), and solubility in diluted HNO3. Three types of species were observed with the maximum peak temperatures as follows: isolated monomeric species (≤770–780 K), polymeric species (810 K), and bulk amorphous V2O5 (852 K). During the reduction, the VO bond of the monomeric species with tetracoordinated vanadium disappears as shown by diffuse reflectance infrared Fourier transform spectroscopy. A formation of new hydroxyl groups with a basic character was observed. The monomeric species was found to be chemically stable with respect to diluted HNO3. Bulk amorphous V2O5 and polymeric vanadia were soluble in HNO3 and removed from the surface. The state of vanadium in the oxidized catalysts was mainly pentavalent, as shown by XPS, and did not change after acid treatment. Reduction by hydrogen of monolayer vanadia in a 0.2-ML V/TiO2 catalyst was studied by temperature-programmed reduction (TPR) at different heating rates. A one-site kinetic model is able to account for the TPR data, in spite of the presence of the monomeric and polymeric species. This indicates that these species could be considered equal with respect to the interaction with hydrogen. The activation energy was determined for the catalyst reduction (98±5 kJ/mol).

Microstructures and electrochemical properties of Pt-doped amorphous V2O5 cathode films for thin film microbatteries

We have investigated the effects of Pt doping on the structural and electrochemical properties of amorphous (a) V2O5 films grown by radio frequency magnetron sputtering. It is shown that the cycling performance of the a-V2O5 films is dependent upon the Pt content. Glancing angle x-ray diffraction and transmission electron microscopy examination shows that the undoped film is amorphous with some short-range order. However, doping of the films at a rf power of 10 W leads to a complete a-V2O5 film which shows the best cycling behavior among the samples. It is shown that the short-range ordered structure reappears when the films are doped by cosputtering at rf powers of 30 and 50 W. It is further shown that doping at a rf power of 50 W results in the formation of amorphous and nanocrystalline PtO5 phases embedded in the a-V2O5 matrix.

V2O5 aerogel-like lithium intercalation host

Thin self-standing films and powders of highly amorphous V2O5 have been prepared via a combined sol-gel and solvent exchange procedure. The amorphous V2O5 is a highly interconnected porous material with very thin solid walls and shows a unique lithium intercalation behavior. Electrochemical tests performed on composite cathodes made by mixing the material with carbon particles demonstrated a very high lithium insertion capacity.

Li Ion diffusion measurements in crystalline and amorphous V2O5 thin-film battery cathodes

ABSTRACTThin films of crystalline and amorphous V2O5 were deposited by pulsed laser deposition (PLD) and the chemical diffusion coefficients, , were measured by the potentiostatic intermittent titration technique (PITT). In crystalline V2O5 films, the maximum and minimum were found to be 1.7 × 10−2 cm2/s and 5.8 × 10−15 cm2/s respectively, with a general trend for to rise in single-phase regions. The changes in correlated well to the known phases in LiV2O5. In amorphous V2O5 films, exhibited a smooth, continuous decrease as the Li concentration increased.

Effect of purification of 2-methyltetrahydrofuran/ethylene carbonate mixed solvent electrolytes on cyclability of lithium metal anodes for rechargeable cells

The effects of the purification of LiAsF6–2-methyl tetrahydrofuran (2MeTHF)/ethylene carbonate (EC) mixed solvent organic electrolytes on the charge–discharge cyclability of lithium metal anodes has been investigated by using an accelerated method for evaluating lithium cycling efficiency. This method involves cycle tests on coin cells with an amorphous V2O5–P2O5 (95:5 molar ratio) cathode and an anode containing a small amount of lithium. Using this method, the cycle life of the cell was determined over a short period simply from the lithium cycling efficiency. The lithium cycling efficiency in LiAsF6–2MeTHF/EC was improved by removing both water and organic impurities such as peroxides. An electrolyte containing less than 14ppm of water and 20ppm of organic impurities had a high lithium cycling efficiency of 97.2%.

Electrical conductivity of amorphous V2O5–Sb2O3 thin films

Amorphous thin films of V2O5–Sb2O3, in the molar ratio of V2O5:Sb2O3 as 100:0, 98:2, 94:6, 92:8 respectively, were obtained by the vacuum–evaporation technique. The dc conductivities of the films were measured in the temperature range of 303–423 K. The experimental data have been analyzed in light of the small polaronic hopping model of Mott and the percolation approach of Triberis and Friedman. The estimated model parameters are found to be consistent with the formation of localized states in these films.