Vanadium Monoxide Research Articles

ExoMol line lists – LV: hyperfine-resolved molecular line list for vanadium monoxide (51V16O)

ABSTRACT The new hyperfine-resolved HyVO line lists for vanadium monoxide (51V16O) are presented. The new line list considers 15 electronic states and comprises almost 59 billion transitions between 3.4 million energy levels, up to 45 000 cm−1. Empirical energy levels obtained through the Marvel procedure are used to update calculated values to experimental accuracy, making the line list well suited for high-resolution experiments. Machine learning is utilized to refine the energies of levels in important bands, where empirical values are not known. These updates allow for the accurate prediction of line positions in regions of astrophysical interest, particularly in strong rovibronic bands in the infrared and visible. Previously published ab initio transition dipole moment curves are adjusted based on experimental lifetime measurements, resulting in increased intensities in key regions. The line list covers levels up to F = 150 and is complete up to 5400 K, making it well suited for studies of ultra-hot Jupiter and M-type stellar atmospheres where VO is believed to be a strong source of opacities. The full line list is made available via www.exomol.com.

A hyperfine-resolved spectroscopic model for vanadium monoxide (51V16O)

Vanadium monoxide ( 51 V 16 O) is believed to play an important role in the atmospheres of hot-Jupiters, but high-resolution studies have so far failed to detect it, at least in part because of the inaccuracy of available lists. It is likely that the large hyperfine splittings in the spectra of VO, arising from the large nuclear spin I = 7 2 of the 51 V atom, has contributed to the non-detections with the current hyperfine-unresolved VOMYT line list. To aid in the production of a new line list, a fully hyperfine-resolved spectroscopic model has been constructed which includes 15 low-lying electronic states (6 quartets and 9 doublets) of VO with the inclusion of hyperfine couplings based on use of the new, hyperfine-resolved version of the diatomic variational nuclear motion programme Duo. The new spectroscopic model is refined against empirical Marvel energies derived from experimental transitions, and hyperfine couplings are fit for the 3 electronic states for which hyperfine effects have been resolved in lab spectra. This model is used to assign some previously identified perturbations.

An empirical spectroscopic model for eleven electronic states of VO

Previously-determined empirical energy levels are used to construct a rovibronic model for the X4Σ−, A′4Φ, A4Π, B4Π, C4Σ−, D4Δ, 12Δ, 12Σ+, 12Φ, 12Π and 22Π electronic states of vanadium monoxide. The spectrum of VO is characterized by many couplings and crossings between the states associated with these curves. The model is based on the use of potential energy curves, represented as extended Morse oscillators, and couplings (spin–orbit, spin–spin, angular momentum), represented by polynomials, which are tuned to the data plus an empirical allowance for spin–rotation couplings. The curves are used as input for the variational nuclear motion code Duo. For the X4Σ−, 12Φ and 12Π states the model reproduces the observed energy to significantly better than 0.1 cm−1. For the other states the standard deviations are between 0.25 and 1.5 cm−1. Further experimental data and consideration of hyperfine effects would probably be needed to significantly improve this situation.

A variational model for the hyperfine resolved spectrum of VO in its ground electronic state.

A variational model for the infra-red spectrum of vanadium monoxide (VO) is presented, which aims to accurately predict the hyperfine structure within the VO XΣ-4 electronic ground state. To give the correct electron spin splitting of the XΣ-4 state, electron spin dipolar interaction within the ground state and the spin-orbit coupling between XΣ-4 and two excited states, AΠ4 and 1Σ+2, are calculated ab initio alongside hyperfine interaction terms. Four hyperfine coupling terms are explicitly considered: Fermi-contact interaction, electron spin-nuclear spin dipolar interaction, nuclear spin-rotation interaction, and nuclear electric quadrupole interaction. These terms are included as part of a full variational solution of the nuclear-motion Schrödinger equation performed using program Duo, which is used to generate both hyperfine-resolved energy levels and spectra. To improve the accuracy of the model, ab initio curves are subject to small shifts. The energy levels generated by this model show good agreement with the recently derived empirical term values. This and other comparisons validate both our model and the recently developed hyperfine modules in Duo.

Density functional theory study of the reaction between VO− and water

The interactions of vanadium monoxide anion (VO−) with water were investigated by density functional calculation. The theoretical calculations confirmed that HOV(OH)− was generated when VO− reacts with one water molecule by overcoming the energy barrier of 7.69 kcal/mol, and HV(OH)3− was generated when the second water molecule is involved. Introducing more water molecules, the structures of stoichiometric VO(H2O)n− (n = 3–7) are characterized by attaching water molecules successively to the HV(OH)3− existing as a core unit. Molecular orbitals analysis suggests that the highest occupied orbitals are most found on the V, with a small fraction on the H/OH connected to V.

Fabrication of VO Nanorings on a Porous Carbon Architecture for High-Performance Li-Ion Batteries.

Vanadium monoxide (VO) is a promising candidate as an anode for lithium-ion batteries due to its high theoretical capacity, low cost, and considerable electronic conductivity. Unfortunately, a large volume change during electrochemical processes obstructs its practical application. In this work, a composite of VO nanorings grown on a porous carbon architecture is prepared via a topochemical self-reduction approach. When used as an anode for lithium-ion batteries, improved redox kinetics from enhanced electronic conduction and the corresponding fast lithium-ion diffusion is observed to greatly promote the electrochemical performance of lithium-ion batteries. The resulting composite delivered a reversible capacity of 336 mA h g-1 after 400 cycles at 10 A g-1 with a capacity retention of 85%, owing to the synergistic effect of VO nanorings and porous carbon in alleviating volume changes that result in a long-term cycling ability at a high current density. At 20 A g-1, the composite anode exhibited a rate capability of 235 mA h g-1, superior to all VO-based electrodes reported in the literature. Furthermore, a full cell was first fabricated by employing VO@C-2 as the anode and LiFePO4 as the cathode, which exhibited a capacity of 213 mA h g-1 after 100 cycles at 0.1 A g-1, indicating the potential of VO as an anode for practical application.

Facile growth of epitaxial vanadium monoxide on SrTiO3 via substrate oxygen scavenging

The deposition of vanadium metal on SrTiO3 results in the spontaneous scavenging of oxygen ions from SrTiO3 to oxidize vanadium to VOx, where x = 0.3–1.1, depending on the deposition temperature. At temperatures above 700 °C, an epitaxial film of the rock salt compound vanadium monoxide is formed on both SrTiO3 (111) and SrTiO3 (100) substrates. Surprisingly, oxygen scavenging and epitaxy persist for thicknesses over 800 Å with no sign of degradation. We describe the growth process and layer-by-layer characterization of the films using in situ reflection high-energy electron diffraction and x-ray photoelectron spectroscopy, as well as ex situ in-plane and out-of-plane x-ray diffraction and cross-sectional scanning transmission electron microscopy. This easy method of growing vanadium monoxide with controlled stoichiometry can provide new opportunities to study this normally hard to synthesize material known for the highly correlated nature of its electronic structure.

Vanadium Monoxide-Based Free-Standing Nanofiber Hosts for High-Loading Lithium-Sulfur Batteries

We report the fabrication of free-standing vanadium monoxide-based carbon nanofiber/sulfur (VO-CNFs/S) cathodes via electrospinning and in situ carbothermal reduction reaction followed by sulfur impregnation using an ultrarapid (<90 s) hybrid solution/melt deposition technique. The assembled Li-S batteries using VO-CNFs/S cathodes delivered a stable capacity of 950 mAh·g–1 with 97% retention after 200 cycles at a 0.5 C rate with a moderate electrolyte/sulfur ratio of 20 mL·g–1 of sulfur. Additionally, we develop cathodes with a high sulfur loading of 8.32 mg·cm–2 that exhibits a stable capacity of 900 mAh·g–1 after a few cycles with near 100% retention over 100 cycles. These cells correspond to a stable aerial capacity of 7.5 mAh·cm–2. We integrate our fabrication and electrochemical performance study with fundamental investigations of Li+ diffusion kinetics, in situ visual monitoring of polysulfides, and ex situ post-mortem X-ray photoelectron spectroscopy study to elucidate polysulfide shuttling and polysulfide–host interactions. The unique integration of oxide and nitride groups in this work results in a synergistic effect of strong Lewis acid–base interactions originating from the vacant d-orbitals of the vanadium monoxide phase and improved conductivity originating from the nitrogen doping in CNFs from a polymeric precursor, thus offering active sites for strong polysulfide binding as well as faster reaction kinetics.

Electrochemical Gas Sensor Integrated with Vanadium Monoxide Nanowires for Monitoring Low Concentrations of Ammonia Emission

An electrochemical sensor for the detection of extremely low concentration of ammonia (1 part per billion, ppb) was fabricated by integrating vanadium monoxide (VOx; x = 0.8–1.2) nanowires on the platinum electrodes. The nanowire-based sensor responds at room temperature non-linearly to a staircase sequence of ammonia from 1 ppb to 100 ppb. The rise and fall time of the nanowire sensor was found to be 10 s and 9 s, respectively. While the immobilization of VO nanowires increased the electrochemical surface area, the defect rich and ionic nature of the VO surface (V2+O2−) facilitated the chemical interaction and adsorption of polar ammonia molecules as evident in the room temperature response of the VO@Pt amperometric electrochemical sensor. The availability of metal centered d-electrons and the semiconductor nature of vanadium monoxide lowered the interfacial resistance of the nanowire-modified sensor enabling the lower detection limit of ammonia. The sensor seems to respond to CH4, H2S and C3H6 as well although the NH3 response is nearly six-fold compared to these common interfering compounds. The results pave the way for a low-cost alternative paper-based sensor to monitor ammonia emissions primarily from confined animal feeding operations (CAFOs).

Photodissociation dynamics and the dissociation energy of vanadium monoxide, VO, investigated using velocity map imaging.

Velocity map imaging has been employed to study multi-photon fragmentation of vanadium monoxide (VO) via the C 4Σ- state. The fragmentation dynamics are interpreted in terms of dissociation at the three-photon level, with the first photon weakly resonant with transitions to vibrational energy levels of the C 4Σ- state. The dissociation channels accessed are shown to depend strongly on the vibrational level via which excitation takes place. Analysis of the evolution of the kinetic energy release spectrum with photon energy leads to a refined value for the dissociation energy of ground state VO of D0(VO) = 53 126 ± 263 cm-1.

Explored vanadium monoxide as a novel anode for lithium storage

Vanadium monoxide (VO) nanoparticles and reduced graphene oxide (rGO) composites with a 3D architecture (GVO) was synthetized by a simple hydrothermal reaction of a mixture of vanadium pentoxide (V2O5) ribbons and graphene oxide (GO). It shows large contact area between electrode and electrolyte, short electron and ion diffusion pathways for LIBs and good electrical conduction. As a result, when the GVO was served as the LIBs anode material, the resulting electrodes exhibited a high specific capacity (909 mAh g−1 at 74.4 mA g−1 after 30 cycles) and remarkable cyclic stability.

Ab initio calculations to support accurate modelling of the rovibronic spectroscopy calculations of vanadium monoxide (VO)

ABSTRACTAccurate knowledge of the rovibronic near-infrared and visible spectra of vanadium monoxide (VO) is very important for studies of cool stellar and hot planetary atmospheres. Here, the required ab initio dipole moment and spin-orbit coupling curves for VO are produced. This data forms the basis of a new VO line list considering 13 different electronic states and containing over 277 million transitions. Open shell transition, metal diatomics are challenging species to model through ab initio quantum mechanics due to the large number of low-lying electronic states, significant spin–orbit coupling and strong static and dynamic electron correlation. Multi-reference configuration interaction methodologies using orbitals from a complete active space self-consistent-field (CASSCF) calculation are the standard technique for these systems. We use different state-specific or minimal-state CASSCF orbitals for each electronic state to maximise the calculation accuracy. The off-diagonal dipole moment controls the intensity of electronic transitions. We test finite-field off-diagonal dipole moments, but found that (1) the accuracy of the excitation energies were not sufficient to allow accurate dipole moments to be evaluated and (2) computer time requirements for perpendicular transitions were prohibitive. The best off-diagonal dipole moments are calculated using wavefunctions with different CASSCF orbitals.

Self-reduced VO/VOx/carbon nanofiber composite as binder-free electrode for supercapacitors

In this work, a free-standing, flexible and highly conductive vanadium-carbon nanofiber composite has been fabricated as electrode for Supercapacitors. Vanadium monoxide (VO) coupled with amorphous vanadium covalent bonds (VOx) are successfully incorporated into carbon nanofibers (VO/VOx/CNF) by electrospinning and heat treatment. A theoretical explanation is proposed for the formation of VO/VOx in CNF composites. The VO and VOx are respectively reduced and formed from vanadium precursors of vanadium dioxide (VO2) and vanadyl (IV) acetylacetonate (VOA) by means of self-reduction method, in which carbon precursors (polyacrylonitrile and polyvinylpyrrolidone), small evolved gas molecules (CO, H2, HCN) and graphitized carbon act as self-reductants. No additional reductants is needed before or after heat treatment, avoiding the secondary contamination. The VO/VOx/CNF electrode has a specific capacitance of 325.7Fg−1 at a current density of 1Ag−1 and is capable of reserving 92% of its initial capacitance after 5000 cycles operating at a current density of 4Ag−1 in a symmetric two-electrode capacitor using 6M KOH as an electrolyte. The superior electrochemical performance of VO/VOx/CNF may be attributed to two advantages. The first is the enhanced conductivity brought upon the incorporation of quasi-metallic VO (∼102Ω−1cm−1) and the network of nanowire, and the second is the rapid ion transfer rate caused by the rich vanadium redox couples VO/VOx and the well-developed pore structure. Notably, this work has also provided a facile method to obtain varaible low valence states from vanadium oxides through self-reduction, which may also be applied to synthesize other metal oxides-carbon nanofiber composites.

Rotationally Selected and Resolved State-to-State Photoelectron Study of Vanadium Monoxide Cation VO(+)(X(3)Σ(-); v(+) = 0-3).

Vanadium monoxide cation VO(+)(X(3)Σ(-)) has been investigated by two-color visible (VIS)-ultraviolet (UV) pulsed field ionization-photoelectron (PFI-PE) methods. The unambiguous rotational assignment of rotationally selected and resolved VIS-UV-PFI-PE spectra thus obtained confirms the ground state term symmetry of VO(+) to be X(3)Σ(-). The rotational analysis also yields the rotational constants Be(+) = 0.5716 ± 0.0012 cm(-1) and αe(+) = 0.0027 ± 0.0005 cm(-1) for VO(+)(X(3)Σ(-)), from which the equilibrium bond distance of VO(+)(X(3)Σ(-)) is determined to be re(+) = 1.557 ± 0.002 Å. This PFI-PE study covers the vibrational bands, VO(+)(X(3)Σ(-); v(+) = 0, 1, 2, and 3) ← VO(X(4)Σ(-); v″ = 0), which has made possible the determination of the vibrational constants for VO(+)(X(3)Σ(-)) to be ωe(+) = 1068.0 ± 0.7 cm(-1) and ωe(+)xe(+) = 5.5 ± 0.7 cm(-1). The present state-to-state measurement also yields a more precise value (58 380.0 ± 0.7 cm(-1) or 7.238 20 ± 0.000 09 eV) for the ionization energy of VO [IE(VO)]. This value along with the known IE(V) has allowed the determination of the difference between the 0 K bond dissociation energy (D0) of VO(+)(X(3)Σ(-)) and that of VO(X(4)Σ(-)) to be D0(V(+)-O) - D0(V-O) = IE(V) - IE(VO) = -3967 ± 1 cm(-1).

The electronic structure of VO in its ground and electronically excited states: A combined matrix isolation and quantum chemical (MRCI) study.

The electronic ground and excited states of the vanadium monoxide (VO) molecule were studied in detail. Electronic absorption spectra for the molecule isolated in Ne matrices complement the previous gas-phase spectra. A thorough quantum chemical (multi-reference configuration interaction) study essentially confirms the assignment and characterization of the electronic excitations observed for VO in the gas-phase and in Ne matrices and allows the clarification of open issues. It provides a complete overview over the electronically excited states up to about 3 eV of this archetypical compound.

Decomposition pathways of formamide in the presence of vanadium and titanium monoxides.

Thermally feasible decomposition pathways of formamide (FM) in the presence of vanadium VO(X(4)Σ(-)) and titanium TiO(X(3)Δ) monoxides are determined using density functional theory (BP86 functional) and coupled-cluster theory (CCSD(T)) computations with large basis sets. These diatomic metal oxides have been shown to be present in the prebiotic conditions. The dehydration, decarbonylation and dehydrogenation reactions of the molecular and dissociative complexes of FM and MO (M = V, Ti) turn out to be more favourable than those of the ground state isolated FM. The effect of addition of one or two water molecules on energy barriers is also probed for these reaction pathways. In some cases, a combined catalytic effect when adding water is observed. This enhanced catalytic effect was not observed in previously reported cases of FM transformation, for example, when adding water molecules into the mineral-catalyzed isomerizations of FM. The dehydration process of MO-FM complexes without the presence of water is found to be more feasible than the decarbonylation and dehydrogenation. The overall energy barrier for the non-water VO-FM dehydration is ∼3 kcal mol(-1) lower than the reference energy of the separated systems, whereas those of the two latter reactions are higher than the reference. Although the TiO-FM dehydration has a larger overall barrier of 14 kcal mol(-1) as compared to the VO-FM counterpart, the two other decomposition pathways still have much higher energy barriers. Direct formation of urea and H2CO from a FM dimer and indirect formation of urea from FM via the intermediate HNCO are also established. Urea formation in an indirect pathway is preferred. These low-energy-barrier pathways leading to the formation of important prebiotic molecules suggest that metal monoxides MO could play an important catalytic role in the prebiotic reactions of FM.

Electrical and optical properties of sputtered amorphous vanadium oxide thin films

Amorphous vanadium oxide (VOx) is a component found in composite nanocrystalline VOx thin films. These types of composite films are used as thermistors in pulsed biased uncooled infrared imaging devices when containing face centered cubic vanadium monoxide phase crystallites, and substantial fractions of amorphous material in the composite are necessary to optimize device electrical properties. Similarly, optoelectronic devices exploiting the metal-to-semiconductor transition contain the room-temperature monoclinic or high-temperature (&amp;gt;68 °C) rutile vanadium dioxide phase. Thin films of VOx exhibiting the metal-to-semiconductor transition are typically polycrystalline or nanocrystalline, implying that significant amounts of disordered, amorphous material is present at grain boundaries or surrounding the crystallites and can impact the overall optical or electronic properties of the film. The performance of thin film material for either application depends on both the nature of the crystalline and amorphous components, and in this work we seek to isolate and study amorphous VOx. VOx thin films were deposited by pulsed dc reactive magnetron sputtering to produce amorphous materials with oxygen contents ≥2, which were characterized electrically by temperature dependent current-voltage measurements and optically characterized by spectroscopic ellipsometry. Film resistivity, thermal activation energy, and complex dielectric function spectra from 0.75 to 6.0 eV were used to identify the impact of microstructural variations including composition and density.

Probing the structural and electronic properties of small vanadium monoxide clusters

The structural evolution and bonding of a series of early transition-metal oxide clusters, V(n)O(q) (n = 3-9, q = 0,-1), have been investigated with the aid of previous photoelectron spectroscopy (PES) and theoretical calculations. For each vanadium monoxide cluster, many low-lying isomers are generated using the Saunders "Kick" global minimum stochastic search method. Theoretical electron detachment energies (both vertical and adiabatic) were compared with the experimental measurements to verify the ground states of the vanadium monoxide clusters obtained from the DFT calculations. The results demonstrate that the combination of photoelectron spectroscopy experiments and DFT calculation is not only powerful for obtaining the electronic and atomic structures of size-selected clusters, but also valuable in resolving structurally and energetically close isomers. The second difference energies and adsorption energies as a function of the cluster size exhibit a pronounced even-odd alternation phenomenon. The adsorption energies of one O atom on the anionic (6.64 → 8.16 eV) and neutral (6.41 → 8.13 eV) host vanadium clusters are shown to be surprisingly high, suggesting strong capabilities to activate O by structural defects in vanadium oxides.

Preparation of nanocrystalline VO y by high-energy ball milling

We report highenergy milling of macrocrysta lline nonstoichiometric cubic vanadium monoxide (VOy) powder in a planetary ball mill lined with stabilized zirconia. The results indicate that milling of macroc� rystalline VOy powder at 500 rpm for more than 2 h considerably broadens diffraction line profiles, with no changes in the crystal structure of the vanadium monoxide, VO1.00. Microstructural examination of vanadium monoxide powder by highresolution scanning electron microscopy and Xray diffraction indicates that high� energy ball milling can be used to produce vanadium monoxide powder with an average crystallite size within 23 nm.

Vacancy distribution in nonstoichiometric vanadium monoxide

Structural vacancy distribution in the crystal lattice of the tetragonal V 52O 64 superstructure which is formed on the basis of disordered superstoichiometric cubic vanadium monoxide VO y ≡V x O z is experimentally determined and the presence of significant local atomic displacements and large local microstrains in a crystal lattice of real ordered phase is established. It is shown that the relaxation of local microstrains takes place owing to the basic disordered cubic phase grain refinement and a formation of ordered phase domains. The ordered phase domains grow in the direction from the boundaries to the centre of grains of the disordered basic cubic phase. Isothermal evolution at 970 K of the average domain size 〈 D〉 in ordered VO 1.29 vanadium monoxide is established. It is shown that the short-range order presents in a metal sublattice of disordered cubic VO y vanadium monoxide. The character of the short-range order is such that vanadium atoms occupying tetrahedral positions are in the environment of four vacant sites of the vanadium sublattice. This means that the disordered VO y vanadium monoxide, especially superstoichiometric VO y>1.0 monoxide, has a cubic (space group F m 3 ¯ m ) D0 3-type structure differing from the B1-type structure typical for most of the strongly nonstoichiometric cubic compounds carbides, nitrides and oxides.