Vanadium Clusters Research Articles

Structure determination of isolated metal clusters via far-infrared spectroscopy.

We present a new method for the size selective structure determination of small isolated metal clusters in the gas phase. The technique is applied to cationic vanadium clusters containing 6 to 23 atoms, whose far infrared absorption spectra are measured in the 140-450 cm(-1) spectral range. The spectra are unique for each cluster size and are true fingerprints of the cluster's structure. By comparing the experimental spectra to spectra obtained from density-functional theory, the geometric cluster structure can be identified.

Solvothermal Synthesis of Paramagnetic Molecular Clusters

AbstractA brief overview is given of the literature of the use of solvothermal techniques − chemistry in superheated solvents − towards paramagnetic cluster synthesis. Although these techniques have been used extensively to prepare infinite lattice magnetic materials, there is much less work on preparing discrete molecular clusters other than the polyoxovanadates and ‐molybdates. This microreview details some of our early success in using these techniques towards high‐nuclearity vanadium(III), chromium(III), manganese(II/III), iron(III) and nickel(II) clusters, including some very high spin ground state clusters (S ⩽ 25), and therefore demonstrates the potential in the synthesis of molecular materials such as single‐molecule magnets. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Infrared spectroscopy of water adsorption on vanadium cluster cations (V x+; x=3–18)

Vanadium cluster cations with one, two or three adsorbed water molecules are investigated with infrared photodissociation spectroscopy in the region of the bending mode of water. In all of these complexes, the bending mode of adsorbed water is measured near the corresponding frequency of the isolated molecule. Dissociation processes are efficient, indicating that these resonances are characteristic of a substantial fraction of all complexes present. This indicates that water is adsorbed without significant dissociation on these clusters. Clusters with two or three water molecules have similar resonances near the bending mode of free water, indicating independent absorption without hydrogen bonding.

Microstructure change of vanadium clusters in ZnO crystalline films by heat-treatment

Vanadium clusters with a size of several nanometres can be dispersivelydistributed in ZnO films by using co-deposition. EELS mapping ofV-L2,3, HRTEM images, SAD patterns and EELS are a good combination for characterizingnanometre-sized V clusters embedded in a ZnO system. There is no obvious orientationrelationship between V and ZnO lattices due to a less good match between ZnO and Vatomic planes. The ratio of the vanadium L-white lines of vanadium clusters embedded inZnO becomes bigger than that of the bulk. The samples were observed in situ from 298 to923 K with a heating holder so as to study the aggregation process of vanadium clusters inthe ZnO crystalline matrix. First, the size of the ZnO crystal grains becomes bigger afterthe sample is heated at 623 K for an hour; second, the size of ZnO and vanadiumparticles grows bigger after the sample is heated at 723 K for half an hour, andanother orientation relationship between ZnO and vanadium appears; third, afterthe sample is heated at 873 K for half an hour, the size of vanadium becomessmaller, while V particles and ZnO begin to react and form another phase; finally,Zn2V O7 phase is formed after the sample is heated at 923 K for half an hour.

Solvothermal syntheses of high-nuclearity vanadium(III) clusters.

Superheating alcohol solutions of simple trimetallic vanadium(III) precursors gives the octa- and decametallic vanadium(III) clusters [V(8)(OEt)(8)(OH)(4)(O(2)CPh)(12)] (1) and [V(10)(OMe)(20)(O(2)CMe)(10)] (2). Cluster 2 is the largest vanadium(III) cluster synthesised to date. Thus solvothermal synthetic techniques are an excellent route to high-nuclearity vanadium(III) clusters. Both 1 and 2 consist of a planar or near-planar array of V(III) ions. The metal ions in 1 are bridged by either a micro(2)-hydroxide and two micro(2)-benzoate groups or two micro(2)-ethoxides and a micro(2)-benzoate groups, the two bridging arrangements alternating around the ring. In 2 each pair of neighbouring metal ions is bridged by two micro(2)-methoxides and a micro(2)-acetate, and this molecule is the V(III) analogue of Lippard's famous "ferric wheel". Preliminary magnetic susceptibility studies show the exchange coupling in both complexes to be antiferromagnetic in nature, with the coupling stronger in 1 than in 2.

Magnetic properties of small vanadium clusters

The magnetism of small free vanadium clusters (V N , N≤59) with a bcc-like structure is studied using a d electron tight binding Hamiltonian. The spin density distribution is calculated self-consistently in the unrestricted Hartree–Fock approximation. We have found that the free V clusters shown different magnetic behavior depending on their size. Results for the local magnetic moments, average magnetic moment and the magnetic order at T=0 K are obtain as a function of N. For the V N clusters μ avg varies as a function of N due to the interplay between the changes in coordination number and local environment; for N≤4 the ground state is ferromagnetic whereas for N≥9 the ground state is mainly antiferromagnetic in agreement with ab initio and tight binding calculations, respectively.

Vanadium on TiO 2( [formula omitted]): adsorption site and sub-surface migration

The initial stages of the growth of vanadium overlayers on TiO 2(1 1 0) at room temperature have been investigated with scanning tunneling microscopy. At very low coverages both individual vanadium adatoms and small vanadium clusters have been imaged with good resolution. The V adatoms adsorb preferentially on the so-called “upper threefold hollow” sites, as revealed by atomically resolved STM images: they are thus bonded to two bridging oxygen atoms and one threefold coordinated basal oxygen atom. At higher coverages the vanadium adlayers grow in form of poorly ordered three-dimensional islands. The number of V clusters at low coverages decreases by gentle annealing or with time even at room temperature. This kinetic effect has been interpreted in terms of sub-surface migration of V adatoms.

Oxidative Degradation of Small Cationic Vanadium Clusters by Molecular Oxygen: On the Way from V+n (n = 2—5) to VO+m (m = 1, 2).

AbstractFor Abstract see ChemInform Abstract in Full Text.

Effect of dynamic strain aging on mechanical properties of vanadium microalloyed steel

In the present work, the dynamic strain aging behaviour in microalloyed steels has been examined using a C-Mn-Al-V-N steel, which occasionally exhibits low toughness in the subcritical heat affected zone (HAZ). This may be attributed to dynamic strain aging, whereby materials show lower ductility and higher yield strength owing to the interaction between mobile dislocations and diffusing solute atoms. The research has shown that the high temperature tensile behaviour of C-Mn-Al-V-N steel depends on the presence of clusters believed to be of vanadium and carbon atoms. The interaction between dislocations and clusters of vanadium and carbon atoms at 200-450°C changes the work hardening rate and contributes to dynamic strain aging as confirmed in the present work. These interactions may also decrease toughness in the subcritical HAZ and lead to the subcritical embrittlement observed in the C-Mn-Al-V-N microalloyed steel.

Oxidative Degradation of Small Cationic Vanadium Clusters by Molecular Oxygen: On the Way from Vn+ (n = 2−5) to VOm+ (m = 1, 2)

Ion-cyclotron resonance mass-spectrometric experiments are used to examine the reactions of small cationic vanadium clusters Vn+ and vanadium-oxide clusters VnOm+ (n ≤ 5, m ≤ 3) with molecular oxygen. Most of these reactions are very fast (k ≥ 3 × 10-10 cm3 molecule-1 s-1) and strongly exothermic. Oxidative degradation of the clusters (decreasing n) is generally more favored than oxygen-transfer resulting in VnOm+ cations (constant n). Consequently, the mononuclear oxide ions VO+ and VO2+ evolve as the major final products at the end of a reaction cascade. For V2+ and V3+, the complete network of elementary steps is explored by MS/MS experiments. Furthermore, some thermochemical brackets are derived, e.g., for the enthalpies of formation of the ions V2Om+ (m = 1−3): 920 kJ/mol ≤ ΔfH (V2O+) ≤ 1130 kJ/mol; 670 kJ/mol ≤ ΔfH (V2O2+) ≤ 880 kJ/mol, and 420 kJ/mol ≤ ΔfH (V2O3+) ≤ 630 kJ/mol.

A two-dimensional framework of novel vanadium clusters bridged by [Ni(en)2]2+: K{V12IVV6VO42Cl[Ni(en)2]3}·8H2O

The hydrothermal reaction of V2O5, H2C2O4·2H2O, en, KOH and NiCl2·6H2O gives a new two-dimensional open-framework solid; the extended structure consists of a two-dimensional network formed by the interconnection of each [V18O42Cl] cage to four other neighboring units via six [Ni(en)2]2+ bridging groups.

Hydrothermal syntheses and crystal structures of new cage-like mixed-valent polyoxovanadates

Three new cage-like mixed-valent polyoxovanadates [Ni(1,10′-phen) 3] 2[V 10O 26] 1, [Zn(2,2′-bipy) 3] 3[V 15O 36Cl]·3H 2O ( 2) and [Co(2,2′-bipy) 3] 3[V 15O 36Cl]·3H 2O ( 3) have been hydrothermally synthesized for the first time and characterized by elemental analyses, IR, EPR spectra, TG analyses and single crystal X-ray diffraction. The polyoxoanion of 1 exhibits an interesting empty ellipsoidal [V IV 2V V 8O 26] 4− ‘host’ shell, while the oxo vanadium clusters of 2 and 3 possess a spherical [V IV 8V V 7O 36Cl] 6− cage with a Cl − ion encapsulated. The structure-directing role of organic templates (1,10′-phen and 2,2′-bipy) on the formation of the polyoxoanion structures is discussed.

Anion photoelectron spectroscopy of VnOm− (n=4–15;m=0–2)

The anion photoelectron (PE) spectra of small mass-selected vanadium oxide clusters VnOm− (n=4–15; m=0–2) are measured at a fixed photon energy of 4.66 eV with the aid of a magnetic bottle photoelectron spectrometer. Cluster anions are generated in a pulsed laser vaporization cluster source. The electronic structure of VnOm− clusters is investigated as a function of size n and composition m with special regard to the increasing oxidation state. The addition of one or two oxygen atoms to the vanadium cluster core induces a change of the electronic structure in the near-threshold binding energy region below 2 eV. Main spectral features are contributed from the transition metal d-derived orbitals, whereas the oxygen 2p contribution induces a hybridization between vanadium and oxygen frontier orbitals in the entire series of the investigated clusters n=4–15. Generally, electron affinities and vertical detachment energies increase with increasing cluster size revealing size-dependent discontinuities. Furthermore, relative dissociation energies for different oxygen-loss channels from the parent VnOm− clusters are determined from thermochemical cycles, and first insights into the stability patterns of neutral and negatively charged vanadium oxide clusters are provided.

Anion Photoelectron Spectroscopy of Vanadium-Doped Cobalt Clusters

Bimetallic cobalt (Co)−vanadium (V) clusters, ConVm- (n = 4−14, m = 0−2), are investigated using anion photoelectron spectroscopy at 4.66 eV photon energy. Electron affinities (EA) and vertical detachment energies (VDE) are determined, and the electronic structures of V- and V2-doped Con clusters are compared to those of pure Con clusters. Bonding and hybridization effects can be explained using a charge-transfer model. A peak shape analysis confirms the high stability of the Co12V cluster with a most plausible icosahedral structure with the V atom in the cage center. Together with ionization potentials (IP), the electronic structures of the clusters are compared to H2 chemisorption rate coefficients reported previously. Thus, this study completes the investigation of electronic properties and reactivity of the bimetallic ConVm clusters as a function of charge, size, and distribution. Although the reactivity of ConVm reveals a better correlation toward the EA rather than toward IP, no improved correlation...

Preparation of vanadium and vanadium oxide clusters by means of inert gas aggregation

Vanadium clusters were prepared by the inert gas aggregation technique by evaporating V of high purity. Structural characterization was performed by high-resolution transmission electron microscopy. Apart from vanadium clusters of bcc structures in different orientations, crystalline VO clusters of NaCl structures were observed, which was attributed to the reaction with free oxygen present on the amorphous carbon substrates. The latter could be detected by electron energy-loss spectroscopy. The exposure of the samples to air caused a change to amorphous structures, which re-crystallized under the electron beam. This effect was interpreted as a reaction with water present in the air.

One-dimensional dynamics of thedelectrons inα′−NaV2O5

We have studied the electronic properties of the ladder compound $\alpha'$-NaV$_{2}$O$_{5}$, adopting a joint experimental and theoretical approach. The momentum-dependent loss function was measured using electron energy-loss spectroscopy in transmission. The optical conductivity derived from the loss function by a Kramers-Kronig analysis agrees well with our results from LSDA+U band-structure calculations upon application of an antiferromagnetic alignment of the V~3$d_{xy}$ spins along the legs and an on-site Coulomb interaction U of between 2 and 3 eV. The decomposition of the calculated optical conductivity into contributions from transitions between selected energy regions of the DOS reveals the origin of the observed anisotropy of the optical conductivity. In addition, we have investigated the plasmon excitations related to transitions between the vanadium states within an effective 16 site vanadium cluster model. Good agreement between the theoretical and experimental loss function was obtained using the hopping parameters derived from the tight binding fit to the band-structure and moderate Coulomb interactions between the electrons within the ab plane.

Ultraviolet-visible photodissociation spectra of Vn+Xe (n=5–8) cluster complex cations

Photodissociation spectra of vanadium cluster ion-xenon atom complexes Vn+Xe (n=5–8) have been measured between 290 and 670 nm. Spectra have been obtained by recording the depletion signal induced on the mass-selected cluster current intensity by the absorption of a photon. Due to the weak interaction between the ionic cluster and the rare-gas atom, photodissociation spectra are regarded as the absorption spectra of the vanadium cluster cations themselves. The absorption bands are broad, but several peaks can be resolved for the smallest sizes. The influence of the rare-gas atom on the electronic structure of the vanadium cluster cation is probed by performing the measurements on krypton instead of on xenon complexes. The features of the spectra do not change, but a blueshift of 0.12 eV is observed from krypton to xenon.

A Kinetic Study of the Effect of Water on the Nitrogen-Fixing Ability of the Mg(OH)2–V(OH)2System

An increase in the limiting oxidation number of V2+ions in the presence of nitrogen in the Mg(OH)2–V(OH)2system was found. This phenomenon was interpreted from the standpoint of the existence of a critical size of vanadium clusters in an ion layer of the mixed hydroxide. The attainment of this critical size is necessary for the reduction of N2and the release of H2. This hypothesis also explains the specific activity of the system as an extremal function of the concentration of vanadium(II) at a constant Mg : V ratio. The effect of solvent (methanol–water) composition on the rate of nitrogen reduction supports the idea that the concentration of free water in the system plays a decisive role in this process. An increase in the intensity of H/D exchange in the presence of nitrogen, which is similar to that observed in biological systems, was found.

All-Electron Mixed-Basis Calculation to Optimize Structures of Vanadium Clusters

As a powerful ah initio method for systems with transition metal elements, all-electron mixed-basis approach which uses both plane waves and atomic orbitals as basis functions is tested to optimize structures of vanadium clusters, A good agreement with several previous calculations is obtained for V 2 and V 4 .

GEOMETRICAL AND ELECTRONIC STRUCTURES OF VANADIUM CLUSTERS, AND THEIR EVOLUTION FROM MOLECULAR TO BULK PHASE 

We have optimized the geometrical structures of the vanadium clusters Vn and discussed their evolution with cluster sizes in the average nearest-neighbor distances, coordination numbers, binding energies, ionization potential, electron affinities, density of states, valence band width, and magnetic moments using the density functional theory.We found that the clusters from V2 to V9 have distinctly molecular behaviour with dramatic size variations; V13, V15, and V19 are the transition regions from the molecular features to the bulk structures, and V27 and V51 possess almost the bulk properties.