Formation, structure and bond dissociation thresholds of gas-phase vanadium oxide cluster ions

Pyrolysis of Mass-Selected (V2O5)NO− (N = 1−6) Clusters in a High-Temperature Linear Ion Trap Reactor

A high-temperature linear ion trap that can stably run up to 873 K was newly designed and installed into a homemade reflectron time-of-flight mass spectrometer coupled with a laser ablation cluster source and a quadrupole mass filter. The instrument was used to study the pyrolysis behavior of mass-selected (V2O5) NO− ( N = 1−6) cluster anions and the dissociation channels were clarified with atomistic precision. Similar to the dissociation behavior of the heated metal oxide cluster cations reported in literature, the desorption of either atomic oxygen atom or molecular O2 prevailed for the (V2O5) NO− clusters with N = 2−5 at 873 K. However, novel dissociation channels involving fragmentation of (V2O5) NO− to small-sized V xO y− anions concurrent with the release of neutral vanadium oxide species were identified for the clusters with N = 3−6. Significant variations of branching ratios for different dissociation channels were observed as a function of cluster size. The kinetic studies indicate that the dissociation rates of (V2O5) NO− monotonically increased with the increase of cluster size. The internal energies carried by the (V2O5) NO− clusters at 873 K as well as the energetics data for dissociation channels have been theoretically calculated to rationalize the experimental observations. The decomposition behavior of vanadium oxide clusters from this study can provide new insights into the pyrolysis mechanism of metal oxide nanoparticles that are widely used in high temperature catalysis.

Inverse Design of Nanoclusters for Light-Controlled CO2-HCOOH Interconversion

With global push of hydrogen economy, efficient scenarios for hydrogen storage, transportation, and generation are indispensable. Here we devise a strategy for controllable hydrogen fuel storage and retrieval via light-switched CO₂-to-HCOOH interconversion. To realize it, palladium sulfide nanocluster catalysts with multiple specific functionalities are directly searched by our home-developed inverse design approach based on genetic algorithm (IDOGA) and ab initio calculations. Over 500 low-energy Pd_xS_y (x + y ≤ 30) clusters are sieved through a multiobjective function combining stability, activity, optical absorption, and reduction capability of photocarriers. The structure-property relationships and key factors governing the trade-off among these stringent criteria are disclosed. Finally, 14 candidate Pd_xS_y clusters with proper sulfidation degree and high stability in an aqueous environment have been screened. Our IDOGA program provides a general approach for inverse search of nanoclusters with any designated elemental compositions and functionalities for any device applications.

Room-Temperature Methane Activation Mediated by Free Tantalum Cluster Cations: Size-by-Size Reactivity

The energetics of small cationic tantalum clusters and their gas-phase adsorption and dehydrogenation reaction pathways with methane are investigated with ion-trap experiments and spin-density-functional-theory calculations. Ta_n⁺ clusters are exposed to methane under multicollision conditions in a cryogenic ring electrode ion-trap. The cluster size affects the reaction efficiency and the number of consecutively dehydrogenated methane molecules. Small clusters (n = 1-4) dehydrogenate CH₄ and concurrently eliminate H₂, while larger clusters (n > 4) demonstrate only molecular adsorption of methane. Unique behavior is found for the Ta⁺ cation, which dehydrogenates consecutively up to four CH₄ molecules and is predicted theoretically to promote formation of a [Ta(CH₂-CH₂-CH₂)(CH₂)]⁺ product, exhibiting C-C coupled groups. Underlying mechanisms, including reaction-enhancing couplings between potential energy surfaces of different spin-multiplicities, are uncovered.

An oxygen-passivated vanadium cluster [V@V10O15]− with metal–metal coordination produced by reacting Vn− with O2

An oxygen-passivated vanadium cluster [V@V₁₀O₁₅]⁻ is reported by reacting V_n⁻ with O₂, giving rise to superatom features of metal–metal coordination and 3D aromaticity.

IR multiple photon dissociation spectroscopy of MO2+ (M = V, Nb, Ta)

A laser vaporization cluster source is coupled to the Fourier-transform ion cyclotron resonance mass spectrometer beamline of the free-electron laser for intracavity experiments. Gas phase metal ions and their oxides (VO₂ ⁺, NbO₂ ⁺, and TaO₂ ⁺) are formed and spectroscopically characterized using IR multiple-photon dissociation spectroscopy via loss of atomic oxygen and overcoming fragmentation energies of 3 eV-6 eV. The signal is observed for all MO₂ ⁺ fundamental modes: the symmetric and anti-symmetric ν₁ and ν₃ stretch modes in the 900 cm^-1-1000 cm^-1 range and the ν₂ bending mode in the 300 cm^-1-450 cm^-1 range. A remarkable substructure is observed for the bending vibration, which is at least partly due to the rovibrational substructure.

The bond dissociation energy of VO measured by resonant three-photon ionization spectroscopy

The predissociation threshold of VO has been measured using resonant three-photon ionization (R3PI) spectroscopy. Given the high density of electronic states in the molecule, it is argued that the molecule dissociates rapidly as soon as the thermochemical bond dissociation energy (BDE) is exceeded, allowing the measured predissociation threshold to be assigned as the BDE. This is the first time a BDE has been measured using the R3PI method. The first photon is provided by an optical parametric oscillator (OPO) laser that promotes VO into a high-energy, discrete vibronic state. A tunable dye laser then excites the molecule further to a resonant state close to the dissociation limit where there is a quasi-continuum of states. A second photon from the same dye laser pulse ionizes the molecule, generating VO⁺ ions. The dye laser is then scanned to higher energies, and when the energy of one OPO photon plus one dye photon exceeds the BDE, the molecule dissociates before another dye photon can be absorbed to induce ionization. The combined photon energy at the sharp drop in the ion signal is assigned as the BDE. The experiment has been repeated using four different intermediate states, all yielding the same BDE, D₀(VO) = 6.545(2) eV. Using thermochemical cycles, a revised value for the BDE of cationic VO is obtained, D₀(V⁺-O) = 6.053(2) eV. The 0 K enthalpy of formation for VO(g) is also derived as Δ_fH_0K ⁰VO(g) = 128.6(1.0) kJ mol^-1. Previous spectroscopic and thermochemical studies of VO are reviewed.

Understanding structure-activity relation in VxOy clusters of varied stoichiometry and sizes through conceptual density functional approach

Computations have been performed on V_xO_y clusters (with x = 1-8, y = 1-21) to explore their structure, stability, and reactivity based on local and global reactivity descriptors defined within the formalism of density functional theory (DFT). The vertical and adiabatic ionization energies and electron affinities are in accordance with Franck-Condon principle and suggest that the V_xO_y clusters are more likely to be electron acceptors than donors. The structure and reactivity of V_xO_y clusters delicately depend on their oxygen content and environment. Distinct active sites have been identified for each cluster species on the basis of coordination, symmetry, and charge distribution. The propensity of all the reactive sites towards an approaching electrophile and/or nucleophile has been studied using local reactivity descriptor. In oxygen-poor clusters, the vanadium atoms are more prone to nucleophilic attack. With an increase in oxygen concentration, the coordination number of vanadium increases and reaches four-fold, the site for nucleophilic attack shifts to terminal oxygens. We conclude that of all the stoichiometries, the stable V_xO_y clusters have the (VO₃)_a(V₂O₅)_b formula unit_. The localization of positive charge density in cubic cage structure of V₈O₂₀ successfully traps halide ions (F^-, Cl^-, and Br^-). In view of increasing use of metal oxide clusters in heterogeneous catalysis, the understanding of structure-activity relationship in vanadium oxides' clusters provided in the current study is highly desirable.

Vanadium(V) oxide clusters synthesized by sublimation from bulk under fully inert conditions

While laser ablation in combination with electron impact mass spectroscopy yield numerous fragments and reaction products, helium-mediated mass analysis reveals the sublimation from bulk in units of (V₂O₅)₂.

Oxide nanoparticles in the size range of a few nanometers are typically synthesized in solution or via laser ablation techniques, which open numerous channels for structural change via chemical reactions or fragmentation processes. In this work, neutral vanadium oxide nanoparticles are instead synthesized by sublimation from bulk in combination with a pickup by superfluid helium droplets. Mass spectroscopy measurements clearly demonstrate the preservation of the bulk stoichiometric ratio of vanadium to oxygen in He-grown nanoparticles, indicating a tendency towards tetrahedral coordination of the vanadium centers in finite geometries. This unexpected finding opens up new possibilities for a combined on-the-fly synthesis of nanoparticles consisting of metal and metal-oxide layers. In comparison to mass spectra obtained via direct ionization of vanadium oxide in an effusive beam, where strong fragmentation occurred, we observe a clear preference for (V₂O₅)_n oligomers with even n inside the He nanodroplets, which is further investigated and explained using the electronic structure theory.

Investigation of direct inkjet-printed versus spin-coated ZrO2 for sputter IGZO thin film transistor

In this work, a low leakage current ZrO2 was fabricated for sputter indium gallium zinc oxide (IGZO) thin-film transistor using direct inkjet-printing technology. Spin-coated and direct inkjet-printed ZrO2 were prepared to investigate the film formation process and electrical performance for different process. Homogeneous ZrO2 films were observed through the high-resolution TEM images. The chemical structure of ZrO2 films were investigated by XPS measurements. The inkjet-printed ZrO2 layer upon IGZO showed a superior performance on mobility and off state current, but a large Vth shift under positive bias stress. As a result, the TFT device based on inkjet-printed ZrO2 exhibited a saturation mobility of 12.4 cm2/Vs, an Ion/Ioff ratio of 106, a turn on voltage of 0 V and a 1.4-V Vth shift after 1-h PBS strain. Higher density films with less oxygen vacancy were responsible for low off state current for the printed ZrO2 device. The mechanism of deteriorated performance on PBS test can be ascribed to the In-rich region formed at the back channel which easily absorbs H2O and oxygen. The absorbed H2O and oxygen capture electrons under positive bias stress, serving as acceptors in TFT device. This work demonstrates the film formation process of direct inkjet-printed and spin-coated oxide films and reveals the potential of direct inkjet-printed oxide dielectric in high-performance oxide TFT device.

Furthering the reaction mechanism of cationic vanadium clusters towards oxygen

We prepared well-resolved V_n⁺ clusters and clarified the reactivity with oxygen involving both etching effect and building block addition.

Multiple Metal-Metal Bond or No Bond? The Electronic Structure of V2 O2

Detailed knowledge of the electronic structure of vanadium oxide clusters provides the basis for understanding and tuning their significant catalytic properties. However, already for the simple four-atom V₂ O₂ molecule, there are contradictory reports in the literature regarding the electronic ground state and a possible vanadium-vanadium bond. We herein show through a combination of experimental (matrix isolation) studies and theoretical results that there is a multiple vanadium-vanadium bond in this benchmark vanadium oxide molecule.

Understanding ligand effects in gold clusters using mass spectrometry

This review summarizes recent research on the influence of phosphine ligands on the size, stability, and reactivity of gold clusters synthesized in solution. Sub-nanometer clusters exhibit size- and composition-dependent properties that are unique from those of larger nanoparticles. The highly tunable properties of clusters and their high surface-to-volume ratio make them promising candidates for a variety of technological applications. However, because "each-atom-counts" toward defining cluster properties it is critically important to develop robust synthesis methods to efficiently prepare clusters of predetermined size. For decades phosphines have been known to direct the size-selected synthesis of gold clusters. Despite the preparation of numerous species it is still not understood how different functional groups at phosphine centers affect the size and properties of gold clusters. Using electrospray ionization mass spectrometry (ESI-MS) it is possible to characterize the effect of ligand substitution on the distribution of clusters formed in solution at defined reaction conditions. In addition, ligand exchange reactions on preformed clusters may be monitored using ESI-MS. Collision induced dissociation (CID) may also be employed to obtain qualitative insight into the fragmentation of mixed ligand clusters and the relative binding energies of differently substituted phosphines. Quantitative ligand binding energies and cluster stability may be determined employing surface induced dissociation (SID) in a custom-built Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS). Rice-Ramsperger-Kassel-Marcus (RRKM) based modeling of the SID data allows dissociation energies and entropy values to be extracted. The charge reduction and reactivity of atomically precise gold clusters, including partially ligated species generated in the gas-phase by in source CID, on well-defined surfaces may be explored using ion soft landing (SL) in a custom-built instrument combined with in situ time of flight secondary ion mass spectrometry (TOF-SIMS). Jointly, this multipronged experimental approach allows characterization of the full spectrum of relevant phenomena including cluster synthesis, ligand exchange, thermochemistry, surface immobilization, and reactivity. The fundamental insights obtained from this work will facilitate the directed synthesis of gold clusters with predetermined size and properties for specific applications.

Compositions and Structures of Vanadium Oxide Cluster Ions VmOn(±) (m = 2-20) Investigated by Ion Mobility Mass Spectrometry

Stable compositions and geometrical structures of vanadium oxide cluster ions, VmOn(±), were investigated by ion mobility mass spectrometry (IM-MS). The most stable compositions of vanadium oxide cluster cations were (V2O4)(V2O5)(m-2)/2(+) and (VO2)(V2O5)(m-1)/2(+), depending on the clusters with even and odd numbers of vanadium atoms. Compositions one-oxygen richer than the cations, such as (V2O5)m/2(-) and (VO3)(V2O5)(m-1)/2(-), were predominantly observed for cluster anions. Assignments of these stable cluster ion compositions, which were determined as a result of collision-induced dissociations in IM-MS, can partly be explained with consideration of spin density distribution. By comparing the experimental collision cross sections (CCSs) obtained from ion mobility measurement with CCSs of the theoretically calculated structures, we confirmed the patterned growth of geometrical structures partially discussed in previous theoretical and spectroscopic studies. We showed that even sized (V2O5)m/2(±) where m = 6-12 had right polygonal prism structures except for the anionic V12O30(-), and for the clusters of odd numbers of vanadium m, cations and anions can either have bridged or pyramid structures. Both of the odd sized structures proposed were derivatives from the even sized right polygonal prism structures. The exception, V12O30(-), which had a CCS almost equal to that of the neighboring smaller V11O28(-), should have a structure of higher density than the right hexagonal prism, in which it was proposed to be a captured pyramid structure, derived from V11O28(-).

Adsorption of a single gold or silver atom on vanadium oxide clusters

The single Au atom can be adsorbed on both V and O sites of vanadium oxide clusters with quite large binding energies, illustrating the stabilization of noble atoms in single-atom catalysts.

On the structural and electronic properties of hexanuclear vanadium oxide clusters V6On(-/0) (n=12-15): is V6O12 cluster planar or cage-like?

Density functional theory (DFT) calculations are carried out to investigate the structural and electronic properties of a series of hexanuclear vanadium oxide clusters V6On(-/0) (n=12-15). Generalized Koopmans' theorem is applied to predict the vertical detachment energies (VDEs) and simulate the photoelectron spectra (PES) for V6On(-) (n=12-15) clusters. Extensive DFT calculations are performed in search of the lowest-energy structures for both the anions and neutrals. All of these clusters appear to prefer the polyhedral cage structures, in contrast to the planar star-like structures observed in prior model surface studies for the V6O12 cluster. Molecular orbitals are performed to analyze the chemical bonding in the hexanuclear vanadium oxide clusters and provide insights into the sequential oxidation of V6On(-) (n=12-15) clusters. The V6On(-) (n=12-15) clusters possess well-defined V(5+) and V(3+) sites, and may serve as molecular models for surface defects. Electron spin density analyses show that the unpaired electrons in V6On(-) (n=12-14) clusters are primarily localized on the V(3+) sites rather than on the V(5+) sites. The difference gas phase versus model surface structures of V6O12 hints the critical roles of cluster-substrate interactions in stabilizing the planar V6O12 cluster on model surfaces.

Gas-phase synthesis of singly and multiply charged polyoxovanadate anions employing electrospray ionization and collision induced dissociation

Electrospray ionization mass spectrometry (ESI-MS) combined with in-source fragmentation and tandem mass spectrometry (MS/MS) experiments were used to generate a wide range of singly and multiply charged vanadium oxide cluster anions including VxOy(n-) and VxOyCl(n-) ions (x = 1-14, y = 2-36, n = 1-3), protonated clusters, and ligand-bound polyoxovanadate anions. The cluster anions were produced by electrospraying a solution of tetradecavanadate, V14O36Cl(L)5 (L = Et4N(+), tetraethylammonium), in acetonitrile. Under mild source conditions, ESI-MS generates a distribution of doubly and triply charged VxOyCl(n-) and VxOyCl(L)((n-1)-) clusters predominantly containing 14 vanadium atoms as well as their protonated analogs. Accurate mass measurement using a high-resolution LTQ/Orbitrap mass spectrometer (m/Δm = 60,000 at m/z 410) enabled unambiguous assignment of the elemental composition of the majority of peaks in the ESI-MS spectrum. In addition, high-sensitivity mass spectrometry allowed the charge state of the cluster ions to be assigned based on the separation of the major from the much less abundant minor isotope of vanadium. In-source fragmentation resulted in facile formation of smaller VxOyCl((1-2)-) and VxOy ((1-2)-) anions. Collision-induced dissociation (CID) experiments enabled systematic study of the gas-phase fragmentation pathways of the cluster anions originating from solution and from in-source CID. Surprisingly simple fragmentation patterns were obtained for all singly and doubly charged VxOyCl and VxOy species generated through multiple MS/MS experiments. In contrast, cluster anions originating directly from solution produced comparatively complex CID spectra. These results are consistent with the formation of more stable structures of VxOyCl and VxOy anions through low-energy CID. Furthermore, our results demonstrate that solution-phase synthesis of one precursor cluster anion combined with gas-phase CID is an efficient approach for the top-down synthesis of a wide range of singly and multiply charged gas-phase metal oxide cluster anions for subsequent investigations of structure and reactivity using mass spectrometry and ion spectroscopy techniques.

On the electronic structure of the low lying electronic states of vanadium trioxide

The electronic structure of transition metal oxides is frequently studied using density functional theory. Nonetheless, the electronic structure of VO3 has been found to be sensitive to the choice of functional. As a consequence, the basic question of whether or not the ground electronic state exhibits a Jahn-Teller distortion has yet to be resolved. Using basis sets of triple zeta quality and multireference configuration interaction wave functions as large as 700 million configuration state functions, we determine that the ground electronic state of VO3 is a (2)A2 state in C3v symmetry. The first two excited electronic states are also characterized and found to be the components of a degenerate (2)E state, in C3v symmetry, which exhibits a small Jahn-Teller distortion. The Jahn-Teller stabilization energy is only 40 cm(-1) and the barrier to pseudo-rotation is 9 cm(-1). This (2)E state exhibits some unexpected properties. In the vicinity of the minimum energy conical intersection, the local topography appears almost quadratic, rather than linear, in the Jahn-Teller active coordinates. This gives rise to three symmetry-related seams of conical intersections in addition to the symmetry-required seam and results in the suppression of the geometric phase effect. These features, attributable to small linear Jahn-Teller parameters, are usually found in states characterized by e(2) (or e(3)e(')) electron configurations rather than the e(3) configuration found here. In addition to its Jahn-Teller minimum, the first excited state exhibits a second minimum with a structure significantly distorted from C3v. A conical intersection with Cs symmetry connects the two minima and puts an upper limit of 190 cm(-1) on the barrier connecting these minima.

Computational study on the molecular structures and photoelectron spectra of bimetallic oxide clusters MW2O9(-/0) (M=V, Nb, Ta)

Density functional theory (DFT) and coupled cluster theory (CCSD(T)) calculations are carried out to investigate the electronic and structural properties of a series of bimetallic oxide clusters MW2O9(-/0) (M=V, Nb, Ta). Generalized Koopmans' theorem is applied to predict the vertical detachment energies (VDEs) and simulate the photoelectron spectra (PES). Theoretical calculations at the B3LYP level yield singlet and doublet ground states for the bimetallic anionic and neutral clusters, respectively. All the clusters present the six-membered ring structures with different symmetries, except that the TaW2O9(-) cluster shows a chained style with a penta-coordinated tantalum atom. Spin density analyses reveal oxygen radical species in all neutral clusters, consistent with their structural characteristics. Moreover, additional calculations are performed to study the oxidation reaction of CO molecule with the W3O9(+) cation and the isoelectronic VW2O9 cluster, and results indicate that the introduction of vanadium at tungsten site can efficiently improve the oxidation reactivity.

Invited review article: laser vaporization cluster sources

The laser vaporization cluster source has been used for the production of gas phase atomic clusters and metal-molecular complexes for 30 years. Numerous experiments in the chemistry and physics of clusters have employed this source. Its operation is simple in principle, but there are many subtle design features that influence the number and size of clusters produced, as well as their composition, charge state, and temperature. This article examines all aspects of the design of these cluster sources, discussing the relevant chemistry, physics, and mechanical aspects of experimental configurations employed by different labs. The principles detailed here provide a framework for the design and implementation of this source for new applications.

Oxidation reactions on neutral cobalt oxide clusters: experimental and theoretical studies

Reactions of neutral cobalt oxide clusters (Co(m)O(n), m = 3-9, n = 3-13) with CO, NO, C(2)H(2), and C(2)H(4) in a fast flow reactor are investigated by time of flight mass spectrometry employing 118 nm (10.5 eV) single photon ionization. Strong cluster size dependent behavior is observed for all the oxidation reactions; the Co(3)O(4) cluster has the highest reactivity for reactions with CO and NO. Cluster reactivity is also highly correlated with either one or more following factors: cluster size, Co(iii) concentration, the number of the cobalt atoms with high oxidation states, and the presence of an oxygen molecular moiety (an O-O bond) in the Co(m)O(n) clusters. The experimental cluster observations are in good agreement with condensed phase Co(3)O(4) behavior. Density functional theory calculations at the BPW91/TZVP level are carried out to explore the geometric and electronic structures of the Co(3)O(4) cluster, reaction intermediates, transition states, as well as reaction mechanisms. CO, NO, C(2)H(2), and C(2)H(4) are predicted to be adsorbed on the Co(ii) site, and react with one of the parallel bridge oxygen atoms between two Co(iii) atoms in the Co(3)O(4) cluster. Oxidation reactions with CO, NO, and C(2)H(2) on the Co(3)O(4) cluster are estimated as thermodynamically favorable and overall barrierless processes at room temperature. The oxidation reaction with C(2)H(4) is predicted to have a very small overall barrier (<0.23 eV). The oxygen bridge between two Co(iii) sites in the Co(3)O(4) cluster is responsible for the oxidation reactions with CO, NO, C(2)H(2), and C(2)H(4). Based on the gas phase experimental and theoretical cluster studies, a catalytic cycle for these oxidation reactions on a condensed phase cobalt oxide catalyst is proposed.