Appearance of bulk properties in small tungsten oxide clusters

Tungsten and molybdenum oxide nanostructures: two-dimensional layers and nanoclusters

W- and Mo-oxides form an interesting class of materials, featuring structural complexities, stoichiometric flexibility, and versatile physical and chemical properties that render them attractive for many applications in diverse fields of nanotechnologies. In nanostructured form, novel properties and functionalities emerge as a result of quantum size and confinement effects. In this topical review, W- and Mo-oxide nanosystems are examined with particular emphasis on two-dimensional (2D) layers and small molecular-type clusters. We focus on the epitaxial growth of 2D layers on metal single crystal surfaces and investigate their novel geometries and structures by a surface science approach. The coupling between the oxide overlayer and the metal substrate surface is a decisive element in the formation of the oxide structures and interfacial strain and charge transfer are shown to determine the lowest energy structures. Atomic structure models as determined by density functional theory (DFT) simulations are reported and discussed for various interface situations, with strong and weak coupling. Free-standing (quasi-)2D oxide layers, so-called oxide nanosheets, are attracting a growing interest recently in the applied research community because of their easy synthesis via wet-chemical routes. Although they consist typically of several atomic layers thick-not always homogeneous-platelet systems, their quasi-2D character induces a number of features that make them attractive for optoelectronic, sensor or biotechnological device applications. A brief account of recently published preparation procedures of W- and Mo-oxide nanosheets and some prototypical examples of proof of concept applications are reported here. (MO₃)₃(M = W, Mo) clusters can be generated in the gas phase in nearly monodisperse form by a simple vacuum sublimation technique. These clusters, interesting molecular-type structures by their own account, can be deposited on a solid surface in a controlled way and be condensed into 2D W- and Mo-oxide layers; solid-state chemical reactions with pre-deposited surface oxide layers to form 2D ternary oxide compounds (tungstates, molybdates) have also been reported. The clusters have been proposed as model systems for molecular studies of reactive centres in catalytic reactions. Studies of the catalysis of (MO₃)₃clusters in unsupported and supported forms, using the conversion of alcohols as model reactions, are discussed. Finally, we close with a brief outlook of future perspectives.

Understanding the Structural and Electronic Properties of Photoactive Tungsten Oxide Nanoparticles from Density Functional Theory and GW Approaches

Tungsten trioxide (WO₃)-derived nanostructures have emerged recently as feasible semiconductors for photocatalytic purposes due to their visible-light harvesting that overcomes the drawbacks presented by TiO₂-derived nanoparticles (NPs). However, applications are still limited by the lack of fundamental knowledge at the nanoscale due to poor understanding of the physical processes that affect their photoactivity. To fill this gap, we report here a detailed computational study using a combined density functional theory (DFT)-GW scheme to investigate the electronic structure of realistic WO₃ NPs containing up to 1680 atoms. Different phases and morphologies are considered to provide reliable structure-property relationships. Upon proper benchmark of our DFT-GW methodology, we use this highly accurate approach to establish relevant rules for the design of photoactive WO₃ nanostructures by pointing out the most stable morphologies at the nanoscale and the appropriate size regime at which the photoactive efficiency is enhanced.

Bond Activation and Hydrogen Evolution from Water through Reactions with M3S4 (M = Mo, W) and W3S3 Anionic Clusters

Transition metal sulfides (TMS) are being investigated with increased frequency because of their ability to efficiently catalyze the hydrogen evolution reaction. We have studied the trimetallic TMS cluster ions, Mo₃S₄^-, W₃S₄^-, and W₃S₃^-, and probed their efficiency for bond activation and hydrogen evolution from water. These clusters have geometries that are related to the edge sites on bulk MoS₂ surfaces that are known to play a role in hydrogen evolution. Using density functional theory, the electronic structures of these clusters and their chemical reactivity with water have been investigated. The reaction mechanism involves the initial formation of hydroxyl and thiol groups, hydrogen migration to form an intermediate with a metal hydride bond, and finally, combination of a hydride and a proton to eliminate H₂. Using this mechanism, free energy profiles of the reactions of the three metal clusters with water have been constructed. Unlike previous reactivity studies of other related cluster systems, there is no overall energy barrier in the reactions involving the M₃S₄ systems. The energy required for the rate-determining step of the reaction (the initial addition of the cluster by water) is lower than the separated reactants (-0.8 kcal/mol for Mo and -5.1 kcal/mol for W). They confirm the M₃S₄^- cluster's ability to efficiently activate the chemical bonds in water to release H_2. Though the W₃S₃^- cluster is not as efficient at bond activation, it provides insights into the factors that contribute to the success of the M₃S₄ anionic systems in hydrogen evolution.

A DFT study of (WO3)3 nanoclusters adsorption on defective MgO ultrathin films on Ag(001)

The structures and electronic properties of (WO₃)₃ nanocluster adsorption on defective MgO ultrathin films supported on Ag(001) metal surfaces have been investigated by means of density functional theory (DFT) calculations including dispersion interactions.

The reactivity of stoichiometric tungsten oxide clusters towards carbon monoxide: the effects of cluster sizes and charge states

Density functional theory (DFT) calculations are employed to investigate the reactivity of tungsten oxide clusters towards carbon monoxide. Extensive structural searches show that all the ground-state structures of (WO3)n(+) (n = 1-4) contain an oxygen radical center with a lengthened W-O bond which is highly active in the oxidation of carbon monoxide. Energy profiles are calculated to determine the reaction mechanisms and evaluate the effect of cluster sizes. The monomer WO3(+) has the highest reactivity among the stoichiometric clusters of different sizes (WO3)n(+) (n = 1-4). The reaction mechanisms for CO with mono-nuclear stoichiometric tungsten oxide clusters with different charges (WO3(-/0/+)) are also studied to clarify the influence of charge states. Our calculated results show that the ability to oxidize CO gets weaker from WO3(+) to WO3(-) as the negative charge accumulates progressively.

Hydroxyl migration in heterotrimetallic clusters: an assessment of fluxionality pathways

Water splitting at the unsaturated metal center and subsequent hydroxyl migration are key steps toward successful H2 liberation from cheap and abundant water using transition metal cluster anions. In this report we initiate a theoretical study (DFT) to assess the efficacy of heterometallic cores instead of the widely studied and well established homometallic cores. To accomplish this goal, one tungsten center in W3O6(-) core has been replaced by different transition metals such as titanium, technetium, and osmium. Introduction of the heterometal makes the core asymmetric and electronically anisotropic. To evaluate the efficiency of these heterometallic cores, fluxionality pathways for hydroxyl migration have been studied in detail. We show that the cores W2TcO6(-) (2) and W2OsO6(-) (3) can exhibit fluxionality for hydroxyl migration and thus can potentially facilitate H2 liberation from H2O. Notably, a new class of low-energy structures generated upon oxide bridge opening process and subsequent structural rearrangement facilitates the hydroxyl migration event. To illustrate the heterometallic effect further, we show that previously inaccessible energy barriers for hydroxyl migration in a homometallic trimolybdenum core become energetically achievable when one of the metals is replaced by a 5d element osmium.

Dehydration, dehydrogenation, and condensation of alcohols on supported oxide catalysts based on cyclic (WO3)3 and (MoO3)3 clusters

The review summarizes recent synthesis and reactivity studies of model oxide catalysts prepared by the deposition of gas phase cyclic (WO₃)₃ and (MoO₃)₃ clusters.

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.

Probing the electronic properties of W3O(x)(-/0) (x = 0-2) and W3(2-) clusters: the aromaticity of W3 and W3(2-)

Density functional theory (DFT) calculations are employed to investigate the structural and electronic properties of bare tritungsten clusters (W3, W3(-), W3(2-)) and tritungsten oxide clusters W3Ox(-/0) (x = 1, 2). Generalized Koopmans' theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra (PES) for W3Ox(-) (x = 0-2) clusters. Extensive DFT calculations are performed in search of the lowest energy structures for both the anions and the neutrals. The bare tritungsten clusters are predicted to be triangular structures with D3h ((3)A1'), C2v ((2)A1) and D3h ((1)A1') symmetry for W3, W3(-) and W3(2-), respectively. For W3O(-) and W3O clusters, the oxygen atom occupies the terminal site, while the next added oxygen atom is found to be a bridging one in both W3O2(-) and W3O2 clusters. Molecular orbital analyses are carried out to elucidate the chemical bonding of these clusters and provide insights into the sequential oxidation from W3(-) to W3O2(-). Partial σ- and δ-aromaticity are revealed in the neutral W3 (D3h, (3)A1'), while the anion W3(2-) (D3h, (1)A1') possesses only δ-aromaticity.

Transition metal oxides for organic electronics: energetics, device physics and applications

During the last few years, transition metal oxides (TMO) such as molybdenum tri-oxide (MoO(3) ), vanadium pent-oxide (V(2) O(5) ) or tungsten tri-oxide (WO(3) ) have been extensively studied because of their exceptional electronic properties for charge injection and extraction in organic electronic devices. These unique properties have led to the performance enhancement of several types of devices and to a variety of novel applications. TMOs have been used to realize efficient and long-term stable p-type doping of wide band gap organic materials, charge-generation junctions for stacked organic light emitting diodes (OLED), sputtering buffer layers for semi-transparent devices, and organic photovoltaic (OPV) cells with improved charge extraction, enhanced power conversion efficiency and substantially improved long term stability. Energetics in general play a key role in advancing device structure and performance in organic electronics; however, the literature provides a very inconsistent picture of the electronic structure of TMOs and the resulting interpretation of their role as functional constituents in organic electronics. With this review we intend to clarify some of the existing misconceptions. An overview of TMO-based device architectures ranging from transparent OLEDs to tandem OPV cells is also given. Various TMO film deposition methods are reviewed, addressing vacuum evaporation and recent approaches for solution-based processing. The specific properties of the resulting materials and their role as functional layers in organic devices are discussed.

Molybdenum oxides versus molybdenum sulfides: geometric and electronic structures of Mo₃X(y)⁻ (X = O, S and y = 6, 9) clusters

We have conducted a comparative computational investigation of the molecular structure and water adsorption properties of molybdenum oxide and sulfide clusters using density functional theory methods. We have found that while Mo₃O₆⁻ and Mo₃S₆⁻ assume very similar ring-type isomers, Mo₃O₉⁻ and Mo₃S₉⁻ clusters are very different with Mo₃O₉⁻ having a ring-type structure and Mo₃S₉⁻ having a more open, linear-type geometry. The more rigid ∠(Mo-S-Mo) bond angle is the primary geometric property responsible for producing such different lowest energy isomers. By computing molecular complexation energies, it is observed that water is found to adsorb more strongly to Mo₃O₆⁻ than to Mo₃S₆⁻, due to a stronger oxide-water hydrogen bond, although dispersion effects reduce this difference when molybdenum centers contribute to the binding. Investigating the energetics of dissociative water addition to Mo₃X₆⁻ clusters, we find that, while the oxide cluster shows kinetic site-selectivity (bridging position vs terminal position), the sulfide cluster exhibits thermodynamic site-selectivity.

Tetratungsten oxide clusters W4O(n)(-/0) (n = 10-13): structural evolution and chemical bonding

Density functional theory (DFT) calculations are carried out to investigate the electronic and structural properties of a series of tetratungsten oxide clusters, W(4)O(n)(-/0) (n = 10-13). Generalized Koopmans' theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra (PES). A large energy gap (approximately 2.9 eV) is observed for the stoichiometric W(4)O(12) cluster, which reaches the bulk value. The calculations suggest that W(4)O(12)(-/0) have the planar eight-membered ring structures, in which each tungsten atom is tetrahedrally coordinated with two bridging O atoms and two terminal O atoms. W(4)O(10)(-/0) and W(4)O(11)(-) can be viewed as removing two and one terminal O atoms from W(4)O(12)(-/0), respectively. The W(4)O(11) neutral is an interesting species, which possesses the pentabridged structure. We show that W(4)O(11)(-) contains a localized W(3+) site, which can readily react with O(2) to form the W(4)O(13)(-) cluster, whereas the corresponding neutral W(4)O(13) can be viewed as replacing a terminal oxygen in W(4)O(12) by a peroxo O(2) unit. Molecular orbital analyses are performed to analyze the chemical bonding in the tetratungsten oxide clusters and to elucidate their electronic and structural evolution.

Carbon nanomaterials as catalysts for hydrogen uptake and release in NaAlH4

A synergistic approach involving experiment and first-principles theory not only shows that carbon nanostructures can be used as catalysts for hydrogen uptake and release in complex metal hydrides such as sodium alanate, NaAlH(4), but also provides an unambiguous understanding of how the catalysts work. Here we show that the stability of NaAlH(4) originates with the charge transfer from Na to the AlH(4) moiety, resulting in an ionic bond between Na(+) and AlH(4)(-) and a covalent bond between Al and H. Interaction of NaAlH(4) with an electronegative substrate such as carbon fullerene or nanotube affects the ability of Na to donate its charge to AlH(4), consequently weakening the Al-H bond and causing hydrogen to desorb at lower temperatures as well as facilitating the absorption of H(2) to reverse the dehydrogenation reaction. In addition, based on our experimental observations and theoretical calculations it appears the curvature of the carbon nanostructure plays a role in the catalytic process. Ab initio molecular dynamics simulation further reveals the time evolution of the charge transfer process.

Cluster reactivity experiments: employing mass spectrometry to investigate the molecular level details of catalytic oxidation reactions

Mass spectrometry is the most widely used tool in the study of the properties and reactivity of clusters in the gas phase. In this article, we demonstrate its use in investigating the molecular-level details of oxidation reactions occurring on the surfaces of heterogeneous catalysts via cluster reactivity experiments. Guided ion beam mass spectrometry (GIB-MS) employing a quadrupole-octopole-quadrupole (Q-O-Q) configuration enables mass-selected cluster ions to be reacted with various chemicals, providing insight into the effect of size, stoichiometry, and ionic charge state on the reactivity of catalyst materials. For positively charged tungsten oxide clusters, it is shown that species having the same stoichiometry as the bulk, WO(3)(+), W(2)O(6)(+), and W(3)O(9)(+), exhibit enhanced activity and selectivity for the transfer of a single oxygen atom to propylene (C(3)H(6)), suggesting the formation of propylene oxide (C(3)H(6)O), an important monomer used, for example, in the industrial production of plastics. Furthermore, the same stoichiometric clusters are demonstrated to be active for the oxidation of CO to CO(2), a reaction of significance to environmental pollution abatement. The findings reported herein suggest that the enhanced oxidation reactivity of these stoichiometric clusters may be due to the presence of radical oxygen centers (W-O) with elongated metal-oxygen bonds. The unique insights gained into bulk-phase oxidation catalysis through the application of mass spectrometry to cluster reactivity experiments are discussed.

Formation and Characterization of the Oxygen-Rich Hafnium Dioxygen Complexes: OHf(η2-O2)(η2-O3), Hf(η2-O2)3, and Hf(η2-O2)4

Hafnium atom oxidation by dioxygen molecules has been investigated using matrix isolation infrared absorption spectroscopy. The ground-state hafnium atom inserts into dioxygen to form primarily the previously characterized HfO(2) molecule in solid argon. Annealing allows the dioxygen molecules to diffuse and react with HfO(2) to form OHf(eta(2)-O(2))(eta(2)-O(3)), which is characterized as a side-on bonded oxo-superoxo hafnium ozonide complex. Under visible light (532 nm) irradiation, the OHf(eta(2)-O(2))(eta(2)-O(3)) complex either photochemically rearranges to a more stable Hf(eta(2)-O(2))(3) isomer, a side-on bonded di-superoxo hafnium peroxide complex, or reacts with dioxygen to form an unprecedented homoleptic tetra-superoxo hafnium complex: Hf(eta(2)-O(2))(4). The Hf(eta(2)-O(2))(4) complex is determined to possess a D(2d) geometry with a tetrahedral arrangement of four side-on bonded O(2) ligands around the hafnium atom, which thus presents an 8-fold coordination. These oxygen-rich complexes are photoreversible; that is, formation of Hf(eta(2)-O(2))(3) and Hf(eta(2)-O(2))(4) is accompanied by demise of OHf(eta(2)-O(2))(eta(2)-O(3)) under visible (532 nm) light irradiation and vice versa with UV (266 nm) light irradiation.

Photodissociation of Chromium Oxide Cluster Cations

Chromium oxide cluster cations, Cr(n)O(m)+, are produced by laser vaporization in a pulsed nozzle cluster source and detected with time-of-flight mass spectrometry. The mass spectrum exhibits a limited number of stoichiometries for each value of n, where m > n. The cluster cations are mass selected and photodissociated using the second (532 nm) or third (355 nm) harmonic output of a Nd:YAG laser. At either wavelength, multiphoton absorption is required to dissociate these clusters, which is consistent with their expected strong bonding. Cluster dissociation occurs via elimination of molecular oxygen, or by fission processes producing stable cation species and/or eliminating stable neutrals such as CrO3, Cr(2)O(5), or Cr(4)O(10). Specific cation clusters identified to be stable because they are produced repeatedly in the decomposition of larger clusters include Cr(2)O(4)+, Cr(3)O(6)+, Cr(3)O(7)+, Cr(4)O(9)+, and Cr(4)O(10)+.