Aluminum Cluster Anions Research Articles

Structural Evolution of Carbon-Doped Aluminum Clusters AlnC- (n = 6-15): Anion Photoelectron Spectroscopy and Theoretical Calculations.

Carbon-doped aluminum cluster anions, AlnC- (n = 6-15), were generated by laser vaporization and investigated by mass-selected anion photoelectron spectroscopy. The geometric structures of AlnC- (n = 6-15) anions were determined by the comparison of theoretical calculations with the experimental results. It is found that the most stable structure of Al6C- is a carbon endohedral triangular prism. The Al7C- anion is a magic cluster with high stability. The structures of Al7-9C- can be viewed as the additional aluminum atoms attached around the triangular prism Al6C-. Two isomers of Al10C- have been detected in the experiments. The most stable one has a planar tetracoordinate carbon structure. The second one derives from Al9C- with the carbon atom located in a pentagonal bipyramid. The Al11C- anion has a bilayer structure composed of one planar tetracoordinate carbon and one aluminum-centered hexagon, in which the major interactions between two layers are multicenter bonds. The structures of Al12-14C- can be viewed as evolving from Al11C- by adding aluminum atoms to interact with the carbon atom. In Al15C-, the carbon atom stays at the surface with a tetracoordinate structure, and an icosahedral Al13 unit can be identified as a part of the geometric structure of Al15C-.

Ligated aluminum cluster anions, LAln- (n = 1-14, L = N[Si(Me)3]2).

A wide range of low oxidation state aluminum-containing cluster anions, LAln- (n = 1-14, L = N[Si(Me)3]2), were produced via reactions between aluminum cluster anions and hexamethyldisilazane (HMDS). These clusters were identified by mass spectrometry, with a few of them (n = 4, 6, and 7) further characterized by a synergy of anion photoelectron spectroscopy and density functional theory (DFT) based calculations. As compared to a previously reported method which reacts anionic aluminum hydrides with ligands, the direct reactions between aluminum cluster anions and ligands promise a more general synthetic scheme for preparing low oxidation state, ligated aluminum clusters over a large size range. Computations revealed structures in which a methyl-group of the ligand migrated onto the surface of the metal cluster, thereby resulting in "two metal-atom" insertion between Si-CH3 bond.

Exploring water adsorption and reactivity in a series of doped aluminum cluster anions.

We present a systematic density functional study of central- and surface-doped aluminum cluster anions Al12X- (X = Mg, B, Ga, Si, P, Sc-Zn), their interactions and reactivity with water. Adsorption of water molecules on central-doped clusters is governed by the cluster electron affinity. Doping introduces a dramatic change in the cluster electronic structure by virtue of different ordering and occupation of super-atomic shells, which leads to the creation of complementary active sites controlling the reactivity with water. Surface doping creates unequal charge distribution on the cluster surface, resulting in the adsorption and reactivity of surface-doped clusters being dominated by electrostatic effects. These results demonstrate the strong influence of the doping position on the nature of the interaction and reactivity of the cluster, and contribute to a better understanding of doping effects.

Probing the Structural Evolution of Gold–Aluminum Bimetallic Clusters (Au2Aln–, n = 3–11) Using Photoelectron Spectroscopy and Theoretical Calculations

We report a combined photoelectron spectroscopy and theoretical study of the structural evolution of aluminum cluster anions doped with two gold atoms, Au2Aln– (n = 3–11). Well-resolved photoelectron spectra have been obtained at several photon energies and are used to compare with theoretical calculations to elucidate the structures of the bimetallic clusters. Global minima of the Au2Aln– clusters were searched using the basin-hopping method combined with density functional theory calculations. Vertical detachment energies were computed for the low-lying isomers with the inclusion of spin–orbit effects and were used to generate simulated photoelectron spectra. Au2Al2– was previously found to exhibit a tetrahedral structure, whereas Au2Al3– is found currently to be planar. Beyond n = 3, the global minima of Au2Aln– are dominated by three-dimensional structures. A robust square-bipyramidal Al6 motif is observed for n = 6–9, leading to a highly stable tubular-like global minimum for Au2Al9–. Compact three-dimensional structures are observed for n = 10 and 11. Except for Au2Al4–, Au2Al6–, and Au2Al7–, the two gold atoms are separated in these digold-atom-doped aluminum clusters due to the strong Au–Al interactions.

Geometries, stabilities, electronic and magnetic properties of small aluminum cluster anions doped with cobalt: A density functional theory study

The geometrical structures, relative electronic and magnetic properties of small Al n Co– (1 ≤ n ≤ 9) clusters are systematically investigated within the framework of density functional theory at the BPW91 level. The single Co doping can dramatically affect the ground state geometries of the 1 Al n+1 - clusters. At the same time, the resulting geometries show that the lowest energy Al n Co– clusters prefer to be three dimensional structures. Here, the relative stabilities are investigated in terms of the calculated average binding energies, fragmentation energies, and second-order energy differences. Moreover, the result of the highest occupiedlowest unoccupied molecular orbital energy gaps indicates that Al6Co– clusters have the highest chemical stability for Al n Co– (1 ≤ n ≤ 9) clusters. Furthermore, the natural population analysis reveals that the charges in Al n Co– clusters transfer from the Al frames to the Co atom. Additionally, the analyses of the local and total magnetic moments of the Al n Co– clusters show that the magnetic effect mainly comes from the Co atom.

Production of warm aluminum cluster anions by femtosecond laser ablation

We report on the production of warm aluminum cluster anions, Al n − (1 ≤ n ≤ 26), after femtosecond laser ablation of an aluminum nitride substrate. Large cluster anions of n ≥ 8 suffer metastable dissociation after their production, which indicates the internal energy of the cluster anions is high enough for the dissociation of an Al atom. We find that the efficiency of metastable dissociation is dependent on the size of cluster anions and the dependence can be rationalized by the dissociation energy of an Al atom from the cluster anions calculated with thermochemical data of the clusters.

Doped aluminum cluster anions: size matters.

The global minima of the cluster anions with the generic chemical formula (XAl₁₂)²⁻, where X = Be, Mg, Ca, Sr, Ba, and Zn, are determined by an extensive search of their potential energy surfaces using the Gradient Embedded Genetic Algorithm (GEGA). All the characterized global minima have an icosahedral-like structure, resembling that of the Al₁₃⁻ cluster. These cages comprise closed-shell electronic configurations with 40 electrons, therefore, in accordance to the jellium model, they are predicted to be highly stable and amenable to experimental detection. The two preferred sites for the dopant species, at the center and at surface of the icosahedral cage, are stabilized depending on the atomic radius of X. Thus, while the small dopants (X = Be, Zn) sit preferably at the center of the cage, the preferred site for X = Mg, Ca, Sr, and Ba is at the surface. Since these dianions are not stable towards electron detachment, one Li cation is added in order to yield stable systems. Our computations show that in the global minimum form of Li(XAl₁₂)⁻, the lithium cation, ionically bonded to the Al atoms, does not change the structure of the (XAl12)²⁻ core.

Boron substitution in aluminum cluster anions: magic clusters and reactivity with oxygen.

We have studied the size-selective reactivity of AlnBm(-) clusters m = 1,2 with O2 to investigate the effect of congener substitution in energetic aluminum clusters. Mixed-metal clusters offer an additional strategy for tuning the electronic and geometric structure of clusters and by substituting an atom with a congener; we may investigate the effect of structural changes in clusters with similar electronic structures. Using a fast-flow tube mass spectrometer, we formed aluminum boride cluster anions and exposed them to molecular oxygen. We found multiple stable species with Al12B(-) and Al11B2(-) being highly resistant to reactivity with oxygen. These clusters behave in a similar manner as Al13(-), which has previously been found to be stable in oxygen because of its icosahedral geometry and its filled electronic shell. Al13(-) and Al12B(-) have icosahedral structures, while Al11B2(-) forms a distorted icosahedron. All three of these clusters have filled electronic shells, and Al12B(-) has a larger HOMO-LUMO gap due to its compact geometry. Other cluster sizes are investigated, and the structures of the AlnB(-) series are found to have endohedrally doped B atoms, as do many of the AlnB2(-) clusters. The primary etching products are found to be a loss of two Al2O molecules, with boron likely to remain in the cluster.

Aluminum Zintl anion moieties within sodium aluminum clusters

Through a synergetic combination of anion photoelectron spectroscopy and density functional theory based calculations, we have established that aluminum moieties within selected sodium-aluminum clusters are Zintl anions. Sodium-aluminum cluster anions, Na(m)Al(n)(-), were generated in a pulsed arc discharge source. After mass selection, their photoelectron spectra were measured by a magnetic bottle, electron energy analyzer. Calculations on a select sub-set of stoichiometries provided geometric structures and full charge analyses for both cluster anions and their neutral cluster counterparts, as well as photodetachment transition energies (stick spectra), and fragment molecular orbital based correlation diagrams.

Gas-phase reactivity of aluminum cluster anions with ethanethiol: Carbon–sulfur bond activation

We report a joint experimental and theoretical study of the gas-phase reactivity of Al cluster anions with ethanethiol (EtSH) in a fast-flow tube reactor. Nearly all Aln- clusters observed are reactive at the presence of sufficient EtSH molecules with minor exceptions. Sulfide species AlnSm- dominate the observed reaction products, indicating C–S bond activation of EtSH. By using DFT calculations we provide an in-depth analysis on the interesting cluster reactivity of Aln- with EtSH. It is demonstrated that the desulfurization leading to AlnSm- products is associated with the dehydrogen processes successively initiated by S–H bond cleavage and C–H bond cracking.

Asymmetric Partitioning of Metals among Cluster Anions and Cations Generated via Laser Ablation of Mixed Aluminum/Group 6 Transition Metal Targets

While high-power laser ablation of metal alloys indiscriminately produces gas-phase atomic ions in proportion to the abundance of the various metals in the alloy, gas-phase ions produced by moderate-power laser ablation sources coupled with molecular beams are formed by more complicated mechanisms. A mass spectrometric study that directly compares the mass distributions of cluster anions and cations generated from laser ablation of pure aluminum, an aluminum/molybdenum mixed target, and an aluminum/tungsten mixed target is detailed. Mass spectra of anionic species generated from the mixed targets showed that both tungsten and molybdenum were in higher abundance in the negatively charged species than in the target material. Mass spectra of the cationic species showed primarily Al(+) and aluminum oxide and hydroxide cluster cations. No molybdenum- or tungsten-containing cluster cations were definitively assigned. The asymmetric distribution of aluminum and Group 6 transition metals in cation and anion cluster composition is attributed to the low ionization energy of atomic aluminum and aluminum suboxide clusters. In addition, the propensity of both molybdenum and tungsten to form metal oxide cluster anions under the same conditions that favor metallic aluminum cluster anions is attributed to differences in the optical properties of the surface oxide that is present in the metal powders used to prepare the ablation targets. Mechanisms of mixed metal oxide clusters are considered.

Ligand-Induced Active Sites: Reactivity of Iodine-Protected Aluminum Superatoms with Methanol

The quantum states in metal clusters are bunched into electronic shells as in atoms. Ligands including halogens or thiols modify the electronic structure through bonding, resulting in stable clusters with filled electronic shells that are resistant to oxygen etching. We demonstrate that the stabilization afforded by ligands is partially confounded because the ligands perturb the charge density of the metallic core, inducing Lewis acid-base sites that make the cluster reactive in a protic environment. We demonstrate the importance of induced active sites by studying the reactivity of methanol with two classes of iodine-passivated aluminum cluster anions: Al(13)I(x)(-), which has a closed geometric shell, and Al(14)I(y)(-), which has an adatom-decorated core. Two adjacent ligands on the closed geometric shell of Al(13)(-) activate the cluster, while in Al(14)I(3)(-) the I induces an active site on the adatom, making the cluster reactive, explaining ligand-protected clusters' preference for closed geometric shells.

Edge-Induced Active Sites Enhance the Reactivity of Large Aluminum Cluster Anions with Alcohols

Understanding the emergence of properties from the size-selective cluster regime to larger nanoparticles is one of the principal goals of nanoscience. We have measured the size-selective reactivity of aluminum cluster anions with alcohols. All clusters with more than 20 atoms are found to be reactive, while Al11(-), Al13(-), and Al20(-) show enhanced resistance to oxidation at smaller sizes. The reactivity of aluminum cluster anions with water, methanol, and tert-butyl alcohol all exhibit patterns that require complementary active sites (Lewis acid, Lewis base) on adjacent atoms. Theoretical investigations reveal that at small sizes, the location of reactive pairs occurs on specific active sites, but at larger sizes the reactive pairs begin to accumulate on the edges between facets, marking the transition from the nonscalable size-dependent regime to the scalable regime where the nanoparticles are universally reactive.

Geometries, stabilities, electronic, and magnetic properties of small aluminum cluster anions doped with iron: A density functional theory study

The geometrical structures, relative stabilities, electronic and magnetic properties of small Al n Fe − (1 ≤ n ≤ 9) clusters, in comparison with pure aluminum clusters have been systematically investigated by means of density functional calculations at the BPW91 level. The resulting geometries show that the frameworks of the lowest energy Al n Fe − clusters prefer to be three-dimensional structures. Here, the relative stabilities are investigated in terms of the calculated average binding energies, fragmentation energies, and second-order difference of energies. Moreover, the result of the highest occupied-lowest unoccupied molecular orbital energy gaps indicates that Al 3Fe − cluster has the highest chemical stability for Al n Fe − (1 ≤ n ≤ 9) clusters. Furthermore, the natural population analysis reveals that the charges in Al n Fe − clusters transfer from the Al frames to the Fe atom. Additionally, the analyses of the local and total magnetic moments of Al n Fe − clusters show that the magnetic influence mainly come from the Fe atom.

Competition between delayed ionization and fragmentation of laser-excited Al4−

A systematic study of the competition between delayed electron and delayed atom emission from photoexcited Al${}_{4}^{\ensuremath{-}}$ clusters was performed using a bent electrostatic ion beam trap, which allows for simultaneous measurement of both decay channels. The aluminum cluster anions, produced with high internal energy in a sputter source, are stored for 100 ms prior to their excitation by a short laser pulse. Delayed ionization and delayed fragmentation were observed for photon energies 1.55 eV $<{E}_{\ensuremath{\lambda}}<$ 2.25 eV. Decay spectra, integrated relative cross sections, and branching ratios were determined as a function of photon energy and storage time. The results show the characteristic pattern expected for a statistical decay ofthe photoexcited Al${}_{4}^{\ensuremath{-}}$ clusters; however, the ratio between fragmentation and ionization is found to increase with decreasing photon energy, even though the threshold energy for fragmentation is calculated to be $~0.5$ eV higher than the adiabatic electron detachment energy. It is suggested that the tetramers, while cooling down in the trap, may be in long-lived excited configurations with lower effective fragmentation barriers; Al${}_{4}^{\ensuremath{-}}$ clusters carrying high rotational angular momenta are discussed as possible candidates. Results obtained for other aluminum clusters Al${}_{n}^{\ensuremath{-}}$ ($5\ensuremath{\leqslant}n\ensuremath{\leqslant}9$) are also reported.

Search for dimer emission from photoexcitedAl4−

Aluminum cluster anions were stored in an electrostatic ion storage ring and irradiated by a laser pulse. Neutral as well as charged decay products emitted $20 \mathrm{\ensuremath{\mu}}\mathrm{s}$ after the excitation were identified and recorded using the daughter ion mass spectrometry method. Besides delayed electron detachment, only monomer emission was observed for ${{\mathrm{Al}}_{4}}^{\ensuremath{-}}$. The branching ratio for the monomer channel was determined to be $8\ifmmode\pm\else\textpm\fi{}2%$, while the dimer channel was found to be $l0.01%$.

Reactivity of Aluminum Cluster Anions with Water: Origins of Reactivity and Mechanisms for H2 Release

The reactivity of aluminum anion clusters with water was found to exhibit variations with size, with some clusters exhibiting negligible reactivity, others absorbing one or more water, while even others releasing H(2) with addition of multiple waters. (Roach, P.J., Woodward, W.H. et al. Science, 2009, 323, 492). Herein, we provide further details on the role of complementary active sites in the breaking of the O-H bond on the cluster. We examine the reactions of Al(n)(-) + H(2)O where n = 7-18, and show how the complementary active sites may be best identified. The clusters with active sites are found to be reactive, and clusters with barriers to reactivity have an absence of paired active sites. The role of charge in the reactivity is considered, which could account for the observed increase in reactivity at large sizes. The H(2) release in the reactivity of Al(17)(-) with multiple water molecules is also studied by comparing multiple reaction pathways, and the selective H(2) production is explained by the first water inducing a new active site. A mechanism for transferring hydroxyl groups on the surface of the cluster is also discussed.

Photoelectron spectroscopy of cold aluminum cluster anions: Comparison with density functional theory results

Photoelectron spectra of cold aluminum cluster anions Al(n)(-) have been measured in the size range n=13-75 and are compared to the results of density functional theory calculations. Good agreement between the measured spectra and the calculated density of states is obtained for most sizes, which gives strong evidence that the correct structures have been found. In particular the results confirm the occurrence of rather different structural motifs in this size range, from fcc-like stacks over fragments of decahedrons to disordered structures. An analysis of the density of states of representatives of the different structural motifs shows that the electronic structure is strongly influenced by the cluster geometry, and that a clear jelliumlike electron shell structure is present only in some exceptional cases.

Reactivity of aluminum cluster anions with ammonia: Selective etching of Al11− and Al12−

Reactivity of aluminum cluster anions toward ammonia was studied via mass spectrometry. Highly selective etching of Al(11)(-) and Al(12)(-) was observed at low concentrations of ammonia. However, at sufficiently high concentrations of ammonia, all other sizes of aluminum cluster anions, except for Al(13)(-), were also observed to deplete. The disappearance of Al(11)(-) and Al(12)(-) was accompanied by concurrent production of Al(11)NH(3)(-) and Al(12)NH(3)(-) species, respectively. Theoretical simulations of the photoelectron spectrum of Al(11)NH(3)(-) showed conclusively that its ammonia moiety is chemisorbed without dissociation, although in the case of Al(12)NH(3)(-), dissociation of the ammonia moiety could not be excluded. Moreover, since differences in calculated Al(n)(-) + NH(3) (n=9-12) reaction energies were not able to explain the observed selective etching of Al(11)(-) and Al(12)(-), we concluded that thermodynamics plays only a minor role in determining the observed reactivity pattern, and that kinetics is the more influential factor. In particular, the conversion from the physisorbed Al(n)(-)(NH(3)) to chemisorbed Al(n)NH(3)(-) species is proposed as the likely rate-limiting step.

Competition between ionization and fragmentation of small aluminum cluster anions

The competition between delayed fragmentation and delayed ionization of laser excited Al-4 clusters was measured by simultaneous detection of charged and neutral fragments.