Bridging the support gap in heterogeneous ultrananocatalysis.

The geometric and electronic structures of Si(n), Si(n) (+), and AlSi(n-1) clusters (2< or =n< or =13) have been investigated using the ab initio molecular orbital theory under the density functional theory formalism. The hybrid exchange-correlation energy function (B3LYP) and a standard split-valence basis set with polarization functions [6-31G(d)] were employed for this purpose. Relative stabilities of these clusters have been analyzed based on their binding energies, second difference in energy (Delta (2)E) and fragmentation behavior. The equilibrium geometry of the neutral and charged Si(n) clusters show similar structural growth. However, significant differences have been observed in the electronic structure leading to their different stability pattern. While for neutral clusters, the Si(10) is magic, the extra stability of the Si(11) (+) cluster over the Si(10) (+) and Si(12) (+) bears evidence for the magic behavior of the Si(11) (+) cluster, which is in excellent agreement with the recent experimental observations. Similarly for AlSi(n-1) clusters, which is isoelectronic with Si(n) (+) clusters show extra stability of the AlSi(10) cluster suggesting the influence of the electronic structures for different stabilities between neutral and charged clusters. The ground state geometries of the AlSi(n-1) clusters show that the impurity Al atom prefers to substitute for the Si atom, that has the highest coordination number in the host Si(n) cluster. The fragmentation behavior of all these clusters show that while small clusters prefers to evaporate monomer, the larger ones dissociate into two stable clusters of smaller size.

Electronic properties of the hetero-structures consisting of silicene, graphene and BN monolayers under the influence of an electric field were investigated using density functional theory. With no electric field, both silicene/graphene and silicene/BN were shown to have a finite gap of about ∼50 meV, though silicene is a zero-gap two-dimensional material. Application of the field perpendicular to the bilayer system was found to facilitate modulation of the band gap, exhibiting an approximately linear relationship with the gap energy, in contrast to what was seen for the constituent monolayers. Also, the degree of the modulation was mainly determined by the Si-pz electronic states at the interface of the silicene/graphene and silicene/BN bilayers.

A combined experimental and theoretical investigation of Ag‐Pt sub‐nanometer clusters as heterogeneous catalysts in the CO→CO2 reaction (COox) is presented. Ag9Pt2 and Ag9Pt3 clusters are size‐selected in the gas phase, deposited on an ultrathin amorphous alumina support, and tested as catalysts experimentally under realistic conditions and by first‐principles simulations at realistic coverage. In situ GISAXS/TPRx demonstrates that the clusters do not sinter or deactivate even after prolonged exposure to reactants at high temperature, and present comparable, extremely high COox catalytic efficiency. Such high activity and stability are ascribed to a synergic role of Ag and Pt in ultranano‐aggregates, in which Pt anchors the clusters to the support and binds and activates two CO molecules, while Ag binds and activates O2, and Ag/Pt surface proximity disfavors poisoning by CO or oxidized species.

In this work, we report the growth behavior of small Pd n clusters (n = 1–7) on the α–Al2O3 (0001) surface using a first principle approach based on the plane wave-pseudopotential method. The results reveal that in general the interaction of Pd n clusters with Al2O3 surface deforms the equilibrium geometry of the isolated clusters. For Pd atom, the most preferred adsorption site is found to be on top of the oxygen atom with an interaction energy of 1.40 eV. For the dimer, binding between two Pd atoms is more favorable than atomic adsorption at a distance. The competition between Pd–Pd and Pd–surface interactions further governs the growth motif of larger clusters. As the size of the Pd cluster increases, it prefers an open structure to maximize the Pd–surface interaction. In case of Pd7, the compact pentagonal bipyramidal structure of the isolated cluster reorganizes into a hexagon with one central atom. Further, a comparison of chemical bonding analysis through electronic density of state (EDOS) between the gas phase and deposited clusters shows that the EDOS of the deposited Pd n cluster is significantly broader, which has been ascribed to the enhanced spd hybridization.

Using state of the art spin-polarized density functional theory it is found that a chemically inert (BN)(36) cluster can be activated by incorporating magnetic nanoparticles inside it. To illustrate this aspect we have calculated the geometries and electronic structure of Fe(BN)(36) and Fe(4)(BN)(36) clusters, which showed the appearance of gap states localized on the impurity atoms. The reaction of O(2) molecules with these clusters results in weak interaction and an elongation of the O-O bond. Further interaction of this complex species with an incoming CO molecule leads to the formation of CO(2). The reaction mechanism has been investigated via Langmuir-Hinshelwood and Elay-Rideal routes, and the minimum energy path calculations are performed using the elastic band method. These results have implications in designing novel materials based on metal nanoparticles for potential applications as industrial catalyst.

We report a systematic theoretical study on the growth pattern and electronic properties of acetonitrile clusters [(CH(3)CN)(n) (n = 1, 9, 12)] using density functional approach at the B3LYP6-31++G(d,p) level. Although we have considered a large number of configurations for each cluster, the stability of the lowest energy isomer was verified from the Hessian calculation. It is found that the lowest energy isomer of the dimer adopts an antiparallel configuration. For trimer and tetramer, cyclic ring structures were found to be favored over the dipole stabilized structure. In general, it is found that the intermolecular CH...N interactions play a significant role in the stabilization of the cyclic layered geometry of acetonitrile clusters. A critical comparison between trimer and tetramer clusters suggests that the three member cyclic ring is more stable than four member rings. The growth motif for larger clusters (n = 5-9, 12) follows a layered pattern consisting of three or four membered rings, which, in fact, is used as the building block. Based on the stability analysis, it is found that clusters with an even number of molecular entities are more stable than the odd clusters, except trimer and nonamer. The exceptional stability of these two clusters is attributed to the formation of trimembered cyclic rings, which have been found to form the building blocks for larger clusters.

Two dimensional nanostructures of group IV elements have attracted a great deal of attention because of their fundamental and technological applications. A graphene-like single layer of tin atoms, commonly called stanene, has recently been predicted to behave like a quantum spin Hall insulator. Here we report the atomic structure, stability and electron transport properties of stanene stabilized on a gold substrate. The optimization of geometry and electronic structure was carried out using a plane-wave based pseudo-potential approach. This work is divided into three parts: (i) the nature of chemical interaction between tin atoms and the gold support, (ii) the geometrical shape and electronic structure of the tin layer on the gold support and (iii) the electron transport behavior of the gold supported tin layer. The results show that tin atoms bind to the gold support through strong chemical bonds and significant electronic charge transfer occurs from tin to the gold support. Remarkably, for a layer of tin atoms, while a buckled structure is preferred in the free state, a planar graphene-like atomic arrangement is stabilized on the gold support. This structural change corroborates the metal-like band structure of the planar stanene in comparison to the semi-metallic buckled configuration. The tunneling current of the supported tin layer shows Ohmic-like behavior and the calculated STM pattern of the supported tin layer shows distinct images of 'holes', characteristic of the hexagonal lattice.