Reaction mechanism between small-sized Ce clusters and water molecules: an ab initio investigation on Cen + H2O.

Reactions of small-sized cerium clusters Cen (n = 1-3) with a single water molecule are systematically investigated theoretically. The ground state structures of the Cen/H2O complex and the reaction pathways between Cen + H2O are predicted. Our results show the size-dependent reactivity of small-sized Ce clusters. The calculated reaction energies and reaction barriers indicate that the reactivity between Cen and water becomes higher with increasing cluster size. The predicted reaction pathways show that the single Ce atom and the Ce2 and Ce3 clusters can all easily react with H2O and dissociate the water molecule. Under UV-irradiation, the reaction of a Ce atom with a single H2O molecule may even release an H2 molecule. The reaction of either Ce2 or Ce3 with a single H2O molecule can fully dissociate the H2O into H and O atoms while it is bonded with the Ce cluster. The electronic configuration and oxidation states of the Ce atoms in the products and the higher occupied molecular orbitals are analyzed by using the natural bond orbital (NBO) analysis method, from which the high reactivity between the reaction products of Cen + H2O and an additional H2O molecule is predicted. Our results offer deeper molecular insights into the chemical reactivity of Ce, which could be helpful for developing more efficient Ce-doped or Ce-based catalysts.

Stability of Metal-Encapsulating Boron Fullerene B40.

The structural stability of MB40 (M = Li, Na, K, Ba, and Tl) is investigated on the basis of density-functional theory calculations at the PBE0 level. Particular attention is placed on the relative stability between the endohedral and exohedral configurations of metalloborospherenes. It is found that the Na and Ba atoms can be stably encapsulated inside the B40 cage, whereas the Li, K, and Tl atoms favor the exohedral configuration where the dopant caps one of heptagons of B40 cage. In-depth analysis of the endohedral versus exohedral configurations with different dopants suggests that besides the comparable atomic size with the cage size, another key factor that can affect stability of endohedral versus exohedral configuration is the interaction between the dopant and B atoms. The infrared (IR) spectra of the endohedral C2v Na@B40 and exohedral Cs Na&B40 clusters are also computed, from which some useful spectral indictors may be used for identification of the structures in the future experiments.

Au60-: The Smallest Gold Cluster with the High-Symmetry Icosahedral Core Au13.

Among coinage metal nanoclusters with 55 atoms, only Ag55- and Cu55- are the geometric magic-number clusters, as both exhibit icosahedral symmetry. Au55-, however, exhibits much lower symmetry due largely to the strong relativistic bonding effect. In this study, we collect a much larger population (>10,000 isomers) of low-energy isomers of Au55- to Au60- by using the combined density-functional theory and basin-hopping global optimization method. We also include the spin-orbit effect in the density-functional theory computation to achieve simulated photoelectron spectra in quantitative fashion. Remarkably, we uncover that the Au13 core with the highest icosahedral ( Ih) symmetry emerges at the size of Au60-. Stability analysis suggests that Au57- with 58 valence electrons, an electronic magic number, is the relatively more stable cluster in the size range considered. Overall, in this size range we reveal a compromise between the trend toward having a perfect icosahedral 13-atom core and the strong relativistic bonding effect.

Structural Evolution of Core-Shell Gold Nanoclusters: Aun- (n = 42-50).

Gold nanoclusters have attracted great attention in the past decade due to their remarkable size-dependent electronic, optical, and catalytic properties. However, the structures of large gold clusters are still not well-known because of the challenges in global structural searches. Here we report a joint photoelectron spectroscopy (PES) and theoretical study of the structural evolution of negatively charged core-shell gold nanoclusters (Aun-) for n = 42-50. Photoelectron spectra of size-selected Aun- clusters are well resolved with distinct spectral features, suggesting a dominating structural type. The combined PES data and density functional calculations allow us to systematically identify the global minimum or candidates of the global minima of these relatively large gold nanoclusters, which are found to possess low-symmetry structures with gradually increasing core sizes. Remarkably, the four-atom tetrahedral core, observed first in Au33-, continues to be highly robust and is even present in clusters as large as Au42-. Starting from Au43-, a five-atom trigonal bipyramidal core appears and persists until Au47-. Au48- possesses a six-atom core, while Au49- and Au50- feature seven- and eight-atom cores, respectively. Notably, both Au46- and Au47- contain a pyramidal Au20 motif, which is stacked with another truncated pyramid by sharing a common 10-atom triangular face. The present study sheds light on our understanding of the structural evolution of the medium-sized gold nanoclusters, the shells and core as well as how the core-shell structures may start to embrace the golden pyramid (bulk-like) fragment.

Probing the structures of gold-aluminum alloy clusters AuxAly(-): a joint experimental and theoretical study.

Besides the size and structure, compositions can also dramatically affect the properties of alloy nanoclusters. Due to the added degrees of freedom, determination of the global minimum structures for multi-component nanoclusters poses even greater challenges, both experimentally and theoretically. Here we report a systematic and joint experimental/theoretical study of a series of gold-aluminum alloy clusters, AuxAly(-) (x + y = 7,8), with various compositions (x = 1-3; y = 4-7). Well-resolved photoelectron spectra have been obtained for these clusters at different photon energies. Basin-hopping global searches, coupled with density functional theory calculations, are used to identify low-lying structures of the bimetallic clusters. By comparing computed electronic densities of states of the low-lying isomers with the experimental photoelectron spectra, the global minima are determined. It is found that for y ≥ 6 there is a strong tendency to form the magic-number square bi-pyramid motif of Al6(-) in the AuxAly(-) clusters, suggesting that the Al-Al interaction dominates the Au-Au interaction in the mixed clusters. A closely related trend is that for x > 1, the gold atoms tend to be separated by Al atoms unless only the magic-number Al6(-) square bi-pyramid motif is present, suggesting that in the small-sized mixed clusters, Al and Au components do not completely mix with one another. Overall, the Al component appears to play a more dominant role due to the high robustness of the magic-number Al6(-) square bi-pyramid motif, whereas the Au component tends to be either "adsorbed" onto the Al6(-) square bi-pyramid motif if y ≥ 6, or stays away from one another if x < y < 6.