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Structure and Reactivity of Isolated Metal Clusters

 

The Trapped Ione Electron Diffraction (TIED) experiment is a hybrid of ion-trap mass spectrometry and the electron diffraction technique. It allows experimental structure determination of cluster ions in gas phase. A high energy electron beam is scattered at a size-selected cluster ion ensemble. A structure assignment is achieved by comparison of experimental and simulated diffraction function from a candidate structure. Candidate structures are obtained from quantum chemical calculations performed either in our group or in collaboration with external theory groups.

TIED-Setup

 

We study the structural development of metal clusters from the dimer to the bulk. Metal clusters show strongly size- dependent physical and chemical properties. Gold clusters, for example, have size-dependent chemical reactivities and may even be catalytically active (e.g. CO-oxidation at low temperatures) while bulk gold is largely inert. As properties and structure strongly correlate, it is crucial to understand the development of structure with size for a better understanding and utilization of these clusters. For systems larger than 20 atoms the currently available computational performance still limits the quality of structural predictions and experimental structure determinations are essential.

 

Structural Evolution of small Ruthenium Cluster Anions

A combination of trapped ion electron diffraction measurements and DFT computations reveal the structural evolution of small isolated ruthenium clusters in the size range Ru4- to Ru20-. J. Chem. Phys. 142, 024319 (2015)

 

Structures of Medium-Sized Ruthenium Clusters: The Octahedral Motif

An unexpected Octahetral Ruthenium Clusterstructural motif is found for ruthenium clusters. Electron-diffraction measurements combined with density functional theory calculations show that free medium-sized ruthenium clusters form close-packed structures based on octahedral geometries. For Ru19 and Ru44 closed-shell octahedra are formed. In the case of Ru28, a double octahedron is obtained. For Ru38, instead of a truncated octahedron we obtain evidence for lower symmetry structures containing a reentrant surface, similar to catalytically active nanoparticle B5 surface sites. ChemPhysChem 15, 862 (2014).

Correlation of cluster and bulk structure

Trapped ion electron diffraction measurements of homonuclear 55-atom transition-metal cluster anions covering essentially all 3d and 4d elements show only four main structure families. Elements with the same bulk lattice morphology generally have a common cluster structure type. The cluster structure types differ in maximum atomic coordination numbers and effective interaction potential in analogy the corresponding bulk lattices. In addition this work describes and proves - for the first time - the occurrence of structural motifs other than the typical 55 atom cluster closed shell geometries icosahedron, decahedron and cuboctahedron discussed in literature so far.

 

Article Angew. Chem. Int. Edit. 52, 6103 (2013).

 

Bismuth cluster

The structures of bismuth cluster cations in the range between 4 and 14 atoms have been assigned by a combination of gas phase ion mobility, TIED and DFT calculations. Above 8 atoms the Bismuth clusters adopt prolate structures. These open structures are more like those seen for clusters of semiconducting-in-bulk elements (such as silicon) rather than resembling the compact structures typical for clusters of metallic-in-bulk elements. An accurate description requires taking spin-orbit coupling into account. For n = 11 we infer that low energy isomers can have fragmentation thresholds comparable to their structural interconversion barriers. This gives rise to experimental isomer distributions which are dependent on formation and annealing histories. More see J. Chem. Phys. 136, 154309 (2012)

Accelerated structure search

The computer time for the search of the global minimum energy structure of a cluster increases exponential with time. Tools for a systematic approach exists, especially genetic algorithms have proven useful. By including experimental electron scattering data (in terms of a weighted profile factor) in addition to the energy of a cluster structure as an additional criterion for evaluating trial structures leads to accelerated convergence behavior and increased robustness. More: Chem. Phys. Lett. 532, 119 (2012) 

 

Tin Clusters

The structures of medium-sized Tin clusters are based on only three repeatedly appearing subunit clusters, the Sn7 pentagonal bipyramid, the Sn9 tricapped trigonal prism and the Sn10 bicapped tetragonal antiprism. These subclusters are either edge or face connected (Sn15-, Sn16-) or combined to dimers (Sn18-, Sn20-) to form the clusters. In larger clusters combined subclusters are realized as in the case of Sn25-. It comprises a Sn10 connected to a Sn15 subunit which closely resembles the ground state of Sn15-. Finally, at the largest cluster size studied, first indications of the appearance of the diamond bulk structure were observed. More: J. Chem. Phys. 2010, 132, 211102 and Phys. Chem. Chem. Phys. 2012, 14, 234

Gold Clusters

The structures of Aun- (n=11-20) have been studied. Gold clusters in this size range show a large structural variability. For Au11- best agreement between experimental data and theoretical model structure was found for a planar structure. The TIED results indicate a 2D to 3D transition at a size of 12 gold atoms. Here a mixture of essentially iso-energetic 2D and 3D structures was found. Au13- to Au15- are flat 3D structures, where a new isomer hitherto unknown, was found for Au14-. Hollow cage structures were found for Au16-, Au17- and Au18-. Au19- and Au20- are tetrahedral structures. More: Phys. Rev. A 2008, 77, 053202 and Phys. Chem. Chem. Phys. 2009, 11, 4344

 

Some gold cluster ions have cage structures which can be filled with a dopant atom. Doping with magnetic atoms Fe and Co was possible without destroying the gold cage leading to endohedral structures like Fe@Au16-. Currently we extend the doping experiments to early transition metal atoms to study a structure forming influence of the dopant atom. Moreover measurements with larger number of dopant atoms are planned to explore the structures of alloyed gold clusters. More: Phys. Rev. B 2009, 79, 033413

Silver Clusters

The structures of mass-selected silver cluster cations Ag19+- Ag79+ have been probed at a temperature of 100 K. For the cluster sizes studied the resulting experimental data are invariably best described by structures based on the icosahedral motif, indicating an icosahedral growth sequence for silver clusters in this size range.

more: Nano Lett. 2005, 5, 1972 and J. Chem. Phys. 2006, 124, 244308

 

TIED publications