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Free keywords:
Cluster, High resolution electron microscopy, Structure, Oxidation
13
Abstract:
the pure Cu MTPs Cu-clusters prepared by means of the inert gas aggregation technique in the size range between 1 and 10 nm diameter have been studied with high resolution transmission electron microscopy (HRTEM) with respect to their structures and their reactivities with oxygen. The preparations were performed from a supersaturated metal-argon vapour. Also small amounts of oxygen were added during the nucleation process, i.e., 10-1-10-3 mbar partial pressures of pure O2. The clusters were deposited on amorphous carbon films of about 3 nm thickness and transferred under argon as protection gas into the Philips 200 kV CM200 FEG microscope in a transfer system which also served as reaction chamber.
The results may be summerized as follows: Pure Cu-clusters smaller than 5 nm diameter under conditions of our preparations show the structures of multiply twinned particles (MTP) with 5-fold symmetries. They exhibit small lattice dilatations of about 2% compared with the bulk caused by sometimes partially asymmetric distributed strains. The dilatation therefore depends on the structure. In particular, larger particles showed the fcc-bulk structure with cuboctahedral morphology. The clusters prepared with additional small amounts of O2 showed deviations from the pure Cu-cluster structures, i.e., fcc-structures even for small particle sizes were stabilized. The creation of suboxides by this technique is favourable, but the presence of oxygen for the preparation with 10-3 mbar O2 partial pressure could not be proved. For larger amounts of oxygen, transitions between pure Cu and Cu2O as cuprite, however, could be visualized by HRTEM. Many not yet explainable intermediate states could be realised. After inspection in the microscope, all samples were exposed to air at room temperature for different time periods and again studied by HRTEM. Clusters, which had been interpreted as non-MTP suboxides with small amounts of oxygen, some of them as cubic structures, showed less reactivity with respect to oxygen than and the larger fcc bulk clusters.
