HREM studies of the structure and the oxidation process of copper clusters created by inert gas aggregation

Structure and reactivity with oxigen of Cu clusters in the size range of 4.5±2.5 nm created by the inert gas aggregation technique were studied by HREM. The pure Cu clusters investigated under clean conditions show the structures of MTP's with a small lattice dilatation of the (111) plane of 1.25%. For icosahedral and decahedral particles this dilatation corresponds to a splitting of the nearest neighbour distance showing two different values, i.e. dilatation of 2.2% and contraction of 2.8% for the two edges of the deformed tetrahedral subunits, respectively. Oxidation at room temperature and air pressure of 1 bar only begins after a few minutes of exposure to air, after having undergone creation of probably non-stoichiometric intermediate states in the cuprite Cu2O structure with the bulk values of the bond lengths

Reaktivität von Cu-Clustern

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

Study of partial oxidation of Cu clusters by HRTEM

Cu clusters of diameters less than 10 nm were selected as models to study oxidation as a function of size, chemical composition, and structure. The investigations were performed with high-resolution electron microscopy. It was shown that beginning with multiply twinned particles as stable structures for particles with diameters less than 5 nm and of cuboctahedra for larger particles, in the transitional state between pure metal and oxide both states can coexist within the same particle; this latter can be proved by HRTEM. It was also demonstrated that this is due to incorporating distortions as step dislocations within grain boundaries which can be shown in electron micrographs of high resolution better than 0.18 nm after image processing. The creation of sub-oxides with less reactivity than the pure metal clusters could also be demonstrated leading to morphologies which are different from those of the pure metal

Electron Microscopy Structural Characterisation of Nano‐Materials: Image Simulation and Image Processing

High resolution electron microscopy is now often used to determine the structure of nano‐materials: very small clusters, multilayers, precipitates etc. The images are often poor, because of the small size of the object, and their complete interpretation is very difficult. Image simulation has to be done in order to obtain a correct interpretation of these experimental images. This simulation is usually done using the multislice theory: however, a cluster or an atomic layer cannot be considered as a 3D‐periodical object, though it is sort of crystallised. Periodisation has already been applied to non periodical structures as defects, interfaces etc in crystallised matter but nano‐samples are different objects as they have only a small and deformed crystalline component. Image processing can then help to interpret the images if one is conscious of all the bugs which can be introduced in the process! In this paper, we want to forewarn newcomers in the field of HREM imaging for nano‐clusters and explain some of the problems linked with simulating and processing nano‐samples, insisting on all the wrong results which can be obtained by applying all the possible logicals which can be bought

Computer simulations of HREM images of metal clusters

Multislice calculations have been performed for Ag, Pd and Au clusters in the size range of ≃5.0 nm diameter of cuboctahedral, icosahedral and decahedral structures. It could be shown that tilt series are necessary for the classification of the structures. Particularly for arbitrary orientations, i.e. deviations from main directions such as 2-, 3- and 5-fold axes, the performance of computer simulations is mandatory. The influence of absorption is also studied for the case of a 100kV microscope by introducing a complex potential

Cu clusters: A HREM study of their oxidation

The reactivity of Cu clusters with a mean particle size of 4.50 {+-} 1.25 nm was studied with respect to oxidation at ambient pressure and temperature. Pure Cu clusters of this size range show icosahedral and decahedral structures with slightly changed bond lengths compared to the bulk. Small amounts of additional O{sub 2}, which were added during cluster nucleation, led to changes in the structure exhibiting, thereby, Cu bulk data. This structure is stabilized through the incorporation of small amounts of oxygen in the Cu clusters. The cubic clusters are less active when compared with the icosahedral and decahedral particles; the latter two mainly consist of deformed (111) net planes at the surface. The final state of oxidation is the Cu{sub 2}O cuprite structure. Intermediate states must be attributed to as yet nonidentified suboxide