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CHARACTERIZATION METHODS. 62 EXAFS can be used to study the impact of pretreatment on the surface composition of bimetallic clusters and the impact of impurities, such as sulfur and carbon, on the segregation of metals at the surface of particles. An EXAFS investigation by Meitzner and coworkers (1985) of the structure of bimetallic Re-Cu, Ir-Cu, and Pt-Cu clusters explored the effect of varying miscibility of the components on the structure of the clusters. These authors showed the interesting result that in the bimetallic clusters the atoms of Re, Ir, and Pt are all coordinated to Cu as extensively as they are to atoms of their own kind. This good coordination to Cu in the cluster configuration is in contrast to the only slight miscibility of Ir and Cu and the complete immiscibility of Re and Cu in the bulk. The miscibility of these metals is attributed to the high dispersion of the metal clusters. TRANSMISSION ELECTRON MICROSCOPY Structural Imaging Because of the ultrafine grain size of nanophase materials, transmission electron microscopy is an important tool for the direct characterization of grain-size distribution and morphologies. In addition, atomic-resolution transmission electron microscopy holds the promise for elucidating the atomic structure of nanophase interfaces in a manner difficult to achieve with other structural probes. A high-resolution electron micrograph of nanophase TiO2 is shown in Figure 16. Figure 16 High-resolution transmission electron micrograph of nanophase TiO2 (rutile) after sintering for one-half hour at 500°C (Siegel and Hahn, 1987).
