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CHARACTERIZATION METHODS. 66 Electron Spectroscopy X-ray photoelectron spectroscopy (XPS) has been widely applied to the characterization of supported catalysts. This technique provides information on elemental composition and chemical environment, since XPS involves the excitation of core electrons. XPS is very surface-sensitive, sampling not more than 6 nm, and can be used for the characterization of multimetallic catalysts. The area under the peaks is a measure of concentration, while peak position provides information on the oxidation state. Limitations of the technique are signal overlap and degradation of the sample in the beam. CALORIMETRY Differential scanning calorimetry is a relatively simple technique that can be used very effectively in the study of the structure and stability of nanometer-scale materials. For example, the finest microstructures, with a grain size on the order of only a few nanometers, give diffraction patterns that contain broad halos and appear featureless in dark-field transmission microscopy. It is therefore often difficult to distinguish them from âtrulyâ amorphous structures, such as liquids and glasses, which have no translational symmetry at all. The kinetics of the transformation of both structures to one with sharp diffraction rings are fundamentally different, however. The amorphous structure transforms by nucleation and growth of new crystals, which is observed as an exothermic peak in isothermal calorimetry. The microcrystalline structure, on the other hand, does not undergo a phase transformation, but simply coarsens by a process of grain growth. This is observed in isothermal calorimetry as a continuously decreasing exothermal signal, corresponding to the elimination of interfacial enthalpy. This has recently been used, for example, to show that sputtered Al-transition metal alloys are microquasicrystalline (Chen and Spaepen, 1988). The high interfacial density of the nanometer-scale materials makes it possible to determine the absolute average interfacial enthalpy directly by calorimetric studies of the grain growth process (either isothermal or by scanning). This could be applied to a number of materials where absolute values of the interfacial energy are not known. At the same time, such studies could also contribute to the understanding of the grain growth process itself, which has recently seen a resurgence of interest. OTHER PROMISING METHODS A number of other recent developments in microstructural and microchemical analysis methods will almost certainly have a significant impact on the characterization of nanophase materials. Because of the ultrafine scale
