The Raft-Like Structure of Supported PtRu 5
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Mat. Res. Soc. Symp. Proc. Vol. 497 © 1998 Materials Research Society
cross-sections, which can then be converted to number of atoms. Accurate measurement of the HAADF detector efficiency is crucial for this technique. There are two methods for measuring the detector efficiency: (1). Directly measuring the current with a Faraday cup, and then tilting the electron beam onto the detector and measuring the detected signal, or (2) using the carbon as an internal standard. The data presented here was obtained by using the carbon as an internal standard. The tilted bean method was also performed in order to confirm the measurements of the number of atoms. RESULTS Simultaneous BF and HAADF images were taken (Figure 1(a) and 1(b)). There existed regions with virtually no clusters and regions with large number of clusters.
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n Figure l(a): BF image of PtRu5.
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Figure l(b): Corresponding HAADF image
The clusters were also observed to be preferentially located at terraces, as can be seen in figure 2. Figure 2(a) is the HAADF image, which is sensitive to the atomic number, Z, of the material. Hence, the PtRu5 clusters appear bright compared to the carbon support. Figure 2(b) is the corresponding secondary electron image, where the surface morphology, such as terraces, of the carbon black substrate can be seen. The arrows point to regions where the clusters are located at terraces. EDS was performed on these specimens. 20 clusters were examined, where the relative concentrations of the Ru and Pt were: Ru mean = 77.6 ± 5%and Pt mean = 22.4 ±5%. EDS spectra taken from the entire region at IM magnification gave: Ru mean = 77.19A and Pt mean = 23±1.7%. Hence, we conclude that the relative atomic concentration of Pt to Ru between each PtRu5 particle is constant. Images similar to Figure I(b) were analyzed by the STEM-based mass-spectroscopic technique. A spread in intensity is visually observable, which correlates to a spread in the measured scattered cross-sections. The average measured scattering cross-section corresponded to 4 PtRu5 groups or 24 atoms, where 30 clusters were analyzed. ,The diameter of these clusters were also measured from the STEM, and the average was 15.6A. Using an average4 number density of Ru and Pt, if the shape of the cluster is spherical, then, for a diameter of 15A, the particle contains 21 PtRu5 groups, whereas for a hemispherical shape, the cluster contains 11 PtRu5 groups. This data suggests that the particles are "raft-like" on the carbon black.
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(b)
(a)
Figure 2(a): HAADF image of the PtRu5 clusters on carbon black and (b) secondary electron image showing the morphology of the carbon black. The white arrows point to regions where the PtRu5 clusters are at terraces. To demonstrate this point, figure 3 is a plot of the cluster diameter vs. the number of atoms per cluster for different 3-dimensional shapes. The two theoretical fits show the number of atoms for a (a) spherical shape with diameter, d, and (b) a hemispherical shape. Clearly, since the number of atoms is considerably less
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