Z-contrast imaging of catalysts in the 300 kV STEM
Z-contrast imaging in the scanning transmission electron microscope has become the accepted technique for imaging sub-nanometer catalyst clusters, utilizing the high angle annular detector introduced by Howie. The lack of coherent phase contrast effects greatly assists the identification of small clusters, especially near the resolution limit of the microscope. The choice of inner detector angle depends on the system being studied. The highest signal to noise ratio is obtained with the smallest inner detector angle, but increasing this angle significantly reduces the contribution of coherently scattered electrons, which is advantageous for crystalline support materials. In this case, small metal clusters may be unambiguosly distinguished from diffracting regions of the support, and their size distributions determined. Fig 1 compares two preparations of 1 wt% Pd on γ-Al2O3, prepared from palladium nitrate solution, and aged at 600°C for (a) 6 hours, and (b) 24 hours. Images were taken with a VG Microscopes HB501UX 100 kV STEM, using a probe size of ∼3Å The narrower size distribution resulting from the longer aging time is clearly observed.