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42361-AC5
In-Situ 3-D Analysis of Nanoscale Catalysts
The aim of this program is to develop novel methods in scanning transmission electron microscopy (STEM) to characterize the atomic and electronic structures of nanocluster heterogeneous catalysts. There are two main areas of research that are involved in accomplishing this goal: the development of a means to quantify the 3-D size, shape and composition of the nanoclusters and the incorporation of an in-situ gas stage into a high-resolution microscope to permit the environment to be accurately controlled during the analysis. Both of these tasks require a significant amount of ramp-up time. The first year of work focused on both the understanding of the reconstruction algorithms needed for 3-D imaging and the design of vacuum transfer and environmental stages. In this second year of research, work has continued to develop a means to quantify the size and composition of nanoclusters with atomic scale accuracy, and these methods have been applied to samples inserted into the microscope using a vacuum transfer stage to avoid sample oxidation.
The ability to quantify the size, shape and composition of individual nanostructures in the STEM is primarily determined by the size of the electron beam (probe). The recent development of aberration correctors for STEM has vastly improved the spatial resolution possible with these instruments, taking resolution to below 0.1nm. However, although a spherical aberration corrected STEM has exceptional spatial resolution, its intense probe can force nanoclusters to move on a support surface, making a quantitative measurement uncertain. Additionally, the intense probe may also cause changes in the structure of the nanoclusters. Reducing the probe intensity (e.g., by changing the optical configuration of the aberration corrected microscope or by using a conventional non-aberration-corrected STEM) limits the cluster movement, but at the expense of resolution and signal intensity (the smallest clusters get lost in the noise of the image). However, in work performed as part of this research program a new approach that permits the quantitative determination of size distributions of supported nanoclusters from conventional STEM HAADF images (low spatial resolution and signal levels) has been developed. The method described here produces accurate results even for the smallest clusters when the image has a low signal-to-noise ratio (S/N) by analyzing all the contributions to the final image intensity.
The method works by using a Gaussian blurring function to smooth out the effect of noise on the image. By convoluting the image with a Gaussian it is possible to determine the root mean square (rms) size of the particles as a function of the width of the Gaussian. By plotting rms size against the Gaussian width it is possible to extrapolate back to an unblurred size that is free from noise. Initial analyses that have been performed on Os10 clusters to test the accuracy of these measurements have found that the size distributions acquired from single STEM images (~30 particles total) agree well with those determined by extensive EXAFS measurements. This approach to size determination does not result in electron beam modification of the size, shape and composition and can readily be extended to 3-D tomographic imaging (work to be performed in the following year).
The analysis of nanoclusters has been extended to Ta on SiO2 supports. Here the samples are sensitive to oxidation effects after the initial fabrication stage. To investigate the effect of oxidation, the Gaussian blurring was used to study samples that were air exposed and samples that were kept in inert conditions and transferred into the microscope using the unique vacuum transfer stage. The analysis of the samples found that oxidation did increase the cluster size in a quantifiable manner. Current work is addressing the rate of oxidation and its effect on nanoscluster size distribution. From comparison with EXAFS data, this analysis is also addressing the local coordination of the atoms in the clusters and determining the shape evolution on the support surface.
In addition to overcoming the usual difficulties of performing high spatial resolution analysis in the STEM, the graduate student supported by the PRF has had to become an expert in instrument design and also in the application of advanced mathematical analysis and reconstruction methods. While this has consumed a great deal of time in the first two years of the program, the research is positioned to be applied to a wide variety of catalyst samples after this first test specimen. The results from the initial analyses have been presented at the 2007 Microscopy and Microanalysis conference in Ft. Lauderdale, FL and publications describing this research are currently being prepared.
