Judith C. Yang Professor, Chemical and Petroleum Engineering; Physics

Surface Reactions (oxidation, catalysis) , Electron Microscopy (in situ)

Keywords: oxidation, heterogeneous catalysis, nanomaterials, TEM, in situ

Dealing with oxidation is a major societal priority, yet oxidation also presents a fascinating challenge in thin film growth.  Classical theories of oxidation were based mostly on thermogravimetric analysis (TGA) that only measures weight change, not structural changes.  Hence, classical theories assume a uniform film growth, yet it is well known that oxides do not develop as uniform films.  Recent developments of in situ experimental tools permit visualization of the dynamic processes at the nanoscale.  Yang’s research group uses in situ ultra-high vacuum transmission electron microscopy (UHV-TEM) to improve the fundamental understanding of oxidation; figure 1 are bright field images of Cu and Cu-5%Ni thin films during oxidation at P(O2) 10-4 Torr in situ, where oxide islands are seen to nucleate and grow and their shapes depend sensitively on temperature.  Heteroepitaxial concepts, based on surface diffusion, strain and structural evolution, describe surprisingly well these initial oxidation stages.

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Surface reactions are critical to heterogeneous catalysis also.  The supported 3-dimensional nanostructure provides the surface where the catalytic reaction occurs.  Yang’s group is part of a coordinated experimental and theoretical effort to determine the structural habits of these nanostructures and the energetic landscapes at the atomistic level that lead to them. For example, Pt nanoparticles (NPs) supported on C and gamma-Al2O3, was studied via combination of high-resolution transmission electron microscopy (HRTEM), synchrotron X-ray absorption spectroscopy (XAS) and first-principle theoretical calculations.  An amorphous-to-crystalline transition of supported Pt NPs in the sub-nm to nm size range has been discovered where theoretical simulations confirmed the observed effects of particle size (NPs <1.2nm are amorphous), adsorbate (H2 stabilizes the fcc structure) and support (changes the width of the transition zone) as shown in figure 2.