The Effect of Substrates / Ligands on Metal Nanocatalysts Investigated By Quantitative Z-Contrast Imaging and High Resol
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The Effect of Substrates / Ligands on Metal Nanocatalysts Investigated By Quantitative ZContrast Imaging and High Resolution Electron Microscopy Huiping Xu1, 2, Laurent Menard3, Anatoly Frenkel4, Ralph Nuzzo3, Duane Johnson5 and Judith Yang1 1
Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261. 2 R.J.Lee Group, Inc., Monroeville, PA 15146 3 Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801. 4 Department of Physics, Yeshiva University, New York, NY 10016. 5 Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 ABSTRACT Our direct density function-based simulations of Ru-, Pt- and mixed Ru-Pt clusters on carbon-based supports reveal that substrates can mediate the PtRu5 particles [1]. Oblate structure of PtRu5 on C has been found [2]. Nevertheless, the cluster-substrate interface interactions are still unknown. In this work, we present the applications of combinations of quantitative zcontrast imaging and high resolution electron microscopy in investigating the effect of different substrates and ligand shells on metal particles. Specifically, we developed a relatively new and powerful method to determine numbers of atoms in a nanoparticle as well as three-dimensional structures of particles including size and shape of particles on the substrates by very high angle (~96mrad) annular dark-field (HAADF) imaging [2-4] techniques. Recently, we successfully synthesize icosahedra Au13 clusters with mixed ligands and cuboctahedral Au13 cores with thiol ligands, which have been shown by TEM to be of sub-nanometer size (0.84nm) and highly monodisperse narrow distribution. X-ray absorption and UV-visible spectra indicate many differences between icosahedra and cuboctahedral Au13 cores. Particles with different ligands show different emissions and higher quantum efficiency has been found in Au11 (PPH3) SC12)2Cl2. We plan to deposit those ligands-protected gold clusters onto different substrates, such as, TiO2 and graphite, etc. Aforementioned analysis procedure will be performed for those particles on the substrates and results will be correlated with that of our simulations and activity properties. This approach will lead to an understanding of the cluster-substrates relationship for consideration in real applications. INTRODUCTION Nanocatalysts we investigate here are really small ( 100 mrad) where the contribution of Bragg electrons is minimized and the electrons collected are predominantly those that are incoherently scattered. This is referred to as high angle annular dark field (HAADF)-STEM or “Z-contrast” microscopy. With careful calibration of the detector efficiency, inner and outer scattering angles of collection, and microscope magnification, the scattering cross-sections of an individual cluster, which is proportional to the absolute scattered intensity, can be calculated. Since the scattering cross-section of a cluster core is simply the sum of the cross-sections of
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