简介: |
Catalytic chemical vapor deposition (C-CVD), using a transition metal catalyst (Ni, Fe, Co, etc.) on SiO2 or Al2O3 support and a carbon-containing precursor (C2H2, C2H4, CH4, CO, etc.), is commonly employed for large scale synthesis of carbon nanotubes (CNTs). However, controlling the synthesis conditions to obtain CNTs with the desired structure and morphology for a specific application has still not been demonstrated. Recently, the effect of synthesis conditions on the structure and morphology of CNTs has been revealed by dynamic observations using environmental scanning-transmission electron microscopy (ESTEM). In situ observations show that the catalyst remains crystalline during the growth, while both in situ and ex situ observations have confirmed that not all catalyst particles are active for CNT growth. Previous reports have shown that the Fe3C structure is formed during CNT growth from Fe catalyst particles, but not much has been reported about the inactive nanoparticles (NPs). We have measured spacings and angles between the lattice planes for both active and inactive nanoparticles using high resolution images, extracted from the same video sequence. Our measurements confirm that the structure of the particles active for CNT nucleation and growth is Fe3C. However lattice plane spacings and angles measured for inactive particles correspond to those of the Fe5C2 structure. This structure was commonly seen in particles that remained inactive during observation as well as in nanoparticles that were enclosed in a graphitic shell. Moreover, we also observed the structure of an active particle change from Fe3C to Fe5C2 upon deactivation. These results show that both the structure and orientation of the catalyst NPs are important for CNT growth: carbon atoms may be bound by surfaces that do not provide easy diffusion paths or a nucleation plane for graphite layers. Complete analysis and models explaining the role of orientation and the formation of carbon coatings on the catalytic activity of such NPs will be presented. A detailed structural analysis of active and inactive particles, subjected to same experimental conditions during dynamic observations will be presented.
报告人简历:
Dr. Renu Sharma is a Project Leader in the Nanofabrication Research Group. She received a B.S. and B.Ed. in Physics and Chemistry from Panjab University, India, and M.S. and Ph.D. degrees in Solid State Chemistry from the University of Stockholm, Sweden, where she had a Swedish Institute Fellowship. Renu joined the CNST in 2009, coming from Arizona State University (ASU), where she began as a Faculty Research Associate in the Department of Chemistry and Biochemistry and the Center for Solid State Science, and most recently served as a Senior Research Scientist in the LeRoy Eyring Center for Solid State Science and as an affiliated faculty member in the School of Materials and Department of Chemical Engineering. Renu has been a pioneer in the development of environmental scanning transmission electron microscopy (E(S)TEM), combining atomic-scale dynamic imaging with chemical analysis to probe gas-solid reactions. She has applied this powerful technique to characterize the atomic-scale mechanisms underlying the synthesis and reactivity of nanoparticles (including catalysts), nanotubes, nanowires, inorganic solids, ceramics, semiconductors, and superconductor materials. Renu has received a Deutscher Akademischer Austauschdienst (DAAD) Faculty Research Fellowship, is a past President of the Arizona Imaging and Microanalysis Society, and has given over 70 invited presentations, and published 3 book chapters and over 160 research articles. At the CNST, Renu is establishing advanced E(S)TEM measurement capabilities for nanoscience research and contributing her research expertise to the operation of a new TEM facility in the NanoFab.
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