An Experimental Estimate of the Free Energy of Formation of Single Walled Carbon Nanotubes
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An Experimental Estimate of the Free Energy of Formation of Single Walled Carbon Nanotubes L.M. Wagg, G.L. Hornyak1, L. Grigorian2, A.C. Dillon, K.M. Jones, J. Blackburn, P.A. Parilla and M.J. Heben National Renewable Energy Laboratory, Nanostructured Materials Research Group 1617 Cole Boulevard, Golden, Colorado 80401 1 University of Denver, Department of Physics and Astronomy, 2199 S. University Boulevard, Denver, Colorado 80208 2 Honda Research Institute USA, 1381 Kinnear Road, Suite 116, Columbus, OH 43212 ABSTRACT Single walled carbon nanotubes (SWNT) were synthesized by methane CVD on a supported mixed transition metal (Fe/Mo) catalyst. Gas feed composition and reaction temperature were varied to identify the threshold conditions for the growth of SWNT. These reaction conditions closely approximate pseudo-equilibrium conditions with some active reaction intermediate (likely chemisorbed carbon atoms) that proceeds to nucleate and grow SWNT. This value also serves as an estimated upper limit of the free energy of formation ∆G*(T)SWNT since the active intermediate proceeds to form SWNT through a process that is thought to be essentially irreversible. The difference relative to graphite is in good agreement with literature values predicted from simulations for SWNT nuclei containing approximately 80 atoms, while considerably larger than that predicted for bulk 5,5 SWNT. Our estimate over the range 700 to 1000 °C of 16.1 to 13.9 kJ/mol is considerably greater than the free energy of formation for diamond (between 5.8 and 6.9 kJ/mol from 700 to 925 °C). INTRODUCTION Single walled carbon nanotubes (SWNT) have been synthesized by a variety of techniques including electric arc evaporation of graphite/metal mixtures,1, 2 laser ablation of carbon/metal targets,3 and chemical vapor deposition (CVD) with a wide variety of gaseous carbon sources.4-7 While growth mechanisms for single and multiwalled carbon nanotubes (MWNT) remain widely debated in the literature, little is known of the thermodynamic properties of these carbon structures. Since both laser ablation and hot wire synthesis methods operate very far from equilibrium, CVD synthesis offers the best opportunity to study formation energetics experimentally. In this work we control the reaction parameters (temperature and reacting gas partial pressures) to provide increasing excess driving force relative to the equilibrium conditions for the deposition of graphite. The conditions that lead to the first appearance of SWNT are identified, and the differential free energy is determined. EXPERIMENTAL DETAILS The SWNT CVD synthesis procedure was reported in our previous work 7 and will not be presented in great detail here. Iron and molybdenum salts were precipitated from an aqueous solution onto a slurry of high surface area fumed alumina support in a manner similar to that reported by A.M. Cassell et al.8 The supported catalyst was dried and ground to a fine powder. Approximately 100 mg of supported catalyst was distributed on a quartz boat and placed in the mid
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