Dependence of Single-Walled Carbon Nanotube Adsorption Kinetics on Temperature and Binding Energy D. S. Rawat, V. Krungleviciute, L. Heroux, M. Bulut, M. M. Calbi, and A. D. Migone* Department of Physics, Southern Illinois UniVersity, Carbondale, Illinois 62901 ReceiVed July 10, 2008. ReVised Manuscript ReceiVed September 8, 2008 We present results for the isothermal adsorption kinetics of methane, hydrogen, and tetrafluoromethane on closed- ended single-walled carbon nanotubes. In these experiments, we monitor the pressure decrease as a function of time as equilibrium is approached, after a dose of gas is added to the cell containing the nanotubes. The measure- ments were performed at different fractional coverages limited to the first layer. The results indicate that, for a given coverage and temperature, the equilibration time is an increasing function of E/(k B T), where E is the binding energy of the adsorbate and k B T is the thermal energy. These findings are consistent with recent theoretical predictions and computer simulations results that we use to interpret the experimental measurements. I. Introduction The adsorption of gases on carbon nanotubes has been the subject of much attention and study in recent years. 1 Practical interest in this topic stems from the fact that nanotubes exhibit characteristics such as high surface area, high binding energy sites, and small pores, all of which are required in order for a material to be successfully used in gas separation (and, perhaps, in gas storage) applications. 2-7 The adsorption of gases on nanotubes is also of fundamental interest to both experimentalists and theorists because this process offers the possibility of realizing low-dimensional matter in the laboratory. 8-14 The majority of the experimental studies of gas adsorption on single-walled nanotubes (SWNTs) have centered on measuring the equilibrium properties of the resulting systems. 1,9-11,13,14 From a dynamical point of view, one of the most investigated aspects of adsorption in nanotubes has been the diffusion and transport of fluids through the interior of the tubes. 15-28 The prediction of exceptionally fast transport of molecules through the pores of SWNTs stimulated a great deal of research conducted to investigate the potential use of carbon nanotubes for membrane-based gas separa- tion. 23-32 Temperature programmed desorption (TPD) experiments have also been conducted on carbon nanotubes. 33-41 In these studies the rate of gas desorption is recorded as a function of increasing temperature, generating the so-called thermal desorption spectra. Recently, TPD studies of various alkanes on nanotube bundles have demonstrated that the different groups of adsorption sites present in the bundles can be resolved through analysis of the desorption spectra. 37,38 Results have also been used to link the morphology of the sample to adsorption rates 39 and to suggest specific desorption mechanisms involving the diffusion of molecules through different * Corresponding author. E-mail: aldo@physics.siu.edu. Telephone: (618) 453-1053. Fax: (618) 453-1056. (1) For a review of recent literature on gas adsorption on carbon nanotubes, see (a) Migone, A. D.; Talapatra, S. In Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H. S., Ed.; American Scientific Publishers: Los Angeles, CA, 2004; Vol. 4, pp 749-767. (2) Yang, R. T. In ADSORBENTS Fundamental and Applications; Wiley Interscience: Hoboken, NJ, 2003. (3) (a) Wang, Q.; Challa, S. R.; Sholl, D.; Johnson, J. K. Phys. ReV. Lett. 1999, 82, 956. (b) Challa, S. R.; Sholl, D. S.; Johnson, J. 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