Study of Nitrogen Adsorbed on Open-Ended Nanotube Bundles Dae-Hwang Yoo, † Gi-Hong Rue, ‡ Moses H. W. Chan, § Yoon-Hwae Hwang, † and Hyung-Kook Kim* ,† Department of Physics and RCDAMP, Pusan National UniVersity, Pusan 609-735, Korea, School of Electronic and Electric Engineering, Kyungpook National UniVersity, Taegue 702-701, Korea, and Department of Physics, PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: September 6, 2002; In Final Form: NoVember 26, 2002 The adsorption of N 2 on the open-ended single-walled carbon nanotube bundles was studied. The amount corresponding to the first coverage adsorbed on open-ended carbon nanotube bundles is three times larger than the amount adsorbed on closed-ended nanotube bundles. The isosteric heat of adsorption was obtained from the adsorption isotherm measurement performed at temperatures ranging from 117 to 130 K. The estimated heat of adsorption of nitrogen on the open-ended nanotube bundles is about twice as great as that on the closed-ended nanotube bundles. This leads directly to the conclusion that the binding energy of nitrogen on the open-ended nanotube is greater than that of nitrogen on the closed-ended nanotube bundles. Introduction Carbon nanotubes (CNTs), which have fine pores and lower dimensionality, are useful materials in both nanoscale and macroscale applications such as ultra-fine probes, 1,2 field emission devices, 3,4 and gas storage devices. 5-8 Studies con- cerning the adsorption of molecules on the single-walled carbon nanotubes (SWNTs) especially, are, attracting fundamental interests and offer important technological information such as separation of mixtures and hydrogen storage. The report by Pederson et al. 5 suggests that carbon nanotubes with a few nanometer-sized diameter should be able to draw liquids up by capillarity. Also, the report by Dillon et al. 7 showing that hydrogen gas can condense to a high density inside SWNTs implies that SWNTs have a large storage efficiency for hydrogen-fueled vehicles. This is a consequence of the higher binding energy inside the nanotube caused by the curvature of the tube’s interior. Recently, some theoretical calculations have been done for the binding energy of H 2 , He, and Ne on the interstitial channel of the nanotube bundles and for the attractive potential energy of H 2 inside the nanotube. 9-11 Some theoretical studies have also predicted the increased adsorption capacity and adsorption binding energy of the open-ended nanotubes. 5,10-12 Weber et al. 13 have shown experimentally that the binding energy of CH 4 on the closed-ended nanotube is 76% larger than that on planar graphite. In our earlier report, 14 the binding energy of nitrogen on closed-ended single-walled carbon nanotubes at lower coverage was also studied. However, it is interesting that there have not yet been any experimental results related to the calculation of the binding energy of the adsorbates inside nanotubes. In this paper, an experimental study of the adsorption of nitrogen on the open-ended SWNTs is reported. Comparison of the amount adsorbed on SWNTs related to the acid treatment and related to the annealing temperature after the acid treatment will be discussed. The isosteric heat of adsorption and the binding energy from isotherm adsorption experiments on open- ended SWNTs below the first coverage were estimated. Experimental Section The apparatus for the isotherm adsorption experiments was composed of a gas handling system and a refrigerator. The gas handling system consisted of 1/4” VCR valves (Nupro) and a capacitance pressure gauge (MKS Baratron 127). A He-recycled refrigerator (CTI model 22 refrigerator) was used. The nanotubes used as a substrate in this experiment were produced at Rice University by the pulsed laser vaporization method. 15 To remove the half-fullerene caps at the ends of the nanotube, an acid treatment was done in the mixture of H 2 SO 4 and HNO 3 with the ratio of 3 to 1 under a sonicating process for 24 h. 16 The acid-treated nanotube samples experienced a filtering process with a membrane filter with the mesh size of 0.45 μm. To compare annealing effects, nanotubes were divided into two pieces. A piece of nanotube was annealed at 873 K and another was annealed at 1073 K for 12 h in a vacuum of 10 -6 Torr pressure, respectively. The nanotube samples were transferred into a copper cell in air and evacuated at 350 K for 24 h before measurements were taken. The mass of the nanotubes used in the experiments was 30 mg. The mean tube diameter of SWNT was 1.2 nm. 15 Temperatures were controlled using a temperature controller (Lakeshore DRC-93CA) with 0.01 K precision. Measurements to obtain the heat of adsorption were performed at five different temperatures; 117.25, 120.28, 123.55, 126.51, and 129.95 K. Results and Discussion Figure 1 shows the isothermal adsorption of nitrogen on the closed-ended SWNTs and the open-ended SWNTs measured at 71 K. The amount adsorbed, y-axis, was represented in mmol/ g. The amount of molecules related to the first coverage, which is believed to correspond to the monolayer of the plane graphite, on the closed-ended SWNTs is about 1 mmol/g. Different from the case of the planar graphite, a clear-cut distinction is not * Corresponding author. E-mail: hkkim@pusan.ac.kr. † Pusan National University. ‡ Kyungpook National University. § Pennsylvania State University. 1540 J. Phys. Chem. B 2003, 107, 1540-1542 10.1021/jp026925d CCC: $25.00 © 2003 American Chemical Society Published on Web 01/29/2003