10730
Langmuir 2005, 21, 10730-10734
Endohedral Condensation and Higher Exohedral Coverage of Kr on Open Single-Walled Carbon Nanotubes at 77 K Zygmunt J. Jakubek* and Benoit Simard Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex Drive, Ottawa, ON K1A 0R6, Canada Received February 25, 2005. In Final Form: July 20, 2005 An isotherm quasi-discontinuity, or a near-vertical step, at 177 µTorr indicative of Kr condensation inside open single-walled carbon nanotubes (SWNT) has been observed at 77 K. The isotherm shows double adsorption-branch structure attributed to the existence of two endohedral phases of confined Kr. Three well-pronounced steps corresponding to the formation of various exohedral phases are present in the high-density (low-pressure) branch. The desorption branch exhibits three rounded steps assigned to higher order exohedral coverage.
Single-walled carbon nanotubes (SWNT), a strongly interacting adsorbent with long, straight, non-interconnected cylindrical nanopores, are believed to be an excellent model system for the investigation of fluid nanoconfinement1 and properties of matter at reduced dimensionality.2 However, because of many experimental limitations including difficulty in obtaining nanotubes of sufficient purity and pore size monodispersity as well as strong adsorption on external polyenergetic sites, they have not been widely utilized in such studies so far. Bundles of individual SWNT held together by van der Waals forces can adsorb gases inside open nanotubes, in external grooves, and on an external surface of peripheral nanotubes.3-17 The smallest adsorptives, such as helium and neon atoms and hydrogen molecules, may also adsorb in interstitial channels of perfect bundles; in real imperfect bundles other gases are believed to adsorb in the interstitial channels as well.6,18-21 Whereas adsorption inside * Corresponding author (e-mail
[email protected]). (1) Gelb, L. D.; Gubbins, K. E.; Radhakrishnan, R.; SliwinskaBartkowiak, M. Rep. Prog. Phys.1999, 62, 1573-1659. (2) Calbi, M. M.; Cole, M. W.; Gatica, S. M.; Bojan, M. J.; Stan, G. Rev. Mod. Phys. 2001, 73, 857-865. (3) Pederson, M. R.; Broughton, J. Q. Phys. Rev. Lett. 1992, 69, 26892692. (4) Kuznetsova, A.; Yates, J. T., Jr.; Simonyan, V. V.; Johnson, J. K.; Huffman, C. B.; Smalley, R. E. J. Chem. Phys. 2001, 115, 6691-6698. (5) Williams, K. A.; Eklund, P. C. Chem. Phys. Lett. 2000, 320, 352358. (6) Stan, G.; Bojan, M. J.; Curtarolo, S.; Gatica, S. M.; Cole, M. W. Phys. Rev. B 2000, 62, 2173-2180. (7) Gatica, S. M.; Bojan, M. J.; Stan, G.; Cole, M. W. J. Chem. Phys. 2001, 114, 3765-3769. (8) Calbi, M. M.; Gatica, S. M.; Bojan, M. J.; Cole, M. W. J. Chem. Phys. 2001, 115, 9975-9981. (9) Talapatra, S.; Migone, A. D. Phys. Rev. Lett. 2001, 87, 2061061-4. (10) Talapatra, S.; Rawat, D. S.; Migone, A. D. J. Nanosci. Nanotechnol. 2002, 2, 467-470. (11) Bojan, M. J.; Steele, W. A. Computer Simulations of Sorption in Model Cylindrical Pores. In Fundamentals of Adsorption; LeVan, M. D., Ed.; Kluwer Academic Publishers: Boston, MA, 1996. (12) Steele, W. A.; Bojan, M. J. Adv. Colloid Interface Sci. 1998, 7677, 153-178. (13) Maddox, M. W.; Gubbins, K. E. Langmuir 1995, 11, 3988-3996. (14) Stan, G.; Cole, M. W. Surf. Sci. 1998, 395, 280-291. (15) Krungleviciute, V.; Heroux, L.; Talapatra, S.; Migone, A. D. Nano Lett. 2004, 4, 1133-1137. (16) Krungleviciute, V.; Heroux, L.; Migone, A. D.; Kingston, C. T.; Simard, B. J. Phys. Chem. B 2005, 109, 9317-9320. (17) Jakubek, Z. J.; Simard, B. Langmuir 2004, 20, 5940-5945. (18) Shi, W.; Johnson, J. K. Phys. Rev. Lett. 2003, 91, 015504. (19) Talapatra, S.; Zambano, A. Z.; Weber, S. E.; Migone, A. D. Phys. Rev. Lett. 2000, 85, 138-141.
nanotubes is limited by the relative diameters of a molecule and a nanotube, there are no adsorptive size restrictions on external adsorption. The suggestion by Williams and Eklund that a groove of the nanotube bundle, that is, a very narrow space between two adjacent peripheral nanotubes, constitutes a strong adsorption site5 gave rise to a series of theoretical and experimental papers. Gatica et al.7 have predicted by grand canonical Monte Carlo (GCMC) simulation the formation of one-channel, three-channel, and monolayer phases of Ar and Kr adsorbed on the external surface of nanotube bundles. Later, Calbi et al.8 carried out a GCMC simulation for Ne, Xe, and CH4 adsorption on nanotube bundles. In addition to the one- and three-channel phases, they have investigated a zigzag phase for Ne, Xe, CH4, Ar, and Kr, which in all cases they found to be unstable. Experimental investigation of the exohedral phases has been carried out by Migone and co-workers,9,10 who have observed the formation of the monolayers as well as oneand three-channel phases of Ar and Xe on the external surface of the nanotube bundles and determined the isosteric heat of adsorption at these adsorption sites. Migone’s group also found evidence for higher coverage phases in Ar10 and Ne.15 Most recently they reinvestigated Ar adsorption in a broad range of temperatures and determined the isosteric heat of adsorption.16 Their observations supported the heterogeneous, as opposed to the homogeneous, bundle model.18 Muris et al.22 studied isothermal adsorption of CH4 and Kr on bundles of SWNT. They observed two broad steps ascribed to adsorption on two distinct classes of adsorption sites, one with higher and the other with lower binding energy as compared with graphite. Adsorption of simple gases inside open SWNT has been the subject of several theoretical studies. Maddox and Gubbins13 have predicted that Ar and N2 fill a carbon nanotube by adsorbing on the internal walls. A monolayer is formed at a very low pressure, which depends on the nanotube diameter, temperature, and other factors. Additional layers may form at higher pressures. The layering (20) Johnson, M. R.; Rols, S.; Wass, P.; Muris, M.; Bienfait, M.; Zeppenfeld, P.; Dupont-Pavlovsky, N. Chem. Phys. 2003, 293, 217230. (21) Wilson, T.; Tyburski, A.; DePies, M. R.; Vilches, O. E.; Becquet, D.; Bienfait, M. J. Low Temp. Phys. 2002, 126, 403-408. (22) Muris, M.; Dufau, N.; Bienfait, M.; Dupont-Pavlovsky, N.; Grillet, Y.; Palmari, J. P. Langmuir 2000, 16, 7019-7022.
10.1021/la050510c CCC: $30.25 Published 2005 by the American Chemical Society Published on Web 09/21/2005
Condensation and Coverage of Kr on SWNT
transitions are reflected by steps in the simulated isotherms. Their isotherms also show hysteresis loops in the simulated condensation/evaporation process. Similarly, Bojan and Steele11 have predicted that Kr adsorbs on an atomically rough internal wall of a carbon nanotube by forming several cylindrical layers up to complete filling of the nanotube. Stan and Cole14 have determined binding energies of simple gases inside nanotubes to be much larger than those on planar graphite. They have also predicted one-, two-, or three-dimensional behavior of adsorbed gases, depending on the nanotube diameter and the temperature. In another study, Stan et al.6 calculated that for simple gases binding energies in the grooves should be larger than in the nanotube interior. More recently Cole’s group23,24 developed a simple model of gas adsorption in nanopores featuring shell and axial phases and evaluated isotherms with mean-field theory and the Monte Carlo simulation techniques. As-grown SWNT, in particular the nanotubes grown in our laboratory, do not show endohedral adsorption, and they are believed to be mostly closed. Mechanical and chemical opening of nanotubes has a dramatic effect on their gas adsorption properties.25-27 Kuznetsova et al.4,28,29 have extensively studied Xe adsorption on both closed and open SWNT using temperature-programmed desorption and other surface analytical techniques and concluded, among others, that chemical opening (sonication in acid environment) and thermal activation (1073 K) of the nanotubes increased Xe adsorption by a factor of 280. The Xe confined inside nanotubes formed a quasi-onedimensional phase. Yoo et al.30 have measured the isosteric heat of adsorption of N2 on acid-treated open SWNT in a broad range of coverages and determined that at low coverage the binding energy was about twice as large as on closed SWNT. Recently, our group investigated the isothermal adsorption of Ar on open SWNT at 77 and 87 K.17 We observed condensation of Ar inside nanotubes, which appeared to be the first-order phase transition. At 77 K, two branches of the isotherm corresponding to two condensed phases, a low-density and a high-density phase, were present. The low-density phase was observable only with Ar dosing not exceeding 0.2 cm3/g STP (0.2 cm3 of adsorptive at standard temperature and pressure per 1 g of adsorbent) per injection. We also estimated the isosteric heat of adsorption of Ar on exohedral sites of bundles (grooves, external surface). The present Kr adsorption project was undertaken to examine the endohedral adsorption of a simple gas at a lower, as compared with that of Ar, value of the reduced temperature Tiso/Ttr, where Tiso is the isotherm temperature and Ttr is the bulk triple-point temperature for the adsorbate. The SWNT used in this study have been grown in our laboratory by a double-laser/oven method.31 The as-grown (23) Trasca, R. A.; Calbi, M. M.; Cole, M. W. Phys. Rev. E 2002, 65, 061607-1-9. (24) Trasca, R. A.; Calbi, M. M.; Cole, M. W.; Riccardo, J. L. Phys. Rev. E 2004, 69, 011605-1-6. (25) Stepanek, I.; Maurin, G.; Bernier, P.; Gavillet, J.; Loiseau, A. Mater. Res. Soc. Symp. Proc. 2000, 593, 119-122. (26) Du, W. F.; Wilson, L.; Ripmeester, J.; Dutrisac, R.; Simard, B.; De´nomme´e, S. Nano Lett. 2002, 2, 343-346. (27) Yang, C.-M.; Kaneko, K.; Yudasaka, M.; Iijima, S. Nano Lett. 2002, 2, 385-388. (28) Kuznetsova, A.; Yates, Jr., J. T.; Liu, J.; Smalley, R. E. J. Chem. Phys. 2000, 112, 9590-9598. (29) Kuznetsova, A.; Mawhinney, D. B.; Naumenko, V.; Yates, J. T., Jr.; Liu, J.; Smalley, R. E. Chem. Phys. Lett. 2000, 321, 292-296. (30) Yoo, D. H.; Rue, G. H.; Chan, M. H. W.; Hwang, Y. H.; Kim, H. K. J. Phys. Chem. B 2003, 107, 1540-1542.
Langmuir, Vol. 21, No. 23, 2005 10731
Figure 1. 77 K isotherm of Kr adsorbed on open SWNT.
material contains ∼93% (mass) of carbonaceous material, the majority of which (∼75%) is SWNT organized into regular bundles with ∼90 nanotubes per bundle. The average diameter of the nanotubes has been estimated to be 1.34(10) nm. A 219 mg sample of as-grown material was prepared for gas adsorption studies by multistep processing, which was described in our recent paper.17 The processing included outgassing the sample to better than 10-6 Torr at room temperature (295 K) for a total of 60 h and at 435 K for 30 min, followed by heating in dry air at 470 K for 2 h and final outgassing in a vacuum at 385 K for 12 h. The processing was monitored with a residual gas analyzer. The mass of the sample decreased to 206 mg after processing. Following completion of Ar adsorption experiments, the sample was stored under Ar at room temperatures. In preparation for the present Kr adsorption measurements, the sample was outgassed for 3 days: 1 day at 383 K, followed by 10 h at 473 K, 6 h at 573 K, 10 h at 373 K, and the rest of the time at room temperatures. Kr isotherms were measured at 77 K (liquid N2 bath). The isotherms were acquired using a porosimeter (Micromeritics, ASAP 2010) equipped with 1 and 10 Torr full-scale capacitance manometers (MKS, Baratron). The resolution of the manometers is 0.0001% of full scale, or 1 and 10 µTorr, respectively. A measurement of an isotherm always started with outgassing of the sample at room temperature (295 K) for 12 h to a pressure of
