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Langmuir 2001, 17, 474-480
Characterization of Microporosity and Mesoporosity in Carbonaceous Materials by Scanning Tunneling Microscopy J. I. Paredes, A. Martı´nez-Alonso,* and J. M. D. Tasco´n Instituto Nacional del Carbo´ n, CSIC, La Corredoria s/n, Apartado 73, 33080 Oviedo, Spain Received June 19, 2000. In Final Form: November 7, 2000 Scanning tunneling microscopy has been employed to characterize the microporous and mesoporous structure of different carbon materials. First, model micropores artificially created on a highly oriented pyrolytic graphite substrate by plasma treatment were studied at the atomic scale: the observed increase in electronic density near the Fermi level around the defect implies an increased adsorptivity of the model micropore region. Second, activated carbon fibers were studied. A spongy mesoporous texture along with slit-shaped microporosity (∼1 nm) was observed, accounting for the high adsorption properties of this material. For comparison, nonporous thermally treated carbon black and nonactivated carbon fibers with a ultramicroporous texture were also investigated. In the former case, as expected, no sign of extensive microporosity or mesoporosity such as that of the activated carbon fibers was encountered, in agreement with its poor sorptive capability. In the latter case, the interpretation of the results was rather troublesome, since the minute pore size rendered a reasonably accurate STM imaging difficult.
Introduction Porosity in carbon materials plays a fundamental role for their application in the adsorption and separation technology of gases and liquids or in heterogeneous catalysis, where the development of an extensive network of narrow pores lending high surface areas is required.1-3 As a particular example, activated carbon fibers have recently been given considerable attention for their potential use as adsorbents in environmental applications, including the removal of a variety of organic contaminants from water4,5 as well as the extraction from air of SOx and NOx, which are byproducts of fossil fuel combustion for energy generation purposes.6 In other cases, such as those of nuclear graphites and some carbon fiber composites employed as first-wall materials in thermonuclear fusion devices,7,8 porosity is not desirable since it promotes, on one hand, the diffusion of oxidizing gases into the inner material, leading to its corrosion and subsequent deterioration and, on the other hand, the outgassing of impurities that contaminate the fusion plasma. Regardless of its specific applications, a thorough knowledge of the physicochemical properties of porous carbon materials is essential for their general use in science and technology. Over the years, a considerable * To whom correspondence may be addressed. Telephone number: (+34) 985 28 08 00. Fax number: (+34) 985 29 76 62. E-mail address:
[email protected]. (1) McEnaney, B.; Mays, T. J. In Introduction to Carbon Science; Marsh, H., Ed.; Butterworth: London, 1989; Chapter 5. (2) Rodrı´guez-Reinoso, F.; Linares-Solano, A. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1989; Vol. 21, Chapter 1. (3) Jaroniec, M.; Choma, J. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1989; Vol. 22, Chapter 3. (4) Brasquet, C.; Le Cloirec, P. Carbon 1997, 35, 1307. (5) Brasquet, C.; Le Cloirec, P. Langmuir 1999, 15, 5906. (6) Mochida, I.; Korai, Y.; Shirahama, M.; Kawano, S.; Hada, T.; Seo, Y.; Yoshikawa, M.; Yasutake, A. Carbon 2000, 38, 227. (7) Moormann, R.; Hinssen, H. K.; Kru¨ssenberg, A.-K.; Stauch, B.; Wu, C. H. J. Nucl. Mater. 1994, 212-215, 1178. (8) Barabash, V.; Akiba, M.; Bonal, J. P.; Federici, G.; Matera, R.; Nakamura, K.; Pacher, H. D.; Ro¨dig, M.; Vieider, G.; Wu, C. H. J. Nucl. Mater. 1998, 258-263, 149.
number of different techniques have been employed to characterize not only the porous structure of these materials but also their surface chemistry. These techniques include gas adsorption (mainly N2 and CO2),9-13 immersion calorimetry,9 small-angle X-ray scattering,13,14 inverse gas chromatography and differential thermal analysis,15,16 Fourier transform infrared spectroscopy,15-17 scanning and transmission electron microscopy (SEM/ TEM)12,18-22 and electron spin resonance and Raman spectroscopy,23 among others. It should be noted that, with the single exception of SEM/TEM, all these techniques only provide indirect information about the porous structure of the materials. In recent years, the development of scanning probe microscopy (SPM), first with the scanning tunneling microscope (STM)24 and shortly thereafter its offspring the atomic force microscope (AFM),25 opened up the (9) Rodrı´guez-Reinoso, F.; Molina-Sabio, M. Adv. Colloid Interface Sci. 1998, 76-77, 271. (10) Valladares, D. L.; Reinoso, F. R.; Zgrablich, G. Carbon 1998, 36, 1491. (11) Nakashima, M.; Shimada, S.; Inagaki, M.; Centeno, T. A. Carbon 1995, 33, 1301. (12) Ryu, S. K.; Kim, S. Y.; Li, Z. J.; Jaroniec, M. J. Colloid Interface Sci. 1999, 220, 157. (13) Bo´ta, A.; La´szlo´, K.; Nagy, L. G.; Copitzky, T. Langmuir 1997, 13, 6502. (14) Cazorla-Amoro´s, D.; Salinas-Martı´nez de Lecea, C.; Alcan˜izMonge, J.; Gardner, M.; North, A.; Dore, J. Carbon 1998, 36, 309. (15) Putyera, K.; Jagiello, J.; Bandosz, T. J.; Schwarz, J. A. Carbon 1995, 33, 1047. (16) Adib, F.; Bagreev, A.; Bandosz, T. J. Langmuir 2000, 16, 1980. (17) Jia, Y. F.; Thomas, K. M. Langmuir 2000, 16, 1114. (18) Huttepain, M.; Oberlin, A. Carbon 1990, 28, 103. (19) Oshida, K.; Kogiso, K.; Matsubayashi, K.; Takeuchi, K.; Kobayashi, S.; Endo, M.; Dresselhaus, M. S.; Dresselhaus, G. J. Mater. Res. 1995, 10, 2507. (20) Oya, A.; Yoshida, S.; Alcan˜iz-Monge, J.; Linares-Solano, A. Carbon 1995, 33, 1085. (21) Le Cloirec, P.; Brasquet, C.; Subrenat, E. Energy Fuels 1997, 11, 331. (22) Brasquet, C.; Rousseau, B.; Estrade-Szwarckopf, H.; Le Cloirec, P. Carbon 2000, 38, 407. (23) Manivannan, A.; Chirila, M.; Giles, N. C.; Seehra, M. S. Carbon 1999, 37, 1741. (24) Binnig, G.; Rohrer, H.; Gerber, Ch.; Weibel, E. Phys. Rev. Lett. 1982, 49, 57.
10.1021/la0008535 CCC: $20.00 © 2001 American Chemical Society Published on Web 12/28/2000
Porosity in Carbonaceous Materials
possibility to directly image the surface structures of an extensive variety of materials26-28 in real space with unprecedented resolution, down to the atomic scale in the most favorable cases. Therefore, SPM appears, a priori, as a promising technique for providing important visual information of the porous structure of carbon materials at scales which are not readily accessible by means of other techniques. However, very few articles have appeared in the literature dealing directly with its application to the study of porosity (especially microporosity) in this type of material,13,22,29-32 the main reason being limitations inherent to the technique that renders the imaging of disordered and rough topographies (such as those typical of porous carbons) difficult to perform. This work is aimed at studying the microporosity and mesoporosity of some carbon materials by means of scanning tunneling microscopy. With that object, STM results for different carbon materials with different degrees and types of porosity are presented and discussed, from highly oriented pyrolytic graphite with artificially created defects (to investigate porosity from a fundamental point of view) to more application-oriented activated carbon fibers with high surface areas. Experimental Section Highly oriented pyrolytic graphite (HOPG) samples, from Union Carbide (grade ZYH), were cleaved in air to expose fresh new surfaces. To develop porosity on their surface, oxygen plasma treatments were carried out. These were accomplished in a Technics Plasma 200-G treatment chamber where the oxygen plasma was generated by means of 2.45 GHz microwave (MW) radiation. Two different etching conditions were employed with the purpose of creating pores of different sizes: in the first, the HOPG was treated at a MW power of 40 W for 6 s, and in the other, the MW power was raised to 100 W and the etching time to 9 min. Activated carbon fibers (ACFs) with a BET specific surface area of 874 m2 g-1, as determined from N2 adsorption at 77 K, were prepared by physical activation of pyrolyzed Kevlar pulp with carbon dioxide at a temperature of 1023 K and with a burnoff degree of 73%. Details of their preparation procedure and textural characterization by gas adsorption are given elsewhere.33 Thermally treated (3000 K) carbon black (Sterling FT N880) with a mean particle diameter of 192 nm34 was obtained from Cabot. N2 adsorption experiments at 77 K yielded a BET surface area of 15.3 m2 g-1. Nonactivated carbon fibers were obtained from the pyrolysis of aramid fibers (Nomex 2.2) in a quartz reactor by heating under Ar at 10 K min-1 to 1173 K, followed by cooling to room temperature under Ar. The carbon fibers thus produced yielded a BET surface area of 0.3 m2 g-1 by N2 adsorption at 77 K. However, their CO2 (273 K) equivalent microporous surface area, calculated by the DRK method, yielded a value of 750 m2 g-1, implying that these fibers, in contrast to the carbon black, present a ultramicroporous texture (pore size
