Langmuir 2005, 21, 8817-8823
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Structural Investigation of Zeolite-templated, Ordered Microporous Carbon by Scanning Tunneling Microscopy and Raman Spectroscopy J. I. Paredes,† A. Martı´nez-Alonso,*,† T. Yamazaki,‡ K. Matsuoka,‡ J. M. D. Tasco´n,† and T. Kyotani‡ Instituto Nacional del Carbo´ n, Consejo Superior de Investigaciones Cientı´ficas, Apartado 73, 33080 Oviedo, Spain, and Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Aoba-Ku, Sendai 980-8577, Japan Received April 20, 2005. In Final Form: July 1, 2005 Scanning tunneling microscopy (STM) and Raman spectroscopy have been employed for a detailed structural characterization of an ordered microporous carbon synthesized in the nanochannels of zeolite Y by a templating approach. The carbon exhibited an exceptionally high adsorption capacity together with a long-range structural organization on the nanometer scale. As revealed by STM, this material exhibited both terrace-like and periodic (∼1.4 nm) stripe-like nanostructures. The vertical separation between contiguous terraces was measured to be also about 1.4 nm and was thus coincident with the structural periodicity deduced by X-ray diffraction. The terraces of the carbon material were shown to consist of arrays of ∼1 nm wide carbon clusters. The carbon clusters displayed only a limited degree of local order within the terraces but not long-range periodicity. Likewise, STM indicated that the micropore structure of this carbon originated from the large number of voids that separate adjacent clusters, being morphologically very different from that commonly found in activated carbons. The range of void sizes measured by STM (0.8-2.3 nm) was in complete agreement with the pore size distribution determined from nitrogen adsorption measurements. The origin of the nanostructural features observed for this microporous carbon was discussed on the basis of the surface structure of the zeolite Y template. Finally, Raman spectroscopy provided evidence that the carbon clusters were made up of nanographenes with a curved topology.
1. Introduction Porous carbons are scientifically and technologically important materials with a range of applications that includes their use as adsorbents for air and water purification,1-3 as catalyst supports,4-6 or as electrodes in electrochemical double-layer capacitors.7,8 The most widespread approach for the synthesis of porous carbons involves pyrolysis of an organic precursor followed by physical activation of the resulting char with CO2 or steam, or the carbonization of the precursor material previously impregnated with a suitable activating agent, such as KOH or H3PO4 (chemical activation).9 The activated carbons thus prepared are of a highly disordered nature, basically composed of randomly oriented, graphite-like crystallites just a few nanometers wide and a few graphenes thick, and where porosity (generally of slit type) * Corresponding author: telephone (+34) 985 11 90 90; fax (+34) 985 29 76 62; e-mail
[email protected]. † CSIC. ‡ Tohoku University. (1) Bashkova, S.; Bagreev, A.; Locke, D. C.; Bandosz, T. J. Environ. Sci. Technol. 2001, 35, 3263-3269. (2) Monneyron, P.; Faur-Brasquet, C.; Sakoda, A.; Suzuki, M.; Le Cloirec, P. Langmuir 2002, 18, 5163-5169. (3) Le Leuch, L. M.; Subrenat, A.; Le Cloirec, P. Langmuir 2003, 19, 10869-10877. (4) Moreno-Castilla, C.; Alvarez-Merino, M. A.; Carrasco-Marı´n, F.; Fierro, J. L. G. Langmuir 2001, 17, 1752-1756. (5) Pe´rez-Cadenas, A. F.; Moreno-Castilla, C.; Maldonado-Ho´dar, F. J.; Fierro J. L. G. J. Catal. 2003, 217, 30-37. (6) Chai, G. S.; Yoon, S. B.; Yu, J.-S.; Choi, J.-H.; Sung, Y.-E. J. Phys. Chem. B 2004, 108, 7074-7079. (7) Jang, J. H.; Oh, S. M. J. Electrochem. Soc. 2004, 151, A571A577. (8) Lee, J.; Kim, J.; Lee, Y.; Yoon, S.; Oh, S. M.; Hyeon, T. Chem. Mater. 2004, 16, 3323-3330. (9) Schu¨th, F.; Sing, K. W. S.; Weitkamp J.; Eds. Handbook of Porous Solids; Wiley-VCH: Weinheim, Germany, 2002.
is made up by the interstices between neighboring crystallites.9-13 As a consequence of such structural configuration, activated carbons typically exhibit very disordered and irregular porous networks, together with relatively broad pore size distributions.12 To increase the functionality of porous materials and attain better control of their properties, the creation of regular porous networks, with pores of uniform size and shape disposed in an orderly arrangement, is highly desirable.14-16 This is also applicable to the case of porous carbons, and for this reason, methodologies for their synthesis alternative to the long-established physical and chemical activation approaches are being actively explored.17-19 In this respect, current research efforts are mainly focused on templating techniques, as they have been shown to enable the synthesis of porous carbons with ordered structure.15,16,19 In such approaches, the carbon material is prepared by infiltration of an appropriate inorganic template with an organic precursor, carbonization of the precursor, and removal of the inorganic template by acid dissolution, so that the derived porous carbon constitutes, in principle, a negative replica of the template. (10) Bradley, R. H.; Rand, B. J. Colloid Interface Sci. 1995, 169, 168-176. (11) Bradley, R. H.; Sutherland, I.; Sheng, E. J. Colloid Interface Sci. 1996, 179, 561-569. (12) Rodrı´guez-Reinoso, F.; Molina-Sabio, M. Adv. Colloid Interface Sci. 1998, 76-77, 271-294. (13) Kaneko, K. Carbon 2000, 38, 287-303. (14) Davis, M. E. Nature 2002, 417, 813-821. (15) Schu¨th, F.; Schmidt, W. Adv. Mater. 2002, 14, 629-638. (16) Stein, A. Adv. Mater. 2003, 15, 763-775. (17) Gogotsi, Y.; Nikitin, A.; Ye, H.; Zhou, W.; Fischer, J. E.; Yi, B.; Foley, H. C.; Barsoum, M. W. Nat. Mater. 2003, 2, 591-594. (18) Inagaki, M.; Kaneko, K.; Nishizawa, T. Carbon 2004, 42, 14011417. (19) Lee, J.; Han, S.; Hyeon, T. J. Mater. Chem. 2004, 14, 478-486.
10.1021/la0510580 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/13/2005
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Langmuir, Vol. 21, No. 19, 2005
Although the templating technique has been known for almost two decades,20 carbons with ordered pore structure have been realized only in recent years, chiefly as a result of advances in the synthesis of suitable templates with periodic porous framework.15,16 This is particularly true for the case of ordered mesoporous carbons, which have enjoyed a steady development since they were first reported in 1999.21-30 By contrast, the synthesis of ordered microporous carbons (pore size
