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4-Nitrobenzylamine Partially Intercalated into Graphite Powder and Multiwalled Carbon Nanotubes: Characterization Using X-ray Photoelectron Spectroscopy and in Situ Atomic Force Microscopy Gregory G. Wildgoose,† Michael E. Hyde,† Nathan S. Lawrence,‡ Henry C. Leventis,† Li Jiang,‡ Timothy G. J. Jones,‡ and Richard G. Compton*,† Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, U.K., and Schlumberger Cambridge Research, High Cross, Madingley Road, Cambridge CB3 0EL, U.K. Received December 28, 2004. In Final Form: March 10, 2005 We report the characterization of partial intercalation of 4-nitrobenzylamine (4-NBA) into edge-plane or edge-plane-like defect sites on the surface of both graphite powder and “bamboo-like” multiwalled carbon nanotubes (MWCNTs) using X-ray photoelectron spectroscopy (XPS). By comparing the XPS spectra of 4-NBA derivatized graphite powder and MWCNTs with that of graphite powder treated with benzylamine in a similar fashion, we conclude that benzylamine itself does not undergo partial intercalation. Using in situ atomic force microscopy, we are able to observe the partial intercalation of 4-NBA into an edgeplane-like “step” defect on the surface of a highly ordered pyrolytic graphite crystal in real time. Together these observations provide further evidence for the partial intercalation of 4-NBA and lead us to propose a new hypothesis to explain this phenomenon.
1. Introduction The intercalation of small molecules and ions, including ammonia,1,2 methylamine,3 alkylamines,4,5 small aromatic molecules,6-8 and alkali metal ions such as lithium, into various forms of graphite and graphitic oxide6-10 is well documented, as is the co-intercalation into graphitic materials of both group I and group II ions.11 As such, graphite intercalation compounds (GICs) have been the subject of much research in recent years, with particular emphasis on their use in battery technology.12 Other methods of derivatizing carbon surfaces, other than intercalation, can be achieved via chemisorption (covalently attaching a modifier to a carbon surface),13-19 * To whom correspondence should be addressed. Tel: +44 (0)1865 275413. Fax: +44 (0)1865 275410. E-mail: richard.compton@ chemistry.ox.ac.uk. † Physical and Theoretical Chemistry Laboratory, University of Oxford. ‡ Schlumberger Cambridge Research. (1) Fan, Y. B.; Solin, S. A.; Neumann, D. A.; Zabel, H.; Rush, J. J. Phys. Rev. B 1987, 36, 3386. (2) Walters, J. K.; Skipper, N. T.; Soper, A. K. Chem. Phys. Lett. 1999, 300, 444. (3) Skipper, N. T.; Walters, J. K.; Lobban, C.; McKewn, J.; Mukerji, R.; Martin, G. J.; de Podesta, M. J. Phys. Chem. B 2000, 104, 10969. (4) Matsuo, Y.; Watanabe, K.; Fukutsuka, T.; Sugie, Y. Carbon 2003, 41, 1545. (5) Matsuo, Y.; Higashika, S.; Kimura, K.; Miyamoto, Y.; Fukutsuka, T.; Sugie, Y. J. Mater. Chem. 2002, 12, 1592. (6) Rashkov, I.; Merle, G.; Maı¨, C.; Gole´, J.; Panayotov, I. C. R. Acad. Sci., Ser. C 1976, 283, 339. (7) Merle, G.; Rashkov, I.; Maı¨, C.; Gole, J. Mater. Sci. Eng. 1977, 31, 39. (8) Kagan, H. B. Pure Appl. Chem. 1976, 46, 177. (9) Kim, Y.-O.; Park, S.-M. J. Electrochem. Soc. 2001, 148, A194. (10) Mizutani, Y.; Abe, T.; Inaba, M.; Ogumi, Z. Synth. Met. 2002, 125, 153. (11) Pruvost, S.; Herold, C.; Herold, A.; Lagrange, P. Carbon 2004, 42(8-9), 1825-1831. (12) Setton, R. Synth. Met. 1988, 23, 467. (13) Downard, A. J. Electroanalysis 2000, 12, 1085. (14) Delamar, M.; Hitimi, R.; Pinson, J.; Save´ant, J.-M. J. Am. Chem. Soc. 1992, 114, 5883.
physisorption (physical adsorption of the modifier to the surface without a covalent bond),20-23 or the use of carbon paste electrodes (where the modifier acts as a dopant within the binder).19,24,25 Derivatized carbon surfaces are of particular interest to electrochemists, leading to the possibility of tailor making electrodes for sensing, catalysis, analysis, or biological applications. Recently we reported that 4-nitrobenzylamine (4-NBA) was found to derivatize the surface of graphite powder and “bamboo-like” multiwalled carbon nanotubes (MWCNTs) when the carbon material was stirred for 2 h in a 10 mM solution of 4-NBA in acetonitrile.26 To explain this phenomenon, we proposed that the 4-NBA molecules partially intercalated into edge-plane-like defects on the surface of graphite powder and MWCNTs. Experimental observation in support of this hypothesis of partial intercalation was provided using voltammetric methods, scanning electron microscopy (SEM), and X-ray powder diffraction (XPD) techniques.26 Graphite powder and MWCNTs derivatized with 4-NBA are important new materials for use in electroanalysis, (15) Barbier, B.; Pinson, J.; Desarmot, G.; Sanchez, M. J. Electrochem. Soc. 1990, 137, 1757. (16) Bahr, J. L.; Tour, J. M. Chem. Mater. 2001, 13, 3823. (17) Strana, M. S.; Dyke, C. A.; Usrey, M. L.; Barone, P. W.; Allen, M. J.; Shan, H.; Kittrell, C.; Hauge, R. H.; Tour, J. M.; Smalley, R. E. Science 2003, 301, 1519. (18) Li, B.; Cao, T.; Cao, W.; Shi, Z.; Gu, Z. Synth. Met. 2002, 132, 5. (19) Wildgoose, G. G.; Pandurangappa, M.; Lawrence, N. S.; Jiang, L.; Jones, T. G. J.; Compton, R. G. Talanta 2003, 60, 887. (20) O’Connell, M. J.; Boul, P.; Ericson, L. M.; Huffman, C.; Wang, Y.; Haroz, E.; Kuper, C.; Tour, J. M.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett. 2001, 342, 265. (21) Chen, J.; Liu, H.; Weimer, W. A.; Halls, M. D.; Waldeck, D. H.; Walker, G. C. J. Am. Chem. Soc. 2002, 124, 9034. (22) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001, 123, 3838. (23) Leventis, H. C.; Streeter, I.; Wildgoose, G. G.; Lawrence, N. S.; Jiang, L.; Jones, T. G. J.; Compton, R. G. Talanta 2004, 63, 1039. (24) Pandurangappa, M.; Lawrence, N. S.; Compton, R. G. Analyst 2002, 127, 1568.
10.1021/la040138l CCC: $30.25 © 2005 American Chemical Society Published on Web 04/15/2005
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Scheme 1. A Cross Section through a MWCNT Illustrating the Difference between Full and Partial Intercalation of 4-NBA
particularly in the field of pH sensing.26 The partial intercalation of 4-NBA into these carbon surfaces is notably distinct from both full intercalation of the 4-NBA molecules between the graphite sheets and previous methods of derivatizing carbon surfaces discussed above. If the 4-NBA were to fully intercalate, then increasing the derivatization time would increase the amount of 4-NBA that could derivatize the surface via diffusion further into the host lattice. However previous studies have shown that the derivatization of graphitic carbon surfaces by 4-NBA is limiting whereby a small amount of 4-NBA derivatizes the surface within 30 min (see Scheme 1). After this period of time has lapsed, the amount of 4-NBA present on the carbon material remains constant.26 This represents, to the best of our knowledge, the first such example of a molecule undergoing partial intercalation. However, analogous materials to 4-NBA including nitrobenzene, 4-nitrotoluene, 4-nitrophenol, and 4-nitrobenzyl alcohol were found not to modify the carbon surface under similar derivatization conditions.26 Thus we speculated that the benzylic amine group was responsible for controlling the remarkable behavior of 4-NBA. Unfortunately we were unable to observe whether benzylamine itself partially intercalated into graphitic carbon materials using voltammetric techniques. In this report we attempt to further our understanding of the mechanism and parameters that control the partial intercalation of 4-NBA into graphite powder and MWCNTs and to investigate whether benzylamine itself partially (25) Tomcˇ´ık, P.; Banks, C. E.; Davies, T. J.; Compton, R. G. Anal. Chem. 2004, 76, 161. (26) Wildgoose, G. G.; Wilkins, S. J.; Williams, G. R.; France, R. R.; Carnahan, D. L.; Jiang, L.; Jones, T. G. J.; Compton, R. G. ChemPhysChem 2005, 6, 352.
intercalates into graphitic materials, using the sensitive surface analysis technique of X-ray photoelectron spectroscopy (XPS).27 Furthermore we present results obtained using in situ atomic force microscopy (AFM) which allowed us to observe in “real time” the effects of the partial intercalation of 4-NBA into an edge-plane “step” defect site on the surface of a highly ordered pyrolytic graphite (HOPG) crystal. 2. Experimental Section 2.1. Reagents. All reagents were obtained from Aldrich (Gillingham, U.K.), with the exception of acetonitrile (synthesis grade, 99.99% anhydrous, Fischer Scientific, Loughborough, U.K.), and were of the highest grade available and used without purification. 4-Nitrobenzylamine (4-NBA) was obtained as the hydrochloride salt. To liberate the free amine, the following procedure was used: 4-Nitrobenzylamine hydrochloride salt (2.0 g, 0.011 mM) was dissolved in water (40 cm3), and sodium hydroxide (20 cm3 of a 1 M aqueous solution) was added. The solution was stirred for 2 h, after which time the solution was washed with diethyl ether (2 × 50 cm3). The combined organic layers were washed with brine (50 cm3), dried over MgSO4, filtered, and concentrated in vacuo to afford 4-nitrobenzylamine (1.2 g, 78% yield) as a red crystalline solid which was used without further purification. The corresponding nuclear magnetic resonance spectrum was recorded and compared with library spectra in order to confirm that we had recrystallized the pure compound. The crystals of 4-NBA were stored in an airtight container at 4 °C prior to use.26 Synthetic graphite powder (2-20 µm diameter) was purchased from Aldrich. “Bamboo-like” multiwalled carbon nanotubes (purity >95%, diameter 10-40 nm, length 5-20 µm) were (27) Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Grant, J. T., Eds.; IM Publications and Surface Spectra Ltd.: Chichester, 2003.
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purchased from NanoLab, Inc. (Brighton, MA), and were used without further purification. The term “bamboo-like” is used to describe the structure of these MWCNTs where the planes of the graphite sheets are at an angle to the axis of the tube. This leads to the tubes periodically closing off into compartments rather like bamboo or a “stack of paper cups” fitted one inside another like so
