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Langmuir 2003, 19, 7026-7029
Al(OH)3/Multiwalled Carbon Nanotube Composite: Homogeneous Coverage of Al(OH)3 on Carbon Nanotube Surfaces Klara Hernadi,*,†,‡ Edina Couteau,† Jin Won Seo,† and La´szlo´ Forro´† Institute of Physics of Complex Matter, Ecole Polytechnique Fe´ de´ rale de Lausanne, CH-1015 Lausanne, Switzerland, and University of Szeged, Department of Applied and Environmental Chemistry, H-6720 Szeged, Rerrich B. te´ r 1., Hungary Received March 13, 2003. In Final Form: June 3, 2003 Carbon nanotube (CNT) based composite materials were obtained by covering the surface of multiwalled carbon nanotubes (MWCNTs) with alumina. The coverage material was prepared by either thermal or chemical decomposition of aluminum sources, such as aluminum trichloride (AlCl3) and aluminum isopropoxide (AlIP). To vary the interaction between the hosting CNTs and the coverage material, MWCNTs were treated with and without surfactants. The direct impregnation technique was found to be successful in the case of AlIP, whereas an adsorbed layer of surfactant provides a better interaction in the case of AlCl3 via their ionic groups. With the latter method, MWCNT-based alumina composites were obtained with a homogeneous coverage never observed before; transmission electron microscopy corroborates these conclusions. By means of energy-dispersive X-ray analysis and X-ray photoelectron spectroscopy, the coverage was identified to be partly Al2O3 but predominantly Al(OH)3, which starts to convert to Al2O3 at 200 °C.
Introduction Carbon nanotubes (CNTs) have attracted considerable attention in recent years, most notably because of their extraordinary mechanical and unique electronic properties. Like CNTs, their composite materials may find applications either as reinforcement fibers or as potential electronic nanodevices.1,2 In particular, since the mechanical properties of CNTs are indisputably better than those of carbon fibers, CNTs can advantageously substitute the latter as reinforcing elements in composites. On the other hand, CNTs can also be used as a template for the synthesis of inorganic nanostructures.3,4 A wide range of possible composite materials have been envisioned using single- or multiwalled CNTs (SWCNTs or MWCNTs) in combination either with different polymers5-7 or with inorganic compounds.8-15 Especially composites with a * Corresponding author. E-mail:
[email protected]. Fax: +36-62-544619. Tel: +36-62-544626. † Ecole Polytechnique Fe ´ de´rale de Lausanne. ‡ University of Szeged. (1) Cochet, M.; Maser, W. K.; Benito, A. M.; Callejas, M. A.; Martinez, M. T.; Benoit, J. M.; Schreiber, J.; Chauvet, O. Chem. Commun. 2001, 16, 1450-1451. (2) Weidenkaff, A.; Ebbinghaus, S. G.; Lippert, T. Chem. Mater. 2002, 14, 1797-1805. (3) Ajayan, P. M.; Stephan, O.; Redlich, P.; Colliex, C. Nature 1995, 375, 564-567. (4) Kiricsi, I.; Fudala, A Ä .; Ko´nya, Z.; Hernadi, K.; Lentz, P.; Nagy, J. B. Appl. Catal. A 2000, 203, L1-L4. (5) Pirlot, C.; Willems, I.; Fonseca, A.; Nagy, J. B.; Delhalle, J. Adv. Eng. Mater. 2002, 4, 109-114. (6) Fan, J.; Wan, M.; Zhu, D.; Chang, B.; Pan, Z.; Xie, S. Synth. Met. 1999, 102, 1266-1267. (7) Gong, X.; Liu, J.; Baskaran, S.; Voise, R. D.; Young, J. S. Chem. Mater. 2000, 12, 1049-1052. (8) Schadler, L. S.; Giannaris, S. C.; Ajayan, P. M. Appl. Phys. Lett. 1998, 73, 3842-3844. (9) Shaffer, M. S. P.; Windle, A. H. Adv. Mater. 1999, 11, 937-941. (10) Haggenmueller, R.; Gommans, H. H.; Rinzler, A. G.; Fischer, J. E.; Winey, K. I. Chem. Phys. Lett. 2000, 330, 219-225. (11) Flahaut, E.; Peigney, A.; Laurent, Ch.; Marlie`re, Ch.; Chastel, F.; Rousset, A. Acta Mater. 2000, 48, 3803-3812. (12) Flahaut, E.; Peigney, A.; Laurent, Ch.; Rousset, A. J. Mater. Chem. 2000, 10, 249-252. (13) Banerjee, S.; Wong, S. S. Nano Lett. 2002, 2, 195-200.
metal matrix reinforced with CNTs are expected to have unique mechanical properties.16 Nevertheless, there are two major obstacles to tackle using CNTs as reinforcement fibers either in CNT/metal or in CNT/polymer composites: (i) the wettability of the CNT surface and (ii) the load transfer from the matrix to the CNTs. A proper coating of their surface and a subsequent heat treatment might be a possible approach. Here we report on our successful attempt to achieve a homogeneous coverage of MWCNTs with alumina (Al2O3) or aluminum hydroxide (Al(OH)3) (hereafter, the term “composite” is used for this assembly). Our results suggest that an effective interfacial bonding between the CNT surface and aluminum precursors is the most important issue for the formation of MWCNT-based composites. Therefore, the initial CNT surface has to be “clean” and free of amorphous carbon for the composite preparation. Moreover, the ionic character of the coating material has to be taken into account in order to establish an interaction via ionic groups. Experimental Methods For the preparation of the composite materials, MWCNTs were synthesized as starting material by the catalytic decomposition of acetylene at 720 °C using a Co,Fe catalyst supported by CaCO3 (5 wt % of Co,Fe). For their purification, pristine MWCNTs were sonicated in 30% HNO3 for 30 min, filtered, washed with distilled water to be acid-free, and finally dried at 120 °C overnight. Details of the synthesis and the purification procedure carried out have been described elsewhere.17 Preparation of MWCNT-Based Composites. Purified MWCNTs were applied as “raw” material as well as impregnated with surfactants for the preparation of composite materials. As (14) Wood, J. R.; Zhao, Q.; Frogley, M. D.; Meurs, E. R.; Prins, A. D.; Peijs, T.; Dunstan, D. J.; Wagner, H. D. Phys. Rev. B 2000, 62, 75717575. (15) Gavalas, V. G.; Andrews, R.; Bhattacharyya, D.; Bachas, L. G. Nano Lett. 2001, 1, 719-721. (16) Kuzumaki, T.; Ujiie, O.; Ichinose, H.; Ito, K. Adv. Eng. Mater. 2000, 2, 416-418. (17) Couteau, E.; Hernadi, K.; Seo, J. W.; Thieˆn-Nga, L.; Forro´, L. Chem. Phys. Lett., submitted.
10.1021/la034432+ CCC: $25.00 © 2003 American Chemical Society Published on Web 07/09/2003
Al(OH)3/MWCNT Composite
Langmuir, Vol. 19, No. 17, 2003 7027
Table 1. Quality of the Surface Coverage Applying Different Impregnation Techniques and Aluminum Sources aluminum source used MWNTs plain MWNT
impregnation technique
AlIP
wet method
almost clean NT surface, rudiments of coverage; alumina nanoparticles (Figure 2b) homogeneous coating of nanotubes, accompanied by needlelike nanocrystals (Figure 2a) preferentially thin, irregular coverage (not shown)
dry method MWNT + SDS
wet method dry method
AlCl3 no nanocrystals, but thin, not continuous coverage (Figure 1b) MWCNTs covered with a thick homogeneous layer of amorphous alumina (Figure 3a,b)
separated nanocrystals sticking to the nanotubes (Figure 1a)
aluminum sources, two compounds, namely, aluminum trichloride (AlCl3‚6H2O, Fluka) and aluminum isopropoxide (AlIP, Al(OC3H7)3, Fluka), were used. In the case of AlCl3, 20 mg of MWCNT was impregnated with a surplus amount of AlCl3 suspended in 2-propanol. The mixture was then dried on a heated magnetic stirrer. Finally, AlCl3 was thermally decomposed at 250 °C for 8 h. In the case of AlIP, MWCNTs were suspended either in 2-propanol or directly in melted AlIP. To avoid confusion, the treatment with 2-propanol is denoted hereafter as the “wet” method whereas the direct impregnation is named as the “dry” method. To be precise, the wet method implied 20 mg of MWCNT impregnated with 50 mg of AlIP in an 2-propanol suspension under nitrogen (hygroscopic!). Distilled water was added in order to hydrolyze AlIP, and the solution was dried on a heated magnetic stirrer. In contrast, 20 mg of purified catalytic MWCNTs and a surplus amount of solid AlIP were directly mixed together under nitrogen for the dry method. Subsequently, the mixture was covered, stirred, and heated to approximately 140 °C (just above the melting point of AlIP, 134-138 °C). At the end of the procedure, AlIP was hydrolyzed by adding a few drops of distilled water, and the final suspension was heated at 250 °C for 8 h. Preparation of MWCNTs Using Surfactants. For the impregnation of MWCNTs with surfactants, two different methods were applied depending on the preparation method of the composite: For the wet method, 120 mg of purified MWCNTs was sonicated in 2-propanol, and then 120 mg of surfactants (sodium dodecyl sulfate, SDS) was added and stirred. For the dry method, 200 mg of purified MWCNTs was sonicated in distilled water and 2 g of SDS was added. This suspension was filtered using a membrane (0.1 µm pore diameter) and dried at 100 °C. Both wet and dry methods were applied using the AlIP source, whereas in the case of AlCl3 only the wet method was used. Since we utilized only one surfactant, the expression “soaptreated” used hereafter involves the treatment with SDS. Characterization of Composite Materials. Transmission electron microscopy (TEM) was performed using a Philips CM20 conventional and a Philips CM-300 high-resolution microscope, both equipped with an energy-dispersive X-ray (EDX) detector. For the TEM-grid preparation, composite samples were sonicated in 2-propanol for 10 min. X-ray photoemission spectra were acquired on a Sigma Probe VGScientific spectrophotometer equipped with a monochromatized Al KR source (E ) 1486.6 eV) and operating at a base pressure of
