Catalytic Effects on Thermal Oxidation of Single‐Walled Carbon Nanotubes by Alkali Metal Chlorides

Chemical reactions on single-walled carbon nanotubes (SWCNTs) are particularly important for purification and application. As-grown SWCNTs contain various forms of other carbons, to which we will refer herein as soot, and residual metal catalyst. One of the common purification procedures is to oxidize soot at high temperatures and to dissolve metals away chemically. For applications, because SWCNTs are difficult to disperse and mix with other materials, chemical modifications are often employed to improve surface properties for further processing. In practice, many chemical reactions involving SWCNTs depend on the tube diameter, defects, and the degree of bundling. There are numerous reports on the diameter dependence of reactions using thin SWCNTs, with diameters typically less than 2 nm, with the consistent result that tubes with smaller diameters are oxidized faster. Such results can be attributed to the increasing strain on carbon– carbon bonds as the tube diameter becomes smaller. Because thick SWCNTs, with diameters larger than approximately 2.5 nm, tend to collapse into flattened configurations as the opposing walls attract each other due to strong van der Waals forces, the carbon–carbon bonds at the folds are severely strained. Although reactions on thick SWCNTs have not been well characterized, we expect that the larger diameter tubes are more reactive for thick SWCNTs. Defects, which include any bonding sites where the carbon hexagonal continuum is broken and surface sites with chemically adsorbed species, are often terminated by various oxide groups and have the effect of localizing p electrons. Some of them can initiate or promote organic reactions, with the consequence that SWCNTs with more defects tend to be more reactive. Due to the close packing of tubes in bundles, reactant molecules cannot access tubes inside the bundles unless they are very small. As a result, bundling tends to lower the reactivity as a whole. In addition to these general tendencies, some reactions are dependent on the electronic states of SWCNTs, which have been applied to separate metallic and semiconducting tubes. Herein we report an exception to these ordinary characteristics. Thermal oxidation in the presence of alkali metal chlorides was found to be independent of these parameters. Using this result, unwanted selectivity during purification, for instance, for an evaluation of “true” chirality distribution of a given as-grown sample, could be minimized. Furthermore, the reaction was catalytic. Carbons were oxidized at much lower temperatures at faster rates. In particular, the reaction did not produce new defects on the SWCNTs. Such “clean” combustion avoids damage to the SWCNTs during purification processes, in addition to helping to save energy and time. The catalytic effects of metal oxide and chloride salts during combustion of soot have been known for some time, in relation to the environmental and health issues of diesel exhaust gases. Because the temperature of the exhaust gas is less than 670 K, it is necessary that combustion catalyst are effective below this temperature. These studies, however, are aimed at lowering the combustion temperatures of soot and the mechanism of catalysis remains largely unknown. Endo et al. investigated thermal oxidation of graphitized multiwalled carbon nanotubes (MWCNTs) in the presence of NaCl. Based on the kinetic study, they proposed two reaction steps; introduction of disorder as the first process and oxidation of the disordered tubes as the second process. They concluded that the main role played by NaCl was to increase the number of defects on the MWCNTs. We examined SWCNTs, which allowed us to obtain detailed information about the diameter, defects, and bundling from Raman spectroscopy. Scanning electron microscopy (SEM) revealed morphological changes taking place during oxidation. X-ray photoelectron spectroscopy (XPS) was used to look for specific interactions between the SWCNTs and the chloride salts. SWCNTs (HiPco) were rendered hydrophilic by sonication in a mixture of H2SO4 and HNO3 (3:1 v/v) for a few hours, followed by thorough washing with water to neutral pH. An aqueous dispersion of the acid-treated SWCNTs was mixed with an alkali metal chloride (NaCl, KCl, CsCl) solution (typically 0.1–10 mm) with brief sonication. The salt caused immediate aggregation of the SWCNTs. The mixture was cast on a glass plate for oxidation experiments. For thermogravimetric analysis (TGA), the solid was collected on a filter and dried. The exact same procedure without the addition of chloride salts was carried out for a control sample. As it turned out, the SWCNTs with chloride salts were oxidized so fast at low temperatures that significant oxidation would have taken place during the heating to the final temperature if we have had used an electric oven. Because the sample was taken out of the oven for each measurement, which was performed at room temperature, and brought back up to temperature repeatedly, the electric oven caused a problem. For this reason, we used a microwave oven for the thermal oxidation of SWCNTs. By microwave heating, temperatures above the combustion point could be reached in several seconds and then brought back down to room temperature on a similar time scale. Because the heating rate is much faster than the rate of oxygen intake, the fast heating and cooling minimized complications due to oxidation during the temperature rise. A drawback of using the microwave oven was that the exact temperature of the SWCNTs was unknown. [a] Y. Kobayashi, Prof. Dr. M. Sano Department of Polymer Science and Engineering, Yamagata University 3-4-16 Jyonan, Yonezawa, Yamagata 992-8510 (Japan) Fax: (+ 81) 238-26-3072 E-mail : mass@yz.yamagata-u.ac. jp