 To whom correspondence should be addressed. Present address: De- partment of Ceramic and Materials Engineering, Rutgers University, 607 Taylor Rd., Piscataway, NJ 08854. Fax: (732) 445-6264. E-mail: suchanek@rci.rutgers.edu.  Present address: Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208.  Present address: Department of Materials Engineering, Drexel Univer- sity, Philadelphia, PA 19104. Journal of Solid State Chemistry 160, 184 } 188 (2001) doi:10.1006/jssc.2001.9220, available online at http://www.idealibrary.com on Behavior of C 60 under Hydrothermal Conditions: Transformation to Amorphous Carbon and Formation of Carbon Nanotubes Wojciech L. Suchanek,* Joseph A. Libera,- Yury Gogotsi,- and Masahiro Yoshimura* *Materials and Structures Laboratory, Center for Materials Design, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503 Japan; and - Department of Mechanical Engineering, University of Illinois at Chicago, M/C 251, Chicago, Illinois 60607-7022 Received December 21, 2000; in revised form April 11, 2001; accepted April 20, 2001; published online June 11, 2001 The behavior of fullerenes C 60 under hydrothermal conditions between 200 and 8003C, and under 100 MPa pressure, in the absence and in the presence of nickel is reported. The highest temperature of fullerene stability in water was 4003C after a 48-h-long treatment. Increasing the temperature and/or time of the hydrothermal treatment resulted in transformation of fullerene to amorphous carbon. At 7003C, high-quality open- ended multiwalled carbon nanotubes were formed in the vicinity of nickel particles. These nanotubes typically had an outer dia- meter of 30+40 nm and a wall thickness of 5 nm, with a graph- itization level similar to that of carbon nanotubes prepared by chemical vapor deposition. The potential for large-scale synthesis of the carbon nanotubes by the hydrothermal technique is dis- cussed. Since the present study was conducted under conditions which are common in the geological environment, our results imply that carbon nanotubes may form in natural hydrothermal systems.  2001 Academic Press Key Words: hydrothermal synthesis; carbon nanotubes; fullerene; C 60 ; amorphous carbon; CVD; hydrothermal systems; geothermometer; hydrothermal mineral deposits. 1. INTRODUCTION Fullerenes (1) and fullerene-related materials, such as carbon nanotubes (2), have many potential applications and thus have attracted great interest of researchers all over the world. Fullerenes can be synthesized under extreme condi- tions, which rarely occur in nature, for example the electric arc method (3), carbon vaporization by pulsed lasers or in focused sunlight, direct inductive heating of carbon, or soot- ing hydrocarbon #ames (4). The major synthesis routes for carbon nanotubes are the carbon arc method (5), laser ablation (6), and chemical vapor deposition (CVD) (7}14). All these techniques require high temperatures (usually at least 700}8003C for the nanotubes and 10003C for the fullerenes), vacuum systems, gas #ow, and complicated equipment. Yields are relatively low and the products often require puri"cation. With the current synthesis of fullerenes and carbon nanotubes being still very expensive and in the absence of known large-scale natural deposits of these carbon allot- ropes, their low-temperature, high-yield synthesis would be of great scienti"c and technological importance. Carbon nanotubes have been synthesized by electrolysis in molten salts (15). It has also been shown that amorphous carbon (16) and even diamond (17) can be prepared under hy- drothermal conditions. Hydrothermal synthesis of carbon "laments, including well-aligned arrays on a substrate (18), demonstrated the potential of the hydrothermal technique for growing complex carbon structures. However, these "laments contained periodic conical cavities and were thicker than typical carbon nanotubes (18). Taking into account the advantages of hydrothermal synthesis in mater- ials processing (19, 20) and the fact that various forms of hydrothermal carbon can be prepared in the temperature range of 300}8003C (100}140 MPa) (16, 17), this method seems to be potentially very promising for synthesis of the new carbon allotropes. It is also worth mentioning that interactions between fullerenes/nanotubes and water have recently attracted at- tention due to e!orts to increase the e$ciency of extrac- tion/puri"cation of these carbon allotropes. Therefore, a study of the hydrothermal behavior of the fullerenes and the carbon nanotubes could give unique information about their formation/stability/transformation conditions, which is of great importance for mastering puri"cation and syn- thesis techniques of these materials. Moreover, such a study could be very useful for geologists looking for natural 184 0022-4596/01 $35.00 Copyright  2001 by Academic Press All rights of reproduction in any form reserved.