Stability of fullerenes under hydrothermal conditions

Wojciech L. Suchaneka) and Masahiro Yoshimura Center for Materials Design, Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503 Japan
Yury G. Gogotsi Department of Mechanical Engineering, М/С 251, University of Illinois at Chicago, Chicago, Illinois 60607-7022 (Received 28 July 1998; accepted 15 September 1998)

Stability of fullerenes C60 under hydrothermal conditions (200-800 °C, 100 MPa, 20 min-168 h) has been investigated. The reaction products have been characterized by Raman spectroscopy and x-ray diffraction. The fullerenes were stable up to 500 °C, but they decomposed immediately at 800 °C into amorphous carbon. In the transition region between 600 and 750 °C, longer times and higher temperatures of the hydrothermal treatment favored decomposition of C6o with the formation of amorphous carbon. Addition of nickel to the C6o-H20 system neither suppressed hydrothermal decomposition of C6o nor induced formation of other phases, except of the amorphous carbon.

Fullerenes and fullerene-related materials in recent years have attracted great interest from researchers all over the world. Since their first discovery in 1985 by Kroto et al.,1 the number of papers related to these materials has increased rapidly.2 Fullerenes and related carbon structures have been the subject of books and review articles.39 Such an interest in this field is due to multiple potential applications of the new carbon allotropes. Applications of fullerenes include energy production (electrodes in batteries, hydrogen storage media),6 superconductors (when doped with alkali met¬als), catalysts, photoconductors, optical limiters, and diamond precursors.56 Additionally, there is a possibility of fabrication of an enormous number of new fullerene-based compounds with still unknown properties and applications because various metal atoms can substitute carbon atoms10 or be imprisoned within the fullerene cage,10 and also multiple functional groups can be at¬tached to the carbon atoms in the fullerene structure.6

Recently, interactions between fullerenes and water in a broad range of temperatures and pressures have attracted the attention of researchers. It has been demonstrated that hydrothermal pretreatment of the fullerene soot increases efficiency of the fullerene extraction.1112 Purification of fullerene nanotubes using the hydrothermal treatment13 or via other routes using aqueous solutions14 has been successfully accomplished. Aqueous electrochemistry of fullerene films/electrodes has been recently intensively investigated.1516 In addi¬tion, research on aqueous chemistry of soluble in waterfunctionalized fullerenes17 has been pursued with a view toward possible biological applications. Even fab¬rication of C6o monolayers on the water surface18 and coating the carbon nanotubes with nickel by electroless plating19 have been reported. However, interaction between fullerenes and water at elevated temperatures and pressures has not been studied.

The purpose of the present work is to report our results on hydrothermal behavior of fullerenes in a broad range of temperatures under moderate pressures. We have investigated the stability region of fullerenes C6o under hydrothermal conditions between 200 and 800 °C, and under 100 MPa pressure. Since nickel has been known to catalyze the formation of a variety of carbon materials,2023 we have also investigated the possible catalytic effect of nickel on the behavior of Сво under hydrothermal conditions.

Fullerene (C6o) powder (purity of 99.95%, provided by Science Laboratories Co., Japan) was used in all experiments. Small samples of this powder (~ 0.020 g) were inserted into golden capsules 3 mm in diameter which were subsequently filled with double-distilled wa¬ter (~ 0.3 g). The capsules were then sealed, placed into autoclave tubes (Tuttle-Roy-type), and heated at 200, 400, 500, 600, 650, 700, 750, and 800 °C for periods between 20 min and 48 h. The nickel powder (purity of 99.8%, provided by Nilaco Co., Japan) was used in hy¬drothermal experiments related to the Сбо-Н20 system with Ni additions. Small samples of these powders were inserted into golden capsules (volume of ~ 0.1-0.2 cm3) which were subsequently filled with double-distilled water. Quantities of water were between 30 and 100% (by weight) of the solid phase. 3% (by weight) of nickel has been added to the solid phase in all cases. Thecapsules were sealed, placed into autoclave tubes, and treated as decribed above, at 400, 500, 600, and 700 °C for 168 h.

All the materials were characterized by Raman spectroscopy, using a spectrometer in a "macro" mode (T64000, Atago-Jobin Yvon, France-Japan; Ar+ laser with the excitation wavelength of 514.5 nm). The laser power was low, about 10 mW, to avoid decomposition of the fullerenes during the measurements. Additionally, x-ray diffraction analysis (XRD, 40 kV-40 mA, Cu Ka radiation, MAC Science Co. Ltd., Tokyo, Japan) was conducted before and after the hydrothermal treatments.

Selected Raman spectra of the hydrofhermally treated fullerene powders in the C6o-H20 system are shown in Fig. 1. The Raman spectrum denoted "as received" corresponds to the fullerene powder which did not undergo any hydrothermal treatment. This spectrum is a characteristic spectrum of C6o, showing all 10 Raman-active modes.24 Raman spectra of the C6o acquired after the hydrothermal treatments in pure water at 200 °C (4 h, 48 h), 400 °C (4 h, 17 h, 48 h), 500 °C (17 h, 48 h), 600 °C (0.3 h), and 650 °C (0.3 h) were almost identical to as-received C6o, indicating that the C60 was stable under hydrothermal conditions in this temperature range for these times (Fig. 1). In the Raman spectra acquired from the fullerenes hydrofhermally treated at 700 °C (0.3 h) and 750 °C (0.3 h), new bands appeared, in addition to weakening of Сбо-derived bands (Fig. 1). Two strong and broad bands at about 1335 and 1608 cm"1 can be ascribed to amorphous carbon.25 These positions of bands of amorphous carbon (down-shifted D-band and up-shifted G-band) are typical for hydrofhermally formed carbon. A shoulder band at about 1200 cm"1 has been observed in hydrofhermally formed carbon, particularly that was grown on diamond substrates, but its origin is not clear.26 After hydrothermal treatment at 800 °C (0.3 h), the Raman spectrum did not show any fullerenes. This demonstrates that Сво was completely decomposed into amorphous carbon under such conditions (Fig. 1).

XRD patterns of the hydrofhermally treated fullerene powders were in a good agreement with the Raman spectra (Fig. 2). The XRD patterns of the C6o meas¬ured after the hydrothermal treatments between 200 and 500 °C, 600 °C (0.3 h, 100 MPa), and 650 °C (0.3 h, 100 MPa) were almost identical to as-received C6o. In the XRD patterns of the fullerenes hydrofhermally treated at 700 °C (0.3 h, 100 MPa) and 750 °C (0.3 h, 100 MPa), two new broad peaks were observed, in addition to weakening Сво peaks (Fig. 2). These new peaks are around 24.5° and 43.5° and can be ascribed to graphitic carbon. [The term "amorphous carbon" has been used in the scientific literature, as well as in our papers, to describe both entirely amorphous material and "disordered graphitic carbon" which is structurally disordered, but in fact contains Basic Structure Units (BSU) of graphite with sp2 bonds.] After the hydrother¬mal treatment at 800 °C (0.3 h, 100 MPa), the XRD pattern did not show any fullerenes, confirming that they were completely decomposed into amorphous carbon (Fig. 2).

Both Raman spectra and XRD patterns of the Сбо hydrofhermally treated in pure water under different conditions show that with increasing temperature and time of the hydrothermal treatment, the C60 fullerenes gradually decompose to amorphous carbon. The approx¬imate stability regions of C6o in water under 100 MPa are graphically illustrated in Fig. 3. Fullerenes seem to be stable in water under the pressure of 100 MPa up to 500 °C but they decompose immediately at 800 °C. The region between 600 and 750 °C is a transition region where longer times and temperatures of the hy¬drothermal treatment favor decomposition of C60 into amorphous carbon.

A catalytic effect of Ni addition to the C-H2O system to enable crystallization of diamond has been suggested by Roy and co-workers in the case of hy¬drothermal synthesis of diamond from glassy carbon.2223 In our case, a source of carbon was the C6o powder. Moreover, the temperature-pressure regime was differlarger size of graphite crystallites. However, this can be explained by a longer duration of experiments with Ni additives. In the C6o-H20 system with Ni additives, the fullerenes were not decomposed at 400 °C as shown in Fig. 4. The level of decomposition of the fullerenes into amorphous carbon between 500 and 700 °C was a function of the H2O: С ratio and temperature (Fig. 4). At a lower H2O:C ratio, the graphite G band domi¬nated Raman spectra [Fig. 4(a)]. At a higher H2O: С ratio, the relative intensity of D-band around 1335 cm"1 increased with increasing temperature [Fig. 4(b)]. The noticeable effect of the water content on the decomposi¬tion of C6o at 500 °C (Fig. 4) suggests that the formation of amorphous carbon occurs by oxidation of fullerene by hydrofhermal fluid with the subsequent deposition of amorphous carbon from the C-H-0 fluid. In the case of direct transformation of fullerene to graphite, we should not see any influence of the water/carbon ratio.

It has been already briefly mentioned in the liter¬ature that fullerenes (C6o-Ci8o) were not decomposed under hydrofhermal conditions at lower temperatures of 100-270 °C for 12-24 h (pressure conditions were not given).1113 However, to the best knowledge of the authors, the stability region of fullerenes under hy¬drofhermal conditions has been established for the first time. Information obtained in this study may be ap¬plied for further improvement of purification techniques of fullerenes and fullerene-based materials; it can be used in hydrofhermal chemistry and electrochemistry of fullerenes, materials synthesis, and even in geology (presence/absence of fullerenes in certain rocks27-28 may indicate occurrence/absence or conditions of previous hydrofhermal activities29).

ACKNOWLEDGMENTS

This research was supported by the "Research for the Future" Program No. 96R06901 of the Japanese Society for the Promotion of Science (JSPS). Y.G.G. was supported by the UIC Campus Research Board.