NANO LETTERS
Irradiation of Single-Walled Carbon Nanotubes with High-Energy Protons
2002 Vol. 2, No. 7 789-791
Vladimir A. Basiuk,*,†,§ Kensei Kobayashi,*,† Takeo Kaneko,† Yoichi Negishi,| Elena V. Basiuk,‡,⊥ and Jose´-Manuel Saniger-Blesa‡ Faculty of Engineering (Department of Chemistry and Biotechnology) and Instrumental Analysis Center, Yokohama National UniVersity, Hodogaya-ku, Yokohama 240-8501, Japan, and Centro de Ciencias Aplicadas y Desarrollo Tecnolo´ gico, UniVersidad Nacional Auto´ noma de Me´ xico, Circuito Exterior C.U., 04510 Me´ xico, D.F., Mexico Received May 1, 2002; Revised Manuscript Received May 19, 2002
ABSTRACT Morphological changes in single-walled carbon nanotubes (SWNTs) upon bombardment with 3MeV protons were monitored by transmission electron microscopy in a wide range of irradiation doses. Evident morphological alterations were observed at >0.1 mC, such as curving of the nanotubes, a loss of their straight shape, and formation of short pieces. During further irradiation (doses approaching 1 mC) SWNTs degraded into an amorphous material, although a significant fraction of them were present as pieces of different lengths.
Effects of different kinds of irradiation on carbon nanotubes (CNTs) are of interest from two main points of view. First, this is a way to modify the physical (including electronic) and chemical properties, by introducing structural defects into the side walls. Second, it allows defining to what extent CNT-based materials can be used under harsh radiation environments, associated with their possible use for nuclear applications and under the conditions of open space. Effects of electron and heavy ion irradiation on CNTs has already received attention in recent publications,1-10 which addressed both atomic-scale damage (defect formation) and morphological changes in CNT structure (coalescence, welding, etc.). Open space is an abundant source of one more important type of high-energy charged particles, namely protons. Their size is intermediate between that of electrons and of heavier positively charged ions, so that any information on the irradiation of CNTs with protons would be a significant addition to our knowledge of the whole spectrum of radiation effects on this important class of materials. In this letter we report on transmission electron microscopic (TEM) observation of morphological changes in single-walled carbon nanotubes (SWNTs) caused by their bombardment with 3MeV protons, in a wide range of irradiation doses. * Corresponding authors. E-mail:
[email protected]; kkensei@ ynu.ac.jp. † Faculty of Engineering (Department of Chemistry and Biotechnology), Yokohama National University. | Instrumental Analysis Center, Yokohama National University. ‡ Universidad Nacional Auto ´ noma de Me´xico. § On sabbatical leave from Instituto de Ciencias Nucleares, Universidad Nacional Auto´noma de Me´xico, Circuito Exterior C. U., 04510 Me´xico, D. F., Mexico (permanent address). ⊥ Also known as Golovataya Dzhymbeeva. 10.1021/nl025601y CCC: $22.00 Published on Web 05/30/2002
© 2002 American Chemical Society
We used open-end SWNTs commercially available from Iljin Nanotech Co., Ltd., Korea (by arc-discharge process, 95%+ purified by thermal oxidation and chemical treatment). To improve their solubility in organic solvents, we performed the amidation of terminal carboxylic groups11-16 with 1-octadecylamine. Although during the industrial purification process SWNTs were treated with strong acids, we secured the presence of carboxylic groups on the nanotube tips by an additional short, 0.5 h boiling step in concentrated nitric acid (this treatment does not result in noticeable SWNT cutting, as we subsequently observed by TEM). To perform the amide derivatization, we employed the gas-phase procedure reported recently by us,16 slightly modified in the following way. SWNTs (100 mg) and 1-octadecylamine (20 mg) were placed together into a glass ampule and evacuated to ca. 10-2 Torr at room temperature, before the ampule was sealed. Then the ampule was placed into an oven preheated at 170 °C. After baking for 2 h, the ampule was opened and the excess of octadecylamine was removed by an additional evacuation with simultaneous heating at 150 °C. Ultrasonication of the SWNTs obtained during 10-20 min in tetrahydrofuran produces a dark-brown solution that is stable during several months (we used it during the first few days). Protons were generated from a Van de Graaff accelerator (at Tokyo Institute of Technology) as a beam of ca. 4-mm diameter. The average current was 0.1 µA. The need to handle very small samples and to ensure their uniform irradiation by so narrow beam led us to the use of SWNTs predeposited from solution/suspension in tetrahydrofuran onto standard TEM grids. (We rejected the idea of irradiating solid as-derivatized SWNTs, and only then dissolving and depositing them onto the grids, since during the latter
Figure 1. Experimental setup for irradiation of SWNTs with highenergy protons. (1) SWNTs deposited onto a TEM copper grid; (2) 1-mm thick microslide glass with (3) 2.7-mm diameter orifice; (4) two microslide glasses; (5) steel spring holders; and (6) proton beam from the Van de Graaff accelerator.
operation an undesirable and unpredictable fractionation of solvent-suspended SWNTs is possible: it would be uncertain whether the deposited fraction adequately represents the entire irradiated sample.) To hold the samples during the irradiation experiments, we used a homemade assembly, schematically shown in Figure 1. The key element is a conventional microslide glass (2), drilled to produce a 2.7mm orifice (3) close to its center. One edge of the orifice is conical, with the bigger diameter of about 3.2 mm, sufficient to adopt a standard 3.05-mm diameter TEM grid (1) with deposited SWNTs. After placing the grid into the orifice, to ensure firm position of the sample and mechanic stability of the entire assembly, the grid was covered with 1-2 unperforated microslide glasses (4). Two steel spring holders (5) kept the glasses fixed with respect to each other. The assembly was adjusted in front of the exit flange of the Van de Graaff accelerator, to provide best coaxiality of the sample and the proton beam (6), and as close as possible to the flange to avoid energy losses due to the passage of protons through the air. The irradiation was performed at ambient temperature.17 Since some probability exists that the sample edge is shadowed by the orifice edge and remains underexposed or unexposed, all TEM measurements were focused on the central part of the grid. A JEOL JEM-2000FX II instrument was used operating at 180 kV. Precautions were taken not to overexpose the specimens to the electron beam and to select image areas as fast as possible, to avoid SWNT alterations due to electron irradiation. The TEM measurements were performed a week after the irradiation, so that they reflect a chemically equilibrated state of SWNTs. We performed the experiments at irradiation times ranging from 1 to 120 min, which correspond to doses from 6 µC to 0.72 mC (or energy deposited of approximately 10-1-101 mJ). Selected TEM microphotographs are shown in Figure 2. Low doses (roughly below 0.05 mC) did not cause any noticeable changes. The images remained essentially the same as for SWNTs without any irradiation, which can be exemplified by Figure 2a (corresponds to 6 µC dose). From this image one can also see that the oxidation procedure employed did not cause significant SWNT cutting: most of them are a few µm long, as the commercially available material was. Due to increasing SWNT solubility through 790
Figure 2. TEM microphotographs of SWNTs irradiated by 3MeV protons for (a) 1 min (dose of 6 µC), (b) 30 min (0.18 mC), (c) 1 h (0.36 mC), and (d) 2 h (0.72 mC). Different samples for different irradiation doses were used.
the amidization, we afforded their relatively uniform deposition onto the grids, although some of them were present in the samples as bundles (right upper corner of Figure 2a). Higher irradiation doses (>0.1 mC) apparently caused radiation damage of the TEM grid material, decreasing the quality of the microphotographs (poor sharpness). Nevertheless, most SWNTs did not exhibit dramatic morphological changes. For example, from Figure 2b, corresponding to 30min irradiation time or 0.18-mC dose, one can clearly see that SWNT length still remains of the order of 1 µm. At the same time, there are some indications of nanotube welding in the places of contact (intersecting as well as parallel SWNTs in bundles). Different kinds of alterations became evident after irradiation for 1 h, or at a dose of 0.36 mC (Figure 2c). SWNTs became curved in an irregular manner, and their thickness was not uniform anymore, thus evidencing on numerous wall defects. A considerable fraction of the nanotubes appeared to be cut into small pieces, of less than 500-nm length: this was not observed in the preceding samples. Although due to decreased image quality it is impossible to definitively Nano Lett., Vol. 2, No. 7, 2002
conclude on the formation of junctions and on coalescence effects, it is most logical to expect these phenomena under the proton-irradiation conditions. With all the changes discussed above, most SWNTs did conserve their integrity as nanotubes. This cannot be said for the last, longest irradiation dose of 0.72 mC (2 h; Figure 2d). While many of them can be identified in the microphotograph as short pieces (of a submicron length), a shapeless material dominates here, which is apparently amorphous carbon. Since all the irradiation experiments were performed under air atmosphere, both this amorphous material and altered SWNTs must possess a high degree of oxidation. We make the following conclusions. (1) Below ca. 0.05 mC, no noticeable changes were observed in SWNT morphology. At these low irradiation doses, high-energy protons can cause the formation of wall defects, which are not yet so numerous as to distort the nanotube shape. At the same time, since electronic properties of SWNTs are very sensitive to any wall defects, this dose range can be used for their modification. (2) Evident morphological alterations observed at higher doses (>0.1 mC) are curving of the nanotubes along with a loss of their straight shape due to accumulating wall defects, as well as the formation of short SWNT pieces, below 500-nm length. Although further high-resolution TEM measurements are required to definitively conclude on welding and coalescence effects, they are most logically expected under the above conditions. The latter phenomena give rise to the formation of a covalently bound threedimensional network and apparently can be used for strengthening mechanical properties of some SWNT materials. (3) At even higher irradiation doses (approaching 1 mC) SWNTs degrade into an amorphous material, although a significant fraction of them are present as pieces of different (mainly submicron) lengths. As a whole, this means a loss of the nanotube physical integrity, and thus puts an upper limit of proton irradiation doses where SWNT materials can be used, regardless of their particular application. This limit, however, is actually very high if to compare flux values in our case (ca. 1013 cm-2s-1) and for those typical for near-Earth space conditions during “quiet sun” periods (of the order of 102 cm-2s-1sr-1 for 10-1-101 GeV protons18). Even upon a direct exposure to proton space irradiation, SWNTs will undergo no detectable alterations for practically unlimited time. Acknowledgment. The authors thank Dr. Katsunori Kawasaki, Tokyo Institute of Technology, for his kind assistance in the operation of the Van de Graaff accelerator, and the DGAPA of the National Autonomous University of Mexico for its support through the IN-106900 project. V.A.B. would like to thank the Japan Society for the Promotion of Science (JSPS) for the JSPS Invitation Fellowship (grant No. L01536). References (1) Kiang, C.-H.; Goddard, W.; Beyers, R.; Bethune, D. J. Phys. Chem. 1996, 100, 3749. (2) Ajayan, P. M.; Ravikumar, V.; Charlier, J.-C. Phys. ReV. Lett. 1998, 81, 1437. (3) Banhart, F. Rep. Prog. Phys. 1999, 62, 1181. (4) Beuneu, F.; l’Huillier, C.; Salvetat, J.-P.; Bonard, J.-M.; Forro, L. Phys. ReV. B 1999, 59, 5945. Nano Lett., Vol. 2, No. 7, 2002
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