Exfoliation of single-walled carbon nanotube bundles under electron beam irradiation

Abstract

Single-walled carbon nanotubes (SWNTs) usually exist in the form of bundles because of strong van der Waals interaction between the tube walls. In many applications for this unique material, individual nanotubes or at least well-dispersed nanotubes are required, so it is quite important to exfoliate or disperse SWNT bundles. Commonly used dispersing routes were involved in a chemical process: dispersing them in solutions through covalent [1,2] or noncovalent [3,4] functionalization. Recently, some successful experiments have been performed to exfoliate graphite. Kaner and co-workers reported that a highly exothermic reaction between potassium intercalated graphite and aqueous solvents caused exfoliation of graphite [5]. Wang and co-workers reported that when red-hot graphite rods were dipped into cold water, graphite was exfoliated and scrolled to form multi-walled carbon nanotubes [6]. These results may have implications for the utilization of extrinsic force to exfoliate SWNT bundles. Irradiation effects on carbon nanostructures have been reviewed [7]. In 2000, Peng et al. reported that under electron beam irradiation, a SWNT with diameter as small as 0.33 nm can be grown from a larger nanotube [8]. We accidentally discovered that SWNT bundles were exfoliated when exposed to electron irradiation in an electron microscope. This phenomenon occurred for purified and shortened SWNT bundles protruding out onto holes in a transmission electron microscope (TEM) specimen grid (A typical TEM specimen grid contains amorphous carbon film with some micro holes on its surface). Exfoliation did not occur for entangled long SWNT bundles or shortened SWNT bundles lying on the amorphous carbon film. The raw soot of SWNTs was produced by direct current arc-discharge method with YNi2 as catalyst [9]. The as-prepared SWNT sample was heated in an air current with a flow rate of 100 sccm at 350 C for 2 h, and the remaining soot was refluxed in 2.6 M HNO3 for 12 h to remove excess metal particles. Then the product was filtered with a / 0.45 lm membrane filters under vacuum to form a paper-like film. We found that it was quite difficult to ultrasonically disperse the SWNT film in solvent. To disperse the SWNTs uniformly, a small amount of the film was ground into fine powder, and then ultrasonically dispersed in ethanol. A drop of uniformly dispersed liquid was put on a typical TEM sample grid and dried in air. Then the so-obtained specimen was observed by TEM at 120 kV and room temperature. TEM provides sufficient resolution and allows in situ observation of local structure transformation of carbon nanotubes under electron beam irradiation. Under electron beam irradiation for a few seconds, some thin bundles protruding out onto holes in the TEM specimen grid were found to be exfoliated from a thick bundle (Fig. 1). Because of thermal effect, the bundle tips vibrated slightly, which caused the blurredness of the thin bundles. The diameters of these thin bundles were below 10 nm, while the diameter of the main thick bundle was about 40 nm. The exfoliation occurred mainly at tips and the exfoliated thin bundles separated from each other as far as they can. To investigate this interesting phenomenon in more details, we performed in situ observation of local structure transformation of a SWNT bundle undergoing electron beam irradiation. Fig. 2(a)–(d) shows TEM images during the exfoliation of the SWNT bundle with an interval of 30 s between the successive images. These images were taken from the same sample and with different magnification as that of Fig. 1. They clearly show continuous and remarkable changes of the SWNT bundle. The main bundle was exfoliated into some thinner bundles (Fig.