Vertically aligned pearl-like carbon nanotube arrays for fiber spinning.

Due to their unique structure and physical properties and numerous potential applications, carbon nanotubes have been extensively studied recently and are expected to attract more attention in the future.1 Morphology control of carbon nanotubes represents one of the most explored directions. Except for the normal linear structure, nanotubes with morphologies of ring,2 rocket,2 branch,3 cone,4 bulb,5 star,6 cup-stacked-type,7 bamboo,8 and helix9 have been widely investigated. Here we first report another new type of nanotubes with a pearl-like morphology. This unique morphology enables nanotubes with much improved fiber spinability. The derived fiber shows excellent mechanical and electrical properties. Furthermore, such nanotubes will be much more effective in reinforcing matrix to produce strong and tough composites.10 Chemical vapor deposition (CVD) was used to synthesize pearllike nanotube arrays (Figure 1). Experimental details are described in the Supporting Information. Arrays with thickness up to hundreds of micrometers can be prepared by this approach. The assynthesized carbon nanotubes are highly aligned with each other in the array, as demonstrated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Figure 2a shows that the top surface of as-synthesized arrays is uniform and flat. The vertically aligned carbon nanotubes can be clearly observed from a side view (inserted image in Figure 2a and Figure S1). The preferentially aligned arrays are further confirmed by XRD.11 When the incident X-ray strikes arrays from the top, only a weak peak of (100) at 42.4° is observed, which corresponds to a crystal spacing of 2.12 nm (Figure S2a); however, when the incident X-ray comes at the side direction of the same sample, a much stronger peak of (002) emerges and corresponds to the interplanar spacing of 0.34 nm (Figure S2b). The XRD results also indicate that the synthesized nanotubes are multiwalled. Nanotube arrays are further characterized by Raman spectroscopy (Figure 2b). For normal linear nanotubes, the strong D-band corresponds to the disorder features due to the finite particle size effect or lattice distortion of the graphite crystals.12 For these pearllike carbon nanotubes, a major contribution also comes from the distortion in the connection segment between “bulb” and “trunk” parts. The pearl-like morphology of synthesized carbon nanotubes is studied by transmission electron microscopy (TEM). As shown in Figure 3a and 3b, the diameter of the trunk part is 23 nm, and the maximum diameter for bulb parts is 39 nm. Diameters of the hollow part in the bulb and trunk are 9 and 15-22 nm, respectively. Highresolution TEM image in Figure 3c further confirms the multiwalled structure of the pearl-like nanotubes. More details can be found in the Supporting Information. Deriving a detailed mechanism for the formation of such a pearllike structure is still a challenge. We have observed that the pearllike structure can be formed only at the temperature range of 800850 °C. The catalyst particles are in a solid-liquid two-phase state in this temperature range.13 It has also been reported that the size and geometry of the catalytic particle largely control the size and morphology of carbon nanotubes.13,14 On the basis of this evidence, we propose a dynamic solid-liquid oscillation mechanism for the formation of the pearl-like nanotubes. According to the C-Fe phase diagram,13 the catalyst particle will transform to the liquid state when the carbon content is high and to the solid state when it is low. It has also been known that carbon atoms diffuse to the catalyst-nanotube interface to sustain the nanotube growth. Assume that the catalyst particle is initially in the solid state. The solid catalyst particle tends to take a spherical shape to lower its surface † Los Alamos National Laboratory. ‡ North Carolina State University. Figure 1. Synthesis of the pearl-like carbon nanotube arrays through a chemical vapor deposition (CVD) process.