Multifunctional Macroarchitectures of Double‐Walled Carbon Nanotube Fibers

Abstract

Many applications are envisioned that will ultimately utilize macroarchitectures fabricated by using carbon nanotubes (CNTs) as building blocks, because these architectures (for example CNT films and fibers) possess a number of superb properties, such as high strength, low density, high specific surface area, and excellent thermal and electrical conductivity. These properties make these macrostructures potential engineering materials for developing applications in a variety of fields, such as composites, field-emission display devices, electrochemical sensing, and electrochemical energy storage. For these reasons, the last several years have seen a growing popularity for spinning CNT fibers. Long CNT strands or fibers could be directly drawn out of a chemical vapor deposition (CVD) furnace. By mimicking the process of drawing silk out of cocoon, continuous CNT yarns were assembled from a vertically aligned multi-walled CNT (MWNT) film. In general, CNT products are powderlike, and fabricating macroscopic fibers from these powders is a challenging task. Several successful attempts for spinning single-walled CNT (SWNT) fibers from CNT solutions (i.e., SWNT/surfactant solution and SWNT/super-acid solution) by a coagulation process have been reported in the literature, however, these processes are costly. In this Communication, we report a simple and cheap process for spinning double-walled CNT (DWNT) fibers out of DWNT cotton. DWNTs consist of two concentric graphene layers, and some of their properties are similar to those of SWNTs. However, DWNTs have a higher structural stability than SWNTs, with the onset temperature for oxidation ca. 200 °C higher than for SWNTs because of the coaxial structure. In our previous research, we have shown that DWNTs can be mass-produced by a sulfur-promoted floating catalyst CVD method, with the as-grown nanotube products in a cottonlike form (nanotube cotton). Developing a technique to spin yarns from the nanotube cotton by mimicking the ancient cotton-spinning process is very practical and promising for the large-scale production of DWNT fibers. Here, a drawing–drying process is reported for the fabrication of DWNT fibers. We have also investigated some of the properties of these fibers to show their multifunctional capabilities. Centimeter-long DWNT strands could be peeled off from the as-grown nanotube cotton. However, it was not easy to pull out a long fiber continuously from the as-grown nanotube cotton, perhaps because of the strong adhesion between the nanotube bundles. After the purification process (as described in the Experimental section), we occasionally found that a long nanotube fiber could be drawn from the wet nanotube cotton. A drawing–drying process to spin nanotube yarns from the wet DWNT cotton was developed (see Experimental section), which was very efficient in producing continuous fibers of DWNT. Figure 1 shows a schematic illustration of the spinning process, and an optical image of the as-spun DWNT fibers. After the spinning process, heat treatment under argon gas was performed for one hour at 150 °C to completely remove water content inside the fibers. This process further compacted the nanotube bundles.