Purification of Carbon Nanotube Sheets

Large volumes of carbon nanotube (CNT) sheets and yarns are required to enable practical applications such as structural reinforcements, electrical conductors, electromagnetic interference shielding, and energy storage. Although these macroscopic assemblages of CNTs are commercially available, reports on their handling and processing have been quite limited. In this article, we report a systematic study of the purification of commercially available CNT sheets produced directly from as‐grown nanotubes. These CNT sheets contain residual metal catalysts encapsulated in graphitic carbon nanoparticles known to contaminate common powdery pristine CNT materials. Although many purification methods effective for powdery CNTs are known, they cannot be directly applied to CNT sheets because these processes often involve sonication and/or reflux, which are too aggressive and deleterious to the integrity of the sheets. Therefore, alternative physically benign processes were chosen and evaluated in detail. Assessment of the purification method included characterization of parameters such as residual metal content, weight retention, and changes in the Raman D‐to‐G intensity ratio. Importantly, the purification methodology was also placed in the context of the targeted application as a structural reinforcement in advanced composites. In this application, the mechanical properties of the CNT sheets are crucial, thus specific strength and modulus were evaluated and compared for the purified CNT sheets and the nanocomposites produced therefrom. Several important processing parameters were identified that resulted in purified CNT sheets with not only low catalyst content but also reasonable retention of mechanical properties. In addition, the scalability of the purification method and a procedure to prevent CNT sheet wrinkling after purification were demonstrated. Assessment criteria of CNT sheet purification techniques toward other potential applications are also discussed.