Controlled Synthesis of Carbon Nanotubes: Past, Present and Future

Carbon nanotubes (CNTs) have received much attention over the past two decades owing to their unique properties such as high tensile strength, mobility, and thermal conductivity as well as clean surface. Hence, CNTs have been widely investigated for many potential applications, for example, as additives in composites and main components of integrated circuits. However, the former application widely used does not exploit their intrinsic properties, while the latter has only been demonstrated at the level of laboratory prototype devices. As the main factor determining future applications of CNTs is the ability to achieve their structure-controlled synthesis, this review first introduces a classification of CNT structures highlighting the potential difficulties associated with fine CNT structure control due to the similarities between different CNTs. Then, advances in the basic research and industrialization of CNTs in the past decades are summarized, including fine structure control, aggregation synthesis, and scale-up production. Catalysts are crucial for controlling the structure of CNTs, as their lifetime determines the CNT length and size (wall number and diameter), while their state and formation affects CNT chirality. Moreover, as the microscopic properties of individual CNTs often differ from their macroscale performance at industrial-scale production, their aggregation state should be carefully taken into consideration. Therefore, several methods were developed to realize different types of aggregates, such as lattice orientation for obtaining horizontally aligned CNT arrays, the use of catalysts with high density for the synthesis of vertical CNT arrays, direct deposition of CNT films, and even fabrication of very complex three-dimensional (3D) macrostructures. Furthermore, many efforts have been invested to promote CNT industrialization and develop various techniques to increase CNT production, including the fluidized bed method and floating method. Finally, the ideal synthesis of CNTs should combine structure control with scale-up preparation. To this aim, further theoretical understanding of the detailed CNT growth mechanism is still needed to clarify, for example, how CNT caps form at the atomic scale, which is the close matching relationship between CNTs and catalysts, and how the growth model affects the chirality preference of single-walled carbon nanotubes (SWNTs). Experimentally, different methods to grow SWNTs with a uniform structure should be further developed, focusing on catalyst design to increase temperature tolerance and achieve epitaxial growth of SWNT segments. On the other hand, the large-scale synthesis of SWNTs should also be reconsidered, for instance, by improving the growth equipment. In order to identify suitable applications for different CNT products, standards should be established and adopted. In addition to improving CNT synthesis, the driving force of the CNT industry in the future will be finding disruptive applications of CNTs, whose functions and contributions are irreplaceable. In conclusion, still much progress is needed to achieve the complete commercialization of CNTs in the future. Nevertheless, the rapid development and continuous attention given to this field may lead to growth opportunities in the CNT industry.