A review of carbon nanotube synthesis via fluidized-bed chemical vapor deposition

Carbon nanotubes (CNTs) are crystalline, tubular, carbon structures with extraordinary mechanical, chemical, optical, and electrical properties. These unique properties make CNTs potentially valuable in a wide range of end-use applications. Currently, research into nanotubes and their applications is hampered by the lack of a suitable technique for manufacturing them in large quantities, which we define here as 10 000 tons per plant per year. Consequently, research into large-scale manufacturing techniques is ongoing. There are three established methods of CNT synthesis: (i) arc discharge, (ii) laser ablation, and (iii) chemical vapor deposition (CVD). Of these, CVD techniques show the greatest promise for economically viable, large-scale synthesis, based upon yields reported in the literature and the inherent scalability of similar technologies, e.g., fluidized catalytic cracking. In particular, the fluidized-bed CVD (FBCVD) technique (where the CVD reaction occurs within a fluidized bed of catalyst particles) has the potential to produce high-quality CNTs, inexpensively in large quantities. In this work, we review the existing literature on CNT synthesis via FBCVD and demonstrate that no systematic study of the key parameters has been undertaken. We demonstrate that there is no clear understanding of the influence of key variables (e.g., temperature, pressure, and carbon source) on CNT properties (e.g., CNT diameter, length, and morphology), which shows that further research is necessary to optimize this process and, ultimately, understand the science behind CNT growth using this technique. We also highlight the assessment of CNT quality as being an issue for further research. CNT quality is usually characterized visually (e.g., from TEM images), and a standard CNT characterization protocol has yet to be elucidated. The successful scale-up of any process requires detailed understanding of key process parameters and their interactions. Hence, further research to optimize the purity, yield, and selectivity of the synthesized CNTs is required before the potential of this technique can be realized.