(615g) Formation, Structure, and Poisoning of Catalyst Nanoparticles During Growth of Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) are generally synthesized by chemical vapor deposition (CVD) techniques through catalytic decomposition of hydrocarbons on metallic nanoparticles. An appropriate selection of CVD process variables has been suggested to be the key of preferential growth of SWCNTs over other kind of carbon species such as amorphous carbon, graphite, and graphene. Temperature, pressure, rate of addition of carbon precursor species and composition of the precursor gas phase are some of the CVD parameters to be adjusted in order to gain SWCNTs selectivity. However, the metallic catalyst nature and geometry are perhaps the most determinant variables, not only on the preferential growth of SWCNTs, but also on the definition of SWCNTs diameter and chirality. In this study, reactive molecular dynamics simulations are carried out with the objective of developing a thorough analysis of the metallic catalyst behavior during SWCNT synthesis, identifying the critical parameters that assure long catalytic life avoiding graphitic poisoning. We have shown that low metal-carbon interactions, high temperatures, and small catalyst particles tend to favor the cap lift-off over graphitic encapsulation.^1-2 Here we discuss how a substrate strongly attracting metal atoms may define the catalyst particle structure and the exposed facets for carbon deposition. Moreover, our simulations show that the metal-substrate strength influences the quality of the growing nanotube and may lead to formation of multiple-walled carbon nanotubes³ on large diameter catalyst particles, in agreement with experimental results.⁴ Finally, we present preliminary results on the clustering process of catalytic nanoparticles during the initial stages of the SWCNT synthesis process, where we observe simultaneous metal atoms agglomeration and catalytic activity. In addition, cluster sintering may occur once carbon structures are already formed on the nanoparticle surface, which may induce sudden diameter/chirality changes during growth.       

References

            (1)        Ribas, M. A.; Ding, F.; Balbuena, P. B.; Yakobson, B. I. The Journal of Chemical Physics 2009, 131, 224501.

            (2)        Burgos, J. C.; Reyna, H.; Yakobson, B. I.; Balbuena, P. B. The Journal of Physical Chemistry C 2010, 114, 6952.

            (3)        Burgos, J. C.; Jones, E.; Balbuena, P. B. The Journal of Physical Chemistry C 2011, 115, 7668.

            (4)        Yamada, T.; Namai, T.; Hata, K.; Futaba, D. N.; Mizuno, K.; Fan, J.; Yudasaka, M.; Yumura, M.; Iijima, S. Nat Nano 2006, 1, 131.