(Invited) In Situ Study of Cobalt-Tungsten and Cobalt Nanocrystals Under Reactive Environment

Abstract

Revealing the catalyst structure in the working environment is crucial for understanding the controlled growth mechanism of single-walled carbon nanotubes (SWNTs). We used the aberration-corrected environmental transmission electron microscope (ETEM) combined with electron energy loss spectroscopy to detect the Co7W6 catalyst structure and composition in the reaction environment. With the quantitative analysis of high-resolution image, we showed that the Co7W6 catalyst structure keep stable with carbon feeding at 700-1100 ○C. Carbon caps were clearly observed to nucleate from the Co7W6 nanocrystals from ETEM. We also established in situ synchrotron X-ray absorption spectroscopy operated in the atmospheric-pressure CVD and demonstrated the stable local coordination structure of Co7W6. No carbides or carbon diffusion induced structural change was observed from these in situ characterization. It reveals that Co7W6 catalysts act as the stable structure template for the chirality controlled growth of SWNTs. Cobalt catalyzed SWNT growth is generally believed to carry out via a vapor–liquid–solid mechanism. We investigated the synergetic role of Co3C in Co catalyzed SWNT growth by ETEM. The carbon atoms disperse from surface into bulk of Co nanoparticle. The metallic Co is effective for catalyzing the formation of a cap, while Co3C, depending on the carbon feeding rate, is essential for subsequently elongating of the carbon cap. The growth of SWNTs is realized only by the cooperation of Co with Co3C in proper carbon feeding condition. These findings are helpful for the rational design of catalysts with high activity for SWNT growth.