Atomic-Scale Evidence of Catalyst Evolution for the Structure-Controlled Growth of Single-Walled Carbon Nanotubes.

Abstract

ConspectusKnowing how nanomaterials nucleate and dynamically evolve at the nanoscale is crucial to understanding and in turn controlling the structure and properties of a wide variety of materials, among which single-walled carbon nanotubes (SWCNTs) with chirality-dependent properties is a typical example. Catalyst takes a central role in guiding the SWCNT growth. An in-depth understanding of the growth mechanism of SWCNTs requires knowledge of the catalyst dynamic behavior during the chemical vapor deposition process, where real-time atomic-scale observations are needed. The high spatial, temporal, and energy resolution makes the state-of-the-art aberration-corrected environmental transmission electron microscope (ETEM) a superior tool for tracking the catalyst evolution and the SWCNT growth.Several key factors and processes, including the catalyst stability, carbon diffusion pathway, nucleation site, and growth modes of nanotubes, greatly influence the structure of SWCNTs. This Account summarizes our recent progress in the ETEM investigation of the dynamic catalyst behavior and nucleation of SWCNTs. We first compare the different growth modes of SWCNTs on two types of catalyst-stable solid intermetallic Co7W6 and unstable monometallic catalysts. Then we address the origin of different growth modes and chirality selectivity by revealing the atomic-scale stability and evolution of catalysts under carbon feed conditions and the observation of the in situ growth of SWCNTs on catalysts. We also discuss the catalyst-support interaction and the possible influence on SWCNT growth. In the end, we summarize the present achievements and future challenges.We carefully compare the difference in the ordinary Co catalyst and Co7W6 catalyst which has shown great chirality selectivity in SWCNT growth. Direct imaging by ETEM demonstrated that solid catalysts initiated the growth of SWCNTs with diameters smaller (dNT) than those of the catalyst particles (dNP) (dNT < dNP), whereas molten catalyst nanoparticles produced SWCNTs with similar diameters (dNT ≈ dNP). ETEM combined with in situ synchrotron X-ray absorption spectroscopy demonstrated that the Co7W6 catalyst maintained a solid stable structure under carbon feed conditions at 700-1000 °C, demonstrating the feasibility in acting as a structure template to grow SWCNTs. By contrast, the state and composition of the Co catalyst were changing during SWCNT growth. The near-surface lattice spacings of Co7W6 remained unchanged under carbon feed condition with carbon diffusion on the surface, whereas the solid Co catalyst underwent dynamic expansion and contraction due to carbon penetration into and precipitation out of Co nanoparticles. These two different pathways of carbon diffusion on or in catalysts indicate the distinctly different growth mechanisms of SWCNTs: the epitaxial growth of SWCNTs with specified chirality on the facets of Co7W6 nanocrystals and the nonselective growth of SWCNTs by the Co catalyst with Co/CoC3 as the active species. Besides the SWCNT-catalyst interface, the catalyst-support interface is also of importance in SWCNT growth. The atomic-scale information on catalyst dynamics provides a deep mechanistic understanding of SWCNT growth and will boost the development of the structure-controlled synthesis of SWCNTs and other nanomaterials.