Conspectus
Knowing 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
(d
NT) than those of the catalyst particles
(d
NP) (d
NT < d
NP), whereas molten catalyst nanoparticles
produced SWCNTs with similar diameters (d
NT ≈ d
NP). 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...
