Haimei Zheng, a staff Scientist in Berkeley Lab’s Materials Sciences Division and DOE Early Career Research Program Awardee, led the observation of how attached nanoparticles evolve into nanorods. (Photo by Roy Kaltschmidt) |
In
the growth of crystals, do nanoparticles act as “artificial atoms”
forming molecular-type building blocks that can assemble into complex
structures? This is the contention of a major but controversial theory
to explain nanocrystal growth. A study by researchers at the U.S.
Department of Energy (DOE)’s Lawrence Berkeley National Laboratory
(Berkeley Lab) may resolve the controversy and point the way to energy
devices of the future.
Led
by Haimei Zheng, a staff scientist in Berkeley Lab’s Materials Sciences
Division, the researchers used a combination of transmission electron
microscopy and advanced liquid cell handling techniques to carry out
real-time observations of the growth of nanorods from nanoparticles of
platinum and iron. Their observations support the theory of
nanoparticles acting like artificial atoms during crystal growth.
“We
observed that as nanoparticles become attached they initially form
winding polycrystalline chains,” Zheng says. “These chains eventually
align and attach end-to-end to form nanowires that straighten and
stretch into single crystal nanorods with length-to-thickness ratios up
to 40:1. This nanocrystal growth process, whereby nanoparticle chains as
well as nanoparticles serve as the fundamental building blocks for
nanorods, is both smart and efficient.”
Zheng is the corresponding author of a paper describing this research in the journal Science. The
paper is titled “Real-Time Imaging of Pt3Fe Nanorod Growth in
Solution.” Co-authors are Hong-Gang Liao, Likun Cui and Stephen
Whitelam.
If
the near limitless potential of nanotechnology is to even be
approached, scientists will need a much better understanding of how
nano-sized particles can assemble into hierarchical structures of
ever-increasing organization and complexity. Such understanding comes
from tracking nanoparticle growth trajectories and determining the
forces that guide these trajectories.
Through
the use of transmission electron microscopy and liquid observation
cells, scientists at Berkeley Lab and elsewhere have made significant
progress in observing nanoparticle growth trajectories, including the
oriented attachment of nanoparticles—the chemical phenomenon that starts
the growth of nanocrystals in solution. However, these observations
have typically been limited to the first few minutes of crystal growth.
In their study, Zheng and her colleagues were able to extend the time of
observation from minutes to hours.
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“The
key to studying the growth of colloidal nanocrystals with different
shapes and architectures is to maintain the liquid in the viewing window
long enough to allow complete reactions,” Zheng says. “We dissolved
molecular precursors of platinum and iron in an organic solvent and used
capillary pressure to draw the growth solution into a silicon-nitride
liquid cell that we sealed with epoxy. The sealing of the cell was
especially important as it helped keep the liquid from turning viscous
over time. Previously, we’d often see the liquids become viscous and
this would prevent the nanoparticle interactions that drive crystal
growth from taking place.”
Zheng
and her colleagues chose to study the growth of platinum iron nanorods
because of the electrocatalytic material’s promising potential for use
in next generation energy conversion and storage devices. They were able
to observe these nanoparticles assemble into nanorod crystals using
powerful transmission electron microscopes at Berkeley Lab’s National
Center for Electron Microscopy, including TEAM 0.5 (Transmission
Electron Aberration-corrected Microscope), which can produce images with
half?angstrom resolution—less than the diameter of a single hydrogen
atom.
“From
what we observed only single nanoparticles exist at the beginning of
crystal growth, but, as growth proceeds, small chains of nanoparticles
become dominant until, ultimately, only long chains of nanoparticles can
be seen,” Zheng says. “Our observations provide a link between the
world of single molecules and hierarchical nanostructures, paving the
way for the rational design of nanomaterials with controlled
properties.”
This research was supported by the DOE Office of Science.
