Berkeley Lab's National
Center for Electron Microscopy (NCEM) provided the technology and a Visiting
Scientist Fellowship that helped a Missouri State University researcher make
a key discovery which should boost efforts to use carbon nanotubes as catalytic
supports in direct ethanol fuel cells.
The TEM image in (a) shows platinum nanoparticles (black specks) on a bundle of single-walled carbon nanotubes. Under higher magnification in (b), the nanotubes start to separate from one another and the configuration of the platinum particles (dashed circles) along the nanotubes is revealed.
Using the advanced characterization capabilities of NCEM’s TEAM 0.5 and
Tecnai microscopes, materials scientist Lifeng Dong found that single-stranded
DNA can be used to disperse bundles of single-walled carbon nanotubes into individual
tubes. The single strands of DNA can also serve as guideposts for synthesizing
platinum nanoparticles onto these tubes.
“Without the Visiting Scientist Fellowship from NCEM, I would not have
had the opportunity to work with NCEM scientists and to use state-of-the-art
microscopes to characterize those samples,” Dong wrote in a letter to
NCEM director Uli Dahmen. Dong acquired his images at TEAM 0.5 and the 200 kV
Tecnai with the help of Berkeley Lab staff at NCEM including Christian Kisielowski,
Thomas Duden, Masashi Watanabe, Zonghoon Lee and ChengYu Song.
Portable fuel cells powered directly by ethanol have the potential to be far
more efficient than ethanol-powered combustion engines and far more practical
than hydrogen fuels cells, as ethanol is easier to store and transport than
hydrogen. What’s been missing for the production of direct ethanol fuel
cells is a good catalyst for oxidizing ethanol.
Platinum-coated single-walled carbon nanotubes (SWCNTs) show bright promise
for this task because of their high electronic conductivity and surface area.
However, it is the nature of these single-walled nanotubes to form bundles.
For them to be effectively used as supporters of platinum catalysts in direct
ethanol fuel cells, efficient ways must be found to separate bundled SWCNTs
into individual tubes and to synthesize platinum nanoparticles on the nanotubes.
“Our images show that platinum nanoparticles selectively grow on carbon
nanotubes in accordance with single-stranded DNA locations,” Dong says.
“The DNA molecules not only effectively disperse SWCNT bundles into individual
tubes, but also provide an address for the formation of platinum nanoparticles
along the nanotube surfaces. This suggests a method to synthesize other types
of carbon nanotube-supported nanoparticles, such as palladium and gold, for
applications in fuel cells and nanoscale electronics.”
The acronym TEAM stands for Transmission Electron Aberration-corrected Microscope.
TEAM 0.5 is capable of producing images with a resolution of one-half angstrom,
which is less than the diameter of a single hydrogen atom. TEAM 0.5 also has
the ability to correct for the image-degrading phenomenon known as spherical
aberration. The 200kV Tecnai microscope is optimized for materials research
that requires either the highest resolution scanning transmission electron microscopy
performance, meaning imaging and spectroscopy, or correlated imaging and analytical
“The biggest challenge for obtaining these images was that our microscopes
remain stable at their top performance levels,” says NCEM staff member
Song, who provides support for the 200 kV Tecnai microscope. “As we image
a sample at the atomic scale, any instability in the microscope is magnified
millions of times with the image. At NCEM we routinely check the performances
of our microscopes and look after any optical, mechanical, or electrical disturbances.”