Scientists can now peer into the inner workings of catalyst nanoparticles
3,000 times smaller than a human hair within nanoseconds.
The findings point the way toward future work that could greatly improve catalyst
efficiency in a variety of processes that are crucial to the world’s energy
security, such as petroleum catalysis and catalyst-based nanomaterial growth
for next-generation rechargeable batteries. The work was performed in a collaborative
effort by Lawrence Livermore
National Laboratory and the University of California at Davis.
Making adjustments to the dynamic transmission electron microscope. From left: Curtis Brown, Thomas LaGrange and Judy Kim.
Using a new imaging technique on Lawrence Livermore’s Dynamic Transmission
Electron Microscope (DTEM), researchers have achieved unprecedented spatial
and temporal resolution in single-shot images of nanoparticulate catalysts.
The DTEM uses a laser-driven photocathode to produce short pulses of electrons
capable of recording electron micrographs with 15-nanosecond (one billionth
of a second) exposure time. The recent addition of an annular dark field (ADF)
aperture to the instrument has greatly improved its ability to time-resolve
images of nanoparticles as small as 30 nanometers in diameter.
“Nanoparticles in this size range are of crucial importance to a wide
variety of catalytic process of keen interest to energy and nanotechnology researchers,”
said UC Davis’ Dan Masiel, formerly of LLNL and lead author of a paper
appearing in the journal, ChemPhysChem. “Time-resolved imaging of such
materials will allow for unprecedented insight into the dynamics of their behavior.”
Previously, particles smaller than 50 nanometers could not be resolved in the
15-nanosecond exposure because of the limited signal and low contrast without
ADF aperature. But by using DTEM’s ADF, almost every 50-nanometer particle
and many 30-nanometer ones became clearly visible because of the fast time resolution
and high contrast.
“The stark difference between these two images clearly demonstrates the
efficacy of annual dark field imaging when imaging samples with feature sizes
near the resolution limit of DTEM,” Masiel said.
The new technique makes it easier to discern significant features when compared
to bright field pulsed imaging. It allows for vastly improved contrast for smaller
particles, widening the range of catalyst systems that can be studied using
DTEM.
DTEM can record images with six orders of magnitude higher temporal resolution
than conventional TEM and can provide important insights into processes such
as phase transformations, chemical reactions and nanowire and nanotube growth.
Co-authors include LLNL’s Bryan Reed, Thomas LaGrange, Geoffrey Campbell,
Ting Guo and Nigel Browning. The work was funded by the Department of Energy’s
Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering
Division.
The article appears in the May 27 online edition of ChemPhys Chem.