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3-D IMAGES OF NANOPARTICLES
Tomography method yields structure of silica-supported nanocatalyst
MICHAEL FREEMANTLE
Z-contrast tomography, a novel analytical technique developed at the University of Cambridge, generates three-dimensional images of catalyst nanoparticles inside the pores of a silica support.
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UNDER THE MICROSCOPE Thomas (left), Weyland, and Midgley create images from high-angle scattered electrons.
PHOTO BY MICHAEL FREEMANTLE |
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The researchers used the technique, which employs a scanning transmission electron microscope (STEM) and a new form of a process known as electron tomography, to create 3-D images of palladium-ruthenium bimetallic catalyst particles inside mesoporous silica [Chem. Commun., 2001, 907].
The work was carried out by lecturer in materials science Paul A. Midgley, Ph.D. student Matthew Weyland, and chemistry professors John Meurig Thomas and Brian F. G. Johnson.
The technique, which is nondestructive, uses an STEM electron beam with a 0.8 nm diameter to scan a specimen of the supported catalyst. A detector collects transmitted electrons that are scattered at high angles. The scan results in an image that is a 2-D projection of the catalyst specimen. The specimen is then tilted in the electron beam through a series of finely spaced angles, and the scans are repeated for each angle in the series. The 3-D image is reconstructed mathematically from the 2-D projections.
The term "electron tomography" is applied to any technique that employs a transmission electron microscope to collect a tilt series of 2-D projections of an object and that uses these projections to reconstruct a 3-D image of the object. It was first used in the biological sciences to reconstruct the morphology of bacteria with a resolution of around 100 nm.
The team calls the new technique "Z-contrast tomography" because the formation of the image depends on the mathematical relationship between the intensity of the scattered electrons and the atomic number (Z) of the scattering atom. In particular, it depends on the contrast between the high-Z particles of the bimetallic catalyst and the low-Z silica.
"To our knowledge, this is the first time high-angle scattered electrons have been used to form images for an electron tomography experiment," Midgley says.
The 3-D images of the bimetallic catalyst reveal that the nanoparticles have a clear size distribution, with larger particles sitting on the outside of the silica and smaller ones within the silica channels. "The larger particles are those in which the original Pd6Ru6 metal clusters have coalesced," the team notes.
Z-contrast tomography is ideal for the study of many similar catalyst structures, Midgley suggests. "By operating in scanning mode, beam damage to the silica is minimized because only the region under the probe is illuminated at any one time," he says. "We can therefore examine specimens for many hours using this technique. This is not possible with conventional fixed-beam parallel illumination."
The work is elegant, according to Pratibha L. Gai, who leads the microstructural competency section at DuPont and is adjunct professor of materials science at the University of Delaware.
"The method is clearly a significant advance in nanostructural analysis and has important applications in nanotechnology in both the inorganic and biological sciences," she says.
The Cambridge group is now using the technique to determine the 3-D structures of other catalysts and other inorganic materials.
"We have applied the new tomography technique to examine the crystal shape, size, and faceting of sub-100-nm magnetite crystals found in magneto-tactic bacteria," Midgley says. "We are also using energy-filtered TEM [EFTEM] to study the 3-D chemical segregation at grain boundaries in steels and other alloys. Such an EFTEM tomography technique may rival atom-probe tomography in the future."
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