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S U R F A C E S C I E N C E
ESPIONAGE AT THE ATOMIC SCALE Microscopy probes nanofiber growth process with angstrom resolution
L IKE INTELLIGENCE AGENTS
armed with high-tech surveillance gear, scientists in
Denmark are using sophisticated lab tools to uncover the clandestine side of chemical reaction dynamics related to the growth of carbon nanofibers. The atomic-scale spying has revealed surprising nanoparticle behavior that may prove useful in understanding a wide range of catalytic surface reactions and other molecular processes.
Growing carbon nanofibers by decomposing hydrocarbon gases on solid catalysts has intrigued researchers for decades. But probing the fundamental steps that drive the growth process has proven difficult to carry out with high resolution, in part because of the high temperature and pressure required to sustain the reaction.
Now, researchers at catalyst manufacturer HaldorTbpsoe and at the Technical University of Denmark, both in Lyngby, have tracked the carbon nanofiber growth process in real time and in unprecedented spatial detail. Using a specially designed transmission electron microscope, the team, which includes Stig Helveg, physics professor Jens K. Nor-skov, and their coworkers, exposed nanometer-sized nickel particles dispersed on MgAl204
to a methane-hydrogen mixture at roughly 500 °C. They recorded the results in angstrom-resolution videos [Nature, 427, 426 (2004)}.
The images capture a lively reaction scenario in which nanofiber growth is seen to be mediated by continuous and sudden changes in the shape of the nickel particles. (Videos can be viewed at C&ENOnline, http://wwwcen-online.org.) Reactions of the gas on the catalyst cause the approx
imately 10-nm spherical particles to become highly elongated, then abruptly return to a spherical shape, while nanofibers grow spontaneously from the particle's surface.
From a detailed inspection of the images and quantum mechanical calculations, the researchers conclude that nucle-ation and growth of the fibers' graphene layers occur at tiny defects in the nickel crystals known as single-atom step edges. The angstrom-sized imperfections are observed to form and then disappear repeatedly during the course of the reaction. Based on the experimental and computational results, the researchers propose that the growth mechanism depends on surface diffusion of carbon and nickel atoms.
Commenting in the same issue of Nature, PulickelM. Ajayan, a professor of materials science and engineering at Rensselaer Polytechnic Institute, remarks that the study "has overcome
EYE OF T H E F IBER An electron micrograph recorded under reaction conditions reveals the dynamic role of 10-nm nickel catalyst particles (blue) in forming carbon nanofibers (green) via hydrocarbon decomposition.
some major experimental stumbling blocks to provide the first direct glimpses of nanofibers as they grow" Ajayan notes that the experiments show the importance of shape transformations in particles used as nanoscale growth catalysts. He adds that the work "should lead to better control over fiber synthesis for nano-technology"—MITCH JACOBY
F E R T I L I Z E R
IMCP Cargill Merge Crop Nutrient Business
IMC Global and agribusiness giant Cargill have signed a definitive agreement to combine their phosphate, potash, and nitrogen
assets to create the second largest global fertilizer operation, with $4.1 billion in annual sales.
When the deal closes later this summer -subject to regulatory approval—the new, as-yet-unnamed publicly owned company wil l be larger than number three Potash Corp. of Saskatchawan, with estimated annual sales of $2.6 billion. Number one Yara, the Norsk Hydro fertilizer business to be spun off to shareholders later this year, has sales of about $4.9 billion.
In return for two-thirds of the new firm's shares, Cargill is to contribute its fertilizer operations with $2 billion in sales and $50
million in debt. IMC turns over a business with $2.1 billion in sales and $2.1 billion in debt. IMC shareholders wil l own one-third of the new firm, which is to be headed by Fritz Corrigan, now Cargill executive vice president.
For IMC, the merger means allying with a strong partner after years of losses caused by industry overcapacity and a farm recession. Executives say the fertilizer industry is poised for a recovery.
For Cargill, the largest U.S. privately held firm, the deal means enlarging its position in fertilizers at the expense of going public—the first time an operation of the secretive family-owned business wil l be open to public scrutiny—MARC REISCH
HTTP: / /WWW.CEN-ONLINE.ORG C & E N / F E B R U A R Y 2 , 2 0 0 4 7
